A process for preparing an oxidic material comprising a zeolitic material having framework type AEI and a framework structure comprising a tetravalent element Y, a trivalent element X, and O, the process comprising preparing a synthesis mixture comprising water, a source of Y, a source of X comprising sodium, an AEI framework structure directing agent, and a source of sodium other than the source of X; and heating the synthesis mixture obtained from (i) to a temperature in the range of from 100 to 180° C. and keeping the synthesis mixture under autogenous pres-sure at a temperature in this range for a time in the range of at least 6 h, obtaining the oxidic material comprising a zeolitic material having framework type AEI and a framework structure comprising a tetravalent element Y, a trivalent element X, and O, comprised in its mother liquor; wherein the AEI framework structure directing agent according to (i) comprises a N, N-diethyl-2,6-dimethylpiperidinium cation.