A comparative study on the effects of alkali metal on the structures, physico-chemical properties and catalytic behaviors of silica-supported vanadium catalysts for the selective oxidation of ethane and the complete oxidation of diesel soot was reported.

The redox and acid–base characters of silica-supported vanadium or alkali-modified vanadium catalysts by varying vanadium loading were studied with the methods of H2-TPR, NH3-TPD and CO2-TPD combining other structural characterization techniques of ESR, UV–vis spectroscopy. The effects of these properties on their catalytic performances for the ethane oxidation by oxygen were investigated. For both the unpromoted V/SiO2 (V:Si = x:Si) and promoted Cs–V/SiO2 (Cs:V:Si = 1:x:100) systems, the reduction extent and the reducibility increase with increasing vanadia loading and the presence of cesium enhances the reduction extent. The structures of the vanadyl species have large effect on the vanadia reducibility and the isolated surface vanadyl species are less reducible than the polymeric vanadyl species including microcrystalline vanadia. For the samples with high vanadia loading (≥2.0%), the reducibility is high and thus the redox characteristic of the catalyst is one of the most important factors that govern the catalytic reactivity. In the samples with low vanadia loading (<0.5%), the reducibility is low. Therefore, the basicity or acidity characteristics of the catalyst become the major factors that control the catalytic reactivity. The cesium promoted samples Cs–V/SiO2 (2.0 and 10.0%) exhibit high reducibilities, which qualitatively suggests that their oxygen mobility is very high and may result in deep oxidation of ethane. In contrast, those samples with low reducibility are selective for the oxidation of ethane including ODH to ethylene and oxygenate formation.

The nanometric La2-xKxCuO4 oxide catalysts with K2NiF4-type structure were prepared by auto-combustion method using citric acid as a ligand and an adjusting agent of particle-size and morphology. The structures and physico-chemical properties of these perovskite-like oxides were examined by means of XRD, FT-IR, H2-TPR and chemical analysis. The catalytic activities for the simultaneous removal of soot and NOx were evaluated by a technique of the temperature-programmed oxidation reaction (TPO). In the La2-xKxCuO4 catalysts, the partial substitution of K for La at A-site leads to the increase of the concentrations of Cu3+ and oxygen vacancy. Thus, it enhances the catalytic activity for simultaneous removal of NOx and diesel soot, and the optimal substitution amount of potassium x is equal to 0.5 among these samples. T10, T50, T90 are 376, 438,487 ℃ and PN2 is 22%, respectively, for simultaneous removal of NOx and soot particulates over the La1.5K0.5CuO4 catalyst under loose contact conditions between the catalyst and soot.

The CrHZSM-5 catalysts with trace amount of Cr were firstly used for catalytic cracking of isobutane, and the effect of Crloading on the catalytic performances of CrHZSM-5 catalysts for the cracking of isobutane was also studied. The results suggested that when the loading of Cr in the CrHZSM-5 catalysts was less than 0.038 mmol/g Cr, especially at Cr loading of 0.004 mmol/g, both the reactivity of isobutane cracking and the selectivity to light olefins of CrHZSM-5 samples were greatly enhanced compared with the unpromoted HZSM-5, and very high yields of olefins(C2+C3) and ethylene were obtained. For instance, the yield of olefins(C2+C3) and ethylene reached 56.1% and 30.8%, respectively, at 625 ℃ when 0.004 mmol/g Cr was loaded on HZSM-5sample.

Vanadium oxides supported on γ-Al2O3, SiO2, TiO2, and ZrO2 were studied on their molecular structures and reactive performances for soot combustion. To investigate the effect of different alkali metals on the structures and reactivities of supported-vanadium oxide catalysts, they were doped into the V4/TiO2 catalyst which had the best intrinsic activity for soot combustion in the selected supported vanadium oxide catalysts. The experimental results demonstrated that the catalytic properties of these catalysts depended on the vanadium loading amount, support nature, and the presence or the absence of alkali metals. The spectroscopic analysis (FT-IR and UV–vis) and H2-TPR results revealed that the higher activity of alkali-promoted vanadium oxide catalysts could be related to the ability of alkali metal promoting the redox cycle of the active vanadyl species. TG results showed that adding alkali to Vm/TiO2 catalyst was beneficial to lowering their melting points. Low melting points could ensure the good surface atom migration ability, which would improve the contact between the catalyst and soot. Due to the alkali metal components promoting the redox ability and the mobility of the catalysts, alkali-modified vanadium oxide catalysts could remarkably improve their catalytic activities for soot combustion. The catalytic activity order for soot combustion followed Li > Na > K > Rb > Cs in the catalyst system of alkali-V4/TiO2, and the reason why it followed this sequence was discussed.

Fe-modified HZSM-5 catalysts were firstly used for catalytic cracking of isobutane, and the influence of Fe-loading on the catalytic performances of FeHZSM-5 catalysts for the cracking of isobutane was also investigated. The results indicated that both the isobutane cracking activity and the selectivity to light olefins of FeHZSM-5 samples with a small amount of Fe were greatly enhanced compared with the unpromoted HZSM-5, and very high yields of total olefins and propylene were obtained. For example, for the FeHZSM-5 sample loading with 0.010 mmol/g Fe the total yield of olefins reached 65.6%, and the yield of propylene was 32.4% at 625℃.

