He continued his studies at the Pontifical Gregorian University in Rome, graduating as a Doctor of Philosophy and Licentiate in Theology and Church History.
In 1940 he was chaplain of a regiment of armoured troops and was taken prisoner of war and held for seven months.
After the promotion of the Constitution on the Liturgy, he became a member of the council to monitor the application of the document.
Bishop van Zuylen made a great effort to put the ideals of the Council into practice in his diocese.
However, the decline in priestly vocations forced him to close the seminary in Liège and to have his seminarians complete their studies elsewhere.
In his old industrial diocese, he was deeply concerned about the deterioration of the living conditions for the workers.
He was outspoken on issues relating to economic recession, the closure of companies and unemployment.
Van Zuylen was trilingual, being fluent in French, Dutch and Walloon, and was also able to speak German.