Decorated multi-walled carbon nanotubes with Sm doped fluorapatites: synthesis, characterization and catalytic activity

Abstract

Novel and sustainable heterogeneous catalysts, namely, multi-walled carbon nanotubes (MWCNT) decorated with Sm doped fluorapatite nanocomposites (MWCNT/Sm-FAp) were prepared by facile co-precipitation method using glutamic acid as an organic modifier with different loadings of Sm (1%, 2%, 3%, 5%, and 7%). The nanocomposites were characterized by powder X-ray diffraction (PXRD), Fourier transform infra-red spectroscopy (FT-IR), microscopic techniques (FESEM, HRTEM), energy dispersive X-ray spectrometer (EDX) and Brunauer–Emmett–Teller method (BET) surface area, and thermogravimetric analysis (TGA). The SEM and TEM results confirmed that with varying Sm loading, the nanocomposites attained varied particles sizes in the nano-grade scale with different morphologies. The N₂ adsorption isotherm results showed that the BET surface area of nanocomposites enhanced from MWCNT/1% Sm-FAp (47.60 m² g⁻¹) to MWCNT/7% Sm-FAp (62.00 m² g⁻¹). The robust catalytic behavior of the MWCNT/Sm-FAp nanocomposites impressively showcased the esthetic results in the highly selective synthesis of the 1,2,4-triazole moiety from aromatic aldehydes. The catalytic efficiency increased with rise in Sm% from 1% to 5% in the nanocomposite. With MWCNTs/5% Sm-FAp nanocomposite as the catalyst, the reaction recorded 96% yield of triazole in short reaction times (10 min).

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1. Introduction

Over the past decade, the coupling of multi-walled carbon nanotubes (MWCNT) with other competent materials has been pursued to ameliorate their existing properties, which was an endorsement of the materials behavior with applications in various fields.^1–4 Nanocomposites based on MWCNT dispersed in other materials received substantial attention to alleviate the obstacles associated with the limited scope of MWCNT towards the widespread utilization.^5–8 MWCNT possess unique properties, such as large surface areas, high aspect ratios, porous nature, mechanical, thermal and electrical properties, which propel their use in every sphere of life.^9,10 Despite the properties of MWCNTs, their excellence in various applications is limited due to propensity of weak dispersivity in different solvents.¹¹ To improve the interactivity of MWCNT, functionalization and conjunction of MWCNTs with other materials are commonly adopted techniques, which help the material take up new roles and pave way for new structures.^12,13 Synthetic fluorapatite [FAp, Ca₁₀(PO₄)₆F₂] shows exceptional physical and chemical properties, which can find applications in various fields as adsorbents, drug and catalyst carriers and clinically essential biomaterials.^14–16 The enveloped traits of low solubility, good compatibility and ease of replacement of ions in FAps have increased the researchers' interest in these materials in heterogeneous catalysis.¹⁷ Moreover, it is important to consider the wider scope of these materials in various organic transformations. Structural engineering of FAps plays a pivotal role in the efficacy as a catalyst.^18,19 Therefore, controlling the crystal growth and morphological features during the preparation of FAps is a huge allurement in the development of well-crafted materials with extreme superiority as heterogeneous catalysts.^20–23 During the synthesis of FAps, additives, such as ethylenediaminetetraacetic acid salt, organic compounds, polysaccharides and surfactant, have been employed in the control of nucleation and crystal growth of materials under various experimental conditions.^24–26 Amino acids can inhibit the growth of crystals through electrostatic interactions on the surfaces of apatite crystals, which predominantly lead to vibrant morphological developments.^27,28 More interestingly, the literature reports have stressed that the glutamic acid-assisted synthesis of apatite crystals has profound significance in the design of materials with different topologies.²⁹ The added advantage of FAps is easily replaceable tendency of its Ca⁺² and PO₄³⁻ ions with other valued metal ions. The incorporation of rare earth elements as promoters can further facilitate the materials for high-efficiency utilization in catalysis. Rare earth elements have the characteristics of unoccupied 4f orbitals and lanthanide contraction, which result in their applicability as catalysts or catalytic components.^30–33 The introduction of rare earth elements in the molecular matrix can enhance the activity, stability, surface area, and selectivity of the catalyst material, which eventually leads to better structural stability as well as higher product yields.^34,35 Herein, the ionic radii from La to Lu are mostly similar to Ca⁺² with meagre variations that simplify the replacement of Ca⁺².³⁶ Sm is quite useful for doping in the solid catalyst lattice because of its ability to control the chelation of Sm intermediate in the transition state.³⁷ Furthermore, the Sm mediated derivatives are less toxic to handle in the chemical processes. Approximately 90% of chemical processes depend on catalysts and the progress of the chemical industry as well as materials science has been dampened due to the lack of innovative catalytic materials. Individually, MWCNTs and Sm-doped FAp materials do not contribute to the development of catalysis, so their combination can divulge a new vista in their enhanced catalytic performance. To the best of our knowledge, MWCNT/Sm-FAp nanocomposites have not been reported in the literature.

