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Article

Catalyst and Process Effects in the Solvent-Free Hydrogenation of p-Cymene to p-Menthane †

by
Ayyoub Selka
and
Mohamed Touaibia
*
Faculty of Science, Université de Moncton, Moncton, NB E1A 3E9, Canada
*
Author to whom correspondence should be addressed.
†
Dedicated to the memory of Professor Farid Chemat.
Appl. Sci. 2025, 15(18), 10287; https://doi.org/10.3390/app151810287
Submission received: 8 August 2025 / Revised: 11 September 2025 / Accepted: 16 September 2025 / Published: 22 September 2025
(This article belongs to the Special Issue Advanced Phytochemistry and Its Applications)
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Abstract

The hydrogenation of p-cymene to p-menthane, a bio-based solvent, over four platinum-group catalysts, was thoroughly investigated in this study. The effect of the support material, pressure, and temperature were explored. Rhodium was the most effective metal, even under normal pressure conditions. Charcoal was a better metal support compared to alumina, offering better selectivity at lower pressure and outstanding recyclability. Hydrogen pressure had no effect on the selectivity; however, the conversion rate was maximal at higher pressure. At higher temperatures, the thermodynamically stable trans-isomer was favored, whereas at lower temperatures the cis-isomer became predominant. Remarkably, Rh/C achieved >99% conversion of p-cymene and maintained stable activity and selectivity over 66 recycling cycles, whereas the same metal-based catalyst on alumina was only recycled twice. These findings demonstrate that the solvent-free hydrogenation of p-cymene can be efficiently achieved using commercially available catalysts, with Rh/C emerging as a promising benchmark for sustainable and green catalytic processes.

1. Introduction

The emergence of sustainable chemistry and the urge for the development of strategies for tackling climate change-related issues have been the driving forces of new innovative chemistry thinking and modern technologies [1,2,3,4,5]. The exploitation of renewable resources and materials appears to be one of the most promising strategies to overcome the challenges faced with conventional limited resources such as petroleum [3,4,5,6,7,8].
Considering feedstock as a valuable and renewable natural resource opens up opportunities for developing innovative approaches or revisiting traditional strategies, offering a potential economic model for the future [8,9,10,11]. Thus, many synthetic processes nowadays are switching to more ecofriendly chemical processes, starting with adopting new green solvents as they represent most of the carbon footprint in different industrial areas [3,5]. Green solvents, particularly those derived from biomass, are emerging, such as limonene, glycerol, γ-valerolactone, cyrene, pinene, and 2-methyl tetrahydrofuran (2-MeTHF), which are gaining traction within the chemical community for various reactions. For example, research has been done into the exploration of 2-MeTHF in Suzuki–Miyaura reactions [12], γ-valerolactone for cross-coupling reactions [13], and glycerol for Sonogashira coupling [14]. On the other hand, some of these biorenewable terpene feedstocks could be transformed into more valuable terpenes via sustainable approaches. For example, the aromatization of limonene leads to p-cymene, a less abundant terpene in essential oil that is commonly obtained by a fastidious process involving hazardous Friedel–Crafts alkylation of benzene derivatives with alkyl halides and AlCl3 as a catalyst [15]. p-Cymene has been explored and demonstrated to hold significant value across various industries, including in fragrances, perfumes, flavorings, and pharmaceuticals [15]. On the other hand, p-cymene can be obtained from waste tires by catalytic processes in order to produce this highly valued terpene [16]. From the same perspective, p-menthane, a hydrogenated version of p-cymene, is an even more highly valued terpene. It has been used as substrate for the synthesis of p-menthane hydroperoxide, which is an excellent initiator for polymerization reactions [17]. On top of that, p-menthane was utilized as a green solvent for cleaning applications [18] and natural product extraction [19]. p-Menthane has a great combustion ability and can play the role of an additive, with p-cymene, in green blend fuel [20,21,22].
p-Menthane is typically produced through the hydrogenation of feedstock using a metal-based catalytic reaction. For example, the reduction of limonene over a Cu–Ni-supported catalytic system produced a mixture of p-menthene and p-menthane (58:42) under the best conditions [23]. On the other hand, menthane was obtained as a mixture with the aromatized p-cymene by hydrogenation of p-menthadienes over palladium [24]. Linalool can also be transformed into a mixture of p-menthane and 2,6-dimethyloctane using commercial catalysts [25]. To the best of our knowledge, only a few works on the synthesis of p-menthane have been reported involving p-cymene hydrogenation over palladium, providing a trans/cis mixture (69/31) [26]. Consequently, developing an eco-friendly method for the synthesis of p-menthane would greatly facilitate deeper research into its potential applications. Such advancements could offer valuable insights into its role in sustainable chemistry and energy solutions. Previously, we investigated the hydrogenation of limonene over various catalysts, and p-cymene was detected at various stages of the reaction kinetics [27]. This p-cymene results from the dehydrogenation of limonene and/or menthene, yet it was never detected at the end of the reaction. Following our preliminary tests of p-cymene hydrogenation as part of our investigation of limonene hydrogenation, we have undertaken a complete investigation of the solvent-free p-cymene hydrogenation to cis- and trans-menthane isomers in this study (Scheme 1).
Four platinum-group metals, palladium (Pd), platinum (Pt), ruthenium (Ru), and rhodium (Rh), were used for their potential in hydrogenation reactions due to their high strong ability to dissociate hydrogen. Additionally, charcoal (C) and alumina (Al2O3) were selected as catalyst supports among a range of other options (e.g., silica, zeolite). This choice was taken in consideration of the morphological characteristics offered by these two supports for enhancing the efficiency of the selected metals in heterogeneous hydrogenation reactions. Fortunately, these catalysts offer the benefit of commercial availability, with comparable metal loading. The metals cited earlier, supported on charcoal (C) or alumina (Al2O3), and were tested for the solvent-free hydrogenation of p-cymene. In this study, the hydrogenation of p-cymene was systematically investigated under different catalytic and process conditions. The effects of metal, support, pressure, and temperature were each examined in detail: the present investigation allows us to provide a comprehensive analysis of the key parameters and their interplay, supported by plausible mechanistic considerations. Thus, the outcome of this study will provide more insights into sustainable hydrogenation for feedstocks, especially the aromatic derivative, looking toward full hydrogenation.

