Insight into a catalytic process for simultaneous production of biodiesel and glycerol carbonate from triglycerides

Accepted Manuscript Title: Insight into a catalytic process for simultaneous production of biodiesel and glycerol carbonate Authors: Manali S. Dhawan, Ganapati D. Yadav PII: DOI: Reference:

S0920-5861(17)30542-4 http://dx.doi.org/10.1016/j.cattod.2017.08.020 CATTOD 10963

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Catalysis Today

Received date: Revised date: Accepted date:

5-4-2017 3-8-2017 10-8-2017

Please cite this article as: Manali S.Dhawan, Ganapati D.Yadav, Insight into a catalytic process for simultaneous production of biodiesel and glycerol carbonate, Catalysis Todayhttp://dx.doi.org/10.1016/j.cattod.2017.08.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Revised: Manuscript ID: CATTOD-D-17-00141

Insight into a catalytic process for simultaneous production of biodiesel and glycerol carbonate

Manali S. Dhawan; Ganapati D. Yadav* Department of Chemical Engineering Institute of Chemical Technology Nathalal Parekh Marg, Matunga, Mumbai-400 019, India Email: [email protected], [email protected] Tel.: +91-22-3361-1001, Fax: +91-22-3361-1020

*Author to whom correspondence should be addressed

1

Graphical abstract

Fuel Triglycerides

Fatty acid methyl esters (Biodiesel)

Methanol

97.3 % conversion 93.2 % selectivity

Glycerol carbonate

Dimethyl carbonate

Synthesis of polymers, Solvent for electrolytes

HIGHLIGHTS  One pot simultaneous co-production of biodiesel and glycerol carbonate.  Reusable hydrotalcite (with Mg:Al 3:1) as base catalyst.  Characterization of uncalcined, calcined and rehydrated forms of hydrotalcite.  Detailed kinetics of the cascade reaction.  Clean and green process

ABSTRACT Biodiesel, a renewable liquid fuel derived from triglycerides is a promising alternative to compensate for the increasing demand of petro-diesel. However, 10 % w/w glycerol is coproduced in the biodiesel production process which reduces the efficiency. The present work deals with the one-pot simultaneous co-production of biodiesel and glycerol carbonate from 2

in-situ coproduced glycerol using hydrotalcite as a catalyst. Uncalcined hydrotalcite, Calcined hydrotalcite and Rehydrated hydrotalcite were screened for their activity towards the reaction in terms of conversion of soybean oil triglycerides and selectivity of glycerol carbonate. Uncalcined hydrotalcite served best for this purpose. At oil to methanol mole ratio of 1:90 and oil to DMC mole ratio of 1:30, the reaction gave 97.3 % conversion of triglycerides and 93.2 % selectivity of glycerol carbonate at 150 oC in 3h at a catalyst loading of 0.0125 g/cm3. The virgin and reused catalysts were well characterized using various analytical techniques which reveals the presence of basic as well as acidic sites with high surface area and ordered pore-size distribution. Effects of various experimental parameters on the conversion of triglycerides and selectivity to glycerol carbonate were studied to interpret the reaction kinetics. The kinetic rate constants and the activation energies were calculated. Reusability study of the catalyst was done up to two cycles and the catalyst was found to be robust and reusable. Keywords: Biodiesel; triglycerides; glycerol; glycerol carbonate; hydrotalcite; transesterification. NOMENCLATURE w

Catalyst loading (g/L)

k

Rate constant, (L2/(mol.g-cat.s))

ri

Rate of consumption or formation of any species ‘i’(mol/ (g-cat.s))

Ci

Concentration of any species ‘i’, (mol/L)

Ea

Activation energy (kcal/mol)

G

Glycerol

M

Methanol

TG

Triglyceride

DG

Diglyceride 3

MG

Monoglyceride

GC

Glycerol Carbonate

DMC

Dimethyl Carbonate

E1, E2, E3

Fatty Acid Methyl Esters

GDC

Glycerol Dicarbonate

1. Introduction Increasing energy requirements and environmental pollution along with the predicted shortages of fossil fuels in near future have accelerated the search for alternative renewable energy sources [1,2]. Biodiesel is an alternative renewable fuel derived from vegetable oils or animal fats. Its properties such as higher flash point and cetane number, biodegradability, low sulfur content and small carbon footprint make it more fuel efficient as compared to petrodiesel [3–5]. Biodiesel is a mixture of fatty acid alkyl esters produced by transesterification of triglycerides with alcohols, usually methanol and ethanol [6,7]. This process is aided by enzymatic as well as chemical alkali or acid catalyzed processes [7–14] or by non-catalytic supercritical processes [15]. Homogeneous base catalysts such as alkali metal carbonates, hydroxides and alkoxides are widely used in commercial processes because they are cost-effective and result in high activity at mild reaction conditions [10,16]. However, these processes require additional steps for recovery of glycerol and purification of waste streams [7]. Thus, many heterogeneous base and acid catalysts such as alkaline metal oxides, hydroxides, hydrotalcites [11–13], heteropolyacids [14] and solid superacid [17] catalysts were developed and evaluated for biodiesel production processes since they could be easily removed from the reaction mixture and no polluting by-products were formed, making the purification step easier. However, 10% w/w glycerol is co-produced in the biodiesel production process, which reduces the process efficiency [18–21]. Increasing demand of biodiesel has led to an increase in the glycerol production simultaneously [19]. Glycerol is treated as an industrial waste in small scale plants. Glycerol can be utilized as a potential platform chemical in large 4

