| Styrene is an important monomer in the chemical industry. Traditionally, benzene is alkylated with ethylene to produce ethylbenzene, which is then dehydrogenated to styrene. Alternatively, the side-chain alkylation of toluene over solid base catalysts is a potential one-step process to styrene.; In this work, alkali-modified zeolites (X, Y, L and {dollar}beta{dollar}), carbon molecular sieves, and metal oxides were studied as catalysts for toluene alkylation with methanol. The effects of alkali occlusion, Group IIIA (B, Al, Ga, In) promotion, zeolite particle size, and pore dimensionality were examined. At 680-690 K, highly basic, micron scale alkali-exchanged zeolites X and Y were active for toluene alkylation but primarily decomposed methanol. Occluding intrazeolitic alkali increased the basicity which, in turn, increased methanol decomposition to carbon monoxide and decreased the aromatic yield. Addition of B to CsX inhibited the decomposition rate, whereas addition of Al, Ga and In had little effect.; Submicron scale zeolites L, {dollar}beta{dollar} and Y were active alkylation catalysts but required higher temperatures to attain reasonable aromatic yields. Their observed activities demonstrated that the dimensionality of the zeolite pore network does not significantly affect alkylation. More importantly, methanol did not decompose to carbon monoxide. Infrared spectroscopic studies and thermogravimetric analyses of adsorbed CO{dollar}sb2{dollar} indicated that the submicron scale zeolites were very weakly basic in comparison to the micron scale zeolites X and Y.; Since cesium-modified magnesia and mesoporous alumina were inactive for toluene alkylation, physical constraints within molecular sieve environments appear to influence the reactivity. These results suggest that the rate and degree of methanol decomposition were strongly related to the acid-base characteristics of the catalysts. In addition, materials with low base site densities and appropriate base strengths selectively alkylate toluene. However, materials that are too basic decompose methanol to carbon monoxide. |