2. Experimental Procedure

2.1. Materials

p-Cymene, hexane, and catalysts (Pd (5% weight percentage (wt. %), Pt (5 wt. %), Ru (5 wt. %), and Rh (5 wt. %)) over C and Al2O3 were purchased from Sigma-Aldrich. (Certificates analysis of used catalysts can be found at: https://www.sigmaaldrich.com/CA/en/documents-search?tab=coa (accessed on 4 August 2025); Pd/C: 75992, lot: BCBJ2096V, Pd/Al2O3: 205710, lot: MKCR3779; Pt/C: 80982, lot: BCBP0634V, Pt/Al2O3: 205974, lot: MKCG0150; Ru/C: 206180, lot: MKBW5890V, Ru/Al2O3: 381152, lot: MKCD9248; Rh/C: 206164, lot: MKCF5073, Rh/Al2O3: 212857, lot: MKCD6696.) The helium gas was purchased from Air Liquide (Moncton, NB, Canada).

2.2. Catalysts Metal Loading

The metal loadings of catalysts, as specified by the commercial supplier, were verified by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) as previously described [27]. The commercial loadings and ICP-MS results are reported in Table 1. It should be noted that the ICP–MS data reported here are to be considered semi-quantitative, since complete dissolution of catalysts in aqua regia is challenging, and this may limit the absolute accuracy.

2.3. General Procedure

The p-cymene was hydrogenated according to the same method used for the hydrogenation of pinenes [28,29,30]. Briefly, in a 45 mL autoclave equipped with a 5 mL conical centrifuge tube containing a stirring bar, p-cymene (1 g, 7.46 mmol) and 0.01 eq. of selected catalyst (5 wt. %, 0.0746 mmol; Pd:158.5 mg; Pt: 291.1 mg; Ru: 150.6 mg; Rh: 153.5 mg) were introduced. Prior to the reaction, the autoclave was purged four times with hydrogen before being pressurized to 2.75 or 0.13 MPa. At low pressure (close atmospheric pressure), p-cymene was hydrogenated in a 5 mL conical centrifuge tube connected with a balloon filled with hydrogen under vigorous stirring. At the end of the reaction or at the end of the required time, hydrogen was vented from the autoclave, and the reaction mixture was centrifugated (centrifugal force applied: 423 g; time: 5 min) and filtered through a Celite pad to obtain p-menthane.
Standard laboratory safety procedures for handling pressurized hydrogen were strictly followed, including the use of appropriate pressure relief devices, protective shields, and continuous monitoring during the reactions. The setup proved reliable and safe throughout the study.

2.4. Temperature-Dependent Screenings

For the temperature-dependent screening experiments, p-cymene (1 g, 7.46 mmol) and 0.01 eq. of selected catalyst (5 wt. %, 0.0746 mmol; Pd:158.5 mg; Pt: 291.1 mg; Ru: 150.6 mg; Rh: 153.5 mg) were charged into the autoclave which was pressurized once the desired temperature had been reached.

2.5. Sample Analysis

All hydrogenation products obtained following Celite filtration were diluted in hexane (10 µL in 15 mL of hexane) and analyzed by gas chromatography/mass spectrometry (GC/MS) using an Agilent 6890 Series GC System coupled to an Agilent 5973 Network mass selective detector and capillary column (Zebron ZB-5MS, 30 m × 0.25 mm × 0.25 μm). Helium was used as the carrier gas at a constant flow rate of 1.2 mL min−1, an inlet temperature of 280 °C, and a detector temperature of 230 °C. The following temperature program was used in the analysis: 40 °C (30 s) to 75 °C (2 °C min−1), 75 °C to 300 °C (100 °C min−1). Retention times: p-Cymene (rt: 13.8 min), cis p-menthane (rt: 11.1 min), trans p-menthane (rt: 10.2 min) (See Supplementary Materials: GC/MS analysis of p-cymene, cis-p-menthane, and trans-p-menthane; Recycling tests of Rh/C.).

2.6. Catalyst Recycling

At the end of each hydrotreating cycle, the hydrotreated product was recovered and analyzed as described above. If GC/MS analysis confirmed the total conversion, p-cymene was added to the catalyst and the reaction was restarted, as described above.

