Catalytic co-pyrolysis of biomass and low-density polyethylene over activated carbon catalyst

This study aims to explore the interaction of biomass components and plastics in the catalytic co-pyrolysis over the Activated Carbon (AC) catalyst. A fixed bed reactor was used to conduct the catalytic pyrolysis of cellulose, xylan, lignin, Douglas Fir (DF) alone, and the catalytic co-pyrolysis of their mixture with Low-Density Polyethylene (LDPE) over AC catalyst. AC catalyst was prepared via phosphoric acid activation followed by microwave carbonization. The obtained AC catalyst was characterized by a Fourier-transform infrared spectroscopy (FTIR), Brunauer-Emmett-Teller (BET), temperature-programmed desorption of ammonia (NH3-TPD), and scanning electron microscopy (SEM). The main surface functional groups of AC were -OH (3 400 cm-1), -C-H (2 950 cm-1), -C=O (1 700 cm-1), -C=C (1 550 cm-1, 880 cm-1), and -CH-Ar (750 cm-1). Notably, the functional groups of -C-O-P (1 150 cm-1) and -P-O (1 050 cm-1) were successfully introduced in the catalyst, providing effectively active sites for the cracking and aromatization reactions to form aromatics. The BET surface area of AC was 1 440.0 m2/g, with a much higher external surface area of 1 412.8 m2/g and a lower micropore surface area of 27.2 m2/g. The total pore volume of AC was 0.86 cm3/g with low micropore volume. The peak at 100-200 ℃ was the weak acid site, which attributed to the weakly absorbed NH3 on the external surface of AC catalyst, whereas, the peak at 200~300 oC corresponded to the medium strength acid sites. The surface morphology of AC catalyst exhibited an irregular pore structure, due mainly to the chemical activation by phosphoric acid created the porosity in biomass matrix via the release of volatiles, shrinkage, fusion, and cracking reactions. Furthermore, the liquid yield was obtained from the catalytic pyrolysis of different feedstocks in the catalysis of AC catalyst. The order was ranked: Cellulose (55.0%) >xylan (36.0%) > DF (32.0%) > lignin (22.5%). The highest yield of char was obtained from the lignin pyrolysis, whereas, the pyrolysis of DF produced the maximum yield of gas. The catalytic pyrolysis of cellulose and xylan produced mainly furans, accounting for 78.6% and 83.2%, respectively. The main products of lignin pyrolysis were sample phenols. CO and CO2 were the main gas components during catalytic pyrolysis of cellulose, indicating that carbonylation and decarboxylation reactions were dominant at the active sites of the AC catalyst. The gas composition of lignin was H2 and CO2, which were from the dehydrogenation and decarboxylation reactions of side chains of lignin structural units. The results were attributed to the different structures and compositions of biomass feedstocks. The catalytic pyrolysis of LDPE produced aromatics and C9-C16 hydrocarbons as the main liquid product and H2 as the main gas products. The experimental liquid yield of four mixtures was reduced by 8.7%-11.4%, while the gas yield increased by 22.6%-64.0%, compared with the simulated. The content of aromatics and light aliphatic hydrocarbons (C9-C16) increased in liquid products, whereas, the content of oxygenates decreased significantly. The H2 content increased, whereas, the contents of CO and CO2 decreased in gas products, indicating that there were interactions between biomass components and LDPE during catalytic co-pyrolysis. The interactions of cellulose /LDPE and hemicellulose /LDPE were mainly Diels-Alder reactions between furans and olefins, while the interaction of lignin and LDPE was mainly hydrogen transfer reaction, which promoted the dehydroxylation and demethoxylation reactions of phenols. These interactions greatly contributed to the formation of aromatic hydrocarbons and light aliphatic hydrocarbons (C9-C16), meanwhile, a large amount of hydrogen (80.6%-91.9%) was released.