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.     

Role of supported metals in the selective reduction of nitrogen monoxide with hydrocarbons over metal/alumina catalysts

The promoting effect of supported metals on alumina catalyst was investigated for the reduction of nitrogen monoxide in oxygen-rich atmospheres. For NO reduction with propene over impregnated CoO/A1₂O₃, the first reaction step was found to be the oxidation of NO to NO₂ probably catalyzed by dispersed cobalt species. The next reaction step, which is the reaction of NO₂ with propene to form N₂, was considered to take place on the alumina surface. Although the activity of impregnated FeO/A1₂O₃ was low because of the presence of large iron oxide particles catalyzing propene oxidation with dioxygen, FeO/A1₂O₃ prepared with sol-gel method showed excellent deNO_x activity.     

Synthesis of cyclic carbonates from epoxides: Use of reticular oxygen of Al2O3 or Al2O3-supported CeOx for the selective epoxidation of propene

Reticular oxygen of Al₂O₃ or CeO_x supported on Al₂O₃ was used for the epoxidation of propene without any double bond cleavage. In batch reaction, Al₂O₃ alone was able to convert propene into propene oxide (PO) with 100% selectivity and 2% conversion of propene with a close to 3:1 ratio with respect to the number of Al(III) reduced to elemental Al. When Ce₂O₃/Al₂O₃ or CeO₂/Al₂O₃ was used, Al remained in its +3 oxidation state, while the Ce oxide was the oxidant as demonstrated by XPS analyses. CeO_x/Al₂O₃ was more active (propene conversion yield of 4–5%) but the selectivity was lower (70%) as PO was isomerized into acetone and propionaldehyde.

Interestingly the use of reticular oxygen very much improves the selectivity with respect to the use of pure O₂. In fact, while propene was more efficiently oxidized (10%) with O₂ in presence of Al₂O₃ or CeO_x/Al₂O₃, the selectivity was as low as 40% because C1 and C2 products were formed. However, the use of reticular oxygen represents a selective two-step technique for the use of molecular oxygen as oxidant of propene. The used oxides can be re-oxidized and the whole process can be further improved towards higher yields.

PO is quantitatively converted into propene carbonate by reaction with CO₂ in presence of Nb₂O₅.     

MoO3/ZrO2催化剂的丙烷氧化脱氢制丙烯催化性能

以十六烷基三甲基溴化铵为表面活性剂,采用溶剂热法制备系列MoO_3/ZrO_2催化剂,采用H2-TPR、N_2吸附-脱附、X射线衍射等对其进行表征,并评价MoO_3/ZrO_2催化剂的丙烷氧化脱氢制丙烯催化性能。结果表明,MoO_3负载于ZrO_2载体上制备的催化剂催化活性增加,MoO_3负载质量分数为20%的MoO_3/ZrO_2催化剂,在反应温度为600℃时,丙烷转化率27.45%,丙烯选择性44.78%,丙稀收率12.29%。     

A study of the catalytic activity of cobalt–zinc manganites for the reduction of NO by hydrocarbons

In this work the catalytic behaviour of pure zinc manganite, ZnMn₂O₄, and cobalt–zinc manganites for the reduction of NO by propane and propene is reported. The NO and N₂O decomposition as well as the reduction of N₂O by propane and propene were also investigated. The catalysts are prepared starting from carbonate monophasic precursors that are decomposed in air at 973 K for 24 h. In all cases a spinel-like phase is obtained. Pure zinc manganite is an efficient catalyst for the NO reduction with both propane and propene and the selectivity to N₂ and CO₂ was almost one. However the presence of cobalt in the catalyst enhances the catalytic activity, in particular when propene is used as reducing agent of NO. All catalysts are stable up to 873 K upon contacting with the propane containing reactant stream whereas in the case of propene they preserve the original spinel structure up to about 773 K. In fact with propene the catalysts start to lose their stability as the reaction temperature increases above 773 K and disaggregate, by reduction of the spinel framework Mn³⁺ cations to Mn²⁺, forming a complex mixture of ZnO and MnO oxides. Despite the collapsing of the spinel phase, the disaggregated polyphasic catalysts still show a good activity and selectivity. An hypothesis for explaining this unusual behaviour is formulated. Finally, the reaction mechanisms presented in literature are consequently revisited on the basis of the results found in this work.     

