# Improving Sorbents for Glycerol Capture in Biodiesel Refinement
## 2 Results
### Glycerol Capture from Aqueous Solution
It is likely that pore collapse contributes significantly to the lack of capacity observed for the ED11a(4) variant; specific surface area is reduced from 478 m\({}^{2}\)/g to 87 m\({}^{2}\)/g following sulfuric acid treatment. Parameters from fitting the isotherm indicate greater affinity in the sulfonated sorbent. When considered on the basis of surface area, the saturation capacities are 81 mg/m\({}^{2}\) for ED11 and 207 mg/m\({}^{2}\) for ED11a(4). Addition of pendant sulfonatophenyl groups (ED11-Ph2(3)) to this sorbent resulted in a distinct improvement in per surface area glycerol binding (452 mg/m\({}^{2}\)) with further reduced per mass capacity (12.2 mg/g) associated with significant additional loss in surface area (27 m\({}^{2}\)/g). The tradeoff between pore collapse and per surface area saturation capacities is further illustrated by the D13 materials. D13b(6) retains 337 m\({}^{2}\)/g following treatment with sulfuric acid; the per surface area binding capacity of this material is six times less than that of D13a(6) or D13c(6) (\(\sim\)3 m\({}^{2}\)/g). This inverse relationship would tend to indicate that more complete sulfonation of the scaffold provides greater capacity, as expected; however, the cost of sorbent materials is driven by the mass of the sorbent, not the total surface area. Because the cost of materials is related to the mass utilized in applications, we have reported both per mass and per surface area capacities in Table 2, facilitating comparison of the materials by both metrics. Adjustment of the sulfonation conditions for the material incorporating pendant phenyl groups on the diethylbenzene bridged material (D13-Ph2(5)) produced a sorbent  
Figure 7: Glycerol binding data and Langmuir-Freundlich isotherms for D13b(6) (**a**), S85-Ph(1) (**b**), ED11-Ph(3) (**c**), and ED11 (**d**), sorbent with no sulfonation. Isotherms for 2.5, 5.0, 10, 15, 20, 25, 30, 35, and 40 mg presented. that retained much of the initial morphological characteristic as well as a high degree of sulfonation. This sorbent provided the highest per mass saturation capacities of the series (552 mg/g) with an affinity coefficient similar to those of the collapsed D13 structures (4.15 mg\({}^{-1}\)). The per surface area saturation capacity for D13-Ph2(5) is more than double that of the D13b(6) sorbent (1780 ug/m\({}^{2}\) versus 810 ug/m\({}^{2}\)). The binding of glycerol from aqueous solution by the sorbents developed under this study was compared to the commercially available sorbents BD Zorbx and DW-R10. Additional points of comparison were desired; however, repeated attempts to obtain Amberlite(tm) BD10 DRY (Dow Chemical Company, Midland, MI, USA) were unsuccessful. Amberlite gel-type ion-exchange resins are based on a styrene divinylbenzene copolymer backbone. With this in mind, two mesoporous poly(divinylbenzenes) were adapted from the literature [30] and sulfonated for comparison to the silicate based materials (Section 3.5; Figure 8). Fitting of the glycerol binding result for these PDVB resins resulted in saturation capacities of 4.7 and 11.2 mg/g for PDVB-2 and PDVB-3, respectively (Table 2). Isotherms are provided in the Supporting Information, Figure S5. Both PDVB resins outperformed the commercially obtained sorbents (Table 2). The Types 3-6 materials, with the exception of D13-Ph1(5), outperformed the PDVB resins under both per mass and per area comparisons. PDVB-3 had a higher saturation capacity than D13-Ph1(5) when compared on a per mass basis.