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-PdNP as the hydrogenation catalyst. Selective and precise transformation of 1 into 1-OTf-PdNP 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-PdNP 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-PdNP 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.