丙烷在负载型V2O5/Zr3(PO4)4催化剂上的氧化脱氢

制备了无定型的磷酸锆Ｚｒ３（ＰＯ４）４载体,采用浸渍法在载体上负载０６％～６０％的Ｖ２Ｏ５．所制备的催化剂在丙烷氧化脱氢反应中具有较好的催化性能,如３０％Ｖ２Ｏ５／Ｚｒ３（ＰＯ４）４催化剂在丙烷转化率为１７０％时,丙烯选择性可达５３８％,丙烯收率达９１％．考察了不同反应条件下催化剂的性能．ＸＲＤ、ＩＲ和Ｒａｍａｎ光谱表明,Ｖ２Ｏ５在Ｚｒ３（ＰＯ４）４载体上主要是以高度分散的钒氧物种存在;ＥＳＲ分析结果证明催化剂中存在Ｖ４＋物种,表明Ｖ５＋／Ｖ４＋参与了氧化还原反应．     

Transient and steady state investigation of selective and non-selective reaction pathways in the oxidative dehydrogenation of propane over supported vanadia catalysts

Mechanistic aspects of the formation of C3H6, CO and CO2 in the oxidative dehydrogenation of propane over VOx/gamma-Al2O3 materials have been investigated by means of steady state and transient isotopic tests. The materials possessed highly dispersed and polymerised VOx species as well as bulk-like V2O5. Propene was primarily formed via oxidative dehydrogenation of propane by lattice oxygen of VOx species. It was suggested that non-selective consecutive propene oxidation is initiated by the breaking of the C-C bond in the molecule by the lattice oxygen, forming formaldehyde as a side product, which is further oxidised to CO and CO2. The following order of initial steady state propene selectivity (at a zero degree of propane conversion) as a function of the nature of VOx species was established: a mixture of bulk-like V2O5 and polymerised VOx>polymerised VOx>highly dispersed VOx species. The low propene selectivity over highly dispersed VOx species was explained by the fact that these species do not fully cover the bare acidic surface of gamma-Al2O3 where propene adsorption and further oxidation take place. Thus, two different locations of COx formation were considered: (i) in the vicinity of acidic sites of the support and (ii) on VOx species. The propene selectivity over samples possessing polymerised VOx species and bulk-like V2O5 strongly decreased with an increasing degree of propane conversion. Contrarily, highly dispersed VOx species showed the lowest ability for consecutive propene oxidation.     

Structural characterization and oxidative dehydrogenation activity of V2O5/Ce(x)Zr(1-x)O2/SiO2 catalysts

The thermal stability of a nanosized Ce(x)Zr(1-x)O2 solid solution on a silica surface and the dispersion behavior of V2O5 over Ce(x)Zr(1-x)O2/SiO2 have been investigated using XRD, Raman spectroscopy, XPS, HREM, and BET surface area techniques. Oxidative dehydrogenation of ethylbenzene to styrene was performed as a test reaction to assess the usefulness of the VOx/Ce(x)Zr(1-x)O2/SiO2 catalyst. Ce(x)Zr(1-x)O2/SiO2 (1:1:2 mol ratio based on oxides) was synthesized through a soft-chemical route from ultrahigh dilute solutions by adopting a deposition coprecipitation technique. A theoretical monolayer equivalent to 10 wt % V2O5 was impregnated over the calcined Ce(x)Zr(1-x)O2/SiO2 sample (773 K) by an aqueous wet impregnation technique. The prepared V2O5/Ce(x)Zr(1-x)O2/SiO2 sample was subjected to thermal treatments from 773 to 1073 K. The XRD measurements indicate the presence of cubic Ce0.75Zr0.25O2 in the case of Ce(x)Zr(1-x)O2/SiO2, while cubic Ce0.5Zr0.5O2 and tetragonal Ce0.16Zr0.84O2 in the case of V2O5/Ce(x)Zr(1-x)O2/SiO2 when calcined at various temperatures. Dispersed vanadium oxide induces more incorporation of zirconium into the ceria lattice, thereby decreasing its lattice size and also accelerating the crystallization of Ce-Zr-O solid solutions at higher calcination temperatures. Further, it interacts selectively with the ceria portion of the composite oxide to form CeVO4. The RS measurements provide good evidence about the dispersed form of vanadium oxide and the CeVO4 compound. The HREM studies show the presence of small Ce-Zr-oxide particles of approximately 5 nm size over the surface of amorphous silica and corroborate with the results obtained from other techniques. The catalytic activity studies reveal the ability of vanadium oxide supported on Ce(x)Zr(1-x)O2/SiO2 to efficiently catalyze the ODH of ethylbenzene at normal atmospheric pressure. The remarkable ability of Ce(x)Zr(1-x)O2 to prevent the deactivation of supported vanadium oxide leading to stable activity in the time-on-stream experiments and high selectivity to styrene are other important observations.     

