Porous Coordination Polymers: The Promising Candidates for Molecular Transformation | Adamas University

Adamas University Chemistry

Porous Coordination Polymers: The Promising Candidates for Molecular Transformation

Introduction:

Many important chemical transformations would not seem to be encountered in our world without suitable catalysts. That at least, not in a way which has some practical application. Catalysts are indispensable in the living systems. For obvious reasons it has become essential in a wide range of industrial application too. However, the study of these systems can offer much more: it can change our understanding of fundamental chemical concepts and at times compelled us to rethink the rules of the chemical world. Research in this area moving very fast: many catalytic reactions now appeared to be easier to the chemists that were remained unachievable a decade ago. More over millions of organic molecules now have their real existences. Many new antibiotic and antiviral drugs have been possible to synthesize to save the world from many pandemic diseases. Metals, especially transition metals play important roles in catalysis. Metals can be found in enzyme as well as in the industrial catalysts. Studies on metal incorporated micro/meso-porous solids in order to use them as heterogeneous catalysts, have received immense attention in last few decades. Porous coordination polymers, i.e., metal organic/inorganic/hybrid framework solids (MOFs or MIFs or MOIFs) being porous have potential to be used in the same manner as in the case of zeolites or aluminosilicate-based catalysts. Heterogeneous catalysis is a major field of interest in material science [1].

Homogeneous to heterogeneous:

Traditionally there are several approaches to heterogenise a homogeneous system supporting over a solid support. In the literature numerous number of homogeneous complexes have been found that are efficiently bound with the support matrix through several pathway like, (i) covalent binding, (ii) electrostatic interaction, (iii) adsorption (iv) encapsulation, and (v) hydrogen bonding, based on the interaction between the catalyst and the solid support matrix.  There may be several advantages and disadvantages regarding the stapling of the homogeneous complexes on any support. But the important question is still asked, is it true heterogeneous? That’s why the modern chemistry tries to construct such a system that can be directly used and the question of true heterogeneity will be solved. Using porous coordination polymer as the catalyst is a good option. Although the field is not so seasoned but several approaches have been established fruitfully and the outstanding properties of porous frameworks (PCPs) have studied recently [1]. The structural porosity of PCP materials places them at the frontier between zeolites and surface metal–organic/inorganic catalysts. PCPs, therefore, appear to be excellent entities for catalysis, with the understanding that their potential still largely in its infancy. Crystalline Porous Polymers (PCPs), especially those exhibiting zeolite-like properties such as high internal surface area and microporosity, comprise a promising emerging class of functional materials. The notion is that MOF-based catalysts may be able to replicate some of the key features of zeolitic catalysts (e.g. single-site reactivity, pore-defined substrate size and shape selectivity, easy catalyst separation and recovery, and catalyst recyclability) while incorporating reactivity and properties unique to molecular catalysts. One important property of many molecular catalysts that has yet to be demonstrated with purely zeolitic catalysts is enantioselectivity.

Synthetic Method of PCPs:

Diverse procedures are available for synthesis of PCPs which are extremely convenient to prepare. Mostly it is prepared through sol-gel process, where the metal salt and ligand solutions are prepared separately and mixed together. Solvent that is used depends on several factors like solubility, redox potential etc., but other methods like solid phase, CVDs, Electrochemical or diffusion methods are also well known. The simple way to design can be depicted as given in the figure 1 and 2.

Figure 1. Preparation of different kinds of PCPs.

Figure 2. Simple overview for the development of porous coordination polymers.

Transition metals are chosen to design redox catalyst. Mainly metals from first transition series, e.g. manganese, copper, iron, vanadium etc. are selected for preparing redox catalysts. Metal having Lewis acidic property are for preparing catalysts that are intended to be used in acid catalysis reactions. Linkers act as bridge between metal centers in PCPs. Heavy metals are introduced within the PCPs for different types of coupling and other tandem catalysis.

