Elementary Steps in the Formation of Hydrocarbons from Surface Methoxy Groups in HZSM-5 Seen by Synchrotron Infrared Microspectroscopy

Abstract

Synchrotron infrared microspectroscopy has identified with high temporal resolution (down to 0.25 s) the initial events occurring when methanol vapor is in contact with a crystal of zeolite HZSM-5. The first alkenes are generated directly from methoxy groups formed at the acid sites via their deprotonation. These alkenes can either desorb directly or oligomerize and cyclize to form dimethylcyclopentenyl cations. The oligomeric and dimethylcyclopentenyl cations are the first major components of the hydrocarbon pool that precede aromatic hydrocarbons and lead to indirect alkene formation. The technique observes these events in real time.

Methanol loading dependent methoxylation in zeolite H-ZSM-5

“…3B . It is clear that the bands assignable to methoxy species emerge at 2980 and 2876 cm –1 , and 2973 and 2868 cm –1 , 8 , 23 , 27 , 30 which imply the occurrence of at least two types of methoxy species. This observation is consistent with our earlier INS and DRIFTS data that show two types of methoxy species 29 , 30 and is in line with earlier infrared and NMR reports.…”

“…The direct role of methoxy species in the rst C-C bond formation in the MTH reaction at 300 C was recently reported by some of us. 23 Moreover, the methylation (which is also known as alkylation) has an impact on a spectrum of petro-and nechemical processes. [18][19][20][21][22][23][24][25][26] Studies based on solid state nuclear magnetic resonance (NMR) and infrared (IR) conclude that methoxylation occurs only at elevated temperatures (>150 C).…”

“…23 Moreover, the methylation (which is also known as alkylation) has an impact on a spectrum of petro-and nechemical processes. [18][19][20][21][22][23][24][25][26] Studies based on solid state nuclear magnetic resonance (NMR) and infrared (IR) conclude that methoxylation occurs only at elevated temperatures (>150 C). 22,27,28 In line with these observations, several computational studies, usually examining congurations with only one methanol molecule per unit cell (i.e.…”

“…The direct role of methoxy species in the first C–C bond formation in the MTH reaction at 300 °C was recently reported by some of us. 23 Moreover, the methylation (which is also known as alkylation) has an impact on a spectrum of petro- and fine- chemical processes. 18 – 26 …”

Effect of steam de-alumination on the interactions of propene with H-ZSM-5 zeolites

“…[5][6][7] Propene can be produced from alternative sources through the methanol-to-olens reaction, 8,9 and may play an important role as a key intermediate in the formation of the initial C-C bonds and the hydrocarbon pool in the methanol-to-hydrocarbons (MTH) process. 10 All of these applications make use of ZSM-5 type acid zeolites as catalysts for the hydrocarbon conversion reactions. 11,12 Acid zeolites are microporous aluminosilicate materials with internal acid sites, with each acid group offsetting the charge from and being associated with a Si / Al substitution within the overall tetrahedral SiO 2 framework.…”