Amorphous microporous mixed oxides as selective redox catalysts

Amorphous microporous homogeneously mixed oxides based on Ti or V in silica can be prepared by an acid catalyzed solgel process. The materials have been characterized by TEM, UV, IR and sorption methods. The glasses can be used for the selective epoxidation of alkenes. The catalytic properties are comparable to those of the well known crystalline Ti-containing zeolites TS-1, Ti-MCM-41 and Ti-Beta. Amorphous microporous homogeneously mixed metal oxides seem to be promising new catalytic materials supplementing the well studied selective zeolites.     

129Xe NMR Investigations for the Textural Characterization of Sol-Gel Derived Amorphous Microporous Silica

The aim of this work was to evaluate the potentialities of 129Xe NMR as an innovative technique for the textural characterization of microporous amorphous materials. We applied this technique to a series of sol-gel derived meso- and microporous silica materials with relatively narrow pore size distributions. The study of the Xe chemical shifts (Xe) as a function of the Xe pressure (PXe) is classically used to extrapolate Xe to zero xenon loading. This parameter can be related to the mean sample pore size using models which cannot however be considered as generally applicable for every type of material. In this study, several original results related to the NMR line shapes and to the slopes, sXe, of the lines Xe = f(PXe) have been obtained. NMR data have been interpreted on the basis of the sample textural characteristics determined by N2 adsorption. Extension of the results to other microporous materials such as zeolites has also been considered. The results showed that sXe is a useful parameter which can be related to the pore hydraulic radius of the different amorphous and zeolitic microporous materials investigated.     

Spectroscopic characterisation of microporous aluminophosphate materials with potential application in environmental catalysis

This review article deals with a spectroscopic characterisation, including FTIR, UV-Vis-NIR and NMR, of acid and redox microporous aluminophosphate catalysts with chabasite-related structure. These materials show high thermal and hydrothermal stability, and for this reason are attractive catalysts for environmental application. An extended investigation of acid SAPO-18 and SAPO-34, which are selective catalysts in methanol-to-olefins (MTO) and oxidative dehydrogenation (ODH) of light alkanes processes, will be presented. These catalysts can also be used as supports for the preparation of metal-containing molecular sieves for De-NO_x reactions. Catalytic studies of NO oxidation to NO₂ and N₂O decomposition performed on cobalt- and copper-containing materials will be illustrated and correlated to spectroscopic results.     

中孔材料在酸催化反应中的应用

随着M41S系列材料成功合成,中孔材料在催化领域的应用备受关注。中孔材料不仅可以作为酸、碱或氧化还原催化反应的催化剂,且由于内表面存在大量硅羟基,可稳定地与有机物或有机金属复合物的基团相结合,还可以作为高分散度的金属或氧化物的载体。对中孔材料在烷基化和裂化两类酸催化反应中的一些重要应用进行分析,指出中孔材料在新催化体系中的应用以制备具有晶体孔壁的、均一的和可调变中孔孔道的中孔材料为发展方向,同时设法降低合成费用,克服在新催化领域的应用障碍。     

多级孔沸石分子筛的制备及其催化应用研究进展

多级孔沸石分子筛在保留了微孔沸石优异的催化活性与择形性的同时,能够从本质上大幅提升沸石分子筛的传质与扩散效率,改善催化剂因积炭问题失活较快的弊端。本文介绍了当前阶段多级孔沸石分子筛的研究现状,主要基于孔径大小的差异,重点综述了微孔-介孔、微孔-大孔以及微孔-介孔-大孔三类具有多级孔道结构的沸石分子筛在制备及催化应用方面的最新研究进展,综合分析了各种制备方法在性能、成本、可操作性及应用上的利弊关系,并且指出,设计并可控地制备出具有多级孔道结构且在三维空间高度贯通的多级孔沸石分子筛材料以最大限度地提高催化效率,将会是未来多级孔沸石分子筛领域的研究重点。     

Transalkylation of Multi-secbutylbenzenes with Benzene over Hierarchical Beta Zeolite

A hierarchical beta zeolite synthesized by quasi-solid phase conversion method was characterized by BET, scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (X...     

