Catalytic co-pyrolysis of biomass and waste polymers using zeolite catalysts for upgrading bio- oils

This research has used catalytic pyrolysis and catalytic co-pyrolysis of biomass and plastics with upgrading of oils to make them more suitable for fuel use or as chemical feedstocks.
Pyrolysis of biomass at 500°C produced char, gas and liquid products, with the liquid containing both oil and water. All of the compounds which were identified in the bio-oil contained oxygen which is the primary cause of poor fuel properties in pyrolysis bio-oils through increased acidity and instability and reduced energy density.
Catalytic pyrolysis using ZSM-5 reduced the yield of liquid products by formation of deoxygenation gases (carbon monoxide and carbon dioxide) and light hydrocarbon gases (C2-C4) through zeolitic cracking. The proportion of oil within the liquid yield was also reduced with further deoxygenation reactions forming water. The Zeolite catalyst acts as a solid acids through two main sites, Strong Brӧnsted acid sites donate protons (H+) whilst weaker Lewis acid sites accept electrons often in the form of a hydride (H-) ion. Deoxygenation reactions include decarbonylation, decarboxylation and hydrodeoxygenation.
Metal-impregnated ZSM-5 catalysts were produced and used during pyrolysis as a pathway towards further upgrading of oils. These metal catalysts improved or maintained the oil yield observed for unmodified ZSM-5 and had varying effects on the oxygenated compounds in the oils. Gallium impregnated ZSM-5 (5 wt.%) produced the lowest proportion of oxygenated compounds. The gallium impregnated zeolites appear to function as bifunctional catalysts with the gallium atoms enhancing the acidity of the Brӧnsted acid sites. This allowed for decarbonylation of furan derived from cellulose into allene compounds which could then be further converted into aromatic compounds.
Catalytic co-pyrolysis using plastics was identified as another method for upgrading bio-oils through hydrogen donation. Plastics produce high oil yields when pyrolysed and these oils have low oxygen content. Co-pyrolysis oils of biomass and plastics, therefore, had a lower abundance of oxygenated compounds and greater oil yields than biomass alone. There were also synergistic effects observed which altered the oil yields beyond what was expected. These synergy effects involved free radical interactions between biomass and the plastics leading to hydrogen donation from the plastics to the biomass as well as natural catalytic effects involving char produced during biomass thermal decomposition. This was particularly notable in co-pyrolysis of biomass and polystyrene where deoxygenation was enhanced during fixed-bed experiments.
Catalytic co-pyrolysis of biomass and polystyrene at different mixing ratios was examined in more detail using metal-loaded ZSM-5 catalysts. The metal impregnated ZSM-5 catalyst were able to reduce the oxygenated compound abundance in the pyrolysis oils compared to unmodified ZSM-5 in many cases. At a 1:1 mixture ratio cobalt-ZSM-5 produced the highest oil yields and joint lowest oxygenated compound abundance. At 4:1 gallium-ZSM-5 produced the highest liquid yield and lowest oxygenated compound abundance. This deoxygenation involved a mixture of decarbonylation, decarboxylation and hydrodeoxygenation reactions. The formation of carbon oxides during decarbonylation and decarboxylation reactions removed oxygen from the oil whilst reducing the oil yield directly as a result of carbon removal. Hydrodeoxygenation to remove oxygen in the form of water could also cause indirect loss of oil yield through carbon loss by enhanced coke formation. This is a particular problem where hydrogen content is depleted such that high yields of alkene and aromatic compounds are not viable. For efficient deoxygenation of bio-oil, the balance of these deoxygenation reactions should lead to maximum oxygen removal with minimal loss of carbon as carbon oxides or coke. Catalytic co-pyrolysis experiments using different metal-impregnated ZSM-5 catalysts gave varied results both for yield and composition of pyrolysis products with variation in the sample composition strongly influencing the effectiveness of a particular catalyst.