We acknowledge our home institution, the &#201;cole Polytechnique F&#233;d&#233;rale de Lausanne (EPFL), for its generous support. This project has received funding from the European Union&#39;s Horizon 2020 Research and innovation program under the Marie Sk&#322;odowska-Curie grant agreement No. 665667. The authors thank Pascal Alexander Schouwink for his help with XRD.

CAUTION: Read carefully the material safety data sheets (MSDS) of the chemicals involved. Some of the chemicals used in the experiment are toxic. The present method involves the synthesis of nanoparticles. Therefore, take appropriate precautions. The entire synthesis procedure must be performed in a well-ventilated fume hood.
NOTE: The details of the instruments, the chemicals, and the materials involved in the synthesis of the MOF films are listed in Table 1.
<p class="jo...

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The standout feature of the ENACT method with respect to the existing methods15 is that the ENACT method enables the synthesis of highly intergrown, ultrathin MOF films on a wide range of porous and nonporous substrates. Any substrate pretreatment is avoided, making this method quite straightforward for the synthesis of MOF films. Although EPD equipment has to be used for the deposition of a nuclei film, the equipment is composed of a power source, a metal electrode, and a beaker, which is quite s...

Thin molecular sieving membranes offer a high-energy efficiency in the separation of molecules and can reduce the overall cost of fuels, CO2 capture, water purification, solvent recovery, etc.1,2. MOFs are a promising class of material for the synthesis of molecular sieving membranes because of the involved isoreticular synthetic chemistry and relatively straightforward crystallization3. To date, MOF membranes comprising of diverse crystalline structures, including that of ZIF-4, -7, -8, -9, -11, -67, -90, and -93, and UiO-66, HKUST-1, and MIL-53 have been reported4,5. These membranes are synthesized by crystallizing high-quality polycrystalline MOF films on a porous support. Generally, to obtain a high separation selectivity, it is necessary to reduce the defects in the polycrystalline MOF film (such as pinholes and grain-boundary defects). A convenient approach to reduce the defects is to crystallize a thick film. Not surprisingly, several of the earlier reported on MOF membranes are extremely thick (over 5 &#181;m). Unfortunately, thick films lead to a long diffusion path, which limits the membrane permeance. Therefore, while selectivity is improved, permeance is sacrificed. To circumvent this trade-off, it is imperative to develop methods to crystallize ultrathin (&#60; 0.5 &#181;m-thick), defect-free MOF films.
ZIF-8 is the most intensively studied MOF for membrane synthesis, due to its exceptional chemical and thermal stability and a simple crystallization chemistry6,7. So far, the reported ultrathin ZIF-8 membranes have been realized by changing the surface chemistry or topology of the underlying porous substrate, favoring the heterogeneous nucleation of ZIF-8, which is essential for an intergrown polycrystalline film. For instance, Chen et al. reported the synthesis of 1 &#181;m-thick ZIF-8 film on (3-aminopropyl)triethoxysilane-modified TiO2-coated poly(vinylidene fluoride) (PVDF) hollow fibres8. They observed a high heterogeneous nucleation density and attributed it to the simultaneous modification of the surface chemistry and nanostructure. The Peinemann group reported an ultrathin ZIF-8 membrane on a metal-chelating, polythiosemicarbazide (PTSC) support9. This unique metal-chelating capability of PTSC led to the binding of zinc ions, promoting the heterogeneous nucleation of ZIF-8 which, subsequently, led to high-performance ZIF-8 membranes. In general, tuning the substrate chemistry and nanostructure facilitates the synthesis of high-performance MOF membranes; however, these methods are quite complex, and usually cannot be reapplied to synthesize MOF membranes from other attractive MOF structures.
Herein, we report the synthesis of ultrathin, highly intergrown ZIF-8 films using a simple and versatile crystallization approach that can be reapplied to form a thin intergrown film of several crystalline materials10. We show examples of ZIF-8 and ZIF-7 films prepared without any substrate pretreatment, which greatly simplifies the preparation process. The ZIF-8 films are prepared on a wide range of substrates (ceramic, polymer, metal, carbon, and graphene). The 500 nm-thick ZIF-8 film on an anodic aluminum oxide (AAO) support displays an attractive separation performance. A high H2 permeance of 8.3 x 10-6 mol m-2 s-1 Pa-1 and attractive ideal selectivities of 7.3 (H2/CO2), 15.5 (H2/N2), 16.2 (H2/CH4), and 2655 (H2/C3H8) are achieved.
The crystallization approach that enables the above-mentioned feat is ENACT. ENACT deposits ZIF-8 nuclei onto a substrate directly from the crystal&#39;s precursor sol. The approach utilizes EPD for a very short period of time (1 - 4 min) right after the induction time (the time when the nuclei appear in the precursor sol). The application of an electric field to the charged MOF nuclei drives them toward an electrode with a flux that is proportional to the strength of the applied electric field (E), the electrophoretic mobility of the colloid (&#956;), and the concentration of nuclei (Cn) as shown in the Equations 1 and 2.
<img alt="Equation 1" src="/files/ftp_upload/58301/58301eq1.jpg" /> 
(Equation 1)
<img alt="Equation 2" src="/files/ftp_upload/58301/58301eq2.jpg" /> 
(Equation 2)
Here, 
v = the drift velocity, 
&#950; = the zeta potential of the nuclei, 
&#949;o = the permittivity of vacuum, 
&#949;r = the dielectric constant, and 
&#951; = the viscosity of the precursor sol.
Therefore, by controlling E and the solution pH (which determines &#950;&#160;), the packing density of nuclei can be controlled. The subsequent growth of the densely-packed nuclei in the precursor sol allows researchers to obtain a highly intergrown polycrystalline film.

