Structural Properties and Catalytic Activity of Binary Poly (vinyl alcohol)/Al₂O₃ Nanocomposite Film for Synthesis of Thiazoles

Abstract

A solution casting technique was applied to prepare a binary poly (vinyl alcohol)/Al₂O₃ nanocomposite. The structural properties of nanocomposite were investigated using Fourier-transform infrared spectra, field emission scanning electron microscope, energy dispersive X-ray analyses, and X-ray diffraction. The hybrid PVA/Al₂O₃ film exhibited a conspicuous catalytic performance for synthesis of thiazole derivatives under mild reaction conditions. Moreover, the optimization of catalytic efficiency and reusability of this nanocomposite have been investigated.

1. Introduction

Polyvinyl alcohol (PVA) is a biodegradable, water-soluble, and environmentally benign polymeric material with unique emulsifying, adhesive, and film-forming properties [1]. It can be used in the synthesis of organic-inorganic hybrid membrane for biodiesel production [2] and has been regarded as appropriate matrix in microbial fuel cell applications [3]. PVA has also been used in production of biomedical hydrogels with high ability to swell [4]. Moreover, diversity of industrial applications of PVA has been reported such as optical and humidity sensors, food packaging, surgical devices, manufacture of polarizing sheet, and production of thin film transistors [5,6]. Recently, many industrial applications of aluminum oxide (Al₂O₃) nanoparticles have been disseminated such as; drilling fluids agent [7], separators for lithium batteries [8], and biosensors for vitamin E [9]. Aluminum oxide nanoparticles were also used to enhance the thermal and mechanical properties of wood fibers [10]. Conspicuously, aluminum oxide nanoparticles, with large surface area and pore-size distribution, have superior catalytic activities for diverse organic reactions [11,12]. Recently, metal oxide nanoparticles embedded within the polymer matrix have attracted growing interest due to the unique properties that displayed by the hybrid nanocomposites. The properties of the created nanocomposites depend on many factors like the method of preparation, the morphological structure of the prepared nanocomposite, nature of metal oxide nanoparticles that incorporated in, the structural features of the polymer used, and the chemical properties of both metal oxide and polymer molecules. The physicochemical properties of the nanosized particles within the hybrid material differ markedly from those of molecular and bulk materials. In our previous work we have concluded that, the synergistic interaction between polymers and inorganic nanoparticles leads to simulative formation of polymer/inorganic nanocomposite with novel catalytic performance for synthesis of different azoles [13,14]. The preparation of PVA/Al₂O₃ composites has received much attention in the past few years for various applications [15,16]. The purpose of this study is the preparation of new PVA/Al₂O₃ thin film nanocomposite by using simple solution casting method (Figure 1). Moreover, the nanocomposite hybrid material was characterized and investigated as powerful recyclable catalyst for synthesis of bioactive azoles.

2. Results and Discussion.

2.1. Characterization of PVA/Al₂O₃ Composite Film

2.1.1. Fourier-Transform Infrared (FTIR) Spectra

The FTIR spectra of virgin PVA [17] and PVA-Al₂O₃ film were measured and the assignment of the most evident absorption bands are shown in Figure 2. The FTIR spectra of virgin PVA revealed an intense and broad stretching band at υ = 3319 cm⁻¹ and another bending vibration at υ = 1634 cm⁻¹ assignable to hydroxyl group. In addition, there is a clear decrease in the intensity of these bands for the hybrid PVA-Al₂O₃ film, which is attributed to the interaction with Al ions [18] and the influence of the drying temperature. Also, the absorption bands at υ = 2945 and 1428 cm⁻¹ appeared as a result of stretching and bending vibrations of (CH) and (CH₂) groups respectively. The broad absorption peak at υ = 1091 cm⁻¹ indicates the C–O stretching vibration [19]. In the finger print region for the spectra of hybrid PVA-Al₂O₃ film, two characteristic peaks at υ = 627 and 579 cm⁻¹ of Al₂O₃ [12] are shown as an evidence for the incorporation of Al₂O₃ molecules in the polymer matrix. Finally, the shift and the intensity decrease in all bands were familiar as result of the interaction between the PVA and metal oxide.

2.1.2. Field Emission Scanning Electron Microscope (FESEM) and Energy Dispersive X-ray (EDX) Analyses

FESEM was applied to study the morphological changes in PVA with the incorporation of Al₂O₃ molecules. FESEM micrographs of the virgin PVA, Al₂O₃ nanoparticles, and the hybrid PVA-Al₂O₃ composite are shown in Figure 3. The surface of the unmodified PVA (Figure 3a) was obtained to be smooth and homogeneous with great extent as compared to the hybrid composite film (Figure 3c). As shown in the micrographs, the surface of polymer was highly deformed upon metal oxide interaction and the metal oxide particles appeared as bright spots that were homogeneously distributed throughout the polymer surface, which has a great similarity to the morphological surface of Al₂O₃ nanoparticles as shown in Figure 3b. Moreover, energy dispersive X-ray (EDX) measurements for the modified PVA-Al₂O₃ composite film confirmed the presence of alumina inside the PVA matrix. From the EDX results, the elemental analysis of modified composite determined the Al₂O₃ content in the matrix to be 10 wt% (Figure 4).

