Influence of acetate, chloride and nitrate ions on the sorption of cobalt by zeolite ZSM-5

Many factors affect the ion exchange process in zeolites. In this work the influence of different anions such as acetate, chloride and nitrate on the ion exchange of cobalt in zeolite ZSM-5 is discussed. After the ion exchange in the presence of those anions no change was found in the zeolite structure, by X ray diffraction and IR spectroscopy. The retention of cobalt by zeolite ZSM-5 at low concentrations (from 0.001 to 0.3N) was higher when the exchange was done with cobalt nitrate and chloride. This behavior was different in the case of 1N cobalt salts, since the highest sorption uptake was found when working with cobalt acetate and its sorption was directly proportional to its concentration.     

Ethylenediamine effect on the sorption of cesium on zeolites ZSM-5 and Y

The sorption of cesium was studied on zeolites ZSM-5 and Y. It was found that the sorption is much higher on zeolite Y than in ZSM-5 and it depends on the crystalline network of the zeolite. It was found as well that the main uptake mechanism involved in both zeolites for cesium was ion exchange.     

Application of quasi-equilibrated thermodesorption of hexane and cyclohexane for characterization of porosity of zeolites and ordered mesoporous silicas

Quasi equilibrated temperature programmed desorption and adsorption (QE-TPDA) of hexane and cyclohexane was applied for characterization of zeolites 5A, ZSM-5, 13X, Y, NaMOR and ordered mesoporous silicas MCM-41, MCM-41/TMB, SBA-15 and HMS. Similar QE-TPDA profiles of hexane and cyclohexane with a single desorption maximum were observed for the wide pore zeolites. No adsorption of cyclohexane for zeolite 5A and a single desorption maximum for ZSM-5 were found, while two-step desorption profiles of hexane were observed for these zeolites. Similar values of the adsorption enthalpy and entropy of hexane and cyclohexane were obtained by fitting the Langmuir model functions for the zeolites X and Y. For NaMOR and ZSM-5 larger differences in these parameters were found. A single desorption peak found at low temperatures in the QE-TPDA profiles of hexane and cyclohexane for the studied silicas was attributed to the multilayered adsorption on their mesopore surface. The adsorption isobars calculated from the thermodesorption profiles were fitted with the BET function. This way values of the specific surface area and the adsorption heat were calculated. Additionally values of the initial heat of adsorption were found by fitting the Henry’s law to the high-temperature sections of the linearized isobars. The largest deviations from the BET and Henry functions and the largest values of the adsorption heats found for SBA-15 indicated the greatest heterogeneity of the adsorption sites on its surface.     

Localization of the copper-containing component in the pore volume of zeolite ZSM-5

The distribution of the copper-containing component in the pore volume of zeolite ZSM-5 has been investigated by H₂ and N₂ adsorption at 77 K and IR spectroscopy. Samples were synthesized by ion exchange and incipient wetness impregnation. Copper-containing clusters are mostly located on the surface of the mesopores formed by packed zeolite nanocrystallites. This causes partial blocking of the volume of microporous channels for N₂ molecules, but these channels remain accessible for H₂ molecules. It has been deduced that no considerable amount of copper located in the structural channels of the zeolite. According to IR spectroscopic data, the sorption of copper ions in the Cu/ZSM-5 catalysts takes place on extraframe-work aluminum, which forms Al-OH-Al bridges and terminal Al-OH groups, and on terminal Si-OH groups located on the zeolite crystal surface.     

氢在沸石上的吸附行为

应用基于Ono-Kondo格子理论得到的通用吸附等温方程, 通过分析氢在不同温度下, 在沸石NaX、CaA、NaA和ZSM-5上的吸附数据, 确定了氢的最大单层吸附容量. 并引入维里吸附方程, 由第二维里吸附系数和圆柱孔的Lennard-Jones(12-6)势模型计算了氢与沸石微孔壁面的作用势. 结果表明, 通用吸附等温方程可较好地描述氢在沸石上的超临界吸附行为, 拟合所得的氢在沸石上的最大单层吸附容量与吸附剂相关, 而与吸附温度无关. 圆柱孔作用势模型计算所得的氢分子在沸石上的吸附作用势与吸附热相近. 氢分子间的作用力表现为吸引力.     

Supercritical CO2 as a superior solvent for the cyclization of diallylmalonate catalyzed by palladium-containing zeolites

Palladium-containing zeolites catalyze the cycloisomerization of diethyl diallylmalonate to dimethylcyclopentenes. When the reaction is carried out in toluene, the performance of the palladium catalyst depends on the pore size of the zeolites. At 60 °C, palladium adsorbed on large pore size Beta zeolite (pore size ∼7.4 Å) is more active than medium pore size ZSM-5 (pore size ∼5.4 Å). This lower activity of ZSM-5 compared to Beta is attributable to the restricted diffusion of reagent and products through the ZSM-5 channels as compared to Beta zeolite. However, due to the gas-like diffusion characteristic of the supercritical state, the activity of ZSM-5 increases and becomes identical to that of Beta zeolite using supercritical CO₂ as medium.     

