Highly efficient, easily recoverable and recyclable Re-SiO2-Fe3O4

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Highly efficient, easily recoverable and recyclable ReSiO2-Fe3O4 catalyst for the fragmentation of lignin Madalina Tudorache, Cristina Opris, Bogdan E. Cojocaru, Nicoleta Georgiana Apostol, Alina Tirsoaga, Simona M. Coman, Vasile I. Parvulescu, Bahir Duraki, Frank Krumeich, and Jeroen Anton van Bokhoven ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04294 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 25, 2018

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ACS Sustainable Chemistry & Engineering

Highly efficient, easily recoverable and recyclable Re-SiO2-Fe3O4 catalyst for the fragmentation of lignin Madalina Tudorache†, Cristina Opris†, Bogdan Cojocaru†, Nicoleta G. Apostol‡, Alina Tirsoaga†, Simona M. Coman†, Vasile I. Parvulescu†*, Bahir Duraki#, Frank Krumeich#, Jeroen A. van Bokhoven#,∫* †

University of Bucharest, Department of Organic Chemistry, Biochemistry and Catalysis, B-dul Regina Elisabeta 4-12, Bucharest 030018, Romania. ‡

National Institute of Materials Physics, Atomistilor 105b, 077125 Magurele-Ilfov, Romania.

#

ETH Zurich, Institute for Chemical and Bioengineering, Vladimir-Prelog-Weg 1-5/10, 8093 Zurich, Switzerland. ∫

Paul Scherrer Institute, 5232 Villigen, Switzerland.

*Email address of corresponding authors: [email protected] [email protected]

ACS Paragon Plus Environment

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ABSTRACT

A series of Fe3O4@SiO2@Re catalysts were prepared by deposition of rhenium by means of the precipitation-deposition and impregnation procedures. Characterization of the catalysts confirmed that the deposition of silica protected the magnetite nanoparticles, resulting in a stable SiO2@Fe3O4 composite, which was not affected by the treatments during the further deposition of rhenium. Rhenium was silent in XRD over the range of concentrations, at which it was deposited. Furthermore, XPS detected rhenium, only in the impregnation series; this may confirm that dispersion was high. As expected, rhenium was not reduced to the metallic state and generated weakly acidic Brønsted-type centers as detected by NH3-TPD. H2-TPD and chemisorption experiments demonstrate the capacity of these catalysts to chemisorb hydrogen. In line with these properties rhenium catalyzed both C-C hydrogenolysis and C-O hydrolysis in successive steps. The performance of these catalysts was checked for a series of lignins of different origin and by means of different separation procedures. A very important finding is that these catalysts were highly stable and easy to recover.

KEYWORDS Catalytic fragmentation of lignin, Re-based catalysts, C-C hydrogenolysis, C-O hydrolysis.

INTRODUCTION Biomass is recognized as the only renewable source of organic carbon1. An attempt was made to replace fossil fuels with biomass and, as a result, to protect the environment2-3. It was also found that the large content of oxygen in biomass makes it even more suitable for the production of chemical commodities4. However, the biomass consists of different kinds and amounts of


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polymeric molecules. Therefore, to achieve chemical valorization of these materials, irrespective of the targeted products, it is compulsory to separate the polymers followed by a depolymerization. Biomass contains large amounts of water5. Thus, processing it in organic solvents to produce chemicals the biomass must be dried, which uses a significant amount of energy. Furthermore, the separation and purification of the solvents are not environmentally friendly processes. Based on these, water emerged as the more preferred solvent6. In addition, water is a green solvent for many types of molecules. Impurities, such as those generated by the fragmentation of biomass, are of no concern. However, the main disadvantage of water as a solvent is the reduced solubility of long-chain substrates7. Lignin is a major component of lignocellulosic biomass, in which aromatic entities are connected by methoxylated phenyl-propane units8, which include C-C and etheric C-O bonds. Recent reports on catalysis indicate that it is possible to disrupt these bonds9-10 and the alkyl-aryl ethers are the most easily cleaved linkages11. The bond energy is important, because it affects the ability to break it. For the ether bonds the energy varies in the following order: α-O-4 (156 203kJ/mol) < β-O-4 (226–303 kJ/mol) < 4-O-5 (~346 kJ/mol)12. Another prerequisite of this procedure is to preserve the aromatic character of the fragments13. Fragmentation of lignin can generate a cocktail of monomers, such as veratryl, vanillyl, sinapyl-, coniferyl- and p-coumaryl alcohols8. It is possible to upgrade these molecules to a number of chemical commodities11. But the fragmentation of lignin encompasses several difficulties generated by its origin, the area of production, the climate, etc. The main difficulty is related to its high structural heterogeneity14, which does not favor depolymerization10. Lignin can be cleaved in the presence of different types of catalysts, including biocatalysts as well as


