Rational design of highly dispersed ultra-small Ru nanoparticles on MgO–Al₂O₃ for efficient and selective aromatic hydrogenation

Abstract

Continuous selective hydrogenation of aromatic compounds exhibits broad application prospects, serving as a key process for the synthesis of high-value-added polymer monomers and pharmaceutical intermediates. The fabrication of heterogeneous catalysts being even more critical to enabling this continuous process. Herein, the traditional preparation protocol of supported Ru-based catalysts was systematically optimized, resulting in a novel Ru/MgO–Al₂O₃ catalyst with highly dispersed, ultra-small Ru nanoparticles. This innovative catalyst demonstrated exceptional catalytic activity and selectivity for the hydrogenation of phenolic compounds to alicyclic alcohols, with preferential aromatic ring hydrogenation and suppressed C–O/C–C bond hydrogenolysis. Leveraging this insight, additional studies revealed its comparable outstanding activity and selectivity in the hydrogenation of aromatic esters and ethers to corresponding alicyclic derivatives. This discovery is critical for realizing aromatic hydrocarbon saturation and non-aromatic residue in chemical processes, thereby endowing it with profound significance in the field of chemical manufacturing.

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1. Introduction

The selective hydrogenation of aromatic compounds represents a crucial route for producing aliphatic derivatives, which serve as key starting materials for polymers, resins, dyes, and fine chemicals.^1–3 As is well established, the intrinsic structure of a molecule determines its properties and applications. During the hydrogenation of aromatic compounds, the saturation of aromatic rings induces structural changes. These changes are reflected not only in physicochemical characteristics (e.g., mechanical strength, thermal stability, reactivity) but also in enhanced safety profiles of materials derived from such hydrogenated intermediates, which pose no toxic hazards to humans. Substances such as o-cresol, bisphenol A (BPA) and hydroquinone are widely used in resin synthesis. However, the unsaturated aromatic groups are commonly labeled as an endocrine disruptor,^4–6 which can cause skin allergy, reproductive toxicity and risk of teratogenicity.^7–9 For instance, resins derived from hydrogenated bisphenol A (HBPA) exhibit stable anti-yellowing properties and biocompatibility, enabling their widespread high-end applications. Traditional phthalate plasticizers face significant challenges under such as the REACH regulations, which mandates residual benzene ring content <50 ppm in human-contact plasticizers.^10–12 Notably, aromatic ring saturation via hydrogenation offers a viable route to fabricate novel eco-friendly plasticizers meeting this stringent requirement.

Hydrogenation reaction modes mainly include batch reactor hydrogenation and continuous hydrogenation. Selective continuous hydrogenation serves as an environmentally friendly, clean, and 100% atom-economical synthetic approach, holding critical importance in chemical industrial production.¹³ Relative to traditional batch hydrogenation processes, it minimizes emissions of hydrogen-containing organic waste gases while enabling precise process control, stable product quality, and simplified operation. These advantages are prominently observed in the synthesis of high-value-added fine chemicals, pharmaceutical intermediates, and human health-related research.¹⁴ Nevertheless, the heterogeneous selective catalysts are even pivotal to this process; catalyst mechanical strength, physicochemical properties, active metal characteristics (type, content, size, dispersion), and preparation methods all exert notable effects on catalytic performance.^15–17 Studies have demonstrated that the dispersion and particle size of the active metal play a crucial role in the activity of hydrogenation catalysts,¹⁸ while the acidity of the support surface is of great importance for the selectivity towards target products during the hydrogenation process. Catalysts commonly employed for the hydrogenation of aromatic compounds include Ni, Co, Ru, Rh, Pd, and Pt,^19–22 while typical supports are activated carbon, Al₂O₃, SiO₂, and TiO₂. Notably, Ru-based^23–25 catalysts exhibit superior aromatic hydrogenation activity while also being the most cost-effective among noble metal catalysts, thus offering great potential for reducing catalyst production costs in industrial applications. Al₂O₃ is widely used in petrochemicals owing to its stable physicochemical properties and favorable pore structure.²⁶

Taking all the aforesaid into account, Ru was selected as the catalytic active center and Al₂O₃ as the catalyst support in this study. Under the continuous fixed-bed hydrogenation mode, the catalytic performance of Ru/MgO–Al₂O₃ catalysts for the hydrogenation of various aromatic compounds was systematically investigated via the optimization of catalyst preparation methods and the regulation of support acidity by MgO modification.^27,28 Their physicochemical properties were comprehensively characterized via XRD, H₂-TPR, TEM, XPS, NH₃-TPD, and ICP-AES, with results correlated to hydrogenation performance. Initially, catalyst precursors were treated with NaOH to form easily reducible oxides, lowering reduction temperature and facilitating the ultra-small Ru nanoparticle formation. Based on this observation, subsequent MgO modification²⁹ tailored Lewis acidity, adjusted the electron density between the active metal and support, and ultimately produced a Ru-based catalyst with highly dispersed, ultra-small nanoparticles. Due to MgO-regulated Lewis acidity, the catalyst suppressed C–O/C–C bond hydrogenolysis. Further tests showed it also exhibited exceptional activity and selectivity in hydrogenating aromatic esters, aromatic ethers, and biomass-derived furans to corresponding alicyclic products.³⁰ This finding is critical for achieving aromatic saturation and non-aromatic residue in chemical processes, holding profound significance for chemical manufacturing.

