Article Cite This: Chem. Mater. 2018, 30, 1301−1315
pubs.acs.org/cm
Extending the Compositional Range of Nanocasting in the Oxozirconium Cluster-Based Metal−Organic Framework NU-1000A Comparative Structural Analysis Wenyang Zhao,#,† Zhao Wang,#,† Camille D. Malonzo,† Thomas E. Webber,† Ana E. Platero-Prats,‡ Francisco Sotomayor,§ Nicolaas A. Vermeulen,∥ Timothy C. Wang,∥ Joseph T. Hupp,∥ Omar K. Farha,∥,⊥ R. Lee Penn,† Karena W. Chapman,‡ Matthias Thommes,§ and Andreas Stein*,† †
Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, Illinois 60439, United States § Quantachrome Instruments, 1900 Corporate Drive, Boynton Beach, Florida 33426, United States ∥ Department of Chemistry and Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ⊥ Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia ‡
S Supporting Information *
ABSTRACT: The process of nanocasting in metal−organic frameworks (MOFs) is a versatile approach to modify these porous materials by introducing supporting scaffolds. The nanocast scaffolds can stabilize metal-oxo clusters in MOFs at high temperatures and modulate their chemical environments. Here we demonstrate a range of nanocasting approaches in the MOF NU-1000, which contains hexanuclear oxozirconium clusters (denoted as Zr6 clusters) that are suitable for modification with other metals. We developed methods for introducing SiO2, TiO2, polymeric, and carbon scaffolds into the NU-1000 structure. The responses of NU-1000 toward different scaffold precursors were studied, including the effects on morphology, precursor distribution, and porosity after nanocasting. Upon removal of organic linkers in the MOF by calcination/pyrolysis at 500 °C or above, the Zr6 clusters remained accessible and maintained their Lewis acidity in SiO2 nanocast samples, whereas additional treatment was necessary for Zr6 clusters to become accessible to pyridine probe molecules in carbon nanocast samples. Aggregation of Zr6 clusters was largely prevented with SiO2 or carbon scaffolds even after thermal treatment at 500 °C or above. In the case of titania nanocasting, NU1000 crystals underwent a pseudomorphic transformation, in which Zr6 clusters reacted with titania to form small aggregates of a Zr/Ti mixed oxide with a local structure resembling that of ZrTi2O6. The ability to maintain high densities of discrete Lewis acidic Zr6 clusters on SiO2 or carbon supports at high temperatures provides a starting point for designing new thermally stable catalysts.
1. INTRODUCTION As an approach for shaping materials, casting has been used for centuries in manufacturing and materials processing. The casting process involves the formation of a mold that predefines the product structure and filling the mold with fluid precursors that are then allowed to solidify. Depending on the shape and size of the mold used for casting, various materials with different structures can be obtained by well-defined processes. The same idea can also be applied to the area of nanomaterials synthesis and nanofabrication. If critical features of the mold are scaled down to the nanometer level, the mold acts as a hard template in a process referred to as nanocasting. Similar to conventional casting, nanocasting provides a relatively general © 2018 American Chemical Society
approach to synthesize materials that maintain certain features of structure and morphology of the original template. It has been applied to prepare porous three-dimensional materials with complex structures that are sometimes difficult to synthesize using direct methods. Ordered mesoporous silica, carbon, and colloidal crystals are some examples of templates for nanocasting.1−3 Products of nanocasting include a wide range of oxides, metals, and other compositions that target applications in catalysis, photonics, and separations.4−10 Received: November 21, 2017 Revised: January 26, 2018 Published: January 29, 2018 1301
DOI: 10.1021/acs.chemmater.7b04893 Chem. Mater. 2018, 30, 1301−1315
Article
Chemistry of Materials
Figure 1. Schematic showing the concept of nanocasting in the MOF NU-1000.
