Effect of different calcination environments on the vanadium phosphate catalysts for selective oxidation of propane and n-butane

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 (23 m² g⁻¹) compared to VPDP (18 m² 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 V^V for VPDB. Furthermore 14.2% of V^III 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 V^V and V^IV phases, respectively. VPDB removed larger amount of active oxygen species linked to V^IV phase which eventually caused a higher conversion rate in the selective oxidation of n-butane and propane to maleic anhydride and acrylic acid, respectively.     

The beneficial effect of cobalt on VPO catalysts

The addition of Co to VPO formulations improves the yield of n-butane to maleic anhydride. In this work, different modes of impregnation and two different organic cobalt salts were used. The equilibrated catalysts were characterized using XRD, ³¹P SEM NMR, FT-IR and acetonitrile adsorption to evaluate Lewis acidity.

The best catalyst was obtained using Co acetyl acetonate for impregnation of the VOHPO₄·0.5H₂O precursor. This catalyst after equilibration had an optimum concentration of very strong Lewis acid sites, very low concentration of isolated V(V) centers, and no V(V) phases.     

Preparation of catalyst precursors for selective oxidation of n-butane by exfoliation–reduction of VOPO4·2H2O in primary alcohol

A new method through intercalation and exfoliation of VOPO₄·2H₂O crystallites in primary alcohol (1-propanol or 1-butanol), followed by reduction with the alcohol, have been investigated for the preparation of catalyst precursor. Lamellar compounds, consisting of V⁴⁺, P⁵⁺ and alkyl group with thin film-like morphology, were formed and was characterized by means of XRD, IR, TG/DTA, and elemental analysis. The chemical formula of the precursor obtained by exfoliation–reduction in 1-butanol was shown to be VO{(n-C₄H₉)_0.16H_0.84}PO₄·0.8H₂O. On the other hand, a direct reduction of VOPO₄·2H₂O in the alcohol gave a mixed phase shown by (VOHPO₄·0.5H₂O)_0.3(VO{(n-C₄H₉)_0.3H_0.7}PO₄·3H₂O)_0.7 comprising plate-like microcrystallites. These precursors transformed to (VO)₂P₂O₇ phase during an activation process at 703 K in the presence of a mixture of n-butane 1.5% and O₂ 17% in He balance. The obtained (VO)₂P₂O₇ through the exfoliation–reduction was well crystallized and consisted of thin flaky crystallites. It was found that (VO)₂P₂O₇ thus prepared through the exfoliation–reduction was highly active and selective for oxidation of n-butane.     

焙烧条件对正丁烷选择性氧化制顺酐VPO催化剂性能的影响

采用有机相还原法制备正丁烷选择性氧化制顺酐VPO催化剂,考察不同焙烧条件对催化剂性能的影响,在固定床反应器上评价催化剂性能,确定催化剂最佳的焙烧条件。结果表明,在反应气氛460 ℃焙烧2 h条件下,正丁烷转化率和顺酐选择性分别为80.9%和71.2%,顺酐收率最高达到57.6%。     

Surface alteration of (VO)2P2O7 by α-Sb2O4 as a route to control the n-butane selective oxidation

The catalytic properties of (VO)₂P₂O₇/α-Sb₂O₄ mixed oxides system for n-butane mild oxidation have been investigated on two mechanical mixtures (M1 and M2) of the same well crystallized (VO)₂P₂O₇ (reference vanadyl pyrophosphate) with two different morphologies of α-Sb₂O₄.The M1 mixture of (VO)₂P₂O₇ with α-Sb₂O₄ (1), prepared by oxidation of Sb₂O₃, leads to the oxidative dehydrogenation (ODH) of n-butane, whereas the M2 mixture of (VO)₂P₂O₇ with a commercial α-Sb₂O₄ (2) (Aldrich) with a different morphology improves the maleic anhydride selectivity as compared to the reference (VO)₂P₂O₇ catalyst (synergetic effect). After reaction, no ternary VPSbO phase is detected by XRD and DTA and it was controlled that the two α-Sb₂O₄ oxides are catalytically inactive.The (VO)₂P₂O₇ reference catalyst which produced only maleic anhydride as mild oxidation product shows by XPS a slightly oxidized surface (14% V⁵⁺–86% V⁴⁺).Contamination of the (VO)₂P₂O₇ phase by migration of Sb species occurs after catalytic reaction in the case of the M1 mixture as shown by XPS, LEIS and TEM–EDX analysis. XPS showed that (VO)₂P₂O₇ is partially superficially reduced (86% V⁴⁺–14% V³⁺). This feature is consistent with the decrease of acidity as observed by pyridine adsorption–desorption.In opposition with the M1 mixture, no contamination of the (VO)₂P₂O₇ phase is observed after catalytic reaction in the case of the M2 mixture. The XPS study shows, in this case, that (VO)₂P₂O₇ is partially oxidized (30% V⁵⁺–70% V⁴⁺) at a higher level than for the reference (VO)₂P₂O₇ catalyst. This situation is associated with the increase of selectivity observed for maleic anhydride (synergetic effect).The difference in the catalytic results for the two M1 and M2 mixtures, as compared to the (VO)₂P₂O₇ reference catalyst, can be explained by the alteration of the surface composition of (VO)₂P₂O₇ and the distribution of vanadium oxidation state due to different interaction between Sb₂O₄and (VO)₂P₂O₇, depending on the orientation of the α-Sb₂O₄ crystals.     

