Surface-mediated Reactions Research Articles

Single Atom Catalyst Anchored on Nitrogen-Doped Porous Carbon As an Effective Sulfur Host for Lithium-Sulfur Batteries

Lithium-sulfur batteries (LSBs) are considered highly promising for next-generation energy storage due to their high theoretical specific capacity and energy density. However, challenges such as the insulating nature of sulfur cathodes, the detrimental shuttle effect of lithium polysulfides (LiPSs), and sluggish conversion kinetics of LiPSs during charge/discharge cycles impede their commercial viability. In this study, we employed a facile ‘dissolution-carbonization’ approach to synthesize nitrogen-coordinated monometallic atomic catalysts anchored on mesoporous carbon to promote surface-mediated reactions of LiPSs. The hierarchical porous carbon support physically confines the sulfur species, while the introduction of single atoms efficiently captures polysulfide intermediates and facilitates their redox conversion kinetics with a lower activation energy barrier. The synergistic effect of the single atom and nitrogen-rich porous carbon (NC) significantly enhances reaction kinetics and sulfur species utilization. Consequently, LSBs incorporating single-atom catalysts (SACs) demonstrate remarkable performance metrics, including high-capacity retention (824.2 mAh g−1), superior Coulombic efficiency (>98.5%), low-capacity decay rate (0.042% per cycle) after 500 cycles at 1 C, and excellent rate capability (776 mA h g–1 at 3 C). This work presents an effective strategy that combines the functions of a nanoporous material host and SACs for lithium-sulfur batteries.

Modeling the distribution of enzymes on lipid vesicles: A novel framework for surface-mediated reactions in coagulation

Blood coagulation is a network of biochemical reactions wherein dozens of proteins act collectively to initiate a rapid clotting response. Coagulation reactions are lipid-surface dependent, and this dependence is thought to help localize coagulation to the site of injury and enhance the association between reactants. Current mathematical models of coagulation either do not consider lipid as a variable or do not agree with experiments where lipid concentrations were varied. Since there is no analytic rate law that depends on lipid, only apparent rate constants can be derived from enzyme kinetic experiments. We developed a new mathematical framework for modeling enzymes reactions in the presence of lipid vesicles. Here the concentrations are such that only a fraction of the vesicles harbor bound enzymes and the rest remain empty. We call the lipid vesicles with and without enzyme TF:VIIa+ and TF:VIIa− lipid, respectively. Since substrate binds to both TF:VIIa+ and TF:VIIa− lipid, our model shows that excess empty lipid acts as a strong sink for substrate. We used our framework to derive an analytic rate equation and performed constrained optimization to estimate a single, global set of intrinsic rates for the enzyme–substrate pair. Results agree with experiments and reveal a critical lipid concentration where the conversion rate of the substrate is maximized, a phenomenon known as the template effect. Next, we included product inhibition of the enzyme and derived the corresponding rate equations, which enables kinetic studies of more complex reactions. Our combined experimental and mathematical study provides a general framework for uncovering the mechanisms by which lipid mediated reactions impact coagulation processes.

Enhanced visible-light-active MOF-based PbO photocatalyst for binary dye decolorization: A comprehensive study

Developing efficient and sustainable wastewater treatment technologies like photocatalysis is essential to combating global water pollution. Despite lead oxide's photocatalytic potential, its high recombination rate and low active sites restrict its application. In the current study, the first MOF-based PbO nanostructure has been successfully synthesized in a facile way. The as-synthesized sample, PbO/Aluminum fumarate metal–organic framework (PbO/AlFu), was characterized in multiple manners and applied to degrade mixtures of methyl green (MeG) and crystal violet (CV) dyes under visible light irradiation. The incorporation of AlFu metal–organic framework as a support for PbO significantly enhanced photo-degradation performance. This enhancement can be attributed to the increased visible light harvesting, adsorption capabilities facilitated by AlFu, and separation of photo-generated electron-hole pairs due to the formation of the step-scheme (S-scheme) heterojunction. Photocatalytic efficiencies and operational parameters were modeled and optimized respectively using the Box-Behnken design (BBD). Under optimized conditions, MeG and CV dyes were removed at 95.6 and 86.1 percent, respectively. •O2– and •OH radicals were identified as key contributors to the photo-degradation of both dyes. Kinetic studies revealed that photo-degradation of both dyes followed the Langmuir-Hinshelwood model, suggesting a surface-mediated reaction mechanism. PbO/AlFu demonstrated outstanding stability and reusability in five sequential photo-degradation cycles. The present findings highlight the potential of PbO/AlFu as a promising photocatalyst for wastewater treatment.

