Investigation of halloysite nanotubes and Schiff base combination with deposited copper iodide nanoparticles as a novel heterogeneous catalytic system

The design, preparation and characterization of a novel composite based on functionalization of halloysite nanoclay with Schiff base followed by immobilization of copper iodide as nanoparticles is revealed. This novel nano composite was fully characterized by utilization of FTIR, SEM/EDX, TGA, XRD and BET techniques. This Cu(I) NPs immobilized onto halloysite was successfully examined as a heterogeneous, thus easily recoverable and reusable catalyst in one of classist organic name reaction so-called “Click Reaction”. That comprised a three component reaction of phenylacetylene, α-haloketone or alkyl halide and sodium azide in aqueous media to furnish 1,2,3‐triazoles in short reaction time and high yields. Remarkably, the examination of the reusability of the catalyst confirmed that the catalyst could be reused at least six reaction runs without appreciable loss of its catalytic activity.

and metalloprotein models and asymmetric catalysts [17][18][19] . Schiff bases are chelated with the most of metal cations, providing the corresponding complexes. Among these metals, copper as Cu-cations have been found the best since their reaction with different Schiff bases give complexes which has been proven to be most efficiently and practically applicable catalysts. Several complexes of Schiff bases as catalytic agents, have been found readily accessible, highly effective and more important very stable in a wide range of organic transformations. Due to their great stability, they are exceptionally useful the catalysts of choice, for the organic reactions, conducted at high temperature (> 100 °C), even under wet conditions [20][21][22][23][24] .
Click chemistry as one of the most prevalent reactions, originally introduced by Sharpless and coworkers in 2001, is performed with high selectivity under mild conditions. This reaction has been used in a wide range of research areas such as, polymers, drug discovery and supramolecular chemistry [25][26][27] . Among click reactions, Huisgen 1,3-dipolar cycloaddition is the most well-known example, in which organic azides and terminal alkynes are combined to provide 1,4-disubstituted 1,2,3-triazole derivatives. 1,2,3-triazoles compound is an significant heterocyclic category of organic compounds with extensive range of applications 28,29 . More recently, novel methods have been used to promote click reaction, including the incorporation of Cu metal into some organic and inorganic materials for preparing heterogeneous or homogeneous catalytic systems [30][31][32][33][34][35][36][37] . Despite all the achievements, some limitations such as high reaction temperature and duration, use of toxic solvents and high amount of the catalyst have been still unsolved. It is therefore necessary to promote efficient methods.
In continuation of interest on using of functionalized HNTs for immobilizing catalytic active species 38-40 and our continuous interest in click reaction from different points of view [41][42][43][44][45] , herein, we wish to report the preparation of a novel Schiff base-halloysite hybrid system, as a catalytic support for immobilization of CuI nanoparticles and reveal our investigation on the aforementioned, hybrid catalyst, CuI@HNT-TSC-PC in one of the classist organic transformations so called "Click Reaction". The new prepared nano composite, CuI@HNT-TSC-PC was fully characterized by employing different techniques such as FTIR, SEM, EDX, XRD, TGA, and ICP-AES. Bruker Tensor 27 instrument was used for recording the FTIR spectra between 4000 and 400 cm −1 and the KBr pellet technique was employed: about 1 mg of the sample and 300 mg of KBr were used to prepare the pellets. SEM/EDS images were recorded by a TESCAN, VEGA 3 SEM instrument and all samples had been coated with a thin gold layer by evaporation. X-ray diffraction patterns of CuI@HNT-TSC-PC, CuI and halloysite were recorded at room temperature by using a Siemens, D5000 diffractometer with Ni-filtered Cu Kα radiation, working at 40 kV and 30 mA, at a scanning speed of 2°/min in the scan range from 5° to 80° 2θ. Thermogravimetric (TG) and differential thermal (DTA) analyses were conducted in a NETZSCH TG 209 F1 Iris thermo gravimetric analysis apparatus from 25 to 600 °C, under nitrogen atmosphere, at a heating rate of 15 °C/min. The BET analysis of the CuI@HNT-TSC-PC was performed using a fully automated BET surface area analyzer (Brunauer-Emmett-Teller, model: Belsorp-mini II) instrument at − 196 °C; 0.2 g of the sample was used. A Perkin-Elmer Optima 3100XL axial viewing ICP-AES equipped with a cyclonic spray chamber and a GemTip cross-flow nebulizer was used for the determination of the trace Cu elements. The CuI@HNT-TSC-PC were introduced into the ICP-AES system at a flow rate of 1.0 mL min −1 . All products were known and identified by comparison of their physical and spectroscopic data with those of authentic compounds reported previously and found being identical and also several of products were identified by NMR analysis.

