Patent Application
20180126337
The present invention relates to the field of membranes.
Membranes are an emerging technology to replace conventional gas separation and purification strategies utilizing absorption or adsorption based processes due to lower energy requirements, less capital cost, and lower physical footprints. Our initial focus has been towards carbon capture applications, but membranes are widely considered to be important technologies in olefin/paraffin separation, nitrogen/oxygen purification, natural gas processing, hydrogen separation to name a few. However, in order for membranes to become performance competitive with adsorption/absorption based processes, membrane permeability and to some extent selectivity need to be greatly improved. Hybrid membranes can achieve this performance enhancement by harvesting gas selective properties of many inorganic materials. Our work focused on developing new materials systems to understand the role of polymer/inorganic interactions on performance enhancements or losses.
We have developed a novel hybrid material system to achieve high permeability for gas separations relevant for carbon capture. MOF presents an additional transport mechanism if used in hybrid membranes because of their rigid porous crystal structure. Our goal was to investigate performance improvements of hybrid membranes when MOF are used.
The foregoing aspects and others will be readily appreciated by the skilled artisan from the following description of illustrative embodiments when read in conjunction with the accompanying drawings.
In the discussions that follow, various process steps may or may not be described using certain types of manufacturing equipment, along with certain process parameters. It is to be appreciated that other types of equipment can be used, with different process parameters employed, and that some of the steps may be performed in other manufacturing equipment without departing from the scope of this invention. Furthermore, different process parameters or manufacturing equipment could be substituted for those described herein without departing from the scope of the invention.
These and other details and advantages of the present invention will become more fully apparent from the following description taken in conjunction with the accompanying drawings.
Our invention addresses the need for higher performing membranes to compete more effectively with absorption/adsorption based gas separation processes.
One alternative method is through use of polymer engineering. By increasing the rigidity and complexity of the polymer backbone through complex synthetic steps, one is able to greatly increase the free volume in the polymer and improve the permeability. However, because polymers are in constant fluid motion, the polymer begins to relax and the free volume can significantly decrease, leading to large loss in performance, within a few days. Our membranes show performance stability after 300 days. A second alternative incorporates a liquid agent in the membrane to help facilitate the transport of gas across the membrane. However, the volatility of the liquid leads to a decrease in performances as the liquid evaporates.
The invention is most immediately translatable to many gas separation industries including but not limited to carbon capture, olefin/paraffin separation, oxygen/nitrogen purification, natural gas processing, and hydrogen separation. This invention of dual transport pathways can also be applied to development of better reverse osmosis and forward osmosis membranes as well as increasing ionic and electronic conductivity of electrolyte membranes and battery separators is appropriate materials are chosen. The invention can be licensed to further the performance of dual transport membranes.
We describe a novel method to develop a high-performing hybrid polymer/inorganic membrane possessing dual transport pathways, which has never been reported before. The presence of dual transport pathways significantly improves the performance of polymer-based membranes. Our current invention shows an 800% increase in performance over the base-polymer used.
Conventional purely polymer or inorganic membranes suffer from permeability/selectivity trade-off or mechanical brittleness, thereby limiting their ultimate performance. By forming a hybrid material system, wherein an inorganic nanomaterials is dispersed in a polymer matrix, we can enhance the gas separation performance over the base polymer system by harvesting the gas selective properties of the inorganic component, metal-organic frameworks (MOFs). The superior performance of these hybrid membranes, is only realized upon surpassing a critical concentration of the MOF, and is an important foundation for this invention.
