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Click-functionalized inverse-opal structured membranes for organocatalytic reactions
Journal Pre-proofs Click-Functionalized Inverse-Opal Structured Membranes for Organocatalytic Reactions Hwanhui Na, Gwan H. Choi, Taejun Eom, Joona Bang, Pil J. Yoo PII: DOI: Reference:
S1383-5866(19)35278-5 https://doi.org/10.1016/j.seppur.2020.116621 SEPPUR 116621
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
16 November 2019 24 January 2020 24 January 2020
Please cite this article as: H. Na, G.H. Choi, T. Eom, J. Bang, P.J. Yoo, Click-Functionalized Inverse-Opal Structured Membranes for Organocatalytic Reactions, Separation and Purification Technology (2020), doi: https://doi.org/10.1016/j.seppur.2020.116621
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© 2020 Published by Elsevier B.V.
Click-Functionalized Inverse-Opal Structured Membranes for Organocatalytic Reactions Hwanhui Na,† Gwan H. Choi,† Taejun Eom,‡ Joona Bang,‡ and Pil J. Yoo†, §,* †
School of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea.
‡ Department § SKKU
of Chemical and Biological Engineering, Korea University, Seoul, 02841, Republic of Korea.
Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University (SKKU), Suwon 16419,
Republic of Korea.
* Corresponding Author. E-mail Address: [email protected] (P. J. Yoo)
Abstract
Three-dimensional (3D) porous polymeric inverse-opal (IO) structures have unique structural characteristics for various applications. The combined benefit of highly interconnected pores with facilitated mass transfer properties imparts a strong potential for use as catalytic reactive membranes. Although research has developed methods for immobilizing catalysts at the internal surface of reactive membranes, there are still problems of catalyst leaching and issues with post-separation of catalysts from reaction products. In this work, we present a straightforward and robust method for generating irreversible chemical bonds by employing click chemistry, using copper-catalyzed alkyneazide 1,3-dipolar cycloaddition in particular. We used a combination of an azide-functionalized 3DIO surface and an alkyne-end-modified 2,2,6,6-Tetramethyl-1-piperidineyloxy (TEMPO) organocatalyst. The TEMPO-embedded 3D-IO structures performed significantly better as catalytic membranes for oxidizing alcohols into their corresponding aldehyde derivatives in the presence of an oxidant and co-catalyst. The synergistic effects contribute to the outstanding membrane performance and high conversion efficiency (> 97% depending on the type of alcohol) while maintaining a 1
permeation flux greater than 300 L/m2h@1 bar. This novel approach can be extended to the design and development of various reactive membranes for biomedical, environmental, and scaled-up synthetic purposes.
KEYWORDS: Inverse-opal, catalytic membranes, click reaction, TEMPO, organocatalytic reactions.
1. Introduction Templated synthesis of porous structures have been investigated extensively, including use of dual surfactants, polymeric templates, and colloidal crystals [1]. Among them, the colloidal crystaltemplated technique has received a lot of attention since it allows for further chemical functionalization within three-dimensional ordered macroporous arrays with structural dimensions from tens to hundreds of nanometers [2]. In particular, three-dimensional inverse-opal (3D-IO) structures are formed by removing the opaline-assembled colloidal template after filling the interconnected internal voids with frame-forming material. This imparts several advantages to its structural properties including a high surface-to-volume ratio, uniformity/tunability in pore size, monolithic pore-interconnectivity, and easy processability [3,4]. These structural features have been successfully implemented in the application areas of biomimetic matrices, photonic devices, biological sensors, thermal insulation, and other energy-storage systems [5-7]. Uniquely ordered macroporous IO structures can also be utilized as reactive membranes due to facilitated internal mass transfer through interconnected pores, accessibility to active sites, and reactive component loading [8,9]. In order to utilize 3D-IOs for membrane applications, it is necessary to process them on a large scale while suppressing occurrences of structural defects. This ensures the separation efficiency and mechanical stability of the membranes, for which organic-based IO scaffold materials have been mainly employed rather than to use inorganic precursors [4,10]. When a liquid-phase inorganic 2
precursor is used as an IO frame-forming material, the volumetric shrinkage during solidification of precursors is significant and the resulting cracked inorganic IO structure cannot work for a feasible membrane. Instead, an organic-based framework, particularly one with a UV-curable prepolymer, could reduce the shrinkage rate upon solidification process to less than ~2%, which would enable large-scale membrane applications [11-13]. Despite the structural benefits of these 3D-IO membranes, their use as reactive membranes is highly challenged, especially in immobilizing reactive functional moieties on the surface of chemically inert cross-linked polymers [14,15]. There have been several recent attempts to modify the inner surface of 3D-IOs using adhesive interlinking agents of polydopamine or physical deposition techniques such as atomic layer deposition [16,17]. After successful modification, reactive agents such as metallic nanoparticles are physically decorated and the resulting functionalized IOs can be used as reactive membranes for mediating targeted chemical reactions [18]. However, physically attaching or anchoring reactive agents such as metal/inorganic nanoparticles or catalytic species will lead to activity loss from membrane leaching due to the lack of strong chemical bonds between the reactive agents and the membrane surface. Therefore, there is an urgent need to develop an irreversible and stable chemical bond between reacting species and the internal surface of membrane frame. Considerable effort has also been devoted to immobilizing heterogeneous molecular units by generating covalent bonds on the target surface [19-22]. One strategy uses click chemistry, whereby a high yield of harmless byproducts can be attained in a modular and stereospecific manner [23,24]. The specific functional groups utilized for the click reaction can impart highly tolerant and stable properties during the reaction, regardless of the type of solvents (i.e. different extents in proticity and polarity) or interfaces (i.e. solid or liquid) used [25]. Because of these merits, click chemistry has been applied to a myriad of chemical and biological species, providing rapid and flexible accessibility to surface manipulation. The ability to use clickable moieties on the solid surface would make it 3
possible to create reaction-mediating membranes with catalyst-immobilized surfaces. Copper-catalyzed azide-alkyne 1,3-dipolar cycloaddition is a highly suitable click reaction approach for solid surfaces due to its ability to anchor the azide moiety [26,27]. However, for the case of polymer-based reactive membranes, there are two major difficulties when using azide-alkyne click chemistry on polymeric membrane surface. First, the azide moiety is highly sensitive to the UV radiation crosslinking process [28]. When generating a 3D-IO structured polymeric frame, the prepolymer in the voids between the assembled colloidal particles needs to be crosslinked and solidified to form the stable skeletal frame of membranes. Conventionally, this is where a UV-curing process is employed. Although azide groups may survive under mild UV irradiation for a short duration [29], they are prone to decomposing into nitrene intermediates upon high UV exposure. Second, the triple bond of alkyne moiety which is complementarily required as a counterpart to the azide group does not generally present at the reactive catalyst. Instead, the alkyne end group needs to be introduced to the catalyst species in a pre-designed way in order to ensure catalytic activity [30]. In this study, we developed a way to implement click-functionalized reactive membranes by combining azide-functionalized 3D-IO structured surfaces and alkyne-end-modified organocatalysts. To circumvent the UV curing issues, we incorporated halogen-ended monomers inside the prepolymer instead of directly attaching the azide groups. After curing, the surface-exposed halide groups were then readily substituted with azides. As a result, the 1,3-dipolar addition between the membrane surface and the targeted organocatalysts occurred successfully without disrupting the polymeric IO framework or the catalytic activity. Although click reaction-based catalyst immobilization has been previously reported for nanoparticles, electrospun nanofibers, and nanotube network [20,30,31], these approaches were only utilized for catalyst support and required postseparation after the reaction. In this study, the reactant flow being in contact with the catalystimmobilized 3D-IO surface facilitates the progress of the targeted reaction with a controllability over the reaction residence time. We employed 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) functional 4
groups over the membrane surface to induce the alcohol oxidation reaction for organocatalysts. The high conversion characteristics, even under mild reaction conditions, allowed the click-functionalized and TEMPO-catalyzed 3D-IO structured membranes to exhibit outstanding reaction membrane performances in both permeability (310 L/m2h (LMH)@1 bar) and reactivity (92.5~99.0% conversion depending on the type of reactant) while securing a membrane reproducibility. The synergistic structural advantage of 3D-IOs and click reaction versatility offers great potential for high performance reaction membrane design and is broadly applicable to environmental remedies, biomedical screening, and fine chemical synthesis.
