https://doi.org/10.1016/j.seppur.2022.121559

CO₂ can be directly hydrogenated into dimethyl ether (DME) via bifunctional catalyst that is consist of metal/metal oxide catalyst (converting CO₂ into methanol) and H-ZSM-5 zeolite catalyst (converting methanol into DME). In order to develop an effective bifunctional catalyst for this purpose, an excellent metal/metal oxide catalyst to produce methanol has to be developed first. Recently, many studies have demonstrated Pd/In₂O₃ as an effective catalyst for the production of methanol via CO₂ hydrogenation. However, there has been no attempt to investigate the potential of Pd/In₂O₃/H-ZSM-5 as bifunctional catalyst for the production of DME. Furthermore, the synthesis of In₂O₃ usually requires either toxic or expensive reagents or templating agents.

Therefore, in this study, carbonized alginate templating was introduced as a potentially green yet low-cost method for the synthesis of In₂O₃ and Pd/In₂O₃. This was done by first producing insoluble indium alginate hydrogel, which was then undergone hydrothermal reaction at various temperature and time. At the optimized hydrothermal conditions of 160 ^oC and 12 h, a microspherical carbonized alginate structure with In³⁺ chemically-anchored onto it was formed. It was then undergone calcination, whereby the carbonized alginate was removed, while the chemically-anchored In³⁺ was converted into In₂O₃. As a result, microspherical-confined nano-In₂O₃ was formed, whereby the localized nucleation growth can be coordinated by the chemically-anchored In³⁺ in the well-distributed alginate structure. It was the used to support a highly well-distributed Pd, resulting in the formation of microspherical-confined nano-Pd/In₂O₃ (labelled as PdIn-160-12) with high specific surface area. This facilitates the formation of more oxygen vacancy onto In₂O₃. As a result, PdIn-160-12 was able to produce high amount of methanol at a space time yield of 458.7 g_Methanol kg_cat^-1 h^-1, which was 803% higher than that of commercial Pd/In₂O₃.

Next, PdIn-160-12 was integrated with H-ZSM-5 to investigate the potential of Pd/In₂O₃/H-ZSM-5 as a bifunctional catalyst for the production of DME via CO₂ hydrogenation. The effects of various integration manners (Dual bed, Granule mixed, Mortar mixed) and various Pd/In₂O₃: H-ZSM-5 (PdIn-160-12: H-ZSM-5) mass ratio on DME production were investigated. It was found that mortar mixed was able to produce the highest amount of DME, as the close proximity causes stronger ion-exchange mechanism between the catalyst components, which facilitates higher oxygen vacancy density in the bifunctional catalyst. Under the optimum Pd/In₂O₃: H-ZSM-5 (PdIn-160-12: H-ZSM-5) mass ratio of 4:1, the highest spacetime yield of 79.7 g_DME kg_cat^-1 h^-1 with CO₂ conversion of 9% and DME selectivity of 44.1% can be obtained. In addition, the spacetime yield is able to be maintained even in 60 h on stream.

Henceforth, an effective, yet stable bifunctional catalyst consists of microspherical-confined nano-Pd/In₂O₃ and H-ZSM-5, with microspherical-confined nano-Pd/In₂O₃ component produced from green synthesized method via carbonized alginate templating for the production of DME via CO₂ hydrogenation has been developed. Furthermore, this study has also successfully expanded the application of Pd/In₂O₃ into bifunctional catalyst. It is believed that such an approach can also be extended to different varieties of bio-materials as template, to produce different varieties of metal/metal oxide, for different varieties of catalytic reaction applications, either as a single or bifunctional catalyst.