Mesoporous carbon confined conversion of silica nanoparticles into zeolite nanocrystals

A novel synthesis method has been developed to synthesize zeolite nanocrystals with controllable size and size distribution through conversion of colloidal silica nanoparticles in a mesoporous carbon matrix. Colloidal silica nanoparticles serving as the silica source as well as the hard template were embedded in the mesoporous carbon via in situ polymerization of furfuryl alcohol in the presence of tri-block copolymer (Pluronic P123), and then reacted with an alkaline aqueous solution (Na₂O–Al₂O₃–H₂O) infiltrated through the mesoporous channels of the carbon matrix. The synthesis of zeolite NaA nanocrystals is demonstrated. SEM images reveal that the sizes of the zeolite NaA nanocrystals obtained are similar to those of the starting silica nanoparticles. As a result, this synthesis method shows the potential for controlling the nanocrystal size and size distribution of silica-containing zeolites by choosing colloidal silica nanoparticles with a desired particle size distribution.     

Perspectives of Micro/Mesoporous Composites in Catalysis

This article covers the recent development in the remarkably growing field of synthesis, characterization and particularly catalytic investigation of zeolite‐based materials combining micro‐ and mesoporous features. New synthetic approaches for preparation of micro/mesoporous composites including recrystallization of originally amorphous matter, utilization of nanocrystalline zeolite seeds and formation of mesoporous zeolite single crystals are the first focus of this article. The advantages and disadvantages of composite materials in comparison with pure micro‐ and mesoporous molecular sieves will be discussed, as well. The relevance of individual experimental techniques for analysis of the composites, i.e., their structure, porosity, chemical composition, morphological features, and so on, are described in the second section of the article. The last Section is focused on the application of micro/mesoporous composites and mesoporous zeolites as catalysts in acid‐catalyzed reactions, oxidation reactions and synthesis of fine chemicals. The potential of the composites in challenging areas of catalysis for future applications is the final objective of the review.     

Investigation on the morphology of hierarchical mesoporous ZSM-5 zeolite prepared by the CO2-in-water microemulsion method

A series of hierarchical mesoporous ZSM-5 zeolites with different morphology were successfully synthesized by the CO₂-in-water microemulsion method, and mesoporosity was formed without organotemplate. The different synthesis conditions, including silica alumina molar ratio, stirring time and compressed CO₂ pressure, were systematically investigated to discuss the influence of these conditions on the morphology of ZSM-5 zeolite. The resulting samples were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), inductively coupled plasma (ICP) and nitrogen adsorption-desorption measurement. XRD results indicated that compressed CO₂ route for the synthesis of MFI zeolites had a fast crystallization rate and good crystallinity. SEM images showed that the ZSM-5 hierarchical mesoporous ZSM-5 zeolite had a uniform chain-like crystal morphology, whereas silicalite-1 displayed a monodisperse crystal morphology. In addition, the nitrogen adsorption-desorption measurement provided sufficient evidence for the presence of hierarchical mesopores in ZSM-5 zeolite.     

Synthesis of ZSM‐5 by hydrothermal method with pre‐mixing in a stirred‐tank reactor

This work proposed a synthesis route of ZSM‐5 via the hydrothermal method with premixing in a stirred tank reactor (STR). Effects of various operating conditions, including pre‐mixing time, molar ratio of SiO₂/Al₂O₃, TPAOH (organic template agents) concentration, NaCl (alkali metal cations) concentration, crystallization temperature, and crystallization reaction time, on the average particle size (PS) and particle size distribution (PSD) were investigated. It was found that the pre‐mixing time in the STR significantly affect the formation of proto‐nuclei in premixing process and crystal growth in hydrothermal reaction process, and consequently influence the PS and PSD of the prepared ZSM‐5. ZSM‐5 with good thermal stability, a PS of 380 nm, PSD of 0.17–0.9 µm, pore diameter of 2.31 nm, pore volume of 0.19 cm³ · g^?1 and specific surface area of 337.25 m² · g^?1 were obtained under the optimal conditions of a crystallization reaction time of 24 h, a crystallization temperature of 130 °C, a molar ratio of SiO₂/Al₂O₃ of 200, a TPAOH concentration of 3.5 mol · L^?1, NaCl concentration of 0.3 mol · L^?1, and a pre‐mixing time of 5 h. This work indicated that the operating conditions including premixing time have a significant effect on its PS and PSD.     

