Silicalite hydrocarbons

Olefins (n-Cg) from mixtures containing Cg branched olefins and cyclic hydrocarbons Silicalite-1 Pentene, 1-butene [163]... 

Arruebo, M., Coronas, J., Menendez, M., and Santamaria, J. (2001) Separation of hydrocarbons from natural gas using silicalite membranes. Sep. Purif. Tech., 25, 275-286. 

V-containing silicalite, for example, has been shown to have different catalytic properties than vanadium supported on silica in the conversion of methanol to hydrocarbons, NOx reduction with ammonia and ammoxidation of substituted aromatics, butadiene oxidation to furan and propane ammoxidation to acrylonitrile (7 and references therein). However, limited information is available about the characteristics of vanadium species in V-containing silicalite samples and especially regarding correlations with the catalytic behavior (7- 6). 

Substituting divalent or trivalent elements for the A1 in the framework has been successfully carried out by several groups yielding novel heterogeneous catalysts (metal-substituted ALPOS, MALPOs Thomas et al 2001) for hydrocarbon oxidation and liquid phase oxidation. MALPO catalysts can be complementary to metal-doped silicalite catalysts. Particularly interesting compounds are MALPOs in which a divalent metal (Me) substitutes for the framework Al +, for example MALPO-36 (where M = Mg, Mn, Zn, Co) and MALPO-34 (M = Mg, Mn, Co etc). 

Supported Co-Mn Fischer-Tropsch Catalysts. F-T synthesis of lower hydrocarbons on silicalite-1 supported Co and Co-Mn catalysts was reported by Das et C03O4 was found to be the only phase present in Mn-free... 

We now consider the diffusion of other hydrocarbons. Most calculations have been performed for n-alkanes, up to and including n-hexane, but alkenes and alkynes have also been considered. Calculations involving larger molecules have been mainly restricted to silicalite. 

Dumont and Bougeard (68, 69) reported MD calculations of the diffusion of n-alkanes up to propane as well as ethene and ethyne in silicalite. Thirteen independent sets of 4 molecules per unit cell were considered, to bolster the statistics of the results. The framework was held rigid, but the hydrocarbon molecules were flexible. The internal coordinates that were allowed to vary were as follows bond stretching, planar angular deformation, linear bending (ethyne), out-of-plane bending (ethene), and bond torsion. The potential parameters governing intermolecular interactions were optimized to reproduce infrared spectra (68). 

Hope et al. (116) presented a combined volumetric sorption and theoretical study of the sorption of Kr in silicalite. The theoretical calculation was based on a potential model related to that of Sanders et al. (117), which includes electrostatic terms and a simple bond-bending formalism for the portion of the framework (120 atoms) that is allowed to relax during the simulations. In contrast to the potential developed by Sanders et al., these calculations employed hard, unpolarizable oxygen ions. Polarizability was, however, included in the description of the Kr atoms. Intermolecular potential terms accounting for the interaction of Kr atoms with the zeolite oxygen atoms were derived from fitting experimental results characterizing the interatomic potentials of rare gas mixtures. In contrast to the situation for hydrocarbons, there are few direct empirical data to aid parameterization, but the use of Ne-Kr potentials is reasonable, because Ne is isoelectronic with O2-. 

More recently phosphorus-containing zeolites developed by Union Carbide (alu-minophosphates, silicoaluminophosphates) were shown to be equally effective in methanol condensation.439-444 ZSM-5 was also shown to exhibit high activity and selectivity in the transformation of Fischer-Tropsch oxygenates to ethylene and propylene in high yields.445 Silicalite impregnated with transition-metal oxides, in turn, is selective in the production of C4 hydrocarbons (15-50% isobutane and 8-15% isobutylene).446... 

Miscellaneous Oxidations. Titanium silicalites (TSs) are molecular sieves that incorporate titanium in the framework. They are able to perform oxygenation of various hydrocarbons under mild conditions by hydrogen peroxide.184,185... 

As has been mentioned in Section III,G, West (101) and Fyfe et al. (102) found that traces of adsorbed species (aromatic hydrocarbons and alkanols) radically change the 29Si MAS NMR spectrum and the XRD pattern of silicalite. It is too early to predict the potential of this fascinating discovery for the structural elucidation of zeolites, but one can speculate about the possible consequential pitfalls. One of them is the extreme sensitivity of the effect, requiring less than one molecule of sorbate per unit cell of the sorbent. Quantitative measurements will therefore have to be carried out under very... 

