Selective adsorption of 2,3-DCDD and 1,2,3,4-TCDD on *BEA,EMT, FAU and MFI-type zeolites as alternative adsorbents for on-line dioxin monitoring

The aim of this study is to evaluate the capabilities of zeolites as selective sorbents for on-line dioxin monitoring. The adsorption isotherms of 2,3-DCDD and 1,2,3,4-TCDD from isooctane on several zeolites with different pore sizes (*BEA, EMT, FAU and MFI-type zeolites) are determined. Selective adsorption is shown for FAU and EMT-type zeolites. 2,3-DCDD is adsorbed by FAU with the highest adsorption capacity and affinity while 1,2,3,4-TCDD is not adsorbed whatever the zeolites. The adsorption of 2,3-DCDD inside the pores of FAU is qualitatively confirmed by diffuse reflectance UV spectrometry and thermogravimetry measurements.     

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.     

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.     

Evaluation of the Secondary Pore Structure of Hydrothermally- and Acid-treated Faujasite Type Zeolites

With regard to H-Y type zeolites dealuminated by hydrothermal and acid treatments, their physical properties were characterized by measurements of 29Si- and 27Al-MAS-NMR, IR, and X-ray diffraction. The secondary pores were quantitatively analyzed by the t-plot method for nitrogen adsorption isotherm at liquid nitrogen temperature and were then compared with the results of samples dealuminated by SiCl4 treatment.The plateau region of the nitrogen adsorption isotherm diminished as hydrothermal and acid treatments proceeded, with the result that the shape of the t-plot changed to that of three straight sections. This t-plot shape suggested that the secondary pores with relatively consistent sizes could develop with the progress of the treatment. On the other hand, SiCl4 treatment was found to produce less secondary pores than hydrothermal and acid treatments. The surface area of micropores calculated from the t-plots gave a considerably higher value than values obtained from the BET equation and from that calculated geometrically. This is attributable to the micropore filling effect.     

Methanol-to-Olefin Conversion over Zeolite Catalysts: Active Intermediates and Deactivation

Methanol-to-olefin (MTO) conversion over various zeolites with different topologies, Si/Al molar ratios, and crystallite sizes were investigated to verify the effects of pore shape and size, acidity, and external surface area on the catalytic activity, product selectivity, and deactivation. The IR and electron spin resonance (ESR) study of zeolite catalysts used in MTO also proceeded to deduce the active intermediates formed in their cages or pores. The zeolites with 8 membered-ring (MR) pore entrances such as CHA, ERI, LTA, and UFI commonly exhibited high selectivity to lower olefins due to their small entrances, but the CHA catalyst with the smallest cage maintained its activity longer than other 8MR zeolites. The slow condensation of polymethylbenzene (PolyMB) to polyaromatic hydrocarbons (PAH) on MOR zeolite with a high Si/Al molar ratio due to its low concentration of strong acid sites resulted in a slow deactivation. The extremely small crystallites of H-SAPO-34 and H-ZSM-5 less than 100 nm showed an adverse effect in MTO; while the large crystallites above 1,000 nm also exhibited poor catalytic performance because of their small external surface. The study of IR regarding the adsorbed and occluded materials on zeolites demonstrated the effect of pore shape and size on the active intermediates: the zeolites with larger pores and cages allowed the formation of alkylbenzenes with long alkyl groups which preferred to be condensated to PAH. The well-resolved hyperfine splitting of ESR spectra observed on H-SAPO-34 used in MTO clearly illustrated the presence of hexamethylbenzenium radical cations. The small intersections of phosphorous-modified H-ZSM-5 allowed the formation of tetramethylbenzenium radical cations in MTO. The formation of PolyMB radical cations, their role as active intermediates and the effect of topology, acidity, and crystallite size of zeolites on their deactivation were discussed.     

