Ammoxidation of methylaromatics over vanadium phosphate catalysts. I. Effect of the size, position and electronic properties of substituents on the catalytic conversion of methylaromatics to the corresponding nitriles

The heterogeneous-catalytic ammoxidation of several mono- and disubstituted methylaromatics and heteroaromatics to their corresponding nitriles was studied. The reaction was carried out on an ammonium oxovanadium pyrophosphate ((NH₄)₂[(VO)₃(P₂O₇)₂]) generated during the catalytic process. The study shows that the catalytic activity and the nitrile selectivity depend mainly on the position and the electronic nature of the particular substituent due to a changed accessibility of the methyl group as well as a possible different electronic stabilization.     

Ammoxidation of propane to acrylonitrile on V---Sb---O catalysts. Role of ammonia in the reaction pathways

Transient experiments in the vacuum on the ammoxidation of propane over VSb₅O_x- and VSb₅O_x(30 wt%)/Al₂O₃ catalysts were performed. Additionally, the interaction of ammonia with the catalysts was investigated by means of TPD and DRIFT spectroscopic studies. The reported results reveal that short lived NH_x.-species, most probably NH_3,ads or NH⁺₄, are active species in the formation of acrylonitrile from propane. These species are also involved in the formation of the non-selective side-product N₂, which is formed via the intermediates NO and N₂O.     

非均相Photo-Fenton反应中催化剂及其光催化机理研究进展

非均相Photo-Fenton反应体系具有反应效率高、有效pH范围宽广以及催化剂可再生利用等优势,是一项极具发展潜力的新型高级氧化工艺。本文简单介绍了近几年来有关非均相Photo-Fenton体系中负载型光催化剂的最新研究成果,主要包括以SiO_2、Al_2O_3、分子筛、碳基材料、黏土材料等无机载体,以及以离子交换树脂和有机纤维材料等有机载体制备的负载型光催化剂,并且在此基础上着重讨论了光催化机理。对目前非均相Photo-Fenton反应存在的一些问题以及未来发展方向作了总结与展望。     

Defined vanadium phosphorus oxides and their use as highly effective catalysts in ammoxidation of methyl aromatics

The heterogeneous catalytic ammoxidation of methyl aromatics and methyl hetero aromatics is a preferred method for the synthesis of aromatic and hetero aromatic nitriles. The resulting nitriles are valuable intermediates for the production of dyestuffs, pesticides, pharmaceuticals and other chemical products. Usually, the ammoxidation is carried out using V-containing oxides (e.g. V/Ti, V/Sn, V/Mo) promoted by further transition metals as catalysts. However, we have shown recently that defined vanadium phosphates (VPO) revealed an excellent ammoxidation performance. The ammoxidation of toluene was used as model reaction. An intense characterisation of some VPO catalysts under pre-treatment and working conditions by the use of various in situ-methods has thrown more light in formation and stability of the used VPO solids as well as the effect of different reactants (ammonia, toluene) on VPO surface and bulk properties.     

Reaction pathway of benzonitrile formation during toluene ammoxidation on vanadium phosphate catalysts

The reaction pathway of the ammoxidation of toluene on (VO)₂P₂O₇ used as catalyst and the interaction of potential intermediates with the pyrophosphate were studied by spectroscopic techniques (FTIR, EPR), temperature-programmed chemisorptions/ reactions (TPD, TPRS) and transient studies such as the temporal analysis of products (TAP) technique. NH₃ is chemisorbed on the catalyst surface, forming three different species, i.e., NH ₄ ⁺ ions located on BrØnsted sites, coordinatively bound NH₃ on Lewis sites and NH ₂ ^– groups, presumably P-NH₂. Toluene that is probably adsorbed on Lewis sites reacts in a first step to a benzyl radical. A subsequent partial oxidation by interaction of V^IV=O groups generates a V...O=CH-C₆H₅ surface structure. This benzaldehyde-like surface species reacts with adsorbed NH₃ according to a Langmuir-Hinshelwood mechanism. TAP experiments on ammonium-containing vanadium phosphates revealed that NH ₄ ⁺ ions could act as potential N-insertion species. No formation of benzylamine as well as the generation of V=NH surface groups as possible intermediates or N-insertion sites were observed.     

