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Preparation and characterization of fluorine modified oxides for transesterifi-cation

Jian Sun, Jingyi Yang, Shaoping Li, Xinru Xu

PII: S1566-7367(14)00372-0DOI: doi: 10.1016/j.catcom.2014.09.014Reference: CATCOM 4051

To appear in: Catalysis Communications

Received date: 7 July 2014Revised date: 25 August 2014Accepted date: 10 September 2014

Please cite this article as: Jian Sun, Jingyi Yang, Shaoping Li, Xinru Xu, Preparationand characterization of fluorine modified oxides for transesterification, Catalysis Commu-nications (2014), doi: 10.1016/j.catcom.2014.09.014

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* Corresponding author. Tel.: +86 021 64252160; fax: +86 021 64252160

E-mail address: jyyang@ecust.edu.cn (Jingyi Yang)

Preparation and characterization of fluorine modified oxides for

transesterification

Jian Sun, Jingyi Yang*, Shaoping Li, Xinru Xu

State Key Laboratory of Chemical Engineering, East China University of Science and

Technology, Meilong Road 130, Shanghai, P.R.China, 200237

Abstract: KF supported Mg3Al and Mg3Al0.6La0.4 oxides were respectively synthesized

by wet impregnation. The catalyst samples were characterized by BET, ICP-AES, XRD,

XPS and CO2-TPD. The results exhibited that the decomposition of KF modified

hydrotalcites resulted in fluoride impregnated oxides. The basicity of KF supported

oxides was improved due to the alkaline fluoride compounds. The catalysts were tested

in the transesterification of waste cooking oil and methanol. A FAME yield (98.9%) was

observed with a methanol to oil molar ratio of 12:1 and 3 wt.% catalyst amount in 8 h at

338 K.

Keywords: solid base, fluoride, transesterification, biodiesel

1. Introduction

The biodiesel is an excellent alternate for fossil diesel because of its advantages

such as free of sulfur, decreasing greenhouse emissions, renewable and environment

friendly. The raw material of biodiesel could be vegetable oil, such as palm oil [1,2],

sunflower oil [3,4], soybean oil [5-7], rap oil [8], as well as animal fat [9], or even waste

cooking oil [10,11]. In order to economically produce biodiesel, unrefined inedible oil is

used as raw oil for biodiesel production in recent years [12].

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The solid base catalyzed transesterification is fast, less corrosive and easy to

separate from product. Hydrotalcites (HTs) and hydrotalcite-like compounds are

considered to be structure controllable layered double hydroxides materials, which have

divalent and trivalent metal cations and intercalated anions [13]. The utilization of

calcined hydrotalcites in the transesterification has been reported because of some

interesting properties of hydrotalcites [14]. The transesterification of triglycerides for

biodiesel production was investigated using MgAl hydrotalcite [15]. In order to improve

catalytic properties, other hydrotalcite-like catalysts containing transition metal [16-17],

rare earth metal [18] and KF [19] were reported. In this paper, KF supported solid base

catalysts were synthesized to improve the basicity of catalysts. The basic sites were

characterized by BET, ICP-AES, XRD, XPS and CO2-TPD. The activity of solid base

catalysts was tested in the transesterification of waste cooking oil and methanol.

2. Experimental

2.1. Catalyst preparation

The Mg3Al1 and Mg3Al0.6La0.4 HTs samples were synthesized with 3 mole ratio of

M2+

to M3+

(M for cation) by co-precipitation method at constant pH. The metal nitrates

aqueous solution (A) containing Mg(NO3)2•6H2O (38.46 g), Al(NO3)3•9H2O (11.25 g)

and La(NO3)3•6H2O (8.66 g) and alkaline solution (B) including NaOH (11.02 g) and

Na2CO3 (7.28 g) were slowly mixed together under constant stirring in a flask for 1 h.

The pH of the mixture was maintained at 9.8-10.2 by controlling titration speed. The

precipitate was left to age at 348 K overnight and subsequently washed with deionized

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water until the washings attained a pH of 7. The solid obtained was dried at 333 K in

vacuum oven for 20 h. The resulting Mg3Al0.6La0.4 HT was calcined at 773 K for 8 h to

obtain oxide. The Mg3Al1 catalyst was prepared by the same process.

