Lianming Zhao

Decomposition of ethanol on Pd(111): a density functional theory study.

Ethanol decomposition over Pd(111) has been systematically investigated using self-consistent periodic density functional theory, and the decomposition network has been mapped out. The most stable adsorption of the involved species tends to follow the gas-phase bond order rules, wherein C is tetravalent and O is divalent with the missing H atoms replaced by metal atoms. Desorption is preferable for adsorbed ethanol, methane, and CO, while for the other species decomposition is preferred. For intermediates going along the decomposition pathways, energy barriers for the C-C, C(alpha)-H, and O-H scissions are decreased, while it is increased for the C-O path or changes less for the C(beta)-H path. For each of the C-C, C-O, and C-H paths, the Bronsted-Evans-Polanyi relation holds roughly. The most likely decomposition path is CH(3)CH(2)OH --> CH(3)CHOH --> CH(3)CHO --> CH(3)CO --> CH(2)CO --> CHCO --> CH + CO --> CO + H + CH(4) + C.

A NbO-type copper metal–organic framework decorated with carboxylate groups exhibiting highly selective CO2 adsorption and separation of organic dyes

A porous Cu metal–organic framework (1) based on a pentacarboxylate ligand and paddlewheel SBU was synthesized and structurally characterized. Complex 1 possesses a NbO-type framework with uncoordinated –COO− groups, resulting in its good selectivity for CO2/N2 (36) and CO2/CH4 (12) as well as a large CO2-uptake capacity of 140 cm3 g−1 at 273 K and 1 bar. Grand Canonical Monte Carlo (GCMC) simulations revealed that strong CO2 adsorption sites exist near the open CuII sites and the uncoordinated –COO− groups. Significantly, complex 1 exhibits water resistance and selective adsorption of cationic methylene blue (MB+) in aqueous solution and the adsorbed MB+ can be released in saturated NaCl solution, making it also a promising porous material for charge and pore-size dependent large-molecule capture and separation. The existence of coordinatively unsaturated metal sites as well as the exposed –COO− groups in the framework of 1 is responsible for its selective gas adsorption and dye separation.

Porous barium–organic frameworks with highly efficient catalytic capacity and fluorescence sensing ability

The current study describes the first barium–organic framework with permanent porosity, efficient catalytic capacity, and highly selective luminescence sensing of DMSO molecules and metal ions. Single-crystal-to-single-crystal transformations (from complex 1 to complexes 2 and 3) were used to thermally generate coordinatively unsaturated metal sites (CUSs) as catalytically active sites (CASs). Complex 3 exhibits efficient catalytic capacity for the cyanosilylation of aldehydes and ketones, and the cycloaddition of CO2 and epoxides. To the best of our knowledge, complex 3 keeps a record among the MOF-based catalysts for the cyanosilylation of aldehydes and ketones. The generation of Ba2+ CUSs with high catalytic activity in a SCSC fashion is responsible for the excellent properties of 3, which is further confirmed by the theoretical calculation. Besides, complex 2 can highly sense DMSO molecules through fluorescence enhancement.

Decomposition of methanthiol on Pt(111): a density functional investigation.

Decomposition of methanthiol on Pt(111) is systematically investigated using self-consistent periodic density functional theory (DFT), and the decomposition network has been mapped out. The most stable adsorption of the involved species tends to form the sp(3) hybridized configuration of both C and S atoms, in which C is almost tetrahedral and S has the tendency to bond to three atoms. Spontaneous dissociation rather than desorption is preferred for adsorbed methanthiol. Based on the harmonic transition state theory calculations, the decomposition rate constants of the thiolmethoxy and thioformaldehyde intermediates are found to be much lower than those for their formation, leading to long lifetimes of the intermediates for observation. Under the ultrahigh vacuum (UHV) condition, the most possible decomposition pathway for methanthiol on Pt(111) is found as CH(3)SH --> CH(3)S --> CH(2)S --> CHS --> CH + S --> C + S, in which the C-S bond cleavage mainly occurs at the CHS species. However, the decomposition pathway is CH(3)SH --> CH(3)S --> CH(3) + S under the hydrogenation condition; the C-S bond scission mainly occurs at CH(3)S. The Brønsted-Evans-Polanyi relation holds for each of the S-H, C-H, and C-S bond scission reactions.

Initial Hydrogenations of Pyridine on MoP(001): A Density Functional Study

The initial hydrogenations of pyridine on MoP(001) with various hydrogen species are studied using self-consistent periodic density functional theory (DFT). The possible surface hydrogen species are examined by studying interaction of H(2) and H(2)S with the surface, and the results suggest that the rational hydrogen source for pyridine hydrogenations should be surface hydrogen atoms, followed by adsorbed H(2)S and SH. On MoP(001), pyridine has two types of adsorption modes, i.e., side-on and end-on; and the most stable η(5)(N,C(α),C(β),C(β),C(α)) configuration of the side-on mode facilitates the hydrogenation of pyridine. The optimal hydrogenation path of pyridine with surface hydrogen atoms in the Langmuir-Hinshelwood mechanism is the formation of 3-monohydropyridine, followed by producing 3,5-dihydropyridine, in which the two-step hydrogenations take place on the C(β) atoms. When adsorbed H(2)S is considered as the source of hydrogen, slightly higher hydrogenation barriers are always involved, while the energy barriers for hydrogenations involving adsorbed SH are much lower. However, the hydrogenation of pyridine should be suppressed by the adsorption of H(2)S, and the promotion effect of adsorbed SH is limited.

