Hydrogenation of Dodecanoic Acid over Iridium-Based Catalysts

Abstract

This study develops iridium (Ir)-based catalysts for the hydrogenation of dodecanoic acid, a medium-chain fatty acid abundant in palm kernel and coconut oils, for producing fatty alcohols and alkanes. Among various supports such as AlOOH, SiO₂, TiO₂, Nb₂O₅, MoO₃, Ta₂O₅, ZrO₂, and WO₃ for 7.5 wt% Ir loading, an Ir-impregnated Nb₂O₅ (Ir/Nb₂O₅) catalyst demonstrated remarkable performance with 100% conversion and a high dodecanol yield (89.1%) under mild conditions (170 °C, 4.0 MPa H₂), while at higher temperatures and pressures (200 °C, 8.0 MPa H₂), Ir-impregnated MoO₃ (Ir/MoO₃) produced dodecane as the main product with a yield of 90.7%. These findings can tailor product selectivity toward desired bio-based chemicals and fuels, offering sustainable pathways for fatty acid hydrogenation by optimizing catalyst supports and reaction conditions in the Ir-based catalyst.

1. Introduction

In recent years, vegetable oils and waste cooking oils have gained attention as cost-effective, renewable resources for the production of fatty alcohols. These alcohols are key components in the manufacturing of plasticizers, cosmetics, surfactants, lubricants, and diesel-range alkanes, which are commonly referred to as second-generation biodiesel. Dodecanoic acid, comprising approximately 50% of palm kernel oil and coconut oil, is a typical bio-based aliphatic fatty acid with a medium carbon chain length (C12). Globally, approximately 3900 thousand tons of fatty alcohols were consumed in 2022, with demand expected to grow steadily at a rate of 4.74% from 2022 to 2030 [1]. Currently, commercial catalysts such as metal sulfides (e.g., NiMoS₄ and CoMoS₄) and copper chromite (CuO/CuCr₂O₄) are employed for the hydrogenation of natural oils to produce diesel-range alkanes or fatty alcohols [2]. However, the use of these catalysts is constrained by the harsh reaction conditions (200–400 °C and high H₂ pressures), as well as the toxic nature of sulfur and chromium compounds, which limit their broader application. This has spurred ongoing research to develop more efficient heterogeneous catalyst systems for fatty acid hydrogenation. Homogeneous catalysts such as sulfuric acid, potassium hydroxide, hydrochloric acid, sodium hydroxide, and sodium methoxide are frequently used due to their high activity and low cost. Nevertheless, these catalysts are non-recoverable, which poses challenges for industrial applications. Additionally, they cause corrosion in equipment and lead to environmental concerns during the purification of end products, which necessitates multiple washing stages [3]. As a result, the search for alternative heterogeneous catalysts, such as metal oxides, has been intensifying. Previous research has shown that noble metal (Pt, Ru, Rh, Pd, Au, and Ir)-supported various metal oxides can act as effective catalysts for fatty acid hydrogenation. Despite this, achieving high selectivity for desired products remains a challenge, necessitating further refinement of catalyst design.

The hydrogenation reaction typically saturates C=C, C=O, and C–O–C bonds in the presence of metal catalysts such as Cu, Ni, Rh, Ru, Pd, and Pt. In the pursuit of improved efficiency, heterogeneous catalysts are being developed to replace conventional homogeneous systems. Commercial fatty alcohol production predominantly utilizes Cu–Cr-based catalysts under stringent conditions (250–350 °C, 10–30 MPa of H₂). This is due to the inherent difficulty of reducing fatty acids [4,5,6]. However, the toxicity and environmental impact of chromium-based catalysts present significant obstacles to their widespread use. Catalysts composed of noble metals such as Ru and Pt have shown promise in the selective hydrogenation of fatty acids. Among various Ru-based catalysts [7], the bimetallic Ru–Sn/Al₂O₃ system has demonstrated the highest selectivity for fatty alcohols, operating at 250 °C and 5.6 MPa of H₂ pressure [8]. Moreover, replacing Al₂O₃ with TiO₂ as the support for Ru–Sn catalysts has enhanced both activity and selectivity due to favorable interactions between the Ru-Ti metal–support pair. Some studies have explored the use of bimetallic catalysts in the hydrogenation of fatty acids. For instance, Ru–Sn catalysts supported [9] on various substrates such as Al₂O₃, SiO₂, ZrO₂, and TiO₂ have been employed to hydrogenate fatty acids at 300 °C and 6 MPa of H₂ pressure, achieving over 95% conversion and approximately 80% selectivity to dodecanol. However, the high reaction temperature remains a limitation. In contrast, hydrogenation of decanoic acid using Pt–Re catalysts supported on TiO₂ at 130 °C and 2 MPa for 4 h resulted in 79% conversion and 75% selectivity to decanol. Similarly, at 300 °C and 3 MPa, octanoic acid was hydrogenated with Ru–Sn/ZnO catalysts to produce octanol with over 99% conversion and 93% selectivity [10].

