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Catalytic transfer hydrogenolysis of lignin derived aromatic ethers promoted by bimetallic Pd/Ni systems Francesco Mauriello, Emilia Paone, Rosario Pietropaolo, Alina Mariana Balu, and Rafael Luque ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01593 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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ACS Sustainable Chemistry & Engineering

Catalytic transfer hydrogenolysis of lignin derived aromatic ethers promoted by bimetallic Pd/Ni systems Francesco Mauriello 1,*, Emilia Paone 1, Rosario Pietropaolo 1 Alina M. Balu2, and Rafael Luque2,3,* 1

Dipartimento DICEAM, Università Mediterranea di Reggio Calabria, Loc. Feo di Vito, I-89122

Reggio Calabria, Italy 2

Departamento de Quımica Organica, Universidad de Cordoba, Edificio Marie-Curie (C-3), Ctra

Nnal IV, Km 396, Cordoba, Spain 3

Peoples Friendship University of Russia (RUDN University), 6 Miklukho-Maklaya str., 117198,

Moscow, Russia

KEYWORDS. catalytic transfer hydrogenolysis, aromatic ethers, heterogeneous catalysis, nickel, palladium, bimetallic catalysts.

* Corresponding Authors - email: [email protected]; [email protected]

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ABSTRACT

Catalytic transfer hydrogenolysis (CTH) of diphenyl ether (DPE), 2-phenethyl phenyl ether (PPE) and benzyl phenyl ether (BPE) - as model molecules of α-O-4 and β-O-4 as well as 4-O-5 lignin linkages - promoted by bimetallic Pd−Ni systems is reported. Pd/Ni (Pd loading of 3 wt%) catalysts were synthesized by using a simple and economic co-precipitation technique and its detailed physico-chemical characterization was performed by means of H2-TPR, XRD, TEM and XPS analysis. In presence of palladium as co-metal, an almost complete conversion of DPE was reached after 90 min at a temperature of 240 °C while BPE and PPE C−O bond breaking could be achieved at milder reaction conditions. Pd/Ni bimetallic systems can be magnetically recovered and efficiently used up to eight consecutive recycling tests in the transfer hydrogenolysis of DPE. The investigated substrates were also tested using analogous Ni monometallic systems. Palladium as cometal present in the catalysts was proved to increase the C−O bond cleavage rates and decreasing aromatic ring hydrogenation selectivity. The catalytic tests on all possible reaction intermediates clearly show that the hydrogenolysis cleavage in etheric C–O bond breaking was the rate determining step under CTH conditions, while hydrogenations only take place in a successive step. Moreover, it has been demonstrated that the hydrogenation of phenol formed from CTH depends on type of aryl groups that form the aromatic ether structure.


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INTRODUCTION One of the major challenges of modern refineries relates to the synthesis of bio-chemicals and bio-materials from non-edible lignocellulosic feedstocks.1-4 Lignin, the most abundant natural phenolic biopolymer, is of particular interest for chemical industries since it may allow a future sustainable production of aromatic compounds and intermediates.5-8 In the last years, several strategies have been proposed for lignin depolimerization into readily available aromatic compounds.3, 9-10 Since the lignin sub-structure is characterized by large amounts of etheric bonds, catalytic hydrogenolysis has received strong attention, allowing the C−O bond breaking by adding molecular hydrogen.11 However, the hydrogenolysis of lignin generally requires harsh reaction conditions due to the high dissociation energies involved in C–O bond cleavage.12,13 Furthermore, the use of high pressure molecular hydrogen leads to undesired aromatic ring hydrogenated products, thus decreasing the process efficiency and lowering the degree of lignin depolymerization (fully saturated products are less susceptible to a further hydrogenolysis process). As a consequence, the preparation of catalysts able to selective cleave the C−O bond in presence of aromatic functionalities together with a deep insight on the basic chemistry of aromatic ethers remains challenging aiming to develop selective chemical processes to produce aromatics from lignin,.

