Etherification on zeolites: MTBE synthesis

Although the use of methyl-tert butyl ether (MTBE) in reformulated gasoline has raised concerns due to its detection in ground water and has led to its gradual phase out in parts of the United States, the use of heavier ethers in gasoline in the future is possible. The synthesis of MTBE provides us with an insight into etherification reactions in general. This article reviews the extensive findings over the past 15 years on the application of acidic zeolites as alternative catalysts for etherification reactions and, in particular, MTBE synthesis, and compares the results with those of the commercially used ion-exchange resins. Although the resin catalysts are very active, they have some significant drawbacks (i.e., thermal fragility, sensitivity to methanol/isobutene ratios, and corrosive/disposal problems). Zeolites have been considered to be potential alternative catalysts for MTBE synthesis due to their excellent properties such as high thermal stability and modifiable acidity. The impact of various zeolite parameters, such as Si/Al ratio, type of zeolite, and the presence of extra-lattice Al, on activity is explored. Although the specific activities of most zeolites have been found to be too low to compete at low temperatures required to avoid thermodynamic limitations, H-beta zeolite has been found to be as active as the current commercial resin catalysts being used for MTBE synthesis. In addition, all zeolites studied have been found to have high selectivities to MTBE and low sensitivities to methanol/isobutene molar ratio, permitting the use of the stoichiometric reactant ratio. Application of zeolites for the synthesis of higher ethers is suggested.