THE DEHYDRATION PROCESS IN THE ZEOLITE LAUMONTITE - A REAL-TIME SYNCHROTRON X-RAY-POWDER DIFFRACTION STUDY

The dehydration process of the natural zeolite laumontite Ca4Si16Al8O4 8. 18 H2O has been studied in situ by means of powder diffraction and X-ray synchrotron radiation. Powder diffraction profiles suitable for Rietveld refinements were accumulated in time intervals of 5 minutes u sing a position sensitive detector (CPS-120 by INEL), while the temper ature increased in steps of about 5 K. The synchronization of accumula tion time and temperature plateau allowed collection of 62 temperature -resolved powder patterns in the range 310-584 K, whose analysis produ ced a dynamic picture of the laumontite structure response to dehydrat ion. The first zeolitic water molecules diffusing out of the channels are those not bonded to the Ca cations and located in the W(1) site, w hose occupancy drops smoothly to 10% during heating to 349 K, while th e sample in the capillary is still submerged in water. The remaining W (1) and 60% of W(5) water molecules are expelled rather sharply at abo ut 370 K. At this temperature all remaining water submerging the powde r crystallites is lost, the structure contains about 13 water molecule s/cell, and the crystal structure is that of leonhardite. On continued heating 80% of the water molecules from the W (2) site are lost betwe en 420 and 480 K, while a small amount of the diffusing water is reins erted in the W(5) site. The occupancy factor of the W(8) site decrease s starting at 480 K, and reaches a maximum loss of 20% at 584 K. The c ombined occupancy of the Ca-coordinated W(2) and W(8) water sites neve r falls much below two, so that the Ca cations in the channels, which are bonded to four framework oxygen atoms, are nearly six-coordinated in the explored temperature range. The water loss is accompanied by la rge changes in the unit cell dimensions. Except at 367 K, where the ex cess surrounding water is leaving, all changes in cell. dimensions are gradual. The loss of the hydrogen bonded W(1) and W(5) water molecule s is related to most of the unit cell volume reduction below 370 K, as shown by the contraction of the a-, b- and c-axes and the increase in the monoclinic angle. Loss of the Ca-coordinated W(2) and W(8) water molecules has a small effect on the unit cell volume as the continued contraction of the a- and c-axes is counter-balanced by a large expans ion in the b-axis and a decrease in the monoclinic beta angle.