J IRAN CHEM SOC DOI 10.1007/s13738-015-0711-z
ORIGINAL PAPER
Highly efficient and recyclable acetylation of phenols and alcohols by nickel zirconium phosphate under solvent‑free conditions Abdol Reza Hajipour1,2 · Hirbod Karimi1,3 · Afshin Kohi1
Received: 6 November 2014 / Accepted: 25 July 2015 © Iranian Chemical Society 2015
Abstract Nickel zirconium phosphate nanoparticles have been used as an efficient catalyst for the acetylation of a wide range of alcohols and phenols with acetic anhydride in good to excellent yields under solvent‐free conditions. The steric and electronic properties of the different substrates had a significant influence on the reaction conditions required to achieve the acetylation. The catalyst used in the current study was characterized by inductively coupled plasma optical emission spectroscopy, X‐ray diffraction, N2 adsorption–desorption, scanning electron microscopy, and transmission electron microscopy. This nanocatalyst could also be recovered and reused at least six times without any discernible decrease in its catalytic activity.
Graphical abstract
O
Solvent-free 40 °C, 10-45 min
X + Ac2O R R=aryl, alkyl, benzyl X=OH, SH, NH2
O P Zr
Ni2+
O P Zr
X + AcOH R
Catalyst regeneration
1. Washed with ethanol and water 2. Dried at 110 oC 3. Activated at 450 oC
Keywords Nickel zirconium phosphate · Nanoparticles · Acylation · Solvent-free · Solid catalyst
Introduction
* Abdol Reza Hajipour
[email protected] 1
Pharmaceutical Research Laboratory, Department of Chemistry, Isfahan University of Technology, 84156 Isfahan, Islamic Republic of Iran
2
Department of Neuroscience, University of Wisconsin, Medical School, Madison, WI 53706‑1532, USA
3
Young Researchers and Elite Club, Shahreza Branch, Islamic Azad University, Shahreza, Iran
α‐Zirconium phosphate (ZP) is one of the most important compounds in inorganic chemistry, and the layered structure of this material has been used in a variety of different fields [1–3]. ZP behaves as a unique ion exchanger because of its exceptionally poor aqueous solubility, high thermal stability, resistance to radiation and abrasive properties [4, 5]. The H+ of the P–OH moiety in ZP can be exchanged for various other ions, which results in the enlargement of the interlayer distance [6–9]. Several studies pertaining to the successful exchange of the H+ of the P–OH group in ZP with various divalent and trivalent cations have been reported in the literature [10–14]. It has also been reported that ZP possesses excellent selectivity towards Pb2+, Zn2+, and Fe3+ as an ion exchanger [15–17]. Furthermore, ZP has been reported to exhibit antibacterial activity when it
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was loaded with Cu2+, Zn2+, or Ce3+ [5, 6, 13, 14]. Several reports have also appeared in the literature concerning the catalytic activities of ion exchanged materials of this type, including the use of zinc zirconium phosphate (ZPZn) as a catalyst in the acetylation of alcohols and phenols and the use of a copper zirconium phosphate (ZPCu) as catalysts in selective oxidation of alcohols [18–24]. Protection and deprotection of organic functions are important processes during multi-step organic synthesis. The choice of a method for the functional group’s transformations depends on its simplicity, high yields of the desired products, short reaction times, low cost of the process, and ease of the work-up procedures [25, 26]. The acetylation of alcohols, phenols, thiols, and amines is one of the most important and frequently used transformations in organic synthesis, especially in the synthesis of natural compounds, biologically active compounds, and polyfunctional molecules such as nucleosides, carbohydrates, chalcones, flavanones, naphthoquinones, pesticides, and steroids. Acetylated groups are also commonly found in cosmetics and foodstuffs, as well as solvents, perfumes, plasticizers, flavors, polymers, and pharmaceuticals [25–27]. One of the most common examples of a compound containing an acetylated group is aspirin, which is produced by the acetylation of salicylic acid with acetic anhydride (AA) in the presence of an acid catalyst [28]. The acetyl group is one of the most inexpensive and commonly used protecting groups in organic chemistry for the protection of –OH, –SH, and –NH2 functional groups because the resulting acetylated compounds are stable to various reaction conditions and reagents. Furthermore, the acetyl group can be readily introduced using inexpensive reagents and easily removed using mild alkaline hydrolysis [29–31]. A variety of different procedures have been developed for the acetylation of alcohols, phenols, amines, and thiols using both homogeneous and heterogeneous catalysts such as VIV(TPP)(OTf)2 [27], La(NO3)3 6H2O [29], B(C6F5)3 [30], CuSO4 5H2O [31], ZnCl2 [32], borated zirconia [33], ZnO2 [34], Ce(OTf)3 [35], SiO2–ZnCl2 [36], H3PW12O40 [37], DMAP HCl [38], Cu(BF4)2 [39], silica‐bonded sulfamic acid [40], Cp2ZrCl2 [41], [TMBSA][HSO4] [42], [bmim] [OTs] [43], [MMPPA][HSO4] [44], SaSA [45], SBNPSA [46], SuSA [47], P(4‐VPT) [48], acylimidazolium acetate [49], polyvinylpolypyrrolidoniume tribromide [50], ZnAl2O4@SiO2 [51], P2O5/Al2O3 [52], [Hmim]HSO4 [53], Yttria‐zirconia [54], CoCl2 [55], MWCNTs–C–PO3H2 [56], NiCl2 [57], Ni/SiO2 [58], DBSA [59], Rice husk [60] anhydrous NiCl2 [61], LaFeO3/SiO2 [62], and Fe/SBA-15 [63]. However, most of these catalysts have advantages and limitations. Despite extensive interest in the development of new methods of acetylation, there is still scope for the development of simple, efficient, inexpensive, widely applicable, reusable, and environmentally benign
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catalysts and procedures capable of promoting the acetylation process. With growing environmental concerns, one of the most promising ways to achieve these goals seems to be the use of green and insoluble catalysts or of ecofriendly solvent-free conditions. When an insoluble catalyst is used, it can be easily recovered from the reaction mixture by simple filtration and recycled and can be reused several times, making the process more economically and environmentally viable. Furthermore, the reported examples have demonstrated that heterogeneous catalysts typically require easier work-up procedures. Also, solvent-free synthetic methods are valuable for environmental and economical reasons [22, 24]. With this in mind, and as part of ongoing work towards the development of efficient green catalysts for organic transformations [64, 65], with particular emphasis on the acetylation and acylation of aromatic compounds [52, 53], we report herein the use of nickel zirconium phosphate (ZPNi) as an efficient catalyst for the mild and convenient acetylation of alcohols and phenols under solvent‐free conditions. This new ZPNi catalyst was characterized by inductively coupled plasma optical emission spectroscopy (ICP‐OES), X‐ray diffraction (XRD), N2 adsorption–desorption, scanning electron microscopy (SEM), and transmission electron microscopy (TEM).
Experimental Catalyst synthesis All of the reagents and solvents used in the current study were purchased from Merck Chemical Company and used without further purification. The catalyst was prepared according to previously published procedures, with minor modifications [2, 8–10]. ZP was prepared according to the following procedure. ZrOCl2 8H2O (5 g) was heated at reflux in a solution of H3PO4 (50 ml, 12 mol/L) for 24 h. The resulting mixture was cooled to ambient temperature to give a suspension, which was filtered, and the filter cake was then washed with a solution of H3PO4 (0.1 mol/L) until the filtrate was free of chloride ions. The filter cake was then washed several times with distilled water until the pH of the filtrate was neutral. The solid was then collected and dried in an oven at 110 °C for 24 h [2]. ZPNi was prepared through an ion‐exchange reaction [8–10]. Briefly, ZP (3 g) was dispersed in deionized water (50 ml) at 50 °C, and the resulting suspension was treated with a solution of Ni(OAc)2 (100 ml, 0.1 mol/L) in water (excess amount of Ni2+). This mixture was then heated at reflux for 4 days. It is noteworthy that the acetate ion performed effectively as a base to keep the hydrogen ion concentration in solution sufficiently low to achieve high loadings of the catalyst [7]. A complete exchange between the cations and the
J IRAN CHEM SOC
Scheme 1 Procedure for the preparation of ZPNi
completion of the reaction (as determined by GC), the catalyst was separated from the reaction mixture by centrifuge, then the supernatant was collected and diluted with 10 % NaHCO3 solution (10 ml) before being extracted with Et2O (2 × 10 ml). The combined organic extracts were washed and then dried over anhydrous CaCl2 before being evaporated to dryness under vacuum to give the desired product. In some cases, it was necessary for the product to be purified by column chromatography over silica gel eluting with a mixture of cyclohexane and ethyl acetate. Recyclability studies of catalyst
hydrogen ions of the P–OH groups could not be achieved in less than 3 days or at temperatures below 80 °C [13]. The resulting slurry was filtered hot to give a light green solid, which was washed with distilled water until no Ni2+ ions could be detected in the filtrate (i.e., until the filtrate was colorless). The solid product was then dried at 100 °C for 24 h before being calcined at 600 °C for 4 h to give the final product, ZPNi, as a pale green solid (Scheme 1).
