High Base Concentrations Research Articles

Concentration‐Dependent Thermodynamics of the Inhibitory Paths in Homogeneous Mn(I) Hydrogenation Catalysis

AbstractMany catalytic reactions suffer from product inhibition, which especially hard to control in homogeneous hydrogenation due to the scaling relation between the inhibited and active states of the catalyst. We recently reported one such pathway in Mn(I) hydrogenation and demonstrated that addition of alkoxide bases could affect the thermodynamic favorability of this reaction and selectively suppress the product inhibition. Since external reaction promotors are formally not involved in reaction thermodynamics, we set to investigate the explicit molecular interactions behind these apparently environmental effects. Herein, we reveal that the thermodynamic landscape of the inhibitory process exhibits a non‐monotonic dependence on the base concentration. This study related this phenomenon to the presence of two dominant mechanisms operating at different base concentrations. Specifically, the base additives can enhance the ionic strength and lower the free energy of the inhibited state at low promotor concentration. At high base concentrations this study suggested the formation of highly labile alcohol‐alkoxide clusters which stabilize the free alcohol and make its addition to the catalyst unfavourable, thereby suppressing the inhibition. While relatively weak, such noncovalent interactions between reactants and reaction environment can cause substantial perturbations to the free energy of catalytic process, ultimately deciding its fate.

Desalination Fuel Cell Stacks: Scaling up the Co-Production of Electricity and Clean Water

The world faces a rising demand for potable water and electricity, while a lack of clean water and use of polluting electricity sources are major hazards1. Nowadays reverse osmosis (RO) is widely used for sea and brackish water desalination2, where RO consumes ~3-4 kWh/m3 for seawater desalination3. A new class of water treatment technologies is emerging that is distinguished from the classical methods by utilizing chemical energy to power both water treatment and electricity generation simultaneously from a single electrochemical cell. When using the hydrogen/oxygen redox couple, such a cell is termed a desalination fuel cell (DFC) which was introduced by our group in 20204.A DFC utilizes a fuel cell anode and cathode to catalyze the chemical-to-electrical energy conversion, as well as a cation and anion exchange membrane to desalinate the feedwater flowing through the cell. A device with a single feed channel (figure a) was able to produce up to 10 kWh/m3 while desalinating water with sea-water level salinity4. In order to for this nascent technology to become practical, scale-up strategies need to be proposed and demonstrated.In this work we show results from the first scaled DFC, where we utilize scaling rules associated with electrodialysis by increasing the number of membrane pairs to allow either two or three feed channels (Figure b). We find the three feed channel device was associated with high voltage loss in the ohmic region and lower limiting current (figure c), but the salt concentration behaved linearly as a function of the current density as expected (figure d). The main voltage losses are clearly emanated from the cathode and the anode sides as the membranes potential loss was proven to be insignificant5. We showed that implementing higher acid concentration in the catholyte and higher base concentration in the anolyte channels can significantly improve performance of the stack. Figure (e) shows results using three different anolyte and catholyte solutions, with highest open circuit voltage (OCV) and improved polarization performance for 0.5M HClO4 and 0.5M NaOH in the catholyte and the anolyte, respectively. We also investigated the feed flow rate impact on DFC polarization performance and salt removal. Overall, we show successful implementation of a scaled-up DFC.

Manipulation of Chemical and Crystal Structures of Multilayered Copper-Hexaaminobenzene Coordination Nanosheets to Modulate Conductivity with Precise Control of Liquid–Liquid Interfacial Synthesis

Multilayered coordination nanosheets made of copper ions and hexaaminobenzene (CuHAB) were synthesized in a liquid–liquid interfacial reaction in the presence of a base and an oxidizing agent. X-ray diffraction studies revealed the suitability of an oxidizing agent with a low oxidation potential for the synthesis of highly crystalline CuHAB. The base concentration affects the chemical and crystal structures of CuHAB. The nanosheets synthesized at low base concentrations exhibit humidity-responsive conductivity due to the ligating amino group. Meanwhile, the nanosheets prepared at high base concentrations have different stacking structures from the CuHABs formed at low base concentrations and show greater conductivity. These results indicate that the liquid–liquid interfacial synthesis with an appropriate oxidizing agent and quantitative addition of base can modulate chemical and stacking structures of coordination nanosheets composed of hexaaminobenzene ligand and their functionalities.

