Multistep Engineering of Synergistic Catalysts in a Metal–Organic Framework for Tandem C–O Bond Cleavage

Abstract

Cleavage of strong C-O bonds without breaking C-C/C-H bonds is a key step for catalytic conversion of renewable biomass to hydrocarbon feedstocks. Herein we report multi-step sequential engineering of orthogonal Lewis acid and palladium nanoparticle (NP) catalysts in a metal-organic framework (MOF) built from (Al-OH)_n secondary building units and a mixture of 2,2'-bipyridine-5,5'-dicarboxylate (dcbpy) and 1,4-benzenediacrylate (pdac) ligands (1) for tandem C-O bond cleavage. Ozonolysis of 1 selectively removed pdac ligands to generate Al₂(OH)(OH₂) sites which were subsequently triflated with trimethyl triflate (Me₃SiOTf) to afford strongly Lewis acidic sites for dehydroalkoxylation. Coordination of Pd(MeCN)₂Cl₂ to dcbpy ligands followed by in situ reduction produced orthogonal Pd NP sites in 1-OTf-Pd^NP as the hydrogenation catalyst. Selective and precise transformation of 1 into 1-OTf-Pd^NP was characterized step-by-step using powder X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, inductively coupled plasma-mass spectrometry, infrared spectroscopy, and X-ray absorption spectroscopy. Hierarchical incorporation of orthogonal Lewis acid and Pd NP active sites endowed 1-OTf-Pd^NP outstanding catalytic performance in apparent hydrogenolysis of etheric, alcoholic, and esteric C-O bonds to generate saturated alkanes via a tandem dehydroalkoxylation-hydrogenation process under relatively mild conditions. The reactivity of C-O bonds followed the trend of tertiary carbon > secondary carbon > primary carbon. Control experiments demonstrated the heterogeneous nature and recyclability of 1-OTf-Pd^NP and its superior catalytic activity over the homogeneous counterparts. Sequential engineering of multiple catalytic sites in MOFs thus present a unique opportunity to address outstanding challenges in sustainable catalysis.