Silicalite concentration

Table 2 Silicalite concentration in washcoated (silica binder) or extruded (alumina binder) honeycomb pieces, 1" (diameter) x 1" (length)...

Effect of concentration of the Si source on the size of silicalite nanocrystais... 

ESI-MS has shown to be able to determine semi-quantitatively the initial dissolution species in silicalite-1. Their identities are further substantiated by monitoring selected fragmentation patterns. Doubly charged species consisting of aggregation of identical anionic silicates were evidenced in very low concentration in agreement with the NMR. 

In the direct ammoxidation of propane over Fe-zeolite catalysts the product mixture consisted of propene, acrylonitrile (AN), acetonitrile (AcN), and carbon oxides. Traces of methane, ethane, ethene and HCN were also detected with selectivity not exceeding 3%. The catalytic performances of the investigated catalysts are summarized in the Table 1. It must be noted that catalytic activity of MTW and silicalite matrix without iron (Fe concentration is lower than 50 ppm) was negligible. The propane conversion was below 1.5 % and no nitriles were detected. It is clearly seen from the Table 1 that the activity and selectivity of catalysts are influenced not only by the content of iron, but also by the zeolite framework structure. Typically, the Fe-MTW zeolites exhibit higher selectivity to propene (even at higher propane conversion than in the case of Fe-silicalite) and substantially lower selectivity to nitriles (both acrylonitrile and acetonitrile). The Fe-silicalite catalyst exhibits acrylonitrile selectivity 31.5 %, whereas the Fe-MTW catalysts with Fe concentration 1400 and 18900 ppm exhibit, at similar propane conversion, the AN selectivity 19.2 and 15.2 %, respectively. On the other hand, Fe-MTW zeolites exhibit higher AN/AcN ratio in comparison with Fe-silicalite catalyst (see Table 1). Fe-MTW-11500 catalyst reveals rather rare behavior. The concentration of Fe ions in the sample is comparable to Fe-sil-12900 catalyst, as well as... 

We synthesized nine silicalites which had different concentrations of defect sites in the zeolite framework determined by isotope exchange method. These silicalites were treated with aluminium trichloride vapor under the same reaction conditions 923 K temperature, 1 h time, 11 kPa aluminium trichloride vapor pressure. Figure 1 shows the plots of the amount of aluminium atoms introduced into the framework against the amount of oxygen atoms on the defect sites. A... 

Other flexible framework calculations of methane diffusion in silicalite have been performed by Catlow et al. (64, 66). A more rigorous potential was used to simulate the motion of the zeolite lattice, developed by Vessal et al. (78), whose parameters were derived by fitting to reproduce the static structural and elastic properties of a-quartz. The guest molecule interactions were taken from the work of Kiselev et al. (79), with methane treated as a flexible polyatomic molecule. Concentrations of 1 and 2 methane molecules per 2 unit cells were considered. Simulations were done with a time step of 1 fs and ran for 120 ps. 

Nicholas et al. (67) have performed MD calculations of propane in sili-calite in which the propane molecule is given complete flexibility. The calculations, which have been detailed previously for methane diffusion, employed a large simulation box with multiple sets of adsorbates to ensure good statistics. The framework was kept fixed and data were collected over a 40-ps run. The results predict diffusion coefficients in very good agreement with the values of Caro et al. (71). The calculated values for a concentration of 4 and 12 propane molecules per silicalite unit cell are 0.12 and 0.005 X 10 8 m2/s, respectively. These values for propane are far lower than those of Nowak et al. (63), the reason for this is that Nicholas et al. used flexible adsorbate molecules, whereas Nowak et al. used rigid ones. 

Activated carbon is by far the most widely used adsorbent. It is available in a wide range of different forms that differ mainly in pore size and pore size distribution. The carbon surface is essentially nonpolar although some polarity can be imparted by surface oxidation or other pre-treatments. It is widely used for removal of low concentrations of organics, either from aqueous streams (for example, decolorization of sugar or water treatment) or from vapor streams (for example, in range hoods and other pollution-control devices). Crystalline silica adsorbents such as silicalite are also organophilic but are substantially more expensive than activated carbon so their application is generally limited to situations where, for some reason, the use of carbon is not appropriate. 

