Synthesis, gas adsorption and reliable pore size estimation of zeolitic imidazolate framework-7 using CO2 and water adsorption

    Synthesis, gas adsorption and reliable pore size estimation of zeolitic imidazolate framework-7 using CO 2 and water adsorption Mahdi Niknam Shahrak, Morteza Niknam Shahrak, Akbar Shahsavand, Nasser Khazeni, Xiaofei Wu, Shuguang Deng PII: DOI: Reference:

S1004-9541(16)30105-7 doi: 10.1016/j.cjche.2016.10.012 CJCHE 696

To appear in: Received date: Revised date: Accepted date:

19 February 2016 10 October 2016 19 October 2016

Please cite this article as: Mahdi Niknam Shahrak, Morteza Niknam Shahrak, Akbar Shahsavand, Nasser Khazeni, Xiaofei Wu, Shuguang Deng, Synthesis, gas adsorption and reliable pore size estimation of zeolitic imidazolate framework-7 using CO2 and water adsorption, (2016), doi: 10.1016/j.cjche.2016.10.012

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ACCEPTED MANUSCRIPT Separation Science and Engineering

Synthesis, gas adsorption and reliable pore size estimation of

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zeolitic imidazolate framework-7 using CO2 and water

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adsorption

Nasser Khazeni3, Xiaofei Wu3, Shuguang Deng4,* 1

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Mahdi Niknam Shahrak1,*, Morteza Niknam Shahrak2, Akbar Shahsavand2,

Chemical Engineering Department, Faculty of Engineering, Quchan university of advanced technology, Quchan, P.O. Box 84686-94717, Iran Chemical Engineering Department, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, P.O. Box 91775-1111, Iran 3 Chemical Engineering Department, New Mexico State University, Las Cruces, New Mexico State 88003, USA 4 Chemical Engineering Department, Arizona State University, USA

Abstract

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Corresponding authors. E-mail address: [email protected] (Mahdi Niknam Shahrak); E-mail address: [email protected] ( Shuguang Deng)

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*

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Reliable estimation of the pore size distribution (PSD) in porous materials such as Metal Organic Frameworks (MOFs) and Zeolitic Imidazolate Frameworks (ZIFs) is crucial for accurately assessing adsorption capacity and corresponding selectivity. In this study, the so-called Zeolitic Imidazolate Framework-7 (ZIF-7) is successfully synthesized via relatively fast and convenient microwave technique. The morphology and structure of the obtained MOF was characterized by XRD, SEM and N2 and CO2 adsorption/desorption isotherms at 77K and 0°C respectively. Then, to determine the PSD of the fabricated MOF, carbon dioxide isotherms are experimentally measured at various temperatures up to atmospheric pressure. Afterward, the experimental CO2 isotherms data are utilized in two recently proposed in-house algorithms of SHN1 and SHN2 to extract the true PSD of manufactured ZIF-7. The obtained results revealed that median pore diameter of the fabricated ZIF-7 is estimated around 0.404nm and 0.370 nm by using CO2 isotherms at 273 K and 298 K respectively. These values are in good agreement with the real pore diameter of 0.42 nm.

Moreover, experimental data of water adsorption

isotherms over four different MOFs, borrowed from literature, are employed to illustrate further effectiveness of the above algorithms on successful determination of the corresponding pore size distributions. All predicted PSDs are proved to be in good agreement with those obtained from independent methods such as topology and morphology studies. Keywords: ZIF-7; Metal-Organic Framework; Size Distribution; Adsorption; CO2 capture; Gas 1

ACCEPTED MANUSCRIPT 1. Introduction Extensive applications of various adsorption processes in numerous chemical engineering industries require much more efficient novel adsorbents. In 1965 Tomic [1] initially introduced the coordination

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polymers or supra-molecular structure and in 1999 Yaghi et al. fabricated the first MOF material known as MOF-5 in a laboratory scale batch [2].

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Afterward, Metal-Organic Frameworks (MOFs) have captured significant attention due to their outstanding properties, such as extremely large surface areas [3], high thermal and mechanical stabilities

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[4], low bulk densities [5], large micro pore volumes and very high porosities [6]. These materials have been used for numerous practical applications such as gas storage (e.g. methane, carbon dioxide and

luminescent and sensors fabrication [2,6,7-17].

