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Computation-Ready, Experimental Covalent Organic Framework for Methane Delivery: Screening and Material Design Minman Tong, Youshi Lan, Zhenglong Qin, and Chongli Zhong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04742 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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The Journal of Physical Chemistry

Computation-Ready,

Experimental

Covalent

Organic

for

Delivery:

Framework

Methane

Screening and Material Design Minman Tong, *‡a Youshi Lan, ‡b Zhenglong Qin, *a and Chongli Zhongb

a

b

School of chemistry and materials science, Jiangsu Normal University, Xuzhou 221116, China. State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical

Technology, Beijing 100029, China.

ACS Paragon Plus Environment

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ABSTRACT

CH4 storage associated with adsorbed natural gas (ANG) technology attracts considerable researches on finding porous materials with remarkable CH4 delivery performance. In this work, we update the online accessible CoRE (computation-ready, experimental) COF (covalent organic frameworks) database with 280 COFs in twelve topologies. All the framework structures are constructed and compiled from the respective experimental studies and are further evaluated for CH4 delivery. The highest deliverable capacity (DC) between 65 and 5.8 bar among the CoRE COFs is 190 v(STP)/v at 298 K achieved by 3D PI-COF-4. Structure-property relationships show that large volumetric surface area generally benefits CH4 delivery. 2D-COFs can also be top performing materials if constructing their pore channels passable in three dimensions, as the volumetric surface area will be increased accordingly. This idea can be realized by enlarging the interlayer spacings of 2D-COFs. We also evaluate the DC of CoRE COFs at condition of 233 K, 65 bar (storage) and 358 K, 5.8 bar (discharge). The highest DC obtained from the CoRE COFs and the designed 2D-COFs are 314 and 337 v(STP)/v, respectively.


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INTRODUCTION Natural gas (NG), primarily comprised of CH4, is an attractive substitute to petroleum as a transportation fuel due to its environmental friendliness, abundant natural reserves and economic advantages. 1,2 For on-board storage in vehicular applications, natural gas must be densified due to the low volumetric energy density of methane under ambient conditions. The densification strategies such as compressed natural gas (CNG) (250 bar) or liquefied natural gas (LNG) (111 K) have been industrialized but yet still cannot motivate the widespread use of natural gas for mass vehicular transport as they face a series problem of cost and practicality. 3,4 An alternative densification strategy of adsorbed natural gas (ANG), in which natural gas is stored in nanoporous materials, provides a promising solution by affording higher energy density at much moderate operating conditions like lower pressure of 65 bar, which can relieve the compression and infrastructure costs and will be much safer than CNG.5 Deliverable capacity of CH4, the difference between the amount of CH4 adsorbed at the storage pressure and at the discharge pressure, is a critical metric that determines the driving range of ANG tanks. To match the performance of a CNG tank, the Advanced Research Projects Agency-Energy (ARPA-E) established a target that ANG systems should have a volumetric energy density of 12.5 MJ/L, or equivalently deliver 315 v(STP)/v to the engine between 65 bar (storage) and 5.8 bar (discharge) considering 25% packing losses.6 To realize the target, significant efforts have been paid on evaluating or designing various kinds of porous materials.7-26 Computational evaluations of ~650 000 advanced hypothetical porous materials, including MOFs (metal-organic frameworks),12,14 porous polymer networks (PPNs),15 COFs (covalent organic frameworks)16 and zeolites,14 and extensive experimental work indicate a maximum deliverable capacity of approximately 200 v(STP)/v using crystal


