Structure and Reactivity of Dehydroxylated Bronsted Acid

STRUCTURE AND REACTIVITY OF DEHYDROXYLATED BRONSTED ACID

SITES IN H-ZSM-5 ZEOLITE:

GENERATION OF STABLE ORGANIC RADICAL CATION AND CATALYTIC

ACTIVITY FOR ISOBUTANE CONVERSION

Jang Ho Yun

A thesis submitted to the Faculty of the University of Delaware in partial fulfillment of the requirements for the degree of Master of Chemical Engineering

Summer 2011

STRUCTURE AND REACTIVITY OF DEHYDROXYLATED BRONSTED ACID

SITES IN H-ZSM-5 ZEOLITE:

GENERATION OF STABLE ORGANIC RADICAL CATION AND CATALYTIC

ACTIVITY FOR ISOBUTANE CONVERSION

Jang Ho Yun

Approved: ______Raul F. Lobo, Ph.D. Professor in charge of thesis on behalf of the Advisory Committee

Approved: ______Norman J. Wagner, Ph.D. Chair of the Department of Chemical Engineering

Approved: ______Babatunde A. Ogunnaike, Ph.D. Interim Dean of the College of Engineering

Approved: ______Charles G. Riordan, Ph.D. Vice Provost for Graduate and Professional Education

TABLE OF CONTENTS

LIST OF TABLES ...... v LIST OF FIGURES ...... vi LIST OF SCHEMES...... viii ABSTRACT ...... ix

Chapter

1 INTRODUCTION ...... 1

1.1 Introduction to Zeolites ...... 1 1.2 Activation Mechanisms of Zeolites as Acid Catalysts ...... 3

1.2.1 Activation of Hydrocarbons on Bronsted Acid Sites...... 3 1.2.2 Heterolytic Pathway of Dehydroxylation of Bronsted Acid Sites ...... 6 1.2.3 Homolytic Pathway of Bronsted Acid Sites Dehydroxylation ...... 8 1.2.4 Properties of the Sites Formed by Dehydroxylation of Bronsted Acid Sites ...... 10

1.3 The FCC Process and Effect of Redox Chemistry ...... 11

1.3.1 FCC Process and Zeolite...... 11 1.3.2 Effect of Redox Chemistry on Hydrocarbon Chemistry ...... 14

1.4 Thesis Outline ...... 15 1.5 References ...... 17

2 EXPERIMENTAL METHOD ...... 20

2.1 Introduction ...... 20 2.2 Zeolite ZSM-5 Synthesis ...... 21 2.3 Sample Pre-treatment ...... 22 2.4 Sample Characterization ...... 25

2.4.1 X-ray Powder Diffraction ...... 25 2.4.2 Scanning Electron Microscopy (SEM) ...... 26 2.4.3 N2 Adsorption Isotherm ...... 27

2.5 UV/vis Spectroscopy ...... 29 2.6 Temperature Programmed Desorption (TPD) of Ammonia ...... 32

iii

2.7 Fourier Transform Infrared Spectroscopy (FTIR) ...... 35 2.8 Gas Chromatography (GC) ...... 37 2.9 Reactor Setup for GC ...... 38 2.10 Summary ...... 39 2.11 References ...... 41

3 GENERATION OF STABLE ORGANIC RADICAL CATIONS IN THERMALLY TREATED ZSM-5 ZEOLITES ...... 43

3.1 Introduction ...... 43 3.2 Generation of Naphthalene Radical Cations ...... 44 3.3 Migration of Electrons and Holes within the Zeolite Framework ...... 53 3.4 Structure of the Dehydroxylated Sites ...... 60

3.4.1 Ammonia TPD ...... 60 3.4.2 IR Spectroscopy and the Thermal Decomposition of Bronsted Acid Sites ...... 63 3.4.3 Effect of Si/Al Ratio ...... 65

3.5 Conclusions ...... 71 3.6 References ...... 73

4 EFFECT OF HIGH TEMPERATURE ON THE CATALYTIC ACTIVITY OF ZEOLITE H-ZSM-5 FOR ISOBUTANE CONVERSION ...... 75

4.1 Introduction ...... 75 4.2 Kinetic Analysis of Isobutane Cracking on Zeolites ...... 78 4.3 Effect of Pre-treatment of the Sample on Isobutane Conversion and Selectivity ...... 81 4.4 Conclusions ...... 92 4.5 References ...... 94

5 CONCLUSIONS AND FUTURE RESEARCH DIRECTIONS ...... 96

5.1 Summary ...... 96

5.1.1 Generation of Organic Radical Cations in Thermally Treated ZSM-5 Zeolites ...... 97 5.1.2 Catalytic Activity of ZSM-5 Zeolite for Isobutane Conversion ...... 97

5.2 Future Research Directions ...... 98

5.2.1 Determination of the Structure of Lewis Acid Sites and Redox Sites ...... 98 5.2.2 Low Temperature CO Oxidation ...... 100

5.3 References ...... 102

LIST OF TABLES

Table 2.1 Temperature-programmed desorption protocol corresponding to the high-temperature treatment of the zeolite samples...... 33

Table 2.2 Temperature-programmed desorption protocol corresponding to the oxygen treated zeolites...... 33

Table 3.1 The desorption areas and the maximum temperatures of TPD signal...... 68

Table 4.1 Thermodynamic properties for cracking and dehydrogenation reaction of isobutane...... 78

Table 4.2 Conversion of isobutane over ZSM5 for three different treatments...... 82

Table 4.3 TOF per aluminum for ZSM5-12 and for ZSM5-18 after treatment 1...... 83

Table 4.4 Contribution of newly generated sites and remaining Bronsted acid sites after treatment 2...... 84

Table 4.5 Measured activation energies for isobutane cracking and dehydrogenation after each treatment...... 87

Table 4.6 Ionization potentials of light hydrocarbons ...... 88

LIST OF FIGURES

Figure 1.1 Poresize of (a) chabazite, (b) ZSM-5, and (c) beta zeolite ...... 2

Figure 1.2 Bronsted acid site ...... 4

Figure 1.4 Schematic diagram of FCC process ...... 13

Figure 2.1 Diagram of the reactor used for sample pre-treatment ...... 24

Figure 2.2 Temperature protocol for the sample pre-treatment ...... 24

Figure 2.3 Experimental protocol for detecting the generation of radical cations ...... 25

Figure 2.4 Sample XRD pattern for ZSM-5 ...... 26

Figure 2.5 (a) SEM image for ZSM-5 and (b) EDAX spectrum analysis ...... 27

Figure 2.6 (a) The schematic layout of UV/vis spectroscopy, (b) optical geometry of the integrating sphere...... 30

Figure 2.7 UV/vis spectrum of naphthalene radical cation in ZSM-5 (Zeolyst, SiO2/Al2O3 = 15) ...... 31

Figure 2.8 Temperature profile with time and the gases during TPD corresponding oxygen treatment...... 34

Figure 2.9 The IR cell – a heater, a reactor, and a gas line ...... 37

Figure 2.10 GC system setup used to study isobutane cracking process ...... 39

Figure 3.1 UV/vis spectra of pure zeolite and naphthalene adsorbed on ZSM-5 zeolite as control experiments: Pure ZSM-5, ZSM-5 in Ar at 500 °C, Pure Silicalite-1, Silicalite-1 in Ar at 500 °C, and 780 °C ...... 46

Figure 3.2 UV/visible spectra for control experiments (ZSM-5 in Ar at 500 °C, Silicalite-1 in Ar at 500 °C and 780 °C) and for naphthalene@ZSM-5 heated to 780 °C in Ar and 500 °C in O2 ...... 48

Figure 3.3 Rescaled UV/vis spectra for naphthalene adsorbed into ZSM-5 heated to 780 °C in Ar and to 500 °C in O2 ...... 49

Figure 3.4 (a) The UV/visible spectra after mixing with the naphthalene and (b) intensity change with time of the 695 nm peak of the samples treated at different temperatures...... 51

Figure 3.5 Mechanism of the single-electron migration in ZSM-5 framework...... 53

Figure 3.6 Time-series evolution of the UV/visible spectra of NPT@ZSM5 after treatment 2 during (a) increasing and (b) decreasing absorption intensity...... 55

Figure 3.7 Time-series evolution of the UV/visible spectra of NPT@ZSM5 after treatment 3 during (a) increasing and (b) decreasing absorption intensity...... 56

Figure 3.8 (a) Pure spectral components and (b) concentration change with time for NPT radical cation and electron-hole pair on the sample after treatment 2 ...... 58

Figure 3.9 (a) Pure spectral components and (b) concentration change with time for NPT radical cation and electron-hole pair on the sample after treatment 3 ...... 59

Figure 3.10 TPD corresponding for (a) treatment under Ar at 780 °C and (b) treatment under O2 at 500 °C...... 62

Figure 3.11 FTIR spectra in the OH vibration region of ZSM-5 measured (a) after heated to 500 and 800 °C, and (b) after heated to 500 and 500 °C in an oxygen flow...... 64

Figure 3.12 TPD of ZSM-5 having Si/Al of 12 compared with TPD of ZSM-5 having Si/Al of 18...... 67

Figure 3.13 UV/visible spectra of ZSM-5 sample with different Si/Al ratios for (a) treatment 2 and (b) treatment 3...... 70

Figure 4.1 The cracking-to-dehydrogenation ratios with temperature for (a) ZSM5-18 and (b) ZSM5-12...... 86

Figure 4.2 Arrhenius plots for isobutane cracking and dehydrogenation (a) ZSM5-18 and (b) ZSM5-12...... 90

Figure 5.1 CO2 production from naphthalene-ZSM-5 catalyst ...... 101

vii

LIST OF SCHEMES

Scheme 1.1 Initial protonation of C-H or C-C bonds of isobutane on Bronsted acid sites in zeolites ...... 5

Scheme 1.2 Heterolytic Bronsted acid site decomposition pathway ...... 8

Scheme 1.3 Bronsted acid sites Homolytic Dehydroxylation ...... 10

Scheme 4.1 Protolytic mechanism of isobutane (cracking and dehydrogenation) ...... 79

Scheme 4.2 Suggested pathway of monomolecular reaction of isobutane over redox sites on ZSM-5...... 92

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ABSTRACT

Zeolites are crystalline aluminosilicate materials that have wide application in industry as solid acid catalyst. Since zeolites have high acidity, high surface area as well as the ability to do shape selectivity, they are used primarily as a solid catalyst in oil refining and petrochemical industries, in processes such as hydrocracking, fluid catalytic cracking (FCC). Bronsted acid sites are described as a hydroxyl group bridged between

Al and Si (Al-OH-Si). It is well known that Bronsted acid sites are significant in a number of hydrocarbon processes, such as alkane cracking and isomerization and many other.

The Bronsted acid sites of zeolites are decomposed at high temperatures, usually above 600 °C. This high temperature condition is commonly found in the fluidized catalytic cracking where catalyst is recycled forth and back between the riser and the regenerator under an oxidative atmosphere. The process of decomposition of hydroxyl group from the initial structure is called dehydroxylation. The dehydroxylation is believed to proceed via a dehydration mechanism of the acid sites. This heterolytic pathway of Bronsted acid site decomposition has been the accepted dehydroxylation path for low-silica zeolites for decades, although the molecular details of the structure remaining inside the zeolites are still unknown. However, our group reported that hydrogen is also formed during the dehydroxylation process. Our group has also

proposed a new pathway to explain the decomposition of Bronsted acid sites of high-

0 silica zeolites and the formation of [AlO4] sties in zeolites.

Oxidized zeolites are known to extract electrons from molecules having small ionization potential. Since oxidation of zeolites is considered to lead to the dehydroxylation of Bronsted acid sites, we suggest that the dehydroxylated Bronsted acid sites are responsible for the electron-transfer process. Using naphthalene as a probe molecule, it can be shown that the new sites have the ability to extract an electron from naphthalene and form stable radical cations. We investigated the formation of these new sites by thermal treatment and oxidation treatment. A series of UV/vis spectra showed that after naphthalene radical cations were generated, single-electron transfers back into the ZSM-5 framework to form a stable electron-hole pair and reform the naphthalene neutral molecule. Using ammonia TPD, IR spectra, and UV/vis spectra of the sample with different Si/Al ratios, the structure of the new generated sites was characterized.

These observations suggest that the most common site generated is different depending on each treatment.

The activation of small alkanes over acid sites has been investigated extensively because of its relevance to technologically important processes such as fluidized catalytic cracking in petroleum refineries, but also because C-H and C-C bond activation is of fundamental scientific interest. The reactivity and selectivity of newly generated sites is investigated using isobutane conversion. The conversions of the samples, which were treated by high temperature treatment and oxygen treatment for dehydroxylation, are

greater than the conversions of the acid catalyst. When conversion is low, the product distribution is limited to the monomolecular cracking of the C-C bond and dehydrogenation of the C-H bond. The cracking-to-dehydrogenation ratio significantly increases after dehydroxylation treatments. Several groups have proposed that carbonium or carbenium ion intermediates on Bronsted acid sites in zeolites play the key role in the activation of isobutane. However, in this thesis, we proposed that the presence of redox sites resulted in radical cation chemistry instead of protolytic chemistry in the propane and isobutane cracking process.

Chapter 1

INTRODUCTION

1.1 Introduction to Zeolites

Zeolites are generally referred to as crystalline aluminosilicate materials with unique three dimensional framework structures and pore size (3-13Å in diameter) [1]. All zeolites have unique framework structures constituted by combining oxide tetrahedra

- such as SiO4 or AlO4 . Framework compositions can be extended by substitution of metal atoms like B, Fe, Ga, Mg, Mn, Ti, and Zn into the tetrahedral positions within the framework.

Zeolites properties such as adsorption capacity, molecular sieving, and catalytic activity are directly related to zeolite structure. The primary building unit of zeolites is the three-dimensional TO4 tetrahedron: By combination of primary building units, other structures, such as squares, pentagons, and octagons, called secondary building units

(SBU) can be formed [2]. The SBU consist of n-ring structures, in which n is commonly

4, 5, 8, 10, or 12. The linkage of SBUs can form cages and channels that are essential elements of all zeolite structures. Due to the large diversity of zeolites reported to data, the zeolite frameworks have been codified into framework type-codes describing only the framework topology, but not framework composition, distribution of the tetrahedral

atoms, or cell dimension. The approved number of the framework types is over 190 species according to the three-letter structure codes established by the International

Zeolite Association [3].

