丙烷氧化脱氢不同硅基载体负载钒氧化物催化剂的TPSR表征

研究了钒负载不同氧化硅载体(Silica-gel,SBA-15,MCM-41,fumed-SiO2,Nano-SiO2)的丙烷氧化脱氢(ODH)催化剂的结构特征和催化性能,结合催化剂的程序升温表面反应(TPSR)的差热热重质谱(TG-DSC-MS)和原位紫外漫反射光谱(UV-vis DRS)等技术,研究钒在载体上的分散度和晶格氧的反应性。结果表明:负载型钒氧化物催化剂的活性取决于钒在不同硅基载体上的分散度,高度分散的隔离的四配位V5+是丙烷氧化脱氢的活性位。C3H6选择性主要与催化剂的平均孔径相关联,平均孔径越小,产物C3H6越易发生深度氧化。另外,不同氧化硅载体晶格氧与钒的结合强度对C3H6的选择性也产生影响,结合力较弱的V-O-Si中的晶格氧是丙烷氧化脱氢的燃烧位,且燃烧温度随晶格氧与钒、硅结合强度的减小而降低。而与钒结合力较强的V=O和V-O-V中的晶格氧是丙烷氧化脱氢的选择氧化位。硅基载体形貌和结构的不同导致负载型钒氧化物催化剂丙烷氧化脱氢活性和选择性发生差异。     

Molecular mechanism of propane oxidative dehydrogenation on surface oxygen radical sites of VO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/TiO<Subscript>2</Subscript> catalysts

Minimum energy pathways of propane oxidative dehydrogenation to propene and propanol on supported vanadium oxide catalyst VO _x /TiO₂ were studied by periodic discrete Fourier transform (DFT) using a surface oxygen radical as the active site. The propene formation pathway was shown to consist of two consecutive hydrogen abstraction steps. The first step includes C_β–H bond activation of propane followed by the formation of a surface hydroxyl group V–O _t H and a propyl radical n-C₃H₇. This step with the activation energy E* = 0.56 eV (54.1 kJ/mol) appears to be rate-determining. The second step involves the reaction of the bridging O _b oxygen atom with the methylene C–H bond of propyl radical n-C₃H₇ followed by the formation of a hydroxylated surface site HO _t –V⁴⁺–O _b H and propene. The initial steps of the C–H bond activation during propane conversion to propanol and propene by ODH on V⁵⁺–(O _t O _b )^? active sites are identical. The obtained results demonstrate that participation of surface oxygen radicals as the active sites of propane ODH makes it possible to explain relatively low activation energies observed for this reaction on the most active catalysts. The presence of very active radical species in low concentration seems to be the key factor for obtaining high selectivity.     

Non-catalytic pyrolysis of ethane to ethylene in the presence of CO2 with or without limited O2

Influence of the presence of CO₂, which is a mild oxidant, on the performance of the thermal cracking of ethane to ethylene in the absence or presence of limited O₂ at different temperatures (750–900‡C), space velocities (1500–9000 h^-1) and CO₂/C₂H₆ and O₂/C₂H₆ mole ratios (0–2.0 and 0–0.3 respectively) has been investigated. In both the presence and absence of limited O₂, ethane conversion increases markedly because of the presence of CO₂, indicating its beneficial effect on the ethane to ethylene cracking. The increased ethane conversion is, however, not due to the oxidation of ethane to ethylene by CO₂; the formation of carbon monoxide in the presence of CO₂ is found to be very small. It is most probably due to the activation of ethane in the presence of CO₂.     

