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 (CH 2 CO) as ar eaction intermediate. The formation of ketene is predicted by detailed DFT calculations and verified experimentally by the observation of doubly deuterated acetic acid (CH 2 DCOOD), when D 2 Ois introduced in the feed during the carbonylation reaction.A number of acidic zeolites are selective catalysts for dimethyl ether (DME) carbonylation, and Mordenite has the highest activity.[1] However,t he zeolite catalysts suffer from rapid deactivation because of ab uild-up of coke and larger carbonaceous species within the zeolite pores. [1c,2] Theframework of Mordenite contains two types of cavities:e ightmembered ring (8-MR) side pockets and 12-MR main channels.I th as been reported that methyl acetate (MA) synthesis takes place within the 8-MR, [3] whereas the 12-MR has been suggested to be responsible for the coke formation which leads to catalyst deactivation. [2a,d] During the initial phase of DME carbonylation, DME reacts with the Brønsted sites,t hus forming methyl groups and water [Eqs. (1) and (2) These reactions,i nw hich the Brønsted acid sites are methylated, give rise to an induction period. Thesteady-state phase involves the reaction of CO with themethyl groups, thus forming acetyl species,which in turnreact with DME to produce MA and regenerate the methyl groups [Eqs. (3) and (4) Experimental studies showed that formation of the acetyl species is the rate-limiting reaction step,a nd that the subsequent reaction between DME and acetyl is comparatively fast.[1a,b] Herein, we present unprecedented insights into the formation of acetyl over Mordenite in the steady-state phase,including the DFT energies and energy barriers for all reaction steps,a nd present experimental verification of the theoretical model by showing that ketene is ar eaction intermediate as predicted by the DFT calculations.We employ the BEEF-vdW functional, [4] which has been shown to quantitatively describe van der Waals interactions of molecules within zeolite pores, [5] as well as reaction kinetics.[6]Formation of the acetyl group was investigated for the 12-MR and 8-MR at the T1-O4 and T3-O3 sites,r espectively (see Figure S1 in the Supporting Information, SI). [7] These sites are the preferred adsorption sites for methyl groups,asshown in Table S1 in the SI. TheT3-O8 site in the 8-MR was found to be the most favorable adsorption site of methyl groups in previous DFT studies employing cluster models, [8] and for comparison we also investigated the formation of acetyl at this site (see Figures S2 and S3).Thereaction path for formation of acetyl, determined by DFT calculations,begins by the reaction of CO with asurface methyl group which yields an acetyl carbocation [Eq. (5)]:Figure 1s hows that the energy barrier for this step is 6kJmol À1 lower at T3-O3 (8-MR) than at T1-O4 (12-MR),

Selective ketonization of acetic acid over HZSM-5: The importance of acyl species and the influence of water

“…While several reactive surface intermediates have been proposed over high lattice reducible oxides involving ketenes, ketene like species, carboxylates, and even anhydrides [16][17][18][19][20][21][22][23][24][25], consensus is slowly growing toward the importance of a β-ketoacid intermediate. In contrast, the reactive intermediates responsible for this transformation over acidic zeolites remains unexplored, while some similar surface intermediates such as ketenes have been reported over Brønsted zeolites [29]. Despite the fact that ketones have been observed as products from the transformation of acetic acid over zeolites since the early 1980's, specific details of the reaction mechanism still remain elusive [30][31][32][33][34].…”

“…Surface acyl species are in equilibrium with acylium ions, as well as gas phase ketenes in low abundance. Ketenes are often proposed as important reactive intermediates for reactions involving carboxylic acids and acyl groups [29]. Specifically for the ketonization reaction, it has been shown that over oxides such as TiO 2 , gas phase ketenes are parallel side products that do not lead to ketone formation as discussed in depth in a recent review on the topic [21].…”