2. Experimental 2.1. Materials Methacryloyl chloride (97%), 6-chlorohexanol (96%), triethylamine (>99%), propargyl bromide solution (80 wt.% in toluene), sodium hydride (60% in mineral oil), 4-hydroxyl-2,2,6,6tetramethylpiperidine-1-oxyl (4-hydroxy TEMPO, 97%), ammonium persulfate (APS, reagent grade 98%), polyvinylpyrrolidone (PVP, 55000 Mw), (+)-sodium L-ascorbate (98%), sodium bromide (99%), and sodium hypochlorite solution (reagent grade, available chlorine 1015%) were obtained from Sigma Aldrich and used as received without further treatments. Dichloromethane (DCM, 95%), N, N´-dimethylformamide (DMF, 99.0%), anhydrous sodium sulfate (99%), n-hexane (95%), ethyl acetate (99.5%), styrene monomer (99.5%), ethyl alcohol (EtOH, 99.9%), and copper(II) sulfate pentahydrate (99%) were obtained from Samchun Chemical (Korea). UV-curable poly(urethane acrylate) (PUA, 311RM) was purchased from Minuta Technology (Korea). 2.2. Synthesis of 6-chlorohexyl methacrylate A mixture of methacryloyl chloride (2.09 g, 20 mmol), 6-chloro-1-hexanol (1.36 g, 10 mmol) and triethylamine (1.51 g, 15 mmol) in DCM was stirred in an ice bath for 3 min and left at room temperature for 8 h. The DCM solution was washed with deionized (DI) water (100 ml) three times 5
after which anhydrous sodium sulfate was mixed in to remove residual water. The separated organic layer from the mixture was dried with a rotary evaporator to remove the DCM and the remaining solution was purified using gel chromatography (with eluents ethyl acetate and n-hexane (1:9 in v/v ratio each)). The eluent was dried to obtain 6-chlorohexyl methacrylate, a pale yellow solution. The 6-chlorohexyl methacrylate was stirred for 24 h with the UV-curable PUA prepolymer with a mixing ratio of 1:20 in w/w. The synthesis was confirmed using 1H NMR (700 MHz, CDCl3) spectra that showed chemical shifts (δ) at 6.018 (d, 1H), 5.481 (d, 1H), 4.089 (t, 2H), 3.479 (t, 2H), 1.910 (s, 3H), 1.758 (tt, 2H), 1.648 (tt, 2H), 1.416 (tt, 2H), and 1.361 (tt, 2H). 13C NMR (700MHz, CDCl3) was also used with δ 167.26, 136.41, 125.05, 64.41, 44.76, 32.39, 28.42, 26.44, 25.45, and 18.19. (Fig. S1a, Fig. S2a) 2.3. Synthesis of propargyl ether-modified TEMPO Sodium hydride (279 mg, 6.97 mmol, 60% in mineral oil) was poured into DMF (5 mL) at room temperature for 50 min and then moved to an ice bath. 4-hydroxy TEMPO (1 g, 5.8 mmol) was added dropwise to the DMF while in the ice bath and then moved to room temperature for 50 min. Propargyl bromide (0.6 mL, 6.97 mmol) was added dropwise to DMF at 0℃ for 30 min and left at room temperature for 10 h. The DMF solution was mixed with 30 ml DI water and the organic layer was extracted using ethyl acetate (50 ml). The extracted ethyl acetate solution was purified with DI water 3 times after which anhydrous sodium sulfate was added to remove any residual water. The separated organic layer from the mixture was dried to remove the ethyl acetate and remaining solution was further purified using gel chromatography (with eluents ethyl acetate and n-hexane (1:5 in v/v ratio each)). The eluent was dried to obtain a reddish powder, propargyl ether TEMPO. The synthesis was confirmed using 1H NMR (700 MHz, CDCl3) spectra showing chemical shifts (δ) at 4.586 (s, 2H) and 2.832 (s, 1H). Not all signals were observable due to paramagnetic broadening by the nitroxide radicals (Fig. S1b, Fig. S2b) [30,32]. 2.4. Synthesis of colloidal nanoparticles 6
The monodispersed polystyrene colloidal particles were synthesized using a dispersion polymerization method [11]. 0.06 g PVP was dissolved in ethanol (99.9%, 150 mL) as the solvent and APS initiator (reagent grade 98%, 0.039 g) dissolved in DI water (18 mL). It was then added dropwise to the PVP ethanol solution. After the addition of the styrene monomer (99.5%, 13.2 mL), the mixture was stirred at 70°C for 12 h. When the polymerization process was over, the synthesized PS colloidal particles were washed with ethanol to remove residual PVP and separated from ethanol using a centrifuge. Washing and separation processes were repeated three times. The PS particles were separated by size to obtain mono-disperse PS particles. 2.5. Three-dimensional inverse-opal (3D-IO) structure fabrication The PS colloidal particles were re-dispersed in a solution of DI water and ethanol (15 wt% in 50:50 v/v%). The colloidal PS particle solution was drop-spread on a glass substrate pre-treated with plasma cleaner for 1 min (PDC-001, Harrick Scientific Corp.) at room temperature to induce the selfassembly of PS particles into an opal structure. The residual solvent within the assembled structure was allowed to dry completely. The PUA and 6-chlorohexyl methacrylate (20:1 in w/w) UV-curable prepolymer mixture was then spread over the assembled opal structure to fill the internal voids between PS particles. The prepolymer mixture was crosslinked using UV-irradiation for 5 h under ambient conditions. After crosslinking and solidification, the cured product was immersed in toluene for 20 min to completely dissolve the PS particles. Finally, after separating the film from the underlying glass substrate using a treatment with 5% hydrofluoric acid solution, the 3D-IO structure with modified chloride end sites was obtained as a free-standing film (thickness with 2225 m). (Fig. S3a) 2.6. Post-modification of 3D-IOs with click reactions After preparing the free-standing 3D-IO film, the chloride groups at the IO surface were substituted with azide moieties through a reaction with sodium azide (1.5 g, 23 mmol) dissolved in DMF (50 mL) at 80°C for 24 h. (Fig. S3b) The azide-substituted 3D-IO membranes were immersed 7
in a solution of DMF/H2O (90 mL /18 mL) with CuSO4·5H2O (420 mg) and L-ascorbic acid sodium salt (670 mg). Propargyl ether TEMPO (353 mg) was added and the solution stirred at room temperature for 72 h. Finally, the TEMPO-catalyzed 3D-IO membranes were washed with DI water and methanol several times and fully dried under vacuum. (Fig. S3c) 2.7. Membrane performance measurement of TEMPO-catalyzed 3D-IOs The membranes were assembled with a homemade filtration system equipped with a pressurized filtration syringe holder (KS 13 and KS 25; Advantec MFS, Inc. Japan). Filtration experiments were performed at room temperature. First, DI water was filtered through the double-IO layer-stacked membrane (thickness with 4550 m) by applying 2.0-bar pressure for 1 h to completely wet the flow passages through the interconnected channels inside the 3D-IO, followed by lowering the pressure to between 0.4 and 1.6 bar to measure the water permeation flux. Dilute alcohol (1 mM, 1.08 g benzyl alcohol in 1 L water) as the feed solution with a co-catalyst (sodium bromide of 2.95 g) and oxidant (sodium hypochlorite of 3.4 mL) were filtered to investigate the oxidative performance of the TEMPO-organocatalyst embedded 3D-IO membranes. To confirm the membrane reactivity for different kinds of alcohols, 1-butanol and 2-methoxyethanol were also tested. For each experiment, 1-butanol (0.74 g) and 2-methoxyethanol (0.76 g) were dissolved with the same amount of co-catalyst and oxidant in DI water (1 L). Also, for further membrane testing with varied amount of alcohol, the concentration of benzyl alcohol was varied in a range of 1 10 mM and the amounts of co-catalyst and oxidant were adjusted accordingly. The permeated solutions were characterized using UV−vis spectroscopy (UV-3600, Shimadzu, Japan) before and after filtration to determine the amount of alcohol oxidized to aldehyde. 2.8. Characterizations The UV polymerization was proceeded by irradiating UV light (40 W). Film thickness and inner surface morphologies of the TEMPO-embedded 3D-IO structured membranes were measured using field emission electron microscopy (FE-SEM; JSM-7600F, JEOL, Japan). 1H NMR spectra were 8
obtained using an Avance Ⅲ 700 MHz spectrometer (Bruker, Germany) and FTIR spectra were obtained using an IFS-66/S, TENSOR 27 (Bruker, Germany). To measure the mechanical properties of the cured PUA films, we used a universal testing machine (Instron 3343, Instron, USA). UV cured PUA polymer samples were prepared in 10 mm 20 mm 150 m pieces. The UTM crosshead speed was adjusted to 1 mm/min during the tensile stretching tests.
