Exchange of tetraethylammonium ion into zeolite Y

Exchange of tetraethylammonium ion into zeolite Y G. T. Kerr

Central Research Division, Mobil Research and Development Corporation, Princeton, New Jersey 08540, USA (Received 13 June 1983) A special technique is described and used to exchange tetraethylammonium ion into zeolite Y, an exchange which is highly difficult or impossible by conventional ion-exchange techniques. That such an exchange is possible sheds light on the mechanism of aluminium removal from zeolite Y using derivatives of H4EDTA. Keywords: Zeolite Y; ion-exchange; tetraethylammonium ion; silver zeolite Y; dealumination; ammonium thioeyanate

INTRODUCTION

In the study of acid zeolites, Barrer I utilized an ingenious technique for preparing acid zeolites in aqueous systems. Silver analcime, for exmnple, was converted at le~t partially to the hydrogen form by the reaction: H20 + MC1 + x A g analcime -+ HAg(x_ 1) (analcime)x + MOH + AgCI ,L where M is a cation of such size that it cannot exchange into the zeolite crystal, for example, alkyl ammonium ions. Barrer stated, 'If a second electrolyte was also present in solution, whose cation was small enough to enter the cryst~d, then hydrogen-ion exchange did not occur at equilibrium. Thus for exchange to take place: (i) all other cations present in solution must be too large to enter the crystal; and (ii) the cations present in the crystal must form a highly insoluble precipitate when they leave it. It is only under these conditions that hydrogen ions, present in aqueous phase at very low concentrations (2H20 ~-H30 + + OH-), can compete for mad occupy cation sites in the crystal as silver is displaced'. Barrer separated the two solid products, the zeolite and silver chloride, by dissolving the silver chloride in an amine which was too large to enter the zeolite crystal, for example, pyridine. For prepm'ing hydrogen zeolite Y as the only solid product, a scheme similar to that of Barfer's was conceived and used: calcium zeolite Y was contacted with a hot aqueous solution of di(tetraethylammonium) dihydrogen ethylenediaminetetraacetate. The formation of the stable, 0144-2449/83/040295-03503.00 © Butterworth & Co. (Publishers) Ltd.

water-soluble Ca-EDTA complex served to force the reaction to completion. The formation of insoluble silver chloride served this purpose in Barfer's method. The tetraethylammonium ion was reported by Barrer I as being too large to exchange into the faujasite crystal. Therefore hydrogen ions, derived from the EDTA acid salt, would exchange for the calcium ion in the zeolite: xCa-Y + [(C2Hs)4NI2H2EDTA -+ H2Ca(x_ ~)Yx + [(C2Hs),~NIzCaEDTA The final product was found to have a significantly higher Si/AI ratio than the initial zeolite, yet there was significant retention of cwstal structure. This unexpected finding led to a long series of studies of aluminum remowd from zeolite Y using H4EDTA , the thermal decomposition of ammonium zeolites, the formation and chemistry of ultrastable faujasitc, etc. 2. On recently reviewing the em-ly work, it was observed that the original dealuminized zeolite Y, prepared by the reaction of di(tetraethylammonium) dihydrogen ethylenediaminetetraacetate with calcium zeolite Y, had 40% of the cation sites occupied by tetracthylammonium ions. The possible significance of this fact was considered. There is now convincing evidence that all tetracoordinate sites in dealuminized zeolite Y prepared by the EDTA method are occupied by either silicon or aluminum a'4. The migration of silicon into vacant T-sites via solution or by a solid state diffusion mechanism has recently been shown to be possible at temperatures as low as 100°C s. Barrer stated, as noted earlier, that tetraethylammonium ion is too large to exchange into the faujasitc structure=. If Barrer is correct, then an obvious and reasonable explanation for occupancy

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of 40% o f the c a t i o n sites in the original dea l u m i n i z e d zeolite Y b y q u a t e r n a r y a m m o n i u m ion would be dissolution and r c c r y s t a l l i z a t i o n during d e a l u m i n i z a t i o n with c o n c o m i t a n t inclusion o1' the q u a t e r n a r y a m m o n i u m ion. The work described here was directed to d e t e r m i n i n g w h e t h e r or n o t t e t r a e t h y l a m n m n i u m could exchange into and o u t of zeolite Y.

