Design and Synthesis of Ionic Liquid-supported Hierarchically Porous

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Kinetics, Catalysis, and Reaction Engineering

Design and Synthesis of Ionic Liquid-supported Hierarchically Porous Zr Metal-organic Framework as a Novel Brønsted-Lewis Acidic Catalyst in Biodiesel Synthesis Changshen Ye, Zhaoyang Qi, Dongren Cai, and Ting Qiu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04107 • Publication Date (Web): 28 Dec 2018 Downloaded from http://pubs.acs.org on January 5, 2019

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Industrial & Engineering Chemistry Research

Design and Synthesis of Ionic Liquid-supported Hierarchically Porous Zr Metal-organic Framework as a Novel Brønsted-Lewis Acidic Catalyst in Biodiesel Synthesis Changshen Ye, Zhaoyang Qi, Dongren Cai, Ting Qiu* Fujian Universities Engineering Research Center of Reactive Distillation, College of Chemical Engineering, Fuzhou University, Fuzhou 350116, Fujian, China ABSTRACT: A novel strategy of approximate ligand substitution was proposed to introduce ILs into the porous framework of MOFs. With this approach, an efficient Brønsted-Lewis acidic catalyst [(CH2COOH)2IM]HSO4@H-UiO-66 was successfully constructed

via

bidentate

coordination

between

one

-COO-

group

of

[(CH2COOH)2IM]HSO4 and two portions of unsaturated Zr ions defeats of the hierarchically porous Zr metal-organic framework (H-UiO-66). The catalyst was systematically characterized by FT-IR, XRD, Nitrogen adsorption-desorption, SEM and TGA, proving the feasibility of this encapsulation mode. The fabricated [(CH2COOH)2IM]@H-UiO-66 catalyst was applied for synthesis of biodiesel, and the reaction conditions were optimized by response surface methodology. The resulting Brønsted-Lewis

acidic

[(CH2COOH)2IM]HSO4@H-UiO-66

catalyst

exhibited

excellent catalytic performance for the esterification of oleic acid with methanol. Under the optimum condition, the predicted yield of biodiesel reached 93.71%, and experimental value was 93.82%, which indicated the synthesized catalyst own high catalytic activity. Moreover, the catalyst could be easily recovered and reused, the yield of biodiesel decreases from 93.82% to 90.95% after 5 runs, indicating good reusability. Besides, based on synergistic effect of Brønsted acidic and Lewis acidic,

*

Corresponding author. E-mail address: [email protected] 1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the catalytic mechanism of [(CH2COOH)2IM]@H-UiO-66 was also discussed. Keywords: Hierarchically porous UiO-66, Supported ionic liquid, Brønsted-Lewis acidic catalyst, Biodiesel, Esterification 1. INTRODUCTION In recent years, with the rapid consumption of fossil fuels, an alternative form of fuel, biodiesel, has been considered as the replaceable resource to alleviate the problems of energy crisis and environmental pollution.1-3 Biodiesel is a kind of biomass energy, which is similar to petrochemical diesel in physical properties, but different in chemical compositions. And the high cetane number, oxygen content and low sulfur content of biodiesel can ensure industrial application and complete combustion. Tests4-6 showed that, compared with the ordinary diesel oil, using biodiesel can significantly reduce the toxicity of emitted gases, and the emissions of carbon dioxide, which could help mitigate global warming. Biodiesel is usually produced from transesterification of triglycerides and lower alcohols, and great progresses have been made in this field,7,8 but the alkali-catalyst will be easy to lost catalyze activity if the raw materials are highly acidic and water-containing.9,10 By comparison, acid-catalyzed esterification reaction has well adaptability for biodiesel production without exact demands on feedstocks.8,11 Therefore, the synthesis of biodiesel by esterification of oleic acid and lower alcohols has a good prospect. The crucial influence factor of esterification to produce biodiesel is the catalyst. The conventional esterification catalysts mainly include inorganic liquid acids and solid acid. However, in practical applications, they have a lot of 2


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problems, such as large amount acid wastes generation, equipment corrosion and non-reusable for inorganic liquid acids, high mass transfer resistance, rigorous reaction conditions and low catalytic activity for heterogeneous solid acid catalyst.12,13 Therefore, it is urgent to develop an environment-friendly, reusable and efficient catalyst for esterification to catalytic synthesis of biodiesel. Functionalized (“task-specific”) ionic liquids (TSILs) known as green and efficient solvent and catalyst have attracted widespread attentions due to their unique properties such as low saturated vapor pressure, multiple catalytic active sites, outstanding designability and superior thermostability.14 In past studies15,16, TSILs have showed excellent catalytic activities for esterification. However, the drawbacks of high viscosity, large consumption and high cost of separation have hampered their industrial applications.17 In order to solve these problems, researchers began to study the immobilization of TSILs on a variety of support materials. Silica gel18, molecular sieves19, magnetic nanoparticles20, zeolites21, and polymers22 were commonly used as support materials. Silica gel and molecular sieves possess high surface area, suitable pore structure and abundant active group Si-OH, but the low hydrothermal stability and expensive coupling agent restrict the widespread application. Polymers have the advantages of large porosity, strong adsorption capacity and low cost, but suffer from the disadvantage of poor catalytic stability. What is more, these carriers are just supporter and the majority of them almost do not have catalytic activity. The only function of those carriers is to support and disperse active components and realize the easy recovery of ionic liquids. So if we can prepare a porous material which carries 3



