PUBLICAÇÕES CIENTÍFICAS NO ÂMBITO DO MOVE2LOWC

ABSTRACT

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Exploration of yeast diversity for the sustainable production of biofuels, in particular biodiesel, is gaining momentum in recent years. However, sustainable, and economically viable bioprocesses require yeast strains exhibiting: (i) high tolerance to multiple bioprocess-related stresses, including the various chemical inhibitors present in hydrolysates from lignocellulosic biomass and residues; (ii) the ability to efficiently consume all the major carbon sources present; (iii) the capacity to produce lipids with adequate composition in high yields. More than 160 non-conventional (non-Saccharomyces) yeast species are described as oleaginous, but only a smaller group are relatively well characterised, including Lipomyces starkeyi, Yarrowia lipolytica, Rhodotorula toruloides, Rhodotorula glutinis, Cutaneotrichosporonoleaginosus and Cutaneotrichosporon cutaneum. This article provides an overview of lipid production by oleaginous yeasts focusing on yeast diversity, metabolism, and other microbiological issues related to the toxicity and tolerance to multiple challenging stresses limiting bioprocess performance. This is essential knowledge to better understand and guide the rational improvement of yeast performance either by genetic manipulation or by exploring yeast physiology and optimal process conditions. Examples gathered from the literature showing the potential of different oleaginous yeasts/process conditions to produce oils for biodiesel from agro-forestry and industrial organic residues are provided.

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Lignocellulosic biomass (LB) is an abundant and renewable raw material that has been explored to produce several added-value compounds. The recalcitrant structure of LB requires preliminary pretreatments to release fermentable sugars, which are used as a carbon source by microbial species. However, some undesired side-products with antimicrobial activity are also formed.

The tolerance of Yarrowia strains to grow in YPD media containing each one of these compounds (acetic acid, formic acid, furfural, and 5-HMF) was demonstrated in 96-well microplate batch cultures. The growth of selected strains (Y. lipolytica W29 and NCYC 2904) was not inhibited in flask batch cultures in a medium containing all the above compounds and glucose (20 gL-1 or 40 gL-1). In bioreactor batch cultures, 35 % (w/w) and 42 % (w/w) lipids content were attained by Y. lipolytica W29 and NCYC 2904, respectively. Maximum lipids contents were not affected when both yeast strains were cultivated in a synthetic medium mimicking eucalyptus bark hydrolysate (glucose 50 gL-1, xylose 7.6 gL-1, acetic acid 5.6 gL-1, formic acid 0.56 gL-1, 5-HMF 0.22 gL-1), although a slight decrease in lipid productivity was observed. Furthermore, the lipids content of Y. lipolytica NCYC 2904 increased to 51 % (w/w), corresponding to 6.3 g·L-1, using a fed-batch culture mode.

These results demonstrate the potential of Y. lipolytica W29 and NCYC 2904 strains to use lignocellulosic biomass hydrolysates without detoxification steps to obtain lipid-rich biomass, which has application in the bioenergy field.

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In this work, microbial lipid production from non-detoxified Eucalyptus bark hydrolysate (EBH) with oleaginous xylose utilizing Ashbya gossypii strains was explored. The best producing strain from a set of engineered strains was identified in synthetic media mimicking the composition of the non-detoxified EBH (SM), the lipid profile was characterized, and yeast extract and corn steep liquor (CSL) were pinpointed as supplements enabling a good balance between lipid accumulation, biomass production, and autolysis by A. gossypii. The potential of the engineered A. gossypii A877 strain to produce lipids was further validated and optimized with minimally processed inhibitor-containing hydrolysate and high sugar concentration and scaled up in a 2 L bioreactor. Lipid production from non-detoxified EBH supplemented with CSL reached a lipid titer of 1.42 g/L, paving the way for sustainable single-cell oil production within the concept of circular economy and placing lipids as an alternative by-product within microbial biorefineries.

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The reverse water–gas shift reaction (RWGS) allows the conversion of CO2 to CO which, mixed with H2, forms syngas, the feedstock of most chemicals and synthetic fuels production. Consequently, it is crucial to develop efficient catalysts for this reaction. To further the development of RWGS catalysts, Cu-based catalysts supported on pristine CNTs and on composites of pristine and functionalized CNTs : ZnO were prepared. ZnO's presence in the catalyst's structure proved to be beneficial, as the CO2 conversion and CO yield reached 49.0% whereas the catalysts supported on pristine CNTs only achieved a CO2 conversion and CO yield of 17.6%, at a temperature of 600 °C. The N-doping of CNTs further improved the CO2 conversion and CO yield to 54.8%, remaining stable at least for 93 h.

