Investigating the effect of mono and bimetallic/zeolite catalysts on hydrocarbon production during bio-oil upgrading from ex-situ pyrolysis of biomass

Catalytic fast pyrolysis of biomass offers an opportunity for upgrading of pyrolysis bio-oils using mono- and bimetallic-supported catalysts, which have been demonstrated to improve the bio-oil qualities. However, the influence of mono- and bimetallic catalysts on different pyrolytic products is less explored. Therefore, this study aimed to examine the effect of mono- and bimetallic catalysts on different pyrolytic products with more emphasis on bio-oil upgrading from ex situ pyrolysis of pine wood biomass. Cu/zeolite and Ni/zeolite were used as the monometallic catalysts, while CuNi/zeolite was used as the bimetallic catalyst in the study. The catalysts were used in ex situ pyrolysis with three different catalyst/biomass ratios: 1, 2, and 3. The results revealed that mono- and bimetallic catalysts with the highest catalyst/biomass ratio of 3 obtained the minimum percentage of oxygen-containing compounds in the bio-oils compared to the sole zeolite. For instance, Cu/zeolite and Ni/zeolite with a catalyst/biomass ratio of 3 (CuZ-2 and NiZ-3) produced the total proportion of hydrocarbons of 50.8 and 41.8%, respectively, while the bimetallic catalyst produced the total hydrocarbons of 54.5% in the bio-oil. It was further revealed that Cu/zeolite favored production of aliphatic hydrocarbons, such as ethylidenecyclobutane and cyclohexene, with CuZ-3 producing 49.6% aliphatic hydrocarbons and 1.25% aromatic hydrocarbons, while Ni/zeolite produced both aromatic and aliphatic hydrocarbons, with NiZ-3 producing 26.8% aromatic hydrocarbons and 15.1% aliphatic hydrocarbons in the bio-oils. The main aromatic hydrocarbons found in the bio-oil were benzene, naphthalene, and phenanthrene. CuNi/zeolite showed better deoxygenation efficiency than monometallic catalysts and produced a comparatively higher percentage of aromatic hydrocarbons at 14.3% and aliphatic hydrocarbons at 39.9%. The main deoxygenation pathway during monometallic catalytic pyrolysis was found to be dehydration and decarboxylation because a higher CO₂ yield was observed during the reaction. The CuNi/zeolite converted the oxygenated compounds into hydrocarbons via dehydration, decarboxylation, and decarbonylation because higher yields of both CO₂ and CO were observed. Overall, CuNi/zeolite catalytic pyrolysis of biomass resulted in improved bio-oil quality when compared to the monometallic counterparts.