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This work investigated the catalytic high-pressure CO

The depletion of fossil resources makes the transition towards renewable ones more urgent. For this purpose, the synthesis of strategic platform-chemicals, such as 5-hydroxymethylfurfural (HMF), represents a fundamental challenge for the development of a feasible bio-refinery. HMF perfectly deals with this necessity, because it can be obtained from the hexose fraction of biomass. Thanks to its high reactivity, it can be exploited for the synthesis of renewable monomers, solvents, and bio-fuels. Sustainable HMF synthesis requires the use of waste biomasses, rather than model compounds such as monosaccharides or polysaccharides, making its production more economically advantageous from an industrial perspective. However, the production of HMF from real feedstocks generally suffers from scarce selectivity, due to their complex chemical composition and HMF instability. On this basis, different strategies have been adopted to maximize the HMF yield. Under this perspective, the properties of the catalytic system, as well as the choice of a suitable solvent and the addition of an eventual pretreatment of the biomass, represent key aspects of the optimization of HMF synthesis. On this basis, the present review summarizes and critically discusses the most recent and attractive strategies for HMF production from real feedstocks, focusing on the smartest catalytic systems and the overall sustainability of the adopted reaction conditions.

5-Hydroxymethyl-2-furaldehyde (HMF) is one of the top 12 biobased chemicals, precursor of renewable monomers and biofuels. This key molecule can be obtained from the dehydration of monosaccharides ...

Coffee is one of the largest consumed drinks all over the world, generating a very significant amount of wastes during the beans processing. Among these by-products, coffee silverskin is a very interesting waste material, being a promising source of bioactive compounds, including valuable polyphenols as chlorogenic acids and caffeine. In order to valorize these valuable components, they should be isolated from the starting matrix through suitable processes, but the current conventional extraction procedures are not suitable. On the contrary, microwave-assisted extraction results particularly efficient, nowadays finding many applications in various fields. On this basis, the aim of the present research is the investigation and the optimization of the solid-liquid extraction of valuable antioxidant polyphenols from coffee silverskin under microwave irradiation. The influence of the main reaction parameters, such as temperature, extraction time and composition of the solvent, consisting of a green binary mixture of water/ethanol, has been assessed, also employing statistical modeling. Maximum yield in total polyphenols of about 45 mgGAE/g, expressed as mg of gallic acid equivalent for g of starting biomass, was experimentally achieved, a promising value, thus leading to an important step forward, in the perspective of improving this waste recycling in a circular economy perspective., Proceedings of the 29th European Biomass Conference and Exhibition, 26-29 April 2021, Online, pp. 699-707

Up to now, on the industrial scale, the acid-catalysed hydrothermal conversion of lignocellulosic biomass has been targeted to the production of levulinic acid (LA), while the lignin fate has been neglected, and its use has been limited at most to the energy recovery. Now, an integrated investigation of the hydrothermal process for the synthesis of LA starting from giant reed (Arundo Donax L.) was studied taking into account the lignin phenol derivatives present in the liquid phase and in the solid hydrochar. The analysis of the hydrochar was carried out adopting coupled pyrolysis techniques, e.g. Py-GC/MS, TGA/FTIR, and EGA-MS, paying a special attention to the contribution of the evolved simple phenolics. The hydrochar washing fractions were analyzed for the total phenolic content (TPC) and antioxidant activity (AA), adopting TEAC, FRAP, and DPPH standard essays, comparing the antioxidant activities with those deriving from the starting LA-rich mother liquor. Two washing cycles allowed the complete recovery of the phenolic compounds of interest, leached from the porous hydrochar. Also the starting LA-rich liquor was of great interest for the same purpose, having the highest content of these compounds, which must be necessarily removed for the production of the commercial pure LA. Promising antioxidant properties were ascertained for the mother liquor and the hydrochar washings, obtaining good linear correlations between total phenolic compounds (TPC) and antioxidant capacity (TEAC, FRAP, and DPPH). This multi-valorisation approach contributes to improving the sustainability of the entire LA process and shifts the attention towards the antioxidants, new niche bioproducts of great interest, which add significant economic value to the overall Biorefinery of the lignocellulosic biomass.

