Selective Synthesis of Aromatics from CO₂

Fossil resources are used as a feedstock for many important processes in the chemical industry—however, they are limited. CO₂, in contrast, could be used as a renewable feedstock for the production of chemicals. CO₂ hydrogenation could provide fuels as well as chemicals such as methanol, olefins, or aromatics. C1 compounds such a methane or methanol can already be selectively synthesized from CO₂ and H₂. A selective hydrogenation of CO₂ to give aromatics, in contrast, had not been achieved so far.

Wenliang Zhu, Zhongmin Liu, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, and colleagues have developed a composite catalyst made from the oxide ZnAlO_x and the zeolite H-ZSM-5 for the selective synthesis of aromatics (example pictured) from CO₂ and H₂. The team synthesized the ZnAlO_x using a co-precipitation method. The zeolite was prepared from commercially available Na-ZSM-5 by ion exchange. The composite catalyst was then prepared by physical mixing of the two components in a 1:1 ratio.

Selective synthesis of aromatics. (Image by ChemistryViews)

Hydrogenation reactions were then performed over the composite catalyst using a gas mixture of CO₂, H₂, and argon (H₂/CO₂/Ar = 3/1/0.2) at a combined pressure of 3 MPa and at a temperature of 593 K. The team achieved up to 9 % CO₂ conversion with a selectivity of aromatics among the carbon products (excluding CO) up to 74 %. The CH₄ selectivity was only 0.4 %. According to the proposed mechanism, intermediates such as methanol and dimethyl ether are first formed on the ZnAlO_x surface and then transmitted to H-ZSM-5 and converted into olefins and aromatics. (ChemistryViews)