Abstract
The performance of bifunctional hybrid catalysts based on phosphotungstic acid (H3PW12O40, HPW) supported on TiO2 combined with Cu-ZnO(Al) catalyst in the direct synthesis of dimethyl ether (DME) from syngas has been investigated. We studied the effect of the HPW loading on TiO2 (from 1.4 to 2.7 monolayers) on the dispersion and acid characteristics of the HPW clusters. When the concentration of the heteropoliacid is slightly higher than the monolayer (1.4 monolayers) the acidity of the clusters is perturbed by the surface of titania, while for concentration higher than 1.7 monolayers results in the formation of three-dimensional HPW nanocrystals with acidity similar to the bulk heteropolyacid. Physical hybridization of supported heteropolyacids with the Cu-ZnO(Al) catalyst modifies both the acid characteristics of the supported heteropolyacids and the copper surface area of the Cu-ZnO(Al) catalyst. Hybridization gives rise to a decrease in the copper surface area and the disappearance of the strong acidic sites typical of HPW nanocrystals, showing all hybrids similar acid sites of weak or medium strength. The activity of the hybrids was tested for direct DME synthesis from syngas at 30 bar and 250 °C; only the hybrids with HPW loading higher than 1.4 monolayers showed activity for the direct synthesis of DME, showing that the sample loaded with 2.7 monolayers of heteropolyacid had higher activity than the reference hybrid representative of the most widely applied catalysts based on the combination of Cu-ZnO(Al) with HZSM-5. In spite of the high activity of the hybrids, they show a moderate loss in the DME production with TOS that denotes some kind of deactivation of the acidity function under reaction conditions.
Highlights
The selective hydrogenation of CO/CO2 into dimethyl ether (DME) is currently one of the most important topics in the research of alternative fuels because DME can be used as a direct and cleaner fuel alternative to conventional diesel [1]
Taking into account that local structure of Keggin units depends on the interaction stablished between the Keggin units with the TiO2 support, in this work we studied the effect of the heteropolyacid loading on the dispersion and acid characteristics of HPAs and its relevance on the hybridization with CZA catalyst applied for the direct synthesis of DME from syngas
The overall methanol and DME yield on CZA-HPW/Ti hybrids were in all cases lower than the methanol yield achieved on the CZA reference
Summary
The selective hydrogenation of CO/CO2 into dimethyl ether (DME) is currently one of the most important topics in the research of alternative fuels because DME can be used as a direct and cleaner fuel alternative to conventional diesel [1]. DME is an important chemical intermediate for the production of widely used chemicals, such as diethyl sulphate, methyl acetate or olefins [2]. DME is considered an ultra-clean transportation fuel because it has a high cetane number 60), low boiling point (−25 ◦ C) and high oxygen content (35 wt %) which allow fast vaporization and higher combustion quality (smokeless operation and 90% less NOx emissions) than other alternative. CO2 -based fuels [2]. DME showed total compliance with the highly strict California ultra-low emission. 22 of of 23 other alternative CO2-based fuels [2]. DME showed total compliance with the highly strict California vehicle (ULEV). Regulations medium-duty vehicles and it has the vehicles highest efficiency of all ultra-low emission vehicle for (ULEV). Regulations for medium-duty and it has thesynthetic highest liquid fuels (e.g., F-T diesel, methanol)
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