3. Results and Discussion
Fig. 1. (a) Schematic procedure of fabricating TEMPO-immobilized 3D-IO via azide-alkyne click reaction. (b) FTIR-spectra of sequentially treated PUA prepolymers: from top to bottom, bare PUA, PUA mixed with 6-chlorohexyl methacrylate, PUA mixed with 6-chlorohexyl methacrylate after azide substitution, and azidated PUA mixture after click reaction with propargyl ether TEMPO. (c) Water contact angle measurements on bare PUA film (top) and on TEMPO-modified PUA (bottom).
9
3.1. TEMPO-incorporated inverse opal structures with PUA fabrication Fig. 1a shows the schematic used to fabricate TEMPO organocatalyst-embedded 3D-IO structured membranes. To generate the clickable surface, 6-chlorohexyl methacrylate-mixed PUA prepolymer was UV-cured first to form the 3D-IO structure as a membrane frame, followed by application of the azide substitution with chloride groups on the inner surface of 3D-IOs. The amount of 6-chlorohexyl methacrylate added to the PUA pre-polymer was adjusted to less than 5 wt%, thus, robust UV-curing and low-volumetric shrinkage characteristics were retained upon crosslinking the PUA [33]. As the pre-polymer mixed with 6-chlorohexyl methacrylate penetrates through the voids in the close-packed-stacked polystyrene (PS) colloidal particles, the excess pre-polymer would cover the outermost surface of colloidal assembly, which needs to be removed for membrane application. Therefore, an ethanol solvent is spin-coated several times on top of the PUA pre-polymer layer until the excess pre-polymer is removed and the upper surface of colloidal assembly is exposed. Next, the pre-polymer is UV-cured and the PS particles embedded within the solidified PUA frame are dissolved using an organic solvent treatment. This generates the 3D-IO structured PUA membranes with chloride-ending functional groups. Next, the membrane is further treated to create the TEMPO catalytic surface. These procedures were monitored using IR spectrometry to confirm the sequential end functional group changes (Fig. 1b). The characteristic band of alkyl chloride end group of 6chlorohexyl methacrylate appears at 810 cm-1. Following the substitution reaction with chloride groups, the azide functional groups were confirmed with a band at 2160 cm-1. After treating the membranes with azide-alkyne 1,3-dipolar cycloaddition, the characteristic azide moiety peak disappears. Due to PUA’s low surface energy (23 dyn/cm), which is required as for molding material [33], the cross-linked PUA surface exhibits fairly hydrophobic characteristics (92.1° water contact angle on flattened PUA surface; upper panel in Fig. 1c). This tendency intensifies when 3D-IOs microporous structures are incorporated. However, considering the targeted application as for a 10
reactive membrane, particularly when using polar solvents (e.g. water or alcohol) as the medium, internal surface properties need to be appropriately modified to be accepting of a waterborne environment. It is desirable that uses of the 6-chlorohexyl methacrylate and piperidine derivatives allow the TEMPO-modified PUA surface to exhibit hydrophilic properties (60.3° water contact angle; lower panel in Fig. 1c) [34]. Therefore, improved affinity for polar solvents are attainable and TEMPO-catalyzed 3D-IO membranes would work as reactive membranes properly.
Fig. 2. Mechanical properties of UV-cured PUA films: bare PUA and PUA mixed with 6-chlorohexyl methacrylate (20:1 in w/w). Stress-strain curves (a) and the estimated Young’s moduli (b). The error bars indicate the standard deviations (n = 3).
The addition of 6-chlorohexyl methacrylate to the PUA pre-polymer influences the mechanical properties of the UV-cured PUA films due to a change in the crosslinking density. The resulting difference in mechanical properties can be evaluated using UTM measurements of the stress-strain relationship (Fig. 2a). The UV-cured PUA film exhibits an enhanced tensile strength of 45.2 MPa, which is two times greater than bare PUA film (22.9 MPa). This enhanced tensile strength improves membrane toughness and structural durability. Young’s modulus of the UV-cured PUA film is estimated to be 1300 MPa, which is slightly lower than bare PUA film (1480 MPa, Fig. 2b). This is because the incorporation of 6-chlorohexyl methacrylate chains hinders denser cross-linking between oligomeric PUA pre-polymers, and results in a slight decrease in Young’s modulus (i.e. relieved brittleness). Therefore, the UV-cured PUA films can endure up to 10 bars of highly pressurized 11
membrane operations without any structural collapse in the 3D-IOs porous architecture.
Fig. 3. Membrane tests of the TEMPO-catalyzed 3D-IO structured films with varying the size of assembling PS colloidal particles from 500 to 800 nm. (a) Diameters of interconnected nano-channels of 3D-IOs. Error bars indicate the standard deviations (n = 33). (Fig. S4) (b) Water permeation fluxes through double-layerstacked 3D-IO membranes with respect to the applied pressure. Error bars indicate the standard deviations (n = 3). (c) Measured permeability of benzyl alcohol solution with respect to the projected area of nano-channels. Error bars indicate the standard deviations (n = 3).
The surface morphology of TEMPO-catalyzed 3D-IO membranes was observed using SEM. As shown in Fig. 3a, no significant morphological changes were observed before or after the organocatalyst immobilization via the click reaction. This is because the sizes of functional moieties - including TEMPO - are negligible compared to the internal pore size of 3D-IOs (Fig. 3a). In addition, the size of the interconnected channels inside the 3D-IOs are enlarged with the increased size of assembling PS colloidal particles. The permeated water flux for 3D-IO membranes fabricated with 800 nm-sized PS colloidal particles is 340 L/m2h (LMH) under 0.4 bar-pressurized conditions. This 12
is equivalent to 880 LMH@1 bar (Fig. 3b). When the particle size decreases to 500 nm, the water flux decreases to 310 LMH@1 bar. Assuming identical membrane volumes, this is indicative of the prolonged residence time of feed solution within the membrane. Overall permeability of the reaction feed of alcohol solution is proportional to the projected area of individual internal nano-channel, which matches well the Hagen-Poiseuille relationship - a general equation describing membrane permeability (Fig. 3c) [12].
Fig. 4. Reaction mechanism of the alcohol oxidation catalyzed with TEMPO moiety.
3.2. Performance Evaluation of Reactive Membranes There have been many studies on metal-free organocatalytic reactions using stable and reusable active sites. N-heterocyclic carbene (NHC) is a versatile ligand because it acts as a strong electron donor and covers a variety of reactions including alcohol/amine oxidation, transesterification, benzoin condensation, and Stetter reactions [34,35]. In this study, an NHC catalyst, 2,2,6,6tetramethyl-1-piperidineyloxy (TEMPO) moiety is immobilized on the inner surface of 3D-IOs and used as a reactive membrane for performing alcohol oxidation reactions in the presence of a primary oxidant (e.g. hypochlorite) and co-catalyst (e.g. sodium bromide) [36]. Fig. 4 shows the catalytic cycle of TEMPO during the alcohol oxidation process involving three mutually transformative forms. 13
TEMPO transforms the original nitroxyl radical state into a nitrosonium ion after the addition of the NaClO oxidant and the NaBr co-catalyst. With the addition of alcohol, the nitrosonium ion changes to an intermediate oxidized state and is then reduced to hydroxylamine along with the alcohol oxidation [37]. This hydroxylamine is re-oxidized to a TEMPO nitrosonium ion with the aid of hypochlorite, completing the catalytic cycle. Therefore, TEMPO-mediated oxidative reaction membranes are highly reproducible with a continuous supply of oxidant and co-catalyst [38].
