Fundamental Insights into the Reactivity and Utilization of Open Metal

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Fundamental Insights into the Reactivity and Utilization of Open Metal Sites in Cu(I)-MFU‑4l Lin Li,† Yahui Yang,† Mona H. Mohamed,‡,§ Sen Zhang,† Götz Veser,† Nathaniel L. Rosi,†,‡ and J. Karl Johnson*,† †

Department of Chemical & Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States § Chemistry Department, Faculty of Science, Alexandria University, P.O. Box 426, Ibrahimia, Alexandria 21321, Egypt Downloaded via BUFFALO STATE on August 22, 2019 at 09:47:29 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Metal−organic frameworks (MOFs) having open metal sites have the potential to approach the activity of homogeneous organometallic complexes, thus combining the advantages of homogeneous and heterogeneous catalysis. We present a fundamental study of the effectiveness of incorporating open metal sites into MOFs. We have modeled the binding of a series of adsorbates in a Cu(I)-substituted MOF, Cu(I)-MFU-4l, using density functional theory and compared the activity of the Cu(I) sites in Cu(I)-MFU-4l with that of two different Cu(I) scorpionate complexes. The computational results confirm the single-site nature of the Cu(I) active site. This is further supported by complementary experiments to measure the chemisorption uptake inside our synthesized samples in order to estimate the amount of active Cu sites present. We observed a level of chemisorption that is roughly half the theoretical maximum, which implies that only half of the Cu atoms incorporated into MFU-4l via metal ion exchange are able to act as binding sites. We speculate that the inactive Cu atoms are coordinately saturated Cu(II) sites. Our work suggests that the performance of Cu(I)-MFU-4l could be significantly increased by optimizing the metal exchange and activation processes.

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various ways,6 and one popular strategy is to modify the secondary building units (SBUs) to include more selective or reactive binding sites.7,8 The SBU in MFU-4l can be functionalized via metal exchange to resemble the active site of a scorpionate-type homogeneous catalyst. In fact, Dinca and co-workers have demonstrated the ability of Ni(II)-MFU-4l9,10 and Co(II)-MFU-4l11 to selectively catalyze polymerization reactions in a fashion similar to that for as the homogeneous Ni and Co scorpionate catalysts. In addition, Jelic et al. have predicted Co(II)-MFU-4l to be active for redox chemistry.12 In this work, we investigate functionalized MFU-4l with an open Cu(I) site via postsynthetic metal exchange. We compare the reactivity of Cu(I)-MFU-4l with the reactivity of similar Cu(I) scorpionate complexes13 having tris(pyrazolyl)borate (Tp) and tris(3-mesitylpyrazolyl)borate (TpMes) ligands using density functional theory (DFT) calculations. Additionally, we also quantify the amount of active Cu sites present in the samples prepared via metal exchange. In order to deduce the

INTRODUCTION Metal atoms play an essential role as binding and catalytic sites in both molecular and heterogeneous catalysts. In a heterogeneous catalyst, the bulk of the metal atoms are unexposed and therefore inactive. In contrast, homogeneous catalysts have high metal utilization, and the ligand environment surrounding the metal center can be tailored to achieve high activity and selectivity. In spite of these advantages, the critical drawback of homogeneous catalysis is the costly separation and catalyst recovery steps.1 One way of overcoming the biggest shortcoming of homogeneous catalysts is incorporating these metal active sites into a high-surface-area support. In this work, we use theory and experiment to carry out a fundamental study of the reactivity of a heterogeneous catalyst with a high degree of metal utilization in the form of a functionalized metal−organic framework (MOF): MFU-4l (metal−organic framework Ulm University-4large).2 MOFs are an ideal platform for generating open metal sites for adsorption, separation, and selective catalysis.3−5 This is because in the ideal case each metal atom may have an identical environment (truly single site) and every metal may be available for adsorption and reaction. How these MOFs interact with different guest molecules can be controlled in © XXXX American Chemical Society

Special Issue: Organometallic Chemistry within Metal-Organic Frameworks Received: May 24, 2019

