Evidence by ac-conductivity for the higher mobility of boron (over aluminium)-associated Brønsted sites in boron-modified HZSM-5

Abstract

A correlation ac-conductivity analysis is performed at 102–105 Hz, 373–613 K and 0.8 V for characterizing the effect of increasing boron incorporation in HZSM-5 lattice on altered electric features in BZSM-5 samples of similar origin. BZSM-5 conducts with decreasing electric impedance in the order: B1ZSM-5>B2ZSM-5>B4ZSM-5>B3ZSM-5. This follows the order of increasing boron content in the zeolite, except for B3ZSM-5. This particular sample, containing less boron than B4ZSM-5, is distinguished by a high B+Al site population. B3ZSM-5 also shows a higher contribution of polarization induced dc-conduction. Therefore, for this particular sample the electric data have been discussed in greater detail. Ruling out the perturbing role of dc-conduction, by analysis at both high frequencies and low temperatures where the dc-conduction is minimum, reveals conductivity increasing in the order: B1ZSM-5<B2ZSM-5<B3ZSM-5<B4ZSM-5 and activation energy decreasing in the order: B1ZSM-5>B2ZSM-5>B3ZSM-5>B4ZSM-5. This corresponds well with the boron content and the B/Al ratio in the zeolite. The higher B-containing BZSM-5 shows the higher conductivity activated at lower energy. Both electric features are considered as evidence for the higher mobility of boron (over aluminium)-associated Brønsted sites; the charge carriers in the boron-modified BZSM-5 samples.