Green and Highly Efficient Strategies for the Straightforward

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Green and Highly Efficient Strategies for the Straightforward Reduction of Carboxylic Acids to Alcohols Using Four Different and Affordable Types of Hydrogen Donors Behzad Zeynizadeh, Farhad Sepehraddin, and Hossein Mousavi* Department of Organic Chemistry, Faculty of Chemistry, Urmia University, Urmia, Iran

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ABSTRACT: Green, simple, and highly efficient various strategies for the straightforward reduction of carboxylic acids to corresponding alcohols have been developed by employing four different types of cost-effective hydrogen donors, namely sodium borohydride (NaBH4), ammonium formate (HCO2NH4), glycerol, and isopropanol (i-PrOH) in the presence of Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2) as a robust and durable nanocatalyst under neat conditions. Also, the mentioned magnetically separable nanocatalyst was recovered and reused at least five times without significant loss in catalytic activity. In comparison with the known protocols for the direct reduction of carboxylic acids to corresponding alcohols, our current methods offer several benefits such as being straightforward (without isolation of aldehyde intermediates), mild reaction conditions, eco-friendly, short reaction times, good to excellent yields of the products, use of inexpensive hydrogen donors, recoverability, and reusability of the catalyst and so on.

1. INTRODUCTION One of the most fundamental issues in organic synthesis (especially in total synthesis), pharmaceuticals, and fine chemical industries as well as biomass conversion is the chemoselective reduction (or hydrogenation) of carboxylic acids to their corresponding aldehydes or alcohols.1 On the other hand, preparation of aliphatic alcohols which are essential building blocks for organic synthesis, life science, and material industry from carboxylic acids which are available in abundance from renewable and nonrenewable sources is valuable from many aspects especially from an economic point of view.2 To date, many catalytic and or noncatalytic procedures have been reported for the reduction or hydrogenation of carboxylic acids to corresponding alcohols by molecular hydrogen (H2) or various hydrogen donor reagents. It is worthwhile to note that most of the reported methods have one or several drawbacks such as implementation of expensive and or toxic catalysts, the addition of auxiliary reagents, and even use of an excess amount of base, harsh reaction conditions, need for a nitrogen (N2) or an argon (Ar) atmosphere, prolonged reaction times, low yields, production of mixture of products (nonselectivity in the production of the final product), use of volatile organic solvents which cause an extremely negative impact on the environment, etc.3 In recent years, demand has increased significantly especially from the different chemical industries for the presentation and © XXXX American Chemical Society

development of new chemical strategies based on green chemistry protocols for various organic transformations particularly in the synthesis of drug and drug-like molecules, natural products, and also fundamental organic reactions such as oxidation and reduction of some functional groups.4 In this regard, the use of environmentally benign reaction medium5 and implementation of highly active, stable, and reusable magnetically nanocatalysts6 are important and favorable subjects in modern organic synthesis. Besides, magnetically earth-abundant transition metal (MEATM) nanoparticles have a pivotal place in green catalytic organic reactions, due to their unique activity, ease of heterogenization, and also recoverability.7 In continuation of our research program8 and because of consideration of all the points mentioned, herein we wish to report various environmentally benign, simple, and highly efficient strategies for the straightforward reduction of carboxylic acids to corresponding alcohols by employing four different types of cost-effective hydrogen donors namely sodium borohydride (NaBH 4 ), ammonium formate (HCO2NH4), glycerol, and isopropanol (i-PrOH) in the Received: April 3, 2019 Revised: August 16, 2019 Accepted: August 19, 2019

A

DOI: 10.1021/acs.iecr.9b01847 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research presence of Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2) as a robust and durable magnetically nanocatalyst under neat conditions (Scheme 1). It is worthwhile to note that to the best of our

Table 1. Optimization of the Reaction Conditions Using NaBH4

Scheme 1. Direct Reduction of Carboxylic Acids to Corresponding Alcohols Catalyzed by Fe3O4@APTMS@ ZrCp2Clx (x = 0, 1, 2) Entry

NaBH4 (mmol)

Catalyst (mg)

1 2 3 4 5 6 7

2 2 2 2 2 2 2

7 7 7 7 7 7 7

8

2

4

9

2

10

10

2

7

11

1

7

Solvent H2O THF PEG-400 DMF CHCl3 CH2Cl2 H2O:THF (1:1) H2O:THF (1:1) H2O:THF (1:1) H2O:THF (1:1) H2O:THF (1:1)

