Vapor-Phase Dicarboxylic Acids and Anhydrides Drive Depolymerization of Polyurethanes

Polyurethane (PU) is the sixth most used plastic in the world. Because many PU derived materials are thermosets and the monomers are valuable, chemical recycling to recover the polyol component is the most viable pathway to utilizing postconsumer PU waste in a closed-loop fashion. Acidolysis is an effective method to recover polyol from PU waste. Previous studies of PU acidolysis rely on the use of dicarboxylic acid (DCA) in high temperature reactions (>200 °C) in the liquid phase and result in unwanted byproducts, high energy consumption, complex separations of excess organic acid, and an overall process that is difficult to scale up. In this work, we demonstrate selective PU acidolysis with DCA vapor to release polyol at temperatures below the melting points of the DCAs (<150 °C). Notably, acidolysis with DCA vapor adheres to the principles of green chemistry and prevents in part esterification of the polyol product, eliminating the need for additional hydrolysis/processing to obtain the desired product. The methodology was successfully applied to a commercial PU foam (PUF) postconsumer waste.

Polyurethane Foam Chemical Recycling: Fast Acidolysis with Maleic Acid and Full Recovery of Polyol

Chemical recycling of polyurethane (PU) waste is essential to displace the need for virgin polyol production and enable sustainable PU production. Currently, less than 20% of PU waste is downcycled through rebinding to lower value products than the original PU. Chemical recycling of PU waste often requires significant input of materials like solvents and slow reaction rates. Here, we report the fast (<10 min) and solvent-free acidolysis of a model toluene diisocyanate (TDI)-based flexible polyurethane foam (PUF) at <200 °C using maleic acid (MA) with a recovery of recycled polyol (repolyol) in 95% isolated yield. After workup (hydrolysis of repolyl ester and separations), the repolyol exhibits favorable physical properties that are comparable to the virgin polyol; these include 54.1 mg KOH/g OH number and 624 cSt viscosity. Overall, 80% by weight of the input PUF is isolated into two clean-cut fractions containing the repolyol and toluene diamine (TDA). Finally, end-of-life (EOL) mattress PUF waste is recycled successfully with high recovery of repolyol using MA acidolysis. The solvent-free and fast acidolysis with MA demonstrated in this work with both model and EOL PUF provides a potential pathway for sustainable and closed-loop PU production.

Supported Platinum Nanoparticles Catalyzed Carbon-Carbon Bond Cleavage of Polyolefins: Role of the Oxide Support Acidity

Supported platinum nanoparticle catalysts are known to convert polyolefins to high-quality liquid hydrocarbons using hydrogen under relatively mild conditions. To date, few studies using platinum grafted onto various metal oxide (MxOy) supports have been undertaken to understand the role of the acidity of the oxide support in the carbon-carbon bond cleavage of polyethylene under consistent catalytic conditions. Specifically, two Pt/MxOy catalysts (MxOy = SrTiO3 and SiO2-Al2O3; Al = 3.0 wt %, target Pt loading 2 wt % Pt ∼1.5 nm), under identical catalytic polyethylene hydrogenolysis conditions (T = 300 °C, P(H2) = 170 psi, t = 24 h; Mw = ∼3,800 g/mol, Mn = ∼1,100 g/mol, Đ = 3.45, Nbranch/100C = 1.0), yielded a narrow distribution of hydrocarbons with molecular weights in the range of lubricants (Mw = < 600 g/mol; Mn < 400 g/mol; Đ = 1.5). While Pt/SrTiO3 formed saturated hydrocarbons with negligible branching, Pt/SiO2-Al2O3 formed partially unsaturated hydrocarbons (<1 mol % alkenes and ∼4 mol % alkyl aromatics) with increased branch density (Nbranch/100C = 5.5). Further investigations suggest evidence for a competitive hydrocracking mechanism occurring alongside hydrogenolysis, stemming from the increased acidity of Pt/SiO2-Al2O3 compared to Pt/SrTiO3. Additionally, the products of these polymer deconstruction reactions were found to be independent of the polyethylene feedstock, allowing the potential to upcycle polyethylenes with various properties into a value-added product.

