Abstract
Formaldehyde is an important intermediate product in the catalytic conversion of methanol to olefins (MTO). Here we show that formaldehyde is present during MTO with an average concentration of ~0.2 C% across the ZSM-5 catalyst bed up to a MeOH conversion of 70%. It condenses with acetic acid or methyl acetate, the carbonylation product of MeOH and DME, into unsaturated carboxylate or carboxylic acid, which decarboxylates into the first olefin. By tracing its reaction pathways of 13C-labeled formaldehyde, it is shown that formaldehyde reacts with alkenes via Prins reaction into dienes and finally to aromatics. Because its rate is one order of magnitude higher than that of hydrogen transfer between alkenes on ZSM-5, the Prins reaction is concluded to be the major reaction route from formaldehyde to produce dienes and aromatics. In consequence, formaldehyde increases the yield of ethene by enhancing the contribution of aromatic cycle.
Highlights
Formaldehyde is an important intermediate product in the catalytic conversion of methanol to olefins (MTO)
It is shown that by increasing the residence time the yield of HCHO increased to a yield maximum of 0.27 C% at ~20% conversion of MeOH, and it decreased gradually with higher conversions to levels below the detection limit. These results directly establish the presence of HCHO in H-ZSM-5 under MTO reaction conditions and its concentration evolution with the conversion of MeOH
To avoid the interference of products directly formed via MeOH routes, we examine these reactions by studying the reaction of 1-butene –chosen as representative of the olefin pool – with HCHO on HZSM-5
Summary
Formaldehyde is an important intermediate product in the catalytic conversion of methanol to olefins (MTO). We show that formaldehyde is present during MTO with an average concentration of ~0.2 C% across the ZSM-5 catalyst bed up to a MeOH conversion of 70%. It condenses with acetic acid or methyl acetate, the carbonylation product of MeOH and DME, into unsaturated carboxylate or carboxylic acid, which decarboxylates into the first olefin. On the basis of these results a methane-formaldehyde mechanism leading to first C–C bond was proposed by Tajima et al.[14] In spite of these first reports, formaldehyde in methanol conversion did not attract much attention until recently.
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