Production of Fuel Bioadditive âTriacetinâ Using a Phosphomolybdic

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Biofuels and Biomass

PRODUCTION OF FUEL BIOADDITIVE "TRIACETIN" BY USING PHOSPHOMOLYBDIC ACID LOADED PVA MEMBRANE IN PERVAPORATION CATALYTIC MEMBRANE REACTOR Derya Unlu, and Nilufer Durmaz Hilmioglu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03344 • Publication Date (Web): 06 Feb 2019 Downloaded from http://pubs.acs.org on February 7, 2019

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PRODUCTION OF FUEL BIOADDITIVE "TRIACETIN" BY USING PHOSPHOMOLYBDIC ACID LOADED PVA MEMBRANE IN PERVAPORATION CATALYTIC MEMBRANE REACTOR Derya Unlu1,a, Nilufer Durmaz Hilmioglu2,b,* (1)Bursa

Technical University, Chemical Engineering Department, 16310, Bursa, Turkey

(2)Kocaeli

University, Chemical Engineering Department, 41380, Kocaeli, Turkey (a) [email protected], (b) [email protected]

*Corresponding author : Nilufer Durmaz Hilmioglu Tel: +90 262 303 35 45 ; Fax: +90 262 359 12 62 E-mail address: [email protected], [email protected] Postal address: Department of Chemical Engineering, Engineering Faculty, Kocaeli University, 41380 Kocaeli, Turkey 1 ACS Paragon Plus Environment

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Abstract The objective of this paper is to synthesize fuel bioadditive triacetin by the innovative membrane process with high yield under mild operating conditions. Phosphomolybdic acid loaded Polyvinyl alcohol (PVA) membrane was prepared as a composite catalytic membrane. The composite catalytic membrane is placed in the membrane reactor cell. Water is separated continuously from the reaction medium by using pervaporation process for the improvement of the glycerol conversion and triacetin selectivity. The effects of the operation conditions on the glycerol conversion, triacetin selectivity, and separation performance are examined in detail. While the glycerol conversion was 53% for 7 h by using the phosphomolybdic acid loaded PVA membrane pieces in the batch reactor, this value was 100% in 7 h by using phosphomolybdic acid loaded PVA membrane under the equal operating circumstances (reaction temperature 75◦C, catalyst concentration 5 wt.% and molar feed ratio 6:1) in the esterification coupled with pervaporation process. Furthermore, while triacetin selectivity was 0.6% in a batch reactor, this value was 76% in esterification coupled with pervaporation process. As a result of the study, esterification coupled with pervaporation process is found as an efficient process for the synthesis of triacetin with high selectivity. Key words: Catalytic membrane, dehydration, glycerol, pervaporation, triacetin.

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1.Introduction Rapid population growth and increasing industrial developments cause the depletion of petroleum reserves, the global warming, and environmental problems while biomass-derived renewable biofuels attract a considerable attention due to the decrement of the environmental pollution by reducing CO2 emissions. Among these biofuels, biodiesel gets attention as a clean and cheap alternative fuel. 1 Biodiesel is one of the developing renewable sources due to properties such as the biodegradability, non-toxicity, and the absence of sulfur or aromatic species.1-3 Glycerol, which is a by-product of the biodiesel production process, greatly affects the process economy. Stoichiometrically, 10 kg of glycerol is produced for each 100 kg of biodiesel4. The increasing of the biodiesel production has incurred to the accumulation of byproduct glycerol. Therefore, the production of valuable chemicals from glycerol is rather important.5 In this study, the esterification reaction of glycerol with acetic acid was investigated. The products of this reaction are monoacetyl glycerol (MAG), diacetyl glycerol (DAG) and triacetyl glycerol (TAG). While the MAG and DAG are used in polyester, cosmetic and medical industries, TAG is used as a cold flow improver and anti-knock additive for biodiesel and gasoline, respectively. 6-9 The esterification reaction of glycerol with acetic acid is a reversible equilibrium reaction. Figure 1 shows the reaction mechanism of the esterification of glycerol with acetic acid.3

Figure 1. Esterification of glycerol with acetic acid 3 ACS Paragon Plus Environment

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There are two basic methods for changing the equilibrium toward the products in reversible reactions: excess reactant usage and reactive distillation.10-12 According to the principle of reversible reaction, water in the reaction medium changes the reaction equilibrium toward the backward side. This situation causes the decrement of selectivity of the desired product TAG. In literature, acetic anhydride is used to obtain high TAG selectivity. The acetic anhydride consumes the water and reaction equilibrium shifts toward the product side.7,

13

However,

acetic anhydride is a high toxicity reactant and the cost of acetic anhydride is four times higher than the cost of acetic acid.7 Pervaporation process is an innovative, alternative process for overcoming the disadvantages such as the use of harmful chemical acetic anhydride and unreusable catalyst in the reaction medium. Pervaporation is a membrane based separation process for the liquid mixtures. In this process, the selective component in the feed permeates through the nonporous membrane. The chemical potential gradient is the driving force of the pervaporationprocess.12, 14-17 In this process, a catalytic membrane is used which has catalytic and separation functions. The reaction mixture consists of the reactants only; the catalyst is placed in the polymeric membrane. In the pervaporation membrane reactor, the reaction and separation processes occur in the same region. The obtainment of products from reactants carry out on the membrane surface as different from the batch reactor. While the catalytic plate of the membrane is responsible for the reaction, the separation plate of the membrane is responsible for the separation of the selective component. Thus, the reaction and separation operations are carried out in one step.18-19Catalytic membranes are one of the heterogeneous catalysts, and they can be obtained by impregnation of catalyst in the polymeric matrix. Polymeric catalytic membranes have higher surface area and easy recovery properties. The chosen of an appropriate polymeric medium is important for the dissolution and diffusion properties of reactants and the increment of the catalytic and separation performance. The 4 ACS Paragon Plus Environment

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expanding diffusion channels in the membrane are responsible for the diffusion of the reactants. As the diffusion channels in the polymer matrix increases, the reactants can be reached to active sites of the catalyst in the membrane easily. The zone between the reaction mixture and the catalytic layer of the membrane is the region in which the reaction takes place. The reaction occurs in this region and high conversion values are obtained with catalytic membrane.

