Temperature Tuning the Catalytic Reactivity of Cu-Doped Porous Metal Oxides with Lignin Models

Reported are the temperature dependencies of the temporal product evolution for lignin model compounds over copper-doped porous metal oxide (CuPMO) in supercritical-methanol (sc-MeOH). These studies investigated 1-phenylethanol (PPE), benzyl phenyl ether (BPE), dihydrobenzofuran (DHBF), and phenol over operating temperature ranges from 280 to 330 °C. The first three model compounds represent the β-O-4 and α-O-4 linkages in lignin as well as the furan group commonly found in the β-5 linkage. Phenol was investigated due to its key role in product proliferation as noted in earlier studies with this Earth-abundant catalyst. In general, the apparent activation energies for ether hydrogenolysis proved to be significantly lower than that for phenol hydrogenation, a major side reaction leading to product proliferation. Thus, temperature tuning is a promising strategy to preserve product aromaticity as demonstrated by the more selective conversion of BPE and PPE at lower temperatures. Rates of methanol reforming over CuPMO were also studied over the temperature range of 280–320 °C since it is this process that generates the reducing equivalents for this catalytic system. In the absence of substrate, the gaseous products H₂, CO, and CO₂ were formed in ratios stoichiometrically consistent with catalyzed methanol reformation and water gas shift reactions. The latter studies suggest that the H₂ production ceases to be rate limiting early in batch reactor experiments but also suggest that H₂ overproduction may contribute to product proliferation

Peroxidative Oxidation of Lignin and a Lignin Model Compound by a Manganese SALEN Derivative

The manganese catalyst, (1<i>R</i>,2<i>R</i>)-(−)-[1,2-cyclohexanediamino-<i>N</i>,<i>N</i>′-bis­(3,5-di-<i>t</i>-butyl­salicylidene)]­manganese­(III) chloride, was used to activate H₂O₂ to oxidize organosolv lignin and a lignin model compound. Oxidation of the β-O-4 lignin model substrate 1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)­propane-1,3-diol (320.3 <i>m</i>/<i>z</i>) and poplar organosolv lignin resulted in both fragmentation and polymerization processes, likely via phenoxy radical formation. Matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS) of the reaction products from the β-O-4 model substrate showed oligomers of the substrate with masses of 661.192, 979.355, and 1297.466 <i>m</i>/<i>z</i> that correspond to a dimer, trimer, and tetramer of the β-O-4 model substrate, respectively. Nuclear magnetic resonance (NMR) shows the formation of 5–5 diphenyl and 4-O-5 linkages in the β-O-4 model substrate oxidation products. Gel permeation chromatography (GPC) detected three peaks, corresponding to the β-O-4 model substrate and its oligomers. Products from the Mn-catalyzed oxidation of poplar organosolv lignin by H₂O₂ were analyzed by GPC, ³¹P NMR, and ¹³C NMR. GPC showed an increase by approximately four in the number-average molecular weight of organosolv lignin upon oxidation. NMR shows that polymerization occurs at positions consistent with phenoxy radical coupling, where the observed changes in guaiacyl subunit chemical shifts are most likely due to the formation of 5–5 biphenyl linkages

Probing the Lignin Disassembly Pathways with Modified Catalysts Based on Cu-Doped Porous Metal Oxides

Described are the selectivities observed for reactions of lignin model compounds with modifications of the copper-doped porous metal oxide (CuPMO) system previously shown to be a catalyst for lignin disassembly in supercritical methanol (Matson et al., <i>J. Amer. Chem. Soc</i>. 2011, 133, 14090–14097). The models studied are benzyl phenyl ether, 2-phenylethyl phenyl ether, diphenyl ether, biphenyl, and 2,3-dihydrobenzofuran, which are respective mimetics of the α-O-4, β-O-4, 4-O-5, 5-5, and β-5 linkages characteristic of lignin. Also, briefly investigated as a substrate is poplar organosolv lignin. The catalyst modifications included added samarium­(III) (both homogeneous and heterogeneous) or formic acid. The highest activity for the hydrogenolysis of aryl ether linkages was noted for catalysts with Sm­(III) incorporated into the solid matrix of the PMO structure. In contrast, simply adding Sm³⁺ salts to the solution suppressed the hydrogenolysis activity. Added formic acid suppressed aryl ether hydrogenolysis, presumably by neutralizing base sites on the PMO surface but at the same time improved the selectivity toward aromatic products. Acetic acid induced similar reactivity changes. While these materials were variously successful in catalyzing the hydrogenolysis of the different ethers, there was very little activity toward the cleavage of the 5-5 and β-5 C-C bonds that represent a small, but significant, percentage of the linkages between monolignol units in lignins

Enhancing Aromatic Production from Reductive Lignin Disassembly: <i>in Situ</i> O‑Methylation of Phenolic Intermediates

The selective conversion of lignin into aromatic compounds has the potential to serve as a “green” alternative to the production of petrochemical aromatics. Herein, we evaluate the addition of dimethyl carbonate (DMC) to a biomass conversion system that uses a Cu-doped porous metal oxide (Cu₂₀PMO) catalyst in supercritical methanol (sc-MeOH) to disassemble lignin with little to no char formation. While Cu₂₀PMO catalyzes C–O hydrogenolysis of aryl–ether bonds linking lignin monomers, it also catalyzes arene methylation and hydrogenation, leading to product proliferation. The MeOH/DMC co-solvent system significantly suppresses arene hydrogenation of the phenolic intermediates responsible for much of the undesirable product diversity via O-methylation of phenolic −OH groups to form more stable aryl-OCH₃ species. Consequently, product proliferation was greatly reduced and aromatic yields greatly enhanced with lignin models, 2-methoxy-4-propylphenol, benzyl phenyl ether, and 2-phenoxy-1-phenylethan-1-ol. In addition, organosolv poplar lignin (OPL) was examined as a substrate in the MeOH/DMC co-solvent system. The products were characterized by nuclear magnetic resonance spectroscopy (³¹P, ¹³C, and 2D ¹H–¹³C NMR) and gas chromatography–mass spectrometry techniques. The co-solvent system demonstrated enhanced yields of aromatic products

High Quantum Yield Molecular Bromine Photoelimination from Mononuclear Platinum(IV) Complexes

Pt­(IV) complexes <i>trans</i>-Pt­(PEt₃)₂(R)­(Br)₃ (R = Br, aryl and polycyclic aromatic fragments) photoeliminate molecular bromine with quantum yields as high as 82%. Photoelimination occurs both in the solid state and in solution. Calorimetry measurements and DFT calculations (PMe₃ analogs) indicate endothermic and endergonic photoeliminations with free energies from 2 to 22 kcal/mol of Br₂. Solution trapping experiments with high concentrations of 2,3-dimethyl-2-butene suggest a radical-like excited state precursor to bromine elimination

High Quantum Yield Molecular Bromine Photoelimination from Mononuclear Platinum(IV) Complexes

Pt­(IV) complexes <i>trans</i>-Pt­(PEt₃)₂(R)­(Br)₃ (R = Br, aryl and polycyclic aromatic fragments) photoeliminate molecular bromine with quantum yields as high as 82%. Photoelimination occurs both in the solid state and in solution. Calorimetry measurements and DFT calculations (PMe₃ analogs) indicate endothermic and endergonic photoeliminations with free energies from 2 to 22 kcal/mol of Br₂. Solution trapping experiments with high concentrations of 2,3-dimethyl-2-butene suggest a radical-like excited state precursor to bromine elimination

Thermodynamic and Kinetic Study of Cleavage of the N–O Bond of N‑Oxides by a Vanadium(III) Complex: Enhanced Oxygen Atom Transfer Reaction Rates for Adducts of Nitrous Oxide and Mesityl Nitrile Oxide

Thermodynamic, kinetic, and computational studies are reported for oxygen atom transfer (OAT) to the complex V­(N­[<i>t</i>-Bu]­Ar)₃ (Ar = 3,5-C₆H₃Me₂, <b>1</b>) from compounds containing N–O bonds with a range of BDEs spanning nearly 100 kcal mol^–1: PhNO (108) > SIPr/MesCNO (75) > PyO (63) > IPr/N₂O (62) > MesCNO (53) > N₂O (40) > dbabhNO (10) (Mes = mesityl; SIPr = 1,3-bis­(diisopropyl)­phenylimidazolin-2-ylidene; Py = pyridine; IPr = 1,3-bis­(diisopropyl)­phenylimidazol-2-ylidene; dbabh = 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]­hepta-2,5-diene). Stopped flow kinetic studies of the OAT reactions show a range of kinetic behavior influenced by both the mode and strength of coordination of the O donor and its ease of atom transfer. Four categories of kinetic behavior are observed depending upon the magnitudes of the rate constants involved: (I) dinuclear OAT following an overall third order rate law (N₂O); (II) formation of stable oxidant-bound complexes followed by OAT in a separate step (PyO and PhNO); (III) transient formation and decay of metastable oxidant-bound intermediates on the same time scale as OAT (SIPr/MesCNO and IPr/N₂O); (IV) steady-state kinetics in which no detectable intermediates are observed (dbabhNO and MesCNO). Thermochemical studies of OAT to <b>1</b> show that the V–O bond in OV­(N­[<i>t</i>-Bu]­Ar)₃ is strong (BDE = 154 ± 3 kcal mol^–1) compared with all the N–O bonds cleaved. In contrast, measurement of the N–O bond in dbabhNO show it to be especially weak (BDE = 10 ± 3 kcal mol^–1) and that dissociation of dbabhNO to anthracene, N₂, and a ³O atom is thermodynamically favorable at room temperature. Comparison of the OAT of adducts of N₂O and MesCNO to the bulky complex <b>1</b> show a faster rate than in the case of free N₂O or MesCNO despite increased steric hindrance of the adducts

