Advanced High-Loaded Ni–Cu Catalysts in Transfer Hydrogenation of Anisole: Unexpected Effect of Cu Addition

Abstract

Binary Ni–Cu heterogeneous catalysts are known to demonstrate excellent activity in conventional hydrogenation of phenolic compounds, and Cu addition facilitates hydrodeoxygenation (HDO). In this study, we aimed to show the effect of Cu on the specific catalytic activity and selectivity of Ni–Cu catalysts in transfer hydrogenation, in which 2-PrOH was used as a solvent and an H donor. Catalytic transformations of anisole were studied in sub- and supercritical alcohol at 150 and 250 °C. The catalysts were prepared using an environmentally friendly supercritical antisolvent coprecipitation method, which makes it possible to obtain well-dispersed particles (less than 7 nm) at high metal loading (up to 50 wt.%). When copper is added, deactivation of the catalyst in transformations of anisole, including HDO, is observed. The experimental data and the appropriate kinetic analysis demonstrate that there is a decrease in the rate of anisole conversion accompanied by an increase in the concentration of acetone formed during the dehydrogenation of 2-PrOH.

1. Introduction

Binary and multicomponent heterogeneous metal catalysts demonstrate excellent results in many chemical processes [1,2,3]. The effect of the second metal can be related to many factors such as improved selectivity [4], thermal and chemical stability [5], particle dispersion [6], and others. An additional compound used to form the catalytically active phase as well as support or reaction conditions expand the variety of factors affecting catalyst performance. This is especially important when working with complex substances, for example, biomass, which includes many chemical compounds with many functional groups. There are many examples where multimetallic catalysts show significant superiority in biomass conversion compared to monometallic ones [7,8].

As is known, there are several main strategies to convert biomass into useful products. The reductive transformations resulting in deoxygenation and dearomatization are extremely important in the production of chemicals or fuel [9,10,11]. Nickel–copper binary catalysts are considered to be very promising in such transformations. For example, copper decreases the reduction temperature of nickel–copper particles [12,13] and increases the selectivity of conventional hydrogenation [13,14]. Tang et al. explained the change in the selectivity of furfural reduction with the electronic structure of the metal surface, which affects the geometry of the adsorption complex [5,14]. Some works also reported that the addition of Cu results in a decrease in the amount of adsorbed hydrogen that also affects the rate of side processes [15,16]. There are some reports demonstrating that copper addition can facilitate deoxygenation when used in H₂ hydrogenation [8,17].

In our opinion, transfer hydrogenation (TH) is a very promising alternative to the conventional processes using H₂ produced from fossils. In some cases, TH can be more effective compared to conventional hydrogenation [18,19]; however, the most important advantage of TH is donors of plant origin such as alcohols, formic acid and its derivatives. This can be used to implement a scheme, such as the one in Figure 1 [20].

Nickel–copper catalysts are active in transfer hydrogenation as well as in conventional processes; however, the number of articles devoted to TH is rather limited [18,21]. For example, a supported Ni–Cu alloy showed a significant synergetic effect compared to single metal catalysts in the transformation of furfural into furfuryl alcohol using 2-propanol (2-PrOH) [22]. Similar results were obtained in other studies where Ni–Cu catalysts demonstrated noticeable activity in TH of furfural and p-cresol [18]. In the presence of Ni–Cu/Al₂O₃ catalyst, the yield of reduction products for the latter substrate at 300 °C under 50 atm of H₂ was about 30–40%, whereas under H₂-free conditions using 2-propanol (2-PrOH) as a hydrogen donor, this value exceeded 95%. The use of a Ni–Cu/Al₂O₃ catalyst in TH results in a higher conversion compared to Ni/Al₂O₃ under the same conditions that is not typical for conventional hydrogenation. In the authors’ opinion, this effect can be explained by the higher oxidation stability of nickel–copper particles compared to metallic nickel. This feature can be very important in TH because the hydrogen donor is a liquid, and air or other oxidants can significantly affect the rate and direction of the whole process [23].

Another example compared hydrogenolysis of the C–O bonds of glycerol under H₂ and 2-PrOH [21]. The process with H₂ led to 100% conversion of glycerol using Ni–Cu/γ-Al₂O₃ and Ni–Cu/TiO₂ as catalysts. The experiments were carried out at 230 °C under 35 atm of H₂ in a flow reactor. At the same time, when hydrogen was replaced with N₂ and 2-PrOH was used as the only H donor, the conversion of the substrate decreased to 78–85%. The addition of copper to nickel promoted an increase in the conversion of glycerol from 76% to 100%, and also markedly increased the part of 1,2-propanediol in the final reaction mixture. There are also several examples where Ni–Cu catalysts were used under acidic conditions; that is possible due to the stability of alloys to oxidation as mentioned above [5,24].

