Organic–Inorganic Hybrid Catalysts Based on Ordered Porous Structures for Carbon–Carbon Bond Forming Reactions

Two types of organic–inorganic hybrid base catalysts are prepared. Organic-functionalized molecular sieves (OFMSs); in particular, “amine-immobilized porous silicates” are designed based on common idea to immobilize catalytic active sites on silicate surface. Silicate–organic composite materials (SOCMs), such as “ordered porous silicate–quaternary ammonium composite materials”, are the precursors of ordered porous silicates obtained during the synthesis. Both the OFMS and the SOCM are used as the catalysts for Knoevenagel condensation and Michael addition reactions. Among the OFMSs, there is clear tendency that the use of molecular sieve with larger pore volume and/or surface area gives the product in higher yield. Aminopropylsilyl (AP)-tethered mesoporous silicate such as AP-MCM-41 gives the Knoevenagel condensation product in high yield under mild conditions. No loss of activity is observed after repeated use for three times. The SOCMs are also active for the same reaction. The OFMSs are effective when the supports have large pore volume and/or surface area and the reaction is carried out in polar solvents ethanol and DMF. However, the activity of the OFMSs is considerably low in a non-polar solvent such as benzene. In contrast, the SOCMs are remarkably active in benzene. The organic cation–MCM-41 composite is more active than the composite of an organic cation and a microporous silicate such as zeolite beta and ZSM-12. In the SOCM catalysts, (SiO)₃SiO⁻(⁺NR₄) moieties located at the accessible sites are considered to play some important roles. The active species are absent in the liquid phase after the reaction. The recycle of the catalyst was possible without significant loss of activity when the substrates are enough reactive. The mechanism of the reaction over SOCM catalyst is discussed.     

A chitosan-templated monolithic siliceous mesoporous-macroporous material

 We synthesised a porous siliceous material via hydrothermal hydrolysis of sodium silicate, using chitosan as a template. As far as we know, this is the first synthesis of siliceous porous material using chitosan as a template in a hydrothermal way. A fibrous material was obtained, whose macroscopic fibres were formed by a spongelike siliceous network with pores having a radius of 0.57 μm. The siliceous walls of the pores were, in turn, of the form of a microporous–mesoporous material; the pore radius distribution was polymodal with maxima at 0.84, 1.0, 1.2 and 1.5 nm and a broad band between 3 and 10 nm. This structure may be due to the aggregation of the hydrated chitosan helices in bundles of parallel fibres with different size and the gelation of the system. The aggregation process might be induced by the addition of silicate. Received: 12 January 2000/Accepted: 7 March 2000     

A novel template-free sol–gel synthesis of silica materials with mesoporous structures and zeolitic walls

Hierarchical porous materials with zeolite structures show great promise in catalysis due to combining the advantages of zeolites and mesoporous materials. Here a novel template-free sol–gel method is developed to synthesize hierarchical porous silica materials. This method involves solvothermal recrystallization of the xerogel converted from uniform silicalite-1 precursor sol via vacuum drying process. The zeolite sol and the solid samples were characterized by laser light scattering, XRD, N₂ adsorption/desorption isotherm, FTIR, SEM, TEM and thermal analysis technologies. The results show that we obtain two novel materials with different mesoporous structures and silicalite-1 walls by using different recrystallization media, one of which has irregular arrays of mesopores, the other possesses 3D wormhole mesoporous structure. We speculate that formation of different mesoporous structures results from different nucleation and growth process of materials     

Multifunctional NO-delivery vessel derived from aminopropyl-modified mesoporous zeolites

A new strategy, releasing nitric oxide (NO) and adsorbing nitrosamines simultaneously by zeolitic materials in the digestive system, is validated in this paper. Three types of moisture-saturated molecular sieves, HZSM-5 zeolite, mesoporous zeolite, and mesoporous silica MCM-41, are used as NO-delivery vessels in mimic gastric juice after modification of γ-aminopropyltriethoxysilane (APTES). APTES modification dramatically increased the capability of zeolite and mesoporous silica in NO release in acidic solution, because more NO can be adsorbed in the composite and stored in the form of nitrite. Some composites released the NO 10 times more than their parent materials, and synchronously captured the carcinogen nitrosamines in mimic gastric juice. The influences of APTES modification on the porous structure and surface state of zeolite and mesoporous silica were investigated by XRD, N(2) adsorption, and FTIR tests, through which the mesoporous zeolite is proven to be the optimal support. With this hierarchical material a controllable APTES modification is realized in which a lot of aminopropyl groups are grafted in mesopores while the zeolitic structure is maintained, so the resulting sample exhibits a high capability in releasing NO and adsorbing nitrosamines. This investigation provides a clue for elevating the efficiency of zeolites in the application of life science.     

