In this study, hydromagnesite sheets have been prepared under simple hydrothermal process treatment of solution of MgCl2 and urea. Calcination of the hydromagnesite sheets at 800 degrees C in air for 4h resulted in porous MgO with polyhedra in the morphology. The high crystalline MgO polyhedra were of side 4-10 mu m length and 2-6 mu m thickness, and consisted of large number of pores, edges and corners, surface hydroxyl groups and low coordinate O-2(-) sites which were regarded as active basic sites in heterogeneous base catalysts. The MgO polyhedra can function as a solid base catalyst for the synthesis of propylene carbonate (PC) from CO2 and propylene oxide (PO). With MgO catalyst calcined at 800 degrees C, the PC yield was 95% and selectivity 99% under mild conditions (1.5 MPa, 10 h). Furthermore, the MgO catalyst can be readily separated and recycled in a test of four cycles.