Interaction of a Zeolite Acidic Site with Methanol

The petrochemical industry heavily relies on zeolite catalysts for petrol refinement and production of fine chemicals. About 80% of the gasoline produced to date has been in contact with zeolites in the refinement process. Furthermore, zeolites are used during the industrial methanol-to-gasoline (or -olefin) process, with which, for example, about 30% of New Zealand's gas supply is produced. They form a class of alumino silicates with large cages linked together with pores. Through these pores only small molecules can pass that must have certain steric properties. Acid sites distributed over the large internal surface activate chemical bonds and thus are responsible for the zeolite's catalytic function. The acid sites in zeolites typically are AlOH groups that replace framework SiO fragments. We have performed dynamical first-principles electronic-structure simulations to elucidate the structures and vibrational spectra of methanol trapped in sodalite, a generic case of methanol in zeolites. Our results reproduce the measured vibrational spectra, and can be generalized for a large class of small polar organic molecules. We find that methanol is not protonated but forms strong hydrogen bonds. Two broad peaks in the measured infrared spectrum that are consistently found for a large class of molecules can be identified for the first time as acid-site hydrogen-stretch vibrations and CH-stretch vibrations, the latter being broadened through interactions with the zeolite framework.