Photocatalytic oxidation of propylene with molecular oxygen over highly dispersed titanium, vanadium, and chromium oxides on silica

Photocatalytic oxidation of propylene with molecular oxygen at room temperature was investigated over various silica-supported metal oxides with low loading. The photocatalytic active site is assumed to be the isolated tetrahedrally coordinated metal oxides in the ligand-to-metal charge-transferred state, such as (Mdelta- -OLdelta+). Photocatalytic epoxidation of propylene into propylene oxide was promoted over silica-supported V and Ti oxides at steady state. Over silica-supported Cr oxide, the propylene oxide formation rate was remarkably decreased with the time course in the reaction. The oxidation state and the coordination environment of the supported Ti, V, and Cr oxide species were determined by diffuse reflectance UV-vis spectroscopy (DRS) and electron spin resonance (ESR). During the photocatalytic oxidation, the oxidation state of the Ti4+ species was not varied. On the other hand, the V5+ species was partially reduced to V4+ and the Cr6+ species was successively reduced to Cr5+ and Cr3+. An isotopic tracer study of the C3H6-18O2 reaction suggests the difference of the active oxygen species between TiO2/SiO2 and V2O5/SiO2. The active oxygen species on TiO2/SiO2 is derived from molecular oxygen. On the other hand, the photogenerated products on V2O5/SiO2 incorporate the lattice oxygen of the surface metal oxide species. It is suggested that the kinds of terminal ligand (hydroxyl or oxo) of the tetrahedrally coordinated metal oxides on silica decide the active oxygen species in the photocatalytic oxidation. A photoinduced hole center on the monohydroxyl (SiO)3Ti-OH species activates molecular oxygen that reacts with propylene. In the case of the monooxo (SiO)3V=O and dioxo (SiO)2Cr=O2 species, the photoactivated lattice oxygen (OL-) directly reacts with propylene.     

高能机械球磨法制备V-Ti-O超细微粒催化剂

负载型V2O5/TiO2氧化物催化剂因具有优良的催化性能而广泛地用于烃类选择性氧化[1,2]和氮氧化物选择性催化还原(SCR)[3]. 迄今,所研究的负载型V2O5/TiO2氧化物催化剂大多是采用浸渍法制备的[2,4～6],通过调整催化剂的组成[2,5]、引入适当的助剂组分[2,6]和选择适宜的反应操作条件[2,5]可进一步优化其催化性能.     

VOx/SBA-15催化剂上甲苯气相部分氧化

采用等体积浸渍法制备了不同负载量的VOx/SBA-15催化剂。UV-Vis和H2-TPR等表征结果表明,在较低钒负载量下,钒物种的分散程度较高,主要以孤立的VO4 3－以及少量聚合体V－O－V形式存在;钒负载量较高时会有大量的聚合体V－O－V甚至晶相V2O5出现,而且,催化剂的酸性随着钒物种的高度分散而降低。甲苯气相部分氧化反应结果表明,随着钒负载量的提高,苯甲醛的选择性先升后降,CO、CO2等选择性逐渐提高。这是由于催化剂存在大量的聚合体V－O－V和晶相V2O5时,聚集态钒物种表面较多的酸量促使苯甲醛深度氧化。在相同钒负载量下,催化剂VOx/SBA-15的钒物种分散状态优于VOx/MCM-41和VOx/SiO2,从而使得催化剂VOx/SBA-15呈现较高的苯甲醛选择性。     

