Ulhas K. Kharul

In situ growth of metal-organic frameworks on a porous ultrafiltration membrane for gas separation

We demonstrate the synthesis of CuBTC and ZIF-8 on a polysulfone based porous asymmetric ultrafiltration (UF) membrane by in situ growth followed by the LBL deposition of crystals without any need for pre-seeding or surface modification of the membrane. In this way, the top surface of the UF membrane pores is completely covered by MOFs; while the remaining part of the membrane offers a flexible support to the MOFs. The pore apertures of the MOF nanoparticles located at the pore opening of the UF membrane act as channels for the entry of penetrants. The remaining porous sublayer of the membrane carries penetrants on the permeate side without significant resistance. These composite membranes were characterized by PXRD and SEM. The gas permeation study was performed using pure gases of industrial significance (H2, C3H6 and CO2). The performance of CuBTC@PSF showed enhanced selectivity, of 7.2 and 5.7 for H2/CO2 and H2/C3H6 respectively, to that of the pristine PSF membrane.

Selective Molecular Sieving in Self‐Standing Porous Covalent‐Organic‐Framework Membranes

Self-standing, flexible, continuous, and crack-free covalent-organic-framework membranes (COMs) are fabricated via a simple, scalable, and highly cost-effective methodology. The COMs show long-term durability, recyclability, and retain their structural integrity in water, organic solvents, and mineral acids. COMs are successfully used in challenging separation applications and recovery of valuable active pharmaceutical ingredients from organic solvents.

Self-Exfoliated Guanidinium-Based Ionic Covalent Organic Nanosheets (iCONs)

Covalent organic nanosheets (CONs) have emerged as functional two-dimensional materials for versatile applications. Although π-π stacking between layers, hydrolytic instability, possible restacking prevents their exfoliation on to few thin layered CONs from crystalline porous polymers. We anticipated rational designing of a structure by intrinsic ionic linker could be the solution to produce self-exfoliated CONs without external stimuli. In an attempt to address this issue, we have synthesized three self-exfoliated guanidinium halide based ionic covalent organic nanosheets (iCONs) with antimicrobial property. Self-exfoliation phenomenon has been supported by molecular dynamics (MD) simulation as well. Intrinsic ionic guanidinium unit plays the pivotal role for both self-exfoliation and antibacterial property against both Gram-positive and Gram-negative bacteria. Using such iCONs, we have devised a mixed matrix membrane which could be useful for antimicrobial coatings with plausible medical benefits.

Improved performance of phosphonated carbon nanotube–polybenzimidazole composite membranes in proton exchange membrane fuel cells

Development of thermally stable polymer electrolyte membranes with higher proton conductivity as well as mechanical stability is a key challenge in commercializing PEM fuel cells operating above 100 °C. Polybenzimidazole membranes are one of the promising candidates in this category although with limited mechanical stability and moderate proton conductivity. Here the incorporation of functionalized MWCNT is shown to increase both these key parameters of the polybenzimidazole membranes. Further, formation of a domain like structure after the incorporation of phosphonated MWCNTs (P-MWCNTs) in phosphoric acid doped polybenzimidazole membranes is demonstrated. The enhanced performance has been attributed to the formation of proton conducting networks that formed along the sidewalls of P-MWCNTs with a domain size of 17 nm as estimated from the small angle X-ray scattering measurements. Membrane electrode assembly (MEA) impedance measurements further reveal that the activation energy of oxygen reduction reaction (ORR) reduced for the composite membranes with enhanced proton conductivity. In addition, the mechanical strength measurements reveal a significant improvement in the yield strength and ultimate strength. Also, the mechanical strength of the composite membrane has been increased significantly as indicated by the improvement in the ultimate strength from 65 MPa to 100 MPa for the pristine and composite membranes, respectively. The optimum loading of P-MWCNTs is found to be 1% as inferred from the polarization measurements carried out using pure hydrogen and oxygen. Thus, this study provides a unique opportunity to tune the properties of polymer electrolytes to prepare application oriented hybrid membranes using CNTs with tailor-made functional groups.

Film forming polymeric ionic liquids (PILs) based on polybenzimidazoles for CO2 separation

PILs are emerging as promising materials for CO2 capture. Film formation, which is a requisite for membrane formation, is induced in a family of PILs by N-substituting a rigid thermo-mechanically stable polybenzimidazole, followed by metathesis. This provided two IL groups per repeat unit of the PIL and enhanced the CO2 separation characteristics.

