Computational and Experimental Assessment of CO2 Uptake in

Subscriber access provided by READING UNIV

Article

Computational and Experimental Assessment of CO2 Uptake in Phosphonate Monoester Metal-Organic Frameworks Benjamin S. Gelfand, Racheal P. S. Huynh, Sean P. Collins, Tom K. Woo, and George K. H. Shimizu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04108 • Publication Date (Web): 22 Nov 2017 Downloaded from http://pubs.acs.org on November 26, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26

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

Chemistry of Materials

Computational and Experimental Assessment of CO2 Uptake in Phosphonate Monoester MetalOrganic Frameworks Benjamin S. Gelfand,a Racheal P. S. Huynh,a Sean P. Collins,b Tom K. Woo*b and George K. H. Shimizu*a a

Department of Chemistry, University of Calgary,

2500 University Drive NW, Calgary, Alberta, T2N 1N4, Canada

b

Department of Chemistry, University of Ottawa, Ottawa, Ontario, K1N 6N5, Canada

Abstract

Phosphonate monoesters (PMEs) as ligands for metal organic frameworks can potentially direct topology, enhance water stability, and modify pore chemistry. Here, we show, experimentally and computationally, that not only is the ratio of phosphonate to phosphonate monoester significant but also that gas sorption depends on the distribution of the monoesters in the structure.

A

phosphonate

monoester

ligand,

1,3,5-tri(4-phosphonato)benzene-

ACS Paragon Plus Environment

1


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

Page 2 of 26

tris(monoethylester), was coordinated to copper(II) to form two different frameworks based on the same copper-phosphonate chain building units, one dense (1) and the other with an experimental surface area over 1000 m2 g-1 (CALF-33-Et3). One of the three phosphonate monoesters in CALF-33-Et3 can be hydrolyzed to make an isostructural material, CALF-33Et2H, with approximately the same surface areas but vastly superior CO2 sorption. Controlling the hydrolysis at this site allowed the partially hydrolyzed variants, CALF-33-Et3-xHx (where 0 < x < 1), to be prepared and their gas sorption studied by experiment and simulation to determine CO2 binding sites and binding energies. These results show that each PME group can impact multiple gas sorption sites meaning that clustering versus random distributions of ester groups gives very different gas uptake. Finally, an algorithm is put forward that allows the CO2 uptake of the hydrolyzed MOF to be simulated by algebraically combining the isotherms of the nonhydrolyzed and fully hydrolyzed forms. This method can be used to assess both the degrees of ester hydrolysis and the distribution of ester groups in the solid.

Introduction Metal-organic frameworks (MOFs), are a class of porous materials formed from metal atoms or clusters linked by organic ligands.1 MOFs have the potential to be used for a variety of applications,2–7 with commercial production and application for some already beginning. An advantage of most MOFs is that they are crystalline – allowing their exact structure to be determined and structure-property relationships to be studied and modelled. However, crystallography only represents an averaged picture and cannot give information on local structure, such as defects or distribution of non-stoichiometric groups. There have been reports of systems containing multiple functional groups on the same ligand core, which show that the functional groups are randomly incorporated into the structures rather than forming large


2

Page 3 of 26

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


domains of a single functional group.8,9 Knowledge of local structure will only increase in importance as MOF materials find broader application. Many MOFs use carboxylate or azolate ligation to balance strong bonding with order, though there are a variety of other coordinating groups that have been successfully employed. Phosphonates are an alternative coordinating group that have been used to make materials with excellent thermal, chemical, and hydrolytic stability.10–13 Unfortunately, in the absence of structure directing components, monophosphonates or linear diphosphonates default to dense materials, or solids without accessible voids because of the phosphonate group’s various protonation states and subsequent numerous accessible coordination modes.14–17 One way of decreasing the number of coordinative variables and directing formation of porous structures is by including a single ester on the phosphonates, a phosphonate monoester (PME), making them more akin to carboxylates in their coordination.18–21 In terms of gas sorption, PMEs have relatively weak interactions with guests, such as CO2, as the esters are typically alkyl groups that have non-specific interactions with guests. One intriguing methodology is to use the PME to direct the structure while removing the esters in situ, leaving a hydrogen phosphonate.22 Previously, we have reported a new framework composed of copper(II) and benzene-1,3,5triphosphonate monoisopropyl ester, CALF-30 (CALF = Calgary Framework). This material had Cu-PME chains linked by the trigonal core to form an ultramicroporous honeycomb structure that adsorbed CO2 but not gases with larger kinetic diameters, such as N2 or CH4.20 Here we report two materials inspired by CALF-30, where additional phenyl spacers have been added to the central benzene ring, H3L-Et3 (Scheme 1). Different synthetic conditions are employed and, despite the inorganic building units being nearly identical, 1, [Cu3(L-Et3)2]n, forms a non-porous material (and so not given a CALF designation), and CALF-33-Et3 [Cu3(L-


3


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

Page 4 of 26

Et3)2]n, a material with surface areas exceeding 1000 m2 g-1. By more subtly altering the synthetic conditions in CALF-33-Et3, it is possible to selectively hydrolyze one of the PMEs to give CALF-33-Et2H, an isostructural material but with vastly improved CO2 binding. CALF33-Et3 and its selective hydrolysis were the subject of a preliminary communication.23 Here, a combination of molecular simulations, experimental adsorption isotherms, and isotherm modelling are used to ascertain the precise binding sites of carbon dioxide in the ester and deesterified forms. The present study also examines the partially hydrolyzed analogues, CALF-33Et3-xHx (where 0 < x < 1). The results from this component of the study give unexpected results in that, for example, the 50% hydrolyzed system behaves almost like the fully esterified material for carbon dioxide capture. These results are interpreted in terms of the individual binding sites and the effect on gas sorption of the distribution of functional groups in the MOF.

