Subcritical H2O Research Articles

Valorization of Elderberry Pomace using Emerging High-Pressure Technologies: Separation of Compounds with Different Polarities using Subcritical H2O, Supercritical CO2, and CO2/H2O/EtOH Mixture

Valorization of Elderberry Pomace using Emerging High-Pressure Technologies: Separation of Compounds with Different Polarities using Subcritical H2O, Supercritical CO2, and CO2/H2O/EtOH Mixture

Formation path and kinetics of caking components from modified lignite in subcritical H2O–CO system

ABSTRACT Modification reaction of lignite in subcritical H2O–CO system can significantly increase its caking index to more than 90. Separating and extracting the caking components from modified lignite, exploring the formation path of caking components, and establishing a kinetic model for the generation of these components are of significant importance for lignite hydrogenation reaction control. The lignite modified reaction was carried out in subcritical D2O–CO systems using the isotope tracer method. The formation path of caking components was defined by solvent extraction–IRMS–GPC combination technology. The results show that the soluble substances of tetrahydrofuran (TS) and benzene (BS) are the main caking components causing the structural transformation of lignite (A). The separation of n-hexane soluble (NS) also has an effect on the caking index of the modified coal. In the hydrogenation caking component formation stage, the lignite pyrolysis fragments combine with active hydrogen generated by water gas shift reaction to form caking substances, that is, A → TS + BS + NS. The formation activation energy of caking components is in the range of 0.39–58.24 kJ/mol. In the polycondensation caking component conversion stage, three soluble substances are converted in the order of TS → BS → NS, and the activation energy for the formation of caking components is 31.710–57.616 kJ/mol.

Supercritical CO2 and Subcritical H2O Analysis Instrument: Automated Lipid Analysis for In Situ Planetary Life Detection.

The search for extraterrestrial extant or extinct life in our Solar System will require highly capable instrumentation and methods for detecting low concentrations of biosignatures. This paper introduces the Supercritical CO2 and Subcritical H2O Analysis (SCHAN) instrument, a portable and automated system that integrates supercritical fluid extraction (SFE), supercritical fluid chromatography (SFC), and subcritical water extraction coupled with liquid chromatography. The instrument is compact and weighs 6.3 kg, making it suitable for spaceflight missions to planetary bodies. Traditional techniques, such as gas chromatography-mass spectrometry (MS), face challenges with involatile and thermally labile analytes, necessitating derivatization. The SCHAN instrument, however, eliminates the need for derivatization and cosolvents by utilizing neat supercritical CO2 with water as an additive. This SFE-SFC-MS method gives efficient lipid biosignature separations with median detection limits of 10 pg/g (ppt) for fatty acids and 50 pg/g (ppt) for sterols. Several free fatty acids and cholesterol were among the detected peaks in biologically lean samples from the Atacama Desert, demonstrating the instrument's potential for in situ life detection missions. The SCHAN instrument addresses the challenges of conventional systems, offering a compact, portable, and spaceflight-compatible tool for the analysis of organics for future astrobiology-focused missions.

Catalyst-free synthesis of 5-hydroxymethylfurfural from fructose by extractive reaction in supercritical CO2 – subcritical H2O two-phase system

An extractive reaction configuration using supercritical carbon dioxide (scCO2) as the extracting solvent was tested for the production of 5-hydroxymethyl furfural (HMF) from a 5 wt% fructose aqueous feed. In this configuration, extraction of HMF by scCO2 prevents HMF degradation in the aqueous phase. Because of water co-extraction by scCO2, the volume of the reactional mixture was maintained by continuous injection of water. Reaction was operated in a 90 mL high pressure reactor, where an HMF maximum yield of 62.4 % was achieved at 160 °C and 25 MPa, with a CO2 flow rate of 20 g.min−1 for 420 min. This is the first time that HMF is reportedly produced with such a yield by a catalyst-and organic solvent-free process. Besides, the separation efficiency reached 97.3 % and the relative purity of HMF in the extract was 95.8 wt%. Therefore, this configuration avoids post reactional purification which is needed in conventional batch processes or in extractive reaction processes using organic solvents. Based on kinetic and thermodynamic studies, modeling of the extractive reaction process was developed to perform a sensitivity analysis for CO2 flow rate and extraction efficiency, upon the HMF yield. As an example, it was shown that for 800 min reaction duration, a CO2 flow rate of 100 g.min−1 or an extraction efficiency increase by a 10-fold factor could theoretically led to HMF yields of 73.0 % and 73.7 %, respectively.

