catalysts
Article Oligomerization of Butene Mixture over NiO/Mesoporous Aluminosilicate Catalyst
Donggun Lee 1, Hyeona Kim 1, Young-Kwon Park 2 and Jong-Ki Jeon 1,*
1 Department of Chemical Engineering, Kongju National University, Cheonan 31080, Korea; [email protected] (D.L.); [email protected] (H.K.) 2 School of Environmental Engineering, University of Seoul, Seoul 02504, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-41-521-9363
Received: 29 August 2018; Accepted: 11 October 2018; Published: 16 October 2018
Abstract: This study is aimed at preparing C8–C16 alkene through oligomerization of a butene mixture using nickel oxide supported on mesoporous aluminosilicate. Mesoporous aluminosilicate with an ordered structure was successfully synthesized from HZSM-5 zeolite by combining a top-down and a bottom-up method. MMZZSM-5 catalyst showed much higher butene conversion and C8–C16 yield in the butene oligomerization reaction than those with HZSM-5. This is attributed to the pore geometry of MMZZSM-5, which is more beneficial for internal diffusion of reactants, reaction intermediates, and products. The ordered channel-like mesopores were maintained after the nickel-loading on MMZZSM-5. The yield for C8–C16 hydrocarbons over NiO/MMZZSM-5 was higher than that of MMZZSM-5 catalyst, which seemed to be due to higher acid strength from a higher ratio of Lewis acid to Brønsted acid. The present study reveals that a mesoporous NiO/MMZZSM-5 catalyst with a large amount of Lewis acid sites is one of the potential catalysts for efficient generation of aviation fuel through the butene oligomerization.
Keywords: butene; oligomerization; aviation fuel; mesoporous aluminosilicate; nickel
1. Introduction The greenhouse gas (GHG) emitted when an aircraft is in operation has negative impacts on the environment, including global warming [1]. To resolve such problems, the International Civil Aviation Organization announced a policy intended to reduce GHG emission by aircraft using biomass-derived fuels for aircraft [2]. Consequently, research about synthesis of aviation fuel from biomass is drawing attention. The Alcohol to Jet (ATJ) process for making alcohol from biomass as feedstock and for making aviation fuel from alcohol has recently attracted attention [3]. This process involves the production of butene through the dehydration of butanol, the synthesis of higher olefins by oligomerization of the newly produced butene, and hydrogenation of the higher olefins [4–6]. Studies on the synthesis of butenes through butanol dehydration in the first step are already under way [7–11]. We have previously reported that silica-based, solid acid catalysts with mesopores and highly dispersed metal oxide catalysts are effective in the production of butene by butanol dehydration [12,13]. Because the demand for aviation fuel is increasing steadily, maximizing the production of aviation fuel is of great interest to oil refining companies [14]. Because butene is an excess in the refineries (FCC), the use of oligomerization to convert butene into aviation fuel is a promising method for the generation of aviation fuel. The butene oligomerization reaction has been studied mainly using zeolites or heterogeneous catalysts [14–19]. Yoon et al. found that ferrierite, zeolite beta, and Amberlyst-35 exhibited high catalytic performance for the trimerization of isobutene [20–22]. It is also known that a catalyst loaded with nickel on Y zeolite or ZSM-5 is effective for trimerization of linear butene [23–26].
Catalysts 2018, 8, 456; doi:10.3390/catal8100456 www.mdpi.com/journal/catalysts Catalysts 2018, 8, x FOR PEER REVIEW 2 of 12
catalytic performance for the trimerization of isobutene [20–22]. It is also known that a catalyst loaded Catalystswith nickel2018, 8on, 456 Y zeolite or ZSM-5 is effective for trimerization of linear butene [23–26]. 2 of 13 We have been studying the application of mesoporous aluminosilicate with zeolite framework for catalyticWe have reaction. been studying In particular, the application we have of published mesoporous research aluminosilicate results in which with zeolite a framework framework was forconstructed catalytic reaction.using commercial In particular, zeolite. we From have publishedthis, mesoporous research aluminosilicate results in which with a framework a well-ordered was constructedstructure was using produced commercial by combining zeolite. a From top-do this,wn mesoporous method and aluminosilicatea bottom-up method with a[13,27]. well-ordered structureThe waspurpose produced of this bystudy combining was to prepare a top-down a particular method fraction and a bottom-up (C8–C16; corresponding method [13,27 to]. aviation fuel) through oligomerization of butene using nickel oxide supported on mesoporous aluminosilicate The purpose of this study was to prepare a particular fraction (C8–C16; corresponding to aviation fuel)with througha well-ordered oligomerization structure. ofWe butene used a using butene nickel mixture oxide having supported the same on mesoporous composition aluminosilicate as the product withobtained a well-ordered from dehydration structure. of Webutanol used as a buteneraw material mixture for having the oligomerization. the same composition High-pressure as the productoligomerization obtained of from the dehydrationbutene mixture of butanol was investigated as raw material in a fixed-bed for the oligomerization. continuous-flow High-pressure reactor. oligomerization of the butene mixture was investigated in a fixed-bed continuous-flow reactor. 