Scheme 1. Procedure for synthesizing H3L-Et3, beginning with an acid-catalyzed trimerization of 4’-bromoacetaphenone to the tribromo- precursor, then a nickel-catalyzed Arbuzov reaction to the tris(phosphonate diethyl ester), and finally hydrolysis to the tris(phosphonate monoethyl ester).


4

Page 5 of 26

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


Results and Discussion The preparation of the two polymorphs, compounds 1 and CALF-33-Et3, are similar. Under solvothermal conditions (48 hours at 120⁰C in ethanol/water-identical to that of the triphosphonatobenzene analogue, CALF-30), the dense structure 1 can be synthesized as large blue crystals. 1 crystallizes in the P21/n space group with one ligand and one and a half copper atoms per asymmetric unit (Figure S1). The overall structure can be described as two different types of inorganic columns bridged by the

1,3,5-triphenylbenzene

cores

of

the

phosphonate monoester ligands. The first inorganic column in 1 has a ladder-like structure where Cu1 adopts a Jahn-Teller Fig. 1 Net structures of 1 in (a) the ball-and-stick model

distorted square pyramidal geometry (Cu-Oeq. =

and (b) space-filling model along with (c) the distorted

1.915(4)-1.962(4) Å, ∑Oeq-Cu-Oeq = 364.49⁰, square pyramidal copper-phosphonate chains and (d) square planar copper-phosphonate chains, which act as

Cu-Oaxial =2.338(4) Å, ∠ave.Oeq-Cu-Oax =

secondary building units. H, C, O, P, and Cu atoms are

95.48⁰). The second inorganic column has

beige, grey, red, pink, and blue, respectively. For clarity,

Cu2, which is half the occupancy of Cu1, with

H atoms have been omitted in (a), (c), and (d) as well as

a square planar geometry (Cu-O = 1.917(5)-

the esters in (c) and (d).

1.933(5) Å, ∑∠O-Cu-O = 360.00⁰) (Fig. S3).


5


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

Page 6 of 26

Each ligand has three inequivalent phosphonate esters with different bridging modes. According to Harris’ notation,24 P1 and P2 each adopt a 2.110 coordination mode and P3 adopts a 3.210 coordination mode, where each uncoordinated oxygen atom is the ester (Fig. S4-S5). Along the a axis (Fig. 1a and b), the square pyramidal Cu1-PME chains bridge four ligands per inorganic unit (Fig. 1c) while the square planar Cu2-PME chains (Fig. 1d) bridge two ligands. The average aryl plane is approximately perpendicular to the square planar copper-PME chains at 80.295⁰ (Fig. S9). There is no apparent porosity in 1 as the crystal structure shows no solvent accessible voids. When probing alternative synthetic conditions, another polymorphic crystalline phase was found. The addition of 1,3-diisopropylbenzene to an otherwise identical preparation results in the formation of CALF-33 (Fig. 2). CALF-33 crystallizes in the P1ത space group also with one ligand and one and a half copper atoms per asymmetric unit (Fig. S6). Again, two types of inorganic columns are observed analogous to 1 and they show isomorphous structural features. Cu1 adopts a distorted Jahn-Teller square pyramidal coordination geometry (Cu-Oeq. = 1.914(4)1.965(5) Å, ∑Oeq-Cu-Oeq = 362.38⁰, Cu-Oaxial =2.319(4) Å, ∠ave.Oeq-Cu-Oax = 95.44⁰) and the half occupied Cu2 adopts a square planar coordination geometry (Cu-O = 1.910(5)-1.912(5) Å, ∑∠O-Cu-O = 360.00⁰) (Fig. S7). Each ligand has three symmetrically inequivalent phosphonates; according to Harris’ notation, P1 and P2 adopt a 2.110 coordination mode and P3 adopts a 3.210 coordination mode (Fig. S4 and S8),23 where each uncoordinated oxygen atom has the ethyl ester attached or is protonated. The net structure (Fig. 2a and b) consists of square pyramidal Cu1-PME (Fig. 2c) chains and square planar Cu2-PME chains (Fig. 2d) running parallel to the a axis while the average aryl plane is approximately parallel to the square planar copper-PME chains at 0.973⁰ (Fig. S9). The overall structure is an open framework with one