Lewis acid sites as supporter, and the Brønsted ionic liquid could be immobilized on it via an efficient strategy, this composite will become an efficient Brønsted-Lewis acidic catalyst. Recently, Zr-terephthalate based metal-organic framework named UiO-6623 has attracted increasing attention owing to its exceptional physical and chemical stability that it can even keep stabilization under strong acidic and high temperature conditions.24 The key to its stability lies in the high topological connectivity of the [Zr6O4(OH)4]12+ secondary building unit (SBU), which is connected with strong Zr-O bonds and 12 terephthalate (BDC) linkers.25 However, the coordination between SBU and BDC is incomplete and the defects are present in the structure. Fortunately, the amount of defeats can be tuned26-29 without compromising the high stability of the material.30,31 Besides, the presence of defects with unsaturated Zr atoms in these materials can provide open Lewis acid sites. Those Lewis acid sites can provide strong catalytic activity, and UiO-66 contain defeats has been regarded as solid acid catalyst for the production of compounds of interest. According to recent studies,32, 33 UiO-66 has been shown to be a certain degree of catalytic activity for the Fischer esterification. However, the catalytic activity isn't high enough, if we only use UiO-66 as solid acid catalyst. Especially it has weak catalytic activity on the synthesis of biodiesel by esterification of oleic acid and lower alcohols, because the pore apertures of 0.6 nm34 approximately of UiO-66 is smaller than the size of oleic acid molecules, so the Lewis acid sites inside the crystal can't provide effective catalytic activity in the absence of oleic acid. Thus, the development of hierarchical porous UiO-66 is of great 4


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significance. In addition, if we can support Brønsted acid TSILs on hierarchical porous UiO-66, TSILs-supported hierarchically porous Zr metal-organic framework catalyst will be constructed. This new type catalyst can own Brønsted and Lewis acidic sites, and the cooperation of Lewis acid and Brønsted acid could improve the activity of the catalyst,35,36 which will performs better catalytic activity for esterification reaction. Herein, we describe a novel strategy of approximate ligand substitution to introduce ILs into UiO-66, and this strategy is that a part of the ligands are removed and provide sites, and the carboxyl functionalized ionic liquid which likes the original ligand of UiO-66, occupy those sites by the same binding force. To demonstrate this strategy, UiO-66 is first etched by propionic acid to fabricate hierarchical porous UiO-66 (H-UiO-66) which could also contain abundant defeats and Lewis acidic sites after ligands and metal clusters are replaced by propionic acid. Then the precursor (CH2COOH)IM(CH2COO-1) of the Brønsted acidic 1, 3-biscarboxymethyl-imidazolium hydrosulfate TSILs ([(CH2COOH)2IM]HSO4) is supported on the H-UiO-66 through an efficient strategy of bidentate coordination between one -COO- group of (CH2COOH)IM(CH2COO-1) and two portions of unsaturated Zr ions defeats of the H-UiO-66, and it will form strong bidentate Zr-O bonds between (CH2COOH)IM(CH2COO-1) and H-UiO-66. [(CH2COOH)2IM]HSO4 supported H-UiO-66 ([(CH2COOH)2IM]HSO4@H-UiO-66) will be obtained via the additive reaction between H2SO4 and (CH2COOH)IM(CH2COO-1). The fabricated Brønsted-Lewis acidic [(CH2COOH)2IM]HSO4@H-UiO-66 catalyst is designed for 5



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synthesis of biodiesel, and the reaction conditions are optimized by response surface methodology. 2. EXPERIMENTAL 2.1 Materials The

chemicals

including

sulfuric

acid

(CAS

7664-93-9

98%),

N,N-Dimethylformamide (CAS 68-12-2 99.5%), terephthalic acid (CAS 100-21-0 99%), methanol (CAS 67-56-1 99.7%), ethanol (CAS 64-17-5 99.5%), oleic acid (CAS 112-80-1 85%), ethylacetate (CAS 141-78-6 99%), propionic acid (CAS 79-09-4 99.5%), heptane (CAS 142-82-5 99.5%), zirconium (IV) chloride (CAS 10026-11-6 99.9%), methyl salicylate (CAS 119-36-8 99%), methyl oleate (CAS 112-62-9 99%), methyl linoleate (CAS 112-63-0 99%), methyl linolenate (CAS 301-00-8 99%), methyl stearate (CAS 112-61-8 99%), formaldehyde solution (CAS 50-00-0 37%), aminoacetic acid (CAS 56-40-6 99%) and glyoxal solution (CAS 107-22-2 40%) were purchased from Aladdin Industrial Corp, and without purification before being used. 2.2 Preparation of precursor of [(CH2COOH)2IM]HSO4 The precursor of [(CH2COOH)2IM]HSO4 was synthesized via known method reported in the literature37 with little modification which contains two steps. Firstly, aminoacetic acid (0.05 mol), acetaldehyde (0.025 mol) and formaldehyde (0.025 mol) were added into a 100 mL three necked flask one by one. Then, distilled water (10 mL) was added dropwise and the mixture was stirred at 90 ℃ for 3h. After filtering the unreacted aminoacetic acid and other impurities, the yellow solid was washed by 6