The potassium fluoride supported Mg3Al1 and Mg3Al0.6La0.4 catalyst was prepared

by wet impregnation. The prepared Mg3Al0.6La0.4 oxide (5.00 g) and potassium fluoride

solution containing KF•2H2O (1.50 g) were mixed together under vigorously stirring.

The Mg3Al0.6La0.4 oxide was rehydrated in potassium fluoride solution at room

temperature for 12 h. The decarbonated deionized water was used to prevent CO2. The

solution was dried at 333 K in vacuum oven for 20 h. The precipitate obtained was

filtered, washed and calcined at 773 K for 8 h to obtain KF/Mg3Al0.6La0.4 catalyst.

KF/Mg3Al1 catalyst was prepared by the same process.

2.2. Characterization method of catalyst

The chemical composition of catalysts was determined by inductively coupled

plasma atomic emission spectrometry (Varian 710ES). Surface area and pore size

distribution were measured by BET method (Micromeritrics ASAP-2400) after

degassing the samples at 77 K for 7 h. Solid catalyst morphologies and crystallite sizes

were analyzed by powder X-ray diffraction (Rigaku D/max2550VB/PC) operating with

CuKα radiation (λ=1.5406 Å), at 100 mA, 45 kV, 2θ scanning range 10-80° and a step

size of 0.02° (2θ). X-ray photoelectron spectroscopy was performed with AlKα

radiation (Thermo Fisher ESCALAB 250Xi). The charging effect was corrected by the

C1s peak at 284.6 eV. The basicity of catalysts was analyzed by the temperature

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programmed desorption (TPD) with CO2 (Micromeritics Autochem 2920). The samples

were activated at 623 K for 1 h and then cooled to 313 K under helium flow. After that,

the samples were saturated with dry CO2 at the same temperature. Subsequently, the

samples were purged with helium flow and the CO2-TPD performer at a rate of 10

K/min to 1273 K.

2.3. Catalytic tests

A certain amount of raw oil (soybean oil or waste cooking oil), anhydrous methanol

and catalysts were added to a glass reactor with condenser and magnetic stirring. After a

certain reaction time, the products were filtered and stratified. The upper layer product

was washed and extracted. The fatty acid methyl ester (FAME) in products was

determined by gas chromatograph, equipped with phenyl-methyl-polysiloxane capillary

column, flame ionization detector, according to EN 14103. A known amount of methyl

heptadecanoate was added as internal standard. The analytical conditions for GC were

as follows: injector temperature 523 K; column initial temperature 353 K; program rate

10 K/min; final temperature 473 K; detector temperature 523 K.

3. Results and discussion

3.1. Catalyst characterization

The textural properties of oxides are displayed in Table 1. ICP-AES analysis

exhibited that the experimental value of metal composition of Mg3Al1, KF/Mg3Al1,

Mg3Al0.6La0.4 and KF/Mg3Al0.6La0.4 catalysts were close to their theoretical value. The

average pore size values were between 21.89 and 24.90 nm. The BET analysis showed

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that the deposition of KF on oxides surface decreased their surface area because of the

decrease of micropore [20].

The XRD patterns of Mg3Al1, KF/Mg3Al1, Mg3Al0.6La0.4 and KF/Mg3Al0.6La0.4

catalysts are shown in Fig. 1. The diffraction peaks of Mg3Al1 oxide (a) were reflected

at 36.9°, 42.9° and 62.3°. Those diffraction peaks were sharp and symmetrical, which

corresponded to MgO crystallites. After KF species were impregnated on Mg3Al1 oxide,

new crystallite KMgF3 (31.7°, 39.1°, 45.4°, 56.5°, 66.2°) could be observed in XRD

pattern of KF/Mg3Al1 oxide (b) in addition to MgO crystallites. Two main crystalline

phases MgO and La2O3 (27.0°, 29.2°, 31.3°) could be observed in XRD pattern of

Mg3Al0.6La0.4 oxide (c). After the impregnation of KF, new diffraction peaks due to

LaOF crystallites appeared at 26.9°, 31.0°, 44.8°, 52.8° in the XRD pattern of

KF/Mg3Al0.6La0.4 oxide (d).

