Inorganic Cation Exchange Properties of Zeolite ZSM-5 - ACS

4 Inorganic Cation Exchange Properties of Zeolite ZSM-5

Downloaded by MONASH UNIV on December 8, 2014 | http://pubs.acs.org Publication Date: May 17, 1983 | doi: 10.1021/bk-1983-0218.ch004

P. C H U and F. G. DWYER Mobil Research and Development Corporation, Paulsboro, NJ 08066

The discovery of the zeolite ZSM-5 has led to the development of catalysts for a wide variety of process applications in petroleum refining, petrochemical manufacture and synthetic fuels. In order to further elucidate the structural characteristics and the basic chemical nature of this most interesting material the ion exchange characteristics have been studied for a wide spectrum of cations using a variety of morphological and compositional forms of the zeolite. The results of this study have shown the ion selectivities to be considerably different from those of the conventional synthetic zeolites A, X and Y. Up to the l i m i t of cation size capable of penetration of the ZSM-5 pore opening, the selectivity for inorganic cations was primarily dependent upon base ion size rather than charge. These cation selectivities are explained i n terms of electrostatic forces and pore dimensions of the zeolite and are related to the shape selective catalytic and adsorptive properties of catalysts made from ZSM-5.

The unique nature of ZSM-5 as a catalyst and as a zeolite has been well documented (1-3), as shown by the wide spectrum of chemistry i t can catalyze as well as the wide range of morphologies and composition i n which i t can be synthesized. In addition, ZSM-5 exhibits ion exchange properties that, in some cases, are very different from other zeolites. Results from the study of these ion exchange characteristics have been used effectively in developing optimal methods for conversion of ZSM-5 into catalytic forms and in supplying information concerning the zeolite structure not discernible by other techniques that could lead to more active, selective and stable catalysts. 0097-6156/83/0218-0059$06.00/0 © 1983 American Chemical Society In Intrazeolite Chemistry; Stucky, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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INTRAZEOLITE CHEMISTRY

The structure of ZSM-5 has been determined by Kokotailo et a l . (4). Its framework contains a novel configuration of linked S i 0 and Al 03 tetrahedra as groups of five-membered rings. The skeletal diagrams of the (010) and (100) faces of a ZSM-5 unit c e l l are shown in Figures l a and lb revealing the two channel structures. The e l l i p t i c a l 10-membered ring apertures, shown in Figure l a , are the entrances to the straight channels which run parallel to [010 ]. The nearly circular 10 membered ring apertures, shown i n Figure l b , are the entrances to the sinusoidal channels which run p a r a l l e l to [100 ]. ZSM-5, i n the as synthesized form, i s not readily susceptible to ion exchange due to the hindrance from bulky quaternary cations and occluded organic materials which are believed to be located i n the channel system. Incoming and outgoing ions a l l have to pass through the channels and apertures to complete the ion exchange process. Therefore, in order to obtain i n t r i n s i c ion exchange information, these bulky organics must be removed and replaced by smaller cations such as Na+ or N H . The method of removal of the organics used in this study was by thermal treatment although chemical treatment i s also an effective mode of removal. Although ZSM-5 has been synthesized with Si02/Al 03 of almost unbounded limits, in this study we investigated materials with Si02/Al 0 of 40 to 200. It should also be noted that even at the lowest S i 0 / A l 0 , the cation exchange capacity of ZSM-5 i s only 0.75 meq/g as compared to 4 to 7 meq/g for zeolites A, X and Y and 3 to 5 meq/g for most cation exchange resins. The extremely high Si0 /Al 03 ZSM-5 materials are less desirable for use i n ion exchange studies due to practical d i f f i c u l t y i n obtaining accurate elemental analysis at such low levels.


