New Technologies and Alternative Feedstocks in Petrochemistry and Refining DGMK Conference October 9 – 11, 2013, Dresden, Germany

Influence of Crystallite Size and Shape of Zeolite ZSM-22 on its Activity and Selectivity in the Catalytic Cracking of n-Octane F. Bager, S. Ernst Department of Chemistry, Chemical Technology, University of Kaiserslautern, Germany

Abstract Light olefins belong to the major building blocks for the petrochemical industry, particularly for the production of polymers. It has become necessary to increase the production of light olefins specifically in the case for propene with so called “on-purpose propene” technologies. One possible route is to increase the amount of propene that can be obtained from Fluid Catalytic Cracking (FCC) by optimizing the catalyst through introducing new additives, which offer a high selectivity to propene. Zeolite ZSM-22 samples with different crystallite sizes and morphologies have been synthesized via hydrothermal syntheses and characterized by powder X-Ray diffraction, nitrogen physisorption, atomic absorption spectroscopy, scanning electron microscopy and solid-state NMR spectroscopy. The zeolites in the Brønsted-acid form have been tested as catalysts in the catalytic cracking of n-octane as a model hydrocarbon. Clear influences of the crystallite size on the deactivation behavior have been observed. Larger crystals of zeolite ZSM-22 produce an increased amount of coke deposits resulting in a faster deactivation of the catalyst. The experimental results suggest that there is probably some influence of pore diffusion on the catalytic activity of the ZSM-22 sample with the large crystallite size. However a noticeable influence on the general product distribution could not be observed.

Introduction Light olefins belong to the major building blocks for the petrochemical industry, in particular for the production of polymers. The high consumption of these olefins leads to a steadily increasing demand. Especially in case of propene, a growth rate of about 5 % per year and therefore a higher growth as compared to ethene with about 4 % per year has been observed in the past decade [1]. Due to this fact, a propene gap is foreseen or even already experienced [1,2]. Over a long period of time, propene has been regarded as a byproduct of two major processes in a petrochemical plant or in a refinery, viz. the steam cracking process and the Fluid Catalytic Cracking Process (FCC) [2]. It has become necessary to develop supplementary, so called on-purpose technologies, which focus on propene as highly desired product. Examples for such processes are the dehydrogenation of propane, metathesis reactions, the methanol-to-olefins (MTO) process and the development of new catalysts and additives for catalytic cracking [1,2].

The aim of our study is to investigate the potential of new zeolites for increasing the propene yield from FCC. The first step of our investigation was to explore the influence of the pore architecture on the product yields. Therefore we tested a series of different ten-membered ring zeolites, which have a pore system different from the standard propene-boosting additive in the FCC catalyst, viz. zeolite ZSM-5. Among others, the one-dimensional zeolite ZSM-22, which has a linear pore system and oval pore openings was studied. Our results show that one-dimensional zeolites, lacking any larger intracrystalline cavities, exhibit an increased selectivity to light olefins, especially to propene and butenes, and a significantly lower selectivity to aromatic compounds like benzene or toluene as compared to zeolite

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ZSM-5 [3]. Furthermore, we studied the influence of the molar silicon-to-aluminum ratio (nSi/nAl) of zeolite ZSM-22 and zeolite ZSM-5. Our results show that with decreasing aluminum content an increasing selectivity to propene and to butenes could be achieved. Moreover, a more pronounced decrease in the n-octane conversion has been observed in the case of zeolite ZSM-22 as compared to the zeolite ZSM-5.

In the part of our work presented here, we studied the influence of the crystallite size and the morphology of zeolite ZSM-22 on its activity and selectivity. In this context the present contribution will also discuss different possibilities to influence the morphology of zeolite ZSM-22 and furthermore the resulting influence on the catalytic cracking of n-octane. Several ways to influence the morphology and the crystallite size of zeolites are known from the literature [4]. Among them there are the use of different mineralizers, the addition of a nucleation suppressing agent like triethanolamine [5,6] and using silica- or alumina-sources with a lower solubility. Furthermore the used structure-directing agent (SDA) and the crystallization conditions can have an influence.

