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Article

Lignin Hydrogenolysis over Bimetallic Ni–Ru Nanoparticles Supported on SiO2@HPS

by
Antonina A. Stepacheva
1,
Oleg V. Manaenkov
1,
Mariia E. Markova
1,
Alexander I. Sidorov
1,
Alexsey V. Bykov
1,
Mikhail G. Sulman
1 and
Lioubov Kiwi-Minsker
2,3,*
1
Department of Biotechnology, Chemistry and Standardization, Tver State Technical University, A. Nikitin str. 22, 170026 Tver, Russia
2
Regional Technological Center, Tver State University, Zhelyabova str. 33, 170100 Tver, Russia
3
Department of Basic Sciences, Ecole Polytechnique Fédérale de Lausanne, ISIC-EPFL, CH-1015 Lausanne, Switzerland
*
Author to whom correspondence should be addressed.
Catalysts 2023, 13(5), 856; https://doi.org/10.3390/catal13050856
Submission received: 23 March 2023 / Revised: 26 April 2023 / Accepted: 4 May 2023 / Published: 8 May 2023
(This article belongs to the Special Issue Heterogeneous Catalysis in Green Chemistry II)
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Abstract

:
Lignin obtained by hydrogenolysis of lignocellulose biomass is a prospective source of valuable green fuels and chemicals such as monophenols. One of the key factors in the chemical decomposition of lignin to monophenols is an efficient catalyst. Inert porous materials such as hypercrosslinked polymers are suitable catalytic supports for the immobilization of noble and transition metal nanoparticles. However, such polymers do not have acidic properties, which are crucial for catalyzing hydrolysis. In this work, we report novel, efficient catalysts for lignin hydrogenolysis to produce valuable monophenolic compounds. The synthesized catalysts contained Ni, Ru, and Ni–Ru nanoparticles supported on SiO2-coated hypercrosslinked polystyrene (SiO2@HPS). Ni-Ru/SiO2@HPS demonstrated remarkable stability without any loss of the metallic phase and a high yield of monophenols (>42 wt.%) at close to full lignin conversion (>95 wt.%). This result was attributed to the synergy between the two metals and the support’s surface acidity. All catalysts were fully characterized by a series of physico-chemical methods.

