High Adsorption Capacities and Two-Step Adsorption of Polar

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High Adsorption Capacities and Two-Step Adsorption of Polar Adsorbates on Cu-BTC Metal-Organic Framework Tom R.C. Van Assche, Tim Duerinck, Juan Jose GutierrezSevillano, Sofia Calero, Gino V. Baron, and Joeri F.M. Denayer J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp405509m • Publication Date (Web): 05 Aug 2013 Downloaded from http://pubs.acs.org on August 8, 2013

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The Journal of Physical Chemistry

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High Adsorption Capacities and Two-Step

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Adsorption of Polar Adsorbates on Cu-BTC Metal-

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Organic Framework

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Tom R.C. Van Assche a, Tim Duerinck a, Juan José Gutiérrez Sevillano b, Sofia Calero b, Gino V.

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Baron a, Joeri F.M. Denayer a,*

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a Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, Brussel 1050,

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Belgium

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b Department of Physical, Chemical and Natural Systems, University Pablo de Olavide, Ctra.

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Utrera Km. 1, Seville 41013, Spain

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ACS Paragon Plus Environment

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KEYWORDS: HKUST-1, MOF, diffusion, methanol

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ABSTRACT

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This work discusses the adsorption of polar and apolar molecules on the Cu-BTC metal-organic

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framework. Vapor phase adsorption isotherms of various polar adsorbates such as methanol,

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ethanol,

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tetrahydrofuran and N,N-dimethylformamide, as well as some apolar adsorbates as n-hexane, n-

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heptane, m-xylene and cyclohexane on the Cu-BTC framework are presented. We show that

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water exposure of the Cu-BTC framework has an adverse effect on the uptake capacity.

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However, with minimized water exposure, we find high adsorption capacities, exceeding 0.65

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cc/g for all adsorbates with exception of water, and show that small polar adsorbates exhibit a

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two-step adsorption behavior. This behavior is further studied using molecular simulation and

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proposed to occur due to the presence of the various Cu-BTC cages. The cages containing the

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exposed coordinatively unsaturated copper sites have a more polar character, whilst the other

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cages behave in a more apolar way, causing a two-step adsorption behavior depending on the

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character of the adsorbate.

1-propanol,

2-propanol,

1-butanol,

1-hexanol,

water,

acetone,

acetonitrile,

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INTRODUCTION

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Metal-organic frameworks (MOF) are a class of materials formed by the coordination of metal

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cations and organic linkers. These often highly porous materials have sparked a large interest in

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the fields of adsorption and catalysis, where advantage is taken of their large internal porosity.

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Within the class of MOFs, copper- benzene-1,3,5-tricarboxylate1 (Cu-BTC) is a well studied

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structure. This structure is also known as HKUST-1 or MOF-199 and commercially available

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under the name Basolite C300. The 3-dimensional Cu-BTC porous framework is formed by the


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coordination of copper cations and benzene-1,3,5-tricarboxylate (BTC) linker molecules which

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form the dimeric copper paddle wheel structural building blocks. The material is often quoted to

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have two types of cages, small pockets and larger central cages.2-5 However, a further distinction

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can be made between the larger cages resulting in the identification of three cage types in the

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structure6-9 as shown on figure 1.

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A first type of cage is the small octahedral pocket (S1 cages) with a diameter of approximately

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5-7 Å.1-2,7-8 The pockets, defined by 4 organic linkers, are quite apolar in nature. In between

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these pockets, two types of large cages are present. The first type of large cages (L3 cages) is

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more polar in nature as the dimeric copper vectors are pointing inwards, exposing the

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coordinatively unsaturated metal sites (CUS) inside these cages. The CUS become available after

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activation when coordinated solvent molecules such as water or N,N-dimethylformamide (DMF)

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are removed. These polar cages are approximately 10-13.5 Å

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pockets each by narrow 3.5-4.6 Å windows.2,7 The second type of large cages (L2 cages) is very

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similar to the L3 cage, yet more apolar since the CUS are not available inside these cages. These

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L2 cages are approximately 11-12 Å 7-8 in diameter and connect to 6 cages of type L3, since the

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large cages L2 and L3 are arranged in alternating fashion, connected by 9 Å windows.2 The L2

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cages do not connect to the small S1 pockets. A Cu-BTC unit cell contains 8 pockets and 4 of

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both large cage types.8

7-8

in diameter and connect to 8


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Figure 1. Three-dimensional view (top) and front view (bottom) of Cu-BTC unit cell with cages

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S1, L2 and L3.

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Various studies have focused on the adsorption of permanent gasses like H2, CH4 and CO2 in

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Cu-BTC for target applications such as hydrogen storage, CO2 capture and gas sweetening.8,10-16

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Adsorption properties of these and various other adsorbates including N2, O2, He, Ar, CO, N2O,

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NO, SF6, CF4 and some saturated and unsaturated isomers of C2-C12 hydrocarbons have been

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reported, either experimentally or by molecular simulation.2-3,6,17-30 The preferential adsorption

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sites for small alkanes, H2, Ar, N2 and O2 are the S1 small octahedral pockets.2-3,10,14-15,19,25,28-29

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For Ar

10,28-29

and N2

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adsorption at low temperatures, a two-step adsorption mechanism has


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been observed, which was attributed to the sequentially filling of the S1 cages followed by the

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filling of the larger cages. Chmelik et al.2 experimentally observed a similar two-step adsorption

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mechanism for small alkanes with an inflection point near 8 molecules per unit cell,

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corresponding to 1 molecule adsorbed per S1 cage. Additionally, weak interactions with the CUS

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have been observed for CO, H2 and CH4.8,20

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Adsorptive properties of more polar adsorbates on Cu-BTC are less documented with notable

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examples as water, thiophenes, methanol, acetonitrile (ACN), SO2, and ammonia.4-5,12,16,18,31-40

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Here the CUS play an important role as they strongly interact with polar adsorbates. The

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interaction of water4,31, NO17 and ammonia38-39 with the copper CUS has been identified. For

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water, several studies have revealed a stepwise adsorption behavior on Cu-BTC.4-5,31 Küsgens et

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al.4 have attributed this behavior to the initial adsorption of water on the CUS, followed by the

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further filling of the L3 cages or the filling of the apolar cages. Castillo et al.32 have showed by

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simulations that no water molecules adsorb in the small octahedral pockets S1, but layerwise

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near the copper CUS in the L3 cage, for loadings up to 42 molecules per unit cell.