This work aimed to demonstrate for the first time the catalytic effect of high-loaded Ni–Cu catalysts in TH of anisole. The catalysts were prepared using an ecofriendly method based on supercritical antisolvent coprecipitation in CO₂. This method makes it possible to obtain highly loaded dispersed nickel–copper catalysts with the structure of a solid substitution solution without phase separation [25,26]. The effect of copper addition to nickel as well as donor activity of 2-PrOH was investigated under sub- and supercritical conditions. The observed time dependence of anisole conversion and the product yield made it possible to analyze in detail the kinetics of the process.

2. Results and Discussion

2.1. Catalyst Properties

Figure 2 shows XRD spectra of the Ni-containing samples. All the samples contained the Ni° phase (PDF No. 04-0850) and a small amount of the NiO phase (PDF No. 047-1049). As mentioned above, the catalysts were passivated under air, therefore, the metal particles were expected to be covered with a NiO film, and XRD confirmed that the size of NiO crystallites was less than 3 nm. The mean size of the Ni° particles defined by XRD is presented in

Table 1. Content of Ni and Cu in the Ni–Cu catalysts measured using XRF. D—mean crystallite size, a—lattice parameter, measured using XRD, S_CO—surface area, measured using CO adsorption.

Sample	Ni, wt.%	Cu, wt.%	D (Ni°), nm	a (Ni°), Å	SCO, m2/g
Ni(36)–SiO2	36.4 ± 0.4	–	6.0 ± 0.5	3.524 ± 0.001	35 ± 3
Ni(41)Cu(4)–SiO2	40.7 ± 0.4	4.0 ± 0.2	5.5 ± 0.5	3.529 ± 0.001	32 ± 3
Ni(36)Cu(14)–SiO2	36.2 ± 0.4	14.4 ± 0.3	6.5 ± 0.5	3.553 ± 0.001	8 ± 1

; it did not change dramatically after the reaction with anisole (see Supplementary Materials) and did not exceed 6.5 nm. The XRD data did not demonstrate the presence of Cu-containing phases in any sample; at the same time, compared to Ni(36)–SiO₂, an increase in the lattice parameter of the bimetallic Ni–Cu catalysts was observed. This fact can be explained by the modification of the Ni° crystal structure with Cu atoms; moreover, the lattice parameter correlates with the Cu content, which also proves our suggestion [27].

The content of Ni and Cu in the samples measured using XRF is given in Table 1. All the samples contained significant amounts of metals. At the same time, the mean size of the metal crystallites remained comparatively small, judging by the XRD data. This is one of the most important advantages of the SAS method, which allows one to synthesize high-loaded catalysts with well-dispersed metal nanoparticles.

The XRD data corroborate those obtained via TEM and EDX mapping (Figure 3). The supported catalyst particles were mostly spherical, with a diameter not exceeding several nanometers. These particles were agglomerated in the larger ones, forming micrometer-sized structures. Interesting and informative results were obtained using EDX mapping, showing uniform distribution of the elements in the samples (Figure 3).

One of our previous studies demonstrated the importance of water addition to obtain monophasic Ni–Cu particles [25]. In this work, this approach was implemented in practice. The addition of water promotes the formation of Ni and Cu carbonates, which are much easier to mix as compared to acetates. As a result, the oxides after thermal decomposition and the metals after reduction in H₂ remain well-mixed as do the initial Ni and Cu carbonates. As we mentioned in the Introduction, the advantages of monophasic Ni–Cu catalysts were demonstrated many times in both transfer and conventional hydrogenation [14,22].

The number of available Ni atoms on the catalyst surface was determined using CO adsorption. The energy of interaction between CO and Cu° is much lower compared to Ni° [28], therefore, the part of Cu° with adsorbed CO is negligible. The data given in Table 1 show that Ni(36)–SiO₂ had the highest specific surface area of Ni° of 35 m²/g. The addition of a small amount of Cu led to a slight decrease in the Ni° surface: the area of available Ni atoms on the surface of Ni(41)Cu(4)–SiO₂ reached 32 m²/g. The further increase in the Cu content resulted in a smaller Ni° surface, so the measured area of Ni(36)Cu(14)–SiO₂ was 8 m²/g only. Cu has a lower surface energy [29], therefore, this metal enriches the surface of monophasic Ni–Cu particles. This explains the significant decrease in the Ni° surface area from 35 m²/g to 8 m²/g when the Cu-to-Ni mole ratio was increased from 0 to 1/2.5. Thus, we can unambiguously conclude that high-loaded and monophasic Ni–Cu catalysts were synthesized.

The properties of the synthesized catalysts were in good accordance with the previous results [8]. Ni(36)–SiO₂ and Ni(41)Cu(4)–SiO₂ had a comparatively developed surface which was approximately two times larger compared to literature data [8]. At the same time, the surface area of Ni° in Ni(36)Cu(14)–SiO₂ was significantly lower due to the high Cu content. The metal particles were well-dispersed and had similar mean crystallite sizes, which were in the range of 5.5–6.5 nm for all the catalysts. This distinguishes the catalysts used in this study from the previous work where the particle distribution was significantly wider [8]. We suggest that these excellent results are related to the influence of water added into the mixture during catalyst preparation.

