ACTIVATION OF NI AL2O3 CATALYSTS THROUGH MODIFICATION OF THE AL2O3 SUPPORT/

The effect of alumina pretreatment on the performance of alumina suppo rted nickel catalysts was demonstrated in gas phase hydrogenation of t oluene to methylcyclohexane. The state of the alumina was changed from pure gamma to pure theta phase through various heat treatments in air . The catalysts were prepared from vapor phase by saturating the acces sible binding sites on the pretreated alumina with the nickel precurso r. The highest number of active sites for hydrogenation was observed f or catalysts prepared on alumina having an incomplete phase transition and a theta/gamma alumina phase ratio between 0.5 and 10. Results fro m temperature programmed desorption (TPD) studies revealed that a maxi mum in weakly chemisorbed hydrogen as well as in total amount of desor bed hydrogen was found for the same catalysts. By hydrogen chemisorpti on studies the total hydrogen uptake was found to correlate with the o bserved hydrogenation maximum. It is suggested that both the chemical and physical properties of the alumina influence the activity. An opti mal metal-support interaction and structural defects on the alumina du e to the phase transition can explain the observed maximum in the numb er of active sites and in hydrogen uptake.