2

2

+

4

2

2

3

2

2

2

3

2

Experimental ZSM-5 Zeolite Synthesis A l l the ZSM-5 zeolites were synthesized by reacting an aluminosilicate gel containing Na and tetrapropylammonium cations. The detailed procedure i s described elsewhere (3). The samples of varying Si0 /Al20 ratio were obtained by adjusting the s i l i c a to alumina ratio i n the starting reaction mixture. A l l zeolite samples were highly crystalline ZSM-5 as determined by x-ray diffraction. Chemical compositions of the ZSM-5 samples were determined by conventional chemical analysis and are shown in Table I. Some typical morphologies of ZSM-5 crystals are presented in Figure 2. 2

3

Pure Ionic Forms of ZSM-5 The "as synthesized" ZSM-5 contains tetrapropylammonium and sodium ions i n i t s pores and structure. The total cation to Al site ratio i s generally greater than one indicating salt occlu-

In Intrazeolite Chemistry; Stucky, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Inorganic Cation Exchange Properties


CHU AND DWYER

Figure l b S k e l e t a l Diagram o f t h e (100) - Face o f t h e ZSM-5 U n i t C e l l t o Show t h e N e a r l y C i r c u l a r Apertures of the S i n u s o i d a l Channels




Figure 2 M i c r o g r a p h o f 0.02M and 2ji ZSM-5 C r y s t a l s


4.

CHU AND DWYER

Inorganic Cation Exchange Properties

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sions, primarily organic salts. Occluded organic salts as well as TPA cations, which are larger than the pore opening of ZSM-5, impedes ionic movement during the ion exchange process, therefore, prior to any ion exchange experiments, the bulky TPA ions and occluded material were removed and a pure cationic form was used as the base form for ion exchange. The pure ammonium or sodium form can be prepared by a 500°C calcination of as synthesized" ZSM-5 i n an ammonia atmosphere followed by repeated ammonium or sodium ion exchange. The exchanged products have excellent crys t a l l i n i t y and a cation to A l ratio of about one. The composition of the various sodium ZSM-5 samples used i n this work i s presented in Table I. Downloaded by MONASH UNIV on December 8, 2014 | http://pubs.acs.org Publication Date: May 17, 1983 | doi: 10.1021/bk-1983-0218.ch004

M

TABLE I Composition of Pure Ionic Forms of ZSM-5 Si0 /Al 0 2

2

3

40

Cation Form Na Composition (mole ratio) A1 0 1.0 Si0 40.4 Na 0 0.901 N0 0.048 (Na+N)/Al 0.95 Crystallinity,% 95 2

3

2

2

2

70 Na

70 Na

70 Na

206 Na

140

140

Na

NH

4

1.0 1.0 1.0 1.0 1.0 1.0 75.4 71.0 77.5 206 142 142 0.945 0.991 0.925 1.053 0.983 0.01 0.056 0.067 0.075 0.092 0.040 1.08 1.00 1.06 1.00 1.08 1.02 1.09 90 100 95 105 100 90

Ion Exchange The data for the ion exchange isotherms were obtained from batch experiments conducted i n a constant temperature agitated system u t i l i z i n g tightly sealed polypropylene bottles. Conventional chemical analyses were used to obtain the cation distribution data i n both zeolite and exchange solution phases. Most of the exchanges were carried out at ambient temperature for 24-72 hours. Preliminary tests had shown that equilibrium was essent i a l l y reached within a few hours. Results and Discussion Ion Exchange Isotherms Equilibrium exchange of some typical monovalent and polyvalent cations were studied mainly at 25°C, with a few cases at 75°C, to show the effect of temperature on the equilibrium. A l l the results are presented i n the form of ion exchange isotherms, Figures 3 to 11. In these figures, the ordinate, Zx, i s the equivalent fraction of the ingoing ion X i n the zeolite phase and the abscissa, Sx, i s that of the exchange solution phase. The equivalent


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f r a c t i o n s Zx, Sx can be d e f i n e d more e x p l i c i t l y as f o l l o w s : e

Inorganic Cation Exchange Properties of Zeolite ZSM-5 - ACS

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