Go read your Constitution. You’ll find a recess appointment is a Constitutionalally enumerated right of the President. Obama used it illegally in that he made recess appointments when the Senate was not technically out of session. He got called on it by SCOTUS, unanimously.
[The President] shall nominate, and by and with the Advice and Consent of the Senate, shall appoint Ambassadors, other public Ministers and Consuls, Judges of the supreme Court, and all other Officers of the United States. -- U.S. Const. art. II, Sec. 2, Cl. 2.The President shall have Power to fill up all Vacancies that may happen during the Recess of the Senate, by granting Commissions which shall expire at the End of their next Session. -- U.S. Const. art. II, Sec. 2, Cl. 3.
Since there is on intention here of a temporary appointment of an AG to replace Sessions, Trump's recess appointment would made in bad faith as was Obama's and he should not do it.