Selective catalytic synthesis of glycerol monolaurate over silica gel-based sulfonic acid functionalized ionic liquid catalysts

Chemical Engineering Journal 359 (2019) 733–745

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Selective catalytic synthesis of glycerol monolaurate over silica gel-based sulfonic acid functionalized ionic liquid catalysts

T

Xiaoxiang Hana, Guangqi Zhua, Yuxi Dinga, Yanli Miaoa, Kuiwu Wanga, Haijiang Zhangb, ⁎ ⁎ Yanbo Wanga, , Shang-Bin Liuc, a

Department of Applied Chemistry, Zhejiang Gongshang University, Hangzhou 310018, China Jiangsu Key Laboratory of Regional Resource Exploitation and Medicinal Research, Huaiyin Institute of Technology, Huaian 223003, China c Institute of Atom and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan b

H I GH L IG H T S

of silica gel-supported sul• Synthesis fonic acid functionalized IL catalysts. SAFIL/SG catalysts show high • Such catalytic activity for ester production. of SAFIL/SG catalyst due to • Activity strong acidity and surface area of

G R A P H I C A L A B S T R A C T

Eco-friendly silica gel-supported sulfonic acid functionalized ionic liquids catalysts with self-separation, facile recovery, and recycling properties shows superior activity for synthesis of glycerol monolaurate.

support.

properties of SAFIL/SG probed • Acidic by the P-TMPO MAS NMR approach. optimization by RSM based on • Process BBD model was performed. 31

A R T I C LE I N FO

A B S T R A C T

Keywords: Esterification Functionalized ionic liquid Acidity Glycerol monolaurate Glycerol Response surface methodology

A series of silica gel supported sulfonic acid functionalized ionic liquid (SAFIL/SG) catalysts have been successfully synthesized through a sol–gel method by varying the organic amine loadings. The catalysts so fabricated were characterized by various physicochemical techniques such as FT-IR, XRD, TGA, FE-SEM, and solidstate 31P NMR probe molecule method and their catalytic performances during esterification of glycerol (GL) with lauric acid (LA) were assessed. The 20% N,N-dimethyl(benzyl)ammonium propyl sulfobetaine hydrosulfate embedded silica gel catalyst (20%[DMBPSH]HSO4/SG) was found to exhibit superior performance and durability during this eco-friendly synthesis of glycerol monolaurate (GML) owing to the high surface area and strong Brønsted acidity observed for the catalyst. Moreover, an experimental design was exploited via response surface methodology (RSM) for the optimization of process variables. Accordingly, a maximum GML yield of 83.9% was achieved for the 20%[DMBPSH]HSO4/SG catalyst under the optimal conditions: amount of catalyst 2.0 wt%, GL/LA molar ratio of 4.1, reaction temperature 418 K, and reaction time 45 min, which was in conformity with those predicted by a mathematical model. Further kinetic studies confirmed that the esterification of GL with LA to GML with an activation energy of 31.51 kJ/mol.

⁎

Corresponding authors. E-mail addresses: [email protected] (Y. Wang), [email protected] (S.-B. Liu).

https://doi.org/10.1016/j.cej.2018.11.169 Received 17 April 2018; Received in revised form 12 November 2018; Accepted 22 November 2018 Available online 23 November 2018 1385-8947/ © 2018 Elsevier B.V. All rights reserved.

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1. Introduction

2. Experimental

As the demands for biodiesel and bio soaps continue to increase, an excessive amount of by-product glycerol (GL) is inevitably accumulated [1,2]. The surplus of low cost GL not only creates a glut in the market but also impairs the overall economy in biodiesel production. Thus, the development of effective and environmental friendly approaches for efficient conversion of GL to value-added chemicals is a challenging and demanding task [3–5]. Among various catalytic reactions that have been applied for conversion of GL, such as hydrogenolysis, reforming, etherification, esterification, oxidation, dehydration, carbonylation, and acetylation [6–14], one of the prospective approaches is the synthesis of glycerol monolaurate (GML) through esterification of GL with lauric acid (LA). Being recognized as a useful non-ionic surfactant, emulsifier, antibacterial agent, as well as thickener, GML has been extensively employed in food, medicine, cosmetic and other industries [15]. As certified by the US Federal Drug Administration (FDA), GML is generally recognized as one of the safe food additives that also exhibits excellent inhibitory effects against bacteria, molds, yeasts, fungi, and other microorganisms [16–18]. The catalytic production of GML normally invokes esterification or transesterification reaction using acidic catalysts such as ion exchange resins, zeolites, enzymes, molecular sieves, ionic liquids and so on [19–24]. Among them, Brønsted acidic ionic liquids (BAILs) have drawn considerable attentions as an environmentally benign catalyst owing to their unique characteristics such as low melting point, negligible volatility, high solubility and thermal stability, and tunable properties [25–27]. Recently, BAILs have been extensively used in hydration/dehydration, oxidation, alkylation, transesterification, and esterification reactions [25–36]. In particular, the strength of Brønsted acid sites could be greatly improved by incorporating sulfonate functional group onto the ionic liquid, which in turn significantly enhances catalyst activity [28–31]. Nevertheless, sulfonated ionic liquid catalysts have many drawbacks of high viscosity, cost, and hydrophilic characteristics, which largely hinder their applications. An effective method to circumvent these problems is to immobilize them on porous inorganic supports with high surface areas such as silica gels, mesoporous silicas, and carbons [37–40]. These organic–inorganic composite catalysts possess unique characteristics not only favorable for diffusion and mass transfer, but also for catalyst separation, recovery, and recycling. Moreover, by immobilizing on porous inorganic supports, the loading of expansive ionic liquid ingredients may be effectively reduced, thereby lowering the overall cost [41]. Such supported ionic liquid catalysts may be prepared by facile sol–gel or impregnation methods [42,43] and have been widely applied in various catalytic reactions for the productions of n-butyl acetate [44], methyl propionate [45], and benzaldehyde ethanediol acetal [46]. Herein, a series of silica gel supported sulfonic acid functionalized ionic liquid (SAFIL/SG) catalysts have been prepared and exploited for the esterification of glycerol (GL) with lauric acid (LA) to produce glycerol monolaurate (GML). These novel SAFIL/SG catalyst samples were thoroughly characterized by different analytical and spectroscopic techniques such as Fourier-transform infrared (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), field-emission scanning electron microscopy (FE-SEM), and nuclear magnetic resonance (NMR) spectroscopy. In particular, their acidic properties are characterized by solid-state 31P magic-angle spinning (MAS) NMR of adsorbed trimethylphosphine oxide (TMPO) [47–51]. It will be shown that these SAFIL/SGs exhibit good catalytic properties with reduced usage of expansive ionic liquids. Moreover, the effects of major reaction variables, such as GL/LA molar ratio, catalyst amount, reaction temperature, and reaction time on yield and selectivity of the main product, namely GML, are elucidated and optimized by means of response surface methodology (RSM) [52] based on a Box-Behnken design (BBD) [53]. The kinetics of the esterification reaction was also established and evaluated under the optimized conditions.