Different types of zeolites have different channel dimensions (Figure 1.1). For example, chabazite (CHA) has small pores formed by 8-membered rings, ZSM-5 (MFI) has medium pores (10-membered rings), and beta zeolite (*BEA) has large pores (12- membered rings). The shape selectivity of different zeolites is due, to a large extent, to differences in pore size. The different pore sizes allow for the selective adsorption of certain reactants, or the selective desorption of certain products and can inhibit or promote different reaction intermediates in catalytic reactions.

(a) (b) (c)

Figure 1.1 Poresize of (a) chabazite, (b) ZSM-5, and (c) beta zeolite

The chemical and physical properties and the application of zeolites are also determined to a great extent by the amount of aluminum in the framework of zeolite. The amount of aluminum in the zeolites framework is typically represented by the (atomic)

Si/Al ratio. The minimum Si/Al ratio is 1 due to the Loewenstein rule, which establishes

- that no Al-O-Al bond exists in a zeolite [4]. Since an alumina tetrahedron (AlO4 ) has a

+ negative charge while a silica tetrahedron (SiO4) is neutral, a counter ion, such as H or alkali-metal ion, must be present to balance the negative charge. The sites compensated by H+ form bridging hydroxyl group (Si-OH-Al) that are chemically and functionally

Bronsted acid sites. The Si/Al ratio affects the acidity of the zeolites: the total number of acid sites increases as the Si/Al ratio decreases but at the same time the acidity becomes weaker. Acid-base reactions, the most common class of industrial chemical reaction, and acid base catalysis can be applied to every area of the chemical industries, including the oil refining industry. Since zeolites have high acidity, high surface area as well as the ability to do shape selectivity, they are used primarily as a solid catalyst in oil refining and petrochemical industries, in processes such as hydrocracking, fluid catalytic cracking

(FCC), aromatization and isomerization [5-7].

1.2 Activation Mechanisms of Zeolites as Acid Catalysts

1.2.1 Activation of Hydrocarbons on Bronsted Acid Sites

The actual details of the hydrocarbon activation mechanism of zeolites in the processes mentioned above have not been completely established, although zeolites are

used as the main catalyst component and as additive in petrochemical processes such as

FCC. Bronsted acid sites are distorted tetrahedral structures of alumina-substituted zeolites (Figure 1.2) with a longer Al-O(H)-Si bond than the three Al-O-Si. Bronsted acid sites are described as a hydroxyl group bridged between Al and Si (Al-OH-Si). The

Bronsted acid form of a zeolite is obtained by exchanging the framework cations with an ammonium solution such as aqueous ammonium nitrate (NH4NO3). The acid site is formed by desorption of ammonia from such materials at elevated temperatures leaving a proton on the zeolite surface.

Figure 1.2 Bronsted acid site

It is well known that Bronsted acid sites are significant in a number of hydrocarbon processes, such as in the synthesis of ethylbenzene from ethylene and benzene, disproportionation of toluene to form xylenes and benzene, alkane cracking and isomerization and many others. In hydrocarbon conversion processes, Bronsted acid sites donate a proton to an absorbed species forming carbonium or carbenium ions, which are considered as transition states in alkane cracking reactions. The carbonium ion

intermediates on Bronsted acid sites in zeolites play an important role in the activation of alkanes: this is called protolytic activation of hydrocarbon. To be specific, on Bronsted acid sites in zeolites, the reaction of isobutane is initiated by protonation of C-H or C-C bonds [8]. First, a pentacoordinated carbonium ion is formed on the Bronsted acid site in zeolite, and then it is decomposed into carbenium ion, such as the t-butyl and propyl carbenium ion. Hydrogen is generated by the t-butyl pathway (dehydrogenation pathway) and methane is formed by the propyl pathway (cracking pathway).

Scheme 1.1 Initial protonation of C-H or C-C bonds of isobutane on Bronsted acid sites in zeolites

As the number of hydrogen atoms attached to the carbon atom from which the hydride ion is abstracted increases, the energy required for the formation of carbonium ion also increases. The high energy of formation decreases the stability of carbonium ions.

For example, the tertiary carbenium ion is so stable that the formation of carbonium ion is easy and prevalent, while there is no formation of carbonium ion since the methyl carbenium ion is the least stable. In a zeolite, the charge separation, which exists when a carbonium ion is formed, may occur over the oxygen atoms so that the micropore in the

zeolite surrounds the carbonium ion. In addition, cracking of heavy hydrocarbon is known to be faster than cracking of light hydrocarbon. For example, cracking of n-C18H38 is 20 times faster than cracking of n-C8H18 [9]. A longer chain length provides more chance to contact the hydrocarbon to the surface of a catalyst. Adsorption equilibrium data for hydrocarbons on zeolite also support this explanation.

1.2.2 Heterolytic Pathway of Dehydroxylation of Bronsted Acid Sites

The acid sites of zeolites are decomposed at high temperatures, usually above

600 °C [10, 11]. The process of decomposition of hydroxyl group from the initial structure is called dehydroxylation. For instance, in a FCC regenerator, the catalysts are treated at temperatures in the range of 670 °C - 720 °C, and dehydroxylation occurs under these conditions. As a result of dehydroxylation, Lewis acid sites are also generated. Since it is difficult to characterize and indentify the molecular structures of

Lewis acid sites, the research about Lewis acid sites is still on-going and there is no consensus as to the structure of the Lewis acid sites. Three-coordinated aluminum units have been typically proposed as the source of Lewis acidity [12, 13]. Other non-

+ +2 + framework aluminium moieties such as AlO , Al(OH) , Al(OH) 2, Al(OH)3, Al2O3 have also been suggested to be the true Lewis acids [14, 15]. The effect of Lewis acid sites on the catalytic activity and selectivity has been investigated and it has been revealed that the turnover rates of cracking and dehydrogenation are not related to Lewis acid sites

concentration [16]. Instead, Lewis acid sites increase the reaction rate by enhancing the adsorption of reactants [17].

Dehydroxylation has been investigated by various spectroscopy techniques including Al nuclear magnetic spectroscopy (NMR), Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) [18-23]. The trigonal structure has not been detected by NMR while octahedral and pentacoordinated aluminum have been observed [22, 23]. For instance, bridged Si-OH-Al hydroxyl groups have an absorption band around the 3600 cm-1 region of IR spectrum, and the intensity of this region decreases after zeolites are heated at elevated temperature above 600 °C [24]. An IR peak at 3720-3750 cm-1 is assigned to silanol groups [25]. The peak at 3666 cm-1 is assigned to aluminum in partially extraframework positions. The aluminum in a three-coordination environment is often proposed as the origin of peak at 3666 cm-1 [25]. CO adsorption studies have revealed that the peak at 3666 cm-1 is less acidic than the normal Bronsted acid sites and more acidic than the silanol group [25]. Extraframework aluminum [26], extra-lattice amorphous materials [12, 27], or the silanol group [28] has been considered to the assignment of the peak at around 3700 cm-1. Bugaev et al. found that by XANES only 5 -

10 % of aluminum has trigonal structure after dehydroxylation of mordenite [13].

The heterolytic dehydroxylation of Bronsted acid sites is illustrated in scheme 1.2

[11, 29], which in this case proceeds by dehydration [29, 30]. In the left side of this figure, the Bronsted acid sites are described as OH-groups. In this reaction two moles of

Bronsted acid sites react to give acid-base and positive-negative site pairs: one mole of aluminum in a trigonal structure (Lewis acid sites) and one mole of aluminum with a symmetric tetrahedral structure. The reverse of dehydroxylation can occur by inducing water from at the highest temperature of dehydroxylation to the temperature which water vapor can be present [31]. This heterolytic pathway has provided the idea that Lewis acid sites are important for the hydrocarbon cracking process at high temperature condition above 600 °C.

Scheme 1.2 Heterolytic Bronsted acid site decomposition pathway

1.2.3 Homolytic Pathway of Bronsted Acid Sites Dehydroxylation

The heterolytic pathway of Bronsted acid site decomposition has been the accepted dehydroxylation path for low-silica zeolites for decades [11, 29], although the molecular details of the structure remaining inside the zeolites are still unknown. In high- silica zeoites, the high energy is needed to decompose the Bronsted acid sites through the heterolytic pathway since the Bronsted acid sites are sparsely placed.

Our group recently examined the dehydroxylation of Bronsted acid sites of high- silica zeolites using mass spectrometry-temperature programmed desorption (MS-TPD)

[11]. We have found that the Bronsted acid sites of high-silica zeolites are decomposed to produce hydrogen and a small amount of water. The MS-TPD of two samples of ZSM-5 heated stepwise to 250 °C, 525 °C and 750 °C was carried out and only until the temperature reaches about 750 °C, a large amount of hydrogen is found. The amount of hydrogen is also related to the Si/Al ratio. With a small Si/Al ratio (high aluminum), less hydrogen gas is produced. An electron hole pair generation for H-ZSM-5 calculated by hybrid quantum mechanics and a shell-model ion-pair potential approach also support our group‟s observation [32]. These results of the MS-TPD experiment and electronic structure calculation show that Bronsted acid sites of high-silica zeolites are decomposed by a redox process, not by dehydration.

Also, it has been known that the acid form of zeolites treated at high temperature

(under dehydroxylation conditions) react with molecules having small ionization potentials to form stable radical cations [29, 33, 34]. These radical cations have been studied frequently by the electron-spin-resonance (ESR) because they give excellent high-resolution ESR spectra [35]. Alkenes, polyaromatics, nitrogen-, oxygen-, and sulfur- containing organic molecules and others have shown to form radical cations in zeolite after heating at high temperatures.

Our group has proposed a new pathway to explain the decomposition of Bronsted

0 acid sites of high-silica zeolites (scheme 1. 3) and the formation of [AlO4] sties in

zeolites [32, 33]. A single electron hole is generated on one of the oxygen atom surrounding aluminum atom by dehydroxylation. The active sites which are formed under the condition of dehydroxylation are considered nonacidic single-electron redox sites [36].

Scheme 1.3 Bronsted acid sites Homolytic Dehydroxylation

1.2.4 Properties of the Sites Formed by Dehydroxylation of Bronsted Acid Sites

Moissette and coworkers reported that oxidized zeolites can extract electrons from molecules having small ionization potential [26]. Since oxidation of zeolites is considered to lead to the dehydroxylation of Bronsted acid sites, we suggest that the dehydroxylated

Bronsted acid sites are responsible for the electron-transfer process. Using naphthalene as a probe molecule, it can be shown that the new sites have the ability to extract an electron from naphthalene and form stable radical cations. This hypothesis is the basis for the experiments described in Chapter 3 and 4.

The sites generated by dehydroxylation of Bronsted acid sites can also activate hydrocarbons and show a different selectivity with respect to the case when Bronsted acid sites activate hydrocarbons, that is, with the pristine zeolite. In our group‟s previous reports, the propane cracking process was investigated using zeolite catalysts, such as

ZSM-5, beta zeolites, and mordenite [37]. In this case, the monomolecular initiation step proceeds by two parallel pathways: cracking of the C-C bond and dehydrogenation of the

C-H bond. Before dehydroxylation, the distribution of products produced by the propane reaction is selective towards cracking by factor of about two. After dehydroxylation at the high temperature, the dehydrogenation rate increased significantly compared to the rate before dehydroxylation. Investigation of the distributions of products and the conversion of isobutane can clarify the structure of the reaction sites after Bronsted acid sites decomposition and the structure-activity relationships in hydrocarbon conversions. To this end, in Chapter 4 we investigate the isobutane reaction over pristine and activated zeolites.

1.3 The FCC Process and Effect of Redox Chemistry

1.3.1 FCC Process and Zeolite

Fluidized catalytic cracking (FCC) is an important process in oil refining and petrochemical industry. The FCC process currently accounts for 47% of the refinery catalyst market value estimated to be a $2.9 billion [38]. In addition, among the catalytic applications of zeolites, the FCC process accounts for over 95% of synthetic zeolites consumption on a per mass basis [39]. The FCC units were in operation at 400 petroleum refineries worldwide in 2006 [40, 41]. During 2009, the FCC units processed at total of

5,000,000 barrels per day of feedstock worldwide [42].

The FCC process converts vacuum distillation oils to high value products such as gasoline, olefinic gases, and other products. Specifically, carbon bonds are broken to convert preheated heavy hydrocarbons to light hydrocarbons, which are used as reactants for alkylation [43]. The cracking process is catalyzed using very short contact times (3 -

5s) over solid catalysts at the lower portion of a FCC riser, a reactor in the FCC process.

Cracked products are separated from the coke and deactivated catalysts at the top of riser.

Solid catalysts are then treated to burn coke on the surface of the catalysts and to be activated again in a regenerator at high temperature around 670 - 720 °C under oxidizing conditions. The heated regenerated catalysts retain the heat balance providing the energy needed for endothermic cracking process. A simplified schematic diagram is depicted in

Figure 1.4.

Catalysts of the FCC process have the following composition: an ultrastable Y zeolite (USY) is a main component (40%) in the FCC process (since 1964), and ZSM-5 is used as an additive to significantly enhance catalytic activity (since 1984). FCC catalysts typically contain a filler (e.g., kaolin clay (Al2Si2O5(OH)4)), a binder, catalytically active and acidic matrix (e.g., -Al2O3, SiO2), and other kinds of additive (such as a CO oxidation promoter) besides the ZSM-5.

Figure 1.4 Schematic diagram of FCC process

Zeolite satisfies the complex requirements of a FCC unit: good thermal stability, high activity, high selectivity to gasoline vs. coke, low coke production, resistance to poisons, low cost, etc [43, 44]. Before zeolite was applied to FCC process, the FCC units were needed to reduce the formation of hydrocarbon on catalysts and to shorten the residence time for control of coke generation. The introduction of zeolites to the FCC units not only solved these issues, but also highly improved both catalytic activity and selectivity. The productivity (yield of gasoline) was increased significantly by 60%. This is related to a tremendous cost savings and a reduction in crude oil consumption [45].

After zeolites (Ultra Stable Y zeolites, USY) were introduced to the FCC process as catalysts in 1962 (by Plank and Rosinski working for Mobil Oil) instead of clay-based synthetic catalysts [44], they have been used as the main catalysts in the FCC process.

