Ni-Pd/ γ -Al2O3 Catalysts in the Hydrogenation of Levulinic Acid and Hydroxymethylfurfural towards Value Added Chemicals

: g -Al 2 O 3 supported Ni-Pd catalysts with different Ni:Pd ratios were studied in the 13 hydrogenation of two industrially-relevant platform molecules derived from biomass, namely 14 levulinic acid and hydroxymethylfurfural. The bimetallic catalysts showed better performances in 15 both processes in comparison to the monometallic counterparts, for which a too strong interaction with 16 the alumina support reduced the activity. The behavior of the bimetallic catalysts was depending on 17 the Ni:Pd ratio, and interestingly also on the targeted hydrogenation reaction. The Pd-modified Ni- 18 rich system behaves like pure Ni catalyst, but with a strongly boosted activity due to a higher number 19 of Ni active sites available, Pd being considered as spectator. This high activity was manifested in the 20 levulinic acid hydrogenation with formic acid used as internal hydrogen source. This behavior differs 21 from the case of the Pd-rich system modified by Ni, that displayed a much higher Pd dispersion on 22 the support compared to the monometallic Pd catalyst. The higher availability of the Pd active sites 23 while maintaining a high surface acidity allows the catalyst to push the HMF hydrodeoxygenation reaction forward towards the green biopolymer precursor 2,5-bis(hydroxymethyl)-tetrahydrofuran, 25 and in consequence to modify strongly the selectivity of the reaction. In that case, residual chlorine 26 was proposed to play a significant role while Ni was considered as spectator. X-ray diffraction (XRD) measurements were performed on a PANalyticalX’Pert Pro MPD diffractometer, using a Cu long-fine focus XRD tube working at 30mA and 40kV as a X-ray source. Data were recorded in the 2θ mode with a 0.0167° step (5–90°). Crystalline phases were identified by referring to the ICDD PDF-2 database (version 2004).


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This resulted from the low amount and small size of the crystallites, as well as from the low crystallinity 123 of the alumina support.

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The increase in the Pd content from 1% to 4% allows to observe a clear and sharp reduction peak 136 with a maximum at 65°C. This peak can be ascribed to the reduction of a mix of PdO and PdOxCly 137 species, therefore it is possible that the Pd is less interacting with the support so that its reduction is 138 facilitated [36].

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The bimetallic Ni-Pd/γ-Al2O3 catalyst reduction profiles are more complex than those of the 140 monometallic catalysts. The shape of the reduction profiles is highly dependent on the Ni-Pd ratio. In 141 all cases, we observe a low-temperature hydrogen consumption peak within the 30-140°C range, 142 associated with the reduction of Pd species. As the palladium content decreases, the low-temperature 143 hydrogen consumption peak shifts towards higher temperature, which may be related to the change of 144 Pd crystallite size and in the interaction between the metals and the support. Large crystalline palladium 145 is able to absorb hydrogen at room temperature within the metal structure, in contrary to small well 146 dispersed palladium species [33]. To better illustrate those differences, the TPR profiles were recorded 147 on the reduced samples ( Figure S2). Only in the case of 4%Pd, a negative TPR peak was observed, which 148 was attributed to the decomposition of the Pd hydride (β-PdH2) formed at room temperature from the 149 absorption of atomic hydrogen within the structure of large size metallic Pd particles [32,35,37,34]. For 150 the bimetallic samples, the absence of the hydride phase may indicate a good dispersion of palladium, 151 staying in interaction with the second metal [35].

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Furthermore, the presence of chlorine derived from the palladium precursor may facilitate the 153 reduction of NiOx and inhibit the formation of the interfacial NiAlxOy spinel [19]. For the 4%Ni-1%Pd content. In addition, the low-temperature region reduction might also correspond to the reduction of a performed for getting information about the catalyst surface and for identifying what kinds of surface 160 species were present as well as their effect on the interactions between the metal(s) and the support (Pdand PdCl -) was higher than that recorded for the monometallic 4%Pd catalyst, which may suggest 163 a better dispersion of palladium in the presence of small amount of nickel and thus may indicate that 164 adding nickel improves the palladium dispersion in a Pd-rich catalyst [17,19]. It was worth noting that 165 the surface of all Pd-containing catalysts contained residual chlorine species coming from the Pd 166 precursor, and that were not completely eliminated during the thermal steps of the catalyst preparation.

