Borosilicates, synthesis

Recently, an essentially unfaulted version of SSZ-33 was synthesized and is referred to as CIT-1. The end member of this family of zeolites was synthesized in a borosilicate composition (22, 23). The CIT-1 corresponds to essentially pure polymorph B. The template used to prepare CIT-1 was the /V, /V, /V- tri m c ill y I - (-) -cis-myrtanylammonium ion (4) shown in Fig. 6. Note that this organocation is chiral but synthesis of CIT-1 does depend on which enantiomer is used. An interesting aspect of the reported CIT-1 syntheses is that the gel was sealed in a silica tube and heated at 175°C for 1 week or at 150°C for 3-5 weeks. It is not clear whether or not CIT-1 has been made any other way, which seems to indicate the silica tube plays a role in the crystallization. 

Although Si—N—B—O materials containing trace amounts of nitrogen (2-3 wt%) have been prepared solely via a processing approach (ammonolysis of a borosilicate gel), complete conversion to Si—N—B was not achieved86. However, there are several literature examples of routes to Si—N—B materials that couple synthesis with a specific processing approach. These include early work on Si—N—B precursors by Seyferth and Plenio, who reacted the now ubiquitous — [MeHSiNH] — polysilazane with H3B SMe287 as shown in equation 26. 

In another glass (borosilicate) microreactor [channel dimensions = 350 pm (wide) x 52 pm (deep) x 2.5 cm (long)], Wiles et al. (2004b) prepared a series of 1,2-azoles, illustrating the synthesis of a pharmaceutically relevant core motif. Reactions were performed using electroosmo-tic flow (EOF) as the pumping mechanism and employed separate... 

Compared with the above examples, whereby an array of pharmaceutically important molecules have been synthesized under pressure-driven flow, Garcia-Egido et al. (2002) reported the synthesis of fanetizole (207), an active compound for the treatment of rheumatoid arthritis, utilizing EOF. Employing a borosilicate glass microreactor fabricated at The University of Hull, the authors demonstrated the first example of a heated EOF-controlled reaction. As Scheme 60 illustrates, using... 

Microreaction technology has also been applied to peptide synthesis. Haswell and coworkers demonstrated that, using a borosilicate glass microreactor, the desired... 

Mordenite. It has been reported that partial substitution of boron in mordenite occurs when boric acid is added to the synthesis gel (6.7). Evidence for boron substitution for aluminum in mordenite samples prepared in borosilicate glass reactors has also appeared (8). We sought to determine the extent of boron substitution into the mordenite framework which can be affected by direct synthesis. 

Elements such as B, Ga, P and Ge can substitute for Si and A1 in zeolitic frameworks. In naturally-occurring borosilicates B is usually present in trigonal coordination, but four-coordinated (tetrahedral) B is found in some minerals and in synthetic boro- and boroaluminosilicates. Boron can be incorporated into zeolitic frameworks during synthesis, provided that the concentration of aluminium species, favoured by the solid, is very low. (B,Si)-zeolites cannot be prepared from synthesis mixtures which are rich in aluminium. Protonic forms of borosilicate zeolites are less acidic than their aluminosilicate counterparts (1-4). but are active in catalyzing a variety of organic reactions, such as cracking, isomerization of xylene, dealkylation of arylbenzenes, alkylation and disproportionation of toluene and the conversion of methanol to hydrocarbons (5-11). It is now clear that the catalytic activity of borosilicates is actually due to traces of aluminium in the framework (6). However, controlled substitution of boron allows fine tuning of channel apertures and is useful for shape-selective sorption and catalysis. 

This review concerns the synthesis, characterization, and catalytic activity of microporous ferrierite zeolites and octahedral molecular sieves (QMS) and octahedral layer (OL) complexes of mixed valent manganese oxides. The ferrierite zeolite materials along with borosilicate materials have been studied as catalysts for the isomerization of n-butenes to isobutylene, which is an important intermediate in the production of methyltertiarybutylether (MTBE). The CMS materials have tunnels on the order of 4.6 to 6.9 A. These materials have been used in the total oxidation of CO to C02, decomposition of H2O2. dehydrogenation of CeHi4, C0H14 oxidation, 1-C4H3 isomerization, and CH4 oxidation. The manuscript will be divided into two major areas that describes zeolites and OMS/OL materials. Each of these two sections will include a discussion of synthesis, characterization, and catalytic activity. 

The synthesis of pentasil-type borosilicates, referred to as BOR-E, from hydrogels containing a binary mixture of Me4N/(n-Pr)4N, Et4N /(n-But)4n or Me4N /(n-But)4N+... 

O. Muraza, E.V. Rebrov, J. Chen, M. Putkonen, L. Niinisto, M.H.J.M. de Croon, J.C. Schouten, Microwave-assisted hydrothermal synthesis of zeolite Beta coatings on ALD-modified borosilicate glass for application in microstructured reactors, Chem. Eng. J. 135 (2008) SI 17. 