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homogeneous and heterogeneous catalysts. Biocatalysis has low reaction rates and is based on expensive systems that cannot be recycled. A main advantage of homogeneous catalysts is that they can penetrate lignin and reach and cleave the linkages, leading to higher reaction rates15. However, it is almost impossible to recover them. Heterogeneous catalysis ensures recyclability, but the penetration of the catalyst through the lignin is very poor, and separation of the catalyst is almost impossible and conversion is never complete. To achieve advanced fragmentation, the catalysts should be able to disrupt both the C-C and the etheric C-O bonds16. Thus, they should exhibit both a metal function to achieve hydrogenolysis of the C-C bond and be acidic one because the C-O linkages are affected by very strong acid and base media17. In accordance, for heterogeneous catalysts, the metal function can be provided either by a transition metal18 or a noble metal19 and the acidity by strong acid materials such as zeolites. We showed that magnetic nanoparticles enable recovery of the catalysts, with rhenium, niobia and cobalt. The acidity of rhenium is attributed to the Brønsted acidity of hydroxylated Re atoms arising from strong Re-O bonds, resulting in a weak O-H bond as well as high electron affinity for the conjugate base20. However, the acidity of the catalysts based on rhenium is strongly affected by the metal support-interaction and by the dispersion of the metallic active species21. Furthermore, the interaction between rhenium oxides and a second metal may affect the surface properties of the catalytic materials22. Utilization of rhenium in bifunctional catalysts leads to an increase in activity of C-O scission reactions and for the hydrogenation of carboxylic acids, in accordance with oxophilic nature of the acids. Hydrodeoxygenation of 2-methoxyphenol is another example of this capacity23. At the same time, metallic rhenium acts as a Lewis acid because of its low occupancy of the d band16.


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Based on the previous research24-25 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 Fe3O4 core embedded in a silica layer (Fe3O4@SiO2). Rhenium was deposited on these catalysts by impregnation and by precipitation-deposition. Other aims were to utilize water as a solvent and to determine its effect on fragmentation of various types of lignin.

EXPERIMENTAL Materials All the reagents were of analytic purity and were used without further purification: ammonium niobate (V) oxalate hydrate (99.99% trace metal basis, Sigma-Aldrich) iron (III) nitrate nonahydratate (ACS reagent, >98%, Sigma-Aldrich), iron (II) chloride tetrahydrate (p.a., SigmaAldrich),

ammonium

hydroxide

solution

33wt%

NH3

(puriss,

Riedel-de

Haen),

hexadecyltrimethylammonium bromide (C19H42BrN) (purum, >96%, Fluka), tetrahydrofuran (inhibitor-free, ChromatosolvRPlus, for HPLC, >99.99%, Sigma-Aldrich), ethanol (Chemical Company),

acetone

(Chemical

Company),

rhenium

(V)

chloride

(Sigma-Aldrich),

tetraethylorthosilicate (98%, Acros Organics), 2-phenethyl phenyl ether (Carbone Scientific CO., LTD), bibenzyl (Sigma-Aldrich).

Preparation of catalysts Magnetic nano-particles Fe3O4@silica@Re were synthesized by a procedure reported earlier24. Rhenium, in the form of rhenium (V) chloride, was deposited on these catalysts by impregnation and precipitation-deposition. Synthesis of Fe3O4@SiO2