2.2. Transfer Hydrogenation of Anisole: Influence of Copper Content, Hydrogen Donor, and Temperature

All the experiments were carried out under sub- and supercritical conditions at 150 °C and 250 °C, respectively. Anisole was chosen as the initial substrate due to its low reactivity compared to molecules containing free OH groups in nickel-catalyzed TH [30,31]. At the same time, transformations of alkylated phenols are more in demand from the practical point of view. In our previous study, high-loaded monometallic Ni catalysts demonstrated excellent results in TH of anisole [26]. Copper addition could significantly improve the performance of these catalysts, including lowering the reduction temperature and increasing the oxidation resistance. Moreover, it has been demonstrated that Cu facilitates HDO of anisole in H₂ hydrogenation [8,32]. The same effect was quite expectable in TH due to the mechanistic similarity between conventional and transfer hydrogenation [11].

Our previous results demonstrated the high activity of the SiO₂-supported Ni-based catalyst in anisole TH conditions [26], therefore, SiO₂ was used as the support in this study. To demonstrate the effect of Cu addition, the activity of Cu-free Ni(36)–SiO₂ was studied first as a reference sample. After 3 h at 250 °C in the experiment with Ni(36)–SiO₂, anisole conversion was found to be 99% (Figure 4A). Methoxycyclohexane (yield, 43%) as well as cyclohexanol (37%) were the main products of anisole TH that showed high activity of Ni(36)–SiO₂ in hydrodearomatization and O–CH₃ bond cleavage. The use of this catalyst also led to the formation of cyclohexane (13%). It seems like benzene, which was found in trace amounts, formed at the first stage. Thus, all the data obtained in the experiment with Ni(36)–SiO₂ are in a good agreement with literature data [26].

The decrease in temperature significantly slows down the reaction rate, which results in low conversion. To overcome this issue, the mass of the catalysts was increased to 0.41–0.42 g in the oxide form and ~0.36–0.37 g after reduction. Another important difference between the experiments at the different temperatures was that at 150 °C the reactions proceeded in the liquid phase, whereas at 250 °C—in a supercritical fluid. The results of anisole transformations over Ni(36)–SiO₂ at 150 °C are shown in Figure 4B. The conversion of anisole after 3 h reached 62%. The formed products included cyclohexane (3%), cyclohexanol (39%), and methoxycyclohexane (18%). In contrast to the mixture obtained under more severe conditions, no isopropyl cyclohexyl ether was observed. An interesting detail is that at 150 °C, cyclohexanol became the main product instead of methoxycyclohexane. It shows that C–O bond cleavage prevails over dearomatization under milder conditions that can be related to the difference in activation energy of the different transformations.

Thus, taking into account the obtained data, we can conclude that the transformation of anisole under TH conditions over Ni(36)–SiO₂ proceeds according to four main pathways (Scheme 1). All the final products did not contain aromatic groups; anisole itself can lose its aromatic system, transforming into methoxycyclohexane (pathway 2). Another option is C–O bond cleavage resulting in the formation of cyclohexane and cyclohexanol—pathways 1 and 3, respectively. These transformations likely proceed through the formation of aromatic compounds such as benzene and phenol found in trace amounts. The final option is replacing the methyl group with an isopropyl one (pathway 4); this process leads to the formation of the isopropyl cyclohexyl ether; however, a small content of the isopropyl phenyl ether was also found as an intermediate product.

Using the scheme of anisole transformations, the experimental data presented in Figure 4 were simulated using a quasi-first-order kinetic model. The formation of all the products was considered as an irreversible process because a large excess of 2-PrOH was used. The good correlation between the modeling kinetic curves (dash lines in Figure 4) and the experimental data (dots in Figure 4) shows that the chosen assumptions describe the observed transformations quite well, especially at 250 °C.

Table 2. Rate constants of product formation in the experiments at 250 °C. The constants were calculated using the least squares method, m(catalyst) = 0.10–0.11 g.

Sample	k1 × 104, s−1	k2 × 104, s−1	k3 × 104, s−1	k4 × 104, s−1	k∑ × 104, s−1
Ni(36)–SiO2	0.73 ± 0.04	1.9 ± 0.1	2.2 ± 0.1	0.27 ± 0.03	5.1 ± 0.2
Ni(41)Cu(4)–SiO2	0.18 ± 0.06	0.95 ± 0.07	0.89 ± 0.07	0.19 ± 0.07	2.2 ± 0.3
Ni(36)Cu(14)–SiO2	0.20 ± 0.14	1.1 ± 0.4	0.20 ± 0.14	–	1.5 ± 0.4
Ni(36)–SiO2 1	0.41 ± 0.04	1.9 ± 0.1	1.2 ± 0.1	0.19 ± 0.04	3.7 ± 0.2
Ni(41)Cu(4)–SiO2 1	0.13 ± 0.1	0.8 ± 0.1	0.5 ± 0.1	–	1.4 ± 0.2

¹ 15 mL of 2-PrOH were replaced with 15 mL of acetone.

and

Table 3. Rate constants of product formation in the experiments at 150 °C. The constants were calculated using the least squares method, m(catalyst) = 0.36–0.37 g.