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density. After pelletization and shaping, the highest deliverable capacity in experiment as far as we know is 172 v(STP)/v attained by monolithic HKUST-1.26 Therefore, it will remain a long lasting challenge to find adsorbents with better performance. COFs are an emerging class of crystalline networks that are covalently constructed from organic components and featured by merits of light weight, high porosity and large surface area.27 So far, ~ 300 COF structures have been experimentally reported, but only some of them have been tested for CH4 storage and the results show that COFs can be promising candidates for ANG technologies.16,28-36 To fast evaluate the CH4 delivery performance of existing ~ 300 COFs, computational methods can be a powerful instrument instead of expensive and time-consuming experimental synthesis and characterization. To date, CH4 delivery performance of these COFs have not been computationally evaluated in a large-scale way, one possible reason may be the difficulty of collecting the massive structure information from their original synthesis work. By virtue of the experience in studying COFs for gas storage and separations,37-41 we built a publicly available CoRE COF database40 which now is updated with 280 experimental COF structures. Assisted by the diverse COFs as the ANG porous materials, we screen CoRE COFs for CH4 delivery between 65 bar (storage) and 5.8 bar (discharge) in this work. Their performance is compared with that of other outperforming materials. Moreover, relationships between performance and geometric and chemical properties are discussed, from which the obtained knowledge provides useful guidance for designing new COFs with better CH4 delivery performance.

METHODS COF structures. The CoRE COF database was initially established with 187 structures in our previous work.40 With the endeavors of experimental chemists, the database now contains 280


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COFs accessible online (https://core-cof.github.io/CoRE-COF-Database/), including 31 3DCOFs with ctn, bor, dia or pts topology, and 249 2D-COFs with triangular, square, hexagonal, octagonal, heteromorphic or hybrid pores. Pore characterizations of LCD (largest cavity diameter), accessible surface area (Sacc) and free volume (Vfree) are performed using the open source software Zeo++ version 0.3.42,43 For each material, Sacc is calculated using a probe molecule with size equal to the kinetic diameter of N2 (3.68 Å). Vfree is computed with a probe size of 0.0 Å, which is the absolute amount of volume unoccupied by the framework atoms. Simulation details. Methane adsorption at different operation conditions of interest is determined through grand canonical Monte Carlo (GCMC) simulations using our in-house HTCADSS code. Methane molecules are modelled as LJ spheres with parameters ( = 3.73 Å and ⁄ = 148.0 K) taken from the TraPPE force field.44 The framework atoms are treated with LJ potentials using parameters from the DREIDING Force Field,45 while the missing parameters for metals Cu and Ni are taken from the Universal force field (UFF).46 The Lorentz−Berthelot mixing rules were used to determine all of the LJ cross potential parameters between adsorbate−adsorbate and adsorbate−COF interactions. In the simulations, molecules involving four types of trials: attempting (i) to translate a molecule (0.25), (ii) to regrow a molecule at a random position (0.25), (iii) to create a new molecule (0.5), and (iv) to delete an existing molecule (0.25). For simulations of methane adsorption, the random swap method has been demonstrated to provide enough samplings to ensure the results accurate.12,47 Figure S1 compares the calculation of methane delivery performance by using the random swap method and configurational-bias Monte carlo insertion method.48 A particle is selected at random and given a random displacement. The maximum displacement is taken such that 50% of the moves is accepted. The acceptance rule is,


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old → new = min {1, ∆ } where Pacc is the probability of acceptance and ∆U = β(Unew – Uold). The numbers of the unit cells contained in the simulation box are COF-dependent, ranging from 1×1×9 to 2×2×10, and no finite-size effects existed by checking the simulations with larger boxes. The cut-off distance for calculating intermolecular interaction energies is set to 14 Å without tail correction. Periodic boundary conditions are considered in all three dimensions. Here, the pressure-activity relation of pure methane (idealized model of natural gas) is given by Peng-Robinson equation of state, which has been demonstrated to reproduce the experimental density of methane well (better than TraPPE model at high pressure)15. For real natural gas storage that contains small amounts of other components (C2H6, N2, CO2, C3H8), more accurate method, such as the Wang-Landau approach, may be needed to convert the pressure to corresponding fugacity.36,49 All GCMC simulations consist of 2×107 steps to ensure the equilibration, followed by 2×107 steps to sample the desired thermodynamic properties. The obtained simulation results are similar to previous work34-36 on modeling CH4 adsorption in COF-5, COF-10, COF-102, COF-103, and are further validated to be reliable by comparing the simulated adsorption isotherms with the experimental data, as shown in Figure S2. The isosteric heats of adsorption (Qst) under the adsorption conditions are calculated using the ensemble fluctuation method as follows, 50 !"# = $% −