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ethyl alcohol (3 ×50 mL), and (CH2COOH)IM(CH2COO-1) was obtained after dried under vacuum. 2.3 Preparation of hierarchical porous UiO-66 H-UiO-66 was prepared following the previously reported method.38 Firstly, 0.170 g of zirconium (IV) chloride, 0.121 g of terephthalic acid and 1.890 g of benzoic acid were dissolved in 90 mL of anhydrous N,N-dimethylformamide to form a mixed solution. The mixed solution underwent sonication for 30 min to become homogeneous solution, and then the homogeneous solution was transferred to the hydrothermal synthesis reactor and maintained at 120 ℃ for 24 h. Afterward, the hydrothermal synthesis kettle was cooled to room temperature naturally and the resulted precipitation of UiO-66 was collected by centrifugation and washed three times using DMF and ethyl alcohol respectively. Secondly, UiO-66 (200 mg) was dispersed in 22 mL propionic acid solution (1.6 mol.L-1), and then the suspension was transferred to the hydrothermal synthesis kettle and maintained at 100 ℃ for 7 h. After the hydrothermal synthesis kettle was cooled to room temperature naturally, the solids were washed three times with water and twice with alcohol. Finally, H-UiO-66 would be obtained, after vacuum drying. 2.4 Preparation of [(CH2COOH)2IM]HSO4@H-UiO-66 [(CH2COOH)2IM]HSO4@H-UiO-66 was constructed based on that a part of the ligands (terephthalic acid) of UiO-66 were removed and provided sites, and the carboxyl functionalized ionic liquid which likes the original ligand of UiO-66, occupy those sites by the same binding force. The specific synthesis method as showed in 7



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Scheme 1, the precursor (CH2COOH)IM(CH2COO-1) (0.612 g) and ammonium hydroxide (3 mmol) were dispersed in distilled water (30 mL), and formed pale yellow solution. Then white powder H-UiO-66 (1.0 g) was added and the mixture was stirred at 80℃for 5 h. The (CH2COOH)IM(CH2COO-1) was attached to the H-UiO-66 through

the

bidentate

coordination

between

one

-COO-

group

of

(CH2COOH)IM(CH2COO-1) and two portions of unsaturated Zr ions defeats of the H-UiO-66. In the end, (CH2COOH)IM(CH2COO-1)@H-UiO-66 was immersed in ethanol solution of sulfuric acid (0.05 mol.L-1) at 70℃ for 2 h and washed by ethyl acetate.

After

filtering

separation

and

vacuum

drying,

[(CH2COOH)2IM]HSO4@H-UiO-66 was prepared.

Scheme 1 The generic tandem procedure for synthesizing [(CH2COOH)2IM]HSO4 supported in H-UiO-66 nanocages by bidentate coordination.

2.5 Catalyst characterization X-ray diffraction (XRD) patterns were acquired on Bruker D8 equipped with CuKa radiation (40 kV, 40 mA). FT-IR spectra of prepared materials were obtained 8


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by a Nicolet iS50 spectrometer (Thermo Fisher) in the range of 400-4000 cm-1. The BET surface area and pore size distribution were measured using N2 adsorption-desorption porosimetry (Micromeritics automatic analyzer ASAP2020). Scanning electron microscopy (SEM) (Hitachi S-4800) was performed to evaluate the morphology and surface structure of materials. By thermal gravimetric analysis (TGA) (Netzsch STA449C), the thermal stability of [(CH2COOH)2IM]HSO4@H-UiO-66 was evaluated. Prior to these measurements, all samples were dried for 10 h under vacuum. 2.6. Esterification Process. In

a

certain

ratio,

oleic

acid,

methanol

and

catalyst

[(CH2COOH)2IM]HSO4@H-UiO-66 were added into a flask. The reaction (Figure 1) was conducted under usual atmospheric pressure (0.1 MPa), and continuous stirring at 80 ℃ for different time. After reaction, the concentration of each component was analyzed by gas chromatography (GC-2014, Shimadzu, Corporation, Japan) equipped with HP-INNOWAX (30 m × 0.320 mm (ID) × 0.25 μm) and a flame ionization detector. To reduce random error, all the samples were performed for three times. O R

O

H

O

H3C OH

R

O

CH3

H2O

Figure 1. Esterification of oleic acid and methanol.

2.7 Experimental design by response surface methodology Response surface methodology (RSM) is an effective method to obtain the optimum reaction condition by analyze the relationship between factors and the response values.13,35 In this work, the software Design-Expert 8.0.6 Trial was adopted 9



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to conduct RSM. And the parameters of time (h), molar ratio (methanol to oleic acid), and catalyst amount (wt%, based on oleic acid) were taken as variables, and the yield of biodiesel (Y, %) was taken as the response to investigate the effects of operating conditions. The multiple regression analysis can be elucidated by quadratic equation model as follows: 2

2

Y  b0  b1 X 1  b2 X 2  b3 X 3  b12 X 1 X 2  b13 X 1 X 3  b23 X 2 X 3  b11 X 1  b22 X 2  b33 X 3

2

（1）

Where Y is the predicted response variable, b0, bi, bii, bij are regression coefficients and X1, X2 and X3 are independent variables. 2.8 Reusability After the esterification, [(CH2COOH)2IM]HSO4@H-UiO-66 was recovered by simple centrifugation and washing, and reused to synthetic biodiesel as catalyst under the optimal reaction conditions. The catalyst will be reused for five times to evaluate the reusability of the [(CH2COOH)2IM]HSO4@H-UiO-66 catalyst. 3. RESULTS AND DISCUSSION 3.1 Catalyst Characterizations 3.1.1 XRD The

