Effect of synthesizing conditions on the activity of zinc-copper aluminate nanocatalyst prepared by microwave combustion method used in the esterification reaction

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Effect of synthesizing conditions on the activity of zinc-copper aluminate nanocatalyst prepared by microwave combustion method used in the esterification reaction Mojgan Hashemzehia, Vahid Pirouzfara, , Hamed Nayebzadehb, Afshar Alihosseinia ⁎

a b

Department of Chemical Engineering, Central Tehran Branch, Islamic Azad University, Tehran, Iran Esfarayen University of Technology, Esfarayen, North Khorasan, Iran

ARTICLE INFO

ABSTRACT

Keywords: Microwave Combustion method Zinc aluminate Copper oxide Ammonium acetate Biodiesel

The effect of microwave radiation power and fuel type as two important parameters on the morphology, properties, and activities of the zinc-copper aluminate spinel fabricated was assessed by microwave combustion method (MCM). Characterization results of the catalysts fabricated at different microwave irradiation power showed faster evaporation of remained water in the precursor gel that can lead to the formation of the sample with better crystalline form and structure. These properties enhance the activity of the nanocatalyst in the esterification reaction. The effect of fuel type (e.g., urea, ammonium acetate, and their mixture) on properties of the nanocatalyst synthesized at the maximum microwave power was also examined. Due to different levels of thermogenesis, the nanocatalyst properties were modified using fuel mixtures a result of increasing the pore diameter of the nanocatalyst from 2.1 to 9.5 nm for the sample with the highest activity. In addition, the nanocatalyst structure such as particle-size distribution and the agglomerated particles was noticeably improved. The best spinel zinc-copper aluminate nanocatalyst was prepared at maximum microwave power and ammonium acetate to urea ratio of 1:3. Also, the yield of 99.1% was obtained at the operating conditions of 180 °C, 9:1 M ratio of methanol to oleic acid, and 3 wt% of catalyst. The catalyst presented high stability such that it can be reused at least for nine cycles. Owing to catalysts activity and reusability, this catalyst can be recommended for a variety of catalyst systems.

1. Introduction

catalysts such as sulfuric acid are suggested for the esterification reaction, these catalysts show high corrosion, toxicity, and environmental problems [7]. Therefore, heterogeneous catalysts have been proposed to overcome the problems of the homogenous catalyst [8]. In this regard, alumina as one of the useful supports is usually reinforced by other activating elements (either base or acid cations) to present higher activity. However, modifying the surface of alumina by active phases showed less stability due to leaching of active phases where doping of these cations into alumina lattice may be a solution. One of the most attractive types of doped materials is Spinel type catalysts, which are produced by doping the divalent component (A) in the lattice of trivalent host component (B) to form (AB2O4). Copper and zinc oxides have attracted much attention in catalytic studies for divalent component applications, because of the high activity, availability, and ease of synthesis. In our previous study, these types of spinal have been synthesized. Comparing the characteristics and activity of zinc aluminate with those of copper-alumina oxide in the esterification reaction shows

Considering that environmental protection is essential for human survival in the current industrialized world, every effort should be made to reduce the emission from fossil fuels, which has been identified as one of the most challenging environmental problems [1–3]. The health testing requirements of the 1990 Clean Air Act (CAA) can be fully met with renewable fuels, particularly biodiesel, which has been proposed to use as the only engine fuel accepted by the Environmental Protection Agency (EPA) for sales and distribution [4,5]. Biodiesel (Alkyl esters) is produced by esterification and transesterification processes, in which a feed reacts with short-chain alcohol in the presence of a catalyst (usually, homogeneous base catalyst). The processes are actually performed to reduce the viscosity and flash point of oil [6]. Since biodiesel is nowadays produced from oils having a high content of free fatty acid, it is necessary to perform the esterification reaction prior to the transesterification reaction. Although the homogeneous acid

⁎

Corresponding author. E-mail address: [email protected] (V. Pirouzfar).

https://doi.org/10.1016/j.fuel.2019.116422 Received 18 June 2019; Received in revised form 14 September 2019; Accepted 11 October 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Mojgan Hashemzehi, et al., Fuel, https://doi.org/10.1016/j.fuel.2019.116422

