Modification of vanadium phosphorus oxides used for n-butane oxidation to maleic anhydride by interaction with niobium phosphate

A new family of vanadium phosphorus oxides (VPO) catalysts has been identified. It consists in a mixture of VPO with niobium phosphate (NbPO). The amorphous NbPO material is introduced during the preparation of the VOHPO₄·0.5H₂O precursor. It is observed that the VPO–NbPO catalyst is more rapidly activated and gives better performances (n-butane conversion and maleic anhydride selectivity) for mild oxidation of n-butane to maleic anhydride. VPO phases and the NbPO material have been identified in the VPO–NbPO precursor and in the VPO–NbPO catalyst. Nb in VPO crystals and V in NbPO particles have been respectively observed by EDX-STEM. No other VPO crystals than the VOHPO₄, 0.5H₂O precursor, and (VO)₂P₂O₇ for the catalyst, have been identified by XRD and ³¹P NMR. ³¹P NMR by spin echo mapping and ³¹P MAS NMR have confirmed an interaction of the VPO precursor with Nb and of the NbPO amorphous material with V, as evidenced by EDX-STEM. This should be the reason for the observed improvement of the catalytic results by a redox effect of niobium on the VPO catalyst modifying the V⁵⁺/V⁴⁺ balance in a favourable way.     

Influence of Bi–Fe additive on properties of vanadium phosphate catalysts for n-butane oxidation to maleic anhydride

The physico-chemical and catalytic properties of three ways of modified catalysts were studied, i.e. (i) the addition of both Bi and Fe (nitrate form) during the refluxing VOPO₄·2H₂O with isobutanol (Catalyst A), (ii) the simultaneous addition of BiFe oxide powder in the course of the synthesis of precursor VOHPO₄·0.5H₂O (Catalyst B) and (iii) the mechanochemical treatment of precursor VOHPO₄·0.5H₂O and BiFe oxide in ethanol (Catalyst C). It was found that surface area of the modified catalysts has increased except Catalyst B. The reactivity of the oxygen species linked to V⁵⁺ and V⁴⁺ was studied by using H₂-TPR, which also affected the catalytic performance of the catalyst. The conversion of n-butane decreases with an increment of oxygen species associated with V⁵⁺.     

Controlling factors in the selective conversion of n-butane over promoted VPO catalysts at low temperature

Unpromoted and 1.2, 2.3 and 4.3 molar % Co:V cobalt-promoted vanadium-phosphorous-oxide (VPO) catalysts were synthesized via an organic route. The catalyst precursors were calcined and then conditioned in a reactor, forming the active vanadyl pyrophosphate (VO)₂P₂O₇ phase, which was confirmed via X-ray diffraction studies (XRD). The effect of co-promotion on the yield of maleic anhydride (MA) from n-butane oxidation was examined at different temperatures and gas hourly space velocities (GHSV). 2.3% Co:V was the optimum promoter loading for a high yield towards MA. Higher GHSV's proved to enhance the selectivity towards MA.

The catalysts were tested over a 200 h period, generally taking some 24 h to reach steady-state performance. The best performing catalyst yielded 45% MA at 275 °C and a GHSV of 7200 ml g⁻¹ h⁻¹, with an n-butane conversion of 73%, whilst all previously reported VPO catalysts produced far lower MA yields at this temperature.     

Relationship between structural/surface characteristics and reactivity in n-butane oxidation to maleic anhydride: The role of V species

V/P/O-based catalysts were prepared by thermal treatments of VOHPO₄0·5H₂O precursors prepared with the organic procedure. Different methods for precursor dehydration led to compounds which were characterized by the prevailing presence of crystalline (VO)₂P₂O₇, but which also contained either V⁵⁺ species or V³⁺ species. The catalytic performance of these compounds in n-butane oxidation under almost-equilibrated conditions was compared. It was found that the presence of either V³⁺ or V⁵⁺ enhances the specific activity in n-butane oxidation, while the selectivity to maleic anhydride at low n-butane conversion (30%) remains substantially unaffected. A fully equilibrated, well-crystallized (VO)₂P₂O₇ was reduced with H₂. The reduced compound was more active than the fully equilibrated vanadyl pyrophosphate, while exhibiting comparable selectivity to maleic anhydride.     

