|In the Yaghi group we are building chemical structures by stitching molecules together into large and extended frameworks within which we can store hydrogen, methane, and separate carbon dioxide. The interior of the crystals is capable of compacting gases under ambient conditions thus foregoing the use of high pressures and low temperatures. In this movie, crystals of metal-organic framework-5 (MOF-5, numbered in roughly chronological order of discovery) are made and their structure is precisely designed to have zinc oxide units linked by organic struts (terephthalate) to make a porous network into which voluminous gases can be compacted and transported. This large open space is currently being used for positioning of organic and organometallic catalysts, charge storage for supercapacitors and binding of biological molecules such as proteins and metabolites. The movie was made by our industry partner BASF (Germany).|
|The six structures placed in the center are materials invented by the Yaghi group. From left to right in the top row are metal-organic frameworks, covalent organic frameworks, and zeolitic imidazolate frameworks, and in the bottom row metal triazolates, metal-organic polyhedra, and metal catecholates. The panel on the left is a figure of a multivariable metal-organic framework found to have sequences of functionalities capable of highly selective separation of carbon dioxide, while the panel on the right is a covalent organic framework composed entirely of light elements (B, O, C) and represents a new class of porous crystals. In the Yaghi group students are free to explore new directions in reticular materials research and experience the thrill of discovery by making new forms of matters and studying their benefit to society.|
The Yaghi group is developing the science and applications of precise assembly of materials by the molecular building block approach. To do this, we use reticular chemistry where molecular building blocks (organic molecules, inorganic clusters and complexes, proteins, peptides, and dendrimers) are linked into extended frameworks using strong bonds. This chemistry allows us to translate the high functionality of molecules into solids without losing the robustness needed for making useful materials, or the dynamics and molecular flexibility required for highly functional materials. Thus in our group new materials are created by 'stitching' metal-ions and metal complexes with organic linkers to make extended porous frameworks called metal-organic frameworks (MOFs), and zeolitic imidazolate frameworks (ZIFs). We also link organic units together to make covalent organic frameworks (COFs). This approach is elaborated to make metal-catecholates (CATs) and metal triazolates (METs). These are all new classes of porous crystals made by our group, and studied for their applications to clean energy storage and generation, clean water generation and delivery, supercapacitors, thermal batteries, ion-conductivity, electronic conductivity, and drug delivery.
|From left to right are illustrations of gases/molecules such as hydrogen, methane, carbon dioxide, and water which the Yaghi group has been studying and has uncovered their selective uptake into metal-organic frameworks (MOFs) and other reticular materials. Highly crystalline and well-shaped crystals can be made in our laboratory. Through our long-standing collaboration with BASF, it has become possible to make these materials in ton quantities. We have shown that fuel tanks filled with our materials can store double, and in many cases, triple the amount of methane and such automobiles were driven 28,000 km in four continents by German explorers without loss of the efficiency of MOF fuel tanks thus demonstrating that MOF fuel tanks are competitive with CNG. On the right are shown crystals of a MOF where our group has been able to expose metal sites and for the first time pin-point their location by single crystal x-ray diffraction techniques and demonstrate their efficiency in capturing carbon dioxide. In the Yaghi group students are working on materials beyond MOFs (e.g. COFs, ZIFs and METs) for clean energy applications.|
The Yaghi group was the first to uncover the porosity of MOFs. In 1998 and 1999, we reported their synthesis and architectural stability. Using gas adsorption isotherms we showed that zinc terephthalate MOFs (MOF-2 and MOF-5) retain their open structure in the absence of guests and that gases can pass in and out of the pores with full retention of the MOF structure. Since then we showed that MOFs can be functionalized and their components (metal containing units and organic links) can be widely varied. This made available a large class of MOF materials whose gas adsorption properties continue to be studied by our group. Specifically, we have several ongoing programs focused on hydrogen storage and methane storage and transport for automobile fueling. We also work extensively on carbon capture from the atmosphere, automobiles and power plants. More recently, we have initiated research programs where it is possible to use amyloids (long known to be important in the onslaught of diseases such as Alzheimer's) for demonstrating that lysine functionalities can trap carbon dioxide in the presence of water. MOFs are also being investigated in our group for their applications to clean water using artificial leaf constructs, and as thermal batteries for heating and cooling automobiles.
