In brief: United Airlines and Rentech have successfully flown a validation flight using a 40:60 mixture of RenJet synthetic jet fuel.
The word

Flying the aircraft, an Airbus 319, using a 40:60 mix of synthetic:Jet A fuel in one engine and standard Jet A in the other engine (for safety), the aircraft successfully made 39,000 feet from the Denver Airport.
An on-board team collected data during the flight, getting information on the fuel's performance during maneuvers. The plane was piloted by Captain Joseph Burns, of United.
The fuel is a synthetic jet fuel called RenJet made by Rentech in their patented Fischer Tropsch process. It is derived from natural gas and is ASTM approved for passenger jet use.
And so ...
Rentech is located in Commerce City, Colorado, near Denver and the airport.
Photo credits: Rentech