氧渗透膜反应器的烷烃氧化脱氢

研究了乙烷和丙烷在催化膜反应器中的氧化脱氢反应．所用的膜材料为La2Ni0.9V0.1O4＋δ和Ba0.5Sr0.5Co0.8Fe0.2O3-δ．为了平衡氧气通量和额外的均匀气相反应,实验选择在中温段进行（550或者650℃）．实验结果表明膜的氧渗透通量和膜表面上主要激活烷烃的活性位在获得高的烷烃转化率过程中起决定性作用．乙烷和丙烷的氧化脱氢实验数据均符合Mars-van Krevelen机理,其中烷烃和膜表面的晶格氧反应产生相应的烯烃．同时证明了气相和膜表面的氧浓度是决定烯烃选择性的关键．     

Interconversion of stannylium ions in the C<Subscript>4</Subscript>H<Subscript>11</Subscript>Sn<Superscript>+</Superscript> system

B3LYP method with the LANL2DZ basis for tin and aug-cc-pVDZ basis for carbon and hydrogen were used to obtain the equilibrium geometry of the main (with a positive charge on the tin) isomers in the C₄H₁₁Sn⁺ system and the transition states at their interconversion. As in the case of silicon and germanium, the cations of lighter elements of the 14th group, the most stable isomer is shown to be the tertiary ion, however, the energy of its complexes with ethane and propane is higher only by several kJ mol⁻¹. Nevertheless, the formation of these complexes from the tertiary ion requires overcoming a rather high barrier (293 and 272 kJ mol⁻¹, respectively). The barrier for isomerization of the secondary ion in the ethane complex is somewhat lower (222 kJ mol⁻¹), but still is significantly greater than the energy gained at the appearance of the nucleogenic ion. The most probable transformation pathways of the nucleogenic stannylium ions are the formation of complexes with ethylene, which requires overcoming barriers of 130 and 117 kJ mol⁻¹ for the tertiary and secondary ions, respectively.     

Oxidative Dehydrogenation of Propane over a VO2‐Exchanged MCM‐22 Zeolite: A DFT Study

The adsorption and the mechanism of the oxidative dehydrogenation (ODH) of propane over VO₂‐exchanged MCM‐22 are investigated by DFT calculations using the M06‐L functional, which takes into account dispersion contributions to the energy. The adsorption energies of propane are in good agreement with those from computationally much more demanding MP2 calculations and with experimental results. In contrast, B3LYP binding energies are too small. The reaction begins with the movement of a methylene hydrogen atom to the oxygen atom of the VO₂ group, which leads to an isopropyl radical bound to a HO? V? O intermediate. This step is rate determining with the apparent activation energy of 30.9 kcal mol^?1, a value within the range of experimental results for ODH over other silica supports. In the propene formation step, the hydroxyl group is the more reactive group requiring an apparent activation energy of 27.7 kcal mol^?1 compared to that of the oxy group of 40.8 kcal mol^?1. To take the effect of the extended framework into account, single‐point calculations on 120T structures at the same level of theory are performed. The apparent activation energy is reduced to 28.5 kcal mol^?1 by a stabilizing effect caused by the framework. Reoxidation of the catalyst is found to be important for the product release at the end of the reaction.     

Selective Hydrogen Oxidation in the Presence of C3 Hydrocarbons Using Perovskite Oxygen Reservoirs

Perovskite‐type oxides, ABO₃, can be successfully applied as solid “oxygen reservoirs” in redox reactions such as selective hydrogen combustion. This reaction is part of a novel process for propane oxidative dehydrogenation, wherein the lattice oxygen of the perovskite is used to combust hydrogen selectively from the dehydrogenation mixture at 550 °C. This gives three key advantages: it shifts the dehydrogenation equilibrium to the side of the desired products, heat is generated, thus aiding the endothermic dehydrogenation, and it simplifies product separation (H₂O vs H₂). Furthermore, the process is safer since it uses the catalysts’ lattice oxygen instead of gaseous O₂. We screened fourteen perovskites for activity, selectivity and stability in selective hydrogen combustion. The catalytic properties depend strongly on the composition. Changing the B atom in a series of LaBO₃ perovskites shows that Mn and Co give a higher selectivity than Fe and Cr. Replacing some of the La atoms with Sr or Ca also affects the catalytic properties. Doping with Sr increases the selectivity of the LaFeO₃ perovskite, but yields a catalyst with low selectivity in the case of LaCrO₃. Conversely, doping LaCrO₃ with Ca increases the selectivity. The best results are achieved with Sr‐doped LaMnO₃, with selectivities of up to 93 % and activities of around 150 μmol O m^?2. This catalyst, La_0.9Sr_0.1MnO₃, shows excellent stability, even after 125 redox cycles at 550 °C (70 h on stream). Notably, the activity per unit surface area of the perovskite catalysts is higher than that of doped cerias, the current benchmark of solid oxygen reservoirs.     

