Molecular Design of Zirconium Tetrazolate Metal–Organic

Publication Date (Web): December 13, 2016 ... This study demonstrates the bottom-up strategy to design Zr-MOFs and suggests MOF-eft-P to be an interes...
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Molecular Design of Zirconium Tetrazolate Metal−Organic Frameworks for CO2 Capture Kang Zhang, Zhiwei Qiao, and Jianwen Jiang* Department of Chemical and Biomolecular Engineering, National University of Singapore, 117576, Singapore S Supporting Information *

ABSTRACT: Zirconium (Zr) metal−organic frameworks (MOFs) have received considerable interest due to their superior hydrolytic, thermal, and chemical stability. On the basis of 40 topological nets, 120 Zr-MOFs with 8-, 10-, and 12-coordination are designed in this study with nitrogen-rich CO2-philic tetrazolate toward CO2 capture. The Zr-tetrazolate MOFs are revealed to exhibit substantially stronger CO2 adsorption than their carboxylate counterparts of identical topologies. By synergizing multiscale modeling from molecular simulation to breakthrough prediction, MOF-eft-P is identified among the 120 Zr-MOFs to possess the largest isosteric heat at infinite dilution and uptake at 1 bar for CO2, as well as the highest CO2/N2 selectivity; it outperforms many typical MOFs and other adsorbents reported in the literature. The breakthrough time of CO2 in MOF-eft-P is predicted to be 25 min and significantly longer than that of N2, indicating excellent CO2/N2 separation. This study demonstrates the bottom-up strategy to design Zr-MOFs and suggests MOF-eft-P to be an interesting candidate for CO2 capture.

1. INTRODUCTION With the explosive growth of energy consumption resulting from anthropogenic activities, the atmospheric CO2 concentration has risen to over 400 ppm.1 As a consequence, the energy balance of the global environmental system is adversely affected. Currently, there is an urgent need to reduce the carbon footprint for environmental protection and sustainable development. In particular, CO2 is required to be captured from flue gas emitted by coal- or oil-fired power plants, which accounts for 13.7 gigatonnes of CO2 emissions yearly.2 The conventional technique for CO2 capture is absorption using aqueous alkanolamine solution.3 The solution is heated up to 120 °C for regeneration, which carries roughly 20−30% energy penalty and needs expensive infrastructure. As an alternative, energetically efficient and economically competitive adsorption technique has been exploited. In the past, porous materials such as carbons and zeolites were examined for CO2 capture; however, they were either not sufficiently selective or difficult to be regenerated.4 Therefore, there is continuous quest for new adsorbents with improved performance. Emerging as a special class of hybrid porous materials, metal−organic frameworks (MOFs) have attracted considerable attention. They are constructed from metal clusters as nodes and polytopic organic ligands as linkers, and their overall architectures are primarily determined by the geometry of metal clusters.5 Because of the existence of a wide variety of metal clusters and organic ligands, the diversity and multiplicity in MOFs are substantially more extensive than any other © XXXX American Chemical Society

porous materials. Consequently, MOFs have been considered as versatile materials for gas storage, separation, catalysis, etc.6 A large number of experimental and simulation studies have been conducted on CO2 adsorption/separation in various MOFs.7−9 While there is a large degree of freedom to tune MOF structures, a major concern is their stability. The stability is primarily governed by the coordination bond between metal cluster and organic linker. In this context, the selection of metal cluster is crucial. It has been recognized that the group-IV metals can interact strongly with oxygen atoms; particularly, zirconium (Zr)-based octahedron cluster is able to construct MOFs with superior hydrolytic, thermal, and chemical stability. The first reported Zr-based MOF is UiO-66 (UiO: University of Oslo) with a face-centered-cubic (fcu) topology, in which each Zr octahedron is bridged with 12 carboxylates and forms 12-coordinated hydroxylated Zr6O4(OH)4(CO2)12 or dehydroxylated Zr6O8(CO2)12.10 Recently, 12-coordinated ZrMOFs with ftw and ith topologies, as well as 10, 8 or 6coordinated with bct, csq, flu, reo, and spn topologies, have also been synthesized.11,12 Toward the high storage and delivery of CH4, Snurr and co-workers computationally designed over 200 Zr-MOFs with Zr octahedron and carboxylate featuring fcu, ftw, csq, and scu topologies.13 Received: September 23, 2016 Revised: December 7, 2016 Published: December 13, 2016 A

