Of borosilicate molecular sieves

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 growing interest in the properties of borosilicate molecular sieves has led to a significant increase in the number of reports which specify a borosilicate molecular sieve as the important component in catalyst compositions (8-11). The range of reactions which have been claimed include a variety of hydrocarbon conversions such as xylene isomerization (8.12),... 

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. 

Methanol Conversion. Methanol conversion reactions based on borosilicate catalysts have been studied extensively (10.15,24,28.33.52-54). During the conversion of methanol, the reaction proceeds through a number of steps, to yield dimethylether, then olefins, followed by paraffins and aromatics. The weaker acid sites of borosilicate molecular sieves relative to those of aluminosilicates require higher reaction temperatures to yield aromatics. The use of less forceful process conditions leads to the formation of olefins selectively, instead of a mixture of paraffins, olefins, and aromatics (10.28.53.54). 

The growth of borosilicate molecular sieve technology during the past 8 years demonstrated the unique properties of borosilicate molecular sieves. The presence of moderately acidic sites in a shape selective environment has led to a number of novel application areas for borosilicate molecular sieves. Additional applications will arise as scientists turn to these interesting materials for their unique properties. 

The syntheses of novel molecular sieves such as aluminophosphates, silicoaluminophosphates (SAPO), gallosilicates, aluminogermanates, ferro-silicates, borosilicates, and chromosilicates, clearly open new vistas for the... 

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. 

The preparation of a borosilicate molecular sieve (termed by the authors "borozeosilite") at relatively low pH, in the neutral to acidic range, has been reported (25). A reaction mixture comprising a silica source, boric acid, tetrapropyl-ammonium bromide and an ammonium fluoride salt was digested hydrothermally at 170°C. Subsequent calcination of the product in air at 550°C was performed to remove organic template and to provide the hydrogen form of the molecular sieve. 

X-Rav Diffraction. Borosilicate molecular sieves have been studied by X-ray diffraction (1.16-20 ). X-ray diffraction techniques have been developed to determine the degree of substitution of the silicate framework by borate tetrahedra (291. The boron-oxygen bond is shorter than the the silicon-oxygen bond, which leads to a contraction of the unit cell for a borosilicate molecular sieve as boron substitution increases. The unit cell volume determined from peak positions in the ranges 2O°<20<35< and 45°<20<5O° or an empirical parameter termed ST (sum of four d spacings) correlated with structural boron content (29). 

Infrared Spectroscopy. Infrared spectroscopy has been used to study borosilicate molecular sieves (22.25.33-361. Vibrational bands associated with trigonal framework boron occur near 900 cm"1 and 1400 cm 1 (22.25.331. The presence of the Si-0-B asymmetric stretching vibration, indicative of tetrahedral framework boron incorporation, cannot be observed directly because it is masked by the strong Si-0-Si band near 1100 cm-1 in the pentasil structures. The tetrahedral Si-O-B vibration has been observed for the borosilicate mineral danburite (.33). Shifts of bands in the framework vibrational region at 550 cm"1 and 560 cm"1 to higher frequencies as a function of boron content has been used to study boron incorporation in the framework of AMS-1B borosilicate (36). 

Evaluation of the hydroxyl region of the spectrum (4000-3000 cm-1) has been reported by several authors for borosilicate molecular sieves (33.35-381. The well-known band near 3610 cm-1 associated with acidic hydroxyls in aluminum-containing ZSM-5 (36-401 is not observed for the borosilicate molecular sieve. Assignment of a band at 3725 cm"1 to hydroxyls associated with structural boron (37.381 was... 

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). 

Proton NMR of a borosilicate molecular sieve was reported by Scholle and co-workers in a study of the hydroxyl groups in borosilicate, silicalite, and ZSM-5 (45). In this study, the silanol protons in both borosilicate and aluminosilicate materials were observed at a chemical shift of -2 ppm relative to Me4Si. A low field resonance attributed to hydroxyls associated with the heteroatom (B or Al) was shifted to higher field for the boron-containing sieve relative to the aluminum-containing sieve (—3.5 ppm vs -6 ppm). This was interpreted as indicating that the borosilicate hydroxyls are less acidic than those of the aluminosilicate, consistent with the IR results reported above. 