Triazoles are valuable organic entities related to fused heterocyclic groups and have numerous emerging applications in the pharmaceutical and other chemical industries.^38,39 Triazole-containing organics possess anti-microbial, anti-malarial, anti-tumor, anti-tubercular, anti-inflammatory, and anti-convulsant properties.^40–42 In addition to the biological importance, triazoles are considered as high energy density materials due to their structure engulfed with more nitrogen, which also has military applications.⁴³ Many of the reported methods for synthesis of triazoles had limitations, such as long reaction times, poor yields, and harsh conditions or elaborate work-up. Earlier, we reported valued added conversions, including Knoevenagel condensations and selective oxidation of n-pentanes using different modified hydroxyapaties.^44–46 We also reported the advantages of using rare earth-loaded mixed oxide catalysts in value-added conversions.⁴⁷

In this manuscript, we report the synthesis and characterization of Sm-FAp in the presence of glutamic acid with a 1% to 7% loading of Sm as well as the control of their morphological features. Secondly, MWCNT/Sm-FAp nanocomposites were synthesized and their catalytic efficacy in the value added transformation of aromatic aldehydes to thione bearing 1,2,4-triazole moieties was evaluated.

2. Experimental

2.1. Materials

All chemicals used were of analytical grade and used as supplied. Calcium nitrate, Ca(NO₃)₃·4H₂O (Merck, Germany), samarium nitrate, Sm(NO₃)₃·9H₂O (Sigma-Aldrich), and glutamic acid (BDH) were used in the preparation. While tri-sodium phosphate (Na₃(PO₄)·12H₂O) and sodium fluoride (NaF) were from Merck (Germany), thiosemicarbazide and the other aromatic aldehydes were from Aldrich. Multi-wall carbon nanotubes (MWCNT, outer diameter 30–50 nm, length 10–20 μm, purity > 95 wt%) were purchased from Shenzhen Nano-Technologies, China and used as received. Absolute ethanol (Merck, Germany) and highly purified deionized water were employed for all the preparations and experiments.

2.2. Synthesis of catalyst

2.2.1. Oxidation of MWCNTs. The MWCNTs were oxidized to enable them to form feasible chemical bonds with the material of interest. An amount of 0.5 g pristine MWCNTs were mixed with a mixture of concentrated H₂SO₄ and HNO₃ in 3 : 1 proportions. Subsequently, the mixture was sonicated for 3 h at 40 °C in an ultrasonic bath to achieve the introduction of carboxylic acid groups on their side walls. After the 3 h stipulated time, the mixer was transferred dropwise to a 50 mL cold distilled water-containing beaker and allowed to stand for 10 min. The resulting oxidized MWCNT were filtered and dried in a vacuum at 90 °C for 6 h.

