Abstract
In this work, sulfated and phosphated metal oxides were studied as catalysts for the Diels–Alder cycloaddition of biomass-derived 2,5-dimethylfuran (DMF) and ethylene to understand the effect of acid strength on the reaction. Four catalysts with varied acidity, namely sulfated SiO2, sulfated TiO2, phosphated SiO2, and phosphated TiO2, were prepared via wet impregnation using sulfuric acid and phosphoric acid as precursors, and their structural and acid properties were examined using X-ray diffraction, Brunauer–Emmett–Teller analysis, Fourier transform infrared spectroscopy, solid state 31P magic angle spinning nuclear magnetic resonance spectroscopy, and temperature programmed desorption of ammonia. The results revealed that the acidity of the catalysts was largely influenced by the type of the acid functional group and the support as well as the calcination temperature. The conversion of DMF and the selectivity toward p-Xylene (PX) were generally correlated with the total acid site density and the acid–metal oxide interaction strength, which in turn affected the acid strength. Overall, phosphated SiO2 and TiO2 calcined at 773 K were identified as the most active and selective catalysts, exhibiting a high PX selectivity of over 70% and DMF conversion of 80% at 523 K after 6 h. The origin of the stability of the highly active phosphated catalysts was also investigated in detail.
Highlights
For the titania-based catalysts, the diffraction peaks at 2θ = 25.2 and 38.0◦ could be assigned to the anatase structure (JCPDS card no. 21-1272), whereas that at 2θ = 28.0◦ could be assigned to the rutile structure (JCPDS card no. 21-1276)
Both the silica and titania catalysts did not show any diffraction peaks corresponding to the sulfate and phosphate framework structure
The activities of the well-characterized sulfated and phosphated catalysts were evaluated for the Diels–Alder reaction of DMF and ethylene
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The Diels–Alder cycloaddition of biomass-derived furans and ethylene is the most atom-efficient route for the production of BTX from biomass [3] The chemistry of this reaction was first proposed by Timothy A. The DMF could be produced from solid biomass using an existing and wellestablished process: the solid biomass is first hydrolyzed into a C6 sugar, such as glucose, which is converted into 2,5-hydroxymethylfurfural (HMF) through dehydration; the HMF is converted into DMF via hydrogenolysis in the presence of copper-based catalysts [5] This cycloaddition chemistry of DMF could be utilized for the production of renewable PX [6]. Achieving the high yields of PX.toward PX over other side products is the key for obtaining high yields of PX
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