Quantitative analyses of products and rates in polyethylene depolymerization and upcycling

Depolymerization and upcycling are promising approaches to managing plastic waste. However, quantitative measurements of reaction rates and analyses of complex product mixtures arising from depolymerization of polyolefins constitute significant challenges in this emerging field. Here, we detail techniques for recovery and analysis of products arising from batch depolymerization of polyethylene. We also describe quantitative analyses of reaction rates and products selectivity. This protocol can be extended to depolymerization of other plastics and characterization of other product mixtures including long-chain olefins. For complete details on the use and execution of this protocol, please refer to Sun et al.1.

Markedly Improved Catalytic Dehydration of Sorbitol to Isosorbide by Sol-Gel Sulfated Zirconia: A Quantitative Structure-Reactivity Study

Isosorbide, a bicyclic C6 diol, has considerable value as a precursor for the production of bio-derived polymers. Current production of isosorbide from sorbitol utilizes homogeneous acid, commonly H2SO4, creating harmful waste and complicating separation. Thus, a heterogeneous acid catalyst capable of producing isosorbide from sorbitol in high yield under mild conditions would be desirable. Reported here is a quantitative investigation of the liquid-phase dehydration of neat sorbitol over sulfated zirconia (SZ) solid acid catalysts produced via sol-gel synthesis. The catalyst preparation allows for precise surface area control (morphology) and tunable catalytic activity. The S/Zr ratio (0.1-2.0) and calcination temperature (425-625 °C) were varied to evaluate their effects on morphology, acidity, and reaction kinetics and, as a result, to optimize the catalytic system for this transformation. With the optimal SZ catalyst, complete conversion of sorbitol occurred in <2 h under mild conditions to give isosorbide in 76% yield. Overall, the quantitative kinetics and structure-reactivity studies provided valuable insights into the parameters that govern product yields and SZ catalyst activity, central among these being the relative proportion of acid site types and Brønsted surface density.

One-Step Production of Renewable Adipic Acid Esters from Mucic Acid over an Ir-ReOx/C Catalyst with Low Ir Loading

The production of adipic acid, a large-volume platform chemical, from biomass represents a petroleum-free route to manufacturing nylon and other polymers more sustainably. In this study, a one-step conversion of...

Tetranuclear rollover cyclometalated organoplatinum-rhenium compounds; C-I oxidative addition and C-C reductive elimination using a rollover cycloplatinated dimer

The novel tetranuclear Pt(IV)-Re(VII) complex [Pt₂Me₄(OReO₃)₂(PMePh₂)₂(µ-bpy-2H)], 4, is synthesized through the reaction of silver perrhenate with a new rollover cycloplatinated(IV) complex [Pt₂Me₄I₂(PMePh₂)₂(µ-bpy-2H)], 3. In complex 4, while 2,2'-bipyridine (bpy) acts as a linker between two Pt metal centers, oxygen acts as a mono-bridging atom between Pt and Re centers through an unsupported Pt(IV)-O-Re(VII) bridge. The precursor rollover cycloplatinated(IV) complex 3 is prepared by the MeI oxidative addition reaction of the rollover cycloplatinated(II) complex [Pt₂Me₂(PMePh₂)₂(µ-bpy-2H)], 2. Complex 2 shows a metal-to-ligand charge-transfer band in the visible region, which was used to investigate the kinetics and mechanism of its double MeI oxidative addition reaction. Based on the experimental findings, the classical S_N2 mechanism was suggested for both steps and supported by computational studies. All complexes are fully characterized using multinuclear NMR spectroscopy and elemental analysis. Attempts to grow crystals of the rollover cycloplatinated(IV) dimer 3 yielded a new dimer rollover cyclometalated complex [Pt₂I₂(PMePh₂)₂(µ-bpy-2H)], 5, presumably from the C-C reductive elimination of ethane. The identity of complex 5 was confirmed by single crystal X-ray diffraction analysis.

Origins of Lithium/Sodium Reverse Permeability Selectivity in 12-Crown-4-Functionalized Polymer Membranes

Direct lithium extraction via membrane separations has been fundamentally limited by lack of monovalent ion selectivity exhibited by conventional polymeric membranes, particularly between sodium and lithium ions. Recently, a 12-Crown-4-functionalized polynorbornene membrane was shown to have the largest lithium/sodium permeability selectivity observed in a fully aqueous system to date. Using atomistic molecular dynamics simulations, we reveal that this selectivity is due to strong interactions between sodium ions and 12-Crown-4 moieties, which reduce sodium ion diffusivity while leaving lithium ion mobility relatively unaffected. Moreover, the ion diffusivities in the membrane, when scaled by their respective solution diffusivities and free ion fractions, can be collapsed to an almost universal relationship depending on solvent volume fraction.