2. Experimental section

2.1 Materials

All chemicals utilized in the experiments are of analytical reagent (AR) grade purity and used without additional purification. Ruthenium(III) chloride hexahydrate (RuCl₃·6H₂O, Ru ≥ 37.0%) is purchased from Shaanxi Kaida Chemical Engineering Co., Ltd (Baoji, China). Sodium hydroxide (NaOH, 96.0%), ethanol (≥99.5%), isopropanol (≥99.7%), methylcyclohexane (≥99.5%), magnesium nitrate hexahydrate (Mg(NO₃)₂·6H₂O, 99.0%) and hydrazine hydrate (N₂H₄ ≥ 80.0%) are acquired from China National Pharmaceutical Group Corporation (Shanghai, China). Aluminum oxide (≥99.0%) is obtained from Sinopec Catalyst Co., Ltd (Yueyang, China). Reactants used in the experiment include hydroquinone(≥99.0%), o-cresol(≥99.0%) are obtained from Sinopharm Group (Beijing, China), whereas bisphenol A (≥99.5%) is supplied by Sinopec Mitsui Chemical Co., Ltd (Shanghai, China), diisooctyl phthalate (DOP ≥ 99.5%) and diisononyl phthalate (DINP ≥ 99.5%) are supplied by Formosa Plastics Group (Taiwan, China).

2.2 Catalyst preparation

The supported Ru/Al₂O₃ catalysts were synthesized via the excessive impregnation method, and the detailed preparation procedure described as following. Firstly, the catalyst support Al₂O₃ was dried at 120 °C for 12 hours, then impregnated with RuCl₃·6H₂O solution at 85 °C for 8 h. Subsequently, the ruthenium catalyst precursor was filtered and dried at 120 °C for 24 h. The catalysts were categorized into different types according to the post-treatment methods applied to the ruthenium catalyst precursor, as illustrated in Fig. 1. Without any prior treatment, catalyst precursors are directly reduced in H₂ atmosphere to yield the catalyst designated as Ru/Al₂O₃-I. The catalyst precursor, first calcined at 450 °C and then reduced in H₂ atmosphere to yield the catalyst designated as Ru/Al₂O₃-II. Washing the catalyst precursor with sodium hydroxide solution and then dried overnight prior to H₂ reduction, which is labeled as Ru/Al₂O₃-III. The catalyst, which is reduced using hydrazine hydrate with the same pre-treated process of Ru/Al₂O₃-III is marked as Ru/Al₂O₃-IV. On the other hand, the MgO modified catalyst support is prepared by incipient wetness impregnation of Mg(NO₃)₂·6H₂O solution with Al₂O₃, then dried at 120 °C for 8 hours and calcined at 450 °C for 6 hours. While the supported Ru/MgO–Al₂O₃ catalyst is prepared by excessive impregnation, and the detailed preparation procedure is same to Ru/Al₂O₃-III, and the MgO modified catalyst is designated as Ru/MgO–Al₂O₃-V. It is worth noting that the effects of the content of active metal Ru and MgO have been thoroughly examined in our previously published research.³¹ Studies also revealed that with a progressive increase in Ru loading, the particle size of Ru (the active component) exhibits a gradual enlargement, accompanied by the aggregation of Ru particles.^32–34 This aggregation-induced decline in the dispersion of active sites ultimately compromises the catalytic activity of the catalyst. For the current study, active metal ruthenium and MgO are present at a content of 3.0 weight percent (wt.%).

Fig. 1 The flowchart for catalyst preparation process.

2.3 Catalyst characterization

Powder X-ray diffraction (XRD) measurements were performed using an Ultima IV diffractometer, employing Ni-filtered Cu Kα radiation (40 kV, 50 mA), catalysts were scanned over a 2θ range of 10–80° at a step size of 0.02°. The Specific surface area and pore volume of the catalysts were determined via dynamic nitrogen adsorption at −196 °C, using a Micromeritics ASAP2400 instrument. Transmission electron microscopy (TEM) images were obtained with a Joel JEM-F200 microscope. For TEM sample preparation: the synthesized catalyst powder was diluted in ethanol, ultrasonicated for 5 minutes, and droplets of the resulting suspension were deposited onto a copper grid. The grid was then dried overnight in a vacuum oven at 100 °C. Particle size distribution statistics were calculated using the Nano Measurer image analysis software. H₂ chemisorption, temperature-programmed reduction (TPR), and NH₃ temperature-programmed desorption (NH₃-TPD) experiments were conducted on a Micromeritics Autochem II 2920 system. X-ray photoelectron spectroscopy (XPS) measurements were carried out with a Thermo Scientific K-Alpha spectrometer, equipped with a hemispherical electron analyzer and a 300 W Al Kα X-ray source (E = 1486.4 eV). For XPS analysis: the powder sample was mounted on double-sided copper adhesive tape, placed on a sample rod in the pretreatment chamber, and subsequently transferred to the analysis chamber. Prior to spectral collection, the sample was held in the analysis chamber until a residual pressure of approximately 5 × 10⁻⁷ Nm⁻² was achieved. Narrow-scan spectra for target elements were recorded at a pass energy of 50 eV. The actual metal ruthenium loading in the catalysts were determined by Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES), using an Agilent 5110 instrument.