Energetic heterogeneity of the surface of a micro/mesoporous zeolite adsorbent

A complex approach based on treatment of calorimetric data using the Guggenheim–Andersen–de Boer and modified Jovanović equations was developed to evaluate the energetic and structural heterogeneity of microporous and mixed micro/mesoporous zeolite adsorbents. The calculated dependence of the hexane adsorption energy distribution function for the Y-US-Ex micro/mesoporous zeolite on the adsorbate pressure is in good accord with the calorimetric curves for the differential heats of adsorption, which indicates the suitability of this theoretical approach.     

The physicochemical properties of micro/mesoporous materials prepared by the recrystallization of zeolite BEA

Micro-mesoporous materials with various ratios between micro- and mesopores were prepared by the recrystallization of zeolite BEA in an alkaline medium in the presence of cetyltrimethylammonium bromide. The materials were characterized by X-ray diffraction, low-temperature nitrogen adsorption, and IR spectroscopy. Recrystallization under mild conditions did not cause substantial changes in the number of acid centers but increased the accessibility of acid centers to large-sized molecules because of the creation of mesopores. An increase in the degree of recrystallization caused first partial and then complete zeolite transformation into MSM-41 mesoporous aluminosilicate, which was accompanied by a decrease in the number of acid centers. The IR spectra were used to determine the diffusion coefficients of cumene in the initial and recrystallized samples. Recrystallization increased the diffusion coefficient by 3–4 times. Original Russian Text ? V.V. Ordomskii, Yu.V. Monakhova, E.E. Knyazeva, N.S. Nesterenko, I.I. Ivanova, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 6, pp. 1150–1155.     

Synthesis and Characterization of Cu Decorated Zeolite A@Void@Et-PMO Nanocomposites for Removal of Methylene Blue by a Heterogeneous Fenton Reaction

Hydrothermal synthesized nano zeolite A has been encapsulated with ethyl bridged periodic mesoporous organosilica(Et-PMO) shell tlirougli a simple modified Stober method and an organosilane-directed growth-induced etching strategy, the obtained yolk-shell structured A@Et-PMO nanocomposite(YS-A@Et-PMO) was further functionalized by the impregnation of copper ions, realizing the composite material with hierarchical porous and catalytic properties. The morphology and metal content of the Cu/A and Cu/YS-A@Et-PMO were fully characterized. As compared to tlie parent material, the composite Cu/YS-A@Et-PMO has an efficient adsorption and catalytic degradation performance on methylene blue(MB), the removal efficiency reached as high as 95% of 60 mg/L MB within 10 min. These novel structured porous composites may have great potential application for the removal of organic dye including waste effluents.     

Periodic Mesoporous Silicas and Organosilicas: An Overview Towards Catalysis

Micelle-templated mesoporous and organic–inorganic hybrid mesoporous materials are important in many fields of material research, especially for hosting catalysts in confined space. Among this class, the recent discovery of periodic mesoporous organosilicas (PMOs) represent an exciting new group of organic–inorganic nanocomposites targeted for a broad range of applications ranging from catalysis to microelectronics. Compared to the earlier generation of organic–inorganic hybrid mesoporous samples, obtained by the cocondensation reaction or by the grafting reaction, PMOs represent the right combination of organic and inorganic groups in the frame wall positions. This article reviews the current state of art in organic–inorganic hybrid mesoporous material research with special emphasis over periodic mesoporous organosilica materials having various redox centers (Ti, V, Cr) suitable for oxidation reactions as well as acidic sites (Al, –SO₃H) for the organic transformation of bulky molecules.     

Adsorption kinetics of hemoglobin onto silicate adsorbents

A comparative study was performed of the adsorption kinetics of hemoglobin onto four silicate adsorbents that differed in parameters of porous structure: industrial silochrome, and three silicates obtained by template synthesis. The existence of a reversible stage of the process preceding irreversible protein binding was shown by kinetic analysis. Rate constants for the reversible stage of the process and the effective constants of first order rate were calculated. It was concluded that the adsorption rate is the highest on silochrome and the lowest on silicate of MCM-41 type, the pore size of which is comparable to the diameter of a hemoglobin molecule. The constant of the desorption rate for the first reversible stage of the process was shown in the case of mesoporous silicates to be lower by a factor of 3.5–4 than in the case of silochrome.     