Further investigations have been conducted previously, and they found that adjusting catalyst supports and/or adding a second metal can improve the selectivity and conversion in the hydrogenation reactions. For instance, crotonaldehyde was converted to crotyl alcohol using an Ir/SiO₂ catalyst at 30 °C and 0.8 MPa hydrogen pressure over 1 h, although the conversion rate was relatively low at 10.9% [11]. The authors have achieved modification of Ir/SiO₂ combined with metal oxides such as MoO_x and ReO_x, affording approximately 95% selectivity in the hydrogenation of crotonaldehyde at 30 °C and 8 MPa of H₂ for 1 h [11]. In another study, the addition of Re to an Ir/SiO₂ catalyst facilitated the production of 1,3-propanediol from glycerol under reaction conditions of 120 °C and 8 MPa hydrogen pressure for 24 h [12]. Despite these modifications, the conversion rate reached only 69%, with product selectivity remaining limited at 47%. Moreover, Ir–Re catalysts supported on activated carbon have been shown to hydrogenate adipic acid with 77% conversion and 52% selectivity under similar reaction conditions [13], although these catalysts still require high H₂ pressure. During the hydrogenation of succinic acid over the Cu–Pd active center, the selection of metal oxide as a support material was significantly contributed to its product selectivity to γ-butyrolactone (by TiO₂), 1,4-butanediol (by hydroxyapatite and SiO₂), or tetrahydrofuran (by γ-Al₂O₃) at the same reaction conditions: 200 °C, 8 MPa H₂, and 96 h [14,15]. These findings emphasize the importance of the combination of the active metal element and catalyst support in the development of efficient catalyst systems for achieving high activity and selectivity in hydrogenation processes.

In this study, first of all, the authors revisit the catalytic performance in the hydrogenation of dodecanoic acid over various supported metal (Au, Pt, Cu, Ir, Ru, Pd, Mo, In, and Ni) catalysts using AlOOH as a support material, particularly for the selective production of dodecanol. Earlier reports described that metal-supported AlOOH catalysts possessed attractive performance not only for the hydrogenation [16,17] but also for various organic reactions [18,19]. After evaluating the catalytic activity, iridium (Ir) was selected as the optimal candidate for further screening. Secondly, the results of the support survey indicated that the Ir-based catalyst had a unique feature in the selective hydrogenation of fatty acids. It was particularly interesting that, when dodecanol was identified as the primary product across most catalysts, the Ir/Nb₂O₅ catalyst showed the highest yield (82.2%) at 200 °C and 8 MPa of H₂ for 17 h, whereas the Ir/MoO₃ catalyst produced dodecane as the main product with a significant yield of 90.7%. Finally, the findings indicated that the simple co-impregnation of Ir onto the Nb₂O₅ and MoO₃ could selectively facilitate the hydrogenation of fatty acids to the corresponding alcohol for chemical an alkane for fuel, respectively. To the best of our knowledge, this is the first report of a heterogeneous Ir-based catalyst for applying the selective hydrogenation of fatty acids. Although the Ir element is expensive and one of the rare metals, these findings hope to open new possibilities of heterogeneous Ir nanoparticle-mediated hydrogenation reactions, particularly for the utility of wasted fatty acids.

