Engineering of coordination polymers for shape selective alkylation of large aromatics and the role of defects

Abstract

For economic and environmental reasons, there is a strong incentive to replace of homogeneous by green and efficient heterogeneous processes in catalysis. The Friedel-Crafts alkylation of aromatics is a marking example. Numbers of homogeneous catalysts (such as HF, H 2SO 3, and AlCl 3) have been replaced by H-form zeolites. However, large pore zeolite can not accommodate bulk aromatic molecules and/or alkylating agents avoiding the reaction to take place within the porous network. Due to their larger pore size, metal-organic frameworks (MOFs) open the doors to the alkylation of very large poly-aromatic compounds. We report different approaches for the design of acid shape selective MOFs. The first refers to a “zeolite mimetic” approach. It deals with the design of porous Zn or Al based MOF exhibiting bridging –OH species (MOF-69, MIL-53 Al) like those found in zeolites [Si–O(H)–Al]. The second approach aims at synthesizing MOF materials having structural defects to generate active catalytic centers. Two different synthetic strategies were investigated, either by fast precipitation or by the partial substitution of dicarboxylic by mono-carboxylic acid linkers. Acid centers have been characterized by solid 1H NMR and Diffuse Reflectance IR. The mono-alkylation of biphenyl with tert-BuCl is achieved with 100% of para-selectivity, well superior to H-MOR and H-BEA reference zeolites.