分子筛催化剂的失活与积炭

分子筛催化剂的失活与积炭刘中民，陈国权，王清遐，梁娟，蔡光宇（中国科学院大连化学物理研究所，大连１１６０２３）关键词分子筛，结炭，催化剂失活，甲醇转化，ＨＺＳＭ－５结炭是酸性分子筛催化剂失活的主要因素．本文通过具有碳链增长、环化、烷基化等多种反应途径...     

Developing the molecular modelling of diffusion in zeolites as a high throughput catalyst screening technique

Molecular modelling techniques have been used to screen zeolite catalysts for their suitability for organic synthesis. For example, we have used these techniques to study the alkylation of aromatic molecules which are important in the fine-chemical and drug industries. A survey of all such efforts is reviewed in this article. The application of molecular modelling techniques in a systematic manner is an efficient first step in the design of zeolite catalysts. As a qualitative screening tool, molecular graphics is used to visualize how well the reactant and product molecules fit inside the pores of the zeolites. Using a hybrid of several molecular modelling methods, which combines molecular dynamics (MD) and Monte Carlo methods with energy minimization, it is possible to determine the minimum energy locations of the molecules inside the zeolites cages. The minimum energy configurations determined by this hybrid method are taken as a starting point for diffusion of the molecules through the zeolite channels. When a molecule is allowed to diffuse through zeolite channel, the molecule attains some maxima and minima in its diffusion energy profile. From the differences between a maximum and a minimum energy configuration, the diffusion energy barrier for the molecule can be calculated in the zeolites. By comparing the diffusion energy barriers for various isomers of a molecule in different zeolites, it is possible to find out the most suitable zeolite for achieving the required shape-selectivity. In addition, factors influencing the diffusivity of the molecules and consequently the shape selectivity are derived. The list of factors and their relative importance are analysed to derive valuable guidelines to design shape-selective zeolite catalysts for a given reaction. Thus, the ultimate aim of these studies is to develop a high throughput computational screening process for the selection of shape-selective zeolite catalysts for various reactions. The dynamic behaviour of molecules inside the pores of zeolites can be studied using MD methods. Since MD is computationally time consuming, it is more efficient to screen the possible zeolite catalysts by energy minimization methods and then perform MD in specific zeolites. More accurate values of diffusivity of the molecules can be calculated using MD methods, and these values can be correlated with the shape-selectivity observed experimentally and /or derived from diffusion energy barrier calculations.     

The influence of pore structure on reaction mechanism of propylene dimerization in zeolite: A theoretical viewpoint

Molecular sieves have been widely used in the petrochemical industry as environment-friendly catalysts. The pore structure is an important factor influencing the catalytic performance of the zeolite. In this work, a combined Our own N-layered Integrated molecular Orbital and Molecular mechanics method was used to study the mechanism of propylene dimerization in four zeolites (ZSM-5, BEA, MCM-22, and MOR) with different pore structures. Comparing the stepwise mechanism and the concerted mechanism, it is found that the two mechanisms compete with each other in the macroporous BEA, MCM-22, and MOR zeolites, and both mechanisms are possible. However, in ZSM-5 zeolite with medium pore size, the propylene dimerization reaction tends to proceed according to the stepwise mechanism. Furthermore, no matter which mechanism is adopted, the activation energy of propylene dimerization reaction in MCM-22 is the smallest in the four zeolites, indicating that its MWW-type structure (A framework type defined by the International Zeolite Association) may be the most favorable pore structure for the reaction and possesses the highest catalytic activity.     

Entropy effects during sorption of alkanes in zeolites

Recent developments in Configurational-Bias Monte Carlo (CBMC) techniques allow the accurate calculation of the sorption isotherms for alkanes, and their mixtures, in various zeolites. The CBMC simulations give new insights into subtle entropy effects affecting mixture adsorption. Three types of entropy effects can be distinguished. (1) Size entropy effects favour the component with the smaller number of C atoms because the smaller molecule finds it easier to fill in the 'gaps' within the zeolite matrix at high molecular loadings. (2) Configurational entropy effects come into play for mixtures of alkanes that differ in the degree of branching. For a mixture of linear and branched alkanes with the same number of C atoms, configurational entropy effects favour the linear isomer because such molecules 'pack' more efficiently within, say, the intersecting channel topology of MFI zeolite. (3) Length entropy effects comes into force for sorption of linear and branched alkanes within the cylindrical channels of say AFI and MOR zeolites; here the double branched alkane has the shortest length and can be packed more efficiently within the channels. We demonstrate that CBMC simulations allow the efficient screening of zeolite structures for a given separation duty and aid the development of novel separation processes exploiting entropy effects.     