2.5 24 24

E

22 21

2o EXPERIMENTAL

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Instnmlent~d techniques used in this s t u d y arc all well-known. A m m o n i u m zeolite Y was c o n v e r t e d to the silver l~{rm by refluxing 10 g o f thc zeolite in 200 ml of 1.0 N silver nitratc s o l u t i o n for 1 11. The resulting silver zeolite was c o m p l e t e l y free of a m l n o n i u m ion. Meanwhile. t c t r a c t h y l a n n n o n i u m t h i o c y a n a t e was p r e p a r e d by mixing 100 ml of 0.87 N t c t r a e t h y l a m n m n i u m h y d r o x i d e ((I.087 moles) with 6.3 g (0.083 moles) o f a n l m o n i u m t h i o c y m l a t e . The m i x t u r e was h e a t e d on a hot plate u n d e r w a t e r a s p i r a t o r v a c u u m ~ , v h e r e u p o l l the a m m o n i u m salt dissolved and the solution was e v a p o r a t e d to a low v o l u m e to effect the reaction:

(Cfl-ls)a NOII + NHaCNS -+ (C2Hs)4NCNS + H20 + NH 3 The final s o l u t i o n was diluted to 100 ml total volume. To 80 ml of the t e t r a e t h y l a m n a o n i u n l t h i o c y a n a t e s o l u t i o n (pH = 8.8) was a d d e d 4.0 g o f silver zeolite Y. A f t e r c o n t a c t at 25°C for 20 rain the m i x t u r e was filtered mid the solid was washed with four 5 ml batches of t e t r a e t h y l a m m o n i u m t h i o c y a n a t e . The filtrate had pH = 12.8. The zeolite was washed with 25 ml of water. The washings had a pH o f 12-13. The solid prod u c t (I) was air-dried mid t h e r m o g r a v i n l c t l i c and evolved amine analyses were c o n d u c t e d simultmleously on the same sample. A n o t h e r p o r t i o n of I was refluxed in excess 1.0 N silver nitrate s o l u t i o n to yield the silver zeolite which was found to hc c o m p l e t e l y Dee of any volatile basic n i t r o g e n c o u s substances. This sih'er zeolite was slurried for 1 h at reflux in excess 1.0 N a n l n l o n i u m thioc y a n a t e solution, filtered, washed first with a m m o n i u m t h i o c y a n a t e s o l u t i o n and then with water to yield p r o d u c t II. T h e n n o g r a v i m e t r i c , ew)lved a m m o n i a mad X-ray d i f f r a c t i o n mlaJyses showed II to be identical in a m m o n i u m ion alld chemical water c o n t e n t s and c w s t a l l i n i t y with the initial a m m o n i u m zeolite Y used in this s t u d y . F r o m the e x p e r i m e n t a l w o r k it is clear that a c o m p l e t e cycle of reactions o c c u r r e d : Ag + NH4-Y NH4CNS l AR h'~

296

ag +

, Ag-Y ~ [C=Hs)4NICNS I

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I

I 200

Figure1

Thermogram

I

I I 400 600 T e m p e r o t u r e (°C)

of product

8O0

1000

I

The c o m p o s i t i o n of I was established by e l e m e n t a l :tnalyscs and d e t e r m i n a t i o n o f volatile basic nitrogcnous m a t e r k d evolved during t h e r m o g r a v i n l e t r i c analysis: 0.320 [(C2Hs),CN]20 • 0. 192H=O. 0.488 Ag20" A1203- 5.19 SiO2. The f o r m u l a weight o f I is 618.3. A f t e r heating in air to elewttcd temperatures ( ~ 700°C) I y i e l d e d ash whose clementtd analysis c o r r e s p o n d s to 0.488 A g 2 0 . A1203" 5.19 SiO2, forlnula weight 526.5. The presence of H30 + ha I (expressed in I as H20 ) was q u a n t i t a t i v e l y shown b y treating I (,vcrnight in a m m ( m i u m h y d r o x i d e , filtering, washing with water, d r y i n g and c o n d u c t i n g a t.g.a, and evolved a m m o n i a analysis. Pr()duct I c()ntained the cquiwtlcnt of 0.120 nnnol NH 3 per 100 mR ash whereas, after a m m o n i a t i o n it c o n t a i n e d 0.220 m m o l NH a per 100 mg ash.