2. Results and Discussion 2. Results and Discussion 2.1. Textural Properties of Catalysts 2.1. Textural Properties of Catalysts The N2 adsorption–desorption isotherm of MMZZSM-5 (Figure 1) showed a type IV(a) physisorptionThe N2 adsorption–desorption isotherm in the IUPAC isotherm classification of MMZ [28,29].ZSM-5 Furthermore,(Figure1) showed the N a2 typeadsorption– IV(a) physisorptiondesorption isotherm isotherm of in MMZ the IUPACZSM-5 exhibited classification the [ 28H4,29 type]. Furthermore, of hysteresis the Nloop,2 adsorption–desorption which is typical of isothermmesoporous of MMZ zeolitesZSM-5 andexhibited micro-mesoporous the H4 type of materials hysteresis [29,30]. loop, which In the iscase typical of NiO/MMZ of mesoporousZSM-5 catalyst, zeolites andthe micro-mesoporouscharacteristics of N materials2 physisorption [29,30]. isotherm In the case were of NiO/MMZ not much ZSM-5differentcatalyst, from the the characteristics N2 adsorption– of Ndesorption2 physisorption isotherm isotherm of MMZ wereZSM-5 not. This much indicates different that from the the geometry N2 adsorption–desorption of the ordered mesopores isotherm ofof MMZMMZZSM-5ZSM-5 was. This not indicates changed that by the NiO geometry loading. of the ordered mesopores of MMZZSM-5 was not changed by NiOThe loading. pore size distribution curves of the three catalysts were compared in Figure 2. As expected fromThe the porenitrogen size adsorption distribution isot curvesherm, of it the was three difficult catalysts to observe were compared the mesopores in Figure of HZSM-5,2. As expected while a frommesopore the nitrogen peak of adsorptionabout 3.0 nm isotherm, could be it wasobserved difficult on toMMZ observeZSM-5. theMoreover, mesopores the ofpore HZSM-5, size somewhat while a mesoporedecreased peakafter ofthe about nickel 3.0 loading nm could of 9 bewt observed%. Howeve onr, MMZ the overallZSM-5. characteristic Moreover, the pore-size pore size distribution somewhat decreasedcurve of the after material the nickel did loadingnot change of 9 after wt %. NiO However, loading. the overall characteristic pore-size distribution curveThe of the BET material surface did areas not of change MMZZSM-5 after and NiO HZSM-5 loading. were 745 and 448 m2/g, respectively (Table 1). 2 3 Furthermore,The BET surfacethe total areas pore ofvolumes MMZZSM-5 of MMZandZSM-5 HZSM-5 and HZSM-5 were 745 were and 0.47 448 mand/g, 0.19 respectively cm /g, respectively. (Table1). 3 Furthermore,Therefore, it was the totalclear pore that volumesthe BET ofsurface MMZ areaZSM-5 andand pore HZSM-5 size of were MMZ 0.47ZSM-5 and catalyst 0.19 cm were/g, much respectively. larger Therefore,than those it of was HZSM-5 clear that catalyst. the BET Meanwhile, surface area the and BET pore surface size of MMZarea andZSM-5 porecatalyst volume were of much MMZ largerZSM-5 thandecreased those slightly of HZSM-5 after catalyst.the NiO loading, Meanwhile, which the was BET du surfacee to the fact area that and the pore nickel volume oxide of was MMZ attachedZSM-5 decreasedto the pore slightly surface after and thepartially NiO loading, blockedwhich a pore was mouth. due to the fact that the nickel oxide was attached to the pore surface and partially blocked a pore mouth.
Adsorption Desorption
NiO/MMZZSM-5 -1
MMZZSM-5 (STP) g 3 /cm a V
HZSM-5
0.0 0.2 0.4 0.6 0.8 1.0 P/P 0
FigureFigure 1.1. NN22 adsorption–desorption isothermsisotherms ofof variousvarious catalysts.catalysts.
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NiO/MMZZSM-5 p /dlogd p dV
MMZZSM-5
HZSM-5
1 10 100 dp/nm Figure 2. Pore size distribution of various catalysts.
Table 1. Textural properties of the catalysts. Table 1. Textural properties of the catalysts. Zeolite Type S (m2 g−1)V (cm3 g−1) a D (nm) b Zeolite Type BET SBET (m2 g−1)VP (cmP 3 g−1) a D P (nm)P b HZSM-5HZSM-5 448448 0.19 0.19 - - MMZ 745 0.47 2.7 ZSM-5MMZZSM-5 745 0.47 2.7 NiO/MMZZSM-5 726 0.43 2.7 NiO/MMZZSM-5 726 0.43 2.7 a b Total pore volume measured at p/p0 = 0.99. Average pore diameter determined by the BJH method. a Total pore volume measured at p/p0 = 0.99. b Average pore diameter determined by the BJH method.
Low-angle XRD patterns of the MMZZSM-5 and NiO/MMZZSM-5 catalysts are presented in Figure3. Low-angle XRD patterns of the MMZZSM-5 and NiO/MMZZSM-5 catalysts are presented in Figure 3. MMZZSM-5 has three diffraction peaks corresponding to 100, 110, and 200 Bragg reflections, respectively. MMZZSM-5 has three diffraction peaks corresponding to 100, 110, and 200 Bragg reflections, In particular, the peak at 2θ of 2.3◦ is the characteristic peak of the 2-D hexagonal structure, and the respectively. In particular, the peak at 2θ of 2.3° is the characteristic peak of the 2-D hexagonal two weak peaks at 3.9◦ and 4.5◦ show the inherent characteristics of the mesoporous material [27]. structure, and the two weak peaks at 3.9° and 4.5° show the inherent characteristics of the The NiO/MMZZSM-5 sample represents the characteristic peaks of MMZZSM-5 with three intense mesoporous material [27]. The NiO/MMZZSM-5 sample represents the characteristic peaks of MMZZSM- peaks at 2θ = 2.3◦, 3.9◦, and 4.5◦, respectively. After nickel loading, the XRD patterns of the material 5 with three intense peaks at 2θ = 2.3°, 3.9°, and 4.5°, respectively. After nickel loading, the XRD showed a slight reduction in peak intensity but maintained the same patterns as those of MMZZSM-5. patterns of the material showed a slight reduction in peak intensity but maintained the same patterns This suggests that the ordered mesoporous structures of the MMZZSM-5 are well retained after the as those of MMZZSM-5. This suggests that the ordered mesoporous structures of the MMZZSM-5 are well nickel loading. The XRD patterns of MMZZSM-5 and NiO/MMZZSM-5 material in the high-angle region retained after the nickel loading. The XRD patterns◦ of MMZZSM-5 and NiO/MMZZSM-5 material in the (Figure4) give a broad line centered at 2 θ ≈ 23 , which indicates that the framework of MMZZSM-5 was high-angle region (Figure 4) give a broad line centered at 2θ ≈ 23°, which indicates that the framework not atomically ordered and the starting source, HZSM-5, was not present in the MMZZSM-5 material. of MMZZSM-5 was not atomically ordered and the starting source, HZSM-5, was not present in the A TEM image is presented in Figure5 to determine whether MMZ ZSM-5 and NiO/MMZZSM-5 MMZZSM-5 material. have hexagonal structures. MMZZSM-5 has a well-ordered structure with mesopores that are hexagonal A TEM image is presented in Figure 5 to determine whether MMZZSM-5 and NiO/MMZZSM-5 have 1-D channels [31]. The NiO/MMZZSM-5 sample maintained the structural features of MMZZSM-5 after hexagonal structures. MMZZSM-5 has a well-ordered structure with mesopores that are hexagonal 1-D the nickel loading. channels [31]. The NiO/MMZZSM-5 sample maintained the structural features of MMZZSM-5 after the nickel loading.
