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Lignin fragmentation onto multifunctional Fe3O4@Nb2O5@Co@Re catalysts: the role of the composition and deposition route of rhenium. Cristina Opris, Bogdan E. Cojocaru, Nicoleta Georgiana Apostol, Madalina Tudorache, Simona M. Coman, Vasile I. Parvulescu, Bahir Duraki, Frank Krumeich, and Jeroen Anton van Bokhoven ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02915 • Publication Date (Web): 10 Mar 2017 Downloaded from http://pubs.acs.org on March 10, 2017

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Lignin fragmentation onto multifunctional Fe3O4@Nb2O5@Co@Re catalysts: the role of the composition and deposition route of rhenium. Cristina Opris†, Bogdan Cojocaru†, Nicoleta Gheorghe‡, Madalina Tudorache†, 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 030016, Romania. [email protected] ‡ National Institute of Materials Physics, Atomistilor 105b, 077125 Magurele-Ilfov, Romania. # ETH Zurich, Wolfgang Pauli Strasse, 8093Zürich, Switzerland. ∫ Paul Scherrer Institute, 5232 Villigen, Switzerland. [email protected] KEYWORDS. Lignin fragmentation, multifunctional ReCo-Nb2O5-Fe3O4 catalysts, cooperative Co-Re effects ABSTRACT: Multifunctional Fe3O4@Nb2O5@Co@Recatalysts with metal loadings in the range from 2 to 7wt% were prepared in a multistep process. Magnetic nanoparticles prepared by co-precipitation were covered with a niobia shell precipitated from an ammonium niobate-oxalate complex in the presence of hexadecyltrimethylammonium bromide. The deposition of cobalt was carried out using a deposition/precipitation procedure. Finally, rhenium has been deposited following three different routes: i) impregnation, ii) deposition/precipitation of rhenium chloride (ImC and PP, respectively) and iii) impregnation with ammonium perrhenate (ImA). The characterization of these catalysts was carried out by XRD, Raman, H2-TPD and NH3-TPD, XPS, and TEM showing the influence of the preparation procedure, reduction and cooperation Re/Co upon the dispersion and reduction degree. ImC and ImA routes led to more reduced catalysts, and the decrease of the cobalt content corresponded to more reduced rhenium. An inverse relation between the acidity and the reduction degree has been evidenced. 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 THF and 14.5 % yield of insoluble LR in THF). Although in a smaller extent, the PP catalysts allowed a more advanced fragmentation of the lignin to fragments with molecular weight between 200 and 400 Da. The catalysts were totally recovered by application of a magnetic field, and recycled for six times without any loss in the activity and selectivity.

The complexity of the chemical world of the hydrocarbons and their derived chemical products and materials reached in over one century is the result of intensive academic and industrial development on research. These achievements were, however, obtained with the cost of the depletion of fossil resources and with the generation of environmental issues as global warming and acid rains1-3. This state of art was well acknowledged by the Report of the World Commission on Environment and Development which, on the basis of its complexity4 recommended the production of renewable fuels and chemicals as a future target5. Besides the renewable character, the main difference between the utilization of biomass and fossil resources is its large content of oxygen which, in the case of the production of fuel, requires an efficient removal solution6,7. In this respect, lignin receives special interest, firstly because is one of the most-abundant biomass components, and secondly because its conversion is expected as a potential route for the production of a large variety of products containing aromatic hydrocarbons (benzene, toluene, xylene and naphthalene), and their oxygenated derivatives, such as phenol8, catechol9, cresol, guaiacol, syringol and vanillin10.

The subtraction of oxygen from these chemical structures requires a hydrogenolysis of carbon-oxygen bonds as a key reaction step11,12. This is not a trivial reaction due to the extended aromatic matrix of lignin incorporating hydroxy and methoxy substituted phenylpropane units13. In fact, the structure of lignin includes at least 14 different chemical bonds, from which the β-O-4 aryl-alkylether bonds are easier to cleave14. Obviously, under hydrogen atmosphere, breaking of the C–O–C linkage is accompanied by hydrogenation of the produced unsaturated groups15. Therefore, the current trend to accomplish this is to combine the hydrogenolysis of the C-O bonds with the hydrogenation of the resulted fragments. Preferably these reactions should be conducted in water16 and in the presence of solid catalysts. Typically, hydrodeoxygenation is a method used to remove oxygen under high hydrogen pressure (4-10 bar) and moderate temperature (300–500°C)15. Many transition metals have been used for this type of reactions, but the main drawback is catalyst deactivation through high coke formation, hydrothermal instability and catalyst sintering17. More stable catalysts that are recyclable and selective to the desired product are being investigated. Such target can only be reached by combining the right active sites with supports and promotes18.

