Atomically decorating of MnOx on palladium nanoparticles towards selective oxidation of benzyl alcohol with high yield
Graphical abstract
Introduction
The selective oxidation of benzyl alcohol is considered to be one of the most fundamental reactions for fine chemicals production in both laboratory and industrial scale [1], [2], [3], [4]. In previous studies, various heterogeneous catalysts have been reported for this model reaction [5], [6]. Considering green chemistry principle, it is important to improve the atomic utilization of the reactants, which could also provide insights for other alcohols oxidation reactions. Supported palladium catalysts are very active for benzyl alcohol oxidation [7], [8], [9], [10], [11]. However, during this process other side reactions also take place and undesired byproducts are formed. Thus, the total yield of target benzaldehyde product decreases. Methods are developed to modify the active sites of the Pd catalysts by introducing promoters and nanotailoring of interfacial perimeter. Typically, metals like Bi [12], Pb [13], Au [14], [15] as well as oxides such as FeOx [16], [17], MnOx [18] etc., are introduced to construct composite configurations to improve the catalytic performance and prevent deactivation. For example, the Pd/Au nanocrystals show enhanced selectivity in the oxidation of alcohols to aldehydes due to the electronic modification of Au to Pd [14]. From previous reports, compared with single MnOx or CeO2 support, the catalytic activity and selectivity for benzyl alcohol are significantly improved by the MnCeOx composite support through the synergistic effect between Pd and MnOx/CeO2 interfaces [19].
Despite extensive studies on Pd catalysts, the description of Pd’s active sites remains controversial. Based on previous researches, metallic Pd0 species are supposed to be the active sites, and the dehydrogenation step on the Pd catalysts during alcohol selective oxidation is universal [20], [21]. As discussed in detail, the activity enhancement of PdO is attributed to its reduction by benzyl alcohol to Pd0 during the reaction [22], [23]. The oxidative dehydrogenation of the alcohol to aldehyde occurred on all exposed Pd surfaces is the main reaction. Meanwhile the undesired decarbonylation formation takes place preferentially at the hollow sites on Pd (1 1 1) facets. Medlin et al. [24], [25] proposed that on clean Pd (1 1 1) facets, the reaction of benzyl alcohol was highly dependent on its adsorption orientation. With low adsorption coverage, benzyl alcohol presented a flat lying structure to produce CO and benzene. While with high coverage, the benzyl alcohol presented an upright structure and produced deoxygenation products such as toluene and H2O. Meanwhile, the aerobic oxidation of alcohols is affected by various coupled factors, including conversion temperature, reaction time length etc. As a result, it is extremely difficult to achieve high selectivity, conversion and reaction rate simultaneously. Generally, the over-oxidation of the reactants leads to the decreasing of target product selectivity [26]. For Au/U3O8 catalyst [27], the selectivity of target product at initial conversion stage was high. However, when increasing the reaction temperature and prolonging the reaction time, the conversion of benzyl alcohol could be increased, but the selectivity of benzaldehyde decreased drastically.
In this work, an atomically manganese oxide decorated Pd nanoparticles structure is designed and fabricated via atomic layer deposition (ALD). In the ALD process, two gaseous precursors are dosed alternately on the substrates. The self-limiting nature provides the advantages of depositing materials with atomic-scale control. Catalysts prepared by ALD show enhanced conversion and selectivity simultaneously, thus a high yield of target product benzaldehyde is obtained. The method enables an effective way to investigate the composite nanoparticles’ structure-property relationship. The reaction pathways are changed on ALD decorated Pd catalysts. Pd (1 1 1) facets are selectively passivated by MnOx deposition to eliminate the byproduct toluene formation. Toluene products are not observed for modified catalysts. On the other hand, the reversed oxides coating structure (compared with the oxides supported catalysts) enriches the active metal-oxide interfaces. Meanwhile, the addition of MnOx can modify the electronic structure of Pd which increases the surface concentration of metallic Pd0 species. Therefore, catalytic activity and reaction rate are also improved. The TOF of MnOx coated Pd catalyst reaches 31,561 h−1, which is 8.7 times larger than that of bare Pd catalyst. Moreover, this structure is also beneficial for inhibiting the decarbonylation reaction of benzaldehyde, further improving the yield of target product. The maximum conversion of benzyl alcohol and yield of benzaldehyde are improved to 84.7% and 76.5% respectively.
Section snippets
Catalysts fabrication
Al2O3 supported palladium catalysts were prepared by wet impregnation method. 18.09 wt% palladium nitrate dihydrate was impregnated into Al2O3 (99.99%, metals basic). The loading of palladium was fixed to 2 wt%. The sample was placed at room temperature for 24 h and dried at 80 °C overnight in a vacuum oven. Then the sample was ground and calcined at 500 °C in the air for 3 h, and naturally cooled to room temperature. The bare Pd catalyst was denoted as Pd/Al2O3.
A rotary ALD reactor coupled
Phase and dispersion of catalysts
The MnOx coating structures are fabricated with 1, 2, 4, 6 MnOx ALD cycles, respectively. With XRF calibration, the coating thickness (ALD cycles) is converted to MnOx loading of 0.1, 0.2, 0.4, 0.6 wt%, respectively. The X-ray diffraction (XRD) patterns of Al2O3, bare Pd nanoparticles and MnOx-decorated Pd nanocatalysts are presented in Fig. 1. Typical diffraction peaks correspond to Al2O3 support (JCPDS 29-1486, 2θ = 37.8, 45.7 and 67.1) are observed in all Pd/Al2O3 samples. Additionally, for
Conclusions
In summary, an atomically MnOx decorated Pd nanoparticles structure is designed and fabricated via ALD. The ALD prepared catalysts show enhanced activity and selectivity simultaneously, thus a high yield of benzaldehyde is obtained. The MnOx coating structures selectively passivate Pd (1 1 1) facets to prevent decarbonylation reaction of benzaldehyde and eliminate the formation of toluene. Meanwhile, MnOx fabricated via ALD forms strong binding with the Pd particles, which increases the MnOx-Pd
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is supported by the National Natural Science Foundation of China (51702106, 51835005, and 51871103, 51911540476). The authors acknowledge the Analytical and Testing Center, the Flexible Electronics Research Center of Huazhong University of Science and Technology.
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