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smaller particle size, higher BET surface area, larger average pore size, a higher acidity, and higher activity (94%) in the former [9]. Recently, the copper and zinc-based nanocomposites, especially in the form of CuO-ZnO-Al2O3, have received much attention and widely used in catalysis reactions. The emerging catalytic properties of these nanoparticles could pave the way to achieve the appropriate performance of the catalyst in many applications. For example, the Zn-Cu/Al mixed metal oxide synthesized at optimum Zn/Cu ratio of 2:3 presented suitable properties and activity in esterification reaction [10]. Goudarzi et al. [11] synthesized CuO/ZnO/Al2O3 (CZA) porous nanostructure via a green method. Through the photocatalytic activity of the catalyst, they found that the CZA nanostructure is a highly efficient photocatalyst in photodegradation of common pollutants. HassanzadehTabrizi et al. [12] prepared ZnO/CuO nanocomposite immobilized on γAl2O3 via heterogeneous precipitation method in order to remove methyl orange dye from aqueous solutions. Saravanakkumar et al. [13] synthesized pure ZnO and ZnO-CuO nanocomposites using a modified perfume spray pyrolysis method and the CuO-ZnO nanoparticles exhibited effective antibacterial activity. Various methods have been utilized for the synthesis of these spinels; e.g., solvothermal [14], corporation [15], hydrothermal [16] and solution combustion method [9,17]. Among these methods, the solution combustion method coupled by microwave irradiation, i.e., “microwave combustion method (MCM)”, has attracted much attention because of the uniform distribution of heat, uniform ignition in all parts of the mixture, the high speed production process, appropriate particle size distribution of catalyst, and no requirement to further heat treatment after synthesis [17]. In this method, physical, chemical and structural properties of the obtained powder strongly depend on the fuel-to-oxidizer ratio [18], fuel type [19], type of precursor, and microwave power level [20]. Ajmaein et al. [20] investigated the effect of fuel/nitrates ratio on the structure and activity of CuO-ZnO-Al2O3 in the steam reforming of methanol reaction. The characteristic analyses results demonstrated that enhancement of the fuel to nitrates ratio from 1 to 2 increased the crystallinity of CuO and ZnO species. However, subsequently, it decreased due to the incomplete combustion reaction. Moreover, enhancement of fuel amount led to a decrease in the specific surface area. Rahmani et al. [18] studied the effect of fuel ratio on the combustion synthesis of MgAl2O4 and reported that the catalyst synthesized with a fuel ratio of 1.5 is the best for biodiesel production. It noteworthy that if the fuel-to-oxidizer ratio is less than the stoichiometric ratio, the heat required to form the spinel will not be available; otherwise, overheating will increase only the crystallite size [21]. Type of fuel is the next factor that leads to producing various amounts of heat and can significantly affect the crystallite size and particle agglomeration. Different fuels have been proposed by researchers for evaluating the fuels used in the combustion synthesis method (e.g., urea, citric acid, glycine, ammonium acetate, and a mixture of oxides). Hossein Ajamein el al. [22] investigated the effect of molecular weight of fuels and their chemical structure on physicochemical properties of synthesized CuO/ZnO/Al2O3 nanocatalyst. They used Sorbitol, propylene glycol, glycerol, diethylene glycol, and ethylene glycol as fuel and showed that among the fuels, sorbitol has the highest polarity and makes to growth of Zn crystals. Nayebzadeh et al. [8] studied the effect of the carbon content of fuels such as urea, ammonium acetate, glycerol, and diethylene glycol in combustion reaction on the properties of KOH/Ca12Al14O33. The results showed that the catalyst synthesized by urea and ammonium exhibited the best structure. Similarly, the radiation can be the next decisive factor in the catalyst synthesis for improving the structure and activity of the catalyst. Allahyari et al. [23] synthesized acetate based CuO–ZnO–Al2O3/HZSM-5 nanocatalyst under high irradiation ultrasound powers. They reported that increasing ultrasound irradiation power yields smaller particles with better dispersion and higher surface area. Shokrani et al. [24] investigated the effect of the fuel to oxidant ratio and sonication on the physiochemical properties and performance of the CuO/ZnO/Al2O3

catalyst in the steam reforming of methanol reaction. The results showed that the ultrasonic mixing of primary gel compared to conventional mixing led to lower crystallite size. To our best knowledge, no research has been carried out the influence of microwave power on the properties and performance of the CuO-ZnO-Al2O3 in biodiesel production. In this regard, the sintering of catalyst has been observed in some studies using only one fuel due to the high local temperature of combustion [25,26]. However, mixed fuel can be a solution to this problem. Therefore, this study aims to synthesize zinc-copper aluminate spinel by the solution combustion method under microwave radiation. Moreover, it was tried to evaluate the effects of microwave power levels and of the type and composition percentage of the two fuels (i.e., urea and ammonium acetate) on the crystal structure, particle size, surface area, and the activity of the nanocatalyst in the esterification reaction. The samples were characterized by X-Ray Diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Brunauer-Emmett-Teller (BET-BJH), scanning electron microscope (SEM), and Transmission Electron Microscopy (TEM) analyses and the stability of the sample fabricated on the optimum conditions was evaluated in the esterification reaction. 2. Experimental and methods 2.1. Catalyst synthesis Solution combustion method is a redox combustion reaction that leads to the production of nanometer oxide powder. In this process, nitrate ions and the intended fuel act as oxidizing and reducing agents, respectively [27]. To fabricate zinc-copper aluminate (ZCA) spinels, a minimum amount of distillated water was used to dissolve zinc nitrate hexahydrate, copper (II) nitrate trihydrate, and aluminum nitrate nonahydrate at a molar ratio of 4:6:10; this set-up was determined as the optimum from our previous work [10]. Then, a certain amount of urea and ammonium acetate was dissolved in the mixture (Table 1). The amount of fuel/oxide ratio was determined to be 1, with respect to the stoichiometric ratio, oxidizing, and reducing agents, and the oxidizer-to-reducer ratio. Then, the raw material solution was stirred at 80 °C until a clear gel was obtained and put in a microwave oven (Daewoo, KOC9N2TB, 900 Watts, 2.45 GHz) for 10 min. The levels of microwave power were varied from 540 to 900 W, as listed in Table 1. After evaporating the remained water in the solution, it was suddenly ignited along with releasing a large volume of smoke. After a while, which was different for each sample, foamy nanocatalysts were synthesized that were labeled as ZCA-X-Y, where X is the microwave power and Y is the ratio of urea used in the reaction. 2.2. Catalyst characterization To identify the phases and crystallite size of catalysts, XRD method Table 1 List of the fuel level and microwave power levels used for the synthesis of samples. Samples

