Steam activation of Mg–Al hydrotalcite. Influence on the properties of the derived mixed oxides So ` nia Abello ´ a , Javier Pe ´rez-Ramı ´rez a,b, * a Laboratory for Heterogeneous Catalysis, Institute of Chemical Research of Catalonia (ICIQ), Av. Paı ¨sos Catalans 16, Tarragona 43007, Spain b Catalan Institution for Research and Advanced studies (ICREA), Pg. Lluı ´s Companys 23, Barcelona 08010, Spain Received 21 April 2006; received in revised form 13 June 2006; accepted 13 June 2006 Available online 4 August 2006 Abstract The influence of steam activation of Mg–Al hydrotalcite on the properties of the resulting mixed oxides has been investigated and compared with the typically performed thermal decomposition under dry conditions. The as-synthesized hydrotalcite was treated at 723 K and 1 bar in nitrogen flow containing steam partial pressures in the range of 0–800 mbar. Thermal analysis indicated that the hydrotalcite decomposition was not affected by the presence of steam in the feed gas. The average crystallite size and morphology of the dry and steam-activated materials, as determined by XRD and TEM, respectively, were very similar too. The porous properties of the resulting oxides present significant differences. The BET surface area decreased ca. 20% when small amounts of steam were present as compared to the dry-decomposed product, but was not further reduced upon increasing the steam partial pressure. This has been attributed to the disappearance of small mesopores (<4 nm) due to the hydrolysis of Al–O–Al bonds from adjacent layers and platelets. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Hydrotalcite; Decomposition; Steam; Mixed oxides; Porosity; N 2 adsorption 1. Introduction Synthetic hydrotalcites are layered double hydroxides with the general formula ½M 2þ 1x M 3þ x ðOHÞ 2 ½X m  x=m  nH 2 O, which consists of brucite-type octahedral layers where M 3+ cations partially substitute for M 2+ cations. The posi- tive charge resulting from this substitution is compensated by anions (often carbonate) and water molecules, located in the interlayer space. These materials find widespread application as ion exchangers, adsorbents, flame retardants, and precursors for catalysts and catalyst supports [1,2]. Properties like high specific surface area, atomic metal dispersion, and memory effect of the multimetallic mixed oxides obtained by thermal treatment make them attractive materials in catalytic applications [3–11]. The decomposi- tion conditions impact on the catalytic performance of the resulting oxide [4,12]. Accordingly, many studies have investigated the thermal decomposition behavior of hydro- talcites, with major emphasis on the Mg–Al–CO 3 system [13–17]. To the best of our knowledge, possibilities of alter- ing the properties of decomposed hydrotalcites by activa- tion in atmospheres other than inert and air have not been explored. High-temperature steam treatment has been widely used to treat catalysts and catalyst supports. Specifically for alu- minum-containing materials, steaming has been success- fully applied to zeolites in order to extract trivalent framework atoms (often Al but also Fe and/or Ga), yield- ing materials with mesopores, higher catalytic activity, and improved (hydro)thermal stability [18–21]. A classical example is the steam treatment of Y zeolite for fluid cata- lytic cracking [22]. High-temperature steaming of alumina led also to an improved thermal stability of the resulting product [23,24]. 1387-1811/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.06.018 * Corresponding author. Address: Laboratory for Heterogeneous Catal- ysis, Institute of Chemical Research of Catalonia (ICIQ), Av. Paı ¨sos Catalans 16, Tarragona 43007, Spain. Tel.: +34 977 920 236; fax: +34 977 920 224. E-mail address: jperez@iciq.es (J. Pe ´rez-Ramı ´rez). www.elsevier.com/locate/micromeso Microporous and Mesoporous Materials 96 (2006) 102–108