Lignin is an aromatic polymer that constitutes over 30 wt% of lignocellulosic biomass and is the most important source of renewable aromatics in nature. The global paper industry generates more than 130 million tons of Kraft lignin annually. Depolymerization of Kraft lignin to value-added monomers can significantly enhance the profitability of biorefinery. However, the method is impeded by the severe condensation of Kraft lignin during the pulping process, which forms robust C-C bonds and results in low monomer yields. In this study, we present a stepwise approach for producing valuable aromatic monomers from Kraft lignin through cleavage of both C-O and C-C bonds. The approach initiated with complete cleavage of C-O bonds between lignin units within Kraft lignin through alcoholysis in isopropanol, resulting in a monomer yield of 8.9%. Subsequently, the selective cleavage of methylene linkages present in the residual dimers and oligomers was achieved with commercial MCM-41 zeolite in the same pot, proceeding with an additional monomer yield of 4.0%, thereby increasing the total monomer yield by 45%. This work provides an avenue for increasing the depolymerization efficiency of Kraft lignin.
Monthly Archives: September 2023
Closing the Gap: Towards a Fully Continuous and Self‐Regulated Kolbe Electrosynthesis
In this article, we address the transition of the Kolbe electrolysis of valeric acid (VA) to n-octane as an exemplary electrosynthesis process from a batch reaction to a continuous, self-regulated process. Based on a systematic assessment of chemical boundary conditions and sustainability aspects, we propose a continuous electrosynthesis including a simple product separation and electrolyte recirculation, as well as an online-pH-controlled VA feeding. We demonstrate how essential performance parameters such as product selectivity (S) and coulombic efficiency (CE) are significantly improved by the transition from batch to a continuous process. Thus, the continuous and pH-controlled electrolysis of a 1M valeric acid, starting pH 6.0, allowed a constantly high selectivity of around 47% and an average Coulomb efficiency about 52% throughout the entire experimental duration. Under otherwise identical conditions, the conventional batch operation suffered from lower and strongly decreasing performance values (Sn-octane, 60min= 10.4%, Sn-octane, 240min= 1.3%; CEn-octane, 60min=7.1%, CEn-octane, 240min= 0.5%). At the same time, electrolyte recirculation significantly reduces wastes and limits the use of electrolyte components.
Carbon Nitride with Single‐Atom Nickel as Co‐Catalyst for Visible‐Light Promoted C—O Coupling
A nickel single-atom heterogeneous photocatalyst was prepared and employed in visible-light-driven C—O cross-coupling reactions under mild conditions, which exceeded the catalytic activity of a semi-heterogeneous catalytic system. The reaction system exhibited good isolated yields for a broad range of aryl bromides (e.g., ketones, aldehydes, esters, and amides) and both primary and secondary alcohols. Experimental characterizations showed that nickel single atoms could facilitate C—O cross-coupling by extending the visible absorption range, improving the charge transfer, and inhibiting the recombination of carriers.
Comprehensive Summary
Solar-driven cross-coupling reactions by dual nickel/photocatalysis under mild conditions have received considerable attention. However, the existing photo/nickel dual catalytic cross-coupling reactions require the addition of expensive photosensitizers and organic ligands, and the catalytic activity is inadequate. Herein, we report a nickel single-atom heterogeneous catalyst supported on mesoporous carbon nitride for photocatalytic C—O coupling reaction between 4-bromobenzonitrile and ethanol, affording 4-ethoxybenzonitrile in excellent yield compared to a semi-heterogeneous catalytic system. The catalytic system exhibits a broad substrate scope including ketones, aldehydes, esters, and amides. This work presents a simple and cost-effective strategy for anchoring metal single atoms onto carbon nitride, providing a new platform for enabling high-performance photocatalytic production of aryl ether compounds.
Alternating Copolymerization of Epoxides and Isothiocyanates to Diverse Functional Polythioimidocarbonates and Related Block Polymers
We study the organoboron catalyzed ring-opening copolymerization of epoxides and isothiocyanates and the performance of the resultant polythioimidocarbonates. Copolymerizations only take place on free -OC(=N)S−, and excess TEB or intramolecular synergy are adverse. Additionally, aromatic isothiocyanates polymerize much faster than aliphatic ones, which facilitated the one-step synthesis of block polymers from mixed monomers. This protocol can deliver sulfur-containing polymers with both high T g and refractive index. Moreover, polythioimidocarbonates can also serve as positive resists for electron beam lithography.