A more recent example of enzyme-catalyzed synthesis performed in micro reactors was reported by Rutjes and co-workers [81], who demonstrated the use of crude enzyme lysates, containing hydroxynitrile lyase, for the enantioselective synthesis of cyanohydrins. Employing a wet-etched borosilicate glass micro reaction channel, containing pillars to promote biphasic laminar flow, the authors evaluated the... 

Porous materials can also be coated with zeolite films by direct synthesis. For example, microcellular SiOC ceramic foams in the form of monoliths were coated on their cell walls with thin films of silicalite-1 and ZSM-5 using a concentrated precursor solution for in situ hydrothermal growth (Fig. 9).[62] The zeolite-coated monoliths show a bimodal pore system and are thermally stable to at least 600 °C. A related strategy is based on the conversion of macroporous Vycor borosilicate glass beads, having pores of about 100 nm, to MFI-type zeolite-containing beads retaining the same macroscopic shape.[63] This conversion was achieved by hydrothermal treatment with an aluminium source and a template such as TPABr. 

Crystalline borosilicate molecular sieves have been the object of an intensive investigation effort since they were reported in the open literature at the Fifth International Conference on Zeolites by Taramasso, et al. (1) A wide range of structures containing framework boron have been synthesized. The physical properties of these borosilicate molecular sieves have been studied by such techniques as X-ray diffraction, infrared and nuclear magnetic resonance spectroscopies, and temperature programmed desorption of ammonia. In addition, the catalytic performance of borosilicate molecular sieves has been reported for such reactions as xylene isomerization, benzene alkylation, butane dehydroisomerization, and methanol conversion. This paper will review currently available information about the synthesis, characterization, and catalytic performance of borosilicate molecular sieves. 

The synthesis of zeolites and zeolitic materials has been pursued for nearly 50 years (2), and the literature is filled with reports of structures, methods of preparation, and uses for these materials. The substitution of aluminum or silicon in the framework structure has been performed using many main group elements (3.41 as well as some transition metals (3.5). New families of molecular sieves which are based on an aluminophosphate framework have been reported recently, some of which are also microporous (6.7). Of the various new materials which have been reported, this review will focus on crystalline borosilicate molecular sieves. 

Borosilicates have been prepared via hydrothermal synthesis in alkaline solutions (1.16-24). Alternatively, synthesis has been successful from neutral or slightly acidic media in the presence of fluoride anions (22). Borosilicate molecular sieves have been prepared through secondary synthesis techniques as reported by Derouane, et al. (26.), in which the aluminosilicate ZSM-5 was treated with boron trichloride to replace aluminum with boron in tetrahedral sites. 

A number of borosilicate molecular sieves have been discovered using these synthesis techniques. By changing the organic compound and other reaction variables, it is possible to prepare various borosilicate structures (1,11.24.28). Modifications of zeolites and molecular sieves with boron compounds which do not lead to tetrahedral (framework) boron will not be addressed in this article. 

Nuclear Magnetic Resonance Spectroscopy. The use of 11B NMR spectroscopy to examine the state of boron in borosilicate molecular sieves has been reported (21.22.24-26.43.441. Scholle and Veeman (43) reported that the boron resonance is characteristic of tetrahedral boron when the samples are hydrated. Dehydration of a borosilicate sample results in a shift to a trigonal environment, as evidenced by the lineshape and peak position. The trigonal boron remains in the framework, and the change between trigonal and tetrahedral environments is reversible. Boron NMR has also been used to show that boron from Pyrex liners can be incorporated in molecular sieve frameworks during synthesis of MFI and MOR structure types (21.44). 

Other Processes. Borosilicates have been used to catalyze a number of other reactions. Among these are dealkylation of cumene by faujasite-type sieves (11). The sieves used for this reaction were prepared by hydrothermal synthesis and contained some aluminum. The catalytic activity increased as the boron content increased. 

The yields of aluminium and silicon are nearly constant for all the experiments, the yield of aluminium loeing slightly higher. The yield of incorporation of boron, instead, is very low when the synthesis gel is aluminium-rich (exp. -4). Boron and aluminium are incorporated at the same extent only when the available boron largely exceeds aluminium (exp. 5, 6). This behaviour can be accounted for by a competition between the Kinetics of incorporation of borosilicate and aluminosilicate units in the zeolite. No rate constants can be established without a knowledge of the partition coefficients of the elements between the liquid and solid phases of the synthesis gel. Anyway, the higher crystallization efficiency of the... 

Partial substitution of boron for aluminium in the synthesis is a suitable tool to control not only the composition, but also the crystal size of the zeolite beta The nucleation flow of the zeolite is a nonlinear function of the composition of the parent gel. The composition of the crystals, instead, depends on the relative rate of sticking of Independent borosilicate and aluminosilicate species. 

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