Theoretical Studies on the Direct Propylene Epoxidation Using Gold-Based Catalysts: A Mini-Review

Abstract

Direct propylene epoxidation using Au-based catalysts is an important gas-phase reaction and is clearly a promising route for the future industrial production of propylene oxide (PO). For instance, gold nanoparticles or clusters that consist of a small number of atoms demonstrate unique and even unexpected properties, since the high ratio of surface to bulk atoms can provide new reaction pathways with lower activation barriers. Support materials can have a remarkable effect on Au nanoparticles or clusters due to charge transfer. Moreover, Au (or Au-based alloy, such as Au–Pd) can be loaded on supports to form active interfacial sites (or multiple interfaces). Model studies are needed to help probe the underlying mechanistic aspects and identify key factors controlling the activity and selectivity. The current theoretical/computational progress on this system is reviewed with respect to the molecular- and catalyst-level aspects (e.g., first-principles calculations and kinetic modeling) of propylene epoxidation over Au-based catalysts. This includes an analysis of H2 and O2 adsorption, H2O2 (OOH) species formation, epoxidation of propylene into PO, as well as possible byproduct formation. These studies have provided a better understanding of the nature of the active centers and the dominant reaction mechanisms, and thus, could potentially be used to design novel catalysts with improved efficiency.

Surface Engineering and Kinetics Behaviors of Au/Uncalcined TS-1 Catalysts for Propylene Epoxidation with H2 and O2

“…In the catalytic Clearly, the Au/TS-1-B-300 catalyst shows greatly lower byproducts' selectivity than that over the Au/TS-1-B-200 catalyst. According to the previous studies, 44,45 the reactive allylic hydrogen atoms in the propylene molecule would easily suffer from the attack from negatively charged oxygen species bound to Au nanoparticles because of their nucleophilic nature, and the partial oxidation of the methyl would contribute to the formation of acrolein. Combined with the XPS results, the higher Au binding energy will make the acrolein formation through (H 2 -assisted) activation of O 2 to be more difficult.…”

Singly Dispersed Bimetallic Sites as Stable and Efficient Single-Cluster Catalysts for Activating N2 and CO2

“…Since the concept of a single-atom catalyst (SAC) was proposed in 2011, its great potentials for high activity, selectivity, and stability have brought it to become the frontier in heterogeneous catalysis, and substantial progress has been made in developing SACs for various applications. − However, SACs may not compose the optimal design of heterogeneous catalysts for certain complex reactions, such as ammonia synthesis, − propylene epoxidation, CO oxidation, and the Fischer–Tropsch process . This may arise from the intrinsic nature of SAC, i.e., the single-atomic active site, for which it is challenging to provide multiple catalytic functionalities required by complex reactions for optimal performances.…”

Solvent Screening and Process Optimization for Separating Propylene Oxide from Direct Propylene Epoxidation with H2 and O2

“…2 The former one suffers from environmental concerns, where a large amount of byproduct CaCl 2 having no commercial value (i.e., 2.2 tons per 1.0 ton of PO) is coproduced, along with toxic chlorinated organic compounds (several hundred grams per ton of PO). 3 The latter one has two variants based on the raw material in use, 2 i.e., the propylene oxide−styrene monomer (PO−SM) process 4−6 and the propylene oxide−tert-butyl alcohol (PO−TBA) process. 7,8 The two processes both produce coproducts (i.e., styrene and tert-butyl alcohol, respectively) in a fixed amount, which account for 2−4 times the amount of PO produced, and make the processed highly dependent on the market of the coproducts.…”

“…Later, hydrogen peroxide (H 2 O 2 ) is used as an oxidant for PO production, called the HPPO process, which is more environmentally friendly with water as the only byproduct. 3 However, the H 2 O 2 and PO have comparable market values on a molar basis, making this process less profitable. An alternative process to produce PO is the direct propylene epoxidation with H 2 and O 2 , which is simple, green, and sustainable 9−12 because it takes only one step to produce PO and the main byproduct is water.…”

“…Propylene oxide (PO) is a versatile and high value-added chemical intermediate, and the increasing demand for PO is driven by its use for production of numerous commercial materials, such as polyether polyols and polyurethanes. , Currently, PO is mainly produced by two kinds of commercial processes, i.e., the chlorohydrin process and the hydroperoxide process . The former one suffers from environmental concerns, where a large amount of byproduct CaCl 2 having no commercial value (i.e., 2.2 tons per 1.0 ton of PO) is coproduced, along with toxic chlorinated organic compounds (several hundred grams per ton of PO) . The latter one has two variants based on the raw material in use, i.e., the propylene oxide–styrene monomer (PO–SM) process − and the propylene oxide– tert -butyl alcohol (PO–TBA) process. , The two processes both produce coproducts (i.e., styrene and tert -butyl alcohol, respectively) in a fixed amount, which account for 2–4 times the amount of PO produced, and make the processed highly dependent on the market of the coproducts.…”