HPA-ZSM-5 nanocomposite as a highly effective and easily retrievable catalyst for the synthesis of furans

INTRODUCTION
Furan derivatives indicate anti-oxidation [1], antimicrobial [2], antimalarial [3], anticancer [4], anti-AIDS [5], anti-inflammatory [6], and anti-diabetic [7] activities. The exploration of effective procedures for the synthesis of furans is a serious challenge. The preparation of furans has been studied using catalysts including K[Al(SO4)2].12H2O [8], N-methyl 2-pyrrolidonium hydrogen sulfate [9], formic acid [10], SnCl2.2H2O [11], β-cyclodextrin [12], tetra-n-butylammonium bisulfate [13], Al(HSO4)3 [14], HY Zeolite [15] and Vitamin B12 [16]. Each of these catalysts may have its own benefits but also suffer apparent disadvantages such as high reaction times, complicated work-up, low efficiency, or unwanted reaction conditions. Despite the use of these procedures, there remains a need for further new ways for the preparation of furans. Nanocatalysts have  obtained  notable  attention  as  effective catalysts  in many  organic  reactions  due  to  their  high  surface-to-volume  ratio  and  coordination  parts  which  create  a  larger  number of  active  sites  per  unit  area  in  comparison  with  their  heterogeneous  counter  sites [17-18]. Heteropolyacids (HPAs) have polyoxometalate inorganic cages, which may adopt the Keggin structure with the common formula H3MX12O40, where X is the heteroatom and M is the central atom. Generally M can be either Si or P, and X = Mo or W [19]. Immobilization of HPAs on silica structures as support results in more stability and increased catalytic activity [20-21]. Heteropolyacids have been heterogenized using immobilization of HPAs on zirconium dioxide [22], titanium dioxide [23], silica [24-25], zeolite [26] and SBA-15 or MCM-41 [27-28]. In this context, among different solid supports, nanocrystalline ZSM-5 zeolite is most preferred owing to its many advantageous properties such as high surface area with different active sites, small pore sizes, short diffusion path, excellent chemical and thermal stability, and good accessibility [29-30]. Herein, we report the use of HPA-ZSM-5 as an efficient catalyst for the preparation of furans by the multi-component reactions of phenylglyoxal, dimethyl acetylenedicarboxylate and primary amines (Scheme 1).

EXPERIMENTAL METHOD
Chemicals and apparatus 
All organic materials were purchased commercially from Sigma-Aldrich and Merck. Powder X-ray diffraction (XRD) was performed on a Philips diffractometer of X’pert Company with monochromatized Cu Kα radiation (λ = 1.5406 Å). Microscopic morphology of the nanocatalyst was visualized by SEM (MIRA3). The IR spectra were recorded on FT-IR Magna 550 apparatus using KBr plates. The energy-dispersive X-ray spectroscopy (EDS) measurement was carried out with the SAMX analyzer. The N2 adsorption/desorption analysis (BET) was performed using an automated gas adsorption analyzer (BEL SORP mini II).

Preparation of ZSM-5
The zeolite precursor was prepared by adding tetrapropylammonium hydroxide (TPAOH) and tetraethyl orthosilicate (TEOS) to a mixed aqueous solution of aluminium isopropoxide [Al(ίPro)3] and NaOH with stirring. The mixture was converted to a gel. The gel was stirred for 20 h. The molar composition of the gel was 1Al2O3:46SiO2:4TPA:5Na2O:2500H2O. The resulting gel was sealed in Teflon-lined autoclaves and heated at 165 ºC for 72h. The solid product was recovered by filtration, washed by deionized water for several times, dried in an oven at 100 ºC overnight. The as-synthesized material was then calcined at 550 ºC for 8h. 

Preparation of HPA-ZSM-5
ZSM-5 Zeolites (1 g) was added to the solution of 0.3 g of phosphomolybdic acid (HPA) in ethanol (25 mL) and the reaction mixture was stirred for 24h. The mixture was filtered, washed by deionized water for several times, and dried in an oven at 100 ºC overnight. The as-synthesized material was subjected to product HPA-ZSM-5 at 400 ºC for 2h. The calcined materials was transferred to desiccator and then converted into fine particles using a mortar and pestle.

General procedure for the preparation of furans (4a-h)
A mixture of amine (1 mmol), dimethyl acetylenedicarboxylate (1 mmol), phenylglyoxal (1 mmol) and HPA-ZSM-5 (6 mg) was stirred in dichloromethane (10 mL) at room temperature. After completion, as indicated by TLC (EtOAc-petroleum ether, 2:8), the nanocatalyst was separated from the mixture using filtration. The solvent was evaporated under vacuum and the products were obtained. The characterization data of the compounds of 4a and 4b are given below.

Methyl 2-benzoyl-4-[(2-methoxybenzyl)amino]-5-oxo-2,5-dihydro-3-furancarboxylate (4a)
Yellow oil, FT-IR (KBr): 3402, 3052, 3004, 1771, 1705, 1675, 1604, 1476, 1122 cm−1 ; 1H NMR δ 8.04-6.92 (m, 9H, ArH), 6.25 (s, 1H, CH), 5.02-4.95 (m, 2H, CH2), 3.87, 3.56 (2s, 6H, 2MeO), 2.20 (s, 1H, NH). 13C NMR δ 192.4, 164.5, 164.4, 157.3, 136.4, 135.2, 129.5, 129.4, 128.4, 128.1, 127.4, 126.7, 124.3, 110.6, 105.3, 76.1, 56.9, 52.5, 42.8. Anal. Calcd. for C21H19NO6: C 66.13, H 5.02, N 3.67. Found: C 66.16, H 4.94, N 3.721.
Methyl 2-benzoyl-4-[(4-methoxybenzyl)amino]-5-oxo-2,5-dihydro-3-furancarboxylate (4b)
Yellow oil, FT-IR (KBr): 3302, 3105, 3006, 1752, 1702, 1675, 1602, 1478, 1468, 1379, 1102 cm-1 ; 1H NMR δ 8.09-6.92 (m, 9H, ArH), 6.42 (s, 1H, CH), 4.92-4.80 (m, 2H, CH2), 3.84, 3.52 (2s, 6H, 2MeO),  2.69 (s, 1H, NH), 13C NMR δ 192.6, 169.1, 162.4, 159.2, 156.4, 155.2, 154.1, 153.2, 135.2, 135.1, 107.2, 105.4, 76.1, 60.2, 57.4, 44.6. Anal. Calcd. for C21H19NO6: C, 66.13; H, 5.02; N, 3.67;. Found: C, 66.15; H, 5.07; N, 3.72. 

