The Constraint Index test revisited: anomalies based upon new zeolite structure types

The Constraint Index (CI) test originally developed in the Mobil laboratories has been an extremely useful catalytic test for investigating the relative pore sizes and shape selective properties of acidic zeolites. The test was designed to allow discrimination between pore systems composed of 8-, 10-, and 12-ring ports. The discovery in recent years of several new zeolite structure types has provided examples where CI data would lead the researcher to incorrect conclusions about the pore size and structure. In this paper we discuss some of the conditions affecting interpretation of el results and present three scenarios in which CI data do not appear to be consistent with structure: 1. Zeolite with 14-rings and pore openings greater than 8 Angstrom cannot be distinguished from traditional large-pore 12-ring zeolites. 2. Zeolites with 10-rings opening into larger cavities did not show intermediate-pore behavior in the CI test (SSZ-25 and SSZ-35). 3. Some zeolites with large internal cavities and pores composed of 8- or 9-rings do not generate CI values higher than the 10-rings, even though the pores of the former are smaller. (C) 2000 Elsevier Science B.V. All rights reserved.