Letter www.acsami.org
pKa‑Dependent Facilitated Transport of CO2 across Hyperthin Polyelectrolyte Multilayers Cen Lin, Erwin R. Stedronsky, and Steven L. Regen* Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015, United States S Supporting Information *
ABSTRACT: Hyperthin (ca. 20−30 nm thick) polyelectrolyte multilayers have been fabricated that are capable of facilitated transport of CO2. These membranes were fabricated from polycations bearing pendant groups of varying basicity plus poly(sodium 4-styrenesulfonate) as a polycounterion. A strong dependency of such transport on the basicity of the pendant groups (i.e., fixed carrier sites) has been found, where pKa values in the range of ca. 5−7 appear optimal.
KEYWORDS: facilitate, transport, hyperthin, polyelectrolyte, carbon dioxide, permeation ne of the major challenges in the fight against global warming has been to develop a cost-effective method for separating CO2 and N2 from flue gasthe major source of man-made CO2. With this goal in mind, considerable effort has been focusing on the use of polymeric membranes because of their minimal energy and capital requirements for processing.1−7 An often used approach to enhance the CO2/N2 permeation selectivity of membranes has been to incorporate carriers and thereby establish facilitated transport pathways.8−10 For example, the use of mobile carriers that can bind CO2 on the high pressure side of a liquid membrane, facilitate its transport via direct carrier-to-carrier exchange to the low pressure side, and then release it is now well-established.8−11 In addition, fixed carriers that are covalently attached to a polymer backbone have also been found capable of facilitating the transport of CO2. For the latter, it has been suggested that the permeant “hops” from one carrier to another as it crosses the membrane (Figure 1).12−18 Because of their greater physical robustness, fixed carrier membranes have attracted special attention from a technological standpoint.12−18 To date, nearly all membranes that exhibit facilitated transport of CO2 have required (i) the presence of a primary, secondary, or tertiary amine (i.e., a strong base), (ii) the presence of water, and (iii) the presumptive participation of bicarbonate ions. Recently, competitive processes have emerged in which flue gas is dried before it is passed through hollowfiber membranes at subambient temperatures.19 Significantly, this “cold membrane operation” has led to a 2- to 4-fold increase in the CO2/N2 selectivity with a minimal decrease in the permeability of CO2, as compared with the use of ambient temperature conditions. Moreover, since the heat that is liberated from the combustion of fossil fuels can be used to
O
© XXXX American Chemical Society
Figure 1. Stylized illustration of a fixed carrier membrane showing hypothetical “hopping” of a permeant molecule from one carrier site to another as it crosses the membrane.
regenerate drying agents, no additional input of energy is necessary. The feasibility of being able to separate CO2 from N2 under dry conditions, economically, raises the possibility that the basicity of fixed carriers could play a significant role in the facilitated transport of CO2. In particular, because “bare” CO2 molecules must bind, reversibly, to the fixed carriers via acid/ base interaction, one might expect that their “on” and “off” rates would be influenced by the basicity of the carrier and that these rates, in turn, could affect the transport of CO2. In general, gas transport across polymer membranes is most commonly described by the solution-diffusion model, which is based on eqs 1 and 2. Here J is the flux of the permeant, P is the permeability coefficient that characterizes a given membrane/ Received: March 29, 2017 Accepted: May 31, 2017 Published: May 31, 2017 A
DOI: 10.1021/acsami.7b04473 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces permeant combination, p is the pressure gradient applied across the membrane, l is the thickness of the membrane and D and S are diffusivity and solubility constants. Because of the inverse relationship between flux and membrane thickness, very thin membranes have an inherent advantage. To the best of our knowledge, the feasibility of carrying out facilitated transport across hyperthin (