Synthesis of high-surface area mesoporous SiC with hierarchical porosity for use as catalyst support

Porous SiC with a hierarchical mesoporous structure is a promising material for high-performance catalytic systems because of its high thermal conductivity, high chemical inertness at high temperature, and oxidation resistance. Attempts to produce high-surface area hierarchical SiC have typically been made by using porous carbon as a template and reacting it with either Si or SiO₂ at high temperature under inert atmosphere. Because the reaction mechanism with Si involves a carbon dissolution step, and the reaction with SiO₂ is highly dependent on C-SiO₂ dispersion, the porous structure of the carbon template is not maintained, and the reaction yields nonporous SiC. In this work, mesoporous SiC has been synthesized using a novel hard-template methodology. SiC was prepared from hierarchical (mesoporous) silica which served as a solid template. Carbon deposition was done by Carbon Vapor Deposition (CVD) using CH₄ as carbon precursor, where different temperatures and reaction times were tested to optimize the carbon coating. The synthesized SiC retained 61 (118 m²/g) and 47% (0.3 cm³/g) of the BET surface area and the mesopore volume of the original SiO₂, which is 10 times higher than the retention reported for other template methods used to produce high surface area SiC.     

Enhanced selectivity to benzaldehyde in the liquid phase oxidation of benzyl alcohol using nanocrystalline ZSM-5 zeolite catalyst

In the present paper, nanocrystalline hierarchical ZSM-5 zeolites were successfully synthesized by the hydrothermal method in the presence of tetrapropylammonium hydroxide as a single template with the gel composition of 58SiO₂:Al₂O₃:20TPAOH:1,500H₂O. The prepared zeolite catalysts were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Nitrogen adsorption–desorption (BET), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HR-TEM) techniques. The formation of pure and highly crystalline ZSM-5 zeolite phase is confirmed by XRD. The IR vibration band at 550 cm^?1 is assigned to the double 5-rings of MFI-type zeolites. N₂ adsorption–desorption isotherms showed that the synthesized product had high BET surface area and possessed composite pore structures with both micro and mesopores. The catalytic performance of hierarchical ZSM-5 zeolite was investigated in the selective oxidation of benzyl alcohol (BzOH) with hydrogen peroxide (H₂O₂) under mild conditions. The results showed that the conversion of BzOH and the selectivity to benzaldehyde were about 94 and about 99 % respectively, when using 0.08 g ZSM-5 catalyst with acetonitrile as the solvent and H₂O₂ as the oxidant at 90 °C. This catalyst can be retrieved and reprocessed for five times without a significant loss in its activity and selectivity.     

Synthesis of hierarchical structured porous MoS2/SiO2 microspheres by ultrasonic spray pyrolysis

Hierarchical structure porous MoS₂/SiO₂ microspheres were prepared by ultrasonic pyrolysis technique. The nanostructured MoS₂/SiO₂ materials were characterised by scanning electron micrograph (SEM), energy dispersive X‐ray spectroscopy (EDX), X‐ray diffraction (XRD), high‐resolution transmission electron microscope (HRTEM), as well as nitrogen isotherm. The MoS₂/SiO₂ microspheres, synthesised using polystyrene latex spheres as a template, showed two pore sizes: 5.8 and 68 nm. The micro‐, meso‐ and macropore volume was also calculated. Effect of PSL:SiO₂ ratio on the hierarchical structure was also investigated. © 2011 Canadian Society for Chemical Engineering     

