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Applied Catalysis A, General (v.376, #1-2)

Contents (pp. v-viii).

Editorial Board (pp. co2).

International VPO Workshop: Preface by Gregory S. Patience Guest Editor; Elisabeth Bordes-Richard (pp. 1-3).

This special issue of Applied Catalysis highlights a research collaboration devoted to vanadium phosphorus oxide (VPO) catalysts from a very broad perspective including catalyst synthesis and characterization, reactor engineering and new applications beyond n-butane partial oxidation to maleic anhydride (MA). The collaboration has its origin in a donation from E.I. du Pont de Nemours & Co. of 3000kg of VPO: 1000kg precursor, 1000kg calcined (activated) catalyst and 1000kg of “equilibrated” catalyst that had been run in a Circulating Fluidized Bed reactor. Catalyst samples have been distributed to over 20 universities and research centers throughout the world for experiments.

Butane oxidation process development in a circulating fluidized bed by Gregory S. Patience; Richard E. Bockrath (pp. 4-12).

DuPont designed and operated a circulating fluidized bed reactor to produce maleic anhydride from n-butane using a vanadium pyrophosphate catalyst encapsulated in a silica shell. Challenges overcome during the commercialization phase included: attrition resistance, heat transfer, and oxygen addition to a vessel with a high butane concentration. Maintaining the catalyst oxidized was critical to catalyst activity and maleic anhydride selectivity.DuPont designed and operated a circulating fluidized bed reactor (CFB) to produce maleic anhydride from n-butane using a vanadium pyrophosphate catalyst (VPP) encapsulated in a silica shell. A fraction of the pyrophosphate was oxidized to the V⁵⁺ state from the V⁴⁺ state in an air fed fluidized bed regenerator. The oxidized VPP was shuttled to a transport bed reactor with a high concentration of butane and oxygen. The gas carried the catalyst up through the bed at velocities of 0.8m/s and, in the commercial plant, solids circulation rates exceeding 7kt/h. Early development work was conducted on an experimental scale facility containing 1kg of catalyst. The pilot plant catalyst inventory exceeded 2000kg and there was 175t in the commercial reactor. Throughout the program, significant advances in catalyst manufacture and process design were achieved. The CFB reactor configuration is being considered for several unrelated processes including chemical looping combustion, methanol-to-olefins and hot gas desulphurization. Improvements in spray drying technology reduced attrition losses by an order of magnitude versus expectation based on the pilot plant. Together with the low attrition losses and good stability, catalyst consumption was reduced by successfully re-spray drying used/attrited catalyst. By modifying the solids entrance and exit configurations, we were able to double initial plant capacity. Operability of the plant was excellent with a turn-down ratio of 5 demonstrated. At a production rate of 65,000t/year of maleic acid – one of the largest single train reactors for a partial oxidation of an alkane – the maximum temperature difference within the bed was less than 20°C. Heat transfer had been a major design consideration but even at this rate, only 1/3 of the total coil surface for cooling was activated.

Keywords: Butane oxidation; Vanadyl pyrophosphate; Circulating fluidized bed; Lattice oxygen contribution; Attrition resistance; Maleic anhydride

Ammoxidation of 3-picoline to nicotinonitrile over VPO catalysts by A. Martin; C. Janke; V.N. Kalevaru (pp. 13-18).

Industrial VPO samples (precursor, fresh and equilibrated) were characterized and used as catalysts for the ammoxidation of 3-picoline to nicotinonitrile. Test runs showed quite good performance for fresh sample; the precursor and equilibrated solid revealed poor performance only mainly due to decrease in BET surface area. Interestingly, solid characterizations showed phase alteration towards NH₄-ion containing VPO phase in case of catalyst precursor compound.Industrial vanadium phosphate (VPO) samples (vanadyl phosphate hemihydrate precursor compound, freshly calcined and equilibrated vanadyl pyrophosphate solids) shaped with polysilicic acid (PSA) were characterized and used as catalysts for the ammoxidation of 3-picoline to nicotinonitrile. Catalyst characterization with N₂-adsorption, XRD, IR spectroscopy, UV–Vis-DRS and XPS provide information on BET surface area and pore size, crystalline properties and vanadium oxidation state data in bulk and near-to-surface region. BET surface area is dramatically increased by calcination of the precursor but catalytic operation drops the surface area again, moreover, deactivation by carbon deposits was observed. The deactivation is also demonstrated by decrease in V⁵⁺ portion in the equilibrated catalyst. Ammoxidation test runs showed quite good performance for the calcined vanadyl pyrophosphate sample; the used and precursor sample revealed poor performance only. Interestingly, characterization of samples used in ammoxidation showed phase alteration in particular for precursor sample proving phase change towards NH₄-ion containing VPO phase.