1. Introduction

Lignin is considered as a prospective green source to produce valuable aromatic and phenolic compounds [1,2,3]. Lignin’s structure mainly contains covalently bonded phenyl-propanoid units [4,5,6]. Selective cleavage of the C–C and C–O bonds in the lignin polymeric network is one of the ways to obtain aromatic and oxygen-containing monomers, such as phenol, benzene, anisole, guaiacol, syringol, eugenol, p-ethylphenol, vanillin, etc. [7,8,9,10,11].
Nowadays, different techniques for lignin depolymerization are used. Among them are acid-catalyzed, base-catalyzed and oxidative depolymerization; pyrolysis; and hydrogenolysis [9,12,13,14,15,16,17,18,19]. Hydrogenolysis seems to be one of the most promising methods for lignin liquefaction [20]. The choice of a proper solvent is one of the main challenges for hydrogenolysis. The solvent should dissolve both lignin and the depolymerization products while preventing condensation reactions which usually lead to coke formation [21,22,23,24,25]. Water and alcohols are the most frequently used solvents for lignin hydrogenolysis [26]. On one hand, water is cheap and non-toxic, attaining over 80% lignin conversion under mild conditions [27]. However, poor lignin and hydrogen solubility in water require the use of either alkaline or acidic additives and also put certain restrictions on the catalyst design [28]. Thus, alcohols are the solvents of choice in lignin hydrogenolysis, which efficiently dissolve lignin and depolymerization products, inhibit char deposition, and provide in situ hydrogen formation [29,30]. Isopropanol is most frequently used for lignin depolymerization compared to primary alcohols because it has a higher hydrogen supply capacity [31,32,33,34,35].
The catalyst is one of the most important factors in lignin hydrogenolysis. It controls the lignin conversion, product yield, and product nature. Metals and their binary compounds have been reported to be effective catalysts. Different metals such as Ru, Rh, Cu, Ni, Pt, and Pd are able to promote the cleavage of different C–O bonds in lignin, resulting in up to a 60 wt.% yield of monomers [36,37,38,39,40,41,42,43,44,45]. For example, highly dispersed Ni allowed up to a 15 wt.% monophenol yield and a high lignin conversion (over 93 wt.%) [46]. However, Ni also tends to catalyze the hydrodeoxygenation of phenols, leading to the formation of aromatic compounds [47]. Pt exhibits a high activity in dehydrogenation reactions, resulting in about a 17 wt.% monophenol yield [48]. Pd is known to effectively catalyze C–O cleavage, resulting in the formation of aromatics [49]. Ru shows a moderate activity in lignin depolymerization but exhibits a high selectivity to monophenols and aromatics due to its defect-rich structure [50,51].
From mechanistic insights, lignin hydrogenolysis into aromatic and phenolic monomers mainly proceeds through the homolytic cleavage of Cα–O, O–CH3, and Cα–Cβ bonds [52,53] or through a pyrolysis mechanism via breaking of β-O-4, β-1, and β-5 bonds [54,55,56]. It was concluded that the homolytic cleavage of C–O bonds is more preferable because of the lower dissociation energy according to DFT calculations [57,58]. The homolytic mechanism depends on the presence of hydrogen, the catalyst’s nature, and the use of hydrogen donor solvents. For example, in an inert solvent, H2 dissociation plays an important role in C–O cleavage [59,60]. The alcohols used as solvents easily form H species on the catalyst surface due to dehydrogenation processes [60,61]. Catalysts containing a metallic active phase are suitable in both cases because of their activity in hydrogen dissociation and solvent dehydration.
The catalytic support also plays an important role in depolymerization processes. Strong Brønsted and Lewis acid sites on the surface of the support promote the activation of C–O bonds and facilitate their cleavage. Acid sites also influence the electron state of the active metal phase through metal–support interactions [62,63,64,65,66,67]. In particular, Ni, Pd, and Ru have been reported to have strong interactions with acidic supports, resulting in a high number of electron-rich sites and a defect-rich crystalline structure [68,69,70]. Moreover, the acidic support can serve as a structural and energetic promoter, facilitating the reduction of the metal active phase [71,72].
Inert porous materials such as hypercrosslinked polymers are promising catalytic supports for the immobilization of noble metals, transition metals, and metal nano-particles. In our recent work, it was shown that the aromatic polymeric matrix can effectively stabilize the metal particles in the pores, providing high stability against aggregation [73,74,75]. However, such polymers do not have acidic sites, which are crucial for hydrogenolysis. Recently, we found that Ni nanoparticles supported on polymers modified by SiO2 effectively catalyze the conversion of polyaromatic compounds into mono-aromatics through the hydrogenolysis of C–C bonds [76].
In this work, we report for the first time the structural properties of Ni, Ru, and bimetallic Ni–Ru nanoparticles supported on hypercrosslinked polystyrene coated by SiO2 (SiO2@HPS) and their catalytic behavior in lignin depolymerization to phenolic monomers. Newly developed Ni-Ru/SiO2@HPS demonstrated a remarkable stability without any loss of the metallic phase and a high yield of monophenols (42 wt.%) at close to full lignin conversion (up to 95%) under optimized reaction conditions.

