High pressure behavior of microporous materials with chabazite topology

The extensive use of microporous materials in the most different sorts of chemical processes has required exhaustive studies about their response to non standard conditions, especially heating and dehydration. On the other hand, their response to high pressure (HP) has been only recently investigated. From these investigations, the crucial role of the extra-framework content on the zeolite response - in terms of deformation mechanisms and compressibility values - emerged clearly. On the contrary, the effect of framework composition on compressibility and HP stability of microporous materials has not yet been systematically investigated. The aim of this work is to understand the influence of both framework and extraframework content of a given framework topology on stability, degree of compressibility and pore deformation under pressure. A series of natural and synthetic microporous materials with chabazite framework topology (CHA), largely applied as heterogeneous catalysts and characterized by a high thermal stability, were selected for this purpose: a) natural chabazite (Vallerano, Italy): (K1.36,Ca1.04,Na0.28,Sr0.4,Ba0.06,Mg0.02) [Si7.17Al4.87O24] • 13.16 H2O [s.g. R-3m] b) SAPO-34 (as synthesized): Si0.9Al6.00P5.10O24 •C4H9ON,C4H10NO+ • 2.7 H2O [s.g. R-3] c) ALPO-34 (as synthesized):Al6(PO4)62F-•2C4H10NO+ [s.g.P-1] d) ALPO-34 (calcinated): Al6(PO4)6 • n H2O [s.g. P2/m] The four phases are characterized by different tetrahedral composition: Si,Al in natural chabazite; Al,P in ALPO-34 and Si,Al,P in SAPO-34, respectively. Moreover, ALPO-34 as synthesized contains one third of the Al atoms in octahedral coordination. Concerning the extraframework content, ALPO-34 and SAPO-34 as synthesized contain morpholinium/morpholine as structure directing agent, while chabazite contains alkaline/earth alkaline cations and water molecules. The selected materials were investigated by means of in-situ synchrotron X-ray powder diffraction from Pamb to 6-7 GPa and upon decompression. HP-XRPD experiments were performed at the Swiss-Norwegian beamline (BM01a) at ESRF (Grenoble, France), with fixed wavelength of 0.71 Å, using a modified Merril-Basset DAC and silicon oil as non-penetrating P-transmitting medium. Patterns were collected by a means of a MAR 345 IP detector, and the two-dimensional images were turned into 2θ-scan with the FIT2D software. The 2θ accessed range is 0 – 47°. No complete amorphization is observed up to the highest investigated pressure for no one of the samples and the P-induced effects on CHA framework topology seem to be completely reversible. The unit cell parameters were extracted from the powder patterns by means of Rietveld or Le Bail methods (Fig. 1, for chabazite from Vallerano ). In the Pamb-4.5 GPa pressure range, a volume reduction of about 6.9%, 11.5%, 9.0% and 12.4% is observed for natural chabazite, SAPO-34, ALPO-34 as synthesized and calcinated, respectively. ALPO-34 as-synthesized undergoes a phase transition from the triclinic P-1 to the monoclinic C2/m s.g. between 3.4 and 3.9 GPa. The lower compressibility of ALPO-34 as synthesized with respect to SAPO-34 can be interpreted as due to the octahedral coordination of part of the framework aluminum by oxygen and fluorine anions in ALPO-34. This coordination geometry leads to a more rigid framework with respect to the conventional tetrahedral Al coordination. The higher compressibility of calcinated ALPO-34 with respect to the as-synthesized one is due to the loss of morpholine molecules used as template upon heating, and the consequent larger free volume in the channels, and to the possibly recovered tetrahedral coordination of the aluminum cations. Natural chabazite has the lowest compressibility among the phases here studied, mainly because of its high and complex extra-framework content.