Conversion of CO2 and C2H6 to Propanoic Acid over a Au‐Exchanged MCM‐22 Zeolite

Finding novel catalysts for the direct conversion of CO₂ to fuels and chemicals is a primary goal in energy and environmental research. In this work, density functional theory (DFT) is used to study possible reaction mechanisms for the conversion of CO₂ and C₂H₆ to propanoic acid over a gold‐exchanged MCM‐22 zeolite catalyst. The reaction begins with the activation of ethane to produce a gold ethyl hydride intermediate. Hydrogen transfers to the framework oxygen leads then to gold ethyl adsorbed on the Brønsted‐acid site. The energy barriers for these steps of ethane activation are 9.3 and 16.3 kcal mol^?1, respectively. Two mechanisms of propanoic acid formation are investigated. In the first one, the insertion of CO₂ into the Au?H bond of the first intermediate yields gold carboxyl ethyl as subsequent intermediate. This is then converted to propanoic acid by forming the relevant C?C bond. The activation energy of the rate‐determining step of this pathway is 48.2 kcal mol^?1. In the second mechanism, CO₂ interacts with gold ethyl adsorbed on the Brønsted‐acid site. Propanoic acid is formed via protonation of CO₂ by the Brønsted acid and the simultaneous formation of a bond between CO₂ and the ethyl group. The activation energy there is 44.2 kcal mol^?1, favoring this second pathway at least at low temperatures. Gold‐exchanged MCM‐22 zeolite can therefore, at least in principle, be used as the catalyst for producing propanoic acid from CO₂ and ethane.     

Ni-V-O催化剂上丙烷氧化脱氢制丙烯的反应

考察了Ｎｉ－Ｖ－Ｏ系列催化剂对丙烷氧化脱氢反应的催化性能,结合ＸＲＤ和电导法等表征结果研究了催化剂的物相结构与仙化活性之间的关系。结果表明：同ｎ型半导体氧化物相比,ｐ型半导体具有优良的催化性能,并且丙烯的选择性与催化剂中ＮｉＯ及Ｎｉ３Ｖ２Ｏ８相的共存有关。适当控制反应条件,４５２℃时该系列催化剂可达到１８．６％的丙烷转化率和４９．９％的丙烯选择性。     

TiO2-ZrO2复合氧化物负载的氧化钒催化剂用于丙烷氧化脱氢反应

 丙烷氧化脱氢反应制备丙稀酸，丙烯醛，由于其巨大的工业价值而成为催化领域研究的热点。该反应也可分为两步实现。即先由丙烷到丙烯，再由丙烯到丙稀酸，丙烯醛。后一步已成功的实现工业化，在低温下( £350℃)，使用钒基催化剂，丙稀酸，丙烯醛的产率可高达80%以上。但是，对于前一步，使用迄今为止最有效的V-Mg-O催化剂，在550℃的高温下，丙烯的产率仅为20%。而在如此高的温度下，在紧接着的第二步反应中，大部分的丙烯会直接转化为深度氧化产物（CO2, H2O）。所以，制备一种能在低温下有效实现丙烷氧化脱氢反应制丙烯的催化剂是由丙烷制备高产率的丙稀酸，丙烯醛的另一种途径。\r\n 催化剂V2O5/TiO2最显著的特点是它具有较高的低温催化活性。但是，作为载体材料，TiO2有一些缺点，如比表面较小，热稳定性较差，机械性能较低，抗磨损性较差等。相比之下，载体ZrO2就具有许多TiO2所不具备的优点。第一，ZrO2具有很大的比表面积（＞300m2/g），并且在高温下它也能保持较高的比表面积。第二，ZrO2的热稳定性，机械性能和抗磨损性都较好。第三，金属氧化物在ZrO2表面能够得到很好的分散。第四，ZrO2非常稳定，在烷烃氧化脱氢反应的条件下是惰性的，不参加反应。所以，如果向TiO2中掺杂ZrO2进行改性，能极大的提高其载体的表面积，热稳定性，机械性能和抗磨损性。那么复合载体TiO2-ZrO2很可能成为在低温下丙烷氧化脱氢制丙烯的极有潜力的催化剂载体材料。在本文中，我们采用溶胶-凝胶法，用廉价的无机盐作为初始材料制备了一系列不同TiO2/ZrO2质量比的TiO2-ZrO2复合氧化物作为催化剂载体并研究了这些催化剂用于丙烷氧化脱氢反应的催化活性。     