Reactivation of sintered Pt/Al2O3 oxidation catalysts

The reactivation of thermally sintered Pt/Al₂O₃ catalysts used in the simultaneous oxidation of CO and propene has been achieved by an oxychlorination treatment. The catalyst can be considered to model the active component of the catalytic converter fitted to diesel driven cars. Platinum crystallites redispersion was verified by XRD, H₂ chemisorption, TEM and FTIR. The extent of regeneration reflects the platinum particle redispersion achieved by such a treatment. Oxychlorination also introduced electronic effects in the Pt particle caused by the presence of chlorine at the Pt-Al₂O₃ interface but no detrimental result of this was observed in the oxidation reactions. The results indicate that the deactivation of the diesel oxidation catalysts (DOCs) can be reverted by this simple treatment resulting in a remarkable recovery of the catalytic activity.     

Progress on the mechanistic understanding of SO2 oxidation catalysts

For almost a century vanadium oxide based catalysts have been the dominant materials in industrial processes for sulfuric acid production. A vast body of information leading to fundamental knowledge on the catalytic process was obtained by Academician [G.K. Boreskov, Catalysis in Sulphuric Acid Production, Goskhimizdat (in Russian), Moscow, 1954, p. 348]. In recent years these catalysts have also been used to clean flue gases and other SO⁻₂ containing industrial off-gases. In spite of the importance and long utilization of these industrial processes, the catalytic active species and the reaction mechanism have been virtually unknown until recent years.

It is now recognized that the working catalyst is well described by the molten salt/gas system M₂S₂O₇–MHSO₄–V₂O₅/SO₂–O₂–SO₃–H₂O–CO₂–N₂ (M=Na, K, Cs) at 400–600°C and that vanadium complexes play a key role in the catalytic reaction mechanism.

A multiinstrumental investigation that combine the efforts of four groups from four different countries has been carried out on the model system as well as on working industrial catalysts. Detailed information has been obtained on the complex and on the redox chemistry of vanadium. Based on this, a deeper understanding of the reaction mechanism has been achieved.     

Methane oxidation over vanadium-modified Pd/Al2O3 catalysts

Three different vanadium-modified Pd/Al₂O₃ catalysts were prepared and tested as catalysts for the deep oxidation of methane. Vanadium was added to the palladium catalyst by incipient wetness of palladium catalyst in order to modify its properties and improve its thermal stability and thioresistance. The behaviour of vanadium-modified catalysts depends on the concentration of this compound, being 0.5 wt.% the optimum amount. However, when strong catalyst poisons are present in the gas (SO₂), these modified catalysts do not show a better performance than unmodified catalyst. Bimetallic catalysts were tested with and without further reduction, being observed that reduced bimetallic catalysts perform worse than the non-reduced ones.     

活化气氛对正丁烷选择性氧化制顺酐催化剂性能的影响

研究了正丁烷选择性氧化制顺酐钒磷氧催化剂的前驱体分别在空气、水蒸汽-空气、正丁烷-空气以及氮气4种活化气氛活化对催化剂性能的影响,采用BET、XRD和XPS等对活化后的催化剂进行表征。结果表明,采用水蒸汽-空气活化的VPO-2催化剂晶相组成较好,钒平均价态为4.19。丁烷氧化评价试验结果表明,VPO-2催化剂的催化性能较好,正丁烷转化率达92.47%,顺酐选择性达60.48%。     