Experimental Section The zeolites used in the present study were synthesized by different modified hydrothermal methods derived from the literature [4,7]. Zeolite ZSM-22 in its typical needle like crystallites with an average length of 1-4 µm was obtained by a standard synthesis composition, viz. with 1,6-diaminohexane as SDA and potassium hydroxide as mineralizer. By changing the SDA to 1-ethylpyridinium bromide large crystallites of zeolite ZSM-22 (size >10 µm) could be synthesized, also with a clearly different morphology. Replacing potassium hydroxide as mineralizer with lithium hydroxide again yields relatively small crystallites (ca. 1 to 2 µm). Under these conditions, zeolite ZSM-22 can be obtained also in the morphology of small needles, comparable with the material synthesized under standard synthesis conditions.

The obtained zeolites were characterized by powder X-ray diffraction, scanning electron microscopy, nitrogen adsorption at 77 K, atomic absorption spectroscopy and solid-state NMR for protons (1H), aluminum (27Al) and silicon (29Si). Modification was done through a threefold ion exchange with a 0.5 M aqueous NH4NO3 solution at 80 °C for 3 h. The proton- containing forms were obtained by calcination of the ammonium forms at 560 °C for 12 h. The catalytic studies were carried out in a fixed-bed flow-type reactor with n-octane as model feed. The reaction was performed at temperatures between 300 °C and 500 °C with a WHSV -1 (weight hourly space velocity) of 0.33 h and a partial pressure pn-octane = 2.4 kPa. Nitrogen was used as carrier gas. The products were analyzed by on-line capillary gas chromatography.

Characterization The powder X-Ray diffraction patterns in figure 1 reveal the purity and crystallinity of all three zeolite materials. The relative intensities and widths of the main diffraction lines (e.g., around 24.5 degree 2θ) indicates the relative size of the different zeolite crystallites. It can be seen, that the large crystallites of zeolite ZSM-22 (B) show a relatively narrow and high diffraction peak as compared to the other catalyst with smaller particle sizes. Atomic absorption spectroscopy revealed a silicon-to-aluminum ratio (nSi/nAl) of 94 (A), 95 (B) and 92 (C) for the three zeolites in their Brønsted-acid form. Due to their similar aluminum content all three catalyst should be quite comparable.

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Figure 1: Powder X-ray diffraction pattern Figure 2: Scanning electron micrograph of of zeolite HZSM-22 synthesized under zeolite ZSM-22 synthesized under different conditions and with different standard reaction conditions according to crystallite sizes of 1-4 µm (A), >10 µm (B), the literature [7]. 1-2 µm (C).

Figure 3: Scanning electron micrograph of Figure 4: Scanning electron micrograph of zeolite ZSM-22 synthesized with 1-ethyl- zeolite ZSM-22 synthesized with 1-ethyl- pyrdinium bromide as SDA. pyridinium bromide as SDA and lithium hydroxide as mineralizer.

Regarding the scanning electron micrographs, clear differences between the different catalysts can be seen. Under standard synthesis conditions, zeolite ZSM-22 is obtained in its typical needle like shape, the major part with an average size of about 1-2 µm, but to a certain amount also with a size/length up to 4 µm (see figure 2) Due to the relatively high silicon-to-aluminum ratio (nSi/nAl) of about 95, the size of the zeolite crystals is slightly increased as compared to materials, that we have been using for our first investigations on the influence of the pore architecture in the cracking of n-octane [3]. This effect is already known from the literature, in particular for all-silica materials [4].

By using 1-ethylpyridinium bromide as the structure-directing agent, the morphology of zeolite ZSM-22 considerably changes (see figure 3). Now, intergrown platelets with a length larger than 10 µm and a width of about 3 µm are obtained. But also a noticeable amount of typical zeolite ZSM-5 crystallites can be observed. Nevertheless the amount of the ZSM-5 contaminations seems to be not significant, which is also confirmed by the powder X-Ray diffraction pattern. These large crystallites of zeolite ZSM-22 occur only at low aluminum contents. With a nSi/nAl ratio of about 30 the zeolites crystallize in the typical needle like shape with an average size of about 1-2 µm, even with 1-ethylpyridinium bromide as SDA (not show in this contribution).