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Magnetite nanoparticles were synthesized by co-precipitation24. Then, a layer of silica was deposited onto the magnetite as follows: 2g magnetite were dispersed in 160 mL ethanol and 40 mL water and then ultrasonicated for 5 min in an inert atmosphere (Ar). Five grams of tetraethyl ortho silicate and 5 mL NH3 (25%) were added; the mixture was stirred for 12h at 40ºC in the same inert atmosphere. The resulting precipitate was washed with water and ethanol until the pH was 7, dried at room temperature and calcined at 180ºCfor 4 h at a heating rate of 1ºC min-1. According to the literature, the silica shell forms in two steps: (i) the hydrolysis of TEOS in ethanol in the presence of ammonium hydroxide as the catalyst, followed by (ii) the polymerization, during which siloxane (Si-O-Si)bonds form and are anchored on the surface of the magnetic particles26. Synthesis of Fe3O4@SiO2@Re by the precipitation-deposition method (PD series) The synthesis methodology of Fe3O4@SiO2@Re PD began with 0.5g SiO2@Fe3O4, which interacted with different amounts of rhenium (V) chloride (0.01, 0.02, 0.03, and 0.04g ReCl5) dissolved in 10 mL distilled water. The precipitation of rhenium oxide was performed with ammonium hydroxide (33 wt% NH3 in water) until pH 9 was reached. To achieve complete precipitation the above mixture was stirred mechanically at 60°C for 4 h. The precipitates were then filtered, washed with distilled water until pH 7, and dried at room temperature for 24 h. Then, the solids were calcined at 500°C with a heating rate of 1°C·min-1 for 3h followed by treatment with a flow of hydrogen at 30 mL·min-1and 450°C (heating rate of 5°C·min-1) for 5h. Cooling was achieved in a flow of 30 mL·min-1 argon. The resulting catalysts are referred to as x%Fe3O4@SiO2@Re PD (x=1-4). Synthesis of Fe3O4@SiO2@Re by impregnation (IMP series)


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Fe3O4@SiO2@Re IMP was synthesized by the dry impregnation of 0.5g Fe3O4@SiO2 with the same amounts of ReCl5 (0.01, 0.02, 0.03, 0.04g ReCl5) dissolved in 3 mL acetone. Drops of the resulting solutions were deposited on the starting material. Prior to use as a support, SiO2@Fe3O4 was dried at 110°C in vacuum for 6h. After impregnation, the drying step was repeated and the samples calcined at 500°C for 4h at a heating rate of 1°C·min-1. Then, reduction of the catalysts was performed in a similar way as for Fe3O4@SiO2@Re PD ones (450°C for 5h in a hydrogen flow of 30 mL·min-1 at a heating rate of 5°C·min-1). Cooling took place in a flow of argon (30 mL·min-1). The resulting catalysts are referred to as x%Fe3O4@SiO2@Re IMP (x=1-4wt% Re).

Catalyst characterization The resulting catalysts were characterized by different techniques. Textural characteristics (surface area and pore diameter) were determined from the adsorption-desorption isotherms of nitrogen at -196°C using a Micromeritics ASAP 2020 Surface Area and Porosity Analyzer. Powder X-ray diffraction (XRD) patterns were recorded with a Schimadzu XRD-7000 diffractometer with Cu Kα radiation (λ = 1.5418 Å, 40 kV, 40 mA) at a step of 0.2o and a scanning speed of 2o·min-1 in the 2theta range of 5 – 90o. Chemical composition was determined by ICP-OES after solubilization. H2-TPD and NH3-TPD measurements were carried out with an AutoChem II 2920 station. The sample, in a U-shaped quartz reactor with an inner diameter of 0.5 cm, was pre-treated under helium (Purity 5.0, from Linde) at 100oC for 30 min and cooled at room temperature. Then, it was exposed to a flow of 5% hydrogen in helium or 1% ammonia in helium at a flow rate of 10 cm3(STP) min-1. After adsorption of hydrogen or ammonia, the sample was purged in a flow of helium (30 cm3(STP) min-1) at room temperature for 2h and then heated at a rate of 10oC·min-1


