Dehydration dynamics and thermal stability of erionite-K: Experimental evidence of the “internal ionic exchange” mechanism
Graphical abstract
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Highlights
► The dehydration dynamics and thermal stability of erionite was studied by XRPD. ► The structural breakdown occurs in the 1113–1193 K thermal range. ► The Ca2 cation migrates toward the center of the base of the erionite cavity. ► The reduction of the s.s of K1 site is due to the internal ion exchange mechanism. ► The full depletion of water molecule sites occurs in the 413–573 K thermal range.
Introduction
Erionite (ERI) is a fibrous zeolite pertaining to the ABC-6 family [1]. It is hexagonal, space group P63/mmc [2] and has an average formula K2(Na,Ca0.5)8[Al10Si26O72]·30H2O [3]. Moreover, a large chemical variability [4], [5], [6] is typical of this zeolite and for this reason it was elevated to group status and its member species were re-defined as erionite-K, -Na, and -Ca [3] according to the most abundant extraframework (EF) cation. Erionite and published erionite group minerals were recently re-evaluated and re-classified [7].
The erionite framework consists of columns of regularly alternating cancrinite, or ε cages ([4665] polyhedra following the IUPAC nomenclature, [8]), and double 6-rings (D6R or [4662] polyhedron). The erionite cavities ([4126586] polyhedra) are formed by the linking at the same level of ε-cages of adjacent columns via single 6-rings [9], [10]. The erionite cage hosts EF cations, as well as all water molecules. Up to four partly mutually excluding cation sites Ca1, Ca2, Ca3, and Mg have been located at or near the sixfold axis running along the cage [6], [11], [12]. Moreover, a further site (K2), located at the center of the boat-shaped 8-member rings (8MR) forming the walls of the erionite cage, has been firstly recognized in dehydrated erionite and assigned to K [13]. The same site has been subsequently reported, with low occupancy, in a few samples [6], [12]. The cancrinite cage of non-thermally treated samples contains a potassium ion at the K1 site that commonly shows a site scattering (s.s.) slightly smaller than expected [6], [11], [12], possibly because of a very limited substitution of Na for K. On the contrary, the cancrinite cage of dehydrated erionite contains a calcium ion because of the so-called “internal ion exchange” mechanism [13], that is the basis of US Patent 3640680 [14], as the Ca ion has been reported to drive the K ion from its position during the dehydration process.
The erionite framework may be also described by the stacking along the z direction of layers made of 6-membered rings of (Si,Al)O4 tetrahedra, following the AABAAC sequence. It is reported the occurrence of epitaxial intergrowths with offretite (OFF) due to the close similarity between the two zeolites [4], [15]. In fact, offretite has an AABAAB sequence and the occurrence of stacking faults in erionite, corresponding to a partial substitution of the C layer with B, has been shown [13].
Much less detailed information is available about the thermal stability of erionite. Erionite has been classified as a category-1 zeolite [16] following the scheme proposed by Alberti and Vezzalini [17]. Category-1 includes zeolites characterized by “…Dehydration with a rearrangement of the extraframework cations and of residual water molecules without considerable changes in the geometry of the framework and in the cell volume…” The cell parameters and volume dependence from temperature has been reported up to 573 K for an erionite sample from Eastgate, Nevada [18]. Such findings have been summarized and compared to those of other selected zeolites by Cruciani [19]. Erionite was reported to shows an anisotropic thermal expansion behavior because the c-parameter increases as a function of temperature while the a-parameter contracts at a faster rate and, as a result, the volume contracts. No significant differences in the thermal expansion behavior were found as a function of the type of EF cations [16].
The present work, which is a part of a broad investigation on the structural features, crystal chemistry relationships, thermal stability, and health related issues of fibrous zeolites, aims to investigate, at a fine temperature scale, the thermal behavior of a sample of erionite-K from Rome, Oregon, USA, to 1253 K from in situ laboratory parallel-beam transmission X-ray powder diffraction data. This instrumental set up has been recently proven to provide temperature-resolved data of excellent quality [20], [21], [22], [23], [24]. More in detail, the paper is aimed at:
- a)
Defining the thermal stability of erionite-K, including the structure breakdown temperature and its classification on the basis of the categories indicated by Alberti and Vezzalini [17];
- b)
Detecting and monitoring the eventual occurrence of the so-called “internal ion exchange” mechanism;
- c)
Correlating the framework modifications occurring as a function of temperature with both the dehydration and cations migration processes;
- d)
Monitoring the mobility of the various cation species.
Section snippets
Data collection
The erionite-K powder, from the same locality of the sample previously characterized at room temperature [12], was loaded and packed in a 0.7 mm diameter quartz-glass capillary, open at both ends, that was fixed to a 1.0 mm diameter Al2O3 tube by means of a high-purity alumina ceramic (Resbond 989). The capillary/tube assembly was subsequently aligned onto a standard goniometer head and diffraction data were collected on a parallel-beam Bruker AXS D8 Advance, operating in transmission in θ–θ
Cell parameters and volume dependence from temperature
Cell parameters and volume at 303 K are in agreement with those obtained at RT for a sample from the same locality [12]. The structural breakdown starts to occur at 1113 K (Tbreak). This process, that can be monitored from the increase of the background counterbalanced by a general intensity reduction of the Bragg reflections of erionite-K, is completed at 1193 K (Fig. 1). Those temperatures are higher than 1023 K reported by Cruciani [19] for erionite. The observation that zeolites with a Si/Al
Conclusions
The dehydration dynamics and the thermal stability of a sample of erionite-K have been investigated, at a fine temperature scale, in the 303–1253 K thermal range by in situ X-ray powder diffraction. Present data extend up to a significantly higher temperature than reference data [21]. Besides, the structural breakdown occurs, due to kinetics, in the 1113–1193 K thermal range, at temperatures significantly higher than that reported by Cruciani [19]. According to the definition of the empirical
Acknowledgments
The work has received financial support from Sapienza Università di Roma.
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