介孔氧化铝负载钒催化剂上丙烷氧化脱氢制丙烯

 采用浸渍法制备了介孔氧化铝 (m-Al2O3) 负载钒催化剂 (V/m-Al2O3), 并考察了其催化丙烷氧化脱氢制丙烯反应活性. 通过 N2 吸附-脱附、透射电镜、X 射线粉末衍射、紫外-可见漫反射光谱、氢-程序升温还原和氨-程序升温脱附对催化剂进行了表征. 结果表明, 介孔氧化铝具有大比表面积、窄孔径分布和两维六方相结构, 在其上负载适量的 V 可实现 V 活性物种的高分散及催化剂的弱酸性, 从而有利于提高丙烷转化率和丙烯选择性. 与共合成法制备的含 V 介孔氧化铝 V/m-Al2O3(C) 和浸渍法制备的 V/?-Al2O3 相比, V/m-Al2O3 表现出更高的催化活性. 这与载体较弱的酸性和较大的比表面积以及 V 物种的高分散有关.     

A DFT Study of Tungsten–Methylidene Formation on a W/ZSM‐5 Zeolite: The Metathesis Active Site

Tungsten–methylidene formation from ethene on either the W^IV, W^V, or W^VI active sites of a W/ZSM‐5 zeolite is investigated by using the M06‐L functional. The reaction is assumed to proceed in two steps; the first step is the [2+2] cycloaddition between ethene and the W?O active site to form an oxametallacycle intermediate. The intermediate is then decomposed to produce the W–methylidene active site from the metathesis reaction. The overall activation barrier of the reaction on W^VI (27.3 kcal mol^?1) is considerably lower than the ones for W^IV and W^V (69.4 and 37.1 kcal mol^?1, respectively). Moreover, the reaction involving the W^VI site also stabilizes intermediates and products to a larger extent than the ones on the W^IV and W^V sites. As a result, we have demonstrated that the reaction of the W–methylidene metathesis active site is both kinetically and thermodynamically favored to occur on the W^VI active site of the zeolite.     

Kinetics of the Oxidative Dehydrogenation of Propane over a VMgO Catalyst

The reaction kinetics of the oxidative dehydrogenation of propane was studied at 475-550℃ over a VMgO catalyst. Vanadium-magnesium-oxides are among the most selective and active catalysts for the dehydrogenation of propane to propylene. Selectivity to propylene up to about 60% was obtained at 10% conversion, but the selectivity decreased with increasing conversion. No oxygenates were detected, the only by-products were CO and CO2. The reaction rate of propane was found to be first order in propane and close to zero order in oxygen, which is in agreement with a Mars van Krevelen mechanism with the activation of the hydrocarbon as the rate determining step. The activation energy of the conversion of propane was found to be 122±6 kJ/mol.     

Kinetics of the Oxidative Dehydrogenation of Propane over a VMgO Catalyst

The reaction kinetics of the oxidative dehydrogenation of propane was studied at 475-550℃over a VMgO catalyst. Vanadium-magnesium-oxides are among the most selective and active catalysts forthe dehydrogenation of propane to propylene. Selectivity to propylene up to about 60% was obtained at10% conversion, but the selectivity decreased with increasing conversion. No oxygenates were detected, theonly by-products were CO and CO2. The reaction rate of propane was found to be first order in propaneand close to zero order in oxygen, which is in agreement with a Mars van Krevelen mechanism with theactivation of the hydrocarbon as the rate determining step. The activation energy of the conversion ofpropane was found to be 122±6 kJ/mol.     

