Journal of the Taiwan Institute of Chemical Engineers 70 (2017) 179–187

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Synthesis of glycerol carbonate from biodiesel by-product glycerol over calcined dolomite Y.T. Algoufi, G. Kabir, B.H. Hameed∗ School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, Nibong Tebal 14300, Penang, Malaysia

a r t i c l e

i n f o

Article history: Received 14 February 2016 Revised 6 October 2016 Accepted 20 October 2016 Available online 3 November 2016 Keywords: Catalyst Dolomite Glycerol Glycerol carbonate Transesterification

a b s t r a c t Glycerol, which threatens the viability of biodiesel industries, can be converted to valuable chemicals and chemical intermediates, such as glycerol carbonate (GC). GC was synthesized by glycerol transesterification with dimethyl carbonate (DMC) on CaO–MgO mixed oxide catalyst prepared from natural dolomite. At calcination temperature of 800 °C, natural dolomite was converted to CaO–MgO mixed oxide to serve as catalyst in the conversion of glycerol to GC. Temperature changes and calcined dolomite surface and textural characteristics caused the glycerol transesterification with DMC to GC. The catalyst with 15< H_<18.4 basic strength yielded a maximum of 97% glycerol conversion and 94% yield of GC at 75 °C, 3 DMC/glycerol ratio, and 6 wt% catalyst dose. Calcined dolomite showed potential for a consecutive cycle of catalytic activity during glycerol transesterification with negligible yield of by-product glycidol. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction Biodiesel is a renewable energy resource that remains a focus of global energy and environmental sustainability policies. Biodiesel market has developed rapidly to encourage production of large quantity of glycerol. Coproduction of glycerol is an important aspect of biodiesel production; normally, 10 wt% of crude glycerol is generated during biodiesel synthesis. Glycerol value depreciates because of excess supply to market from biodiesel synthesis via transesterification with methanol. Cost of storage and handling of excess glycerol threaten the viability of biodiesel industries. An alternative strategy is the conversion of glycerol to valuable chemicals and chemical intermediates [1,2]. Glycerol carbonate (GC) [3,4] and polyglycerol are typical valuable products from glycerol transesterification and etherification, respectively [5,6]. GC is a nontoxic, biodegradable, and nonflammable solvent suitable as environmentally friendly source of chemical intermediate [7,8]. A series of valuable chemicals, such as dihydroxyacetone, mesoxalic acid, 1,3-propanediol, 1,3dichloropropanol, and glyceryl ether, is produced from GC [9]. Furthermore, GC serves as a precursor for synthesis of dyes, lacquers, pharmaceuticals, detergents, adhesives, cosmetics, and biolubricants [10]. Catalytic transesterification of glycerol with dimethyl carbonate (DMC) produces GC via decarboxylation mechanism. Phosgene, urea, carbon monoxide, and carbon dioxide are prominent decarboxylation agents for GC synthesis, but they are restricted because ∗

Corresponding author. Fax: +6045941013. E-mail address: [email protected] (B.H. Hameed).

of environmental and handling issues [3,4,11]. DMC remains a reputable carboxylating agent that promotes direct production of GC negligible coproducts [7,12,13]. Heterogeneous basic catalysts mostly assist glycerol transesterification with DMC to produce the desired quantity and quality of GC; thus, catalysts should have sufficient basicity and basic strength distribution [3]. Bulk heterogeneous basic catalysts of alkali and alkaline earth metals oxides possess outstanding catalytic characteristics that are selective toward GC synthesis. The catalysts are inexpensive and readily available, but poor recovery and reusability during cycles of transesterification are their major drawbacks [14,15]. Synthetic heterogeneous base catalysts are explored to ascertain their influence on the upgrade of glycerol to GC. Catalysts synthesized from mixed oxide are often basic in nature; they support the progression of transesterification reaction mechanisms that produce glycerol derivative compounds, such as GC [3,15]. Mixed oxide catalysts, such as MgO–ZrO2 [16], Ca–Al hydrocalumite [17], Mg1+x Ca1−x O2 [13], LiNO3 /Mg4 AlO5.5 [18], and Mg-La [19], are efficient. However, industrial application of catalyst faces some drawbacks, such as leaching, reusability during cycles of reactions, costliness, and unhealthy synthesis routes [7,12]. Dolomite is a mineral material that consists of solid solution of calcium and magnesium carbonates (MgCa(CO3 )2 ). Calcined dolomite decomposes to CaO–MgO mixed oxide that serves as efficient catalyst in reformation and gasification processes [20], as well as transesterification processes for biodiesel synthesis [21,22]. CaO and MgO in calcined dolomite exist as stable phases that exhibit an efficient catalytic activity, which is better than that of bulk calcium oxide [15].

http://dx.doi.org/10.1016/j.jtice.2016.10.039 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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The present work aims to synthesize GC from glycerol using CaO–MgO mixed oxide catalysts prepared from natural dolomite. The study also examines the reusability of catalyst to establish catalyst stability during many consecutive reaction cycles. Dolomite, as a catalyst precursor for glycerol transesterification, can further consolidate the environmental friendliness of GC production and reduce financial constraint associated with synthetic catalysts.

accelerated at 300 kV. Elemental composition of catalysts was analyzed using energy-dispersive X-ray spectroscopy (EDX) mounted on microscope. Base strength (H_) of catalysts was studied using Hammett indicator method, and neutral red (H_ =6.8) of Hammett indicators was used for basicity analysis.

