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

Summary

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.

Abstract

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 [Al₂H_12-x(PMP)₃]Cl_x∙6H₂O (H₄PMP = N,N '-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 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∙xHCl (x = 4 or 6).

Introduction

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 application^1,2,3. Therefore, MOFs contai....

Protocol

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 .......

Discussion

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.......

Materials

Name	Company	Catalog Number	Comments
AlCl3·6H2O	Grüssing	N/A	99%
Filter block for filtration of max. 48 reaction mixtures	In-house made	N/A	Technical drawings in the supplementary files
Hydrochloric acid	Honeywell	258148	Conc. 37 %, p.a.
Multiclaves with 24 individual Teflon inserts	In-house made	N/A	Technical drawings in the supplementary files
N,N ‘-piperazine bis(methylenephosphonic acid	Prepared by coworkers	N/A	H4PMP,  Prepared by coworkers with the method reported by Villemin et al.: D. Villemin, B. Moreau, A. Elbilali, M.-A. Didi, M.’h. Kaid, P.-A. Jaffrès, Phosphorus Sulfur Silicon Relat. Elem. 2010, 185, 2511.
Sample Plate for PXRD	In-house made	N/A	Technical drawings in the supplementary files
Sodium hydroxide	Grüssing	N/A	99%
Stoe Stadi P Combi	STOE	Stadi P Combi	Cu-Kα1 radiation (λ = 1.5406 Å); transmission geometry; MYTHEN2 1K detector; opening angle 18°; curved  monochromator; xy-table
Forced convection oven	Memmert	UFP400