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. 2025 Apr 10;14(6):e202400484. doi: 10.1002/open.202400484

Effects of Catalyst Synthesis Methods on the Physicochemical Properties of Silica‐Supported Au–Ru Bimetallic Catalysts and their Influence on the Oxidation of Phenols with H2O2

Tumisang Lekgetho 1, Matshawandile Tukulula 2, Letlhogonolo Fortunate Mabena 1, Mabuatsela Virginia Maphoru 1,✉
PMCID: PMC12138045  PMID: 40211580

Abstract

Herein, silica‐supported Au–Ru catalysts with 5% loading for each metal were prepared by microwave‐assisted loading (MW) and deposition (DP) methods. Au–Ru nanoparticles are obtained on MW‐5Au5Ru while short Au–Ru nanochains are obtained on DP‐5Au5Ru. The performance of the catalysts is tested through the oxidation of 2,3,5‐trimethylhydroquinone (TMHQ) and 4‐methoxy‐1‐naphthol (MNL) with H2O2, in which 2,3,5‐trimethyl‐1,4‐benzoquinone (TMBQ) and 4,4′‐dimethoxy‐2,2′‐binaphthalenylidene‐1,1′‐dione (BNP) are produced as main products, respectively. Catalytic data obtained for the oxidation of TMHQ demonstrate that the structures of the catalysts, type of solvent, and reaction temperatures used have a significant influence on the activities and selectivities of the catalysts. When MeOH and MeNO2 are used at room temperature (RT) in the oxidation of TMHQ on MW‐5Au5Ru catalyst, 58.2% and 100% conversions of TMHQ are achieved, respectively. Both MW and DP‐synthesized catalysts are highly active in the oxidation of TMHQ. Similar to TMHQ, the catalytic outcomes on the oxidative coupling of MNL highly depend on the temperature and structure of the catalyst. For example, 34% and 96% conversions of MNL are achieved at RT and 60 °C, respectively, over MW‐5Au5Ru catalyst in MeOH. However, MNL conversion of 82% is achieved on DP‐5Au5Ru catalyst in MeOH at RT.

Keywords: binaphthones, oxidations, phenols, quinones, Ru–Au catalysts

The catalytic oxidative conversion of 2,3,5‐trimethylhydroquinone and 4‐methoxy‐1‐naphthol using gold–ruthenium catalysts supported on SiO2 results in the formation of varying quantities of 2,3,5‐trimethyl‐1,4‐benzoquinone and 4,4’‐dimethoxy‐2,2’‐binaphthalenylidene‐1,1’‐dione, respectively. The catalysts, which are synthesized using microwave‐assisted loading and deposition techniques, exhibit varying morphological characteristics, which have a high impact on the product yields and selectivities.

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1. Introduction

Hydroxyarenes are key substrates in the synthesis of an array of biologically active organic compounds.[ 1 , 2 ] Their chemical reactions are essential in the fabrication of biaryl and triaryl aromatic structures, which are central building blocks for many synthetic and natural macromolecules.[ 3 ] Quinones, for example, are fundamental building blocks for a wide variety of biologically active compounds and a vast family of vitamins that are often used as antioxidants.[ 4 , 5 ] The reactions of hydroxyarene derivatives, such as phenols and binaphthols, are known to yield monoquinones, binaphthoquinones, binaphthols, and binaphtho‐furans.[ 6 , 7 ] Naphthols derivatives serve as key intermediates in the synthesis of biologically active synthetic and natural products such as dihydrodiosindigo B and diosindigo B (Figure  1 ).[ 8 ] Diosindigo B is a naturally existing binaphthone often extracted from the heartwood of Diospyros melanoxylon and used as a herbal remedy for the treatment of whooping cough, menstrual problems, and abdominal pains.[ 9 ] Doxorubicin and mitomycin C are natural and synthetic compounds that can be isolated from Streptomyces peucetius and Streptomyces caespitosus, respectively.[ 10 ] They are used for the treatment of breast, bladder and lung cancers, bone and soft tissue sarcomas. Moreover, they are employed in the treatment of ovary and acute lymphocytic leukemia and thyroid carcinoma, among other illnesses.[ 11 , 12 ] Some binaphthoquinones are used for the treatment of Candida albicans due to their antifungal activities.[ 13 ]

Figure 1.

Figure 1

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Examples of natural binaphthols, binaphthones, quinones, and binaphthoquinones.