Engineering Li/Na selectivity in 12-Crown-4-functionalized polymer membranes

Lithium is widely used in contemporary energy applications, but its isolation from natural reserves is plagued by time-consuming and costly processes. While polymer membranes could, in principle, circumvent these challenges by efficiently extracting lithium from aqueous solutions, they usually exhibit poor ion-specific selectivity. Toward this end, we have incorporated host-guest interactions into a tunable polynorbornene network by copolymerizing 1) 12-crown-4 ligands to impart ion selectivity, 2) poly(ethylene oxide) side chains to control water content, and 3) a crosslinker to form robust solids at room temperature. Single salt transport measurements indicate these materials exhibit unprecedented reverse permeability selectivity (∼2.3) for LiCl over NaCl-the highest documented to date for a dense, water-swollen polymer. As demonstrated by molecular dynamics simulations, this behavior originates from the ability of 12-crown-4 to bind Na+ ions more strongly than Li+ in an aqueous environment, which reduces Na+ mobility (relative to Li+) and offsets the increase in Na+ solubility due to binding with crown ethers. Under mixed salt conditions, 12-crown-4 functionalized membranes showed identical solubility selectivity relative to single salt conditions; however, the permeability and diffusivity selectivity of LiCl over NaCl decreased, presumably due to flux coupling. These results reveal insights for designing advanced membranes with solute-specific selectivity by utilizing host-guest interactions.

Organosolv Fractionation of Walnut Shell Biomass to Isolate Lignocellulosic Components for Chemical Upgrading of Lignin to Aromatics

Renewable carbon sources are a rapidly growing field of research because of the finite supply of fossil carbon. The lignocellulosic biomass walnut shell (WS) is an attractive renewable feedstock because it has a high lignin content (38-44 wt %) and is an agricultural waste stream. Lignin, a major component of lignocellulosic biomass that is currently a waste stream in pulping processes, has unique potential for chemical upgrading because its subunits are aromatic. In the interest of improving the sustainability and reducing the environmental impact of biomass processing, valorization of agricultural waste streams is important. Herein, three lab-scale, batch organosolv procedures are explored in the interest of optimal isolation of protected WS lignin (WSL). One system uses acetic acid, one MeOH, and the final EtOH as the primary solvent. The optimal condition for protected WSL isolation, which resulted in a 64% yield, was methanol and dilute sulfuric acid with formaldehyde to act as a protecting group at 170 °C. Select samples were upgraded by hydrogenolysis over a nickel catalyst. Protected lignin recovered from the optimal condition showed 77% by weight conversion to monomeric phenols, demonstrating that the protected WSL can selectively afford high value products. One key finding from this study was that MeOH is a superior solvent for isolating WSL versus EtOH because the latter exhibited lignin recondensation. The second was that the Ni/C-catalyzed reductive catalytic fractionation (RCF) directly of WS biomass was not selective relative to RCF of isolated WSL; conversion of raw WS to monomers produced significantly more side products.

Lignin-Derived Non-Heme Iron and Manganese Complexes: Catalysts for the On-Demand Production of Chlorine Dioxide in Water under Mild Conditions

A lignin-derived ligand, bis(2-hydroxy-3-methoxy-5-propylbenzyl)glycine (DHEG), was synthesized from 2-methoxy-4-propylphenol (dihydroeugenol (DHE)) and the amino acid glycine. Two mononuclear iron and manganese complexes of DHEG were prepared, characterized, and employed for the oxidation of chlorite to chlorine dioxide in aqueous solution. Peroxyacetic acid (PAA) was used as a "green" oxidant in the redox reactions to initiate the formation of high-valent Fe and Mn (IV)-OH intermediates. EPR studies verified the formation of a high-valent Mn^IV species. Both Fe and Mn complexes catalyzed chlorite oxidation with bimolecular rate constants of 32 and 144 M^-1 s^-1, respectively, at pH 4.0 and 25 °C. The Mn complex was found to be more efficient for chlorite oxidation with a turnover frequency of 17 h^-1 and remained active during subsequent additions of PAA. The rate of ClO₂ decomposition with PAA/Mn-DHEG was first order in PAA and increased significantly as pH increased. A mechanism that accounts for all observations is presented.

