Comprehensive study of surface chemistry of MCM-41 using Si-29 CP/MAS NMR, FTIR, pyridine-TPD, and TGA

A comprehensive study was conducted on mesoporous MCM-41. Spectroscopic examinations demonstrated that three types of silanol groups, i.e., single, (SiO)(3)Si-OH, hydrogen-bonded, (SiO)(3)Si-OH-OH-Si(SiO)(3), and geminal, (SiO)(2)Si(OH)(2), can be observed. The number of silanol groups/nm(2), alpha(OH), as determined by NMR, varies between 2.5 and 3.0 depending on the template-removal methods. All these silanol groups were found to be the active sites for adsorption of pyridine with desorption energies of 91.4 and 52.2 kJ mol(-1), respectively. However, only free silanol groups (involving single and geminal silanols) are highly accessible to the silylating agent, chlorotrimethylsilane. Silylation can modify both the physical and chemical properties of MCM-41.

Organophilicity of MCM-41 adsorbents studied by adsorption and temperature-programmed desorption

In this paper, the organophilic property of MCM-41 was studied and compared with hydrophobic silicalite-l using adsorption and temperature-programmed desorption (TPD) methods. The surface heterogeneity of MCM-41 was evaluated in terms of activation energy for desorption (E-d) and isosteric heat of adsorption (q(st)). Results show that MCM-41 has a higher affinity to polar organic compounds than to non-polar organics while silicalite-l has a higher affinity to non-polar organic compounds than to polar organics. This organophilic behaviour of MCM-41 is attributed to its surface heterogeneity. (C) 2001 Elsevier Science B.V. All rights reserved.

Characterisation of templated xerogels for molecular sieve application

This paper presents the results of the characterisation of templated silica xerogels as precursor material for molecular sieve silica membranes for gas separation. The template agent integrated in the xerogel matrix is a methyl ligand covalently bended to the siloxane network in the form of methyltriethoxysilane (MTES). Several surface and microstructural characterisation techniques such as TGA, FTIR, NMR, and nitrogen adsorption have been employed to obtain information on the reaction mechanisms involved in the sol-gel processing of such molecular sieves. The characterisation results show the effects of processing parameters such as heat treatment temperature, and the concentration of the covalently bonded template on the development of the pore structure. It was found that calcination temperature significantly enhanced the condensation reactions thus resulted in more Si-O-Si groups being formed. This was also confirmed with the data of FTIR characterisation showing enhanced silicon bands at higher heat treatment temperatures. As a result of the promoted densification and shrinkable pore network the micropore volume also reduced with increasing methyl ligand molar ratio. However, the mean pore diameter does not change significantly with calcination temperature. While the contribution of the templates towards controlling pore size is less precise, increasing the methyl ligand molar ratio results in the broadening of the pore size distribution and lower pore volume. Higher template concentration induces the collapse of the xerogel matrix due to capillary stress promoting dense xerogels with low pore volume (C) 2001 Elsevier Science B.V. All rights reserved.

Influence of synthesis parameters on the formation of mesoporous SAPOs

The synthesis and characterization of high-quality mesoporous silicoaluminophosphates (SAPOs) with a hexagonally arranged pore structure and a good thermal stability are described. The influence of some important synthesis parameters including temperature, time, and Si content in the synthesis gel was examined. The local environments of Al, P, and Si were investigated using MAS NMR spectroscopy. The acidity of the mesoporous SAPOs was studied and compared with those of aluminosilicate MCM-41 and SAPO-5. Results show that both the synthesis temperature and time have a significant impact on the formation of mesoporous SAPOs, whereas the presence of Si in the synthesis gel has a direct influence on the structure type and the quality of the resulting mesoporous SAPO materials. High-quality mesoporous SAPOs can be synthesized from the synthesis gels with Si/Al ratio smaller than 0.5 in the presence of cationic surfactants in a weakly basic aqueous solution. The mesoporous SAPO materials show interesting acidity properties, possessing both strong and mild sites. (C) 2002 Elsevier Science Inc. All rights reserved.

