S. Khaja Masthan

Ammoxidation of 3-picoline over V2O5/TiO2 (anatase) system. II. Characterisation of the catalysts by DTA, SEM, FTIR, ESR and oxygen and ammonia chemisorption

Abstract In an earlier communication the ammoxidation activity of V 2 O 5 /TiO 2 catalysts with V 2 O 5 loadings in the range 0.4–9.9 mol% was correlated to the average oxidation number of vanadium in the catalysts. In the present work, these catalysts were characterised by SEM, FTIR, ESR, DTA techniques and chemisorption of NH 3 and O 2 . The scanning electron micrographs of the catalysts indicate that deposition of vanadium is taking place inside the mesopores of titania (anatase) up to 3.4 mol% V 2 O 5 corresponding to a monolayer coverage. Beyond this loading neddle-like and bulk structures of vanadia appear probably on the external surface of the catalysts. The bands at 1010–1020 cm −1 appearing in the FTIR spectra of fresh catalysts are characteristic of highly dispersed monomeric VO x units and two-dimensional structures. The FTIR spectra of the used catalysts are altogether different from those of the fresh catalysts suggesting that the active phase has been drastically modified during the course of the reaction. The ESR spectrum of 0.4 mol% V 2 O 5 shows an eightfold well resolved hyperfine structure indicating that V 4+ is in diluted conditions on anatase surface. As V 2 O 5 content increases the hyperfine structure of ESR spectrum gets progressively smeared out due to strong coupling between V 4+ dipoles. The results indicate that vanadium is in a highly dispersed distorted octahedral or square pyramidal geometry at 3.4 mol% corresponding to a monolayer coverage. The DTA curves contain endothermic peaks at 100–150°C and 630–675°C corresponding to desorption of adsorbed water and melting of vanadia particles and loss of oxygen from vanadia. Chemisorption of NH 3 and O 2 is observed to exhibit maximum at the monolayer V 2 O 5 loading just as the ammoxidation activity of the catalysts.

Ammoxidation of 3-picoline over V2O5/TiO2 (anatase) system : I. Relationship between ammoxidation activity and oxidation state of vanadium

Abstract A series of titania (anatase)-supported vanadia catalysts ranging in V 2 O 5 content from 0.4 to 9.9 mol% was prepared by wet impregnation technique, characterized by BET surface area measurement and X-ray diffraction, and evaluated for ammoxidation of 3-picoline. The average oxidation number of vanadium in the fresh and used catalysts was determined by titrimetric methods. The ammoxidation activity and the average oxidation number were observed to increase with vanadia loading up to 3.4 mol% in the catalyst which corresponds to a monolayer coverage. The phase transformation of anatase to rutile after the reaction was observed at a V 2 O 5 loading of 5.9 mol%. The slow decrease of ammoxidation activity beyond 3.4 mol% V 2 O 5 was attributed to the coverage of active monomeric VO x species on the support by bulk vanadia and by other oxides, and also to compound formation with ammonia.

Influence of V2O5 content on ammoxidation of 3-picoline over V2O5/AlF3 catalysts

V2O5/AlF3 catalysts with V2O5 loadings ranging from 2 to 15 wt% were prepared by the conventional wet impregnation method, using nonporous AlF3·3H2O sample as the support for impregnating NH4VO3. It was found that the catalysts evolve porous structures upon calcination at 723 K. The influence of V2O5 content was studied on ammoxidation of 3-picoline on the reduced catalysts. The catalyst with 15 wt% V2O5 exhibited the highest selective ammoxidation acitivity towards nicotinonitrile. The XRD and oxygen chemisorption studies revealed that vanadia is in a highly dispersed state in the catalysts.

Reactivity of V2O5/MgF2 Catalysts for the Selective Ammoxidation of 3-Picoline

V2O5/MgF2 catalysts with V2O5 contents ranging from 2.1 to 15.7 wt% were prepared, and the influence of the V2O5 content of the V2O5/MgF2 catalyst on the structure and activity for the ammoxidation of 3-picoline was investigated. XRD data indicate that V2O5 is in a highly dispersed state though segregation of V2O5 into tiny crystallites occurs at and above 8 wt% V2O5. The 3-picoline ammoxidation activity increased with an increase in V2O5 content due not only to the species arising out of interaction of V2O5 and MgF2, but also to the presence of V2O5 microcrystals in the catalysts.

Highly active titania supported ceria catalysts for ammoxidation of picolines

TiO2 supported ceria catalysts with 20 wt% CeO2 were prepared by impregnating rutile and anatase polymorphs in aqueous solution of ammonium ceric nitrate [(NH4)2Ce(NO3)6], characterized by BET surface area measurement and low temperature oxygen chemisorption (LTOC) at −78°C and evaluated for ammoxidation of β- and γ-picolines. Anatase supported catalyst has exhibited higher ammoxidation activities than rutile supported one and pure ceria. Turnover frequencies calculated from ammoxidation rates and irreversible O2 uptakes in respect to a particular picoline isomer were almost equal for pure supports on one hand and for supported ceria catalysts on the other. The two structural isomers, β- and γ-picolines seem to have exhibited marked steric effect in their reactivity in catalysis on the catalysts.