6

Page 7 of 26

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


dimensional

channels

measuring

approximately 16.1 Å ൈ 7.2 Å, including van der Waals radii. In analyzing the single

crystal

of

CALF-33,

when

examining the residual electron density around the phosphonate oxygen atoms, it was only possible to locate two of the three ethyl ester groups. Initially, this was assumed to be because of a highly fluxional and disordered ester but was later determined, via 31P NMR, to be a result of a selective in situ hydrolysis of one ester position.24 Despite many attempts, it has not yet been possible to grow single Fig. 2 Net structures of CALF-33: (a) ball-and-stick model;

crystals of the full ethyl ester form of

(b) space-filling model; (c) the distorted square pyramidal

CALF-33.When comparing the structures

copper-phosphonate chains; (d) square planar copper-

of 1 and CALF-33, it is evident that the

phosphonate chains, which act as secondary building units. H,

two

materials

are

similar

in

their

C, O, P, and Cu atoms are beige, grey, red, pink, and blue, respectively. For clarity, H atoms have been omitted in (a),

composition and building units but adopt

(c), and (d) as well as the esters in (c) and (d). The absent

very different topologies. A summary of

ester has been highlighted by an orange circle in (a).

the structural similarities and differences

can be found in Table 1. Both materials have one ligand and one and a half copper atoms per asymmetric unit. Each ligand has three unique phosphonates – two adopt a 2.110 coordination


7


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

Page 8 of 26

mode and the third adopts a 3.210 coordination mode (Fig. S4). Each structure also contains two different copper atoms, one in a square planar geometry and the other in a distorted square pyramidal geometry (Fig. S3 and S7). This combination of phosphonate modes and copper coordination geometries results in two distinct building units, a square pyramidal copperphosphonate chain (Fig. 1c, 2c) and a square planar copper-phosphonate chain (Fig. 1d and 2d).


8

Page 9 of 26

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


Table 1 Structural comparison of 1 and CALF-33. 1

CALF-33-Et3†

[Cu3(L-Et3)2]

[Cu3(L-Et3)2]

Monoclinic

Triclinic

P21/n

P1ത

a, b, c (Å)

5.039(1), 27.642(6), 23.662(5)

5.0813(1), 15.1585(3), 29.3246(7)

α, β, γ (⁰ )

90, 94.95(3), 90

83.882(1), 85.728(1), 82.705(2)

3283.54(11)

2223.42(8)

0

0.118

1.462

1.080

2.110, 2.110, 3.210

2.110, 2.110, 3.210

Jahn-Teller square pyramidal

Jahn-Teller square pyramidal

1.915(4)-1.962(4)

1.914(4)-1.965(4)

364.49

362.38

2.338(4)

2.319(4)

95.48

95.44

Geometry

Square planar

Square planar

Cu-O (Å)

1.917(5)-1.933(5)

1.910(5)-1.912(5)

360.00

360.00

80.295

0.973

8.151, 83.971,

109.367, 179.835,

Unit cell formula Crystal system Space group

V (Å3) §

Fractional porosity

-3

Evacuated density (g cm ) Ar-PO3R coordination modes Geometry Cu-Oeq. (Å) Cu1

∑∠Oeq-Cu-Oeq. (⁰ ) Cu-Oaxial (Å) ∠ave.Oeq-Cu-Oaxial (⁰ )

Cu2

∑∠O-Cu-O (⁰ ) angle between the inorganic ribbon and organic layers (⁰ ) C-P-O-Cusquare planar torsion (⁰ )

-83.971, -8.151 -179.835, -109.367 This is for the structure of CALF-33 with the ethyl ester added in silico30 for a more consistent comparison. § Calculated from the solvent accessible surface area with a probe radius of 1.8 Å (approximating N2).30

†

Both 1 and CALF-33 can be described as the linking of dimeric Cu1 columns and monomeric Cu2 columns. The Cu1 columns are bridged by two PO3 groups of L in one direction and then these assemblies are further linked to the monomeric Cu2 column by the remaining PO3 group on each ligand. In Figures 2 and 4, the general shape of each inorganic column with respect to the protruding ligands can be seen. For both 1 and CALF-33, the dimeric Cu1 columns adopt a


9


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

Page 10 of 26

pinwheel-like shape and the monomeric Cu2 columns have ligands extending in a linear fashion. However, whereas in CALF-33, the Cu1 columns are doubly bridged by L molecules to form pores, in 1, the Cu1 columns are only singly bridged by L molecules (Fig. 3). The remaining coordination is occupied by L molecules bridging to the Cu2 column, effectively filling the pore that exists in CALF-33.

Fig. 3

Simplified nets found in 1 (left) and CALF-33 (right). Aryl rings, dimeric Cu1 columns and monomeric

Cu2 columns are represented by grey, green, and orange, respectively.

The ester group plays a vital role in the formation of both 1 and CALF-33. In the absence of the esters (i.e. the triphosphonic acid as ligand), a poorly crystalline material is formed in analogous synthetic conditions to both 1 and CALF-33 (Fig. S10). The ester restricts the protonation states that are possible and likely also acts to kinetically slow the self-assembly. As previously reported with simple diphosphonate coordination, using PMEs rather than phosphonic acids results in chains rather than layers as building units,

18,20,25

which allows, with appropriate

guest molecules, for the highly porous CALF-33 to be formed. The preparations of dense 1 and porous CALF-33 differ only by the addition of 1,3-diisopropylbenzene. In this system, 1,3diisopropylbenzene can act as a structure directing agent in a few different ways:26 (1) it can act


