Boosting Higher Selectivity for Thymol Hydrogenation Reaction over Ni/Ce Catalyst

Abstract

The production of menthol via thymol hydrogenation is an industrial technology but is challenging due to the unsatisfied selectivity to menthol. Herein, Ni/Ce catalysts were prepared and used in thymol hydrogenation. A high selectivity of menthol was achieved over Ni₄/Ce₁ catalysts under the optimized reaction condition. Ce incorporation can improve both the activity of Ni catalyst and the selectivity to menthol. To reveal the functions of Ce, catalyst characterizations were conducted. The catalytic activity improvement may be related to the remarkable increase in the surface area of the catalyst and the lower crystalline sizes of Ni that take place when a tiny amount of Ce is incorporated into Ni. Higher selectivity to menthol may be related to the modification of the acidity of an Ni catalyst. In addition, the stability of the Ni₄/Ce₁ catalysts was also evaluated, and after five recycles, the Ni₄/Ce₁ catalysts exhibited outstanding catalytic activity and stability.

1. Introduction

Menthol, one of the most important fragrances, is widely used in the cosmetic and pharmaceutical industries. The international demand is 34,000 metric tons per year [1]. For menthol synthesis, the starting materials should be highly available. One of the most successful synthetic routes to produce menthol is starting from citral [2]. However, the drawback of this synthetic route is the use of chiral and homogeneous catalysts, leading to recycling difficulties. Another important synthetic route to produce menthol was developed by Symrise, starting with the alkylation of m-cresol with propene to form thymol, then transformed to menthol via hydrogenation [3,4,5]. The advantage of this process is the use of a heterogeneous catalyst, which can be easily separated from the reaction system or be used in a fixed bed reactor. Thymol hydrogenation is one of the most important reactions in the synthetic route to produce menthol. Many kinds of heterogeneous catalysts have been reported for thymol hydrogenation. Noble metal catalysts have shown an encouraging performance for the thymol hydrogenation reaction, but the cost of precious metal catalysts is very high. Based on the kinetics study of thymol hydrogenation over a supported Pt catalyst in a 0.3 L autoclave with a magnetic stirrer, the main side product, menthane, generated via a hydrogenolysis reaction, is unavoidable, even at 100 °C [6]. In addition to noble metal, base metal, such as Fe, Co, Ni and Cu, have been widely researched due to their low cost [7,8,9,10]. Co/Mn mixed oxides have been reported for thymol hydrogenation. High thymol conversion can be obtained at 170–220 °C and 20 MPa via a fixed bed reactor, and d/l-menthol selectivity of 59% can be obtained. However, unfortunately, the side product is more than 4% [11]. The development of cobalt nanoparticles supported on silica as selective catalysts for arene hydrogenations has been reported, where the yield of menthol isomers is 93% [12]. Raney nickel, the most important base metal hydrogenation catalyst, has showed high activity for the hydrogenation reaction, but the selectivity to target products can be poor, and the storage and transportation of Raney nickel are complex [13,14]. Therefore, supported Ni catalysts have been reported, where 55% of d/l-menthol could be achieved at 190 °C and 1.5 MPa. The kinetics of thymol hydrogenation over Ni/Cr₂O₃ have been investigated in a batch reactor, and unstable menthol isomers were formed at the initial stage; menthol can be obtained only at high thymol conversion rates [10].

Menthol is a commercial product. Conversion and selectivity improvements are very important for reducing the cost of menthol. A high conversion of thymol can easily be obtained using commercial noble metal or base metal hydrogenation catalysts. The major challenge is to improve the selectivity to menthol and keep the side product below 1%.

CeO₂ is often used as a catalyst or support for oxidation reactions, owing to its high thermal stability and excellent redox property [15,16,17]. However, it is hardly employed for hydrogenation reactions [18,19]. Herein, we report a composite of Ni/Ce as a catalyst for the hydrogenation of thymol to menthol. The activity of the Ni/Ce catalyst was evaluated. The reaction condition was optimized and the stability of the Ni/Ce catalyst was tested. A series of characterizations were conducted to study the reason why the Ce introduced into the Ni can improve the performance of the hydrogenation catalyst.

