Synthesis and characterization of a green composite of H3PW12O40 and starch-coated magnetite nano particles as a magnetically-recoverable nano catalyst in Friedel-Crafts alkylation

Accepted Manuscript Title: Synthesis and characterization of a green composite of H3 PW12 O40 and starch-coated magnetite nano particles as a magnetically-recoverable nano catalyst in Friedel-Crafts alkylation Author: Ezzat Rafiee Maryam Khodayari PII: DOI: Reference:

S1381-1169(15)00006-0 http://dx.doi.org/doi:10.1016/j.molcata.2015.01.005 MOLCAA 9388

To appear in:

Journal of Molecular Catalysis A: Chemical

Received date: Revised date: Accepted date:

21-9-2014 23-12-2014 4-1-2015

Please cite this article as: Ezzat Rafiee, Maryam Khodayari, Synthesis and characterization of a green composite of H3PW12O40 and starchcoated magnetite nano particles as a magnetically-recoverable nano catalyst in Friedel-Crafts alkylation, Journal of Molecular Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2015.01.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis and characterization of a green composite of H3PW12O40 and starch-coated magnetite nano particles as a magnetically-recoverable nano catalyst in Friedel-Crafts alkylation

Ezzat Rafiee 1, Maryam Khodayari Faculty of Chemistry, Razi University, Kermanshah, 67149, Iran

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Highlights

Preparation and characterization of a magnetically composite catalyst.

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It was fully characterized by TEM, Z. analyzer, FTIR, VSM and elemental analysis.

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Tungstophosphoric acid and starch coated magnetite as efficient nano catalyst.

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There was no considerable loss in catalytic activity after several reaction cycles.

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Abstract

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It used as heterogeneous separable catalyst for alkylation process.

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Preparation and characterization of a magnetic composite of 12-tungstophosphoric acid

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(HPW) and starch coated magnetite nano particles (SMNs) were reported. Magnetite nano particles coated with starch were prepared using controlled chemical coprecipitation of

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magnetite phase from aqueous solution in a polymeric starch matrix. HPW was immobilized on this nano particles in order to produce a composite of SMNs and HPW (HPW/SMNs) as a nano catalyst. The as-prepared HPW/SMNs catalyst was characterized by transmission electron microscopy, laser particle size analyzer, Fourier transform infrared spectroscopy, 1

Corresponding author: Tel/Fax: +98 831-4274559; E-mail: [email protected], [email protected]

vibrating sample magnetometer and chemical composition of the HPW/SMNs was estimated by elemental analysis. The results show that HPW/SMNs had a well-defined composite structure and an average size of approximately 29 nm. The characterization data derived from FT-IR reveal that basic structure and geometry of the Keggin anion are preserved after synthesis of HPW/SMNs. Activity of the catalyst was probed through alkylation of aromatic compounds from benzhydrol. The excellent conversions show that the catalyst has strong acidity, which are responsible for its catalytic performance. The catalyst could be recovered

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simply by using an external magnetic field and reused several times without appreciable loss

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of its catalytic activity. Keywords

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Composite materials, Magnetically recoverable catalyst, Heteropoly acid, Starch, Friedel-

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Crafts alkylation.

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1. Introduction

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In recent years, in order to replace problematic mineral acid catalysts (e.g. H2SO4 and

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HCl) widely applied in a number of chemical processes, solid acid catalysts such as clays,

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zeolites, sulfated metal oxides or carbons and heteropoly acids (HPAs) have already attracted

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extensive research interests [1-4]. Among these solid acid catalysts, HPA compounds are unique due to their strong Brönsted acidity, lower corrosivity and higher catalytic activity,

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etc. The HPA compounds have been used as acid catalysts in several large scale industrial

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processes [5-7]. However, the HPAs in the bulk form possess very low surface area and high solubility in polar medium, which limits the exertion of potentially catalytic performance and makes some difficulties in catalyst recovery [8]. For overcoming these disadvantages, considerable research endeavors have been devoted to improve the catalytic efficiency and stability of HPAs by using different strategies such as pillaring layered clays with polyanions,

dispersing of HPAs on solid supports with high surface area, changing counter cations in HPAs, immobilizing HPAs into an organic polymer and prepare nano scale polyoxometalate (POM) particles [5, 9-11]. However, for those potentially promising reactions to be used in practice, complete recovery and reduction of the amount of rather expensive POM catalysts used will be required. During recent years, the advances in nano science and nano technology have led to a new research interest in employing nano meter-sized particles to construct a magnetically recyclable nano catalyst system in the heterogeneous catalysis [12, 13]. The

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magnetic nano particles are often used to immobilize catalytic materials in this system.

