The oil refinery at Grangemouth is currently in the process of being shut down with more than 400 jobs lost as a result.
The site, owned by Manchester United owner Sir Jim Ratcliffe's company Ineos, is also home to a petrochemicals plant.
Unite the Union said a new report on the site's future would take years to implement and wouldn't help refinery workers already losing their jobs.
The Project Willow report, jointly commissioned by the UK and Scottish Governments, set out nine options for the future of Scotland's largest industrial site. None would start until after 2030 and the report says significant private capital investment of around £3.5billion would be required.
Options considered by Project Willow include production of sustainable aviation fuel, which would begin in 2035 employing up to 270 staff. It would require capital expenditure of up to £2.1billion.
But Unite general secretary Sharon Graham said: "It asked the wrong questions then failed to provide the answers Grangemouth refinery workers need.
"The UK and Scottish governments need to act rapidly to protect skilled well-paid employment.
"There are projects like SAF production which can be swiftly enacted to protect jobs and those opportunities must not be lost. This would pave the way for the UK to become a world leader in green aviation."
But First Minister John Swinney said he was "confident" public and private money could create new jobs and ...