Hydrogen release from ammonia borane dissolved in an ionic liquid

Hydrogen release from ammonia borane (AB) dissolved in an ionic liquid (IL) is analyzed using batch, isothermal reaction data. It is found that an Avrami-Erofeyev type model, normally used in solid–gas systems, can be modified for this two-step reaction to describe the measured kinetics of hydrogen release and the observed double peaks in release rates. The kinetic model indicates that temperatures in excess of 200 °C are needed for complete conversion (release of 2.35 H2-equivalents) in a reasonably compact flow reactor with <20 s AB/IL residence time. A non-isothermal, plug flow reactor model indicates that heat transfer to ethylene glycol at 80 °C alone is not sufficient to control the peak temperature to <250 °C in the exothermic decomposition reaction. The peak temperature, however, can be controlled by partial recycling of the spent AB/IL mixture. The reactor can also be operated adiabatically to obtain complete conversion while limiting the peak temperature through recycle. In a flow reactor where the feed stream is heated by mixing with the recycle stream there is a minimum recycle ratio for complete conversion. This minimum recycle ratio is a function of the reactor outlet temperature, which is determined by heat transfer.

► On-board hydrogen storage using ammonia borane in ionic liquid. ► Dehydrogenation kinetics data fitted to an Avrami-Erofeyev type model. ► Temperatures in excess of 200 °C are needed for complete conversion. ► The peak temperature can be controlled by partial recycling of the spent AB–IL mixture. ► There is a minimum recycle ratio for complete conversion of AB.