Jeong F. Kim

Sustainability assessment of organic solvent nanofiltration: from fabrication to application

Can Organic Solvent Nanofiltration (OSN) be considered green? Is OSN greener than other downstream processing technologies? These are the two main questions addressed critically in the present review. Further questions dealt with in the review are as follows: What is the carbon footprint associated with the fabrication and disposal of membrane modules? How much solvent has to be processed by OSN before the environmental burden of OSN is less than the environmental burden of alternative technologies? What are the main challenges for improving the sustainability of OSN? How can the concept of Quality by Design (QbD) improve and assist the progress of the OSN field? Does the scale have an effect on the sustainability of membrane processes? The green aspects of OSN membrane fabrication, processes development and scale-up as well as the supporting concept of QbD, and solvent recovery technologies are critically assessed and future research directions are given, in this review.

Sustainable wastewater treatment and recycling in membrane manufacturing

It is widely accepted that membrane technology is a green and sustainable process; however, it is not well known that the membrane fabrication process itself is quite far from green, with more than 50 billion liters of wastewater being generated every year contaminated with toxic solvents such as DMF and NMP. This urgent challenge is often overlooked and recent attempts to improve the sustainability of membrane fabrication have been limited to the replacement of toxic solvents with greener alternatives. Our recent survey from membrane industries indicates that such wastewater contributes to more than 95% of the total waste generated during the membrane fabrication process, and their disposal is considered cumbersome. Hence, recycling wastewater in the membrane industry is a pressing challenge to be resolved to augment the rapidly growing membrane market. In this work, a continuous wastewater treatment process is proposed and the quality of the recycled water was validated through membrane fabrication and performance tests. Seven different classes of adsorbents—graphene, polymers with intrinsic microporosity, imprinted polymers, zeolites, metal organic frameworks, activated carbon, and resins—were evaluated. The isotherm and kinetic behaviors of the best adsorbents have been fully characterized and the adsorbent regenerability without any performance loss has been confirmed for up to 10 wastewater treatment cycles. It has been demonstrated that over 99% of the organic impurities in the wastewater can be successfully removed and the recycled water can be reused without adverse effects on the final membrane performance. The proposed wastewater treatment technique can reduce the process mass intensity (PMI) of membrane fabrication by 99.9% per m2 of the membrane produced. The required energy duty for different regeneration methods and wastewater treatment methods revealed that the adsorption technology is the most effective method, with the lowest energy requirement of about 1200 kJ per m2 of the membrane produced.

Increasing the sustainability of membrane processes through cascade approach and solvent recovery ? pharmaceutical purification case study

Membrane processes suffer limitations such as low product yield and high solvent consumption, hindering their widespread application in the pharmaceutical and fine chemicals industries. In the present work, the authors propose an efficient purification methodology employing a two-stage cascade configuration coupled to an adsorptive solvent recovery unit, which addresses the two limitations. The process has been validated on purification of active pharmaceutical ingredient (API) from genotoxic impurity (GTI) using organic solvent nanofiltration (OSN). The model system selected for study comprises roxithromycin macrolide antibiotic (Roxi) with 4-dimethylaminopyridine (DMAP) and ethyl tosylate (EtTS) as API and GTIs, respectively. By implementing a two-stage cascade configuration for membrane diafiltration, the process yield was increased from 58% to 95% while maintaining less than 5 ppm GTI in the final solution. Through this yield enhancement, the membrane process has been “revamped” from an unfeasible process to a highly competitive unit operation when compared to other traditional processes. The advantage of size exclusion membranes over other separation techniques has been illustrated by the simultaneous removal of two GTIs from different chemical classes. In addition, a solvent recovery step has been assessed using charcoal as a non-selective adsorbent, and it has been shown that pure solvent can be recovered from the permeate. Considering the costs of solvent, charcoal, and waste disposal, it was concluded that 70% solvent recovery is the cost-optimum point. Conventional single-stage diafiltration (SSD) and two-stage diafiltration (TSD) configurations were compared in terms of green metrics such as cost, mass and solvent intensity, and energy consumption. It was calculated that implementation of TSD, depending on the batch scale, can achieve up to 92% cost saving while reducing the mass and solvent intensity up to 73%. In addition, the advantage of adsorptive solvent recovery has been assessed revealing up to 96% energy reduction compared to distillation and a 70% reduction of CO2 footprint.

Highly conductive and durable poly(arylene ether sulfone) anion exchange membrane with end-group cross-linking

Here, we demonstrate the improved electrochemical performance and stability of end-group cross-linked anion exchange membranes (AEM) for the first time via the introduction of imidazolium groups in poly(arylene ether sulfone) (Imd-PAES). A novel feature of the cross-linking reaction is that basic additives are not required to prevent gelation with the cationic functional groups. In this work, the sodium salt of 3-hydroxyphenylacetylene acted directly as the end-group cross-linker, and it was cross-linked by thermal treatment at 180 °C. The gel fraction and hydroxide conductivity of the cross-linked membranes (XE-Imds) depended on the cross-linking temperature and time. The prepared XE-Imd70 (70 refers to the degree of functionalization) membranes with an ion exchange capacity (IEC) of 2.2 meq g−1 achieved a high hydroxide conductivity (107 mS cm−1). This material also showed good single cell performance (XE-Imd70: 202 mA cm−2 at 0.6 V and a maximum power density of 196.1 mW cm−2) at 80 °C, 100% relative humidity (RH), and improved durability and alkaline stability. The excellent hydroxide conductivity and electrochemical performance of XE-Imd70 was due to the fact that the ion cluster size of XE-Imd membranes was larger (12.1–14 nm) than that of E-Imd (5.5–8.14 nm), indicating that XE-Imd membranes have a closely associated ion-clustered morphology, which was confirmed by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) measurements.

