Mo/MCM-22分子筛碳化钼活性中心结构及甲烷活化机理的密度泛函理论研究

 应用密度泛函理论研究了Mo/MCM-22分子筛上碳化钼活性中心的几何结构和电子结构,以及甲烷在该活性中心上的活化机理. 设计了两种结构的活性中心模型: Mo(CH2)2(模型A)和Mo(CH)CH2(模型B); 它们都嫁接在MCM-22分子筛超笼边缘的T4位的Brnsted-酸性位上,用3T簇模型代替分子筛的骨架,对所设计的模型进行了几何结构优化和电子结构分析. 结构优化结果显示, Mo与CH2端基以双键结合,键长为0.18～0.19 nm, 而Mo与CH端基以叁键结合,键长为0.17 nm. 通过自然键轨道分析,证明中心钼原子以配位键与骨架氧原子结合. 根据前线分子轨道的分析,预测了甲烷活化反应将发生在甲烷分子的HOMO和钼活性中心的LUMO之间,即 C-H 键的电子流向 Mo-C 键的π*轨道. 甲烷 C-H 键发生异裂, H+和H3C-基团分别与 Mo-C 键上的Mo和C成键. 在模型A上,甲烷活化反应的活化能为119.97 kJ/mol; 在模型B上,甲烷的H原子可以分别结合到CH2端基和CH端基上,对应的活化能分别为91.37和79.07 kJ/mol.     

ZSM-5分子筛氮化的密度泛函理论研究

利用量子化学中的密度泛函理论,基于ZSM-5分子筛的8T簇模型,在Gaussian 98程序中采用B3LYP方法和6-311G(d,p)基组计算了ZSM-5分子筛中氮的最佳取代位置.计算结果表明,分子筛骨架中氧原子被氮原子取代的最佳位置为O11和O21位.由于位于B酸位上的O11原子是氮原子的最佳取代位置之一,所以氮化可以减弱分子筛表面的B酸强度.     

含氮ZSM-5分子筛骨架中氮取代位置的密度泛函计算

运用Gaussian 98程序包, 采用密度泛函理论B3LYP方法, 基于ZSM-5分子筛的8T模型, 分别通过6-31G, 6-31G(d)和6-311G(d,p)基组计算了ZSM-5分子筛中氮原子取代前后各O原子和各N原子的能量, 从而得到各O原子与各N原子在骨架中的稳定性及其对氮化取代反应的影响. 计算结果表明, N原子在骨架中的稳定性对氮取代反应的影响较大. ZSM-5分子筛晶体结构中与B酸位处于同一个四面体的O11位置, 为氮原子的最佳取代位置, 因此氮化后分子筛表面的B酸强度得到较大程度的减弱.     

不同载体成型的 Mo/ZSM-5 催化剂上甲烷无氧芳构化反应

 以不同氧化物为载体, 通过挤条成型法制备了 Mo/ZSM-5 催化剂, 考察了其催化甲烷无氧芳构化反应性能. 采用吸附吡啶红外光谱、氨程序升温脱附和氢程序升温还原等方法对催化剂进行了表征. 结果表明, ZnO 的添加使催化剂的强酸量减少, 强 B 酸比例降低, Mo 物种还原能力提高, 因而催化剂表现出较高的甲烷芳构化活性和较低的积炭选择性.     

蒸气相法ZSM-5分子筛的合成及其负载的Mo催化剂在甲烷芳构化中的应用

采用蒸气相法使无定形凝胶在水蒸气中结晶形成ZSM-5分子筛,通过X射线衍射、红外光谱、热重、扫描电镜以及能量色散型X射线荧光分析对合成的分子筛进行了表征.结果表明,蒸气相法比水热法更易合成出粒径均匀、形貌规则的小颗粒分子筛(粒径为150～250nm),将其负载Mo后用于甲烷无氧芳构化反应中,表现出更好的催化性能,初期甲烷转化率达17.5%,同时表现出更高的稳定性和容炭能力.     

Methane Direct Conversion on Mo/ZSM-5 Catalysts Modified by Pd and Ru

The effect of addition of Ru and Pd to Mo/HZSM-5 catalysts used in the dehydroaromatiza-tion of methane was investigated. Catalytic tests and temperature-programmed oxidation results showed that Pd-based catalysts were more selective to naphthalene and suffered strong deactivation. The presence of Ru improved the activity and stability, with a decrease in the carbonaceous deposit probably because of a mechanism of protection of the Mo2C surface.     