In literature, the esterification reaction of glycerol with acetic acid was carried out by using high temperature, high catalyst amounts and high molar feed ratio in the batch reactor.20, 21However,

the esterification reaction of glycerol with acetic acid coupled with pervaporation

process has not been studied in the literature, yet. Higher conversion and selectivity values were observed under low reaction conditions such as low reaction temperature, low catalyst amount and low molar feed ratio in pervaporation membrane reactor according to the studies in the literature that was used the batch reactor. Also, the phosphomolybdic acid loaded PVA membrane was utilized only for separation process in pervaporation in the literature

22,23.

Phosphomolybdic acid loaded PVA membrane was also act as a catalyst besides the separation property in the pervaporation membrane reactor in only this study, so this study is a rather original study. Some studies of esterification coupled with pervaporation membrane reactor that is used the catalytic membrane in literature have been reviewed. The studies in the literature about the impacts of pervaporation operation on the enhancement of reaction yield have been ordered below. Qing et al. investigated the synthesis of butyl acetate by Zr(SO4)2.4H2O doped PVA/PES catalytic composite membrane. A roughly 43 % of conversion increase was obtained in the membrane reactor according to the batch reactor.24Zhang et al. synthesized lipase loaded biocatalytic PVA/PES membrane for the reaction of stearic acid with lauryl 5 ACS Paragon Plus Environment

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alcohol. Due to the separation of water by the membrane from the reaction medium in the pervaporation membrane reactor, conversion value was more 40% than the batch reactor.25 Zhang et al. prepared resin loaded polyvinyl alcohol/PES catalytic membranes for the synthesis of n-butyl acetate. Pervaporation enhanced the conversion from 71.9% to 91.4%.26 Zhu and Chen present a synthesis of butyl acetate by the catalytically active membrane. Zr(SO4)2.4H2O and PVA were used as a catalyst and polymeric material for the catalytic membrane, respectively. While the 95% of acid conversion was observed with pervaporation at 90◦C, 10.6 g/L of catalyst amount and 50% of acid conversion was obtained without pervaporation under the same reaction conditions.27 Figueiredo et al. examined the acetic acid and ethanol esterification reaction in a batch reactor and in a PVCMR. Amberlyst 15 coated PVA membrane was synthesized for the usage at the pervaporation membrane reactor. The reaction was attempted at 90◦C, molar feed ratio of 2:1, the catalyst amount of 3 wt.%. Under these circumstances, 45% and 96% conversion values are achieved with batch reactor and with pervaporation membrane reactor, respectively.28Chandane et al. synthesized the polyvinyl alcohol–polyethersulphone (PVA–PES) membrane for the production of isobutyl propionate. Experiments occurred in both of the batch reactor and PVCMR. The conversion of propionic acid reaches 89.82 % at 90oC by using the catalytic membrane in pervaporation membrane reactors, while that is around 65.17 % in the batch reactor.29 The overall study objective was to attain the highest triacetin selectivity and glycerol conversion under the mild operating conditions. The composite catalytic membrane was prepared and used as a catalyst both of batch reactor and esterification coupled with pervaporation process. Polyvinyl alcohol and phosphomolybdic acid were chosen as polymeric membrane material and catalyst, respectively. Conversion of glycerol, selectivities of the reaction products of MAG, DAG and TAG were calculated. The flux and selectivity values were used to determination of separation performance of esterification coupled with 6 ACS Paragon Plus Environment

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pervaporation process. The conversion values in esterification coupled with pervaporation process were compared with the obtained conversion values in the batch reactor. Esterification coupled with pervaporation process was found as an efficient process for enhancement of conversion values. 2. Experimental 2.1. Materials Polyvinyl alcohol (PVA), Phosphomolybdic acid, glutaraldehyde were obtained from Sigma Aldrich. Glycerol and acetic acid were purchased from Merck Chemicals. 2.2. Phosphomolybdic acid loaded catalytic composite PVA membrane synthesis The composite membrane with catalytic properties comprises of catalytic and separation layers. Firstly, the separation layer of the composite membrane was synthesized. Polyvinyl alcohol as polymer was preferred due to its hydrophilic properties. 10 wt.% of PVA solution was prepared at 90 ◦C to form a homogeneous solution by stirring.

The obtained

homogeneous solution was cast on the glass petri dish, so the separation layer of the membrane was formed. 5 wt.% of PVA solution was prepared and stirring at 90 ◦C for the catalytic layer of the membrane. 0.3 ml of glutaraldehyde was added to the polymeric membrane solution for the in situ crosslinking. For the preparation of composite membrane, the nonporous PVA separation layer was coated with catalytic membrane solution containing different ratios of PMA such as 5, 10, and 15 wt. %. These ratios were defined as the PMA amount with respect to the mass of the polymer were used in the catalytic layer of the membrane. In this situation; while the 5 gr PVA is used for the preparation of the catalytic layer of the membrane; 5, 10, and 15 wt. % of PMA means 0.25, 0.50 and 0.75 g PMA amount immobilized in the membrane, respectively. The prepared composite catalytic 7 ACS Paragon Plus Environment

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membrane was left to dry at ambient temperature. The thickness of the phosphomolybdic acid loaded PVA membrane was measured at about 100 μm using a micrometer. Also, the active surface area of the membrane was 28.26 cm2. 2.3. Esterification reaction of glycerol with acetic acid The esterification reaction of glycerol with acetic acid was carried out by using phosphomolybdic acid loaded catalytic composite membrane in the batch reactor and in the pervaporation catalytic membrane reactor. Phosphomolybdic is a Keggin-type heteropolyacid. It has high catalytic activity, stability and selectivity. Therefore, phosphomolybdic acid was chosen as a catalyst. While the catalytic membranes were cut into 4 mm2small pieces by using membrane scissors and used as a catalyst in the batch reactor, whole composite catalytic membranes were placed in the membrane chamber for esterification coupled with pervaporation process. Temperature, catalyst concentration and the molar feed ratio of acetic acid to glycerol as the reaction parameters were examined for the reaction and separation performance.


