Theoretical Investigation of Propylene Epoxidation on Ag(111) by Molecular Oxygen: Na(K,Cl) Effects

Abstract

As an attractive and environmentally friendly propylene oxide (PO) production process, the direct epoxidation of propylene by metallic catalysts such as Ag and Cu with molecular oxygen has attracted extensive attention, but this process remains one of the biggest challenges in chemistry because the selective oxidation of propylene is influenced by the competitive reactions. It is well known that NaCl(KCl) can promote PO selectivity significantly, and in order to explore the possibility to enhance the PO selectivity by Na, K, and Cl additives, we used first-principles calculations in this work to explore the PO formation mechanism on pure and Na-, K-, and Cl-modified Ag(111). We have reported two parallel reaction pathways on the Ag(111) surface in detail: dehydrogenation and epoxidation. On the surface of Ag( 111), acrolein is obtained by two H-stripping reactions in the process of dehydrogenation. In addition, in the epoxidation reaction, PO and acetone/propanal can be obtained through a propylene oxametallacycle (OOMMP) intermediate. The crucial competitive steps of the selectivity of PO are the competitive reactions between the dehydrogenation reaction and the OOMMP intermediate formation reaction and the formation of PO and acetone/propanal. The present calculation results showed that the addition of alkali metal atoms (Na/K) had almost no effect on the first-step competitive reaction (allylic hydrogen stripping vs OOMMP) but could improve the selectivity of PO formation in the second-step competitive reaction, among which the Na atom had a better effect than the K atom. The addition of a Cl atom can not only improve the selectivity of OOMMP in the first competitive reaction but also improve the selectivity of PO in the second competitive reaction, and it is more significant in the first competitive reaction. The promotion effect for the PO formation selectivity follows the trend of Cl > Na > K. Through calculation, the selectivity of the target product (PO) was improved, and the insight obtained could provide theoretical guidance for further development of catalysts for propylene epoxidation.

The Lord of the Chemical Rings: Catalytic Synthesis of Important Industrial Epoxide Compounds

“…However, these methods show several drawbacks: corrosion of the equipment, environmental problems and the formation of many byproducts [33], in fact the coproducts of chlorohydrin process are chlorine salts, considered harmful. In the second aforementioned synthesis route, hydroperoxide is formed and it is used for the formation of propylene oxide using Mo-based homogeneous catalysts or Ti on silica as heterogeneous catalysts [30,32].…”

“…All these adverse aspects have led to the development of new ways of synthesis of propylene oxide, using an environmentally friendly oxidant [33,34].…”

“…Among the catalysts tested in the HPPO (hydrogen peroxide propene oxide) process, the most promising was titanium silicalite-1 (TS-1) which permit to operate under mild condition [30,38]. This process permits a higher propylene conversion and propylene oxide selectivity to be reached but with the drawback of the high price of the catalyst and hydrogen peroxide [33]. A summary of the main processes for the propylene oxide synthesis is reported in Table 2.…”

“…Many studies were focused on the propylene oxidation catalyzed by metals or metal oxides. Some of them demonstrated that these catalysts, for example Au, Ag, Cu or Cu 2 O, showed catalytic activity in this type of reaction [33,34,44].…”

“…Ag-based catalysts were widely studied thanks to their good properties, even if the selectivity towards propylene oxide was very low [33,49]. A way to improve this value is to modify these catalysts.…”