Synthesis and Reactivity of Al-MMM-2: A New Microporous/Mesoporous Catalyst for the Alkylation of Toluene

Porous, mixed-phase aluminosilicate materials containing a microporous MFI phase and a mesoporous Al-MCM-48 phase were synthesized using a one-pot synthesis method. Initially, the gemini surfactant “18-12-18” was used to form Al-MCM-48, and then the MFI template tetrapropylammonium (TPA⁺) was added to the reaction mixture at a later time. Subsequent crystallization at 150 °C led to the formation of a series of mixed-phase materials. These new materials, called “Al-MMM-2” (microporous/mesoporous materials), were characterized using powder X-ray diffraction and ²⁹Si MAS-NMR. Consistent with previous results, the amount of the microporous phase formed was dependent on the crystallization time; thus, a range of materials could be formed from a single reaction mixture. In the alkylation of toluene with benzyl alcohol, Al-MMM-2 materials showed greater structural stability, catalytic activity, and product selectivity during repeated reaction cycles, compared to either pure Al-MCM-48 or pure MFI. This article is dedicated to Professor Christopher Allen, for contributions to inorganic chemistry at the University of Vermont and for his advice, mentorship, and friendship.     

Fe-based electrocatalysts made with microporous pristine carbon black supports for the reduction of oxygen in PEM fuel cells

Fe-based catalysts for the oxygen reduction reaction at the cathode of polymer electrolyte membrane (PEM) fuel cells have been prepared using several highly microporous (defined as pores having a size <2 nm) carbon supports. The aim is to produce better performing catalysts as it is known that catalytic sites are hosted in the micropores of the carbon supports. All catalysts were loaded with a nominal Fe content of 0.2 wt% and were obtained by heat-treatment at 950 °C in pure NH₃ atmosphere. It is demonstrated, however, that the use of highly microporous carbon supports does not lead to improved catalytic activity, as originally expected, since the surface of these micropores is devoid of the nitrogen functionalities necessary to build the catalytic sites. Also, it is shown that for these microporous carbon supports, it is only the new micropores, i.e. those created during NH₃ etching at high temperature, that are capable of hosting catalytic sites.     

Recent Advances in Solid State NMR of Small Molecules in Confinement

In the past years, porous media have become a major focus of materials science, due to their versatile properties, such as high surface area, low specific weight, high surface functionality, and the ability to customize their surface properties. Applications of porous media range from catalysis to separation media to gas storage. All of the mentioned applications involve the introduction of guest molecules into the pores. For efficient application of the materials, it is essential to know the behavior of these introduced molecules in the confined state. Solid state (ss) NMR gives a unique insight into the dynamics, the guest-host interactions, and the binding sites of porous materials and is probably the most powerful characterization method for probing a huge variety of real-life systems. Recent results in research of microporous zeolites and periodically mesoporous silica (PMS) materials using NMR will be highlighted.     

Facile synthesis of hydrophobic microporous silica membranes and their resistance to humid atmosphere

The ethylene-modified silica membranes were prepared by the acid-catalyzed co-hydrolysis and condensation reaction of tetraethylorthosilicate (TEOS) and ethylenetriethoxysilane (TEVS) in ethanol and the final materials were characterized by scanning electron microscope (SEM), water contact angle measurement, solid-state ²⁹Si magic angle spinning nuclear magnetic resonance (²⁹Si MAS NMR), and N₂ adsorption. The modification leads to a transform from superhydrophilicity for the unmodified silica membranes to hydrophobicity for the modified materials. The ethylene-modified silica membranes are much less water sensitive than the unmodified materials because the hydrophobic ethylene groups replace a portion of the hydrophilic hydroxyl groups on the pore surface. The modified materials process a microporous structure with a narrow pore size distribution centered at 1.1 nm. Such a microporous structure can be stabilized after exposured to humid atmosphere for 450 h, in intense contrast to the collapse of the micropores in the unmodified silica membranes.     

First evidence of interconnected micro and mesopores in CMK-3 materials

The porous structure and the pore interconnections of mesoporous carbon of CMK-3 type have been studied by means of hyperpolarized ¹²⁹Xe NMR spectroscopy.Parallel studies of purely microporous carbon and CMK-3 have unambiguously revealed the presence of micropores inside the CMK-3 structure. In addition, ¹²⁹Xe 2D-exchange NMR experiments have clearly shown direct exchange of Xe atoms between the micro and the mesoporosity. This indicates the presence of micropores inside the carbon rods constituting the mesoporous structure of CMK-3 materials.     

