Some catalytic properties of silica-supported base and metal porphyrins for hydrocarbon cracking and hydrogenation

A base porphyrin, etioporphyrin (EPI), has been synthesised and a number of metal-etioporphyrin compounds have been derived from EPI by metal insertion, these being nickel, vanadyl, palladium and platinum. The metal-etioporphyrins were supported on silica gel with loadings of 0.5-5.0% (w/w) to be employed as catalysts for hydrocarbon cracking and to a minor extent for hydrogenation. The porphyrins themselves were characterised using temperature programmed decomposition (TPD), temperature programmed reduction (TPR), mass spectroscopy (MS) and infra-red (IR) spectroscopy. TPD studies up to 550 degreesC indicated complete stability and TPR studies (20-500 degreesC) showed interaction with hydrogen, nickel-EPI and Pd-EPI especially showing strong interaction. MS studies showed that metal insertion had occurred for VO-EPI and Ni-EPI and Pd insertion was demonstrated to have occurred using an analytical method. IR spectroscopy carried out on VO-EPI and Ni-EPI showed an absence of-NH linkages, again confirming metal insertion. The behaviour of the catalysts for hydrocarbon cracking was studied using 2,2-dimethylbutane (2,2-DMB) as the model reactant in the temperature range 440-550 degreesC and thermally in the temperature range 440-600 degreesC and at 1 at, m (101.3 kPa) pressure. All porphyrins, even the base porphyrin, exhibited cracking activity and the catalysed reaction had an energy of activation, depending on the porphyrin, in the range 78-113 kJ/mol(-1), compared with a value of 210 kJ mol(-1) for the thermal reaction. The product distribution was dominated by C-1 and C-2 hydrocarbons and is typical of a free radical reaction, the thermal reaction giving a similar product distribution, so that the porphyrin catalyst acts as a free radical initiator. Hydrogenation studies using hex-1-ene at 150 degreesC and at I atm. pressure showed that Pd-EPI/SiO2 was an active and possibly stable hydrogenation catalyst, whereas Ni-EPI/SiO2 while of only slightly lower activity initially, lost that activity so that the Pd-EPI catalyst was over 16 times more active at the end of a 2 h period. (C) 2001 Society of Chemical Industry.