Altered thermodynamic and kinetic properties of MgH(2) infiltrated in microporous scaffold

MgH(2) nanoparticles with a size of <3 nm were formed by direct hydrogenation of Bu(2)Mg inside the pores of a carbon scaffold. The activation energy for the dehydrogenation was lowered by 52 kJ mol(-1) compared to the bulk material, and a significantly reduced reaction enthalpy of 63.8 ± 0.5 kJ mol(-1) and entropy (117.2 ± 0.8 J mol(-1)) was found for the nanoconfined system.     

The application of Py-GC/MS for the catalytic upgrading of oil separated from summer food waste leachate

The catalytic cracking of oil fractions separated from summer food waste leachate was investigated over BEA zeolite and Al-SBA-15 catalysts. In this study, a mixture of food waste oil fractions and catalyst was directly introduced to pyrolysis gas chromatography/mass spectrometry (Py-GC/MS), with the resulting vapor phase products being simultaneously analyzed. Various acid compounds, including oleic acid, produced by the non-catalytic pyrolysis of food waste leachate were reformed into valuable compounds, such as oxygenates, hydrocarbons, and aromatics. The BEA zeolite catalyst showed higher selectivity for hydrocarbon compounds, especially aromatics, within the gasoline range due to its superior cracking ability originating from its highly acidic sites. Conversely, the cracking performance of the Al-SBA-15 catalyst, possessing mild acidic sites, was lower than that of the BEA zeolite. Increasing the amount of Al-SBA-15 catalyst enhanced the cracking activity and resulted in higher selectivity for hydrocarbons.     

Interpretation of water isotherm hysteresis for an activated charcoal using stochastic pore networks

Water vapor adsorption equilibria on activated carbons typically exhibit hysteresis. The size and shape of the hysteresis loop which separates the adsorption and desorption branches is a strong function of the pore size and interconnectivity of the pores. Neither conventional pore filling models nor statistical thermodynamics approaches provide a means for predicting the extent of hysteresis from only adsorption measurements. This work uses the Kelvin Equation in conjunction with the structural concept of a stochastic pore network to describe measured water isotherms on BPL carbon. Using a pore segment distribution function determined from the adsorption branch, it is shown that totally random assemblies underestimate the extent of hysteresis. It is possible, however, to closely fit the measured BPL-water hysteresis loop using a patchy heterogeneity in which a proportion of the larger pores are preferentially located on the exterior, mid-range pores are concentrated in a sub-surface layer and some large pores form shielded voids behind much smaller pores.Nomenclature p vapor phase partial pressure of sorbate - p _sat saturation vapor pressure of sorbate - R gas constant - r pore radius - T absolute temperature - t adsorbed layer thickness - V _L molar volume of adsorbed phase - surface tension - contact angle     

UV Raman spectrum of 1,3-dimethylcyclopentenyl cation adsorbed in zeolite H-MFI

The first Raman spectrum of an adsorbed carbenium ion has been measured: The 1,3-dimethylcyclopentenyl cation adsorbed in zeolite H-MFI. 1,3-Dimethylcyclopentenyl cation has been observed as a component of the hydrocarbon pool formed during the methanol-to-gasoline process catalyzed by zeolite H-MFI. The Raman shifts recorded for 1,3-dimethylcyclopentenyl cation are in remarkable agreement with computer calculations of the vibrational band positions for the isolated cation. This agreement suggests that the cation is unperturbed by interactions with the zeolite pore walls so that Raman spectra of free or solution-phase hydrocarbons can be used to identify these same species adsorbed in zeolite pores.     

Structure and phase behavior of a confined nanodroplet composed of the flexible chain molecules

A polymer density functional theory has been employed for investigating the structure and phase behaviors of the chain polymer, which is modelled as the tangentially connected sphere chain with an attractive interaction, inside the nanosized pores. The excess free energy of the chain polymer has been approximated as the modified fundamental measure-theory for the hard spheres, the Wertheim's first-order perturbation for the chain connectivity, and the mean-field approximation for the van der Waals contribution. For the value of the chemical potential corresponding to a stable liquid phase in the bulk system and a metastable vapor phase, the flexible chain molecules undergo the liquid-vapor transition as the pore size is reduced; the vapor is the stable phase at small volume, whereas the liquid is the stable phase at large volume. The wide liquid-vapor coexistence curve, which explains the wide range of metastable liquid-vapor states, is observed at low temperature. The increase of temperature and decrease of pore size result in a narrowing of liquid-vapor coexistence curves. The increase of chain length leads to a shift of the liquid-vapor coexistence curve towards lower values of chemical potential. The coexistence curves for the confined phase diagram are contained within the corresponding bulk liquid-vapor coexistence curve. The equilibrium capillary phase transition occurs at a higher chemical potential than in the bulk phase.     