XRD

diffractograms

of

synthesized

UiO-66,

H-UiO-66

and

[(CH2COOH)2IM]HSO4@H-UiO-66 are shown in Figure 2. The diffraction peaks of UiO-66 (Figure 2 II) are consistent with the simulated pattern obtained with Mercury 3.3 software using the corresponding CIF file (Figure 2 I). The main diffraction peaks of H-UiO-66 (Figure 2 III) is weaker than UiO-66, indicating that H-UiO-66 have a lower crystallinity. The slight change of crystallinity might be caused by the 10


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decomposition and deformation of partial skeleton of UiO-66 after acid etching to transform into hierarchical porous H-UiO-66. Almost all the main diffraction peaks of [(CH2COOH)2IM]HSO4@H-UiO-66 (Figure 2 IV) can be observed in accordance with the H-UiO-66. These results clearly indicate that the crystalline structure of the UiO-66 is unchanged and retain intact during the acetic acid etching and [(CH2COOH)2IM]HSO4 functionalization process.

Ⅳ

Ⅲ

Ⅱ

Ⅰ 10

20

30

40

50

2 degree

Figure 2. The experimental XRD patterns of (I) Simulated UiO-66, (II) UiO-66, (III) H-UiO-66 and (IV) [(CH2COOH)2IM]HSO4@H-UiO-66.

3.1.2 SEM The morphology of UiO-66, H-UiO-66 and [(CH2COOH)2IM]HSO4@H-UiO-66 were observed by SEM, and the results are presents in Figure 3. Compared to UiO-66, the regular crystal morphology of H-UiO-66 and [(CH2COOH)2IM]HSO4@H-UiO-66 was maintained. This could be explained by the fact that the inner layer of the MOF crystal is chemically feebler than the outer layer and can be preferentially etched in the process of pore broadening.38 Besides, UiO-66 has exceptional physical and 11



chemical stability, so it can maintain the original morphology and skeleton structure in the process of [(CH2COOH)2IM]HSO4 functionalization. The results obtained by SEM agree with the results of structural analysis from XRD. a1

200nm

a2

EHT = 10.00 kV Mag = 50.00 KX Aperture Size = 30.00 um WD = 4.8 mm Signal A = InlensDUO

EHT = 10.00 MagkV = 50.00 Aperture = 30.00 um EHTkV = 10.00 MagKX = 100.00 KX Size Aperture Size = 30.00 um 100nm WD = 4.8WD mm= 4.8Signal = InlensDUO mm A Signal A = InlensDUO

b1

200nm

b2


100nm EHT = 10.00 kV WD = 5.0 mm

c2

c1

200nm

Mag = 100.00 KX Aperture Size = 30.00 um Signal A = InlensDUO


100nm


Figure 3. The different magnified SEM images of UiO-66 (a1 and a2), H-UiO-66 (b1 and b2) and [(CH2COOH)2IM]HSO4@H-UiO-66 (c1 and c2). 12


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3.1.3 FT-IR The FT-IR spectra for the UiO-66, H-UiO-66, [(CH2COOH)2IM]HSO4 and [(CH2COOH)2IM]HSO4@H-UiO-66 are shown in Figure 4.

a

b

Ⅳ

Ⅱ

Ⅲ

Transmittance / %

650

Transmittance / %

1734

Ⅱ 2925

2925

Ⅰ Ⅰ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3156 3500

3116 3000

1601

1167 1027

1500 -1

Wavenumbers (cm )

1000

3100

500

3050

3000

2950

2900

2850

2800

2750

-1

Wavenumbers (cm )

Figure 4. (a) The FT-IR spectra of (I) UiO-66, (II) H-UiO-66, (III) [(CH2COOH)2IM]HSO4 and (IV) [(CH2COOH)2IM]HSO4@H-UiO-66, and (b) the enlarged view from 2750-3100 cm-1 of (I) UiO-66 and (II) H-UiO-66.

Compared to UiO-66, the exclusive characteristic peak at 2925 cm-1 in Figure 4b which attribute to the stretching vibration of -CH3 in H-UiO-66 indicate that a part of propionic acids replaced the ligands of UiO-66. The other characteristic peaks of H-UiO-66 are consistent with the UiO-66, indicating that the coordination structure of the UiO-66 was unchanged and remained intact during the propionic acid etching. All peaks of H-UiO-66 can be correspondingly found in the spectrum of [(CH2COOH)2IM]HSO4@H-UiO-66, interpreting survival of the MOF structure. Furthermore, compared to H-UiO-66, the characteristic peaks at 650 cm-1, 1167 cm-1 and 1027 cm-1 are obviously broader indicates the exist of the sulfuric acid group. Besides, The characteristic peak at 3153 cm-1 and 3076 cm-1 belong to symmetric and 13



asymmetric stretching vibration of =C-H in imidazole ring, and the characteristic peak at 1734 cm-1 corresponds to the symmetric and asymmetric stretching of the C=O groups indicates the exist of -COOH groups in [(CH2COOH)2IM]HSO4@H-UiO-66. All of those are strong evidence for the incorporation of [(CH2COOH)2IM]HSO4 within H-UiO-66. 3.1.4 Nitrogen adsorption-desorption To investigate the structural characteristics, N2 adsorption–desorption isotherms for UiO-66, H-UiO-66 and [(CH2COOH)2IM]HSO4@H-UiO-66 were tested and shown in Figure 5 (a).

