Atomic Layer Deposition in a MetalâOrganic Framework: Synthesis

Article pubs.acs.org/cm

Atomic Layer Deposition in a Metal−Organic Framework: Synthesis, Characterization, and Performance of a Solid Acid Martino Rimoldi,† Varinia Bernales,‡ Joshua Borycz,‡ Aleksei Vjunov,§ Leighanne C. Gallington,∥ Ana E. Platero-Prats,∥ I. S. Kim,⊥ John L. Fulton,§ A. B. F. Martinson,⊥ Johannes A. Lercher,§,# Karena W. Chapman,∥ Christopher J. Cramer,‡ Laura Gagliardi,*,‡ Joseph T. Hupp,*,† and Omar K. Farha*,†,∇ †

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States Department of Chemistry, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States § Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States ∥ X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, Illinois 60439, United States ⊥ Materials Science Division, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, Illinois 60439, United States # Department of Chemistry and Catalysis Research Institute, TU München, Lichtenbergstrasse 4, 85748 Garching, Germany ∇ Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 23218, Saudi Arabia

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‡

S Supporting Information *

ABSTRACT: NU-1000, a zirconium-based metal−organic framework (MOF) featuring mesoporous channels, has been postsynthetically metalated via atomic layer deposition in a MOF (AIM) employing dimethylaluminum iso-propoxide ([AlMe2OiPr]2, DMAI), a milder precursor than widely used trimethylaluminum (AlMe3, TMA). The aluminum-modified NU1000 (Al-NU-1000) has been characterized with a comprehensive suite of techniques that points to the formation of aluminum oxide clusters well dispersed through the framework and stabilized by confinement within small pores intrinsic to the NU-1000 structure. Experimental evidence allows for identification of spectroscopic similarities between Al-NU-1000 and γ-Al2O3. Density functional theory modeling provides structures and simulated spectra, the relevance of which can be assessed via comparison to experimental IR and EXAFS data. The catalytic performance of Al-NU-1000 has been benchmarked against γ-Al2O3, with promising results in terms of selectivity.

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INTRODUCTION Atomic layer deposition1−4 (ALD) in metal−organic frameworks (MOFs) (referred to as AIM) has recently been utilized as a postsynthetic modification technique.5 Because of their tunability, crystallinity, high surface area, and stability, MOFs can serve as ideal materials in several fields, including applications as catalysts.6−9 In particular, thermal stability and availability of hydroxyl functional groups are of exceptional importance to the success of the AIM process. AIM was first successfully demonstrated in NU-1000, a zirconium based MOF featuring mesoporous channels (ca. 3 nm) with zirconium oxide nodes and 1,3,6,8tetrakis(p-benzoic acid)pyrene linkers (Scheme 1). Its Zr6O8 © 2017 American Chemical Society

nodes contain various types of protons (terminal and bridging hydroxyls and hydrogen bonding coordinated H2O molecules) that were thoroughly investigated both experimentally and theoretically.10 The various proton environments make these nodes intriguing from both a coordination chemistry perspective and as a potential support for numerous metals. The transfer of the ALD technique typically employed on flat surfaces to mesoporous crystalline materials represents a significant advance in the preparation of highly functionalized, Received: September 13, 2016 Revised: November 28, 2016 Published: January 5, 2017 1058

DOI: 10.1021/acs.chemmater.6b03880 Chem. Mater. 2017, 29, 1058−1068

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Chemistry of Materials Scheme 1. Schematic Representation of NU-1000a

([AlMe 2 (O i Pr)] 2 , DMAI) has been introduced as an alternative, milder precursor.23−25 From a molecular perspective, DMAI significantly differs from TMA (Scheme 2). TMA exists in a monomer−dimer Scheme 2. Molecular Structures of TMA and DMAI

equilibrium, with the monomer probably being the major isomer at high temperatures in the gas phase.26 In an alkyl aluminum species such as TMA, the dimerization products obtained via bridging methyl groups are based on three center− two electron bonds; an aluminum alkoxide species such as DMAI exhibits a three center−four electron bond including the dative bond involving the oxygen atom lone pair and the aluminum center. DMAI, like most of the branched-chain dimethyl aluminum alkoxides, was found to exist exclusively in the dimeric form.27 Also, dimethylaluminum tert-butoxide, an alkoxide structurally related to DMAI, is known to retain its dimeric structure in the gas-phase, as revealed by gas-phase electron diffraction studies.28 These differences confer to DMAI its typically higher stability and milder reactivity compared to TMA. In view of the reduced reactivity of aluminum alkoxide species compared to aluminum alkyl complexes, the use of DMAI as an alternative precursor to TMA may be expected to reduce or eliminate degradation of the MOF. Herein we present the postsynthetic modification of NU1000 by Al-AIM using DMAI as a precursor that does exhibit ALD-like behavior. Its chemical and physical characterization is achieved with techniques able to probe coordination environment and local structure and to spatially locate within the framework the sites associated with ALD modification; the interpretation of the experimental data is facilitated through comparisons to computationally determined structures and associated spectra. The aluminum-modified MOF is benchmarked against γ-Al2O3 in ethanol dehydration, an acidcatalyzed reaction of industrial interest.

