Reac Kinet Mech Cat (2016) 118:509–521 DOI 10.1007/s11144-016-0986-9

Zn(II) coordination polymer as a bifunctional catalyst for biodiesel production from soybean oil Faezeh Farzaneh1 • Faezeh Moghzi1 Elnaz Rashtizadeh1

•

Received: 24 September 2015 / Accepted: 20 January 2016 / Published online: 27 January 2016 Ó Akade´miai Kiado´, Budapest, Hungary 2016

Abstract In this study, a Zn coordination polymer with formula [Zn(4,40 bipy)(OAc)2]n, designated as compound 1 was prepared with Zn acetate and 4,40 -bipyridine in ethanol. It was characterized using X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, NH3 and CO2-TPD techniques. Compound 1 was found to be an efficient catalyst for biodiesel production. The effect of reaction parameters on the yield of fatty acid methyl esters (FAMEs or biodiesel) including the reaction temperature, time, molar ratios of methanol to oil and catalyst amount were investigated. Obtaining the highest biodiesel yield up to 98 % within 2 h in the presence of 2 % of compound 1 as catalyst (based on the soybean oil weight) together with its stability and reusability is promising. Due to insolubility of compound 1 in methanol and methyl esters, it can be easily separated and reused as catalyst. Therefore, the stability and reusability of 1 makes it a good alternative for biodiesel production. Keywords polymer

Biodiesel  Soybean oil  Heterogeneous catalyst  Zn coordination

Introduction Alternative and renewable sources of energy have become more attractive in recent years due to the depletion of world petroleum reserves, increasing energy demand and increasing environmental concerns due to rising greenhouse gas emissions. Biodiesel or Electronic supplementary material The online version of this article (doi:10.1007/s11144-016-0986-9) contains supplementary material, which is available to authorized users. & Faezeh Farzaneh [email protected]; [email protected] 1

Department of Chemistry, Faculty of Physics and Chemistry, Alzahra University, Vanak, P.O. Box 1993891176, Tehran, Iran

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FAMEs, an important biofuel with an estimated global production as well as promising non-toxic and biodegradable renewable fuel and essentially free of sulphur and aromatics makes it a cleaner burning fuel than petroleum diesel [1–3]. As vegetable oils and animal fats cannot be used for fuel application due to undesirable properties like high viscosity, poor volatility and atomization or polymerization in combustion chamber leading to fuel line and filter clogging and injection fouling, conversion into fuels with desirable properties is required [4, 5]. Transesterification or methanolysis is commonly catalyzed by acids or bases or carried out in the presence of enzyme catalysts [6]. Commercially, biodiesel is produced by the transesterification of triglycerides (the main constituent of vegetable oil) with methanol in the presence of an alkaline liquid catalyst such as sodium or potassium methoxide in a stirred tank reactor [7–9]. The main disadvantage of this method is the formation of soaps due to either reaction of the alkaline catalyst with free fatty acids or saponification of the triglycerides and biodiesel [10]. These undesired reactions consume the catalyst and hinder phase separation of the biodiesel product from the glycerol side-product, which in practice results in reduced yields. On the other hand, the formation of the glycerol salt by-product generated from the neutralization of the catalyst is another disadvantage of the method. As such, a more costly purification process of saleable glycerol is needed. These disadvantages could, however, be removed by using a heterogeneous catalyst [11]. Recall that not only the heterogeneous catalysis systems are environmentally benign, but also operative in continuous processes [12, 13]. Moreover, the catalyst can also be retained in the reactor by simple filtration with no need to be neutralized by quenching the reaction as is conventional in related technologies. In addition, heterogeneous catalysts can be reused and regenerated [14, 15]. Nowadays, a wide variety of heterogeneous catalysts including microporous materials like zeolites [16], mesoporous materials [17], metal oxides or mixed oxides [18–31] and layered silicates like clays [32] have been investigated for transesterification reactions. The applications of coordination polymers and metal–organic frameworks (MOFs) or coordination porous polymers as a new family of organic–inorganic hybrid materials in the field of catalysis have attracted attentions during the past few years [33–36]. Based on our knowledge, there are a few reports on using coordination polymers as catalyst for transesterification of vegetable oil with various alcohols [37–40]. In this study, preparation of FAMEs (biodiesel) via transesterification of soybean oil with methanol using Zn coordination polymer as heterogeneous catalyst with good stability and high conversion activity has been investigated. The optimization of reaction parameters such as the amount of catalyst, reaction time, temperature and molar ratio of oil to methanol is described.

