Active form of 12-vanadomolybdophosphoric acids in n-butane selective oxidation

Abstract

The catalytic behaviour of H5PV, Mo, , 0, , heteropoly acid in the oxidation of n-butane to maleic anhydride is analysed in relation to that of its partially salified Cs salt and their FTlR spectra, UV-VIS diffuse reflectance spectra, thermogravimetry and reducibility tests as well as solid state 51V NMR and EPR characterizations are also compared. At temperatures around 573 K the acid sample dehydrates with partial reduction and the formation of lacunar 0x0-anions but with preservation of the Keggin unit structure. The creation of inter-Keggin V-0-V bonds during dehydration is suggested. The presence of the alkali metal in the secondary structure inhibits these effects. The superior catalytic performance of the acid sample in butane transformation is discussed in terms of the role of the presence of V5+ sites associated with a n inter-Keggin interaction.

The structure of molybdenum-heteropoly acids under conditions of gas-phase selective oxidation catalysis: a multi-method in situ study

“…All spectroscopic measurements were carried out with Spectralon ® (Labsphere) as reference material. HPA samples (7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19) …”

“…An essential advance in the studies of optical spectra of catalysts of different stages of their transformation was achieved by applying the in situ diffuse reflectance UV/Vis/near-IR spectroscopy (DRS) [10,11], which proved to be a suitable technique for probing both d-d and charge transfer transitions at reaction temperature and under realistic gas compositions [12]. A number of groups have reported exsitu optical spectra [13,14,15,16,17]. It is known from these studies that the DRS method detects even small changes in spectral features connected with water loss or chemical reduction.…”

“…Mixed phases of partially salified HPA have been proposed forming a core-shell system of the Cs 3 A-salt and the free acid under catalytic conditions [25]. Furthermore, it has been suggested that the molybdenum cations [28] as well as the vanadium cations [29,30] migrate from the Keggin anion to cationic positions in the structure [31,32] and that the heteropoly acids become partially reduced under reaction conditions [33,34]. This migration of cations from the Keggin anion has been speculated to be responsible for the catalytic activity of the heteropoly oxomolybdates with Keggin structure [25,29,30,33,35,36,37].…”

UV/Vis/near-IR spectroscopic characteristics of H4–xCsxPVMo11O40 (x = 0, 2) catalyst under different temperatures and gas atmospheres

“…Besides the oxygen ligand to metal charge transfer band (LMCT) there are absorption bands which can develop or disappear due to the optical d-d transitions in V 4+ and Mo 5+ and intervalence V 4+ -Mo 6+ and/or Mo 5+ -Mo 6+ optical transitions. Up to now, this spectroscopic method has been applied to the investigation of heteropoly compounds by some work groups, but all spectra were recorded ex situ [17][18][19][20][21]. The heteropoly compounds alter the spectroscopic behaviour due to the loss of water [22,23].…”

Chemistry, Spectroscopy and the Role of Supported Vanadium Oxides in Heterogeneous Catalysis

“…A related technique is chemical vapor deposition (CVD), which makes use of a volatile inorganic or organo-metallic compound that is deposited at the surface of a support oxide by reaction with the support hydroxyl groups. Interesting vanadium precursor compounds for this preparation method are VCl 4 and VOCl 3 , and their interaction with SiO 2 , TiO 2 and Al 2 O 3 has been studied in detail [154][155][156][157]. A similar synthesis method is atomic layer deposition (ALD) and series of V/SiO 2 and V/Al 2 O 3 catalysts were prepared according to this gas phase preparation technique [158].…”