Periodic mesoporous organosilica nanoparticles: Morphology control and sorption properties

The synthesis of various periodic mesoporous organosilica nanoparticles (nanoPMOs) from the corresponding organo-bridged bis(triethoxy)silanes is described. A strong influence of the sodium hydroxide concentration is observed, leading to various sizes or morphologies depending on the precursor. In particular, in the case of the ethenylene linker, the morphology evolves from small flakes to elongated particles, while in the case of the phenylene precursor, small well-organized arms start to form at high base concentration. A mechanistic study shows that in all cases the nanoparticles nucleate before the condensation reaction commences and at very low conversions of the hydrolysis reaction of ethoxysilanes. The resulting hybrid nanoPMOs were compared for their adsorption properties towards rhodamine B (RB), which highlights large differences between the nanoPMOs with different linkers, and evidences a very strong affinity of the phenylene PMO with the poly-aromatic dye. Similarly, the sorption of cyclohexane and water revealed a much higher lipophilicity of the phenylene-bridged nanoPMO compared to the other studied linkers, despite a similar hydrophilicity. These results should help to better design nanoPMOs as nanovectors for drugs.

Stabilization of alkaline 5-HMF electrolytes via Cannizzaro reaction for the electrochemical oxidation to FDCA

This work investigates the degradation of 5-hydroxymethylfurfural (HMF) in an alkaline electrolyte, enabling “indirect” electrooxidation of HMF at unprecedentedly high substrate and base concentrations at improved atom economy.

Unexpectedly efficient ion desorption of graphene-based materials

Ion desorption is extremely challenging for adsorbents with superior performance, and widely used conventional desorption methods involve high acid or base concentrations and large consumption of reagents. Here, we experimentally demonstrate the rapid and efficient desorption of ions on magnetite-graphene oxide (M-GO) by adding low amounts of Al3+. The corresponding concentration of Al3+ used is reduced by at least a factor 250 compared to conventional desorption method. The desorption rate reaches ~97.0% for the typical radioactive and bivalent ions Co2+, Mn2+, and Sr2+ within ~1 min. We achieve effective enrichment of radioactive 60Co and reduce the volume of concentrated 60Co solution by approximately 10 times compared to the initial solution. The M-GO can be recycled and reused easily without compromising its adsorption efficiency and magnetic performance, based on the unique hydration anionic species of Al3+ under alkaline conditions. Density functional theory calculations show that the interaction of graphene with Al3+ is stronger than with divalent ions, and that the adsorption probability of Al3+ is superior than that of Co2+, Mn2+, and Sr2+ ions. This suggests that the proposed method could be used to enrich a wider range of ions in the fields of energy, biology, environmental technology, and materials science.

Photoacid-induced aqueous acid–base reactions probed by femtosecond infrared spectroscopy

Acid-base reactions involving an excited photoacid have typically been investigated at high base concentrations, but the mechanisms at low base concentrations require clarification. Herein, the dynamics of acid-base reactions induced by an excited photoacid, pyranine (DA), were investigated in the presence of azide ion (N3-) in D2O solution using femtosecond infrared spectroscopy. Specifically, the spectral characteristics of four species (DA, electronically excited DA (DA*), the conjugate base of DA* (A*-), and the conjugate base of DA (A-)) were probed in the spectral region of 1400-1670cm-1 in the time range of 1ps-1μs. This broad timescale encompassed all the acid-base reactions initiated by photoexcitation at 400nm; thus, reactions related to both DA* and A- could be probed. Furthermore, changes in the populations of N3- and DN3 were monitored using the absorption bands at 2042 and 2133cm-1, respectively. Following excitation, approximately half of DA* relaxed to DA with a time constant of 0.44 ± 0.04ns. The remainder underwent an acid-base reaction to produce A*-, which relaxed to A- with a time constant of 3.9 ± 0.3ns. The acid-base reaction proceeded via two paths, namely, proton exchange with the added base or simple deuteron release to D2O (protolysis). Notably, all the acid-base reactions were well described by the rate constant at the steady-state limit. Thus, although the acid-base reactions at low base concentrations (< 0.1M) were diffusion controlled, they could be described using a simple rate equation.

