Coking and regeneration of H3PW12O40/SiO2 catalysts Original Research Article

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 31P and 13C 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 flow 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°C) but is unable to remove hard coke (with the TGA removal range of 370–570°C). Ozone treatment can remove both soft and hard coke at 150°C. The aerobic burning of coke on the undoped PW/SiO2 proceeds to completion in the temperature range centred at 500–560°C, 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°C without loss of catalytic activity.