Bulk V-Nb-O, Mo-Nb-O, Te-Nb-O, and V-Mo-Te-Nb-O mixed metal oxides were synthesized and characterized with Raman spectroscopy, XRD, and BET methods. The interaction of the V and Mo cations with the Nb2O5 lattice followed three stages: (1) cations were initially incorporated into the Nb2O5 lattice forming a solid solution or compound as well as on the Nb2O5 surface, (2) a two-dimensional surface cation overlayer was formed after saturation of the solid solution, and (3) microcrystalline metal oxide phases (e.g., V2O5 and MoO3) were formed after completion of the two-dimensional surface cation monolayer. The catalytic properties of these bulk mixed metal oxides were investigated for the oxidative dehydrogenation (ODH) of propane to propylene, and their activity follows the trend: V-Nb-O > Mo-Nb-O>>Nb2O5 > Te-Nb-O. The highest propane conversions and propylene yields were found when the two-dimensional surface metal oxide monolayers were formed, which suggests that the surface metal oxide species are the surface active sites in these bulk mixed metal oxide catalysts for propane ODH. Furthermore, the number of surface active sites present in the bulk mixed metal oxides was determined by comparative studies between the bulk mixed metal oxides and the corresponding model Nb2O5-supported metal oxides. These numbers can be used further for the calculation of the TOF (turn-over-frequency) values and quantitative comparison of the catalytic behavior of the different bulk mixed metal oxide catalysts for propane ODH. The catalytic results over the model Nb2O5-supported metal oxides demonstrate that the propane ODH reaction is structure insensitive because the TOF is independent of the number and the structure of surface active sites. The composition and calcination temperature of the bulk mixed metal oxide catalysts affects the surface density of the active sites, which controls their catalytic behavior for propane ODH.

Selective oxidation of ethane by oxygen was examined over silica catalysts supporting cesium and bismuth. Aldehydes of C1–C3 were obtained with total selectivity of about 40% at ethane conversion lower than 5%. The catalytic performance was strongly dependent upon the bismuth loading on the catalysts. The higher turn-over-rate and the higher selectivity to aldehydes were observed at the Bi loadings lower than 0.5%. Characterization of the catalysts showed that isolated or highly dispersed Bi on the catalyst surface was indispensable for the high aldehyde yield. Cesium also plays an important role for the high ethane reactivity and the high aldehyde selectivity. A reaction pathway was proposed in which ethane is oxidized into acetaldehyde, but not through ethylene, and acrolein is formed through a cross-condensation of acetaldehyde and formaldehyde.

The oxidation of ethane by oxygen was studied over silica catalysts supporting different amounts of vanadium with and without cesium. Three different catalytic properties of the product selectivity were observed, aldehyde formation, oxidative dehydrogenation (ODH), and combustion, depending upon the vanadium loading amount and the presence or the absence of cesium. A very low loading of vanadium (V:Si=0.02–0.1 at.%) and the addition of Cs (Cs:Si=1 at.%) on silica were found to be important for the formation of aldehyde. Not only acetaldehyde but also acrolein were observed in the aldehyde formation from ethane. On the other hand, catalysts with medium and high vanadium loadings (V:Si=0.5–20 at.%) gave a dehydrogenated product, ethene, when Cs was not added to the catalysts. The addition of cesium to the catalysts with medium and high vanadium loadings changed the catalytic property from ODH to combustion. The different types of vanadyl species were identified by UV–visible and IR measurements in samples with different vanadium loadings. It was estimated that isolated vanadyl species with tetrahedral coordination, which were found mainly on the catalysts with vanadium loading lower than 0.5 at.%, became the active site for the aldehyde formation through the interaction with Cs. As a plausible reaction path giving acrolein from ethane, cesium-catalyzed cross-condensation between acetaldehyde and formaldehyde, formed in the reaction, was proposed. Polymeric vanadyl species with octahedral coordination and vanadium–oxygen clusters with dioxo tetrahedral coordination were detected in the samples with medium (0.5–5.0 at.%) and high (10 and 20 at.%) vanadium loadings, respectively. Both species show the ODH catalytic property without cesium, but they bring about a deep oxidation of ethane if cesium is added to the catalysts.

Many samples containing different elements M in the system of M/SiO2 (M:Si=1:1000, molar ratio) or Cs/M/SiO2 (Cs : M : Si=10:1:1000, molar ratio) were prepared and screened for the oxidation of ethane by use of oxygen as oxidant. It has been found that the elementsM(M=V, Bi, In, Ga, P, Zr, Zn, La) can give good aldehyde yields in the Cs/M/SiO2 system. The promoting effects of different alkali metals (Li, Na, K, Rb, Cs) on the catalytic performance of V/SiO2 (V:Si=1:1000) in ethane oxidation were investigated. Among them, cesium gave the best promoting effect on V/SiO2 for aldehyde formation. The presence of alkali metals increases the basicity and neutralizes the acid site of catalyst; thus it enhances the selectivity to acetaldehyde and controls the formation of formaldehyde. Increase in basicity promotes the cross-aldol condensation of acetaldehyde and formaldehyde to give acrolein. The pathway for acrolein formation is mainly through the cross-aldol condensation of acetaldehyde and formaldehyde over Cs/V/SiO2 catalysts.