MnOx/Pt/Al2O3 catalysts for CO oxidation in H2-rich streams

The catalytic activity of Pt on alumina catalysts, with and without MnO_x incorporated to the catalyst formulation, for CO oxidation in H₂-free as well as in H₂-rich stream (PROX) has been studied in the temperature range of 25–250 °C. The effect of catalyst preparation (by successive impregnation or by co-impregnation of Mn and Pt) and Mn content in the catalyst performance has been studied. A low Mn content (2 wt.%) has been found not to improve the catalyst activity compared to the base catalyst. However, catalysts prepared by successive impregnation with 8 and 15 wt.% Mn have shown a lower operation temperature for maximum CO conversion than the base catalyst with an enhanced catalyst activity at low temperatures with respect to Pt/Al₂O₃. A maximum CO conversion of 89.8%, with selectivity of 44.9% and CO yield of 40.3% could be reached over a catalyst with 15 wt.% Mn operating at 139 °C and λ = 2. The effect of the presence of 5 vol.% CO₂ and 5 vol.% H₂O in the feedstream on catalysts performance has also been studied and discussed. The presence of CO₂ in the feedstream enhances the catalytic performance of all the studied catalysts at high temperature, whereas the presence of steam inhibits catalysts with higher MnO_x content.     

Effects of the structural and cationic properties of AV2P2O10 solids on propane selective oxidation

The activity performances of five AV₂P₂O₁₀ compounds of either orthorhombic (A=Cd, Ca), or monoclinic (A=Cd, Ba, Pb) symmetry types, have been compared for propane partial oxidation. They present a rather good activity (9% of propane conversion at 460°C) and a high selectivity (60% of propene selectivity). Results are discussed as a function of the differences in the structural properties. Kínetic studies were also performed for propane and propene oxidation reaction.     

Catalytic oxidation of butane to maleic anhydride enhanced yields in the presence of CO2 in the reactor feed

The addition of CO₂ to the reaction mixture for the oxidation of butane over a VPO catalyst gives rise to significantly improved yields of maleic anhydride.     

碳纳米管负载CuO-CeO_2催化剂用于CO选择性氧化

分别以碳纳米管(CNTs)和68%浓HNO3处理的CNTs为载体,采用超声辅助的浸渍法制备负载型Cu O-CeO_2复合氧化物催化剂,用于富氢气中CO选择氧化。采用XPS和LRS对预处理前后CNTs管的结构与表面性质进行研究。采用XRD和H2-TPR对催化剂结构进行表征。结果表明,经浓HNO3处理的CNTs载体表面含氧官能团—COOH相对含量提高了约68%,且表面缺陷增多,有助于催化剂活性组分的沉积和分散。以此负载的Cu O-CeO_2催化剂上Cu O物种具有较好的分散性,晶粒尺寸较小,催化剂表现出强的低温氧化还原能力,且表面CO氧化活性位增多,对CO选择性氧化具有低温高活性,T50低至90℃,反应温度低于140℃保持高选择性,且CO完全转化反应温度窗口拓宽宽至30℃。     

Polymers oxidation with VO(acac)2 complex

The introduction of epoxy groups in the polymer backbone is one of the most promising methods of modifying polydienes. In this work the performance of the classical catalytic system VO(acac)₂/tert-butylhydroperoxide was evaluated for the epoxidation of polydienes. The polymers investigated were a hydroxylated poly(butadiene), two polybutadienes with high and with low 1,4-content, a poly(isoprene), and a styrene–butadiene copolymer. The epoxidized polymers were characterized by IR, ¹H and ¹³C NMR and GPC techniques. The vanadium system was active for the epoxidation reaction and the reactivity decreases in the order HTPB>PI>1,4-PB>1,2-PBSBR. The occurrence of secondary reactions to low extension could be detected by the presence of hydroxyl, carbonyl and alcohol signals in the spectra of epoxidized polymers. Incomplete mass recovery (70–90%) was observed probably due to the chain degradation or modification of the polymer polarity.     

The promoting effect of Nb2O5 addition to Pd/Al2O3 catalysts on propane oxidation

Pd/Nb₂O₅/Al₂O₃ catalysts were investigated on propane oxidation. Diffuse reflectance spectroscopy (DRS) and X-ray photoelectron spectroscopy (XPS) analysis suggested that monolayer coverage was attained between 10 and 20 wt.% of Nb₂O₅. Temperature programmed reduction (TPR) evidenced the partial reduction of niobium oxide. The maximum propane conversion observed on the Pd/10% Nb₂O₅/Al₂O₃ corresponded to the maximum Nb/Al surface ratio. The presence of NbO_x polymeric structures near to the monolayer could favor the ideal Pd⁰/Pd²⁺ surface ratio to the propane oxidation which could explain the promoting effect of niobium oxide.     

