Enzymatic treatment of brown coal following ionic liquid pretreatment

As a low-rank coal, lignite (brown coal) is usually well treated by chemical or enzymatic methods prior to its utilization for energy and materials. Following ionic liquids (ILs)-pretreatment, this study focuses on the treatment of lignite by oxidoreductase-type of enzymes including horseradish peroxidase (HRP), alcohol dehydrogenase (ADH), and laccase. We observed some synergistic effects of IL-pretreatment and enzymatic actions on lignite coal. The combination of IL-pretreatment and HRP-/ADH-treatment enabled smaller and more uniform coal particles than IL-pretreatment alone or that without HRP. However, laccase-treatment led to large coal aggregates likely due to the crosslinking effect. Both HRP- and ADH-treatments induced a lower aliphatic and a higher aromatic content of lignite, particularly when the coal was pretreated by an IL prior to the enzymatic process. Both ADH- and laccase-treatments produced more hydrogen bonds in the coal. Therefore, the combined treatments of lignite by ILs and oxidoreductases dedicate low-rank coal structures.</p

Elucidating Interactions Between Ionic Liquids and Polycyclic Aromatic Hydrocarbons by Quantum Chemical Calculations

Using quantum mechanical calculations performed at the density functional level of theory, the present study explores the binding energetics, orbital energies, and charge transfer behavior accompanying sorption of 12 different ionic liquids (ILs) onto 6 archetypal polyaromatic hydrocarbons (PAHs). The ILs were based on combinations of three different onium cations (i.e., 1-butyl-3-methylimidazolium, 1-butylpyridinium, 1-butyl-1-methylpyrrolidinium) paired with four common anions, that is, bromide, tetrafluoroborate, hexafluorophosphate, and bis­(trifluoromethylsulfonyl)­imide. In general, the size of the anion as well as interaction of the butyl side chain present on the cation with the paired anion exerted significant influence over the cation ring orientation with respect to the PAH surface. A smaller highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) energy band gap was observed for pyridinium-based ILs upon adsorption on the PAH surface in comparison to imidazolium and pyrrolidinium analogs, hinting at stronger interactions between PAHs and pyridinium ILs. Of the 12 ILs investigated, 1-butyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide displays the least favorable free energy of adsorption with PAHs whereas PAH interactions with 1-butyl-1-methylpyrrolidinium bis­(trifluoromethylsulfonyl)­imide are the most favored thermodynamically. Charges determined from a Mulliken population analysis were consistent with charge transfer (CT) from the IL to the PAH. On the contrary, charges determined via electrostatic potential using the more reliable grid based analysis method (i.e., CHELPG) suggested the reverse direction of CT from the PAH to the IL. The direction of the CT occurring from the HOMO of the PAH to the LUMO of the IL, as shown by CHELPG analysis, is consistent with the physical location of the orbitals and the negative shift in the Fermi energy level observed for the IL–PAH complex. A more favorable enthalpy of adsorption for ILs onto a PAH is observed with an increase in the size of the PAH considered. The free energy of adsorption, however, does not change significantly with an increase in the PAH surface area. The adsorption of an IL on the PAH surface leads to a small change in the entropy of the adsorbate/adsorbent system. The thermochemistry computed at variable temperature indicates a significant increase in the free energy of adsorption (i.e., a less favorable adsorption) as temperature rises. Additionally, decomposition of the entropic contribution suggests a greater contribution from translational and rotational entropies upon cooling, again consistent with stronger association at lower temperatures. Overall, the thermochemical analyses suggest an entropically driven process of desorption of an IL from the PAH surface, generally leading to fairly weak interactions between ILs and ordinary PAHs under normal laboratory conditions

Sum Frequency Generation Spectroscopy of Imidazolium-Based Ionic Liquids with Cyano-Functionalized Anions at the Solid Salt–Liquid Interface