负载PtSn金属助剂的镁铝水滑石上的丙烷脱氢反应研究

我们研究了以镁铝水滑石作为载体,利用水滑石层间阴离子的可交换性,负载活性金属铂和锡的丙烷脱氢反应.在镁铝水滑石载体中加入Ga能够影响丙烷脱氢活性,当镓的含量为1%时催化剂丙烷脱氢反应活性最高,反应初始时,丙烷转化率为46.5%,反应2 h后,丙烷转化率仍有37.5%.当以Mg(Ga)(Al)O-1%为载体时,考察了不同H_2/C_3H_8摩尔比对丙烷脱氢活性的影响,结果表明当H_2/C_3H_8摩尔比为0.5∶1时,丙烷脱氢反应具有最佳的反应活性,即当在原料气中加入H_2时,能够使得丙烷脱氢的转化率大幅度提升,且选择性也有所提升.烷烃脱氢是一个吸热反应,同时考察了温度对烷烃脱氢反应性能影响,结果表明温度越高,丙烷脱氢反应具有更高的转化率.对催化剂进行长时间寿命实验考察,发现当反应经过40 h后,丙烷的转化率仍有23.5%,说明Pt Sn-Mg(Ga)(Al)O-1%催化剂具有较好的稳定性.     

Photo–thermo Catalytic Oxidation over a TiO2-WO3-Supported Platinum Catalyst

Photo–thermo catalysis, which integrates photocatalysis on semiconductors with thermocatalysis on supported nonplasmonic metals, has emerged as an attractive approach to improve catalytic performance. However, an understanding of the mechanisms in operation is missing from both the thermo- and photocatalytic perspectives. Deep insights into photo–thermo catalysis are achieved via the catalytic oxidation of propane (C₃H₈) over a Pt/TiO₂-WO₃ catalyst that severely suffers from oxygen poisoning at high O₂/C₃H₈ ratios. After introducing UV/Vis light, the reaction temperature required to achieve 70 % conversion of C₃H₈ lowers to a record-breaking 90 °C from 324 °C and the apparent activation energy drops from 130 kJ mol⁻¹ to 11 kJ mol⁻¹. Furthermore, the reaction order of O₂ is −1.4 in dark but reverses to 0.1 under light, thereby suppressing oxygen poisoning of the Pt catalyst. An underlying mechanism is proposed based on direct evidence of the in-situ-captured reaction intermediates.     

IR Spectra of Ethane Adsorbed on the Hydrogen,Sodium, and Zinc Forms of a Y-Type Zeolite: Interpretation Using <Emphasis Type="Italic">ab initio</Emphasis> Quantum Chemical Calculations

Ethane adsorption on the hydrogen, sodium, and zinc forms of faujasite brings about polarization anisotropy of the C-H stretching vibrations. This anisotropy shows itself most clearly as perturbation of the vibrational mode analogous to the breathing C-H mode ν₁ of free ethane. The relative intensity and the low-frequency shift of the absorption band due to the distorted ν₁ mode increase with increasing perturbation caused by the C₂H₆-adsorption site interaction. The polarizing power of adsorption sites increases in the order H⁺ < Na⁺ < Zn²⁺. The C-H stretching vibrations in ethane adsorbed on the cationic forms of the Y zeolite are not symmetry-forbidden; accordingly, adsorbed ethane gives more absorption bands than gaseous ethane. The interaction between ethane and zinc cations in the Y zeolite structure eliminates not only the symmetry forbiddenness but also the twofold degeneracy of the C-H stretches.__________Translated from Kinetika i Kataliz, Vol. 46, No. 3, 2005, pp. 434–440.Original Russian Text Copyright © 2005 by Pidko, Kazanskii.     