晶种导向剂法制备纳米ZSM-5沸石

在水热合成体系中添加自制晶种导向剂成功制备了纳米ZSM-5沸石,考察了晶种导向剂、晶化温度和合成体系硅铝物质的量比对合成纳米ZSM-5沸石的影响。制备的晶种导向剂是全硅的Silicalite-1沸石纳米颗粒、沸石初级或次级结构单元的SiO₂以及模板剂TPAOH的混合胶体溶液,结果表明,在ZSM-5沸石制备体系中添加晶种导向剂可有效降低有机模板剂的使用量,缩短晶化时间,并能得到纳米尺寸的ZSM-5沸石,降低晶化温度和合成体系硅铝比有利于减小纳米ZSM-5沸石晶体尺寸。     

Replication of jute stem for porous cellular ceramics/composites in the SiO2–SiC–C System

The present work demonstrates that carbonaceous template preforms derived from jute stem by thermal and microwave processing can be infiltrated at room temperature under vacuum with silica sol, obtained by acid catalyzed hydrolysis of tetra ethyl orthosilicate (TEOS). The microwave processed carbon-preforms are better suited for exercising control of SiO₂ infiltration. The oven-dried hybrid C/SiO₂ material accumulates silica as SiO₂·nH₂O (n lying approximately between 1 and 3) and may be suitably converted, depending on the modes of processing, into macroporous cellular SiO₂ or SiC ceramics or SiC/SiO₂/C or SiC/C or SiC/SiO₂ composites with tailored composition and microstructures. The processes for synthesis of the specific products have been developed and their phase compositions and microstructures have been examined through characterization by bulk density measurements, X-ray diffraction and scanning electron microscopy.     

Magnesiothermal synthesis of nanostructured SiC from natural zeolite (clinoptilolite) and mesoporous carbon CMK-1

Magnesiothermal synthesis of nanosized SiC has been successfully achieved from a mixture of the natural zeolite clinoptilolite and a high-surface area mesoporous carbon CMK-1, synthesized by impregnating a mesoporous silica template MCM-48 with sucrose, followed by carbonization in argon and removal of the silica template. Magnesium powder was used to initiate the self-combustion reaction. Removal of the alkaline and alkaline earth exchangeable cations from the clinoptilolite by ion exchange with NH₄⁺ was essential for a good yield of product, which is also dependent on the use of excess of carbon. TEM confirmed the nanostructure and size of the 15–25 nm SiC product crystallites.     

Mesoporous H‐ZSM‐5 for the Conversion of Dimethyl Ether to Hydrocarbons

The potential of hierarchical H‐ZSM‐5 zeolites was studied for the conversion of DME to fuel‐compatible hydrocarbons. For this purpose, hierarchical H‐ZSM‐5 zeolites have been prepared from commercial H‐ZSM‐5 by desilication and organosilane‐directed hydrothermal synthesis. The zeolites were characterized by X‐ray diffraction, NH₃‐TPD, DRIFTS, and N₂ physisorption measurements. The catalysts have been tested in a tube reactor (1 bar, 648 K). The results indicate important structural changes in framework and acidic sites, which are significant for the synthesis of gasoline‐range hydrocarbons.     

Synthesis of SUZ-4 zeolite by a dry gel conversion method

SUZ-4 zeolite was synthesized by the dry gel conversion (DGC) process with water vapor as gas phase, and characterized by XRD, SEM and N₂ adsorption. The dry gel was prepared with the assistance of a small amount of crystalline seed and organic template tetraethylammonium hydroxide (TEAOH). Molar ratios of SiO₂/Al₂O₃, KOH/SiO₂, TEAOH/SiO₂ and H₂O/SiO₂, amounts of seed and dry gel, types of silica sources, and crystallization temperature and time, were changed to optimize synthesis conditions. The results show that the DGC method leads to formation of SUZ-4 zeolite in a broad range of crystallization temperature 120–180 °C. Under the optimal conditions, i.e., SiO₂/Al₂O₃ = 22.5, KOH/SiO₂ = 0.44, TEAOH/SiO₂ = 0.044, H₂O/SiO₂ = 22.2, seed amount = 0.1 wt%, fumed silica as the silica source, 160 °C and 5 days, SUZ-4 zeolite is obtained with a high crystallinity. Compared to hydrothermal synthesis, the present DGC approach employs far less amount of organic template and allows using inert fumed silica as silica source, producing smaller rod-like SUZ-4 zeolite crystals.     