Transition metals such as Ag(I), Cr(IV), Cu(II), Fe(III), Mn(III), and Pb(IV) were often used in their stoichiometric amounts in the oxidation of hydroxyarenes.[ 14 ] One typical example is the oxidation of phenols to 1,4‐benzoquinones using stoichiometric amounts of copper(I) chloride alongside O2.[ 15 ] The authors reported low substrate conversions and product selectivities for the reactions of mono‐ and unsubstituted phenols.[ 15 ] Kholdeeva and co‐workers also conducted the oxidative transformation of 2‐methyl‐1‐naphthol with O2 as an oxidant.[ 16 ] The authors obtained low‐to‐moderate yields of menadione due to the presence of several side products that lowered the selectivity of the target product. Separation of major products from the byproducts remains one of the major problems since it increases the production costs of these typical reactions. Furthermore, handling the hazardous reduced inorganic oxidants from stoichiometric oxidation reactions of phenolic compounds is one of the major environmental challenges.[ 17 , 18 , 19 ] Strategic transformation of these reactions using green methods that align with the United Nations‐Sustainable Development Goals is one of the major goals in the production of value‐added chemical compounds.[ 20 ]

The use of metal oxides and supported metals as catalysts for the oxidation of hydroxyarenes has been reported by other researchers over the past years.[ 21 , 22 ] Kholdeeva and co‐workers studied Au/C and Au/TiO2‐catalyzed oxidation of 2‐methyl‐1‐naphthol with O2 as an oxidant.[ 23 ] They achieved 2‐methyl‐1‐naphthol conversions that range between 79% and 94% given the reaction conditions used.[ 23 ] Jawale and co‐workers also obtained exceptional yields of the targeted 1,4‐benzoquinones in the oxidation of 1,4‐dihydroxybenzene derivatives using AuCNT (CNT: carbon nanotubes) in the presence of NaHCO3 and K2CO3 bases.[ 7 ] Previously, our research group reported on the coupling of 4‐methoxy‐1‐naphthol (MNL) and 2‐methyl‐1‐naphthol over Bi‐ and Sb‐promoted Pt/AC (AC: activate carbon) catalysts in various solvents using H2O2 as an oxidant.[ 24 , 25 , 26 ] It was observed that the structures of the catalysts, which were influenced by their synthesis method, have detrimental effects on their catalytic outcomes.[ 24 , 25 , 26 ] Furthermore, the reaction conditions, such as temperature and solvent, greatly impacted the outcomes of the catalytic reactions.[ 24 , 25 , 26 ] In addition, the monometallic 5%Pt/AC catalyst contained agglomerated particles in comparison to dispersed metal NPs obtained on the bismuth‐promoted catalyst.[ 25 ] This resulted in MNL conversion of 33.1% from 5%Pt/AC relative to 54.1% conversion obtained from 5%Pt‐5%Bi/AC under the same reaction conditions.[ 24 ] This shows the importance of promoters on the overall activity of the catalyst and also on the structural properties and stability of the catalyst. Recently, our group reported on the oxidation of TMHQ on 5%Ru‐5%Bi/AC catalysts with various metal loadings.[ 26 ] It was established, apart from metal loading, that the solvent and structural properties of the catalysts have an impact on the outcome of the catalytic reaction due to the interaction of the metals on the catalysts.[ 26 ] A yield of 86.5% of TMBQ was obtained in MeNO2 at room temperature (RT) from deposition (DP)‐prepared 5%Ru‐5%Bi/SiO2 catalyst relative to 98.6% yield obtained from microwave (MW)‐synthesized 5%Ru–5%Bi/SiO2 catalyst. It should be noted that the noble metals are active metal catalysts in these reactions and Bi serves as a promoter.[ 22 , 24 , 25 , 27 ] The use of noble metal alloys and bimetallic catalysts has been applied in many catalytic oxidation systems and found to be highly active due to the synergistic effects between the noble metals.[ 25 , 28 , 29 , 30 ] This study examines the influence of catalyst fabrication methods, MW‐assisted loading, and DP method, on the morphologies of silica‐supported 5Au5Ru catalysts. The influence of their morphologies on their catalytic outcome in the oxidation of TMHQ and MNL will be determined. Furthermore, the impact of solvents and reaction temperatures on substrate conversion, product yields, and selectivities will be thoroughly discussed in this article.