a

View (a) perpendicular to and (b) parallel to the c-axis and (c) expanded view of the linker and node with the most stable proton topology.

engineered materials with potential applications in gas separations and catalysis. In addition, MOFs typically represent a highly tunable platform with possibilities in varying the organic linkers and metal nodes to control the topology of the framework, to tune relevant properties, and to introduce active sites responsive to ALD modification.11−13 Aluminum oxides and aluminates are ubiquitous in catalysis, both as supports and as active species themselves.14−16 Aluminum-based acid catalysts such as aluminas,15 zeolites,17−19 and aluminates in general are especially noteworthy due to their remarkable and well-studied properties.20 Hence, the potential of an aluminum-modified MOF warranted deeper investigation into AIM. Recent works5,21 showed that aluminum oxide ALD in MOFs (Al-AIM) can be achieved using trimethylaluminum (AlMe3, TMA), a commonly implemented ALD precursor.22 However, by this method, NU-1000 was metalated with poor control over the aluminum loading, which was observed to exceed the theoretically predicted (monolayer) limit based on the degree of −OH functionalization of the framework. Subsequent powder X-ray diffraction (PXRD) showed loss of the MOF crystallinity and possible degradation of the NU-1000 structure.21 The unexpectedly high metal loading and the loss of the framework integrity was attributed to the highly reactive nature of TMA, which can moreover lead to large increases in local temperature that may also contribute to poor control over the ALD process. Although TMA is ubiquitous in Al2O3 thin film preparations by ALD, dimethylaluminum iso-propoxide

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RESULTS AND DISCUSSION Initially, the self-limiting reactivity of DMAI in ALD was investigated. Figure 1 shows the Al loading per Zr6 node as a function of the number of DMAI doses. As is typical of a self-limiting reaction, the amount of deposited aluminum plateaus at a certain value despite the increasing number of precursor doses. In particular, the maximum aluminum loading in Al-NU-1000 was determined to be approximately 7 Al/Zr6 (node). As a result, the selflimiting regime during Al-AIM conducted with DMAI assures an atomic level of control of metal deposition. Scanning electron microscopy−energy dispersive X-ray spectroscopy (SEM-EDS) revealed a uniform distribution of aluminum through the NU-1000 crystallites and confirmed the retention of crystal morphology (Figure 2a). The aluminummodified material is also shown to be crystalline by powder X1059


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Chemistry of Materials

volume was accompanied by highly anisotropic changes in the individual lattice dimensions. This involved a 1.60% expansion along the a-axis and a 2.81% contraction along the c-axis. Nitrogen physisorption measurements confirmed retention of porosity in the modified NU-1000 material (Figure 2c) supporting the stability of the framework to the postsynthesis modification. This set of characterizations is consistent with the effectiveness of the AIM method described here. Diffuse reflectance infrared Fourier transform (DRIFT) spectra taken after AIM revealed a high level of hydration (mostly physisorbed water) as expected after the water cycle in the ALD process. However, to explore the nature of Al-NU1000 and in particular to determine the identity of the hydroxo functionalities, we performed variable temperature DRIFT measurements. Most of the physisorbed water was removed upon heating up to 70−80 °C under argon flow, as confirmed by the decreasing of a broad band centered at 3300 cm−1. A significant rearrangement of the hydroxyl-related peaks in the 3800−3600 cm−1 region occurred upon heating up to 100 °C, indicating that further dehydration took place (Figure 2d, left). The latter dehydration process is presumed to lead to removal of chemisorbed water as assessed by solid-state 27Al MAS NMR spectroscopy (Figure 4).

Figure 1. Self-limiting nature of DMAI in the AIM process. Typical reaction conditions: precursor temperature = 80 °C; deposition chamber temperature = 120 °C. DMAI: pulse time = 0.1 s; exposure time = 60 s; purge time = 75 s. H2O: pulse time = 0.015 s; exposure time = 60 s; purge time = 75 s.

ray diffraction (PXRD) (Figure 2b); however, comparison with the parent NU-1000 shows peak broadening and a reduction in Bragg scattering intensity of higher angle reflections, indicating some reduction in crystallinity. A slight increase in the lattice