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Although the Cu-BTC material has a well-defined crystalline structure, large discrepancies

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exist between various published results on pore volume (0.32-0.828 cc/g), adsorbate capacities or

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BET surface areas (692.2-1635 m²/g). 1,10,12,17-18,25,41-42 These differences can be attributed mainly

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to crystal defects and contamination by varying synthesis, washing and activation procedures.

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This underlines the need for a reliable MOF sample. In this work, we used a commercial Cu-

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BTC sample and strict sample storage procedures, which allowed to reveal the negative effect of

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water exposure for the Cu-BTC adsorption capacity. Furthermore, we present the vapor phase

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adsorption isotherms of various polar adsorbates including alcohols, water and common solvents

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such as acetone, ACN, tetrahydrofuran (THF) and DMF on Cu-BTC. Some of these polar


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solvents (water, methanol, ethanol, DMF) are employed as solvents or washing agents during

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Cu-BTC synthesis and can strongly influence the outcome of the synthesis in terms of

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morphology, composition or internal porosity.41,43-45 The adsorption behavior of these molecules

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plays a fundamental role in stabilizing the crystal structure during synthesis. Better

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understanding of the adsorptive properties of these solvents can enhance the knowledge in this

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respect. Special attention is given to the appearance of steps in the uptake isotherm of small polar

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adsorbates, pointing at a change in adsorption behavior, and an adsorption mechanism is

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proposed. Additionally, isotherms of the apolar adsorbates n-hexane, cyclohexane, n-heptane and

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m-xylene are presented.

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EXPERIMENTAL SECTION

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Adsorbent

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A 10 g batch of commercial Cu-BTC material Basolite C300 produced by BASF was

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purchased from Sigma-Aldrich. This material was delivered in activated state and stored under

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nitrogen atmosphere. The entire batch was periodically (ca. every 3 weeks) outgassed in a

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vacuum oven at 333K. The samples were inserted in activated state (recognizable by the purple

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color) in the measuring equipment while minimizing the exposure time to the atmosphere. These

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rigorous steps are required for the sake of reproducibility. Regeneration was performed by

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heating the samples to 453K at 1 K/min and remaining at 453K for 6 hours, all under nitrogen

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flow.

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Isotherms

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Vapor phase isotherms were measured using 5-10 mg samples from the same 10 g batch,

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without sample reuse. These measurements were performed on a gravimetric uptake device

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(SGA-100H, VTI Corporation) where the vapor phase was generated by flowing nitrogen


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through a temperature controlled saturator. The vapor phase isotherms were measured at 323K

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unless otherwise stated. The vapor pressure was controlled by either controlling the dilution rate

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or the saturator temperature. Low vapor pressures were obtained by diluting the saturator flow at

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the lowest saturator temperature with a flow of nitrogen gas. Due to the rapid response of the

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dilution valves, the increase of vapor pressure with time at a change in partial pressure point can

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be approximated as a step function. This allows kinetic uptake data to be retrieved from the valve

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controlled isotherms point measurements. In this work, we cannot exclude the interference of

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film transfer resistance, internal surface barriers or bed diffusion. The vapor pressure of water

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was determined using a dew point analyzer (Dew Prime I, EdgeTech). Argon isotherm

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measurements were performed on a Quantasorb Autosorb AS-1 (Quantachrome Instruments,

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USA) at 87K. Activation was achieved at 10-3 Torr and 453K. Isotherm results refer to Cu-BTC

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handled as described above (water exposure prevented) unless otherwise stated.

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Water stability

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The water stability of Cu-BTC was studied by comparing a sample stored in activated state

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(sample A), a sample aged for several (>10) months exposed to the atmosphere and from a

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different batch (sample B), a sample submerged in liquid water for approximately 24 hours at

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room temperature (sample C), and a sample submerged in liquid water for approximately 24

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hours at 363K (sample D).

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Simulations

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Grand Canonical Monte Carlo (MC) simulations were performed at 313-343K for methanol

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using the Cu-BTC crystal structure of Chui et al1. For each methanol adsorption isotherm, NVT

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Monte Carlo simulations were performed at 16 pressures in the 557-41846 Pa range,

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corresponding to the experimental data. The Cu-BTC framework was considered rigid and


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dehydrated. The Lennard-Jones parameters were taken from the DREIDING46 forcefield with

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exception of Cu, were parameters were taken from the UFF forcefield. Mixed Lennard-Jones

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parameters were calculated by Lorentz-Berthelot mixing rules. The atomic charges and adsorbate

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parameters were taken from Calero et al.9,32 In a selected number of simulations, adsorbate

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molecules were prevented from entering one or more types of cages by defining inert blocking

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structures at these specific sites. Besides methanol isotherm simulations, energies of adsorption

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and Henry coefficients were obtained from MC simulations in the NVT ensemble for longer 1-

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alcohols, water, ACN, acetone, THF and DMF. The Widom particle-insertion method was used

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to obtain the chemical potentials and adsorption energies. These simulations consist of minimum

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2.105 equilibration cycles and 2.106 production cycles. In each cycle regrow, rotation or

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translation, trial moves were randomly selected. Lennard-Jones and electro-static cutoffs of 12 Å

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were used. Coulombic interactions were computed using the Ewald summation technique with a

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relative precision of 10-6. More details on these simulation techniques can be found elsewhere.47

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For acetone, ACN, THF, DMF and alcohols, we used united atom models based on TraPPE force

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fields.48-50 All the models are flexible except those for THF and DMF molecules, which are rigid.