Preparation of nano composite. Preparation of Cl-functionalized halloysite: HNT-Cl. To functionalize
HNTs, initially, HNTs (1.5 g) was dispersed in 40 ml dry toluene and then CPTES (4 ml) was added dropwise to the mixture. The resulting suspension was refluxed at 110 °C for 24 h. At the end of the process, the resulting precipitate was separated by simple filtering, washed and dried at 80 °C overnight.
Synthesis of Schiff base: TSC-PC. Schiff base was prepared according to the previously reported procedure 46 . Briefly, pyridine-2-carbaldehyde (10 mmol, 1.05 ml) and thiosemicarbazide (10 mmol, 0.94 g) were dissolved in water (10 ml) and heated at 75 °C for 6 h. Upon completion of the reaction, the yellow precipitate was filtered off, and washed with EtOH (10 ml) and dried at 70 °C. www.nature.com/scientificreports/ Synthesis of HNT-TSC-PC. Conjugation of HNT-Cl and Schiff base was accomplished through the following procedure: first, HNT-Cl (1.5 g) was well-dispersed in dry toluene by using ultrasonic irradiation for 20 min. then, Schiff base (TSC-PC) (1.5 g) and TEA (0.1 ml) were added and the resulting mixture refluxed overnight. At the end of the reaction, the obtained product was filtered off, washed with toluene and dried in oven.

Immobilization of CuI NPs on the HNT-TSC-PC: Synthesis of CuI@HNT-TSC-PC.
HNT-TSC-PC (1 g) was dispersed in toluene and a solution of CuI (0.095 g, 0.5 mmol) in CH 3 CN was then added into the suspension and the obtained mixture was stirred under N 2 atmosphere for 10 h. Finally, the product was collected, washed with toluene and dried at 70 °C for 12 h (Fig. 1).
Regioselective synthesis, of 1,4-disubstituted 1,2,3-triazoles via click reaction: general procedure. Alkyne (1 mmol) and α-haloketone or alkyl halide (1.0 mmol) and sodium azide (1.3 mmol) were mixed in 5.0 ml water:ethanol (1:1) mixture in the presence of CuI@HNT-TSC-PC (0.03 g) and the resulting mixture was refluxed for appropriate time. Upon completion of the reaction (monitored by TLC), the mixture was filtered off, and the filtrated was washed with deionized water and purified by recrystallization with hot ethanol. The residue catalyst was washed and dried at 80 °C for the next reaction runs.