TiO2-SiO2 and V2O5/TiO2-SiO2 catalyst: Physico-chemical characteristics and catalytic behavior in selective catalytic reduction of NO by NH3

TiO₂-SiO₂ with various compositions prepared by the coprecipitation method and vanadia loaded on TiO₂-SiO₂ were investigated with respect to their physico-chemical characteristics and catalytic behavior in SCR of NO by NH₃ and in the undesired oxidation of SO₂ to SO₃, using BET, XRD, XPS, NH₃-TPD, acidity measurement by the titration method and activity test. TiO₂-SiO₂, compared with pure TiO₂, exhibits a remarkably stronger acidity, a higher BET surface area, a lower crystallinity of anatase titania and results in allowing a good thermal stability and a higher vanadia dispersion on the support up to high loadings of 15 wt% V₂O₅. The SCR activity and N₂ selectivity are found to be more excellent over vanadia loaded on TiO₂-SiO₂ with 10–20 mol% of SiO₂ than over that on pure TiO₂, and this is considered to be associated with highly dispersed vanadia on the supports and large amounts of NH₃ adsorbed on the catalysts. With increasing SiO₂ content, the remarkable activity decrease in the oxidation of SO₂ to SO₃, favorable for industrial SCR catalysts, was also observed, strongly depending on the existence of vanadium species of the oxidation state close to V⁴⁺ on TiO₂-SiO₂, while V⁵⁺ exists on TiO₂, according to XPS. It is concluded that vanadia loaded on Ti-rich TiO₂-SiO₂ with low SiO₂ content is suitable as SCR catalysts for sulfur-containing exhaust gases due to showing not only the excellent de-NO_x activity but also the low SO₂ oxidation performance.     

Effect of Re loading on the structure, activity and selectivity of Re/C catalysts in hydrodenitrogenation and hydrodesulphurisation of gas oil

A series of Re-containing catalysts supported on activated carbon, with Re loading between 0.74 and 11.44 wt.% Re₂O₇, was prepared by wet impregnation and tested in the simultaneous hydrodesulphurisation (HDS) and hydrodenitrogenation (HDN) of a commercial gas oil. Textural analysis, XRD, X-ray photoelectron spectroscopy (XPS) and surface acidity techniques were used for physicochemical characterisation of the catalysts. Increase in the Re concentration resulted in a rise in the HDS and HDN activity due to the formation of a monolayer structure of Re and the higher surface acidity. At Re concentrations >2.47 wt.% Re₂O₇ (0.076 Re atoms nm⁻²) the reduction in the catalytic activity was related to the loss in specific surface area (BET) due to reduction in the microporosity of the carbon support. The magnitude of the catalytic effect was different for HDS and HDN, and depended strongly on the Re content and reaction temperature. The apparent activation energies were about 116–156 kJ mol⁻¹ for HDS and 24–30 kJ mol⁻¹ for HDN. This led to a marked increase in the HDN/HDS selectivity with decreasing temperature (values >3 at 325 °C), due to the large differences in the apparent activation energies of HDS and HDN found for all catalysts. A gradual increase in the HDN/HDS selectivity with increased Re loading was also found and related to the observed increase of catalyst acidity. The results are compared with those obtained for a series of Re/γ-Al₂O₃ catalysts.     

Chemical deactivation of V2O5/WO3–TiO2 SCR catalysts by additives and impurities from fuels, lubrication oils and urea solution: Part II. Characterization study of the effect of alkali and alkaline earth metals

A characterization study on a practice-oriented V₂O₅/WO₃–TiO₂ SCR catalyst deactivated by Ca and K, respectively, was carried out using NH₃-TPD, DRIFT spectroscopy, and XPS as well as theoretical DFT calculations. It was found from NH₃-TPD experiments that strongly basic elements like K or Ca drastically affect the acidity of the catalysts. Detailed DRIFT spectroscopy experiments revealed that these poisoning agents mostly interact with the Brønsted acid sites of the V₂O₅ active phase, thus affecting the NH₃ adsorption. Moreover, these experiments also indicated that the V⁵⁺ = O sites are much less reactive on the poisoned catalysts. XPS investigations of the O 1s binding energies showed that the oxygen atoms of the V⁵⁺ = O sites are affected by the presence of the poisoning agents. Based on these results and on DFT calculations with model clusters of the vanadia surface, the poisoning mechanism is explained by the stabilization of the non atomic holes of the (0 1 0) V₂O₅ phase as a result of the deactivation element. Consequently, V–OH Brønsted acid sites and V⁵⁺ = O sites are inhibited, which are both of crucial importance in the SCR process. The deactivation model also gives an explanation to the very low concentrations of potassium needed to deactivate the SCR catalyst, since one metal atom sitting on such a non-atomic hole site deactivates up to four active vanadium centers.     