2.4 Catalytic hydrogenation evaluation

The hydrogenation reaction testing for the synthesized catalysts is carried out on a fixed-bed reactor shown in Fig. 2. Catalysts are loaded into the middle section of the reactor, with inert packing material placed above and below, the inner diameter of the reactor is 22 mm, and the fixed-bed reactor is heated using an electric heater. Prior to the hydrogenation reaction, catalysts are all pretreated under hydrogen atmosphere at 80–250 °C for 8 h. The reaction feed is pumped at a controlled flow rate, mixed with hydrogen, and preheated before entering the catalyst-packed reactor. The hydrogenated mixtures are quantitatively analyzed using a gas chromatograph equipped with a 30 m SE-54 capillary column to evaluate the performance of the synthesized catalysts. Meanwhile, product identification is conducted via gas chromatography-mass spectroscopy (GC-MS) on an Agilent 7890 C instrument.

Fig. 2 Process flow diagram for catalyst hydrogenation reaction testing.

Feedstock conversion (Conv./%) and selectivity (Sel./%) toward the target product are most critical metrics for assessing the overall performance of the catalyst, which are calculated as follows:

The Weight Hourly Space Velocity (WHSV) is a critical parameter for characterizing a catalyst's processing capacity. This important parameter reflects the catalyst's capacity to process reactants, representing the amount of raw material that a specified quantity of catalyst can convert within a set timeframe. It is calculated as follows:

The hydrogen–oil molar ratio represents the molar ratio of H₂ to the raw material per unit time, and it is precisely measured using a flowmeter during the experiment. It is also an important parameter of the hydrogenation reaction, which is calculated as follows:

3. Results and discussion

3.1 Characterization of catalysts

The porous parameters of these materials calculated from the isotherms, and the size of ruthenium active centers nanoparticles of the studied catalysts observed by TEM micrographs are compiled in Table 1. As summarized in Table 1, the Al₂O₃ support exhibits a BET surface area (S_BET) of 210 m² g⁻¹, a total pore volume (PV) of 0.68 cm³ g⁻¹, and an average pore diameter of 10.24 nm. Modifying the support with MgO exerted no appreciable influence on its pore size and pore volume. Nevertheless, a slight decrease in specific surface area is observed, which may be attributed to the deposition of MgO nanoparticles (causing partial pore blockage) and interactions between Al₂O₃ support and MgO.³⁵ As showed in Fig. S1, with the exception of Ru/Al₂O₃-II, the catalysts exhibited negligible changes in pore volume relative to the catalyst support, accompanied by a slight reduction in pore size and a substantial increase in specific surface area. Following NaOH treatment, the Ru/Al₂O₃-III, Ru/Al₂O₃-IV, and Ru/MgO–Al₂O₃-V catalysts exhibited notably higher specific surface areas compared with the untreated Ru/Al₂O₃-I and Ru/Al₂O₃-II catalysts, which is 225 m² g⁻¹ and 210 m² g⁻¹ respectively. This observation is likely due to the etching action of NaOH on the catalyst support.

Table 1 Physicochemical properties of the samples

Catalysts	Pore parameters			Diameter^a/nm	Ru^b/wt.%	ρs^c/µg m^−2
	SBET/m^2 g^−1	PV/cm^−3 g^−1	D/nm
a Calculated from TEM results.b Ru content are determined by ICP-AES.c Surface density are calculated from Ru content and the surface area of catalysts.
Al2O3	210.55	0.68	10.24	—	—	—
MgO/Al2O3	205.10	0.66	10.24	—	—	—
Ru/Al2O3-I	212.18	0.66	10.05	1.86	2.95	139.03
Ru/Al2O3-II	208.78	0.69	10.34	7.13	2.89	138.42
Ru/Al2O3-III	223.54	0.67	9.97	2.32	2.86	127.94
Ru/Al2O3-IV	225.05	0.65	9.63	2.09	2.84	126.19
Ru/MgO–Al2O3-V	228.55	0.63	9.57	1.72	2.82	123.39