Result and discussion
Catalyst characterization. First, to study the structure of CuI@HNT-TSC-PC and to confirm the appropriate progress of each stage of catalyst synthesis, the FTIR spectra of HNTs and the product of each step of catalyst synthesis (TSC-PC, HNT-Cl, HNT-TSC-PC, CuI@HNT-TSC-PC) were recorded (Fig. 2). The FTIR spectra of HNTs showed the characteristic bands at 1649 cm −1 (Si-O stretching, 539 cm −1 (Al-O-Si vibration), and 3621-3696 cm −1 (inner -OH groups) 47 . The similarity between FTIR spectrum of HNT-Cl and pure HNTs, indicates the stability of the halloysite structure after functionalization with organosilane. The FTIR spectrum of the HNT-TSC-PC showed the characteristic bands of HNTs included, 1649, 539 and 3621-3696 cm −1 as well as two additional band at 1326 cm −1 , which is representative of C=S stretching and the strong absorption peaks of the C=N band at 1627 cm −1 confirms the successful formation of Schiff base (Fig. 2c, TSC-PC). Moreover, in the FTIR spectrum of the catalyst not only all characteristic bands were observed, but also two bands at around 1415 www.nature.com/scientificreports/ and 1627 cm −1 of Schiff base were shifted slightly to right (1359 and 1617 cm −1 ), indicates successful interaction between Cu and C=N and C=S groups. The morphology of the catalyst was investigated by SEM/EDS techniques (Fig. 3). The SEM images of CuI@ HNT-TSC-PC are shown in Fig. 3B and C. As depicted, the short tubes of HNTs closed together to form aggregates. Compared to the tubular morphology of pure HNTs (Fig. 3A), CuI@HNT-TSC-PC exhibited a distinguished morphology, which is more compact than pure HNTs. This observation can be attributed to the presence of Schiff base on the surface of HNTs and the possibility of electrostatic interactions among the tubes. This may bring the tubes closer and facilitate formation of aggregates. As depicted in the SEM image, the average diameter size of synthesized CuI@HNT-TSC-PC was 27 nm. The EDS analysis of the catalyst is presented in Fig. 3D. The observation and concentration of Si (2.81%), Al (2.98%) and O (26.83%) in the EDS of CuI@HNT-TSC-PC can indicate HNTs structure. The presence of C (27.96%), N (28.35%) and S (7.66%) atoms can confirm the incorporation of Schiff base that the high concentration observed in C and O atoms may be related to the holder. Additionally, the presence of Cu (1.48%) and I (1.93%) can be representative of CuI in the structure of the catalyst.
Moreover, the pure HNTs show a diffraction peak at 2θ = 12.25° (001), which is related to its tubular morphology, high degree of disorder, small crystal size, and interstratifications of layer with various hydration states. In the XRD pattern of the catalyst, this peak is shifted to a lower 2θ value. The d-spacing of catalyst is 0.716 nm at a 2θ of 12.18°. The evidence of intercalation between Schiff base and the HNTs is strongly supported by the 2θ reductions of the increases in the basal spacing of the HNTs in catalyst, which confirms the formation of CuI@ HNT-TSC-PC. In relation to the two additional diffraction peaks displayed in the XRD pattern for pure HNTs at 2θ of 20.06° (020) and 24.94° (002), the subsequent XRD patterns for the catalyst revealed that the pure HNTs peak at 2θ at around 20.06°, for the catalyst, had shifted markedly lower, and the peak at 2θ at around 24.94°, for the catalyst, had almost completely vanished. These results support the existence of intercalation of the organic linker into the structure of the HNTs.
To further characterization of the catalyst, the thermal stability of pure HNTs, HNT-Cl and CuI@HNT-TSC-PC were studied using TGA (Fig. 5). As shown, pure HNTs exhibited two weight losses, the first one, around 150 °C, is because of losing water and the second one, around 500 °C, is due to the dehydroxylation of the HNTs www.nature.com/scientificreports/ matrix 50 . To estimate the content of the organosilan, Cl, on the surface of HNTs, the thermogram of HNT-Cl was obtained and compared with that of pure HNTs, Fig. 5a and b. The results showed that the content of organosilan was about 4 w/w%. Next, the thermograms (TG and DTG analyses) of catalyst were recorded. According to Fig. 5c, three steps weight losses were exhibited, which are attributed to the loss of water (130 °C), decomposition of organic groups (270 °C) and dehydroxylation of Hal (480 °C). Moreover, to estimate the content of Schiff base and CuI on the catalyst, the thermogram of CuI@HNT-TSC-PC was considered and compered with that of HNT-Cl. The comparison of the thermograms of HNT-Cl and CuI@HNT-TSC-PC, showed that incorporation of Schiff base and CuI can alter the thermogram obviously, that this can be due to degradation of copper iodide. The calculation showed that the content of Schiff base was about 34.2 wt%. Finally, BET analysis was performed to confirm that ligand and copper nanoparticles were located on the surface of Hal and examined the textural of the catalyst. The CuI@HNT-TSC-PC nitrogen adsorption-desorption isotherm is illustrated in Fig. 6 and Table 1. As shown, the recorded isotherm of type II isotherms is similar to pure HNTs 51 . As shown, the recorded isotherm is a type II isotherm and is similar to the pure HNTs sample. Using BET, the specific surface area of CuI@HNT-TSC-PC was measured 8.48 m 2 g −1 , which is much lower than the pristine Hal (51 m 2 g −1 ). These observations indicate that ligand and CuI nanoparticles are located on the outer surface of the Hal.
Cu loading of CuI@HNT-TSC-PC was measured by ICP-AES analysis. Sample was prepared for ICP analysis as follow: CuI@HNT-TSC-PC (0.02 g) was digested in a mixture (1:3) of concentrated nitric acid and hydrochloric acid by continuous stirring for 24 h. The extract was then analyzed by ICP-AES. The Cu content of catalyst was measured at about 3.6 wt%.    www.nature.com/scientificreports/ (Table S1). The results indicated that the highest yield of the model product was achieved at room temperature in the presence of 40 mg catalyst. The generality of these conditions was then examined Using different raw materials to produce different 1,2,3-Triazoles ( Table 2). The results confirm that CuI@HNT-TSC-PC can catalyze the click reaction of all substrates to achieve the corresponding 1,2,3-triazole compounds within short reaction times and in high yields.
Catalyst recyclability. To achieve a comprehensive study, the recyclability and copper leaching of this catalyst were surveyed (Fig. S1). The importance of such determinations is because of clarifying the heterogeneous nature of the catalyst and its potential to be used in scale-up processe in industries. Therefore, the yield of the model reaction was determined in the presence of both fresh CuI@HNT-TSC-PC and the recycled ones. It was found that the catalyst can be used up to 6 cycles giving the product with no significant drop of yield. It proves high recyclability of CuI@HNT-TSC-PC.

Conclusion
In summary, a novel heterogeneous nano composite, CuI@HNT-TSC-PC, was designed and prepared through Cl-functionalization of HNTs followed by reaction with Schiff base and formation of imine functionality and incorporation of CuI. The catalyst was successfully used for promoting Click reaction of α-haloketone or alkyl halide, terminal alkyne and sodium azide in aqueous media and mild reaction condition for the synthesis of 1,2,3-triazoles. Notably, the results of recyclability of the catalyst confirmed its good recyclability and low leaching of Cu species. Moreover, the above-mentioned catalyst was reused up to 6 cycles with slight descent of product yield and Cu leaching.  www.nature.com/scientificreports/  www.nature.com/scientificreports/

Data availability
The raw/processed data that supports the findings of this study is available from the corresponding author upon reasonable request.