Host-guest chemistry immobilized nickel nanoparticles on zeolites as efficient catalysts for amination of 1-octanol
Graphical abstract
Introduction
Alkylamines are extensively used as building blocks in a variety of high value-added chemicals including pharmaceuticals, polymers, dyes, pigments, and plasticizing agents, which play essential roles both in bulk and fine chemical industries [1], [2]. In the huge alkylamine family, aliphatic primary amines are of particular importance, serving as intermediates for further derivatization reactions, which stimulates researchers to develop eco-efficient synthesis route for the primary amines [3]. Nevertheless, the established traditional strategies for the primary amine synthesis, such as hydroamination of alkene or alkynes, and amination of aryl halides, encounter the disadvantages of poor selectivity, low atom-economy and toxic byproducts [4], [5]. As an alternative way, the reductive amination of the corresponding alcohols, producing water as the sole stoichiometric side product, emerges as an atom-economic and environmental-benign method for the production of amines [6].
The reductive amination of aliphatic alcohols proceeds via nucleophilic substitution reaction by means of borrowing hydrogen mechanism [7]. As described in Scheme 1, alcohol is firstly dehydrogenated to alkanal, which then translates to imine by condensation with ammonia, and finally the hydrogen returns to imine, forming primary amine [8], [9]. However, as the primary amine is more nucleophilic than ammonia to react with electrophiles, it is prone to over-alkylate the alcohol producing secondary amines and tertiary amines. Besides, the imine intermediate easily dehydrogenates to the corresponding nitrile or condensates with the primary amine giving rise to N-alkyl-imine and ammonia. Thus, the main challenge is to break thermodynamic equilibrium and minimize the selectivity of the nitrile and undesirable amines, in which case the choice of an effective catalyst is highly desired to achieve high activity and selectivity towards primary amine. The premier catalysts mainly focus on a variety of homogeneous pincer Ru complexes [10], [11], [12] and heterogeneous noble metals, such as supported Pd [13], Ru [14], Pt [15] and bimetallic catalysts [16], [17]. However, the above catalysts suffer from the problems of the catalyst recovery and the high cost of noble metals in practical industrial applications. Recent years have witnessed the development of a series of heterogeneous supported transition metal catalysts [18], [19], [20]. Amongst, Co-based catalysts are always performed in bimetallic format such as Ag-Co [21], [22], Co-Pd [17], Co-Ru [23], and Pt-Co catalysts [24], while the supported Cu catalyst was only reported to produce secondary amines [25]. Raney Ni exhibits excellent catalytic performance in alcohol amination yet the operation security hiders the wide application [26], [27]. Notably, supported-Ni catalysts can be employed as fascinating alternatives for the amination of a series of alcohols owing to the outstanding activity and stability [[28], [29], [30], [31]].
Shimizu and coworkers screened a series of metal-supported catalysts for the amination of 2-octanol and obtained the highest performance over Ni/γ-Al2O3 catalyst. They demonstrated the small-sized metallic Ni0 NPs on the alumina surface acting as the active species and acid-base sites of γ-Al2O3 support were indispensable for the high activity [32]. Similar conclusions that the reaction rate decreased with the increase of Ni particle size for alcohol amination were reported [33], [34]. For supported Ni catalyst, the major disadvantage lies in easy aggregation and poor dispersion in particular under high Ni content conditions. On this basis, Tomer et al. fabricated a series of Ni/Al2O3 catalysts with higher dispersion and narrower size distributions of Ni particles using cyclodextrins as the metal complex, which enhanced the catalytic properties in alcohol amination [35]. Whereas the interaction between Ni NPs and Al2O3 support was weak in above method and the participation of organic additives may result in environmental problems. In order to achieve highly efficient and stable catalyst for alcohol amination, it is highly desirable to develop green synthesis route to prepare the Ni-supported catalysts with high dispersion and strong metal species-support interaction.
Crystalline aluminosilicates possessing high surface area, intrinsic acidity and unique microporosity, have been utilized as important supports and catalysts for a variety of industrialized reactions [36], [37]. As one of the most used industrial catalysts, Y zeolite with the merits of adjustable acidity and feasible availability has been employed to catalyze the gas-phase 1-octanol amination [38]. Our group previously reported a host-guest chemistry for the in-situ formation of metal silicate and layered double-hydroxides (LDHs) precursors on Si-rich and Al-rich zeolite respectively, where the smaller particle size and narrower size distribution of metal particles were obtained arising from the Ni-O-Si and Ni-O-Al hetero-condensation/polymerization on the surface, resulting in the improved catalytic activity and stability in hydrogenation reactions [39], [40], [41].
On the basis of aforementioned researches, we here applied USY@Ni catalysts, synthesized by the above mentioned in-situ growth route, in the reductive amination of 1-octanol reaction. A series of USY zeolites with various Si/Al ratios were used as precursors to interact with Ni precursors in different ways, with high Si content ones to form nickel silicate and high Al content ones to NiAl-LDHs, which were then reduced to produce the USY@Ni catalysts. Detailed and thorough characterizations by spectroscopy and microscopy were employed to investigate the distinct USY precursors and the reduced USY@Ni catalysts, and the relationship between the structure properties and the catalytic performance was established preliminarily in the reductive amination of 1-octanol.
Section snippets
Synthesis of USY zeolites with different ratios
The parent USY zeolites with Si/Al molar ratios of ca. 3 and 6 were purchased from Wenzhou Huahua Group Co., Ltd, China. While USY zeolites with higher Si/Al molar ratios were synthesized by performing the acid treatments over the USY zeolite according to previous literatures [42]. The USY zeolite (Si/Al = 6) was firstly calcined in air at 600 °C for 5 h and then refluxed in 6 M HNO3 solution with a solid-to-liquid ratio of 1 g : 50 mL for 1 h and 18 h, to produce delauminated USY with the
Preparation and characterization of USY@Ni-P and USY@Ni materials
All the parent USY zeolites exhibited the well-defined reflections attributed to the typical FAU topology (Fig. S1). With the increase of Si/Al ratio, the reflection peak around 6.3° attributed to [1 1 1] plane gradually shifted to higher angle correspondingly (Fig. S1B), due to the lattice shrinkage after removing larger Al ions. Upon hydrothermal treatment in aqueous solution of nickel nitrate and base, Ni-Al layered hydrotalcite (NiAl-LDHs) were formed for USY@Ni-3-P and USY@Ni-6-P samples,
Conclusion
In summary, we fabricated well-dispersed Ni NPs robustly immobilized on USY zeolites with various Si/Al ratios through a facile in-situ growth method. Specifically, the NiAl-LDHs and nickel silicate precursors could be fabricated respectively on the Al-rich and Si-rich USY zeolites, from which highly dispersed Ni NPs were confined on the Al2O3 and SiO2 matrix upon reduction. Among the comparison of supported Ni zeolites with different Si/Al ratios and topologies in the reductive amination of
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.
Acknowledgement
This study is financially supported by China Ministry of Science and Technology under contract of 2016YFA0202804 and the National Natural Science Foundation of China (Grant No. 21872052, 21533002 and 21603075).
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