2. Results and Discussion

2.1. Catalyst Characterisation

To study the structure and morphology of the synthesized SiO2@HPS support, low-temperature nitrogen physisorption (BET), X-ray photoelectron spectroscopy (XPS), and X-ray powder diffraction (XRD) was carried out.
During preparation of the SiO2@HPS sample, APTES is hydrolyzed by the subcritical water, leading to the formation of silica gel on the polymer surface. Moreover, some covalent and van der Waals interactions between the tert-amino groups of HPS and APTES can also take place, as shown previously [77]. The results of the low-temperature nitrogen physisorption showed that the deposition of the silicon-containing phase on the surface of HPS leads to a decrease in the total specific surface area and the surface area of micropores by about 200 m2/g compared to the as-synthesized sample. This is due to the partial HPS pore blockage while using the APTES as a surface modifier [78]. Moreover, during SiO2@HPS synthesis, APTES partially undergoes hydrolysis with water, forming an amorphous polysiloxane, as already reported [77,79]. Heating under a nitrogen flow, in contrast, increases the surface area, particularly by increasing the mesopores. Such an increase can be correlated with the formation of a SiO2 phase during the calcination and additional decomposition of non-hydrolyzed APTES (see Table 1). The formation of xerogel-like structures with an additional porosity on the inert support while using ethoxysilane as a modifier has also been discussed elsewhere [76,80].
To analyze the changes in the textural properties of the sample after heating, a comparison of the nitrogen adsorption–desorption isotherms was carried out (Figure 1). The presented isotherms can be associated with the Via type [81] for both samples. An increase in the adsorbed nitrogen volume can be observed at high Ps/P0 ratios. This is correlated with an increase in the capillary absorption process in pores with a diameter of 6–8 nm. For the sample obtained after heating at 300 °C, the hysteresis loop is more significant, indicating the intensification of capillary condensation in mesopores.
An XPS study of the SiO2@HPS samples showed the presence of C, O, Cl, and Si on the surface of the synthesized support (see Figure S1). The initial polymer possesses an aromatic polymeric matrix containing about 4–5 wt.% of O and 0.05–0.1 wt.% of Cl. The presence of N (about 1–2 at.%) is due to the tert-amino groups in the polymer structure. The surface composition of SiO2@HPS samples is presented in Table 2. Heating of the SiO2@HPS sample at 300 °C leads to a two-fold increase in the surface concentration of Si and a 1.3-fold increase in the surface concentration of O in comparison with the as-synthesized one. This is related to SiO2∙xH2O migration from the polymer volume to the surface with the capillary water during its evaporation, as well as the decomposition of SiO2∙xH2O leading to SiO2 shell formation [80]. An analysis of the high-resolution XPS spectra in the Si and O region showed that in the as-synthesized sample, the silica-containing phase is presented by SiOx, and in the sample after heating, the formation of SiO2 can be observed (Figure S2) [82]. However, a shift in the binding energies due to the differential charging of the polymer surface should be noted.
An XRD analysis of the sample (Figure 2) shows that in the as-synthesized SiO2@HPS sample, SiO2 was found to be amorphous, with a wide peak at ca. 22 2θ. For the sample after heating at 300 °C, XRD peaks at ca. 23, 29, 32, 36, 43, 45, and 48 2θ are observed and are associated with the developed tridymite structure [83]. The possibility of low-temperature calcination of ethoxysilanes leading to the formation of a crystalline phase of SiO2 was studied by Ayu Lestari et al. in [80]. This confirms the results obtained in the current study, showing the change in crystallinity of the silica shell after heating the SiO2@HPS sample at 300 °C. However, the low peak intensity, probably due to a high dispersion of SiO2, should be mentioned [84,85,86].
Three catalysts (Ni, Ru, and Ni-Ru) using a SiO2@HPS support were synthesized according to the procedure described in Section 3.3. The catalysts were studied by low-temperature nitrogen physisorption (BET), X-ray photoelectron spectroscopy (XPS), small-angle X-ray scattering (SAXS), and NH3 chemisorption. The results are reported in Table 3. The deposition of the metal-containing phase on the SiO2@HPS surface leads to a decrease in the specific surface area caused by the blockage of micropores by active phase formation. After the reaction, the surface area of the catalysts slightly decreased due to the adsorption of the reaction products on the surface. The adsorption of reaction components is also indicated by a decrease in acidity. The acidity of the initial support was found to be 1012 and 986 μmol/g for the as-synthesized SiO2@HPS and SiO2@HPS after heating, respectively. Metal deposition on the support slightly decreases the acidity due to formation of the metal-containing phase on both the polymer and SiO2 surface. For the catalysts after the reaction, the acidity was found to be 0.7–0.8 mmol/g, which is probably related to the adsorption of phenolic and aromatic products on acid sites, along with their further condensation [87]. Coke formation can lead to catalyst deactivation; however, as no metal carbonization was found on the catalyst after the reaction (see Figure 3), the active phase seems to be stable. One can suppose that the coke is mostly formed on surface acid sites.
Regarding the active phase particle size (see Table 3, column 4, and Figure S3), it should be noted that for Ni-SiO2@HPS, the formation of large particles was observed. After the reaction, a further increase in particle size of the Ni-containing samples was seen, indicating that aggregation took place. For Ru-SiO2@HPS, smaller particles were formed, while no aggregation during the reaction was noticed. The addition of Ru to Ni leads to a decrease in the particle size as compared to pure Ni, and almost no aggregation during the reaction was observed.
An XPS study of the as-synthesized catalysts confirmed the formation of metal oxide particles on the catalyst surface. However, after the reaction, the presence of a metallic phase due to the partial in situ reduction of the metal oxides was detected (see Figure 3).

