Selectivities in Post-Synthetic Modification of Borosilicate Zeolites

The chemistry of post-synthetic modification of borosilicate zeolites, and their understanding and use in catalytic applications are reviewed. First, a survey is given on the practical application of solid state NMR to follow the structural properties of boron. Then, the hydrolytic chemistry of boron in zeolites is summarized with a focus on selective reactivities of trigonal boron created by counterions, and stepwise extraction of boron from the framework. The reinsertion of other heteroatoms (mainly Al is discussed here) can be carried out by three different methods with different selectivities. First, the extent of Al reinsertion can be tailored quantitatively by controlling boron removal and vacancy formation with appropriate counterions. Secondly, the substitution of Al for B can be kinetically retarded by using uncalcined as-made zeolites with organic structure directing agents still occluded in the pores. Although these zeolites are not porous, quantitative replacement of framework atoms (Al for B) is possible, most likely by a solid state diffusion mechanism. Finally, Al reinsertion can be efficiently achieved in aqueous Al(NO3)3 solution under acidic conditions and is pore selective. This is due to the finding that the hydrated aluminum cations are too bulky for medium-sized pores, so that preferably 12-membered ring pores can be aluminated, and 10-membered ring pores do not substitute Al for B. This substitution can be quantitatively followed by the pH change in the zeolite/Al(NO3)3 slurry. A variety of catalytic test reactions are used to characterize the catalytic properties of the aluminosilicate zeolites prepared via post-synthetic treatment of their borosilicate counterparts.