Ligand-Controlled Csp2-H versus Csp3-H Bond Formation in Cycloplatinated Complexes: A Joint Experimental and Theoretical Mechanistic Investigation

The cyclometalated platinum(II) complexes [PtMe(C^∧N)(L)] [1_PS: C^∧N = 2-phenylpyridinate (ppy), L = SMe₂; 1_BS: C^∧N = benzo[h]quinolate (bhq), L = SMe₂; 1_PP: C^∧N = ppy, L = PPh₃; and 1_BP: C^∧N = bhq, L = PPh₃] containing two different cyclometalated ligands and two different ancillary ligands have been investigated in the reaction with CX₃CO₂H (X = F or H). When L = SMe₂, the Pt-Me bond rather than the Pt-C bond of the cycloplatinated complex is cleaved to give the complexes [Pt(C^∧N)(CX₃CO₂)(SMe₂)]. When L = PPh₃, the selectivity of the reaction is reversed. In the reaction of [PtMe(C^∧N)(PPh₃)] with CF₃CO₂H, the Pt-C^∧N bond is cleaved rather than the Pt-Me bond. The latter reaction gave [PtMe(κ¹N-Hppy)(PPh₃)(CF₃CO₂)] as an equilibrium mixture of two isomers. For L = PPh₃, no reaction was observed with CH₃CO₂H. The reasons for this difference in selectivity for complexes 1 are computationally discussed based on the energy barrier needed for the protonolysis of the Pt-C_sp³ bond versus the Pt-C_sp² bond. Two pathways including the direct one-step acid attack at the Pt-C bond (S_E2) and stepwise oxidative-addition on the Pt(II) center followed by reductive elimination [S_E(ox)] are proposed. A detailed density functional theory (DFT) study of these protonations along with experimental UV-vis kinetics suggests that a one-step electrophilic attack (S_E2) at the Pt-C bond is the most likely mechanism for complexes 1, and changing the nature of the ancillary ligand can influence the selectivity in the Pt-C bond cleavage. The effect of the nature of the acid and cyclometalated ligand (C^∧N) is also discussed.

Ligand-Mediated C-Br Oxidative Addition to Cycloplatinated(II) Complexes and Benzyl-Me C-C Bond Reductive Elimination from a Cycloplatinated(IV) Complex

Reaction of the Pt(II) complexes [PtMe₂(pbt)], 1a, (pbt = 2-(2-pyridyl)benzothiazole) and [PtMe(C^N)(PPh₂Me)] [C^N = deprotonated 2-phenylpyridine (ppy), 1b, or deprotonated benzo[h]quinoline (bhq), 1c] with benzyl bromide, PhCH₂Br, is studied. The reaction of 1a with PhCH₂Br gave the Pt(IV) product complex [PtBr(CH₂Ph)Me₂(pbt)]. The major trans isomer is formed in a trans oxidative addition (2a), while the minor cis products (2a' and 2a″) resulted from an isomerization process. A solution of Pt(II) complex 1a in the presence of benzyl bromide in toluene at 70 °C after 7 days gradually gave the dibromo Pt(IV) complex [Pt(Br)₂Me₂(pbt)], 4a, as determined by NMR spectroscopy and single-crystal XRD. The reaction of complexes 1b and 1c with PhCH₂Br gave the Pt(IV) complexes [PtMeBr(CH₂Ph)(C^N)(PPh₂Me)] (C^N = ppy; 2b; C^N = bhq, 2c), in which the phosphine and benzyl ligands are trans. Multinuclear NMR spectroscopy ruled out other isomers. Attempts to grow crystals of the cycloplatinated(IV) complex 2b yielded a previously reported Pt(II) complex [PtBr(ppy)(PPh₂Me)], 3b, presumably from reductive elimination of ethylbenzene. UV-vis spectroscopy was used to study the kinetics of reaction of Pt(II) complexes 1a-1c with benzyl bromide. The data are consistent with a second-order S_N2 mechanism and the first order in both the Pt complex and PhCH₂Br. The rate of reaction decreases along the series 1a ≫ 1c > 1b. Density functional theory calculations were carried out to support experimental findings and understand the formation of isomers.