A surface-sensitive nonlinear vibrational spectroscopic technique, sum frequency generation (SFG), has been used to study cyano-containing ionic liquids in contact with two different solid salt surfaces. Specifically, the interfacial chemistry of BaF₂(111) single-crystal and solid NaCl{100} surfaces in contact with ionic liquids such as [BMIM]­[SCN], [BMIM]­[DCA], [BMIM]­[TCM], and [EMIM]­[TCB] has been investigated. Spectral features in both C–H and C–N stretching regions were assigned, with a detailed discussion of the nature of surface interactions and ordering of the ionic liquid ions at the interface of the different crystals. Results showed that [BMIM]⁺ cations adhered closely via Coulombic interactions to the negatively charged NaCl{100} surface, while [SCN]⁻, [TCM]⁻, and [DCA]⁻ anions revealed a strong electrostatic affinity to the positively charged BaF₂(111) surface. Ions of the ionic liquid adsorbed to the solid salt surface to form a Helmholtz-like electric double layer. The linear [SCN]⁻ anion has a particularly strong affinity to the BaF₂(111) surface, resulting in a first layer of anions directly in contact with BaF₂(111) containing an effective negative surface excess charge. This promoted ordering of the cations in the second layer to counter the charge excess. At the BaF₂(111)–[EMIM]­[TCB] interface, however, a strongly bound layer of anions populating the first layer resulted in a much larger counterion charge delivered near the crystal salt surface than required to effectively neutralize the initial surface charge from the crystal. As a result, strong resonances from the cation were observed at the BaF₂(111) surface, suggesting a more complicated structure of the double layer at the interface than a simple Helmholtz-type model

Halide effects on the performance of equimolar choline halide: guanidinium thiocyanate deep eutectic solvents as dye-sensitized solar cell electrolytes

We have characterized the deep eutectic solvent (DES) comprising a 1:1 equimolar mixture of choline chloride and guanidinium thiocyanate (‘guaniline–Cl’), alongside its halide counterparts wherein the bromide or iodide salts of choline are paired with guanidinium thiocyanate (‘guaniline–Br’ and ‘guaniline–I’), as electrolytes for solar photoconversion. These pared-down electrolyte systems containing only DES (up to 90 wt. %), water (up to 40 wt. %), and the I–/I3– redox couple were explored within dye-sensitized solar cells. Average device performance generally increased with a decrease in hydrogen bond affinity (Cl– – – ), with guaniline–I yielding markedly higher photocurrents relative to guaniline–Cl and guaniline–Br. Indeed, 70 and 80 wt. % guaniline–I exceeded power conversion efficiencies of 2.0% despite producing lower photovoltages and fill factors than the corresponding Cl/Br-based electrolytes. Attributed in part to solution viscosity, this effect arises chiefly from the fact that guaniline–I contains substantial organic iodide built into the DES proper. Significantly, this self-contained electrolyte can function as the sole iodide source for forming the required electrolyte redox couple, yielding performance essentially equivalent to lower-viscosity electrolytes containing supplemental inorganic iodide, in spite of significant differences in solution viscosity.</p

Differential Microscopic Mobility of Components within a Deep Eutectic Solvent

From macroscopic measurements of deep eutectic solvents such as glyceline (1:2 molar ratio of choline chloride to glycerol), the long-range translational diffusion of the larger cation (choline) is known to be slower compared to that of the smaller hydrogen bond donor (glycerol). However, when the diffusion dynamics are analyzed on the subnanometer length scale, we find that the displacements associated with the localized diffusive motions are actually larger for choline. This counterintuitive diffusive behavior can be understood as follows. The localized diffusive motions confined in the transient cage of neighbor particles, which precede the cage-breaking long-range diffusion jumps, are more spatially constrained for glycerol than for choline because of the stronger hydrogen bonds the former makes with chloride anions. The implications of such differential localized mobility of the constituents should be especially important for applications where deep eutectic solvents are confined on the nanometer length scale and their long-range translational diffusion is strongly inhibited (e.g., within microporous media)

Bimolecular Electron Transfer in Ionic Liquids: Are Reaction Rates Anomalously High?

