Porous alumina nanosheet-supported asymmetric platinum clusters for efficient diboration of alkynes†

Yan Gao‡ ^a, Huilong Geng‡ ^b, Jinlong Ge ^a, Linlin Zhu ^a, Zhiyi Sun ^b, Ziwei Deng ^b and Wenxing Chen *^b
aAnhui Provincial Engineering Research Center of Silicon-based Materials, Bengbu University, Bengbu 233030, China
^bEnergy & Catalysis Center, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China

First published on 22nd August 2024

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Noble metal catalysts supported on suitable materials are crucial for various significant chemical reactions because of their exceptional reaction activity and selectivity.^1–4 However, the high cost and limited availability of these metals have led to efforts to reduce their use.^5,6 Cluster catalysts have become a popular area of study in the field of catalysis due to their higher reactivity compared to traditional supported metal nanoparticle (NP) catalysts.^7–9 Clusters catalysts exhibit distinct chemical and physical properties that hold potential for novel applications and enhancing the atom efficiency of noble metals. Moreover, the asymmetric cluster structure can provide more active sites, improve the reaction rate, and thus improve the activity of the catalyst.^10,11 Additionally, the interaction between the metal and supports can significantly affect catalytic performance, where factors such as electron transfer, dispersion, stability, and surface properties all play crucial roles.¹² Nevertheless, synthesizing these catalysts is challenging because they are highly mobile and susceptible to coalescence during their operation, especially when dealing with oxide-based cluster catalysts.^13,14 Although impregnation is the primary synthesis strategy for oxide-based cluster catalysts, stabilizing these catalysts using a defect-rich nonreducible oxide support has been rarely achieved and is not widely reported.^15,16

In many fields, including materials science, organic synthesis, and biomedicine, boronic acids and their derivatives are highly valuable chemical compounds.^17–19 In recent decades, several supported transition metals have been employed as catalysts for the production of these compounds. One notable approach is Pt-catalyzed diboration of alkynes, extensively studied since its development by Miyaura and Suzuki et al.^20,21 However, despite the development of numerous homogeneous transition-metal catalysts, there has been limited focus on the creation of heterogeneous catalysts for diboration reactions.²² Homogeneous catalysts offer excellent catalytic activity, high selectivity, and atom efficiency, but their practical application in industry is limited due to challenges in recycling and product separation.²³ Conversely, heterogeneous catalysts demonstrate stability and high activity but often lack selectivity and atom efficiency.²⁴ In this context, cluster catalysts provide a promising solution by merging the benefits of heterogeneous and homogeneous catalysts.²⁵ These catalysts immobilize the active centers of homogeneous catalysts onto insoluble supports, and possess the benefits of stability, high efficiency, reproducibility, and multiple active sites, providing higher selectivity and catalytic activity, reducing the amount of catalyst used and the reaction temperature, and improving reaction efficiency. The crucial requirements for diboration reactions include the development and utilization of asymmetric cluster catalysts that possess high stability, exceptional catalytic activity, and convenient recyclability.²⁶

In this study, we prepared asymmetric Pt cluster catalysts supported on defective porous alumina nanosheets (Pt clu/dp-Al₂O₃) using a pyrolysis technique coupled with the wet impregnation method. Al₂O₃, known for its cost-effectiveness, high stability, and environmental friendliness, is a commonly favored option for catalyst supports in various environmental and industrial fields. The Pt clu/dp-Al₂O₃ catalyst demonstrated excellent efficiency in alkyne diboration, with the conversion yield and selectivity reaching up to 97% and 95%, correspondingly. Thorough analysis using X-ray techniques such as X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and X-ray photoelectron spectroscopy (XPS) indicates that the enhanced catalytic activity and cycling performance result from the heightened activity site, the robust anchoring impact of defective porous alumina nanosheets on asymmetric Pt, and the combined effect between Pt clusters and the dp-Al₂O₃ support at the atomic interface.

The crystal AlOOH nanosheets were first synthesized by a solvothermal method to prepare the catalyst with asymmetric Pt clusters anchored to the defect rich porous alumina sheets (Fig. 1(a)). Based on observations using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) (Fig. S1, ESI†), it can be inferred that the AlOOH exhibits a homogeneous nanosheet structure. The XRD patterns (Fig. S2, ESI†) reveal the presence of a well-crystallized AlOOH nanosheet structure. To select the optimal pyrolysis temperature while maintaining the morphology of the nanosheets, we performed TGA (thermogravimetric analysis) and the heating rate is 10 °C min⁻¹ (Fig. S3, ESI†). We can see that the TGA curve revealed a 5% weight reduction between 20 and 100 °C, attributed to the evaporation of physically absorbed H₂O (stage A). Stage B represents the desorption of molecules from the surfaces of 1-butanesulfonic acid sodium salt, leading to a cumulative weight reduction of 25%. Stage C was associated with the boehmite phase changing to γ-Al₂O₃, resulting in an overall weight decrease of 18%, aligning with the expected weight loss when transitioning from γ-AlOOH to γ-Al₂O₃. In light of these results, we selected 500 °C as the appropriate pyrolysis temperature. The pyrolysis of AlOOH nanosheets yielded highly crystalline porous Al₂O₃ nanosheets (Fig. S4, ESI†). Fig. S5 (ESI†) presents the nitrogen adsorption–desorption isotherms and pore-size analysis for the obtained alumina nanosheets. The BET specific surface area of Pt clu/dp-Al₂O₃ is 249.8 m² g⁻¹. The pore diameter distributions of dp-Al₂O₃ aligned with TEM analysis, around 3.1 and 12.9 nm. These findings further demonstrate that the acquired Al₂O₃ displays favourable porous characteristics and high surface area, strongly stabilising asymmetric platinum clusters.

