Selectivity engineering in catalysis by ruthenium nanoparticles supported on heteropolyacid-encapsulated MOF-5: one-pot synthesis of allyl 4-cyclohexanebutyrate and kinetic modeling

Abstract

Functionalized metal-organic framework containing a heteropolyacid such as dodecatungstophosphoric (DTP)-encapsulated MOF-5 was synthesized by an in situ method and then ruthenium was incorporated by incipient wetness impregnation. A multifunctional heterogeneous catalyst, 1% Ru-15%DTP@MOF-5 with active acid and the metal sites make it the most efficient catalyst. For acid sites, dodecatungstophosphoric acid (DTP) was encapsulated into MOF-5 and ruthenium was loaded as metal sites. Its activity was examined in the one-pot synthesis of allyl 4-cyclohexanebutyrate, a flavoring agent, by esterification of 4-phenylbutyric acid with allyl alcohol followed by aromatic ring hydrogenation using molecular hydrogen. Esterification of 4-phenylbutyric acid with allyl alcohol gives allyl 4-phenylbutyrate which is further hydrogenated to give allyl 4-cyclohexanebutyrate. The octahedral cubic morphology of MOF-5 was retained even after DTP encapsulation and loading of ruthenium. Catalyst screening for esterification step was carried out by varying loadings of DTP (10, 15, and 20%) on MOF-5. Among these, 15% DTP-loaded MOF-5 showed the best catalytic activity. For selective aromatic ring hydrogenation, different metals such as Pd, Re, Ru, and Rh were examined and it was found that the Ru-based catalyst resulted in the highest conversion of allyl 4-phenylbutyrate (89.63%) and selectivity for allyl 4-cyclohexanebutyrate (96.52%). 1% Ru-15% DTP@MOF-5 catalyst was thermally stable and five times reusable. For both the steps, the kinetics was studied using the Langmuir-Hinshelwood-Hougen-Watson (LHHW) mechanism and the apparent activation energy for esterification was calculated as 13.34 kcal/mol and that for hydrogenation as 14.87 kcal/mol.