Hydronium-Ion-Catalyzed Elimination Pathways of Substituted Cyclohexanols in Zeolite H-ZSM5

Abstract

Hydronium ions in the pores of zeolite H-ZSM5 show high catalytic activity in the elimination of water from cyclohexanol in aqueous phase. Substitution induces subtle changes in rates and reaction pathways, which are concluded to be related to steric effects. Exploring the reaction pathways of 2-, 3-, and 4-methylcyclohexanol (2-McyOH, 3-McyOH, and 4-McyOH), 2-and 4-ethylcyclohexanol (2-EcyOH and 4-EcyOH), 2-n-propylcyclohexanol (2-PcyOH), and cyclohexanol (CyOH) it is shown that the E2 character increases with closer positioning of the alkyl and hydroxyl groups. Thus, 4-McyOH dehydration proceeds via an E1-type elimination, while cis-2-McyOH preferentially reacts via an E2 pathway. The entropy of activation decreased with increasing alkyl chain length (ca. 20 J mol −1 K −1 per CH 2 unit) for 2-substituted alcohols, which is concluded to result from constraints influencing the configurational entropy of the transition states.

Influence of Intracrystalline Ionic Strength in MFI Zeolites on Aqueous Phase Dehydration of Methylcyclohexanols

“…In contrast, the concerted E2 mechanism was concluded to dominate in the dehydration of cis ‐2‐McyOH, which almost exclusively resulted in the formation of the Saytzeff‐product 1‐methylcyclohexene (1‐MCH) (Scheme 1). [13, 15] The cis isomer shows a 30 kJ mol −1 lower activation barrier than the trans isomer and, therefore, is converted preferentially in a racemic mixture of both isomers. The reaction order in 2‐ and 4‐McyOH was zero for MFI zeolites; consequently, it is assumed that the measured activation parameters are representing intrinsic values, i.e., the energy difference between transition state and sorbed substrate (Supporting Information, Figures S1,S2 and Tables S2,S3) [13] …”

Influence of Intracrystalline Ionic Strength in MFI Zeolites on Aqueous Phase Dehydration of Methylcyclohexanols

“…Two isomers of methylcyclohexanol were chosen for the study, i.e., 4-methylcyclohexanol (4-McyOH) and cis-2-methylcyclohexanol (cis-2-McyOH). The former is shown to dehydrate via an E1 mechanism and can access to all micropore space in MFI channels, while the latter dehydrates via an E2 mechanism [13] and can only access part of the MFI micropore space due to its bulkiness. The comparison of the dehydration rates of the two substrates catalyzed by a series of MFI zeolites with varying BAS concentrations allows to address the role of steric challenges and of the nature of the transition state.…”

“…Previous investigations have established that trans-2-McyOH and 4-McyOH (cis/trans) dehydrate via carbenium ion intermediates following an E1 mechanism. [13] 4-McyOH is thereby predominately dehydrated to 4-methylcyclohexene (4-MCH), which represents the Hofmann-product (Scheme 1). [13,14] In contrast, the concerted E2 mechanism was concluded to dominate in the dehydration of cis-2-McyOH, which almost exclusively resulted in the formation of the Saytzeff-product 1-methylcyclohexene (1-MCH) (Scheme 1).…”

“…[13] 4-McyOH is thereby predominately dehydrated to 4-methylcyclohexene (4-MCH), which represents the Hofmann-product (Scheme 1). [13,14] In contrast, the concerted E2 mechanism was concluded to dominate in the dehydration of cis-2-McyOH, which almost exclusively resulted in the formation of the Saytzeff-product 1-methylcyclohexene (1-MCH) (Scheme 1). [13,15] The cis isomer shows a 30 kJ mol À1 lower activation barrier than the trans isomer and, therefore, is converted preferentially in a racemic mixture of both isomers.…”

“…[13,14] In contrast, the concerted E2 mechanism was concluded to dominate in the dehydration of cis-2-McyOH, which almost exclusively resulted in the formation of the Saytzeff-product 1-methylcyclohexene (1-MCH) (Scheme 1). [13,15] The cis isomer shows a 30 kJ mol À1 lower activation barrier than the trans isomer and, therefore, is converted preferentially in a racemic mixture of both isomers. The reaction order in 2and 4-McyOH was zero for MFI zeolites; consequently, it is assumed that the measured activation parameters are representing intrinsic values, i.e., the energy difference between transition state and sorbed substrate (Supporting Information, Figures S1,S2 and Tables S2,S3).…”