We report (i) isomorphous substitution of boron, by secondary synthesis, into silicalite and into highly siliceous (Si/Al>400) ZSM-5 and (ii) an improved direct synthesis of zeolite (Si,B) -ZSM-5. The chemical status of B in die boronated products depends upon reaction conditions. Careful control of the concentration of the base, the borate species and of die duration of treatment, allows the preparation of samples containing only 4-coordinated B or a mixture of 3- and 4-coordinated B in various relative concentrations. The products were characterized by magic-angle-spinning (MAS) NMR and infrared (IR) spectroscopies and by powder x-ray diffraction (XRD). 

Table 11 gives the absolute intensities of NMR signals and the concentration of tetrahedral and trigonal boron sites. The boron content of the samples was calculated from spectral intensities. It follows that the amounts of boron introduced into silicalite/ZSM-5 during hydrothermal treatment are relatively small, between 0.17-0.50 B atoms per unit cell. Of this amount up to 0.36 B/u.c. was found in tetrahedral coordination (sample 6, treated with 0.16M KOH for 42 h). There is also a systematic decrease of the (tetrahedral) boron line width from 115 to 91 kHz (Table I) with the increase alkalinity of the treatment. 

As predicted, at low loadings, argon and nitrogen are adsorbed in a very similar manner on pure Silicalite. Thus, in each case the adsorption energy remains almost constant until TV" = 20 molec uc 1. This suggests that localized adsorption is taking place with very little adsorbate-adsorbate interaction. The adsorbed molecules are mainly located in the channels and at a lower concentration in the intersections. 

For mono-methyl paraffm separation, two pulse test techniques, one with and one without iso-octane pre-pulse, were developed (2,3). In each test the feed was a mixture containing equal volumes of 3,3,S-trimethyl heptane, 2,6-dimethyl octane, 2-methyl nonane, n-decane, and I,3,S-trimethyl benzene. The pulse test column had a volume of 70 cc and was held at a temperature of 120 C in the experiments shown. The flow rate through the column was 1.2 ml/min. The adsorbent was silicalite and the desorbent was a 30/30 volume % mixture of n-hexane/cyclohexane. Test I was run without a pre-pulse and test 2 was run with a pre-pulse of 40 ml of iso-octane injected into the test loop immediately before the feed mixture was injected. Iso-octane pre-pulse diluted the n-hexane concentration at the adsorption zone and increased the adsorbent selectivity for mono-methyl paraffin. 

In Figure 23 the contribution of the gas phase and the adsorbed phase concentration of a number of gases to the total concentration in silicalite is presented. 

Figure 23. Contribution of gas phase (soiid line) and adsorbed phase concentration (symbols A= Hj, += Nj, = CH4, =C02) to the total concentration in silicalite-1 (dashed lines) as a function of temperature (pi = 1 atm). Adsorption data are taken from [71].

On the other hand, organics may be removed fi"om aqueous solutions by applying a high silica membrane of a suitable pore size. An application might be the continuous removal of ethanol by e.g. a Silicalite-1 based membrane from carbohydrate fermentation liquids. The Microorganisms used are deactivated at an alcohol content > 12%. By continuous removal of the alcohol produced its concentration may be kept at a level below 12% enabling a higher final conversion of the carbohydrate feed. 

In Fig. 2 the fluxes of several light hydrocarbons through a silicalite-1 membrane are shown as a function of their partial pressure on the feed side. The trend that can be deduced from this figure is that as the molecules get larger, their flux becomes lower. This decrea.se in flux is, however, smaller than expected on the basis of differences in diffusion coefficients [14]. The increase in the size of the molecule results in a lower mobility in the pores, but this effect is partly compensated by the higher concentration in the membrane, due to better adsorption of the larger molecules. This compensation effect is also the reason that at low partial pressures, ethane permeates faster through the membrane than does methane. 

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