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Water molecules are modeled using the Tip5pEw model that was parameterized for use with the

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Ewald summation method.51 MC simulations were performed using our in-house code RASPA52.

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This code has been extensively tested and validated with a large number of experimental and

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simulation data concerning the computing of adsorption and diffusion properties of gases in

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confined systems.53-56

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Data fitting

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The isotherms of the various adsorbates were fitted using a Langmuir-Sips isotherm model (eq.

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1), being a combination of a Langmuir and Sips isotherm model. The Langmuirian part allows


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fitting of a convex shape (favorable isotherm), while the Sips isotherms also allows fitting of

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concave isotherms. This combination allows to fit the experimental isotherms measured on Cu-

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BTC where both concave as convex parts are present, with a steep transition in between for some

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cases. K1. p K2. pn + qsat ,2 (1) 1 + K1. p 1 + K2 . pn

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q = qsat ,1

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Fitting was performed on the adsorption branch only when large hysteresis occurred. No

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isotherm fit was performed for water, due to the even more complex isotherm shape. The 5

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parameter Langmuir-Sips model provides a good description of the isotherms in the experimental

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measured vapor range. Since many isotherms could not be measured at low Cu-BTC loadings,

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care should be taken when extrapolating the isotherm model to low loadings where type I

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adsorption (represented by the Langmuir model) is not experimentally confirmed.

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Additionally the thermodynamic factor Γ was estimated from the fitted isotherm equations using eq. 2.

d ln p (2) d ln q

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Γ≈

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Kinetic data was analyzed using a Fickian diffusion model (supporting information) for

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spherical particles with an 8 µm radius based on the d50 value of the manufacturer. This allowed

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determination of the apparent Fick diffusion coefficient D. The apparent Maxwell-Stefan

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diffusion coefficient DMS was calculated according to eq. 3.

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D = D MS Γ (3)

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Isotherm and kinetic data fitting were performed using a non-linear least square solver in

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Athena Visual Studio 14.0.


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The specific isosteric heat of adsorption Qads of methanol on Cu-BTC was determined by use of eq. 4.

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 ∂ ln p  Qads  ∂(1 / T )  ≈ − R  q

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The isosteric vapor pressures were calculated by linear interpolation between the measured

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(4)

isotherm points to eliminate effects of isotherm model fittings. Adsorbates

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Methanol (≥99.9%, Sigma-Aldrich), ethanol (≥99.8%, Sigma-Aldrich), 1-propanol (≥99.8%,

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Fluka), 2-propanol (≥99.95%, Biosolve), 1-butanol (≥99.7%, Sigma-Aldrich), 1-hexanol (>98%,

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Sigma-Aldrich), acetonitrile (ACN) (≥99.8%, Sigma-Aldrich), acetone (≥99.8%, Sigma-

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Aldrich), tetrahydrofuran (THF) (≥99.9%, Merck), N,N-dimethylformamide (DMF) (≥99.9%,

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Sigma-Aldrich), n-hexane (≥96%, Biosolve), cyclohexane (≥99.5%, Sigma-Aldrich), n-heptane

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(≥99%, Across Chemicals) and meta-xylene (≥99%, Across Chemicals) were used as adsorbates.

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Pure water was produced from distilled water using a Millipore Simplicity.

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RESULTS/DISCUSSION

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1. Water exposure

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The stability of Cu-BTC in the presence of water has been the topic of discussion in various

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studies. Several authors have shown a loss of crystallinity, decreased CO2 capacity or irreversible

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hysteresis in adsorption isotherms upon exposure to water4-5,57-60, while others mention a better

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stability or hydrocarbon uptake of the Cu-BTC framework at various humid conditions.18,61 In

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our work, long-term reproducibility of the measured isotherms was achieved when the Cu-BTC

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was stored in activated (dehydrated) state. Figure 2 shows normalized ethanol isotherms on 4

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Cu-BTC samples, exposed to water in various conditions. The highest value measured (100%)


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was 573 mg/g of ethanol adsorption. A significant loss in adsorption capacity upon water

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exposure is revealed.

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Figure 2. Normalized (100% being 573 mg/g) ethanol vapor phase isotherms (323K) for Cu-

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BTC, stored in activated state (A), after prolonged atmosphere exposure (B), after liquid water

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contact at room temperature (C) and after liquid water contact at 363K (D). Full symbols indicate

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the adsorption branch, open symbols the desorption branch of the isotherms.

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Especially direct contact with liquid water (samples C and D) causes a rapid loss of adsorption

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capacity, reducing the original capacity with as much as 60% when Cu-BTC is contacted with

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water at 363K for approximately 24h. Besides the large losses in uptake capacity, larger

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hysteresis between the adsorption and desorption branch of the isotherms is observed. The step

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observed near 1 kPa ethanol vapor pressure fades away with intensified water contact. This

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indicates a loss in crystallinity of the Cu-BTC sample. Reduced capacities were also found for

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methanol, 1-propanol, 2-propanol, 1-butanol, DMF and ACN adsorbates on Cu-BTC stored in

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non-activated state (supporting information). It is clear that the presence of water is detrimental


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for the adsorption properties of Cu-BTC, hence adsorption based applications in the presence of

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water are at least questionable. However, the addition of strongly adsorbing molecules in water

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might help to stabilize the structure as Cu-BTC was found to be stable in a 7:1 water:DMF

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mixture.57 In the following sections only Cu-BTC samples with rigorous water exposure

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prevention, as described in the experimental section, are used.

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2.Vapor phase isotherms

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2.1 Alcohols

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Vapor phase isotherms of methanol, ethanol, 1-propanol, 2-propanol, 1-butanol and 1-hexanol

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at 323K are shown in figure 3.

