The effect of boron on ZSM-5 zeolite shape selectivity and activity II. Coincorporation of aluminium and boron in the zeolite lattice

ZSM-5 zeolite samples have been synthesized in the presence of NaAlO2 and NaBO2 to yield a crystalline phase of almost unchanged Al + B site population, but with increased BAl ratio. The presence of boron in the zeolitic lattice is evident by MAS NMR. Transformation of boron (upon NH3 sorption) within trigonal and tetrahedral symmetries, which is revealed by IR spectroscopy as persistence and disappearance of the trigonal BO stretching mode at 1385 cm−1, together with the appearance of the &z.tbnd;SiOB skeletal vibration at 920 cm−1 for the zeolite as synthesized, confirms the incorporation of boron by synthesis with no evidence of occluded amorphous boron oxide phase. IR spectroscopy reveals a distinct absorption at 3700 cm−1 assigned to Brønsted sites associated with lattice boron. NH3 sorption, studied by IR spectroscopy and microcalorimetry, reveals weak acidity for boron-associated sites; however, they do certainly sorb ammonia. Kinetic data of 3-Me-pentane sorption suggest a role for lattice boron in modifying the interior part of the zeolite, which is also indicated by catalysis of toluene disproportionation as improved selectivity toward p-xylene formation. Zeolite modification by the present method, although exhibiting a minor effect on ZSM-5 shape selectivity, has a major role in drastically decreasing the zeolite acidity and hence activity.