Production of xylooligosaccharides from Camellia oleifera Abel fruit shell using a shell-based solid acid catalyst

This study aimed to produce xylooligosaccharides (XOS) from Camellia oleifera Abel fruit shell (CFS) using a shell-based solid acid derived from CFS (CFS-BSA). CFS-BSA preparation was optimized by incomplete carbonization at 450 °C for 1 h, followed by sulfonation at 130 °C for 8 h to yield a -SO3H functional group concentration of 1.04 mmol/g. When CFS-BSA was used to hydrolyze CFS with a 1:5 ratio of CFS-BSA to CFS at 170 °C for 20 min, a maximum XOS yield (X2-X5) of 51.41 % was achieved, which was notably higher than when using subcritical H2O solely. CFS-BSA can be recycled and reused at least six times by sieving without a substantial loss in its catalytic activity. CFS-BSA can also be used to produce XOS from other lignocellulosic materials such as corncob (41.04 %), sugarcane bagasse (45.03 %), corn stalk (45.89 %), birchwood (46.05 %), and poplar (40.10 %).

Production of 5-Hydroxymethylfurfural from Fructose by Extractive Reaction in Supercritical CO2 – Subcritical H2O Two-Phase System

Production of 5-Hydroxymethylfurfural from Fructose by Extractive Reaction in Supercritical CO2 – Subcritical H2O Two-Phase System

Efficient and green production of manno-oligosaccharides from Gleditsia microphylla galactomannans using CO2 and solid acid in subcritical water

This study aimed to produce manno-oligosaccharides (MOS) from Gleditsia microphylla galactomannans (GMG) using CO2 and solid acid (Amberlyst-35) in subcritical water. The optimal condition for MOS preparation was 3 MPa CO2, 0.1 g/g solid acid (relative to GMG) at 150 °C for 40 min. The maximum MOS yield with a degree of polymerization from 2 to 4 (M2-M4) was 52.19%, which doubled the yield of MOS compared to either using solely CO2 or solid acid. Solid acid showed excellent performance in producing MOS under subcritical H2O–CO2 condition, due to the enhanced mass transfer efficiency and increased H+ concentration in the reaction system. The solid acid can be easily separated and reused. Comparing with traditional methods used to produce MOS, this approach has many merits such as higher galactomannan hydrolysis efficiency (largely reduced time and higher MOS yield), purer M2-M4 product, lower costs, and more environmental-friendly.

Sequence of supercritical CO2 extraction and subcritical H2O extraction for the separation of tobacco waste into lipophilic and hydrophilic fractions

A tobacco solid waste was extracted by a sequence of supercritical CO2 (SC-CO2) extraction and subcritical water extraction (SWE). The tobacco waste lipophilic compounds (targeted fatty acids, free base nicotine and VOCs) were extracted using the series of the main operation parameters for the SC-CO2 extraction (pressure: 100–300bar, temperature: 40–80°C). The obtained SC-CO2 extracts were rich in nicotine (0.362–1.585%), neophytadiene (0.082–0.256%) and 4,8,13-duvatriene-1,3-diol (0.144–0.590%). Other VOCs and targeted fatty acids (palmitic, stearic, oleic, linoleic and linolenic) were also determined. Optimized condition for the SC-CO2 extraction of tobacco waste was found at 300bar and 61.22°C. The residues (after SC-CO2 extraction) were re-extracted with SWE under the solvent/solid ratio: 28mL/g, temperature: 150°C, time: 23min. SWE extracts (hydrophilic fractions) were characterized by high concentrations of nicotine (1.61–4.67%), nicotinamide (0.198–0.428%), nicotinic acid (0.118–0.239%), phenolic compounds: chlorogenic acid (0.458–0.740%), 3,4-dihydroxybenzoic acid (0.236–0.403%), neochlorogenic acid (0.240–0.534%), and rutin (0–1.025%). They contained total phenols in the amount 8.03–10.11% and exhibited DPPH antiradical scavenging activity in the range 77.89 to 93.35%. As the results of applied higher temperature 5-HMF (0.076–0.11%), furfural (0.510–0.740%) and 5-MF (0.0292–0.0453%) were also detected in SWE extracts. In some experiments, the obtained values of detected compounds were considerable higher than those obtained from the raw scrap (without SC-CO2 extraction pretreatment) implying that two-stage extraction process (SC-CO2 extraction followed by SWE) can enhance the extraction efficiency due to the elimination of fats during SC-CO2 extraction which enables better dissolution of the other compounds in subcritical H2O.

