Mixed silicalite

The increasing volume of chemical production, insufficient capacity and high price of olefins stimulate the rising trend in the innovation of current processes. High attention has been devoted to the direct ammoxidation of propane to acrylonitrile. A number of mixed oxide catalysts were investigated in propane ammoxidation [1]. However, up to now no catalytic system achieved reaction parameters suitable for commercial application. Nowadays the attention in the field of activation and conversion of paraffins is turned to catalytic systems where atomically dispersed metal ions are responsible for the activity of the catalysts. Ones of appropriate candidates are Fe-zeolites. Very recently, an activity of Fe-silicalite in the ammoxidation of propane was reported [2, 3]. This catalytic system exhibited relatively low yield (maximally 10% for propane to acrylonitrile). Despite the low performance, Fe-silicalites are one of the few zeolitic systems, which reveal some catalytic activity in propane ammoxidation, and therefore, we believe that it has a potential to be improved. Up to this day, investigation of Fe-silicalite and Fe-MFI catalysts in the propane ammoxidation were only reported in the literature. In this study, we compare the catalytic activity of Fe-silicalite and Fe-MTW zeolites in direct ammoxidation of propane to acrylonitrile. 

Guo, G., Long, Y., and Sun, Y. (2001) Hydrophobic silicalite method for liquid-phase selective adsorption, separation and mixing of dichlorobenzene. C.N. Patent 1,315,217. 

Jia and coworkers prepared thin-film composite zeolite-filled silicone rubber membranes by a dip-coating method [82]. The membranes have a thin silicalite-1/ silicone rubber mixed-matrix selective layer on top of a porous polyetherimide support. 

The direct hydroxylation of benzene and aromatics with a mixture of 02 and H2 have been performed by simultaneously mixing benzene, oxygen and hydrogen in the liquid phase using a very complicated system containing a multi-component catalyst, a solvent and some additives. Besides the possibility of an explosive gas reaction, these hydroxylations gave only very low yields, 0.0014—0.69% of phenol and aromatic alcohols. For example, Pd-containing titanium silicalite zeolites catalyzed... 

As part of a systematic investigation of the adsorption of single and mixed gases by zeolites, Rees and his co-workers used an isosteric approach (i.e. by measuring a series of p-T isosteres) to compare the adsorptive properties of Silicalite I and zeolite NaY (Hampson and Rees, 1993). They concluded that, in contrast to NaY, Silicalite I was energetically homogeneous with respect to the adsorption of both propane and ethene. The results also indicate that the ethene-Silicalite interaction was largely non-specific. 

Vibrational spectroscopies give rise to interesting information on the microscopic structure of soUd-solution mixed oxides. For example, the state of vanadium in soUd solution in Ti02 anatase catalysts [59], the partial ordering of cations in comndum-type Fe-Cr oxides [60], the real presence of Ti" in the silicalite framework of TSl catalysts [58] and the solubility of AT ions in the NiO rock-salt structure [61] have been objects of IR spectroscopic studies. 

More promising from an industrial perspective, however, is the separation of the oxidation zone from the aqueous one effected by the catalytic material itself, through the selective adsorption of the reagents. The introduction of Titanium Silicalite-1 (TS-1), in which the hydrophobic properties of the pores protect the active sites from the inhibition of the external aqueous medium, was a demonstration of the concept. The catalyst, the substrate and the aqueous soluhon of hydrogen peroxide can, in this case, be mixed together, with a great simplification of the process and also a reduction of the hazards. Three commercial processes. 

Jia et al. [1993] have prepared thin, dense pure silicalite zeolite membranes on porous ceramic supports by an in-situ synthesis method. A sol consisting of silica, sodium hydroxide, tctrapropylammonium bromide and water is prepared with thorough mixing. A ceramic support is immersed in the sol which is then heated and maintained at 180X... 

Even small amounts of carbon monoxide or other contaminants can poison a fuel cell. Sandia National Laboratories (SNL) is working on developing gas-selective thin film membranes to improve and lower the cost for hydrogen purification. Defect-free aluminosilicate and silicalite zeolite thin films supported on commercially available alpha and gamma alumina disk substrates were developed. In tests using SNL s permeation unit, which can test both pure and mixed gases from room temperature to 250°C, excellent separation values for hydrogen were achieved. 

