Undesired metal nanoparticles located outside zeolite nanoboxes (hollow zeolites) can be formed during the preparation of zeolite-embedded metal nanoparticles. The present work demonstrates that it is possible to use citric acid to selectively leach outmost of the external Ni nanoparticles from a Ni@silicalite-1 material. The leached sample exhibited an improved selectivity in the hydrogenation of toluene as compared to that of the bulkier mesitylene. (C) 2017 Elsevier B.V. All rights reserved.