Silicalite benzene

Beyer and Belenykaia (27) have investigated the sorption properties of DAY zeolites prepared from Y zeolite and SiCl vapors. They reported a very low adsorption capacity for water and ammonia, similar to that of the almost aluminum-free silicalite (49). The low adsorption capacity for water is indicative of a hydrophobic zeolite surface. The adsorption isotherms for n-butane, benzene and n-hexane obtained on the aluminum-deficient zeolite have a shape similar to those obtained on NaY zeolite and are characteristic for micropore structures. They show the absence of secondary pores in this DAY zeolite. 

Let us recall that by the sol-gel method one can obtain very efficiently very well-defined systems such as Ti silicalite, which can be considered as a single site system where titanium is tetracoordinated in a zeolitic matrix and undergoes epoxidation of propylene or hydroxylahon of benzene to phenol. Bear in mind that it took industry more than 20 years to realize such an industrial processes (Dow-BASF process) [1]). 

The incorporation of vanadium(V) into the framework positions of silicalite-2 has been reported by Hari Prasad Rao and Ramaswamy . With this heterogeneons oxidation catalyst the aromatic hydroxylation of benzene to phenol and to a mixtnre of hydroqninone and catechol conld be promoted. A heterogeneons ZrS-1 catalyst, which has been prepared by incorporation of zirconinm into a silicalite framework and which catalyzes the aromatic oxidation of benzene to phenol with hydrogen peroxide, is known as well in the literature. However, activity and selectivity were lower than observed with the analogous TS-1 catalyst. 

Possibly the earliest theoretical study of diffusion of aromatics in zeolites was published in 1987 by Nowak et al. (89), who considered diffusion of benzene and toluene in the pores of silicalite and theta-1. Theta-1 (90) has a unidimensional medium-sized pore opening bounded by 10-rings. In this study, only the straight channel of silicalite was considered, making the... 

Similar energy minimization calculations were reported for benzene and p-xylene in silicalite (92). Diffusion coefficients were estimated from minimum energy paths through the pore. The value for benzene, 27.6 kJ/mol, is in good agreement with that of Pickett et al. (91). For the bulkier p-xylene molecule, the activation barrier was predicted to be slightly lower (23 kJ/... 

The predicted diffusion behavior of benzene showed the familiar anisotropy observed for other molecules in silicalite, although evolving over a far longer time period. The calculated diffusion coefficients at temperatures between 200 and 500 K varied by 6 orders of magnitude. The orientationally averaged value at 300 K is 1.1 X 10-12 m2/s, approximately 1-2 orders of... 

The complexity of modeling the adsorption of benzene in silicalite has already been discussed in the section concerned with diffusion. A TST study by Snurr et al. (106) led to the identification of 27 unique sorption minima in the asymmetric unit. Given this result, it is unsurprising that there have been relatively few simulation studies of this system. However,... 

Silica-alumina particles coated with a permselective silicalite membrane is almost completely selective in the formation of p-xylene in the disproportionation of toluene.402 Friedel-Crafts alkylations were performed in ionic liquids. The strong polarity and high electrostatic fields of these materials usually bring about enhanced activity.403 404 Easy recycling is an additional benefit. Good characteristics in the alkylation of benzene with dodecene were reported for catalysts immobilized on silica or MCM-41 405... 

A very interesting development was the discovery (101) that the 29Si spectra of silicalite are highly sensitive to even small amounts of sorbed organics such as ethanol, 1-propanol, n-decane, and especially benzene (Fig. 29). The transformation of the spectra is accompanied by a distortion of the silicate framework, as demonstrated by X-ray diffractometry (102). 

Fig. 29. 29Si MAS NMR spectra (101) at 39.76 MHz of silicalite (Si/Al) > 4400) containing sorbed organic molecules. (A), parent material (B), containing ethanol (C), containing 1-propanol (D), containing n-decane (E), containing benzene.

Meiler and Pfeifer (493) measured 13C and H NMR spectra of carbon monoxide, carbon dioxide, and benzene adsorbed on ZSM-5 and silicalite. The 13C signal from benzene was a superimposition of two lines corresponding to relatively mobile molecules (narrow Lorentzian line) and strongly adsorbed molecules (broad asymmetric line similar to that in polycrystalline benzene). Quantitative interpretation of the spectrum was possible via the measurement of the transverse proton relaxation times, T2, as a function of temperature and coverage. Recent work involving 13C NMR studies of sorbed species is summarized in Table XX. 

