[acs symposium series] zeolite synthesis volume 398 || nonaqueous synthesis of silica sodalite

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Chapter 15 Nonaqueous Synthesis of Silica Sodalite D. M. Bibby, N. I. Baxter, D. Grant-Taylor, and L. M. Parker Chemistry Division, Department of Scientific and Industrial Research, Private Bag, Petone, New Zealand The synthesis of ethylene glycol-silica sodalite from the non-aqueous systems SiO 2 -Na 2 O-ethylene glycol and SiO 2 -Na 2 CO 3 -ethylene glycol is described. Addition of water up to a water/ethylene glycol ratio of 0.05 has no effect, above this sodium metasilicate is produced in increasing amounts until it is the only product at a water/ethylene glycol ratio 0.5. Each sodalite cage contains one ethylene glycol molecule which can be removed by a combination of pyrolysis and high pressure oxidation. These oxidation products can be decapsulated at low pressures to produce a pure silica sodalite. We describe here the synthesis of silica sodalite containing ethylene glycol in the sodalite cages, a material referred to as ethylene glycol-silica sodalite, EG-SS. We also describe a method for removing the ethylene glycol to leave a pure silica sodalite. Unlike the usual systems for synthesis of zeolitic materials ^) these are non-aqueous, namely silica-sodium hydroxide-ethylene glycol and silica-sodium carbonate-ethylene glycol. Our first syntheses were described elsewhere ^ 2 ) . Subsequently, a wide range of allowable reagent compositions for successful EG-SS production have been studied and the crystallisation fields are presented here. The available evidence suggests that the material as synthesised contains one ethylene glycol molecule in each sodalite cage. The high temperature weight loss in thermogravimetric analysis corresponded to nearly two ethylene glycol molecules per unit cell, with a carbonaceous residue in the black calcined material. A neutron diffraction study (3) produced data of such a quality that essentially one ethylene glycol molecule must be present in each cage. With a free diameter of the sodalite cage of about 6 angstrom, the size of the molecules that can be accommodated is limited. Ethylene glycol, propanol and also tetramethyl ammonium ions (5) can be easily encapsulated. However, molecules such as 0097-6156/89/0398-0209$06.00/0 ο 1989 American Chemical Society Downloaded by UCSF LIB CKM RSCS MGMT on November 23, 2014 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0398.ch015 In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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Page 1: [ACS Symposium Series] Zeolite Synthesis Volume 398 || Nonaqueous Synthesis of Silica Sodalite

Chapter 15

Nonaqueous Synthesis of Silica Sodalite

D. M. Bibby, N. I. Baxter, D. Grant-Taylor, and L. M. Parker

Chemistry Division, Department of Scientific and Industrial Research, Private Bag, Petone, New Zealand

The synthesis of ethylene glycol-silica sodalite from the non-aqueous systems SiO2-Na2O-ethylene glycol and SiO2-Na2CO3-ethylene glycol is described. Addition of water up to a water/ethylene glycol ratio of 0.05 has no effect, above this sodium metasilicate is produced in increasing amounts until it is the only product at a water/ethylene glycol ratio 0.5. Each sodalite cage contains one ethylene glycol molecule which can be removed by a combination of pyrolysis and high pressure oxidation. These oxidation products can be decapsulated at low pressures to produce a pure silica sodalite.

We describe here the synthesis of s i l i c a s o d a l i t e containing ethylene g l y c o l i n the s o d a l i t e cages, a mate r i a l r e f e r r e d to as ethylene g l y c o l - s i l i c a s o d a l i t e , EG-SS. We also describe a method f o r removing the ethylene g l y c o l to leave a pure s i l i c a s o d a l i t e . Unlike the usual systems f o r synthesis of z e o l i t i c materials ^ ) these are non-aqueous, namely s i l i c a - s o d i u m hydroxide-ethylene g l y c o l and s i l i c a - s o d i u m carbonate-ethylene g l y c o l . Our f i r s t syntheses were described elsewhere ^ 2 ) . Subsequently, a wide range of allowable reagent compositions f o r s u c c e s s f u l EG-SS production have been studied and the c r y s t a l l i s a t i o n f i e l d s are presented here.