In conventional hybrid systems, membranes are not mechanically stable at high MOF concentrations (>30 wt %), and as a result, cannot perform under stresses required for membrane gas separation. Taking advantage of positive interactions between the polymer, polysulfone, and the MOF, UiO-66-NH2, we create a novel material system where high concentrations of MOF can be achieved (50 wt %) while still maintaining mechanical stability. This was the biggest technical challenge to overcome because mechanical properties are weakened when MOF concentration increases and the membrane becomes more brittle. We overcome this challenge through appropriate selection of both the polymer and the MOF, which interact favorable with each other. The fabrication of the MOF is done using solvent casting approaches (see
Membranes with MOF concentrations at or below 30 wt % have limited performance because the overall gas separation properties are governed by the polymer and solution-diffusion principles. Upon exceeding a percolation threshold (determined by the concentration and shape factor of the MOF, e.g. ellipsoid with an aspect ratio of 2 has critical percolation concentration of 31 vol %), a continuous channel through the MOF exists across the membrane. This continuous channel serves as a secondary, but more efficient, transport pathway across the membrane allowing gas molecules to entirely bypass contact with the polymer. The secondary MOF pathway enables significantly higher rates of permeation because the rigid pore structure enables separation through molecular sieving rather than solution diffusion.
The example materials system polysulfone/UiO-66-NH2 exhibit an 800% increase in permeability over pure polysulfone once the critical percolation threshold was surpassed. Without the presence of the secondary pathway the maximum improvement in permeability observed was only 280%. The permeability improvement in the hybrid system, combined with maintaining selectivity of the pure polymers, shows the possibility to break permeability/selectivity trade-offs of conventional polymer membranes. The method of utilizing a hybrid system to enable performance enhancements is a more fruitful pathway over polymer engineering strategies because it does not depend on complex synthetic reaction steps to achieve the necessary material properties desired. The general concept of producing dual transport pathways in membranes can be applied across many applications to improve gas separation, water purification and electrical conductivity performances.
Membranes have been targeted as an energy-efficient method to improve carbon capture technology because of their passive and continuous nature of operation (i.e. no regeneration steps needed). However, current commercialized membranes do not meet the performance standards to replace traditional pressure- and temperature-swing adsorption processes. Hybrid membranes composed of organic and inorganic materials offer new opportunities to achieve higher performance metrics due to performance advantages unique to each phase. However, while hybrid membranes display transport properties higher than their pure counterparts, there remains much headspace to further improve the separation efficiency.
In conventional mixed matrix systems, the inorganic phase often cannot be incorporated in sufficient quantity to establish a percolative network. Thus, their transport behavior can be easily understood using simple effective-medium approximations and is constrained by conventional solution-diffusion principles. If instead, the inorganic phase can exist continuously across the membrane, there is an opportunity to reach new non-classical transport regimes governed by dual transport pathways. Here, we demonstrate, for the first time, the ability to engineer dual transport pathways in polysulfone and UiO-66-NH2 MOF hybrid membranes by achieving very high loadings of MOF (50 wt %), which result in an 8-fold improvement in CO2 permeability from the pure polymer. These results enable new approaches towards designing hybrid membranes to become more competitive in carbon capture processes.
While historical trends indicate the gradual decarbonization of fuel sources over time, the global economy in its present state remains heavily dependent on fuels with high carbon content such as coal, oil, and natural gas. Consequently, carbon emissions are reaching record levels and are identified as contributing to recent patterns of global climate change. Mitigating carbon emissions to reverse or curb climate change using traditional amine scrubbing techniques is not scalable due to the large energy consumption and physical footprint required. Membrane separation processes have emerged as a promising technology because of the passive nature of its operation and relative ease of scalability. Unfortunately, many commercialized membranes have not been optimized for the stringent purification metrics required for carbon capture applications. These membranes, typically derived from polymers, suffer from an inherent trade-off between permeability and selectivity as popularized by Robeson and his eponymous plot. The central dilemma is that many polymers provide either high permeability or high selectivity but not both, which limits the industrial utility of these systems.
Hybrid membranes, which typically contain an organic polymer phase and a dispersed inorganic phase, have been shown to significantly improve separation performance over pure polymer systems in a variety of applications including carbon capture, hydrogen purification, and petrochemicals. The inorganic phase can be a nonporous materials such as nanoparticles or porous materials such as carbon molecular sieves, zeolites, and metal-organic frameworks. When integrated with an organic polymer into a hybrid system, the competitive advantages of each individual phase can be realized, such as the processibility of polymers and molecular selectivity of inorganics, while also fostering new properties and functionalities through synergistic enhancements.