scale plants but this requires further purification processes [20,21]. Even though glycerol has many applications in fields such as textile, cosmetics, pharmaceuticals, and food industries, but it stills remains an excessively supplied chemical in the market [21,22]. Several valueadded chemicals could be produced using glycerol as a platform molecule [22–27]. If glycerol formed in the reaction as a co-product could be utilized in-situ to produce another value added chemical simultaneously, then the whole biodiesel production process could be made more efficient. Valorization of glycerol to produce glycerol carbonate by chemical, enzymatic and supercritical methods has been extensively researched over the years [28–33]. Glycerol carbonate (GC) is a colorless and stable value added chemical derived from glycerol. It has varied applications such as synthesis of polymers, surfactants, solvents for electronic electrolytes, novel gas separation membranes, cosmetics, and a green substitute for propylene carbonate and ethylene carbonate, etc. [34]. Recently, there have been studies on coproduction of biodiesel and GC using triglycerides with dimethyl carbonate as reactant. Dimethyl carbonate (DMC) is an eco-friendly, noncorrosive and non-toxic chemical with versatile chemical reactivity [35,36]. Enzymatic co-production of biodiesel and GC using lipase as a catalyst has been reported under mild reaction conditions but it requires long reaction time [37–40]. Tan et al. [32] and Ilham and Saka [41–44] reported simultaneous coproduction of biodiesel and GC by supercritical processes but it requires high temperature and pressure. Most literatures on DMC-mediated production of biodiesel are either centred on supercritical methods or lipase catalysed processes. These methods are expensive and difficult to commercialize. Various research works have been reported on coproduction of biodiesel and GC using homogenous catalyst such as hydroxides [45–47] and methoxides [48–50] with high conversions of triglycerides. However, these processes require additional extraction and purification procedures [48]. Recently, Tang et al. [51] reported biodiesel and GC production using tricomponent mixture of oil/methanol/DMC using CaO as a catalyst. A very few reports have been published on simultaneous co-production of biodiesel and GC using a heterogeneous catalyst. Hydrotalcites were intended to do this thing. Hydrotalcites are clay minerals represented by general formula M2+1-xMx3+ (OH2)x+ (Ax/n) n-.yH2O, where M2+ and M3+ represent divalent and trivalent cations, respectively and An-is an intercalated anion with charge n and x usually has a value between 0.25 and 0.33. In Mg-Al hydrotalcites, some of the Mg2+ ions are replaced 5

by Al3+ ions. The charge balancing anions, usually CO32- ions along with the water molecules are located in the interlayer region between the brucite-like layers. Calcination of Mg–Al hydrotalcite at appropriate temperatures gives rise to Mg–Al mixed oxide possessing high surface area and Lewis basicity. The structure of hydrotalcite is recovered by rehydration yielding Bronsted basicity because the anions are replaced by hydroxyls [52,53]. In the present work, one-pot simultaneous co-production of biodiesel and GC was aimed using hydrotalciteas a heterogeneous base catalyst. The co-product of conventional transesterification reaction - glycerol, was in situ utilized to produce a value added coproduct- GC, simultaneously. This is an example of cascade engineered synthesis of biodiesel and GC whereby the cost of processing is reduced using a single pot catalytic reaction. Also, a concentration profile of the reactants, intermediates and the products was derived and on the basis of that reaction kinetics was studied. The kinetic rate constants and the activation energies of all the individual reaction steps were calculated. The virgin and reused catalysts were well characterized using various analytical techniques. 2. Material and methods 2.1. Chemicals Refined vegetable soybean oil was procured from local market. The composition of fatty acids in the soybean oil was as follows: palmitic acid 11.9%, stearic acid 5.6%, oleic acid 26.8%, linoleic acid 49.7%, linolenic acid 5.9% and 0.1 % other fatty acids, according to Gas Chromatograph (Chemito model GC 1000) analysis. The chemicals were obtained from following sources: DMC, methanol, sodium hydroxide, sodium carbonate, glycerol(S.D. Fine Chemicals Ltd, Mumbai),Magnesium nitrate hexahydrate, aluminum nitrate nonahydrate, concentrated sulphuric acid (98 %), methanol HPLC grade, isopropyl alcohol HPLC grade, nhexane HPLC grade (Thomas Baker (Chemicals), Mumbai), methyl palmitate (97%), methyl oleate (96%), methyl stearate (99%), methyl linoleate (Alfa Aesar, Mumbai), glycerol carbonate (Sigma Aldrich). 2.2. Catalyst synthesis Uncalcined hydrotalcite (HT) of Mg:Al ratio 3:1 was synthesized by co-precipitation method. Solution of magnesium nitrate (38.3 g, 0.15 mol) and aluminium nitrate (18.15 g, 0.05 mol) in 150 mL distilled water was prepared. Solution of sodium hydroxide (18 g, 0.45 mol) and anhydrous sodium carbonate (15.9 g, 0.132 mol) in 150 mL distilled water was prepared. 6

