Carbon Films for CO2-Sensing at High Pressure | ACS

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MIL-53(Al)/Carbon Films for CO- Sensing at High Pressure Pascal Freund, Lukas Mielewczyk, Marcus Rauche, Irena Senkovska, Sebastian Ehrling, Eike Brunner, and Stefan Kaskel ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05368 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019

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ACS Sustainable Chemistry & Engineering

MIL-53(Al)/Carbon Films for CO2- Sensing at High Pressure Pascal Freund#, Lukas Mielewczyk#, Marcus Rauche+, Irena Senkovska#, Sebastian Ehrling#, Eike Brunner+, Stefan Kaskel#* #Inorganic

Chemistry I, Technische Universität Dresden, Bergstraße 66, 01062 Dresden,

Germany +Bioanalytical

Chemistry, Technische Universität Dresden, Bergstraße 66, 01062 Dresden,

Germany *Corresponding Author. E-mail: [email protected]

KEYWORDS: metal-organic frameworks, percolation threshold, gas sensor, composites, high pressure, CO2/CH4

ABSTRACT

Chemiresistive threshold sensor films based on the switchable metal-organic framework (MOF) MIL-53(Al) (MIL = Matériaux de l’Institut Lavoisier) and conductive carbon additives were developed, characterized and successfully applied for selective detection of CO2 in mixtures with methane at high pressure (up to 25 bar).

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Two transitions of the crystal structure, from the lp (large pore) form to the np (narrow pore) form at CO2 partial pressure below 0.5 bar and back to the lp form at ca. 6 bar, also known as breathing, result in two detectable stepwise resistance changes of sensor film, if CO2 is present in the mixture with methane. The sensor shows a rapid response of the specific resistance (response time ca 25 s), which directly correlates to the expected structural change of the switchable MOF. Two films containing carbon or carbon nanotubes as conductive component were compared regarding the influence of the additive particles geometry on sensing performance, showing benefits of the isotropic particle shape. Cyclic sensing measurements proved the durability of the composites.

INTRODUCTION Global warming and the associated climate change caused by the emission of greenhouse gases plays a crucial role for a sustainable society as it poses a threat to the entire global ecosystem.1-4 Despite CO2 has a lower global warming potential than other greenhouse gases, such as ozone, nitrous oxide or halogenated hydrocarbons,5 it plays a major role in climate change simply because of the huge emission amount. Fossil fuel, one of the main sources of CO2 emission, currently ensures the production of about 85% of global energy.6 There are two main strategies discussed for the effective reduction of CO2 emission. On one hand, by capture and sequestration processes, the CO2 can be recaptured from the emitting power plants and stored geologically under high pressure.7-8 On the other hand, switching to more clean energy sources, such as natural gas, would reduce CO2 emissions. One of the major impurities present in natural gas is CO2 (up to 65%). The separation of CO2 under high pressure is important to overcome the corrosion limitations during transport and


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distribution.9-10 For separation applications it is important to be able to detect (to determine) the exact amount of CO2 in the gas mixture under high pressures. Devices for detection under these conditions are generally based on infrared techniques and are space-consuming and costly or still under development.11-12 Metal-organic frameworks (MOFs) have been proposed among others as active materials for sensor devices, due to their outstanding porosity and highly selective interactions with gases.13-19 The majority of MOF sensors that have been reported are based on luminescence quenching or color change.20-22 Sensor applications could thereby be implemented by the use of additional, for example optical detectors, but would require a higher preparative effort. In some MOFs, the so-called 3rd generation MOFs,23 the interaction with specific analytes can lead to the selective structural changes of the framework, which result in changes of a wide variety of properties such as density, color, porosity, permeance, refractive index, fluorescence and more.24-27 In MOFs showing phenomena such as gating,28 breathing29 or the recently discovered negative gas adsorption (NGA),30 the structural change caused by gas adsorption is accompanied, among others, by the change in volume of the crystals. All these property changes can be potentially used as sensor response signal. However, despite the numerous properties of MOFs that suggest them as attractive chemo-sensory materials, their implementation has been largely limited by one major challenge: signal transduction.21 The conversion of this chemical response directly into an electrical signal is a big step towards smaller and mobile sensors. Although, in the last years several materials with demonstrated conductivity or charge mobility were reported,16 there are no conductive, switchable MOFs developed so far, to the best of our knowledge. A conceptual approach developed recently by us31 makes use of the step-wise change of conductivity near the percolation threshold in any switchable MOF/carbon composite.