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DOI: 10.1021/acs.organomet.9b00351 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Cu utilization in the synthesized samples, one must first establish the maximum uptake limits in the ideal system. Using DFT calculations, we have shown that a saturation occurs at one adsorbate per Cu(I) site, and that Cu(I) sites in Cu(I)MFU-4l are not influenced by factors such as confinement, extent of substitution, lattice strain, and adsorbate−adsorbate interaction on neighboring binding sites. A meaningful prediction from theory depends on the reliability of the computational method. For DFT calculations, the reactivity of isolated metal atoms is especially difficult to model due to self-interaction errors,14 which originate from an inaccurate description of the electron density. In this work, we rigorously benchmark the performance of different families of exchange-correlation functionals for adsorption energy calculation. In addition, we also apply uncertainty estimation15 to generate error bars for the computational results and compare calculations with experimental measurements for a series of different adsorbates.

energies between the Cu(I) site in MFU-4l and the Cu(I) scorpionate complexes is shown in Figure 2. Here, the

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RESULTS AND DISCUSSION The systems used in our DFT calculations are shown in Figure 1. In the functionalized MFU-4l, we replace the peripheral Zn-

Figure 2. Comparison of binding of six different adsorbates on Cu(I) sites in three different environments. The diamonds (squares) represent PBE+D3 calculated binding energies on Cu(I) scorpionate complex with Tp (TpMes) ligands. The x values represent binding energies calculated in Cu(I)-MFU-4l with a single Cu substitution.

adsorption energies were calculated using the PBE17 functional with D3 correction.18 Our DFT results consistently predict adsorbates to bind more strongly on the scorpionate complexes relative to that in MFU-4l, in the absence of solvent effects. The difference in reactivity could be due to the difference in bite angles of the Cu center between having the three ligands attached via Zn or B. In a more simplistic picture, the N−Cu−N angle is 109.2° in Cu(I)-MFU-4l and only 100.5° in Cu(I)-Tp, where a smaller angle appeared to correlate with greater reactivity. Surprisingly, there was a minimal difference between the binding energies on Cu(I)-Tp and Cu(I)-TpMes. In fact, these adsorbates seem to be slightly more strongly bound on Cu(I)-TpMes despite the bulker TpMes ligand. Overall, the relative ordering in the binding strength of these adsorbates is preserved in these three systems. More importantly, these results imply that the reactivity of Cu(I)MFU-4l is similar to that of the Cu(I) scorpionate complex and no additional topological effects, such as confinement,19 influence chemisorption in Cu(I)-MFU-4l. In other words, it is reasonable to assume that the 3-fold ligand provides a similar steric environment around the Cu(I) center in both the scorpionate complex and Cu(I)-MFU-4l. We note that Magdysyuk et al.20 ascribed the lack of difference between Xe and Kr adsorption in MFU-4l and Cu(I)-MFU-4l to steric effects around the Cu(I) site. However, our calculations (see the Supporting Information) show that the lack of interaction with the Cu(I) center is the primary reason for the weak adsorption of these noble gases in these MOFs, which is consistent with the observed and calculated weak Xe binding on Cu surfaces.21 Our DFT calculations also predict that the reactivity of Cu(I) binding site is invariant to many experimental variables. For example, we do not expect the binding energy to change due to variations in Cu loading during synthesis or fluctuations in temperature or partial pressure of adsorbates during the measurements. We have investigated the two extremes in the

Figure 1. Calculated structures: (a−c) the freestanding Cu(I)-Tp, Cu(I)-TpMes, and Cu(I) metal modified scorpionate clusters, respectively; (d) the Cu(I) binding site in Cu(I)-MFU-4l. The Cu binding site is shown in yellow. Other atoms are color coded as C (black), N (dark blue), Zn (light blue), B (red), and H (white).

Cl with an open Cu(I) site. Here, the reactivity is represented by the binding energy of a series of adsorbates, and the reactivity of the Cu(I) site incorporated in MFU-4l is compared against Cu(I) scorpionate complexes with Tp and TpMes ligands.13 We also use a metal-modified scorpionate complex to represent the Cu(I) site in the periodic MOF. A similar cluster was used by Reuter et al.12,16 However, in this work, we validated the cluster model by directly comparing with the full periodic framework, illustrated in Figure 1d. The results of our benchmark calculations are shown in Figure S1. By using a periodic system, we are able to investigate the effects of coverage and lattice strain, which cannot be studied using an isolated cluster. First, we discuss how the Cu(I) sites in MFU-4l differ from the Cu(I) scorpionate complexes. A comparison of binding B

