Thana Maihom

Mechanistic studies on the transformation of ethanol into ethene over Fe-ZSM-5 zeolite.

Ethanol, through the utilization of bioethanol as a chemical resource, has received considerable industrial attention as it provides an alternative route to produce more valuable hydrocarbons. Using a density functional theory approach incorporating the M06-L functional, which includes dispersion interactions, a large 34T nanocluster model of Fe-ZSM-5 zeolite in which T is a Si or Al atom is employed to examine both the stepwise and concerted mechanisms of the transformation of ethanol into ethene. For the stepwise mechanism, ethanol dehydration commences from the first hydrogen abstraction of the ethanol OH group to form the ethoxide-hydroxide intermediate with a low activation energy of 17.7 kcal mol(-1). Consequently, the ethoxide-hydroxide intermediate is decomposed into ethene through hydrogen abstraction from the ethoxide methyl carbon to either the OH group of hydroxide or the oxygen of the ethoxide group with high activation energies of 64.8 and 63.5 kcal mol(-1), respectively. For the concerted mechanism, ethanol transformation into the ethene product occurs in a single step without intermediate formation, with an activation energy of 32.9 kcal mol(-1).

A DFT Study of Tungsten–Methylidene Formation on a W/ZSM‐5 Zeolite: The Metathesis Active Site

Tungsten-methylidene formation from ethene on either the W(IV) , W(V) , or W(VI) active sites of a W/ZSM-5 zeolite is investigated by using the M06-L functional. The reaction is assumed to proceed in two steps; the first step is the [2+2] cycloaddition between ethene and the W-O active site to form an oxametallacycle intermediate. The intermediate is then decomposed to produce the W-methylidene active site from the metathesis reaction. The overall activation barrier of the reaction on W(VI) (27.3 kcal mol(-1) ) is considerably lower than the ones for W(IV) and W(V) (69.4 and 37.1 kcal mol(-1) , respectively). Moreover, the reaction involving the W(VI) site also stabilizes intermediates and products to a larger extent than the ones on the W(IV) and W(V) sites. As a result, we have demonstrated that the reaction of the W-methylidene metathesis active site is both kinetically and thermodynamically favored to occur on the W(VI) active site of the zeolite.

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.

Adsorption and decarbonylation of furfural over H-ZSM-5 zeolite: a DFT study

The conversion of low cost biomass and its derivatives is receiving considerable and growing attention as an alternative feedstock for fuel and chemicals production. Herein, the furfural adsorption and decarbonylation to furan on H-ZSM-5 zeolite are studied by density functional theory calculations. The furfural interacts with the zeolite active site through its three sites of methine carbon atoms, rings oxygen and carbonyl group with the adsorption Gibbs free energies of −5.5, −8.3 and −15.7 kcal mol−1, respectively. Three pathways are considered for the decarbonylation of furfural. In the first pathway, the reaction starts with the protonation of a alpha-carbon leading to the arenium-ion like intermediate. This intermediate then eliminated the CO group to form the furan product. The earlier step is considered to be the rate-determining step of this pathway with an activation energy of 22.1 kcal mol−1. In the second pathway, it commences by protonation of furfural at the beta-carbon to produce the arenium-ion like intermediate. This intermediate then undergoes a 1,2-hydride shift and is subsequently decarbonated to eliminate the CO molecule and form the furan product. The first step of the furfural protonation has the highest activation energy, 27.7 kcal mol−1, and is therefore rate-determining. For the last path, the protonation at the furfural carbonyl group to the surface hydroxyalkyl species formation is the first step of this pathway. The reaction then proceeds through the migration of H to form the tertiary carbocation intermediates followed by the 1,2-hydride shift to form the secondary carbocation species. Finally the CO molecule is eliminated to generate the furan product. The rate-determining step of this pathway is the second step of the H migration with the activation energy of 45.8 kcal mol−1. Since the first pathway requires a lower rate-determining step activation barrier compared with the second and third pathways, the first pathway is therefore preferred for the furfural decarbonylation on H-ZSM-5 zeolite. The effect of the zeolite framework is also highlighted to greater stability of the intermediates and also transition state complexes.

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.

Chemical Adsorption and Physical Confinement of Polysulfides with the Janus-faced Interlayer for High-performance Lithium-Sulfur Batteries

We design the Janus-like interlayer with two different functional faces for suppressing the shuttle of soluble lithium polysulfides (LPSs) in lithium-sulfur batteries (LSBs). At the front face, the conductive functionalized carbon fiber paper (f-CFP) having oxygen-containing groups i.e., -OH and -COOH on its surface was placed face to face with the sulfur cathode serving as the first barrier accommodating the volume expansion during cycling process and the oxygen-containing groups can also adsorb the soluble LPSs via lithium bonds. At the back face, a crystalline coordination network of [Zn(H2PO4)2(TzH)2]n (ZnPTz) was coated on the back side of f-CFP serving as the second barrier retarding the left LPSs passing through the front face via both physical confinement and chemical adsorption (i.e. Li bonding). The LSB using the Janus-like interlayer exhibits a high reversible discharge capacity of 1,416 mAh g−1 at 0.1C with a low capacity fading of 0.05% per cycle, 92% capacity retention after 200 cycles and ca. 100% coulombic efficiency. The fully charged LSB cell can practically supply electricity to a spinning motor with a nominal voltage of 3.0 V for 28 min demonstrating many potential applications.

