Due to the considerable effort increasement to lower CO2 emission from liquid fuel production and also from the notorious decrease on fossil sources like gas, coal and petroleum; it has been really evident the need to search for new sustainable and efficient alternatives (Rashid Miandad et al., 2019). In this context, gasification, thermal and catalytic pyrolysis are technologies with various operational and environmental advantages for energy recovery, which are becoming alternative methods for clean energy production, especially in processes involving plastic waste. In this way, the use and reuse of plastic waste through its anaerobic thermochemical decomposition at high temperature (pyrolysis) has allowed the production of biofuels. This environmental technology is carried out with or without the presence of a catalyst, however, the use of different types of catalysts has improved the pyrolysis process of plastic waste and the efficiency of the process, thus obtaining biofuels of quality greater than or equal to that of conventional fuels (Nizami et al., 2015). Also, in the pyrolysis process with an inert atmosphere (nitrogen), the oxygen in the molecules is removed and consequently, it is possible to convert plastic into liquid oils (short-chain hydrocarbons), solids (carbon), and gases (hydrocarbon isomers between C1 and C5). Thermal degradation takes place at high temperatures, achieving decomposition of the original chain of the compound into a wide variety of new compounds with smaller carbon numbers, similarly to fuels (gasoline and diesel). Not only the temperature, but also the heating rate, plastic-type and class of reactor (Anuar Sharuddin et al., 2016).
When a catalytic phase (Zeolite-based catalysts) is incorporated into the degradation process, it helps to produce lighter liquid fractions with proper characteristics to use in engines, furthermore decreasing the total energy consumed. (R. Miandad et al., 2016) In the catalytic pyrolysis of polyolefins such as polyethylene (PE) and PP, studied between 420 to 510 ◦C (Abbas-Abadi et al., 2014), the researchers found that the yield was 92.3% for the highest liquid fraction at 450°C. While Aisien et al.(Aisien et al., 2021) reported a liquid fraction of 83.3% using FCC catalysts in PP pyrolysis at different temperatures and 77.6% using FCC spent catalyst at 450°C. The influence of the catalysts on the obtained fractions depends on the porous surface of the catalyst and its acidity. If the acidity is excessively high it can give an uncontrolled fractionation reducing the liquid fraction and increasing the produced gases.
The fractions distribution of the pyrolytic products in catalytic cracking is influenced mainly by the catalyst used. The catalysts in the pyrolysis of plastics can be classified into three types: silica-alumina, zeolite, and FCC processes catalysts. ZSM-5, red mud, HY zeolites have been studied in pyrolysis [6,7], obtaining high selectivity and a liquid fraction mainly in diesel-like compounds. In addition, other researchers (Onwudili et al., 2019), studied the catalytic pyrolysis in a mixture of plastics using FCC and ZSM-5 type zeolites with similar conclusions regarding the orientation of the products, however, there was a decrease of 10% in the liquid fraction.
Several investigations have been carried out in catalytic pyrolysis, however, the regeneration methods of catalysts from oil refining processes are limited. For example, Ding et al., studied the solvent method to remove sulphur using carbon disulfide, ethanol and benzene as solvents,[9,10] the results showed better yield with carbon disulfide followed by ethanol. Another method to remove sulphur and carbon is by the gasification of catalysts at 450°C (Su et al., 2019), where they are satisfactory converted into sulphur dioxide and carbon dioxide. The leaching process is another interesting method for this purpose, lanthanum for example, is a catalyst that can be recovered applying acidic leaching followed by a heat treatment at 750°C (Zhao et al., 2017).
Based on all the previous information, in this work a regenerated FCC catalyst from a petrochemical industry applied to a catalytic degradation of polypropylene has been investigated. First, a chemical regeneration of the catalyst with three solvents at different contact times and a thermal regeneration by gasification using two heating ramps have been developed. Then, the catalyst was characterized by analyzing its surface area. Then, the behaviour of the regenerated catalyst compared to a commercial one was analyzed by thermogravimetry. Finally, the regenerated catalyst was investigated to determine the efficiency of pyrolysis processes.