RESULTS AND DISCUSSION
The prepared catalyst was characterized by XRD, FE-SEM, EDS, FT-IR and BET analyses.
FE-SEM images of ZSM-5 and HPA-ZSM-5 are provided in Fig. 1. After the immobilization, the surfaces of the catalyst, covered with a white translucent substance, became smoother. The particles became larger in size and their profiles clearer, indicating that HPA was immobilized on the surface of ZSM-5. The evaluation of the used catalyst structure by FE-SEM provides evidence that the morphology of the catalyst remained unchanged after the 5th cycle (Fig. 1b, 1c).
EDX analysis (Fig. 2) of the catalyst demonstrated the presence of Al, P, O, Si, and Mo elements, confirming the formation of the catalytic system as visualized. Elemental mapping images of the catalyst showed uniform distribution of the elements P and Mo in the desired catalytic system.
The XRD patterns of ZSM-5 and HPA-ZSM-5 are shown in Fig. 3. In pattern (a),
the peaks of high intensity at 23.4°, 24.1°, and 24.6° are the characteristic diffraction peaks of ZSM-5, indicating good crystallinity of our synthesized ZSM-5. Compared with the ZSM-5 pattern, the HPA-ZSM-5 pattern exhibits all the diffraction peaks of ZSM-5, and the shape and intensity of the diffraction peaks have negligible changes, indicating that the prepared catalysts maintained the good crystallinity of ZSM-5 after the immobilization of the HPA onto ZSM-5. The particle size of HPA-ZSM-5 calculated by the Debye–Scherer equation is about 42 nm.
N2-sorption isotherms at 77 K of ZSM-5 and HPA-ZSM-5 were indicated in Fig. 4. As shown in Fig. 4, all the isotherms exhibited a typical type IV isotherm with an H1 hysteresis loop starting from P/P0 = 0.5. The results demonstrate that the BET specific surface area of ZSM-5 increased from 170 to 240 m2/g after modification with HPA. Nanostructures exhibit good catalytic activity due to their large surface area and active sites which are mainly responsible for their catalytic activity [31-36].
FT-IR studies on zeolite ZSM-5 and HPA-ZSM-5 were conducted (Fig. 5). ZSM-5 has bands at the following wavenumbers (cm-1): 548, 796, 1096 (absorptions of SiO2), 1630 (adsorbed H2O) and 3448 (O-H). The bands at 787 and 962 cm-1 are typical for Keggin’s structure of heteropolyacids and corresponds to νasMo-O-Mo vibrations [25, 37].
At first, to find the optimum conditions, the one-pot reaction of 2-methoxybenzylamine, dimethyl acetylenedicarboxylate and phenylglyoxal in the presence of the diverse catalysts and solvents was selected as the model reaction for the preparation of 5-oxo-2,5-dihydro-3-furancarboxylates. The best results were obtained in dichloromethane and we found that the reaction provided convincing results in the presence of HPA-ZSM-5 (6 mg) at room temperature (Table 1). In this reaction, the nanocatalyst in non-polar solvents such as CH2Cl2 and CHCl3 shows more activity. After that, the obtained optimal conditions were applied to perform the reaction of different primary amines in the presence of HPA-ZSM-5 (6 mg) as catalyst, in order to afford the corresponding products in high to excellent yields (Table 2).
The reusability of the nanocatalyst was studied for the model reaction, and it was found that the product yields lessened only to a very small extent on each reuse (run 1, 95%; run 2, 95%; run 3, 94%; run 4, 94%; run 5, 93%, run 6, 93%). After the completion of the reaction (as determined by TLC), since HPA-ZSM-5 was insoluble in CH2Cl2,  it could be obtained by simple filtration. The catalyst was washed four times with ethanol and dried at room temperature for 15h prior to re-use.
Scheme 2 shows a plausible mechanism for this reaction in the presence of HPA-ZSM-5. At first, the nucleophilic attack by the amine on dimethyl acetylenedicarboxylate generates aminobutendioate I as an electron-rich enaminone. The subsequent nucleophilic attack of aminobutendioate I to the aldehyde carbonyl group of the phenylglyoxal would yield iminium–oxoanion intermediate II, which can be tautomerized to intermediate III. γ -Lactonization of intermediate III would produce the 5-oxo-2,5-dihydro-3-furancarboxylates.

CONCLUSIONS
In conclusion, we have developed a simple way for the synthesis of furans using HPA-ZSM-5 as an efficient catalyst at room temperature in dichloromethane. The zeolite catalyst has been characterized by XRD, FE-SEM, EDS, FT-IR, and N2-adsorption analysis. The structures of the products were deduced from their 1H NMR, 13C NMR, FT-IR, and elemental analyses. The advantages of this method include its simplicity, the reusability of the catalyst, low catalyst loading, and easy separation of products. 

CONFLICTS OF INTEREST 
The authors declare that there are no conflicts of interest regarding the publication of this paper.