|The Yaghi group is working extensively on developing materials capable of counting and sorting molecules as well as coding for properties. The figure shows how our research group can achieve the assembly of multiple building units having multiple functionalities into a structure to make multivariate metal-organic frameworks (MTV-MOFs). These materials have pores decorated with sequences of functionalities, which we believe, code for specific properties. The bottom two structures show how such functionalities can be arranged in small intermingled clusters while other functionalities can be thoroughly mixed to give highly specific carbon capture properties, and by-product free catalysis.|
Beyond MOFs, COFs and ZIFs, and related porous crystals mentioned above we study ways of creating new materials based on the flexibility with which the building units can be varied. Among these are multivariate metal-organic frameworks (MTV-MOFs): 3D structures in which we introduce a large number of functionalities such that a MOF incorporates a multivariable arrangement of these functional groups and yields heterogeneity only found in biological systems. Since we can vary the type and ratio of functionalities, MTV-MOFs can be designed to have regions of a targeted functionality intermingled with regions of another and therefore have polar and non-polar regions juxtaposed as found in proteins. These systems raise interesting questions dealing with controlled complexity: what is the sequence of functionality within the MTV-MOFs and could these sequences be identified and designed to code for specific properties?
|The Yaghi group has taken metal-oxide clusters from the almost forgotten inorganic literature and succeeded in linking them with organic molecules to make porous metal-organic frameworks. Students in our group experience inorganic synthesis of clusters using elements from almost every corner of the periodic table, and their assembly using solution and solid-state techniques. We also functionalize the MOF structure with coordination and organometallic complexes for metal-based catalysis (e.g. activation of methane and water splitting for hydrogen generation).|
Our research is focused on using metal complexes and metal-oxide clusters as building units in the synthesis of MOFs. These inorganic units are copolymerized with organic linkers to make MOFs and related porous crystals. An important direction pertains to exposing metal coordination sites within the pores. Here, after the MOF structure formation, terminal ligands are found on the metal connectors. These ligands can be removed with full preservation of the structure, thereby leading to open metal sites in low coordinated metals. The electronic and steric nature of these open metal sites makes them ideal for enhanced gas storage and Lewis acid-based catalysis.
|Organic chemistry is central to the work being done in the Yaghi group. Typically, projects involve developing synthetic strategies for making organic links and functionalities capable of gas separation, carbon dioxide capture, methane storage, and catalysis. We also carry out extensive organic reaction chemistry in our efforts to post-synthetically modify the interior of the pores to produce precisely shaped architectures for highly specific binding of substrates. The example shown above is representative: progressively longer linkers were synthesized and made into MOFs having the largest pores ever made, enabling exceptional hydrogen storage, capture of carbon dioxide from flue gas, and inclusion of large molecules such as green fluorescent proteins, Vitamin B12, and hemoglobin. We are also pursuing the development of new forms of carbon, and new reactions for linking organic building units to make covalent organic frameworks.|
We work extensively on the synthesis of organic linkers for their use in metal-organic frameworks. These organic units are made from simple starting materials using efficient synthetic protocols. Their linking functionality is typical carboxyl groups but we are also interested in catecholes, beta-diketonates, imidazolates, triazolates and the like. Since these functionalities are charged, they form strong bonds to metal ions in the formation of extended porous networks. Another aspect of this work deals with the covalent organic chemistry we carry out on the frameworks after they form in order to functionalize the interior of the frameworks. Of course, all the reaction chemistry and the assembly of the covalent organic frameworks (COFs) involve intimate knowledge of organic reactions leading to C-B, C-N, C-O, and C-C bond formation.
|The control in composition, structure and metrics of frameworks is being translated into the nano-regime. Our approach is to mix variously shaped nanocrystals into superlattices for light conversion to fuels, supercapacitors and biomedical applications. Students in this program learn how to navigate between diverse fields such as colloids, organic, inorganic and materials, and become well-versed in many related physical techniques such as scanning electron microscopy, high resolution tunneling electron microscopy, small angle x-ray scattering, and powder x-ray diffraction.|
We have recently succeeded in making nanocrystals of a large number of MOFs. The research in the group encompasses the synthesis of nanocrystals of various porous multivariate (MTV) MOFs in which the pores are decorated with a large number of organic functional groups. This research is expanding in several directions including: (a) control of composition, atomic structure, and metrics of the nanocrystals to design devices capable of hybrid properties such as electronic conductivity and solar energy conversion. (b) The fact that the pores of these MTV-MOF nanocrystals have a large variation of functionalities leads us to investigate whether the sequence of the functionalities are unique and thus could be used to code for specific properties. (c) Many of these nanocrystals are of the same size regime as proteins and other biological molecules, an aspect that is helping us in designing MTV-MOF nanocrystals for biomedical imaging and drug transport.
