Macadamia Carpel Pyrolysis Catalyzed by Natural and Modified Clinoptilolite in a Fixed-Bed Reactor

The macadamia carpel, an agro-industrial byproduct, can be valorized as a feedstock for bio-oil production via pyrolysis. However, bio-oils derived from lignocellulosic biomass often exhibit a high oxygen content, which reduces their fuel quality. In this study, natural clinoptilolite (NC) and its modified form (modified clinoptilolite (MC)) were evaluated as catalysts in a fixed-bed pyrolysis reactor to enhance both the liquid yield and the bio-oil quality. The MC catalyst was prepared via ion exchange using 1.0 M NH₄Cl, followed by calcination at 773.15 K, which improved its textural properties by increasing the surface area from 24.99 m²/g to 101.11 m²/g, the pore volume from 0.088 m³/g to 0.135 m³/g, and the average pore diameter from 9.05 to 10.67 nm. Notably, this study is the first to systematically compare macadamia carpel pyrolysis under noncatalytic conditions using both NC and MC, with a focus on enhancing bio-oil quality. Three full factorial experimental designs were conducted, considering reaction temperatures (773.15, 873.15, and 973.15 K); heating rates (10, 30, and 50 K/min); and catalyst concentrations (0, 10, and 20 wt %). Desirability function analysis identified the optimal conditions as a reaction temperature of 873.15 K, a heating rate of 50 K/min, and a catalyst concentration of 20 wt % MC, resulting in a maximum liquid yield of 58.20%. Moreover, GC–MS analysis revealed that the use of MC significantly enhanced the production of phenolic compounds, while reducing undesired oxygenated species. These findings demonstrate that MC improves not only the yield but also the quality of bio-oil, offering a promising strategy for the valorization of macadamia carpel waste.