Effect of heterogeneous zeolite catalyst on hydrothermal liquefaction of pulp and paper mill mixed sludge

Pulp and paper mill sludge (PPMS) is a major biogenic waste stream whose valorization is limited by its high moisture and inorganic content. Among thermochemical routes suitable for wet feedstocks, hydrothermal liquefaction (HTL) enables conversion of biomass into energy-dense bio-oil without prior drying. This study evaluates the effect of a low-cost heterogeneous protonated zeolite (PZ) catalyst on HTL performance of PPMS. Experiments assessed the influence of temperature (255–285 °C), residence time (22.5–47.5 min), solids loading (5–10 wt.%), and catalyst loading (2.5–7.5 wt.% of dry solids) on product yields and bio-oil quality. The zeolite enhanced bio-oil yield and heating value under mild HTL conditions. At 255 °C and short residence time, increasing catalyst loading to 7.5 wt.% resulted in bio-oil yield (dry ash free) up to 10 wt.%. In contrast, at higher temperatures, higher catalyst loading promoted gasification and char formation, reducing bio-oil yields from 17.6 wt.% (low catalyst loading) to 6.2 wt.% under otherwise identical conditions. These results indicate that protonated zeolite shows limited effectiveness under higher-temperature HTL conditions, suggesting alternative catalysts may provide improved performance. The study contributes experimental evidence for catalyst screening in HTL and supports development of more efficient valorization pathways for PPMS.