Uranium accumulation in environmentally relevant microplastics and agricultural soil at acidic and circumneutral pH

The co-occurrence of microplastics (MPs) with potentially toxic metals in the environment stresses the need to address their physicochemical interactions and the potential ecological and human health implications. Here, we investigated the reaction of aqueous U with agricultural soil and high-density polyethylene (HDPE) through the integration of batch experiments, microscopy, and spectroscopy. The aqueous initial concentration of U (100 μM) decreased between 98.6 and 99.2 % at pH 5 and between 86.2 and 98.9 % at pH 7.5 following the first half hour of reaction with 10 g of soil. In similar experimental conditions but with added HDPE, aqueous U decreased between 98.6 and 99.7 % at pH 5 and between 76.1 and 95.2 % at pH 7.5, suggesting that HDPE modified the accumulation of U in soil as a function of pH. Uranium-bearing precipitates on the cracked surface of HDPE were identified by SEM/EDS after two weeks of agitation in water at both pH 5 and 7.5. Accumulation of U on the near-surface region of reacted HDPE was confirmed by XPS. Our findings suggest that the precipitation of U was facilitated by the weathering of the surface of HDPE. These results provide insights about surface-mediated reactions of aqueous metals with MPs, contributing relevant information about the mobility of metals and MPs at co-contaminated agricultural sites.

Mechanistic insights into C-C coupling in electrochemical CO reduction using gold superlattices

Developing in situ/operando spectroscopic techniques with high sensitivity and reproducibility is of great importance for mechanistic investigations of surface-mediated electrochemical reactions. Herein, we report the fabrication of highly ordered rhombic gold nanocube superlattices (GNSs) as substrates for surface-enhanced infrared absorption spectroscopy (SEIRAS) with significantly enhanced SEIRA effect, which can be controlled by manipulating the randomness of GNSs. Finite difference time domain simulations reveal that the electromagnetic effect accounts for the significantly improved spectroscopic vibrations on the GNSs. In situ SEIRAS results show that the vibrations of CO on the Cu2O surfaces have been enhanced by 2.4 ± 0.5 and 18.0 ± 1.3 times using GNSs as substrates compared to those on traditional chemically deposited gold films in acidic and neutral electrolytes, respectively. Combined with isotopic labeling experiments, the reaction mechanisms for C-C coupling of CO electroreduction on Cu-based catalysts are revealed using the GNSs substrates.

Stability and Speciation of Hydrated Magnetite {111} Surfaces from Ab Initio Simulations with Relevance for Geochemical Redox Processes.

Magnetite is a common mixed Fe(II,III) iron oxide in mineral deposits and the product of (anaerobic) iron corrosion. In various Earth systems, magnetite surfaces participate in surface-mediated redox reactions. The reactivity and redox properties of the magnetite surface depend on the surface speciation, which varies with environmental conditions. In this study, Kohn-Sham density functional theory (DFT + U method) was used to examine the stability and speciation of the prevalent magnetite crystal face {111} in a wide range of pH and Eh conditions. The simulations reveal that the oxidation state and speciation of the surface depend strongly on imposed redox conditions and, in general, may differ from those of the bulk state. Corresponding predominant phase diagrams for the surface speciation and structure were calculated from first principles. Furthermore, classical molecular dynamics simulations were conducted investigating the mobility of water near the magnetite surface. The obtained knowledge of the surface structure and oxidation state of iron is essential for modeling retention of redox-sensitive nuclides.