2.2. Catalyst Testing

The catalytic performance of the synthesized samples was tested in lignin hydrogenolysis according to the procedure described in Section 3.3. The catalytic performance was characterized by lignin conversion and the total monophenol yield (see Table 4). All synthesized catalysts showed lignin conversion into liquid and gaseous products over 70 wt.%. The highest conversion (over 85 wt.%) was observed when using the bimetallic Ni-Ru-SiO2@HPS sample. For monometallic Ni and Ru catalysts, the conversion was also sufficiently high (>75 wt.%). An increase in the lignin conversion over the bimetallic catalyst can be assigned to synergy between the two metals, leading to an increased electron density on Ni, which facilitates metal reduction due to an increased hydrogen and electron transfer [18,21,28,32,33,35,39]. Ru is known to provide high selectivity to monophenols and aromatics due to its defect-rich structure [50,51].
The gaseous phase contained light hydrocarbons (mainly methane and ethylene), methanol, methyl aldehyde, and CO. As hydrogenolysis mainly targets liquid products, the composition of the gas phase was estimated only qualitatively. Lignin hydrogenolysis results in the formation of a wide range of products, including monophenols (phenol, anisole, guaiacol, syringol, eugenol, p-ethylphenol, and p-hydroxyphenol), arenes (toluene and benzene), cyclohexanes (cyclohexane and methylcyclohexane), and soluble oligomers (see Figure 4). Monophenols were chosen to be the target product in this study as they have a wide range of applications. The yield of monophenols was over 20 wt.% for Ni-containing catalysts, and even exceeded 32 wt.% for Ni-Ru-SiO2@HPS. The chosen catalyst seems to provide an improved monophenol yield in comparison with the literature data for bimetallic Ni-based catalysts, which showed ca. 20–35 wt.% yield of monophenols at 50–80 wt.% lignin conversion [33,34,37,40,41,46,88]. The synergistic effect can be related to the higher Ni dispersion obtained by the addition of Ru [33,89]. In Section 2.1, it was shown that the addition of Ru to Ni leads to a diminished mean particle size of the active phase and a decreased polydispersity (Figure S3). Moreover, the addition of noble metals to Ni is known to increase the number of electron-rich sites, resulting in a higher catalytic activity in C–O and C–C bond cleavage [90,91]. Besides, the high acidity of the catalyst can play an important role because the acid sites can effectively adsorb the isopropanol and polyphenols, facilitating their further transformation.
To estimate the effect of HPS and SiO2 coating on the lignin hydrogenolysis, the catalysts Ni-Ru-HPS and Ni-Ru-SiO2 were synthesized by the impregnation method and tested in the process at the same conditions (lines 4 and 5 in Table 4). It is seen that the HPS-supported catalyst showed relatively low lignin conversion and monophenol yield in comparison with the developed catalysts. As it is well known that the presence of Lewis acid sites enhances the C–O cleavage and increases both lignin conversion and the product yield [64,65,66,67]. HPS, in turn, does not have acidic sites, thus leading to low lignin conversion in comparison with other supports [92,93,94]. For the SiO2-based catalyst, the lignin conversion was over 50 wt.%; however, the monophenol yield did not exceed 16 wt.%. This catalyst mainly formed oligomers with a molecular weight of 250–500 m/z, which is in a good agreement with other studies [8,10,11,31]. Despite the high acidity of the support, for the indicated catalyst, the specific surface area was significantly lower (ca. 120 m2/g) and aggregation of the active phase was observed (mean particle diameter 31 ± 8 nm), leading to a lower activity in C–O bond cleavage. Moreover, a high carbonization degree (ca. 21 wt.% in one run) was observed for the Ni-Ru-SiO2 catalyst. Thus, HPS coated by SiO2 is a promising support for effective lignin hydrogenolysis catalysts.

2.3. Ni-Ru-SiO2@HPS Stability Tests

The catalyst Ni-Ru-SiO2@HPS showed a higher lignin conversion and monophenol yield and so was tested in several consecutive reaction runs to evaluate its stability during lignin depolymerization. The catalyst was removed from the reaction mixture by filtration and washed twice with chloroform to dissolve the residuals. After washing, the catalyst was used in the next depolymerization experiment with a fresh portion of lignin. Ni-Ru-SiO2@HPS did not show any decrease in lignin conversion or monophenol yield during eight consecutive runs (see Table 5). Afterwards, a slight decrease in both parameters (ca. 2%) was observed (see the 10th cycle). Since no leaching of silica or the active metal was detected, the decrease in the catalyst efficiency can be attributed to a slight aggregation of the metal particles and a reduction in acidity.

2.4. Process Conditions Optimization

To increase the monophenol yield, the optimization of lignin hydrogenolysis conditions in the presence of the Ni-Ru-SiO2@HPS catalyst was carried out. Three parameters were varied: temperature (from 260 to 300 °C), hydrogen pressure (from 2.0 to 5.0 MPa), and the lignin/catalyst ratio (from 1000 to 2000 g/g). The results are shown in Table 6.
An increase in the lignin hydrogenolysis temperature (Table 6, lines 1–5) led to a higher lignin conversion due to the acceleration of the reaction rate. The monophenol yield also increased in the temperature range of 260–280 °C and reached up to 40 wt.%. A further temperature rise resulted in a decrease in the monophenol yield, probably due to the intensification of side reactions such as the deoxygenation and hydrogenation of the products [10,18,19,28]. This was confirmed by an increase in the concentration of aromatic and cyclic hydrocarbons in the reaction mixture. When the partial hydrogen pressure was increased from 2.0 to 3.0 MPa at the optimal reaction temperature (280 °C), rises in both the lignin conversion and the monophenol yield were observed (Table 6, lines 6–9). However, a further increase in hydrogen pressure decreased the monophenol yield, favoring the formation of hydrocarbons (aromatic and cyclic). Besides the lignin depolymerization, the hydrogen pressure also affects the deoxygenation process [40,41,62,88]. Varying the catalyst loading, an augmentation in the lignin conversion was observed when the lignin/catalyst ratio decreased (Table 6, lines 10–12). Meanwhile, the monophenol yield decreased in this case because of the intensification of side reactions. Thus, the highest yield of target monophenols (up to 42 wt.%) was obtained at a temperature of 280 °C, a partial hydrogen pressure of 3.0 MPa, and a lignin/catalyst ratio of 2000 g/g.

