Hydrogenation of CO2 to Dimethyl Ether over Tandem Catalysts Based on Biotemplated Hierarchical ZSM-5 and Pd/ZnO

The chemical transformation of CO2 to value-added chemicals and fuels remains a daunting research challenge, requiring the development of more efficient catalysts. The present work offers highly active integrated nanocatalysts for CO2 hydrogenation to dimethyl ether (DME) through tandem catalysis, that is, Pd/ZnO converts CO2 to methanol, and bio-ZSM-5 further catalyzes methanol dehydration to DME. Notably, the hierarchically porous bio-ZSM-5 with high pore connectivity was prepared by using low-cost rice husk and luffa sponge as biotemplates. The formation of bio-ZSM-5 involved three consecutive steps, including (i) diffusion of precursor species, (ii) confined crystal growth, and (iii) calcination treatment to remove the residual biomass. Interestingly, the obtained bio-ZSM-5 possesses abundant hierarchical porosity, including the intrinsic micropores from ZSM-5 frameworks, the mesopores from intracrystalline void space or pinholes in the intergrown zeolite layers, and the macroscopic channels from the original biomass architecture. It was found that the tetrapropylammonium hydroxide amount and Si/Al ratio were two critical factors affecting the crystallinity and porosity of the bio-ZSM-5. The silicon content in biomass (8.3 wt % in rice husk and 0.016 wt % in luffa sponge) also played a crucial role in the synthesis of bio-ZSM-5. Different spatial configurations of Pd/ZnO and bio-ZSM-5 significantly affected the DME yield, wherein spatial distances between the bio-ZSM-5 and Pd/ZnO components were adjusted by controlling the size of Pd/ZnO or the integration manners. The current results indicated that the closest proximity of the bifunctional catalyst was unfavorable for DME production (selectivity < 3%), because of which low-valent Zn cations from Pd/ZnO could displace the Bronsted acid sites on bio-ZSM-5 by ion exchange, and the powder mixing in the mortar was the best combination configuration of Pd/ZnO and bio-ZSM-5, promoting the CO2 conversion (10.8%) with a DME selectivity of 31% under the reaction of 300 degrees C and 30 bar. Furthermore, as compared to the traditional ZSM-5 based catalyst, the obtained Pd/ZnO/bio-ZSM-5 bifunctional catalysts showed excellent long-term stability in 60 h on stream with a less amount of coke formed in the spent catalysts.