Keywords: Ammoxidation; 3-Picoline; Nicotinonitrile; VPO catalysts; Vanadium oxidation state

Exfoliated VOPO₄·2H₂O dispersed on alumina as a novel catalyst for the selective oxidation of cyclohexane by Parijat Borah; Arunabha Datta (pp. 19-24).

The VOPO₄·2H₂O phase exfoliated in 2-propanol and subsequently dispersed on alumina has been found to be a very efficient catalyst for the oxidation of cyclohexane preferentially to cyclohexanol with a cyclohexanol/cyclohexanone ratio of 71 in acetonitrile medium at 333K, using H₂O₂ as the oxidant.The VOPO₄·2H₂O phase has been exfoliated in 2-propanol and the exfoliated solution dispersed on alumina in the ratio 1:1 (VPO-ED). A sample of exfoliated VOPO₄·2H₂O was also obtained by recovering the solid from the exfoliated solution of VOPO₄·2H₂O (VPO-E). Both these samples were characterized by XRD, FTIR, TGA/DTA, and SEM studies and it was shown that VPO-ED and VPO-E have the original VOPO₄·2H₂O structure although the morphology, crystallinity and crystal size and surface areas of the samples are affected both on exfoliation and subsequent dispersion on alumina. These phases have been shown to be active catalysts for the oxidation of cyclohexane preferentially to cyclohexanol. The conversion of cyclohexane increased markedly in the case of the sample dispersed on alumina. Thus, using VPO-ED as the catalyst, a cyclohexanol/cyclohexanone ratio of 71 can be obtained even at a high cyclohexane conversion of 83.3%. The present work therefore not only represents a novel approach for preparing dispersed VPO phases but also demonstrates the efficiency of the VOPO₄·2H₂O phase in catalyzing the oxidation of cyclohexane selectively to cyclohexanol.

Keywords: VOPO; ₄; ·2H; ₂; O; Exfoliation; Dispersion on alumina; Cyclohexane oxidation; Selectivity for cyclohexanol

Catalytic performance of vanadium pyrophosphate oxides (VPO) in the oxidative dehydration of glycerol by Feng Wang; Jean-Luc Dubois; Wataru Ueda (pp. 25-32).

Vanadium pyrophosphate oxides prepared by treating VOHPO₄·0.5H₂O at high temperature were investigated in glycerol dehydration with adding oxygen. The sample treated at 800°C exhibits the best result with 100% glycerol conversion and more than 95% selectivity for useful products, including acrolein, acetaldehyde, acidic acid and hydroxyacetone. The addition of oxygen avoids coke formation and reduces side reactions.A series of vanadium pyrophosphate oxide catalysts were prepared by thermally treating catalyst precursor VOHPO₄·0.5H₂O in the temperature range of 500–900°C in nitrogen and their catalytic activities were evaluated in glycerol dehydration with molecular oxygen. The catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, thermogravimetric–differential thermal analysis and temperature programmed desorption of ammonia. The dehydration activity depends on treatment temperature on the catalyst precursor; the catalyst treated at 800°C exhibits the best result with 100% glycerol conversion and more than 95% selectivity for useful products, including acrolein (64%), acetaldehyde, acetic acid and hydroxyacetone. We observed that the addition of molecular oxygen into reaction could greatly reduce the selectivity for by-products from 20% to 4% and can avoid coke formation. The dehydration activities of the catalysts were correlated with the characteristics of the catalysts.

Keywords: Dehydration; Glycerol; Acrolein; Vanadium pyrophosphate oxide; Molecular oxygen

Ordering of point defects in VPO precursors: Effect on doping and catalyst performance by Andreas Delimitis (pp. 33-39).

The systematic existence of kinematically forbidden reflections in experimental diffraction patterns of hemihydrate VPO precursors is attributed to ordered vanadium vacancies. Such modified precursor structures are shown to effectively facilitate promoter incorporation into the bulk and surface of the crystal and significantly enhance the activity and selectivity of VPO catalysts.The kind and percentage of point defects in commercial as well as laboratory prepared hemihydrate VPO precursors have been analysed using a combination of electron diffraction experiments and diffraction pattern simulation. Experimental patterns reveal the systematic existence of kinematically forbidden reflections that are attributed to vacant lattice sites in the VOHPO₄·1/2H₂O matrix. The vacancies are associated with vanadium ions and are distributed into the bulk and surface of hemihydrate in an ordered fashion. The amount of vacancies is in similar proportion between the two precursors studied. A considerable enhancement in the forbidden reflections intensity in doped hemihydrate samples has been experimentally observed, which is attributed to the partial substitution of dopant ions into V vacant sites. The beneficial effect of such a modified hemihydrate structure on the resulting VPO catalyst performance is demonstrated, since the facilitation of promoter incorporation in V-based vacant active sites brings upon an enhanced activity and selectivity for the catalyst.

