Tailoring Polyurethane Membranes With Nanofillers: A Pathway to Enhanced Gas Separation Efficiency

Polyurethane-based gas separation membranes: A review and perspectives

Mixed matrix membranes (MMMs) are hybrid membranes, which have been intensively studied and expected to overcome the drawbacks of both polymeric and inorganic counterparts. In fact, MMMs are still facing great challenges, mostly due to the poor compatibility and adhesion between the fillers and polymer matrix, which considerably reduce MMMs separation performance. To address that issue, the work in this thesis focus on modification methods in order to improve the interfacial adhesion between polymer/fillers in the MMMs and consequently enhance the gas separation efficiency of the MMMs.In the first part of experiment, a non-porous nano-size filler, nanodiamond (ND) was introduced into Pebax copolymer to fabricate the MMM. While being promising filler, the non-porous structure and susceptible to agglomeration of ND are still the issues in gas separation membrane. This chapter proposes an efficient approach as grafting polyethyleneimine (PEI) onto the surface of ND before embedding into the polymer matrix to fabricate the MMMs for CO2/N2 separation. The presence of PEI layer on ND surface significantly improved the interfacial adhesion and dispersion of ND in the Pebax matrix, which were clearly indicated by SEM and FIB-SEM observation. The improvement of interfacial interaction led to the increment in CO2/N2 selectivity compared to the pristine polymer membranes and the non-PEI MMMs as well. The CO2/N2 selectivity of the Pebax/oxND-PEI 0.5 wt.% increased 25% compared to the neat polymer and 43.66% compared to the Pebax/oxND. This chapter has contributed to a simple but effective method to improve the dispersion of the non-porous nanofiller, as well as enhance the gas separation performance of the MMMs.The next chapter studied the effects of different morphologies of filler on the dispersion, interfacial interaction and gas separation performance of the MMMs. Three types of filler morphologies: conventional polyhedral (P-ZIF), nanorod (R-ZIF) and leaf-shaped nanosheet (L-ZIF) were introduced and investigated. The change in morphology can alter the interfacial interaction between polymer and fillers due to the different aspect ratio and surface structure. The L-ZIF and R-ZIF showed better compatibility with the 6FDA-durene polymer matrix compared to the polyhedral ZIF. L-ZIF improved the gas selectivity of CO2/N2 (30.3%), CO2/CH4 (40%) compared to the neat polymer, while the R-ZIF enhance the CO2 permeability (41%) with comparable gas selectivity to the neat polymer. This chapter's results suggested that the nanorod and nanosheet morphologies are more effective in enhancing the interfacial adhesion between polymer/filler and contributed to the guidance in filler morphology selection to achieve improved gas separation performance.In the following chapter, ZIF nanorod (R-ZIF) was further investigated as the filler and was coated with two types of ILs before incorporated in the the 6FDA-durene matrix. In the previous chapter, while showing compatibility with the 6FDA-durene matrix at low filler loading (10 wt.%), R-ZIF still formed aggregates in the membrane at high loading (20 wt.%) which decrease the gas separation performance of the MMMs. The ionic liquid decoration improved the interfacial interaction between R-ZIF and the polymer matrix leading to the enhancement in gas separation performance of the PR/IL MMMs which was intensively investigated by conventional SEM, FTIR, single and mix gas tests. The most significant improvements were the increment of 50% in CO2/CH4 selectivity, while maintaining the CO2 permeability of the 10 wt.% R-ZIF/IL MMM. The improvement in gas separation efficiency of the IL-incorporated MMMs compared to the non-IL MMMs was still observed even at high loading of filler (20 wt.%). The contribution in this part is to confirm that IL-decoration is an effective approach to enhance the interfacial issues and improve the gas separation efficiency of the MMMs.In the last experiment section, micron size polyhedral shape ZIF (P-ZIF) was coated with 3 different ILs and dispersed in 6FDA-durene matrix to prepare the MMMs. As investigated in previous experiment section, P-ZIF exhibited the worst interfacial interaction with the polymer matrix among 3 different morphologies. Thus, it is more challenging to obtain excellent filler/polymer contact between micron-sized P-ZIF and polymer matrix and achieve improvement in gas separation efficiency. Acting as the interfacial binder, IL layer has effectively reduced the non-selective interfacial voids in the MMM and enhanced the polymer/P-ZIF adhesion. The vol.% of interfacial voids of the pristine PZ MMM has been reduced from 1.17% to 0.35%, 0.33% and 0.49% with the PZ/IL1, PZ/IL2 and PZ/IL3 MMM, respectively, leading to a significant improvement in gas separation performance, particularly with the CO2/CH4 separation performance surpassing the 2008 upper bound. Additionally, the PZ/IL MMMs also showed enhancement in gas separation performance for the CO2 - CH4 mix gas (50:50) compared to the non-IL MMMs and the neat polymer membrane. The contribution of this chapter is that it further evidenced the effectiveness of using IL as a interfacial binder to minimize the interfacial defects in MMMs as well as enhance the gas separation performance in both ideal and real conditions.

