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CO2 have weak inter molecular spaces due to which it occurs as gas at room temperature where as SiO2 has metallic character, more the metallic character greater is the inter molecular spaces. Hence SiO2 exist as a solid in room temperature.

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The chemical bonds between the atoms of oxygen and carbon in C O2 are weaker than the bonds between silicon and oxygen in silicon dioxide Si O2.  Further the molecules of SiO2 are closely packed and have more attraction with the neighbouring molecules.

In order to break the attractive forces and bonds between the molecules, we need to supply more energy.  Then they separate more and become liquids from solids or, gases from liquids.  Thus SiO2 molecules need more kinetic energy to separate and become free molecules in gaseous form.

Internal heat energy in a substance = K_B T , where K_B = Boltzmann constant and T is the absolute temperature.

The internal heat energy in CO2 at room temperature T=300 Kelvin  is enough to overcome attractive forces between molecules.   However, the SiO2 molecules need much higher energy and so need a much higher temperature to melt and to vaporize.

Thus the boiling point and melting points of SiO2 are higher than that of CO2.