3. Results and Discussion

3.1. Metal, Support, and Pressure Screening

In a first set of experiments, the influence of the metal, the support, and the pressure were investigated at room temperature (Table 2, entries 1–24).
Among the four metals investigated, Pd was the least efficient (Table 2, entries 1–6). Whether on charcoal or alumina, the conversion rate of p-cymene to menthane over Pd does not exceed 1%, even after 24 h of reaction. The same applies to pressure: whether at high or low pressure, the conversion rate of p-cymene to menthane does not exceed 1%. Except for Pd/Al2O3 at 0.13 MPa (Table 2, entry 5), hydrogenation of p-cymene over Pd resulted in the trans-menthane being favored over the cis-isomer (Table 2, entries 1–4 and 6). This result is in agreement with the result reported by Stalzer et al. [26] and allowed us to clearly identify the cis-p-menthane and trans-p-menthane isomers [26].
When platinum was supported on charcoal, complete conversion was achieved (Table 2, entries 7 and 8), except at low pressure (Table 2, entry 9); however, when platinum was supported on alumina, the catalyst exhibited low conversion (Table 2, entries 10–12). This lower performance may arise from the strong influence of the support and hydrogen partial pressure on the activity, as previously reported for the hydrogenation of aromatic substrates such as phenol over Pt/Al2O3 [31].
On the other hand, both ruthenium and rhodium showed complete conversion of the starting material (Table 2, entries 13, 14, 16, 17, 19, 20, 22, and 23), except at low hydrogen pressure (Table 2, entries 15, 18, 21, and 24), independently of the support. Thus, comparing the activities of these two metals belonging to the platinum group, the differences can be explained by the percentage of d-character and the π-complex with the aromatic ring of the p-cymene.
At full conversion, no significant variation in selectivity was observed; nevertheless, a slight preference toward trans-p-menthane was consistently observed, regardless of the metal (Pt, Ru, Rh), the support, or the hydrogen pressure (Table 2). However, Rh stands out from the other three metals, with the highest conversion rate of cymene to menthane at low pressure (Table 2, entries 15, 18, 21, and 24).
To sum up, the d-character of the metal likely plays an important role in determining catalytic activity, as metals with lower d-character (Pd, Pt) generally show lower activity compared with metals bearing a stronger d-character (Ru, Rh). However, other factors such as metal–support interactions and reaction conditions may also influence the results. For Pt, our observations suggest that high conversion is primarily achieved when supported on charcoal at standard pressure, whereas Pt on alumina shows lower conversion under the same conditions (Table 2, entries 7–12). This may indicate that the support affects Pt’s activity, potentially through differences in surface interactions rather than merely surface area. In contrast, Ru and Rh appear less sensitive to the nature of the support, which could be related to their higher intrinsic activity.

3.2. Effect of Temperature

The effect of the temperature on the reaction was also a part of our study, and the reactions were conducted at low temperatures (3–5 °C, Table 3) as well as at high temperatures (100 °C, Table 4). Expectedly, at low temperature, Pd led to a very low conversion rate using both supports (Table 3, entries 1 and 2). On the other hand, all other metals—Pt, Ru and Rh—supported on charcoal led to total conversion of p-cymene, with a slightly better selectivity toward cis-p-menthane (Table 3, entries 3, 5, and 7). When supported on alumina, all the metals led to low conversion (Table 2, entries 4 and 6), except for Rh where the conversion was complete (Table 2, entry 8), with similar selectivity as when supported on charcoal (Table 3, entry 7).
On the other hand, when the reaction was conducted at high temperature, almost a full conversion was obtained using Pd, independently of the support (Table 4, entries 1 and 2). However, using other metals led to full conversion (Table 4, entries 3–8). In terms of selectivity, Pd on both supports (Table 4, entries 1 and 2) as well as rhodium on alumina (Table 4, entry 8) clearly promoted the formation of trans-p-menthane as the major isomer. However, in all the other cases, almost no selectivity was observed. In the case of Pt, the same selectivity was obtained independently of the support, and very low selectivity toward the trans-p-menthane was detected (Table 4, entries 3 and 4).
On the contrary, Ru supported on charcoal or alumina (Table 4, entries 5 and 6) and Rh on charcoal (Table 4, entry 7) led to opposite selectivity, and cis p-menthane was the major isomer. To sum up, the temperature plays a crucial role in achieving full conversion, even when the catalyst is almost not active at room temperature. In terms of selectivity, in most cases cis-p-menthane was the major isomer at room temperature; however, at high temperature the selectivity switched slightly toward the trans-isomer.
The temperature effect on isomer selectivity reveals the interplay between thermodynamic and kinetic control. At low temperature, the cis-p-menthane (kinetic product) dominates across all catalysts except for Pt. When the temperature increases, thermodynamic factors favor trans-p-menthane formation, reducing cis selectivity. Interestingly, for Ru/C, Ru/Al2O3, and Rh/C, the cis product remains slightly predominant even at high temperature, suggesting residual kinetic stabilization. This trend highlights how temperature and catalyst nature jointly govern product distribution in p-cymene hydrogenation.