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until 450oC and 400oC, respectively. Pulse H2 chemisorption was performed with the same AutoChem II 2920 station. The samples were pre-treated in helium (Purity 5.0 from Linde) at 100oC for 30 min and cooled at room temperature. At room temperature, pulses of 5vol% hydrogen in helium were introduced until the peak areas were the same. Raman spectra were collected with a Horiba JobinYvon – Labram HR UV–Visible–NIR (200– 1600 nm) Raman Microscope Spectrometer with a laser with a wavelength of 633 nm. The spectra were collected from 10 scans at a resolution of 2 cm-1. XPS measurements were performed at normal emission in a Specs setup by using Al Kα -monochromated radiation (hν = 1486.7 eV) from an X-ray gun operating at 300 W (12 kV/25 mA) power. A flood gun with an electron acceleration of 1 eV and electron current of 100 µA was used to avoid charge effects. Photoelectrons were energy-recorded at normal emission by using a Phoibos 150 analyzer, operating at a pass energy of 30 eV. The XPS spectra were fitted by Voigt profiles combined with their primitive functions for inelastic backgrounds24. The Gaussian width of all the lines and thresholds was the same for one spectrum and did not differ considerably from one spectrum to another, being always in the range of 2 eV. Analyses focused on the characterization of the O1s, Si2p,Re4f and Fe2p3/2 energy levels. To limit reoxidation, the reduced samples were transferred from the reduction set-up to the Raman and XPS apparatus under isooctane. Transmission electron microscopy (TEM) was done on a Tecnai F30 (FEI, FEG) microscope, operated at an acceleration potential of Uacc = 300 kV. Samples were dispersed in ethanol and some droplets deposited on a C-foli supported on a Cu grid. EDS mappings were recorded at a Talos (FEI, 200 KV, high brightness gun (XFEG), 4 EDX spectrometers (Bruker)).

Lignin characterization


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For fragmentation the lignin was of different types: L1- Miscanthus lignin extracted under alkali conditions, L2- Miscanthus lignin extracted under acidic conditions , L3- Beech lignin extracted by organosolv approach, L4- Beech lignin extracted by Klason approach, L5- Beech lignin extracted by acid-H2O2 treatment, L6- Pine lignin extracted by organosolv approach, L7Pine lignin extracted by soda delignification followed by acidification with sulfuric acid, L8Pine lignin extracted under alkali conditions, and L9- Pine lignin extracted by Klason approach. These lignins were characterized by NMR (1H-NMR spectroscopy, BrukerAvance III 500 MHz spectrometer in DMSO-d6 solvent and Me4Si as the internal standard) and by DTA-DTG (nitrogen atmosphere, 5 mL·min-1 gas flow, temperature in gradient with 10oC·min-1rate until a temperature of 900oC).

Lignin fragmentation Lignin fragmentation was carried out in batch experiments while stirring in a 16 mL autoclave (HEL), starting from 100 mg lignin as the raw material dispersed in 5 mL water together with 10 mg catalyst. The reactions were carried out from 100 to 230oC at 5 to 15 bars H2; the reaction time in the range 1 to 24 h. The autoclave was flushed several times with hydrogen before to pressurize.

Analytical measurements After the reaction, the reacted mixture was separated by centrifugation into two phases (liquid and solid). Both phases were evaluated to identify and quantify the lignin fragments (Scheme 1). The liquid phase was characterized in a HPLC modular system (Agilent Technology 1290 instrument equipped with Zorbax C18 column (4.6 x 150 mm, 5 µm) with DAD detector


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(Agilent Technology, 1260 Infinity) and MS detector (6400 Series Triple Quadrupole LC/MS)). The conditions in the HPLC were: gradient of the mobile phase (MeOH/H2O) starting from 5 % MeOH up to 70 % MeOH in 60 min, a flow rate of 0.8 mL·min-1 and 50oC. The absorbance was read at 280 nm (DAD detection). HPLC- DAD and MS/MS analyses were performed for the quantification of the fragmentation products. MS spectra were analysed with Xcalibur MS data base27. TOC system (1200 Thermo Analyzer) was used to investigate total carbon (lignin) content of the liquid phase. Based on the analytical measurements we calculated the yield of the light fragments (LF) with a molecular mass in the range of 200-400, 400-600 and 600-850 (e.g. C20-28, C29-37 and C38-40, respectively). After centrifugation, the solid phase (lignin residue – LR) was dried at 70°C under vacuum until constant mass in order to quantify the lignin introduced in the catalytic reaction and the fragmentation products. LR was re-dispersed in 1 mL THF while stirring for 1 h leading to a THF solution of soluble lignin fragments (LR soluble in THF). After separation from the THF phase by centrifugation, the insoluble fraction of the solid (LR insoluble in THF) was acetylated according to published protocols28. The analysis of LR (soluble/ insoluble in THF) was carried out by GPC (Gel Permeation Chromatography) using an Agilent Technologies instrument (1260 model) equipped with two columns (Zorbax PSM 60-S, 6.5 x 250 mm, 5 µm and Polargel-M, 300 x 7.5 mm) and a multi-detection unit (260 GPC/SEC MDS containing RID, LS and VS detectors). GPC analysis was run under the following conditions: flow rate of THF mobile phase of1 mL·min-1, injection volume of 100 µL and temperature of 35°C. The calibration of the GPC system was performed in the range of 162 to1,000,000 gmol-1 polystyrene with good accuracy of the measurements for MW >1000. GPC chromatograms enabled the calculation of the average


