Quantum chemical calculations for the H free radical chemisorption with different chain models during oil shale pyrolysis

Abstract

In this paper, the authors conduct an in-depth investigation for the thermochemical reaction of organic carbon and H free radical during oil shale retorting process. A set of carbon chain and reaction models were constructed and calculated through the transition state theory (TST) of quantum chemistry with Gaussian-16 package. The effect of functional groups during the reactions was explored through the density functional theory (DFT) at the B3LYP/6-311G (d) level. To make the calculation results more trustful and reliable, we choose reasonably simplified straight carbon chain models and construct the corresponding multiple reaction paths, which could elucidate the mechanisms of the thermochemical reaction. In the presence of H free radical, the cracking process of carbon chains in kerogen is promoted because of lower energy barriers. The charge re-distribution causing by the functional group also changes the H adsorption capacity of the organic carbons in different positions (α, β, γ, etc.). Moreover, different measuring methods of the rate constants are evaluated in this paper. The results could provide direct insight into the limitation of the conventional transition state theory (TST), which deserves more attention in further theoretical research.