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NiO/MMZZSM-5 NiO/MMZZSM-5
NiO/MMZZSM-5 Intensity (a.u.) Intensity Intensity (a.u.) Intensity Intensity (a.u.) Intensity
MMZZSM-5 MMZZSM-5
MMZZSM-5 246810 2θ (degree) 246810
2θ (degree) ZSM-5 ZSM-5. Figure 3. Low-angle XRD patterns of MMZZSM-5 and NiO/MMZ ZSM-5. Figure 3. Low-angle XRD patterns of MMZZSM-5 and NiO/MMZZSM-5. Figure 3. Low-angle XRD patterns of MMZZSM-5 and NiO/MMZZSM-5.
HZSM-5
Intensity (a.u.) HZSM-5 Intensity (a.u.)
MMZ Intensity (a.u.) ZSM-5 MMZZSM-5
MMZZSM-5
NiO/MMZZSM-5 NiO/MMZZSM-5
20 40 60NiO/MMZ 80 20 40 60 80 ZSM-5 θ 2 θ (degree) 20 40 60 80
Figure 4. High angle XRD patterns of2 θHZSM-5, (degree) MMZZSM-5, and NiO/MMZZSM-5. FigureFigure 4. 4.High High angle angle XRD XRD patterns patterns of of HZSM-5, HZSM-5, MMZ MMZZSM-5ZSM-5,, andand NiO/MMZNiO/MMZ ZSM-5.
Figure 4. High angle XRD patterns of HZSM-5, MMZZSM-5, and NiO/MMZZSM-5. (a) (b) (a) (b)
ZSM-5 ZSM-5 FigureFigure 5. 5.TEM TEM image image of of (a ()a MMZ) MMZZSM-5ZSM-5 andand ((bb)) NiO/MMZNiO/MMZZSM-5. .
2.2. Acid Properties of CatalystsFigure 5. TEM image of (a) MMZZSM-5 and (b) NiO/MMZZSM-5.
The acidic characteristics of HZSM-5, MMZ , and NiO/MMZ were analyzed using ZSM-5 ZSM-5 NH3-TPD (Figure6). HZSM-5 exhibits two peaks at the desorption temperature of ammonia near
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2.2. Acid Properties of Catalysts
The acidic characteristics of HZSM-5, MMZZSM-5, and NiO/MMZZSM-5 were analyzed using NH3- Catalysts 2018, 8, 456 5 of 13 TPD (Figure 6). HZSM-5 exhibits two peaks at the desorption temperature of ammonia near 180 and 380 °C, indicating that weak strength acid sites and medium strength acid sites are widely distributed. 180The andNH 3803-TPD◦C, curve indicating of MMZ thatZSM-5 weak shows strength thatacid the sitespeakand intensity medium is greatly strength reduced acid sites compared are widely to distributed.HZSM-5. This The indicates NH3-TPD that MMZ curveZSM-5 ofMMZ has bothZSM-5 theshows weak acid that sites the peak near 170 intensity °C and is medium greatly reducedstrength ◦ comparedacid near 340 to HZSM-5.°C, but the This total indicates amount that of acid MMZ sitesZSM-5 overhas MMZ bothZSM-5 the weakis greatly acid reduced sites near compared 170 C and to ◦ mediumHZSM-5. strengthThe reason acid could near be 340 that C,aluminum but the is total relatively amount less of exposed acid sites to overthe amorphous MMZZSM-5 frameworkis greatly reducedthat constructs compared the pore to HZSM-5. walls of The MMZ reasonZSM-5. couldOn the be other that aluminumhand, in the is case relatively of the lessMMZ exposedZSM-5 catalyst, to the amorphousthe peak temperature framework of that NH constructs3-TPD shifted the to pore a lower walls temperature of MMZZSM-5 than. On that the of other HZSM-5, hand, which in the casemeans of thethat MMZ the acidZSM-5 strengthcatalyst, was the also peak weakened. temperature of NH3-TPD shifted to a lower temperature than that of HZSM-5,For the which NiO/MMZ means thatZSM-5 the catalyst, acid strength the desorption was also weakened.peak at a low temperature shifted to high temperatureFor the NiO/MMZ(210 °C), indicatingZSM-5 catalyst, that the the acid desorption strength peak of NiO/MMZ at a low temperatureZSM-5 is higher shifted than tothat high of ◦ temperatureMMZZSM-5 catalyst. (210 ItC), is indicatingnoticeable that that an the additional acid strength high-temperature of NiO/MMZ (540ZSM-5 °C)is peak higher appeared than that on the of ◦ MMZNH3-TPDZSM-5 profilecatalyst. of Itthe is NiO/MMZ noticeable thatZSM-5 ancatalyst, additional which high-temperature means that nickel (540 oxideC) peakgenerates appeared a new on type the NHof acid3-TPD site. profile This ofhigh-temperature the NiO/MMZZSM-5 peakcatalyst, is known which to be means attributable that nickel to Lewis oxide acid generates sites from a new nickel type ofoxide acid [32]. site. This high-temperature peak is known to be attributable to Lewis acid sites from nickel oxide [32].
