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 ∼ -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 βO4 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.

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.

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.

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.

An Interesting Class of Porous Polymer--Revisiting the Structure of Mesoporous α-D-Polysaccharide Gels

The processes involved in the transformation of non-porous, native polysaccharides to their highly porous equivalents introduce significant molecular complexity and are not yet fully understood. In this paper, we propose that distinct changes in polysaccharide local short-range ordering promotes and directs the formation of meso- and micro-pores, which are investigated here using N2 sorption, FTIR, and solid-state (13)C NMR. It is found that an increase in the overall double helical amylose content, and their local association structures, are responsible for formation of the porous polysaccharide gel phase. An exciting consequence of this local ordering change is elegantly revealed using a (19)F NMR experiment, which identifies the stereochemistry-dependent diffusion of a fluorinated chiral probe molecule (1-phenyl-2,2,2-trifluoroethanol) from the meso- to the micro-pore region. This finding opens opportunities in the area of polysaccharide-based chiral stationary phases and asymmetric catalyst preparation.

PCDDs, PCDFs and PCNs in products of microwave-assisted pyrolysis of woody biomass--Distribution among solid, liquid and gaseous phases and effects of material composition

Microwave-assisted pyrolysis (MAP) of lignocellulosic biomass is a technique that could potentially be used to produce and upgrade renewable energy carriers. However, there is no available information about the formation of dioxins and other organic pollutants in MAP treatment of woody biomass. In this study, MAP experiments were conducted in lab-scale using virgin softwood, bark, and impregnated wood as feedstocks. The non-condensable gas, liquid (fractionated into aqueous and oil phases), and char fractions generated during pyrolysis were collected and analysed for polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs) and naphthalenes (PCNs). The concentrations of PCDDs, PCDFs and PCNs in the pyrolysis products ranged from 0.52 to 43.7 ng kg(-1). All investigated compound groups were most abundant in the oil fraction, accounting for up to 68% (w/w) of the total concentrations. The highest PCDD, PCDF and PCN concentrations were found from the pyrolysis of bark, which has relatively high contents of chlorine and mineral matter, followed by impregnated wood, which contains organic and metal-based preservatives. The homologue profiles of all three compound groups were dominated by the less chlorinated homologues. The homologue abundance decreased as the degree of chlorination increased. This trend was observed for all three feedstocks.

Processed Lignin as a Byproduct of the Generation of 5-(Chloromethyl)furfural from Biomass: A Promising New Mesoporous Material

The lignin by-product of the conversion of lignocellulosic biomass to 5-(chloromethyl)furfural (CMF) has been characterised by thermogravimetric analysis, N2 physisorption porosimetry, attenuated internal reflectance IR spectroscopy, elemental analysis and solid-state NMR spectroscopy. The lignin (LCMF) has a moderate level of mesoporosity before thermal treatment and a surface area of 63 m(2)  g(-1) , which increases dramatically on pyrolysis at temperatures above 400 °C. An assessment of the functionality and textural properties of the material was achieved by analysing LCMF treated thermally over a range of pyrolysis temperatures. Samples were sulfonated to test their potential as heterogeneous acid catalysts in the esterification of levulinic acid. It was shown that unpyrolysed catalysts gave the highest ester yields of up to 93 %. To the best of our knowledge, this is the first example of mesoporous lignin with an appreciable surface area that is produced directly from a bio-refinery process and with further textural modification of the material demonstrated.

A sustainable freeze-drying route to porous polysaccharides with tailored hierarchical meso- and macroporosity

Bio-derived polysaccharide aerogels are of interest for a broad range of applications. To date, these aerogels have been obtained through the time- and solvent-intensive procedure of hydrogel fomation, solvent exchange, and scCO2 drying, which offers little control over meso/macropore distribution. A simpler and more versatile route is developed, using freeze drying to produce highly mesoporous polysaccharide aerogels with various degrees of macroporosity. The hierarchical pore distribution is controlled by addition of different quantities of t-butanol (TBA) to hydrogels before drying. Through a systematic study an interesting relationship between the mesoporosity and t-butanol/water phase diagram is found, linking mesoporosity maxima with eutectic points for all polysaccharides studied (pectin, starch, and alginic acid). Moreover, direct gelation of polysaccharides in aqueous TBA offers additional time savings and the potential for solvent reuse. This finding is a doorway to more accessible polysaccharide aerogels for research and industrial scale production, due to the widespread accessibility of the freeze drying technology and the simplicity of the method.

Other Approaches and the Commercialisation of Sustainable Carbonaceous Material Technology

To conclude the book, this chapter aims to provide the reader with an overview of a number of developing approaches to the production of porous carbons from sustainable precursors. Discussion will focus predominantly on the production of carbon-based materials from bacterial cellulose, lignins, tannins and finally to examine the possibility of employing ionic liquids. The relative merits of the approaches discussed will also be highlighted. The use of the resulting carbons synthesised based on these approaches in applications including energy storage, energy generation and purification/remediation will also be briefly discussed. Finally, the chapter will conclude with an overview of the latest developments regarding the commercialisation of the approaches to the synthesis of porous carbons from sustainable precursors discussed in this book will also be provided.