Hysteresis during ammonia synthesis over promoted ruthenium catalysts supported on carbon-covered alumina☆

Dissociative chemisorption of nitrogen is the rate-determining step in the synthesis of ammonia. The nitrogen molecule donates a pair of electrons to the empty d-orbitals of the metal catalyst, which in turn donates the delectrons to the antibonding orbit& of the nitrogen [ 1, 21. The strength of adsorption and the extent of dissociation of nitrogen depend presumably on the extent of back-donation of the electrons from metal to nitrogen. Aika et al. [3-S] and various other investigators [g-12] have carried out extensive work on Ru catalysts using various supports and promoters in order to obtain better ammonia synthesis activity at atmospheric pressure. Very recently, it has been found that the addition of alkali ionic promoters, Cs and K, and some alkaline earth metals such as Ba [8], to the Ru supported on active carbon form effective catalysts. There has been renewed interest in identifying novel promoters and supports which increase the electron density on the metal. As a part of this process, the authors have recently studied the synthesis reaction on Cs-Ru and Cs-Ru-Ba catalysts supported on carbon-covered alumina and have reported the influence of the support and the Ba addition to it on the steady state activity [ 12, 151. In the present study the activity of the catalysts with increasing and decreasing reaction temperature has been determined. During these temperature cycles, a rate hysteresis is observed on these catalysts, just as Richard and Vanderspurt [ 131 observed on their triply-promoted Fe catalyst at 30 and 90 atm. Carbon-covered alumina (CCA) was prepared by the pyrolysis of ethylene (Matheson, USA) in Nz (IOLAR-1, India) at 600 “C over commercial y-A1203 (Harshaw, Al-3996 R, l&/25 BSS mesh). The flow rates of ethylene and nitrogen were 2 ml min’ and 15 ml min’ respectively per gram of alumina. The procedure followed was similar to that of Vissers et al. [ 141. The carbon

Sorption properties of modified silicoaluminophosphate(SAPO)-5 and SAPO-11 molecular sieves

Abstract The pore structures of silicoaluminophosphates (SAPO), SAPO-5, SAPO-11 and their transition metal substituted modifications were studied by physisorption of nitrogen at 77 K. The pore volume distributions in these materials were computed by applying the expanded form of the BJH equation to the N 2 desorption data. The N 2 adsorption–desorption hysteresis loops, the pore volume distributions and other physical properties show that these molecular sieves contain nonuniform pores spreading over a wide range of pore radii upto 300 A with appreciable contributions from micropores (pore radii

Vapour phase ammoxidation of mesitylene to 1,3,5-tricyanobenzene on V-Sn-O catalysts

A series of vanadium-tin mixed oxide catalysts have been prepared by the solid-state reaction of V2O5 and SnO2 at 1250°C. The fresh and the used catalysts have been characterized by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). High temperature oxygen chemisorption (HTOC) has been employed to titrate the coordinatively unsaturated vanadia sites. Ammoxidation of mesitylene has been carried out on these catalysts and an optimum composition of vanadium is arrived at. Better yields of tricyanobenzene (TCB) compared to literature values are obtained. A good correlation between the oxygen uptake and the TCB yield extends the applicability of HTOC to the fused oxide system.

Derivation of the Expanded Form of the BJH Equation and its Application to the Pore Structure Analysis of Mesoporous Adsorbents

The BJH equation, v n = R n ( Δ V ) n − R n Δ t n ∑ i = 1 n − 1 c i A i , used for pore structure analyses via its application to the N2 desorption isotherms at 78 K of porous materials, has been modified by substituting the value of ci and expanding the last term. The quantity vn is the volume of the pores involved in the nth desorption step and is given in terms of the volume of N2, (ΔV)n, exuded from the porous material, the constants Rn and ci which depend on the average pore size and average thickness of the physically adsorbed multilayer, Δtn being the decrease in thickness of the multilayer as a result of the nth desorption step and Ai the surface area of the set of pores involved in the ith desorption step. A derivation of the modified equation is presented. It has been applied to the N2 desorption data of bone char used by Barrett, Joyner and Halenda (BJH). The effect of the multilayer thickness (t) values given by Pierce and de Boer on the pore size distribution of bone char has been studied. The pore size distribution data are compared with those obtained by the original BJH and Pierce methods. The pore system conforms very well with the idealised open-ended cylindrical pore model. The expanded BJH equation has also been applied to the desorption isotherms of fresh and sintered silica-alumina cracking catalysts over the entire range of silica/alumina ratios. The multilayer thickness (t) values given by de Boer have been used in the calculations. The pore systems of the catalysts, vacuum-dried at 400°C, are observed to conform to the cylindrical pore model as revealed by the excellent agreement between the cumulative surface areas obtained in P.S.D. calculations and the BET surface areas. The pore systems of the catalysts, sintered at different temperatures in a dry oxygen stream, deviate considerably from the cylindrical pore model. Enlargement of the pores and their nonconformity with the ideal pore model are attributed to modifications of the pore structures brought about by the phenomenon known as ‘self-steaming’ with the water vapour released from the catalysts.

The influence of metal loading on the structure-sensitive chemisorption of nitrogen on Ru/γ-Al2O3 catalysts

Nitrogen chemisorption has been studied on five Ru/γ-Al2O3 catalysts ranging in Ru loading from 0.5–5.0 wt.% at different temperatures from 25°C to 300°C and at different pressures up to 1 atm in order to obtain an insight into the influence of the morphological structure of ruthenium on the amount and nature of nitrogen chemisorption. The chemisorption of N2 at ambient temperature increased steadily with pressure on all the catalysts studied so that the resulting isotherms were straight lines. When the adsorption temperature was raised, N2 chemisorption decreased and the corresponding isotherms developed three adsorption steps. These three adsorption steps are attributed to structure-sensitive nitrogen chemisorption on the three low Miller index planes of ruthenium, i.e. (101), (100) and (001). When the nitrogen uptakes at 740 Torr as taken from the adsorption isotherms were plotted against the Ru loadings, a maximum was obtained at an Ru loading of 2.5 wt.% which corresponds to an Ru crystallite size of 4 nm, suggesting that N2 chemisorption is a highly structure-sensitive phenomenon on these catalysts.