Solid-state n.m.r, and catalytic studies of mildly hydrothermally dealuminated HZSM-5 E. Brunner, H. Ernst, D. Freude,* M. Hunger, C.B. Krause and D. Prager Sektion Physik der Karl-Marx-Universitiit Leipzig, Linn~strasse 5, DDR-7010 Leipzig, GDR and W. Reschetilowski Sektion Chemie der Karl-Marx-Universitiit Leipzig, Liebigstrasse 18, DDR-7010 Leipzig, GDR and W. Schwieger and K.-H. Bergk Sektion Chemie der M artin-Luther-Unwersitiit Halle-Wittenberg, Schlossberg 2, D D R-402 0 Halle, GDR Magic angle spinning (MAS) n.m.r, investigations and n-hexane cracking were carried out on HZSM-5, synthesized free of template, after mild hydrothermal dealumination. The maximum catalytic activity was obtained after 2.5-h treatment with water vapor at 50 Torr at 540°C. The n.m.r, data show that the enhanced activity is neither due to enhanced Br6nsted acidity nor to a significant number of paired aluminium sites in the framework. The concentrations of the different OH groups and aluminium species suggest that the interaction of the molecule with both a bridging hydroxyl group and a nonframework aluminium species is the reason for the enhanced catalytic activity. Keywords: Zeolite HZSM-5; catalytic activity; MAS n.m.r. INTRODUCTION Hydrothermal treatment of zeolites usually causes a dealumination of the framework (cf. Ref. 1-3). In certain circumstances, this dealumination gives rise to an enhancement of the catalytic activity (cf. Ref. 4-8). Miradatos and Barthomeuf~ found enhanced activity in mordenites after hydrothermal treatment and explained it by an interaction of bridging hydroxyl groups with nonframework aluminium species. Lago et al. ° observed an enhanced activity of HZSM-5 after mild steaming at 540°C and developed a kinetic model on the basis of paired framework AI atoms, which are partially hydrolysed, so that one aluminium atom in a nontetrahedral symmetry acts as a'strong electron withdrawing center for the remaining tet- rahedral AI, thus creating a stronger Br6nsted site. Sendoda et al. 8 proved the enhanced activity of ZSM-5 after mild hydrothermal treatment for the conversion of propane. All investigators agree that the enhanced-activity sites must be the result of an interaction of Br6nsted and Lewis sites rather than the result of a decrease in This paper is dedicated to Professor Harry Pfeifer on the occasion of his 60th birthday. Address reprint requests to Dr. D. Freude at the Sektion Physik der KarI-Marx-Universitiit Leipzig, Linn6strasse 5, DDR-7010, Leipzig, GDR. Received 13 June 1988; accepted 18 July 1988 © 1989 Butterworth Publishers the concentration of framework aluminiunl. The question arises, however, as to the nature of these sites. The existence of 3-coordinated A1 in the framework after dehydroxylation" could be dis- proved by X-ray spectroscopy I° and MAS n.m.r. It There is no evidence of nontetrahedral symmetry of the aluminium atoms in the framework. Scherzer I considered a variety of charged or neutral nonfi'ame- work aluminium species. Jacobs and Beyer 12 propose the existence of A10 +. MAS n.m.r, proved, however, the existence of tetrahedrally coordinated nonframe- work aluminium in the hydrated dealuminated zeolites. ~ s In the present study, we describe the mild hyd- rothermal dealumination of HZSM-5 synthesized free of template. The framework Si/AI ratio wwies from 15 to 65, dependent on the temperature T, the water vapor pressure p, and the duration t of the steaming process. For the specific conditions T = 540°C, t = 2.5 h,.and varying water vapor pressure from 0 to 700 Torr, the samples have been investi- gated in detail by MAS n.m.r, and a catalytic test reaction to determine the nature of the enhanced- activity sites. EXPERIMENTAL Zeolite synthesis and hydrothermal treatment The zeolite was synthesized without template 282 ZEOLITES, 1989, Vol 9, July