Name: discovery and synthesis optimization of isoreticular al(iii) phosphonate-based metal-organic framework compounds using high-throughput methods
Uploaded: 2023-10-06T14:20:00.000+00:00
Duration: 7 min 20 s
Description: Watch this Scientific Journal Video about Accelerating Discovery in Microporous Material Chemistry at JoVE.com

The work was supported by the Christian-Albrechts-University, the State of Schleswig-Holstein, and the Deutsche Forschungsgemeinschaft (especially STO-643/2, STO-643/5 and STO-643/10).
Norbert Stock would like to thank the B.Sc., M.Sc., and doctoral students, as well as the cooperation partners who have carried out many interesting projects using the high-throughput methodology, in particular Prof. Bein from the Ludwig-Maximilians-Universit&#228;t in Munich, who played a major role in the development of the reactors.

In this protocol, the HT investigation of chemical systems to discover new crystalline materials, using Al-CAU-6029 as an example, is described.
1. Design of Experiment (DOE)
NOTE: The first step is to set up a synthesis plan, which requires knowledge of the reactor setup (Figure 2), reactants, and solvents used. This synthesis planning procedure is adapted to performing 24 or 48 reactions under a specific temperature-time program, for which in-house made steel multiclaves are used to perform 24 (Figure 2A) or 48 reactions (Figure 2B) at once. The reactors are in-house made PTFE inserts with a used reagent/solvent volume of 1 mL (PTFE reactor for carrying out 24 reactions in the steel multiclave) or 100 &#181;L (PTFE reactor for carrying out 48 reactions in the steel multiclave). The technical drawings of the reactor setup can be found in Supplementary File 1 and Supplementary File 2 respectively.
<ol>
	<li>First, determine the parameter space to be investigated. Therefore, make decisions about an initial number of reactions, metal source, and linker molecule, as well as the use of additives and solvent.
	<ol>
		<li>For the chosen example of Al-CAU-60, carry out 24 reactions using AlCl3&#8729;6H2O as a metal source and N, N&#8242;-piperazine-bis(methylenephosphonic acid) (H4PMP) as a linker molecule. Furthermore, use aqueous solutions of NaOH and HCl as additives to study the influence of the pH of the reaction mixture on product formation.1 
		NOTE: The choice of parameters is usually based on published synthesis procedures or principles based on fundamental chemical knowledge. However, for the successful discovery of new materials, a broader variation of the reaction parameters should be applied (i.e., a certain degree of diversity of the reaction parameters should be considered). The number of parameters to be varied and the type of variations can be based on different principles. In the simplest form, only one parameter should be changed at a time. For example, a fixed metal salt concentration in combination with varying linker molecule concentrations can be used to investigate different linker-to-metal ratios. However, the investigation can also use different molar ratios of the linker to the metal and other solvents or additives. The accessible parameter space is limited by the solubility of the starting materials (amount and solvent type) in cases where only solutions are used21. The dosing of solids extends the accessible parameter space20.</li>
	</ol>
	</li>
	<li>Specify the parameter space. For this purpose, choose and calculate quantities of starting materials (molar ratios) and solvent volumes.
	<ol>
		<li>For the chosen example of Al-CAU-60, vary the molar ratio of H4PMP to Al3+ between 4:1 and 0.3:1 in six steps: 4:1, 3:1, 2:1, 1:1, 0.5:1, 0.3:1. Carry out all six syntheses with different additive ratios; study one molar ratio of NaOH to Al3+ (1:1) and two molar ratios of HCl to Al3+ (20:1 and 40:1), as well as one without any additive. Use a spreadsheetto calculate the quantities of starting materials required for this, which can be found in the additional information.</li>
	</ol>
	</li>
</ol>
2. Dosing and solvothermal synthesis
<ol>
	<li>Prepare the stock solutions in a fume hood by following the standard protocol for preparing stock solutions of the reagents. 
	CAUTION: H4PMP, AlCl3&#8729;6 H2O, HCl, and NaOH are corrosive substances, which cause severe skin burns and eye damage on contact. Wear personal protective equipment when working with these substances.
	<ol>
		<li>For the chosen example of Al-CAU-60, prepare the following reagents according to the spreadsheet in the supporting information (Supplementary Table 1): hydrochloric acid solution with a concentration of 10 mol/L, sodium hydroxide solution with a concentration of 1 mol/L, and an AlCl3&#8729;6H2O solution with a concentration of 1 mol/L. 
		NOTE: Product formation can also depend on the aggregation state of the added reagents. For solids, the particle size can have an effect due to the dissolution rate. A decision should be made at the beginning of the study whether to use solids or solutions to allow for systematic evaluation.</li>
	</ol>
	</li>
	<li>Insert the discs into the sample plate (Figure 3A).</li>
	<li>Transfer reagents, additives, and solvents into the PTFE inserts (Figure 3B).
	<ol>
		<li>For the chosen example of Al-CAU-60, first add the linker H4PMP as a solid to the PTFE inserts, then add the aluminum chloride solution, the demineralized water, and the solution of additives (NaOH or HCl) with a pipette in accordance with the values calculated in the spreadsheet in the supporting information (Supplementary Table 1). 
		NOTE: The order in which the PTFE inserts are filled can also influence product formation; therefore, the order of starting materials should be chosen in advance and kept the same throughout the study to allow a systematic evaluation.</li>
	</ol>
	</li>
	<li>Insert the filled PTFE inserts into the sample plate.</li>
	<li>Mark the ground plate of the reactor in a way that allows the identification of the PTFE inserts later. Insert the sample plate with the filled PTFE inserts into the ground plate of the reactor (Figure 3C).</li>
	<li>Prepare two PTFE sheets (with a thickness of 0.1 mm) to cover the sample plates.</li>
	<li>Place the PTFE sheets on the sample plate (Figure 3D).</li>
	<li>Ensure that the PTFE sheet is correctly positioned and fits the head plate using the guide pins (Figure 3E), add the screws, and tighten them by hand.</li>
	<li>Seal the initially closed reactor with the help of, for example, a mechanical or hydraulic press (Figure 4A), far enough that the spring-loaded pressure pieces still have 2 mm of free space (Figure 4B). Then, tighten the screws by hand again (Figure 4C). Be aware that over-tightening can damage (bend) the multiclaves.</li>
	<li>Place the multiclave in a programmable forced convection oven (Figure 4D), and then set and start the selected temperature-time program. It is advisable to use a convection oven to ensure uniform heating.
	<ol>
		<li>For the discovery of Al-CAU-60, set the following temperature-time-program: Heat the oven to 160 &#176;C in 12 h, maintain the target temperature for 36 h, and cool to room temperature (RT) in 12 h. 
		NOTE: The choice of the temperature-time program can influence product formation30. This includes the phases formed, but more often the crystal size and morphology30.</li>
	</ol>
	</li>
</ol>
3. Isolation and workup
<ol>
	<li>Remove the multiclave from the oven when the temperature reaches room temperature.</li>
