Evaluation of the BET Theory for the Characterization of Meso and Microporous Mofs
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REVIEW BET Theory www.small-methods.com Evaluation of the BET Theory for the Characterization of Meso and Microporous MOFs Filip Ambroz, Thomas J. Macdonald,* Vladimir Martis, and Ivan P. Parkin* together by organic bridging ligands to Surface area determination with the Brunauer–Emmett–Teller (BET) method form 1D, 2D, or 3D structures as illus- [30] is a widely used characterization technique for metal–organic frameworks trated in Figure 1. Since there are a (MOFs). Since these materials are highly porous, the use of the BET theory variety of different metal ions and organic linkers, essentially an infinite number of can be problematic. Several researchers have evaluated the BET method to possible combinations exist.[31] gain insights into the usefulness of the obtained results and interestingly, Furthermore, since MOFs are also their findings are not always consistent. In this review, the suitability of the relatively simple to prepare, building BET method is discussed for MOFs that have a diverse range of pore widths components can be well designed which can result in the production of targeted below the diameters of N2 or Ar and above 20 Å. In addition, the surface area of products.[33–35] Computational calcula- MOFs that are obtained by implementing different approaches, such as grand tions have proven to be useful in pre- canonical Monte Carlo simulations, calculations from the crystal structures dictions of interactions between guest or based on experimental N2, Ar, or CO2 adsorption isotherms, are compared hosts and the framework leading to pos- and evaluated. Inconsistencies in the state-of-the-art are also noted. Based on sible synthesis routes for MOFs with [36,37] the current literature, an overview is provided of how the BET method can give required properties. They are often [38–40] useful estimations of the surface areas for the majority of MOFs, but there are compared to zeolites; however cer- tain characteristics are often not present some crucial and specific exceptions which are highlighted in this review. in zeolites including big pore sizes, high sorption capacities, and complex sorption behavior.[6,41] New MOF structures with 1. Introduction diverse properties are still emerging with ever increasing spe- cific surface areas, that have to be determined in the course of Metal–organic frameworks (MOFs) are one of the most MOF characterization.[42–44] In this regard, correct measure- highly porous materials and since their discovery, have been ments of the surface areas are valuable as this is a very impor- thoroughly investigated as a result of their specific properties tant characteristic of microporous materials. Usually the sur- that are exceptional among known materials.[1–3] They combine face areas of MOFs are predicted by implementing the theory two disciplines, namely organic and inorganic chemistry[4] of Brunauer–Emmett–Teller (BET) where the surface areas are and possess many favorable characteristics, for example high derived from gas adsorption isotherms at the boiling point of a [45,46] [47,48] surface area, high porosity, tenability, reproducibility, high gas. Typically, N2 gas is used for this purpose. How- sorption capacities, facile syntheses, and good possibilities for ever, other gases, for instance Ar or CO2 or even organic vapors scale up.[5–8] Therefore, such materials can be used in numerous (dynamic vapor sorption) may also be used.[49–52] While there [9–11] [12–14] applications, for instance catalysis, gas storage, gas have been attempts to use H2, they remained in the form of separation,[15–17] drug delivery,[18–20] luminescence,[21–23] solar discussion.[53] Surface area can also be reported as the Lang- cells,[24–26] and batteries.[27–29] This class of porous polymeric muir surface area that is defined in terms of the monolayer material is assembled by the connection of a metal ion linked being the limit of adsorption.[54,55] Many researchers have investigated the suitability of the BET theory since it is based F. Ambroz, Dr. T. J. Macdonald, Prof. I. P. Parkin on several assumptions that may not be ideal for determining Department of Chemistry the surface area of microporous materials such as MOFs.[56–58] University College London London WC1E 6BT, UK Many concerns were focused on the pore-filling mechanism E-mail: [email protected]; [email protected] and uneven monolayer formation which is discussed in the Dr. V. Martis sections below. For amorphous porous materials, the most con- Surface Measurement Systems Ltd. venient way to determine the surface area is from adsorption Unit 5 Wharfside Rosemont Road, Middlesex, HA0 4PE, UK isotherms as opposed to crystalline materials where as a result The ORCID identification number(s) for the author(s) of this article of a known crystal structure, geometric methods can regularly can be found under https://doi.org/10.1002/smtd.201800173. be applied.