largely maintained in the target materials, but the composition is modified to provide new functionality. Nanocasting MOFs followed by template removal is another promising approach to modify MOFs, which may lead to products with different structures and morphologies. Early nanocasting studies focused on the synthesis of porous carbon materials using a variety of MOFs as the sacrificial template, such as MOF-5, Al-PCP and ZIF-8.31−33 Various carbon sources can be selected for nanocasting as well, including furfuryl alcohol, glycerol, glucose, phenolic resins and resorcinol.31,33−35 Pyrolysis is generally applied to effect the conversion, given that both the precursors and the organic linkers from MOFs can be carbonized when treated at a high temperature. Apart from carbon, the metal oxide titania has also been reported as a product from nanocasting in MOFs, where microporous titania was synthesized using HKUST-1 as a template.36 Both an acid and an oxidant were employed to remove the metal nodes from the MOF, with only titania left in the final product as the inverse replica. One important point to address is that the cost of synthesizing MOFs is often much higher than the value of the corresponding templated products, so that the economics of using MOFs merely as sacrificial templates may be unfavorable. A significant component of the MOF that has been overlooked in this sacrificial approach is the metal or metaloxo node. These nodes are connected by the organic linkers and evenly distributed throughout the entire MOF particles. Together with the well-defined pores and channels in their structures, they provide important functionality, such as high selectivity and activity toward catalytic reactions.18,21 However, MOFs are not suited for high-temperature gas-phase reactions because they are usually stable only below 350 °C, especially in oxidizing environments.37 At higher temperatures, the organic linkers in MOFs decompose and the metal nodes eventually
Typically, in nanocasting, the template consists of a single component (e.g., carbon) that is completely removed after the nanocasting process, with only structural information being utilized and replicated. One approach to utilize a template more efficiently involves multicomponent templates, so that it is possible to selectively remove one component at a time, possibly leaving another component intact in the final product to provide a specific function.11 Metal−organic frameworks (MOFs), also known as porous coordination polymers (PCPs), are organic−inorganic hybrid materials that consist of organic linkers as ligands and metal ions or metal-oxo clusters as coordination centers. As one of the important categories of porous materials, MOFs have attracted much attention in the past two decades.12−14 Owing to the large variety of organic linker molecules, metal species, and complex coordination environments between them, MOFs with different three-dimensional structures have been synthesized and characterized. These materials have porous structures with pore sizes ranging from a few angstroms to several nanometers.15 The well-defined pores provide high surface areas, leading to potential applications in gas storage,16,17 separation,18,19 heterogeneous catalysis,20−23 and chemical sensors.24 MOFs can be further altered by postsynthetic modifications, including decorations of the metal nodes with other active metals or exchange of the linkers.25,26 For example, atomic layer deposition in MOFs (AIM) and solvothermal deposition in MOFs (SIM) have been reported as efficient ways to introduce active species onto the metal nodes, so that novel properties can be created that extend the applications of the original MOFs.27−29 Furthermore, MOFs can also act as sacrificial templates for pseudomorphic transformations, during which the components of the MOFs are converted to other materials while the general morphology is conserved.30 In this way, the porous structure and high surface areas of MOFs are 1302
DOI: 10.1021/acs.chemmater.7b04893 Chem. Mater. 2018, 30, 1301−1315
Article
Chemistry of Materials
and forms ZrxTi1−xO2, a mixed oxide phase of ZrO2−TiO2 that is stable at high temperature.48 The accessibility of the Zr6 nodes in SiO2 and carbon nanocast samples and the presence of Lewis acidic sites in TiO2 nanocast sample was verified by pyridine adsorption experiments. These nanocasting approaches are deemed suitable for the preparation of singlesite catalysts with high thermal stability.
