Application of periodic operation to maleic anhydride production

Two-step and multi-step feed composition cycling of the partial oxidation of butadiene to maleic anhydride have been studied. Multi-step cycles were created by N₂ flushes between steps containing just one reactant. Both production and selectivity to maleic anhydride are decreased over a cycle in this type of periodic operation. Selectivity to furan is increased, however. N₂ flushing concentrates production of maleic anhydride in the butadiene step, which may have process advantages.     

Investigation of the catalytic oxidation of benzene to maleic anhydride by wave front analysis with respect to the application of periodic operation

A kinetic analysis of the partial oxidation of benzene to maleic anhydride over a Merck, Sharp and Dohme developed V₂O₅-MoO₃/TiO₂ catalyst shows that partial oxidation proceeds through surface reduction, induction and collapse phases. In each, surface species and reactions are different. The reduction/induction phases correspond to a “ignited” state whereas the collapse phase is similar to an extinguished state familiar in the catalytic oxidation of CO. Selectivity to maleic anhydride is lower in the extinguished state. Periodic operation should offer a means to keep selectivity high provided the cycle period does not exceed just a few minutes. Improvement in selectivity of close to 100% has been attained by periodic switching of the feed composition between mixtures containing oxygen and benzene in ratios of 2.4 and 4.6 with periods between 1/2 and 6 minutes. Benzene conversion, however, is lower.     

Oxidation of butenes to maleic anhydride on MnMoO4 based catalyst

The oxidation of n-butene-1 was carried out in a stirred tank reactor and in a pulse reactor using MnMoO₄ as a catalyst. This catalyst exhibits a fairly good selectivity to maleic anhydride. MnMoO₄ shows a polyfunctional nature; it is possible to distinguish the properties of isomerization, dehydrogenation, oxidation with oxygen insertion, and complete oxidation by varying parameters such as temperature, oxygen concentration and contact time.The compositions of the products in the oxidation of n-butene-1 carried out in a pulse reactor are completely different in the presence and in the absence of oxygen, respectively. In the absence of oxygen, MnMoO₄ is a very selective catalyst in the dehydrogenation of n-butene-1 to butadiene. In the presence of oxygen, CO and CO₂ are the main products together with small amounts of maleic anhydride.The selectivity of MnMoO₄ to butadiene formation has been attributed to the presence of MoO bonds which are responsible for dehydrogenation reactions.A monocenter oxidation mechanism, accounting for the formation of CO, CO₂, and maleic anhydride, has been proposed in which the gaseous oxygen is considered to be adsorbed on the same center of the hydrocarbon.     

On the mechanism of the selective oxidation of n-butane, but-1-ene and but-1,3-diene to maleic anhydride over a vanadyl pyrophosphate catalyst

The mechanism of the selective partial oxidation of n-butane, but-1-ene and but-1,3-diene over a vanadyl phosphate catalyst has been investigated by temperature-programmed desorption (TPD) and by anaerobic temperature-programmed oxidation (TPO). TPD showed lattice oxygen to be desorbed in two states at 998 and 1023 K. The anaerobic TPO of n-butane produced butene and butadiene at 1020 K; anaerobic TPO of but-1-ene produced butadiene and furan at 990 K and dehydrofuran at 965 K, while anaerobic TPO of but-1,3-diene produced dehydrofuran at 970 K, furan at 1002 K and maleic anhydride at 1148 K. The total amount of oxygen removed from the lattice in these anaerobic selective partial oxidations was the same as that evolved from the vanadyl phosphate catalyst by TPD. This, and the fact that the selective oxidation reactions occurred at the same temperature at which the oxygen evolves from the lattice, suggests that the lattice oxygen is uniquely selective when it appears at the surface of the catalyst. (Under identical conditions of flow rate, weight of catalyst, heating rate etc., the reaction of n-butane or of but-1,3-diene in air produced only CO₂ and H₂O.) This revised version was published online in July 2006 with corrections to the Cover Date.     

Selectivity improvement using alternate flushing and reactant cycle steps

When a reactor is operated by alternately exposing the catalyst to hydrocarbon and oxygen, selectivity has been found to increase, but not for all partial oxidation products and not for all systems. In this study, multi-step periodic operation in which the catalyst is alternately exposed to hydrocarbon and air with an inert gas pulse between either or both of the reactant gases is examined. Partial oxidation of butadiene over a promoted vanadia molybdate catalyst was used. The influence of cycle period as well as the location and duration of the inert flush upon oxygenate production and yield was studied. Multi-step periodic operation using an inert gas for flushing was found not to be an attractive option for maleic anhydride from butadiene unless an increase in the yield of furan is desired.     