2. Results and Discussion

2.1. Structure, Composition and Acidity

XRD patterns were collected to investigate the crystal structure of NiO and CeO₂. In the XRD patterns of the pure NiO catalyst, there were characteristic diffraction peaks of NiO (110), (200) and (220) facets at 2θ = 37.2°, 43.2° and 62.8° (JCPDS 78-0643) [20]. No obvious changes in the position of characteristic diffraction peaks can be observed in Ni_x/Ce_x catalysts, suggesting that a partial replacement of Ni²⁺ by Ce⁴⁺ in the framework of Ni_x/Ce_x did not take place. As depicted in Figure 1, with an increasing amount of CeO₂, the peaks corresponded to NiO becoming weaker, especially for the Ni₁/Ce₁ sample. This was possibly caused by the dispersion improvement in NiO when introducing CeO₂. The weak peak suggests that the crystalline size of NiO was very small. The average crystalline size of each Ni_x/Ce_x catalyst was determined using the Scherrer equation [21] and is listed in Figure 2. It was noted that pure NiO possessed the largest average particle size of 7.6 nm. With an increasing amount of CeO₂, the crystalline size of NiO decreased sharply and had the smallest average crystalline size of 4~5 nm. These low crystalline sizes of NiO occur when a tiny amount of CeO₂ is incorporated into the NiO [22].

The nitrogen adsorption–desorption isotherms of the Ni_x/Ce_x catalysts at −196 °C are presented in Figure 3. It can be found that each catalyst exhibited a classical Type IV isotherm with a notable hysteresis loop in a relative pressure range of 0.7 < P/P₀ < 1.0, which indicated the formation of irregular mesoporous characteristics [23]. In addition, the textural properties of the Ni_x/Ce_x catalysts, including the specific surface area (SBET), the average pore diameter and pore volume, are summarized in

Table 1. Textural properties of the Ni_x/Ce_x catalysts.

Catalysts	SBET (m2/g)	Pore Volume (cm3/g)	Pore Diameter (nm)
NiO	64.1	0.14	7.6
Ni16/Ce1	104.0	0.26	9.9
Ni8/Ce1	97.5	0.24	10.3
Ni4/Ce1	91.0	0.37	15.7
Ni2/Ce1	93.3	0.25	11.7
Ni1/Ce1	55.7	0.25	17.6
CeO2	50.9	0.18	14.4

. Obviously, the specific surface area of the pure NiO catalyst was relatively low, about 64 m²/g. With the incorporation of CeO₂, the specific surface area of Ni₁₆/Ce₁ catalysts increased to higher than 100 m²/g. Ce incorporation can improve the thermal stability of NiO and maintain the higher specific surface area of the Ni_x/Ce_x catalysts after calcination [24]. A higher specific surface area in the catalyst may provide more active sites for thymol hydrogenation, boosting the catalytic activity for thymol hydrogenation.

Figure 4 shows the Raman spectra of the Ni_x/Ce_x catalysts. For the pure NiO catalyst, there is a weak band at 500 cm⁻¹, which is assigned to the Ni-O stretching mode [25,26]. With the incorporation of Ce, two new bands at 450 cm⁻¹ and 570 cm⁻¹, respectively, can be observed for the Ni_x/Ce_x catalysts. The strong band at 450 cm⁻¹ is assigned to the Ce-O stretching mode. The new band at 570 cm⁻¹ may be caused by the interaction between Ni and Ce [27,28].

Acidity is an important factor for thymol hydrogenation because it is generally believed that acid is beneficial for hydrolysis reactions and will result in a more significant side product [29,30]. Therefore, NH₃-TPD was also carried out to test the acidity of Ni_X/Ce_X catalysts. As shown in Figure 5 and

Table 2. NH₃-TPD of Ni_x/Ce_x catalysts.