Modification of the adsorption and catalytic properties of micro- and mesoporous materials by reactions with organometallic complexes

This review describes the work of two laboratories in the field of the modification of micro- and mesoporous molecular sieves through reactions with organometallic complexes. The modification of zeolites can occur inside the pore channels or on the external surface, depending on the size of the organometallic complex. When the modification occurs on the external surface, it results in a decrease of the pore entrance, which will lead in turn to a modification of the sorption properties of the zeolite, by decreasing the rate of the adsorption (mainly by a kinetic control). Such a material can be also used in catalysis, because the external acid sites, which are responsible for side-reactions, have been removed upon grafting. When small organometallic complexes are used, they can fill the channels and cages of the zeolite and react with internal hydroxyl groups. Due to the high acidity of zeolites, the reaction occurs very easily (for example at ?100 °C on faujasite), in contrast to what is observed on the external surface, therefore leading to high metal loadings. In that case, the modification of the sorption properties will be mainly related to a thermodynamic control. The resulting materials can be useful in catalysis, by combining the activity of the organometallic complex and properties (for example shape-selectivity) of the zeolite. Modification of mesoporous molecular sieves occurs always in the pores and results in altering of the sorption properties of the solid, by changing the interaction type between the sorbent and the sorbate. For example the sorption isotherm of alkanes is changed from type II to type III according to the IUPAC nomenclature.     

沸石分子筛上程序升温脱附谱的Monte Carlo模拟研究 Ⅰ．模型的建立

沸石分子筛因其特殊的孔道结构而致使其ＴＰＤ谱图的分析比其它催化剂更为困难，目前尚无一种切实可行的理论分析方法。本文发展了一种较为简单的模拟分子筛上ＴＰＤ谱图的ＭｏｎｔｅＣａｒｌｏ方法，模拟结果表明，在同一组脱附动力学参数下，沸石分子筛上的峰形和峰位置与其它催化剂都有明显的差异，并且峰温的差别与脱附活化能和指前因子有关。同时表明，对于普通催化剂可以根据峰最大时的覆盖度θ＿Ｍ来判断脱附级数，而对于沸石分子筛，θ＿Ｍ却随Ｅ＿ｄ线性变化。     

FER型沸石的合成、结构与吸附性质

在TMEDA(四甲基乙基二胺)-Na2O-SiO2-Al2O3-H2O体系(I),Na2O-K2O-Al2O3-SiO2-H2O-HCO3^--CO3^2^-体系(II)及Py(吡啶)-PrNH2(正丙胺)-HF-SiO2-H2O体系(III)中, 分别合成了纯相FER沸石及FER硅沸石。用粉末XRD, FT-IR, 29Si MAS NMR及TG/DTA等表征其结构性质, 并用超微量电子真空吸附天平测定这些沸石样品对正己烷, 甲醇和水的吸附等温线。结果表明: 各体系合成的样品虽然结晶度高, 呈现出FER沸石的典型结构特征, 但由于它们的组成和晶格微结构不同, 热稳定性与吸附性质有明显的差异。在(I)体系中合成的FER沸石层错缺陷少, 晶格完美, 正己烷与甲醇的吸附量可达到理论值, 结构破坏温度为1190℃。红外精细谱及29Si MAS NMR高分辨谱证明FER硅沸石具有十分完美的骨架结构。由于晶胞收缩, 它对正己烷与甲醇吸附量略低于理论值, 并呈现出高度的疏水性。它的结构破坏温度高于1300℃。在(II)体系中合成的FER型沸石结构缺陷多, 沸石孔中的钾离子不易被质子完全交换。它的正己烷与甲醇吸附量均较低, 而水的吸附量相对较高。吸附现象表明, 正己烷和甲醇都被吸附于FER沸石的十元环主孔道中, 分压较高时, 甲醇可通过八元环进入小笼, 而水的吸附性质则主要与各样品的Si-OH缺陷及骨架中的阳离子含量有关。     

Surface processes during sorption of aromatic molecules on medium pore zeolites

The elementary steps of sorption and transport of benzene, toluene, and o- and p-xylene from the gas phase to hydroxy groups of zeolite H/ZSM-5 on the outer surface (SiOH groups) and in the pores (SiOHAl groups) were studied using pressure modulations followed by fast time-resolved IR spectroscopy. Sorption on these acid sites occurs via a common physisorbed state on the outer surface. The equilibration of the molecules in this state is fast compared to the sorption rates on SiOH and SiOHAl groups. The relative rates of equilibration of functional groups with the aromatic molecules suggest that the aromatic molecules move freely on the surface of the outer surface before reversibly binding to OH groups, entering the micropores or desorbing. Molecules able to enter into the pores (benzene, toluene, p-xylene) adsorb faster on SiOHAl groups than on SiOH groups. If the access of the molecules into the pores is sterically constrained (o-xylene), the rate of adsorption on the remaining accessible SiOH groups is strongly enhanced.     

