On the strength of solid acids

Abstract

The objective of the present work was to establish the relative strengths of some common solid acids and to put them on a scale familiar to chemists. A modified spectrophotometric approach involving Ho and HR indicators was developed. The red-shifts of spectra from the adsorbed neutral forms of 4-nitrotoluene and 4-nitrofluorobenzene were used to deduce the equivalent H 2S0 4 concentrations. Thus, values of 70 to 80%, and 90 to 98% H 2S0 4 were determined for silica-alumina and HY preparations, respectively. HM preparations were found to be mild superacids (stronger than 100% H 2S0 4). On the other hand, H 2S0 4/ZrO 2 preparations had surface acidities about equivalent to 100% H 2S0 4, and SbF 5/silica-alumina was a superacid with − 14.5 < H 0 < − 13.7. These data were correlated with the catalytic activities of the solid acids expressed either as In (reaction rate) at 370°C or as the temperature required to obtain 0.5% conversion of isobutane. The extent and nature of the secondary reactions also varied with the acidity. These results demonstrate that the intensive factor of the acidity (acid strength) dominated over the extensive factor (surface concentration of Brønsted sites) in controlling the acidity, and therefore the catalytic properties, of these solids.