Mesoporous under pressure

Important physical properties of catalysts include the particle size and shape, surface area, pore volume, pore size distribution, and strength to resist cmshing and abrasion. Measurements of catalyst physical properties (43) are routine and often automated. Pores with diameters <2.0 nm are called micropores those with diameters between 2.0 and 5.0 nm are called mesopores and those with diameters >5.0 nm are called macropores. Pore volumes and pore size distributions are measured by mercury penetration and by N2 adsorption. Mercury is forced into the pores under pressure entry into a pore is opposed by surface tension. For example, a pressure of about 71 MPa (700 atm) is required to fill a pore with a diameter of 10 nm. The amount of uptake as a function of pressure determines the pore size distribution of the larger pores (44). In complementary experiments, the sizes of the smallest pores (those 1 to 20 nm in diameter) are deterrnined by measurements characterizing desorption of N2 from the catalyst. The basis for the measurement is the capillary condensation that occurs in small pores at pressures less than the vapor pressure of the adsorbed nitrogen. The smaller the diameter of the pore, the greater the lowering of the vapor pressure of the Hquid in it. 

Pore volumes are determined by forcing N2 (for micro- and mesoporous materials) or Hg (macroporous materials) under pressure into the pores. The quantity of N2 or Hg entering the catalyst is directly related to the pressure and the radius of the pores. The Kelvin equation describes this ... 

M S. Zhurko, F.V. (2006a). Formation and decomposition of ethane, propane, and carbon dioxide hydrates in silica gel mesopores under high pressure. J. Phys. Chem. B, 110 (39), 19717-19725. 

Type IV isotherms are characterized by the presence of a hysteresis loop (i.e., adsorption and desorption branches are not coincident) due to the capillary condensation on the mesopores. They are characteristic of adsorbents that have a wide proportion of mesopores (i.e., compacted carbon blacks under pressure, nanostructured carbons prepared using mesoporous silica as templates, etc.). 

Mercury porosimetry is the most widely used technique for characterizing macroporosity in solids this technique covers a wide range of pore sizes, which also includes the majority of mesopores. Mercury porosimetry is based on the penetration of mercury, under pressure, into porosity. As mercury does not wet the carhon surface, pressui e is required to force the mercury into the structure. The relationship between pore radius, r, and mercury intrusion follows the so-called Washburn equation, already suggested in 1921 ... 

Ceramic membranes normally have an asymmetrical structure composed of at least two, normally three, different porosity levels. Indeed, before applying the active top layer, a mesoporous intermediate layer is often appHed in order to reduce the surface roughness. A macroporous support ensures mechanical stahility. Ceramic membranes generally show a higher chemical, structural and thermal stability. They do not deform under pressure, do not swell and are deaned easily [13]. 

The hydroamination of alkenes has been performed in the presence of heterogeneous acidic catalysts such as zeolites, amorphous aluminosilicates, phosphates, mesoporous oxides, pillared interlayered clays (PILCs), amorphous oxides, acid-treated sheet silicates or NafioN-H resins. They can be used either under batch conditions or in continuous operation at high temperature (above 200°C) under high pressure (above 100 bar). 

Prior to nitrogen adsorption experiment to determine surface properties, ACC sample was degassed at 130°C under vacuum (up to 10 torr) for 12 h. The adsorption data were obtained at the Central Laboratory of Middle East Technical University (METU) with a Quantachrome Autosoib-l-C/MS apparatus over a relative pressure ranging from 10" to 1. The BET specific surface area, total pore volume, micropore volume, mesopore volume, and pore size distribution, PSD, of ACC were yielded by using the software of the apparatus. 

The mechanical stability of PSM and AMM-5 samples was investigated by pressing the sample in a die (having a diameter of 16 mm) under different pressures for 15 min. The effects of compression on the surface areas and pore properties of the materials are shown in Table 1. It can be seen that the surface areas of both PSM and AMM-5 samples decrease under high pressure compression. The decrease of surface area, which is proportional to the pressure exerted on the samples, is accompanied with the decrease of pore volume, with no obvious decrease of the pore diameter for both samples. The results indicate that, under high pressure compression, some of the mesoporous channels of MCM-41 have collapsed completely and not constricted to pores of smaller diameter. 

Recently, the Horvath-Kawazoe (HK) method for slit-like pores [40] and its later modifications for cylindrical pores, such as the Saito-Foley (SF) method [41] have been applied in calculations of the mesopore size distributions. These methods are based on the condensation approximation (CA), that is on the assumption that as pressure is increased, the pores of a given size are completely empty until the condensation pressure corresponding to their size is reached and they become completely filled with the adsorbate. This is a poor approximation even in the micropore range [42], and is even worse for mesoporous solids, since it attributes adsorption on the pore surface to the presence of non-existent pores smaller than those actually present (see Fig. 2a) [43]. It is easy to verify that the area under the HK PSD peak corresponding to actually existing pores does not provide their correct volume, so the HK-based PSD is not only excessively broad, but also provides underestimated volume of the actual pores. This is a fundamental problem with the HK-based methods. An additional problem is that the HK method for slit-like pores provides better estimates of the pore size of MCM-41 with cylindrical pores than the SF method for cylindrical pores. This shows the lack of consistency [32,43]. Since the HK-based methods use CA, one can replace the HK or SF relations between the pore size and pore filling pressure by the properly calibrated ones, which would lead to dramatic improvement of accuracy of the pore size determination [43] (see Fig. 2a). However, this will not eliminate the problem of artificial tailing of PSDs, since the latter results from the very nature of HK-based methods. 

The reaction of epoxides with C02 affords either CCs or polymers [119], and many reports have been made [120-125] and different active catalysts described [126-130] such as alkyl ammonium-, phosphonium-salts and alkali metal halides, in this respect. The main drawbacks here are the need for a high catalyst concentration, a high pressure (5 MPa of C02), and a temperature ranging from 370 to 400 K. The recovery of the catalysts for reuse is also a key issue, and in order to simplify the recovery process various hybrid systems have been developed, an example being that prepared by coupling 3-(triethoxysilyl)propyltriphenylphosphonium bromide with mesoporous silica [131]. In this case, the reaction was carried out in the absence of solvent, under very mild conditions (1 MPa, 263 K, 1 mol% loading of catalyst, 6h), such that the hybrid catalyst could be recovered and recycled several times. 

Usually, two 100 cm long monolithic columns were prepared from the same reaction mixture, and two-four 33 cm long columns were obtained from the two 100 cm long silica capillaries containing silica monolith. The capillary columns (100 pm I.D.) showed 10,000-12,000 theoretical plates for the effective length of 25 cm under optimized conditions in a pressure-driven mode, and up to 40,000 plates in the CEC mode. The use of smaller-sized capillaries, e.g., 50 pm I.D., and the modification of the preparation method of mesopores, resulted in a monolithic silica column of higher efficiency and higher mechanical stability [25-27], Under optimized conditions, 80,000 plates were obtained with a 25 cm column in CEC. 

The simplest method is to pelletize the zeolite under low pressure then to crush the pellets into small particles (0.02-0.04 mm). This treatment causes no significant agglomeration of the small crystals and the organic molecules continue to enter easily the zeolite micropores through interparticular mesopores. 

Several non-carbonyl cobalt sources used recently show high efficiency in the catalysis of the PKR. Chung has reported different reusable catalysts like cobalt supported on mesoporous silica or on charcoal that work under high CO pressures [ 127]. Most recently they have described milder conditions with the use of colloidal cobalt nanoparticles, which react at lower CO pressures and can be used in aqueous media [128]. 

Since the capillary condensate in a particular mesopore is in thermodynamic equilibrium with the vapour, its chemical potential, p°, must be equal to that of the gas (under the given conditions of T and p). As we have seen, the difference between p° and p1 (the chemical potential of the free liquid) is normally assumed to be entirely due to the Laplace pressure drop, Ap, across the meniscus. However, in the vicinity of the pore wall a contribution from the adsorption potential, 0(z), should be taken into account. Thus, if the chemical potential is to be maintained constant throughout the adsorbed phase, the capillary condensation contribution must be reduced. 

Unlike the thermal and hydrothermal stabilities, the mechanical stability seems less dependent on the nature of mesoporous materials. All materials gradually collapse with the increase of pressure, accompanied with the decrease of surface area and pore volume. Recent studies show that cubic SBA-1 and MCM-48 are more mechanically stable than hexagonal mesoporous materials such as MCM-41 and SBA-15. Hydrolysis of Si-O-Si bonds by water adsorbed onto the silanol groups under compression was found as the main reason for mechanical instability. Organically functionahzed materials are more hydrophobic than unmodified counterparts, and thus show enhanced mechanical stability due to the water repelling ability. " ... 

Therefore, the aim of the present work is to deepen into the accepted exclusive mesoporous character of MCM-41 by carefiilly analyzing the physical adsorption data of N2 at 77 K and CO2 at 273 K, covering in both cases a wide range of relative pressures (from 10 to near unity) and comparing the adsorption data of a sample of MCM-41 with a precipitated silica, prepared under identical conditions but without the templating agent. 

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