Efficient hydrothermal deoxygenation of tall oil fatty acids into n-paraffinic hydrocarbons and alcohols in the presence of aqueous formic acid

Hydrothermal deoxygenation of tall oil fatty acids (TOFA) was investigated in the presence of aqueous formic acid (0.5–7.5 wt%) as a H2 donor in the presence of subcritical H2O pressure (569–599 K). Pd and Ru nanoparticles supported on carbon (5% Pd/CSigma, 5% Ru/CSigma, 10% Pd/CO850_DP, and 5% Ru/COPcomm_DP) were found to be efficient catalysts for deoxygenation of TOFA. The reaction pathway was mainly influenced by the concentration of formic acid and the catalyst. In case of Pd catalysts, in the presence of 0–2.5 wt% formic acid, decarboxylation was the dominant pathway producing n-paraffinic hydrocarbons with one less carbon atom (heptadecane yield up to 94 wt%), while with 5–7.5% formic acid, a hydrodeoxygenation/hydrogenation mechanism was favored producing C18 deoxygenation products octadecanol and octadecane as the main products (yields up to 70 wt%). In contrast, Ru catalysts produced a mixture of C5-C20 (n-and iso-paraffinic) hydrocarbons via decarboxylation, cracking and isomerization (up to 58 wt% C17 yield and total hydrocarbon yield up to 95 wt%) irrespective of formic acid concentration. Kinetic studies showed that the rates of deoxygenation displayed Arrhenius type behavior with apparent activation energies of 134.44 ± 31.36 kJ/mol and 148.92 ± 3.66 kJ/mol, for the 5% Pd/CSigma and 5% Ru/CSigma catalyst, respectively. Furthermore, the experiments with glycerol tristearate, rapeseed oil, sunflower oil, rapeseed biodiesel, and hydrolyzed rapeseed oil produced identical products confirming the versatility of the aforementioned catalytic systems for deoxygenation of C18 feedstocks.

Supercritical CO2–subcritical H2O system: A green reactive separation medium for selective conversion of glucose to 5-hydroxymethylfurfural

The supercritical CO2 (scCO2)-subcritical H2O (subH2O) system offers a metal- and additive-free reaction solvent that can be used for organic synthesis. The enhanced formation of carbonic acid and ionization of water under elevated pressures and temperatures promote the production of hydronium and hydroxyl ions that can catalyze the conversion of a number of organic compounds. Furthermore, the biphasic nature of the system allows for the simultaneous extraction of less polar products from the aqueous solution by a flowing scCO2 stream. In this study, we investigated the use of scCO2-subH2O as a reactive separation medium for the conversion of glucose to 5-hydroxymethylfurfural (HMF). The highest HMF yield achieved was 32.6 % with a selectivity of 52.7 %. These values corresponded to a 20.25 % and 41.9 % increase in the yield and selectivity that were achieved in the experiments performed under batch operations. The improvement was primarily attributed to the extraction of some of the HMF from the solution. The highest separation efficiency so far was 12.7 %. We also performed semi-empirical quantum calculations to determine a possible mechanism of the production of HMF from glucose. The two-step process, which involved the isomerization of glucose to fructose and the dehydration of fructose to HMF, was shown to proceed with only the OH– and H3O+ that are inherent in the solvent system as catalysts. Insights from both kinetic data and quantum calculations revealed that the reaction is highly favored in the acidic environment of the scCO2-subH2O system.