The a v a i l a b l e evidence suggests that the mate r i a l as synthesised contains one ethylene g l y c o l molecule i n each s o d a l i t e cage. The high temperature weight lo s s i n thermogravimetric a n a l y s i s corresponded to nearly two ethylene g l y c o l molecules per u n i t c e l l , with a carbonaceous residue i n the black c a l c i n e d m a t e r i a l . A neutron d i f f r a c t i o n study

(3) produced data of such a

q u a l i t y t h a t e s s e n t i a l l y one ethylene g l y c o l molecule must be present i n each cage.

With a free diameter of the s o d a l i t e cage of about 6 angstrom, the s i z e of the molecules tha t can be accommodated i s l i m i t e d . Ethylene g l y c o l , propanol and also tetramethyl ammonium ions (5) can be e a s i l y encapsulated. However, molecules such as

0097-6156/89/0398-0209$06.00/0 ο 1989 American Chemical Society

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210 ZEOLITE SYNTHESIS

propylene g l y c o l or g l y c e r o l , which might als o be s u i t a b l e s olvents, would be too large to f i t comfortably i n the cage.

Experimental

The reagents were r e d i s t i l l e d ethylene g l y c o l (EG) (laboratory reagent), fumed s i l i c a (Degussa A e r o s i l 200), NaOH (laboratory grade) and Na2C03 (laboratory grade). A l l reactions were c a r r i e d out a t a temperature of 170°C, f o r periods of up to 21 days, i n s t a i n l e s s s t e e l vessels with a g i t a t i o n when the v i s c o s i t y was low enough. In syntheses using NaOH where the S 1 O 2/EG r a t i o was less than 0.1, the hydroxide was added as the s o l i d as i t r a p i d l y d i s s o l v e d i n the EG at the r e a c t i o n temperature. For SÎ02/EG r a t i o s between 0.1 and 1.0, the NaOH was generally d i s s o l v e d i n the EG p r i o r to mixing with the Si02« For r a t i o s above 1.0, NaOH was di s s o l v e d i n methanol and a s l u r r y with the S 1 O 2 was prepared. The methanol was then removed at low temperature (below 50°C) i n vacuo, and the appropriate amount of EG was added. In syntheses using Na2C03 the carbonate was mixed i n t o the r e a c t i o n mixture i n a f i n e l y ground anhydrous form.

Thermogravimetric a n a l y s i s was performed on a Stanton Redcroft thermobalance i n a flowing oxygen atmosphere at a heating rate of 10°C/minute. Thermal desorption/mass spectrometry (TD/MS) was c a r r i e d out using an Extranuclear quadrupole mass spectrometer.^) To analyse the evolved gases a mass range of 1 to 100 was scanned, while the sample was heated i n a p a r t i a l vacuum (ca. 0.1 bar argon) at 10°C/minute.

The high pressure oxidation studies were c a r r i e d out i n t e s t v essels (American Instrument Co.) with a working pressure l i m i t of 1000 bar at room temperature and only s l i g h t l y lower at a temperature l i m i t of 400°C. High pressure oxygen (commercial grade) was introduced from a c y l i n d e r with up to 140 bar a v a i l a b l e at room temperature. The oxygen pressure at 400°C could be c a l c u l a t e d from values of the c o m p r e s s i b i l i t y f a c t o r a n d the f i l l i n g pressure at room temperature. Sample powders were spread t h i n l y along the wa l l of the h o r i z o n t a l v e s s e l to reduce any l o c a l heating to a minimum.

Sodium analyses were c a r r i e d out by flame photometric methods a f t e r d i s s o l u t i o n of the sample i n HF. X-ray d i f f r a c t i o n was c a r r i e d out on standard powder d i f f r a c t i o n equipment.

