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The management of coffee and peapod waste presents significant environmental challenges, with millions of tons generated annually, leading to disposal issues and resource inefficiencies. Hydrothermal processes offer a promising valorization method, though biomass characteristics significantly influence the resulting products. Biomass characterization revealed distinct profiles for coffee cherry waste (moisture: 10.94%, ashes: 7.79%, volatile matter: 79.91%, fixed carbon: 1.36%, cellulose: 27.6%, hemicellulose: 12.5%, and lignin: 13.7%) and peapods (moisture: 7.77%, ashes: 4.22%, volatile matter: 74.18%, fixed carbon: 13.0%, cellulose: 20.2%, hemicellulose: 17.4%, and lignin: 5.0%). Experiments were conducted in 100 mL and 500 mL hydrothermal reactors with varying conditions for temperature (120–260 °C), time (1–4 h), stirring (none and at 5000 and 8000 rpm), biomass/water ratio (1:5, 1:10, 1:20, and 1:40), particle size (0.5–5 mm), and catalysts (acids and bases). The results showed that peapods produced over 30 times more platform chemicals than coffee. High temperatures (over 180 °C) degraded peapods, whereas coffee yields increased. Both biomasses were influenced similarly by reaction conditions: lower biomass/water ratios, smaller particle sizes, acid catalysts, and no stirring increased yields. Peapods consistently had higher yields than coffee in all conditions. Biochar analysis revealed anthracite from coffee and coal from peapods.

Pea pod cultivation spans various regions and climates, with a global production of around 20 million tons. The pea peel wastes, which make up 30–40% of the total weight of the peas, are freely available in large quantities. The biomass used was characterized via ultimate, proximate, and structural analysis, obtaining 20.2%w of cellulose and 17.4%w of hemicellulose, which, via valorization processes, can be transformed into platform chemicals. Hydrothermal valorization presents itself as a clean form of treatment for these wastes, ranging from 120 to 180 °C (LHW) and from 180 to 260 °C (HTC). The use of LHW can lead to the production of sugars (up to 70%w yield) and levulinic acid (4%w yield), while the use of HTC leads to formic acid (40%w yield) and levulinic acid (4%w yield). The use of LHW for longer periods favors the production of HMF and furfural. The use of homogeneous catalysts (H2SO4, CH3COOH, KOH, and NaHCO3) was implemented, and their selectivity was described. Solid fractions of LHW and HTC were characterized via FTIR and elemental analysis, and the change in their structure was described as they shifted from biomass to biochar. Optimal conditions for each platform chemical were reported to best utilize the pea pod waste.

Coffee cherry waste, a byproduct of coffee production, presents significant environmental challenges due to its large volume—approximately 20 million tons annually. The disposal of this waste, which includes pulp and mucilage, often leads to pollution of land and water systems, contributing to environmental degradation. Additionally, the high acidity and organic content of coffee cherry waste complicate its management, making it crucial to find sustainable solutions for its valorization and reuse in order to mitigate these ecological impacts. The purpose of this study is to investigate the efficiency and selectivity of various organic acid catalysts in the hydrothermal valorization of coffee cherry waste. The procedures were conducted using the liquid hot water (LHW) treatment for one hour with a 1:20 biomass/catalyst ratio and a 1 mm biomass particle size at 180 °C in 500 mL batch reactors modifying 10 different organic acids at 0.02 M. Concentrations of the valorized products (HMF, furfural, levulinic acid, formic acid, and sugars) were measured using HPLC-IR. Among the catalysts tested, adipic acid demonstrated the highest efficiency, with a total yield of 53.667%, showing significant selectivity towards formic acid (19.663%) and levulinic acid (11.291%). In contrast, butyric acid was the least efficient catalyst, yielding a total of 17.395% and showing minimal selectivity towards other compounds. Chloroacetic acid and benzoic acid were notable for their high selectivity towards sugars. Other catalysts, such as anthranilic acid, propanoic acid, and succinic acid, displayed moderate efficiency and selectivity, with balanced yields across various compounds. These findings highlight the importance of catalyst selection in optimizing the hydrothermal process for desired product outcomes.