2.1. Catalyst preparation A series of silica gel supported sulfonic acid functionalized ionic liquid (SAFIL/SG) were prepared following the procedure outlined in literature [28,54]. In brief, desirable amounts of 1,3-propane sultone and N,N-dimethyl(benzyl)ammonium (DMB) were first dissolved in ethyl acetate. Then, the solution mixture was stirred continuously at 323 K for 2 h. Subsequently, the zwitterion precursor was washed with ethyl acetate thrice, followed by mixing with a stoichiometric amount of sulfuric acid. The mixture was stirred at 363 K for 4 h to obtain the sulfonic acid functionalized ionic liquid, which was mixed with tetraethyl orthosilicate (TEOS) and ethanol under stirring condition before it was further acidified with concentrated hydrochloric acid. After stirring at 333 K for 24 h, the final mixture was aged at room temperature for 12 h, followed by drying at 473 K for 5 h to obtain the silica gel (SG) supported sulfonic acid functionalized ionic liquid catalyst. The catalysts so synthesized were denoted as x%[DMBPSH]HSO4/SG, where x represents the molar contents of [DMBPSH]HSO4 (x = 5, 10, 20 and 30). Similar method was used to prepare other supported SAFIL catalysts with x = 20, namely 20%[XILH]HSO4/SG with various acidic proton-bonded cationic organic amine groups (XIL) such as N,N-dimethyl(phenyl)ammonium (DMP), triethylamine (TEA), N-methyl imidazole (MIM), and pyridinium propyl sulfobetaine (PPS), as summarized in Table 1. In addition, a SG supported tetrapropane sulfonic hexamethylenetetramine (Tshx) tetrahydrosulfate, namely 20%[Tshx] [HSO4]4/SG was also prepared. Moreover, analogous BAILs with the same cationic groups, i.e., [XILH]Z with XIL = DMBPS were prepared with different anionic components (Z) of HSO4−, p-CH3C6H4SO3− (denotes as PTSA), H2PO4−, and CF3COO− together with their SG supported counterparts, namely x%[DMBPSH]Z/SG with x = 20 and Z = HSO4−, p-CH3C6H4SO3−, H2PO4−, and CF3COO−. The structural compositions of these Brønsted acidic ionic liquids have been verified by solid-state 1H and 13C NMR spectroscopy reported elsewhere [14,24,34,36,55]. All research grade chemicals were used without further purification unless stated otherwise. 2.2. Catalyst characterization The structural properties of various SAFIL/SG catalysts were characterized by FT-IR spectroscopy. All FT-IR spectra were recorded using the KBr pellet method on a Bruker Vertex 70 spectrometer with a resolution of 4 cm−1 over the range of 4000–400 cm−1. The XRD experiments were performed on a DX-2700 diffractometer (Dandong Kemait NDT Co., Ltd.) operating at 40 kV and 30 mA using Cu Kα radiation (0.15418 nm). Each XRD profile was recorded over a 2θ angle of 5–90° at an interval of 0.04°/min. The thermal properties of various SAFIL/SG catalysts were measured by TGA on a TGA/DSC-1 (Mettler Toledo) analyzer. All thermogravimetric and differential thermogravimetric (TG-DTG) curves were measured by heating each catalyst sample under a dynamic N2 atmosphere from 303 to 873 K at a rate of 10 K/ min. The textural properties of various catalysts were studied by N2 adsorption/desorption isotherms using a physisorption apparatus (Micromeritics, ASAP 2020). Field-emission scanning electron microscopy (FE-SEM) experiments were conducted on a Hitachi SU 8010 electron microscope. Elemental analyses were carried out on an ICPOES CID spectrometer (iCAP 6500, Thermo Scientific). On the other hand, the acidic properties of various catalysts were characterized by solid-state 31P MAS NMR probe molecule technique using adsorbed trimethylphosphine oxide (TMPO) as the probe [47–51]. Such solid-state 31P-TMPO NMR approach [49–51] has been shown as a powerful technique for probing detailed acid features, namely type (Brønsted vs Lewis), concentration (amount), and strength of acid sites. It has been shown that a linear correlation between the observed 31P NMR chemical shifts (δ31P) of TMPO with proton affinity 734

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Table 1 Catalytic performances of the esterification of glycerol (GL) with lauric acid (LA) over various catalysts.a Entry

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

Catalystb

Blank SG [DMBPSH]HSO4 [DMBPSH]PTSA [DMBPSH]H2PO4 [DMBPSH]CF3COO [DMBPSH]+(½Cu2+)SO42− 5%[DMBPSH]HSO4/SG 10%[DMBPSH]HSO4/SG 20%[DMBPSH]HSO4/SG 30%[DMBPSH]HSO4/SG 20%[DMBPSH]PTSA/SG 20%[DMBPSH]H2PO4/SG 20%[DMBPSH]CF3COO/SG 20%[MIMPSH]HSO4/SG 20%[TEAPSH]HSO4/SG 20%[Tshx][HSO4]4/SG 20%[PPSH]HSO4/SG 20%[DMPPSH]HSO4/SG 20%[DMBPSH]+ (½Cu2+)SO42−/SG

Selectivity (%)c GML

GDL

GTL

84.6 85.3 76.3 68.4 72.1 71.1 71.4 85.0 81.8 84.3 82.4 81.8 82.5 80.5 84.1 83.2 82.5 83.4 83.5 84.1

15.4 14.7 23.7 31.6 27.9 18.9 28.6 5.0 18.2 15.7 17.6 18.2 17.5 19.5 15.9 16.8 17.5 16.6 16.5 15.9

nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil nil

Conversion (%)d

Yield (%)e

47.0 53.8 89.2 90.5 85.4 85.0 84.7 81.3 90.1 95.3 96.5 92.3 90.7 90.1 94.6 94.5 95.1 93.9 93.7 66.9

39.7 45.9 68.1 61.9 61.6 60.4 60.5 69.1 73.7 80.3 79.5 75.5 74.8 72.5 79.6 78.6 78.5 78.3 78.2 56.3

Reaction conditions: GL/LA = 4 (mol/mol); catalyst amount (relative to LA) 2 wt%; reaction time 45 min; reaction temperature 423 K. SG = silica gel; PTSA = p-CH3C6H4SO3−. Analyzed by GC; GML = glycerol monolaurate; GDL = glycerol dilaurate; GTL = glycerol trilaurate. LA conversion. GML yield.