3. Materials and Methods

3.1. Materials

Hypercrosslinked polystyrene (HPS) Macronet MN100 (functional groups: tert-amino groups, specific surface area: 790 m2/g) was purchased from Purolite Int. Ltd. (Llantrisant, UK) and used as received as a catalytic support. (3-aminopropyl)triethoxysilane (APTES, 99.9%, Sigma Aldrich, St. Louis, MO, USA) was used for SiO2 shell formation on HPS. Acetates of nickel and ruthenium (Aurat, Moscow, Russia) were used as catalyst active phase precursors. Tetrahydrofuran (chemical grade, Sigma Aldrich, St. Louis, MO, USA) and methanol (chemical grade, Sigma Aldrich, St. Louis, MO, USA) were used as received for catalyst preparation. Kraft lignin alkali (water soluble, 5 wt.% of water, pH 6.5) (Sigma Aldrich, St. Louis, MO, USA) was used for catalyst testing in the hydrogenolysis process. Isopropyl alcohol (IPA) (chemical grade, Kupavna Reactive, Moscow, Russia) was used as a solvent in the hydrogenolysis process. H2 (99.9%, GasProduct, Tver, Russia) and N2 (99.9%, GasProduct, Tver, Russia) were used as received in the hydrogenolysis process.

3.2. SiO2@HPS Preparation

Amounts of 3.0 g of HPS, 1.17 mL of APTES (calculated to get 10 wt.% of SiO2 per 1 g of HPS), and 10.0 mL of distilled water were loaded into a PARR 4307 reactor (Parr Instruments Ltd., Moline, IN, USA). The reactor was sealed and purged with nitrogen to remove air. Then, the reactor was heated up to 200 ± 1 °C under a nitrogen atmosphere. After the temperature reached the set value, the reaction mixture was held for 1 h at constant stirring (500 rpm). Then, the reactor was cooled down to room temperature. The suspension was filtered and washed with 10 mL of distilled water. The sample was dried in air at 105 ± 5 °C for 4 h and heated in a nitrogen flow at 300 ± 5 °C for 5 h. The resulting sample was called SiO2@HPS.

3.3. Catalyst Preparation

The catalyst was prepared by wet impregnation of 3.0 g SiO2@HPS with the calculated amount of metal precursor in a solution containing 5.0 mL of tetrahydrofuran, 1.0 mL of methanol, and 3.0 mL of distilled water. The suspension was continuously stirred for 10 min. Then, the resulting granules were dried at 75 ± 5 °C for 1 h. The catalyst was washed with 10.0 mL of an aqueous solution containing 0.084 g of sodium bicarbonate and dried again at 100 ± 5 °C for 3 h. The dry sample was heated up to 300 ± 5 °C for 5 h under a nitrogen flow to form metal oxides. The synthesized catalysts and their Si and metal contents are listed in Table 7.

3.4. Catalyst Characterisation

The specific surface area, porosity, and pore size distribution of the catalyst sample (initial, after saturation with hydrogen, and after catalysis) were determined by nitrogen low-temperature adsorption using a Beckman Coulter SA 3100 (Coulter Corporation, Brea, CA, USA) analyzer. Before the analysis, the samples were degassed in a Beckmann Coulter SA-PREP (Coulter Corporation, Brea, CA, USA) apparatus for sample preparation at 120 °C in a vacuum for 1 h. To estimate the specific surface area and total pore volume, t-plots and Brunauer–Emmett–Teller models were used. The pore size distribution was evaluated using the Harkins–Jura equation.
X-ray photoelectron spectroscopy (XPS) analysis was performed using an ES-2403 spectrometer (Institute for Analytical Instrumentation RAS, St. Petersburg, Russia) modified with a PHOIBOS 100 analyzer produced by SPECS GmbH (Berlin, Germany) equipped with a MgKα/AlKα XR-50 X-ray radiation source. The spectra were acquired at an X-ray power of 200 W and an energy step of 0.1 eV. Before the analysis, the samples were degassed for 180 min. Data analyses were performed in CasaXPS (Casa Software Ltd., Teignmouth, UK).
X-ray powder diffraction (XRD) patterns were collected on an Empyrean from PANalytical (Malvern, UK). X-rays were generated from a copper target with a scattering wavelength of 1.54 Å. The step size of the experiment was 0.02.
To obtain the SAXS data, an S3 MICRO diffractometer (Hecus X-Ray Systems GmbH, Graz, Austria) with point collimation and copper radiation (Cu Ka, 50 W) was used. The measurements were performed in the range of vectors q from 0.01 to 0.6 Å−1, where q = 4πsinθ/λ. The samples for the study were placed in a 1.5 mm glass capillary with a wall thickness of 0.01 mm. To exclude the influence of residual scattering from porous hypercrosslinked polystyrene, the sample was impregnated with a contrast agent with a known excess in terms of moisture capacity. Data analyses were performed in ATSAS data analysis software (EMBL, Hamburg, Germany) using a spherical form factor.
NH3 chemisorption was carried out using an AutoChem HP (Micromeritics Ltd., Norcross, GA, USA). The analysis was performed in a temperature range of 30–300 °C with a heating rate of 5 °C/min and following this the temperature was maintained at 300 °C for 1 h. The quantity of the desorbed gas was estimated using calibration curves.

