Bilateral Causality Effects Between Product Distribution and Reaction Exotherm in the Conversion of Syngas

Abstract

In a heterogeneous catalytic reaction with strong heat release, the reaction exotherm causes a temperature increment and further has potential effects on product selectivity. This work focuses on syngas conversion, a representative strong exothermic reaction, to reveal a causation effect between the product distribution and the reaction exotherm. Owing to the thermodynamic characteristics that lead to higher heat release for methane or C₂₊ hydrocarbon formation than that for methanol or C₂₊ alcohol formation, the decrease in methanol or C₂₊ alcohol selectivity but increase in methane or C₂₊ hydrocarbon selectivity could increase the heat release and temperature increment (ΔT). It has been found that the distinguishing activation energies result in different kinetic sensitivities to heat. By decreasing ΔT, carbonyl insertion/C–C coupling reactions are boosted and hydrogenation of dissociated CO is suppressed, affording a significant decrease in methane selectivity and increase in C₂₊ alcohol and C₂₊ hydrocarbon selectivity. The activation energy of hydrogenation of nondissociated CO places in the middle among various reactions, leading to an insensitivity of methanol selectivity to the reaction exotherm in this system. The change in product distribution could further aggravate/weaken the heat release, showing a bilateral causality effect between the product distribution and reaction exotherm. Moreover, an optimized model has been developed for correlating the product selectivity (methane, C₂₊ hydrocarbon, methanol, or C₂₊ alcohol) with ΔT at a known setting temperature, which well predicts the sensitivity of the reaction exotherm to product distribution. This work innovates an approach to manipulate product distribution in intensely exothermic reactions via thermal management.