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.

The recitation of the Quran from one’s memory (in one’s heart) does not need ablution, but it is an obligation on the one who touches it to perform ablution. Allah Says (what means): {In a Register well-protected; None touch it except the purified.} [Quran 56:78-79]

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

Allah knows best.