Synthesis, characterization and catalytic evaluation of supported catalysts (PdNPs/TiO2, PdNPs/Al2O3, PdNPs/SiO2) for esterification of levulinic acid to levulinate esters

Abstract

• The manuscript describes the synthesis of supported (Silica, titania, and alumina) Pd nanoparticles via wet-impregnation method. • The size of supported Pd nanoparticles determines their catalytic activity. • Indicators such as induction period and kinetics of de/activation were used to determine the possibility of Pd surface atom rearrangement during catalytic reaction. • The alumina-supported Pd nanoparticles proved to be more robust and activate at later reaction cycles. • The manuscript contributes towards the development of sustainable and environmentally friendly biobased value-added chemicals. In this work, we report on the synthesis of supported-palladium nanoparticles (Pd NPS /TiO 2 , Pd NPS /SiO 2 , Pd NPS /Al 2 O 3 ) and their catalytic performance in the esterification of levulinic acid (LA). The N 2 physisorption technique (BET) was used to determine the surface properties of Pd NPS and gave surface areas ranging between 69.53 – 596.5 m 2 g -1 for the set of catalysts, and the high-resolution transmission electron microscopy (HRTEM) whose images were used to measure the average nanoparticle sizes (2.2 ± 0.2 – 8.0 ± 2.0 nm) of all supported Pd NPS . Metal loading ranged from 0.5 to 5 Pd wt.% as determined by the inductively coupled plasma-optical emission spectroscopy (ICP-OES). The highest LA conversion, 96% was achieved with Pd/Al 2 O 3 . An interesting observation for 0.5 – 5% Pd NPS /TiO 2 catalytic systems is that LA's turnover frequency (TOF) decreases with increasing metal loading and/or size of the Pd NPS on TiO 2 . The Pd NPS supported on TiO 2 exhibit an induction period during the first hour of the reaction and later reported 83% LA conversion with 98% selectivity towards isopropyl levulinate (IL). Therefore, we postulate a TiO 2 support-induced surface atom rearrangement of Pd NPS prior to the catalytic reaction that is responsible for the induction period observed.