ORIGINAL PAPER Reinventing rice husk ash: derived NaX zeolite as a high- performing CO 2 adsorbent N. Gargiulo 1 • K. Shibata 2 • A. Peluso 1 • P. Aprea 1 • T. Valente 3 • G. Pezzotti 2 • T. Shiono 2 • D. Caputo 1 Received: 23 December 2016 / Revised: 7 July 2017 / Accepted: 5 September 2017 Ó Islamic Azad University (IAU) 2017 Abstract In this work, rice husk ash was used as silica source to synthesize NaX zeolite potentially suitable for CO 2 adsorption. The produced material, denoted NaX- RHA, was characterized employing X-ray diffraction, scanning electron microscopy and gas adsorption porosimetry, in order to verify the occurred production of well-crystallized NaX zeolite with a significant degree of purity. CO 2 adsorption isotherms on NaX-RHA were vol- umetrically evaluated in the 298–348 K temperature range up to standard pressure, revealing performances that are higher than those reported for commercial similar sub- strates. The experimental data regarding CO 2 adsorption on NaX-RHA were very satisfyingly fitted by the semiem- pirical Sips model. Analyzing the best fitting values of model parameters allowed to conclude that the synthesized adsorbent could be quite suitable for applications like CO 2 capture from flue gas. Keywords NaX zeolite Á Synthesis Á Rice husk ash Á CO 2 capture Á Adsorption Á Modeling Introduction Among the different anthropogenic air contaminants, car- bon dioxide can be currently considered the most relevant one, mainly due to its contribution to the greenhouse effect. Even though cutting CO 2 emissions has become the objective of significant efforts by both the scientific com- munity and many national governments, the effectiveness of such efforts is still lacking, mainly because of the per- sisting use of fossil fuels as the global primary energy source (Victor et al. 2014). Indeed, this is also a crucial drawback of the most common technologies for the pro- duction of ‘‘clean’’ energy carriers such as hydrogen (Halmann and Steinberg 1999). One feasible way to block fossil fuel—related CO 2 emissions is to separate this compound from the gas mixtures that are produced during fossil fuel processing. Absorption, with or without the implementation of chemical reactions, is the most common unit operation for CO 2 removal from gas mixtures. Nev- ertheless, throughout the last decades, many efforts have been made in order to identify better performing, less expensive unit operations for carrying out this separation process. In this sense, adsorption, i.e., the transfer of gas- eous compounds on solid surfaces, seems to be the best alternative unit operation (Gargiulo et al. 2014a). The effectiveness of adsorption processes relies on the adsorptive–adsorbent mutual interactions, which in turn depend on the nature of the adsorptive and its thermody- namic activity, on the nature of the adsorbent and on the working values of the state variables (e.g., temperature). Microporous materials, i.e., materials having pore sizes Editorial responsibility: Agnieszka Galuszka. & D. Caputo domenico.caputo@unina.it 1 ACLabs - Laboratori di Chimica Applicata, Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Universita ` degli Studi di Napoli Federico II, P.le Tecchio 80, 80125 Naples, Italy 2 Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan 3 Dipartimento di Ingegneria Chimica Materiali Ambiente, Universita ` di Roma La Sapienza, Via Eudossiana 18, 00184 Rome, Italy 123 Int. J. Environ. Sci. Technol. DOI 10.1007/s13762-017-1534-5