10

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values of the molecular mass of the lignin fragments. Thus, the yield of the lignin fragments and residue were based on the following equations (1-4). ! " #$%

, % "& & " !!! % ' 100

%

%! *+',-

(2)

.//

01/34 , %

*5 6! #7+ "& &

081/ 34 , %

' 100

*5 6! #7+ "&&

(1)

' 100

(3)

(4)

where: i =C20-28, C29-37 or C38-40, and Si is the selectivity of i fraction determined by HPLC-DAD and MS/MS. Weight of coke was determined by weighing the residue after acetylation of the LR, which is insoluble in THF.

Scheme 1. Pre-treatment and analysis of the reacted mixture.


11


The solid product was analyzed by 1H- and

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C NMR spectroscopy (BrukerAvance III 500

MHz spectrometer, in DMSO-d6 solvent and Me4Si as the internal standard). The crude product was separated by distillation of water under vacuum and analyzed by 1H - 1H COSY NMR, 1H 13

C HMQC NMR and 1H - 13C HMBC NMR (500 MHz, DMSO-d6). Spectra of the raw material

(lignin) and fragmentation products took place after the extraction of the crude product in methanol.

RESULTS AND DISCUSSION Table 1 compiles the textural properties of the investigated catalysts. The samples prepared according to the PD protocol showed slightly higher surface areas then the corresponding IMP. In general, an increase of the Re loading led to a lower surface area.

Table 1. Textural properties of the reduced x%Fe3O4@SiO2@Re catalysts Fe3O4@SiO2@Re

Surface area

Preparation

wt% Re

m2g-1

Fe3O4

-

127

Fe3O4@SiO2

-

12

PD

1

67.2

PD

2

65.9

PD

3

60.9

PD

4

57.4

IMP

1

48.9

IMP

2

46.6

IMP

3

41.2

IMP

4

38.7


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Figure 1 depicts the XRD patterns of the 2wt%Fe3O4@SiO2@Re catalysts before and after reduction with hydrogen. Only the characteristic XRD pattern of magnetite was observed for all the samples. The lines at 2theta = 30.1, 35.5, 43, 53, 57, and 62.5o correspond to the (220), (311), (400),(422), (511)and (440)facets of Fe3O429. No reduced iron phases were identified in these diffractograms, confirming the protective role of the silica shell. Silica was not detected in these diffractograms, which, in accordance with the weak intensity at 2theta 20-30o, corresponds to an amorphous phase. No reflections of therhenium phases were observed as a result of their very

(620) (533)

(440)

(422) (511)

(400)

(220)

(111)

(311)

small particle size.

d

Intensity, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


c

b a a

10

20

30

40

50

60

70

80

90

2Theta, degrees Figure 1. XRD patterns of the 2wt%Fe3O4@SiO2@Re catalysts: a) PD; b) reduced PD; c) IMP; d) reduced IMP.


13


Figure 2 presents the Raman spectra with lines (687, 482, 341 cm-1) assigned to octahedral rhenium oxide (Figure 2A)30-31. After covering the magnetic core nanoparticles with a silica shell the lines at 660 and 288cm-1, attributed to Fe2+ and Fe3+ species in magnetite, were very weak3233

. The reduction with hydrogen preserves these profiles (Figure 2B), suggesting that the rhenium

oxidation state is preserved. Raman spectra of the PD catalysts (Figure 2C) corresponded to much weaker signals than those recorded for the IMP series, again in accordance with the dispersion of rhenium on these catalysts. Re

Re

Re

Re

Re

A

Re

B

a

b

Intensity, a.u.