Kinetics of Oxidative Dehydrogenation of Propance over a VMgO Catalyst

The reaction kinetics of the oxidative dehydrogenation of propane was studied at 475-550oC over a VMgO catalyst. Vanadium-magnesium-oxides are among the most selective and active catalysts for the dehydrogenation of propane to propylene. Selectivity to propylene up to about 60% was obtained at 10% conversion, but the selectivity decreased with increasing conversion. No oxygenates were detected, the only by-products were CO and CO2. The reaction rate of propane was found to be first order in propane and close to zero order in oxygen, which is in agreement with a Mars van Krevelen mechanism with the activation of the hydrocarbon as the rate determining step. The activation energy of the conversion of propane was found to be 122±6 kJ/mol.     

不同孔道结构的氧化硅负载钒氧化物催化丙烷氧化脱氢

采用固定床微型反应装置,结合催化剂的原位电子自旋共振光谱、程序升温表面反应和紫外漫反射光谱等技术,研究了丙烷氧化脱氢的介孔氧化硅负载钒氧化物催化剂的性能和表面氧物种的状态及其反应性.结果表明,催化剂载体孔结构是影响钒氧物种分散状态乃至催化性能的一个重要因素.SBA-15负载钒氧化物催化剂因具有较大的比表面积和较大的孔径,不仅具有较高的丙烷氧化脱氢催化活性,而且具有较高的丙烯选择性.复合型钒氧化物催化剂表面与V离子相连的晶格氧物种是丙烷氧化脱氢牛成内烯的主要活性物种,载体表面高度分散的钒氧物种具有较高的丙烷氧化脱氢催化活性.负载型钒氧化物催化剂晶格氧物种是丙烷氧化脱氢转化为丙稀的主要活性物种,CO_2分子可以再生钒氧化物催化剂的晶格氧物种,同时它对丙烯的深度氧化作用较弱,因此在负载型钒氧化物催化剂上CO_2氧化丙烷可高选择性地生成丙烯.     

CO2氧化丙烷脱氢制丙烯用Pd-Cu/V2O5-SiO2催化剂的研究

 采用等体积浸渍法制备了V2O5-SiO2负载的Pd-Cu双金属催化剂,以程序升温还原/程序升温氧化、红外光谱、程序升温脱附和微反技术表征了Pd-Cu/V2O5-SiO2对CO2和丙烷的化学吸附性能及对CO2部分氧化丙烷脱氢反应的催化性能. 结果表明,在催化剂表面金属活性位(Pd,Cu)和邻近的Vn+协同下形成的CO2卧式吸附态可在172和284 ℃断裂形成CO和晶格氧,以甲基氢和亚甲基氢双位吸附在V=O上的丙烷分子吸附态可在238 ℃脱氢生成丙烯. 在600 ℃,CO2/C3H8体积比为1和空速为1?286 h-1的条件下,丙烷转化率为35.22%,丙烯选择性为85.44%. 催化剂V=O中的晶格氧参与了丙烷氧化脱氢过程.     

正丁烷在VMgO和Ni-VMgO催化剂上氧化脱氢

 采用4种方法制备了VMgO催化剂样品(w(V2O5)=30%,w(MgO)=70%),并将其用于正丁烷氧化脱氢气固相反应. 结果表明,MgO经蒸馏水回流和焙烧处理后再用NH4VO3溶液浸渍所制得的VMgO,对正丁烷氧化脱氢生成丁烯和丁二烯反应具有更好的催化性能. 这是由于用该法制备的VMgO催化剂中存在较多的Mg3V2O8物种. 通过添加Ni对VMgO催化剂进行了改性. 结果表明,适量添加Ni(n(Ni)/n(V)=0.3)有利于催化剂中Mg3V2O8的生成,而Ni以Ni3V2O8的形态存在. 由此明显改善了VMgO催化剂对正丁烷氧化脱氢生成丁烯和丁二烯反应的催化性能.     