a

700

UiO-66-ad UiO-66-de H-UiO-66-ad H-UiO-66-de [(CH2COOH)2]HSO4@H-UiO-66-ad

500

3

-1

Volume adsorption (cm g )

600

[(CH2COOH)2]HSO4@H-UiO-66-de 400 300 200 100 0

0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (p/p0)

b

0.2 0.020

0.015

0.010

-1

dV/dlogD (cm g Å )

-1

0.005

0.000

3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 36

50

100

150

200

Pore diameter (Å)

UiO-66 H-UiO-66 [(CH2COOH)2HSO4@H-UiO-66

0.0

5

10

15

Pore diameter (Å)

14


20

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Figure 5. (a) N2 adsorption-desorption isotherms, and (b) pore size distributions of UiO-66, H-UiO-66 and [(CH2COOH)2IM]HSO4@H-UiO-66.

All of the three isotherms exhibit mixed type I/IV isotherms with hysteresis loop, which at low relative pressure and high relative pressure belong to disparate type of isotherms is characteristic of solids with microporous windows and partial mesoporous cages.39 The specific surface area of H-UiO-66 (879 m2/g) and [(CH2COOH)2IM]HSO4@H-UiO-66 (748 m2/g) is lower than parent UiO-66 (1160 m2/g). This could be explained by the fact that in this process of UiO-66 is etched by propionic acid to prepare H-UiO-66, partial ligands and metal clusters are replaced and washed by propionic acid. So it led to an obvious decrease in specific surface area. Besides, the decrease of [(CH2COOH)2IM]HSO4@H-UiO-66 is probably ascribable to

the

filling

of

partial

micropore

and

mesoporous

of

H-UiO-66

by

[(CH2COOH)2IM]HSO4. The calculation by the Horvath-Kawazoe (H-K) and Barrett-Joyner-Halenda (BJH) methods presents pore size distribution (Figure 5b) of UiO-66, H-UiO-66 and [(CH2COOH)2IM]HSO4@H-UiO-66. The number of mesoporous of H-UiO-66 is significantly greater than UiO-66, indicating propionic acid etching has successfully created hierarchically porous. Compared with the parent H-UiO-66, the [(CH2COOH)2IM]HSO4@H-UiO-66 demonstrated a significant decrease in the pore diameter, suggesting the encapsulation of IL guest inside H-UiO-66 rather than the surface. 3.1.5 TG TG curves of UiO-66, H-UiO-66 and [(CH2COOH)2IM]HSO4@H-UiO-66 are presents in Figure 6 which were tested to further demonstrate partial ligands are 15



Page 16 of 36

replaced during etching process and the [(CH2COOH)2IM]HSO4 is in the pores and cages of H-UiO-66. UiO-66, H-UiO-66 and [(CH2COOH)2IM]HSO4@H-UiO-66 were heat treated at 100℃ in order to remove water and solvent on their surface. All of the three materials have a spot of mass loss before 120℃ due to the exit of water and low boiling solvent. With the increase of temperature, DMF trapped in the three materials, bound water and propionic acid molecules adsorbed in the defeats are released, which caused further mass loss. The parent UiO-66 and H-UiO-66 is stable up to 470 ℃ , and indicates the creation of hierarchical porous have no obvious influence to the heat stability, and the further reduction is induced by decomposition of metal−organic framework component. Besides, the average number of defects of H-UiO-66 (7.46) increase from 6.28 of UiO-66 of each metal cluster and the mass loss of H-UiO-66 (22.5%) increase from 28.5% of UiO-66 also indicates a mass of ligands

are

replaced

and

washed

during

etching

process.

The

[(CH2COOH)2IM]HSO4@H-UiO-66 shows much weight loss after 300 ℃ , because the [(CH2COOH)2IM]HSO4 in the pores and cages started to decompose with the increase of temperature. Meanwhile, the decomposition temperature of metal−organic framework component in [(CH2COOH)2IM]HSO4@H-UiO-66 is lower than H-UiO-66, indicating the coordination of [(CH2COOH)2IM]HSO4 with defeats of H-UiO-66 will decrease the heat stability of metal−organic framework slightly.

16


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100 90 80

Weight loss (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


28.5%

22.5%

70 60 50 40 30

UiO-66 [(CH2COOH)IM]HSO4@H-UiO-66 H-UiO-66 100

200

300

400

500

600

700

800

Temperature (°C)

Figure 6. TG curves of UiO-66, H-UiO-66 and [(CH2COOH)2IM]HSO4@H-UiO-66.

3.2 RSM analysis for the esterification of oleic acid According to previous studies,13,35 the parameters of reaction temperature (℃), molar ratio (methanol to oleic acid), catalyst amount (wt% of oleic acid) and reaction time (h) have significant influence on the yield of biodiesel. However, the effect of reaction temperature on the yield depicted in Figure 7 indicates that reaction temperature has little effect on the reaction. It can be explained that the [(CH2COOH)2IM]HSO4@H-UiO-66 has high acid content (1.13 mmol.g-1), so it can own high catalytic activity when [(CH2COOH)2IM]HSO4@H-UiO-66 as a catalytic. Hence, the reaction temperature was fixed at 80℃.

17



100 95 90 85

Yield / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 36

80 75 70 65 60

60

65

70

75

80

85

90

Temperature / ℃

Figure 7. Effect of reaction temperature: oleic acid: 5.649 g; molar ratio (methanol to oleic acid) 10:1; catalyst 5% based on oleic acid.

