Methanol Adsorption on HKUST-1 Coatings Obtained by Thermal

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Methanol Adsorption on HKUST-1 Coatings Obtained by Thermal Gradient Deposition (TGD) Sebastian-Johannes Ernst, Felix Jeremias, Hans-Jörg Bart, and Stefan K. Henninger Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03637 • Publication Date (Web): 30 Nov 2016 Downloaded from http://pubs.acs.org on December 3, 2016

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Industrial & Engineering Chemistry Research

Methanol adsorption on HKUST-1 coatings obtained by Thermal Gradient Deposition (TGD) Sebastian-Johannes Ernsta,b*, Felix Jeremiasa, Hans-Jörg Bartb and Stefan K. Henningera a

Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110 Freiburg, Germany b

Chair of Separation Science and Technology, TU Kaiserslautern, Postfach 3049, 67653 Kaiserslautern, Germany

ABSTRACT:

Adsorption driven heat transformation has been reported as one of the most promising areas to bring metal-organic frameworks (MOFs) into industrial application. The vast majority of evaluated systems apply water as working fluid. Since the use of water comes with many drawbacks, short-chained alcohols have been proposed as an alternative working fluid. In this contribution the working flow from adsorbent (HKUST-1) characterisation up to technical application is presented. Beginning with the thorough characterisation of the process of direct crystallisation on different structures, the results of methanol adsorption measurements are presented. Coatings are evaluated applying the Dubinin-Astakhov-approach for description of adsorption data and calculating a coefficient of performance (COP). Maximum methanol uptake of the coatings was approximately 0.6 gMeOH/gMOF leading to a COP of 1.5 at given temperatures. With respect to the cyclic application, stability of HKUST-1 and methanol has also been investigated.

ACS Paragon Plus Environment

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INTRODUCTION Metal-organic frameworks (MOFs), three dimensional and porous networks consisting of metal nodes and bridging organic ligands, have been investigated for numerous potential applications like catalysis,1,2 gas separation,3 and storage,4 drug delivery,5,6 sensor applications7,8 and many others.9,10 Only recently their application in adsorption heat pumps emerged.11–16 Water seems to be the working fluid of choice because it is not toxic, easily available and has a high enthalpy of vaporisation. Especially aluminium fumarate17 and the aluminium isophthalate CAU-10-H15 show very promising results regarding adsorption performance as well as stability over many adsorption-desorption cycles. Unfortunately, the use of water as a working fluid brings some disadvantages along with its comparably high freezing point. Many MOFs, that exhibit promising water sorption behaviour, suffer from poor hydrothermal stability over multiple adsorption-desorption cycles.13,19 To circumvent these problems, alternative working fluids like ammonia, ethanol or methanol can be used. The phase transition properties of methanol are especially advantageous with regard to typical working conditions of adsorption chillers. The low freezing temperature and high vapour pressure facilitate refrigeration applications using ambient air for driving the evaporator. Methanol adsorption has already been investigated for many common micro- and mesoporous adsorbents like silicagel,20 activated carbon,21–25 zeolites and related materials,26 and most recently MOFs.12,27 As to this, copper trimesate HKUST-1 has been reported to show a type-I adsorption isotherm for methanol and a loading lift of approximately 0.5 g / g.12 These properties identify HKUST-1 as a very interesting MOF for the use in methanol-based heat transformation applications.


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Besides the aforementioned material properties, optimal shaping for the application is a very important aspect, due to its strong influence on the adsorption-desorption cycle time and hence on the performance of any adsorption heat pump or chiller. A way to improve heat and mass transfer to and from the adsorbent consists in producing coatings of the adsorbent directly on heat exchanger surfaces, obtained for instance by (binder-based) dip-coating.28–30 Since binders are also susceptible to degradation and introduce additional thermal mass into the system, our group came up with a more elegant method to produce highly accessible, binder-free MOF coatings using thermal gradient deposition (TGD).17,31 Another aspect is the stability of the working fluid over many thousand cycles when considering industrial practice. One of the few experimental results can be found in Hu, when investigating the decomposition of methanol in the presence of activated carbon, copper and aluminum.37 He reports the formation of 1 to 1.5 mole dimethyl ether per mole methanol after 25 days at a constant temperature of 110 °C. Only recently Sapienzia et al. highlighted the slowdown of an adsorption process due to non-adsorbable gases like hydrogen in systems using water as a working fluid.38 In this contribution, we present the fabrication of coatings of HKUST-1 on two different structures obtained by TGD as well as the respective methanol adsorption properties and the stability of the adsorbent as well as that of the working fluid. Subsequently, the capability of these coatings for adsorption heat pumps at different boundary conditions is discussed in detail. SAMPLE PREPARATION AND CHARACTERISATION Preparation of the samples. Two types of copper substrates were coated: one copper sheet (50 x 50 x 1 mm3, mass: 22.079 g) sanded on both sides and a copper fibre (50 x 50 x 4.2 mm3, porosity: 62 %, mass:


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31.957 g) provided by Fraunhofer IFAM, sanded on one side to improve the connection between the substrate and the heating device. For description and characterisation of the fibres see reference 32. Both substrates were rinsed with isopropanol, etched in sulfuric acid (10 %, 120 s), rinsed again with deionized water and dried in air at 80 °C. For the coating procedure, a custom-made heating device was equipped with the substrates as reported elsewhere.31 For a typical synthesis, a solution of copper nitrate trihydrate (Carl Roth, ≥ 98 %, 8.75 g) and trimesic acid (ABCR, 98 %, 4.2 g) in dimethyl formamide (VWR Prolabo, ≥ 99.9 %, 250 mL) was filled into a stainless steel beaker (inner diameter: 65 mm) being held constant at 30 °C. The whole device was immersed into the solution and heated to 150 °C for 2.5 h. After cooling, the substrates were rinsed with and soaked overnight in dimethyl formamide and ethanol consequently, then dried in air at 120 °C and stored under inert conditions. Smaller sheets (25 x 25 x 0.5 mm3) were prepared in the same way but immersed in the solution for short times (1 min up to 45 min) only to investigate the progress of the crystallisation of HKUST-1 on the substrate. For comparison, a commercially available HKUST1, namely Basolite™ C300 supplied by Sigma-Aldrich, was used. All chemicals were used as received. Characterisation of the samples. Microscopic images were obtained on a Keyences® VHX reflected-light microscope and a 3D Laser-scanning microscope Olympus LEXT OLS 4000, respectively. Powder diffractograms were obtained on a Bruker D8 Advance using Cu-Kα radiation and a LYNXEYE detector on a Newport stage, with Bragg-Brentano geometry and 1s/0.02° per step. The distribution of the meso- and macropores has been determined by mercury intrusion (Quantachrome Poremaster®). The porosimetry were calculated with contact angles of 140° for mercury/HKUST-1 and 116°


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for mercury/copper, respectively. True density measurements were performed with a helium pycnometer (Quantachrome Ultrapyc 1200e). In order to remove contaminants or trapped air, the samples were purged and evacuated before analysis. Density has been determined by measuring a flake removed from the substrate with Keyence and weighing it afterwards. A magnetic suspension balance (Rubotherm) with a resolution of 10 µg working under 7.4 kPa pure methanol pressure was used to measure equilibrium sorption characteristics gravimetrically, while isothermal equilibrium uptake was measured by a Quantachrome Autosorp. For the investigation of the multi-cycle stability samples have been cyclic charged and discharged in a custom built test setup under pure methanol vapour atmosphere (pS(8 °C) = 6.5 kPa) as described elsewhere.33 Applied temperatures during adsorption and desorption were 30 °C and 130 °C, respectively. Cycle time has been 3 min. For the analysis of the stability of the adsorbent powder diffractograms of the coated sheets were measured after 500 and 1 500 cycles. For the analysis of the atmosphere gas samples were taken after 3 500 and 10 000 cycles with a coated copper sheet, after one month of resting and after 3 000 cycles with a blank sheet. Gas samples had to be pressurised with nitrogen subsequently for GC analysis (Agilent 7890 GC). The GC system is calibrated on carbon dioxide, water, dimethyl ether, methanol, formaldehyde, oxygen and argon, nitrogen, methane and carbon monoxide. RESULTS AND DISCUSSION Process of layer formation. Figure 1 shows HKUST-1 coatings on the copper plates obtained by direct crystallisation after 1, 2, 5, 10, 15, 22 and 45 min. As can be seen, crystallisation begins immediately after immersion in the solution at the smallest diameters on the plate and proceeds very quickly. Originating from these first nuclei the next generation of crystals grow as agglomerates on already existing ones