Figure 2. (a) SEM micrograph and zirconium−aluminum EDS line scan plot of a Al-NU-1000 crystallite; (b) PXRD patterns of Al-NU-1000 (top, red) and pristine NU-1000 (bottom, blue); (c) nitrogen physisorption isotherms (adsorption branch) of Al-NU-1000 (red) and pristine NU-1000 (blue). Inset: pore size distribution analysis of Al-NU-1000 (red) and pristine NU-1000 (blue); and (d) (left) variable temperature DRIFT spectra of Al-NU-1000 and (right) DRIFT spectra of γ-Al2O3 dehydrated at 400 °C under argon flow (black, top), of Al-NU-1000 after treatment at 100 °C under argon flow (red, middle), and of bare NU-1000 (blue, bottom). 1060


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Chemistry of Materials In particular, the hydroxyl region in the final spectrum (Figure 2d right) consists of three major peaks at 3769, 3718, and 3675 cm−1 and a minor one at 3642 cm−1. Peaks are likely composed of multiple lines as a result of both Al- and Zrhydroxyl and/or H2O moieties. Due to the extent of the changes observed in the variable temperature DRIFT spectra, we investigated the effect of the thermal treatment on the stability of the framework. PXRD and nitrogen physisorption measurements, as well as SEM images, confirmed the retention of crystallinity, porosity, and morphology after dehydration. Interestingly, IR peaks observed in Al-NU-1000 show strong similarities with those reported for γ-Al2O3: at a dehydroxylation temperature of 450 °C, major peaks are observed at 3764, 3728, and 3675 cm−1, with hydrogen bonded hydroxyl bands at 3561 and 3512 cm−1.29 While there may not be a direct relation between Al-NU-1000 and γ-Al2O3, an empirical understanding of the aluminum-modified NU-1000 structure can come from a comparison with a known aluminum oxide. Although various models have been proposed to describe the nature of hydroxyl species characterizing γ-Al2O3, it is generally accepted that the −OH band frequencies follow the trend μ1 > μ2 > μ3 and that each kind of −OH is observed within certain characteristic ranges.30,31 Hence, we can tentatively assign the peaks at 3769, 3718, and 3675/3642 cm−1 to μ1−OH, μ2−OH, and μ3−OH, respectively, with the assignment of the first and second type of hydroxyls being the most certain. The peak at 3675 cm−1 is very close to the terminal −OH specific to bare NU-1000 (3674 cm−1),10 making the interpretation of this band nontrivial, as it could be assigned to hydroxyls pertaining either to the zirconium node or to the deposited aluminum. It is worth mentioning that Al-NU-1000 shows relatively sharp hydroxyl bands when treated under conditions considerably milder (100 °C) than those required in the dehydration protocols of aluminum oxides (temperatures of up to 500 °C).32 Difference envelope density (DED) analysis revealed the localization of additional electron density in the NU-1000 framework following Al-AIM.33 Structure envelopes were generated from NU-1000 and Al-NU-1000 powder diffraction data as described previously.34−37 A difference envelope density reflecting the distribution of deposited Al was generated via scaled subtraction of the structure envelope of the parent material from Al-NU-1000. ALD with DMAI resulted in an increase in electron density between the Zr6 nodes parallel to the c-axis (Figure 3). This suggests that aluminum preferentially locates on two of the nodal faces (and spans the small pore along the c-axis) instead of distributing over the full node. DRIFT measurements, pointing to spectroscopic similarities between Al-NU-1000 and γ-Al2O3, along with DED analyses evidencing clustering upon modification by Al-AIM, suggest that the overall ALD process results in the synthesis of aluminum oxide clusters that are homogeneously distributed through the NU-1000 framework, as shown by SEM-EDS. Moreover, considering the self-limiting behavior of the deposition technique in use, with a metal incorporation of maximum 7 Al/Zr6 as determined by elemental analysis, and the size of the pores (approximately 8−9 Å) in which the extra electron density has been located, we propose that these Al2O3 clusters consist of only a few Al atoms. Alumina nanoparticles are currently of much interest in materials research.39−41 However, in the nano regime, the synthetic protocols are challenged by aggregation phenomena promoted by hydrogen bonding, electrostatic, and van der

Figure 3. View of NU-1000 lattice (left) perpendicular to and (right) parallel to c-axis. A difference envelope indicating extra electron density present in Al-NU-1000 is represented in magenta. Zr6O8 clusters are represented in cyan.38

Waals forces.42 Here, we notice that, in the present system, where AlxOy clusters are confined in the small pores of NU1000, their subnanometer structure is preserved. To elucidate further the structure of Al-NU-1000 and, in particular, to probe the coordination environment of the aluminum atoms, we used solid-state 27Al MAS NMR spectroscopy. 27Al NMR chemical shifts are strongly sensitive to geometry and coordination number, and their use to differentiate between six-coordinated (AlO6), five-coordinated (AlO5), and four-coordinated (AlO4) units is well-established.43−45 The spectrum of the as-synthesized Al-NU-1000 reveals three distinct chemical shifts centered at δ 4.5, 35, and 68. The major signal at δ 4.5 is close to δ 0 indicating an octahedral geometry (AlO6) whereas the two signals at δ 35 and δ 68 are attributed to AlO5 and AlO4 units, respectively (Figure 4a). A

