Effect of the coverage of modulated Au(Pd) atoms over bimetallic Pd-Au catalysts on catalytic performance for direct oxidative esterification of methacrolein into methyl methacrylate
Introduction
As an important industrial chemical raw materials, methyl methacrylate (MMA) has been widely studied in the production of polymer dispersions for acrylic plastics and other coatings [1], [2], [3]. The traditional methods for the synthesis of MMA include acetone cyanohydrin (ACH) and isobutene (IB) oxidation, which not only require multistep process, but also corrode the equipment and produce a large number of by-products [4,5]. Recently, a green and sustainable production route for the synthesis of MMA by direct oxidative esterification of methacrolein (MAL) with methanol (CH3OH) in the presence of molecular oxygen (O2) has been discovered and investigated [6], [7], [8]. Direct oxidative esterification has been reported on Pd-based catalysts, but the selectivity of MMA was lower because Pd was easy to be oxidized [9], [10], [11], [12], [13], [14]. In addition, it was generally recognized that MMA formation via dehydrogenation of MAL following coupling with CH3O group in the process of direct oxidative esterification of MAL to MMA on Pd-based catalysts [15,16].
Au-based catalysts have attracted more and more attention in heterogeneous catalytic reaction [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]. Marsden et al. [27] reported that supported Au catalysts have high yield of methyl acrylate in aerobic oxidation of aldehydes. Casanova et al. [29] investigated the oxidative esterification of 5-hydroxymethyl-2-furfural into 2, 5-dimethylfuroate on Au/CeO2 catalysts, in which the alcohol oxidation to aldehyde was the rate-limiting step of the reaction. Ke et al. [30] confirmed that the catalytic activity of supported Au catalysts depended on the diameter of Au particle. However, these Au-based catalysts are affected by instability duo to the Au aggregation in the reaction process [26]. Therefore, it is still a challenge to regulated Pd-based and Au-based catalysts with high catalytic performance for direct oxidative esterification reaction.
Nowadays, bimetallic Pd-Au catalysts have received increasing attention because of the electronic and geometric effects between Au and Pd atoms. It is well known that bimetallic Pd-Au system mainly concentrated on the Au-doped Pd catalysts and Pd-doped Au catalysts. For example, Enache et al. [31] showed that the addition of Pd to Au for the formation of PdAu catalysts increased the oxidation activity of alcohol and the selectivity for the formation of aldehyde. Pritchard et al. [32] observed that the catalytic performance for direct synthesis of hydrogen peroxide and benzyl alcohol oxidation on AuPd catalysts with optimum ratio of Au and Pd (1:1.85) gradually improved. A more detailed study about the effect of thermal treatment on the surface composition and catalytic activity of Pd-Au catalysts was also reported [33]. Yang et al. [34] studied the diffusion mechanism of metal atoms in bimetallic Pd-Au catalysts under CO atmosphere using molecular dynamic. Xu et al. [35] reported CO oxidation on a compositional series of Pd-Au supported on SiO2 catalysts, and confirmed that Pd-Au alloy catalysts showed significant activity and stability toward low-temperature CO oxidation. In addition, bimetallic Pd-Au catalysts have also been widely applied in the decomposition of HCOOH, showing higher catalytic activity for the activation of HCOOH [36], [37], [38], [39], [40].
A large number of experiments have pointed to the fact that the catalytic performance for heterogeneous reactions of bimetallic Pd-Au catalysts depended on the ratio of Au and Pd on the surface of the catalysts, namely, the catalytic performance for heterogeneous reactions of bimetallic Pd-Au catalysts is particularly sensitive to the surface coverage of modulated Au(Pd) atoms. However, as far as we know, direct oxidative esterification mechanism on bimetallic Pd-Au catalysts has not been mentioned. Therefore, up to now, due to the lack of incomplete information, the fundamental understanding on the effect of the coverage of modulated Au(Pd) atoms over bimetallic Pd-Au catalysts on the stability of Pd-Au catalysts and catalytic performance for direct oxidation esterification are still not unclear.
This work is designed to solve above issues based on density functional theory methods, the effect of the coverage of modulated Au(Pd) atoms over bimetallic Pd-Au catalysts on H2O activation, CH3OH activation and the formation of MMA would be systematically investigated, which are also as probe for insight into the catalytic performance for direct oxidative esterification of bimetallic Pd-Au catalysts including Au modulated Pd and Pd modulated Au catalysts. Meanwhile, the electronic properties of bimetallic Pd-Au catalysts with different coverage of modulated Au(Pd) atoms are carried out to obtain a deeper understanding the effect of surface composition. Further, the effect of coverage of modulated Au(Pd) atoms on bimetallic Pd-Au catalysts on catalytic performance for direct oxidative esterification would be confirmed. This study is of great significance for designing and optimizing bimetallic Pd-Au catalysts with higher stability and catalytic performance for direct oxidative esterification reaction.
Section snippets
Computational methods
DFT calculations were performed using Dmol3 program package of Materials Studio 8.0 [41,42]. GGA-PBE functional was used to presented the exchanged correlation effect [43,44]. The valence electron function was carried out by DNP [45,46]. All electron basis set was treated for small molecules, and effective core potential was used for the inner electrons of Pd and Au atoms [47,48], and a smearing of 0.001 Ha was applied for all the calculations. A 4 × 4 × 1 k-points was selected to the surface
Possible reaction mechanism for direct oxidative esterification of MAL with CH3OH into MMA
As shown in Scheme 1, the possible reaction mechanism for direct oxidation esterification of MAL with CH3OH in the presence of O2 on Pd-based and Au-based catalysts were summarized based on previous experimental and computational investigations in detail [[9], [10], [11],[15], [16], [17]]. Direct oxidation esterification was H2O-containing system, resulting in the decrease of the stability of the catalysts because of deactivation by the formed water film. The direct dissociation of H2O is very
Conclusion
In this work, the stability for resistance towards deactivation induced by H2O and activity for MMA formation in direct oxidation esterification on Au modulated Pd (AuPd) and Pd modulated Au (PdAu) have been investigated to reveal the effect of the coverage of modulated Au(Pd) atoms using DFT calculations. The results illustrated that Pd modulated Au catalysts exhibited higher stability for resistance toward deactivation induced by H2O deposition via O+H2O+*→2OH than Au modulated Pd and pure
CRediT authorship contribution statement
Baohe Wang: Investigation, Resources, Funding acquisition, Supervision, Software, Formal analysis. Zheng Luo: Investigation, Data curation, Validation. Qiaoyun Qin: Investigation. Honggang Dong: Data curation. Hongxia Liu: Conceptualization, Investigation, Methodology, Formal analysis, Funding acquisition, Writing – review & editing.
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.
Acknowledgment
This work is financially supported by the National Natural Science Foundation of China (No.21406153, No.21076156).
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H bond activation of the CH2C(CH3)CHOOCH3 intermediate. Therefore, the interface-rich Au/t-ZrO2 catalyst achieved nearly 90 % yield of MMA and excellent catalyst stability without the incorporation of second promoter. The outcome of this work could provide some insights into developing an efficient Au-based catalyst in the oxidative esterification system.