Dispersion state and catalytic properties of vanadia species on the surface of V_2O_5/TiO_2 catalysts

The dispersion state and catalytic properties of anatase-supported vanadia species are studied by means of X-ray diffraction (XRD), laser Raman spectroscopy (LRS), H2 temperature-programmed reduction (TPR) and the selective oxidation of o-xylene to phthalic anhydride. The almost identical values of the experimental dispersion capacity of V2O5 on anatase and the surface vacant sites available on the preferentially exposed (001) plane of anatase suggest that the highly dispersed vanadium cations are bonded to the vacant sites on the surface of anatase as derived by the incorporation model. When the loading amount of V2O5 is far below its dispersion capacity, the dispersed vanadia species might mainly consist of isolated VOx species bridging to the surface through V-O-Ti bonds. With the increase of V2O5 loading the isolated vanadia species interact with their nearest neighbors (either isolated or polymerized vanadia) through bridging V-O-V at the expenses of V-O-Ti bonds, resulting in the increase of the ra     

On the Role of Vanadia Species for VOx／SiO2 in the Selective Oxidation of Methane

Various VOx/SiO2 catalysts were prepared by the methods of physical mixing, conventional wetness impregnation and ultrasonication-assistant impregnation. The catalysts were characterized by XRD, UV-Vis DRS, Raman, TPR, ESR and TPSR techniques and the nature of the vanadium species were correlated to their catalytic performance in the reaction of direct conversion of methane to formaldehyde. It is concluded that highly dispersed monomeric and low oligomeric vanadia species are formed on the sample prepared with both traditional wetness impregnation method and ultrasonication-assistant impregnation, whereas in the latter case, the amount of oligomeric vanadia species is much smaller. The V2O5 microcrystallines are the dominant species on the material prepared by physical mixing method. During the selective oxidation of methane, Vv species are reduced to V^IV paramagnetic species and both microcrystalline V205 species and oligomeric vanadia species are found to further disperse and transform into tetrahedral vanadia species. Based on the results of UV Raman spectroscopy and TPSR, C02 is suggested to be formed via two different routes, in which one is from the sequence reaction of CH4→ HCHO→ CO → CO2 over monomeric vanadia species, and the other is from the direct oxidation of methane to CO2 over oligomeric vanadia species. Oligomeric vanadia species is more active than monomeric vanadia species for methane activation.     

异丁烷部分氧化V-P-O/SiO2催化剂的研究

The V2O5/SiO2 and V2O5-P2O5/SiO2 catalysts were prepared by the Sol-gel method. The surface composition,structure,chemisorption and reactivity of the catalysts were systematically studied by using the techniques of XRD,TPR,IR,TPD and icro-reactor. The results showed that the active component was dispersed uniformly on the surface of amorphous SiO2 . There existed Lewis base sites（V=O and V-O-V）and Lewis acid sites （Vn+）on the surface of these catalysts. An addition of PO43- could lower the activity of surface terminal oxygen of Lewis base sites V=O. Iso-butane could chemisorbed on surface terminal oxygen of Lewis base sites V=O by one or two H atoms in -CH3 to form molecular adsorption states. Reaction products of i-C4H10 on the V-P-O/SiO2 catalyst were mainly i-C4H8,MAL,MAA and COx . Conversion of i-C4H10 was 5% and selectivity of partial oxidation products is above 72% at 300℃. The addition of PO43- could improve the selectivity of partial oxidation products on the VSiO catalyst because PO43- could reduce the activity of lattice oxygen in V = O bond,and weaken the adsorption intensity of i-C4H10 .     