Figure 6.6. NH33-TPD profiles profiles of various catalysts.
Figure7 7 shows the the results results of ofthe the pyridine-FTIR pyridine-FTIR for the for NiO/MMZ the NiO/MMZZSM-5 sample.ZSM-5 Assample. the desorption As the ◦ −1 desorptiontemperature temperature increased to increased 300 °C, to the 300 bandC, the intensity band intensity at 1596 at 1596cm−1 cmdiminisheddiminished drastically drastically and ◦ anddisappeared disappeared at temperatures at temperatures higher higher than than150 °C, 150 indicatingC, indicating a very a veryweak weak H acid H site acid [33,34]. site [33 Four,34]. well- Four −1 ◦ well-resolvedresolved bands bands can be can observed be observed at (1450, at (1450,1491, 1542, 1491, and 1542, 1610) and cm 1610)−1 at cm higherat than higher 150 than °C. Two 150 ofC. −1 −1 Twothese of bands these bandsare attributed are attributed to Lewis to Lewis sites sites (1450 (1450 and and 1610 1610 cm cm−1), ),while while the the band band at 15421542 cmcm−1 −1 corresponds toto thethe BrönstedBrönsted sitessites [[3333,34].,34]. AdditionalAdditional bandband nearnear 14911491 cmcm−1 isis associated with a Lewis site and Brönsted site simultaneously.simultaneously. The pyridine-FTIRpyridine-FTIR resultsresults ofof HZSM-5, HZSM-5, MMZ MMZZSM-5ZSM-5,, andand NiO/MMZNiO/MMZZSM-5 catalystscatalysts are are shown in −1 −1 −1 Figure8 8.. HZSM-5 HZSM-5 is is rich rich in in Brönsted Brönsted sites sites (1542 (1542 cm cm−1)) andand also also has has Lewis Lewis sites sites (1450 (1450 cm cm−1,, 16101610 cm cm−1).). In MMZMMZZSM-5 also,also, both both Brönsted Brönsted and and Lewis Lewis acidity acidity can can be be observed. observed. It It should should also be noted that for −1 the NiO/MMZ ZSM-5ZSM-5 catalyst,catalyst, the the band band corresponding corresponding to the to theLewis Lewis acidacid site (1450 site (1450 cm−1) cm was much) was muchlarger larger than that associated with the Brönsted site (1542 cm−1). That is, the NiO loading shows that the
Lewis site is relatively increased. The pyridine-FTIR and NH3-TPD results suggest that a new acid site is generated by loading of NiO over MMZZSM-5, which is attributable to the Lewis site. Catalysts 2018, 8, x FOR PEER REVIEW 6 of 12 Catalysts 2018, 8, x FOR PEER REVIEW 6 of 12 than that associated with the Brönsted site (1542 cm−1). That is, the NiO loading shows that the Lewis sitethan is thatrelatively associated increased. with the The Brönsted pyridine-FTIR site (1542 and cm NH−1). 3That-TPD is, results the NiO suggest loading that shows a new that acid the site Lewis is generatedsite is relatively by loading increased. of NiO The over pyridine-FTIR MMZZSM-5, which and isNH attributable3-TPD results to the suggest Lewis thatsite. a new acid site is generated by loading of NiO over MMZZSM-5, which is attributable to the Lewis site. Catalysts 2018, 8, 456 6 of 13
1450 a 1450 b a 1596 c db 1491 c 1610 1596 a e d b 1542 1491 f 1610 a e b 1542 f Absorbance Absorbance Absorbance Absorbance c d ec f d e f 1650 1600 1550 1500 1450 1400 -1 1650 1600 Wave1550 number1500 (cm ) 1450 1400 Wave number (cm-1) Figure 7. Pyridine-FTIR spectra over NiO/MMZZSM-5 under a vacuum (a: room temperature, b: 100 °C, ◦ c:FigureFigure 150 °C, 7.7. d: Pyridine-FTIRPyridine-FTIR 200 °C, e: 250 spectra spectra °C, and over over f: 300 NiO/MMZ NiO/MMZ °C). ZSM-5ZSM-5 underunder a vacuum (a: room temperature,temperature,b: b: 100100 °C,C, ◦ ◦ ◦ ◦ c:c: 150150 °C,C, d:d: 200 °C,C, e: 250 °C,C, and and f: f: 300 300 °C).C).