Fig. 5. Catalytic reaction performances of the TEMPO-embedded 3D-IO membranes for alcohol oxidation reactions: benzyl alcohol (a), 1-butanol (b), and 2-methoxyethanol (c). Upper row shows UV-vis absorption spectra before and after treating the oxidative reaction of alcohols. (d) Conversion of benzyl alcohol in response to the applied pressure with varying the pore size of 3D-IO reactive membranes fabricated with 500 nm, 700 nm, and 800 nm-sized PS colloidal particles. (e) Variation of conversion performance of 3D-IO membranes with increasing the reaction time. The interval of sampling is every 12 h. (f) Percent oxidation of benzyl alcohol versus the permeated volume of the feed filtered through TEMPO-incorporated 3D-IO membranes. (g) 14
Conversion of benzyl alcohol with different concentration through 3D-IO membrane fabricated with 500 nmsized PS colloidal particles. (h) Residence time of the feed solution within 3D-IO membranes with varying the size of templating colloidal particles. (i) Plots of ln([alcohol]0 –[aldehyde]t) versus residence time for different feed concentration of benzyl alcohol.
The degree of oxidation reaction of benzyl alcohol can be quantitatively monitored using UVvis absorption spectra analysis (Fig. 5a). Although a strong absorption band appears at 200220 nm with a slight shoulder at 255 nm, these peaks are located out of the range of conventional UV-vis spectra [39]. After the oxidation reaction, benzaldehyde in the effluent is visible in the absorption band at 250270 nm with a shoulder at 285 nm. This can be monitored and concentration increases with extended reaction time (i.e. increasing the residence time inside the 3D-IO membranes) [37]. In general, the degree of TEMPO-mediated oxidation reactions is proportional to the residence time within the reactive membrane unless the immobilized TEMPO catalysts do leach out. For example, when a feed flow of alcohol, NaBr, and NaClO in water passes through the 3D-IO fabricated with 800 nm-sized particles (average channel size ~275 nm), the alcohol conversion only reaches 57.7% under 1.6 bar pressure (flux with 1410 LMH). On the other hand, when the channel size narrows with the use of 500 nm-sized particles (average channel size ~170 nm), the conversion increases to 95.3% under 1.6 bar pressure (flux with 513 LMH). Using the results shown in Fig. 5d, we chose the 3D-IO membranes fabricated with 500 nm-sized particles as optimized conditions exhibiting both high permeability and high conversion efficiency. We also investigated long term durability of the reactive membranes. When the operational pressure was adjusted to 0.6 bar, the conversion was retained at ~ 99% with a permeation flux of 188 LMH. These feed flowing conditions were maintained while the reaction continued for 60 h (Fig. 5e). We sampled the effluent every 12 h over the course of this experiment. The catalytic activity of 3D-IO membranes was maintained for 48 h (with an average conversion rate of 99.4%) and decreased slightly after 60 h (with a conversion rate of 97.6%). This confirms that using 3D-IO membranes is reproducible and continuous reactions are attainable without significant reduction in reaction efficiency. Continuously monitoring the reaction conversion (Fig. 15
5f) verified the reactivity of TEMPO-catalyzed oxidation of benzyl alcohol while retaining a high conversion rate is located in the range of 97.8%99.9%. Along with aromatic alcohols, aliphatic alcohols can also be oxidized by TEMPOincorporated 3D-IO membranes, although the reaction time required to obtain the same conversion rate is prolonged instead. Two kinds of aliphatic alcohols were studied: 1-butanol and 2methoxyethanol. The former yielded an average conversion rate of 92.5% under 0.6 bar pressure (Fig. 5b and Fig. S5b; butyraldehyde peak at 270 nm), and the latter yielded a conversion efficiency of 96.7% under the same conditions (Fig. 5c and Fig. S5a; methoxyacetaldehyde peak at 280 nm). Due to differences in electron withdrawing strength, the stability of the alcohol-TEMPO intermediates was also different and effected the rate of oxidation and reaction conversion. In addition to the variation in band intensity of aldehyde groups, characteristic oxidant band of NaClO can be used as an indicator of the oxidation reaction. Feed solutions containing NaClO initially exhibit a strong peak at 330 nm (Figs. 5a, 5b, and 5c) which disappears after oxidant consumption. To elaborate on the performance of reactive membranes, the alcohol oxidation tests were carried out with varying the concentration of benzyl alcohol from 1 mM to 10 mM while fixing the pore size of 3D-IO membrane using 500 nm-sized PS colloidal particles. As shown in Fig. 5g, very high conversion (> 99%) was still retained up to 3.0 mM of benzyl alcohol with applying 0.4pressurized condition, whereas the conversion decreased to ~ 94.3% under 1.0 bar. On the other hand, for highly concentrated condition of 10 mM, the conversion values were substantially reduced to 94.3% and 92.4% in response to the applying pressure variation from 0.4 to 1.0 bar, respectively. Basically, the kinetics of TEMPO-mediated alcohol oxidation is known to conform the following reaction for a fixed amount of TEMPO catalyst [40], which typically exhibits the first-order kinetic behavior. R ― CH2OH + TEMPO + + OH ―
𝑘1
R ― CHO + TEMPO ― H + H2O
[CH2OH]𝑡 = [CH2OH]0exp ( ― 𝑘1t) 16
𝑙𝑛 ([CH2OH]0 ― [R ― CHO]t) = ― 𝑘1𝑡 + 𝑙𝑛 [CH2OH]0
(1)
Therefore, from experimentally obtained values for the concentration of reactant and product along with the residence time of the feed within membranes, the kinetic constant of k1 can readily be estimated using Eq. (1). The double-layer-stacked 3D-IO membranes (thickness with 4550 m) used in this study represent the internal volumetric porosity of 74%, which is the packing ratio of the closepacked structure with colloidal spheres [11]. Accordingly, corresponding residence time of reactant alcohol inside the membrane is calculated to be placed in a range between 0.34 s and 2.15 s, depending on the variation of the pore size and the applied pressure (Fig. 3b and Fig. 5h) [41]. As manifested in Fig. 5i, it clearly shows that a linear relationship is retained between ln([CH2OH]0[R-CHO]t) and the residence time (t) for different concentration of benzyl alcohol, confirming the reaction to follow the known kinetics of TEMPO-mediated alcohol oxidation. As a result, the corresponding reaction constants (k) for varied alcohol concentration from 1.0 to 2.0, 3.0, and 10 mM are estimated to be 3.38 s-1, 1.97 s-1, 1.45 s-1, and 0.24 s-1, respectively. It needs to be noted that the estimated value of reaction constant for high feed concentration (e.g. 10 mM of benzyl alcohol) is observed to be relatively smaller, which is presumably associated with inadequate contact of the reactant alcohol with the surface-immobilized TEMPO catalyst due to thin membrane thickness. Finally, the stability of TEMPO-incorporated 3D-IO membranes was also investigated. Unlike physically adsorbed catalyst particles in other studies, the organocatalyst of TEMPO moieties is chemically anchored to the PUA surface via a 1,2,3-triazole ring group formed between the alkyne end site of TEMPO and the azide group embedded on the inner surface of 3D-IO as a result of the click reaction. Therefore, when the membrane permeation effluents were subject to post-UV-vis spectral analysis, the TEMPO moieties were not found in the permeate (Fig. S6). This clearly confirmed that immobilized TEMPO moieties are not leached out from the membrane surface within the 10 ppb detection limit.
17
4. Conclusions In summary, a TEMPO-immobilized 3D-IO structure was newly created using a click reaction for use in catalytic membrane applications. Conventionally, despite large-scale demonstrations of 3DIO structures with porous membranes using a colloidal template method, the chemically inert and hydrophobic UV-cured PUA membrane frame has hindered its use for developing reactive membranes. To overcome this hurdle, here, we modified the 3D-IO membranes surface by introducing clickable moieties for alkyne-azide cycloaddition. This process chemically anchored the organocatalytic TEMPO functional groups via 1,2,3-triazole ring formation on the poreinterconnected internal surface of 3D-IO structured membranes. To control catalytic reactivity, the size of the internal pores was adjusted to ensure extended residence time for the reaction while retaining a high permeation flux. As a result, the TEMPO-immobilized 3D-IO structured membranes exhibited outstanding reactivity for alcohol oxidation reactions (e.g. benzyl alcohol, 1-butanol, and 2-methoxyethanol) while securing long-term operability and stability of TEMPO catalysts. This method yielded a highly reproducible reactive membrane for various applications.
Acknowledgments. This work was supported by research grants of the NRF (2020R1A2B5B02002483) funded by the National Research Foundation under the Ministry of Science, ICT & Future, Korea. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/
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Graphical abstract
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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TEMPO-functionalized inverse opal (IO) structured membranes are implemented. Modification of IO framework is proceeded by employing alkyne-azide click reaction. Pore size, permeability, and mechanical properties of IO membranes are regulated. Catalytic IO membranes exhibit high performance in alcohol oxidation reactions.