Selective photocatalytic oxidation of light alkanes over alkali-ion-modified V2O5/SiO2; kinetic study and reaction mechanism

Alkali-ion-modified silica-supported vanadium oxides are photocatalysts available in the field of photooxidation of light alkanes using molecular oxygen. The photooxygenated reaction was promoted over the catalyst under irradiation at steady state. Acetone formation on the photooxidation of propane was investigated over a rubidium-ion-modified silica-supported vanadium oxide that is the most effective catalyst. The kinetic analysis demonstrated that the rate-determining step is the reaction of propane on the lattice oxygen of the photoexcited VO4Rb species to yield the vanadium isopropoxide species. It was suggested that the photocatalytic active sites are occupied by photogenerated acetone during photoreaction at 333 K. Heating the photocatalyst bed drastically enhanced not only product yield but also the selectivity to propionaldehyde that is a minor product in the photooxidation at 333 K. The product distribution of photoassisted oxidation of propane was described by Boltzmann's distribution of stabilization energy of the intermediates: an isopropoxide-like one for the precursor of acetone and an n-propoxide-like one for the precursor of propionaldehyde.     

V-P-O／TiO2-SiO2催化剂的制备、表征及其在丙烷氧化制取丙烯酸反应中的应用研究

通过湿磨法,将钒-磷-氧(V-P-O)氧化物负载于钛硅干凝胶(TiO_2-SiO_2)上,制备了V-P-O/TiO_2-SiO_2氧化物催化剂,并运用BET,XRD,TEM及XPS等技术考察了润湿剂,V-P-O的含量及焙烧条件对催化剂结构的影响.由BET方法发现,以异丁醇为润湿剂对催化剂表面积影响较小.XRD和TEM考察结果表明,所制备的催化剂有较高的热稳定性.通过XPS分析,证实钒元素主要以V(Ⅳ)价态存在于催化剂的表面.V-P-O/TiO_2-SiO_2催化剂在丙烷氧化反应中显示了良好的催化性能.     

V2O5-MoO3-SiO2表面复合氧化物催化剂的制备与表征

采用表面改性法制备了V2O5-SiO2,MoO3-SiO2,V2O5-MoO3-SiO2复合氧化物催化剂,并用TPR和IR技术研究了催化剂的表面结构及V=O,M0=O的活性,同时用化学吸附IR技术研究了催化剂样品对异丁烷的化学吸附性能.实验结果表明:这些复合氧化物催化剂对异丁烷都有较高的化学吸附能力;SiO2能缓解表面Lewis碱位V=O和Mo=O的氧化能力,有利于选择氧化.     

Preparation and characterization of vanadium(IV) oxide supported on SBA-15 and its catalytic performance in benzene hydroxylation to phenol using molecular oxygen

Preparation of dispersed transition metal oxides catalyst with low oxidation state still remains a challenging task in heterogeneous catalysis.In this study,vanadium oxides supported on zeolite SBA-15 have been prepared under hydrothermal condition using V 2 O 5 and oxalic acid as sources of vanadium and reductant,respectively.The structures of samples,especially the oxidation state of vanadium,and the surface distribution of vanadium oxide species,have been thoroughly characterized using various techniques,including N 2-physisorption,X-ray diffraction(XRD),transmission electron microscopy(TEM),X-ray photoelectron spectroscopy(XPS),UV-visible spectra(UV-Vis) and UV-visible-near infrared spectra(UV-Vis-NIR).It is found that the majority of supported vanadium was in the form of vanadium(IV) oxide species with the low valence of vanadium.By adjusting hydrothermal treatment time,the surface distribution of vanadium(IV) oxide species can be tuned from vanadium(IV) oxide cluster to crystallites.These materials have been tested in the hydroxylation of benzene to phenol in liquid-phase with molecular oxygen in the absence of reductant.The catalyst exhibits high selectivity for phenol(61%) at benzene conversion of 4.6%,which is a relatively good result in comparison with other studies employing molecular oxygen as the oxidant.     

