Bifunctional Molybdenum Polyoxometalates for the Combined Hydrodeoxygenation and Alkylation of Lignin‐Derived Model Phenolics

Abstract

Reductive catalytic fractionation of biomass has recently emerged as a powerful lignin extraction and depolymerization method to produce monomeric aromatic oxygenates in high yields. Here, bifunctional molybdenum-based polyoxometalates supported on titania (POM/TiO ) are shown to promote tandem hydrodeoxygenation (HDO) and alkylation reactions, converting lignin-derived oxygenated aromatics into alkylated benzenes and alkylated phenols in high yields. In particular, anisole and 4-propylguaiacol were used as model compounds for this gas-phase study using a packed-bed flow reactor. For anisole, 30 % selectivity for alkylated aromatic compounds (54 % C-alkylation of the methoxy groups by methyl balance) with an overall 72 % selectivity for HDO at 82 % anisole conversion was observed over H PMo O /TiO at 7 h on stream. Under similar conditions, 4-propylguaiacol was mainly converted into 4-propylphenol and alkylated 4-propylphenols with a selectivity to alkylated 4-propylphenols of 42 % (77 % C-alkylation) with a total HDO selectivity to 4-propylbenzene and alkylated 4-propylbenzenes of 4 % at 92 % conversion (7 h on stream). Higher catalyst loadings pushed the 4-propylguaiacol conversion to 100 % and resulted in a higher selectivity to propylbenzene of 41 %, alkylated aromatics of 21 % and alkylated phenols of 17 % (51 % C-alkylation). The reactivity studies coupled with catalyst characterization revealed that Lewis acid sites act synergistically with neighboring Brønsted acid sites to simultaneously promote alkylation and hydrodeoxygenation activity. A reaction mechanism is proposed involving activation of the ether bond on a Lewis acid site, followed by methyl transfer and C-alkylation. Mo-based POMs represent a versatile catalytic platform to simultaneously upgrade lignin-derived oxygenated aromatics into alkylated arenes.

A Chromium Hydroxide/MIL‐101(Cr) MOF Composite Catalyst and Its Use for the Selective Isomerization of Glucose to Fructose

“…Metal–organic frameworks (MOFs) are a class of crystalline materials with high surface areas and well‐defined pore systems that hold promise for uses as adsorbents and membranes . The ability to tune their composition also provides opportunities in heterogeneous catalysis, especially when bifunctional catalysts are desirable, such as in the catalysis of tandem reactions . One such reaction is the isomerization of glucose to fructose in the presence of alcohols .…”

Differences in S/G ratio in natural poplar variants do not predict catalytic depolymerization monomer yields

“…1, a β-O-4 content of 69% only yields a maximum monomer yield of 36%. Monomeric units from lignin depolymerization have been shown to be highly valuable as they offer a diverse platform to synthesize chemicals 10–20 and functional replacements for conventional polymers 21–25 .

Fig. 1Lignin structure overview.

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Operando NAP-XPS unveils differences in MoO3 and Mo2C during hydrodeoxygenation

“…Hydrodeoxygenation (HDO) is an important upgrading strategy for converting lignocellulosic biomass to fuels and chemicals that relies on molecular H 2 to selectively remove oxygen in the form of water. [1][2][3][4][5][6][7][8][9][10][11][12][13] While noble metal catalysts have dominated the HDO landscape, molybdenum trioxide (MoO 3 ) [14][15][16][17][18][19][20] and molybdenum carbide (Mo 2 C) [21][22][23][24][25][26][27][28][29][30][31] have recently emerged as promising earth-abundant alternatives that can selectively cleave C-O bonds under mild conditions. In contrast to state-of-the-art noble metal catalysts, MoO 3 and Mo 2 C catalysts produce unsaturated hydrocarbons, while concurrently minimizing both the saturation of C=C double bonds and the cleavage of C-C bonds.…”