Temperature Programmed Desorption. Ammonia has been used as a probe molecule in a number of studies of crystalline borosilicate molecular sieve (22,33,45). It has been shown that the ammonia is desorbed from the borosilicate samples at low temperature, 465°K, indicating the weak acidity of the hydroxyls (33). The hydroxyls have been shown to have higher acidity than silanol groups and lower acidity than those of ZSM-5 (45). The results are consistent with IR and calorimetric data for NH3 adsorption/desorption (35.). ... 

Although adsorption processes represent an extremely large application of molecular sieves (49), applications in the area of heterogeneous catalysis have received the most attention for borosilicate molecular sieves. Due to the inherently weaker acidity of borosilicates relative to aluminosilicates, a number of advantages in using borosilicates have been reported due to improved product distributions or reaction selectivities. 

Borosilicate catalysts provide high approach to thermodynamic equilibrium of the xylenes, and offer high selectivity in the conversion of ethylbenzene (8.12.22.50 ). In addition, they have been shown to be less prone to the effects of thermal and steam treatments than corresponding aluminosilicate zeolite catalysts (51). The catalytic activity of borosilicate catalysts was demonstrated to be a function of the structural boron content of the molecular sieve (22.36,50). In addition, the by-product distribution obtained from a borosilicate catalyst in a xylene isomerization/ethylbenzene conversion process was found to be distinctive (50), with high transethylation reactivity relative to transmethylation. 

Various borosilicates have been reported in the methanol conversion process. In a study reported in 1984, Holderich gave details for the preparation of propene selectively from methanol using a borosilicate molecular sieve of the MFI structure type (10 ). Autocatalysis was observed when small amounts of olefin were added to the feed. Modification of the borosilicates using HF, HC1, or extrusion with amorphous silica-alumina led to changes in the observed product distribution to yield more C2-C4 olefins. Use of borosilicates of the MOR and ERI structure types for methanol conversion was reported by lone, et al. (28). The selectivity to olefins was improved for borosilicates with these structures relative to the silicate of the same structure and aluminum impurity level. 

According to U S Patent 4 672 049 (5jj), a borosilicate molecular sieve of the beta structure can be used as the active component in a hydrocracking catalyst. The advantage offered by the use of a borosilicate component in a hydrocracking catalyst was the ability to operate the process under low pressure, specified as 100-1000 psig (broad) or 300-770 psig (preferred). 

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. 

Substitution of either A1 or Si with various heteroatoms changes acid strength from the extremely weak acidity of borosilicates to the superacid-like strength of certain aluminosilicates. The acid sites of Ga- and Fe-silicates are weaker than those of their Al-analogs [35]. Several shape selective commercial processes use hetwoatom substituted molecular sieve catalysts. Iron-substituted pentasils (Encilite) are used for xylene isomerization and for producing ethylbenzene fi om benzene and ethanol [36,37]. 

M. Taramasso, G. Perego, and B. Notari, Molecular Sieve Borosilicates. Proceedings of the 5th International Zeolite Conference, Heyden and son, London, 1980, 40-48. 

Felthouse and Mills (321) report the amination of methyl ferf-butyl ether (MTBE) and isobutene to fert-butylamine using alumino- and borosilicate pentasil molecular sieve catalysts. The ether and alkene amination reactions were found to proceed preferentially under SCF conditions at temperatures on the order of 330°C and pressures greater than 193 bar. The smdy showed that MTBE can be used as a substitute raw material for terf-butylamine manufacture, but MTBE decomposition products of isobutene, methanol, and methanol conversion products are produced that require a more complicated product separation process than with isobutene as the only C4 substrate. 

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