Synthesis and characterization of ZSM‐8‐type zeolite crystals

Abstract

Procedure for hydrothermal synthesis of ZSM-8-type zeolite crystals, using TEA-OH as a template, in the system having molar compositionat 170 "C is described. Results of characterization of these crystals by XRD, IR, EDAX, SEM, TGA, and adsorption studies are described and discussed. X-ray diffraction studies establish that the crystals are ZSM-8-type with structure similar to that of ZSM-5-type crystals. Crystals have prismatic bipyramidal morphology with average size 18-25 microns. Degased calcined samples adsorb xylenes in the order p > na > 0. Pore length per unit cell, estimated from n-hexane adsorption studies, is 46 A.

Pitfalls in the powder diffraction analysis of zeolites ZSM-5 and ZSM-8

“…Recalling that the linear fit shown in Figure 2(a) and (b) is based on ZSM-5 and silicalite data points only, there appears to be a coincidence of all three ZSM-8 lattice parameters, a, b, and c with ZSM-5 average data. This observation is in contrast to the data given by Joshi and Prabhu (1988), where a and c rep- Joshi and Prabhu (1988) are marked with an asterisk. Data correspond to Table III. resented by the open and filled circles at 5.5 Al/u.c.…”

“…Careful evaluation of the published data reveal that the powder diagram given for ZSM-8 is in part incorrectly indexed and thus yields wrong lattice constants. Reindexing based on d values published by Joshi and Prabhu (1988) gives lattice parameters in the normal range known for ZSM-5 after least-squares refinement (Figure 2). The corrected data points are represented by open and filled diamond symbols in Figure 2(a) and (b).…”

“…For reindexing of the powder diffraction data given by Joshi and Prabhu (1988) and refinement of the lattice constants, the program NBS*AIDS80 (Mighell et al, 1981) was used. Powder diffraction data of ZSM-8 and ZSM-5 samples studied here are calculated with a modified Rietveld program (Kassner, 1993).…”

“…However, some criteria indicate distinct differences between the crystal structures which support the statement in the patent literature (Plank et al, 1973) that the characteristic X-ray diffraction pattern of ZSM-8 indicates that "ZSM-8 possesses a definite distinguishing crystalline structure." These criteria are as follows: (a) The lattice constant a reported for ZSM-8 (Joshi and Prabhu, 1988) is significantly smaller than the corresponding value for ZSM-5 with similar aluminum content (Figs. 2 and 3); (b) the peak at 23° 20 is reported to be split in ZSM-8 diagrams and to be unsplit in ZSM-5 diagrams (Lechert, 1984); and (c) sorption studies (e.g., Chon et al, 1985;Akolekar and Choudhary, 1987) show distinct differences between ZSM-5 and ZSM-8 in the adsorption capacities of organic molecules.…”

“…The crystal structure of ZSM-5 has been studied extensively by single crystal and powder diffraction methods (see, e.g., van Koningsveld et al, 1987; Koningsveld, 1990;Baerlocher, 1984) yielding precise data on the framework structure and template position. The crystal structure of ZSM-8 has not been described yet, but it has been suspected that it is closely related to the crystal structure of ZSM-5 due to similarities in the powder diffraction patterns (see, e.g., Joshi and Prabhu, 1988;Akolekar and Choudhary, 1987). However, some criteria indicate distinct differences between the crystal structures which support the statement in the patent literature (Plank et al, 1973) that the characteristic X-ray diffraction pattern of ZSM-8 indicates that "ZSM-8 possesses a definite distinguishing crystalline structure."…”