Saowapak Choomwattana

Formaldehyde Encapsulated in Lithium‐Decorated Metal‐Organic Frameworks: A Density Functional Theory Study

The stability of monomeric formaldehyde encapsulated in the lithium-decorated metal-organic framework Li-MOF-5 was investigated by means of density functional calculations with the M06-L functional and the 6-31G(d,p) basis set. To assess the efficiency of Li-MOF-5 for formaldehyde preservation, we consider the reaction kinetics and the thermodynamic equilibrium between formaldehyde and its trimerized product, 1,3,5-trioxane. We propose that trimerization of encapsulated formaldehyde 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 addition, the reaction energy of the system studied herein is endothermic by 6.1 kcal  mol(-1) and the Gibbs free energy (ΔG) of the reaction becomes positive (11.0 kcal  mol(-1)). Consequently, the predicted reverse rate for the trimerization reaction in the Li-MOF-5 is significantly faster than the forward rate. The calculations 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 formaldehyde in its monomeric form.

Comparison of Cu‐ZSM‐5 Zeolites and Cu‐MOF‐505 Metal‐Organic Frameworks as Heterogeneous Catalysts for the Mukaiyama Aldol Reaction: A DFT Mechanistic Study

The density functional theory (DFT) model ONIOM(M06L/6-311++G(2df,2p):UFF was employed to reveal the catalytic activity of Cu(II) in the paddle-wheel unit of the metal-organic framework (MOF)-505 material in the Mukaiyama aldol reaction compared with the activity of Cu-ZSM-5 zeolites. The aldol reaction between a silyl enol ether and formaldehyde catalyzed by the Lewis acidic site of both materials takes place through a concerted pathway, in which the formation of the CC bond and the transfer of the silyl group occurs in a single step. MOF-505 and Cu-ZSM-5 are predicted to be efficient catalysts for this reaction as they strongly activate the formaldehyde carbonyl carbon electrophile, which leads to a considerably lower reaction barrier compared with the gas-phase system. Both MOF-505 and Cu-ZSM-5 catalysts stabilize the reacting species along the reaction coordinate, thereby lowering the activation energy, compared to the gas-phase system. The activation barriers for the MOF-505, Cu-ZSM-5, and gas-phase system are 48, 21, and 61 kJ mol(-1) , respectively. Our results show the importance of the enveloping framework by stabilizing the reacting species and promoting the reaction.

Ethylene Epoxidation with Nitrous Oxide over Fe‐BTC Metal‐Organic Frameworks: A DFT Study

The epoxidation of ethylene with N2 O over the metal-organic framework Fe-BTC (BTC=1,3,5-benzentricarboxylate) is investigated by means of density functional calculations. Two reaction paths for the production of ethylene oxide or acetaldehyde are systematically considered in order to assess the efficiency of Fe-BTC for the selective formation of ethylene oxide. The reaction starts with the decomposition of N2 O to form an active surface oxygen atom on the Fe site of Fe-BTC, which subsequently reacts with an ethylene molecule to form an ethyleneoxy intermediate. This intermediate can then be selectively transformed either by 1,2-hydride shift into the undesired product acetaldehyde or into the desired product ethylene oxide by way of ring closure of the intermediate. The production of ethylene oxide requires an activation energy of 5.1 kcal mol-1 , which is only about one-third of the activation energy of acetaldehyde formation (14.3 kcal mol-1 ). The predicted reaction rate constants for the formation of ethylene oxide in the relevant temperature range are approximately 2-4 orders of magnitude higher than those for acetaldehyde. Altogether, the results suggest that Fe-BTC is a good candidate catalyst for the epoxidation of ethylene by molecular N2 O.

A mechanistic study of ethanol transformation into ethene and acetaldehyde on an oxygenated Au-exchanged ZSM-5 zeolite

Ethanol transformation to ethene and acetaldehyde over low- and high-spin state oxygenated Au-exchanged ZSM-5 zeolite has been investigated using a well-validated density functional method, M06-L. The reaction initiates from the ethanol O–H bond dissociation leading to the formation the ethoxide–hydroxide intermediate with the activation energy of 9.5 kcal mol−1. This intermediate can be then decomposed to either ethene or acetaldehyde products. In the ethene production pathway, the decomposition of the ethoxide–hydroxide intermediate proceeds via the β-H–C scission with the activation energy of 40.5 kcal mol−1. For the acetaldehyde production pathway, the ethoxide–hydroxide intermediate transforms to acetaldehyde via α-H–C scission with the activation barrier of 10.6 kcal mol−1 which is significantly lower than the ethene pathway. The reaction rate for acetaldehyde formation is also found to be higher than the ethene one. The results suggest that the acetaldehyde product is thermodynamically and kinetically favored over ethene for the transformation of the ethanol on oxygenated Au-exchanged ZSM-5 zeolite.