agglomerate into larger metal or metal oxide crystallites. This irreversible process is thermodynamically favored and reduces the amount of accessible metal sites, thus compromising the catalytic activity of MOFs. Nanocasting in MOFs is a promising strategy to stabilize the structure and maintain the activities of MOF-derived catalytic sites at high temperatures (see Figure 1). It has been demonstrated through the formation of a silica skeleton in the mesopores of the MOF NU-1000, which was able to keep the catalytically active oxozirconium clusters isolated and accessible even after high temperature treatment at 500 °C or higher.11,38 In order to extend the concept of nanocasting in MOFs to other skeletal compositions, an understanding of the precursors, the interactions between the MOFs and the precursors, and the ability of the skeleton to stabilize the structure at high temperature is necessary. This article provides a comprehensive study of these factors. Here we chose the MOF NU-1000 as the template for nanocasting and studied products obtained with different precursors, including tetramethyl orthosilicate for SiO2 nanocasting, tetrakis(dimethylamido) titanium and titanium ethoxide for TiO2 nanocasting, and furfuryl alcohol for polymer/ carbon nanocasting. NU-1000 consists of oxozirconium clusters ([Zr6(μ3-O)4(μ3-OH)4(OH)4(OH2)4]8+, also denoted as Zr6 node) and 1,3,6,8-tetrakis(p-benzoate)pyrene linkers (TBAPy4−). The Zr6 nodes are unsaturated and 8-connected by the pyrene linkers, forming regular arrays of hexagonal and triangular channels along the c-axis of the crystal.39 NU-1000 is suitable for nanocasting because of its relatively large hexagonal channels (diameter ∼ 3.1 nm) that can accommodate a variety of precursors during infiltration processes. These mesopores also provide sufficient capillary forces to prevent precursors from leaching out after infiltration. The precursors inside the channels of NU-1000 are further transformed into a network structure either by gelation or polymerization, with the aim of providing secondary supports that can prevent the Zr6 nodes from aggregating or forming into large crystallites at high temperature. Subsequent calcination (SiO2 and TiO2 nanocasts) or pyrolysis (carbon nanocast) converts the precursors to the corresponding oxide or carbon scaffolds and removes the linkers from the original NU-1000. The scaffold composition can alter the chemical environment of the Zr6 nodes. While the silica scaffold is amorphous and relatively inert, a titania scaffold can, in principle, be crystalline and exhibit redox activity and strong metal−support interactions that alter catalytic properties.40−42 Because of these metal−support interactions, titaniasupported clusters, for example, Pt/TiO2,43 Pd/TiO2,44 and Au/TiO2,45,46 exhibit increased catalytic activity compared to the metal clusters themselves. Carbon was chosen as another possible scaffold to introduce electrical conductivity, because a conductive scaffold may enable electrocatalytic processes, such as the conversion from methane to methanol under mild conditions.47 Here we compare the differences in structure, morphology, and composition of the nanocast materials derived from NU1000 crystallites that were infiltrated with the different scaffold components listed above. We investigated the interactions between the precursors and the NU-1000 host during nanocasting, which led to different distributions of the precursors in the pores of NU-1000 and in some cases altered the nature of the Zr6 nodes. We also demonstrate that, by applying appropriate nanocasting methods, SiO2 and carbon scaffold materials can stabilize Zr6 nodes and prevent their aggregation at high temperature, whereas TiO2 reacts with Zr6