Hydrolysis and butanolysis of lignocellulosic biomass are efficient routes to produce two valuable bio-based platform chemicals, levulinic acid and n-butyl levulinate, which find increasing applications in the field of biofuels and for the synthesis of intermediates for chemical and pharmaceutical industries, food additives, surfactants, solvents and polymers. In this research, the acid-catalyzed hydrolysis of the waste residue of Cynara cardunculus L. (cardoon), remaining after seed removal for oil exploitation, was investigated. The cardoon residue was employed as-received and after a steam-explosion treatment which causes an enrichment in cellulose. The effects of the main reaction parameters, such as catalyst type and loading, reaction time, temperature and heating methodology, on the hydrolysis process were assessed. Levulinic acid molar yields up to about 50 mol % with levulinic acid concentrations of 62.1 g/L were reached. Moreover, the one-pot butanolysis of the steam-exploded cardoon with the bio-alcohol n-butanol was investigated, demonstrating the direct production of n-butyl levulinate with good yield, up to 42.5 mol %. These results demonstrate that such residual biomass represent a promising feedstock for the sustainable production of levulinic acid and n-butyl levulinate, opening the way to the complete exploitation of this crop.

5-hydroxymethylfurfural (HMF), accessible from various feedstocks, represents an important renewable platform-chemical, precursor for valuable biofuels and bio-based chemicals. In this work, the continuous hydrogenation of an aqueous solution of HMF to give strategic monomers, 2,5-bis(hydroxymethyl)furan (BHMF) and 2,5-bis(hydroxymethyl)tetrahydrofuran (BHMTHF) was investigated in a continuous flow reactor adopting a commercial Ru/C (5 wt%) as catalyst. The influence of the main process variables on products yield and selectivity was studied and optimized. The highest BHMF and BHMTHF yields of 87.9 and 93.7 mol%, respectively, were achieved by tuning the catalyst contact time, keeping all other variables constant (temperature, pressure, hydrogen flow rate, initial HMF concentration). Intraparticle diffusion limitation for hydrogen and HMF was shown to occur at some of the tested conditions by performing the HMF hydrogenation with different catalyst particle sizes, confirmed by calculations. Constant catalyst activity was observed up to 6 h time-on-stream and then gradually reduced. Fresh and spent catalyst characterization showed no significant sintering and negligible leaching of ruthenium during time-on-stream. A decrease of the specific surface area was observed, mainly due to humin deposition which is likely the reason for catalyst deactivation. Catalyst performance could be restored to initial values by a thorough washing of the catalyst.

5-hydroxymethylfurfural (HMF) is one of the most important renewable platform-chemicals, a very valuable precursor for the synthesis of bio-fuels and bio-products. In this work, the hydrogenation of HMF to two furan diols, 2,5-bis(hydroxymethyl)furan (BHMF) and 2,5-bis(hydroxymethyl)tetrahydrofuran (BHMTHF), both promising renewable monomers, was investigated. Three commercial catalysts, Ru/C, Pd/C and Pt/C, were tested in the hydrogenation of aqueous HMF solutions (2–3 wt%), using a metal loading of 1 wt% respect to HMF content. By appropriate tuning of the process conditions, either BHMF or BHMTHF were obtained in good yields, and Ru/C resulted the best catalyst for this purpose, allowing us to obtain BHMF or BHMTHF yields up to 93.0 and 95.3 mol%, respectively. This catalyst was also tested for in the hydrogenation of a crude HMF-rich hydrolyzate, obtained by one-pot the dehydration of fructose. The influence of each component of this hydrolyzate on the hydrogenation efficiency was investigated, including unconverted fructose, rehydration acids and humins, in order to improve the yields towards each furan diol. Moreover, ICP-OES and TEM analysis showed that the catalyst was not subjected to important leaching and sintering phenomena, as further confirmed by catalyst recycling study.

Improvement of processes starting from renewable feedstocks to produce chemicals and biofuels up to now derived from fossil carbon feedstocks is paramount to creating environmentally sustainable industrial processes. Guerbet condensation of ethanol to give n-butanol represents a turning point not only respect to the traditional petrochemical “oxo” process, but also to the bio-chemical path (Acetone Butanol Ethanol “ABE” process). In this work, different Cu/Mg/Al hydrotalcite-derived mixed oxides catalysts, with Cu atomic content in the range 7.6–1.0%, were synthetised, characterized, and tested for the Guerbet reaction. The adopted catalysts are easy to produce, and based on available, cheap and non-toxic metals. Tailoring Cu-content in hydrotalcite-derived Cu/Mg/Al mixed oxides allowed us to selectively address this reaction towards n-butanol formation, highlighting how a low Cu-content favours the condensation to n-butanol and higher alcohols, in particular n-hexanol, decreasing the ester formation. The Guerbet selective co-production of [n-butanol + n-hexanol] is economically even more attractive than that of the sole n-butanol, being both saleable products. Good catalytic performances to n-butanol and n-hexanol were achieved, in a batch process carried out under subcritical conditions. These catalysts have required a simple procedure of recovery, showing good recyclability, in term of catalytic performances to give the target products, thus opening the way to applicative perspectives.

