Enhanced microwave assisted pyrolysis of waste rice straw through lipid extraction with supercritical carbon dioxide.

A combination of supercritical carbon dioxide (scCO2) extraction and microwave-assisted pyrolysis (MAP) have been investigated for the valorisation of waste rice straw. ScCO2 extraction of rice straw led to a 0.7% dry weight yield of lipophilic molecules, at elevated temperatures of 65 °C and pressures of 400 bar. Lipid compositions (fatty acids, fatty alcohol, fatty aldehydes, steroid ketones, phytosterols, n-alkanes and wax esters) of the waxes obtained by scCO2 were comparable to those obtained Soxhlet extraction using the potentially toxic solvent n-hexane. ScCO2 extraction positively influenced the pyrolysis heating rate, with a rate of 420 K min-1 for particles of 500-2000 µm, compared to 240 K min-1 for the same particle size of untreated straw. Particle size significantly affected cellulose decomposition and the distribution of pyrolysis products (gaseous, liquid and char), highlighting the importance of selecting an adequate physical pre-treatment. TG and DTG of the original rice straw and resulting biochar produced indicated that cellulose was completely decomposed during the MAP. While a rapid pressure change occurred at ∼120 °C (size > 2000 µm) and ∼130 °C (size 500-2000 µm) during MAP and was associated with the production of incondensable gas during cellulose decomposition, this takes place at significantly lower temperatures than those observed with conventional pyrolysis, 320 °C. Wax removal by scCO2 influences the dielectric properties of the straw, enhancing microwave absorption with rapid heating rates and elevated final pyrolysis temperatures, illustrating the benefits of combining these sustainable technologies within a holistic rice straw biorefinery.

Efficient adsorption of bulky reactive dyes from water using sustainably-derived mesoporous carbons.

Hazardous reactive dyes can cause serious environmental problems, as they are difficult to remove from water using conventional adsorbents due to their large molecular sizes and bulky structures. Sustainable mesoporous carbons derived from alginic acid demonstrated promising adsorbent capacity for several representative industrial bulky reactive dye molecules that account for almost 30% of the global textile dye market: Procion Yellow H-XEL (PY), Remazol Black (RB), Procion Crimson H-XEL (PC) and Procion Navy H-XEL (PN). These new adsorbents showed high mesoporosity (>90%) and large pore diameters (>20 nm) facilitating more straightforward and efficient adsorption and desorption processes when compared with predominately microporous activated carbon (AC), Norit, of similar surface chemistry, or with Silica gel (Sgel) that shows good mesoporosity but is hydrophilic. Their adsorption capacity was also significantly higher than that of both AC and Sgel, verifying suitability for bulky dye elimination from wastewater. Adsorption kinetic studies showed a best fit with the Elovich model, indicating a heterogeneous surface adsorption process. The adsorption isotherm data was best represented via the Toth model for almost all adsorbent/dye systems (R2 ≥ 0.98), validating the results of the Elovich model whereby the adsorbent is structurally heterogenous with multilayer dye coverage. From thermodynamic analysis, the derived parameters of ΔG (-11.6 âˆ¼ -6.2 kJ/mol), ΔH and ΔS demonstrate a spontaneous, enthalpy controlled adsorption process that was exothermic for RB (-10.0 kJ/mol) and PC (-23.9 kJ/mol) and endothermic for PY (3.9 kJ/mol) and PN (13.2 kJ/mol). Overall these alginic acid based mesoporous carbons are cost-effective, sustainable and efficient alternatives to current predominantly microporous adsorbent systems.

Rapid and efficient adsorption of methylene blue dye from aqueous solution by hierarchically porous, activated starbons®: Mechanism and porosity dependence.

Hierarchically porous activated Starbons® derived from starch are found to make excellent adsorbents for methylene blue, even in the presence of other dyes and inorganic salts, highlighting their potential to be used in water purification. The optimal material (S950C90) has a methylene blue adsorption capacity (891 mg g-1) almost nine times higher than that of unactivated S800 and four times higher than that of commercial activated carbon at 298 K. The adsorption of methylene blue onto optimal materials (S950C90 and S800K4) reaches equilibrium within 5 min. Adsorption data for all the adsorbents show a good fit to the Freundlich isotherm which allows the Gibbs free energies of adsorption to be calculated. The adsorption capacities increase as the pH of the methylene blue solution increases, allowing the dye to be desorbed by treatment with acidic ethanol and the Starbon® materials reused. Porosimetry and SEM-EDX imaging indicate that methylene blue adsorbs throughout the surface and completely fills all the micropores in the Starbon® adsorbent. The methylene blue adsorption capacities show excellent correlations with both the BET surface areas and the micropore volumes of the materials.

Electrical conductivity of beech sawdust using graphite catalytic coating: unlocking the microwave-assisted thermolysis efficiency of lignocellulosic biomass.

