Regeneration of Coked Zeolite from PMMA Cracking Process by Ozonation

At the highest temperature, decoking takes place according to a shrinking core process as already observed by Hutchings et al. [19]: cross-section views (c) show a black core of coked catalyst surrounded by a very white layer of regenerated catalyst.

At 140 ◦C the rate of ozone decomposition is much faster than its diffusion rate. Radical species generated are known to have a short lifetime, and they should be consumed (either by reaction with remaining coke in the close vicinity or by radical recombination reactions) before reaching the coke deposit within the particles. This makes the overall process highly limited by diffusion.

At 100 ◦C, the smaller difference in grey level contrast between the core and the surface of the particles was due to the slower ozone decomposition rate: ozone had more time to diffuse within the particle before being converted into highly reactive species. Therefore the overall carbon removal is improved as shown in Fig. 6.

The regenerated zeolites were subsequently examined for acid properties and activity in PMMA degradation in batch reactor.

3.3.1. Acid properties

As abovementioned, acid sites are known to promote C C bond scission. Moreover, the strength and nature (Brønsted or Lewis) of the acid sites influence both the activity and selectivity of the cracking reaction: the stronger the sites, the lighter hydrocarbons obtained [3,29].

From NH₃-TPD analysis, a distribution of the acid sites according to their strength could be provided: Fig. 10a and b gives, for different regeneration conditions, the relative amount of each category of acid sites (weak, medium and strong) with respect to the total amount of acid sites in the fresh ZSM-5 extrudate (2.09 mmol g⁻¹ from calibration with ASA).

With respect to fresh zeolite, the amount of sample A acid sites decreased by about a factor 3, almost regardless of their strength.