Energy-efficient separation of gases has attracted intensive attentions both in research and in industry.nThe separation of gases by membranes is more effective and energy saving with lower productionnand equipment cost than some traditional gas separation methods, such as adsorption or distillation.nHowever, most of the polymeric membranes suffer from the trade-off between mass transport ratesnand separation efficiency. Polymeric membranes show high gas permeation flux but low selectivity,nand vice versa. To overcome such weakness, mixed matrix membranes (MMMs) can providenpromising potentials in high performance gas separation, by combining high separation properties ofnthe inorganic filler with low cost and flexible of the polymers. For filler selection, metal-organicnframeworks (MOFs) are promising adsorbents for gas storage and separation due to their high surfacenarea and porosity, adjustable pore sizes and controllable surface functionality.This thesis is focused on developing novel MOFs-based MMMs for gas separation with highnpermeability and selectivity, as well as good thermal and chemical stability. The studies includenfabrication and optimization of MOFs-based MMMs, designing MMMs with good filler/polymerninteraction and good interfacial morphology, as well as evaluating the permeation and selectivitynperformance of all the prepared membranes. It aims to establish the guidance for the MOF/polymernpair selection and interface manipulation in the fabrication of MMMs to greatly increase thenmembrane permeability and selectivity.In the first part of the experimental chapters, novel MMM with dispersed MOF into polyimide matrixnwere fabricated and the derived membranes are employed for gas separation. MMMs werensynthesized from 2,2-bis(3,4-anhydrodicarboxyphenyl) hexafluoropropane (6FDA) and 4,4(-diaminodiphenyl ether (oxydianiline, ODA) into which were incorporated Cd-6F MOF filler. Cd-6Fnwas synthesized by using 4,4p-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) as the organicnlinker which is also one of the monomers in the 6FDA-ODA synthesis. A specific interfacialninteraction between MOF crystals and polymer chains was innovatively targeted with this specificnMOF/polymer pair by controlling the in-situ 6FDA-ODA polymerization procedure with thenexistence of MOF particles. The enhanced adhesion between filler particles and polymer phase wasnachieved and the improved interfacial interaction between MOF and polymeric matrix was confirmednby FTIR, NMR and XPS. Moreover, it was found that the MOF/polymer interfacial morphologynstrongly affects the gas permeability and selectivity of the membrane. The MMM prepared by in-situnpolymerization displays excellent interfaces between micron-ized Cd-6F crystals and polymernmatrix as well as increased permeability and selectivity compared to pure 6FDA-ODA polymericmembrane. The interaction between MOF crystals and polymeric matrix can be controlled for theninterfacial voids elimination and optimal membrane transport properties.The second part of the experimental chapters focuses on MMMs with MOF/CNT composite. NovelnMMM filler CNT-MIL was synthesized by in-situ growth of NH2-MIL-101(Al) on the externalnsurface of carbon nanotubes (CNT) and applied to fabricate MMMs for CO2/CH4 separation. Extranamino groups and active sites were introduced to the surface of CNT. Compared to pure polymericnmembrane, MMMs containing synthesized MOF/CNT composite displayed significantly increasednCO2 permeance (up to 150%) and selectivity (up to 37.5%). The separation performance of derivednMMMs clearly transcends the 1991 upper bound and being on the 2008 upper bound for polymericnmembrane performance. MMMs for efficient C3H6/C3H8 separation were fabricated by embeddingnZIF-8/CNT composite into 6FDA-durene polymer matrix. The volume of filler and voids in thenpolymeric matrix as well as the distribution of the fillers and their contact with the polymeric matrixnwere quantitatively evaluated by using the tomographic focused ion beam scanning electronnmicroscopy (FIB-SEM). The dispersion of ZIF-8 in 6FDA-durene polymer was enhanced by growthnof ZIF-8 on the CNT surfaces. Meanwhile, good adhesion between the synthesized MOF/CNT fillersnand polymer matrix was observed, only 0.086% voids volume fraction is determined, even at a highnfiller loading. The improved ZIF-8 dispersion and filler/polymer interface lead to the efficient C3H6separation in MMMs. MMMs containing synthesized MOF/CNT composite displayed significantlynincreased C3H6 permeability (up to 105%) and selectivity (up to 96%) compare to pure 6FDA-durenenmembrane.The third part of the experimental chapters focuses on elimination of interfacial voids bynincorporating ionic liquid (IL) as the MOFs/polymer interfacial binder into MMM. Thin layer of ILnhas been fabricated on CuBTC fillers for MMMs fabrication. With the aid of ionic liquid, theninterfacial voids of the CuBTC-IL MMM was significantly reduced due to the favourable MOF/ILnand IL/polymer adhesion, thus improving CO2 selectivity compare to pure polymer membrane andnMMMs with untreated CuBTC. The performance of 10% CuBTC-IL/6FDA-durene MMM clearlyntranscends the 2008 upper bound for polymer membrane. The strategy of IL decoration method cannprovide an effective way to eliminate interfacial voids and enhance CO2 selectivity and can be appliednin large sized fillers with better gas separation properties.n

