The activation of a dimethylgallium/ZSM-5 precursor to well-defined reduced and oxidized species is studied by in situ Ga K edge XANES. The precursor is prepared by chemical implanting of trimethylgallium on acidic HZSM-5. Subsequent reduction leads to charge-compensating Ga + species. Direct oxidation of the trimethylgallium precursor leads to various forms of gallium oxide and regeneration of Brønsted acid protons. Oxidation of the reduced Ga + species yields predominantly to GaO + species. The GaO + species exhibit a much higher H 2 /D 2 exchange activity than reduced Ga + species.

A series of Ga-substituted hydrotalcite-like compounds, [Mg1 - x Ga x (OH)2] (CO3) x /2·mH2O (where 0.07 ≤ x ≤ 0.36; GaHTs), were obtained in order to characterize their structural and crystal properties by means of X-ray diffraction and conventional/high-resolution electron microscopy. A linear relationship between Mg/Ga ratios and the a parameter (3.123−3.086 Å) was found for all x values, indicating that Ga3+ cations incorporate into the layered structure. The interlayer spacing, i.e. the c parameter, shrinks (8.173−7.574 Å) as the Ga content increases, due to a greater electrostatic attraction between layers and interlayers. However, for Ga-poor materials (Mg/Ga = 7.7, 12.9) the c parameter remains practically constant (8.173 and 8.169 Å), probably due to the high dilution of Ga in the brucite-like layers. GaHTs are made up of hexagonal crystallites. High-resolution observations of the layered stacking structure point out a defective configuration containing dislocations, stacking faults, weaving planes, and disruptions in planes. Outermost layers in a crystal plate present interlayer distances (3.0−3.8 Å) greater than those in the bulk (2.4 Å), suggesting that peripheral layers are probably more loosely bonded to each other than the later ones.

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A series of Ga-substituted hydrotalcite-like materials (GaHTs), [Mg 1-x Ga x (OH) 2 ](CO 3 ) x/2 ‚mH 2 O where x ) 0.12-0.36, were prepared in order to examine their textural and basic properties upon different Mg/Ga contents and calcination temperature. Specific surface area increases from 46 to 156 m 2 g -1 (Mg/Ga ) 7.7) when calcining from 523 to 673 K as a consequence of dehydroxylation and decarbonation of the material. The total number of base sites increases upon higher calcination temperatures until reaching a maximum at 873 K and then decreases. CO 2 thermal programmed desorption indicates that different Mg/Ga relative contents have a considerable effect on the amount of base sites. For instance, a GaHT with Mg/Ga ) 7.7 exhibits 2.4 times more total base sites than that of Mg/Ga ) 2.9. Additionally, in the former, the generation of medium and strong base sites is considerably higher than in the latter. By means of CDCl 3 adsorption and FTIR, several types of base sites were identified. The base sites strength (pK a ) -1 to +19) and proton affinity (815-987 kJ mol -1 ) values were estimated considering the ν CD shift of adsorbed CDCl 3 and correlating it with the pK a of bases that form hydrogen-bonded complexes.

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Several reconstructions of catalytic active sites in gallium-exchanged mordenite have been analyzed using periodic density functional theory method. It is found that a number of structures in which gallium is present as Ga (III) H x , Ga (II) Ga (II) H x , and Ga (III) Ga (I) H x can be reached under alkane dehydrogenation conditions. The transition barrier for the rate determining step in alkane dehydrogenation are evaluated. The values indicate that these gallium structures can catalyze alkane dehydrogenation and suggest an alternative reaction route in addition to those that have been proposed before.

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Formation of Base Sites on Calcined Mg-Ga Hydrotalcite-Like (Mg1-xGax( OH)2)(CO3)x/2·mH2O.-The base properties of a series of title hydrotalcites with x ranging from 0.12 to 0.36, prepared by a coprecipitation method, are studied as a function of both the calcination temp. and the Mg/Ca molar ratio using TPD of CO2, FTIR analyses, and CDCl3 adsorption. Calcination in the temp. range 523 to 673 K causes an increase of the specific surface area from 46 to 156 m2/g due to dehydroxylation and decarbonation. The total number of base sites increases with calcination temp. and reaches a maximum at 873 K. The amount of base sites, which depends on the Mg/Ga ratio, is 2.4 times higher at a ratio of 7.7 than at 2.9. The relative base site distribution depends on both the calcination temp. and the Mg/Ga ratio. Higher temp. favor a greater population of medium and strong base sites and Mg-richer compounds exhibit stronger base sites. -(LOPEZ-SALINAS, E.; GARCIA-SANCHEZ, M.; LLANOS-SERRANO, M. E.; NAVARRETE-BOLANOZ, J.; J.