2. Materials and methods

Double-necked flask (50 mL) coupled with a stirrer, condenser, and heating mantle was used to produce GC from glycerol transesterification reaction with DMC. Glycerol (8 mL), DMC (32 mL), and catalyst (1 wt%–7 wt% based on the glycerol amount used) were fed to the reactor. The resulting mixture was stirred at 10 0 0 rpm and heated on heating mantle to temperatures of 60–85 °C for 30– 120 min. The catalyst was separated from the liquid phase by using a centrifuge at the end of reaction time. The liquid phase was decanted, and the spent catalysts were regenerated with methanol for the next cycle of glycerol transesterification under the same condition. The catalysts were discarded when their activity declined to the lowest level.

2.1. Materials Acros and Sigma–Aldrich (Malaysia) supplied reagent-grade (99%) glycerol and DMC, respectively. Dolomite was acquired from a mining site in Kuala Lumpur, Malaysia. Merck Malaysia supplied analytical reagents (>99.5%) pyridine (99%) and HPLC-grade methanol. 2.2. Catalyst preparation The dolomite was ground to conglomerate of particles with different sizes using mortar and pestle. The particles were sieved through 125–250 μm mesh sizes and calcined in muffle furnace at 800 °C for 3 h. The calcined dolomite was used as catalyst in GC synthesis via glycerol transesterification with DMC. Natural dolomites fuse calcium and magnesium carbonates (CaMg(CO3 )2 ) and a few impurities that are not suitable catalysts. The carbonate decomposes to stable CaO–MgO mixed oxides during total calcination that releases CO2 [21].

2.4. Catalytic activity test

2.5. Product analysis Products of glycerol transesterification reactions were analyzed with a gas chromatograph (Model: GC-2010 Plus, Shimadzu, Japan) equipped with flame ionization detector capillary column, split/splitless injection unit and a capillary column (Zebron, ZB5HT, 30 m, 0.25 mm, 0.25 μm). The procedures for sample analysis were reported in our previous work [23].

2.3. Catalyst characterization 3. Results and discussions Dolomite decomposition profile was determined using thermogravimetric analysis (TGA) under air at heating ramping rate of 10 °C/min using a thermobalance SETARAM. Brunauer–Emmett– Teller technique was used to estimate the catalyst textural and surface characteristics using porosity analyzer (Micromeritics Instruments Corporation, USA) and surface area micromeritics (ASAP 2020), respectively. Phase structure of calcined dolomite (catalyst) was determined with X-ray diffractometer (Model:Bruker D8 Focus, Bruker, Germany) and presented as X-ray diffraction (XRD) patterns over 10° ≤ 2θ ≤ 90° using Cu kα radiation (λ=1.5406°). Fourier transform infrared (FTIR) spectrophotometer was used to analyze the functional groups on the surface of calcined dolomite. The spectra were recorded in the range of 40 0 0– 400 cm−1 using Perkin–Elmer System 2000 spectrometer. Catalyst surface morphology was studied using scanning electron microscopy (SEM) (Philips XL30S) with FEI as a source of electrons,

3.1. Catalyst characterization 3.1.1. TGA Fig. 1 depicts the devolatilization spectra of natural dolomite as per weight loss versus temperature. Dolomite decomposes at steady rate at temperature of 570–800 °C to obtain high weight losses during the exothermic decomposition. The weight loss attains a minimal value of 47.6% at 800 °C during the devolatilization reaction. The thermal decomposition of dolomite can be explained according to the decomposition reaction in Eqs. (1) and (2) [24]. Theoretical weight loss at 800 °C is 52%, as estimated from the reaction stoichiometry; this value correlates with 47.6% obtained during the TGA experiment. Moreover, Fig. 1 exhibits the thermograph for the derivate weight loss of dolomite thermal decomposition by TGA under air.

Fig. 1. Thermogravimetric analysis of natural dolomite.

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Table 1 Textural characteristics for raw and calcined dolomite at 800 °C. Characteristic

Dolomite-natural

Dolomite-calcined at 800 °C

BET surface area (m2 /g) Total pore volume (cm3 /g) Average pore diameter (nm)

1.23 1.5 4.85

9.3 98 12.4

The thermograph shows a single peak at 800 °C to signify the temperature for maximum weight loss during the devolatilization reaction. Subsequently, the decomposition temperature of 800 °C is the optimum temperature for total devolatilization of dolomite during calcination.

