Influence of Crystallite Size and Shape of Zeolite ZSM-22 on Its Activity and Selectivity in the Catalytic Cracking of N-Octane F

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 DGMK-Tagungsbericht 2013-2, ISBN 978-3-941721-32-6 171 New Technologies and Alternative Feedstocks in Petrochemistry and Refining 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. 172 DGMK-Tagungsbericht 2013-2 New Technologies and Alternative Feedstocks in Petrochemistry and Refining 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). DGMK-Tagungsbericht 2013-2 173 New Technologies and Alternative Feedstocks in Petrochemistry and Refining 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].

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