Acidic Properties of Cage-Based, Small-Pore Zeolites with Different Framework Topologies and Their Silicoaluminophosphate Analogues

Acidic properties of cage-based, small-pore aluminosilicate zeolites with CHA, AFX, RHO, LEV, ERI, and LTA topologies and their silicoaluminophosphate (SAPO) analogues were measured by means of an ammonia IRMS (infrared/mass spectroscopy)-TPD (temperature-programmed desorption) method. All SAPO molecular sieves studied here showed weaker Bronsted acid strength (11-26 kJ mol(-1) lower in the lot heat of ammonia desorption) than their aluminosilicate counterparts. The density functional theory (DFT) calculations of the ammonia desorption energy were in good agreement with experiments; the difference in the energy of ammonia desorption was less than 10 kJ mol(-1). DFT also showed that the introduction of Al into the SiO2 framework to form aluminosilicate zeolites resulted in large changes to the distance between atoms close;100 to the acid site, while Si substitution into the AlPO4 framework to form SAPO materials I 91 predominantly modified the angles between atoms relatively far from the acid site. The introduction of Al into SiO2 frameworks causes higher compression of the Al-OH-Si bridge, inducing strongly acidic behavior, while the more flexible Al-O-P bond relaxed the compression in SAPO frameworks.