Effects of microwaves on selective oxidation of toluene to benzoic acid over a V2O5/TiO2 system

Two series of catalysts, V₂O₅/TiO₂ and modified V₂O₅/TiO₂, were prepared with a conventional impregnation method. They were tested in the selective oxidation of toluene to benzoic acid under microwave irradiation. The reaction conditions were optimized over V₂O₅/TiO₂. It was found that in the microwave catalytic process the optimum reactor bed temperature of the titled reaction decreases to 500 K (600 K in the conventional process). The modification of V₂O₅/TiO₂ with MoO₃, WO₃, Nb₂O₅ or Ta₂O₅, which has no negative influence on the reaction in the conventional catalytic process, can greatly promote the catalytic activities in the microwave process, leading to a high yield of benzoic acid (41%). The effects of microwave electromagnetic field on the catalysts are discussed.     

Partial oxidation of ethanol over Pd/CeO2 and Pd/Y2O3 catalysts

The effect of the support nature on the performance of Pd catalysts during partial oxidation of ethanol was studied. H₂, CO₂ and acetaldehyde formation was favored on Pd/CeO₂, whereas CO production was facilitated over Pd/Y₂O₃ catalyst. According to the reaction mechanism, determined by DRIFTS analyses, some reaction pathways are favored depending on the support nature, which can explain the differences observed on products distribution. On Pd/Y₂O₃ catalyst, the production of acetate species was promoted, which explain the higher CO formation, since acetate species can be decomposed to CH₄ and CO at high temperatures. On Pd/CeO₂ catalyst, the acetaldehyde preferentially desorbs and/or decomposes to H₂, CH₄ and CO. The CO formed is further oxidized to CO₂, which seems to be promoted on Pd/CeO₂ catalyst.     

The mechanisms and topologies of Ru-based water oxidation catalysts: A comprehensive review

A water-splitting device is based on the activity of water oxidation catalysts. Water oxidation catalysts have been designed using different strategies to form multiple topologies. The catalysts vary in their function depending on their coordination flexibility, steric hindrance, and carboxylate ligands. The onset potential of the water-oxidation catalysts can be lowered by the addition of functional groups with negative charges. The normal configuration of the transition metal complex should be distorted to form an open site that can conveniently bind to the substrates. In this review, we have discussed the mechanism of Ru-based water oxidation catalysts with different topologies beneficial for the conversion of the substrate.     

Reactivation of sulphated Pt/Al2O3 catalysts by reductive treatment in the simultaneous oxidation of CO and C3H6

A Pt/Al₂O₃ catalyst prepared by incipient wetness impregnation was used as a diesel oxidation model catalyst and tested in the simultaneous total oxidation of CO and C₃H₆. Sulphur incorporation by wet impregnation results in deactivation of the Pt/Al₂O₃ catalyst in both oxidation reactions. Characterization of the catalysts by evolved gas analysis by mass spectrometry (EGA-MS), X-ray diffraction (XRD), isotherm of adsorbed nitrogen, X-ray photoelectron spectroscopy (XPS), infrared spectroscopy of probe molecules (pyridine and carbon monoxide) and finally temperature-programmed surface reaction (O₂-TPSR of chemisorbed CO) demonstrated that the formation of aluminium sulphate modifies the acidic properties of the support and the electronic properties of the platinum particles. Thus, new Brønsted acid sites are formed and, moreover, the capacity of the Pt particles to chemisorb CO and O₂, the latter as strongly chemisorbed O species, is seriously deteriorated. The alteration of the electronic properties of the particles (they become electronically deficient) is related to the modification of the acidic properties of the support. Treatment of the deactivated catalysts by a reductive treatment at 873 K resulted in the removal of the sulphur due to decomposition of the aluminium sulphate. Thus, the original acidic properties of the support and the electronic properties of the Pt particles were largely recovered and a high degree of catalytic reactivation was achieved.     

Comparison of the activity of Ru and Pt catalysts for the oxidation of carbon by NO2

The role of two catalysts Pt/Al₂O₃ and Ru/NaY on the oxidation of carbon by NO₂ was investigated in the temperature range 300–400 °C. In the case of Pt/Al₂O₃ no significant catalytic effect on the carbon oxidation rate is observed although decomposition of NO₂ takes place on the noble metal and leads to the formation of NO. This result suggests that the amount of the oxygen atoms transferred from the metallic surface sites to the carbon surface to form C(O) complex is negligible. In contrast, in presence of Ru/NaY the oxidation rate of carbon by NO₂ is markedly increased. Hence, a significant part of the formed O through catalytic decomposition of NO₂ on Ru surface sites is transferred to the carbon surface leading to a larger amount of C(O) complexes on the carbon surface. Thus, the ruthenium surface is a generator of active oxygen species that are spilled over on the carbon surface at 350 °C.