Sample	k1 × 104, s−1	k2 × 104, s−1	k3 × 104, s−1	k4 × 104, s−1	k∑ × 104, s−1
Ni(36)–SiO2	0.03 ± 0.01	0.37 ± 0.03	0.70 ± 0.03	–	1.1 ± 0.4
Ni(41)Cu(4)–SiO2	0.06 ± 0.01	0.46 ± 0.03	0.29 ± 0.03	–	0.81 ± 0.4
Ni(36)Cu(14)–SiO2	0.04 ± 0.01	0.34 ± 0.02	0.12 ± 0.02	–	0.50 ± 0.03

show the rate constants calculated according to the kinetic data (Figure 4) within the scheme of catalytic transformations of anisole and the formed products (Scheme 1).

Table 4. Specific catalytic activity of the catalysts in TH of anisole at 250 °C.

Sample	k1/SCO × 105, 1/(s × m2)	k2/SCO × 105, 1/(s × m2)	k3/SCO × 105, 1/(s × m2)	k4/SCO × 105, 1/(s × m2)	k∑/SCO × 105, 1/(s × m2)
Ni(36)–SiO2	2.0 ± 0.1	5.3 ± 0.3	6.2 ± 0.3	0.74 ± 0.09	15.0 ± 0.4
Ni(41)Cu(4)–SiO2	0.5 ± 0.2	2.7 ± 0.2	2.5 ± 0.2	0.5 ± 0.2	6.3 ± 0.3
Ni(36)Cu(14)–SiO2	2.0 ± 1.4	12 ± 4	2.0 ± 1.4	–	16 ± 4
Ni(36)–SiO2 1	1.1 ± 0.1	4.9 ± 0.3	3.1 ± 0.2	0.5 ± 0.1	9.6 ± 0.4
Ni(41)Cu(4)–SiO2 1	0.4 ± 0.3	2.3 ± 0.3	1.4 ± 0.3	–	4.1 ± 0.5

¹ 15 mL of 2-PrOH were replaced with 15 mL of acetone.

and

Table 5. Specific catalytic activity of the catalysts in TH of anisole at 150 °C.

Sample	k1/SCO × 105, 1/(s × m2)	k2/SCO × 105, 1/(s × m2)	k3/SCO × 105, 1/(s × m2)	k4/SCO × 105, 1/(s × m2)	k∑/SCO × 105, 1/(s × m2)
Ni(36)–SiO2	0.020 ± 0.007	0.30 ± 0.02	0.54 ± 0.02	–	0.86 ± 0.03
Ni(41)Cu(4)–SiO2	0.050 ± 0.008	0.39 ± 0.03	0.25 ± 0.02	–	0.69 ± 0.04
Ni(36)Cu(14)–SiO2	0.14 ± 0.04	1.4 ± 0.1	0.42 ± 0.07	–	2.0 ± 0.1

collect the data about the specific catalytic activity of the catalysts used at 250 and 150 °C. The total activity of Ni(36)–SiO₂ at 250 °C was almost 20 times higher compared to 150 °C. At the same time, the anisole transformation into cyclohexane demonstrated to be the most sensitive reaction to temperature because the catalytic activity decreased by two orders of magnitude. This is consistent with the known literature data indicating a strong dependence of HDO on temperature [33]. Cyclohexanol becomes the main product at 150 °C, which is also mirrored in the kinetic constants: k₃ contributes the most to the total activity under subcritical conditions.

The addition of 4 wt.% of Cu resulted in a lower conversion of anisole and significantly decreased the yield of cyclohexane (5%) and cyclohexanol (29%, Figure 5), although the yield of methoxycylohexane reached 44%. In contrast to H₂ hydrogenation [8], the obtained data show that copper does not speed up C–O cleavage in TH; moreover, HDO was slowed down compared to Ni(36)–SiO₂. Another interesting detail is that the kinetic model suitable for transformations over a Cu-free catalyst does not show the same convergence in the case of Ni(41)Cu(4)–SiO₂ (dash lines in Figure 5) because the molar fraction profiles of the products obtained over Ni(41)Cu(4)–SiO₂ tend to stabilize. At 150 °C (Figure 5B), the conversion of anisole decreased to 49% vs. 62% over Ni(41)Cu(4)–SiO₂ and Ni(36)–SiO₂, respectively, and the selectivity remained very similar compared to 250 °C. In addition, the used kinetic model did not fit the experimental data very well (dash lines in Figure 5B). We suggested that saturation of aromatic rings could be reversible [19,34], which means methoxycyclohexane can be transformed into anisole and vice versa. In order to demonstrate reversibility, an experiment with methoxycyclohexane as the initial substrate was carried out. No traces of anisole or any other aromatic compounds were found, therefore, the idea of reversibility was declined.