	<li>Place the multiclave, for example, in a mechanical or hydraulic press and gently compress it until the screws can be loosened by hand (Figure 5A).</li>
	<li>Place the multiclave in a fume hood and remove the head plate of the reactor, then remove the PTFE sheets and remove the sample plate with the PTFE inserts from the ground plate of the reactor (Figure 5B).</li>
	<li>Inspect the PTFE inserts and check for crystals (Figure 5C). If present, isolate some of them together with some mother liquor.</li>
	<li>Next, assemble the in-house high-throughput filtration block (Figure 6A): connect the filter block to a vacuum pump via two wash bottles, and place two filter papers between two silicone sealing mats with the corresponding recesses (Figure 6B-D) in the filter block. Place the PTFE filling block on top, making sure that the appropriate recesses match with the sealing mats and the filter block (Figure 6E). Tighten the layers using the clamping frame, which is held in place by four stud bolts. To properly seal the unit, use wing nuts on the stud bolts and tighten by hand (Figure 6F). 
	NOTE: The technical drawings of the filtration block are shown in the supporting information (Supplementary File 3). If a filter block is not available, the products can also be filtered individually.</li>
	<li>Close the recesses of the filling block that are not to be filled with plugs (Figure 6F).
	<ol>
		<li>Later in the process, seal the recesses that have already been drained. This allows the other wells to be drained as well.</li>
	</ol>
	</li>
	<li>Turn on the membrane vacuum pump and set it to a mode in which it will pump down to the best possible vacuum (5-12 mbar).</li>
	<li>Using disposable pipettes, transfer the contents of the PTFE inserts into the designated wells of the filling block (Figure 7A). 
	NOTE: If harmful solvents (e.g., dimethylformamide ) are used, the products should be washed with ethanol or another less toxic and more volatile solvent to reduce contact with harmful substances during the following steps.</li>
	<li>After all the inserts are empty, take a second look for crystals and isolate them if there are any (Figure 7B).NOTE: It is recommended to use an optical microscope with the possibility to use different magnifications in order to determine the size of the crystallites.</li>
	<li>Carefully disassemble the filtration block once all the wells are drained (Figure 7C).</li>
	<li>A so-called &#34;product library&#34; is now available on the filter paper (Figure 7D).</li>
	<li>Dry the product library by allowing it to air-dry in a fume hood; in the case of non-toxic and non-corrosive solvents, the PXRD measurements can be performed with wet products.</li>
</ol>
4. Characterization
NOTE: For the discovery of new crystalline compounds, the products obtained are characterized by HT-PXRD. New crystalline phases are identified and used for further characterization. Working with the powder X-ray diffractometer follows a standard procedure, which can be found in the operating manual. A standard powder X-ray diffractometer can also be used, which makes the characterization more tedious.
<ol>
	<li>Place the product library between two metal plates (base plate and cover plate; Figure 7E and Supplementary File 4) in a way that the recesses in the plates match the product locations to allow examination by PXRD. Carefully align the plates and secure them with two screws (Figure 7F).</li>
	<li>Insert the product library into the sample holder of the diffractometer (Figure 8A,B). 
	NOTE: Other sample holders may require different brackets. Refer to the user manual for further information.</li>
	<li>Carefully place the loaded sample holder into the xy stage of the diffractometer and close the instrument (Figure 8C).</li>
	<li>The diffractometer is controlled via WinXPOW software31. In the Diffractometer Control window, set the measuring mode by clicking on the Ranges menu and choose Scan Mode. A new window opens; herein, choose Scan Mode: Transmission, PSD Mode: Moving, Scan Type: 2Theta, and Omega Mode: Fixed and confirm the dialog.</li>
	<li>To set the measuring parameters, click on the Ranges menu and choose Scan Range.
	<ol>
		<li>A new window opens; herein, click on the Plus Icon and edit the appearing standard settings by double-clicking on it.</li>
		<li>To characterize the product library, perform a short 4 min measurement of each sample with the following settings: (a) 2Theta(Begin, End): 2, 47, (b) Step: 1.5, (c) Time/PSD Step [s]: 2, (d) Omega: 0. Confirm both dialogs.</li>
	</ol>
	</li>
	<li>To choose the samples to be measured on the xy stage, click on the Ranges menu and choose Scan Usage.
	<ol>
		<li>A new window opens; herein, set the Scan Usage to Multiple Samples and check the option Individual Ranges/Files.</li>
		<li>Next, click on the button Ranges/Files; a new window (&#34;HT_Editor&#34;) with 48 selectable sample positions opens. Select all positions with samples on the sample plate by clicking on the position with the &#39;control&#39; key pressed.</li>
		<li>To activate the positions, use the right-click on Measure Samples. Confirm both dialogs.</li>
	</ol>
	</li>
	<li>Save the files by clicking on File in the menu and choose Save As. After choosing a directory and a filename, click on the Save button.</li>
	<li>Start the measurement by clicking on Measure in the menu and choose the first entry, Data Collection. A new window opens; click on the Ok button to start the measurement. 
	NOTE: The default settings and the procedure for calibrating the diffractometer should be taken from the user manual. The choice of measurement parameters (scanning angle, step size, time per scanning step) is also dependent on the density of the material, the weight of the diffracting atoms, etc., and may have to be adjusted. Absorption of the X-rays can be a problem if too much sample is formed and heavy elements are used.</li>
</ol>
5. Data evaluation
NOTE: An in-house procedure is used to evaluate the data; other procedures are conceivable. The PXRD data is obtained in &#34;.raw&#34; file format. To evaluate the diffractograms in other software, this file format must be converted, for example, to &#34;.xyd&#34; file format.
<ol>
	<li>Open the WinXPOW software31. To open the powder X-ray diffractograms, use the Raw Data menu and choose Raw Data Handling. A new window opens.</li>
	<li>Click on the icon for Batch Open and select all the files via Add Files. After selecting all the files, click on Open and confirm with Ok.</li>
	<li>Normalize the intensities to a maximum value of 10,000 by clicking on the Ranges and choosing Adapt Intensities; a new window opens. Choose the option Normalize Intensities to max. Int. and write 10000. Click Ok. 
	NOTE: WinXPOWsoftware31 overwrites the raw data when the data is changed; make sure to work on copies of the data.</li>
	<li>Export the files via the Export icon in a file format suitable for evaluation programs. Choose an output directory and use the X/Y file format. Click Ok to finish the export.</li>
	<li>Display the PXRD data in a stacked or separated view in a suitable program. Identify the most crystalline products by examining the number of reflections, half-widths (full width at half maximum [FWHM]), and signal-to-noise ratio. 
	NOTE: For a first analysis, WinXPOWsoftware31 with the Graphics subroutine and the Search and Match function can also be used.</li>
</ol>

