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Measuring water adsorption processes of metal-organic frameworks for heat pump applications via optical calorimetry
Accepted Manuscript Measuring water adsorption processes of metal-organic frameworks for heat pump applications via optical calorimetry Michelle Wöllner, Nicole Klein, Stefan Kaskel PII:
S1387-1811(18)30595-X
DOI:
https://doi.org/10.1016/j.micromeso.2018.11.024
Reference:
MICMAT 9200
To appear in:
Microporous and Mesoporous Materials
Received Date: 24 August 2018 Revised Date:
14 November 2018
Accepted Date: 15 November 2018
Please cite this article as: M. Wöllner, N. Klein, S. Kaskel, Measuring water adsorption processes of metal-organic frameworks for heat pump applications via optical calorimetry, Microporous and Mesoporous Materials (2018), doi: https://doi.org/10.1016/j.micromeso.2018.11.024. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT Measuring water adsorption processes of metal-organic frameworks for heat pump applications via optical calorimetry
Michelle Wöllnera,b, Nicole Kleina, Stefan Kaskela,b a
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Fraunhofer Institute for Material and Beam Technology, Chemical Surface and Reaction Technology, Winterbergstraße 28, 01277 Dresden, Germany b Dresden University of Technology, Inorganic Chemistry I, Bergstraße 66, 01069 Dresden, Germany
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Corresponding author: [email protected] (phone: +49 35146333632, fax: +49 35146337287)
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Abstract
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Keywords
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Optical calorimetry (broadband IR-detection of the released heat of adsorption, “InfraSORP”) during water adsorption processes is used for an accelerated assessment of porous materials performance in heat pump applications. Metal-organic frameworks (MOFs) are screened as a highly promising class of materials for water adsorption driven heat exchangers with high water adsorption capacities. Based on a proper calibration, optical calorimetry is demonstrated to allow for rapid estimation of the total water adsorption capacity at a given relative humidity. In a dynamic mode, full water adsorption isotherms can be measured using a step-wise increase of the relative humidity. As cycling stability is among the most critical issues for the integration of new porous materials into systems, the InfraSORP methodology provides a valuable and inexpensive tool for accelerated cycling and stability testing. The InfraSORP technique is demonstrated to provide a significantly accelerated automated and easy-to-acquire alternative as compared to conventional characterization methods.
Metal-organic frameworks, water adsorption, adsorption heat pump, optical calorimetry, InfraSORP
1. Introduction In recent years, metal-organic frameworks (MOFs) have become a focus of research for potential application in water adsorption heat pumps.[1–3] These crystalline coordination polymers are made of metal ions or metal oxide clusters connected by functional organic linkers and hence offer tunable pore structures and properties.[4–6] High specific surface areas, high pore volumes, narrow pore size distributions and a tunable hydrophilicity by open metal sites or polar linkers are unique properties that feature MOFs as attractive candidates for water adsorption applications such as adsorption-driven heating or cooling. MOFs
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surpass established materials for adsorptive heating or cooling such as zeolites, silica gel or metal aluminum-phosphates in terms of working capacity and regeneration conditions: For instance, zeolites require high regeneration temperatures due to the strong hydrophilicity.[2] In contrast, silica gel has a more hydrophobic character which results in low desorption temperatures but low water uptake in the necessary relative pressure range below 0.4, although the overall water uptake of silica gel is high.[7,8] Crystalline metal aluminumphosphates (aluminum-phosphate, AlPO or silica aluminum-phosphates, SAPO) are very attractive for heat exchanger applications due to the S-shaped water adsorption isotherm. This special type of isotherm leads to high working capacities within a small relative pressure range and low regeneration temperatures (< 100 °C).[9,10] However, these materials are more expensive than zeolites or silica gels due to their complex synthesis.[7] Several MOFs such as MIL-100 (Al, Fe)[8], MIL-101(Cr)[11–13] or Al-fumarate[14] outperform established heat exchanger materials in terms of higher working capacities, an S-shape isotherms in an appropriate p/p0 range, low regeneration temperatures and low synthesis costs (in contrast to AlPOs oder SAPOs). However, another critical issue of an appropriate material is the hydrothermal stability regarding several thousand water adsorption-desorption cycles which are performed during the lifetime of an adsorption heat exchanger.[7,15,16] One of the first MOFs that was tested in regard to water sorption heat exchanger was Al-fumarate. This MOF shows high hydrothermal stability up to several thousand water adsorption cycles.[14] In recent years, several MOFs were investigated for their suitability as heating or cooling materials.[1–3] In most cases, the materials were evaluated by their water isotherms in terms of working capacity and thermogravimetric cycle stability tests. However, these characterization methods are very time-consuming and often limit rapid discovery, screening and process quality assessment. In respect to the rapid development of novel materials, especially MOF structures, a screening method would be beneficial to identify simply the materials with the best performance related to the application-relevant working conditions. To speed up and simplify the characterization of porous materials, the InfraSORP technology was recently developed.[17,18] The InfraSORP technique is based on the optical detection of the temperature change of a sample due to the released heat of adsorption when it is exposed to an adsorptive. The magnitude of the resulting thermal response signal reflects the total heat of adsorption. In recent years, the InfraSORP technology was established successfully for characterizing various materials for adsorbent key parameters[18–20] or industrially relevant applications such as filter evaluation for the removal of toxic gases[21,22]. So far, the InfraSORP was used mostly for screening of n-butane adsorption capacity[19,22] and cycling stability studies of flexible MOFs[23]. Water adsorption is often kinetically hindered due to slow equilibration and hydrogen bonding rearrangements. This leads to long equilibration times. The InfraSORP technology offers enormous time-saving advantages for this purpose. Due to the low sample mass required for InfraSORP measurements (only a few milligram), the equilibration time is significantly shorter than for competing methods which require more than 10 times of the sample quantity for a precise measurement. Herein, we introduce InfraSORP technology for an accelerated evaluation of MOFs for water adsorption-based heat exchanger applications. Several MOFs and commercially used heat exchange materials as benchmark materials are characterized regarding their water uptake and cycling stability in respect to repeated water adsorption/desorption stress by the InfraSORP technology. The results are compared to volumetric physisorption experiments.