Another possible explanation of the observed result is catalyst deactivation. XRD analyses of the spent catalysts showed that the phase content and the mean crystallite size did not change significantly during the reaction (see Supplementary Materials); furthermore, noticeable carbon deposition was not observed. Thus, we suggested that the found catalyst deactivation can be related to competitive adsorption of 2-PrOH, anisole, and products of their transformations on the catalyst surface. It is well-known that Cu promotes adsorption of carbonyl compounds [22,35]; therefore, we suggested that acetone formed via 2-PrOH dehydrogenation could have a negative influence on catalyst performance. This effect should be taken into account in kinetic calculations. To do this, quasi-equilibrium between 2-PrOH on the one hand and acetone on the other was added into the kinetic model. A part of the formed acetone was adsorbed on the catalyst surface, blocking it. This process was described as a quasi-equilibrium between acetone in the solution and on the catalyst surface (see Supplementary Materials). The solid lines in Figure 5 show the dependence of anisole and product concentrations on the time calculated with acetone influence. It is clearly seen that this kinetic model describes the experimental data including the concentration of acetone much better.

The rate constants as well as specific catalytic activity describing the anisole transformations over Ni(41)Cu(4)–SiO₂ were calculated according to the model taking into account the competitive adsorption of acetone. At 250 °C, copper addition resulted in the decrease in all the rate constants compared to the Ni(36)–SO₂-catalyzed transformations. At the same time, Table 4 shows that the values of k₁ and k₃ were the most sensitive to the presence of Cu. This correlates with the significant decrease in the cyclohexane and cyclohexanol yield discussed above. At 150 °C, the difference between Ni(36)–SiO₂ and Ni(41)Cu(4)–SiO₂ was not so significant compared to the supercritical conditions. For example, k_∑/S_CO calculated for a copper-containing catalyst reached 0.81 × 10⁻⁵ s⁻¹m⁻², whereas for Ni(36)-SiO₂, this value was 1.1. × 10⁻⁵ s⁻¹m⁻² (Table 5). The calculation of k₂/S_CO showing catalyst activity in the reduction of anisole to methoxycyclohexane was noticeably higher in the case of Ni(41)Cu(4)–SiO₂. At the same time, copper affected the formation of cyclohexane (k₁) and cyclohexanol (k₃) negatively, which is in good agreement with the data obtained at 250 °C (Table 3).

In order to demonstrate the negative effect of acetone on catalyst performance, experiments with pre-addition of acetone were conducted using Ni(36)–SiO₂ and Ni(41)Cu(4)–SiO₂. For this, 15 mL of 2-PrOH were replaced with 15 mL of acetone. The results of the anisole transformations are presented in Figure 6. The addition of acetone affected TH over both catalysts. In case of Ni(36)–SiO₂, the conversion of anisole reached almost 100% after 3 h, which corroborates the experiment in which acetone was not added. At the same time, the yield of methoxycyclohexane and cyclohexanol after 3 h reached 31% and 43%, whereas without acetone addition, these values were 37% and 49%, respectively. The addition of acetone decreased the product yields, which was related to the higher acetone concentration exceeding 1.1 mol/L after 3 h (Figure 6B). At the same time, when acetone was not pre-added, this value almost reached 0.80 mol/L.

The kinetic analysis of the experiments with pre-added acetone demonstrated that the transformations over Ni(36)–SiO₂ could be accurately described with the model which did not include acetone influence (Figure 6A). At the same time, the presence of acetone should be considered when Ni(41)Cu(4)–SiO₂ is used (Figure 6B). In the case of the Cu-free catalyst, the addition of acetone did not affect k₂ which is responsible for aromatic group saturation. The negative influence of ketone on other constants was more obvious: k₁ and k₃ decreased from 0.73 × 10⁻⁴ s⁻¹ and 2.2 × 10⁻⁴ s⁻¹ to 0.41 × 10⁻⁴ s⁻¹ and 1.2 × 10⁻⁴ s⁻¹, respectively (Table 2). This effect cannot be related to the dilution of 2-PrOH, therefore, we conclude that acetone prevents transformations of C–O bonds over Ni(36)–SiO₂. It seems to be quite logical that acetone and other oxygen-containing compounds compete for the same sites on the catalyst surface. We assume that the relatively high concentration of acetone as well as its carbonyl group promote adsorption of ketone. Thus, some amount of active centers remains to be blocked and does not contribute to transformations of anisole. Addition of copper facilitates this effect because this metal promotes adsorption of carbonyl compounds [22,36]. In the case of Ni(41)Cu(4)–SiO₂, this is indicated by the lower rate constants which are sensitive to the higher acetone concentration (Figure 7).