〈()) *〉 − 〈()) 〉〈*〉 〈()- *〉 − 〈()- 〉〈*〉 − 〈* , 〉 − 〈*〉〈*〉 〈* , 〉 − 〈*〉〈*〉

where the brackets denote the ensemble average, R is the gas constant, and N is the number of molecules adsorbed. The first and second terms are the contributions from the molecular thermal energy and adsorbate-adsorbate interaction energy Uff, respectively, while the remaining term is the contribution from the adsorbate-adsorbent interaction energy Ufm. In the following,


6

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the contributions for the Qst from the second and third terms are denoted as Qst,ff and Qst,fm, respectively. More details of the simulation methods can be found elsewhere.51 Compared with MOFs, COFs are considered as more robust ‘‘organic zeolites’’ as the organic groups are connected via covalent bonds.27 Even for MOFs, structural phase transition rarely happens in methane storage.24 To the best of our knowledge, in the 280 CoRE COFs, only LZU-30152 has been reported to show a tetrahydrofuran-induced reversible crystal-structure transformation, and none of the CoRE COFs have been reported to exhibit gate-opening behavior for CH4 adsorption. Therefore, COFs are held as rigid frameworks in the simulations as others do.16,29,3336

. To ensure the accuracy of our results, CH4 delivery performance of LZU-301 is not estimated

here.

RESULTS AND DISCUSSION Materials. At present, all the COFs are based on a total of twelve topologies. Figure 1 illustrates the distributions of the topologies with cyan and orange histograms representative of 2D and 3D nets. For 2D-COFs, there are eight possible topologies in which hcb and sql account for the majority part, while the other six types are newly discovered in recent three years.53-58 For 3DCOFs, there are four possible topologies; obviously, the total number of 3D-COFs is much smaller than 2D-COFs. Distributions of geometric properties for the 280 COFs are also calculated as they usually have direct relations with the performance of a target application. As shown in Figure 2, all the geometric properties span a wide range of values, while the LCD of 8~32 Å, void fraction of 0.6~0.85, volumetric surface area of 800~1600 m2/cm3, and gravimetric surface area of 1000~2500 m2/g are the areas the geometric properties of CoRE COFs concentrate on.


7


0.7

2D-COFs 3D-COFs

0.6

Proportion

0.5 0.4 0.3 0.2 0.1 0.0 hcb sql dia ctn pts kgm bor bex hxl fes fxl kgd

Topology

Figure 1. Topology distribution of the 280 CoRE COFs. (a)

0.20

(b)

Proportion

Proportion

0.35

0.28

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Proportion

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0.21 0.14 0.07

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φ

Figure 2. Distributions of geometric properties for the 280 CoRE COFs. (a) largest cavity diameter, (b) volumetric surface area, (c) gravimetric surface area and (d) void fraction. Screening and design of COFs for methane delivery. The volumetric deliverable capacity at 298 K between 65 and 5.8 bar is calculated and correlated with the geometric and chemical properties, as shown in Figure 3. The highest deliverable capacity observed is 190 v(STP)/v of PI-COF-4,59 a two-fold interpenetrated 3D-COF with dia topology. The value is close to the