Both the solutions were added simultaneously to the round bottom flask (500 mL) with overhead stirrer at a speed of 300 rpm immersed in oil bath at temperature 30˚C. White precipitate of HT obtained was further digested at 60˚C for 12 h. HT obtained was washed with distilled water to get neutral pH of the supernatant solution. Then HT was dried at 100 ˚C for 24 h to obtain dry lumps. HT lumps were crushed in mortar to get fine powder of weight 14.2 g. HT powder was calcined at 500˚C for 6 h to get calcined HT (CHT) [54]. The CHT was sonicated in deionized water for 1 h and subsequently filtered and dried at 100 ˚C to obtain rehydrated hydrotalcite (RHT) [55]. 2.3. Synthesis of Glycerol Dicarbonate (GDC) as Model Compound Glycerol (2 g, 0.021 mol), DMC (19.5 g, 0.217mol) and K2CO3 as a catalyst (0.3 g, 0.002 mmol) were placed into a 50 ml glass reactor and immersed in the oil bath equipped with impeller, water-cooled condenser and temperature controller. The reaction took place at 75 ˚C for 72 h (Scheme 1). The residual methanol and excess DMC was distilled off using rotary evaporater [39]. GDC was confirmed by GC-MS (Thermo Scientific Trace Gas Chromatograph equipped with an ISQL LT single quadrupole Mass Spectrometer) using RTX-5 column (150mm x 0.25 mm, 0.25 µm). 2.4. Reaction Procedure All transesterification experiments were carried out in 100 cm3 high pressure autoclave (Amar Equipments, Mumbai) equipped with bladed pitched turbine impellor, temperature controller (±1˚C), pressure indicator (kg/cm2) and speed of agitation (±5 rpm). The reaction was solvent free. The control experiment consisted of 0.0045 mol soybean oil, 0.403 mol methanol (1:90 molar ratio) and 0.135 mol DMC (1:30 molar ratio) (total volume = 32cm3) with catalyst loading 0.0125 g/cm3 (0.4 g) at a speed of agitation of 1000 rpm at 150˚C. The samples were withdrawn periodically and then filtered to remove the catalyst particle remaining in the sample, if any. 2.5. Analysis Conversion of soybean oil triglycerides was monitored by HPLC (Agilent 1200 Infinity series) using Agilent C18 reverse phase column (250 mm x 4.6 mm, 5 µm) with gradient elution of methanol and isopropyl alcohol-hexane 5:4 (v/v). Analysis was done using UV detector at a wavelength 205 nm with injection volume 10 µL, phase flow rate 1.0 ml min-1 and temperature 40ºC. The calculations were based on limiting reactant soybean oil 7

triglyceride. The details about the gradient method of elution used are given in supplementary information (Table SI). Analysis of the concentration of GC and GDC formed was done by HPLC (Agilent 1200 Infinity series) using PL Hi Plex-H column (300 mm × 7.5 mm) with isocratic elution of 0.005 M sulphuric acid. Analysis was done using RI detector. The calculations were made on the basis of standard calibration curve of pure GC compound. The products were confirmed by GC-MS (Thermo Scientific Trace 1300 Gas Chromatograph equipped with an ISQL LT single quadrupole Mass Spectrometer) using RTX-5 column (150 mm x 0.25 mm, 0.25 µm) (Supplementary Information). 3. Results and discussion 3.1. Catalyst characterization The fresh and spent catalyst was characterized by different techniques such as XRD, FTIR, TPD, SEM, TEM, nitrogen BET surface area and DSC-TGA. The details about characterization method can be found in earlier literature[56,57] and have been provided in supplementary information (ESI). 3.1.1. XRD XRD patterns of HT, CHT, RHT and reused HT are shown in Figure 1. The XRD pattern of HT showed sharp reflections at 11.72º, 23.22º, 34.8º, 38.68º, 45.65º, 60.7º, and 61.98º which corresponds to (003), (006), (012), (015), (018), (110), (113) planes respectively (JCPDS 22700). These planes are characteristic of a highly crystalline Mg-Al layered double hydroxide structure. Crystallite size of HT was found to be 16.73 nm obtained from Scherrer equation. Reused HT is showing similar reflections as shown by HT which confirms the catalyst stability after reuse. The XRD pattern of CHT suggested that after calcination of the HT material at 500 ˚C for 6 h, the layered double hydroxide was converted to Mg(Al)O mixed oxide depicted by the reflections at 43.43º, 62.88º which correspond to the (200) and (220) planes (JCPDS 45-0946). In calcined samples, intensity of peaks decreases which reveal amorphous form of material. The XRD pattern of RHT showed similar reflections as of HT catalyst. RHT is showing the characteristic peaks of hydrotalcite. This indicates that after rehydration of CHT, the layered double hydroxide structure was recovered. This memory effect is a characteristic of hydrotalcites [52]. 3.1.2. FT-IR

