Mechanistic Analysis of the Selective Hydrogenation of Acetylene Over NiZn Surface

Open Access
Author:
Held, Jacob T
Area of Honors:
Chemical Engineering
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • Michael John Janik, Thesis Supervisor
  • Michael John Janik, Honors Advisor
  • Robert Martin Rioux Jr., Faculty Reader
Keywords:
  • DFT
  • catalysis
  • NiZn
  • Hydrogenation
  • ethylene
  • polyethylene
  • acetylene
Abstract:
Ethylene produced via steam cracking is contaminated with a small amount of acetylene which poisons ethylene polymerization catalysts in the production of polyethylene. Current technology utilizes precious metal catalysts to purify this stream via the selective hydrogenation of acetylene to ethylene. Thus, the development of selective base-metal catalysts has the potential to greatly reduce the cost of this process. Recently, some attention has been given to bimetallic nickel-zinc as a potential base-metal catalyst for this reaction. Experimental results pertaining to the selectivity of NiZn catalysts have been inconclusive, motivating the scrutiny of this reaction using DFT methods. In the present study, we established an optimized crystalline NiZn structure, modeled the energetics of the hydrogenation of acetylene, and used Langmuir-Hinshelwood kinetics to analyze the results. This analysis was then used to evaluate under what conditions the criteria of preferential ethylene desorption relative to hydrogenation is sufficient to conclude that a catalyst is selective for hydrogenation of acetylene to ethylene. Based on DFT results and under typical selective hydrogenation reaction conditions, both nickel and nickel-zinc should be selective to the hydrogenation of ethylene with respect to production of ethane, with nickel-zinc orders of magnitude more selective. Future work should consider the selectivity with respect to oligomerization products, since the formation of C-C bonds leading to higher hydrocarbons may also represent a selectivity challenge for the use of Ni-based catalysts for this reaction.