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The percolating conductive carbon networks in such composites is disrupted by expansion of the crystals during gas adsorption, so the volume change upon gating of switchable MOFs can be exploited in principle for use in gas sensor technology at ambient pressure.31 In this case, the chemical signal can be converted directly into an electrical signal (chemiresistive), eliminating the need for additional detectors or larger installations. In this work we present a new carbon nanotube based MOF-film containing the switchable metal-organic framework MIL-53(Al) as active component. For the first time we demonstrate repeated detection of CO2 in high pressure gas mixtures in composite-based chemiresistive sensors.

EXPERIMENTAL SECTION Materials: All used reagents were purchased from commercial suppliers and used without further purification. The solvents were at least of analytical grade. The Carbon Black (Fig. S6b) purchased from ABCR (density 0.2 g/cm3, average particle diameter of 42 nm , surface area 64 m2/g) was sieved using a 250 µm sieve to suppress agglomeration before use. SW-CNT (TUBALLTM) had the following specifications: Carbon content > 85 wt.%, CNT content ≥ 75 wt.%, number of layers 1-2, outer mean diameter 1.8 ± 0.4 nm, length > 5 µm and metal impurities < 15 wt.% (Fig. S6a). Synthesis of MIL-53(Al). According to a procedure published by Loiseau et al.29, Al(NO3)2·9H2O (14.13 g, 37.6 mmol) and terephthalic acid (H2bdc, TPA) (3.13 g, 18.8 mmol) were mixed in 55 mL deionized water in a 250 mL Teflon-inlet stainless steel autoclave and heated at 493 K for 24 h. After cooling to room temperature, the crystalline solid was separated and washed with water (3 times) before suspending in 150 mL DMF and heating at 403 K


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overnight.32 To remove residual terephthalic acid, the MOF was washed again with 150 mL DMF, filtered and calcined for 72 h at 603 K in air. Percolation Threshold Estimation. The conductivity percolation threshold is specified as the critical concentration of conductive additive in the MOF based composite, at which the overall electrical conductivity shows a drastic change from the isolating behavior of the pure MOF to the high conductivity of the added carbon source (carbon clack (CB), single-walled carbon nanotubes (SW-CNTs)) forming the percolating network in the composite. To determine this value defined amounts of either carbon black or SW-CNTs were added to the MOF suspension and processed (more details are given below) into the film, the resistance of which was measured. In the case of carbon black, the series of composites was prepared starting with the lowest additive concentration of 2 wt.%, which was stepwise increased by 0.1 wt.%. In case of SW-CNTs the lowest concentration was 0.1 wt.%, which was stepwise increased by 0.05 wt.%. Preparation of Sensing Films. Carbon Black (2.9 mg) or SW-CNTs (0.15 mg) were added to 4 mL of ethanol and sonicated for at least 30 min until the suspension looks homogeneous. Under further sonication, MIL-53(Al) (45 mg) was added and mixed for 5 more minutes. Afterwards the suspension was stirred at 353 K to evaporate the solvent almost completely, before addition of approximately 5 mg of polytetrafluoroethylene (PTFE) suspension (60 wt.% PTFE in H2O). During further stirring, the mixture was heated again (353 K) to obtain a doughlike, dried blend that was subsequently rolled to a film of rather 2 cm2 in size and 500 µm thick. Finally, pieces of around 2 by 4 mm were cut out and fixed on a glass slide with silver conductive paint (Figure S1). Ambient Pressure Resistance Measurements. The prepared sensing films were activated under vacuum at 373 K for 10 h, transferred in the measuring setup (Figure S2) under argon


