Benzaldehyde (BzH) has long been used in the chemical industry as an important fine chemical and intermediate (Guo et al., 2014). It plays an important role in our daily lives and is often used in the production of medical treatments, dyes, spices, resins, etc. (Cánepa et al., 2017). Studies in recent years have reported the transformation of benzyl alcohol (BnOH) to benzaldehyde by selective oxidation using molecular oxygen to avoid the production of toxic substances, with pure oxygen or even air used as oxidants under environmental-friendly conditions, thus meeting the concept of green chemistry (Feng et al., 2015; Tang et al., 2019).

In recent years, noble metal catalysts have attracted attention due to their high catalytic performances in a variety of heterogeneous reactions (Chen et al., 2021; Jia et al., 2022). Among them, Au catalysts have shown outstanding efficiency for the selective oxidation of primary alcohols compared to other noble metals, although mostly in the presence of an additional alkali (Yu et al., 2013; Adnan and Golovko, 2019). Ferraz et al. (2016) reported the use of different basic promoters to enhance the catalytic activity of Au/TiO₂ for the selective oxidation of benzyl alcohol. The presence of K₂CO₃ was indispensable for increasing BnOH conversion from 2% to 72%. The addition of alkali leads to challenges including environmental pollution and equipment corrosion, which hinder the use of Au catalysts in industrial-scale development.

To adapt to the demand for alkali-free processes, researchers have reported promotion strategies using basic metal oxides as supports or dopants; e.g., CeO₂, MgO, NiO, and CoO_x (Wang et al., 2015; Parmeggiani et al., 2017). The density of surface alkalinity can be tuned to benefit the fracture of C-H bonds with improved oxidation behavior for primary alcohols (Chen et al., 2010). Su et al. (2008) prepared a series of binary mesomorphic Ga_xAl_6-xO₉ solid solutions to support Au nanoparticles, which facilitated the oxidation of benzyl alcohol without any external alkali source. Xu et al. (2020) synthesized an Ni₃Al-LDH support to anchor small gold clusters via electrostatic adsorption and specific basicity for alkali-free oxidation.

In a previous study, we used carbon-supported gold nanoparticles in a colloid immobilization method (Luo et al., 2022a; Luo et al., 2022b). Our results showed that the surface of carbon materials could be easily tuned by introducing hetero-atoms, which provided rapid reaction rates and stability under appropriately designed surfaces and structures. However, the Au/C-based catalysts required the presence of a strong base such as NaOH or Na₂CO₃ to act as a proton acceptor for alcohol oxidation, although the carbon supports showed superiority with large reserves and controllable micro-surfaces. As an alternative, metal-organic frameworks (MOFs) have attracted interest in recent decades, in which a basic metal (oxide) can be confined in carbon layers (Lu et al., 2022), thus providing more possibilities for metal-modulated carbons as promising supports for gold in the alkali-free oxidation of primary alcohols. Zhu et al. (2014) prepared a Zr-MOF-coated Au/UIO-66 catalyst, which showed a 53.7% benzaldehyde yield at 80 °C. Liu et al. (2018) also suggested that the unique micro reactor type structure in Ag@Au/ZIF-8 improved AuNP dispersion and the reactant adsorption speed, revealing the advantages of MOF precursors.

Herein, we propose a new strategy to synthesize CoO_x-doped carbon-supported Au nanoparticles with structure promotion. Co-ZIF-67 was synthesized in the presence of Stöber silica to obtain carbon layer-encapsulated silica spheres. A well-developed mesoporous structure was obtained after NaOH etching. The resulting abundant-mesoporous carbon material could be used as an efficient support for gold nanoparticles, which displayed surprisingly high alkali-free activity for benzyl alcohol oxidation under very mild conditions (80 °C, atmospheric pressure). Characterization, including XRD, XPS, SEM, TEM, and N₂ adsorption-desorption, was performed for an in-depth understanding of the influences of the promoted structure and the chemical composition.

In this work, a new type of carbon material–supported Au catalyst with promoted structure was synthesized, as illustrated in Scheme 1. The XRD patterns of typical samples during the synthesis of Au/(Si)C are shown in Figure 1A. Silica was synthesized by the Stöber method and used as a hard template with monodispersed spheres approximately 90 nm in diameter (Figure 1B) according to previous work (Luo et al., 2013).

The SiO₂@ZIF sample synthesized in the presence of silica presented XRD patterns similar to those for the simulated ZIF-67 phase, suggesting the successful formation of the ZIF-67 structure (Zhou et al., 2020). After thermal treatment in Ar at 800 °C, most of the diffraction peaks disappeared in the SiO₂@ZIF-Ar, and two additional peaks at 2θ of 44.2° and 51.5° appeared, corresponding to the Co (111) and (200) crystal faces (PDF#15-0806) (Bai et al., 2019). Moreover, after thermal treatment in Ar, no CoO_x phase was detected, suggesting the full self-reduction of Co²⁺ in the high-temperature Ar atmosphere. After NaOH etching, the diffraction peaks of Co were greatly weakened and broadened. Co₃O₄ was observed at 2θ of 31.3°, 36.9°, 38.6°, 59.5°, and 65.3°, relating to the (220), (311), (222), (511), and (440) crystal planes of Co₃O₄ (PDF#76-1802), likely due to the re-oxidation of metallic Co exposed to both the strong basic solution and air. After gold loading and thermal calcination, all diffraction peaks broadened, suggesting the possible re-dispersion of metal species in the Au/(Si)C sample before and after calcination.