Steady-state and picosecond time-resolved emission spectroscopy are used to monitor the bimolecular electron transfer reaction between the electron acceptor 9,10-dicyanoanthracene in its S₁ state and the donor <i>N</i>,<i>N</i>-dimethylaniline in a variety of ionic liquids and several conventional solvents. Detailed study of this quenching reaction was undertaken in order to better understand why rates reported for similar diffusion-limited reactions in ionic liquids sometimes appear much higher than expected given the viscous nature of these liquids. Consistent with previous studies, Stern–Volmer analyses of steady-state and lifetime data provide effective quenching rate constants <i>k</i>_q, which are often 10–100-fold larger than simple predictions for diffusion-limited rate constants <i>k</i>_D in ionic liquids. Similar departures from <i>k</i>_D are also observed in conventional organic solvents having comparably high viscosities, indicating that this behavior is not unique to ionic liquids. A more complete analysis of the quenching data using a model combining approximate solution of the spherically symmetric diffusion equation with a Marcus-type description of electron transfer reveals the reasons for frequent observation of <i>k</i>_q ≫ <i>k</i>_D. The primary cause is that the high viscosities typical of ionic liquids emphasize the transient component of diffusion-limited reactions, which renders the interpretation of rate constants derived from Stern–Volmer analyses ambiguous. Using a more appropriate description of the quenching process enables satisfactory fits of data in both ionic liquid and conventional solvents using a single set of physically reasonable electron transfer parameters. Doing so requires diffusion coefficients in ionic liquids to exceed hydrodynamic predictions by significant factors, typically in the range of 3–10. Direct, NMR measurements of solute diffusion confirm this enhanced diffusion in ionic liquids

Quantum Chemical Evaluation of Deep Eutectic Solvents for the Extractive Desulfurization of Fuel

Sulfur compounds in fuels are converted to SO_<i>x</i> during combustion, poisoning automotive catalytic converters and creating serious environmental concerns (e.g., acid rain). The efficient desulfurization of liquid fuel is thus a critical step toward minimizing SO_<i>x</i> emissions and their associated environmental impact. To address this problem, governments worldwide have passed stringent legislation regulating the maximal sulfur levels allowable in fuels. In the petroleum refining industry, the conventional method for removing sulfur from fuel is catalytic hydrodesulfurization which, while highly efficient for removing mercaptans, thioethers, and disulfides, shows limited performance in removing aromatic organosulfur compounds exemplified by dibenzothiophene. To meet these strict environmental targets, innovative strategies beyond hydrodesulfurization for the deep desulfurization of fuel are sought. One key strategy entails the oxidation of refractory organosulfur compounds in liquid fuel, coupled with efficient liquid/liquid extraction of the oxidized sulfur compounds using an immiscible solvent phase (i.e., oxidative desulfurization). In this study, we employ computational chemistry to gain atomistic-level insight into the specific interactions responsible for the extraction of key organosulfur compounds and their oxidation products from fuel using deep eutectic solvents (DESs). Specifically, we perform quantum chemical calculations involving the well-studied DESs reline (1:2 choline chloride:urea) and ethaline (1:2 choline chloride:ethylene glycol) to characterize the intermolecular interactions, charge transfer behavior, and thermodynamics associated with their application for organosulfur extraction. We observe that the model aromatic sulfur compounds (ASCs) benzothiophene and dibenzothiophene interact with choline and the hydrogen bond donor (HBD; i.e., urea or ethylene glycol) of the DES via a plurality of weak noncovalent interactions. However, the chloride ion is essentially noninteractive with the ASC due to retention of the conventional hydrogen bond network existing within the initial DES. Oxidation of the model ASCs to their respective sulfoxide and sulfone products was shown to enhance interactions with the DES components, particularly the HBD species due to its propensity for forming multiple hydrogen bonds. We further demonstrate that, upon oxidation, the ASCs exhibit significant and favorable free energies of solvation, suggesting that oxidation will aid in the partition of these sulfur compounds from liquid fuel to a conventional DES phase

Artifacts and Errors Associated with the Ubiquitous Presence of Fluorescent Impurities in Carbon Nanodots