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atmosphere and flushed with 100 vol.% n-butane for 5 min, followed by 100 vol.% synthetic air to obtain MIL-53(Al) in the np form, before exposed to a 50 mL min-1 syn. air flow containing defined amounts of n-butane. High Pressure Resistance Measurement. High pressure measurements were performed for carbon dioxide (CO2), methane (CH4) and a mixture of both with a ratio of 3:1 (CO2/CH4) up to 25 bar. Therefore, a stainless steel autoclave was prepared with two ceramic feedthroughs to ensure electrical contact only to the sensor and not to the autoclave itself (Figure S3). The sensor was placed inside the measuring cell without previous heating and then installed in the setup (Figure S4). Afterwards, the cell was evacuated for 3 h before gradual increasing the pressure. To ensure that CO2 adsorption/desorption equilibrium is reached, after increasing/decreasing the pressure at each point the sample was equilibrated for 6 minutes before recording the value for the resistance. Electrical resistance changes were recorded by a connected multimeter. Characterization. n-Butane physisorption measurements (n-butane purity: 99.95 %) were performed at 298 K up to 1 bar using a BELSORP-max (Microtrac BEL). The volumetric CO2 and CH4 high-pressure physisorption measurements in the pressure range up to 20 bar were carried out at 298 K on the BELSORP-HP device, a thermostat F25-MA from JOLABO was used for temperature control. Powder X-ray diffraction patterns were collected using a STOE STADI P diffractometer operated with monochromatic CU Kα1 radiation (λ = 0.15405 nm, 40 kV, 30 mA), equipped with a 0D gas-filled detector, measured in transmission geometry with a scan speed of 15 s step-1 and a step size of 0.3°. Scanning electron microscopy (SEM) was performed on a HITACHI SU 8020 instrument. Thermogravimetric analysis (TGA) were carried out on a STA 409 PC Luxx (Netzsch) thermal analyzer.


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RESULTS AND DISCUSSION Study the influence of the composite ingredients on sensor performance Switchable MOFs show a structural transition as response to the exposure to certain gases at defined pressures and temperature often accompanied by a change in the crystallite volume. This behavior is rather specific for each MOF and could be shown to be an useful feature in sensing architectures.31 The working principle of switchable chemiresistive sensors proposed by us, based on the conversion of a mechanical signal into an electrical one, is illustrated in Scheme 1. Since, there are no conductive, switchable MOFs developed so far showing major changes in their crystallite volume under gas adsorption, conductive additives have to be added to the composite to guarantee the electrical conductivity.

Scheme 1. Principle of the chemiresistive sensing: Gas adsorption leads to a distinct volume change of the switchable MOF crystals, which results in an increased resistivity of the MOF/conductive additive composite by disrupting the percolating network of conductive additive. To improve the sensor performance and processing of the device, rather thin active films are beneficial instead of pellet formulations or powder as reported by us before.31 To create such


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films, the carbon black and SW-CNTs (Fig. S6) were used as conductive additives to study the influence of the intrinsic characteristics of these additives to the final sensor performance. It should be mentioned, that there are many other conductive additives potentially conceivable, such as silver nanowires or graphene.33-36 Those form a percolating network through the composite at a certain concentration (percolation threshold) and thus give a pathway for electrons to flow. Since the MOF crystals expand during gas adsorption at a defined pressure, this network is broken up and the overall specific resistance of the material increases (pronounced drop in conductivity). For real applications, the produced sensing material should be chemically and mechanically stable as well easy to process (an installation of a loose powder in a sensor is not desirable). We focused on switchable MIL-53(Al), a representative of the so-called breathing MOFs, showing change in crystallite volume upon the transition from the large pore (lp) to the narrow pore (np) form and back. It is highly stable against chemical influences (such as solvents or humidity) and against mechanical effects (such as high pressure, repetitive adsorption or even ball milling).31, 37 In addition, it is inexpensive to produce (in regards to the starting regents) and easy to store (no need of protective inert atmosphere). To transform this MOF into a stable and manageable film, PTFE was used as binder. It can be seen in Figure 1 that after rolling, a compact layer is formed (a) in which the MOF crystals are interwoven with PTFE fibrils in order to keep the composite in shape but still guarantee flexibility (b). Hereby, an important characteristic is weather and how the binder influences the adsorption of the gas of interest. nButane physisorption experiments performed on the films obtained show that the adsorption profile is not significantly affected by formulation (Figure2).


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Figure 1. SEM images of the prepared sensor film that picture a compact packing of the MOF crystals (a) that are interwoven and held together by PTFE fibrils (b).