2. Results and Discussion

2.1. Catalyst Characterization

MW‐5Ru5Au and DP‐5Ru5Au catalysts were prepared by MW‐assisted loading and DP methods (Supporting Material 1, Supporting Information). The samples were characterized with X‐Ray diffraction (XRD), Brunauer–Emmett–Teller (BET) nitrogen physisorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy‐dispersive X‐Ray (EDX), X‐ray photoelectron spectroscopy (XPS), and inductively coupled plasma–optical emission spectroscopy (ICP‐OES) (Figure S2, Supporting Information). The XRD patterns for silica and silica‐supported catalysts shown in Figure  2 have diffraction peaks at 2θ = 20.64, 26.41, 36.35, 39.27, 40.69, 42.24, 45.59, 49.94, 54.71, 59.79, and 68.01°, which correspond to the (100), (101), (110), (012), (111), (200), (201), (134), (022), (211), and (122) lattice planes of crystalline hexagonal α‐SiO2 (quartz) support, respectively (COD 9 005 022). The XRD pattern of MW‐5Au5Ru possesses diffraction peaks at 2θ = 38.11, 44.32, 64.52, 77.52, and 81.63°, which represent (111), (200), (220), (311), and (222) lattice planes of face‐centered‐cubic (FCC) Au phase (COD 1 011 175), respectively. XRD peak reflections of Ru at 2θ = 38.30 and 77.55° on MW‐5Au5Ru are assigned to the hexagonal phase Ru (COD 1 539 052). Similar to the MW‐5Au5Ru, the XRD pattern of DP‐5Au5Ru in Figure 2 showed that it contains FCC and hexagonal phases of Au and Ru, respectively. There are no new peaks or shift in the diffraction patterns of either Ru or Au metals, which implies that the synthesized MW‐ and DP‐5Au5Ru is truly bimetallic.

Figure 2.

Figure 2

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XRD patterns of SiO2,[ 26 ] MW‐5Au5Ru, and DP‐5Au5Ru.

Nitrogen physisorption data and metal loadings for SiO2 and SiO2‐supported 5Au5Ru catalysts prepared by MW‐assisted loading and DP methods are given in Table  1 . The BET SA (Surface area) of the commercial SiO2 support was found to be 6.6 m2 g−1. There was no significant change observed on the surface area of MW‐5Au5Ru from the support, but its average pore diameters (APD) increased slightly after metal loading. This is probably due to the blockage of the smaller pores by metal NPs, which are most likely to target the small pores and leaving large pores unoccupied. The SEM and TEM analysis of this catalyst in Figure  3a shows that it contains small metal NPs with the average particle size (APS) of 2.58 nm. Contradictory to MW‐5Au5Ru, an increase in the BET SA was observed on DP‐5Au5Ru relative to the SiO2 support. The considerable increase in the BET SA on this catalyst may be due to the presence of the agglomerated nanochains that are isolated from the support, which may be adsorbing nitrogen (see SEM and TEM results in Figure 3b). Jin and co‐workers prepared Pd–Ni chains and obtained the surface areas of 39.2, 37.6, 67.1, and 72.3 m2 g−1 for Pd–Ni chains with molar ratios of 1:3, 2:3, 1:1, and 3:1, respectively.[ 31 ] On the DP‐5Au5Ru, the surface area contribution from the chains is very low due to low metal loading on this catalyst. In addition, the catalyst fabrication method has the potential to increase the BET SA of the catalyst relative to that of the support due to the restructuring or formation of amorphous silica on the support surface during the catalyst preparation step.[ 32 ] This was also observed on the MW‐prepared Ru‐Bi/SiO2 catalysts previously reported by us.[ 26 ] Metal loadings on both catalysts are close to the target value of 5% for each of the metals (Table 1).

Table 1.

Nitrogen physisorption data of silica and silica‐supported 5Au5Ru catalysts (drying temperature: 120 °C, drying time: 24 h).

Catalyst BET SA [t]‐Plot [S] BJH [S] PV APD Ru Au
[m2 g−1] [m2 g−1] [m2 g−1] [cm3 g−1] [nm] [mass%]
Silica 6.6 0.55 5.1 0.03 20.6 – –
MW‐5Au5Ru 6.4 0.29 4.4 0.03 24.9 4.5 4.6
DP‐5Au5Ru 10.2 0.23 8.6 0.05 22.8 4.8 4.8
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Figure 3.