2.2.2. Preparation of Sm-FAp. As a protocol, to 25 mL of deionized water in 50.0 mL beaker, fixed amounts of tri-sodium phosphate dodecahydrate (1.5 mM, 570 mg) and glutamic acid (2 mmol, 330 mg) were added in sequence with continuous stirring. Sodium fluoride (0.5 mM, 20 mg) was added to the mixture and stirred until a clear solution was obtained. Then, calcium nitrate tetra hydrate (2.5 mmol, 590 mg) was added to the mixture and vigorous stirring was continued for 15 min. The required amount of samarium nitrate hexahydrate (Sm: 1% = 15 mg, 2% = 30 mg, 3% = 45 mg, 5% = 15 mg, 7% = 105 mg) was added slowly and the mixture was stirred continuously for 6 h. To facilitate crystallization, the solution mixture was left undisturbed for 2 h. The formed crystals were then separated by centrifugation plus washings with deionized water to obtain the high purity crystals. The material was calcined for 3 h at 350 °C under continuous air-flow conditions. A total five samples with different Sm loadings of were synthesized and characterized.

2.2.3. Preparation of MWCNT/Sm-FAp nanocomposite catalyst. In 25.0 mL of ethanol solvent, a series of Sm-FAp samples from 1% to 7% of Sm were mixed independently with MWCNT. The mixture containing 20 mg of oxidized MWCNT and 120 mg of Sm-FAp was exposed to ultra-sonication at room temperature for 45 min. After the reaction was complete, the excess ethanol was evaporated under mild temperature conditions. The five nanocomposites (MWCNT/1% Sm-FAp, MWCNT/2% Sm-FAp, MWCNT/3% Sm-FAp, MWCNT/5% Sm-FAp, and MWCNT/7% Sm-FAp) were characterized to confirm their structural architectures.

2.3. Instruments

A powder X-ray diffractometer (Bruker D8 Advance diffractometer with Ni filtered Cu Kα radiation, V = 45 kV, I = 40 mA) was employed to analyze the crystallinity and phase. The spectra were obtained at scan rate of 2° min⁻¹ and a step size of 0.02° in a 2θ range from 10° to 70° at room temperature. A Perkin Elmer, spectrum 100 spectrometer was used to obtain the Fourier transform infrared spectra (FT-IR) over the 4000–400 cm⁻¹ range with 4 cm⁻¹ resolution to ascertain the formation of fluorapatites, by confirming the existence of phosphate bands. A field emission scanning electron microscope (FESEM, Zeiss Ultra Plus, Germany) operating at 5–20 kV and a high resolution transmission electron microscope (HRTEM, JEOL 2100, Japan) with an accelerating voltage of 200 kV were used to investigate the size, morphology, and nanostructure of the composites. For FESEM, an Au film was used to coat all the samples analyzed to achieve a good resolution results. Elemental analysis was performed employing an energy dispersive X-ray spectrometer (EDX, Aztec software, Oxford instruments, UK) unit attached to FESEM for the confirmation of presence of fluorine and Sm in the nanocomposites. N₂ sorption isotherm (Micromeritics Tristar II 3020, USA) at 77 K was derived to determine the surface area and pore properties. For the sorption experiments, to clear the spaces occupied by unwanted gases, all the samples were degassed for 16 h at 180 °C. To determine the thermal stability of the materials, TG analysis was performed with an SDT Q 600 apparatus at a heating rate of 10 °C min⁻¹ under N₂ flow. A Bruker Advance 400 spectrometer at ambient temperature was used to obtain the ¹H, ¹⁵N and ¹³C NMR spectra to identify and confirm the products of the organic reactions.

3. Results and discussion

3.1. Powder XRD analysis

Fig. 1 illustrates the powder XRD pattern of MWCNT/Sm-FAp nanocomposites, which shows that the materials were very crystalline. All the five materials displayed similar patterns with trivial variations in the relative intensities. The characteristic peaks of FAp matrix well matched the standard fluorapatite card of JCPDS # 15-0876 (ref. 48) and the reflections were indexed to the (002), (102), (210), (211), (300), (202), (310), (222), (213), and (004) planes. The XRD data were examined using X'Pert Pro high Score plus software and the FAp phase was found to have aggregable purity. The high purity of the phases without the scope for impurities proved the elegance of the synthetic procedure applied for the synthesis of catalysts. The relative intensity of the reflections and the peak broadening varied slightly according to the Sm content. MWCNT/3% Sm-FAp exhibited sharp and high intense peaks compared to other catalysts. The main reflections appearing at 2θ = 23.26° of MWCNT could be seen in the 2% and 3% Sm doped catalysts and disappeared in the higher Sm loaded catalysts due to the influence of the FAp phase. No reflections corresponding to Sm phases were observed. The minute deviations in the reflections might be due to the presence of various morphological features attained by the catalysts in the presence of glutamic acid as well as the addition of Sm.