Gas-Phase and Surface-Initiated Reactions of Household Bleach and Terpene-Containing Cleaning Products Yield Chlorination and Oxidation Products Adsorbed onto Indoor Relevant Surfaces.

The use of household bleach cleaning products results in emissions of highly oxidative gaseous species, such as hypochlorous acid (HOCl) and chlorine (Cl2). These species readily react with volatile organic compounds (VOCs), such as limonene, one of the most abundant compounds found in indoor enviroments. In this study, reactions of HOCl/Cl2 with limonene in the gas phase and on indoor relevant surfaces were investigated. Using an environmental Teflon chamber, we show that silica (SiO2), a proxy for window glass, and rutile (TiO2), a component of paint and self-cleaning surfaces, act as a reservoir for adsorption of gas-phase products formed between HOCl/Cl2 and limonene. Furthermore, high-resolution mass spectrometry (HRMS) shows that the gas-phase reaction products of HOCl/Cl2 and limonene readily adsorb on both SiO2 and TiO2. Surface-mediated reactions can also occur, leading to the formation of new chlorine- and oxygen-containing products. Transmission Fourier-transform infrared (FTIR) spectroscopy of adsorption and desorption of bleach and terpene oxidation products indicates that these chlorine- and oxygen-containing products strongly adsorb on both SiO2 and TiO2 surfaces for days, providing potential sources of human exposure and sinks for additional heterogeneous reactions.

Soft Deposition of Organic Molecules Based on Cluster-Induced Desorption for the Investigation of On-Surface and Surface-Mediated Reactions.

Desorption/ionization induced by neutral clusters (DINeC) was employed for the soft transfer of organic and biomolecules, such as porphyrins and peptides, from a bulk sample onto any substrate of choice. Qualitative analysis of the deposition technique was performed by means of mass spectrometry, demonstrating that the deposited molecules remained intact due to the soft nature of the transfer process. Deposition rates were studied quantitatively using a quartz crystal microbalance; layers of intact biomolecules ranging from the submonolayer regime up to a few monolayers in thickness were realized. Mixed layers of molecules were deposited when two different sources of molecules were employed. The samples which were prepared based on this soft deposition method were used for the investigation of reactions of the deposited molecules with either coadsorbates on the surface or the surface itself. Examples include adduct formation of peptides with alkali metals on SiO2, the oxidation of peptides exposed to oxygen, as well as the metallization of porphyrins in interaction with the substrate.

Computational study on the adsorption of small molecules to surface-supported Ni-porphyrins

With their flexible oxidation and spin states, surface-supported metalloporphyrins are attractive building blocks for tuneable, periodic spin interfaces. A technique to manipulate the spin in such systems is based on the adsorption of external gas ligands to the chelated metal center. In the search for promising interfaces, computational studies allow for a convenient scan through potential ligand candidates. In this contribution, we perform density functional theory calculations to investigate the coordination of six exemplary gases (CH4, H2O, NH3, CO, NO and NO2) to nickel tetraphenylporphyrin interfaces. Analyzing the formed complexes, we distinguish four major interaction mechanisms between metal center and ligand: non-interacting, weak field d-splitting, competing surface trans-effect and redox reaction. We find that a surface-mediated redox reaction between a strong π-acceptor (e.g.: NO2) and an unusually reduced metal (e.g.: Ni d9) has the strongest potential to induce spin changes at ambient conditions. Furthermore, we demonstrate how commonly chosen theoretical approximations may influence the interpretation of DFT calculations for metalloporphyrins and their complexes. By highlighting the limitations and possibilities of theoretical approaches, we hope to contribute to a better understanding and more selective harnessing of the special properties of surface-supported metalloporphyrins.