Figure 3

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SEM micrographs, TEM micrographs, and PSD curve of a) MW‐5Au5Ru and b) DP‐5Au5Ru catalysts (PSD of DP‐5Au5Ru cannot be calculated for DP‐5Au5Ru due to its nanochain morphology).

The SEM micrographs of the MW‐5Au5Ru catalyst in Figure 3a i show the presence of spherical Au and Ru NPs that are dispersed on the surface of the support. Its TEM images further confirmed this observation (Figure 3a ii). The APS of Au–Ru NPs on MW‐5Au5Ru was found to be 2.58 nm with a narrow particle size distribution (PSD) that ranges between 1.3 and 4.2 nm (Figure 3a iii). The presence of both Au and Ru was confirmed by SEM–EDX imaging (Figure  4b and Figure S3, Supporting Information) and “point and shoot” analysis (Figure S4, Supporting Information). EDX imaging analysis (Figure 4a) showed that Ru dispersed on the surface of the support with agglomerated Au particles (Figure 3a ii). The Au agglomerated particles were also observed on its TEM image (Figure 3a ii). Contrary to what was observed on the SEM images of the MW‐5Au5Ru, DP‐5Au5Ru (Figure 3b i) contains mostly isolated nanochains that consist of clusters of interlaced Au–Ru particles on the catalyst surface. The short nanochains containing Au and Ru species were also confirmed by TEM analysis in Figure 3b ii,biii. It is clear from the TEM images that some of the nanochains are completely isolated from the support materials and they are likely to adsorb nitrogen during the nitrogen physisorption analysis. This explains the slight increase in the BET SA obtained on DP‐5Au5Ru relative to the support (Table 1). The EDX “point and shoot” and imaging analysis in Figure  5a,b and Figure S5, Supporting Information) confirmed that the nanochains contain both Au and Ru species. The striking morphological differences between MW‐ and DP‐5Au5Ru catalysts demonstrate the impact of catalyst preparation methods on the final structure of the catalysts. Therefore, it can be concluded that MW preparation method promoted high metal dispersion than DP method. The formation of nanochains was also observed on 5%Pt‐5%Sb/AC and 5%Ru‐5%Bi/SiO2 catalysts that were prepared by DP methods, while highly dispersed metal nanoparticles were obtained on MW‐synthesized 5%Pt‐5%Sb/AC catalyst and less‐prominent Ru–Pd nanochains or agglomerated particles were obtained on 5%Ru‐5%Bi/SiO2.[ 25 , 26 ] Catalysts preparation by conventional DP methods are known to support the agglomeration of particles while MW‐based catalyst preparation methods are known to increase metal dispersion.[ 25 , 33 , 34 , 35 ]

Figure 4.

Figure 4

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a) EDX imaging and b) EDX spectrum of MW‐5Au5Ru catalyst.

Figure 5.

Figure 5

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a) EDX “point‐and‐shoot” and b) EDX imaging of DP‐5Au5Ru catalyst.

The XPS survey spectrum of MW‐5Au5Ru in Figure  6a complements the EDX analysis in Figure 4. The prominent signal at binding energy (BE) of 103.21 eV is characteristic of Si 2p electrons of silica, which validates the existence of the silicon backbone in the catalyst support (Figure 6b). The BEs of 280.88 and 282.53 eV for two deconvoluted Ru 3d5/2 peaks were detected for two distinct Ru species (Figure 6c). Ru BEs between 280.9 and 281.2 eV are characteristic of RuO2,[ 36 ] while weak peaks at higher BEs of 282.3–282.5 eV are characteristic of oxides of Ru in a higher oxidation state such as RuO3.[ 37 , 38 ] The signals with BE around 285.0–285.4 eV and 286.4–286.6 eV are characteristic of Ru 3d3/2 electrons in RuO2 and RuO3, respectively. The XRD patterns of this catalyst in Figure 2 confirmed the hexagonal phases of the reduced Ru metal present. This implies that the bulk sample contains metallic Ru with Ru metal oxides on its surface. It is widely known that metallic Ru oxidizes readily on its surface, even at RT, to form stable Ru oxide species.[ 39 ] Hence, the observation made in Figure 6c is common. The low intensity doublet peaks were observed at BEs of 463.54 and 486.06 eV for Ru 3p3/2 and Ru 3p1/2, respectively. They indicate the presence of oxidized Ru species, RuO2, on the catalyst surface.[ 40 ] The C 1s peaks on the XPS spectrum of MW‐5Au5Ru are due to carbon that originates from organic solvents, often present in high quantities during the catalyst synthesis process. In this case, the source of carbon may be coming from acetone used during the catalyst synthesis process or other carbon contaminants from the air. There are no peaks observed for Au on the XPS survey spectra of the MW‐5Au5Ru catalyst. The difficulty in detecting Au by XPS may be due to its poor dispersion (Figure 6a) and uneven distribution of its particles over the catalyst surface as shown on its TEM and EDX analysis (Figure 3a ii,4 and Figure S3, Supporting Information). The detection of an oxygen signal at 529.90 eV confirms the existence of silica and supported metal oxides on this catalyst (Figure 6d). Similar to MW‐5Au5Ru, the same Ru metal and oxides species were observed on DP‐5Au5Ru catalyst. The XPS spectrum of Au 4f for the DP‐5Au5Ru catalyst shows double bands at 83.9 and 87.6 eV BE for 4f7/2 and 4f5/2 electrons (Figure  7c) that correspond to the BE of pure bulk Au.[ 41 ] This indicates that the electronic interactions between SiO2 and Au are weak since the BE of 4f7/2 is close to that of bulk gold metal.[ 42 ]