The location of F1s levels are illustrated in Fig 2. Three peaks in KF/Mg3Al1 spectra

(a) were assigned to F- anions of AlF3 (687.6 eV), KMgF3 (686.3 eV) and KF (684.0

eV). The replacement of Al3+

by La3+

in Mg3Al1 oxide led to disappearance of AlF3

peak and appearance of LaOF (685.2 eV) peaks in KF/Mg3Al0.6La0.4 spectra (b).

According to the XRD results, the deposition of KF on precursor oxides surface led to

fluorides. The Schottky defects resulted surface fluorine unsaturated.

The basicity of catalysts was characterized by CO2-TPD, which is presented in Fig.

3. For Mg3Al1 oxide (a), there were three desorption peaks around 373K, 473K and

673K, which was similar to the literatures [4,6]. The three basic sites were OH- groups

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(373 K), M-O pairs (473 K) and O2-

anions (673 K), respectively [21]. After KF specie

was impregnated on Mg3Al1 oxide (b), there was a new significant desorption peak

around 973 K, which indicated the increase of new basic sites. The new desorption peak

around 973 K could be due to the unsaturated F- anions according to XRD and XPS

results. In the CO2-TPD pattern of Mg3Al0.6La0.4 oxide (c), the desorption peaks around

673K became broad and intense after the substitution of Al3+

cations by La3+

cations. It

was found that the basicity of oxides could be improved via La3+

cations modification

[22]. Similarly, new broad desorption peaks appeared around 973 K in the CO2-TPD

pattern of KF/Mg3Al0.6La0.4 oxide (d) after KF modification. The new adsorption peaks

around 973 K could be due to the unsaturated F- anions from LaOF crystallite according

to XRD and XPS results. The desorption peaks in higher temperature proved that the

unsaturated F- anions exhibited much more basic than O

2- anions.

3.2. Catalytic tests

Fig. 4 (a) illustrates the result of methanolysis of soybean oil catalyzed by Mg3Al1,

KF/Mg3Al1, Mg3Al0.6La0.4 and KF/Mg3Al0.6La0.4 oxides. Mg3Al1 oxide showed low

active in the transesterification at low reaction temperature. The FAME yield catalyzed

by Mg3Al1 oxide was 45.8% in 5h. After La3+

cation modification, Mg3Al0.6La0.4 oxide

exhibited more active because of the defects in oxides [23]. The FAME yield reached

98.3% in 5 h. After alkaline fluoride modification, the catalysts exhibited high active

because of the new basic sites. The FAME yield catalyzed by KF/Mg3Al1 and

KF/Mg3Al0.6La0.4 oxide respectively reached 99.3% and 99.6% in 5h. The result was

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higher than other literatures [8,15] and our previous research [23].

Fig.4 (b) illustrates the comparison of FAME yields catalyzed by Mg3Al1,

KF/Mg3Al1, Mg3Al0.6La0.4 and KF/Mg3Al0.6La0.4 oxides in the transesterification of

waste cooking oil and methanol. The waste cooking oil, which was pretreated to

remove the major water, was collected after frying. The properties of soybean oil and

waste cooking oil are compared in table 2. The primary chemical changes in soybean

oil during cooking could be due to pyrolysis, oxidation and hydrolysis [12]. The

changes demonstrated the decrease of unsaturated fatty acids and increase of acid value.

Fig. 4 (b) shows that the Mg3Al1 and Mg3Al0.6La0.4 oxides exhibited low active in the

reaction because of high acid value in unrefined feedstock. The FAME yields catalyzed

by Mg3Al1 and Mg3Al0.6La0.4 were respectively 10.6% and 22.8% in 8 h. However,

acceptable FAME yields catalyzed by KF/Mg3Al1 (77.5%) and KF/Mg3Al0.6La0.4

(98.9%) could be reached in 8 h. The catalytic properties of fluorine modified oxides in

unrefined feedstock exhibited higher activity than other reports [10,11]. According to

XRD results (Fig. 1), KMgF3 and LaOF species formed after the impregnation of KF on

Mg3Al1 and Mg3Al0.6La0.4 oxides surface. The defects resulted surface fluorine

unsaturated. The unsaturated F- anions became strong base sites in KF supported oxides

(Fig. 3). Because of higher basicity, the abstraction of proton from alcohol could more

easily happen for unrefined feedstock.