Keywords: VPO; n-Butane selective oxidation; TEM; Electron diffraction; Simulation; Doping

Direct grafting of alkoxide promoters on vanadium hydrogen phosphate precursors: Improved catalysts for the selective oxidation of n-butane by Kostantinos Kourtakis; Lin Wang; Elizabeth Thompson; Pratibha L. Gai (pp. 40-46).

New and highly reactive vanadyl phosphate (VPO) catalysts for the oxidation of n-butane to maleic anhydride were synthesized by grafting reactive metal alkoxides onto pre-formed vanadyl phosphorus oxide precursor, VO(HPO₄)1/2H₂O. Large reactivity enhancements were also obtained by contacting the alkoxides with vanadyl phosphate catalyst precursors which had been spray dried with polysilicic acid to form attrition resistant microspheres.New vanadyl phosphate (VPO) catalysts for the oxidation of n-butane to maleic anhydride were synthesized by grafting reactive metal alkoxides onto pre-formed vanadyl phosphorus oxide precursor, VO(HPO₄)1/2H₂O, followed by a calcination and activation protocol. A series of precursors grafted with these alkoxides was used to generate promoted catalysts containing molybdenum and mixtures of molybdenum with main group or transition metal promoters. For many catalysts, a large increase (greater than twofold) in oxidation activity for n-butane was observed and the high selectivity to maleic anhydride was not compromised. For these catalysts, the N₂ BET surface areas do not change significantly with respect to unpromoted catalysts. Large reactivity enhancements were also obtained by contacting the alkoxides with vanadyl phosphate catalyst precursors which had been spray dried with polysilicic acid to form attrition resistant microspheres.Promoted VPO catalysts containing molybdenum combined with other promoters such as Sn, Ti and, in catalysts without silica binder, Fe, Mn, and Zn show further performance improvements compared to the single promoter system containing molybdenum. Hence, the relative ordering of molybdenum co-promoters for VPO catalysts without silica binder is Sn>Fe>Zn>Mn∼Bi>Co∼Ni in order of decreasing activity. For catalysts spray dried with silica binder, the relative ordering of co-promoters is Ti>Sn>Cu>Al∼Fe∼Zr. The reactive grafting technique of promoters with VPO is well suited for rapid, high throughput screening methods for the preparation of materials in which a vanadyl phosphate precursor “scaffold” is used to generate the new materials. This technique may be extended to generate a variety of new catalysts.

Keywords: Selective oxidation of; n; -butane; Maleic anhydride; Vanadyl phosphate; Grafted catalysts; High throughput synthesis; Combinatorial chemistry

Structural evolution and catalytic performance of DuPont V-P-O/SiO₂ materials designed for fluidized bed applications by Nicholas F. Dummer; Weihao Weng; Carol Kiely; Albert F. Carley; Jonathan K. Bartley; Christopher J. Kiely; Graham J. Hutchings (pp. 47-55).

An industrial V-P-O catalyst (DuPont), comprising vanadyl pyrophosphate crystallites enclosed by a silica shell, has been investigated for butane oxidation and studied by standard characterization techniques. Additionally, the core–shell particles have been non-destructively examined with a novel X-ray ultramicroscopy (XuM) technique which reveals the internal structure. The microstructural and surface compositional changes occurring during the industrial lifespan of the catalyst in a fluidized bed reactor are described.An industrial V-P-O catalyst (DuPont), comprising vanadyl pyrophosphate crystallites encapsulated by a silica shell, has been investigated. Reaction evaluation for butane oxidation was conducted over (i) the catalyst precursor, (ii) the calcined precursor and (iii) equilibrated material (2 years on-line) in a fixed-bed reactor. Characterization was conducted on these three samples by XRD, SEM, TEM and XPS. The core–shell particles have also been examined with a novel X-ray ultramicroscopy (XuM) technique which reveals the internal structure without having to section the material. The microstructural and surface compositional changes that have occurred over the course of its industrial lifespan in a fluidized bed reactor are correlated with the catalytic activity measurements.

Keywords: V-P-O catalyst; n; -Butane oxidation; Maleic anhydride; XuM; Silica shell

Attrition resistance of a VPO catalyst by A. Thon; J. Werther (pp. 56-65).

