Reaction Mechanism for Direct Cyclization of Linear C₅, C₆, and C₇ Alkenes over H‐ITQ‐13 Zeolite Investigated Using Density Functional Theory

Abstract

Although dienes or trienes have been shown to be possible precursors for cyclization, direct cyclization of alkenes or alkoxides has not been systematically studied yet. Thus, the reaction mechanism of cyclization of linear alkenes over H-ITQ-13 was investigated here by density functional theory considering dispersive interactions (DFT-D). The similar free energy of different linear alkoxides of the same carbon number suggests that they can co-exist in the H-ITQ-13 intersection at 673.15 K during the methanol to olefins (MTO) process. The formation of linear alkenes by olefins methylation with methoxyl groups (ZOCH ), trimethyloxonium ions (TMO ), and methanol are kinetically more favorable than by dimerization of olefins. Linear alkoxides or alkenes prefer direct cyclization to cycloalkanes rather than hydride transfer to diene. This study provides new insight into the alkene cyclization and aromatization mechanisms in MTO process.

Aromatics Production via Methanol-Mediated Transformation Routes

“…This was consistent with experimental observations where cyclization occurs predominantly for C 8+ aliphatics . The cyclization of C 5 –C 7 liner alkenes was investigated by Fan et al over a H-ITQ-13 zeolites . Compared to the dimerization of light olefins, the formation of linear alkenes by olefins methylation was kinetically more favorable.…”

“…As illustrated in Figure , the cyclization may occur at various reaction steps, either prior to dehydrogenation or after that, and sometimes it is difficult to identify the real sequence via experimental approaches. , Alternatively, theoretical computational studies are able to provide more fruitful insights into the cyclization process. Joshi and Thomson have investigated the cyclization of hexadiene through 1,5-cyclization and 1,6-cyclization pathways on H-ZSM-5 (Figure g) .…”

“…100 The cyclization of C 5 −C 7 liner alkenes was investigated by Fan et al over a H-ITQ-13 zeolites. 184 Compared to the dimerization of light olefins, the Disclosing the nature of species that undergo cyclization is essential for understanding the formation of aromatics. Using a combination of Kerr-gated Raman spectroscopy and molecular simulations, Beale and co-workers recently further identified that polyenes are strongly related to the aromatics formation and deactivation of MTH catalysts.…”

Dimerization of Linear Butenes and Pentenes in an Acidic Zeolite (H‐MFI)

“…There may be other alkene and alkane isomers formed, but this will not change the difference between alkene and cyclic alkane formation and, hence, will not change the conclusion. Cyclization of linear alkenes to (alkyl‐substituted) cyclopentanes and ‐hexanes has been studied by DFT as part of the hydrocarbon pool mechanism [16] . Future mechanistic studies should consider a two‐step mechanism in which formation of the C 8 or C 10 alkene at the BAS is followed by cyclization as sketched in the Supporting Information.…”

H2‐free Synthesis of Aromatic, Cyclic and Linear Oxygenates from CO2

“…The cyclisation of surface-adsorbed carbonaceous species has also been identified as ak ey step in the formation of coke deposits on zeolites. [63] The tendency to form larger-carbon-number rings in HY compared with H-ZSM-5 may be ascribed to the largerp ore size of the former.D etailed theoretical studies of hydrocarbon oligomerisation-cyclisation reactions over zeolite catalysts have previously provided insights into the reactionm echanism, [64][65][66] with hydrocarbonsp roposed to adsorb at framework oxygen speciesi nt he zeolite structure. Subsequent dehydrogenation of the cyclic products through ah ydride transfer mechanism can yield aromatic products such as those observed in this work under alternative reaction conditions.…”