Controlling Nanosheet Spacing of ZnAl-Layered Double Hydroxide Assemblages for High-Efficiency Hydrogenation of CO2 to Methanol

Abstract

Three different morphological variations of monodispersed Zn-Al-containing layered double hydroxide (ZnAl-LDH) microspheres were synthesized, with varying degrees of spacing between the nanosheets. The ZnAl-LDH materials were calcined into Zn-Al-containing layered double oxide (ZnAl-LDO) microspheres and loaded with Cu to be used as catalyst candidates for CO2 hydrogenation to methanol under high pressure and constant flow. Two different low-cost facile Cu loading methods were also employed, namely, wet impregnation and ion exchange. The different morphological variations were then evaluated and compared by computational fluid dynamics (CFD) simulation (ANSYS Fluent) as well as the hydrogenation reaction of CO2 to methanol with the purpose of exploring the effect of nanosheet spacing on catalyst performance in terms of the overall catalytic activity and methanol selectivity. The different metal loading methods have also led to different stability results, where there is a slight drop in performance of the incipient wet-impregnated catalyst, while the ion-exchanged catalyst retains much better performance over the same period of 40 h. Based on our simple CFD modeling, an increase in the nanosheet spacing improves convection-driven vortices within the wider channels of the LDH assemblage, with the vortices reaching deeper into the interior core of the microsphere. The simulated vortex phenomena explain the higher catalytic activities observed in our experimental results. This work indicates that catalysts with intricate morphological structure engineering would significantly enhance their catalytic activities.