浸渍法制备的Mo/ZSM-5催化剂中Mo的分布状态

 通过多晶XRD利用Rietveld方法测定和精修了浸渍法制备的Mo/ZSM-5催化剂样品的物相和晶体结构. 结果表明,Mo物种在ZSM-5分子筛中以3种形式存在. 一种是以［Mo5O12］6+形式存在于ZSM-5分子筛的孔道内,每个分子筛晶胞中平均含0.4个［Mo5O12］6+单元. 这些单元覆盖了分子筛孔道内的部分B酸中心,使其B酸量及总酸量减少,但使L酸量增加; ［Mo5O12］6+团中Mo的含量约占Mo/ZSM-5催化剂质量的3.03%. 其余的两种Mo物种分别以α-MoO3晶相和无定形氧化钼形式存在,修饰分子筛的外表面.     

同晶置换改性FAU分子筛结构及吸附性能的密度泛函理论研究

采用密度泛函理论(DFT)方法,考察了八面沸石(FAU)型分子筛β笼孔道结构内含氧化合物(甲醇、二甲醚、丙醛)的吸附,并进一步计算研究了Zn,Ca同晶置换改性的作用机理.研究结果表明,β笼孔道结构内,Al原子为甲醇、二甲醚和丙醛的吸附活性位,Si原子无吸附活性.Zn,Ca掺杂的β笼结构内,正2价的Zn和Ca掺杂替换正3价的Al,导致邻近的Si原子位置形成缺电子空穴,增强了甲醇、二甲醚和丙醛的吸附,而杂原子Zn和Ca本身并没有吸附活性.     

处理条件对Mo/HZSM-5催化剂结构及性能的影响

使用付立叶变换红外光谱和固体高分辨核磁技术，考察了在不同处理条件下，甲烷无氧芳构化催化剂Ｍｏ／ＨＺＳＭ５分子筛骨架结构的变化情况．结果显示，较高的焙烧温度和较高的预处理温度，都会造成催化剂中担载的钼物种对分子筛骨架铝的严重抽提，特别是在钼物种含量较高时，这种情况更明显．以不同担体及不同钼物种构成的前驱态催化剂的催化反应评价结果表明，以微晶状态存在的ＭｏＯ３和与担体间以相对较弱作用力存在的高分散钼物种，在反应过程中被活化成具有催化活性钼物种的几率更大；而晶相状态的Ａｌ２（ＭｏＯ４）３以及与载体间存在较强相互作用力的钼物种，在甲烷无氧芳构化过程中不起主要催化活性作用．计算得出，甲烷无氧脱氢芳构化反应的表观活化能为８９．８ｋＪ／ｍｏｌ．     

用密度泛函理论研究NO在H-ZSM-5分子筛不同酸性位的吸附

利用密度泛函理论(DFT),基于7T簇模型,在B3LYP/6-31G(d,p)水平上研究了NO分子在H-ZSM-5分子筛孔道中α,β,γ酸性位的吸附.在计算过程中,首先对H-ZSM-5的α,β,γ酸性位进行优化计算,然后对NO分子η1-N和η1-O两种吸附模式的红外光谱和吸附能进行计算.计算结果表明,NO分子以η1-N模式吸附于H-ZSM-5分子筛酸性位上,不同酸性位对NO分子的吸附能力排序为:α酸性位>β酸性位>γ酸性位.此外,H-ZSM-5分子筛直型孔道更有利于NO分子的吸附和扩散,因而可更有效地促进NO分子催化分解反应的进行.     

Methane Activation by Diatomic Molybdenum Carbide Cations

Metal carbide species have been proposed as a new type of chemical entity to activate methane in both gas‐phase and condensed‐phase studies. Herein, methane activation by the diatomic cation MoC⁺ is presented. MoC⁺ ions have been prepared and mass‐selected by a quadrupole mass filter and then allowed to interact with methane in a hexapole reaction cell. The reactant and product ions have been detected by a reflectron time‐of‐flight mass spectrometer. Bare metal Mo⁺ and MoC₂H₂⁺ ions have been observed as products, suggesting the occurrence of ethylene elimination and dehydrogenation reactions. The branching ratio of the C₂H₄ elimination channel is much larger than that of the dehydrogenation channel. Density functional theory calculations have been performed to explore in detail the mechanism of the reaction of MoC⁺ with CH₄. The computed results indicate that the ethylene elimination process involves the occurrence of spin conversions in the C?C coupling (doublet→quartet) and hydrogen atom transfer (quartet→sextet) steps. The carbon atom in MoC⁺ plays a key role in methane activation because it becomes sp³ hybridized in the initial stages of the ethylene elimination reaction, which leads to much lower energy barriers and more stable intermediates. This study provides insights into the C?H bond activation and C?C coupling involved in methane transformation over molybdenum carbide‐based catalysts.     