The partial oxidation of crotonaldehyde over V–Mo–O catalyst

The partial oxidation of crotonaldehyde, furan, crotonic acid and maleic anhydride was carried out over V–Mo–O _x catalyst in the temperature range 260–360 °C. The dependence of selectivity to main reaction products on the conversion of crotonaldehyde confirmed the parallel character of furan and maleic anhydride formation. The effect of water vapor on furan formation was explained by ability of the catalyst to isomerize crotonaldehyde. The reaction scheme of the oxidation of crotonaldehyde was suggested based on the experimental data.     

Investigation of the nature of the oxidant (selective and unselective) in/on a vanadyl pyrophosphate catalyst

The anaerobic oxidation of CO by a (VO)₂P₂O₇ catalyst has been used to investigate the nature of the oxidant (selective and unselective) in/on that material. Three peaks were observed in the rate of production of CO₂ - at 993, 1073 and 1093 K. The temperature of the maximum in the rate of production of the first CO₂ peak and the amount of oxygen associated with it are the same as that observed in the selective anaerobic oxidation of n-butane to butene and butadiene, but-1-ene to butadiene and furan and but-1,3-diene to dihydrofuran, furan and maleic anhydride. The interaction of CO with the (VO)₂P₂O₇ catalyst forming CO₂ at 993 K is therefore concluded to be with the selective oxygen. The total amount of oxygen removed by the CO from the (VO)₂P₂O₇ lattice (>5 monolayers) is about six times greater than that of the selective oxygen. The higher activation energies for the removal of the unselective oxygen accounts for the high selectivities (~80%) encountered commercially for the anaerobic oxidation of n-butane to maleic anhydride. Re-oxidation of the CO reduced (VO)₂P₂O₇ by N₂O quantitatively replaces all of the lattice oxygen removed by the formation of CO₂, but does not restore the original morphology. This revised version was published online in July 2006 with corrections to the Cover Date.     

瞬变应答法研究苯催化氧化的反应机理

采用瞬变应答法研究了苯在钒催化剂上催化氧化的反应机理。通过对应答曲线的分析,证明氧首先被催化剂表面吸附,然后以较慢的速率转化为晶格氧,这一步是苯催化氧反应速率的控制步骤。吸附氧是生成 CO 和 CO_2的氧源;晶格氧是生成顺酐的氧源。气相苯不直接参与反应,但以较快的速率可逆吸附于催化剂上,吸附的苯与晶格氧反应生成中间物 IN,IN 进一步氧化生成顺酐。顺酐可逆吸附于催化剂上,其介吸速率较慢。中间物和吸附的顺酐对反应起阻滞作用。     

The vapor-phase oxidation of benzene over a vanadium oxide catalyst

The kinetics of the reaction of benzene with oxygen over a vanadium oxide/potassium sulphate-promoted catalyst have been studied in a differential flow reactor. Rates of oxidation of benzene to maleic anhydride, p-benzoquinone and carbon dioxide were measured at temperatures from 350 to 400°C, at benzene concentrations from 2.0 to 4.33 × 10–⁸ moles/1, and at oxygen concentrations from 1.6 to 15.0 × 10–³ moles/1. The rate data were correlated by the steady state adsorption model and, by comparison with previous published results, it was concluded that this model provides a valuable means of correlating and interpreting catalytic oxidation rate data. Earlier data for benzene oxidation on the same catalyst were critically evaluated and a possible source of error was suggested.     

The reaction network of the selective oxidation of n-butane on (VO)2P2O7 catalysts: Nature of oxygen containing intermediates

The reaction network of the partial oxidation of n-butane to maleic anhydride on (VO)₂P₂O₇ has been investigated using steady-state and transient experiments in a Temporal-Analysis-of-Products (TAP) reactor under vacuum conditions to identify by mass spectrometry possible intermediate products and in a tubular fixed bed reactor at atmospheric pressure to derive information on the role of the detected and other potential intermediates in the reaction network. The oxidation of butane, butadiene, tetrahydrofuran, dihydrofuran, and furan has been studied in the TAP reactor and, additionally to these compounds, crotonaldehyde, crotonlactone, and malealdehydic acid were oxidized in the tubular flow reactor. From the results obtained it can be concluded that the main reaction pathway from butane to maleic anhydride proceeds via the intermediate products n-butenes, butadiene, crotonaldehyde, dihydrofuran, furan, and crotonlactone.     

