Ketene as a Reaction Intermediate in the Carbonylation of Dimethyl Ether to Methyl Acetate over Mordenite

Abstract

Unprecedented insight into the carbonylation of dimethyl ether over Mordenite is provided through the identification of ketene (CH2CO) as a reaction intermediate. The formation of ketene is predicted by detailed DFT calculations and verified experimentally by the observation of doubly deuterated acetic acid (CH2DCOOD), when D2O is introduced in the feed during the carbonylation reaction.

Bridging the Gap between the Direct and Hydrocarbon Pool Mechanisms of the Methanol‐to‐Hydrocarbons Process

“…[35] In the vicinity of Brønsted acidic sites,a na cetyl group is nothing but ap hysisorbed protonated ketene (that is, CH 2 CO + H + !CH 3 CO + ,w ith av ery small energy barrier of 17 kJ mol À1 and al arge energy gain of > 50 kJ mol À1 ). [16,24,36] Thus,t he steady-state concentration of ketene is not only low (particularly at the higher reaction temperature), but its existing rapid equilibrium to acetyl might fundamentally prohibit its direct detection by experimental methods in the present study. [24,36,33,37] It is worth mentioning that ketene was experimentally observed previously,d uring the conversion of syngas to light olefins by Fischer-Tropsch synthesis using bifunctional catalysts.…”

“…[16,24,36] Thus,t he steady-state concentration of ketene is not only low (particularly at the higher reaction temperature), but its existing rapid equilibrium to acetyl might fundamentally prohibit its direct detection by experimental methods in the present study. [24,36,33,37] It is worth mentioning that ketene was experimentally observed previously,d uring the conversion of syngas to light olefins by Fischer-Tropsch synthesis using bifunctional catalysts. [38] In the present case,t he identification of acetic acid could be considered as indirect support for the existence of ketene (CH 2 CO + H 2 O!CH 3 CO 2 H).…”

“…[38] In the present case,t he identification of acetic acid could be considered as indirect support for the existence of ketene (CH 2 CO + H 2 O!CH 3 CO 2 H). [24] Moreover,the existence of ketene during the zeolite-catalyzed Koch-type carbonylation was hitherto theoretically predicted by multiple research groups. [16,17,24,[33][34][35]39] Thus,C 2 -C 4 olefins were initially formed by decarbonylation of ketenes,a sp roposed by Plessow and Studt (F/G in Scheme 2; Supporting Information, Figure S1).…”

“…[24] Moreover,the existence of ketene during the zeolite-catalyzed Koch-type carbonylation was hitherto theoretically predicted by multiple research groups. [16,17,24,[33][34][35]39] Thus,C 2 -C 4 olefins were initially formed by decarbonylation of ketenes,a sp roposed by Plessow and Studt (F/G in Scheme 2; Supporting Information, Figure S1). [16,17] Subsequently,s maller olefins oligomerize into bigger olefinic and aromatic species as ap art of the alkene and arene cycle of the dual-cycle/HCP mechanism, respectively,t og overn the autocatalytic part of the reaction.…”

“…From ad ifferent perspective,o ur earlier proposed Kochtype carbonylation direct mechanism for the MTH reaction (Scheme 1a)h as conceptual resemblance to the Monsanto and BPsCativa processes (that is,toform acetic acid/methyl acetate from methanol/dimethyl ether (DME)). [21][22][23][24] Although industry currently uses Rh/Ir organometallic catalysts for this process,z eolites have recently appeared as an alternative option to overcome existing disadvantages (for example,n on-recyclability of expensive catalysts,t edious separation steps,a nd the use of corrosive water-insoluble additives). [23] Thus,m echanistic understanding of the fate of methyl acetate over zeolite-based materials will also be beneficial for unravelling other zeolite-catalyzed hydrocarbon conversion processes.…”