## 4 Conclusions  
In this study, the discarded surgical and FFP2 face masks were pyrolyzed as a strategy to produce high value-added by-products from a new increasing waste stream, such as personal protective equipment used against COVID-19. The influence of temperature on the products was studied in a typical pyrolysis temperature range (450-550 \({}^{\circ}\)C). Temperature significantly affects the fractions obtained; we found that the highest amount of liquid fraction for the thermal pyrolysis of surgical and FFP2 masks was obtained at 500 \({}^{\circ}\)C and the maximum syngas yield was at 550 \({}^{\circ}\)C. The percentage of solid drops significantly when the temperature increases from 400 to 440 \({}^{\circ}\)C, to remain practically constant. Furthermore, the interaction of a low-cost catalyst, such as sepiolite, during pyrolysis was investigated. Results showed that, in general, the use of sepiolite increased the amount of syngas at the expense of biooil. Additionally, in catalytic pyrolysis, the olefin and isoparaffin content in biooil obtained increased to the detriment of naphthenes and paraffins. However, with the successive use of the catalyst, it is seen that this trend is reversed due to the degradation that occurs during use. Aromatic compounds were not found in either thermal or catalytic pyrolysis. Regarding the solid fraction, the results found are similar for all the biochar samples, which indicate that the use of the catalyst does not have an appreciable influence on the characteristics of the biochar. Elemental analysis showed a high carbon content (from 50.0 to 70.9%), lower nitrogen and hydrogen, and no sulfur. The high heating values of the biochar samples obtained in this work suggested that biochar could be used as a solid fuel. The gases derived from the thermal and catalytic pyrolysis of both surgical and FFP2 masks had average hydrocarbon contents between 77.8 and 85.5% and high calorific values, so they could be used as fuels for different applications. In all tests, the major compound was methane (CH\({}_{4}\)), with an average content of 47.9%. In addition, in the thermal pyrolysis and in the first use of the catalyst in the catalytic pyrolysis of both masks, methane increased to a maximum at 30 min and then decreased, while ethane had the opposite behavior. For the second and the third use of catalyst, the highest composition of methane was reached at the end of the experiment at the expense of ethane. Finally, characteristics measured on the catalyst before and after its use in the pyrolysis test showed that a good regeneration was performed. Hence, it is concluded that the face mask waste can be used in the pyrolysis processes to convert them into useful products. However, before large-scale application, some additional research is needed to solve the following aspects:  
* The construction of detailed kinetic models. * Understanding of the pyrolysis mechanism over sepiolite to adequately modify this material to develop more selective sepiolite-based catalysts towards specific high-value products. * The analysis of the socio-economic and environmental impacts. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app13053188/s1](https://www.mdpi.com/article/10.3390/app13053188/s1)]([https://www.mdpi.com/article/10.3390/app13053188/s1](https://www.mdpi.com/article/10.3390/app13053188/s1)), Figure S1: Syngas component yield in catalytic pyrolysis of surgical masks. Figure S2: Syngas component yield in catalytic pyrolysis of FFP2 masks. For the experiments and collected the experimental data; wrote the original draft of manuscript; M.A.M.-L.: conceived and designed the experiments; performed the analysis of the data; wrote the advanced versions of the manuscript; funding acquisition; supervision; H.J.P. : revised and edited the final version of the manuscript; M.Z. : revised and edited the final version of the manuscript; M.C.