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hydrogen), gas separation (e.g. nitrogen recovery from air), catalysis, drug delivery, membranes,

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Regarding the variety of the metals and ligands, over 38000 MOF structures are listed up to now in the Cambridge Structure Database (CSD). Several reviews have also been addressed this fast growing area, for example, over 2460 articles were recorded in MOF related subjects up to 2008 [6, 18].The most

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comprehensive ones are given by Kitagawa and Rowsell and Kupplera et al. [6,19,20]. More recently, in

metal-organic frameworks [21].

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March 2015, a review article has explained fundamentals and wide array of potential applications of

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In general, pore size distribution (PSD) is probably the most significant adsorbent specification that directly affects many other adsorption properties, such as surface area, bulk density and especially adsorption capacity [22-25]. Bastos-Neto et al related various textural properties (such as: PSD, micropore volume and solid surface area) to methane adsorption capacities for a number of activated carbon samples produced from different raw materials [22]. They reported that “the textural parameters per se do not unequivocally determine natural gas storage capacities. Surface chemistry and methane adsorption equilibria must be taken into account in the decision-making process of choosing an adsorbent for gas storage”. They also concluded that “for carbons produced from the same source, those which have higher surface area, higher micro-pore volume and narrower PSD within the range of 8–15 A°, show better methane adsorption properties”. Finally, they concluded that “although textural parameters provide an easy and useful tool for initial screening of activated carbons for natural gas storage, they do not allow ranking of these samples accurately”. In 2007, Saha et al. synthesized three different MOF-5s by dispersion in dimethylformamide (DMF), to investigate the effects of various synthesis operating conditions on their crystal structure, pore textural properties and the corresponding hydrogen adsorption performances [25]. They also proposed a relationship between the PSDs of various porous MOF-5 materials and their hydrogen adsorption capacities. Finally they concluded that higher order of crystallinity in the MOF-5 materials leads to better 2

ACCEPTED MANUSCRIPT adsorbent with larger crystal sizes, higher specific surface areas, more uniform PSDs, higher hydrogen adsorption capacities and faster hydrogen diffusions. PSD estimation methods are usually classified into two main groups of independent and dependent

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methods. The former ones are more accurate but usually very cost demanding and are often used for comparison or validation purposes [26]. Mercury porosimetry, X-ray diffraction (XRD), small angle X-

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ray scattering (SAXS), small angle neutron scattering (SANS) and nuclear magnetic response (NMR) are a few samples of such independent methods [27-33]. Mercury porosimetry is more common for PSD

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determination of conventional adsorbents, such as active carbon and control-pore glasses (CPG) [27], while other methods are usually more adequate for extremely well structured materials such as MOFs and

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MCM-41(Mobil Composition of Matter No. 41 (mesoporousmolecular sieves))[28,29,32,34]. The latter dependent group for PSD estimation are often less expensive and can be applied to almost all

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adsorbents. Almost most of these techniques [e.g. BJH(Barret, Joyner and Halenda), HK(Horvath– Kawzoe), KJS(Kruk–Jaroniec–Sayari), ND(Nguyen and Do (Do and co-workers)), DFT(Nonlocal density functional theory) and DBdB(Derjaguin-Broekhoff-de Boer)] require experimental adsorption (or

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condensation) isotherms coupled with some theoretical or analytical background [35-46]. Most of these

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techniques suffer from some shortcomings and sometimes unrealistic assumptions some of which are discussed in more details elsewhere [47,48]. Amongst the above methods, SAXS and HK procedures have received more attention for PSD estimation of MOF materials via independent and dependent

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methods, respectively.

In 2007, Tsao et al. employed SAXS method to determine various real structural details of MOF-5, including pore surface characteristics, pore shape, PSD, specific surface area, and pore-network structure [34]. They confirmed that SAXS method provides adequate textural properties such as pore surface, pore shape, PSD and pore volume, while, BJH and HK methods may fail for some adsorbents. The HK method (presented in 1983), has been widely applied for several PSD estimations of various MOF materials [17,25,41,45,49]. This method uses average potential function inside the slit-shape pores by employing the Kelvin equation [50] at the scale of molecule dimensions. Similar to other conventional PSD estimation techniques, HK method also requires a variety of detailed informations (such as diameter, polarizability and susceptibility of both adsorbent and adsorbate molecules and liquid density of adsorbate). Some of these data may not be readily available for many adsorption systems. Other conventional techniques (such as BJH) have also been extensively used to recover the PSD of various MOF materials [42,43]. In contrast to traditional dependent PSD recovery techniques, two recently proposed in-house methods [SHN1(Stands for Shahsavand – Niknam first method) and SHN2 (Stands for Shahsavand – Niknamsecond method)] require only adsorbate surface tension and its liquid molar volume which are 3