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2.1 Materials The used MOFs (Cu3BTC2, MIL-101(Cr), MIL-100(Fe), MIL-100(Al), CAU-10, Al-fumarate, Cu-pymo, UiO-66, DUT-4, ZIF-8) were purchased from Materials Center Dresden (www.metal-organic-frameworks.eu). The reference materials AlPO-5 (AQSOA Z01) and SAPO-34 (AQSOA Z02) were kindly provided by Mitsubishi Chemical Corporation (www.mchemical.co). Prior to testing, all materials were pretreated at temperatures of 150 to 200 °C under vacuum for 12 hours. The activation conditions are given in the Supplementary Information (Table S1) in particular.
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2.2 Screening by InfraSORP technology The InfraSORP measurement is realized in a single cell setup at 298 K and ambient pressure (1 atm). The sample is filled in the sample holder (2 mm in height and 8 mm in diameter) and activated in an external activation apparatus. After determination of the weight loss, the sample holder is placed in the InfraSORP device and purged with a nitrogen flow of 140 cm3 min-1 until the thermal equilibrium is observed. This purging process takes 10 min. The subsequent InfraSORP measurement is performed by using a humid gas flow of 140 cm3 min-1 nitrogen equivalent. The humidity is adjusted by a nitrogen flow through a water filled bubbler system at 295 K and a downstream nitrogen flow for dilution. For water capacity screening, the first adsorption thermal response is measured and the specific peak area A/m [Ks/mg] is determined by integration of the thermal response over a linear base line by using OriginPro 2017 software and deviation by the sample mass. Cycling experiments are performed automatically by specifying the adsorption and desorption time. Adsorption is realized by a 140 cm3 min-1 nitrogen flow of 80% RH and desorption by a pure nitrogen flow of 140 cm3 min-1 without further heating. Adsorption and desorption are carried out until a temperature equilibrium is reached, with the consideration that the adsorption and desorption times are constant within a 10-cycle test. Furthermore, a 100-cycle test of Al-fumarate is performed by using 400 cm3 min-1 80% RH nitrogen flow at 298 K.
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2.3 Volumetric physisorption experiments The volumetric physisorption experiments are performed at a Belsorp-max apparatus (Bel Japan Inc). The specific surface areas are determined by nitrogen physisorption experiments at 77 K by using the multi-point BET plot (0.05 < p/p0 < 0.2). Water isotherms are measured at 298 K. The reference water capacity is obtained from the water adsorption isotherms at a certain partial pressure related to the InfraSORP experiments. 2.4. Powder X-ray diffraction (PXRD) Powder X-ray diffraction data are collected in transmission geometry on a STOE STADI P diffractometer with Cu-Kα1 radiation (λ = 1.5405 Å) at room temperature.