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Using 1-ethylpyridinium bromide as SDA and lithium hydroxide as mineralizer instead of potassium hydroxide results in zeolite ZSM-22 with an average size of 1-2 µm (see figure 4). This synthesis route has been performed in order to check, whether the different SDA has an influence not only on the crystallite size but also on the aluminum distribution inside the zeolite particles, viz. the distribution over different crystallographic T positions, which could, in principle, be seen by solid-state NMR spectroscopy [8]. The activity and the product distribution can be influenced if the aluminum is distributed differently over the variable crystallographic T-positions, since the different T-sites in the zeolite may have a different relative acidity [9]. Zoning, which could also influence the catalytic activity and selectivity, cannot be ruled out at the present stage of the investigation [8,10]. For example, if the aluminum is concentrated in the shell of the zeolite particle, the deactivation by coke formation may be faster due to the increased number of aluminum on the external surface.

Figure 5: 1H and 27Al Solid State NMR spectra of zeolite ZSM-22 obtained under different synthesis routes and with different morphologies (1-4 µm (A), >10 µm (B) and 1-2 µm (C)).

The 27Al Solid State NMRs plotted in figure 5 prove the good quality of the materials. In all cases only minor amounts of octahedral coordinated aluminum could be detected, which is indicated by the small signal at about 0 ppm. Almost all of the aluminum is therefore incorporated inside the zeolite structure, which is revealed by the large signal between 55 and 68 ppm. However, regarding the 1H-NMR spectroscopic results, noticeable differences can be seen between the zeolites synthesized via different methods.

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The large signals at 2.5 and 2.9, respectively, and also the signal at 0.8 ppm are assigned to terminal silicon hydroxide groups or defect sides inside the zeolite structure. The signals at 3.0 ppm (A), 3.7 ppm (B) and 3.1 ppm (C) can be assigned to aluminum hydroxide groups of non-framework aluminum species. All signals above 4.9 ppm are most probably due to Brønsted-acid sites. In case of the standard synthesized zeolite ZSM-22 (A) a sharp signal at 4.9 ppm and a broad signal at 7.1 ppm could be detected, indicating several Brønsted-acid sites with a different relative acidity. In case of the larger crystals of zeolite ZSM-22 (B) the signal at 4.9 ppm is slightly wider and the signal at 6.4 ppm seems to be slightly more intensive as compared to zeolite (A). Nevertheless both catalysts show quite comparable proton NMR spectra. Regarding zeolite ZSM-22 synthesized with LiOH and 1-ethylpyridinium bromide, only one signal for Brønsted-acid sites could be detected at 6.5 ppm, the signal at 4.9 ppm does not occur, it may also be covered by the by the broad signal at 3.1 ppm. However it seems that the use of lithium hydroxide as mineralizer causes a noticeable different distribution of aluminum over the unequal T sites inside of the zeolite structure.

Results and Discussion In figure 6, the time-on-stream behavior of the three catalysts is shown. Clear differences regarding the deactivation behavior and activity at the beginning of the experiments can be seen. The larger crystallites of zeolite ZSM-22 show a more pronounced activity loss with increasing reaction time. The deactivation in case of the two catalysts with smaller crystals is definitely slower. This observation can have different reasons: One explanation is that a faster mass transport is possible in smaller zeolite crystallites due to the shorter diffusion path. In addition to that, smaller crystallites offer a larger external surface area and therefore also an increased number of pore entrances, which may lead to a faster mass transport into and from the pore system. Rownaghi et al. reported similar results in the catalytic cracking of n-hexane over zeolite ZSM-5 with different crystallite sizes [11]. They concluded that with a faster mass transport, the formation of coke is reduced, which leads to an increased stability of the conversion with time-on-stream.