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maximum deliverable capacity of 196 v(STP)/v found by computational screening of 650 000 porous materials. Similarly, the materials with the best experimentally measured deliverable capacities to date are MOF-5 (185 v(STP)/v),60 HKUST-1 (181 v(STP)/v),60 UTSA-76a (189 v(STP)/v),61 NJU-Bai 3 (195 v(STP)/v)62 and Co(bdp) (197 v(STP)/v)24. As can be seen from Figure 3, neither the single property of LCD, φ or Qst can be used to identify a top performing COF, as a given value of the properties may correspond to COFs with quite different performance. Top performed COFs should have the properties of LCD of 8~12 Å, φ of 0.75~0.85 and Qst of 13~18 kJ/mol simultaneously. The optimum LCD is similar to the value screened by ~130 000 hypothetical MOFs for CH4 delivery.47 For isosteric heats of adsorption (Qst), if we inspect the constituents of Qst, it will be found that the interaction between CH4 and framework atoms (Qst,fm) are higher than the intermolecular forces between CH4 molecules (Qst,ff) for all COFs, as shown in Figure 4. This finding can help us understand why the deliverable capacities intuitively increase with increasing volumetric Sacc (Figure 3c), as the framework surface plays quite an important role in recruiting CH4 molecules. The strong correlations between deliverable capacity and volumetric Sacc agree with the trend of hypothetical COFs and PPNs for CH4 delivery screened by Martin and Haranczyk.15,16 200

200

(a)

Deliverable capacity (v(STP)/v)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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Qst (kJ/mol)

Sacc (m /cm )

Figure 3. Relationship between CH4 deliverable capacity and (a) largest cavity diameter, (b) void fraction, (c) volumetric surface area, (d) isosteric heats of adsorption at 65 bar and 298 K. The operation condition is 298K, 65 bar (storage) and 298 K, 5.8 bar (discharge). Orange and purple circles represent 2D-COFs and 3D-COFs, respectively. 20

Qst,fm-Qst,ff (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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15

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No. of structures

Figure 4. Difference between Qst,fm (interaction between CH4 and framework atoms) and Qst,ff (intermolecular forces between CH4 molecules) at 298 K and 65 bar. The red line represents the zero baseline. With regard to the performance ranking, four 3D-COFs and one 2D-COF rank in the top 5, as shown in Table 1. The good performance of 3D-COFs is as expected as the three dimensional developed frameworks are inclined to exhibit higher volumetric surface area than 2D-COFs. The four 3D-COFs show topologies of dia, ctn and pts. Among them, the two-fold interpenetrations in PI-COF-4 (dia-net) and 3D-Py-COF 63 (pts-net) reduce the pore sizes to an optimum region


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while increase the volumetric surface area for CH4 delivery. bor net is excluded in the top performing 3D-COFs as the bor net usually leads to 3D frameworks with relatively large pores. 64-67

The highest deliverable capacity achieved by bor net in the CoRE COFs is 120 v(STP)/v of

COF-10867 which has a LCD of 27 Å and volumetric Sacc of 1089 m2/cm3. The only 2D-COF ascended in the top 5 is ILCOF-1-AB,30 inquisitively, how ILCOF-1-AB can stand out from the other 2D-COFs? If this question can be answered, the obtained knowledge may help us to design more 2D-COFs with top performance. As CH4 delivery performance of hypothetical 3D-COFs has been explored by Martin and Haranczyk,16 in this work, we will put our attention on improving the performance of 2D-COFs. Table 1. Structure characteristics of the top 5 COFs for CH4 delivery at 298 K.