Oxygen‐Assisted Hydroxymatairesinol Dehydrogenation: A Selective Secondary‐Alcohol Oxidation over a Gold Catalyst

Selective dehydrogenation of the biomass‐derived lignan hydroxymatairesinol (HMR) to oxomatairesinol (oxoMAT) was studied over an Au/Al₂O₃ catalyst. The reaction was carried out in a semi‐batch glass reactor at 343 K under two different gas atmospheres, namely produced through synthetic air or nitrogen. The studied reaction is, in fact, an example of secondary‐alcohol oxidation over an Au catalyst. Thus, the investigated reaction mechanism of HMR oxidative dehydrogenation is useful for the fundamental understanding of other secondary‐alcohol dehydrogenation over Au surfaces. To investigate the elementary catalytic steps ruling both oxygen‐free‐ and oxygen‐assisted dehydrogenation of HMR to oxoMAT, the reactions were mimicked in a vacuum over an Au₂₈ cluster. Adsorption of the involved molecular species—O₂, three different HMR diastereomers (namely, one SRR and two RRR forms), and the oxoMAT derivative—were also studied at the DFT level. In particular, the energetic and structural differences between SRR‐HMR and RRR‐HMR diastereomers on the Au₂₈ cluster were analyzed, following different reaction pathways for the HMR dehydrogenation that occur in presence or absence of oxygen. The corresponding mechanisms explain the higher rates of the experimentally observed oxygen‐assisted reaction, mostly depending on the involved HMR diastereomer surface conformations. The role of the support was also elucidated, considering a very simple Au₂₈ charged model that explains the experimentally observed high reactivity of the Au/Al₂O₃ catalyst.     

纳米碳纤维的微观结构及对丙烷氧化脱氢反应的催化性能

 采用化学气相沉积法制备了具有不同微观结构的纳米碳纤维,并用丙烷程序升温吸脱附和热重实验对纳米碳纤维进行了表征,考察了纳米碳纤维在丙烷氧化脱氢反应中的催化性能. 结果表明,纳米碳纤维表面的含氧基团可能是催化活性中心; 纳米碳纤维不仅具有较好的催化性能,而且在反应条件下具有较高的热稳定性. 在550 ℃下,鱼骨式纳米碳纤维上丙烷转化率为44.9%时,丙烯选择性为33.0%,其催化性能与V-Mg-O等金属氧化物催化剂相当. 不同微观结构纳米碳纤维的催化性能相差较大,这是由于其表面化学性质不同所致.     

Oxidative Dehydrogenation of Propane to Propene over Barium Promoted Ni-Mo-O Catalyst

Oxidative dehydrogenation of propane to propene was investigated over Ba- promoted Ni_0.9MoO₄ catalysts (molar ratio of Ba/Mo: 1%, 3%, 6%, 9%, 12%, 15%). The selectivity for propene increases with the increasing number of basic sites on the catalyst, and molybdenum ions play an important role in propane activation. No significant deactivation of the 6% Ba- Ni_0.9MoO₄ and Ni_0.9MoO₄ at 773 K for 80 h and for 40 h, respectively, appeared, indicating that the 6% Ba- Ni_0.9MoO₄ has great advantage over the Ni_0.9MoO₄. This revised version was published online in June 2006 with corrections to the Cover Date.     

Identification of tert‐Butyl Cations in Zeolite H‐ZSM‐5: Evidence from NMR Spectroscopy and DFT Calculations

Experimental evidence for the presence of tert‐butyl cations, which are important intermediates in acid‐catalyzed heterogeneous reactions, on solid acids has still not been provided to date. By combining density functional theory (DFT) calculations with ¹H/¹³C magic‐angle‐spinning NMR spectroscopy, the tert‐butyl cation was successfully identified on zeolite H‐ZSM‐5 upon conversion of isobutene by capturing this intermediate with ammonia.     

