Gas Transport across Hyperthin Membranes

Abstract

The use of organic polymeric membranes to separate gaseous mixtures provides an attractive alternative to other methods such as selective adsorption and cryogenic distillation. The primary advantages of membrane-based separations are their relative energy efficiency and lower costs. Because the flux of a gas across a membrane is inversely proportional to the membrane's thickness, this method relies on fabricating membranes that are as thin as possible. However, as researchers have tried to produce "hyperthin" membranes (less than 100 nm), these membranes often form defects and lose their permeation selectivity. In this Account, we review some of the progress in our laboratories at Lehigh University to create hyperthin membranes with high permeation selectivities. We focus special attention on gaseous permeants that are relevant for the production of clean energy (H2 and CO2 formed from CH4) and the reduction of global warming (CO2 and N2, the major components of flue gas). Our studies make extensive use of Langmuir-Blodgett (LB) methods and porous surfactants derived from calix[6]arenes. We specially designed each surfactant to form cohesive monolayers and multilayers, and we introduced a "gluing" technique, where we cross-link porous surfactants containing quaternary ammonium groups ionically with polymeric counterions. Using ellipsometry, atomic force microscopy, X-ray photoelectron spectroscopy, monolayer isotherm, surface viscosity, and permeation measurements, we have characterized these hyperthin films. While molecular sieving appears to make a significant contribution to the permeation selectivity of some of these membranes, solution-diffusion pathways predominate. We also describe initial studies in which we formed hyperthin films from poly(ethylene glycol)-based polyelectrolytes using layer-by-layer deposition (LbL) methods. We have found remarkably high H2/CO2 and CO2/N2 permeation selectivities with these LB- and LbL-based hyperthin membranes. These results suggest that such materials may lead the way to materials that researchers can exploit to purify hydrogen produced from CH4 and to capture CO2 from flue gas.

Ultrathin Composite Polymeric Membranes for CO2/N2 Separation with Minimum Thickness and High CO2 Permeance

“…This is of particular importance for largescale gas separations such as carbonc apture membranes, where as urface area of the membrane of many square kilometers is required. [3] In this study,w eh ave shown that PIM-EA-TB(H 2 )f orms homogeneous and stable monolayersa tt he air-wateri nterface (see the Supporting Information and Figure S1) that can be transferred onto differents ubstrates by using the LS method.Each PIM-EA-TB(H 2 )L Sm onolayer deposited had at hickness of approximately 1nma nd severalm onolayers could be successivelyd eposited by using this procedure to obtain an ultrathin selective layer with the desired thickness. Highly rigid PIMs composed of bridged bicyclic units, such as ethanoanthracene (EA) and Trçger's Base (TB), [7] are of particular interest due to their highers electivity.…”

“…In this contribution, composite membranes have been obtained by the successive deposition of approximately 1nmt hickm onolayers of ap olymer of intrinsic microporosity (PIM) on top of dense membranes of the ultra-permeable poly[1-(trimethylsilyl)-1-propyne] (PTMSP). [3] For composite multilayer membranes, the cost of the polymer selectivel ayer would be significantly reduced by using ultrathin films, therefore allowing the use of high-cost, high-performance materials. [1] An ideal membrane should be as thin as possible, to maximize the flux (i.e.,permeance) across the membrane, as selectiveaspossible, to achieve an efficient separation, and mechanically robust.…”

Recent developments in membranes for efficient hydrogen purification

“…As a result, the CO 2 permeance was below the detection limit and the estimated minimum H 2 /CO 2 selectivity was 190. It should be noted that these as-developed polyelectrolyte multilayer membranes showed the exceptional H 2 /CO 2 selectivity combined with the extremely low gas permeance, even though the membrane thickness was only a dozen or even a few nanometers [166]. Therefore, these membranes show great potential in gas barrier applications, however, they are generally unattractive in large-scale hydrogen purification.…”

Membrane gas dehydration in a pressure-electric coupled field

“…However, it is difficult to fabricate a defect-free ultrathin film on a large scale for gas separations. [18][19][20] There have been advances on development of novel membrane materials, membrane structures and membrane formation processes to increase the separation performance of membranes, but much more work is still needed to apply them in practice.…”