2. Results and Discussion

The performance of different metals supported on AlOOH for the hydrogenation of dodecanoic acid to dodecanol was investigated, as listed in Table S1 of the Supplementary Materials (SM). Among the nine elements of Ir, Pt, Ru, Cu, Ni, Pd, In, Au, and Mo monometallic catalysts, the highest yield of dodecanol (52.2%) was obtained over the Ir-based catalyst with 72.5% conversion. The Pt-based catalyst gave a 27.1% yield with 56.0% conversion, and the Ru- and Cu-based catalysts showed slight yield values of 3–5%. In contrast, other elements such as Ni-, Pd-, In-, Au-, and Mo-based catalysts exhibited poor performance with low conversion (<25.0%) and no dodecanol formation (0% yield). As a result, the Ir was selected as the metal center for further investigations of metal-supported catalysts in the hydrogenation of dodecanoic acid. Previously, there were some reports for the hydrogenation reaction over heterogeneous monometallic Ir catalysts [11,20,21,22,23,24,25], particularly Ir/SiO₂; therefore, focusing on the Ir-based catalyst in the hydrogenation of fatty acids is an attractive subject.

To obtain insight into the role of Ir metal, eight types of catalyst support were prepared and tested for the hydrogenation of dodecanoic acid. The results are shown in Figure 1. The details of the activity are listed in Table S2 of the SM. The catalytic activity of the various monometallic Ir-based catalysts for the hydrogenation of dodecanoic acid exhibited a wide range. There were mainly three categories, i.e., dodecane formation, highly-selective dodecanol production, and inactive, among the eight types of Ir-based catalyst performances. Notably, under the same reaction conditions outlined in Figure 1, quantitative conversions of dodecanoic acid (100% conversion) were achieved with Ir-supported MoO₃, Ta₂O₅, and Nb₂O₅, while poor activities (<10% conversions) were observed with Ir-supported ZrO₂, SiO₂, and WO₃. Among the catalysts tested, the Ir/Nb₂O₅ catalyst demonstrated the highest yield of dodecanol (82.2%) and complete conversion of dodecanoic acid, highlighting its considerable potential for the sustainable production of fatty alcohols from bio-based sources. Ir-supported AlOOH and TiO₂ (anatase) also gave good yields for the dodecanol production as 52.2% and 48.7%, respectively. Under identical conditions, interestingly, the Ir/MoO₃ catalyst also achieved complete conversion (100%), with dodecane, constituting 90.7% of the main product, with low yield towards dodecanol production (9.3%). The Ir/Ta₂O₅ gave both capacity for the production of dodecanol with 39.5% yield and 60.5% yield with 100% conversion of dodecanoic acid. Accordingly, the types of support materials for the Ir-based catalysts could alter the reactivity, and Nb₂O₅ and MoO₃ gave a significant trend with high selectivity for dodecanol and dodecane, respectively, at the same reaction conditions. Additionally, it was very interesting that the yield to dodecane decreases drastically at lower reaction temperature (170 °C) and lower H₂ pressure (4.0 MPa) over the Ir/MoO₃; it decreased from 90.7% to 5.7%, while Ir/Nb₂O₅ maintained a good yield for dodecanol (89.1% yield) under identical conditions. It indicated that optimizing not only the catalyst support material but also reaction conditions was a crucial factor in the Ir-based catalyst for the selective hydrogenation of fatty acids.