An overview of supercritical fluid extraction in Chinese herbal medicine: From preparation to analysis

A review concerning the use of supercritical fluid extraction (SFE) in the preparation and analysis of Chinese herbal medicine (CHM) is presented. The literature review foresees the trend of increasing use of SFE in CHM preparation. The application examples in the preparation of useful ingredients and analysis of pesticide residues are discussed. The use of SFE CO2 to replace traditional organic solvent is justified and promising. Consideration is given to the coupling of sub-critical H2O and supercritical CO2 to extract more compounds and to use the dual role of extracting useful ingredients and removing pesticide residues. Careful integration of laboratory SFE results into the design and implementation of factory production is beneficial in ensuring the successful use of SFE in CHM.

Preparation of Activated Carbons Originated from Orange Peel Waste by Subcritical H2O Activation Method

Low-cost and environmental friendly activated carbons were synthesized from orange peel waste by carbonization followed by activation process using subcritical water. The carbonization process of orange peel waste was conducted in the electrical furnace at temperature of 800°C for 2 h. Activation process of the impregnated orange peel was carried out in the tubular furnace for 1 h at activation temperature of 200°C and pressure variation of 120, 170 and 220 bar. Activated carbon with highest surface area of 491 m2/g was obtained by water pressure of 170 bar. The activated carbons were then utilized as adsorbents for removal of methylene blue (MB) from aqueous solution. The batch adsorption study was carried out by varying the initial concentration of MB solution (2, 4, 6, 8 and 10 ppm). Experimental results showed that the adsorption kinetic of MB fitted the pseudo-second-order rate equation, whereas for the adsorption isotherm model followed two models i.e. the Dubinin-Radushkevich and Freundlich model. The adsorption mechanism was found to be governed by the intraparticle and surface diffusion mechanism.

Structural Evolution and Formation Mechanisms of Caking Components of Modified Lignite in Subcritical H2O–CO Systems

A new method for the alkali catalytic modification of lignite in subcritical H2O–CO systems is presented, which can greatly improve its caking properties (caking index, >90). Soxhlet extraction tec...

Improvement of the sequential extraction procedure based on supercritical CO2 and subcritical H2O solvents for the estimation of the environmentally mobile potentially toxic element fractions of sediments and soils

The estimation of environmental risk caused by pollution with potentially toxic elements (PTE) is usually carried out using the (3+1) step sequential extraction procedure suggested in 1993 by the Community Bureau of Reference (BCR). In the 1st step the water-soluble, exchangeable and carbonate-bound element content is extracted with acetic acid. In 2002 a fractionation procedure based on the application of supercritical CO2, subcritical H2O and of a mixture of subcritical H2O/CO2 was proposed, which allowed the water-soluble and carbonatebound element contents to be extracted separately from sediment or soil samples weighed into the preparative column of a supercritical fluid extractor and diluted with quartz sand in a mass ratio of 1:20. The aim of the present study was to develop a new reduced-size column construction with which this dilution rate could be decreased to 1:2. A kinetic study was performed to determine the extraction time necessary for samples with different carbonate contents and the extracted element contents were compared to the results of the BCR sequential procedure on the same samples. It was established that fractionation using the reduced-size column may be a rapid way to obtain more reliable information on the easily mobilizable (watersoluble and carbonate-bound) PTE content of soils and sediments than was previously available to supplement BCR fractionation.

Process analysis of hydrothermal carbonization of corn Stover with subcritical H2O

This work investigates the influence of temperature on the performance of hydrothermal carbonization of corn Stover between 175 and 250 °C, 240 min, 2.0 °C/min, and biomass-to-water ratio of 1:10. The performance analyzed for yields, energy densification and recovery. Aqueous phase analyzed for pH, acetic acid equivalent (HAc), HPLC, and GC, solid phase for dry and organic matter, ash, and elemental analysis. Solid yield (62.92–35.82%) increases with temperature, showing an inflection region between 200 and 225 °C, while that of liquid (35.43–54.59%) decreases in a similar behavior, and gas yield (1.49–9.59%) increases exponentially. GC identified alcohols, carboxylic acids, and furans. The concentration of furfural and Hidroxymethylfurfural (HMF) decreases exponentially and as a sigmoid with temperature, being not detectable at 250 °C. Carbon content increases, while oxygen and hydrogen decreases with temperature. The high heating values (HHV) increase, and the energy efficiency decreases with temperature.