ACCEPTED MANUSCRIPT readily available for almost all adsorbates [51]. It should be emphasized that in contrast to many conventional techniques, both SHN1 and SHN2 methods do not require any information about the form of local adsorption isotherm or kernel. A detailed comparison of various PSD estimation techniques can

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be found in our previous articles [47,48]. In the current article, a fast microwave technique is described for synthesis of a Zeolitic Imidazolate

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Frameworks, ZIF-7. Afterwards, the collected CO2 experimental isotherms data along with four other data sets of water adsorption borrowed from literature [52], over four different MOF materials (HKUST-1,

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ZIF-8, MIL-101 and MIL-100(Fe)) are used to extract the corresponding PSDs via various in-house and conventional methods. The PSD prediction results are then validated by other independent techniques. It

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is worthwhile to mention that the most important reason for selection of ZIF-7 nano-porous material is related to its gate opening characteristic. The ZIF-7 sample can be opened or closed while it is faced with

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some gases such as CO2 in different pressure ranges. So, because of this capability we selected this structure to investigate its PSD determination via theoretical methods.

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2. Experimental Synthesis of Metal-Organic Frameworks can be carried out using several different methods such as

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solvothermal, microwave, sonochemical, electrochemical and mechanochemical [18,53,54]. The

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microwave method has been employed in this work to produce ZIF-7 adsorbent as fast as possible.

2.1. Synthesis of ZIF-7 The ZIF-7 was synthesized by a microwave-assisted procedure, following the procedures given elsewhere [55-56]. In this method, benzimidazole (C7H6N2, 98%, from Aldrich) was used as linker, while, zinc nitrate (Zn (NO3)2.6H2O, from Fluka) was employed as metal source and N, N-dimethylformamide (DMF) (99%, Aldrich) is recruited as dispersant in a solvothermal reaction. In a typical experiment, 0.2347 g (2 mmol) benzimidazole and 0.8025 g (2.7 mmol) Zn (NO3)2.6H2O were dissolved in 75 ml DMF under sonication for 10 minutes. The homogeneous solution was then evenly transferred to two 80-ml reaction vessels, with about 37.5 ml each. The reaction vessels are capped tightly and kept in the microwave reaction system (Multiwave 3000/synthos 3000, Anton Paar). Synthesis was carried out at 130 °C with a heating rate of 5 °C·min-1. The reaction was performed under autogenous pressure for 200 min and, then, the product was removed from the reaction vessels and allowed to cool to the room temperature. The mother liquor was carefully decanted from the product and replaced with methanol. Fresh methanol was used to exchange the DMF for 48 hours at room temperature. After decanting the extra methanol and drying in air for 24 hours, white crystals was obtained. The guest molecules in the crystals were removed under a dynamic vacuum at 150 °C for 12 h.

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2.2. Characterization of ZIF-7 The pore textural properties including the specific Langmuir area, Brunauer–Emmet–Teller (BET) surface

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area and pore volume were performed by analyzing adsorption and desorption N2 isotherms at 77K with Micromeritics_ASAP 2020 built-in software. Also Micromeritics_ASAP 2020 built-in software coupled

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with HK method was employed to calculate pore size of ZIF-7.

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2.3. Adsorption of carbon dioxide on ZIF-7 at low pressures Carbon dioxide adsorption equilibrium on ZIF-7 sample was measured in the Micromeritics_ ASAP 2020 adsorption apparatus at different temperatures and CO2 pressure up to 800 mmHg. Ultra-high purity

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carbon dioxide (99.999%) was utilized for the adsorption measurements. About 0.1 g of adsorbent was used for the gas adsorption studies. The initial degassing process was carried out at 150 °C for 12 h under

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a 0.0001 mmHg vacuum pressure.

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3. Results and Discussion The above experimental techniques and corresponding apparatuses were used to collect the following

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measurements.

