Introduction of Zn, Ga, and Fe into HZSM-5 Cavities by Sublimation:  Identification of Acid Sites

Abstract

M/ZSM-5 zeolites with M = Ga, Fe, or Zn were prepared by subliming volatile compounds onto HZSM-5. They were characterized by XRD, H2-TPR, CO-TPR, FTIR, adsorption of pyridine and acetonitrile-d 3 and, in the case of Fe/ZSM-5, also by ESR spectroscopy. Immediately after sublimation, the concentration of Brønsted acid sites is low, but with Ga/ZSM-5 and Fe/ZSM-5, it rises again after hydrolysis. This dissipation of multipositive ions to monopositive ions, each located near the negative countercharge in the zeolite matrix, is favored by a considerable gain in Coulomb energy. In Ga/ZSM-5, some of the regenerated Brønsted sites are stronger than those in the original HZSM-5. In Fe/ZSM-5 and Zn/ZSM-5, such Brønsted sites are weaker, as evidenced by CD3CN adsorption. However strong Lewis acid sites are formed, they are characterized by NH3-TPD, and pyridine and CD3CN adsorption. Zeolites with a high Si/Al ratio contain lattice defects with internal silanol groups. These defects disappear when metal ions are added by sublimation, and some of the ions end up in distorted tetrahedra carrying their own Brønsted acid sites. For Ga/SM-5 and Fe/ZSM-5, FTIR and TPR evidence shows the presence of at least three kinds of species: (i) (GaO)+ and binuclear Fe3+ ions located in cationic exchange positions and contributing to the perturbation of T−O−T vibrations; (ii) neutral clusters of Ga2O3 and Fe2O3; (iii) Ga3+ or Fe3+ ions in tetrahedral coordination. In the case of Fe/ZSM-5, the presence of binuclear ions with bridging oxygen is also identified by ESR spectroscopy. In contrast to Fe and Ga, Zn seems to be stable in cationic exchange positions either compensating the charge of two Al-centered tetrahedra or ligated to one internal silanol or OH group.

Nitrous Oxide Decomposition on the Binuclear [FeII(μ-O)(μ-OH)FeII] Center in Fe-ZSM-5 Zeolite

“…Entropic values include the contributions of the transitional, rotational, vibrational, and electronic partition function for zeolite cluster and for all gaseous species involved in the reaction. 29,57,58 and similar to the so-called "diamond core" structure of the active site in the MMO enzyme. 59 Prins and co-workers 25 have proposed various shell models consisting of different back scattering pairs fitting the EXAFS spectrum of iron sites in ZSM-5 zeolite.…”

“…According to numerous experimental studies on the nature of iron sites in Fe-ZSM-5 regarding the zeolite preparation/ activation steps, 25,29,41 it has been concluded that iron first exists as isolated Fe species with tetrahedral coordination, which upon subsequent steaming and calcination are transformed into hydroxylated binuclear complexes. 25,33 In this structure, iron cations are in the oxidation state Fe III and are completely inactive for the N 2 O decomposition.…”

“…The formation and high activity of oxygen-bridged binuclear species were suggested by numerous experimental studies, [25][26][27][28][29] but only few theoretical approaches have been reported in the literature. Investigations of binuclear iron sites in the ZSM-5 extra-framework position have considered the reactivity of a In contrast with these theoretical results, Kiwi-Minsker and co-workers 5 have reported a different N 2 O kinetic behavior "suggesting that the high activity of HZSM-5 with a high Si/ Al ratio and low iron content could be explained by the presence of binuclear iron sites".…”

Silicalite-1 zeolite acidification by zinc modification and its catalytic properties for isobutane conversion

“…5B, in addition to the apparent bands in the range 3350-3200 cm À1 , which were attributed to ammonia coordination on Lewis acid sites, two weak yet clear absorption peaks at 3220 and 3190 cm À1 emerged in the N-H stretching region. 26,28 It was clear that Zn modication not only produced a number of Lewis [14][15][16]30,31 can provide Lewis acid sites in Znmodied ZSM-5 or beta zeolite. Similarly, we speculated that the Lewis acid sites in Zn x /S-1 were also associated with their Zn species.…”

Benzene hydroxylation over FeZSM-5 catalysts: which Fe sites are active?

“…The catalytic performance is due to iron in the zeolites [5][6][7][8][9]. Different forms of iron have been identified in the FeZSM-5 zeolites [10][11][12][13][14][15][16]: (1) isolated ions isomorphously substituted in the MFI framework; (2) ions in cation-exchange positions; (3) iron complexes of low nuclearity in extraframework positions; (4) Fe oxide nanoparticles of < 2 nm size in the micropores; and (5) large Fe oxide particles (2-25 nm) located at the external surface of zeolite crystals. However, only a small part of the total iron is usually active in the hydroxylation of benzene [5,7,8].…”