Boron-substituted mordenites

Table 1. Chemical Composition of Boron Substituted Mordenite Samples...

Figure 2. Boron content versus SiC /A Oj ratio for boron substituted mordenites.

The lattice constants of a boron substituted mordenite sample with Si<>2/Al203 = 25.3 and Si02/(Al203 + B2O3) = 22.5... 

Figure 3. Boron substitution level versus gel composition for boron substituted mordenites.

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... 

Figure 4. Solid state NMR spectra of boron substituted mordenite (a), and extra-lattice boron occluded in mordenite (b).

The synthetic boron substituted mordenites have alpha values similar to the aluminosilicate analogues. This is not surprising since boron replaces only 10% of the aluminum in these materials. For boron to affect the alpha values of these samples, the acid strength of a B-OH proton would have to be much greater than for a A1-0H proton, which is clearly not the case. 

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... 

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. 

A1 saturated mordenite (x = 8 in Formula 1), the product contains only trace amounts of boron. As the Si02/Al2O3 ratio of the gel increases, and insufficient A1 is present to form the aluminous mordenite end-member, the boron content of the product rises dramatically. The upper limit on the Si02/Al2(>3 level at Which pure mordenite forms, which is approximately 25, determines the upper limit on boron substitution level. 

The amount of boron substitution achieved using this post-synthetic method is approximately six times higher than the most heavily substituted mordenite prepared by direct synthesis from gels (vide supra). If this post-synthetic treatment... 

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. 

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. 

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