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3.1.Physical properties of ZIF-7 scanning electron microscopy (SEM) images of the ZIF-7 crystals prepared in this work are shown in Figure 1. As it can be observed, the crystals are in flower shape. Furthermore, the powder X-ray diffraction (XRD) patterns of the ZIF-7 sample are shown in Figure 2. The main peaks are identified by comparing the observed pattern with calculated pattern from the established crystal structure data. It can be seen from Figure 2 that the experimentally observed pattern agrees well with the simulated pattern, indicating that the bulk sample is the same as the single crystal. Carbon dioxide adsorption was employed for the sample pore textural properties. The adsorption isotherms of Carbon dioxide at three temperatures (273, 298, and 323K) and gas pressure up to 800 mmHg(1 mmHg=133.322Pa) are demonstrated in Figure 3. All temperatures are achieved by using a Dewar with a circulating jacket connected to a thermostatic bath with a precision of ±0.01 °C. As it has been shown, CO2 isotherm shapes at 273 and 298 K are like type IV BDDT(Brunauer, Deming, Denting, Teller) [50] or IUPAC [57] Classification of adsorption isotherms. By using the adsorption isotherm of N2 at 77K, the textural properties were calculated as it is demonstrated in Table 1. The Langmuir and BET specific surface area, maximum pore volume and HKmedian pore size were calculated by the ASAP 2020 analyzer’s built-in software.

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ACCEPTED MANUSCRIPT Evidently, the value of pore size of ZIF-7, shown in Table 1, suggests that the synthesized sample lies in micro-porous material category. The mean diameter of 0.4310 nm reported by Yaghi et al. [55] for ZIF-7 is in good agreement with our measurements presented in Table 1. These obtained and reported values

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using HK method and Yaghi et al. [55] article will be compared with the PSD estimation via SHN methods in next sessions.

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4. PSD determination As mentioned earlier, pore size distribution of adsorbents is one of the most significant textural properties that directly affects on the other adsorbent specifications. Various methods could be applied for PSD calculation however choosing the most convenient procedure is vital to achieve reliable size of pores. In

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this section at the first, SHN2 method would be utilized to extract the mean size of pores of the produced ZIF-7. Then, the obtained results will be compared with available performances of the HK method

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(presented in Table 1) and reported one by Yaghi et al. Subsequently, both SHN1 and SHN2 method are applied for PSD determination of four different MOFs borrowed from literature via applying water

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adsorption isotherm data [52].

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4.1. Theoretical estimation of optimal PSD

Detailed description of SHN1 and SHN2 are presented in our previous articles [47, 48]. A brief overview

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will be presented here to familiarize the reader with the essence of the proposed methods. The following integrals are usually used to find the PSD of a heterogeneous solid adsorbent (f(r)) from a set of noisy measured isothermal data available for the amount of adsorbed material at a given sets of pressures (Pi; i=1,…,n):

Ψ ( Pi ) =

rK ( Pi )

∫ 0

Ψ ( Pi ) =

rk ( Pi )

∫ 0

f ( r )dr

f ( r )dr + t

rmax

∫

rk ( Pi )

2 f (r ) dr r

(mere condensation)

(1)

(condensation with a prior adsorbed layer)

(2)

The ill-posed problems of finding f(r) from the above integrals can be replaced with a set of linear algebraic equations {( RT R + λ Β TΒ ) f (r ) = RTψ } , using a combination of inverse theory and linear regularization technique. In this equation, the N × M t coefficient matrix R ∈ ℜ

N    N × M ( Pi )  i =1  

∑

( M ( Pi ) is the

number of discretized intervals between rK ( Pi −1 ) and rK ( Pi ) and N is the number of data points of any isotherm) has usually much more columns than its rows, the overall PSD column vector ( f ∈ ℜ

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N   M ( Pi )×1  i =1 

∑

) has

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Ψ is a [N × 1] vector and λ is the regularization parameter. Detailed

descriptions of various R and B matrices have been received proper attention elsewhere [47, 48].

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After construction of matrix R and using proper order of regularization technique (with appropriate form of matrix B), the optimal level of regularization parameter (λ*) should be selected to establish the best

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stabilization of the solution vector f(r). Our other recently published article reviews various stabilization criteria (e.g. LOOCV(Leave One Out Cross Validatio), LC(L-Curve method), MLC(Modified L-Curve

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method), UC(U-Curve method) and MUC(Modified U-Curve method)) for automatic selection of optimum regularization parameter (λ*) [58].