I.'tleurc 1 is the t h e r m o g r m n of p r o d u c t I run in helium from 25 ° to 650°C mid in air from 650 ° to 825°C. The ash c o m p o s i t i o n is 0.488 A g 2 0 . AI203 - 5.19 SiOz based on elemental maalyses with f o r m u l a weight 526.6. The f o r m u l a weight o f 553.6 at 6 1 0 ° - 6 2 0 ° C c o r r e s p o n d s to 0.320(C2Hs)20 0.192 H20 - 0.488 AgzO - AIzO 3 • 5.19 SiO 2 which is o b t a i n e d as an i n t e r m e d i a t e since the tetrac t h y l a m m o n i u m ion d e c o m p o s e d initially to yield d i e t h y l m n i n e . This n i t r o g e n o u s basic gas was cw~lved using a helium purge from 340 ° to 530~C. The f o r m u l a weight o f 618.3 at 1 9 0 ° - 2 0 0 ° C is the calculated vzdue for the f o r m u l a p r e s e n t e d above for p r o d u c t I, t e t r a e t h y l a m m o n i u m h y d r o gen sih'er zeolite Y.

RESULTS A N D DISCUSSION All these d a t a show that a significant q u a n t i t y o f t c t r a e t h y l a m n l o n i u m ion e x c h a n g e d into the faujasitc s t r u c t u r e , c o n t r a r y to Barrer's findings. Of spccial s i ~ l i f i c a n c e is the fact that the quaternary a m m o n i u m ions can be exchmlged o u t of the zeolite u n d e r c o n d i t i o n s which dissolution mad r e o y s t a l l i z a t i o n m'e highly unlikely. Hence, a

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mcchanism inw>lving dissolution and recrystallization nccd not be inw~kcd to cxplain the prcscnce of quaternary a m m o n i u m ion in tbc originzd dcaluminized zeolite Y. Barrer's attempted exchange consisted of simply contacting sodium zeolite X with 1 N tetraethyla m m o n i u m chloride solution. Apparently these conditions were not sufficient to o v c r c o m e thc vel-}' tight fit of the cation into the zcolitc channels. In thc work rcported hcrc, adwmtagc was takcn of the t'onnation of the highly stable di(thiocyanato) silveratc ion which prccludcs equilibrium bctwcen silver ions in thc cryst~d mad in the aqueous phasc. This complexing of silver was similar to the precipitation of silver chloride in the Barrer h y d r o n i u m zeolite preparation. However, ttnlikc the Barrer reaction, where the Ha O+ ion c o n c e n t r a t i o n was 10 -7 g-ions 1-t and the tctrametlaylananaonium ion is absolutely too large to entcr the zeolite, in the l)rcscnt case the H3O+ concentration was about 10 -9 g-ions 1-l but thc tetracthylamnaonium ion, present in a c o n c e n t r a t i o n 10 9 greater than H3 O+, can with s(mae difficulty, fit into the zeolite ctlanncls. Even thou~.:h therc is 10 9 times as much quaternary ion as H3 O+, still one H3O+ exchanged into the zeolite for every two quaternary ammoniuna ions indicating that exchange of the

quaternary is slow due to its size relative to channel size. Although the dealuminized materi~d had 40% of the cation sites occupied by quaternary ions compared with only 32% in the material prepm'ed in the present study, both samples contained 1 7.2 -+ 0.1 tctraethylammoniuna ions per unit cell (2.1 per supercage) as the de~fluminized zeolite had fewer total cations per unit cell than the normal zeolite Y. Barrer and guthcrland reported that a m a x i m u m of 2.8 tool of i-octane or 3.1 mol of n-octane can be sorbed per supercage in zeolite X s. The vMue for neopentane is 3.4. Thus, the value found in this study for the quaternary is reasonable if it is assumed the supercages are thoroughly loadcd with the quaternary.

REFERENCES 1 Barrer, R. M. Proc. Chem. Soc. 1958, 00, 99 2 Kerr, G. T. Adv. Chem. Ser. 1973, 121, 219 3 Scherzer, J. and Bass, J. L.J. CataL 1973,28, 101 4 Gallezot, P., Beaumont, R, and Barthomeuf, DJ. Phys. Chem. 1974,78, 1550 5 yon Ballmoos, R. and Meier, W. M.J. Phys. Chem. 1982,86, 2698 and reference 15 therein 6 Barrer, R. M. and Sutherland, J. W. Proc. Roy. Soc. 1956, A237, 439

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