PdZn intermetallic on a CN@ZnO hybrid as an efficient catalyst for the semihydrogenation of alkynols
Graphical abstract
Introduction
Vitamin E, as one of the vital nutrients that organisms require but cannot synthesize on their own in sufficient amounts, plays a significant role in controlling the fertility of human beings [1]. One of prevalent approaches to synthesis of Vitamin E involves the reaction between isophytol and 2,3,5-trimethylhydroquinone [2], the former of which mainly comes from alkenols such as linalool and 2-methyl-3-buten-2-ol (MBE). Thus, facile industrial access to those alkenols, via semihydrogenation of relevant alkynols in particular, has caught more and more attention in recent years [3], [4], [5], [6], [7].
Conventionally, semihydrogenation of alkynes was often carried out on Lindlar catalysts [8]. However, Lindlar catalysts go through deactivation even after the first run in aqueous media [3]. Other attempts have been made to resolve this problem. Single palladium catalysts based on carbon materials [9], [10], metal oxides [11], [12], zeolites [13], [14], pumice [15], borate monoliths [16], and hydrotalcite [17] have been reported to be applicable to the semihydrogenation of alkynes. Nonetheless, these supports may suffer from such problems as complex pretreatments, poor water dispersion, or high costs of raw materials. To avoid the high costs of palladium-based catalysts, non-palladium-metals-based catalytic systems such as nickel phosphides [18] and gold [19], [20]-based catalytic systems have also exhibited intriguing progress in this area. However, those catalytic systems are usually associated with either more rigorous reaction conditions or less satisfactory activity. Increasing interest has been witnessed recently in developing alloys (PdAu [21], [22], PdCu [23], [24], PdPb [25], PdSi [26], PdRhP [27], etc.) – or intermetallics (Pd2Ga [28], PdZn [3], [29], [30], [31])-based catalysts for reactions of selective hydrogenation. Further study has demonstrated that involvement of other inert components such as Zn in the Pd structure enables palladium catalysts to weaken their adsorption of the reactant, thereby improving the selectivity [30].
Recent years have witnessed amazing breakthroughs in developing carbon-based materials converted from naturally available low-cost biomass precursors [32], [33], [34]. Particularly, N-doped carbon (CN) has demonstrated great merits in various heterogeneous catalytic reactions, including modified electronic state, basicity, excellent water dispersion, and stability [35], [36]. Nonetheless, due to the high activity of Pd catalysts supported on CN (Pd/CN), the hydrogenation of alkene, as a side reaction to the semihydrogenation of alkyne, could take place easily on Pd/CN [37]. With rising interest in unique hybrids as a catalyst support possessing enhanced catalytic performance and bifunctional features [38], [39], [40], it is rather illuminating for us to design a novel hybrid catalyst to deal with those challenges.
Considering the advantages of CN in promoting reaction activity and Zn in controlling reaction selectivity for Pd-based alkynols semi-hydrogenations, a PdZn/CN@ZnO catalyst was produced by thermal condensation of biomass-derived glucosamine hydrochloride (GAH) and ZnO, followed by wet impregnation of Pd nanoparticles and subsequent high-temperature reduction under H2. To test its performance in alkynol hydrogenation, selective hydrogenation of 2-methyl-3-butyn-2-ol (MBY) to 2-methyl-3-buten-2-ol (MBE) (Scheme 1) was employed as a model reaction. Compared with PdZn/ZnO, PdZn/CN@ZnO presented sixfold higher activity and equally high selectivity (92%). Through proper adjustment of reaction conditions and introduction of additives, the final yield of MBE could reach above 95%. To further probe the function of PdZn intermetallics in the system, the ZnO-free CN-supported Pd catalysts PdZn/CN and Pd/CN were also tested as control experiments, and results showed that the presence of the PdZn phase was the key to high MBE selectivity. Therefore, the synergic effect of the PdZn phase and the incorporated CN ensured superior performance of the PdZn/CN@ZnO in this reaction. Furthermore, the present catalyst could be reused at least eight times without significant loss of activity and selectivity. When applied to other industrially relevant alkenols, the present catalyst still offers excellent performance.
Section snippets
Materials
PdCl2 (50–60 wt.%), 2-methyl-3-butyn-2-ol (98%), zinc oxide (50 nm, 99.8% metals basis), D(+)-glucosamine hydrochloride (99%), polyvinylpyrrolidone (PVP, MW = 58,000), γ-Al2O3, quinoline (99%), pyridine (99%), thiophene (99%), and 3,6-dithia-1,8-octanediol (97%) were used as received from Aladdin Chemistry Co., Ltd. Dehydrolinalool and dehydroisophytol were purchased from Xinhecheng Co., Ltd.
Synthesis of CN@ZnO
In a crucible, 2 g glucosamine hydrochloride (GAH), 20 mL water, and 1 g ZnO were thoroughly mixed and heated
Results and discussion
To get basic information on the successful fabrication of PdZn/CN@ZnO, various characterizations were conducted. Elemental analysis illustrated successful doping of nitrogen into the carbon structure (5.2% nitrogen in mass proportion). Thermogravimetric analysis (TGA) of CN@ZnO under O2 flow revealed that the mass ratio of ZnO was 39.2% (Fig. S3), demonstrating successful hybridization of CN with ZnO.
To uncover the detailed components of the resulting catalysts, XRD patterns of PdZn/CN@ZnO,
Conclusions
In summary, a novel hybrid catalyst, PdZn/CN@ZnO, was fabricated to be employed in the semihydrogenation of 2-methyl-3-butyn-2-ol with water as the solvent. Sixfold higher TOF and identically high selectivity were achieved for PdZn/CN@ZnO compared with PdZn/ZnO. Various evidence showed that the presence of CN promoted its hydrophilic properties, and PdZn and CN interacted, thus improving the catalytic activity; and the substitution effect of corner and edge Pd atoms helped to improve the
Acknowledgments
Financial support from the National Natural Science Foundation of China (21622308, 91534114, 21376208), the MOST (2016YFA0202900), and the Fundamental Research Funds for the Central Universities is greatly appreciated. We also appreciate the computing time supported by the Special Program for Applied Research on Super Computation of the NSFC–Guangdong Joint Fund (the second phase).
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