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Figure 2. Process systems (a) Batch reactor and (b) Esterification coupled with pervaporation (1) thermometer, (2) stirrer, (3) reflux condenser, (4) reactor, (5) catalytic membrane, (6−7) cold traps, and (8) vacuum pump. Batch reactor and esterification coupled with pervaporation systems were shown in Figure 2 (a) and (b). Batch reactor studies were performed in a 100 mL three-necked flask equipped with a thermometer, a reflux condenser, and a stirrer. Heating was provided with a heater. Initially, the reactants were heated separately to the desired reaction temperature and fed to the reactor with a feed pump, then mixed. Small catalytic membrane pieces were added to the reaction mixture and so the reaction was assumed to have started. Pressure value during the reaction is atmospheric pressure in the batch reactor. Collected reaction samples by using a pasteur pipette were analyzed at hourly intervals. The same reaction was also carried out in esterification coupled with pervaporation process. The catalytic composite membrane was assembled in the membrane cell. The reactants were heated to the desired reaction temperature separately and fed to the membrane reactor. The reaction temperature was kept constant with the heater jacket in which a heated water flow around the reactor. The diffusion of the selective component through the membrane was provided by a vacuum pump. The feed solution was supplied to the membrane at atmospheric pressure while the permeate side was kept at 2000 Pa. The differences in the pressure both side of the membrane create driving force. This pressure difference caused the phase change. The selective component was obtained as vapor and it was condensed in liquid nitrogen by traps. Reaction performance was determined by glycerol conversion and product selectivity at both of the reactor. Conversion of glycerol was calculated by Equation 2.30


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Conversion of glycerol =

Number of glycerol moles consumed x100 Initial moles of glycerol

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(2)

The selectivity values of MAG, DAG and TAG were calculated using Equations 3, 4 and 5.3

Product Selectivity =

MAG (mole) x100 MAG + DAG + TAG (mole)

(3)


DAG (mole) x100 MAG + DAG + TAG (mole)

(4)


TAG (mole) x100 MAG + DAG + TAG (mole)

(5)

The separation performance of the esterification coupled with pervaporation process is characterized by flux and separation selectivity. Equation 6 and 7 are used for the calculation flux and separation selectivity, respectively.

J=

m A.t

(6)

m defines the mass of the compound in the permeate stream, A is the active surface area of the membrane, t is the time.

α=

Pa / Pb Fa / Fb

(7)

P and F display the concentration of components as weight fraction in the feed side and permeate side. For the water selectivity, a and b show the water and acetic acid in the sample, respectively.31 The membrane has a higher affinity for the water due to the hydrophilic nature of the polymeric membrane material. Membrane does not permit transfer of the MAG, DAG and TAG as organic components due to the hydrophobicity and molecular size of them. A


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little amount of acetic acid as organic component is diffused through the diffusion channels expanded of the membrane. Therefore, component b is assumed only acetic acid in Eq. 7 at this study.

Conversion, product selectivity, and separation selectivity values were determined by the gas chromatography (GC). The GC (Agilent, 7820A) was coupled with a flame ionization detector and a DB-1 column.

The repeatability of conversions was within ±1.5 %. The repeatability of the product selectivity was about ±10 % level. The reproducibility of flux and selectivity values were also within ±2 %. Experimental studies were replicated three times and glycerol conversion, flux and selectivity parameters were determined. Also, analyses were replicated three times for every experiment. It means that every sample was examined three times for the determination of conversion, flux and selectivity values. 2.4. Characterization of phosphomolybdic acid loaded catalytic composite PVA membrane 2.4.1. Scanning electron microscopy (SEM) The surface and cross-section structures of phosphomolybdic acid loaded PVA membrane were viewed with the JEOL/JSM-6510-LV Scanning electron microscope. Catalytic composite membrane samples were fractured at liquid nitrogen temperature and covered by a thin gold layer. Thequalitative chemical analysis in the Energy Dispersive X-Ray Analysis (EDX)was performed with an Oxford Instruments X-Max 80 Aztec EDX system.


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2.4.2. Fourier transform infrared spectroscopy (FTIR) The Fourier transforms infrared spectroscopy was used to analyze the differences in the chemical bond structure between the pristine and composite catalytic membrane. The analyses were performed by means of a Thermo Nicolet 5700 spectrometer, in the range between 400 and 4000 cm-1 with a resolution of 4 cm-1. 2.4.3. Thermogravimetric Analysis (TGA) The thermal properties of catalytic composite membranes were determined by thermogravimetric analysis (TGA). Thermal stability of membranes was analyzed in a programmed temperature range of 25 to 600°C using Mettler Toledo thermal analyzer with the scanning rate of 10°C. 2.4.4. Acid values of the catalytic membranes The acid values of the catalytic membranes were calculated by a titration method. The catalytic membranes were cut into the small pieces. These catalytic membrane samples were left in the aqueous NaCl solution for a day. After 24 h, the solution was analyzed by titration with 0.1 mol/L sodium hydroxide (NaOH). The acid values of the catalytic membranes were found by Eq. (1).

Q=

C NaOH .VNaOH Wdry

(1)

where Q, acid capacity value (meq/g); CNaOH, the concentration of NaOH (mol/L); VNaOH, the volume of NaOH (mL); Wdry, the mass of the dry catalytic membrane (g).32


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3. Results and discussion 3.1. Characterization of catalytic composite membrane 3.1.1. FTIR The FTIR spectra of the pristine and catalytic PVA membranes are shown in Figure 3.

