Extraframework Gallium Species Research Articles

Ethylene Dehydroaromatization over Ga-ZSM-5 Catalysts: Nature and Role of Gallium Speciation.

Bifunctional catalysis in zeolites possessing both Brønsted and Lewis acid sites offers unique opportunities to tailor shape selectivity and enhance catalyst performance. Here, we examine the impact of framework and extra-framework gallium species on enriched aromatics production in zeolite ZSM-5. We compare three distinct methods of preparing Ga-ZSM-5 and reveal direct (single step) synthesis leads to optimal catalysts compared to post-synthesis methods. Using a combination of state-of-the-art characterization, catalyst testing, and density functional theory calculations, we show that Ga Lewis acid sites strongly favor aromatization. Our findings also suggest Ga(framework)-Ga(extra-framework) pairings, which can only be achieved in materials prepared by direct synthesis, are the most energetically favorable sites for reaction pathways leading to aromatics. Calculated acid site exchange energies between extra-framework Ga at framework sites comprised of either Al or Ga reveal a site-specific preference for stabilizing Lewis acids, which is qualitatively consistent with experimental measurements. These findings indicate the possibility of tailoring Lewis acid siting by the placement of Ga heteroatoms at distinct tetrahedral sites in the zeolite framework, which can have a marked impact on catalyst performance relative to conventional H-ZSM-5.

Influence of the acid sites density in the acetone transformation over bifunctional Pt/H[Ga]ZSM5 catalysts

The purpose of this work is to elucidate the effect of acid sites density on the catalytic behaviour of a series of bifunctional catalysts prepared by supporting platinum (about 1.00 wt.%) on MFI-type gallosilicates (H[Ga]/ZSM5 with Si/Ga atomic ratio = 16, 46 and 145). The XPS characterization studies of the bifunctional catalysts allowed to detect the presence of framework as well as extraframework gallium species for the catalyst with highest density of acid sites (Si/Ga = 16), while only framework Ga species could be detected for catalysts with lower acid site density. It was also verified in all the bifunctional catalysts the presence of three types of metal sites: Pt (0), Pt–O ads and PtO species. The results obtained in the synthesis of MIBK from acetone with the different bifunctional solids, indicate that as the density of the acid sites increases, the solids start to behave solely as acid catalysts. This phenomenon is probably related to the presence of extraframework gallium species in the catalyst with higher surface acidity, capable to interact with the platinum species and thus provoking the passivation of the metallic phase, while in the solids with lower surface acidity it seems to exist a better balance between the acidic–metallic functionalities.

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 +, GaH 2 + , 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 C H 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 GaH 2 + and GaH +2 sites, these paths are not favored. Decomposition of GaH 2 + 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 Al Al 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 C H bond cleavage reaction.

Insight into the dry-gel synthesis of gallium-rich zeolite [Ga]Beta

The conversion of a dry gel with an n Si/ n Ga ratio of 8.5 to zeolite [Ga]Beta (structure-type BEA) with a framework n Si/ n Ga ratio of 11.6 was studied by scanning electron microscopy, X-ray diffraction (XRD), and multi-nuclear solid-state NMR spectroscopy. XRD indicated a fast formation of the long-range order while, as revealed by NMR spectroscopy, the rearrangement of the local structure occurs during a period of up to 65 h. This rearrangement of the local structure during the dry-gel conversion (DGC) process consists of different concerted mechanisms: The dominating step at the beginning of the DGC is a breakage of chemical bonds leading to a strong increase in the concentration of defect SiOH groups as indicated by 1H MAS NMR spectroscopy. After a conversion time of 16 h, the intensity of the 1H MAS NMR signals of defect SiOH groups and of the 29Si MAS NMR signals of Q 1, Q 2, and Q 3 silicon species decreases significantly, indicating a condensation of terminal bonds. High-field ( B 0=17.6 T) 71Ga MAS NMR investigations evidenced that the silanol condensation is accompanied by an incorporation of gallium into the zeolite framework. This finding is corroborated by the increase of the 29Si MAS NMR signals of Si(1Ga) and Si(2Ga) species. Two-dimensional 71Ga MQMAS NMR spectroscopy shows the presence of two types of tetrahedrally coordinated gallium atoms at crystallographically non-equivalent framework positions, but also distorted tetrahedrally coordinated extra-framework gallium species. The existence of the latter species explains the difference between the bulk (determined by AES/ICP) and the framework (determined by 29Si MAS NMR spectroscopy) n Si/ n Ga ratios. 1H and 13C MAS NMR spectroscopy indicate that the template molecules (tetraethylammonium hydroxide) are not affected by the DGC process, excluding a loss of mobility in zeolite [Ga]Beta obtained after a DGC time of 65 h.

