The hydration enthalpies of allophane and imogolite have been measured by means of the adiabatic vapor absorption calorimeter. The hydration enthalpy (ΔHh) of allophane and imogolite were -63.2kJ/mol and -59.8kJ/mol, respectively, which were almost the same as those of a zeolite group. The total heat of hydration (Q) of allophane was 409kJ/kg at the dehydration temperature of 80°C, and was 1.5 times larger than that of the Mg-A type zeolite, The Q-value of imogolite was 405kJ/kg at the dehydration temperature of 40°C, and was 2.2 times larger than that of the Mg-A type zeolite. Allophane and imogolite are thus characterized by larger Q-values than that of Mg-A type zeolite when they are dehydrated at low temperature below 100°C. This shows that allophane and imogolite are suitable for heat exchange absorbents of zeolite-water heat pumps for low temperature heat sources.