The contribution of homogeneous and non-oxidative side reactions in the performance of vanadyl pyrophosphate,catalyst for the oxidation of n-butane to maleic anhydride,under hydrocarbon-rich conditions

The reactivity of vanadyl pyrophosphate, catalyst for the selective oxidation of n-butane to maleic anhydride, was examined under n-butane-rich conditions, simulating a feed in which oxygen is the limiting reactant, and a process in which the unconverted n-butane is recycled. A lower selectivity to maleic anhydride was found with respect to the hydrocarbon-lean conditions, due to the higher selectivity to carbon oxides and to the formation of C₈ by-products: tetrahydrophthalic and phthalic anhydrides. The latter compounds formed by a consecutive reaction of maleic anhydride with the unsaturated C₄ intermediates. This occurred under conditions of total oxygen conversion, due to the decreased catalyst oxidizing property. A relevant contribution of radicalic, homogeneous reactions was also observed, which mainly led to the formation of carbon oxides and olefins. This contribution decreased in the presence of the catalyst, which acted as a radical scavenger, but nevertheless remained important at temperatures higher than 400 °C. When conditions were used under which the conversion of oxygen was not total, olefins generated in the gas phase reacted at the catalyst surface yielding maleic anhydride. This homogeneously initiated heterogeneous process led to an unusual effect, of a relevant increase of maleic anhydride yield over 400 °C.     

Catalytic oxidation of benzene to maleic anhydride in a continuous stirred tank reactor

The kinetics of the vapor phase oxidation of benzene has been studied over an industrial catalyst in a continuous stirred tank reactor in the temperature range from 280 to 430°C and at atmospheric pressure. The products obtained are maleic anhydride, carbon oxides and water. The rate of the overall reaction (disappearance of benzene) is represented by the following expression based upon a steady state adsorption model The rate of formation of maleic anhydride is correlated by the equation which allows for a homogeneous depletion of maleic anhydride. The rate constants k_B, k_O, k_2(g) were found to follow Arrhenius behavior.      

Modified Mo/V/W‐Mixed Oxides for Catalytic Tar Removal from Biosyngas via Oxidation

In view of the growing energy demand and the current climate problems, renewable energy sources are becoming increasingly popular. The so‐called biosyngas obtained from the gasification of dry biomass can be used to synthesize fuels and basic chemicals. Besides gaseous by‐products and salts, this gas contains tar which needs to be removed before downstream processing. Catalysts like Mo/V/W‐oxides were found to be inert towards the oxidation of CO and H₂ but are able to activate the tar model compound naphthalene highly selective. Unfortunately, besides the total oxidation, the partial oxidation to the undesired intermediates phthalic acid anhydride and maleic acid anhydride takes place. Modification of the catalyst with bases leads to synergetic effects on the catalyst surface, the total oxidation is promoted, and the formation of intermediates decreases.     

Hydrocarbon Selective Oxidation on Vanadium Phosphorus Oxide Catalysts: Insights from Electronic Structure Calculations

The ability of the vanadium phosphorus oxide (VPO) catalyst to selectively activate n-butane and then perform subsequent selective oxidation to maleic anhydride was investigated using electronic structure calculations. Both active site cluster models and periodic surface models, including explicit consideration of surface relaxation and hydration, led to the same qualitative conclusions about the reactivity of the (VO)₂P₂O₇ (1 0 0) surface in substrate adsorption and oxidation. Density functional theory (DFT) reactivity indices and Density of States (DOS) plots show that, whether stoichiometric or phosphorus-enriched, strained or relaxed, bare or hydrated, covalent reactivity at the (1 0 0) surface is controlled by vanadium species, their dual acid–base attack giving selective activation of n-butane via methylene C–H bond cleavage. 1-butene is predicted to chemisorb at the surface using a π-cation complex, the strength of which makes 1-butene an unlikely intermediate in the production of maleic anhydride from n-butane. Coordinatively-unsaturated surface P–O and in-plane P–O–V oxygen species are the most nucleophilic surface oxygens, which may explain the surface-enrichment in phosphorus always seen in industrial catalysts for maleic anhydride synthesis and also recent in situ microscopy images of surface oxygen transfer to n-butane. The resistance of the maleic anhydride selective oxidation product to further transformation was shown to be dependent on its orientation in the active site, and simulation of surface hydration indicated that dissociative adsorption of water may serve to regenerate the catalyst, replenishing its supply of selective nucleophilic oxygen species for mild oxidation.     

