Many chemical reactions can take advantage of supercritical CO2 as solvent. But will this affect their kinetics? Here we predict the rates of the title reaction in CO2 bath. Two mechanisms were investigated: one involves the hydrogen transfer in bimolecular complex alone (R1), and the other one includes CO2 molecule in the reactive complex (R2). We applied DFT+D3 and ab initio CBS-M11 model chemistry levels to study potential energy surfaces and based on activation barriers ruled out the third mechanism where CO2 participates covalently in the hydrogen transfer. CO2 molecule stabilizes the transition state in mechanism R2 and reduces the activation energy from 3.04 to 0.16 kcal/mol. This indicates a possible catalytic effect of CO2. We further applied Transition State Theory (TST), Rice-Ramsperger-Kassel-Marcus theory (RRKM) and solved master equations in steady-state approximation. We assumed equilibrium between reactants and pre-reactive complexes (PRC) and calculated rates. The equilibrium constants for bimolecular PRC1 formation in R1 and trimolecular PRC2 formation in R2 KPRC1 = [PRC1] / [OH][CH2O]; KPRC2 = [PRC2] / [OH][CH2O][CO2] and the high-pressure rate constants kR1∞ and kR2∞ were computed by TST. Branching fractions for chemically activated isomerization were predicted with RRKM in order to account for pressure dependence. The unimolecular rate coefficients kR1 and kR2 were calculated by multiplying k∞ and the forward reaction branching fraction. The reaction rates rate R1 = kR1 [PRC1] = k1 [OH][CH2O]; rate R2 = kR2 [PRC2] = k2 [OH][CH2O] and bimolecular rate constants were k1 = kR1 KPRC1; k2 = kR2 KPRC2 [CO2] so that k2 depends on CO2 pressure. Comparison found the catalyzed R2 mechanism is faster (k2&gt;k1) at higher CO2 pressures (300 atm) and lower temperatures (&lt;300 K), when pre-reactive complexes have non-negligible concentration. Therefore, this catalytic effect may be important for this reactive process in room temperature CO2 solvent, but is unlikely to play a role during oxy-combustion.

Catalytic effect of CO2 environment on OH + CH2O → H2O + CHO reaction rate

2019 Spring National Meeting

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