Figure 2. n-Butane physisorption isotherms showing a loss in adsorbed volume for the films (CB - blue circles, SW-CNT - red diamonds) compared to the pure MIL-53(Al) (black squares). Closed symbols refer to adsorption and open symbols to desorption branches.


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However, the uptake is reduced by ca. 20% due to the presence of non-porous components. The nitrogen adsorption isotherms of films show additional hysteresis below 30 kPa, characteristic for swelling behavior or diffusion limitation, typically observed for porous polymers38 (for nitrogen adsorption data at 77 K see Fig. S1, Tab. S1). Powder X-ray diffraction (PXRD) data shown in Figure 3 outline that the crystal structure of the MOF remains intact even after the mechanically demanding preparation procedure. Aside from the MOF and binder used to prepare the composite, the influence of the conductive additive on the final sensor performance was investigated as well. Additives that have too large particles would hamper the formation of a percolating network and reduce the effect of the expansion of the crystals.

Figure 3. PXRD pattern of: a) MIL-53(Al)/CB film; b) MIL-53(Al)/SW-CNTs film; c) Parent MIL-53(Al) used for film preparation; d) Theoretical pattern of MIL-53(Al)_np calculated from the crystal structure (CCDC 220477). All samples were not additionally activated and are in np form.


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An exact specification of the ideal particle size ratio of active component (MOF) to conductive additive is a very own topic and is therefore not discussed in this work. An increase in steric demand of the additive also leads to a deterioration of sensor performance as could be shown in the measurements (Figure 4).

(a)

(b)

(c)

(d)

Figure 4. Relative resistance changes of the films for different volume fractions of n-butane in synthetic air (above) and sensing studies at 50 vol.% n-butane in synthetic air for up to ten cycles with corresponding response times for each cycle (below). (a, c) Film containing CB; (b, d) Film containing SW-CNTs.


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To show that sensing in gas mixtures is feasible with those films, resistance measurements with n-butane in synthetic air were done under ambient pressure first. Comparing the performance of two films produced using two different conductive additives (Figure 4) but similar relative resistance changes gives a more precise view on the influence of additives. Both show the expected step-wise increase in the resistance at the corresponding volume fraction of n-butane in the gas mixture leading to a structural change of the MOF from the np to the lp form. Due to their high aspect ratio, the film made with SW-CNTs require significantly less conductive additive to be added (appr. one order of magnitude) to obtain these resistance changes. Sensor operation in the gas mixture at ambient pressure However, it also can be seen in the cycling studies (Figure 4c, 4d) that the fluctuation in the relative resistance changes of the individual cycles varies between the additives and the recorded response time for the individual cycles is significantly higher for the film produced with SWCNTs than for the sensor made with CB (tr in Figure 4c,d). This is due to the fact that the long CNTs (Fig. S6a) have to be pushed much further apart before they lose contact and the resistance increases. In contrast for small, spherical CB particles (Fig. S6b) even slight volume changes are sufficient to break up the conductive network. Although the structural change of the MOF occurs spontaneously, diffusion effects of the gas through the film influence the switching kinetic and cause a delay in complete volume change of a few minutes. Thus, sensor measurements in gas mixtures are feasible with both conductive additives, wherein CB is cheaper and shows a faster response.


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Detection of CO2 in CO2/CH4 mixtures at high pressure To further explore applications of the concept for separations under higher pressure, in addition to measurements under atmospheric conditions, high pressure measurements of gas mixture were carried out. High pressure physisorption measurements performed by Llewellyn et al. have shown a selective adsorption of CO2 over methane for MIL-53(Al).39 As a result, sensor applications for CO2 under high pressure should be possible as long as the active material can withstand these conditions.

Figure 5. Resistance measurements of the CB containing performed with (a) CO2 and (b) methane at 298 K up to high gas pressure. Red diamonds indicate the relative resistance change inside the material and black squares show associated physisorption data. Closed symbols refer to adsorption and open symbols to desorption branches.