3.5. Lignin Hydrogenolysis Procedure

Lignin hydrogenolysis experiments were carried out in a stainless-steel batch reactor (Parr Series 5000 Multiple Reactor System) (Parr Instrument, Moline, IN, USA) with a cell volume of 50 mL equipped with a magnetic stirrer. In a typical procedure, 1.0 g of lignin, a calculated amount of catalyst, and 30 mL of isopropanol were loaded into the reactor cell. The catalyst loading was calculated as 2000 g of lignin per 1 g of metal in the catalyst. The cell was sealed and the air was replaced by nitrogen by flushing three times. The reactor was heated up to 260 °C under a nitrogen pressure of 2.0 MPa. After the temperature reached 260 °C, nitrogen was replaced by hydrogen. Hydrogenolysis was performed for 3 h at a constant stirring (1200 rpm). Then, the reactor was cooled to room temperature.
The gaseous phase was sampled after cooling of the reactor and analyzed by GC using a Crystallux 4000 M (Meta-Chrom, Yoshkar Ola, Russia) equipped with a flame ionization detector, a katharometer, and a packed column filled with granules of polymer adsorbent MN-270 (Purolight Inc., Llantrisant, UK) with a fraction of 125–250 µm (length: 2.5 m, diameter: 3.0 mm). The analysis was carried out under the following conditions: the initial temperature of the column was 40 °C, which was maintained for 4 min, then the temperature was raised to 250 °C at a heating rate of 15 °C/min; the temperatures of the evaporator and the detector were 260 °C; the carrier gas was helium; and the total flow of He was 30.0 mL/min. A quantitative analysis of the gaseous phase was performed based on calibration curves using standard compounds.
The liquid phase was separated from the solid residue by centrifugation. An analysis of the liquid phase was performed by GCMS using a GC-2010 gas chromatograph and a GCMS-QP2010S mass spectrometer (Shimadzu, Kyoto, Japan) equipped with a HP-1MS chromatographic column (length: 30 m, diameter: 0.25 mm, film thickness: 0.25 µm). An analysis was carried out under the following conditions: an initial temperature of 120 °C, which was maintained for 5 min, then the column was heated up to 250 °C at a rate of 5 °C/min and maintained at 250 °C for 5 min. Helium (volumetric velocity of 20.8 mL/s, pressure of 253.5 kPa) was used as the gas carrier. The injector temperature was 280 °C, the ion source temperature was 260 °C, and the interface temperature was 280 °C. Methylene diamine (Sigma Aldrich, St. Louis, MO, USA) was used as an external standard for the quantitative estimation. For the proper quantitative analysis, calibration regarding the main monophenolic compounds was performed.
Lignin conversion was calculated based on Equation (1). The yield of monophenolic compounds was calculated according to Equation (2).
C o n v e r s i o n = m L − m s m L ⋅ 100 %
M o n o p h e n o l   y i e l d = m m o n o m L ⋅ 100 %
where mL—the weight of lignin taken for the experiment, g; ms—the weight of residual after the hydrogenolysis, g; and mmono—the total weight of monophenols, g.

4. Conclusions

Catalysts containing Ni, Ru, and Ni–Ru nanoparticles supported on SiO2@HPS were synthesized for the first time and tested in a lignin hydrogenolysis process, targeting catalyst stability and a high yield of monophenols. Coating of HPS with amorphous SiOx and subsequent heating in a nitrogen flow at 300 °C led to the formation of tridymite-type SiO2 on the surface, presenting a high SSA of up to 950 m2/g. The synthesized catalysts contain metal/metal oxide nanoparticles as an active phase and have a surface acidity of ca. 1 mmol/g. The best catalyst was found to be bimetallic Ni-Ru-SiO2@HPS, which showed a high lignin conversion (up to 95%) and a high monophenol yield (42 wt.%) under optimized reaction conditions. Moreover, the catalysts demonstrated a remarkable stability in ten consecutive runs without any loss of the active metal phase.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13050856/s1, Figure S1: Survey XPS spectra of the as-synthesized (a) and heated (b) SiO2@HPS; Figure S2: High-resolution XPS spectra of Si 2p for the as-synthesized (a) and heated (b) SiO2@HPS, and O 1s for the as-synthesized (c) and heated (d) SiO2@HPS; Figure S3: Particle size distribution for the as-synthesized Ni-SiO2@HPS, Ru-SiO2@HPS, and Ni-Ru-SiO2@HPS catalysts.