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The size of the active center Ru was statistically analyzed by transmission electron microscopy, with the results summarized in Table 1. The average Ru nanoparticle diameters for Ru/Al₂O₃-I, Ru/Al₂O₃-II, Ru/Al₂O₃-III, Ru/Al₂O₃-IV, Ru/MgO–Al₂O₃-V are 1.86 nm, 7.13 nm, 2.32 nm, 2.09 nm, and 1.72 nm, respectively. It is particularly noteworthy that Ru/Al₂O₃-II exhibits substantially larger Ru nanoparticles compared with the other four catalysts. This is primarily attributed to the sintering and agglomeration of RuO_x species formed during the 450 °C high-temperature calcination of the RuCl₃ precursor, leading to an increase in particle size.³⁶ TEM images further clearly reveal the agglomeration of Ru nanoparticles, showed in Fig. 4b. More notably, the MgO-modified catalyst possesses the smallest active metal particle size and the largest specific surface area, both of which are crucial for improving catalytic performance. This observation can be ascribed to the acid–base property regulation of Al₂O₃ upon MgO modification, which facilitates the formation of highly dispersed Ru nanoparticles³⁷ and ultimately results in ultra-small Ru particles. Furthermore, literature reports^38–40 indicate that MgO modification diminishes the catalyst's acidity, thereby inhibiting the cleavage of C–OH and C–C bonds. Such inhibition suppresses the generation of diverse hydrogenation by-products, leading to improved selectivity for the target product, which in agreement with subsequent findings. The active metal Ru content and the catalyst's surface density are also shown in Table 1. During the catalyst synthesis, the loading of active ruthenium (Ru) was uniformly controlled at 3 wt.% across all samples. ICP-OES analyses confirmed that the Ru contents in Ru/Al₂O₃-I and Ru/Al₂O₃-II—prepared without precursor pretreatment—were marginally higher than those in catalysts derived from NaOH-pretreated precursors (Ru/Al₂O₃-III, Ru/Al₂O₃-IV, Ru/MgO–Al₂O₃-V). This discrepancy arises primarily from partial leaching of the active metal during NaOH washing. Additionally, the surface area densities of these catalysts fell within the range of 125–139 µg m⁻². Notably, the catalysts prepared without NaOH pretreatment exhibited a slightly lower specific surface area, which consequently led to a moderately higher surface area density relative to their counterparts.

X-ray diffraction (XRD) serves as a key analytical technique for obtaining critical insights into material composition and the atomic/molecular-level structural or morphological characteristics. The crystal structure of the catalyst support and supported ruthenium catalysts are probed by XRD, which showed in Fig. S2. The XRD patterns of Al₂O₃ and MgO/Al₂O₃ are nearly identical, with only the characteristic reflections of Al₂O₃ being observed. No distinct diffraction peaks attributable to MgO are detected, primarily due to the high dispersion and relatively low loading of MgO. With the exception of Ru/Al₂O₃-II, the XRD patterns of the various catalysts reveal that there are no distinct diffraction peaks corresponding to the metallic ruthenium phase are observed for the other catalysts. This observation suggests that ruthenium is highly dispersed on the support, which is well consistent with the phenomenon observed in the TEM images showed in Fig. 4a and c–e. In contrast to the catalysts reduced under a hydrogen atmosphere, the X-ray diffraction pattern of the Ru/Al₂O₃-IV catalyst reduced with hydrazine hydrate exhibits two relatively weak characteristic peaks of RuO₂ observed at 2θ = 29.3 (110), 53.8(211),⁴¹ in agreement with the JCPDS file (PDF file # 40-1290). This result indicating the presence of partially unreduced ruthenium oxide under this reduction protocol. Nevertheless, a distinct characteristic peak attributed to Ru metal observed in the XRD spectrum of the Ru/Al₂O₃-II catalyst at 2θ = 44.0°, which can be indexed to the (101) lattice plane of metallic ruthenium with a hexagonal close-packed (hcp) structure, in agreement with the JCPDS file (PDF file # 06-0663). This observation is fully consistent with the findings from H₂ temperature-programmed reduction (H₂-TPR) and transmission electron microscopy (TEM) analyses, which showed in Fig. 3 and 4b. TEM images clearly reveal sintering and agglomeration of metallic ruthenium, while the H₂-TPR profile shows that this catalyst exhibits the highest reduction temperature. These results collectively indicate that the ruthenium catalyst prepared via precursor calcination possesses low dispersion and relatively large particle sizes.

Fig. 3 H₂-TPR patterns of the supported ruthenium catalysts.

Fig. 4 TEM micrographs of the supported ruthenium catalysts: Ru/Al₂O₃-I (a), Ru/Al₂O₃-II (b), Ru/Al₂O₃-III (c), Ru/Al₂O₃-IV (d) and Ru/MgO–Al₂O₃-V (e and f). HAADF-STEM images of Ru/MgO–Al₂O₃-V (f) and EDS mapping images of the Ru/MgO–Al₂O₃-V (g).

Furthermore, the Ru/Al₂O₃-II, Ru/Al₂O₃-IV, Ru/MgO–Al₂O₃-V catalysts obtained after the pretreatment step involving washing the catalyst precursor with NaOH solution exhibited two distinct diffraction peaks around 20.0°, which are characteristic of NaAlO₂.⁴² This phase is primarily generated via the reaction between NaOH and the catalyst support Al₂O₃ during the pre-treated process. Notably, for the catalyst prepared using the MgO-modified support, the intensity of these NaAlO₂ peaks is significantly reduced. This observation may be attributed to the inhibiting effect of MgO on NaAlO₂ formation, or alternatively, to the formation of MgAl₂O₄ ref. 43 spinel at overlapping diffraction positions, leading to peak broadening and attenuation in the XRD pattern. NaAlO₂ and MgAl₂O₄ are both alkaline in nature, and their formation can effectively reduce the acidity of the catalyst support. Moreover, these phases exhibit relatively high chemical stability, which may contribute to enhancing the overall stability of the catalyst.^44,45