2. EXPERIMENTAL METHODS 2.1. Materials. The chemicals used in this study were obtained from the following sources: ethanol (anhydrous, 200 proof) from Pharmco-AAPER; titanium(IV) ethoxide (TEOT, 95%) and titanium(IV) tetrakis(dimethylamide) (TDMAT, >99%) from Gelest; N,Ndimethylformamide (DMF, certified ACS, 99.9%), acetone (certified ACS, 99.7%), pentane (certified, >98%), and sodium hydroxide solution (50% w/w, certified) from Fisher Chemical; tetramethyl orthosilicate (TMOS, 98%), furfuryl alcohol (98%), methanol (>99.8%), hydrochloric acid (ACS reagent, 37%), benzoic acid (ACS reagent, 99.5%), zirconium(IV) oxychloride octahydrate (reagent grade, 98%), and pyridine (99.8%) from Sigma-Aldrich; and dimethyl sulfoxide-d6 (D, 99.9%) from Cambridge Isotope Laboratories, Inc. 1,3,6,8-Tetrakis(p-benzoic acid)pyrene (H4TBAPy) was synthesized following a previously reported procedure.39 Deionized water was purified to a minimum resistivity of 18.2 MΩ· cm with a Milli-Q PLUS reagent-grade water system and was used in all experiments. 2.2. Nanocasting NU-1000 with Different Precursors. Silica Nanocasting. The synthesis of NU-1000 and nanocasting of NU-1000 with SiO2 were carried out following previously reported methods.11 Details are provided in the Supporting Information. Titania Nanocasting. Two different organotitanium precursors were used for nanocasting NU-1000, TEOT and TDMAT. For TEOT, 15 mg of activated NU-1000 was placed in a 2 mL centrifuge tube and 1 mL of TEOT was added. The centrifuge tube was then sealed with parafilm and placed on a vortex mixer until all the solid was suspended in the liquid precursor. After settling for 24 h, the mixture was centrifuged to separate solids and excess precursor material. The residue was washed either once or twice with ethanol, each time followed by centrifugation to isolate the solid. After the last centrifugation step, the centrifuge tube was left open to dry in air for 24 h and then placed in a vacuum oven (200 mTorr) at 80 °C for 2 h for further drying. A yellow powder was obtained. The resulting material is denoted as TEOT1@NU-1000 for samples washed once or TEOT2@NU-1000 for samples washed twice. Nanocasting with the air-sensitive and moisture-sensitive alternate titania precursor, TDMAT, was conducted in a glovebox under a nitrogen atmosphere. 100 mg of TDMAT was added to 30 mg of activated NU-1000 and allowed to infiltrate the MOF for 24 h. The infiltrated sample was then washed with pentane, separated by vacuum filtration, exposed to air for 24 h, and then dried in a vacuum oven (200 mTorr) at 80 °C for 2 h. The resulting material is referred to as TDMAT@NU-1000. All three samples were heated in static air to 500 °C in a tube furnace with a heating ramp rate of 2 °C/min and maintained at that temperature for 1 h. The resulting white powders are referred to as TEOT1_Zr6@ TiO2, TEOT2_Zr6@TiO2, and TDMAT_Zr6@TiO2, respectively. Carbon Nanocasting. For nanocasting NU-1000 with carbon, 30 mg of NU-1000 was placed in an uncapped vial, and the vial was put into an autoclave containing 10 mL of furfuryl alcohol (FA), taking care to prevent direct contact between the liquid FA and the NU-1000 powder. The autoclave was heated to 90 °C to create FA vapor. Different heating times (12 h, 24 h, 48 h, 72 h, and 144 h) were examined for the infiltration step because the exposure time to the FA vapor affected the amount of FA loaded inside the pores of NU-1000. The NU-1000 product loaded with FA is denoted as FA@NU-1000. Polymerization of FA inside the pores of NU-1000 was performed in a tube furnace at 250 °C for 6 h using a 5 °C/min temperature ramp rate and a N2 flow of 1000 mL/min. NU-1000 with polymerized poly(furfuryl alcohol) inside the pores is denoted as PFA@NU-1000xh (x represents the different infiltration times in hours). Pyrolysis of 1303
DOI: 10.1021/acs.chemmater.7b04893 Chem. Mater. 2018, 30, 1301−1315
Article
Chemistry of Materials PFA@NU-1000 was performed in a tube furnace at 600 °C for 1 h using a 5 °C/min temperature ramp rate and an N2 flow of 1000 mL/ min. The product formed from pyrolysis is denoted as Zr6@C-xh. 2.3. Pyridine Adsorption. Pyridine was used as the probe molecule to test the presence and accessibility of Lewis and Brønsted acid sites. The Zr6@SiO2 and Zr6@TiO2 samples were first activated at 200 °C for 1 h under dynamic vacuum (