Intensity, u.a.

b

a

c

1400

Fe3O4

1200

1000

800

600

400

200

Fe3O4

Fe3O4

Fe3O4

1400

1200

1000

-1

Raman Shift, cm

800

600

400

200

-1

Raman Shift, cm

Re Re

C Re

b

Intensity, u.a.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 41

a Fe3O4

1400

1200

1000

800

600

400

200

-1

Raman Shift, cm


14

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Figure

2.

Raman

spectra

of

(A)

unreduced

a)

1%Fe3O4@SiO2@Re

IMP,

b)

3%Fe3O4@SiO2@Re IMP, c) 4%Fe3O4@SiO2@Re IMP; (B) reduced a) 2%Fe3O4@SiO2@Re IMP, b) 4%Fe3O4@SiO2@Re IMP and (C) reduced a) 2%Fe3O4@SiO2@Re PD, and b) 4%Fe3O4@SiO2@Re PD catalysts. Table S1 and Figures S1-3 give the results of the XPS analysis. The binding energy of the O1s level is specific to oxygen in oxides in all the samples. Furthermore, core catalysts in the XPS investigation had binding energies corresponding to iron in Fe3O4and to silicon in SiO2. Previous reports indicated that the spectrum of Fe 2p3/2 for Fe3O4 does not have a satellite peak34. The absence of the satellite peak was also confirmed in this study (Figure S2 A), 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 magnetite34. 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)35. Silicon with a binding energy in the range of 101.7 to 102.9 eV confirmed its presence in a tetrahedral configuration surrounded by oxygen atoms36. The presence of rhenium was detected only in the IMP series. According to Iwasawa

37

the peak at 42.8 eV, is assigned to the Re 4f7/2 level for

Re4+ species, that at 48.3 eV, to the Re 4f5/2 level for the Re6-7+ species, and the most intense peak around 45.7 eV to the sum of the Re 4f7/2 peak for the Re6-7+ species and the Re 4f5/2 peak for the Re4+ species. Compared to unreduced catalyst (Figure S3B b) the exposure to hydrogen at 450oC (Figure S3B c) changes the ratio of Re4+/ Re6-7+ in the favor of the reduced one. The spectrum of the unreduced catalyst does not contain the band located at 42.8 eV. In the PD series the dispersion of Re was beyond the limits of detection by XPS. However, its presence has been confirmed by the EDS analysis (Figure S4). The comparative XPS surface and the chemically


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determined composition, taking into account the Re/Si atomic ratios, confirm the very good distribution of rhenium. Table 2 lists the H2-TPD and H2-chemisorption results for the Fe3O4@SiO2@ReIMP and PD catalysts. Hydrogen is desorbed from 100 to 450oC. Based on other literature studies, it is inferred that the hydrogen atoms derived from those bound to metallic sites (such as Re0) or from atomic hydrogen inserted into ReOx38. Based on the XPS results for the reduced catalysts, Re preserved its (VI) oxidation rate, even after reduction, which may suggest that hydrogen exists indeed in the inserted state. This model also accounts for the desorption of hydrogen, which occurs irrespective of the Re loading or the method of preparation. H2 chemisorption data are in agreement with the XPS results. The Fe3O4@SiO2@Re PD series had higher metal dispersion and a larger metallic surface area compared to the IMC series. The data presented in Table 2 should be correlated to the particle size. The decrease in the rhenium loading corresponded to a decrease in the particle size, which afforded a higher number of adsorbed H2 molecules per atom Re. Furthermore, these data suggest that the IMP procedure led to larger particles of Re compared to the PD procedure.

Table 2. H2 chemisorption and TPD results for the Fe3O4@SiO2@Re catalysts. H2-uptake (mmol g-1)