Coupling covariance matrix adaptation with continuum modeling for determination of kinetic parameters associated with electrochemical CO2 reduction

Coupling covariance matrix adaptation with continuum modeling for determination of kinetic parameters associated with electrochemical CO2 reduction

Steering CO2 electrolysis selectivity by modulating the local reaction environment: An online DEMS approach for Cu electrodes

Electrochemical CO2 reduction is a typical surface-mediated reaction, with its reaction kinetics and product distributions largely dependent on the dynamic evolution of reactive species at the cathode–catholyte interface and on the resultant mass transport within the hydrodynamic boundary layer in the vicinity of the cathode. To resolve the complex local reaction environment of branching CO2 reduction pathways, we here present a differential electrochemical mass spectroscopic (DEMS) approach for Cu electrodes to investigate CO2 mass transport, the local concentration gradients of buffering anions, and the Cu surface topology effects on CO2 electrolysis selectivity at a temporal resolution of ∼400 ​ms. As a proof of concept, these tuning knobs were validated on an anion exchange membrane electrolyzer, which delivered a Faradaic efficiency of up to 40.4% and a partial current density of 121 ​mA ​cm−2 for CO2-to-C2H4 valorization. This methodology, which bridges the study of fundamental surface electrochemistry and the upgrading of practical electrolyzer performance, could be of general interest in helping to achieve a sustainable circular carbon economy.

Ambient Carbon-Neutral Ammonia Generation via a Cyclic Microwave Plasma Process.

A novel reactor methodology was developed for chemical looping ammonia synthesis processes using microwave plasma for pre-activation of the stable dinitrogen molecule before reaching the catalyst surface. Microwave plasma-enhanced reactions benefit from higher production of activated species, modularity, quick startup, and lower voltage input than competing plasma-catalysis technologies. Simple, economical, and environmentally benign metallic iron catalysts were used in a cyclical atmospheric pressure synthesis of ammonia. Rates of up to 420.9 μmol min-1 g-1 were observed under mild nitriding conditions. Reaction studies showed that both surface-mediated and bulk-mediated reaction domains were found to exist depending on the time under plasma treatment. The associated density functional theory (DFT) calculations indicated that a higher temperature promoted more nitrogen species in the bulk of iron catalysts but the equilibrium limited the nitrogen converion to ammonia, and vice versa. Generation of vibrationally active N2 and, N2+ ions is associated with lower bulk nitridation temperatures and increased nitrogen contents versus thermal-only systems. Additionally, the kinetics of other transition metal chemical looping ammonia synthesis catalysts (Mn and CoMo) were evaluated by high-resolution time-on-stream kinetic analysis and optical plasma characterization. This study sheds new light on phenomena arising in transient nitrogen storage, kinetics, effect of plasma treatment, apparent activation energies, and rate-limiting reaction steps.

Action at a distance: organic cation induced long range organization of interfacial water enhances hydrogen evolution and oxidation kinetics.

Engineering efficient electrode-electrolyte interfaces for the hydrogen evolution and oxidation reactions (HOR/HER) is central to the growing hydrogen economy. Existing descriptors for HOR/HER catalysts focused on species that could directly impact the immediate micro-environment of surface-mediated reactions, such as the binding energies of adsorbates. In this work, we demonstrate that bulky organic cations, such as tetrapropyl ammonium, are able to induce a long-range structure of interfacial water molecules and enhance the HOR/HER kinetics even though they are located outside the outer Helmholtz plane. Through a combination of electrokinetic analysis, molecular dynamics and in situ spectroscopic investigations, we propose that the structure-making ability of bulky hydrophobic cations promotes the formation of hydrogen-bonded water chains connecting the electrode surface to the bulk electrolyte. In alkaline electrolytes, the HOR/HER involve the activation of interfacial water by donating or abstracting protons. The structural diffusion mechanism of protons in aqueous electrolytes enables water molecules and cations located at a distance from the electrode to influence surface-mediated reactions. The findings reported in this work highlight the prospect of leveraging the nonlocal mechanism to enhance electrocatalytic performance.