Results and Discus s i o n

Synthesis of Ethylene G l y c o l - S i l i c a S o d a l i t e (EG-SS)· The c r y s t a l l i s a t i o n f i e l d f o r the system Si02~Na0H-EG at 170°C i s shown i n Figure 1. At low values of the r a t i o S 1 O 2-EG the products were e i t h e r c l e a r s o l u t i o n s or translucent g e l s . At low Na20/EG r a t i o s and high S 1 O 2/EG r a t i o s no obvious r e a c t i o n products were found and the m a t e r i a l was s t i l l amorphous a f t e r r e a c t i o n . At Na20/EG r a t i o s above 0.025 and S 1 O 2/EG r a t i o s between 0.05 and 0.5 both 3-sodium s i l i c a t e and c r i s t o b a l i t e were formed. Pure EG-SS was formed a t S 1 O 2/EG r a t i o s between 0.05 and 3.0 and Na20/EG r a t i o s from 0.007 t o 0.4. Mixed EG-SS and amorphous mat e r i a l was produced up to a S 1 O 2/EG r a t i o of ca.10.

Assuming that each s o d a l i t e cage i s occupied by one EG

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molecule (a u n i t c e l l composition Si-j 2 Ο 2 4 · 2EG), the S 1 O 2/EG r a t i o of the pure EG-SS i s 5.81. Above t h i s r a t i o i t i s i n e v i t a b l e that EG-SS w i l l be mixed with other products, u s u a l l y unreacted m a t e r i a l . In p r a c t i c e t h i s work shows that pure EG-SS can be formed up to Si0 2/EG = 3 w i t h i n the r e a c t i o n times used here.

As Figure 1 shows, as the amount of EG i n the system f a l l s , the r a t i o Na20/EG has to increase f o r s u c c e s s f u l EG-SS synthesis. For S 1 O 2/EG r a t i o s below 0 . 4 the c a l c u l a t e d S i 0 2 / N a 2 0 r a t i o v a r i e s from ca. 3 t o ca.10. For Si0 2/EG above 0 . 4 the S i 0 2 / N a 2 0 r a t i o can increase from ca. 10 t o a maximum of ca. 50. Thus as the amount of EG i n the system f a l l s , the proportion of Na20 required f o r s u c c e s s f u l synthesis of EG-SS a l s o f a l l s .

For values of S Î 0 2 / E G above 1 the reactant mixture has the appearance of a dry powder. Although EG has a high vapour pressure at 170°C and was r a p i d l y and uniformly d i s t r i b u t e d throughout the reactants, i t was c l e a r from preliminary experiments i n these "dry" systems that the NaOH could not be uniformly d i s t r i b u t e d . I t was f o r t h i s reason that the procedure of mixing the S 1 O 2 with the appropriate amount of NaOH d i s s o l v e d i n methanol was adopted. The presence of any small r e s i d u a l amounts of methanol i n the reactants a f t e r drying i n vacuo d i d not appear to be detrimental.

In the system S i02~Na0H-EG, water may be present i n trace amounts as an impurity or as a product of a r e a c t i o n between NaOH and EG to form sodium g l y c o l a t e . In an i n v e s t i g a t i o n i n t o the p o s s i b i l i t y that water acts as a c a t a l y s t , EG-SS was synthesised from a system where sodium metal was d i s s o l v e d i n r e d i s t i l l e d EG before being mixed with S 1 O 2 p r e v i o u s l y d r i e d at 500°C. No d i f f e r e n c e s were observed i n the rate of formation of the EG-SS or i n the p u r i t y of the product. However, the experimental d i f f i c u l t i e s of ensuring complete removal of water from the system are such that, as yet, a c a t a l y t i c r o l e f o r water cannot be e n t i r e l y e l i m i n a t e d .

The e f f e c t the d e l i b e r a t e a d d i t i o n of water on the c r y s t a l i z a t i o n of s i l i c a s o d a l i t e was i n v e s t i g a t e d i n a separate s e r i e s of experiments where i n c r e a s i n g amounts of water were added to the system. A d d i t i o n of water up to water/ethylene g l y c o l r a t i o s of 0.05 had no observable e f f e c t , x-ray d i f f r a c t i o n showing 100 percent s i l i c a s o d a l i t e . Above t h i s r a t i o i n c r e a s i n g amounts of meta sodium s i l i c a t e were formed u n t i l t h i s became the only product a t water/ethylene g l y c o l r a t i o s above 0.5.

