An Efficient Synthesis of 1,8-Dioxo-Octahydroxanthenes Derivatives Using Heterogeneous Ce-ZSM-11 Zeolite Catalyst

The Ce-ZSM-11 zeolite has been used as an efficient catalyst for the one pot synthesis of 1,8-dioxooctahydroxanthene derivatives from aromatic aldehyde and 5,5-dimethyl-cyclohexane-1,3-dione under reflux condition. The catalyst was characterized by Powder X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), Fourier Transform Infrared Spectroscopy (FTIR), Brunauer-Emmer-Teller (BET) surface area analysis, and Temperature Programmed Desorption (TPD) techniques. This method provides several advantageous such as use of inexpensive catalyst, simple work-up procedure, high yield of desired product and reusability of catalyst. Copyright © 2018 BCREC Group. All rights reserved


Introduction
The significant challenge in the synthetic chemistry is to develop environmentally benign, efficient and economical methods for the synthesis of biologically active compounds [1]. Multi-component reactions (MCRs) have become one of the best tools for modern synthesis of chemically and biologically important compounds because of their high atom economy, simple procedure and excellent yields [2][3][4]. Xanthene and its derivatives have received con-siderable attention in the field of pharmaceutical and medicinal chemistry because of their wide range of biological and pharmacological properties such as antibacterial [5], analgesic [6], anti-inflammatory [7], and anti-cancer properties [8]. In addition, these heterocyclic compounds can be used as pH-sensitive fluorescent materials [9], in laser technology [10], and as luminescent dyes [11]. In view of this wide applicability of xanthene derivatives, the development of synthetic procedure is important. Several methods that have been reported for the synthesis of xanthene derivatives catalyzed by Fe3O4 nanoparticle [12], SiCl4 [13], CaCl2 [14], Imidazol-1-yl-acetic acid [15], CAN [16], DSIMHS ionic liquid [17], ZnO nanoparticle [18], Ru@SH-MWCNT [19], and Fe-Cu/ZSM-5 [20] catalyst. However, some of these protocols suffer from one or more drawbacks, such as: use of hazardous solvent, high temperature, long reaction time, tedious workup procedure and use of expensive catalyst. Therefore, the development of alternative route for the synthesis of 1,8-dioxo-octahydroxanthene derivatives is highly desirable.
Zeolites are microporous, crystalline, aluminosilicate compounds composed of staggered framework of [AlO4] 5¯ and [SiO4] 4¯ tetrahedra which generates a network of pores and cavities having molecular dimensions. They finds extensive applications in heterogeneous catalysis due to their tunable pore sizes, high surface area, framework acid/base properties and provides greener alternatives to homogeneous catalysts [21]. ZSM-11 belongs to pentasil family of zeolites which has straight micropore with MEL framework topology [22]. ZSM-11 possesses unique properties, such as high-surface area, acidity and shape selectivity characters [23] hence it has been utilized as heterogeneous catalyst for conversion of methanol to hydrocarbon, aromatization and isomerization of 1-hexene, cracking of pentene to C2-C4 olefins and conversion of methanol into light olefins [24][25][26][27]. The transition metal doped ZSM-11 possesses greater Lewis acidity compared to parent ZSM-11 and showed better catalytic performance in some reaction such as degradation of Dichlorvos, decomposition of N2O, conversion of methane (C1) into higher hydrocarbons, dehydrogenation-aromatization of alkane and decomposition of low-density polyethylene [28][29][30][31][32].