3.3. Recycling of Catalysts

The recyclability was systematically evaluated over consecutive hydrogenation cycles by replenishing fresh p-cymene after each run. Conversion and cis/trans selectivity were monitored throughout, providing an indirect but reliable measure of catalyst stability. The recycling of catalysts was performed using the best catalysts at room temperature and at high hydrogen pressure (2.75 MPa). Rh/C was by far the most efficient catalyst, achieving over 66 cycles with constant full conversion and selectivity (Table 5, entry 4).
Cycles 67 and 68 showed the beginning of decreased conversion (see Supporting Information), indicating the onset of deactivation. This gradual loss of activity may be attributed to slight metal particle agglomeration, subtle changes in metal–support interactions, or surface site deactivation.
The second-best catalyst was Pt/C, achieving 24 cycles with the same constant performance (Table 5, entry 1). Similarly, Ru/Al2O3 had almost the same performance for 20 cycles (Table 5, entry 3). On the contrary, the recycling ability of Ru/C and Rh/Al2O3 was very low. Only three cycles were achieved by Ru/C, and the reaction time had to increase to achieve full conversion (Table 4, entry 2). Rh/Al2O3 was only recycled twice, with significant decreases in TOF and TON (Table 5, entry 5). Only 28% of the starting material was converted in the third cycle (Table 5, entry 5). The low recyclability of alumina-supported catalysts compared to charcoal support is likely due to alumina’s Lewis acid character, as well as to their different characteristics (Table 6) [27]. The difference in surface area between the two Rh-based catalysts seems to explain this wide disparity in their recyclability (Table 6). With a surface area more than 5 times that of Rh/Al2O3 [27], Rh/C offers more active sites and probably better sorption/desorption processes for p-cymene and p-menthanes. On the other hand, in the case of Rh/Al2O3 (Table 5, entry 5), the conversion in the third cycle was significantly reduced, possibly due to catalyst deactivation caused by the known high interaction of the alumina and the metal, which can lead to aggregate formation [32]. However, it should be noted that such aggregation has been mainly reported at higher temperatures (>600 °C), while under milder conditions, metal–support interactions may still alter the accessibility of active sites without necessarily leading to particle growth [32].
The high activity of Rh/C is due to the ability of the charcoal support to provide a favorable condition for the active component to form well-dispersed species compared to with metal oxides or alumina. Additionally, the charcoal support is known to produce more active centers and developed pore structures [33,34].
Rh/C exhibits a remarkably high surface area (815 m2/g) and large pore volume (0.7 cm3/g), which provide extensive exposure and accessibility of Rh active sites. Although the metal dispersion is similar to Rh/Al2O3 (~23%), the narrow pore diameter (10.8 Å) in Rh/C suggests a high density of accessible micropores that may enhance substrate–catalyst interactions and diffusion efficiency. Enhanced hydrogen spillover effects with charcoal support could explain the superior activity of Rh/C [35]. These features facilitate effective hydrogenation under mild conditions and contribute to improved catalytic turnover. Furthermore, the TPR profile of Rh/C (Figure 1) shows a broad and intense reduction peak centered around 400–450 °C, reflecting a more extensive and progressive reduction of Rh species. This suggests that a greater proportion of Rh is reduced to its catalytically active metallic state (Rh0). Combined with the inert nature of the carbon support, which minimizes strong metal–support interactions, Rh/C exhibits enhanced catalytic activity and stability. The larger the area of the hydrogen consumption reduction peak, the larger the amount of the reducible species, which confirms the superiority of Rh/C [36].
It is worth mentioning that the ICP-MS analyses performed on the p-menthane obtained after the first and the tenth cycle with Rh/C revealed Rh concentrations of 0.3 µg/L and 0.2 µg/L, respectively. The low leaching (cycle 1: 0.000002%; cycle 10: 0.000001%) of the initial Rh quantity confirms the strong heterogeneous nature of the hydrogenation of p-cymene over Rh/C, as well as the stability and cost-effectiveness of this catalyst.

3.4. Hydrogenation of Cymene over Rh/C

Having all these results in hand, we selected Rh/C as our best candidate to further investigate the free solvent hydrogenation of p-cymene and thus find the best conditions for optimum efficiency. The kinetics of the reaction were studied under the same conditions, and the full conversion of the starting material was achieved after 4 h (Figure 2) with the same selectivity (trans/cis: 42:58) mentioned earlier (Table 2, entry 19).
At the same concentration and conditions, the other five catalysts selected for recycling tests showed conversion rates of less than 6% after 2 h of reaction, highlighting the superior performance of Rh/C. Notably, Rh/C achieved a TOF value nearly 25 times higher than most catalysts for solvent-free p-cymene hydrogenation (Table 5).
On the other hand, the increase in the catalyst amount from 0.001 eq. to 0.01 eq. led to complete conversion after 1 h of reaction, with a slightly different selectivity (trans/cis: 35:65) than at lower catalyst loading (Figure 3). Therefore, the increase in the catalyst amount affects only the reaction time; thus, the catalyst loading (0.01 eq.) is the optimum catalyst equivalent with a reaction time of 50 min.
A comparison of the two kinetic profiles highlights that at 0.01 eq Rh/C loading, the hydrogenation rate remains constant over time, confirming a zero-order reaction due to the excess of active sites relative to p-cymene. Conversely, at 0.001 eq Rh/C loading, the reaction rate progressively decreases with p-cymene consumption, which is typical of a first-order process.
The investigation of optimum conditions using Rh/C was furthered by screening different hydrogen pressures and temperatures (Table 7). The effect of hydrogen pressure at room temperature was explored, leading to a full conversion and a slight selectivity toward the cis-p-menthane (Table 7, entries 2–6). From 0.34 to 2.75 Mpa the conversion was complete, with almost identical selectivity (Table 7, entries 2–6). At ballon pressure (BP), the conversion did not exceed 14%, with an absence of selectivity, since the two isomers were obtained in equal proportions (Table 7, entry 1).
Although hydrogen pressure had little impact on the selectivity of p-cymene hydrogenation at room temperature, this result is significant as it highlights the effectiveness of Rh/C to fully hydrogenate p-cymene at low pressures.
At variable temperatures (4–150 °C) and fixed high pressure (2.75 MPa), the conversion was complete, with variable trans/cis selectivity (Table 7, entries 7–12). At low temperature (4 °C), the hydrogenation led to high selectivity toward the cis-p-menthane, which is the kinetic isomer (Table 7, entry 7). By increasing the temperature, this selectivity decreased, leading to very high selectivity at 150 °C, reaching 91% conversion to trans-p-menthane (Table 7, entry 12). The increase in the temperature in steps of 25 °C increased the selectivity toward trans p-menthane slowly but not significantly, as shown in Table 7 (entries 8–11). To sum up, temperature has a major effect on selectivity, providing high selectivity for one isomer at low temperature and high selectivity for the other at high temperature, allowing access to highly enriched mixtures of trans- or cis-p-menthane which could be studied separately for different applications.