工业Fe-Mn催化剂上基于详细反应机理的F-T合成动力学模型Ⅱ.不同校正方法的动力学模型分析

 通过平衡闪蒸模拟催化剂孔道液体组成、烯烃物理吸附和虚拟烯烃分压等方法,考察了化学反应以外的非本征因素对F-T合成动力学模型的校正. 平衡闪蒸模拟催化剂孔道中烯烃组成的校正计算结果表明,在烯烃浓度出现峰值前,溶解度效应对烯烃再吸附及参与二次反应起主导作用,而在烯烃浓度出现峰值后,烯烃的扩散和物理吸附等效应可能起主导作用. 分析烯烃添加的反应器模拟结果发现,考虑烯烃物理吸附作用的动力学模型校正方法不能够正确反映烯烃添加实验的定性规律,而虚拟烯烃分压校正方法能够正确反映烃分布规律并可定量预测烯烃添加对产物分布规律的影响,这对需要尾气循环的F-T合成工业操作具有重要意义.     

Selective generation of organic aggregates included within metal-organic micropores through vapor adsorption

The ethanol vapor adsorption behavior and the inclusion crystal structure of a 1D-transformable coordination polymer host were characterized. The adsorption jump was observed during phase transition or two-phase equilibrium with abnormal adsorption enthalpy caused by the nature of "mass induced phase transition." The included ethanol guests selectively form O-H...O hydrogen bonded pairs inside channels, suggesting selective construction of a specific cluster/aggregate in pores under control of thermodynamic factors and cooperative intermolecular interactions among the guest and channel surface.     

Modeling of monolayer adsorption of hydrogen on ZSM‐5 zeolite by lattice density functional theory

The adsorption isotherms of hydrogen on microporous zeolite ZSM‐5, at supercritical conditions, have been modeled using the monolayer lattice density functional theory (LDFT) models, where the simple cubic lattice, face‐centered cubic lattice, body‐centered cubic lattice and tetragonal lattice structures are assumed for the arrangements of the adsorption sites inside pores based on the size and shape of the zeolite. The results indicate that the monolayer LDFT models appear to be effective in describing hydrogen adsorption on zeolite ZSM‐5 at supercritical conditions, and the calculated adsorption isotherms agree well with the experimental isotherms measured previously. The layer density of adsorbed phase is presented versus the bulk density and temperature. It is found that the densities of adsorbed phase on adsorbent surface are much higher than the bulk density for temperature range under study. However, in the core region, the layer densities are close to the bulk density. The monolayer adsorption is suitable for hydrogen on ZSM‐5 zeolite. Copyright © 2013 John Wiley & Sons, Ltd.     

Quantitative description of the hydrophobic effect: the enthalpic contribution

A new method of experimental determination of the hydrophobic effect enthalpy is proposed. The method is based on regarding the hydration enthalpy as the sum of the nonspecific hydration enthalpy, specific hydration enthalpy, and the hydrophobic effect enthalpy. The hydrophobic effect enthalpies of noble and simple substance gases, alkanes, arenes, and normal aliphatic alcohols are determined. For the noble gases and alkanes, the hydrophobic effect enthalpy is found to be negative and independent of the size of molecule. For aromatic hydrocarbons, it is positive and grows up with the size of the hydrocarbon. The hydrophobic effect enthalpies of normal aliphatic alcohols are determined by assuming that the specific interaction enthalpies of alcohols in water and in methanol are equal. The hydrophobic effect enthalpy values for the aliphatic alcohols (-10.0 +/- 0.9 kJ.mol(-1)) were found to be close to the alkanes hydrophobic effect enthalpies (-10.7 +/- 1.5 kJ.mol(-1)).     

