Double catalyst for the direct conversion of syngas to lower olefins
21 March 2016
The light olefins ethylene, propylene, and butylene—usually made from petroleum—are key building blocks for chemical industry, and are starting materials for making plastics, synthetic fibers, and coatings. In the journal Angewandte Chemie, Chinese scientists report on a new bifunctional catalyst that converts syngas to lower olefins (C2-C4) with high selectivity. This could make it more attractive to make olefins from alternative sources of carbon, such as biomass, natural gas, or coal.
The design of bifunctional catalysts could result in further breakthroughs in developing one-step processes for selective production of fuels and chemicals such as gasoline, diesel, and aromatics from synthesis gas.
Currently, the production of lower olefins primarily involves the thermal cracking of lighter fractions of petroleum. Increasing demand and decreasing oil reserves are causing scientists to turn their attention to coal, natural gas, shale gas, and biomass as sources of raw material.
The first step in such a process would involve the conversion of these materials into syngas through either gasification or steam reforming. Using Fischer–Tropsch synthesis, the resulting synthesis gas is then catalytically converted to paraffins (saturated hydrocarbons), olefins (unsaturated hydrocarbons), and alcohols.
However, the FT process follows an Anderson–Schulz–Flory (ASF) distribution that limits the ability to form the desired lower olefins.
An alternative method is the conversion of synthesis gas in two process steps. First the CO and H2 are converted to methanol. In the second process, methanol is converted to lower olefins with relatively high selectivity by a methanol to olefins (MTO) process, which forms carbon–carbon bonds. Yet, a one-step process that combines these two processes is more desirable because it would be more energy- and cost-efficient.
Scientists working with Qinghong Zhang and Ye Wang at Xiamen University developed a special bifunctional catalyst with active components for both reaction steps: SAPO-34, a silicon aluminum phosphate molecular sieve, is an outstanding catalyst for the MTO reaction. Zirconium oxide and zinc oxide nanoparticles in a 2:1 ratio catalyze the methanol synthesis reaction with a preference for lower olefins.
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| Reaction coupling for the direct synthesis of lower olefins from syngas by the integration of active components for CO activation and C-C coupling. (a) = adsorbed. Cheng et al. Click to enlarge. |
The way in which the two components of the combined catalyst are united is of critical importance. They must be in close contact, but if they are too close then the newly formed olefins can too easily come into contact to the catalytic centers that are intended to convert the CO and hydrogen to methanol. The olefins can also bind hydrogen, which causes them to lose their double bond and thus forms more paraffins. The best results came from grinding the two catalysts together in a mortar.
At about 400 °C, the bifunctional catalyst resulted in a selectivity of 74% for lower olefins (C2–C4) with a CO conversion of about 11%—an excellent result for the direct conversion of synthesis gas to lower olefins.
Resources
Kang Cheng, Bang Gu, Xiaoliang Liu, Jincan Kang, Qinghong Zhang, Ye Wang (2016) “Direct and Highly Selective Conversion of Synthesis Gas to Lower Olefins: Design of a Bifunctional Catalyst Combining Methanol Synthesis and Carbon–Carbon Coupling” Angewandte Chemie http://dx.doi.org/10.1002/anie.201601208
We have an ethylene glut in the US as it is, and little need to process more conventional petroleum for what we need. The source is oil/gas liquids. The "selectivity" described in the above article seems fairly meaningless within a complex refinery or FT operation, and there are numerous FT variants more specifically attuned to gas and shale oil feedstock. This may be more suited to coal conversion to fuel, but I have my doubts that lower coal grades will pass muster due to sulfur contamination, despite the seeming low cost of the catalyst.
On the other hand there is an an approach called chemical coupling that might be adaptable to produce heat and hydrocarbons in the same pot.
Posted by: kalendjay | 28 March 2016 at 05:05 PM
It is pointless to focus research efforts towards utilizing fossil fuels for upgraded products and the world is definitively not moving towards restricted access to oil.
The fossil fuel age is not going to end due to lack of fossil fuels but simply because fossil fuels are not competitive in the long run. (Not really many decades into the future.)
Renewable electricity is already cheap enough to outcompete some oil wells across the globe and not that far from being able to outcompete even cheap oil from Saudi Arabia.
If you produce Synfuels based upon excess electricity, excess CO2 and water you will no longer need any fossil fuel for any purpose today used in modern civilizations.
Posted by: Jens Stubbe | 02 April 2016 at 02:19 AM