Use of water as a probe of zeolitic properties: interaction of water with HZSM-5

The adsorption of water in ZSM-5 has been used to study the crystal chemist ry of the family of HZSM-5 materials. Several of the features discussed are generic in nature, applying to all high-silica zeolites. The strongest int eraction of water is with the acidic protons associated with framework alum inum atoms. The quantity of water adsorbed is proportional to the framework aluminum content and this water adsorbs rapidly. Heats of adsorption are s hown to be a monotonic function of the number of water molecules adsorbed p er H(A1) unit and independent of the framework aluminum content of the zeol ite. Weaker adsorption occurs on external surface silanol groups and on cen ters ascribed to intracrystalline defects created as charge balancing cente rs for the charged template, tetrapropylam-monium (TPA) ions. An adsorption isotherm is presented for external surface adsorption. For a high-silica Z SM-5, adsorption ascribed to defect sites corresponds to four H2O molecules per unit cell, equal in number to the number of TPA(+) ions and hence the number of these sites. (C) 2000 Elsevier Science B.V. All rights reserved.