3. Results and discussion To evaluate a potential heat exchanger material, the knowledge of the water capacity and related integral heat release is essential. Using the InfraSORP technology, the integrated peak area A (in K⋅s) is a relative measure of the total heat released during adsorption/desorption.[19] The mass related peak area A/m is a specific quantity and reflects
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the specific water adsorption capacity if the average heat of adsorption is assumed to be constant. While the differential heat of adsorption may vary significantly for different materials, the integral heat of adsorption, in particular at higher coverage is dominated by fluid-fluid interactions and thus less prone to variations. In such cases A/m shows a linear correlation to the adsorption capacity Cads. Various MOFs were measured at different humidity values (30% RH, 60% RH and 80% RH at 298 K) in the InfraSORP device. Fig. 1a shows the resulting specific peak areas A/m which are in a direct correlation to the respective water capacities Cads with an excellent regression quality factor of 0.97; A/m [K s g-1] = 0.4416 · 106 · Cads [g g-1] (eq. 1). The correlation is little affected by varying relative humidity values or the measurement of MOFs with different structural properties such as hydrophilicity, availability of open metal sites or pore structure. The impact of various adsorptive concentrations on the thermal response signal is insignificant as it is empirically supported by the overall linear correlation between specific peak area and water capacity. The heat capacity of humid air in the measured humidity range is between 1.02 kJ kg-1 K-1 (30% RH) and 1.03 kJ kg-1 K-1 (80% RH)[24,25] and hence, does not affect the thermal response signal. Adsorption enthalpy has only a significant impact on the thermal response if different adsorptives are used.[26] The effect of varying porosity on the adsorption enthalpy and hence the thermal response signal is also negligible which was shown in previous work.[19,26] Especially for water adsorption in MOFs, the major heat effect is caused by the condensation enthalpy which is around 50 ± 3 kJ mol-1.[1] Small enthalpy effects due to open metal sites for instance are negligible, especially in terms of heat resolution and measuring uncertainty of the InfraSORP technology.[26,27] Nevertheless, the good linear correlation between specific peak area and reference water capacity can be used to demonstrate that InfraSORP technology is ideally suited for the screening of MOFs for their water capacity. The measurement uncertainty is only 5% for water adsorption. The accuracy is determined by the average deviation of specific peak areas from repeated measurement (four times) of adsorption of 80% RH (298 K) for Al-fumarate as an example. The deviation is comparable to values reported in the past.[17,18,26] To verify the capacity evaluation by InfraSORP technology, the reference heat exchanger materials SAPO-34 and AlPO-5 are tested. For calibration, the linear fit eq. 1 from A/m – Cads plot is used. Fig. 1b shows the water adsorption capacities (80% RH, 298 K) which are determined by InfraSORP technology and reference physisorption experiments. The deviation of both values is with ca. 7% very low. Fig. 1. (a) Linear correlation between water capacity from reference physisorption experiments and specific peak area from InfraSORP technology of various MOFs measured at different humidity values at 298 K, raw data are available in Supplementary Information, Table S2 (b) Water capacity (80% RH, 298 K) from InfraSORP experiments (by using eq. 1) and physisorption measurements for Al-fumarate and reference materials AlPO-5 and SAPO-34.
The fact of a concentration-independent capacity evaluation allows a dynamic measurement of adsorption isotherms using InfraSORP technology. For this purpose, the sample is exposed to a gas flow of different humidity levels. To demonstrate the feasibility of this method, Al-fumarate and Cu3BTC2 as examples are measured with increasing humidity concentration flow, starting with an activated material and exposed in 10% RH steps up to 80% RH at 298 K. The thermal response signal is recorded after each concentration step. Once the temperature equilibrium is reached, a step-wise concentration increase is set without an intermediate purging step. The overall thermal response signal in the measured
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water concentration range is depicted in the Supporting Information (Fig. S8 and Fig. S9). The resulting specific peak areas are converted to a water capacity by applying the previous calibration equation eq. 1, cumulated and plotted against the respective humidity value. The resulting dynamic water adsorption isotherms are shown in Fig. 2 in comparison to the reference isotherm from common physisorption experiments. The water adsorption isotherm from the InfraSORP measurement is in a good correlation with the reference isotherm for both MOFs. There are only minor deviations, which are within the measurement accuracy of the InfraSORP method. In general, a comparison of isotherms from static and dynamic measurements should be critically considered due to the slow equilibration of water adsorption isotherms.[28,29]
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Fig. 2. Water adsorption isotherm from static physisorption experiments (open symbols) and dynamic InfraSORP experiments (closed symbols) at various humidity values at 298 K for Cu3BTC2 and Alfumarate.
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These initial observations demonstrate the capability of InfraSORP technology for isotherm measurements. The knowledge of the isotherm shape for evaluation of a potential waterbased heat exchanger material is as important as that of water capacity to find an appropriate p/p0 range for the desired application. Common physisorption experiments take a long time, especially for water adsorption. While the measurement of a water isotherm takes several days, the adsorption thermal response is obtained in only a few hours, depending on the material. The 8-step isotherm measurement of Cu3BTC2 and Al-fumarate using InfraSORP technology takes only 5 h and is thus more than four times faster than conventional physisorption experiments – and requires only a low sample mass of a few milligram.
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Water cycling stability studies on different MOFs are performed in the InfraSORP by repeated adsorption with a humid gas flow (80% RH) and desorption with dry nitrogen at ambient conditions (298 K, 1 atm). To demonstrate the capability of InfraSORP technique for cycling studies, 10 adsorption-desorption cycles of some selected materials are performed. The resulting cycling plots are depicted in Fig. 3a. All results from InfraSORP cycling studies are confirmed by water physisorption experiments (Fig. 3b, Fig. S2). Furthermore, nitrogen physisorption and PXRD measurements are performed before and after water cycling to evaluate the samples regarding porosity and crystallinity (see Fig. S3 – Fig. S5). The stability of MIL-101(Cr) and Al-fumarate is proven by the constant specific peak areas as a measure of the water capacity (by applying eq. 1) over 10 cycles. The deviation is about 2% over 10 cycles, which is confirmed by water physisorption experiments and hydrodynamic and hydrothermal stability studies in the literature, too.[12,14,30,31] The capacity of UiO-66 decreases approximately 15% during the cycling experiment which is equal to the water capacity loss from reference isotherm measurement. Jeremias et al. reported a capacity loss of about 20% after 20 hydrothermal water sorption cycles[16] which is very close to the data reported by InfraSORP tests. As it is shown in Fig. 3a, the main decrease in capacity loss is obtained from the first to the second cycle, while the capacity decreases only slightly (ca. 2%) with further cycling progress. In contrast, Cu3BTC2 exhibits a constant capacity loss of overall 50% within 10 cycles. A similar capacity loss is reported in the literature.[16,32]
ACCEPTED MANUSCRIPT To further verify the InfraSORP method, AlPO-5 and SAPO-34 are tested as standard heat pump materials regarding their cycling stability. As expected, the InfraSORP and physisorption studies demonstrate a constant water capacity over 10 cycles.