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Figure 1. From left to right: (Zr6O8) cuboctahedron, (Zr6O8)(TZ)12, (Zr6O8)(TZ)10(OH)4, and (Zr6O8)(TZ)8(OH)8. TZ = tetrazole. Color code: Zr, cyan polyhedron; O, red; N, blue; C, gray; H, white.

Figure 2. Illustration for the construction of MOF-urr. Color code: Zr, cyan polyhedron; O, red; N, blue; C, gray; H, white.

2. METHODOLOGY

The Zr-MOFs mentioned above consist of carboxylate. It has been demonstrated that MOFs with nitrogen-rich (N-rich) ligands exhibit superior performance in selective CO 2 adsorption.14 Particularly, tetrazole possesses several uncoordinated N-rich donors, which are electron dense centers with strong affinity for polar guest molecules (e.g., CO2); moreover, the tetrazole ring has a strong electron-withdrawing inductive effect. Therefore, a larger number of studies have been reported on tetrazolate-based MOFs. For instance, Banerjee and coworkers demonstrated, by both experiment and simulation approaches, a zeolitic tetrazolate framework with amino groups exhibiting high adsorption capacity for CO2.15 Eddaoudi and co-workers synthesized a series of fcu-MOFs by assembling rare-earth metals with various organic ligands including carboxylate and single tetrazolate substitution; compared with the carboxylate counterparts, the MOFs with tetrazolate were found to possess higher CO2-framework interaction strength.16 Dong et al. incorporated multiple tetrazolate ligands into a water-stable MOF with rich N-donors inside the framework channels, leading to high selective adsorption of CO2.17 It is also interesting to note, however, Ma and co-workers demonstrated that the local electric field would be more favorable for CO2 capture than exposed N atoms.18 In this study, we design 120 Zr-MOFs from 40 topological nets using Zr6O8 and tetrazolate as basic building units, and subsequently simulate and evaluate their performance for CO2 capture. Following this introduction, design strategy, simulation methodology, and breakthrough prediction are outlined in Section 2. In Section 3, CO2 adsorption isotherms in Zrtetrazolate and Zr-carboxylate MOFs are first compared; the isosteric heats of CO2 in 120 Zr-MOFs are estimated at infinite dilution and 1 bar, and further compared with those in literature reported adsorbents; finally, adsorption selectivities for a CO2/N2 mixture in 120 Zr-MOFs are presented, and breakthrough curves are predicted in the best Zr-MOF. In Section 4, concluding remarks are summarized.