Utilizing the calcination method coupled with a feasible adsorption (Fig. 1(a)), the Pt clusters are loaded onto porous alumina nanosheets rich in defects. The Pt clu/dp-Al₂O₃ catalyst was characterized using various techniques. The TEM image of the catalyst (Fig. 1(b)) revealed a porous nanosheet morphology, with many fine and visible Pt clusters on it. The XRD (Fig. S6, ESI†) analysis confirmed the presence of the (311) plane on the surface of the dp-Al₂O₃. The size distribution of the measured Pt species ranged from 0.74 nm to 2.33 nm (Fig. 1(c)). The average Pt particle size is estimated at 1.40 nm, suggesting that the Pt clusters could enhance catalytic activity by increasing the surface area of Pt and improving the Pt utilization efficiency. Energy dispersive X-ray spectroscopy (EDS) results indicated an even distribution of Pt, Al, and O throughout the architecture (Fig. 1(d) and Fig. S7, ESI†). Additionally, the Pt loading in Pt clu/dp-Al₂O₃ was measured to be 1.6 wt% using inductively coupled plasma optical emission spectrometry (ICP-OES). FT-IR spectra (Fig. S8, ESI†) of various samples, including AlOOH, dp-Al₂O₃, Pt NPs/c-Al₂O₃, and Pt clu/dp-Al₂O₃, exhibited characteristic bands corresponding to specific vibrations and stretching modes. The band at 1070 cm⁻¹ corresponds to the stretching vibrations of Al–O–H. The peaks at 3,447 and 1,630 cm⁻¹ are associated with the stretching and bending vibrations of the –OH groups, and the peak at 574 cm⁻¹ corresponds to the Al–O–Al vibrations.²⁷

Additional confirmation of the coordination distribution of Pt clu/dp-Al₂O₃ was provided by X-ray absorption spectrometric (XAS) studies. Fig. 3(a) exhibits the Pt L₃-edge X-ray absorption near edge structure (XANES) spectra of the Pt clu/dp-Al₂O₃ and the references (Pt foil, and PtO₂). The absorption edge in the spectra provided information on the oxidation state, with a decreasing trend observed at approximately 11567 eV in the order of Pt foil < Pt clu/dp-Al₂O₃ < PtO₂. The above results indicated a positive charge on the Pt species in the Pt clu/dp-Al₂O₃ catalyst. Furthermore, Fig. 2(b) depicts the Pt L₃-edge Fourier transformed (FT) k³-weighted extended X-ray absorption fine structure (EXAFS) spectra of Pt clu/dp-Al₂O₃, along with the reference spectra of PtO₂ and Pt foil. The spectra of Pt clu/dp-Al₂O₃ (Fig. 2(b)) displayed an enhanced peak at 1.7 Å with a lower peak at 2.4 Å in R space. The enhanced peak came from the contribution of Pt–O, and the lower peak corresponding to the Pt–Pt bond contribution revealed the existence of Pt clusters in Pt clu/dp-Al₂O₃. To further clearly conform the Pt clusters, wavelet transform (WT) analysis was conducted on the Pt L₃-edge EXAFS oscillations, as it offers high resolution in both R and k space. By analyzing the wavelet transform (WT) EXAFS results of PtO₂ and Pt foil (Fig. 2(d)), it was evident that the intensity peaks at 6.4 and 12.4 Å corresponded to the Pt–O and Pt–Pt bonding. Notably, two high-intensity zones at 6.0 and 10 Å were observed in the WT EXAFS results of Pt clu/dp-Al₂O₃, indicating the presence of Pt–O and Pt–Pt bonding. Quantitative analysis using EXAFS fitting was performed to extract the structural parameters (Fig. 2(c), (e), Fig. S9 and Table S1, ESI†), revealing the asymmetry in the internal structure of the Pt clusters. The fitted structural parameters indicate that there is a difference in bond length between the pathways Pt–Pt₁ (2.76 Å) and Pt–Pt₂ (2.73 Å) compared to the initially fitted Pt–Pt bond length. As a comparison, the fitting curves for Pt foil are presented in Fig. S10 (ESI†).