229 230

Figure 3. Vapor phase adsorption isotherms of methanol, ethanol, 1-propanol, 2-propanol, 1-

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butanol and 1-hexanol on Cu-BTC at 323K. Full symbols indicate the adsorption branch, open

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symbols the desorption branch of the isotherms.

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All adsorbates show high uptake capacities, over 520 mg/g. Although Cu-BTC has polar

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properties due to its CUS, longer and thus less polar alcohols have a higher uptake at lower vapor

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pressures. For 1-butanol and 1-hexanol, the isotherms are limited to higher vapor pressures due


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to very long equilibration times needed at low vapor pressures, indicating reduced uptake

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kinetics for these larger molecules. The isotherms of the propanol isomers are comparable,

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although 2-propanol is expected to be more sterically hindered inside the framework. A

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remarkable two-step uptake is noticed for the smaller alcohols in the measured vapor pressure

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range. This effect is clearly noticeable for methanol and ethanol, but can also be distinguished

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for 1-propanol, 2-propanol and 1-butanol. During desorption only small to negligible hysteresis

243

can be noticed over the second step, being the most pronounced for methanol. The methanol

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isotherm on Cu-BTC has a similar shape but a higher capacity than the isotherms measured

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previously by our group45 as a result of the more rigorous water exposure prevention.

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2.2 Other polar adsorbates

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Vapor phase isotherms of water, acetone, ACN, THF and DMF on Cu-BTC at 323K are shown

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in figure 4.

249 250

Figure 4. Vapor phase adsorption isotherms of water, ACN, acetone, THF and DMF on Cu-BTC

251

at 323K. Full symbols indicate the adsorption branch, open symbols the desorption branch of the

252

isotherms.

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These polar adsorbents show similar behavior as the alcohols with uptake capacities exceeding

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530 mg/g for all adsorbates. Especially THF and DMF molecules have a large affinity for Cu-

256

BTC as these adsorbates approach saturation at low vapor pressures. For the smaller adsorbates

257

water and ACN, a second adsorption step can be noticed in the measured vapor pressure range,

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similar to the behavior of small alcohols. A two-step adsorption behavior is also expected for

259

acetone as suggested by the shape of the isotherm, although lower vapor pressure points could

260

not be measured. Small to no hysteresis is noticed for the polar adsorbates, with the exception of

261

water where a large hysteresis loop exists after the first adsorption step. The water isotherm

262

shape and capacity should be treated with care as differences in experimental water isotherms

263

exist.4-5,18,31,40,60 The prolonged exposure of an initially dehydrated Cu-BTC sample to humid

264

conditions is expected to cause partial structural degradation of the framework. Küsgens et al.

265

noticed a severe reduction in surface area from 1340 to 647 m²/g after a water isotherm

266

measurement.4

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2.3 Apolar adsorbates

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Figure 5 shows vapor phase isotherms of n-hexane, n-heptane, cyclohexane and m-xylene.

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Figure 5. Vapor phase adsorption isotherms of n-hexane, cylohexane, n-heptane and m-xylene

271

on Cu-BTC at 323K. Full symbols indicate the adsorption branch, open symbols the desorption

272

branch of the isotherms.

273 274

For the alkanes, uptake capacities over 430 mg/g are found. Due the favorable interaction of

275

alkanes with the host-structure, the apolar adsorbates fill the Cu-BTC structure at low vapor

276

pressures, allowing only measurement near conditions of saturation. Despite the fact that polar

277

molecules adsorb strongly in the Cu-BTC structure, the material also shows a quite apolar nature

278

due to the presence of the aromatic linker. High uptake capacities are also found for larger cyclic

279

molecules such as m-xylene and cyclohexane, indicating that these molecules are not sterically

280

prevented from entering the Cu-BTC structure. In contrast to the other apolar adsorbates, m-

281

xylene contains π-electrons, which have favorable interactions with the copper sites in the

282

framework.19,21,24

283

3. Pore filling

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The measured maximum capacities for all adsorbates are remarkably high exceeding a 53 wt.%

285

uptake for all polar adsorbates, while lower maximum capacities of 43-49 wt.% are measured for

286

non-aromatic apolar adsorbates. These capacities are measured at high relative pressures at

287

which the adsorbates condense into a liquid like state inside the framework. By converting the

288

gravimetric uptake results into volumetric units, using the liquid density at 323K, the occupied

289

pore volume can be estimated. All adsorbates, with the exception of water, reach an occupied

290

pore volume between 0.65 and 0.76 cc/g (Table 1).

291

Table 1. Pore volume of Cu-BTC probed with various adsorbates. Values are based on the

292

maximum gravimetrical uptake measured at 323K and liquid density at 323K.


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

adsorbate methanol ethanol 1-propanol 2-propanol 1-butanol

pore volume (cc/g) 0.731 0.760 0.684 0.695 0.685

adsorbate 1-hexanol water acetone ACN THF

pore volume (cc/g) 0.664 0.542 0.726 0.735 0.670

Page 16 of 40

adsorbate DMF cyclohexane n-hexane n-heptane m-xylene

pore volume (cc/g) 0.706 0.653 0.684 0.681 0.707

293 294

The volumetric capacity values of most adsorbates lie within relative close proximity of each

295

other and approximate or even exceed the micropore volume of 0.66 cc/g as calculated by the

296

BET method from argon porosimetry at 87K (supporting information). This suggests that the Cu-

297

BTC micropores are nearly fully saturated with the various types of adsorbates, both polar and

298

non-polar in nature. The maximum measured uptake for the various adsorbates in function of

299

their diameter of gyration is shown in figure 6.

300 301

Figure 6. Pore volumes obtained for various adsorbates on Cu-BTC at 323K versus their

302

diameter of gyration (2x radius of gyration). The triangles represent the first saturation level

303

before occurrence of the second isotherm step, obtained from Langmuir-Sips model fitting. The

304

grey area represents the Cu-BTC uptake limit for the various adsorbates.