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Several investigations have been carried out in the field of superparamagnetic iron oxide nano particles because of their high magnetic susceptibility and relatively low cytotoxicy [14].

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Superparamagnetic nano particles possess an advantage in that they do not retain any

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magnetization after removal of an external magnetic field [15]. In particular, magnetic

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materials are the most commonly selected substrates as affinity probes because of the ease of

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isolation. A recurrent problem, however, is that the direct use of these particles result in

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formation of agglomerates. Agglomeration of iron oxide nano particles reduces the

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superparamagnetic properties [16]. Agglomeration was occurred because of the high surface

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area to volume and consequently high surface energy of iron oxide nano particles [17].

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Researchers have been studying different techniques to overcome these difficulties by engineering the surface of iron oxide nano particles [18, 19]. Several methods have been

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reported for surface modification of superparamagnetic iron oxide nano particles. Most works

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focused on synthesis of nano composite materials with magnetic particles dispersed in organic or inorganic matrices. Recently, various studies on nano metric magnetic particles have been reported including ultrafine metallic iron and iron alloy particles, as well as nano metric iron, and iron oxide particles, which were embedded in inorganic matrices (e.g. Al2O3, SiO2) [20–23] and

polymer matrices (e.g. ion-exchanging resin) [24-26]. Among the natural polymers, starch is an ideal material to this purpose because of the abundant availability of starch, low cost, renewability, biocompatibility, biodegradability, and nontoxicity [27]. Furthermore starch nano crystals have also been found to be excellent reinforcements [28, 29]. In this study, starch coated magnetic nano particles (SMNs) were used as supports for the immobilization of HPAs. 12-Tungestophosphoric acid (HPW) on SMNs (HPW/SMNs) can be employed to derive a novel heterogeneous catalyst system that possesses both high separation efficiency

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and relatively high surface area to maximize catalyst loading and activity. Then, reactivity of

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the produced catalyst was investigated in alkylation of aromatic compounds with benzhydrol. The Friedel-Crafts alkylation is a well-known widely uses organic reaction for C-C bond

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formation between aromatic compounds and alkyl halides, alcohols, or carbonyl compounds.

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The ideal Friedel-Crafts reaction would employ an alcohol as the electrophile, and therefore

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generate water as the only byproduct. Several systems for the alkylation of aromatic

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compounds with alcohols have been reported so far [30-35]. To the best of our knowledge

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this is the first attempt on using of starch coated magnetic nano particles as a green material

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2. Experimental

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coated magnetite nano particles as a support for HPW.

2.1. Preparation of starch-coated iron oxide nano particles (SMNs)

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FeCl3 .6H2O (99%) and FeSO4 .7H2O (99%), NaOH (98%), HCl (37%), starch, HPW

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(>99), other reagents and solvents used in this work were obtained from Merck, Aldrich or Fluka without further purification. SMNs were prepared in a polymeric starch matrix, by controlled chemical coprecipitation of magnetite phase from aqueous solution containing suitable salts of Fe2+ and Fe3+ mainly based on the van der Waals interaction between starch and magnetite nano particles. Starch (6

g) was dissolved in 40 mL of deionized water at 80 °C. Starch solution was added to an aqueous solution of Fe3+ (0.8 M) ions and Fe2+ (0.4 M) ions in 40 mL H2O under vigorous stirring. Then a solution of NaOH (1.0 M) was added dropwise into the mixture of starch and iron salts until the pH of the solution achieved 9–11. After 2 hours, the resulting black suspension was neutralized with HCl (0.1 M). The suspension was centrifuged for 15 min at 1,500 rpm. Finally, the SMNs were washed with deionized water several times to remove little free starch exists. The resulting aqueous suspension of SMNs was used for the next step

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synthesis of the catalyst.