Time (min)

Yield (%)

rt rt rt rt rt rt rt

60 60 70 100 120 110 30

75 80 88 84 81 83 95

rt

60

85

rt

30

96

50 °C

30

94

rt

110

78

Condition

slight decrease in the desired product yield (Table 1, entry 10). To show the importance and effect of the as-prepared nanocatalyst on the reduction of carboxylic acids (particularly, benzoic acid) in the presence of NaBH4 (2 mmol), we carried out some control experiments (Table 2). The control knowledge use of HCO2NH4 and glycerol as a hydrogen donor (reducing agent) for the reduction of carboxylic acids to alcohols is reported for the first time. Also, to date, only one article existed that showed an application of isopropanol as the hydrogen donor for the aforementioned reduction reaction, which was published by Takahashi and co-workers in 1989.9 In spite of this, it should be noted that the synthetic protocol, which was disclosed by Takahashi’s research group, does not exhibit good selectivity and affords a mixture of products (alcohol and ester).

Table 2. Control Experiments

2. RESULTS AND DISCUSSION First, the reduction of benzoic acid to corresponding benzyl alcohol using NaBH4 was selected as a model reaction for the initial investigation in the presence of various catalytic amounts of Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2). In order to screen the effect of solvent (Table 1, entries 1−7), the model reaction carried out in the presence of 7 mg of mentioned magnetically heterogeneous catalyst at room temperature in various solvents such as H2O, THF, PEG-400, DMF, CHCl3, CH2Cl2, and also H2O:THF revealed that H2O:THF (1:1) was the effective solvent for this desired transformation (Table 1, entry 7). Next, the effect of catalyst loading on efficiency on the catalyst was examined, and a 7 mg loading of the Fe3O4@APTMS@ ZrCp2Clx (x = 0, 1, 2) was observed to be optimal, providing an excellent yield (95%) of the desired alcohol (Table 1, entry 7). Notably, the use of a small amount of mentioned catalyst (4 mg) caused a decrease of yield along with an increase of reaction time (Table 1, entry 8). Also, the higher loading of the catalyst (10 mg) has no significant effect (only 1%) on the yield of the reaction, and it was not able to diminish the reaction time (Table 1, entry 9). It is necessary to note that increasing the reaction temperature (50 °C) caused a very

Entry

NaBH4 (mmol)

1 2 3 4 5

2 2 2 2 2

6

2

7

2

8

2

9

3

10

2

Catalyst Catalyst-free Fe3O4 (20 mol %) APTMS (20 mol %) Cp2ZrCl2 (20 mol %) Fe3O4 (20 mol %) + APTMS (20 mol %) Fe3O4 (20 mol %) + Cp2ZrCl2 (20 mol %) APTMS (20 mol %) + Cp2ZrCl2 (20 mol %) Fe3O4 (20 mol %) + APTMS (20 mol %) + Cp2ZrCl2 (20 mol %) Fe3O4 (20 mol %) + APTMS (20 mol %) + Cp2ZrCl2 (20 mol %) Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2) (7 mg)

Time (min)

Yield (%)

1440 180 180 180 180

N.R. N.R. N.R. N.R. N.R.

180

N.R.

180

N.R.

180

N.R.

180

N.R.

30

95

experiments clearly showed that the reduction reaction could not take place in the absence of the as-synthesized MNPs (Table 2, entry 1) and also in the presence of the component parts of the mentioned nanocatalyst (viz. Fe3O4 NPs, APTMS, and Cp2ZrCl2), in the single or double and or triple forms (Table 2, entries 2−8), even in the presence of 3 mmol of NaBH4 (Table 2, entry 9). Furthermore, despite having the very appropriate reaction conditions in hand, due to the high B