4. Conclusions

This study evaluated the performance of four catalysts, each on two different supports, for the hydrogenation reaction of p-cymene. Among the four investigated platinum-group catalysts, Rh/C was the most efficient. This catalyst fully converted p-cymene to p-menthane with two stereoisomers at both low (4 °C) and high (150 °C) temperatures, thereby offering excellent reactivity. The effect of the temperature on the selectivity was also observed: at 4 °C, the kinetically favored cis-p-menthane isomer dominated (72%), while at 150 °C, the thermodynamically favored trans-p-menthane isomer dominated (91%). These results highlight the importance of the temperature not only for complete conversion, but also for controlling stereoselectivity, allowing the reaction to be directed towards the desired product depending on the chosen experimental conditions.
Whether the reaction was carried out at medium or high pressure, conversion remained complete, accompanied by a marked preference for the cis-p-menthane isomer, with selectivity up to 65%. This highlights not only the robustness of the catalyst, but also its effectiveness in maintaining pure, well-defined products, which is crucial in industrial applications.
A particularly remarkable aspect of this study was the stability of Rh/C, which demonstrated excellent recyclability, with over 60 cycles of reuse without any significant loss of performance. This durability confirms that Rh/C is not only efficient but also cost-effective for large-scale processes. To the best of our knowledge, the use of Rh/C for the hydrogenation of p-cymene under solvent-free conditions has not been previously reported. Our study highlights the high efficiency and recyclability of Rh/C under green conditions, which makes it particularly attractive for the sustainable valorization of this natural aromatic compound.
In conclusion, this research establishes Rh/C as a catalyst of choice for the hydrogenation of p-cymene to cis- and trans-p-menthane, combining efficiency, selectivity, and recyclability, essential evaluation criteria for potential industrial applications.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/app151810287/s1, GC/MS analysis of p-cymene, cis-p-menthane, and trans-p-menthane; Recycling tests of Rh/C.

Author Contributions

Conceptualization: M.T.; methodology, A.S. and M.T.; validation, A.S. and M.T.; formal analysis, A.S. and M.T.; investigation, A.S. and M.T.; visualization, A.S. and M.T.; writing—original draft preparation, A.S. and M.T.; writing—review and editing, A.S. and M.T.; supervision, M.T.; project administration, M.T.; funding acquisition, M.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially supported by the National Sciences and Engineering Research Council of Canada (RGPIN-2022-03950) and Université de Moncton, NB, Canada.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data used to support the findings of this study are included within the manuscript.

Acknowledgments

Authors would like to thank Simon Lamarre for generously providing access to his GC/MS instrument for the analysis of the hydrogenation products, and Olivier Clarisse for the ICP-MS analyses.

Conflicts of Interest

The authors declare that there are no conflicts of interest.