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2.2. Preparation of HPW/SMNs

For the preparation of HPW/SMNs a solution of HPW (2 g in 10 mL water) was added to

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the aqueous solution of SMNs (3.3 g in 10 mL water to produce 60 wt.% of PW to support)

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that thoroughly dispersed by sonication and stirred overnight at room temperature. Finally,

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the solvent was removed by using rotary evaporator and dried. After preparation, the catalyst

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2.3. Catalyst characterization

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calcinated at different temperature (100, 150, 200, 250, 300 °C) for 2 h.

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Transmission electron microscopy (TEM) was obtained using a TEM microscope (Jeol

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JEM-2100 with an accelerating voltage of 200 kV). The size distribution of the samples was

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obtained using a laser particle size analyzer (HPPS 5001, Malvern, UK). Fourier transform infrared (FT-IR) spectra were recorded with KBr pellets using a FT-IR spectrometer ALPHA.

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UV–vis spectra were obtained with an Agilent (8453) UV-vis diode-array spectrometer using

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quartz cells of 1 cm optical path. NMR spectra were recorded on a Bruker Avance 200 MHz NMR spectrometer with CDCl3 as solvent and TMS as internal standard. Elemental analyses (C, H and N) were performed using a Heraeus Elemental Analyzer CHN-O-Rapid (Elementar-Analyses system, GmbH). Magnetic properties of SMNs and HPW/SMNs were measured using a BHV-55, Riken, Japan vibrating sample magnetometer (VSM).

2.4. Catalytic experiments The solid acid catalyst (0.25 g), was added to a mixture of benzhydrol (1 mmol) and various aromatic compounds (2.5 ml) at 75 °C. The reaction was preceded for a short period of time. Progress of the reaction was monitored by thin-layer chromatography (TLC). At the end of the reaction, the catalyst was separated from the product solution using an external magnet, followed by decantation of reaction mixture. The solvent was evaporated to generate the crude product. The crude product was purified by column chromatography on silica gel

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using hexane/ethyl acetate 4:1 as eluent. All products were identified by comparing their

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spectral data with those of the authentic samples [30-35].

The remaining catalyst was washed with diethyl ether, dried under vacuum and reused in a

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subsequent reaction. More than 85 wt.% of the catalyst could usually be recovered from each

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run.

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To check the leaching stability of the catalyst, 0.001 g of the catalyst was stirred in 5 mL

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methanol for 30 min. UV–vis spectra of the diluted solution was recorded after removal of

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solid. The content of HPW in solution was determined with the aid of calibration curves.

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2.5. Large scale synthesis

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Reaction of benzhydrol and benzene was selected for large-scale synthesis. The reaction of

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1a (20.0 mmol) with 2b (50 ml), in the presence of HPW/SMNs (4.8 g) was done at 75 °C.

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3. Results and discussion

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The purpose nano catalyst, HPW/SMNs, were prepared in two steps, Scheme 1 presents

the synthetic strategy for HPW/SMNs. (1) SMNs were prepared by controlled chemical coprecipitation of magnetite phase from aqueous solutions containing suitable salts of Fe2+ and Fe3+.

(2) Immobilization of SMNs with a water solution of HPW occurs via formation of hydrogen bonding between hydroxyl group on the surface of starch and HPW molecule. Fig. 1 (a, b, c, d) show the TEM images, size histogram and electron diffraction (ED) pattern for the HPW/SMNs. As shown in Fig. 1 (a, b) TEM observation indicated that HPW/SMNs had well-defined composite structure composed of several magnetite particles, HPW and starch chains which surrounds these particles. Histogram of size distribution by means of microstructure measurement software was obtained by measuring several spot from

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TEM image. As shown in Fig. 1 (c) the particles had an average diameter of about 29 nm.

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The ED pattern of the composite structure of HPW/SMNs shows amorphous diffraction (Fig.

Fe―OH O

Magnetite Fe―OH nanoparticles

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O

O

O

O

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O

O

O

O

O

O

O

O

O

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Fe―O―H…..