Figure 3. FTIR spectra of the membranes (a) Pristine PVA membrane (b) phosphomolybdic acid loaded catalytic PVA membrane Figure 3.(a) shows the FTIR spectrum of the pristine PVA membrane. The characteristic – OH, C–H, C=O, and C–O bands in PVA can be seen at 3278, 2924, 1710 and 1085 cm−1 for the pristine membrane. Figure 3.(b) illustrates the FTIR spectrum of the catalytic PVA membrane. The bands at 807, 957, 1079, 1420, 1653, 1740, 2926, 2975 cm−1indicate the presence of phosphomolybdic acid in the catalytic membrane. The band intensity of –OH around 3288 cm−1demonstrates a decrement of the intensity compared to the pristine PVA 13 ACS Paragon Plus Environment

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membrane. This situation indicates the interaction between phosphomolybdic acid and –OH bonds in PVA membrane through (Mo Ot), (Mo–Oc–Mo) and (Mo–Oe–Mo).33 3.1.2. TGA The thermal decomposition of the catalytic membrane was determined by thermal gravimetric analysis (TGA). TGA profile of the phosphomolybdic acid loaded catalytic PVA membrane was presented in Figure 4.

Figure 4. TGA curves for the phosphomolybdic acid loaded catalytic composite PVA membrane. The TGA of phosphomolybdic acid loaded catalytic membrane shows mass loss between 180200°C. The weight loss which has been occurred before 180oC, is related to the evaporation 14 ACS Paragon Plus Environment

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of water molecules. A decrease in the weight of the catalytic membrane between 250 and 420◦C is due to decomposition or oxidation of the side and main chains. The mass loss in the temperature range from 420°C to 500°C is thought to be related to the reaction H3PMo12O40→(1/2) P2O5 + 12MoO3 + (3/2)H2O (Mo=W).33 3.1.3. SEM The surface image of the phosphomolybdic acid loaded catalytic PVA membrane is shown in Figure 5.

Figure 5. SEM images of the phosphomolybdic acid loaded catalytic PVA membrane (a) surface view and SEM-EDX analysis (b) cross-section view. The presence of phosphomolybdic acid catalysts on the membrane surface is seen as the small white particles in Figure 5(a). Catalysts had distributed uniformly in the catalytic layer of the membrane. To verify the presence of phosphomolybdic acid into the membrane, the SEMEDX was utilized. The P, Mo and O elements in the membrane were seen clearly in the EDX analysis in Figure 5 (a). Figure 5(b) is the cross-section image of the catalytic membrane.


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Catalytic layer and separation layer have seen clearly. While the top layer of the composite structure is the catalytic layer, the bottom layer is the dense separative layer. 3.2. Determination of the reaction time The experiments of determination of reaction time were carried out in the esterification coupled with pervaporation process, because the obtainment of high TAG selectivity value is difficult in the batch reactor. Reaction conditions are as follows: the reaction temperature 75oC, molar ratio of acetic acid to glycerol 3:1, 5wt.% phosphomolybdic acid loaded catalytic PVA membrane. Figure 6 represents the change in the conversion of glycerol and selectivity values of MAG, DAG, and TAG with the reaction time.

Figure 6. Effect of reaction time on conversion of glycerol and selectivity of products in esterification coupled with pervaporation process (T =75°C, Ccat=5 wt.%, M=3 mol/mol). 16 ACS Paragon Plus Environment

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Literature studies have indicated that MAG selectivity is higher during the first hours of the reaction. This situation has been verified by experimental studies.3,32-33 The reaction was carried out in esterification coupled with pervaporation process for 7 hours. At the end of the first hour of the experiment, the MAG has high selectivity value. After that hour, while the selectivity of MAG decreased due to the reaction of MAG with acetic acid, the selectivity of DAG and TAG increased. As the reaction progressed, the selectivity of the DAG decreased due to the conversion of DAG to TAG. Glycerol conversion was also showed the close conversion values in the last reaction hours. For these reasons, reaction time was determined as 7 h. 3.3. Effect of temperature on reaction and separation performance To determine temperature effect on glycerol conversion and selectivity, pervaporation catalytic membrane reactor and batch reactor tests were realized by a 5 wt.% phosphomolybdic acid catalyst loaded catalytic PVA membrane at the temperature of 65, 75 and 85oC. The molar ratio of glycerol to acetic acid was 3:1 and reaction time was 7 h. Figure 7 shows the effect of the reaction temperature on the glycerol conversion in the batch reactor and in the esterification coupled with pervaporation process.


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Figure 7. Effect of the reaction temperature on the glycerol conversion (Ccat=5 wt.%, M=3 mol/mol) Temperature is a significant parameter for the reaction conversion and product selectivity.34 The high temperature results in high reaction conversion, reaction rate and ester production distribution.35 Figure 7 displays that the temperature is increased the conversion of glycerol both in the batch reactor and in esterification coupled with pervaporation process. When the reaction temperature was risen from 65 to 85oC, conversion value of glycerol increased from 47% to 61% in the batch reactor and increased from 89% and 100% in the esterification coupled with pervaporation process. High temperature accelerates ester and water formation. Furthermore, temperature facilitates the mobility of the polymer chains in the membrane, diffusion of the 18 ACS Paragon Plus Environment

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reactants and separation of water from the reaction mixture. Reaction equilibrium shifts toward the product side. Thus, esterification coupled with pervaporation process gives the higher conversion value according to the batch reactor.36 Table 1 presents the variations in selectivity of MAG, DAG, and TAG versus operating temperature in the batch reactor and in the esterification coupled with pervaporation process. The esterification reaction of glycerol with acetic acid is a consecutive endothermicreaction and the high temperature is required.32Therefore, forward reaction rate accelerates with the temperature due to the endothermic properties of reaction.37-38 Table 1. Effect of temperature on MAG, DAG and TAG selectivity (Ccat=5wt.%, M = 3mol/mol, reaction time=7 h) Reactor Type