COF name

type

topology

LCD (Å)

φ

Sacc (m2/cm3)

Other Features

CH4 deliverable capacity (v(stp)/v)

PI-COF-4

3D

dia

8.2

0.76

2795

two-fold interpenetration

190

COF-102

3D

ctn

9.0

0.78

2157

189

COF-103

3D

ctn

9.7

0.80

2066

185

ILCOF-1-AB

2D

sql

11.1

0.82

2284

AB stacking with interlayer spacing of 6.7 Å

184

3D-Py-COF

3D

pts

13.5

0.84

2045

two-fold interpenetration

173

ILCOF-1-AB has a suitable LCD of 11.1 Å, and especially, as a 2D-COF, ILCOF-1-AB shows the second largest volumetric Sacc of 2284 m2/cm3 in the CoRE COFs. The advantageous preconditions instruct us to visit the pore channels of ILCOF-1-AB, as shown in Figure 5a. Commonly, 2D-COFs exhibit only one dimensional pore channels, i.e., passable in c direction but closed for CH4 molecules in a and b directions like Figure 5b shows. However, for ILCOF-1AB, the proper stacking modes and relatively large interlayer spacing together open the channel in a and b directions for CH4 molecules, making the pore channels passable in three dimensions.


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Therefore, although ILCOF-1-AB is a 2D-COF, it has a three dimensional pore channel just like 3D-COF, and this generates the much larger volumetric Sacc of ILCOF-1-AB than most other 2DCOFs.

Figure 5. Pore channels of (a) ILCOF-1-AB, (b) common 2D-COFs with narrow interlayer spacings, (c) idealized design of common 2D-COFs with enlarged interlayer spacings. (up: top view, bottom: side view). To verify the inference, as well as to explore better CH4 delivery performance, interlayer spacings of five 2D-COFs with different geometric properties are regulated for making their one dimensional pore channels passable in three dimensions, as depicted in Figure 5c. The idealized 2D-COFs are constructed by increasing the distance of interlayer spacing in the c-axis of a unit cell, while the ab plane coordinates of all the framework atoms are fixed. The selected pristine 2D-COFs have LCD ranged from 6 to 15 Å with CH4 deliverable capacity ranged from 30 to 130 v(STP)/v. As the interlayer spacings are enlarged, LCD also increases steadily (Figure 6a), while volumetric Sacc rise first then fall (Figure 6b-f). The hollow squares in Figure 6b-f represent the properties of the pristine 2D-COFs. CH4 delivery performance of the idealized 2D-COFs is all improved compared with the original ones. Especially for CTF-FUM,68 CTF-169 and ATFGCOF70 which have LCD smaller than 13 Å, their performance can be enhanced to the top level for CH4 delivery. Take ATFG-COF as an example, when the distance of interlayer is increased


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to 6.8 Å, the ATFG-COF-6.8 Å shows a CH4 deliverable capacity (also represented by DC in Figure 6d) of 204 v(STP)/v with LCD of 11.6 Å and volumetric Sacc of 2573 m2/cm3. For CTFFUM, the deliverable capacity can be enhanced to 192 v(STP)/v, which is 520% larger than the performance of original CTF-FUM. Pristine TDFP-171 and T-COF 272 respectively show LCD of 13.1 Å and 14.9 Å, the increasing volumetric Sacc resulting from enlarged interlayer spacings can help recruit more CH4 molecules, but the appendant increasing LCD are beyond the optimum region of 8~12 Å for CH4 delivery. Therefore, 2D-COFs with large pore size are not proper to be constructed into top candidates by increasing interlayer spacings. From the perspective of experimental synthesis, interlayer spacings can be enlarged by adopting three-dimensional building blocks in the assembly like CCOF-273 (see Figure S3), which is a reported 2D-COF that has an interlayer spacing of 14.9 Å, or by introducing supports to prop up the plane layered structures to get 2D-COFs with wide interlayer spacing.

