Isomerization of Meta‐Xylene Over Pentasil‐Type Microporous Gallosilicates

Abstract

The isomerization of meta-xylene under normal pressure was studied in the temperature range 570-770 K on three catalyst series, oiz. (i) gallosilicates with a constant Si/Ga ratio of 50, but different synthesis times ranging from 5 to 96 h, (ii) gallosilicates with a higher (100) and lower (25) Si/Ga ratio after 48 h and 96 h crystallization time, and (iii) a series that included a ZSM-5 zeolite, a gallosilicate, and a ferrisilicate, all with comparable concentrations of Brernsted acid sites. The characterization of the catalysts is based on XRD, electron microscopy (TEM, SEM), ammonia thermodesorption, thermoanalysis and n-hexane adsorption. Synthesis products that were characterized by X-ray diffraction to have 100% crystallinity, aquire further acidity when the synthesis time is prolonged. This effect is due to an additional insertion of galtium in framework positions from a reservoir of nonfrarnework gallium species. -The isomerization on gallosilicates proceeds with negligible disproportionation to toluene. The activity is intermediate, compared to the ZSM-5 zeolite and to the ferrisilicate. The shape selectivity of the gallosilicates is inversely related to the crystallinity. -The kinetics of the isomerization can be described by a parallel two step model, in good agreement with experimental results. The significance of internal mass transport effects is discussed.Sample C2 is identical with sample A6. Sample B3 is identical with sample A6.' ' Sample B4 is identical with sample A7. Ber. Bunsetiyes. Phys. Chem. 96 ( I 992) No. 4 Ber. Burisunyr.~. Phys. Chnri. 96 / 1992) No. 4 I'CH Vr~ltrys~/esr//schrI/i mhH, W-6940 Weiriheim. lYY2 0005-902ljU2J0404-O~Y7 $ 3.50-k .25jO

Incorporation of Gallium into Zeolites: Syntheses, Properties and Catalytic Application

“…For this reaction, relative activity is linearly correlated to the strength and concentration of Brønsted acid sites, at least for zeolite ZSM-5. A similar trend was found for the m -xylene conversion over H[Ga]-ZSM-5 obtained after different crystallization times up to 24 h, the borderline value of Ga insertion into the framework (Figure ) 44 Conversion of m- xylene over [Ga]-ZSM-5 at 623 K (×), 673 K (□), and 723 K (○) vs crystallization time.…”

“…Reaction conditions: sample weight 1 g, flow rate 10 L/h (1 vol % aromatics). (Reprinted with permission from ref . Copyright 1992 Wiley-VCH.…”

“…They form columns of the stacking sequence ABAB... which are interconnected by oxygen bridges between the cancrinite units. Thereby supercages (gmelinite units) of 6.3−6.6 Å diameter and 7.39 Å length with oxygen-8-ring openings of 4−5 Å diameter are formed 54 Framework structure of offretite (OFF).…”

“…Steady values after 5 h time on stream. (Reprinted with permission from ref . Copyright 1992 Wiley-VCH.)…”

“…A similar trend was found for the m-xylene conversion over H [Ga]-ZSM-5 obtained after different crystallization times up to 24 h, the borderline value of Ga insertion into the framework (Figure 44). 242 In principle, other reactions are suited equally well provided that the molecular size of the reactant(s) is small enough to avoid diffusional limitations of the activity. Bifunctional conversion of n-decane following modification of the zeolite with noble metals aims at the elucidation of structural features of the zeolite besides its acid characteristics.…”