Author Contributions

Conceptualization, L.K.-M. and M.G.S.; methodology, A.I.S.; investigation, M.E.M., A.V.B. and O.V.M.; writing—original draft preparation, A.A.S.; writing—review and editing, A.A.S. and L.K.-M.; project administration, A.A.S. All authors have read and agreed to the published version of the manuscript.

Funding

The financial support for the work was provided by the Russian Science Foundation (grant 22-79-10096).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 13 00856 g001 550
Figure 1. BET study of SiO2@HPS support: (a) adsorption isotherms; (b) pore size distribution.
Figure 1. BET study of SiO2@HPS support: (a) adsorption isotherms; (b) pore size distribution.
Catalysts 13 00856 g001
Catalysts 13 00856 g002 550
Figure 2. XRD pattern for as-synthesized SiO2@HPS (a) and SiO2@HPS after heating (b): ●—literature data.
Figure 2. XRD pattern for as-synthesized SiO2@HPS (a) and SiO2@HPS after heating (b): ●—literature data.
Catalysts 13 00856 g002
Catalysts 13 00856 g003 550
Figure 3. High-resolution Ni 2p and C 1s XPS spectra of the Ni-Ru-SiO2@HPS catalyst before (a,c) and after (b,d) reaction.
Figure 3. High-resolution Ni 2p and C 1s XPS spectra of the Ni-Ru-SiO2@HPS catalyst before (a,c) and after (b,d) reaction.
Catalysts 13 00856 g003
Catalysts 13 00856 g004 550
Figure 4. Product composition of lignin hydrogenolysis over SiO2@HPS-supported catalysts.
Figure 4. Product composition of lignin hydrogenolysis over SiO2@HPS-supported catalysts.
Catalysts 13 00856 g004
Table
Table 1. BET study of supports.
Table 1. BET study of supports.
SampleVpores, cm3/gSBET, m2/gt-Plot Surface Area, m2/g
HPS (MN-100)0.52 ± 0.01814 ± 1External 208 ± 1
Micropore 696 ± 1
SiO2@HPS as synthesized0.40 ± 0.01611 ± 1External 197 ± 1
Micropore 435 ± 1
SiO2@HPS after heating0.65 ± 0.01951 ± 1External 380 ± 1
Micropore 597 ± 1
Table
Table 2. Elemental composition of SiO2@HPS samples.
Table 2. Elemental composition of SiO2@HPS samples.
SampleElemental Composition, wt.%
CONSiCl
SiO2@HPS as-synthesized86.2 ± 0.36.9 ± 0.13.1 ± 0.12.8 ± 0.11.0 ± 0.1
SiO2@HPS after heating83.8 ± 0.38.7 ± 0.10.6 ± 0.16.0 ± 0.10.8 ± 0.1
Table
Table 3. Catalyst characterization.
Table 3. Catalyst characterization.
SampleSBET, m2/gMetal Compound *Dm, nmTotal Acidity, μmol/g
Ni-SiO2@HPS584 ± 1NiO17 ± 5965 ± 5
Ni-SiO2@HPS after reaction521 ± 1NiO, Ni21 ± 6782 ± 5
Ru-SiO2@HPS736 ± 1RuO25 ± 3903 ± 5
Ru-SiO2@HPS after reaction694 ± 1RuO2, Ru5 ± 3726 ± 5
Ni-Ru-SiO2@HPS628 ± 1NiO, RuO27 ± 4942 ± 5
Ni-Ru-SiO2@HPS after reaction589 ± 1NiO, RuO2, Ni, Ru7 ± 4788 ± 5
* According to XPS.
Table
Table 4. Catalyst performance in lignin hydrogenolysis.
Table 4. Catalyst performance in lignin hydrogenolysis.
SampleLignin Conversion, wt.%Monophenol Yield, wt.%
Ni-SiO2@HPS75.4 ± 0.224.8 ± 0.1
Ru-SiO2@HPS78.3 ± 0.117.8 ± 0.2
Ni-Ru-SiO2@HPS85.7 ± 0.332.7 ± 0.2
Ni-Ru-SiO254.3 ± 0.215.3 ± 0.2
Ni-Ru-HPS37.4 ± 0.19.7 ± 0.1
Process conditions: solvent—isopropanol; temperature—260 °C; hydrogen pressure—2.0 MPa; lignin/catalyst ratio—2000 g/g; stirring rate—2000 rpm; process duration—3 h.
