Article pubs.acs.org/cm
Cite This: Chem. Mater. 2017, 29, 9628-9638
Confining Metal−Organic Framework Nanocrystals within Mesoporous Materials: A General Approach via “Solid-State” Synthesis Ignacio Luz, Mustapha Soukri,* and Marty Lail RTI International, Post Office Box 12194, Research Triangle Park, North Carolina 27709-2194, United States S Supporting Information *
ABSTRACT: Here we report the selective growth of well-dispersed metal−organic framework (MOF) nanocrystals within mesoporous materials via novel “solid-state” synthesis. The ability to control and direct the growth of MOFs on confined surfaces (pores) paves the way for new prospective applications of such hybrid systems and could unlock the full potential of these materials. As confirmed by a combination of different characterization techniques, an outstanding high loading of mesoporous cavities (≤40 wt %) by the smallest MOF crystals yet reported (4.5 ± 1 nm) leads to several improved properties, including diffusion, attrition resistance, and handling.
■
INTRODUCTION Metal−organic frameworks (MOFs) have emerged as a special class of solid-state materials, because of their modularity, ultrahigh surface area, organic−inorganic hybrid composition, crystalline nature, and readily functionalized pore structures. Additionally, the ability to direct their assembly from judiciously preselected molecular building blocks offers potential to deliberately construct functional porous MOFs that can address various challenges pertinent to energy, security, and environmental suitability, e.g., gas separation, hydrogen storage, carbon dioxide capture, and catalysis.1−6 To facilitate applications of MOFs to key market sectors, it is important to develop new synthetic methodologies allowing for practical and controlled integration of MOFs with other solid materials resulting in functional MOF-based hybrid materials.7 Specifically, such control in MOF chemistry will facilitate (i) construction and/or growth of uniform MOF nanoparticles within the pores of a given solid support and (ii) incorporation and immobilization of nanosized MOF crystals in a suitable polymeric matrix.8 Rational design of sophisticated hybrid materials based on MOFs as functional species blended with different supports has emerged as an advanced strategy for integrating their most interesting physicochemical properties9,10 while avoiding their weakness as single components (handling, mechanical, thermal, and chemical stability,11,12 or conductivity13), and further © 2017 American Chemical Society
adding additional synergistic properties that arise from the intimate interactions and complex hierarchical architectures of the resulting composites, including micro/meso-porosity or multifunctionality.14,15 There is an ever-increasing awareness, however, that the insoluble micrometer-sized crystals typically obtained from traditional MOF synthesis reactions (e.g., solvothermal methods) are not necessarily the best configuration for the applications such as gas adsorption/ separation,16−18 drug delivery,19 conductivity,20 sensors,21 optoelectronics,22 heterogeneous catalysis,23 and chromatography.24 Consequently, there are now a growing number of reports of methods for preparing MOF nanoparticles, thin films or coatings, and composites, which have been recently reviewed.25,26 Hence, a general approach for preparing hybrid materials in which MOF nanocrystals are selectively and homogeneously confined and protected in one continuous mesoporous material (MPM) exhibiting specific morphology and pore architecture is needed.27 Those hybrid compounds are expected to show excellent performance on several promising applications in which bulk MOFs have been successfully applied, as mentioned above. The methods explored to date for the preparation of Received: May 17, 2017 Revised: August 18, 2017 Published: August 21, 2017 9628
DOI: 10.1021/acs.chemmater.7b02042 Chem. Mater. 2017, 29, 9628−9638
Article
Chemistry of Materials
