Full Length ArticleDesign of a difunctional Zn-Ti LDHs supported PdAu catalyst for selective hydrogenation of phenylacetylene
Introduction
The selective hydrogenation of alkynes to alkenes is one of the most important and fundamental reactions in the bulk and fine chemicals [1], [2]. Conventionally, this process is preformed over Lindlar’s catalyst (Pd/CaCO3 treated by Pb salts) [3]. However, the disadvantages of this catalyst such as low selectivity of alkene, high toxicity of Pb species and catalyst deactivation limit its application. Thus, a series of alternative Pb-free catalysts as Pd [4], Ru [5], Cu [6] and Au [7] have been investigated. Previously, much effort has been made to obtain high selectivity of alkene at relatively high alkyne conversion. However, the over-hydrogenation of alkene could not be suppressed with increasing reaction time when a complete conversion was obtained. The high selectivity was generally obtained just under a particular reaction time (batch reactor) or space time (fixed-bed reactor), namely that a decrease in selectivity of alkene with increasing reaction/space time was observed at complete conversion [8], [9]. In industrial production, the changes of selectivity in a broad range of time at complete conversion can lead to a severe decrease in the quality of products. Thus, it is very important for industrial operation to maintain high selectivity of alkene for a long time at complete conversion, which is to eliminate the influence of factors like backmixing and dead space.
Recently, it had been found that Pd-Pb alloy octahedral nanocrystals dramatically diminished the rate of subsequent hydrogenation of styrene after phenylacetylene was depleted, maintained high selectivity of styrene even after a long reaction time [10], because the addition of Pb could suppress the formation of β-palladium hydride (β-PdH) phase [11]. So the over-hydrogenation of alkyne could be restrained by suppressing the formation of metal-H. Besides, inhibiting the contact between metal-H and alkene could also restrain the over-hydrogenation of alkyne. As reported, the excellent alkene selectivity was maintained over core-Pd/shell-Ag and Pd@Pd-Au nanocrystals catalyst even when the reaction time was prolonged after full conversion of alkyne [12], [13]. However, these unsupported catalysts have the disadvantage of low intrinsic activity. So the development of the supported catalysts with high intrinsic activity is necessary.
The Pd catalysts are widely used with a promoter to improve alkene selectivity, such as Ag, Rh, Au, Cu, Zn, Cr, V and Si [8], [14], [15], [16], [17], [18], [19], due to the induced electronic effect and/or the geometric effect. Wherein, the supported Pd-Au alloy catalysts have attracted substantial interests on account of high alkene selectivity [16], [20], while the selectivity of alkene could not be hold as alkyne was complete converted. Thus, only the formation of alloy is insufficient. In the catalytic deoxygenation of epoxides with H2, the alkaline hydrotalcite-supported gold catalyst was employed, which exhibited excellent alkene selectivity [21]. The hydrogenation of alkene was suppressed, which was attributed to a concerted effect between the basic sites of hydrotalcite and the Au nanoparticles. Namely, heterolytic cleavage of H2 occurs at the interface between them, which is inactive for the hydrogenation of C
C bonds of alkene. Therefore, it is reasonable to believe that an alkaline difunctional supported Pd-Au catalyst is a promising catalyst in the selective hydrogenation of alkyne.
As known, layered double hydroxides (LDHs) are a large class of typical inorganic layered host materials with cation tunability of the brucite-like layers. Large amount of hydroxyl ions lie in the layers, making that LDHs has unique alkaline properties. Herein, the difunctional Zn-Ti LDHs supported bimetallic PdAu catalyst was designed and synthesized by a photochemical reduction method. In the selective hydrogenation of phenylacetylene, high selectivity of styrene was maintained even after a long time reaction as phenylacetylene was depleted, which was attributed to the synergistic effect between bimetallic alloy nanoparticles and Zn-Ti LDHs.
Section snippets
Catalysts preparation
The Zn-Ti LDHs with Zn2+/Ti4+ molar ratio of 2:1 was prepared by co-precipitation of zinc and titanium salts from homogeneous solution [22]. Typically, 0.22 mL of TiCl4, 1.19 g of Zn(NO3)2·6H2O and 3.0 g of urea were dissolved in 100 mL of deionized water under vigorous stirring for 0.5 h at room temperature. The resulting reactant was aged in an autoclave at 130 °C for 48 h. The precipitate was centrifuged and washed thoroughly with water and finally dried overnight at 60 °C.
The supported
Results and discussion
XRD patterns of the Zn-Ti LDHs and its supported catalysts are shown in Fig. 1. The reflections of (0 0 3), (0 0 6), (0 0 9), (1 0 0), (1 0 1), (0 1 2), (1 1 0) and (1 1 3) were observed for all samples, which could be indexed to typical LDHs materials [22]. While no reflections corresponding to Au or Pd were detected after intercalation by noble metal, this could be attributed to high dispersion or low loading amount. Cell parameters c and a of the triple hexagonal unit cell, calculated
Conclusions
In summary, the Zn-Ti LDHs supported monometallic and bimetallic nanoparticles catalysts were designed and prepared by photochemical reduction method. Compared to the Zn-Ti LDHs, the interlayer distance was increased due to the incorporation of the metal nanoparticles into the interlayer region of LDHs. Additionally, a strong interaction between nanoparticles and LDHs was also identified by XPS measurement. It was found that the interaction intensity was increased in the order of Au/ZnTi < Pd9Au
Acknowledgements
We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (21503028, 21506148).
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