Fig. 1 Powder XRD pattern of (a) MWCNT/1% Sm-FAp (b) MWCNT/2% Sm-FAp (c) MWCNT/3% Sm-FAp (d) MWCNT/5% Sm-FAp (e) MWCNT/7% Sm-FAp.

3.2. FT-IR results

Fig. 2 shows the FT-IR spectra of the catalysts, which interpreted the conformation of structural integrity of five catalysts. The MWCNT/Sm-FAp with various Sm doping concentrations displayed similar absorptions peaks without any obvious changes. The major characteristic peaks observed at 1065 cm⁻¹, 605 cm⁻¹, and 565 cm⁻¹ represent the phosphate group's stretching and bending vibrations. The peaks appeared at 605 and 565 cm⁻¹ were from the bending asymmetric modes of triply degenerated O–P–O bonds of phosphate group, whereas the strong band at 1065 cm⁻¹ was due to P–O asymmetric stretching vibrational mode of the phosphate group.⁴⁹ Glutamic acid added for the purpose of nucleation control acted as constraint in crystal growth with no presence in the final product. This was supported by no absorption frequencies corresponding to the amino acid in the spectra. No predominant change was observed in the spectrum with increasing Sm doping concentration. The absorption peaks corresponding to carboxyl groups present on the sidewalls of MWCNT were much less intense and were overlapped by the absorption peaks of Sm-FAp. As shown in Fig. S1,† the FT-IR spectrum of oxidized MWCNT showed peaks at 3392, 1693, 1195, and 1052 cm⁻¹, which corresponding to –OH, C=O, C–O–C and CO–C bonding vibrational frequencies of carboxyl functional group, respectively. These peaks represent the successful generation of –COOH groups on the MWCNTs compared to the pristine MWCNTs.

Fig. 2 FT-IR spectra of (a) MWCNT/1% Sm-FAp (b) MWCNT/2% Sm-FAp (c) MWCNT/3% Sm-FAp (d) MWCNT/5% Sm-FAp (e) MWCNT/7% Sm-FAp.

3.3. SEM and TEM microscopic analysis of MWCNT/Sm-FAp nanocomposites

Fig. 3 displays the typical SEM, TEM and EDX images of series of MWCNT/Sm-FAp nanocomposite catalysts. The morphology attained by the catalysts has a great role in showcasing the catalytic performance. The glutamic acid assisted synthesis of MWCNT/Sm-FAp with various % of Sm contributed to the changes in the morphological structures. The surface of the MWCNT appeared with a partial covering by Sm-FAp. The Sm-FAp with a 1% Sm content displayed the flower bud-like in morphology (Fig. 3a). It is evident from the TEM images that as the Sm content was changed from 1% to 7%, the nano-sized particles were initially well dispersed and flower bud-like and were modified to a disturbed structure. The decoration of MWCNTs with Sm-FAp was achieved with an uneven distribution of Sm-FAp around the circumference of the MWCNTs. TEM images shows that the Sm-FAp particles in the nanocomposite possessed irregular sizes with about 60 nm average diameter, confirming the formation of the MWCNT/Sm-FAp nanocomposite through the control of nucleation and growth with the assistance of glutamic acid.

Fig. 3 FESEM, EDX, HRTEM micrographs of (a–c) MWCNT/1% Sm-FAp (d–f) MWCNT/2% Sm-FAp (g–i) MWCNT/3% Sm-FAp (j–l) MWCNT/5% Sm-FAp (m–o) MWCNT/7% Sm-FAp.

