# Green Production of Glycerol Ketals with a Clay-Based Heterogeneous Acid Catalyst
## 1 Introduction  
With the advent of biorefineries, glycerol has become into a platform chemical, and not only a final product with a plethora of applications. In particular, new processes, or routes, and products are being sought for this polyol, while environmentally-friendly catalysts are created focusing on their simplicity on the one hand, and an enhanced selectivity and activity on the other. In the recent past, traditional industry usually used the homogeneous catalyst acids such as H\({}_{2}\)SO\({}_{4}\), HF and H\({}_{3}\)PO\({}_{4}\) for numerous processes [1; 2]. However, homogenous acid catalysis is not environmentally friendly due to catalyst toxicity, its corrosive nature and its difficulty to be recycled [3], and need for further removal operation. For those reasons, their application is limited. Thereby, the proper choice of heterogeneous catalyst constitutes a major scientific challenge in organic chemistry, in particular for glycerol recycling and upgrading. Several studies have focused on the valorization of glycerol as a byproduct in biodiesel industry [4; 5], as it has attracted an appreciable attention. Glycerol is an organic alcohol; an odorless, colorless and viscous liquid and it is fully miscible in water. It is mostly obtained from animal and plant sources by transesterification of triglycerides,their hydrolysis or by saponification processes conducted on them. This polyol has well known applications in many fields such as pharmaceutical which is used in toothpaste, mouthwashes or skin care products, food and beverage, tobacco, pulp and paper, leather, textile, detergent and explosive industries. Glycerol can also be transformed into high value-added products including polyethers, polycarbonates or alkyl resins. Moreover, due to its low toxicity and inflammability, it is exploited in medicinal applications [6]. Additionally, large quantities of glycerol are available as a reaction by-product because of the development of biodiesel production by transesterification of vegetable oils [7]. Numerous main pathways of glycerol transformation have been reported in literature involving hydrogenolysis [8], dehydration [9], oxidation [10], carbonation [5; 11], etherification [12; 13], esterification [14; 15], acetalization [14; 16] and ketalization [17; 18]. Therefore, a considerable number of chemical intermediates and commodities can be obtained from glycerol through optimized catalytic routes, as shown in Figure 1[19]. Glycerol acetals and ketals can be synthesized through the acid-catalyzed reaction of glycerol with aldehydes and ketones, respectively. Glycerol ketals are interesting as fuel additives (oxygenates) [20; 21; 22], fuel antioxidants [23] and as reactive oxygen species (ROS) scavengers able to act in-vitro and in-vivo [24]. Solketal, in particular, is known to reduce gum formation and particulate emissions, enhance fuel flow in cold conditions, improve stability towards oxidation, and boost octane number when it is added to gasoline, which contributes to reducing the environmental impact (the LD\({}_{50}\) for fish is higher than 3100 ppm for solketal, while for MTBE it is 1000 ppm) [25]. Some heterogeneous catalysts that have been applied for the acetalization of glycerol are Amberlyst-46, Amberlyst-36 [26], Amberlyst-15 [27], beta-zeolite [28], montmorillonite [18], acidic clay supported heteropolycatalyst [29] and acidic VMCM-41 [6]. Within this context, we studied the application of acid-activated clay as a heterogeneous catalyst owing to easy external adsorption and desorption of chemicals during the reaction, its low price, the small effort to apply for its separation from the liquid media and its low environmental impact [30]. Clays and modified clays are widely included in chemical reactions such as synthesis and rearrangements of heterocycles, additions reactions, condensations reactions, Diels-Alders reaction, imine synthesis, epoxidation and several more [29; 31; 32].