V. Kanazirev

Ga2O3/HZSM-5 propane aromatization catalysts: Formation of active centers via solid-state reaction

Abstract An active and very selective catalyst for propane conversion to aromatics near atmospheric pressure and 750–850 K is formed by ball-milling 2–10 wt% Ga as Ga 2 O 3 with HZSM-5 followed by hydrogen reduction at about 850 K for 2 h. Microbalance TPR experiments and X-ray diffraction studies along with catalytic testing have helped establish that the intimate physical mixture of Ga 2 O 3 with HZSM-5, the acidity of the zeolite, and the hydrogen reduction are all necessary to generate the highly active form of the catalyst. The active species is probably Ga(I) as a zeolitic cation and is not incorporated in the zeolite lattice.

Methanol conversion to light alkenes over SAPO-34 molecular sieves synthesized using various sources of silicon and aluminium

Depending on the various sources of Si and Al used for the synthesis, SAPO-34 materials with different degrees of crystallinity and density of acid sites, as shown by TPD of ammonia and TPD-TGA of 1-propanamine have been prepared. The effect of the different acidities on the activity, selectivity and lifetime of the catalysts in the reaction of methanol conversion to light alkenes has been studied.

The oxidation state of Ga in Ga/ZSM-5 light paraffin aromatization catalysts

Abstract Gallium nitrate-impregnated NaZSM-5 and HZSM-5 light paraffin aromatization catalysts have been reduced via hydrogen in a microbalance, and a disparity in the degree of reduction of gallium was determined. This result suggests that catalytic activity differences in the two materials may be associated with the degree of gallium reduction.

IR study of the active sites formed by H2 treatment of Ga/HZSM-5 catalysts

Abstract Results obtained by IR spectroscopy using carbon monoxide as a probe molecule provide evidence for the formation of a new Ga active site after the hydrogen treatment of Ga/HZSM-5 catalysts prepared by different techniques such as ball-milling, impregnation and substitution in the zeolite framework. A band at 2155 cm −1 arising after CO adsorption on reduced samples is assigned to a Ga + CO complex localized mainly in the interior of the zeolite crystallites.

X.p.s. study on the effect of hydrogen treatment on the state of gallium in Ga2O3/HZSM-5 mechanically mixed catalysts

Abstract Mechanically mixed Ga2O3/HZSM-5 catalysts have been examined via X.p.s. and X-ray diffraction both before and after reduction with H2. Results are consistent with the postulate that there is a strong interaction of Ga2O3 with HZSM-5, which results in a transfer of Ga into the zeolite upon reduction.

Gallium-loaded zeolites for light paraffin aromatization : evidence for exchanged gallium cation active centers

Abstract It is shown that certain metal-loaded zeolites (e.g., Ga-MFI) initially prepared as separate metal-containing and zeolite phases are reducible, resulting in solid-state transfer of the metal to the zeolite. The final material is similar to those initially prepared as ‘unsegregated’ by methods such as aqueous impregnation. Reduction occurs either through appropriate pretreatment or onstream in certain hydrocarbon reactions. First, preparation methods leading to one or the other type of material are examined. Then it is shown that reduced catalysts give far higher ratios of dehydrogenation to cracking rates (for alkanes) than either segregated or unsegregated but unreduced systems. The product distributions of the propane reaction at low partial pressure and low conversion are also reviewed, and new data presented for unsegregated, reduced catalysts. There are great differences in the distribution obtained using reduced 1/1 metal/A1 catalysts from corresponding distributions for the H-form zeolites or for segregated systems. The differences suggest a mechanism which may be entirely independent of catalysis by protons.

Thermal analysis of adsorbed propanamines for the characterization of GaMFI zeolites

Gallium-containing forms of MFI zeolite have been subjected to adsorption of 1- and 2-propanamine, and the results of subsequent thermal treatment have been analyzed and compared to literature reports for propanamines adsorbed on proton forms of MFI. Zeolitic gallium cations show completely different behavior from zeolitic protons, including a propanamine decomposition mechanism which is different from Hofmann elimination. In this mechanism NH3 is desorbed at temperatures as much as 160 K below that of HMFI. Major differences in the decomposition chemistry of the two amines were also observed when Ga cations were present.

Thermochemical and acidic properties of the zeolites offretite, omega and ZSM—5

Abstract The thermochemical and acidic properties of offretite, omega and ZSM—5 have been compared by thermal and X-ray analysis, infrared spectroscopy and temperatureprogrammed desorption of ammonia. It has been established that the temperature range of the decomposition of the organic cation depends on the zeolite structure, and is wider in the case of offretite. The decomposition of the NH 4 -forms of the three types of zeolites proceeds at higher temperatures than in the case of NH 4 -Y and also for the decomposition of the organic cation. The appearance of a high-temperature part in the t.p.d. curves of the samples is attributed to their stronger acidic properties after deammoniation. With respect to the strength of ammonia adsorption at 573 K, the decationized forms of the zeolites showed the sequence omega ∼ offretite>ZSM—5.

Temperature programmed desorption studies on the penetration of ammonia into the sodalite cages of A, X and Y type zeolites

The adsorption of ammonia on sodium forms of A, X and Y type synthetic zeolites and mordenite are examined in the 423–573 K temperature interval in a flow apparatus using helium as carrier gas. Diffusion controlled penetration of ammonia into the sodalite cage, most marked for Y type zeolites, is found to take place by temperature programmed desorption up to 823 K. It is shown that the accessibility of the sodalite cages of faujasite type zeolites to ammonia strongly diminishes on increasing the Si/Al ratio. The assumption is made that this effect is due to small structural changes undergone by the zeolite. It is not excluded that these changes could also affect the Na + location.

Propane conversion on Cu-MFI zeolites

Abstract A highly exchanged copper-containing MFI zeolite can be prepared by thermal treatment of CuO H-MFI mechanical mixtures at 973 K in an O2 free environment. This material effectively catalyzes aromatization reactions of light paraffins and olefins. An unusually high selectivity for benzene is observed during propane conversion. The preparation technique appears to allow replacement of virtually all zeolitic protons with Cu+ ions. The high temperatures and presence of H-MFI favor the conversion of Cu2+ to Cu+ so the process is termed ‘reductive solid-state ion-exchange’. The participation of Cu+ cations in the aromatization mechanism is discussed.