The zeolite-catalyzed alkylation of isobutane with butene

Isobutane/butene alkylation is used to produce high octane, low vapor pressure blending components for gasoline. Currently, this reaction is catalyzed by one of two extremely toxic liquid acids, HF or H{dollar}sb2{dollar}SO{dollar}sb4{dollar}. The danger associated with HF leaks, in particular, has refineries scrambling to come up with alternatives. A number of researchers have looked into using benign solid acids, in particular zeolites. Though zeolites are initially very active towards catalyzing alkylation, within a few minutes of time on stream they become inactive and must be regenerated. This regeneration step is costly and has become a barrier for the incorporation of solid acids into alkylation processes. In the work described in this thesis, a fundamental approach has been taken to trying to understand why the zeolites deactivate rapidly and to determine how this problem may be mitigated. Kinetics experiments using a high pressure flow reactor were performed to elucidate the alkylation mechanism using USHY, a commercially-available faujasitic zeolite. To negate the effect of deactivation on the results, effluent samples were taken rapidly to facilitate an extrapolation of the kinetic data to time-zero.; A simplified kinetic model was then formulated based upon the reaction mechanism and effect of intraparticle diffusion. The values of the kinetic parameters extracted suggest that slow hydride transfer from isobutane to surface carbocations is the cause of catalyst deactivation. Repeated olefin additions to these carbocations cause large, immobile molecules to form within the zeolite crystals, thus covering acid sites and plugging up the pores. A mathematical reactor model confirms that site coverage via high molecular weight carbenium ions is a viable explanation for the decay of catalytic activity. Neither zeolite modifications nor changes in the operating conditions were found to alleviate this problem. lt is speculated that a novel solid acid catalyst is needed--one which alleviates the problem of steric hindrance. Guidelines are given for designing such a catalyst.; One of the most important findings of this thesis is that increasing the I/O ratio improves the catalytic activity, selectivity and, resistance to deactivation. Such a finding suggests that a CSTR or distributed feed would be a better reactor system to employ than a fixed bed reactor.; Another critical finding in this thesis is that the reaction is severely intraparticle diffusion-limited. One implication of this result is that vapor phase experiments will involve higher effectiveness factors than liquid phase ones. The drawback of vapor phase experiments is poor selectivity which can be remedied by lowering the reaction temperature.; Considering both that high I/O ratios are needed to run the reaction efficiently and that the reaction is severely diffusion limited in the liquid phase, another possibility raised by this thesis is that the reaction could be run in the liquid phase with a high concentration of olefin, provided that the active catalyst is imbedded a significant distance into the catalyst pellets. The effect of diffusion in this instance is a positive one, decreasing the local concentration of olefin over the active catalyst. (Abstract shortened by UMI.)...