MgCa(CO3 )2 → MgO + CaCO3 + CO2

(1)

MgO+CaCO3 → MgO + CaO + CO2

(2)

Fig. 2. Nitrogen adsorption/desorption isotherms of raw and calcined dolomite at 800 °C.

CO2 and other volatile contaminants desorb to transform the dolomite into Ca and Mg mixed oxide material during calcination. Calcined dolomite is free of materials with strong tendencies to interfere with the active sites required for catalytic activity. The characteristic features of the calcined dolomite are consistent with those of synthesized mixed oxide material used for catalytic activity reported in the literature [24].

amount of nitrogen that begins at P/Po=0.8–1.0 in the narrow pore size distribution. However, the large diameter pore requires high relative partial pressure at P/Po ≈ 1.0. According to the relative partial pressure values, the nitrogen adsorption isotherms indicate the complete filling of the pore network of the calcined and raw dolomites.

3.1.2. Nitrogen adsorption/desorption isotherm Table 1 shows the surface and textural properties of raw and calcined dolomite at 800 °C. Natural dolomite exhibits lower surface area than that of the calcined dolomite. CO2 from carbon combustion and other volatile contaminants desorb to free a large number of active surface on calcined dolomite for catalytic activity, in addition to changes in its textural properties. The calcined dolomite has 9.3 m2 g-1 surface area, which is eightfold higher than that of the natural dolomite. Calcined dolomite possesses a substantially higher pore volume than that of natural dolomite at 98 cm3 g-1 and an average pore diameter of 12.4 nm. The calcined dolomite presents sufficiently larger diameter in the range of 0.33– 0.47 nm compared with that of the reactant molecules (Table 1). The available channel in the catalyst structure can allow the mass transport of the reactants and resultant products. In addition, the reactants can easily access active sites on the dolomite surface to enhance the selectivity for the desired product and complete conversion of glycerol. Evidently, dolomite shows an enhanced surface and textural properties induced by the thermal treatment propagated by solid-state diffusion. Volatile materials and CO2 desorb from the main structure of dolomite by diffusion, and pore networks are developed to support the catalyst activity. Fig. 2 presents nitrogen adsorption/desorption isotherms for raw and calcined dolomite at 800 °C. Calcination facilitates cracking of dolomite surface because of the devolatilization of CO2 to proliferate mesopores that stem through the calcined dolomite. The isotherms present a reliable pore structures that stem through the calcined dolomite in the form of channel-like pores. The higher nitrogen uptake of calcined dolomite compared with natural dolomite can justify the presence of an extensive network of mesopores in the material. The isotherms are typical type IV isotherms, which contain hysteresis loops typical to the type H2. The isotherms of calcined dolomite exhibit pore structure with extensively connected channel-like pores as well as size and shape in disordered distributions [13]. The hysteresis loops occur in the isotherms because of the nitrogen desorption caused by pore constrictions. Additionally, Fig. 2 infers the sharp increase in the adsorbed nitrogen propagates because of the increasing pore diameter that needs a large partial pressure. The calcined and raw dolomites exhibit a relatively sharp increase in the adsorbed

3.1.3. XRD XRD analyzed crystal structure and phase change that occur between the natural and calcined dolomites. Fig. 3 depicts the X-ray diffractogram for natural and calcined dolomite, as well as calcined dolomite used for 4th reaction cycle. The diffractogram reveals that natural dolomite has diffraction peaks at 2θ 30.76°, 41°, and 50.9° that are characteristic peaks of MgCa(CO3 )2 structure. Diffraction peaks of calcined dolomite have no peaks that correspond to those of the natural dolomite. Other new peaks appear as the characteristic peaks for calcined dolomite to infer complete decomposition of the natural dolomite. New peaks are detected at 2θ 32.04°, 37.2°, and 53.72° to signify the presence of cubic calcium oxide generated by (110), (200), and (220) reflections, respectively [25]. Conversely, peaks at 2θ 42.74° and 62.16° caused by (200) and (220) reflections [26] are cubic magnesium oxide characteristic peaks. According to Scherrer’s formula, the CaO crystallite size in the calcined dolomite and MgO is 33 nm and 21 nm, respectively. Calcium carbonate peaks dominate the diffractogram of the 4th used cycle catalyst with some amount of CaO, and MgO is almost depleted. Peaks that correspond to CaCO3 result from carbonation of CaO that relatively becomes unstable after the depletion of the MgO in calcined dolomite. Regeneration exposes spent catalyst to atmospheric CO2 that eventually carbonizes CaO to CaCO3 after each reaction. The catalyst becomes less active because of the decline in basicity, which leads to substantial decrease in yield and selectivity of GC. 3.1.4. FTIR Fig. 4 presents the FTIR spectra of raw dolomite (a) calcined at 800 °C. The spectra for natural dolomite show three peaks at 730, 870, and 1413 cm−1 . The peak designates an in-plane bending (ν 4) mode of CO3 −2 in the dolomite structure at 730 cm−1 . However, the peak at 870 cm−1 wavelength band is significant to the carbonate bending mode (ν 2). The peak at 1413 cm−1 wavelength band attributes to C–O stretching vibration mode of ν 3 (CO−2 ). All the 3 peaks associated with CO3 −2 disappear after calcination at 800 °C to infer that dolomite decomposes to oxides of calcium and magnesium. Sharp peak at 3640 cm−1 is the stretching vibration of surface O–H groups. The functional groups with the O–H bonds are often attached to the metals and un-dissociated water molecules