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Fig. 3. (a) Water adsorption capacities over 10-cycle InfraSORP experiments at 80% RH at 298 K for various MOFs and reference material AlPO-5 and SAPO-34, (b) Water capacities (80% RH, 298 K) from reference isotherm measurement for various MOFs and reference materials AlPO-5 and SAPO34, which are performed before and after the InfraSORP cycling tests.
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Although hydrothermal cycle tests are more application-oriented, especially with regard to a potential heat pump application, InfraSORP cycling measurements provide an insight into the hydrolytic stability of potential materials in only a fraction of the time. A complete regeneration in nitrogen flow is possible for the selected samples - also for highly hydrophilic materials such as Cu3BTC2 or SAPO-34. To finally demonstrate the capability of InfraSORP technology for hydrolytic stability testing, 100 adsorption-desorption cycles were performed for Al-fumarate at 80% RH at 298 K and optimized flow rate. Due to a gas flow differing from that used in the calibration measurement, eq. 1 cannot be applied to the InfraSORP results and hence only the specific peak area as a measure of adsorption capacity is plotted over the cycle number (Fig. 4). The specific peak areas are constant over 100 cycles with an overall deviation of only 1%. Small deviations in the specific peak areas – especially in the beginning and the middle of the cycle test – are caused by deviations in the measuring temperature of ca. 2 K during the cycle tests. Nevertheless, a high hydrolytic stability of Al-fumarate is confirmed by the InfraSORP cycling tests, which is also supported by a reference measurement (see Fig. S6 and Fig. S7).
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Fig. 4. Specific peak areas from adsorption thermal response of 100 cycles of Al-fumarate, measured 3 -1 with 400 cm min 80% RH nitrogen flow at 298 K.
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Cycle stability tests are usually performed gravimetrically coupled with moisture dosing or by repeated volumetric physisorption experiments.[14,32] In contrast to these methods InfraSORP technology offers a much faster, simpler, automated and easy-to-acquire alternative to conventional methods. The shorter measuring time is caused on the one hand by the low calorimeter constant in the InfraSORP instrument (in comparison to conventional heat flux calorimeters or DSC), since the thermal resistance is eliminated by the optical temperature measurement directly on the sample surface (thermal radiation). If the adsorption rate is limited by a slow equilibration (which is often the case for water adsorption), the advantage of the low time constant of the optical calorimetry is compensated. However, due to the low sample mass required for InfraSORP measurements (only a few milligram), the equilibration time is significantly shorter than for competing methods which require more than 10 times of the sample quantity for a precise measurement.
4. Conclusion Herein, we demonstrate the InfraSORP technology as a direct optical method to evaluate various materials regarding their water adsorption properties. Using MOFs as highly promising materials for water-based heat exchanger applications, the InfraSORP technology is demonstrated for screening regarding water adsorption capacity, measurement of
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Acknowledgements
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adsorption isotherms and cycling stability testing. The method is verified by classical water physisorption experiments and the measurement of benchmark heat exchanger materials as reference. An excellent linear correlation of the integral heat released (A/m) and water capacity is observed. The measurement at higher coverage is little affected by the materials structural properties (hydrophilicity, availability of open metal sites, etc.) nor by differing relative humidity values. Using a cell calibration measurement, even the measurement of dynamic water adsorption isotherms is possible by the InfraSORP method with only few milligrams of material by recording optical heat signals in parallel to a step-wise increase of the relative humidity. The resulting adsorption isotherms reflect well the reference water adsorption isotherms from volumetric physisorption experiments but the InfraSORP method is four times faster. By performing 10-cycle InfraSORP experiments of selected MOFs and reference materials, the hydrodynamic stability can be estimated. While a water capacity loss of up to 50% is recorded for Cu3BTC2 and UiO-66, Al-fumarate and MIL-101(Cr) as well as the heat exchanger reference materials remain stable during a 10-cycle measurement. This result is confirmed by the literature as well as by water capacity, surface area and crystallinity analysis before and after cycling experiments. To further demonstrate the capability and speed of the InfraSORP cycling tests, hydrodynamic stability of Al-fumarate regarding 100 water adsorption-desorption cycles is successfully confirmed using the InfraSORP method within only three days. Thus, the InfraSORP technology is a promising supplementary technique in contrast to timeconsuming and apparatus-complex physisorption or thermogravimetric experiments. The InfraSORP method allows to rapidly characterizing small amounts of potential materials for usage in water adsorption applications such as adsorption-driven heat exchangers, drying, and humidity management systems and hence to speed up research and development processes.
