Transesterification of rapeseed oil to biodiesel over Zr-dopped MgAl hydrotalcites
Graphical abstract
Introduction
Fatty acids methyl esters (FAME), widely known as biodiesel, are one of the key biocomponents commercially produced from vegetable oils. Because of its physicochemical properties, similar to diesel fuels, biodiesel is used as a biocomponent of diesel-type engine fuels. Biodiesel is produced by methanolysis of vegetable oils in the presence of a number of catalysts, including basic and acidic, homogeneous or heterogeneous and also is produced biotechnologically. The industrial processes of the synthesis of biodiesel most often use homogeneous catalysts, such as alkali hydroxides, alkali methoxides or mineral acids. However, the use of strong alkaline catalysts causes the formation of considerable amounts of waste water and soaps. In the case of using mineral acids the reaction medium is highly corrosive. Technological problems, that occurred during the transesterification of vegetable oils by methanol were described in details in the literature [1], [2], [3], [4]. A promising alternative for homogenous catalysts in synthesis of biodiesel are heterogeneous catalysts [5], [6], [7], [8], [9], [10], [11], [12], [13].
The most commonly used heterogeneous catalysts for the synthesis of biodiesel are Mg, Ca, Sr and Cs oxides [14], alkali hydroxides, alkali metal salts immobilized on supports [15], [16] and zeolites modified with K and Cs cations [17]. MgAl hydrotalcites also might be used as catalysts suitable for methanolysis of glycerides. They have a characteristic double-layered structure build with Mg(OH)x and Al(OH)y structural fragments. Their catalytic properties are determined by the presence of Brønsted and Lewis basic centers. The highest activity is observed for Mg to Al molar ratio 3:1 (calculated as metal). Fresh prepared and dried hydrotalcites are characterized in low catalytic activity, but after calcination at temperatures of 723–823 K result in a significant increase of their specific surface area up to 300 m2/g and catalytic activity. Calcined hydrotalcites are widely used as basic catalysts, also in biodiesel synthesis [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. The detailed impact of the structure of hydrotalcites on the synthesis of biodiesel was studied by Cantrell [19]. Results of the studies on the use of MgAl hydrotalcites in biodiesel synthesis show, that they are characterized by an average activity and allow only for a relatively low yield of biodiesel. In the one of the earlier Leclercq paper only 34% conversion of glycerides [18] were reported after using MgAl hydrotalcite for 22 h at 340–344 K (reaction mixture reflux). In the later studies, Xie obtained only 67% conversion of soybean oil at reflux temperature for molar ratio methanol to oil 15:1 [20]. Zeng achieved 90% of conversion after 12 h for MgAl hydrotalcite (Mg to Al molar ratio 3:1), at reflux temperature [23]. A similar conversion was reported by Li at 433 K for the molar ratio methanol to oil 48: 1 [24]. Di Serio and Alvarez independently carried out systematic studies of transesterification of glycerides at 473 K with the use of MgAl hydrotalcites as catalysts [21], [28]. Although they have obtained 96% conversion of glycerides, the process required a long reaction time, up to 36 h. Despite of satisfactory conversion rate the selectivity toward biodiesel was not so high and did not exceed 88%. A considerable improvement of selectivity was reported Barakos using gradual removing the glycerol phase during the reaction [25]. The procedure allowed for a significant increase in the selectivity to FAME up to 95%. The high conversion of glycerides of 96% obtained also Trakarnpruk by conducting the reaction at temperature of 373 K for 35 h [22].
MgAl hydrotalcites were often modified by incorporation of a third atom into their structure at the synthesis step. These can be either MII, MIII or MIV atoms. Interesting materials, in the context of their possibility of using as heterogeneous catalysts, could present MgAl hydrotalcites modified with Zr atoms [29], [30], [31], [32]. Intissar suggested, that in the process of co-precipitation of Mg, Al and Zr salts Zr(IV) hydroxide (ZrO2 hydrate) is formed, which partially incorporates itself into the structure of hydrotalcite, but only into its surface layer. A detailed analysis of XRD diffraction patterns showed, that Zr atoms are covalently bonded at the surface of MgAl hydrotalcite framework and part of the ZrO bonds is pointing outwards (as ZrOH). As described by Li the presence of free OH groups in the hydrotalcites have positive impact on the transesterification reaction of vegetable oils with methanol [24]. In addition, as it is apparent from McNeff's work, introduction to the structure of MgAl hydrotalcites Ti and Zr atoms improves the mechanical strength of the formed catalyst [33].
Section snippets
Materials
Mg and Al nitrates (99.8% purity) used in the synthesis were purchased from Scharlau. Zirconium (IV) oxynitrate hydrate (99% purity, Zr content ca. 27 wt%) was purchased from Fluka. Methanol (99% purity), NaOH (99% purity) and Na2CO3 (99% purity) were purchased from POCh (Poland). Refined rapeseed oil was commercially available and used without further purification.
Catalysts preparation
ZrMgAl hydrotalcites were synthesized using a well-known co-precipitation method based on the procedure described by Tichit [31]. Zr
Catalysts characterization
Fig. 1 shows XRD patterns of HTZr5 (a) and HTZR15 (b) samples, as-synthesized and calcined at 773 K for 3 h. As shown in Fig. 1, the diffractograms recorded for samples HTZr5 and HTZr15 are very similar. The recorded reflexes take a form of broad bands, very different from the typical diffraction patterns recorded for MgAl hydrotalcites [35]. The appearance of additional reflexes, in particular at 30 theta may indicate the presence in the structure of the hydrotalcite crystallites ZrO2·H2O. The
Conclusions
In the present study Zr-dopped MgAl hydrotalcites have been synthesized and their catalytic performance in the transesterification of rapeseed oil with methanol have been investigated and also compared with MgAl hydrotalcite as a reference catalyst. ZrMgAl hydrotalcites are very active as basic catalysts for transesterification process. Their high activity could be attributed to the presence of strong basic sites on the surface and OH group attached to the Zr atoms presents in separate grains
References (38)
- et al.
Renev. Sust. Energy Rev.
(2000) - et al.
Catal. Today
(2004) - et al.
Fuel
(2009) - et al.
Appl. Catal. A: Gener.
(2009) - et al.
Appl. Catal. A: Gener.
(2010) - et al.
Bioresour. Technol.
(2011) - et al.
Fuel
(2011) - et al.
Appl. Energy
(2012) - et al.
Catal. Today
(2008) - et al.
J. Mol. Catal. A: Chem.
(2006)