Hydrothermal Valorization of Peapods and Coffee Cherry Waste: Comparative Analysis of Organic and Inorganic Acid Catalysis and Evaluation of Biomass’ Influence on Catalytic Efficiency

Abstract

Hydrothermal processing has emerged as a promising clean technology for managing the substantial amounts of agro-industrial waste generated worldwide. This study aims to introduce a clean technology approach to biomass valorization processes by exploring the hydrothermal conversion of two distinct biomass feedstocks, peapods and coffee cherries, into valuable platform chemicals through the use of homogeneous acid catalysts. The hydrothermal valorization experiments were conducted in a 500 mL reactor at 180 °C for 1 h with a 1:20 biomass–acid solution ratio, utilizing a set of organic and inorganic acids as catalysts. The chemical compositions of the biomass feedstocks were analyzed, revealing significant differences in their cellulose (20.2 wt% in peapods; 27.6 wt% in coffee cherries), hemicellulose (17.4 wt% in peapods; 12.5 wt% in coffee cherries), and lignin (5.0 wt% in peapods; 13.7 wt% in coffee cherries) contents. Without the use of catalysts, peapods yielded 45.128 wt% platform chemicals, outperforming coffee cherries, which produced 32.598 wt%. The introduction of various acid catalysts influenced the yields and selectivity of platform chemicals. Sulfuric acid enhanced sugar production, yielding 62.936 wt% from peapods and 51.236 wt% from coffee cherries. Hydrochloric acid selectively favored sugar production but resulted in decreased overall yields. Nitric acid facilitated the conversion of both biomass types, yielding 35.223 wt% from coffee cherries and 40.315 wt% from peapods. Adipic acid achieved the highest overall yields, with 53.668 wt% for coffee cherries and 65.165 wt% for peapods, while also increasing levulinic acid production. Acetic acid significantly increased sugar yields, which reached 50.427 wt% with peapods. The findings highlight the potential of hydrothermal valorization as a clean technology for biomass conversion and underscores the importance of tailoring catalyst selection and process conditions to optimize the valorization of biomass feedstocks.