RSM was applied to research the influence of the parameters of reaction time, molar ratio and catalyst amount in order to obtain the optimum esterification condition of oleic acid and methanol, and the levels of independent variables and experimental ranges are set in Table 1. The detailed experimental matrix and corresponding results are listed in Table 2. The second-order polynomial equation model was obtained as follows: 2 2 2 Y  89.02  7.26 X 1  5.89 X 2  4.69 X 3  2.23 X 1 X 2  4.06 X 1 X 3  1.82 X 2 X 3  3.63 X 1  6.42 X 2  2.73 X 3 （2）

Where Y is the yield of biodiesel, X1, X2, and X3 are the coded values of variables of reaction time, molar ratio, and catalyst amount, respectively. Table 1. Factors and Levels of RSM Coding level -1 0 1

X1

X2

X3

Reaction time / h

Molar ratio

Catalyst amount / wt% of oleic acid

2 4 6

6 9 12

3 5 7

18


Page 19 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Experimental Design and Response Values of Experiment and Calculate Experimental variables NO.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 a

Yield / % Experiment a

X1

X2

X3

4 6 6 6 2 4 4 4 2 4 4 6 2 4 4 2 4

9 9 12 9 6 9 6 12 12 9 12 6 9 9 6 9 9

5 7 5 3 5 5 7 7 5 5 3 5 3 5 3 7 5

Average values

Standard dev.

Predicted

88.20 91.23 89.69 88.35 63.81 88.80 75.71 91.80 79.36 90.82 80.40 83.04 65.98 87.20 71.61 85.12 91.10

1.21 0.97 0.54 0.91 1.79 0.87 1.08 0.50 1.58 1.70 0.54 1.18 0.73 0.85 0.60 1.06 1.48

89.02 90.55 89.89 89.30 63.60 89.02 76.85 92.27 79.83 89.02 79.25 74.70 66.66 89.02 71.12 84.16 89.02

Heating temperature: 80℃, oleic acid: 5.649 g, normal pressure, 500 r / min.

Analysis of variance (ANOVA) of the fitted polynomial quadratic model is shown in Table 3. The model F-value of 66.61 implies the model is significant. And the values of “Prob > F” of independent variables (X1, X2, X3), interaction terms (X1X2, X1X3, X2X3) and quadratic terms (X12, X22, X32) are all less than 0.0500, indicates all of them are significant model terms to affect the yield of biodiesel. Besides, the “Lack of Fit F-value” of 1.01 and its p-value of 0.4758 are greater than 0.05, which proved the chosen model is acceptable. The “Pred R2” of 0.9102 is in reasonable agreement with the “Adj R2” of 0.9736, which shows that the model is crucial. “Adeq Precision” of 25.795 confirms the validity of the model again. Residual analysis for the model is illustrated graphically in Figure 8. Data points in 19



Page 20 of 36

the following plot are linear, indicating good regression results and quality of sample data. The above results proved that the established second-order polynomial equation model could be effectively applied for optimization of oleic acid esterification. Table 3. ANOVA and Statistical Criteria Source

Sum of squares

df

Mean square

F value

p-value Prob > F

Model X1(Temperature) X2(Mole ratio) X3(Catalyst amount) X1X2 X1X3 X2X3 X12 X22 X32 Residual Lack of fit Pure error Std. dev. 0.26

1259.03 421.37 277.30 175.78 19.80 65.93 13.32 55.41 173.41 31.32 14.70 6.33 8.37 R2 0.9885

9 1 1 1 1 1 1 1 1 1 7 3 4 Adj R2 0.9736

139.89 421.37 277.30 175.78 19.80 65.93 13.32 55.41 173.41 31.32 2.10 2.11 2.09 Pred R2 0.9102

66.61 200.64 132.04 83.70 9.43 31.40 6.34 26.38 82.57 14.91

< 0.0001 < 0.0001 < 0.0001 < 0.0001 0.0180 0.0008 0.0399 0.0013 < 0.0001 0.0062

1.01

0.4758

Mean 83.01

Adeq Precision 25.795

Figure 8. Normal probability plot of residuals for yield of biodiesel.

The 3D response surfaces were applied to research the yield varies with two factors where the third one was set as constant, and the effect results of the three process variables on response are illustrated in Figure 9. 20


Page 21 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9. Response surface graphs for yield of biodiesel: a) reaction time (h) and mole ratio, b) reaction time (h) and catalyst amount (wt / %), c) mole ratio and catalyst amount (wt / %).

Figure 9a represents the interaction of reaction time and mole ratio on the yield of biodiesel under the catalyst amount was set at 5% which is based on the weight of oleic acid. At low methanol / oleic acid mole ratios, the yield increases with the rise of 21



Page 22 of 36

reaction time, however, it decreases slightly in the late stage of reaction at high methanol / oleic acid mole ratios. It can be explained that the yield increases with the rise of mole ratios, and meanwhile the water content increase correspondingly, however, too much water will reduce the Lewis acid activity. Accordingly, with the extended of reaction time, there is more water in the reaction system with higher methanol

