Mechanism of Dehydrocyclization of 1-Hexene to Benzene on Cu₃Pt(111)

Abstract

Even though dehydrocyclization is widely practiced in heterogeneous catalysis for the conversion of straight chain hydrocarbons into aromatic compounds, knowledge of the mechanism of this process remains limited, largely because it has not previously been possible to carry out the reaction under conditions amenable to detailed mechanistic studies. We report here ultrahigh vacuum studies of the dehydrocyclization of submonolayer coverages of 1-hexene to benzene on a Cu3Pt(111) single-crystal surface. On the basis of temperature-programmed reaction/desorption (TPR/D) studies of dehydrocyclization of 1-hexene as compared to the reactions of cyclohexene, 1,3-cyclohexadiene, 1,4-cyclohexadiene, benzene, 1,3-hexadiene, and 1,3,5-hexatriene with a Cu3Pt(111) surface, it is found that a rate-determining step in the overall reaction is cyclization. The obtained results show that at low coverages of mono- and bi-unsaturated cyclic compounds, benzene is the only gas-phase hydrocarbon product of reaction of these compounds with a Cu3Pt(111) surface, while after a certain threshold in coverage, molecular desorption of these compounds commences. The temperature of benzene evolution for all the cyclic compounds studied is between 200 and 300 K, whereas for linear chain hydrocarbons this temperature is ∼400 K. TPR/D studies of product hydrogen evolution show that all cyclic compounds evolve hydrogen upon reaction with the surface at the temperatures close to that of hydrogen recombination−desorption reaction, 220−290 K. On the other hand, 1-hexene evolves hydrogen upon reaction with this surface at two different temperatures: ∼270 and ∼405 K. Combining these results with the studies of hydrogen evolution from 1,3,5-hexatriene, which occurs at 400 K, we suggest that cyclization is a rate-determining step and that mono- and bi-unsaturated C6 cyclic compounds are not the reaction intermediates for the dehydrocyclization of 1-hexene to benzene.

Dehydrocyclization of peripheral alkyl groups in porphyrins at Cu(100) and Ag(111) surfaces

“…2B) of the Co-PP which are from porphyrin ring bending, breathing, and C-H wagging. AES C, XPS C 1s, and XPS N 1s analysis before and after annealing show less than 5% loss of signal, so no significant desorption occurred at 500 K. It is likely that the dehydrocyclization mechanism here is similar to that derived for hexenes on a Cu 3 Pt surface [25,30]. The first dehydrogenation on the ethyl group is likely at the more acidic allylic α carbon hydrogen.…”

“…The temperature of dehydrocyclization on Ag(111) at 600 K is higher than measurements on the (111) surfaces of metals in the same group; dehydrocyclization has been reported for Fe-OEP on Cu(111) at 430 K [59] and for H 2 -OEP on Au(111) at 530 K [58]. Dehydrocyclization on each of those surfaces occurs at significantly higher temperatures than on Pt(111) (300 K) [31] or on Cu 3 Pt(111) (393 K) [26,29,30]. for Pt-OEP / Cu(100),and (Fig.…”

“…Dehydrocyclization reactions, a subset of dehydrogenation reactions that involve the further step of ring formation, are of high interest for the oil refining industry as a method to increase the octane number of fuels [18]. Dehydrocyclization requires high temperature (873 K for 1,3-pentadiene to cyclopentadiene [19]) or a catalyst, such as Ir or Ru complexes [20,21]; zeolite materials loaded with Co, Pt, Cu, Zn, Ga, or In metals [22][23][24]; or on Rh, Ir, Ni, Pd, Pt, Cu 3 Pt, or W carbide surfaces [25][26][27][28][29][30][31][32][33][34][35][36]. Most of the dehydrocyclization surface studies rely on surfaces that can catalyze the reaction at a temperature below thermal desorption [37][38][39].…”

Controlling Hydrogenation Selectivity by Size: 3-Hexyne on Supported Pt Clusters

“…Our previous work has studied the thermal chemistry of 3-and 1-hexyne on Pt(111) in detail, concluding that cyclization follows several dehydrogenation steps for both molecules similar to a copper platinum alloy. 35,36 Furthermore, this dehydrocyclization path is also open for the corresponding hexenes regardless of the initial stereochemistry. The dehydrogenation mechanism followed by ring-closure seems to be applicable to platinum clusters, as dehydrogenation, as observed by H 2 -hexene desorption in Figure 2, has vanished below 400 K, which is significantly lower than benzene desorption.…”

Effect of hydrogen pressure on intentional deactivation of unsupported Pt catalyst: Catalytic properties and physical characterization

“…Using ethylene reactant, C 4 and C 6 products appeared in the gas phase, indicating commencing polymerization [8]. ''Polyene'' and ''C 1 '' pathways of carbonization have been reported: the former would involve polymerization of trans-polyenes produced parallel with cispolyenes, being intermediates of aromatization [9,10]. The C 1 pathway involves formation of C-C bonds between CH x fragments [11,12].…”