Comprehensive Summary
Synthesis of diverse polythioimidocarbonates via ring-opening copolymerization of epoxides and isothiocyanates catalyzed by organoboron catalyst was reported herein. Both aromatic and aliphatic isothiocyanates underwent successful copolymerization with terminal and internal epoxides, allowing for the precise tuning of the performance of the resultant copolymers over a broad range. The wide scope of available isothiocyanates and epoxides enables the direct construction of sulfur-containing functional polymers featuring both high glass transition temperature and refractive index. Additionally, it was observed that aromatic isothiocyanates polymerize much faster than aliphatic ones, and the reactivity difference facilitated the one-step synthesis of block polymers from mixed aromatic isothiocyanates, aliphatic isothiocyanates and epoxides due to the preferential incorporation of aromatic isothiocyanates over the aliphatic analogues during their alternating copolymerization with epoxides. The produced polythioimidocarbonates can be used as positive resists for electron beam lithography (sensitivity of 130 μC/cm2 and contrast of 1.53 for poly(CHO-alt-EITC)). Coupling with their high refractive index (1.58—1.68), polythioimidocarbonates might find functional applications in optics. These results render ring-opening copolymerization of epoxides and isothiocyanates a facile route to enrich functional polymer library.
The Potential Game Changer: a Concept‐to‐Proof Study on D:A Heterojunction‐Free Organic Photovoltaics
Comprehensive Summary
Since 1986, the donor-acceptor (D:A) heterojunction has been regarded a necessity for high-efficiency organic photovoltaics (OPVs), due to its unique advantage in compensating the intrinsic limitations of organic semiconductors, such as high exciton binding energy and poor ambipolar charge mobility. While this adversely causes tremendous non-radiative charge recombination and instability issues, which currently become the most critical limits for commercialization of OPVs. Here, we present a concept-to-proof study on the potential of D:A heterojunction free OPV by taking advantage of recent progress of non-fullerene acceptors. First, we demonstrate that the “free carriers” can be spontaneously generated upon illumination in an NFA, i.e. the 6TIC-4F single layer. Second, the 6TIC-4F layer also exhibits good ambipolar charge transporting property. These exceptional characteristics distinguish it from the traditional organic semiconductors, and relieve it from the reliance of D:A heterojunction to independently work as active layer. As a result, the subsequent OPV by simply sandwiching the 6TIC-4F layer between the cathode and anode yields a considerably high power conversion efficiency ~ 1%. Moreover, we find the D:A heterojunction free device exhibits two order of magnitude higher electroluminescence quantum efficiency and significantly reduced VOC loss by 0.16 eV compared to those of the D:A BHJ structure, validating its promise for higher efficiency in the future. Therefore, our work demonstrates the possibility of using D:A heterojunction-free device structure for high performance, that can potentially become the next game changer of OPV.
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Enhancing the Circularly Polarized Luminescence of Self‐Assembled Cyanostilbenes through Extended π‐Conjugation
Main observation and conclusion
Chiral supramolecular assembly of π-conjugated luminophores provides a promising avenue for enhancing circularly polarized luminescence. In this study, we shed light on the impact of π-conjugation length on circularly polarized luminescent performance of the resulting supramolecular assemblies, by designing a tetra-cyanostilbene monomeric compound alongside two dicyanostilbene control compounds. These cyanostilbene-based compounds possess the ability to form chiral supramolecular polymers in toluene, driven by a synergistic combination of intermolecular hydrogen bonding and π–π stacking interactions. The extended π-aromatic skeleton brings bathochromic-shifted fluorescence and enhanced intermolecular stacking capability for the tetra-cyanostilbene compound. Consequently, chiral supramolecular assemblies formed by the tetra-cyanostilbene compound demonstrate a remarkable two-fold increase in g lum values relative to the assemblies formed by the dicyanostilbene compounds. Overall, this study provides valuable insights into the relationship between π-conjugation length and the circularly polarized luminescent performance of π-conjugated supramolecular assemblies.
This article is protected by copyright. All rights reserved.
Recent Progress on Carbon‐Based Electrocatalysts for Oxygen Reduction Reaction: Insights on the Type of Synthesis Protocols, Performances and Outlook Mechanisms
This review explores carbon-based catalysts for oxygen reduction (ORR) in acidic and alkaline electrolytes, focusing on their mechanism, performance modulation strategies such as functionalization engineering, and doping strategies. Carbon-based materials are cost-effective, highly conductive, and have a wide range of allotropes. However, no specific review distinguishes between ORR activity, mechanism, and fuel cell performance in acidic and alkaline media for carbon functionalized and doped nanomaterials. That aspect is outlined in this review.