Synthesis and Discovery of Phosphonate-Based MOFs: A Solvothermal Approach

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M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Targeted+synthesis+of+an+highly+stable+aluminium+phosphonate+metal-organic+framework+showing+reversible+HCl+adsorption.">Targeted synthesis of an highly stable aluminium phosphonate metal-organic framework showing reversible HCl adsorption.</a> Angewandte Chemie. , (2023).</li><li>Biemmi, E., Christian, S., Stock, N., Bein, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-throughput+screening+of+synthesis+parameters+in+the+formation+of+the+metal-organic+frameworks+MOF-5+and+HKUST-1.">High-throughput screening of synthesis parameters in the formation of the metal-organic frameworks MOF-5 and HKUST-1.</a> Microporous and Mesoporous Materials. 117 (1), 111-117 (2009).</li><li>STOE &amp; Cie GmbH. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=WinXPOW+v.3.1.">WinXPOW v.3.1.</a> STOE &amp; Cie GmbH. , (2016).</li><li>Groom, C. R., Bruno, I. J., Lightfoot, M. P., Ward, S. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Cambridge+structural+database.">The Cambridge structural database.</a> Acta Crystallographica Section B, Structural Science. Crystal Engineering and Materials. 72, 171-179 (2016).</li><li>Bruno, I. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=New+software+for+searching+the+Cambridge+Structural+Database+and+visualizing+crystal+structures.">New software for searching the Cambridge Structural Database and visualizing crystal structures.</a> Acta Crystallographica. Section B, Structural Science. 58, 389-397 (2002).</li><li>Hermer, N., Wharmby, M. T., Stock, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> CCDC 1499757: Experimental Crystal Structure Determination. , (2017).</li><li>Cawse, J. N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Experimental Design for Combinatorial and High Throughput Materials Development. , (2003).</li><li>Dhanumalayan, E., Joshi, G. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Performance+properties+and+applications+of+polytetrafluoroethylene+(PTFE)-a+review.">Performance properties and applications of polytetrafluoroethylene (PTFE)-a review.</a> Advanced Composites and Hybrid Materials. 1, 247-268 (2018).</li><li>Lenzen, D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Scalable+green+synthesis+and+full-scale+test+of+the+metal-organic+framework+CAU-10-H+for+use+in+adsorption-driven+chillers.">Scalable green synthesis and full-scale test of the metal-organic framework CAU-10-H for use in adsorption-driven chillers.</a> Advanced Materials. 30 (6), 1705869 (2018).</li></ol>

The authors have nothing to disclose.