5-Hydroxymethyl-2-furaldehyde (HMF) is a key bio-based platform for the production of renewable monomers and bio-fuels. However, most of its syntheses are carried out under not sustainable conditions. In this work, the production of HMF from fructose and inulin was investigated following the Green Chemistry principles, adopting aqueous medium, appreciable substrate concentration (10 wt%), low loading of heterogeneous acid catalyst (niobium or zirconium phosphate) and microwave heating. Both the catalysts resulted very active and promising, in particular zirconium phosphate and the performances were related to their different acid characteristics. The optimization of HMF synthesis with zirconium phosphate was also supported by a statistical modelling, which shows that the highest yield to HMF (about 40 mol%) is ascertained at high temperature (190 °C) and short reaction time (8 min). The catalysts resulted recyclable maintaining their starting activity almost unchanged.

Recycled paper needs a lot of mechanical/chemical treatments for its re-use in the papermaking process. Some of these ones produce considerable rejected waste fractions, such as "screen rejects", which include both cellulose fibers and non-fibrous organic contaminants, or "stickies", these last representing a shortcoming both for the papermaking process and for the quality of the final product. Instead, the accepted fractions coming from these unit operations become progressively poorer in contaminants and richer in cellulose. Here, input and output streams coming from mechanical screening systems of a papermaking plant using recycled paper for cardboard production were sampled and analyzed directly and after solvent extraction, thus confirming the abundant presence of styrene-butadiene rubber (SBR) and ethylene vinyl acetate (EVA) copolymers in the output rejected stream and cellulose in the output accepted one. Despite some significant drawbacks, the "screen reject" fraction could be traditionally used as fuel for energy recovery within the paper mill, in agreement with the integrated recycled paper mill approach. The waste, which still contains a cellulose fraction, can be also exploited by means of the hydrothermal route to give levulinic acid, a platform chemical of very high value added.

The characterization of the solid residue coming from the acid hydrolysis of giant reed (Arundo Donax L.) herbaceous biomass was performed and compared with the characteristics of two commercial technical lignins, i.e. a soda and a sulphate (kraft) one, in order to ascertain their differences, and thus better address their successive possible applications. The investigated hydrolysis residue derived from an hydrothermal treatment with dilute hydrochloric acid under harsh conditions, addressed to the production of levulinic acid, and contains both lignin and humin units. These last components are not present in the compared lignins that are industrially obtained by different extraction processes. A. Donax L. hydrolysis residue still contains aromatic hydroxyl groups, and carbonyl moieties deriving from lignin and carbohydrates degradation reactions, whilst the aliphatic hydroxyl and methoxyl groups were almost completely degraded. The elemental composition of this residue is similar to that of brown coal, highlighting that biomass carbonization has occurred. The solubilisation of the A. Donax L. hydrolysis residue was investigated in different solvents: only a little amount (∼10 wt%) was dissolved. The condensed nature of the hydrolysis residue was confirmed by thermogravimetric analysis. This approach allows the contemporary production of levulinic acid and the recovery of the A. Donax L. hydrolysis residue, hitherto considered only the waste fraction of the hydrothermal process, thus making this last a promising starting material for future green applications.

Alkyl levulinates (ALs) represent outstanding bio-fuels and strategic bio-products within the context of the marketing of levulinic acid derivatives. However, their synthesis by acid-catalyzed esterification of pure levulinic acid, or by acid-catalyzed alcoholysis of furfuryl alcohol, although relatively simple, is still economically disadvantageous, due to the high costs of the pure precursors. The direct one-pot alcoholysis of model C6 carbohydrates and raw biomass represents an alternative approach for the one-step synthesis of ALs. In order to promote the market for these bio-products and, concurrently, the immediate development of new applications, it is necessary to speed up the intensification of their production processes, and this important achievement is onlypossible by using low-cost or, even better, waste biomasses, as starting feedstocks. This review provides an overview of the most recent and promising advances on the one-pot production of ALs from model C6 carbohydrates and real biomasses, in the presence of homogeneous or heterogeneous acid catalysts. The use of model C6 carbohydrates allows for the identification of the best obtainable ALs yields, resulting in being strategic for the development of new smart catalysts, whose chemical properties must be properly tuned, taking into account the involved reaction mechanism. On the other hand, the transition to the real biomass now represents a necessary choice for allowing the next ALs production on a larger scale. The improvement of the available synthetic strategies, the use of raw materials and the development of new applications for ALs will contribute to develop more intensified, greener, and sustainable processes.

