Adsorption in mesopores: A molecular-continuum model with application to MCM-41

The classical model of surface layering followed by capillary condensation during adsorption in mesopores, is modified here by consideration of the adsorbate solid interaction potential. The new theory accurately predicts the capillary coexistence curve as well as pore criticality, matching that predicted by density functional theory. The model also satisfactorily predicts the isotherm for nitrogen adsorption at 77.4 K on MCM-41 material of various pore sizes, synthesized and characterized in our laboratory, including the multilayer region, using only data on the variation of condensation pressures with pore diameter. The results indicate a minimum mesopore diameter for the surface layering model to hold as 14.1 Å, below which size micropore filling must occur, and a minimum pore diameter for mechanical stability of the hemispherical meniscus during desorption as 34.2 Å. For pores in-between these two sizes reversible condensation is predicted to occur, in accord with the experimental data for nitrogen adsorption on MCM-41 at 77.4 K.

Modification of MCM-41 by surface silylation with trimethylchlorosilane and adsorption study

Siliceous MCM-41 samples were modified by silylation using trimethylchlorosilane (TMCS). The surface coverage of functional groups was studied systematically in this work. The role of surface silanol groups during modification was evaluated using techniques of FTIR and Si-29 CP/MAS NMR. Adsorption of water and benzene on samples of various hydrophobicities was measured and compared. It was found that the maximum degree of surface attachments of trimethylsilyl (TMS) groups was about 85%, corresponding to the density of TMS groups of 1.9 per nm(2). The degree of silylation is found to linearly increase with increasing pre-outgassing temperature prior to silylation. A few types of silanol groups exist on MCM-41 surfaces, among which both free and geminal ones are responsible for active silylation. Results of water adsorption show that aluminosilicate MCM-41 materials are more or less hydrophilic, giving a type IV isotherm, similar to that of nitrogen adsorption, whereas siliceous MCM-41 are hydrophobic, exhibiting a type V adsorption isotherm. The fully silylated Si-MCM-41 samples are more hydrophobic giving a type III adsorption isotherm. Benzene adsorption on all MCM-41 samples shows type IV isotherms regardless of the surface chemistry. Capillary condensation occurs at a higher relative pressure for the silylated MCM-41 than that for the unsilylated sample, though the pore diameter was found reduced markedly by silylation. This is thought attributed to the diffusion constriction posed by the attached TMS groups. The results show that the surface chemistry plays an important role in water adsorption, whereas benzene adsorption is predominantly determined by the pore geometry of MCM-41.

Irreversible change of pore structure of MCM-41 upon hydration at room temperature

The pore structure stability of MCM-41 materials upon hydration/dehydration was studied by XRD, Si-29 MAS NMR, and gravimetric adsorption techniques. Results demonstrated that collapses of the pore structure of MCM-41 occurred upon rehydration at room temperature due to the hydrolysis of the bare Si-O-Si(Al) bonds in the presence of water vapor. Full structure collapses of MCM-41 were found to occur when a MCM-41 sample was left in air for three months. It is also suggested that care must be taken when XRD is used to evaluate the structure property of MCM-41 materials to avoid the possible adverse effects of water vapor.

Experimental and theoretical investigations of adsorption hysteresis and criticality in MCM-41: Studies with O-2, Ar, and CO2

MCM-41 materials of six different pore diameters were prepared and characterized using X-ray diffraction, transmission electron microscopy, helium pycnometry, small-angle neutron scattering, and gas adsorption (argon at 77.4 and 87.4 K, nitrogen and oxygen at 77.4 K, and carbon dioxide at 194.6 K). A recent molecular continuum model of the authors, previously used for adsorption of nitrogen at 77.4 K, was applied here for adsorption of argon, oxygen, and carbon dioxide. While model predictions of single-pore adsorption isotherms for argon and oxygen are in satisfactory agreement with experimental data, significant deviation was found for carbon dioxide, most likely due to its high quadrupole moment. Predictions of critical pore diameter, below which reversible condensation occurs: were possible by the model and found to be consistent with experimental estimates, for the adsorption of the various gases. On the other hand, existing models such as the Barrett-Joyner-Halenda (BJH), Saito-Foley, and Dubinin-Astakhov models were found to be inadequate, either predicting an incorrect pore diameter or not correlating the isotherms adequately. The wall structure of MCM-41 appears to be close to that of amorphous silica, as inferred from our skeletal density measurements.

VOC removal: Comparison of MCM-41 with hydrophobic zeolites and activated carbon

The recently discovered mesoporous molecular sieve MCM-41 was tested as an adsorbent for VOC removal. Its adsorption/desorption properties were evaluated and compared with other hydrophobic zeolites (silicalite-1 and zeolite Y) and a commercial activated carbon, BPL. The adsorption isotherms of some typical VOCs (benzene, carbon tetrachloride, and n-hexane) on MCM-41 are of type IV according to the IUPAC classification, drastically different from the other microporous adsorbents, indicating that VOCs, in the gas phase, have to be at high partial pressures in order to make the most of the new mesoporous material as an adsorbent for VOC removal. However, a proper modification of the pore openings of MCM-41 can change the isotherm types from type IV to type I without remarkable loss of the accessible pare volumes and, therefore, significantly enhance the adsorption performance at low partial pressures. Adsorption isotherms of water on these adsorbents are all of type V, demonstrating that they possess a similar hydrophobicity. Desorption of VOCs from MCM-41 could be achieved at lower temperatures (50-60 degrees C), while this had to be conducted at higher temperatures (100-120 degrees C) for microporous adsorbents, zeolites, and activated carbons.