New mesoporous carbon materials synthesized by a templating procedure

The new porous carbon materials were obtained by templating procedure using mesoporous silica (SBA-15) as template. The ordered mesoporous silica materials were synthesized by using Pluronic P123 (non-ionic triblock copolymer, EO₂₀PO₇₀O₂₀). SBA-15/cryogel carbon composites were obtained by sol–gel polycondenzation of resorcinol and formaldehyde in the presence of different amount of SBA-15. The polycondenzation was followed by freeze drying and subsequent pyrolysis. One set of SBA-15/sucrose carbon composites was prepared by using sucrose as carbon source. The silica template was eliminated by dissolving in hydrofluoric acid (HF) to recover the carbon material. The obtained carbon replicas were characterized by nitrogen adsorption–desorption measurements, X-ray diffraction and scanning electron microscopy (SEM). It was revealed that the samples have high specific surface (533–771 m² g^?1), developed meso- and micro-porosity and amorphous structure. Porous structure of carbon replicas was found to be a function of the carbon source, properties of SBA-15 and silica/carbon ratio. Room temperature adsorption of nitrogen and adsorption of phenol from aqueous solutions were investigated.     

Synthesis of ZSM-5 type zeolites with and without template and evaluation of physicochemical properties and aniline alkylation activity

ZSM-5 zeolite containing SiO₂/Al₂O₃ = 28 was synthesised by hydrothermal process with and without template. Characterisation of the zeolites was done using XRD, IR, NMR, SEM and BET surface area measurements. Acidity of the zeolites was measured by ammonia adsorption-desorption. Catalytic activity of HZSM-5 zeolites was evaluated by aniline alkylation, cumene dealkylation, alkylation of benzene with ethylene and propylene and isomerisation ofm-xylene. Catalytic activity is discussed from the point of view of zeolite acidity and morphology.     

多级孔HZSM-5分子筛的制备及其催化丙烷脱氢反应性能

以十八水合硫酸铝、硅溶胶、四丙基氢氧化铵(TPAOH)、硝酸铵(NH_4NO_3)为原料,合成硅铝物质的量比为100的HZSM-5分子筛。采用乙酸钠溶液处理HZSM-5分子筛后加入TPAOH溶液进行二次晶化,通过对处理前、后的HZSM-5分子筛负载Pt,制得Pt/HZSM-5催化剂用于丙烷脱氢反应。利用XRD、SEM、XRF、BET、NH_3-TPD和H_2化学吸附对处理前、后的HZSM-5分子筛进行了表征,考察了不同浓度的TPAOH溶液对HZSM-5分子筛的晶体结构、表面形貌、孔结构、表面酸性及丙烷脱氢性能的影响。结果表明,采用乙酸钠溶液处理后的HZSM-5分子筛在TPAOH溶液中发生了二次晶化,不仅形成了微孔-介孔多级孔结构,还能调变分子筛的表面酸性;当TPAOH溶液浓度为0.3 mol/L、晶化温度为170℃、晶化时间为24 h时,得到的Pt/HZSM-5催化丙烷的初始转化率和丙烯选择性均达到最高值,分别为34.2%和99.0%。     

Perspectives of Micro/Mesoporous Composites in Catalysis

This article covers the recent development in the remarkably growing field of synthesis, characterization and particularly catalytic investigation of zeolite-based materials combining micro- and mesoporous features. New synthetic approaches for preparation of micro/mesoporous composites including recrystallization of originally amorphous matter, utilization of nanocrystalline zeolite seeds and formation of mesoporous zeolite single crystals are the first focus of this article. The advantages and disadvantages of composite materials in comparison with pure micro- and mesoporous molecular sieves will be discussed, as well. The relevance of individual experimental techniques for analysis of the composites, i.e., their structure, porosity, chemical composition, morphological features, and so on, are described in the second section of the article. The last Section is focused on the application of micro/mesoporous composites and mesoporous zeolites as catalysts in acid-catalyzed reactions, oxidation reactions and synthesis of fine chemicals. The potential of the composites in challenging areas of catalysis for future applications is the final objective of the review.     