assessed by the esterification of glycerol (GL) with lauric acid (LA). As illustrated in Scheme 1, the primary product of fatty acid esters of glycerol, namely glycerine laurate esters, including glycerol monolaurate (GML), glycerol dilaurate (GDL), and glycerol trilaurate (GTL). The yields and selectivities depended not only on the type of catalyst used but also on relevant reaction variables such as reactant (GL/LA) molar ratio, catalyst amount, reaction time, and temperature. In a typical experiment, a known amount of catalyst (0.5–3.0 wt% relative to LA) was mixed with a fixed amount of LA (0.05 mol) and a desirable amount of GL (0.1–0.3 mol) in a 100 mL three-necked flask reactor equipped with a stirrer and a condenser. The reaction was carried out at 393–443 K for a desired period of time (30–180 min) in an oil bath under continuous stirring condition. Upon completion, the reaction mixture was cooled down to room temperature (RT; 298 K). The spent catalyst was filtered, washed with ethyl acetate, and recycled. Chemical analyses of reactants and products were carried out using gas chromatography (GC; Agilent 7890B) equipped with a flame ionization detector (FID) and a HP-5 capillary column (30 m × 0.22 mm i.d. × 0.25 μm) while ramping the temperature from 373 to 553 K at a rate of 25 K/min, then, maintained at the final temperature for additional 15 min (see Fig. S1 of the Supplementary Information). All

(or deprotonation energy) [47,48], which facilitated a convenient scale for acidic strength. In other words, the higher the observed δ31P of the adsorbed TMPO, the higher the acidic strength of the corresponding acid sites. Moreover, such 31P-TMPO NMR approach also rendered qualitative and quantitative characterization of acidic sites while performing the experiments in conjunction with spectral analysis and elemental analysis, respectively, making it possible to determine distribution and concentration of acidic sites. All NMR spectra were recorded on a Bruker Avance III 500 spectrometer at a Larmor frequency of 202.46 MHz using a single-pulse sequence with a pulse length of 1.5 μs (ca. π/6 pulse), recycle delay of 10 s, and a sample spinning rate of 12 kHz. Moreover, all δ31P values were referenced to 85% H3PO4 aqueous solution (δ31P = 0 ppm). Prior to each NMR measurement, the catalyst was first dehydrated under vacuum (< 10−5 Torr) at 423 K, followed by adsorption of the TMPO probe molecule and post sample treatments, relevant standard operation procedure has been elaborated in several review articles [49–51]. 2.3. Esterification reaction The catalytic performances for various SAFIL/SG catalysts were

Scheme 1. Illustration of production of glycerine laurate esters by SAFIL/GC catalyzed esterification of glycerol with lauric acid. 735

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The reaction rate equation can be expressed as:

Table 2 List of symbols for different process variables and corresponding coded levels and ranges used in the experimental designs. Variable (unit)

Symbol

GL/LA ratio (mol/mol) Amount of catalyst (wt%) Reaction temperature (K)

x1 x2 x3

− rLA = −

dCLA = k1CLA dt

(5)

Level and range

rGML =

−1

0

1

3.0 1.0 403

4.0 2.0 413

5.0 3.0 423

dCGML = k1 CLA − k2 CGML dt

(6)

where rLA and rGML denote reaction rates, CLA, CGML and CGDL represent the concentrations of LA, GML, and GDL, respectively, and k1 and k2 denote the rate constants for LA toward GML and GML toward GDL, respectively. As such, Eqs. (5) and (6) may be integrated to Eqs. (7)–(9).

reactants and products were identified by means of authentic samples while using methyl laurate as the internal standard. Accordingly, the product yield was calculated by:

CLA = e−k1 t CLA0

Yield (%) = Conversion (%) × Selectivity (%)

ln

(1)

(7)

CLA = −k1 t CLA0

(8)

2.4. Experimental design and mathematical model

CGML k1 [e−k2 t − e−k1 t ] = CLA0 k1 − k2

Response surface methodology (RSM), which compiles both statistical and mathematical methods [28,52,56], has been used to optimize the reaction process and product yield during the esterification reaction. A Box-Behnken design (BBD) [53] was applied to study the effects of process variables on LA conversion and GML yield during the esterification reaction. Accordingly, three independent process variables, namely GL/LA molar ratio (x1), amount of catalyst (x2), and reaction temperature (x3), were chosen at three coded levels of experimental design with designated range, as depicted in Table 2. As such, a 33 fullfactorial central composite design with three coded levels was exploited, leading to a total of 17 experimental sets, including 12 factorial points, and 5 centering points (vide infra). The aforementioned three experimental variables were tested at levels coded with either a plus sign (+1; higher value), zero (0; central value), or a minus sign (−1; lower value). A Design-Expert Version 8.0.7.1 software (Stat-Ease, USA) was used for both design of experiments and analysis of experimental data. The coded values of these factors were obtained by the equation:

The values of k1 at different reaction temperatures may be obtained by simple linear fittings of the curves in the ln(CLA/CLA0) vs t plot. Whilst Eq. (9) may be simplified as:

xi =

Y = (eat − 1)/a

where Y a = k1–k2. The values of k2 at different reaction temperatures may also be obtained by facile non-linear fittings of the curves in the Y vs t plot. Accordingly, the variation of reaction rate with temperature may be expressed by the Arrhenius equation:

ln k = ln k 0 −

3.1. Catalyst characterization

3 i=1

The FT-IR spectra of the unsupported SAFIL, namely [DMBPSH] HSO4, and various supported x%[DMBPSH]HSO4/SG catalysts with different [DMBPSH]HSO4 loadings (x = 5, 10, 20, and 30) were depicted in Fig. 1. As shown in Fig. 1a, the [DMBPSH]HSO4 exhibited main characteristic absorption bands near 3446, 1660, 1583, 1477, 1043, and 1000–700 cm−1. The bands near 3446 cm−1 may be assigned to stretching vibration of hydroxyls (eOH) functional group [58], the peaks at 1600 and 1477 cm−1 were due to C]C stretching of aromatics, whereas peaks at 1583, 1043, and 700–1000 cm−1 were due to CeN, S]O, and SeO stretching vibrations, respectively [59]. Upon anchoring the SAFIL onto the silica gel support, the IR spectra of various x%[DMBPSH]HSO4/SG (x = 5, 10, 20, and 30) catalysts showed not only the anticipated features of the unsupported [DMBPSH]HSO4 (Fig. 1a) but also characteristic absorption bands arising from the SG support (Fig. 1b), as illustrated in Fig. 1c–g. This may be verified by the presences of absorption bands at 1043–1200, 958, and 462 cm−1, which may be ascribed due to asymmetric stretching vibrations of SieOeSi, vibrations of SieOH, and symmetric stretching of SieOeSi, respectively [60]. In particular, the broad absorption bands at 3650–3400 cm−1, which were attributed to stretching vibrations of eOH groups, represented the amount of Brønsted acid sites of the supported catalysts. Overall, the above results clearly indicated that the SAFIL, namely [DMBPSH]HSO4, was successfully immobilized on the SG support. Similar conclusions may also be drawn from the XRD results, as