Chemicals and Instrument
All the chemicals were purchased from Merck, Avra, and Spectrochem and were used without purification. Thin layer chromatography was performed on Merck pre-coated silica gel 60F254 aluminum sheets as adsorbent. Melting points were taken in an open capillary and are uncorrected. FT-IR spectra were recorded on Thermo Nicolet; Avatar 370. 1 H NMR spectra were recorded on a 400 MHz using DMSO-d6 as solvent and tetramethylsilane (TMS) as an internal standard. The X-ray diffraction (XRD) patterns were recorded on Bruker AXS D8 Advance X-ray diffractometer using monochromatic Cu-Kα radiation having wavelength λ = 1.5406 Å. Scanning Electron Microscope image (SEM) was obtained on JS-6390 LV operated at 30.0 KV. Surface area and porosity (BET) of catalyst was measured on micro-meritics, ASAP 2010 instrument. Temperature Programmed Desorption (TPD) ammonia was measured on Micromeritics instrument.

Synthesis of ZSM-11 Zeolite Catalyst
The ZSM-11 zeolite was synthesized by hydrothermal method. Tetraethyl orthosilicate and sodium aluminate were used as silicon and aluminum source respectively and tetra propyl ammonium bromide was used as structure directing agent. In a typical synthesis, tetraethyl orthosilicate (20.8 g) was mixed with 50 mL deionized water and stirred at room temperature for 2 h to obtain silica sol. Sodium aluminate (0.246 g) and 2 g NaOH were dissolved in 50 mL deionized water and added drop-wise to stirred solution of silica sol. Tetra propyl ammonium bromide (2 g) was dissolved in 20 mL of deionized water and added drop wise to above solution. The resulting mixture was stirred at room temperature for 12 h. The viscous homogeneous gel was transferred into Teflon lined stainless steel autoclave and treated hydrothermally under static condition and autogenous pressure at 150 °C for 24  For the enhancement of acidic strength in the catalyst, the cerium ion containing ZSM-11 was prepared by ion exchange method. In this procedure 2 g ZSM-11 zeolite and 20 mL 0.1 M ceric ammonium nitrate solution were stirred at 50 °C for 12 h. The resulting product (Ce-ZSM-11) was filtered, washed with deionized water and dried in oven at 100 °C for 5 h. To a mixture of aromatic aldehyde (1 mmol), 5,5 dimethyl-cyclohexane-1,3-dione (2 mmol) and Ce-ZSM-11 catalyst (0.1 g) in water (10 mL) as solvent were added and refluxed for the time shown in (Table 4). The progress of the reaction was monitored by TLC (petroleum ether: ethyl acetate = 7:3 as eluent). After completion of the reaction, the reaction mixture was filtered, the catalyst was separated and the crude product obtained was recrystallized from ethanol to afford pure product (3a-k) and representative compound characterized by FT-IR, 1 H, and 13 C NMR spectroscopy.   .04 is the characteristic peak for the MEL type framework topology which indicates the presence of ordered tetragonal crystal structure of ZSM-11 material [35]. Figure 1 (b) shows decreased peak intensities as compared Figure 1 (a) this may be due to insertion of cerium ion inside the channels of parent ZSM-11. However, crystal structure of parent ZSM-11 is retained after ion exchange.

Scanning electron microscopy-energy dispersive spectrometry analysis
The surface morphology and chemical composition of synthesized catalyst was analyzed by SEM-EDS spectroscopy. The Figure 2 shows uniform sphere shaped crystals which is characteristic morphology of ZSM-11. The SEM images show that prepared catalyst has uniform particle size and ordered morphology. The elemental composition Ce-ZSM-11 is shown in Figure 3 which confirms the presence of Si, Al, O and Ce with atomic wt. % 29.82, 1.11, 67.82, 1.25, respectively.

Fourier Transforms Infrared Spectroscopy analysis
The FT-IR spectrum of the ZSM-11 and Ce-ZSM-11 zeolite is shown in Figure 4. The FT-IR spectrum shows sharp peak at 451, 550, 798, 1104, and 1229 cm -1 which are characteristic adsorption bands of ZSM-11. The band at 798, 1229 assigned for external symmetric and asymmetric stretching vibration and sharp band at 1104 cm -1 is assigned to internal asym-metric stretching of T-O-T (T= Si or Al) unit in ZSM-11 framework. The band at 550 cm -1 confirms the presence of double five member ring which is secondary building unit of pentasil family zeolites framework (MFI or MEL) [36]. The peak at 1635 cm −1 is attributed to deformation mode of Si-OH bond. The band appear between 3455 cm −1 confirms presence of bridged hydroxyl group, i.e. Brönsted acidic center.

