Understanding the interactions between cellulose and polypropylene during fast co-pyrolysis via experiments and DFT calculations

Co-pyrolysis of lignocellulosic biomass with waste plastics is a promising option to produce high quality liquid fuels. During co-pyrolysis the molecular level interactions between the intermediates produced from biomass with polymers play a crucial role in altering the distribution of various organics in bio-oil. Recently, it was shown that interactions between cellulose and polypropylene during fast co-pyrolysis leads to the formation of C8-C20 long chain alcohols in bio-oil. The formation of alcohols was proposed to occur via reaction of hydroxyl radicals from cellulose pyrolysis with polypropylene radicals. This study is an attempt to unravel the formation mechanism of long chain alcohols during co-pyrolysis using quantum chemical calculations. The reactions of propylene trimer (2,4,6-trimethyl heptane) and its primary, secondary and tertiary radicals with hydroxyl radical (•OH) and water molecule are investigated at B3LYP/6-31G(d,p) level of theory using Gaussian 09. The reaction of •OH with propylene trimer readily leads to the formation propylene trimer radicals with the liberation of water. The Arrhenius activation energy of this reaction is in the range of 11-14 kcal/mol. It is shown that the reaction of propylene trimer radical (primary, secondary or tertiary) with a water molecule readily leads to the formation of respective alcohols with Arrhenius activation energy of 10-14 kcal/mol. This reaction competes with the barrierless recombination of polypropylene radical with •OH. However, the presence of •OH is limited by the high reaction barrier for its abstraction from cellulose. Therefore, the reaction of polypropylene radicals with water molecules formed via cellulose dehydration is shown to be a plausible pathway for the formation of long chain alcohols duirng fast co-pyrolysis of cellulose and polypropylene. The mass loss profiles during fast co-pyrolysis for different cellulose:polypropylene compositions were obtained in a Pyroprobe® reactor. The first order rate constants of decomposition were evaluated, and they follow the order: 0.044 s^-1 (cellulose:polypropylene 100:0) > 0.041 s^-1 (75:25) ≈ 0.042 s^-1 (50:50) > 0.032 s^-1 (25:75) > 0.028 s^-1 (0:100).