Inhibition of platelet-surface-bound proteins during coagulation under flow I: TFPI

Blood coagulation is a self-repair process regulated by activated platelet surfaces, clotting factors, and inhibitors. Tissue factor pathway inhibitor (TFPI) is one such inhibitor, well known for its inhibitory action on the active enzyme complex comprising tissue factor (TF) and activated clotting factor VII. This complex forms when TF embedded in the blood vessel wall is exposed by injury and initiates coagulation. A different role for TFPI, independent of TF:VIIa, has recently been discovered whereby TFPI binds a partially cleaved form of clotting factor V (FV-h) and impedes thrombin generation on activated platelet surfaces. We hypothesized that this TF-independent inhibitory mechanism on platelet surfaces would be a more effective platform for TFPI than the TF-dependent one. We examined the effects of this mechanism on thrombin generation by including the relevant biochemical reactions into our previously validated mathematical model. Additionally, we included the ability of TFPI to bind directly to and inhibit platelet-bound FXa. The new model was sensitive to TFPI levels and, under some conditions, TFPI could completely shut down thrombin generation. This sensitivity was due entirely to the surface-mediated inhibitory reactions. The addition of the new TFPI reactions increased the threshold level of TF needed to elicit a strong thrombin response under flow, but the concentration of thrombin achieved, if there was a response, was unchanged. Interestingly, we found that direct binding of TFPI to platelet-bound FXa had a greater anticoagulant effect than did TFPI binding to FV-h alone, but that the greatest effects occurred if both reactions were at play. The model includes activated platelets’ release of FV species, and we explored the impact of varying the FV/FV-h composition of the releasate. We found that reducing the zymogen FV fraction of this pool, and thus increasing the fraction that is FV-h, led to acceleration of thrombin generation.

Mechanistic Impacts of Metal Site and Solvent Identities for Alkene Oxidation over Carboxylate Fe and Cr Metal–Organic Frameworks

Isolated Fe(III) and Cr(III) sites contained in nanoporous voids of isoreticular carboxylate MIL-101(Fe) and MIL-101(Cr) are interrogated for their reactivity and selectivity of liquid-phase styrene oxidation by hydrogen peroxide (H2O2). Batch kinetic measurements in acetonitrile (MeCN) at 323 K showcase that both metal-normalized oxygenate production and H2O2 consumption rates are O(101) higher for MIL-101(Fe) than MIL-101(Cr). Thermodynamically consistent reaction pathways, constructed through spiking experiments, reveal complex interconnectivities between primary (styrene oxide, benzaldehyde) and secondary (styrene glycol, benzoic acid, phenylacetaldehyde) oxygenates. Though benzaldehyde is the majority product for both MIL-101(Fe) and MIL-101(Cr), isoconversion (Xstyrene = 7%) product distributions suggest intrinsic differences in preferred reaction pathways. Apparent energy barriers for all pathways are lower over MIL-101(Fe) than for MIL-101(Cr), conferred by metal electron affinity differences for primary oxygenate selectivity, while secondary (inter)conversion rates trend with acid site densities. Fitted rate laws, radical trapping, adsorption experiments, and complementary DFT calculations indicate surface-mediated reactions by H2O2-derived surface species that outcompete bound styrene, product oxygenate, solvent, and water molecules for both MIL-101(Fe) and MIL-101(Cr) in MeCN. Extracted enthalpic and entropic effects from temperature-dependent experiments (318–328 K) in MeCN and MeOH showcase the ability of hydrogen bonding solvents in locally hydrophilic environments to selectively stabilize primary oxygenate transition states and indicate additional confinement effects from microporous substructures in MIL-101(Fe) and MIL-101(Cr). Simplified rate expressions are expanded to encompass first-order catalyst deactivation rates through temporal metal leaching experiments and assert that metal leaching dominates MIL-101(Cr) catalyst inefficiencies, while a combination of metal leaching and other (ir)reversible site changes are present for MIL-101(Fe). Overall, this work combines kinetic, spectroscopic, numerical, and computational approaches to rigorously define reaction and deactivation mechanisms for styrene oxidation by H2O2 over isoreticular MIL-101(Fe) and MIL-101(Cr).