10

Page 11 of 26

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


as a filler, having a stabilizing energetic contribution to make the less dense structure, CALF-33, favorable; (2) it can pre-organize the constituent species, such as allowing L-Et3 dimers or oligomers to nucleate in the more open arrangement by providing more hydrophobic groups with which the ligand could interact rather than the polar solvents present; or (3) it can act as a template where L-Et33- is better solubilized by additional C-H···π interactions, which results in the formation of the 1-D channels occupied by solvent molecules.27 Though the exact nature of this structure directing agent has not been confirmed, it is possible that these different effects are happening in conjunction. By subtle variation of the synthetic conditions, CALF-33 can be made with different and controllable levels of ester hydrolysis at the susceptible site (on Cu2, circled in Fig. 2a). Although the reason why the hydrolysis occurs at just this one site is not fully understood, the hydrolysis occurs in solution followed by incorporation into the framework. Hydrolysis of the ester with retention of the network does not occur in any condition tested (see Fig. S11 and S12 and the supporting information). Given the voids evident in CALF-33, both the unhydrolyzed (CALF-33-Et3) and monohydrolyzed (CALF-33-Et2H) frameworks were synthesized to determine the impact of hydrolysis on the structure. Powder x-ray diffraction (PXRD) (Fig. S13) confirms that the unhydrolyzed and monohydrolyzed frameworks form isostructural materials by comparing with the simulated pattern from crystallography. As the two bookend structures have been previously communicated, their sorption behavior will only be summarized here prior to presenting more detailed analysis of the system.24 From N2 sorption at 77K (Fig. S14), CALF-33-Et3 has a Langmuir surface area of 1017 m2/g (BET (Brunauer–Emmett–Teller) = 842 m2/g), which compares well to the simulated surface area range of 916 – 1021 m2/g (1.8 Å probe radius); the


11


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

Page 12 of 26

range arises as uptake depends on the orientation of the esters. Despite the high surface area, CALF-33-Et3 has a low CO2 uptake of 0.92 mmol/g at 278K and 1200 mbar (Fig. S17). From N2 sorption at 77K (Fig. S18), CALF-33-Et2H has a Langmuir surface area of 950 m2/g (BET = 810 m2/g), corresponding well to the simulated surface area of 969 m2/g (1.8 Å probe radius). Hydrolysis of a single ester results in nearly double the CO2 uptake to 1.83 mmol/g at 278K and 1200 mbar (Fig. S21). This outcome is congruent with the removal of a hydrophobic group and a new surface with an acidic site as well as possibly more exposed metal sites. To probe water stability, a freshly activated sample of CALF-33-Et3 was exposed to water, yielding a dramatic structural change by PXRD (Fig. S22). Even exposure to ambient air caused a structural change and decrease in Langmuir surface area of 287 m2/g (Fig. S22 and S23), giving CALF-33-Et3, a hydrolytic stability classification of 1C.28


12

Page 13 of 26

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


After observing the changes that full ester hydrolysis at the Cu2 site has on gas sorption properties, experiments were performed to determine the effect of partial hydrolysis at the Cu2 site. By controlling the water content in the synthesis Fig. 4 (a) Considering the ethyl ester as freely rotating, it is

of

CALF-33,

intermediate

amounts of ester hydrolysis at the

confined to a cone in CALF-33-Et3. (b) The cone radius is

susceptible

~2.2 Å (dark grey circle) and ~4.2 Å (lighter grey circle),

CALF-33-Et3-xHx, 0 < x < 1). Initially,

excluding and including van der Waal radii, respectively. (c)

site

are

possible

giving

CALF-33-Et2.53H0.47 was synthesized and

Despite the ester cones’ volume, they are not sufficiently

structural retention confirmed (Fig. S25 large to block a hydrogen (darker beige and lighter beige are without and with van der Waal radii, respectively. C, O, P,

and S26). N2 sorption at 77K (Fig. S40)

and Cu atoms are grey, red, pink, and blue, respectively. H

indicates that CALF-33-Et2.53H0.47 has a

atoms are omitted for clarity.

Langmuir surface area of 1076 m2/g (BET

= 895m2/g), larger than the surface areas observed for either the full ester or monohydrolyzed materials. This increased surface area could be a result of two factors: 1) additional pore texturing, whereby the presence of some ethyl esters causes additional surfaces for adsorption as they protrude into the pore, and; 2) the lower molecular weight caused by losing some ethyl ester, when compared to CALF-33-Et3. While a 50% hydrolyzed material may have been expected to give CO2 uptake halfway between CALF-33-Et3 and CALF-33-Et2H. Surprisingly, the CO2 sorption for CALF-33-Et2.53H0.47 was only slightly higher than the unhydrolyzed analogue. Further study was needed to understand this phenomenon.


13


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

Page 14 of 26

Previously, it had been assumed that the increased CO2 uptake in CALF-33-Et2H over CALF33-Et3 was a result of Lewis acid/base interactions, involving the hydrogen phosphonate and/or the Cu center in the hydrolyzed materials. In a situation where half the ethyl esters are hydrolyzed, there may be a potential for the remaining ethyl esters to still block the hydrogen phosphonate group from interacting with CO2. A simple geometrical rotation of the ethyl esters about the C-O single bond (Fig. 4a) shows that they have a maximum lateral radius of approximately 4.8 Å, including van der Waals radii (Fig. 4b).29 However, based on this simple model, the ethyl esters are not sufficiently large to block Lewis acid/base interactions between the hydrogen phosphonate on the framework and CO2 even if the hydrogen phosphonate has ethyl esters on both sides (Fig. 4c).