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Among many metals, rhenium is known for its ability to catalyse hydrogenolysis of heteroatomic X-C bonds19-21 but also as a selective hydrogenation catalyst22-26. It is also able to promote reactions involving C-C and aromatic C-H cleavages27. Another remarkably property of rhenium oxide is its ability to reduce selectively carboxyl groups without affecting the aromatic ring28. The utilization of rhenium as active species dispersed on a support29 was supposed to have two effects; (i) as scavenger of the strongly chemisorbed species, and ii) more importantly, to disrupt the second metal in much smaller and more active catalytically species30. For the C-O hydrogenolysis it demonstrated such abilities31 with the disadvantage of its volatility at high temperature32. Other studies explained the activity and selectivity of bimetallic catalysts containing rhenium by a reduction of the adsorption enthalpy of hydrogen on the metal surface, which promotes the desorption of the products leaving thus more accessible sites for the catalytic reaction33. As it was mentioned above, rhenium is typically used in concert with a second noble metal such as ruthenium, platinum and palladium, both deposited over a proper support (carbon, silica and alumina). Rhenium promotes platinum/carbon catalysts for glycerol conversion, which suggests that its presence facilitates hydrogenolysis of the C–O bonds due to its ability to preferentially bond hydroxyl groups34,35. Then, these species will react in the second step with hydrogen dissociatively adsorbed and activated on the second noble metal36,37, resulting in an enhancement of both the activity and selectivity28. The Lewis acid effect exerted by rhenium in the hydrogenolys is of oxygenated compounds has also been demonstrated by the enhancement induced in the selectivity of the bimetallic rhenium-ruthenium catalyst18. The role of the support is also very important to achieve a high activity of rhenium by controlling its particle size22. The support effects on it ensuring an improved catalyst stability, lifetime and selectivity. It is also the reason for which many commercial reformer catalysts use rhenium as promoter for platinum supported on alumina37. The aim of this study was to determine the role of rhenium as promoter for lignin fragmentation reaction overFe3O4@Nb2O5@Co catalysts having magnetite as a magnetic core. Fe3O4@Nb2O5@Co catalysts are effective catalysts for the fragmentation of lignin following a concerted hydrolysis/hydrogenolysis mechanism. Catalysts characterization Figure 1 shows comparative XRD patterns of reduced Fe3O4@Nb2O5@Co@Re catalysts with different loadings of cobalt prepared via the three different deposition procedures. These patterns show the characteristic lines of the pure Fe3O4 at 2 theta 18.3, 30.3, 35.5, 43.5, 53.4, 57.2 and 62.5 corresponding to the crystallographic facets with Miller indices (111) (220), (311), (400), (422), (511) and (440) (PDF 00-0190629 and 38). No distinct lines of cobalt or niobia could be distinguished in these patterns. This could be attributed to the high dispersion of these species. Characteristic lines of Co3O4 at 2 theta 31.2, 36.9, 43.5, 53.4, 60.0 and 65.04 corresponding to crystallographic facets with Miller index (220), (311), (400), (422), (511) and (440) (PDF 00-042-1467) are superposed with those of Fe3O4. The same patterns were observed for the original supports (not shown) demonstrating the integ-

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rity of the magnetic core after reduction with hydrogen. Also, irrespective of the route rhenium chloride was deposited no reflections that correspond to rhenium compounds were pointing to the high dispersion and small particle size. Because of its higher abundance, we attribute their reflections to Fe3O4. The impregnation with ammonium perrhenate led to a different behavior. The preparation and the subsequent reduction changed the structure of the support. A new phase had been identified indicating the formation of the metallic iron (PDF 00-006-0696) (Figures 1A, b and 1B, b) with lines at 2 theta 44.6, 64.9 and 82.3 corresponding to a Miller index of (110), (200) and (211), respectively). Table 1.Textural analysis of catalysts Catalyst 1

Fe3O4

BET *m2/ g 127

P1 nm

P2 nm

8.9

-

2

Fe3O4@Nb2O5

10

14.2

-

3

Fe3O4@Nb2O5@2%Co

49

1.9

-

4

Fe3O4@Nb2O5@3%Co

37

2.2

-

5

Fe3O4@Nb2O5@4%Co

2

2.6

-

6

Fe3O4@Nb2O5@3%ReImC

53

7

Fe3O4@Nb2O5@2%Co@3%ReImC

47

3.5; 8.9; 12.9 12.5

8


40

12.3

9

9


37

14.7

9

10

Fe3O4@Nb2O5@3%Co@2%ReImA

44

12.1

7

11


41

12.7

9

12


46

18.1

8

13


31

17.8

7

14


31

17.8

7

15

Fe3O4@Nb2O5@2%Co@3%RePP

53

10

16


54

17


46

3.6; 6.2 3.7; 7.4 3.5; 6.1

8 9

9 8

* BET porous size; P1-Average pore size; P2-particle size at 2θ, 35.5 nm.

Figure 2 shows patterns collected after the re-calcination and re-reduction of catalysts prepared by impregnation with ammonium perrhenate. Re-calcination led to the oxidation of iron (lines at 2 theta 44.6, 64.9 and 82.3 disappeared) and further re-reduction was not regenerating the formation of the reduced iron species. Repeating this treatment did not change the profile of the pattern presented in Figure 2,c. Indeed, the line at 82.3 demonstrated the presence of reduced iron, whose formation was “catalysed” by rhenium. The XRD patterns in Figure 3 illustrate the influence of the rhenium loading on the phase composition for reduced catalysts prepared by impregnation with ammonium perrhenate.