Due to the good availability and the competitive market price of nickel precursors with respect to other transition metals, significant interest was mainly devoted to address the design of homogeneous and heterogeneous Ni-based catalysts for hydrogenolysis of aromatic ethers.14 While homogeneous nickel catalysts were found highly efficient in reductive C−O bond cleavage of arylalkyl ethers and diaryl ethers under mild reaction conditions,15-16 their application in lignin depolymerization has been partially limited for their high separation costs, reusability and handling. At the same time, in literature, a remarkable development of heterogeneous Ni-based catalysts for the hydrogenolysis of aromatic ethers as well as for the reductive valorization of lignin was 3 ACS Paragon Plus Environment


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reported. Lercher and co-workers, for example, deeply investigated the conversion of aromatic ethers in aqueous phase elucidating kinetics and reaction pathways over several supported Ni catalysts showing the possibility to cleave C−O bonds via parallel hydrogenolysis, hydrolysis and hydrogenation/hydrodeoxygenation integrated steps.17-19 In addition, a series of bimetallic Ni catalysts have been proved to show better performances towards aromatic products and stability in the hydrogenolysis of lignin model compounds if compared with analogous monometallic Nisystems.20-25 Recently, the research group of Prof. Cai prepared and successfully applied some bimetallic Pd-Ni systems (BMNPs) in lignin hydrogenolysis as well as model compounds using low H2 pressure as well as under self-hydrogenolysis conditions.26-28 Among other heterogeneous bimetallic Ni-based catalysts, RANEY® Ni was proposed by the Rinaldi research group for C−O bond

cleavage of phenols, aromatic ethers and organosolv lignin under catalytic transfer

hydrogenolysis (CTH) conditions.29-32

CTH reactions represent a valid green substitute of molecular H2 for sustainable catalysis.33-35 This because, at present, molecular hydrogen is currently mainly produced from fossil feedstocks36 while a lot of H-donor molecules - including simple aliphatic alcohols such as methanol and ethanol - can be now easily obtainable from renewable resources.37, 38 Among many, 2-propanol - a simple model molecule for secondary alcohols found in nature - has been efficiently adopted as H-donor source in several CTH reactions. Nonetheless of importance, the use of an indirect H-source can minimize safety risks also reducing costs related to purchase, transportation and storage of H2. Therefore, the lignin depolymerization and the cleavage of the C−O bond in aromatic ethers through the CTH approach has been demonstrated to be a valid alternative to classic hydrogenolysis procedures. Since the pioneering studies of Ford on dihydrobenzofuran C−O bond cleavage, a lot of research groups have, so far, presented efficient transfer hydrogenolysis protocols for lignin and model molecules conversion ecules.39-44


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In this context, we prepared a bimetallic Pd/Ni catalyst, synthesized by a simple and economic co-precipitation preparation method, with the aim to test its catalytic performance in the hydrogenolysis of aromatic ethers that can mimic most common lignin linkages. Diphenyl Ether (DPE), 2-phenethyl phenyl ether (PPE) and benzyl phenyl ether (BPE) were chosen as simplest model molecules of lignin linkages and tested in CTH reactions using isopropanol as H-donor (Scheme 1). We primarily focused our attention on DPE that has a dissociation C−O bond energy of 314 kJ*mol/l, being among the strongest structural links in lignin that, in order to be broken, generally requires harsh reaction conditions. An analogous monometallic Ni substrate was also prepared and tested for comparative purposes. The physico-chemical characterization of all investigated catalysts was performed through H2-TPR, SEM, TEM, XRD and XPS analysis. If compared with the Ni monometallic system prepared with the similar synthetic procedure, the Pd‐Ni bimetallic catalyst shows higher activity and, at the same time, a lower tendency in the reduction of the aromatic ring. Furthermore, the Pd/Ni system could be re-utilized eight times without any noticeable activity loss and easily magnetically separated from the reaction medium. Finally, a thorough examination of the reaction mechanism is also proposed.

Scheme 1. Diphenyl Ether (DPE), 2-phenethyl phenyl ether (PPE) and Benzyl phenyl ether (BPE) as lignin model compounds.