At this point, molecular simulations were performed to elucidate the interactions that allow CALF-33-Et2H to perform much better than its unhydrolyzed analogue and give insight towards developing a model to explain the results from the partially hydrolyzed materials.29 GCMC (Grand Canonical Monte Carlo) simulations of CO2 adsorption in each material were performed at 0.15-1.2 bar, 298 K and the binding sites localized from analyzing the resulting probability distributions29. Full details about the GCMC simulations are given in the supporting information. The CO2 uptake of CALF-33-Et3 and CALF-33-Et2H at 1 bar and 278 K were found to be 1.04 mmol/g and 1.70 mmol/g, respectively, in reasonable agreement with experiment. Importantly, simulations reproduce the large increase in CO2 uptake upon hydrolysis of a single ester. In the hydrolyzed material, CALF-33-Et2H, a single dominant CO2 binding site was identified that runs nearly parallel to the coordination plane of the Cu atoms as shown in Fig. 5a. In this binding site, one of the CO2 O atoms was within 2.71 Å of the Cu atom, while the carbon


14

Page 15 of 26

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


atom of the CO2 was found to be within 2.90 Å of the oxygen atom of the hydrolyzed ester group. The binding site was found to have a CO2 binding energy of 30.5 kJ/mol, where 45% of the net attractive interaction can be attributed to electrostatics with the remainder be due to dispersion. The large electrostatic component of the interaction is consistent with the OCO2-Cu and CCO2-OMOF distances identified.

In the material with no hydrolysis, CALF-33Et3, two unique binding sites were identified as shown in Figure 5b. In both binding sites, the CO2 lies in the pockets formed by hydrophobic aryl groups and the ethyl groups of the esters. There are no significant interactions between the Cu and the CO2 guest molecules with the closest Cu-OCO2 distance being 6.82 Å. The binding sites were found to have binding energies of 24.8 and 20.1 kJ/mol, which are notably lower than the binding energies found in CALF-33-Et2H. Interestingly, these two binding sites are dominated by Fig. 5 CO2 binding sites identified in a) CALF-33-

dispersion interactions with only ~8% or less of

Et2H and b) CALF-33-Et3 from simulation. H, C, O, P, Cu atoms are beige, grey, red, pink, and blue,

the net binding coming from electrostatics. These

respectively. In a) CO2 and atoms interacting with it

results suggest that the ethyl ester in CALF-33-

are shown as ball and stick representation. Aryl H

Et3 provides enough steric hindrance to prevent

atoms are omitted for clarity.

CO2 from strongly interacting with the Cu center


15


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

Page 16 of 26

or the bridging oxygen atoms, as compared to in the hydrolyzed structure CALF-33-Et2H, which in turn leads to weaker CO2 binding in CALF-33-Et3. To examine the partially hydrolyzed structure CALF-33-Et2.53H0.47, we created a structural model in which the PMEs and hydrogen phosphonates alternate along the square planar copperphosphonate building units. Most interestingly, this model for CALF-33-Et2.53H0.47 also showed a CO2 uptake comparable to CALF-33-Et3 as was observed experimentally. Analysis of the binding sites (Fig. S28) identified from the GCMC simulations also show no direct CO2-Cu interactions (O···Cu > 3 Å). Instead, CO2 primarily maximizes non-specific, dispersion dominated interactions in a similar manner to that observed for CALF-33-Et3. A simple predictive model to gauge the distribution of the esters based on CO2 sorption was sought. As the clustered versus isolated phosphonate ester groups impact CO2 sorption differently, it was hypothesized that by making a linear combination of the CO2 isotherms of CALF-33-Et3 and CALF-33-Et2H could give insight to the extent of clustering (Equation 1), where ܰ(‫݌‬, ܶ) is the adsorption of the framework at a given temperature and pressure and ܽ is the contribution of CALF-33-Et3. ܰ௠௜௫௘ௗ = (ܽ)ܰ஼஺௅ிିଷଷିா௧మ ு + (1 − ܽ)ܰ஼஺௅ிିଷଷିா௧య

(1)

There are four possible environments with respect to the adjacent potentially open copper sites and the ester groups (Fig. 6): (a) PME + PME, (b) PO3H + PME, (c) PME + PO3H, or (d) PO3H + PO3H. Based on these four environments, it is possible to propose the distribution of PMEs and hydrogen phosphonates. If a clustered distribution is assumed (i.e.: large sections of PMEs and large sections of hydrogen phosphonates), then the probability for (a), (b), (c), and (d) would be 1 − ‫ݔ‬, ~0, ~0, and ‫ݔ‬, respectively, where ‫ ݔ‬is the portion of hydrolyzed sites. Conversely, if a random distribution of PMEs and hydrogen phosphonates is assumed, then the probability for