2.1. Design Strategy. From the Reticular Chemistry Structural Resource,19 the topological nets with 12-, 10-, and 8-coordinated vertices were first identified. The nets unable to form cuboctahedron were excluded by examining the connectivity of vertices. In total, there were 40 topologies including 12 nets with 12-coordinated vertices (fcu, ftw, ith, llk, ttv, urr, xag, xaj, xal, xbn, xij, and xxv), 3 nets with 10-coordinated vertices (bct, cco, and xau), and 25 nets with 8coordinated vertices (bcu, bcu-b, csq, cut, cuz, eft, fla, flt, flu, hbr, reo, scu, sea, seh, the, tsq, tty, uri, urj, urt, xak, xbg, xbm, xbv, and xlz). These nets were used as templates to construct Zr-MOFs by assembling Zr6O8 cluster and tetrazolate. As illustrated in Figure 1, the Zr6O8 cluster was built from an octahedral Zr6 unit capped by 8 μ3O ligands. If 12-coordinated, all the 12 octahedral edges were connected with tetrazolate to form (Zr6O8)(TZ)12; while 2 and 4 edges were occupied by 4 and 8 terminal hydroxyl groups to form (Zr 6 O8)(TZ)10 and (Zr 6 O8 )(TZ)8 if 10- and 8-coordinated, respectively. On the other hand, the organic linker was formed by connecting various organic building units (Figure S1). Using the reversed topological approach,20,21 as illustrated in Figure 2 for MOFurr, the end points of organic linker were connected to tetrazolate and further bridged to Zr6O8 cluster. The template nets were scaled proportionally in terms of the sizes of (Zr6O8)(TZ)n (n = 12, 10, and 8) and organic linker. The preformed (Zr6O8)(TZ)n was positioned at the net vertice with the edge pointing along the net connectivity. Then, the net edge was replaced by organic linker with proper orientation to connect with (Zr6O8)(TZ)n. Thereafter, the structure was optimized using the Forcite module in Materials Studio22 based on the Universal force field (UFF).23 By this strategy, 120 Zr-MOFs were computationally constructed and structurally optimized (available in the Supporting Information). The void fractions of these Zr-MOFs were estimated with RASPA package24 using the helium atom as a probe. 2.2. Simulation Methodology. For 120 designed Zr-MOFs, the atoms were described by Lennard−Jones (LJ) plus electrostatic potentials:

⎡⎛ ⎞12 ⎛ ⎞6 ⎤ σij σij ∑ 4εij⎢⎢⎜⎜ ⎟⎟ − ⎜⎜ ⎟⎟ ⎥⎥ + r ⎝ rij ⎠ ⎦ ⎣⎝ ij ⎠

∑

qiqj 4πε0rij

(1)

where εij and σij are the well depth and collision diameter, respectively, rij is the distance between atoms i and j, qi is the atomic charge of atom B

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Figure 3. CO2 adsorption at 298 K in (a) PCN-222 and MOF-csq-222, (b) DUT-67 and MOF-reo, (c) NU-1000 and MOF-csq-1000, (d) NU-TPE and MOF-scu. The insets in (a) show the structures of MOF-csq-222 and PCN-222. i, and ε0 = 8.8542 × 10−12 C2 N1− m−2 is the permittivity of vacuum. Table S1 lists the LJ parameters adopted from the UFF. The atomic charges of MOFs were estimated by the electrostatic-potentialoptimized charge scheme (MEPO-QEq), which can rapidly generate atomic charges of periodic structures and has been validated for CO2 adsorption in thousands of MOFs.25 For gas molecules (CO2 and N2), as shown in Figure S2, the potential parameters were adopted from the TraPPE force field.26 The Lorentz−Berthelot combining rules were employed to calculate cross interactions. Adsorption of pure CO2, N2, and a CO2/N2 mixture was simulated using Grand Canonical Monte Carlo (GCMC) method. The isosteric heats at infinite dilution were estimated in a canonical ensemble with a single gas molecule. The frameworks were assumed to be rigid with framework atoms frozen during simulation. The simulation cells were expanded to at least 25.6 Å along each dimension and periodic boundary conditions were exerted. A spherical cutoff of 12.8 Å with long-range correction was used to calculate the LJ interactions, whereas the electrostatic interactions were calculated using the Ewald sum. In each MOF, the simulation ran for 10 000 cycles with the first 5000 cycles for equilibration and the last 5000 cycles for ensemble averages. Each cycle consisted of N trial moves (N: the number of adsorbate molecules), including translation, rotation, regrowth, and swap. Further increasing the number of cycles was found to have an insignificant effect on adsorption. All the simulations were conducted using the RASPA package.24 To validate the atomic models used in simulations, gas adsorption in four Zr-MOFs was simulated and compared with available experimental data. These include N2 adsorption in NU-1000,27 MOF-841,28 and DUT-6728,29 at 77 K, and CO2 adsorption in UiO6630 at 298 K. As shown in Figure S3, the simulated isotherms agree fairly well with experimental data. The slightly higher simulation results are plausibly due to the existence of impurities (e.g., residual solvents and ligands) in experimental samples, which reduce the free volume and lead to a lower uptake. For N2 adsorption in NU-1000, it is worthwhile to note that there are deviations between this study and literature simulation results.27 This is because our model of NU-1000