H₂-TPR analyses are conducted to investigate the reducibility of the supported ruthenium catalysts, which can significantly influence catalytic performance. With the exception of Ru/Al₂O₃-II, a single well-defined hydrogen consumption peak below 150 °C was observed for the prepared catalysts Ru/Al₂O₃-I, Ru/Al₂O₃-III and Ru/MgO–Al₂O₃-V (Fig. 3). Studies^46,47 have shown that the reduction peaks appearing below 200 °C in Ru-based supported catalysts are universally assigned to the characteristic reduction of surface RuO_x species. In contrast, in the case of Ru/Al₂O₃-II catalyst displays two distinct reduction peaks: one centered near 200 °C (198.7 °C) and another at a higher temperature of 588.7 °C. Complementary TEM characterization results provide insight into this unique reduction behavior: the active RuO_x species in Ru/Al₂O₃-II undergo significant sintering and agglomeration,^48,49 resulting in larger RuOx particle sizes. The increased particle size, coupled with enhanced metal-support interaction (MSI) between Ru species and the Al₂O₃ support, elevates the reduction energy barrier for these RuO_x aggregates—manifested as the reduction peak at 198.7 °C. Notably, the high-temperature reduction peak at 588.7 °C is proposed to originate from a specific Ru-containing species formed during the 450 °C calcination step. It is hypothesized that large RuO_x particles, derived from the catalyst precursor, interact strongly with the Al₂O₃ support to form a stable Ru–O–Al interfacial structure. This Ru–O–Al species exhibits high thermal stability, requiring a much higher reduction temperature to break the strong Ru–O–Al bonds and achieve complete reduction to metallic Ru⁰.

Among these prepared catalysts, the variation trend of Ru species reduction temperatures (Ru/MgO–Al₂O₃-V < Ru/Al₂O₃-III < Ru/Al₂O₃-I < Ru/Al₂O₃-II) implies a corresponding descending order of H₂ dissociation capability for the catalysts, i.e., Ru/MgO–Al₂O₃-V > Ru/Al₂O₃-III > Ru/Al₂O₃-I > Ru/Al₂O₃-II. This sequence of H₂ dissociation capability is well-correlated with the catalytic activity results presented in Table 3, where an identical activity order (Ru/MgO–Al₂O₃-V > Ru/Al₂O₃-III > Ru/Al₂O₃-I > Ru/Al₂O₃-II) is observed. Notably, the Ru/MgO–Al₂O₃-V catalyst which modification of MgO results in a significant decrease to 78.9 °C. In our previously published works,³¹ a systematic investigation was conducted to elucidate the correlation between MgO loading (0–5 wt%) and the reduction behavior of RuO_x for the Ru/MgO–Al₂O₃ catalysts. Results showed that the reduction temperature decreased initially before increasing with the incorporation of MgO into the catalyst support, with the lowest reduction temperature attained at a MgO loading of 3 wt%. This pronounced reduction temperature lowering is primarily ascribed to MgO facilitates the dispersion of active Ru species, promoting the formation of ultra-small Ru nanoparticles, which substantially lowers the energy barrier for H₂-induced reduction. Furthermore, MgO modulates the electronic state of the active Ru species to a certain extent—likely via electron transfer at the Ru–MgO interface or the induction of a local electron-rich environment around Ru sites—which further weakens the Ru–O bond strength and thus contributes to the reduced reduction temperature.⁵⁰ The TEM results showed that the Ru/MgO–Al₂O₃-V catalyst presented the minimum size of 1.72 nm among the prepared ruthenium catalysts, which is well consistent with the above observation results.

Ammonia Temperature-Programmed Desorption (NH₃-TPD) is a well-established characterization technique for probing the surface acid properties of catalytic materials,⁵¹ Fig. S3 presented the NH₃-TPD patterns of the studied catalysts. The as-fabricated catalysts display three distinct characteristic signals at 100 °C, 250 °C and 400 °C, assigned to weak, medium-strong, and strong acid sites, respectively.⁵² The catalyst supports Al₂O₃ and MgO/Al₂O₃ exhibited signal peaks solely at 100 °C and 400 °C, whereas the 250 °C signal in the catalyst can attributed to ruthenium (Ru) catalyst formation.⁵³ Compared with catalyst support's intrinsic acidity, Ru/Al₂O₃-I and Ru/Al₂O₃-II which prepared via direct H₂ reduction and post-calcination of RuCl₃ precursor exhibited significantly higher acidity, primarily due to Lewis acid HCl introduced during synthesis. NaOH washing of the precursor induces a marked acidity decline for Ru/Al₂O₃-III, whereas MgO modification affords the lowest acidity for Ru/MgO–Al₂O₃-V. For the hydrazine hydrate-reduced Ru/Al₂O₃-IV catalyst, three distinct peaks in 100–300 °C range stems mainly from the decomposition of residual hydrazine hydrate during characterization. Generally, NaOH pretreatment decreases catalyst acidity, and MgO modification further diminishes its intrinsic acidity. This reduced acidity may effectively inhibits C–C and C–OH bond cleavage in the hydrogenation process, thus improving selectivity toward the aromatic ring-saturated target product.⁵⁴