Temperature (oC)

Batch Reactor

Esterification coupled with

Product Selectivity (%)

MAG

DAG

TAG

65

93

6.8

0.1

75

63

37

0.2

85

53

46

0.3

65

3.6

46

51

75

2.4

38

59

85

1.0

26

73

pervaporation process


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The selectivity values of the products also depend on the temperature. The formation of MAG was higher at low temperatures as 65oC in the batch reactor at the beginning. TAG selectivity was fairly low in the batch reactor under the equal operating circumstances. As the temperature increased, MAG selectivity decreased and the TAG selectivity increased in both of the batch reactor and pervaporation membrane reactor.34 Besides the temperature, the effect of the pervaporation process is also important in this case. The pervaporation process removes the water during the esterification reaction and increases TAG formation.7 While the TAG selectivity was obtained as 0.3 in the batch reactor, it was 73 in the esterification coupled with pervaporation process. The separation capacity of the membrane is also influenced by the reaction temperature. The flux and selectivity values of the membrane were demonstrated in Table 2. Table 2. Effect of the reaction temperature on flux and separation selectivity (Ccat=5wt.%, M=3mol/mol) Flux (kg/m2.h) Temperature

Separation Selectivity

Total

Water

Acetic acid

Water

Acetic acid

65

0.58

0.58

0.0

3.7x104

8x10-5

75

0.78

0.78

0.0

5.4x103

2 x10-3

85

1.10

1.08

0.02

1.6 x103

0.04

(oC)


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The membrane has a higher affinity for the water due to the hydrophilic nature of the polymeric membrane material. It does not allow the transport of the MAG, DAG and TAG. A little amount of acetic acid is only diffused. The obtained values show that the membrane exhibits excellent separation performance. As the temperature increased, separation selectivity decreased and total flux increased. At low temperatures, only water molecules diffused through the membrane. As for the higher temperature, polymer chains had more elastic due to the plasticization effect. Thus, total flux increased and water selectivity decreased.35 While the total flux increased from 0.58 to 1.10, the water selectivity decreased from 3.7x104 to 1.6 x103 with an increase in the temperature from 65∘C to 85∘C. 3.4. Effect of catalyst concentration on reaction and separation performance The different ratios of phosphomolybdic acid were loaded on the membrane (5-10-15 wt.%) to observe the effect of catalyst concentration on the reaction and pervaporation operation. Experimental studies were carried out at 75oC of temperature, M=3 (mol/mol) of molar feed ratio. The conversion values of glycerol were shown in Figure 8.


Energy & Fuels 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 8. Effect of catalyst concentration on the glycerol conversion (75oC, M=3 mol/mol) Catalyst concentration is described as the weight percentage of the PVA amount in the membrane solution. The reaction accelerates by increasing the concentration of the catalyst because the amount of catalyst is directly proportional to the amount of catalyst.39 The increment of catalyst concentration on the membrane surface led to an increase in the number of acid sites. Thus, reactants contact easily with catalytically active regions of the membrane.32 The conversion of glycerol increased with the increase of the catalyst concentration both in the batch reactor and in the esterification coupled with pervaporation process. Esterification coupled with pervaporation process combines catalytic reaction and separation functions. Both the reaction and the separation behavior have a significant function in the determination of the performance of the reactor. An increase in the amount of catalyst 22 ACS Paragon Plus Environment

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affects the formation and removal of water.39 Therefore, the higher conversion value was obtained in esterification coupled with pervaporation process according to the batch reactor. While the glycerol conversion for the catalyst concentration of 5%, 10%, and 15% were 48%, 54%, and 58% in the batch reactor at the end of the 7 hours. Under the same reaction conditions, conversion values were obtained as 88%, 96%, 100% in the esterification coupled with pervaporation process. These results are also related to the acidity values of catalytic membranes. The presence of acidic groups informs about the acid values of the catalytic membranes. The catalytic activity of membrane is related to the acid values of the catalytic membranes. Acid values of 5, 10, 15 wt. % phosphomolybdic acid loaded catalytic membranes are 0.58, 0.108 and 0.176 meq/g, respectively. The membrane in which catalyst concentration is 15 wt. % has the highest acidity, because of the number of phosphomolybdic acids amounts in the membrane structure increases. Therefore, conversion value has the highest value on the membrane in which catalyst concentration is 15 wt. %. The reaction was also carried out by the homogeneous catalyst and blank PVA membrane without acid. While the 37 % conversion value was obtained by using the homogeneous catalyst, the 61% conversion value was obtained by using the blank PVA membrane without acid. These conversion values are rather low according to the catalytic membrane. The change of selectivity values of MAG, DAG and TAG with the catalyst concentration is shown in Table 3.


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Table 3. Effect of catalyst concentration on MAG, DAG and TAG selectivity (75oC, M=3mol/mol, reaction time=7 h) Catalyst Reactor Type

concentration


MAG

DAG

TAG

5

67

32

0.2

10

57

43

0.3

15

51

48

0.5

5

4

43

53

10

1

41

58

15

0.9

29

70

(wt.%)

Batch Reactor

Esterification coupled with pervaporation process

As the catalyst concentration increases, the concentration of H+ ions increases. These H+ ionsprotonate the acetic acid molecules and activate. Thus, acetic acid becomes more reactive and formation of DAG and TAG increases.40 Esterification coupled with pervaporation process is an important alternative process due to its properties such as the absence of additional chemicals, the removal of water by an environmentally friendly process, low production and investment cost and high reaction and separation yield. The highest TAG selectivity value was obtained as 70 in the esterification


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Energy & Fuels

coupled with pervaporation process while 0.5 of TAG selectivity was obtained in the batch reactor. The success of the pervaporation process The success of the pervaporation process is specified by the flux and selectivity parameters. Table 4. gives the influence of catalyst concentration on the flux and selectivity values. Table 4. Effect of the catalyst concentration on flux and separation selectivity (75oC, M=3 mol/mol) Flux(kg/m2.h) Catalyst