The increase in the Cu content resulted in the lowest total rate constant; at the same time, this catalyst also had a low Ni° surface area which reached 8 m²/g for Ni(36)Cu(14)–SiO₂ (Table 1). These two factors resulted in the total activity of 16 × 10⁻⁵ s⁻¹m⁻² and 2.0 × 10⁻⁵ s⁻¹m⁻² at 250 °C and 150 °C, respectively (Table 3 and Table 5). It is important to notice that the transformations of anisole over Ni(36)Cu(14)–SiO₂ strongly deviated from the quasi-first order kinetic model that resulted in a larger error in rate constant calculations. The abnormally high values of the rate constants compared to Ni(41)Cu(4)–SiO₂ were related to the calculations in which Ni atoms were considered as the active centers. The increase in the Cu content from 4.0 wt.% to 14.4 wt.% led to the significant enrichment of the catalyst surface with Cu atoms that reduced the Ni° area four times (Table 1). Cu is known to demonstrate limited activity in TH [37], and we assume that its activity could be noticeable in the case of Ni(36)Cu(14)–SiO₂.

Hydrodearomatization of anisole resulting in methoxycyclohexane formation made the most significant contribution to the total activity of Ni(36)Cu(14)–SiO₂. The yield of this product achieved 48% and 22% at 250 °C and 150 °C, respectively. At the same time, the yields of cyclohexane (2% and 3% at 150 °C and 250 °C, respectively) and cyclohexanol (10% at both temperatures) remained very low, which proves the negative influence of Cu on the transformations of C–O bonds. Figure 8 shows that the kinetic model which considers the influence of acetone fits the experimental data very well (solid lines), whereas the exclusion of acetone from calculations results in a lower convergence. Interestingly, at 250 °C, the usage of Ni(36)Cu(14)–SiO₂ led to a higher acetone content compared to other catalysts. We assume this difference could be related to the different activation barriers of dehydrogenation over the catalysts. Earlier, it was demonstrated that Ni addition to Cu decreases the activation energy of alcohol dehydrogenation [38].

In contrast to the data obtained in the experiments with 2-PrOH, the use of ethanol (EtOH) as an H donor did not lead to noticeable anisole conversion over all the catalysts used in this study. This fact is consistent with the previous results showing the low donor activity of primary alcohols compared to the secondary ones in TH catalyzed by Ni-based heterogeneous catalysts [39,40,41]. There are several possible explanations for this fact in literature. First, the presence of two alkyl substituents in secondary alcohols increases the electron donation effect that promotes alcohol dehydrogenation [11]. Second, aldehydes formed from primary alcohols block the catalyst surface, preventing TH [41]. Third, formed aldehydes can lose carbon atoms under TH conditions, transforming into CO, which is a well-known poison of Ni-based catalysts [42]. We assume that all the factors mentioned above could affect the activity of EtOH as an H donor in this study.

During the experiments at 250 °C, significant amounts of gas formed, its pressure reached 0.4–2.0 MPa after cooling down the reactor to 40 °C. The qualitative analysis of the formed gas mixtures showed that they included a lot of H₂. At the same time, the analysis of liquids demonstrated the presence of acetone and the products of its condensation. It showed that dehydrogenation of 2-PrOH to acetone and H₂ occurred. Interestingly, molecular hydrogen as well as diethyl acetal were also found in the gas and liquid mixtures in the case of EtOH use, however, anisole transformations were not observed. These facts confirmed dehydrogenation of both primary and secondary alcohols under the used conditions, resulting in H₂ formation. Furthermore, the analysis of gases after the experiments with 2-PrOH demonstrated the presence of CH₄; however, its content remained extremely low compared to H₂. The possible mechanism of methane formation is reductive cleavage of the O–CH₃ bond. Another product of this process is phenol which was also found in the reaction mixtures in a trace amount. This molecule quickly transformed into cyclohexanol under the used conditions.

Thus, Ni and Ni–Cu catalysts prepared using an advanced supercritical antisolvent coprecipitation method demonstrate excellent results in TH of anisole used as a model compound of bio-oil. The prepared catalysts had important advantages such as a monophasic Ni–Cu active component and high metal content (up to 50 wt.%), at the same time remaining metal particles to be well-dispersed after high-temperature reduction. The addition of Cu decreased the anisole conversion and affected the selectivity of the process as the presence of Cu significantly slowed down the transformations of the oxygen-containing group. We suggest that this property can be useful for selective hydrogenation of aromatic groups under TH conditions.

Many reports published earlier show that Cu facilitates deoxygenation in conventional hydrogenation of anisole when added to Ni-based catalysts [32,38]. The authors suggest Cu promotes the interaction between the oxygen-containing group of anisole and the catalyst surface. It causes better adsorption and faster transformations of anisole. At first glance, the results of this study contradict the previous ones because Cu affects C–O cleavage negatively. However, we demonstrated that this important difference is based on the nature of the reductant and the products, in particular, acetone. Literature data show that Cu can promote interaction between carbonyl compounds and the catalyst surface [22,37]. In this study, we demonstrated the negative influence of acetone on the catalytic activity of Cu-containing systems that is important from the theoretical and practical points of view.