1451 1491 1542 1451 1610 1542 1491 1610 NiO/MMZZSM-5
NiO/MMZZSM-5
MMZZSM-5
MMZZSM-5 Absorbance (a.u.) Absorbance Absorbance (a.u.) Absorbance
HZSM-5
HZSM-5
1650 1600 1550 1500 1450 1400 -1 1650 1600 Wave1550 number1500 (cm ) 1450 1400 -1 Figure 8. Pyridine-FTIR spectra atWave 200 ◦numberC under (cm a vacuum) over various catalysts. Figure 8. Pyridine-FTIR spectra at 200 °C under a vacuum over various catalysts. 2.3. OligomerizationFigure of 8. Mixed-Butene Pyridine-FTIR spectra at 200 °C under a vacuum over various catalysts. 2.3. Oligomerization of Mixed-Butene The HZSM-5 catalyst showed good stability for butene oligomerization (Figure9). In previous 2.3. Oligomerization of Mixed-Butene studiesThe on HZSM-5 trimerization catalyst of showed iso-butene, good it stability was proposed for butene that oligomerization the catalytic performance (Figure 9). ofIn solidprevious acid studiescatalystsThe on andHZSM-5 trimerization zeolites catalyst could of showediso-butene, be altered good byit wasstability their proposed pore for geometry butene that theoligomerization [21 catalytic,22]. The performance most (Figure common 9). of In solid finding previous acid is catalyststhatstudies medium-pore onand trimerization zeolites molecular could of beiso-butene, sieves, altered such by it as theirwas zeolite proposedpore MFI geometry and that zeolite the [21,22]. catalytic beta, The were performancemost shown common to be of suitable findingsolid acid foris thatoligomerizationcatalysts medium-pore and zeolites of molecular iso-butene could besieves, or altered n-butene such by as intotheir zeolite dieselpore MFI geometry and and gasoline zeolite [21,22]. fuels beta, The due were most to shown their common shape-selective to be finding suitable is forpropertiesthat oligomerization medium-pore [20,35]. Besides molecularof iso-butene that, sieves, these or zeolite suchn-butene as catalysts zeolite into MFI werediesel and proved and zeolite gasoline to havebeta, high fuelswere coke-resistance dueshown to totheir be suitableshape- due to limitedfor oligomerization polyaromatic of generation iso-butene in or their n-butene pore structure. into diesel Based and on gasoline similar fuels analogies, due to the their stability shape- of HZSM-5 catalyst during the conversion of the butene mixture into C8–C16 hydrocarbon could be due to the coke-resistance resulting from the pore geometry. Catalysts 2018, 8, x FOR PEER REVIEW 7 of 12
selective properties [20,35]. Besides that, these zeolite catalysts were proved to have high coke- resistance due to limited polyaromatic generation in their pore structure. Based on similar analogies, the stability of HZSM-5 catalyst during the conversion of the butene mixture into C8–C16 hydrocarbon could be due to the coke-resistance resulting from the pore geometry. Catalysts 2018, 8, 456 7 of 13
100
80
60
40
20 HZSM-5 Conversion of butene (%) butene of Conversion MMZZSM-5
NiO/MMZZSM-5 0 123456 Time on stream (h) FigureFigure 9. 9.Conversion Conversion of ofbutene butenethrough through the theoligomerization oligomerization of of a abutene butene mixture mixture over over various various catalysts catalysts ◦ −1 (Reaction(Reaction condition: condition: pressure pressure 1.5 1.5 MPa, MPa, temperature temperature 350 350 C,°C, WHSV WHSV 10 10 h h−1).
After the reaction had progressed for six hours, the MMZ catalyst showed much higher After the reaction had progressed for six hours, the MMZZSM-5ZSM-5 catalyst showed much higher butene conversion and C –C yield than those with HZSM-5 (Figures9 and 10). From the NH -TPD butene conversion and C8 8–C1616 yield than those with HZSM-5 (Figures 9 and 10). From the NH3 3-TPD and Pyridine-FTIR results, MMZ was found to have fewer and weaker acidities than HZSM-5 and Pyridine-FTIR results, MMZZSM-5ZSM-5 was found to have fewer and weaker acidities than HZSM-5 catalyst. Thus, it is presumed that the reason why the MMZ catalyst is beneficial for butene catalyst. Thus, it is presumed that the reason why the MMZZSM-5ZSM-5 catalyst is beneficial for butene oligomerization, despite the fact that the acidity of MMZ is inferior to HZSM-5, is its pore oligomerization, despite the fact that the acidity of MMZZSM-5ZSM-5 is inferior to HZSM-5, is its pore geometry. MMZ has an average pore size of 2.7 nm, which is much larger than the pore size geometry. MMZZSM-5 has an average pore size of 2.7 nm, which is much larger than the pore size (0.5– (0.5–0.6 nm) of HZSM-5. Consequently, it can be suggested that the mesoporous structure of MMZ 0.6 nm) of HZSM-5. Consequently, it can be suggested that the mesoporous structure of MMZZSM-5ZSM-5 playsplays an an important important rolerole in in overcoming overcoming the the internal internal pore pore diffusion diffusion of of the the molecules molecules participating participatingin in thethe reaction. reaction. Moreover, the yield for C –C hydrocarbons over NiO/MMZ was higher than that over the Moreover, the yield for 8C8–C1616 hydrocarbons over NiO/MMZZSM-5ZSM-5 was higher than that over the MMZ (Figures9 and 10). The pore size distributions of the two catalysts were not significantly MMZZSM-5ZSM-5 (Figures 9 and 10). The pore size distributions of the two catalysts were not significantly different. On the other hand, according to the NH -TPD and pyridine-FTIR analysis, it can be confirmed different. On the other hand, according to the3 NH3-TPD and pyridine-FTIR analysis, it can be that when NiO is loaded on MMZ , the Lewis acid sites become more abundant and the acid confirmed that when NiO is loadedZSM-5 on MMZZSM-5, the Lewis acid sites become more abundant and strength becomes stronger. Therefore, the higher butene conversion over NiO/MMZ catalysts the acid strength becomes stronger. Therefore, the higher butene conversion over ZSM-5NiO/MMZZSM-5 seemedcatalysts to seemed be due to to be higher due to acid higher strength acid strength resulting resulting from a higherfrom a Lewishigher site-to-BrønstedLewis site-to-Brønsted site ratio. site Consequently,ratio. Consequently, it may beit may summarized be summarized that mesoporous that mesoporous structures, structures, as well as as the well high as ratiothe high of Lewis ratio toof Brønsted acid, were beneficial for production of C –C hydrocarbons through butene oligomerization. Lewis to Brønsted acid, were beneficial for production8 16 of C8–C16 hydrocarbons through butene oligomerization.The carbon number distribution in the aviation fuel range (C8–C16 hydrocarbons) was not significantly influenced by the catalysts (Figure 11). In our previous study, it was found that dimers The carbon number distribution in the aviation fuel range (C8–C16 hydrocarbons) was not weresignificantly the main influenced reaction products by the atcatalysts a low butane (Figure conversion 11). In our over previous the zeolite study, catalysts it was found [26]. In that this dimers study, becausewere the the main butene reaction conversion products was at high a low over butane three co catalysts,nversion the over carbon the zeolite number catalysts distribution [26]. inIn thethis rangestudy, of because C8–C16 thehydrocarbon butene conversion became broader,was high which over three is ascribed catalysts, tothe thecontribution carbon number of consecutive distribution cracking and subsequent oligomerization pathways. in the range of C8–C16 hydrocarbon became broader, which is ascribed to the contribution of consecutive cracking and subsequent oligomerization pathways.
