Place the teflone tube in aqua regia i.e HCl + HNO3, or in strong base solution like NaOH, .

Muhammad Zahid please suggest the concentration of NaOH.

I'd be interested in the chemical equation for the 'digestion' of MnO2 with NaOH.....

Further, HCl should be sufficient. Adding HNO3 makes aqua regia a strong oxidizing agent and this won't dissolve the MnO2.....

I agree with Alan F. Rawle ― Manganese dioxide dispersed nanoparticles should oxidize hydrochloric acid to free chlorine, thus reactively dissolving as manganese(II) chloride: MnO2 + 4 Cl- + 4 H3O+ → Cl2 + MnCl2 + 6 H2O.

But it did not work. even i put teflon in acid and place in sonicator

OK, then the only other course of action is a new Teflon autoclave.

How did you conclude it is contaminated MnO2? if you used that Teflon for MnO2 preparation, very likely it is oxidized carbon containing material (either from Teflon or organics you used in synthesis), so HCl will not work and HNO3 may further burn you Teflon. Get new one, or if you wont a cost efficient approach, then take it to mechanical machining service to remove 1-2 mm of inner wall.

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I agree with Alan F

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