Is it obligatory to perform wudu before reading the qur'an, because if i don't peform it i am still on my tahara because the najasa is another thing?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Recitation of the Quran by heart (from one’s memory) does not need Ablution but it is an obligation for one who touches it to be in a state of purity (i.e. with ablution). Allah Says (what means): {In a Register well-protected. None touch it except the purified [i.e. the angels].} [Quran 56:78-79]

The purification referred to in this verse contains the purification from all ritual impurities.

For more benefit on the nullifiers of ablution, please refer to Fatwa 84093.

For more benefit on the detailed description of ablution, please refer to Fataawa 82845 and 83164.

Allah knows best.