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Fig. 3. Diffractograms of raw and calcined dolomite at 800 °C.

Fig. 4. Fourier transform infrared spectra of raw and calcined dolomite at 800 °C.

Table 2 Elemental analysis for raw and calcined dolomite at 800 °C. Element Calcium Magnesium Carbon Oxygen Total

Dolomite-raw

Dolomite-calcined at 800 °C

21.5 9.6 15.64 53.26

28.8 16.8 7.2 47.2

100.00

100.00

that form the hydrated layer on the catalyst. However, the peak at 3640 cm−1 wavelength appears because of the moisture adsorbed by dolomites during handling before the FTIR analysis. 3.1.5. EDX-SEM Table 2 reveals the elemental compositions of raw and calcined dolomites. The calcium content of the calcined dolomite is higher than that of natural dolomite. Such high calcium content is caused by the natural aggregation of dolomite with calcite. Calcination segregates the magnesium phases of the dolomite during decarbonization [22] and ultimately slightly increases the magnesium content that stabilizes the calcium in the calcined dolomite. Burning dolomite carbon content (15.64 wt%) increases the magnesium and calcium compositions in calcined dolomite. Carbon dioxide desorbs through the pore structure of dolomite during calcination. Evidently, the raw dolomite surface and textural character-

istics mutate during calcination to yield developed pores that increase the surface. Heat burns out a substantial part of the inherent carbon in dolomite during calcination at 800 °C. Eventually, Table 1 shows that carbon content of calcined dolomite becomes 7.2 wt% less than 15.64 wt% found in natural dolomite. The catalytic potential of calcined dolomite during glycerol transesterification depends on the extent of carbon and metal carbonate decomposition during calcination. Fig. 5 depicts the SEM images for natural and calcined dolomite at 50 0 0× magnifications. The images present surface morphology of both natural and calcine dolomite. The rough surface on calcined dolomite indicates that calcination subdues the surface morphology of natural dolomite to crack, which develops into pores through which CO2 is desorbed. The surface morphology induced on dolomite during calcination can be observed in Fig. 5 Calcined dolomite exhibits higher pore volume and surface area compared with those of natural dolomite. The structural characteristics positioned calcined dolomite as a good catalyst for catalytic glycerol transesterification reaction. 3.2. Effect of calcination temperature Table 3 presents the effect of calcination temperature on the catalytic activity of calcined dolomite during transesterification of glycerol with DMC to GC. Calcination changes the topology and

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that of natural dolomite. The pores encourage the catalytic activity of calcined dolomite during glycerol transesterification. Table 3 highlights the effect of calcination temperature on basicity of natural dolomite and calcined dolomites (catalysts). Natural dolomite exhibits basic strength of H_<6.2, which promotes glycerol transesterification reaction to insignificant glycerol conversion and GC yield. Dolomite calcined at 600 °C has basic strength of 6.2< H_ <9.2, which causes low GC yield and glycerol conversion. An exceptional high conversion of glycerol and selectivity of GC occur at a corresponding maximum basic strength of 15< H_ <18.4 for dolomites calcined at 70 0–90 0 °C. Similarly, the authors in [1] reported that different amounts of active basic sites available on the catalyst depend on the calcination temperature during dolomite decarbonation. The basicity of dolomite calcined at 800 °C is sufficient in encouraging transesterification reaction to 97% and 94% maximum conversion of glycerol and GC yield, respectively. Increasing calcination temperature to 900 °C the MgO and CaO phases sintered to decrease the pores on the catalysts; the reactants’ access to sufficient active sites is restricted, which subsequently decreases the GC yield. Calcination decarbonizes both the MgCO3 and CaCO3 phases of dolomite to stable mixed CaO–MgO oxide to inculcate abundant basic sites on the catalysts. MgO oxide presents basicity insufficient to catalyze the glycerol transesterification reaction, but CaO augments to increase the catalyst basicity to a maximum of 15< H_ <18.4. CaO from CaCO3 decarbonation at different calcination temperatures increases the basicity of calcined dolomite. Basicity is enhanced with increasing CaO phase on the calcined dolomite topology that favors the high yield of GC. 3.3. Catalytic activity test

Fig. 5. Scanning electron microscopy image of raw (a) and calcined dolomite (b) at 800 °C at 5000 × magnifications.