Characterization of nanometer-scale porosity in reservoir carbonate rock by focused ion beam-scanning electron microscopy

Sedimentary carbonate rocks are one of the principal porous structures in natural reservoirs of hydrocarbons such as crude oil and natural gas. Efficient hydrocarbon recovery requires an understanding of the carbonate pore structure, but the nature of sedimentary carbonate rock formation and the toughness of the material make proper analysis difficult. In this study, a novel preparation method was used on a dolomitic carbonate sample, and selected regions were then serially sectioned and imaged by focused ion beam-scanning electron microscopy. The resulting series of images were used to construct detailed three-dimensional representations of the microscopic pore spaces and analyze them quantitatively. We show for the first time the presence of nanometer-scale pores (50-300 nm) inside the solid dolomite matrix. We also show the degree of connectivity of these pores with micron-scale pores (2-5 μm) that were observed to further link with bulk pores outside the matrix.     

Solute-solvent interactions in cryosolutions: a study of halothane-ammonia complexes

The formation of C-H···N bonded complexes of halothane with ammonia has been studied using infrared and Raman spectroscopy of solutions in the liquid rare gases argon, krypton and xenon, of supersonic jet expansions and of room temperature vapor phase mixtures. For the solutions and for the vapor phase experiments, the formation of complexes with 1:1 and 1:2 stoichiometry was observed. The complexation enthalpy for the 1:1 complex was determined to be -20 (1) kJ mol(-1) in the vapor phase, -17.0 (5) kJ mol(-1) in liquid xenon and -17.3 (6) kJ mol(-1) in liquid krypton. For the 1:2 complex in liquid xenon, the complexation enthalpy was determined to be -31.5 (12) kJ mol(-1). Using the complexation enthalpies for the vapor phase and for the solutions in liquid xenon and krypton, a critical assessment is made of the Monte Carlo Free Energy Perturbation approach to model solvent influences on the thermodynamical properties of the cryosolutions. The influences of temperature and solvent on the complexation shifts of the halothane C-H stretching mode are discussed.     

晶化时间对ZSM-5分子筛物化性质及催化性能的影响

 考察了ZSM－5分子筛在晶化过程中的变化规律及其在苯与乙烯气相烷基化制乙苯反应中的催化性能．结果表明,当晶化时间为70h时,分子筛晶体开始出现;晶化90h时无定形物相基本消失．晶化时间从90h再延长至150h,ZSM－5分子筛晶粒的大小、形貌和体相硅铝比都基本不变,但分子筛表面的硅铝比逐渐降低．以晶化时间为90h的ZSM－5分子筛原粉为活性组元制备的催化剂,对苯与乙烯气相烷基化制乙苯反应表现出最佳的催化性能．     

动物油在不同催化剂上的转化

在固定床微反装置内,对动物油在USY,HZSM-5,ANA,Al2O3和SiO2催化剂上的反应行为进行了研究.结果表明,脂肪酸酯分子一般先发生脱羧和脱羰反应(主要是C-O键的断裂),初始裂化反应产生的大分子烃类及其含氧衍生物会在催化剂的酸性位上发生二次裂化和脱氧等反应.USY和HZSM-5分子筛的酸性强,有利于二次裂化,生成较多的液化气和汽油;在具有弱酸位、大孔的Al2O3催化剂上,发生温和的二次裂化反应,转化率略低;而在非酸性的大孔SiO2催化剂及无孔的ANA分子筛上,二次裂化反应被显著抑制,转化率更低,产物中存在大量的含氧衍生物.热裂化反应会产生较多的CO2,而催化裂化反应会产生相对较多的CO.     

Vapor Pressure and Solid Phases of Methanol below Its Triple Point Temperature

We present an experimental work devoted to study of the thermodynamical properties of solid methanol. We combine Fourier transform infrared spectroscopy (FTIR) and mass spectrometry (MS) to measure, for the first time, the vapor pressure of various methanol solid phases and determine their Clausius-Clapeyron equations. We perform our experiments between T = 130 K and the triple point temperature T(t) = 175.61 K. When methanol is condensed from its vapor below T(t), we observe three different solid phases depending on temperature. A condensation at T = 130 K forms a metastable phase with an enthalpy of sublimation deltaH(metastable-vapor) = 42.9 +/- 0.5 kJ.mol(-1). Upon heating, this phase transforms itself at T approximately 145 K to the alpha-phase that has an enthalpy of sublimation deltaH(alpha-vapor) = 46.9 +/- 0.2 kJ.mol(-1). Cooling the alpha-phase does not lead back to the metastable phase, whereas heating this alpha-phase leads to the beta-phase occurrence at T(alpha-beta) = 157.36 K. This latter one is stable until T(t) and has an enthalpy of sublimation deltaH(beta-vapor) = 44.2 +/- 0.5 kJ.mol(-1).     