Transmission electron microscopy (TEM) images are acquired to characterize the morphology and particle size distribution of the as-prepared catalyst, as shown in Fig. 4. Notably, all catalysts except Ru/Al₂O₃-IV exhibit uniform dispersion of active metal nanoparticles on the Al₂O₃ support, with an average size of ∼2.0 nm. As can be observed from Fig. 4a, c and d, the average particle size of ruthenium metal for Ru/Al₂O₃-I, Ru/Al₂O₃-III and Ru/Al₂O₃-IV is 1.86 nm, 2.32 nm and 2.09 nm, respectively. For the Ru/Al₂O₃-IV catalyst, Ru particles show a significant size increase to 7.13 nm, with distinct metal clusters visible in TEM images, primarily attributed to high-temperature sintering during preparation. Notably, the Ru/MgO–Al₂O₃-V catalyst exhibits the smallest average Ru particle size (1.72 nm), suggesting that MgO modification promotes Ru active site dispersion, thereby forming ultra-small Ru nanoparticles and enhancing catalytic activity. More interestingly, the average particle size of ruthenium for the prepared catalysts correlates positively with their H₂-TPR reduction temperatures: larger particles necessitate higher reduction temperatures.

X-ray photoelectron spectroscopy (XPS) is employed to characterize the valence and electronic properties of Ru for the target catalysts, shown in Fig. 5. To avoid overlap between Ru 3d_3/2 and C 1s peaks at 284.8 eV, Ru 3p XPS spectra were analyzed to determine the valence states and electronic properties of Ru.⁵⁵ The Ru 3p spin–orbit splits into Ru 3p_3/2 and Ru 3p_1/2, the fitted peaks at 462.4 eV, 484.1 eV are attributed to the Ru (0) species,⁵⁶ while the peaks at higher binding energies (466.7, 488.3 eV) correspond to Ru(IV).⁵⁷ XPS qualitative analysis results for the catalysts are summarized in Table 2. Despite the nearly consistent total content of active Ru species during catalyst preparation, results observed from Table 2 reveal that the peak area and the distributions of Ru (0) and Ru(IV) vary among the samples. The Ru (0)/Ru(IV) peak area ratio follows an identical order in both 3p_3/2 and Ru 3p_1/2 regions: Ru/MgO–Al₂O₃-V > Ru/Al₂O₃-III > Ru/Al₂O₃-I > Ru/Al₂O₃-II > Ru/Al₂O₃-IV. Notably, Ru/MgO–Al₂O₃-V displays the highest ratios (5.97 and 4.93, respectively), corroborating its elevated content of reduced metallic Ru and enhanced reducibility—consistent with its lowest H₂-TPR reduction temperature and the ultra-small, highly dispersed particles observed via TEM images. In contrast, the hydrazine hydrate-reduced catalyst Ru/Al₂O₃-IV exhibits the lowest Ru (0)/Ru(IV) ratios (2.4 and 2.19), attributed to incomplete reduction during synthesis. Ru/Al₂O₃-II also shows relatively low ratios, correlating with its large particle size and significant aggregation. XPS characterization offers profound insights into the surface valence distribution of the catalysts, with all findings consistent with H₂-TPR and TEM results.

Fig. 5 Wide-scan XPS spectra of the supported ruthenium catalysts. (a) XPS spectra of Ru 3p for the supported ruthenium catalysts: (b) Ru/Al₂O₃-I, (c) Ru/Al₂O₃-II, (d) Ru/Al₂O₃-III, (e) Ru/Al₂O₃-IV and (f) Ru/MgO–Al₂O₃-V.

Table 2 The XPS qualitative analysis of the supported ruthenium catalysts

Catalyst	Ru	Ru 3p3/2 peak area	Ru 3p3/2 Ru (0)/Ru (IV)	Ru 3p1/2 peak area	Ru 3p1/2 Ru (0)/Ru (IV)
Ru/Al2O3-I	Ru (0)	8535.31	4.87	5512.45	4.06
	Ru (IV)	1751.77		1357.75
Ru/Al2O3-II	Ru (0)	4910.4	3.05	2833.96	3.78
	Ru (IV)	1608.56		750.36
Ru/Al2O3-III	Ru (0)	5841.16	3.18	2860.97	2.96
	Ru (IV)	1834.01		966.55
Ru/Al2O3-IV	Ru (0)	9808.47	2.40	5073.69	2.19
	Ru (IV)	4090.98		2320.92
Ru/MgO–Al2O3-V	Ru (0)	4809.22	5.97	2553.98	4.93
	Ru (IV)	806.07		518.12