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until the whole sheet is covered by a layer of HKUST-1. The grown crystals are indeed HKUST1 as is confirmed by the powder diffractograms shown in the supporting information (Figure S1). The difference in the intensities of the first four reflexes referring to the different orientations of the crystals indicate that the thinner coatings obtained during reaction times of 1 – 5 min (a, b, c) show a slight preference for the (222) orientation as well as the powder form does. With proceeding reaction time the crystal growth becomes increasingly undirected. This leads to an inhomogeneous layer thickness over the sheet during this state of process as well as to the loss of a preferred orientation of the crystals. In a following step, gaps and valleys are being closed directed by the thermal gradient until a uniform coating thickness is obtained.


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Figure 1: LEXT-pictures of HKUST-1 coatings obtained by direct crystallisation after 1, 2, 5, 10, 15, 22 and 45 min. From left to right: microscope picture, laser scanning picture, height profile. White bars represent 20 µm. The MOF load per area of the copper sheet was determined by weighing the coated copper sheets. Coating yield was 0.060 g (0.003 gadsorbent / gcopper) for one sheet. The samples have been dried for 24 h at 120 °C under vacuum before weighing. Afterwards the area covered with HKUST-1 was determined from the height profiles shown in Figure 1. Both values are plotted over process time in Figure 2. The covered area per area of copper sheet shows a steep rise reaching complete coverage after 22 min. A linear trend line is drawn for the HKUST-1 load to guide the eye. A rate of layer growth of 50 mg / (mm² h) as reported earlier31 was confirmed by this trend as well as by long term experiments for 2 and 3.5 hours, respectively. As to this, it can be concluded that the layer thickness obtained via thermal gradient deposition can easily be controlled by process time.

Figure 2: HKUST-1 covered area (right y-axis, ) and HKUST-1 load per sheet-area (left yaxis, ) over reaction time.


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Entering the third dimension. Following the characterisation of the layer formation, the next step towards an industrial application consists of scaling up to larger and more complex structures like the copper fibres which show both: good thermal conductivity and a high specific surface area and therefore a large coatable surface. Figure 3 shows reflective-light microscopic images for the full sample in Figure 3a and excerpts in Figure 3b and c. The pictures indicate a uniform and dense coating over

the

full

fibre.

The

coating

yield

determined

by

weighing

was

0.914 g

(0.029 gadsorbent / gcopper) for the fibre, increasing the mass ratio by 10 times compared to the coating on a sheet. The blank spots that can be seen in Figure 3b are caused by the connection between substrate and heating device. The pictures were taken directly after drying. The deep blue colour of dry HKUST-1 can be seen as well as turquoise spots at the edges indicating the beginning adsorption of water.

Figure 3: Copper fibre blank (a) and coated (b) by TGD (50 x 50 mm², 5x magnification); and (c) densely coated fibres (50x magnification). The blank spots are caused by the connection to the heating device. The porosity of the coated fibre was determined by mercury intrusion and He-pycnometry, respectively. The pore size distribution of the coated fibre shows a bimodal character with one mean pore size at approx. 350 µm and another very small one at about 50 nm (see Figure S2 for


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the pore size distribution). The porosity accessible by helium is 63 %, the one by mercury is 37 %. The volume fraction of HKUST-1 per metal fibre can be calculated as the difference of these two values to