To examine the recyclability of the catalyst, the used ZPNi was recovered from the reaction media and re-used. For recycling, after the first use, the catalyst was separated from the reaction mixture by centrifugation, washed sequentially with ethanol and water before being dried at 110 °C for 2 h, and then activated at 450 °C for 2 h.
Catalyst characterization
Results and discussion
The chemical composition of the ZPNi catalyst was evaluated at different stages of the reaction (i.e., before and after the catalytic reaction) by ICP-OES using an Optima 7300 V ICP-OES spectrometer (PerkinElmer). The samples were ground into a fine powder and analyzed by XRD on a Philips X’pert X-ray diffractometer. The specific surface areas of the samples were determined from their N2 adsorption–desorption isotherms using the Brunauer-EmmettTeller (BET) method on a Quantachrome ChemBET 3000 instrument. Each sample was degassed at 400 °C for 2 h before being analyzed to remove any adsorbed species from their surfaces. The BET surface areas of the materials were estimated from their N2 adsorption–desorption isotherms. The surface morphologies of the ZP and ZPNi materials were studied by SEM on a Philips XL scabbing electron microscope (Philips). TEM images of ZPNi were obtained on a CENTRA 100 TEM system (Zeiss).
Catalyst characterization results
General experimental procedure for the acetylation of substrates under solvent‐free conditions ZPNi (1 mol %) was added to a mixture of alcohol (1 mmol) and AA (2 mmol), and the resulting mixture was stirred at 40 °C for the specified time (Scheme 2). Upon
The ICP-OES analyses of ZP and ZPNi are shown in Table 1. The results obtained in the current study for ZPNi were compared with those reported previously in the literature [8–10]. Our results revealed that there was a negligible leach of nickel ions into the reaction media after the reaction (i.e., following the first use of the catalyst). Figure 1 shows the powder XRD patterns of the ZP and ZPNi materials. The results show some characteristic reflections in the 2θ range of 5°–40°. The diffraction peak in ZP at 2θ–12° was assigned to a d002 basal spacing of 7.5 Å between the planes, which was consistent with the patterns previously reported for ZP and its derivatives with a hexagonal crystal system [2]. It shows that the d-spacing of the (002) plane of ZPNi had increased, which indicated that the Ni2+ ions had intercalated into the interlayer of ZP and increased the d002 basal interlamellar spacing of ZP from 7.5 to 9.8 Å. It is well known that the ion radii of Ni2+ Table 1 Element contents of ZPNi (atm. %) Entry
Sample
Cu
O
Zr
P
1 2 3
ZP ZPNi ZPNia
– 11.7 11.6
63.1 58.7 59.3
13.6 10.9 10.4
23.3 18.7 18.7
4
ZPNib
5.9
62.5
12.2
19.4
a
After run 1
Scheme 2 Summarized procedure for acetylation of phenols
b
After run 7
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Fig. 1 XRD patterns of powder ZP (down), ZPNi fresh (middle) and ZPNi after 7th run (up)
Fig. 2 N2 adsorption–desorption isotherm of ZPNi
(0.69 Å) and hydrated Ni2+ (4.04 Å) are smaller than the basal spacing of ZP (7.5 Å) [66, 67]. These results therefore indicated that Ni2+ ions had inserted into the interlayer of ZP and increased the basal spacing of the modified ZP after the exchange [8–10]. Taken together, these data indicated that ZPNi had been formed successfully. The XRD pattern of the ZPNi catalyst after the 7th run showed that the basal spacing of ZP was about 10.1 Å, which was only a little larger than that of the fresh ZPNi catalyst. This increase may have occurred because of the presence of less Ni2+ on the surface of ZP, and an increase in the number of water molecules between the layers following the seventh run (i.e., Ni2+ ions may have been washed off during the regeneration of the catalyst, see “General experimental procedure for the acetylation of substrates under solvent-free conditions” and Table 1). Figure 2 shows the N2 adsorption–desorption isotherm of ZPNi, as a representative example, in the relative pressure range (p/p0) of 0.1–1.0. The surface area of ZPNi was determined to be 103.1 m2/g. The isotherm for
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ZPNi shows three adsorption stages. The first of these stages was observed at p/p0