Polymorphic transitions in n-hydrocarbon-water and n-alcohol-water binary systems

The mixing of hydrocarbons and alcohols in an excess of water is explained by polymorphic transitions similar to crystallization in an ensemble of water clusters. Enthalpies of transitions of 4.90 ± 0.07 and 2.2 ± 0.3 kJ/mol are obtained for solutions of hydrocarbons and alcohols in an excess of water, respectively. It is concluded that the mixing of water in an excess of hydrocarbons and alcohols is similar to evaporation (the breaking of H-bonds) with an enthalpy of 34 ± 1.4 kJ/mol. It is established that a polymorphic transition occurs between two binodals, and is accompanied by the emergence of microphases (concentration fluctuations) of alcohols in water. Binodals and spinodals in an excess of water and alcohol coincide for butyl and other higher alcohols.     

用于费托合成的骨架Co@HZSM-5核壳催化剂

以骨架Co 为内核, 通过水热合成在其表面包覆HZSM-5 分子筛膜, 制备了具有核壳结构的骨架Co@HZSM-5 催化剂. 采用元素分析、氮物理吸附、粉末X射线衍射、扫描电子显微镜、氨脱附等手段对催化剂的物理化学性质进行了表征. 在气相费托合成反应中, 骨架Co@HZSM-5 核壳催化剂显示了比物理混合的骨架Co-HZSM-5催化剂更好的催化裂解作用, 故C₅-C₁₁汽油段产物选择性高. 通过改变水热时间, 对分子筛膜厚进行了调节, 发现适当的分子筛膜厚在保证催化剂具有较高活性的前提下, 使长链费托合成产物完全裂解, 高选择性地得到汽油段产物. 提高反应温度有利于费托合成反应的进行以及分子筛上裂解效率的提高, 但产物分布向短链烃方向移动. 在水热4天制备的骨架Co@HZSM-5核壳催化剂上及反应温度为250 ℃时, 得到了最佳反应结果, 汽油段产物选择性达79%, 说明费托合成活性中心与催化裂解酸中心之间形成了良好的协同作用.     

Formation Mechanism of the First Carbon–Carbon Bond and the First Olefin in the Methanol Conversion into Hydrocarbons

The elementary reactions leading to the formation of the first carbon–carbon bond during early stages of the zeolite‐catalyzed methanol conversion into hydrocarbons were identified by combining kinetics, spectroscopy, and DFT calculations. The first intermediates containing a C?C bond are acetic acid and methyl acetate, which are formed through carbonylation of methanol or dimethyl ether even in presence of water. A series of acid‐catalyzed reactions including acetylation, decarboxylation, aldol condensation, and cracking convert those intermediates into a mixture of surface bounded hydrocarbons, the hydrocarbon pool, as well as into the first olefin leaving the catalyst. This carbonylation based mechanism has an energy barrier of 80 kJ mol^?1 for the formation of the first C?C bond, in line with a broad range of experiments, and significantly lower than the barriers associated with earlier proposed mechanisms.     

Selective Water Sorbents for Multiple Applications. 11. CaCl2 Confined to Expanded Vermiculite

This paper presents the water sorption properties of a new selective water sorbent based on expanded vermiculite as a host matrix and calcium chloride as a hygroscopic salt. Sorption isobars, isosters and isotherms at T = 30–150°C and vapor partial pressure 8.2–42.0 mbar clearly show that at low water contents crystalline hydrates with 0.33, 1 and 2 molecules of water per 1 molecule of CaCl₂ are formed in the pores. These hydrates are stable over a temperature change of 20–30°C and exhibit kinetically slow transformations. At higher water uptake, the vapor absorption leads to the formation of a CaCl₂ aqueous solution inside the pores, which properties are close to those in the bulk. Isosteric sorption heat was found to depend on water sorption and change from 76.3 kJ/mol for solid hydrates to 39.1–46.6 kJ/mol.     