Figure 6.

Figure 6

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a) XPS survey spectrum and high‐resolution spectra of b) Si 2p, c) Ru 3d, C 1s, and d) O 1s for MW‐5Au5Ru.

Figure 7.

Figure 7

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a) XPS survey spectrum and high‐resolution spectra of b) Ru 3d, C 1s c) Au 4f, and d) O 1s for DP‐5Au5Ru.

2.2. Oxidation of 2,3,5‐Trimethylhydroquinone and 4‐Methoxy‐1‐Naphthol

The substrates, 2,3,5‐trimethylhydroquinone (TMHQ) and 4‐methoxy‐1‐naphthol (MNL), were oxidized with H2O2 over MW‐ and DP‐5Au5Ru catalysts (Scheme  1 ) according to the procedure described in Figure S6, Supporting Information.[ 26 ] Product obtained for the oxidation of TMHQ is 2,3,5‐trimethyl‐1,4‐benzoquinone (TMBQ) while MNL gave 4,4′‐dimethoxy‐2,2′‐binaphthalenylidene‐1,1′‐dione (BNP). The reactions were carried out in either MeOH or MeNO2 at RT and/or in refluxing solvent. The conversions, selectivities, and yields obtained in these reactions are reported in Table  2 , Figure  8 and  9 .

Scheme 1.

Scheme 1

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Oxidation of a) TMHQ and b) MNL over SiO2‐supported 5Au5Ru catalysts.

Table 2.

Oxidation of TMHQ and MNL on SiO2‐supported 5Au5Ru catalysts in MeOH or MeNO2.

Entry Catalyst substrate Solv. T [°C] t [h] Conv. [%] Sel. [%]
TMBQ BNP
1 a) MW‐5Au5Ru TMHQ MeOH r.t 2 58.2 91.3
2 a,b) DP‐5Au5Ru TMHQ MeOH r.t 2 100 92.9
3 c) MW‐5Au5Ru TMHQ MeOH 60 3 100 100
4 a) DP‐5Au5Ru TMHQ MeOH 60 2 100 97.9
5 a) MW‐5Au5Ru TMHQ MeNO2 r.t 2 100 98.9
6 DP‐5Au5Ru TMHQ MeNO2 r.t 2 100 100
7 MW‐5Au5Ru TMHQ MeNO2 96 2 100 100
8 a) DP‐5Au5Ru TMHQ MeNO2 96 2 100 99.3
9 MW‐5Au5Ru (used) TMHQ MeNO2 96 2 100 100
10 MW‐5Au5Ru MNL MeOH r.t 2 34.0 100
11 d,e) DP‐5Au5Ru MNL MeOH r.t 1 82 88.7
12 MW‐5Au5Ru MNL MeOH 60 1 96 100
13 e) DP‐5Au5Ru MNL MeOH 60 1 ≤99.0 90.2
14d, e,f) MW‐5Au5Ru MNL MeNO2 r.t 1 47.1 52.0
15c, d,e) MW‐5Au5Ru MNL MeNO2 96 1 ≤99.0 55.0
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a)

A brown polymeric compound was identified (13C NMR);[ 26 ]

b)

A purple minor product was observed;

c)

Conversion of MNL after 2 h was very low;

d)

Multiple minor byproducts with comparable retention factors were not separated by column chromatography;

e)

Traces of a brown product were obtained;

f)

Trace amounts of BNL (binaphthol) was formed.

