Design of heterogeneous catalysts for CO2 conversions to value-added chemicals is highly desirable. Montmorillonite and other clay minerals have been used widely in catalytic reactions including CO2 hydrogenation, while a molecular-level understanding remains lacking. In this study, periodic density functional theory calculations are employed and a comprehensive understanding about montmorillonite-catalyzed CO2 hydrogenation to formic acid is given, including active site, mechanism, influence factors, competitive reaction paths, and origin of superior catalysis. Catechol that is readily available and can also be considered as a fragment of abundantly distributed humic substances is an effective hydrogen source. The penta-coordinated (M2+) sites of edge surfaces are active sites, and reactions occur preferentially at M2+ rather than M3+ sites. The catalytic activities depend strongly on the identity of M2+ (M3+) cations, and all reaction paths follow the concerted mechanisms transferring two hydrogen atoms in one step, with those producing formate being highly preferred. M2+/M3+ substitutions and substituent effects are two critical factors to affect catalytic activities, and with synergy of Mg2+/Al3+ substitutions and -NMe2 substituent, reactions are exergonic (-0.09 eV) and activation barriers are so low (0.48 eV) that formate can be facilely produced at ambient conditions. Edge surfaces of clay minerals are bifunctional catalysts, with M2+ cations showing Lewis acids and -M-OH groups playing similar effects as basic additives. Results provide new insights about heterogeneous catalysis of CO2 hydrogenation and other reactions. (C) 2019 Elsevier Inc. All rights reserved.