THE USE OF WATER AS A PROBE MOLECULE IN THE STUDY OF ZEOLITES
Dr. David H. Olson
Department of Chemical Engineering
University of Pennsylvania
Wednesday, November 13, 1996 3:30 pm
Ben Bandy Conference Room
Center for Applied Energy Research
Water plays a dominant role in the crystallization of high alumina zeolites such as zeolite A and X where transient water clusters may have a templating effect. Possible structures of some of these clusters will be presented. For high silica zeolites such as ZSM-5, organic moieties take on the role of templating where they appear to define, direct and fill zeolitic pore structure. For the latter materials, water is a useful probe of crystal chemistry; water adsorption "capacity" is governed not by the total zeolite's pore volume but by the concentration of hydrophillic centers.
Recently obtained evidence for the formation of water dimers and tetramers around hydrophillic acidic sites, Al-O(H)-Si units, will be shown. The high alumina zeolite, zeolite X, is highly sensitive to acidic conditions and rapidly dissolves at low pH. However, under controlled exchange, 1/3 of the total sodium ions in NaX(Si/Al=1) can be exchanged with hydronium ions with no evidence for acid attack on the zeolite structure. The nature of this exchange and the specific location of hydronium ions will be discussed.