1.3.2 Effect of Redox Chemistry on Hydrocarbon Chemistry

From the description of FCC process in the previous section, it can be seen that a large fraction of the Bronsted acid sites of zeolites would be dehydroxylated at the point of contact of catalysts and hydrocarbons. The catalytic chemistry can be greatly affected by radical cation intermediates [46] formed by interaction with redox sites [47]. The potential role of redox chemistry has been suggested by McVicker et al. [48] where the selectivity patterns revealed a „radical-like‟ pattern rather than an acid-like pattern. In addition, our previous observation leads a necessity for new plausible mechanism that can explain formation and role of redox sites in the hydrocarbon chemistry.

The main catalysts of FCC process are not high-silica zeolites. However, USY are dealuminated during contacting high temperature steam in the modern FCC process, and then act as high-silica zeolites. Steam at high temperature causes the dealumination of zeolites and the increase of Si/Al ratio [49]. Thus, understanding the mechanisms of high- silica zeolites is considered relevant a very important issue in the FCC process.

The motivation of this research starts from the observation of the unexpected thermal decomposition of Bronsted acid sites of high-silica zeolites. If a redox process is

revealed as contributing significantly to the chemistry of a FCC unit, this opens new possibilities to improve hydrocarbon chemistry including the cracking process. The aim of this thesis is to assess the chemistry and structure of these redox sites and determine if indeed, they can play a substantive role in the catalytic cracking of hydrocarbons.

1.4 Thesis Outline

The main objective of this thesis is to investigate the structure, composition, and properties of the sites generated by dehydroxylation of Bronsted acid sites in ZSM-5 and

ZSM-5 like materials and their roles in electron-transfer and reactivity on hydrocarbon chemistry.

Chapter 2 describes the experimental methods and techniques used in this thesis.

This chapter includes ultraviolet/visible light spectroscopy (UV/vis), temperature programmed desorption (TPD) of ammonia, gas chromatograph (GC), and the reactors set up.

Chapter 3 discusses that the formation of the sites can extract an electron from naphthalene and form stable radical cations on differently treated ZSM-5 samples. The formation of naphthalene radical cations is detected by UV/vis spectroscopy and time- series spectra are recorded to study various electron-transfer processes that occur within the zeolite over time.

Chapter 4 examines the catalytic activity of ZSM-5 samples pretreated at different conditions adopting isobutane as a model reactant. The distributions of products, the conversions of isobutane, and the kinetic parameters including activation energy are investigated using GC.

Chapter 5 summarizes the main findings in this thesis, describes the main conclusions from our investigation and outlines possible directions for the future studies.

1.5 References

[1] Lobo, R.F., Introduction to the structural chemistry of zeolite, in Handbooks of Zeolite Science and Technology, K.A.C. Scott M. Auerbach, Prabir K. Dutta, Editor. 2003, Marcel Dekker.

[2] Meier, W.M.a.U., J.B, Molecular Sieves. 1973, Washington, D.C.: American Chemical Society.

[3] Database of Zeolite Structure. Available from: http://www.iza- structure.org/databases/.

[4] Loewenstein, W., Am. Miner., 1954. 39(1-2): p. 92-96.

[5] Corma, A., Chem. Rev., 1995. 95(3): p. 559-614.

[6] Garcia, H. and H.D. Roth, Chem. Rev., 2002. 102(11): p. 3947-4007.

[7] Flanigen, E.M., Pure Appl. Chem., 1980. 52(9): p. 2191-2211.

[8] Engelhardt, J., J. Catal., 1996. 164(2): p. 449-458.

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Chapter 2

EXPERIMENTAL METHOD

2.1 Introduction

The experimental methods and techniques used in this thesis are described in this chapter. The synthesis of the zeolites ZSM-5 is described first, followed by a section describing the treatment protocols used for dehydroxylation of Bronsted acid sites of this zeolite. The key characterization techniques, such as X-ray diffraction (XRD), N2 adsorption, scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR) are described to the extent needed for understanding their application in this thesis. Ultraviolet/visible light spectroscopy (UV/vis) is used extensively and the set up as well as the diffusive-reflectance cell and its application are described in detail. The technique of temperature programmed desorption (TPD) of ammonia is described in relation to its application in zeolites. The final section describes the reactor setup, the analysis of the product gas composition by gas chromatography

(GC) and the experimental protocols used for the iso-butane cracking and dehydrogenation reaction (Chapter 4).

2.2 Zeolite ZSM-5 Synthesis

ZSM-5 is most often prepared using an organic structure-directing agent (SDA) such as tetrapropylammonium hydroxide. Here, however, we use a completely inorganic synthesis gel to avoid the calcination step needed to remove the organic SDA in the typical ZSM-5 synthesis. The all-inorganic synthesis of zeolites ZSM-5 is the method used to prepare this zeolites in the industry mainly because it is inexpensive. This synthesis thus has the added advantage that the results can be directly applicable to ZSM-

5 samples used in industrial catalytic reactors.

ZSM-5 with different framework composition (Si/Al ratios) is synthesized from the following molar batch compositions: x Na2O: y Al2O3: 100 SiO2: z H2O. Based on

100 mol of SiO2, x has a range from 7 to 14, y varies from 1.5 to 5, and z from 2250 to

4200 [1]. The compositions are calculated according to the described Si/Al ratios. Two samples with high and low Si/Al ratios are used throughout the thesis. The sample ZSM-

5-18 with a Si/Al ratio of ~18 is synthesized using a gel of composition 12 Na2O: 2.86

Al2O3: 100 SiO2: 3000 H2O. The sample with a Si/Al ratio of 12.5 (ZSM-5-12, higher alumina content) is synthesized with a synthesis gel of composition 9 Na2O: 4 Al2O3: 100

SiO2: 3000 H2O. Colloidal silica (Ludox AS-40, Sigma-Aldrich) and sodium aluminate

(NaAlO2, EM Science) are used as the silica and alumina sources, respectively.

The reactant solution is prepared in two containers (polypropylene beakers).

Colloidal silica with 5M NaOH solution and deionized water are dissolved in container 1.

Sodium aluminate with 5M NaOH solution and deionized water are dissolved in

container 2. After mixing for 1 hour separately, the solutions are combined. After one additional hour of continuous stirring, the final solution is loaded into Teflon-lined Parr autoclaves and heated at 190 °C for 2 days under rotation. The Parr autoclaves are cooled to room temperature in air and the zeolites samples are separated from the solution by using vacuum filtration. The separated samples are washed using deionized water, and dried at 80 °C overnight. Ammonium exchange of these samples is performed twice in

0.1 M NH4NO3 solution at 80 °C for 1 day to convert the sodium form of the zeolites to the ammonium form.

2.3 Sample Pre-treatment

A total of ~0.25g of the ammonium-exchanged form of zeolite is put in a quartz vertical tube reactor (ID = 19 mm), designed to flow gas through the sample space. The reactor has a porous (4-15 m) fritted disc in the middle where the sample is placed

(Figure 2.1). The reactor is heated by using a ceramic radiant heater (Omega Engineering,

OMEGALUX® CRFC). A gas manifold system allows an inert (Ar) and/or oxygen gas to flow through the reactor. The sample is first dehydrated at 200 °C in an Ar gas for 2 hours, and then the ammonium ions are decomposed to form acid zeolites by heating the sample at 450 °C in Ar for 4 hours. Three different treatments are carried out on three different zeolite samples. In the first treatment the sample is heated at 500 oC in Ar, in the second treatment the sample is heated at 780 oC in Ar, and in the third treatment the sample is heated at 500 oC in an oxygen (99.999%) flow. The purpose of the second

treatment is to dehydroxylate the BAS of the zeolite and the purpose of the third treatment is to generate redox sites by exposing the sample to a highly oxidizing environment as it has been shown before that oxygen treatment forms redox sites in H-

ZSM-5 [2]. The first treatment using an inert gas is the base-case scenario where the zeolite is expected to be in a „pristine‟ state without any changes induced by the heat treatment (See Figure 2.2).

After the sample treatment process, the samples are cooled down to room temperature in an inert atmosphere. A weighed amount of naphthalene (~0.005g, Sigma

Aldrich, ≥ 99.7%), corresponding to ~1 molecule per unit cell of zeolites, is mixed with the treated sample using a mortar and pestle in a glove bag (Glove bagTM Inflatable Glove

Chambers) filled with dry argon. To create an inert atmosphere in the glove bag, the inert gas is repeatedly filled and purged into and from the glove bag. After this step, time- series UV/visible spectra are measured at 0.5 nm of resolution over a wavelength range of 220 - 850 nm. The time-series spectra are recorded for 2 days right after mixing the pre-treated sample with naphthalene. These experiment steps are depicted in Figure 2.3.

Figure 2.1 Diagram of the reactor used for sample pre-treatment

Figure 2.2 Temperature protocol for the sample pre-treatment

Figure 2.3 Experimental protocol for detecting the generation of radical cations

2.4 Sample Characterization

2.4.1 X-ray Powder Diffraction

Synthesized samples were characterized using X-ray powder diffraction (XRD), which is a widely used characterization technique for polycrystalline materials. XRD provides information about the atomic structure of the crystal and the dimensions and symmetry of the periodic three dimensional lattice structure of the material. XRD patterns are recorded on a Phillips X‟Pert X-ray diffractometer using a Cu K radiation.

The patterns are collected from 5 ° to 50 ° 2 using a step size of 0.02 ° and 2s per step.

Figure 2.4 is an example of an XRD pattern of the ZSM-5 used in this study. By

comparison to published patterns we establish that the samples are pure ZSM-5 and that they do not contain any detectable impurities or amorphous material.

Figure 2.4 Sample XRD pattern for ZSM-5

2.4.2 Scanning Electron Microscopy (SEM)

A scanning electron microscope (SEM, JEOL JSM 7400F) is used to obtain electron microscopy images of the ZSM-5 samples. The detector collects emitted electrons and photons by hitting the sample with an electron beam, and thus obtaining an image. A sample example for ZSM-5 is provided in Figure. 2.5. EDAX spectrum analysis

shows that the ZSM-5 structure contained Si and Al in approximately 94.89:5.11 (Si/Al ratio is 18.57).

(a) (b)

Figure 2.5 (a) SEM image for ZSM-5 and (b) EDAX spectrum analysis

2.4.3 N2 Adsorption Isotherm

Physisorption of nitrogen is frequently used to determine surface area, pore volume, pore diameter, and pore size distributions of catalysts. Nitrogen adsorption isotherms are measured using Micrometrics ASAP 2010 instrument at 77K. The surface area of a material is commonly found using the Brunauer, Emmett, and Teller (BET) equation. The BET equation assumes the adsorption potential from one wall and the subsequent layers of adsorption are controlled by condensation [3, 4]. Since adsorption

potential from both walls affect the adsorption in materials with micropores, the BET cannot be used for materials such as zeolites.

Micropore volume, surface area, and mesopore volume of zeolites are determined by the deBoer t-plot method. In the t-plot method, the statistical thickness of the adsorption layer is plotted against the adsorbed volume measured from the N2 adsorption isotherm. The statistical thickness is estimated from a semi-empirical formula, such as

Harkins-Jura equation, which is commonly used for the analysis of zeolites. This equation is based on adsorption on nonporous Al2O3. The intercept is related to the micropore volume and the slope is related to the external surface area using following equations;

3 Vmp  cm/ g  intercept 0.001547

2 Sext  m/ g  slope 15.47

where, Vmp represents the micropore volume and Sext indicates the external surface area.

The ZSM-5 micropore volume is reported in the range 0.13 - 0.23 cm3/g [5-7]. The micropore volumes of the samples used in this thesis are consistent with the range (~ 0.15 cm3/g).

2.5 UV/vis Spectroscopy

UV/vis spectroscopy is used to study molecular structure and dynamics through electronic transitions and vibrations in the ultra-violet and visible range (200 – 800nm wavelength) of the electromagnetic spectrum. Molecules in the ground state absorb a specified range of UV/vis light to induce electronic transitions or vibrations and show an absorption spectrum [8]. The wavelength of the absorption bands depends on the atomic or molecular structure and composition, and the intensity of absorbance determines the concentration of the molecule or absorbing species. A schematic layout of UV/vis used in this study is illustrated in Figure 2.6(a) where the traditional transmission mode is presented. While UV/vis spectroscopy is a powerful tool to detect and identify organic species that absorb radiation in UV/vis energy range in the liquid phase, it is not applicable to directly obtain the spectrum of a powdered sample since transmission of the light through the powdered sample can be very low and limited due to light scattering.

Instead of transmission, a diffuse-reflective method is used for powdered samples. To this end, an integrating sphere is used as shown in Figure 2.7(b). The most widely used theory of diffuse-reflectance is the Kubelka-Munk theory which assumes the radiation is composed of two oppositely directed radiation flux through a continuous medium. Using this theory, diffusive reflectance UV/vis spectra were translated by the Kubelka-Munk function:

2 1 R K FR   2RS

where, R indicates the ratio of the diffuse reflectance of the sample to reference material

(spectralon made of polytetrafluoroethylene (PTFE) or barium sulfate), K an absorption coefficient, S the scattering coefficient of the powder.

Figure 2.6 (a) The schematic layout of UV/vis spectroscopy, (b) optical geometry of the integrating sphere.

Figure 2.7 shows a UV/vis spectrum of naphthalene radical cations adsorbed in

ZSM-5. The spectra show that there are peaks in the range between 550 and 680 nm and in the range between 350 and 400 nm. The UV/vis spectrum of the naphthalene radical cation is well known [2, 9-13]. It is characterized by strong vibronic bands at 675 nm (8 peaks at 675, 653, 635.5, 616.5, 598, 586, 567, and 551 nm) and weak visible absorptions at 380 nm (2 peaks at 382 and 366.5 nm) in Ar [12, 13].

Figure 2.7 UV/vis spectrum of naphthalene radical cation in ZSM-5 (Zeolyst, SiO2/Al2O3 = 15)

The time-series spectra recorded for 2 days are analyzed to extract to the spectra of pure species and the concentration change of each species by using an interactive self- modeling mixture analysis program (SIMPLISMA). This approach extracts pure component spectra without any basic information about components while a conventional principal component analysis needs some basic initial information to deconvolute the multivariable spectra [2, 14, 15]. The SIMPLISMA approach uses the average value of multivariable spectra and the difference between average value and mixed value to calculate the pure spectra for the first component [14, 15]. The contribution of the first component to the multivariable spectra is removed using the difference. The pure spectra

for the next component can then be calculated by repeating this procedure. The number of components and the noise level are chosen as the parameters [14, 15].