3. Materials and Methods

3.1. Materials

Ethanol (EtOH, ≥99.9%, Merck KGaA, Darmstadt, Germany, 2-propanol (2-PrOH, ≥99.8%, EKOS-1, Moscow region, Russia), anisole (99%, Solvay, Clamecy, France), methoxycyclohexane (>95%, freshly distilled), acetone (98%, EKOS-1, Moscow region, Russia), dodecane (≥99%, Shandong Lanhai Industry Co., Ltd., Jinan, China), tetraethoxysilane (TEOS, 98%, Gihi Chemicals Co., Hangzhou, China), Ni(OAc)₂·4H₂O, (99% extra, Biochem Pharma, Cosne-Cours-sur-Loire, France), Cu(OAc)₂·H₂O, (98%, Biochem Pharma, Cosne-Cours-sur-Loire, France), methanol (HPLC gradient grade, Baker Petrolite, Stavanger, Norway), CO₂ (99.8%, Promgazservis, Novosibirsk, Russia).

3.2. Catalyst Preparation and Characterization

The synthesis of SiO₂ sol: to prepare partially hydrolyzed TEOS, 15 mL of methanol, 2.2 mL of water, and 0.3 mL of 0.1 N HCl were added to 20 mL of TEOS in this order. The resulting solution was left with stirring for 24 h. To carry out complete hydrolysis, distilled water was added to the resulting pre-hydrolyzed solution at a ratio of 0.3 mL water per 1 mL of the solution and left to mix for 1 h. The concentration of SiO₂ in the sol was 0.11 g/mL.

The catalysts were synthesized using supercritical antisolvent precipitation (SAS) of the Ni and Cu acetates and SiO₂ sol (Figure 9). Experimental parameters: CO₂ flow, 80 g/min; solution flow, 2 mL/min; temperature, 40 °C; nozzle, 0.004” (0.1 mm); pressure, 150 bar. The total concentration of the nickel and copper acetates was 15.5 mg/mL in the methanol solution containing 9 vol.% water. The typical volume of the solution of Ni and Cu acetates in methanol was 120 mL. To obtain the certain SiO₂ concentration in the samples, the required volume of the SiO₂ sol was also added to the solution. Three catalysts were synthesized with a mass Ni content of ~40 wt.%, while the Cu content varied: 0, 4, and 14 wt.%. The samples were marked as Ni(X)Cu(Y)–SiO₂, where X and Y are the mass fractions of Ni and Cu, respectively. The Cu-free catalyst was marked as Ni(36)–SiO₂. The samples after precipitation were calcined at 300 °C for 3 h with the ramp rate of 3 °C/min in static air to give the oxide phase.

The elemental composition of the catalysts passivated in air was determined using an X-ray fluorescence spectrometer ARL Advant’X 2247 (Thermo Fisher Scientific, Waltham, MA, USA) equipped with an Rh anode as an X-ray source. Mass percentage of Ni and Cu was estimated using QuantAS software. Phase analysis using XRD was performed using a D8 Advance X-ray diffractometer (Bruker, Billerica, MA, USA) equipped with a linear LynxEye (1D) detector. The measurements were carried out using Cu Kα radiation (λ = 1.5418 Å) with a step of 2θ = 0.05° and accumulation time of 3 s at each point. Before the analysis, all the samples were reduced in H₂ flow then they were passivated and analyzed. After the treatment at 250 °C in the reaction, the samples of the catalysts were also passivated and analyzed. TEM studies were performed using a ThemisZ electron microscope (Thermo Fisher Scientific) operated at 200 kV. For electron microscopy studies, the samples were deposited on perforated carbon substrates attached to aluminum grids using an ultrasonic dispersant. Images were recorded using a Ceta 16 CCD sensor (Thermo Fisher Scientific). CO pulse chemisorption measurements were carried out using a Chemosorb analyzer (Modern Laboratory Equipment). Fifty mg of each catalyst were placed inside a U-shaped quartz tube and treated in H₂ flow (100 mL/min) upon heating up. After the reduction, pulses of CO were fed to the tube (100 µL) until the amount of CO in the outlet stopped changing according to the thermal conductivity detector. Thereafter, the amount of chemisorbed CO was estimated.

3.3. Batch Experiments

Before the experiment, the samples of the oxidized catalysts (0.41–0.42 g or 0.11–0.12 g) were reduced in H₂ flow (30 l/h) for 45 min, the activation was carried out in a quartz tube at 400 °C, the Cu-free sample was reduced at 450 °C. The completeness of reduction was controlled by ex situ XRD. After reduction, the catalysts were pulled out of the furnace and cooled down to room temperature in H₂ flow. After that, the sample in the tube was purged with Ar and carefully replaced to a glass filled with 30 mL of 2-PrOH, avoiding contact with air.

