Effect of surface hydroxyls on DME and methanol adsorption over γ-Al2O3 (hkl) surfaces and solvent effects: a density functional theory study

Scavenging performance and antioxidant activity of γ-alumina nanoparticles towards DPPH free radical: Spectroscopic and DFT-D studies

The radical scavenging performance and antioxidant activity of γ-alumina nanoparticles towards 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical were investigated by spectroscopic and computational methods. The radical scavenging ability of γ-alumina nanoparticles in the media with different polarity (i.e. i-propanol and n-hexane) was evaluated by measuring the DPPH absorbance in UV-Vis absorption spectra. The structure and morphology of γ-alumina nanoparticles before and after adsorption of DPPH were studied using XRD, FT-IR and UV-Vis spectroscopic techniques. The adsorption of DPPH free radical on the clean and hydrated γ-alumina (1 1 0) surface was examined by dispersion corrected density functional theory (DFT-D) and natural bond orbital (NBO) calculations. Also, time-dependent density functional theory (TD-DFT) was used to predict the absorption spectra. The adsorption was occurred through the interaction of radical nitrogen N and NO₂ groups of DPPH with the acidic and basic sites of γ-alumina surface. The high potential for the adsorption of DPPH radical on γ-alumina nanoparticles was investigated. Interaction of DPPH with Brønsted and Lewis acidic sites of γ-alumina was more favored than Brønsted basic sites. The following order for the adsorption of DPPH over the different active sites of γ-alumina was predicted: Brønsted base < Lewis acid < Brønsted acid. These results are of great significance for the environmental application of γ-alumina nanoparticles in order to remove free radicals.

DME synthesis from methanol over hydrated γ-Al2O3(110) surface in slurry bed using continuum and atomistic models

The possible paths of dimethyl ether (DME) synthesis from methanol over hydrated [Formula: see text]-Al2O3(110) in vacuum and liquid paraffin have been investigated by using density functional theory (DFT). Over hydrated [Formula: see text]-Al2O3(110), the three possible paths of methanol dehydration to DME have been investigated by the DFT method in vacuum and liquid paraffin. DME synthesis from methanol is carried out along the same pathway 2CH3OH(g) [Formula: see text] 2* [Formula: see text] 2CH3OH* [Formula: see text] 2CH3O* [Formula: see text] 2H* [Formula: see text] CH3OCH3* [Formula: see text] H2O* in vacuum and liquid paraffin, and the step of highest energy barrier is the reaction of 2CH3O* [Formula: see text] CH3OCH3* [Formula: see text] O*. The energy barrier of the step in liquid paraffin is higher than that in vacuum by 0.33[Formula: see text]eV. The surface acid strength in liquid paraffin decreases over [Formula: see text]-Al2O3(110) surface comparing with vacuum, showing that stronger surface acid strength benefits to DME synthesis. Our result is in consistent with the experiment results.

Density functional theory studies of methanol adsorption and decomposition mechanism on Al13 clusters

The adsorption and decomposition of the CH3OH molecule on Al13 clusters were investigated by generalized gradient approximation of the density functional theory. The strong attractive forces between the CH3OH molecule and aluminum atoms induce the breaking of the H–O and C–O bonds of CH3OH. Subsequently, the dissociated CH3O and OH radical fragments oxidize the aluminum clusters. The largest adsorption energy is –205.4 kJ/mol. We also investigated five reaction pathways of the CH3OH molecule on the Al13 clusters. The activation energies are in the range of 10.3−113.1 kJ/mol. Compared with the bond dissociation energies of the C–O and O–H bonds in the isolated methanol, Al13 performs very well in decreasing the bond break barrier of CH3OH. In addition, although the C–O bond is slightly weaker than the O−H bond, the O−H bond is even easier to decompose on the Al13 surface. The rate constants of five adsorption paths over the temperature range 300−700 K are presented.

Effect of surface hydroxyls on dimethyl ether synthesis over the γ-Al₂O₃ in liquid paraffin: a computational study

In a recent paper (Zuo et al., Appl Catal A 408:130-136, 2011), the mechanism of dimethyl ether (DME) synthesis from methanol dehydration over γ-Al2O3 (110) was studied using density functional theory (DFT). Using the same method, the effect of surface hydroxyls on γ-Al2O3 in liquid paraffin during DME synthesis from methanol dehydration is investigated. It is found that DME is mainly formed from two adsorbed CH3O groups via methanol dehydrogenation on both dehydrated and hydrated γ-Al2O3 in liquid paraffin. No close correlation between catalytic activity and acid intensity was found. Before and after water adsorption at typical catalytic conditions (e.g., 553 K), the reaction rate is not obviously changed on γ-Al2O3(100) surface in liquid paraffin, but the reaction rate decreases by about 11 times on the (110) in liquid paraffin. Considering the area of the (110) and (100) surfaces under actual conditions, the catalytic activity decreased mainly because the Al3 sites on the (110) surface gradually become inactive. Catalytic activity decreased mainly due to surface hydrophilicity. The calculated results were consistent with the experiment.