(458c) Fundamentals of Competitive Adsorption Phenomena within Dilute, Multi-Component Aqueous Mixtures

Increasing global energy demand combined with decreasing fossil fuel supplies serve as the impetus for the development of alternative sources of fuels and chemical intermediates. Toward this goal, many biologically based processes have been recently invented to convert biomass into short chain oxygenated molecules that can be subsequently converted to fuels and chemicals. The primary advantage of these biological processes are their high selectivity and mild operating conditions. However, the primary drawback is that the final product is typically produced as a very dilute aqueous solution. The main objective of this study is to investigate the major factors that affect the performance of solid adsorbent materials for the extraction of small chain alcohols, aldehydes, and ketones from dilute aqueous solutions that contain a wide range of molecules and ions, similar to those derived from fermentation processes. To understand the fundamentals of adsorption using microporous, aluminosilicate materials, adsorption isotherms were collected for alcohols and aldehydes on proton and Na-exchanged forms of BEA, MOR, FER, FAU and MFI zeolites. Langmuir and Freundlich adsorption models were used to develop a quantitative understanding of the relationship between adsorbent structure and performance by determining the effect of solution composition and adsorbent structure on the parameters of each model (equilibria constants, saturation capacities, and interaction parameters.)

The effects of different zeolite structure on the parameters for the Langmuir model were initially probed using butanol as the adsorbate. Maximum adsorption capacities were similar for all zeolites tested (2.5 mmol/g on FAU, 1.8 mmol/g on MOR, 2.2 mmol/g on MFI, and 2.2 mmol/g on BEA). The adsorption equilibrium coefficient parameter was lower for FAU and MOR (30 L/mol) compared to BEA and MFI (140 L/mol). For the Freundlich model, the adsorption equilibrium coefficient was inversely proportional to zeolite pore size whereas the adsorption capacity parameter was directly proportional to molecule size.

Multi-component mixtures were used to probe competitive adsorption on BEA zeolite. These systems were analyzed using the Langmuir competitive adsorption model. For an equimolar binary system of butanol with smaller alcohols (propanol or ethanol) on BEA zeolite, two phenomena were observed, competitive adsorption between butanol and propanol and co-adsorption of ethanol and butanol. Maximum capacities for propanol were unaffected by the presence of butanol (~2.2 mmol/g), however, the maximum capacity for ethanol increased compared to its value in a single component system (from 0.9 to 2.5 mmol/g). The value of the adsorption equilibrium coefficient was the same for butanol in both binary (with propanol) and single component systems (134 mol/g), however competitive adsorption with propanol (with an adsorption coefficient of 22 L/mol) caused a decrease in its maximum capacity. In the presence of ethanol, the adsorption equilibrium coefficient for butanol increased from 134 to 241 L/mol, which led to a corresponding increase in butanol uptake.