Molecular sieving silica

Adsorbents are natural or synthetic materials of amorphous or microcrystalhne structure. Those used on a large scale, in order of sales volume, are activated carbon, molecular sieves, silica gel, and activated alumina [Keller et al., gen. refs.]. 

The separation factors are relatively low and consequently the MR is not able to approach full conversion. With a molecular sieve silica (MSS) or a supported palladium film membrane, an (almost) absolute separation can be obtained (Table 10.1). The MSS membranes however, suffer from a flux/selectivity trade-off meaning that a high separation factor is combined with a relative low flux. Pd membranes do not suffer from this trade-off and can combine an absolute separation factor with very high fluxes. A favorable aspect for zeoHte membranes is their thermal and chemical stability. Pd membranes can become unstable due to impurities like CO, H2S, and carbonaceous deposits, and for the MSS membrane, hydrothermal stability is a major concern [62]. But the performance of the currently used zeolite membranes is insufficient to compete with other inorganic membranes, as was also concluded by Caro et al. [63] for the use of zeolite membranes for hydrogen purification. 

Packed-bed conversion. Membrane reactor conversion. Molecular Sieve Silica. 

Microwaves are frequently used in the laboratory by synthetic organic chemists for regeneration and activation of solids such as molecular sieves, silica gel or alumina when fast and complete drying is required. 

Acetone purified by these means, or the analytical reagent grade, may have the water content reduced by storage over a Type 4A molecular sieve. Silica gel or alumina should not be used as an aldol type reaction is initiated with the formation of water as a by-product. Pure acetone has b.p. 56.2 °C/760mmHg and is highly flammable. 

Conventional high pressure NICI spectra were obtained using a Hewlett-Packard 5985B quadrupole GC/MS, as described previously (1). Methane was used as the Cl reagent gas and was maintained in the source at 0.2-0.4 torr as measured through the direct inlet with a thermocouple gauge. A 200 eV electron beam was used to ionize the Cl gas, and the entire source was maintained at a temperature of 200° C. Samples were introduced into the spectrometer via the gas chromatograph which was equipped with a 25 meter fused silica capillary column directly interfaced with the ion source. For all experiments, a column coated with bonded 5% methyl phenyl silicon stationary phase, (Quadrex, Inc.) was used and helium was employed as the carrier gas at a head pressure of 20 lbs. Molecular sieve/silica gel traps were used to remove water and impurities from the carrier gas. 

The book explores various examples of these important materials, including perovskites, zeolites, mesoporous molecular sieves, silica, alumina, active carbons, carbon nanotubes, titanium dioxide, magnesium oxide, clays, pillared clays, hydrotalcites, alkali metal titanates, titanium silicates, polymers, and coordination polymers. It shows how the materials are used in adsorption, ion conduction, ion exchange, gas separation, membrane reactors, catalysts, catalysts supports, sensors, pollution abatement, detergency, animal nourishment, agriculture, and sustainable energy applications. 

In summary, the main goal of the present work is the development of a hydrothermally stable microporous silica membrane with prescribed transport properties. Preferably, these steam stable membranes should have very high permselectivities. Because the permselectivity of a molecular sieving silica membrane will drop to the Knudsen value of the y-alumina supporting membrane when the silica membrane deteriorates under steam reforming conditions, a selectivity of the silica layer higher than the Knudsen selectivity is sufficient. In this way the measurement of the permselectivity is a powerful tool to assess the hydrothermal stability of a supported microporous membrane. 

The improvements that have been made in the preparation of molecular sieving silica membranes started with the development of high quality membrane supports, because quality of the supporting system is of crucial importance for the quality of the final molecular sieving membrane. To this end, the synthesis of the supports was performed by means of colloidal proc-... 

Finally, in chapter 9, conclusions are drawn and suggestions made for further research on (steam-stable) molecular sieving silica membranes or mesoporous y-alumina membranes. Though not all of the project objectives were obtained, progress was made in the synthesis of micro- and mesoporous membranes. Especially the development of steam stable membranes may be a large step forward in the development of ceramic membranes. 

N.K. Raman and C.J. Brinker, Organic Template Approach to Molecular Sieving Silica Membranes , J. Membrane Sci., 105 273-79 (1995). 

