Acidic Properties of Cage-Based, Small-Pore Zeolites with Different Framework Topologies and Their Silicoaluminophosphate Analogues
journal contributionposted on 22.02.2016, 15:00 by Naonobu Katada, Kazuma Nouno, Jun Kyu Lee, Jiho Shin, Suk Bong Hong, Miki Niwa
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 Brønsted acid strength (11–26 kJ mol–1 lower in the 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 to the acid site, while Si substitution into the AlPO4 framework to form SAPO materials 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.