Marked influence of support on the catalytic performance of PdSb acetoxylation catalysts: Effects of Pd particle size, valence states, and acidity characteristics

The influence of support on the catalytic performance of PdSb catalysts for the gas-phase acetoxylation of toluene was investigated using various support materials with different acid strengths and surface areas. The supports used were TiO2, ZrO2, SiO2, and γ-Al2O3. The catalysts were characterized by N2 adsorption (BET-SA and pore size distribution), XRD, TEM, XPS, and FTIR (pyridine adsorption), and their performance in the selective acetoxylation of toluene to benzyl acetate (BA) was evaluated. Electron micrographs of spent solids exhibited considerably larger Pd particles compared with the corresponding fresh solids, indicating growth of Pd particles during the course of reaction, depending on the support. Such Pd growth was accompanied by enhanced catalytic performance. XPS revealed that the nature of support had a clear influence on the reducibility of Pd. The samples prepared with reducible supports (i.e., TiO2, ZrO2) displayed a substantial decrease in surface Sb and Pd/metal ratios in the used samples compared with fresh samples. In contrast, no such changes occurred with nonreducible supports (SiO2 and γ-Al2O3). Acidity measurements by means of pyridine adsorption disclose that all of the PdSb samples, irrespective of support-exhibited bands, correspond solely to Lewis sites; no evidence of Brønsted sites was found. The present study was aimed at determining the acidic strength of the catalysts based on catalytic performance. Changes in catalytic activity with time on stream were found to be dependent on the type of support used. Achieving steady-state conditions took longer in TiO2-supported catalyst (ca. 10 h) than with other supports (ca. 3 h). The best results were obtained over a 10Pd8Sb/TiO2 catalyst (10 wt% Pd, 8 wt% Sb; XTOL=68%XTOL=68%, SBA=85%SBA=85%) compared with all other catalysts of this series. Evaluating the nature of the supports revealed very pronounced differences in terms of Pd particle size, valence states, and acidity characteristics, which in turn showed a significant effect on the catalytic performance. However, growth of Pd particles and higher Lewis acidity represent a major requisite for obtaining better catalytic performance.