Figure 4. Solid-state 27Al MAS NMR (104.19 MHz) spectra of assynthesized Al-NU-1000 (top, black) and after treatment at 100 °C under argon flow (bottom, red). Dashed line indicates δ 0.

sample treated at 100 °C shows the same set of peaks (Figure 4b). However, in addition to slight deviation in the chemical shifts, relative intensities are significantly changed. The intensity of the peak corresponding to AlO6, now at δ 0, decreases with a concomitant increase of the peak belonging to AlO5, now at δ 30 (Figure 4). These findings are consistent with an increased 1061


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Figure 5. (a) XANES normalized xμE spectra for Al-NU-1000 as well as the aqueous Al(H2O)63+ (octahedral Al) and Al(OH)4− (tetrahedral Al) references are shown. All spectra are corrected for fluorescence self-absorption; (b) the EXAFS Img[χ̃(R)] spectra for Al-NU-1000 (black) and the fit (magenta) obtained from the aluminozirconate (painite) model are shown. The experimental spectrum is corrected for self-absorption (Booth correction), and the fits are performed using a k-weighting of 3. The fit details, e.g., chosen scattering paths with respective fit atom distances and Debye−Waller factors (DWFs), are reported in the Supporting Information.

concentration of five-coordinate aluminum atoms relative to the six-coordinate. Formation of AlO5 units is explained by the removal of a water molecule from the coordination environment of an octahedral aluminum (AlO6). This observation is consistent with the removal of chemisorbed water during the dehydration process which was previously followed by DRIFT spectroscopy. Upon exposing a dehydrated sample to water, the effect of the thermal treatment was reversed and the intensity of the AlO6 signal recovered. Al2O3 is known to have several crystalline phases, including α- and γ-Al2O3, but it also occurs as an amorphous material.42 AlO4 and AlO5 were determined to be the predominant species in amorphous alumina, with AlO6 accounting only for a minor fraction.46 In crystalline alumina, the α-phase is made of AlO6 units, whereas the 27Al NMR spectra of the γ-phase show predominantly features of AlO4 and AlO6.47 Recent works, including surface selective NMR techniques and differential PDF, showed that γ-Al2O3 also contains five-coordinate aluminum atoms (AlO5) localized at the surface.14,48−51 Comparison of Al-NU-1000 with known aluminum oxides can be instructive, especially their IR and NMR characterization, in establishing structural similarities. However, the very small size of the AlxOy clusters under examination defines their bulk-free nature resulting in all aluminum atoms being exposed to the surface. Overall, Al-NU-1000 reveals spectroscopic signatures that resemble those of γ-Al2O3, but as demonstrated by variable temperature DRIFTS, the dehydration process requires drastically lower temperatures than bulk γ-Al2O3. This peculiar behavior is supported by NMR measurements showing a significant amount of AlO5 units, typically observed on aluminum oxides only under a certain degree of dehydration. Probing Local Structure. In order to further study the nature of Al-NU-1000, Al XAS measurements (XANES and EXAFS) were performed. Figure 5a shows the normalized xμE XANES plot for the MOF and the chosen reference compounds, the aqueous Al(H2O)63+ (octahedral Al) and Al(OH)4− (tetrahedral Al) ions. Al-NU-1000 exhibits a major peak at ∼1569 eV, which is typical for octahedral Al species.52 Indeed, the MOF spectrum is nearly identical to that of octahedral Al(H2O)63+ (Figure 5a), which strongly suggests

that the Al-species in the as-synthesized Al-NU-1000 is predominantly 6-coordinate. In order to provide a quantitative insight of the first shell Al coordination, a XANES linear combination fit was performed using the above-mentioned reference compounds to define tetrahedral and octahedral Al. The best fit indicates that ∼80% of Al in the MOF is sixcoordinate and only a minor (∼20%) fraction of Al is present as tetrahedral species. Three different models were tested to fit the experimental EXAFS data. We started with a model based on a mineral aluminozirconate, painite,53 a naturally occurring material with an elemental structure related to Al-NU-1000. Painite contains 6-coordinated Al and Zr atoms forming distorted edge-shared octahedra. The mineral also contains B, which replaces Zr in an alternating (1:1) pattern, and Ca, which is present in cationic form to balance the negative framework charge. The fit to the MOF using this model is shown in Figure 5b. When performing these fits, the Al−O first shell coordination number was defined as 5.7. This value corresponds to the ratio of octahedral/tetrahedral Al determined from the XANES analysis. The first shell Al−O bond distance is ∼1.83 Å, which is only 0.02 Å longer than that previously determined for corundum,54 α-Al2O3, where all Al atoms are octahedrally coordinated. The fit also suggests that the Al species in the MOF form predominantly symmetric Al−O octahedra, because the Al−O multiple scattering (MS) features are essential to fit the experimental EXAFS data. A similar phenomenon was previously reported when studying the nature of aqueous ions in water, e.g., Al55 and Ni.56 In fitting to the painite model, inclusion of the Al−Zr scattering path only slightly improved the quality of the fit so that it was not possible to detect with certainty the existence of Al−O−Zr linkages. This would either be due to the limited number of linkages for the Al clusters or due to the spatial disorder in the Al−Zr distances. A more simplified model without the Al−Zr paths showed sensitivity for the Al−Al scattering path that corresponds to an Al−Al distance of ∼2.9 Å. Although this distance is somewhat longer than that in painite or corundum,53,54 we suggest that it is reasonable for the Al-NU-1000 especially considering the extent of disorder (cf. Debye−Waller factors (DWFs), Table S1). 1062