Regeneration studies of redox catalysts

Cited advantages of circulating fluidized bed reactors (CFB) include higher selectivity and conversion together with the ability to optimize the process conditions of each vessel independently—temperature, gas partial pressure and residence time. DuPont commercialized a CFB process to produce maleic anhydride in which a vanadium pyrophosphate (VPO) was cycled between a fast bed riser and an air fed regenerator. Together with VPO, we examined two other redox catalyst systems—MoVSb (acrylic acid from propane) and FeMoO (methanol to formaldehyde).The lattice oxygen capacity of the FeMoO catalyst was about five times higher than either the VPO or MoVSb with little adsorbed carbon but a significant quantity of chemisorbed water. Above 350 °C, carbon deposition was detected and increased with increasing temperature. Carbon deposition decreased with increasing temperature for the MoVSb system and its lattice oxygen capacity was slightly higher than for VPO. The carbon deposition pattern for VPO was the opposite of the MoVSb and increased with temperature. Based on a hydrogen and carbon mass balance during the catalyst re-oxidation treatment, the molecular composition of the adsorbed species were C₄H₆ and C₃H₃—like for the VPO and MoVSb, respectively.Based on the high lattice oxygen capacity, the formaldehyde reaction appears to be ideally suited for development in a CFB. Whereas the lattice oxygen contribution of the MoVSb is equivalent to VPO, less oxygen is required to produce acrylic acid (compared to maleic anhydride) so the incentive of developing a CFB process should be greater than for butane oxidation to maleic anhydride.     

Oxidation of Benzene to Maleic Anhydride in a Fluidized Bed Reactor

In this project, the selective oxidation of benzene to maleic anhydride (MAN) was studied. Gas phase catalytic oxidation of benzene was carried out in a laboratory scale fluidized bed reactor on six different types of catalysts, which have different compositions. Effects of temperature, flow rates of benzene and air and catalyst type on the reaction selectivity were investigated at atmospheric pressure. The experiments were performed over a temperature range of 325 to 400 °C, a space‐time (W/F_A0) range from 11.28 × 10⁵ to 31.9 × 10⁵ g s mol^–1, and benzene/air mole ratio changes between 0.0109 and 0.0477. It was seen that conversion of benzene to MAN increased with increasing temperature for the catalysts supported by silica gel, aluminum oxide and titanium oxide. From the results it was found that conversion increased with increasing flow rate of air. When the comparison of the catalysts were made, it could be said that catalysts supported by silica gel showed higher MAN conversions. So it can be concluded that catalysts supported by silica gel were more suitable catalysts for benzene oxidation to MAN in a fluidized bed reactor.     

Assessment of micro-structured fixed-bed reactors for highly exothermic gas-phase reactions

The application of micro-structured fixed-bed reactors for highly exothermic partial oxidation reactions and their comparison to established multi-tubular fixed-bed reactors was investigated by numerical simulation. As examples, the partial oxidations of butane to maleic anhydride and of o-xylene to phthalic anhydride were chosen. The simulation results revealed that the reactor productivity, i.e. the amount of product per unit of reactor volume, achievable in micro-structured fixed-bed reactors is between 2.5 and 7 times higher than in conventional multi-tubular fixed-bed reactors without the danger of excessive pressure drop. For the partial oxidation of butane to maleic anhydride this can be explained by the increased reactor efficiency caused by lower efficiency losses through heat and mass transfer limitations. In addition, maleic anhydride selectivities and yields are higher in micro-structured fixed-bed reactors. In the case of o-xylene oxidation to phthalic anhydride the main advantage is that egg-shell catalysts in the conventional fixed-bed reactor can be replaced by bulk catalysts in the micro-structured fixed-bed reactor. For this reaction, product selectivities are very similar for all reactor configurations. Thus the catalyst inventory and reactor productivity are strongly increased. This study underlines, that micro-structured fixed-bed reactors exhibit the potential to intensify large scale industrial processes significantly.     

