| Dimethoxymethane (DMM) is an important methanol derivative with good solubility, high oxygen content and low toxicity. It is a good solvent, a diesel additive and a raw material for the synthesis of highly concentrated formaldehyde. In addition, it might be reformed to produce H2 for fuel cells. Traditionally, DMM is synthesized via the condensation reaction between methanol and formaldehyde in the presence of acid catalysts. Recently, attention has been paid on the direct synthesis of DMM through the selective oxidation of methanol. According to our previous work, the V2O5/TiO2 catalysts doped with sulfate exhibited good performance for the selective oxidation of methanol to DMM. The catalytic performance can be further improved by increasing surface areas of supports and by using improved preparation methods. In this thesis, different ingredients were added to the surfaces of V2O5/TiO2 catalysts to modify the surface structures and properties, so as to improve the activity for the selective oxidation of methanol to DMM. The main results obtained in this work are summarized below:(1) Effect of acidic promoters on the V2O5/TiO2 catalysts for the selective oxidation of methanol to dimethoxymethane:Mesoporous TiO2 nanotubes (TNT) with high surface areas (e.g.,305 m2g-1) were prepared by an alkaline hydrothermal method, and used to support vanadia species. The V2O5/TiO2 catalysts were modified with sulfuric, phosphoric and phosphotungstic acids for the selective oxidation of methanol to dimethoxymethane (DMM). Temperature programmed reduction (TPR), microcalorimetric adsorption of ammonia and isopropanol probe reaction were used to characterize their surface acidic and redox properties, which were two important parameters determining the reactivity of selective oxidation of methanol to DMM. It was found that the addition of phosphoric and phosphotungstic acids might enhance the surface acidity of V2O5/TiO2, but weakened its redox ability, and therefore had the negative effect for the target reaction. Without calcination, the addition of sulfuric acid had the similar negative effect since it weakened redox ability, too. Only the V2O5/TiO2 modified with sulfuric acid and calcined at 673 K exhibited the significantly enhanced selectivity to DMM with high methanol conversions. The calcination created some sulfate groups strongly interacted with vanadium species which enhanced the strengths of surface acidity without weakening the redox ability of vanadium sites.(2) Effect of silica on the selective oxidation of methanol to dimethoxymethane over V2O5/TiO2 catalysts:V-Ti-O and V-Ti-Si-O complex oxides containing sulfate were prepared by co-precipitation method using Ti(SO4)2, VOSO4 and TEOS as precursors. These catalysts possessed acidic and redox bifunctional characters. The introduction of SiO2 into V-Ti-O did not increase the surface area, but decreased the oxidation ability and surface acidity, but significantly improved the selectivity to DMM above 453 K.93% selectivity to DMM with 66% conversion of methanol was obtained over the Si-V-Ti-O catalyst at 483 K.(3) Effect of alumina on the V2O5/TiO2 catalysts for the selective oxidation of methanol to dimethoxymethane:A series of Al-V-Ti-O oxides containing SO42-were prepared by a co-precipitation method using Al2(SO4)3·18H2O, VOSO4·xH2O (structure water content:50%) and Ti(SO4)2 as precursors. The samples were characterized by means of X-ray diffraction, FT-IR, Raman spectra, O2 chemisorption and N2 adsorption-desorption at liquid N2 temperature. It was found that the surface areas of the Al-V-Ti-O continuously increased with the addition of alumina content. Beyond the alumina content of 5%, vanadia could be well dispersed (>70% dispersion degree). The sample 5A125VTi and 10A125VTi exhibited highest surface acidity and both methanol and DMM were strongly adsorbed on these samples. High DMM yields (56-62% methanol conversion with 84-87% DMM selectivity) could be achieved on the Al-V-Ti-O samples with alumina contents of 1-10%. |
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