Nowadays, the solvothermal conversion of biomass has reached a good level of development, and now it is necessary to improve the process intensification, in order to boost its further growth on the industrial scale. Otherwise, most of these processes would be limited to the pilot scale or, even worse, to exclusive academic investigations, intended as isolated applications for the development of new catalysts. For this purpose, it is necessary to improve the work-up technologies, combining, where possible, reaction/purification unit operations, and enhancing the feedstock/liquid ratio, thus improving the final concentration of the target product and reducing the work-up costs. Furthermore, it becomes decisive to reconsider more critically the choice of biomass, solvent(s), and catalysts, pursuing the biomass fractionation in its components and promoting one-pot cascade conversion routes. Screening and process optimization activities on a laboratory scale must be fast and functional to the flexibility of these processes, exploiting efficient reaction systems such as microwaves and/or ultrasounds, and using multivariate analysis for an integrated evaluation of the data. These upstream choices, which are mainly of the chemist’s responsibility, are fundamental and deeply interconnected with downstream engineering, economic, and legislative aspects, which are decisive for the real development of the process. In this Editorial, all these key issues will be discussed, in particular those aimed at the intensification of solvothermal processes, taking into account some real case studies, already developed on the industrial scale.

Lignocellulosic biomass represents one of the most important feedstocks for future biorefineries, being a precursor of valuable bio-products, obtainable through both chemical and biological conversion routes. Lignocellulosic biomass has a complex matrix, which requires the careful development of multi-step approaches for its complete exploitation to value-added compounds. Based on this perspective, the present work focuses on the valorization of hemicellulose and cellulose fractionsof giant reed (Arundo donax L.) to give second-generation sugars, minimizing the formation of reaction by-products. The conversion of hemicellulose to xylose was undertaken in the presence of the heterogeneous acid catalyst Amberlyst-70 under microwave irradiation. The effect of the main reaction parameters, such as temperature, reaction time, catalyst, and biomass loadings on sugars yield was studied, developing a high gravity approach. Under the optimised reaction conditions (17 wt% Arundo donax L. loading, 160 &deg, C, Amberlyst-70/Arundo donax L. weight ratio 0.2 wt/wt), the xylose yield was 96.3 mol%. In the second step, the cellulose-rich solid residue was exploited through the chemical or enzymatic route, obtaining glucose yields of32.5 and56.2 mol%, respectively. This work proves the efficiency of this innovative combination of chemical and biological catalytic approaches, for the selective conversion of hemicellulose and cellulose fractions of Arundo donax L. to versatile platform products.

Eucalyptus globulus wood samples were subjected to preliminary aqueous processing to remove water-soluble extractives and hemicelluloses, and the resulting solid (mainly made up of cellulose and lignin) was employed as a substrate for converting the cellulosic fraction into mixtures of levulinic and formic acid through a sulfuric acid-catalyzed reaction. These runs were carried out in a microwave-heated reactor at different temperatures and reaction times, operating in single-batch or cross-flow modes, in order to identify the most favorable operational conditions. Selected liquid phases deriving from these experiments, which resulted in concentrated levulinic acid up to 408 mmol/L, were then employed for γ-valerolactone production by levulinc acid hydrogenation in the presence of the commercial 5% Ru/C catalyst. In order to assess the effects of the main reaction parameters, hydrogenation experiments were performed at different temperatures, reaction times, amounts of ruthenium catalyst and hydrogen pressure. Yields of γ-valerolactone in the range of 85–90 mol % were obtained from the hydrogenation of the wood-derived solutions containing levulinic acid, obtained by single-batch operation or by the cross-flow process. The negative effect of co-produced formic acid present in crude levulinic acid solutions was evidenced and counteracted efficiently by allowing the preliminary thermal decomposition of formic acid itself.

