Regeneration of coked zeolite from PMMA cracking process by ozonation

Abstract

Regeneration of coked ZSM-5 zeolite was performed by oxidation with ozone at low temperature range (<150°C) so that to restore catalytic activity. Physicochemical properties of the samples were characterized by several techniques: thermogravimetry (nature of coke deposit), elemental analysis (carbon content), porosimetry (surface area and pore size), ammonia temperature-programmed desorption and pyridine adsorption followed by infrared spectroscopy (acidity). Reactions were carried out at various temperatures, gas hourly space velocities and inlet concentrations of ozone. They showed that partially coked samples (containing 3 wt.% of C) can be successfully regenerated by ozone with carbon removal up to 80%. Carbon removal is improved by increasing the inlet ozone concentration in the range 16-50 g/m3, with almost linear trend, and by increasing time on stream until it plateaus after 2 h. Coke oxidation with O₃ starts at low temperature and exhibits an optimum at about 100°C. At higher temperatures, the rate of ozone decomposition becomes much faster than its pore diffusion rate, so that radical species are no longer available for the coke deposit within the particles and the overall oxidation yield decreases. Indeed, catalytic decomposition of ozone is found to occur significantly above 100°C: O₃ decomposition reaches 90% with fresh ZSM-5 catalyst. Thus regeneration of coked zeolite particles involves both complex chemical reactions (coke oxidation and O₃ decomposition to active but unstable species) and transport processes (pore diffusion to the internal coked surface). Ozonation can restore both textural and acidic properties, allowing the catalyst to almost recover its initial activity in poly(methyl metacrylate) cracking. The activity results are well correlated with the carbon removal efficiency.