Atomic XAFS as a tool to probe the reactivity of metal oxide catalysts: quantifying metal oxide support effects

The potential of atomic XAFS (AXAFS) to directly probe the catalytic performances of a set of supported metal oxide catalysts has been explored for the first time. For this purpose, a series of 1 wt % supported vanadium oxide catalysts have been prepared differing in their oxidic support material (SiO2, Al2O3, Nb2O5, and ZrO2). Previous characterization results have shown that these catalysts contain the same molecular structure on all supports, i.e., a monomeric VO4 species. It was found that the catalytic activity for the selective oxidation of methanol to formaldehyde and the oxidative dehydrogenation of propane to propene increases in the order SiO2 < Al2O3 < Nb2O5 < ZrO2. The opposite trend was observed for the dehydrogenation of propane to propene in the absence of oxygen. Interestingly, the intensity of the Fourier transform AXAFS peak decreases in the same order. This can be interpreted by an increase in the binding energy of the vanadium valence orbitals when the ionicity of the support (increasing electron charge on the support oxygen atoms) increases. Moreover, detailed EXAFS analysis shows a systematic decrease of the V-Ob(-M(support)) and an increase of a the V-O(H) bond length, when going from SiO2 to ZrO2. This implies a more reactive OH group for ZrO2, in line with the catalytic data. These results show that the electronic structure and consequently the catalytic behavior of the VO4 cluster depend on the ionicity of the support oxide. These results demonstrate that AXAFS spectroscopy can be used to understand and predict the catalytic performances of supported metal oxide catalysts. Furthermore, it enables the user to gather quantitative insight in metal oxide support interactions.     

Elucidation of the molecular structure of hydrated vanadium oxide species by X-ray absorption spectroscopy: correlation between the V...V coordination number and distance and the point of zero charge of the support oxide

The effect of the point of zero charge (PZC) of the support oxide (Al(2)O(3), Nb(2)O(5), SiO(2) and ZrO(2)) on the molecular structure of hydrated vanadium oxide species has been investigated with EXAFS spectroscopy for low-loaded vanadium oxide catalysts. It was found that the degree of clustering (i.e., the V[dot dot dot]V coordination number) and the V...V distance increase with decreasing PZC of the support oxide; i.e., Al(2)O(3) (8.7) < ZrO(2) (7) < Nb(2)O(5) (3.3) < SiO(2) (2). Upon hydration the silica-supported vanadium oxide exhibited a clear alteration in the position of the oxygen atoms surrounding the central vanadium atom and the number of oxygen atoms around vanadium increased to five. In contrast, only minor changes in the molecular structure were detected for the alumina-, niobia- and zirconia-supported vanadium oxide catalysts. Based on a detailed analysis of the EXAFS data a semi-quantitative distribution of vanadium oxide species present on the surface of the different support oxides can be obtained, which is in good agreement with earlier characterization studies primarily making use of Raman spectroscopy.     

CO2氧化丙烷脱氢制丙烯V-Cr/SBA-15催化剂的研究

以介孔分子筛SBA-15为载体，V和Cr为活性组分，采用浸渍法制备了不同V和Cr质量分数的V-Cr/SBA-15催化剂，研究了其对二氧化碳氧化丙烷制丙烯反应的催化性能，采用XRD、BET、TPR等分析测试技术对催化剂的结构进行了表征。结果表明，催化剂中V组分的质量分数较大时以V2O5物相存在，Cr组分以Cr2O3物相存在，它们对SBA-15分子筛的介孔特征影响不大；V、Cr单组分和双组分催化剂都具有较好的CO2氧化丙烷脱氢制丙烯的催化性能，V和Cr质量分数相同的双组分催化剂比单组分催化剂具有更高的催化活性；在V-Cr/SBA-15催化剂中，V和Cr之间存在一定的相互作用，进而改变了催化剂的氧化还原性能，提高了催化剂的催化性能。     

Temperature-programmed desorption study of the selective oxidation of alcohols on silica-supported vanadium oxide

The partial oxidation of methanol and ethanol on silica-supported vanadium oxide catalysts was studied using temperature-programmed desorption (TPD), Raman spectroscopy, and diffuse reflectance infrared spectroscopy (DRIFTS). Methanol TPD results for V2O5/SiO2 samples as a function of vanadia loading in conjunction with X-ray diffraction data and Raman spectra indicated that dispersed vanadia on silica agglomerates into vanadia crystallites during a CH3OH TPD experiment. For ethanol-dosed samples, agglomeration of the dispersed vanadia was less severe, and it was possible to measure the activation energy for the dehydrogenation of adsorbed ethoxides to produce CH3CHO. Assuming a preexponential factor of 10(13) s(-1), the activation energy for this reaction was estimated to be 132 kJ/mol. The results of this study further demonstrate that there is a relatively weak interaction between vanadia and silica and suggest that adsorbed methoxide species help facilitate agglomeration of dispersed vanadia.     