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The increase of the loading gave no signals for rhenium but generated more intense lines for the new iron phase with lines at 2 theta 44.6, 64.9 and 82.3o.

reduced at 450oC; b) Fe3O4@Nb2O5@3%Co@2%Re -recalcined at 500oCand c) Fe3O4@Nb2O5@3%Co@2%Re -re-reduced 450oC.(* - Fe3O4 (magnetite), PDF 00-019-0629; #Fe, PDF 00006-0696).

Figure 3. The X-Ray powder diffraction patterns of the impregnated reduced catalysts. Catalysts were prepared by impregnation with ammonium perrhenate (ImA series): a) Fe3O4@Nb2O5@3%Co@2%Re, b)Fe3O4@Nb2O5@3%Co@3%Re, c) Fe3O4@Nb2O5@4%Co@3%Re, d) Fe3O4@Nb2O5@4%Co@5%Re (* - Fe3O4 (magnetite), PDF 00-019-0629; #Fe, PDF 00-0060696).

Figure 1. The X-Ray powder diffraction patterns of the reduced catalysts containing Fe3O4@Nb2O5@3%Co@3%Re (A) and Fe3O4@Nb2O5@4%Co@3%Re (B) prepared by a) PP, b) ImA and c) ImC approaches (* - Fe3O4 (magnetite), PDF 00-019-0629; #Fe, PDF 00-006-0696 ).

Figure 2.The X-Ray powder diffraction patterns of the recalcined and re-reduced catalysts prepared by impregnation with ammonium perrhenate. a) Fe3O4@Nb2O5@3%Co@2%Re -

Table 1 shows the textural characteristics of the prepared catalysts. These data demonstrate the influence of both the composition and the preparation procedure. Adding niobium to the Fe3O4 nanoparticles corresponded to a decrease of the surface area from 127 m2/g to 10 m2/g (Entry 2). Further addition of cobalt or rhenium to the Nb2O5@Fe3O4 nanoparticles led to the partial recovery of the surface area (Entries 3-6). The addition of cobalt, and then of rhenium, to the support nanoparticles has almost no influence on the surface area (Entries 8-17). Thus, the correlation of these results with the XRD and TEM analyses, suggest that, indeed, the values of the determined pore sizes do not correspond to an intrinsic porosity of the very tinny nano-particles, but to intraparticle porosity. Figure 4 shows Raman spectra of 3wt% rhenium catalysts with different loadings of cobalt prepared following different procedures. The samples with 3wt% cobalt (Figure 4, A) exhibited only the lines corresponding to octahedral rhenium (687, 482, 341 cm-1)21. All spectra showed a band at about 1300 cm-1 that typically is due to Fe3O439. In addition, like XRD, the spectra of ImA samples showed other bands at 660, 288 and 220 cm-1 that were assigned to Fe3O4 species in different environments40-42. The increase of the cobalt loading to 4wt% (Figure 4, B) led to apparition of shoulders at about 800 cm-1, which could be assigned to CoOOH43. A typical Co3O4 Raman spectrum corresponds to five Raman bands at about 191, 470, 510, 608, and 675 cm−1 44, which were not observed in our spectra. Figure 5 shows the Raman spectra of the catalysts prepared via the impregnation with ammonium perrhenate. Along the band

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at 1300 cm-1, the presence of iron was identified from bands located at 660, 288 and 220 cm-1 which were also confirmed by XRD patterns (Figure 1 (A and B) and Figure 3). Spectra of catalyst with higher cobalt loading showed the apparition of shoulders at around 800 cm-1.

Figure 5. Raman Spectra of ImA prepared samples: a) Fe3O4@Nb2O5@3%Co@2%Re, b)Fe3O4@Nb2O5@3%Co@3%Re, c) Fe3O4@Nb2O5@4%Co@3%Re, d) Fe3O4@Nb2O5@4%Co@5%Re. # - octahedral rhenium; * - Fe3O4 (magnetite); ^ - CoOOH

Figure 4. Raman spectra of the 3wt% rhenium catalysts with different loading of cobalt (A: Fe3O4@Nb2O5@4%Co@3%Re;B: Fe3O4@Nb2O5@4%Co@3%Re) prepared following the a) PP, b) ImA and c) ImC procedures.# - octahedral rhenium; * - Fe3O4 (magnetite); ^ - CoOOH

Figure 6. XPS spectra of the Re4f levels for the reduced catalysts.

Core catalysts XPS investigation indicated binding energies corresponding to iron in Fe3O445 and to niobium in Nb2O546,47 .Table 2, Figure 6 and FigureS1 also show the results of the XPS analysis of the reduced catalysts. Fe2p3/2 showed the species, iron(0) and iron(III). Iron(0) peak appears at 706.7 eV and iron (III) around 710.5-711.7 eV44. Synthesis of the catalyst Fe3O4@Nb2O5@2%Co@3%Re via the ImA route shifted