Typical SEM images (Figures 1B–E) also revealed the changes in morphology during synthesis. The SiO₂@ZIF prepared in the presence of silica spheres displayed irregular morphology, different from the commonly reported dodecahedron structure of ZIF-67 (Saliba et al., 2018). Small silica spheres were surrounded or inserted on the flat carbon blocks, forming a hybrid composition. After thermal treatment in Ar, the main structure shrank and surface spherical bulging was maintained in the SiO₂@ZIF-Ar. After NaOH etching, the removal of silica from the (Si)C-NaOH was evidenced by the weakened contrast of the spherical shells.

An additional Au/(Si’)C sample supported by 350 nm silica spheres (Supplementary Figure S1) was also synthesized for comparison. However, SEM images of the SiO₂(350nm)@ZIF showed phase separation of the ZIF structure from silica (Supplementary Figure S2), mainly caused by the incompatibility of the large spheres. In addition, large Au diffraction peaks were observed in the sample. The sharp diffraction peaks of CoO_x were also observed in the Au/(Si’)C (Figure 1A). Thus, the Au/(Si)C and Au/(Si’)C synthesized using silica spheres with different diameters greatly influenced the crystallization of metal particles. Stöber silica may act as a structure promoter and enhance the surface distribution of both Au and CoO_x, but only with the proper diameter.

TEM images of SiO₂@ZIF-Ar and (Si)C-NaOH are shown in Figures 2A and B. Combined with the CoO_x diffraction peaks in XRD, the black spots in these images could be ascribed to the co-existence of CoO_x particles after NaOH etching, as denoted by the dotted circles. However, it was surprising that the average particle size was much smaller: only 4.1 nm after Au loading and calcination of the Au/(Si)C. The CoO_x may have been re-dispersed during the etching and calcination process in the presence of Au nanoparticles, consistent with the broadened peaks in the XRD patterns. We observed no metal aggregates in the final Au/(Si)C. Small metal particles with an interplanar distance of 0.235 nm related to the Au (111) crystal plane (Luo et al., 2019) confirmed the surface distribution of the Au particles. The irregular sheet-like carbon structure (arrows in Figures 2C and D) was ascribed to the destroyed multilayer carbon shells after silica etching. The fine dispersion and re-arrangement of the metal species during calcination provided more possibilities to construct new interfaces between Au particles and Co^δ+ species in oxidative states, as revealed by their much-broadened XRD diffraction peaks.

Stöber silica was used as a hard template to promote a structure with better texture properties and diffusion effects for liquid-phase reactions. N₂ adsorption-desorption curves were studied to understand the porosity modified by the hard template. Figure 3 shows the isotherm profiles of several typical samples during the synthesis and their pore size distributions. All the tested samples displayed Type IV isotherms exhibiting a small H1 hysteresis loop, corresponding to the existence of mesopores (Zhao et al., 2019). The sharp increase at low p/p₀ (<0.1) confirmed the co-existence of micropores (Li et al., 2018). The pore sizes were in the range of 1–10 nm with mesopores dominating in each sample. The etching process caused an increase in the surface area from 152.8 m²/g in SiO₂@ZIF-Ar to 216.7 m²/g in (Si)C-NaOH, with the volume greatly increasing to 0.57 cm³/g in the latter sample (Supplementary Table S1). The subsequent loading of Au nanoparticles and calcination resulted in pore shrinkage or blockage by Au particles, with the surface area decreasing to 110.1 m²/g in Au/(Si)C (Yan et al., 2018).

XPS spectra of the (Si)C-NaOH support and the final Au/(Si)C samples were analyzed to understand their surface composition and the possible metal interactions in the gold catalyst. Spectra of the Au 4f core level are shown in Figure 4A. The characteristic peaks of Au 4f7/2 can be deconvoluted into two peaks at 84.3 eV and 85.5 eV, corresponding to surface Au⁰ and Au^δ+ species, respectively (Luo J. J. et al., 2022). The binding energy of Au⁰ in the Au/(Si)C shifted to a slightly higher value compared to the standard value in the literature (84.0 eV), which is commonly ascribed to electron transfer induced by metal-support synergy (Duan et al., 2020). The surface Au^δ+ species reached 16% in the Au/(Si)C, which is reportedly beneficial for many selective oxidation processes (Luo et al., 2017).