Due to the complexity of the HT method, the individual steps and the method itself are discussed in the following sections. The first part covers the critical steps for each working step of the HT workflow (Figure 1), possible modifications, and limitations of the technique, where applicable. At the end, a general discussion also including the significance of the HT method with respect to existing methods and future applications is presented. 
In the first step of...

Metal-organic frameworks (MOFs) are porous, crystalline compounds whose structures consist of metal-containing nodes, like metal ions or metal-oxygen clusters, which are connected by organic molecules (linkers)1. By varying the metal-containing nodes as well as the linker, a variety of compounds can be obtained that exhibit a wide range of properties and therefore have potential applications in different fields1.
The stability of a material is important for its application1,2,3. Therefore, MOFs containing tri- or tetravalent metal ions, such as Al3+, Cr3+, Ti4+, or Zr4+, with carboxylate2 or phosphonate4 linker molecules have been the focus of many investigations5,6,7. In addition to the direct synthesis of stable MOFs, the enhancement of stability through post-synthetic modifications as well as the formation of composites is a field of interest2. Phosphonate-based MOFs have been less often reported compared to carboxylate-based MOFs8. One reason is the higher coordination flexibility of the CPO32- group compared to the -CO2- group, which often leads to the formation of dense structures and greater structural diversity8,9,10,11. In addition, phosphonic acids often must be synthesized, as they are rarely available on the market. While some metal phosphonates exhibit exceptional chemical stability10, systematic access to isoreticular metal phosphonate MOFs, which allows the tuning of properties, is still a topic of high relevance12,13. Different strategies for the synthesis of porous metal phosphonates have been investigated, such as incorporating defects into otherwise dense layers, for example, by partially replacing phosphonate with phosphate ligands4,14. However, as defective structures are poorly reproducible, and the pores are not uniform, other strategies have been developed. In recent years, the use of sterically demanding or orthogonalized phosphonic acids as linker molecules have emerged as a suitable strategy for the preparation of porous metal phosphonates4,8,10,11,13,15,16,17,18. However, a universal synthesis route for porous metal phosphonates has not yet been discovered. As a result, the synthesis of metal phosphonates is often a process of trial and error, requiring the investigation of many synthesis parameters.
The parameter space of a reaction system includes chemical and process parameters and can be vast19. It consists of parameters such as the type of starting material (metal salt), molar ratios of starting materials, additives for pH adjustment, modulators, type of solvent, solvent mixtures, volumes, reaction temperatures, times, etc.19,20. A moderate number of parameter variations can easily result in several hundred individual reactions, making a carefully considered synthesis plan and well-chosen parameter space necessary. For example, a simple study using six molar ratios of the linker to the metal (e.g., M:L = 1:1, 1:2, ... to 1:6) and four different concentrations of an additive and keeping the other parameter constant, leads already to 6 x 4 = 24 experiments. Using four concentrations, five solvents, and three reaction temperatures would necessitate carrying out the 24 experiments 60 times, resulting in 1,440 individual reactions.
High-throughput (HT) methods are based on the concepts of miniaturization, parallelization, and automation, to varying degrees depending on the scientific question being addressed19,20. As such, they can be used to accelerate the investigation of multi-parameter systems and are an ideal tool for the discovery of new compounds, as well as synthesis optimization19,20. HT methods have been used successfully in different fields, ranging from drug discovery to materials science20. They have also been used for the investigation of porous materials such as zeolites and MOFs in solvothermal reactions, as recently summarized20. A typical HT workflow for solvothermal synthesis consists of six steps (Figure 1)19,20,21: a) selection of the parameter space of interest (i.e., the design of experiment [DOE]), which can be done manually or by using software; b) dosing of the reagents into the vessels; c) solvothermal synthesis; d) isolation and workup; e) characterization, which is typically done with powder X-ray diffraction (PXRD); and f) data evaluation, which is followed by step one again.
Parallelization and miniaturization are achieved in solvothermal reactions through the use of multiclaves, often based on the well-established 96-well plate format most commonly used in biochemistry and pharmacy19,20,22,23. Various reactor designs have been reported and several groups have constructed their own reactors19,20. Reactor choice depends on the chemical system of interest, especially the reaction temperature, (autogenous) pressure, and reactor stability19,20. For example, in a systematic study of zeolitic imidazolate frameworks (ZIFs), Banerjee et al.25 used the 96-well glass plate format to perform over 9600 reactions24. For reactions under solvothermal conditions, customized polytetrafluoroethylene (PTFE) blocks, or multiclaves with 24 or 48 individual PTFE inserts, have been described among others by the Stock group19,20.&#160;They are routinely employed, for example, in the synthesis of metal carboxylates and phosphonates. As such, Reinsch et al.25 reported the advantages of the methodology in the field of porous aluminum MOFs25. The in-house made HT reactor systems (Figure 2), which allow 24 or 48 reactions to be studied simultaneously, contain PTFE inserts with a total volume of 2.655 mL and 0.404 mL, respectively (Figure 2A,B). Usually, no more than 1 mL or 0.1 mL, respectively, is used. While these reactors are used in conventional ovens, microwave-assisted heating using SiC blocks and small glass vessels has also been reported26.
The automation of studies leads to time savings and improved reproducibility, as influence of the human factor is minimized20. The degree to which automation has been used varies strongly19,20. Fully automated commercial systems, including pipetting20 or weighting capabilities20, are known. A recent example is the use of a liquid-handling robot to study ZrMOFs, reported by the group of Rosseinsky27. Automated analysis can be performed by PXRD using a diffractometer equipped with an xy stage. In another example, a plate reader was used to screen solid-state catalysts, mainly MOFs, for HT screening of nerve agent degradation28. Samples can be characterized in a single run without the need for manual sample or position changes. Automation does not eliminate human error, but it reduces the possibility of its occurring19,20.
Ideally, all steps in a HT workflow should be adapted in terms of parallelization, miniaturization, and automation to eliminate possible bottlenecks and maximize efficiency. However, if it is not possible to establish a HT workflow in its entirety, it may be helpful to adopt selected steps/tools for one&#39;s own research. The use of multiclaves for 24 reactions is particularly useful here. The technical drawings of the in-house made equipment used in this study (as well as others) are published for the first time and can be found in Supplementary File 1, Supplementary File 2, Supplementary File 3, and Supplementary File 4.