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Various real case studies will be employed to investigate the capability of SHN1 and SHN2 methods for determination of correct pore size distributions for various MOF materials. These methods are more

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suitable for types IV and II (SHN2) or V and III (SHN1) isotherms of BDDT or IUPAC classification. Various measured PSD’s of ZIF-7 at different temperatures (as presented in Figure 3) and borrowed experimental PSD’s [52] of HKUST-1, ZIF-8, MIL-101, MIL-100(Fe) will be used to validate the

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successful performances of SHN1 and SHN2 methods on capturing the true PSD from real adsorption isotherms. It is worthwhile to mention that the predicted PSD’s reported by Yaghi et al. for ZIF-7 [55],

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are computed via independent method of topology study. Finally, two isotherms of carbon dioxide adsorption over ZIF-7 (Figure 3) at two different temperatures and four water adsorption isotherms on

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different MOF materials (Figure 4) are used to calculate the corresponding PSD’s of five MOFs by resorting to SHN1 and/or SHN2 methods.

4.2. Optimal calculation of PSD As SHN1 and SHN2 methods are based on regularization technique the order of regularization and also the value of regularization parameter (level) have crucial effect on the overall performances. Figure 5 (a) and (b) illustrate the efficient effect of the order and level of regularization on PSD calculation of one of the studied samples (e.g. ZIF-8) respectively (The effect of these two parameters for other MOFs are depicted in supplementary material). As it has been shown in this Figure, three order of regularization, namely first, second and third show the same behavior. It proves that these methods can be readily used at various orders of regularization; however, the cost of time will increase with increasing the order of regularization. Furthermore, almost always the zero order of regularization technique provides relatively non-smooth solutions, while other regularization orders lead to very similar PSDs. It has to be noted, regularization orders more than three can be easily developed and successfully used for PSD recovery [58].

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ACCEPTED MANUSCRIPT On the other hand, regularization level is more important parameter that could adversely affect the produced results, if they are selected improperly. The effect of this factor on the PSD performance via SHN methods (Figure 5 (b)), was reproduced in four levels (λ). Based on final conclusion of

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regularization order (last paragraph) and also to minimize the number of figures without missing any vital information, the effect of regularization level were investigated for the first order of regularization.

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As it can be seen, the too small values of λ could not predict the accurate values of PSD and result in highly oscillatory solutions. On the other hand, extremely large values of regularization parameter

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ignore the information embedded in the coefficient matrix [47,48] used in SHN methods and pushes the final solution, f(r) toward a priori information sought for the specific order of regularization (a constant

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for first order regularization). Therefore, finding the optimal regularization parameters (λ*) have important rule in accurate PSD recovery. Since the value of regularization level can vary in the entire

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domain of positive real numbers there should exist an optimal value of regularization parameter for each isotherm which provides the optimal PSD recovery performance. Evidently, the optimal value of λ* tends to zero for well-behaved RTR (coefficient) matrices. On the contrary, the value of λ* will

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increase to infinity when the corresponding matrix RTR becomes severely ill-conditioned, i.e. singular

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[47,48].

Furthermore, the optimal regularization level can be selected by visual method when the actual PSD is known. Otherwise, more advanced techniques like leave one out cross validation (LOOCV), L-curve,

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U-curve and Modified U-curve are required for automatic selection of the optimal regularization level. This point has been received more attention in the literature by Shahsavand and Niknam [58]. Finally some important notes are remembered as follows. (a) Shown results in Figure 5(a), were depicted at the optimal regularization values (λ*) that are stated in parenthesis. (b) SHN1 was used for PSD determination in ZIF-8 sample and (c) Because of uniformity of pore sizes in MOF materials, the median point of calculated pore size distribution should be considered as a dominant size of pores. Consequently, the first order of regularization and LOOCV criterion were employed for PSD estimation of all MOF samples using adsorption isotherms (Figure 3 and 4). Figure 6 shows the PSD of ZIF-7 extracted from SHN2 method. As expected, the obtained results from CO2 isotherms at 273 and 298 K don't show significant difference. As a result, any adsorbate at any arbitrary temperature which exhibits isotherm shapes like type IV and II or V and III can be used for PSD recovery via SHN methods. Moreover, the obtained results from SHN2 and HK methods have been demonstrated and compared together in Figure 6. As mentioned in introduction, the HK method is one of the mostapplied procedures to calculate PSD of MOFs in which usually N2 adsorption isotherm at 77 K is used [34].