Polyethylene upcycling to long-chain alkylaromatics by tandem hydrogenolysis/aromatization

The current scale of plastics production and the accompanying waste disposal problems represent a largely untapped opportunity for chemical upcycling. Tandem catalytic conversion by platinum supported on γ-alumina converts various polyethylene grades in high yields (up to 80 weight percent) to low-molecular-weight liquid/wax products, in the absence of added solvent or molecular hydrogen, with little production of light gases. The major components are valuable long-chain alkylaromatics and alkylnaphthenes (average ~C30, dispersity Ð = 1.1). Coupling exothermic hydrogenolysis with endothermic aromatization renders the overall transformation thermodynamically accessible despite the moderate reaction temperature of 280°C. This approach demonstrates how waste polyolefins can be a viable feedstock for the generation of molecular hydrocarbon products.

Advanced Paramagnetic Resonance Studies on Manganese and Iron Corroles with a Formal d4 Electron Count

Metallocorroles wherein the metal ion is Mn^III and formally Fe^IV are studied here using field- and frequency-domain electron paramagnetic resonance techniques. The Mn^III corrole, Mn(tpfc) (tpfc = 5,10,15-tris(pentafluorophenyl)corrole trianion), exhibits the following S = 2 zero-field splitting (zfs) parameters: D = -2.67(1) cm^-1, |E| = 0.023(5) cm^-1. This result and those for other Mn^III tetrapyrroles indicate that when D ≈ - 2.5 ± 0.5 cm^-1 for 4- or 5-coordinate and D ≈ - 3.5 ± 0.5 cm^-1 for 6-coordinate complexes, the ground state description is [Mn^III(Cor^3-)]⁰ or [Mn^III(P^2-)]⁺ (Cor = corrole, P = porphyrin). The situation for formally Fe^IV corroles is more complicated, and it has been shown that for Fe(Cor)X, when X = Ph (phenyl), the ground state is a spin triplet best described by [Fe^IV(Cor^3-)]⁺, but when X = halide, the ground state corresponds to [Fe^III(Cor^•2-)]⁺, wherein an intermediate spin (S = ³/₂) Fe^III is antiferromagnetically coupled to a corrole radical dianion (S = ¹/₂) to also give an S = 1 ground state. These two valence isomers can be distinguished by their zfs parameters, as determined here for Fe(tpc)X, X = Ph, Cl (tpc = 5,10,15-triphenylcorrole trianion). The complex with axial phenyl gives D = 21.1(2) cm^-1, while that with axial chloride gives D = 14.6(1) cm^-1. The D value for Fe(tpc)Ph is in rough agreement with the range of values reported for other Fe^IV complexes. In contrast, the D value for Fe(tpc)Cl is inconsistent with an Fe^IV description and represents a different type of iron center. Computational studies corroborate the zfs for the two types of iron corrole complexes. Thus, the zfs of metallocorroles can be diagnostic as to the electronic structure of a formally high oxidation state metallocorrole, and by extension to metalloporphyrins, although such studies have yet to be performed.

Discovery and mechanistic investigation of Pt-catalyzed oxidative homocoupling of benzene with PhI(OAc)2

Pt-catalyzed direct coupling of benzene to biphenyl using PhI(OAc)₂ as an oxidant in the absence of any acid as a co-solvent or co-catalyst was mechanistically investigated.

Isolation and characterization of cellulose and α-cellulose from date palm biomass waste

Towards the utilization of different parts of date palm biomass waste, low-concentration acid-alkali treatment was used to isolate the contained cellulose and α-cellulose. The cellulose yields achieved from the rachis, leaflet, and fiber parts of the biomass were 74.70%, 71.50%, and 73.82%, respectively, while the corresponding α-cellulose yields were 78.63%, 75.64%, and 70.40%, respectively. The cellulose samples were bleached and characterized by thermogravimetric analysis (TGA), Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD). The XRD results revealed high crystallinity of both the cellulose and α-cellulose samples, while the TGA thermograms indicated that the alkali treatment completely removed lignin and hemicelluloses from the rachis. The results of this study demonstrate the promise of using date palm biomass waste as raw material to produce cellulose and α-cellulose.