MFI分子筛限域空间内Pd催化剂上甲烷燃烧

甲烷是一种重要的温室气体,其开发利用过程中不完全燃烧所残留的气体排放到大气中会造成严重的环境问题,因此提高甲烷燃烧效率显得尤为重要.与传统燃烧方式相比,催化燃烧在低温区表现出高的燃烧效率,成为甲烷燃烧理想的选择.在实际应用时,甲烷燃烧催化剂应在低温区具备高的催化活性,同时在过量水蒸气存在下具备好的稳定性.负载型Pd基催化剂是当前研究最多的甲烷燃烧催化剂,Pd粒子尺寸、载体类型、酸性位点以及金属与载体的相互作用是影响甲烷燃烧活性与稳定性的关键因素.本文设计了原位水热合成路线将孤立的Pd离子稳定封装于MFI分子筛孔道内(Pd@MFI),以期获得高活性、高稳定性的甲烷燃烧催化剂,并揭示其反应机理与构效关系.通过X射线粉末衍射、高分辨透射电子显微镜以及球差校正扫描透射电子显微镜分析了Pd@MFI催化剂的基本结构,并直接观测了Pd物种在分子筛晶体中的分布;进而利用氨气程序升温脱附、固体核磁共振、氢气程序升温还原、X射线光电子能谱(XPS)和CO吸附红外光谱等表征技术研究了催化剂的酸性以及Pd在分子筛中的存在状态.表征结果证实,通过原位水热合成方法可将Pd物种以pd2+和Pd(OH)+的形式封装在MFI分子筛孔道内,孤立的Pd离子与分子筛骨架之间存在着强相互作用,有效稳定Pd离子并实现贵金属Pd的最大化利用.在甲烷燃烧反应中,Pd@H-ZSM-5在高空速下表现出较好的催化活性与较低的表观活化能(70.7 kJ/mol).热稳定性及耐水性测试结果表明,Pd@H-ZSM-5在400℃下连续反应100 h后甲烷燃烧活性无明显下降,且反应后Pd物种在分子筛孔道内仍保持高度分散,说明该催化剂在甲烷燃烧过程中具备优异的稳定性和抗烧结性能.通过反应动力学、程序升温脱附以及原位红外光谱等技术手段研究了甲烷催化氧化机理,结果表明,Brφnsted酸性位点的存在有利于甲烷吸附并促进其在相邻Pd位点上活化,在MFI分子筛限域空间内形成Pd位点和Brφnsted酸性位点的有效协同.原位近常压XPS分析结果表明,Pd@H-ZSM-5催化的甲烷燃烧过程中存在着pd2+-pdn+-pd2+的可逆氧化还原循环.综合分析上述结果,最终可阐明Pd@H-ZSM-5模型催化剂上甲烷燃烧的反应机理.     

In-flight kinetic measurements of the aerosol growth of carbon nanotubes by electrical mobility classification

We describe an on-the-fly kinetic study of gas-phase growth of multiwalled carbon nanotubes. The methodology employs electrical mobility classification of the CNT, which enables a direct measure of CNT length distribution in an aerosol reactor. The specific experiment employs two mobility classification steps. In the first step we mobility classify the catalyst particle, in this case Ni, created by pulsed laser ablation, to generate a stream of monodisperse particles. This then determined the diameter of the CNT, when a hydrocarbon/H2 mix is added in a heated aerosol reactor. A second electrical mobility classification step allows us to determine the length distribution of the CNTs. We found that CNT growth from ethylene required the addition of small amounts of water vapor, whereas growth from acetylene did not. We show that acetylene, which always has small amounts of acetone present when purchased, can provide the oxygen source to prevent catalyst coking. By varying the temperature of the growth, we were able to extract Arrhenius growth parameters. We found an activation energy for growth approximately 80 kJ mol(-1) from both acetylene and ethylene, which is considerably lower than previous works for substrate-grown CNTs (E(a) = 110-150 kJ mol(-1)). Furthermore, we observed that our aerosol CNT growth rates were about 2 orders of magnitude higher than those for substrate-grown CNTs. The dominant growth mechanism of CNT previously proposed is based upon bulk diffusion of carbon through nickel particles. However, on the basis of the lower activation energy found in this work, we proposed that the possible mechanism of gas-phase growth of CNT is correlated with both surface (E(a) = 29 kJ mol(-1)) and bulk diffusion (E(a) = 145 kJ mol(-1)) of carbon on nickel aerosol particles. Finally, the experimental approach described in this work should be amenable to other nanowire systems grown in the aerosol phase.