The coating of the beech sawdust using a catalytic amount of graphite (as low as 0.25 wt%) allowed a step improvement in the microwave-assisted thermolysis. Results demonstrated that the pyrolysis performance was linked to an electrical conductivity threshold of the coated samples rather than a gradual increase. With as low as 0.13 mS m-1 of electrical conductivity, the 0.75 wt% graphite coated sawdust (250-500 µm) was efficiently gasified with up to 43 wt% of gas (30 wt% of carbon monoxide, 25 vol% of hydrogen). Initial particle size impacted the thermolysis performance where optimal size (250-500 µm) provided high heat homogeneity due to efficient graphite coating and low temperature gradient between the outer and inner part of the sawdust. The small initial particle size (75-250 µm) was unsuitable for microwave pyrolysis, exhibiting a too large surface area for efficient coating with 0.75 wt% of graphite which was confirmed by the absence of electrical conductivity (<0.003 mS m-1). The electrical conductivity can be used as a marker to evaluate the suitability of the sample for microwave-assisted pyrolysis. Unlike simple graphite mixing, the mechanical coating allowed more than 20-fold decrease of susceptor quantity, providing more homogeneous samples with higher reproducibility.

Microwave-assisted catalytic depolymerization of lignin from birch sawdust to produce phenolic monomers utilizing a hydrogen-free strategy.

Catalytic hydrogenolysis of lignin to obtain value-added phenolic chemicals is a sustainable and cost-effective strategy for the efficient valorization of biomass derived wastes. Herein, an innovative approach by using a single-step microwave assisted depolymerization of lignin from birch sawdust without external hydrogen in the mixture of water-alcohol (methanol, ethanol, isopropanol) co-solvents over commercial catalysts (Pd/C, Pt/C, Ru/C) was investigated. A 65 wt% yield of phenolic monomers was obtained based on 43.8 wt% of delignification (190 °C, 3 h). The solid residues retained 92.0 wt% of cellulose and 57.3 wt% of hemicellulose, which could be further used for fermentation or in the pulp industry. Analysis of the lignin oil revealed that in-situ hydrogen generated from methanol decomposition promoted the hydrogenolysis of ßO4 ether linkage and selective hydrogenation of unsaturated side-chains of phenolic monomers. This work introduces new perspectives for the efficient and cost-effective production of value-added phenolic compounds from lignin in agro-industrial wastes without external hydrogen assisted by microwave heating.

Recent Advances in the Catalytic Depolymerization of Lignin towards Phenolic Chemicals: A Review.

The efficient valorization of lignin could dictate the success of the 2nd generation biorefinery. Lignin, accounting for on average a third of the lignocellulosic biomass, is the most promising candidate for sustainable production of value-added phenolics. However, the structural alteration induced during lignin isolation is often depleting its potential for value-added chemicals. Recently, catalytic reductive depolymerization of lignin has appeared to be a promising and effective method for its valorization to obtain phenolic monomers. The present study systematically summarizes the far-reaching and state-of-the-art lignin valorization strategies during different stages, including conventional catalytic depolymerization of technical lignin, emerging reductive catalytic fractionation of protolignin, stabilization strategies to inhibit the undesired condensation reactions, and further catalytic upgrading of lignin-derived monomers. Finally, the potential challenges for the future researches on the efficient valorization of lignin and possible solutions are proposed.

Synthesis and application of tuneable carbon-silica composites from the microwave pyrolysis of waste paper for selective recovery of gold from acidic solutions.

Microwave pyrolysis bio-oil from waste paper and K60 silica gel has successfully been utilised to synthesise mesoporous carbon-silica composites with uniquely tuneable surface properties, where functionality and structural characteristics can be altered and even enhanced by curing at different temperatures. This temperature-dependence resulted in composites ranging from highly oxygenated polymerised bio-oil composites at 300 °C to aromatic carbonaceous materials covering the silica surface at 800 °C, making them attractive materials for gold recovery from mining wastewater. The composite materials exhibit exceptional ability and selectivity to recover gold from dilute solutions. Metal adsorption on the surface of these composites proceeded via both chemisorption and physisorption leading to the reduction of Au(iii) to Au(0), resulting in high recovery capacities for gold. Composite material prepared at 500 °C demonstrated the optimum combination of surface functionality and porosity, allowing for an adsorption capacity of 320 mg g-1 of gold and with 99.5% removal being achieved at concentrations mimicking those of real-life mine tailing wastes. All materials pioneered in this research display great potential as selective adsorbents for the recovery of gold from acidic media.

Selective Microwave-Assisted Pyrolysis of Cellulose towards Levoglucosenone with Clay Catalysts.

