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- Highly selective valorisation of D-xylose from corncobs hydrolysates catalysed by C-scorpionate gold(III) complexesPublication . Van-Dúnem, Vanmira; Duarte, Luís; Martins, Pedro L.; Lapa, Hugo; Martins, Luisa Margarida; Carvalheiro, FlorbelaABSTRACT: D-xylonic acid is a multifunctional, environmentally friendly, and versatile platform chemical, and one of the most promising conversion products of D-xylose for replacing several petrochemicals that is still not produced commercially. Although microbial conversion is the most widely studied process, the low efficiency and cost of these methods create opportunities for the development of chemical routes. In this work, we report for the first time the direct chemical conversion of real hemicellulose-derived pentose-rich hydrolysates into D-xylonic acid. Furthermore, a novel chemical oxidation route based on C-scorpionate gold(III) complexes was explored. Two specific complexes are used as model catalysts, the cationic [AuCl2(kappa 2-Tpm)]Cl (1, Tpm = hydrotris(pyrazol-1-yl)methane, HC(C3N2H3)(3)) or the neutral [AuCl2(kappa 2-Tpms)] complex (2, Tpms = tris(1H-pyrazol-1-yl)methane sulfonate, [SO3C(C3H3N2)(3)](-)). The catalytic operational conditions (i.e., reaction time, temperature, type and concentration of alkali and oxidant agent) were initially optimised for D-xylose solutions using cationic complex 1. Under the best conditions (100 degrees C, 2 mmol NaOH and 20 mmol H2O2) nearly 100% conversion of D-xylose and a 94% D-xylonic acid yield were achieved, with minimal formation of secondary products, when using 6 g/L D-xylose as sole carbon substrate. The direct catalytic conversion of real hemicellulosic hydrolysate, without any previous purification was possible and up to 36 g/L of D-xylose, regardless of the presence of other sugars, aliphatic acids, furans and phenolic compounds. Comparing similar catalytic conditions among model D-xylose solutions and real hydrolysates with similar titres, the later presented better performance, even though D-xylose conversion and D-xylonic acid yield decreased slightly for higher D-xylose concentrations. The catalysis was further improved by using the neutral gold(III) C-scorpionate complex 2, which further increased the D-xylose conversion and selectivity.
- Advances of Hydrothermal Biomass Liquefaction Using Microalgae: Process Parameters and Biocrude Upgrading MethodsPublication . Martins, Marta; Fernandes, Marcelo Domingos; Rodrigues, Alda J.; Costa, Paula; Gírio, FranciscoABSTRACT: The ReFuelEU Aviation Regulation introduces mandatory targets for sustainable aviation fuels (SAF) from 2025 to 2050. However, hydrotreated esters and fatty acids (HEFA) technology based on waste oils alone is insufficient to meet targets beyond 2030, highlighting the need for alternative biocrude feedstocks to increase SAF production in the EU. Microalgae are promising feedstocks due to their biochemical composition and CO2-utilization potential, although their high moisture content and nitrogen and oxygen levels require energy-efficient conversion technologies. Hydrothermal liquefaction (HTL) is a suitable process for converting wet microalgal biomass into biocrude, with an optimal temperature window of approximately 300-330 degrees C and typical biocrude yields ranging from 20 to 70 wt%, depending on feedstock composition and operating conditions. However, microalgal HTL remains at TRL 5-7 and faces challenges related to the high heteroatom content of the resulting biocrude. Hydrodeoxygenation (HDO) is a key upgrading step for converting biocrude into drop-in aviation fuels and commonly operates at approximately 250-400 degrees C and 10-30 MPa H2 pressure. Nevertheless, few studies have addressed the HDO of microalgae-derived biocrude. This review examines microalgal HTL, pilot and demonstration facilities, biocrude yields and quality, and upgrading strategies for producing synthetic drop-in aviation biofuels.