Time-dependent reaction profiles of the hydrogenation of dodecanoic acid over the Ir/Nb₂O₅ (at 8.0 MPa H₂) and Ir/MoO₃ (at 4.0 MPa H₂) catalysts at 200 °C are shown in Figure 2. Note that the H₂ pressure for Ir/MoO₃ was decreased to 4.0 MPa because its reactivity is so significant, which allowed us to easily trace the reaction profile. As shown in Figure 2a, over the course of the reaction with the Ir/Nb₂O₅ catalyst, the conversion of dodecanoic acid increased steadily from 8 to 17 h. The conversion remained constant at 100% for up to 40 h of reaction time. The yield of dodecanol reached its maximum yield of 86.2% at 24 h; however, it progressively declined after 40 h to 58.9% yield. This decrease inversely correlated with a corresponding increase in the yield of dodecane from 13.8% to 41.1%. Therefore, the production of dodecane occurred via dodecanol stepwise. Under the milder H₂ pressure condition at 4.0 MPa, the conversion of dodecanoic acid over the Ir/MoO₃ catalyst gave the same profile: an increase and then decrease trend of dodecanol production with a linear increase of dodecane yield, as shown in Figure 2b. Complete conversion was achieved within 40 h, with a high yield of dodecane (85.3%), a saturated aliphatic alkane, emerging as the dominant product. Note that the performance of Ir/MoO₃ was very significant and gave 91.3% yield for dodecane at 6 h under 8.0 MPa of H₂. The results are summarized in Table S3 of the SM. Accordingly, the Ir/MoO₃ catalyst led to the activation of more reactant molecules much faster than the Ir/Nb₂O₅ catalyst to dodecane. To emphasize the good yield for dodecanol production over Ir/Nb₂O₅, time-dependent reaction profiles of the hydrogenation of dodecanoic acid over Ir/Nb₂O₅ under the milder reaction conditions (at 170 °C and 4.0 MPa H₂) were further examined; however, the yield of dodecanol peaked at 17 h at 89.1% yield, after which it progressively declined with extended reaction time (see Table S3 of SM).

The 1.0 g scale of reaction (10 times larger scale) was also compared over Ir/Nb₂O₅ and Ir/MoO₃ catalysts (see Table S4 of SM). The Ir/Nb₂O₅ catalyst predominantly produced dodecanol as the major product, achieving a high conversion rate of 99.8% with 68.2% yield of dodecanol and 30.5% yield of dodecane even at 40 h of reaction. In contrast, the Ir/MoO₃ catalyst demonstrated an exceptional yield of dodecane, approximately 91.7%, with complete conversion (100%) at 24 h of reaction. At this time, GC-TOFMS analysis revealed the presence of dodecyl laurate, which can be synthesized from the substrate (dodecanoic acid) and the alcoholic product (dodecanol) through esterification, as illustrated in Scheme 1. Therefore, figuring out how to prohibit the side-reaction of the esterification is crucial for the application of a large-scale reaction to afford the corresponding alcoholic product with high yield in the hydrogenation of fatty acids. The utility of the ester form as the starting material is a well-known approach [9,26]; however, large-scale applications still seem to be challenging.

In the current study, Nb₂O₅ was selected as the catalyst support due to its reported superior performance in facilitating the selective hydrogenation of dodecanoic acid to the alcoholic form (dodecanol). To evaluate the stability and reusability of the catalyst, the Ir/Nb₂O₅ system was subjected to a straightforward regeneration process. The catalyst was centrifuged, thoroughly washed with ethanol solvent, and subsequently dried under vacuum before being employed in subsequent reaction cycles. Figure 3 demonstrates that Ir/Nb₂O₅ catalyst could maintain its high activity and good selectivity for dodecanol in the hydrogenation of dodecanoic acid even after the fifth reuse. Detailed values are listed in Table S5 of the SM. Note that the recycling test was examined at H₂ (4.0 MPa), 170 °C, and 17 h, which was the best condition for Ir/Nb₂O₅ to afford high yield for dodecanol (see Table S3 of the SM). This stability implied the strong interaction between metallic iridium and the Nb₂O₅ support, which is crucial for the efficient conversion of dodecanoic acid to dodecanol. However, the selectivity toward dodecanol exhibited some fluctuations across the recycling runs.