2.6 Temperature Programmed Desorption (TPD) of Ammonia

TPD is one of the most frequently used techniques to characterize the acidity of materials. Here this technique is used to obtain information about the initial state (number and strength) of the acidic sites of the sample. The measurement is repeated then for both the high temperature and oxygen treated samples and by difference, we can quantify the effect of the treatment on the concentration of BAS. A 6.35mm diameter U-shape quartz flow reactor (Quartz Plus) is connected to a piping network integrated in a catalyst characterization system (Altamira Instruments, AMI-200i). The reactor is installed in a clam-shell style furnace to control temperature. Temperature is measured using a K-type thermocouple and automatically controlled by the control software (Altamira Instruments,

AMI-5200). A bed of quartz wool and quartz chips (Quartz Plus) are placed in the reactor, and 30 mg of ammonium-exchanged ZSM-5 is put on the bed. The corresponding TPD protocol for the high temperature treatment and the oxygen treatment is presented in

Table 1 and 2. Figure 2.8 shows the temperature profile with time during the corresponding TPD experiment to the oxygen treatment.

Table 2.1 Temperature-programmed desorption protocol corresponding to the high-temperature treatment of the zeolite samples.

No. Ramp to Rate Treatment gas Flow rate Procedure 1 200 oC +10 oC/min Inert (He) Dehydrate

2 550 oC +20 oC/min Inert (He) Desorption of ammonium

3 100 oC -30 oC/min Inert (He) Cool down

o 4 100 C Ammonia (NH3) 20cc/min Regenerate ammonium exchanged ZSM-5 by flowing NH3

5 200 oC +10 oC/min Inert (He)

6 780 oC +20 oC/min Inert (He) Desorption of ammonium and dehydroxylate

7 100 oC -30 oC/min Inert (He) Cool down

o 8 100 C Ammonia (NH3) 20cc/min Regenerate ammonium exchanged ZSM-5 by flowing NH3

9 200 oC +10 oC/min Inert (He)

10 550 oC +20 oC/min Inert (He) Desorption of ammonium

Table 2.2 Temperature-programmed desorption protocol corresponding to the oxygen treated zeolites.

No. Ramp to Rate Treatment gas Flow rate Procedure

1 200 oC +10 oC/min Inert (He) Dehydrate

2 550 oC +20 oC/min Inert (He) Desorption of ammonium

3 100 oC -30 oC/min Inert (He) Cool down

o 4 100 C Ammonia (NH3) 20cc/min Regenerate ammonium exchanged ZSM-5 by flowing NH3

5 200 oC +10 oC/min Inert (He)

6 550 oC +20 oC/min Inert (He) Desorption of ammonium

o 7 550 C Oxygen (O2) 30cc/min Oxygen treatment

8 100 oC -30 oC/min Inert (He) Cool down

o 9 100 C Ammonia (NH3) 20cc/min Regenerate ammonium exchanged ZSM-5 by flowing NH3

10 200 oC +10 oC/min Inert (He)

11 550 oC +20 oC/min Inert (He) Desorption of ammonium

Figure 2.8 Temperature profile with time and the gases during TPD corresponding oxygen treatment.

In ammonia TPD, both BAS and LAS can be observed. BAS interact with ammonia to form ammonium ion, and LAS interact with the unpaired electrons on the nitrogen of ammonia. The ammonia in LAS is desorbed at lower temperatures than that in BAS. Different peaks can usually be observed in the TPD trace [16-18].

Ammonia TPD can provide useful information about the initial state of the acidic site of the sample. However, it is recognized that some limitations of ammonia TPD technique hinder a quantitative analysis of Bronsted acid sites densities [19, 20]. It was

reported that ammonia adsorbs more strongly on CaO than on a USY zeolite [21]. In this sense, zeolites contain non-framework alumina or other species, for instance Lewis acid sites, and ammonia can be adsorbed on such non-BAS. In addition, the temperature observed the peak maxima can be strongly affected by the conditions used for the measurement. The heat of adsorption can be estimated from the temperature of the desorption peak using a simple kinetic model [19]. However, the desorption kinetics of crystalline materials are much complicated because of molecular interactions [22]. The application of TPD is assumed that adsorption and desorption are in local equilibrium where diffusion limits the desorption process. However, this assumption is not valid since desorption and adsorption occur simultaneously with diffusion in these kind of microporous materials [19]. Therefore, using the TPD of ammonia experiment, strict quantitative analysis of the obtained profile is not possible.

2.7 Fourier Transform Infrared Spectroscopy (FTIR)

Infrared (IR) spectroscopy has been frequently used to identify and study molecules and materials, especially to investigate the functional groups present in the samples. Specific energies are absorbed at specific frequencies matching the normal mode of vibrating bonds or functional groups. When the dipole moment changes due to the vibration, the bonds or groups of molecules are observed by IR spectroscopy.

Different molecules vibrate in different ways. The types of vibrational modes are categorized as symmetrical and asymmetrical stretching, scissoring, rocking, wagging,

and twisting. When the size of a molecule is large, the observed peaks in an IR spectrum are so many that the analysis becomes complicated. An IR spectrum gives exact information of the position, shape, and height of the abdorption bands. From the spectral information, we can infer the nature and the concentration of the functional group or bond and the conditions (environment) where the molecules are placed.

IR spectra are recorded by Fourier Transform Infrared (FTIR) spectroscopy. FTIR guarantees a fast measurement and good repeatability because all wavelengths simultaneously pass through the sample. In FTIR, a Michaelson interferometer, which consists of a beamsplitter, fixed mirror, and moving mirror, is adopted as the optical component. The interfered beams are transformed by using the Fourier transform, and then the IR spectrum can be recorded.

In this thesis, the hydroxyl groups in zeolites are observed to establish the effect of dehydroxylation by the FTIR. Figure 2.9 shows the in-house-build IR cell which is designed to heat the sample, to keep the sample under low pressure by evacuation, and to introduce gases to the IR chamber for pre-treatment. Using this IR cell, the sample is dehydrated first, and then, dehydroxylated at around 800 °C in vacuum and at 500 °C in an oxygen atmosphere. The sample is then cooled to room temperature and placed across the beam path. The measurements are recorded in the transmission mode.

Figure 2.9 The IR cell – a heater, a reactor, and a gas line

2.8 Gas Chromatography (GC)

Gas Chromatography (GC) is used for identifying chemical compounds in mixture of unknown compositions. GC passes a sample containing a mixture of compounds through a column, which is a thin tube, and electronically detects each component as it reaches, at different times, at the end of a column. Two detectors, a flame

ionization detector (FID) and a thermal conductivity detector (TCD) are used in this work.

These are the two most common detectors for GC. FID is more sensitive to hydrocarbons than TCD, thus is generally used for identifying hydrocarbons. TCD is more frequently used for detecting hydrogen and inert gases. The GC was calibrated using two mixture calibration gases (two point calibration).

The GC instrument used for this research is GC model 2014 (Shimadzu) with two columns. One column is a molesieve connected to a TCD detector and another column is a RT-alumina connected to a FID detector. The catalytic reaction rate were determined using a quartz tube plug flow reactor (ID = 5mm). Differential reaction conditions were used whenever possible to measure reaction rates without the assumption of a reaction rate expression model.

2.9 Reactor Setup for GC

The reactor, gas connection, and GC (MS) are connected as depicted in Figure

2.10. The reactant gases, including inert gases, oxygen and alkanes, flow through the quartz tube reactor heated at the specified temperature. The temperature inside the reactor is monitored by a K-type thermocouple and controlled by a temperature controller

(NC74000, Omega Engineering). Ammonium form of catalysts on the quartz bed inside the reactor is heated to 200 °C for dehydration, and then it is heated to 450 °C to convert the ammonium form to the acid form of the zeolite. The samples are treated at 500 °C

(treatment 1) and 800 °C in an inert gas flow (treatment 2) and 500 °C in oxygen flow

(treatment 3), respectively. After each treatment, the temperature is lowered to 450 °C for the reaction with isobutane. The products, produced by contacting the reactant gas to the pre-treated catalyst, are separated and recorded by GC.

C4H10

Figure 2.10 GC system setup used to study isobutane cracking process

2.10 Summary

We have described briefly the experimental methods and techniques used in this thesis. First, the synthesis of zeolite was described followed by the three different sample treatment protocols. The use of some important characterization techniques was briefly illustrated. As main experimental techniques, UV/vis spectroscopy for detecting organic radical cations, ammonia TPD for determining structures of acid sites, and GC with the reactor setup for measuring catalytic activity and selectivity were explained. The

generation of sites which have an ability to extract an electron from naphthalene will be addressed in chapter 3, and catalytic activity and selectivity related to the newly generated sites will be examined in chapter 4.

2.11 References

[1] Kim, S.D., et al., Microporous Mesoporous Mat., 2004. 72(1-3): p. 185-192.

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[8] Skoog, D.A. and J.J. Leary, Principles of Instrumental Analysis. 4th ed. 1992: Saunders College Publishing.

[9] Andrews, L. and T.A. Blankenship, J. Am. Chem. Soc., 1981. 103(19): p. 5977- 5979.

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[11] Kelsall, B.J. and L. Andrews, J. Chem. Phys., 1982. 76(10): p. 5005-5013.

[12] Szczepanski, J., et al., J. Phys. Chem., 1992. 96(20): p. 7876-7881.

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[14] Windig, W. and J. Guilment, Anal. Chem., 1991. 63(14): p. 1425-1432.

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[19] Gorte, R.J., Catal. Today, 1996. 28(4): p. 405-414.

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[21] Juskelis, M.V., et al., J. Catal., 1992. 138(1): p. 391-394.

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Chapter 3

GENERATION OF STABLE ORGANIC RADICAL CATIONS IN THERMALLY TREATED ZSM-5 ZEOLITES

3.1 Introduction

It has been shown before that when zeolite ZSM-5 is activated by thermal treatment in an inert gas flow as well as in oxygen, adsorption of organic molecules with relatively low ionization energy spontaneously give an electron to the zeolite framework forming stable occluded radical species. These observations point to the generation of new reaction sites upon thermal treatment. The classical conception of a zeolite catalyst is based on the presence of Bronsted acid sites in the zeolite pores, sites that play a key role as catalytically active sites. We observe, as many others have in the past, catalytic activation of hydrocarbons by Bronsted acid sites when using the first (mild) thermal treatment described in Chapter 2 (500 °C in Ar). The zeolite activation by the second and the third thermal treatments leads to important differences in selectivity from the typical reactions catalyzed by the Bronsted acid sites (the subject of Chapter 4 of this thesis). It is clear that new catalytic sites are formed by dehydroxylation of Bronsted acid sites in

ZSM-5 by these two treatments. Using naphthalene as a probe molecule, it can be shown that the new sites have the ability to extract an electron from naphthalene and form stable

radical cations. This observation suggests that the new sites can also activate hydrocarbons by a redox mechanism.

In this chapter, we investigated the formation of these new sites, sites that can extract an electron from naphthalene and form stable radical cations on the differently treated ZSM-5 samples. Differences between the sites generated by treatments 2 and 3 can be revealed by comparing the UV/vis spectra of each sample upon adsorbing naphthalene. Surprisingly, naphthalene radical cations can be generated on samples treated at temperatures as low as 200 °C. We also observe that after naphthalene radical cations were generated, single-electron transfers back into the ZSM-5 framework to form a stable electron-hole pair and reform the naphthalene neutral molecule. Using ammonia

TPD, IR spectra, and UV/vis spectra of the sample with different Si/Al ratios, the structure of the new generated sites was characterized.

3.2 Generation of Naphthalene Radical Cations

We treated the H-ZSM-5 samples at different conditions as described in chapter 2, and then in a glove bag flushed with dry argon, mixed the zeolite with the solid naphthalene using a pestle and mortar. The amount of naphthalene is added enough to adsorb one molecule of naphthalene per unit cell of the MFI framework. In a very short time a change of color of the naphthalene-zeolite mixture is evident. At this point, a portion of the sample is loaded into a UV/vis diffuse reflectance sample cell and the time evolution of the sample was followed using UV/vis spectroscopy. The first treatment was

heating the sample at 500 °C in Ar, the second treatment was heating the sample at

780 °C in Ar to dehydroxylate the Bronsted acid sites of the zeolite. Since earlier research has shown that oxygen treatment also leads to the formation of the naphthalene radical cation in H-ZSM-5[1], the third treatment was heating the sample at 500 °C in oxygen to generate electron-abstracting sites.

Figure 3.1 shows that no reaction occurred between naphthalene and all-silica

ZSM-5 (silicalite-1) treated in Ar at both 780 °C and 500 °C. The samples mixed with naphthalene commonly have a very large peak at 270 nm (data not shown), which is attributed to sorption of naphthalene in the ground state within the pore or channel of the zeolites. The spectra of silicalite-1 treated in Ar at both 780 °C and 500 °C are flat just like the spectrum of pure silicalite-1 at wavelengths above 270 nm. This result shows that aluminum is a key component of the sample needed to form electron-abstracting sites, that is framework aluminum and Bronsted acid sites must be present in the sample before treatment in order for the initial electron transfer to occur [1]. Also, Figure 3.1 shows that no reaction occurred between naphthalene and the pre-treated H-ZSM-5 in Ar at 500 °C

(treatment 1). The spectrum is also entirely flat like the spectra of pure silicalite-1 and pure ZSM-5. The lack of reactivity means that the naphthalene radical cation does not form on ZSM-5 by reaction with the classical Bronsted acid sites (Si-OH-Al), external silanol groups, or internal silanol groups. This temperature is deemed too low to start decomposing the Bronsted acid sites [2-4].

The acid form of ZSM-5 treated in Ar at 780 °C (treatment 2) and treated in O2 at

500 °C (treatment 3) reacted with the naphthalene to form naphthalene radical cations.

This is clearly indicated by the absorption peaks observed in Figure 3.2. The spectra show that there are peaks in the 550 and 680 nm range and in the 350 and 400 nm range.