- Enhanced CO2 Fixation Through Continuous Cultivation of Microalgae in a Two-Stage Photobioreactor SystemPublication . Tavares, João; Paixão, Susana M.; Silva, Tiago; Alves, LuísABSTRACT: The integration of microalgae cultivation into biorefinery systems represents a promising strategy to enhance carbon dioxide (CO2) sequestration and support sustainable biomass production for diverse biotechnological applications. This study reports the CO2 fixation performance and biomass productivity of a two-stage photobioreactor (PBR) system employed for the continuous cultivation of Haematococcus pluvialis, utilizing biogenic CO2 generated from a heterotrophic culture. The first stage (ST1) operated as an autotrophic chemostat (ST1 PBR) with variable dilution rates (0.18–1.00 d−1), while the second stage (ST2) consisted of sequential high-irradiance columns that promoted biomass and carotenoid accumulation. The two-stage PBR (ST1 + ST2 PBR) achieved a maximum CO2 assimilation capacity of 0.747g/L/d and up to 94% CO2 fixation efficiency. The daily CO2 assimilation rate reached 6.53 g/d, representing a 10% improvement compared with the single-stage system (ST1 PBR). The maximum overall biomass productivity reached 0.332 g/L/d when both PBR stages were operated together, whereas the second stage alone achieved up to 1.18 g/L/d under continuous operation and a maximum biomass concentration of 3.33 g/L during batch-induced carotenogenesis. Nutrient uptake analysis revealed an increasing fraction of unconsumed major nutrients with higher dilution rates. Continuous operation under high irradiance did not induce pigment accumulation, whereas batch mode successfully triggered carotenogenesis over 13 days, resulting in visible pigment production and high biomass yield. Overall, the two-stage PBR system demonstrated high CO2 capture efficiency, enhanced biomass productivity, and operational flexibility, offering a scalable and sustainable approach for integrating microalgal cultivation into biorefineries while contributing to carbon mitigation and circular bioeconomy goals.
- Catalytic Upgrading of Microalgae-Based Bio-Oils for Sustainable Jet Fuel ProductionPublication . França, Bruna Thomazinho; Paradela, Filipe; Martins, Marta; Fernando, Ana Luisa; Reis, Alberto; Costa, PaulaABSTRACT: The transition to sustainable energy systems has intensified the search for renewable alternatives to reduce greenhouse gas emissions and reliance on fossil fuels. In this context, microalgae have emerged as a promising third-generation feedstock for biofuel production due to their rapid development, high lipid content, and ability to grow in wastewater without competing with freshwater resources. In this study, the hydrotreatment of biocrudes derived from C. vulgaris, T. obliquus, and a mixed microalgal culture cultivated in domestic wastewater is investigated. Catalytic upgrading was applied using sulphided CoMo/Al2O3 (sCoMo) and Pt/Al2O3 catalysts. The results demonstrated that catalytic upgrading enhanced the upgraded bio-oils’ quality compared to non-catalysed reactions, confirming the crucial role of catalysts in improving bio-oil properties. Compared with the Pt catalyst, sCoMo produced higher yields of upgraded bio-oil, greater enrichment in carbon and hydrogen, and higher heating value (HHV), while effectively enhancing nitrogen and oxygen removal. However, when compared with the non-sulphided CoMo, the sulphiding treatment did not significantly improve denitrogenation and treated oil yields. The highest fraction of components within the jet fuel boiling range (37.7%) was obtained using a Pt catalyst, while the non-catalysed process yielded the lowest (26.6%). In this sense, catalytic upgrading of microalgae-based biocrude represents an important step towards the production of advanced and environmentally sustainable fuels.
- Biogas Production from the Anaerobic Co-Digestion of Elephant Grass Juice and CattleWastewater: Batch and Semi-Continuous Performance in Anaerobic ReactorsPublication . da Silva, Guilherme Henrique; Otenio, Marcelo; Reis, Alberto; Martins, Marcio; Borges, Alisson; Lopes, Tiago; Renato, NataliaABSTRACT: Considering the limited information available on the use of elephant grass juice (EGJ) as a liquid co-substrate, this study aimed to evaluate its feasibility and biogas production potential for anaerobic co-digestion with dairy cattle wastewater (DCW), focusing on substrate ratios and reactor operating conditions. Initially, batch tests were conducted at different temperatures to evaluate the effects of varying substrate mixture ratios (EGJ/DCW). In the reactor with 20% EGJ/80% DCW at 39 °C, a biogas volume of 414.35 mL was generated, which was considerably higher than that in the test at 25 °C, where 258.7 mL was generated over the 30-day experimental period. Greater efficiency in organic matter removal and other analyzed parameters was also observed under these conditions. The best configuration was selected for the semi-continuous tests. To this end, an Upflow Anaerobic Sludge Blanket (UASB) reactor was operated with hydraulic retention times of 10, 6, 2, and 1 d and volumetric organic loading rates of 2.06, 3.39, 10.40, and 20.70 kg VS m−3 d−1, respectively. Volatile solids were removed by 29–58%, with biogas yields ranging from 0.78 to 2.28 m3 m−3 d−1 and a maximum CH4 concentration of 72% (v/v). The co-digestion of the analyzed agricultural waste substrates proved to be a promising approach for bioenergy recovery. These results provide useful guidelines for optimizing co-digestion systems, improving reactor performance, and waste treatment processes.