Microwave Power (Watt)

CH4N2O (g/St. ratioa)

C2H3O2NH4 (g/St. ratioa)

ZCA-540-1 ZCA-630-1 ZCA-720-1 ZCA-810-1 ZCA-900-1 ZCA-900-0.75 ZCA-900-0.5 ZCA-900-0.25 ZCA-900-0

540 630 720 810 900 900 900 900 900

3.2/1 3.2/1 3.2/1 3.2/1 3.2/1 2.4/0.75 1.6/0.5 0.4/0.25 0/0

0/0 0/0 0/0 0/0 0/0 0.56/0.25 1.12/0.5 0.84/0.75 1.12/1

St. ratio: the amount of fuel used in the mixture as compared to stoichiometric ratio. a

2

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was used by a UNISANTIS/XMP 300 with Cu radiation at 1.54 Å over the 2θ range from 20° to 70° with a scan speed of 10° per minute. FTIR spectroscopy was also used with a SHIMADZU 4300 spectrophotometer (Japan) in the range of 400–4000 cm−1 to detect the surface functional groups. The pore diameter, porosity, and surface area of each catalyst were measured by the BET method using a Belsorp-mini II (BEL Japan, Inc.) apparatus. In addition, an SEM analysis using Hitachi model S4160 was carried out to identify the morphology of nanocatalysts, appropriately. Finally, the catalyst with the highest activity was assessed by TEM analysis (LEO model 912AB).

ZnAl2O4 , Cu2Al4O7 CuO * Al2O3

(e) ZCA-900-1

(d) ZCA-810-1

2.3. Catalytic activity The activity of the samples was evaluated during the esterification process. The reaction was performed in a closed system with a stainlesssteel reactor under identical conditions (i.e., 180 °C, the molar ratio of methanol to oleic acid of 9, 3 wt% of catalyst, and 6 h of reaction time). The oil bath was used to adjust a reaction temperature with the error of ± 3 °C by means of a K-type thermocouple. Once the reaction was completed, the reaction mixture was centrifuged to remove the catalyst, and then water and excess methanol were removed by heating the mixture. Afterward, the conversion of oleic acid to methyl ester was calculated by reducing the acid index of product toward oleic acid by performing a standard titration method using KOH.

(c) ZCA-720-1

*

*

*

*

(b) ZCA-630-1

*

*

*

* (a) ZCA-540-1 ZnAl2O4: Cubic, 73-1961

3. Results and discussion

Cu2Al4O7: Cubic, 83-1476

3.1. Evaluating the effect of microwave power on the catalyst properties

CuO: Monoclinic, 05-0661

Fig. 1 shows the XRD patterns of the ZCA samples synthesized under different microwave powers (540, 630, 720, 810, and 900 W). According to Fig. 1, the peaks at 31.2°, 36.8°, 44.8°, 49°, 55.6°, 59.3°, 65.2°, 74.1°, and 77.3° were related to (2 2 0), (3 1 1), (4 0 0), (3 3 1), (4 2 2), (5 1 1), (4 4 0), (6 2 0), and (5 3 3) crystallography of zinc and/ or copper alumina spinel, respectively [28,29]. In addition, the peaks of copper oxide can be observed at 35° and 38° [30]. The peaks corresponding to zinc and copper alumina can be found in almost all samples. The crystallinity of zinc alumina spinel is decreased by reducing the intensity of microwave power. Here, the intensity of the peaks at 31°, 36.55°, and 59° are considerably reduced at the microwave radiation power of 540 W [27]. The broad reflections in ZCA-540-1 reveal that the particle sizes of CuO and ZnO are much smaller and the copper and zinc exhibit amorphous-like or less ordered structural features [31]. By increasing the intensity of microwave power, more uniform distribution can be found in alumina-based zinc and copper crystals. In this structure, the peaks corresponding to copper and zinc oxides have substantially the same height and more width. Here, a considerable enhancement of the crystallization is observed by increasing the applied microwave power [32], which can be attributed to the shorter period of heating used for the preparation crystals over the high power levels. Besides, applying microwave to the starting solution, homogeneous nucleation occurs followed by the growth of the crystals and subsequent aggregation. It is of note that once higher microwave power is applied to synthesize nanocatalyst, a good synthesis mixture homogenization is achieved without any impurities and with a high crystallization level. Meanwhile, the ZCA-540-1 catalyst contained impurities or an excess of amorphous material related to insufficiently crystalline structure [33]. Table 2 shows the crystallite size of the samples calculated using the Scherrer equation from the peak of 38.6° for zinc/copper aluminate spinel and the average of two peaks at 2θ = 35° and 38° for copper oxide. The results showed that the ZCA-900-1 catalyst has the smallest crystallite size of zinc aluminate against copper oxide crystals. This result can be explained by the higher diffusion of copper cations into alumina lattice and growth of remained copper oxide crystals, which caused to increase the copper oxide crystal and decrease the spinels crystal size. Moreover, this can be referred to rapidly starting the