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S1383-5866(19)35278-5 https://doi.org/10.1016/j.seppur.2020.116621 SEPPUR 116621
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Separation and Purification Technology
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16 November 2019 24 January 2020 24 January 2020
Please cite this article as: H. Na, G.H. Choi, T. Eom, J. Bang, P.J. Yoo, Click-Functionalized Inverse-Opal Structured Membranes for Organocatalytic Reactions, Separation and Purification Technology (2020), doi: https://doi.org/10.1016/j.seppur.2020.116621
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Click-Functionalized Inverse-Opal Structured Membranes for Organocatalytic Reactions Hwanhui Na,† Gwan H. Choi,† Taejun Eom,‡ Joona Bang,‡ and Pil J. Yoo†, §,* †
School of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea.
‡ Department § SKKU
of Chemical and Biological Engineering, Korea University, Seoul, 02841, Republic of Korea.
Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University (SKKU), Suwon 16419,
Republic of Korea.
* Corresponding Author. E-mail Address: [email protected] (P. J. Yoo)
Abstract
Three-dimensional (3D) porous polymeric inverse-opal (IO) structures have unique structural characteristics for various applications. The combined benefit of highly interconnected pores with facilitated mass transfer properties imparts a strong potential for use as catalytic reactive membranes. Although research has developed methods for immobilizing catalysts at the internal surface of reactive membranes, there are still problems of catalyst leaching and issues with post-separation of catalysts from reaction products. In this work, we present a straightforward and robust method for generating irreversible chemical bonds by employing click chemistry, using copper-catalyzed alkyneazide 1,3-dipolar cycloaddition in particular. We used a combination of an azide-functionalized 3DIO surface and an alkyne-end-modified 2,2,6,6-Tetramethyl-1-piperidineyloxy (TEMPO) organocatalyst. The TEMPO-embedded 3D-IO structures performed significantly better as catalytic membranes for oxidizing alcohols into their corresponding aldehyde derivatives in the presence of an oxidant and co-catalyst. The synergistic effects contribute to the outstanding membrane performance and high conversion efficiency (> 97% depending on the type of alcohol) while maintaining a 1
permeation flux greater than 300 L/m2h@1 bar. This novel approach can be extended to the design and development of various reactive membranes for biomedical, environmental, and scaled-up synthetic purposes.
KEYWORDS: Inverse-opal, catalytic membranes, click reaction, TEMPO, organocatalytic reactions.
1. Introduction Templated synthesis of porous structures have been investigated extensively, including use of dual surfactants, polymeric templates, and colloidal crystals [1]. Among them, the colloidal crystaltemplated technique has received a lot of attention since it allows for further chemical functionalization within three-dimensional ordered macroporous arrays with structural dimensions from tens to hundreds of nanometers [2]. In particular, three-dimensional inverse-opal (3D-IO) structures are formed by removing the opaline-assembled colloidal template after filling the interconnected internal voids with frame-forming material. This imparts several advantages to its structural properties including a high surface-to-volume ratio, uniformity/tunability in pore size, monolithic pore-interconnectivity, and easy processability [3,4]. These structural features have been successfully implemented in the application areas of biomimetic matrices, photonic devices, biological sensors, thermal insulation, and other energy-storage systems [5-7]. Uniquely ordered macroporous IO structures can also be utilized as reactive membranes due to facilitated internal mass transfer through interconnected pores, accessibility to active sites, and reactive component loading [8,9]. In order to utilize 3D-IOs for membrane applications, it is necessary to process them on a large scale while suppressing occurrences of structural defects. This ensures the separation efficiency and mechanical stability of the membranes, for which organic-based IO scaffold materials have been mainly employed rather than to use inorganic precursors [4,10]. When a liquid-phase inorganic 2
precursor is used as an IO frame-forming material, the volumetric shrinkage during solidification of precursors is significant and the resulting cracked inorganic IO structure cannot work for a feasible membrane. Instead, an organic-based framework, particularly one with a UV-curable prepolymer, could reduce the shrinkage rate upon solidification process to less than ~2%, which would enable large-scale membrane applications [11-13]. Despite the structural benefits of these 3D-IO membranes, their use as reactive membranes is highly challenged, especially in immobilizing reactive functional moieties on the surface of chemically inert cross-linked polymers [14,15]. There have been several recent attempts to modify the inner surface of 3D-IOs using adhesive interlinking agents of polydopamine or physical deposition techniques such as atomic layer deposition [16,17]. After successful modification, reactive agents such as metallic nanoparticles are physically decorated and the resulting functionalized IOs can be used as reactive membranes for mediating targeted chemical reactions [18]. However, physically attaching or anchoring reactive agents such as metal/inorganic nanoparticles or catalytic species will lead to activity loss from membrane leaching due to the lack of strong chemical bonds between the reactive agents and the membrane surface. Therefore, there is an urgent need to develop an irreversible and stable chemical bond between reacting species and the internal surface of membrane frame. Considerable effort has also been devoted to immobilizing heterogeneous molecular units by generating covalent bonds on the target surface [19-22]. One strategy uses click chemistry, whereby a high yield of harmless byproducts can be attained in a modular and stereospecific manner [23,24]. The specific functional groups utilized for the click reaction can impart highly tolerant and stable properties during the reaction, regardless of the type of solvents (i.e. different extents in proticity and polarity) or interfaces (i.e. solid or liquid) used [25]. Because of these merits, click chemistry has been applied to a myriad of chemical and biological species, providing rapid and flexible accessibility to surface manipulation. The ability to use clickable moieties on the solid surface would make it 3
possible to create reaction-mediating membranes with catalyst-immobilized surfaces. Copper-catalyzed azide-alkyne 1,3-dipolar cycloaddition is a highly suitable click reaction approach for solid surfaces due to its ability to anchor the azide moiety [26,27]. However, for the case of polymer-based reactive membranes, there are two major difficulties when using azide-alkyne click chemistry on polymeric membrane surface. First, the azide moiety is highly sensitive to the UV radiation crosslinking process [28]. When generating a 3D-IO structured polymeric frame, the prepolymer in the voids between the assembled colloidal particles needs to be crosslinked and solidified to form the stable skeletal frame of membranes. Conventionally, this is where a UV-curing process is employed. Although azide groups may survive under mild UV irradiation for a short duration [29], they are prone to decomposing into nitrene intermediates upon high UV exposure. Second, the triple bond of alkyne moiety which is complementarily required as a counterpart to the azide group does not generally present at the reactive catalyst. Instead, the alkyne end group needs to be introduced to the catalyst species in a pre-designed way in order to ensure catalytic activity [30]. In this study, we developed a way to implement click-functionalized reactive membranes by combining azide-functionalized 3D-IO structured surfaces and alkyne-end-modified organocatalysts. To circumvent the UV curing issues, we incorporated halogen-ended monomers inside the prepolymer instead of directly attaching the azide groups. After curing, the surface-exposed halide groups were then readily substituted with azides. As a result, the 1,3-dipolar addition between the membrane surface and the targeted organocatalysts occurred successfully without disrupting the polymeric IO framework or the catalytic activity. Although click reaction-based catalyst immobilization has been previously reported for nanoparticles, electrospun nanofibers, and nanotube network [20,30,31], these approaches were only utilized for catalyst support and required postseparation after the reaction. In this study, the reactant flow being in contact with the catalystimmobilized 3D-IO surface facilitates the progress of the targeted reaction with a controllability over the reaction residence time. We employed 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) functional 4
groups over the membrane surface to induce the alcohol oxidation reaction for organocatalysts. The high conversion characteristics, even under mild reaction conditions, allowed the click-functionalized and TEMPO-catalyzed 3D-IO structured membranes to exhibit outstanding reaction membrane performances in both permeability (310 L/m2h (LMH)@1 bar) and reactivity (92.5~99.0% conversion depending on the type of reactant) while securing a membrane reproducibility. The synergistic structural advantage of 3D-IOs and click reaction versatility offers great potential for high performance reaction membrane design and is broadly applicable to environmental remedies, biomedical screening, and fine chemical synthesis.