contains terminal −OH groups at metal clusters, which were not present in the literature simulation.27 The existence of terminal −OH reduces the free volume of NU-1000, thus yields a lower N2 uptake. The four Zr-MOFs (NU-1000, MOF-841 and DUT-67 and UiO-66) are carboxylate based and not among the 120 designed Zr-MOFs. However, the good agreement between simulation and experiment implies that the atomic models can be also used for tetrazolate-based Zr-MOFs. 2.3. Breakthrough Prediction. A fixed bed packed with adsorbent is the primary unit in an industrial adsorption process, and knowing its breakthrough profiles is insightful for adsorbent selection and process optimization. Figure S4 illustrates the separation of a CO2/N2 mixture in a fixed bed. After identifying the best Zr-MOF based on CO2/N2 selectivity from GCMC simulation, we predicted the breakthrough curves of a CO2/N2 mixture. The adsorbate concentration at a position of the fixed bed can be obtained by solving the mass-balance equations31

cT

(1 − εb) ∂v =− ∂z εb

DL

∂ 2ci ∂z

2

−

∑

∂qi ∂t

(2)

∂(civ) ∂c (1 − εb) ∂qi − i = ∂z ∂t εb ∂t

(3)

i

where cT is the total concentration of fluid, v is interstitial fluid velocity, z is axially dispersed plug flow direction, εb is the bed voidage, qi(z,t) is the loading of component i into adsorbent at time t, ci(z,t) is the concentration of component i in fluid phase, and DL is axial dispersion coefficient. The mass transfer rate into adsorbent can be expressed using the linear driving force model

∂qi ∂t

= ki(qi* − qi)

(4)

where q*i is the equilibrium loading calculated from dual-site Langmuir isotherm model and ki is the overall effective mass transfer coefficient C

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Figure 4. Qost of CO2 adsorption at 298 K simulated in (a) 120 Zr-MOFs (b) literature reported adsorbents.33−36

Figure 5. (a) Electrostatic potential map and (b) distribution of CO2 in MOF-eft-P. of component i. Specifically, ki is related to external film, macropore, and micropore resistances

do so, CO2 adsorption was simulated in four Zr-tetrazolate MOFs: MOF-scu, MOF-csq-1000, MOF-reo, and MOF-csq222, as well as four Zr-carboxylate MOFs: NU-TPE,13 NU1000,27 DUT-67, 29 and PCN-222.32 For each pair of counterparts (e.g., MOF-csq-222 and PCN-222), they share the same topology, Zr6O8 cluster and organic building unit; the only difference is that either tetrazolate or carboxylate is coordinated with Zr6O8. As shown in Figure 3, there is dramatic increase of CO2 uptake in each tetrazolate MOF compared with its carboxylate counterpart. At 1 bar, the increase is almost 120% in Figure 3a and 35−70% in Figure 3b−d. This confirms that N-rich tetrazolate can enhance CO2 adsorption. 3.2. CO2 Adsorption at Infinite Dilution and 1 bar. The interaction strength of adsorbate−adsorbent can be quantified by the isosteric heat Qost at infinite dilution. Figure 4 plots the Qost of CO2 adsorption simulated in 120 Zr-MOFs, as well as in literature reported typical adsorbents.33−36 Generally, the highest Qost is observed in 8-coordinated Zr-MOFs, then 12and 10-coordinated counterparts. In 8-coordinated Zr-MOFs, four octahedral edges are occupied by eight terminal −OH, which are strongly interacting with CO2; consequently, 8coordinated Zr-MOFs possess high CO2-framework affinity. Moreover, the less coordination leads to easy access for CO2 molecules toward the metal center, which further enhances CO2 adsorption. Comparing 10- and 12-coordinated Zr-MOFs, the former contain four terminal −OH per metal cluster and exhibit lower Qost than 12-coordinated without terminal −OH. This is attributed to the symmetrically oriented tetrazolate ligands in 12-coordinated Zr-MOFs, which act as two arms stabilizing CO2 molecules near the metal cluster. Therefore, the −OH groups and symmetry contribute in a synergistic way to high Qost. Among 120 Zr-MOFs, MOF-eft-P has the highest Qost