<table>
	<tbody>
		<tr>
			<td>Zinc nitrate hexahydrate</td>
			<td>Sigma-Aldrich</td>
			<td>96482-500G</td>
			<td>98% purity</td>
		</tr>
		<tr>
			<td>2-Methylimidazole</td>
			<td>Sigma-Aldrich</td>
			<td>M50850-500G</td>
			<td>99% purity</td>
		</tr>
		<tr>
			<td>Benzimidazole</td>
			<td>TCI</td>
			<td>B0054-500G</td>
			<td>98% purity</td>
		</tr>
		<tr>
			<td>Tape</td>
			<td>DuPont</td>
			<td>KPT-1/8</td>
			<td></td>
		</tr>
		<tr>
			<td>Epoxy</td>
			<td>GC Electronics</td>
			<td>19-823</td>
			<td></td>
		</tr>
		<tr>
			<td>Copper foil</td>
			<td>Alfa Aesar</td>
			<td>13380.CV</td>
			<td>99.9% purity</td>
		</tr>
		<tr>
			<td>Power source for EPD</td>
			<td>Gamry Instruments</td>
			<td>Interface 1000E Potentiostat</td>
			<td></td>
		</tr>
		<tr>
			<td>Ultrasonic cleaner</td>
			<td>MTI corporation</td>
			<td>VGT-1860QTD</td>
			<td></td>
		</tr>
		<tr>
			<td>AAO</td>
			<td>GE Healthcare Life Sciences&#8206;</td>
			<td>6809-7013</td>
			<td></td>
		</tr>
		<tr>
			<td>PAN</td>
			<td>Shandong MegaVision</td>
			<td></td>
			<td>The molecular weight cut-off is 100 kDa</td>
		</tr>
	</tbody>
</table>

electrophoretic crystallization of ultrathin high-performance metal-organic framework membranes

We report the synthesis of thin, highly intergrown, polycrystalline metal-organic framework (MOF) membranes on a wide range of unmodified porous and non-porous supports (polymer, ceramic, metal, carbon, and graphene). We developed a novel crystallization technique, which is termed the ENACT approach: the electrophoretic nuclei assembly for the crystallization of highly intergrown thin films (ENACT). This approach allows for a high density of heterogeneous nucleation of MOFs on a chosen substrate via the electrophoretic deposition (EPD) directly from the precursor sol. The growth of well-packed MOF nuclei leads to a highly intergrown polycrystalline MOF film. We show that this simple approach can be used for the synthesis of thin, intergrown zeolite imidazole framework (ZIF)-7 and ZIF-8 films. The resulting 500 nm-thick ZIF-8 membranes show a considerably high H2 permeance (8.3 x 10-6 mol m-2 s-1 Pa-1) and ideal gas selectivities (7.3 for H2/CO2, 15.5 for H2/N2, 16.2 for H2/CH4, and 2655 for H2/C3H8). An attractive performance for C3H6/C3H8 separation is also achieved (a C3H6 permeance of 9.9 x 10-8 mol m-2 s-1 Pa-1 and a C3H6/C3H8 ideal selectivity of 31.6 at 25 &#176;C). Overall, the ENACT process, owing to its simplicity, can be extended to synthesize intergrown thin films of a wide range of nanoporous crystalline materials.

A homemade EPD set-up was used to synthesize the MOF films (Figure 1). Scanning electron microscopy (SEM) images and X-ray diffraction (XRD) patterns were collected for the ZIF-8 nuclei film (Figure 2). SEM was used to image the surface and cross-sectional morphologies of the AAO support, ZIF-8/AAO membrane, PAN support, ZIF-8/PAN membrane, ZIF-8/graphene film, and ZIF-7/AAO membrane (Figure 3). The ...

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Electrophoretic Crystallization of Ultrathin High-performance Metal-organic Framework Membranes

A simple, reproducible, and versatile approach for the synthesis of intergrown, polycrystalline metal-organic framework membranes on a wide range of unmodified porous and non-porous supports is presented.

We report the synthesis of thin, highly intergrown, polycrystalline metal-organic framework (MOF) membranes on a wide range of unmodified porous and ...

