DRIFTS study of the chemical state of modifying gallium ions in reduced Ga/ZSM-5 prepared by impregnation I. Observation of gallium hydrides and application of CO adsorption as a molecular probe for reduced gallium ions

Reduction in hydrogen at 773 K of gallium-modified HZSM-5 zeolite prepared by incipient wetness impregnation results in complete substitution of acidic hydroxyl groups. The modification of the zeolite with gallium suppresses dealumination of the hydrogen form due to substitution of protons by gallium species. When the reduction at 773 K is followed by evacuation, coordinatively unsaturated Ga+ ions are formed. These species can be reversibly oxidized by nitrous oxide at 673 K. Alternatively, these Ga+ ions adsorb molecular hydrogen at lower temperatures resulting in several types of hydride species. The latter species are completely decomposed only in vacuum at relatively high temperatures (773 K). We propose that the oxidation of gallium upon cooling of reduced samples to room temperature in hydrogen can be attributed to an oxidative addition of H2 resulting in the formation of gallium dihydrides

On the nature of the sites of dihydrogen molecular and dissociative adsorption in ZnHZSM-5. II. Effects of sulfidation

The nature of zinc ions in ZnHZSM-5 zeolite prepd. by incipient wetness impregnation was studied by DRIFT spectroscopy using dihydrogen adsorbed at low temp. as a mol. probe. The results obtained indicate the appearance of low-coordinated zinc ions following dehydroxylation at high temp. It is concluded that the ions that most strongly perturb adsorbed hydrogen are localized on the surface of nanometric ZnO clusters that are formed in the channels of the zeolite framework upon high temp. pretreatment. This was demonstrated by the inhibition of dihydrogen adsorption by high temp. sulfidation of the zeolite with H2S. The latter presumably converts ZnO particles into ZnS clusters. This was also confirmed by UV diffuse reflectance spectra of the sulfided samples. By contrast, sulfidation did not influence DRIFT spectra of hydroxyl groups in either HZSM-5 or HY zeolites, indicating that substitution of framework oxygen by sulfur does not occur in these materials. [on SciFinder (R)

A comprehensive density functional theory study of ethane dehydrogenation over reduced extra-framework gallium species in ZSM-5 zeolite

The stability of various gallium species (Ga+, , and GaH+2) as models for the active sites in reduced Ga/ZSM-5 and the possible reaction paths of alkane dehydrogenation were studied using a density functional theory cluster modeling approach. In general, alkanes are preferentially activated via an alkyl mechanism, in which gallium acts as an acceptor of the alkyl group. A comparison of the computed energetics of the various reaction paths for ethane indicates that the catalytic reaction most likely proceeds over Ga+. The initial step of CH activation is the oxidative addition of an alkane molecule to the Ga+ cation, which proceeds via an indirect heterolytic mechanism involving the basic oxygen atoms of the zeolite lattice. Although the catalytic reaction can also occur over and GaH+2 sites, these paths are not favored. Decomposition of leading to formation of Ga+ during the catalytic cycle is more favorable than regeneration of these sites. The reactivity of GaH+2 ions is strongly dependent on the distance between the stabilizing aluminum-occupied oxygen tetrahedra. In cases of greater AlAl distances, the stability of the GaH+2 species is very low, and it decomposes to Ga+ and a Brønsted acid site, whereas when Al atoms are located more closely, the charge-compensating GaH+2 ions are the most stable and exhibit the lowest activity for the initial CH bond cleavage reaction

Characterization and reactivity of Ga+ and GaO+ cations in zeolite ZSM-5

The reduction of Ga(CH3)3/ZSM-5 was closely followed by Fourier transform infrared spectroscopy and Ga K-edge X-ray absorption near-edge spectroscopy. Chemical vapor deposition of trimethylgallium on HZSM-5 (TMG/ZSM-5) resulted in the replacement of nearly all Brønsted acid protons by dimethylgallium species. Removal of the methyl ligands from the cationic Ga clusters gave charge-compensating Ga+ and species. At high temperatures and in the absence of hydrogen, the Ga+ species were the most stable, although decomposition of the species was very slow. Ga+ ions can be oxidized by nitrous oxide at low temperature (473 K), resulting in the formation of gallyl (GaO+) cations. A detailed comparison of the reactivity of Brønsted acid protons (HZSM-5) and Ga+ ions (reduced TMG/ZSM-5) in propane dehydrogenation showed that the former converted propane via protolytic cracking with methane, ethane, and propene as hydrocarbon products, whereas monovalent Ga+ ions produced propene almost exclusively. The reaction data suggest that propane was converted over Ga+ cations but not over cations. The initial rate of propane dehydrogenation was highest for GaO+ ions, although rapid deactivation was observed, due to the higher barrier for regeneration of GaO+ ions than for formation of less active Ga+ ions. ERRATUM : Journal of Catalysis, Volume 240, Issue 1, 15 May 2006, Page 85

Cluster model DFT study of CO adsorption to gallium ions in Ga/HZSM-5

Cluster model DFT calcns. of CO adsorption on various possible forms of gallium in Ga/HZSM-5 zeolites have been performed. CO was found to only weakly interact with Ga+, (GaO)+, and (Ga(OH)2-nHn)+ (n = 1, 2) cationic clusters. The resulting shifts of the CO stretching frequency (.nu.CO) are only very small. On the other hand, CO coordination to small mononuclear and binuclear Ga3+ hydride/hydroxide/oxide species results in pos. shifts in the stretching frequency in the range .nu.CO = 20-40 cm-1. Larger shifts (.nu.CO = 70-90 cm-1) are assocd. with CO coordination to Ga ions at the corners of small three-dimensional Ga-oxide clusters. The exptl. obsd. changes in the IR spectrum of adsorbed CO over Ga/HZSM-5 zeolites upon reductive and oxidative treatments are interpreted with these insights. Possibilities for the formation of such polynuclear oxide species in the zeolite micropore space are discussed. On the basis of recent literature insights, we suggest that large shifts derive from CO coordination to oligomeric Ga cationic complexes stabilized by the neg. zeolite charge