<table><tbody><tr><td>AlCl&lt;sub&gt;3&lt;/sub&gt;&amp;middot;6H&lt;sub&gt;2&lt;/sub&gt;O</td><td>Gr&amp;uuml;ssing</td><td>N/A</td><td>99%</td></tr><tr><td>Filter block for filtration of max. 48 reaction mixtures</td><td>In-house made</td><td>N/A</td><td>Technical drawings in the supplementary files</td></tr><tr><td>Hydrochloric acid</td><td>Honeywell</td><td>258148</td><td>Conc. 37 %, p.a.</td></tr><tr><td>Multiclaves with 24 individual Teflon inserts</td><td>In-house made</td><td>N/A</td><td>Technical drawings in the supplementary files</td></tr><tr><td>N,N &amp;lsquo;-piperazine bis(methylenephosphonic acid</td><td>Prepared by coworkers</td><td>N/A</td><td>H4PMP,&amp;nbsp; Prepared by coworkers with the method reported by Villemin et al.: D. Villemin, B. Moreau, A. Elbilali, M.-A. Didi, M.&amp;rsquo;h. Kaid, P.-A. Jaffr&amp;egrave;s, Phosphorus Sulfur Silicon Relat. Elem. 2010, 185, 2511.</td></tr><tr><td>Sample Plate for PXRD</td><td>In-house made</td><td>N/A</td><td>Technical drawings in the supplementary files</td></tr><tr><td>Sodium hydroxide</td><td>Gr&amp;uuml;ssing</td><td>N/A</td><td>99%</td></tr><tr><td>Stoe Stadi P Combi</td><td>STOE</td><td>Stadi P Combi</td><td>Cu-K&lt;sub&gt;&amp;alpha;1&lt;/sub&gt; radiation (&amp;lambda; = 1.5406 &amp;Aring;); transmission geometry; MYTHEN2 1K detector; opening angle 18&amp;deg;; curved&amp;nbsp; monochromator; xy-table</td></tr><tr><td>Forced convection oven</td><td>Memmert</td><td>UFP400</td><td></td></tr></tbody></table>

discovery and synthesis optimization of isoreticular al(iii) phosphonate-based metal-organic framework compounds using high-throughput methods

High-throughput (HT) methods are an important tool for the fast and efficient screening of synthesis parameters and the discovery of new materials. This manuscript describes the synthesis of metal-organic frameworks (MOFs) from solution using an HT reactor system, resulting in the discovery of various phosphonate-based MOFs of the composition [Al2H12-x(PMP)3]Clx&#8729;6H2O (H4PMP = N,N &#39;-piperazine bis(methylenephosphonic acid)) for x = 4, 6, denoted as Al-CAU-60-xHCl, containing trivalent aluminum ions. This was accomplished under solvothermal reaction conditions by systematically screening the impact of the molar ratio of the linker to the metal and the pH of the reaction mixture on the product formation. The protocol for the HT investigation includes six steps: a) synthesis planning (DOE = design of&#160;experiment) within the HT methodology, b) dosing and working with in-house developed HT reactors, c) solvothermal synthesis, d) synthesis workup using in-house developed filtration blocks, e) characterization by HT powder X-ray diffraction, and f) evaluation of the data. The HT methodology was first used to study the influence of acidity on the product formation, leading to the discovery of Al-CAU-60&#8729;xHCl (x = 4 or 6).

The PXRD data is shown in Figure 9. For the first evaluation, the results obtained are linked to the synthesis parameters of the investigated parameter space. The investigation was carried out using six different molar ratios of linker to metal and four different molar ratios of NaOH/HCl to Al3+. By linking this information with the obtained PXRD data (Figure 9), it can be seen that products of low crystallinity were obtained from syntheses at a molar...

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Author Spotlight: Accelerating Discovery in Microporous Material Chemistry 

The targeted synthesis of new metal-organic frameworks (MOFs) is difficult, and their discovery depends on the knowledge and creativity of the chemist. High-throughput methods allow complex synthetic parameter fields to be explored quickly and efficiently, accelerating the process of finding crystalline compounds and identifying synthetic and structural trends.

Discovery and Synthesis Optimization of Isoreticular Al(III) Phosphonate-Based Metal-Organic Framework Compounds Using High-Throughput Methods

High-throughput (HT) methods are an important tool for the fast and efficient screening of synthesis parameters and the discovery of new materials. This ...

