On the direct imaging of offretite, cancrinite, chabazite and other related ABC-6 zeolites and their intergrowths

Abstract

Zeolites that belong to the so-called ABC-6 family may be regarded as having been assembled architecturally from individual sheets, each of thickness ca . 2.6 Å (1 Å = 10 -10 m = 10 -1 nm). The sheets consist of macro-anions of corner-linked SiO 4- 4 and AlO 5- 4 tetrahedra (Si: Al ranging from 1- 4 ), charge-compensating cations, typically Na + , Ca 2+ and K + and occluded water. Cancrinite (idealized formula Na 5 Al 6 5 Si 6 O 24 . m H 2 O), in which the stacking sequence of the sheets may be symbolized AB, has a 5.1 Å repeat along the direction of stacking; offretite (idealized formula (Na 2 Ca) 2 Al 4 Si 14 O 36 . 14 H 2 O ) has AAB stacking and a repeat distance of 7.6 Å and sodalite (NaAl 6 Si 6 O 24 . m H 2 O) has ABC stacking with a repeat distance of 7.7 A. This pattern continues; gmelinite (AABB), losod (ABAC), erionite (AABAAC), chabazite (AABBCC), TMA-E(AB) (ABBACC), liottite (ABABAC), afghanite (ABABACAC), levyne (AABCCABBC) and franzinite (ABCABCBACB), the repeat distance of the latter being 26.6 Å. It has long been suspected, largely on the basis of chemical and diffusive behaviour, that naturally occurring and synthetic analogues of this class of zeolites can, depending upon the nature of their genesis, form intergrowths at the unit-cell level. For example, there is a sliver of erionite or sodalite in offretite. The presence of such features would profoundly affect the catalytic and adsorptive performance of the parent zeolite and for this reason there is a pressing need for a technique to identify such intergrowths. Hitherto no direct method has been available that is capable of probing the nature of these intergrowths and their atomic detail. We show here that high-resolution electron microscopy (h. r. e. m.) combined with computer-generated images can solve this problem, provided that the samples are first ‘de-aluminated’ under conditions close to those used to activate many of these zeolites before their use as catalysts. The consequence of dealumination, which greatly increases the Si:Al ratio (to more than 10) is to jettison most of the compensating cations originally present and to render the resulting specimens (after annealing to heal the individual sheets) more resistant to electron irradiation. The resulting structure is quite an open one, amenable to imagining by h. r. e. m. and to simple optical simulation. The reliability of the h. r. e. m. -based method of reading off stacking sequences in this family of de-aluminated zeolites was tested by imaging offretite and comparing the result with images produced by rigorous ‘multi-slice’ calculations and simple optical simulation. From such work it was established that the siting of the six- and eight-membered apertures present in the structures gives unequivocal information regarding the sequencing of the sheets. On the basis of this information, computed images of 29 distinct kinds of stacking sequences are derived, encompassing both the structurally regular (de-aluminated) zeolites cancrinite, offretite, sodalite, gmelinite, erionite, chabazite and TMA-E(AB) and also unit-cell intergrow ths (i) within erionite of cancrinite, offretite, sodalite, gmelinite and chabazite; (ii) within offretite of cancrinite, sodalite, erionite, gmelinite and chabazite; (iii) within chabazite of offretite, sodalite, gmelinite and erionite; (iv) within gmelinite of offretite, erionite, chabazite and TMA-E(AB) and (v) within cancrinite of sodalite, offretite and erionite. This compilation can now serve as a reference library th at should aid future work in recognizing and characterizing intergrowths recorded by h. r. e. m.

Synthesis of free-standing chabazite-type films

“…In gmelinite, the D6R units are stacked in the sequence of ABAB instead of ABCABC [23]. Chabazitegmelinite intergrowths have been reported by several researchers [22,23,26,27], indicating that these two frameworks may share similarly structured nuclei. Moreover, Daniels et al [22] observed that with increase in the Si/Al ratio of the composition, chabazite is the preferred product.…”

Hierarchically Structured Zeolites: From Design to Application

“…Hierarchically porous zeolites can also be prepared without the employment of secondary hard or soft templates. Several methods including engineering of intergrowth, ,− additive-assisted crystallization, ,,− seed-induced synthesis, − ,− a controlled crystallization process, − ,, and designing special structure-directing agents − have been reported for the synthesis of hierarchically porous zeolites in the absence of organic templates.…”

“…In particular, these OSDAs favored the generation of ultrathin plates with perpendicularly rotational intergrowth, resulting in the formation of the hierarchically porous structure. The synthesis of hierarchical zeolite formed by the intergrowth of two zeolite crystal structures has been explored in other systems, such as FAU/EMT, EMT/FAU, ETS-4/ETS-10, CAN/SOD, MFI, and CHA . However, only the SPP morphology has been reported to contain zeolite nanosheets of single-unit-cell thickness …”

Sandwich-Type Zeolite Intergrowths with MFI and the Novel Extra-Large Pore IDM-1 as Ordered End-Members

“…The presence of stacking faults may affect important zeolite properties and, hence, their performance in catalysis and separation applications. For instance, in the so-called ABC-6 family, disorder may result in pore blockage, and it may also alter the catalytic performance of the disordered material . The degree of intergrowth in MFI/MEL catalysts and membranes may impact their catalytic − as well as their permeation and selectivity properties, respectively .…”