Al2O3: Cubic, 47-1292 20

25

30

35

40

45

50

55

2 (degree)

60

65

70

75

80

Fig. 1. XRD patterns of Zn0.4Cu0.6Al2O4 nanocatalyst prepared via microwaveassisted combustion method using urea as fuel at different levels of microwave power.

solution combustion synthesis reaction (after 2 min) at the maximum microwave power, while about 40 min was required for the synthesis of the ZCA-540-1 nanocatalyst. As a result, it led to a lower diffusion of the copper cation in alumina lattice in ZCA-540-1 nanocatalyst and the growth of copper oxide. In other words, the lower microwave power can delay the process of converting gel into powder and lead to an increased tendency of the core to agglomeration and decreasing the time for the synthesis of nonporous materials. These reasons lead to achieving significantly narrower distributions of crystallite dimensions, more rapid nucleation of the initial crystallites, and an increase in the average size of nanostructures in spinel form [33,34]. Fig. 2 presents the FTIR analysis of ZCA nanocatalyst fabricated by urea as fuel at different microwave power levels. The broad peak in the regions of 3000–3500 cm−1 and 1600 cm−1 is related to the stretching and bending vibration bonds of –OH group, respectively; probably due to the hydrophilic property of catalysts in the presence of moisture in the air [34]. Moreover, some diffraction peaks of carbonyl group can be observed in the region of 1700–2300 cm−1, which can be related to organic impurities and/or reaction of surface of nanocatalyst with CO2 of in the air [35]. The vibrations of zinc aluminate in the band of 492 cm−1 are associated with octahedral vibration bonds of Moctra-O metal. Moreover, the peaks at 557 cm−1 and 668 cm−1 are associated with the intrinsic stretching vibration bands of the metal at the tetrahedral sites of Mtetra-O [30,34]. The broad peak corresponding to copper oxide in the region of 883 cm−1 overlapped with the zinc aluminate bands [36]. The intensity of the bands belonging to metal oxides was reduced when applying the 3

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Table 2 Physicochemical properties of Zn0.4Cu0.6Al2O4 nanocatalyst prepared via microwave-assisted combustion. Catalyst

Microwave Power (W)

U/AA ratio

RCa (%)

Crystalline size (nm) ZnAl2O4

ZCA-540-1 ZCA-630-1 ZCA-720-1 ZCA-810-1 ZCA-900-1 ZCA-900-0.75 ZCA-900-0.5 ZCA-900-0.25 ZCA-900-0

e

3640

f

– 19.5 15.0 15.8 14.7 14 14.2 15 15.8

Dpf (nm)

Conversion (%)

18.62 – 15.77 – 4.91 8.38 9.27 96.86 47.56

0.003 – 0.006 – 0.001 0.002 0.002 0.007 0.009

6.9 – 6.6 – 8.6 9.5 8.9 2.9 2.1

87.2 90.2 93.9 95.0 96.9 99.1 96.7 90.1 89.3

c

24.6 31.2 31.5 32.1 32.3 – 33.1 30.7 –

± ± ± ± ± ± ± ± ±

2.1 1.8 1.2 0.8 0.4 0.5 0.9 1.6 2.0

catalysts. It inhibits the formation of the sample with high crystallinity of spinel structure and metal oxides and leads to a greater surface area of the nanocatalysts. Although a longer period of microwave radiation can lead to the increased surface area of the nanocatalyst, mean pores diameter was reduced due to more pores produced by the huge volume of combustion gases created during the microwave-assisted combustion synthesis [20,37]. Moreover, lower surface area in ZCA-900-1 nanocatalysts can be due to the rapid nucleation of the initial crystallites and growth process during enhancement in microwave power, which is in good accordance with XRD results [33]. It is noteworthy that surface area is not considered the only important factor in the esterification reactions, because the pore diameter of nanocatalysts must be large enough to facilitate the penetration of large molecules of fatty acids so as to increase the connection of the molecules reacting with active acid sites [38,39]. Because of the better properties of ZCA-900-1 nanocatalyst, the sample showed higher catalytic activity in the esterification process than nanocatalysts prepared at the power levels of 540, 630, 720, and 810 (Table 2). Fig. 3 represents the N2 adsorption-desorption isotherm and the pore size distribution of the ZCA nanocatalysts prepared at the power levels of 540, 720, and 900 W. In Fig. 3a, all the synthesized samples indicate a type-IV hysteresis plot. However, different types of hysteresis loops are seen in terms of microwave intensity. For example, the ZCA540-1 sample gives an H2 type hysteresis loop, which is usually related to mesoporous samples of a complex structure, while H3-type hysteresis loops, which are evidence of aggregates of plate-like particles and open pores with a uniform diameter, are seen in other samples [36,37]. The pore size distribution of the ZCA-540-1 sample represents two peaks with the largest pore volume relating to holes with a diameter of 2.1 nm while those with a diameter of 1.8 nm may provide an appropriate volume, as well. Diameter and volume of a hole with the maximum volume (i.e., 1.6 nm) decrease as the microwave power increases to 720 W. Although the diameter of the pore with the largest volume does not change such that with further increase in microwave power, the pore volume is significantly reduced, probably due to the lower diameter. Fig. 4 shows the surface morphology of the ZCA-540-1, ZCA-720-1, and ZCA-900-1 nanocatalysts. Formation of uniform nanoparticles can be anticipated by microwave-assisted heating by which uniform heating is provided. When the sample is irradiated with a microwave power of 540 W (Fig. 4a), it is contributed to the formation of irregular particles, which is confirmed using the XRD analysis. By increasing the applied microwave power to 720 W, formations of spherical nano-particles can be observed, notwithstanding the fact that aggregated particles are still