Figure 8.

Figure 8

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Conversions and yields obtained for the oxidative transformation of TMHQ to TMBQ on 5Au5Ru catalysts in different solvents and under different reaction temperatures (refl.: reflux).

Figure 9.

Figure 9

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Conversions and yields obtained for the catalytic oxidation of MNL to BNP over silica‐supported 5Au5Ru catalysts in different solvents and under different reaction temperatures.

Exceptional catalytic activities were obtained when TMHQ was oxidized on either MW‐ or DP‐5Au5Ru catalyst. A complete conversion of the TMHQ on both catalysts was obtained (Table 2, entries 2‐8) except for MW‐5Au5Ru in MeOH at RT, in which only 58.2% of TMHQ was converted to TMBQ (Table 2, entry 2). Both catalysts gave relatively comparable yields and selectivities for TMBQ (Table 2, Figure 8). Despite its high selectivity for TMBQ, MW‐5Au5Ru yielded a low amount of TMBQ due to the partial conversion of TMHQ (Table 2, entry 1 and Figure 8). The DP‐5Au5Ru catalyst is prone to minimal side reactions that led to the formation of multiple side products in comparison to MW‐5Au5Ru (Table 2, entries 2, 4 and 8). The emergence of byproducts in the reaction carried out on DP‐5Au5Bi led to a slight reduction of TMBQ selectivity on DP‐5Au5Ru. The SEM and TEM images of this catalyst showed that some of the Ru–Au nanochains, which are the active species, are not in contact with the support (Figure 3b). This surface heterogeneity may lead to diverse active sites with a wide spectrum of activities.

In the oxidation of MNL, both MW and DP‐5Au5Ru catalysts in MeOH at RT and at 60 °C always gave BNP as the major product (Table 2, entries 10‐15, Figure 9). Although high selectivities of BNP were attained on MW‐5Au5Bi catalyst, high MNL conversions were obtained on DP‐5Au5Ru catalyst in MeOH (Figure 9). The same trend was also observed for the oxidation of TMHQ (Figure 8). The selectivity in the oxidation reactions of MNL is affected by the multiple side products emerging from parallel reactions, which results in poor product selectivity. Maphoru and co‐workers also observed the formation of side products in the oxidative coupling of MNL with hydrogen peroxide over 5%Pt–5%Bi/AC catalyst.[ 32 ]

It is evident from the results obtained for the oxidation of TMHQ and MNL on MW‐ and DP‐5Au5Ru that the structure of the catalyst has a solid influence on the outcome of the reaction, especially in the oxidation of MNL. Between the two catalysts prepared by MW and DP methods, DP‐5Au5Ru catalyst proved to be the most active catalyst in comparison to MW‐5Au5Ru (Table 2, entries 2, 6 and 11). However, the MW‐5Au5Ru catalyst is more selective compared to the most active DP‐5Au5Ru catalyst. The selectivity on DP‐5Au5Ru was lowered by the formation of multiple side products on the catalyst surface. The occurrence of side reactions from the most active catalyst is quite common in these typical reactions.[ 6 , 24 ] In the study conducted on the oxidative coupling of 2‐methyl‐1‐naphthol on 5%Pt‐5%Bi/AC, it was established that the association of Pt and Bi has a direct impact of the outcome of the reaction. 5%Pt‐5%Bi/AC with associated metal were highly active due to the electronic effects and synergistic effects between the two metals.[ 25 ] In this study, both catalysts contain Au and Ru, which are active metal catalysts in these reactions, but with different structural properties. DP‐5Au5Ru contains Au and Ru metals that are associated in the nanochains (Figure 3, 4, 5). Therefore, electronic and synergistic effects are at play between Ru and Au, which may be responsible for the high MNL conversions on this catalyst. This may be through an increase in the electron density on Au from Ru. However, a highly active catalyst is often prone to side reactions, as it was observed on DP‐5Au5Ru (Table 2). In addition, XPS and XRD analysis showed that there is a presence of oxidized Ru species on the catalyst surface with Ru metal in the bulk sample (Figure 2 and 7b ). Different metal species may possess different catalytic activities and routes, which are expected to reduce the selectivity of the reaction. Multiple reaction routes on the mechanism that lead to formation of monoquinones, binaphthofurans, and polymeric compounds are very common in the reactions of phenolic compounds.[ 6 , 43 , 44 , 45 , 46 , 47 ] In contrast to DP‐5Au5Ru, some Au agglomerates were dissociated from Ru NPs on MW‐5Au5Ru, which might have reduced the activity of the catalyst. Furthermore, Au is not very effective in dissociating hydrogen peroxide to peroxy and hydroxy radicals that are required for the oxidation of phenoxy and naphthoxy radicals.[ 22 , 37 ] Similar to the dissociation of hydrogen peroxide, it was previously proposed that the coupling of MNL occurs when the active metal catalyst cleaves the hydroxy group of the naphthol by abstracting its hydrogen.[ 22 ] This is followed by the stabilization of the naphthoxy radicals by chemisorption on the metal surface. The ability of the metal to cleave and chemisorp the intermediates phenoxy and naphthoxy radical species is key to the activity and selectivity of the reaction. Therefore, partial isolation of Au from the ruthenium particles in this catalyst may be one of the contributing factors towards its low catalytic activities since each metal will function on its own. This may, in general, affect the kinetics and also the reaction route. Most of the oxidation reactions of the phenolic compounds follow this reaction mechanism.[ 6 , 43 , 44 , 45 ]