Chelating and Bridging Roles of 2-(2-Pyridyl)benzimidazole and Bis(diphenylphosphino)acetylene in Stabilizing a Cyclic Tetranuclear Platinum(II) Complex

The reaction of complex [Pt(Me)(DMSO)(pbz)], 1, (pbz = 2-(2-pyridyl)benzimidazolate) with [PtMe(Cl)(DMSO)₂], B, followed by addition of bis(diphenylphosphino)acetylene (dppac), gave the novel tetranuclear platinum complex [Pt₄Me₄(μ-dppac)₂(pbz)₂Cl₂], 2, bearing both the pbz and dppac ligands. In this structure, the pbz ligands are both chelating and bridging to stabilize the tetraplatinum framework. The tetranuclear Pt(II) complex was fully characterized by NMR spectroscopy, X-ray crystallography, and mass spectrometry, and its electronic structure was investigated and supported by DFT calculations.

Synthesis and Properties of Quinoxaline-Containing Benzoxazines and Polybenzoxazines

The object of this work is to prepare quinoxaline-based benzoxazines and evaluate thermal properties of their thermosets. For this object, 4,4'-(quinoxaline-2,3-diyl)diphenol (QDP)/furfurylamine-based benzoxazine (QDP-fu) and 4,4',4″,4‴-([6,6'-biquinoxaline]-2,2',3,3'-tetrayl)tetraphenol (BQTP)/furfurylamine-based benzoxazine (BQTP-fu) were prepared. The structures of QDP-fu and BQTP-fu were successfully confirmed by FTIR and ¹H and ¹³C NMR spectra. We studied the curing behavior of QDP-fu and BQTP-fu and thermal properties of their thermosets. According to DSC thermograms, QDP-fu and BQTP-fu have the attractive onset exothermic temperatures of 181 and 186 °C, respectively. The onset temperature is approximately 45 °C lower than that of a bisphenol A/furfurylamine-based benzoxazines. According to DMA TMA and TGA thermograms, the thermoset of BQTP-fu shows impressive thermal properties, with a T _g value of 418 °C, a coefficient of thermal expansion of 39 ppm/°C, a 5% decomposition temperature of 430 °C, and a char yield of 72%.

Overcoming cellulose recalcitrance in woody biomass for the lignin-first biorefinery

BACKGROUND

Low-temperature swelling of cotton linter cellulose and subsequent gelatinization in trifluoroacetic acid (TFA) greatly enhance rates of enzymatic digestion or maleic acid-AlCl₃ catalyzed conversion to hydroxymethylfurfural (HMF) and levulinic acid (LA). However, lignin inhibits low-temperature swelling of TFA-treated intact wood particles from hybrid poplar (Populus tremula × P. alba) and results in greatly reduced yields of glucose or catalytic conversion compared to lignin-free cellulose. Previous studies have established that wood particles from transgenic lines of hybrid poplar with high syringyl (S) lignin content give greater glucose yields following enzymatic digestion.

RESULTS

Low-temperature (- 20 °C) treatment of S-lignin-rich poplar wood particles in TFA slightly increased yields of glucose from enzymatic digestions and HMF and LA from maleic acid-AlCl₃ catalysis. Subsequent gelatinization at 55 °C resulted in over 80% digestion of cellulose in only 3 to 6 h with high-S-lignin wood, compared to 20-60% digestion in the wild-type poplar hybrid and transgenic lines high in guaiacyl lignin or 5-hydroxy-G lignin. Disassembly of lignin in woody particles by Ni/C catalytic systems improved yields of glucose by enzymatic digestion or catalytic conversion to HMF and LA. Although lignin was completely removed by Ni/C-catalyzed delignification (CDL) treatment, recalcitrance to enzymatic digestion of cellulose from the high-S lines was reduced compared to other lignin variants. However, cellulose still exhibited considerable recalcitrance to complete enzymatic digestion or catalytic conversion after complete delignification. Low-temperature swelling of the CDL-treated wood particles in TFA resulted in nearly complete enzymatic hydrolysis, regardless of original lignin composition.