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The authors thank the Materials Center Dresden (TU Dresden, Germany) for providing the MOFs and Mitsubishi Chemical Corporation (Japan) for providing the AQSOATM samples. M. Sc. Claudia Schenk (TU Dresden, Germany) is acknowledged for measuring the PXRD data. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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Direct optical method for fast and simple screening to evaluate various materials regarding water adsorption properties Screening of metal-organic frameworks (MOFs) as highly promising materials for water adsorption heat pumps Four times faster measurement of dynamic water adsorption isotherms compared to standard physisorption experiments Rapid evaluation of hydrolitic stability of metal-organic frameworks
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DOI:
https://doi.org/10.1016/j.micromeso.2018.11.024
Reference:
MICMAT 9200
To appear in:
Microporous and Mesoporous Materials
Received Date: 24 August 2018 Revised Date:
14 November 2018
Accepted Date: 15 November 2018
Please cite this article as: M. Wöllner, N. Klein, S. Kaskel, Measuring water adsorption processes of metal-organic frameworks for heat pump applications via optical calorimetry, Microporous and Mesoporous Materials (2018), doi: https://doi.org/10.1016/j.micromeso.2018.11.024. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT Measuring water adsorption processes of metal-organic frameworks for heat pump applications via optical calorimetry
Michelle Wöllnera,b, Nicole Kleina, Stefan Kaskela,b a
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Fraunhofer Institute for Material and Beam Technology, Chemical Surface and Reaction Technology, Winterbergstraße 28, 01277 Dresden, Germany b Dresden University of Technology, Inorganic Chemistry I, Bergstraße 66, 01069 Dresden, Germany
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Corresponding author: [email protected] (phone: +49 35146333632, fax: +49 35146337287)
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Abstract
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Keywords
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Optical calorimetry (broadband IR-detection of the released heat of adsorption, “InfraSORP”) during water adsorption processes is used for an accelerated assessment of porous materials performance in heat pump applications. Metal-organic frameworks (MOFs) are screened as a highly promising class of materials for water adsorption driven heat exchangers with high water adsorption capacities. Based on a proper calibration, optical calorimetry is demonstrated to allow for rapid estimation of the total water adsorption capacity at a given relative humidity. In a dynamic mode, full water adsorption isotherms can be measured using a step-wise increase of the relative humidity. As cycling stability is among the most critical issues for the integration of new porous materials into systems, the InfraSORP methodology provides a valuable and inexpensive tool for accelerated cycling and stability testing. The InfraSORP technique is demonstrated to provide a significantly accelerated automated and easy-to-acquire alternative as compared to conventional characterization methods.
Metal-organic frameworks, water adsorption, adsorption heat pump, optical calorimetry, InfraSORP
1. Introduction In recent years, metal-organic frameworks (MOFs) have become a focus of research for potential application in water adsorption heat pumps.[1–3] These crystalline coordination polymers are made of metal ions or metal oxide clusters connected by functional organic linkers and hence offer tunable pore structures and properties.[4–6] High specific surface areas, high pore volumes, narrow pore size distributions and a tunable hydrophilicity by open metal sites or polar linkers are unique properties that feature MOFs as attractive candidates for water adsorption applications such as adsorption-driven heating or cooling. MOFs
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surpass established materials for adsorptive heating or cooling such as zeolites, silica gel or metal aluminum-phosphates in terms of working capacity and regeneration conditions: For instance, zeolites require high regeneration temperatures due to the strong hydrophilicity.[2] In contrast, silica gel has a more hydrophobic character which results in low desorption temperatures but low water uptake in the necessary relative pressure range below 0.4, although the overall water uptake of silica gel is high.[7,8] Crystalline metal aluminumphosphates (aluminum-phosphate, AlPO or silica aluminum-phosphates, SAPO) are very attractive for heat exchanger applications due to the S-shaped water adsorption isotherm. This special type of isotherm leads to high working capacities within a small relative pressure range and low regeneration temperatures (< 100 °C).[9,10] However, these materials are more expensive than zeolites or silica gels due to their complex synthesis.[7] Several MOFs such as MIL-100 (Al, Fe)[8], MIL-101(Cr)[11–13] or Al-fumarate[14] outperform established heat exchanger materials in terms of higher working capacities, an S-shape isotherms in an appropriate p/p0 range, low regeneration temperatures and low synthesis costs (in contrast to AlPOs oder SAPOs). However, another critical issue of an appropriate material is the hydrothermal stability regarding several thousand water adsorption-desorption cycles which are performed during the lifetime of an adsorption heat exchanger.[7,15,16] One of the first MOFs that was tested in regard to water sorption heat exchanger was Al-fumarate. This MOF shows high hydrothermal stability up to several thousand water adsorption cycles.[14] In recent years, several MOFs were investigated for their suitability as heating or cooling materials.[1–3] In most cases, the materials were evaluated by their water isotherms in terms of working capacity and thermogravimetric cycle stability tests. However, these characterization methods are very time-consuming and often limit rapid discovery, screening and process quality assessment. In respect to the rapid development of novel materials, especially MOF structures, a screening method would be beneficial to identify simply the materials with the best performance related to the application-relevant working conditions. To speed up and simplify the characterization of porous materials, the InfraSORP technology was recently developed.[17,18] The InfraSORP technique is based on the optical detection of the temperature change of a sample due to the released heat of adsorption when it is exposed to an adsorptive. The magnitude of the resulting thermal response signal reflects the total heat of adsorption. In recent years, the InfraSORP technology was established successfully for characterizing various materials for adsorbent key parameters[18–20] or industrially relevant applications such as filter evaluation for the removal of toxic gases[21,22]. So far, the InfraSORP was used mostly for screening of n-butane adsorption capacity[19,22] and cycling stability studies of flexible MOFs[23]. Water adsorption is often kinetically hindered due to slow equilibration and hydrogen bonding rearrangements. This leads to long equilibration times. The InfraSORP technology offers enormous time-saving advantages for this purpose. Due to the low sample mass required for InfraSORP measurements (only a few milligram), the equilibration time is significantly shorter than for competing methods which require more than 10 times of the sample quantity for a precise measurement. Herein, we introduce InfraSORP technology for an accelerated evaluation of MOFs for water adsorption-based heat exchanger applications. Several MOFs and commercially used heat exchange materials as benchmark materials are characterized regarding their water uptake and cycling stability in respect to repeated water adsorption/desorption stress by the InfraSORP technology. The results are compared to volumetric physisorption experiments.
