Name: Solvothermal Synthesis and Isolation of the Crystalline Material Al-CAU-60
Uploaded: 2023-10-06T14:20:00
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solvothermal synthesis and isolation of the crystalline material al-cau-60

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

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.

Author Spotlight: Accelerating Discovery in Microporous Material Chemistry 

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

I see the periodic table as a huge playground, and in our research, we focus on the synthesis of new materials, porous materials, such as this here. That's a structural model, and we see holes in the framework. and that can be used to store molecules or also to separate molecules based on the size of these.And in addition to that kind of studies, we also develop new reactor systems in order to accelerate the discovery process. The current experimental challenges are to further accelerate the synthesis and characterization process with additional high throughput instrumentation, such as high throughput absorption characterization device. The lack of detail in publications regarding the synthesis and characterization procedure affecting reproducibility is a well-known problem in chemistry.By providing a thorough description of our method and setup, we aim to encourage other groups to adopt the described methodology, thereby increasing the reproducibility of experimental studies. We do not know what the future will bring since we work on the discovery of new materials, and we never know what kind of properties these materials will have. Nevertheless, I think that the use of linker molecules from renewable feedstocks will play a major role.And regarding the application, the use in sensing and the separation of gas molecules will be very important.

Watch this Scientific Journal Video about Solvothermal Synthesis and Isolation of the Crystalline Material Al-CAU-60 at JoVE.com

Solvothermal Synthesis and Isolation of the Crystalline Material Al-CAU-60  

To begin, add the linker H4PMP as a solid to the polytetrafluoroethylene, or PTFE, inserts for Al-CAU-60. Then use a pipette to add the aluminum chloride solution, demineralized water, and the solution of additives to the inserts. Insert the discs into the sample plate and place the filled PTFE inserts into the sample plate.Mark the ground plate of the reactor in a way that allows the identification of the PTFE inserts and insert the sample plate with the filled PTFE inserts into the ground plate. Prepare two PTFE sheets and place them on the sample plates ensuring full coverage. Ensure that the PTFE sheet is correctly positioned and fits the head plate using the guide pins.Add the screws and tighten them by hand. Then seal the initially closed reactor with the help of a mechanical or hydraulic press and tighten the screws by hand again. Place the multiclave in a programmable forced convection oven.For Al-CAU-60, set the temperature time program shown on the screen. Then start the temperature time program. Remove the multiclave from the oven.Once it cools down to room temperature, place it on a mechanical or hydraulic press and gently compress it until the screws can be loosened by hand. Place the multiclave inside a fume hood and remove the head plate of the reactor. Then remove the PTFE sheets and the sample plate with the PTFE inserts from the ground plate of the reactor.Now assemble the high-throughput filtration block by connecting the filter block to a vacuum pump via two wash bottles. Place two filter papers between two silicone sealing mats with the corresponding recesses in the filter block. Then place the PTFE filling block on top, ensuring that the appropriate recesses match the ceiling mats and the filter block.Tighten the layers using the clamping frame, which is held in place by six stud bolts. To properly seal the unit, use wing nuts on the stud bolts and tighten them by hand. Close the recesses of the filling block that are not to be filled with plugs.Turn on the membrane vacuum pump and set it to a mode in which it will pump down to the best possible vacuum. Transfer the contents of the PTFE inserts into the designated wells of the filling block with disposable pipettes. Once all the inserts are empty, take a second look for crystals and isolate them if present.Carefully disassemble the filtration block once all the wells are drained. A product library is now available on filter paper. Allow it to air dry inside the fume hood.Place the product library between the base plate and the cover plate to characterize the obtained product by powder x-ray diffraction later. Ensure that the recesses and the plates match the product locations. After aligning the plates, secure them with two screws.Insert the product library into the sample holder of the diffractometer. Carefully place the loaded sample holder into the XY stage of the diffractometer. Close the instrument before starting the measurements.The parameter space is shown in this figure. The synthesis with sodium hydroxide as an additive are highlighted in blue, the synthesis with no additive are highlighted in green, and the synthesis with the additive hydrochloric acid are highlighted in orange and red. The molar ratios of the reagents are shown on the left side and on the top.The stacked plot of all the measured powder X-ray diffraction patterns is shown in this figure. Here, the additive sodium hydroxide is highlighted in blue, no additive is highlighted in green, and the additive hydrochloric acid is highlighted in red. The sample names are shown here which correlate to the molar ratios of the starting materials used in the synthesis mixture.These results suggest that the products with low crystallinity were obtained from the synthesis where the molar ratio of sodium hydroxide to aluminum was one-to-one, or no sodium hydroxide or hydrochloric acid was present. Products of higher crystallinity were obtained from synthesis where hydrochloric acid was used as an additive.

solvothermal-synthesis-isolation-crystalline-material-al-cau

Research

JoVE Journal

Chemistry

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 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.

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