3.4. Textural properties and TG analysis

The nitrogen adsorption isotherm was used to determine the surface area of MWCNT/Sm-FAp nanocomposite. A perusal of the obtained isotherms of the nanocomposites indicate that they belong to type II isotherms and the amount of N₂ adsorbed also increased with increasing % loading of Sm. The BET-surface area results showed that the surface area of MWCNT/Sm-FAp nanocomposite increased with increasing Sm content from 1% to 7%. The MWCNT/1% Sm-FAp had the lowest surface area 47.60 m² g⁻¹, whereas the MWCNT/7% Sm-FAp recorded the highest surface area of 62.0 m² g⁻¹. The incorporation of Sm increased the surface area of the catalyst. Sm may act as a good structural promoter through developing additional surface planes, leading to an increase in surface area. With increasing Sm loading, the concentration of dopant induced roughness to the surface of the composite, which possibly increases the surface area. In the current case, the beneficial synthetic procedure has hindered the clogging of the pores during dopant loading. Fig. 4 illustrates the BET-surface area, pore size and pore volume attained by the five catalysts under various Sm contents. The average pore volume was high for the MWCNT/7% Sm-FAp catalyst, and only marginal changes were observed with 1%, 2%, and 3% Sm MWCNT/Sm-FAp nano-composites. The MWCNT/Sm-FAp composites were mesoporous (pore size, 15 to 20 nm) and MWCNT/1% Sm-FAp had the highest porosity. Unambiguously, the influence of glutamic acid and Sm content on the textural characteristics of nanocomposites was observed.

Fig. 4 N₂ sorption isotherms with surface area and pore properties.

Thermogravimetric analysis over the temperature range, 25 °C to 1000 °C, showed that the Sm-FAp/MWCNT nanocomposites displayed good thermal resistance and the onset temperature for structural fragmentation was 600 °C. The first weight loss was observed between 200 and 250 °C, which corresponded to the free water content adsorbed on the surface of the composite. At the temperature range between 500 and 600 °C, the oxidized MWCNTs underwent decomposition. Moreover, the oxidized MWCNTs effectively retarded the thermal decomposition compared to pristine MWCNTs due to the presence of hydrogen bonds between the carboxyl groups present on the walls of the tubes.⁵⁰ The preservation of MWCNTs in the composite structure is believed to improve the adsorptive and mechanical properties of Sm-FAp/MWCNT composite. The Sm loaded fluorapatites in the composite started decomposing above 900 °C (Fig. S2†). Only 15% weight loss was observed up to 1000 °C. The Sm loadings on FAp have not shown any predominant changes in their thermal stability. The literature reports that fluorapatites are more stable compounds due to their highly ordered and fluorine content in their structure.⁵¹

4. Catalytic studies

4.1. Procedure for the synthesis of 1,2,4-triazoles with MWCNT/Sm-FAp nanocomposite as a catalyst

To the ethanol solvent (5.0 mL) in a 10.0 mL round bottom flask at room temperature, aromatic aldehyde (1.0 mmol), thiosemicarbazide (1.0 mmol) and MWCNT/Sm-FAp (30 mg) were added with continuous stirring using a magnetic stirrer. The reaction progress was monitored by TLC. After the completion of the reaction, sufficient ethanol was added to dissolve the organic compound formed and the solid catalyst was recovered by filtration. The final product was recovered after the evaporation of ethanol under vacuum. The reaction product was positively identified and confirmed by IR, ¹H-NMR, and ¹³C-NMR spectral data. The progression of the reaction in the presence of the MWCNT/Sm-FAp nanocomposite may be described using the following scheme.

An examination of the IR spectra of products illustrated in Fig. 5 indicates that all the reactions catalyzed by the differently loaded Sm nanocomposites gave the same product. The main absorption frequencies appeared at 3443, 3321, and 3175 cm⁻¹ were the stretching vibrations belonging to the three –NH groups. The stretching vibrational peaks appearing at 1276 cm⁻¹ confirms the existence of –C=S bonding. The identity of triazole and the presence of three –NH secondary amine protons were established by the ¹HNMR signals observed at δ 7.91, 8.10, 11.39 ppm (Fig. 6) and the signal at δ 3.82 ppm validates the presence of methoxy protons. In ¹³C NMR, the signal at δ 177 ppm indicates the thiocarbonyl entity in the structure (Fig. 7). The ¹⁵N spectrum (Fig. 8) further endorses the presence of three N in the triazole moiety.

