One-step valorization of soda lignin in supercritical ethanol using a CuMgAlOx catalyst results in high monomer yield (23 wt%) without char formation. Aromatics are the main products. The catalyst combines excellent deoxygenation with low ring-hydrogenation activity. Almost half of the monomer fraction is free from oxygen. Elemental analysis of the THF-soluble lignin residue after 8 h reaction showed a 68% reduction in O/C and 24% increase in H/C atomic ratios as compared to the starting Protobind P1000 lignin. Prolonged reaction times enhanced lignin depolymerization and reduced the amount of repolymerized products. Phenolic hydroxyl groups were found to be the main actors in repolymerization and char formation. 2D HSQC NMR analysis evidenced that ethanol reacts by alkylation and esterification with lignin fragments. Alkylation was found to play an important role in suppressing repolymerization. Ethanol acts as a capping agent, stabilizing the highly reactive phenolic intermediates by O-alkylating the hydroxyl groups and by C-alkylating the aromatic rings. The use of ethanol is significantly more effective in producing monomers and avoiding char than the use of methanol. A possible reaction network of the reactions between the ethanol and lignin fragments is discussed.
In
this Perspective, we highlight the main challenges to be addressed
in the development of heterogeneous catalysts for the direct functionalization
of methane. Along with our personal view on current developments in
this field, we outline the main mechanistic, engineering, and catalyst
design issues that have hampered implementation of new technologies
and highlight possible paths to overcome these problems.
Gold nanoparticles (AuNPs) supported on MgCuCr2O4-spinel are highly active and selective for the aerobic oxidation of ethanol to acetaldehyde (conversion 100%; yield ∼95%). The catalyst is stable for at least 500 h. The unprecedented catalytic performance is due to strong synergy between metallic AuNPs and surface Cu(+) species. X-ray photoelectron spectroscopy shows that Cu(+) is already formed during catalyst preparation and becomes more dominant at the surface during ethanol oxidation. These Cu(+) species are stabilized at the surface of the ternary MgCuCr2O4-spinel support. Further kinetic measurements indicate that the Cu(+) species act as sites for O2 activation.
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This journal isObtaining renewable fuels and chemicals from lignin presents an important challenge to the use of lignocellulosic biomass to meet sustainability and energy goals. We report on a thermocatalytic process for the depolymerization of lignin in supercritical ethanol over a CuMgAlO x catalyst. Ethanol as solvent results in much higher monomer yields than methanol. In contrast to methanol, ethanol acts as a scavenger of formaldehyde derived from lignin decomposition. Studies with phenol and alkylated phenols evidence the critical role of the phenolic -OH groups and formaldehyde in undesired repolymerization reactions. O-alkylation and C-alkylation capping reactions with ethanol hinder repolymerization of the phenolic monomers formed during lignin disassembly. After reaction in ethanol at 380 °C for 8 h, this process delivers high yields of mainly alkylated mono-aromatics (60-86 wt%, depending on the lignin used) with a significant degree of deoxygenation. The oxygen-free aromatics can be used to replace reformate or can serve as base aromatic chemicals; the oxygenated aromatics can be used as low-sooting diesel fuel additives and as building blocks for polymers.
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The isomerization of glucose to fructose in the presence of Sn-containing zeolite BEA (beta polymorph A) was studied by periodic DFT calculations. Focus was placed on the nature of the active site and the reaction mechanism. The reactivities of the perfect lattice Sn(IV) site and the hydroxylated SnOH species are predicted to be similar. The isomerization activity of the latter can be enhanced by creating an extended silanol nest in its vicinity. Besides the increased Lewis acidity and coordination flexibility of the Sn center, the enhanced reactivity in this case is ascribed to the reaction environment that promotes activation of the confined sugar intermediates through hydrogen bonding. The resulting multidentate activation of the substrate favors the rate-determining hydrogen-shift reaction. These findings suggest the important role of defect lattice sites in Sn-BEA for catalytic glucose isomerization.
A novel route for the introduction of well-defined zinc
species
into ZSM-5 zeolite via chemical vapor deposition of dimethylzinc (CVD(DMZ))
is explored. The structural properties and catalytic reactivity of
the synthesized material (Zn/ZSM-5-CVD(DMZ)) are investigated against
a set of Zn/ZSM-5 catalysts prepared by incipient wetness impregnation
(IWI), ion exchange (IE), and high-temperature reaction with zinc
vapor (CVD(m)). The materials are characterized by a range of physicochemical
methods including temperature programmed reduction (TPR), in situ
FTIR, magic-angle spinning (MAS) NMR, and X-ray absorption spectroscopy
(XAS). The catalysts are tested for their activity in the dehydrogenation
of propane. Catalysts prepared by IE and IWI exhibit a high degree
of heterogeneity of extraframework zinc species. These include, besides
isolated Zn2+ cations, multinuclear oxygenated zinc clusters
and bulk zinc oxide aggregates. The CVD(m) method results in quantitative
replacement of all Brønsted acid protons by isolated Zn2+. In CVD(DMZ) the Brønsted acid sites (BAS) react stoichiometrically
with dimethylzinc Zn(CH3)2 (DMZ) yielding grafted
[Zn-CH3]+ species, which can further be transformed
to isolated Zn2+ ions by reduction in hydrogen. The presence
of zinc in ZSM-5 enhances the rate of alkane dehydrogenation. The
initial activity of Zn/ZSM-5 prepared by IWI and IE correlates with
the Zn content. The samples with a more heterogeneous distribution
of extraframework Zn species are more active than the samples with
isolated Zn2+. The activity of reduced Zn/ZSM-5-CVD(DMZ)
containing predominantly isolated Zn2+ ions can be substantially
increased by oxidation prior to the reaction. However, the resulting
oxygenated complexes easily decompose during the reaction. Propane
dehydrogenation and catalyst stability of Zn/ZSM-5-CVD(DMZ) can be
improved by addition of steam to the hydrocarbon feed. This rate enhancement
is ascribed to an increase of the steady-state concentration of the
reactive oxygenated sites.
Aberration-corrected transmission electron microscopy and high-angle annular dark field imaging was used to investigate the surface structures and internal defects of CeO2 nanoparticles (octahedra, rods, and cubes). Further, their catalytic reactivity in the water-gas shift (WGS) reaction and the exposed surface sites by using FTIR spectroscopy were tested. Rods and octahedra expose stable (111) surfaces whereas cubes have primarily (100) facets. Rods also had internal voids and surface steps. The exposed planes are consistent with observed reactivity patterns, and the normalized WGS reactivity of octahedra and rods were similar, but the cubes were more reactive. In situ FTIR spectroscopy showed that rods and octahedra exhibit similar spectra for -OH groups and that carbonates and formates formed upon exposure to CO whereas for cubes clear differences were observed. These results provide definitive information on the nature of the exposed surfaces in these CeO2 nanostructures and their influence on the WGS reactivity.
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