SYNTHESIS OF MOLECULAR-SIEVE [B]-BEA AND MODIFICATION OF THE BORON SITE

The synthesis field and product composition of the [B,Al]-BEA molecular sieve were studied in order to prepare highly crystalline essentially aluminum-free (Si/Al > 400) boron-containing Beta, using. tetraethylammonium hydroxide as templating agent. Fully crystalline [B.Al]-BEA was obtained with 4-6.8 trivalent ions per unit cell, i.e., Si/(B + Al) = 8.4-15. BEA is proposed to be nucleated and grown by aluminosilicate species for Si/Al < 21 in the synthesis mixture. In this case, the boron content in the product is low. For Si/Al > 21, both boro- and aluminosilicate entities provide charge compensation for the template ions and, hence, stabilization of the structure. Consequently, the boron incorporation efficiency is much higher for lower aluminum contents of the gel. Essentially aluminum-free [B]-BEA could be synthesized in the narrow compositional range, 5.6-6.6 B/unit cell, i.e., Si/B = 8.7-10.4. Calcination and deboronation of [B]- and [B,Al]-BEA, silanol nest reoccupation of deboronated BEA, and direct Si reversible arrow B substitution in calcined boron-containing BEA were studied by variable temperature. FTi.r. spectroscopy, X-ray diffraction, and B-11, Al-27, and Si-29 MAS n.m.r. spectroscopy techniques. By performing template removal in an ammonia atmosphere, boron hydrolysis and extraction as well as T-atom reorganizations could be avoided. Subsequently, mild extraction of lattice boron (> 90%) could be achieved by leaching with aqueous hydrochloric acid (pH < 5) at room temperature. The deboronated materials showed 20% XRD crystallinity loss, but no micropore volume or surface area reduction and could withstand thermal treatments up to 300 degrees C in nitrogen. Preliminary investigations on silanol nest reoccupation and direct Si reversible arrow B replacement by exposure to silicon-introducing reagents showed only partial incorporation of silicon. Partial silylation of the silanol nests was achieved in the liquid phase using dichloromethylsilane (DCMS) or tetramethyl orthosilicate (TMOS) at 62 and 110 degrees C, respectively. Direct substitution of framework boron by silicon was carried out by a gas phase reaction (300 degrees C) using tetrachlorosilane (TCS).