Improving of Micro Porous Layer based on Advanced Carbon Materials for High Temperature Proton Exchange Membrane Fuel Cell Electrodes

This work reports the study of four different carbon materials for their application as carbon material in microporous layers for high temperature proton exchange membrane fuel cells electrodes. The microporous layers were prepared with carbon black (a commercial one, Vulcan XC72), two different carbon nanofibers, CNF, (Ribbon and Platelet structure) and carbon nanospheres, all of them prepared in our lab. The microporous layers were characterized by XRD. The hydrophobicity, electrical conductivity, and permeability to different gases were also evaluated. The stability is an important issue to be overcome in the field of proton exchange membrane fuel cells. Thus, accelerated thermal and electrochemical degradation tests in phosphoric acid media were carried out to evaluate the stability of the different advanced materials tested under the same conditions. From all the performed essays, the carbon nanospheres were the best nano‐carbon materials because of the lower degradation degree shown by the microporous layer prepared with them and the good conductivity and permeability achieved, whereas CNF with a Platelet structure showed a low electrochemical stability due to their greater edge plane exposure which favors their corrosion.     

Hybrid zeolitic-mesostructured materials as supports of metallocene polymerization catalysts

The potential application of hybrid ZSM-5/Al-MCM-41 zeolitic-mesostructured materials as supports of metallocene polymerization catalysts has been investigated and compared with the behaviour of standard mesoporous Al-MCM-41 and microporous ZSM-5 samples. Hybrid zeolitic-mesostructured solids were prepared from zeolite seeds obtained with different Si/Al molar ratios (15, 30 and 60), which were assembled around cetyltrimethylammonium bromide (CTAB) micelles to obtain hybrid materials having a combination of both zeolitic and mesostructured features. (nBuCp)₂ZrCl₂/MAO catalytic system was impregnated onto the above mentioned solid supports and tested in ethylene polymerization at 70 °C and 5 bar of ethylene pressure. Supports and heterogeneous catalysts were characterized by X-ray powder diffraction, nitrogen adsorption-desorption isotherms at 77 K, transmission electron microscopy, ²⁷Al-MAS-NMR, ICP-atomic emission spectroscopy and UV-vis spectroscopy.Catalysts supported over hybrid ZSM-5/Al-MCM-41 (Si/Al = 30-60) exhibited the best catalytic activity followed by those supported on Al-MCM-41 (Si/Al = 30-60). However, catalyst supported on ZSM-5 gave lower polymerization activity because of its microporous structure with narrower pores and lower textural properties than hybrid and mesoporous materials.Although higher acid site population shown by hybrid materials could contribute to the stabilization of the metallocene system on the support, in this case their better catalytic performance is mainly ascribed to the larger textural properties.     

Inhibition of the catalytic oxidation of carbon/carbon composite materials by an aluminophosphate coating

The oxidation protection property of an aluminophosphate coating used to increase the lifetime of C/C composite materials has been investigated. This coating has already shown its ability to limit the thermal oxidation at high temperatures of C/C materials and we demonstrate in this paper its capacity to inhibit the catalytic oxidation. This catalytic oxidation, induced at low temperature by alkali and alkaline-earth elements (Na, K, Ca), is a crucial issue for aeronautical applications of C/C composite materials because of the presence of these catalytic elements in de-icing solutions used in airports. In addition, new information about the structure of the coating, prepared through the thermal treatment of a commercial aluminium phosphate solution, has been derived from XRD and solid-state NMR investigations.     

Recent Advances in the Control and Characterization of the Porous Structure of Pillared Clay Catalysts

Several methods have been developed and applied in recent years in the characterization of the structure of microporous materials, in general, and of pillared clays, in particular. In the present article, the latest results obtained in the control of the microstructure developed in alumina-pillared clays and various approaches used to the evaluation of the porosity of these materials are reviewed. These include the control of the microstructure developed in pillared clays by adjusting the several parameters involved in the synthesis process, and the evaluation of the porosity of pillared clays. Emphasis is given to the methods to evaluate the pore size and pore size distribution (PSD) of these materials. Two separate sections are devoted to review mathematical modeling studies and ¹²⁹Xe NMR spectroscopy of physisorbed Xe to investigate the microporous structural properties of these materials.     