Solvents have a significant influence in the oxidation of TMHQ on both catalysts. It was established that MeOH often favors the formation of side products than MeNO2 (Table 1, entries 1‐8). Overall, conversions, yields, and selectivities obtained for this reaction are relatively comparable for both solvents, even though MeNO2 is slightly better than MeOH. It can be concluded that MeNO2 is the best solvent for oxidation reactions of TMHQ since it offers complete conversions with selectivities of over 99% (Table 2) on both 5Au5Ru catalysts at RTand under reflux. MeOH has a higher donor number and polarity than MeNO2.[ 48 , 49 ] The donor number (DN) is used to determine the basicity of the solvent or its ability to donate electrons.[ 50 , 51 ] MeOH has a DN of 19.0 kcal mol−1, which is significantly higher that the DN of 2.7 kcal mol−1 for MeNO2. Oxidation of phenolic compounds often takes place in basic conditions. This was demonstrated by Jawale and co‐workers in the oxidation of 1,4‐dihydroxybenzene using AuCNT catalysts.[ 7 ] Solvents with high donor numbers are expected to increase the basicity of the reaction medium, which promotes the oxidation of hydroxyarenes.[ 7 , 38 ] Furthermore, solvents with high polarity are known to stabilize the intermediates in the oxidation reactions and therefore facilitate the formation of the products.[ 49 ] However, highly oxidative environments promote the formation of byproducts from side reactions, overoxidation, and multiple coupling reactions, which result in product selectivity reduction.[ 6 , 25 ] A similar trend was noted in the oxidation of MNL when reactions were conducted under identical conditions (Table 2, compare entries 10 and 12 to 13 and 14). Although low conversions were observed at RT over MW‐5Au5Ru catalyst in both MeOH and MeNO2, the conversions obtained in MeNO2 were higher than those obtained for reactions carried out in MeOH as a solvent (Table 2). Takeya and co‐workers[ 52 ] reported that for SnCl4‐mediated coupling reactions of 1‐naphthols, solvents with low donor numbers proved to be particularly favorable, which is a trend observed in the comparison between MeOH and MeNO2 in this reaction. However, higher yields and selectivities of BNP were obtained in MeOH than in MeNO2. The formation of multiple minor side products resulting from coupling reactions leads to low selectivities obtained in MeNO2 (Table 2). The reduction of the BNP yield in MeNO2 is due to the stabilization of intermediate species such as binaphthol (BNL), which was also observed in the oxidation of MNL on 5%Pt‐5%Bi/AC catalyst.[ 6 ] BNL is the intermediate product toward the formation of BNP.[ 22 ]

It is very common to obtain high substrate conversion but with low product selectivities in reactions that are carried out at high temperatures due to side reactions competing with the formation of the target product.[ 6 ] This was the case for the oxidation of MNL in refluxing MeOH (Table 2, entries 9‐12). There was an insignificant increase in BNP selectivity when the reaction was carried out under refluxing MeNO2 on MW‐5Au5Ru (Table 2, entries 13‐14). On the other hand, an increase in temperature in the oxidation of TMHQ led to a minimal increase in the selectivity of TMBQ (Table 2, entries 1‐8). A used MW‐5Au5Ru catalyst was tested for the oxidation of TMHQ under refluxing MeNO2 and 100% conversion of TMHQ was obtained with a 100% yield of TMBQ (Table 2, entry 9). This shows that this catalyst is stable and recyclable, and can be reused without or with minimal loss of its activity.