Processing of Oil Refinery Gases: Oxidative Dehydrogenation of the Ethane–Ethylene Fraction

Oxidative transformations of the ethane–ethylene fraction of oil refinery gases, containing 20 vol % C₂H₄, on VMoTeNb oxide catalyst in the temperature interval 330–450°C were studied. Comparison with oxidative transformations of the individual components (oxidative dehydrogenation of C₂H₆ and oxidation of C₂H₄) shows that ethylene does not noticeably influence the ethane conversion, whereas ethane strongly suppresses the ethylene conversion. The maximal yield of ethylene from the ethane–ethylene fraction is close to that reached in oxidative dehydrogenation of ethane under similar conditions and amounts to 70–72%.     

Transformation of Metallic Ti to TiH2 Phase in the Ti/MgH2 Composite and Its Influence on the Hydrogen Storage Behavior of MgH2

The current work explores the in-situ formation of TiH₂ additive in a Ti/MgH₂ nanocomposite system. Mild mechanical milling leaves Ti chemically unchanged, while formation of stable TiH_2–x occurs upon strong mechanical milling. TiH_2–x further transforms to TiH₂ upon recycling the powder (dehydrogenation and subsequent hydrogenation) and lowers the activation energy of MgH₂ to 89.4 kJ (mol H₂)⁻¹ [E_a of as-received MgH₂ is 153 kJ (mol H₂)⁻¹]. This work also reiterates that metallic Ti additive mixed MgH₂ requires strong mechanical milling for better H₂ ab/de-sorption performance. The current observations support the view that lattice strain may be an important factor in the catalysis of additives incorporated MgH₂ hydrogen storage systems.     

Photo–thermo Catalytic Oxidation over a TiO2‐WO3‐Supported Platinum Catalyst

Photo–thermo catalysis, which integrates photocatalysis on semiconductors with thermocatalysis on supported nonplasmonic metals, has emerged as an attractive approach to improve catalytic performance. However, an understanding of the mechanisms in operation is missing from both the thermo‐ and photocatalytic perspectives. Deep insights into photo–thermo catalysis are achieved via the catalytic oxidation of propane (C₃H₈) over a Pt/TiO₂‐WO₃ catalyst that severely suffers from oxygen poisoning at high O₂/C₃H₈ ratios. After introducing UV/Vis light, the reaction temperature required to achieve 70 % conversion of C₃H₈ lowers to a record‐breaking 90 °C from 324 °C and the apparent activation energy drops from 130 kJ mol^?1 to 11 kJ mol^?1. Furthermore, the reaction order of O₂ is ?1.4 in dark but reverses to 0.1 under light, thereby suppressing oxygen poisoning of the Pt catalyst. An underlying mechanism is proposed based on direct evidence of the in‐situ‐captured reaction intermediates.     

Catalytic properties of the VO<Subscript><Emphasis Type="Italic">х</Emphasis></Subscript>/Ce<Subscript>0.46</Subscript>Zr<Subscript>0.54</Subscript>O<Subscript>2</Subscript> oxide system in the oxidative dehydrogenation of propane

Ce_0.46Zr_0.54O₂ solid solution prepared using a cellulose template was employed as a carrier for vanadium catalysts of the oxidative dehydrogenation of propane. The properties of VO _х /Ce_0.46Zr_0.54O₂ catalyst (5 wt % vanadium) are compared with the properties of the neat support. The carrier and catalyst are studied by means of BET, SEM, DTA, XRD, and Raman spectroscopy. It is shown that the CeVO₄ phase responsible for the ODH process is formed upon interaction between vanadate ions and cerium ions on the surface of the solid solution. The catalytic properties of the catalyst and the support are studied in the propane oxidation reaction at temperatures of 450 and 500°C with pulse feeding of the reagent. It is found that the complete oxidation of propane occurs on the support with formation of CO₂ and H₂O. Three products (propene, CO₂, and H₂O) form in the presence of the vanadium catalyst. It is suggested that there are two types of catalytic centers on the catalyst’s surface. It is concluded that the centers responsible for the complete oxidation of propane are concentrated mainly on the carrier, while the centers responsible for propane ODH are on the CeVO₄.     