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Industrial & Engineering Chemistry Research importance of providing and developing green, highly efficient, and cost-effective methods for straightforward reduction of carboxylic acids to corresponding alcohols, we decided to investigate some other types of very inexpensive hydrogen donors namely HCO2NH4, glycerol, and also i-PrOH for this mentioned valuable transformation. In this regard, we repeated all the optimization steps on the model reaction in the presence of the newly hydrogen donors instead of NaBH4 (Table S1). Fortunately, as shown in Table S1, all three new hydrogen donors could reduce benzoic acids to benzyl alcohol with high efficiency under green conditions. In the next step of the current research program, the optimized reaction conditions (which were obtained for all four reduction systems) were applied to the direct reduction of various carboxylic acids to corresponding alcohols. It was observed that the reduction processes were uniform and irrespective of the type of carboxylic acid (Table 3). It should be mentioned that the actual mechanism for the direct reduction of carboxylic acids to alcohols using listed hydrogen donors in the presence of the catalytic amount of Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2) as the magnetically heterogeneous nanocatalyst is unclear. Despite this issue, we introduced two different plausible reaction mechanisms (one mechanism to illustrate the role and behavior of NaBH4 and HCO2NH4 (Scheme 2) and also another reaction mechanism (Scheme 3) for glycerol and i-PrOH in the presence of asprepared MNPs based on our observations and reliable literature data.10 To further clarify the value of the current work, we compared it with other previously reported methods which existed in the literature. As the results in Table 4 demonstrate, our work is simple, highly efficient, and environmentally benign compared to most of the reported protocols. Finally, recyclability of the as-prepared nanocatalyst was evaluated in the presence of all mentioned hydrogen donors on the reduction of benzoic acid to benzyl alcohol. As can be seen from Figure 1, the desired reduction reactions were repeated for five consecutive runs without significant loss of catalytic activity of the as-synthesized MNPs. It is worthwhile to note that the magnetic nanocatalyst which was used in this research was prepared in a three-step procedure (Scheme 4). Also, the as-prepared Fe3O4@ APTMS@ZrCp2Clx (x = 0, 1, 2) magnetic nanocomposite was fully characterized by various well-known methods such as Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, inductively coupled plasma-optical emission spectrometry (ICP-OES), alternating gradient magnetometry (AGFM), thermogravimetric analysis (TGA), and also Brunauer−Emmett−Teller (BET) gas (nitrogen) adsorption−desorption studies. Figure 2 shows the FT-IR spectra for the Fe3O4, Fe3O4@ APTMS and also Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2) NPs. The spectrum for Fe3O4 alone (Figure 2, blue curve) shows the absorption band for the stretching vibration of the Fe−O bond at approximately 588 cm−1, and a broad band appeared at 3370 cm−1 for the surface OH groups while bending vibrations of these mentioned OH groups appeared at 1621 and 1401 cm−1. Figure 2 (orange curve) for the Fe3O4@ APTMS shows the characteristic peak at approximately 3442 cm−1 for the stretching vibrations of the OH and NH2 groups along with peaks at 1621 and 1272 cm−1 for the bending vibrations of these mentioned groups, respectively. Also, the

Table 3. Substrate Scope for the Direct Reduction of Carboxylic Acids to Corresponding Alcohols under Optimized Reaction Conditionsa

a

System A: Hydrogen donor = NaBH4; solvent = H2O:THF (1:1). System B: Hydrogen donor = HCO2NH4; solvent = H2O:PEG-400 (1:1). System C: Hydrogen donor = glycerol; solvent = H2O. System D: Hydrogen donor = i-PrOH; solvent = H2O. T: Time. Y: Yield (isolated yield). Z: Yield refer to GC yield.

peaks at 2933−2850 and 1040 cm−1 are attributed to the stretching vibration of CH2 and Si−O, respectively. Furthermore, Figure 2 (gray curve) indicates a new sharp peak at C


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Industrial & Engineering Chemistry Research Scheme 2. Plausible Mechanism for the Direct Reduction of Carboxylic Acids to Alcohols Using NaBH4 or HCO2NH4 Catalyzed by Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2) MNPs

Scheme 3. Proposed Mechanism for the Direct Reduction of Carboxylic Acids to Alcohols Using Glycerol or i-PrOH Catalyzed by Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2) MNPs

1616 cm−1, which is due to the stretching vibration of the aromatic CC which belongs to the cyclopentadienyl (Cp) groups. The XRD pattern of the Fe3O4@APTMS@ ZrCp2Clx (x = 0, 1, 2) is shown in Figure 3. In the XRD pattern of the mentioned as-prepared core−shell nanocatalyst, all the diffraction peaks are consistent with the nine diffraction peaks at (111), (220), (311), (400), (422), (511), (440), (620), and (553) by comparison with Joint Committee on Powder Diffraction Standards (JCPDS card, file Nos. 79-0418, 653107, 74-2402, and 98-007-7842), which are indexed to the cubic spinel phase of the Fe3O4. It should be of note that the characteristic peaks of the pure zirconium11 in the XRD pattern of the as-prepared zirconocene-containing core−shell MNPs were overlapped with the peaks of the Fe3O4 NPs. The particle size and morphology of the as-prepared zirconocene moiety core−shell MNPs were estimated by scanning electron microscopy (SEM). As shown in Figure 4, SEM images of the Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2) MNPs with various magnifications show that the mentioned magnetically nanocomposite was nanometer-sized at less than about 15 nm, having an approximately spherical morphology. The energy dispersive X-ray spectroscopy (EDX) analysis exhibits that all the elemental [namely, zirconium (Zr), iron (Fe), nitrogen (N), silicon (Si), chlorine (Cl), carbon (C), and oxygen (O)] composition of the Fe 3 O 4 @APTMS@ ZrCp2Clx (x = 0, 1, 2) MNPs exist in the final nanocomposite