Effect of doping of TiO2 support with altervalent ions on physicochemical and catalytic properties in oxidative dehydrogenation of propane of vanadia–titania catalysts

Vanadia phase (one monolayer) was deposited on TiO₂ anatase doped with Ca²⁺, Al³⁺, Fe³⁺ and W⁶⁺ ions and the catalysts thus obtained (VMeTi) were characterized by XPS, work function technique, decomposition of isopropanol (a probe reaction for acido–basic properties) and tested in oxidative dehydrogenation of propane. The doping of the TiO₂ support modifies physicochemical and catalytic properties of the active vanadia phase with respect to the undoped TiO₂. The specific activity in the propane oxydehydrogenation decreases in the order: VFeTi>VWTi>VTi>VAlTi>VCaTi (3), whereas the selectivity to propene follows the sequence: VWTiVTi>VFeTi>VAlTi>VCaTi. This implies that the lower is the surface energy barrier for transfer of electrons from the catalyst to the reacting molecules the higher is the selectivity to the partial oxidation product. It is argued that owing to the decrease in this energy barrier the reoxidation step in the catalytic reaction, involving such a transfer: O₂+4e→2O²⁻ is fast, thus, preventing the presence of intermediate non-selective electrophilic oxygen species on the surface.     

Evaluation of V2O5–WO3–TiO2 and alternative SCR catalysts in the abatement of VOCs

Two different commercial SCR catalysts belonging to the V₂O₅–WO₃–TiO₂ system, and different alternative catalysts based on Mn, Fe, Cr, Al and Ti oxides have been tested in the conversion of VOCs in excess oxygen in a temperature range typical of the SCR process (500–700 K). Propane, propene, isopropanol, acetone, 2-chloropropane and 1,2-dichlorobenzene have been fed with excess oxygen and helium. The industrial catalysts are poorly active in the conversion of propane, giving mainly rise to propene by oxy-dehydrogenation. The conversion of propene is higher with CO as the predominant product. In any case, the oxidation activity depends on the vanadium content of the catalyst. Isopropanol is mainly converted into acetone and propene, while acetone is burnt predominantly to CO. Mn- and Fe- containing systems are definitely more active in the conversion of hydrocarbons and oxygenates, giving rise almost exclusively to CO₂. 2-Chloropropane is selectively dehydrochlorinated to propene and HCl starting from 350 K, propene being later burnt to CO on the industrial V₂O₅–WO₃–TiO₂ catalysts, whose combustion activity is, apparently, not affected by chlorine. On the contrary, chlorine strongly affects the behavior of Mn-based catalysts, that are active in the dehydrochlorination of 2-chloropropane, but are simultaneously deactivated with respect to their combustion catalytic activity. The conversion of 1,2-dichlorobenzene gives rise to important amounts of heavy products in our experimental conditions with relatively high reactant concentration.     

Temperature programmed desorption/surface-reaction study of an anodic alumina supported Ag catalyst for selective catalytic reduction of nitric oxide with propene

On an anodic alumina supported silver catalyst with a low Ag loading (1.68 wt.%), NO_x (NO/He, NO/O₂/He, NO₂/He) adsorption measurements and NO_x-temperature programmed decomposition (TPD)/temperature programmed surface-reaction (TPSR) measurements in different gas streams (He, C₃H₆/He, C₃H₆/O₂/He) were conducted to investigate the formation, consumption and reactivity of surface adsorbed NO_x species.

During NO adsorption, no noticeable uptake of NO was detected. Introducing oxygen greatly improved the formation of ads-NO_x species. A greater quantity of surface nitrate species was found after NO₂ adsorption, accompanied with gaseous NO release. The result of TPSR demonstrates the surface nitrate species can be effectively and preferentially reduced by propene. When introducing oxygen into the propene gas stream of TPSR test, the significantly increased amount of reacted nitrate undoubtedly shows the importance of oxygen in activating propene. The pathway for the selective reduction of NO_x in the presence of excess oxygen is proposed to pass through the selective reduction of the adsorbed nitrate species with the activated propene.

The enhanced NO_x conversion when replacing NO with NO₂ was attributed to the stronger NO_x adsorption capacity and oxidation ability of NO₂, than those for NO. With increasing oxygen concentration, the difference between NO and NO₂ would gradually decrease, and finally disappear in a high excess of oxygen.     