Catalysts	NH3 Desorption Peak Area
NiO	325
Ni16/Ce1	268
Ni8/Ce1	243
Ni4/Ce1	241
Ni2/Ce1	143
Ni1/Ce1	142
CeO2	226

, there is one main peak at 168 °C for the pure NiO catalyst. Rare earth oxides, such as CeO₂, La₂O₃ and Eu₂O₃, are regarded as basicity oxides, and the acidity of rare earth oxides is weaker than transition metal oxides [31,32]. As expected, with the incorporation of Ce in the Ni_x/Ce_x catalysts, the main desorption peak shifts to lower temperature, which may be caused by the Ni-O-Ce interaction, and led to a decrease in acidic sites and weakened the acidic strength. The acidic sites promote the hydrogenolysis reaction and result in lower selectivity for the hydrogenation reaction [33]. Thus, Ni_x/Ce_x catalysts can show much better selectivity than the Ni catalyst.

H₂-TPD was also carried out to study H₂ adsorption and activization over Ni_x/Ce_x catalysts. As shown in Figure 6 and

Table 3. Peak area of H₂ desorption.

Catalysts	Peak Area
NiO	8.8
Ni16/Ce1	29.4
Ni8/Ce1	426.7
Ni4/Ce1	607.2
Ni2/Ce1	466.8
Ni1/Ce1	90.3
CeO2	71.0

, there is a weak H₂ desorption peak at 100–150 °C for the pure NiO catalyst. With the incorporation of Ce in the Ni_x/Ce_x catalysts, one broad and strong H₂ desorption peak can be observed. Ce incorporation can improve the H₂ adsorption. The Ni₄/Ce₁ catalysts possessed a high H₂ adsorption capability, which is favorable for higher catalytic activity for thymol hydrogenation reactions.

TEM was employed to observe the particle size and the structure of the Ni₄/Ce₁ catalyst. As shown in Figure 7a–c, the average size of the Ni₄/Ce₁ catalyst was about 5 nm. The EDX element mappings (Figure 7d–g) of Ni₄/Ce₁ catalyst indicate that the nanoparticles of the Ni₄/Ce₁ catalyst were composed of both Ni and Ce elements, which were uniformly dispersed, and non-isolated Ni and Ce nanoparticles.

2.2. Evaluation of Catalysts

Thymol hydrogenation reactions were carried out under reaction conditions of 190 °C and 6 Mpa H₂, with a reaction time of 6 h. The performances of Ni_x/Ce_x catalysts are listed in

Table 4. Catalytic performance of different catalysts for thymol hydrogenation reaction.

Entry	Catalysts	Conversion/% Thymol	Selectivity% Menthol + Isomer	Selectivity% Menthol	Selectivity% Side Product
1	NiO	92.3	90.5	47.7	9.5
2	Ni16-Ce1	>99	96.8	53.4	3.2
3	Ni8-Ce1	>99	99.5	55.8	0.5
4	Ni4-Ce1	>99	99.6	56.3	0.4
5	Ni2-Ce1	>99	99.5	55.8	0.5
6	Ni1-Ce1	65.0	99.6	36.2	0.4
7	CeO2	<1	~	~	~

Reaction conditions: 0.8 g of catalysts, 40 g thymol, 190 °C, 6.0 h; 6 MPa H₂ pressure.

. The pure Ni catalyst showed some activity for the thymol hydrogenation reaction, but the selectivity of the side product, menthane, was about 9.5%, which is unsatisfied. Ni catalysts show higher activity for hydrogenation reactions, but the selectivity should be improved through catalyst modification [34]. As a result of the incorporation of Ce, higher selectivity of the target products can be achieved over the Ni₄/Ce₁ catalyst, and the selectivity of the side product can be controlled at below 0.4%. With an increase in the molar ration of Ce/Ni, the selectivity of the side product decreased sharply from 9.5% to 0.4%. Based on the NH₃-TPD results, the incorporation of Ce can lead to a decrease in acidic sites’ amount and weaken the acidic strength, resulting in higher selectivity for thymol hydrogenation. Fortunately, the conversion of thymol can also be improved over the Ni_x/Ce_x catalyst. With an increase in the molar ration of Ce/Ni, the conversion of thymol increases gradually and then decreases sharply. Based on the XRD and specific surface area results, a higher specific surface area and small crystalline size of Ni can be obtained over the Ni₄/Ce₁ catalyst, which can provide more active sites for thymol hydrogenation, boosting the catalytic activity for thymol hydrogenation. However, too much Ce incorporation will result in low catalytic activity for thymol hydrogenation. Thus, the molar ratio of Ni/Ce should be controlled to maintain the best performance for the thymol hydrogenation reaction.