|A transparent film of a MOF made into a supercapacitor device exhibits exceptional capacity and lifetime. The facility with which MOF materials can be designed allows the Yaghi group to survey the landscape of structure space in its effort to identify the best performing MOF. This also leads to identification of specific structure attributes, metrics and functionalities most favored for this application.|
Electrochemical capacitors or supercapacitors are an important class of energy storage devices because of their high power density. Porous carbon materials are commercial supercapacitors that operate by storing charge on electrochemical double layers. In contrast pseudocapacitors, typically made from metal oxides, store charge by redox reactions. Each of these classes of supercapacitor has advantages and disadvantages: carbon-based materials operate at very high charge/discharge rate with long lifecycle but have low capacitance, while metal oxide materials have high capacitance but their redox reactions lead to low lifecycle. Our group is working on ways of bridging the gap separating these two classes of supercapacitors using nano-MOFs. We examine different MOF compounds made in their nanocrystalline form. The measurements are done on thin film devices prepared from these nanocrystalline MOFs.
|The Yaghi group is working on several areas where concepts from biology are transferred into synthetic materials. On the left, MOFs are designed to be capable of trapping large proteins and ordering them in the pores to identify their structure and function. This applies also to crystallization of metabolites and peptides. On the right are functional groups (histidine) covalently arranged in the pore of a MOF to yield an intricate pocket for fixing carbon dioxide.|
MOF chemistry has matured to the point where the composition, structure, functionality, porosity, and metrics of a metal-organic structure can be designed for a specific application. This precise control over the assembly of materials is expected to propel this field further into new realms of synthetic chemistry in which far more sophisticated materials may be accessed. In our group we have begun to use concepts known in biology to make highly sophisticated materials. Research is focused on making and studying having (a) compartments linked together to operate separately, yet function synergistically; (b) dexterity to carry out parallel operations; (c) ability to count, sort, and code information; and (d) capability of dynamics with high fidelity.
|A gas adsorption isotherm for some of the most reticular materials that indicate permanent porosity and ability to use the interior of MOFs for organic catalysis and positioning organometallic catalysts strategically such that reactions can be carried out with higher selectivity and efficiency.|
Our research group works closely with BASF-The Chemical Company on large-scale production of MOFs, COFs and ZIFs, and related materials. At present, MOFs have been commercialized by BASF under the label Basocubes and Basolites. Our studies in this direction take advantage of a long history of work on gas adsorption in MOFs, which originally was developed in our lab in 1998. We continue to develop MOFs and study their properties and applications in a wide range of fields as mentioned above but more recently we found they are ideally suited as catalysts for organic transformations..
|The Yaghi lab is equipped with high-throughput equipment, which is used to discover new materials and screen for various properties as mentioned above. In the example shown here, over 30 new zeolitic imidazolate frameworks were discovered using this technique from only 9,600 reactions.|
During the past decade, interest has grown tremendously in the design and synthesis of crystalline materials constructed from molecular clusters linked by extended groups of atoms. Most notable are MOFs, in which polytopic organic linkers join polyatomic inorganic metal-containing clusters. The realization that MOFs could be designed and synthesized in a rational way from molecular building blocks led to the emergence of a discipline that we call reticular chemistry. Because of the explosive growth in this area, a need has arisen for a universal system of nomenclature, classification, identification, and retrieval of these topological structures. We are developing a system of symbols for the identification of three periodic nets of interest, and this system is now in wide use. In reticular chemistry, we now can, for a given set of building units, predict with some certainty, the structures which might result from the assembly of those building blocks. We then use high-throughput synthesis to survey the possible structure space and 'zoom-in' on the desirable phase and use for a particular application as explained above.