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EXPERIMENTAL SECTION 2-phenethyl phenyl ether (PPE) was purchased from Frinton Laboratories and used without further purification. All other chemicals and Pd/C catalyst were acquired from Alfa-Aesar. Ni and Pd/Ni (with a nominal palladium loading of 3 wt %) catalysts were synthetized using a coprecipitation technique using inorganic salts (nickel II nitrate nonahydrate and palladium nitrate hydrate as metal precursors). Before catalytic tests, Ni, Pd/Ni and Pd/C catalysts were reduced at 250°C for 2h under a flow of hydrogen. PdO catalyst was reduced “in-situ” (inside the autoclave Parr Instrument 4560 Mini reactor system) in 2-propanol (40 ml) at 250°C for 2 hours in a H2 atmosphere (initial pressure: 40 bar). XRD data were collected using a Brucker D2 PHASER (Ni β-filtered Cu Kα radiation with a λ of 1.54 A) in the 2 theta range of 20–80° (scan speed of 0.5°min−1). Transmission Electron Microscopy (TEM) measurements were carried out using a JEM-2100F (200 kV). Mean particle size and relative distributions were calculated from the expression: dn= Σnidi/ni (ni = number of particles having a diameter di). H2-TPR reductions were carried out using a TPR instrument equipped with TCD detector. The dried catalyst (50mg) was heated at a constant rate of 10 °C min−1 from 0 to 1000 °C in a 5 vol % of H2/Ar mixture at a flow rate of 20 cm3 min−1. XPS data were collected using a JPS-9010MC photoelectron spectrometer (radiation source = Al Kα - 1486.6 eV). The catalyst was previously reduced and then inserted into the chamber instrument, preventing any contact with air. XPS data of “in situ-reduced” catalysts were obtained after a further in-situ thermal treatment in a secondary reduction chamber at 200°C under a hydrogen pressure of 100 Pa for 4 hours. All data were always obtained using a room temperature, and the binding energies (BE) were calibrated taking as reference the C 1s peak (284.6 eV). Reactions were performed at 500 rpm in a 160 mL stainless steel autoclave (Parr Instrument 4560 Mini reactor system). The reactor was loaded with the catalyst (0.2 g) suspended in a solution containing both the chosen substrate and 2-propanol (60 mL, 4% wt). Any trace of air present in the 6 ACS Paragon Plus Environment

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system was eliminated by fluxing three times N2 (99.99%). The reactor was subsequently pressurized at the desired gas (N2 or H2) pressure and heated at the final reaction temperature. At the end of the reaction, the system was cooled down at room temperature, the pressure released and the analysis of the organic phase was done by gas chromatography (Agilent 6890N equipped with CP- WAX 52CB, 60 m, i.d. 0.53 mm) For every recycling test, after any run, the catalyst was magnetically recovered, thoroughly washed with 2-propanol and reused under the same reaction conditions. The conversion and product selectivity in the liquid phase were calculated on the basis of the following equations:

Conversion [%]=

mol of reacted substrate mol of substrate feed

Liquid phase selectivity [%]=

× 100

mol of specific product in liquid phase sum of mol of all products in liquid phase

(1)

× 100

(2)

The product and aromatic yield was, respectively, calculated and defined as:



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RESULTS AND DISCUSSION

Catalysts synthesis and characterizations Ni-based catalysts were obtained starting from a solution containing nickel (II) nitrate nonahydrate and palladium nitrate hydrate, upon addition to a Na2CO3 (1M) aqueous solution under moderate stirring at room temperature. After stirring for two hours, the heterogeneous mixture was allowed to stand overnight and the final solid was recovered by filtration and washed several times using deionized water till neutral pH. The material was stored overnight at 120°C and subsequently calcined at 300°C (3 hours) under static air. Calcined material was finally conditioned under hydrogen flow (to ensure reduction) at 250°C for 2 hours.

The main surface and textural properties of Ni and Pd/Ni systems are reported in Table 1.

Table 1. Main characteristic of Ni and PdNi catalysts Catalyst notation

S.A.

Nickel dn

Palladium Loading

Palladium dn

[m2/g]

[nm]

[%]

[nm]

PdNi

9.2

21.4

3.0

3.8

Ni

9.0

22.5

-

-

S.A. = surface area; Nichel dn = mean cristallite size from XRD; Palladium loading: amount of Pd (determined by XRF); Palladium dn = mean particles size (determined by TEM).

XRD spectra of Pd/Ni and Ni catalysts, before and after reduction at 250°C, are included in Figure 1 and Figure S1. Unreduced catalysts exhibited clearly noticeable NiO diffraction lines.45 Comparatively, reduced samples confirmed the reduction of NiO into metallic nickel in both materials (Figure 1 vs S1). 45 Typical diffraction peak of metallic palladium (111), located at 2θ = 40.18, can be hardly observed for Pd/Ni, suggesting that extremely small and well dispersed Pd8 ACS Paragon Plus Environment

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particles are present (Figure 1).46, 47 Diffraction linesat 2θ values of 44.4, 51.8, and 76.2 – could be observed at slightly higher angles as compared to those of classic metallic nickel. These results indicated the presence of a Pd–Ni alloy phase, in good agreement with previous reports.48-50

Figure 1. XRD patterns of unreduced (down) and reduced (up) Pd/Ni catalysts.

