Effective silica supported Sb–V mixed oxide catalyst for selective oxidation of methanol to formaldehyde
Introduction
Selective oxidation of methanol to formaldehyde, which is an important building block for some complex chemicals, is one of the dominant oxidation processes in industry [1]. The current industrialized processes for this transform from methanol to formaldehyde are based on silver catalysts and iron molybdate catalysts. Recently, supported vanadia catalysts were found to be promising catalysts for this reaction [2], [3], [4], [5]. Vanadia catalysts have been found to be structurally sensitive for this reaction [2], [6], [7], [8], [9]. The active species of supported vanadia catalysts are proposed to be isolated tetrahedrally coordinated VO4 species [2] or adjacent VO4 units [8] with VO bands as remarkable characteristic. The polymerized VOx species with VOV bonds are suggested not to be involved in methanol selective oxidation but usually relevant to deep oxidation [8]. The formation of aggregated or crystalline vanadia species results in the drops in both methanol conversion and formaldehyde selectivity [5], [7]. Kim and Wachs found that, in the methanol oxidation catalyzed by V2O5/Al2O3 catalysts, the crystalline V2O5 nanoparticles above monolayer coverage were relatively inactive and served only to decrease the number of exposed catalytic active surface vanadia sites by covering them [3].
However, the aggregation of highly isolated vanadia species under reaction conditions is always a puzzle. Consequently, high specific area materials like MCM-41 [10], MCM-48 [7] and SBA-15 [5], [11], [12] were employed as supports to decrease the surface concentration of vanadia species and then promote the well dispersion of vanadia species. In addition, some novel grafting methods were adopted together with the high specific area supports in order to benefit the isolation of vanadia species [5], [7]. As another option within the efforts to develop active catalyst for the selective oxidation of methanol, mixed oxides have been of great interest because of the opportunity to adjust the variation in active sites [9], [13], [14].
Sb–V mixed oxide catalysts have been widely used for selective oxidation of hydrocarbons [15], [16]. Benvenutti and Gaushikem found that the deep oxidation of methanol or formaldehyde can be depressed with the presence of antimony species [17]. Additionally, Spengler et al. found that the aggregation of vanadia species can be interrupted in Sb–V mixed oxide by means of the formation of VOSbOV species [18]. In our previous work, we found that VO in common with SbOV sites prevail in the Sb–V mixed oxides dispersed on MSU-2 [19] or amorphous silica [20] and the speciation of supported Sb–V mixed oxides can be tuned by changing Sb/V ratio. Furthermore, the surface acidity of supported Sb–V mixed oxide catalysts is adjustable due to the interaction of antimony and vanadium atoms [14]. It has been found that, in methanol selective oxidation, the production of formaldehyde needs bi-functional catalysts with acid–base character [21], [22], [23]. It can be expected that Sb–V mixed oxide catalysts may show promising activity and selectivity in methanol selective oxidation.
In this work, we prepared and investigated silica supported Sb–V mixed oxide catalysts (VSbOx/SiO2) in the selective oxidation of methanol with O2 as oxidant. As the comparison of VSbOx/SiO2 catalysts, silica supported vanadia catalysts (VOx/SiO2) with corresponding loading of vanadia to that for the VSbOx/SiO2 catalysts were prepared and investigated under same reaction conditions. In VSbOx/SiO2 catalysts, the framework of VOx species is interrupted by the incorporation of Sb atoms and isolated VOx species can be stabilized in the framework of Sb–V mixed oxide. In the methanol selective oxidation with O2 as oxidant, VSbOx/SiO2 catalysts exhibit very stable catalytic performance with high selectivity to formaldehyde but low selectivity to COx. One-pass yield of formaldehyde at 91% can be achieved on VSbOx/SiO2 catalyst. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies exhibit that the VSbOx/SiO2 catalysts can greatly depress the overoxidation to form COx.
Section snippets
Catalyst preparation
VSbOx/SiO2 catalysts were prepared by a two-step incipient wetness impregnation method. Silica (Qingdao Mandarin Chemical Group, China) was firstly impregnated with SbCl5 (Acros, 99.5%)–ethanol solution [24], and then dried at room temperature. Antimony precursors can be well dispersed on silica surface after this impregnation step [17], [18], [24], [25]. The dried solid product was then impregnated for the second time with aqueous solution of NH4VO3 (Shanghai Chemical Reagent Co., Ltd.) [20]
Textural and chemical properties of catalysts
In Fig. 1a we can see that the silica support and all catalysts show narrow distributions of pore size. The pore sizes of both VOx/SiO2 catalysts and VSbOx/SiO2 catalysts become smaller with increasing loading of V2O5. This trend is also indicated by the pore diameter values given in Table 1. For SbOx/SiO2 catalysts, SbOx species are well dispersed on silica surface and the pore size and specific area just exhibit little drop even the loading of Sb2O5 reaches 20 wt% [24]. The specific areas of
Conclusions
Silica supported Sb–V mixed oxide catalysts and vanadia catalysts have been prepared and evaluated in methanol selective oxidation with O2 as oxidant. The active phase of VSbOx/SiO2 catalysts is Sb–V mixed oxide, in which the framework of VOx species is interrupted by the incorporation of Sb atoms so isolated tetrahedrally coordinated VOx species can be stabilized in the framework of Sb–V mixed oxide. The relative amount of monomeric VOx species in VSbOx/SiO2 catalysts is higher than VOx/SiO2
Acknowledgments
The financial supports from the National Basic Research Program of China (Grant No. 2003CB615806 & 2005CB221407) and the National Natural Science Foundation of China (NSFC 20673115, 20773118) are acknowledged.
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