Jakkapan Sirijaraensre

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).

Direct oxidation of methane to methanol on Fe–O modified graphene

Introduction of functional groups to graphene can be used for the rational design of catalysts for the oxidation of hydrocarbons to alcohols. We have employed the PBE-D2 level of theory to study the direct oxidation of CH4 to CH3OH on a Fe–O active site generated on graphene by the decomposition of nitrous oxide (N2O) over Fe-embedded graphene. Restricted and unrestricted spin state of systems were also taken into account. The calculations show that FeO/graphene provides excellent reactivity for the oxy-functionalization of methane to methanol. The oxygen-centered radicals (O−˙) on the catalyst can activate the strong C–H bond of methane leading to its homolytic cleavage. The C–H bond activation requires an energy of 17.5 kcal mol−1, which is comparable with the barrier on traditional effective catalysts. Comparing the molecular adsorption complex, the formation of the iron coordinated fragments of C–H bond activation on the graphene support is found to be less energetically stable than on the Fe sites in the zeolite support. As a result, the conversion of the grafted species to the methanol product in the second step of the reaction is much more facile than for Fe-exchanged zeolite catalysts. An activation energy of 16.4 kcal mol−1 is required to yield the methanol product. Fe–O modified graphene materials could be promising catalysts for the partial oxidation of methane with N2O as an oxidant.

Mechanisms of the ammonia oxidation by hydrogen peroxide over the perfect and defective Ti species of TS-1 zeolite.

The catalytic performances of titanium species in TS-1 zeolite for the hydroxylamine formation have been investigated using the density functional theory with the ONIOM scheme. The reaction process for making hydroxylamine is divided into two steps: (i) the H2O2 decomposition over the Ti species to produce the peroxo titanium species and (ii) the NH3 oxidation over the generated oxidizing species. Our results indicated that defective Ti species in the TS-1 zeolite are the dominant catalytic sites for H2O2 decomposition rather than perfect Ti species, leading to the formation of ≡Ti-OOH species as oxygen-donating intermediates for NH3 oxidation reaction. The energetic profiles for the ammonia oxidation over the ≡TiOOH species and the catalytic effect from water were fully investigated, consisting of three proposed mechanisms. The most favored pathway was found to be: the adsorption of ammonia (NH3/η(1)≡TiOOH) → ammonia oxide complex (NH3O/≡TiOH) → hydrated-titanium-oxyamine species (H2O/≡TiONH2) → hydroxylamine product (NH2OH/≡TiOH), in which the highest energy barrier is 16.3 kcal mol(-1). Besides the hydrolysis of titanium-oxyamine species, the hydroxylamine was also generated through the second H2O2 decomposition over the titanium-oxyamine species whereas the activation energy for this step was slightly decreased to be 15.7 kcal mol(-1).

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.

Reaction mechanism of methanol to formaldehyde over Fe- and FeO-modified graphene.

We employed periodic DFT calculations (PBE-D2) to investigate the catalytic conversion of methanol over graphene embedded with Fe and FeO. Two possible pathways of dehydrogenation to formaldehyde and dehydration to dimethyl ether (DME) over these catalysts were examined. Both processes are initiated with the activation of methanol over the catalytic center through O-H cleavage. As a result, a methoxo-containing intermediate is formed. Subsequently, H-transfer from the methoxy to the adjacent ligand leads to the formation of formaldehyde. Conversely, the activation of the second methanol over the intermediate gives DME and H2O. Over Fe/graphene, the dehydration process is kinetically and thermodynamically preferable. Unlike Fe/graphene, FeO/graphene is predicted to be an efficient catalyst for the dehydrogenation process. Oxidative dehydrogenation over FeO/graphene takes place through two steps with free energy barriers of 5.7 and 10.2 kcal mol(-1).

Methane activation on Fe- and FeO-embedded graphene and boron nitride sheet: role of atomic defects in catalytic activities

Methane activation and direct oxidation to methanol on graphene (GP) and boron nitride sheet (BN) embedded Fe and FeO have been carefully studied by means of dispersion corrected DFT (PBE-D2). The strong orbital interactions between methane and the Fe active center through σ-donation and π-backdonation were found to facilitate the C–H bond dissociation. In the Fe-BN system, the π-backdonation is more dominant than that in the Fe-GP resulting in the facile C–H bond breaking with a lower energy barrier of 10.0 kcal mol−1, compared to that of 20.2 kcal mol−1. As a result, the methane C–H bond cleavage is kinetically and thermodynamically favorable on the Fe-BN system. For methane oxidation to methanol on FeO-BN compared to FeO-GP (results from RSC Adv., 2014, 4, 12572), the results reveal that the oxygen-center radical can activate the C–H bond in methane through a homolytic cleavage mechanism with reaction barriers of 20.9 kcal mol−1 and 17.5 kcal mol−1 for FeO-BN and FeO-GP, respectively. These barriers are comparable with reports on effective enzymatic systems. For methanol formation through the combination of methyl- and hydroxyl-grafted Fe-BN intermediate, the product derived from the C–H bond cleavage, required a very large energy barrier of 44.9 kcal mol−1, whereas in the Fe-GP system, the barrier was only 16.4 kcal mol−1 owing to its intermediate being less energetically stable. As a result, the conversion of methane to methanol over FeO-BN would be impeded by the incorrect stability of the intermediate. Overall, the supports play a significant role in the catalytic activity of Fe and FeO active sites for methane C–H bond cleavage and direct oxidation to methanol. This implies that the activity of the catalyst could be suitably designed by the selection of appropriate supports.

Carbon dissolution and segregation in platinum

Recent experimental studies showed evidence for C dissolution in Pt nanoparticles after CH4 decomposition, and the posterior low temperature segregation to form surface graphene, highlighting graphene growth from below. There are indications of an easier C transfer between surface and subsurface regions at Pt grain boundaries, although the ultimate atomistic mechanism remains unclear. A plausible explanation is provided here by exploring and comparing C incorporation in Ni, Pd, and Pt(111) surfaces by density functional (DF) calculations on slab models under a low coverage regime, evaluating the energetic stability and subsurface sinking kinetic feasibility. Four DF functionals have been used, avoiding possible biased results. All functionals showed that C atoms occupy octahedral subsurface (oss) sites in Ni(111), with high sinking energy barriers of 80–90 kJ mol−1, whereas both oss and tetrahedral subsurface (tss) sites can be occupied in Pd(111), with low sinking energy barriers of 20–50 kJ mol−1. The oss sites are strongly disfavoured on Pt(111), whereas the tss sites are found to be isoenergetic to surface sites, with low subsurface sinking energy barriers of 27–41 kJ mol−1. Calculations on Pt79 and Pt140 nanoparticle models reveal how tss sites are more stabilized at low-coordinated sites, where subsurface sinking energy barriers drop to values of ∼17 kJ mol−1. These results explain the experimentally observed C dissolution and segregation in Pt systems, more favoured at grain boundaries, as well as the graphene growth from below and the formation of double layer models. In addition, the present results open a gate for profiting from the small quantities of C placed at the subsurface region in order to tune the surface catalytic activity of Pt nanoparticle based catalysts.