Fig. 5 FT-IR spectra of 5-(2-methoxyphenyl)-1,2,4-triazolidine-3-thione with different Sm loading MWCNT/Sm-FAp nanocomposite.

Fig. 6 ¹H-NMR spectrum of 5-(2-methoxyphenyl)-1,2,4-triazolidine-3-thione.

Fig. 7 ¹³C spectrum of 5-(2-methoxyphenyl)-1,2,4-triazolidine-3-thione.

Fig. 8 ¹⁵N spectrum of 5-(2-methoxyphenyl)-1,2,4-triazolidine-3-thione.

4.1.1. Spectral data of 5-(2-methoxyphenyl)-1,2,4-triazolidine-3-thione. ¹H NMR (400 MHz, DMSO-d₆) δ 3.82 (s, 3H, OCH₃), 6.95 (t, J = 7.48 Hz, 1H, ArH), 7.05 (d, J = 8 Hz, 1H, ArH), 7.35–7.39 (m, 1H, CH), 7.91 (s, 1H, NH), 8.07 (dd, J = 7.76 Hz, 1.68 Hz, 1H, ArH), 8.10 (s, 1H, NH), 8.39 (s, 1H, CH), 11.39 (s, 1H, NH): ¹³C NMR (100 MHz, DMSO-d₆): 55.64, 111.61, 120.52, 122.09, 126.03, 131.30, 137.92, 157.71, 177.77; ¹⁵N NMR (40.55 MHz, DMSO-d₆) δ 7.91 (s, 1H, NH), 8.10 (s, 1H, NH), 11.39 (s, 1H, NH).

4.2. Catalytic performance of MWCNT/Sm-FAp nanocomposite with respect to Sm content

All the experiments were conducted at room temperature and with ethanol as the solvent. In the initial experiments in the absence of a catalyst, after 80 min reaction time, the observed yield was 60%. The scope of the five prepared MWCNT/Sm-FAp nanocomposites as a catalyst in the same reaction was examined. The catalyst, MWCNT/1% Sm-FAp quantitatively measured to 30 mg was introduced in the reaction mixture. In presence of the catalyst, the reaction was complete in the short interval time of 15 min with an 86% yield of the desired product. Under the same experimental conditions, the reaction was examined with other nanocomposites with different Sm loadings. The reactions using 2%, 3% and 5% Sm doped MWCNT/Sm-FAp nanocomposites occurred smoothly and completed with excellent yields in 10 min. While the product yield with MWCNT/2% Sm-FAp was 89%, MWCNT/3% Sm-FAp and MWCNT/5% Sm-FAp nanocomposites yielded 94% and 96%, respectively. When Sm was increased from 5% to 7%, the yield dropped to 85% and reaction time increased to 20 min (Table 1). Although the surface area increased with increasing Sm loading from 1% to 7%, the catalytic efficiency was higher with 5% Sm loading. Based on the results, 5% Sm was considered ideal. The nature of the supports, metal/support ratio, metal-support interaction is known to influence the catalytic activity predominantly.⁵² In addition to the surface area, the surface morphologies, particle size, and how the dispersion of metal takes place on the surface of the host matrix grab the attention in the selectivity of catalyst.⁵³ The catalytic activity is not always proportional to the metal content as its superb distribution and balance of the active sites of the support and the metal accelerate the reaction with high proficiency. A higher metal loading could disturb such a balance, leading to lower efficiency. Sm loading, when increased from 5% to 7% led to detrimental catalytic activity, which could be due to the increased coverage of Sm on the support surface, which ultimately decreased the amount of adsorbed precursors. Hence, the proposed model reaction (scheme) was investigated with MWCNT/5% Sm-FAp as a catalyst for optimal performance. In order to enumerate the amount of catalyst required, the reaction was conducted with varying catalyst amounts. At catalyst amounts below 30 mg, the reaction yields reduced to 75% (ESI, Table 1,† entries 1–3), whereas with more than 30 mg, the yields were more identical with marginal changes (ESI, Table 1,† entries 4 and 5). Thus, MWCNT/5% Sm-FAp catalyst (30 mg) was chosen as the ideal quantity.