Vapor-solid interfacial reaction and polymerization for wafer-scale uniform and ultrathin two-dimensional organic films

Chemical vapor deposition is a conventional synthesis method for growing large-scale and high-quality two-dimensional materials, such as graphene, hexagonal boron nitride, and transition-metal dichalcogenides. For organic films, solution-based methods, such as inkjet printing, spin coating, and drop and micro-contact printing, are commonly used. Herein, we demonstrate a general method for growing wafer-scale continuous, uniform, and ultrathin (2–5 nm) organic films. This method is based on a copper (Cu) surface-mediated reaction and polymerization of several equivalent bromine (Br)-containing π-conjugated small molecules (C12S3Br6, C24H4O2Br2, and C24H12Br2N4), in which local surface-mediated polymerization and internal π-π interactions among organic molecules are responsible for the dimension and uniformity control of the thin films. Specifically, the growth rate and morphology of thin films were found to be Cu-facet-dependent, and single-crystal Cu(111) surfaces could improve the uniformity of thin films. In addition, the number of Br groups and size of organic molecules were critical for crystallinity and thin-film formation. This method can be used to fabricate heterostructures, such as organic film/graphene, giving room for various functional materials and device applications.

Insights into interactions of chlorine-based cleaning products with indoor relevant surfaces

Environmental context The chemistry that occurs in indoor environments and the role that indoor surfaces play have recently received increased attention in the scientific community. Here we have investigated the chemistry of chlorine-based cleaning products and their interactions with indoor relevant surfaces and find that these surfaces react with these cleaning products to yield surface adsorbed chlorine oxides and other surface-bound species. Rationale Indoor chemistry has recently received increased attention in the scientific community due to the fact that there is relatively little known given its unique environment including point combustion sources (candles, gas stoves, etc.) resulting in high aerosol concentrations, high surface to volume ratios and the impact of humans on indoor air quality. Recently, surface-initiated reactions during chlorine cleaning events have been proposed. Methodology In this study, we probe the interaction of bleach headspace gas with high surface area silica as a proxy for window glass – an ‘inert’ and impervious surface – using attenuated total reflectance Fourier Transform infrared (ATR-FTIR) spectroscopy, atomic force microscopy photothermal infrared (AFM-PTIR) spectroscopy and transmission electron microscopy (TEM) to observe surface chemical and physical changes. Results The results suggest chemical transformations occur at the silica surface forming surface adsorbed chlorine oxides (ClOx). Conductivity and ion chromatography methods support the presence of adsorbed chloride after surfaces have been exposed to bleach and HOCl. Discussion Interactions between HOCl and indoor surfaces have not been previously studied with molecular based techniques. The possibility of surface-mediated reactions has been relatively unexplored on indoor surfaces and this study shows the chemistry of chlorine-containing cleaning products on indoor relevant surfaces.

Synthesis of Mackinawite (FeSm) and its heterogeneous Fenton-like catalytic degradation performance of rhodamine B.

In this work, Mackinawite (FeSm) was synthesized by homogeneous precipitation method, and flower-like nanoparticles formed by the aggregation of nanosheets with a preferred orientation along the (001) plane. The heterogeneous Fenton-like degradation performance of FeSm on rhodamine B (RhB) was investigated, results illustrated that RhB degradation was the synergistic effect of adsorption, Fenton, and a heterogeneous Fenton-like reaction. In repeated experiments, the reduction of reactivity was attributed to the oxidation of FeSm into lepidocrocite, whereas lepidocrocite has relatively low hydroxyl radicals (•OH) production reactivity. Thus, it showed excellent degradation effects in long-time degradation of RhB. Photoluminescence technology and free radical capture experiments demonstrated that •OH produced on the surface of catalyst was the main active species to remove RhB. In addition, the Fe species on the surface of FeSm was the main active center for surface-mediated reactions. A total organic carbon test revealed that the degradation was not complete and degradation intermediates were formed. Liquid chromatography-mass spectrometry technology was used to identify the degradation intermediates. On this basis, possible degradation pathways were proposed.

