Biodiesel is an important renewable biofuel which can be produced from animal fat and vegetable oil via transesterification. The application of biodiesel can help reduce the dependence on fossil energy. Glycerol is a byproduct in the production of biodiesel, and the rapidly rising production of biodiesel leads to a large surplus of glycerol. The catalytic conversion of glycerol to valuable products has become a hot research topic [1, 2, 3, 4]. Several products, such as propanediols [5, 6, 7, 8, 9, 10], dihydroxyacetone [11, 12, 13], acrolein [14, 15, 16], and glycerol carbonate (GC) [17, 18, 19], can be synthesized from glycerol. Among these products, GC is one of the most attractive derivatives of glycerol because of its high reactivity with alcohols, amines, carboxylic acids, ketones, and isocyanates, which can yield a wide range of valuable products [20]. GC itself can be used as a solvent in NMR analysis and organic synthesis [20]. In addition, it has been reported that GC is an important alternative solvent in lithium batteries, cosmetics, surfactants, and polymer synthesis [20]. Several routes, such as phosgenation between glycerol and phosgene [21], direct carboxylation of glycerol with CO2 [22, 23], and the glycerolysis of urea [24, 25, 26, 27, 28], have been proposed for the production of GC from glycerol. Among these, the direct reaction between glycerol and CO2 is an attractive route, but it must be performed at high pressure and the yield of GC is low [29, 30].
Transesterification of glycerol with dimethyl carbonate (DMC) is also an attractive way to produce GC under mild conditions [17, 18]. It was reported that K2CO3 [31], CaO [32], K-zeolite derived from coal fly ash [33], Mg/Zr/Sr [34] and Mg/Al/Zr [35] mixed oxides were effective for this reaction. The rate of this reaction depended on the basicity of the catalyst [36], but the selectivity of GC decreased when a strong base (such as NaOH, KOH, and K2CO3) was used because of the formation of glycidol [37]. More recently, Liu et al. [19] disclosed that the activity of transition metal doped hydrotalcites (HT-M) for the transesterification between glycerol and DMC depended on the surface density of basic sites.
Hydrocalumite is a layered double hydroxide (LDH) with a well ordered Ca-Al distribution in the hydroxide layers, while the anions and water are highly ordered in the interlayer spaces [38, 39]. The calcined hydrocalumite-like compound mainly contains Ca12Al14O33 and CaO, and it shows strong basicity in CO2 adsorption [40] and in the production of biodiesel via transesterification [41, 42]. In this work, a series of Ca-Al hydrocalumite with different Ca/Al ratios (1-6) were synthesized and used in the transesterification of glycerol with DMC to GC under mild conditions. The activity and structure of the calcined hydrocalumite were discussed.
Ca-Al hydrocalumite (Ca/Al = 1-6) was prepared by a conventional coprecipitation method. CaCl2 (12.5-75 mmol) and AlCl3·6H2O (12.5 mmol) were dissolved in 75 mL deionized water (solution A) [43]. Solution B was an aqueous solution of 2 mol/L NaOH. Solutions A and B were simultaneously added to a 250 mL three-necked flask under a purified N2 flow at 30 °C with the pH of the mixture controlled at 11 during the addition [44]. The resulting suspension was filtered and washed thoroughly with deionized water until the pH of the filtrate reached about 7.0. The precipitate was dried at 110 °C overnight and calcined at 400−1000 °C for 4 h under a purified N2 flow. The composition of the Ca-Al hydrocalumites was determined using inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The obtained Ca-Al hydrocalumite was denoted as CaxAl-y, in which x represented the molar ratio of Ca/Al and y denoted the calcination temperature.
X-ray diffraction (XRD) patterns were collected on a Rigaku D/MAX-2500 diffractometer with a 2q range of 5°−80° using Cu Kα radiation (λ = 0.15406 nm). Thermogravimetric differential scanning (TG-DSC) analysis of the samples from room temperature to 800 °C was carried out on a Netzsch STA409 thermobalance system using a heating rate of 10 °C/min under N2 nitrogen flow. Scanning electron microscopic (SEM) images were obtained on a Leo Evo Series SEM (VP 1430, Germany). Samples were coated with gold to avoid charging. Analysis was carried out at an accelerating voltage of 15 kV. The N2 adsorption-desorption isotherm was measured at -196 °C using a TriStar II analyzer after pretreatment of the sample at 150 °C for 10 h. The basicity of the catalyst was determined by temperature-programmed desorption of CO2 (CO2-TPD). In this experiment, the sample was first treated in Ar at 600 °C for 30 min, cooled to 50 °C and exposed to 20% CO2 (50 mL/min, Ar in balance) for 30 min. It was then purged with Ar for 1 h at 100 °C and heated linearly at 15 °C/min to 800 °C in 50 mL/min Ar. CO2 (m/e = 44) in the effluent was recorded continuously as a function of temperature.
