分子筛催化cis-2-丁烯的双键异构反应机理的DFT研究

基于含有两个Si和一个Al的分子筛3T簇模型, 利用密度泛函方法(DFT)研究了分子筛催化1-丁烯双键异构为cis-2-丁烯的反应机理. 在B3LYP/6-31G(d,p)计算水平上对反应各驻点进行了全优化, 并计算了反应的活化能. 研究发现, 分子筛上的酸性OH基团首先通过物理吸附靠近1-丁烯的双键, 形成了π配位复合物后, 丁烯双键的端基C原子逐渐抽取这个质子, 同时相邻酸性位的一个O原子也抽取丁烯碳链上的一个H原子, 形成吸附态的cis-2-丁烯, 最后通过脱附形成产物, 使分子筛复原, 反应按照协同反应机理发生. 计算得到的表观活化能是55.9 kJ/mol, 与实验结果接近.     

ZSM-5分子筛催化1-己烯生成cis-2-己烯的理论研究

基于54T团簇模型, 采用ONIOM分层计算方法, 研究了1-己烯在ZSM-5分子筛上进行顺式双键异构的反应机理. 计算结果表明, 1-己烯的顺式双键异构反应通过只有分子筛Brønsted酸部分起作用的机理进行. 首先, 1-己烯与分子筛的Brønsted酸性位形成π配位复合物. 接着, 酸质子发生迁移使1-己烯的双键端基碳原子被质子化, 同时双键的另一碳原子与失去质子的Brønsted酸羟基的氧原子成键, 形成稳定的烷氧基中间体. 然后, 烷氧基中间体中的C―O共价键被打断, 同时Brønsted酸羟基的氧原子从C₆H₁₃基团提取一个氢原子还原分子筛的酸性位, 并且生成cis-2-己烯. 这一反应路径与借助于分子筛活性位的酸-碱双功能性质的反应路径是相互竞争的. 计算得到的表观活化能是59.37 kJ·mol^-1, 该值与实验值非常接近. 这一结果合理解释了双键异构过程中的能量特征, 并且扩展了对分子筛活性位本质的理解.     

DFT法研究分子筛催化trans-2-丁烯的双键异构

利用一个3T簇模型模拟分子筛催化剂的酸性位, 采用密度泛函理论(DFT)的 B3LYP/6-31G(d, p)方法, 研究了分子筛催化1-丁烯双键异构为trans-2-丁烯的反应机理. 对反应各驻点进行了全优化, 经过零点能校正后, 得到了反应的活化能. 研究表明, 反应分三步进行：物理吸附→化学反应→物理脱附. 分子筛的酸性位OH基团首先吸附1-丁烯的双键形成了π配位复合物, 然后按协同反应机理发生双键异构反应, 生成吸附态的trans-2-丁烯, 最后脱附成产物. 计算得到的表观活化能为57.1 kJ•mol^-1, 与实验结果接近.     

乙烷部分氧化制C2氧化物用Fe－Al－P－O催化剂的研究Ⅱ．Fe－Al－P－O催化剂的吸附及催化性能

 用化学吸附－IR，化学吸附－TPD和微反技术研究了超细Fe－Al－P－O催化剂的化学吸附性能及对乙烷部分氧化反应的催化性能．结果表明，乙烷能够以－CH3中的H原子吸附于催化剂表面P＝O键的端氧上形成分子吸附态，并且随着吸附温度的升高，对乙烷的吸附强度逐渐增大；乙烯则主要是以C＝C双键吸附在催化剂的Lewis酸位Fe3＋上．乙烷部分氧化反应的主要产物为乙烯和COx，但在反应物中引入氢的条件下，乙烷部分氧化反应的性能大为改善，并可生成乙醇和乙醛等含氧化合物．     

V2O5/SiO2催化剂的异丁烷选择氧化反应性能

 采用溶胶－凝胶法和浸渍法制备了V2O5／SiO2催化剂,并用XRD,IR,TPD和活性评价等手段对催化剂的表面构造、化学吸附性能和异丁烷选择氧化反应性能进行了研究．结果表明：催化剂表面由Lewis碱位V＝O双键的端氧和Lewis酸位V5＋构成,异丁烷分子主要通过甲基中的H双位吸附在催化剂表面的Lewis碱位上,异丁烯分子可通过甲基的H吸附在催化剂表面的Lewis碱位,也可通过C＝C双键吸附在催化剂表面的Lewis酸位上．在常压条件下,异丁烷选择氧化产物主要有异丁烯、甲基丙烯醛和甲基丙烯酸,其中深度氧化产物CO2主要由通过C＝C吸附的异丁烯继续反应生成．     

乙烷部分氧化制C2氧化物用Fe-A1-P-O催化剂的研究 Ⅱ．Fe-A1-P-O催化剂的吸附及催化性能

用化学吸附-IR，化学吸附-TPD和微反技术研究了超细Fe-Al-P-O催化剂的化学吸附性能及对乙烷部分氧化反应的催化性能．结果表明，乙烷能够以-CH3中的H原子吸附于催化剂表面P=O键的端氧上形成分子吸附态，并且随着吸附温度的升高，对乙烷的吸附强度逐渐增大；乙烯则主要是以C=C双键吸附在催化剂的Lewis酸位：Fe3 上．乙烷部分氧化反应的主要产物为乙烯和COx，但在反应物中引入氢的条件下，乙烷部分氧化反应的性能大为改善，并可生成乙醇和乙醛等含氧化合物．     

镧改性β分子筛的制备及催化合成ETBE的活性研究

采用离子交换法和浸渍法将稀土元素镧引入到Hβ分子筛中,制备了负载型催化剂x%La/Hβ(x表示镧负载量),通过XRD、FT-IR、NH3-TPD、Py-IR等考察了镧对Hβ分子筛结构及表面酸性的影响.结果表明,两种方法负载镧后Hβ分子筛的骨架结构没有变化,但结晶度都明显降低;两种方法都使得Hβ分子筛的B酸量增加,而交换法增加较多.将其用于乙醇和叔丁醇为原料合成乙基叔丁基醚(ETBE)的反应,结果发现,交换法负载镧后Hβ分子筛的催化活性较高,而浸渍法负载镧后Hβ分子筛的活性较低.说明B酸量的增加有利于此醚化反应的进行.     

单晶Au表面和Au/TiO2（110）模型催化剂表面CO氧化反应机理和活性位

在分子尺度上介绍了Au/TiO2(110)模型催化剂表面和单晶Au表面CO氧化反应机理和活性位、以及H2O的作用.在低温(<320 K), H2O起着促进CO氧化的作用, CO氧化的活性位位于金纳米颗粒与TiO2载体界面(Auδ+–Oδ––Ti)的周边. O2和H2O在金纳米颗粒与TiO2载体界面边缘处反应形成OOH,而形成的OOH使O–O键活化,随后OOH与CO反应生成CO2.300 K时CO2的形成速率受限于O2压力与该反应机理相印证.相反,在高温(>320 K)下,因暴露于CO中而导致催化剂表面重组,在表面形成低配位金原子.低配位的金原子吸附O2,随后O2解离,并在金属金表面氧化CO.     

Ga/HZSM-5分子筛上乙烯二聚反应的理论研究

应用理论计算方法研究了Ga/HZSM-5及Al/HZSM-5 分子筛上乙烯二聚生成1-丁烯的反应历程, 比较了分子筛酸性对反应能量的影响. 计算采用分为两层的76T簇模型, 应用量子力学和分子力学联合的 ONIOM2 (B3LYP/6-31G(d, p):UFF) 方法. 乙烯二聚过程可按照分步机理和协同机理进行, 均得到表面丁基烷氧中间产物. 计算结果表明, 与在Al/HZSM-5分子筛上的反应过程相比, 乙烯在 Ga/HZSM-5分子筛上的吸附能低 20.62 kJ·mol^-1, 但质子化反应的活化能只高出1.26 kJ·mol^-1; 而乙基烷氧中间体与乙烯分子结合过程的活化能高出 62.55 kJ·mol^-1, 原因是Ga 原子半径大, 降低了六元环过渡态的稳定性. 若按协同机理, 质子转移和C―C键聚合同时进行, 在 Ga/HZSM-5分子筛上的活化能较Al/HZSM-5的高16.44 kJ·mol^-1. 因此乙烯二聚按照协同机理有利. 研究还表明, 表面丁基烷氧中间体脱质子, 生成1-丁烯并吸附在复原的分子筛酸性位上. 该反应在两种酸中心上的活化能几乎相同, 但明显高于其他各步的活化能, 因此成为整个反应的速度控制步骤.     

固体多核 NMR 研究 Mo/Hβ-Al2O3 催化剂在烯烃歧化反应中的积炭

 采用固体多核 13C, 27Al 及 29Si MAS NMR 结合吸附气体 Xe 后的 129Xe NMR, 详细研究了乙烯和 2-丁烯歧化生成丙烯反应过程中 6%Mo/Hβ-Al2O3 催化剂上积炭的类型和落位. 结果表明, 积炭以饱和烷烃为主, 并随着反应的进行生成更多的支链烷烃, 积炭主要分布在 Hβ 分子筛上, 并主要落位在分子筛的交叉孔道处.     

Kinetics of gas phase synthesis of ethyl-tert-butyl ether (ETBE) on Wells–Dawson catalyst

Mechanism and kinetics of gas phase synthesis of ethyl-tert-butyl ether (ETBE) over Wells–Dawson heteropolyacid H₆P₂W₁₈O₆₂ in an anhydrous system have been discussed. The rates of ETBE synthesis were measured in a differential reactor for ethanol–isobutene system, far from the limitation by chemical equilibrium. In the pressure range studied, kinetic measurements show that isobutene has an enhancing effect on the reaction while ethanol has an inhibitory one.

Catalytic and sorption experiments allowed to formulate the mechanism of ETBE synthesis which assumed isobutene adsorption and oligomerization at the surface of heteropolyacid crystallites and the formation of carbocation using protons supplied from the bulk of catalyst. Kinetic equations were deduced indicating that it is the concentration of protons forming inter-anionic bonds between heteropolyacid anions which determines the catalytic activity of heteropolyacid.     

Gas-Phase Synthesis of Ethyl tert-Butyl Ether (ETBE) on H-ZSM-5 Catalyst in Continuous Fixed-Bed and Fluidized-Bed Reactors

The gas-phase synthesis of ETBE from ethanol and isobutene has been carried out over a H-ZSM-5 catalyst in two types of continuous-flow catalytic reactors, fixed-bed and fluidized-bed. We have studied the influence of temperature, molar ratio ethanol/isobutene and weight hourly space velocity on the yield of ETBE.     

Effect of Phosphoric Acid on HZSM-5 Catalysts for Prins Condensation to Isoprene from Isobutylene and Formaldehyde

A series of HZSM-5 zeolites modified with different amounts of phosphoric acid(P/HZSM-5) was prepared. The physicochemical features of the P/HZSM-5 catalysts were characterized via X-ray diffraction(XRD), N₂ adsorption-desorption, NH₃-temperature programmed desorption(NH₃-TPD) and Fourier tranform infrared(FTIR) spectra of the adsorbed pyridine, and the performances of the catalysts for Prins condensation to isoprene from isobutylene and formaldehyde were investigated. The maximum isobutene conversion and isoprene selectivity were 10.3% and 94.6% on the HZSM-5 catalyst with a Si/Al molar ratio of 600 using 5%(mass fraction) phosphoric acid. The phosphoric acid modification not only modulated the amount of acidic sites but also regulated the acid type. An appropriate amount of weak Lewis and Brönsted acid sites served as the active sites for the condensation of isobutene with formaldehyde, and the strong acid sites could cause side reactions and coke deposition.     