/

oleic

acid

mole

ratios,

and

the

catalytic

performance

of

[(CH2COOH)2IM]HSO4@H-UiO-66 will be weakened. The effect of interaction between reaction time and catalyst amount on yields was presented in Figure 9b. In the case of constant mole ratios, the yield increases with the rise of catalyst amount and reached a high yield with catalyst amount around 5.5%, nevertheless, a slight decreases occurred when catalyst amount exceeds 5.5%. The reason for this is that when there are enough active sites in reaction system, it plays a positive part in yield, but negative effect appears when there are overmuch Lewis acid sites. It is speculated that the excess Lewis acid sites will adsorb product biodiesel with enough reaction time by the unsaturated Zr atoms coordinate with the carbonyl group of biodiesel. The circular contour line indicated that the interaction effect of the reaction time and catalyst amount was significant, which was matched with the P value (0.0008) of the interaction X1 X3 term. Figure 9c depicts the influence of interaction between mole ratio and catalyst amount on the yield. At different catalyst amount, the increasing of mole ratio had obvious positive effects on the yield and reached a high value around 10.4, but negative effect appears when it is beyond 10.4. It can be explained that the excess methanol dilutes active site concentration of catalyst, so that it slows down the 22


Page 23 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


reaction rate. All the three response surfaces were convex surfaces that we can get the maximum response from them. Table 4. Yield of Biodiesel at Optimum and Experimental Parameters parameters predicted experimental

Reaction time

Molar ratio

Catalyst amount

Yield

(h)

(methanol to oleic acid)

(wt / % based on oleic acid )

(%)

5 5

10.39 10.39

6.28 6.28

93.71 93.82

Standard dev.

0.62

The optimum process conditions of oleic acid esterification for biodiesel production can be obtained by optimize the regression equation (Eq 2). The optimum conditions (Table 4) are 5 h (reaction time), 10.39:1 (mole ratio, methanol to oleic oil) and 6.28% (catalyst amount, based on oleic acid), respectively. Under the above condition, the predicted yield of biodiesel is 93.71%. To check the accuracy of the predicted value, a verification experiment was carried out with three experiments at same experimental conditions to compare with the predicted value. The experimental value of 93.82% presented in Table 4 is the average value of three parallel experiments. The good match between the predicted value and experimental value indicates the validity of the model for predicting the yield of biodiesel. 3.3 Catalytic Performance Comparison Compared

with

reported

immobilized

ionic

liquid

catalysts,

[(CH2COOH)2IM]HSO4@H-UiO-66 exhibited high catalytic activity and stability, and the roles of different components of [(CH2COOH)2IM]HSO4@H-UiO-66 in catalyzed esterification reaction for biodiesel production were investigated based on the oleic acid esterification in Table 5.

23



Page 24 of 36

Table 5. Catalytic Performance Comparison NO.

Catalyst

Alcohol

Molar ratio of alcohol to oleic acid

1

Blank

Methanol

10.39:1

2

UiO-66

Methanol

3

H-UiO-66

4

[(CH2COOH)2IM]HSO4

5 6 7 8 9 10

[(CH2COOH)2IM]HSO4@H -UiO-66 [HVIm-(CH2)3SO3H]-HSO4 @HKUST-1 POSS-[VMPS][H2SO4] DAIL-Fe3O4@NH2-MIL88B(Fe) MPEG-350-[SO3H-(CH2)4 -HIM][HSO4] [SO3H-(CH2)3HIM]3PW12O40@MIL-100 a

Catalyst amount a

Yield / %

ref

0

4.5

this work

10.39:1

6.28

25.6

this work

Methanol

10.39:1

6.28

78.6

this work

Methanol

10.39:1

6.28

89.2

this work

Methanol

10.39:1

6.28

93.8

this work

Ethanol

12.00:1

15.00

92.1

ref 40

Methanol

20.00:1

6.64

94.1

ref 41

Ethanol

10.50:1

8.50

93.2

ref 42

Methanol

10.00:1

4.86

84.5

ref 3

Ethanol

11.00:1

15.00

94.6

ref 39

Catalyst amount: wt / % based on oleic acid.

UiO-66 shows a certain degree of catalytic activity compared with the blank test, but it is weaker than H-UiO-66 (hierarchical porous UiO-66), because the relatively small pore apertures of approximately 0.6 nm of UiO-66 is smaller than oleic acid, so the Lewis acid sites inside the crystal can't provide catalytic activity without direct contact

with

the

carbonyl

of

oleic

acid.

As

a

homogeneous

catalyst,

[(CH2COOH)2IM]HSO4 has a high catalytic activity of about 89.2%. However, [(CH2COOH)2IM]HSO4@H-UiO-66 catalyst showed better catalytic performance due to it contains Brønsted and Lewis acid sites simultaneously. In addition, it can be separated conveniently and efficiently through centrifugation. 3.4 Catalyst reusability and analyze of catalytic mechanism A series of recycle experiments were conducted to investigate the reusability of 24


Page 25 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[(CH2COOH)2IM]HSO4@H-UiO-66 under the optimum conditions (reaction time is 5 h, mole ratio of methanol to oleic acid is 10.39 and catalyst amount is 6.28%, based on oleic acid). After each run, [(CH2COOH)2IM]HSO4@H-UiO-66 was recovered by simple centrifugation and washing by ethyl acetate, then the ethyl acetate was removed by filtration. The yield of biodiesel decreased slightly from 93.82% to 90.95% after reusing for 5 times and the trend had stabilized, as presented in Figure 10. The slight decrease of catalyst activity might be caused by leaching of a spot of active sites from framework structure, which leads to the decrease of acid content from 1.13 mmol.g-1 to 1.09 mmol.g-1 after reusing for 5 times, in the high temperature reaction

and

drying

process.