850 685 480

d

39 77 77 83 75 93 75 83 100

CuO

Vpe (cm3/g)

Relative Crystallinity determined based on the peak at 2θ = 37°. Crystalline size was measured for the peak at 2θ = 37°. Crystalline size is average of two peaks at 2θ = 35.6° and 38.7°. BET Surface area. Pore volume. Pore diameter.

1680 1550

c

1/0 1/0 1/0 1/0 1/0 3/1 1/1 1/3 0/1

2770 2550 2350

b

3330

a

540 630 720 810 900 900 900 900 900

b

SBETd (m2/g)

(e) ZCA-900-1

(d) ZCA-810-1

(c) ZCA-720-1

(b) ZCA-630-1

(a) ZCA-540-1

4000 3500 3000

2500 2000 1500 1000 500 Wavenumber (cm-1)

Fig. 2. FTIR spectra of Zn0.4Cu0.6Al2O4 nanocatalyst prepared via microwaveassisted combustion method using urea as fuel at different levels of microwave power.

microwave power of 540 W, suggesting the formation of less intended groups. Table 2 also shows the BET surface area, pore diameter, and pore volume of the nanocatalysts synthesized at different levels of microwave power. As noted above, the time of starting the combustion reaction was reduced with increasing microwave power so that the combustion synthesis started after 2 min of microwave radiation at 900 W. In comparison, about 40 min is needed for the synthesis of other 4

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Va/cm3(STP)g-1

20

(a) ZCA-540-1

15 10

(b) ZCA-720-1

5 0

(a)

(c) ZCA-900-1 0

0.2

0.4

P/P0

0.6

0.8

1

Fig. 3. (a) Nitrogen adsorption-desorption isotherm and (b) pore size distribution of Zn0.4Cu0.6Al2O4 nanocatalyst prepared via microwave-assisted combustion method using urea as fuel at different levels of microwave power.

seen. Once sufficient heat of combustion to prepare the sample is provided at the microwave power of 900 W, the nucleation rate increases and more uniform distribution of particles is achieved. At the same time, at 900 W, the particles are smaller compared to those observed for the lower microwave power at 720 and 540 W.

ammonium acetate is not an appropriate fuel to use individually. As the amount of urea is increased, the combustion reaction is converted from a smoldering into a flaming. Urea can provide the heat of combustion to transform the materials from nitrate to crystal form. Formation of suitable uniform distribution of copper oxide crystal on the surface of ZnAl2O4 can be observed. This result is also proved by the changing intensity of peaks at 2θ = 35° and 38°, which is due to the control of combustion temperature. The observed transformation from amorphous to crystal form with a lower size of crystallite can act as an active phase and contribute to increasing the activity of catalyst [30]. In this case, a flame is held to it for a longer period of time, which provides sufficient temperature [20,21]. However, it is observed that using the urea/ammonium acetate in the ratio of 3:1, the copper oxide peaks were clearly decreased. Such a decrease is attributed to sufficient reaction temperature and duration form complete diffusion of copper cations into alumina lattice. At this ratio, the reaction temperature not only was very high to accelerate the sintering problem, as seen for ZCA-900-1 catalyst, but also the copper peaks were sharply reduced due to doping Cu2+ into Al3+ as host structure. Therefore, it seems that low amounts of ammonium acetate can be added to urea to control its high local combustion temperature, leading to a sintering problem [21]. The crystalline size of the catalyst is specified in Table 2. It firstly decreases with an increase in ammonium acetate due to controlling the reaction temperature and duration and preventing the sintering of the samples. However, when the ammonium acetate is used as the only fuel, the maximum crystallite size can be achieved due to the uneven distribution of the temperature to form crystals. It can be proved by the

3.2. Evaluating the fuel type and composition After determining the optimal microwave power for nanocatalyst synthesis, a combination of the two fuel types (urea and ammonium acetate) with different molar ratios was used to achieve to the maximum catalytic activity in the esterification reaction. Fig. 5 illustrates the XRD patterns of the ZCA-900 sample synthesized at different ratios of urea/ammonium acetate as fuel. When only the auxiliary fuel (ammonium acetate) was used for the synthesis of nanocatalyst, the combustion is converted into a smoldering reaction without visible flame. Here, the smoldering combustion can lead to two phenomena. Firstly, the combustion temperature and the sintering of the sample are clearly reduced such that the copper oxide peak with less intensity is observed. In fact, sintering of the final powder by overheating caused egression of copper cations from alumina lattice while the copper and zinc cations were completely diffused into alumina lattice to form spinel structure using ammonium acetate. Secondly, low amounts of precursors were converted from amorphous to crystal form due to the low temperature of the combustion medium. The light brown color of the final powder can be related to the insufficient decomposition of precursor nitrate salts caused by low combustion temperature. It can prove the

