Gallium, isomorphous substitution

Difluorobenzenes are isomerized under gas-phase conditions in the presence of metallosilicates, containing the structure of pentasil zeolites with isomorphic substitution of some silicon atoms by aluminum, gallium, or iron.4 A German patent describes the isomerization of l-bromo-2,4-difluorobenzene to l-bromo-3,5-difluorobenzene in pentasil-type zeolites in an autoclave at 320 C and 25 x 105 Pa for 1 h, giving 29% conversion and 73% selectivity.5... 

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. 

Tunable sfrengfh and concentration of the acid sites (both Bronsted and Lewis types), as well as hydrophobicity by isomorphous substitution of frivalenf cations (i.e., aluminum, gallium, iron, boron, efc.) info fhe silicate framework. 

Thomsonlte samples in which Si was replaced by Ge in the zeolite framework. Again in the fifties appeared the remarkable work of Barrer et al. [3] Thomsonite, zeolite A, Faujasite and Harmotome were obtained having gallium and/or germanium in the lattice. These works were followed by a worldwide scientific effort to identify zeolites in which isomorphous substitution could give rise to ... 

The modification of acidic properties of zeolite catalysts through the isomorphous substitution of trivalent species into tetrahedral framework positions has generated great interest recently fl-3). Gallium analogues of ZSM-5, in particular, have been shown to act as acid catalysts for several reactions L4-6J. However, the knowledge about the temporal course of the crystallization process is still incomplete. The present paper reports on the influence of the crystallization time on the synthesis of gallosllicates (Ga-ZSM 5) and on their catalytic properties. 

There is considerable interest in isomorphous substitution of aluminium in the zeolite framework by other elements and some papers have described the synthesis of MFI zeolites containing boron, gallium, titanium and iron as lattice elements (ref.1-3). The replacement of Al ions with the ions of another element can modify both the acidity and pore size features of the zeolite (ref.4, 5), resulting in modification of the catalytic property of zeolite catalysts (ref.6-8). 

The isomorphous substitution of silicon within the zeolitic framework is an important problem and a challenge for elements different from aluminium. Although the introduction of boron, gallium, or iron is relatively easy and well documented [1], few studies are devoted to the introduction of Co(II) into the framework of zeolites [2]. As both the framework and the extraframework Co-species seem to be active in catalysis [3], it is of paramount importance to synthesize and well characterize Co-containing zeolites [4],... 

Recently, there has been considerable interest in the isomorphous substitution of tetrahedral aluminium in zeolite frameworks with catalytically active elements such as iron, gallium and boron. These materials have acidities Afferent from the corresponding aluminosilicates leading to altered activity, selectivity and stability. Mdssbauer spectroscopy has been used to study the iron incorporated into zeolites during synthesis. Fe(III) can be present on tetrahedral framework sites as Fe " cations acting as counterions and as Fe(III) oxides precipitated in or on the zeolite crystals. The most common iron oxide is a-Fe203 which contains iron only in octahedral coordination. 

Figure 2.16 Hydroxyl stretching region of FTIR spectra of ZSIVI-5 zeolites isomorphously substituted with iron, gallium, or aluminum. The position of the bridging hydroxyls is labeled. Reproduced with permission from Ref. (99). Copyright 1985 American Chemical Society.

Microporous silicates synthesised with isomorphous substitution of elements such as boron and gallium for silicon show similar demetallation behaviour, where the heteroatoms leave the structure more readily than aluminium atoms. In particular, the behaviour upon calcination of boron-containing solids has been examined by and Si MAS NMR. Boron is observed to move from tetrahedral to trigonal coordination upon the formation of the protonic boro-silicate form, and studies on the protonated form of zeolite B-Beta have shown that the boron may be removed from the framework stepwise by hydrolysis of Si-O-B bonds, ultimately giving boric acid. This is lost from the structure if put into contact with aqueous solution. 

The isomorphous substitution of tetrahedral A1 in zeolite structure with elements such as gallium, boron or iron has been described in a large number of papers. In the case of Fe for instance, the presence of Fe h jp the framework gives rise to new infrared bands assigned to Si-O-Fe bonds (8). Mossbauer spectroscopy, which is a very valuable tool in this case,... 

For quite a number of crystalline aluminosilicates the gallium- and iron-silicate analogues have been synthesized in the past as well. Therefore, it is not too surprising that the isomorphous substitution of gallium [60] or iron [61] in the MWW framework has recently been reported. Again, as in the case of zeolite ZSM-58, this offers the opportunity of fine-tuning the strength of the Bronsted-acid sites in such materials. 

Gallium has been successfully introduced into munerous zeolite frameworks (Beta, MFI, offretite, faujasite,. ..). The Ga " " ions in zeolites can occupy tetrahedral framework sites (T) and non-framework cationic positions. The isomorphous substitution of gallium into aluminosilicate zeolites results in modified acidity and subsequently modified catalytic activity such as enhanced selectivity towards aromatic hydrocarbons. 

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