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Chemistry of Materials The large errors reported for the metal−metal scattering paths are tentatively explained by the disorder in the Al structure as well as the complexity of accurately separating and identifying the contributions of Al−Al and Al−Zr scattering signals. Results from the XANES and the EXAFS fitting suggest an octahedral symmetric coordinationin agreement with the NMR dataand the occurrence of small (a few Al atoms) AlxOy clusters, as also determined from DED analyses. To gain a deeper understanding of the structure of Al-NU1000, analysis of pair distribution functions extracted from Xray total scattering data was performed. The elevated temperatures used during synthesis are known to contribute to a local phase change in the structure of the Zr6(O)8 nodes;57 therefore, a differential PDF obtained by direct subtraction of the PDF of pristine NU-1000 from that for Al-NU-1000 will reflect not only new correlations associated with the deposited Al species but also any distortions that may occur owing to high local temperatures during the ALD process. A differential PDF containing only ALD species framework and ALD−ALD species correlations was obtained by use of an NU-1000 control heated to 135 °C, which exhibited the same node distortion as Al-NU-1000 (Figure 6).

Figure 7. (a) γ-Al4 and (b) γ-Al8 cluster models. Aluminum atoms are pink, oxygen atoms are red, and hydrogen atoms are white.

considered. In the “flat” configuration (Al8-flat), as shown in Figure 8b, the aluminum oxide cluster is oriented closer to the

Figure 8. (a) γ-Al4 cluster attached to small Zr6 cluster model. (b) γAl8 cluster model attached to small Zr6 cluster model in the flat configuration. Aluminum atoms are pink, zirconium atoms are light blue, oxygen atoms are red, carbon atoms are gray, and hydrogen atoms are white.

zirconium oxide node. A separate “vertical” configuration of the same aluminum oxide cluster was considered, now oriented vertically with respect to the node (Al8-vertical, Figure S8). To validate the structure of the DFT optimized models in Figure 8, IR and EXAFS spectra were simulated to compare to the experimental data. Clusters obtained from γ-Al2O3 were quite energetically stable and provided models that showed good agreement to the O−H stretching regions of the IR spectra and the Al−O and Al−Al peaks of the EXAFS data. For modeling purposes, the zirconium oxide nodes, instead of bearing the entire linker, were terminated with benzoate groups to compute the IR frequencies and simulate EXAFS data. Simulated IR spectra generated from the optimized models are compared with the experimental DRIFT spectrum of AlNU-1000. Qualitatively, based on frequency values and peak intensities, the γ-Al4 model (Figure S11) agrees less well with the experimental IR pattern; while comparing the two Al8 clusters, flat vs vertical, the flat version matches slightly better the DRIFT spectrum of Al-NU-1000 (Figure 9 and Figures S11 and S12). Assignment of the computed frequencies is not entirely straightforward as many OH bonds are present, but peaks at frequencies higher than 3750 cm−1 are associated primarily with terminal Al−OH stretches while stretches in the region between 3700 and 3750 cm−1 are more associated with Zr−μ1−OH, Zr−μ3−OH, Al−μ2−OH, and H2O bound to

Figure 6. Pair distribution functions of Al-NU-1000, NU-1000, NaAlO2, and Al2O3 extracted from total scattering data.

This differential PDF contains a new feature at 1.86 Å which does not correspond to any node distortion phenomena.57 This peak can be attributed to new Al−O correlations, which closely match the value obtained from the EXAFS analysis of 1.83 Å. The new Al−O distances most closely match those found in an Al2O3 standard. Computational Modeling. After various attempts to obtain a model of the aluminum oxide cluster confined within the pores of NU-1000, in light of the spectroscopic similarities with γ-Al2O3, we elected to build a model starting from the known structure of γ-Al2O3. The aluminum oxides clusters were cut directly from periodic γ-Al2O3,58 with the charges neutralized by adding protons, and attached to models of the zirconium oxide node of NU-1000. Most of the oxygen atoms come directly from the periodic structure, but undercoordinated aluminum atoms were saturated with H2O molecules or −OH groups to reproduce the predominantly octahedral coordination environment of fully hydrated Al-NU1000 as determined by NMR and EXAFS. We generated and investigated two models with different size, an Al4 and an Al8 oxide cluster as presented in Figure 7. As a result of the large size of the Al8 structure, which nearly occupies the entire pore, two possible orientations have been 1063