Fluorescent carbon dots have attracted tremendous attention owing to their superlative optical properties which suggest opportunities for replacing conventional fluorescent materials in various application fields. Not surprisingly, the rapid pace of publication has been accompanied by a host of critical issues, errors, controversies, and misconceptions associated with these emergent materials, which present significant barriers to elucidating their true nature, substantially hindering the extensive exploitation of these nanomaterials. Of particular interest are expedient, bottom-up pathways to carbon dots starting from molecular precursors (e.g., citric acid, amino acids, and alkylamines), although such routes are associated with generation of a ubiquity of small molecular weight or oligomeric fluorescent byproducts. A primary obstacle to progress is the inadequacy of purification in reported studies, an omission which gives rise to misconceptions about the nature and characteristics of the carbon dots. In this work, we conducted a series of carbon dot syntheses using facile hydrothermal and microwave routes employing citric acid (paired with urea or ethylenediamine as a nitrogen source), followed by dialysis or ultrafiltration purification steps. Careful comparison and analysis of the optical properties of the resulting purification products (i.e., dialysate/filtrate versus retentate fractions) affirms the formation of molecular fluorophores (potentially oligomeric or polymeric in nature) during the bottom-up chemical synthesis which contribute a majority of the emission from carbon dot samples. We provide clear evidence showing that the fluorescent impurities produced as byproducts of carbon dot synthesis must be rigorously removed to obtain reliable results. On the basis of our findings, the inadequate purification in many reports calls into question published work, suggesting that many previous studies will need to be carefully revisited using more rigorous purification protocols. Of course, deficiencies in purification in prior studies only add to the ongoing debate on carbon dot structure and the origin of their emission. Moving forward, rigorous and consistent purification steps will need to be uniformly implemented, a tactical change that will help pave the way toward the development of carbon dots as next-generation agents for cellular imaging, solid-state and full-color lighting, photovoltaics, catalysis, and (bio)­sensing

Contrasting Behavior of Classical Salts versus Ionic Liquids toward Aqueous Phase J-Aggregate Dissociation of a Cyanine Dye

The effect of addition of ionic liquids (ILs) on the aggregation behavior of a cyanine dye, 5,5′,6,6′-tetrachloro-1,1′-diethyl-3,3′-di(4-sulfobutyl)-benzimidazolocarbocyanine (TDBC), was investigated. In basic aqueous buffer solutions (pH ≥ 10), TDBC preferably exists in its J-aggregated form. Addition of hydrophilic ILs > 5 wt % is observed to disrupt the TDBC J-aggregates, converting them to monomer form most likely because of the interaction between bulky IL cation and the J-aggregates in a time-dependent fashion. This is evidenced by the observed increase in monomer band absorbance at the expense of the absorbance band due to J-aggregates over time. Inorganic salts at similar molar concentrations do not cause this phenomenon but instead induce TDBC precipitation. At low concentrations (<5 wt %), the added IL acts similarly to the inorganic salts, reducing the overall absorbance of TDBC in the solution most likely due to cation exchange causing TDBC precipitation. Addition of a molecular solvent, ethanol, at 15 wt % results in an initial increase in monomer absorbance, albeit to a much lesser extent than for the corresponding molar fraction of IL, which then decreases over time with recovery of J-aggregate absorbancequite opposite the time-dependent behavior seen for TDBC in PB at pH 12.0 with >5 wt % IL. The unique and dual behavior of ILs as an additive toward affecting cyanine dye aggregation is demonstrated

Ionic Liquid Anion Controlled Nanoscale Gold Morphology Grown at a Liquid Interface

Two different ionic liquids comprising the tetrabutylphosphonium cation ([P₄₄₄₄]) paired with the strongly coordinating anions 6-aminocaproate ([6-AC]) or taurinate ([tau]) were prepared and employed in an aqueous/organic liquid bilayer system to generate nanoscale gold by Au­(OH)₄^– photoreduction. Generally, as the concentration of ionic liquid in the organic phase was increased, the resulting quasi-spherical gold nanoparticles were smaller in size and presented less aggregation, leading to marked increases in the catalytic efficiency for 4-nitrophenol reduction using borohydride. The diffusion of the ionic liquids across the liquid/liquid interface was also investigated, revealing partition coefficients of 6.0 and 7.6 for [P₄₄₄₄]­[6-AC] and [P₄₄₄₄]­[tau], respectively. Control studies elucidated that biphasic interfacial reduction was necessary to achieve stable nanoparticles possessing high catalytic activity. When the ionic liquid anion was instead replaced by the weakly coordinating bis­(trifluoro­methyl­sulfonyl)­imide ([Tf₂N]), photoreduction of Au­(OH)₄^– led to holey, wavy gold nanowires instead of spherical nanoparticles, indicating the dramatic morphological control exerted by the coordination strength of the ionic liquid anion. This strategy is straightforward and simple and opens up a number of intriguing avenues for controllably preparing plasmonic colloids for a range of applications from catalysis to optical sensing