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.
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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