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2.1 Materials The used MOFs (Cu3BTC2, MIL-101(Cr), MIL-100(Fe), MIL-100(Al), CAU-10, Al-fumarate, Cu-pymo, UiO-66, DUT-4, ZIF-8) were purchased from Materials Center Dresden (www.metal-organic-frameworks.eu). The reference materials AlPO-5 (AQSOA Z01) and SAPO-34 (AQSOA Z02) were kindly provided by Mitsubishi Chemical Corporation (www.mchemical.co). Prior to testing, all materials were pretreated at temperatures of 150 to 200 °C under vacuum for 12 hours. The activation conditions are given in the Supplementary Information (Table S1) in particular.
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2.2 Screening by InfraSORP technology The InfraSORP measurement is realized in a single cell setup at 298 K and ambient pressure (1 atm). The sample is filled in the sample holder (2 mm in height and 8 mm in diameter) and activated in an external activation apparatus. After determination of the weight loss, the sample holder is placed in the InfraSORP device and purged with a nitrogen flow of 140 cm3 min-1 until the thermal equilibrium is observed. This purging process takes 10 min. The subsequent InfraSORP measurement is performed by using a humid gas flow of 140 cm3 min-1 nitrogen equivalent. The humidity is adjusted by a nitrogen flow through a water filled bubbler system at 295 K and a downstream nitrogen flow for dilution. For water capacity screening, the first adsorption thermal response is measured and the specific peak area A/m [Ks/mg] is determined by integration of the thermal response over a linear base line by using OriginPro 2017 software and deviation by the sample mass. Cycling experiments are performed automatically by specifying the adsorption and desorption time. Adsorption is realized by a 140 cm3 min-1 nitrogen flow of 80% RH and desorption by a pure nitrogen flow of 140 cm3 min-1 without further heating. Adsorption and desorption are carried out until a temperature equilibrium is reached, with the consideration that the adsorption and desorption times are constant within a 10-cycle test. Furthermore, a 100-cycle test of Al-fumarate is performed by using 400 cm3 min-1 80% RH nitrogen flow at 298 K.
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2.3 Volumetric physisorption experiments The volumetric physisorption experiments are performed at a Belsorp-max apparatus (Bel Japan Inc). The specific surface areas are determined by nitrogen physisorption experiments at 77 K by using the multi-point BET plot (0.05 < p/p0 < 0.2). Water isotherms are measured at 298 K. The reference water capacity is obtained from the water adsorption isotherms at a certain partial pressure related to the InfraSORP experiments. 2.4. Powder X-ray diffraction (PXRD) Powder X-ray diffraction data are collected in transmission geometry on a STOE STADI P diffractometer with Cu-Kα1 radiation (λ = 1.5405 Å) at room temperature.