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the binding energy to lower values leading to a small shoulder at 706.7 eV. This catalyst showed higher metallic character compared to the catalyst synthesized using the ImC route. The catalysts synthesized via ImA route also showed iron(0) species. Nb3d5/2 exhibited a slightly shift to the lower binding energy at this point. Co2p3/2 shows peak from 778.2 eV (cobalt(0)) to 783 eV (cobalt (II)) irrespective of the cobalt loading. After the deposition of rhenium, different binding energies are present as: 40.07-40.16 eV for rhenium(0), 41.9142.16 eV for rhenium(IV), 45.68-45.94 eV for rhenium(VI) and 46.24-46.48 eV for rhenium (VII). According to Table 2, the catalysts Fe3O4@Nb2O5@2%Co@2%ReImA, Fe3O4@Nb2O5@2%Co@3%ReImC, Fe3O4@Nb2O5@3%Co@3%ReImC showed rhenium(0) species. Rhenium(VII) species were observed for all catalysts synthesized via ImA and PP route except for Fe3O4@Nb2O5@2%Co@3%Re ImA. Also, in all catalysts, rhenium(IV) and rhenium(VI) were observed regardless the synthesis route in various oxidation states48. Samples prepared via ImC and ImA routes appeared to be more reduced. The binding energy of around 40 eV accounted for rhenium in metallic state, that around 42 eV to rhenium (IV), and around 45.8 to rhenium (VI). The decrease of the cobalt content increased the content of the reduced rhenium species. For the PP catalysts, although reduced under the same conditions, no metallic rhenium has been evidenced. Like for the ImC catalysts, the increase of the cobalt content had no positive effect on the reduction of rhenium. Even more, the decomposition of the spectrum of the 3%Co@2%Re catalyst indicated the presence of a band that can be assigned to rhenium (VII)48. Like for rhenium, the analysis of the Co2p indicated a mixture of reduced and oxidized metal47,49,50. Samples prepared via ImC and ImA routes appeared to be more reduced, but the decrease of the cobalt content corresponded to less reduced cobalt. For the ImA catalysts the cobalt (0) to cobalt (II) ratio was smaller than 0.08. After reduction with hydrogen, the samples preserved rhenium. Table 3 presents the comparative XPS surface and chemical determined compositions taking into consideration the Nb/Fe, Co/Fe and Re/Fe atomic ratios. These data confirm a very good distribution of both cobalt and rhenium. These ratios are influenced by the loading of these elements and less influenced by the metal deposition route. The slightly higher XPS Nb/Fe ratios compared to analytic results are attributed to the fact that magnetite constitutes the core of these nanoparticles that is covered by a niobia shell.

Figure 7. H2-TPR profiles as function of rhenium loading and preparation procedure.

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Figure 7 shows the H2-TPR profiles of catalysts with different loadings of cobalt and rhenium, and synthesized following different preparation procedures. The reduction of Fe3O4@Nb2O5 shows a broad peak starting from 430°C and ending at 650°C. Adding 3wt% cobalt, an additional peak appears around 380°C to the broadened peak starting from 430°C to 650°C49,50. Introducing rhenium into the support, the broad peak gets shorter (430°C – 650°C) and two additional peaks appear one at 320°C and the other at 380°C. The shortened peak shows the introduction of rhenium into the Fe3O4@Nb2O5 texture since the reduction stops earlier than without the use of rhenium. Using the PP method to produce the 2%Co@3%RePP catalyst, one broad peak is observed, which starts from 350°C up to 720°C. Using this catalyst composition but different precipitation method (2%Co@3%ReImA and 2%Co@3%ReImC), the H2-TPR profiles looks similar, except a huge reduction peak appearing at 390°C. To prove that rhenium has an influence on the Fe3O4@Nb2O5 support impregnation, a catalyst with a higher amount of rhenium ie. 2%Co@5%ReImC was also measured. In this case the reduction starts around the same temperature as the other catalysts (350°C), but stops earlier at 670°C. The consumption hydrogen volume increases by covering the support with rhenium, cobalt or both metals. Introducing both metals on the support generated a synergistic effect and the H2 consumption volume increases more than for each metal. Using the PP method, the H2 consumption volume is the lowest compared to the other precipitation method. The volume is in general higher for the ImC method than for the ImA method. For Fe3O4@Nb2O5@2%Co@3%Re catalyst, the H2 consumption volume is the highest with 127 cm3/g STP. Table 4 quantifies the hydrogen consume for a series of catalysts. Table 2. XPS binding energies of the Nb3d5/2, Fe2p3/2, Co2p3/2, Re3d5/2, and O1s for reduced catalysts Catalyst Nb 3d5/2

Fe2p3/2

Co2p3/2

Re4d5/2

O1s

Fe(III) Co(0) Co(II) Re(0) Re(IV) Re(VI) Re(VII) Fe(0)

Fe3O4@Nb2O5@3%ReImC

207.1

-

711.4


206.9

706.7


206.8


-

-

41.91

45.92

-

529.8

710.7

778.4 782.8 40.16 42.25

45.71

46.24

529.9

706.7

710.7

778.3 782.7

-

42.56

45.68

46.46

530.3

206.9

-

711.6

778.3 782.7

-

41.97

45.89

46.48

530.1


206.8

706.7

710.5

778.2 787.5

42.07

45.70

-

530.2


206.9

-

711.7

778.6 783.0 40.11 41.94

45.88

-

530.1


206.9

-

711.6

778.5 782.9 40.07 41.92

45.85

-

530.1


206.9

706.7

710.5

778.5 782.8

-

42.56

45.68

46.46

530.2


206.8

-

711.5

778.4 782.8

-

41.98

45.92

46.48-

530.3


206.8

-

711.5

778.2 782.7 40.15 41.96

45.89

-

530.1

6


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Table 3. XPS surface composition versus bulk determined chemical composition has similar shape with that of the pure niobia but the desorbed Catalyst XPS M/Fe atomic ratio Analytic M/Fe atomic ratio Nb/Fe