Table 3 Transesterification reaction using dolomite catalyst calcined at different temperatures. Calcination temperature (°C)

Basicity (H_)

Glycerol conversion (%)

Glycerol Carbonate yield (%)

Uncalcined 600 700 800 900

H_<6.2 6.2< H_ <9.2 15< H_ <18.4 15< H_ <18.4 15< H_ <18.4

0.0 12.6 72 97 93.4

0.0 9.8 68.7 94 91.3

textural nature of dolomite to impose relevant properties beneficial to glycerol transesterification on calcined dolomite. XRD diffractograms, SEM images, and FTIR spectra all present evidence that links the change in dolomite textural and topology to calcination temperature. The structures induced in calcined dolomite are beneficial for catalytic activity during glycerol transesterification. The structural phase in calcined dolomite implants outstanding basicity for selective production of GC from glycerol transesterification reaction. Nitrogen adsorption/desorption isotherm predicted that pore network within calcined dolomite has developed higher than

3.3.1. Effect of catalyst loading Fig. 6 displays the dependency of glycerol transesterification with DMC to GC on catalyst loading range of 1 wt%–7 wt% based on glycerol weight. Glycerol conversion and GC yield increase gradually with catalyst increasing from 1 wt% to 6 wt%. Similarly, the GC yield increases proportionally with increasing amount of catalyst in the reaction system. Maximum yield of 94% GC is accomplished with 6 wt% catalyst loading. Further increase in catalyst loading causes negligible conversion of glycerol or GC yield. The GC yield declines at a 7 wt% catalyst loading to produce glycidol. The mechanism of decarboxylation reaction converts part of GC to glycidol over the available excess basic sites on the catalyst to reduce glycerol conversion and GC yield. The decrease comes from the excess amount of requisite catalysts providing excess basic sites sufficient to propagate the reversion of the desired forward reaction during the transesterification reaction. The catalytic activity of calcined dolomite derived from CaO/MgO basic oxide shows dynamic characteristics in propagating glycerol transesterification. Catalyst availability provides the necessary basic sites for glycerol transesterification; thus, glycerol transesterification reaction proceeds to equilibrium. The basic sites induce the transesterification reaction by abstracting proton from the glycerol molecule [10,27]. 3.3.2. Effect of glycerol/DMC molar ratio Fig. 7 shows the effect of DMC/glycerol molar ratio on glycerol conversion and GC yield at 75 °C. Glycerol conversion and GC yield are below 40% at DMC/ glycerol molar ratio of 1. The transesterification of glycerol with DMC is a reversible reaction; the DMC/glycerol molar ratio of 1 shifts the chemical equilibrium toward glycerol synthesis. Therefore, a higher than 1 DMC/glycerol molar ratio is required to shift the chemical equilibrium toward the GC yield [11].

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Fig. 6. Effect of catalyst loading on glycerol transesterification with DMC. Reaction conditions: molar ratio, 3:1; reaction temperature, 75 °C; and reaction time, 90 min.

Fig. 7. Effect of DMC/glycerol molar ratio on glycerol transesterification with DMC. Reaction conditions: catalyst loading, 6 wt%; reaction temperature, 75 °C; and reaction time, 90 min.

DMC/glycerol ratio of 2–3 increases the conversion of glycerol and GC yield. The GC yield attains a maximum of 94% at DMC/glycerol molar ratio of 3, and glycerol conversion reaches a maximum of 97%. Thus, high molar ratios of DMC to glycerol shift the chemical equilibrium to favor high GC yield. A DMC/glycerol ratio of 4 results in the declined conversion and yield of glycerol and GC respectively. The chemical equilibrium shifts to impedes the forward reaction of the transesterification and then catalyst support the yield of unwanted products. The reaction mechanism for the GC yield is subdued to favor the yield of glycerol and other by-products [28]. The DMC/glycerol ratio of 3 as an effective ratio for the maximum yield of and maximum conversion of glycerol is consistent with that reported in literature [24,27]. DMC is an environmentally safe alternative to CO2 and urea that act as carboxylating agent during the glycerol transesterification reaction. Carboxylation with DMC occurs at 70–85 °C to limit the synthesis of an elusive product [3,11]. 3.3.3. Effect of reaction temperature Fig. 8 depicts the effect of temperature on the conversion and yield of GC. Glycerol conversion is 37% at the reaction temperature of 60 °C. Subsequently, the GC yield and glycerol conversion increase dramatically from 65 °C to 80 °C. The glycerol transesterification reaction attains a maximum GC yield and glycerol

