Catalytic and adsorbent properties of solid acid catalysts studied by ammonia adsorption microcalorimetry

Solid acid catalysts are becoming of great importance within the chemical industry and their
acidity is of great interest, as this determines their application, plus many of their catalytic
properties can also be directly related with their acidity. There has been a drive towards
heterogeneous solid acid catalysts because of the environmental concerns with safe handling
and disposal of mineral acid homogeneous catalysts such as H2SO4, and their separation from
the product.
Objectives of this work have been to study a range of solid acid catalysts and establish a
relationship between catalyst strength, activity, and structure, and then identify the influence of
solvent and type of reaction on the catalytic properties of the catalysts to be studied. Acid
catalysts have been chosen to represent a cross-section of the various types of catalysts in use.
The solid acid catalysts being investigated include sulfonated polystyrene ion exchange resins,
acid activated clays, zeolites, and heteropoly acid (H3PW12O40) supported on carbon and
mesoporous silica. Supported heteropoly acids have been prepared by Dr A Lapkin, University
of Bath in the collaborative part of the project.
Catalysts have been characterised in terms of their surface areas, pore diameters, pore volumes,
and crystallinity from nitrogen adsorption, powder x-ray diffraction, cation exchange capacity,
and elemental analysis. The acidity of these catalysts has been studied by NH3 adsorption
microcalorimetry. NH3 is assumed to adsorb stoichiometrically on surface acid sites and the
molar enthalpy of ammonia adsorption is assumed to reflect the strength of the acid sites. The
catalytic activities of the catalysts have been measured using two Brønsted acid catalysed test
reactions (rearrangement of α-pinene and the hydrolysis of ethyl acetate). The correlation
between characterisation results and catalytic data has been examined with emphasis being
placed on the relationship between acidity measurements and the reaction medium or solvent.
Conclusions that can be drawn from this work are that NH3 adsorption microcalorimetry is a
useful technique for studying surface acidity of solid acids and that it does allow for some
correlation to be drawn between catalytic activity and acidity, with the aid of additional catalyst
characterisation techniques.