The effects of H₂ pressure and reaction temperature were investigated, and the results are displayed in

Table 5. Effect of reaction temperature.

Entry	Reaction Temperature (°C)	Conversion/% Thymol	Selectivity% Menthol + Isomer	Selectivity% Menthol	Selectivity% Side Product
1	150	37.6	99.9	30.0	0.1
2	160	41.1	99.8	37.8	0.2
3	170	71.4	99.7	39.4	0.3
4	180	81.3	99.7	41.7	0.3
5	190	>99	99.6	56.3	0.4
6	200	>99	99.4	54.2	0.6

Reaction conditions: 0.8 g of catalysts, 40 g thymol, 6.0 h, 6 MPa H₂ pressure.

and

Table 6. Effect of H₂ pressure.

Entry	H2 Pressure (MPa)	Conversion/% Thymol	Selectivity/% Menthol + Isomer	Selectivity/% Menthol	Selectivity/% Side Product
1	2	31.1	99.8	28.6	0.2
2	3	54.8	99.6	34.9	0.4
3	4	64.3	99.8	37.5	0.2
4	5	74.3	99.9	46.2	0.1
5	6	>99	99.6	56.3	0.4

Reaction conditions: 0.8 g of catalysts, 40 g thymol, 6.0 h, 190 °C.

. The reaction rate of the hydrogenation reaction is fast at higher temperatures and higher H₂ pressures. However, the selectivity of the target product is dissatisfactory under higher reaction temperatures and higher H₂ pressures [35]. As depicted in Table 5, the Ni₄/Ce₁ catalyst showed low activity at a lower reaction temperature, and thymol cannot be totally converted when the reaction temperature is below 190 °C for 6 h. A thymol conversion of 99.0% can only be obtained when the reaction temperature is higher than 190 °C. As the reaction temperature increased from 150 °C to 200 °C, the selectivity of the side product slightly increased. Even when the reaction temperature increased to 200 °C, the selectivity of the side product, menthane, was only 0.6%, which suggests that Ni₄/Ce₁ catalysts have better selectivity for thymol hydrogenation, even at higher reaction temperatures. From Table 6, it can be seen that high H₂ pressure is beneficial for improving the conversion of thymol. Thymol can be totally converted at an H₂ pressure 6 MPa. The selectivity of menthol was almost unchanged when the H₂ pressure increased from 2 MPa to 6 MPa.

In addition to the selectivity to the target product and activity of the catalysts, the stability of heterogeneous catalysts is another significant characteristic for heterogeneous catalysts [36,37]. However, the conversion of thymol is close to 100% at 190 °C regarding the Ni₄/Ce₁ catalyst, which would be confusing in the assessment of catalyst stability. In general, the stability test should be carried out at lower conversion rates. Therefore, we conducted the catalyst stability test at a reaction temperature of 190 °C and reaction time of 4 h. The conversion of thymol can be controlled at below 90%. It can be seen from

Table 7. The recycle stability of Ni₄/Ce₁ catalyst.

Entry	Recycle Number	Conversion/% Thymol	Selectivity/% Menthol + Isomer	Selectivity/% Side Product
1		89	99.6	0.4
2	1	88	99.7	0.3
3	2	89	99.6	0.4
4	3	89	99.7	0.3
5	4	87	99.6	0.4
6	5	88	99.6	0.4

Reaction conditions: 0.8 g of catalysts, 40 g thymol, 4.0 h, 6 MPa H₂ pressure, 190 °C.

that, after five recycle runs, the catalyst could still perform well, with only a slight decrease in the conversion of thymol, indicating that the Ni₄/Ce₁ catalyst had good stability.

3. Experimental

3.1. Materials

The chemical reagents used included nickel acetate, cerium acetate and Na₂CO₃. All the above reagents of analytical grade were purchased from Adamas and used as received without pretreatment.