6 8

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9

10

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13

1070

2415

TDFP-1

DC: 149 v(STP)/v dint: 6.8 Å LCD:13.5 Å 2 3 Sacc: 1968 m /cm

140

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13.0

3

Sacc (m /cm )

2

3

50

1020

9

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11

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13

14

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180

LCD: 11.6 Å 2 3 Sacc: 2573 m /cm

3100

2

200

DC: 204 v(STP)/v dint: 6.8 Å

3

ATFG-COF

8

2

Sacc: 2046 m /cm

LCD (Å)

Sacc (m /cm )

(d) 220

3

0

12

dint (Å)

LCD: 13.0 Å 100

13.5

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14.5

LCD (Å)

1275

(f) 180

2560

TCOF 2


160

LCD: 15.9 Å 2 3 Sacc: 2066 m /cm

140

2

6


4

50

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2

LCD: 11.6 Å 2 3 Sacc: 2192 m /cm


150

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3150

CTF-1

Sacc (m /cm )

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(c) 200 Deliverable capacity (v(STP)/v)

12

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3380

CTF-FUM

Sacc (m /cm )

16

LCD (Å)

(b) 200

TCOF 2 TDFP-1 ATFG-COF CTF-1 CTF-fuma

18

Sacc (m /cm )

20


(a)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


120

100 14

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LCD (Å)

Figure 6. (a) Function of the distance of interlayers (dint) and largest cavity diameter (LCD), (b-f) relationships between LCD and CH4 deliverable capacity of idealized 2D-COFs with enlarged


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interlayer spacings, scaled by volumetric Sacc. The COF names are written on the top left corner of each figure. To date, increasing number of experimental studies and computational high throughput screenings show that ~200 v(STP)/v is the limit of present porous materials (including experimental and hypothetical materials). Delicate material designs are on going for better CH4 delivery performance; however, the ARPA-E methane deliverable target is often considered too high to be reached.14, 26, 47, 74 To evaluate if the ARPA-E target is achievable by delicate material design, Gómez-Gualdrón et al. made a try and found that the distance between each adjacent methane pockets should be close to 5.3 Å to reach the 315 v(STP)/v target.23 However, it is apparent that such low distance is impossible to obtain when actual atoms are involved in the creation of porous materials. To cross the ~35 % gap from the target, current operation conditions have been proposed to be adjusted by several research groups.14,23,47 In the announcement of ARPA-E, the sorbent materials will experience low temperature (233 K) at initial stages of refuelling from a CNG station and high temperature (not higher than 358 K) for methane desorption. As the low storage temperature can increase the CH4 uptake and the high discharge temperature can increase the CH4 desorption amount, we are wondered if the performance of porous materials can be greatly enhanced if methane are adsorbed at 233 K and discharged at 358 K? Take the CoRE COFs as a class of representatives, the CH4 delivery performance are evaluated at condition of adsorption at 65 bar and 233 K and desorption at 5.8 bar and 358 K. The highest CH4 deliverable capacity is 314 v(STP)/v achieved by PI-COF-4, which almost fulfills the 315 v(STP)/v target. Other top 5 COFs are ILCOF-1-AB, 3D-Por-COF, PI-COF-5 and COF-102 with greatly improved CH4 deliverable capacities of 311, 308, 305 and 300 v(STP)/v, respectively,


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showing the possible advantages of the new operation condition if the operation costs are increased moderately. Detailed information of the top 5 COFs are shown in Table S1. Under this operation condition, the ARPA-E target is very promising to be surpassed as there are hundreds of thousands of advanced porous materials like MOFs, PPNs, future COFs, etc. Figure 7 depicts the properties of the top performed CoRE COFs that are LCD of 8~16 Å, φ of 0.75~0.90 and Qst of 10~20 kJ/mol. As the adsorption temperature is decreased to 233 K, there are 96 % CoRE COFs showing increasing intermolecular forces between CH4 molecules (Qst,ff) compared with that at 298 K, indicating that the role of Qst,ff becomes important in improving the CH4 delivery performance when decreasing the adsorption temperature (Figure S4). Similar to the situation at 298 K, COFs with large volumetric Sacc incline to exhibit better CH4 delivery performance. Notice that ILCOF-1-AB still shows a high CH4 deliverable capacity of 311 v(STP)/v, the idealized 2D-COFs with enlarged interlayer spacings designed before are reevaluated under this condition. As depicted in Figure S5, CH4 deliverable capacities of the selected 2D-COFs are largely improved. The best performance can be high to 337 v(STP)/v achieved by ATFG-COF with LCD of 13.7 Å, φ of 0.87 and volumetric Sacc of 2056 m2/cm3. The results show the effectiveness of the increased interlayer spacings in enhancing the deliverable capacity, which may be a new strategy of designing remarkable 2D-COFs for CH4 delivery.