To evaluate the versatility of the catalytic activity, the scope of hydrogenation using the Ir/Nb₂O₅ catalyst was extended to a range of other aliphatic carboxylic acids, including hexadecenoic acid (C₁₆H₃₀O₂), tetradecanoic acid (C₁₄H₂₈O₂), and oleic acid (C₁₈H₃₄O₂), with the results summarized in Table S6 of the SM. In all cases, the catalyst exhibited high activity, achieving complete conversion (100%) across all three substrates. The catalytic hydrogenation of aliphatic carboxylic acids consistently demonstrated the highest selectivity toward the corresponding alcohols. The production yields of hexadecanol and oleyl alcohol were, respectively, obtained as 80.8% and 78.0% at 200 °C in 8.0 MPa H₂. Although the production of tetradecanol was 52.0% yield with the corresponding alkane (48.0%), the optimized reaction conditions, including temperature, pressure, and hydrogen availability, would arrange the partial reduction of the carboxyl group to alcohols rather than complete hydrogenation to alkanes.

Powder XRD patterns for the Ir/Nb₂O₅ and Ir/MoO₃ catalysts are presented in Figure 4, comparing the calcined and reduced catalysts. No significant differences were observed in the peak positions of metallic iridium among the catalysts; however, variations in intensity were significant. Both Ir/Nb₂O₅ and Ir/MoO₃ catalysts exhibited similar patterns across all calcined samples with their respective calcined supports themselves. Following reduction, metallic Ir peaks became prominent in the Ir/Nb₂O₅ catalyst, contributing to its enhanced performance in the hydrogenation of dodecanoic acid. In contrast, the Ir species in the Ir/MoO₃ catalyst were not clearly discernible after reduction, likely due to their smaller size and higher dispersion on the MoO₃ support, with only a faint peak observed at 2θ = 41.7°. Additionally, MoO₂ species were distinctly visible in the XRD pattern of Ir/MoO₃ at 2θ = 25.9° and 37.2°, corresponding to the (110) and (200) planes, respectively, with intensities increasing post-reduction.

To further validate the differences between Ir/Nb₂O₅ and Ir/MoO₃, TEM analysis was conducted to investigate the particle distribution and morphology of the catalysts. Iridium species have been effectively dispersed on the surfaces of Nb₂O₅ and MoO₃ supports, as illustrated in Figure 5, where numerous bright black metal nanoparticles are observed on both Ir/Nb₂O₅ and Ir/MoO₃ catalysts. The Ir/Nb₂O₅ catalyst exhibits a particle size distribution ranging from 1.4 to 3.0 nm and mean diameter is to be 2.38 nm (Figure S1 of SM). In contrast, the Ir/MoO₃ catalyst displays a large number of fine particles, with an average size that cannot be accurately determined because the distinctions between Ir and MoO_x support were difficult to identify. In the H₂-TPR profile of calcined samples, both Ir/Nb₂O₅ and Ir/MoO₃ gave a reducing peak at around 200–300 °C, and the H₂ consumption amount of Ir/MoO₃ (6.43 mmol g⁻¹) was much larger than that of Ir/Nb₂O₅ (0.43 mmol g⁻¹), as shown in Figure S2 of the SM. This also suggested the presence of a unique interaction between Ir and Mo, which would be generated after the reduction process under an H₂ flow at 500 °C.

Figure 6a–d present the XPS analysis of the Ir 4f region results for the reduced and used catalyst samples in Ir/Nb₂O₅ and Ir/MoO₃, respectively. The deconvolution of the Ir 4f peaks revealed the presence of two distinct iridium species on the catalyst surface. Figure 6a,b for the Ir/Nb₂O₅ illustrate the two doublets corresponding to Ir 4f_5/2 and Ir 4f_7/2. The binding energy (BE) for the metallic iridium species in the Ir 4f_7/2 orbital of Ir/Nb₂O₅ is observed at 60.8 eV, while a value of 63.8 eV is recorded for the Ir 4f_5/2 orbital. Differences are observed as +0.3 eV between the (a) fresh and (b) used Ir/Nb₂O₅ catalysts in the Ir 4s region. In contrast, the Ir/MoO₃ catalyst displays iridium oxide species (Ir⁴⁺) as the shoulder peaks in both (c) reduced and (d) spent catalyst. Additionally, there is evidence of coordination between Ir–O and C=O species in the O 1s region, as well as interactions between Ir and oxygen-rich C 1s. However, differences before and after the reaction showed no trends in the Ir states. One can consider that partial oxidation by air during sample transportation is the reason for its difficulty in analysis. Importantly, there were significant differences between Ir/Nb₂O₅ and Ir/MoO₃; the former was mainly composed of the Ir metallic state, whereas the latter contained a mixture of Ir metal and oxides. The deconvolution of the Mo 3d peaks for the Ir/MoO₃ catalyst after reduction revealed the presence of Mo⁶⁺ species (232.5 eV and 235.7 eV) and Mo⁴⁺ species (229.7 eV and 233.0 eV), as shown in Figure S3 of the SM. Both XRD and XPS analyses indicated a transition from the MoO₃ (Mo⁶⁺) to MoO₂ (Mo⁴⁺) phase during this process. Accordingly, the presence of reduced Mo species facilitates the formation of oxygen vacancies and/or Ir–Mo alloying, thereby contributing to an enhancement in catalytic activity. The importance of these factors on the hydrogenation reaction has been discussed in previous papers [27,28]. Interestingly, Ir and Mo combinations are likely attracted to many researchers for hydrogenolysis and hydrodeoxygenation reactions over heterogeneous catalysts by pressurized H₂ [29,30].