The valorization of coffee cherry waste through hydrothermal carbonization (HTC) was investigated using various organic and inorganic acid catalysts to produce platform chemicals. This study aimed to evaluate the effectiveness of these catalysts for enhancing reaction rates, improving yields, and promoting selectivity. The results showed that sulfuric acid and adipic acid were the most effective, each resulting in a 20% increase in the total yield, demonstrating the potential of organic acids as efficient catalysts in HTC. Other catalysts, such as benzoic acid and phenylacetic acid, also showed promising results, while butyric acid significantly decreased the total yield. The most abundantly produced platform chemicals were sugars, followed by formic acid, levulinic acid, HMF, and furfural. These findings highlight the potential of coffee cherry waste as a valuable resource for producing key chemicals, and the feasibility of hydrothermal carbonization as a sustainable approach for biomass valorization. This study emphasizes the importance of selecting the appropriate catalysts to optimize the conversion process and maximize the extraction of valuable chemicals. The environmental and economic implications of these findings are significant, as they can contribute to the development of sustainable and efficient biomass utilization technologies that could transform agricultural waste into high-value products while reducing waste and promoting a circular economy.

Agro-industries produce over 2 billion tons of agricultural waste annually, including by-products like bagasse, molasses, seeds, stems, leaves, straw, and shells. The use of agro-industrial waste is a way to reduce the impact of industrial processes on the environment. The pea pod is a biomass with a high concentration of cellulose, hemicellulose and some lignin; therefore, it can be used to produce platform chemicals by means of a hydrothermal process. There is limited research on the hydrolysis of pea biomass, but it has been shown to obtain high yields. This study analyzed the effectiveness and selectivity of the hydrothermal process using pea pod biomass with a particle size of 0.5 mm at 180 °C for one hour. A 500 mL reactor was used, with a biomass-to-acid solution ratio of 1:20. The concentration of the acid solution was 0.02 M. The concentrations of sugar, formic acid, levulinic acid, HMF, and furfural produced were measured. Among the catalysts studied, adipic acid catalysis showed the highest yield of 65.16%, with 37.09% of sugar, 16.37% of formic acid, and 11.71% of levulinic acid. On the other hand, the catalysts with chloroacetic acid, butyric acid, anthranilic acid, and phthalic acid were less effective but demonstrated selectivity for sugar production, proving that the liquid phase obtained using the catalyst with those acids can be used as carbon sources for a fermentation process. In general, when comparing the process with or without the use of a catalyst, it is observed that with a catalyst in the reaction, the amount of HMF and furfural produced is reduced and the selectivity with respect to sugar production is increased.

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.

The replacement of traditional and non-renewable resources by shifting towards renewable-based strategies is a strategy implemented by the European Union for a circular economy-based society. Among the various methods to produce renewable biofuels, hydrothermal carbonization is promising in terms of waste management. This technology involves thermochemical treatment of wastes at lower emperatures than other common thermal treatments such as pyrolysis or gasification. Here we review hydrothermal carbonization for managing a wide variety of biomass–wastes and obtaining value-added products. Three types of products are obtained: gases, liquids and solids. The solid carbonized product is the main product, which can be used for many applications, highlighting its use as substitute for fossil coal. Nonetheless, actual commercial plants hardly reach profitability by considering only the solid products. Therefore, this review focus on the integration of other processes. For instance, anaerobic digestion, catalytic conversion of gaseous streams and membrane technology for liquid concentration appear as the most interesting options to valorize all the products generated during hydrothermal carbonization.

