Date of Award
Open Access Thesis
Epoxidation of ethylene with oxygen over promoted silver catalyst is one of the most important examples of heterogeneous catalysis to date with a multi-billion dollar annual market. Ethylene oxide (EO) can be used directly as a sterilizer, disinfectant, and fungicide, or as an intermediate chemical for producing ethylene glycol, detergents, antifreeze, polyesters, as well as a variety of other commercial chemicals. Thus, ethylene oxide is known as one of the highest volume chemicals produced in the chemical industry, accounting for approximately 40-50% of the total value of organic chemicals produced by heterogeneous oxidation. Considering the application of EO in detergents and sterilizers, the importance of EO production has been increased more than before due to the COVID19 pandemic. Preventing the total oxidation and improving the yield of selective products can be counted as one of the major challenges in selective oxidation reactions.
It has been shown in the literature that the selectivity of ethylene oxide, among many other factors, depends on silver faceting of the catalyst nanoparticles. However, it is not exploited in industry, where the silver is mainly being used in nanoparticles with (111) facets due to the preparation and stability issues of the other structures. Thermodynamically, the Ag(111) facet is the most stable but the least active facet among the others ,while Ag(100) has been shown theoretically to exhibit the highest selective facet for EO formation. In this thesis, silver nanowires catalysts were synthesized via hydrothermal method and were deposited on 𝛼 -aluminum oxide supports using wet impregnation method. Obtaining a pure nanowires sample in not possible, however, some synthesis parameters such as time, temperature, and chemicals’ concentrations that are parameters possibly affecting the morphology, yield, length, and diameter of nanowires were varied to increase the nanowires yield. Low yield, thin nanowires resulted from a shorter synthesis time while a mixture of nanorods and nanospheres were fabricated for longer synthesis times. An optimum time (24hours) was selected for synthesis time as one of the most effective synthesis parameters.
Unpromoted supported silver nanowires catalyst with an average diameter of less than 100nm and unpromoted semi-spherical silver catalyst with (111) facets that was synthesized using wet impregnation method, were tested in a single channel reactor at atmospheric pressure and for different temperatures. Structure of silver nanowires thinner than 100 nm were changed at the reaction condition while thicker nanowires remined intact.
Shariat, K.(2021). Shape-Selective Silver Catalysts for Ethylene Epoxidation. (Master's thesis). Retrieved from https://scholarcommons.sc.edu/etd/6830