The XPS spectra of the Co 2p core level suggested the presence of both Co³⁺ and Co²⁺ species with binding energies of 780.2 and 782.1 eV, respectively, in both the (Si)C-NaOH support and Au/(Si)C (Wei et al., 2021). After gold loading and calcination, the percentage of Co³⁺ decreased slightly from 55% to 51% (Supplementary Table S2). Considering the clear peak shift of Au⁰ toward a higher binding energy and the generation of Au^δ+ species, Au likely specifically interacted with CoO_x on the support, especially neighboring fine CoO_x. Metallic Co⁰ species was not present in the tested samples, consistent with the XRD patterns. For better elucidation, O 1s spectra were also obtained (Figure 4C). The single broad peak was deconvoluted into three peaks at 530.0, 531.5, and 533.5 eV, which were related to the presence of lattice oxygen species in the metal oxide, oxygenated groups (C=O) on the carbon materials, and adsorbed –OH on the surface (Dong et al., 2020). The percentages of oxygen species (Supplementary Table S2) suggested the promoted formation of lattice oxygen in the Au/(Si)C sample, mostly provided by reducible CoO_x on the surface according to the XRD patterns. Surface lattice oxygen is frequently reported to contribute to the activation and supplementation of active oxygen species during oxidation (Ha et al., 2018). Considering the XRD and the XPS spectra, these findings suggest possible metal interactions and electron delivery between Au and CoO_x-doped carbon. After calcination, both the Au nanoparticles and CoO_x were redispersed on the carbon surface with abundant Au^δ+ and lattice oxygen species at a reasonable efficiency for the selective oxidation of alcohol.

The catalytic performances for alkali-free benzyl alcohol (BnOH) oxidation by different catalysts are shown in Figure 5. The as-synthesized Au/(Si)C successfully transformed BnOH at only 80 °C in the absence of alkali with 40.3% conversion. The selectivity of benzaldehyde (BzH) reached 90.4% with a yield of 36.4%. Pure carbon materials (e.g., activated carbon and carbon nanotubes) did not display any detectable activity in the absence of alkali. Au/ZIF and Au/CoO_x, without any structure promotion by Stöber silica, showed BnOH conversion rates as low as 28.5% and 22.3%, respectively, with poor yields toward BzH and other by-products such as toluene. Pure CoO_x–supported Au, the ZIF-67–supported gold, or the (Si)C-NaOH without gold loading did not perform as efficiently as the Au/(Si)C catalyst. The co-existing CoO_x species also affect active Au/(Si)C. We further treated the Au/(Si)C with an aqueous HCl solution to eliminate the influence of CoO_x; however, after CoO_x removal, the Au/(Si)C-Co only showed 19.2% BnOH conversion, illustrating the hindered catalytic process in the absence of CoO_x. The catalytic behavior of Au/(Si)C was greatly enhanced after structure promotion. The promoted structure of Au/(Si)C with finely dispersed metal species and modulated Au-support synergy demonstrated more significant effects on the catalytic performances.

The appropriate hard template for Au/(Si)C is 90 nm Stöber silica; however, the template diameter also requires consideration. Using 350 nm Stöber silica resulted in poor activity with only 27.1% BnOH conversion by Au/(Si’)C. The similar BnOH conversion of the Au/ZIF (without template) suggested the lack of usefulness of large silica spheres in Au/(Si’)C, mainly due to the incompatibility of such spheres with the ZIF-67 structure in the Au/(Si)C precursor, as shown in Supplementary Figure S2. Consequently, the XPS spectra showed no clear peak shift of Au⁰ (Supplementary Figure S4) and only a few lattice oxygen contributed by CoO_x.

The superior catalytic behavior of Au/(Si)C can be viewed as the combined result of the synergy between metals and the promoted structure. The former was reflected by the XPS and XRD results, with metal re-dispersion during synthesis and peak shifts with electron transfer from Au to the support. The fine dispersion of CoO_x on the surface was also revealed by the large amounts of lattice oxygen species, which could help activate and supplement the active oxygen species during BnOH oxidation. The largely promoted mesoporous structure caused by the etching of silica spheres created channels for reactant transportation and diffusion with faster reaction rates. After optimizing the reaction conditions, the alkali-free conversion of BnOH reached approximately 89.3% with a BzH yield of 74.5% at only 80 °C, if the BnOH/Au ratio was kept at 55 for economic reasons (Figure 5B). The catalyst was generally stable, with consistent catalytic performance (Figure 5C) for at least three reaction cycles without post-treatment after drying.

We synthesized a series of modified Co-ZIF-67 materials with tunable morphology to support fine Au nanoparticles for the aerobic oxidation of benzyl alcohol. Structure promotion was performed using Stöber silica as a hard template, which was removed by NaOH etching before gold immobilization. The texture structure of Au/(Si)C was greatly improved and revealed by the increased surface area and volume. Consequently, CoO_x was introduced into the carbon shell during the formation of the Co-ZIF-67 precursor. XRD, XPS, and TEM images demonstrated the redispersion of both Au and CoO_x as well as the electronic delivery between metals due to bimetallic synergy. Analysis of the chemical and surface composition suggested a surface rich in Au^δ+ with abundant lattice oxygen, which benefited the transformation rate of benzyl alcohol even in alkali-free conditions. The Au/(Si)C with finely dispersed Au particles showed excellent reactivity in the alkali-free environment, with 89.3% benzyl conversion and 74.5% benzaldehyde yield at only 80 °C.