IR Spectral Investigation of the Multistage Single Catalytic Process of CH₄ Conversion into Higher Hydrocarbons over HZSM-5 Structurally Modified with an Oxidative Element

Abstract

The acidic zeolite HZSM-5 is modified by partial isomorphous substitution of the framework aluminum (Al) with an oxidative element during the zeolite synthesis. This is to develop an oxidative-acidic mediated catalyst suitable for the direct conversion of methane (or natural gas) into higher hydrocarbons in a multistage single catalytic process. The pure and modified ZSM-5 materials are highly crystalline of high morphology, which is shown by XRD and SEM. Pure ZSM-5 shows a single decomposition TGA band for the TPA+ cations associated with the anionic framework Al. Besides, modified ZSM-5 shows a lower temperature band revealing weaker interaction of TPA+ cations with the modifier element. The infrared spectrum of modified ZSM-5 shows a high frequency 3685 cm-1 band assigned to Bronsted sites weakly interacting with the modifier in addition and compared to the well-established more acidic Al-associated Bronsted sites absorbing at 3610 cm-1. Thus, XRD, TGA, and IR show evidence of incorporating the modifier element in the framework. The modifier site catalyzes the CH4 oxidation into H2CO using molecular O2 near 1 bar and 373 K. This is explicit in the spectral appearance of υC  O at 1711 cm-1 and δC - H at 1420 and 1375 cm-1. This stage of the catalytic conversion is so peculiar to modified ZSM-5 that can never be catalyzed at any circumstance over ordinary ZSM-5. Bronsted sites associated with Al and modifier element interact strongly with H2CO yielding unsaturated species defined by the weak υC  C at 1675 cm-1. Evidence of such interaction is shown as a downward shift of 23 cm-1 in the υC  O of the gaseous H2CO at 1734 cm-1 to 1711 cm-1 on the surface and absence of the Bronsted sites absorbing at 3610 and 3685 cm-1. In effect, the latter Bronsted bands recover at the end of the catalytic process. The unsaturated species are higher aldehydes equilibrated in a keto−enol tautomeric structure. They eventually decompose in the temperature 473−573 K range into a mixture of hydrocarbons identified by υC - H at 2960, 2935, 2913, and 2868 cm-1 and δC - H at 1469 and 1385 cm-1 (aliphatics) and by υring at 1600 and 1510 cm-1 (aromatics). The IR spectral profile of the products is similar to that of Mobil methanol-to-gasoline MTG process. The mechanistic implications of this process are proposed on the basis of the IR spectral findings observed in the reaction gas phase and on the surface that shows no evidence for the MTG route, however.

(1H AND31P)-NMR, UV, AND FT-IR SPECTRAL ANALYSES FOR SYMMETRICTRIS-PHTHAL ANDHEXA-SALICYL DERIVATIVES OF CYCLOTRIPHOSPHAZATRIENE

“…Water drives boron out of the zeolite rendering the modified catalyst eventually as original. [15] The previous efforts correlating the spectral and structural features [1715] have continued in the present work to shed light on diverse molecular structures of cyclotriphosphazatrienes that have been achieved upon substitution with similar aldehydes of rather different drives. [10] Variable temperature Ft-ir spectroscopy afforded key structural information of importance that helped in comprehending long debated low activity of protons, albeit are highly acidic.…”