There is, as is well known, a close similarity between the crystalline and porous structures of silicalite-1 and silicalite-2. The same similarity therefore exists between TS-1 and TS-2, and it appears logical that they should have very similar catalytic properties. TS-2 has been evaluated as a catalyst for many different reactions, such as Beckmann rearrangement of cyclohexanone oxime with vapor-phase reactants H202 oxidation of phenol, anisole, benzene, toluene, n-hexane, and cyclohexane and ammoximation of cyclohexanone. As described in detail in Section V.C.3, differences that had been claimed between the catalytic properties of TS-1 and those of TS-2 have not been substantiated. Later investigations have shown that, when all the relevant parameters are identical, the catalytic activities of TS-1 and TS-2 are also identical. The small differences in the crystalline structure between the two materials have no influence on their catalytic properties (Tuel et al., 1993a). 

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... 

In earlier work from this laboratory (14,15), we have shown that several aromatic ketones show phosphorescence when included in the channel structure of Silicalite. In particular, the behaviour of 8-phenylpropiophenone is virtually identical to that of acetophenone by contrast, in homogeneous solution in benzene at room temperature the two ketones show very different behaviour (16). 

Examination of molecular models indicates that 8-phenylpropio-phenone can readily adopt a conformation whose kinetic diameter is comparable to that of benzene, which is known to be included in the channels of Silicalite. 

Amorphous Ti/SiCL oxides and crystalline Ti zeolites are two classes of well-studied solid Ti catalysts (11-14). In both classes, a Lewis-acidic Ti atom is anchored to the surrounding siliceous matrix by Si-O-Ti bonds. The oxidant of choice for Ti zeolites such as titanium silicalite 1 (TS-1) and 11-/1 is H2O2, whereas the amorphous, silica-based materials function optimally with organic peroxides such as /-butyl hydroperoxide (/-BuOOH) or ethyl benzene hydroperoxide. However, there are strictly no homogeneous analogues of these materials, and they therefore do not fit within the context of anchoring of homogeneous catalysts. 

Benzene hydroxylation to give phenol has been performed with Mo-substituted mesoporous silicas and H2O2 in the absence of solvent (267). However, as explained earlier, reports of anchoring of Mo in an inorganic support must be treated with great caution, particularly if there is no clear concept for immobilizing both Mo and peroxo Mo. The same holds true for the Mo silicalite MoS-1, which has been used for sulfide oxidation with H2O2 (268). 

Closer to industrial application however, is the gas phase hydroxylation with nitrous oxide as the oxidant (Equation 39). The reaction is carried out at 350°C with a selectivity to phenol of 98%, at 27% benzene conversion. The catalyst is Fe-ZSM-5 a zeolite containing A1 and Fe in the silicalite-1 framework. Active sites are thought to be binuclear clusters of iron oxide, formed in the channels by the migration of Fe, during thermal treatments of the zeolite. Selectivity is of... 

For the most part, the UV Raman spectra of adsorbed molecules are similar to those of their free-molecule counterparts. A significant exception, reported here, is that of benzene adsorbed in silicalite (the all-silica form of zeolite MFI) (56). 

The UV Raman spectra of benzene as a liquid and adsorbed in silicalite are plotted in Fig. 10. The liquid benzene UV Raman spectrum, in the spectral region... 

Fig. 10. UV Raman spectra of liquid benzene and benzene adsorbed in silicalite. The band at 1550cm is assigned to vibrationally resolved fluorescence. The bands at 1075, 1483, and 1648 cm are assigned to resonance enhancement of combinations of non-totally symmetric fundamentals (56).

Although it appears that the new peaks observed for benzene adsorbed in silicalite can be explained by resonance enhancement of a distorted benzene... 

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. 

Li J.-M. and Talu O., Effect of structural heto-ogeneity on multicomponent adsorption benzene and p-xylene mixture on silicalite, in M. Suzuki (ed.) Proc. IV Int. Corf, on Fundamentals of Adsorption, (Elsevio", Amsto dam, 1993) pp. 373-380. Meininghaus C. K.W. and Prins R., Sorption of volatile organic compounds on hydrophobic zeolites, Microporous and Mesoporous Materials 35-36 (2000) pp. 349-365. 

In a more recent membrane model study [11] a large (twinned) silicalite-1 crystal (100 100 300 p.m) was embedded in an epoxy matrix, using an aluminium gasket as a support. Polishing improved the crystal surface exposure. Mcropore diffiisitivities were measured for benzene, toluene and the xylenes. 

C. Shao, X. Li, S. Qiu, F. Xiao, and O.Terasaki, Size-controlled Synthesis of Silicalite-1 Single Crystals in the Presence of Benzene-1,2-diol. Microporous Mesoporous Mater., 2000,... 

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