# Improving Sorbents for Glycerol Capture in Biodiesel Refinement
## 1 Introduction  
Biodiesel is produced by transesterification of animal fat, vegetable oil, or waste cooking oil with an alcohol. A number of studies have focused on evaluating the economic viability of biodiesel [1; 2; 3; 4]. Production costs are approximately 0.5 USD/L, 1.5 times that of petroleum-based diesel, with virgin feedstocks comprising 50 to 70% of costs [2; 3]. The feedstock costs can be reduced through the use of waste cooking oils and similar products, but these materials increase the complexity of synthesis and, often, the presence of contaminants [4]. In addition to providing a renewable source of energy, biodiesel is attractive as an alternative fuel due to related reduced environmental impact; use of biofuels decreases emission of atmospheric pollutants [5]. Procurement of water together with liquid and solid waste disposal comprise up to 10% of manufacturing costs [2; 3] while also influencing the environmental impact of this alternative fuel source. As an example, multiple wash steps for biodiesel purification can generate gallons of waste water for each gallon of usable biofuel. Biodiesel contaminants include glycerol, alcohols, free fatty acids, surfactants, and residual catalyst. Glycerol is a significant contaminant in biodiesel, but it can be recovered and purified to aid in offsetting the costs of biodiesel production [3].  
Currently, dry wash media use is largely limited to small operations and hobbyists. These media can be used for biodiesel contaminant removal without excessive waste water production. This approach also reduces production time and requires less space when compared to water washing. BD ZorbX and DudaLite (DW-R10(r)) ion exchange resin are commercial examples of these types of materials. BD ZorbX is a mixture of cellulose fibers from hardwood intended to absorb a variety of contaminantsincluding surfactants, glycerol, and catalyst; Eco2 Pure(tm) is a similar material. DW-R10 is intended as a polishing step following bulk glycerol removal. Claims for this sorbent are removal of surfactants, glycerol, catalyst, and water. Drionex, Purolite(r) PD206, and Magnesol D-Sol are intended for similar applications [6].  
Nanoporous scaffolds within a macrotextured structure offer an alternative to currently available commercial dry wash media. The materials under consideration for this study are hybrid inorganic-organic materials [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17] in which inorganic and organic moieties are mixed on the molecular scale through the use of bridged polysilsesquioxane precursors [18]. The materials offer large macropores with a templated mesopore structure [7; 12; 17; 19]. While a number of applications have been reported for mesoporousorbents, additional applications can be facilitated through the use of these hierarchical structures; the macroscale features reduce diffusion limitations and increase access to the mesopore volumes [20; 21]. We have previously described materials of this type for the capture of nitrogenergetic and organophosphate targets [20; 22]. The materials of the current study were developed for adsorption and removal of glycerol, which can result in fuel tank deposits, injector corrosion, and higher emissions of aldehydes. The materials are synthesized with varying pore wall composition then sulfonated to promote hydrogen bonding interactions with glycerol. While sulfonation of the material provides a similar functionality to that present in several of the commercial dry wash media mentioned above, incorporation of the sulfonate groups on and within the scaffold and adjacent to varying structures provides an opportunity to evaluate the potential for improving sorbent performance through scaffold tailoring.