Fig. S11 (ESI†) illustrates the survey scan and Pt 4f XPS spectra of P clu/dp-Al₂O₃. The survey spectrum of Pt clu/dp-Al₂O₃ (Fig. S11a, ESI†) revealed the presence of Al, C, and O elements. In the case of Pt clu/dp-Al₂O₃, the measured binding energy of 72.4 eV (Pt 4f_7/2 peak) indicated the presence of an ionic Ptⁿ⁺ (0 < n < 2) nature within the support. This value fell below the reported value for Pt²⁺ (73.6 eV) and exceeded that of Pt⁰ (71.4 eV).²⁸ This observation suggested an improved support-metal interaction resulting from charge transfer within the supports and metal. Additionally, it demonstrated the influence of electron transformation on the system.

As shown in Fig. 3(a), upon utilizing the synthesized Pt clu/dp-Al₂O₃ catalyst for the diboration reaction of phenylacetylene with bis(pinacolato)diboron, it was observed that nearly complete conversion of phenylacetylene was achieved within six hours at a temperature of 100 °C. Furthermore, the Pt clu/dp-Al₂O₃ catalyst displayed an impressive selectivity of up to 97% (Fig. 3(b)), devoid of any observed accessory substance such as hydroborylated products. To facilitate comparison, Pt nanoparticles (Fig. S12, ESI†) and Pt clusters were synthesized and analyzed when supported on commercial Al₂O₃, and the catalysts were employed under identical conditions. As shown in Fig. 3(b), the catalytic performance of Pt clu/c-Al₂O₃ was better than that of Pt NPs/c-Al₂O₃, but lower than that of Pt clu/dp-Al₂O₃, indicating that the prepared Al₂O₃ support and the interaction between the Pt clusters and the support contributed to this result. Additionally, the catalytic performance of the c-Al₂O₃ and dp-Al₂O₃ supports was evaluated. The catalytic activity of the inert support was found to be negligible, suggesting its lack of involvement in the reaction. Subsequently, a recyclability assessment was conducted on the highly effective Pt cluster catalyst to assess its stability. The selectivity remained relatively constant while the conversion experienced a minor decline after five cycles, highlighting the significant stabilizing influence of alumina nanosheets on Pt clusters (Fig. 3(c)). Conversely, Pt NPs/c-Al₂O₃ and Pt clu/c-Al₂O₃ showed limited cycling stability (Fig. S15, ESI†). The XRD patterns and microscopic images of the recovered Pt catalysts after five cycles at either moderate or full conversions indicated that Pt clu/dp-Al₂O₃ retained its crystalline structure after catalysis (Fig. 3(d) and Fig. S13, S14, ESI†). As depicted in Fig. S16 (ESI†), the catalyst facilitates the diboration reaction through three primary steps.

The synthetic strategy employed in this study readily allowed for the generalization of other metal elements (M = Ru, Pd, Ir, and Au). Fig. 4 provides the FT-EXAFS curves of M clu/dp-Al₂O₃ (the XANES, TEM images and fitting results of M clu/dp-Al₂O₃ are shown in Fig. S17, S18 and Table S2, ESI†), which clearly suggest the characteristics of the metal species and indicate that these M clu/dp-Al₂O₃ were typically coordinated by O atoms, the same as the Pt clu/dp-Al₂O₃. The content of Ru was 1.53 wt%, Pd was 1.62 wt%, Ir was 1.48 wt%, and Au was 1.33 wt%. The XAS analysis and TEM images were consistent with ICP-OES measurements.

In conclusion, the wet impregnation method successfully achieved the formation of positively charged Pt clusters on the dp-Al₂O₃ support. The resulting catalyst exhibited exceptional performance in chemoselective diboration reactions, with a conversion rate of 97%, selectivity of 95%, and reusability for five cycles. Several factors contribute to the cycling properties and superior catalytic activity of Pt clu/dp-Al₂O₃: a strong support-metal interaction, the presence of positively charged Pt centers, and the stabilizing effect of dp-Al₂O₃ on Pt species. This study also introduces the possibility of using nonreducible oxides to stabilize cluster catalysts, presenting a promising approach for their preparation.

This work was supported by the Beijing Natural Science Foundation (Grant No. 22201262 and 2212018), Natural Science Foundation of Henan Province (222300420290), Beijing Institute of Technology Research Fund Program for Young Scholars (3090012212219), National Natural Science Foundation of China (51902013), the Excellent Innovative Scientific Research Team of Silicon-based Materials (Grant No. 2022AH010101) and the University Synergy Innovation Program of Anhui Province (GXXT-2023-096).

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Experimental details and supplementary characterization. See DOI: https://doi.org/10.1039/d4cc01226g

‡ These authors have contributed equally to this work.

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