Levulinic acid is a platform chemical obtained from acid-catalyzed hydrothermal conversion of cellulose-rich biomass. The low amounts of solid biomass which can be handled in the reactor limit the levulinic acid concentration in the aqueous stream, making the economic viability of the aqueous phase process unsuitable for large scale applications. Now a novel approach to biphasic process has been proposed, where a mineral oil has been used as non-solvent for levulinic acid, thus concentrating it in the water phase, reducing the water volume to be processed downstream but at the same time maintaining enough liquid phase to sustain the slurry processability. The work has studied: i) the optimization of the biphasic hydrolysis of corn grain to levulinic acid; ii) the characterization of the recovered oil; iii) the evaluation of the energetic properties of the recovered hydrochar for its exploitation, thus smartly closing the biorefinery cycle.

A semiquantitative analysis by means of midinfrared FT-IR spectroscopy was tuned for the simultaneous determination of cellulose, hemicellulose, and lignin in industrial crops such as giant reed (Arundo donaxL.) and switchgrass (Panicum virgatumL.). Ternary mixtures of pure cellulose, hemicellulose, and lignin were prepared and a direct correlation area/concentration was achieved for cellulose and lignin, whereas indirect correlations were found for hemicellulose quantification. Good correspondences between the values derived from our model and those reported in the literature or obtained according to the official Van Soest method were ascertained. Average contents of 40–45% of cellulose, 20–25% of hemicellulose, and 20–25% of lignin were obtained for different samples of giant reed species. In the case of switchgrass, a content of 36% of cellulose, 28% of hemicellulose, and 26% of lignin was achieved. This analysis was also carried out on giant reed and switchgrass residues after a mild hydrolysis step carried out with dilute hydrochloric acid for the production of furfural with good yield. Reasonable compositional data were obtained, thus allowing an indirect monitoring which helps the optimization of the hydrothermal pretreatment for furfural production from hemicellulose fractions.

For the first time, the exploitation of hazelnut shells for the combined production of levulinic acid (LA) and hydrochar was investigated. The optimization of the catalytic hydrothermal treatment was performed both in autoclave and microwave reactor, approaching a maximum LA yield of ∼9-12wt%. Hydrochars recovered with high yield (∼43-47wt%) were characterized by different techniques, including elemental and proximate analysis, heating value, FT-IR, XPS, XRD, SEM-EDX, and SAA. Their "lignite-like" energetic properties make them suitable for the energy recovery within the same biorefinery plant for LA production, thus partially offsetting the cost of the entire process. Alternatively, since the synthesized hydrochars maintain high levels of oxygenated groups, they could be smartly exploited as natural sorbents for environmental applications. The proposed integrated approach makes possible to fully exploit this waste, smartly closing its biorefinery cycle in a sustainable development perspective.

Flexible polyurethane foams were prepared from solid waste residue derived from the hydrothermal acid treatment of the Arundo donax L. herbaceous biomass, which produced a very high yield of levulinic acid. An innovative, sustainable, and green liquefaction route was adopted to produce lignin-based flexible polyurethane foams by partially replacing fossil-fuel source polyols with an abundant and renewable hydroxyl source, the Arundo donax L. “lignin-like” residue. Lignin liquefaction was performed in polyolic solvents using microwave irradiation, saving time and energy while ensuring a more sustainable and green approach. Foam production was performed with controlled expansion using the “one-shot” technique. Water was adopted as the only blowing agent, and the isocyanate index (NCO/OH) was kept to less than 100, which reduced the cross-linking degree of the desired foam and increased its flexibility. About 7 wt.% of the conventional petrochemical polyether polyol was replaced with the Arundo donax L. hydrolysis residue. The chemical and mechanical properties of the synthesized foams were compared with those obtained by using a pure technical soda lignin, ProtoBind 1000. The results were characterized by satisfactory mechanical properties, thus closing the biorefinery cycle of Arundo donax L. exploitation.

5-Hydroxymethyl-2-furaldehyde (HMF) is a biomass-derived intermediate for the sustainable production of monomers and biofuels. However, most of its syntheses are performed under not environmentally and/or economically green conditions. In this work, the dehydration of fructose/inulin to HMF was optimized, employing aqueous medium, high substrate concentrations (10 and 20 wt%), low loading of commercial acid resin (Amberlyst-70) and microwave heating. The influence of substrate/catalyst ratio, time and temperature on the products yields was investigated. Amberlyst-70 resulted a very active and recyclable system: HMF yields up to 46 mol% were ascertained, the best result up to now reached under these reaction conditions.