Co/Fe

Re/Fe

Nb/Fe

Co/Fe

Re/Fe

Fe3O4@Nb2O5@3%ReImC

1.62

-

0.51

0.50

-

0.14


1.55

0.14

0.13

0.49

0.19

0.07


1.48

0.15

0.19

0.49

0.33

0.07


1.47

0.16

0.21

0.51

0.34

0.07


1.52

0.17

0.26

0.50

0.20

0.11


1.50

0.17

0.27

0.50

0.20

0.12


1.18

0.19

0.44

0.49

0.34

0.11


1.14

0.16

0.39

0.50

0.34

0.11


1.20

0.21

0.42

0.51

0.35

0.12


1.08

0.28

0.49

0.50

0.46

0.12

Table 4. H2-TPR consume for selected catalysts Sample Fe3O4@Nb2O5

Quantity (cm³/g STP) 15

Fe3O4@Nb2O5@3%Co

49

Fe3O4@Nb2O5 @3%Re

60

Fe3O4@Nb2O5@2%Co@3%RePP Fe3O4@Nb2O5@2%Co@3%ReImC

99


105

94


86


81


96


118

NH3 is smaller reflecting a certain interaction with the Fe3O4 nanoparticles (Table 5). The addition of cobalt (see the Fe3O4@Nb2O5@3%Co catalyst) (Table 5) led to an additional desorption of ammonia at 268 oC that might be attributed to cobalt species. However, for the monometallic Fe3O4@Nb2O5@Co catalysts the total desorbed NH3 was very similar to that of Nb2O5@Fe3O4 indicating the fact the reduced cobalt does not affect its acidity. For Fe3O4@Nb2O5@3%Re catalyst the peak attributed to ammonia desorbed from rhenium shifted to 276 oC and was double compared to the similar peak onto the Fe3O4@Nb2O5@3%Co catalyst (Table 5). The measured concentration of the acid sites demonstrate that the presence of the Re species enhance the catalyst’s acidity. Finally, the presence of the two elements (cobalt and rhenium) suggest a synergistic effect leading to a much higher concentration of the acid sites as compared to the monometallic catalysts and reaching the maximum for the catalysts with 3wt%Co and 2wt%Re (Table5). An increase of the metal loading over 5wt% caused a decrease of the concentration of the acid sites. The Co/Re ratio and the preparation method also influence the acidity of these catalysts. ImC series showed higher concentrations of acid sites compared to PP and ImA series. These acidity results were in an inverse order compared to the hydrogen consume (Table 4). In fact, these data confirm previous reports attributing the catalysts acidity to the less reduced rhenium species51. Also, they correlate to the XPS results indicating the presence of Re partially inhibits the reduction of Co, thus increasing the Lewis acidity. In terms of the acidity strength, all the Re/Co samples exhibited medium and strong acid sites (Table 5).

Figure 8 shows the temperature-programmed desorption (TPD) profiles for NH3as a function of Re loading and preparation procedure while Figure S2 the evolution of the weak, medium, strong and total concentration of the acid sites corresponding to these catalysts. The profile of the Nb2O5@Fe3O4


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Figure 8. NH3-TPD profiles as function of rhenium loading and preparation procedure

Table 5. NH3-TPD for reduced samples Catalyst

Temperature at maximum (°C) 103 315 439

Fe3O4@Nb2O5

Total acid sites concentration Fe3O4@Nb2O5@3%CoPP

268 315 439 Total acid sites concentration

Fe3O4@Nb2O5@3%RePP


Fe3O4@Nb2O5@4%Co@3%Re-PP

274 351 381 454 Total acid sites concentration

Fe3O4@Nb2O5@4%Co@3%Re-ImC


Fe3O4@Nb2O5@3%Co@2%Re-ImC Total acid sites concentration

274 316 377 436

Acid sites concentration (µmol/g) 4.91 6.70 11.2 22.81 6.70 5.36 9.37 21.43 18.75 5.36 8.93 33.04 18.30 21.87 16.52 12.95 69.64 22.80 35.20 20.13 78.13 52.68 54.46 41.96 33.93

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Fe3O4@Nb2O5@3%Co@3%Re-ImC






Fe3O4@Nb2O5@3%Co@3%Re-ImA

281 390 Total acid sites concentration

183.03 37.50 59.82 30.80 128.12 73.66 54.02 28.57 156.25 34.37 25.89 34.82 95.08 30.36 46.43 76.79

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A

B

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difficult, in particular since the cobalt Kα line is overlapping with the iron Kβ line. The EDX spectra extracted for two areas (marked in the EDX map by square 1 and 2) show that in square 1 almost exclusively niobium (oxide) is present. In square 2, the iron intensity is high but niobium is still observable, which is not in agreement with previous results reported elsewhere47. The rhenium distribution was analysed using HAADF-STEM imaging giving atomic number (Z) contrast (Figure 9, B). Bright patches of a few nm diameter and even sub-nm spots can be observed in thin areas. As the rhenium has the highest scattering potential here (Z = 75) compared to niobium (Z = 41) and iron (Z = 26), the white dots represent rhenium. The composition in the indicated area was measured by EDXS and indeed the presence of rhenium was confirmed.