Synthesis,analysis and characterization of ordered mesoporous TiO2/SBA-15 matrix: Effect of calcination temperature

Mesoporous TiO₂/SBA-15 matrix was prepared by the sol–gel synthesis of TiO₂ in previously prepared SBA-15 particles. Nonionic surfactant was used as liquid template and Na₂SiO₄ as SiO₂ precursor for the synthesis of mesoporous silica SBA-15 with high specific surface area. Different calcination temperature was used for the synthesis and analysis of TiO₂/SBA-15 matrix. The synthesized titania/silica composites were characterized by X-ray diffraction, FTIR, TEM, UV–vis spectroscopy, etc. TEM micrographs showed titania is successfully embedded in SBA-15 channel. Different calcination temperature indicates different size of particle formation and different photocatalytic properties. The activity test indicated that TiO₂/SBA-15 composite prepared by this method had better photocatalytic performance than pure TiO₂. The preparation method and the textural characteristics of mesoporous materials have great influence for the photocatalytic activity.     

Co-adsorption of nitrogen monoxide and nitrogen dioxide in zeolitic de-NOx catalysts

Room temperature adsorption and temperature programmed desorption (TPD) of NO, NO with O₂, and NO after NO₂ saturation were investigated for Na/Y, Na/ZSM5, Cu/ZSM5 and SiO₂ gel. The adsorbates were characterized by TPD, using mass spectrometry to identify the desorbed molecules. In the presence of O₂, the adsorption of NO and NO₂ is non-additive; co-desorption of nearly equal amounts of NO and NO₂ takes place suggesting the formation of a complex with an overall composition N₂O₃ within the zeolite. Since SiO₂ gel does not adsorb NO nor NO₂, the adsorption capacity of the zeolites is a function of their specific structure. The oxidation of NO to NO₂ is catalyzed by Na/Y and Na/ZSM5.     

Dramatic influence of carbamate-linked double chain organosilane with different length on zeolite morphology control

A series of rationally designed carbamate-linked organosilanes with double alkyl chain lengths from C₈ to C₁₄ were synthesized as novel template agents to prepare hierarchically porous zeolites. The variable chain lengths had strong affects on zeolite morphology control process in the given synthesis condition as indicated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and nitrogen adsorption–desorption analyses. The addition of the organosilanes with C₈, C₁₀ and C₁₄ lengths (5 mol % of the total silica source) into the synthesis compositions gave zeolites with isolated crystals. While the organosilane with C₁₂ length caused dramatic change in the product morphology and gave mesoporous ZSM-5 crystals with a chain-like morphology. XRD patterns, nitrogen sorption isotherm and SEM images of the samples taken during crystallization clearly showed the change of crystallinity, texture properties and forming process of this morphology.     

Ethanol perm‐selective B‐ZSM‐5 zeolite membranes from dilute solutions

Boron‐substituted MFI (B‐ZSM‐5) zeolite membranes with high pervaporation (PV) performance were prepared onto seeded inexpensive macroporous α‐Al₂O₃ supports from dilute solution and explored for the separation of ethanol/water mixtures by PV. The effects of several parameters on microstructures and PV performance of the B‐ZSM‐5 membranes were examined systematically, including the seed size, synthesis temperature, crystallization time, B/Si ratio, H₂O/SiO₂ ratio and silica source. A continuous and compact B‐ZSM‐5 membrane was fabricated from solution containing 1 tetraethyl orthosilicate/0.2 tetrapropylammonium hydroxide/0.06 boric acid/600 H₂O at 448 K for 24 h, showing a separation factor of 55 and a flux of 2.6 kg/m² h along with high reproducibility for a 5 wt % ethanol/water mixture at 333 K. It was demonstrated that the incorporation of boron into mobile five (MFI) structure could increase the hydrophobicity of B‐ZSM‐5 membrane evidenced by the improved contact angle and amount of the adsorbed ethanol, and thus enhance the PV property for ethanol/water mixtures. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2447–2458, 2016