Fragmentation of allylmethylsulfide by chemical ionization: dependence on humidity and inhibiting role of water.

We report on a previously unknown reaction mechanism involving water in the fragmentation reaction following chemical ionization. This result stems from a study presented here on the humidity-dependent and energy-dependent endoergic fragmentation of allyl methyl sulfide (AMS) upon protonation in a proton transfer reaction-mass spectrometer (PTR-MS). The fragmentation pathways were studied with experimental (PTR-MS) and quantum chemical methods (polarizable continuum model (PCM), microhydration, studied at the MP2/6-311+G(3df,2p)//MP2/6-31G(d,p) level of theory). We report in detail on the energy profiles, reaction mechanisms, and proton affinities (G4MP2 calculations). In the discovered reaction mechanism, water reduces the fragmentation of protonated species in chemical ionization. It does so by direct interaction with the protonated species via covalent binding (C3H5+) or via association (AMS·H+). This stabilizes intermediate complexes and thus overall increases the activation energy for fragmentation. Water thereby acts as a reusable inhibitor (anticatalyst) in chemical ionization. Moreover, according to the quantum chemical (QC) results, when water is present in abundance it has the opposite effect and enhances fragmentation. The underlying reason is a concentration-dependent change in the reaction principle from active inhibition of fragmentation to solvation, which then enhances fragmentation. This amphoteric behavior of water is found for the fragmentation of C3H5+ to C3H3+, and similarly for the fragmentation of AMS·H+ to C3H5+. The results support humidity-dependent quantification efforts for PTR-MS and chemical ionization mass spectrometry (CIMS). Moreover, the results should allow for a better understanding of ion-chemistry in the presence of water.

Furfural to Furfuryl Alcohol: Computational Study of the Hydrogen Transfer on Lewis Acidic BEA Zeolites and Effects of Cation Exchange and Tetravalent Metal Substitution

The hydrogen transfer of furfural to furfuryl alcohol with i-propanol as the hydrogen source over cation-exchanged Lewis acidic BEA zeolite has been investigated by means of density functional calculations. The reaction proceeds in three steps. First the O-H bond of i-propanol is broken to form a propoxide intermediate. After that, the furylmethoxy intermediate is formed via hydrogen transfer process, and finally furylmethoxy abstracts the proton to form the furfuryl alcohol product. The second step is rate-determining by requiring the highest activation energy (23.8 kcal/mol) if the reaction takes place on Li-Sn-BEA zeolite. We find that the catalytic activity of various cation-exchanged Sn-BEA zeolites is in the order Li-Sn-BEA > Na-Sn-BEA > K-Sn-BEA. The lower activation energy for Li-Sn-BEA compared to Na-Sn-BEA and K-Sn-BEA can be explained by the larger charge transfer from the carbonyl bond to the catalyst, leading to its activation and to the attraction of the hydrogen being transferred. The larger charge transfer in turn is due to the smaller gap between the energies of furfural HOMO and the zeolite LUMO in Li-Sn-BEA, compared to both Na-Sn-BEA and K-Sn-BEA. In a similar way, we also compare the catalytic activity of tetravalent metal centers (Sn, Zr, and Hf) substituted into BEA and find in the order Zr ≥ Hf > Sn, based on activation energies. Finally we investigate statistically which property of the reactants is a suitable descriptor for an approximative prediction of the reaction rate in order to be able to quickly screen promising catalytic materials for this reaction.

Collaborative design of Li–S batteries using 3D N-doped graphene aerogel as a sulfur host and graphitic carbon nitride paper as an interlayer

Lithium–sulfur batteries (LSBs) have been widely investigated due to their high energy densities; however, their practical applications have still been limited by their poor cycling stability owing to the shuttle mechanism effect, volume expansion, soluble polysulfides, and the poor electrical conductivity of sulfur and Li2S. To address these issues, sulfur was loaded into a conductive 3D nitrogen-doped reduced graphene oxide aerogel (NGae) host with a finely tuned nitrogen doping content. In addition, an interlayer of graphitic carbon nitride coated on flexible and conductive carbon fiber paper (g-C3N4/CFP) was inserted between the cathode and the polymer separator to trap the soluble polysulfides. It was found that the as-fabricated LSB using the NGae host with 4.2% N doping content and the g-C3N4/CFP interlayer can provide a specific capacity of 1271 mA h g−1 at 0.1C with excellent stability over 400 cycles. The capacity fading is rather small (only 0.068% per cycle) while the coulombic efficiency is rather high (ca. 100%). This battery may be practically used in high-energy applications.

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.