While conventional mixed matrix systems display improved separation properties, the inorganic phase often is not present in sufficient quantity to establish a percolative network, and thus their transport behavior is limited by classical solution-diffusion principles. If hybrid membranes can be designed to possess continuity of both organic and inorganic phases, there is an opportunity to reach new non-classical transport regimes governed by dual transport pathways. In this dual transport regime, the inorganic phase will act as a molecular transport highway. However, achieving dual transport pathways is no easy feat as high loadings of the inorganic phase are required to achieve percolation.
Only a few studies have reported inorganic loadings in hybrid membranes surpassing 40 wt % due to mechanical failure of the membrane. This is primarily a result of poor interphase interactions, which lead to the formation of voids, commonly referred to as “sieves-in-a-cage,” in hybrids containing porous inorganic materials. Under these circumstances, molecular diffusion can circumvent the inorganic sieve and instead transport through the less selective voids at the interface. Thus, precise control of both the polymer and inorganic phase is critical to maximize separation performance.
Metal-organic frameworks (MOFs) are a relatively new class of 3-D porous crystalline inorganic materials that are ideal candidates to incorporate in hybrid membranes and design dual transport pathways. Their chemical flexibility provides opportunities to tune and optimize interfacial interactions between the MOF crystal and a polymer, thus reducing chances for mechanical failure. Further, large internal surface areas, tunable but rigid pores, and chemical functionalities of MOFs (accessible through functionalization of the organic linkers or Lewis acid open metal site) can simultaneous improve diffusive size selectivity and adsorption uptake of gases in membranes. While inclusion of the MOF as a dispersed phase can be expected to improve gas transport properties, the full benefit of hybrid MOF membranes is only realized when a continuous phase exists, which would lead to a percolative transport highway.
Here, we report on the design and characterization of robust hybrid membranes possessing dual transport pathways using UiO-66-NH2 MOF and polysulfone for relevant carbon capture applications. UiO-66-NH2 is a zirconium based MOF, comprised of Zr6O4(OH)4 octahedral clusters and 2-amino-1,4 benzenedicarboxalate linkers. UiO-66-NH2 is a well-studied MOF and exhibits high thermal stability, water stability, and carbon dioxide adsorption. We selected the amine derivative over its non-functionalized counterpart (UiO-66) to maximize interactions with polar backbone groups in the polysulfone polymer, which is critical to avoid mechanical failure as we increase the MOF loading beyond what is normally considered high loadings (i.e. 30 wt %). This hybrid system successfully maintains structural integrity at very high loadings. We demonstrate, to the best of our knowledge, the first hybrid system possessing dual transport pathways.
UiO-66-NH2 is prepared following a modified version of a microwave synthetic technique. Zirconium tetrachloride (99.5%) is supplied by Alfa Aesar, 2-amino-1,4-benzenedicarboxylic acid (99%) and dimethylformamide (99%) is supplied by Sigma-Aldrich.
35 mmol of ZrCl4 (8.12 g) and 0.11 mmol of nanopure water (2 ml) are added to 148 mmol (400 mL) of DMF.
The solid is allowed to fully dissolve. Separately, 35 mmol (6.28 g) of 2-aminoterephthalic acid is dissolved in 148 mmol DMF.
The solutions are combined and heated using microwave irradiation (Anton Paar) in sealed vessels at 1500 W for two hours at 120° C.
The resulting pale yellow powder is filtered and washed with methanol in a Soxhlet extractor overnight. The final product is dried in air overnight and finally in an oven at 65° C. to remove residual solvent.