HPW

Fe―O―H……

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O

Fe―OH

O

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O

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1 (d)).

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Scheme 1. Schematic illustration for preparation of HPW/SMNs nano catalyst.

Size distribution of the SMNs and HPW/SMNs derived from a laser particle size analyzer, illustrated in Fig. 2 (a, b), indicated that the mean diameter of them is 368 and 476 nm, respectively. As shown in Fig. 2, aggregation of individual nano particles was occurred. Such

aggregations cause the difference between particle size derived from TEM analysis and the measurements made using the laser particle size analyzer. The CHN analyses of SMNs and HPW/SMNs are given in Table 1. Loading percentage of HPW on the SMNs is 53.9 wt.%. HPW content was slightly lower than the amount was

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expected from the preparation stoichiometry [36].

Fig. 1. (a, b) TEM images, (c) particle size distribution histogram and (d) ED pattern of

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HPW/SMNs.

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Fig. 2. Grain size distribution of (a) SMNs and (b) HPW/SMNs.

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The FT-IR spectra of starch, HPW, HPW/SMNs and SMNs after drying are presented in

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Fig. 3 (a, b, c, d). In the spectrum of starch (Fig. 3a), several absorbances can be distinct at

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1083, 1158 and 1013 cm-1, which are due to C−O bond stretching [39]. The band stretch

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around 3400 cm-1 is assigned to hydrogen bonded OH on the starch molecules. There are several additional characteristics absorption bands at 929, 857, 763 and 578 cm-1 which are

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because of the entire anhydroglucose ring stretching vibrations [38]. HPW Keggin structure

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is well known and consists of a PO4 tetrahedron surrounded by four W3O13 groups formed by edge-sharing octahedra. These groups are connected to each other by corner-sharing oxygens

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[39]. This structure gives rise to four types of oxygen, being responsible for the fingerprint bands of the Keggin ion between 700 and 1200 cm-1. HPW shows typical bands for absorptions at 1080 (P-O), 984 (W=O), 896 and 814 (W-O-W) cm-1 (Fig. 3b). In HPW/SMNs, the characteristic bands are at the same wavenumbers with SMNs with a small shift according to the interaction with the support (Fig. 3c, 4d).

The magnetic properties of SMNs and HPW/SMNs were characterized using VSM (Fig. 4). The Ms of SMNs is about 17 emu/g (Fig. 4a). However, the saturation magnetization of HPW/SMNs is about 10 emu/g (Fig. 4b), which is significantly smaller than that of SMNs.

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There is no hysteresis, suggesting that HPW/SMNs are superparamagnetic.

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Fig. 3. FT-IR spectra of the (a) Starch, (b) HPW, (c) HPW/SMNs and (d) SMNs.

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Fig. 4. Magnetization measurements for (a) SMNs and (b) HPW/SMNs.

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The reaction of benzhydrol (1 mmol) and benzene (2.5 ml) was carried out as a model

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system for studying the various factors on the product yield (Scheme 2). First of all, leaching

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stability of the catalyst was investigated according to the weight percent of HPW to the catalyst (Fig. 5). Leaching of the HPW in methanol was investigated without reactants, since

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methanol was used in the catalytic tests. It should be mentioned that according to the results in Fig. 5 leaching amount is 16 and 8 wt.% in the cases of 40 and 60 wt.% respectively.

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Therefore, leaching amount was decreasing when HPW loading was increasing from 40 to 60

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wt.%. Catalytic activities of the samples were also investigated. It should be noted that reaction time was 30 min in all cases in Fig. 5. As can be seen, the catalytic activity increased

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with increasing the weight percentage of HPW. Thus 60 wt.% HPW/SMNs was selected as optimized weight percentage of HPW in the catalyst. The HPW was well interacted with the support surface and leaching was not high. It seems that, the 60 wt.% of HPW/SMNs catalyst could be recovered and reused several times; only 8 % of the initial HPW content leached into the reaction mixture. Furthermore, effect of sonication in the preparation of the catalyst

was investigated. According to the result in Fig. 5, when the 60 wt.% of HPW/SMNs prepared without sonication (60 b in Fig. 5), leaching of the HPW was increased and thus catalyst reactivity was decreased. To understand the effect of calcination temperature on the leaching of the HPW, the catalyst was calcined at different temperatures (100, 150, 200, 250, 300 °C), no decrease in

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leaching were observed (Fig. 6).