Catalytic tests Determination of molecular weight of lignin still remains a challenge due to non-uniformity of the chain length and composition52 and low solubility in most of the common solvents. Reactions led to water soluble and insoluble fragments. According to the analyses, the soluble fragments are those in which the oxygen functionalities predominate, while the insoluble ones are the larger fragments and those in which the aromaticity predominate. This has also been confirmed by the supplementary tests with bisphenol and guaiacol. The identification of molecular weight of fragments obtained after reaction was realized by HPLC and GPC analysis and sizeexclusion chromatography (SEC)53,54. Influence of catalysts synthesis protocol

Figure 9. EDX mapping (A) and HAADF-STEM image (B) of Fe3O4@Nb2O5@4%Co@3%Re catalyst synthesized using the ImC route. In (A), EDX spectra were extracted from the measured data for the Nb-rich area 1 (red) and the Fe-rich 2 (green). The EDXS of the frame area in (B) confirms the presence of Re at a bright spot.

Figure 9, A shows the EDX mapping of the elements niobium and iron in the catalyst Fe3O4@Nb2O5@4%Co@3%Re. Iron and niobium were inhomogeneously distributed in the observed region. A local accumulation of niobium can sometimes be observed (square 1). The formation of a typical coreshell structure with niobium oxide cannot be detected in this region. The small amount of cobalt makes its observation

Figure 10. Influence of the catalyst preparation method on the product distribution of lignin fragmentation identified in water soluble (Y(LF), orange), THF soluble (Y(inTHF), green) and THF insoluble (Y(insol THF), blue) fragments. The x-axis represents the precipitation method and the insets the wt% of rhenium/cobalt. (0.02 g catalyst, 0.01g lignin, 2.5 mL H2O, 180 oC, 10 bar, 6 h).

Figure 10 presents product yield for lignin fragmentation over ImC, ImA and PP catalysts with different ratios of rhenium and cobalt. ImC exhibited better catalytic activity compared to ImA and PP catalysts at low total metal loadings. Thus, 2%Co@3%Re afforded the most-advanced lignin fragmentation with a 85 % yield of LF, 15.5 % yield of soluble LR in THF and 14.5 % yield of insoluble LR in THF. Higher loadings produced smaller fragmentation yields.

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Figure 11. Influence of the catalyst preparation method on the mass distribution of water soluble (A) and insoluble (B) fragments. (A) represents the mass distribution for C20-C28 (blue), C29C37 (red) and C38-C40 (green). The x-axis represents the precipitation method and the insets the wt% of rhenium/cobalt. (B) represents the mass range for the different catalyst composition. The yaxis represents the precipitation method and the inset the wt% of rhenium/cobalt. (0.02 g catalyst, 0.01g lignin, 2.5 mL H2O, 180 o C, 10 bar, 6 h).

Figure 11, A & B presents an evaluation of the mass fragments composition for both phases. The ImA catalyst showed high selectivity for the fragments C29-C37. For ImC series, small fragments (C20-C28) dominated the liquid phase for a metal loading of 3wt% Re and 2wt%Co. Further increase of the cobalt content from 2wt% to 4wt% catalyst led to a decrease of the selectivity in the favor of C29-C37 fragments (Figure 11, A). Very important, the PP catalysts yielded a more advanced fragmentation in which the dominants were the C20C28fragments. Irrespective of the preparation route, the solid phase was composed by entities with masses in the range 4001000 Da (Figure 11, B). Influence of the operational conditions Figure S3 depicts the influence of the solvent of the fragmentation of the lignin over the 2wt%Co@3wt%Recatalyst. While acetone afforded poor Y(LF) values, the use of water, ethanol and methanol corresponded to Y(LF) values higher than 80%. Such a behavior has also been confirmed by the Y(insol THF) values. Figure S4shows the mass distribution of the produced fragments. Working in ethanol and methanol led to excellent

