Synthesis of glycerol carbonate from biodiesel by-product glycerol over calcined dolomite
Graphical abstract
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 non-flammable solvent suitable as environmentally friendly source of chemical intermediate [7], [8]. A series of valuable chemicals, such as dihydroxyacetone, mesoxalic acid, 1,3-propanediol, 1,3-dichloropropanol, 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 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+xCa1−xO2 [13], LiNO3/Mg4AlO5.5 [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].
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.
Section snippets
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.
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
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
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
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).
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