改性Y沸石的孔结构与催化性能

测定了不同方法改性的Y沸石样品的N_2吸附和脱附等温线, 并计算了样品的微孔、大孔和二次孔的孔容和表面积, 以及样品的二次孔分布, 证实改性方法对样品的孔结构有显著的影响。同时, 还考察了不同尺码探针分子在改性Y沸石样品上的酸催化反应活性, 将所得数据与样品的酸量、酸强度和二次孔容相关联, 取得了满意的结果。说明对大尺码反应分子, 改性过程中生成的大孔径二次孔, 对提高沸石催化剂的反应活性是有利的。     

Structure of the first silicate molecular sieve with 18-ring pore openings, ECR-34

The three-dimensional microporosity of zeolite frameworks have allowed their widespread use in industry as heterogeneous catalysts, absorbents, and ion-exchangers. While the phosphate analogues of zeolites having up to 24 tetrahedral atoms in the pore openings are known, silicate-based zeolites have, until now, been limited to 14-membered ring pore openings. We now disclose the structure and characterization of the synthetic zeolite ECR-34, which can be prepared from a mixed alkali metal reaction gel containing tetraethylammonium (TEA) cations. Its structure has been determined from powder diffraction data and shows ECR-34 to be hexagonal with the dimensions a, b = 21.030(1) A, c = 8.530(1) A, containing one-dimensional, 18-ring pores with 10 A diameter free openings. ECR-34 is stable to 800 degrees C and is able to absorb and ion-exchange large organic molecules. The existence of ECR-34 suggests the potential of preparing other thermally stable silicate molecular sieves with extra-large pores.     

Availability and interconnectivity of pores in mesostructured ZSM-5 zeolites by the adsorption and diffusion of mesitylene

Probing the mesopore architecture in mesoporous zeolites is of importance for large scale applications of the materials. In this work, the adsorption and diffusion of mesitylene with larger molecule size in mesoporous ZSM-5 zeolites were carried out, in order to acquaint the availability and interconnectivity of mesopores in zeolite crystals. The comparisons of the shape of adsorption isotherms and the mesopore volume calculated from mesitylene and N₂ adsorption in mesoporous ZSM-5 zeolites with different mesoporosities can be used to discriminate two cases of mesopores: accessible mesopores connected to external surface of the zeolite crystals and non-accessible meso-voids that are occluded in the microporous matrix. Furthermore, the effective diffusivity and activation energy of mesitylene in mesoporous ZSM-5 extracted from ZLC desorption curves as a function of mesopore volume calculated from mesitylene adsorption reveal the enhancement of mesopore interconnectivity to molecule diffusion in zeolite crystals.     

Diffusion of Ethane-Hydrogen Mixtures in LiLSX and CaA Zeolites Studied by Diffuse Reflectance IR Spectroscopy

The kinetics of the adsorption of ethane-hydrogen mixtures at low pressures (5–20 Torr) on the zeolites LiLSX and CaA are studied by diffuse reflectance IR spectroscopy. The adsorption of ethane mixed with hydrogen on the granulated zeolites is two orders of magnitude slower than the adsorption of pure ethane. Comparison of the adsorption kinetics of ethane-hydrogen mixtures on granulated samples and thin pellets shows that the transport of ethane into the microcrystals of the granular zeolites is retarded primarily by the counter-diffusion of ethane and hydrogen in the intercrystallite pores.     

Untersuchungen an Nickel-Zeolith-Trägerkatalysatoren. I. Herstellung und Charakterisierung von Nickel-Zeolith-Trägerkatalysatoren

By means of cation exchange nickel zeolites of the types A, X and Y were prepared with different (0.5–12.5% of weight) nickel, contents. Nickel zeolites of the A type up to 14% exchange and zeolites of the X type up to 27% exchange do not show changes in structure. Nickel zeolite catalysts were prepared by reduction of the nickel zeolites with hydrogen. Analyses of nickel metal in these catalysts showed that only part of the nickel cations is reduced. By means of hydrogen chemisorption it could be proved that the metallic nickel does not exist in atomic dispersion and is deposited on the external surface as well as inside the pores.     

Multipore Zeolites: Synthesis and Catalytic Applications

In the last few years, important efforts have been made to synthesize so‐called “multipore” zeolites, which contain channels of different dimensions within the same crystalline structure. This is a very attractive subject, since the presence of pores of different sizes would favor the preferential diffusion of reactants and products through those different channel systems, allowing unique catalytic activities for specific chemical processes. In this Review we describe the most attractive achievements in the rational synthesis of multipore zeolites, containing small to extra‐large pores, and the improvements reported for relevant chemical processes when these multipore zeolites have been used as catalysts.     

