Structural and Catalytic Activity of V 2 O 5 -Supported on AlPO 4 Catalysts (original) (raw)

A Facile Synthesis of Vanadium Phosphate: An Efficient Catalyst for Solvent Free Esterification of Acetic Acid

Catalysis Letters, 2010

Abstract This paper reports the facile synthesis of vanadium phosphate (VPO) by the decomposition of VOHPO4·0.5H2O which was prepared by reduction of dihydrate VOPO4·2H2O with isobutanol. The material was promoted with aluminum by impregnation method. The catalysts were unambiguously characterized by N2 adsorption–desorption, XRD, FT-IR techniques, UV–Vis DRS and the total amount of the acidity of the catalysts was estimated by NH3-TPD. The catalytic activities were checked in the heterogeneous catalytic esterification of acetic acid with 1° alcohol (n-butanol) in a solvent free medium. The optimization of reaction was carried out by varying temperature from 75 to 150 °C, molar ratio (butanol:acetic acid) from 1:1 to 1:4. Under optimum conditions, the catalytic esterification runs revealed a significant effect of the VPO giving 62% conversion with 100% selectivity to butyl acetate. Graphical Abstract Vanadium Phosphate is prepared in aqueous medium by taking vanadium pentoxide, o-phosphoric acid and isobutanol. The catalyst is found to be robust enough to achieve high degree of esterification under solvent free condition.

Microcalorimetric and infrared studies of the acid-base properties of V2O5/?-Al2O3 catalysts

Appl Catal a Gen, 1994

The acid-base properties of V,OJ-y-A&O, catalysts were characterized by ammonia, pyridine and sulphur dioxide adsorptions using microcalorimetry and diffuse reflectance Fourier transform Ir spectroscopy (DRIFT). For vanadium content less than 10 wt.-% V,O, (4.9 pmol( V205) m 2), the vanadium cations were found to be well spread as vanadate compounds over alumina. Such vanadate species developed Bransted and Lewis-type acidity as shown by a DRIFT spectroscopy study of pyridine adsorption, but did not exhibit a basic character. Sulphur dioxide adsorption allowed to differentiate a vanadate layer from free alumina. Ammonia and sulphur dioxide adsorptions also showed that at low vanadium coverage, a large part of the vanadate layer was bound to acid-base pairs of alumina. Vanadium pentoxide crystallites were detected at less than complete monolayer coverage with vanadate compounds, but did not contribute to the development of the acidic surface character. At low vanadium coverage, ( < 3 wt.-% V20s, 1.2 pmol( V205) mm2), the acidic character of the V,OJ 'y-Al,O, catalysts is ascribed to vanadium-free alumina, whereas it is largely attributed to vanadate compounds at higher vanadium coverage.

Preparation, characterization, and activity of Al 2 O 3-supported V 2 O 5 catalysts

A series of activated alumina-supported vanadium oxide catalysts with various V 2 O 5 loadings ranging from 5 to 25 wt% have been prepared by wet impregnation technique. A combination of various physicochemical techniques such as BET surface area, oxygen chemisorption, X-ray diffraction (XRD), temperature-programmed reduction (TPR), thermal gravimetric analysis (TGA), and Fourier transform infrared (FTIR) were used to characterize the chemical environment of vanadium on the alumina surface. Oxygen uptakes were measured at 370 • C with prereduction at the same temperature, which appears to yield better numerical values of dispersion and oxygen atom site densities. XRD and FTIR results suggest that vanadium oxide exists in a highly dispersed state below 15 wt% V 2 O 5 loading and in the microcrystalline phase above this loading level. TPR profiles of V 2 O 5 /Al 2 O 3 catalysts exhibit only a single peak at low temperature up to 15 wt% V 2 O 5 . It is suggested that the low-temperature reduction peak is due to the reduction of surface vanadia, which has been ascribed to the tetrahedral coordination geometry of the V ions. TPR of V 2 O 5 /Al 2 O 3 at higher vanadia loadings exhibited three peaks at reduction temperatures, indicating that bulk-like vanadia species are present for these catalysts only at higher vanadia loadings, with V ions in an octahedral coordination. The TPR profiles of V 2 O 5 /Al 2 O 3 catalysts indicate that at loadings lower than 15% vanadia forms isolated surface vanadia species, while two-dimensional structure and V 2 O 5 crystallites become prevalent in highly loaded (>15% V 2 O 5 ) systems. Liquid-phase oxidation of ethylbenzene to acetophenone has been employed as a chemical probe reaction to examine the catalytic activity. Ethylbenzene oxidation results reveal that 15%V 2 O 5 /Al 2 O 3 is more active than higher vanadia loading catalysts.