- A molecular 'LEGO®' approach to high-spin triangular {MnIIILn2} clusters from {MnIII} and {Ln2} metalloligandsPublication . Pantelis, Konstantinos N.; Cunha-Silva, Luís; Saber, Mohamed R.; Dunbar, Kim R.; Alexandropoulos, Dimitris I.; Stamatatos, TheocharisABSTRACT: A stepwise "LEGO (R)-brick" strategy has been employed for the targeted assembly of rare low-nuclearity MnIII/LnIII clusters from preformed molecular building units. The mononuclear MnIII metalloligand (Pr2NH2)[MnIII(sacb)2] (1; sacbH2 = N-salicylidene-2-amino-5-chlorobenzoic acid) was combined with newly isolated dinuclear lanthanide complexes [Ln2(sacb)(sacbH)3L(MeOH)] [LnIII = Dy (2), Gd (3); L- = 2-amino-5-chlorobenzoate], affording the trinuclear heterometallic compounds [MnIIILn2(sacb)3(sacbH)2L] [LnIII = Dy (4), Gd (5)]. Single-crystal X-ray diffraction studies revealed that 4 and 5 possess triangular {MnIIILn2(mu-O2CR)3}6+ cores, representing the first structurally characterized triangular {MnIIILn2} complexes and rare examples of triangular {MLn2} (M = any 3d-metal ion) clusters. Magnetic studies have demonstrated that the MnIII-free {Dy2} precursor 2 displays weak field-induced slow relaxation of magnetization, whereas incorporation of the Jahn-Teller active MnIII ion leads to ferromagnetically coupled {MnIIILn2} species. In particular, the {MnIIIGd2} analogue 5 exhibits a high-spin ground state of S = 9, with weak ferromagnetic MnIII & ctdot;GdIII and GdIII & ctdot;GdIII exchange interactions. Although no slow magnetic relaxation was observed for the trinuclear complexes, the results demonstrate that controlled integration of a MnIII metalloligand can promote ferromagnetic exchange in preassembled {Ln2} units and provide a direct route to structurally defined high-spin 3d/4f triangles.
- Acid-Based Deep Eutectic Solvents for Structural Modification of Sulphite Pulp Cellulose: A Potential Route Toward Advanced MaterialsPublication . Moran, Guadalupe; Costa Trigo, Iván; Domínguez, José Manuel; Vilaseca, FabiolaABSTRACT: The transition toward renewable and environmentally responsible materials has intensified interest in cellulose-based systems for use in sustainable packaging applications. Although cellulose offers biocompatibility, structural versatility, and tuneable physicochemical properties, conventional modification routes rely on harsh chemicals and generate environmentally burdensome effluents. In this study, an efficient and a potentially green strategy for cellulose modification was developed using acid-based deep eutectic solvents (DES) composed of choline chloride and lactic, acetic, or citric acid at different molar ratios. Under mild conditions (110 degrees C, 4 h), DES pretreatment reduced glucan content in sulphite pulp from 99% to 79-93%, depending on the hydrogen bond donor (HBD), while suggesting an apparent increase in relative crystallinity, from approximately 82% to 90%, as estimated by the Segal method. FTIR, XRD, and morphological analyses revealed the disruption of the hydrogen bonding network, enhanced fibrillation, and residual DES-derived functional groups detectable by FTIR. Although DES pretreatment increased structural order, it also reduced enzymatic digestibility due to the higher proportion of crystalline domains. Overall, the results demonstrate that acidic DES constitutes a sustainable and recyclable medium capable of modulating cellulose structure and generating materials with enhanced physicochemical properties. These findings suggest that DES-modified cellulose could serve as a potential reinforcement platform for future biodegradable packaging and bioplastic formulations, enabling the development of high-performance, renewable, and environmentally compliant packaging materials.