3.2. Preparation of NiO/CeO₂ Catalyst

NiO/CeO₂ was prepared via coprecipitation method. As is typical, nickel acetate and cerium acetate were dissolved in 100 mL deionized water under stirring at room temperature; then, they were slowly added into the solution of Na₂CO₃ under stirring at room temperature for 1 h. After aging at room temperature for 2 h, the precipitate was separated by filtering and washed with deionized water three times. After drying the filter cake at 110 °C for 12 h and then calcining at 400 °C for 4 h, the back NiO/CeO₂ composite powder was obtained. Different molar ratios of NiO/CeO₂ can be obtained via the above method. The method was utilized, changing the molar ratio of NiO/CeO₂ with 16:1, 8:1, 4:1, 2:1 and 1:1, denoted as Ni₁₆/Ce₁, Ni₈/Ce₁, Ni₄/Ce₁, Ni₂/Ce₁ and Ni₁/Ce₁, respectively.

3.3. Catalyst Evaluation

The hydrogenation of thymol was performed in a stainless-steel autoclave. The catalysts were pretreated by H₂ at 350 °C for 4 h before being used in hydrogenation reaction. Typically, 0.8 g of catalysts and 40 g thymol were added into the autoclave reactor. The autoclave reactor was purged with N₂ 3 times and finally charged with H₂ to a certain pressure at room temperature, and then heating was started, the stirring was switched and stirring speed was kept at 500 r/min.

After the reaction, the liquid products were separated by centrifuging and analyzed by gas chromatography of GC9790 series with SE-54 column.

3.4. Catalyst Characterization

XRD patterns were recorded on Bruker D2 PHASER, using Cu Kα radiation and operating at 40 kV and 40 mA. The scanning range was set from 10° to 80° (2θ), with a step size of 5°/min.

The Brunauer–Emmett–Teller (BET) specific surface areas of typical products were obtained using Micrometrics ASAP2020 HD88 multi-function nitrogen adsorption instrument. Firstly, the test sample was degassed in vacuum at 200 °C for 8 h and then determined at −196 °C. The specific surface area data were obtained according to the Brunauer–Emmett–Teller (BET) equation.

Raman experiment was adopted using HORIBA Scientific LabRAM HR Evolution instrument, with an excitation wavelength of 514 nm.

NH₃-TPD experiments were performed in a Micromeritics AutoChem II 2920 with a thermal conductivity detector. A 50 mg sample was pre-treated at 400 °C under He flow and then cooled down to room temperature. It was filled with NH₃ to achieve adsorption saturation, and then the sample was heated from 50 °C to 800 °C.

H₂-TPD experiments were performed in a quartz micro-reactor with a thermal conductivity detector. A 100 mg sample was pre-treated at 350 °C under H₂ flow for 4 h and then cooled down to room temperature under N₂. It was filled with H₂ to achieve adsorption saturation at 50 °C for 1 h, followed by N₂ flushing for another 1 h to remove the physical absorption H₂, and then the sample was heated from 50 °C to 500 °C under N₂. Thermal conductivity detector recorded the signal. Transmission electron microscope (TEM) images were obtained with HRTEM 2010 JEOL (200 kV). The average Ni particle sizes were determined using ImageJ software.

4. Conclusions

In conclusion, Ni_x/Ce_x catalysts were prepared and applied in thymol hydrogenation. Ce incorporation can improve both the activity of Ni catalysts and the selectivity to menthol. Under the optimized conditions, a thymol conversion of 99.0% and menthol + isomer selectivity of 99.6% were harvested over the Ni₄/Ce₁ catalyst. Combining the characterizations of the catalysts, Ce incorporation can improve the thermal stability of Ni and maintain the higher specific surface area of the Ni_x/Ce_x catalysts after calcination. Benefitting from the larger specific surface area and smaller crystalline size of the catalyst, the Ni₄/Ce₁ catalyst presented a superior catalytic performance. Ce incorporation led to a decrease in acidic sites’ amount and weakened the acidic strength, resulting in high menthol selectivity. In addition, the Ni₄/Ce₁ catalyst had high stability and still performed well after five recycle runs. This study supplies a strategy for developing efficient and stable Ni-based catalysts in terms of hydrogenation reaction.