3. Results and discussion To evaluate a potential heat exchanger material, the knowledge of the water capacity and related integral heat release is essential. Using the InfraSORP technology, the integrated peak area A (in K⋅s) is a relative measure of the total heat released during adsorption/desorption.[19] The mass related peak area A/m is a specific quantity and reflects
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the specific water adsorption capacity if the average heat of adsorption is assumed to be constant. While the differential heat of adsorption may vary significantly for different materials, the integral heat of adsorption, in particular at higher coverage is dominated by fluid-fluid interactions and thus less prone to variations. In such cases A/m shows a linear correlation to the adsorption capacity Cads. Various MOFs were measured at different humidity values (30% RH, 60% RH and 80% RH at 298 K) in the InfraSORP device. Fig. 1a shows the resulting specific peak areas A/m which are in a direct correlation to the respective water capacities Cads with an excellent regression quality factor of 0.97; A/m [K s g-1] = 0.4416 · 106 · Cads [g g-1] (eq. 1). The correlation is little affected by varying relative humidity values or the measurement of MOFs with different structural properties such as hydrophilicity, availability of open metal sites or pore structure. The impact of various adsorptive concentrations on the thermal response signal is insignificant as it is empirically supported by the overall linear correlation between specific peak area and water capacity. The heat capacity of humid air in the measured humidity range is between 1.02 kJ kg-1 K-1 (30% RH) and 1.03 kJ kg-1 K-1 (80% RH)[24,25] and hence, does not affect the thermal response signal. Adsorption enthalpy has only a significant impact on the thermal response if different adsorptives are used.[26] The effect of varying porosity on the adsorption enthalpy and hence the thermal response signal is also negligible which was shown in previous work.[19,26] Especially for water adsorption in MOFs, the major heat effect is caused by the condensation enthalpy which is around 50 ± 3 kJ mol-1.[1] Small enthalpy effects due to open metal sites for instance are negligible, especially in terms of heat resolution and measuring uncertainty of the InfraSORP technology.[26,27] Nevertheless, the good linear correlation between specific peak area and reference water capacity can be used to demonstrate that InfraSORP technology is ideally suited for the screening of MOFs for their water capacity. The measurement uncertainty is only 5% for water adsorption. The accuracy is determined by the average deviation of specific peak areas from repeated measurement (four times) of adsorption of 80% RH (298 K) for Al-fumarate as an example. The deviation is comparable to values reported in the past.[17,18,26] To verify the capacity evaluation by InfraSORP technology, the reference heat exchanger materials SAPO-34 and AlPO-5 are tested. For calibration, the linear fit eq. 1 from A/m – Cads plot is used. Fig. 1b shows the water adsorption capacities (80% RH, 298 K) which are determined by InfraSORP technology and reference physisorption experiments. The deviation of both values is with ca. 7% very low. Fig. 1. (a) Linear correlation between water capacity from reference physisorption experiments and specific peak area from InfraSORP technology of various MOFs measured at different humidity values at 298 K, raw data are available in Supplementary Information, Table S2 (b) Water capacity (80% RH, 298 K) from InfraSORP experiments (by using eq. 1) and physisorption measurements for Al-fumarate and reference materials AlPO-5 and SAPO-34.
The fact of a concentration-independent capacity evaluation allows a dynamic measurement of adsorption isotherms using InfraSORP technology. For this purpose, the sample is exposed to a gas flow of different humidity levels. To demonstrate the feasibility of this method, Al-fumarate and Cu3BTC2 as examples are measured with increasing humidity concentration flow, starting with an activated material and exposed in 10% RH steps up to 80% RH at 298 K. The thermal response signal is recorded after each concentration step. Once the temperature equilibrium is reached, a step-wise concentration increase is set without an intermediate purging step. The overall thermal response signal in the measured
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water concentration range is depicted in the Supporting Information (Fig. S8 and Fig. S9). The resulting specific peak areas are converted to a water capacity by applying the previous calibration equation eq. 1, cumulated and plotted against the respective humidity value. The resulting dynamic water adsorption isotherms are shown in Fig. 2 in comparison to the reference isotherm from common physisorption experiments. The water adsorption isotherm from the InfraSORP measurement is in a good correlation with the reference isotherm for both MOFs. There are only minor deviations, which are within the measurement accuracy of the InfraSORP method. In general, a comparison of isotherms from static and dynamic measurements should be critically considered due to the slow equilibration of water adsorption isotherms.[28,29]
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Fig. 2. Water adsorption isotherm from static physisorption experiments (open symbols) and dynamic InfraSORP experiments (closed symbols) at various humidity values at 298 K for Cu3BTC2 and Alfumarate.
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These initial observations demonstrate the capability of InfraSORP technology for isotherm measurements. The knowledge of the isotherm shape for evaluation of a potential waterbased heat exchanger material is as important as that of water capacity to find an appropriate p/p0 range for the desired application. Common physisorption experiments take a long time, especially for water adsorption. While the measurement of a water isotherm takes several days, the adsorption thermal response is obtained in only a few hours, depending on the material. The 8-step isotherm measurement of Cu3BTC2 and Al-fumarate using InfraSORP technology takes only 5 h and is thus more than four times faster than conventional physisorption experiments – and requires only a low sample mass of a few milligram.
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Water cycling stability studies on different MOFs are performed in the InfraSORP by repeated adsorption with a humid gas flow (80% RH) and desorption with dry nitrogen at ambient conditions (298 K, 1 atm). To demonstrate the capability of InfraSORP technique for cycling studies, 10 adsorption-desorption cycles of some selected materials are performed. The resulting cycling plots are depicted in Fig. 3a. All results from InfraSORP cycling studies are confirmed by water physisorption experiments (Fig. 3b, Fig. S2). Furthermore, nitrogen physisorption and PXRD measurements are performed before and after water cycling to evaluate the samples regarding porosity and crystallinity (see Fig. S3 – Fig. S5). The stability of MIL-101(Cr) and Al-fumarate is proven by the constant specific peak areas as a measure of the water capacity (by applying eq. 1) over 10 cycles. The deviation is about 2% over 10 cycles, which is confirmed by water physisorption experiments and hydrodynamic and hydrothermal stability studies in the literature, too.[12,14,30,31] The capacity of UiO-66 decreases approximately 15% during the cycling experiment which is equal to the water capacity loss from reference isotherm measurement. Jeremias et al. reported a capacity loss of about 20% after 20 hydrothermal water sorption cycles[16] which is very close to the data reported by InfraSORP tests. As it is shown in Fig. 3a, the main decrease in capacity loss is obtained from the first to the second cycle, while the capacity decreases only slightly (ca. 2%) with further cycling progress. In contrast, Cu3BTC2 exhibits a constant capacity loss of overall 50% within 10 cycles. A similar capacity loss is reported in the literature.[16,32]
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Fig. 3. (a) Water adsorption capacities over 10-cycle InfraSORP experiments at 80% RH at 298 K for various MOFs and reference material AlPO-5 and SAPO-34, (b) Water capacities (80% RH, 298 K) from reference isotherm measurement for various MOFs and reference materials AlPO-5 and SAPO34, which are performed before and after the InfraSORP cycling tests.