Figure 1. Schematic representation of the solid-state synthesis of MOFs within mesoporous materials for (Cr)MIL-101/Silica(A). The first step is ligand salt impregnation (a), the second step gas phase acidification (b), the third step metal salt impregnation (c), and the final step application of synthesis conditions and crystallization of MOF nanocrystals (d). degassed at 120 °C overnight under vacuum to remove the adsorbed water. The particle size and morphology of these materials are shown in Figures S1 and S2. Ligand Salt Precursors. Na2BDC and Na3BTC ligand salt precursors were prepared from their acid form in water with the stoichiometric amount of NaOH necessary to deprotonate the carboxylic acid of the organic linker followed by a purification step via precipitation in acetone. Alternatively, ligand salt precursor solutions for H2BDC(NH2), H2BpyDC, H4TCPP, and H4TBAPy were directly prepared with the stoichiometric amount of TEA, thereby skipping the step of isolating the ligand salt. H2BDC(SO3Na) and HMeIM were directly dissolved in water. H4DOBDC was dissolved in hot THF because of the insolubility in water of sodium 2,5-dioxyterephthalate coordination polymers, and the use of triethylammonium salts did not give rise to the targeted MOF-74 structure. Bulk-Type MOFs. For the purpose of comparison, the following MOFs were prepared and activated according to the reported literature: (Cr)MIL-10136 and -53,37 (Cr)MIL-100,38 (Cr)MIL101(SO3H),39 (Al)MIL-100,40 (Al)MIL-53(NH2),41 (Co,Ni)MOF74,42,43(Zr)UiO-66(H,NH2),44 (Zr)UiO-67(Bpy),45 (Ru)HKUST-1,46 (Zn)ZIF-8,47 (Zr)PCN-222,48 (Zr)NU-1000,34 and Co2(dobpdc).49 Fourier transform infrared spectroscopy (FTIR) spectra of these MOFs were used as a reference for MOF/MPM hybrid materials. N2 isotherms and the pore distribution for (Cr)MIL-101(SO3H) are included in Figure 1. The rest of the characterization data for bulk-type MOFs was not included in this work. Solid-State Synthesis of a 19.1 wt % (Cr)MIL-101(SO3H) Precursor Solution on Mesoporous Silica(A). One hundred milliliters of an aqueous solution containing 20 g of H2BDC(SO3Na) was impregnated in 50 g of evacuated mesoporous silica(A) and dried at 50 °C under vacuum in a rotavapor for 2 h. Subsequently, the resulting dry material [H2BDC(SO3Na)/Silica(A)] was placed in a tubular calcination reactor where it was first treated with a nitrogen flow saturated with concentrated HCl (37%) for 2 h at room temperature and then purged with a nitrogen flow for 2 h to remove the excess HCl. Afterward, 75 mL of an aqueous solution containing 15 g of Cr(NO3)3·9H2O in 75 mL of H2O was impregnated in the compound [H2BDC(SO3H)/Silica(A)]. The resulting solid [Cr(NO3)3/H2BDC(SO3H)/Silica(A)] was finally dried at 50 °C under high vacuum in a rotavapor for 2 h. All the impregnation steps were performed via incipient wetness impregnation. The solid [Cr(NO3)3/ H2BDC(SO3H)/Silica(A)] was separated in two 125 mL stainless steel Parr autoclaves (>40% void space) at 190 °C for 24 h after adjusting the water level of the solid to 15−20 wt %. After the autoclave had been cooled, the resulting products were thoroughly washed with distilled water in a filtration funnel. Subsequently, the material was washed overnight in a Soxhlet with MeOH. All the materials were activated overnight at 120 °C under vacuum. More detail about the general procedure for solid-state synthesis of MOFs within mesoporous materials is included in the Supporting Information.
MOF nanocrystals confined within MPMs have been mainly based on solvothermal approaches, which unfortunately suffer from several drawbacks, such as inhomogeneity and difficulty in regulating and controlling crystal growth.28−33 These shortcomings encouraged us to develop a novel, highly efficient, scalable, environmentally friendly, and inexpensive strategy for rendering MOFs into an applied form that may be useful for fluidizable catalyst and sorbent processes. Beyond the obvious applications, the methodology further provides a universal tool for confining most well-known MOFs within a broad spectrum of mesoporous materials. This is an enabling step for MOFs, facilitating a new direction in their development and giving the research field the means to create and investigate new materials. Herein, we demonstrate a vapor-assisted “solid-state” synthesis technique, different than the conventional “solvothermal” approaches, that ensures a high loading and well-dispersed growth of a large collection of MOF nanostructures within a series of commercially available MPMs exhibiting cavities larger than MOF cages, such as silica, alumina, carbon, and polymer. The absence of a liquid phase during crystallization restricts the crystal growth, size, and mobility to the void space where the precursors are confined, thereby overcoming the limitations found when the current solvothermal methods are applied. This study is, to the best of our knowledge, the first reported “solidstate” synthesis technique for incorporating any kind of MOF within the pores of any mesoporous network solid.