structure (Figure 5). Also, the exact amounts of Fe and Zr in the as-prepared magnetically nanocomposite were determined as 52.75% and 3.72%, respectively by inductively coupled plasma-optical emission spectrometry (ICP-OES). Magnetic properties of the Fe3O4 and Fe3O4@APTMS@ ZrCp2Clx (x = 0, 1, 2) NPs are demonstrated from the alternating gradient force magnetometer (AGFM) data at room temperature. The saturation magnetization (Ms) value for the Fe3O4 and Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2) was measured at 0.13 a.u. and 0.095 a.u., respectively (Figure 6). The significant decrease in the Ms value for the as-prepared Fe3O4@ APTMS@ZrCp2Clx (x = 0, 1, 2) with respect to the nonmodified Fe3O4 shows the successful immobilization process on the Fe3O4 NPs surface. To investigate the thermal stability of the as-prepared magnetically nanocatalyst, the thermogravimetric analysis (TGA) in the range of 25−800 °C was conducted under a nitrogen atmosphere. In this regard, the TGA curves for the prepared (a) Fe3O4, (b) Fe3O4@APTMS, and (c) Fe3O4@ APTMS@ZrCp2Clx (x = 0, 1, 2) MNPs are depicted in Figure 7. As can be seen, the TGA curve of the as-prepared Fe3O4@ APTMS@ZrCp2Clx (x = 0, 1, 2) core−shell nanocomposite has two weight-losing stages. The first stage which appears at temperatures below 200 °C demonstrates the elimination of surface hydroxyl groups and or surface absorbed water molecules, while the second stage (200−800 °C) represents release of the organic moieties (viz. silylpropylamine and zirconocene groups). Notably, the comparison between the D


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Industrial & Engineering Chemistry Research

Table 4. Comparison of the Current Protocols for the Direct Reduction of Benzoic Acid to Benzyl Alcohol with Other Reported Methods

Entrya

Reaction Condition

Time

Yield (%)

Ref

1 2 3 4 5b 6 7c 8e 9 10 11f 12 13 14 15 16c 17 18g 19h 20g

Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2) (7 mg)/NaBH4 (2 mmol)/H2O:THF (1:1)/rt Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2) (8 mg)/HCO2NH4 (2 mmol)/H2O:PEG-400 (1:1)/rt Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2) (10 mg)/glycerol (2 mmol)/H2O/rt Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2) (10 mg)/i-PrOH (2 mmol)/H2O/rt Co(BF4)2·6H2O (5 mol %)/triphos (5 mol %)/H2 (80 bar)/THF/100 °C ZrCl4 (1.2 mmol)/NaBH4 (5 mmol)/THF/rt Ru(acac)3 (4 mol %)/triphos (5 mol %)/Al(OTf)3 (10 mol %)/H2 (60 bar)/THF:H2O:CH3OH (2:0.1:0.3)/160 °C [RuCl(PPh3)2(3-phenylindenyl)] (1 mol %)/PhSiH3 (2−3 equiv)/THF/60 °C TFA (1 mmol)/Zn(BH4)2 (4 mmol)/DME/rt NaBH4/diglyme/162 °C Benzyltriethylammonium borohydride (4 mmol)/chlorotrimethylsilane (4 mmol)/CH2Cl2/rt (Py)Zn(BH4)2 (2 mmol)/THF/Reflux Cu(OTf)2 (5 mol %)/TMDS (4 mmol)/2-Me THF/80 °C Ti(OiPr)4 (1 mmol)/PMHS (10 mmol)/THF/N2 atm/Reflux Pd (2 mol %)−Re (7 mol %)−C/MS 4 Å (200 mg)/n-hexane (0.2 M)/H2 (20 bar)/130 °C [RhCl(cod)]2 (0.25 mmol)/4 PPh3 (0.10 mmol)/Ph2SiH2 (8 mmol)/THF/NaOH/rt KBH4−MgCl2 (2 mmol)/THF/66 °C Catechol (20 mmol)/CF3COOH (10 mmol)/NaBH4 (20 mmol)/THF/N2 atm/0 °C HfCl4 (1 mmol)/KBH4 (4 mmol)/THF/N2 atm/40 °C 50% T3P in EtOAc (20 mmol)/DIPEA (11 mmol)/NaBH4 (10 mmol)/THF/0 °C