Measured amounts of DMC (45 mmol) and glycerol (45 mmol) were mixed in a 10 mL round bottomed glass reactor fitted with a magnetic stirrer and a reflux condenser. The mixture was first heated under stirring to 70 °C, and the catalyst (0.15 g) was added to start the reaction. After reacting for 3 h, the solid catalyst was removed by centrifugation, and the supernatant liquid was analyzed using a gas chromatograph (Shimadzu, 14B) equipped with a 30-m capillary column (DB-WAX 52 CB, USA) and a flame ionization detector. All products detected in the liquid were identified by a gas chromatography-mass spectrometry system (GC-MS, Agilent 6890) and quantified by an external calibration method. The product selectivity was calculated on a carbon basis.
Figure 1 shows the XRD patterns of the Ca-Al samples. All the diffraction peaks of the (002), (004), (010), and (006) planes of hydrocalumite (Ca4Al2O6Cl2∙10H2O, JCPDS 00-031- 0245) [44, 45] were detected. This result indicates that a hydrocalumite-like compound formed and a high degree of crystallinity was detected in all those samples with Ca/Al = 2-6. On the other hand, Al(OH)3 (JCPDS 00-033-0018) was formed in Ca1Al due to the high content of Al [40].
Figure 2 shows typical SEM images of the Ca-Al hydrocalumites. Solid Ca-Al lamellas were formed in all samples. A clear image of separated lamellas was detected in Ca2Al and Ca3Al. The platelet in Ca6Al was smaller and an amorphous solid (in white circle) was formed in Ca1Al. The peony-shaped crystal in Ca2Al showed the rough outline of hydrocalumite. The high resolution image of this sample is shown in Fig. 2(c). We can also see some “rosette” particles in Ca3Al (Fig. 2(d)) [46].
Figure 3 presents the TG-DSC curves of the hydrocalumite (Ca2Al). The TG curve indicated that there were four stages of weight losses at 50−152, 265−329, 420-494, and 600−750 °C. The first stage was attributed to the removal of adsorbed water, the second stage was due to the loss of bound water or chloride in the interlayer of the hydrocalumite [47], and the third and the fourth stage was attributed to the further removal of hydroxyl groups. At the same time, an exothermic peak was detected at 786 °C without obvious weight loss, which was attributed to the decomposition of layered Ca4Al2O7 to Ca12Al14O33 and CaO [48, 49].
Figure 4 shows the XRD patterns of Ca-Al hydrocalumite calcined at different temperatures for 4 h. The peaks assigned to the (002), (004), and (006) planes of hydrocalumite disappeared with the loss of water, chloride, and hydroxide after calcination at 400 °C. A broad peak at 25°-35° in the sample calcined at 600 °C was attributed to the formation of amorphous CaO due to the partial collapse of layered structure. Calcination above 800 °C led to the generation of mayenite Ca12Al14O33 (JCPDS 00-048-1882) [39, 50]. These results fitted well the TG-DSC analysis.
Figure 5 shows the typical SEM image of hydrocalumite (Ca2Al) calcined at 800 °C for 4 h. It is obvious that the layered structure was broken and the lamellas had accumulated as one hard stone.
Figure 6 presents the CO2-TPD profiles of Ca2Al hydrocalumite calcined at different temperatures. The desorption profiles were deconvoluted to incorporate two kinds of basic sites. The first (250 to 600 °C) was ascribed to the desorption of CO2 adsorbed on the surface of LDH lamella and Lewis basic sites of Ca in the LDH framework [49, 50]. The proportion of these basic sites was higher in the samples calcined at 400 and 600 °C. The second peak (desorbed above 600 °C) was attributed to the desorption of CO2 adsorbed on CaO and Ca12Al14O33. The proportion of these basic sites was calculated and summarized in Table 1. The calculated amount of total basic sites decreased along with the surface area when the calcination temperature was increased from 400 to 1000 °C. But it is interesting to note that the proportion of strong basic sites reached a maximum value (56.9 µmol/g) when the sample was calcined at 800 °C.