R pqi*0 τpR p2qi*0 r2 1 1 1 1 = + + = + + c ki k film k macro k micro 3k f ci0 15εpDmci0 15Dc (5) where kf is the mass transfer coefficient across the external film around adsorbent particles, ci0 is the concentration of component i in the feed, qi0* is the equilibrium loading at feed concentration, Dm and Dc are molecular and intracrystalline diffusivities, rc is crystal size, and Rp, εp, and τp are particle radius, porosity, and tortuosity, respectively. It is commonly acceptable to assume that the micropore (intracrystalline diffusion) resistance is negligible, particularly when rc is small and Dc is large. The set of partial differential equations 2−4 were solved numerically by reducing into ordinary differential equations via the finite difference technique for spatial terms. Finally, the ordinary differential equations subject to appropriate initial and boundary conditions were solved. Table S2 lists the parameters used in the breakthrough prediction.

3. RESULTS AND DISCUSSION First, we compare the performance of tetrazolate and carboxylate based Zr-MOFs for CO2 adsorption. Then, the isosteric heats of CO2 adsorption in 120 designed Zr-MOFs are estimated at infinite dilution and 1 bar, and compared with those in other adsorbents. Finally, the adsorption of a CO2/N2 mixture in 120 Zr-MOFs is examined, and the best Zr-MOF is identified; moreover, the breakthrough curves are predicted in the best Zr-MOF. 3.1. Comparison between Tetrazolate and Carboxylate Based Zr-MOFs. As mentioned above, most synthesized Zr-MOFs consist of carboxylate. It is intriguing to compare the performance of carboxylate and tetrazolate based Zr-MOFs. To D

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Figure 6. CO2 uptakes at 298 K and 1 bar simulated in (a) 120 Zr-MOFs and (b) literature reported adsorbents.33−37

Figure 7. CO2/N2 selectivities at 298 K and 1 bar simulated in (a) 120 Zr-MOFs (b) literature reported MOFs.35,37,38

highest CO2 uptake is 137.6 cm3(STP)/cm3 in MOF-eft-P, and the second highest is 128.4 cm3(STP)/cm3 in MOF-eft. These uptakes are higher than in most adsorbents in Figure 6b (except Mg-MOF-74). Both MOF-eft-P and MOF-eft are 8-coordinated, which further suggests that 8-coordinated Zr-MOFs outperform 10- and 12-coordinated counterparts. A similar trend is observed for Qst at 1 bar, as shown in Figure S5. The highest Qst of 33.7 kJ/mol is found in MOF-eft, followed by 31.4 kJ/mol in MOF-eft-P. The reason for higher Qst in MOFeft than MOF-eft-P is that the strong adsorption sites in MOFeft-P are fully occupied at 1 bar, and CO2 molecules start to accommodate at the less favorable sites (Figure S6). 3.3. CO2/N2 Separation. To examine the capability for CO2 capture from flue gas, Figure 7 shows the adsorption selectivities of CO2/N2 at 298 K and a total pressure of 1 bar simulated in 120 Zr-MOFs and literature reported MOFs.35,37,38 The selectivity is defined as Si/j = (xi/xj)/(yi/ yj), where xi and yi are the compositions of component i in adsorbed and bulk phase, respectively. The bulk composition of CO2/N2 is 0.15:0.85, which mimics dry flue gas in a postcombustion CO2 capture process and can be achieved by removing moisture from emitted flue gas. Again, 8-coordinated Zr-MOFs generally perform better than 10- or 12-coordinated, not only in CO2 uptake and isosteric heat but also in adsorption selectivity. Among 120 Zr-MOFs, MOF-eft-P exhibits the highest selectivity of 68.4, which is approximately 2−3 times of