- A two-stage cultivation strategy for the industrial production of the novel microalga Chlorococcum amblystomatisPublication . Correia, Nádia; Trovão, Mafalda; Santo, Gonçalo E.; Guerra, Inês; Cunha, P.; Fonseca, Joana; Pereira, Hugo; Costa, Monya M.; Ferreira, Sara; Santos, Tamára; Barros, Ana; Cardoso, Helena; Silva, Joana; Gouveia, Luisa; Varela, JoãoABSTRACT: This study presented the industrial-scale validation of a two-stage growth strategy for a poorly explored yet industrially relevant chlorophyte microalga, Chlorococcum amblystomatis, addressing the gap between laboratory research and practical application. By using cultures grown heterotrophically as inoculum for industrial-scale photoautotrophic reactors, this approach significantly accelerated scale-up. The proposed strategy reduced the overall scale-up time by approximately 5.5-fold compared to the conventional pipeline, without compromising productivity or biochemical quality. A maximum biomass concentration of 70.96 +/- 0.35 g L-1 was achieved in 7-L benchtop fermenters under heterotrophic conditions, representing the highest value reported to date for this microalga. The biomass obtained presented a protein content of 32.42 +/- 2.30% of dry weight, which increased to 54.55 +/- 2.04% upon cultivation under photoautotrophic conditions, as well as polyunsaturated fatty acids, which increased from 50.12 +/- 3.27 to 67.74 +/- 0.35% of total fatty acids, with alpha-linolenic acid as the predominant fatty acid. Conversely, the saturated fatty acids content (38.26 +/- 1.63% of TFA) decreased to 22.37 +/- 0.02% of TFA in the phototrophic stage. Pigment contents also increased significantly during the phototrophic stage, with chlorophylls rising from 10.04 +/- 1.68 to 44.91 +/- 5.11 mg g(-)1 and carotenoids reaching notable concentrations, including lutein (5.34 +/- 0.10 mg g-1), beta-carotene (5.92 +/- 0.39 mg g-1), and neoxanthin (3.53 +/- 0.75 mg g-1). Among the industrial photoautotrophic systems tested, the closed tubular photobioreactors were the most suitable configuration for C. amblystomatis cultivation, achieving higher biomass productivity, enhanced protein content and pigment accumulation compared to open raceway ponds. Overall, this work demonstrates a scalable, efficient, and time-saving cultivation strategy for C. amblystomatis, supporting its potential for industrial biotechnological applications.
- A Strategy to Improve Porous MOF Structural Stability for the Effective Removal of S/N Pollutants from (Bio)fuelsPublication . Silva, Dinis; Mirante, Fátima; Santos-Vieira, Isabel; Granadeiro, Carlos; Silva, Luis Cunha; Balula, Salete S.ABSTRACT: The need to mitigate environmental pollution drives the continuous quest for advanced desulfurization and denitrogenation methods in refineries. The presence of sulfur and nitrogen compounds in fossil fuels and biofuels contributes largely to atmospheric polllution. Oxidative catalysis presents a promising way for addressing these challenges, offering efficient and environmental friendly pathways. This work investigates the heterogeneous catalytic performance of the polyoxometalates, plenary Keggin PW12 structure and the derivative lacunary PW11 structure, supported on the porous MIL-100(Fe) metal-organic framework, for simultaneous oxidative desulfurization and denitrogenation. Both PW11@MIL-100(Fe) and PW12@MIL-100(Fe) catalysts demonstrated rapid and complete desulfurization within 60 min of reaction using a sustainable H2O2 oxidant. Remarkably, PW11@MIL-100(Fe) exhibited higher catalytic efficiency and demonstrated high recycle capacity. In fact, notable results were attained to remove the most noxious pollutants in (bio)fuels, offering insights for the development of effective catalytic materials with high viability for the energy industry.
- Microalgae-Based Treatment of Winery WastewaterPublication . Lopes da Silva, Teresa; Lopes, Tiago; Reis, AlbertoABSTRACT: Winery wastewater (WWW) is a high-strength effluent characterized by high organic load, nutrient content, and seasonal variability, posing significant environmental challenges. This review critically evaluates microalgae-based systems for WWW treatment, addressing their performance, scalability, and role within circular bioeconomy frameworks. Analysis of recent studies shows that microalgae-based systems can achieve high pollutant removal efficiencies, including up to ~ 90–92% chemical oxygen demand removal, high removal of total organic carbon and nitrogen (typically above 80%), and up to 91–95% ammonium removal. These systems also enable the production of biomass rich in proteins (up to ~ 58.8% dry weight) and valuable compounds such as pigments, supporting multiple valorization pathways. A key finding is that, despite strong technical performance, large-scale implementation remains limited. The main constraints are associated with wastewater variability, presence of inhibitory compounds, and operational challenges in biomass recovery, particularly energy-intensive harvesting and downstream processing. Techno-economic and life cycle analyses indicate that standalone systems are rarely economically viable under current conditions, and that feasibility depends on integration into multiproduct biorefineries and existing infrastructures. Regulatory uncertainty and the need for stakeholder acceptance further constrain deployment. Overall, microalgae-based WWW treatment represents a promising but system-dependent solution, requiring integrated technological, economic, and policy frameworks to achieve scalable implementation.