Structural, acidic and redox properties of V2O5/NbP catalysts

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Abstract

V2O5 supported on niobium phosphate (NbP) with V2O5 loadings from 5 to 25 wt% were investigated by using different techniques. The structural properties were characterized by O2 chemisorption, X-ray diffraction (XRD), Raman spectroscopy (LRS) and X-ray photoelectron spectroscopy (XPS). The surface acidity was determined by the techniques of microcalorimetry and infrared spectroscopy (FTIR) using NH3 as a probe molecule. Isopropanol (IPA) and methanol probe reactions in the presence of O2 were employed to provide the information about the surface acidity and redox property simultaneously. V2O5 is well dispersed on the surface of NbP according to the results from XRD and LRS with a V2O5 loading lower than 15 wt%. The vanadium and niobium elements in V2O5/NbP catalysts were present in the +5 oxidization state as observed from XPS. Chemical adsorption of O2 showed that vanadia dispersed on NbP with about 60% dispersion. The results from NH3 adsorption microcalorimetry suggested that the loaded V2O5 weakened the surface acidity of NbP while increased the proportion of weak acid sites. The results from IPA conversion reaction pointed out that IPA only converted to dehydration products (propylene (PPE) and diisopropyl ether (DIPE)) on NbP. The addition of V2O5 on NbP led to an oxidative product acetone (ACE), but not as much as on bulk V2O5. These results indicated that NbP possessed only surface acidity and weakened the redox property of supported V2O5. Accordingly, NbP produced only dimethyl ether (DME) from the dehydration of methanol owing to the lack of redox property, while the V2O5/NbP catalysts produced mainly dimethoxymethane (DMM) due to its acidic-redox bi-functional character. Specifically, methanol was first oxidized on the redox sites of V2O5/NbP to produce formaldehyde (FA) which was then condensed with additional methanol on the acidic sites of V2O5/NbP to form DMM.

Graphical abstract

A series of V2O5/NbP catalysts were prepared by the wetness impregnation method. The results showed that V2O5 could be well dispersed on the surface of NbP. The strength of surface acidity of NbP was decreased while the proportion of weak acid sites was increased upon the addition of V2O5 on NbP. The isopropanol conversion reaction and methanol oxidation reaction indicated that NbP possessed only surface acidity and weakened the redox properties of supported V2O5.

Introduction

Supported vanadia are extensively used as industrial catalysts for various processes such as selective oxidation of methanol to formaldehyde (FA) [1] and methyl formate (MF) [2], selective oxidation of o-xylene to phthalic anhydride [3], [4], ammoxidation of alkyl aromatic hydrocarbons [5], [6], removal of NOx and SOx [7], oxidation of SO2 to SO3 [8], selective catalytic reduction (SCR) of nitric oxides [9], etc.

Due to the restrictions of thermal stability, mechanical strength and surface area of bulk V2O5, it is generally not used directly as a catalyst in industry. It is usually supported on different carriers for different purposes. With a suitable support, its surface area, thermal stability and mechanical strength could be improved [10]. Studies indicated that the intensive interactions between a support and V2O5, and the dispersion and surface structures as well as the redox and acid-base properties of V2O5 would be modified by supports [11], [12]. Hence, the supported V2O5 are sometimes used as model catalysts [13]. In fact, by employing different supports (such as SiO2, Al2O3 and TiO2) and loadings, the surface structure [14], [15] and number of surface sites as well as the surface acid-base [16], [17], [18] and redox properties [19] can be intentionally monitored. The catalytic properties of the active vanadia phase can be greatly influenced by the nature of support and the dispersion of active components.

Nb2O5 is a solid acid that can be used as a support or a promoter in various catalysts [20]. The surface acidity could be enhanced by addition of P to form niobium phosphate (NbP) [21]. In recent years, the dispersion and structure of V2O5/Nb2O5 catalysts have been studied by using different spectroscopic techniques, such as X-ray diffraction (XRD), 51V NMR [22], [23], [24], X-ray photoelectron spectroscopy (XPS) [25], infrared spectroscopy (FTIR) [26], Raman spectroscopy (LRS) [26], EPR [27] and LEIS [28]. Their catalytic behavior in selective oxidation reactions was also studied [25], [26], [27], [28], [29], [30], [31]. In this work, we used niobium phosphate to support vanadia. The acidic and redox properties as well as the structure of the catalysts with different loadings were characterized. Specifically, the surface acidity was characterized by employing NH3 adsorption microcalorimetry and infrared spectroscopy, while the surface acidic and redox properties were probed by the reactions of isopropanol conversion and methanol oxidation in the presence of O2. The purpose of this work was to determine the relation between acidic-redox properties and dispersion of V2O5 on NbP.

Section snippets

Catalyst preparation

The niobium phosphate supports were prepared as described in the literature [32]. Specifically, 2.73 g of NbCl5 (Alfa Aesar, 99%) was partially hydrolyzed in 50 ml of H2O, followed by the addition of 2.30 g of H3PO4 (Aldrich, 85% aqueous solution) to initiate a vigorous hydrolytic reaction. An additional 50 ml of H2O was then added, and the reaction mixture was stirred for 30 min. The pH of the reaction mixture was adjusted to 2.60 with an ammonia solution. After stirring, the slurry was filtered

Surface structures

The XRD patterns of NbP and V2O5/NbP samples are presented in Fig. 1. It can be seen from the figure that all the samples showed two broad peaks with 2θ located around 25 and 52° typical of amorphous NbP [34]. No other crystalline niobia phases were observed in this work. The absence of XRD peaks due to V2O5 for 5V/NbP and 15V/NbP samples indicates that vanadium oxide is present in a highly dispersed manner on the surface of NbP, while weak signs of V2O5 were observed for 25V/NbP sample,

Conclusions

A series of V2O5/NbP catalysts with different V2O5 loading were synthesized by the wetness impregnation method. The results from XRD and LRS showed that V2O5 could be well dispersed on the surface of NbP. XPS showed that both vanadium and niobium were present in oxidized state of +5 in the samples. Adsorption of O2 showed about 60% dispersion of V2O5 on NbP. The results of microcalorimetry and FTIR for NH3 adsorption showed that the strength of surface acidity of NbP was decreased, while the

Acknowledgements

We acknowledge the financial supports from the French Ministry of Education, the CNRS–France, NSFC (20373023) and MSTC (2004DFB02900 and 2005CB221400).

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