3 i
(3)

where Y represented the predicted response (i.e., GML yield), xi and xj were the coded levels of the independent variables, and β0, βi, βii, and βij denoted the regression coefficient of the offset, linear, quadratic, and interactive term for the variables, respectively. 2.5. Kinetic study A Langmuir-Hinshelwood-Hougen-Watson (LHHW) kinetic model was adopted for the esterification reaction, during which both internal and external mass transfer limitations over the SAFIL/SG catalyst may be ignored [57]. In the case of excessive glycerol (GL), which favors enhancement of reaction rate as well as formation of esters, the reverse reaction may be ignored. Given that the concentration of GL is higher than that of LA, the former could be deemed as constant. Thus, the reaction equation can be simplified as: k1

k2

LA→GML→GDL

(11)

(2)

∑ βi xi + ∑ βii xi2 + ∑ βij xi xj i=1

Ea 1 R T

where Ea represents the activation energy during esterification of GL with LA (or GML with LA) over the SAFIL/SG catalyst, R and T denotes the gas constant and reaction temperature, respectively, and k0 is the pre-exponential factor.

where xi, Xi, and X0 (i = 1–3) represent the coded, real, and central value of the independent variables, respectively, and ΔXi indicates the step-change values of the associated variable. To optimize the reaction process and to predict the yield of GML, a second-order RSM model, including interactive effects between process variables, may be expressed by the quadratic equation: 3

(10)

= exp(k1t)k1−1(CLA/CLA0),

3. Results and discussion

Xi − X0 ΔXi

Y = β0 +

(9)

(4) 736

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Fig. 2. XRD patterns of the (a) bare SG, and various supported x%[DMBPSH] HSO4/SG catalysts with different [DMBPSH]HSO4 loadings, x, of (b) 5 wt%, (c) 10 wt%, (d) 20 wt%, and (e) 30 wt%.

Fig. 1. FT-IR spectra of the (a) pristine [DMBPSH]HSO4, (b) bare SG, and various supported x%[DMBPSH]HSO4/SG catalysts with different [DMBPSH] HSO4 loadings, x, of (c) 5 wt%, (d) 10 wt%, (e) 20 wt% (fresh), (f) 20 wt% (spent), and (g) 30 wt%.

revealed that the textural properties of the supported catalyst played an important role during the catalytic reaction. The above notion was supported by the BET surface area (SBET) obtained from N2 adsorption/ desorption isotherm measurements (Fig. S2, Supplementary Information) and BJH pore size distributions (Fig. S3, Supplementary Information) of assorted x%[DMBPSH]HSO4/SG with different SAFIL loading (x). Comparing to the SBET of the bare SG support (536 m2/g), the surface areas observed for various x%[DMBPSH]HSO4/SG catalysts were found to decrease with increasing loading of [DMBPSH]HSO4; a value of 383, 214, 76.2, and 9.8 m2/g was obtained for 5%[DMBPSH] HSO4/SG, 10%[DMBPSH]HSO4/SG, 20%[DMBPSH]HSO4/SG, and 30%[DMBPSH]HSO4/SG catalysts, respectively. Moreover, a progressive increase in pore size with increasing SAFIL loading was also observed. The above results indicated a progressive anchoring of the SAFIL in pore channels of the silica gel support. The morphology of the bare SG and 20%[DMBPSH]HSO4/SG was also investigated. As revealed by FE-SEM images in Fig. 4, the presence of mesoscopic pores on the surfaces of the bare SG support was evident (Fig. 4a). Upon incorporating 20%[DMBPSH]HSO4 IL onto the SG support (Fig. 4b), a notable deposition of IL on the surfaces of the catalyst was evident, suggesting a successful immobilization of [DMBPSH]HSO4 IL in the porous SG support [54]. The acid properties of various catalysts were characterized by solidstate MAS NMR in conjunction with a probe molecule technique, namely by the 31P-TMPO NMR approach, which has been shown as a powerful tool for probing detailed acid features of solid acid catalysts [47–51]. Upon adsorption, the basic TMPO molecule tended to interact with the Brønsted acidic proton (H+) to form TMPOH+ complex, whose 31 P chemical shift (δ31P) value has been shown to depend linearly with increasing acidic strength [47,48]. Moreover, through spectral

shown in Fig. 2 for the bare and supported x%[DMBPSH]HSO4/SG (x = 5, 10, 20, and 30) catalysts. The bare SG (Fig. 2a) revealed a broad diffraction peak centering at 23.3°, in excellent agreement with literature report [41]. Upon incorporating various amounts of [DMBPSH] HSO4 IL onto the SG support (Fig. 2b–e), additional diffraction peaks at 18.5, 26.5, 29.2 and 35.8° were observed in addition to the broad diffraction peak responsible for the SG support. These results clearly indicate successful anchoring of the sulfonic acid functionalized ILs in the pore channels of the SG support. The thermal properties and structural stabilities of various catalysts were further characterized by the TG-DTG technique [61,62]. As shown in Fig. 3(a,b), the pristine Brønsted acidic ionic liquid (BAIL) DMBPS precursor and SAFIL (i.e., [DMBPSH]HSO4) exhibited similar TG-DTG profiles. The weight-loss at ca. 300–400 K was mainly due to removal of free and surface-adsorbed water, and decomposition of small molecules. The weight-loss at 500–600 K may be attributed to the decomposition of the DMBPS. Likewise, the TG-DTG profile of the 20%[DMBPSH]HSO4/ SG catalyst (Fig. 3c) also showed two main weight-loss peaks at ca. 335 and 605 K, which may be ascribed to the desorption of physisorbed water and decomposition of organic components in the supported catalyst. A slightly lower decomposition temperature was observed for the [DMBPSH]HSO4 (Fig. 3b) compared to its SG-supported counterpart (Fig. 3c), indicating that the supported catalyst indeed showed improved thermal stability. Moreover, as it will be shown later that the 20%[DMBPSH]HSO4/ SG catalyst also exhibited superior catalytic performance compared to other SAFIL/SG catalysts. Additional N2 physisorption study further 737

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Fig. 3. TG-DTG profiles of the (a) pristine DMBPS, (b) [DMBPSH]HSO4 (i.e., SAFIL), and (c) 20%[DMBPSH]HSO4/SG.

increased with increasing SAFIL loading and that the concentration of the latter was notably higher than the former. However, as the SAFIL loading (x) exceeded 30%, notable changes in peak intensities of the 31P resonances responsible for both Brønsted acid sites were observed (Fig. 5e). This was attributed to the blockage of pore channels of the SG support by excessive amount of immobilized ILs, making superacidic Brønsted acid sites residing in the pores inaccessible to the TMPO probe molecule (and hence, GL and LA reactants; vide infra). The above trend observed for acidity of the supported [DMBPSH]HSO4/SG catalyst coincided with that observed for their catalytic activities (Table 1, entries 8–11). In addition, as illustrated by the spectra obtained from the 20%[DMBPSH]HSO4/SG catalyst, a slightly lower acidic concentration was observe for the spent catalyst (Fig. 5f) compared to its fresh counterpart (Fig. 5d). Moreover, by incorporating a Lewis metal center onto SAFIL [35,36], for example, by replacing H+ in [DMBPSH]HSO4 with Cu2+, 31P resonances observed for the supported 20%[DMBPSH] (½Cu2+)SO42−/SG catalyst showed δ31P identical to that of other supported SAFIL/SG catalyst but with much weaker peak intensities (i.e., concentrations; Fig. 5g). These results indicated that Brønsted acid played a decisive role in acidity and activity of the catalyst.