Aromatization of propane over GaHMFI catalysts. Reaction scheme, nature of the dehydrogenating species and mode of coke formation

The kinetic modelling of propane aromatization over a Ga/HMFI catalyst and the comparison of Ga₂O₃-HMFI mixtures with different compositions has allowed us to specify the respective roles of dehydrogenating species and of protonic sites. Propane aromatization over Ga/HMFI catalyst occurs mainly through a conventional bifunctional reaction scheme (with desorbed intermediates) in which the dehydrogenating gallium species are responsible for dehydrogenation of propane, of oligomers and of cyclic olefins and diolefins while protonic sites are responsible for oligomerization, cyclization and cracking. This bifunctional transformation is kinetically limited by propane dehydrogenation. Unfortunately, gallium species also catalyze alkane cracking and hydrogenolysis as well as alkene hydrogenation with formation of the undesired products methane and ethane. From the comparison of the catalytic properties of MFI gallosilicates calcined under air flow at various temperatures, it can be concluded that tetrahedral gallium is inactive in dehydrogenation reactions. Dehydrogenation occurs through a bifunctional mechanism involving the joint participation of gallium oxide (extra framework species) and of protonic sites. Coke formed during propane aromatization causes a blockage of the access to the protonic sites of the zeolite which is responsible for deactivation. Ga decreases the coking/aromatization rate ratio of HMFI and facilitates the coke removal through oxidative treatment, both effects being more pronounced after Ga/HMFI pretreatment at high temperature under hydrogen because of the better dispersion of the Ga species.     

In situ-electron spin resonance: a useful tool for the investigation of vanadium phosphate catalysts (VPO) under working conditions

Differently prepared (VO)₂P₂O₇ phases and an amorphous V³⁺PO catalyst were investigated under conditions of the selective n-butane oxidation and the toluene ammoxidation, respectively, using a self-constructed in situ-ESR flow reactor in the X-band. By examining the temperature dependence and the line shape of the ESR signals exchange integrals as well as the 2nd and the 4th moment were obtained. These parameters characterize the spin—spin exchange behaviour and, thus, structural and electronic disorder of the catalysts. Increasing structural disorder was found to improve the catalytic performance in the n-butane oxidation. For both catalytic processes a significant reversible alteration of the ESR line shape was observed under working conditions which is discussed in terms of a perturbation of exchange interactions between neighbouring vanadyl centres near the surface the oxidation state of which is assumed to fluctuate between +4 and +5 during the catalytic reaction.     

Role and importance of oxidized nitrogen oxide adspecies on the mechanism and dynamics of reaction over copper-based catalysts

The formation of nitrate and NO₂ adspecies over Cu/MFI and copper-on-alumina catalysts and their role in the mechanism of reaction is discussed on the basis of FT-IR results and catalytic tests in unsteady-state conditions. Three specific cases are discussed: (i) reduction of NO by propane/O₂ over Cu/MFI, (ii) conversion of NO by NH₃/O₂ over copper-on-alumina catalysts and (iii) oxygen-promoted reduction of NO in the absence of reductants over Cu/MFI. The formation of nitrate species leads to self-deactivation, but Cu²⁺-NO₂ like adspecies are suggested to be a key intermediate in the reduction of NO to N₂ in all three cases examined.     

Ethylbenzene isomerization over bifunctional platinum alumina–EUO catalysts: Location of the active sites

The transformation of ethylbenzene (EB) was carried out on intimate mixtures of Pt/Al₂O₃ (PtA) and HEUO (Si/Al = 15) catalysts with different composition under the following conditions: fixed bed reactor, temperature 683 K, pressure of hydrogen and ethylbenzene equal to 8 and 2 bar respectively, weight hourly space velocity between 5 and 60. Products result from the following transformations of ethylbenzene or/and of the reaction products: the bifunctional desired isomerization of EB into xylenes (X), the acidic reactions of disproportionation, dealkylation of EB (followed by ethylene hydrogenation) and EB–X transalkylation, the hydrogenation of EB followed by ethylcyclohexane isomerization and cracking. The selectivity to isomers, which is very low on the fresh catalysts, increases significantly during an initial period of fast deactivation, essentially at the expense of disproportionation and dealkylation by products. This deactivation as well as the increase in the selectivities to isomers were shown to result from the blockage by carbonaceous deposits of the access to the inner sites of the EUO micropores. The selectivity to isomers of EB transformation over these inner sites is very low, which could be due to the trapping of ethylcyclohexene intermediates within the monodimensional channels of EUO zeolite. In contrast, over the stabilized catalysts for which only the protonic sites of large side pockets located on the outer surface can be active, the selectivity to isomers is very high, which could be related to the environment of these sites as well as to their easy access by cycloalkene intermediates.     