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Although hydrothermal cycle tests are more application-oriented, especially with regard to a potential heat pump application, InfraSORP cycling measurements provide an insight into the hydrolytic stability of potential materials in only a fraction of the time. A complete regeneration in nitrogen flow is possible for the selected samples - also for highly hydrophilic materials such as Cu3BTC2 or SAPO-34. To finally demonstrate the capability of InfraSORP technology for hydrolytic stability testing, 100 adsorption-desorption cycles were performed for Al-fumarate at 80% RH at 298 K and optimized flow rate. Due to a gas flow differing from that used in the calibration measurement, eq. 1 cannot be applied to the InfraSORP results and hence only the specific peak area as a measure of adsorption capacity is plotted over the cycle number (Fig. 4). The specific peak areas are constant over 100 cycles with an overall deviation of only 1%. Small deviations in the specific peak areas – especially in the beginning and the middle of the cycle test – are caused by deviations in the measuring temperature of ca. 2 K during the cycle tests. Nevertheless, a high hydrolytic stability of Al-fumarate is confirmed by the InfraSORP cycling tests, which is also supported by a reference measurement (see Fig. S6 and Fig. S7).
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Fig. 4. Specific peak areas from adsorption thermal response of 100 cycles of Al-fumarate, measured 3 -1 with 400 cm min 80% RH nitrogen flow at 298 K.
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Cycle stability tests are usually performed gravimetrically coupled with moisture dosing or by repeated volumetric physisorption experiments.[14,32] In contrast to these methods InfraSORP technology offers a much faster, simpler, automated and easy-to-acquire alternative to conventional methods. The shorter measuring time is caused on the one hand by the low calorimeter constant in the InfraSORP instrument (in comparison to conventional heat flux calorimeters or DSC), since the thermal resistance is eliminated by the optical temperature measurement directly on the sample surface (thermal radiation). If the adsorption rate is limited by a slow equilibration (which is often the case for water adsorption), the advantage of the low time constant of the optical calorimetry is compensated. However, due to the low sample mass required for InfraSORP measurements (only a few milligram), the equilibration time is significantly shorter than for competing methods which require more than 10 times of the sample quantity for a precise measurement.
4. Conclusion Herein, we demonstrate the InfraSORP technology as a direct optical method to evaluate various materials regarding their water adsorption properties. Using MOFs as highly promising materials for water-based heat exchanger applications, the InfraSORP technology is demonstrated for screening regarding water adsorption capacity, measurement of
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Acknowledgements
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adsorption isotherms and cycling stability testing. The method is verified by classical water physisorption experiments and the measurement of benchmark heat exchanger materials as reference. An excellent linear correlation of the integral heat released (A/m) and water capacity is observed. The measurement at higher coverage is little affected by the materials structural properties (hydrophilicity, availability of open metal sites, etc.) nor by differing relative humidity values. Using a cell calibration measurement, even the measurement of dynamic water adsorption isotherms is possible by the InfraSORP method with only few milligrams of material by recording optical heat signals in parallel to a step-wise increase of the relative humidity. The resulting adsorption isotherms reflect well the reference water adsorption isotherms from volumetric physisorption experiments but the InfraSORP method is four times faster. By performing 10-cycle InfraSORP experiments of selected MOFs and reference materials, the hydrodynamic stability can be estimated. While a water capacity loss of up to 50% is recorded for Cu3BTC2 and UiO-66, Al-fumarate and MIL-101(Cr) as well as the heat exchanger reference materials remain stable during a 10-cycle measurement. This result is confirmed by the literature as well as by water capacity, surface area and crystallinity analysis before and after cycling experiments. To further demonstrate the capability and speed of the InfraSORP cycling tests, hydrodynamic stability of Al-fumarate regarding 100 water adsorption-desorption cycles is successfully confirmed using the InfraSORP method within only three days. Thus, the InfraSORP technology is a promising supplementary technique in contrast to timeconsuming and apparatus-complex physisorption or thermogravimetric experiments. The InfraSORP method allows to rapidly characterizing small amounts of potential materials for usage in water adsorption applications such as adsorption-driven heat exchangers, drying, and humidity management systems and hence to speed up research and development processes.
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The authors thank the Materials Center Dresden (TU Dresden, Germany) for providing the MOFs and Mitsubishi Chemical Corporation (Japan) for providing the AQSOATM samples. M. Sc. Claudia Schenk (TU Dresden, Germany) is acknowledged for measuring the PXRD data. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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Direct optical method for fast and simple screening to evaluate various materials regarding water adsorption properties Screening of metal-organic frameworks (MOFs) as highly promising materials for water adsorption heat pumps Four times faster measurement of dynamic water adsorption isotherms compared to standard physisorption experiments Rapid evaluation of hydrolitic stability of metal-organic frameworks
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