Which one is faster 2.52 or 2.12 in knots and by how much in mintues

2.52 because that equals 0.04833274 miles/min.
2.12 equals 0.04066087 miles/min

2.52 knots = 4.66704 kilometers per hour
and 2.12 knots = 3.92624 kilometers per hour
Therefore 2.52 knots is faster.

You have asked for a difference in time in your question, when really the only difference is speed. The difference in speed for this is:
(4.67 / 60) - (3.93 / 60) = 0.012 kilometers per minute difference.