[59,60] © 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA In this review, the applicability of the BET method for deter- Weinheim. This is an open access article under the terms of the Creative mining surface areas of MOFs with various pore widths is Commons Attribution License, which permits use, distribution and discussed. Reported drawbacks and limitations of the BET reproduction in any medium, provided the original work is properly cited. theory which can lead to unreliable results are also covered. For DOI: 10.1002/smtd.201800173 evaluation of the BET method, experimental data have to be Small Methods 2018, 1800173 1800173 (1 of 17) © 2018 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.small-methods.com Filip Ambroz is currently an Engineering Doctorate (EngD) student in the Department of Chemistry at University College London (UCL). He received his M.S. degree in molecular mod- eling and materials science from UCL and B.S. degree in Figure 1. Assembly of 1D, 2D, and 3D network structures of MOFs. Reproduced with permission.[32] Copyright 2003, Elsevier. chemical engineering from the University of Maribor, compared. Hence, the surface areas of MOFs obtained with dif- Slovenia. Before starting his ferent approaches, namely the values calculated from the crystal EngD, he did research for a year at the University of South structures and from the simulated isotherms (applying BET Australia under the supervision of Prof. Thomas Nann. theory), are examined and compared against the experimental His research is closely linked with the industrial company, results. Surface Measurement Systems Ltd. In addition, his work includes the synthesis and characterization of microporous materials that can be used for renewable energy generation. 2. BET Theory Thomas J. Macdonald is The fundamental element of BET theory is associated with the [61–63] currently a Ramsay Memorial adsorption of a gas on the material’s surface. This phenom- Fellow in the Chemistry enon is caused by van der Waals forces that are created by a film Department at University of the adsorbate, which consists of atoms, ions, or molecules College London. He is also on the surface of a substance that adsorbs these particles. The [64,65] a visiting researcher in the process of adsorption can be physical or chemical. While Chemistry Department at physical adsorption is related to van der Waals forces, chemical Imperial College London. adsorption is a result of the chemical reaction between the solid Prior to this, he was a and the adsorbate (gas).[66,67] The amount of the adsorbed gas on [68] Research Associate in the the adsorbent material can be correlated with its surface area. group of Prof. Ivan Parkin There are several parameters that have influence on this pro- working on the synthesis of cess for instance, temperature, pressure, characteristics of the [69,70] gold nanomaterials. He obtained his Ph.D. at the University material, etc. BET theory is closely related to Langmuir of South Australia in 2016 working in the research group theory. The latter assumes that gas molecules form a monolayer [71,72] of Prof. Thomas Nann where he studied the synthesis of adsorption which is an ideal situation. In such formations, inorganic nanomaterials for water splitting and photovol- all the molecules that are adsorbed are in contact with the sur- taics. His research interests are in the synthesis of functional face of the adsorbent’s material. This coverage can occur in a nanomaterials for renewable energy conversion and storage. closed-packed structure where molecules are tightly next to each other or they can also be spread around the surface. The gas molecules are held on the surface by gas–solid forces. On the Ivan P. Parkin obtained his other hand, with multilayer adsorption more than one layer of B.S. and Ph.D. (supervisor gas molecules are formed, consequently not all of them are in Prof. Woollins) degrees from contact with the surface layer of the adsorbent. Therefore, vapor- Imperial College London and phase interaction between gas molecules occurs. These vapor did his postdoctoral research phase interaction energies are similar to gas–solid interaction at Indiana University with energies leading to the phenomenon of gas adsorption on the Prof. Chisholm FRS. He was top of gas molecules that are already adsorbed on the surface of appointed to a temporary lec- a solid. As a result of that, a gas condenses to a liquid phase.[73] tureship at the Open University Multilayer adsorption is a typical phenomenon when tempera- before moving to UCL in ture of the absorbent’s surface is lower than the critical tem- 1993. Having been head of the perature of gas molecules. When layers of adsorbate molecules Chemistry Department for six are formed, the pressure is increasing until it reaches a value and a half years, he has recently been appointed as Dean of similar to the bulk vapor pressure when the bulk condensation the Faculty of Mathematical and Physical Sciences.