30 min 60 min 20 min 60 min 22 h 5h 24 h 16 h 48 h 15 h 5h 1.5 h 16 h 16 h 18 h 48 h 24 h 12 h 17 h 25 min

95 97 97 96 65 89 42d 84 55 97 92 96 84 69 72d 62 67.7 20 61 83

− − − − 3a 3b 3c 3d 3e 3f 3g 3h 3i 3j 3k 3l 3m 3n 3o 3p

a In all cases, the amount of benzoic acid is 1 mmol (except those specified). bAmount of benzoic acid = 0.15 M. cAmount of benzoic acid = 0.5 mmol. dGC yield. eAmount of benzoic acid = 0.25 mmol. fAmount of benzoic acid = 2 mmol. gAmount of benzoic acid = 10 mmol. hAmount of benzoic acid = 0.8 mmol.

Figure 1. Reusability of the as-synthesized MNPs in the presence of (a) NaBH4, (b) HCO2NH4, (c) glycerol, and also (d) i-PrOH as the costeffective hydrogen donors on the reduction of benzoic acid to benzyl alcohol.

E


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Industrial & Engineering Chemistry Research Scheme 4. Preparation of the Fe3O4@APTMS@ ZrCp2Clx (x = 0, 1, 2) MNPs

Figure 3. XRD pattern of the Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2) MNPs.

Figure 2. FT-IR spectra of the Fe3O4, Fe3O4@APTMS, and Fe3O4@ APTMS@ZrCp2Clx (x = 0, 1, 2). Figure 4. SEM images of the Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2).

thermogravimetric curves of the Fe3O4@APTMS (red curve) and Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2) (blue curve) at 200−800 °C confirms that the existence of the zirconocene moiety on the structure of the final magnetic nanocomposite based on the fact that in the mentioned area (200−800 °C) the losing weight of the Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2) is too much as compared to Fe3O4@APTMS. The surface area (SBET) and total pore volumes (Vtotal) for the prepared Fe3O4 and Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2) MNPs were studied using Barrett−Joyner−Halenda (BET) analysis (Table S2). Also, Figure 8 represents the nitrogen adsorption−desorption isotherms of the Fe3O4 NPs and zirconocene-containing nanocomposite, which approximately classifies as type IV and H1 type hysteresis looped according to the IUPAC standards.

3. EXPERIMENTAL SECTION 3.1. Reagents, Samples, and Apparatus. All reagents, samples, and also solvents are commercially available (purchased from Merck, Fluka, and Sigma−Aldrich chemical companies) and were used as received without further purification. Infrared spectra were recorded on a Bruker VRTEX 70 model FT-IR spectrophotometer, measured using KBr disks in the range 400−4000 cm−1. 1H and 13C NMR spectra were recorded on a Bruker Avance 300 MHz spectrometer in CDCl3 with tetramethylsilane (TMS) as an internal standard at 300 and 75 MHz, respectively. Progress of the reactions and purity of the products were monitored by thin-layered chromatography (TLC) using silica gel 60 F254 aluminum sheets. The crystalline structure of the as-prepared magnetically nanocatalyst was surveyed by X-ray diffraction (XRD) on a Philips PANalytical X’PertPro diffractometer F


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Figure 5. EDX spectrum of the Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2) nanocomposite.

