Clean gasoline from natural gas

Today's commercial MTG process converts methanol to high octane gasoline directly. One drawback in the existing process is the carcinogenic aromatics content in the produced gasoline. While the aromatics content of MTG gasoline is not particularly high compared to standard gasoline, it is still higher than what is desired, due to the potential human risks.

inGAP researchers have, through rational design based on their exstensive knowledge in the field, been able to develop a new catalyst for the reaction, and the new product is a high-octane gasoline virtually without aromatics.

The cyclic molecule (behind) does not fit into the ZSM-22 channel, while the branched chain (in front) can diffuse through the structure.

The catalysts employed in the methanol-to-gasoline (MTG) process are zeolites. These are porous, crystalline solid materials with channels of molecular size. The channels can be considered tunnels inside the crystal into which small molecules may migrate in and out. In methanol conversion, methanol molecules travel into the tunnel where they react, and the products can then travel out of the tunnel again. Thus it is not surprising that the size and form of these tunnels play a major role in determining what products are actually formed. 

The dimensionality of zeolites.

In one-dimensional zeolites, the tunnnels go from one side to the other without crossing. In two-dimensional zeolites tunnels cross so that molecules can diffuse in two directions. Three-dimensional zeolites are the most complicated as the tunnels intersect so that molecules can diffuse in from one side of the crystal and diffuse out from any other side. The intersections have great influence on the product distribution, as they often make larger spaces or cavities where reactions may occur.

The dual cycle mechanism which shows that in the conventional industrial catalyst, ZSM-5, two almost independent catalytic cycles are operating, was identified by inGAP researchers. An important factor of this mechanism is that the aromatic molecules only participate in one of the cycles. The idea was that if the aromatic cycle could be suppressed by choosing the appropriate zeolite architecture, we could obtain a product free of aromatics.

It worked!

The conventional catalyst for the process, ZSM-5, is a three-dimensional zeolite. By instead choosing a one-dimensional zeolite, ZSM-22, a new product spectrum was obtained: A high octane gasline without aromatics. This was an exciting result, also because previous reports using the same zeolite indicated that the catalyst was completely inactive in the MTG reaction.

 

Published Feb. 28, 2012 10:57 AM - Last modified Mar. 1, 2012 4:55 PM