deconvolution, detailed quantitative features such as distribution and concentration of Brønsted and Lewis acid sites may readily be determined [49–51]. The 31P NMR spectra of adsorbed TMPO in the bare SG and various SG supported SAFIL catalysts are depicted in Fig. 5. The bare SG exhibited a single 31P resonance with δ31P of 53 ppm (Fig. 5a), indicating the presence of Brønsted acid sites with weak acidic strength comparable to typical zeolitic catalysts [49–51]. Upon incorporating SAFIL (i.e., [DMBPSH]HSO4) onto the SG support, two 31P resonance peaks corresponding to two types of TMPOH+ complexes with δ31P of 93 and 81 ppm were observed (Fig. 5b–e), revealing the presences of Brønsted acid sites with strong acidity. Among them, the peak at down field whose δ31P (93 ppm) exceeded the threshold for superacidity (86 ppm) [47–51], indicating the presence of Brønsted acid sites with superacidity. The 31P resonance observed at 81 ppm reveals the presence of Brønsted acid sites with acid strength stronger compared to most zeolitic catalysts (typically ca. 55–70 ppm) [49–51]. The incorporation of [DMBPSH]HSO4 onto the SG support therefore greatly enhanced the acidic strength of the catalyst. In addition, the intensities of the two 31P resonances observed for various x%[DMBPSH]HSO4/SG catalysts increased simultaneously with increasing [DMBPSH]HSO4 loading from 5% to 20% (Fig. 5b–d), indicating the anticipated increase in acid amounts. It is indicative that the amounts of Brønsted acid sites with strong (81 ppm) and ultra-strong (93 ppm) acidic strengths both

Fig. 4. FE-SEM images of the (a) bare SG and (b) 20%[DMBPSH]HSO4/SG catalysts. 738

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DMBPSH]H2PO4 ≈ [DMBPSH]CF3COO > [DMBPSH]+(½Cu2+)SO42− (entries 3–6, Table 1), indicating that sulfonic acid functionalized ionic liquid (SAFIL) indeed showed better acidity and catalytic activity compared to other BAILs and BLAIL. The above results were consistent with that found in a previous study [28]. To surmount the high viscosity nature of ILs, to facilitate product separation and catalyst recycle, to further improve the yield of ester, several silica gel supported SAFIL catalysts were prepared and screened. Their catalytic performances for the esterification reaction were also summarized in Table 1 (entries 8–20). Among various x%[DMBPSH] HSO4/SG (x represented the molar contents of SAFIL; x = 5–30) catalysts examined (entries 8–11), it was obvious that the 20%[DMBPSH] HSO4/SG catalyst (i.e., x = 20) showed the best performance. Thereby, a LA conversion and a GML yield of 95.3% and 80.3% were achieved, respectively, in excellent agreement with the higher concentration and strength of Brønsted acid sites observed compared to other supported SAFIL/SG catalysts (vide supra, Fig. 5). Moreover, by comparing to the pristine SAFIL (entry 3) and bare SG (entry 2), it was conclusive that the catalytic activity may be enhanced significantly upon anchoring SAFIL onto SG. This also indicated that the specific surface area of the SG support indeed played an important role during the esterification reaction. Furthermore, the catalytic activities of other supported catalysts (with SAFIL loading x = 20) were also assessed, namely 20%[XILH]Z/ SG with different acidic proton-bonded cationic (XIL = DMBPS, MIMPS, TEAPS, PPS, and DMPPS) and anionic (Z = HSO4−, p-CH3C6H4SO3−, H2PO4−, and CF3COO−) components (entries 10, 12–16, 18, 19, Table 1) as well as the supported tetrapropane sulfonic hexamethylenetetramine tetrahydrosulfate (20%[Tshx][HSO4]4/SG, entry 17) catalyst. For SG supported SAFIL catalysts with the same cationic group (XIL = DMBPS), namely 20%[DMBPSH]Z/SG, their catalytic properties with the following descending order of anionic groups (Z) were observed: [HSO4]− > [p-CH3C6H4SO3]− (PTSA) > [H2PO4]− > [CF3COO]− (entries 10, 12–14, Table 1), indicating that the strength of Brønsted acidity played an important role during the production of GML. Likewise, for x%[XILH]Z/SG catalysts with the same BAIL loading (x = 20) and anionic group (Z = HSO4−), similar catalytic performance with varied cationic components (XIL) on the SAFIL were observed, typically a LA conversion of ca. 94–95% and a modest descending GML yield from 80.3 to 78.2% with different XIL of the order: DMBPS (80.3%) > MIMPS (79.6%) > TEAPS (78.6%) > PPS (78.3%) > DMPPS (78.2%) may be inferred (entries 10, 15, 16, 18, and 19, Table 1). This is ascribed to the larger volume possesses by the DMBPS IL precursor. Similar results were obtained for IL-modified heteropolyacid composite catalyst during the oxidation of benzyl alcohol, as proposed by Ouyang et al. [32]. Thus, the collaborated effects of high surface area and ultra-strong Brønsted acidity facilitated by the 20%[DMBPSH]HSO4/SG catalyst were responsible for the superior performance observed during the catalytic production of GML. On the other hand, a slightly lower GML selectivity and yield was observed for the 30%[DMBPSH]HSO4/SG (entry 11) compared to the 20%[DMBPSH]HSO4/SG catalyst. This is ascribed due to blockage of SG pore channels by excessive amount of immobilized IL, making superacidic Brønsted acid sites residing in the pores inaccessible to the reactants (vide supra). Moreover, the 20%[DMBPSH]HSO4/SG catalyst also exhibited catalytic performances surpassing other SAFIL/SG and BLAIL/SG catalysts, including those observed for the 20%[Tshx] [HSO4]4/SG (entry 17) and the 20%[DMBPSH]+-(½Cu2+)SO42−/SG (entry 20). The presences of Brønsted acid sites with ultra-strong acidic strength (vide supra, Fig. 5d) facilitated by the sulfonic acid functional group as well as the high surface area available by the SG support were responsible for the superior performances observed for the 20%[DMBPSH]HSO4/SG catalyst. Accordingly, this catalyst was adopted for process optimization and kinetic studies (vide infra).

Fig. 5. Solid-state 31P MAS NMR spectra of TMPO adsorbed on the (a) bare SG and various supported x%[DMBPSH]HSO4/SG catalysts with different [DMBPSH]HSO4 loadings, x, of (b) 5 wt%, (c) 10 wt%, (d) 20 wt% (fresh), (e) 30 wt%, (f) 20 wt% (spent), and (g) 20%[DMBPSH] + (½Cu2+)SO42−/SG. The asterisks denote spinning sidebands.