CuO改性商用选择性催化还原催化剂催化氧化单质汞性能

燃煤烟气中单质汞（Hg⁰）的高效氧化是提高燃煤电厂脱汞效率的关键,为提高传统选择性催化还原（SCR）催化剂氧化Hg⁰的性能,本文采用溶液浸渍法制备了CuO-SCR催化剂,在固定床反应装置上考察了其脱硝协同氧化单质汞性能,并借助BET、XRD和XPS等分析技术对催化剂进行表征。结果表明,CuO的掺杂显著提高了商用SCR催化剂的Hg⁰氧化性能和低温脱硝活性;在200~400℃内,随着反应温度升高和NO+NH₃浓度增大,NH₃对CuO-SCR催化剂的Hg⁰氧化性能抑制作用加强;在350℃、模拟燃煤烟气条件下,随着空速比的减小,催化剂的Hg⁰氧化性能显著提高,NH₃对Hg⁰氧化的抑制作用明显减弱。催化剂表征结果表明,CuO-SCR催化剂表面存在氧化还原反应V⁴⁺+Cu²⁺?V⁵⁺+Cu⁺,增强了催化剂的催化活性。Hg⁰在CuO-SCR催化剂表面的氧化过程遵循Mars-Maessen机制,能够有效增强催化剂的Hg⁰氧化性能。     

Progress of vanadium phosphorous oxide catalyst for n-butane selective oxidation

The utilization of lighter alkanes into useful chemical products is essential for modern chemistry and reducing the CO₂ emission. Particularly, n-butane has gained special attention across the globe due to the abundant production of maleic anhydride (MA). Vanadium phosphorous oxide (VPO) is the most effective catalyst for selective oxidation of n-butane to MA so far. Interestingly, the VPO complex exists in more or less fifteen different structures, each one having distinct phase composition and exclusive surface morphology and physiochemical properties such as valence state, lattice oxygen, acidity etc., which relies on precursor preparation method and the activation conditions of catalysts. The catalytic performance of VPO catalyst is improved by adding different promoters or co-catalyst such as various metals dopants, or either introducing template or structural-directing agents. Meanwhile, new preparation strategies such as electrospinning, ball milling, hydrothermal, barothermal, ultrasound, microwave irradiation, calcination, sol–gel method and solvothermal synthesis are also employed for introducing improvement in catalytic performance. Research in above-mentioned different aspects will be ascribed in current review in addition to summarizing overall catalysis activity and final yield. To analyze the performance of the catalytic precursor, the reaction mechanism and reaction kinetics both are discussed in this review to help clarify the key issues such as strong exothermic reaction, phosphorus supplement, water supplement, deactivation, and air/n-butane pretreatment etc. related to the various industrial applications of VPO.     

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。     

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.     

Vanadium species in new catalysts for the selective oxidation of methane to formaldehyde: Activation of the catalytic sites

New vanadium oxide supported on mesoporous silica catalysts for the oxidation of methane to formaldehyde were investigated by infrared and Raman spectroscopies to identify and characterize the molecular structure of the most active and selective catalytic sites. In situ and operando experiments have been conducted in order to understand the redox and hydroxylation/dehydroxylation processes of the vanadium species. (SiO)₂VO(OH) species were identified in these catalysts in reaction conditions and shown to undergo a deprotonation at 580 °C under vacuum, leading to a site giving a photoluminescence band at 550 nm attributed to reverse radiative decay from the excited triplet state:

(V⁴⁺–O⁻)^*  (V⁵⁺O²⁻). An activation mechanism of vanadium monomeric species with electrophilic oxygen species is proposed.     

In situ XPS investigations of Cu1−xNixZnAl-mixed metal oxide catalysts used in the oxidative steam reforming of bio-ethanol

A series of CuNiZnAl-multicomponent mixed metal oxide catalysts with various Cu/Ni ratios were prepared by the thermal decomposition of Cu_1−xNi_xZnAl-hydrotalcite-like precursors and tested for oxidative steam reforming of bio-ethanol. Dehydrogenation of EtOH to CH₃CHO is favored by Cu-rich catalyst. Introduction of Ni leads to CC bond rupture and producing CO, CO₂ and CH₄. H₂ yield (selectivity) varied between 2.6–3.0 mol/mol of ethanol converted (50–55%) for all catalysts at 300 °C. The above catalysts were subjected to in situ XPS studies to understand the nature of active species involved in the catalytic reaction. Core level and valence band XPS as well as Auger electron spectroscopy revealed the existence of Cu²⁺, Ni²⁺ and Zn²⁺ ions on calcined materials. Upon in situ reduction at reactions temperatures, the Cu²⁺ was fully reduced to Cu⁰, while Ni²⁺ and Zn²⁺ were partially reduced to Ni⁰ and Zn⁰, respectively. On reduction, the nature of ZnO on Cu-rich catalyst changes from crystalline to amorphous, relatively inert and highly stabilized electronically. Relative concentration of the Ni⁰ and Zn⁰ increases upon reduction with decreasing Cu-content. Valence band results demonstrated that the overlap between 3d bands of Cu and Ni was marginal on calcined materials, and no overlap due to metallic clusters formation after reduction. Nonetheless, the density of states at Fermi level increases dramatically for Ni-rich catalysts and likely this influences the product selectivity.     