The

relatively

satisfactory

results

indicate

[(CH2COOH)2IM]HSO4@H-UiO-66 catalyst possess high stability and catalytic activity. In order to clarify the esterification process of oleic acid and methanol, the catalytic

mechanism

of

the

prepared

Brønsted-Lewis

acidic

[(CH2COOH)2IM]HSO4@H-UiO-66 catalyst was analyzed in Figure 11. The catalytic process contains two parts. For part I, the Brønsted acidic ionic liquid [(CH2COOH)2IM]HSO4 can provide the H+, and the H+ attack the carbonyl group of oleic acid. Then protonation of the carbonyl group leads to generate of carbocation. In the end, after the nucleophilic attack of methanol molecule, a tetrahedral intermediate is formed,36 and it can be decomposed into biodiesel and H+. For part II, the H-UiO-66 part produces the unsaturated Zr atoms, and unsaturated Zr atoms can produce coordination with the carbonyl group of oleic acid. Then protonation of the 25



carbonyl group leads to generate of carbocation. In the end, after the nucleophilic attack of methanol molecule, a tetrahedral intermediate is formed, and it can be decomposed into biodiesel and unsaturated Zr atoms. Furthermore, the exit of Lewis acid sites can improve the acidity of adjacent Brønsted acid sites, so the cooperation of Lewis acid and Brønsted acid improved the activity of the catalyst.35,36 In short, the synergistic

effect

of

Brønsted

and

Lewis

acidic

parts

makes

[(CH2COOH)2IM]HSO4@H-UiO-66 catalyst perform high catalytic activity in the esterification reaction. 100 95

93.82

92.58

92.71

91.52

run 3

run 4

90.95

90 85

Yield / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 36

80 75 70 65 60 55 50

run 1

run 2

run 5

Times

Figure 10. The reusability of [(CH2COOH)2IM]HSO4@H-UiO-66 catalyst.

26


Page 27 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


R

C

C R

R

OH

OH

OH

OH

O + H

C

OH

O

H3C

Ⅰ

R

OH

C

H3C

H

OH OH2

-H2O

C R

O

CH3 O

H+ O [(CH2COOH)2IM]HSO4@UiO-66

Zr+ O

Ⅱ

O

O +

O

O

O

O O

R

C

C

CH3OH

Zr O

R

H3C + OH O R Zr OH O

R

H3C

OH O

O

O

O

O

Zr O

CH3 O

O Zr+ O O

O

R

R

O

OH2 + -H2O

O

O

C

Zr O O HC 3

OH O

O

O

O

O

O

O

Figure 11. The catalytic mechanism of [(CH2COOH)2IM]HSO4@H-UiO-66 in the esterification.

4.CONCLUSIONS In this work, via an efficient strategy of bidentate coordination between one -COO- group of [(CH2COOH)2IM]HSO4 and two portions of unsaturated Zr ions defeats

of

the

H-UiO-66,

the

Brønsted-Lewis

acidic

[(CH2COOH)2IM]HSO4@H-UiO-66 catalyst was designed and prepared. The catalyst was systematically characterized by FT-IR, XRD, Nitrogen adsorption-desorption, SEM and TGA, proving ionic liquid was grafted in the carrier. The fabricated [(CH2COOH)2IM]HSO4@H-UiO-66 catalyst was applied for synthesis of biodiesel, and exhibited excellent catalytic performance for the esterification of oleic acid with methanol. Under the optimum condition of 5 h (reaction time), 10.39:1 (mole ratio, methanol to oleic oil) and 6.28% (catalyst amount, based on oleic acid), the predicted yield of biodiesel reached 93.71%, and experimental value is 93.82%, which indicated high catalytic activity, and based on synergistic effect of Brønsted acidic and Lewis acidic, the catalytic mechanism of [(CH2COOH)2IM]HSO4@H-UiO-66 was also 27



discussed. Moreover, the catalyst could be easily recovered and reused for five times, the yield of biodiesel decreases from 93.82% to 90.95%, indicating well reusability. In conclusion, the novel approximate ligand substitution method is feasible to introduce TSILs into the MOFs, and the [(CH2COOH)2IM]HSO4@H-UiO-66 as probe was synthesized by this method is a new kind of heterogeneous catalyst that combines the advantages of the TSILs and hierarchically porous Zr metal-organic framework, and reveals the enormous potential as efficient catalyst for esterification. AUTHOR INFORMATION Corresponding Authors *Ting

Qiu: e-mail, [email protected]

ORCID Ting Qiu: 0000-0001-7737-8640 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS

We acknowledge the financial support for this work from the National Natural Science Foundation of China (Project no. 21878054 and 21576053), and the Natural Science Foundation of Fujian Province (Project no. 2016J01689). REFERENCES (1) Huang, G. H.; Chen, F.; Wei, D.; Zhang, X. W.; Chen, G. Biodiesel production by microalgal biotechnology. Appl. Energy. 2010, 87, 38-46. (2) Qiu, T.; Guo, X. T.; Yang, J. B.; Zhou, L. H.; Li, L.; Wang, H. X.; Niu, Y. The synthesis of biodiesel from coconut oil using novel Brønsted acidic ionic liquid 28


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Design and Synthesis of Ionic Liquid-supported Hierarchically Porous

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