3.2. Esterification reaction To assess the catalytic activity of various catalysts during the esterification of GL with LA, the performances of different SAFIL/SG catalysts were compared with those from blank catalyst, bare SG, as well as assorted unsupported BAILs and Brønsted-Lewis acidic IL (BLAIL) [35,36], as summarized in Table 1. All reactions were carried out under the conditions: GL/LA = 4 (mol/mol), catalyst amount (relative to LA) = 2 wt%, reaction time = 45 min, and reaction temperature = 423 K. It is noteworthy that all data depicted in Table 1 were average results from three parallel runs. In the absence of a catalyst (blank), a conversion of LA and GML yield of 47.0% and 39.7% were obtained, respectively (entry 1), which was inferior compared to those catalyzed by bare SG (entry 2), unsupported BAILs (entries 3–6) and BLAIL (entry 7), and supported BAIL/SG (entries 8–19) and BLAIL/SG (entry 20), as expected. Likewise, in the absence of acidic IL, the bare SG also exhibited a low LA conversion of 53.8% and GML yield of 45.9% (entry 2), which were only slightly better than that obtained from the blank experiment. On the other hand, a notable increase in reaction activity was observed over unsupported acidic ILs and supported IL/SG catalysts. The catalytic activities of various unsupported ILs followed the trend: [DMBPSH]HSO4 ≈ [DMBPSH]PTSA > [ 739

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Fig. 6. Variations of LA conversion and GML yield during the esterification of GL with LA over the 20%[DMBPSH]HSO4/SG catalyst as a function of (a) catalyst amount, (b) GL/LA molar ratio, (c) reaction temperature, and (d) reaction time. While varying each experimental variable, the other parameters were kept constant at their predicted values: catalyst amount = 2 wt%, GL/LA = 4.0, reaction temperature = 423 K, and reaction time = 45 min.

2.0–6.0 while other reaction parameters were kept constant. Initially, the catalytic performance was found to increase with increasing GL/LA ratio (see Fig. 6b). However, the GML yield reached a maximum (80.3%) with a LA conversion of 95.3% at GL/LA = 4.0 mol/mol. Moreover, the LA conversion reached a plateau while the GML yield decreased as the GL/LA ratio exceeded 4.0 mol/mol. This may be attributed to the dilution of the reaction system in the presence of excessive GL and the esterification of GML with GL to GDL. Fig. 6c displayed the influence of temperature (over the range of 393–443 K) on the LA conversion and GML yield while other reaction parameters were kept fixed. Again, it was found that the catalytic activity initially increased with increasing temperature, then, the GML yield reached a maximum (82.8%) at T = 413 K with a LA conversion of 94.3%. As the temperature was further increased, the LA conversion gradually levelled off and the GML yield descended rapidly. In general, a higher reaction temperature should favor enhancements in reaction rate as well as conversion efficiency. However, undesirable site reactions were prone to occur at elevated temperatures (T > 413 K), which led to a lowering of GML selectivity. Thus, it was indicative that the esterification of GL with LA over the 20%[DMBPSH]HSO4/SG catalyst would be eminent at a reaction temperature of 413 K under other given reaction conditions. The effect of reaction time on LA conversion and GML yield was also investigated under the reaction conditions: catalyst amount = 2 wt%, GL/LA = 4.0 mol/mol, and reaction temperature = 413 K. As shown in Fig. 6d, the LA conversion quickly reached a plateau within a reaction time exceeding 60 min while the GML yield reached a maximum of 82.8% (corresponding to a LA conversion 94.3%) at a reaction time of ca. 45 min. Since selective esterification of GL with LA was a consecutive reaction, the GML and/or GDL products may react with excessive GL during the reaction to spoil the selectivity and yield of GML at prolonged reaction time (> 45 min). The decreases in yield and selectivity of GML may be ascribed due to the enhanced formation of byproducts. Thus, in view of production cost and energy consumption, a

3.3. Effects of reaction parameters The influences of experimental variables such as catalyst amount, reactant molar ratio, reaction temperature, and reaction time during the esterification of GL with LA were further investigated by using 20%[DMBPSH]HSO4/SG as the model catalyst. The experiments were carried out by varying one variable while keeping other variables fixed under the following conditions: catalyst amount = 2 wt%, GL/ LA = 4.0, reaction temperature = 423 K, and reaction time = 45 min, as shown in Fig. 6. The effect of catalyst amount was assessed by varying the catalyst/ reactant (LA) mass ratio from 0.5 to 3 wt%, an initial GL/LA molar ratio of 4.0 (mol/mol), while performing the esterification reaction at 423 K for 45 min. As shown in Fig. 6a, the LA conversion increased almost linearly with increasing the catalyst amount up to ca. 3 wt%, then gradually levelled off afterward. Meanwhile, the GML yield also increased with increasing catalyst amount initially, reaching a maximum (80.3%) at a catalyst amount of 2 wt% in 45 min, and then decreased progressively when the amount of catalyst was further increased. It is anticipated that the amount of active acid sites available for the esterification reaction should increase with increasing catalyst amount. However, an excessive catalyst (or acid) amount tended to provoke formation of undesirable side-products such as diglyceride (GDL) or triglyceride (GTL). Thus, as the catalyst amount exceeded 2 wt%, the selectivity of GML was spoiled; these results were in a good agreement with the literature reported earlier [24]. Thus, a catalyst amount of 2.0 wt% should be desirable for the esterification of GL with LA to warrant an optimal catalytic performance (i.e., LA conversion and GML yield). Since esterification is a reversible reaction, an excessive amount of GL than LA is normally added in the reactant mixture, not only to enhance the reaction rate but also to shift the reaction equilibrium towards formation of esters. Herein, the effect of initial amount of reactants on catalytic activity was examined over a GL/LA molar ratio of 740

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reaction time of 45 min was most suitable for the selective esterification of GL with LA for the production of GML.

Table 4 List of ANOVA data for prediction of GML yield by the proposed quadratic model.

3.4. Process optimization Response surface methodology (RSM) [52] was exploited to optimize the process variables for enhancing GML yield and to explore the interactions between different variables based on a Box-Behnken design (BBD) [53]. Accordingly, three independent process variables, namely GL/LA molar ratio (x1), amount of catalyst (x2), and reaction temperature (x3), were chosen at three coded levels of experimental design with designated range, as depicted in Table 2. By means of multiple regression analysis, the response value Y (i.e., GML yield) may be predicted by means of an empirical model based on Eq. (3):

Y = 82.83 + 0.63x1 − 1.01x2 + 4.88x3 − 4.63x12 − 6.23x 22 − 4.53x 32 (12)