The UV/vis spectrum of the naphthalene radical cation is well known [1, 5-9]. It is characterized by strong vibronic bands at 675 nm (8 peaks at 675, 653, 635.5, 616.5, 598,

586, 567, and 551 nm) and weak visible absorptions at 380 nm (2 peaks at 382 and 366.5 nm) in Ar matrix [8, 9]. According to Hückel molecular orbital energy levels of

naphthalene cation [10, 11], the vibronic transition corresponding to 670 nm wavelength

22 is BDXAD3gu 2   0  and that corresponding to 380 nm wavelength is

22 BDXAD2gu 4   0  [8, 9]. Our observed peaks are consistent with the UV/visible spectra of naphthalene radical cations in an Ar matrix.

The intensity is directly related to the molar concentration of the component based on Lambert-Beer Law [12];

A  dc , AIIlog / 0  where, A represents the absorbance,  the molar extinction coefficient, d pathlength in cm, c molar concentration, I intensity of the transmitted light, and I0 intensity of the incident light. The concentration of the radical cations on the sample treated in O2 at 500 °C is clearly higher than the concentration of the radical cations on the sample treated in Ar at

780 °C, although the shapes and positions of the characteristic peaks of both samples are similar.

Figure 3.2 UV/visible spectra for control experiments (ZSM-5 in Ar at 500 °C, Silicalite-1 in Ar at 500 °C and 780 °C) and for naphthalene@ZSM-5 heated to 780 °C in Ar and 500 °C in O2

Besides the intensity, there are other differences between the two spectra. For ease of comparison, the spectra of the sample treated in O2 at 500 °C and the sample treated in

Ar at 780 °C were rescaled in Figure 3.3. The broad band from 400 to 550 nm is different between treatments 2 and 3. The sample treated in O2 at 500 °C has a prominent peak at

460 nm in the range of the broad band while the sample treated in Ar at 780 °C does not.

Since this band is related to electronic transitions associated to the electron-hole pair and electron traps in the zeolite [1], the difference can be related to the structural difference between sites generated by treatments 2 and 3.

Figure 3.3 Rescaled UV/vis spectra for naphthalene adsorbed into ZSM-5 heated to 780 °C in Ar and to 500 °C in O2

A number of possible models can explain the differences between spectra after the two treatments: 1) two different sites were generated by the different treatments

(treatments 2 and 3); 2) the sites generated in the sample treated in O2 were much more reactive than the sites in the sample treated in Ar.

To understand better the differences between the samples, the naphthalene adsorption experiment was repeated at temperatures lower than 500 °C (always in the presence of oxygen). As shown in Figure 3.4(a), when the sample was treated at 200 °C,

300 °C, and 400 °C, naphthalene radical cations were readily observed immediately after

the sample was mixed with the naphthalene. Even though the treatment temperatures were low, the intensity levels seemed to be similar for all the temperatures above 200 °C.

When the samples were treated at temperatures less than 100 °C, some small peaks around 650 and 675 nm were observed, but the characteristic peaks of the radical cation did not appear over the entire spectral range. It is surprising that naphthalene radical cations can be generated on the ZSM-5 samples treated at temperatures as low as 200 °C.

The intensity at 675.5 nm – the wavelength of the adsorption peak with the highest intensity for the naphthalene radical cation – was tracked as a function of time to investigate the effect of the temperature during the oxygen treatment on the appearance and disappearance of the radical cations. Since it is assumed that the oxygen-treated

ZSM-5 powder and the naphthalene were completely and evenly mixed, the intensity at each of the different temperatures was normalized to the intensity of the first spectrum obtained five minutes after mixing. Initially, the intensities of all the samples increased, and then after about 100 min the intensity began to decrease. This observation is thought to be due to the migration of the electrons of the naphthalene radical cations into the

ZSM-5 framework [1].

(a)

(b)

Figure 3.4 (a) The UV/visible spectra after mixing with the naphthalene and (b) intensity change with time of the 695 nm peak of the samples treated at different temperatures.

There were some differences between samples treated at different temperatures, but the intensity changes with time were essentially the same. Therefore, we can conclude that the active sites and the number of active sites resulting from the oxygen treatment were nearly the same by heating at temperatures above 200 °C. The reactivity of the active sites or the migration mechanism of electrons was not very different, regardless of the temperature.

Naphthalene radical cations with a single-electron were generated on new sites in pre-treated ZSM-5 (treatments 2 and 3). It is reported that the electron-transfer from neutral naphthalene is responsible to the formation of trapped electrons, not the zeolite framework [13]. It can be considered that the trapped electron is generated by electron- transfer from neutral naphthalene to a site that can extract a single-electron. During the time that naphthalene radical cations are formed by electron-transfer, an increase in the intensity of the absorption spectrum is detected by UV/vis spectroscopy. Then, as the unpaired electrons migrate to the ZSM-5 framework, the concentration of electron-hole pairs increases. As a result, the concentration of the naphthalene radical cations begins to decrease, and concurrently the intensity of the absorption spectrum also starts to decrease.

After the electron-hole pairs are stabilized, the concentrations of both components remain stable.

3.3 Migration of Electrons and Holes within the Zeolite Framework

It is known that the generation and migration of electrons and holes within zeolites occur after n-acene adsorption, including naphthalene (i.e., n=2) [1, 14].

Moissette et al. have reported that the naphthalene radical cation was generated by spontaneous ionization after mixing of solid naphthalene with H-ZSM-5 [1, 15]. Then, electron-hole pairs were generated by electron transfer between the naphthalene radical cation and the zeolite framework [1, 15]. Finally, after gentle warming at 400K for several days, the electron-hole pairs disappeared and naphthalene was reformed as the form of neutral naphthalene adsorbed into the zeolite framework by the charge recombination from the framework different from the one that was initially lost [1, 15].

The mechanism of single-electron migration is illustrated in Figure 3.5 [1].

Figure 3.5 Mechanism of the single-electron migration in ZSM-5 framework.

A series of UV/visible spectra were recorded to trace single-electron migration from the ZSM-5 structure to the naphthalene radical cations during a 48-hour period after the sample was mixed with the naphthalene. Figure 3.6 and 3.7 show the series of UV/vis spectra after treatments 2 and 3, respectively. The characteristic peaks of the naphthalene radical cation increase with time, and then they begin to decrease at some point. It is difficult to determine the precise time at which the intensity begins to decrease, because the cusp of the curves is relatively flat (Figure 3.4(b)) and the times at which the intensity decrease began were different for different samples and conditions. For instance, the times at which the intensity begins to decrease were 50 min and 12 hr for the same ZSM-

5 treated at 500 °C in O2 (data not shown). Note that the change of intensity can be observed for samples treated in both Ar and O2.

There are, however, some important differences between the samples treated in

Ar and O2. First, the times when we start to observe a decrease in the absorption intensity, was around one hour for the oxygen treated samples (Figure 3.7), but was approximately

15 hours (Figure 3.6) for the sample treated in Ar at 780 °C. While the times when we start to observe a decrease in the absorption intensity for the oxygen treated samples vary greatly (50 min - 12 hr) for different samples or different conditions, the times when we start to observe a decrease in the intensity for the samples after treatment 2 are between

12 - 15 hr. In addition, the broad band from 450 nm to 550 nm of the sample treated at

780 °C in Ar, which is related to the formation electron-hole pairs in the zeolite framework, still has the peak at 460 nm 2 days after mixing. This band for the sample treated in O2 relatively becomes much broader and less defined.

(a)

(b)

Figure 3.6 Time-series evolution of the UV/visible spectra of NPT@ZSM5 after treatment 2 during (a) increasing and (b) decreasing absorption intensity.

(a)

(b)

Figure 3.7 Time-series evolution of the UV/visible spectra of NPT@ZSM5 after treatment 3 during (a) increasing and (b) decreasing absorption intensity.

To investigate the contribution of specific absorption features to the changes in the spectra with time, the Simple-to-use Interactive Self-modeling Mixture Analysis

(SIMPLISMA) program was used. To start, the spectrum with peaks in the range of 350 -

400 nm and in the range of 550 - 680 nm was resolved by the SIMPLISMA program, as shown in Figure 3.8(a) for treatment 2 and in Figure 3.9(a) for treatment 3. The peaks in this spectrum are the ones assigned to the naphthalene radical cations [1, 5-9]. The second spectrum, with broad peaks around 500 nm, has been explained by assuming that there is a trapped electron associated with the hole (electron-hole pair) formed upon recapture of an electron by the naphthalene radical cation, as was concluded in previous works with biphenyl [1, 16-18]. As we pointed out above, the difference between the spectra of the samples after treatments 2 and 3, the second pure component, is related to the electron-hole pair. The sample after treatment 2 has much clear band than the second pure component of the sample after treatment 3. This difference in the second component between two samples is related to the structural difference between sites generated by treatments 2 and 3. The SIMPLISMA methodology provides the changes in the relative concentration of species with time. The concentration of the naphthalene radical cations and the electron-hole pairs are illustrated in Figure 3.8(b) for treatment 2 and in Figure

3.9(b) for treatment 3. The concentrations in Figure 3.8(b) clearly reflect the delay in the onset of the neutral naphthalene regeneration.

(a)

(b)

Figure 3.8 (a) Pure spectral components and (b) concentration change with time for NPT radical cation and electron-hole pair on the sample after treatment 2

(a)

(b)

Figure 3.9 (a) Pure spectral components and (b) concentration change with time for NPT radical cation and electron-hole pair on the sample after treatment 3

3.4 Structure of the Dehydroxylated Sites

3.4.1 Ammonia TPD

The temperature-programmed desorption (TPD) of ammonia experiment can provide information about the initial state of the acidic properties of zeolites. An ammonia TPD experiment for ZSM-5 was performed on a sample after the three thermal treatments (see Figure 3.10). In what follows, we use the term “Before” to indicate that the ammonia desorption is performed after subjecting the sample to treatment 1

(increasing temperature to 500 °C in an inert gas). The term “After” means that the ammonia is adsorbed on and desorbed from the samples after treatment 2 or 3.

In all cases TCD signal profiles with one peak maximum were recorded, as shown

+ in Figure 3.10. This peak is consistent with the typical profile of ammonia TPD for NH4 form of ZSM-5 prepared by ammonium exchange [19]. Typical ammonia TPD for ZSM-

5 shows two chemisorptions peaks at a low and at a high temperature. The peak at low temperature is attributed to non framework aluminum (e.g., Lewis acid sites). The peak at high temperature is known to be due to Bronsted acid sites in the zeolite [19-21]. Since aqueous ammonium nitrate, which is the fluid used for the ammonium exchange of ZSM-

5, fills the Lewis acid sites with water rather than ammonia, we observe only one peak at high temperature for the sample used in this thesis. However, it still cannot be excluded that some desorption of ammonium from strong Lewis acid sites can be part of (but not been resolved) in the high temperature peak [21].

We have found that treatment 1 does not change the number or character of the acid sites in either of the two samples (data not shown). That is this treatment has no effect on the structure or composition of the sample. However, after dehydroxylation at

780 °C, the amount of ammonia desorbed decreases by approximately 50% (see Figure

3.10). This decrease indicates that some of the acid sites initially involved in the adsorption of ammonia were destroyed by treatment 2. In the other words, the Bronsted acid sites, which are the site for ammonia chemisorption, were eliminated at high temperature. In addition, the position of the peak maximum was shifted to a slightly lower temperature. The temperature of the peak maximum is known to be affected by the number of acid sites in the zeolite, the change in the zeolite structure, the carrier gas flow rate, or the amount of the sample [21]. Since the carrier gas flow rate and the amount of the sample are the same, it can be considered that the decrease of the temperature of the peak maximum reflects the reduced number of acid sites after dehydroxylation at 780 °C.

In contrast, oxygen treatment at 500 °C did not affect the adsorption of ammonia on the ZSM-5 sample. Dehydroxylation of Bronsted acid sites can proceed by dehydration and dehydrogenation channels [2]. We consider that treatment 2 is attributed to the dehydroxylation of Bronsted acid sites through the dehydrogenation channel [2], while the dehydration channel is preferred for the dehydroxylation by treatment 3. The dehydration channel generates Lewis acid sites and an Al tetrahedron in the zeolite framework containing a negative charge. The Lewis acid sites, can adsorb ammonia, while the latter sites, being basic, can not. However, the same amount of desorption of ammonia indicates that certain sites, which can react with ammonia, are generated by

decomposition of the Bronsted acid sites. Thus, the combination of the remaining

Bronsted acid sites and the newly generated sites by treatment 3 (oxygen treatment) makes the amount of desorbed ammonia the same. With this information, we can understand better the UV/vis spectra of naphthalene@ZSM-5 after treatments 2 and 3.

The intensity observed after treatment 3 (oxygen treatment) was higher than that of treatment 2. This means that the reactivity of the sites generated by treatment 3 with the naphthalene to form the naphthalene radical cation is stronger than the reactivity of the redox sites, which are formed by dehydrogenation channel (treatment 2) [2].

(a)

Tmax=414 ° C

Tmax=393 ° C

Figure 3.10 TPD corresponding for (a) treatment under Ar at 780 °C and (b) treatment under O2 at 500 °C.

(b)

Tmax=410 ° C

Figure 3.10 Continued

3.4.2 IR Spectroscopy and the Thermal Decomposition of Bronsted Acid Sites

Figure 3.11(a) shows the FTIR spectra of dehydrated ZSM5-18 sample. The samples have Bronsted acid sites and silanol groups after heating at 500 °C. Bronsted acid sites have an absorption band around the 3600 cm-1 region of IR spectrum [22]. The

IR peak at 3720-3750 cm-1 is assigned to silanol groups [23]. The peak at 3666 cm-1 is assigned to hydroxyl groups attached to extraframework aluminum [23, 24]. Low temperature CO adsorption studies using FTIR have revealed that the peak at 3666 cm-1 is less acidic than the normal Bronsted acid sites but more acidic than the silanol group

[23]. Extraframework aluminum [1], extra-lattice amorphous materials [25, 26], or the silanol group [27] has been considered to the assignment of the peak at around 3700 cm-1.

The intensity of the Bronsted acid sites stretching vibration decreases after heating to 780 °C (Figure 3.11(a)). This is evidence of the dehydroxylation of the

Bronsted acid sites during high-temperature treatment. However, the spectrum of the sample after heating to 500 °C in oxygen flow does not show the decrease of the intensity.