ZrO2‐Based Alternatives to Conventional Propane Dehydrogenation Catalysts: Active Sites,Design, and Performance

Non‐oxidative dehydrogenation of propane to propene is an established large‐scale process that, however, faces challenges, particularly in catalyst development; these are the toxicity of chromium compounds, high cost of platinum, and catalyst durability. Herein, we describe the design of unconventional catalysts based on bulk materials with a certain defect structure, for example, ZrO₂ promoted with other metal oxides. Comprehensive characterization supports the hypothesis that coordinatively unsaturated Zr cations are the active sites for propane dehydrogenation. Their concentration can be adjusted by varying the kind of ZrO₂ promoter and/or supporting tiny amounts of hydrogenation‐active metal. Accordingly designed Cu(0.05 wt %)/ZrO₂‐La₂O₃ showed industrially relevant activity and durability over ca. 240 h on stream in a series of 60 dehydrogenation and oxidative regeneration cycles between 550 and 625 °C.     

Mechanistic Studies on the Transformation of Ethanol into Ethene over Fe‐ZSM‐5 Zeolite

Ethanol, through the utilization of bioethanol as a chemical resource, has received considerable industrial attention as it provides an alternative route to produce more valuable hydrocarbons. Using a density functional theory approach incorporating the M06‐L functional, which includes dispersion interactions, a large 34T nanocluster model of Fe‐ZSM‐5 zeolite in which T is a Si or Al atom is employed to examine both the stepwise and concerted mechanisms of the transformation of ethanol into ethene. For the stepwise mechanism, ethanol dehydration commences from the first hydrogen abstraction of the ethanol OH group to form the ethoxide‐hydroxide intermediate with a low activation energy of 17.7 kcal mol^?1. Consequently, the ethoxide‐hydroxide intermediate is decomposed into ethene through hydrogen abstraction from the ethoxide methyl carbon to either the OH group of hydroxide or the oxygen of the ethoxide group with high activation energies of 64.8 and 63.5 kcal mol^?1, respectively. For the concerted mechanism, ethanol transformation into the ethene product occurs in a single step without intermediate formation, with an activation energy of 32.9 kcal mol^?1.     

丙烷氧化脱氢M-Fe-O催化剂的研究

采用微波加热草酸盐共沉淀法制备了一系列M-Fe-O(M=V、Cr、Mn、Co、Ni、Cu、Zn)催化剂,考察了其对丙烷氧化脱氢制丙烯反应的催化性能,并对催化剂进行了BET、XRD、H2-TPR、电导测量等表征.实验结果表明V-Fe-O和Cr-Fe-O催化剂表现出较好的丙烷氧化脱氢制丙烯催化性能.反应温度为873 K时,以V-Fe-O为催化剂时丙烷转化率34.46%,丙烯选择性30.91%;在Cr-Fe-O为催化剂上丙烷转化率36.31%,丙烯选择性34.22%.     

Structural,Electronic, and Bonding Properties of Zeolite Sn‐Beta: A Periodic Density Functional Theory Study

The structural, electronic, and the bonding properties of the zeolite Sn‐beta (Sn‐BEA) have been investigated by using the periodic density functional theory. Each of the nine different T‐sites in BEA were substituted by Sn atoms and all the nine geometries were completely optimized by using the plane‐wave basis set in conjunction with the ultra‐soft pseudopotential. On the basis of the structural and the electronic properties, it has been demonstrated that the substitution of Sn atoms in the BEA framework is an endothermic process and hence the incorporation of Sn in the BEA is limited. The lowest unoccupied molecular orbitals (LUMO) energies have been used to characterize the Lewis acidity of each T‐site. On the basis of the relative cohesive energy and the LUMO energy, the T2 site is shown to be the most favorable site for the substitution Sn atoms in the BEA framework.