DOI: 10.1002/cphc.201100642 Formaldehyde Encapsulated in Lithium-Decorated Metal- Organic Frameworks: A Density Functional Theory Study Thana Maihom, [a, b] Saowapak Choomwattana, [a, b] Pipat Khongpracha, [a, b, c] Michael Probst, [d] and Jumras Limtrakul* [a, b, c] 1. Introduction Formaldehyde, a well-known volatile organic compound (VOC), is widely used as an industrial feedstock for the production of fine chemicals, resins, and several domestic products such as paints. Nevertheless, its applicability is limited by its low boil- ing point of 19.5 8C. Moreover, formaldehyde rapidly self- polymerizes into chain polymers such as paraformaldehyde or into oligomers such as the cyclic trioxane. Its preservation in the monomeric form is difficult. Na X and Na Y zeolites have been reported to be capable of storing formaldehyde. [1] These materials stabilize and also activate formaldehyde to undergo carbonyl–ene reactions with a variety of olefins as has also been clarified by theoretical studies. [2] In recent years, metal-organic frameworks (MOFs) were investigated as promising candidates for gas storage (e.g. CO 2 and H 2 ) [3] because of their flexibility and the possibility to tune their surface composition and pore structures by chang- ing the metal center or the organic linker. Computational and experimental investigations [4–5] have shown that metal atoms or cations that either decorate the linkers or are incorporated into them can enhance the capacity of MOFs for gas storage. The most frequently used metal is Lithium (Li) because deco- rating MOF structures with it is especially simple. Li on MOF linkers forms an accessible open metal site that can interact well with incoming molecules. It can therefore be expected to interact with formaldehyde in a way similar to the Na cation in zeolites, which is known to prevent formaldehyde from self- polymerizing. To the best of our knowledge, such a system has not yet been studied in detail. Therefore, we investigated it herein by means of density functional theory (DFT) calculations with the M06-L functional. We calculated the reaction mechanism and the corresponding energy profile defined by the structures of the adsorbed starting complex (three formaldehydes coordi- nated to Li), the transition state and the product trioxane. We also compared our findings to those obtained for the bare system. 2. Results and Discussion 2.1. Decoration of MOF-5 with Li and its Subsequent Appli- cation as Formaldehyde Adsorption Complex Herein, MOF-5 [6] (also called IRMOF-1) was chosen as host ma- terial. It consists of Zn 4 O clusters connected to 1,4-benzenedi- The stability of monomeric formaldehyde encapsulated in the lithium-decorated metal-organic framework Li-MOF-5 was in- vestigated by means of density functional calculations with the M06-L functional and the 6-31G(d,p) basis set. To assess the ef- ficiency of Li-MOF-5 for formaldehyde preservation, we consid- er the reaction kinetics and the thermodynamic equilibrium between formaldehyde and its trimerized product, 1,3,5-triox- ane. We propose that trimerization of encapsulated formalde- hyde takes place in a single reaction step with an activation energy of 34.5 kcal mol 1 . This is 17.2 kcal mol 1 higher than the corresponding activation energy in the bare system. In ad- dition, the reaction energy of the system studied herein is en- dothermic by 6.1 kcal mol 1 and the Gibbs free energy (DG) of the reaction becomes positive (11.0 kcalmol 1 ). Consequently, the predicted reverse rate for the trimerization reaction in the Li-MOF-5 is significantly faster than the forward rate. The calcu- lations show that the oligomerization of formaldehyde in Li- MOF-5 is a reversible reaction, suggesting that such a zeolite might be a good candidate material for preserving formalde- hyde in its monomeric form. [a] T. Maihom, S. Choomwattana, Dr. P. Khongpracha, Prof.Dr. J. Limtrakul Laboratory for Computational and Applied Chemistry Department of Chemistry Faculty of Science and Center of Nanotechnology Research and Development Institute Kasetsart University Bangkok 10900 (Thailand) Fax: (+ 66) 2-562-5555 E-mail : Jumras.l@ku.ac.th [b] T. Maihom, S. Choomwattana, Dr. P. Khongpracha, Prof.Dr. J. Limtrakul NANOTEC Center of Excellence National Nanotechnology Center Kasetsart University Bangkok 10900 (Thailand) [c] Dr. P. Khongpracha, Prof. Dr. J. Limtrakul Center for Advanced Studies in Nanotechnology and its Applications in Chemical, Food, and Agricultural Industries Kasetsart University Bangkok 10900 (Thailand) [d] Prof. Dr. M. Probst Institute of Ion Physics and Applied Physics University of Innsbruck 6020 Innsbruck (Austria) ChemPhysChem 2012, 13, 245 – 249  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 245