2.1.3. Structure Analysis

X-ray diffraction (XRD) patterns of the control PVA (obtained under the same conditions but in absence of alumina powder) and the PVA-Al₂O₃ thin film composite were shown in Figure 5. The pattern of control PVA contains only a characteristic sharp peak at 2Ɵ = 20° as reported in literature [20]. For the PVA-Al₂O₃ composite pattern there was a clear broad hump that indicates the majority of the amorphous nature of virgin PVA. It is obviously seen that the relatively higher sharpness of the diffraction peaks a bit with the incorporation of Al₂O₃ particles, which indicates the interaction with alumina results in a clear decrease in the PVA crystallinity. Moreover, the disappearance of the characteristic Al₂O₃ peaks (ASTM 75-1862) and the lower crystallinity of the PVA-Al₂O₃ nanocomposite are attributed to the strong interaction between Al₂O₃ and PVA [16].

2.2. Synthesis of Thiazole Derivatives

The reaction conditions between thiosemicarbazones and α-haloketones or α-haloesters were optimized based on the model reaction of 2-benzylidenehydrazine-1-carbothioamide (1a) and ethyl 2-chloro-3-oxobutanoate (2a) to estimate the proper catalytic loading and time (Scheme 1 and

Table 1. Optimization of catalytic loading of PVA-Al₂O₃ nanocomposite as a basic catalyst.

Entry	Loading Catalyst (Wt%; *)	Time (min)	Yield (%)	Entry	Loading Catalyst (Wt%; *)	Time (min)	Yield (%)
1	2.5	60	40	7	10	30	40
2	5	60	50	8	10	60	60
3	10	60	60	9	10	90	72
4	15	60	60	10	10	120	80
5	20	60	59	11	10	150	87
6	25	60	56	12	10	180	92

(*) Method A, by using 10 wt% of PVA/Al₂O₃ nanocomposite film as basic catalyst.

).

As shown in Table 1, the reactions were proceeded in dioxane under thermal conditions with a loading weight percentage of (2.5%, 5%, 10%, 15%, 20%, and 25%) PVA-Al₂O₃ nanocomposite as a basic catalyst. The results indicated that, the optimal loading catalyst was 10 wt% (entry 3) for the reaction time 60 min. In addition, the reactions were screened to estimate the optimal time. It was noted that, the formation of thiazole product 3a was significantly increased with time (entries 7–12). Thus, the maximum yield percent was obtained after 180 min (entry 12), which produced the product 3a with 92% yield. Moreover, the recyclability of PVA-Al₂O₃ nanocomposite as a basic catalyst was also investigated. The catalyst was reused three times without significant loss of its catalytic efficiency (

Table 2. Recyclability of PVA-Al₂O₃ nanocomposites as basic catalyst.

State of Catalyst	Fresh Catalyst	Recycled (1)	Recycled (2)	Recycled (3)
Yield (%) of product 3a (*)	92	91	90	90

(*) Method A, by using 10 wt% of PVA/Al₂O₃ nanocomposite film as basic catalyst.

) under optimum conditions (10 wt% and 180 min).

Subsequently, a wide array of 2-arylidenehydrazine-1-carbothioamides (1a–g), bearing electron-donating or electron-withdrawing group, and α-haloesters 2a or α-haloketones 2b were refluxed in dioxane for 3h, under different conditions, and provided 2-hydrazinothiazoles 3a–j (Scheme 2 and

Table 3. Yield percent of products 3a–j.

Compd. No.	R1	R2	R3	R4	Yield (%)			M.P. (°C)	Ref.
					No Catalyst	Al2O3 (*)	PVA/Al2O3 (**)
3a	H	H	H	COOEt	78	90	92	195–197	[21]
3b	H	H	Cl	COOEt	77	87	90	205–207	[21]
3c	H	H	NO2	COOEt	-	-	94	260–262	[21]
3d	H	H	N(CH3)2	COOEt	-	-	95	230–232	[21]
3e	H	H	OCH3	COOEt	-	-	95	180–182	[21]
3f	Cl	H	Cl	COOEt	-	-	92	222–224	[21]
3g	H	OCH3	OH	COOEt	-	-	97	226–228	[21]
3h	H	H	H	COCH3	-	-	93	222–224	[22]
3i	H	H	OCH3	COCH3	-	-	91	214–216	[22]
3j	Cl	H	Cl	COCH3	-	-	96	240–242	[22]

(*) Method B in experimental part. (**) Method A in experiment part.