Reaction Optimization of a Suzuki‐Miyaura Cross‐Coupling Using Design of Experiments

AbstractThe combination of lab automation and design of experiments for the execution of screening experiments increases productivity and reduces error‐prone manual work. A self‐developed software tool allows for creating fractional‐factorial experimental design (FFED). Application of FFED on the screening of a Suzuki‐Miyaura cross‐coupling leads to a 93 % reduced design compared to full‐factorial design. The resulting regression model qualitatively shows the positive effect of educt concentrations, time, and temperature and reveals the decrease in conversion at high base concentrations.

Performance Improvement of Gold Electrode towards Methanol Electrooxidation in Akaline Medium: Enhanced Current Density Achieved with Poly(aniline-co-2-hydroxyaniline) Coating at Low Overpotential.

Electronically conducting poly (aniline-co-2-hydroxyaniline) (PACHA), a copolymer of aniline and 2-hydroxyaniline (2HA), was electrochemically coated on gold substrate for methanol electrooxidation in alkaline media. The electrochemical behavior of PACHA coated gold electrode towards methanol electrooxidation was investigated via cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) for application in an alkaline fuel cell. Methanol electrooxidation was observed at two different electrode potentials depending on the concentration of the base. At the PACHA coated gold electrode, the methanol oxidation peak was observed at lower overpotential (at 0.19 V) in a solution of high base concentration (1.8 M NaOH), which was 30 mV lower than the peak for the uncoated gold electrode. In addition, the Faradic current Imax obtained on the PACHA coated electrode (20 mA) was two times higher as compared to the Faradic current Imax of the un-modified gold electrode (10 mA). In solution of lower base concentration (0.06 M NaOH), the electrooxidation of methanol became sluggish on both electrodes, as indicated by peak shifting towards positive potential and with reduced faradaic current (at 0.74 V on PACHA coated electrode; Imax 10 mA). The electrooxidation of methanol at both lower and higher electrode potentials was analyzed mechanistically and discussed in light of the literature. EIS results were interpreted using Nyquist and Bode plots. The charge transfer resistance was decreased and pseudo-capacitive behavior changed to conductive behavior when external applied potential was increased from 0.1 V to 0.4 V.

On manipulating the thermoelectric potential of p-type ZnO by nanostructuring

Throughout this work, ZnO nanoparticles were produced by a simple co-precipitation method, in which the volume of the basic solution (NaOH, 5–30 ml) was varied to understand its effect on the properties of the nanoparticles. A structural and morphological effect was observed, namely a change in reflections between 2θ=31.75°(100) and 2θ=34.4°(002) peaks. At the same time, nanoparticles changed from a hexagonal prism shape to a desert-rose stones structure for high base concentrations. The transport measurements were performed from room temperature (RT) up to 340 K. At RT, a correlation between the Seebeck coefficient and the electrical conductivity was achieved, whereas the thermal conductivity behaves distinctly. A thermal conductivity value of 0.5 W m-1 K−1 was observed, which is two orders of magnitude lower than the bulk counterpart. The behaviours of the PF and ZT also reveal the different influences of the electrons and the phonons for the different nanostructures, since the electronic phenomena predominate in the hexagonal prism structures, the phonon phenomena predominate in the desert-rose stones, due to the reduced dimension of these structures. For higher temperatures, a thermal activation model was used to determine the Activation Energy (Ea), leading to values in the hundredths of meV range, with higher values for the volumes of NaOH where a mixture of the two types of structures is observed, where the presence of different interfaces created a barrier for the carriers to pass from grain to grain.