305


16

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306

Here, the diameter of gyration is used as measure for adsorbate size, equal to two times the

307

radius of gyration. The radii of gyration were taken from Yaws.62 The grey zone in figure 6

308

illustrates the general decreasing trend of the maximum adsorbed volume with the diameter of

309

gyration of the adsorbate. Only water, the smallest molecule in figure 6, breaks this trend. For

310

water, a cluster based adsorption mechanism on the CUS can explain the formation of interstitial

311

voids when initially formed water clusters make contact near hydrophobic surfaces, resulting in a

312

lower degree of pore filling.4 However, the water saturation pore volume is obscured by factors

313

like structural degradation and the steepness of the water isotherm at high partial water pressure,

314

making it difficult to compare water with the other adsorbates.

315

4. Two-step isotherm

316

For methanol, ethanol, 1-propanol, 2-propanol, water and ACN, two steps are clearly observed

317

in their adsorption isotherm in the measured pressure range (Figs. 3 and 4). All these adsorbates

318

are characterized by their large dipole moment and small molecular size. The adsorption and

319

desorption isotherms of these adsorbates show a slight hysteresis. In general, both the step height

320

of the second step and the observed hysteresis diminishes with increasing molecular size. Since

321

the occurrence of the second step occurs at low relative pressures (P/Ps< 0.2), it is not the result

322

of mesoporosity in the sample. In contrast to alkanes which display an isotherm step at low

323

fractional loadings2, these polar adsorbates display the onset of the second step at high fractional

324

pore occupancies.

325

For water, two steps are observed in the adsorption isotherm on Cu-BTC, similar to previous

326

studies.4 Figure 7 includes the water isotherm in terms of molecules per unit cell, where several

327

distinct zones can be recognized.


17


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Page 18 of 40

328 329

Figure 7. Isotherm of water, methanol, ethanol, 1-propanol, 1-butanol and ACN on Cu-BTC at

330

323K, expressed as molecules of adsorbate adsorbed per Cu-BTC unit cell in function of the

331

relative pressure. Dashed lined represent linear extrapolation of the isotherm lines in a

332

semilogaritmic graph.

333 334

Initially the 48 copper CUS per unit cell start adsorbing water molecules, followed by the

335

further adsorption of water near the copper atoms32, creating a polar environment. At a capacity

336

of approximately 150 water molecules per unit cell (≈3 water molecules per Cu), an intermediate

337

saturation plateau is observed. This adsorbed amount corresponds to around 40-50% of the

338

micropore volume being filled with water, which also approximates the relative pore volume of

339

the L3 cages (supporting information). At higher vapor pressures the isotherm gradually

340

increases corresponding to type III behavior associated with multilayer adsorption.4 This

341

behavior could be explained by the initial clustering and pore filling in the L2 cages, resulting in

342

a large hysteresis (Fig. 4) loop due to the large difference in cage and molecule diameter.

343

Alcohols adsorb more strongly than water on the Cu-BTC framework, as evidenced by the

344

large loading at relative vapor pressures as low as 0.01 (Fig. 7). The isotherms of methanol and


18

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345

ethanol, when plotted in terms of molecules per unit cell versus the logarithm of relative pressure

346

(P/Ps) yield linear functions in the low vapor pressure range. When these isotherms are

347

extrapolated, the two linear functions converge at 47.1 molecules per unit cell. This is

348

remarkably close to the number of copper CUS in a Cu-BTC unit cell. A plausible explanation

349

lies in the strong specific interaction of the OH-group of the alcohols with the CUS, causing

350

significant adsorption at low relative pressures. Since the L3 cages holding the copper CUS, are

351

not completely filled with 48 of these small adsorbates molecules, further adsorption in these

352

polar cages can occur with increasing vapor pressure. Further uptake, including the second

353

isotherm step, should accordingly be attributed to further filling of the L3 cages as well as

354

clustering and pore filling in the apolar cages. For methanol, the step occurs near 100 molecules

355

per unit cell (≈2 methanol molecules per Cu).

356

A similar observation regarding the isotherm extrapolation of methanol and ethanol is made

357

for ACN, 1-propanol and 1-butanol (Fig. 7). The consecutive filling of polar and apolar cages

358

can explain the occurrence of a two-step isotherm as well as the occurrence of hysteresis over the

359

second step. For larger alcohols, the adsorption step occurs at lower pressures while hysteresis

360

and the step height of the second isotherm step become increasingly smaller, shown in figure 3.

361

Since these adsorbates are more amphiphilic, they can adsorb in both polar as apolar cages,

362

reducing the difference in adsorption behavior between the different types of cages and resulting

363

in a more continuous adsorption behavior.

364

5. Molecular simulation

365

To investigate the hypotheses of sequential filling of the polar cage L3 and the apolar cages L2

366

and S1, molecular modeling was performed for the adsorption of methanol on Cu-BTC. The

367

individual contribution of the different cages (S1, L2, L3) was evaluated by blocking the other


19


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Page 20 of 40

368

respective cages in the simulation. Figure 8 shows the simulated methanol isotherm for the

369

unblocked Cu-BTC structure (S1+L2+L3) and the isotherms for cages S1 (L2 and L3 blocked),

370

L2 (S1 and L3 blocked) and L3 (S1 and L2 blocked) respectively.

371 372

Figure 8. Molecular simulation of methanol adsorption on (unblocked) Cu-BTC framework,

373

partially blocked Cu-BTC with only the S1, L2 or L3 cages available and experimental isotherm

374

at 323K. The grey area marks the first experimental saturation plateau.

