Danhong Zhou

On configuration of exchanged La3+ on ZSM-5: A theoretical approach to the improvement in hydrothermal stability of La-modified ZSM-5 zeolite

Density functional calculations have been employed to investigate the locating and binding of lanthanum cation, i.e., La(OH)2+, on HZSM-5 zeolite. Through geometry optimization, it was determined that lanthanum ions are favorably accommodated in the two 6-T rings of the straight channels (Clusters 1 and 2, see Sec. III A for details). Cluster 1 was found to exist in prior to Cluster 2 due to the preference of Al substitution in the T11 site (Cluster 1) rather than in the T8 site (Cluster 2). Geometry-optimization of Cluster 1 containing another two lanthanide ions Nd3+ and Yb3+ was also carried out and it was found that a monotonic decrease in Ln–O bond length will take place as the atomic number increases, conforming well to the rule of lanthanide contraction. Some of the optimized parameters are comparable to the corresponding experimental values in Y zeolite, which confirms that the optimized configurations are acceptable. The average frequencies of hydroxyls attached to La3+ or Yb3+ in Cluster 1 fall ...

Study with density functional theory method on methane C–H bond activation on the MoO2/HZSM-5 active center

Density functional theory is used to describe the reaction profile for methane activation on the MoO2/HZSM-5 active center. The physisorbed state of methane on the Mo center and the transition state structure of C-H bond dissociation were obtained, and the barrier energy was calculated. The local and nonlocal density functional activation barriers were 91 and 158 kJ/mol, respectively. In the dissociative state the methyl moiety was connected to molybdenum, and the dissociated hydrogen combined with the extraframework oxygen to form a hydroxyl group. The mechanism of methane activation on the MoO2/HZSM-5 active center is discussed

Towards Guest–Zeolite Interactions: An NMR Spectroscopic Approach

Guest(metal)-zeolite interactions in a two component heterogeneous catalyst have been investigated by high-field and high-speed (27)Al MAS NMR, and two-dimensional (27)Al MQ MAS NMR experiments as well as ab initio DFT methods. It was established that strong interactions between guest and zeolite occur in a metal/zeolite system, with the metal anchored to the tetrahedral aluminum framework site through two oxygen bridges. It disturbs the tetrahedral environment of associated aluminum framework, changing AlO(4) geometry from near T(d) to C(2v); this enables us to resolve this species from the undisturbed aluminum framework species in high-field (27)Al MAS NMR and two-dimesional (27)Al MQ MAS NMR experiments.

A simulation study on the absorption of molybdenum species in the channels of HZSM-5 zeolite

The Monte Carlo calculation method was used for the adsorption of mobile molybdenum oxide in ZSM-5 zeolite pores. Two models of mobile Mo species were designed and their adsorptions in ZSM-5 zeolite pores were investigated, respectively. The simulation calculation results suggest that the tetrahedral coordinated MoO2(OH)(2) should be the possible mobile Mo species in ZSM-5 zeolite pores. The maximum loading of MoO2(OH)(2) molecules per unit cell of ZSM-5 was determined as 5, and this sorbate prefers to locate at the intersections of the straight and zigzag channels of ZSM-5 zeolite. The average adsorption energy and isotherm energy per MoO2(OH)(2) molecule in ZSM-5 zeolite was -5.80 kcal/mol and 1.54 kcal/mol K, respectively, at 773 K. The interaction between MoO2(OH)(2) and ZSM-5 framework was dominated by the van der Waals energy

Study with density functional theory method on methane dehydro-aromatization over Mo/HZSM-5 catalysts I: Optimization of active Mo species bonded to ZSM-5 zeolite

A study of the structure of active Mo species on HZSM-5 zeolite was carried out by applying the density functional theory (DFT). The interaction of the MoO2+ unit with the zeolite framework had been verified by an optimization procedure based on the DFT. The result of calculation reveals a dicoordinated bridging geometry in which the molybdenum coordinated simultaneously to both of the two lattice oxygen atoms, leading to a quadrangular geometry with an O(24)–Al(12)–O(20) bridge structure. The Mo ions having O(24)–Al(12)–O(20) bridges exist in identical planes, with equal bond distances of 1.72 A to both the O(24) and O(20) atoms and an interatomic distance of 2.86 A to lattice Al. The molybdenum has a tetrahedral geometry and a formal charge of +5. This kind of active Mo center possesses a typical C2v symmetry. Approximation vibrational analysis indicates that the vibrations of Mo–O (framework oxygen) bonds occur at the frequencies between 580 and 928 cm−1, while the vibrations of the Mo–O (extra-framewo...

Density Functional Theory Study on Structure of Molybdenum Carbide and Catalytic Mechanism for Methane Activation over ZSM-5 Zeolite

Abstract Density functional theory (DFT) calculation was employed to investigate the geometric and electronic structure of molybdenum carbide loaded on ZSM-5 zeolite and the catalytic mechanism for methane C-H bond dissociation. Four active center models of the monomer and dimer models were proposed, which were Mo(CH2)2/ZSM-5, Mo(CH2)2CH3/ZSM-5, Mo2(CH2)4/ZSM-5, and Mo2(CH2)5/ZSM-5. The monomer model was located at the Bronsted acid site of the T6 site positioned at the intersection of the channels of ZSM-5 zeolite. The dimer model was constructed at the T6–––T6 Bronsted acid sites. Mo-carbene in the form of Mo=CH2 was formed in both the monomer and dimer models, and the optimized bond lengths of Mo-C were in reasonably good agreement with the corresponding experimental values. The frontier molecular orbitals in the active center were assigned to the p orbitals of the Mo=CH2 bonds in all four models. The catalytic activity of the Mo carbide active centers were investigated. It was found that the C-H bond of methane was heterogeneously dissociated with the H+ and the H3C− moieties bonded on the C and Mo atoms of the Mo=CH2 bond, respectively, and the p bond was broken simultaneously. The calculated activation energies of the methane C-H bond in the four models were between 106 and 196 kJ/mol. The Mo2(CH2)5/ZSM-5 model showed the best activity for methane C-H bond dissociation.