Levoglucosenone (anhydrosugar) is one of the most promising chemical platforms derived from the pyrolysis of biomass. It is a chiral building block for pharmaceuticals as well as an intermediate in the production of solvents and polymers. Therefore, the development of cost-efficient, low-energy production methods is vital for a future sustainable biorefinery. Here, a novel, green approach to the production of levoglucosenone was developed by using a microwave (MW)-assisted pyrolysis of cellulose in the presence of readily available clays. It was shown that natural and pillared clays in the presence of MW irradiation significantly increase the yield of levoglucosenone from cellulose. Both the water content and the presence of acid centres are critical characteristics that influence the yield and distribution of catalysed products. A unique experiment was designed by using a synergetic effect between different types of catalysts, which enhanced the levoglucosenone yield to 12.3 wt % with 63 % purity.

Production and characterization of lignin and cellulose fractions obtained from pretreated vine shoots by microwave assisted alkali treatment.

This work deals with the optimization of the second stage of a biorefinery scheme to separate simultaneously cellulose and lignin from hydrothermally pre-treated vine shoots. For this, the suitability of the microwave-assisted alkaline delignification was assessed and optimized through a Box-Wilson experimental design. The optimum conditions (150 °C, 6 wt% NaOH, 30 min) allowed maximizing the lignin removal (82 wt%) and minimizing the loss of the cellulose (35 wt%) present in the pre-treated vine shoots. A thorough characterization of the two fractions obtained at optimum conditions was performed: the cellulose rich solid was analyzed by XRD and FTIR and the lignin was subjected to HPSEC, Py/GC-MS, 13C- and 1H NMR. This purposed second stage would allow performing an integral biorefinery with low energy requirements and environmentally friendly conditions. This approach aligns with the circular economy and the zero waste production philosophies, promoting the sustainable development.

Geminal Diol of Dihydrolevoglucosenone as a Switchable Hydrotrope: A Continuum of Green Nanostructured Solvents.

The addition of water to dihydrolevoglucosenone (Cyrene) creates a solvent mixture with highly unusual properties and the ability to specifically and efficiently solubilize a wide range of organic compounds, notably, aspirin, ibuprofen, salicylic acid, ferulic acid, caffeine, and mandelic acid. The observed solubility enhancement (up to 100-fold) can be explained only by the existence of microenvironments mainly centered on Cyrene's geminal diol. Surprisingly, the latter acts as a reversible hydrotrope and regulates the polarity of the created complex mixture. The possibility to tune the polarity of the solvent mixture through the addition of water, and the subsequent generation of variable amounts of Cyrene's geminal diol, creates a continuum of green solvents with controllable solubilization properties. The effective presence of microheterogenieties in the Cyrene/water mixture was adequately proven by (1) Fourier transform infrared/density functional theory showing Cyrene dimerization, (2) electrospray mass-spectrometry demonstrating the existence of dimers of Cyrene's geminal diol, and (3) the variable presence of single or multiple tetramethylsilane peaks in the 1H NMR spectra of a range of Cyrene/water mixtures. The Cyrene-water solvent mixture is importantly not mutagenic, barely ecotoxic, bioderived, and endowed with tunable hydrophilic/hydrophobic properties.

Unexpected nitrile formation in bio-based mesoporous materials (Starbons®).

The bio-based mesoporous materials made from polysaccharides, Starbons® can be modified by two different routes to give high levels of N-content, unexpectedly including significant quantities of nitrile groups which can improve the materials performance in applications including metal capture.

Fast microwave-assisted acidolysis: a new biorefinery approach for the zero-waste utilisation of lignocellulosic biomass to produce high quality lignin and fermentable saccharides.

Generally, biorefineries convert lignocellulosic biomass into a range of biofuels and further value added chemicals. However, conventional biorefinery processes focus mainly on the cellulose and hemicellulose fractions and therefore produce only low quality lignin, which is commonly burnt to provide process heat. To make full use of the biomass, more attention needs to be focused on novel separation techniques, where high quality lignin can be isolated that is suitable for further valorisation into aromatic chemicals and fuel components. In this paper, three types of lignocellulosic biomass (softwood, hardwood and herbaceous biomass) were processed by microwave-assisted acidolysis to produce high quality lignin. The lignin from the softwood was isolated largely intact in the solid residue after acidolysis. For example, a 10 min microwave-assisted acidolysis treatment produced lignin with a purity of 93% and in a yield of 82%, which is superior to other conventional separation methods reported. Furthermore, py-GC/MS analysis proved that the isolated lignin retained the original structure of native lignin in the feedstock without severe chemical modification. This is a large advantage, and the purified lignin is suitable for further chemical processing. To assess the suitability of this methodology as part of a biorefinery system, the aqueous phase, produced after acidolysis of the softwood, was characterised and assessed for its suitability for fermentation. The broth contained some mono- and di-saccharides but mainly contained organic acids, oligosaccharides and furans. While this is unsuitable for S. cerevisiae and other common ethanol producing yeasts, two oleaginous yeasts with known inhibitor tolerances were selected: Cryptococcus curvatus and Metschnikowia pulcherrima. Both yeasts could grow on the broth, and demonstrated suitable catabolism of the oligosaccharides and inhibitors over 7 days. In addition, both yeasts were shown to be able to produce an oil with a similar composition to that of palm oil. This preliminary work demonstrates new protocols of microwave-assisted acidolysis and therefore offers an effective approach to produce high purity lignin and fermentable chemicals, which is a key step towards developing a zero-waste lignocellulosic biorefinery.

