High conversion rates, zeolite

Gadolinia, conversion rates, 27 35 Gallium arsenide, high Miller index, 26 12 Gallosilicate zeolite, Si MAS NMR studies of, 33 233-236... 

Another property of zeolites is the high conversion rates in the channel system. It was also observed that with different spatial configurations of channels, cavities, windows, etc, the catalytic properties are changed and the selectivity orientates toward less bulky molecules due to limitation in void volume near the active sites or to resistance to diffusivity. This feature termed shape-selectivity, was first proposed by McBain (20) demonstrated experimentaly by Weisz et al (21) and reviewed recently (22). For instance CaA zeolite was observed to give selective dehydration of n-butanol in the presence of more bulky i-butanol (23) while CaX non selective zeolite converted both alcohols. In a mixture of linear and branched paraffins, the combustion of the linear ones was selectively observed on Pt/CaA zeolite (24). Moreover, selective cracking of linear paraffins was obtained from petroleum reformate streams resulting in an improvement of the octane number known to be higher for branched paraffins and for aromatics than for linear paraffins. Shape selectivity usually combines acidic sites within... 

The gases used were purchased premixed in aluminum cylinders to avoid carbonyl formation. The high purity gas mixture was further purified by a zeolite water trap and a copper carbonyl trap. The gas pressure in the reactor was measured with a capci-tance manometer and the fTow monitored with a mass fTow controT-ler. The typical gas flow rates were 15 cc/min (STP) and the maximum conversion was 1% based on integration of hydrocarbon products. The hydrocarbon products were analyzed by gas chromatography (temperature programmed chromosorb 102, FID). 

The results in Table 3 show that H-mordenite has a high selectivity and activity for dehydration of methanol to dimethylether. At 150°C, 1.66 mol/kg catal/hr or 95% of the methanol had been converted to dimethylether. This rate is consistent with that foimd by Bandiera and Naccache [10] for dehydration of methanol only over H-mordenite, 1.4 mol/kg catal/hr, when extrt lat to 150°C. At the same time, only 0.076 mol/kg catal/hr or 4% of the isobutanol present has been converted. In contrast, over the HZSM-5 zeolite, both methanol and isobutanol are converted. In fact, at 175 X, isobutanol conversion was higher than methanol conversion over HZSM-5. This presents a seemingly paradoxical case of shape selectivity. H-Mordenite, the zeolite with the larger channels, selectively dehydrates the smaller alcohol in the 1/1 methanol/ isobutanol mixture. HZSM-5, with smaller diameter pores, shows no such selectivity. In the absence of methanol, under the same conditions at 15(fC, isobutanol reacted over H-mordenite at the rate of 0.13 mol/kg catal/hr, higher than in the presence of methanol, but still far less than over H M-5 or other catalysts in this study. 

Toluene alkylation with isopropyl alcohol was chosen as the test reaction as we can follow in a detail the effect of zeolite structural parameters on the toluene conversion, selectivity to cymenes, selectivity to para-cymene, and isopropyl/n-propyl ratio. It should be stressed that toluene/isopropyl alcohol molar ratio used in the feed was 9.6, which indicates the theoretical toluene conversion around 10.4 %. As you can see from Fig. 2 conversion of toluene over SSZ-33 after 15 min of T-O-S is 21 %, which is almost two times higher than the theoretical toluene conversion for alkylation reaction. The value of toluene conversion over SSZ-33 is influenced by a high rate of toluene disproportionation. About 50 % of toluene converted is transformed into benzene and xylenes. Toluene conversion over zeolites Beta and SSZ-35 is around 12 %, which is due to a much smaller contribution of toluene disproportionation to the overall toluene conversion. A slight increase in toluene conversion over ZSM-5 zeolite is connected with the fact that desorption and transport of products in toluene alkylation with isopropyl alcohol is the rate controlling step of this reaction [9]... 

In toluene disproportionation the highest toluene conversion was achieved over SSZ-33 due to a high acidity combined with 3-D channel system. High toluene conversion over SSZ-35 results from its strong acidity and large reaction volumes in 18-MR cavities. Toluene conversion in the alkylation with isopropyl alcohol is influenced by a high rate of competitive toluene disproportionation over SSZ-33. ZSM-5 exhibits a high p-selectivity for /7-isopropyl toluene, which seems to be connected with diffusion constraints in the channel system of this zeolite. 

A variety of solid acids besides zeolites have been tested as alkylation catalysts. Sulfated zirconia and related materials have drawn considerable attention because of what was initially thought to be their superacidic nature and their well-demonstrated ability to isomerize short linear alkanes at temperatures below 423 K. Corma et al. (188) compared sulfated zirconia and zeolite BEA at reaction temperatures of 273 and 323 K in isobutane/2-butene alkylation. While BEA catalyzed mainly dimerization at 273 K, the sulfated zirconia exhibited a high selectivity to TMPs. At 323 K, on the other hand, zeolite BEA produced more TMPs than sulfated zirconia, which under these conditions produced mainly cracked products with 65 wt% selectivity. The TMP/DMH ratio was always higher for the sulfated zirconia sample. These distinctive differences in the product distribution were attributed to the much stronger acid sites in sulfated zirconia than in zeolite BEA, but today one would question this suggestion because of evidence that the sulfated zirconia catalyst is not strongly acidic, being active for alkane isomerization because of a combination of acidic character and redox properties that help initiate hydrocarbon conversions (189). The time-on-stream behavior was more favorable for BEA, which deactivated at a lower rate than sulfated zirconia. Whether differences in the adsorption of the feed and product molecules influenced the performance was not discussed. 

ECC catalyst is subject to hydrothermal deactivation. This occurs when the A1 atom in the zeolitic cage is removed in the presence of water vapor and temperature. The result is a loss of activity and unit conversion. The effect of temperature on this process is nonlinear. The deactivation rate increases exponentially with temperature. Units that experience high afterburn have attributed high rates of catalyst deactivation on the higher dilute phase temperatures. This phenomenon is more apparent on units with high combustion air superficial velocities. The high velocity not only increases afterburn, but also increases catalyst entrainment to the cyclones and dilute area. COP is used to decrease afterburn and minimize catalyst deactivation. 

Therefore, rather than varying the test temperature, a much simpler and more accurate procedure is to actuaily perform aii measurements at the standard temperature of 538°C (1000°F), as has been proposed previously [2], By an appropriate choice of F, W and conversion (e.g. 0.3-60%) rate constants differing by over four orders of magnitude can be readily measured. It has been found that even for the medium pore zeolite ZSM-5, of high activity, no diffusion limitations exist even at this relatively high temperature [8,12] except for very large crystals exceeding 40 pm [13]. At 538°C it is also easy to woh<... 

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