375 376

The simulated isotherm reaches a methanol saturation capacity comparable with the

377

experimentally measured value, indicating the high quality of the Cu-BTC sample. In contrast to

378

the experimental results, no inflection behavior (two-step adsorption) is noticed for the

379

unblocked Cu-BTC structure. The simulation with only the L3 cages available for adsorption

380

suggests the underestimation of methanol adsorption at low vapor pressures near the copper

381

CUS, resulting in a more continuous overall adsorption behavior obtained by simulation.

382

Despite the model failing to accurately reproduce the two-step isotherm over the entire pressure

383

range, the individual contributions reveal the polar character of the L3 cages and the more apolar


20

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384

character of cages S1 and L2. The L3 cages have a strong attraction for methanol and start

385

reaching saturation at 2 methanol molecules per CUS as the L2 cages start filling. The good

386

correspondence between the simulated saturation capacity of the L3 cages and the first

387

experimental saturation plateau suggests almost complete filling of L3 cages before adsorption in

388

the L2 cages initiates. While blocking of cages in the simulation could prevent effects such as

389

adsorption in the windows between the various cages to manifest fully and hereby obscure the

390

detailed adsorption mechanism, the simulation confirms sequential filling of the different types

391

of cages to play an important role in the two-step isotherm behavior of methanol.

392

The simulated Gibbs free energy change upon adsorption of methanol and other polar

393

molecules on the Cu-BTC cages at 323K is shown on figure 9. A detailed table of these, and

394

other thermodynamical values can be found in the supporting information. For all molecules,

395

adsorption in the polar L3 cages is significantly more favorable than in the L2 cages, as indicated

396

by the values of the change in Gibbs free energy at zero loading in figure 9. However, this does

397

not necessarily gives rise to a detectable two-step adsorption behavior. For a marked two-step

398

adsorption isotherm to be experimentally (at the experimental conditions) noticed, at least two

399

conditions must be fulfilled. A first requirement is that the difference in Gibbs free energy

400

change between the main adsorption sites, here the L2 and L3 cages, must be substantial. For all

401

molecules, the Gibbs free energy change upon adsorption in the Cu-BTC framework is at least -

402

2.3 kJ/mol larger in the L3 cages than the L2 cages, for methanol and ethanol this is -11.3 kJ/mol

403

and -10.7 kJ/mol respectively. Second, at least one of the involved cages should display a

404

relatively small Gibbs free energy change upon adsorption. In the case of Cu-BTC, when the L2

405

cages show a simulated Gibbs free energy change of adsorption lower than -15 kJ/mol, a clear

406

two-step adsorption isotherm was measured. This is the case for water, methanol, ACN and


21


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Page 22 of 40

407

ethanol (Figs. 3, 4,9). The order of adsorption affinity (Gibbs free energy) of these molecules in

408

the L2 cages matches the order of adsorption as determined experimentally (Figs. 3, 4, 7, S.6),

409

where a higher relative pressure for the inflection point corresponds with a lower Gibbs free

410

energy change. In these cases, the L3 cages will have started reaching saturation before any

411

significant adsorption in the L2 cages occurs, resulting in a well pronounced two-step isotherm.

412

Propylalcohols, acetone and 1-butanol have simulated Gibbs free energy changes upon

413

adsorption of -15 kJ/mol to -25 kJ/mol in the L2 cages, resulting in a less pronounced two-step

414

adsorption behavior at the experimental conditions. Finally, the simulated Gibbs energy change

415

for adsorption of DMF and 1-hexanol in the L2 cages exceeds -25 kJ/mol corresponding to

416

stronger adsorption. Accordingly, any two-step adsorption behavior for these molecules is not

417

noticeable in the experimentally measured vapor pressure range and at the temperature of 323K,

418

as both L2 and L3 cages reach saturation at low vapor pressures.

419


22

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420

Figure 9. Simulated Gibbs free energy change upon adsorption at zero loading of various polar

421

molecules in the S1, L2 and L3 cages of the Cu-BTC framework at 323K, ordered in function of

422

the L2 cages Gibbs free energy change. The blue zone indicates molecules which show a low L2

423

Gibbs free energy change and pronounced two-step adsorption behavior. The red zone indicates

424

molecules which show a high L2 Gibbs free energy change without an experimentally measured

425

two-step adsorption behavior. The purple zone is an intermediate zone with average L2 Gibbs

426

free energy change and poorly pronounced two-step isotherms.

427 428

6. Methanol isosteric heat

429

The isosteric heat of adsorption can provide an additional fingerprint of the adsorption

430

mechanism. Vapor phase isotherms of methanol on Cu-BTC at 313K, 323K, 333K and 343K are

431

given in figure 10. The isosteric heat of adsorption obtained from this data is given in function of

432

the methanol loading in figure 11. The loading is expressed as a fractional loading with regard to

433

the maximal measured methanol adsorption capacity.

434

435


23


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Page 24 of 40

436

Figure 10. Methanol adsorption isotherms at various temperatures on Cu-BTC framework. Full

437

symbols indicate the adsorption branch, open symbols the desorption branch of the isotherms.

438

439 440

Figure 11. Isosteric heat of methanol adsorption on Cu-BTC (313K-343K) (solid line) and 95%

441

confidence intervals (grey lines) given in function of the fractional loading, and inverse

442

thermodynamic factor (dashed line).

443 444

Additionally, the inverse of the thermodynamic factor (based on the Langmuir-Sips model) is

445

given in figure 11, as a convenient manner to identify the occurrence of the step near fractional

446

loadings of 0.5-0.6. The isosteric heat of methanol in Cu-BTC varies between 39 and 51 kJ/mol

447

with an average of 48 kJ/mol, which is significantly more than the heat of vaporization being

448

approximately 35 kJ/mol. In comparison, the isosteric heat of adsorption on Cu-BTC was found

449

equal to 7 kJ/mol for H2, 13 kJ/mol for N2, 21 kJ/mol for CH4, 29 kJ/mol for CO2

450

simulations resulted in 46-51 kJ/mol for water, being about 10 kJ/mol more than its heat of

451

evaporation.32 The average heat of adsorption of 48 kJ/mol for methanol is higher than the

452

simulated value at zero loading of 41.7 kJ/mol (see supporting information), but within the 95%

12

and


24

Page 25 of 40

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453

confidence intervals. In the L3 cages, near the CUS where the methanol molecules adsorb first,

454

the heat of adsorption is simulated to be 42.7 kJ/mol.