gamma-alumina nanofibers prepared from aluminum hydrate with poly(ethylene oxide) surfactant

Introducing poly(ethylene oxide) surfactant to aluminum hydrate colloids can effectively direct the crystal growth of boehmite and the crystal morphology of final gamma-alumina crystallites. Fibrous crystallites of gamma-alumina about 3-4 nm thick and 30-60 nm long are obtained. They stack randomly, resulting in a structure with a low contact area between the fibers but with a very large porosity. Such a structure exhibits strong resistance to sintering when heated to high temperatures. A sample retains a BET surface area of 68 m(2)/g, after being heated to 1473 K. The surfactant molecules form micelles that interact with the colloid particles of aluminum hydroxide through hydrogen bonding. This interaction is not sufficient to change the intrinsic crystal structure of boehmite, but induces profound changes in the morphology of boehmite crystallites and their growth. The surfactant-induced fiber formation (SIFF) process has distinct features from templated synthesis but shows similarities in some respects to biomineralization processes in which inorganic crystals with complex morphological shapes can be formed in biological systems. SIFF offers an effective approach to create new nanostructures of inorganic oxide from aqueous media.

Characterization of the acidic properties of mesoporous aluminosilicates synthesized from leached saponite with additional aluminum incorporation

The acidic properties of hexagonal mesoporous aluminosilicates synthesized via a new successful short time synthesis route using leached saponite and a low concentration of surfactant are thoroughly investigated. The resulting aluminosilicate mesoporous materials with high Si/Al ratios of around 11 have a maximal surface area of 1130 m(2)/g, a pore volume of 0.92 cm(3)/g, and a narrow pore size distribution at around 3 nm. The replacement of the sodium ions, present as counterions in the synthesized aluminosilicates, with protons imparts useful catalytic acidity. This acidity is extensively studied with FTIR spectroscopy after adsorption of ammonia and cyclohexylamine, while deuterated acetonitrile differentiates between Bronsted and Lewis acidity. Al-27 NMR spectroscopy determined the coordination of the aluminum in the FSM materials. Simultaneously the effect of an additional Al incorporation, utilizing sodium aluminate, aluminum nitrate, and aluminum isopropoxide is studied. From an acidic point of view, the incorporation with Al(NO3)(3) appears to be the most optimal, as the sample has a very high amount of acid sites (1.3 mmol/g). Investigating the nature of the acid sites it is found that in all samples except the one incorporated with Al(NO3)(3), more Bronsted than Lewis sites are present, both sites being quite acidic as they resist desorption temperatures up to 300 degreesC. Probing the coordination and location of the Al atoms, all the catalysts appeared to have mostly tetrahedral aluminum, up to 95% of the total Al amount for the proton exchanged AI(NO3)(3) incorporated sample.

Prediction of multilayer adsorption and capillary condensation phenomena in cylindrical mesopores

MCM-41 periodic mesoporous silicates with a high degree of structural ordering are synthesized and used as model adsorbents to study the isotherm prediction of nitrogen adsorption. The nitrogen adsorption isotherm at 77 K for a macroporous silica is measured and used in high-resolution alpha(s)-plot comparative analysis to determine the external surface area, total surface area and primary mesopore volume of the MCM-41 materials. Adsorption equilibrium data of nitrogen on the different pore size MCM-41 samples (pore diameters from 2.40 to 4.92 nm) are also obtained. Based on the Broekhoff and de Boer' thermodynamic analysis, the nitrogen adsorption isotherms for the different pore size MCM-41 samples are interpreted using a novel strategy, in which the parameters of an empirical expression, used to represent the potential of interaction between the adsorbate and adsorbent, are obtained by fitting only the multilayer region prior to capillary condensation for C-16 MCM-41. Subsequently the entire isotherm, including the phase transition, is predicted for all the different pore size MCM-41 samples without any fitting. The results show that the prediction of multilayer adsorption and total adsorbed amount are in good agreement with the experimental isotherms. The predictions of the relative pressure corresponding to capillary equilibrium (coexistence) transition agree remarkably with experimental data on the adsorption branch even for hysteretic isotherms, confirming that this is the branch appropriate for pore size distribution analysis. The impact of pore radius on the adsorption film thickness and capillary coexistence pressure is also investigated, and found to agree with the experimental data. (C) 2003 Elsevier Inc. All rights reserved.