Hydrothermal carbonization enables the valorization of biomass via thermochemical conversion into various products. Today, this technology is experiencing a situation similar to that experienced in the past by other process technologies. Of these technologies, some have become important industrial realities, such as reverse osmosis, while others have never been able to establish themselves fully. This paper presents a brief overview of this technology’s current status, highlighting its strengths and various drawbacks. The primary purpose of the research activity is to identify a possible future scenario toward which this technology is heading. Hydrothermal carbonization has already been established on a laboratory scale for some time, and now it is in a transitional phase between pilot-scale and industrial-scale applications. The interest that HTC has aroused and continues to arouse is evidenced by the growing number of publications and patents published. In particular, the uniform percentage of patents filed in various countries testifies to the worldwide interest. This technology has advantages but also some bottlenecks that have yet to be overcome. Process integration, higher-capacity plants, and the use of Industry 4.0 technologies seem to be the most interesting options to overcome the last limiting factors and make hydrothermal carbonization an established industrial reality.

Approximately 15 million dry tons of food waste is produced annually in the United States (USA), and 92% of this waste is disposed of in landfills where it decomposes to produce greenhouse gases and water pollution. Hydrothermal liquefaction (HTL) is an attractive technology capable of converting a broad range of organic compounds, especially those with substantial water content, into energy products. The HTL process produces a bio-oil precursor that can be further upgraded to transportation fuels and an aqueous phase containing water-soluble organic impurities. Converting small oxygenated compounds that partition into the water phase into larger, hydrophobic compounds can reduce aqueous phase remediation costs and improve energy yields. HTL was investigated at 300 °C and a reaction time of 1 h for conversion of an institutional food waste to bio-oil, using either homogeneous Na2CO3 or heterogeneous CeZrOx to promote in situ conversion of water-soluble organic compounds into less oxygenated, oil-soluble products. Results with food waste indicate that CeZrOx improves both bio-oil higher heating value (HHV) and energy recovery when compared both to non-catalytic and Na2CO3-catalyzed HTL. The aqueous phase obtained using CeZrOx as an HTL catalyst contained approximately half the total organic carbon compared to that obtained using Na2CO3—suggesting reduced water treatment costs using the heterogeneous catalyst. Experiments with model compounds indicated that the primary mechanism of action was condensation of aldehydes, a reaction which simultaneously increases molecular weight and oxygen-to-carbon ratio—consistent with the improvements in bio-oil yield and HHV observed with institutional food waste. The catalyst was stable under hydrothermal conditions (≥16 h at 300 °C) and could be reused at least three times for conversion of model aldehydes to water insoluble products. Energy and economic analysis suggested favorable performance for the heterogeneous catalyst compared either to non-catalytic HTL or Na2CO3-catalyzed HTL, especially once catalyst lifetime differences were considered. The results of this study establish the potential of heterogeneous catalysts to improve HTL economics and energetics.

Worldwide, carbon fiber (CF) demand has been rising over the last decade, which contrasts with the fact that up to 30–50% of composite materials in aircraft production are scrapped. This situation highlights the increasing need for recycling methods to reduce fabrication costs and global warming potential. Emerging technologies focus on recovering long CFs, as they represent the most valuable form but are also the most difficult to reclaim using conventional recycling methods. Hydrothermal treatments offer a promising alternative to valorize this waste by decomposing the polymer matrix under subcritical and supercritical conditions without significantly damaging the fibers. Water, isopropanol, and mixtures of water/isopropanol or water/acetone were tested as solvents, with and without the addition of zinc chloride (ZnCl2) as a homogeneous catalyst. The influence of temperature, pressure, and solvent composition on resin degradation was evaluated. In this work, degradation rates of up to 92% were achieved at 415 °C, 233 bar, 120 min, 5 wt.% IPA, and ZnCl2 0.1 M. It should be noted that ZnCl2 caused reactor corrosion. Furthermore, the recovered fibers retained their morphology, including the sizing layer, and showed mechanical properties similar to the original material, while a small H2-rich gaseous fraction was generated as a byproduct of the hydrothermal degradation. Using water–isopropanol solutions resulted in the reactor being significantly cleaner than when using water alone, which can be advantageous for future scale-up and for reducing maintenance requirements. These results confirm the potential of hydrothermal processing as an efficient and selective method for the recycling and valorization of carbon-fiber-reinforced composites from the aeronautical industry.