Physicochemical properties of some metal supports for hydrotreating catalysts preparation Original Research Article

Nitrogen sorption, 27Al MAS NMR, FTIR spectroscopy of pyridine thermal desorption, together with microcalorimetry experiments with both ammonia and pyridine, have been used to study the physicochemical properties of commercially used catalyst supports. An amorphous aluminosilicate such as AAA-alumina (containing about 78.2% SiO2 and 21.7% Al2O3) and a silica–alumina mixture (SAM, containing about 49.3% SiO2 and 52.3% Al2O3), have mostly cylindrical mesopores (<20 nm in size), and surprisingly similar surface area. Pore volume is largest in SAM samples. The main difference in the pore structure of these two supports is the presence of large (>10 nm) mesopores and even macropores in SAM that are absent in AAA-alumina samples. Reference silica is mainly a microporous solid while reference alumina (catapal B) is mainly a mesoporous material.The 27Al MAS NMR spectrum of AAA-alumina contains two resonances near 55 ppm and near 0.0 ppm indicative of the presence of Al(IV)- and Al(VI)-species. In contrast, the SAM spectrum contains only one resonance near 0.0 ppm. As a result, FTIR spectra of chemisorbed pyridine in the C–C stretching region, indicate that only AAA-alumina contains both Brønsted and Lewis acid sites. In SAM, acidity is only of the Lewis type. In either solid, acid site strength is lower than the one measured in pillared clays and zeolites.Differential heat profiles obtained by microcalorimetry show that, as expected, the reference silica is a very weak solid acid and that the Al-containing supports contain a small population of high energy sites and a larger one of sites with a heterogeneous strength distribution. Acid strength, as measured from initial heat of adsorption, increases in the following order: silica≪SAM