8

FTIR patterns of HT, CHT, RHT and reused HT are shown in Figure 2. The hydrotalcite catalysts showed -OH stretching vibration in the brucite like layer at approximately 3500 cm-1 range, presence of water at 1635-1660 cm-1 and carbonates at 1250-1500 cm-1, bands for Mg-O and Al-O at 423 cm-1 and 657 cm-1 respectively. CHT showed decrease in intensity of FTIR bands of 1635-1660 cm-1, 1250-1500 cm-1, 3500 cm-1 region. This is attributed to the loss of water and carbonated species upon calcination [53]. RHT showed increase in the intensity of FTIR bands of 3500 cm-1 and 1635-1660 cm-1 due to the presence of high water content. Reused HT showed similar pattern of FTIR spectrum as of HT. 3.1.3. Surface area analysis Surface area and pore size were analyzed by nitrogen adsorption-desorption method. The Brunauer-Emmett-Teller (BET) surface area, pore volume and pore size of HT, CHT, RHT and reused HT are mentioned in Table 1. After calcination, an increase in surface area and pore volume was observed which is due to the loss of carbonate anions. The adsorption– desorption isotherms of HT and CHT showed type IV isotherm which is a characteristic of a mesoporous solid with hysteresis loop of type H1 indicating pores open at both ends and narrow necked (Figure 3). RHT also showed type IV isotherm but hysteresis loop of type H3, indicating wide pores with narrow openings. The decrease in surface area of RHT may be attributed to the closure of the mesopores after rehydration as reported in some of the earlier literatures [58]. Reused HT shows similar textural properties as that of fresh HT. This shows the fidelity of the catalyst after reuse. 3.1.4. SEM SEM micrographs of HT, CHT, RHT and reused HT are shown in Figure 4. The particles show irregular morphology. Reused HT shows similar agglomerates of particles as that of fresh HT. 3.1.5. TEM The TEM images of HT show particles of somewhat less irregular morphology with more uniformity in their respective crystallites (Figure 5). 3.1.6. EDAX

9

Elemental composition of HT, CHT, RHT and reused HT catalyst was analyzed by EDAX. Actual mole ratios are matching with theoretical mole ratio taken for catalyst preparation (Table 2). 3.1.7. DSC-TGA The DSC-TGA analysis was carried out to see the thermal stability of samples. DSC curve for HT is shown in Figure 6. The DSC curve of HT showed two endotherms. The endotherm at approximately 228 ˚C is attributed to the loss of water and CO2. The endotherm at approximately 410 ˚C is due to the loss of hydroxide and interlamellar ions [13]. Figure 7 shows the TGA analysis of catalysts HT, CHT, RHT and reused HT respectively. HT shows total mass loss of over 42 % and CHT shows total mass loss of over 14.9 %. Thus, the mass loss decreases after calcination. RHT shows total mass loss of 88.5 %. Reused HT shows a similar mass loss (42.4 %) as compared to fresh HT. 3.1.8. TPD TPD analysis was carried out using NH3 and CO2 as probe molecule for finding acidity and basicity of samples, respectively. TPD analysis of the catalysts confirmed presence of both acidic and basic sites. Low temperature CO2 desorption peaks were observed around 160-220 ºC corresponding to weak basic sites of concentration 1.35 and 1.33 mmol g-1 in HT and CHT respectively (Figure 8). Low temperature NH3 desorption peaks were observed around 190-250 ˚C corresponding to weak acidic sites of concentration 0.84 and 0.87 mmol g-1 in HT and CHT respectively (Figure 9). 3.2. Simultaneous co-production of biodiesel and GC The overall reaction is depicted by Scheme 2 in which Step 1 is the transesterification reaction of triglyceride with methanol to produce biodiesel and glycerol and Step 2 is the transesterification reaction of glycerol and DMC to produce GC. When transesterification of soybean oil was carried out using methanol alone, both biodiesel and glycerol were produced in the reaction. However, when DMC was used solely as a reactant, the reaction did result in biodiesel production but GC was not observed. This brings us to the conclusion that the initiation of reaction with methanol was required. Thus, methanol and DMC were both used as reactant and solvent in the reaction. Various combinations of mole ratios were examined for the reaction by keeping DMC in excess at one time and methanol at the other. When 10