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3.4K Views.  Kiel University. The targeted synthesis of new metal-organic frameworks (MOFs) is difficult, and their discovery depends on the knowledge and creativity of the chemist. High-throughput methods allow complex synthetic parameter fields to be explored quickly and efficiently, accelerating the process of finding crystalline compounds and identifying synthetic and structural trends.

Video: Author Spotlight: Accelerating Discovery in Microporous Material Chemistry 

Synthesis and Characterization of Functionalized Metal-organic Frameworks

Metal-organic frameworks have attracted extraordinary amounts of research attention, as they are attractive candidates for numerous industrial and technological applications. Their signature property is their ultrahigh porosity, which however imparts a series of challenges when it comes to both constructing them and working with them. Securing desired MOF chemical and physical functionality by linker/node assembly into a highly porous framework of choice can pose difficulties, as less porous and more thermodynamically stable congeners (e.g., other crystalline polymorphs, catenated analogues) are often preferentially obtained by conventional synthesis methods. Once the desired product is obtained, its characterization often requires specialized techniques that address complications potentially arising from, for example, guest-molecule loss or preferential orientation of microcrystallites. Finally, accessing the large voids inside the MOFs for use in applications that involve gases can be problematic, as frameworks may be subject to collapse during removal of solvent molecules (remnants of solvothermal synthesis). In this paper, we describe synthesis and characterization methods routinely utilized in our lab either to solve or circumvent these issues. The methods include solvent-assisted linker exchange, powder X-ray diffraction in capillaries, and materials activation (cavity evacuation) by supercritical CO2 drying. Finally, we provide a protocol for determining a suitable pressure region for applying the Brunauer-Emmett-Teller analysis to nitrogen isotherms, so as to estimate surface area of MOFs with good accuracy.

Synthesis, activation, and characterization of intentionally designed metal-organic framework materials is challenging, especially when building blocks are incompatible or unwanted polymorphs are thermodynamically favored over desired forms. We describe how applications of solvent-assisted linker exchange, powder X-ray diffraction in capillaries and activation via supercritical CO2 drying, can address some of these challenges.

Metal-organic frameworks have attracted extraordinary amounts of research attention, as they are attractive candidates for numerous industrial and ...

Synthesis of a Water-soluble Metal&#8211;Organic Complex Array

We demonstrate a method for the synthesis of a water-soluble multimetallic peptidic array containing a predetermined sequence of metal centers such as Ru(II), Pt(II), and Rh(III). The compound, named as a water-soluble metal-organic complex array (WSMOCA), is obtained through 1) the conventional solution-chemistry-based preparation of the corresponding metal complex monomers having a 9-fluorenylmethyloxycarbonyl (Fmoc)-protected amino acid moiety and 2) their sequential coupling together with other water-soluble organic building units on the surface-functionalized polymeric resin by following the procedures originally developed for the solid-phase synthesis of polypeptides, with proper modifications. Traces of reactions determined by mass spectrometric analysis at the representative coupling steps in stage 2 confirm the selective construction of a predetermined sequence of metal centers along with the peptide backbone. The WSMOCA cleaved from the resin at the end of stage 2 has a certain level of solubility in aqueous media dependent on the pH value and/or salt content, which is useful for the purification of the compound.

A potential general method for the synthesis of water-soluble multimetallic peptidic arrays containing a predetermined sequence of metal centers is presented.

We demonstrate a method for the synthesis of a water-soluble multimetallic peptidic array containing a predetermined sequence of metal centers such as ...

Synthesis of Platinum-nickel Nanowires and Optimization for Oxygen Reduction Performance

Platinum-nickel (Pt-Ni) nanowires were developed as fuel cell electrocatalysts, and were optimized for the performance and durability in the oxygen reduction reaction. Spontaneous galvanic displacement was used to deposit Pt layers onto Ni nanowire substrates. The synthesis approach produced catalysts with high specific activities and high Pt surface areas. Hydrogen annealing improved Pt and Ni mixing and specific activity. Acid leaching was used to preferentially remove Ni near the nanowire surface, and oxygen annealing was used to stabilize near-surface Ni, improving durability and minimizing Ni dissolution. These protocols detail the optimization of each post-synthesis processing step, including hydrogen annealing to 250 &#176;C, exposure to 0.1 M nitric acid, and oxygen annealing to 175 &#176;C. Through these steps, Pt-Ni nanowires produced increased activities more than an order of magnitude than Pt nanoparticles, while offering significant durability improvements. The presented protocols are based on Pt-Ni systems in the development of fuel cell catalysts. These techniques have also been used for a variety of metal combinations, and can be applied to develop catalysts for a number of electrochemical processes.

&#160; The protocol describes the synthesis and electrochemical testing of platinum-nickel nanowires. Nanowires were synthesized by the galvanic displacement of a nickel nanowire template. Post-synthesis processing, including hydrogen annealing, acid leaching, and oxygen annealing were used to optimize nanowire performance and durability in the oxygen reduction reaction.

Platinum-nickel (Pt-Ni) nanowires were developed as fuel cell electrocatalysts, and were optimized for the performance and durability in the oxygen ...

