Report on combined catalyst synthesis
Creators: name: French National Centre for Scientific Research
Publisher: name: Zenodo
Year: 2022
Subjects: zeotypes
catalyst synthesis
catalyst characterization
tandem catalysts

description: This deliverable presents the results obtained at this stage in task 1.5 involving combined catalyst synthesis and their catalytic activity in CO2 conversion to light hydrocarbons. As the targeted reaction in this project consists of two steps (methanol synthesis from CO2 and methanol to olefin/hydrocarbon), and existing systems for this cascade reaction comprise a mechanical mixture of two catalysts, it is believed that a material having both active phases in molecular proximity may increase the catalytic performance.
The general strategy is to immobilize the active phase in methanol synthesis (PdZn nanoparticles) into zeolite/zeotype materials (H-ZSM-5, H-SAPO-34, H-SAPO-18). The surface organometallic chemistry approach was employed. This synthesis methodology allows a controlled deposition of the desired phase through grafting of a suitable organometallic precursor. In this work, the heterobimetallic PdZn(OOCCH3)4 complex was synthesized and used. Another advantage with this precursor is to ensure formation of the PdZn alloy with a preferred stoichiometry (1:1). The first attempt was carried out by directly grafting of the precursor onto the selected zeolite/zeotype materials. The catalysts were characterized by DRIFT, XRD, BET, ICP, STEM/EDX. However, those materials were able to hydrogenate CO2 (at 30 bar, 380 °C), but with the undesired CO (> 90%) as main product. Additional efforts were then dedicated to understanding the behavior of the methanol synthesis catalyst (supported PdZn nanoparticles) mechanically mixed with commercial SAPO-34. Based on our investigations in this project (WP1), it seems that the support of the PdZn phase has a critical role in the cascade reaction to finally yield light hydrocarbons. Literature usually employs basic or redox supports. The reason is probably related to their ability to adsorb/activate CO2. We found CeO2-ZrO2 to be the most promising support. Hence, our next attempt was to decorate the zeolite/zeotype materials with CeO2 or ZrO2 prior to immobilization of the PdZn nanoparticles. Two strategies to functionalize the zeolite/zeotype materials with oxide clusters were applied: i) aqueous impregnation of inorganic salt precursors of Zr and Ce followed by calcination; ii) combination of surface organometallic chemistry and non-hydrolytic sol-gel. The first approach resulted in degradation of the zeolite/zeotype materials as revealed by XRD and BET, probably due to the release of substantial amounts of water (as the form of steam) during the calcination. Furthermore, the catalytic test results also contained small amount of methanol in the products (CO remained to be the main product), confirming the methanol to olefin/hydrocarbon catalysts were inactive. Applying the second strategy, the crystallinity of the zeolite/zeotype materials was retained and the PdZn alloy was successfully deposited onto the support (containing CeO2 or ZrO2 decorated zeolite/zeotype materials). The catalysts were characterized by DRIFT, XRD, BET, ICP, STEM/EDX. Those materials were indeed able to hydrogenate CO2 with a decent conversion, but the main product was still the undesired CO (> 80%) along with minor amount of light hydrocarbons (saturated C1-C3).
This work has been able to provide a synthesis method to achieve combined catalyst containing both active phases in molecular proximity. The catalytic performance of the available combined catalysts at this stage exhibited low productivity of the desired products (light hydrocarbons, notably C3). Nevertheless, there are still a lot of unexplored area, which can improve the current system. For example, additional efforts will be dedicated to the challenging deposit CeO2-ZrO2 mixed oxide by using non-hydrolytic sol-gel onto the zeolite/zeotype materials as this mixed oxide has previously shown to be most promising support for the methanol synthesis catalyst. Moreover, additional work will be dedicated to modify supported CeO2-ZrO2 in zeolite by other promotor.