Methylene diisocyanate - aided tailoring of nanotitania for dispersion engineering through polyurethane mixed matrix membranes: Experimental investigations

Nanostructured membranes containing UiO-66 (Zr) and MIL-101 (Cr) for O2/N2 and CO2/N2 separation

Study on the advantageous effect of nano-clay and polyurethane on structure and CO2 separation performance of polyethersulfone based ternary mixed matrix membranes

MOF-based MMMs breaking the upper bounds of polymers for a large variety of gas separations

Recent advances of inorganic fillers in mixed matrix membrane for gas separation

ABSTRACTPolymer/nanoparticle mixed matrix membranes (MMMs) is one of the most important topics in gas separation field. In this study, to improve gas separation efficiency, methoxy poly(ethylene glycol) methacrylate (MPEG) was grafted on TiO2 surface and was used for synthesis of poly (methyl methacrylate) (PMMA) MMMs. Gas permeation and separation properties of PMMA/PMPEG-TiO2 MMMs were studied for CO2, CH4, O2, and N2 gases. The results showed that the MMM filled with 5 wt% PMPEG-TiO2 nanoparticle exhibited optimal separation performance with CO2 permeability of 32.48 Barrer and CO2/N2 selectivity of 56.98, which are higher than pure polymer (2.75 Barrer and 36.71).

A review on the recent advances in mixed matrix membranes for gas separation processes

A comprehensive study on transport behaviour and physicochemical characteristics of PU/based 3-phase mixed matrix membranes: Effect of [HNMP][HSO4] ionic liquid and ZnO nanoparticles

Tailoring the CH4/CO2/N2 separation performance of ultrapermeable polymeric composite membranes by altering the concentration of Pd/g-C3N4

Toward rational design of ionic liquid/Metal-Organic Framework composites for efficient gas separations: Combining molecular modeling, machine learning, and experiments to move beyond trial-and-error

Mixed matrix membranes for gas separations: A review

Non-porous polyurethane (PU) membranes and porous PU thin films are used as sample supports for MALDI-TOFMS. Mass spectra obtained are compared with those acquired using metal targets and the crushed matrix method. The compounds characterized are wheat proteins which consist of moderately water-soluble gliadins, and of water-insoluble low molecular weight (LMW) and high molecular weight (HMW) glutenins. Mass spectra obtained using the PU supports are in general of good quality, and this method of sample preparation is the most convenient for sample handling. In the case of gliadins and LMW glutenins, the spectra obtained on PU are comparable with those obtained using metal supports. Isolation of the LMW and HMW wheat proteins characterized in this study requires the use of buffers incompatible with MALDI. Spectra of samples containing buffer components on PU supports are of better quality than those obtained using the crushed matrix method. This effect is attributed to stronger protein binding onto the PU supports, which allows for extensive washing and removal of water soluble buffer components. The PU film, when cast onto a MALDI probe, is porous and flat in topology. The differences in surface characteristics between the PU film and the PU membrane result in slight variations in the mass spectra. The extent of surface charging, observed significantly using 50 µm thick PU membranes, decreases with 25 µm membranes and becomes insignificant with PU thin films. An important advantage of using the PU supports is the possibility of preparing samples on the film or membrane in the field and of analysing them at a later time. This is especially important when samples are susceptible to chemical degradation in solution. These proteins are known to degrade while stored in solution. We have thus incorporated the use of PU membrane–film supports into our routine analysis of these proteins.Key words: gliadins, glutenins, MALDI, membrane supports, polymeric supports, time-of-flight analysis.