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Figure 9. Experimental and theoretical infrared spectra in the O−H stretching region. The theory peaks were plotted using a width of 8 cm−1 at half height and a frequency scaling factor of 0.956. Comparison with other models is shown in Figures S11 and S12.

either Zr or Al, all in close proximity. These findings agree with assigned values for γ-Al2O3. To assess sensitivity to the dehydration extent, we also optimized a structure containing two extra water molecules forming hydrogen bonds with the γAl8-NU-1000 cluster in the flat configuration (γ-Al8 flat + 2H2O). As seen in Figure 9, the positions of the highest frequency peaks are quite sensitive to the presence or absence of the two water molecules that we speculate may also contribute to the experimentally observed peak broadening (see Supporting Information for further details). Al-EXAFS spectra were compared to simulated spectra obtained using Al scattering paths based on the DFT-optimized models, an approach previously utilized in Al-EXAFS analysis of zeolite samples.54 The comparison was performed in both the EXAFS k- and R-spaces (Figure 10). Compared to the Al4 model, both Al8 models mimic better the full variation in the EXAFS spectral features, especially in the R-plot, but it is not possible to discriminate between flat and vertical configuration; also, simulations of the Al8-flat models either with or without water molecules resulted in nearly identical spectra (Figures S13 and S14). Considering the complexity of the Al-NU-1000 structure, insights from EXAFS fitting, and DFT structural optimizations, we suggest that Al-NU-1000 contains small (4−8 Al atoms) aluminum oxide clusters that possibly vary in size and orientation in the small pores and are bound to the Zr6nodes of NU-1000. The models developed here represent realistic proposals of structures that show good spectroscopic agreement with experimental data. The actual, macroscopic material may well incorporate a range of structures, as a consequence of the existence of numerous possible local minima, but pore confinement and local Al2O3-like structure renders the structures similar to one another. We speculate that the interaction of DMAI with NU-1000 might produce local heating that may be further responsible for slight alteration of the clusters in structure from one pore to another. Evaluation of Catalytic Performance. Alcohol dehydration is a typical reaction to benchmark solid acids, and typical

Figure 10. k2-weighted Img[χ(R)] (top) and χ(k) (bottom) spectra of the measured Al-NU-1000 (black) and calculated EXAFS spectra for three different models.

substrates comprise cyclohexanol or isobutanol. Ethanol dehydration is more challenging, as it requires higher operating temperatures, and the selectivity for ethene poses issues at times. Various byproducts can be obtained, and diethyl ether a product of partial dehydrationis the most favorable one. However, as ethene is one of the most relevant raw materials in the petrochemical industry, conversion of ethanol to ethene remains a reaction of importance, and it is practiced at the industrial production level.59 The constantly increasing production of biomass-derived ethanol makes this process an attractive option for further engineering. Over the years, various catalysts have been employed, either homogeneous or heterogeneous. Up to date, several solid acids have been found to catalyze ethanol dehydration, of which γAl2O3 is one of the most efficient. For the high conversion and selectivity achieved, as well as for good stability, it is the catalyst of choice in the industry, with operating temperatures commonly set between 400 and 500 °C to maximize the efficiency.59,60 The interesting features of Al-NU-1000 make its catalytic properties worth exploring and prompted us to investigate the performance in ethanol dehydration and to benchmark its selectivity against γ-Al2O3. Catalytic tests were performed in a vertical tubular quartz reactor equipped with mass flow controllers, and the major products (ethene and diethyl ether) were analyzed and quantified by online gas chromatography. The reactions were conducted at 300 °C saturating the carrier gas (argon) with ethanol by means of a bubbler at a 1064


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Chemistry of Materials constant ethanol concentration and varying the total flow rate. Al-NU-1000 and γ-Al2O3 were tested under the same experimental conditions and over the same conversion range to ensure comparability between the two systems. Conversion vs selectivity plots for both Al-NU-1000 and γ-Al2O3 are shown in Figure 11.

of AlO5 units. The concomitance of these related properties contributes to the superior catalytic performance of Al-NU1000.