Synthesis and catalytic properties of ZSM-5 zeolite with hierarchical pores prepared in the presence of n-hexyltrimethylammonium bromide

ZSM-5 samples with hierarchical pores (macropores, mesopores and micropores) were synthesized in the presence of n-hexyltrimethylammonium bromide (HTAB) and tetrapropylammonium hydroxide (TPAOH). The effect of synthesis conditions including the Si/Al ratio, crystallization temperature and time, and the amount of HTAB added to the synthesis system on the final products was examined. The catalytic properties of the hierarchical zeolite were investigated in reactions of Claisen–Schmidt condensation of benzaldehyde and acetophenone, self-condensation of cyclohexanone and methanol conversion. The hierarchical zeolite exhibits superior catalytic performance in Claisen–Schmidt condensation of benzaldehyde and acetophenone and self-condensation of cyclohexanone and has a remarkably high selectivity for dimethyl ether in the methanol conversion reaction at relatively low temperatures, which was attributed to the fast mass transport in the three-dimensional hierarchical pore network. A cooperative assembly mechanism accounting for the formation of the hierarchical zeolite was proposed based on experimental results.

Theoretical analysis of the conversion mechanism of acetylene to ethylidyne on Pt(111)

The conversion of acetylene to ethylidyne on Pt(111) has been comprehensively investigated using self-consistent periodic density functional theory. Geometries and energies for all of the intermediates involved as well as the conversion mechanism were analyzed. On Pt(111), the carbon atoms in the majority of stable C(2)H(x) (x = 1-4) intermediates prefer saturated sp(3) configurations with the missing H atoms substituted by the adjacent metal atoms. The most favorable conversion pathway for acetylene to ethylidyne is via a three-step reaction mechanism, acetylene → vinyl → vinylidene → ethylidyne. The first step, acetylene → vinyl, depends on the availability of surface H atoms: without preadsorbed H the reaction occurs via the initial disproportionation of acetylene, which resulted in adsorbed vinyl; with an abundance of preadsorbed H, acetylene could transform to vinyl via both the disproportionation and hydrogenation reactions. Conversions through initial dehydrogenation of acetylene and isomerizations of acetylene and vinyl are unfavorable due to high energy barriers along the relevant pathways. The conversion rate involving vinylidene as an intermediate is at least 100 times larger than that involving ethylidene.

Theoretical Investigation of the Oxidation of Propane by FeO

We report herein a comprehensive theoretical study of the oxidation of propane by FeO(+) on both the sextet and quartet potential energy surfaces (PESs) using density functional theory. The geometries and energies of all the stationary points involved are located. Interaction of FeO(+) with propane could account for four types of encounters (i.e., alpha,beta,gamma-, 2alpha,beta-, 3alpha-eta(3), and 2alpha,2gamma-eta(4)) complexes. Various mechanisms leading to the loss of CH(3), H(2)O, C(3)H(7)OH (H(2)O + C(3)H(6)), and C(3)H(6) are analyzed in terms of the topology of the PES. The reaction of FeO(+) with propane involves initial C-H activation, while initial C-C activation is indeed unlikely to be important. The loss of CH(3) takes place adiabatically on the sextet PES via the simple C(alpha)-to-O H shift from eta(4)-OFe(+)(C(3)H(8)) followed by CH(3) shift. The C(3)H(7)OH elimination proceeds via direct C(alpha)-to-O H shift followed by C-O coupling, while the loss of H(2)O, C(3)H(6), and (H(2)O + C(3)H(6)) proceeds via the alpha,beta-H and beta,alpha-H abstraction mechanisms from all the eta(3) complexes. The most favorable channel is the alpha,beta-H abstraction mechanism for the H(2)O loss because it not only is energetically and dynamically favorable but also has a high crossing probability between the sextet and quartet PESs. The computational results are in concert with the available experimental information and add new insight into the details of the individual elementary steps.

On the Gas-Phase Co+-Mediated Oxidation of Ethane by N2O: A Mechanistic Study

The potential energy surface (PES) corresponding to the Co(+)-mediated oxidation of ethane by N(2)O has been investigated by using density functional theory (DFT). After initial N(2)O reduction by Co(+) to CoO(+), ethane oxidation by the nascent oxide involves C-H activation followed by two possible pathways, i.e., C-O coupling accounting for ethanol, Co(+)-mediated β-H shift giving the energetically favorable product of CoC(2)H(4)(+) + H(2)O, with minor CoOH(2)(+) + C(2)H(4). CoC(2)H(4)(+) could react with another N(2)O to yield (C(2)H(4))Co(+)O, which could subsequently undergo a cyclization mechanism accounting for acetaldehyde and oxirane and/or a direct H-abstraction mechansim for ethenol. Loss of oxirane and ethenol is hampered by respective endothermicity and high kinetics barrier, whereas acetaldehyde elimination is much energetically favorable. CoOH(2)(+) could facilely react with N(2)O to form OCoOH(2)(+), rather than Co(OH)(2)(+) or CoO(+).

The competitive O-H versus C-H bond activation of ethanol and methanol by VO2(+) in gas phase: a DFT study.

The activation of ethanol and methanol by VO2(+) in gas phase has been theoretically investigated by using density functional theory (DFT). For the VO2(+)/ethanol system, the activation energy (ΔE) is found to follow the order of ΔE(C(β)-H) < ΔE(C(α)-H) ≈ ΔE(O-H). Loss of methyl and glycol occurs respectively via O-H and C(β)-H activation, while acetaldehyde elimination proceeds through two comparable O-H and C(α)-H activations yielding both VO(H2O)(+) and V(OH)2(+). Loss of water not only gives rise to VO(CH3CHO)(+) via both O-H and C(α)-H activation but also forms VO2(C2H4)(+) via C(β)-H activation. The major product of ethylene is formed via both O-H and C(β)-H activation for yielding VO(OH)2(+) and VO2(H2O)(+). In the methanol reaction, both initial O-H and C(α)-H activation accounts for formaldehyde and water elimination, but the former pathway is preferred.