Udel P-1700 polysulfone is generously supplied by Solvay Plastics. Polysulfone (PSF) is dried overnight in a vacuum oven at 110° C. prior to use. PSF is dissolved in chloroform (BDH Chemicals) to form a 5 wt % solution and subsequently filtered with a 0.45 μm PVDF filter. For hybrid membranes containing up to 50 wt % UiO-66-NH2, the MOF is first dispersed in chloroform by sonication. Once dispersed, the MOF is “primed” by adding a portion of the PSF solution equal to 35 wt % of the total MOF mass and subsequently sonicated. Priming the MOF is believed to increase interaction and homogeneity between the MOF and polymer by coating the MOF with a thin polymer layer. The remaining PSF is then added to the MOF mixture and sonicated. To mitigate MOF settling during casting, the solution is concentrated by gentle purging with nitrogen gas to evaporate the solvent until the solids concentration reaches 25-30 wt %. The solution is then cast into a casting plate, loosely covered, and allowed to dry under atmospheric conditions over the course of two days. The dried membranes are then placed into a vacuum oven at 110° C. overnight to remove any residual solvent and water. The target thickness of each film is 65 μm. The thickness of each film is measured individually using a micrometer.
Nitrogen adsorption measurements of the MOF are performed at 77 K using a Tristar II Surface Area Analyzer (Micromeritics). Surface area values are calculated following Brunauer-Emmett-Teller method over a relative pressure range, p/po, of 0.05 to 0.25. Carbon dioxide adsorption isotherms of the MOF and membranes are collected using an ASAP 2020 Physisorption Analyzer (Micromeritics) at 20° C. up to a pressure of 1 bar. Before adsorption measurements are carried out, all samples are heated under vacuum at 110° C. for 12 hours to remove residual solvent in the pores.
X-ray diffraction patterns of the MOF powder and hybrid membranes are collected at ALS Beamline 12.2.2 on a Perkin Elmer amorphous silicon detector using synchrotron radiation monochromated by silicon(111) to a wavelength of 0.4978(1) Å. Distance and wavelength calibrations were done, using a NIST LaB6 diffraction standard, with the program Dioptas, which was also employed for radial integration. Simulated powder diffraction patterns of UiO-66-NH2 are calculated using Mercury 3.6 software (Cambridge Crystallographic Data Centre). Glass transition temperatures of the membranes are determined using a Q200 Differential Scanning calorimeter (TA Instruments). The samples are heated under vacuum at 110° C. for two hours to remove water vapor before scanning to 250° C. at a scan rate of 20° C./min. Density measurements of the bulk hybrid films are performed using hydrostatic weighing with a density determination kit (Mettler Toledo). Heptane is used as the secondary liquid. Cross-sectional images of the hybrid films are acquired with a Zeiss Gemini Ultra-55 Analytical Scanning Electron Microscope using an accelerating voltage of 5 keV. Prior to imaging, the films are cryofractured after immersion in liquid N2 to provide a clean surface.
Pure gas permeability of PSF/UiO-66-NH2 membranes for nitrogen, methane, and carbon dioxide are measured using a custom built constant volume/variable pressure apparatus. The films are masked with brass discs to accurately define an area through which gas transport could occur. Prior to testing, the films are degassed within the apparatus. A fixed pressure is applied to the upstream side of the membrane, while the gas flux is recorded as a steady-state pressure rise downstream of the membrane. Permeability values are calculated as follows:
where VD is the downstream volume (cm3), 1 is the film thickness (cm), p2 is the upstream pressure (cmHg), A is the exposed area of the film (cm2), R is the gas constant, T is the absolute temperature (K), and dp1/dt steady state pressure rise downstream at fixed upstream pressure (cmHg/sec). The measurements are obtained under isothermal conditions at 308 K.
Diffusivity and solubility of the hybrid membranes are calculated through permeation time lag experiments described in detail elsewhere and analyzed employing the solution-diffusion model.