Formation and characterization of phosphate-modified silicate materials derived from sol–gel process

Phosphate-containing silicate materials prepared using sol–gel method from Si(OC₂H₅) were investigated at the variation of the amount of phosphate modifier from 5 to 50 wt% in term of P₂O₅. Chemical composition, textural and structural properties of these materials were characterized by FTIR-spectroscopy, TEM, X-ray diffraction and nitrogen adsorption. It was shown that the materials posse monomodal pore size distribution of 5–20 nm for the samples dried at 100 °C and 40–60 nm for the specimens calcined at 600 °C. The mean pore size and surface area depended on the amount of phosphoric acid. Before the stage of high temperature treatment phosphoric acid, introduced into the structure of the materials as a modifying agent, was uniformly distributed inside a porous space of the material and was not chemically bonded with silicate. After high temperature treatment both chemical interaction of silicate with phosphate, providing the formation of silicate-phosphate structures, as well as redistribution of free modifier from the bulk of granules to their surface took place. The polyphosphate layer is formed on the material surface closing the internal porous space. However, in this case a part of the phosphate modifier remains chemically unbound to SiO₂ structure.     

Calorimetric evolution of the early pozzolanic reaction of natural zeolites

The pozzolanic reaction between natural zeolite tuffs, portlandite and water was investigated over the course of the early reaction period up to 3 days. Isothermal conduction calorimetry experiments supplemented by TG/DTG and XRD analyses assisted in the elucidation of the sequence of reaction processes taking place. The calorimetry experiments clearly showed the dependence of the pozzolanic reaction rate and associated heat release on the fineness of the zeolite tuff. Higher external surface areas of pozzolans yield higher total heat releases. Also the exchangeable cation content of the zeolites influences the reaction rate. Release of exchangeable alkalis into solution promotes the pozzolanic reaction by raising the pH and zeolite solubility. The appearance of an exotherm after approximately 3 h of reaction is more conspicuous when alkali-rich zeolites are reacted. This exotherm is conceived to be related to a transformation or rupture of initially formed reaction products covering the zeolite grains. The formation of substantial amounts of ‘stable’ calcium silicate hydrate (C–S–H) and calcium aluminate hydrate (C–A–H) reaction took place after an induction period of more than 6 h. The openness of the zeolite framework affects the proneness of the zeolite to dissolution and thus its reactivity. Open framework zeolites such as chabazite were observed to react much more rapidly than closed framework zeolites such as analcime.     

General and Facile Syntheses of Metal Silicate Porous Hollow Nanostructures

Porous hollow nanostructures have attracted intensive interest owing to their unique structure and promising applications in various fields. A facile hydrothermal synthesis has been developed to prepare porous hollow nanostructures of silicate materials through a sacrificial‐templating process. The key factors, such as the concentration of the free metal cation and the alkalinity of the solution, are discussed. Porous hollow nanostructures of magnesium silicate, nickel silicate, and iron silicate have been successfully prepared by using SiO₂ spheres as the template, as well as a silicon source. Several yolk–shell structures have also been fabricated by a similar process that uses silica‐coated composite particles as a template. As‐prepared mesoporous magnesium silicate hollow spheres showed an excellent ability to remove Pb²⁺ ions in water treatment owing to their large specific surface and unique structures.     

Direct electrochemistry and biocatalysis of glucose oxidase immobilized on magnetic mesoporous carbon

A magnetic mesoporous carbon material (i.e., mesoporous iron oxide/C, mesoFe/C) is synthesized for protein immobilization, using glucose oxidase (GOx) as model. Transmission electron microscopy images show that mesoFe/C has highly ordered porous structure with uniform pore size, and iron oxide nanoparticles are dispersed along the wall of carbon. After adsorption of GOx, the GOx-mesoFe/C composite is separated with magnet. The immobilized GOx remains its natural structure according to the reflection–absorption infrared spectra. When the GOx-mesoFe/C composite is coated on a Pt electrode surface, the GOx gives a couple of quasireversible voltammetric peaks at −0.5 V (vs. saturated calomel electrode) due to the redox of FAD/FADH₂. The electron-transfer rate constant (k _s) is ca. 0.49 s⁻¹. The modified electrode presents remarkably amperometric response to glucose at 0.6 V. The response time (t _95%) is less than 6 s; the response current is linear to glucose concentration in the range of 0.2–10 mM with a sensitivity of 27 μA mM⁻¹ cm⁻². The detection limit is 0.08 mM (S/N = 3). The apparent Michaelis–Menten constant (K _m^app) of the enzyme reaction is ca. 6.6 mM, indicating that the GOx immobilized with mesoFe/C has high affinity to the substrate.     

Influence of the composition of the reaction mixture and the synthesis conditions on the formation of mesomorphous substances in the presence of glucose oxidase

The possibility of synthesizing mesoporous substances in silicate and aluminosilicate systems in the presence of combinations of glucose oxidase and cetyltrimethylammonium bromide has been demonstrated. Highly crystalline mesoporous molecular sieves of type MCM-41 with a high aluminum content were obtained from the aluminosilicate system with a reaction mixture pH close to neutral. L. V. Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences of Ukraine, 31 Prospekt Nauki, Kiev 252039, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 35, No. 3, 190–193, May–June, 1999.     

