Obtaining Zeolites from Natural Materials of Volcanic Origin for Application in Catalytic Pyrolysis for the Sustainable Chemical Recycling of Polymers

The present investigation studies the use of three natural precursors of volcanic origin (pozzolana, ignimbrite and pumice) in the synthesis of low-cost and environmentally friendly zeolites. The developed zeolites were evaluated as sustainable catalysts for the catalytic pyrolysis process in the chemical recycling of polypropylene. A zeolite was synthesized from each precursor. The hydrothermal treatment was performed with NaOH (3M) at 160 °C for 72 h and NH4Cl (1M) was added to convert it into proton form. The synthesized zeolites were characterized by FTIR, XRD, SEM and BET. The evaluation of the catalytic ability of the obtained zeolites was carried out with polypropylene mixed with a 4, 6 and 8 wt.% catalyst in a ceramic crucible. Pyrolysis was always carried out at 450 °C and for 30 min in a tubular furnace with a continuous flow rate of 250 L·min−1 of gaseous nitrogen. The gases generated were captured in the cooling system. The characterized zeolites show a resemblance to the ZSM-5 commercial zeolite, especially for the ignimbrite and pozzolan zeolites. Likewise, in pyrolysis, liquid products, gases and waxes were obtained. As the amount of catalyst was increased (from 4 to 8%), the yield of the desired liquid–gas products was also increased. The synthesized zeolites showed similar pyrolytic characteristics to ZSM-5, although they did not reach the same pyrolytic efficiency. Zeolites improved the pyrolysis products, especially at 8 wt.%, when compared to thermal pyrolysis. This study highlights the potential of the developed zeolite catalysts to efficiently convert PP into valuable light olefins, advancing sustainable polyolefin recycling technologies.