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

In this work, we report on the synthesis of supported-palladium nanoparticles (PdNPS/TiO2, PdNPS/SiO2, PdNPS/ Al2O3) and their catalytic performance in the esterification of levulinic acid (LA). The N2 physisorption technique (BET) was used to determine the surface properties of PdNPS and gave surface areas ranging between 69.53 - 596.5 m2g- 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 PdNPS. 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/Al2O3. An interesting obser-vation for 0.5 - 5% PdNPS/TiO2 catalytic systems is that LA's turnover frequency (TOF) decreases with increasing metal loading and/or size of the PdNPS on TiO2. The PdNPS supported on TiO2 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 TiO2 support-induced surface atom rearrangement of PdNPS prior to the catalytic reaction that is responsible for the induction period observed.

Automated summary

In this study, the synthesis of supported-palladium nanoparticles and their catalytic performance in esterification reaction were investigated. The results showed that Pd/Al2O3 exhibited the highest conversion rate of levulinic acid. Interestingly, PdNPS supported on TiO2 showed an induction period in the initial stage of the reaction, followed by a high conversion rate and selectivity towards isopropyl levulinate.