Thermally rearranged polymer membranes for desalination

Herein, we demonstrate thermally rearranged polybenzoxazole-co-imide (TR-PBOI) electrospun nanocomposite membranes for membrane distillation and membrane crystallization applications. We seek to demonstrate that a synergistic combination of TR polymers, porous nanofibrous membranes, and particle coating improves the long-term stability while maintaining high porosity and water flux. The fabricated membranes exhibit an excellent water flux (80 kg m−2 h−1) and NaCl rejection (>99.99%) with steady performance over more than 186 hours. In addition, for the first time, controlling the heterogeneous nucleation phenomena in membrane crystallization was clearly demonstrated using TR membrane morphology.

Exploring and exploiting the effect of solvent treatment in membrane separations

It is well-known that solvent treatment and preconditioning play an important role in rejection and flux performance of membranes due to solvent-induced swelling and solvent adsorption. Investigations into the effect of solvent treatment are scarce and application specific, and were limited to a few solvents only. This study reveals the trend in solvent treatment based on solvent polarity in a systematic investigation with the aim to harness such effect for intensification of membrane processes. Nine solvents with polarity indices ranging from 0.1 to 5.8 (hexane to acetonitrile) were used as treatment and process solvents on commercial Borsig GMT-oNF-2, Evonik Duramem 300, and emerging tailor-made polybenzimidazole membranes. TGA-GCMS, HS-GC-FID, and NMR techniques were employed to better understand the effect of solvent treatment on the polymer matrix of membranes. In this work, apart from the solvent treatments direct effect on the membrane performance, a subsequent indirect effect on the ultimate separation process was observed. Consequently, a pharmaceutical case study employing chlorhexidine disinfectant and antiseptic was used to demonstrate the effect of solvent treatment on the nanofiltration-based purification. It is shown that treatment of polybenzimidazole membranes with acetone resulted in a 25% increase in product recovery at 99% impurity removal. The cost of the process intensification is negligible in terms of solvent consumption, mass intensity, and processing time.

Liquid-Phase Synthesis of 2'-Methyl-RNA on a Homostar Support through Organic-Solvent Nanofiltration.

Due to the discovery of RNAi, oligonucleotides (oligos) have re-emerged as a major pharmaceutical target that may soon be required in ton quantities. However, it is questionable whether solid-phase oligo synthesis (SPOS) methods can provide a scalable synthesis. Liquid-phase oligo synthesis (LPOS) is intrinsically scalable and amenable to standard industrial batch synthesis techniques. However, most reported LPOS strategies rely upon at least one precipitation per chain extension cycle to separate the growing oligonucleotide from reaction debris. Precipitation can be difficult to develop and control on an industrial scale and, because many precipitations would be required to prepare a therapeutic oligonucleotide, we contend that this approach is not viable for large-scale industrial preparation. We are developing an LPOS synthetic strategy for 2′-methyl RNA phosphorothioate that is more amenable to standard batch production techniques, using organic solvent nanofiltration (OSN) as the critical scalable separation technology. We report the first LPOS-OSN preparation of a 2′-Me RNA phosphorothioate 9-mer, using commercial phosphoramidite monomers, and monitoring all reactions by HPLC, 31P NMR spectroscopy and MS.

A compact and scalable fabrication method for robust thin film composite membranes

Membrane-based liquid filtration systems have been highlighted as a key green process engineering platform for process intensification. While these systems have been successfully commercialized in desalination and micro/ultra/nanofiltration, it is yet challenging to expand the limited operating conditions (ambient temperature in an aqueous environment) of the currently available membranes, and to solve the exacerbating environmental burden during the fabrication of polymeric membranes. Herein, a new green and compact fabrication method is proposed for both (i) fabricating a robust membrane that can function beyond the typical operating conditions of conventional membranes and (ii) minimizing waste creation and production time. In this work, contrary to the conventional phase inversion method, a spray-coating technique was employed to fabricate a thin composite membrane with high temperature stability and fast solvent permeability. The proposed compact fabrication method can reduce the carbon footprint and the usage of polymers and solvents more than twofold, and eliminate the time-consuming coagulation and washing steps, both of which generate an enormous amount of solvent-contaminated wastewater (up to 50 billion liters per year). Furthermore, the process used dimethyl sulfoxide to bypass conventional N,N-dimethylformamide and N-methyl-2-pyrrolidone as a green solvent, and can simultaneously fabricate a membrane ready to use within a half-day. The resulting membrane exhibited remarkable stability and performance for the separation of solutes in N,N-dimethylformamide, even at elevated temperatures not feasible with conventional polymeric membranes, and so it is a candidate for potential use in the pharmaceutical and fine chemical industries.