Copper aluminum mixed oxide (CuAl MO) catalyst: A green approach for the one-pot synthesis of imines under solvent-free conditions
Graphical abstract
Introduction
The search for new environmentally benign reagents and catalysts with high efficiency and minimum waste production has become one of the most important goals in chemistry. In this sense one-pot, tandem, or cascade reactions involving multiple catalytic transformations reduces the number of synthetic steps by leading sequential catalytic processes into one-synthetic operation with minimum workup, or change in conditions [1]. These processes have become an important area of research since they improve atom economy and decrease energy consumption and raw materials consumption by avoiding intermediate separation and purification steps [2]. Catalysis of two or more mechanically distinct processes by a single catalyst is known as auto tandem catalysis. Searching for such direct synthetic pathways are key for green and sustainable chemistry as they avoid side product formation, loss of starting material, separation and purification of intermediates therefore producing an economical as well as environmental benefits [3], [4].
Synthesis of imines is a very important reaction in chemistry as well as in biology as they are crucial intermediates in the synthesis of biologically active nitrogen compounds, nitrogen heterocycles, fine chemicals, and pharmaceuticals [5], [6]. In general, imines are synthesized by the condensation of primary amines with carbonyl compounds. However, several other synthetic strategies have been developed such as dimerization of primary amines under oxidative conditions, transition metal-promoted hydrogen transfer from secondary amines, oxidation of secondary amines, and the direct reaction of nitro arenes and primary alcohols in the presence of hetero bimetallic catalysts [7], [8], [9].
The synthesis of imines through the cross-coupling between amines and alcohols is a very useful and appropriate preparation method due to their wide substrate scope. In this method, aldehydes and ketones are obtained from alcohols by stoichiometric oxidation. Compared to carbonyl compounds, alcohols are less toxic, readily available, more stable, inexpensive, and easier to handle and produce only hydrogen or water as a byproduct [10]. Therefore this is an environmentally attractive method for the synthesis of imines. Several catalytic systems have been reported for this reaction sequence based on precious metals such as platinum, iridium, and palladium under homogeneous conditions [11], [12], [13] or other systems like graphene oxide [14] and copper [15]. But the pricing, limited availability, toxicities, and non-recoverable nature have forbidden their large-scale usage [5], [16], [17]. Therefore it is highly desirable to develop mild, efficient, more environmentally and economically friendly alternatives for the synthesis of imines [18], [19]. Furthermore, these processes require dehydrating agents, additional bases, activated aldehydes, prolonged reaction time, high temperatures, and stoichiometric oxidants [20]. However, the use of oxidants such as chromate, permanganate or 2-iodoxybenzoic acid [21] has several drawbacks, which are the difficulty in product separation and the formation of undesirable toxic waste products.
Mixed metal oxides that contain transition metals can play a significant catalytic role in a large variety of heterogeneous chemical processes. Among them Cu and Al based oxides can be considered as potential catalysts as they are relatively cheap and can readily be prepared by diverse synthetic routes. Moreover, it has been put forward that the presence of acidic sites is very important for these types of reactions and mixed oxides show much stronger acidic properties compared to single oxides. In the present study, we report the tandem synthesis of imines directly from alcohols and amines under mild conditions using an inexpensive CuAl mixed oxide (MO) catalyst and air as an oxidant. CuAl MO acts as a bifunctional catalyst in the process to oxidize the alcohol to the corresponding carbonyl compound and then acts as a Lewis acid to form the imine. Here, the CuAl mixed hydroxide material was precipitated from a homogeneous solution of metal salts by the thermally induced hydrolysis of urea. Then the mixed hydroxide has been converted into the metal oxide by thermal treatment. The synthesis was performed in the presence of activated carbon which subsequently helps to tune the surface properties of the resultant metal oxide. The catalytic process is done under solvent free conditions and does not form any harmful byproducts. In this reaction, air is used as the oxidant which is considered as the most economical and green oxidant among different oxidizing agents. Furthermore, no base additives, dehydrating agents, or any special apparatus are needed.
Section snippets
Synthesis of activated carbon/mixed hydroxide composites and corresponding metal oxide catalysts
Activated carbon was purchased from Calgon Carbon Corporation. Activated carbon/CuAl mixed hydroxide composite (Cu/Al molar ratio = 3) was synthesized through a urea hydrolysis process in the presence of activated carbon. Synthesis was done by adding 1.5 × 10−3 mol of Cu(NO3)2∙6H2O, 0.5 × 10−3 mol of Al(NO3)3∙9H2O, 4.0 × 10−3 mol of urea and a known amount of activated carbon into 60 mL deionized water under vigorous stirring at room temperature. The mixture was refluxed at 100 °C for 15 h and the final pH
Structural characterization of activated carbon/mixed hydroxide composites and corresponding metal oxides
XRD patterns of as synthesized CuAl-Cx% mixed hydroxide materials are displayed in Fig. S1. Samples synthesized with 0%, 5% and 10% activated carbon showed a CuAl mixed hydroxide phase while the XRD pattern of the sample with 20% activated carbon corresponded to a copper nitrate hydroxide phase (ICDD 75-1779). Activated carbon acts as a template and it remains as a separate phase in all the compositions. However, above 20%, some inhomogeneity appears in the reaction mixture; therefore the
Discussion
The application of highly abundant, relatively cheap carbon materials as hard templates has been widely used to synthesize structurally ordered materials with enhanced performances [28], [29], [30], [31]. In the current study, activated carbon has been used to alter surface properties of the material and acts as an efficient green catalyst for the synthesis of imines under solvent free conditions. Here, synthesis of mixed metal hydroxide/carbon composite has been achieved through
Conclusions
In conclusion, an efficient and a green catalytic process to synthesize imines directly from alcohols and amines using CuAl MO catalyst have been reported. Activated carbon has been successfully used as a template for the synthesis of CuAl MO catalysts with modified surface properties. CuAl MO20%C material exhibited a uniform pore size distribution (pore diameter 40 Å) with the highest surface area (161 m2/g) and showed the best catalytic performance. CuAl MO catalyzes two distinct processes,
Acknowledgments
We thank the Chemical sciences, Geosciences, and Biosciences Divisions of the Office of Basic Energy Sciences, Office of Science, and U.S. Department of Energy for supporting this work under grant DE-FG02-86ER13622-A000. We also thank Wei Zhong for her help with XPS.
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