Abstract
Due to their low cost, accessibility of resources, and improved stability and durability, carbon-based nanomaterials have attracted significant attention as cathode materials for oxygen reduction reactions. These materials also exhibit intrinsic physical and electrochemical features. However, their potential for use in fuel cells is constrained by low ORR activity and slow kinetics. Carbon nanomaterials can be functionalized and doped with heteroatoms to change their morphologies and generate a large number of oxygen reduction active sites to lessen the problems. Doping the carbon lattice with heteroatoms like N, S, and P and functionalizing the carbon structure with −OCH3, −F, −COO−, −O− are two of these modifications that can change specific properties of the carbon nanomaterials like expanding interlayer distance, producing a large number of active sites, and enhancing oxygen reduction activity. When compared to pristine carbon-based nanomaterials, these doped and functionalized carbon nanomaterials, including their composites, exhibit accelerated rate performance, outstanding stability, and higher methanol tolerance. This article summarizes the most recent developments in heteroatom-doped and functionalized carbon-based nanomaterials, covering different synthesis approaches, characterization methods, electrochemical performance, and oxygen reduction reaction mechanisms. As cathode materials for fuel cell technologies, the significance of heteroatom co-doping and transition metal heteroatom co-doping is also underlined.
High‐temperature platinum‐catalyzed hydrosilylation and dehydrocoupling cross‐linking of silicones
We propose to use the platinum(II) C,N-cyclometalated complex (PCC) to catalyze the hydrosilylation and dehydrocoupling reactions of high molecular weight polysiloxanes at elevated temperatures (above 100°C). PCC was prepared via a three-step procedure consisting of the 2-hydrazinopyridine synthesis, its treatment with 3-methoxy-4-(prop-2-yn-1-yloxy)benzaldehyde, and coupling of the obtained product with cis-[PtCl2(CNXyl)2]. This complex exhibits thermal stability up to 150°C even at heating in air. PCC allows carrying out the cross-linking of vinyl-terminated polydimethylsiloxane (V-PDMS) and polymethylhydrosiloxane (PMHS) by hydrosilylation, as well as PMHS dehydrocoupling cross-linking at 150 and 120°C, respectively. The coupling (cross-linking) patterns were successfully confirmed by 1H, 13C, and 29Si solid-state NMR spectroscopy. The thermal and swelling characteristics and the transparency of the obtained silicone materials indicate the absence of aggregation of platinum particles.
Use of Hydrothermal Carbonization to Improve the Performance of Biowaste‐Derived Hard Carbons in Sodium Ion‐Batteries
From Waste to Anode Material: Hard carbons are produced from waste biomass (spent coffee grounds, sunflower seed shells and rose stems) by two methods: direct pyrolysis and by combined hydrothermal carbonization and pyrolysis. Electrochemical performance of as-obtained hard carbons using hydrothermal carbonization combined with pyrolysis is improved with up to 76 % ICE and 280 mAh g−1 at C/5.
Abstract
Over the last years, hard carbon (HC) has been the most promising anode material for sodium-ion batteries due to its low voltage plateau, low cost and sustainability. In this study, biomass waste (spent coffee grounds, sunflower seed shells and rose stems) was investigated as potential material for hard carbon preparation combining a two-step method consisting of on hydrothermal carbonization (HTC), to remove the inorganic impurities and increase the carbon content, and a subsequent pyrolysis process. The use of HTC as pretreatment prior to pyrolysis improves the specific capacity in all the materials compared to the ones directly pyrolyzed by more than 100 % at high C-rates. The obtained capacity ranging between 210 and 280 mAh g−1 at C/15 is similar to the values reported in literature for biomass-based hard carbons. Overall, HC obtained from sunflower seed shell performs better than that obtained from the other precursors with an initial Coulombic efficiency (ICE) of 76 % and capacities of 120 mAh g−1 during 1000 cycles at C with a high capacity retention of 86–93 %.
Kraft Lignin: A Valuable, Sustainable Resource, Opportunities and Challenges
To be or not to be burnt: that's the question. Read more about kraft lignin: the potential, the chemistry of how it is formed, and stateof-the-art applications in both fuels and materials. A technoeconomic discussions discloses two important economic incentives to recover lignin from pulp production.
Abstract
Kraft lignin, a by-product from the production of pulp, is currently incinerated in the recovery boiler during the chemical recovery cycle, generating valuable bioenergy and recycling inorganic chemicals to the pulping process operation. Removing lignin from the black liquor or its gasification lowers the recovery boiler load enabling increased pulp production. During the past ten years, lignin separation technologies have emerged and the interest of the research community to valorize this underutilized resource has been invigorated. The aim of this Review is to give (1) a dedicated overview of the kraft process with a focus on the lignin, (2) an overview of applications that are being developed, and (3) a techno-economic and life cycle asseeements of value chains from black liquor to different products. Overall, it is anticipated that this effort will inspire further work for developing and using kraft lignin as a commodity raw material for new applications undeniably promoting pivotal global sustainability concerns.