Dehydration dynamics and thermal stability of erionite-K: Experimental evidence of the “internal ionic exchange” mechanism

Abstract

The dehydration dynamics and the thermal stability of a sample of erionite-K, (Na_2.00K_2.87Mg_0.80Ca_0.86) [Al_6.95Si_29.05O_72.64]·30.5H₂O, have been investigated in the 303–1253 K thermal range by in situ X-ray powder diffraction. The structural breakdown starts to occur at 1113 K and is completed at 1193 K. The dependence of cell parameters and volume from temperature is fairly complex. As general trends, the c-parameter expands whereas the a-parameter contracts at a faster rate and, as a net result, the cell volume contracts. The smallest cell volume is observed at 1093 K just before the incoming of the structural breakdown. Microstrain ε₀ is remarkably constant from 303 K up to approximately 1000 K. At this temperature, ε₀ starts to increase, at a fast rate, from ca. 0.05 to a maximum value of ca. 0.09 at 1113 K, in correspondence of the starting of the structural breakdown. As the structural breakdown proceeds, microstrain is increasingly released until the complete structure disruption is attained. The most relevant framework modifications with increasing temperature are: (a) the squeezing of the 8-member ring (8MR) leading to a reduction of the Crystallographic Free Area (CFA) from ca. 14 to ca. 12.4 Å²; (b) the expansion of the 6-member ring (6MR) shared between the cancrinite cage and the erionite cavity permitting the cation diffusion between them; (c) the expansion of the base of the erionite cavity to host the migrating Ca2 cation. The evolution of the distribution of the site scattering (s.s.) of the extra framework (EF) cations as a function of temperature is quite complicated. It may be roughly described as a general depletion of the Ca1, Ca2, and K1 sites counterbalanced by an increase of the K2 s.s. The observed reduction of the site scattering of the K1 site has been explained by invoking an “internal ion exchange” mechanism whose detailed description is difficult because of the simultaneous occurrence of three cationic species (Na, K, and Ca). In fact, the process should lie anywhere between two extremes, the partial substitution of Na for K (at a 50:50 K:Na proportion at high temperatures) and the complete substitution of Na + Ca for K (at a 55:45 Na:Ca proportion at high temperatures). The full depletion of the various water molecule sites is attained in the 413–573 K thermal range, in good agreement with TG data, the only exception being OW7 that shows significant s.s. up to the structural breakdown. This fact has been justified considering OW7 as a site partially occupied by both water molecules and EF cations.

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,Ca_0.5)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:

• 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];

• Detecting and monitoring the eventual occurrence of the so-called “internal ion exchange” mechanism;

• Correlating the framework modifications occurring as a function of temperature with both the dehydration and cations migration processes;

• Monitoring the mobility of the various cation species.