A comprehensive evaluation of factors affecting the reactivity of FeS towards hexabromocyclododecane diastereoisomers

Reactivity of iron sulfide (FeS) towards hexabromocyclododecane (HBCD) was explored under conditions of varying temperature, pH, inorganic ion and dissolved organic matter (DOM) in this study. Results show that the reduction of HBCD by FeS has an activation energy of 29.2 kJ mol−1, suggesting that the rate-limiting step in the reduction was a surface-mediated reaction. The reduction of HBCD by FeS was a highly pH-dependent process. The optimal rate for HBCD reduction by FeS was observed at a pH of 6.2. All the tested inorganic ions suppressed the reduction of HBCD by FeS. XPS analysis confirmed that both Fe(II)-S and bulk S(-II) on FeS surface could be impacted by solution pH and inorganic ions and were responsible for the regulation of HBCD reduction. Some DOMs (i.e., fulvic acid, humic acid, salicylic acid, catechol and sodium dodecyl sulfate) were found to hinder the reduction via competing with HBCD for active sites on FeS surface. However, the presence of 2,2′-bipyridine, triton X-100 and cetyltrimethyl ammonium bromide was able to significantly enhance the rate of HBCD reduction by 5.8, 9.0 and 20 times, respectively. Different factors could influence the reduction efficiency of HBCD diastereoisomers to different extent, but not the reaction orders of HBCD diastereoisomers (α-HBCD < γ-HBCD < β-HBCD). Moreover, FeS could completely remove HBCD diastereoisomers in sediments with multiple factors within 9 d reaction. Our results contribute to give a better understanding on the performance of FeS towards HBCD under real and complex conditions and facilitate the application of FeS in remediation sites.

Electrochemical Valorization of Biomass-Derived Feedstocks: Levullinic Acid to 4-Hydroxyvaleric Acid

Fossil fuel-based sources of energy are being gradually replaced by renewable electricity (such as solar, wind, etc), while there is a rising interest in biomass as a sustainable platform to produce fuels and other carbon-based chemicals.1,2 Electrochemistry is thus an attractive approach for the valorization of biomass-derived feedstocks. This approach has shown some advantages compared to traditional thermal- or bio-based chemical conversion such as the amenability to operate directly on aqueous biomass hydrolysate streams, and operability near ambient conditions, which also facilitates distributed scale production.2 This work discusses the electrochemical conversion of levulinic acid (LA) to 4-hydroxyvaleric acid (HVA). HVA is a versatile compound that can be used as a monomer for the production of biodegradable and biocompatible polyesters, in addition to diverse commodities and fine chemicals.3,4 It is projected that the biopolymers industry will reach a global market size of USD 27.9 billion by 2025.5 While prior work has demonstrated that LA can be electrochemically converted into valeric acid (VA) and γ-valerolactone (GVL), among other compounds,2, HVA has only previously been fund as a trace-level intermediate of LA electrochemical reduction into GVL.Here, we show a maximum HVA production rate higher than 40 g L-1 h-1 (i.e., 200 kg L-1 m-2 h-1) at 100% selectivity, and conversion and faradaic efficiency of 81%, while complete LA conversion could also be achieved under lower rates. Our characterization indicates this can further be optimized by reactor design (particularly the mass transport properties) and electrolyte engineering. Though evaluation of different pHs, aqueous supporting electrolytes, temperature, electrode potential, stirring rates, and initial concentration of LA we also elaborate on mechanistic underpinnings of the reaction selectivity, proposing that the formation of VA and GVL is through surface-mediated reactions, while HVA is formed via an outer sphere electron transfer (OSET) route (Fig.1). References (1) npj Clim. Atmos. Sci. 2019, 2 (1), 35.(2) ACS Energy Lett. 2021, 1205–1270. https://doi.org/10.1021/acsenergylett.0c02692.(3) . J. Biotechnol. 2015, 210, 38–43.(4) J. Agric. Food Chem. 2019, 67 (9), 2540–2546.(5) Global Bioplastics & Biopolymers Market Outlook 2020-2025 https://www.globenewswire.com/news-release/2020/04/17/2017839/0/en/Global-Bioplastics-Biopolymers-Market-Outlook-2020-2025.html (accessed April 22, 2021). Figure 1