2. Experimental 2.1. Materials Methacryloyl chloride (97%), 6-chlorohexanol (96%), triethylamine (>99%), propargyl bromide solution (80 wt.% in toluene), sodium hydride (60% in mineral oil), 4-hydroxyl-2,2,6,6tetramethylpiperidine-1-oxyl (4-hydroxy TEMPO, 97%), ammonium persulfate (APS, reagent grade 98%), polyvinylpyrrolidone (PVP, 55000 Mw), (+)-sodium L-ascorbate (98%), sodium bromide (99%), and sodium hypochlorite solution (reagent grade, available chlorine 1015%) were obtained from Sigma Aldrich and used as received without further treatments. Dichloromethane (DCM, 95%), N, N´-dimethylformamide (DMF, 99.0%), anhydrous sodium sulfate (99%), n-hexane (95%), ethyl acetate (99.5%), styrene monomer (99.5%), ethyl alcohol (EtOH, 99.9%), and copper(II) sulfate pentahydrate (99%) were obtained from Samchun Chemical (Korea). UV-curable poly(urethane acrylate) (PUA, 311RM) was purchased from Minuta Technology (Korea). 2.2. Synthesis of 6-chlorohexyl methacrylate A mixture of methacryloyl chloride (2.09 g, 20 mmol), 6-chloro-1-hexanol (1.36 g, 10 mmol) and triethylamine (1.51 g, 15 mmol) in DCM was stirred in an ice bath for 3 min and left at room temperature for 8 h. The DCM solution was washed with deionized (DI) water (100 ml) three times 5
after which anhydrous sodium sulfate was mixed in to remove residual water. The separated organic layer from the mixture was dried with a rotary evaporator to remove the DCM and the remaining solution was purified using gel chromatography (with eluents ethyl acetate and n-hexane (1:9 in v/v ratio each)). The eluent was dried to obtain 6-chlorohexyl methacrylate, a pale yellow solution. The 6-chlorohexyl methacrylate was stirred for 24 h with the UV-curable PUA prepolymer with a mixing ratio of 1:20 in w/w. The synthesis was confirmed using 1H NMR (700 MHz, CDCl3) spectra that showed chemical shifts (δ) at 6.018 (d, 1H), 5.481 (d, 1H), 4.089 (t, 2H), 3.479 (t, 2H), 1.910 (s, 3H), 1.758 (tt, 2H), 1.648 (tt, 2H), 1.416 (tt, 2H), and 1.361 (tt, 2H). 13C NMR (700MHz, CDCl3) was also used with δ 167.26, 136.41, 125.05, 64.41, 44.76, 32.39, 28.42, 26.44, 25.45, and 18.19. (Fig. S1a, Fig. S2a) 2.3. Synthesis of propargyl ether-modified TEMPO Sodium hydride (279 mg, 6.97 mmol, 60% in mineral oil) was poured into DMF (5 mL) at room temperature for 50 min and then moved to an ice bath. 4-hydroxy TEMPO (1 g, 5.8 mmol) was added dropwise to the DMF while in the ice bath and then moved to room temperature for 50 min. Propargyl bromide (0.6 mL, 6.97 mmol) was added dropwise to DMF at 0℃ for 30 min and left at room temperature for 10 h. The DMF solution was mixed with 30 ml DI water and the organic layer was extracted using ethyl acetate (50 ml). The extracted ethyl acetate solution was purified with DI water 3 times after which anhydrous sodium sulfate was added to remove any residual water. The separated organic layer from the mixture was dried to remove the ethyl acetate and remaining solution was further purified using gel chromatography (with eluents ethyl acetate and n-hexane (1:5 in v/v ratio each)). The eluent was dried to obtain a reddish powder, propargyl ether TEMPO. The synthesis was confirmed using 1H NMR (700 MHz, CDCl3) spectra showing chemical shifts (δ) at 4.586 (s, 2H) and 2.832 (s, 1H). Not all signals were observable due to paramagnetic broadening by the nitroxide radicals (Fig. S1b, Fig. S2b) [30,32]. 2.4. Synthesis of colloidal nanoparticles 6
The monodispersed polystyrene colloidal particles were synthesized using a dispersion polymerization method [11]. 0.06 g PVP was dissolved in ethanol (99.9%, 150 mL) as the solvent and APS initiator (reagent grade 98%, 0.039 g) dissolved in DI water (18 mL). It was then added dropwise to the PVP ethanol solution. After the addition of the styrene monomer (99.5%, 13.2 mL), the mixture was stirred at 70°C for 12 h. When the polymerization process was over, the synthesized PS colloidal particles were washed with ethanol to remove residual PVP and separated from ethanol using a centrifuge. Washing and separation processes were repeated three times. The PS particles were separated by size to obtain mono-disperse PS particles. 2.5. Three-dimensional inverse-opal (3D-IO) structure fabrication The PS colloidal particles were re-dispersed in a solution of DI water and ethanol (15 wt% in 50:50 v/v%). The colloidal PS particle solution was drop-spread on a glass substrate pre-treated with plasma cleaner for 1 min (PDC-001, Harrick Scientific Corp.) at room temperature to induce the selfassembly of PS particles into an opal structure. The residual solvent within the assembled structure was allowed to dry completely. The PUA and 6-chlorohexyl methacrylate (20:1 in w/w) UV-curable prepolymer mixture was then spread over the assembled opal structure to fill the internal voids between PS particles. The prepolymer mixture was crosslinked using UV-irradiation for 5 h under ambient conditions. After crosslinking and solidification, the cured product was immersed in toluene for 20 min to completely dissolve the PS particles. Finally, after separating the film from the underlying glass substrate using a treatment with 5% hydrofluoric acid solution, the 3D-IO structure with modified chloride end sites was obtained as a free-standing film (thickness with 2225 m). (Fig. S3a) 2.6. Post-modification of 3D-IOs with click reactions After preparing the free-standing 3D-IO film, the chloride groups at the IO surface were substituted with azide moieties through a reaction with sodium azide (1.5 g, 23 mmol) dissolved in DMF (50 mL) at 80°C for 24 h. (Fig. S3b) The azide-substituted 3D-IO membranes were immersed 7
in a solution of DMF/H2O (90 mL /18 mL) with CuSO4·5H2O (420 mg) and L-ascorbic acid sodium salt (670 mg). Propargyl ether TEMPO (353 mg) was added and the solution stirred at room temperature for 72 h. Finally, the TEMPO-catalyzed 3D-IO membranes were washed with DI water and methanol several times and fully dried under vacuum. (Fig. S3c) 2.7. Membrane performance measurement of TEMPO-catalyzed 3D-IOs The membranes were assembled with a homemade filtration system equipped with a pressurized filtration syringe holder (KS 13 and KS 25; Advantec MFS, Inc. Japan). Filtration experiments were performed at room temperature. First, DI water was filtered through the double-IO layer-stacked membrane (thickness with 4550 m) by applying 2.0-bar pressure for 1 h to completely wet the flow passages through the interconnected channels inside the 3D-IO, followed by lowering the pressure to between 0.4 and 1.6 bar to measure the water permeation flux. Dilute alcohol (1 mM, 1.08 g benzyl alcohol in 1 L water) as the feed solution with a co-catalyst (sodium bromide of 2.95 g) and oxidant (sodium hypochlorite of 3.4 mL) were filtered to investigate the oxidative performance of the TEMPO-organocatalyst embedded 3D-IO membranes. To confirm the membrane reactivity for different kinds of alcohols, 1-butanol and 2-methoxyethanol were also tested. For each experiment, 1-butanol (0.74 g) and 2-methoxyethanol (0.76 g) were dissolved with the same amount of co-catalyst and oxidant in DI water (1 L). Also, for further membrane testing with varied amount of alcohol, the concentration of benzyl alcohol was varied in a range of 1 10 mM and the amounts of co-catalyst and oxidant were adjusted accordingly. The permeated solutions were characterized using UV−vis spectroscopy (UV-3600, Shimadzu, Japan) before and after filtration to determine the amount of alcohol oxidized to aldehyde. 2.8. Characterizations The UV polymerization was proceeded by irradiating UV light (40 W). Film thickness and inner surface morphologies of the TEMPO-embedded 3D-IO structured membranes were measured using field emission electron microscopy (FE-SEM; JSM-7600F, JEOL, Japan). 1H NMR spectra were 8
obtained using an Avance Ⅲ 700 MHz spectrometer (Bruker, Germany) and FTIR spectra were obtained using an IFS-66/S, TENSOR 27 (Bruker, Germany). To measure the mechanical properties of the cured PUA films, we used a universal testing machine (Instron 3343, Instron, USA). UV cured PUA polymer samples were prepared in 10 mm 20 mm 150 m pieces. The UTM crosshead speed was adjusted to 1 mm/min during the tensile stretching tests.
3. Results and Discussion
Fig. 1. (a) Schematic procedure of fabricating TEMPO-immobilized 3D-IO via azide-alkyne click reaction. (b) FTIR-spectra of sequentially treated PUA prepolymers: from top to bottom, bare PUA, PUA mixed with 6-chlorohexyl methacrylate, PUA mixed with 6-chlorohexyl methacrylate after azide substitution, and azidated PUA mixture after click reaction with propargyl ether TEMPO. (c) Water contact angle measurements on bare PUA film (top) and on TEMPO-modified PUA (bottom).
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3.1. TEMPO-incorporated inverse opal structures with PUA fabrication Fig. 1a shows the schematic used to fabricate TEMPO organocatalyst-embedded 3D-IO structured membranes. To generate the clickable surface, 6-chlorohexyl methacrylate-mixed PUA prepolymer was UV-cured first to form the 3D-IO structure as a membrane frame, followed by application of the azide substitution with chloride groups on the inner surface of 3D-IOs. The amount of 6-chlorohexyl methacrylate added to the PUA pre-polymer was adjusted to less than 5 wt%, thus, robust UV-curing and low-volumetric shrinkage characteristics were retained upon crosslinking the PUA [33]. As the pre-polymer mixed with 6-chlorohexyl methacrylate penetrates through the voids in the close-packed-stacked polystyrene (PS) colloidal particles, the excess pre-polymer would cover the outermost surface of colloidal assembly, which needs to be removed for membrane application. Therefore, an ethanol solvent is spin-coated several times on top of the PUA pre-polymer layer until the excess pre-polymer is removed and the upper surface of colloidal assembly is exposed. Next, the pre-polymer is UV-cured and the PS particles embedded within the solidified PUA frame are dissolved using an organic solvent treatment. This generates the 3D-IO structured PUA membranes with chloride-ending functional groups. Next, the membrane is further treated to create the TEMPO catalytic surface. These procedures were monitored using IR spectrometry to confirm the sequential end functional group changes (Fig. 1b). The characteristic band of alkyl chloride end group of 6chlorohexyl methacrylate appears at 810 cm-1. Following the substitution reaction with chloride groups, the azide functional groups were confirmed with a band at 2160 cm-1. After treating the membranes with azide-alkyne 1,3-dipolar cycloaddition, the characteristic azide moiety peak disappears. Due to PUA’s low surface energy (23 dyn/cm), which is required as for molding material [33], the cross-linked PUA surface exhibits fairly hydrophobic characteristics (92.1° water contact angle on flattened PUA surface; upper panel in Fig. 1c). This tendency intensifies when 3D-IOs microporous structures are incorporated. However, considering the targeted application as for a 10
reactive membrane, particularly when using polar solvents (e.g. water or alcohol) as the medium, internal surface properties need to be appropriately modified to be accepting of a waterborne environment. It is desirable that uses of the 6-chlorohexyl methacrylate and piperidine derivatives allow the TEMPO-modified PUA surface to exhibit hydrophilic properties (60.3° water contact angle; lower panel in Fig. 1c) [34]. Therefore, improved affinity for polar solvents are attainable and TEMPO-catalyzed 3D-IO membranes would work as reactive membranes properly.
Fig. 2. Mechanical properties of UV-cured PUA films: bare PUA and PUA mixed with 6-chlorohexyl methacrylate (20:1 in w/w). Stress-strain curves (a) and the estimated Young’s moduli (b). The error bars indicate the standard deviations (n = 3).
The addition of 6-chlorohexyl methacrylate to the PUA pre-polymer influences the mechanical properties of the UV-cured PUA films due to a change in the crosslinking density. The resulting difference in mechanical properties can be evaluated using UTM measurements of the stress-strain relationship (Fig. 2a). The UV-cured PUA film exhibits an enhanced tensile strength of 45.2 MPa, which is two times greater than bare PUA film (22.9 MPa). This enhanced tensile strength improves membrane toughness and structural durability. Young’s modulus of the UV-cured PUA film is estimated to be 1300 MPa, which is slightly lower than bare PUA film (1480 MPa, Fig. 2b). This is because the incorporation of 6-chlorohexyl methacrylate chains hinders denser cross-linking between oligomeric PUA pre-polymers, and results in a slight decrease in Young’s modulus (i.e. relieved brittleness). Therefore, the UV-cured PUA films can endure up to 10 bars of highly pressurized 11
membrane operations without any structural collapse in the 3D-IOs porous architecture.
Fig. 3. Membrane tests of the TEMPO-catalyzed 3D-IO structured films with varying the size of assembling PS colloidal particles from 500 to 800 nm. (a) Diameters of interconnected nano-channels of 3D-IOs. Error bars indicate the standard deviations (n = 33). (Fig. S4) (b) Water permeation fluxes through double-layerstacked 3D-IO membranes with respect to the applied pressure. Error bars indicate the standard deviations (n = 3). (c) Measured permeability of benzyl alcohol solution with respect to the projected area of nano-channels. Error bars indicate the standard deviations (n = 3).
The surface morphology of TEMPO-catalyzed 3D-IO membranes was observed using SEM. As shown in Fig. 3a, no significant morphological changes were observed before or after the organocatalyst immobilization via the click reaction. This is because the sizes of functional moieties - including TEMPO - are negligible compared to the internal pore size of 3D-IOs (Fig. 3a). In addition, the size of the interconnected channels inside the 3D-IOs are enlarged with the increased size of assembling PS colloidal particles. The permeated water flux for 3D-IO membranes fabricated with 800 nm-sized PS colloidal particles is 340 L/m2h (LMH) under 0.4 bar-pressurized conditions. This 12
is equivalent to 880 LMH@1 bar (Fig. 3b). When the particle size decreases to 500 nm, the water flux decreases to 310 LMH@1 bar. Assuming identical membrane volumes, this is indicative of the prolonged residence time of feed solution within the membrane. Overall permeability of the reaction feed of alcohol solution is proportional to the projected area of individual internal nano-channel, which matches well the Hagen-Poiseuille relationship - a general equation describing membrane permeability (Fig. 3c) [12].
Fig. 4. Reaction mechanism of the alcohol oxidation catalyzed with TEMPO moiety.
3.2. Performance Evaluation of Reactive Membranes There have been many studies on metal-free organocatalytic reactions using stable and reusable active sites. N-heterocyclic carbene (NHC) is a versatile ligand because it acts as a strong electron donor and covers a variety of reactions including alcohol/amine oxidation, transesterification, benzoin condensation, and Stetter reactions [34,35]. In this study, an NHC catalyst, 2,2,6,6tetramethyl-1-piperidineyloxy (TEMPO) moiety is immobilized on the inner surface of 3D-IOs and used as a reactive membrane for performing alcohol oxidation reactions in the presence of a primary oxidant (e.g. hypochlorite) and co-catalyst (e.g. sodium bromide) [36]. Fig. 4 shows the catalytic cycle of TEMPO during the alcohol oxidation process involving three mutually transformative forms. 13
TEMPO transforms the original nitroxyl radical state into a nitrosonium ion after the addition of the NaClO oxidant and the NaBr co-catalyst. With the addition of alcohol, the nitrosonium ion changes to an intermediate oxidized state and is then reduced to hydroxylamine along with the alcohol oxidation [37]. This hydroxylamine is re-oxidized to a TEMPO nitrosonium ion with the aid of hypochlorite, completing the catalytic cycle. Therefore, TEMPO-mediated oxidative reaction membranes are highly reproducible with a continuous supply of oxidant and co-catalyst [38].
Fig. 5. Catalytic reaction performances of the TEMPO-embedded 3D-IO membranes for alcohol oxidation reactions: benzyl alcohol (a), 1-butanol (b), and 2-methoxyethanol (c). Upper row shows UV-vis absorption spectra before and after treating the oxidative reaction of alcohols. (d) Conversion of benzyl alcohol in response to the applied pressure with varying the pore size of 3D-IO reactive membranes fabricated with 500 nm, 700 nm, and 800 nm-sized PS colloidal particles. (e) Variation of conversion performance of 3D-IO membranes with increasing the reaction time. The interval of sampling is every 12 h. (f) Percent oxidation of benzyl alcohol versus the permeated volume of the feed filtered through TEMPO-incorporated 3D-IO membranes. (g) 14
Conversion of benzyl alcohol with different concentration through 3D-IO membrane fabricated with 500 nmsized PS colloidal particles. (h) Residence time of the feed solution within 3D-IO membranes with varying the size of templating colloidal particles. (i) Plots of ln([alcohol]0 –[aldehyde]t) versus residence time for different feed concentration of benzyl alcohol.
