Tungstophosphoric acid/zirconia composites prepared by the sol–gel method: An efficient and recyclable green catalyst for the one-pot synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes
Graphical abstract
Highlights
► Zirconia modified with tungstophosphoric acid samples were synthesized. ► The SBET decreases and the microporosity increases with the increase of TPA content. ► The species present in the samples are [PW12O40]3−, [P2W21O71]6− and [PW11O39]7− anion. ► Aryl-14H-dibenzo[a,j]xanthenes have been synthesized by a one-pot condensation. ► Condensation of 2-naphthol and aryl aldehydes catalyzed by TPA/zirconia composites.
Introduction
Organic reactions catalyzed by inorganic solid materials are gaining much importance due to the advantages of heterogeneous catalysis, such as simplified product isolation, mild reaction conditions, easy recovery and catalyst reuse, and reduction of waste by-products [1], [2], [3].
In particular, catalysis by heteropolyacids (HPA) and related compounds is a field of increasing significance worldwide. Many developments have been carried out both in basic research and in fine chemistry processes [4]. HPA have been used as valuable and versatile acid catalysts for some acid-catalyzed reactions because of their strong acidic properties [5], [6]. They can be used as bulk or supported materials in both homogeneous and heterogeneous systems. Furthermore, HPA have several advantages, such as much flexibility in the modification of the acid strength, ease of handling, nontoxicity and environmental compatibility [7].
Zirconium oxide (zirconia) is an interesting material to be used as catalyst support due to its thermal stability, and its basic and acid properties. The latter can be modified by the addition of cationic or anionic substances. For example, the addition of sulfate or tungstate ions has been widely studied, obtaining materials with high acidity [8], [9]. However, the addition of Keggin heteropolyacids has not been studied so much [10], [11], [12], [13], [14]. The most common methods to obtain zirconia are: sol–gel, micellar or mechanochemical synthesis [15]. The preparation of HPA supported on zirconia employing the micellar method, with zirconyl chloride as oxide precursor, has been more extensively reported than the sol–gel method using a zirconium alkoxide [13]. In addition, different types of ionic and neutral surfactants have been employed to obtain mesoporous materials with high specific surface area. More recently, nonsurfactant, low-cost organic compounds, such as urea, have started to be used as pore-forming agents [16].
Conversely, the preparation of organic compounds involving greener processes under solvent-free conditions has been investigated worldwide due to stringent environmental regulations. The implementation of several transformations in a single operation is highly compatible with the principles of Green Chemistry [17], [18], [19].
Xanthenes and benzoxanthenes are interesting compounds for the pharmaceutical industry due to their wide range of biological and therapeutic properties [20], such as anti-inflammatory, antibacterial, antiviral, and stimulating activity of the central nervous system [21], [22], [23], [24]; they are also used in photodynamic therapy [25] and for antagonism of the paralyzing action of zoxazolamine [26].
Different routes have been reported for the synthesis of xanthenes and benzoxanthenes, such as the reaction of aryl oxomagnesium halides with triethylorthoformate, trapping of benzynes by phenols, cyclization of polycyclic aryltriflate esters, and cyclocondensation between 2-hydroxy aromatic aldehydes and tetralone [27], [28], [29], [30]. However, one of the best routes is the synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes from 2-naphthol and aldehydes in the presence of a catalyst [31] such as AcOH-H2SO4, p-TSA, Amberlist-15, silica-sulfuric acid, and bulk or silica-supported heteropolyacids [32], [33], [34], [35], [36], [37].
But, although these methods showed different degrees of success, some of them had limitations, such as large reaction times, low yields, use of toxic solvents, and harsh reaction conditions [37], [38]; that is why the development of alternative clean procedures for the synthesis of benzoxanthenes and related compounds is a challenge. In this regard, our research group has experience in the friendly synthesis of heterocyclic compounds using HPA, such as coumarins, dihydropyrimidinones, azlactones, and flavones [39], [40], [41], [42].
Continuing with the studies directed toward the development of highly expedient methods and the synthesis of diverse heterocyclic compounds, we are herein reporting a new one-pot synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes from aldehydes and 2-naphthol under solvent-free conditions (Scheme 1), catalyzed by tungstophosphoric acid-modified mesoporous zirconia obtained from zirconium propoxide as precursor and polyethylene glycol as pore-forming agent.
Section snippets
Catalyst preparation
Zirconium propoxide (Aldrich, 26.6 g) was mixed with absolute ethanol (Merck, 336.1 g) and stirred for 10 min to obtain a homogeneous solution under N2 at room temperature. Then 0.47 cm3 of 0.28 M HCl aqueous solution was dropped slowly into the above mixture to catalyze the sol–gel reaction.
After 3 h, an appropriate amount of polyethylene glycol (PEG)–alcohol–water solution (1:5:1 weight ratio) was added to the hydrolyzed solution under vigorous stirring to act as template. The amount of added
Catalyst characterization
The textural properties of the samples were determined from the N2 adsorption–desorption isotherms at the liquid–nitrogen temperature. The isotherms obtained (Fig. 1) showed the main characteristics assigned to mesoporous materials and can be classified as type IV. The hysteresis was hardly visible, which is attributed to an ordered arrangement of the mesopores present in the material, as was reported for MCM-41 and mesoporous titania modified with tungstophosphoric acid [43], [44].
The specific
Conclusions
Tungstophosphoric acid/zirconia composite materials were prepared using PEG as pore-forming agent, via sol–gel reactions.
FT-IR and 31P MAS-NMR results indicated that the main species present in the samples is the [PW12O40]3− anion, which was partially transformed into [P2W21O71]6− and [PW11O39]7− anions during the synthesis and drying steps. The prepared catalysts presented acid and textural properties adequate for their use as catalysts in heterogeneous acid reactions.
ZrTPA composites have
Acknowledgements
The authors thank ANPCyT, CONICET and Universidad Nacional de La Plata, for financial support, and L. Soto, L. Osiglio, D. Peña and N. Firpo for their collaboration in the experimental measurements.
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