process for the preparation of a porous , pure silica which is particularly suitable as a catalyst support for a chromium - oxide catalyst used in polymerizing of olefins . the process comprises the steps of converting a silicon - halogen compound to a silica and then admixing with water to form a silica gel . the gel is then spray - dried to achieve the required particle size and pore volume . the silica can then be made into a catalyst by placing on the support a chromium compound and then heating in a non - reducing atmosphere .