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CONCLUSION NU-1000, a zirconium-based MOF, has been postsynthetically functionalized with aluminum oxide by means of a well established atomic layer deposition technique that preserved its mesoporosity, crystallinity, and high surface area. Multiple experimental techniques were used to characterize the Almodified metal−organic framework and demonstrated the spectroscopic similarities of incorporated AlxOy clusters with γAl2O3. Small pores in the NU-1000 framework were found responsible for the stabilization of unprecedentedly small aluminum oxide nanoclusters, which were determined to be less than 1 nm in size. Calculated models of Al-NU-1000 support this finding with simulated IR and EXAFS spectra closely matching experimental measurements. Such well-dispersed aluminum oxide clusters made of only a few Al atoms represent a unique platform for under-coordinated aluminum centers. Based on its intriguing structural properties, Al-NU-1000 was tested for ethanol dehydration and found to be remarkably more selective than γ-Al2O3. In addition to its application in an acid-catalyzed reaction of industrial interest, we envisage the potential use of Al-NU-1000 as a new class of support for various metal-based catalysts. This promising approach is currently under investigation.

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EXPERIMENTAL METHODS

Synthesis of Al-NU-1000. In a typical procedure 60 mg of NU1000 in a custom-made powder sample holder at 120 °C were dosed with vapors obtained by heating the aluminum precursor ([AlMe2(OiPr)]2, DMAI) at 80 °C. Atomic layer deposition was carried out in a Savannah 100 ALD reactor (Cambridge Nanotech) using the following conditions: precursor pulse time, 0.1 s; precursor exposure time, 60 s; and purge time, 75 s. The H2O cycle was performed at the same temperature adopting the following conditions: H2O pulse time, 0.015 s; exposure time, 60 s; and purge time, 75 s. NMR Spectroscopy. Solid-state 27Al MAS NMR spectra were recorded on a 400 MHz Varian VNMRS spectrometer with a conventional triple resonance 5 mm probe-head in double resonance mode. Samples were filled into a ZrO2 rotor in a glovebox and tightly closed. The spinning frequency was set at 10.00 kHz. Chemical shifts (δ) are given in ppm and referenced to Al(NO3)3 (1 M in HNO3). Spectra were baseline-corrected. Al K-Edge XAFS. The Al K-edge XAFS spectra were measured at the Phoenix I elliptical undulator beamline at the Swiss Light Source (SLS) at the Paul Scherrer Institute. To calibrate the energy, we have set the inflection point of an Al foil spectrum to 1559.6 eV. The double-crystal monochromator contained a set of KTiOPO4 (011) crystals that provide an energy resolution of ∼0.6 eV for an Al K-edge scan from 1500 to 2150 eV. To provide the cutoff for higher harmonics, two Ni-coated mirrors were set at an angle of 1.45°. The measurements were performed using an unfocused 1.0 × 1.0 mm beam with a flux of ∼109 photons/s. During data acquisition, the sample chamber pressure was maintained at ∼2.5 × 10−4 mbar. All spectra were acquired in fluorescence mode. I0 was determined as the total electron yield signal from an 0.5 μm thin polyester foil coated with 50 nm of Ni. We have placed this I0 detector in a small vacuum chamber (2.9 × 10−6 mbar) that was separated by a thin Kapton foil from the main (experiment) chamber. A 4-element Vortex Si-drift diode detector was used to count the X-ray fluorescence. iXAFS software package61,62 containing ATHENA was used to perform the background processing necessary in order to extract the χ(k) data from the background function. A Fourier filter cutoff distance, Rbkg, of 1.0 Å was used. The XAFS data were weighted by k2 and truncated using a

Figure 11. Conversion vs selectivity toward ethene (blue diamond) and diethyl ether (red square) in ethanol dehydration catalyzed by AlNU-1000 and γ-Al2O3.

Interestingly we found that selectivity toward ethene, the desired product, is higher using Al-NU-1000 as catalyst. In particular, we determined selectivity values of approximately 20% and 5% with Al-NU-1000 and γ-Al2O3, respectively, with only a marginal variation over the explored conversion range. Concomitantly, Al-NU-1000 shows significantly lower selectivity toward diethyl ether with respect to the γ-Al2O3-catalyzed reaction (Figure 11). We additionally confirmed that unmodified NU-1000 induced negligible ethanol conversion and was nonselective for ethene. The newly prepared aluminum-modified NU-1000 demonstrates, under the adopted catalytic conditions, an improved selectivity toward ethene, the desired product of ethanol dehydration. As discussed above, Al-NU-1000 shows a set of properties that makes it an unprecedented material. In particular, it contains AlxOy particles with an unusually small size, a feature that we speculate is preserved due to their confinement in the small framework pores. Additionally, a significant degree of dehydration can be achieved at a moderate temperature, which points to the existence of a high population 1065



