Vanadium phosphorus oxides VPO

Vanadium phosphorus oxides (VPO) are commercially used as catalysts for the s5mthesis of maleic anhydride from the partial oxidation of n-butane. The phase constitution and the morphology of the catalyst are found to be dependent on the preparation routes and the applied solvent [78]. Recently, a method to prepare VPO catalysts in aqueous solution at elevated temperature was reported [79]. In addition to the linear relationship between specific activity and surface area, a small group of catalysts exhibit enhanced activity, which could be due to the combination of a higher proportion of V phases in the bulk of vanadyl pyrophosphate (V0)2P207 catalyst [79, 80]. With high relevance to the catalytic properties, the microstructure characterisation of VPO therefore is of great importance. 

Diacetyl (DA) is used as a flavour enhancer in the food industry and is currently manufactured from methyl ethyl ketone (MEK) in homogeneous systems via an oxime intermediate (ref.1). In principle, DA can also be manufactured by the selective oxidation of MEK and several reports have appeared in the literature which apply heterogeneous catalysts to this task (refs. 2-4). A number of reports have specified the importance of basic or weakly acidic sites on the catalyst surface for a selectively catalysed reaction and high selectivities to DA at moderate conversions of MEK have been reported for catalysts based on C03O4 as a pure oxide and with basic oxides added conversely scission reactions have been associated with acidic oxide additives (refs. 2-4). Other approaches to this problem have included the application of vanadium phosphorus oxide (VPO) catalysts. Ai (ref. 5) has shown that these catalysts also catalyse the selective oxidation of MEK to DA. Indeed this catalyst system, used commercially for the selective oxidation of n-butane to maleic anhydride (ref.6), possesses many of the desired functionalities for DA formation from MEK, namely the ability to selectively activate methylene C-H bonds without excessive C-C bond scission. 

Vanadium phosphorus oxides (VPO) are good catalysts for selective O- and N-insertion reactions of aliphatics and methylaromatics, in particular for the oxidation of -butane to maleic anhydride (MA) and for the ammoxidation of methylaromatics and heteroaromatics to the corresponding benzonitriles (47,48). VOHPO4 O.5H2O is a well-defined crystalline compound used as a precursor of... 

While the techniques described above are widely applied in the study of metal oxide catalysts, they are not necessary suitable for systems that contain paramagnetic nuclei. Paramagnetic nuclei, as discussed in Section 5.3.1.2, are not observable by conventional NM R techniques and furthermore can increase the linewidths, sometimes to beyond observable limits, of neighboring nuclei. This poses a problem when studying catalytic metal oxides that contain paramagnetic centers. Vanadium phosphorus oxides (VPO), for instance, contain paramagnetic V. ... 

The abundance and low cost of light alkanes have generated in recent years considerable interest in their oxidative catalytic conversion to olefins, oxygenates and nitriles in the petroleum and petrochemical industries [1-4]. Rough estimates place the annual worth of products that have undergone a catalytic oxidation step at 20-40 billion worldwide [4]. Among these, the 14-electron selective oxidation of -butane to maleic anhydride (2,5-furandione) on vanadium-phosphorus-oxide (VPO) catalysts is one of the most fascinating and unique catalytic processes [4,5] ... 

Dupont has recently commercialized the reaction of n-butane to maleic anhydride using a vanadium phosphorus oxide (VPO) catalyst in a recirculating solid reactor system. The overall reaction is ... 

The greatest use of MAN is in the formation of unsaturated polyester resins. The polymeric resin is formed from the reaction between MAN, ethylene glycol, and a vinyl monomer. The production of MAN is carried out in most of the existing processes by a selective oxidation of n-butane over a vanadium-phosphorus oxide (VPO) catalyst. The main selective and nonselective reactions that occur are... 