催化剂上丙烷氧化脱氢制丙烯

以浸渍法制备VMo/γ-Al₂O₃和VMoMg/γ-Al₂O₃催化剂,考察其催化丙烷氧化脱氢制丙烯的反应活性,采用XRD、UV-Vis DRS和In suit IR对催化剂进行表征。结果表明,V负载质量分数为3%、Mo负载质量分数为7%时的3V7Mo/γ-Al₂O₃催化剂表现出较好的催化性能;添加Mg后催化剂的催化性能有所改善,反应温度500 ℃时,丙烷转化率为18.19%,丙烯选择性74.76%。丙烷和丙烯在3V7Mo/γ-Al₂O₃和3V₇Mo₄Mg/γ-Al₂O₃催化剂上吸附后,C—H键的H与催化剂活性中心的晶格氧发生作用形成H—O键,且3V₇Mo₄Mg/γ-Al₂O₃催化剂上出现C—O键的温度比3V₇Mo/γ-Al₂O₃催化剂高,表明加入Mg有利于提高丙烯选择性。     

Factors controlling the selectivity of V2O5 supported catalysts in the oxidative dehydrogenation of propane

The surface properties of a series of V₂O₅ catalysts supported on different oxides (Al₂O₃, H–Na/Y zeolite, MgO, SiO₂, TiO₂ and ZrO₂) were investigated by transmission electron microscopy and FTIR spectroscopy augmented by CO and NH₃ adsorption. In the case of the V₂O₅/SiO₂ system TEM images evidenced the presence of V₂O₅ crystallites, whereas such segregated phase was not observed for the other samples. VO_x species resulted widely spread on the surface of Al₂O₃, H–Na/Y zeolite, MgO and SiO₂, whereas on TiO₂ and ZrO₂ they are assembled in a layer covering almost completely the support. Furthermore, evidences for the presence in this layer of V–OH Brønsted acid sites close to the active centres were found. It is proposed that propene molecules primarily produced by oxydehydrogenation of propane can be adsorbed on this acid centres and then undergo an overoxidation by reaction with redox centres in the neighbourhood. This features could account for the low selectivity of V₂O₅/TiO₂ and V₂O₅/ZrO₂ catalysts.     

Removal of NO by absorption and the subsequent reduction method using YBa2Cu3Oy and Ce-ZSM-5

The effectiveness of the ab(ad)sorption and the subsequent reduction (abbreviated as ASR hereafter) system for the removal of highly dilute NO has been examined using a flow-type reaction system. The ASR system comprises two serial processes: first, NO is absorbed into (and/or adsorbed on) the system in the presence of O₂ until saturation, and then the absorbed NO is decomposed, for example, by reduction at a higher temperature in the presence of propene and oxygen. YBa₂Cu₃O_y (YBC) and Ce-ZSM-5 were chosen in this study as materials for absorption and reduction, respectively. Efficient removal of NO was attained when the first layer consisting of a mixture of YBC and Ce-ZSM-5 was combined with the second layer of Ce-ZSM-5, and propene was additionally introduced between the two layers during the reductive decomposition. By this ASR system, the amount of NO absorption reached 1.5 mol per mole of YBC at 573 K and 60% of the NO absorbed was selectively decomposed to N₂ at 623 K in 9 h (N balance, 75%; N₂ selectivity, 79%).     

Reactivity of Ru-based catalysts in the oxidation of propene and carbon black

The relationship between the state of Ru on different supports and catalytic activity in the oxidation of propene and carbon black was investigated for catalysts prepared by different impregnation methods. It is demonstrated that the addition of ruthenium to ceria (CeO₂), alumina (Al₂O₃) and ceria–alumina significantly improves the reactivity: the temperature of carbon black oxidation decreases by 100–140 °C. It is also shown that the addition of Ru to the different supports is very beneficial for the total oxidation of propene. Temperature programmed reduction (TPR) experiments of the catalysts showed that the oxygen species of ruthenium oxides are reduced at low temperatures which is the main reason of its high reactivity in oxidation reactions.     