Density functional theory study on structure of molybdenum carbide loaded on MCM-22 zeolite and mechanism for methane activation

Abstract Density functional theory was employed to study the geometric and electronic structure of molybdenum carbide loaded on the MCM-22 zeolite and to predict the mechanism for CH4 activation. Two models of active center, Mo(CH2)2 (Model A) and Mo(CH)CH2 (Model B), were designed; they were located on the T4 site of the supercage in MCM-22. The geometry optimization and the electronic structure analysis of the models were performed based on the 3T cluster model. The optimized geometry showed that Mo was bonded to the CH2 terminal group by a double bond (bond length of 0.18–0.19 nm) and with the CH terminal group by a triple bond (bond length of 0.17 nm). The natural bond orbital calculation revealed that Mo was bonded to framework oxygen through a σ bond. In terms of the composition and the energy of the frontier orbitals, it was suggested that the CH4 activation on the Mo carbide active center would happen between the HOMO of the CH4 molecule and the LUMO of the Mo carbide. Namely, electrons preferred to transfer from the σ orbital of the C–H bond to the π* orbital of the Mo–C bond. After heterogeneous splitting of the C–H bond, the H3C− group was bonded to Mo and the H+ was bonded to C in Mo carbide species. The calculated activation energy on Model A was 119.97 kJ/mol. On Model B, there were two possible pathways, in which the hydrogen in CH4 could attach to CH2 and to the CH terminal groups. The corresponding activation energy was 91.37 and 79.07 kJ/mol, respectively.

Framework stability and Brønsted acidity of isomorphously substituted interlayer-expanded zeolite COE-4: a density functional theory study.

COE-4 zeolites possess a unique two-dimensional ten-ring pore structure with the Si(OH)2 hydroxyl groups attached to the linker position between the ferrierite-type layers, which has been demonstrated through the interlayer-expansion approach in our previous work (H. Gies et al. Chem. Mater. 2012, 24, 1536). Herein, density functional theory is used to study the framework stability and Brønsted acidity of the zeolite T-COE-4, in which the tetravalent Si is isomorphously substituted by a trivalent Fe, B, Ga, or Al heteroatom at the linker position. The influences of substitution energy and equilibrium geometry parameters on the stability of T-COE-4 are investigated in detail. The relative acid strength of the linker position is revealed by the proton affinity, charge analysis, and NH3 adsorption. It is found that the range of the ⟨T-O-Si⟩ angles is widened to maintain the stability of isomorphously substituted T-COE-4 zeolites. The smaller the ⟨O1-T-O2⟩ bond angle is, the more difficult is to form the regular tetrahedral unit. Thus, the substitution energies at the linker positions increase in the following sequence: Al-COE-4 < Ga-COE-4 < Fe-COE-4 < B-COE-4. The adsorption of NH3 as a probe molecule indicates that the acidity can affect the hydrogen-bonding interaction between (N-H⋅⋅⋅O2) and (N⋅⋅⋅H-O2). The relative Brønsted-acid strength of the interlayer-expanded T-COE-4 zeolite decreases in the order of Al-COE-4 > Ga-COE-4 > Fe-COE-4 > B-COE-4. These findings may be helpful for the structural design and functional modification of interlayer-expanded zeolites.

Structures and vibrational spectra of Ti-MWW zeolite upon adsorption of H2O and NH3: A density functional theory study

Abstract The structures and vibrational spectroscopic features of framework Ti(IV) species in Ti-MWW zeolite upon adsorption of H2O and NH3 were investigated by density functional theory. The calculations were carried out on cluster models up to 36 tetrahedra at the B3LYP/6-31G(d,p) level of theory. The calculated results indicate that both Ti(OSi)4 and Ti(OSi)3OH species can interact with H2O or NH3 molecules to form five-coordinated complexes. The Ti(OSi)3OH species has higher Lewis acidity and adsorbs the ligands more easily than Ti(OSi)4. The Ti-specific band is attributed to the collective vibration of the antisymmetric stretching of Ti–O–Si bonds. The vibrational frequencies of coordinated Ti species can be divided into two regions: the Ti-specific vibration region and the hydroxyl group vibration region. After adsorption of H2O, the Ti-specific band of the Ti(OSi)4 species shifted from 960 to 970 cm−1, and the Ti-specific bands of the Ti(OSi)3OH species shifted from 990 cm−1 (T1 site) and 970 cm−1 (T3 site) to 980 cm−1. The frequencies of the corresponding NH3 adducts were about 5 cm−1 higher. The Ti(OSi)3OH species can also form hydrogen bonded complexes with H2O and NH3 through Ti–OH, resulting in the hydroxyl stretching band of Ti–OH red shifting by 500–1100 cm−1 and appearing in the 2700–3200 cm−1 region.