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This was particularly pronounced on the 1%Ni-4%Pd bimetallic catalyst that contained a strongly 168 higher amount of chlorine compared to that remaining in the corresponding 4%Pd monometallic 169 counterpart, so that modifying the 4%Pd catalyst with 1% Ni unfavored the removal of the chlorine 170 species during the thermal treatment, the highest amount of both PdCland Clions being recorded on 171 the Pd-rich bimetallic catalyst. However, the intensity of NiCl2 -ions was similar for both bimetallic 172 catalysts. By contrast, in the case of the Ni-rich catalyst, i.e. when Pd is only present as dopant, no 173 difference in terms of chlorine ions intensity was observed compared to the monometallic Pd 174 counterpart. Although the intensity of Ni ions was similar to that observed for the monometallic catalyst, the presence of additional NiCl2 -ions suggested that Ni is also partially interacting with Cl 176 species. The lower intensity of Pd in this case can suggest that Pd is partially covered by Ni species or 177 that Pd is present as larger crystallites compared to the monometallic Pd counterpart. 180 Figure 2 shows the palladium Pd 3p region, the Ni 2p3/2 region and the Cl 2p region of XPS spectra 181 recorded on the catalysts, while the wide scan survey spectra are reported as Figure S3. In general, the 182 Ni 2p3/2 orbital spectra exhibit a complex feature with a broad multi-contribution envelope containing 183 the core level peaks and shake-up satellite peaks corresponding to a multi-electron excitation and being 184 characteristic of Ni species in oxidized states [37,38,39].

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The two lower energy contributions were assigned to metallic and oxidized Pd surface species,

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FTIR spectra of CO adsorbed on the monometallic and bimetallic catalysts recorded under pressure 221 are shown in Figure 3A and 3B, respectively. The spectra of nickel catalysts obtained under pressure 222 showed only the band at 2056 cm -1 attributed to CO linearly adsorbed on Ni 0 or physically adsorbed on 223 nickel as tetracarbonyl Ni(CO)4 band [48,49] being more intense in the spectrum of the 4% Ni catalyst.

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The strong decrease of the band intensity recorded after CO evacuation suggests the weak adsorption 225 of CO on the nickel crystallites ( Figure S4).

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and that at the higher wavenumber of 2114 cm -1 was indicated to characterize the linear adsorption of 237 CO on highly dispersed cationic Pd δ+ [54].

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Moreover, although on both monometallic Pd catalysts there is bridged CO adsorption on both 239 facet and corner/edge sites, the adsorption is more intense on corner/edge sites for 1%Pd, and on facets 240 for 4%Pd. This can be related with different sizes for the metal crystallites and with their different 241 surface features being dominated by low index planes or corner/edge atoms [55]. to one nickel atom, and that is not observed for the monometallic nickel catalysts [56].

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In addition, FTIR studies can be associated with CO chemisorption performed at ambient pressure 257 ( Table 2). As shown above, after pressure release, the CO adsorption on Ni crystallites is weak, unlike 258 what is observed for Pd adsorption centers ( Figure S4). Therefore Ni-rich catalysts showed lower CO 259 chemisorption uptakes than the Pd-based catalysts, however the chemisorption capacity was higher on 260 the Ni-rich bimetallic catalyst (4%Ni-1%Pd) than on the monometallic Ni counterpart.

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The acidity of the catalysts was derived from the temperature programmed desorption of NH3 and 270 expressed as the molar amount of NH3 adsorbed per gram of catalyst (Table 2). It can be seen that the 271 acidity of the monometallic Ni catalysts is lower than that of the support itself and is decreasing with 272 the increase in the metal loading, which is related with the coverage of the acid sites of the support [57].

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In contrast to the nickel catalysts, the acidity of Pd-based catalysts (mono and in particular 274 bimetallic) does not decrease or is even higher than for bare γ-Al2O3. There are several factors that can Secondly, the interaction between chlorine and palladium results in the electron-deficient character 9 metallic palladium and nickel are also considered as NH3 adsorption site and this adsorption is 284 facilitated in the case of bimetallic catalysts due to the decrease of the Ni/support interaction [65].

