The older process is conducted in the gas phase at 300-350°C using, e. g., boron phosphate catalysts with a great excess Of NH,. The disadvantage is the decomposition of adipic acid on evaporation, which limits the selectivity to only 80% and also necessitates a catalyst regeneration. The main improvement in this process has been the conversion from a fixed bed to a fluidized bed with an H,P0,/Si02 catalyst and continuous regeneration. The selectivity to adiponitrile can reach 90%. However, many plants have been taken out of operation in recent years for economic reasons.