Examples of Organic Reactions on Zeolites: Methanol to Hydrocarbon Catalysis

Solid (heterogeneous) catalysts (1–4) are favored in industrial processes because they eliminate the need to separate the catalyst from the products. Heterogeneous catalysts can be solid acids, bases, supported metals, mixed metal oxides, or multifunctional materials. Commercial processes based on solid acids outnumber all others, and zeolites (and closely related materials) are usually the solid acids of choice. Many solvents and bulk chemicals are produced using zeolites as are important consumer goods such as gasoline. Although zeolites are not used to synthesize high polymers, they are essential for the production of the corresponding monomers. For example, p-xylene is obtained in high yield by toluene disproportionation or by the reaction of benzene and methanol in zeolite HZSM-5. Affordable p-xylene allows production of p-terpthalic acid and hence polyethylenetelephthalate (PET) polymer (plastic soda bottles). Methanol conversion on modified HZSM-5 or various silicoaluminophosphate catalysts produces propene and ethylene in high yields, and the production of polyolefins by way of methanol will soon be common. Zeolite catalysts such as HBEA (beta) are used to convert benzene and propene to cumene, which in turn is converted to acetone and phenol by selective oxidation (cumene hydroperoxide chemistry). Other heteroatom-containing products are produced directly by reactions in zeolites. For example, pyridine and picolines are produced on HZSM-5 from a mixture of ammonia and aldehydes or ketones, especially formaldehyde, acetaldehyde, and acetone. In-depth coverage of all organic reactions on zeolites would require a dedicated volume. We have chosen instead to focus on the process we know best, and that is methanol conversion to hydrocarbons (5–10). Through this process we will at least briefly touch on many important zeolite-catalyzed reactions, such as ether synthesis, cracking, isomerization, and so forth. This approach will also permit an in-depth treatment of one important reaction mechanism. We will see that many mechanisms have been proposed for this chemistry, which in turn have motivated many experimental and theoretical studies. This is not unique to methanol conversion; there is no consensus on the mechanisms of most organic reactions in zeolites, even seemingly simple ones such as butane isomerization to isobutane. This lack of insight into mechanism does not prevent