Catalytic activation of ceramic filter elements for combined particle separation, NOx removal and VOC total oxidation

The development of a catalytically active filter element for combined particle separation and NO_x removal or VOC total oxidation, respectively, is presented. For NO_x removal by selective catalytic reduction (SCR) a catalytic coating based on a TiO₂–V₂O₅–WO₃ catalyst system was developed on a ceramic filter element. Different TiO₂ sols of tailor-made mean particle size between 40 and 190 nm were prepared by the sol–gel process and used for the impregnation of filter element cylinders by the incipient wetness technique. The obtained TiO₂-impregnated sintered filter element cylinders exhibit BET surface areas in the range between 0.5 and 1.3 m²/g. Selected TiO₂-impregnated filter element cylinders of high BET surface area were catalytically activated by impregnation with a V₂O₅ and WO₃ precursor solution. The obtained catalytic filter element cylinders show high SCR activity leading to 96% NO conversion at 300 °C, a filtration velocity of 2 cm/s and an NO inlet concentration of 500 vol.-ppm. The corresponding differential pressures fulfill the requirements for typical hot gas filtration applications. For VOC total oxidation, a TiO₂-impregnated filter element support was catalytically activated with a Pt/V₂O₅ system. Complete oxidation of propene with 100% selectivity to CO₂ was achieved at 300 °C, a filtration velocity of 2 cm/s and a propene inlet concentration of 300 vol.-ppm.     

The activity and characterization of sol–gel Sn/Al2O3 catalyst for selective catalytic reduction of NOx in the presence of oxygen

Catalytic performance of Sn/Al₂O₃ catalysts prepared by impregnation (IM) and sol–gel (SG) method for selective catalytic reduction of NO_x by propene under lean burn condition were investigated. The physical properties of catalyst were characterized by BET, XRD, XPS and TPD. The results showed that NO₂ had higher reactivity than NO to nitrogen, the maximum NO conversion was 82% on the 5% Sn/Al₂O₃ (SG) catalyst, and the maximum NO₂ conversion reached nearly 100% around 425 °C. Such a temperature of maximum NO conversion was in accordance with those of NO_x desorption accompanied with O₂ around 450 °C. The activity of NO reduction was enhanced remarkably by the presence of H₂O and SO₂ at low temperature, and the temperature window was also broadened in the presence of H₂O and SO₂, however the NO_x desorption and NO conversion decreased sharply on the 300 ppm SO₂ treated catalyst, the catalytic activity was inhibited by the presence of SO₂ due to formation of sulfate species (SO₄²⁻) on the catalysts. The presence of oxygen played an essential role in NO reduction, and the activity of the 5% Sn/Al₂O₃ (SG) was not decreased in the presence of large oxygen.     

The mechanism of SO2 effect on NO reduction with propene over In2O3/Al2O3 catalyst

An In₂O₃/Al₂O₃ catalyst shows high activity for the selective catalytic reduction of NO with propene in the presence of oxygen. The presence of SO₂ in feed gas suppressed the catalytic activity dramatically at high temperatures; however it was enhanced in the low temperature range of 473–573 K. In TPD and FT-IR studies, the formation of sulfate species on the surface of the catalyst caused an inhibition of NO_X adsorption sites, and the absorbance ability of NO was suppressed by the presence of SO₂, and the amount of ad-NO₃⁻ species decreased obviously. This leads to a decrease of catalytic activity at higher temperatures. However, addition of SO₂ enhanced the formation of carboxylate and formate species, which can explain the promotional effect of SO₂ at low temperature, because active C₃H₆ (partially oxidized C₃H₆) is crucial at low temperature.     

Effect of rhodium on the properties of bifunctional MxOy/ZrO2 catalysts in the reduction of nitrogen oxides by hydrocarbons

The catalytic properties of transition metal oxides (Cr, Ce, and Co) supported on ZrO₂ synthesized by various methods, as well as the effect of rhodium on the performance of the M_xO_y/ZrO₂ oxide systems in NO reduction with hydrocarbons (methane, propane–butane mixture, and propene) were studied. Scanning electron microscopy, ammonia thermoprogrammed desorption (NH₃-TPD), XPS, and IR spectroscopy were used to study the physicochemical indices of rhodium-promoted M_xO_y/ZrO₂ oxide catalysts. The enhancement of the redox properties of the oxide catalysts upon the introduction of rhodium does not alter their bifunctional nature in SCR activity: these catalysts have both redox and strong acid Brønsted-sites.     