DOI: 10.1021/acs.organomet.9b00351 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

not detected in pristine MFU-4l (see Figure S2 in the Supporting Information), which verifies that the reactivity is due to the exchanged Cu sites. On the basis of repeated C2H4 TPD over the same sample, we estimate the experimental error to be ∼5%. With this estimation, the largest error is only ∼60 meV, which is very close to the chemical accuracy of ∼50 meV25 and much smaller than the typical errors in DFT. For example, the uncertainty in our DFT calculations for these binding energies falls in the range of 230−350 meV. In spite of the poor precision, DFT can still correctly predict trends in binding energy in Cu(I)-MFU-4l. We have reduced the system size to the metal-modified scorpionate cluster (Figure 1c) in order to carry out higherlevel (i.e., B3LYP26) calculations. In addition to PBE+D3 and B3LYP+D3, we have also computed the binding energies using BEEF-vdW,15 SCAN,27 and LDA28 functionals, shown in Figure 5. We have also used the BEEF ensemble for uncertainty quantification for the range of errors in DFT. The calculated values are compared with experimental measurements (on the x axis) in Figure 5. Experimental binding energies for CO, C2H4, and C3H6 were obtained from our TPD experiments; the other compounds are reported isosteric heats from adsorption isotherms.16,29 The overall results show that B3LYP and BEEF-vdW calculations can accurately reproduce experimental measurements. Additionally, we also included one spin-polarized adsorbate, NO, in our experimental and computational study. The binding energy of NO was computed using spin-polarized B3LYP calculations. The calculated binding energy (−0.66 eV) is in good agreement with our TPD measurements (−0.73 eV). In contrast, LDA and PBE functionals systematically overestimate the binding strength. Surprisingly, the SCAN functional, which satisfies all theoretical limiting constraints in DFT, does not appear to significantly outperform PBE. The difference between B3LYP and PBE is consistent with self-interaction errors in DFT,30 which leads to errors in calculated electron density. Furthermore, the results from LDA, PBE, and SCAN lying outside the error bar from the BEEF-ensemble also indicates that the difference is due to the electron density instead of the functional form.31 Despite this error, the overall trend and ordering of binding strength is consistent among these methods. Overall, our calculations suggest a chemisorption saturation limit of one adsorbate per Cu(I) site. At a Cu:Zn ratio of 3:2, and assuming that each Cu atom forms an open Cu(I) site by replacing a peripheral Zn-Cl group in a defect-free framework, the ideal uptake limit for chemisorption is 58.5 cm3/g at STP. The actual uptake in Cu(I)-MFU-4l was measured by chemisorption experiments. Chemisorption uptake results are shown in Figure 6. The amount of adsorbate uptake was calculated on the basis of the difference between the known dosed amount and the integrated area of the peaks. Each pulse has an injection volume of 0.11 cm3 at STP, and each C2H4 and C3H6 pulse (undiluted ultrahigh purity) contained ∼5 times as many chemisorbing gas molecules as each CO and NO pulse (20% in He). The first five CO doses and first three NO doses were completely adsorbed in the experiments shown in Figure 6. None of the gas pulses were completely adsorbed for the undiluted C2H4 and C3H6. The measured uptakes are 24.97−25.12, 24.02−26.25, 19.74−20.45, and 21.54−22.96 cm3/g for C2H4, CO, NO, and C3H6, respectively. The samples used for C2H4 and CO chemisorption were from a fresh batch of Cu(I)-MFU-4l. The NO chemisorption was measured using

extent of metal exchange, and we compare systems with only a single substitution to the peripheral site and those having all four peripheral Zn-Cl groups replaced by Cu. We have observed only an ∼1% change in the optimized lattice constant between these Cu loadings. This suggests that strain effects appear to be minor for this system, which was reflected by adsorption energy calculations. In Figure 3, the horizontal lines

Figure 3. Effects of coverage in Cu(I)-MFU-4l. The horizontal lines represent binding energies calculated in Cu(I)-MFU-4l with a single substitution in the unit cell. The points represent differential binding energies calculated in Cu(I)-MFU-4l with four open Cu sites per SBU. Here, the coverage represents the final coverage per SBU after adsorption.