3. Materials and Methods

3.1. Catalyst Preparation

Iridium-based catalysts were synthesized using the impregnation method. In a typical procedure, iridium (III) chloride hydrate (Aldrich) was added as the metal precursor into a glass cylinder tube, followed by the addition of approximately 6 mL of distilled water and a catalyst support (1.0 g). At present, 7.5 wt% Ir was loaded in theory (i.e., 0.42 mmol/g). The resulting mixture was stirred using a magnetic stirrer for 24 h at ambient temperature. Afterward, the mixture was subjected to centrifugation at 80 °C for 6 h to isolate the solid phase. The obtained solid was then dried overnight at 110 °C in an oven, ground, and subsequently calcined at 500 °C for 7 h in a furnace. Following calcination, the sample was ground again and reduced in a 5 vol% H₂/N₂ flow (50 mL min⁻¹) for 2 h at 500 °C. Note that these temperatures (i.e., 500 °C) were determined experientially on the basis of our previous study in the Cu–Pd-catalyzed hydrogenation of succinic acid [14,15]. Bohemite (AlOOH; SASOL Germany GmbH, Humburg, Germany), SiO₂ (G-6, 3 μm; Fuji Silysia Chemical Ltd., Aichi, Japan), and Nb₂O₅ (JRC-NBO-2; Companhia Brasileira de Metalurgia e Mineração (CBMM), State of Minas Gerais, Brasil) were commercially available. MoO₃, Ta₂O₅, ZrO₂, WO₃, and TiO₂ (anatase) were supplied from Kanto Chemical Co. Inc., Tokyo, Japan. All chemicals and materials were used without any purification.

3.2. Catalyst Test

The hydrogenation of dodecanoic acid was carried out in a stainless-steel autoclave reactor (TAIATSU Technol., Tokyo, Japan) with a 50 mL glass vessel. In a typical reaction, 10 mL of 1,4-dioxane (FUJIFILM Wako, Osaka, Japan) was added as the solvent, along with 0.1 g of dodecanoic acid (98%, Sigma-Aldrich, St. Louis, MO, USA) and 0.1 g of the reduced catalyst. The reactor was purged six times with high-purity hydrogen (>99.999%) and pressurized to 8.0 MPa at room temperature (approximately 27 °C). The reactor was then placed in an aluminum block bath (Zodiac CCX, EYELA, Tokyo, Japan), heated, and stirred at 200 °C for 17 h. After the reaction, the reactor was cooled to room temperature, and the contents were centrifuged to separate the catalyst and products. The resulting products were analyzed using gas chromatography with a flame ionization detector (GC-FID) (GC-2014, Shimadzu, Kyoto, Japan) and gas chromatography–time of flight mass spectrometry (GC-TOFMS) (AccuTOF GCX, JEOL, Tokyo, Japan). The GC column (Agilent, DB-1, 50 m) temperature was increased at a rate of 20 °C min⁻¹ from 50 °C (held for 2 min) to 250 °C. The injector and detector temperatures were set to 300 °C. Dodecanoic acid conversion was determined using an internal standard method, employing naphthalene as the reference.