Experimental Soybean oil (FFA%, 0.7 %) was obtained from Sigma (with No. S7381-1L, Germany). Dichloromethane, methanol, n-hexane, ethanol, dimethylformamide, zinc acetate dehydrate, zinc nitrate-hexahydrate, terephthalic acid, 4,40 -bipyridine, 1,3,5-

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511

benzenetricarboxylic acid and methyl heptadecanoate ([99 wt%) as the GC standard were purchased from Merck Chemical Company and used without further purification. Preparation of [Zn(4,40 -bipy)(OAc)2]n (1) as a catalyst Compound 1 was prepared on the basis of the previously reported procedure [41]. 4,40 -Bipyridine (0.80 mmol, 0.125 g in 4 mL ethanol) was added dropwise over 30 min to a magnetically stirred (with rate 500 rpm) solution of Zn(CH3COO)22H2O (0.50 mmol, 0.107 g in 6 mL H2O) at room temperature. The reaction mixture was stirred for 1 h. [Zn(4,40 -bipy)(OAc)2]n Was obtained as a white solid by slow evaporation of solvent at room temperature. It was then washed with acetone and air dried. Preparation of coordination porous polymers Zn (BTC) Zn(BTC) designated as compound 2 was prepared from Zn(CH3COO)22H2O (205 mmol, 0.05 g in 4 mL water) and 1,3,5-benzene tricarboxylic acid (BTC) (205 mmol, 0.052 g in 8 mL ethanol) at 200 °C within 2 days in an Teflon-lined autoclave (20 mL) [42].The solid product was filtered and washed with ethanol for several times. General procedure for biodiesel production Transesterification reactions were carried out in an autoclave with a mechanical stirrer. The desired amount of catalyst (0.05 g) was dispersed in MeOH (3.2 mL), followed by the addition of soybean oil (5 mL) with the average molecular weight of 881 g/mol (calculated from the saponification value (S.V. = 190 mg KOH/g, with acid value and water contents of 0.4 mg KOH/g and 96.7 mg/kg, respectively) was then added to the reaction mixture. After stirring for 2 h at 180 °C, the reaction mixture was cooled to room temperature. The mixture was then centrifuged and the solid separated with decantation, washed with n-hexane and the solvent removed by rotary evaporator at 60 °C. Two layers containing methyl esters, soybean oil, mono and diglycerides in the upper and glycerol in the lower phases were separated by a decanter. The FAME content was determined by GC and GC-Mass. Catalyst characterization and analysis of biodiesel yield The X-ray diffractions (XRD) patterns of samples were recorded on a Philips PW1800 diffractometer using monochromatic nickel-filtered CuKa radiation (k = 0.15405 nm). The X-ray generator was run at 40 kV and 30 mA and the diffractograms were recorded in the 2h range of 4–90°. The phases were identified using the powder diffraction file (PDF) database (JCPDS, International Centre for Diffraction Data). Temperature-programmed desorption (TPD) of CO2 and NH3 from the catalysts were examined using Bellcut instrument type A (made by Bell company of Japan) as follows. First, 0.1 g catalyst was charged into a quartz tube and then activated with helium at 100 °C for 2 h. The samples were subsequently

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exposed under CO2 or NH3 for 1 h at 100 °C until saturation and then purged with helium flow at 100 °C for 30 min. Sample temperatures of the were increased to 700 °C at a ramping rate of 5 °C/min to detect the desorbed CO2 or NH3. The composition of biodiesel was determined using GC (Agilent series 6890) equipped with a 5 % phenylmethylsiloxane capillary column and flame ionization detector (FID). Nitrogen was used as the carrier gas (with a flow rate of 1 mL/min.), and the products were characterized by GC-MS analysis (complex mass selective detector Agilent 5973 network and GC Agilent 6890 network). After the completion of reactions, the liquid product was centrifuged, filtered to separate the catalyst and the excess solvent then removed under reduced pressure. A mixture containing two layers was separated by a decanter. The FAME content was determined by GC using the European regulated procedure EN14103 by dissolving 250 mg of ester layer in 5 mL of n-hexane containing methyl heptadecanoate with equal response factor to those of FAME (10 g/L in n-hexane) as the internal standard. The conversion percent of species i was calculated using Eq. 1: Conversion% ¼ ðCalculatedX weight of methyl esters /weight of methyl ester phaseÞ  100½A= Ai   ðC  V/WÞ  100 ð1Þ P Here Ai is the total peak area of soybean oil methyl esters, A, C and V are the area, concentration (10 mg/mL) and volume in mL of methyl heptadecanoate solution respectively and W is the weight in mg of the sample [14].