Production and Characterization of Energy Materials with Adsorbent Properties by Hydrothermal Processing of Corn Stover with Subcritical H2O

This work aims to investigate the effect of temperature on the process performance of hydrothermal processing (HTC) of corn Stover with subcritical H2O and on the morphology of solid products. The experiments were carried out at 200, 225 and 250 ºC, reaction time of 240 minutes, heating rate of 2.0 ºC/min, and biomass to water ratio of 1:10, using a pilot scale stirred tank reactor (STR) of 5 gallon, operating in batch mode. The process performance analyzed by computing the yields of solid and liquid reaction products (RLP). The aqueous phase (H2O + RLP) was physicochemical analyzed for pH and total carboxylic acids, expressed as total acetic acid content. The chemical compositions of carboxylic acids, furfural, and hydroxymethylfurfural (HMF) in the aqueous phase determined by GC-MS and HPLC. The results showed solid yields ranging from 57.39 to 35.82% (wt.), and liquid reaction products (RLP) yields ranging from 39.53 to 54.59% (wt.). The solid phase products were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). The chemically activated (2.0 M NaOH) solid phase energy material obtained by HTC at 250 °C, applied as adsorbent to investigate the capacity and/or efficiency to adsorb acetic acid from 1.0 to 4.0 g/L model solutions at 25 °C. The solid phase yield decreases along with the temperature, showing an inflection region between 200 and 225 °C, whereas a drastic change takes place, while that of liquid phase increases, showing also a drastic change between 200 and 225 °C. The total acetic acid content of aqueous phase varied from 4064 to 5387 mg/L, while the pH from 3.77 to 3.91. The GC analysis identified the presence of volatile carboxylic acids, particularly acetic acid, in concentrations between 4020 and 5040 mg/L. HPLC identified the presence of furfural and hydroxymethylfurfural, whose concentrations decrease exponentially and linearly along with the temperature between 686.7 and 0.0, and 443.9 and 0.0 mg/L, respectively, being both compounds not detectable at 250 °C. The elemental/ultimate analysis of solid products shows that carbon content increases, while the oxygen and hydrogen contents decrease, along with the temperature. The H/C and O/C ratios decrease linearly as process temperature increases, and the high heating value (HHV) of solid reaction products, an energy densified material, changes sharply between 200 and 225 °C, showing an increase with temperature. The SEM, EDX, and XDR indicates a change on the morphology and mineralogical phases present in solid reaction products with temperature, particularly at 250 °C. The activated solid phase has proven to be very selective to adsorb acetic acid, showing that recovery of acetic acid from hydrothermal carbonization/liquefaction aqueous solutions is feasible by using a multistage-stage adsorption process in series.

Effective conversion of maize straw wastes into bio-hydrogen by two-stage process integrating H2 fermentation and MECs.

The enhanced H2 production from maize straw had been achieved through the two-stage process of integrating H2 fermentation and microbial electrolysis cells (MECs) in the present work. Several key parameters affecting hydrolysis of maize straw through subcritical H2O were optimized by orthogonal design for saccharification of maize straw followed by H2 production through H2 fermentation. The maximum reducing sugar (RS) content of maize straw reached 469.7 mg/g-TS under the optimal hydrolysis condition with subcritical H2O combining with dilute HCl of 0.3% at 230 °C. The maximum H2 yield, H2 production rate, and H2 content was 115.1 mL/g-TVS, 2.6 mL/g-TVS/h, and 48.9% by H2 fermentation, respectively. In addition, the effluent from H2 fermentation was used as feedstock of MECs for additional H2 production. The maximum H2 yield of 1060 mL/g-COD appeared at an applied voltage of 0.8 V, and total COD removal reached about 35%. The overall H2 yield from maize straw reached 318.5 mL/g-TVS through two-stage processes. The structural characterization of maize straw was also carefully investigated by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) spectra.