Fig. 4. SEM images of ZCA catalyst prepared by MCM with urea as fuel at the microwave power levels of (a) 540 W (b) 720 W and (c) 900 W. 5

1680 1550 1360

2550

2770 2550

3330

3640

ZnAl2O4 , Cu2Al4O7 CuO * Al2O3

980 850 685 480

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(e) ZCA-900-1

(e) ZCA-900-1

(d) ZCA-900-0.75

(d) ZCA-900-0.75

(c) ZCA-900-0.5

(c) ZCA-900-0.5 (b) ZCA-900-0.25

(b) ZCA-900-0.25

(a) ZCA-900-0 ZnAl2O4: Cubic, 73-1961 Cu2Al4O7: Cubic, 83-1476

(a) ZCA-900-0

CuO: Monoclinic, 05-0661 Al2O3: Cubic, 47-1292 20

25

30

35

40

45

50

55

2 (degree)

60

65

70

75

4000 3500 3000

80

2500 2000 1500 1000 500 Wavenumber (cm-1)

Fig. 6. FTIR spectra of Zn0.4Cu0.6Al2O4 nanocatalyst prepared via microwaveassisted combustion method using at high microwave power with different composition of fuels.

Fig. 5. XRD patterns of Zn0.4Cu0.6Al2O4 nanocatalyst prepared via microwaveassisted combustion method using at high microwave power with different composition of fuels.

identical and within the range of 2–8 nm, which was an appropriate range for heterogeneous catalysts used in the production of biodiesel [43]. The adsorption/desorption hysteresis plots of the samples showed IV type based on IUPAC classification, corresponding to a material with pore diameters of 1.5–100 nm. The hysteresis plots of ZCA-900-0.25, ZCA-900-0.5, and ZCA-900-0.75 samples are of the H3-type hysteresis loop related to slit-shaped particles. Meanwhile, the H2 type hysteresis loop was found for ZCA-900-0 and ZCA-900-1, which corresponds to cylindrical pores. The two parallel lines observed in the ZCA-900-0.25, ZCA-900-0.5 and ZCA-900-0.75 samples show open pores of the uniform radius. In addition, the hysteresis curve of the samples is closed in the partial pressure of 0.8 to 1, showing the presence of large mesopores. Moreover, the curved shape of the hysteresis loop is associated with the uniform distribution of the pore diameter [37,44]. According to Fig. 7b, pore diameter ranges between 1.5 and 10 nm. The pore size distribution of samples synthesized using the fuel of ammonium acetate shows three peaks at 1.7, 2.1, and 2.4 nm, which are weakened with an increase in the urea content. Among the sample, ZCA-900-0.5 and ZCA-900-0.75 present broader pore size that has a positive influence on the activity of the sample in the liquid medium reactions. The results of BET analysis are listed in Table 2. As can be seen, among the synthesized samples, ZCA-900-0 and ZCA-900-0.25 represent the much larger surface area and pore diameter than other samples. The large difference can be as a result of their morphology and crystallinity, as mentioned in the XRD section. It is noteworthy that by increasing the amount of ammonium acetate, the crystals may not form or a small amount of catalyst would crystalize due to the incomplete

crystalline size of copper oxide, wherein the particle size is significantly reduced due to inaccessibility to sufficient temperature for growth of copper cations. Regarding the main aim of this study, which is to synthesize a spinel nanocatalyst with high activity, ZCA-900-0.75 was used for our purpose. The FTIR spectra of the ZCA-900 samples synthesized by different composition of fuels are illustrated in Fig. 6. The peaks of O–H and/or unreacted organic groups, and the peak corresponding to the vibrations of zinc and copper oxides can be detected for all samples (Fig. 2) [36,40,41]. The main difference can be assigned to the intensity of metal oxides peaks in the range of 500–1000 cm−1. These peaks were sharply enhanced using the mixing of fuel at the ratios of 3:1 and 1:1, which can be related to the good formation of metal cations from amorphous to crystal form and diffusion of them in alumina lattice. The suitable reaction temperature along with appropriate duration was provided for good interaction between dopant cation and host lattice (alumina) to form spinel. To obtain the surface area, pore diameter, and pore volume of the synthesized catalysts, BET-BJH analysis was carried out. The obtained results are shown in Table 2 and Fig. 7 in where p/po denotes relative pressure, Rp shows the radius of the mesoporous pores, and dVp/dRp represents the change of the adsorption volume with respect to the size of the pores. The adsorption-desorption hysteresis and pore size distribution plots of the catalysts are illustrated in Fig. 7 Here, hysteresis loops, which appear in the multilayer range of physisorption isotherms, are generally associated with the filling and emptying of mesopores [42]. According to the Fig. 7, the largest pore size distributions of the samples synthesized with different fuel proportions were almost 6

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60

10 1

10

rp (nm)

