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I'm not 100% sure about this, so I just wanted to double-check.

I think it has something to do with each carbon atom in Graphene having a delocalized electron and this creates carbon ions that can exert an electrostatic attraction. This would reduce the inter-particle distances between carbon atoms in Graphene relative to Diamond, hence Graphene's higher melting point. But again, I'm not quite sure.

Tom Brooks
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    Neither graphene nor diamond have a "melting point". https://de.wikipedia.org/wiki/Kohlenstoff#/media/File:Carbon-phase-diagramp_german.svg – Karl Jan 01 '17 at 15:46

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This is a poorly defined question. Melting is a change in stare, and it's based on thermodynamics. But at room pressure, neither diamond nor graphene are stable allotropes: graphite is. They are metastable.

So you start to mix in kinetics with the thermodynamics.

It's a bit like asking why does ice melt at a lower temperature than water.

If you look at the effects of pressure, then you can construct a phase diagram, and then the "melting" point of any given allotrope is defined by the boundary between solid and liquid.

AlanJ
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Graphite(collection of graphene layers) has higher melting point than diamond, because of the partial double bond character of C-C bonds. So, the bond in each layer(graphene) is strong. Also, there are attractive forces between successive layers of graphene, increasing the melting point of Graphite

Bhavya
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