Another explanation could be that in case of medium-pore zeolites, viz. porous catalysts with relatively narrow pore architecture, the coke formation primarily takes place on the external surface of the zeolite particles [12]. Due to the increasing external surface with a decreasing crystallite size, these smaller particles should be less influenced by the coke formation as compared to larger crystals which are faster completely covered by coke deposits blocking the pore entrances. Konno et al. came to a similar conclusion when they studied the kinetics of the catalytic cracking of n-hexane over zeolite ZSM-5 with a special focus on the influence of different crystallite sizes [13]. Furthermore they predicted that especially in case of nano- sized zeolite crystals the kinetics of the reaction is almost entirely reaction-controlled, whereas in case of larger crystals pore diffusion and diffusion limitations can influence the overall kinetics. Our results could also indicate a diffusion limitation in case of the large zeolite ZSM-22 crystals (B). The activity at the beginning of the experiment is decreasing in the order (A) > (C) > (B), with conversion of 50.5 %, 45.9 % and 38.7 % respectively. This effect may only in part be explained by the slightly different aluminum content in the catalyst. Furthermore, we have observed noticeable differences in the amount of coke deposits after 24 h time-on-stream, which have been analyzed by thermo gravimetric analysis (TGA). The amount of coke is increasing in the following order: (B) > (A) > (C) with amounts of 2.2 wt.-%, 1.6 wt.-% and 1.1 wt.-% respectively. Hence, a clear trend could be observed, viz. the amount of coke is decreasing with a decreasing crystallite size. This result matches well with the study of Rownaghi et al. [11] but is in contrast to the study of Konno et al. [13]. This can probably be explained by the differing reactions conditions between the two studies, especially regarding the reaction temperature.

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In contrast to results of Rownaghi et al. [11] and Konno et al. [13] we could not observe an increased stability of the n-octane conversion with time-on-stream for the smallest zeolite particles (C). The two catalysts with relatively small crystals, viz. (A) and (C) show a similar deactivation behavior. The reason for that could be that the difference in the average crystallite size is too small to show a significant effect. Furthermore the catalyst (A) does not have a sharp crystallite size distribution, rather the size of the crystallites varies between 1 and 4 µm. As a whole our results suggest that the catalytic stability could still be enhanced if nano-sized crystallites in the range of < 0.1 µm would be used.

Figure 6: Time-on-stream behavior at Figure 7: Product distribution with zeolite 500 C and WHSV = 0.33 h-1 of HZSM-22 HZSM-22 synthesized with KOH and 1,6- with different crystallite sizes. diaminohexane (crystallite sizes of 1-4 µm, T = 500 °C, WHSV = 0.33h-1).

Figure 8: Product distribution with zeolite Figure 9: Product distribution with zeolite HZSM-22 synthesized with KOH and 1- HZSM-22 synthesized with LiOH and 1- ethylpyridinium bromide (crystallite sizes ethylpyridinium bromide (crystallite sizes of >10 µm, T = 500 °C, WHSV = 0.33h-1). of 1-2 µm, T = 500 °C, WHSV = 0.33h-1).

The product distributions presented in figures 7 to 9 where determined after 15 minutes time- on-stream. It can be seen, that all catalysts exhibit a comparable distribution. In all cases, C3 hydrocarbons are the main product followed by C4 and C2 hydrocarbons. The two catalysts with smaller crystals, viz. (A) and (C), yield comparable product distributions. In case of the large crystals of zeolite ZSM-22 (B), a slightly decreasing overall yield of products is observed, resulting from the lower activity of this catalyst. However, this catalyst also gave the highest yield of aromatic compounds like benzene or toluene, the precursors of coke formation (viz. formation of polycyclic aromatic compounds), which is in agreement with the higher amount of coke determined for this catalyst.

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Conclusions Zeolite ZSM-22 has been synthesized with different crystallite sizes between 1-2 µm and >10 µm and different morphologies by changing the structure directing agent from 1,6- diaminohexane to 1-ethylpyridinium bromide and varying the mineralizer. The characterization by solid-state NMR spectroscopy, powder X-Ray diffraction and atomic absorption spectroscopy proved that, except their crystallite size, all three obtained catalysts are comparable to each other. The time-on-stream experiments reveal a clear difference in the deactivation behavior of the catalysts with different crystallite sizes. The activity loss by coke formation is clearly more pronounced on larger zeolite crystals, which is tentatively attributed to their smaller external surface. This effect is supported by the increasing amount of coke deposits with an increasing crystallite size and by other studies known from the literature [11-13]. Furthermore, the results with the larger crystallites also suggest a certain influence of pore diffusion on the overall activity of the catalyst. However, the crystallite size seems not to have a clear influence on the relative product distribution, since for all catalysts studied almost similar product distributions are observed.

Acknowledgements

Financial support of this work from NanoKat (Center for Nano-Structured Catalysts at the University of Kaiserslautern) is gratefully acknowledged.

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