Silicon (IV) oxide or SiO2 has a high melting point because it is a solid. In comparison, carbon dioxide is a gas at room temperature. Even though both carbon and silicon are members of group 14 of periodic table of elements, their oxides have different states (solid vs. gas).
This difference is due to the structures of these oxides. Carbon dioxide molecule consists of double bonds between carbon and oxygen atoms. This provides the molecule a linear shape. The main force of attraction between the atoms is weak van der Waals force. In comparison, the SiO2 molecule is very different. Each silicon atom is surrounded by 4 oxygen atoms and there are only single bonds between silicon and oxygen. This silicon-oxygen linkage provides it with a networked structure where many atoms are connected to each other (that is, a single oxygen atom is shared between multiple silicon atoms).
The melting of SiO2 requires breaking of a large number of covalent bonds and hence its melting point is high.
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