Accurate Measurements of NH₃ Differential Adsorption Heat Unveil Structural Sensitivity of Br?nsted Acid and Br?nsted/Lewis Acid Synergy in Zeolites

Differential adsorption heats of NH3 on a series of zeolites, including MOR, MFI, FER, and BEA, are accurately measured to probe their acidity using flow-pulse adsorption microcalorimetry. Initial adsorption heats of NH3 at Bronsted acid sites (BAS) vary between 105 to 136 kJ/mol, depending on framework aluminum amounts and topography structures of zeolites. A Bronsted/Lewis acid synergy between BAS and proximate tricoordinated framework-associated aluminum species is identified to generate super acid sites with initial adsorption heats of NH3 around 150 kJ/mol, but occurs only in the MFI zeolites and sensitively depends on the Si/Al ratio. These accurate data of NH(3 )differential adsorption heats unveil structural sensitivity of BAS and Bronsted/Lewis acid synergy in zeolites and provide experimental benchmark data for fundamental understanding of acidity and acid-catalysis of zeolites.