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Hanning window with dk = 1.0 Å−1 in the range of 1.5 < k < 8.0 Å−1. Two different reference compounds α-Al2O3 and Na2Al2O4 were also evaluated using the ARTEMIS software package.61,62 Theoretical standards for these compounds were derived from FEFF9.61,62 The aqueous reference compound measurements were performed using a liquid cell with a 1.5 mm diameter, 800 nm thick CVD diamond X-ray window (Applied Diamond, Inc., Wilmington, DE). The cell design has been reported previously.63 A 2 M aqueous AlCl3 solution was used as the octahedral Al(H2O)3+ reference,54 and an aqueous mixture of 7 M Al(OH)3 and 12 M NaOH was used as the tetrahedral Al(OH)4− reference55 for the Al XAFS analysis. To generate the EXAFS spectrum of the DFT-optimized structure f irst all of the atom positions and CNs are adopted from the simulated structure. This produces the primary input for the ab initio EXAFS scattering code (FEFF9 code)64 that includes all the single and multiple scattering paths out to 6 Å. Next, an approximate treatment of the bond disorder at 300 K is applied by setting a universal value of DWF of 0.003. While this is a good estimate of the first shell disorder, it will tend to overestimate the order in the higher shells that will manifest as slight overprediction of the amplitudes. The σ2 could be exactly calculated from ab initio molecular dynamics (MD) as previously described.54,56 The obtained spectra are then averaged, and the E0 is applied to match experimental values (oscillations in χ(k) converge at k = 0). Pair Distribution Function Analysis. X-ray total scattering data suitable for extraction of pair distribution functions (PDFs) were collected at beamline 11-ID-B at the Advanced Photon Source using high energy X-rays (58.6 keV) in combination with an amorphous silicon-based area detector. Data reduction and calibration of detector distance, rotation, tilt, and beam center with CeO2 were performed using FIT2D.65 Pair distribution functions were extracted from onedimension data using PDFgetX2.66 Synchrotron Powder Diffraction. Ambient temperature powder diffraction data were collected at beamline 17-BM-B at the Advanced Photon Source using 17.04 keV X-rays. Diffraction data were collected for capillary-loaded samples using an amorphous silicon-based area detector and processed using QXRD.67 Subsequent calibration of sample to detector distance, beam center, detector tilt and rotation with Na2Ca3Al2F14 (NAC), and reduction of data to one-dimensional patterns were performed using GSAS-II.68 Lattice parameters were extracted from powder diffraction data via Le Bail whole pattern fitting.69−71 The previously reported crystal structure for NU-1000 (P6/mmm) was used as a starting model for this analysis.5 Hexagonal lattice and pseudo-Voigt profile parameters were refined. Computational Methods. Density functional theory (DFT) has been applied in previous work with Al-AIM in NU-1000.21 Here, cluster models were first cut from a periodically optimized NU-1000 structure.5 The cutting points were then terminated with hydrogen atoms. A large Zr6 cluster model with benzoic acid groups in place of the linkers was used to compare to XAFS data and to compute frequencies for comparison to experimental IR spectra (Figure S6). For the optimization of the cluster model the carbon atoms in the para position of the benzoic acids were fixed to maintain the overall structure of periodic NU-1000. The structures were optimized in the gas phase with the M06-L density functional in Gaussian 09.72 The optimizations were performed with the 6-31G(d,p)73 basis set on C, H, and O and the Stuttgard/Dresden effective core potential and basis (SDD)74 on Zr. Frequency calculations were then performed at the same level of theory to compute the IR spectra.

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Article

AUTHOR INFORMATION

Corresponding Authors

*(L.G.) E-mail: [email protected]. *(J.T.H.) E-mail: [email protected]. *(O.K.F.) E-mail: [email protected]. ORCID

Martino Rimoldi: 0000-0002-2036-3648 Leighanne C. Gallington: 0000-0002-0383-7522 Ana E. Platero-Prats: 0000-0002-2248-2739 Johannes A. Lercher: 0000-0002-2495-1404 Christopher J. Cramer: 0000-0001-5048-1859 Laura Gagliardi: 0000-0001-5227-1396 Omar K. Farha: 0000-0002-9904-9845 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported as part of the Inorganometallic Catalysis Design Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0012702. This work made use of the IMSERC facility supported by the National Science Foundation (NSF, DMR-0521267); the J.B. Cohen X-ray Diffraction Facility at the Materials Research Center of NU supported by the MRSEC (NSF, DMR1121262); the EPIC facility of the NUANCE Center at NU, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF, NNCI-1542205); the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. Metal analysis was performed at the NU Quantitative Bio-element Imaging Center. Gas flow reactions were performed at the NU CleanCat Core facility. A.V., J.L.F., and J.A.L. acknowledge the support by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Work done at Argonne was performed using the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Authors thank Dr. T. Huthwelker for support during Al XAFS measurements at the Swiss Light Source (PSI, Switzerland). M.R. was supported by the Swiss National Science Foundation with an “Early Postdoc.Mobility Fellowship”. A.E.P.-P. acknowledges a Beatriu de Pinós fellowship (BP-DGR 2014) from the Ministry of Economy and Knowledge (Catalan Government).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b03880. Methods, additional characterization data, XAFS details, and DFT optimized models (PDF) 1066


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