Dechlorination of chlorinated hydrocarbons by catalytic steam reforming

Past research in this laboratory on catalytic steam reforming of chlorinated hydrocarbons demonstrated extremely high levels of destruction (0.99999+) at 600–750 °C, with GHSVs as high as 2.5 × 10⁵ h⁻¹. Feasible operation was demonstrated with chlorinated alkanes, alkenes, aromatics and PCBs using Pt/γ-Al₂O₃ catalysts. The major mechanism for deactivation with trichloroethylene was sintering of the γ-Al₂O₃ support and encapsulation of Pt crystallites.

Evidence is presented here that ZrO₂ is a superior support for steam reforming of trichloroethylene (TCE), due to its low acidity and ability to store oxygen. Formulations of 0.8 wt.% Pt/ZrO₂ tested at a GHSV of 20,000 h⁻¹ and a H₂O/C ratio of 20 operated for 42 days at 750 °C, with only slight carbon deposits in the first 15% of the catalyst bed. No pyrolysis was found, and the product CO/CO₂ ratio was at equilibrium, indicating high water gas shift activity with very low CO concentrations. Kinetic measurements revealed a pseudo-first order rate equation, sintering of the support and Pt was much less than with γ-Al₂O₃ supports, and no encapsulation was detected. Slow deactivation occurred due to deposition of catalytic carbon. This carbon was removed by combustion with air, and the rate of deactivation indicated the 42-day run would have lasted seven months.     

Hexagonal mesoporous silicas with and without Zr as supports for HDS catalysts

Hexagonal mesoporous materials (HMS) with and without zirconium were used as supports for preparation of HDS catalysts. The catalysts prepared by modification of the supports with 12-phosphomolybdic acid (HPMo) were characterised by nitrogen adsorption, IR spectroscopy, TPD of ammonia, TPR and catalytic activity in thiophene hydrodesulphurisation. Parent silicas showed mesoporous structure with BET surface area between 1200 and 800 m² g⁻¹, pore size diameter around 3.3 nm and acidity around 0.3 mmol NH₃ g⁻¹. Molybdenum catalysts possess stronger acidity than the supports used. The catalytic activities in hydrodesulphurisation of thiophene of the molybdenum containing catalysts prepared with HMS were higher than the activity of the catalyst prepared with amorphous silica. Higher acidity of Zr–HMS supports lead to lower stability in the thiophene conversion.     

水热处理和钨添加对低钒催化剂高温脱硝性能的影响

利用真空过量浸渍法在TiO₂和WO₃-TiO₂载体上负载0.5%钒（质量分数）制备了V₂O₅-TiO₂ （VTi）和V₂O₅/WO₃-TiO₂（VWTi）催化剂,并在含有10%（体积分数）水蒸气的空气氛下750℃水热处理24h得到处理态的VTi-A和VWTi-A催化剂。研究了水热老化和钨掺杂对催化剂脱硝效率的影响,并通过XRD、TEM、NH₃-TPD、H₂-TPR、XPS和Raman等表征方法对催化剂的结构和表面特性进行了表征分析。研究表明,水热处理和钨的协同作用使催化剂中产生更多的聚合态VO_x和低价态钒（V⁴⁺+V³⁺）。这两类活性物质所含有的氧物种具有较强SCR反应活性,可改善VWTi催化剂的高温脱硝性能,即使在酸性位数量减少的情况下仍有提升,这是低钒含量催化剂可满足燃气机组高温脱硝需求的主要原因。     

Engineering the electronic and geometric structure of VOx/BN@TiO2 heterostructure for efficient aerobic oxidative desulfurization

