Coking and regeneration of H3PW12O40/SiO2 catalysts

The coking during propene oligomerisation over silica-supported heteropoly acid (HPA) H3PW12O40 (PW) and its palladium-doped form (1.6-2.5 wt.% Pd) and subsequent catalyst regeneration have been studied. Coke formation has been found to cause rapid deactivation of the catalysts. The coked versus fresh catalysts have been characterised by P-31 and C-13 MAS NMR, XRD, XPS and TGA/TPO to reveal that the Keggin structure of the catalysts was unaffected by coke deposition in both undoped and Pd-doped PW/SiO2. The Pd doping has been shown to affect the nature of coke formed, inhibiting the formation of polynuclear aromatics. Addition of water, methanol or acetic acid to the propene how causes the formation of oxygenated products at the expense of propene oligomers. These additives have been found to inhibit the coking, water being the most effective inhibitor. The removal of coke from HPA catalysts has been attempted using solvent extraction, ozone treatment and aerobic oxidation. The extraction (e.g. with CH2Cl2) allows removing soft coke (with the TGA removal range of 170-370 degreesC) but is unable to remove hard coke (with the TGA removal range of 370-570 degreesC). Ozone treatment can remove both soft and hard coke at 150 degreesC. The aerobic burning of coke on the undoped PW/SiO2 proceeds to completion in the temperature range centred at 500-560 degreesC, exceeding the temperature of PW decomposition. Doping the catalyst with Pd significantly decreases this temperature to allow catalyst regeneration at temperatures as low as 350 degreesC without loss of catalytic activity. (C) 2001 Elsevier Science B.V. All rights reserved.