Fig. 5. Leaching in methanol and catalytic activity of HPW/SMNs in the model reaction after

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30 min at 75 °C; 60 b refer to the catalyst with 60 wt.% of HPW to the catalyst was prepared

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without sonication.

OH

HPW@SMNs

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2a

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Scheme 2. Model reaction.

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in methanol.

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Fig. 6. Effect of calcination temperature on the leaching stability of the 60 wt.% of HPW/SMNs

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The reusability of the catalyst was studied by employing recovered catalyst in a model

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reaction. The yield of the product was not substantially changed after the fourth run (10% difference) but the reaction time for each run slightly increased (Fig. 7). A decrease in catalytic activity of the catalyst mainly is due to the leaching from the support. Also, the catalyst deactivation is caused by formation of carbonaceous deposit (coke) on the catalyst surface. Coke deposition may be lead to limit the reactant access to catalytic sites and

therefore, decreased activity. It was also observed that this catalyst could be recovered more than 85 wt.% from the first run to forth one. With these results, it seems that this nano catalyst is active in organic reactions. Alkylation of the aromatic hydrocarbons is one of the most important processes used on an industrial scale. Di-, tri-, and tetraarylmethanes are important substructures in a variety of substances with biological relevance as well as diverse classes of functional materials [40, 41]. The activity of this catalyst was probed through alkylation of the aromatic compounds from

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benzhydrol.

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Fig. 7. Reusability of the HPW/SMNs in the model reaction at 75 °C.

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Effects of the catalyst loading and reaction temperature on the product yield were

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investigated in the model reaction (Table 2). Control experiment showed that the substrates did not react in the absence of the catalyst and in the presence of the SMNs (Table 2, entries 1 and 2). When HPW was used as catalyst 70 % yield of the product was obtained after 1 h (Table 2, entry 3). The lowest yield of the product was obtained in the presence of HPW/SMNs at room temperature (Table 2, entry 4). Whereas by increasing the temperature

to 75 °C, a significant improvement was observed and yield of the product was increased to 95% after 30 min. Thus 75 °C was chosen as reaction temperature in further investigations (Table 2, entry 5). The quantity of the catalyst used in this reaction was optimized. Decreasing in the catalyst quantity from 0.25 to 0.15 g led to increasing in time of the reaction. Further increase from 0.25 to 0.30 g in the catalyst quantity showed no improvement in the yield or reaction time (Table 2, entries 6 and 7). Encouraged by these results; various aromatic compounds were employed as a substrate for the reaction with

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benzhydrol and the results obtained were summarized in Table 3. The experimental results

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showed that aromatic compounds with different substitution performed to afford the

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corresponding products in excellent yields.

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Table 3. Alkylation reaction of various aromatic compounds with benzhydrol.a

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OH

ArH

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Entry

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1

NHPW/SMNs

3a-3k

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2a-2k

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1

Ar

Ar

2a

Time (min)

30 (45)

Product

Yield (%)

3a

95 (90)

2b

40

3

2c

40

3b

3c

85

96

4

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2

40

3d

91

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2d

40

A

5

3e

95

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2e

OMe

O Me

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6

a

7

30

3f

84

Cl Cl

2g

60

3g

93

Reaction conditions: benzhydrol (1 mmol), ArH (2.5 ml), catalyst (0.25 g), 75 °C.

b

Isolated yield, results in parentheses refer to large scale syntheses, All products

were identified by comparing their spectral data with those of the authentic samples [30-35].

From these results, a possible mechanism of the alkylation of aromatic compounds with benzhydrol in the presence of HPW/SMNs as a Brönsted acid catalyst is proposed (Scheme 3). By action of the catalyst benzhydrol was protonated to generate a stable cation after

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dehydration. Under the present conditions, due to the resonance of π-bonding electrons, the

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aromatic compound as a nucleophile combined with the stable carbocation to produce, after

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releasing of H+, the final triarylmethanes product.

..