The cryogenic air separation process starts with air entering the plant through air filters, where it is compressed and cooled. The air is passed through heat exchangers for further cooling and for removal of water vapor and carbon dioxide by freezing. Solid adsorbents, such as molecular sieves, silicas, and aluminas can also be used to remove the water and carbon dioxide by adsorption before separation of... 

The O2 isotherms for these AlPO molecular sieves are essentially Type I, typical of micropore filling (12,13). In Figure 16 the O2 isotherms of AlP0i.-5, -11, -17 are compared with those of a typical zeolite, NaX, and a silica molecular sieve, silica-lite. A1P04-5 and -17 have saturation pore volumes of approximately 0.2 cm /g similar to that of silicalite, while A1P0 -11 has a relatively small O2 pore volume (ca. 0.1 cm3/g). 

In gas-solid chromatography, the stationary phase is an active solid. These solids may be inorganic materials, e.g. synthetic zeolite molecular sieve, carbon molecular sieve, silica gel, or graphitised carbon, or they may be oiganic polymers. They are generally used for the separation of low molecular weight materials, i.e. gases and liquids. 

Nair BN, Yamagushi T, Okubo T, Suematsu H, Keizer K, and Nakao S-I. Sol-gel synthesis of molecular sieving silica membranes. J. Membr. Sci. 1997 135 237-243. 

Dinis da Costa JC, Lu GQ, Rudolph V, and Lin YS. Novel molecular sieve silica (MSS) membranes characterisation and permeation of single-step and two-step sol-gel membranes. J. Membr. Sci. 2002 198 9-21. 

Kodakari N, Tomita K, Iwata K, Katada N, Niwa M (1998) Molecular sieving silica overlayer on gamma-alumina The structure and acidity controlled by the template molecule. Langmuir 14 4623... 

Yamakita S, Katada N, Niwa M (2005) Shape-selective adsorption of substituted benzalde-hyde isomers by a molecular sieving silica overlayer prepared by the chemical vapor deposition method using organic template on tin oxide. Bull Chem Soc Jpn 78 1425... 

Specificity is the most important requirement in gas analysis. Techniques dependent on the physical properties of the gas molecules, such as thermal conductivity, density, viscosity, and sound velocity, generally have insufficient specificity to differentiate a single gas in a mixture of gases, and therefore must incorporate in the procedure some type of preliminary separation. Vapor phase fractionation (gas chromatography) is an example of a popular analytical technique based upon a physical property (thermal conductivity) of the gas that requires preliminary separation of the gases by means of special columns (molecular sieve, silica gel, etc.). 

Table II. Pore Volumes at p/p° = 0.95 of Molecular Sieve Silicas...

Fig 6 shows the single-stage system, which is referred to as plasma-driven catalysis [77]. In the PDC process, catalysts arc directly placed in the NTP reactor. These catalysts arc activated by NTP at low temperature region, where the thermal catalysis docs not occur. The shape of catalyst is cither of honeycomb, foam or pellet. In contrast to the PEC system, all reactions of gas-phase, surface and their interaction lake place simultaneously. In this sense, it is quite complicate to understand and optimize the chemical reactions in the PDC system. In an early USA patent, Henis proposed a PDC reactor for NO.r removal. Figure 7 shows the schematic diagram of the PDC reactor proposed by Henis [78], which is quite similar to those used in recent studies. The gases arc introduced to the reaction zone through the contact materials for heat transfer purpose. The catalysts listed in the patent are alumina, zirconium silicate, cobalt oxide, Thoria, activated carbon, molecular sieves, silica gel etc. 

Nair BN, Keizer K, Suematsu H, Suma Y, Ono S, Okubo T, et al. Synthesis of gas and vapor molecular sieving silica membranes and analysis of pore size and connectivity. Langmuir. 2000 16(10) 4558-662. 

Raman NK, Brinker CJ. Organic template approach to molecular sieving silica membranes. J Memb Sci. 1995 105 273-9. 

Supported reagent. One limitation to potassium permanganate is the limited solubility in organic solvents. Solubilization with a crown ether is one solution (4,143) another is impregnation on an inorganic solid support such as a molecular sieve, silica gel, and certain clays. Although the factors responsible for the enhanced reactivity of this form of KMn04 are not understood, such materials are useful for oxidation. ... 

Recently, investigation of silica membranes was also reported in the literature. Giessler et al. [32] were the first who investigated molecular sieve silica membrane for WGS reaction. They prepared membrane by using tetra ethyl... 

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