Comparison of cupric ion location and adsorbate interactions in Cu(II)-exchanged H-SAPO-5 and H-SAPO-11 molecular sieves determined by electron spin resonance and electron spin echo modulation spectroscopies

Abstract

The locations and interactions of cupric ion exchanged into H-SAPO-11 and H-SAPO-5 molecular sieves with water, methanol, ethanol, ammonia, ethylene and pyridine have been compared by electron spin resonance and electron spin echo modulation spectroscopies. After dehydration, equilibration with ethylene and pyridine adsorbates was slower compared with that for more polar adsorbates. One major difference between these two molecular sieves in cupric ion coordination to adsorbates is that two waters are coordinated in CuH-SAPO-11 while three waters are coordinated to cupric ion in CuH-SAPO-5. This indicates different coordination sites for the cupric ion in these two molecular sieves of rather similar structure. By considering a particular location of the cupric ion, these coordination numbers can be understood, however. Another major difference involves coordination to ammonia; four ammonias are coordinated to cupric ion in CuH-SAPO-11 consistent with the cupric ion being located in the center of a 10-ring channel, while only three ammonias are coordinated in cupric ion to CuH-SAPO-5. This indicates a different coordination site for cupric ion between these two molecular sieves also with ammonia adsorbate. The coordination site in the SAPO-5 material for cupric ion is suggested to be the same for ammonia coordination and water coordination and is different from the site for water coordination in the SAPO-11 material.