30

10

dvp/drp

14

Va/cm3(STP)g-1

12 10

4

P/P0

0.6

0.8

0

1

(c) ZCA-900-0.5

10

100

rp (nm)

6 4

Adsorption

2 0

0

0.2

0.4

P/P0

0.6

0.8

0.2

P/P0

0.6

1

1

2 1

6

10

100

rp (nm)

4

0

0.8

(d) ZCA-900-0.75

4

0

8

0.4

Adsorption

2

Desorption 0

Desorption

10

1

100

Adsorption

12

2 0

8

0.4

10

rp (nm)

10

Desorption 0.2

1

20

Adsorption

0

10

30

20

0

40

100

(b) ZCA-900-0.25

20 0

Va/cm3(STP)g-1

0

40

50

dvp/drp

20

Va/cm3(STP)g-1

Va/cm3(STP)g-1

50

dvp/drp

60

30

(a) ZCA-900-0

dvp/drp

30

Desorption 0

0.2

0.4

P/P0

0.6

0.8

1

Fig. 7. Nitrogen adsorption-desorption isotherm (insert image: pore size distribution) of Zn0.4Cu0.6Al2O4 nanocatalyst prepared via microwave-assisted combustion method at high microwave power with different composition of fuels.

combustion of fuel. Hence, the samples are more likely to represent the higher surface area and pore volume. Besides, during the combustion of fuel, a huge amount of smoke is released from the reaction medium that increases the pore diameter of the final powder. If the reaction does not perform well, a catalyst with an amorphous structure, brown color, and low pore diameter will be obtained. The addition of some acetate ammonium causes the surface area to increase up to 96.863 m2/g because of the better combustion during the catalyst synthesis. However, the lower pore diameter in both catalysts is a negative factor in the catalytic activity [38]. If the urea content is further increased up to 50%, the surface area is greatly reduced because of the ignition of the reaction mixture, the severe exhaust of gases, and more formation and adhesion of the catalyst particle. However, the sudden exhaust of gases causes a significant increase in the pore diameter, allowing the reactants to easily access all active sites of the catalyst. The surface area and the average pore diameter are reduced with a further increase in the urea content. In other words, the urea/ammonium acetate composition at the ratios of 1:1 and 1:3 leads to flare in less time than a fuel completely composed of urea during the combustion synthesis. Accordingly, the crystal growth is inhibited and those with a larger surface area are formed. The activity of the nanocatalyst, as shown in Table 2, proves the results of the analysis. In this regard, the ZCA-900-0.75 and ZCA-900-0 samples showed the highest and lowest catalytic activity in the esterification process, respectively. From these results, it can be stated that when the second fuel is added to synthesize nanocatalysts, not only a better dispersion is found for the alumina-based crystals of copper and zinc oxides but also the catalytic activity can be improved by the higher pore diameter in the catalysts [45]. In fact, these samples have a larger surface area than the sample synthesized only with urea. On the other

hand, the latter sample has a greater particle size, because the higher heat release rate of the synthesis reaction allows the particles to grow larger. However, according to the BET analysis, the ZCA-900-0.5 catalyst was expected to have the highest activities, while ZCA-900-0.75 catalyst showed greater catalytic activity. It seems that an increase in the surface area of catalysts is accompanied by a decrease in the average pore size. As a result, reactants find it difficult to have access to internal areas of catalyst and it is more likely to reduce catalyst’s activity. The result can be detected in the SEM analysis (Fig. 8). When the urea content to synthesize nanocatalyst is increased, the higher heat of reaction provides a suitable medium for the growth of crystals. For the sample with a higher amount of ammonium acetate as fuel, it seems that lower combustion temperature during preparation caused to lower crystallinity and dense structure. Meanwhile, the formation of homogeneous, spherical, and well-crystallized nanoparticles with minimal agglomeration can be identified at higher urea/ammonium acetate ratio. By further increasing the urea amount, the uniform distribution of particles can be found on the surface, which could be the reason for the sufficient solution temperature during synthesis. However, when urea was only used as fuel, the higher heat is associated with the increased temperature of the aqueous solution in the reaction. As a result, the core is more likely to grow and can lead to an increase in the degree of agglomeration and sintering problem [46]. Accordingly, the ZCA-9000.75 nanocatalyst was selected as the best nanocatalyst for utilizing in the esterification reaction because of its almost single crystalline phase, large pore size, and high activity. The TEM images of ZCA-900-1 and ZCA-900-0.75 are shown in Fig. 9 to compare the particle size of the sample when the sintering problem is solved. The ZCA-900-1 shows some large dark particles that 7

Fuel xxx (xxxx) xxxx

M. Hashemzehi, et al.

(a) ZCA-900-0.25

(b) ZCA-900-0.5

(c) ZCA-900-0.75

(d) ZCA-900-1

Fig. 8. SEM image of Zn0.4Cu0.6Al2O4 nanocatalyst prepared via microwave-assisted combustion method using at high microwave power with different composition of fuels.