Synthesis of a Thiol Building Block for the Crystallization of a Semiconducting Gyroidal Metal-sulfur Framework

We present a method for preparing thioester molecules as the masked form of the thiol linkers and their utilization for accessing a semiconducting and porous metal-dithiolene network in the highly ordered single crystalline state. Unlike the highly reactive free-standing thiols, which tend to decompose and complicate the crystallization of metal-thiolate open frameworks, the thioester reacts in situ to provide the thiol species, serving to mitigate the reaction between the mercaptan units and the metal centers, and to improve crystallization consequently. Specifically, the thioester was synthesized in a one-pot procedure: an aromatic bromide (hexabromotriphenylene) reacted with excess sodium thiomethoxide under vigorous conditions to first form the thioether intermediate product. The thioether was then demethylated by the excess thiomethoxide to provide the thiolate anion that was acylated to form the thioester product. The thioester was conveniently purified by standard column chromatography, and then used directly in the framework synthesis, wherein NaOH and ethylenediamine serve to revert in situ the thioester to the thiol linker for assembling the single-crystalline Pb(II)-dithiolene network. Compared with other methods for thiol synthesis (e.g., by cleaving alkyl thioether using sodium metal and liquid ammonia), the thioester synthesis here uses simple conditions and economical reagents. Moreover, the thioester product is stable and can be conveniently handled and stored. More importantly, in contrast to the generic difficulty in accessing crystalline metal-thiolate open frameworks, we demonstrate that using the thioester for in situ formation of the thiol linker greatly improves the crystallinity of the solid-state product. We intend to encourage broader research efforts on the technologically important metal-sulfur frameworks by disclosing the synthetic protocol for the thioester as well as the crystalline framework solid.

Here, we present a one-pot, transition-metal-free synthesis of thiols and thioesters from aromatic halides and sodium thiomethoxide, followed by the preparation of single crystals of a metal-dithiolene network using thiol species generated in situ from the more stable and tractable thioester.

We present a method for preparing thioester molecules as the masked form of the thiol linkers and their utilization for accessing a semiconducting and ...

Surface Functionalization of Metal-Organic Frameworks for Improved Moisture Resistance

Metal-organic frameworks (MOFs) are a class of porous inorganic materials with promising properties in gas storage and separation, catalysis and sensing. However, the main issue limiting their applicability is their poor stability in humid conditions. The common methods to overcome this problem involve the formation of strong metal-linker bonds by using highly charged metals, which is limited to a number of structures, the introduction of alkylic groups to the framework by post-synthetic modification (PSM) or chemical vapour deposition (CVD) to enhance overall hydrophobicity of the framework. These last two usually provoke a drastic reduction of the porosity of the material. These strategies do not permit to exploit the properties of the MOF already available and it is imperative to find new methods to enhance the stability of MOFs in water while keeping their properties intact. Herein, we report a novel method to enhance the water stability of MOF crystals featuring Cu2(O2C)4&#160;paddle-wheel units, such as HKUST (where HKUST stands for Hong Kong University of Science &#38; Technology), with the catechols functionalized with alkyl and fluoro-alkyl chains. By taking advantage of the unsaturated metal sites and the catalytic catecholase-like activity of CuII ions, we are able to create robust hydrophobic coatings through the oxidation and subsequent polymerization of the catechol units on the surface of the crystals under anaerobic and water-free conditions without disrupting the underlying structure of the framework. This approach not only affords the material with improved water stability but also provides control over the function of the protective coating, which enables the development of functional coatings for the adsorption and separations of volatile organic compounds. We are confident that this approach could also be extended to other unstable MOFs featuring open metal sites.

Robust functional catechol coatings were produced in one step by direct reaction of the material known as HKUST with synthetic catechols under anaerobic conditions. The formation of homogeneous coatings surrounding the entire crystal is ascribed to the biomimetic catalytic activity of Cu(II) dimers on the external surface of the crystals.

Metal-organic frameworks (MOFs) are a class of porous inorganic materials with promising properties in gas storage and separation, catalysis and sensing. ...

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 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 ...

Design, Synthesis, and Photochemical Properties of Clickable Caged Compounds

Caged compounds enable the photo-mediated manipulation of the cell physiology with high spatiotemporal resolution. However, the limited structural diversity of currently available caging groups and the difficulties in synthetic modification without sacrificing their photolysis efficiencies are obstacles to expanding the repertoire of caged compounds for live cell applications. As the chemical modification of coumarin-type photo-caging groups is a promising approach for the preparation of caged compounds with diverse physical and chemical properties, we report a method for the synthesis of clickable caged compounds that can be modified easily with various functional units via the copper(I)-catalyzed Huisgen cyclization. The modular platform molecule contains a (6-bromo-7-hydroxycoumarin-4-yl)methyl (Bhc) group as a photo-caging group, which exhibits a high photolysis efficiency compared to those of the conventional 2-nitrobenzyls. General procedures for the preparation of clickable caged compounds containing amines, alcohols, and carboxylates are presented. Additional properties such as the water solubility and cell targeting ability can be readily incorporated into clickable caged compounds. Furthermore, the physical and photochemical properties, including the photolysis quantum yield, were measured and were found to be superior to those of the corresponding Bhc caged compounds. The described protocol could therefore be considered a potential solution for the lack of structural diversity in the available caged compounds.

A protocol for the synthesis and measurement of the photochemical properties of modular caged compounds with clickable moieties is presented.

Caged compounds enable the photo-mediated manipulation of the cell physiology with high spatiotemporal resolution. However, the limited structural ...

