Green catalyst-based cardboard waste conversion into biogas†

Abstract

Biofuels, such as biogas, are crucial for a sustainable and green energy future. Biogas can be generated from abundant biomass wastes, e.g., corrugated cardboard waste (CBW), which has surged owing to e-commerce growth. Anaerobic digestion (AD) of CBW to generate biogas is challenged by the rate-limiting step of enzymatic hydrolysis and solid mass transport constraints. To address these issues, acid hydrolysis of carbohydrate components of CBW to a water-soluble sugar mixture is a more efficient alternative. Typical acid hydrolysis employs fossil fuel-derived sulphuric acid as the catalyst, which is unsustainable and may inhibit downstream bioprocesses. Herein, we report a novel mechanochemical-microwave pretreatment strategy that effectively overcomes the mass transport constraint of enzymatic hydrolysis. We replace the commonly used sulphuric acid with a renewable organic acid (e.g., oxalic acid) as a green catalyst. Our new process with a green catalyst exhibits several advantages. The new mechanochemical treatment leveraging the catalytic effect during milling efficiently shrinks the CBW size, disintegrates its structure, and disperses the acid catalyst into CBW particles. The addition of oxalic acid facilitates dual-effect transformation of cellulose by reducing its degree of polymerization and converting crystalline cellulose into reactive amorphous cellulose through esterification of the C6–OH group, which disrupts intra- and intermolecular hydrogen bonds. Building on this, microwave-assisted hydrolysis further breaks down cellulose, where oxalic acid undergoes deprotonation to form hydronium ions, facilitating the cleavage of β-1,4 glycosidic bonds and releasing glucose products. This integrated process enhances overall efficiency and enables a much greener pretreatment. As a result, CBW's recalcitrant complex structure is effectively broken down, achieving a remarkable sugar yield of 52.3 g per 100 g dried CBW. The AD of the treated CBW, without additional separation steps, produced biogas with a high methane content of 69.9%, which is comparable to that of the control pure glucose feed. Notably, this whole-mixture approach significantly simplifies the process and boosts initial methane production without compromising long-term methane production potential. Moreover, life cycle assessment reveals that the process has a global warming potential comparable to that of traditional waste management processes, and the oxalic acid catalyst has a lower environmental impact than sulphuric acid, thus showing promise for enhanced sustainability.