The attrition propensity of a VPO catalyst subjected to mechanical stress in a gas cyclone and in a bubbling-fluidized bed with and without submerged gas jet has been investigated. The material specific attrition rate constants were compared with available literature data. The investigated VPO catalyst has a low attrition propensity which is comparable to FCC-catalyst.The attrition propensity of used and fresh vanadium phosphorous oxide (VPO) catalyst subjected to mechanical stress in a gas cyclone and in a bubbling-fluidized bed with and without submerged gas jet has been investigated. Attrition models for the investigated sources of attrition have been adopted from literature. The attrition tests confirmed the relationship between the operating conditions and the steady-state attrition rates under abrasion conditions. The material specific attrition rate constants for the three sources of attrition have been determined. These constants were compared with those measured for other catalyst materials in previous attrition tests. It is found, that the investigated VPO catalyst has a low attrition propensity comparable to FCC-catalyst, when subjected to mechanical stress in gas cyclone and in fluidized bed with submerged gas jet. The attrition rate constant of the VPO catalyst subjected to mechanical stress in the bubbling-fluidized bed seems to be slightly higher than those measured for FCC-catalysts. However, the deviation is in the range of the experimental error. In agreement with previous findings for other catalysts the cyclone attrition rate constant for fresh VPO catalyst is higher than for used catalyst.

Keywords: Abbreviations; FCC; fluidized cracking catalyst; PSD; particle size distribution; SEM; scanning electron microscope; VPO; vanadium phosphorous oxideAttrition; Abrasion; Cyclone; Fluidized bed; VPO catalyst

Transient reactivity of vanadyl pyrophosphate, the catalyst for n-butane oxidation to maleic anhydride, in response to in-situ treatments by Fabrizio Cavani; Davide De Santi; Silvia Luciani; Axel Löfberg; Elisabeth Bordes-Richard; Carlotta Cortelli; Roberto Leanza (pp. 66-75).

The nature of the active layer in vanadyl pyrophosphate catalysts depends on the P/V atomic ratio, temperature and partial pressure of steam. A slight excess of P, with respect to the stoichiometric requirement for vanadyl pyrophosphate, is necessary for the development of the selective compound, δ-VOPO₄, over the entire range of reaction temperatures.Two vanadyl pyrophosphate catalysts of n-butane oxidation to maleic anhydride differing slightly by their P/V atomic ratio (P/V=1.00 or ≈1.05) were first equilibrated for 100h at 400°C to reach the steady-state. Then, their transient reactivity was studied during oxidizing or hydrolyzing in-situ treatments aimed at forcing changes in the characteristics of the active surface. The catalytic properties were monitored after the treatment to check if the latter has caused relevant changes of the active layer features with respect to those of the equilibrated samples. Treatments were carried out at 380°C, a temperature at which the two catalysts exhibit rather different steady-state reactivity behaviour, and at 440°C, where the two catalysts behave similarly. Accordingly, the transient catalytic behaviour of the two catalysts differed at 380°C whereas at 440°C only minor changes were observed. The comparison between transient reactivity tests, Raman spectra recorded in-situ and X-ray photoelectron spectra allowed to gain insights into how the P/V ratio affects the nature of the catalytically active layer and how the latter is modified after in-situ treatments. The major reversible variations of catalytic performance were observed after hydration and dehydration treatments. They are related to the distribution of vanadium phosphates (δ-VOPO₄ or α_I-VOPO₄) and/or [VO_y+(PO₄)_n] or VOPO₄·2H₂O on the catalyst surface. The nature of the formed compound depends on P/V ratio when an intermediate temperature, i.e., 380°C, is used to carry out the treatments.

Keywords: Vanadyl pyrophosphate; Maleic anhydride; n-Butane oxidation; Transient reactivity; In-situ; Raman spectroscopy

Partial oxidation of n-butane to maleic anhydride over VPO in a simulated circulating fluidized bed reactor by José R. Fernández; Aurelio Vega; Fernando V. Díez (pp. 76-82).

