Lignin Fragmentation onto Multifunctional Fe₃O₄@Nb₂O₅@Co@Re Catalysts: The Role of the Composition and Deposition Route of Rhenium

Abstract

Multifunctional Fe3O4@Nb2O5@Co@Re catalysts with metal loadings in the range of 2–7 wt % were prepared via a multistep process. Magnetic nanoparticles prepared by coprecipitation were covered with a niobia shell precipitated from an ammonium niobate–oxalate complex in the presence of hexadecyltrimethylammonium bromide. The deposition of cobalt was performed using a deposition/precipitation procedure. Finally, rhenium has been deposited following three different routes: (i) impregnation, (ii) deposition and precipitation of rhenium chloride (ImC and PP, respectively), and (iii) impregnation with ammonium perrhenate (ImA). The characterization of these catalysts was achieved by X-ray diffraction, Raman, H2 and NH3 temperature-programmed desorption, X-ray photoelectron spectroscopy, and transmission electron microscopy showing the influence of the preparation procedure, reduction, and Re/Co cooperation on the degree of dispersion and reduction. ImC and ImA routes led to more reduced catalysts, and the decrease in cobalt content corresponded to more reduced rhenium. An inverse relation between the acidity and the degree of reduction has been found. The screening of these catalysts in the fragmentation of the lignin confirmed the role of the structural characteristics and solvent. ImC catalysts exhibited better catalytic activity, especially for low metal loadings [2%Co@3%Re, 85% yield of LF, 15.5% yield of LR in tetrahydrofuran (THF), and 14.5% yield of insoluble LR in THF]. Although the extent was smaller, the PP catalysts allowed a more advanced fragmentation of the lignin to fragments with molecular weights between 200 and 400 Da. The catalysts were totally recovered by application of a magnetic field and recycled six times without any loss of activity or selectivity.

Highly Efficient, Easily Recoverable, and Recyclable Re–SiO2–Fe3O4 Catalyst for the Fragmentation of Lignin

“…34 The absence of the satellite peak was also confirmed in this study (Figure S2A), representing an additional argument in the favor of the presence of magnetite. The peak positions of Fe 2p3/2 and Fe 2p1/2 at 710.7−711.7 eV and 726.0−721.1 eV (Table 2) 25 also correspond to those previously reported for magnetite. 34 However, besides these bands, there was a very weak signal at 706.5−706.9 eV, indicating a very small reduction of magnetite to iron(0).…”

“…Higher M W than the raw material is an effect of the aggregation tendency, as reported previously. 25,44 Lignin fragmentation was monitored for 24 h (Figure 7). The yield in LF reached maximum values after the first 3 h. Then, the yield of LF was constant, while a slight enhancement of the yield of LRs/THF was achieved against the yield of LR Insol /THF (Figure 7A).…”

“…23 At the same time, metallic rhenium acts as a Lewis acid because of its low occupancy of the d band. 16 On the basis of the previous research 24,25 for the concerted hydrolysis and hydrogenolysis of lignin, the aim of this study was to simplify the catalyst by depositing rhenium directly onto a magnetic Fe 3 O 4 core embedded in a silica layer (Fe 3 O 4 @ SiO 2 ) . Rhenium was deposited on these catalysts by impregnation and by precipitation-deposition.…”

Functionalised heterogeneous catalysts for sustainable biomass valorisation

“…Various coating materials, such as SiO 2 , g-Al 2 O 3 , Nb 2 O 5 -SiO 2 , organosilica, carbon, polymers, and hydroxyapatite have been used for the preparation of core-shell iron oxide based magnetic catalysts. 347,348 Primary applications of these catalysts in biomass valorisation refer to cellulose transformation, lignin fragmentation, and further upgrading of hereby obtained depolymerised (model) compounds.…”

“…The enzyme immobilised magnetic Fe 3 O 4 @SiO 2 catalysts exhibited reasonably good efficiencies for the production of biodiesel via transesterification of triglycerides with alcohols. Opris et al 347 studied the role of the composition and deposition method of rhenium on the catalytic efficiency of a multifunctional Fe 3 O 4 @Nb 2 O 5 @Co@Re catalyst for lignin fragmentation. The catalyst (2%Co@3%Re) synthesised via an impregnation method showed the best performance with a 85% yield of lignin-derived fragments.…”