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International audienceIn this article, we considered all aspects of acidity (nature of acid sites, strength, density, etc.) in solid catalysts and in zeolites in particular. After reminding the definition of acidity in liquid and solid acids, we emphasized acidity characterization by the most used physical techniques, such as Hammett's indicator titration, microcalorimetry of adsorbed probe molecules (ammonia, pyridine or other amines for acidity characterization and CO2 or SO2 for basicity characterization), ammonia or any amine thermodesorption, IR spectroscopy of hydroxyl groups and of several probe molecules adsorbed (ammonia, pyridine, piperidine, amines, CO, H2, etc.), MAS-NMR of 27Al, 29Si, 1H elements and of 1H, 13C, 31P, etc. of adsorbed probe molecules, and model catalytic reactions

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