𝑉coating 𝑉fibre

= 0.26 and with the densities of the HKUST-1 coating determined to

1.1 g / cm3 and the fibre material CuSn1 8.9 g / cm3 a mass ratio can be determined to: 𝑚coating 𝜌coating = 0.26 ⋅ = 0.032 g/g 𝑚fibre 𝜌fibre As can be seen, this mass ratio differs from the mass ratio determined by weighing the substrate before and after coating (0.028 gMOF / gsheet) and is slightly lower than the mass ratio determined via porosimetry. Besides some weighing errors it can be concluded that copper ions from the fibre dissolve and help to form the HKUST-1-layer. This behaviour benefits the thermal coupling of the layer. Methanol adsorption For a first comparison of the produced material’s methanol uptake, the coated structures as well as the C300-powder were measured under isobaric conditions at 7.4 kPa. The isobars are depicted in Figure S3. The isobars of all three materials show the same shape as well as a very similar maximum uptake of about xMeOH = 0.52 gMeOH / gMOF at prel = 0.6. The uptake of the sheet is slightly higher than the one of C300 due to the slightly higher inner surface and pore volume (AC300 = 1599 m2 / gMOF, VC300 = 0.686 cm3 / gMOF vs Asheet = 1665 m2 / gMOF, Vsheet = 0.710 cm3 / gMOF). However, it is very likely that the HKUST-1 coated on the fibre has a slightly lower surface and pore volume (N2-adsorption was not measured because it was not possible to collect enough powder for N2-adsorption). Since HKUST-1 obtained via coating gives the same equilibrium uptake behaviour as powdery material, the following experiments were only performed using the powder.


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Data of equilibrium methanol uptake of HKUST-1 are depicted in Figure 4. Measurements have been performed isothermally at 25 °C and 40 °C and isobaric at 5.6 kPa and 7.5 kPa referring to evaporator temperatures of 5 and 10 °C respectively. To avoid condensation isobaric methanol uptake at 5.6 kPa has only been measured up to relative pressure of approx. 0.2. All equilibrium curves show a steep rise starting from the origin. The isotherms show maximum uptakes of approx. 0.5 and 0.6, respectively, already at a relative pressure of prel = 0.6 and neglegible hysteresis of less than 0.04 in methanol uptake. The isosteric heat of adsorption was determined to ΔHads = 38.43 kJ / mol from the isotherms using the equation by ClausiusClapeyron. The two isobars show the same shape, and, as expected, methanol uptake is slightly lower at a lower pressure.

Figure 4: Equilibrium methanol uptake of HKUST-1 versus relative pressure, isothermally measured at 25 °C () and 40 °C () and isobaric at 5.6 kPa () and 7.5 kPa (). The obtained p-X-T data were transformed according to the Dubinin-Astakhov approach as described elsewhere 23: 𝑊(𝐴) = 𝑊0 ⋅ exp[−(𝑏𝐴)𝑛 ]

(1)


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This approach defines an adsorbed volume W to 𝑊 = 𝑋/𝜌l,MeOH and an adsorption potential A to 𝐴 = −𝑅𝑇 ⋅ ln 𝑝

𝑝 . S (𝑇)

The coefficients W0, b and n were fitted to experimental data. Figure 5

shows the transformed experimental data and the curve according to the Dubinin-Astakhovequation. Estimated parameters are listed in Table 1. As can be seen, Dubinin-Astakhov-equation does not describe the experimental data perfectly. Nonetheless it is used in the following calculation as a first figure of merit.

Figure 5: Experimental data transformed according to the Dubinin-Astakhov-approach () and the curve of the fitted Dubinin-Astakhov-equation (). A potential working window defined by the operating temperatures (TM = 40 °C, Tevap = -5 °C, Tdes = 120 °C) is highlighted. Table 1: Parameters for calculation of COP values for HKUST-1 (this contribution) in comparison of two activated carbons

Adsorbent

Model parameter W0

n

b

cm3 / g

-

g/J

Density

cp

g / cm³

J / ( g K)


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HKUST-1

0.74

1.676

0.0025

1.1

1.04 31

CarboTech A35 23

0.786

1.76

0.00666

0.33

0.95

G32-H 23

0.482

2.59

0.00405

0.37

0.95

Multi-cycle stability One requirement for the application of a material in cyclic heat transformation is its stability over many cycles. For this reason, a coated copper sheet was subjected to multiple isobaric adsorption / desorption cycles with methanol in a custom made multi-cycle test apparatus. As depicted in Figure 6 the x-ray crystallinity in terms of intensity of the characteristic peaks of the sample remains virtually unchanged, even after 1500 adsorption / desorption cycles with methanol vapour. The augmentation of the reflection at 2 = 6° can be attributed to different states of methanol load of the samples during the measurements.39 There were neither optical alterations nor mechanical failure of the sample.