Traditional MOF synthesis relies on conventional solvothermal techniques, which usually requires prolonged reaction times that range from hours to several days. Microwave assisted synthetic techniques are an emerging method to rapidly synthesize MOFs and other microporous materials within a matter of minutes to a few hours without compromising crystallite quality. This technique is not only advantageous for its short reaction time, but also for its scalability and particle size control. We employ microwave synthesis for UiO-66-NH2 for the reasons listed above and to minimize the risk of batch-to-batch variation. All hybrid membranes investigated contained MOFs from a single large-scale batch. In
By controlling the MOF-polymer interface using the techniques described previously, robust polysulfone membranes containing up to 50 wt % UiO-66-NH2 are successfully fabricated. Few MOF-polymer membranes at such high loadings have been reported; mainly a result of mechanical failure of the membrane at these loadings, due to poor interphase interactions. Thus, when undertaking the design of hybrid membranes, it is imperative to select materials which are not only individually good materials for CO2 capture, but also with mutual chemical affinities to maximize interphase adhesion and solubility and minimize the onset of sieve-in-a-cage morphology, which deleteriously impacts the gas selectivity. One diagnostic used to understand the magnitude and type of interfacial interactions in hybrid soft/hard systems are shifts in the glass transition temperature (Tg). Favorable interactions are noted by a Tg shift towards higher temperatures. This positive shift is due to reduced polymer chain mobility and rigidification as the polymer becomes adsorbed onto the MOF surface, resulting in a more mechanically robust membrane. Opposite trends (i.e. reductions in Tg relative to that of the homopolymer) are observed when unfavorable interactions are present. Glass transition temperatures as measured by differential scanning calorimetry of the hybrid membranes are presented in Table 1. The Tg of the neat homopolymer is 176° C. The incorporation of 10 wt % UiO-66-NH2 has a minor influence on the Tg, shifting it by only 4° C. As MOF is further added to the membrane, we observe a larger Tg shift of 10-12° C. to a maximum Tg of 188° C. The Tg shifts at all loadings indicates that favorable interactions are present, and we speculate this is due to hydrogen bonding interactions between the amine groups of the MOF and sulfonyl groups in the polymer. The interactions are sufficiently strong that any post-synthetic surface modification of the MOF to promote interaction is not required.
Physical confirmation of good interfacial interactions as indicated by the aforementioned Tg shifts can be seen through cross-sectional imaging of the hybrid membranes. SEM cross-sections of the membranes are shown in
In addition to detecting strong interfacial interactions, the SEM cross-sections reveal another significant characteristic of these hybrid membranes advantageous for gas transport. At MOF loadings of 30 wt % or below, the MOF can be clearly discerned from the polymer as seen in
Transmittance X-ray diffraction was used to determine the presence of UiO-66-NH2 in hybrid membranes and the diffraction patterns are shown in
CO2 adsorption isotherms of the hybrid membranes are collected at 25° C. as shown in
Pure gas permeability and selectivity of N2, CH4, and CO2 at 35° C. and 3 bar are shown in
In order to understand the permeability trends in the hybrid MOF membranes, we perform analysis using a simple effective medium model. Such models often capture the physical behavior of a broad range of systems with a continuous phase and a dispersant. Specifically, permeability in heterogeneous two-phase materials are frequently modeled by Maxwell's model (Eqn. 1),
which consider the volume loading of the dispersed phase (i.e. MOF), ϕd, and the geometry shape factor of the dispersed phase, n, as the only adjustable parameters. PMaxwell, Pp, and Pd are the permeability of the hybrid membrane, polymer, and dispersed phase, respectively. While the simplicity of the model allows for quick comparison of experimental data to the predicted values, the model does not consider the effects of interphase interactions, is typically applied only to systems below 20 vol % loading, and usually assumes the dispersed phase has spherical geometry (n=⅓). In this scenario, Maxwell's model collapses into the more common Maxwell's equation used in hybrid or composite membrane analysis. From our density measurements, we calculated the bulk density of UiO-66-NH2 to be 1.53 g/cm3, which is close to the estimated density of 1.3 g/cm3 for a perfect crystal and ideal unit cell. Assuming the experimental density, we find that hybrid membranes contain a maximum of 45 vol % MOF, which is well beyond the 20 vol % threshold for typical applications of Maxwell's model.