The intensity after treatment 3 looks lower than the intensity before treatment 3. However, all peaks still remain with relatively small intensities. The IR spectra and the TPD signals in the previous section suggest that certain unidentified sites, which behave like Bronsted acid sites in TPD experiment and IR spectroscopy, are generated after treatment 3.

(a)

Figure 3.11 FTIR spectra in the OH vibration region of ZSM-5 measured (a) after heated to 500 and 800 °C, and (b) after heated to 500 and 500 °C in an oxygen flow.

(b)

Figure 3.11 Continued.

3.4.3 Effect of Si/Al Ratio

The Si/Al ratio affects the acidity of the zeolites. Since Al is directly associated with the formation of Bronsted acid sites in zeolites, the number of acid sites per unit cell increases, as the Si/Al ratio decreases. At the same time the sites become weaker although this does not occur until Si/Al ratios below 10 [28, 29]. The effect of the Si/Al ratio on the amount of ammonia desorption and naphthalene adsorption was investigated.

In the previous sections, the Si/Al ratio of the sample investigated was 18. Here we compare these results to a zeolite with a lower Si/Al ratio of 12 (higher alumina concentration).

Figure 3.12 compares the TCD signals of ammonia TPD of ZSM-5 with Si/Al ratio of 18 and ZSM-5 with Si/Al ratio of 12. The TCD signals were rescaled to show the differences more clearly. The amount of ammonia desorption of ZSM-5 having Si/Al ratio of 12 is larger than that of ZSM-5 with Si/Al ratio of 18 because the sample with low Si/Al ratio has more acid sites. The areas below the desorption peak were calculated between 220 and 560 °C of the temperature range for quantitative comparison and tabulated in Table 3.1. The areas of ZSM5-12 before treatments 2 or 3 are 1.5 and 1.2 times larger than the areas of ZSM5-18. Since ZSM5-12 has more acid sites, the maximum peak temperatures (420, 401, 411 °C) are also higher than the maximum temperatures of ZSM5-18 (414, 393, 410 °C). The desorption areas reduced by 40.7 % and 36.5% after treatment 2 while the areas reduced by 4.7% and 3.4% after treatment 3.

But generally the effect of dehydroxylation is qualitatively the same: the intensity of the second TPD traces decreases and the position of the peak maximum shifts to lower temperatures.

The most remarkable difference between the UV/vis spectra of the sample with a high Si/Al ratio and the sample with low Si/Al ratio is the level of the naphthalene cation absorption intensity. For both treatments 2 and 3, the sample with Si/Al ratio of 18 has higher intensities than the sample with Si/Al ratio of 12. This is counterintuitive because the sample with a low Si/Al ratio definitely has more acid sites, as confirmed by the TPD experiments. However, the reaction by which the naphthalene radical cations are formed occurs less frequently when the total acid sites increase as the aluminum content

increases. We suggest that this is likely due to differences in the Bronsted acid sites decomposition mechanism.

ZSM-5 (Si/Al=18) ZSM-5 (Si/Al=12)

Treatment 2 Treatment

Treatment 3 Treatment

Figure 3.12 TPD of ZSM-5 having Si/Al of 12 compared with TPD of ZSM-5 having Si/Al of 18.

Table 3.1 The desorption areas and the maximum temperatures of TPD signal.

Sample Process Area Max. Temp., °C Before 554.5 414 Treatment 2 After 328.9 393 Reduction, % 40.7 ZSM5-18 Before 476.7 410 Treatment 3 After 454.4 410 Reduction, % 4.7

Before 849.1 420 Treatment 2 After 539.2 401 Reduction, % 36.5 ZSM5-12 Before 588.1 411 Treatment 3 After 567.9 411 Reduction, % 3.4

From the TPD experiments, we concluded that Lewis acid sites were generated in ZSM-5 after treatment 3 (oxygen treatment) while redox sites were generated in ZSM-

5 treated at 780 °C in Ar (treatment 2). However, this does not mean that only one type of sites is generated by each treatment. We can consider that the sites generated by treatments 2 and 3 or that the third and fourth type of site is formed. From the UV/vis spectra of naphthalene adsorbed in ZSM-5 with Si/Al ratio of 18, we know the sites generated by treatment 3 are much more reactive with naphthalene than the sites formed by treatment 2. In this sense, at high Si/Al ratio of 18, the decomposition pathway leads predominantly to the same type of sites generated by treatment 3, even at the high temperature treatment (treatment 2). At low Si/Al ratio of 12, it seems that the formation

of the same type of sites formed by treatment 2 in ZSM-5-18 becomes dominant. The adsorption of aromatic molecules can be affected by the concentration of guest molecules or surface diffusion based on structures [13, 14]. Since the sample with a low Si/Al ratio has more acid sites, their density in zeolite sample is considered to be high. The interaction between the generated sites by decomposition of Bronsted acid sites or between naphthalene molecules (e.g, form naphthalene anion [13]) can be also considered to be a possible explanation. In addition, we know that the more Bronsted acid sites remain in ZSM-5-12 than in ZSM-5-18 from the TPD experiments. The remaining

Bronsted acid sites could have some effect on the reaction with naphthalene. Further research, however, is needed to decide what the exact chemical identities of these sites are. This is further discussed in the next chapter using the hydrocarbon cracking reaction.

(a)

(b)

Figure 3.13 UV/visible spectra of ZSM-5 sample with different Si/Al ratios for (a) treatment 2 and (b) treatment 3.

3.5 Conclusions

Now sites, that can extract a single-electron from a neutral organic molecule, are generated by dehydroxylation of Bronsted acid sites in ZSM-5 framework. Two treatments, which are the high temperature treatment in an inert gas (Ar) and the oxygen treatment, are used for dehydroxylation of Bronsted acid sites. Naphthalene radical cations are formed as a result of the electron transfer from neutral naphthalene molecule to the newly generated sites into the ZSM-5 framework after mixing naphthalene with the treated ZSM-5.

The characteristic peaks of the naphthalene radical cation were detected by

UV/visible spectroscopy on the samples of the ZSM-5 zeolite after two different thermal treatments. The spectra of the sample after treatment 3 show higher intensity than the spectra of the sample after treatment 2. The broad band from 400 to 550 nm is different between treatments 2 and 3. For treatment 3, naphthalene radical cations can be generated on the ZSM-5 samples treated at temperatures as low as 200 °C. These observations suggest that different sites are generated by different treatments for dehydroxylation.

A series of UV/visible spectra were recorded to trace single-electron migration from the ZSM-5 structure to the naphthalene radical cations during a 48-hour period after the sample was mixed with the naphthalene. The characteristic peaks of the naphthalene radical cation increase with time, and then they begin to decrease. This observation and deconvoluted component spectra by SIMPLISMA algorithm support the generation and migration of electrons and holes within zeolites.

Ammonia TPD experiment shows that the newly generated sites in ZSM-5 after treatments 2 and 3 are different sites. The sites generated in the sample treated in O2 are much more reactive than the sites in the sample treated in Ar. At high Si/Al ratio of 18, the decomposition pathway leads dominantly to the same type of sites generated by treatment 3, even in treatment 2. At low Si/Al ratio of 12, it seems that the formation of the same type of sites formed by treatment 2 in ZSM-5-18 becomes dominant.

These observations suggest that the most common site generated is different depending on each treatment. We cannot establish the identity of newly generated sites by decomposition of Bronsted acid sites. In the next chapter, this conclusion is validated using a different approach, i.e., isobutane conversion.

3.6 References

[1] Moissette, A., et al., J. Phys. Chem. B, 2003. 107(34): p. 8935-8945.

[2] Nash, M.J., et al., J. Am. Chem. Soc., 2008. 130(8): p. 2460-2462.

[3] Stamires, D.N. and J. Turkevich, J. Am. Chem. Soc., 1964. 86(5): p. 749-&.

[4] Uytterhoeven, J.B., L.G. Christner, and W.K. Hall, J. Phys. Chem., 1965. 69(6): p. 2117-2126.

[5] Andrews, L. and T.A. Blankenship, J. Am. Chem. Soc., 1981. 103(19): p. 5977- 5979.

[6] Andrews, L., B.J. Kelsall, and T.A. Blankenship, J. Phys. Chem., 1982. 86(15): p. 2916-2926.

[7] Kelsall, B.J. and L. Andrews, J. Chem. Phys., 1982. 76(10): p. 5005-5013.

[8] Szczepanski, J., et al., J. Phys. Chem., 1992. 96(20): p. 7876-7881.

[9] Salama, F. and L.J. Allamandola, J. Chem. Phys., 1991. 94(11): p. 6964-6977.

[10] Clark, P.A., F. Brogli, and E. Heilbronner, Helv. Chim. Acta, 1972. 55(5): p. 1415-1428.

[11] Obenland, S. and W. Schmidt, J. Am. Chem. Soc., 1975. 97(23): p. 6633-6638.

[12] Ingle, J.D.J.a.C., S.R., Spectrochemical Analysis. 1988, New Jersey: Prentice Hall.

[13] Hashimoto, S., et al., J. Chem. Soc. Faraday Trans., 1996. 92(19): p. 3653-3660.

[14] Turro, N.J., et al., J. Am. Chem. Soc., 2000. 122(47): p. 11649-11659.

[15] Marquis, S., et al., C. R. Chim., 2005. 8(3-4): p. 419-440.

[16] Gener, I., A. Moissette, and C. Bremard, Chem. Comm., 2000(17): p. 1563-1564.

[17] Moissette, A., et al., Angew. Chem. Int. Ed. Engl., 2002. 41(7): p. 1241-1244.

[18] Gener, I., G. Buntinx, and C. Brémard, Angew. Chem. Int. Ed. Engl., 1999. 38(12): p. 1819-1822.

[19] Woolery, G.L., et al., Zeolites, 1997. 19(4): p. 288-296.

[20] Topsøe, N.-Y., K. Pedersen, and E.G. Derouane, J. Catal., 1981. 70(1): p. 41-52.

[21] Kapustin, G.I., et al., Appl. Catal., 1988. 42(2): p. 239-246.

[22] Szanyi, J. and M.T. Paffett, Microporous Mater., 1996. 7(4): p. 201-218.

[23] Zecchina, A., et al., J. Chem. Soc. Faraday Trans., 1992. 88(19): p. 2959-2969.

[24] Kustov, L.M., et al., J. Phys. Chem., 1987. 91(20): p. 5247-5251.

[25] Borade, R., et al., J. Phys. Chem., 1990. 94(15): p. 5989-5994.

[26] Jacobs, P.A. and R. Von Ballmoos, J. Phys. Chem., 1982. 86(15): p. 3050-3052.

[27] Védrine, J.C., et al., J. Catal., 1979. 59(2): p. 248-262.

[28] Shirazi, L., E. Jamshidi, and M.R. Ghasemi, Cryst. Res. Technol., 2008. 43(12): p. 1300-1306.

[29] Gayubo, A.G., et al., J. Chem. Technol. Biotechnol., 1996. 65(2): p. 186-192.

Chapter 4

EFFECT OF HIGH TEMPERATURE ON THE CATALYTIC ACTIVITY OF ZEOLITE H-ZSM-5 FOR ISOBUTANE CONVERSION

4.1 Introduction

We have shown in the previous chapters that Bronsted acid sites in zeolites are decomposed by thermal treatment into redox sites and Lewis acid sites. Since zeolite acid catalysts are known to catalyze a number of hydrocarbon reactions, it is of scientific interest and of potential technological value to investigate the impact of the newly- generated sites on hydrocarbon reactions. To be specific, the effects of these new sites on hydrocarbon cracking processes will be investigated by determining the distributions of product formed in the cracking process, the cracking-to-dehydrogenation ratio, and the activation energy of the reactions.

The activation of small alkanes over acid sites has been investigated extensively because of its relevance to technologically important processes such as fluidized catalytic cracking in petroleum refineries, but also because C-H and C-C bond activation is of fundamental scientific interest [1, 2]. In our group‟s previous work, the propane cracking process was scrutinized using various zeolite catalysts, such as ZSM-5, beta zeolites, and mordenite [3]. The distribution of products produced by propane is controlled by

cracking on the classical Bronsted acid sites (after mild activation such as treatment 1) [1,

3]. After dehydroxylation was performed on the acid zeolite by a high temperature treatment (treatment 2), the dehydrogenation rate increased significantly compared to the rate before dehydroxylation [3]. When the sample was treated in an oxygen flow

(treatment 3), the distribution of products changed slightly; only a small decreased cracking-to-dehydrogenation ratio is observed.

The isobutane cracking process is the topic of investigation in this chapter. There are several reasons to look beyond the propane cracking process, discussed previously, into the isobutane cracking process. An important aspect of our investigation is to examine the initial step for isobutane cracking. The protonation of C-H or C-C bonds, which is referred to as a protolytic reaction mechanism, has been considered by many researchers to be the initial step of hydrocarbon activation in acid zeolites. Several groups have proposed that carbonium or carbenium ion intermediates on Bronsted acid sites in zeolites play the key role in the activation of isobutane [1, 4-7]. The effect of Lewis acid sites on activity and selectivity has also been investigated. While Lewis acid sites enhance the adsorption of the reactant and increase the strength of Bronsted acid sites by withdrawing electron density [2], turnover frequency (TOF) does not correlate with

Lewis acid sites concentration [1]. Thus, we can consider that Lewis acid sites, by themselves, are not responsible for the activation of alkanes. In contrast to this view,

McVicker et al. have suggested that radical cation intermediates play an important role in the conversion of isobutane [8]. They also suggested that highly acidic catalysts crack isobutane by a combination of radical cation and carbonium ion routes [8].

Different reactivity has been observed in the cracking of isoalkanes and the cracking of normal alkanes (n-alkanes) [1, 9]. For n-alkanes, monomolecular dehydrogenation is preferred rather than the cracking process on acid catalysts [1].

However, the cracking process, rather than dehydrogenation, is the preferred reaction channel for isobutane [9]. Moreover, the relationship between the cracking-to- dehydrogenation ratio and temperature is different for the two classes of compounds, with a direct relationship for one class (n-alkanes) and an inverse relationship for the other class (iso-alkanes). Based on our observation for propane activation on thermally treated zeolites, the study of the behavior of isobutane is a clear next step. According to several reports on light hydrocarbon conversions [1, 9-11], the rate of the reaction of isobutane is higher than the rate of the reaction of propane. Thus, small amounts of catalysts (5 - 10 mg in this thesis) are used for isobutane conversion rather than propane reaction (70 mg in our previous work).