Importance:

Open-structure PCPs bear prospects for designing of new heterogeneous catalysts that are spatially accessible for directing the reactions to the site-isolated and structurally well-defined active centers like enzymatic catalysts. Heterogeneous catalysts have potential in commercial uses due to easy recovery and recycling of catalysts as well as minimizing the undesired and/or toxic side products. Catalysts can be designed through various approaches. One of the new approaches is to incorporate redox metal centers into the frameworks porous solid. The active center can be metal ions or metal containing entity such as complex with designed ligands. Site-isolation is of paramount importance in achieving maximum catalytic efficiency. Often such types of molecular assemblies with porous structure have potential in the application of various heterogeneous catalysis through the small molecule activation and conversion. A rapid growth in the study of these materials has arisen from the realization that these frameworks synthesis offers considerable flexibility and control over structure and properties, thereby offering wide pathways to rational materials design. By designing the MOFs or porous coordination polymer it has been possible to separate and isolate the closely similar small organic molecules and to convert them to the desired products [2]. It is now well established that large cavities are catalytically important for several reasons. Large cavities allowing for more complex functional groups are to be added to the pores, similar to catalytically active hybrid materials consisting of functional complexes tethered to metal oxide surfaces [3]. It is well reported that bifunctional dicarboxylates served to bridge various metal ions producing a lot of highly porous and catalytically active frameworks [4], Yaghi group has developed a family of compounds based on one structural arrangement in which the pore size may be tuned over a wide range. By using longer or shorter organic molecules of similar geometry, Yaghi et al. have now prepared 16 variations on the MOF-5 structure, with pores varying from 3.8 to 28.8 Å in diameter [5]. So, by tuning the porous channel one can rationalize the activation of organic molecules within the pore for catalysis. In this case the effective heterogeneous conversion of small molecules is the most extensively employed processes for prevailing desired organic materials in industry and the laboratory. PCPs which include metallosilicates or phosphates (ALPO, SAPO, VAPO etc.) are rather more well studied as regards the catalytic study [6], however, reports on heterogeneous catalytic reactions over PCPs are scanty in the literature. Fujita reported the shape selective cyanosilylation reaction over Cd-based MOF [7]. Horcajada et. al. employed hydrothermally synthesized iron(III) carboxylate zeotype metal-organic framework in Friedel-Crafts benzylation catalysis [8]. Metalloporphyrin encapsulated into zeolite-like three-dimensional metal-organic frameworks efficiently catalyze oxidation of cyclohexane with tert-BuOOH [9]. Cd containing MOF catalyzes the Knoevengel condensation in heterogeneous medium [10]. Notably this catalytic system shows size selectivity of the reactants. So, the flexibility of the frameworks originates the enormous structural and chemical diversities afforded by molecular systems, features that are less prevalent in many other branches of materials chemistry.  The innovation of a new porous compound with desired properties based on pore and surface functionalities is extremely important in this advanced scientific era. Hence, in the present project, thrust will be given to study more on catalytic reactions besides the preparation and characterization of the catalysts. As environmental protection laws in all over the world are becoming more stringent the chemical companies are searching for processes that will be less hazardous than the processes being followed in the industry at present.

Future Scope:

These types of catalytic systems are promising in this count. These materials show promise for applications in heterogeneous catalysis. The area of advanced materials research has very broad scope and potential applications. Advanced materials outperform conventional materials with superior properties showing particular predetermined features. In this regard, the capability of incorporating a variety of organic single molecules into the nano cavity makes PCPs superior candidates as nanosized reactors, which strongly compete the traditional zeolites or other porous metal-oxide materials. And remarkably the simple modification over the interior cavities of the PCPs allows us the selective reactions in the channels. So, by systematic functionalization of the pore surface we can easily control the desired reactions this methodology can afford new vistas for the designing of new functional PCPs by which not only they can maintain the catalytic reactions but also, they can meet the preinstalled specific information. This field has a quite good future for industrial applications. 

Reference:

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