methanol was added in excess, an ample amount of glycerol was created in the reaction for DMC to act upon. The simultaneous co-production of biodiesel and GC was sought out in this work. The effect of various parameters on the conversion of soybean oil triglycerides and concentration of GC were studied with triglyceride as a limiting reactant. Concentration profiles of reactants, intermediates and products were plotted as a function of time. 3.2.1. Catalyst screening The transesterification of soybean oil with methanol and DMC for simultaneous coproduction of biodiesel and GC was carried out using HT, CHT and RHT as catalyst. Typical reaction conditions were maintained for 3 h as oil to methanol mole ratio of 1:90, oil to DMC mole ratio of 1:30 with constant total volume (32 cm3) at 150 oC in 3 h with stirring speed of 1000 rpm at a catalyst loading of 0.0125 g/cm3. The hydrotalcite catalysts were found to have appreciable activity towards the transesterification reaction with varied selectivities towards GC. The activity of these catalysts was evaluated for the reaction in terms of conversion and selectivity towards the desired products under similar experimental conditions (Figure 8). Order of catalysts conversion and selectivity towards GC was as follows: HT (97.3 %, 93.2 %) > CHT (82.4 %, 68.5 %) > RHT (99.7%, 29.9 %). The selectivity of GC was calculated by taking into consideration GC and GDC. After 75 min, there is a decrease in the concentration of GC due to its further conversion into GDC. The formation of GDC was confirmed by GC-MS by comparing it with the mass spectra of synthesized GDC (Figure S8, Supplementary Information). HT was found to have better activity as compared to other catalysts. This is due to high basicity of the uncalcined catalyst because of the presence of more hydroxyl groups. RHT showed the best activity towards the conversion of triglycerides and formation of biodiesel but the selectivity towards GC was tremendously low. Upon rehydration of the calcined catalyst, all the oxide groups were converted to the hydroxyl groups which favoured high activity for the transesterification of triglycerides to biodiesel. However, excess amount of water present in RHT confirmed by TGA analysis (Figure 7) might have hindered the transesterification of glycerol to GC. HT was selected as the best catalyst for the reaction considering both the activity and selectivity of the catalyst towards the desired product. Hence it was used in further experimental studies. 3.2.2. Effect of speed of agitation The effect of speed of agitation was evaluated in the range from 600–1000 rpm with catalyst loading of 0.0125 g/cm3, mole ratio of soybean oil to methanol 1:90, mole ratio of soybean 11

oil to DMC 1:30 with constant total volume (32 cm3) at 150˚C (Figures 9 and 10). The conversion profile of triglycerides and concentration profile of GC at 800 and 1000 rpm showed no significant change thereby indicating the absence of external mass transfer resistance for transport of reactants and products. Thus, further experiments were performed at a speed of 1000 rpm. 3.2.3. Effect of catalyst loading The catalyst loading was varied from 0.00625 to 0.01875 g/cm3 keeping the total volume and the other variables constant. With increase in catalyst loading, the conversion of triglycerides increased (Figure11). This is because of the increase in the number of active sites with increase in catalyst loading. However, a decrease in concentration of GC was noticed after 75 min in the reaction (Figure 12). This may be because of the increase in active sites which favours the formation of GDC. Hence 0.0125 g/cm3 catalyst loading was chosen for further studies. 3.2.4. Effect of Mole Ratio 3.2.4.1. Effect of Mole Ratio of Soybean oil to Methanol The mole ratio of soybean oil triglycerides to methanol was varied from 1:60 to 1:120 keeping the catalyst loading (0.0125 g/cm3) and mole ratio of oil to DMC constant (1:30) at 150 °C (total volume = 32 cm3) (Figures 13 and 14). Increase in mole ratio of soybean oil to methanol resulted in high conversion of triglycerides. The excess of methanol was required to initiate the reaction and also it acts as a solvent to dissolve the intermediate glycerol, product GC besides making the initial reaction mixture less viscous. There was not much increase in the final conversion of triglyceride and concentration of GC when mole ratio was increased from 1:90 to 1:120. Hence, mole ratio of 1:90 (oil:methanol) was found optimum and used for further reactions. 3.2.4.2. Effect of Mole Ratio of Soybean oil to DMC The mole ratio of soybean oil triglycerides to DMC was varied from 1:20 to 1:40 with a catalyst loading of 0.0125 g/cm3 at 150 °C keeping the mole ratio of oil to methanol constant (1:90) ( Figures 15 and 16). High mole ratio of soybean oil to DMC favours the reaction by shifting the reaction equilibrium towards the products. There was no significant increase in the final conversion of triglyceride and concentration of GC when mole ratio was increased 12

from 1:30 to 1:40. Hence, mole ratio of 1:30 (oil:DMC) was found optimum and used for further studies. 3.2.5. Effect of temperature The effect of temperature was observed on the reaction in the range of 150-180 ºC keeping the other parameters constant (Figures 17 and 18). The rate of reaction increased with increase in temperature from 150 to 180 ˚C. However, high temperature favoured the formation of GDC as soon as GC was formed. This led to a decrease in the selectivity of GC at higher temperatures. Hence, 150 ºC was selected as the optimum temperature for the reaction. 3.2.6. Development of kinetic model The various reactions taking place with base catalyst are depicted below (Scheme 3 and Scheme 4).

