Highly active Pd-based catalysts with hierarchical pore structure for toluene oxidation: Catalyst property and reaction determining factor

A series of micro/mesoporous hybridized materials with coke-resistant ZSM-5 and three-dimensional large pore KIT-6 as their micro- and mesoporous components were synthesized and investigated for toluene deep oxidation. The characterization results show that most of Al atoms are tetrahedrally coordinated in the framework of the samples, and the ratio of ZSM-5 to KIT-6 can be systematically controlled by varying the Si/Al molar ratio in the synthesis mixture. The acid sites over the supports are favorable for Pd nanoparticle dispersion and metallic Pd oxidation due to their electrophilic characters. All Pd supported composite materials are found to be powerful catalysts for the total oxidation of toluene. Particularly, Pd/ZK-50 possesses the highest catalytic activity with 90% toluene decomposition at 197 °C, this temperature is much lower than that of Pd/KIT-6 (T90 = 234 °C). CO2 desorption capability has positive effects on catalytic activity, while the influence of toluene adsorption property is negligible. The superior activity of the composite catalysts can be ascribed to the cumulative of specific surface area, support acidity, Pd particles dispersion and CO2 desorption capability.

► ZSM-5/KIT-6 materials with 3-D hierarchical pore structure were synthesized under simple reaction system. ► Ratio of micro- to mesoporous phase can be adjusted systematically. ► Controllable acidity and enhanced hydrothermal stability. ► Superior low temperature activity and high CO2 selectivity. ► Reaction process and decisive factors were elucidated and discussed in detail.