Total

Water

Acetic acid

Water

Acetic acid

5

0.54

0.54

1x10-4

2.7x104

2x10-4

10

0.79

0.79

7.9 x10-5

9.2x104

1x10-4

15

0.87

0.87

0

∞

0

Concentration (wt. %)

The water amount in reaction mixture increases as the concentration of the catalyst increases. High water concentration in the reaction mixture creates a driving force for water removal, flux increases.41


Energy & Fuels 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

Alongside catalyst properties in the reaction, phosphomolybdic acid acts as a filler and crosslinking agent in the membrane. There are some studies in the literature where phosphomolybdic acid is added to the membrane as filler for the separation of binary mixtures.42-43 The presence of phosphomolybdic acid in the structure of the membrane increases the crosslinking density and causes a decrease in membrane swelling. The protons in the structure of the phosphomolybdic acid provide the interaction with the hydroxyl groups in the PVA polymer. The diffusion channels in the polymer matrix are decreased and thus the swelling degree is decreased. It only forms hydrogen bonds with water and allows only permeation of water.43 Therefore, the high catalyst concentration results in high partial flux and water selectivity. Transport of acetic acid decreases with the increment of the concentration of phosphomolybdic acid and reaches to zero at 15 wt. % phosphomolybdic acid concentration. The water flux rises from 0.54 to 0.87 when the phosphomolybdic acid concentration increases from 5 wt. % to 15 wt. %. 3.5. Effect of molar feed ratio on reaction and separation performance Batch reactor and esterification coupled with pervaporation process tests were realized by using phosphomolybdic acid catalyst loaded PVA catalytic membranes at the different molar feed ratio of acid / alcohol (M = 1, M = 3 M = 6), keeping the other reaction parameters constant (Ccat =15wt. % and T = 70 °C). Figure 9 presents the change of glycerol conversion with time at different molar feed ratio.


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Energy & Fuels

Figure 9. Effect of molar feed ratio on the glycerol conversion (Ccat=5 wt.%, T=75oC) The usage of excess reactant shifts the equilibrium to the forward side of the reaction.43 The excess acetic acid results in an increment of conversion of glycerol. However, this operation increases the cost of the process due to the post-reaction additional separation step and requirement of large reactor volume.38, 44 For these reasons, M = 1, M = 3 and M = 6 were chosen as the molar feed ratios in this study. M=1 is a value under the stoichiometry of the reaction. Therefore, conversion and selectivity values were very lower at M=1 molar feed ratio.45As seen in Figure 9, the conversion increases with molar feed ratio in both batch reactor and esterification coupled with pervaporation process. These obtained results are the expected results for reversible reactions.


Energy & Fuels 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

As the water amount in the reaction medium increased by conversion, removal of water from esterification coupled with pervaporation process was increased and glycerol conversion in an esterification coupled with pervaporation process was obtained higher than in a batch reactor. While conversion of glycerol was calculated as 53% in the batch reactor, conversion of glycerol was achieved as 100% in the esterification coupled with pervaporation process under the same reaction conditions. The molar feed ratio considerably affects the distribution of the product. The formation of more DAG and TAG are observed with increasing molar ratio. The selectivities of MAG, DAG and TAG are given in Table 5 by the increase of the molar feed ratio in the batch reactor and in esterification coupled with pervaporation process.


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Energy & Fuels

Table 5. Effect of molar feed ratio(mol/mol) on MAG, DAG and TAG selectivity (Ccat=5wt.%, T=75oC, reaction time=7 h) Reactor Type

Molar Feed


Ratio

Batch Reactor

Esterification coupled with

MAG

DAG

TAG

1

89

11

0.1

3

61

38

0.2

6

47

52

0.6

1

2

50.1

48

3

0.3

45

55

6

0.2

23

76

pervaporation

The use of excess acetic acid acts as an extra acetylation agent and results in the formation of the high amount of glycerol acetate.3 The other advantage is the low viscosity value. The mass transfer resistance between reactants and the catalyst is eliminated.35 Thus, the selectivity values increase with the conversion. In esterification coupled with pervaporation process, besides the molar feed ratio, the removal of water also causes a double effect for the improvement of selectivity, and the selectivity values of DAG and TAG increase as well. Therefore, the highest TAG selectivity value was obtained as 76 at M=6:1 in the esterification coupled with pervaporation process.


Energy & Fuels 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

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The flux and selectivity values in the esterification coupled with pervaporation process are given in Table 6. Table 6. Effect of the molar feed ratio on flux and separation selectivity (Ccat=5wt.%, T=75oC) Flux(kg/m2.h)

Molar Feed Ratio


Total

Water

Acetic acid

Water

Acetic acid

1

0.39

0.39

1x10-4

4.3x104

3x10-3

3

0.51

0.50

9x10-3

4.7x103

0.02

6

0.72

0.56

0.16

1.4x103

0.10

The water concentration in the reaction solution increases at the high molar feed ratio due to the increment of conversion. As the reaction progresses, the accumulation of water in the reaction mixture causes the driving force for the removal of the water. Thus, the membrane has high water removal rate, that is, the flux value increases. The high water amount on the upper side of the membrane causes the swelling of the membrane. In addition, the use of excess acid causes the diffusion of acetic acid with water together and the selectivity of the water decreases, and the selectivity of the acetic acid increases. While the water selectivity decreases from 4.3x104 to 1.4x103, the acetic acid selectivity increases from 3x10-3 to 0.10.


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Energy & Fuels

3.6. Reusability of catalytic PVA membrane loaded with phosphomolybdic acid The reusability of the catalytic PVA membrane was tested for five times under the equal operating conditions of 65◦C, acetic acid to glycerol molar ratio of 3:1, catalyst concentration of 5 wt.% in esterification coupled with pervaporation process.