In our previous studies of butane oxidation over silica-supported vanadium-phosphorus oxide (VPO) catalysts, it was observed that the selectivity for maleic anhydride increased with the PA ratio, from about 25% for catalysts with a PA ratio of unity to about 50% for a PA7 ratio of about two [2,3]. In order to more fully understand the properties of VPO catalysts with P/V ratios near 2, we have prepared and characterized silica-supported and unsupported catalysts derived from V0(H2P04)2, the precursor to VOfPOjjj, and studied the selective oxidation of a series of alkanes ranging from CjHg to CjHij over the bulk catalyst. 

DuPont s new tetrahydrofuran plant, which is slated to start up in the second quarter of 1996 in Asturias, Spain, is based on a new process that is claimed to be environmentally friendly [133], The process is harnessed with a circulating fluidized-bed (CFB) reactor for the partial oxidation of n-butane. The catalyst used for this stage is vanadium phosphorus oxide (VPO). To enhance the catalytic life in the CFB reactor, attrition-resistant catalyst particles have been developed. A highly selective catalyst for hydrogenation of maleic acid to THF also has been developed. This THF conversion reaction is carried out in a bubble column reactor under relatively mild conditions. This process has several significances ... 

The modified process is based on the oxidation of butane (see Fig. 9.32). A major advantage of C4 over benzene is that no carbon is lost in the reaction. The yield from butane process is 30% greater than that from the benzene process. Also, C4 is much cheaper than benzene. Benzene is a known carcinogen. So, the fixed bed process with n-butane as the raw material has been the only MA route used commercially since 1985 in the United States. In the fixed bed process, a low concentration of butane is passed over the catalyst at 400 80°C and 0.3-0.4MPa. The process generates more water than the benzene process. Unsupported vanadium phosphorus oxide (VPO) catalysts with promoters such as lithium, zinc, and molybdenum are commonly used. 

It can be argued that some mixed metal oxides can also be technically considered as supported metal oxide catalysts because the surface is discernibly different from the underlying mixed metal oxide in terms of composition and molecular structure. For example, the vanadium phosphorus oxide (VPO) catalyst is used in the commercial production of maleic anhydride from butane [12]. The most active crystal phase is the vanadium pyrophosphate (VO)2P207, and the surface structure proposed to be the active phase is a nanometer-thick amorphous VPO layer enriched in phosphorus [12,15]. As another example, Wachs and coworkers [16]... 

Among various vanadia-based catalysts, the vanadium phosphorus oxides (VPOs) have been proved to be excellent catalysts for selective O- and N-insertion reactions of aliphatic and methylaromatics, in particular for the oxidation of -butane to maleic anhydride and the ammoxidation of methylaromatics and heteroaromatics to their corresponding aldehydes and nitriles [12,44,59-65], Various VPO precursors of different structure and vanadium valence state were studied in recent years with an aim to elucidate their reaction behavior and to improve their catalytic performance in the earlier mentioned processes. Regarding the nature of active phase in these catalysts and the way its structure influences the catalytic activity and selectivity of these catalysts, comprehensive investigations were made by direct observation of the catalysts under reaction conditions with the help of various spectroscopic in situ methods. More recently, a comprehensive picture of structure-reactivity relationship for the industrially important ammoxidation of toluene to benzonitrile was demonstrated experimentally for VPO catalysts [5] ... 

The selective oxidation of ra-butane to give maleic anhydride (MA) catalyzed by vanadium phosphorus oxides is an important commercial process (99). MA is subsequently used in catalytic processes to make tetrahydrofurans and agricultural chemicals. The active phase in the selective butane oxidation catalyst is identified as vanadyl pyrophosphate, (V0)2P207, referred to as VPO. The three-dimensional structure of orthorhombic VPO, consisting of vanadyl octahedra and phosphate tetrahedra, is shown in Fig. 17, with a= 1.6594 nm, b = 0.776 nm, and c = 0.958 nm (100), with (010) as the active plane (99). Conventional crystallographic notations of round brackets (), and triangular point brackets (), are used to denote a crystal plane and crystallographic directions in the VPO structure, respectively. The latter refers to symmetrically equivalent directions present in a crystal. 