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3.2 Catalytic performance

The selective hydrogenation of o-cresol is performed on a fixed-bed reactor to systematically evaluate the catalytic activity and selectivity of the catalysts, with the results compiled in Table 3. The desired product is aromatic ring-saturated o-methylcyclohexanol. During hydrogenation, C–C and C–OH bond cleavage-induced side reactions take place, generating methylcyclohexane, cyclohexanol, and cyclohexane as byproducts. Notably, o-methylcyclohexanone is identified in the hydrogenation products under reduced catalyst activity, which is likely resulting from the isomerization of the semi-hydrogenation intermediate cyclohexenol. As clearly reflected in Table 3, catalysts exhibit a hydrogenation activity order of Ru/MgO–Al₂O₃-V > Ru/Al₂O₃-III > Ru/Al₂O₃-I > Ru/Al₂O₃-II > Ru/Al₂O₃-IV, well consistent with the trends deduced from physicochemical characterizations (H₂-TPR, XPS, TEM). Their selectivity towards the target product o-methylcyclohexanol follows a distinct sequence: Ru/MgO–Al₂O₃-V > Ru/Al₂O₃-IV > Ru/Al₂O₃-III > Ru/Al₂O₃-II > Ru/Al₂O₃-I, which correlates positively with the catalyst basicity as probed by NH₃-TPD. Notably, the Ru/MgO–Al₂O₃-V catalyst delivers the superior catalytic activity and selectivity, accompanied by the minimal formation of byproducts from C–C and C–OH bond cleavage. Conversely, the catalysts without NaOH treatment (Ru/Al₂O₃-I and Ru/Al₂O₃-II) show the highest yield of such cleavage-derived byproducts. Collectively, optimization of the preparation protocol and MgO modification not only promotes the formation of highly dispersed ultrasmall Ru nanoparticles but also modulates the catalyst's acid–base properties, thereby enhancing catalytic activity, suppressing side reactions, and ultimately improving the overall catalytic performance.

Table 3 Catalytic performance for o-cresol hydrogenation over synthesized catalysts ^a

Catalysts	Conv. (%)	Product distribution (% selectivity)
		P1	P2	P3	P4	P5
a Reaction conditions: catalyst (15 g), temperature (120 °C), H2 (5.0 MPa), H/O (Mol) = 6 : 1, WHSV (0.30 h^−1), ethanol used as solvent, o-cresol concentration 20 wt.%. Products were quantified by GC.
Ru/Al2O3-I	92.42	82.41	0.63	1.95	10.19	4.82
Ru/Al2O3-II	86.39	85.37	0.86	1.60	7.86	4.31
Ru/Al2O3-III	98.67	90.63	0.27	1.06	5.48	2.56
Ru/Al2O3-IV	85.86	92.49	0.32	0.83	4.19	2.17
Ru/MgO–Al2O3-V	100.00	99.74	0.01	0.05	0.12	0.08

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In this study, a high-performance Ru/MgO–Al₂O₃-V hydrogenation catalyst is developed, which delivers exceptional catalytic performance in o-cresol hydrogenation. The catalyst features robust aromatic ring saturation activity while efficiently suppressing C–C and C–OH bond cleavage, thus holding substantial application potential. Herein, aromatic compounds with diverse functional groups are utilized as substrates to assess the catalyst's performance, with the results compiled in Table 4. As clearly reflected in the data, the catalyst exhibits outstanding catalytic efficacy in the hydrogenation of other aromatic compounds bearing varied functional groups—particularly for bisphenol-type compounds, where it shows remarkable effectiveness in inhibiting C–OH bond cleavage, and the selectivity toward the target products in the hydrogenation of hydroquinone and bisphenol A both exceeds 98.5% (Entry 1, 2). Additionally, the catalyst demonstrates high selectivity exclusively toward aromatic ring saturation, irrespective of the presence of aldehyde, ester, ether, or nitrile moieties on the aromatic ring (Entry 3,5–7). Furthermore, the catalyst's catalytic performance in the hydrogenation of biomass-derived furan compounds is further explored. Intriguingly, it also displays favorable furan ring hydrogenation capability (Entry 3). Across all the investigated hydrogenation reactions, the catalyst sustains strong aromatic/furan ring saturation activity while suppressing the hydrogenolysis and hydrogenation of functional groups, including C–C, C–O–C, C–OH, –CHO, –C=O, and –CN. The optimization of the catalyst preparation protocol and MgO modification are the pivotal factors underpinning the catalyst's exceptional performance.

Table 4 Exploration of the universality of the Ru/MgO–Al₂O₃-V^a

Entry	Substrate	Product	Reaction conditions	Conv. (%)	Sel, (%)
a Reaction conditions: catalyst (15 g), H2 (5.0 MPa), H/O (mol) = (^b,d,e 6 : 1,^c 10 : 1), substrate concentration (20 wt.%), solvent (^b ethanol, ^c,d isopropanol, ^e isooctanol). Products were quantified by GC.
1^b			100 °C, 0.35 h^−1	100.00	98.52
2^c			120 °C, 0.3 h^−1	100.00	98.53
3^b			120 °C, 0.3 h^−1	100.00	99.76
4^b			100 °C, 0.35 h^−1	100.00	99.16
5^e			120 °C, 0.15 h^−1	100.00	96.84
6^e			120 °C, 0.2 h^−1	100.00	99.52
7^e			120 °C, 0.2 h^−1	100.00	99.48

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3.3 Proposed reaction mechanism

