*2.1.4 Catalytic reaction method*

The etherification reaction of glycerol to produce value-added diglycerol has been extensively studied either using homogeneous or heterogenous alkali or acid catalysts. The etherification reaction is normally conducted under solvent-free condition for economical and environmental reasons [7]. Etherification is a condensation process whereby two glycerol molecules will react to form ether bond in between the molecules by removing water molecule. This reaction is also called etherification since the final product is in ether form as shown in **Figure 2**. Glycerol oligomers can be in linear, branched, or cyclic form [11]. The formation of different isomers is affected by the reaction conditions such as temperature, time, catalyst type, catalyst loading as well as the starting raw material for diglycerol synthesis. Diglycerols are mostly obtained from the oligomerization of glycerol catalyzed by acid or based catalysts.

deprotonated, yielding the respective diglycerol [1]. Acid-catalyst oligomerization used sulfuric acid at 280°C in 2 hours giving more than 90% conversion of glycerol, and the main oligomers were triglycerol and tetraglycerol which only make the 20% of overall component, showing other side products were dominant in the reaction

*SN1-type mechanism for glycerol oligomerization in acid-catalyzed homogeneous reaction.*

*Glycerol Conversion to Diglycerol via Etherification under Microwave Irradiation*

Reported studies suggest that homogeneous acid-catalyzed reaction is generally fast but not selective for diglycerol. This could be due to the dehydration or oxidation of glycerol as secondary reactions to other undesired products. These secondary reactions may also result in the deterioration of the main product quality by chang-

The reaction with glycerol conversion under basic homogeneous catalyst is proposed to follow an SN2 mechanism as shown in **Figure 4**. In SN2, the interaction of the base OH with glycerol weakens one of the glycerol OH bonds and enhances nucleophilic character of the hydroxyl oxygen. The attack of this polarized glycerol

to a carbon of a second glycerol with simultaneous split off water results in

*SN2-type mechanism for glycerol oligomerization in base-catalyzed homogeneous reaction.*

Several homogeneous bases have also been studied as catalysts in glycerol etherification. Depending on their basicity and solubility in glycerol, the following order of activity was reported under reaction temperature of 260°C at 4 hours with

K2CO3>Li2CO3 > Na2CO3>KOH > NaOH > CH3ONa>LiOH > MgCO3>CaCO3*:*

Based on solubility measurements, the higher activity of carbonates than that of hydroxides was indeed ascribed to a better solubility of the former in glycerol and in the polyglycerol mixture at elevated temperature [6]. However, there are several studies presented that contradict the results from the proposed theory on solubility. A study conducted by [9] of glycerol conversion using Cs2CO3, CsOH, and CsHCO3 showed 20% glycerol conversion and 100% diglycerol selectivity; hence, different anions did not alter the reaction characteristics. From the research carried out by

ing its color, making the final product darker in appearance.

which were hardly identified [6].

*DOI: http://dx.doi.org/10.5772/intechopen.90513*

*2.2.2 Homogeneous base catalyst*

diglycerol.

**Figure 4.**

**121**

**Figure 3.**

2.5 mol% of catalyst:
