Research Article www.acsami.org
Nanoparticle-Directed Metal−Organic Framework/Porous Organic Polymer Monolithic Supports for Flow-Based Applications María del Mar Darder,† Shima Salehinia,†,‡ José B. Parra,§ José M. Herrero-Martinez,∥ Frantisek Svec,⊥ Víctor Cerdà,† Gemma Turnes Palomino,† and Fernando Maya*,† †
Department of Chemistry, University of the Balearic Islands, Carretera de Valldemossa km 7.5, E-07122 Palma de Mallorca, Spain Department of Analytical Chemistry, Faculty of Chemistry, Kashan University, 87317-51167 Kashan, Iran § Instituto Nacional del Carbon, INCAR-CSIC, P. O. 73, 33080 Oviedo, Spain ∥ Department of Analytical Chemistry, University of Valencia, C. Doctor Moliner 50, E-46100 Burjassot, Valencia, Spain ⊥ Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China ‡
S Supporting Information *
ABSTRACT: A two-step nanoparticle-directed route for the preparation of macroporous polymer monoliths for which the pore surface is covered with a metal−organic framework (MOF) coating has been developed to facilitate the use of MOFs in flowbased applications. The flow-through monolithic matrix was prepared in a column format from a polymerization mixture containing ZnO-nanoparticles. These nanoparticles embedded in the precursor monolith were converted to MOF coatings via the dissolution−precipitation equilibrium after filling the pores of the monolith with a solution of the organic linker. Pore surface coverage with the microporous zeolitic imidazolate framework ZIF-8 resulted in an increase in surface area from 72 to 273 m2 g−1. Monolithic polymer containing ZIF-8 coating was implemented as a microreactor catalyzing the Knoevenagel condensation reaction and also in extraction column format enabling the preconcentration of trace levels of toxic chlorophenols in environmental waters. Our approach can be readily adapted to other polymers and MOFs thus enabling development of systems for flow-based MOF applications. KEYWORDS: porous organic polymers, zinc oxide nanoparticles, metal−organic frameworks, monolithic columns, flow-through support, flow chemistry, catalysis, pollutant extraction
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topics.18−21 Recent interesting approaches comprise conversion of precursor metal oxides to MOFs. The metal oxides can also be attached to a variety of solid supports.22−30 Most of the attention here has been focused on the zeolitic imidazolate frameworks (ZIFs), a subclass of MOFs, typically prepared via coordination of metals with imidazolate linkers.31−33 Porous organic polymer monoliths (PMs) prepared as a single piece are materials containing a large number of interconnected large through pores. They have proven to be excellent supports in flow-through applications.34−37 PMs can be prepared in a variety of formats such as columns,38 microfluidic channels,39,40 thin layers, and coatings.41−43 The main advantages of PMs are their excellent stability, low cost, ease of preparation, and very low resistance to flow. The flip side of polymer monoliths is their small surface areas due to the lack of microporosity. Recently, PM-containing MOFs have been reported.44 These MOF-PMs are typically prepared using addition of preformed MOF crystals to the polymerization mixture, 45−47 or
INTRODUCTION The feasibility to integrate micro- or nanocrystalline materials in suitable porous supports is critical for the development of flow-based applications.1,2 Metal−organic frameworks (MOFs), also known as porous coordination polymers, are an emerging class of porous materials,3−8 with a significant potential for liquid phase separation and catalysis.9−12 MOFs are constructed from metal ions or clusters linked by bi- or multidentate organic ligands shaping extended networks. The synthesis of MOFs is highly versatile, and thousands of MOF structures have been already discovered. However, the progress of flowbased applications of MOFs is sluggish mostly due to small particle size and inappropriate morphology of MOFs making them unsuitable for packing in flow-through devices.13 Flow-based methods are crucial for the development of highly desirable applications including organic synthesis,14 or the development of automated methods of analysis.15 Flowbased techniques enabled the miniaturization and automation of heterogeneous catalysis,16 or the fast and reproducible preconcentration of trace levels of environmental pollutants.17 As a result, MOF engineering for specific applications is a burgeoning research field. Developing new strategies for growth of supported MOF on solid supports is one of the hot © 2016 American Chemical Society
Received: August 31, 2016 Accepted: December 14, 2016 Published: December 14, 2016 1728
DOI: 10.1021/acsami.6b10999 ACS Appl. Mater. Interfaces 2017, 9, 1728−1736
Research Article
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the procedure described by Rouquerol et al.55 The results are summarized in Table S1 included in the Supporting Information. High-performance liquid chromatography (HPLC) was used for quantification of the water pollutants in extraction experiments and for following the conversion process in the catalytic reaction. A Jasco HPLC instrument composed of a high-pressure pump (PU-4180), a manual injector, and a photodiode-array detector (MD-4017) was used. Separations were carried out at room temperature using a Phenomenex Kinetex EVO C18 100A core−shell column (150 mm length × 4.6 mm i.d., 5 μm) with a guard column (5 mm length × 4.6 mm i.d.) from the same material. A Chemyx Inc. Fusion 2000 syringe pump was used for functionalization of capillaries, and for flowthrough catalysis experiments. Preparation of Bulk ZnO-Nanoparticle Polymer Monoliths. The preparation was carried out at a scale of 1−10 mL. No differences in the textural properties and porosity of the final monolith were observed based on the volume of the monolith. The desired weight of ethanolic dispersion of zinc oxide nanoparticles (ZnO-NPs) was added to a 20 mL scintillation vial and dried at 70 °C in a conventional oven. Solid ZnO-NPs were re-dispersed in an appropriate volume of MAA using sonication for 15 min. The cross-linker EDMA was then added to the mixture. The volumetric MAA/EDMA ratio in the monomer mixture was kept at 1:9. The macropores forming solvent (porogen) consisted of a methanol/1-dodecanol (1:9 (v/v)) mixture. These porogens were added to the monomer mixture to achieve a final volumetric MAA-EDMA/porogen ratio of 30:70. The initiator AIBN was then added (1 wt % with respect to monomers). The mixture was sonicated for 5 min and purged with nitrogen for 5 min, and the capped vial was sealed using paraffin film. Polymerization was carried out in a water bath kept at 60 °C for 24 h. The glass vial was then carefully crushed and the monolith transferred in a cellulose extraction thimble. Pore forming solvents and other unreacted components were removed using Soxhlet extraction with methanol for 16 h. Typically, the nanoparticle monoliths (NPMs) were produced on a gram scale. In this case, 250 mg of ZnO-NP dispersion (40 wt %) was dried resulting in 100 mg of ZnO-NPs that were re-dispersed in 0.1 mL of MAA, and after homogenization, 0.9 mL of EDMA and 2.3 mL of a 1:9 methanol−1-dodecanol mixture were added followed by addition of 10 mg of AIBN. The same procedure except for addition of ZnONPs was applied for the preparation of reference PMs without ZnO. Preparation of NPMs in Capillary Format. Functionalization of the internal surface of the capillary with methacrylate groups was carried out first to ensure strong attachment of monolithic polymer to the capillary walls. Using a syringe pump, the capillary was rinsed thoroughly with acetone, followed by water until acetone was completely removed. An aqueous sodium hydroxide solution (0.2 mol L−1) was pumped through the capillary at 0.25 μL min−1 for 30 min. The capillary was rinsed with water until neutral pH and then flushed with HCl (0.2 mol L−1) at a flow rate of 0.25 μL min−1 for another 30 min. The capillary was rinsed again with water until neutral pH, followed with ethanol. A 20 wt % ethanol solution of 3(trimethoxysilyl)propyl methacrylate pH 5 adjusted with acetic acid was pumped through the capillary at a flow rate of 0.25 μL min−1 for 1 h. The capillary was washed with acetone, dried in a nitrogen stream, and left at room temperature overnight before use. The functionalized capillary was filled with the polymerization mixture detailed above for the preparation of bulk monoliths. Both ends of the capillary were sealed with a rubber septum, and polymerization was carried out in a water bath kept at 60 °C for 24 h. The unreacted components and pore forming solvents were removed by pumping acetonitrile through the capillary with at a flow rate of 1 μL min−1 for 8 h. Preparation of NPMs in Short Column format. Four polyether ether ketone (PEEK) 5 mm length and 4.6 mm i.d. O-rings were placed in a 20 mL scintillation vial. The vial was filled with the polymerization mixture to completely immerse the O-rings. After the polymerization was completed, the vial was crushed and the filled Orings were embedded in the bulk polymer separated into individual pieces with a razor blade. The polymer adhering to the external surface of the rings was carefully removed using a razor cutter, and the top and bottom were cut straight thus producing homogeneous monolithic
alternatively via functionalizing the pores with specific functionalities followed by coating them with a thin layer of MOF using a sequential layer-by-layer (LbL) approach.48−50 Using the MOF admixing approach, many porous crystals are buried within the polymer scaffold and not accessible. Thus, a significant percentage of the MOF initially added to the polymerization mixture may be inactive in the application. In contrast, the more tedious LbL approach enables a better control over the growth of MOF on the pore surface of the porous materials.51 The LbL approach is well-suited for creation of MOF thin films. However, it is a time-consuming procedure that requires substantial amounts of organic solvents. It is worth noting that the LbL approach including different types of supports was recently miniaturized and automatized.52−54 The development of a fast and efficient approach for the preparation of PMs which pores are well-covered with MOF is highly desirable to translate benefits of MOFs in flow-through applications. Herein we describe our metal oxide conversion approach that is an efficient route for the preparation of MOF coating of pores in monolithic polymer support. We added MOF precursor containing the metal source to the polymerization mixture and prepared the monolith. Applications of these conjugate supports are demonstrated with heterogeneous catalysis and enrichment of environmental micropollutants from water.
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EXPERIMENTAL SECTION
Materials. Methacrylic acid (MAA, 99%), ethylene dimethacrylate (EDMA, 98%), methanol (≥99.8%), 1-dodecanol (≥98.0%), 2,2′azobis(isobutyronitrile) (AIBN, 98%), zinc oxide nanoparticles (ZnONPs, 40 wt % dispersion in ethanol, 18 MΩ cm; Millipore Iberica, Madrid, Spain) was used in the experiments. Capillary reactors were prepared in polyimide-coated 75 μm inner diameter (i.d.) fused silica capillaries purchased from Polymicro Technologies (Phoenix, AZ, USA). Columns for the extraction of pollutants were prepared in homemade polyether ether ketone tubes (5 mm length × 4.6 mm inner diameter). A Phenomenex Onyx guard column holder was used in the extraction experiments. Instrumentation. Powder X-ray diffraction (XRD) patterns were obtained using Cu Kα radiation on a Siemens D5000 diffractometer. The morphology and elemental composition were analyzed by a scanning electron microscope (SEM) Hitachi S-3400N, equipped with a Bruker AXS Xflash 4010 energy dispersive X-ray spectroscopy (EDS) system. Thermogravimetric analysis (TGA) was carried out in air atmosphere using a TA Instrument SDT 2960 simultaneous DSCTGA. Nitrogen adsorption isotherms were measured at 77 K using a TriStar 3020 (Micromeritics) gas adsorption analyzer at pressures up to atmospheric. Samples were first outgassed under a dynamic vacuum (ca. 10−5 mbar) at 393 K overnight. Ultrahigh-purity nitrogen gas (99.992%) was supplied by Air Products. Data were analyzed using the Brunauer−Emmett−Teller (BET) model to calculate the specific surface areas. The proper experimental range of relative pressures in which to apply the BET model for each sample was determined using 1729
DOI: 10.1021/acsami.6b10999 ACS Appl. Mater. Interfaces 2017, 9, 1728−1736
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Figure 1. (a) Schematic representation of the preparation procedure of nanoparticle-containing polymer monolith (NPM) and its conversion to zeolitic imidazolate framework/nanoparticle polymer monolith (ZIF-NPM). (b) Image of ZnO-nanoparticles dispersion (10 wt %) in the monomer/ porogen mixture, and the NPMs obtained when prepared in column format (5 mm length × 4.6 mm i.d.).
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filling of the inner O-ring space. These columns were then placed in a commercial HPLC guard column holder and the unreacted components removed by pumping acetonitrile at a flow rate of 1 mL min−1 for 30 min using a HPLC pump. Solvothermal Conversion of NPM to Zeolitic Imidazolate Framework−ZnO Nanoparticle Polymer Monoliths. The procedure for the preparation of bulk material containing ZIF-8 was adapted from the literature.26 In a scintillation vial, 0.25 g of ground monolith was weighed and 150 mg of 2-methylimidazole dissolved in 5 mL of aqueous DMF (50:50 (wt %)) solution was added. The mixture was sonicated for 5 min to allow the solution to enter the pores of the monolith. The capped vial was sealed with paraffin film and reaction carried out in a water bath at 70 °C for 20 h. The product was dispersed in methanol and then separated by centrifuging at 4000 rpm for 2 min. This procedure was repeated 4 times and the solid then dried at 60 °C overnight. For the bulk preparation of zeolitic imidazolate framework−ZnO nanoparticle polymer monoliths (ZIFNPMs) containing ZIF-7, 2-methylimidazole was substituted with an equimolar amount of benzimidazole, and the procedure was carried out as stated above. For preparation of ZIF-containing monolith in capillary, the previous 2-methylimidazole solution was pumped through the capillary at a flow rate of 1 μL min−1 for 2 h. The capillary was sealed with rubber septa and the reaction carried out in a water bath at 70 °C for 20 h. The capillary was washed by pumping methanol at a flow rate of 1 μL min−1 for 2 h. The short PEEK columns were prepared using the 2-methylimidazole solution that was pumped through the monolith using a syringe pump at a flow rate of 1 mL min−1 for 5 min. The short column was placed in a scintillation vial, immersed in 2methylimidazole solution, and the reaction carried out as detailed above. After the reaction was completed, the column was placed back in the guard column holder and flushed with methanol at a flow rate of 1 mL min−1 for 15 min using a HPLC pump. Room Temperature Conversion of NPM to ZIF-NPM. The following procedure for modification of bulk monolith was adapted from the literature.24 A 250 mg amount of NPM was placed in a 20 mL liquid scintillation vial and 10 mL of 1 mol L−1 2-methylimidazole solution in methanol added. The system was left at room temperature for 24 h. The monolith was washed following the same procedure as in the previous section. This procedure was also performed in the presence of 0.1 mol L−1 butylamine solution. In that case, the reaction time was 1 h at room temperature. Alternatively, the preparation in the presence of butylamine was carried out by just covering the polymer with the methanolic 2-methylimidazole/butylamine solution and the solvent evaporated upon heating at 60 °C for 15 min. Each time, the polymer was washed following the same procedure as in the previous section. The column format was as follows: columns were filled with the organic linker mixture, reacted, and washed following the procedures stated above.
RESULTS AND DISCUSSION Preparation and Characterization of Zeolitic Imidazolate Framework−ZnO Nanoparticle Polymer Monoliths. Bulk ZnO-nanoparticles (NPs) can be partially converted to ZIF-8 by reaction with 2-methylimidazole under mild conditions using a 2-methylimidazole:Zn(II):butylamine mixture at a ratio of 5:2:0.5. Reaction was carried out at room temperature for 24 h using methanol as solvent. TEM confirmed the coexistence of ZIF-8 crystals with the remaining ZnO-NPs (Figure S1). XRD also revealed the coexistence of both ZIF-8 diffraction peaks at lower deviation angles and the diffraction peaks of (001), (002), and (101) facets of the remaining ZnO-NPs (Figure S2). We prepared metal oxide nanoparticles-containing polymer monoliths (NPMs) using copolymerization of MAA and EDMA in the presence of ZnONPs.56 ZnO-NPs have a dual function: (i) to act as a structuring agent of the polymer monolith tuning the size of the microglobules that are the building blocks of the macroporous structure and (ii) to participate in the solid−liquid dissolution− precipitation reaction to produce the ZIF metal−organic framework when the macropores are filled with a solution of the organic linker. The ZnO-NPs remaining at the pore surface acted as an intermediate ligand connecting the polymer scaffold and the ZIF-8 crystals, thus leading to the coating of the surface of the macropores of the NPM with a layer of the ZIF. The polymerization mixture for the preparation of the NPMs consisted of a dispersion of ZnO-NPs in a small amount of MAA, a larger quantity of EDMA, macropore forming solvents (methanol and 1-dodecanol) known as porogens, and a free radical initiator (AIBN). The polymerization was then carried out in a vial and in columns with different internal diameters, obtaining highly cross-linked NPMs. Figure 1a, shows a schematic representation of the preparation of a zeolitic imidazolate framework/nanoparticle/polymer monolith (ZIFNPM). The first step of the preparation includes mixing ZnONPs with the monovinylic methacrylic acid and enables the interaction of the carboxylic acid groups of the monomer with the nanoparticles. The cross-linker (EDMA) polymerizes and is incorporated in the polymer faster thanks to its excess in the mixture and the two double bonds present in its structure. Therefore, the core of the microglobules which form the monolith (see below) consist mostly of EDMA, while the MAA-ZnO-NPs are incorporated later thus residing at the surface of the macropores. As it can be observed in Figure 1b, the ZnO-NPs dispersion in the polymerization mixture is stable for several days, even at a high load of 10 wt %. This feature 1730
DOI: 10.1021/acsami.6b10999 ACS Appl. Mater. Interfaces 2017, 9, 1728−1736
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Figure 2. (a) SEM image of the globular structure of a polymer monolith prepared in the absence of ZnO-nanoparticles. (b) SEM image of a NPM prepared in the presence of 10 wt % ZnO-nanoparticles. (c−f) SEM images of the NPM shown in Figure 1b after reaction with 2-methylimidazole at different magnifications. Scale bars, 1 μm. Conversion of ZnO-NPs to ZIF-8 performed using DMF:H2O (50:50 (wt %)) at 70 °C for 20 h.
The weight of the solid residue obtained after the calcination in air at 600 °C was 9 wt % and corresponds to ZnO. While after conversion to ZIF-8 the amount of Zn in the material decreased slightly (8 wt %). ZnO is well-distributed in the NPM as observed by Zn elemental mapping using energy dispersive Xray spectrometry (Figure S4, Supporting Information). Figure 3 shows that the bare monolith was amorphous with no observable XRD diffraction peaks. When the monolith was
enabled the homogeneous incorporation of ZnO-NPs in the PM. Figure 1b also shows the appearance of the ZIF-NPMs after carrying out the preparation in a short column (5 mm length × 4.6 mm i.d.). The bare monoliths had a globular structure with a microglobule size in the micrometer range as shown in Figure 2a. When the monolith was prepared in the presence of 10 wt % ZnO-NPs, the size of the globules in PM decreased as it is demonstrated in Figure 2b. The reaction of the ZnO-NPs present on the pore surface with the 2-methylimidazole in solution filling the pores did not change the original globular structure, and only a thin layer of ZIF crystals of a few micrometers thick was formed on the pore surface (Figure 2c,d). In this case, the ZIF-8 coating is predominantly formed by sub-micrometric crystals, although some larger crystals can be found within the ZIF-NPM structure (Figure 2e,f). The presence of 10 wt % ZnO-NPs in the monolith did not affect the excellent flow-through properties typical of organic polymer monoliths prepared in the absence of ZnO. The in situ solvothermal conversion of the ZnO-NPs to ZIF-8 within the pores using 2-methylimidazole dissolved in a mixture of DMF:H2O did not reduce the excellent permeability either. Use of moderate flow rates was achieved using low-pressure syringe pumps and ensured that a sufficient amount of ZnONPs was present to obtain conversion to ZIF-8. The use of lowpressure instrumentation is also desirable in further applications with these supports. No leaching of ZIF-8 crystals was observed when water or methanol at a flow rate of 1−5 mL min−1 was applied. In the NPM, ZnO-NPs are embedded in the polymer scaffold, acting simultaneously as metal source and anchoring seeds for the growth of ZIF-8 crystals. In the case of complete conversion, the carboxylic groups of the methacrylic acid present in the polymer monolith can also contribute to the strong attachment of the ZIF-8 crystals in the resulting ZIFNPM. Thermogravimetric analysis of the PM, NPM, and ZIF-NPM (Figure S3, Supporting Information) showed that the thermal stability of the modified monolith slightly increased by the addition of 10 wt % ZnO-NPs to the polymerization mixture.
Figure 3. Powder X-ray diffraction patterns of the PM, NPM, and ZIFNPM. Conversion of ZnO-NPs to ZIF-8 performed using DMF:H2O (50:50 (wt %)) at 70 °C for 20 h.
prepared in the presence of ZnO-NPs, the corresponding diffractogram showed three weak diffraction peaks in the 2θ range of 30−40. After reaction of the NPM with 2methylimidazole new intense diffraction bands, which matched well with those of ZIF-8, appeared at lower diffraction angles, confirming the conversion of the ZnO-NPs to the metal− organic framework. Figure 4 shows the textural characterization by means of nitrogen adsorption−desorption isotherms at 77 K. The bare monolithic polymer, prepared in the absence of ZnO-NPs, was mainly macroporous, exhibiting a type II isotherm, and a low 1731
DOI: 10.1021/acsami.6b10999 ACS Appl. Mater. Interfaces 2017, 9, 1728−1736
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etching and the possible remaining ZnO-NPs and is in agreement with the reported surface area values. Our simply prepared NPM is a versatile building block that can be easily converted to different MOFs. This option was demonstrated via preparation of ZIF-7 by reaction of the NPM with an equimolar amount of benzimidazole. Corresponding XRD pattern and SEM image (Figures S5 and S6, Supporting Information) show the presence of MOF crystals within the monolithic matrix, confirming that the developed approach is applicable to other imidazole-based ligands useful for the preparation of other MOF-NPMs. Heterogeneous Catalysis in a Capillary Microreactor Format. To exemplify application of our ZIF-NPM, we prepared a ZIF-NPM in a 75 μm I.D. capillary and used it as microfluidic flow-through reactor for heterogeneous catalysis. Detailed conditions are described in the Supporting Information. We first prepared a single 30 cm long ZIF-NPM capillary and then cut it to three reactors each 10 cm long. The simple Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate in flow mode was used as a model reaction. We demonstrated that the presence of ZIF in the monolith significantly accelerated the reaction rate and enabled one to achieve a conversion of 97% after only 110 min, i.e., more than twice as high compared to that observed with both the precursor NPM in flow-through mode and bulk ZIF-8 microcrystals using batch conditions (Figure 5a). The catalytic activity remained high even after 18 h of continuous reaction under flow-through conditions with a decrease of 30% (Figure 5b). The decrease in catalytic performance can be attributed to both the acidity of the benzoic acid and the increased temperature of 70 °C leading to a possible etching of some of the ZIF-8 crystal facets.57−59 In this proof of concept, we expect that catalytic activity is mostly on the surface of the ZIF-8 crystals. We believe that the activity of ZIF-NPMs could be enhanced after postsynthetic ligand exchange that would expand the pore size of the ZIF.60,61 Furthermore, nanoparticle encapsulation in ZIF-NPM can also extend the range of catalytic applications of this type of material.62 Preconcentration of Micropollutants from Water in Short Column Format. Another application of NPMs is the rapid preconcentration of toxic environmental micropollutants. For this application, a short 5 mm length × 4.6 mm i.d. column,
Figure 4. Nitrogen adsorption−desorption isotherms of the PM, NPM, and ZIF-NPM. Conversion of ZnO-NPs to ZIF-8 performed using DMF:H2O (50:50 (wt %)) at 70 °C for 20 h.
surface area (27 m2 g−1). Addition of ZnO-NPs to the polymerization mixture (NPM) led to an increase in the surface area up to 72 m2 g−1. The increment of the surface area in this step is attributed to the action of the ZnO-NPs as seeds for polymer nucleation, limiting further growth of the polymer globules and resulting in an increase of the overall surface area of the NPM. The contribution of the intrinsic porosity of the ZnO-NPs is negligible due to their low surface area (