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+

D

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OH2 +

-H2O

N

HPW/SMNs

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OH

+ R R

-H+

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H

Scheme 3. Proposed mechanism for the alkylation of aromatic compounds with benzhydrol.

Acknowledgement The authors thank the Razi University Research Council for support of this work.

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A

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Table 1. Content of carbon, hydrogen and nitrogen of SMNs and HPW/SMNs. CHN

SMNs

HPW/SMNs

C (%)

26.74

11.95

H (%)

4.503

2.436

N (%)

0.000

0.000

Table 2. Optimization of the reaction conditions.

Temp. (°C)

Time (min)

Yield (%)a

1

―

75

90

―

2

SMNs (0.25 g)

75

60

―

3

HPW (0.25 g)

75

60

70

4

HPW/SMNs (0.25 g)

25

60

10

5

HPW/SMNs (0.25 g)

75

30

95

6

HPW/SMNs (0.15 g)

75

50

94

7

HPW/SMNs (0.30 g)

75

30

97

Model reaction conditions: benzhydrol (1 mmol), benzene (2.5 mL).

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Catalyst

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a

Entry

The applicability of this catalyst for the large-scale synthesis was investigated in the

U

reaction of 1 (20.0 mmol) with 2b (20.0 mmol) in the presence of HPW/SMNs (4.8 g) under

N

the optimized conditions (Table 3, entry 1). Results show that HPW/SMNs is a good

M

(45 min) compared to small scale.

A

candidate for large-scale synthesis and yield of the reaction was 90% in larger reaction time

D

The reaction data, along with some literature data for comparison, are given in Table 4.

TE

Obviously, HPW/SMNs catalyst is efficient for the synthesis of triarylmethanes from every

EP

point including catalyst loading, yield of the product, reaction time and magnetic

CC

recoverability.

A

Table 4. Comparison of the reaction data with other reported methods

Entry

Substrate

Catalyst

Reaction

Yield (%)

conditions 1

Benzhydrol, benzene

HPW/SMNs (0.25 g)

75 °C, 30 min

95

2

Benzhydrol, methyl

NaHSO4/SiO2 (1.0 g)

80 °C, 30 min

99

benzene[32] 3

4

Benzyl alcohol, p-xylene

InCl3.4H2O (5 mol%),

120 °C, 6 h

92

[34]

Hacaca (15 mol %)

Benzhydrol, acethyl acetone

Fe(ClO4)3. xH2O (5 mol %)

60 °C, 2.5 h

98

Benzhydrol, methyl benzene

[Ir(COD)Cl]2-SnCl4 (4

90 °C, 30 min

71

[30]

mol%)

Benzhydrol, methoxy

I2 (10 mol%)

80 °C, 6 h

60

5

6

PT

[31]

p-Toluenesulfonic acid

diphenylprop-2-en-1-ol[35]

monohydrate (5 mol%)

CH3CN, 20 °C

86

Hacac: Acetylacetonate

N

U

a

1-Methyl-4-nitrobenzene,

SC

7

RI

benzene [33]

A

4. Conclusion

M

The results described in this study present an efficient magnetic catalyst based on starch-

D

coated iron oxide nano particles used as an ideal support for immobilization of HPW. The

TE

SMNs are composed of iron oxide and starch chains coating the magnetic nano particles. The

EP

commercially available and low cost of starch allow carrying out synthesis of HPW/SMNs on macro scale. High surface area of the SMNs support leading to high dispersion of HPW and a

CC

high HPW loading. The TEM study clearly demonstrates that HPW/SMNs had a well-defined

A

composite structure, almost spherical in shape, and of diameter 29 nm. Superparamagnetism of this nano catalyst is confirmed from VSM studies. Moreover, the magnetic properties of iron oxide nano particles enable complete recovery of the HPW/SMNs catalyst using an external magnetic field; this is an important advantage of the use of a magnetically separable catalyst. The isolated catalyst was reused at least four times with negligible loss of its

catalytic activity. The HPW/SMNs catalyst was used in a novel methodology for the

A

CC

EP

TE

D

M

A

N

U

SC

RI

PT

syntheses of triarylmethanes, which are biologically interesting compounds.