Investigation of the active Bronsted acid site for the DME carbonylation reaction in chabazite-type zeolites

Abstract

Chabazite-type (CHA) zeolites (SSZ-13s) are active acid catalysts in the carbonylation of di-Me ether (DME), an important C-C bond forming reaction involved in the conversion of syngas derivs. to valuable chem. intermediates. We observe a max. in the formation rate of Me acetate, the target product, as a function of Si/Al in the SSZ-13s that cannot be explained from the standpoint of total acid site d. While the active sites for this reaction have been better studied in other zeolite topologies (MOR, FER), to date, the origin of activity in CHA remains unclear. To investigate this issue, we synthesized SSZ-13s with a range of Si/Al (5 < Si/Al < 75) to vary both Bronsted acid site d. and siting (e.g., paired vs. isolated), and evaluated the impact of these parameters on the DME carbonylation activity. Further, because the CHA structure has only one T-site, differentiating Bronsted acid sites via 1H NMR may be feasible. Three possible extraframework, monovalent cation sites exist. Only two of these sites are accessible by reactants, reducing the active Bronsted acid site candidates to the proton that sits in the 6-membered ring (MR) window, and the one oriented in the 8MR window. We detect three distinct resonances in the high-spin 1H NMR spectra on dehydrated SSZ-13s. From the reactivity and NMR analyses, coupled with divalent cation exchange expts. for specific titrn. of paired (framework Al sepd. by no more than 3 Si-O units) acid sites, we developed a model for the location of the active site in CHA. Given the unique framework topol. of CHA compared to other active, DME carbonylation zeolite topologies, this investigation provides further insights into this useful C-C bond forming reaction.