Multisite Porphyrinic Metal–Organic Frameworks for Biomass Valorization: Computational Design and Mechanistic Investigation

Abstract

Renewable chemicals and fuels have received significant attention for sustainable economic development. Our aim in this work is to investigate a porphyrinic metal–organic framework (MOF), namely, MOF-525 for the transfer hydrogenation of methyl levulinate (ML) to γ-valerolactone (GVL) through density functional theory calculations. We computationally design five multisite defect-engineered MOF-525(M) (M = CrIII, FeIII, RuIII, RhIII, and IrIII) and examine their catalytic performance for the transformation of ML to methyl valerate (MV) via catalytic transfer hydrogenation (CTH), subsequently for the cyclization of MV to GVL by anchoring a SO3H group as a Brønsted acid site on the metallated linker M-TPP-Cl. The CTH involves three pathways (two concerted and one stepwise) on the defective Zr node and two pathways (one stepwise and one concerted) on the metallated linker, respectively. It is revealed that a stepwise Meerwein–Ponndorf–Verley pathway is energetically preferred on the defective Zr node. A similar stepwise pathway on Ru-TPP-Cl is also found to be favorable, while a concerted pathway is more favorable on Rh-TPP-Cl and Ir-TPP-Cl. The rate-determining activation barriers on M-TPP-Cl are rationalized by geometry analysis, second-order perturbation theory analysis, and the noncovalent interactions. Among the five metallated MOF-525(M), Ru, Rh, and Ir are predicted to be active for the transformation of ML to MV. However, the defective Zr-SBU is found to have a lower activation energy barrier compared to M-TPP-Cl, suggesting its dominant role in CTH. In addition, the Brønsted acid-catalyzed cyclization appears to be rate-determining on the defective Zr node, Rh-TPP-Cl, and Ir-TPP-Cl. This study suggests that MOF-525 might be potentially used as a multisite catalyst platform for the conversion of ML to GVL and would assist in the future rational design of new MOFs for biomass valorization.