Influence of Rare-Earth and Bimetallic Promoters on Various VPO Catalysts for Partial Oxidation of n-Butane

Vanadium phosphorous oxide (VPO) catalyst was prepared using dihydrate method and tested for the potential use in selective oxidation of n-butane to maleic anhydride. The catalysts were doped by La, Ce and combined components Ce + Co and Ce + Bi through impregnation. The effect of promoters on catalyst morphology and the development of acid and redox sites were studied through XRD, BET, SEM, H₂-TPR and TPRn reaction of n-butane/He. Addition of rare-earth element to VPO formulation and drying of catalyst precursor by microwave irradiation increased the fall width at half maximum (FWHM) and reduced the crystallite size of the Vanadyl hydrogen phosphate hemihydrate (VOHPO₄ · 1/2 H₂O, VHP) precursor phase and thus led to the production of final catalysts with larger surface area. The Ce doped VPO catalyst which, assisted by the microwave heating method, exhibited the highest surface area. Moreover, the addition of promoters significantly increased catalyst activity and selectivity as compared to undoped VPO catalyst in the oxidation reaction of n-butane. The H₂-TPR and TPRn reaction profiles showed that the highest amount of active oxygen species, i.e., the V⁴⁺–O^? pair, was removed from the bimetallic (Ce + Bi) promoted catalyst. This pair is responsible for n-butane activation. Furthermore, based on catalytic test results, it was demonstrated that the catalyst promoted with Ce and Bi (VPOD1) was the most active and selective catalyst among the produced catalysts with 52% reaction yield. This suggests that the rare earth metal promoted vanadium phosphate catalyst is a promising method to improve the catalytic properties of VPO for the partial oxidation of n-butane to maleic anhydride.     

Effect of support material on the adsorption structures of furan and maleic anhydride on the surface of V2O5/P2O5 catalysts : II. Results of In Situ Infrared Spectroscopic Studies

The structures of furan and maleic anhydride (MA) adsorbed on surfaces of Al₂O₃-supported and TiO₂-supported V₂O₅/P₂O₅ catalysts (V₂O₅/P₂O₅/support=5/5/90 mass-%) were investigated by IR spectroscopy. For furan four different adsorption structures could be observed at temperatures below 500 K: a cationic allyl complex, that interacted in 2-position with the catalyst, an oxyfuran complex, a 2-(5H)-furanone complex and an aldehyde maleate complex. Between 573 and 648 K adsorbed maleic anhydride and maleate complexes were observed additionally. Aldehyde maleate and maleate were favored on the surface of the Al₂O₃,-supported catalyst. At temperatures below 423 K maleic anhydride interacted with hydroxyl groups of the catalyst surface via hydrogen bonds; a maleate complex as well as a maleic acid complex and an acid maleate complex were identified. Maleate was favored on the Al₂O₃-supported catalyst when furan was adsorbed. Between 427 and 573 K the maleate complex was observed only on the surface of the Al₂O₃-supported catalyst. Ring opening facilitated by hydroxyl groups existing on the Al₂O₃ support is assumed to be responsible for the non-selective oxidation of furan and for the oxidative degradation of MA on the Al₂O₃-supported catalyst. The results have been confirmed by IR spectroscopic investigation of the structures of MA adsorbed on γ-Al₂O₃ and TiO₂ alone. MA adsorption was strongest on γ-Al₂O₃; also the formation of maleate structures were favoured on this support.     

不同P与V比的Mo/VPO催化剂物相组成及其催化性能

采用有机相法制备了不同P与V物质的量比的Mo掺杂VOHPO₄·0.5H₂O前驱体,并通过体积分数为50%空气-40%氮气-10%水蒸汽混合气氛活化得到Mo/VPO催化剂,采用固定床反应器评价其催化正丁烷氧化制顺酐的性能。结果表明,Mo/VPO催化剂催化活性随P与V物质的量比的增大而降低,但顺酐选择性与P与V物质的量比并不呈线性关系,P与V物质的量比为0.9的Mo/VPO催化剂具有最佳的催化性能。XRD分析表明,Mo/VPO催化剂催化正丁烷氧化制顺酐的主要活性物相为(VO)₂P₂O₇和钒磷云母相,形成的主要因素不是P与V物质的量比,而是由焙烧条件决定。低P与V物质的量比的Mo/VPO中存在的少量V₂O₅物相能够提升催化剂的活性和顺酐选择性,但含量过高会因深度氧化降低催化性能。催化剂中存在的钒磷云母相有利于缩短催化剂稳定时间并提升催化性能。