The unusual thermal behaviour of boron-ZSM-5 probed by ‘‘in situ’’ time-resolved synchrotron powder diffraction Lara Leardini a,b,⇑ , Annalisa Martucci b , Giuseppe Cruciani b a Dipartimento di Fisica e Scienze della Terra, Viale Ferdinando Stagno d’Alcontres 31, I-98166 Messina – S. Agata, Italy b Dipartimento di Fisica e Scienze della Terra, Sezione di Mineralogia, Petrografia e Geofisica, Via Saragat 1, I-44100 Ferrara, Italy article info Article history: Received 9 October 2012 Received in revised form 25 January 2013 Accepted 29 January 2013 Available online 13 February 2013 Keywords: B-ZSM-5 Thermal behaviour Rietveld refinement In situ X-ray powder diffraction abstract The thermal behaviour of boron substituted ZSM-5 synthesized in the presence of ethylenediamine as an organic template was highlighted for the first time by in situ time-resolved synchrotron powder diffrac- tion in the 25–900 °C temperature range. The evolution of the structural features was followed by full profile Rietveld refinements at up to 730 °C. Above this temperature, a structural breakdown occurs and B-ZSM-5 is converted into a b-cristobalite phase. A continuous increase in unit-cell parameters as a function of temperature is observed both during calcination (between 200 and 500 °C) and afterwards. This behaviour is completely unusual when compared to that assumed by other boron-free MFI-type materials both in their as-synthesized as well as calcined forms. On the basis of all these results, B-ZSM-5 represents the first example of positive thermal expansion in a material with an MFI type topology. Ó 2013 Elsevier Inc. All rights reserved. 1. Introduction Zeolites are an important class of microporous materials with good stability, large void volumes and well-defined tailorable cav- ities of uniform size. Their potential is great since these are pre- cisely the properties required for catalysis, separation and storage/release applications. Moreover, their catalytic properties may be appropriately tuned by isomorphously substituting Al and/or Si with other tetrahedrally coordinated heteroatoms such as B 3+ , Ga 3+ , Fe 3+ , Mn 3+ , Ti 4+ and Zr 4+ . In particular, the boron incor- poration into the tetrahedral sites of zeolites provides new materi- als, known as ‘‘boralites’’ or ‘‘borosilicates’’, which demonstrate lower acidity than their parent aluminosilicates [1]. In recent years, boralites have attracted special attention since their weak acidity makes them suitable catalysts for reactions requiring low acid strength [1–6]. For this reason, the incorpora- tion, acidity and stability of B in zeolite frameworks has been the object of several experimental and theoretical studies (see Ref. [1] and references therein) employing various methods such as so- lid state NMR [7–12] and IR spectroscopy [12,13] XRD analysis [7,14–20], quantum chemical calculations [10,21], density func- tional theory (DFT) [22], and the Car–Parrinello approach [23,24]. These studies have unambiguously proved the boron incorporation into the tetrahedral sites of several zeolite framework types [1]. In addition, boron can change its coordination from tetrahedral to tri- gonal upon heating depending on the nature of the counterions (protons, large cations, organic cationic structure directing agent (SDA)) and on the hydration/dehydration level (see Ref. [20] and references therein). This tetrahedral-to-trigonal boron conversion is expected to have important implications on the catalytic func- tion of these microporous materials. Catalysts need to have sufficient thermal stability to withstand the extreme conditions frequently involved in their use and regen- eration. In comparison to zeolites, boralites display lower thermal stability and framework deboronation often occurs during the cal- cination steps which are required to eliminate organic molecules trapped within their pores [1]. Consequently, knowledge of the structural modifications induced by temperature after B incorpora- tion into the framework of zeolites is of prime importance to as- sure their effectiveness in technological applications. As one of the most famous solid acid catalysts largely used in the field of catalysis and fine chemistry, considerable interest has been shown in the incorporation of B into ZSM-5 zeolite (MFI-type topology [25]). The pore system of the MFI framework structure is three-dimensional and presents two intersecting sets of tubular channels, a linear one parallel to the (0 1 0) direction, and a sinusoi- dal one parallel to the (1 0 0) direction. Both channels are limited by 10-membered ring-openings of TO 4 tetrahedra. The MFI frame- work is very flexible, and presents at least four known space group symmetries: at room temperature, the empty hydrogen-containing ZSM-5 is known to be monoclinic space group P2 1 /n, whereas, the hydrogen free MFI type material is orthorhombic, with a Pnma space group [26,27]. At higher temperatures or at low and 1387-1811/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2013.01.036 ⇑ Corresponding author. Address: Dipartimento di Scienze della Terra, Università di Messina, Viale Ferdinando Stagno d’Alcontres 31, I-98166 Messina – S. Agata, Italy. Tel.: +39 (0)90 6765096; fax: +39 (0)90 392333. E-mail address: lleardini@unime.it (L. Leardini). Microporous and Mesoporous Materials 173 (2013) 6–14 Contents lists available at SciVerse ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso