A persistent carbenium ion on the methanol-to-olefin catalyst HSAPO-34: Acetone shows the way

In situ C-13 NMR with both CAVERN and pulse-quench methods was used to unde rstand the chemistry of acetone on the silico-aluminophosphate catalyst HSA PO-34. The isotropic shift of [2-C-13]acetone shows that most of the acid s ites on this catalyst are weaker than those on aluminosilicate zeolites, bu t a minority site was resolved with a shift higher than that on zeolite HZS M-5. At elevated temperatures, acetone dimerized to diacetone alcohol and d ehydrated to mesityl oxide. Mesityl oxide cracked in the presence of water to acetic acid and isobutylene. Trimerization of butenes formed a significa nt amount of a persistent carbenium ion with unusual spectroscopic properti es, and this is proposed to be the heptamethylcyclopentenyl cation, the fir st observation of a persistent carbenium ion on a SAPO catalyst. Chemical s hift calculations at the GIAO-MP2 level revealed a discrepancy for one sign al of the proposed cation but ruled out an alternative assignment. Methylar omatic formation coincided with cyclopentenyl cation synthesis, supporting a mechanistic proposal for aromatic synthesis on zeolites. Conventional flo w reactor studies revealed a high selectivity for C-4 olefins and rapid dea ctivation with acetone as feed. The results of this investigation are inter preted in the context of methanol-to-olefin (MTO) chemistry on HSAPO-34.