Boron-substituted zeolites

Characterization of Substituted Boron. We used solid state -B NMR and X-ray diffraction data to distinguish occluded borates from boron substituted into the zeolite framework. When an element replaces aluminum or silicon in a zeolite structure, the local coordination environment changes to accommodate the new ion. Since B + is a much smaller ion than Al "1", the unit cell axes contract when boron replaces aluminum in the framework. The ionic radii of trivalent B and A1 in a tetrahedral environment are 0.25 and 0.53, respectively (1). The magnitude of the contraction is dependent upon the level of substitution (17). 

The NMR and X-ray diffraction data are only consistent with substitution of boron into the framework structure of the mordenite. Although we prepared boron substituted mordenite directly from modified gels, direct synthesis has severe limitations. The solution chemistry of the substituting element can interfere with zeolite nucleation and crystallization, as... 

Zeolite Y. We also substituted boron into dealuminated zeolite Y. We dealuminated zeolite Y by EDTA treatment using standard methods (4). The presence of hydroxyl nests in the product was confirmed using 29Si CPMAS NMR spectroscopy. The dealuminated material incorporated 33 times more boron than zeolite Y when treated with KOH/B2O3. These data are summarized in Table 3. The boron substituted faujasite exhibits a single sharp resonance in the NMR spectrum, consistent with structural substitution. Since the substitution level was low and would not be expected to cause large shifts in the diffraction pattern, no corrected XRD data were obtained on substituted zeolite Y. 

Zeolite Y does not recrystallize in KOH solutions (24). Our results are in agreement with this for Y, but for dealuminated Y zeolite there is a decrease in the Si/Al ratio irtien treated with B2O3 in KOH solution (Table 3). We found a similar trend in the mordenite system. Apparently these zeolite structures are more susceptible to recrystallization when dealuminated. Preparation of boron substituted zeolite Y by post- synthetic substitution demonstrates that this method may be used to prepare materials which are not readily available by direct synthetic procedures. 

B. n-Butene Isomerization with Boron Substituted Zeolites and Ferrierites. 

We prepared boron substituted mordenite by direct synthesis from gel precursors and by post- synthetic substitution into dealuminated mordenite. Direct substitution is favored in aluminum deficient gels, but exacting crystallization requirements for mordenite formation limit the amount of boron that can be incorporated into the framework structure. Higher substitution levels were achieved using a post-synthetic treatment. Boron substituted zeolite Y could not be prepared by a similar direct synthetic method, but post-synthetic methods were effective at providing low substitution levels. This demonstrates the more general utility of post-synthetic substitution methods. The hexane cracking activity of... 

Because most of the microporous open structures are metastable thermodynamically, many zeolites and microporous aluminophosphates can transfer to more stable structures under heating, which can be used to prepare some specific zeolite structures. For example, boron-substituted zeolite [> was used to prepare boron-substituted microporous SSZ-24 via crystal transition. 

Further catalytic uses of zeolites and related materials include polymerisation of alkenes as well as the development of basic zeolitic materials generated by the incorporation of alkali metal ions. Gallium- and boron-substituted zeolites have already been shown to be useful catalysts in wide variety of reactions (Table 2.2) and undoubtably these will be followed by novel zeotypes including mesopor-ous materials with other catalytically active elements within their frameworks.40... 

B MAS NMR yields quantitative information about the incorporation of boron into zeolite frameworks. H MAS NMR and IR spectroscopy show that OH groups introduced into the framework by boron substitution are non-acidic. 2D proton spin diffusion measurements of the zeolite SAPO-5 reveal that defect OH groups are adjacent to acidic bridging hydroxyl groups and do not exist in an amorphous phase. Strongly adsorbed water molecules in mildly steamed zeolites H-Y can be explained by Lewis sites. 

The synthesis of SSZ-24 is usually described to occur from aluminum-free gels. On the other hand, the preparation of an SSZ-24-type material which could be converted to its Bronsted-acid form suitable for catalysis requires the presence of at least a minor amount of aluminum in the framework. This however, as well as the isomorphous substitution of other atoms for framework Si by direct synthesis, turned out to be difficult. This has changed with the recently reported technique of substituing boron for silicon using a calcined form of boron-substituted zeolite Beta as the boron source or even as a combined source for boron... 

A co-incorporation of aliuninum and boron in the zeolite lattice has revealed weak acidity for boron-associated sites [233] in boron-substituted ZSM-5 and ZSM-11 zeolites. Ammonia adsorption microcalorimetry gave initial heats of adsorption of about 65kJmol for H-B-ZSM-11 and showed that B-substituted pentasils have only very weak acidity [234]. Calcination at 1073 K increased the heat of NH3 adsorption to about 170kjmol by creation of strong Lewis acid sites. The lack of strong BrOnsted acid sites in H-BZSM-11 was confirmed by poor catalytic activity in methanol conversion and in toluene alkylation with methanol. 

The Hb NMR spectrum of this sample contains a single narrow resonance centered at -3.2 ppm, which is characteristic of boron in a tetrahedral coordination environment in the framework structure. The Si nmr spectra of a synthetically prepared siliceous mordenite with the same Si/Al ratio is shown in Figure 8. No CP resonances are present, Which indicates that hydroxyl nest concentration in this material is very low compared to the acid treated sample. These data confirm that hydroxyl nests, generated by the removal of A1 from the zeolite structure, are reactive sites for isomorphous substitution. Aluminum deficient, preformed zeolites which do not contain hydroxyl nests, i.e. synthetically prepared samples, do not undergo isomorphous substitution when treated in a similar fashion. 

Hydrothermal Isomorphous Substitution of Boron in Zeolite ZSM—5/Silicalite... 

We report (i) isomorphous substitution of boron, by secondary synthesis, into silicalite and into highly siliceous (Si/Al>400) ZSM-5 and (ii) an improved direct synthesis of zeolite (Si,B) -ZSM-5. The chemical status of B in die boronated products depends upon reaction conditions. Careful control of the concentration of the base, the borate species and of die duration of treatment, allows the preparation of samples containing only 4-coordinated B or a mixture of 3- and 4-coordinated B in various relative concentrations. The products were characterized by magic-angle-spinning (MAS) NMR and infrared (IR) spectroscopies and by powder x-ray diffraction (XRD). 

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. 

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. 

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