High pressure resistance measurements upon CO2 adsorption were performed on activated films (the MOF is in the lp form at this conditions). As can be seen in Figure 5a, the film shows rapid changes of resistance at two defined pressures in pure CO2. Initially the resistance decreases at


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low pressures up to 0.5 bar due to the lp – np transformation of the MOF (Figure 5a inlet). It reduces the volumes of the individual crystals and brings the carbon particles closer together to gain higher conductivity. Higher amounts of CO2 should lead to a second transition step at 6 bar from the np to the lp form and another abrupt decrease in the resistance. This in the resistance change fits well to the second step of the physisorption isotherm. At this point the conductive particles are pushed apart and thus conductive paths are broken which results in a relative resistance change of approximately 15%. Until the end of the measurement at 20 bar, the resistance increases rather linear. This can be explained by the inclusion of gas molecules between the carbon particles due to the rising pressure. Here, the gas molecules act as a dielectric medium and thus reduce the conductivity of the composite. A comparison with the similar measurements in methane atmosphere (Figure 5b) clearly demonstrates that the stepwise volume change of the composite in case of CO2 adsorption causes the resistance change due to the structural transitions. For methane, however, only a minor, linear increase in resistance can be observed, which is caused by the inclusion of gas molecules. The high pressure CO2-desorption measurements demonstrate that also other properties of the physisorption isotherm, such as the occurrence of a hysteresis, are directly reflected in the progression of the resistance. In the CO2/CH4 gas mixture (3:1) the same step-wise resistance changes are observed but the gate opening pressure shifts from 6 to 8 bar (Figure 6). This is in exact correlation to the ratio of the gases in the mixture used and is therefore attributable to the change in the partial pressure of CO2.


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Figure 6. Resistance change measurements of the film in a CO2/CH4 mixture (ratio 3:1) at 298 K against the total gas pressure. The relative resistance change is represented by red diamonds, related physisorption isotherms are shown in grey: circles – methane, squares – carbon dioxide. Adsorption branches relate to closed symbols and desorption branches to open symbols.

CONCLUSION

Sensor films containing the switchable metal organic framework MIL-53(Al) as an active material, were developed and tested for their suitability in gas mixture sensing at high pressure, as well as their durability for up to ten sensing cycles. It was shown that using the demonstrated sensor concept, the structural change of the MOF during the selective adsorption of only very specific gases from the gas mixture can be converted into an electrical signal. Different conductive additives showed characteristic response time resulting from different degrees of anisotropy of the added particles. Small isotropic particles of carbon black effect shorter response times in comparison to highly anisotropic carbon nanotubes. The response times for several cycles of sensing for film containing CB are under 30 seconds and with this in an application-related range. High pressure measurements of CO2 in methane demonstrated the


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versatility of the material applications and gave a deeper insight into the exact dependency of the sensor principle on the transition of the MOF. It could be shown that the conversion of lp to np form causes a first sudden change in resistance at relatively low pressures (0.5 bar), whereby the detection of even low concentrations of CO2 in methane is feasible. This and a second structural transformation, with accompanying resistance change of the material, at higher partial pressures of CO2 (~ 6 bar) could be used for high pressure sensing in a wide range of concentrations at different pressures. The development of new switchable MOFs could further expand this concept. New structures with different gate pressures or new, volume changing phenomena would lead to the detection of a multitude of gases both in gas mixtures and under high pressure conditions. Hereby, mechanical and chemical stability are key features for a later application. Due to the compact design this integrated sensor concept is ideal for the integration into separation columns or natural gas transport lines which are operated under high pressure and require monitoring of the CO2 concentration by producing an alarm signal above a critical threshold that should not be surpassed.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge: Schematic illustration of sensing set-up at ambient pressure and high pressure, draft of the ceramic feedthrough build into the autoclave, image of prepared film (PDF)


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AUTHOR INFORMATION Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Financial support by DFG (FOR 2433, Switchable MOFs) is gratefully acknowledged. ABBREVIATIONS PXRD, powder X-ray diffraction; MIL, Matériaux de l’Institut Lavoisier; np, narrow pore form; lp, large pore form; CB, carbon black; SW-CNT, single-wall carbon nanotubes; MOF, metalorganic framework; DMF, N,N-dimethylformamide; NGA, negative gas adsorption; TPA, terephthalic acid; PTFE, polytetrafluoroethylene.

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TOC/Abstract Graphic – For Table of Content Use Only

Chemiresistive sensor films based on switchable MOF composites for potential application in sensing of CO2 in natural gas at high pressure.


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Carbon Films for CO2-Sensing at High Pressure | ACS

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