Self‐Propagated para‐Fluoro‐Thiol Reaction

We introduce a protecting-group-free concept for the powerful para-fluoro-thiol reaction (PFTR) employing a source of fluoride ions as base. The reaction is shown to be self-propagating, with under-stoichiometric amounts of base, thus effectively foregoing the need for high base concentrations. Careful tuning of the solvent-thiol combination allows for quantitative conversion, in some cases within a short timeframe, when only minimal amounts of base are used, allowing the PFTR reaction to essentially proceed base-free.

The Role of H-Bonding in Non-Concerted Proton-Coupled Electron Transfer: Explaining the Voltammetry of Phenylenediamines in the Presence of Weak Bases in Acetonitrile

The kinetics of many proton-coupled electron transfer (PCET) reactions cannot be adequately described by step-wise proton and electron transfer. Concerted electron-proton transfer (CPET) is another possibility, but examples exist where step-wise mechanisms are not viable yet there is no compelling evidence for CPET. This study investigates such a reaction, the oxidation of an NH-containing phenylenediamine radical cation, H2PD+, in the presence of pyridines in acetonitrile. H2PD+ is formed by a net one electron oxidation of 2,3,5,6-tetramethylphenylenediamine in acetonitrile via a rather complicated mechanism that likely proceeds through a H-bonded dimer. Once formed, it can be further oxidized at more positive potentials to the quinoidal dication, H2PD2+, which will be considerably more acidic than the radical cation. Indeed, the E1/2 for the radical oxidation jumps to a considerably more negative potential upon addition of 1 equivalent of the weak base pyridine. The CV wave broadens but stays chemically reversible. Further addition of pyridine leads to smaller E1/2 shifts with continued reversibility. This behavior could possibly be explained by stabilization of H2PD2+ through H-bonding or proton transfer to pyridine, however, UV-vis spectroelectrochemical experiments provide definitive evidence for proton transfer. Furthermore, CV studies with 4-substituted pyridine derivatives of weaker basicity show that the observed E1/2 with 1 equivalent of the pyridine depends in a Nernstian fashion on the pKa of the conjugate acid of the pyridine, indicating that all the pyridines studied deprotonate H2PD2+ to give the quinoidal cation, HPD+. The continued E1/2 shift observed upon further addition of the pyridines can then be completely explained by application of the Nernst equation to the overall reaction H2PD+ + pyr → HPD+ + Hpyr+ + e−. However, while this explains the thermodynamics, the classic step-wise proton-electron transfer mechanism for this reaction cannot explain the observed reversibility at high base concentration. In contrast, the reversibility can be explained by a “wedge” scheme mechanism in which electron and proton transfer occur within the H-bond complex formed as an intermediate in proton transfer. In this case, the electron-proton transfer could be concerted, however, the kinetics for the second oxidation in the presence of pyridine show no significant isotope effect. Furthermore, a new reduction peak that appears at faster scan rates suggests the presence of an intermediate in the electron-proton transfer. Both results strongly suggest that the electron-proton transfer is step-wise, not concerted, within the H-bond complex. This result points to the important role H-bonding may play in PCET even without CPET.

Iridium–PNP Pincer Complexes for Methanol Dehydrogenation at Low Base Concentration

AbstractA catalytic system based on an iridium–PNP complex was developed for the aqueous‐phase reforming of methanol. Investigations revealed higher activity at low base concentrations and higher stability at high base concentrations as an opposing trend relative to that of known Ru and Fe PNP‐pincer‐containing systems for methanol dehydrogenation. During mechanistic investigations it was possible to identify a CO containing derivate of the deactivation species.

Storage Stability of Bio-oils Derived from the Catalytic Conversion of Softwood Kraft Lignin in Subcritical Water

The stability of lignin-derived bio-oil obtained from a continuous process [base (K2CO3)-catalyzed, using phenol as a capping agent] under subcritical conditions of water (25 MPa, 290–370 °C) was investigated. The lignin-derived bio-oil obtained was stored at ambient temperature for 2 years. Our results show that the base concentration in the feed solution affects the stability of this lignin-derived bio-oil during its long-term storage. It was found that, at low base concentrations (i.e., 0.4%–1.0%), the yields of all lignin-derived bio-oil fractions were relatively stable. At high base concentrations (i.e., 1.6%–2.2%), however, the yield of high-molecular-weight (high-Mw) structures increased and that of low-molecular-weight (low-Mw) structures decreased after storage. This indicated that the low-Mw materials had been polymerized to form high-Mw materials. In addition, it was found that the yield of gas chromatography-mass spectrometry (GC-MS)-identified compounds (excluding phenol) in this lignin-deriv...