Table 1 Catalytic performance of MWCNT/Sm-FAp nanocomposite with various Sm loadings for model reaction (scheme)^a

Catalyst	Reaction time (min)	Yield (%)
a Reaction conditions: 2-methoxybenzaldehyde (1.0 mmol), thiosemicarbazide (1.0 mmol) and catalyst and ethanol (5.0 mL) were stirred at room temperature.
No catalyst	80	65
MWCNT/1% SmFAp	15	86
MWCNT/2% SmFAp	10	89
MWCNT/3% SmFAp	10	94
MWCNT/5% SmFAp	10	96
MWCNT/7% SmFAp	20	85

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By using these optimal conditions, the efficacy of the MWCNT/5% Sm-FAp catalyst for the synthesis of other 1,2,4-triazole derivatives was investigated employing various substituted aromatic aldehydes as precursors. Reactions with all substituted aromatic aldehydes with MWCNT/5% Sm-FAp as a catalyst worked well and produced the 1,2,4-triazole derivatives with excellent yields (Table 2, entries 1–4). The literature survey shows a preparation method for 5-aryl-1,2,4-triazolidine-3-thione derivatives using polyethylene glycol as a solvent.⁵⁴ The synthesis of 1,2,4-triazolidine-3-thiones have also been reported using substituted thiosemicarbazide and ionic liquid, [C₁₆MPy]AlCl₃Br.⁵⁵ 5-Aryl-1,2,4-triazolidine-3-thiones have been prepared by Mane et al. using a mixture of aldehyde, hydrazine hydrate, and trimethylsilyl isothiocyanate, and sulfamic acid as a catalyst.⁵⁶ 1,2,4-Triazolidine-3-thione derivatives synthesized in excellent yields in this study are novel compounds and have not been reported previously. There are no reports about the use MWCNT/Sm-FAp nanocomposites as catalysts in any organic synthesis, making it a novel catalyst.

Table 2 Synthesis of 1,2,4-triazolidine-3-thione moiety with different substituted aromatic aldehydes with MWCNT/5% Sm-FAp nanocomposite as a catalyst^a

Entry	Aldehyde	Product	Yield (%)	Time (min)
a Reaction conditions: substituted benzaldehyde (1.0 mmol), thiosemicarbazide (1.0 mmol) catalyst and ethanol (5.0 mL) were stirred at room temperature.
1			93	10
2			95	15
3			90	15
4			96	10

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4.3. Catalyst recycling

After the completion of the reaction, the catalyst material was recovered thorough vacuum filtration followed by washing several times with water and drying in a vacuum for 2 h at 50 °C. The activity of the used catalyst was examined for further cycles. The recycled catalyst retained its activity for up to 6 runs and slight depletion in the yield and increase in reaction time was observed in the consequent run.

5. Conclusions

While the surface area of the catalyst is important, the structural modification of Sm dispersed over the surface of the host matrix is paramount for the desired catalytic performance. This study demonstrates that the decoration of MWCNTs with Sm-doped fluorapatite is a novel heterogeneous catalyst to promote the synthesis of 1,2,4-triazole derivatives. The MWCNT/Sm-FAp nanocomposite provides a simple solution for the technical glitches in the synthesis of 1,2,4-triazole moieties from aromatic aldehydes with excellent yields in short reaction times at room temperature using an eco-friendly solvent. Finally, the efficient, inexpensive, reusable MWCNT/Sm-FAp nanocomposite provided a facile protocol in valued organic transformations.

Acknowledgements

The authors thank the National Research Foundation, Pretoria, South Africa, and the University of KwaZulu-Natal, South Africa, for financial assistance and research facilities.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra08733g

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