Fundamental studies of methanol to olefin (MTO) catalysis

Methanol to olefin (MTO) reaction mechanisms on zeolite HZSM-5 and HSAPO-34 were studied experimentally and theoretically. Experimental studies were performed with solid state NMR and gas chromatograph (GC) using a pulse-quench reactor. On both catalysts “Carbon Pool” type mechanisms, which require a kinetic induction period, were confirmed. On HZSM-5, 1,3-dimethylcyclopentenyl carbenium ion is an intermediate in the MTO reactions and is a precursor to aromatic compounds. The presence of 1,3-dimethylcyclopentenyl cation on HZSM-5 reduced the induction period of the MTO reactions. On HSAPO-34 catalyst, the organic reaction centers were methylbenzenes which were synthesized through “ship in a bottle” mechanisms from methanol. The average number of methyl groups per benzene ring governs the selectivity of ethylene and propene. Ethylene was favored with lower average methyl number per ring. The SAPO-34 was modified by adding phosphorous compounds into the SAPO-34 cages. A phosphorous treated catalyst showed higher selectivity towards ethylene.; The chemistry of carbenium cations on zeolites was studied. Pentamethyl benzenium ion was synthesized on HZSM-5 from methanol and benzene with a pulse quench reactor. The Heptamethylcyclopentenyl cation was synthesized on HSAPO-34 from acetone with CAVERN techniques. The presence of both cations was verified by theoretical calculations. The reactions between 1,3-dimethylcyclopentenyl cation and several bases were also studied. Theoretical calculations predicted that a base with gas phase proton affinities higher than 215 kcal/mol, such as trimethyl phosphine and pyridine, will deprotonate the cation to form a neutral cyclic diene and a protonated base; while weak bases can only form hydrogen bonds with the cation. Some bases can also react with the cation to form onium ion by nucleophilic addition. All theoretical predictions were verified by experimental results.