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) an d subsequent catalyst regeneration have been studied. Coke formation has be en found to cause rapid deactivation of the catalysts. The coked versus fre sh 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 unaffecte d by coke deposition in both undoped and Pd-doped PW/SiO2. The Pd doping ha s 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, w ater being the most effective inhibitor. The removal of coke from HPA catal ysts has been attempted using solvent extraction, ozone treatment and aerob ic 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 har d coke (with the TGA removal range of 370-570 degreesC). Ozone treatment ca n remove both soft and hard coke at 150 degreesC. The aerobic burning of co ke on the undoped PW/SiO2 proceeds to completion in the temperature range c entred at 500-560 degreesC, exceeding the temperature of PW decomposition. Doping the catalyst with Pd significantly decreases this temperature to all ow catalyst regeneration at temperatures as low as 350 degreesC without los s of catalytic activity. (C) 2001 Elsevier Science B.V. All rights reserved .