Role of Catalyst Domain Size in the Hydrogenation of CO₂ to Aromatics over ZnZrO_<i>x</i>/ZSM-5 Catalysts

Abstract

The direct conversion of carbon dioxide into aromatic compounds may provide an environmentally friendly resource for a family of chemical building blocks with high global demand aromatic chemicals. An array of bifunctional catalysts composed of ZnZrO&lt;sub&gt;x&lt;/sub&gt; for the conversion of CO&lt;sub&gt;2&lt;/sub&gt; to methanol and ZSM-5 for methanol aromatization is evaluated considering multiple operational and catalytic parameters, with a focus on the particle size of each catalyst functionality. Aromatic yields generally increase as the ZSM-5 domain size decreases for different configurations of catalyst domains mixed powders, mixed pellets, and separate beds. However, for a physical mixture of the catalyst components prior to pelletization (mixed powder case), varying the ZSM-5 crystal size inversely impacts the particle size of ZnZrO&lt;sub&gt;x&lt;/sub&gt;, leading to enlarged metal oxide domains and enhanced intermediate diffusion barriers for very small crystals of ZSM-5. Larger crystals of ZSM-5 can be employed to obtain a higher concentration of benzene, toluene, and xylene (BTX) among aromatics. Upon employing a physical mixture of the ZnZrO&lt;sub&gt;x&lt;/sub&gt;/ZSM-5 catalyst with a Si/Al ratio of 300 and a ZSM-5 crystal size of 300 nm, a total aromatics selectivity of 44% at CO&lt;sub&gt;2&lt;/sub&gt; conversion of 6% was obtained at 320 °C, WHSV of 7200 mL g&lt;sub&gt;catalyst&lt;/sub&gt; &lt;sup&gt;–1&lt;/sup&gt; h&lt;sup&gt;–1&lt;/sup&gt;, and H&lt;sub&gt;2&lt;/sub&gt;/CO&lt;sub&gt;2&lt;/sub&gt; ratio of 3.