CONCLUSIONS

Genetic modification of lignin composition can enhance the portfolio of aromatic products obtained from lignocellulosic biomass while promoting disassembly into biofuel and bioproduct substrates. CDL enhances rates of enzymatic digestion and chemical conversion, but cellulose remains intrinsically recalcitrant. Cold TFA is sufficient to overcome this recalcitrance after CDL treatment. Our results inform a 'no carbon left behind' strategy to convert total woody biomass into lignin, cellulose, and hemicellulose value streams for the future biorefinery.

The reaction of activated esters with epoxides for self-curable, highly flexible, A2B2- and A3B3-type epoxy compounds

To achieve high-T_g and low-dissipation epoxy thermosets, bis(2-methoxy-4-(oxiran-2-ylmethyl)phenyl)isophthalate (2) and tris(2-methoxy-4-(oxiran-2-ylmethyl)phenyl)benzene-1,3,5-tricarboxylate (3) were prepared.

Arene C–H bond activation and methane formation by a methylplatinum(ii) complex: experimental and theoretical elucidation of the mechanism

A combined experimental/computational investigation reveals that the cyclometalation of [PtMe₂(DMSO)₂], 1, by HC^N ligands proceeds via HC^N coordination through the N donor atom, oxidative addition of the arene C–H bond, and final dissociation of methane from a platinum hydride complex.

Which is the Stronger Nucleophile, Platinum or Nitrogen in Rollover Cycloplatinated(II) Complexes?

The rollover cyclometalated platinum(II) complexes [PtMe(2,X'-bpy-H)(PPh₃)], (X = 2, 1a; X = 3, 1b; and X = 4, 1c) containing two potential nucleophilic centers have been investigated to elucidate which center is the stronger nucleophile toward methyl iodide. On the basis of DFT calculations, complexes 1b and 1c are predicted reacting with MeI through the free nitrogen donor to form N-methylated platinum(II) complexes, while complex 1a reacts through oxidative addition on platinum to give a platinum(IV) complex, which is in agreement with experimental findings. The reasons for this difference in selectivity for complexes 1a-1c are discussed based on the energy barrier needed for N-methylation versus oxidative addition reactions.

Mild, Selective Sulfoxidation with Molybdenum(VI) cis-Dioxo Catalysts

Three molybdenum(VI) cis-dioxo catalysts (8-10) were synthesized with the goal of developing stable and selective oxidation catalysts for sulfoxidation. Their reactivities were investigated with a variety of substrates. We have demonstrated the usefulness of these catalysts for the chemoselective sulfoxidation of sulfides in the presence of reactive moieties, which has important applications for total synthesis processes. Notably, these catalysts are able to oxidize compounds analogous to sulfur mustard and can be used as an alternative to sodium periodate or meta-chloroperoxybenzoic acid (m-CPBA) for the oxidation of various organic sulfides without sacrificing total conversion. As the catalysts are tolerant of water and hydrogen peroxide, they allow for the design of completely green oxidation reactions, particularly for sulfur-containing amino acids.

Mechanism of Me-Re Bond Addition to Platinum(II) and Dioxygen Activation by the Resulting Pt-Re Bimetallic Center

Unusual cis-oxidative addition of methyltrioxorhenium (MTO) to [PtMe₂(bpy)], (bpy = 2,2'-bipyridine) (1) is described. Addition of MTO to 1 first gives the Lewis acid-base adduct [(bpy)Me₂Pt-Re(Me)(O)₃] (2) and subsequently affords the oxidative addition product [(bpy)Me₃PtReO₃] (3). All complexes 1, MTO, 2, and 3 are in equilibrium in solution. The structure of 2 was confirmed by X-ray crystallography, and its dissociation constant in solution is 0.87 M. The structure of 3 was confirmed by extended X-ray absorption fine structure and X-ray absorption near-edge structure in tandem with one- and two-dimensional NMR spectroscopy augmented by deuterium and ¹³C isotope-labeling studies. Kinetics of formation of compound 3 revealed saturation kinetics dependence on [MTO] and first-order in [Pt], complying with prior equilibrium formation of 2 with oxidative addition of Me-Re being the rate-determining step. Exposure of 3 to molecular oxygen or air resulted in the insertion of an oxygen atom into the platinum-rhenium bond forming [(bpy)Me₃PtOReO₃] (4) as final product. Density functional theory analysis on oxygen insertion pathways leading to complex 4, merited on the basis of Russell oxidation pathway, revealed the involvement of rhenium peroxo species.