Synthesis and Characterization of ZSM-5/β Co-Crystalline Zeolite

ZSM-5/β co-crystalline zeolites with different content of ZSM-5 have been synthesized by adding different amount of ZSM-5 to the synthetic system of β zeolite with NaAlO2, silica sol as the source of aluminum and silica, respectively, and TEA as the template under controlled condition of the synthesis. The ZSM-5/β co-crystalline zeolite was studied by XRD, SEM, BET and NH3-TPD. The reaction activity of toluene alkylation was investigated with a mixture of toluene-methanol as the feedstock in a pulse micro-reactor over the ZSM-5/β co-crystalline zeolite. It is found that ZSM-5/β co-crystalline zeolite has two kinds of zeolite structure including ZSM-5 and β zeolite, not in the form of a physical mixture. The pore structure of ZSM-5/β co-crystalline zeolites is different from that for β zeolite, ZSM-5 and their physical mixture. In addition, the peaks of both high and low temperature desorption of ammonia over the ZSM-5/β co-crystalline zeolite shift 23 ℃ to lower temperatures and the acid amount of its strong acid is 3% more than the physical mixture. So the ZSM-5/β co-crystalline zeolite produces the highest content of xylene, which is 10.4% higher than the physical mixture. And the ZSM-5/β co-crystalline zeolite has better selectivity for toluene alkylation and weaker de-methylation than β zeolite, ZSM-5 and their physical mixture.     

Conjugate Conversion of Methanol and Lower Alkanes C3-C4 on Zeolite-Containing Catalysts

Simultaneous conversion of methanol and lower alkanes C₃-C₄ on high-silica zeolites of ZSM-5 type of various compositions (SiO₂/Al₂O₃ 30, 50, 90) was studied. The catalytic activity of zeolite catalysts and their physicochemical properties (acidity, sorption capacity, and pore structure) were evaluated.     

The influence of mechanical activation and the nature of a carrier on the structural and diffusion properties of zeolite-containing catalysts

The influence of mechanical activation and the nature of a carrier on the structural and diffusion properties of zeolite-containing catalysts, including the microporous and mesoporous structure of zeolites, was studied. Zeolite-matrix contacts were found to provide the accessibility of the inside volume of zeolite pores. In addition, new pores of a larger size appeared. The IR spectra of adsorbed molecules were used to determine the effective diffusion coefficients of methanol in the porous system of zeolites. It was shown that there was some optimum zeolite pore radius at which the largest diffusion coefficient was attained.     

沸石分子筛上程序升温脱附谱的Monte Carlo模拟研究Ⅱ．不同晶粒的沸石分子筛

沸石分子筛上程序升温脱附谱的ＭｏｎｔｅＣａｒｌｏ模拟研究＊Ⅱ．不同晶粒的沸石分子筛王建国李永旺陈诵英彭少逸（中国科学院山西煤炭化学研究所，太原０３０００１）关键词沸石分子筛，程序升温脱附，ＭｏｎｔｅＣａｒｌｏ模拟沸石分子筛被广泛用作吸附剂和催化剂的根...     

FTIR Studies on NO Adsorption on [Co+Mg(Sr)]/ZSM-5

IntroductionThe FTIR spectroscopy of NO adsorption on[Co +Mg(Sr) ]/ZSM- 5 has not been fullyelucidated yet.Previous researchers working onNO adsorption over Co/ZSM- 5 stated that thebands at 1 81 0 and 1 890 cm-1are due to dinitrosylspecies adsorbed on Co2 +[1] ,while the band at193 5 cm-1is attributable to mononitrosyl species.Aband near1 85 8cm-1(w) may be attributable tothe species of Co3O4 —NO[2 ] .Further,Zhu etal.[2 ]reported the bands at1 81 3 ,1 896 and1 93 9cm-1assigned to …     

温度对沸石酸性的影响

用红外光谱研究温度对沸石酸性的影响,结果表明,温度使沸石表面酸性(酸强度)增强,酸性增强的规律是大孔道或大孔腔中的酸性羟基>表面硅醇基(或缺陷羟基)>小笼中的酸性羟基,在所研究的温度范围内(290～510K)酸性随温度的变化呈线性关系。