The solid base catalysts were separated and collected from reaction after

transesterification. The recyclability of KF/Mg3Al0.6La0.4 oxide was investigated in

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other runs of transesterification of waste cooking oil and methanol. As shown in Fig. 5,

some acceptable FAME yield from 97.6% to 73.2% was observed after 4 recycles. The

acceptable deactivation could be attributed to the leaching of active species [24].

4. Conclusions

KF supported Mg3Al1 and Mg3Al0.6La0.4 oxides exhibited high active in the

transesterification of waste cooking oil and methanol. The basicity of oxides was

improved accounting for the fluorides. The unsaturated F- anions became the primary

active sites in the transesterification. The order of basicity of the samples was

KF/Mg3Al0.6La0.4＞KF/Mg3Al1＞Mg3Al0.6La0.4＞Mg3Al1 oxides. The maximum FAME

yield catalyzed by KF/Mg3Al0.6La0.4 oxide was 98.9 % when the transesterification of

waste cooking oil and methanol was carried out at 338 K with a methanol to oil molar

ratio of 12:1, 8 h and 3 wt.% catalyst amount.

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O: MgO; #: KMgF3; △: La2O3; □: LaOF

Fig. 1 XRD patterns of Mg3Al1 (a), KF/Mg3Al1 (b), Mg3Al0.6La0.4 (c) and KF/Mg3Al0.6La0.4 (d)

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Fig.2 XPS spectra of KF/Mg3Al1 (a) and KF/Mg3Al0.6La0.4 (b) in the region of F1s levels

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Fig. 3 CO2-TPD profiles of Mg3Al1 (a), KF/Mg3Al1 (b), Mg3Al0.6La0.4 (c) and KF/Mg3Al0.6La0.4 (d)

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Fig. 4 Methanolysis of soybean oil (a) and waste cooking oil (b) catalyzed by different catalysts

Reaction condition: 338 K, 12:1 methanol to oil, 3wt% catalyst amount

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Fig. 5 Effect of recycle number on FAME yield

Reaction condition: 338 K, 8h reaction time,12:1 methanol to oil, 3wt% catalyst amount

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Table 1

Chemical composition of catalysts and textural characterization of catalysts

Sample Mg/Al/La BET Surface

area(m2/g)

Pore

size(nm) Theoretical Experimental

Mg3Al1 3:1 3:1 82.18 21.89

KF/Mg3Al1 3:1 3:1 54.63 24.43

Mg3Al0.6La0.4 3:0.6:0.4 3:0.6:0.3 53.14 20.96

KF/Mg3Al0.6La

0.4 3:0.6:0.4 3:0.6:0.3 21.86 24.90

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Table 2

Properties of soybean oil and waste cooking oil

Property Soybean oil Waste cooking oil

Fatty acid components

Palmitic acid C16:0 15.0 12.7

Stearic acid C18:0 6.8 14.2

Oleic acid C18:1 26.6 27.9

Linoleic acid C18:2 46.3 45.2

Linolenic acid C18:3 5.3

Viscosity (mm2/s)

313 K 33.8 32.5

373 K 7.8 7.8

Iodine value (g I2/100g) 130 125

Saponification number mg(KOH)/g 201.6 196.3

Acid value mg(KOH)/g 0.7 4.9

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Graphical abstract

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Highlights

Fluorine modified Mg3Al1 and Mg3Al0.6La0.4 oxides were synthesized.

The basicity of catalysts was improved due to alkaline fluoride compounds.

The unsaturated F- anions became strong basic sites after modification.

The maximum FAME yield from waste cooking oil was 98.9% at 338K in 8 h.