多级结构ZSM-5沸石分子筛的合成及其Mo基催化剂在甲烷无氧脱氢芳构化中的应用

以经盐酸预处理的碳纳米管为第二模板,在不添加其它有机溶剂的情况下,仅通过控制晶化条件,即采用变温水热晶化法合成具有多级结构的ZSM-5分子筛.通过x射线衍射、红外光谱测试、透射电镜和N2吸附对合成的分子筛进行了表征,结果表明,该合成分子筛呈近球形,是由纳米棒自组装形成的具有多级结构的亚微米球.该分子筛改性后用于甲烷无氧脱氢芳构化反应,显示出良好的催化性能,甲烷转化率最初达到19%,反应至24 h时甲烷转化率仍保持在10%左右,并且保持了较高的芳香物选择性(达到50%以上).     

Catalytic Combustion of Methane over High Copper-Loading ZSM-5 Catalysts

Two series of Cu/ZSM-5 catalysts,loading from 5 to 20 wt% CuO,were prepared by the deposition-precipitation and impregnation methods,respectively.The catalysts prepared by the impreg- nation method showed better catalytic performances than those prepared by the deposition-precipitation method and the increase of copper loading favored methane conversion.20Cu(I)/ZSM-5 had the highest activity with T_(90%)of 746 K,and for 20Cu(D)/ZSM-5,T_(90%)was as high as 804 K.The characteriza- tion of X-ray diffraction(XRD),temperature-programmed reduction(TPR),temperature-programmed desorption(TPD),and X-ray photoelectron spectroscopy(XPS)revealed that the dispersion of cop- per species could be improved by using the deposition-precipitation method instead of the impregnation method,but the fraction of surface CuO,corresponding to active sites for methane oxidation,was larger on 20Cu(I)/ZSM-5 than 20Cu(D)/ZSM-5.The results of Pyridine-Fourier transform infrared spectrum (Py-FT-IR)showed that a majority of Lewis acidity and a minority of Brφnsted acidity were present on Cu/ZSM-5 catalysts.20Cu(I)/ZSM-5 presented more Lewis acid sites.The number of Lewis acid sites changed significantly with preadsorption of oxygen.Adsorption of methane and oxygen on acid sites was observed.The properties of Cu/ZSM-5 catalysts were correlated with the activity for methane oxidation.     

金属-有机骨架材料中甲烷吸附机理的密度泛函理论研究

采用密度泛函理论研究了甲烷在MOF-5中的吸附位置、吸附构型和吸附能. 结果表明: 吸附位置主要有四种, Zn₄O簇为最佳吸附位, 其吸附能为17.38 kJ•mol^－1, 高于沸石中的甲烷吸附能. 从吸附能与MOF-5的结构关系分析得出: 在苯环中引入给电子基团, 有利于增强甲烷与MOFs的吸附作用; 引入含氧等极性官能团, 将增加甲烷吸附位, 有利于提高吸附储存量.     

Theoretical Study of Ethanethiol Adsorption on HZSM-5 Zeolite

The density functional theory and the cluster model methods have been employed to investigate the interactions between ethanethiol and HZSM-5 zeolites. Molecular complexes formed by the adsorption of ethanethiol on silanol H3SiOH with two coordination forms, model Bronsted acid sites of zeolite cluster H3Si(OH)Al(OH)2SiH3 interaction with ethanethiol, aluminum species adsorbed ethanethiol have been comparatively studied. Full optimization and frequency analysis of all cluster models have been carried out using B3LYP hybrid method at 3-21G basis level for hydrogen atoms and 6-31G(d) basis set level for silicon, aluminum, oxygen, carbon, and sulfur atoms. The structures and energy changes of different coordination forms of H3Si(OH)Al(OH)2SiH3-ethanethiol, silanol-ethanethiol and Al(OH)3-ethanethiol have been studied. The calculated results showed the nature of interactions was van der Waals force as exhibited by not much change in geometric structures and properties. The preference order of ethanethiol adsorbed on HZSM-5 zeolite may be residual aluminum species, bridging hydroxyl groups and silanol OH groups from the adsorption heat. The adsorbed models of protonized ethanethiol on bridging hydroxyl OH groups and linear hydrogen bonded ethanethiol on bridging OH groups suggested in literature might not exist as revealed by this theoretical calculation. Possible adsorption models were obtained for the first time.