Cascade Process for Coupling Molecular H2-free Hydrogenation of CO2 with Alkylation of Aromatics Using a Single-Bed Catalytic System

A single-step, molecular H2-free, integrated catalytic process for producing xylene-rich alkylated aromatics (AAs) using CO2 as an alkylating reagent has been developed, wherein methylcyclohexane (MCH) is used as a liquid organic hydrogen carrier (LOHC) as well as a reactant, and CO2 as a carbon source to generate alkylating species. A multifunctional catalyst has been developed through the metal-functionalization of zeolite. Physico-chemical properties of the catalyst were studied using characterization techniques, like XRD, FTIR, NH3-TPD, pyridine-IR, N2-adsorption–desorption, ICP-OES, UV–vis absorption spectroscopy, HR-TEM, HR-SEM, TGA, etc. The coexistence of Lewis and Bro̷nsted acid sites in a single catalyst facilitates three types of reaction in a single pass, i.e., the dehydrogenation of MCH to produce toluene and hydrogen, the hydrogenation of CO2 to form active alkylating species, and the alkylation of toluene to produce AAs. Using LOHC not only sidestepped the storage, transport, and safety issues associated with the molecular H2, but also conferred almost four times higher product yield and carbon utilization compared to the molecular H2-based process. In a single pass, >70% MCH conversion and ∼58% (wt/wt) selectivity for AAs in the liquid product were achieved, which is equivalent to the space–time yield of ∼390 mg gcat–1 h–1. The catalyst could effectively suppress the toluene disproportionation and the methanol-to-olefins reaction. The values of reaction free energy calculated using the DFT method indicate that the LOHC-based route is thermodynamically more favorable than the molecular H2-based route. Moreover, the in situ utilization of the dehydrogenated species of LOHC as a reactant makes the process atom-economical and evades the process complexity associated with the separation of dehydrogenated LOHC.