+ 1.17x1 x2 + 0.39x1 x3 + 0.76x2 x3

where x1, x2 and x3 are the coded values (cf. Table 2) derived from the three experimental variables based on Eq. (2). To verify the validity of the model, a total of 17 experiment runs, including 12 factorial points and 5 center points, were adopted for the production of GML through the esterification of GL with LA over the 20%[DMBPSH]HSO4/SG catalyst. The experimental values were depicted in Table 3 along with the experimental and predicted GML yields. It was noteworthy that each set of experimental run was repeated 3 times to afford estimation of experimental error. It was clear that no significant differences were found between the experimental and predicted values of GML yields, indicating the validity of the quadratic model in Eq. (12). Moreover, the fitting quality of the quadratic model was further evaluated by analysis of variance (ANOVA) and the results were summarized in Table 4. Since an F-value of 197.74 was much higher than its tabular counterpart (F0.01, 9, 7 = 6.71), the model was highly significant. This was also evidenced by the obtained P-value (< 0.0001), which justified that the probability for ascribing such a large model Fvalue observed due to noise was highly unlikely (< 0.01%). In addition, the obtained “Lack of fit” F-value of 2.32 also implied that it was insignificant relative to the pure error. As revealed by the coefficient of determination (R2 = 0.9961), the model was highly reliable in predicting the responses, > 99.6% of the experimental variables were covered. The above results coincided with the large “Adjusted R2” value of 0.9910 observed. Likewise, an “Adeq precision” of 39.442 observed was much greater than the desirable value of 4, revealing that the

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

Variable and value x2

x3

Experimental

Predicted

3.0 5.0 3.0 5.0 3.0 5.0 3.0 5.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0

1.0 1.0 3.0 3.0 2.0 2.0 2.0 2.0 1.0 3.0 1.0 3.0 2.0 2.0 2.0 2.0 2.0

413 413 413 413 403 403 423 423 403 403 423 423 413 413 413 413 413

73.25 72.97 68.61 73.02 68.89 68.56 77.98 79.22 68.87 65.61 77.01 76.78 82.62 83.16 83.46 82.46 82.43

73.51 72.43 69.15 72.75 68.54 69.02 77.52 79.57 68.95 65.42 77.20 76.70 82.83 82.83 82.83 82.83 82.83

DFa

Mean square

F-value

Prob > F

Significantb

Model x1 x2 x3 x12 x22 x32 x1x2 x1x3 x2x3 Residual Lack of Fit Pure Error Cor Total

589.38 3.18 8.16 190.71 90.43 163.38 86.38 5.50 0.62 2.30 2.32 1.47 0.85 591.70

9 1 1 1 1 1 1 1 1 1 7 3 4 16

65.49 3.18 8.16 190.71 90.43 163.38 86.38 5.50 0.62 2.30 0.33 0.49 0.21

197.74 9.59 24.64 575.86 273.05 493.35 260.81 16.60 1.86 6.93

< 0.0001 0.0174 0.0016 < 0.0001 < 0.0001 < 0.0001 < 0.0001 0.0047 0.2148 0.0338

**

2.32

0.2172

NS

DF denotes degree of freedom. Represents highly significant; nificant. b **

* ** ** ** ** ** **

*

*

represents significant; NS = not sig-

model was performed with desirable signal-to-noise ratio and adequate precision. Moreover, a relatively low coefficient of variation (C.V.) of 0.67% was observed, confirming that the experiments were reproducible and were conducted reliably. In summary, the three independent variables as well as their mutual interactions were mostly highly significant (except for the x1x3 term) to the esterification reaction. The correlations between mutual pair of process variables and their influence on GML yield may also be revealed by the three-dimensional (3D) response plots and contour plots (Fig. 7) obtained from the predicted model while keeping the other variable at a constant level of 0 (cf. Table 2). In general, the presence of a convex 3D response surface plot indicated the existence of a maximum in the response value (in this case, the GML yield). Whilst, the interaction between a pair of reaction variables may also be inferred from the shape of contour plots. The presence of an elliptical or saddle shape contour plot would indicate that the interaction between the two independent variables was significant. On the other hand, contour plots having nearly circular shape would reflect the weak or insignificant interaction between the variable pair. Moreover, the density of the response surface contour also reflected the influence of corresponding pair of variables on the response value. A denser contour curves would indicate a greater impact on the response value. At fixed reaction temperature and time, the correlation between GL/ LA molar ratio (x1) and catalyst amount (x2) and their influence on GML yield were shown in Fig. 7(a,d). The observed convex 3D response curve and corresponding elliptical contour plot revealed both interaction between x1 and x2 and their influence on the GML yield were both significant. In this context, the circular contour plot observed for x1 vs reaction temperature (x3) in Fig. 7e reflects that the correlation between them was insignificant, in excellent agreement with the ANOVA data (Table 4). Moreover, on the basis of the 3D response surface plot (Fig. 7b), it was clear that the influence of x3 on GML yield was more significant than x1. Likewise, the influences of x2 and x3 on GML yield may also be inferred from Fig. 7(c,f). It was clear that the influence of x3 on GML yield was more significant than x2. As such, it may be concluded that reaction temperature (x3) had the most significant impact on GML yield during the esterification reaction compared to x1 and x2. Additionally, in terms of interactions between each variable pairs and their effect on GML yield, the (x3, x2) pair showed more significant effects compared with that of (x3, x1) pair. The above results were also in excellent agreement with the ANOVA results shown in Table 4. On the basis of results obtained from RSM studies, which gave rise to predicted responses closely resembling the experimental values, an

GML yield (%)

x1

Sum of squares

a

Table 3 List of experimental designs and response values for the esterification of GL with LA over the 20%[DMBPSH]HSO4/SG catalyst. Run

Source

741

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Fig. 7. (a-c) 3D response surface and (d-f) contour plots showing variations between a pair of experimental variables (see Table 2) on predicted GML yield while keeping the other variable at a constant level of 0. Assignments: interactions between (a, d) GL/LA molar ratio (x1) vs catalyst amount (x2); (b, e) x1 vs reaction temperature (x3); (c, f) x2 vs x3.

(catalyst amount relative to LA) = 1.96 wt%, and x3 (reaction temperature) = 418.4 K. To further verify the accuracy of the proposed model, three sets of parallel experiments were carried out under the following conditions: x1 = 4.1 mol/mol, x2 = 2.0 wt%, and x3 = 418 K. These parameters led to an average experimental GML yield of 83.9% with LA conversion of 95.1%, which coincide with the predicted values. The above results therefore validated the accuracy of the model, which also reflected the influence of different process variables on GML yield during the esterification reaction. 3.5. Stability and recycling of the catalyst As mentioned earlier, one of the major advantages of the supported catalysts studied herein is their feasibility for catalyst separation and recycling. As an illustration, the stability of the 20%[DMBPSH]HSO4/ SG catalyst was tested for six consecutive experimental runs under the optimal operation conditions mentioned above, namely GL/LA molar ratio = 4.1 mol/mol; catalyst amount (relative to LA) = 2.0 wt%; reaction temperature = 418 K; reaction time = 45 min. After each run, the catalyst retreated from the reaction system was washed with ethyl acetate, and then dried under vacuum at 343 K for 10 h before reuse. As shown in Fig. 8, the 20%[DMBPSH]HSO4/SG catalyst exhibited good stability; the LA conversion decreased marginally from 95.1% of the