Results and discussion Catalyst characterization The Zn(4,40 -bipy)(OAc)2]n coordination polymer (compound 1) as biodiesel catalyst was prepared based on the previously reported method [41] from zinc acetate and 4,40 -bipyridine (scheme 1).

Scheme 1 The XRD patterns of [Zn(4,40 -bipy)(OAc)2]n before and after using as catalyst indicated in Fig. 1 exhibits typical diffractions at 2h = 5.36°, 5.89°, 7.60°, 10.83°, 11.21°, 11.59°, 12.65°, 18.39°, attributed to the crystal (according to ICDD). Inset is the XRD pattern of the theoretical calculated [Zn(4,40 -bipy)(OAc)2]n. The FTIR spectrum of compound 1 presented in Fig. S1 (see supplementary) includes bands appearing at 2925 and 3073 cm-1 corresponding to the C–H stretching vibrations of CH3 and Ar–H, respectively. The bands displaying in the range of 1217–1562 cm-1 is due to pyridine ring vibrations. The two rather strong bands appearing at 1604 and 1433 cm-1 are perhaps due to the asymmetric and symmetric COO vibrations of acetate group.

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Zn(acetate).4H2O + 4,4'-bipyridine

513

Zn(4,4´-bipy)(OAc)2]n

Compound 1

Compound 1 Scheme 1 Preparation of Zn coordination polymer with formula Zn(4,4´-bipy)(OAc)2] as compound 1

Fig. 1 XRD patterns of [Zn (OAc)2(4,40 -bipy)]n as compound 1 a before and b after esterification reaction and c is the XRD pattern of the theoretical calculated of compound 1

Thermogravimetric (TG) measurements were carried out under nitrogen flow atmosphere in the range of 30–700 °C using 10 °C min-1 heating rate. No melting of [Zn(4,40 -bipy)(OAc)2]n was observed up to 200 °C and the TG curve exhibited a

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one-step weight loss due to the release of 4,40 -bipyridine molecule. The acetate anion was decomposed at 200–300 °C with an endothermic effect at 250 °C (Fig. S2, see supplementary). Optimization of the reaction conditions Initially, the molar ratio of methanol to oil was selected as 15:1 on the basis of our own previously reported experience in other biodiesel productions [18, 19]. Although no significant soybean oil conversion to biodiesel was observed when it was heated at reflux in methanol in the presence of 1 % catalyst, but 20 % of oil was converted to FAME within 2 h upon heating the reaction mixture in an autoclave starting at 120 °C (Fig. 2). Notably, marginally increasing temperature was accompanied with increase in conversion (Fig. 2), so that 78 % of oil consumed within 2 h at the upper limit of 180 °C. Leveling off the conversion curve above 180 °C indicates that no further FAME is produced (Fig. 2). The biodiesel conversion within the time range of 4 h at 180 °C using similar methanol to oil molar ratio and catalyst amount reveals that whereas 65 % of the oil conversion occurs within the initial 60 min and proceeds to 78 % conversion after 2 h, beyond which the conversion curve levels off indicating no more oil consumption (Fig. 3). The transesterification reaction requires three moles of alcohol and one mole of triglyceride to afford three moles esters and one mole glycerol. Due to the reversibility of reaction, excess of methanol is required to drive the reaction to completion. Therefore, the biodiesel reaction was studied using different methanol to oil molar ratios based on the soybean oil weight and 1 wt% catalyst (Fig. 4). As seen, the introduction of an excess amount of methanol starting from 6:1 to 18:1 shifts the equilibrium toward the desired product, so that maximum amount of biodiesel yield was obtained using methanol/oil molar ratio of 12:1 within 2 h. It

Fig. 2 Influence of reaction temperature on biodiesel yield, using [Zn (OAc)2(4,40 -bipy)]n compound 1 as catalyst (reaction conditions: 0.05 g catalyst, 3.2 mL methanol, 5 mL oil, Time 2 h, Temperature 120–180 °C)

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Fig. 3 Influence of reaction time on biodiesel yield, using [Zn (OAc)2(4,40 -bipy)]n compound 1 as catalyst (reaction conditions: 0.05 g catalyst, 3.2 mL methanol, 5 mL oil, Time 30–240 min, Temperature 180 °C)