Fluorescent polymeric ionic liquids for the detection of nitroaromatic explosives

We report, for the first time, applicability of film forming polymeric ionic liquids (PILs) possessing pyrene and anthracene fluorophores for the detection of nitroaromatics (NACs). These functionalized PILs were synthesized via post modification of thermo-chemically and mechanically stable polybenzimidazole (PBI). Formed PILs were evaluated for their ability to detect NACs through quenching of fluorescence intensity. Quenching of fluorescence intensity in solution state for NACs, such as nitrobenzene (NB), 2,4,6-trinitrotoluene (TNT) and picric acid (PA), was found to be rapid in both the PILs possessing pyrene and anthracene. The solution-phase Stern–Volmer quenching constants for PA were higher than for other NACs. After these promising results, self-standing films (∼12 μm thick) were also evaluated for fluorescence quenching by NACs as well as possible interferents of different nature. These films also exhibited rapid and selective fluorescence quenching when exposed to the saturated vapors of NACs at ambient temperature and pressure. Fluorescence emission of PIL films was affected little by the presence of commonly found interferents. Furthermore, fluorescence intensity could be recovered after the quenching, enabling the reuse of these PIL films for detection of NACs. Smart performance of these films and ease of preparation qualify them as attractive candidates in developing sensor devices for sensitive NACs detection in presence of possible interferents.

Investigation of gas permeation properties of systematically modified polybenzimidazoles by N-substitution

Abstract Gas permeation behavior of a series of thermally stable N -substituted polybenzimidazoles by systematically varying bulk and flexibility of the substituent was investigated. Two different PBIs having variation in their acid moiety, viz., PBI-I (based on isophthalic acid) and PBI-BuI (based on 5- tert -butyl isophthalic acid) were selected for N -substitution by alkyl groups possessing different bulk and flexibility. These substituent groups were methyl, n -butyl, methylene trimethylsilane and 4- tert -butylbenzyl. Pure gas sorption and permeability using H 2 , N 2 , O 2 , CH 4 and CO 2 were investigated and correlated with physical properties of formed polymers. Estimation of dual-mode sorption parameters, coefficients of sorption, permeability and diffusion for different gases provided an insight towards effects of nature of a substituent group and parent PBI on governing gas sorption and permeation properties. By changing the substituent group, diffusivity coefficients was found to vary to a larger extent than the solubility coefficient. This significantly enhanced gas permeability for different gases by 1.2–129 times than that of parent PBIs. The permselectivity P O 2 / P N 2 was increased (up to 237%), while for other gas pairs, it was decreased to a different extent.

Pore surface engineering in porous, chemically stable covalent organic frameworks for water adsorption

Herein, we have explored the possibility of a class of covalent organic frameworks (COFs) as water adsorbing materials. We have selected, synthesized 12 chemically stable functionalized Schiff base COFs and thoroughly studied their water uptake behaviour. Further, a deep understanding was developed with these COFs towards the effects of condensation pressure of water and hydrophilic/hydrophobic groups present in the COF pores on water absorption capacity and ultimately, their recyclability. Among all reported COFs, TpPa-1 shows the highest water uptake of 30 wt% (368 cm3 g−1; 17 mmol g−1) at P/P0 = 0.3, which is also comparable with the recently reported carbon materials and few well known MOFs. This study also reveals that the overall water uptake of COFs can be tuned systematically based on chemical functionality and pore size in a wider window of relative pressures.

Selective Molecular Separation by Interfacially Crystallized Covalent Organic Framework Thin Films

Exponential interest in the field of covalent organic frameworks (COFs) stems from the direct correlation between their modular design principle and various interesting properties. However, existing synthetic approaches to realize this goal mainly result in insoluble and unprocessable powders, which severely restrict their widespread applicability. Therefore, developing a methodology for easy fabrication of these materials remains an alluring goal and a much desired objective. Herein, we have demonstrated a bottom-up interfacial crystallization strategy to fabricate these microcrystalline powders as large-scale thin films under ambient conditions. This unique design principle exploits liquid-liquid interface as a platform, allowing simultaneous control over crystallization and morphology of the framework structure. The thin films are grown without any support in free-standing form and can be transferred onto any desirable substrate. The porous (with Tp-Bpy showing highest SBET of 1 151 m2 g-1) and crystalline thin films, having high chemical as well as thermal stability, also hold the merit to tune the thickness as low as sub-100 nm. These nanostructured thin COF films demonstrate remarkable solvent-permeance and solute-rejection performance. A prominent instance is the Tp-Bpy thin film, which displays an unprecedented acetonitrile permeance of 339 L m-2 h-1 bar-1.

ZIF-8@PBI-BuI composite membranes: elegant effects of PBI structural variations on gas permeation performance

The composites of metal organic frameworks (MOFs) and polymers look promising as membrane materials for gas separation, provided benefits of both the components can be shown successfully. This work shows that the structural architecture of polybenzimidazole (PBI) is highly advantageous in offering attractive gas permeation properties of its composites with MOFs. PBI-BuI and its N-substituted (methyl and 4-tert-butylbenzyl) derivatives were blended with ZIF-8. In general, homogeneous blend formation of ZIF-8 was achieved with all three polymers, as supported by SEM. Wide angle X-ray diffraction, mechanical property analysis and density measurements of the composite membranes were performed in order to understand the effects of physical blending of MOFs and polymers. Gas permeability analysis of the composite membranes revealed that the properties of MOFs as well as those of polymers arising from their structural architecture are responsible for governing the permeability and selectivity of the resulting composites.