The net rate of methanol consumption by the reactions mentioned in Scheme 3 and 4 is as follows: rM  

dCM  w.{(k1CM CTG  k2CM CDG  k3CM CMG )  (k1 ' CE1 CDG  k2 ' CE2 CMG  k3 ' CE3 CG )  dt (k4CG CDMC  k4 ' CGC CM 2 )  (k5CGC CDMC  k5 ' CGDC CM )}

(1)

The net rate of consumption of DMC is given by the following equation: rDMC  

dCDMC  w.{(k4CGCDMC  k4 ' CGC CM 2 )  (k5CGC CDMC  k5 ' CGDC CM )} dt

(2)

The rates of formations of the methyl esters E1, E2 and E3 and the corresponding rates of reactions of the tri-, di- and mono-glycerides are as follows:

rTG  

rE2 

dCE1 dCTG  rE1   w.(k1CM CTG  k1 ' CE1 CDG ) dt dt

dCE2 dt

 w.(k2CM CDG  k2 ' CE2 CMG )

(3)

(4)

13

rDG 

dCDG  w.(k1CM CTG  k1 ' CE1 CDG  k2CM CDG  k2 ' CE2 CMG ) dt

rE3 

dCE3

rMG 

dt

 w.(k3CM CMG  k3 ' CE3 CG )

dCMG  w.(k2CM CDG  k2 ' CE2 CMG  k3CM CMG  k3 ' CE3 CG ) dt

(5)

(6)

(7)

Where, - rTG is the rate of reaction of triglyceride, whereas rDG , rMG , rE1 , rE2 and rE3 are rates of formation of diglyceride, monoglyceride, methyl esters E1, E2 and E3, respectively. The net rate of consumption of glycerol (G) is given by the following equation: rG  

dCG  w.{(k4CGCDMC  k4 ' CGC CM 2 )  (k3CM CMG  k3 ' CE3 CG )  (k5CGC CDMC  k5 ' CGDC CM )} (8) dt

The rate of formation of GC and GDC is given by the following equation: rGC 

dCGC  w.(k4CGCDMC  k4 ' CGC CM 2 ) dt

rGDC 

(9)

dCGDC  w.(k5CGC CDMC  k5 ' CGDC CM ) (10) dt

Where, - rG is the rate of reaction of glycerol, whereas rGC and rGDC are rates of formation of GC and GDC, respectively. The rate of reaction of methanol (- rM ), DMC (- rDMC ) and glycerol (- rG ) are in mol/(g-cat.s), the concentrations in mol/L, catalyst loading w in g/L and the rate constants in L2/(mol.gcat.s). The individual rate constants were calculated on the basis of the concentration profiles (Figure 19 and Table 3). The activation energy values (Ea) were calculated using the value of kinetic rate constants at different temperatures (Figure 20 and Table 4). 3.2.7. Catalyst reusability HT was tested for its reusability upto two cycles (Table 5). The catalyst was recovered by filtration at the end of the reaction and washed with methanol to remove adsorbed impurities. 14

It was dried at 100 ºC for 12 h. The catalyst loss after filtration was made up with the fresh catalyst before each experiment. There was no major decrease in conversion of triglycerides and selectivity of the desired product (marginal decrease by 3 %). The reused catalyst was characterized by techniques such as BET surface area analysis, XRD, DSC-TGA, and FTIR as shown above. The results indicate that the catalyst was stable and it maintained high activity after subsequent reuses. 4. Conclusions The simultaneous co-production of biodiesel and GC was studied using hydrotalcite as the solid base catalyst. The co-product of conventional transesterification reaction - glycerol, was in situ utilized to produce a value added co-product- GC, simultaneously. HT showed the best activity for the reaction in terms of conversion and selectivity towards the product. The optimum conditions for this reaction are found at speed of agitation of 1000 rpm, a catalyst loading of 0.0125 g/cm3, oil to methanol mole ratio of 1:90, oil to DMC mole ratio of 1:30, a temperature of 150 oC. The experimental data was interpreted for the reaction kinetics. The kinetic rate constants and the activation energies were calculated. The catalyst was fully characterized before and after use by using various analytical techniques to determine its stability. Reusability study of the catalyst was done up to two cycles and the catalyst was found to be active and reusable. Conflict of Interest Statement The authors declare no conflict of interest.

ACKNOWLEDGEMENT Manali Dhawan acknowledges All India Council for Technical Education (AICTE), India for the award M.Tech. Research Fellowship. G.D. Yadav acknowledges support from R.T. Mody Distinguished Professor Endowment and J.C. Bose National Fellowship of Department of Science and Technology, Govt. of India.

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19

Figure Captions and Figures Figure 1

XRD of catalysts (a) HT, (b) CHT, (c) RHT, (d) Reused HT.

Figure 2

FT-IR spectra of catalysts (a) HT, (b) CHT, (c) RHT, (d) Reused HT.

Figure 3

N2 adsorption-desorption isotherms of (a) HT, (b) CHT, (c) RHT, (d) Reused HT.

Figure 4

SEM images of (a) HT, (b) CHT, (c) RHT, (d) Reused HT.

Figure 5

TEM image of catalyst HT (a) 30000x, (b) 40000x.

Figure 6

DSC analysis of catalyst HT.

Figure 7

TGA analysis of (a) CHT, (b) Reused HT, (c) HT, (d) RHT.

Figure 8

CO2-TPD patterns of (a) HT, (b) CHT.

Figure 9

NH3-TPD patterns of (a) HT, and (b) CHT.

Figure 10

Effect of hydrotalcite catalysts on the conversion of oil and concentration of GC. Soybean oil: 4.0 g, oil:methanol mole ratio: 1:90, oil:DMC mole ratio: 1:30, speed of agitation: 1000 rpm, temperature: 150°C, catalyst loading: 0.0125 g/cm3, total volume 32 cm3, reaction time 180 min.