The appearance of adsorption ability of modified zeolites for sodium dodecylsulfate from its aqueous solution

Hydrophobic faujasite-type zeolites were prepared by the treatment of hydrophilic Na-Y_4.6 with silicon tetrachloride (SiCl₄) by way of dealumination-silicon exchange reaction. Hydrophilic-hydrophobic characters of these zeolite surfaces were evaluated by measurements of immersional heats into water orn-hexane. Adsorption rates of sodium dodecylsulfate (SDoS) on these zeolites from its aqueous solution and their adsorption characteristics were investigated, and compared with those on ZSM-5 with various Si/Al ratios.The adsorption ability of SDoS into zeolite micropores developed on the zeolite only when it was modified to some extent. The adsorption rates of SDoS were very slow and proved to be affected by molecular diffusion in zeolite pores. They depended on hydrophilic-hydrophobic character of zeolites as well as on pore structures. Adsorption isotherms of SDoS were of Langmuir type, which indicates that the adsorption occurred typically into zeolite pores. Adsorbed amount of SDoS depended on the balance of hydrophilic-hydrophobic character and number of cation site of zeolite surfaces, and had a maximum in case of ZSM-5 zeolites.     

Photoluminescence response of terbium-exchanged MFI-type materials to Si/Al ratio, texture, and hydration state

Terbium-exchanged MFI zeolite type materials, i.e., microporous-mesoporous Zeotile-1 with the Si/Al ratio in the range 33-200, Zeogrid with the Si/Al ratio of 75, and nanocrystalline MFI with the Si/Al ratio of 75, were prepared via an ion exchange procedure. All of these zeolites were investigated by means of time-resolved photoluminescence techniques in various hydration states: as-synthesized (hydrated), calcined (heated at 450 degrees C in air), and rehydrated (after a six-month exposure to the atmospheric moisture). The photoluminescence decays and spectra were analyzed by discrete exponential fitting, distribution lifetimes analysis, and area-normalized time-resolved photoluminescence spectra. The results sustained a single average terbium species coordinated to both water molecules and framework oxygens in the hydrated zeolites. The framework contribution increased with the Si/Al ratio in Zeotile-1 and was greatest for the nanocrystalline MFI zeolite. For the calcined Zeotile-1 and Zeogrid, two main terbium species of different environments were found. For the nanocrystalline Tb3+-MFI, a distinct number of species could not be inferred, indicating a more heterogeneous distribution. Rehydration further differentiated among the Tb3+-exchanged zeolites. Photoluminescence line shape and decay of Tb3+-Zeotile-1 were between those of the hydrated and calcined states indicating a slow rehydration rate in contrast with the photoluminescence properties of Tb3+-MFI, which fully recovered the values of the hydrated state. Tb3+-Zeogrid presented an intermediate case: while the PL line shape was fully restored to that measured for the hydrated sample, the decay was still longer than that measured with the hydrated sample. Terbium photoluminescence response related to zeolite texture, Si/Al ratio, and hydration state suggest different sitting and location of terbium in Zeotile-1, Zeogrid, and nanocrystalline MFI materials. In mesoporous Zeotile-1 and Zeogrid, the results sustained two types of terbium sites: one on the internal surface of mesopores, the other inside the pores, while for the nanocrystalline MFI, terbium sites inside the pores predominate.     

Sustainable Synthesis of Pure Silica Zeolites from a Combined Strategy of Zeolite Seeding and Alcohol Filling

Currently, the synthesis of pure silica zeolites always requires the presence of organic structure‐directing agents (OSDAs), which direct the assembly pathway and ultimately fill the pore space. A sustainable route is now reported for synthesizing pure silica zeolites in the absence of OSDAs from a combined strategy of zeolite seeding and alcohol filling, where the zeolite seeds direct crystallization of zeolite crystals from amorphous silica, while the alcohol is served as pore filling in the zeolites. Very importantly, the alcohol could be fully washed out from zeolite pores by water at room temperature, which completely avoids calcination at high temperature for removal of OSDAs in the synthesis of pure silica zeolites.     

Porous structure of natural and modified clinoptilolites

The evaluation of the pore-size distribution (PSD) of natural and modified mesoporous zeolites, i.e., clinoptilolites is presented. We demonstrate the SEM results showing that the pores of fracture-type from 25-50 nm to 100 nm in size between clinoptilolite grains, as well as pores between crystal aggregates up to 500 nm in size are present in the studied material. The detailed distribution of pore sizes and tortuosity factor of the above-mentioned materials are determined from the adsorption-desorption isotherms of nitrogen measured volumetrically at 77 K. To obtain the reliable pore size distribution (PSD) of the above-mentioned materials both adsorption and desorption branches of the experimental hysteresis loop are described simultaneously by recently developed corrugated pore structure model (CPSM) of Androutsopoulos and Salmas. Evaluated pore size distributions are characterized by well-defined smooth peaks placed in the region of the mesoporosity. Moreover, the mean pore diameter calculated from the classical static measurement of nitrogen adsorption at 77 K correspond very well to the pore diameters from SEM, showing the applicability of the CPSM for characterization of the porosity of natural zeolites. We conclude that classical static adsorption measurements combined with the proper modeling of the capillary condensation/evaporation phenomena are a powerful method which can be applied for pore structure characterization of natural and modified clinoptilolites.     