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ACCEPTED MANUSCRIPT Dashed line in Figure 6 illustrates the actual(real) pore diameter of ZIF-7, obtained via structure study [52]. As it can be seen, recovered PSD via SHN2 method is in good agreement with real diameter and estimated pore diameters using HK method.

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The calculated PSD for other samples by using SHN methods are depicted in Figure 7. Water adsorption isotherms at 298K (Figure 4) were used for their PSD recovery.

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Evidently, Figure 7 confirms the remarkable performance of SHN methods to predict the actual pore sizes. It is important to know that the SHN1 method was employed for ZIF-8 (because its water isotherm

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exhibits type V BDDT classification) and SHN2 was utilized for HKUST-1, MIL-101, MIL-100(Fe) (because their water isotherms show type IV BDDT classification).

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As before, in this Figure, dashed lines reflect the real (actual) PSDs extracted from literature (via topology or morphology studies) [52]. Obviously, all predicted PSDs by using SHN methods reveal the highest

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agreement with actual data.

As it was concluded, the first order of regularization was used for all PSD calculations. The optimum values of regularization levels for each adsorbent calculated via LOOCV criterion are presented in Table

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2. If other regularization orders are employed, the optimum values change as shown in Table S.1 in

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supporting information.

Table 3 also includes all required information necessary in PSD recovery via SHN methods. It should be pointed out that successful application of SHN methods for adsorbents like AC, CMC and CPG has

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already been proved [47,48].

5. Conclusions A typical zeolitic Imidazolate Frameworks, known as ZIF-7, was successfully synthesized using fast microwave fabrication technique. The collected Zif-7 is then characterized using various experimental facilities such as SEM and CO2 adsorption apparatus. Carbon dioxide isotherms at three different temperatures were used coupled with two in-house (SHN) methods to extract the unique PSD of ZIF-7. Furthermore, four other experimental isotherms for adsorption of water vapor at 298 K on various MOF materials were borrowed from literature and the corresponding PSDs were successfully predicted via same SHN procedures. Both of these in-house methods do not require any restrictive pre-assumptions and only need minimum information to predict PSD for any shape of pores with any range of pore size. Since both SHN1 and SHN2 techniques have deep roots in advanced mathematical topics such as inverse theory and linear regularization method, therefore, the effect of regularization order and the corresponding optimal regularization level were discussed in sufficient details. The computation results clearly indicated that the order of regularization has minor effect on the overall performance of the SHN methods. On the other hand, the optimal value of the regularization parameter has crucial effect on the 9

ACCEPTED MANUSCRIPT final extracted PSD. The leave One Out Cross Validation (LOOCV) technique was successfully used for efficient calculation of the optimal regularization level for any given order of regularization.

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List of Tables

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Table 1. Pore textural properties of ZIF-7

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Table 2. The optimum levels of regularization (λ*) for all adsorbents at first order of regularization.

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Table 1. Pore textural properties of ZIF-7 Langmuir surface

HK- median pore

/m2·g-1

area /m2·g-1

diameter /nm

281.97

526.62

0.4206

Maximum pore

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0.0904

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volume /cm3·g-1

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Table 2. The optimum levels of regularization (λ*) for all adsorbents at first order of regularization. First order of regularization Adsorbent ZIF-8 MIL-100(Fe) MIL-101 HKUST-1 ZIF-7 * ିସ ିହ ିଽ 0.66 7.1 × 10 5.7 × 10 4.1 × 10 4.3 × 10ି଺ λ

Description

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Table3: The only required physical properties for PSD recovery via SHN methods [51].

Σ

N· m-1

Adsorbate liquid molar volume

vM

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Value ૝. ૞ૢ × ૚૙ି૜ ૞. ૠૡ × ૚૙ି૝ ૠ. ૛૟ × ૚૙ି૛ ૝. ૠ૞ × ૚૙ି૞ ૟. ૚ૡ × ૚૙ି૞ ૚. ૡ૙ × ૚૙ି૞

Remarks CO2 at 273K CO2 at 298K H2O at 298 CO2 at 273K CO2 at 298K H2O at 298K