Polysaccharide-derived mesoporous materials (Starbon®) for sustainable separation of complex mixtures.

The recovery and separation of high value and low volume extractives are a considerable challenge for the commercial realisation of zero-waste biorefineries. Using solid-phase extractions (SPE) based on sustainable sorbents is a promising method to enable efficient, green and selective separation of these complex extractive mixtures. Mesoporous carbonaceous solids derived from renewable polysaccharides are ideal stationary phases due to their tuneable functionality and surface structure. In this study, the structure-separation relationships of thirteen polysaccharide-derived mesoporous materials and two modified types as sorbents for ten naturally-occurring bioactive phenolic compounds were investigated. For the first time, a comprehensive statistical analysis of the key molecular and surface properties influencing the recovery of these species was carried out. The obtained results show the possibility of developing tailored materials for purification, separation or extraction, depending on the molecular composition of the analyte. The wide versatility and application span of these polysaccharide-derived mesoporous materials offer new sustainable and inexpensive alternatives to traditional silica-based stationary phases.

Subtle Microwave-Induced Overheating Effects in an Industrial Demethylation Reaction and Their Direct Use in the Development of an Innovative Microwave Reactor.

A systematic study of the conventional and microwave (MW) kinetics of an industrially relevant demethylation reaction is presented. In using industrially relevant reaction conditions the dominant influence of the solvent on the MW energy dissipation is avoided. Below the boiling point, the effect of MWs on the activation energy Ea and k0 is found nonexistent. Interestingly, under reflux conditions, the microwave-heated (MWH) reaction displays very pronounced zero-order kinetics, displaying a much higher reaction rate than observed for the conventionally thermal-heated (CTH) reaction. This is related to a different gas product (methyl bromide, MeBr) removal mechanism, changing from classic nucleation into gaseous bubbles to a facilitated removal through escaping gases/vapors. Additionally, the use of MWs compensates better for the strong heat losses in this reaction, associated with the boiling of HBr/water and the loss of MeBr, than under CTH. Through modeling, MWH was shown to occur inhomogeneously around gas/liquid interfaces, resulting in localized overheating in the very near vicinity of the bubbles, overall increasing the average heating rate in the bubble vicinity vis-à-vis the bulk of the liquid. Based on these observations and findings, a novel continuous reactor concept is proposed in which the escaping MeBr and the generated HBr/water vapors are the main driving forces for circulation. This reactor concept is generic in that it offers a viable and low cost option for the use of very strong acids and the managed removal/quenching of gaseous byproducts.

Importance of Micropore-Mesopore Interfaces in Carbon Dioxide Capture by Carbon-Based Materials.

Mesoporous carbonaceous materials (Starbons®) derived from low-value/waste bio-resources separate CO2 from CO2 /N2 mixtures. Compared to Norit activated charcoal (AC), Starbons® have much lower microporosities (8-32 % versus 73 %) yet adsorb up to 65 % more CO2 . The presence of interconnected micropores and mesopores is responsible for the enhanced CO2 adsorption. The Starbons® also showed three-four times higher selectivity for CO2 adsorption rather than N2 adsorption compared to AC.

Mechanistic evaluation of polychlorinated dibenzo-p-dioxin, dibenzofuran and naphthalene isomer fingerprints in microwave pyrolysis of biomass.

Isomer distribution patterns of polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs) and naphthalenes (PCNs) were investigated in microwave-assisted pyrolysis (MAP) products of woody biomass. The feedstocks included bark and impregnated wood. The results indicated that isomer distributions in MAP are more selective compared to those reported from wood burning and waste incineration. Favored formation of 4-MoCDF and highly selective chlorine substitution at the 2,4-position observed during MAP suggested a preferred formation pathway of PCDFs involving (chloro)phenol precursors followed by subsequent chlorination. The PCDD distribution was dominated by isomers typically formed from chlorophenol condensation at relatively low temperature. The PCN isomer distributions showed a tendency for sequential chlorination from non-substituted naphthalene at successive positions. The presence of isomers such as 1-MoCDD, 4-MoCDF, 1,2,3-TriCN with low thermodynamic stability indicates that kinetic factors may be important in the MAP process.