Synthesis, Characterization and Ammoxidation Functionality of Silica Supported Vanadium Incorporated Ammonium Salt of 12-Molybdophosphoric Acid Catalyst

A series of silica supported vanadium incorporated ammonium salts of 12-molybdophosphoric acid (AMPV) catalysts, with 5–25 wt% of active component were prepared. These catalysts were characterized by XRD, FTIR, BET-Surface Area, TGA/DTA and acidity by potentiometry techniques. The characterization data derived from XRD, FT-IR techniques reveal that the AMPV on silica support exists in the form of Keggin structure. These catalysts are studied for the vapor phase ammoxidation of 2-methyl pyrazine (MP) to cyanopyrazine (CP). The conversion of methyl pyrazine increased with increase in loading on silica showing a maximum value of conversion at 20 wt% AMPV loading. The ammoxidation activity of these catalysts with and without vanadium is studied and the vanadium incorporation led to substantial improvement in the cynopyrazine selectivity.     

Decane reforming reaction over Pt,Ir, Pt-Ir and Pt-Ni bimetallic catalysts supported on Y-zeolite

Pt, Ir, Pt-Ir and Pt-Ni bimetallic catalysts supported on NaY- and HY-zeolite were examined as a catalyst for producing gasoline from n-decane via simultaneous reforming and cracking. The catalysts were prepared by calcining and reducing metal-ion-exchanged Y-zeolite with O₂ and H₂ at 300°C., respectively. Thus prepared catalysts were characterized by hydrogen chemisorption and temperature programmed desorption of ammonia. Pt-Ni/NaY and Pt-Ir/NaY bimetallic catalysts offered the improved activity maintenance compared to Pt/NaY monometallic catalyst. The catalysts supported on HY-zeolite showed higher selectivity toward C₅–C₇ and skeletal isomers of C₅–C₇- and C₈–C₁₀ than those of the catalysts supported on NaY-zeolite, which is a desired characteristic for increasing octane value of gasoline these days. However, deactivation with reaction time was much more pronounced on HY-zeolite-supported catalyst. When the catalyst was prcsulfided with H,S, the stability with time on stream was enhanced and the selectivity was quite different from that of the catalyst before presulfiding. The acidity of Y-zeolite and presulfiding of catalyst greatly influenced the activny, selectivity and stability of Pt, Ir, Pt-Ir and Pt-Ni bimetallic catalysts supported on Y-zeolite in n-decane reforming reaction.     

Particle size distribution and its effect on the olefin polymer reactor dynamics over olefin polymerization catalysts with multiple active sites

Mass and energy balances in a reactor have been derived to study the effect of particle size distribution for multiple active site catalyst systems on the reactor dynamics. It was found that multiple active sites in a Ziegler-Natta catalyst affect neither the reactor dynamics nor the particle size distribution, as opposed to a system which uses single site catalysts. It was discovered that a simple reaction model with a single type of active site dominant adequately explains the reactor dynamics and the particle size distributions for a continuous stirred-bed reactor for polymerization of propylene over a Ziegler-Natta catalyst.     

On the reaction mechanism of ammoxidation of propane over vanadium containing oxide catalysts

Vacuum transient pulse and continuous flow investigations on the ammoxidation of propane over VSb_xO_y (x = 1, 2 and 5) and (VO)₂P₂O₇ catalysts reveal that propene and acrolein are reaction intermediates over both types of oxides. However, the N-insertion step and the formation of total oxidation products proceed significantly different. Short-lived NH₃ or NH₄ ⁺ species are involved in the N-insertion step over V-Sb-O, whereas strongly bound, long-lived NH_x species are participating in the reaction on V-P-O. Only lattice oxygen is involved in the hydrocarbon reaction pathway over V-Sb-O, whereas additionally adsorbed oxygen species lead to total oxidation over V-P-O. This revised version was published online in July 2006 with corrections to the Cover Date.     