represent binding energies at the low Cu loading limit and the points represent the differential binding energy as a function of coverage on fully substituted Cu(I)-MFU-4l. There is a minimal difference (1 eV.22 These results suggest that, regardless of the extent of metal exchange in the MOF and the uptake, for a given adsorbate, all bound species will have adsorption energies within a narrow range of values. This prediction was also verified by temperature-programmed desorption (TPD) experiments (see below). Having established the single-site nature of Cu(I)-MFU-4l, we compare the accuracy and precision of our theoretical results with experiments. We adapted a literature synthetic procedure2,23 and synthesized Cu(I)-MFU-4l with a Cu:Zn ratio of 3:2, as measured by elemental and ICP-OES analyses (see the Supporting Information for details). For CO, C2H4, C3H6, and NO, the desorption energy was determined via TPD, using the variable heating rate method,24 shown in Figure 4. In addition to TPD, pulse chemisorption experiments and adsorption isotherms measurements were also performed. The measured desorption energies from TPD experiments are 1.12, 0.83, 0.72, and 0.66 eV for CO, C2H4, C3H6, and NO, respectively, in Cu(I)-MFU-4l. In contrast, chemisorption was C

DOI: 10.1021/acs.organomet.9b00351 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Figure 4. TPD spectra of (a) C2H4, (b) CO, (c) NO, and (d) C3H6 over Cu(I)-MFU-4l with different heating rates. The variable heating rate method was used to determine the desorption energy, as shown in Figure S3 of the Supporting Information.

uptakes from the chemisorption measurements from the fresh batch implies that the saturation limit does not change on the basis of the identity of the adsorbate, as predicted by theory. The decrease in uptake for the sample stored for 3 months indicates that there may be a loss of activity upon extended exposure to the atmosphere. We have also measured C2H4 adsorption isotherms for two different batches of Cu(I)-MFU-4l and have repeated measurements for one of the samples. These experiments show a steep increase in C2H4 adsorption at low pressure, which corresponds to chemisorption (Figure S4 in the Supporting Information). The uptake from these adsorption measurements falls between 29 and 32 cm3/g (STP), as shown in Figure S4 in the Supporting Information. The overall Cu utilization from chemisorption measurements is hence around 41−45%, versus 50−55% from C2H4 adsorption isotherms. The difference in uptake between the two methods is likely due to loss of sample during transfer for chemisorption, which can cause up to 20% uncertainty in the sample mass as noted in Methods. Thus, the results from pulse chemisorption and adsorption isotherms are in reasonable agreement for the amount of active Cu sites in the prepared sample. We speculate that the unavailability of roughly half the Cu sites for gas adsorption is mainly due to the presence of Cu(II) in the sample, which could be due to either oxidation or incomplete conversion to Cu(I). Additionally, the amount of active Cu sites decreases over time as the sample is exposed to ambient air, as noted above. The Cu(I) in our samples may have oxidized16 to Cu(II) over this period of time. The decrease in chemisorption coinciding with a decrease in the number of Cu(I) sites also suggests that Cu(I) sites are the

Figure 5. Performance of different functionals vs experiment. The calculations were performed on the Cu(I) metal modified scorpionate cluster using the methods listed. The experimental values were from TPD experiments from this work (CO, C2H4, C3H6) and isosteric heats from the literature.16,29

the same batch 3 months later, stored in air. The sample for C3H6 chemisorption was a combination of the old batch and a newly synthesized batch. The close agreement between the D

DOI: 10.1021/acs.organomet.9b00351 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Figure 6. Pulse chemisorption uptake results for (a) C2H4, (b) CO (20% in helium), (c) NO (20% in helium), and (d) C3H6 over Cu(I)-MFU-4l at 273 K. Subsequent pulses of adsorbates were injected into a fixed bed of the sample until full breakthrough (i.e., identical peak height) was observed.

binding sites, as observed in both adsorption isotherms and pulse chemisorption experiments. We attribute at least part of the loss in Cu utilization to the presence of Cu(II) in the MOF. These results reveal additional opportunities for improving the performance of this class of materials via improvements in synthetic control over the metal sites during or post metal ion exchange.

active sites for chemisorption. Hence, one should aim to incorporate and maintain a high loading of Cu(I) sites in this material.

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CONCLUSION

Cu(I)-MFU-4l is a promising open metal site material, with potential uses in separations and catalytic reactions. We have verified the single-site nature of Cu(I)-MFU-4l through a combination of theory and experiments. Although the absolute binding energy differed depending on the functionals used in DFT calculations, the trend in adsorption strength stayed consistent at the LDA through the hybrid (B3LYP) level. The estimated error for DFT binding energy (∼0.3 eV) was found to be much larger than experimental errors (