In {TAIL ~ another preferred embodiment} of {HEAD ~ the present invention} the X zeolite selected for the second adsorbent layer comprises a NaX zeolite with a Si/Al elemental ratio of the zeolitic phase between 0.9 and 1.3, preferably 0.9 and 1.15, and especially being 0.95 to 1.05 
In { TAIL ~ another preferred embodiment } of { HEAD ~ the present invention } the X zeolite selected for the second adsorbent layer comprises a NaX zeolite with a Si / Al elemental ratio of the zeolitic phase between 0.9 and 1.3 , preferably 0.9 and 1.15 , and especially being 0.95 to 1.05
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