Fig. 4 Influence of methanol amount on biodiesel yield using [Zn (OAc)2(4,40 -bipy)]n compound 1 as catalyst (reaction conditions: 0.05 g catalyst, with different methanol/oil molar ratio, 5 mL oil, Time 2 h, Temperature 180 °C)

can be concluded that the rate acceleration may be the result of decreasing soybean oil viscosity via higher dilution in methanol as well as promotion of mass transfer. However, the utilization of methanol in high excess is not favorable due to higher energy consumption. The amount of the catalyst is an effective parameter on the biodiesel conversion. As indicated in Fig. 5, using catalyst amounts in the range of 0.05–2.5 wt% at 180 °C using 12:1 molar ratio of methanol to oil within 2 h increases the oil conversion to 98 %. Therefore, the reaction is optimized by using 2 wt% of catalyst, beyond which no more oil is consumed. Obviously, recovering the catalyst from reaction mixture and recycling it is economically important for biodiesel production. Therefore, after the reaction proceeded to completion under optimized conditions (180 °C within 2 h using

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Fig. 5 Influence of catalyst amount on biodiesel yield using [Zn (OAc)2(4,40 -bipy)]n compound 1 as catalyst (reaction conditions: 5 mL oil, methanol/oil molar ratio:12/1, Time 2 h, Temperature 180 °C)

Fig. 6 The effect of recyclability of the catalyst on biodiesel yield using [Zn (OAc)2(4,40 -bipy)]n compound 1 as catalyst (reaction conditions: 2 % catalyst, 5 mL oil, methanol/oil:12/1 Time 2 h, Temperature 180 °C)

methanol to oil molar ratio of 12:1), the catalyst was recovered, washed with methanol and dichloromethane, dried at 120 °C before reusing it in another transesterification. It was observed that the activity of the catalyst did not decreased significantly after five consecutive runs (Fig. 6). The observation of a slight drop in catalyst activities within first to fifth reuses is notable. To check whether or not leaching of catalyst occurs during the reaction, a suspension of compound 1 and methanol was heated up to 180 °C under stirring for 2 h. The methanol phase was subsequently separated from the catalyst by filtration and fresh soybean oil was added to the methanol solution. The mixture was reheated up to 180 °C under stirring for 2 h. No transesterification was observed, which clearly indicated the heterogeneous character of the Zn coordination polymer 1. The stability of the heterogeneous catalyst was another important property to be studied. Comparison of the XRD patterns of the fresh and recycled catalyst used in the second run indicated

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Table 1 The effect of various coordination polymers on conversion of oils to biodiesel Catalyst

Conversion (%)

Product distribution (%) Methyl palmitate

Methyl linoleate

Methyl oleate

Methyl stearate

Methyl linoleate

[Zn2(OAc)4(4,40 -bipy)2]n

98.1

19.3

72

6.6

2.1

0.5

Zn (BTC)

16.8

20.7

74.8

4.5

–

–

Reactions conditions catalyst amount 2 wt%, temperature 180 °C, methanol/oil molar ratio, 12/1, reaction time, 2 h

in Fig. 1b revealed that no significant change in the catalyst structure occurred during the biodiesel production. Influence of the different coordination polymer In order to compare the catalytic activities of [Zn(4,4-bipy)(OAc)2]n coordination polymer 1 with Zn (BTC) (2), it was prepared on the basis of the previously reported procedure [42], and then examined as catalyst for biodiesel production under the optimized reaction conditions. As indicated in Table 1, compared to compound 1, which exhibited 98 % conversion, compound 2 exhibited slight activity when it was used as catalyst. To cast light on the different behavior of the catalytic activities of compounds 1 and 2 as the most and the least active catalysts, their acidities and basicities were determined using NH3 (Fig. 7) and CO2-TPD (Fig. 8) techniques. Whereas compound 1 showed a strong peak at 327 °C, corresponding to 70 mmol/g of the adsorbed NH3, compound 2 exhibited a broad peak with low intensity at 477 °C

Fig. 7 NH3-TPD paatterns of (A) [Zn (OAc)2(4,40 -bipy)]n as compound 1 and (B) Zn(BTC) as compound 2

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Fig. 8 CO2-TPD patterns of a [Zn (OAc)2(4,40 -bipy)]n compound 1 and b Zn(BTC) compound 2