A new way to obtain acid or bifunctional catalysts: V. Considerations on bifunctionality of the propane aromatization reaction over [Ga,Al]-ZSM-5 catalysts

The as-synthesized alkali-free TPA, MeNH 3-[Ga,Al]-ZSM-5 zeolites were steamed at different steam pressures (68 kPa≤ P≤92 kPa), reaction times (0 h≤ t≤9 h), and temperatures (530°C≤ T≤800°C). The solids obtained were characterized by XRD, FT-IR, N 2 adsorption, 27 Al and 71 Ga MAS-NMR. Acidity of the solids was characterized using n-heptane cracking ( T=350°C, P N 2 / P C 7 =4, P=100 kPa) as a model reaction. Acidic or bifunctional behavior of these catalysts was determined using propane aromatization (530°C, P C 3 =100 kPa) as a model reaction. Steaming of these solids leads, even at low temperatures (530°C), to a profound and progressive demetalization (degalliation and/or dealumination). Catalysts obtained under mild steaming conditions show propane aromatization properties similar to those of catalysts prepared by means of direct calcination under dry air at higher temperatures ( T>700°C), or by conventional impregnation methods. A clear synergistic effect is observed between acidic sites in the structure and dehydrogenating species composed by extraframework gallium species (EFGS). Linear dependence of initial activity in propane aromatization per protonic acidic site ( A 0/H +), as a function of EFGS/H + relation, shows that this aromatization reaction should be kinetically limited by the initial dehydrogenation step produced on EFGS, which demonstrates bifunctionality of this reaction. Finally, inactivation of acidic sites of the structure of the solid (H +) is produced by EFGS, and the numeric ratio between them shows that EFGS present on galloaluminosilicates, under reaction conditions, should be species with a positive charge equal to 1.

High catalytic activity for synthesis of aniline from phenol and ammonia found on gallium-containing MFI

Gallium-containing zeolitic materials, i.e., MFI-type gallosilicate and gallium-loaded ZSM-5 zeolite, exhibited quite high activity for vapor-phase reaction of phenol and ammonia into aniline. Co-presence of the extra-framework gallium species and the ion-exchange site ascribed to framework gallium or aluminum atom is considered to generate the high activity.

Coordination state of gallium in MFI structures prepared by direct synthesis and by postsynthetic modification of boralites

High-field 71 Ga MAS n.m.r. spectroscopy shows that gallium occupies preferentially the framework positions in Ga-MFI structures prepared by direct synthesis from a gel. However, modifications of boralite with Ga 3+ in autoclave experiments provide structures with extraframework gallium species and only partially occupied boron vacancies in framework positions. Evidence of gallium oxohydrates in the modified samples was given by X-ray analysis. Extraframework gallium species with a quadrupole coupling constant of 8.8 MHz could be detected, but part of the extraframework gallium remains n.m.r. invisible.

Spectroscopic and catalytic evidence for the incorporation of gallium in the AEL framework

71Ga MAS n.m.r. analysis and the catalytic behavior during the transformation of 1-butene and n-butane strongly suggest the incorporation of Ga into the AlPO4-11 (AEL) framework during the synthesis of GaAPSO-11. A unique clear signal at around +120 ppm is proposed to be associated with the presence of tetrahedral Ga in the AEL framework of the unmodified GaAPSO-11. Migration of structural Ga atoms to the silicon domain of the AEL framework seems to occur as a result of a mild hydrothermal treatment, giving rise to an additional signal at +156 ppm, previously associated with tetrahedral gallium in gallosilicates with the MFI topology. The fact that GaAPO-11 and GaAPSO-11 behaved in a way similar to their counterparts AlPO4-11 and SAPO-11 during the skeletal isomerization of n-butenes reinforces the idea of an isomorphous substitution of Al(III) by Ga(III) in the AEL framework. The transformation of n-butane was shown to be a valuable test for detecting the presence of small amounts of hydro-dehydrogenating extraframework gallium species (EFGS) in Ga-supported SAPO-11 (Ga/SAPO-11). The fact that the sample of GaAPSO-11 was completely inactive for this transformation leads us to believe that the incorporation of gallium into the tetrahedral positions of the AEL framework was almost complete.