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CatalystsCatalysts2018 2018, 8, 8 456, x FOR PEER REVIEW 88 of of 13 12 100 conversion of butene 100 selectivity to C8-C16 90 conversion of butene yield of C8-C16 selectivity to C8-C16 90 80 yield of C8-C16
80 70
70 60
60 50
Conversion, and (%) selectivity, yield 50 40
Conversion, and (%) selectivity, yield NiO/MMZ HZSM-5 MMZZSM-5 ZSM-5 40 HZSM-5 MMZCatalyst NiO/MMZZSM-5 ZSM-5 Catalyst Figure 10. Conversion of butene, selectivity to C8–C16 fraction, and yield of C8 –C16 fraction over Figurevarious 10. catalystsConversion (reaction of condition: butene, selectivity pressure 1.5 to MPa, C8–C temperature16 fraction, and350 °C, yield WHSV of C 108–C h−161, andfraction T-O-S Figure 10. Conversion of butene, selectivity to C8–C16 fraction, and yield of C8–C16 fraction over over6 h). various catalysts (reaction condition: pressure 1.5 MPa, temperature 350 ◦C, WHSV 10 h−1, various catalysts (reaction condition: pressure 1.5 MPa, temperature 350 °C, WHSV 10 h−1, and T-O-S and T-O-S 6 h). 6 h).
25 NiO/MMZZSM-5 20 25 NiO/MMZ 15 ZSM-5 1020 155 100 255 MMZZSM-5 200 25 15 MMZZSM-5 1020 15
Selectivity (%) Selectivity 5 100
Selectivity (%) Selectivity 5 25 HZSM-5 200 1525 HZSM-5 1020 155 100 5 8 9 10 11 12 13 14 15 16 0 Carbon Number 8 9 10 11 12 13 14 15 16 Figure 11. Product distribution of the C8–C16 rangeCarbon through Number the oligomerization of a butene mixture Figure 11. Product distribution of the C8–C16 range through the oligomerization of a butene mixture over various catalysts (reaction condition: pressure 1.5 MPa, temperature 350 ◦C, WHSV 10 h−1, over various catalysts (reaction condition: pressure 1.5 MPa, temperature 350 °C, WHSV 10 h−1, T-O- T-O-SFigure 6 h). 11. Product distribution of the C8–C16 range through the oligomerization of a butene mixture S 6 h). over various catalysts (reaction condition: pressure 1.5 MPa, temperature 350 °C, WHSV 10 h−1, T-O- ◦ CatalyticS 6 h). stability of the NiO/MMZZSM-5 catalyst was evaluated under 1.5 MPa at 350 C for 30 h Catalytic stability of the NiO/MMZZSM-5 catalyst was evaluated under 1.5 MPa at 350 °C for 30 h on stream (Figure 12). The conversion of butene was more than 80% and did not decrease significantly on stream (Figure 12). The conversion of butene was more than 80% and did not decrease significantly for 18 hCatalytic on stream. stability However, of the the NiO/MMZ NiO/MMZZSM-5ZSM-5 catalystcatalyst was exhibitedevaluated a under decrease 1.5 inMPa butene at 350 conversion °C for 30 h for 18 h on stream. However, the NiO/MMZZSM-5 catalyst exhibited a decrease in butene conversion afteron stream 18 h of (Figure time-on-stream 12). The conversi (TOS) whileon of havingbutene was no significant more than drop 80% and in selectivity did not decrease to C8–C significantly16 fraction after 18 h of time-on-stream (TOS) while having no significant drop in selectivity to C8–C16 fraction withfor 18 TOS. h on As stream. a result, However, the yield the of NiO/MMZ C8–C16 hydrocarbonZSM-5 catalyst remained exhibited abovea decrease 63% in for butene 18 h TOS, conversion then with TOS. As a result, the yield of C8–C16 hydrocarbon remained above 63% for 18 h TOS, then graduallyafter 18 h decreased of time-on-stream to 48.0% after(TOS) 30 while h TOS. having no significant drop in selectivity to C8–C16 fraction gradually decreased to 48.0% after 30 h TOS. withTo TOS. quantify As a the result, coke the deposits yield of formed C8–C16 on hydrocarbon the NiO/MMZ remainedZSM-5 catalyst above 63% after for reaction 18 h TOS, for 30 then h, To quantify the coke deposits formed on the NiO/MMZZSM-5 catalyst after reaction for 30 h, the thegradually weight contents decreased of theseto 48.0% deposits after 30 were h TOS. determined by TGA. According to Figure 13, two ranges of weight contents of these deposits were determined by TGA. According◦ to Figure 13, two ranges of weightTo losses quantify can be the recognized: coke deposits a low-temperature formed on the regionNiO/MMZ at <100ZSM-5 catalystC and a after high-temperature reaction for 30 region h, the weight losses can be recognized: a low-temperature region at <100 °C and a high-temperature region atweight >400 ◦ C,contents which couldof these be deposits attributed were to volatile determined organic by compoundsTGA. According and coke to Figure deposits, 13, two respectively. ranges of at >400 °C, which could be attributed to volatile organic compounds and coke deposits, respectively. Consequently,weight losses itcan was be foundrecognized: that about a low-temperature 4 wt % of coke region was formed at <100 on°C and the NiO/MMZa high-temperatureZSM-5 catalyst region Consequently, it was found that about 4 wt % of coke was formed on the NiO/MMZZSM-5 catalyst after afterat >400 TOS °C, of 30which h. could be attributed to volatile organic compounds and coke deposits, respectively. TOS of 30 h. Consequently, it was found that about 4 wt % of coke was formed on the NiO/MMZZSM-5 catalyst after TOS of 30 h.
