Functionalized ammonia-borane mediated synergistic metal-free strategy for the Water-Gas shift reaction: A systematic DFT study.

Abstract

Syngas, a mixture of carbon monoxide (CO) and hydrogen (H2), is an important feedstock for the sustainable production of fuels and value-added oxygenated chemicals. In this work, density functional theory (DFT) calculations were employed to investigate a series of functionalized ammonia-borane derivatives, NH(R1)2BH(R2)2, as metal-free hydrogen-storage reagents for the sequential conversion of CO through coupled hydrogenation and water-gas shift reaction (WGSR) pathways. The complete reaction network comprises ammonia-borane-mediated hydrogenation of CO to formaldehyde (HCHO), hydrolysis of the ammonia-borane derivative, hydrolytic conversion of CO to formic acid (HCOOH), and decomposition of HCOOH to CO2 and H2 for catalyst regeneration. The generated HCHO can subsequently undergo further hydrogenation to methanol (CH3OH), thereby establishing an integrated route for syngas valorization. A systematic structure-activity relationship study was performed by varying the substituents on both the nitrogen and boron centers of the ammonia-borane framework, and the reaction energetics were analyzed at the level of individual elementary steps as well as for the complete one-pot catalytic cycle. The calculations reveal that functionalization of the ammonia-borane scaffold strongly influences the thermodynamic and kinetic characteristics of CO activation, hydrogen transfer, hydrolysis, and catalyst regeneration processes. Among the investigated systems, cyclic π-conjugated aromatic ammonia-borane derivatives bearing electron-donating substituents on boron and electron-withdrawing substituents on nitrogen exhibit the most favorable balance of activation barriers, reaction free energies, and regeneration efficiency. Overall, this study establishes a mechanistic framework for the metal-free conversion of syngas-derived CO into formaldehyde and methanol through ammonia-borane-mediated hydrogenation coupled with WGSR chemistry and provides valuable molecular design principles for developing sustainable and cost-effective catalysts for carbon monoxide utilization.

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