Silicalite isomorphous substitution

An extremely versatile catalyst for a variety of synthetically useful oxidations with aqueous hydrogen peroxide is obtained by isomorphous substitution of Si by Ti in molecular sieve materials such as silicalite (the all-silica analogue of zeolite ZSM-5) and zeolite beta. Titanium(IV) silicalite (TS-1), developed by Enichem (Notari, 1988), was the progenitor of this class of materials, which have become known as redox molecular sieves (Arends et al., 1997). 

TS-l and titanium silicalite-2 (TS-2) are microporous solid materials made of Si02 and Ti02 that have silicalite structures (TS-1 has the ZSM-5 structure and TS-2, the ZSM-11 structure) modified by isomorphous substitution of Si(IV) with Ti(IV). TS-1 and TS-2, the former being most studied, show similar properties in catalysis of H202 oxidations. 

In the present study isomorphic substitution of Al with Sn [11-13] in the silicalite framework was attempted in order to induce catalytic functionalities and these catalysts was studied for Baeyer-Villiger oxidation of 2-adamantanone and norboranone. 

A new microporous solid material has been obtained made of Ti02 and Si02 (TS-1) which has a silicalite-1 structure modified by isomorphous substitution of Si(IV) with Ti(IV), Its synthesis takes place in the presence of tetraalkylammonium bases under carefully controlled conditions,... 

A unique titanium(IV)-silica catalyst prepared by impregnating silica with TiCLt or organotitanium compounds exhibits excellent properties with selectivities comparable to the best homogeneous molybdenum catalysts.285 The new zeolite-like catalyst titanium silicalite (TS-1) featuring isomorphous substitution of Si(IV) with Ti(IV) is a very efficient heterogeneous catalyst for selective oxidations with H2C>2.184,185 It exhibits remarkable activities and selectivities in epoxidation of simple olefins.188,304-306 Propylene, for instance, was epoxidized304 with 97% selectivity at 90% conversion at 40°C. Shape-selective epoxidation of 1- and 2-hexenes was observed with this system that failed to catalyze the transformation of cyclohexene.306 Surface peroxotitanate 13 is suggested to be the active spe-... 

Recently, there has been a growing interest into niobium- and tantalum-containing molecular sieves. The introduction of niobium into mesoporous molecular sieves has been studied by Ziolek et al [3,4], while Antonelli and Ying reported the synthesis of mesoporous niobium oxide [5], The synthesis and characterization of niobium- and tantalum-containing silicalite-1 (NbS-1 and TaS-1) was published recently [6,7,8] and some evidence has been presented for isomorphous substitution [6,8] of Nb and Ta into the silicalite-1 framework. The synthesis of NbS-2 (MEL) [9] and a new molecular sieve named NbAM-11 have been reported as well [10],... 

MCM-41 and silicalite-1 can be synthesized in the presence of niobium- and tantalum-containing compounds. The results indicated that Nb(V) and Ta(V) are well dispersed in the framework of silicalite-1 and in the amorphous walls of MCM-41 y-irradiation of activated niobium and tantalum molecular sieves show two radiation induced hole centers (V centers) located on Si-O-Si and M-O-Si (M = Nb, Ta) units. True isomorphous substitution as suggested in the literature for Ti(IV), however, is unlikely to be present Nevertheless, interesting chemical and catalytic properties can be expected from these systems and are subject to further studies... 

The isomorphous substitution of Siiv by Ti,v was claimed by Taramasso, Perego, and Notari in 1983 for a new material with the composition xTi02(l - x)Si02 (0.0 x 0.04 M). This has the crystalline structure of silicalite-1 (or MF1) with Tilv in framework positions it was named titanium silicalite-1 or TS-1 (Taramasso el al., 1983). The occurrence of isomorphous substitution was deduced from the regular increase in unit-cell parameters with the degree of substitution and from the good agreement between the observed and calculated values of the Si—O and Ti—O distances. The same type of evidence had already been obtained by the same authors in the synthesis of crystalline microporous boron silicates, where the smaller B—O distance relative to Si—O causes a decrease in unit-cell parameters (Taramasso et al., 1980). 

Fig. 15. Crystalline structure of TS-1 obtained by isomorphous substitution of Silv by Tilv in framework positions of silicalite-1.

The isomorphous substitution of T atoms by other elements produces novel hybrid atom molecular sieves with interesting properties. In the early 1980s, the synthesis of a zeolite material where titanium was included in the MFI framework of silicalite, that is, in the aluminum-free form of ZSM-5, was reported. The name given to the obtained material was titanium silicalite (TS-1) [27], This material was synthesized in a tetrapropylammonium hydroxide (TPAOH) system substantially free of metal cations. A material containing low levels (up to about 2.5 atom %) of titanium substituted into the tetrahedral positions of the MFI framework of silicalite was obtained [28], TS-1 has been shown to be a very good oxidation catalyst, mainly in combination with a peroxide, and is currently in commercial use. It is used in epoxidations and related reactions. TS-1, additionally an active and selective catalyst, is the first genuine Ti-containing microporous crystalline material. 

Hydrothermal Isomorphous Substitution of Boron in Zeolite ZSM—5/Silicalite... 

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). 

