The invention relates to a medium or large pore synthetic zeolite comprising at least 0.02 wt %, based on the weight of the zeolite, of a catalytic metal selected from the group consisting of Ru, Rh, Pd, Ag, Os, Ir, Pt, Au, Mo, W, Re, Co, Ni, Zn, Cr, Mn, Ce, Ga, Cu and combinations thereof, which is present as catalytic metal particles, wherein at least 60% by number of said catalytic metal particles have a particle size of 2.0 nm or less, and at least 0.005 wt %, based on the weight of the zeolite, of an alkali metal or alkaline earth metal selected from the group consisting of Li, Na, K, Cs, Ca, Mg, Ba and Sr and combinations thereof, wherein, if the zeolite comprises in the zeolite framework a trivalent element Y selected from the group consisting of Al, B, Ga Fe and combinations thereof, the SiO2:Y2O3 ratio is greater than 200:1.