References

  1. Salgado-Ramos, M.; Tabasso, S.; Gaudino, E.C.; Cravotto, G. Process Intensification for Sustainable Microwave-Assisted Bio p -Cymene Production from Limonene. Ind. Eng. Chem. Res. 2024, 63, 7507–7518. [Google Scholar] [CrossRef]
  2. Horváth, I.T. Introduction: Sustainable Chemistry. Chem. Rev. 2018, 118, 369–371. [Google Scholar] [CrossRef]
  3. Clarke, C.J.; Tu, W.-C.; Levers, O.; Bröhl, A.; Hallett, J.P. Green and Sustainable Solvents in Chemical Processes. Chem. Rev. 2018, 118, 747–800. [Google Scholar] [CrossRef] [PubMed]
  4. Dutta, S.; Zhang, Q.; Yu, I.K.M.; Tsang, D.C.W. Green Solvent-Mediated Catalysis for Efficient Biorefinery: Unravelling the Kinetic and Thermodynamic Impacts. ACS Sustain. Chem. Eng. 2024, 12, 9222–9230. [Google Scholar] [CrossRef]
  5. Shi, X.; Wang, Z.; Liu, S.; Xia, Q.; Liu, Y.; Chen, W.; Yu, H.; Zhang, K. Scalable production of carboxylated cellulose nanofibres using a green and recyclable solvent. Nat. Sustain. 2024, 7, 315–325. [Google Scholar] [CrossRef]
  6. Jessop, P.G. Searching for green solvents. Green. Chem. 2011, 13, 1391. [Google Scholar] [CrossRef]
  7. Oliveira, A.d.N.d.; Melchiorre, M.; da Costa, A.A.F.; da Silva, L.S.; Paiva, R.d.J.; Auvigne, A.; Ouyang, W.; Luque, R.; Filho, G.N.d.R.; Noronha, R.C.R.; et al. Glycerol: A green solvent for synthetic chemistry. Sustain. Chem. Pharm. 2024, 41, 101656. [Google Scholar] [CrossRef]
  8. Rahmawati, Z.; Santoso, L.; Abdullah, W.N.W.; Hamid, A.; Jamari, N.L.A.; Sugiarso, D.; Ni’MAh, Y.L.; Widati, A.A. Biomass as an alternative feedstock to oleochemicals. RSC Adv. 2024, 14, 28827–28843. [Google Scholar] [CrossRef]
  9. Naeem, A.; Saeed, B.; AlMohamadi, H.; Lee, M.; Gilani, M.A.; Nawaz, R.; Khan, A.L.; Yasin, M. Sustainable and green membranes for chemical separations: A review. Sep. Purif. Technol. 2024, 336, 126271. [Google Scholar] [CrossRef]
  10. Najaf-Abadi, M.K.; Ghobadian, B.; Dehghani-Soufi, M. A review on application of deep eutectic solvents as green catalysts and co-solvents in biodiesel production and purification processes. Biomass Conv. Bioref 2024, 14, 3117–3134. [Google Scholar] [CrossRef]
  11. Nguyen, B.-N.T.; Lim, J.Y.C. Emerging green approaches for valorization of plastics with saturated carbon backbones. Trends Chem. 2024, 6, 100–114. [Google Scholar] [CrossRef]
  12. Mondal, M.; Bora, U. Eco-friendly Suzuki–Miyaura coupling of arylboronic acids to aromatic ketones catalyzed by the oxime-palladacycle in biosolvent 2-MeTHF. New J. Chem. 2016, 40, 3119–3123. [Google Scholar] [CrossRef]
  13. Strappaveccia, G.; Ismalaj, E.; Petrucci, C.; Lanari, D.; Marrocchi, A.; Drees, M.; Facchetti, A.; Vaccaro, L. A biomass-derived safe medium to replace toxic dipolar solvents and access cleaner Heck coupling reactions. Green. Chem. 2015, 17, 365–372. [Google Scholar] [CrossRef]
  14. Raglione, V.; Palmeri, F.; Mattioli, G.; Porcelli, F.; Caschera, D.; Zanotti, G. Glycerol as Reaction Medium for Sonogashira Couplings: A Green Approach for the Synthesis of New Triarylamine-Based Materials with Potential Application in Optoelectronics. Chem.—Eur. J. 2024, 30, e202402901. [Google Scholar] [CrossRef]
  15. Martín-Luengo, M.; Yates, M.; Domingo, M.M.; Casal, B.; Iglesias, M.; Esteban, M.; Ruiz-Hitzky, E. Synthesis of p-cymene from limonene, a renewable feedstock. Appl. Catal. B Environ. 2008, 81, 218–224. [Google Scholar] [CrossRef]
  16. Zhang, Y.; Meng, X.; Jiang, J.; Ragauskas, A.J.; Wang, J. Sustainable valorization of waste tires: Selective hydrotreating for renewable p-cymene production. Fuel 2024, 364, 131137. [Google Scholar] [CrossRef]
  17. Yang, W.; Tao, N.; Guo, C. Preparation of p-menthane hydroperoxide from p-menthane in presence of metalloporphyrins. J. Cent. South. Univ. Technol. 2007, 14, 660–665. [Google Scholar] [CrossRef]
  18. Prasad, W.; Wani, A.D.; Khamrui, K.; Hussain, S.A.; Khetra, Y. Green Solvents, Green solvents, potential alternatives for petroleum based products in food processing industries. Clean. Chem. Eng. 2022, 3, 100052. [Google Scholar] [CrossRef]
  19. Madji, S.; Hilali, S.; Fabiano-Tixier, A.-S.; Tenon, M.; Bily, A.; Laguerre, M.; Chemat, F. para-Menthane as a Stable Terpene Derived from Orange By-Products as a Novel Solvent for Green Extraction and Solubilization of Natural Substances. Molecules 2019, 24, 2170. [Google Scholar] [CrossRef] [PubMed]
  20. Woodroffe, J.-D.; Harvey, B.G. High-Performance, Biobased, Jet Fuel Blends Containing Hydrogenated Monoterpenes and Synthetic Paraffinic Kerosenes. Energy Fuels 2020, 34, 5929–5937. [Google Scholar] [CrossRef]
  21. Yang, X.; Li, T.; Tang, K.; Zhou, X.; Lu, M.; Ounkham, W.L.; Spain, S.M.; Frost, B.J.; Lin, H. Highly efficient conversion of terpenoid biomass to jet-fuel range cycloalkanes in a biphasic tandem catalytic process. Green. Chem. 2017, 19, 3566–3573. [Google Scholar] [CrossRef]