Ab initio and microcalorimetric investigations of alkene adsorption on phosphotungstic acid

The adsorption of ethene, propene, 1-butene, trans-2-butene, and isobutene on phosphotungstic acid has been characterized by density functional theory (DFT) calculations and microcalorimetric experiments. The DFT-calculated chemisorption energies to form the corresponding alkoxides for ethene, propene, 1-butene, trans-2-butene, and isobutene were -86.8, -90.3, -102.6, -79.9, and -91.4 kJ mol(-1), respectively (for their most-favorable binding modes). The relative chemisorption energies to form the alkoxides are dictated by the strength of interaction of the acidic proton with the carbon atom of the double bond that becomes protonated. The activation barrier for chemisorption was greatest for alkenes with primary (1 degrees) carbenium-like transition states followed by secondary (2 degrees) and tertiary (3 degrees) transition states. The adsorption enthalpy established from microcalorimetric experiments with propene and isobutene was approximately -100 kJ mol(-1), which is close to the DFT-calculated values. Chemisorption of ethene on phosphotungstic acid during microcalorimetric experiments was minimal, presumably because of the large activation barrier associated with a 1 degrees carbenium-like transition state. The results from this study are compared with those in the literature for the adsorption of alkenes on zeolites, which have a similar adsorption mechanism. Our results suggest that alkene adsorption is stronger on phosphotungstic acid than on zeolites, as supported by the more exothermic chemisorption energies. Additionally, activation barriers for alkene adsorption are lower over phosphotungstic acid than over zeolites.     

A germanium isocyanide complex featuring (n → π*) back-bonding and its conversion to a hydride/cyanide product via C-H bond activation under mild conditions

Reaction of the diarylgermylene Ge(Ar(Me(6)))(2) [Ar(Me(6)) = C(6)H(3)-2,6-(C(6)H(2)-2,4,6-(CH(3))(3))(2)] with tert-butyl isocyanide gave the Lewis adduct species (Ar(Me(6)))(2)GeCNBu(t), in which the isocyanide ligand displays a decreased C-N stretching frequency consistent with an n → π* back-bonding interaction. Density functional theory confirmed that the HOMO is a Ge-C bonding combination between the lone pair of electrons on the germanium atom and the C-N π* orbital of the isocyanide ligand. The complex undergoes facile C-H bond activation to produce a new diarylgermanium hydride/cyanide species and isobutene via heterolytic cleavage of the N-Bu(t) bond.     

Application of semiempirical long-range dispersion corrections to periodic systems in density functional theory

Ewald summation is used to apply semiempirical long-range dispersion corrections (Grimme, J Comput Chem 2006, 27, 1787; 2004, 25, 1463) to periodic systems in density functional theory. Using the parameters determined before for molecules and the Perdew-Burke-Ernzerhof functional, structure parameters and binding energies for solid methane, graphite, and vanadium pentoxide are determined in close agreement with observed values. For methane, a lattice constant a of 580 pm and a sublimation energy of 11 kJ mol(-1) are calculated. For the layered solids graphite and vanadia, the interlayer distances are 320 pm and 450 pm, respectively, whereas the graphite interlayer energy is -5.5 kJ mol(-1) per carbon atom and layer. Only when adding the semiempirical dispersion corrections, realistic values are obtained for the energies of adsorption of C(4) alkenes in microporous silica (-66 to -73 kJ mol(-1)) and the adsorption and chemisorption (alkoxide formation) of isobutene on acidic sites in the micropores of zeolite ferrierite (-78 to -94 kJ mol(-1)). As expected, errors due to missing self-interaction correction as in the energy for the proton transfer from the acidic site to the alkene forming a carbenium ion are not affected by the dispersion term. The adsorption and reaction energies are compared with the results from M?ller-Plesset second-order perturbation theory with basis set extrapolation.     

Ethers on Si(001): A Prime Example for the Common Ground between Surface Science and Molecular Organic Chemistry

By using computational chemistry it has been shown that the adsorption of ether molecules on Si(001) under ultrahigh vacuum conditions can be understood with classical concepts of organic chemistry. Detailed analysis of the two‐step reaction mechanism—1) formation of a dative bond between the ether oxygen atom and a Lewis acidic surface atom and 2) nucleophilic attack of a nearby Lewis basic surface atom—shows that it mirrors acid‐catalyzed ether cleavage in solution. The O−Si dative bond is the strongest of its kind, and the reactivity in step 2 defies the Bell–Evans–Polanyi principle. Electron rearrangement during C−O bond cleavage has been visualized with a newly developed method for analyzing bonding, which shows that the mechanism of nucleophilic substitutions on semiconductor surfaces is identical to molecular S_N2 reactions. Our findings illustrate how surface science and molecular chemistry can mutually benefit from each other and unexpected insight can be gained.