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The catalytic activity of the mono-and bi-metallic catalysts with different Ni-Pd ratios in the LA 287 hydrogenation using FA as a hydrogen source is reported in Table 3

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The conversion and the product yields in hydrogenation of HMF obtained for the mono and 10 In the case of the monometallic Ni catalyst, the reaction is not selective, and the main products 310 observed were BHMTHF and MTHFA, giving also the highest number of other by-products. The lack Ni catalysts [66], or for Raney Nickel in milder hydrogenation conditions. 16 According to the literature,

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the presence of a support with high isoelectric point (like alumina) can additionally enhance the 317 reduction of the aromatic ring [26].

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In contrast, the monometallic 4%Pd shows a high yield of BHMTHF (57%), with the formation of 319 significant amount of DMF (32%

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Nickel was also shown by the group of Tomishige for systems supported on silica, but they did not 334 observed this behavior related with the selectivity issue was however not observed by his group on 335 silica supported materials [18].

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Adding 1%Pd to the 4%Ni catalyst allowed the formation of equal amounts of BHMTHF and 337 DMTHF, avoiding the formation of by-products as in the case of monometallic Ni. Once again similar 338 effect was observed for the Ni-rich bimetallic system. We observed typical behavior in this reaction for 339 monometallic Ni, namely ring hydrogenation and BHMTHF formation and further hydro-genolysis via 340 MTHFA to DMTHF. The difference was however related to the fact that this reaction proceeds to much 341 higher extend, as much higher yields to BHMTHF and DMTHF were obtained with the suppression of 342 by-products. In this case, Ni was considered as active site, similarly to what is described in the literature 343 for Ni-Au systems, for which the role of Au was to alter the Ni properties, e.g. by preventing from 344 surface poisoning due to a beneficial change in the adsorption energy of reaction substrates.

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The Pd-modified Ni-rich system behaves therefore like the pure Ni catalyst but with a strongly 376 boosted activity, and the role of Pd can be considered as spectator. This is especially visible in the 377 HMF hydrodeoxygenation, and this stronger activity could be related to the higher number of Ni 378 active sites. This higher amount of active sites is also manifested in FALA reaction, providing the 379 highest activity is the reaction.

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The situation is different in the case of the Ni-modified Pd-rich system, for which a higher 381 dispersion of Pd was manifested compared to the monometallic Pd catalyst (ToF-SIMS, XPS study).

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A higher number of active sites while preserving the very high acidity of the surface of the catalyst 383 help to push the HMF hydrodeoxygenation forward towards BHMTHF [10]. Pd generally favors the 384 hydrogenation of the furan ring because of the strong interaction between the narrow d band of Pd 385 and π bonds [5]. Additionally, the Pd-supported catalyst is quite selective towards BMHTHF, 386 especially at high temperatures [73]. Higher activity of this bimetallic palladium-rich catalyst in       diffractometer, using a Cu long-fine focus XRD tube working at 30mA and 40kV as a X-ray source.
[74] and a S-shaped Shirley type background [75] were used, while the surface atomic ratios 456 and the surface atomic concentrations were obtained using the appropriate experimental 457 sensitivity factors as determined by Scofield [76].

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The liquid products were analysed by high-performance liquid chromatograph (Agilent

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The catalysts were tested in the hydrogenation of 5-(hydroxymethyl)furfural.  Ni active sites, while Pd is proposed to be a spectator species for the reaction. By contrast, the much 503 higher Pd active site availability in the Ni-modified Pd-rich catalyst in comparison to its 504 monometallic Pd counterpart associated to the maintain of a very high surface acidity was proposed 505 to push the HMF hydro-deoxygenation forward and consequently to achieve a higher BHMTHF 506 selectivity. Beside the role of metals, residual chlorine was suggested to positively influence the 507 metals/support interactions and consequently to alter the catalyst properties.

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Forthcoming investigations will study the influence of both nature and the morphology of the 509 oxide support on the catalytic behavior of NiPd catalysts, with focus of the establishment of new 1D 510 morphology-induced metal/support interactions [77].

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By testing the catalysts in two hydrogenation reactions that require different site-specific 512 properties, we showed also the broader aspect of this work which can be easily extended for other 513 high impact hydrogenation processes of high industrial relevance, for which activity and even more 514 selectivity aspects remain crucial.

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Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: XRD 516 patterns of mono-and bi-metallic catalysts, Figure S2: TPR profiles of the mono-and bi-metallic catalysts after 517 the reduction step, Figure S3: Wide scan survey XPS spectra of the mono and bimetallic catalysts. Figure

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Conflicts of Interest: The authors declare no conflict of interest.