SBA-15 负载 β-Mg2V2O7 催化剂的制备及其催化丙烷选择氧化脱氢的性能

 以 MgO 修饰的 SBA-15 为载体, 采用浸渍法制备了负载 β-Mg2V2O7 催化剂, 并运用 X 射线衍射、拉曼光谱、紫外-可见漫反射光谱和 H2 程序升温还原等技术对催化剂 V 中心的结构和还原性能进行了表征. 结果表明, β-Mg2V2O7 具有与 α-Mg2V2O7 相同的结构单元, 但其催化丙烷氧化脱氢 (ODH) 反应的初始活性和初始选择性均低于后者. 与体相 β-Mg2V2O7 相比, 负载的 β-Mg2V2O7 上 V 中心分散度以及丙烷 ODH 反应活性和选择性更高, 520 oC 时丙烷 ODH 反应的初始活性提高了约 20 倍, 丙烯初始选择性也从体相的 88.3% 提高到 94.1%, 接近于 α-Mg2V2O7 (94.6%), 并且在 20% 的丙烷转化率时也表现出相似的规律. 这与表征催化剂选择性的两个本征动力学参数 k1/k2 (丙烷初级 ODH 和燃烧反应速率常数之比) 和 k3/k1 (次级丙烯燃烧和初级丙烷 ODH 反应速率常数之比) 反映出的规律一致. 这些对体相和负载的 Mg2V2O7 催化剂催化丙烷 ODH 反应本征特性的认识将有助于设计合成更高效的 Mg-V-O 催化剂, 如基于 α-Mg2V2O7 结构的高分散催化剂, 以获得更高的丙烷 ODH 反应活性和选择性.     

Spectroscopic identification of adsorption properties of Zn2+ ions at cationic positions of high-silica zeolites with distant placing of aluminium ions

For Zn²⁺ cations in ZnZSM-5 zeolite unusual type of cationic positions, formed by two distantly placed framework aluminium atoms, is considered. Some extent of structural destabilization of cations in these cationic positions in comparison with traditional localization should result in promoted Lewis activity and adsorption activity of these sites. The last ones are manifested in the significantly increased IR low frequency shifts for adsorbed molecules and in their ability for heterolytic dissociation at elevating temperature. DFT cluster quantum chemical modeling of light alkane adsorption on Zn²⁺ in ZnZSM-5 zeolites confirms these conjectures in full agreement with recent experiments. Similar to the previously considered dihydrogen and methane molecule adsorption, we present here the calculations of ethane molecular and dissociative adsorption on these sites. It is shown that the unusually large ethane IR frequency shift recently observed in ZnZSM-5 zeolite can result from adsorptive interaction of C₂H₆ with Zn²⁺ stabilized in a cationic position with distantly placed aluminium ions. The dissociative adsorption of ethane molecules with the formation of bridged hydroxyl group and Zn–C₂H₅ structure is considered and an activation energy of ethylene formation from the alkyl fragment is evaluated.     

Propane dehydrogenation over extra-framework In(i) in chabazite zeolites

Indium on silica, alumina and zeolite chabazite (CHA), with a range of In/Al ratios and Si/Al ratios, have been investigated to understand the effect of the support on indium speciation and its corresponding influence on propane dehydrogenation (PDH). It is found that In₂O₃ is formed on the external surface of the zeolite crystal after the addition of In(NO₃)₃ to H-CHA by incipient wetness impregnation and calcination. Upon reduction in H₂ gas (550 °C), indium displaces the proton in Brønsted acid sites (BASs), forming extra-framework In⁺ species (In-CHA). A stoichiometric ratio of 1.5 of formed H₂O to consumed H₂ during H₂ pulsed reduction experiments confirms the indium oxidation state of +1. The reduced indium is different from the indium species observed on samples of 10In/SiO₂, 10In/Al₂O₃ (i.e., 10 wt% indium) and bulk In₂O₃, in which In₂O₃ was reduced to In(0), as determined from the X-ray diffraction patterns of the product, H₂ temperature-programmed reduction (H₂-TPR) profiles, pulse reactor investigations and in situ transmission FTIR spectroscopy. The BASs in H-CHA facilitate the formation and stabilization of In⁺ cations in extra-framework positions, and prevent the deep reduction of In₂O₃ to In(0). In⁺ cations in the CHA zeolite can be oxidized with O₂ to form indium oxide species and can be reduced again with H₂ quantitatively. At comparable conversion, In-CHA shows better stability and C₃H₆ selectivity (∼85%) than In₂O₃, 10In/SiO₂ and 10In/Al₂O₃, consistent with a low C₃H₈ dehydrogenation activation energy (94.3 kJ mol⁻¹) and high C₃H₈ cracking activation energy (206 kJ mol⁻¹) in the In-CHA catalyst. A high Si/Al ratio in CHA seems beneficial for PDH by decreasing the fraction of CHA cages containing multiple In⁺ cations. Other small-pore zeolite-stabilized metal cation sites could form highly stable and selective catalysts for this and facilitate other alkane dehydrogenation reactions.

