Promising Developments in Methane to Ethylene Technology 

  
Dalian Institute of Chemical Physics (DICP), SABIC, and China National Petroleum Corporation (CNPC) recently signed a Memorandum of Understanding (MOU) to jointly develop the new technology of "direct, non-oxidative conversion of methane to ethylene, aromatics, and hydrogen." We consider this a significant step forward for the petrochemicals industry.  China, the world's second largest economy, has been relentlessly pursuing non-fossil based energy (e.g. solar, wind etc.) and alternate feedstocks (e.g. coal-to-olefins, shale gas, biomass etc.).  As stipulated in China's 13th five-year plan, the development of clean and economical feedstock is a mandate as it is crucial to the sustainability of China's petrochemical and chemical industries. The direct conversion of methane to ethylene and aromatics is a shining example of this endeavor. 
 
This joint development program stemmed from the encouraging results first reported in Science, 344, 616-619 (2014).  In this article, researchers at Dalian Institute of Chemical Physics (DICP), Chinese Academy of Sciences (CAS), reported a new iron/silica catalyst which achieved total carbon selectivity exceeding 99% (which means ethylene benzene, naphthalene, and hydrogen are the only products and carbon utilization almost reaches 100%).  At reactor temperature at 1,090 C, the methane conversion rate was up to 48.1% and ethylene selectivity peaked at 48.4% (the rest are two aromatics). The single iron catalyst sites are more stable, showing no deactivation during a 60-hour test.
 
The key of this exciting breakthrough is a catalyst system which uses a silica matrix to "confine" single iron  sites,  generating methyl radicals followed by a series of gas-phase reactions. It can be best described as radical mechanism. The catalyst activates C-H bonds, suppresses complete dehydrogenation, and avoids over oxidation. One unique feature is that there is no adjacent iron sites which prevents catalytic C-C coupling and further oligomerization, Coke deposition is better controlled by this route. Contrary to the oxidative coupling methane (OCM), no oxidant was required for C-H bond activation.
 
For OCM, there has been an intensive research effort since the early 1980s in developing the OCM process which uses oxygen. This was driven by the potential of OCM technology to reduce costs, energy and environmental emissions in the production of ethylene; however, a viable catalyst has not been identified for commercial operations. Siluria Technologies has shown promising results and has advanced to a demonstration plant.
 
In the oxidative coupling reaction, methane (CH4) and oxygen (O2) react over a catalyst to produce ethylene (C2H4) water (H2O) and heat:
2 CH4 + O2  --  C2H4 + 2 H2O + Heat 
 
The reaction is exothermic (H = -280 kJ/mol) and occurs at high temperatures (750-950 C). Methane (CH4) is activated on a heterogeneous catalyst surface, forming methyl free radicals, which then combine to form ethane C2H6 in the gas phase. The ethane subsequently undergoes dehydrogenation to form ethylene (C2H4).
 
The main challenges include catalyst durability and ethylene selectivity at high temperature (about 850 C ). Recently Siluria Technology has demonstrated technical feasibility and economics at pilot plant level. They are working hard to move it up to commercial level. On May 11, 2016,  Siluria announced that their demonstration plant in La Porte, Texas has successfully completed eighteen test campaigns since early 2015 to replicate custom-specific production conditions including change of temperature, pressure, flow rate, and inlet gas compositions, etc.  
 
It will be interesting to see in the next couple of years how this non-oxidative technology stacks up against the more established OCM in terms of catalyst endurance, selectivity, and economics.   
 
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