Effect of solvent on the hydrothermal liquefaction of macro algae Ulva fasciata

Hydrothermal liquefaction is an attractive approach for the conversion of aquatic biomass like algae as it does not require the energy intensive drying steps. The objective of the study is to understand the effect of various solvents (H2O, CH3OH and C2H5OH) on product distribution and nature of products of hydrothermal liquefaction of macro algae Ulva fasciata (MAUF). Hydrothermal liquefaction of MAUF was performed using subcritical H2O (300°C) as well as supercritical organic solvents CH3OH and C2H5OH (300°C). The use of alcoholic solvents significantly increased the bio-oil yield. The bio-oil yield was 44% and 40% in case of liquefaction with CH3OH and C2H5OH respectively whereas the bio-oil yield was 11% with H2O. Use of alcoholic solvents converted the acids obtained in bio-oil to the corresponding methyl and ethyl esters. 1H NMR data showed that use of alcoholic solvents (C2H5OH and CH3OH) increased aliphatic content of bio-oil1 (ether/methanol/ethanol fraction). FTIR and SEM results showed the difference in the bio residue obtained using alcoholic solvents and H2O. The results showed that liquefaction with supercritical alcohols is an effective way to produce functional hydrocarbons for chemical feedstock.

Subcritical Water Reactions of Lignin-Related Model Compounds with Nitrogen, Hydrogen, Carbon Monoxide, and Carbon Dioxide Gases

Experiments using subcritical H2O (365 °C) and various industrial gases (500 psi cold pressure of N2, H2, CO, or CO2) have been considered previously for the conversion of Organosolv lignin (representative of lignin-derived compounds generated from the production of bioethanol from woody biomass sources) into value-added products. The findings indicated that subcritical H2O can be used to depolymerize lignin into GC amenable compounds with or without added gas pressures, though adding gas further modified the types of products generated. However, the roles of the combined effect of subcritical H2O and the gases were not confirmed, as the reaction chemistry for lignin depolymerization at these conditions is not intuitive. Thus, experiments with subcritical H2O, the aforementioned industrial gases and lignin-related model compounds (i.e., aromatic aldehydes represented by vanillin and syringaldehyde, aromatic ketones represented by acetovanillone and acetosyringone, and aromatic ethers represented by dibenzyl ether and 2-phenethyl phenyl ether), were completed to study this. From these results, the suggested reaction pathway of Organosolv lignin reactions in subcritical H2O with and without added pressure appears to proceed in two stages. The first stage appears to be a hydrolysis reaction that breaks the ether linkages between lignin aromatic monomers with typical methoxy and hydroxyl functional groups and either an aldehyde or ketone (i.e., syringaldehyde, acetophenone). A second stage appears to remove the aldehyde/ketone so that the main products are methoxy phenols.

Subcritical Water Reactions of a Hardwood Derived Organosolv Lignin with Nitrogen, Hydrogen, Carbon Monoxide, and Carbon Dioxide Gases

Subcritical H2O at 365 °C is considered for lignin conversion, because H2O exhibits unusual properties at higher temperatures (i.e., decreased ion product and static dielectric constant), such that there is a high solubility for organic compounds. This high solubility for organic compounds is expected to apply to lignin for its conversion into high value transportation fuels, which may prove the effectiveness of integrated biorefineries. Experiments were conducted with hardwood derived Organosolv lignin, subcritical H2O (defined here as H2O at 365 °C and autogenous pressure), and various industrial gases (N2, H2, CO, and CO2 at a cold pressure of 500 psi) for 30 min to determine both lignin’s potential to generate value-added products (e.g., monomer compounds and methanol) without the need for a catalyst and the roles (if any) of the H2O and the gases in the reactions. The behavior of H2O at temperature (365 °C) and pressure within this research is expected to be similar to the behavior of supercritical H2O (374 °C and 3205 psi), without the need to maintain supercritical conditions. Different characterization techniques were used for the products collected including primarily gas chromatography with flame ionization detection and thermal conductivity detection (GC/FID-TCD) of the evolved gases, GC/MS analysis of the organic liquids, solid phase microextraction analysis of the recovered H2O, and solid state 13C NMR analysis of the solid residues. The reactor pressure at temperature was shown to influence the outcome of products, and the highest conversions (≈54–62%) were obtained when adding gas. The collected solids from the N2, H2, and CO reactions appeared to be the most reacted (i.e., the most changed from the unreacted lignin) according to solid state 13C NMR analysis, and the widest variety of products (methoxy-substituted phenolic compounds) were also obtained when using CO, according to GC/MS analysis.