A novel method for tailoring the pore-opening size of MCM-41 materials

A novel pore tailoring method is proposed by which the pore-opening sizes of MCM-41 materials can be finely tuned without significant loss in pore volume and surface area.

Characterization of the structural and surface properties of chemically modified MCM-41 material

Mesoporous Mobil catalytic materials of number 41 (MCM-41) silica was chemically modified using both inorganic and organic precursors and characterized using the techniques, XRD, XPS, MAS NMR, FTIR, W-Vis, and physical adsorption of nitrogen, hydrocarbons (hexane, benzene, acetone, and methanol) and water vapor. Modification using organic reagents was found to result in a significant loss in porosity and a shape change of surface properties (increased hydrophobicity and decreased acidity). With inorganic modifying reagents, the decrease in porosity was also observed while the surface properties were not significantly altered as reflected by the adsorption isotherms of organics and water vapors. Chemical modifications can greatly improve the hydrothermal stability of MCM-41 material because of the enhanced surface hydrophobicity (with organic modifiers) or increased pore wall thickness (with inorganic modifiers). (C) 2000 Elsevier Science B.V. All rights reserved.

Comprehensive structural characterization of MCM-41: From mesopores to particles

In the present work the meso- and macro-structural characteristics of the mesoporous adsorbent MCM-41 have been estimated with the help of various techniques. The structure is found to comprise four different length scales: that of the mesopores, the crystallites, the grains and of the particles. It was found that the surface area estimated by the use of small angle scattering techniques is higher, while that estimated by mercury porosimetry is much lower, than that obtained from gas adsorption methods. Based on the macropore characterization by mercury porosimetry, and the considerable macropore area determined, it is seen that the actual mesopore area of MCM-41 may be significantly lower than the BET area. TEM studies indicated that MCM-41 does not have an ideal mesopore structure; however, it may still be treated as a model mesoporous material for gas adsorption studies because of the large radius of curvature of the channels.

Copper/MCM-41 as catalyst for the wet oxidation of phenol

A heterogeneous copper catalyst supported on mesoporous MCM-41 was developed. The parent MCM-41 has a large pore area of over 1400 m(2)/g. Copper was chosen as the active element of catalyst and loaded into MCM-41 by adsorption at ambient temperature. The prepared catalysts were evaluated in the catalytic wet oxidation of phenol solution with an initial concentration of 1,300 ppm at 150 and 200 degreesC. The catalyst was found to be of high catalytic activity. It is also shown that the catalyst with a higher copper loading exhibits higher ability of accelerating the catalytic reaction to certain extent but reaches its constant level afterwards. (C) 2001 Elsevier Science B.V. All rights reserved.

Copper/MCM-41 as catalyst for photochemically enhanced oxidation of phenol by hydrogen peroxide

Heterogeneous copper catalyst was developed using the mesoporous molecular sieve MCM-41 as the catalyst support. Copper was impregnated onto the support. Catalysts with different copper loadings were obtained. The performance of the developed catalysts was evaluated in photochemically enhanced oxidation of phenol using hydrogen peroxide as the oxidant. The catalyst was found to significantly increase the oxidation rate and enhance the removal level of phenol with UV light present. The effects of copper loading on the catalyst, photo (UV), H2O2 concentration, and catalyst dosage on the photo-oxidation of phenol were studied. (C) 2001 Elsevier Science B.V. All rights reserved.

Improved comparison plot method for pore structure characterization of MCM-41

MCM-41 samples of various pore dimensions are synthesized. Plotting of nitrogen adsorption data at 77 K versus the statistical film thickness (comparison plot) reveals three distinct stages, with a characteristic of two points of inflection. The steep intermediate stage caused by capillary condensation occurred in the highly uniform mesopores. From the slopes of the sections before and after the condensation, the surface area of the mesopores is calculated. The linear portion of the last section is extrapolated to the adsorption axis of the comparison plot, and this intercept is used to obtain the volume of the mesopores. From the surface area and pore volume, average mesopore diameter is calculated, and the value thus obtained is in good agreement with the pore dimension obtained from powder X-ray diffraction measurements. The principle of the calculation as well as problems associated are discussed in detail.

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.

Recent advances in processing and characterization of periodic mesoporous MCM-41 silicate molecular sieves

The discovery of periodic mesoporous MCM-41 and related molecular sieves has attracted significant attention from a fundamental as well as applied perspective. They possess well-defined cylindrical/hexagonal mesopores with a simple geometry, tailored pore size, and reproducible surface properties. Hence, there is an ever-growing scientific interest in the challenges posed by their processing and characterization and by the refinement of various sorption models. Further, MCM-41-based materials are currently under intense investigation with respect to their utility as adsorbents, catalysts, supports, ion-exchangers, and molecular hosts. In this article, we provide a critical review of the developments in these areas with particular emphasis on adsorption characteristics, progress in controlling the pore sizes, and a comparison of pore size distributions using traditional and newer models. The model proposed by the authors for adsorption isotherms and criticalities in capillary condensation and hysteresis is found to explain unusual adsorption behavior in these materials while providing a convenient characterization tool.

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.