The partial oxidation of n-butane to maleic anhydride catalyzed by VPO, was studied in a simulated circulating fluidized reactor (CFB), consisting of a fluidized bed operated alternating oxidizing and reducing feeds. The performance of the simulated CFB is strongly affected by the reduction feed composition and the cycle's duration. When compared with a conventional fluidized bed, the simulated CFB shows slightly worse combination of butane conversion-MA selectivity, although cycling operation provides better MA productivity for a wide range of operating conditions, and CFB presents more possibilities for optimization.The reaction of partial oxidation of n-butane to maleic anhydride, catalyzed by VPO, was experimentally studied in a fluidized bed reactor, operated alternating oxidizing and reducing feeds, simulating in this way a circulating fluidized reactor. The influence of feed composition and oxidation/reduction time on butane conversion, maleic anhydride selectivity and maleic anhydride production per unit catalyst weight and time was studied. Results for the simulated circulating fluidized bed were compared with that of a conventional fluidized bed.

Keywords: Maleic anhydride; VPO catalyst; Fluidized bed reactor; Circulating fluidized bed

Effect of feed nozzle configuration on n-butane to maleic anhydride yield: From lab scale to commercial by Ali Shekari; Gregory S. Patience; Richard E. Bockrath (pp. 83-90).

We have studied the effect of several feed sparger configurations on the reactor performance in a lab scale fluidized bed as well as in the DuPont commercial CFB reactor for the production of maleic anhydride. In the commercial reactor, maleic acid production rate increased by about 15% when oxygen was switched to a lower sparger in the reactor. Sparger studies in the lab scale fluidized bed highlighted the importance of supplying sufficient oxygen to the reactor to improve the reaction yield by maintaining the catalyst in an oxidized state.In the process to produce maleic anhydride via the partial oxidation of n-butane, selectivity is sensitive to feed gas configuration of both oxygen and n-butane. Based on laboratory scale fluidized bed experiments, selectivity was superior when the n-butane was co-fed together with oxygen. When the oxygen and n-butane were fed separately through a distributor and a sparger, selectivity was highest when the sparger was closest to the distributor (independent of whether the n-butane was fed through the sparger and the oxygen through the distributor or vice versa). Various feed gas configurations were tested in a 4.2m diameter commercial circulating fluidized bed reactor equipped with 926 spargers at three different levels. Maleic acid production rate increased by about 15% when oxygen was fed to a lower sparger 0.45m above the distributor compared to when it was fed at a height of 1.9m. These observations indicate that maintaining the catalyst in an atmosphere containing oxygen is important for overall n-butane conversion and maleic anhydride selectivity.

Keywords: Maleic anhydride; n-Butane partial oxidation; Vanadyl pyrophosphate; Fluidized bed; Circulating fluidized bed; Oxygen sparger

Parametric study of n-butane oxidation in a circulating fluidized bed reactor by Keith W. Hutchenson; Concetta La Marca; Gregory S. Patience; Jean-Philippe Laviolette; Richard E. Bockrath (pp. 91-103).

DuPont's n-butane oxidation process uses VPP catalyst and CFB technology to produce maleic anhydride. Reactor operating conditions fall within the butane flammability envelope, and localized combustion reactions were experienced during operation. Experimental and theoretical investigations of local reaction behavior demonstrated pronounced pressure, temperature, and residence time effects on the onset of combustion and led to operating strategies to improve reactor productivity.DuPont's n-butane oxidation process uses a proprietary attrition-resistant vanadyl pyrophosphate (VPP) catalyst and circulating fluidized bed (CFB) technology for the selective oxidation of n-butane to maleic anhydride. In the redox mode, n-butane feed plus recycle gas are fed to the riser side of the CFB, and air is fed to the regenerator. To optimize both activity and selectivity, oxygen may be fed to the riser side together with n-butane. These conditions fall within the n-butane flammability envelope, and localized combustion reactions were experienced during operation of a pilot plant utilizing this reactor technology. Complementary experimental and theoretical investigations were conducted using a laboratory fluidized bed reactor and an elementary step kinetic model to study the local reaction behavior near an oxygen sparger placed in a fluidized bed of solid VPP catalyst and to evaluate the effect of operating conditions on the onset of combustion in the homogeneous gas phase regions of the reactor. The laboratory fluidized bed reactor was operated at temperatures of 320–390°C, pressures of 3–4bar, freeboard residence times of 4–8s, and n-butane and oxygen compositions of 4–10 and 1–6%, respectively. For the low solids regions of the reactor, both experimental and modeling results demonstrated pronounced pressure and temperature effects on the onset of combustion in this range of operating conditions and that decreasing the residence time delays the onset of complete combustion to longer time or higher relative molecular oxygen compositions. Kinetic model predictions also showed a significant influence of reactant composition and reactor pressure on oxidation induction time.

Keywords: Autoignition; n; -Butane oxidation; Circulating fluidized bed; Combustion; Induction time; Maleic anhydride; Oxidation; Vanadyl pyrophosphate

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Applied Catalysis A, General (v.376, #1-2)