Steam-Induced Coarsening of Single-Unit-Cell MFI Zeolite Nanosheets and Its Effect on External Surface Brønsted Acid Catalysis

Commonly used methods to assess crystallinity, micro-/mesoporosity, Brønsted acid site density and distribution (in micro- vs. mesopores), and catalytic activity suggest nearly invariant structure and function for aluminosilicate zeolite MFI two-dimensional nanosheets before and after superheated steam treatment. Yet, pronounced reaction rate decrease for benzyl alcohol alkylation with mesitylene, a reaction that cannot take place in the zeolite micropores, is observed. Transmission electron microscopy images reveal pronounced changes in nanosheet thickness, aspect ratio and roughness indicating that nanosheet coarsening and the associated changes in the external (mesoporous) surface structure are responsible for the changes in the external surface catalytic activity. Superheated steam treatment of hierarchical zeolites can be used to alter nanosheet morphology and regulate external surface catalytic activity while preserving micro- and mesoporosity, and micropore reaction rates.     

Zeolite nanocrystals inside mesoporous TUD-1: a high-performance catalytic composite

A hierarchically structured composite material with interconnecting meso- and micropores has been developed with the aim to optimize zeolite performance. A general synthetic method has been developed that, in a controlled manner, allows for various types of nanosized zeolite to be incorporated into a three-dimensional mesoporous matrix. Nanosized zeolite Beta was used to exemplify this new approach, resulting in a system in which zeolite Beta shows a higher cracking activity per gram of zeolite than pure nanosized zeolite Beta for the model feed n-hexane. Additionally, FTIR studies of CO and NH3 adsorption revealed that the nature of the acid sites in the nanozeolite has been partially modified due to the interactions with the mesoporous matrix, TUD-1.     

X-rays to study,induce, and pattern structures in sol–gel materials

X-rays investigations have been shown to reveal important information regarding material features and the formation mechanism of mesostructured materials. Small angle X-ray scattering (SAXS) analysis performed using a synchrotron source has been very important in the optimization of the organization of mesoporous coatings obtained by evaporation induced self-assembly (EISA). The interaction between X-rays and ordered mesoporous films has only recently been reported, and new knowledge has been developed to use this external radiation source to tune the local material properties. Here we discuss the recent developments in X-ray lithography combined with sol–gel synthesis to pattern mesostructured and hierarchical porous coatings including the ability to tailor functionalized surfaces.     

Structural and sorption characteristics of the products from zeolitization of sba-15 in the presence of tetraalkylammonium hydroxides

Micro–mesoporous materials combining the structural and sorption characteristics of a mesoporous molecular sieve (MMS) and zeolite BEA were obtained by the “dry gel conversion” method – partial zeolitization of silica MMS SBA-15 in the presence of tetraethylammonium hydroxide. The volume of the mesopores reaches 0.65 cm³/g, while that of the micropores is in the region of 0.1 cm³/g. The acidity of the obtained zeolitized materials differs from that of BEA; the total concentration of medium-strength acid centers (maximum thermal desorption of ammonia at ~315 °C) amounts to 0.15 mmol/g.     

Catalytic activity of H-ZSM-5 and H-ZSM-5/AL-MCM-41 in the conversion of ethanol to gasoline fraction hydrocarbons

H-ZSM-5 (Si/Al = 10.6 and 20) efficiently catalyzes the transformation of ethanol into C₅-C₁₂ gasoline hydrocarbons in 27–33 mass % yield at 320°C and feed rate 20 mmol C₂H₅OH/(gcat·h). Only ethylene is produced on the mesoporous zeolite H-ZSM-5/Al-MCM-41 with 100% ethanol conversion. This discrepancy may be attributed to blockage of the ZSM-5 micropores in the mesoporous zeolite structure.     

Carbon and SiC macroscopic beads from ion-exchange resin templates

A method for preparing carbon and SiC macroscopic beads using ion-exchange resins as a macrotemplate that determines the macroshape and the pore structure of the product materials is reported. First, silicates are ion-exchanged into the resins to prevent the resin from collapsing during subsequent carbonization and allow them to be used as precursors for SiC formation. SiC is prepared via carbothermal reduction of carbon/silica composite beads obtained upon carbonization of the resin/silicate in an inert atmosphere. Finally, silica is removed by HF etching. Very high-surface area (1670-2026 m2 g-1) micro- or micro-/mesoporous carbon beads and relatively high-surface area (35-63 m2 g-1) macro- and meso-/macroporous SiC beads were prepared by the described method. The pore structure and the macroshape of the particles were controlled by the type of ion-exchange resins employed, gel or macroreticular.