16

Page 17 of 26

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


(a), (b), (c), and (d) would be (1 − ‫)ݔ‬ଶ , ‫ ݔ‬− ‫ ݔ‬ଶ (from x(1-x)), ‫ ݔ‬− ‫ ݔ‬ଶ , and ‫ ݔ‬ଶ , respectively, again where ‫ ݔ‬is the portion of hydrolyzed sites. In the case of a random distribution, it was shown previously that (b) and (c), those copper sites surrounded by a single PME and a single hydrogen phosphonate, behave similarly to (a) – PME+PME, so the probabilities can be simplified to 1 − ‫ ݔ‬ଶ and ‫ ݔ‬ଶ for (a) and (d). Thus, having an experimental isotherm for a partially hydrolyzed sample, one can gain insights to the extent of hydrolysis and/or the distribution of the esters by combining the equations of the isotherms with appropriate weightings to match the experimental data. That is, it is possible to simulate the isotherm by a fractional combination of gas sorption isotherms (FCI). Using Equation 1 and the Fig. 6 A statistical distribution of different environments

assumptions

outlined

above,

it

becomes

around a Cu center in partially hydrolyzed CALF-33 structures. H, C, O, P, Cu atoms are beige, grey, red,

possible to differentiate between clustered

pink, and blue. The large spheres indicate the space that

(where ܽ = ‫ )ݔ‬and random distribution (where

ethyl or protons can occupy, including van der Waals

ܽ = ‫ ݔ‬ଶ ) using Equations 2 and 3, respectively.

radii.

ܰ௠௜௫௘ௗ = (‫ܰ)ݔ‬஼஺௅ிିଷଷିா௧మ ு + (1 − ‫ܰ)ݔ‬஼஺௅ிିଷଷିா௧య (2)


17


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

Page 18 of 26

ܰ௠௜௫௘ௗ = (‫ ݔ‬ଶ )ܰ஼஺௅ிିଷଷିா௧మ ு + (1 − ‫ ݔ‬ଶ )ܰ஼஺௅ிିଷଷିா௧య Based

on

these

(3) equations

and

assumptions, if the degree of hydrolysis is known, the differences between clustered

and

random

distribution

should be easily identified (see Fig. 7 and 8). Conversely, if assumptions can be made about the degree of clustering, Fig. 7 Experimental 278K CO2 sorption isotherms for CALF-33-

then this method could be used to

Et2H (purple), CALF-33-Et3 (orange) and mech-CALF-33Et2.71H0.29 (black x), the standard employed for clustered ester

determine the extent of hydrolysis. This

distribution. FCI generated isotherms are shown for random

may be much more useful in systems

distributions

which are more robust and NMR study

(green)

and

clustered

distributions

(blue),

confirming the match with the clustered standard. FCI calculations used ‫ = ݔ‬0.29 as determined from NMR analysis, for

of degraded solids is not an option. As a proof of concept, a mechanical

the random distribution.

mixture of CALF-33-Et3 and CALF33-Et2H was made by selecting arbitrary amounts of the two solids. CO2 isotherms were collected for this sample at four different temperatures. As, in this experiment, the ester groups are known to be in the clustered form, Equation 2 was employed and ‫ ݔ‬varied until a curve of best fit was obtained for isotherms at four different temperatures. Figure 7 shows the outcome of this fitting for one temperature. Considering the data for all four temperatures (Fig. 7 and S31S33) a degree of hydrolysis of 0.26 ± 0.02 is obtained from FCI corresponding to a composition of CALF-33-Et2.74H0.26. NMR studies on the degraded solid sample gave ‫ = ݔ‬0.29 ± 0.02 (Fig. S26), a result in good agreement. As a separate check, Figure 7 also shows the curve estimated by the random model, Equation 3, which clearly does not fit. A similar clustered control test was


18

Page 19 of 26

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


performed with CALF-33-Et2.90H0.10, which gives the hydrolysis amount to be 0.10 ± 0.01 for both FCI and NMR (see Fig. S38-41). Thus, this validated that the clustered FCI model gave predictions that fit clustered experimental data and that the fittings also matched the extent of hydrolysis. The FCI method was then applied to a sample hydrolyzed in situ, a procedure that would

presumably

generate

randomly

distributed ester groups. In this experiment, the solid was degraded and the degree of hydrolysis measured by NMR. NMR gave a degree of hydrolysis of 0.47±0.03. FCI was Figure 8 Experimental 278K CO2 sorption isotherms for CALF-33-Et2H (purple), CALF-33-Et3 (orange) and

performed on CALF-33-Et2.53H0.47 at four temperatures (Fig. 8 and S49-S51). Curves

CALF-33-Et2.53H0.47 (black x), the standard employed for

of best fit with the random hydrolysis model

clustered ester distribution. FCI generated isotherms are

estimated the degree of hydrolysis as 0.43 ±

shown for random distributions (green) and clustered

0.02. As further confirmation, a second

distributions (blue), confirming the match with the random distribution. FCI calculations used ‫ = ݔ‬0.47, as determined from NMR analysis, for the clustered distribution.

sample

with

a

different

amount

of

hydrolysis, CALF-33-Et2.09H0.91, was also synthesized and characterized. For CALF-

33-Et2.09H0.91, FCI calculations give the random distribution of hydrolysis to be 0.88 ± 0.01 (Fig. S55-S58), which matches very well to the 0.91 ± 0.05 from 31P NMR. There have been notable examples of mixed functional group MOFs in the past.8,9 These frameworks have been characterized both crystallographically and by NMR, similar to the