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Figure 7. Relationship between CH4 deliverable capacity and (a) largest cavity diameter, (b) void fraction, (c) volumetric surface area, (d) isosteric heats at 65 bar and 233 K. The operation condition is 233K, 65 bar (storage) and 358 K, 5.8 bar (discharge). Blue and red circles represent 2D-COFs and 3D-COFs, respectively.

CONCLUSION In this work, CoRE COFs have been screened for ANG CH4 delivery. The highest deliverable capacity at 298 K between 65 and 5.8 bar observed is 190 v(STP)/v achieved by PI-COF-4, close to the limit of ~200 v(STP)/v found by the extended experimental work and large-scale computational screenings. COFs with large volumetric Sacc incline to exhibit better CH4 delivery performance. Besides, LCD of 8~12 Å and φ of 0.75~0.85 are also premises for achieving high deliverable capacity. In the top performing COFs, 2D-COFs are scarce as they usually have


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small volumetric Sacc. 2D ILCOF-1-AB is an exception which shows a deliverable capacity of 184 v(STP)/v. A specialty of ILCOF-1-AB is that it has a 3D pore channel like 3D-COF, and this generates the much larger volumetric Sacc of ILCOF-1-AB than most other 2D-COFs. Inspired from ILCOF-1-AB, interlayer spacings of several 2D-COFs are regulated to making their 1D pore channels passable in three dimensions. With the enlarged interlayer spacings, CH4 deliverable capacities of the 2D-COFs are all improved. Take ATFG-COF as an example, when the distance of interlayer is increased from 3.4 Å to 6.8 Å, it shows a CH4 deliverable capacity of 204 v(STP)/v with LCD of 11.6 Å and volumetric Sacc of 2573 m2/cm3. The regulation of interlayer spacings may be feasible in experiment by using three-dimensional building blocks in the assembly, or by introducing supports to prop up the plane layered structures. Deliverable capacities of CoRE COFs at condition of 233K, 65 bar (storage) and 358 K, 5.8 bar (discharge) are also evaluated. Under this condition, the highest CH4 deliverable capacity of the CoRE COFs is 314 v(STP)/v achieved by PI-COF-4; moreover, performance of the idealized 2D-COFs with enlarged interlayer spacings even can surpass the ARPA-E target. The findings and the strategy developed in this work will provide new ideas for ANG technology with high volumetric energy density, which is not limited to COFs but may also be transferrable for other nanoporous materials. For real natural gas storage that contains small amounts of other components, our work shows reference meaning for future extended studies. The absolute value of CH4 deliverable capacities may show little degree of deviation in real natural gas, and future work that considers the effect of impurities are needed to get a definite conclusion; however, the proposed material design strategy for enhancing methane storage is still instructive for the real natural gas system through precise control of the interlayer spacings.


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ASSOCIATED CONTENT Supporting Information. Validation of the simulation methods and force fields; comparison of 2D-COFs with 2D and 3D building blocks; performance of CoRE COFs and designed 2D-COFs under condition of 233K, 65 bar (storage) and 358 K, 5.8 bar (discharge). The material is available free of charge on the ACS Publications website. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; [email protected]

Author Contributions ‡ M.T. and Y.L. contributed equally to this paper. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the Natural Science Foundation of China (Nos. 21706106 and 21536001), the Natural Science Foundation of Jiangsu Normal University (16XLR011) and Priority Academic Program Development of Jiangsu Higher Education Institutions. Notes The authors declare no competing financial interest.


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