The batch reactor (AISI 316L, 285 mL) previously purged with Ar was charged by the suspension of the catalyst and 30 mL of 2-PrOH or EtOH. Then, a solution of 3.2 g anisole (29.6 mmol), 3.38 g of methoxycyclohexane, 0.3 g of dodecane (internal standard), and 80 mL of 2-PrOH or EtOH was added. Two additional experiments were carried out using Ni(36)–SiO₂ and Ni(41)Cu(4)–SiO₂ where 15 mL of 2-PrOH were replaced with the same volume of acetone. After that, the reactor was purged with argon under stirring (mechanical agitator MagneDrive^®), closed, and heated up to 150 °C for 20 min or to 250 °C for 32 min under permanent stirring at 800 rpm. The self-pressure of the alcohol, anisole, and reaction products achieved 7.3–8.1 MPa at 250 °C and 0.8–1.1 MPa at 150 °C. During the reaction, the probes of the reaction mixture were collected. To calculate the conversion, yield, and specific catalytic activity, the following equations were used:

$$Conversion=\frac{C{\left(anisole\right)}_{in}-C{\left(anisole\right)}_{fin}}{C{\left(anisole\right)}_{in}}\times 100\%$$

(1)

$$Yield=\frac{C\left(product\right)}{\sum C\left(product\right)}\times 100\%$$

(2)

$$Specific catalytic activity=\frac{k_i}{S_{CO}}$$

(3)

where k_i is the rate constant and S_CO is the surface area of Ni° measured using CO adsorption. C(anisole)_in, C(anisole)_fin—the initial and final concentrations of anisole, C(product)—the concentration of a certain product at any moment.

3.4. Product Analysis

The liquid products of anisole transformations were analyzed using a Shimadzu GCMS–QP2010 SE chromato-mass spectrometer equipped with an autosampler. A GsBP-INOWAX capillary chromatographic column (crosslinked polyethylene glycol) was used (length, 30 m; internal diameter, 0.32 mm; stationary phase thickness, 0.25 μm). The temperature mode of column conditioning was as follows: 55 °C for 3 min, programmed heating up to 200 °C at a rate of 15 °C per minute, and then up to 250 °C at the rate of 25 °C per minute. The evaporator temperature was 270 °C, split—1:30; helium was used as a carrier gas. The constant flow rate of the carrier gas was 1.8 mL min^–1. The products were identified using the peak retention time and the mass spectrum of the substance, which were compared with the corresponding data of the pure compounds or with the data from the NIST and Wiley7 electronic mass spectral libraries. The conversion of anisole and the yield of the products were evaluated using an internal standard method with dodecane as the internal standard.

To determine the qualitative composition of the formed gases, the reactor was cooled to 40 °C when the pressure dropped to 0.4–2.0 MPa. Then, part of gas was taken from the reactor using a 150 mL syringe. Before the analysis, the gas in the syringe was diluted 20 times with air. The chromatographic analysis was preformed using a Chromos GC 1000 equipped with a chromatography column (length, 2 m; internal diameter, 3 mm; stationary phase—NaX zeolite) and a thermal conductivity detector. Ar was used as the carrier gas. The temperature mode of column conditioning was as follows: 30 °C for 3 min, programmed heating up to 120 °C at the rate of 24 °C/min, then 5 min at 120 °C.

3.5. Calculations of Kinetic Data

The experimental dependence of anisole conversion was described by quasi-first-order kinetics with respect to anisole and the intermediate products. The change in the molar fraction was calculated using the following simplified model:

$$\raisebox{1ex}{$d{C}_i$}\!\left/ \!\raisebox{-1ex}{$t$}\right.=k_i\times C_i$$

(4)

where k_i is the first-order rate constant and C_i is the molar fraction of anisole and the reaction products. The detailed procedure is described in one of our recent studies [27].

We also tried to develop a kinetic model that would take into account the effect of acetone concentration on the rate of catalytic transformations. In this model, the k_i value is not a constant, but a function of acetone concentration and is determined using the following equation:

$$k_i=k_i^0-K_{acetone}\times C_{acetone}$$

(5)

where $k_i^0$ is the first-order rate constant at the initial time, K_acetone is the acetone effect constant, C_acetone is the acetone concentration. The calculated values of constants in this model are presented in Supplementary Materials.

4. Conclusions

This study shows that, in contrast to H₂ hydrogenation, in transfer hydrogenation of anisole catalyzed by Ni-based nanoparticles, Cu addition negatively affects the reaction rates and selectivity of HDO. For example, the kinetic analysis demonstrated that at 250 °C in 2-PrOH, k_∑ of anisole consumption decreased from 5.1 × 10⁻⁴ s⁻¹ to 2.2 × 10⁻⁴ s⁻¹ when Ni(36)–SiO₂ and Ni(41)Cu(4)–SiO₂ were used. At the same time, the yield of cyclohexanol dropped from 37 mol.% to 29 mol.%. The kinetic model and the experimental data demonstrated that this effect was associated with acetone formed via 2-PrOH dehydrogenation. Likely, Cu facilitates adsorption of a carbonyl compound that blocks active sites on the catalyst surface at least partially. Thus, competitive adsorption of H donors, H acceptors, and products of their transformation should be avoided. In case of anisole transfer hydrogenation catalyzed by Ni–Cu systems, the use of oxygen-free donors, for example, of cyclohexane or tetralin, could be more preferable.