Catalytic Sequences and Kinetic Requirements for C–O Bond Scission and Formation in Methanol and Trioxane on Solid Polyoxometalate Clusters

This study unravels the catalytic rate couplings and mechanistic similarities and differences in C–O bond scission and formation catalysis in methanol and trioxane, catalyzed by Brønsted acidic polyoxometalate clusters (POM). Turnovers of methanol dehydration, trioxane decomposition, and methanol–formaldehyde acetalization that form dimethyl ether (CH 3 O–CH 3 ), formaldehyde (CH 2 O), and dimethoxymethane (CH 3 O–CH 2 O–CH 3 ), respectively, vary over six orders of magnitude (308–423 K), yet these reactions share several common mechanistic requirements─their catalytic sojourns proceed via the formation of carbonium ion type transition states, occurred on Brønsted sites occupied by a quasi-equilibrated dynamic pool of monomeric, dimeric, and trimeric methanol species, etc., with their relative surface abundance varied with temperature and methanol pressure. These common mechanistic features allow their rates to be captured with a generalized rate expression and their activation enthalpies required for the evolvement of the carbonium ion type transition states with the same Born–Haber thermochemical framework. This framework decouples the elementary C–O bond scission and formation enthalpies into deprotonation enthalpy of the protons, proton affinity and structure rearrangement energy of reactants, ion-pair interaction energy between the carbocationic transition state analogs and anionic POM clusters, and adsorption enthalpy between gaseous reactants and the precursors of transition states. In these reactions, the similar requirements of proton transfer from POM clusters to the transition states lead their C–O bond scission and formation elementary rate constants to increase with decreasing deprotonation enthalpy. Among these reactions, the ability of reacting fragments to delocalize the charge would stabilize their carbocationic transition states, i.e., the more localized positive charge on the small methyl group in the transition state of methanol dehydration ([(CH 3 OH··· CH 3 + ···H 2 O)···POM – ] ‡ ) leads the C–O bond formation in dimethyl ether to occur only at a kinetically significant rate at higher temperatures (383–423 K). In contrast, the larger methoxy-like groups in the transition states of trioxane decomposition ([(O··· + CH 2 OCH 2 OCH 2 O···H)···POM – ] ‡ ) and methanol–formaldehyde acetalization ([(CH 3 OH··· CH 3 O CH 2 + ···H 2 O)···POM – ] ‡ ) are able to disperse the charge. This is the primary factor that leads to these C–O bond scission and formation turnovers are five to six orders of magnitude larger and occur at much lower temperatures (308–348 K). This work describes a generalized kinetic framework accounting for dynamic interconversion of surface species and putting the catalytic requirements of Brønsted acid-catalyzed C–O bond scission and formation of small oxygenate units (methanol and trioxane) within the context of transition state theory. The concepts and framework established here could also be applied to rationalize the turnovers of C–O bond scission in ethers and alcohols and of C–O bond formation between other alcohols and aldehydes.

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