In situ FTIR Spectroscopy as a Tool for Investigation of Gas/Solid Interaction: Water-Enhanced CO2 Adsorption in UiO-66 Metal-Organic Framework

In situ infrared spectroscopy is an inexpensive, highly sensitive, and selective valuable tool to investigate the interaction of polycrystalline solids with adsorbates. Vibrational spectra provide information on the chemical nature of adsorbed species and their structure. Thus, they are very useful for obtaining molecular level understanding of surface species. The IR spectrum of the sample itself gives some direct information about the material. General conclusions can be drawn concerning hydroxyl groups, some stable surface species and impurities. However, the spectrum of the sample is &#34;blind&#34; with respect to the presence of coordinatively unsaturated ions and gives rather poor information about the acidity of surface hydroxyls, species decisive for the adsorption and catalytic properties of the materials. Furthermore, no discrimination between bulk and surface species can be made. These problems are solved by the use of probe molecules, substances that interact specifically with the surface; the alteration of some spectral features of these molecules as a result of adsorption provides valuable information about the nature, properties, location, concentration, etc., of the surface sites.
The experimental protocol for in-situ IR studies of gas/sample interaction includes preparation of a sample pellet, activation of the material, initial spectral characterization through the analysis of the background spectra, characterization by probe molecules, and study of the interaction with a particular set of gas mixtures. In this paper we investigate a zirconium terephthalate metal organic framework, Zr6O4(OH)4(BDC)6 (BDC = benzene-1,4-dicarboxylate), namely UiO-66 (UiO refers to University of Oslo). The acid sites of the UiO-66 sample are determined by using CO and CD3CN as molecular probes. Furthermore, we have demonstrated that CO2 is adsorbed on basic sites exposed on dehydroxylated UiO-66. Introduction of water to the system produces hydroxyl groups acting as additional CO2 adsorption sites. As a result, CO2 adsorption capacity of the sample is strongly enhanced.

In situ FTIR Spectroscopy as a Tool for Investigation of Gas/Solid Interaction: Water-Enhanced CO2 Adsorption in UiO-66 Metal-Organic Framework

The use of FTIR spectroscopy for investigation of the surface properties of polycrystalline solids is described. Preparation of sample pellets, activation procedures, characterization with probe molecules and model studies of CO2 adsorption are discussed.

In situ infrared spectroscopy is an inexpensive, highly sensitive, and selective valuable tool to investigate the interaction of polycrystalline solids ...

Synthesis of Single-Crystalline Core-Shell Metal-Organic Frameworks

Because of their designability and unprecedented synergistic effects, core-shell metal-organic frameworks (MOFs) have been actively examined recently. However, the synthesis of single-crystalline core-shell MOFs is very challenging, and thus a limited number of examples have been reported. Here, we suggest a method of synthesizing single-crystalline HKUST-1@MOF-5 core-shells, which is HKUST-1 at the center of MOF-5. Through the computational algorithm, this pair of MOFs was predicted to have the matched lattice parameters and chemical connection points at the interface. To construct the core-shell structure, we prepared the octahedral- and cubic-shaped HKUST-1 crystals as a core MOF, in which the (111) and (001) facets were mainly exposed, respectively. Via the sequential reaction, the MOF-5 shell was well-grown on the exposed surface, showing a seamless connect interface, which resulted in the successful synthesis of single-crystalline HKUST-1@MOF-5. Their pure phase formation was proved by optical microscopic images and powder X-ray diffraction (PXRD) patterns. This method presents the potential of and insights into the single-crystalline core-shell synthesis with different kinds of MOFs.

Here, we demonstrate a protocol for the two-step synthesis of single-crystalline core-shells using a non-isostructural metal-organic framework (MOF) pair, HKUST-1 and MOF-5, which have well-matched crystal lattices.

Because of their designability and unprecedented synergistic effects, core-shell metal-organic frameworks (MOFs) have been actively examined recently. ...

Synthesis of Low-Valent MOFs from Multitopic Phosphine Linkers

Experimental Approaches for the Synthesis of Low-Valent Metal-Organic Frameworks from Multitopic Phosphine Linkers

Metal-organic frameworks (MOFs) are the subject of intense research focus due to their potential applications in gas storage and separation, biomedicine, energy, and catalysis. Recently, low-valent MOFs (LVMOFs) have been explored for their potential use as heterogeneous catalysts, and multitopic phosphine linkers have been shown to be a useful building block for the formation of LVMOFs. However, the synthesis of LVMOFs using phosphine linkers requires conditions that are distinct from those in the majority of the MOF synthetic literature, including the exclusion of air and water and the use of unconventional modulators and solvents, making it somewhat more challenging to access these materials. This work serves as a general tutorial for the synthesis of LVMOFs with phosphine linkers, including information on the following: 1) the judicious choice of the metal precursor, modulator, and solvent; 2) the experimental procedures, air-free techniques, and required equipment; 3) the proper storage and handling of the resultant LVMOFs; and 4) useful characterization methods for these materials. The intention of this report is to lower the barrier to this new subfield of MOF research and facilitate advancements toward novel catalytic materials.

Author Spotlight: Experimental Approaches for the Synthesis of Low-Valent Metal-Organic Frameworks from Multitopic Phosphine Linkers  

Here, we describe a protocol for the synthesis of low-valent metal-organic frameworks (LVMOFs) from low-valent metals and multitopic phosphine linkers under air-free conditions. The resulting materials have potential applications as heterogeneous catalyst mimics of low-valent metal-based homogeneous catalysts.

Metal-organic frameworks (MOFs) are the subject of intense research focus due to their potential applications in gas storage and separation, biomedicine, ...

Discovery and Synthesis Optimization of Isoreticular Al(III) Phosphonate-Based Metal-Organic Framework Compounds Using High-Throughput Methods

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