China tightened its controls on the export of fentanyl precursors to the United States, Canada and Mexico, adding two new chemicals to its list requiring export licenses on Tuesday, one day ahead of President Xi Jinping’s announced visit to the US. China’s Ministry of Commerce, together with four other government agencies, added two N-phenethyl piperidine esters to part one of China’s Catalogue for the Administration of Exports of Precursor Chemicals to Specific Countries (Regions), according to...