3. Conclusion

The structural properties of the catalysts vary according to their fabrication methods. Both MW‐5Au5Ru and DP‐5Au5Ru contain FCC Au and hexagonal phases of Ru metal in their bulk samples with ruthenium metal oxides on their surfaces (XRD and XPS studies). MW‐5Au5Ru has agglomerated Au particles with a high Ru metal dispersion and APS of 2.58 nm while DP‐5Au5Ru contains interlaced Au–Ru nanochains on the surface of the silica support. Furthermore, the catalyst preparation methods appear to influence the BET surface areas, as evidenced by the increase in the surface area of the DP‐5Au5Ru catalyst relative to the support, which is possibly attributable to the restructuring or formation of amorphous silica on the surface of the support during the catalyst preparation step. An increase in the surface area could also be possibly due to the isolated agglomerated Au–Ru nanochains present on this catalyst (SEM and TEM).

The oxidation of TMHQ and oxidative coupling of MNL were carried out over silica‐supported 5Au5Ru catalysts prepared by MW or DP, with H2O2 as an oxidant to yield TMBQ and BNP as products, respectively. Both MW‐5Au5Ru and DP‐5Au5Ru exhibited outstanding catalytic activities in the oxidation of TMHQ, in which TMHQ conversions of over 100% were achieved. However, MW‐5Au5Ru catalyst is a notable exception with TMHQ conversion of 58.2% in MeOH at RT. The catalytic activity data indicated that reactions in both MeOH and MeNO2 promote high TMHQ conversions, with MeNO2 providing slightly higher yields and selectivities of TMBQ than MeOH. The slight reduction of TMBQ selectivities in MeOH is due to the formation of byproducts, which seemed to be prominent in MeOH than in MeNO2. This may be due to the high donor number and polarity of MeOH. For the oxidation of MNL, low conversion of MNL and low yields of BNP at RT for both DP‐5Au5Ru and MW‐5Au5Ru in MeOH were obtained. Increasing the temperature from RT to 96 °C in MeNO2 increased MNL conversion but did not enhance the selectivity of the BNP significantly. In general, it can be concluded that DP‐5Au5Ru catalyst is more active than MW‐5Au5Ru catalyst. Furthermore, MeOH supports the formation of byproducts in comparison to MeNO2 in the reaction of TMHQ. However, the opposite was observed in the oxidative coupling of MNL. It can therefore be concluded that the interaction of the solvent with the substrate is one of the key influences in the catalytic outcome of the reaction.

In conclusion, the catalyst synthesis method, solvent, and reaction temperature (RT or reflux) have an influence on the outcome of the reaction. These parameters impact substrate conversion, as well as the yield and selectivity of the intended product. The silica‐supported 5Au5Ru catalysts utilized in this study are highly efficient for the production of TMBQ from TMHQ, as well as BNP from MNL. This catalytic approach is anticipated to play a significant part in creating more chemically viable and ecologically friendly strategies for the production of chemicals with economical value.

Supporting Information

The materials that were used to support the findings of this study are given in the supporting information file.

Conflict of Interest

The authors declare no conflict of interests.

Supporting information

Supplementary Material

OPEN-14-e202400484-s001.pdf (210.6KB, pdf)

Acknowledgements

The authors’ sincere gratitude goes to the CSIR National Centre for Nano‐Structured Materials staff, Sharon Eggers, Rhandzu Rikhotso, and Solly Motaung, for microscopy characterization and Brunauer–Emmett–Teller analysis, as well as Werner Jordaan from NMISA for the X‐ray photoelectron spectroscopy analysis. The authors are also grateful for the financial support received from National Research Foundation of South Africa through Dr. MV Maphoru's NRF‐Thuthuka Funding with grant no: 121881, as well as TUT for the infrastructure, resources, and financial support. T.L. is thankful for the bursary he received from NRF Thuthuka Grantholder‐Linked Masters Scholarship Program.

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Supplementary Material

OPEN-14-e202400484-s001.pdf (210.6KB, pdf)

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.

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