Improvement of catalytic activity and sulfur-resistance of Ag/TiO2–Al2O3 for NO reduction with propene under lean burn conditions

Ag-based catalysts supported on various metal oxides, Al₂O₃, TiO₂, and TiO₂–Al₂O₃, were prepared by the sol–gel method. The effect of SO₂ on catalytic activity was investigated for NO reduction with propene under lean burn condition. The results showed the catalytic activities were greatly enhanced on Ag/TiO₂–Al₂O₃ in comparison to Ag/Al₂O₃ and Ag/TiO₂, especially in the low temperature region. Application of different characterization techniques revealed that the activity enhancement was correlated with the properties of the support material. Silver was highly dispersed over the amorphous system of TiO₂–Al₂O₃. NO₃⁻ rather than NO₂⁻ or NOx reacted with the carboxylate species to form CN or NCO. NO₂ was the predominant desorption species in the temperature programmed desorption (TPD) of NO on Ag/TiO₂–Al₂O₃. More amount of formate (HCOO⁻) and CN were generated on the Ag/TiO₂–Al₂O₃ catalyst than the Ag/Al₂O₃ catalyst, due to an increased number of Lewis acid sites. Sulfate species, resulted from SO₂ oxidation, played dual roles on catalytic activity. On aged samples, the slow decomposition of accumulated sulfate species on catalyst surface led to poor NO conversion due to the blockage of these species on active sites. On the other hand, catalytic activity was greatly enhanced in the low temperature region because of the enhanced intensity of Lewis acid site caused by the adsorbed sulfate species. The rate of sulfate accumulation on the Ag/TiO₂–Al₂O₃ system was relatively slow. As a consequence, the system showed superior capability for selective adsorption of NO and SO₂ toleration to the Ag/Al₂O₃ catalyst.     

Pt/TiO2/ZSM-5催化剂的制备及其催化转化正丁烷

采用溶胶-凝胶法和等体积浸渍法,分别对ZSM-5分子筛进行TiO₂改性和Pt负载,获得了具有脱氢-裂解双功能的Pt/TiO₂/ZSM-5催化剂,采用XRD、N₂吸附-脱附、TEM、XPS和NH₃-TPD对样品的晶体结构,孔结构、形貌、活性金属价态和酸性质等进行了表征,并研究了正丁烷在此催化剂上催化转化制备低碳烯烃的反应规律。研究结果表明,TiO₂的引入,一方面使得改性后的ZSM-5分子筛获得了额外的酸性中心,特别是强酸性位含量的增加,有助于促进正丁烷的活化;另一方面Pt与TiO₂之间存在“金属-载体”强相互作用（SMSI）,在H₂还原气氛下,Pt能够促进TiO₂的还原,生成Ti³⁺物种,而Ti³⁺的存在增加了Pt周围的电荷密度,降低了Pt对低碳烯烃（C₂⁼～C₃⁼）的吸附能力,抑制了深度脱氢和生焦反应,从而提高双功能催化剂对烯烃的选择性。当H₂还原温度为450℃时,Pt/10TiO₂/ZSM-5催化剂在625℃下的正丁烷转化率为76.1%,低碳烯烃（C₂⁼～C₃⁼）收率为50.9%,分别比Pt/ZSM-5催化剂提高了16.7%和12.6%。     

Propane oxydehydrogenation reaction on a VPO/TiO2 catalyst. Role of the nature of acid sites determined by dynamic in-situ IR studies

A VPO/TiO₂ catalyst tested in the oxydehydrogenation reaction (ODH) of propane between 300 and 400°C shows satisfactory performances (up to 80% of propene selectivity at 2% of propane conversion at 300°C or 56% of propene selectivity at 9% of propane conversion at 400°C). Addition of water or pyridine in the feed gas tends to decrease the propane conversion and enhances the propene selectivity. It is shown that water increases the number of Brönsted surface acid sites by dissociative adsorption which, in turn, enhances propene selectivity at the expense of the CO_x selectivity. These results are in good agreement with spectroscopic IR observations performed under catalytic conditions showing that the Lewis acid sites are linked to CO_xformation, whereas it seems that Brönsted sites would rather be linked to propene formation.