Figure 6: Powder diffractograms of a coated copper sheet before and after several methanol adsorption / desorption cycles.


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Besides the stability of the adsorbent and the coating, the stability of the working fluid methanol has to be addressed. The chromatograms of the samples taken after 10 000 cycles on the coated sheet and after 3 500 cycles on the blank sheet are shown in Figure 7. As can be seen, the methanol peak and two peaks for water and dimethyl ether occur after approx. 6 min. Additionally, a large amount of nitrogen/air and carbon dioxide occurs from contamination due to dilution and the GC-system.

Figure 7: Chromatogram of methanol after 10 000 cycles of ad- and desorption on HKUST-1 () and 3 500 cycles on a blank copper sheet (). The amounts of methanol, carbon dioxide, water and dimethyl ether were calculated from the peaks normalised to the determined amount of methanol. The vol-% normalised to methanol for different numbers of cycles is depicted in Figure 8. The content of possible reaction products


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rises over the number of cycles. The reaction of dimethyl ether to methanol follows the chemical equation: 2 CH3 OH→CH3 OCH3 +H2 O The content of water should be stoichiometric correlated to the content of dimethyl ether. As it is shown in Figure 8 the measured water content is more than twice the content (around 4vol-%) of dimethyl ether. As to this, it can be concluded that the main amount of water can be attributed to contamination due to condensation. The origin of the carbon dioxide is also not fully clear. Since dimethyl ether and carbon dioxide are non-condensable gases, they will certainly hinder adsorption and desorption and therefore afffect the working cycle of an adsorption heat pump.

Figure 8: Formation of dimethyl ether (), water () and CO2 () normalised on methanol content over the numbers of cycles

Performance evaluation The process cycle of an adsorption heat pump and the calculation of the COP are described in the Supporting Information (SI-4). Given the aimed application in domestic heating, the middle temperature level is set to TM = 40 °C and TM = 50 °C, respectively. The lower temperature can


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be used in new buildings with a sufficient thermal insulation, the higher temperature has to be chosen in older buildings with lower standards. The calculated values of COPHP plotted versus desorption temperature for different evaporator temperatures are depicted in Figure 9. In general, higher values of COPHP can be achieved for higher evaporator temperatures and still COP remains sufficiently high when the evaporator temperature drops during winter. The COPHP values for the scenario with TM = 40 °C are higher than for the higher temperature since more fluid exchange can be achieved at larger temperature differences. Methanol exchange and therefore COP remain at a high level above 1.4 for high desorption temperatures of above 120 °C at TM = 40 °C and responds very sensitively to lower desorption temperatures. In case of the higher medium temperature level, the COP values are lower and spread broader for different evaporator temperatures. The maximum COP value was calculated to 1.42 for TM = 50 °C and 1.5 for TM = 40 °C. With respect to efficiency heat is preferably supplied at a medium temperature level of 40 °C.

Figure 9: COP as a function of provided desorption temperature and different evaporation temperatures. Medium temperature level is kept constant at TM = 40 °C (a) and TM = 50 °C (b).


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Figure 10: Comparison of COP values of HKUST-1 and two activated carbon (CarboTech A35 and G32-H, experimental data see reference

23

.) Evaporator temperatures: 2 and -10 °C,

middle temperature level: 40 °C (a) and 50 °C (b).

Since MOFs are still in the preindustrial stage, a realistic pricing has not been possible yet. Nonetheless, activated carbon is a very cheap material that has already been commercialised and can be applied in adsorption-driven heat transformation using methanol as working fluid. For comparison, calculated COP values of HKUST-1 and two activated carbons (CarboTech A35 and G32-H, characterisation see reference

23

) are plotted versus desorption temperatures for two

different evaporator temperatures (Figure 10, 2 °C filled symbols and -10 °C open symbols). Underlying values for the calculation are listed in Table 1. The two activated carbons show slightly higher COP values for higher desorption temperatures but respond more sensitively to lower desorption temperatures. For evaporator temperatures of 2 °C the difference between the values is about 0.15 in the maximum at 40 °C medium temperature level and 0.07 at 50 °C, respectively. The COP values of the three compared materials converge for falling evaporator temperatures. Accordingly, with respect to these COP values, material of choice would be an


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activated carbon. However, the application of HKUST-1 coatings yields multiple benefits compared to the use of activated carbon in adsorption-based heat transformation: -

The density of HKUST-1 coatings is triple the bulk density of the compared activated carbon. Therefore an adsorber module equipped with a bulk of activated carbon would end up much larger than a HKUST-1 coated adsorber.