Brunauer-Emmer-Teller Surface Area analysis
The presence of intercrystalline pore was studied by N2 adsorption-desorption isotherm. It is observed that surface area, micropore volume and average pore diameter of Ce-ZSM-11 decreased as compared to ZSM-11 zeolite which implies that cerium ions are well dispersed in framework (Table 1).

Temperature Programmed Desorption (Ammonia) analysis
The Figure 5a and 5b shows NH3-TPD profile of ZSM-11 and Ce-ZSM-11 zeolite. The single broad peak at 662.5 °C for ZSM-11 and 623.4 °C for Ce-ZSM-11 is due to desorption of ammonia from strong acidic sites. The total acidity was found 1.8195 mmol.g -1 and 2.6178 mmol.g -1 for ZSM-11 and Ce-ZSM-11, respectively. It was observed that the acidity of ZSM-11 zeolite increases after ion exchange.

Optimization of solvent and amount of catalyst
In order to investigate suitable solvent and optimum loading of catalyst amount for the synthesis of 1,8-dioxo-octahydroxanthene derivatives, 3-nitro benzaldehyde (1 mmol) and 5,5-dimethyl-cyclohexane-1,3-dione (2 mmol) were taken as model substrate and various solvents were screened on model reaction. Initially the model reaction was performed under solvent free condition low yield of desired product was obtained. The same reaction was performed using different solvents such as, EtOH, MeOH, THF, and H2O under reflux condition. Among all these solvents in H2O maximum yield was obtained, hence H2O was selected as  No  120  30  2  EtOH  120  30  3  MeOH  120  20  4  THF  120  35  5 H2O 120 90 Table 1. BET surface area, micropore volume, and average pore diameter for ZSM-11 and Ce-ZSM-11 a Reaction conditions: 3-Nitro benzaldehyde (1 mmol) and 5, 5 dimethyl-cyclohexane-1,3-dione (2 mmol) and catalyst with different solvents (10 ml) at reflux condition. b Isolated yields.  (Table 2). Therefore, further optimization of catalyst was carried out in water solvent. It was observed that 0.1 g of catalyst is sufficient for the reaction and use of excessive catalyst had no effect on either the rate of reaction or on the product yield (Table 3). The scope and efficiency of the present method was studied by reacting various substituted aromatic aldehyde with 5,5-dimethylcyclohexane-1,3-dione under optimize reaction conditions and results are summarized in Table  4. In all cases, it was found that aromatic aldehydes having electron withdrawing or electron donating group reacted effectively and gave the product in good yields. It is observed that the substituents in aromatic ring of aldehydes have little effect on the reaction time as well as yield of the product.
The main advantage of present method is reusability of Ce-ZSM-11 zeolite catalyst. The catalyst was separated after completion of reaction by diluting reaction mixture by hot ethanol and filtration. The recovered catalyst was washed with acetone and dried at 100 °C for 3 h before the next catalytic run. Reusability of the catalyst was investigated on model reaction for three times and it was found that the catalyst has retained almost consistent activity (   To specify the advantages of present methods, results of different reported methods are compared with present work and summarized in Table 5. It is observed that, Ce-ZSM-11 zeolite promote reaction more effectively than other reported catalyst.

Conclusions
In summary, we described the synthesis, characterization of ZSM-11 and Ce-ZSM-11 zeolite catalyst. The Ce-ZSM-11 zeolite was found efficient catalyst for the synthesis of 1,8-dioxooctahydroxanthene derivatives. The distinguished advantageous of present method are use of inexpensive catalyst, simple reaction workup, good to excellent yield, and reusability of catalyst.