Bipolar membrane electrodialysis for generation of hydrochloric acid and ammonia from simulated ammonium chloride wastewater

Simulated ammonium chloride wastewater was treated by a lab-scale bipolar membrane electrodialysis for the generation of HCl and NH3·H2O and desalination. The influence of initial concentration of NH4Cl, current density, salt solution volume, initial concentration of acid and base and membrane stack structure on the yields of HCl and NH3·H2O was investigated. The current efficiency and energy consumption were also examined under different conditions. The results showed that, at the current density of 48 mA/cm2, the highest concentration of HCl and NH3·H2O with initial concentration of 110 g/L NH4Cl was 57.67 g/L and 45.85 g/L, respectively. Higher initial concentration of NH4Cl was favor to reduce unit energy consumption and increase current efficiency of the BMED system. The membrane stack voltage of BMED increased quickly under constant current when the concentration of NH4Cl contained in the solution of salt compartment was depleted below the “inflection point concentration” about 8000 mg/L. It means that the concentration of NH4Cl below 8000 mg/L was no longer suitable for BMED because of higher energy consumption. The HCl and NH3·H2O concentration increased more quickly following the increase of current density. When increasing the volume of NH4Cl, the concentration of HCl and NH3·H2O also increased. The high initial concentration of acid and base could improve the final concentration of them, while the growth rate was decreased. Compared with the BMED system with three compartments, the growth rate of HCl concentration with the two compartments was higher and its unit energy consumption was lower. It meant that the performance of the BMED system could be improved by optimizing operation conditions. The application feasibility of the generation of HCl and NH3·H2O and desalination of ammonium chloride wastewater by BMED was proved.

Barium content of paleosols from Central Italy

A paper published in 2013 concerning the elemental composition of 50 paleosols from Central Italy (Tuscany and Abruzzo) noted that Barium values were exceptionally high, a fact which could be not justified by the presence of neighbouring outcrops of igneous rocks. This current paper aims to explain the origin and significance of barium enrichment in these Pleistocene paleosols (Xerults, Xeralfs, Udalfs and Xerepts) formed on alluvial siliciclastic deposits mainly derived from sandstones and shales and, in a few cases, insoluble residues of Mesozoic carbonate rocks. Ba content was considered in relation to the molecular weathering ratio, soil pH and total Mg content. A qualitative inverse correlation was suggested between Mg and Ba for most soils. Maximum Ba values were found both in acid and alkaline soils, while minimum values were observed in neutral soils. This suggests the existence of different pedoclimatic environments. The acid soil solution supports the Ba release from minerals and soil components and its precipitation in the presence of SO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;2-&lt;/sup&gt;, clays and Fe/Mn nodules, whereas other bases are removed. In contrast, the soils during seasonal low humidity periods or extended events of drought have a different pH: the soil solution then has a higher base concentration and therefore a greater likelihood of Ba precipitation bound to clays, metal oxides or even as insoluble sulphate (barite). Thus the soils with higher and lower pH are enriched in Ba both directly and indirectly in relation to ionic strength of the soil solution. The significant enrichment in Ba is ascribed to intense and very long pedogenetic processes during past climatic conditions of a wet- warm subtropical type, which acted on the Italian paleosols of Pleistocene age. The recurring shrink-swell pattern would have increased the rate of chemical weathering, the rate of barium release in the soil solution and its subsequent immobilisation in neoformed complexes as barite and others.

