Isobutane/2-butene alkylation using large-pore zeolites: Influence of pore structure on activity and selectivity

Abstract

The alkylation of isobutane with 2-butene was performed in the liquid phase at 80degreesC and 300 psi. This study constitutes a detailed comparison under identical conditions of 12-membered pore zeolites with different dimensionalities, namely USY, mordenite, beta, LTL, and ZSM-12 zeolite. Particular attention was paid to the role of the structural characteristics of the catalysts. In the mechanism studies, very-low-olefin feed (i/o molar ratio of 1000, olefin WHSV of 0.01 h(-1)) was used to keep real alkylation without undesirable oligomerizations. For selectivity toward the individual TMP and DMH compounds, pore structure plays a significant role. At the initial stage (5 min on stream), it was found that the relative value for TMP selectivities described by the configurational ratio of 2,2,4-/2,2,3-TMP to 2,3,3-/2,3,4-TMP decreased in the following order: beta > ZSM-12 > mordenite > USY > LTL. Beta and ZSM-12 showed higher values due to their specific pore structure (the absence of large expansions at either the channel intersections or within the channel itself), in which bulky TMPs could diffuse out without structural restriction. In order to support this, the ratio of diffusivity for selected TMP isomers (2,2,4- to 2,3,4-TMP) was obtained. The value of diffusivity ratio of 2,2,4-TMP to 2,3,4-TMP decreased in the sequence beta > USY > LTL. This difference was reflected clearly in the different distribution of TMP isomers. The comparable trends were also observed for the selectivity of DMHs, ratio of 2,5-/2,4-/2,3-DMH to 3,4-DMH. Most of cracking products, 2-methyl butane, 2,3-dimethyl butane, and 2,3- and 2,4-dimethly pentane, were formed by type A beta-scissions from [C12H25](+) and [C16H33](+). For other studies with different feed conditions (i/o molar ratio of 98, olefin WHSV of 0.1 h(-1)), it was also observed that the pore structure of zeolites plays a significant role in the activity of olefin conversion. Beta and ZSM-12 were shown to outperform other zeolites because their favorable pore structures limited the accumulation of carbonaceous compounds that lead to pore plugging. This is in contrast to the common belief that three-dimensional zeolites are less susceptible to pore plugging than one-dimensional zeolites; we observed dramatically better coke tolerance over ZSM-12 (one dimensional) than USY (three dimensional). It was observed that samples synthesized with low Si/Al ratios resulted in higher activity and selectivity to the desired products. This was mainly because of the high hydrogen transfer (HT) capability of the low Si/Al samples. However, the dealumination procedure used strongly affected the catalyst performance; samples dealuminated with ammonium hexafluorosilicate were significantly less active and selective to TMPs than samples either dealuminated with HCl or synthesized directly. This was primarily because of the ability of the latter dealuminating agent to remove surface aluminum as well as partial pore blocking by the silicate salts deposited. (C) 2002 Elsevier Science (USA).