A VOx/meso-TiO2 catalyst for methanol oxidation to dimethoxymethane

doi:10.1016/j.catcom.2009.07.029

Catalysis Communications

Volume 10, Issue 15, 15 September 2009, Pages 2051-2055

https://doi.org/10.1016/j.catcom.2009.07.029 Get rights and content

Abstract

Mesoporous TiO₂ was prepared by simply controlling the hydrolysis of Ti(OBu)₄ with the help of acetic acid. The mesoporous TiO₂ had a well-crystallized anatase phase and a high surface area of 290 m² g⁻¹ with a pore size of about 4 nm. The anatase phase and the mesoporous structure were maintained in the VO_x/TiO₂ catalyst with a monolayer dispersion of V₂O₅, however, the surface area decreased to 126 m² g⁻¹. The catalyst was highly active and selective for methanol oxidation, giving about 55% conversion of methanol and 85% selectivity to dimethoxymethane at 423 K.

Introduction

TiO₂ is one of the most important metal oxides in heterogeneous catalysis. In addition to be used as a typical photocatalyst, it is also an essential component in oxidation catalysts [1]. For example, the reaction rate of methanol oxidation over a VO_x/TiO₂ catalyst is at least one order of magnitude larger than those on VO_x/SiO₂ and VO_x/Al₂O₃ catalysts at near monolayer coverage of V₂O₅ [2]. However, the surface area of the currently used TiO₂ material is usually less than 100 m² g⁻¹, hindering the preferred monolayer dispersion of vanadia, particularly at higher loading (>10% V₂O₅). Mesoporous TiO₂ materials synthesized using structure-directing agents have high surface area up to 1000 m² g⁻¹, but they are usually amorphous or semi-crystalline [3], [4], [5]. As a result, the mesoporous structure tends to collapse due to crystal growth upon thermal treatment at relatively higher temperatures (723–823 K) [4], [5]. Hydrolysis of titanium alkoxide in the presence of mineral acids (HNO₃ or H₃PO₄) produced mesoporous TiO₂ of anatase crystalline with a surface area of 106–294 m² g⁻¹ after calcination above 723 K [6], [7], but the mineral acids are difficult to be removed completely. Deshpande et al. [8] have recently reported that hydrolysis of Ti(OBu)₄ in the presence of acetic acid produced anatase mesoporous TiO₂ with a surface area of 125 m² g⁻¹ after calcination at 673 K. However, the process proceeded very slowly and the synthetic period was up to 120 days for obtaining the mesoporous structure. Therefore, a simple and effective method for preparing crystalline mesoporous TiO₂ material with a high surface area is of high importance.

In this work, we synthesized mesoporous TiO₂ with anatase crystalline by simply controlling the hydrolysis process of Ti(OBu)₄. The TiO₂ material was used to support VO_x species as a catalyst for the selective oxidation of methanol to dimethoxymethane (DMM).

Section snippets

Materials preparation

The mesoporous TiO₂ was prepared by hydrolysis of Ti(OBu)₄. About 50 g Ti(OBu)₄ and 10 ml acetic acid were dissolved into 200 ml ethanol at 273 K, and added to 700 ml ethanol aqueous solution (90%) at the same temperature under stirring. The mixture was kept at 298 K for 2 h and 323 K for 2 h, respectively. 45 ml deionized water was then added at 323 K and the mixture was kept at this temperature for 13 h. The precipitate was filtered, washed with deionized water and ethanol, and dried at 383 K for 12 h

Mesoporous TiO₂

Fig. 1 shows the XRD pattern of the as-prepared TiO₂ material. It exhibited typical diffraction peaks of crystalline anatase structure (JCPDS #86-1157). The crystalline size estimated from the (1 0 1) diffraction peak was about 7 nm. Fig. 2 shows the N₂ adsorption–desorption isotherms and the pore size distribution (inset) of the as-prepared TiO₂ material. It displayed type IV adsorption isotherm with H₂ hysteresis loop, characteristics of mesoporous structure. This kind of pore structure may be a

Conclusions

Mesoporous anatase TiO₂ was prepared by simply controlling the hydrolysis process of Ti(BuO)₄ in the presence of acetic acid. The presence of acetic acid not only favored the formation of mesoporous structure but also accelerated the crystallization of TiO₂. The resultant VO_x/TiO₂ catalyst had a monolayer dispersion of VO_x even the loading was as high as 16% (in V₂O₅), and exhibited high activity and selectivity for methanol oxidation to DMM.

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A VOx/meso-TiO2 catalyst for methanol oxidation to dimethoxymethane

Abstract

Introduction

Section snippets

Materials preparation

Mesoporous TiO2

Conclusions

Catal. Today

Micropor. Mesopor. Mater.

Micropor. Mesopor. Mater.

Mater. Chem. Phys.

Catal. Today

J. Catal.

J. Catal.

Appl. Catal. A

Appl. Catal.

J. Solid State Chem.

Catal. Today

A VO_x/meso-TiO₂ catalyst for methanol oxidation to dimethoxymethane

Mesoporous TiO₂