From Polysaccharides to Starbons®

Many commercially employed carbon materials are typically hydrophobic, chemically inert and microporous. Therefore, with an eye to the future, there is a need to develop new, carbon-based porous materials, the properties of which can be easily tuned to address the catalytic and separation challenges of future energy and chemical provision schemes (e.g. the Methanol Economy or Biorefinery schemes). In this regard, the synthesis of such materials must be conducted in as sustainable a manner as possible, ideally providing a flexible platform upon which to tailor properties such as functionality, porosity at different length scales (e.g. micro-, meso-, and macroporosity), hydrophilic character and macrophology (e.g. monoliths, particulates, etc.) amongst others. This chapter therefore aims to introduce one top-down synthetic approach to this end, the Starbon® materials concept. An accompanying material development history will be provided followed by a review of the variety of interesting functionally rich, highly mesoporous, high surface area (e.g. &gt; 0.5 cm3 g–1; &gt; 200 m2 g–1) carbonaceous materials that are accessible via the development of porous polysaccharide-derived materials and their subsequent carbonaceous derivatives. The chapter intends to provide the reader with an overview of the exciting opportunities that are open to the carbon materials chemist based on the discussed synthetic approach.

Bulk and Surface Analysis of Carbonaceous Materials

It is difficult to fully characterise the surface chemistry and properties of the complex materials that are carbons. These can range from amorphous-based activated carbons to organised graphene, carbon nanotubes and other forms. However, a combination of techniques, such as, TG supplemented by TGIR, XPS and Boehm titration, bromination with various solid-state NMR methodologies can permit a comprehensive understanding of both their bulk and surface characteristics. The application of these techniques in the characterisation of both the bulk and surface features of carbon-based materials will be presented and discussed ADDIN EN.REFLIST .

Sustainable carbon materials

Carbon-based structures are the most versatile materials used in the modern nanotechnology. Therefore there is a need to develop increasingly more sustainable variants of carbon materials.

Microwave assisted extraction as an important technology for valorising orange waste

Kinetics of microwave-assisted d-limonene extraction demonstrates a five-fold increase in yield over conventional extraction in a two stage process.

NMR and IR study of fluorobenzene and hexafluorobenzene adsorbed on alumina

The adsorption of fluorobenzene (C6H5F) and hexafluorobenzene (C6F6) onto the surface of neutral alumina is investigated by reflectance IR spectroscopy, near-IR spectroscopy, and measurement of 19F NMR chemical shift values. Chemical shifts are dependent on surface coverage and reveal multiple peaks where different adsorption environments occur. C6H5F appears to be adsorbed through polar interactions with surface hydroxyls, whereas C6F6 shows separate resonances for the first layer of coverage and outer layers. Available surface areas are estimated, and reorientation of adsorbed hexafluorobenzene is proposed to account for chemical shift behavior and differences between calculated and measured monolayer coverage.

19F Magic angle spinning NMR reporter molecules: empirical measures of surface shielding, polarisability and H-bonding

Magic Angle Spinning (MAS) (19)F NMR spectra have been obtained and chemical shifts measured for 37 molecules in the gas phase and adsorbed on the surfaces of six common materials: octadecyl- and octyl-functionalised chromatography silicas, Kieselgel 100 silica, Brockmann neutral alumina, Norit activated charcoal and 3-(1-piperidino)propyl functionalised silica. From these six surfaces, octadecyl-silica is selected as a non-polar reference to which the others are compared. The change in chemical shift of a fluorine nucleus within a molecule on adsorption to a surface from the gas phase, Deltadelta(gas)(surface), is described by the empirical relationship: Deltadelta(gas)(surface) = delta(s) + (alpha(s)+pi(s))/alpha(r) (Deltadelta(gas)(reference) - delta(r)) + delta(HBA) + delta(HBD), where delta(s) and delta(r) are constants that describe the chemical shift induced by the electromagnetic field of the surface under investigation and reference surface, alpha(s) and alpha(r) are the relative surface polarisability for the surface and reference, pi(s) is an additional contribution to the surface polarisabilities due to its ability to interact with aromatic molecules, and delta(HBA) and delta(HBD) are measurements of the hydrogen acceptor and donor properties of the surface. These empirical parameters are measured for the surfaces under study. Silica and alumina are found to undergo specific interactions with aromatic reporter molecules and both accept and donate H-bonds. Activated charcoal was found to have an extreme effect on shielding but no specific interactions with the adsorbed molecules. 3-(1-Piperidino)propyl functionalised silica exhibits H-bond acceptor ability, but does not donate H-bonds.

Surface energy and surface area measurements by 19F MAS NMR of adsorbed trifluoroacetic acid

Trifluoroacetic acid, when adsorbed on the surface of inorganic materials, is a useful (19)F NMR probe molecule for studying surface properties including surface energy and surface area.

Chemical reactions of double bonds in activated carbon: microwave and bromination methods

The reaction of localised C[double bond, length as m-dash]C bonds on the surface of activated carbons has been shown to be an effective method of chemical modification especially using microwave-assisted reactions.