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Some catalytic properties of silica‐supported base and metal porphyrins for hydrocarbon cracking and hydrogenation
Author(s) -
Caga I T,
Carnell I D,
Winterbottom J M
Publication year - 2001
Publication title -
journal of chemical technology and biotechnology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.64
H-Index - 117
eISSN - 1097-4660
pISSN - 0268-2575
DOI - 10.1002/jctb.344
Subject(s) - catalysis , hydrocarbon , porphyrin , nickel , metal , chemistry , palladium , platinum , thermal decomposition , inorganic chemistry , atmospheric temperature range , cracking , temperature programmed reduction , photochemistry , organic chemistry , physics , meteorology
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 °C indicated complete stability and TPR studies (20–500 °C) 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 °C and thermally in the temperature range 440–600 °C 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 °C and at 1 atm. pressure showed that Pd–EPI/SiO 2 was an active and possibly stable hydrogenation catalyst, whereas Ni–EPI/SiO 2 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. © 2001 Society of Chemical Industry