Published: August 23, 2011 r2011 American Chemical Society 8582 dx.doi.org/10.1021/es2019725 | Environ. Sci. Technol. 2011, 45, 8582–8588 ARTICLE pubs.acs.org/est Evaluating the Efficacy of Amino Acids as CO 2 Capturing Agents: A First Principles Investigation M. Althaf Hussain, Yarasi Soujanya,* and G. Narahari Sastry* Molecular Modeling Group, Organic Chemical Sciences, Indian Institute of Chemical Technology, Tarnaka, Hyderabad 500607, India b S Supporting Information ’ INTRODUCTION Carbon dioxide capture and storage from the flue exhaust is one of the major environmental challenges and plays a significant role in the mitigation of atmospheric CO 2 . 1 Although there are number of technologies available for CO 2 capture, an ideal strategy however should be economical and have a minimal environmental impact. In this regard, particular attention is directed toward the design of materials for the capture of CO 2 with minimal impact on environment. Alkanolamines are the most commonly used chemical solvents for the CO 2 capture in the process of chemical absorption. 2À5 Due to problems asso- ciated with alkanolamines, such as their degradation in oxygen- rich atmospheres, considerable energy need for solvent regen- eration and recycling efficiency, the quest to find alternative candidates is of outstanding necessity. Application of other amine-based molecules with lower regeneration temperatures have thus been explored for chemical absorption of CO 2 albeit with limited success. In the process of physisorption of CO 2 ,a variety of solid physical adsorbents have been developed ranging from microporous to mesoporous materials, such as zeolites, zeolitic imidazole frameworks (ZIFs), 6 metal organic frame- works (MOFs), activated carbon, metal oxides to name a few. 1 Amino acids, we consider to be in the domain of both (a) chemical absorption and (b) physical adsorption. The dimensions of pores or channels present in mesoporous materials essentially deter- mine the accommodating ability of CO 2 , which in turn depends on the linker structural features. Thus, the existence of a variety of different modes of interaction for each type of AAs for CO 2 is expected to contribute favorably to adsorption process. While several MOF structures based on amino acid backbone are known for their chiral properties, 7À14 their adsorption data are scarce. 10,11 In a recent report, AAs as building blocks of dipep- tide-functionalized solid materials have shown remarkable and selective adsorption of methane, carbon dioxide and hydrogen. 13,14 Although theoretical calculations have been performed to under- stand the nature of CO 2 interaction 15 with functionalized ben- zenes and N-containing organic heterocycles, 16À19 to our knowl- edge there are no systematic studies available for CO 2 interaction with AAs. The major difficulty for CO 2 removal is the weak acidity of CO 2 , causing it to react incompletely with weak bases. Never- theless, this weak interaction does provide a means for stripping the CO 2 from acidÀbase complex at high temperatures. There- fore one has to resort for a judicious strategy to look for materials with multiple adsorption sites rather than looking for only en- hancement in the interaction. The side chain present in AAs differ in their element composition, structure, size, shape, charge, acidity, functional group present, hydrogen-bonding ability, and chemical reactivity. As a result the presence of diverse functionalities Received: April 1, 2011 Accepted: August 23, 2011 Revised: August 16, 2011 ABSTRACT: Comprehension of the basic concepts for the design of systems for CO 2 adsorption is imperative for increasing interest in technology for CO 2 capture from the effluents. The efficacy of 20 naturally occurring amino acids (AAs) is demonstrated as the most potent CO 2 capturing agents in the process of chemical absorption and physisorption through a systematic computational study using highly parametrized M05À2X/6-311+G(d,p) method. The ability of AAs to bind CO 2 both in the noncovalent and covalent fashion and presence of multiple adsorption sites with varying magnitude of binding strengths in all 20 AAs makes them as most promising materials in the process of physisorption. The binding energies (BEs) estimating the strength of noncovalent interaction of AAs and CO 2 are calculated and results are interpreted in terms of the nature and strength of the various types of cooperative interactions which are present. The study underlines the possi- bility to engineer the porous solid materials with extended networks by judiciously employing AA chains as linkers which can substantially augment their efficacy. Results show that a significant increase in the CO 2 333 AA affinity is achieved in the case of AAs with polar neutral side chains. Furthermore, the study proposes AAs as effective alternatives to alkanolamines in chemical dissolution of CO 2 .