Figure 11

Effect of speed of agitation on conversion of oil. Soybean oil: 4.0 g, oil:methanol mole ratio: 1:90, oil:DMC mole ratio: 1:30, catalyst HT, temperature: 150°C, catalyst loading: 0.0125 g/cm3, total volume 32 cm3, reaction time 180 min. (♦) 1000 rpm, (■) 800 rpm, (▲) 600 rpm

Figure 12

Effect of speed of agitation on concentration of GC. Soybean oil: 4.0 g, oil:methanol mole ratio: 1:90, oil:DMC mole ratio: 1:30, catalyst HT, temperature: 150°C, catalyst loading: 0.0125 g/cm3, total volume 32 cm3, reaction time 180 min. 20

(♦) 1000 rpm, (■) 800 rpm, (▲) 600 rpm Figure 13

Effect of catalyst loading on conversion of oil. Soybean oil: 4.0 g, oil:methanol

mole ratio: 1:90, oil:DMC mole ratio: 1:30, speed of agitation: 1000 rpm, temperature: 150°C, total volume 32 cm3, reaction time 180 min. (♦) 0.01875 g/cm3, (■) 0.0125 g/cm3, (▲) 0.00625 g/cm3 Figure 14

Effect of catalyst loading on concentration of GC. Soybean oil: 4.0 g, oil:methanol mole ratio: 1:90, oil:DMC mole ratio: 1:30, speed of agitation: 1000 rpm, temperature: 150°C, total volume 32 cm3, reaction time 180 min. (♦) 0.01875 g/cm3, (■) 0.0125 g/cm3, (▲) 0.00625 g/cm3

Figure 15

Effect of oil to methanol mole ratio on conversion of oil. Soybean oil: 4.0 g,

oil:DMC mole ratio: 1:30, catalyst HT, temperature: 150°C, catalyst loading: 0.0125 g/cm3, total volume 32 cm3, reaction time 180 min. (♦) 1:120, (■) 1:90, (▲) 1:60 Figure 16

Effect of oil to methanol mole ratio on concentration of GC. Soybean oil: 4.0 g,

oil:DMC mole ratio: 1:30, catalyst HT, temperature: 150°C, catalyst loading: 0.0125 g/cm3, total volume 32 cm3, reaction time 180 min. (♦) 1:120, (■) 1:90, (▲) 1:60 Figure 17

Effect of oil to DMC mole ratio on conversion of oil. Soybean oil: 4.0 g,

oil:methanol mole ratio: 1:90, speed of agitation: 1000 rpm, temperature: 150°C, catalyst loading: 0.0125 g/cm3, total volume 32 cm3, reaction time 180 min. (♦) 1:40, (■) 1:30, (▲) 1:20 Figure 18

Effect of oil to DMC mole ratio on concentration of GC. Soybean oil: 4.0 g,

oil:methanol mole ratio: 1:90, speed of agitation: 1000 rpm, temperature: 150°C, catalyst loading: 0.0125 g/cm3, total volume 32 cm3, reaction time 180 min. (♦) 1:40, (■) 1:30, (▲) 1:20

21

Figure 19

Effect of temperature on conversion of oil. Soybean oil: 4.0 g, oil:methanol mole

ratio: 1:90, oil:DMC mole ratio: 1:30, speed of agitation: 1000 rpm, catalyst loading: 0.0125 g/cm3, total volume 32 cm3, reaction time 180 min. (♦) 180C, (■) 170C, (▲) 160C, (×) 150C Figure 20

Effect of temperature on concentration of GC. Soybean oil: 4.0 g, oil:methanol

mole ratio: 1:90, oil:DMC mole ratio: 1:30, speed of agitation: 1000 rpm, catalyst loading: 0.0125 g/cm3, total volume 32 cm3, reaction time 180 min. (♦) 180C, (■) 170C, (▲) 160C, (×) 150C Figure 21

Concentration profile at 150 ˚C. Monoglycerides [(MG) (♦)], Diglycerides [(DG) (■)], Fatty acid methyl ester [(FAME) (▲)], Glycerol [(G) (×)], Glycerol Carbonate [(GC) (ӿ)], Glycerol Dicarbonate [(GDC) (●)] and Triglyceride [(TG) (+)]

Figure 22

Arrhenius plot (●) k1, (●) k2, (●) k3, (●) k4, (●) k5

22

Figure 1: XRD of catalysts (a) HT, (b) CHT, (c) RHT, (d) Reused HT

Figure 2: FT-IR spectra of catalysts (a) HT, (b) CHT, (c) RHT, (d) Reused HT

Figure 3: N2 adsorption-desorption isotherms of (a) HT, (b) CHT, (c) RHT, (d) Reused HT

Figure 4: SEM Images of (a) HT, (b) CHT, (c) RHT, (d) Reused HT

Figure 5: TEM image of catalyst HT (a) 30000x, (b) 40000x

Fig ure 6: DSC analysis of catalyst HT

Figure 7: TGA analysis of (a) CHT, (b) Reused HT, (c) HT, (d) RHT

Figure 8

CO2-TPD patterns of (a) HT, (b) CHT

Figure 9

NH3-TPD patterns of (a) HT, and (b) CHT

Figure 10: Effect of hydrotalcite catalysts on the conversion of oil and concentration of GC