19


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

Page 20 of 26

methods here. In those reports, varying not just the types of functional groups but also their distributions were shown to drastically affect the materials’ selectivity over the single component parent frameworks in a synergistic manner. The inorganic building units in CALF-33 are similar to those in many phosphonate ester MOF materials in that they are 1-D chains. The present study presents a method to assess both the extent of hydrolysis of phosphonate ester groups and also their distribution in the MOFs if pure isotherms of the non- and fully hydrolyzed forms are accessible. The present results show that the FCI method will be most valuable in systems that are insoluble (excluding NMR determination of hydrolysis) and/or amorphous. Moreover, the potential for FCI extends beyond phosphonate ester MOFs to larger classes of mixed porous solids.

Conclusions Here, two new frameworks have been synthesized from a trigonal planar phosphonate monoester and Cu(II). 1 possesses no porosity while CALF-33-Et3 has a Langmuir surface area of over 1000 m2 g-1, despite being composed of nearly identical building units. In CALF-33-Et3, one ester per ligand can be hydrolyzed, resulting in an isostructural material (CALF-33-Et2H) with approximately double the uptake of CO2 at ambient conditions. Simulations are employed to determine the binding sites of CO2 and understand the effect of the hydrolysis. Finally, a methodology involving the fractional combination of sorption isotherms was developed to determine the distribution of hydrolysis in CALF-33-Et3-xHx (where x = the amount of hydrolysis). These results confirmed that random and clustered distributions give very different gas uptake and determined the in situ hydrolysis to be random in nature.


20

Page 21 of 26

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


ASSOCIATED CONTENT Supporting Information. Synthetic details for 1 and CALF-33. NMR of hydrolysis experiments. CIF files, PXRD data and gas sorption analyses for all studies on partially hydrolyzed samples.

AUTHOR INFORMATION Corresponding Authors *[email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources We thank Alberta Innovates Technology Futures for a Strategic Grant and the Natural Sciences and Engineering Research Council (NSERC) of Canada for funding the CREATE Carbon Capture Initiative and an Alexander Graham Bell Canada Graduate Scholarship to BSG.

ABBREVIATIONS BET, Brunauer–Emmett–Teller; CALF, Calgary Framework; MOF, metal-organic framework; PME, phosphonate monoester; FCI, Fractional Combination of Isotherms; GCMC, Grand


21


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

Page 22 of 26

Canonical Monte Carlo; PXRD, powder X-ray diffraction; NMR, nuclear magnetic resonance; CIF, crystallographic information file

References (1)

Batten, S. R.; Champness, N. R.; Chen, X.-M.; Garcia-Martinez, J.; Kitagawa, S.; Öhrström, L.; O’Keeffe, M.; Suh, M. P.; Reedijk, J. Terminology of Metal-Organic Frameworks and Coordination Polymers (IUPAC Provisional Recommendation); Research Triangle Park, NC, 2013.

(2)

Sumida, K.; Rogow, D. L.; Mason, J. a; McDonald, T. M.; Bloch, E. D.; Herm, Z. R.; Bae, T.-H.; Long, J. R. Carbon dioxide capture in metal-organic frameworks. Chem. Rev. 2012, 112 (2), 724–781.

(3)

Suh, M. P.; Park, H. J.; Prasad, T. K.; Lim, D.-W. Hydrogen Storage in Metal?Organic Frameworks. Chem. Rev. 2012, 112 (2), 782–835.

(4)

Rieter, W. J.; Taylor, K. M. L.; An, H.; Lin, W.; Lin, W. Nanoscale Metal−Organic Frameworks as Potential Multimodal Contrast Enhancing Agents. J. Am. Chem. Soc. 2006, 128 (28), 9024–9025.

(5)

Shekhah, O.; Liu, J.; Fischer, R. a; Wöll, C. MOF thin films: existing and future applications. Chem. Soc. Rev. 2011, 40 (2), 1081–1106.

(6)

Ma, L.; Lin, W. Designing metal-organic frameworks for catalytic applications. Top. Curr. Chem. 2010, No. 293, 175–205.


22

Page 23 of 26

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


(7)

Ramaswamy, P.; Wong, N. E.; Shimizu, G. K. H. MOFs as proton conductors – challenges and opportunities. Chem. Soc. Rev. 2014, 43 (16), 5913–5932.

(8)

Deng, H.; Doonan, C. J.; Furukawa, H.; Ferreira, R. B.; Towne, J.; Knobler, C. B.; Wang, B.; Yaghi, O. M. Multiple Functional Groups of Varying Ratios in Metal-Organic Frameworks. Science (80-. ). 2010, 327 (5967), 846–850.

(9)

Zhang, Y. B.; Furukawa, H.; Ko, N.; Nie, W.; Park, H. J.; Okajima, S.; Cordova, K. E.; Deng, H.; Kim, J.; Yaghi, O. M. Introduction of functionality, selection of topology, and enhancement of gas adsorption in multivariate metal-organic framework-177. J. Am. Chem. Soc. 2015, 137 (7), 2641–2650.

