For the first time in ethanol conversion, steady state experimental observations have been complemented with UV/VIS catalyst characterization after reaction and Temporal Analysis of Products. This combination provides crucial information on the reaction mechanism and the active species during the reaction.
In the course of the past decades, alcohol conversion on zeolites focused primarily on methanol as feed. Recently, the interest in bio-ethanol as a versatile platform molecule for the production has increased. While its direct use as a engine fuel requires an energy intensive water removal, aqueous ethanol may present a direct starting basis for the production of olefins such as ethylene and propylene. The reaction mechanism of its conversion to hydrocarbons is still a matter of debate [1-3], particularly with respect to the role of water and ethylene.
The present work focuses on reaction mechanism elucidation for ethanol conversion on HZSM-5 (Si/Al = 40) between 573 and 623 K. Site times below 1 molH+ s mol-1 resulted in both ethylene and diethylether as dehydration products, indicating the importance of monomolecular as well as bimolecular dehydration. Complete ethanol conversion is observed at higher site times with full selectivity towards ethylene because of the thermodynamic dehydration equilibrium, which is in favor of ethylene.
Comparative catalytic experiments between ethylene and ethanol showed a higher C2-conversion, XC2, the latter defined as the combined conversion of ethanol and ethylene to C3+ hydrocarbons, when feeding ethylene compared to ethanol at identical site times. When feeding ethanol, a part of the catalyst bed and, hence, the site time, is used for ethanol dehydration. This part becomes smaller with increasing temperature due to the correspondingly higher ethanol dehydration rate. The production rate of C3+ hydrocarbons, however, is not affected by water. At an identical C2-conversion (XC2 = 10 and 30 %), no differences in product distribution are observed between ethanol and ethylene as feed. The conversion of aqueous ethanol (20 wt.% water) exhibited no activity nor selectivity differences compared to pure ethanol when care was taken to have an identical ethanol partial pressure.
The selectivity to olefins at the investigated reaction conditions exceeds 90 % which indicates only a minor occurrence of aromatization and hydride transfer reactions. Thermodynamic equilibrium was found to be established between double bound and branched C4 and C5 olefin isomers. This indicates that for olefinic species the isomerization and branching reactions occur on a smaller timescale than the alkylation and cracking reactions . Important to note is also the significant amount of olefins with an odd-carbon number (C3, C5 and C7). A delplot analysis  showed propylene as a primary product during C3+ hydrocarbon production which is unexpected using ethanol and indicate the existence of a secondary mechanism.
An induction period prior to the production of C3+ hydrocarbons has been observed for the first time in ethylene conversion. It is indicative of a kinetically significant role in the reaction mechanism of other species inside the catalyst pores formed during the reaction, in addition to acid catalyzed elementary steps such as alkylation and cracking. This autocatalytic step was further explored using UV-VIS and Temporal Analysis of Products.
Three different regions have been identified in the catalyst bed during ethanol conversion: a dehydration, an induction and a C3+ production region. These regions could be visually observed by zones in the catalyst bed exhibiting color differences ranging from white over yellow to gray and quantified using UV-VIS spectroscopy . Initially no color differences are observed within the catalyst bed which corresponds to dehydration being the only reaction. It is followed by an increase in intensity in the region of 400 nm which is attributed to the formation of polysubstituted benzene rings and linked to the induction period. Finally an additional increase occurs in the region between 500 and 600 nm, which is associated with polyaromatic species, that could be located on the external surface.
Performing multi-pulse experiments at 623 K with the Temporal Analysis of Products setup , more information related to the induction period was achieved. Initially C4 and C6 hydrocarbons were formed, followed by the production of aromatics. Isotopic labeling experiments with alternating pulses of 12C2H4 and 13C2H4 show a scrambled product distribution with time delays smaller than 0.01 s. At 1 s time delay between pulses, this scrambling is no longer observed. This indicates the formation of short-life time species in the pores formed during the induction period which could participate in the production of C3+ hydrocarbons.
Using this combined approach, it can be concluded that ethanol and ethylene conversion to C3+ hydrocarbons proceeds according to an autocatalytic reaction mechanism consisting of acid catalyzed steps, such as alkylation and cracking, etc. complemented by a secondary mechanism involving short-life time species accumulated during the induction period which lies at the origin of the formation of propylene as a primary product during C3+ hydrocarbon formation.
This work was supported by the ‘Long Term Structural Methusalem Funding by the Flemish Government’, the Interuniversity Attraction Poles Programme (P7/K5) - Belgian State – Belgian Science Policy and the Fund for Scientific Research Flanders (FWO).
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