of approximately 30 kJ/mol. Although this value is lower than 38.9 kJ/mol in Mg-MOF-7436 uniquely containing a large density of open metal sites, it is higher than the Qost in many typical adsorbents (e.g., UiO-66, MFI , and IRMOF-1). To provide microscopic insight into CO2 adsorption in MOF-eft-P, Figure 5 plots the electrostatic potential map and distribution of CO2. MOF-eft-P has a pore aperture of 4.9 Å, which is slightly larger than the molecular size of CO2. A MOF with such a pore aperture has strong affinity for CO2 due to potential overlap. In MOF-eft-P, Zr atoms are positively charged, while the tetrazolate and terminal −OH are negatively charged. The positively charged and negatively charged regions face two different pore centers, respectively, which are exposed to CO2 molecules for their easy access. The distribution indicates that the favorable adsorption sites of CO2 in MOF-eftP coincide with the positions of the highest charge density. Specifically, the negatively charged O atoms of CO2 align with electron positive Zr atoms; CO2 molecules are perpendicular to terminal −OH groups. The small pore aperture and the optimal framework strongly facilitate CO2 adsorption in MOF-eft-P, thus leading to the highest Qost. There are experimental data reported for CO2 adsorption in various adsorbents at 1 bar. Figure 6 presents CO2 uptakes simulated in 120 Zr-MOFs and other MOFs, zeolites, and activated carbon.33−37 Remarkably, 9 out of 120 Zr-MOFs exhibit CO2 uptake larger than 100 cm3(STP)/cm3. The E

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with MOF-eft-P, the breakthrough time is 25 min for CO2 and 0.9 min for N2. Such a large difference implies that CO2/N2 separation in MOF-eft-P can be achieved well. Nevertheless, we should note that the study aims to design possible Zrtetrazolate MOFs for high-performance CO2 capture. The practical feasibility to synthesize the designed Zr-tetrazolate MOFs, as well as their chemical and thermal stabilities, should be tested experimentally. We hope that the study can trigger experimental studies toward this end and further facilitate the development of new MOFs.

those in most MOFs (except Mg-MOF-74). It should be noted that a large number of literature reported selectivities are based on single-component adsorption, and such selectivities do not incorporate the competition effect of gas mixture and thus are not accurate. In addition, high selectivities in some MOFs are originated from chemisorption. Nevertheless, only physisorption is considered in this study. The fair comparison with literature data suggests that MOF-eft-P is a promising candidate for CO2 capture from flue gas. 3.4. Breakthrough Profiles. Among 120 Zr-MOFs, MOFeft-P exhibits the highest CO2 uptake and CO2/N2 selectivity. To mimic a practical adsorption process, breakthrough profiles were predicted in a fixed bed packed with MOF-eft-P for a CO2/N2 mixture at 298 K and 1 bar. As shown in Figure 8, the

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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.cgd.6b01405. Organic building units; structures of 120 Zr-tetrazolate MOFs; Lennard−Jones parameters of Zr-MOFs; atomic models of CO2 and N2; simulation and experimental adsorption isotherms of pure N2 and CO2; fixed-bed parameters for breakthrough prediction; Qst of CO2 adsorption at 298 K and 1 bar; CO2 distributions in MOF-eft and MOF-eft-P (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jianwen Jiang: 0000-0003-1310-9024 Figure 8. Breakthrough curves for a CO2/N2 mixture in MOF-eft-P at 298 K. The inlet pressure is 1 bar for CO2/N2 (0.15:0.85).

