Search

A precipitation-deposition method in an organic medium was used to prepare vanadium phosphorus oxide (VPO) catalysts supported on SBA-15 and TUD-1. Oxidation of cyclohexane has been examined over mesoporous silica supported VPO catalyst. The supported VPO catalyst has been characterized by XRD, N 2 physisorption, SEM, HRTEM, H 2 -TPR and NH 3 -TPD techniques. The characteristic X-ray diffraction patterns of bulk VPO catalyst and VPO catalysts supported on mesoporous SBA-15 and TUD-1 showed that each of them are having well-crystallized vanadyl pyrophosphate [(VO) 2 P 2 O 7 ] phase. The vanadium redox centers were easily accessible for supported VPO catalyst. The TUD-1 supported VPO catalyst had higher acidity as found in NH 3 -TPD studies. The conversion of cyclohexane increased in the case of VPO dispersed on TUD-1 mesoporous material. From SEM and HRTEM results it is found that the nature of the support and VPO loading had a profound influence on the crystallinity, morphology and chemical property of the dispersed VPO phase. The changes in all the property had a prominent role on its catalytic activity for the oxidation of the cyclohexane. [ABSTRACT FROM AUTHOR]

Abstract: The formation of dispersed VPO phases on a lignocellulosic-based activated carbon results in a catalyst that is selective for the partial oxidation of propane, and stable under oxidizing conditions. The use of a stable activated carbon as a catalytic support for active VOx species during the partial oxidation of propane is described for the first time. [Copyright &y& Elsevier]

Abstract: Cycling catalyst between a net oxidizing and reducing gaseous environment has been practiced commercially to produce maleic anhydride from n-butane over vanadium pyrophosphate. Typically, the oxidation period is less than 1min to minimize catalyst inventory. In this study, the effect of the oxidation period on maleic anhydride productivity was assessed in the range of 0.3–10min. Irrespective of the feed gas composition during the reduction period, the productivity increased linearly with the oxidation soak time in air. A full range of reducing conditions was examined from the pure redox mode (10% n-butane in argon) to highly oxidizing conditions typical of fixed bed operation (1.4% n-butane and 18.1% oxygen). On average, maleic anhydride yield increased by up to 50% when the oxidation time was extended from 0.3 to 10min. The maleic anhydride yield was lowest under redox mode and it was highest when the feed composition was close to equimolar in n-butane (∼6%) and oxygen. Our results show that industrial CFB reactor performance may be improved considerably by efficient regeneration of the catalyst and optimization of the reducing feed gas composition. [ABSTRACT FROM AUTHOR]

Vanadium phosphorus oxide (VPO) catalysts synthesized via hydrothermal method were introduced with some different dopants, Cr, Ni, Fe, and Mn during the preparation of the precursors, VOHPO4 · 0.5H2O. All modified precursors were subsequently transformed under reaction conditions to give the active phase, (VO)2P2O7. Several techniques were used to characterize the physicochemical properties of the catalysts, such as X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), H2-temperature programmed reduction (TPR), laser Raman spectroscopy (LRS), and chemical analysis. The catalytic performance of the catalysts for selective oxidation of n-butane to maleic anhydride has been carried out by using a fixed bed microreactor (673 K, gas hourly space velocity [GHSV] = 2,400 hr-1). The results show that the addition of dopants into the VPO catalysts increased the surface area of the catalysts and induced the formation of V5+ phases as shown in XRD and LRS spectra that promoted the catalytic performance. A further water reflux treatment to all the precursors gave catalysts with only (VO)2P2O7 phase. However, the activity and selectivity of these catalysts were reduced markedly. [ABSTRACT FROM AUTHOR]

Abstract: Vanadium phosphate catalysts were synthesized via VOPO4·2H2O and were calcined in two different hydrocarbon reaction environments, i.e. n-butane/air and propane/air. Both catalysts are denoted VPDB and VPDP, respectively. Both catalysts exhibited a good crystalline with characteristic peaks of pyrophosphate phase. However, the peaks for VPDP are shown to be more prominent than those of VPDB. BET surface area showed that VPDB gave higher surface area (23m2 g−1) compared to VPDP (18m2 g−1). The average V valence state for VPDP is 4.08 and the higher V valence state for VPDB is 4.26 due to higher amount of VV for VPDB. Furthermore 14.2% of VIII was found for VPDP but none for VPDB. SEM micrographs clearly revealed that the morphologies of both catalysts composed of plate-like crystallite that was arranged into the characteristic of rosette cluster. However, the catalyst calcined in n-butane/air environment (VPDB) resulted in an increment of the amount of plate-like crystal formed in the rosette rosebud agglomerates. TPR in H2 profiles of both catalysts gave two reduction peaks corresponding to two kinetically different oxygen species which were associated with VV and VIV phases, respectively. VPDB removed larger amount of active oxygen species linked to VIV phase which eventually caused a higher conversion rate in the selective oxidation of n-butane and propane to maleic anhydride and acrylic acid, respectively. [Copyright &y& Elsevier]

In unsupported V-containing catalysts, V4+ sites are frequently present in amorphous oxidic clusters and/or crystalline paramagnetic bulk phases in which they are coupled by effective spin–spin exchange interactions. This work presents two EPR procedures for evaluating these interactions and relating their strength to the degree of disorder and catalytic performance, namely (1) evaluating permanent structural disorder by calculating exchange energies and integrals and (2) monitoring transient electronic disorder caused by catalyst–reactant interaction in working catalysts using the ratio of the fourth and the square of the second moment of the EPR signal. Selected application examples comprise VPO catalysts such as (i) (VO)2P2O7, in which a certain degree of structural disorder revealed to be essential for high catalytic performance in the selective oxidation of n-butane to maleic anhydride, (ii) (NH4)2(VO)3(P2O7)2 in which transient perturbation of spin–spin exchange sensitively reflects the strength of reactant adsorption during ammoxidation of toluene and (iii) (NH4)2VOP2O7 in which an amorphous VO2+-containing phase could be identified as active component besides inactive crystalline phases. Based on operando-EPR measurements, reasons for activity differences in MoVTeNb oxide catalysts could be suggested. [ABSTRACT FROM AUTHOR]

In situ analysis of the surface of a working selective oxidation catalyst is an essential yet rarely conducted experiment in attempts to derive structure–function relationships. The case study of n-butane oxidation over vanadyl pyrophosphate (VPP) is used to develop a general working hypothesis and to illustrate that the molecular properties of the substrate sets boundary conditions on the surface chemical properties of the catalyst. Experiments using in situ X-ray photoelectron spectroscopy (XPS) and in situ low-energy X-ray absorption spectroscopy are used to derive compositional, electronic, and geometric structural information of the surface of the working VPP. These data allow the conclusion that a surface phase different from VPP must be present covering at least part of the active material. The recent data together with literature observations are used to derive a scenario explaining the function of VPP as a unique catalytic system. [ABSTRACT FROM AUTHOR]