Catalysts 2018, 8, x FOR PEER REVIEW 9 of 12
CatalystsCatalysts2018 2018,,8 8,, 456 x FOR PEER REVIEW 99 of 1312 100
100 80
80 60
60 40
40 20 conversion of butenes selectivity to C8-C16
20 yieldconversion of C8-C of16 butenes Conversion, Selectivity, and (%) Yield 0 selectivity to C8-C16 0 3 6 9 12 15 18 21 24 27 30 yield of C8-C16 Conversion, Selectivity, and (%) Yield 0 Time on stream (h) 0 3 6 9 12 15 18 21 24 27 30 Time on stream (h) Figure 12. Conversion of butene, selectivity to C8–C16 fraction, and yield of C8–C16 fraction over NiO/MMZZSM-5 catalyst for TOS up to 30 h (reaction condition: pressure 1.5 MPa, temperature 350 °C, Figure 12. Conversion of butene, selectivity to C8–C16 fraction, and yield of C8–C16 fraction over Figure 12. Conversion of butene, selectivity to C8–C16 fraction, and yield of C8–C16 fraction over and WHSV 10 h−1). ◦ NiO/MMZZSM-5 catalyst for TOS up to 30 h (reaction condition: pressure 1.5 MPa, temperature 350 C, NiO/MMZZSM-5 catalyst for TOS up to 30 h (reaction condition: pressure 1.5 MPa, temperature 350 °C, and WHSV 10 h−1). and WHSV 10 h−1). 100 600
100 500600 98
400500 98 96 300400 96 94 Weight(%) 200300 Temperature (℃) Temperature 94 weight Weight(%) 92 100200 temperature Temperature (℃) Temperature 92 weight 90 0100 temperature 0 102030405060 90 Time (minute) 0 0 102030405060 Figure 13. Degradation temperature measurement of the spent NiO/MMZ catalyst. Figure 13. Degradation temperature measurementTime (minute) of the spent NiO/MMZZSM-5ZSM-5 catalyst. 3. Materials and Methods 3. MaterialsFigure and Methods 13. Degradation temperature measurement of the spent NiO/MMZZSM-5 catalyst. 3.1. Preparation of Catalysts 3.1.3. Materials Preparation and of MethodsCatalysts The procedure of the catalyst preparation consists of the top–down and bottom–up approaches [13,27]. The procedure of the catalyst preparation consists of the top–down and bottom–up approaches A3.1. cationic Preparation surfactant of Catalysts and HZSM-5 zeolite were used as the template and the framework sources, [13,27]. A cationic surfactant and HZSM-5 zeolite were used as the template and the framework respectively.The procedure At first, of HZSM-5 the catalyst is rent preparation into nano-unit consists in alkaliof the solution top–down (top–down and bottom–up approach). approaches NaOH sources, respectively. At first, HZSM-5 is rent into nano-unit in alkali solution (top–down approach). (0.375[13,27]. mol) A cationic was dissolved surfactant in 76.5 and mL HZSM-5 of deionized zeolite water were toused prepare as the NaOH template aqueous and solutionthe framework (4.9 M, NaOH (0.375 mol) was dissolved in 76.5 mL of deionized water to prepare NaOH aqueous solution 80.0sources, mL), respectively. and then 33.75 At g first, of HZSM-5 HZSM-5 (Zeolyst, is rent into SiO2 nano-unit/Al2O3 mole in alkali ratio =solution 50) was (top–down mixed into approach). the NaOH (4.9 M, 80.0 mL), and then 33.75 g of HZSM-5 (Zeolyst, SiO2/Al2O3 mole ratio = 50) was mixed into aqueousNaOH (0.375 solution mol) and was stirred dissolved for 1 in h. 76.5 Subsequently, mL of deionized this zeolitic water nano-unit to prepare is NaOH assembled aqueous into orderedsolution the NaOH aqueous solution and stirred for 1 h. Subsequently, this zeolitic nano-unit is assembled mesoporous(4.9 M, 80.0 mL), material and usingthen 33.75 the templateg of HZSM-5 (bottom–up (Zeolyst, approach). SiO2/Al2O3 Hexadecyltrimethylammoniummole ratio = 50) was mixed into into ordered mesoporous material using the template (bottom–up approach). bromidethe NaOH (0.189 aqueous mol) solution was dissolved and stirred into 1050 for 1 mL h. ofSubsequently, deionized water, this zeolitic and the nano-unit HZSM-5 solutionis assembled was Hexadecyltrimethylammonium bromide (0.189 mol) was dissolved into 1050 mL of deionized water, addedinto drop-by-drop.ordered mesoporous This mixture material was stirred usin forg 24the h, andtemplate the pH was(bottom–up controlled toapproach). 10 using and the HZSM-5 solution was added drop-by-drop. This mixture was stirred for 24 h, and the pH CHHexadecyltrimethylammonium3COOH solution (50%). After bromide pH adjustment (0.189 mol) and was mixing dissolved were into repeated 1050 mL three of deionized times, a water, white was controlled to 10 using CH3COOH solution (50%). After pH adjustment◦ and mixing were repeated◦ precipitateand the HZSM-5 was obtained. solution The was resulting added productsdrop-by-drop. were driedThis atmixture 110 C was overnight stirred and for calcined24 h, and at the 550 pHC three times, a white precipitate was obtained. The resulting products were dried at 110 °C overnight forwas 4 controlled h in an air to atmosphere 10 using CH to3COOH obtain solution a mesoporous (50%). aluminosilicateAfter pH adjustment material. and mixing Subsequently, were repeated it was and calcined at 550 °C for 4 h in an air atmosphere to obtain a mesoporous+ aluminosilicate material. ion-exchangedthree times, a white using precipitate NH4Cl aqueous was obtained. solution (1The M) re tosulting convert products to the NH were4 form dried and at 110 again °C convertedovernight Subsequently,+ it was ion-exchanged using NH4Cl aqueous◦ solution (1 M) to convert to the NH4+ form toand H calcinedform by at calcination 550 °C for in4 h an in airan atmosphereair atmosphere at 550 to obtainC for a 3 mesoporous h. The newly aluminosilicate prepared material material. was and again converted to H+ form by calcination in an air atmosphere at 550 °C for 3 h. The newly namedSubsequently, MMZZSM-5 it was, meaning ion-exchanged mesoporous using material NH4Cl aqueous from ZSM-5. solution (1 M) to convert to the NH4+ form prepared material was named MMZZSM-5, meaning mesoporous material from ZSM-5. and again converted to H+ form by calcination in an air atmosphere at 550 °C for 3 h. The newly prepared material was named MMZZSM-5, meaning mesoporous material from ZSM-5.