conversion at 75 °C. The reaction mixture at 60 °C shows sufficient viscosity that limits reactant miscibility and diffusion into the catalyst pores, which host substantial active sites [29]. The low glycerol conversion and low GC yield become evident. Miscibility between the reactants increases when the reaction mixture is maintained at 65–80 °C. Frequency of collision between the reactants molecules reduces the reaction activation energy. The reactants easily diffused into the catalyst pores to access the active sites to increase the rate of transesterification reaction. Consequently, the reaction rapidly develops equilibrium to achieve higher maximum GC yield than that at low temperature. The subsequent increase in temperature to 80 °C results in no reasonable increase in GC yield. The reaction temperature convincingly shows significant influence on glycerol conversion and GC yield during transesterification with DMC over catalyst. Simanjuntak et al. [30] reported similar findings that show a good agreement with this study and other studies using different heterogeneous catalysts [23,31]. 3.3.4. Effect of reaction time Appropriate time is required to achieve maximum conversion of glycerol and yield of GC during transesterification on calcined dolomite catalysts. Fig. 9 illustrates the effect of reaction time on GC yield from transesterification of glycerol with DMC on calcined dolomite. The reaction time varies from 30 to 120 min, but the GC yield is low at 25% at 30 min. Glycerol conversion and GC yield dramatically increase to 97% and 94% at 90 min, respectively. The threshold time of 90 min is adequate for the adsorption of reactant and desorption of products that decline in catalyst activity. Moreover, the time is sufficient to subdue the mass transfer resistance that militates against accessing the active catalyst sites by the reactants. GC yield and glycerol conversion show no significant increase when the reaction continues for 120 min. Glycerol, as the limiting reactant, depletes, and the active sites of the catalyst are poisoned by the products and excess DMC. 3.4. Catalyst reusability test The stability of calcined dolomite in glycerol transesterification is critical to its reusability in successive reaction cycles. The reusability confirms the catalyst stamina in a consecutive glycerol transesterification. Fig. 10 shows the catalytic activity of calcined dolomite used in four consecutive cycles of glycerol transesterification for GC production. The catalyst activity is marked with a

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Fig. 8. Effect of reaction temperature on glycerol transesterification. Reaction conditions: catalyst loading: 6 wt%; glycerol: DMC molar ratio, 1:3; and reaction time, 90 min.

Fig. 9. Effect of reaction time on glycerol transesterification with DMC. Reaction conditions: catalyst loading: 5 wt%; DMC/glycerol molar ratio: 3:1; and reaction temperature, 75 °C.

Fig. 10. Reusability study of dolomite catalyst. Reaction conditions: catalyst loading, 6 wt%; DMC/glycerol: molar ratio, 3:1; reaction temperature, 75 °C; and reaction time, 90 min.

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progressive decline in activity from the first to the fourth recycle during GC synthesis. The catalyst sustains about 84% glycerol conversion and 79% GC yield during the fourth reaction cycle. The decline in the catalyst activity after cycles of consecutive reactions is an obvious occurrence, whose typical case is that of rehydrated Mg/Al hydrotalcite catalyst during glycerol transesterification with DMC [10]. Conversely, when the catalyst is recalcined at 800 °C after recovery for reuse in the next reaction cycle, the catalyst assumed the same activity equivalent to that of the fresh catalyst. FTIR shows spectra changes in the catalyst before and after the fourth cycle of transesterification reaction. The spectra (Fig. 4) for the recovered catalyst after fourth cycle of reaction (Used4th cycle) exhibit peaks similar to those of natural dolomite. Intense peak at 1450 cm−1 is attributed to C–H stretching vibration, and the less intense peak at 880 cm−1 wavelength corresponds to CO3 −2 . The spectra of the 4th used cycle infer that the catalyst acquires the chemical phases of natural dolomite during handling via contamination with carbon dioxide. XRD pattern of 4th used cycle of calcined dolomite is depicted in Fig. 3. The spectra show changes on the catalyst topology and textural characteristics during fourth cycle of transesterification reaction. The spectra resemble those of natural dolomite as the CaO and MgO phases almost disappear. Calcium oxide phase becomes unstable and reacts with atmospheric CO2 to proliferate inactive carbonate phase during reaction cycle. CaO is a high activity catalyst, but it is easily deactivated during transesterification of glycerol with DMC. However, the stability of CaO extruded with activated alumina exhibits better stability than the bulk CaO. Subsequently, the reusability of CaO is enhanced significantly than that of bulk CaO in transesterification reaction of glycerol with DMC to produce GC [19]. Magnesium oxide phase enhances the stability of CaO phase; consequently, calcined dolomite can be reused in several cycles of reaction compared with unstable bulk CaO [30]. In addition, Conesa et al. [32] reported calcined dolomite and waste seashell as catalysts for methanolysis of palm oil to biodiesel; these catalysts are more stable than the bulk CaO obtained from a seashell. Moreover, most of CaO obtained from seashell transforms to calcium glyceroxide, whereas calcined dolomite is less sensitive to glycerol. The role of MgO in enhancing the stability of CaO active phase provides researchers the leverage to synthesize stable CaO–MgO metal oxides and stabilize CaO catalyst in biodiesel production [2,33].