Thermodynamic and Kinetic Study of Cleavage of the N–O Bond of N‑Oxides by a Vanadium(III) Complex: Enhanced Oxygen Atom Transfer Reaction Rates for Adducts of Nitrous Oxide and Mesityl Nitrile Oxide

Thermodynamic, kinetic, and computational studies are reported for oxygen atom transfer (OAT) to the complex V­(N­[<i>t</i>-Bu]­Ar)₃ (Ar = 3,5-C₆H₃Me₂, <b>1</b>) from compounds containing N–O bonds with a range of BDEs spanning nearly 100 kcal mol^–1: PhNO (108) > SIPr/MesCNO (75) > PyO (63) > IPr/N₂O (62) > MesCNO (53) > N₂O (40) > dbabhNO (10) (Mes = mesityl; SIPr = 1,3-bis­(diisopropyl)­phenylimidazolin-2-ylidene; Py = pyridine; IPr = 1,3-bis­(diisopropyl)­phenylimidazol-2-ylidene; dbabh = 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]­hepta-2,5-diene). Stopped flow kinetic studies of the OAT reactions show a range of kinetic behavior influenced by both the mode and strength of coordination of the O donor and its ease of atom transfer. Four categories of kinetic behavior are observed depending upon the magnitudes of the rate constants involved: (I) dinuclear OAT following an overall third order rate law (N₂O); (II) formation of stable oxidant-bound complexes followed by OAT in a separate step (PyO and PhNO); (III) transient formation and decay of metastable oxidant-bound intermediates on the same time scale as OAT (SIPr/MesCNO and IPr/N₂O); (IV) steady-state kinetics in which no detectable intermediates are observed (dbabhNO and MesCNO). Thermochemical studies of OAT to <b>1</b> show that the V–O bond in OV­(N­[<i>t</i>-Bu]­Ar)₃ is strong (BDE = 154 ± 3 kcal mol^–1) compared with all the N–O bonds cleaved. In contrast, measurement of the N–O bond in dbabhNO show it to be especially weak (BDE = 10 ± 3 kcal mol^–1) and that dissociation of dbabhNO to anthracene, N₂, and a ³O atom is thermodynamically favorable at room temperature. Comparison of the OAT of adducts of N₂O and MesCNO to the bulky complex <b>1</b> show a faster rate than in the case of free N₂O or MesCNO despite increased steric hindrance of the adducts

Thermodynamic and Kinetic Study of Cleavage of the N–O Bond of N‑Oxides by a Vanadium(III) Complex: Enhanced Oxygen Atom Transfer Reaction Rates for Adducts of Nitrous Oxide and Mesityl Nitrile Oxide

Thermodynamic, kinetic, and computational studies are reported for oxygen atom transfer (OAT) to the complex V­(N­[<i>t</i>-Bu]­Ar)₃ (Ar = 3,5-C₆H₃Me₂, <b>1</b>) from compounds containing N–O bonds with a range of BDEs spanning nearly 100 kcal mol^–1: PhNO (108) > SIPr/MesCNO (75) > PyO (63) > IPr/N₂O (62) > MesCNO (53) > N₂O (40) > dbabhNO (10) (Mes = mesityl; SIPr = 1,3-bis­(diisopropyl)­phenylimidazolin-2-ylidene; Py = pyridine; IPr = 1,3-bis­(diisopropyl)­phenylimidazol-2-ylidene; dbabh = 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]­hepta-2,5-diene). Stopped flow kinetic studies of the OAT reactions show a range of kinetic behavior influenced by both the mode and strength of coordination of the O donor and its ease of atom transfer. Four categories of kinetic behavior are observed depending upon the magnitudes of the rate constants involved: (I) dinuclear OAT following an overall third order rate law (N₂O); (II) formation of stable oxidant-bound complexes followed by OAT in a separate step (PyO and PhNO); (III) transient formation and decay of metastable oxidant-bound intermediates on the same time scale as OAT (SIPr/MesCNO and IPr/N₂O); (IV) steady-state kinetics in which no detectable intermediates are observed (dbabhNO and MesCNO). Thermochemical studies of OAT to <b>1</b> show that the V–O bond in OV­(N­[<i>t</i>-Bu]­Ar)₃ is strong (BDE = 154 ± 3 kcal mol^–1) compared with all the N–O bonds cleaved. In contrast, measurement of the N–O bond in dbabhNO show it to be especially weak (BDE = 10 ± 3 kcal mol^–1) and that dissociation of dbabhNO to anthracene, N₂, and a ³O atom is thermodynamically favorable at room temperature. Comparison of the OAT of adducts of N₂O and MesCNO to the bulky complex <b>1</b> show a faster rate than in the case of free N₂O or MesCNO despite increased steric hindrance of the adducts