Figure 8. N2 absorption−desorption isotherms for the Fe3O4 and Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2) MNPs.

scanning electron microscopy (SEM) images with FE-SEM TESCAN MIRA3. The chemical contents (especially Fe and Zr) of the as-prepared MNPs contents were detected by inductively coupled plasma-optical emission spectrometry (ICP-OES). The magnetic properties of the prepared materials were measured on an alternating gradient force magnetometer (AGFM) at room temperature. The thermogravimetric analysis curves were performed using a Shimadzu DTG-60 instrument under a nitrogen atmosphere. The N2 adsorption− desorption isotherms were measured by a Belsorp-Max (BEL Japan, Inc.) 3.2. Preparation of the Fe3O4 NPs. In a two-neck roundbottom flask, a solution of FeCl3·6H2O (5.838 g) and FeCl2· 4H2O (2.147 g) in distilled water (100 mL) was prepared. The solution was stirred vigorously by a mechanical magnetic stirrer for 5 min at 85 °C under a N2 atmosphere. In the next step, aqueous NH3 (25%, 10 mL) was quickly added to the prepared solution. Immediately after the mentioned addition, the blackcolored nanoparticles of Fe3O4 were precipitated. The resulting mixture was continuously stirred for 30 min at 85 °C. After cooling the mixture to room temperature, the prepared magnetically nanoparticles were collected by an external magnet and washed with distilled water and then with a solution of 0.02 M sodium chloride (NaCl). Ultimately, the wet prepared nanoparticles dried under air atmosphere to obtain the pure magnetic nanoparticles of Fe3O4. 3.3. Preparation of the Fe3O4@APTMS Core−Shell MNPs. The Fe3O4@APTMS core−shell MNPs was synthesized by refluxing prepared Fe3O4 (1.5 g) with 3-aminopropyltrimethoxysilane (APTMS) (2 g) in n-hexane (100 mL) under a N2 atmosphere for 1 day (24 h). After the titled reaction completed, the resulting Fe3O4@APTMS core−shell MNPs were collected, washed with ethanol, and then dried under vacuum at room temperature (25 °C). 3.4. Preparation of the Fe3O4@APTMS@ ZrCp2Clx (x = 0, 1, 2) MNPs. The prepared Fe3O4@APTMS MNPs (1 g) were dispersed in ethanol by sonication for 10 min. Afterward, Cp2ZrCl2 (1 g) was added, and the resulting mixture was stirred mechanically at reflux for 10 h. After completion of the reaction, the mixture was cooled to room temperature. Next, the Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2) MNPs were collected by an external magnet and washed with acetone and then deionized water. Finally, the obtained

Figure 6. Magnetization curve for the (a) Fe3O4 and (b) Fe3O4@ APTMS@ZrCp2Clx (x = 0, 1, 2) MNPs.

Figure 7. TGA curves for the (a) Fe3O4, (b) Fe3O4@APTMS, and also (c) Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2) MNPs.

(Netherlands) at 40 kV and 30 mA with monochromatized Cu Kα radiation (λ = 1.5418 Å) in a range of Bragg’s angle (2θ = 10°−80°) at room temperature. The size and morphology of the magnetically nanoparticles (MNPs) were investigated by G


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zirconocene containing core−shell MNPs were dried under vacuum for 6 h. 3.5. General Procedure for the Direct Reduction of Carboxylic Acids to Corresponding Alcohols. As a representative example, in a round-bottom flask equipped with a magnetic stirrer, a mixture of benzoic acid (1 mmol), Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2) MNPs (7 mg), and NaBH4 (2 mmol) was stirred in 3 mL of H2O:THF (1:1) as a solvent at room temperature for 30 min. After completion of the reaction (checked by TLC), the catalyst was separated using an external magnet. Next, the product was extracted with EtOAc (5 × 3 mL). The extracts were dried with MgSO4, and then evaporation of the solvent afforded the crude product. In the next step, the crude product was purified by the TLC plate [silica-gel, n-hexane:EtOAc (3:1)].

4. CONCLUSIONS In conclusion, we have developed green and highly efficient protocols for the direct reduction of carboxylic acids to corresponding alcohols using four different types of hydrogen donors namely NaBH4, HCO2NH4, glycerol, and i-PrOH in the presence of Fe3O4@APTMS@ZrCp2Clx (x = 0, 1, 2) as the reusable magnetic nanocatalyst in good to excellent yields. Undoubtedly, these practical and environmentally benign strategies can open a new avenue for the green and convenient reduction of carboxylic acids to alcohols. Notably, work on the other catalytic applications of the as-prepared Fe3O4@ APTMS@ZrCp2Clx (x = 0, 1, 2) is underway in our laboratory and will be reported in due course.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b01847.

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Optimization data and NMR spectra (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hossein Mousavi: 0000-0003-4292-3762 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful to the Research Council of Urmia University for the partial support of this work. REFERENCES

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