Portfolio: One afternoon earlier this year, Rex Tillerson, the chairman and CEO of Exxon Mobil Corp., and Barack Obama, then president-elect, laid out very different visions of America’s energy future. With the days counting down to his inauguration, Obama told a crowd at George Mason University of his plan to double U.S. production of renewable energy by 2012–"to finally spark the creation of a clean-energy economy,’ he said, to screams from the students in the audience. That pledge was a small part …