3.3. Catalyst Characterization

The crystal structure of the catalyst was determined using powder X-ray diffraction (XRD) with a MiniFlex diffractometer (Rigaku Co., Tokyo, Japan), employing Cu Kα radiation (λ = 0.154 nm) at 40 kV and 30 mA. The diffraction patterns were analyzed using the ICDD (International Centre for Diffraction Data) database. The morphological characteristics of the catalyst were observed via transmission electron microscopy (TEM) using an H-7650 microscope (Hitachi Ltd., Tokyo, Japan), operated at 100 kV. X-ray photoelectron spectroscopy (XPS) analysis, performed with equipment from Shimadzu Co., Kyoto, Japan and Kratos Analytical Ltd., Manchester, UK (Axis-Ultra DLD spectrometer), was used to investigate the electronic state of the iridium-based catalysts. Binding energies were calibrated using the C 1s spectrum of adventitious carbon contamination, with a reference value of 284.8 eV. H₂-TPR profile was conducted by the BELCatII (MicrotracBEL Corp., Osaka, Japan) under a 10 vol% H₂/Ar flow (30 mL min⁻¹) at 50–800 °C in a ramping rate of 10 °C min⁻¹. The TCD signal was quantified by the CuO (99.999%) standard taken under the same conditions. Nitrogen adsorption-desorption measurements were conducted using a BELSORP-mini analyzer (MicrotracBEL Corp., Osaka, Japan), with pre-treatment of the sample at 150 °C before analysis. The surface area of the samples was determined using the Brunauer–Emmett–Teller (BET) theory. Ir/Nb₂O₅ and Ir/MoO₃ possessed a specific surface area of 7.3 m² g⁻¹ and 2.5 m² g⁻¹, respectively, without any unique porosity (Figure S4 of SM).

4. Conclusions

This study described the hydrogenation of dodecanoic acid to dodecanol and dodecane in a 1,4-dioxane solvent using iridium (Ir)-based catalysts. Based on the iridium catalysts survey results, it was found that support materials could contribute to their reactivity for the hydrogenation of dodecanoic acid. Notably, Ir/Nb₂O₅ exhibited the highest yield of dodecanol (89.1%) under the milder reaction conditions of 170 °C, 4.0 MPa of H₂, and 17 h, making it a promising catalyst for the selective production of dodecanoic acid hydrogenation. In contrast, the Ir/MoO₃ catalyst primarily produced dodecane as the main product, achieving a significant yield of 90.7% under higher temperature and H₂ pressure conditions (200 °C, 8.0 MPa). Based on the time-dependent reaction progress, this difference in catalytic behavior lies in the reactivity: Ir/Nb₂O₅ favors alcohol formation because it possesses milder reactivity than Ir/MoO₃, which promotes alkane production. In other words, in the case of Ir/Nb₂O₅, the moderate reactivity of hydrogenation allowed a high yield of intermediate alcoholic compounds by adjusting the temperature and H₂ pressure more easily, while in the case of Ir/MoO₃, it was also indicated that activation of its nature needs a higher operation temperature and H₂ pressure. These might be related to the genesis of oxygen vacancies and/or Ir–Mo alloying during the reaction. Consequently, the underlying reaction mechanism remains an area of ongoing investigation; however, the current understanding of the catalyst system lays a solid foundation for controlling the hydrogenation of not only dodecanoic acid but also a broad range of substrates, including other fatty acids and carboxylic acids. Overall, the process underscores the synergistic interaction between the iridium species and the catalyst support, which collectively facilitates the efficient hydrogenation of dodecanoic acid. The iridium acts as the active site for hydrogenation, while the support material enhances catalyst stability, dispersion, and selectivity, ultimately contributing to the high conversion and selective production of desired products. Mechanistic study on the synergy between iridium and support is an attractive subject. From the viewpoint of its elemental rarity and price, determining how to decrease the usage of iridium loading should be the next subject. It is believed that the unique behaviors of iridium reported here open new insights into the hydrogenation of medium-chain fatty acids.