A new way to obtain acid or bifunctional catalysts. III. Steaming of as-synthesized TPA, MeNH 3-[Ga,Al]-ZSM-5 zeolites obtained from alkali-free media

The steaming of as-synthesized alkali-free TPA-MeNH 3-[Ga 0.99Al 1.99]-ZSM-5 zeolite was carried out at temperatures of 530°C (St-530), 550°C (St-550), and 600°C (St-600). Physical characterization of the solids was carried out by XRD, IR, N 2 adsorption, and 27Al and 71Ga MAS-NMR. n-Heptane cracking ( T=350°C; P N 2 / P nC 7 =4; P=1 atm) was used as a model reaction to characterize the acidity of the catalysts. Propane aromatization ( T=530°C; P=1 atm) was used to characterize the acid or the bifunctional behavior of solids. Results show that steaming produces less acid solids due to degalliation and dealumination of the zeolite structure. Because the presence of steam reduces the temperature at which the formation of extra framework gallium species occurs, bifunctional “Ga 2O 3”/H-[Ga,Al]-ZSM-5 catalysts are formed at lower temperature ( T=530°C) in comparison when calcination is carried out under dry air. Catalysts are less selective to BTX as steaming temperature increases because of a strong reduction of Bronsted acid sites.

A new way to obtain acid or bifunctional catalysts. II. Straightforward calcination of as-synthesized [Ga,A1]-ZSM-5 zeolites obtained from alkali-free media

The acid or bifunctional behavior of catalysts obtained by the straightforward calcination (530–800°C) of as-synthesized [GaA1]-ZSM-5 zeolite, obtained from alkali-free media, was determined using propane, cyclohexane and cyclohexene transformations ( T = 530°C, p = 1 atm) as reaction models. Solids were characterized by XRD, FTIR, N 2 adsorption, 27A1 and 71Ga MAS NMR and chemical analysis. The catalyst acidities were evaluated using n-heptane cracking and m-xylene isomerization as reaction models. Results show that calcination temperatures (T c)between 530 and 550°C, causes only the removal of organic molecules used for the synthesis of the zeolite, leading then to pure acidic catalysts. At 700 ⩽ T c ⩽ 800°C, simultaneously with the removal of those organic molecules, a significant chemical change of the zeolite framework composition take place (degalliation and dealumination). The presence of extraframework gallium species (EFGS), led to acid gallium promoted bifunctional catalysts with catalytic properties for propane and naphthenes aromatization almost identical to those for gallium promoted zeolites obtained by conventional methods (ion exchange and/or impregnation).

Conversion of Light Alkanes into Aromatic Hydrocarbons .VII. Aromatization of Propane on Gallosilicates: Effect of Calcination in Dry Air

The aromatization of propane at 530°C on three MFI gallosilicates (Si/Ga = 30, 50 and 180) and on a MFI aluminosilicate (Si/Al = 30) was investigated. After pretreatment at 530°C under dry air flow, 30 Al MFI was seven times more active than 30 Ga MN. This can be related to the greater strength of the acid sites of the aluminosilicate sample, which was shown by a calorimetric study of ammonia adsorption and by ammonia thermodesorption. On all the catalysts propene and methane + ethene in quasi-equimolar amounts appeared as primary products, whereas ethane, butenes, butanes, C + 5 olefinic and naphthenic compounds, and C 6-C 8 aromatics were secondary products. However, large differences were found in the product distribution, namely a lower initial cracking/ dehydrogenation ratio on 30 Ga MFI, greater selectivity to aromatics, and lower selectivity to alkanes (C 1, C 2, C 4) at high conversion, which can be explained by a dehydrogenating effect of gallium. The product distribution depended also on the Si/Ga ratio of the gallosilicate, a better selectivity to aromatics being found with 30 Ga MFI. The selectivity differences are mainly due to differences in the acid site density. The treatment of gallosilicates at 700-800°C under dry air flow caused a significant increase in their activity and in their selectivity for propane aromatization. This could be related to the formation of a small amount of extraframework gallium species, well dispersed in the crystallites and highly active for dehydrogenation of propane and of naphthene intermediates. On these catalysts propane aromatization occurred mainly through a bifunctional scheme and the formation of unwanted C 1-C 2 products, through acid cracking was very limited. The effect of the pretreatment temperature on the acid and dehydrogenating activity of the 30 Ga MFI sample showed that framework gallium species were much less active for dehydrogenation than extraframework species and indeed are probably inactive.

Analytical method for the determination of intra- and extraframework gallium species in zeolite catalyst

The aim of this work is to report a very simple analytical method for the quantitative determination of intra- and extralattice gallium species in zeolitic material using flame atomic absorption spectroscopy. The method was developed by adjusting the sample treatment severity. Whereas a HF treatment of the zeolite sample provides the total concentration of gallium, a controlled digestion with HCl allows one to detect only the nonframework species of this element. Furthermore, the HCl digestion does not modify the zeolite structure. The application of this digestion scheme to different zeolitic support with gallium in framework and nonframework position produced results that were in good aggreement with the values expected from the synthesis and preparation of these materials.