-

Coatings of activated carbons on heat exchangers have been published only using additives like binders. This would decrease the mass ratio of adsorbent to passive mass and therefore the COP of the system.

-

Coating of activated carbons would result in another manufacturing step and therefore increase the costs of the total adsorber module.

CONCLUSIONS AND OUTLOOK A complete path from design to application for the use of HKUST-1 in sorption-based heat transformation has been presented. Beginning with the comprehensive characterisation of the process of coating via direct crystallisation it has been shown that this process can be transferred to complex 3D-structures optimised for heat and mass transfer with a mass ratio 10 times higher compared to a coating on a flat sheet. Methanol uptake of the coated structures has been measured to approximately 0.6 gMeOH / gMOF at 40 °C in the maximum leading to a COP of 1.5 for given temperatures. Further, the use of such structures in sorption-based heat transformation using methanol as a working fluid is discussed for the first time following the whole process including the stability of the adsorbent and of the working fluid. It could be shown that over many thousand cycles, methanol reacts to dimethyl ether not more than 4 percent by volume and the adsorbent coating stays fully stable. Summing up, it can be concluded that the working pair HKUST-1 and methanol is a suitable working pair for application in adsorption-based heat


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pumps. Direct crystallisation leads to an optimal shaped coating without any additives and further manufacturing steps like dip coating. In a next step the coating process via thermal gradient deposition will be applied to full scale heat exchangers that can be characterised as part of a complete industrial scale adsorber module. Dynamic modelling of the adsorber module will be informative with respect to optimisation potential regarding cycle times, coating layer thickness and fibre geometry. Further research is needed regarding the impact of the formation of dimethyl ether and other reaction products of methanol during cycles as well.

ASSOCIATED CONTENT Supporting Information. Powder x-ray diffractograms, methanol uptake curves of coatings, pore size distribution determined by mercury intrusion, description of COP calculation. AUTHOR INFORMATION Corresponding Author *[email protected], Tel. +49 761 4588 5907 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 Funding by the Fraunhofer Zukunftsstiftung under grant HARVEST is gratefully acknowledged.


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Max Baumgartner for thermogravimetric, Mohamed Ouda for GC, Philipp Hügenell for volumetric measurements and Dominik Fröhlich for XRD support and Harry Kummer for further hints are acknowledged. ABBREVIATIONS COP, coefficient of performance; HKUST-1, metal-organic framework Cu3(btc)2 (btc = 1,3,5benzenetricarboxylate); LEXT, 3D measuring laser microscope; MOF, metal-organic framework; SI supporting information, TGD, thermal gradient deposition. REFERENCES (1) Lee, J.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S. T.; Hupp, J. T. Metal– organic framework materials as catalysts. Chem. Soc. Rev. 2009, 38 (5), 1450. DOI: 10.1039/B807080F. (2) Corma, A.; García, H.; Llabrés i Xamena, F. X. Engineering Metal Organic Frameworks for Heterogeneous Catalysis. Chem. Rev. 2010, 110 (8), 4606–4655. DOI: 10.1021/cr9003924. (3) Li, J.-R.; Sculley, J.; Zhou, H.-C. Metal–Organic Frameworks for Separations. Chem. Rev. 2012, 112 (2), 869–932. DOI: 10.1021/cr200190s. (4) Furukawa, H.; Cordova, K. E.; O‘Keeffe, M.; Yaghi, O. M. The Chemistry and Applications of Metal-Organic Frameworks. Science 2013, 341 (6149), 1230444. DOI: 10.1126/science.1230444.


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Methanol Adsorption on HKUST-1 Coatings Obtained by Thermal

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