Microcalorimetric and infrared studies of the acid-base properties of V2O5/γ-Al2O3 catalysts

Applied Catalysis A: General, 1994

The acid-base properties of V,OJ-y-A&O, catalysts were characterized by ammonia, pyridine and sulphur dioxide adsorptions using microcalorimetry and diffuse reflectance Fourier transform Ir spectroscopy (DRIFT). For vanadium content less than 10 wt.-% V,O, (4.9 pmol(V205) m 2), the vanadium cations were found to be well spread as vanadate compounds over alumina. Such vanadate species developed Bransted and Lewis-type acidity as shown by a DRIFT spectroscopy study of pyridine adsorption, but did not exhibit a basic character. Sulphur dioxide adsorption allowed to differentiate a vanadate layer from free alumina. Ammonia and sulphur dioxide adsorptions also showed that at low vanadium coverage, a large part of the vanadate layer was bound to acid-base pairs of alumina. Vanadium pentoxide crystallites were detected at less than complete monolayer coverage with vanadate compounds, but did not contribute to the development of the acidic surface character. At low vanadium coverage, (< 3 wt.-% V20s, 1.2 pmol(V205) mm2), the acidic character of the V,OJ 'y-Al,O, catalysts is ascribed to vanadium-free alumina, whereas it is largely attributed to vanadate compounds at higher vanadium coverage.

Effect on the structure and morphology of vanadium phosphates of the addition of alkanes during the alcohol reduction of VOPO4·2H2O

Journal of Materials Chemistry, 2010

Vanadium phosphate catalysts were prepared by the reduction of VOPO 4 2H2OwithanalcoholandcharacterisedusingacombinationofpowderXRD,BETsurfaceareameasurement,scanningelectronmicroscopyandtransmissionelectronmicroscopy.Theeffectoftheadditionofanalkaneco−solventduringtherefluxstageofthepreparationwasinvestigated.TheadditionofC4−C16n−alkaneswasobservedtoaffectthestructureofthevanadiumphosphateproductssignificantly.WithoutthealkanetheproductisVOHPO42H 2 O with an alcohol and characterised using a combination of powder XRD, BET surface area measurement, scanning electron microscopy and transmission electron microscopy. The effect of the addition of an alkane co-solvent during the reflux stage of the preparation was investigated. The addition of C 4 -C 16 n-alkanes was observed to affect the structure of the vanadium phosphate products significantly. Without the alkane the product is VOHPO 4 2H2OwithanalcoholandcharacterisedusingacombinationofpowderXRD,BETsurfaceareameasurement,scanningelectronmicroscopyandtransmissionelectronmicroscopy.Theeffectoftheadditionofanalkanecosolventduringtherefluxstageofthepreparationwasinvestigated.TheadditionofC4C16nalkaneswasobservedtoaffectthestructureofthevanadiumphosphateproductssignificantly.WithoutthealkanetheproductisVOHPO40.5H 2 O which is the precursor to the industrial catalyst. Addition of the alkane leads to the formation of VO(H 2 PO 4 ) 2 , with its characteristic block-shaped crystallites, and the alkane/alcohol liquid phase solubilises the excess vanadium. The amount of alkane required to induce these changes decreased with increasing carbon number of the n-alkane. The effect of the addition of the alkane co-solvent is thought to effect the rate of reduction of V 5+ to V 4+ which then reacts with phosphoric acid to give either VOHPO 4 0.5H2OorVO(H2PO4)2.IfthereductionstepisfastthenthelocalP:V4+ratioisapproximately1andVOHPO40.5H 2 O or VO(H 2 PO 4 ) 2 . If the reduction step is fast then the local P : V 4+ ratio is approximately 1 and VOHPO 4 0.5H2OorVO(H2PO4)2.IfthereductionstepisfastthenthelocalP:V4+ratioisapproximately1andVOHPO40.5H 2 O is the major product. However, if the reduction step is slow then the local P : V 4+ ratio is much higher and VO(H 2 PO 4 ) 2 is preferentially formed.

Molybdena–vanadia supported on alumina: Effective catalysts for the esterification reaction of acetic acid with n-butanol

Journal of Molecular Catalysis A: Chemical, 2013

The paper describes the preparation of alumina-supported molybdena-vanadia catalysts, their structural and textural characterization using XRD, N 2 adsorption, UV-vis and NH 3-TPD techniques as well as their catalytic properties in the esterification reaction of acetic acid with n-butanol. The effects of esterification conditions including reaction time, catalyst loading and acid-to-alcohol mole ratio and of reactant preadsorption on the conversion were investigated. The catalytic activity correlated well with the number of strong acid sites which increased by increasing the vanadia content. In optimized conditions, conversions higher than 85% with 100% selectivity for n-butyl acetate can be obtained. Reactant pre-adsorption experiments suggested that the reaction follows the Langmuir-Hinshelwood mechanism. A good reusability of the catalysts after three reaction cycles was observed. A local interaction between molybdenum and vanadium on the catalyst surface has been evidenced.

