The U.S. Court of Appeals for the Federal Circuit in Washington, D.C., on Friday vacated and remanded a Patent Trial and Appeal Board decision that rejected an application by Stepan Co., a Northfield-based chemical manufacturer, for a patent relating to the formulation of a weed killer.The appeals court found in a split 2-1 decision that the trial and appeal board failed to explain why it rejected Stepan’s application and it improperly placed the burden to prove patentability on Stepan.The company filed a patent …