Figure 10. Catalytic membrane reusability and stability in esterification coupled with pervaporation process (T=65°C, Ccat=5 wt.%,, M=3 mol/mol) In the reusability tests of the PMA loaded catalytic PVA membrane; it was found that the membrane maintained its catalytic activity during the tests. As seen in Figure 10, while conversion of glycerol was obtained as 92% after the first use, 88% of glycerol conversion was obtained after the fifth use. These results were also supported by acidity value of catalytic membranes. The acidity values of catalytic membranes have the same value during the first to 31 ACS Paragon Plus Environment

Energy & Fuels 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

fifth runs. While the acid capacity of the membrane was 0.176 meq/g after the first usage, it was found as 0.175 meq/g after the fifth usage.

Figure 11. SEM images of the used catalytic membrane (a) surface view (b) cross-section view During reusability experiments, it was not observed structural deformation. The catalytic membrane was tested with SEM analysis after 5 times usage. Catalyst particles can be seen easily on the surface and cross-section images of the membrane in Figure 11. This situation shows that the membrane has a stable catalytic activity as well as a strong mechanical resistance. 3.7. Comparison of conversion and selectivity values with literature Table 7 compares the obtained conversion and selectivity values from the esterification reaction of acetic acid with glycerol with literature studies. There is no esterification coupled with pervaporation application for this reaction in the literature.


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Energy & Fuels

Table 7. Comparison of conversion and selectivity values reported in the literature for the reaction of acetic acid with glycerol. Reaction Conditions

Selectivity

Conversion

Catalyst T

C

M

t

55

0.25

0,01

24

100

0.1

10

120

0.3

60

References Glycerol

MAG

DAG

TAG

SO4-2/ZrO2

62.1

98.3

1.7

-

20

8

MoO3/SiO2

97

40

38

22

46

6

7

H3PW12O40/Si

99

24

70

5

21

0.06

3

8

H3PW12O40

62

68

32

0

47

60

0.06

3

8

H3SiW12O40

100

42

53

5

47

60

0.06

3

8

FePMo12O40

57

28

72

0

47

60

0.03

3

8

H3PW12O40

96

66

34

0

40

60

0.03

3

8

PTSA

85

86

8

0

40

105

0.03

8

12

Amberlyst-15

95.6

70.3

4.5

0

2

110

0.4

6

3

SBAH-15

100

14

67

19

6

120

0.8

8

3

91

38

29

34

6

97.1

7.8

47.7

44.5

35

Activated carbon 110

-

9

5

Amberlyst-15 H3PMo12O40

This study

loaded PVA 85

0.25

3

7

61

53

catalytic

46

0.3 (BR)

membrane


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This study

H3PMo12O40 loaded PVA 85

0.25

3

7

100

1.0

26

73

catalytic

(Esterification coupled with

membrane

pervaporation)

H3PMo12O40 This study

loaded PVA 75

0.25

6

7

53

47

52

0.6

catalytic

(BR)

membrane This study

H3PMo12O40 loaded PVA 75

0.25

6

7

100

0.2

0.1723

76

catalytic

(Esterification coupled with

membrane

pervaporation)

*T:Temperature (oC), C:Catalyst amount (g), M:Molar feed ratio (Acid/Alcohol), t:time(h)

As can be seen in Table 7, the obtained selectivity values in this study are higher than the studies in the literature under similar conditions. The esterification coupled with pervaporation process is also eco-friendly and economical alternative to the traditional processes due to the advantages such as the absence of additional chemicals (such as acetic anhydride) and the carrying out of experimental studies under the mild reaction conditions (minimum molar feed ratio, low temperature and low amount of catalyst). 4. CONCLUSION The productive conversion of glycerol to triacetin with composite catalytic membrane has been successfully performed in esterification coupled with pervaporation process. The composite catalytic membrane was synthesized by using the phosphomolybdic acid heteropolyacid catalyst. PVA was preferred as a polymeric material for membrane due to its 34 ACS Paragon Plus Environment

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hydrophility, high chemical, thermal and mechanical stability. Phosphomolybdic acid loaded PVA membrane has been shown superior catalytic activity and purification performance. The influences of reaction time, temperature, catalyst concentration and molar feed ratio of acid to alcohol were studied on the reaction and pervaporation performance of phosphomolybdic acid loaded PVA membrane in the PVCMR process. Under mild process conditions, PVCMR has higher reaction conversion and separation selectivity values according to the batch reactor in the literature. The results exhibit the effect of water separation from the reaction medium on reaction conversion. A 100% conversion of glycerol and 76% selectivity of triacetin are obtained using the phosphomolybdic acid loaded PVA catalytic composite membrane catalyst under the reaction conditions as reaction temperature 75◦C, catalyst concentration 5 wt.% and molar feed ratio 6:1. The selectivity value of triacetin in esterification coupled with pervaporation process is 30% higher than the values which were obtained from the literature under the same operating conditions. In addition, the catalytic composite membrane was displayed excellent separation properties. Only water and a very little amount acetic acid from the reaction mixture diffused across the membrane. These results show that phosphomolybdic acid loaded PVA catalytic composite membrane has good catalytic and separation properties. The obtained results exhibit that the esterification coupled with pervaporation process is very promising in the synthesis of triacetin by the esterification reaction of glycerol with acetic acid. Acknowledgements The authors are grateful to Kocaeli University Scientific Research Projects Coordination Unit for the financial support under the project number 2017/067.