Indium-containing chabazite zeolites show better stability and C₃H₆ selectivity for propane dehydrogenation than In₂O₃, In/SiO₂ and In/Al₂O₃. Extra-framework In⁺ is identified as the stable active site upon reduction of an impregnated sample.     

Selective Oxidative Dehydrogenation of Ethane and Propane over Copper-Containing Mordenite: Insights into Reaction Mechanism and Product Protection

Copper(II)-containing mordenite (CuMOR) is capable of activation of C−H bonds in C₁-C₃ alkanes, albeit there are remarkable differences between the functionalization of ethane and propane compared to methane. The reaction of ethane and propane with CuMOR results in the formation of ethylene and propylene, while the reaction of methane predominantly yields methanol and dimethyl ether. By combining in situ FTIR and MAS NMR spectroscopies as well as time-resolved Cu K-edge X-ray absorption spectroscopy, the reaction mechanism was derived, which differs significantly for each alkane. The formation of ethylene and propylene proceeds via oxidative dehydrogenation of the corresponding alkanes with selectivity above 95 % for ethane and above 85 % for propane. The formation of stable π-complexes of olefins with Cu^I sites, formed upon reduction of Cu^II-oxo species, protects olefins from further oxidation and/or oligomerization. This is different from methane, the activation of which proceeds via oxidative hydroxylation leading to the formation of surface methoxy species bonded to the zeolite framework. Our findings constitute one of the major steps in the direct conversion of alkanes to important commodities and open a novel research direction aiming at the selective synthesis of olefins.     

Specific Processes and Scrambling in the Dehydrogenation of Ethane and the Degenerate Hydrogen Exchange in the Gas‐Phase Ion Chemistry of the Ni(C,H3,O)+/C2H6 Couple

A mechanistically unprecedented situation characterizes the gas‐phase ion chemistry of Ni(C,H₃,O)⁺ when reacted under thermal, single‐collision conditions with ethane. A dehydrogenation channel leading to Ni(C₃,H₇,O)⁺ is to 90% preceded by a complete loss of positional identity of all nine H‐atoms of the encounter complex (‘scrambling’), whereas ca. 10% of the reaction exhibit a selective C? H bond activation of the alkane. In addition, a degenerate H exchange between ethane and the (C,H₃,O) unit occurs as a side reaction, the mechanistic details of which remain unknown for the time being.     

Features of propane conversion in the presence of SmVO<Subscript>3</Subscript> and SmVO<Subscript>4</Subscript>

Features of propane conversion in the presence of samarium vanadite and samarium vanadate, both produced via solid-phase synthesis, are studied. It is shown that SmVO₃ catalyzes mainly the propane cracking process to form methane and ethylene, while SmVO₄ equally accelerates both cracking and the dehydrogenation of propane. Based on the results from catalytic experiments, energies of activation are calculated for the thermal cracking of propane (104 kJ/mol) and the conversion of propane in the presence of SmVO₃ (39 kJ/mol) and SmVO₄ (42 kJ/mol). The thermal stability of SmVO₄ in a hydrogen atmosphere is studied via temperature-programmed reduction, while SmVO₃ stability in an oxidizing environment is studied by DTA. Energies of activation for the reduction of SmVO₄ (75 kJ/mol) and the oxidation of SmVO₃ (244 kJ/mol) are calculated using the Kissinger method.