The above procedures for catalyst preparation have generally provided excellent results. Especially important are surface-sensitive reactions. With supported catalysts in which the active components have a narrow particle-size distnbution, the optimum particle size for a demanding reaction can be established. Major improvements of supported catalysts, e.g. with respect to carbon deposition and ammonia decomposition, can be achieved by preparing catalysts with a narrow par-ticle-size distribution. Also, the preparation of catalysts in which the active components have a uniform chemical composition is highly important One instance is the preparation of supported vanadium oxide phosphorus oxide (VPO) catalysts for the selective oxidation of w-butane to maleic anhydride, which has been carried out using vanadium(III) deposition onto silica [31]... 

The similar structural and catalytic properties of the SiOj-supported and unsupported samples prepared from the same precursor suggest that the same active surface is formed on both types of samples. The higher conversions obtained with the supported samples could be attributed to higher dispersions of the VPO compounds. The slightly lower maleic anhydride selectivity observed for catalyst A than B or the bulk catalyst could be due to some phosphorus atoms interacting with the silica surface rather than with vanadium atoms, such that the P/V ratio is less than two in the VPO compounds. Addition of phosphorus to catalyst B replenished this lost phosphorus. Previous studies of supported vanadium-phosphorus oxides have shown that some phosphorus atoms can be associated with the silica [2,8]. The catalytic properties of the supported samples as well as the LRS are similar to the SiOj-supported PA =2 VPO samples prepared previously [2,3]. These earlier samples were prepared by adding H3PO4 to PA =1 samples synthesized by various synthesis routes. Thus, for the supported samples, the method of preparation is much less important than the composition. 

Despite widespread commercial use and considerable research efforts, the vanadium-phosphorus oxide multifunctional catalytic system remains poorly understood It is generally accepted that (V0)2P2O7 bulk phase is the most selective one to produce maleic anhydride (MA) from n-butane [1], However, in spite of the large number of studies on the VPO catalysts there are stiU many unclear things and disagreements in scientific hterature. The catalyst surface composition and structure under reaction conditions are difficult to stu and stiU remains unresolved... 

Bruckner, A., Martin, A., Steinfeldt, N., Wolf, G.-U., and Liicke, B. Investigation of vanadium phosphorus oxide catalysts (VPO) during toluene ammoxidation new mechanistic insights by in-situ EPR. J. Chem. Soc., Faraday Trans. 1996, 92, 4257- 263. 

DuPont recently commercialized a new process for the oxidation of butane to maleic anhydride using a CSR. The maleic anhydride is scrubbed from the reaction zone as maleic acid and then hydrogenated to tetrahydrofuran. The advantages are well documented in the references. A key to this process was the development of an attrition-resistant catalyst obtained by spray-drying a solution of micronized vanadium-phosphorus-oxygen (VPO) catalysts in polysilicic acid. In the spray dryer, a porous shell of very hard silica is formed to protect the soft VPO catalyst. 

VPO catalyst, they are assumed to be V2Ok units made up of pairs of distorted edge-sharing V05 square pyramids. The assumption of these active sites, especially for the VPO catalyst, was discussed in detail in Ref. 56, in view of the fact that the most selective VPO catalyst for butane oxidation to maleic anhydride contained a slight excess of phosphorus over the stoichiometric ratio for vanadyl pyrophosphate, the phosphorus was concentrated on the surface (57-61), and the average vanadium valence of the catalyst under reaction conditions was about 4.1 (57, 58). 

Bidk vanadium pentaoxide is quite active but low selective catalyst of hydrocaibons partial oxidation. It was established by XRD that the higher content of the phosphorus additive in it [12] the weaker peaks attributed to V2O) in bulk catalysts and, simultaneously, P-VOPO4 phase reflections appeared. The latter became the major component of VPO catalyst at fp > 0.67. The constituents of the prepared sample were found to be also 6 0)2P207, VO(POj)2 and some amorphous compounds. At this takes place, all the cations were considered by authors [12] to be bonded in vanadyl groiqis V=0 and phosphorus atoms form Bronsted acidic center each. It has been found an increased concentration of phosphorus over the surface as compared to the biilk and the higher phosphorus content in the sample the grown bulk concentration of the reduced vanadium ions were observed. 

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