SHANGHAI—China’s central bank kept some of its policy rates unchanged in a liquidity operation on Tuesday, dashing expectations for a cut, although investors believe policymakers may resume monetary easing soon to prop up the cooling economy. The surprise decision comes a day before the U.S. Federal Reserve is expected to deliver its first interest rate hike in three years and analysts say Beijing may want to avoid widening policy divergence for the time being. The People’s Bank of China (PBOC) said it would keep the rate on 200 billion yuan ($31.44 billion) worth of one-year medium-term lending facility (MLF) loans to some financial institutions unchanged at 2.85 percent from the previous operation. The operation resulted in a net injection of 100 billion yuan in fresh funds, replacing the 100 billion yuan due to mature on Tuesday. The PBOC attributed the move to “maintaining banking system liquidity reasonably ample,” according to …