However, we can still apply Maxwell's equation (n=⅓) in hybrid membranes containing below 30 wt % (25 vol %) UiO-66-NH2 as shown in Fig. S3 of the ESI. The equation severely underestimates permeability values by up to 44%, even when assuming an infinitely permeable dispersed phase. Gas permeability of UiO-66-NH2 has not been previously measured before, and thus, there is not an accepted literature value to use as Pd. We speculate that the poor prediction of Maxwell's model could arise from one of two possibilities. First, poor interactions between the polymer and MOF can result in interphase voids, which act as fast non-selective diffusive pathways. However, our Tg measurements and SEM images indicate the opposite, where good interfacial interactions are present. Second, our approximation of spherical geometry of the dispersed phase could be incorrect. This scenario is more likely than the former and is at least suggested by partial aggregation observed in SEM images (
The dramatic increase in permeability in hybrid membranes when loading is increased from 30 to 40 wt % UiO-66-NH2 is postulated to arise from the formation of a percolative network of MOF crystals throughout the membrane, whose effect is not captured in effective medium models. Percolation is reached when the dispersed phase surpasses a threshold volume fraction, forming an interconnected network, which spans across the entire system. Below this value, no interconnectivity across the entire system is present. The percolation threshold depends on the dimensionality of the system as well as the shape and aspect ratio of the discontinuous phase. A shape factor, n, of 0.14 suggests that the ellipsoids will have an aspect ratio between 2 and 3 and this is suggested by SEM images of MOF nanoparticles (see
The selectivity of CO2 over N2 and CH4 is shown in
To better understand the changes in the transport mechanism, we investigate diffusivity values for hybrid membranes as a function of weight % as shown in
The solution-diffusion model is most commonly used to describe molecular transport through dense polymer membranes From this model, one defines the permeability of a gas through a polymer (P) as the product of both a kinetic term (diffusivity, D) and a thermodynamic term (solubility, S), or simply P=DS. While the model may not be appropriate towards understanding transport mechanisms in hybrid membranes, it can still be a useful tool to provide qualitative solubility trends in hybrid membranes. Using the permeability and diffusivity data collected, we can employ the P=DS relationship to calculate S, which is plotted as a function of weight % in
We demonstrate for the first time the formation of dual transport pathways in hybrid polymer/MOF membranes and investigate its evolution as it relates to percolation transition. The formation of dual transport pathways requires high loading of the inorganic phase, which often leads to mechanical failure of the membrane. Using polysulfone and UiO-66-NH2, we are able to maintain structural integrity of the membranes even at very high loadings (50 wt %). Further, the transport properties associated with dual transport membranes are distinctively different than conventional mixed-matrix membranes, which contain discontinuity with the inorganic phase. Thus, in dual transport membranes, gas transport through the MOF acts as a molecular transport highway and complements classical solution-diffusion through the polymer. Below the percolation threshold, permeation properties of the hybrid membranes are higher than the pure polymer and could easily be fitted to a classical effective medium model. However, above the percolation threshold, permeation properties far exceed what the model could predict and signify a new, non-classical dual transport regime. We find for our hybrid system that dual transport pathways develop between 30 and 40 wt % UiO-66-NH2. In the percolative regime, CO2 permeability rises to a remarkable 46 barrers; a 8-fold increase over pure polysulfone. Additional evidence of dual transport pathways is found in a similar phenomenon in CO2 diffusion as we surpass the percolation threshold. Furthermore, our hybrid membranes deviate from conventional permeability/selectivity trade-off relationships as selectivity over methane and nitrogen remained near that of polysulfone at 22 and 25, respectively. The unique discovery of engineering dual transport pathways enables new approaches towards designing hybrid membranes to significantly improve gas separation performance.
This US application claims priority to U.S. Provisional Application Ser. No. 62/417,954 filed Nov. 4, 2016, which application is incorporated herein by reference as if fully set forth in their entirety.
The invention described and claimed herein was made in part utilizing funds supplied by Grant Number FA9550-11-C-0028, Department of Defense, National Defense Science & Engineering Graduate (NDSEG) Fellowship, and the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 between the U.S. Department of Energy and the Regents of the University of California for the management and operation of the Lawrence Berkeley National Laboratory. The government has certain rights in this invention.
| Number | Date | Country | |
|---|---|---|---|
| 62417954 | Nov 2016 | US |
