An ONIOM study of amines adsorption in H-[Ga]MOR

A comparative computational study on the synthesis prescriptions, structures and acid properties of B-, Al- and G-incorporated MTW-type zeolites

The synthesis prescriptions, structures and acid properties of B-, Al- and Ga-incorporated MTW-type zeolites were studied at the DFT-D2 level.

Adsorption of Small Aun (n = 1−5) and Au−Pd Clusters Inside the TS-1 and S-1 Pores

We used a hybrid quantum-mechanics/molecular-mechanics (QM/MM) approach to simulate the adsorption of Au(n)() (n = 1-5), AuPd, and Au(2)Pd(2) clusters inside the TS-1 and S-1 pores. We studied nondefect and metal-vacancy defect sites in TS-1 and S-1 for a total of four different environments around the T6 crystallographic site. We predict stronger binding of all clusters near Ti sites in Ti-substituted framework compared to adsorption near Si sites-consistent with the experimental finding of a direct correlation between the Ti-loading and the Au-loading on the Au/TS-1 catalysts with high Si/Ti ratio. The cluster binding is also stronger near lattice-metal vacancies compared to fully coordinated, nondefect sites. In all the cases, a trend of binding energy (BE) versus Au cluster size (n) shows a peak at around n = 3-4. Our results show that there is enough room for the attack of H(2)O(2) on the Ti-defect site even with Au(1-4) adsorbed-a result that supports the possibility of H(2)O(2) spillover from the Au clusters to the adjacent Ti-defect sites. Mulliken charge analysis indicates that in all the cases there is electron density transfer to adsorbed clusters from the zeolite lattice. In the case of both gas-phase and adsorbed Au-Pd clusters, all the Pd atoms were positively charged, and all the Au atoms were negatively charged due to the higher electron-affinity of Au. We also found a correlation between the BE and the charge transfer to the clusters (the higher the charge transfer to the clusters, the higher the BE), and a universal correlation was found for Au(2-5) when BE and charge transfer were plotted on a per atom basis. A relatively larger charge transfer to the adsorbed clusters was found for the Ti sites versus the Si sites, and for the defect sites versus the nondefect sites. The trends in the BE were corroborated using Gibbs free energy of adsorption (DeltaG(ads)), and the implications of DeltaG(ads) in sintering of Au clusters are also discussed. Our results confirm that electronic factors such as cluster-charging are potentially important support effects for the Au/TS-1 catalyst.

Combined DFT Theoretical Calculation and Solid-State NMR Studies of Al Substitution and Acid Sites in Zeolite MCM-22

The distributions of Brönsted acidic protons and their acid strengths in zeolite H-MCM-22 have been characterized by density functional theory (DFT) calculations as well as magic angle spinning (MAS) NMR experiments. The embedded scheme (ONIOM) that combines the quantum mechanical (QM) description of active sites and semiempirical AM1 treatment of the neighboring environment was applied to predict the aluminum substitution mechanism and proton affinity (PA), as well as adsorption behaviors of acetone and trimethylphosphine oxide (TMPO) onto the zeolite. Our theoretical results indicate that the Al substitution takes place in the order of Al1-OH-Si2 > Al8-OH-Si8 > Al5-OH-Si7. The DFT theoretical and NMR experimental results suggest that the acid strength of the three Brönsted acid sites in H-MCM-22 zeolite is slightly lower than that of H-ZSM-5 zeolite and the accessible Brönsted acidic protons most likely reside in both the supercages (at the Al8-OH-Si8 and Al1-OH-Si2 sites) and external surface pocket (at the Al8-OH-Si8 site) rather than in the sinusoidal channels (Al5-OH-Si7), with the Al1-OH-Si2 site having the strongest acid strength (as probed by TMPO). This may partially explain the special selectivity of acid-catalyzed reactions occurring inside the channels of H-MCM-22 zeolite.