4. Conclusion GC is synthesized via transesterification of glycerol with DMC on calcined dolomite. Calcination changes structural phases in dolomite to CaO–MgO mixed oxide catalyst with basicity appropriate for glycerol transesterification. Calcined dolomite possesses basicity and basic strength that favor high yield and selectivity to GC. The catalyst prepared at calcination temperature of 800 °C displays maximum basicity of 15< H_ <18.4 that promotes glycerol conversion to 97% and GC yield to 94%. The catalyst exhibits a decline in catalytic activity after each reuse during GC synthesis. The catalyst promotes the transesterification reaction to conversion of 84% glycerol and yield of 79% GC after the fourth reaction cycle. The catalyst shows remarkable potential for several cycles of catalytic activity for the synthesis of GC from transesterification with DMC.

Acknowledgment The authors acknowledge the financial support provided by the Ministry of Education, Malaysia under the Transdisciplinary Research Grant Scheme (TRGS) Phase 2/2014 (203/PJKIMIA/6762002).

References [1] Quispe CAG, Coronado CJR, Carvalho JA Jr. Glycerol: production, consumption, prices, characterization and new trends in combustion. Renew Sustain Energy Rev 2013;27:475–93. [2] Avhad MR, Marchetti JM. A review on recent advancement in catalytic materials for biodiesel production. Renew Sustain Energy Rev 2015;50: 696–718. [3] Teng WK, Ngoh GC, Yusoff R, Aroua MK. A review on the performance of glycerol carbonate production via catalytic transesterification: effects of influencing parameters. Energy Convers Manag 2014;88:484–97. [4] Liu J, Li Y, Zhang J, He D. Glycerol carbonylation with CO2 to glycerol carbonate over CeO2 catalyst and the influence of CeO2 preparation methods and reaction parameters. Appl Catal A Gen 2015;513:9–18. [5] Chang J-S. Optimization on the etherification of glycerol with tert-butyl alcohol. J Taiwan Inst Chem Eng 2011;42:760–7. [6] Gholami Z, Abdullah AZ, Lee K-T. Glycerol etherification to polyglycerols using Ca1+x Al1-x Lax O3 composite catalysts in a solventless medium. J Taiwan Inst Chem Eng 2013;44:117–22. [7] Hu K, Wang H, Liu Y, Yang C. KNO3 /CaO as cost-effective heterogeneous catalyst for the synthesis of glycerol carbonate from glycerol and dimethyl carbonate. J Ind Eng Chem 2015;28:1–10. [8] Pan S, Zheng L, Nie R, Xia S, Chen P, Hou Z. Transesterification of glycerol with dimethyl carbonate to glycerol carbonate over Na–based zeolites. Chin J Catal 2012;33:1772–7. [9] Lanjekar K, Rathod VK. Utilization of glycerol for the production of glycerol carbonate through greener route. J Environ Chem Eng 2013;1:1231–6. [10] Alvarez MG, Segarra AM, Contreras S, Sueiras JE, Medina F, Figueras F. Enhanced use of renewable resources: transesterification of glycerol catalyzed by hydrotalcite-like compounds. Chem Eng J 2010;161:340–5. [11] Sonnati MO, Amigoni S, Taffin de Givenchy EP, Darmanin T, Choulet O, Guittard F. Glycerol carbonate as a versatile building block for tomorrow: synthesis, reactivity, properties and applications. Green Chem 2013;15: 283–306. [12] Zuhaimi NAS, Indran VP, Deraman MA, Mudrikah NF, Maniam GP, Taufiq-Yap YH, et al. Reusable gypsum based catalyst for synthesis of glycerol carbonate from glycerol and urea. Appl Catal A Gen 2015;502:312–19. [13] Khayoon MS, Hameed BH. Mg1+x Ca1−x O2 as reusable and efficient heterogeneous catalyst for the synthesis of glycerol carbonate via the transesterification of glycerol with dimethyl carbonate. Appl Catal A Gen 2013;466: 272–281. [14] Simanjuntak FSH, Choi JS, Lee G, Lee HJ, Lee SD, Cheong M, et al. Synthesis of glycerol carbonate from the transesterification of dimethyl carbonate with glycerol using DABCO and DABCO-anchored Merrifield resin. Appl Catal B Environ 2015;165:642–50. [15] Ochoa-Gómez JR, Gómez-Jiménez-Aberasturi O, Maestro-Madurga