Figure 11: Effect of speed of agitation on conversion of oil

Figure 12: Effect of speed of agitation on concentration of GC

Figure 13: Effect of catalyst loading on conversion of oil

Figure 14: Effect of catalyst loading on concentration of GC

Figure 15: Effect of oil to methanol mole ratio on conversion of oil

Figure 16: Effect of oil to methanol mole ratio on concentration of GC

Figure 17: Effect of oil to DMC mole ratio on conversion of oil

Figure 18: Effect of oil to DMC mole ratio on concentration of GC

Figure 19: Effect of temperature on conversion of oil

Figure 20: Effect of temperature on concentration of GC

Figure 21: Concentration profile

Figure 22: Arrhenius plot

O

OH O OH

+

H3C

O

O

K2CO3

CH3

O O

+

O

OH

O

H3C

OH

CH3

O

Scheme 1: Reaction of glycerol and DMC to produce GDC

O O

O R1

H3C

O

O O

R2

+

3H3C

OH

Catalyst

OH

O H3C

O

H3C

O

O O

R1

R2

+

OH

O R3

R3

OH

Step1: Transesterification reaction of triglyceride with methanol to produce biodiesel and glycerol OH

O O

OH

+

H3C

O

O

CH3

Catalyst

O

OH

O

+

2 H3C

OH

OH

Step 2: Transesterification reaction of glycerol and DMC to produce GC Scheme 2: Simultaneous co-production of biodiesel and GC

23

O O

R1

OH

O O

R2

+

H3C

OH

k1 k1'

O H3C

O

O R1

+

O

O O

(1)

R2 O

R3

O

TG

M

E1

R3

DG OH

OH

O O

R2

+

H3C

OH

k2 k2'

O H3C

O

R2

+

O

O O

(2)

OH

O

R3

DG

M

E2

R3

MG OH

OH

+

OH

H3C

OH

k3 k3'

O H3C

O

R3

+

(3)

OH

O O

MG

OH

R3

M

E3

G

Scheme 3: Consecutive reactions in transesterification of vegetable oil with methanol

24

OH

O O

+

OH

H3C

O

O

CH3

OH

G

k4

O

+

2 H3C

(4)

OH

OH

DMC

GC

M

O

O O O

O

k4'

O

+

H3C

O

O

OH

CH3

k5 k5'

(5)

O O

+

O

O

H3C

OH

CH3

O

GC

DMC

GDC

M

Scheme 4: Transesterification reaction of glycerol with DMC to produce GC and GDC

25

List of tables Table 1

Surface area pore volume and pore diameter analysis

Table 2

Elemental analysis of hydrotalcite catalysts

Table 3

Value of kinetic rate constants at different temperatures

Table 4

Activation energies for the transesterification of vegetable oil with methanol and DMC

Table 5

Catalyst reusability studies

26

Table 1: Surface area pore volume and pore diameter analysis HT

CHT

RHT

Reused HT (after 2nd reuse)

BET Surface Area (m² g-1)

71.5

202.8

131.2

60.3

BJH Pore Volume (cm³ g-1)

0.6

0.6

0.3

0.6

BJH Average Pore Size (Å)

377.1

122.9

92.9

425.9

Table 2: Elemental analysis of hydrotalcite catalysts Element (mole

HT

CHT

RHT

Reused HT

Mg

74.3

73.7

74.2

73.5

Al

25.6

26.2

25.6

26.2

%)

Table 3: Value of kinetic rate constants at different temperatures Reaction Rate Constant [L2/(mol.g-cat.s)]

Reaction

T = 150 ºC

T = 160 ºC

T = 170 ºC

T = 180 ºC

TG

DG

k1 = 0.5 X 10-5

k1 = 0.6 X 10-5

k1 = 0.9 X 10-5

k1 = 2.2 X 10-5

DG

MG

k2 = 2.7 X 10-5

k2 = 3.5 X 10-5

k2 = 6.6 X 10-5

k2 = 20 X 10-5

MG

G

k3 = 13.5 X 10-5

k3 = 18.2 X 10-5

k3 = 36.9 X 10-5

k3 = 143 X 10-5

G

GC

k4 = 14.1 X 10-5

k4 = 18.3 X 10-5

k4 = 24.8 X 10-5

k4 = 65.1 X 10-5

GC

GDC k5 = 17.5 X 10-5

k5 = 21.9 X 10-5

k5 = 33.9 X 10-5

k5 = 51 X 10-5

Table 4: Activation energies for the transesterification of vegetable oil with methanol and DMC Reaction

Ea (kcal/mol)

TG

DG

18.2

DG

MG

24.8

MG

G

29.4

G

GC

18.5

GC

GDC

13.8

27

Table 5: Catalyst reusability studies Sr. No.

Catalyst reusability

1 2 3

Fresh 1st reuse 2nd reuse

Conversion of oil (%) 97.3 96.2 94.5

Selectivity of GC (%) 93.2 92.8 90.7

28