).

As shown in Table 3, the comparative study for the yield percentage of the products 3a and 3b under different reaction conditions, namely blank experiments (without catalyst), control experiments (Al₂O₃ nanoparticles), and PVA/Al₂O₃ nanocomposite film was investigated. The yield was enhanced in the presence of the later basic catalysts. Although the catalytic performance of both basic catalysts were quite similar, PVA/Al₂O₃ nanocomposite film was more superior due to the ease of its recyclability and also a relatively smaller amount of Al₂O₃ was required in the catalyst preparation.

A plausible mechanism for synthesis of 2-hydrazinothiazoles 3a–j using PVA/Al₂O₃ nanoparticles was depicted in Scheme 3.

Al₂O₃ nanoparticles act as a Bronsted base [11] via deprotonation of the thiol group of 2-arylidenehydrazine-1-carbothioamide (1a–g). The producing thiolate anion attacks α−halocarbonyl compounds (2a,b) with displacement of chlorine atom to give non-isolable intermediate. Cyclocondensation of the latter intermediate afforded the authenticated thiazole derivatives 3a–j.

3. Materials and Methods

PVA (molecular weight = 22,000 g/mol, density = 1.19 g/cm³), Al₂O₃ nanoparticles (Merck 642991; 30–60 nm particle size (TEM); 20 wt% in water), and ethylenediamine (ED) were purchased by Sigma Aldrich. Thiosemicarbazones 1a–g were prepared as reported in literature [23,24,25,26,27]. Melting points of authenticated samples 3a–j were measured on an electrothermal Gallenkamp capillary apparatus (Leicester, UK). The FTIR spectra of nanocomposite were recorded on a Pye-Unicam SP300 Instrument (Cambridge, UK) in potassium bromide discs. FESEM analyses were measured on a high resolution scanning electron microscope (model HRSEM, JSM 6510A, Jeol, Tokyo, Japan). XRD measurement was carried out on Philips Diffractometer (Model: X’Pert-Pro MPD; Philips, Eindhoven, The Netherland).

3.1. Preparation of PVA-Al₂O₃ Composite Films

The PVA and Al₂O₃ hybrid composite was prepared using the simple solution casting method. Initially, 0.5 g of PVA was dissolved in deionized water (50 mL) under stirring for 6 h at 80 °C then, 0.1 g of Al₂O₃ nanoparticles was added to the latter PVA solution and the mixture was kept under magnetic stirring for 1 h till a dispersion viscous solution was formed. The produced viscous solution was casted in a pre-cleaned Teflon Petri-dish and this solution was annealed in an oven adjusted at 150 °C for 1 h. The final PVA-Al₂O₃ films for the study was 125 mm × 12 mm × 1.5 mm in dimensions. Furthermore, a pure PVA film, without Al₂O₃, was prepared in a similar way for comparative studies (Figure 6).

3.2. General Procedures for the Synthesis of 2- Hydrazonothiazoles 3a-j

Method A: A mixture of thiosemicarbazones 1a–g (1 mmol) in dry dioxane (20 mL), containing PVA/Al₂O₃ film (0.1 g, 10% wt relative to thiosemicarbazones), and ethyl 3-chloro-2-oxobutanoate (2a) or 3-chloro-2,4-pentanedione (2b) (1 mmol of each) was refluxed until all the starting material was consumed (3 h. as monitored by TLC). After completion of the reaction, the hot solution was filtered to remove the film and the filtrate was poured into 50 mL ice/50 mL, 6 M HCl mixture. The precipitate was filtered, washed with water, and crystallized from appropriate solvent to give authenticated thiazole derivatives 3a–j [21,22]. The film was washed with hot ethanol and dried in an oven adjusted at 150 °C for 1 h to be reused following the same procedure in method A.

Method B: the same procedure in method A was applied using Al₂O₃ nanoparticles instead of PVA-Al₂O₃ film. After the completion of the reaction, the solution was poured into 50 mL ice/50 mL, 6 M HCl mixture to get rid of Al₂O₃ nanoparticles.

4. Conclusions

In this work, a new hybrid PVA/Al₂O₃ nanocomposite was prepared via simple solution casting method and the obtained hybrid nanocomposite film exhibited an obvious catalytic performance for synthesis of 2-hydrazonothiazoles with an excellent yield. The optimal loading catalyst was 10 wt% and the catalyst was reused effectively three times without significant loss of its catalytic efficiency.