Immobilization of aluminum chloride on MCM-41 as a new catalyst system for liquid-phase isopropylation of naphthalene

A great deal of effort has been made at searching for alternative catalysts to replace conventional Lewis acid catalyst aluminum trichloride (AlCl3). In this paper, immobilization of AlCl3 on mesoporous MCM-41 silica with and without modification was carried out. The catalytic properties of the immobilized catalyst systems for liquid-phase isopropylation of naphthalene were studied and compared with those of H/MCM-41 and H/mordenite. The structures of the surface-immobilized aluminum chloride catalysts were studied and identified by using solid-state magic angle spinning nuclear magnetic resonance (MAS NMR), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), nitrogen adsorption, and X-ray diffraction (XRD) techniques. The catalytic activity of the immobilized catalysts was found to be similar to that of acidic mordenite zeolite. A significant enhancement in the selectivity of 2,6-diisopropylnaphthalene (2,6-DIPN) was observed over the immobilized aluminum chloride catalysts. Immobilization of aluminum chloride on mesoporous silica coupled with surface silylation is a promising way of developing alternative catalyst system for liquid-phase Friedel-Crafts alkylation reactions. (C) 2002 Elsevier Science B.V. All rights reserved.

Study of hexane adsorption in nanoporous MCM-41 silica

We study here the adsorption of hexane on nanoporous MCM-41 silica at 303, 313, and 323 K, for various pore diameters between 2.40 and 4.24 nm. Adsorption equilibria, measured thermogravimetrically, show that all the isotherms, that are somewhat akin to those of type V, exhibit remarkably sharp capillary adsorption phase transition steps and are reversible. The position of the phase transition step gradually shifts from low to high relative pressure with an increase in the temperature as well as the pore sizes. The isosteric heats of adsorption derived from the equilibrium information using the Clapeyron equation reveal a gradual decrease with increasing adsorbed amount because of the surface heterogeneity but approach a constant value near the phase transition. A decrease in the pore size results in an increase in the isosteric heat of adsorption because of the increased dispersion forces. A simple strategy, based on the Broekhoff and De Boer adsorption theory, successfully interprets the hexane adsorption isotherms for the different pore size MCM-41 samples. The parameters of an empirical expression, used to represent the potential of interaction between the adsorbate and adsorbent, are obtained by fitting the monolayer region prior to capillary condensation and the experimental phase transition simultaneously, for some pore sizes. Subsequently, the parameters are used to predict the adsorption isotherm on other pore size samples, which showed good agreement with experimental data.

Hydrothermal seeded synthesis of mesoporous titania for application in dye-sensitised solar cells (DSSCs)

A modified-templated- hydrothermal technique was used to prepare mesoporous titania powders through the interaction of tiny anatase seeds (

Application of density functional theory to capillary phenomena in cylindrical mesopores with radial and longitudinal density distributions

In this paper, we applied a version of the nonlocal density functional theory (NLDFT) accounting radial and longitudinal density distributions to study the adsorption and desorption of argon in finite as well as infinite cylindrical nanopores at 87.3 K. Features that have not been observed before with one-dimensional NLDFT are observed in the analysis of an inhomogeneous fluid along the axis of a finite cylindrical pore using the two-dimensional version of the NLDFT. The phase transition in pore is not strictly vapor-liquid transition as assumed and observed in the conventional version, but rather it exhibits a much elaborated feature with phase transition being complicated by the formation of solid phase. Depending on the pore size, there are more than one phase transition in the adsorption-desorption isotherm. The solid formation in finite pore has been found to be initiated by the presence of the meniscus. Details of the analysis of the extended version of NLDFT will be discussed in the paper. (C) 2004 American Institute of Physics.

