Grain boundary defects are normally formed in zeolite membranes during membrane preparation and calcination processes. In thiS work, a siloxane polymer coating with an imidazole group as grafted on the surface of defective SSZ-13 membranes by chemical liquid deposition to seal the defects. The parameters, such as silanization time, polymerization time, monomer type, and concentration, were optimized. Characterizations including Fourier transform infrared spectroscopy, field-emission scanning electron Microscopy, and energy-dispersive X-ray spectroscopy showed that siloxane polymers were coated on the surfaces of SSZ-13 crystals and membrane. Six modified membranes showed decreased CO2 permeance by only 21 +/- 5% [average CO2 permeance of 1.9 X 10(-7) mol/(m(2) s Pa)] and increased CO2/CH4 selectivity by a factor of 9 3 (average CO2/ CH4 selectivity of 108) for an equimolar CCYCH4 mixture at 298 K. CO2/CH4 and CO2/N-2 selectivities of the modified membrane -decreased with pressure and temperature. Membrane stability Was inveStigated by a long-time test and exposures to watet-vapor at temperatures up to 378 K and to sOme organic solutions. This modification method is also effective in sealing the defects of other Zeolite membranes, such as AlPO-18 membranes.