等级孔分子筛的可控合成、扩散研究及催化应用

等级孔分子筛是一类具有两种或多种以特定形式排布的孔结构的分子筛材料. 多层等级的孔结构使得分子筛孔道内的分子扩散得到显著改善, 进而提升了其在吸附和非均相催化等领域的应用性能. 等级孔分子筛的制备策略通常有两种, 即“自上而下”后处理法(如对母样分子筛进行脱铝、 脱硅产生介孔)和“自下而上”合成法(如软模板、 硬模板法). 本文主要对近20年来等级孔分子筛的合成方法进行了梳理, 并着重介绍了具有较高应用潜力的“自上而下”制备法. 鉴于合成等级孔分子筛的主要目的是提高分子的晶内扩散, 对近年来客体分子在等级孔分子筛内扩散的实验研究也进行了简要综述. 此外, 本文还综合评述了等级孔分子筛与传统分子筛在催化应用中的对比, 以展示前者在提升催化性能方面(如活性、 选择性等)的独特优势.     

苯在酸碱处理多级孔Y分子筛上的吸附及传质行为

利用H_4EDTA-NaOH共处理的方法制备了具有不同孔径分布的多级微-介孔NaY分子筛。运用XRD、N_2吸附、SEM、TEM对其结构进行了表征。采用频率响应(FR)和智能重量分析仪(IGA)技术研究了苯在改性后的多级孔NaY分子筛及微孔NaY分子筛上的吸附和传质性能。结果表明,适当的酸碱处理不会改变分子筛的晶体结构,但可调变NaY分子筛的精细结构;介孔的引入降低了分子在孔道中的扩散阻力,较大的孔径和较好的孔道贯通性有利于扩散和吸附中心的可接近性;对于微孔NaY分子筛,苯在分子筛上的吸附过程为其传质过程的速控步骤,对于酸碱处理的多级孔NaY分子筛,分子筛颗粒中微/介孔内的扩散过程及分子筛微-介孔孔道间的分子交换过程是传质过程的速控步骤。     

Dissolution Behavior and Varied Mesoporosity of Zeolites by NH4F Etching

The mesopores formation in zeolite crystals has long been considered to occur through the stochastic hydrolysis and removal of framework atoms. Here, we investigate the NH₄F etching of representative small, medium, and large pore zeolites and show that the zeolite dissolution behavior, therefore the mesopore formation probability, is dominated by zeolite architecture at both nano- and sub-nano scales. At the nano-scale, the hidden mosaics of zeolite structure predetermine the spatio-temporal dissolution of the framework, hence the size, shape, location, and orientation of the mesopores. At the sub-nano scale, the intrinsic micropore size and connectivity jointly determine the diffusivity of reactant and dissolved products. As a result, the dissolution propensity varies from removing small framework fragments to consuming nanodomains and up to full digestion of the outmost part of zeolite crystals. The new knowledge will lead to new understanding of zeolite dissolution behavior and new adapted strategies for tailoring hierarchical zeolites.     

不同来源的钛硅沸石的孔结构,酸性和催化性能

用气固相类质同晶取代和水热方法合成了Ｔｉ－ＺＳＭ－５和ＴＳ－１沸石，对它们的组成、孔结构、酸性和催化活性进行了比较。以季铵盐为模板剂水热合成的无铝ＴＳ－１沸石骨架Ｔｉ含量高，表面无强酸位，且随骨架Ｔｉ含量增加，沸石晶内由缺陷造成的二次孔数量增多。类质同晶取代的Ｔｉ－ＺＳＭ－５沸石骨架Ｔｉ含量有限，样品内尚保留部分骨架Ａｌ，沸石表面强弱酸位同时存在。增加沸石骨架Ｔｉ含量和晶内二次孔对提高苯酚过氧化氢