Diffusion of n-decane in mesoporous MCM-41 silicas

We investigate here the diffusion of n-decane in nanoporous MCM-41 silicas with pore diameters between 3.0 and 4.3 nm, and at various temperatures and purge flow rates, by the Zero Length Column method. A complete-time-range analysis of desorption curves is used to derive the diffusion coefficient, and the effect of pore size, purge flow rate and temperature on the diffusion character is systematically studied. The results show that the calculated low-coverage diffusivity values are strongly dependent on temperature but only weakly dependent on pore size. The study reveals that transport is controlled by intracrystalline diffusion and dominated by sorbate-sorbent interaction, with the experimental isosteric heat matching the potential energy of flat-lying n-decane molecules on the surface, determined using a united atom model. The diffusion activation energy and adsorption isosteric heat at zero loading for the different pore size MCM-41 samples vary in a narrow range respectively, and their ratio is essentially constant over the pore size range studied. The study shows that the ZLC method is an effective tool to investigate the diffusion kinetics of hydrocarbons in mesoporous MCM-41 materials. (c) 2005 Elsevier Inc. All rights reserved.

Adsorption of argon and nitrogen in cylindrical pores of MCM-41 materials: application of density functional theory

Adsorption of argon and nitrogen at their respective boiling points in cylindrical pores of MCM-41 type silica-like adsorbents is studied by means of a non-local density functional theory (NLDFT), which is modified to deal with amorphous solids. By matching the theoretical results of the pore filling pressure versus pore diameter against the experimental data, we arrive at a conclusion that the adsorption branch (rather than desorption) corresponds to the true thermodynamic equilibrium. If this is accepted, we derive the optimal values for the solid–fluid molecular parameters for the system amorphous silica–Ar and amorphous silica–N2, and at the same time we could derive reliably the specific surface area of non-porous and mesoporous silica-like adsorbents, without a recourse to the BET method. This method is then logically extended to describe the local adsorption isotherms of argon and nitrogen in silica-like pores, which are then used as the bases (kernel) to determine the pore size distribution. We test this with a number of adsorption isotherms on the MCM-41 samples, and the results are quite realistic and in excellent agreement with the XRD results, justifying the approach adopted in this paper.

Modeling nitrogen adsorption in spherical pores of siliceous materials by density functional theory

Adsorption of nitrogen in spherical pores of FDU-1 silica at 77 K is considered by means of a nonlocal density functional theory (NLDFT) accounting for a disordered structure of pore walls. Pore size distribution analysis of various FDU-1 samples subject to different temperatures of calcination revealed three distinct groups of pores. The principal group of pores is identified as ordered spherical mesopores connected with each other by smaller interconnecting pores and irregular micropores present in the mesopore walls. To account for the entrances (connecting pores) into spherical mesopores, a concept of solid mass distribution with respect to the apparent density was introduced. It is shown that the introduction of the aforementioned distribution was sufficient to quantitatively describe experimental adsorption isotherms over the entire range of relative pressures spanning six decades.

Modeling of adsorption and nucleation in infinite cylindrical pores by two-dimensional density functional theory

In this paper, we present an analysis of argon adsorption in cylindrical pores having amorphous silica structure by means of a nonlocal density functional theory (NLDFT). In the modeling, we account for the radial and longitudinal density distributions, which allow us to consider the interface between the liquidlike and vaporlike fluids separated by a hemispherical meniscus in the canonical ensemble. The Helmholtz free energy of the meniscus was determined as a function of pore diameter. The canonical NLDFT simulations show the details of density rearrangement at the vaporlike and liquidlike spinodal points. The limits of stability of the smallest bridge and the smallest bubble were also determined with the canonical NLDFT. The energy of nucleation as a function of the bulk pressure and the pore diameter was determined with the grand canonical NLDFT using an additional external potential field. It was shown that the experimentally observed reversibility of argon adsorption isotherms at its boiling point up to the pore diameter of 4 nm is possible if the potential barrier of 22kT is overcome due to density fluctuations.