|Research Areas and Focus:
Our research primarily focusses on adsorption of gases on solids. We deal with adsorption at all scales i.e. from the synthesis of nanoporous adsorbents to their use in design and development of industrial scale processes. Interest in liquid phase adsorption using adsorbents as drug delivery vehicles is a more recent addition.
We synthesize high surface area adsorbents such as metal organic frameworks, zeolites, carbons etc. and evaluate their adsorption characteristics to understand structure property relationships. High pressure gravimetric and volumetric type adsorption apparatus are used to measure adsorption characteristics of these materials. Pressure (and vacuum) swing and temperature swing adsorption units are used to evaluate the performance of adsorbents on a larger scale under cyclic steady state conditions. In addition, we use instruments such as BET surface area analyzer, XRD, SEM, TEM, GC/GC-MS, FTIR, TGA etc. for our materials characterization needs.
We also have expertise in analysis of adsorption equilibrium data, design and development of adsorption process and molecular simulations for adsorption equilibrium. Please see the projects section for more details.
A brief account of various research activities currently pursued by our group is given below.
- Synthesis and Characterization of Adsorbent Materials [ 20, 14 , 13, 12 , 11, 4, 3]
While thousands of Metal Organic Frameworks (MOFs) with high porosity and surface area are reported in literature, only a few of them are being currently investigated for their adsorptive properties. One of the goals of our research is to understand the relationship between the structural properties of MOFs and their adsorption characteristics (which is necessary for design better adsorbents). In order to understand this relationship, we perform a systematic investigation of chosen MOFs for their adsorption ability of select gases (with different physical properties) to understand this relationship.
One of the goals of above study would be to develop adsorbents for CO2 separation; a good adsorbent for CO2 separation needs to have high capacity at relatively low partial pressures (and moderate temperatures), good selectivity and low heat of adsorption (for easy regeneration). MOFs are being investigated as potential candidates due to their good CO2 capacities and relatively low heats of adsorption. However, several characteristics of MOFs still need to be understood and fine-tuned before the processes can be commercialized.
Key outcomes of the work in the area so far include understanding the effect of the metal cation, its coordinative saturation [ 20
] and variation of organic linkers in the DABCO and DOBDC metal organic frameworks [14
] and comparison of adsorption characteristics of Cu-BTC and Cr-BDC frameworks [12
At present, we are working on synthesis of MOFs with different organic linkers and surface functionalities to design better adsorbents for natural gas storage and carbon dioxide capture (in collaboration with department of Chemistry).
Figure 1: Variation of CO 2/CO selecitivity with change of constituent metal in DOBDC MOFs. This work has been published in Mishra et al., Journal of Physical Chemistry C , 118 (13), 6847-6855 (2014).
- Flexibility of Porous Solids [21, 16 ]
Some of the porous solids are known to exhibit flexible framework structure and the solid framework is known to change its configuration from one phase to another during adsorption of a "suitable" sorbate or change in other physical conditions (temperature, mechanical pressure etc.). For example, a framework known as MIL-53(Al) is known to change its structure from lp to np phase upon adsorption of CO2 at 1 bar; it transforms back into the lp phase at about 4 bar. Both the phases are known to exhibit different adsorption characteristics for various gases. Several research groups have made attempt to theoretically model and explain this behavior.
In an effort to exploit this behavior for improving adsorption characteristics of the framework, we have demonstrated that the history of the sample significantly affects its adsorption characteristics. We were also able to successfully transform the phase of the sample into a more desirable np phase by changing its history and thereby improving its CO2 adsorption capacity and selectivity in the region of interest (less than 1 bar); all "non-tuned" samples hitherto reported were in the lp phase at ambient conditions.
At present, we have developed a Pressure/Vacuum Swing Adsorption Unit (PVSA) for cyclic adsorption/column dynamics studies. At present we are working on demonstrating the efficiency of the proposed tuning method of flexible MIL-53 (Al) for CO2 separation in a cyclic adsorption unit. In future, we plan to use this PVSA unit for cyclic adsorption studies on other adsorbent materials also.
Figure 2: Tuning of MIL-53 structure based on its history. This work has been published in Mishra etal., Langmuir, 29, 12162-12167 (2013).
- Porous Materials for Hydrogen Storage 
Hydrogen storage is one of the key challenges for migration into hydrogen economy. One of the attractive ways to store hydrogen on-board is by absorption/adsorption on solids. While conventional adsorbents like zeolites and carbons have very low storage capacity for hydrogen, some metal composites have desired storage capacity. However, drawbacks of using metal hydrides for hydrogen storage include slow kinetics and large binding energies (and thereby high regeneration temperatures). MOFs with tunable surface functionalities and large pore volumes may be potential candidates as hydrogen storage materials. However, none of the MOFs reported so far have desirable hydrogen storage capacities to meet the DoE target.
Our attempts have been to enhance the hydrogen storage capacity of MOFs through improved synthesis procedures and doping of metal nano particles into the frameworks. We observed that doping select MOFs with light metals such as Li significantly enhance the hydrogen storage capacity of select MOFs (compared to modest increase in capacities when doped with heavy metals such as Pt and Pd). The results of doping are however very sensitive to handling and experimental procedures; in addition we were also unable to dope a large number of MOFs (with different organic linkers) using the existing protocol.
At present we are working in collaboration with department of Chemistry to understand the doping phenomenon in more detail and extend our ability to dope various families of MOFs with light weight metals such as Li. A theoretical study is also underway to understand the effect of metal doping results we have obtained so far.
Figure 3: Tank volume necessary to store 1 kg of H2 at 100 bar. This work has been published in Sahu et al., Journal of Chemical and Engineering Data, 58(6), 1606-1612 (2013).
- Fundamentals of Adsorption [9, 5, 1]
Figure 4: Schematic illustrating various reference states for definition of adsorption. (a) represents a typical scenario at equilibrium between gas and a porous solid. (b), (c) and (d) are reference states for absolute, excess and net adsorption respectively. This work has been published in Gumma and Talu, Langmuir, 26, 17013-17023 (2010).
- Desulfurization and Drug Delivery Applications:
Our research groups interest and experience in synthesis and characterization of MOFs has lead us to initiate work in areas related to adsorptive desulfurization of liquid fuels, controlled drug delivery studies using MOFs and MOFs coated on magnetic metal nanoparticles such as Fe3O4. We see this as an active direction for our research group in near future.