A method for determining the effect of particle size on the effective permeability values of zeolite-polymer mixed matrix membranes has been developed in this study. The model presented is a modified form of the effective medium theory, including the permeability and thickness of an additional phase, the interphase, which is assumed to surround the zeolite particles in the polymer environment. The interphase thickness and permeability values were determined by taking into consideration the assumptions that in case the size of the zeolite particles is held constant, the interphase thickness should be equal for different gases and in case the zeolite particle size is varied, the interphase permeability should remain constant for the same gas. The model seems to fit the experimental permeability data for O2, N2 and CO2 in the silicalite-PDMS mixed matrix membranes well. 

Ol-P-14 - Synthesis of titanium, niobium, and tantalum silicalite-1 by microwave heating of the mixed oxide xerogel precursors... 

The first method uses the controlled concurrent hydrolysis of tetraethoxytitanium (IV) and tetraethoxysilane. This procedure has been labeled the mixed alkoxide method. Frequently, the acronyms TET (tetraethyltitanate) and TEOS (tetraethyl orthosilicate or tetraethoxysilane) are used for the respective reactants these are derived from the alternative names tetraethyl titanate and tetraethylorthosilicate. In examples described in patents, the synthesis involves adding TET to TEOS and then combining the alkoxide mixture with an aqueous solution of a SDA, which is typically tetrapropylammonium hydroxide (TPAOH). The resulting precursor mixture is then heated to a temperature of 175 °C to initiate crystallization. Subsequent washing of the crystallized solid with water, drying, and air calcination produces framework titanium-containing silicalite. 

The CS-based mixed matrix membranes are also suitable for pervaporation application. Patil and Aminbhavi [96] prepared mixed matrix membranes of CS by incorporating sili-calite zeolite particles in 5 and 10 wt.% for the pervaporation separation of toluene/methanol and toluene/ethanol feeds in compositions of 10-40 wt.% of toluene at 30°C. The membranes were toluene selective than alcohol selective. Flux of toluene/methanol and toluene/ethanol mixtures decreased, but selectivity increased with increasing alcohol content of the feed. Toluene permeated preferentially with a selectivity of 264 and fluxes of 0.019-0.027 kg/m h for toluene/ methanol mixture. Selectivity of 301 with fluxes ranging from 0.019 to 0.026 kg/m h was observed for tolnene/etha-nol mixtures. Flux increased, while selectivity decreased with increasing toluene content of the feeds. An increase in silicalite content of the MMMs gave increased pervaporation performances. 

Mixtures of supported platinum and aluminophosphate-based molecular sieves have been tested for w-hexane rearrangement reactions under reforming conditions (900°F, 200 psig). Data were collected for the large, medium, and small pore SAPO molecular sieves mixed with a Pt-alumina catalyst. Data for Pt mixed with chlorided alumina or mixed with silicalite were also taken for reference. The data show that the catalyst consisting of Pt and large pore SAPO-5 is less active than reference Pt-chlorided alumina however, the catalyst appears... 

The preparation of defect-free zeolite layers on a large scale is extremely difficult and it seems doubtful that this will ever be achieved at a competitive price. However, the combination of the superior gas selectivities of molecular sieves with the processibility of polymeric membranes has attracted many researchers. The hybrid membranes consisting of inorganic molecular sieves and polymers are often referred to as mixed-matrix membranes. The term mixed-matrix membrane has been introduced by Kulprathipanja et al. [309], who performed pioneering work in the field of polymer/zeolite hybrid membranes. Kulprathipanja showed that the CO2/H2 selectivity of cellulose acetate could be reversed by addition of silicalite. The siHcaHte-CA membrane had a CO2/H2 selectivity of 5.1, whereas the pure CA membrane exhibited a selectivity of 0.77. 

Hydrothermal synthesis has also been used to prepare mixed-metal oxide catalysts. The group of Maier presented already in 1998 the first hydrothermal high-throughput preparation method for such catalytic materials [93]. Corma et al. used a hydrothermal treatment of sol-gel synthesized Ti-silicalite catalyst precursors to accelerate the crystallization [94]. 

Several mixed matrix membranes made of silicalite-1 crystals of different size and shape embedded in Teflon AF 1600, Teflon AF 2400 and Hyflon AD 60X have been prepared. The gas permeation properties of the membranes have been tested with pure gases, and some experiments with n-C4Hio/CH4 mixtures have also been carried out on silicalite-l/Teflon AF 2400 membranes. 

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