Notes

breakthrough curves are characterized by different regimes. First, nearly pure N2 is exited from the bed, whereas CO2 is adsorbed. After a certain time, CO2 breakthrough occurs and gradually reaches its feed composition; meanwhile, N2 concentration at the outlet drops to its feed composition. The outlet CO2 concentration is 0.001% (10 ppm) at 22 min and 0.01% (100 ppm) at 25 min. To quantify, breakthrough time τ is defined as the time when gas concentration at the outlet is 0.01%. The τ is 0.9 min for N2 and 25 min for CO2. Thus, the breakthrough time for CO2 is dramatically different from N2 and the CO2/N2 mixture can be efficiently separated. The difference in breakthrough time between CO2 and N2 is 24.1 min, much longer than 9.9 min predicted in Cu-TDPATN.37 This indicates that MOF-eft-P is superior for CO2/N2 separation.

ACKNOWLEDGMENTS The authors gratefully acknowledge the National University of Singapore and the Ministry of Education of Singapore for financial support (R-279-000-474-112 and R261-508-001-646/ 733).

The authors declare no competing financial interest.

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REFERENCES

(1) http://co2now.org/, accessed on 07 December 2016. (2) Chu, S. Science 2009, 325, 1599−1599. (3) Haszeldine, R. S. Science 2009, 325, 1647−1652. (4) Aaron, D.; Tsouris, C. Sep. Sci. Technol. 2005, 40, 321−348. (5) O’Keeffe, M.; Yaghi, O. M. Chem. Rev. 2012, 112, 675−702. (6) Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M. Science 2013, 341, 1230444. (7) Sumida, K.; Rogow, D. L.; Mason, J. A.; McDonald, T. M.; Bloch, E. D.; Herm, Z. R.; Bae, T.-H.; Long, J. R. Chem. Rev. 2012, 112, 724− 781. (8) Li, J. R.; Sculley, J.; Zhou, H. C. Chem. Rev. 2012, 112, 869−932. (9) Jiang, J. W. Metal-Organic Frameworks for CO2 Capture: What Are Learned from Molecular Simulations. In Coordination Polymers and Metal Organic Frameworks; Ortiz, O. L.; Ramírez, L. D., Eds.; Nova Science Publishers: New York, 2012. (10) Cavka, J. H.; Jakobsen, S.; Olsbye, U.; Guillou, N.; Lamberti, C.; Bordiga, S.; Lillerud, K. P. J. Am. Chem. Soc. 2008, 130, 13850−13851. (11) Kim, M.; Cohen, S. M. CrystEngComm 2012, 14, 4096−4104. (12) Bai, Y.; Dou, Y.; Xie, L. H.; Rutledge, W.; Li, J. R.; Zhou, H. C. Chem. Soc. Rev. 2016, 45, 2327−2367. (13) Gomez-Gualdron, D. A.; Gutov, O. V.; Krungleviciute, V.; Borah, B.; Mondloch, J. E.; Hupp, J. T.; Yildirim, T.; Farha, O. K.; Snurr, R. Q. Chem. Mater. 2014, 26, 5632−5639. (14) Zhang, J. P.; Zhang, Y. B.; Lin, J. B.; Chen, X. M. Chem. Rev. 2012, 112, 1001−1033.

4. CONCLUSIONS By assembling Zr6O8 clusters and N-rich tetrazolates, 120 ZrMOFs with 40 different topologies have been designed from the bottom-up and applied for CO2 capture. A dramatic increase of CO2 uptake is found in Zr-tetrazolate MOFs against Zr-carboxylate counterparts. CO2 adsorption is governed synergistically by the terminal −OH groups surrounding the Zr6O8 clusters and the symmetrical tetrazolate ligands. The 8coordinated Zr-MOFs generally perform better than the 10- or 12-coordinated ones, not only in isosteric heat and CO2 uptake but also in adsorption selectivity. Among 120 Zr-MOFs, MOFeft-P contains a small pore aperture and optimal structure, thus possesses the highest CO2 adsorption capacity, which is superior to many typical adsorbents. In a fixed bed packed F

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Crystal Growth & Design

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DOI: 10.1021/acs.cgd.6b01405 Cryst. Growth Des. XXXX, XXX, XXX−XXX