455

7. Adsorption kinetics

456

Apparent Fick diffusion coefficients obtained from experimental uptake data are given in

457

figure 12 for the various adsorbates displaying two-step adsorption behavior.

458 459

Figure 12. Apparent Fick diffusion (open symbols) and thermodynamically corrected apparent

460

Maxwell-Stefan diffusion coefficients (full symbols) in function of the fractional loading on Cu-

461

BTC at 323K for methanol (A), ethanol (B), 1-propanol (C), 2-propanol (D), 1-butanol (E) and

462

ACN (F). The inverse thermodynamic factor is given by the dashed lines (left axis).

463 464

Diffusion coefficients are given in function of fractional loading, where the corresponding

465

fractional loading is the average of the initial and final loading for each imposed pressure step. It


25


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Page 26 of 40

466

is clear that the apparent Fick diffusion coefficients strongly vary with fractional loading,

467

generally increasing with the fractional loading. Remarkably, near the occurrence of the second

468

isotherm step, the apparent Fick diffusion coefficients bend off and even decrease with further

469

fractional loading for the alcohols. The apparent Fick (and Maxwel-Stefan) diffusion coefficients

470

for apolar adsorbates rise more monotonically (supporting information). When the apparent Fick

471

diffusion coefficients are corrected for the thermodynamic factor, apparent Maxwell-Stefan

472

coefficients can be obtained, giving a more fundamental insight in the concentration dependence

473

of the uptake kinetics. For the alcohols, the apparent Maxwell-Stefan diffusion coefficients are

474

lowered compared to the apparent Fick diffusion coefficients, while the same loading

475

dependency trends hold.

476

The lowering of the apparent Maxwell-Stefan diffusion coefficients near the saturation of the

477

initial adsorption sites (first adsorption plateau) suggests a change in diffusion mechanism. For

478

large pore microporous structures, the Maxwell-Stefan diffusivities are expected to show a

479

reasonable correlation with the inverse thermodynamic factor, as the inverse thermodynamic

480

factor is a good approximation of the vacancy available for the adsorbate molecules to hop

481

forward.2 However, for polar adsorbates in Cu-BTC, the mechanism might be more complex due

482

to the variety of cages (small or large, polar or apolar) present. For example, as the L3 cages

483

reach saturation near the isotherm inflection point, it is possible that strongly adsorbed molecules

484

obstruct the framework as the L2 and L3 cages are arranged in alternating order.

485

Recently, Tsotsalas et al.63 have noticed a drop in the Fick diffusion coefficient for methanol,

486

ethanol and 1-butanol during desorption experiments. The drop in methanol diffusion coefficient

487

at loadings close to the inflection point was attributed to a metastable state, which we find to be

488

near 2 methanol molecules per Cu, present in the L3 cages.


26

Page 27 of 40

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489

With increasing adsorbate size, represented by the diameter of gyration, the apparent diffusion

490

coefficients decrease as shown on figure 13 for linear alcohols and linear alkanes. n-hexane and

491

n-heptane have higher apparent diffusion coefficients than the shorter 1-butanol, suggesting a

492

different transport mechanism for alkanes and alcohols as a result of the stronger interactions

493

between the alcohols and the Cu-BTC framework.

494 495

Figure 13. The averaged apparent Maxwell-Stefan coefficients found for C1-C4 linear alcohols

496

and C6-C7 linear alkanes on Cu-BTC at 323K, plotted in function of their diameter of gyration.

497

The grey area is representative for the measured diffusion coefficient range (with varying

498

fractional loading).

499 500

The apparent diffusion coefficients obtained for n-hexane and n-heptane (supporting

501

information) do not show the same fractional occupancy dependency trends as previous studies

502

with alkanes2-3, and are several orders of magnitude lower than those of small alkanes in Cu-

503

BTC as determined by IR microscopy2 and PFG NMR.3 To clarify these differences, as well as

504

to identify which mechanisms lie behind this fractional occupancy dependency of alcohols, a

505

further study of the diffusion mechanism of both polar and apolar adsorbates, beyond the scope

506

of this work, is required.


27


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Page 28 of 40

507

8. Adsorption of polar molecules on apolar MOFs

508

The S-shaped isotherms, as obtained by simulation for the apolar L2 cages in the present work,

509

were also obtained in experimental and simulation studies for the adsorption of various polar

510

adsorbates on apolar MOFs.4,64-66 For example, figure 14 compares the second adsorption step of

511

Cu-BTC with experimental data for hydrophobic ZIF-84,66 at 323K for water, methanol and

512

ethanol. The second adsorption step for methanol and ethanol on Cu-BTC was described by the

513

Sips part of the fitted Langmuir-Sips model. The Sips part of the isotherm model represents

514

adsorption in the apolar Cu-BTC cages, further referred to as Cu-BTC*. A similar approach,

515

ascribing isotherm model parts to specific adsorption sites on Cu-BTC was also used by Chmelik

516

et al.2 For water, the experimental isotherm part above the first saturation plateau near 300 mg/g

517

was chosen to represent the apolar Cu-BTC* structure.

518 519

Figure 14. Adsorption isotherms of water4, methanol and ethanol66 at 323K on ZIF-8 and on the

520

apolar part (identified with the second isotherm step) of Cu-BTC (Cu-BTC*). Closed symbols

521

represent the adsorption branch, open symbols the desorption branch.