  22. Wu, S.; Kang, D.; Xiao, R.; Boehman, A.L. Autoignition Characteristics of Bio-Based Fuels, Farnesane and TPGME, in Comparison with Fuels of Similar Cetane Rating. Proc. Combust. Inst. 2021, 38, 5585–5595. [Google Scholar] [CrossRef]
  23. Valencia, D.; Martinez-Hernandez, E.; García, A.; Aburto, J. Sustainable hydrogenation of limonene to value-added products using Cu–Ni catalysts supported on KIT-5. J. Clean. Prod. 2024, 434, 140356. [Google Scholar] [CrossRef]
  24. Tibbetts, J.D.; Bull, S.D. p-Menthadienes as Biorenewable Feedstocks for a Monoterpene-Based Biorefinery. Adv. Sustain. Syst. 2021, 5, 2000292. [Google Scholar] [CrossRef]
  25. Keller, C.L.; Doppalapudi, K.R.; Woodroffe, J.-D.; Harvey, B.G. Solvent-free dehydration, cyclization, and hydrogenation of linalool with a dual heterogeneous catalyst system to generate a high-performance sustainable aviation fuel. Commun. Chem. 2022, 5, 113. [Google Scholar] [CrossRef]
  26. Stalzer, M.M.; Nicholas, C.P.; Bhattacharyya, A.; Motta, A.; Delferro, M.; Marks, T.J. Single-Face/All- cis Arene Hydrogenation by a Supported Single-Site d 0 Organozirconium Catalyst. Angew. Chem. Int. Ed. 2016, 55, 5263–5267. [Google Scholar] [CrossRef]
  27. Ndongou Moutombi, F.J.; Fabiano-Tixier, A.S.; Clarisse, O.; Chemat, F.; Touaibia, M. Partial and Total Solvent-Free Limonene’s Hydrogenation: Metals, Supports, Pressure, and Water Effects. J. Chem. 2020, 2020, 5946345. [Google Scholar] [CrossRef]
  28. Yara-Varón, E.; Selka, A.; Fabiano-Tixier, A.S.; Balcells, M.; Canela-Garayoa, R.; Bily, A.; Touaibia, M.; Chemat, F. Solvent from forestry biomass. Pinane a stable terpene derived from pine tree byproducts to substitute n-hexane for the extraction of bioactive compounds. Green. Chem. 2016, 18, 6596–6608. [Google Scholar] [CrossRef]
  29. Selka, A.; Levesque, N.A.; Foucher, D.; Clarisse, O.; Chemat, F.; Touaibia, M. A Comparative Study of Solvent-Free and Highly Efficient Pinene Hydrogenation over Pd on Carbon, Alumina, and Silica Supports. Org. Process Res. Dev. 2017, 21, 60–64. [Google Scholar] [CrossRef]
  30. Touaibia, M.; Selka, A.; Levesque, N.A.; St-Onge, P.A. Green hydrogenation: Solvent-free hydrogenation of pinenes for an undergraduate organic chemistry laboratory. J. Chem. Educ. 2020, 97, 2296–2301. [Google Scholar] [CrossRef]
  31. Talukdar, A.K.; Bhattacharyya, K.G.; Sivasanker, S. Hydrogenation of phenol over supported platinum and palladium catalysts. Appl. Catal. A Gen. 1993, 96, 229–239. [Google Scholar] [CrossRef]
  32. Yao, H. Surface interactions in the system Rh/Al2O3. J. Catal. 1977, 50, 407–418. [Google Scholar] [CrossRef]
  33. Xie, H.; Zhao, X.; Zhou, G.; He, X.; Lan, H.; Jiang, Z. Investigating the performance of CoxOy/activated carbon catalysts for ethyl acetate catalytic combustion. Appl. Surf. Sci. 2015, 326, 119–123. [Google Scholar] [CrossRef]
  34. Xie, H.; Shen, Y.; Zhou, G.; Chen, S.; Song, Y.; Ren, J. Effect of preparation conditions on the hydrogen storage capacity of activated carbon adsorbents with super-high specific surface areas. Mater. Chem. Phys. 2013, 141, 203–207. [Google Scholar] [CrossRef]
  35. Su, F.; Lv, L.; Lee, F.Y.; Liu, T.; Cooper, A.I.; Zhao, X.S. Thermally reduced ruthenium nanoparticles as a highly active heterogeneous catalyst for hydrogenation of monoaromatics. J. Am. Chem. Soc. 2007, 129, 14213–14223. [Google Scholar] [CrossRef]
  36. Pozan, G.S. Effect of support on the catalytic activity of manganese oxide catalyts for toluene combustion. J. Hazard. Mater. 2012, 221–222, 124–130. [Google Scholar] [CrossRef]
Applsci 15 10287 sch001
Scheme 1. Hydrogenation of p-cymene to trans- and cis-p-menthane.
Scheme 1. Hydrogenation of p-cymene to trans- and cis-p-menthane.
Applsci 15 10287 sch001
Applsci 15 10287 g001
Figure 1. H2—TPR profiles of Rh/C and Rh/Al2O3.
Figure 1. H2—TPR profiles of Rh/C and Rh/Al2O3.
Applsci 15 10287 g001
Applsci 15 10287 g002
Figure 2. Kinetics of solvent-free p-cymene hydrogenation of cymene over Rh/C. Experimental conditions: p-cymene (1.0 g); Rh/C (5 wt. %: 0.001 eq; 15.35 mg); PH2: 2.75 MPa, rt.
Figure 2. Kinetics of solvent-free p-cymene hydrogenation of cymene over Rh/C. Experimental conditions: p-cymene (1.0 g); Rh/C (5 wt. %: 0.001 eq; 15.35 mg); PH2: 2.75 MPa, rt.
Applsci 15 10287 g002
Applsci 15 10287 g003
Figure 3. Kinetics of solvent-free p-cymene hydrogenation of cymene over Rh/C. Experimental conditions: p-cymene (1.0 g); Rh/C (5 wt. %: 0. 01 eq; 153.5 mg); PH2: 2.75 MPa, rt.
Figure 3. Kinetics of solvent-free p-cymene hydrogenation of cymene over Rh/C. Experimental conditions: p-cymene (1.0 g); Rh/C (5 wt. %: 0. 01 eq; 153.5 mg); PH2: 2.75 MPa, rt.
