# Advances and Challenges in the Creation of Porous Metal Phosphonates
## 1 Introduction  
There has been a surge in the pursuit to create inorganic-organic hybrid materials in the past two decades, with the success of metal-organic frameworks (MOFs) and periodic mesoporous organosilacs (PMOs). Hybrid materials are interesting for material chemists and in application as the integration of both organic and inorganic functionalities allows to modify the matrix and interface properties of materials. The unique combination of inorganic and organic features creates particular properties and control with respect to the structure, physical properties and/or specific chemical reactivity and/or interactions. This has indeed resulted in the extensive and intensive creation of hybrid porous materials such as MOFs [1; 2; 3; 4; 5] and PMOs [6; 7; 8]. An interesting, more specific and less studied class of materials are porous metal phosphonates, built up from the interaction between metals and organophosphonic units. These hybrid porous metal phosphonate materials are particularly interesting where stable performance under sometimes harsh conditions and specific surface interactions are an important requirement for application, like the sorption of metal ions, heterogeneous catalysis, thin-film-based devices, chromatography or membranes technology. We describe in this review the evolution of metal phosphonates over the past couple of decades from layered metal phosphonates to phosphonate-based MOFs and finally to templated (supramolecular) mesoporous metal phosphonates. All of these materials fall under the category of coordination polymeric networks and can be subdivided further based on their long-range order and/or pore morphology into: phosphonate MOFs, layered metal phosphonates and templated (supramolecular) metal phosphonates, as illustrated by Figure 1. This classification is thus only based on the finalstructure/texture of the porous coordination networks, which is primarily determined by the interaction of the metal precursors and phosphonic moieties in solution during synthesis, resulting in structures featuring a variety of coordination modes. We would like to point out that this is not a detailed review concerning each of these classes of materials, but rather an attempt to put together some of the similarities and differences when it comes to the choice of metal, metal valency and phosphonic linkers with respect to specific interesting morphologies of porous coordination networks that can be obtained. The critical aspects required to obtain particular metal phosphonate architectures are recognized by analyzing the reaction parameters and their consequences on the porous hybrid frameworks obtained. For a more detailed reading on the synthesis and/or applications for each class of metal phosphonates, we refer the readers to some of the interesting reviews [9; 10; 11; 12; 13; 14; 15; 16; 17]. Many of the interesting advances and developments in the field of porous metal phosphonates hybrids have taken place in the last couple of decades. A substantial part of the progress in porous metal phosphonates has been built on the knowledge stemming from the pioneering works from [PERSON] et al. and [PERSON] et al. on layered metal phosphates/phosphonates [9; 10; 18; 19; 20]. We describe an overview of the different synthesis methods undertaken for various transition metals and then funnel down towards the metal (IV) phosphonate system. The focus of this review is addressing the challenges posed for creating porous metal (IV) phosphonate networks. We restrict ourselves to materials synthesized purely from transition metal precursors and organic precursors containing only phosphonate moieties. Therefore, mixed ligand systems such as the metal carboxphosphonates are not a part of this review [21]. This review consists of two parts, where we first summarize on the synthesis methods employed for obtaining porous metal phosphonates and subsequently provide an overview of the potential benefit of porous metal phosphonates in some key areas of application.