Biomass Hydropyrolysis in Molten Chloride Salts: Thermocatalytic Stability of Recycled Salts

Abstract

Pyrolysis of biomass using molten chloride salts offers a novel approach to biomass valorization. This process benefits from the rapid heat transfer provided by the molten salts and their catalytic activity. In this study, we investigated the effects of the pyrolysis atmosphere (inert gas versus pressurized hydrogen), biomass type, and temperature, primarily at the microscale, while also addressing industrially relevant considerations, particularly on the recyclability of the molten salts. Given that an excess of salt is needed with respect to the biomass, achieving high recovery rates of the salts is critical for process viability. To this end, we developed a recovery process involving water extraction, filtration, and drying to recycle the eutectic chloride salts. Achieved results demonstrated recovery yields of 95–100 wt % with minimal impact on the thermal and catalytic properties of the salts. The catalytic performance of the recycled salts remained largely consistent; the total volatiles’ yields decreased only ca. 4% points upon 3 salt recycling series. Hydropyrolysis of pinewood using recycled salts led to an increase in the production of gas chromatography (GC)-detected volatiles (from ca. 17 wt % using fresh salts to 29 wt % using spent salts from 3 recycling cycles), which in majority were furans and ketones. This effect may be attributed to the retention of calcium and magnesium from the biomass in the recycled salts, which potentially enhances the formation of these compounds. This study provides the first comprehensive evaluation of chloride molten salt recycling in biomass hydropyrolysis, demonstrating the potential for sustainable and efficient biomass valorization through molten salt reuse. The findings suggest that recycled salts can maintain high catalytic performance and may even enhance certain product yields, underscoring the promise of this approach.