Particle size governs the electronic and geometric structure of metal nanoparticles (NPs), shaping their catalytic performances in heterogeneous catalysis. However, precisely controlling the size of active metal NPs and thereafter their catalytic activities remain an affordable challenge in ultra-deep oxidative desulfurization (ODS) field. Herein, a series of highly-efficient VO_x/boron nitride nanosheets (BNNS)@TiO₂ heterostructures, therein, cetyltrimethylammonium bromide cationic surfactants serving as intercalation agent, BNNS and MXene as precursors, with various VO_x NP sizes were designed and controllably constructed by a facile intercalation confinement strategy. The properties and structures of the prepared catalysts were systematically characterized by different technical methods, and their catalytic activities were investigated for aerobic ODS of dibenzothiophene (DBT). The results show that the size of VO_x NPs and V⁵⁺/V⁴⁺ play decisive roles in the catalytic aerobic ODS of VO_x/BNNS@TiO₂ catalysts and that VO_x/BNNS@TiO₂-2 exhibits the highest ODS activity with 93.7% DBT conversion within 60 min under the reaction temperature of 130 °C and oxygen flow rate of 200 mL·min^–1, which is due to its optimal VO_x dispersion, excellent reducibility and abundant active species. Therefore, the finding here may contribute to the fundamental understanding of structure-activity in ultra-deep ODS and inspire the advancement of highly-efficient catalyst.     

Fe2O3对V2O5-WO3/TiO2催化剂表面性质及其性能的影响

催化剂是选择性催化还原(SCR)脱硝技术的核心,研究Fe对钒钛系SCR催化剂脱硝活性及SO₂/SO₃转化率的影响具有重要意义。采用等体积浸渍法制备了不同Fe/V质量比的Fe₂O₃-V₂O₅-WO₃/TiO₂催化剂,并进行表征,研究Fe对钒钛系SCR催化剂脱硝活性及SO₂/SO₃转化率的影响,并讨论Fe对于钒钛系SCR催化剂表面性质的影响。结果表明,随着催化剂表面Fe₂O₃含量增加,催化剂的脱硝效率及二氧化硫氧化率均是先上升后下降,当Fe/V质量比为3.0时,催化剂的脱硝效率和二氧化硫氧化率均达到最大值91.78%、1.01%。XPS及H₂-TPR结果表明,随着Fe₂O₃含量增加,催化剂表面钒活性组分的相对含量及V⁴⁺/V⁵⁺比减小,催化剂表面吸附氧(O_α)浓度增加,催化剂的氧化能力增强。NO-TPD结果表明,随着Fe₂O₃含量增加,催化剂表面吸附NO的能力增强。     

Kinetic studies of CH4 oxidation over Pt–NiO/δ-Al2O3 in a fluidised bed reactor

The oxidation of CH₄ over Pt–NiO/δ-Al₂O₃ has been studied in a fluidised bed reactor as part of a major project on an autothermal (combined oxidation–steam reforming) system for CH₄ conversion. The kinetic data were collected between 773 and 893 K and 101 kPa total pressure using CH₄ and O₂ compositions of 10–35% and 8–30%, respectively. Rate–temperature data were also obtained over alumina-supported monometallic catalysts, Pt and NiO. The bimetallic Pt–NiO system has a lower activation energy (80.8 kJ mol⁻¹) than either Pt (86.45 kJ mol⁻¹) and NiO (103.73 kJ mol⁻¹). The superior performance of the bimetallic catalyst was attributed to chemical synergy. The reaction rate over the Pt–NiO catalyst increased monotonically with CH₄ partial pressure but was inhibited by O₂. At low partial pressures (<30 kPa), H₂O has a detrimental effect on CH₄ conversion, whilst above 30 kPa, the rate increased dramatically with water content.     

Vanadium oxide supported on mesoporous Al2O3: Preparation, characterization and reactivity

The application of different techniques (diffuse reflectance-UV–vis, ⁵¹V NMR, FT-IR of adsorbed pyridine and TPR-H₂) in the characterization of vanadia supported on mesoporous Al₂O₃ catalysts shows that the nature of the vanadium species depends on the V-loading. At V-content lower than 15 wt.% of V-atoms (30% of the theoretical monolayer), vanadium is mainly in a tetrahedral environment. Higher V-contents in the catalyst leads to the formation of octahedral V⁵⁺ species and V₂O₅-like species. Both XRD and textural results indicate that the mesoporous structure of the support is mostly maintained after the vanadium incorporation, and therefore high surface areas were obtained on the final catalysts. Al₂O₃-suppported vanadia catalysts are active and selective in the oxidative dehydrogenation of ethane, although the catalytic behavior depends on the V-loading. High rates of formation of ethylene per unit mass of catalyst per unit time have also been observed as a consequence of the high dispersion of V-atoms on the surface of the support.