VOx/SiO2-Al2O3催化剂催化苯羟基化制苯酚

以硅溶胶为硅源、偏铝酸钠为铝源及环己胺合成SiO2-Al2O3复合氧化物,并采用水热合成法合成了VOx/SiO2-Al2O3催化剂。采用XRD、SEM、TEM、XPS、NH3-TPD、N2物理吸/脱附等手段对此催化剂和复合氧化物微观结构进行表征分析。由表征结果可知钒活性中心能够高分散于此复合氧化物表面,VOx/SiO2-Al2O3催化剂呈现层片积雪状;当钒的含量为4%时,催化剂中V4+含量最大且总酸量契合于此反应所需要的催化酸环境。探究了4%VOx/SiO2-Al2O3催化剂对苯羟基化制苯酚反应的催化性能,结果表明：基于高分散活性中心、V4+含量最大及载体酸性环境等优点而表现出良好的催化性能,苯转化率为48.1%,苯酚选择性为99.6%。     

Molecular structure of a supported VO4 cluster and its interfacial geometry as a function of the SiO2, Nb2O5, and ZrO2 support

The influence of the support oxide on the molecular structure of a VO(4) cluster and its interfacial geometry has been determined for SiO(2), Nb(2)O(5), and ZrO(2) as supports. Raman, IR, UV-vis-NIR diffuse reflectance, electron spin resonance, and extended X-ray absorption fine structure (EXAFS) spectroscopies were used to characterize the supported vanadium oxide clusters after dehydration. It has been found that for all supports under investigation the vanadium ion is tetrahedral coordinated and consists of one V=O and three V-O bonds. For a VO(4)/SiO(2) catalyst it has been established that only one O neighbor is shared with the SiO(2) support via a V-O(b)-Si(support) bond with an angle of approximately 101 degrees (+/-0.5 degrees ) and a V...Si distance of 2.61 A. The absence of a second vanadium atom in the vicinity of the vanadium oxide cluster further subverts the classical assignment of the 920 cm(-1) Raman band to a V-O-V related vibration. The EXAFS results combined with structural modeling using Cerius(2) software lead to structural constraints, which imply a similar V-O(b)-M(support) interaction for Nb(2)O(5) and ZrO(2) as well. The V-O(b) and the V...M(support) distances depend on the geometry of each support surface. The results show that the classical model with three V-O(b)-M(support) bonds could not be experimentally observed with EXAFS under the applied measuring conditions. Additional experiments with IR and Raman spectroscopy under experimental conditions mimicking those of the EXAFS measurements reveal the presence of V-OH groups, giving further support for the presence of a O=V(OH)(2)-O(b)-M moiety at the support surface.     

Ir/SiO2上甲烷部分氧化制合成气反应的原位时间分辨红外和原位Raman光谱表征

采用原位时间分辨红外光谱和原位显微Raman光谱技术对Ir/SiO₂上甲烷部分氧化(POM)制合成气反应的初级产物和反应条件下催化剂表面物种进行了跟踪考察,实验结果表明,在H₂预还原的新鲜Ir/SiO₂表面,CO是V(CH₄):V(O₂):V(Ar)=2:1:45混合气反应的初级产物,因而甲烷的直接氧化过程是CO生成的主要途径;而在稳态反应条件下,CO生成的途径可能主要来自CO₂和H₂O与催化剂表面积碳物种(CHx)和/或CH₄的反应.催化剂上生成的积碳可能是导致稳态条件下Ir/SiO₂上POM反应机理不同于H₂预还原的新鲜催化剂的主要原因.