Studies of oxygen reduction reaction active sites and stability of nitrogen-modified carbon composite catalysts for PEM fuel cells

A non-precious nitrogen-modified carbon composite (NMCC) catalyst is synthesized by the pyrolysis of cobalt, iron-ethylenediamine-chelate complexes on silica followed by chemical and pyrolysis treatments. Pyrolysis temperature and time have a remarkable impact on the content and the type of the nitrogen-containing functional groups in the NMCC catalysts, which affect their catalytic activity and stability. Based on the analysis of the nitrogen functional groups before and after the stability tests, the ORR active sites of the NMCC catalysts are proposed to be pyridinic-N and quaternary-N functional groups. However the pyridinic-N group is not stable in the acidic environment due to the protonation reaction.     

Oxydehydrogenation of propane over Mg-V-Sb-oxide catalysts. II. Reaction kinetics and mechanism

Recently we reported that Mg₄V₂Sb₂O_x is selective for propane andn-butane Oxydehydrogenation at low hydrocarbon conversion, and that propane is oxidized in parallel reactions to propylene and CO_x. We report now on the kinetics of propane and propylene oxidations over this catalyst. The partial oxidations of propane and propylene and zero-order in oxygen, whereas deep oxidations of both hydrocarbons are half-order. This difference in reaction order indicates that different forms of reactive oxygen are involved in the partial and deep oxidation reactions. Presumably, nucleophilic lattice oxygen partakes in the partial oxidation, while electrophilic dissociatively adsorbed oxygen is involved in deep oxidation. A single activated surface adsorbed state of the hydrocarbons is thought to be involved in both the partial and deep oxidation reactions. An interpretation of the observed reaction kinetics in context of the Mg₄V₂Sb₂O_x solid state chemistry, and the partial oxidation literature in general, suggests that selective oxydehydrogenation of propane occurs on isolated (Sb-O-V-O-Sb) sites, deep oxidation on multiple vicinal vanadium sites (Sb-O-V-O-V-O-Sb), and partial oxidation of propylene to acrolein on subsurface V-promoted antimony sites (Sb-O-Sb). Therefore, unproved selectivity of desired intermediates (propylene/acrolein) should be achieved by further lowering the vanadium concentration and/or through key solid state positioning of the vanadium in the catalyst lattice. Alternatively, selective doping to electronically decrease the electrophilicity of the waste forming sites and its appended oxygen should also help depress the waste forming reaction channels in favor of the desired partial oxidation channels. Finally it is anticipated that higher useful product yields would be attained with a compositionally optimized Mg-V-Sb-oxide catalyst by opting for a more stable, isolatable intermediate, e.g., acrylonitrile, by reacting propane in the presence of ammonia and oxygen (air) over this catalyst.     

废SCR催化剂钠化焙烧回收钨和钒的机理探究

文章从热力学角度,分别以温度和碳酸钠加入量为变量,用热力学软件HSC Chemistry计算了蜂窝状废SCR催化剂钠化焙烧的过程,得到了相应的相图,并分析了其对钨和钒浸出率的影响。研究了不同动力学条件下钨和钒的浸出率,得到最佳焙烧条件为：碳酸钠加入量为30%、焙烧温度为800 ℃、粒度为75～100 μm、焙烧时间为2～2.5 h。采用XRD和SEM进行物相和形貌分析。从理论和实验上探究了失效SCR催化剂钠化焙烧过程的机理。     

Oxidative dehydrogenation of ethane to ethylene over alumina-supported vanadium oxide catalysts: Relationship between molecular structures and chemical reactivity

