Multicomponent reactions (MCRs) are defined as attractive and powerful synthetic protocols for producing highly complex molecules and biological significance molecules owing to the formation of C-C and C-heteroatom bonds in a one-pot manner through an easy tandem synthetic method with step-efficiency and atom-economy (1–10). Polyhydroquinoline (PHQ) derivatives as a significant class of nitrogen heterocycle compounds can be converted into biological compounds, displaying promising pharmaceutical and biological properties, including antitumor, antidiabetic, platelet anti-aggregation, bronchodilator, antibacterial, and neurotropic (11–14). Thus, the production of PHQ derivatives is of great importance. The new techniques have been developed to improve the reaction efficiency in the preparation of PHQ derivatives in the presence of catalysts including [CholineCl][ZnCl2]3 (15), SBA-15@Glycine-Cu (16), Fe3O4@SiO2/ZnCl2 (17), ascorbic acid (18), NiAlTi LDH (19), and CNNs-Bu-SO3H (20). However, some of these synthetic methods suffer from the usage of toxic organic solvents, long reaction time, harsh reaction conditions, a great amount of catalyst, and low yields. Consequently, there is further improvement toward more sustainable protocol for the fabrication of PHQ derivatives. Lately, remarkable attention has been developed to designing eco-friendly catalysts and synthetic procedures for the Hantzsch reaction. The environmentally benign processes comprise the use of effective, biodegradable, and economical catalysts and non-toxic systems such as solvent-free conditions, water, and supercritical fluids (21–25).
The substantial advance in nanotechnology during the last decades has led to the development of a large variety of nanomaterials with outstanding catalysis applications. It is possible to design and construct numerous nanomaterials suitable as heterogeneous catalysts (26–37). The support material selection possesses a key role in the overall efficiency of the catalyst because these materials impact the catalytic properties of nano-scale catalysts (38, 39). The materials for catalyst supports indicate the high surface area, capability to disperse the supported metal, and chemical stability. Amongst the various support materials, mesoporous silica materials (MSMs) are promising materials owing to their thermally and chemically stability, large surface area, easy surface functionalization, good biocompatibility, and can be produced with tunable micro/meso porosity (40, 41). MSMs are amorphous inorganic materials composed of silicon and oxygen elements in their framework with pore diameters ranging from 2 to 50 nm. The well-defined pore structure of porous silica can function as a molecular sieve at small sizes and may ultimately be utilized to control substrate access to the catalyst which is very important in improving/tuning the selectivity (42, 43). These materials have proved their versatility in separation (44), sensor (45), drug delivery (46), and catalysis (47). Carbon nanostructures are especially of attention owing to their promising properties including high specific surface area, excellent mechanical strength, high conductivity, and fascinating physicochemical features. Among these, graphitic carbon nitride (g-C3N4) as a free metals material is especially of attention, owing to its unique crystal structure, nontoxic, cost-effectiveness, high thermal and chemical stability, and resistance to acidic and basic conditions (48). C3N4 has a stacked two-dimensional structure and can be synthesized easily from low-cost precursors such as urea, thiourea, melamine, and cyanamide via pyrolysis. Owing to its promising features, g-C3N4 and its composites are applied in a variety of photocatalytic applications (49). So far, g-C3N4 has been utilized as a catalyst or catalyst support in various organic reactions (50–55). However, the practical application of g-C3N4 is limited by its low surface area, insufficient light absorption, reduction potential, inappropriate rapid recombination, and large diffusion resistance of charges. The g-C3N4 can enhance the surface area, promote charge transfer and mass diffusion through nanostructure materials design.
Copper hydroxide nitrate, [Cu2(OH)3NO3], is a basic copper(II) salt with a layered structure, that have applications in vehicle airbags, catalyst, and ion exchangers (56–60). [Cu2(OH)3NO3] exists as two structurally related dimorphs, a synthetic metastable monoclinic phase and a natural orthorhombic phase occurring in the mineral gerhardtite. The structure can be observed as layers of copper octahedra stacked with each other. The Cu octahedral form layers of stoichiometry [Cu2(OH)3]+, and NO3- ions stand in between the positive layers for charge balance, which are linked to the hydroxyl groups via hydrogen bonding belonging to the copper octahedra layers.
In this study, g-C3N4/MSN was fabricated and utilized as support to load copper nitrate hydroxide (CNH) (Cu2(OH)3NO3) and emerged as a competent heterogeneous nanocatalyst for the Hantzsch reaction.