This method can help answer the key question in the synthesis of polycrystalline MOF films such as how to reproducibly synthesize MOF membranes. The main advantage of this technique is that we can control the heterogeneous nucleation over a wide range of substrates leading to ultrathin, pinhole-free MOF films. With the help of crystal engineering involving controlled nucleation and growth we aim to simplify the synthesis protocol of MOF films.First cut a high purity copper foil into four by four centimeter pieces. For ease of clamping the cut foil with a set of copper crocodile clips draw a line 0.5 centimeters from one of the edges of each square foil. Then flatten each foil using a cylindrical roller on a clean surface.Clean the copper foils thoroughly by bath sonication in acetone for 15 minutes followed by bath sonication in isopropanol for 15 minutes. Then dry the copper foils in a glass dish. Carefully position the desired substrate on the center of the copper electrode using tape.Rinse the substrate electrode assembly for one minute with water followed by isopropanol and again with water. Following this attach a bare copper electrode to an anode. Then attach the substrate electrode assembly to a cathode.Place the two electrodes in a 100 milliliter glass beaker and adjust the separation between them to one centimeter. Add 31.6 grams of zinc hexahydrate solution and 35 grams of benzimidazole solution to a 100 milliliter beaker and stir for 30 seconds at room temperature to form the precursor sol. Transfer the precursor sol to the beaker containing the electrodes.Then immerse both the electrodes up to the 3.5 centimeter mark by adjusting the height of the beaker. Carry out electrophoretic deposition with the deposition voltage of one volt for four minutes by turning on the power of the power source. At the end of the deposition lower the beaker slowly to avoid disturbing weak adhesion between the freshly deposited nuclei and the substrate.After drying the substrate transfer it to a microscopic glass slide using tape to hold the substrate in place. For crystal growth mix 31.6 grams of zinc hexahydrate solution and 35 grams of 2-Methylimidazole solution in a 100 milliliter beaker. Place the microscopic glass slide with the substrate vertically in the precursor solution and leave it undisturbed for 10 hours at 30 degrees Celsius.After 10 hours of crystal growth rinse the substrate with water for 30 minutes. Then dry the substrate in a clean atmosphere. To prepare the sealant thoroughly mix an equal proportion of epoxy and hardener and leave the mixture for one hour.Place the ZIF-8 membrane on a 24 millimeter wide steel disc with a five millimeter diameter hole at the center. Apply the epoxy along the edges of the substrate and subsequently cover the substrate except for the five millimeter diameter hole at the center. After allowing the epoxy to dry overnight use a stereomicroscope to scan the membrane along with the known reference scale.Use graphical software to calculate the exposed area of the membrane from the scanned image. Next place the steel disc with the membrane in the stainless steel permeation cell and place the cell in an oven. Ensure a leak tight fit by placing Viton O-rings above and below the steel disk and fastening the screws.Set the gas flow rates on the feed and the sweep sides to 30 milliliters per minute. To remove the absorbed water during synthesis heat the membrane cell using hydrogen as the feed gas and argon as this sweep gas at 130 degrees Celsius for two hours. Maintain a pressure of 0.1 megapascal at the feed and the sweep side by adjusting the needle valves on the on the retentate and the permeate side, respectively.Change the gas flow in the feed side to target gas and set the gas flow rate to 30 milliliters per minute. Set the temperature of the oven to the desired temperature. Then calculate the permeance in Excel once a steady state is established according to the data from the mass spectrum.The SEM images and XRD patterns show that the ZIF-8 nuclei film is compact and demonstrates that the ENACT method is quite efficient in controlling the heterogenous nucleation density over a substrate. Post growth the film morphology observed by SEM is compact and pinhole free and appears to be highly intergrown. The surface and cross sectional morphologies of the AAO support, ZIF-8/AAO membrane, PAN support, and ZIF-8/PAN membrane are shown here.The gas permanence data of ZIF-8/AAO and ZIF-8/PAN membranes show that their hydrogen propane selectivity are 2700 and 190, respectively, proving that the MOF film is nearly defect free. The ZIF-8/AAO membrane shows ultra high hydrogen permeance because of the ultra thin thickness. While attempting this procedure it is important to wait for the crystal induction to occur before applying electrophoretic deposition.For example crystal induction can be ratified by studying the precursor sol in a transmission electron microscope. This protocol allows one to precisely control the heterogeneous nucleation density and the thickness of the nuclei film by simply controlling the electric field and the electrophoretic deposition time and can be extended to synthesize polycrystalline films of various structures. After the development of this technique it paved the way for us to conduct systematic studies on engineering MOF films of thickness less than 100 nanometers.Don't forget working with methylimidazole and zinc nitrate can be hazardous and precautions such as wearing gloves and operating in a fume hood should always be taken while performing this procedure.

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9.8K Views.  École Polytechnique Fédérale de Lausanne (EPFL). This method can help answer the key question in the synthesis of polycrystalline MOF films such as how to reproducibly synthesize MOF membranes. The main advantage of this technique is that we can control the heterogeneous nucleation over a wide range of substrates leading to ultrathin, pinhole-free MOF films. With the help of crystal engineering involving controlled nucleation and growth we aim to simplify the synthesis protocol of MOF films.First cut a high purity copper foil into fo...