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Biomass gasification produces syngas composed mainly of hydrogen, carbon monoxide, carbon dioxide, methane, water, and higher hydrocarbons, till C4, mainly ethane. The hydrocarbon content can be upgraded into richer hydrogen streams through the steam reforming reaction. This study assessed the steam reforming process at the thermodynamic equilibrium of five streams, with different compositions, from the gasification of three different biomass sources (Lignin, Miscanthus, and Eucalyptus). The simulations were performed on Aspen Plus V12 software using the Gibbs energy minimization method. The influence of the operating conditions on the hydrogen yield was assessed: temperature in the range of 200 to 1100 °C, pressures of 1 to 20 bar, and steam-to‑carbon (S/C) molar ratios from 0 (only dry reforming) to 10. It was observed that operating conditions of 725 to 850 °C, 1 bar, and an S/C ratio of 3 enhanced the streams' hydrogen content and led to nearly complete hydrocarbon conversion (>99%). Regarding hydrogen purity, the stream obtained from the gasification of Lignin and followed by a conditioning phase (stream 5) has the highest hydrogen purity, 52.7%, and an hydrogen yield of 48.7%. In contrast, the stream obtained from the gasification of Lignin without any conditioning (stream 1) led to the greatest increase in hydrogen purity, from 19% to 51.2% and a hydrogen yield of 61.8%. Concerning coke formation, it can be mitigated for S/C molar ratios and temperatures >2 and 700 °C, respectively. Experimental tests with stream 1 were carried out, which show a similar trend to the simulation results, particularly at high temperatures (700–800 °C).

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Currently, the most common catalysts for CO2 methanation reaction are based on Ni supported on metal oxides. However, such catalysts require high operation temperatures and present stability issues, which have been tackled by the use of expensive metal oxides such as CeO2 or ZrO2. In this study, the decrease in the amount of ZrO2 in ZrO2-based catalysts was addressed through the preparation of composites of ZrO2 and carbon materials (activated carbon (AC) and carbon nanotubes (CNTs)). The optimization of the carbon:ZrO2 ratio demonstrated an optimal value of 50:50 in the case of AC:ZrO2, and 70:30 for CNT:ZrO2. With this composite composition, the possibility of enhancing the performance of the catalyst by functionalizing the carbon material was evaluated, and it was demonstrated that reduced AC (AC-R) with increased Lewis basic sites showed the best performance for AC-based composites, achieving a CO2 conversion of 79 % and CH4 selectivity of 98.9 % at 400 °C, whereas the catalysts supported on the pristine CNT:ZrO2 composite presented the highest CO2 conversion of 82.1 % and CH4 selectivity of 99.3 % at 400 °C. Notably, promotion with Fe was studied in the best performing support (CNT:ZrO2 (70:30) and it was shown that it enabled an improvement in terms of CO2 conversion and optimal temperature, achieving a CO2 conversion of 85 % and CH4 selectivity of 99.5 % at a lower temperature of 370 °C. This catalyst demonstrated to be highly stable for 70 h of time on stream with no apparent modifications on its chemical composition or microstructure. This work demonstrates that combining the properties of carbon materials and ZrO2 can be an interesting approach to obtain high performing catalysts for CO2 methanation.

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The use of hydrogen (H2) as an energy carrier may have an important role in the near future. Nowadays, H2 is mostly produced from fossil fuels, thus, it is urgent to prove the viability of alternative and sustainable processes to obtain H2. Gasification of renewable sources like biomass that are widely available, inexpensive and carbon-neutral is considered a viable technical, economic and environmental alternative for H2 production. The first step of biohydrogen (bio-H2) production through biomass gasification is the production of syngas (gasification gas). Syngas cleaning and upgrading procedures are essential to remove undesirable compounds and to increase bio-H2 content. Steam reforming and water gas shift reactions are also crucial to increase bio-H2 content in syngas. Next, bio-H2 separation from other gaseous compounds is needed to finally obtain bio-H2 at the required purification level. Although some of the mentioned technologies are proven and available on the market for similar applications, some drawbacks and technological challenges for the overall process need to be overcome to improve the overall techno-economic feasibility. This paper aims to analyse and discuss different bio-H2 production routes from biomass gasification to c the most promising ones.