(10)

Wharmby, M. T.; Mowat, J. P. S.; Thompson, S. P.; Wright, P. A. Extending the pore size of crystalline metal phosphonates toward the mesoporous regime by isoreticular synthesis. J. Am. Chem. Soc. 2011, 133 (5), 1266–1269.

(11)

Clearfield, A.; Demadis, K. D. Metal Phosphonate Chemistry; Clearfield, A., Demadis, K., Eds.; Royal Society of Chemistry: Cambridge, 2011.

(12)

Mah, R. K.; Gelfand, B. S.; Taylor, J. M.; Shimizu, G. K. H. Reconciling order, stability, and porosity in phosphonate metal–organic frameworks via HF-mediated synthesis. Inorg. Chem. Front. 2015, 2 (3), 273–277.

(13)

Hermer, N.; Stock, N. The new triazine-based porous copper phosphonate [Cu 3 (PPT)(H 2 O) 3 ]·10H 2 O. Dalt. Trans. 2015, 44 (8), 3720–3723.

(14)

Poojary, D. M.; Zhang, B.; Clearfield, A. Synthesis and structures of barium arylbisphosphonates derived from X-ray powder data. An. química 1998, 94 (6), 401–405.


23


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

(15)

Page 24 of 26

Poojary, D. M.; Zhang, B.; Bellinghausen, P.; Clearfield, A. Synthesis and X-ray Powder Structures of Two Lamellar Copper Arylenebis(phosphonates). Inorg. Chem. 1996, 35 (17), 4942–4949.

(16)

Poojary, D. M.; Zhang, B.; Bellinghausen, P.; Clearfield, A. Synthesis and X-ray Powder Structures of Covalently Pillared Lamellar Zinc Bis(phosphonates). Inorg. Chem. 1996, 35 (18), 5254–5263.

(17)

Taddei, M.; Costantino, F.; Vivani, R.; Sabatini, S.; Lim, S.-H.; Cohen, S. M. The use of a rigid tritopic phosphonic ligand for the synthesis of a robust honeycomb-like layered zirconium phosphonate framework. Chem. Commun. 2014, 50 (43), 5737–5740.

(18)

Iremonger, S. S.; Liang, J.; Vaidhyanathan, R.; Martens, I.; Shimizu, G. K. H.; Daff, T. D.; Aghaji, M. Z.; Yeganegi, S.; Woo, T. K. Phosphonate monoesters as carboxylate-like linkers for metal organic frameworks. J. Am. Chem. Soc. 2011, 133 (50), 20048–20051.

(19)

Iremonger, S. S.; Liang, J.; Vaidhyanathan, R.; Shimizu, G. K. H. A permanently porous van der Waals solid by using phosphonate monoester linkers in a metal organic framework. Chem. Commun. (Camb). 2011, 47 (15), 4430–4432.

(20)

Gelfand, B. S.; Lin, J. Bin; Shimizu, G. K. H. Design of a Humidity-stable metal-organic framework using a phosphonate monoester ligand. Inorg. Chem. 2015, 54 (4), 1185–1187.

(21)

Zhang, J.-W.; Zhao, C.-C.; Zhao, Y.-P.; Xu, H.-Q.; Du, Z.-Y.; Jiang, H.-L. Metal–organic frameworks with improved moisture stability based on a phosphonate monoester: effect of auxiliary N-donor ligands on framework dimensionality. CrystEngComm 2014, 16 (29), 6635–6644.


24

Page 25 of 26

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


(22)

Yamada, T.; Kitagawa, H. Syntheses of metal–organic frameworks with protected phosphonate ligands. CrystEngComm 2012, 14 (12), 4148–4152.

(23) Gelfand, B. S.; Huynh, R. P. S.; Mah, R. K.; Shimizu, G. K. H. Mediating Order and Modulating Porosity by Controlled Hydrolysis in a Phosphonate Monoester MetalOrganic Framework. Angew. Chem., Int. Ed. 2016, 55 (47), 14614–14617. (24)

Harris, S. G. Crystallographic and modelling studies of organic ligands on metal surfaces, Ph.D. Thesis, The University of Edinburgh, 1999.

(25)

Gelfand, B. S.; Taylor, J. M.; Shimizu, G. K. H. Extracting structural trends from systematic variation of phosphonate/phosphonate monoester coordination polymers. CrystEngComm 2017, 19 (27), 3727–3736.

(26)

Davis, B. H.; Sing, K. S. W. Handbook of Porous Solids - Volume 1. In Handbook of Porous Solids - Volume 1; Schüth, F., Sing, K. S. W., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, Germany, 2002.

(27)

Jorgensen, W. L.; Severance, D. L. Aromatic-Aromatic Interactions: Free Energy Profiles for the Benzene Dimer in Water, Chloroform, and Liquid Benzene. J. Am. Chem. Soc. 1990, 112 (13), 4768–4774.

(28)

Gelfand, B. S.; Shimizu, G. K. H. Parameterizing and grading hydrolytic stability in metal–organic frameworks. Dalt. Trans. 2016, 45 (9), 3668–3678.

(29)

See supporting information for full details.


25


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

(30)

Page 26 of 26

Accelrys Software Inc. Materials Studio v. 6.0.

Graphic for the ToC:


26

Computational and Experimental Assessment of CO2 Uptake in

Recommend Documents