The invention relates to a catalyst that comes in the form of grains, each grain consisting of a core covered by at least one external layer, the core consisting of an acidic porous solid and having a size of between 0.1 micron and 0.4 millimeter, in which the external layer has a uniform thickness with a uniformity criterion C, which is less than 0.30, whereby said uniformity criterion C is defined as being equal to an average, on a number N of catalyst grain samples, of the ratio of the difference between the maximum thickness, Eimax, of the external layer and the minimum thickness, Eimin, of this same layer to the average of these two thicknesses Eimax and Eimin.