Effect of Chemical Composition on the Structure and Catalytic Behaviour of AlPO4 and Al2O3–AlPO4 Mixed Catalysts

Adsorption Science & Technology, 2002

AlPO4 and Al2O3–AlPO4 mixed catalysts of different composition (Al/P > 1) were prepared and calcined in the temperature range 350–650°C. Such catalysts were characterized by DTA and X-ray diffraction methods, and by nitrogen adsorption studies at −196°C. Their acidity was determined using a calorimetric titration method while their catalytic activity towards the dehydration of isopropanol was determined using a pulse microcatalytic technique. The data obtained from XRD studies showed that pure AlPO4 when calcined at 650°C had a rather low crystallinity with its crystalline structure (which is of the α-cristobalite type) being characterized by poorly developed peaks. However, significant changes in the texture, surface acidity and catalytic activity were observed as a result of changing the chemical composition of the solid, with the surface area, total pore volume and surface acidity generally increasing with increasing alumina content. Sintering commenced above 550°C leading to ...

Effect of varying reflux durations on the physico-chemical and catalytic performance of vanadium phosphate catalysts synthesized via vanadyl hydrogen phosphate sesquihydrate

Applied Catalysis A: General, 2012

A series of vanadyl pyrophosphate, (VO) 2 P 2 O 7 , catalysts prepared via vanadyl hydrogen phosphate sesquihydrate precursors (VOHPO 4 •1.5H 2 O) was calcined in a reaction flow of 0.75% n-butane in air mixture at 733 K for 18 h. The precursors have been synthesized by refluxing vanadyl phosphate dihydrate (VOPO 4 •2H 2 O) with 1-butanol for different lengths of time, i.e. 8, 15 and 24 h, and the produced catalysts were denoted as VPO s-R8, VPO s-R15 and VPO s-R24, respectively. X-ray diffraction (XRD) patterns of the three catalysts showed similar diffraction pattern, comprised of a well-crystallized (VO) 2 P 2 O 7 phase. Brunauer-Emmett-Teller (BET) surface area measurements showed that VPO s-R24 has the highest specific surface area, i.e. 31 m 2 g −1 followed by 27 m 2 g −1 and 19 m 2 g −1 for VPO s-R15 and VPO s-R8, respectively. Inductively coupled plasma (ICP) analyses indicated that the P/V atomic ratios of these catalysts were in the optimum range in producing (VO) 2 P 2 O 7 phase. A small increment in the average oxidation number of the vanadium was observed as the precursor reflux duration increased. Scanning electron microscope showed the secondary structures of the catalysts with plate-like crystals in different sizes, which were agglomerated into rosette-shape clusters. The total amount of oxygen desorbed from the catalysts increased as the precursor reflux duration increased. Temperature-programmed reduction (TPR) in H 2 profiles of all the catalysts gave three reduction peaks. VPO s-R8 gave the highest total amount of oxygen removed from V 5+ /V 4+ phase followed by VPO s-R15 and VPO s-R24. Catalytic tests revealed that the catalyst with lower precursor reflux duration exhibited higher selectivity but lower activity and vice versa.

Oxidation of ethanol to acetaldehyde over Na-promoted vanadium oxide catalysts

Applied Catalysis A: General, 2007

Sodium-promoted vanadium oxide catalysts supported on MCM-41 and TiO 2 (anatase) were investigated for the partial oxidation of ethanol to acetaldehyde. The catalysts were prepared by incipient wetness impregnation with a vanadium oxide content of 6 wt.%. The experimental characterization was performed by X-ray diffraction (XRD), N 2 adsorption, temperature-programmed reduction (TPR), and diffuse reflectance UV-vis. Temperature-programmed oxidation (TPO) was also used to identify carbon deposits on the spent catalysts. The presence of sodium plays a strong role in the dispersion and reducibility of the vanadium species as detected by TPR analysis and optical absorption spectroscopy. While sodium addition increases the dispersion of the VO x species, its presence also decreases their reducibility. Additionally, TPO of the spent catalysts revealed that an increase in the Na loading decreases the carbon deposition during reaction. In the case of the catalysts supported on MCM-41, these modifications were mirrored by a change in the activity and selectivity to acetaldehyde. Additionally, on the VO x /TiO 2 catalysts the catalytic activity decreased with increasing sodium content in the catalyst . A model in which sodium affects dispersion, reducibility and also acidity of the supported-vanadia species is proposed to explain all these observations. #