Acidity studies of fluid catalytic cracking catalysts by microcalorimetry and infrared spectroscopy

Isoprene by Prins Condensation over Acidic Molecular Sieves

“…Steaming results in the preferential loss of Brønsted acid sites. This conclusion agrees with earlier work (24) indicating that the sites of intermediate strength in Y zeolites are predominantly Brønsted acids and their number and strength are reduced with steaming. As Brønsted acid strength decreases, the fractional carbenium ion coverage on USY decreases; strong Brønsted acidity is indeed necessary to stabilized carbenium ions on the surface.…”

“…Macedo et al (22), as well as other authors (23,24), studying HY zeolites dealuminated by steaming, found that the strength of intermediate sites decreased with increased dealumination for Si/Al ratios from 8 to higher than 100. Also, Yaluris et al (25) found from differential heats of ammonia adsorption that the acid site strength distribution shifts toward weaker sites when the catalyst is steamed.…”

Determination of Acid/Base Properties by Temperature Programmed Desorption (TPD) and Adsorption Calorimetry

“…In particular, the correlation observed in the above-mentioned study by Auroux et al [181] between an increasing steam dealumination and a decrease in the number of sites exhibiting intermediate strength is confirmed by the results of Chen et al [62]. Moreover, the heat of ammonia adsorption at zero coverage, measured at 423 K, first increased with Si/Al ratio and then decreased upon further dealumination beyond Si/Al ratios of 10.…”

“…This constant value depends on the nature of the probe (i.e., of its enthalpy of vaporization at the adsorption temperature). However, there is also evidence in the literature for weak Lewis acidity giving rise to low heats of adsorption [62]. The heat then falls to a value close to that of the heat of liquefaction of the adsorbate.…”

“…Curve 3: number of acid sites with Q > 120 kJ mol À1 (+, Â, □) from [178] Differential molar heats of ammonia adsorption were measured at 423 K Steam dealumination of H-Y zeolite is known to cause a progressive destruction of weak and intermediate sites while producing new stronger sites. Microcalorimetric measurements of ammonia and pyridine adsorption have shown the presence in samples containing extra-framework aluminum of sites with adsorption heats that are much higher than those observed for samples containing only framework Al [62,88,181,182]. Moreover, steamed samples possess sites presenting a wide distribution of acid strength.…”