We have earlier addressed the problem of the post-synthesis insertion of aluminium in zeolites ZSM-5 (12) and Y (Hamdan, H. Sulikowski, B. Klinowski, J. T.Phvs.Chem.. (in press)). The substitution of gallium in silicalite-n has also been achieved (13). It was therefore of considerable interest to establish whether boron can also be incorporated into silicate frameworks after the completion of synthesis. We report isomorphous substitution of boron into zeolite ZSM-5 by mild hydrothermal treatment with borate species. 

Titanium silicalite (TS-1), an isomorphously substituted molecular sieve [215], is a truly heterogeneous catalyst for oxidations with 30% aq. H202, including the oxidation of alcohols [216]. 

TS-1 Titanium silicalite-1 material obtained by the isomorphous substitution of Ti for Si in the lattice of the pure silica Silicalite-1 (S-1). 

Unlike aluminum, titanium is tetravalent and can exhibit different oxidation states. Thus TS-1 is non-acidic if isomorphously substituted. TS-1 is relatively difficult to synthesize, which is probably one of the reasons the site structure was under debate for quite some time. TS-1 can only be synthesized with a maximum of 3 wt% Ti if more Ti is added extra-framework titanium is formed. Inihally, it was suggested from XAS that octahedral sites are formed in the silicalite framework [60]. However, later more and more groups suggested tetrahedral isomorphous substitution of the titanium sites in the MFl framework [61-63], It is now more or less generally accepted that the Ti is four-coordinate and has a Ti—O distance of 1.79-1.81 A. This is a significantly increased distance compared to the Si—O distance 1.605 A, which is constant for a vast number of oxides [4]. The increase in... 

Both types of molecular sieve catalysts, i.e., those containing exchangeable metal cations and those with metal ions isomorphously substituted into the framework are difficult to tailor with respect to activity and selectivity, and with the exception of Ti silicalite [161] none of the catalysts is used in a commercial process. 

In general, when carrying out a new separation, the kinetic diameter and the heat of adsorption of the gases, which compose the mixture, are the main variables used to select the most adequate zeolite. MFI, FAU, LTA, SOD, ANA, DDR, MOR, BEA, CHA, FER, KFI are zeolite structures widely used as membranes for different separations. In gas separation, MFI zeolite membranes (silicalite-1, ZSM-5, and with Al, Fe, B, and Ge isomorphously substituted into their stmctures) are the most commonly used membranes because their pores (-0.55 nm diameter) are in the size range of many industrial mixtures furthermore, their synthetic chemistry is well established in the literature. 

K. S. Smirnov and M. V. Kudryashova, Microporous Mater., 5,9 (1995). Computer Simulation Study on Isomorphously Substituted Silicalites. 

The catalytically active sites of isomorphous substituted MFI structures have been characterized by inirared spectroscopy and microcalorimetric measurements using ammonia and acetonitrile as probe. Due to decreasing heats of NH, adsorption, the NH, TPD peak positions, the positions of the IR OH stretching frequencies and their shifts upon adsorption of acetonitrile the Bronsted acid site strength of the modified MFI decreases from Al>Fe>In> >silicalite. In addition to those strong sites weaker Lewis centres due to the non-framework material have been found. For TS-1 comparatively low heats of adsorption due to coordinatively bonded ammonia have been detected. The amounts of adsorption with heats higher than found for silicalite correlates with the amount of Ti in the sample. 

Based on the quite remarkable results obtained with TS-1 we expect that many more examples of microporous solids modified by isomorphous substitution with redox metal ions in the crystal lattice will be described in the future (see Fig. 12). Indeed, the scope for developing unique oxidation catalysts based on the concept of site-isolation in zeolites, silicalites, alpos and sapos is enormous [37], In addition to varying the redox metal the size and hydrophobicity of the cavity can be tuned by, for example, varying Si/Al ratios to provide a variety of unique heterogeneous catalysts for liquid phase oxidation. 

The isomorphous substitution of Si" " by Ti and Sn" ", V"" in the silicalite-2 (ZSM-11, MEL) framework has been the subject of some recent reports [1-4]. The possibility of substituting silicon by zirconium in the framework of ZSM-5 (MFI) by hydrothermal analysis has been reported but not really substantiated by experimental evidence [5-9]. We have recently attempted the isomorphous substitution of Si" by Zr ions in the MFI and MEL framework by hydrothermal synthesis by using ZrCU as a source [10,11]. In these reports, we have shown that about 0.6 - 1.0 Zr atom per unit cell could be incorporated in the framework positions and that those well dispersed Zr ions are capable of catalyzing hydroxylation of phenol with aqueous H2O2. This report describes the hydrothermal synthesis of Al- free zirconium silicate molecular sieves with MEL structure using two different sources viz., zirconium (IV) tetrachloride and zirconium (IV) acetylacetonate and their physico-chemical characterization and a comparison of the samples. 

Powder X-ray profiles of silicalite-1 prepared by various research workers, during their preparation of molecular sieves by isomorphously substituting the T element by titanium, tin, zirconium, vanadium, chromium, molybdenum, differ in their crystallinity (80-95%) and their unit cell volume values (Table 1). It is obvious from the Table that whenever a metal ion is being substituted, a metal free all silica polymorph should also be prepared for deciphering the unit cell expansion. The variation in the unit cell volume obtained by various workers is an indication of the inherent problems in synthesising zeolites and molecular sieves with repeatable metal substitution in the framework. 

Silicalite samples prepared during the isomorphous substitution of M (Metal atoms with the maximum concentration used)... 

---