Re loading, %

Fe3O4@SiO2@Re IMP series

Fe3O4@SiO2@Re PD series

H2chemisorption

H2 TPD

H2chemisorption

H2 TPD

1

1.1

0.94

21.0

1.10

2

1.7

1.41

17.4

1.65

3

2.1

2.01

15.7

1.70

4

2.4

2.23

14.6

0.46


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The amount of desorbed H2 and the maximum desorption temperature are dependent on the concentration of Re and the precursors used for the preparation of the catalysts (see Table 2). These affected both the oxidized state of Re and its structural, electronic, and adsorption characteristics thus explaining the differences in the chemisoption properties of the samples prepared by the two methods. In the same time, the different temperatures of H2 desorption are associated to the existence of several chemisorbed forms39. According to literature reports, both surface and volume defects are formed during the thermal reduction. These can serve as traps for hydrogen that is released at higher temperatures. This model explains the step desorption of hydrogen irrespective of the Re loading and preparation route. Redox reaction involving the participation of metallic Re were also assumed (Me0 + nH+ → Mem + H2) 40. However, since the XPS characterization of our samples did not reveal metallic Re, this possibility can be ruled out. The high adsorbed hydrogen/rhenium ratios could be explained as an effect of a dimensional factor and a spill-over effect correlated with the acidity of the samples41. Even after the reduction in hydrogen, rhenium was not reduced to metallic state. The oxidized species can transfer the hydrogen due to a spill-over effect. Also, the volume defects created in this process can serve as traps for the H2 which penetrates under the surface and then is released at higher temperatures. On the other hand, the aggregation of Re is enhanced during the thermal treatment, and increases with the Re content. This leads to a decrease of the number of defects capable of trapping hydrogen, as it can be observed in both the surface area trend and H2-TPD. Also, the acidity of the samples (the capacity of “capturing” protons) is decreasing with the increase of the Re content as demonstrated by NH3-TOD experiments. Table 3 and Figure 3 show results of the NH3-TPD experiments. With the exception of the 1wt% Fe3O4@SiO2@Re PD catalyst, the desorption of NH3 started at 100oC and ended below


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250oC and is characterized by broad desorption peaks. For the 1wt% Fe3O4@SiO2@Re PD catalyst the onset of NH3 desorption occurred at a slightly lower temperature. According to the literature, this feature can be assigned to an acid site related to the presence of the Re-OH species42. The increase in the Re loading was accompanied by a decrease in the number of the adsorbed NH3 molecules over the Re atom, which may also be attributed to the decrease in the dispersion.

Table 3. NH3-TPD results for the Fe3O4@SiO2@Re IMP and PD catalysts

Temperature (°C)

Adsorbed NH3 molecules (mmol/g)

Adsorbed NH3 molecules/Re Temperature atoms (°C)

1wt% Fe3O4@SiO2@Re IMP 100-250

0.04

0.03

0.74

50-200

0.02

0.30

100-250

0.01

0.56

0.02

0.19

3wt% Fe3O4@SiO2@RePD 0.12

100-250

4wt% Fe3O4@SiO2@ReIMP 100-250

0.03

2wt% Fe3O4@SiO2@Re PD

3wt% Fe3O4@SiO2@ReIMP 100-250

Adsorbed NH3 molecules/Re atoms

1wt% Fe3O4@SiO2@Re PD

2wt% Fe3O4@SiO2@Re IMP 100-250

Adsorbed NH3 molecules (mmol/g)

0.01

0.07

4wt% Fe3O4@SiO2@RePD 0.04-

100-250

0

0


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TCD Signal (a.u.)

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d c b a

50

100

150

200

250

300

350

400

Temperature (°C)

Figure 3. NH3-TPD profiles on reduced: a) 1wt% Fe3O4@SiO2@RePD; b) 2wt% Fe3O4@SiO2@Re; c) 3wt% Fe3O4@SiO2@Re; d) 4wt% Fe3O4@SiO2@Re


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Figure 4. TEM picture of the 4wt%Fe3O4@SiO2@Re catalyst synthesized by the IMP (A, B and C) and by PD method (D) method. The amorphous SiO2 surrounds the crystalline Fe3O4 core like a shell. The 10 nm scale bar is valid for A, B and C.


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The TEM pictures in Figure 4 are of the 4wt%Fe3O4@SiO2@Re catalyst synthesized by the IMP (A, B and C) and by the PD (D) method. The crystalline Fe3O4 core and silica shell are clearly visible in A, B and D. In conclusion, the characterization results showed that the deposition of silica protected the magnetite nanoparticles very well during the deposition/reduction of rhenium onto the Fe3O4@SiO2 composite. TEM and surface area experiments revealed a compact, nonporous layer of SiO2 around the magnetite nanoparticles. This layer suppresses the permeability of H2 molecules to the magnetite core. As demonstrated from the XPS spectra of the recycled catalyst only "a very weak signal at 706.5-706.9 eV has been evidenced, that confirms a very small reduction of magnetite to iron(0)”. Further deposition of Re resulted in catalysts with a fairly large surface area and mesopores, on which rhenium is highly dispersed. Thus, it was silent in XRD and XPS, demonstrating Re/Si ratios very close to those determined analytically. As expected, Re did not reduce to the metallic state and generated a weak Brønsted-type acidity, as suggested by NH3-TPD. H2-TPD experiments demonstrate the capacity of these catalysts to chemisorb hydrogen.