As reported in the literature, the acylation of aromatic hydrocarbons can be carried out by using zeolites as catalysts and carboxylic acids or acyl chlorides as acylating agents. Thus toluene can be acylated by carboxylic acids in the liquid phase in the presence of cation exchanged Y-zeolites (ref. 1). The acylation of phenol or phenol derivatives is also reported. The acylation of anisole by carboxylic acids and acyl chlorides was obtained in the presence of various zeolites in the liquid phase (ref. 2). The acylation of phenol by acetic acid was also carried out with silicalite (ref. 3) or HZSM5 (ref. 4). The para isomer has been generally favoured except in the latter case in which ortho-hydroxyacetophenone was obtained preferentially. One possible explanation for the high ortho-selectivity in the case of the acylation of phenol by acetic acid is that phenylacetate could be an intermediate from which ortho-hydroxyacetophenone would be formed intramolecularly. 

Krishna and Paschek [91] employed the Maxwell-Stefan description for mass transport of alkanes through silicalite membranes, but did not consider more complex (e.g., unsaturated or branched) hydrocarbons. Kapteijn et al. [92] and Bakker et al. [93] applied the Maxwell-Stefan model for hydrocarbon permeation through silicalite membranes. Flanders et al. [94] studied separation of C6 isomers by pervaporation through ZSM-5 membranes and found that separation was due to shape selectivity. 

Of interest with respect to this hypothesis is the significant difference in heat of paraffin adsorption between the medium-pore zeolite silicalite and large-pore, de—aluminated faujasite. The heat of paraffin adsorption is much smaller in the case of the de-aluminated faujasite, which has so far had to be prepared by an indirect route, than for silicalite, which can be synthesized direct in the presence of an organic molecule. The difference, which increases with chain length, is of the order of 5 kJ/mol per CH2 unit, and may be ascribed to the optimum fit of hydrocarbon and channel in the case of the medium-pore zeolite (H, 12). ... 

For silicalite all the simulations (MC, MD and MM) were carried out with the zeolite structure held rigid inasmuch as, according to the results of Tiltiloye et al., [24], for n<4 the effect of the zeoUte relaxation is negligible (< 0.2 kcal/mol). In addition, full relaxation was allowed to aU hydrocarbon molecules. However, for ZSM-5 the effect of the framework relaxation on the adsorption energy is already noticeable. Therefore, because the computer time requirements for... 

Diffusion of aromatic hydrocarbons in silicalite has been widely studied by several different methods although these systems are not amenable to NMR measurements... 

Jama, M.A. E)elmas, M.P.F., Ruthven, D.M., Diffusion of linear and branched C6 hydrocarbons in silicalite studied by the wall-coated capillary diromatographic Method. Zeolites 18 (1997) pp. 200-204. 

Kapieijn F., Bakker W.J.W., Zheng G., Poppe J. and Moulijn J.A., Permeation and separation of light hydrocarbons through a silicalite-1 membrane Application of the generalized Max-well-Stefan equations, Cherru Eng. J. 57 145 (1995). 

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. 

Figure 2 Flux of methane ( ), ethane, ( ), propane ( ), n-buiane ( ), and iso>butane (A) through a silicalite-1 membrane as a function of partial pressure on the feed side (T = 298 K, = 100 kPa). Feed was composed of hydrocarbon and balance helium sweep gas used was helium. There was no absolute pressure difference across the membrane. (Adapted from Ref. 14.)...

Figure 6b Fluxes of mixtures of ethane (A) and ethene (9) through a silicalite-1 membrane as a function of mol fraction in the feed (P = 101.3 kPa, T = 297 K). Feed was composed of 100% hydrocarbon (mol fraction of ethene = 1 - mol fraction of ethane) sweep gas used was helium. The measured separation selectivity toward ethane is also given (+) together with the separation selectivity predicted from single-component fluxes for identical partial pressures ratios ( ).

The same model was applied to permeation of lighter hydrocarbons (C1-C3) through the silicalite-1 membrane [50]. In the case of methane, ethane, and ethene, some concentration dependence of the Maxwell-Stefan diffusivity was observed. This can be caused either by the importance of interfacial effects, which are not taken into account, or by the contribution of activated-gas translational diffusion to the net flux. The diffusivities calculated from these permeation experiments were, however, in rather good agreement with diffusivity values from the literature, which implies that these zeolitic membranes could also be a valuable tool for the determination of diffusion coefficients in zeolites. 

As an example. Figure 10.15 compiles the permeance evolution with temperamre for single hydrocarbons (from Ci to C3) over silicalite-1 membranes supported on stainless steel tubes. A specific interaction with hnear hydrocarbons appears, as it could be expected due to the organophilic character of SIL-1 membranes. For methane, the temperature used is too high to find... 

---