28

Page 29 of 40

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522 523

Despite the difference in chemical composition and pore size for Cu-BTC* and ZIF-8, the

524

same adsorption trends can be noticed: (1) S type isotherms shifting towards lower vapor

525

pressure when the adsorbate becomes more apolar (methanol, ethanol), without any large

526

hysteresis. (2) type III adsorption of water with larger hysteresis. Nalaparaju et al. have made

527

similar observations for the adsorption of methanol and ethanol on hydrophobic ZIF-71, and

528

found a large hysteresis for water adsorption.67 These observations for polar adsorbates,

529

combined with their favorable adsorption at low pressures on Cu-BTC, are a further indication of

530

the dual (polar, apolar) character of the Cu-BTC framework.

531

CONCLUSIONS

532

Isotherms of various adsorbates on the metal-organic framework Cu-BTC are presented,

533

showing high uptake capacities (>0.65 cc/g) for both polar and apolar adsorbates, with the

534

exception of water. The combination of apolar cages and copper CUS, give Cu-BTC both polar

535

and apolar properties. For small polar adsorbates a two-step uptake behavior was noticed and

536

attributed to different adsorption mechanisms in the different types of cages. Molecular

537

simulation has identified the polar cages L3 to show a favorable adsorption of methanol, while

538

the apolar part of structure tends towards an S-shaped isotherm. The combined behavior is

539

proposed to cause a two-step adsorption behavior.

540

We have also shown that prolonged exposure to water has a detrimental effect on the

541

adsorption performance of the material. Applications for air purification and VOC removal,

542

including both polar and apolar molecules, seem viable in low humidity conditions.

543

ASSOCIATED CONTENT


29


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Page 30 of 40

544

Argon porosimetry, cage pore volume estimation, capacity reduction for atmosphere exposed

545

Cu-BTC samples, adsorption isotherms in function of relative pressure, experimental and

546

simulated maximal Cu-BTC loading, fitted isotherm parameter values, apparent diffusion

547

coefficients of various adsorbates, values for thermodynamic parameters obtained by molecular

548

simulation. This material is available free of charge via the Internet at http://pubs.acs.org.

549

AUTHOR INFORMATION

550

Corresponding Author

551

* Email: [email protected], Tel.: +32 2 6291798, Fax.: +32 2 6293248

552

Author Contributions

553

The manuscript was written through contributions of all authors. All authors have given approval

554

to the final version of the manuscript

555

ACKNOWLEDGEMENT

556

This research is funded by a Ph.D. grant (T.R.C. Van Assche) of the Agency for Innovation by

557

Science and Technology (IWT) Flanders. We would like to thank Prof. Stefan Kaskel for kindly

558

providing us ZIF-8 water isotherm data.

559

REFERENCES

560 561

(1) Chui, S.S.Y.; Lo, S.M.F.; Charmant, J.P.H.; Orpen, A.G.; Williams, I.D. A Chemically Functionalizable Nanoporous Material [Cu3(TMA)2(H2O)3]n. Science 1993, 283, 1148-1150.

562

(2) Chmelik, C.; Kärger, J.; Wiebcke, M.; Caro, J.; van Baten, J.M.; Krishna, R. Adsorption

563

and Diffusion of Alkanes in CuBTC Crystals Investigated using Infra-red Microscopy and

564

Molecular Simulations. Microporous Mesoporous Mater. 2009, 117, 22-32.


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565

(3) Wehring, M.; Gascon, J.; Dubbeldam, D; Kapteijn, F.; Snurr, R.Q.; Stallmach, F. Self-

566

Diffusion Studies in CuBTC by PFG NMR and MD Simulations. J. Phys. Chem. C 2010, 114 ,

567

10527-10534.

568

(4) Küsgens, P.; Rose, M.; Senkovska, I.; Fröde, H.; Henschel, A.; Siegle, S.; Kaskel, S.

569

Characterization of Metal-Organic Frameworks by Water Adsorption. Microporous Mesoporous

570

Mater. 2009, 120, 325-330.

571

(5) Liu, J.; Wang, Y.; Jakubczak, P.; Willis, R.R.; LeVan, M.D. CO2/H2O Adsorption

572

Equilibrium and Rates on Metal-Organic Frameworks: HKUST-1 and Ni/DOBDC. Langmuir

573

2010, 26, 14301-14307.

574

(6) Calero, S.; Gutiérrez-Sevillano, J.J.; García-Pérez, E. Effect of the Molecular Interactions

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on the Separation of Nonpolar Mixtures using Cu-BTC Metal-Organic Framework. Microporous

576

Mesoporous Mater. 2013, 165, 79-83.

577 578

(7) Yang, L.; Naruke, H.; Yamase, T. A Novel Organic/Inorganic Hybrid Nanoporous Material Incorporating Keggin-type Polyoxometalates. Inorg. Chem. Commun. 2003, 6, 1020-1024.

579

(8) Getzschmann, J.; Senkovska, I.; Wallacher, D.; Tovar, M.; Fairen-Jimenez, D.; Düren, T.;

580

van Baten, J.M.; Krishna, R.; Kaskel, S. Methane Storage Mechanism in the Metal-Organic

581

Framework Cu3(btc)2: An in situ Neutron Diffraction Study, Microporous Mesoporous Mater.

582

2010, 136, 50-58.

583

(9) Gutiérrez-Sevillano, J.J.; Vicent-Luna, J.M.; Dubbeldam, D.; Calero, S. On the Molecular

584

Mechanisms for Adsorption in Cu-BTC Metal Organic Framework, J. Phys. Chem. C, 2013, 117,

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11357-11366.


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(10) Krawiec, P.; Kramer, M.; Sabo, M.; Kunschke, R.; Fröde, H.; Kaskel, S. Improved

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Hydrogen Storage in the Metal-Organic Framework Cu3(BTC)2. Adv. Eng. Mater. 2006, 8, 293-

588

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High Adsorption Capacities and Two-Step Adsorption of Polar

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