Adsorption of carbon monoxide in H-ZSM-5 and Li-ZSM-5 zeolites: an embedded ab initio cluster study

The absorption of carbon monoxide with H-ZSM-5 and metal-substituted Li-ZSM -5 zeolites has been investigated by using both cluster and embedded cluste r approaches at the HF/6-31G(d,p) level of theory. For the H-ZSM-5 zeolite, the binding energy of CO on a 3T quantum cluster is predicted to be 2.25 k cal/mol for the C-bound complex. The O-bound complex was found to be less s table by about 0.84 kcal/mol. inclusion of the Madelung potential was found to increase the acidity of the Bronsted acidic site and the GO-binding ene rgy to 4.95 kcal/mol, consequently, it leads better agreement with experime ntal observation. Similar results were also obtained for the Li-ZSM-5/CO co mplex. The Madelung potential field from the zeolite framework was found to reverse the order of relative stability of C-bound and O-bound adducts in comparison to the Li+-CO system. (C) 2000 Elsevier Science B.V. All rights reserved.