B, Pesquera-Rodríguez A, Ramírez-López C, Lorenzo-Ibarreta L, et al. Synthesis of glycerol carbonate from glycerol and dimethyl carbonate by transesterification: catalyst screening and reaction optimization. Appl Catal A Gen 2009;366: 315–324. [16] Varkolu M, Burri DR, Kamaraju SRR, Jonnalagadda SB, Van Zyl WE. Transesterification of glycerol with dimethyl carbonate over nanocrystalline ordered mesoporous MgO–ZrO2 solid base catalyst. J Porous Mater 2015;23(1):185–93. [17] Zheng L, Xia S, Hou Z, Zhang M, Hou Z. Transesterification of glycerol with dimethyl carbonate over Mg–Al hydrotalcites. Chin J Catal 2014;35:310–18. [18] Liu Z, Wang J, Kang M, Yin N, Wang X, Tan Y, et al. Structure-activity correlations of LiNO3 /Mg4 AlO5.5 catalysts for glycerol carbonate synthesis from glycerol and dimethyl carbonate. J Ind Eng Chem 2015;21:394–9. [19] Simanjuntak FSH, Widyaya VT, Kim CS, Ahn BS, Kim YJ, Lee H. Synthesis of glycerol carbonate from glycerol and dimethyl carbonate using magnesium-lanthanum mixed oxide catalyst. Chem Eng Sci 2013;94:265–70. [20] Conesa JA, Domene A. Gasification and pyrolysis of Posidonia oceanica in the presence of dolomite. J Anal Appl Pyrolysis 2015;113:680–9. [21] Jaiyen S, Naree T, Ngamcharussrivichai C. Comparative study of natural dolomitic rock and waste mixed seashells as heterogeneous catalysts for the methanolysis of palm oil to biodiesel. Renew Energy 2015;74:433–40. [22] Ilgen O. Dolomite as a heterogeneous catalyst for transesterification of canola oil. Fuel Process Technol 2011;92:452–5. [23] Algoufi YT, Hameed BH. Synthesis of glycerol carbonate by transesterification of glycerol with dimethyl carbonate over K-zeolite derived from coal fly ash. Fuel Process Technol 2014;126:5–11. [24] Shajaratun Nur ZA, Taufiq-Yap YH, Rabiah Nizah MF, Teo SH, Syazwani ON, Islam A. Production of biodiesel from palm oil using modified Malaysian natural dolomites. Energy Convers Manag 2014;78:738–44. [25] a Olutoye M, Lee SC, Hameed BH. Synthesis of fatty acid methyl ester from palm oil (Elaeis guineensis) with Ky (MgCa)2x O3 as heterogeneous catalyst. Bioresour Technol 2011;102:10777–83. [26] Simanjuntak FSH, Lim SR, Ahn BS, Kim HS, Lee H. Surfactant-assisted synthesis of MgO: characterization and catalytic activity on the transesterification of dimethyl carbonate with glycerol. Appl Catal A Gen 2014;484:33–8. [27] O’Neill RE, Vanoye L, De Bellefon C, Aiouache F. Aldol-condensation of furfural by activated dolomite catalyst. Appl Catal B Environ 2014;144:46–56. [28] Esteban J, Ladero M, Molinero L, García-Ochoa F. Liquid–liquid equilibria for the ternary systems DMC–methanol–glycerol, DMC–glycerol carbonate–glycerol and the quaternary system DMC–methanol–glycerol carbonate–glycerol at catalytic reacting temperatures. Chem Eng Res Des 2014;92:2797–805.

Y.T. Algoufi et al. / Journal of the Taiwan Institute of Chemical Engineers 70 (2017) 179–187 [29] Simanjuntak FSH, Widyaya VT, Kim CS, Ahn BS, Kim YJ, Lee H. Synthesis of glycerol carbonate from glycerol and dimethyl carbonate using magnesium-lanthanum mixed oxide catalyst. Chem Eng Sci 2013;94: 265–270. [30] Simanjuntak FSH, Kim TK, Lee SD, Ahn BS, Kim HS, Lee H. CaO-catalyzed synthesis of glycerol carbonate from glycerol and dimethyl carbonate: isolation and characterization of an active Ca species. Appl Catal A Gen 2011;401:220–5.

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[31] Liu P, Derchi M, Hensen EJM. Promotional effect of transition metal doping on the basicity and activity of calcined hydrotalcite catalysts for glycerol carbonate synthesis. Appl Catal B Environ 2014;144:135–43. [32] Conesa JA, Domene A. Gasification and pyrolysis of Posidonia oceanica in the presence of dolomite. J Anal Appl Pyrolysis 2015;113:680–9. [33] Kouzu M, Hidaka JS. Transesterification of vegetable oil into biodiesel catalyzed by CaO: a review. Fuel 2012;93:1–12.