In this chapter, we examine the catalytic activity of the ZSM-5 samples treated under different conditions by using isobutane as a reactant. The samples were treated 1) at 500 °C in He, 2) 800 °C in He, and 3) 500 °C in O2 before contacting the catalyst with the isobutane stream. The products were separated and quantified using gas chromatography (GC). The isobutane reaction on Bronsted acid sites and on dehydroxylated sites can be discussed in relation to our group‟s previous work on propane. Investigation of the distributions of products and the conversion of isobutane may help to clarify the structures after Bronsted acid sites after decomposition and the structure-activity relationships in hydrocarbon conversions.

The cracking and the dehydrogenation of isobutane can be analyzed using thermodynamics including the enthalpy of reaction, Gibbs free energy of reaction, equilibrium constants, and conversions. Table 4.1 shows the relevant thermodynamic information of the isobutane reaction [12, 13]. The enthalpies of reaction change slightly as the temperature increases from 480 to 560 °C: the enthalpies of cracking reaction decrease ~1 kJ/mol, while those of dehydrogenation reaction increase ~0.2 kJ/mol. The

Gibbs free energies of both cracking and dehydrogenation reactions decrease ~12 kJ/mol.

The equilibrium conversions of both cracking and dehydrogenation process increase with temperature because the reactions are endothermic. The cracking of isobutane shows higher equilibrium conversion than the dehydrogenation of isobutane. The information can provide us an insight into the further analysis in this chapter.

Table 4.1 Thermodynamic properties for cracking and dehydrogenation reaction of isobutane.

Cracking Dehydrogenation

Temp. iC4 H 10 iC 3 H 6 CH 4 iC4 H 10 iC 4 H 8 H 2 (°C) H G K rxn rxn eq Conversion Conversion kJ/mol % kJ/mol % 480 76.3 -38.1 436.7 99.989 122.1 16.8 0.0678 67.502 560 75.3 -50.2 1397.7 99.997 122.3 5.7 0.4420 91.034

4.2 Kinetic Analysis of Isobutane Cracking on Zeolites

Several groups have studied the isobutane cracking reaction [4, 8, 9, 14, 15].

These studies conclude that the initial step of the reaction associated with isobutane

conversion, which proceeds by monomolecular and bimolecular channels, occurs primarily on Bronsted acid sites. At low conversion, the monomolecular reaction dominates the product distribution. Scheme 4.1 illustrates the rate-limiting step, which is the monomolecular cracking of the C-C bond and dehydrogenation of the C-H bond. The reaction rate expressions for cracking and dehydrogenation are similar and are expressed by equation 1. At low conversion and high temperatures, the rate expression can be simplified to a first order rate equation (equation 2) since most of the sites are vacant.

The intrinsic activation energy and entropy are represented by equations 3 and 4. From transition state theory, the intrinsic rate constant can be expressed in term of the transition state enthalpy and entropy as shown in equation 5 [1, 9]. The measured entropy is scaled by the number of C-C bonds for cracking and H-C bonds for dehydrogenation (equation

6).

Scheme 4.1 Protolytic mechanism of isobutane (cracking and dehydrogenation)

KP rk 1 CH4 10 1 3 1KPKPKP   1C4 H 10 4 c C 3 H 6 4 d C 4 H 8 r k K P k P 2 31C4 H 10 app C 4 H 10

EEHint meas   ads 3

SSSint  meas   ads 4

Smeas  Rln A meas  ln k B T / h 6

The product distribution for the protolytic monomolecular reaction mechanism is represented as the ratio of the rate constants (k3c/k3d). The measured activation energy and entropy are estimated from the Arrhenius plot of the rate data obtained at 450 -

560 °C. The weight hour space velocity (WHSV) is set at ~30 h-1 for a short residence time because low conversion is needed to limit the reaction only to the monomolecular mechanism. Equimolar ratios of methane to propylene, which confirm that only the monomolecular reaction occurs, were observed for all experiments.

4.3 Effect of Pre-treatment of the Sample on Isobutane Conversion and Selectivity

Isobutane conversions and product distributions were measured in a plug flow quartz reactor (ID = 4 mm) with a bed supported by fine quartz chips. The ammonium form of catalyst (5 - 10mg) was loaded in the reactor and activated at 500 °C under He

(99.999%) flow of 100 sccm (treatment 1); at 800 °C under He flow of 100 sccm

(treatment 2); and at 500 °C under O2 (99.999%) flow of 70 sccm (treatment 3). Small amount of catalysts was used for the reaction because of higher rate of the isobutane reaction than the propane reaction. Different experiments were performed on separate samples for each treatment. The flow rate of isobutane (Matheson tri-gas, research grade,

99.995%) was fixed so that WHSV was ~30 h-1 in order to minimize the residence time and decrease conversion. The WHSV is calculated using the following relationship;

WHSV = mass flow / catalyst mass

When WHSV is low, the reaction intermediates have enough time to react through bimolecular reaction mechanisms. However, when WHSV is high, the contact time of the reaction intermediates with the catalysts is decreased. In order to obtain high WHSV, the mass flow rate of the reactant needs to increase and the catalyst mass needs to be reduced as much as possible. The temperature measured at the catalyst bed was controlled between 450 and 560 °C and the products were separated and recorded by the GC. The

GC instrument used is GC model 2014 (Shimadzu) with two columns. One column is a molesieve connected to a TCD detector for H2 and N2 detection and another column is a

RT-alumina connected to a FID detector for hydrocarbon analysis.

Table 4.2 Conversion of isobutane over ZSM5 for three different treatments.

Conversion, %

ZSM5-18 ZSM5-12 Temp., °C Treatment 1 Treatment 2 Treatment 3 Treatment 1 Treatment 2 Treatment 3

450 0.199 0.069 0.171 0.076

480 0.028 0.718 0.116 0.081 0.408 0.124

500 0.065 1.275 0.174 0.183 0.868 0.205

530 0.192 3.333 0.445 0.557 2.300 0.538

560 0.577 7.299 0.911 1.525 5.670 1.504

Table 4.2 shows the conversions for each treatment. For both samples (ZSM5-18 and ZSM5-12), the conversions for treatment 2 were significantly greater than the conversions for treatment 1. For treatment 3, the conversions were slightly different from the conversions of treatment 1, but the differences were not significant. As discussed in chapter 3, dehydroxylation at high temperatures leads predominantly to the formation of redox sites, and these sites produce significantly increases in the conversions of the isobutane reaction. The dehydroxylation obviously leads to the loss of Bronsted acid sites.

When we compared sample ZSM5-18 with sample ZSM5-12 for treatment 1, the conversions of ZSM5-12 were greater than the conversions of ZSM5-18 because ZSM5-

12 has more acid sites. Table 4.3 also shows turn-over frequencies (TOF) per aluminum for ZSM5-12 are higher than those for ZSM5-18 after treatment 1.

Table 4.3 TOF per aluminum for ZSM5-12 and for ZSM5-18 after treatment 1.

TOF (mol/s Al mol) × 10-5 Temp. ZSM5-18 ZSM5-12 °C Cracking Dehydrogenation Cracking Dehydrogenation 450 1.89 3.24 480 1.71 1.87 3.68 3.30 500 3.43 4.10 7.22 8.36 530 10.78 11.61 24.00 22.90 560 35.17 30.95 73.07 54.24

For treatment 2, the observed reaction rate is the sum of the reaction rate of the dehydroxylated Bronsted acid sites (new sites) and the remaining Bronsted acid sites. The contribution of newly generated sites can be separated from the contribution of remaining

Bronsted acid sites using the information from the TPD experiments in Chapter 3. The remaining Bronsted acid sites are still responsible to the protolytic process while the newly generated sites are responsible to the redox chemistry process. The reaction rates at reaction temperature 500 °C were calculated for both ZSM-5 samples in Table 4.4. The reaction rate contributed by the newly generated sites was significantly larger than the reaction rate on the remaining Bronsted acid sites. It is assumed that there is a certain effect to enhance the isobutane conversion after treatment 2 contributed by the newly generated sites.

Table 4.4 Contribution of newly generated sites and remaining Bronsted acid sites after treatment 2.

Reaction rate at 500 °C, r [mol/s∙gcat] × 108 Cracking Dehydrogenation Remaining BAS New sites Remaining BAS New sites

ZSM5-18 Treatment 1 3.01 3.60 133.03 28.28 Treatment 2 1.78 131.25 2.13 26.25 59.3% 59.3% ZSM5-12 Treatment 1 8.96 10.37 91.45 25.63 Treatment 2 5.69 85.76 6.58 19.05 63.5% 63.5%

The conversions of ZSM5-18 after treatment 2, however, exceeded those of

ZSM5-12. As was noted in Chapter 3, at high Si/Al ratio, the decomposition of the

Bronsted acid sites pathway produces more redox sites that the low Si/Al ratio sample.

Therefore, the conversion of ZSM5-12 after treatment 2 was lower than the conversion of

ZSM5-18. From the Chapter 3 and literature [16, 17], it is expected that Lewis acid sites were formed mainly by the decomposition of the Bronsted acid sites in treatment 3. Even if the Lewis acid sites enhance the adsorption of reactant and increase Bronsted acid sites strength by withdrawing electron density [2], the effect on conversion was small compared to the redox sites generated by treatment 2.

The product distributions are shown in Figure 4.1. For treatment 1, the cracking- to-dehydrogenation ratios were approximately 1. However, for treatment 2, the cracking- to-dehydrogenation ratios showed a significant increase. This observation is opposite to the change in product distribution in the propane activation process, for which the cracking-to-dehydrogenation ratios decrease by treatment 2 [1, 3]. This opposite tendency also was reported in a recent report by Gounder and Iglesia [1, 9]. It was observed by

Gounder and Iglesia that the selectivity of the isoalkane and n-alkane cracking reactions on acid catalysts change in opposition to each other [9]. For treatment 3, even if the cracking-to-dehydrogenation ratios increase with temperature, it was considered that the results are due to the small number of redox sites generated by treatment 3, rather than the effect of the Lewis acid sites.

(a)

(b)

Figure 4.1 The cracking-to-dehydrogenation ratios with temperature for (a) ZSM5-18 and (b) ZSM5-12.

Measured activation energies (Table 4.5) were calculated assuming first-order kinetics. For treatment 1, the measured activation energies were consistent with values found in literature [9]. The activation energy for cracking was ~ 197 kJ/mol, and that for dehydrogenation was ~ 182 kJ/mol. Measured activation energies determined in this work were 15 - 16 kJ/mol larger for cracking than those determined for the dehydrogenation of isobutane for both ZSM-5 samples. These differences were also consistent with values found in literature [7, 9, 14]. These differences are related to the high cracking-to-dehydrogenation ratios. It is explained that the transition states for cracking is less stable than the transition states for dehydrogenation since the activation energies for cracking is higher than the activation energies for dehydrogenation [1, 9].

Table 4.5 Measured activation energies for isobutane cracking and dehydrogenation after each treatment.

Cracking Dehydrogenation Difference Sample Ea (kJ/mol) Ea (kJ/mol) Ea,c – Ea,d ZSM-5-18 – treatment 1 197.8 182.4 15.4 ZSM-5-18 – treatment 2 163.5 142.2 21.3 ZSM-5-18 – treatment 3 183.2 92.4 90.8

ZSM-5-12 – treatment 1 196.8 180.8 16 ZSM-5-12 – treatment 2 164.3 142.7 21.6 ZSM-5-12 – treatment 3 192.6 124.3 68.3

After treatment 2, the measured activation energies for both ZSM-5 samples decreased significantly to ~ 164 kJ/mol for cracking and to ~142 kJ/mol for dehydrogenation. We observed that the redox sites in zeolites have the ability to extract a single-electron from neutral organic molecules in Chapter 3. Likewise, the redox sites can also oxidize molecules that have high ionization potential such as ethylene (IP = 10.52 eV) [18]. The ionization potentials of isobutane, isobutylene, propane, and propylene are tabulated as Table 4.6 [19]. The IP of isobutane is 10.57 eV and therefore, the reaction of isobutane over the dehydroxylated sample after treatment 2 could proceed by radical cation chemistry [20]. The IP of propylene is 9.73 eV and the IP of isobutylene is 9.23 eV.

The isobutylene is less stable than the propylene. Therefore, the selectivity to the formation of propylene is much higher than the formation of isobutylene when the radical cation chemistry plays a kinetically controlling role. The combination effect of the stability of transition state and the difference of ionization potential between the isobutylene and the propylene significantly increase the selectivity on cracking rather than dehydrogenation.

Table 4.6 Ionization potentials of light hydrocarbons

Isobutane Isobutylene Propane Propylene

Ionization potential (eV) 10.57 9.23 11.07 9.73

The measured activation energies for cracking after treatment 3 were slightly less than those after treatment 1. However, the measured activation energies for dehydrogenation decreased very significantly to 92.4 kJ/mol for ZSM5-18 and 124.3 kJ/mol for ZSM5-12. From Chapter 3, the generated sites by treatment 3 also have the ability to extract a single-electron from neutral organic molecules. In addition, it is expected that the transition state for cracking is less stable. However, the selectivity is slightly different from the selectivity of the sample after treatment 1. Even if there is a combination effect like the case of treatment 2, the activation barriers for dehydrogenation are so small that the selectivity on cracking cannot significantly increase as was observed on the sample after treatment 2.

The rate constants as a function of reciprocal temperature after treatments 1 and 2 are shown in Figure 4.2. When the sample was treated at 500 °C in He, the rate constants for cracking and for dehydrogenation were of similar magnitude. After treatment 2, the rate constants for cracking became larger than those for dehydrogenation, while the rate constant for both processes were greater than those for treatment 1. After treatment 3, the rate constants for dehydrogenation were greater than those for cracking at low temperatures. At high temperatures, the rate constants for cracking became larger than those for dehydrogenation. This observation is basically the same to the rate constants after treatment 1, while the effect of treatment 3 magnifies the difference between the rate constants for cracking and dehydrogenation. The changes that were observed for the activation energies and the rate constants suggest that the generated sites have a different nature altogether from Bronsted acid sites.

(a)

(b)

Figure 4.2 Arrhenius plots for isobutane cracking and dehydrogenation (a) ZSM5-18 and (b) ZSM5-12.