Catalysts 2018, 8, 456 10 of 13
Nickel nitrate hexahydrate (98.0%) was impregnated on MMZZSM-5 catalyst by the dry impregnation method. The loading amount of nickel metal was 3 wt %. After calcination in an ◦ air atmosphere at 550 C for 3 h, the NiO/MMZZSM-5 catalyst was obtained.
3.2. Characterization of Catalysts The nitrogen physisorption isotherms of the catalysts were obtained using a BELSORP-mini II device (BEL Japan Inc., Toyonaka, Japan) at −196 ◦C. XRD patterns of the catalysts were collected at 40 kV and 40 mA using a D/MAX-2200V Diffractometer (Rigaku Co., The Woodlands, TX, USA), and data were reduced using the JADE program. TEM images were collected using a JEM-3010 (JEOL, Tokyo, Japan) operated at 300 kV. The NH3-TPD was performed using BEL-CAT-B device (BEL Japan Inc., Toyonaka, Japan). The sample was outgassed at 550 ◦C in a He gas flow (50 mL/min) for an hour. The ammonia (5% in He) was adsorbed on the sample, and then the physically adsorbed ammonia was desorbed in a He gas flow (50 mL/min) for 2 h. Subsequently, the amount of ammonia desorbed was detected by a thermal conductivity detector while increasing the temperature of sample holder from 100 to 550 ◦C at a heating rate of 10 ◦C/min. FTIR spectra of adsorbed-pyridine were attained with a Spectrum GX device (Perkin Elmer Co., Waltham, MA, USA) equipped with an HgCdTe detector and a custom-made cell. FTIR spectra of pyridine adsorbed on the catalyst disk were measured while the temperature of the cell was increased to 300 ◦C[12,13]. In order to determine the amount of the coke over the spent catalyst, a TGA analysis was carried out using a SDT Q600 device (TA Instruments, New Castle, DE, USA). N2 was injected through a mass flow controller (model 5850E from Brooks, Hatfield, PA, USA) operated using a flow pressure controller (ATOVAC, GMC 1200, Yongin, Korea). The sample was loaded onto a Pt holder, and the volume changes and energy flow were recorded while running N2 (60 mL/min) and increasing the temperature up to 600 ◦C at a heating rate of 10 ◦C/min.
3.3. Oligomerization of Butene Mixture The oligomerization reaction of the butene mixture was conducted in a fixed-bed continuous-flow reactor [26]. The reactor set-up and the experimental procedures for butene oligomerization were described in detail in the literature [26]. In our previous research on dehydration reaction of butanol, a mixture of butene (butene-1:butene-2 = 1.0:1.3) could be generated through dehydration of 2-butanol over a solid acid catalyst [13]. Accordingly, a mixture composed of 43.5% of butene-1 (99%, Rigas Korea Co., Daejeon, Korea) and 56.5% of butene-2 (11.8% of cis-2-butene and 87.5% of trans-2-butene, Rigas Korea Co., Daejeon, Korea) was used as a model reactant. The particle size of the catalyst was 25 µm or less. The reaction pressure and temperature were 15 bar and 350 ◦C, respectively, unless otherwise specified. In this study, most of the reaction products were alkene compounds. Needless to say, in order to use this product as a fuel, it must be converted to a paraffin compound via a hydrogenation reaction. The selectivity to C8–C16 hydrocarbons was calculated based on the alkenes in the range of C8–C16. The formulae for the conversion of butene, the selectivity to C8–C16 hydrocarbons, and the yield of C8–C16 hydrocarbons are as follows:
Conversion of butene (wt %) = (consumed butene/fed butene) ∗ 100 (1)
Selectivity of C8-C16 (wt %) = (C8-C16 product/consumed butene) ∗ 100 (2)
Yield of C8-C16 (wt %) = (C8-C16 product/fed butene) ∗ 100 (3) Catalysts 2018, 8, 456 11 of 13
4. Conclusions Mesoporous aluminosilicate with a well-ordered structure was successfully prepared from HZSM-5 by combining a top-down method and a bottom-up method. The ordered channel-like mesopores were maintained after the nickel-loading on MMZZSM-5 catalyst. The NH3-TPD and pyridine-FTIR results revealed that a new kind of acid site (Lewis acid sites) was generated by NiO loading on MMZZSM-5. The MMZZSM-5 catalyst showed much higher butene conversion and C8–C16 yield in the butene oligomerization reaction than those with HZSM-5. This is attributed to the pore geometry of MMZZSM-5, which is more beneficial for internal diffusion of reactants, reaction intermediates, and products. Moreover, the yield for C8–C16 hydrocarbons over NiO/MMZZSM-5 was higher than that over the MMZZSM-5, which seemed to be due to higher acid strength resulting from a higher ratio of Lewis sites to Brønsted sites. The present study reveals that mesoporous NiO/MMZZSM-5 catalyst with high concentration of Lewis acid sites is a potential catalyst for production of C8–C16 hydrocarbons through the oligomerization of a butene mixture.
Author Contributions: D.L. and H.K. performed experiments; D.L., H.K., Y.-K.P., and J.-K.J. collected and analyzed data; and D.L. and J.-K.J. wrote the manuscript. Funding: This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2015R 1D 1A 1A01058354). This work was also supported by “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20174010201560). Conflicts of Interest: The authors declare no conflict of interest.
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