The influence of vanadium oxide loading in the supported VO_x/Al₂O₃ catalyst system upon the dehydrated surface vanadia molecular structure, surface acidic properties, reduction characteristics and the catalytic oxidative dehydrogenation (ODH) of ethane to ethylene was investigated. Characterization of the supported VO_x/Al₂O₃ catalysts by XPS surface analysis and Raman spectroscopy revealed that vanadia was highly dispersed on the Al₂O₃ support as a two-dimensional surface VO_x overlayer with monolayer surface coverage corresponding to 9 V/nm². Furthermore, Raman revealed that the extent of polymerization of surface VO_x species increases with surface vanadia coverage in the sub-monolayer region. Pyridine chemisorption-IR studies revealed that the number of surface Brønsted acid sites increases with increasing surface VO_x coverage and parallels the extent of polymerization in the sub-monolayer region. The reducibility of the surface VO_x species was monitored by both H₂-TPR and in situ Raman spectroscopy and also revealed that the reducibility of the surface VO_x species increases with surface VO_x coverage and parallels the extent of polymerization in the sub-monolayer region. The fraction of monomeric and polymeric surface VO_x species has been quantitatively calculated by a novel UV–Vis DRS method. The overall ethane ODH TOF value, however, is constant with surface vanadia coverage in the sub-monolayer region. The constant ethane TOF reveals that both isolated and polymeric surface VO_x species possess essentially the same TOF value for ethane activation. The reducibility and Brønsted acidity of the surface VO_x species, however, do affect the ethylene selectivity. The highest selectivity to ethylene was obtained at a surface vanadia density of 2.2 V/nm², which corresponds to a little more than 0.25 monolayer coverage. Below 2.2 V/nm², exposed Al support cations are responsible for converting ethylene to CO. Above 2.2 V/nm², the enhanced reducibility and surface Brønsted acidity appear to decrease the ethylene selectivity, which may also be due to higher conversion levels. Above monolayer coverage, crystalline V₂O₅ nanoparticles are also present and do not contribute to ethane activation, but are responsible for unselective conversion of ethylene to CO. The crystalline V₂O₅ nanoparticles also react with the Al₂O₃ support at elevated temperatures via a solid-state reaction to form crystalline AlVO₄, which suppresses ethylene combustion of the crystalline V₂O₅ nanoparticles. The molecular structure–chemical characteristics of the surface VO_x species demonstrate that neither the terminal VO nor bridging VOV bonds influence the chemical properties of the supported VO_x/Al₂O₃ catalysts, and that the bridging VOAl bond represents the catalytic active site for ethane activation.     

Surface science studies of Ziegler-Natta olefin polymerization system: Correlations between polymerization kinetics, polymer structures, and active site structures on model catalysts

The surface composition and structure of model Ziegler-Natta catalysts, polymerizing α-olefins to produce polyolefins, have been studied using modern surface science techniques and compared with their polymerization behaviors. Two types of thin films — TiCl_x/MgCl₂ and TiCl_y/Au — were fabricated on an inert gold substrate, using chemical vapor deposition methods, to model the high-yield catalysts of MgCl₂-supported TiCl₄ and TiCl₃-based catalysts, respectively. The model catalysts could be activated by exposure to triethylaluminum (AlFt₃) vapor. Once activated, both catalysts were active for polymerization of ethylene and propylene in the absence of excess AlEt₃ during polymerization. The model catalysts had polymerization activities comparable to the high-surface-area industrial catalysts. Though both catalysts were terminated with chlorine at the surface, each catalyst assumed different surface structures. The TiCl_x/MgCl₂ film surface was composed of two structures: the (001) basal plane of these halide crystallites and a non-basal plane structure. The TiCl_y/Au film surface assumed only the non-basal plane structure. These structural differences resulted in different tacticity of the polypropylene produced with these catalysts. The TiCl_x/MgCl₂ catalyst produced both atactic and isotactic polypropylene, while the TiCl_y/Au catalyst without the MgCl₂ support produced exclusively isotactic polypropylene. The titanium oxidation state distribution did not have a critical role in determining the tacticity of the polypropylene.     

甲醇羰基合成碳酸二甲酯反应机理及多相催化剂研究进展

针对甲醇羰基合成碳酸二甲酯（DMC）反应体系,在综合分析以氯化亚铜、氯化铜以及铜分子筛为催化剂的氧化羰基化反应机理的基础上,着重介绍了多相催化剂的种类、结构、活性中心以及制备工艺的研究进展,并对各类催化剂的催化性能进行了分析和评价。通过溶胶-凝胶技术和分子筛包覆的方法制备的钴系催化剂,其活性和稳定性得到了提高;钯铜系催化剂经过载体改性和加入适当的助剂,甲醇转化率增大并在一定程度上抑制了氯离子的流失;无氯铜系催化剂避免了对设备的腐蚀,但催化活性和DMC的选择性普遍较低。最后指出利用各种现代表征手段以及分析技术研究表面过程深刻了解甲醇羰基合成机理,才能在催化剂的设计和开发上取得创新和突破。