with 22 mmol/g of adsorbed NH3 (Fig. 7). On the other hand, whereas the CO2TPD of compound 1 revealed a very strong peak at 337 °C with 452 mmol/g of the adsorbed CO2 and two rather weak peaks at 450 and 560 °C sites, compound 2 exhibited two broad peaks with low intensities at 230 and 475 °C with 97 mmol/g of the adsorbed CO2 (Fig. 8). The distribution of the basic sites at 50–177 °C and 177–700 °C should be due to the weak and medium to strong basicities, respectively [43, 44]. Based on the obtained results indicated in Table 1 and Figs. 7 and 8, the higher basicity of compound 1 in comparison with that of compound 2 was concluded. Therefore, the higher basicity of compound 1 as the key factor in proposing reaction mechanism should be taken into consideration [45]. As such, the methoxide anion generated initially via adsorption of solvent H? by the nitrogen atoms of compound 1 attacks the carbonyl carbon of the triglyceride. In the next step, the generated intermediate reacts with methanol to regenerate the methoxide anion. Finally, rearrangement of the intermediate results in the formation of biodiesel (Scheme 2). Therefore, the transesterification reaction catalyzed by compound 1 has proceeded more efficiently due to its more basic property. The Zn containing compound 2 with different coordination in their unit cell (Scheme S1) has lower activity than that of Zn in compound 1. As reported before [45], aminofunctionalized mesoporous MCM-41 containing different amine groups such as piperazine and guanidine have been used as catalyst for biodiesel production. The catalytic activity of the latter was found to be higher than the former due to the intrinsic basicity of guanidine [45]. Therefore, one can conclude that compound 1 exhibits higher activity than 2 due to the presence of nitrogen atoms of 4,40 bipyridine linkage ligand [45]. The fuel properties of the prepared biodiesel under optimized process conditions have been found to comply with the ASTM and EN standards as presented in

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CH3OH + Comound 1 (Catalyst)

1)

2)

519

R1OCO

CH2

R2OCO

CH

+

CH2

O

CH3O

CH3O C

+ H Compound 1 (Catalyst) R1OCO

CH2

R2OCO

CH

R

OCH3

CH2

O

R3

O

3)

R1OCO

CH2

R2OCO

CH CH2

O R1OCO

O

4)

R3

CH H2C

R3COOCH3 Biodiesel

R1OCO +

+

H2C

CH2

R2OCO

CH

R2OCO

O R1OCO

CH2

OCH3

H Compound 1 (Catalyst)

R2OCO

O CH2 CH

H 2C

O

Comound 1 (Catalyst)

+ OH

Scheme 2 Proposed mechanism for biodiesel formation using Zn-coordination polymer as catalyst (compound 1)

Table 2. As seen, the produced biodiesel using compound 1 as catalyst meets the ASTM and EN standard limits. The presence of the highly basic character of compound 1 relative that of 2 makes it more active as catalyst in biodiesel production processes. Compared to the Zif 8 [37], which has been reported recently for biodiesel production with 90 % Table 2 Comparison of the physical properties of the prepared biodiesel with standard biodiesel Test property

Unit

EN 14212

ASTM D6751

Measured value

Test method

Flash point

°C

120

130

137

ASTM D93

Kinematic viscositya

mm2/s

3.5–5.0

1.9–6.0

4.173

ASTM D445

Pour point

°C

–

–

0

ASTM D97

Cloud point

°C

–

–

0

ASTM D2500

Densityb

kg/m3

860–900

–

876.5

ASTM D4052

Sulfur content

%

–

0.05 max

\0.01

ASTM D4294

Acid value

mg KOH/g

0.5 max

0.8 max

0.26

ASTM D664

Prepared biodiesel within 2 h in the presence of 1 % catalyst 1 (based on the soybean oil weight), with methanol/oil molar ratio 12/1 at 180 °C a

Determined at 40 °C

b

Determined at 15 °C

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biodiesel formation using 3 % catalyst loading and methanol to oil ratio of 27.5:1 at 200 °C, the utilization of 2 % of compound 1 in this work within the reaction time of 2 h with methanol to the oil ratio of 12:1 and formation of biodiesel in 98 % yield with no significant desorption after 5 recycling runs is notable.

Conclusions In this study, Zn coordination polymers [Zn(4,4-bipy)(OAc)2]n and Zn(BTC), designated as compounds 1 and 2, respectively, were prepared and used as catalyst for conversion of soybean oil to biodiesel. Notably, only compound 1 exhibited significant catalytic activity for biodiesel production under optimum conditions. This coordination polymer was found to be a stable catalyst and 98 % catalytic activity was observed together with its stability and repeatedly excellent reusability examined in five runs, which is promising. Particularly significant is the higher basicity of 1 than that of 2, although both show acidic properties. Although both catalysts contain Zn2? ions, it seems that the catalytic activity is due to the 4,4bipyridine present within the lattice of compound 1. Finally, we hope that this work could have opened a new perspective on the use of metal coordination polymers as catalyst for biodiesel production. Acknowledgments The financial support from the Alzahra University is gratefully acknowledged.

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