Photocatalytic Oxidative Dehydrogenation of Propane for Selective Propene Production with TiO₂

Abstract

Oxidative dehydrogenation of propane (ODHP) as an exothermic process is a promising method to produce propene (C 3 H 6 ) with lower energy consumption in chemical industry. However, the selectivity of the C 3 H 6 product is always poor because of overoxidation. Herein, the ODHP reaction into C 3 H 6 on a model rutile(R)-TiO 2 (110) surface at low temperature via photocatalysis has been realized successfully. The results illustrate that photocatalytic oxidative dehydrogenation of propane (C 3 H 8 ) into C 3 H 6 can occur efficiently on R-TiO 2 (110) at 90 K via a stepwise manner, in which the initial C−H cleavage occurs via the hole coupled C−H bond cleavage pathway followed by a radical mediated C−H cleavage to the C 3 H 6 product. An exceptional selectivity of ∼90% for C 3 H 6 production is achieved at about 13% propane conversion. The mechanistic model constructed in this study not only advances our understanding of C−H bond activation but also provides a new pathway for highly selective ODHP into C 3 H 6 under mild conditions.

Photocatalytic ethane conversion on rutile TiO2(110): identifying the role of the ethyl radical

“…1 shows the typical TPD spectra of the mass-to-charge ratios ( m / z ) of 15 (CH 3 + ), 18 (H 2 O + ), 26 (C 2 H 2 + ), 27 (C 2 H 3 + ), 28 (C 2 H 4 + and CO + ), 29 (C 2 H 5 + and CHO + ), 30 (C 2 H 6 + ), 31 (CH 2 OH + ) and 43 (CH 3 CO + ) collected on the oxidized R-TiO 2 (110) surfaces after adsorbing 0.28 ML (1 ML = 5.2 × 10 14 molecules per cm 2 ) of C 2 H 6 followed by 355 nm irradiation for 0 (black lines) and 10 min (red lines). The oxidized R-TiO 2 (110) surfaces were prepared by exposing the reduced surfaces to 200 L of O 2 at 300 K. 25,27,28 After surface oxidation, the bridging oxygen vacancies (O v ) will be healed, leaving oxygen atoms on the five coordinated Ti 4+ sites (Ti 5c , O Ti ). 25 Before irradiation, only one desorption peak at 129 K appeared in the TPD traces of m / z = 15, 26, 27, 28, 29, 30, and 31, which was attributed to the desorption of the molecular C 2 H 6 on the Ti 5c sites (C 2 H 6(Ti) ).…”

Low-Temperature Ethylbenzene Conversion on Rutile TiO2(100) via Photocatalysis: The Strong Photon Energy Dependence

“…The phonons may also couple to molecules to realize energy transfer. Similar to the initial step of photocatalytic dehydrogenation of hydrocarbons and alcohols on R−TiO 2 (110), 24,35,40,44 the initial α-C−H bond cleavage of C 6 H 5 C 2 H 5 on R−TiO 2 (100) may follow the proton-coupled hole transfer (PCHT) mechanism, leading to the formation of the C 6 H 5 CHCH 3 • radical. However, as shown in Figure 5, only the β-C−H cleavage is strongly energy-dependent, which may be affected by the deep hole or the local heat produced by the deep hole relaxation.…”

“…In addition, when one methyl group on the end of the C 3 H 8 molecule is substituted by a phenyl group, C 6 H 5 C 2 H 5 forms. However, low-temperature propene formation from photocatalytic C 3 H 8 conversion occurs on R−TiO 2 (110) efficiently, 24 but low-temperature C 6 H 5 CHCH 2 production from photocatalytic C 6 H 5 C 2 H 5 conversion on R−TiO 2 (110) 26 and R− TiO 2 (100) does not, indicating that the high energy barrier of the second C−H bond cleavage may not be the main reason for the low efficiency of LT C 6 H 5 CHCH 2 formation.…”

“…In comparison to the various thermocatalytic C 6 H 5 C 2 H 5 conversion techniques (DDH and ODH), photocatalysis, as a potential alternative, may achieve low-temperature C 6 H 5 C 2 H 5 conversion. Recently, photocatalysis has been successfully applied for the conversion of light alkanes [such as methane (CH 4 ), ethane (C 2 H 6 ), and propane (C 3 H 8 )] with TiO 2 -based catalysts under mild conditions. − Further, photocatalytic C 6 H 5 C 2 H 5 conversion on the model rutile (R)–TiO 2 (110) surface was also systematically investigated. , The results suggested that photocatalytic C 6 H 5 C 2 H 5 conversion into C 6 H 5 CHCH 2 occurs via a stepwise manner, in which the α-C–H bond is dissociated first with a very low energy barrier (∼0.3–0.4 eV) and is then followed by the β-C–H bond cleavage with a high energy barrier (∼1.2 eV) . Actually, the energy of photogenerated charge carriers in R–TiO 2 (110) (≥3.0 eV) , is much higher than the barrier of the β-C–H bond cleavage of C 6 H 5 C 2 H 5 ; thus, realizing the β-C–H bond cleavage should be very easy.…”

“…On the basis of the desorption temperature of HT C 6 H 5 CHCH 2 , the energy barrier of the second C–H bond cleavage on the ground state is about 1–1.3 eV. The energies of photogenerated electrons/holes by 257 nm (∼4.8 eV) and 343 nm (∼3.6 eV) light are much higher than the energy barrier, while a series of theoretical works on photocatalytic C–H bond activation on R–TiO 2 (110) ,, suggest that the formation of an excited radical intermediate by trapping a hole will largely decrease the energy barrier of the C–H bond cleavage and then facilitate the C–H bond cleavage. In addition, when one methyl group on the end of the C 3 H 8 molecule is substituted by a phenyl group, C 6 H 5 C 2 H 5 forms.…”