We have proposed that a single electron deficient site (a redox site) was generated by the high temperature treatment (treatment 2) of the acid zeolites. The electron deficient site leads to catalytic chemistry and is responsible for the variations in the kinetic parameters and for variations in the distribution of products observed above. In addition, they suggest a different reaction mechanism from the classical protolytic mechanism. In our group‟s previous work [3], it was suggested that the presence of redox sites resulted in radical cation chemistry instead of protolytic chemistry in the propane cracking process [21]. Other researchers also have proposed this radical chemistry as the mechanism for the cracking of isobutane and n-butane [8, 22]. Therefore, we believe that the reaction of isobutane over the zeolite sample that had redox sites proceeded through the radical cation chemistry mechanism. Scheme 4.2 represents a feasible mechanism for the unimolecular reaction of isobutane when redox sites are available.

Scheme 4.2 Suggested pathway of monomolecular reaction of isobutane over redox sites on ZSM-5.

4.4 Conclusions

In addition to the catalytic effect of typical acid catalyst, Lewis acid sites and redox sites, which are generated by high temperature treatment and oxygen treatment for dehydroxylation, also promote alkane cracking processes. The conversions of the samples after treatments 2 and 3 are greater than the conversions of the samples after treatment 1

(acid catalyst). For treatment 1, the conversions of ZSM5-12 are greater than the conversions of ZSM5-18 because ZSM5-12 has more acid sites. However, the conversions of ZSM5-18 become greater than the conversion of ZSM5-12 after treatments 2 and 3.

When conversion is low, the product distribution is limited to the monomolecular cracking of the C-C bond and dehydrogenation of the C-H bond. In the propane conversion reaction, the dehydrogenation process is preferred over the cracking process on the zeolite samples after treatments for the dehydroxylation. However, in the isobutane conversion in this thesis, the cracking-to-dehydrogenation ratio significantly increases after dehydroxylation treatments.

The measured activation energies are larger for cracking than those for the dehydrogenation of isobutane for both ZSM-5 samples. The transition states for the cracking process are less stable due to the higher measured activation energies. The sites generated by dehydroxylation have the ability to form organic radical cations. The effect of the stability of transition state and the radical cation chemistry affect the selectivity on cracking process. The change of the rate constants after treatments 2 is much larger that the change of the rate constants after treatment 1.

While Lewis acid sites enhance the reactivity of Bronsted acid sites and rarely affect the conversions and selectivity of the isobutane reaction, the redox sites result in a reaction that is based on radical cation chemistry. We proposed that the presence of redox sites resulted in radical cation chemistry instead of protolytic chemistry in the propane and isobutane cracking process. In the industrial hydrocarbon cracking processes, such as the FCC process, the existence of redox sites can affect the distribution of products in the way that have not been considered by the zeolite community to this point.

4.5 References

[1] Gounder, R. and E. Iglesia, J. Am. Chem. Soc., 2009. 131(5): p. 1958-1971.

[2] van Bokhoven, J.A., et al., J. Catal., 2004. 224(1): p. 50-59.

[3] Al-majnouni, K.A., HIGH TEMPERATURE DECOMPOSITION OF BRONSTED ACID SITES: STRUCTURES FORMED AND THEIR CATALYTIC ACTIVITY TOWARD SMALL ALKANES ACTIVATION, in Chemical Engineering. 2011, University of Delaware: Newark.

[4] Engelhardt, J., J. Catal., 1996. 164(2): p. 449-458.

[5] Xu, B., et al., J. Catal., 2006. 244(2): p. 163-168.

[6] Sendoda, Y. and Y. Ono, Zeolites, 1988. 8(2): p. 101-105.

[7] Corma, A., P.J. Miguel, and A.V. Orchilles, J. Catal., 1994. 145(1): p. 171-180.

[8] McVicker, G.B., G.M. Kramer, and J.J. Ziemiak, J. Catal., 1983. 83(2): p. 286- 300.

[9] Gounder, R. and E. Iglesia, Angew. Chem. Int. Ed. Engl., 2010. 49(4): p. 808-811.

[10] Hoobler, R.J., B.J. Opansky, and S.R. Leone, The Journal of Physical Chemistry A, 1997. 101(7): p. 1338-1342.

[11] Copeland, L.R., et al., J. Chem. Phys., 1992. 96(8): p. 5817-5826.

[12] Sandler, S.I., Chemical, Biochemical, and Engineering Thermodynamics. 4 ed. 2006: John Wiley & Sons, Inc.

[13] NIST Chemistry Webbook. Available from: http://webbook.nist.gov/chemistry/.

[14] Yaluris, G., et al., J. Catal., 1995. 153(1): p. 65-75.

[15] Sanchez-Castillo, M.A., et al., J. Catal., 2002. 205(1): p. 67-85.

[16] Moissette, A., et al., J. Phys. Chem. B, 2003. 107(34): p. 8935-8945.

[17] Marquis, S., et al., C. R. Chim., 2005. 8(3-4): p. 419-440.

[18] Yoon, K.B., Chem. Rev., 1993. 93(1): p. 321-339.

[19] Occupational Training Inc.; Available from: http://www.otrain.com/OTI_MSDS(IP)800.html.

[20] McAdoo, D.J., S. Olivella, and A. Solé, The Journal of Physical Chemistry A, 1998. 102(52): p. 10798-10804.

[21] Narbeshuber, T.F., et al., J. Catal., 1997. 172(1): p. 127-136.

[22] Bizreh, Y.W. and B.C. Gates, J. Catal., 1984. 88(1): p. 240-243.

Chapter 5

CONCLUSIONS AND FUTURE RESEARCH DIRECTIONS

5.1 Summary

Bronsted acid sites are known to be the active sites in many hydrocarbon conversions processes by forming carbonium or carbenium ions as reaction intermediates.

In some industrial hydrocarbon processes, such as fluidized catalytic cracking (FCC),

Bronsted acid sites are decomposed by dehydroxylation at high temperatures under an oxidizing environment. After Bronsted acid sites are decomposed, new sites having a different „chemical‟ nature from Bronsted acid sites are generated. The catalytic chemistry of the zeolites must be affected by the newly generated sites. In this thesis, the

Bronsted acid sites are decomposed by a high temperature pretreatment and an oxygen- rich pretreatment. The structures and the reactivity of the newly generated sites by decomposition of Bronsted acid sites were investigated and compared with the Bronsted acid sites.

5.1.1 Generation of Organic Radical Cations in Thermally Treated ZSM-5 Zeolites

The newly generated sites can extract single-electrons from neutral organic molecules having small ionization potential, such as naphthalene, to form radical cations.

The adsorbed naphthalene, the naphthalene radical cations, electron-hole pairs, and reformed naphthalene were observed by time-series UV/vis spectroscopy inside the

ZSM-5 framework. Ammonium TPD experiment provided information about how many

Bronsted acid sites are left, and some information about the structure of the newly generated sites by the different treatments. The sites generated in the sample treated in O2 are much more reactive than the sites in the sample treated in Ar towards naphthalene.

The amount of ammonia desorbed from ZSM-5 decreased after the high temperature treatment while there is no significant change after the oxygen treatment. At high Si/Al ratio of 18, the decomposition pathway leads dominantly to the same type of sites generated by treatment 3, even in treatment 2. At low Si/Al ratio of 12, it seems that the formation of the same type of sites formed by treatment 2 in ZSM-5-18 becomes dominant. These observations suggest that the most frequently generated site is different depending on each treatment.

5.1.2 Catalytic Activity of ZSM-5 Zeolite for Isobutane Conversion

The reactivity and selectivity of isobutane conversion on these sites are also revealed to be different depending on treatment. The conversion of isobutane significantly increased after treatment 2 while the conversion of isobutane only slightly

increased after treatment 3. The product distribution is categorized into the monomolecular cracking of the C-C bond and dehydrogenation of the C-H bond. Before dehydroxylation (acid catalyst), the cracking-to-dehydrogenation ratio is around 1.

However, the cracking-to-dehydrogenation ratio significantly increased after the high temperature treatment while the ratios are slightly affected by the oxygen treatment. The measured activation energies are larger for cracking than those for dehydrogenation. The higher activation energies are related to less thermodynamically stable transition states.

Thus, the cracking process is preferred to the dehydrogenation process for isobutane on the new reaction sites. The kinetic parameters suggest that the newly generated sites after treatments 2 and 3 have a different nature from Bronsted acid sites and each other. We have proposed that the presence of redox sites resulted in radical cation chemistry instead of protolytic chemistry in the propane and isobutane cracking process.

5.2 Future Research Directions

5.2.1 Determination of the Structure of Lewis Acid Sites and Redox Sites

To elucidate the structure of newly generated sites (assuming Lewis acid sites and redox sites) after the high temperature treatment in an inert gas (treatment 2) and the oxygen treatment (treatment 3), further experiments are necessary.

We have collected structural information using ammonia TPD experiment and

FTIR spectroscopy in this thesis. To reveal the actual Lewis acid sites, other TPD

experiment, such as TPD of isopropylamine should be explored. To elucidate the structure of the acid sites after dehydroxylation, spectroscopic techniques, such as X-ray absorption spectroscopy, would also be helpful. Al K edge XANES has been applied to investigate the local coordination environment of aluminum in several zeolites [1-4].

Bugaev et al. reported that the aluminum atoms remain in the zeolite framework at high temperatures [5]. We expect that the structure of acid sites in zeolite after dehydroxylation and the pathway for dehydroxylation of high-silica and low-silica zeolites can be revealed by the X-ray spectroscopy.

In this thesis we have used ZSM-5 zeolite as the main catalyst. ZSM-5 has a high silicon aluminum ratio and it is one of most versatile heterogeneous catalysts known. In addition, due to the pore size (~5.6 Å ) of ZSM-5, it shows a good confinement for naphthalene (~5 Å along its short dimension), our probe molecule for the formation of radical cation. However, ZSM-5 is known to have 24 crystallographically different tetrahedron sites [6, 7] in the monoclinic setting. This complexity of the structure of

ZSM-5 makes the analysis of dehydroxylation of Bronsted acid sites very difficult although all the sites retain their original coordination sequences and the distortion is subtle [6, 7]. We need to examine other kinds of zeolites with a simpler structure than

ZSM-5. Chabazite (CHA) and Ferrierite (FER) are possible alternatives for further experiments. Chabazite has only one topologically distinct T-site and only 4 non- equivalent oxygen atoms that upon protonation from four non-equivalent Bronsted acid sites [8, 9]. Ferrierite has 4 non-equivalent tetrahedron atoms [10]. As simpler structures are investigated, in addition to ZSM-5, we can identify the structural characteristics that

lead to Lewis acid sites and redox sites. Since Chabazite is a small-size pore zeolite, other probe molecules other than naphthalene will be needed to observe the generation of the sites that can extract a single-electron from a neutral organic molecule. Since Ferrierite is a medium pore size zeolite, newly generated sites by decomposition of Bronsted acid sites can be observed using naphthalene in the same way used in this thesis. For catalyst activation, Chabazite and Ferrierite are also expected to provide much information about the Lewis acid sites and redox sites present in the sample. Based on our previous report

[3, 11] and this thesis, propane and isobutane reactions can be examined using both catalysts and analyzed to clarify the reaction pathway and the local structures involved in the catalytic activity.

5.2.2 Low Temperature CO Oxidation

Low temperature oxidation is an important academic and industrial issue because the high temperatures are required for oxidation by current catalysts, such as platinum, limit its applicability for purification processes such as CO removal from hydrogen gas.

For example, currently catalysis by Au is of high interest since it was discovered that novel gold catalysts are highly active for H2 and CO oxidation at low temperature as low as 70 °C [12]. It would be interesting if we could oxidize CO at low temperatures using catalysts without the need for expensive transition metals.

In a few preliminary experiments, using the generated electron-hole pair from the interactions of naphthalene with ZSM-5 after treatments for dehydroxylation, CO

100

oxidation was observed at low temperatures (close to the room temperature). CO2 was generated from the oxidation of CO with O2 (300% excess) at temperature between 40 to

80 °C even though the generation of CO2 was not sustainable over long periods of time.

Above 80 °C, no further oxidation was observed, probably because the naphthalene was destroyed. We still need further experiments to prove that the generated electron-hole pair is involved to the formation of CO2, and to clarify chemistry behind this process.

However, there is the potential using other molecules besides naphthalene, to carry out this reaction under conditions in which long-term catalytic stability is obtained.

Figure 5.1 CO2 production from naphthalene-ZSM-5 catalyst

101

5.3 References

[1] van Bokhoven, J.A., A.M.J. van der Eerden, and D.C. Koningsberger, J. Am. Chem. Soc., 2003. 125(24): p. 7435-7442.

[2] Xu, B., et al., Journal of Catalysis, 2006. 241(1): p. 66-73.

[3] Nash, M.J., et al., J. Am. Chem. Soc., 2008. 130(8): p. 2460-2462.

[4] Joyner, R.W., et al., A soft X-ray EXAFS study of the local structure of tetrahedral aluminium in zeolites, in Recent Advances in the Science and Technology of Zeolites and Related Materials, Pts a - C. 2004. p. 1406-1410.

[5] Bugaev, L.A., et al., J. Phys. Chem. B, 2005. 109(21): p. 10771-10778.

[6] Cejka, J., A. Corma, and S. Zones, Zeolites and catalysis : synthesis, reactions and applications. 2010, Weinheim; Chichester: Wiley-VCH ; John Wiley, distributor].

[7] Fyfe, C.A., et al., Nature, 1982. 296(5857): p. 530-533.

[8] Stoyanov, S.R., et al., J. Phys. Chem. C, 2008. 112(17): p. 6794-6810.

[9] Lo, C. and B.L. Trout, J. Catal., 2004. 227(1): p. 77-89.

[10] Pinar, A.B., et al., J. Catal., 2009. 263(2): p. 258-265.

[11] Al-majnouni, K.A., HIGH TEMPERATURE DECOMPOSITION OF BRONSTED ACID SITES: STRUCTURES FORMED AND THEIR CATALYTIC ACTIVITY TOWARD SMALL ALKANES ACTIVATION, in Chemical Engineering. 2011, University of Delaware: Newark.

[12] Haruta, M., et al., J. Catal., 1989. 115(2): p. 301-309.

102