selectivities to small C20-C28 fragments. Contrarily, in water, the fragmentation stops at large C38-C40 lignin fragments. However, the analysis of the solid residue confirmed ethanol as the best solvent. In methanol, the average weight measured was larger than in ethanol (Figure S4, B). Figure S5 shows the influence of the temperature in the range between 150 °C and 230 °C on the fragmentation of lignin using 2%Co@3%Re as catalyst. Temperature higher than 180 °C corresponded to an increase of the yield in water soluble fragments at the expense of those insoluble in water (including also the fragments insoluble in THF). Concerning selectivity, the increase of the temperature led to a decrease in selectivity to fragments with a low molecular weight and favored the formation of C29-C37 fragments, which was the dominant component at temperatures in the range 200-230°C (Figure S6, A). The analysis of the LR fractions indicated a decrease of the weight average for temperatures higher than 180 °C (Figure S6, B). The lignin concentration (glignin/lsolvent) exhibited only a small influence upon the yields of the fragments in the two phases (Figure S7). However, it influenced the distribution of the produced fragments. Higher concentration, which corresponds to 4 glignin/lsolvent favored a limited fragmentation (including also C20-C28 fragments), while concentrations of 2 g/l and 1 g/l stopped the fragmentation to intermediate C29-C37 fragments (Figure S8, A). This behavior was also supported by the analysis of the solid residue (Figure S8, B). Figure S9 shows the distribution of the molecular fragments after the fragmentation of the lignin over the 3wt%Re@2wt%Co catalyst under different pressures. The increase of pressure from 5 to 10 bars led to larger yields in water-soluble fragments. Further increase to 12 bars did not improve the yield for the water soluble fragments. However, increasing the pressure led to higher yields in C20-C28 fragments. Pressures of 5 and 12 bar favor the C29-C37 and disfavor C38-C40 fragments. At 10 bar pressure, C38-C40 fragments are dominant (Figure S10, A). The analysis of the solid residue indicated a slightly increase in masses in the range 130-2000 Da at 10 bars. The lowest mass fragments, ie. 200-1700 Da, were observed for 5 bars (Figure S10, B). Figure S11 shows the time evolution of the molecular fragments identified in the fragmentation of the lignin over the Fe3O4@Nb2O5@2%Co@3%Re catalyst. It presents a continuous increase in the yields of water-soluble fragments up to 6 h that parallel led a reduction in the yield of insoluble residue. Further increase of the reaction time led to an enhanced formation of the insoluble residue. In terms of the compounds class distribution, the increase of the reaction time up to 6 h led to a decrease of the content in the water-soluble C29-C37 fragments that paralleled the formation of the C20-C28 fragments. Further increase of the reaction time recovered the C29C37 fragments (Figure S12, A). The increase of the reaction time also determined a decrease of the weight of the insoluble residue (Figure S12, B). In order to confirm the capability of these catalysts to afford hydrogenolysis and hydrolysis of the C-C and C-O bonds, respectively, separate tests have been carried out under optimized catalytic conditions with bisphenol (Scheme 1) and guaiacol (Scheme 2) as probe molecules, using the most performing catalyst, ie 2%Co@3%ReImC. Thus, catalytic tests with bisphenol led to the fragmentation to phenol, 1,4-diisopropylbenzene and biphenyl, with a conver-

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sion of 25% after 6h reaction (Figure S13) and a selectivity close to the stoichiometry of this reaction.

Scheme 1. Fragmentation of Bisphenol.

The reaction of guaiacol occurred with a conversion of 21% after 4h resulting in phenol and in a less extent (smaller than 5%) oligomers of the coupling of the resulted phenol with guaiacol (Figure S14). OH

OH O

+

oligomers

Scheme 2. Fragmentation of guaiacol.

General assessment The prepared catalysts showed pore sizes large enough to allow the access of the lignin fragments to react at the active sites (Table 1). Also, the deposition of rhenium and cobalt in the total metal loadings up to 7wt% did not generate significant differences in the textural properties irrespective of the deposition route of rhenium. Both cobalt and rhenium were in a very high dispersion state. The deposition of rhenium chloride either by impregnation or deposition/precipitation did not affected the structure of the Fe3O4@Nb2O5@Co and the magnetic core remains intact. The very small thickness of the layer of niobia protects this core. In a different way, the impregnation with ammonium perrhenate caused some changes in the structure of the support and in addition to Fe3O4 a new phase was identified indicating the formation of metallic species. They were visible from the XPS analysis (Table 2, Figure S1).The increase of the rhenium loading led to more intense lines of this phase suggesting the “catalytic” role of rhenium in this process. XPS showed that after reduction with hydrogen, the samples preserved rhenium in various oxidation states. However, the samples prepared via ImC and ImA routes appeared to be more reduced and the decrease of the cobalt content has as an effect an increase in the content of the reduced rhenium. For the PP catalysts, although reduced under the same conditions no metallic rhenium has been evidenced. Like for rhenium, the XPS analysis of cobalt indicated a mixture of reduced and oxidized species. Samples prepared via ImC and ImA routes appeared as well to be more reduced, but the decrease of the cobalt content corresponded to less reduced cobalt. These results have been also confirmed by H2-TPR, NH3-TPD, and TEM experiments. H2-TPR profiles confirmed an increased H2-uptake for ImC and ImA samples while NH3uptakes confirmed an increased acidity for the rhenium-based catalysts prepared via PP route. Also, the profiles of these samples indicated desorption at higher temperatures, thus suggesting increased acidity strengths. Catalytic tests indicated a good correlation with these structural characteristics confirming the role of the composition and the route through which rhenium has been deposited. Thus the catalysts prepared by impregnation with rhenium (V) chloride