From above observations, it can be summarized that a novel Ru/MgO–Al₂O₃ catalyst with highly dispersed ultra-small nanoparticles for aromatic selective hydrogenation was developed via precise precursor design and optimization. The Ru/MgO–Al₂O₃ catalyst enables highly selective aromatic ring hydrogenation while suppressing C–C, C–OH and others bond hydrogenolysis. The proposed reaction mechanism is illustrated in Fig. 6. As can be observed from the selective hydrogenation of o-cresol to o-methylcyclohexanol, catalysts synthesized via different methods show distinct catalytic activity and selectivity. Notably, the exceptional hydrogenation activity of the optimal catalyst originates from highly dispersed active Ru centers and the formation of ultra-small Ru nanoparticles (minimum size: 1.72 nm). Analysis of hydrogenation products indicates that MgO-free catalysts with relatively strong acidity produce more by-products, attributed to C–O and C–C bond cleavage. In contrast, the Ru/MgO–Al₂O₃ catalyst exhibits a more prominent ability to suppress such bond cleavage. Further studies confirm that the Ru/MgO–Al₂O₃ catalyst enables efficient aromatic ring hydrogenation for aromatic compounds bearing various benzene-attached functional groups, while also effectively inhibits the hydrogenolysis of such functional groups (e.g., C–C, C–O) on the aromatic ring. Overall, the proposed reaction mechanism might be the modification by MgO resulted in changes in their electronic properties, which enhance the adsorption of phenolics and ultimately resulted in superior catalytic activity. Furthermore, the acidity regulation by MgO reduced catalyst's acidic sites, not only enhanced the dispersion of active ruthenium species and facilitated the formation of ultra-small Ru nanoparticles but also inhibits the cleavage of C–OH, C–C and others bonds under acidic environments. Ultimately, this synergistic effect enabled the development of the aromatic compounds hydrogenation catalyst with outstanding activity and selectivity.

Fig. 6 Proposed reaction mechanisms for aromatic compounds hydrogenation over Ru/MgO–Al₂O₃ catalyst.

3.4 Catalytic stability

A kiloton-scale production unit for o-methylcyclohexanol has been successfully established. A long-term catalytic stability test (Fig. 7) was conducted under optimal reaction conditions, where the Ru/MgO–Al₂O₃-V catalyst operated continuously in industrial equipment more than 4000 hours. Throughout the test, o-cresol conversion remained above 99.96%, while the selectivity towards the target product o-methylcyclohexanol is stably maintained at over 99.5%. Additionally, HPLC analysis revealed that residual o-cresol in the hydrogenation products is consistently below 400 ppm. These results confirm the catalyst's excellent stability and suitability for long-term service in continuous fixed-bed reactors. Notably, prior studies⁵⁸ demonstrated that MgO can enhance the electron cloud density around active metal sites (e.g., Ni) and effectively suppress catalyst coking and poisoning. For the Ru/MgO–Al₂O₃-V catalyst, its outstanding catalytic performance is closely associated with MgO modification. This modification not only facilitates the formation of highly dispersed ultra-small Ru nanoparticles but also reduces the catalyst's acidity. The diminished acidity inhibits the cleavage of C–OH and C–C bonds, which leads to the formation of various hydrogenation by-products—thereby significantly improving the selectivity towards the target product.

Fig. 7 Stability of Ru/MgO–Al₂O₃-V catalyst. Reaction conditions: the amount of catalyst 500 kg, ethanol used as solvent, o-Cresol concentration 20 wt.%. H₂ 4.0–5.0 MPa, H/O (mol) 6–10 : 1, reaction temperature 115–135 °C, WHSV 0.25–0.45 h⁻¹.

4. Conclusion

A novel heterogeneous Ru-based catalyst featuring highly dispersed ultra-small Ru nanoparticles was successfully fabricated. This catalyst delivers exceptional catalytic performance in the continuous fixed-bed hydrogenation of o-cresol to o-methylcyclohexanol. Furthermore, investigations demonstrate that the catalyst also delivers outstanding performance in the hydrogenation saturation of other aromatic compounds while suppressing the hydrogenolysis and hydrogenation of functional groups, irrespective of the presence of aldehyde, ester, ether, or nitrile moieties on the aromatic ring. Overall, tailoring of the catalyst synthesis route and MgO modification stand as the pivotal elements responsible for the outstanding catalytic performance. In particular, MgO modification promotes the homogeneous dispersion of active centers, the formation of ultra-small Ru nanoparticles, and the precise fine-tuning of acid-base properties—thereby suppressing the generation of undesirable side reactions and highlighting its considerable potential for industrial implementation.

Author contributions

Yongjun Wang: conceptualization, data curation, writing – original draft, investigation, project administration, validation, visualization. Cheng Han: conceptualization, methodology, formal analysis, investigation, writing – review and editing, Funding acquisition. Xiaohan Wu: formal analysis, investigation, visualization. Minglin Xiang: investigation, project administration, resources, funding acquisition. Tao Liu: formal analysis, investigation, visualization. Xiaoshan Zhang: formal analysis, supervision, validation. Bing Wang: methodology, formal analysis, supervision, validation. Yingde Wang: conceptualization, supervision, methodology, formal analysis, investigation, writing – review and editing, funding acquisition.

Conflicts of interest

There are no conflicts to declare.

Data availability

All data generated or analyzed in this study are included in this article.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d5ra08531d.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 52202031), the National Key R&D Program (Grant 2021YFC3001103) and the Hunan Provincial Key R&D Project (No. 2023GK2076).

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