Recent Advances in the Control and Characterization of the Porous Structure of Pillared Clay Catalysts

Several methods have been developed and applied in recent years in the characterization of the structure of microporous materials, in general, and of pillared clays, in particular. In the present article, the latest results obtained in the control of the microstructure developed in alumina‐pillared clays and various approaches used to the evaluation of the porosity of these materials are reviewed. These include the control of the microstructure developed in pillared clays by adjusting the several parameters involved in the synthesis process, and the evaluation of the porosity of pillared clays. Emphasis is given to the methods to evaluate the pore size and pore size distribution (PSD) of these materials. Two separate sections are devoted to review mathematical modeling studies and ¹²⁹Xe NMR spectroscopy of physisorbed Xe to investigate the microporous structural properties of these materials.     

Ordered Mesoporous Materials with Improved Stability and Catalytic Activity

Microporous crystals of zeolites such as Y, Beta, and ZSM-5 are widely used commercial catalysts, but their applications are strongly limited by their small pore sizes. Recent progress in solving this is used to ordered mesoporous materials such as MCM-41, HMS, and SBA-15. These mesoporous materials have pore diameters of 2.0–30 nm and exhibit good catalytic properties for the catalytic conversion of bulky reactants. However, when compared with microporous crystals of zeolites, the catalytic activity and hydrothermal stability are relatively low, which severely hinders their practical applications in industrial catalytic reactions such as petroleum cracking. The relatively low catalytic activity and hydrothermal stability can be attributed to the amorphous nature of the mesoporous walls. In this account, we systemically review the routes for improving catalytic activity and hydrothermal stability of mesoporous materials, which include (1) acidic sulfated zirconia supported in mesoporous materials; (2) strongly acidic and thermally stable mesostructured sulfated zirconia with tetragonal crystalline phase; (3) strongly acidic and hydrothermally stable mesoporous aluminosilicates synthesized in alkaline media; (4) strongly acidic and hydrothermally stable mesoporous aluminosilicates synthesized in strongly acidic media; (5) hydrothermally stable mesoporous titanosilicates with catalytically active titanium species in oxidations; (6) high-temperature generalized synthesis of ultrastable ordered mesoporous silica-based materials by using fluorocarbonhydrocarbon surfactant mixtures.     

SAPO分子筛及其应用

磷酸硅铝分子筛(SAPO)是由SiO2、AlO2-、PO2 三种四面体单元构成的微孔型晶体,其孔结构可通过改变不同的合成条件及硅含量来控制.由于骨架呈负电性,具有可交换的阳离子,同时呈现出质子酸性,因此SAPO分子筛被广泛用作吸附剂、催化剂和催化剂载体,是一种具有优异择形选择性、热稳定性和湿热稳定性的新型催化材料.     

Chemical vapor deposition of pyrolytic carbon on carbon nanotubes. Part 2. Texture and structure

In a previous study we showed both the formation of genuine vapor grown carbon fibers (VGCFs) and of new and peculiar carbon nanotube-supported morphologies using a chemical vapor deposition process. Briefly, the latter are an association of beads (or fiber segments) with a more or less rough surface and more or less extended cone-based sub-morphologies with a smooth surface. The investigation of these materials regarding their texture, nanotexture and structure by transmission electron microscopy is reported, as a first step to understanding the formation mechanisms. It is shown that VGCFs exhibit a concentric texture, however with a variable microporous character and nanotexture quality. On the other hand, beads and related morphologies have a coarsely concentric microporous texture, as opposed to the cones and related morphologies, which exhibit a perfectly concentric and dense texture similar to that of perfect multiwall carbon nanotubes. Cross-sections performed with ultra-microtomy have revealed the spatial and textural relationship between cones and beads.     

A titanium containing micro/mesoporous composite and its catalytic performance in oxidative desulfurization

A novel Ti-containing meso–microporous composite molecular sieve, namely Ti-HMS/TS-1, has been successfully synthesized through a two-step crystallization process. The products were characterized by X-ray diffraction, transmission electron microscopy, FT-infrared, UV–visible and N₂ physical adsorption–desorption techniques. The catalytic performances of the composite materials were investigated by means of sulfur compounds oxidation. It has been shown that the Ti-HMS/TS-1 simultaneously possesses a disordered mesoporous phase and a crystalline zeolitic phase and the two phases are composite rather than the physical mixture. UV–visible analysis indicates the existence of framework tetracoordination titanium species (Ti⁴⁺). Composite Ti-HMS/TS-1 exhibits excellent activities in the oxidation of both small and bulky molecular sulfur compounds, which is superior to the pure materials of TS-1 and Ti-HMS.