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(ImC) and by deposition-precipitation (PP) led to a superior behavior compared to catalysts prepared by impregnation with ammonium perrhenate (ImA). From the structural point of view this corresponded to catalysts in which rhenium coexisted in different oxidation states (including rhenium(0)) and cobalt as well. The role of rhenium in the fragmentation of lignin is obvious. In a previous report using monometallic Fe3O4@Nb2O5@Co the maximum yield in water soluble fragments was around 40% for the catalysts containing 4wt% Co.The addition of rhenium increased the yield of light fragments (LF) and is accompanied by a complete recovery of the catalyst. The comparison of bimetallic Fe3O4@Nb2O5@2%Co@3%Re with monometallic catalysts with 3wt%Re and 2wt%Co under similar reaction conditions suggests a synergetic effect of the two metals. The bimetallic catalyst afford a LF yield of 85% that is superior to the sum achieved with the monometallic catalysts (see Figure S15 in supporting information section). Recent studies concerning the role of rhenium in activation of platinum for the glycerol reforming and hydrogenolysis presented evidences on the activation of the C-O bonds by dispersed rhenium oxide species in the proximity of platinum55. These species were assumed to act as Brønsted acid sites56. However, in the present study, none of the bimetallic catalysts presented Brønsted acid sites. All concentrations led to Lewis acid sites, distributed as week, medium and strong acid sites. This distribution (Table 5, Figure S2) is controlled by the metal loading and preparation route. Besides this, XPS analysis showed that the ImC series, ie. the most active, contained metallic rhenium. Thus, this complex synergism between the reduced and oxidized species of the two metals affords catalysts exhibiting higher activity to soluble fragments with small molecular mass. However, this positive effect is limited to a maximum loading of 3wt% of each element. Higher loadings led to an opposite effect. All these results led us to propose the mechanism described in the Scheme S1. The reaction conditions (solvent, reaction temperature, pressure, reaction volume) also exerted importance in this reaction allowing the optimization of the parameters. Conclusions The deposition of rhenium and cobalt onto Fe3O4@Nb2O5 led to multifunctional catalysts in which Fe3O4 ensured the total recoverability of the samples at the end of the reaction. Niobium and rhenium provided the acidity required for the disruption of the etheric groups, while cobalt and rhenium cooperated for the hydrogenolysis of the C-O bonds and hydrocracking of the C-C bonds. After the deposition of all these species, the particle size of the resulted composites were preserved in the range of the order of tens of nanometer and the pore sizes were enough large to allow the access of the lignin fragments to the active surface. Cooperation between cobalt and rhenium in the reduction process of the metals has been evidenced. The cobalt content provides the optimal Re-oxidation state distribution for this process and is correlated to the deposition procedure of rhenium. The screening of the ImC, ImA and PP catalysts in the fragmentation of the lignin confirmed the role of the structural

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ACS Catalysis

characteristics. ImC exhibited better catalytic activity compared to ImA and PP for low total metal loadings. Thus, the 2%Co@3%Re ImC catalyst afforded the most advanced lignin fragmentation (e.g. 85 % yield of LF, 15.5 % yield of LR in THF and 14.5 % yield of insoluble LR in THF). Except, the 2%Co@3%Re ImC for other ImC and ImA catalysts, the water soluble molecular fragments prevailed in compounds with a molecular weight between 400 and 600 Da. PP catalysts allowed a more advanced fragmentation of the lignin leading mostly to water soluble fragments with molecular weight between 200 and 400 Da. This confirmed the synergism between the specific acidity induced by the rhenium species in these catalysts and the participation of the reduced metals (see Tables 4 and 5). Dominated compounds with Mw between 400 and 1000 Da were identified in the solid phase for all of the tested catalysts. The solvent is important since it carries out the fragments. The use of ethanol led to the best yields. As another very important feature of these catalysts, they were totally recovered by application of a magnetic field, and after a simple washing with water they were recycled for six times without any loss in the activity and selectivity.

Supporting Information Supporting Information available: Experimental section; Fe2p, Nb3d, and Co2p XPS spectra; Concentration of the acid sites and strength distribution for the investigated catalysts; Influence of solvent, temperature, lignin concentration, pressure and reaction time on the product distribution of lignin fragmentation; Confirmation of the capability of the catalysts to afford hydrogenolysis and hydrolysis of the C-C and C-O bonds using bisphenol and guaiacol as probe molecules. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgement This work was financially supported by PNII-ID-RSRP-201216 (contract no. 16RO-CH/RSRP/1.01.2013) and POSDRU/159/1.5/S/137750. References (1) Beckham, G. T.; Johnson, C. W.; Karp, E. M.; Salvachúa, D.; Vardon, D. R. Curr. Opin. Biotechnol. 2016, 42, 40-53. (2) Carey, D. E.; Yang, Y.; McNamara, P. J.; Mayer, B. K. Bioresour. Technol. 2016, 215, 186-198. (3) Galkin, M. V.; Samec, J. S. M. ChemSusChem 2016, 9, 1544-1558. (4) Brundtland, G. H. Our Common Future, Oxford University Press, Oxford, 1987. (5) Linger, J. G.; Vardon, D. R.; Guarnieri, M. T.; Karp, E. M.; Hunsinger, G. B.; Franden, M. A.; Johnson, C. W.; Chupka, G.; Strathmann, T. J.; Pienkos, P. T.; Beckham, G. T. Proc.Natl. Acad. Sci. USA 2014, 111, 12013-12018. (6) Alonso, D. M.; Bond, J. Q.; Dumesic, J. A. Green Chem. 2010, 12, 1493-1513. (7) Kornstaje, T. J. PhD Thesis, Utrecht University, 2013.

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