Applsci 15 10287 g003
Table 1. Metal loadings of commercial catalysts determined by ICP–MS.
Table 1. Metal loadings of commercial catalysts determined by ICP–MS.
MetalSupportLoading According to Commercial Supplier (%)Loading Measured by ICP-MS (%) a
PdC55
Al2O354.8
PtC54.3
Al2O354.5
RuC53.1
Al2O353.8
RhC51.4
Al2O353.2
a: Incomplete digestion of catalysts may have occurred. The metal loading determined by ICP-MS should be interpreted as the amount of metal extractable in aqua regia, rather than the total metal content.
Table 2. Hydrogenation of p-cymene: Metal, support, and pressure effects.
Table 2. Hydrogenation of p-cymene: Metal, support, and pressure effects.
EntryMetalSupportPH2 (MPa)Conversion (%)trans/cis (%)
1PdC2.75166/34
20.13163/37
3BP *171/29
4Al2O32.75154/46
50.13149/51
6BP *171/29
7PtC2.7510041/59
80.1310038/62
9BP *651/49
10Al2O32.75727/73
110.131326/74
12BP *160/40
13RuC2.7510038/62
140.1310040/60
15BP *195/5
16Al2O32.7510038/62
170.1310040/60
18BP *158/42
19RhC2.7510041/59
200.1310037/63
21BP *1450/50
22Al2O32.7510042/58
230.1310047/53
24BP *3456/44
Experimental conditions: p-cymene (1.0 g); catalyst (5 wt. %: 0.01 eq; Pd/support: 158.5 mg; Pt/support: 291.1 mg; Ru/support: 150.6 mg; Rh/support: 153.5 mg); rt, 24 h. * Balloon pressure.
Table 3. Hydrogenation of p-cymene: Low-temperature effect.
Table 3. Hydrogenation of p-cymene: Low-temperature effect.
EntryMetalSupportConversion (%)trans/cis (%)
1PdC<172/28
2Al2O3<168/32
3PtC10031/69
4Al2O3327/73
5RuC10033/67
6Al2O31236/64
7RhC10028/72
8Al2O310029/71
Experimental conditions: p-cymene (1.0 g); catalyst (5 wt. %: 0.01 eq; Pd/support:158.5 mg; Pt/support: 291.1 mg; Ru/support: 150.6 mg; Rh/support: 153.5 mg); rt: 24h; PH2: 2.75 Mpa; T: 3–5 °C.
Table 4. Hydrogenation of cymene: High-temperature effect.
Table 4. Hydrogenation of cymene: High-temperature effect.
EntryMetalSupportConversion (%)trans/cis (%)
1PdC9472/28
2Al2O39169/31
3PtC10056/44
4Al2O310056/44
5RuC10041/58
6Al2O310046/54
7RhC10048/52
8Al2O310067/33
Experimental conditions: p-cymene (1.0 g); catalyst (5 wt. %: 0.01 eq; Pd/support: 158.5 mg; Pt/support: 291.1 mg; Ru/support: 150.6 mg; Rh/support: 153.5 mg); rt: 24 h; PH2: 2.75 Mpa; T: 100 °C.
Table 5. Catalyst recycling.
Table 5. Catalyst recycling.
EntryMetalSupporttrans/cis (%) aTOF(h−1) bCyclesTON c
1 PtC56/4429242396
2 RuC41/58103 d300
3 Al2O346/548202000
4 RhC42/58249666593
5 Al2O333/67102 e200
Experimental conditions: p-cymene (1.0 g); catalyst (5 wt. %: 0.01 eq; Pt/support: 291.1 mg; Ru/support: 150.6 mg; Rh/support: 153.5 mg); PH2, 2.75 Mpa; rt, 24 h. a: Last cycle ratio. b: TOF = number of moles of consumed cymene/(moles of catalyst × reaction time); catalyst (5 wt. %: 0.001 eq); PH2: 2.75 MPa; rt: 2 h. c: TON = total moles of p-cymene converted over all recycling cycles/moles of active metal. Reported TON values correspond to the cumulative turnover after n cycles. d: Cycle 1: 24 h; cycle 2: 48 h; cycle 3: 72 h. e: Cycle 3: 28% of conversion.
Table 6. Rh/Al2O3 and Rh/C characteristics [27].
Table 6. Rh/Al2O3 and Rh/C characteristics [27].
CharacteristicsRh/Al2O3Rh/C
Surface area (m2/g)150815
Metal dispersion (%)24.023.4
Pores volume (cm3/g)0.40.7
Pore diameter (Å)74.510.8
Particle size (nm)0.760.93
Catalyst loading (%) a4.94.9
a: Specified by the commercial supplier.
Table 7. Hydrogenation of p-cymene over Rh/C (5 wt. %).
Table 7. Hydrogenation of p-cymene over Rh/C (5 wt. %).
EntryT (°C)PH2 (MPa)Conversion (%)trans/cis
1rtBP *1450/50
2rt0.3410038/62
3rt0.6710035/65
4rt1.3710037/63
5rt2.0710035/65
6rt2.7510041/59
742.7510028/72
8502.7510037/63
9752.7510043/57
101002.7510048/52
111252.7510056/44
121502.7510091/9
Experimental conditions: p-cymene (1.0 g), Rh/C (5 wt. %: 0.01 eq; 153.5 mg), 24 h. * Balloon pressure.
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Selka, A.; Touaibia, M. Catalyst and Process Effects in the Solvent-Free Hydrogenation of p-Cymene to p-Menthane. Appl. Sci. 2025, 15, 10287. https://doi.org/10.3390/app151810287

AMA Style

Selka A, Touaibia M. Catalyst and Process Effects in the Solvent-Free Hydrogenation of p-Cymene to p-Menthane. Applied Sciences. 2025; 15(18):10287. https://doi.org/10.3390/app151810287

Chicago/Turabian Style

Selka, Ayyoub, and Mohamed Touaibia. 2025. "Catalyst and Process Effects in the Solvent-Free Hydrogenation of p-Cymene to p-Menthane" Applied Sciences 15, no. 18: 10287. https://doi.org/10.3390/app151810287

APA Style

Selka, A., & Touaibia, M. (2025). Catalyst and Process Effects in the Solvent-Free Hydrogenation of p-Cymene to p-Menthane. Applied Sciences, 15(18), 10287. https://doi.org/10.3390/app151810287

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