Atomic-Level Structure Characterization of Biomass Pre- and Post-Lignin Treatment by Dynamic Nuclear Polarization-Enhanced Solid-State NMR

Lignocellulosic biomass is a promising sustainable feedstock for the production of biofuels, biomaterials, and biospecialty chemicals. However, efficient utilization of biomass has been limited by our poor understanding of its molecular structure. Here, we report a dynamic nuclear polarization (DNP)-enhanced solid-state (SS)NMR study of the molecular structure of biomass, both pre- and postcatalytic treatment. This technique enables the measurement of 2D homonuclear ¹³C-¹³C correlation SSNMR spectra under natural abundance, yielding, for the first time, an atomic-level picture of the structure of raw and catalytically treated biomass samples. We foresee that further such experiments could be used to determine structure-function relationships and facilitate the development of more efficient, and chemically targeted, biomass-conversion technologies.

Identification of the Phenol Functionality in Deprotonated Monomeric and Dimeric Lignin Degradation Products via Tandem Mass Spectrometry Based on Ion-Molecule Reactions with Diethylmethoxyborane

Conversion of lignin into smaller molecules provides a promising alternate and sustainable source for the valuable chemicals currently derived from crude oil. Better understanding of the chemical composition of the resulting product mixtures is essential for the optimization of such conversion processes. However, these mixtures are complex and contain isomeric molecules with a wide variety of functionalities, which makes their characterization challenging. Tandem mass spectrometry based on ion-molecule reactions has proven to be a powerful tool in functional group identification and isomer differentiation for previously unknown compounds. This study demonstrates that the identification of the phenol functionality, the most commonly observed functionality in lignin degradation products, can be achieved via ion-molecule reactions between diethylmethoxyborane (DEMB) and the deprotonated analyte in the absence of strongly electron-withdrawing substituents in the ortho- and para-positions. Either a stable DEMB adduct or an adduct that has lost a methanol molecule (DEMB adduct-MeOH) is formed for these ions. Deprotonated phenols with an adjacent phenol or hydroxymethyl functionality or a conjugated carboxylic acid functionality can be identified based on the formation of DEMB adduct-MeOH. Deprotonated compounds not containing the phenol functionality and phenols containing an electron-withdrawing ortho- or para-substituent were found to be unreactive toward diethylmethoxyborane. Hence, certain deprotonated isomeric compounds with phenol and carboxylic acid, aldehyde, carboxylic acid ester, or nitro functionalities can be differentiated via these reactions. The above mass spectrometry method was successfully coupled with high-performance liquid chromatography for the analysis of a complex biomass degradation mixture. Graphical Abstract ᅟ.

Lewis-Acid-assisted Hydrogen Atom Transfer to Manganese(V)-Oxo Corrole through Valence Tautomerization

The kinetics of formation of the valence tautomers (tpfc^⋅)Mn^IV(O−LA)]ⁿ⁺ [where LA=Zn^II, Ca^II, Sc^III, Yb^III, B(C₆F₅)₃, and trifluoroacetic acid (TFA); tpfc=5,10,15‐tris(pentafluorophenyl) corrole] from (tpfc)Mn^V(O) were followed by UV/Vis spectroscopy, giving second‐order rate constants ranging over five orders of magnitude from 10⁻² for Ca to 10³  m⁻¹ s⁻¹ for Sc. Hydrogen atom transfer (HAT) rates from 2,4‐di‐tert‐butyl phenol (2,4‐DTBP) to the various Lewis acid valence tautomers of manganese oxo corrole complexes were evaluated and compared. For LA=TFA, Sc^III, or Yb^III, the rate constants of HAT were comparable to unactivated (tpfc)Mn^V(O). However, with LA=B(C₆F₅)₃, Zn^II, and Ca^II, 6‐, 21‐, and 31‐fold rate enhancements were observed, respectively. Remarkably, [(tpfc^⋅)Mn^IV(OCa)]²⁺ gave the most enhancement despite its rate of formation being the slowest. Comparisons of HAT rate constants among the various Lewis acid tautomers revealed that both size and charge are important. This study underscores how valence may affect the reactivity of high‐valent manganese‐oxo compounds and sheds light on nature's choice of Ca in the activation of Mn‐oxo in the oxygen‐evolving complex.