Figure 7 shows the activity of Ca2Al hydrocalumite calcined at different temperatures for the transesterification reaction between glycerol and DMC at 70 °C. The conversion of glycerol increased steadily from 27% to 75% when the calcination temperature rose from 400 to 800 °C, and the selectivity of GC was higher than 97%. The conversion of glycerol and selectivity of GC over the hydrocalumite calcined at 1000 °C decreased to 63% and 92%, respectively. These results indicated that calcined Ca-Al hydrocalumites were effective catalysts for the transesterification reaction between glycerol and DMC, and strong basic sites were more active. The conversion of glycerol depended mainly on the proportion of strong basic sites (Table 1).
Reaction conditions: catalyst (CaxAl hydrocalumite calcined at 800 °C for 4 h) 0.15 g, glycerol 45 mmol, DMC 45 mmol, 70 °C, 3 h.
Table 2 summarizes the activity of calcined (at 800 °C for 4 h) CaxAl hydrocalumites with different Ca/Al ratios for the transesterification reaction between glycerol and DMC at 70 °C. The conversion of glycerol increased from 51% to 75% when the Ca/Al ratio increased from 1 to 2. The selectivity to GC remained higher than 96%. However, the conversion of glycerol and the selectivity of GC decreased slowly on the samples with higher Ca/Al ratios (> 3). The characterization disclosed that Ca12Al14O33 and CaO were formed in the calcined (at 800 °C for 4 h) Ca/Al hydrocalumites. Although CaO can catalyze this reaction, the selectivity of the desired product was low (62%). At the same time, CaO can combine with glycerol and DMC as Ca(C3H7O3)(OCO2CH3) during the reaction [51]. On the basis of these results, we conclude that Ca12Al14O33 derived from calcined Ca/Al hydrocalumite is more selective to GC.
Table 3 summarizes the activity of calcined hydrocalumite (Ca2Al, at 800 °C for 4 h) for the transesterification reaction between glycerol and DMC at 70 °C with different feed composition (DMC/glycerol ratio). The conversion of glycerol reached 93% when the DMC/glycerol molar ratio was 3. Further increase in the amount of DMC in the feed did not promote the conversion of glycerol because the intersolubility of DMC and glycerol is low.
Reaction conditions: catalyst (Ca2Al hydrocalumite calcined at 800 °C for 4 h) 0.15 g, glycerol 45 mmol, 70 °C, 3 h.
Figure 8 presents the performance of a recycled catalyst (Ca2Al) for the transesterification reaction between glycerol and DMC at 70 °C. The reused catalyst was collected and dried in vacuum for several hours. The conversion of glycerol decreased from 84% to 38% as the recycle number increased. XRD analysis of the 6-time recycled catalyst disclosed that the crystalline structure of Ca12Al14O33 in the recycled catalyst was similar to that of the fresh calcined sample, but the CaO crystalline phase had disappeared (Fig. 9). ICP analysis also confirmed that the content of Ca2+ decreased from 12.2 mmol/g (in fresh catalyst) to 9.64 mmol/g (in the 6-times recycled catalyst). These results indicated that the deactivation of the calcined Ca-Al hydrocalumite was due to the leaching of Ca and the loss of catalyst during the recycle experiment (the weight of the catalyst decreased from the initial 0.150 to 0.075 g after 6 recycles).
Ca-Al hydrocalumite (Ca/Al = 2-6) with high crystallinity was synthesized by a conventional coprecipitation method in N2 atmosphere. Calcined Ca-Al hydrocalumite was an effective catalyst for the transesterification of glycerol with DMC and gave a high selectivity towards GC under mild conditions. The highest conversion of glycerol at 70 °C reached 93% with 97% selectivity to GC over the calcined hydrocalumite Ca2Al-800. Characterization disclosed that Ca12Al14O33 was stable, but the CaO crystalline phase disappeared during recycle experiments and the leaching of CaO brought deactivation.
2015, Vol. 36 Issue (10): 1759-1765
PDF (1777 KB)
2015, Vol. 36 Issue (10): 1759-1765
PDF (1777 KB)