In Situ Observation of Stepwise C–H Bond Scission: Deciphering the Catalytic Selectivity of Ethylbenzene-to-Styrene Conversion on TiO₂

Abstract

The conversion of light alkanes to olefins is crucial to the chemical industry. The quest for improved catalytic performance for this conversion is motivated by current drawbacks including: expensive noble metal catalysts, poor conversion, low selectivity, and fast decay of efficiency. The in situ visualization of complex catalysis at the atomic level is therefore a major advance in the rational framework upon building the future catalysts. Herein, the catalytic C−H bond activations of ethylbenzene on TiO 2 (110)-(1 × 1) were explored with highresolution scanning tunneling microscopy and first-principles calculations. We report that the first C−H bond scission is a two-step process that can be triggered by either heat or ultraviolet light at 80 K, with near 100% selectivity of β-CH bond cleavage. This work provides fundamental understanding of C−H bonds cleavage of ethylbenzene on metal oxides, and it may promote the design of new catalysts for selective styrene production under mild conditions.

Low-Temperature C–H Bond Activation via Photocatalysis: Highly Efficient Ethylbenzene Dehydrogenation into Styrene on Rutile TiO2(110)

“…Correspondingly, a shoulder peak appears at ∼298 K. The relative ratio of the 298 K peak in the TPD traces of m / z = 78, 91, and 106 is the same with that of the 245 K peak, illustrating that the 298 K peak is also due to the desorption of C 6 H 5 C 2 H 5 . According to the recent work about toluene (C 6 H 5 CH 3 ) and C 6 H 5 C 2 H 5 photochemistry on R-TiO 2 (110), , the shoulder peak at 298 K can be attributed the desorption of C 6 H 5 C 2 H 5 via the recombination of dissociated C 6 H 5 C 2 H 4 groups and H atoms on the bridging oxygen sites (O b ). In addition, two other obvious desorption peaks appear at 553 K ( m / z = 18) and 458 K ( m / z = 51, 77, 78, 103 and 104) after UV irradiation.…”

“…The above dilemma is caused by the high endothermic C–H activation in C 6 H 5 C 2 H 5 dehydrogenation. Alternatively, photocatalysis has been emerging as a promising approach for the activation and selective conversion of hydrocarbons into various types of products under mild conditions. , Recently, the photocatalytic conversion of light alkanes, such as methane and ethane, has been achieved over TiO 2 -based catalysts under mild conditions. − Moreover, the activation of the C–H bond of C 6 H 5 C 2 H 5 on rutile­(R)-TiO 2 (110) under ultraviolet (UV) light has also been reported by Lin et al Although these works illustrate that TiO 2 can be used as a good photocatalytic material for photocatalytic C–H bond activation, high efficiency of C 6 H 5 CHCH 2 production via photocatalytic C 6 H 5 C 2 H 5 dehydrogenation under mild conditions has not been realized yet. Moreover, the fundamental mechanism of the photocatalytic process is also ambiguous.…”

“…Because C 6 H 5 C 2 H 5 is the only reactant on the surface, the 290 K peak is likely due to the C 6 H 5 CHCH 2 formation as well, which is attributed to C 6 H 5 CHCH 2 at the Ti 5c sites (C 6 H 5 CHCH 2(Ti) ) . Recently, Lin and co-workers reported that the initial C–H bond cleavage of C 6 H 5 C 2 H 5(Ti) on R-TiO 2 (110) can occur efficiently under UV irradiation, forming C 6 H 5 C 2 H 4 groups and H b . …”

“…However, the energy barrier of the second C–H bond cleavage is very high (∼1.20 eV), thus inhibiting the further dehydrogenation of C 6 H 5 C 2 H 4 groups under UV irradiation. Here, the two desorption peaks of C 6 H 5 CHCH 2 product at 290 and 458 K indicate that the second C–H bond cleavage of C 6 H 5 C 2 H 4 groups may occur via different pathways to produce C 6 H 5 CHCH 2 .…”

Photocatalytic C–H Bond Activation of Toluene on Rutile TiO2(110)

“…Furthermore, no observable peak appears in the TPD traces of m/z = 18 with/without UV irradiation, suggesting that no OH br exists on the R-TiO 2 (110) surface above 400 K, that is, the R-TiO 2 (110) surface is not hydroxylated above 400 K. 45 However, it is difficult to identify the origin of the toluene desorption at 310 K. Two reasons may account for the appearance of the 310 K peak. According to the results of ethylbenzene photochemistry on R-TiO 2 (110), 46 photocatalytic dehydrogenation of ethylbenzene can occur at 100 K, forming phenethyl groups and OH br . Similarly, photocatalytic dehydrogenation of toluene is likely to occur in a similar manner, forming benzyl groups and OH br .…”