IRMS-TPD of ammonia: Direct and individual measurement of Brønsted acidity in zeolites and its relationship with the catalytic cracking activity

By using an improved method of infrared mass spectrometry/temperature-programmed desorption (IRMS-TPD) of ammonia, the number and strength of Brønsted acid sites on various zeolites were precisely measured. Using this method, it is possible to measure the property of acid site directly and individually when the multiply overlapped acid sites exist. Thus, measured ΔH (enthalpy change of ammonia adsorption) as a parameter of strength was roughly correlated with the IR band position of hydroxide, as long as the OH was located in a large pore consisting of an 8- to 12-member ring; that is, the lower the wave number of the OH band position, the stronger the ΔH. Under the conditions of monomolecular reaction, the rate of octane cracking was measured on these zeolites, and the turnover frequency (TOF) was calculated from the reaction rate and number of Brønsted acid sites. Among the distributed acid sites, the Brønsted acid site with the stronger acidity located in a large pore was considered the active site. Thus measured TOF was related strongly with the ΔH of the Brønsted acid sites. Therefore, it is concluded that the Brønsted acid sites play a predominant role in the cracking of hydrocarbons.