The degree of oxidation reaction of benzyl alcohol can be quantitatively monitored using UVvis absorption spectra analysis (Fig. 5a). Although a strong absorption band appears at 200220 nm with a slight shoulder at 255 nm, these peaks are located out of the range of conventional UV-vis spectra [39]. After the oxidation reaction, benzaldehyde in the effluent is visible in the absorption band at 250270 nm with a shoulder at 285 nm. This can be monitored and concentration increases with extended reaction time (i.e. increasing the residence time inside the 3D-IO membranes) [37]. In general, the degree of TEMPO-mediated oxidation reactions is proportional to the residence time within the reactive membrane unless the immobilized TEMPO catalysts do leach out. For example, when a feed flow of alcohol, NaBr, and NaClO in water passes through the 3D-IO fabricated with 800 nm-sized particles (average channel size ~275 nm), the alcohol conversion only reaches 57.7% under 1.6 bar pressure (flux with 1410 LMH). On the other hand, when the channel size narrows with the use of 500 nm-sized particles (average channel size ~170 nm), the conversion increases to 95.3% under 1.6 bar pressure (flux with 513 LMH). Using the results shown in Fig. 5d, we chose the 3D-IO membranes fabricated with 500 nm-sized particles as optimized conditions exhibiting both high permeability and high conversion efficiency. We also investigated long term durability of the reactive membranes. When the operational pressure was adjusted to 0.6 bar, the conversion was retained at ~ 99% with a permeation flux of 188 LMH. These feed flowing conditions were maintained while the reaction continued for 60 h (Fig. 5e). We sampled the effluent every 12 h over the course of this experiment. The catalytic activity of 3D-IO membranes was maintained for 48 h (with an average conversion rate of 99.4%) and decreased slightly after 60 h (with a conversion rate of 97.6%). This confirms that using 3D-IO membranes is reproducible and continuous reactions are attainable without significant reduction in reaction efficiency. Continuously monitoring the reaction conversion (Fig. 15
5f) verified the reactivity of TEMPO-catalyzed oxidation of benzyl alcohol while retaining a high conversion rate is located in the range of 97.8%99.9%. Along with aromatic alcohols, aliphatic alcohols can also be oxidized by TEMPOincorporated 3D-IO membranes, although the reaction time required to obtain the same conversion rate is prolonged instead. Two kinds of aliphatic alcohols were studied: 1-butanol and 2methoxyethanol. The former yielded an average conversion rate of 92.5% under 0.6 bar pressure (Fig. 5b and Fig. S5b; butyraldehyde peak at 270 nm), and the latter yielded a conversion efficiency of 96.7% under the same conditions (Fig. 5c and Fig. S5a; methoxyacetaldehyde peak at 280 nm). Due to differences in electron withdrawing strength, the stability of the alcohol-TEMPO intermediates was also different and effected the rate of oxidation and reaction conversion. In addition to the variation in band intensity of aldehyde groups, characteristic oxidant band of NaClO can be used as an indicator of the oxidation reaction. Feed solutions containing NaClO initially exhibit a strong peak at 330 nm (Figs. 5a, 5b, and 5c) which disappears after oxidant consumption. To elaborate on the performance of reactive membranes, the alcohol oxidation tests were carried out with varying the concentration of benzyl alcohol from 1 mM to 10 mM while fixing the pore size of 3D-IO membrane using 500 nm-sized PS colloidal particles. As shown in Fig. 5g, very high conversion (> 99%) was still retained up to 3.0 mM of benzyl alcohol with applying 0.4pressurized condition, whereas the conversion decreased to ~ 94.3% under 1.0 bar. On the other hand, for highly concentrated condition of 10 mM, the conversion values were substantially reduced to 94.3% and 92.4% in response to the applying pressure variation from 0.4 to 1.0 bar, respectively. Basically, the kinetics of TEMPO-mediated alcohol oxidation is known to conform the following reaction for a fixed amount of TEMPO catalyst [40], which typically exhibits the first-order kinetic behavior. R ― CH2OH + TEMPO + + OH ―
𝑘1
R ― CHO + TEMPO ― H + H2O
[CH2OH]𝑡 = [CH2OH]0exp ( ― 𝑘1t) 16
𝑙𝑛 ([CH2OH]0 ― [R ― CHO]t) = ― 𝑘1𝑡 + 𝑙𝑛 [CH2OH]0
(1)
Therefore, from experimentally obtained values for the concentration of reactant and product along with the residence time of the feed within membranes, the kinetic constant of k1 can readily be estimated using Eq. (1). The double-layer-stacked 3D-IO membranes (thickness with 4550 m) used in this study represent the internal volumetric porosity of 74%, which is the packing ratio of the closepacked structure with colloidal spheres [11]. Accordingly, corresponding residence time of reactant alcohol inside the membrane is calculated to be placed in a range between 0.34 s and 2.15 s, depending on the variation of the pore size and the applied pressure (Fig. 3b and Fig. 5h) [41]. As manifested in Fig. 5i, it clearly shows that a linear relationship is retained between ln([CH2OH]0[R-CHO]t) and the residence time (t) for different concentration of benzyl alcohol, confirming the reaction to follow the known kinetics of TEMPO-mediated alcohol oxidation. As a result, the corresponding reaction constants (k) for varied alcohol concentration from 1.0 to 2.0, 3.0, and 10 mM are estimated to be 3.38 s-1, 1.97 s-1, 1.45 s-1, and 0.24 s-1, respectively. It needs to be noted that the estimated value of reaction constant for high feed concentration (e.g. 10 mM of benzyl alcohol) is observed to be relatively smaller, which is presumably associated with inadequate contact of the reactant alcohol with the surface-immobilized TEMPO catalyst due to thin membrane thickness. Finally, the stability of TEMPO-incorporated 3D-IO membranes was also investigated. Unlike physically adsorbed catalyst particles in other studies, the organocatalyst of TEMPO moieties is chemically anchored to the PUA surface via a 1,2,3-triazole ring group formed between the alkyne end site of TEMPO and the azide group embedded on the inner surface of 3D-IO as a result of the click reaction. Therefore, when the membrane permeation effluents were subject to post-UV-vis spectral analysis, the TEMPO moieties were not found in the permeate (Fig. S6). This clearly confirmed that immobilized TEMPO moieties are not leached out from the membrane surface within the 10 ppb detection limit.
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4. Conclusions In summary, a TEMPO-immobilized 3D-IO structure was newly created using a click reaction for use in catalytic membrane applications. Conventionally, despite large-scale demonstrations of 3DIO structures with porous membranes using a colloidal template method, the chemically inert and hydrophobic UV-cured PUA membrane frame has hindered its use for developing reactive membranes. To overcome this hurdle, here, we modified the 3D-IO membranes surface by introducing clickable moieties for alkyne-azide cycloaddition. This process chemically anchored the organocatalytic TEMPO functional groups via 1,2,3-triazole ring formation on the poreinterconnected internal surface of 3D-IO structured membranes. To control catalytic reactivity, the size of the internal pores was adjusted to ensure extended residence time for the reaction while retaining a high permeation flux. As a result, the TEMPO-immobilized 3D-IO structured membranes exhibited outstanding reactivity for alcohol oxidation reactions (e.g. benzyl alcohol, 1-butanol, and 2-methoxyethanol) while securing long-term operability and stability of TEMPO catalysts. This method yielded a highly reproducible reactive membrane for various applications.
Acknowledgments. This work was supported by research grants of the NRF (2020R1A2B5B02002483) funded by the National Research Foundation under the Ministry of Science, ICT & Future, Korea. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/
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Graphical abstract
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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TEMPO-functionalized inverse opal (IO) structured membranes are implemented. Modification of IO framework is proceeded by employing alkyne-azide click reaction. Pore size, permeability, and mechanical properties of IO membranes are regulated. Catalytic IO membranes exhibit high performance in alcohol oxidation reactions.
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