Results of SEM-EDS analysis of reduced Ni and PdNi catalysts, including elemental mapping, are depicted in Figure S2. No specific description of the shape and size of nickel particles was possible to define; importantly, metallic species (Ni and Pd) could be observed to be nicely mixed and homogeneously dispersed in Pd/Ni.

Representative TEM microphotographs of Pd/Ni, reduced at 250°C for 2 h, are presented in Figure 2: faceted Ni metal particles and small Pd ensembles with a narrow particles size distribution



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(the obtained size distribution histograms of palladium nanoparticles are displayed in Figure 2 as an inset corner) can be observed.

Figure 2. TEM microphotographs and metal particle size distribution histograms of Pd/Ni catalyst H2-TPR proﬁle of NiO (Figure 3) displays an intense reduction peak at ca. 350°C corresponding to the reduction step of nickel oxide, with a shoulder at 310°C assigned to the reduction of oxygen species adsorbed on the surface.51-52 On the other hand, H2-TPR profile of PdNi (Figure 3) is analogous to that of the NiO sample although the main reduction peak is shifted to around 290°C and displays the formation of both Pd and Ni metals indicative of a promoting effect of well dispersed Pd ions on NiO reduction.47, 53


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Figure 3 PdNi and Ni H2-TPR profiles. XPS results of unreduced, reduced and in-situ reduced samples were analyzed by curve-ﬁtting and results are summarized in Table 2.

Table 2. Ni 2p3/2 and Pd 3d5/2 Binding energy values of investigated Ni-based systems Catalyst

PdNi

Ni

Binding Energy (eV) Ni 2p3/2

Pd 3d5/2

852.2

335.6

reduced

852.2; 855.2; 856.5

335.6

unreduced

853.7; 855.2; 856.5

336.8

852.6

-

reduced

852.6; 853.6; 855.2; 856.5

-

unreduced

853.6; 855.2; 856.5

-

in-situ reduced

in-situ reduced



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In the Supporting Information (Figure S3), Ni 2p core level spectra of unreduced and reduced Ni and Pd/Ni systems are reported. Unreduced Ni and Pd/Ni samples display, in the Ni 2p3/2 area, peaks centered at about 853.6, 855.2 and 856.5 eV corresponding to NiO and Ni(OH)2 species.

54

In the reduced Ni catalyst, the

peak at 852.6 eV, typical of metallic Ni, appears together with those attaining to Ni oxide and hydroxide components. After in situ reduction at 200°C, the typical Ni 2p3/2 XPS spectrum was registered. Instead, the binding energy of Ni 2p3/2 of reduced and in-situ reduced PdNi samples is characterized by a shift to lower B.E values of about 0.5 eV with respect to that of pure nickel, that can be ascribed to an electron density transfer from Pd to Ni species (Figure 4).55 Accordingly, in reduced and in-situ reduced PdNi systems the binding energy of the Pd 3d5/2 level was found at 335,6 eV, about 0,5 eV higher than that reported for metallic palladium (Figure 4), indicating formation of a bimetallic Pd-Ni ensemble that might be an alloy.56

Figure 4. XPS of Ni 2p3/2 and Pd 3d5/2 core level relative to the in-situ reduced Pd/Ni catalyst.


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CTH of lignin model compounds In principle, DPE catalytic transfer hydrogenation may occur via alternative pathways (Scheme 2): (i) direct C−O bond cleavage, affording phenol (PHE) and benzene (BEN) as primary products that may be further hydrogenated, respectively, into cyclohexane (CHX) and cyclohexanol (CXO) or (ii) a previous hydrogenation of DPE into cyclohexyl phenyl ether (CPE) that, in a successive step, may undergo hydrogenolysis allowing the formation of BEN and CXO (Scheme 1). Obviously, C-O bond hydrolysis rendering two phenol molecues should be ruled out for the absence of water in the reaction medium.

Scheme 2. Possible catalytic pathways in the CTH of diphenyl ether. Values relative to catalytic activities of Pd/Ni and Ni systems in the CTH of DPE have been included in Table 3.



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Table3. Catalytic transfer hydrogenolysis of diphenyl ether promoted by Ni-based systems

Catalyst

Chemoselectivity [%] Aromatic Yield Temperature Time Conversion [min] [°C] [%] [%] BEN CHX PHE CXO

Pd/Ni

240

90

96

45

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