Simultaneous regeneration of inorganic acid and base from a metal washing step wastewater by bipolar membrane electrodialysis after pretreatment by crystallization in a fluidized pellet reactor

This study aims to investigate the feasibility of an integrated system consisting of a fluidized pellet reactor and bipolar membrane electrodialysis (EDBM) for regeneration of acid and base from wastewater with a high concentration of calcium and nickel ions. The fluidized pellet reactor was used to reduce the potential of scaling on the membranes by crystallization. EDBM was applied to regenerate acid/base from this pretreated wastewater. The results show that calcium and nickel can be removed by the fluidized pellet reactor with sufficient efficiency (74% and 73% without filtration, 74% and 94.4% with filtration, respectively). After pretreatment, it was feasible to regenerate mixed acid and base using EDBM. The current efficiency was 69% for the acid, 80% for the base; the energy consumption was 5.5kWh/kg acid and 4.8kWh/kg base at a current density of 60mA/cm2 and an initial concentration of 0.2N. In a long-term experiment, the pretreatment increased cation exchange membrane performance and a high concentration of acid and base up to 1.76N and 2.41N was produced with only slight scaling on the cation exchange membrane surface. This integrated process is not only technically but also environmentally feasible, reducing water pollution and yielding valuable products to achieve sustainable development.

Baseline BTMA Degradation as a Standard for Cationic Degradation in AEMFCs

Alkaline membranes are of high interest as hydroxide conducting polymers in electrochemical devices. Benzyltrimethylammonium (BTMA) has historically been the cation most often employed in anion exchange membranes. Fuel cell literature often compares cation stabilities under alkaline conditions to that of BTMA. However throughout the literature each study employs a different experimental method when determining these stabilities. This variety of methods makes cross comparisons of novel cations difficult if at all possible. Additionally, we have found that several methods reported have inherent flaws in their findings due to the difficulty of performing stability experiments with high base concentrations. Herein we report a reliable, standardized method of determining cation stability under alkaline conditions that mimic those expected under operating conditions of electrochemical devices. During development, several common experimental pitfalls are addressed and avoided. The standardized method has been employed to determine BTMA stabilities at varying concentrations (0.01, 0.1, and 0.5 M BTMA) at 120, 140 and 160 °C in 2 M KOH in H2O, and have found BTMA to be very stable at 80 °C.

Liquid phase reforming of woody biomass to hydrogen

This work concentrates on the production of H2 directly from raw biomass through liquid phase reforming in the presence of a liquid base and a solid catalyst. Both precious metal and base metal catalysts were found to be active for the liquid phase hydrolysis and reforming of wood. Pt-based catalysts, particularly Pt–Re, were shown by atomistic modeling to be more selective toward breaking C–C bonds, resulting in a higher selectivity to hydrogen versus methane. Ni-based catalysts were found to prefer breaking C–O bonds, favoring the production of methane. The results showed that at a constant wood concentration, increasing the concentration of base (base to wood ratio) in the presence of Raney Ni catalysts resulted in greater selectivity toward hydrogen. The amount of wood converted to gas was lower due to increased production of undesirable organic acids from the wood at higher base concentrations. It was shown that by modifying Ni-based catalysts with dopants, it was possible to reduce the base concentration while maintaining the selectivity toward hydrogen and increasing wood conversion to gas versus organic acids.

Degradation Behaviour of Thin Polymer Films of Poly(Amide Ester) Hydrogel Using Quartz Crystal Microbalance

Quartz Crystal Microbalance (QCM) has been typically used as a mass sensor in the study of monolayer and multilayer depositions and dissolution, mass transport in polymer films on electrodes, corrosion processes at electrodes and mass changes caused by protein adsorption at electrodes. Thin films of an aromatic poly (amide ester) hydrogel with hydrolyzable cross links were prepared in-situ on a gold-coated quartz crystal which is the transducing element of a Quartz Crystal Microbalance (QCM). Here we report the synthesis and degradation behavior of an aromatic poly (amide ester) hydrogel with hydrolyzable cross links using QCM. Degradation of the hydrogel films was found to proceed with Δf increment, indicating mass decrease and is in line with the theory proposed by Sauerbrey. Films with a higher cross-link density underwent partial degradation with swelling; and are depicted as a decrease in the Δf values with time. Deviation from the general base catalysis of ester hydrolysis was observed at higher base concentration due to the biphasic environment of the hydrolysis reaction.