Energy & Fuels 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

NOMENCLATURE α: Selectivity A: Active surface area of membrane (cm2 ) Ccat: Catalyst concentration (wt.%) DAG: Diacetin Fa, Fb: Concentration of water and acetic acid in the feed J: Flux (kg/m2.h) m: Permeate mass (g) M: Molar feed ratio MAG: Monoacetin Pa, Pb: Concentration of water and acetic acid in the permeate PVA: Polyvinyl alcohol t: Time (h) TAG: Triacetin


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(36) Chandane, V.S.; Rathod, A.P.; Wasewar, K. L. Coupling of in-situ pervaporation for the enhanced esterification of propionic acid with isobutyl alcohol over cenosphere based catalyst. Chem. Eng. Process. Process Intensif. 2017, 119, 16–24. (37) Karakus, S. Productıon of ıso-butyl acrylate by pervaporatıon-esterıfıcatıon hybrıd process. Master Thesis, Ege University, Graduate School of Natural and Applied Sciences, İzmir, 2014. (38) Sert, E.; Atalay, F.S. N-Butyl acrylate production by esterification of acrylic acid with nbutanol combined with pervaporation. Chem. Eng. Process. Process Intensif. 2014, 81, 4147. (39) Jyoti, G.; Keshav, A.; Anandkumar, J. Review on Pervaporation :Theory, Membrane Performance, and Application to Intensification of Esterification Reaction. Journal ofEngineering. 2015, 2015, 1-24. (40) Goncalves, C.E.; Laier, L.O.; Cardoso, A.L.; Silva, M.J.D. Bioadditive synthesis from H3PW12O40-catalyzed glycerol esterification with HOAc under mild reaction conditions. Fuel Process. Technol. 2012, 102, 46–52. (41) Torabi, B.; Ameri, E. Methyl acetate production by coupled esterification-reaction process using synthesized cross-linked PVA/silica nanocomposite membranes. Chem. Eng. J.2016, 288, 461–772. (42) Magalad, V.T., Gokavi, G.S.; Raju, K.V.S.N.; Aminabhavi, T.M. Mixed matrix blend membranes of poly(vinyl alcohol)-poly(vinyl pyrrolidone) loaded with phosphomolybdic acid used in pervaporation dehydration of ethanol. J. Memb. Sci. 2010, 354, 150–161. (43) Zhu, M. H.; Feng, Z. J.; Hua, X. M.; Hu, H.; Xia, S.L.; Hu, N.; Yang, Z.; Kumakiri, I.; Chen, X.S.; Kita, H. Application of a mordenite membrane to the esterification of acetic acid and alcohol using sulfuric acid catalyst. Microporous Mesoporous Mater. 2016, 233, 171– 176.


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(44) Qing, W.; Chen, J.; Shi, X.; Wu, J.; Hu, J.; Zhang, W. Conversion enhancement for acetalization using a catalytically active membrane in a pervaporation membrane reactor. Chem. Eng. J. 2016, 313, 1396–1405. (45) Wang, Z.Q.; Zhang, Z.; Yu, W.J.; Li, L.D.; Zhang, M.H.; Zhang, Z.B. A swelling changeful catalyst for glycerol acetylation with controlled acid concentration. Fuel Process. Technol. 2016, 142, 228–234. (46) Kotbagi, T.V.; Pandhare, S.W.; Dongare, M.K.; Umbarkar, S.B. In situ Formed Supported Silicomolybdic Heteropolyanions: Efficient Solid Catalyst for Acetylation of Glycerol. Int. J. Environ. Anal. Chem. 2015, 2, 1-5. (47) Silva, M.J.D.; Liberto, N.A.; Leles, L.C.D.A.; Pereira, U.A. Fe4(SiW12O40)3-catalyzed glycerol acetylation: Synthesis of bioadditives by using highly active Lewis acid catalyst. J. Mol. Catal. A Chem. 2016, 422, 69-83.


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Energy & Fuels

FIGURE CAPTIONS Figure 1. Esterification of glycerol with acetic acid. Figure 2. Process systems (a) Batch reactor and (b) Esterification coupled with pervaporation (1) thermometer, (2) stirrer, (3) reflux condenser, (4) reactor, (5) catalytic membrane, (6−7) cold traps, and (8) vacuum pump. Figure 3. FTIR spectra of the membranes (a) Pristine PVA membrane (b) phosphomolybdic acid loaded catalytic PVA membrane Figure 4. TGA curves for the phosphomolybdic acid loaded catalytic composite PVA membrane. Figure 5. SEM images of the phosphomolybdic acid loaded catalytic PVA membrane (a) surface view and SEM-EDX analysis (b) cross-section view. Figure 6. Effect of reaction time on conversion of glycerol and selectivity of productsin esterification coupled with pervaporation process(T =75°C, Ccat=5wt.%, M=3mol/mol). Figure 7. Effect of the reaction temperature on the glycerol conversion (Ccat=5wt.%, M=3mol/mol). Figure 8. Effect of catalyst concentration on the glycerol conversion (75oC, M=3mol/mol). Figure 9. Effect of molar feed ratio on the glycerol conversion (Ccat=5wt.%,T=75oC). Figure 10. Catalytic membrane reusability and stability in esterification coupled with pervaporation process (T=65°C, Ccat=5wt.%,, M=3mol/mol). Figure 11. SEM images of the used catalytic membrane (a) surface view (b) cross-section view. 43 ACS Paragon Plus Environment

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TABLE CAPTIONS Table 1. Effect of temperature on MAG, DAG and TAG selectivity (Ccat=5wt.%, M = 3mol/mol) Table 2. Effect of the reaction temperature on flux and separation selectivity (Ccat=5wt.%, M=3mol/mol) Table 3. Effect of catalyst concentration on MAG, DAG and TAG selectivity (75oC, M=3mol/mol) Table 4. Effect of the catalyst concentration on flux and separation selectivity (75oC, M=3mol/mol) Table 5. Effect of molar feed ratio(mol/mol) on MAG, DAG and TAG selectivity (Ccat=5wt.%, T=75oC) Table 6. Effect of the molar feed ratio on flux and separation selectivity (Ccat=5wt.%, T=75oC) Table 7. Comparison of conversion and selectivity values reported in the literature for reaction of acetic acid with glycerol.


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