■
EXPERIMENTAL SECTION
Chemicals. All chemicals were used as received from SigmaAldrich without further purification: Cr(NO3)3·9H2O, CrCl3·6H2O, Al(NO3)3·9H2O, AlCl3·xH2O, Co(NO3)2·6H2O, Ni(NO3)2·6H2O, ZrOCl2·8H2O, RuCl3·xH2O, Zn(NO3)3·9H2O, 1,4-benzenedicarboxylic acid (H2BDC), 1,3,5-benzenetricarboxylic acid (H3BTC), 2aminoterephthalic acid [H2BDC(NH2)], monosodium 2-sulfoterephthalate [H 2 BDC(SO 3 Na)], 2,5-dihydroxyterephthalic acid (H4DOBDC), 2,2′-bipyridine-5,5′-dicarboxylic acid (H2BpyDC), 2methylimidazole (HMeIM), and tetrakis(4-carboxyphenyl)porphyrin (H4TCPP). 1,3,6,8-Tetrakis(p-benzoic acid)pyrene (H4TBAPy) was synthesized according to the published procedure.34 Triethylamine (TEA), N,N-dimethylformamide (DMF), tetrahydrofuran (THF), and methanol (MeOH) were of analytical grade (Sigma-Aldrich). Mesoporous Materials. Silica(A) (75−250 μm), Silica(B) (200− 500 μm), Silica(C) (75−200 μm), and Silica(D) (75−150 μm) were kindly supplied by our commercial partner. SBA-15 was prepared according to the published procedure.35 MCM-41 was provided by Claytec, γ-Al2O3 by Sasol, TiO2 by Sachtleben, and ZrO2 by Mel Chemicals. Mesoporous carbon and HayeSep A (Supelco) (100−120 μm) were supplied by Sigma-Aldrich. All mesoporous materials were 9629
DOI: 10.1021/acs.chemmater.7b02042 Chem. Mater. 2017, 29, 9628−9638
Article
Chemistry of Materials Table 1. Versatility and Scope of the Solid-State Synthesis of MOFs within Silica(A) (HyperMOF-X)a code (X)b
MOF
metal
ligand
synthesis conditions
additive (wt %)c
MOF (wt %)d
SBET (m2/g)
A1a A2a B1a B2a C1a C2a D1a D2a E1a E2a F1a G1a H1a I1a J1a K1a
MIL-101 MIL-101 MIL-100 MIL-100 MIL-53 MIL-53 MOF-74 MOF-74 UiO-66 UiO-66 UiO-67 ZIF-8 HKUST-1 PCN-222 NU-1000 Co2(DOBPDC)
Cr Cr Cr Al Cr Al Co Ni Zr Zr Zr Zn Ru Zr Zr Co
BDC BDC(SO3H) BTC BTC BDC BDC(NH2) DOBDC DOBDC BDC BDC(NH2) BpyDC MeIM BTC TCPP TBAPy DOBPDC
220 °C, 1 h 190 °C, 1 day 200 °C, 2 h 200 °C, 8 h 220 °C, 1 day 120 °C, 12 h RT,e 1 h RT,e 1 h 120 °C, 2 h 120 °C, 2 h 120 °C, 2 h RT,e 1 h 160 °C, 1 day 120 °C, 12 h 120 °C, 12 h RT,e 1 h
15% H2O 15% H2O 15% H2O 15% H2O 15% H2O 15% DMF Et3N vap. Et3N vapor 15% H2O 15% H2O 15% H2O Et3N vapor 15% H2O 15% DMF 15% DMF Et3N vapor
30.8 19.1 35.0 20.4 22.7 28.7 27.3 27.7 30.0 37.6 22.6 34.1 11.0 9.8 12.8 13.4
584 486 647 364 377 417 323 386 363 434 366 346 258 348 364 344
Silica(A) (SBET = 256 m2/g); precursors loaded on Silica(A) (SBET = 100 ± 50 m2/g). bCode corresponding to the material data sheet (MDS) in the Supporting Information. cPer weight of resulting MOF precursors loaded on silica. dDetermined by XRF. eRoom temperature.
a
Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM). FIB-SEM samples were prepared in a DualBeam FEI Quanta 3D FEG microscope that combines a high-resolution field emission gun SEM column with a high-current Ga liquid metal ion gun FIB column. The procedure followed for sample preparation is described and illustrated in Figure S5. Transmission Electron Microscopy (TEM). Transmission electron microscopy (TEM) experiments were performed in a JEOL JEM-2000FX S/TEM microscope with a LaB6 emitter at 200 kV with a 120 μm condenser lens aperture and a 80 μm objective lens aperture inserted. N2 Sorption Isotherms. The samples were analyzed in a Micromeritics ASAP (accelerated surface area and porosimetry) 2020 System. Samples were weighed into tubes with seal frits and degassed under vacuum (