Table
Table 5. Ni-Ru-SiO2@HPS catalyst performance in consecutive cycles.
Table 5. Ni-Ru-SiO2@HPS catalyst performance in consecutive cycles.
Number of CyclesLignin Conversion, wt.%Monophenol Yield, wt.%Element Concentration *, wt.%Dm, nmTotal Acidity, μmol/g
SiMetal
185.7 ± 0.332.7 ± 0.29.84.5 (Ni), 4.7 (Ru)7 ± 4942 ± 5
285.6 ± 0.232.5 ± 0.39.84.5 (Ni), 4.7 (Ru)7 ± 4788 ± 5
385.8 ± 0.332.6 ± 0.29.84.5 (Ni), 4.7 (Ru)7 ± 4782 ± 5
585.5 ± 0.232.6 ± 0.39.84.5 (Ni), 4.7 (Ru)7 ± 4766 ± 5
885.2 ± 0.332.5 ± 0.29.84.5 (Ni), 4.7 (Ru)7 ± 4735 ± 5
1083.6 ± 0.231.3 ± 0.39.84.5 (Ni), 4.7 (Ru)8 ± 4702 ± 5
* according to XFA. Process conditions: solvent—isopropanol; temperature—260 °C; hydrogen pressure—2.0 MPa; lignin/catalyst ratio—2000 g/g; stirring rate—2000 rpm; process duration—3 h.
Table
Table 6. Influence of hydrogenolysis conditions on the lignin conversion monophenol yield.
Table 6. Influence of hydrogenolysis conditions on the lignin conversion monophenol yield.
EntrySampleLignin Conversion, wt.%Monophenol Yield, wt.%
Temperature, °C
Process conditions: solvent—isopropanol; hydrogen pressure—2.0 MPa; lignin/catalyst ratio—2000 g/g; stirring rate—2000 rpm; process duration—3 h.
126085.7 ± 0.332.7 ± 0.2
227088.1 ± 0.235.2 ± 0.1
328092.8 ± 0.139.8 ± 0.2
429094.4 ± 0.234.7 ± 0.1
530095.2 ± 0.129.4 ± 0.2
Hydrogen pressure, MPa
Process conditions: solvent—isopropanol; temperature—280 °C; lignin/catalyst ratio—2000 g/g; stirring rate—2000 rpm; process duration—3 h.
62.092.8 ± 0.139.8 ± 0.2
73.094.5 ± 0.242.1 ± 0.1
84.096.2 ± 0.137.4 ± 0.2
95.097.8 ± 0.234.8 ± 0.1
Catalyst loading, g (lignin)/g (catalyst)
Process conditions: solvent—isopropanol; temperature—280 °C; hydrogen pressure—3.0 MPa; stirring rate—2000 rpm; process duration—3 h.
10100097.8 ± 0.229.4 ± 0.1
11150096.3 ± 0.134.9 ± 0.2
12200094.5 ± 0.242.1 ± 0.1
Table
Table 7. List of the synthesized catalysts.
Table 7. List of the synthesized catalysts.
SampleElement Concentration *, wt.%
SiMetal
Ni-SiO2@HPS9.84.6
Ru-SiO2@HPS9.84.3
Ni-Ru-SiO2@HPS9.84.5 (Ni), 4.7 (Ru)
* according to XFA.
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MDPI and ACS Style

Stepacheva, A.A.; Manaenkov, O.V.; Markova, M.E.; Sidorov, A.I.; Bykov, A.V.; Sulman, M.G.; Kiwi-Minsker, L. Lignin Hydrogenolysis over Bimetallic Ni–Ru Nanoparticles Supported on SiO2@HPS. Catalysts 2023, 13, 856. https://doi.org/10.3390/catal13050856

AMA Style

Stepacheva AA, Manaenkov OV, Markova ME, Sidorov AI, Bykov AV, Sulman MG, Kiwi-Minsker L. Lignin Hydrogenolysis over Bimetallic Ni–Ru Nanoparticles Supported on SiO2@HPS. Catalysts. 2023; 13(5):856. https://doi.org/10.3390/catal13050856

Chicago/Turabian Style

Stepacheva, Antonina A., Oleg V. Manaenkov, Mariia E. Markova, Alexander I. Sidorov, Alexsey V. Bykov, Mikhail G. Sulman, and Lioubov Kiwi-Minsker. 2023. "Lignin Hydrogenolysis over Bimetallic Ni–Ru Nanoparticles Supported on SiO2@HPS" Catalysts 13, no. 5: 856. https://doi.org/10.3390/catal13050856

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