are related to sintered materials as detected in the XRD pattern while ZCA-900-0.75 presents a smaller particle size. The small particle size of the catalysts can provide an appropriate surface area for the interaction between the reactant as mentioned in BET analysis. Furthermore, the image of ZCA-900-1 shows nearly uniform particles with a similar shape. In comparison, ZCA-900-0.75 represents that the difference in the diameter between the ZnAl2O4 and CuO allowed CuO to occupy inside the pores, as confirmed by the appearance of the dark particles. Similar observations were reported by Kurhade et al., as well [47].

stability and reusability have been the subject of intense research because they can justify the high cost of catalyst. In other words, the high stability of the produced catalysts allows reusing catalyst several times through desire reaction. Thus, the reusability of catalyst was studied by performing the circulating catalytic experiments. After each run, catalysts were collected and reused in the next run without any posttreatment. The results are presented in Fig. 10. The activity of the ZCA900-0.75 nanocatalyst was insufficiently reduced after the 9th runs. This reduction of esterification efficiency during the reuse can be due to various reasons. Deactivation of active sites owing to the accumulated water and alcohol decreases the esterification efficiency [48]. Deposition of reactants, intermediates, and products on the surface and the pores of the catalyst also reduces the activity due to the barrier of

3.3. Stability of the optimum catalyst From an economic point of view, the catalysts that benefit from high 8

Fuel xxx (xxxx) xxxx

M. Hashemzehi, et al.

(a) ZCA-900-1

(b) ZCA-900-0.75

Fig. 9. TEM image of (a) ZCA-900-1 and (b) ZCA-900-0.75 nanocatalysts prepared via microwave-assisted combustion method with different composition of fuels.

80

fuel to synthesize catalyst under proper microwave power contributes to achieving a more suitable structure, which may be a prominent factor to enhance the catalyst activity. The other important aspect of this study is that the procedure used for catalyst preparation (MCM) is more efficient and takes less time than other methods. Thus, it is suitable for preparing catalysts with high pore diameter, activity, and reusability.

70

4. Conclusion

Conversion (%)

100

99.1

98.4

98

97.3

96.7

95.8

95.2

94.6

93.7

90

60

1

2

3

4

5 Cycle

6

7

8

In summary, zinc/copper aluminate spinel was successfully synthesized via a microwave-assisted solution combustion process. Next, two major factors influential on the combustion synthesis namely microwave intensity and various mixture of urea and ammonium acetate as fuel were examined for achieving spinel with lower crystallite size and less agglomerated particles. The characteristics of powders were identified by different analyses. The results showed that a higher microwave power improved the morphology and structure of nanocatalyst and, in turn, contributed to better catalytic activity in the esterification process. The growth of pore diameter was resulted by increasing microwave power, in which fatty acid molecules would have easy access into the porosity of catalyst and better binding with the catalyst surface would be achieved. In addition, the type of fuel used in the combustion synthesis was the second major influential factor on the combustion synthesis. The lower heat of reaction contributed to increasing the amount of ammonium acetate and thus inhibited the growth of crystals. There was a gradual rise in the pore diameter to 9.5 nm such that the acetate ammonium amount increased to 25%. However, it declined to 2.1 nm with further acetate addition. Notably, the mixture of fuels yielded smaller crystallites and better activity in esterification reaction

9

Fig. 10. Reusability of ZCA-900-0.75 nanocatalyst in the esterification reaction (180 °C, 9 M ratio of methanol/oleic acid, 3 wt% of catalyst, 6 h reaction time).

actives sites [49]. Fouling of some porosities of the sample after each run by reactant and product can also reduce the activity of the catalyst. Therefore, the results of reusability and activity tests indicated that ZCA-900-0.75 nanocatalyst provides excellent reusability during the catalytic reaction. 3.4. Comparison with literature Some of the alumina-based catalysts used in the esterification, transesterification, and glycerol esterification are given in Table 3, these catalysts represent different catalyst preparation methods, characterization of final products, and activity of the catalyst in their desired reaction. As can be inferred, recognition of an appropriate type of

Table 3 List of similar improvement catalyst structure projects in order to use in biodiesel production process. Catalyst

Preparation methods (Fuel)

Type of reaction1

CPT2 (h)

PS3 (nm)

Pd4 (nm)

Yield (%)

Stability

Ref.

ZCA-900-0.75 SO4-2/Co–Al MgO-MgAl2O4 KOH/Calcium Aluminate Cu-Zn-Al ZnxCu1-xAl2O4 Mg-Zn-Al

MCM (AA + U) Combustion (U) Combustion (U) MCM (U) Coprecipitation MCM (Urea) Coprecipitation

E E T T T E T

3 3 3 3 48 3 18

14 28.5 100 35 – 10 –

9.5 7.4 6.44 7.4 5.5 8.6 10.2

99.1 96.7 96.5 97.5 70 96.9 94

9 5 7 4 – – 5

This study [50] [51] [17,52] [53] [10] [54]

1 2 3 4

Reaction type: E = Esterification, T = Transesterification. CPT = Catalyst Preparation Time. Particle size. Pore diameter. 9

Fuel xxx (xxxx) xxxx

M. Hashemzehi, et al.

compared to the products of combustion fabricated by single fuel. The optimum urea/ammonium acetate ratio to prepare a nanocatalyst with appropriate properties and performance was found to be 3:1. The esterification reaction was carried out using this catalyst, by which the resultant yield was up to 99%.

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10