Fig. 8. Stability of the 20%[DMBPSH]HSO4/SG catalyst during the esterification of GL with LA. Reaction conditions: GL/LA = 4.1 mol/mol; catalyst amount = 2.0 wt%; reaction temperature = 418 K; reaction time = 45 min.

optimal GML yield of 84.19% was obtained for the esterification of GL with LA over the 20%[DMBPSH]HSO4/SG catalyst under the optimized reaction conditions: x1 (GL/LA molar ratio) = 4.08 mol/mol, x2 742

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first run to 86.8% after six consecutive running cycles, while the GML product yield gradually descended from 83.9 to 76.8%, respectively. Additional measurements by FT-IR spectroscopy revealed that the spent catalyst (Fig. 1f) obtained after six cyclic runs retained all characteristic bands observed for the fresh 20%[DMBPSH]HSO4/SG catalyst (Fig. 1e) except for slight decreases in their peak intensities. The decreases in catalytic activity may be due to the loss of [DMBPSH]HSO4 during recovery and regeneration processes. The spent 20%[DMBPSH]HSO4/SG catalyst after six runs was also examined by elemental analysis with ICP as well as BET surface area measurement. It was found that the content of the S element decreased from 10.2 wt% of the fresh catalyst to 8.5 wt % in the regenerated spent catalyst. In addition, the specific surface area of the 20%[DMBPSH]HSO4/SG catalyst also decreased from 76.2 to 55.4 m2/g after six consecutive runs. These together with the notable decrease in acidic strength observed for the spent catalyst (Fig. 5f) compared to it fresh counterpart (Fig. 5d), indicate that the lowering of catalytic activity in the spent catalyst may be attributed to the loss of [DMBPSH]HSO4 IL during the recovery and regeneration processes. The above results confirmed the stability and recyclability of the supported SAFIL/SG catalysts.

Table 5 List of k and R2 values at different temperatures during the esterification of GL with LA over the 20%[DMBPSH]HSO4/SG catalyst. Temperature (K)

k1

R12

k2

R22

403 408 413 418

0.04395 0.04856 0.05409 0.06173

0.9879 0.9896 0.9892 0.9915

0.04148 0.04598 0.05126 0.05866

0.9932 0.9897 0.9919 0.9885

obtained for the same reaction over the [DMBPSH]H3SiW12O40 (39.5 kJ/mol) [24], HSO3-functionalized SBA-15 (42 kJ/mol) [63], and zinc carboxylate (51 kJ/mol) [64], and esterification of ethylene glycol and acetic acid for the production of ethylene glycol diacetate using silica supported IL as the catalyst (67.6 kJ/mol) [65]. This indicates that the 20%[DMBPSH]HSO4/SG catalyst may be regarded as a highly effective catalyst for the esterification of GL with LA to GML.

4. Conclusions A series of silica gel (SG) supported sulfonic acid functionalized ionic liquid (SAFIL) catalysts were successfully synthesized and exploited for the production of glycerol monolaurate (GML) via esterification of glycerol (GL) with lauric acid (LA). Various SAFILs, namely [XILH]Z with different acidic proton-bonded cationic organic amine groups (XIL) such as N,N-dimethyl-(phenyl)ammonium (DMP), triethylamine (TEA), N-methyl imidazole (MIM) and pyridinium propyl sulfobetaine (PPS), and different anionic components (Z) of HSO4−, pCH3C6H4SO3−, H2PO4− and CF3COO− were prepared along with their SG supported counterparts, i.e., [XILH]Z/SG. Among various pristine [XILH]Z and [XILH]Z/SG catalysts examined, the supported catalyst with 20% SAFIL loading, namely 20%[DMBPSH]HSO4/SG was found to exhibit superior catalytic activity with a LA conversion of 95.3% and a GML yield of 80.3%, which were in close resemblance with the experimental values (LA conversion 95.1%, GML yield 83.9%) obtained under reaction conditions optimized by RSM based on the Box-Behnken design model. The relevant optimized process variables included: reactant molar ratio of GL/LA = 4.1 mol/mol, catalyst amount (relative to LA) = 2.0 wt%, reaction temperature = 418 K, and reaction time = 45 min. In addition, additional variable-temperature studies confirmed that the selective esterification of GL with LA to GML over the 20%[DMBPSH]HSO4/SG catalyst with an activation energy of 31.51 kJ/mol, surpassing other supported acid catalysts. Moreover, the supported SAFIL/SG catalysts were also found to possess desirable stability and recyclability rendering practical industrial applications for not only selective productions of esters but also a wide range of other acid-catalyzed reactions.

3.6. Kinetic model In order to establish the kinetic model for the 20%[DMBPSH]HSO4/ SG catalyst during the esterification of GL with LA, additional experiments were carried out under reaction conditions optimized by RSM, namely GL/LA = 4.1 mol/mol and catalyst amount = 2.0 wt% under different temperatures (403, 408, 413, and 418 K) and varied reaction time. During the reaction, ca. 1 mL sample was withdrawn from the mixture for analysis at reaction time of 5, 15, 25, 35, and 45 min, respectively. As a result, the corresponding rate constant during the esterification reaction under varied concentrations of LA (CLA) versus reaction time at various temperatures may be recorded based on Eq. (7). Taking the value of k1 got as an example, the plot of ln (CLA/CLA0) vs time at different temperatures fitted Eq. (8) well with a correlation, and the results were shown in Fig. 9a. Accordingly, the k1 values and the correlation coefficient R12 so deduced from various temperatures were summarized in Table 5. Similarly, the reaction rate constant k2 and its correlation coefficient R22 can be obtained by non-linear fitting of each experimental data CGML/CLA0 vs time according to Eq. (9) and Eq. (10), as shown in Fig. 10a and summarized in Table 5. Accordingly, a preexponential factor k0 = 5.34 × 102 L/mol⋅min and an activation energy Ea1 = 31.51 kJ/mol may be derived from the slope and intercept of the Arrhenius plot shown in Fig. 9b based on Eq. (11). Similarly, an activation energy Ea2 = 20.70 kJ/mol may also be derived from Fig. 10b. The E1 value so observed for the esterification of GL with LA to GML over the 20%[DMBPSH]HSO4/SG catalyst was lower than those

Fig. 9. (a) Variations of ln (CLA/CLA0) vs time and (b) Arrhenius plot for the esterification of GL with LA to GML over the 20%[DMBPSH]HSO4/SG catalyst. Reaction conditions: GL/LA = 4.1 mol/mol, and catalyst amount = 2.0 wt% under varied reaction time (5–45 min) and temperature (403–418 K). 743

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Fig. 10. (a) Variations of Y vs time and (b) Arrhenius plot for the esterification of GML with LA to GDL over the 20%[DMBPSH]HSO4/SG catalyst. Reaction conditions: GL/LA = 4.1 mol/mol, and catalyst amount = 2.0 wt% under varied reaction time (5–45 min) and temperature (403–418 K).

Acknowledgment

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