Lignin fragmentation over the synthesized catalysts Figures 5 and 6 show the catalyst screening for the fragmentation of lignin L1. The yield of the fragmentation process is considered to be the sum of the Y(LF) and Y(in THF); Y(insol THF) corresponds to the lignin residues and coke (Figure 5). The fragmentation yield varied from 40% for Fe3O4 to 60% for Fe3O4@SiO2 and reached a maximum (98%) for the 2%Fe3O4@SiO2@Re PD catalyst. Control run with Fe3O4@Re exhibited only 47 % of fragmentation yield of lignin, while SiO2@Re yielded around 70 % of lignin fragmentation. Indeed, SiO2@Re exhibited


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relative high activity compared toFe3O4@SiO2@Re catalysts. However, the conversion of the lignins is never total, and using SiO2@Re, at the end of the reaction the catalyst is mixed with the residual lignin and really lost. The only possibility to recover it is the calcination. As an effect, at the end, the recovered catalyst changes the properties due to several reasons (changes in the oxidation state and dispersion, covering with carbonaceous species, etc). Core-shell ensures an easy recoverability by simple washing in the presence of the magnet. Under these conditions the specific properties of the catalysts are well preserved as confirmed by the characterization of the recovered samples and the recycling tests. In both series of catalysts (PD and IMP) the presence of Re led to high activity. The PDsynthesized catalyst showed higher conversion. For the PD series, a loading of Re of 2wt% led to the maximum yield of lignin fragmentation, while for the IMP series, there is a tendency for the fragmentation yield to increase with the Re loading. The efficiency of the PD series was confirmed by the distribution of the lignin fragments in the aqueous phase (Figure 6). It is well known the poor solubility of lignin in water. Even though, the experiments carried out in this study demonstrate the possibility to perform the fragmentation of lignin in this solvent. As observed, it ensures a dispersion of the lignins and a partial recoverability of the reaction products. Literature suggested that water-solvent mixture can exhibit beneficial effects in this process24. However, both the cost and environmental issues do not justify the use of such mixtures.


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Figure 5. Yields of lignin fragmentation over the investigated materials. Experimental conditions: 0.01 g catalyst, 0.1 g L-1, 5 mL solvent, 180°C, 10 bars and 6 h.

Figure 6. Yields of LF fragments over the investigated materials. Experimental conditions: 0.01 g catalyst, 0.1 g L-1, 5 mL solvent, 180°C, 10 bars and 6 h.


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Table 4. Lignin fragments and residue in the fraction soluble/insoluble in THF related to the lignin fragmentation over different catalysts. (MWLRs/THF < 1000 Da)

Catalyst

MWLRinsol/THF (Da)

Fe3O4

4300

Fe3O4@SiO2

6300

Fe3O4@Re

4500

SiO2@Re

4700

1% Fe3O4@SiO2@RePD

6200

2% Fe3O4@SiO2@RePD

4500

3% Fe3O4@SiO2@RePD

2500

4% Fe3O4@SiO2@RePD

2000

1% Fe3O4@SiO2@ReIMP

6300

2% Fe3O4@SiO2@ReIMP

6500

3% Fe3O4@SiO2@ReIMP

4700

4% Fe3O4@SiO2@ReIMP

5300

As a general trend, the PD preparation provided the best fragmentation with higher yields in the THF soluble/insoluble fractions. Consequently, the 2wt% Fe3O4@SiO2@Re PD catalyst was chosen for further experiments. GPC analysis allowed the determination of the average molecular weight of the lignin fragments (Table 4). For L1 (Figure S5), the starting raw material showed a distribution curve (dw/dLogM vs MW)43 with an average molecular weight of about 3500 Da for the soluble fraction in THF (non-acetylated) and 4300 Da for the insoluble fraction in THF (acetylated).


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After fragmentation, lignin fragments with MW

Highly efficient, easily recoverable and recyclable Re-SiO2-Fe3O4

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