The e f f e c t of Na2C03 on the S i02~Na20-EG system was i n v e s t i g a t e d because of the reported ( Ί) synthesis of c a n c r i n i t e from hydrothermal systems containing Na2C03. However, we found no product here other than EG-SS. Figure 2 shows the c r y s t a l l i s a t i o n f i e l d f o r EG-SS i n the system S i02-Na2C03-EG. Within the composition range studied, there appears to be no upper boundary to the allowable Na20/EG r a t i o over a range of values of S 1 O 2/EG from ca. 0.03 to ca.2. Considering the l i m i t e d s o l u b i l i t y of Na2C03 i n EG, t h i s i s perhaps not s u r p r i s i n g . The EG i s e s s e n t i a l l y saturated at room temperature at Na20/EG r a t i o s >0.02( 7). Above the s a t u r a t i o n r a t i o the Na2C03 acts as an i n e r t d i l u e n t . Although r e a c t i o n periods were l i m i t e d to 21 days, there was no i n d i c a t i o n that products other than EG-SS would form.

As seen i n Figure 3, powder X-ray d i f f r a c t i o n showed that EG-SS

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BIBBY ET A L Nonaqueous Synthesis of Silica Sodalite

Figure 2. SiO /EG versus Na O/EG for the system SiO :Na CO :EG.

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15. BIBBY ET A L Nonaqueous Synthesis of Silica Sodalite 215

products from both systems (NaOH and Na2C03) were extremely c r y s t a l l i n e . Inspection of a pure EG-SS sample by scanning e l e c t r o n microscopy showed regular cuboctahedra up to 25 microns i n diameter (Figure 4). Chemical a n a l y s i s of the products from e i t h e r system showed e s s e n t i a l l y no i n c o r p o r a t i o n of sodium unless aluminium was present, when sodium was present i n amounts appropriate to balance the framework charge, as has been reported p r e v i o u s l y ( 2 , 8 ) #

I t i s d i f f i c u l t to determine the r o l e of the EG i n these systems. C l e a r l y i t i s a reactant since, at the l i m i t , the amount of EG present governs the y i e l d of EG-SS. At low S 1 O 2/EG r a t i o s i t would appear that EG had a s i m i l a r s o l v a t i o n behaviour to that of water i n t y p i c a l hydrothermal z e o l i t e syntheses. However, a t high S 1 O 2/EG r a t i o s a p o s s i b l e comparison would be with pneumatolysis^) where reactions take place not i n excess solvent but i n the presence of small amounts of r e a c t i v e v o l a t i l e s which transport the reactants to the c r y s t a l l i s i n g surface. This i s supported by the formation of r e l a t i v e l y large regular c r y s t a l s of EG-SS, suggesting that they grow from s o l u t i o n , and i n d i c a t e s that the EG does a c t as a solvent to t r a nsport the s i l i c a to the c r y s t a l l i s i n g surface. This solvent need not n e c e s s a r i l y be more than a t h i n layer over the growing surface as long as there i s s u f f i c i e n t to bridge the amorphous s i l i c a p a r t i c l e s and the EG-SS c r y s t a l s .

As the S 1 O 2/EG r a t i o i n the reactants approaches the compositional l i m i t of 5.81, and a large proportion of the EG i n i t i a l l y present becomes incorporated i n t o the EG-SS s o l i d , the Na20/EG r a t i o increases r a p i d l y . Eventually the Na20/EG r a t i o r i s e s to the po i n t where apparently no f u r t h e r EG-SS formation occurs. At t h i s point, the remaining EG i s presumably saturated with sodium s i l i c a t e and conditions become unfavourable f o r furth e r conversion of the amorphous s i l i c a .

I t should be noted that z e o l i t e s w i l l form from reactant gels with very low water c o n t e n t s a l t h o u g h r e a c t i o n rates are low. Also, the conversion of the z e o l i t e chabazite i n the absence of added water to give a range of other more compact framework s t r u c t u r e s has been reported. P a r t i c u l a r l y r e l evant was the conversion of a s i l i c a c e o u s sodium-form of chabazite to nosean ( s o d a l i t e ) at ca. 3 0 0 ° C . ( 1 2 )

R e m o v a l o f O c c l u d e d E t h y l e n e G l y c o l . Since the smallest cross s e c t i o n of the EG molecule i s greater than the free dimension of the 6-window of the s o d a l i t e cage, the removal of occluded EG t o y i e l d a pure S 1 O 2 s o d a l i t e was not straightforward.

I n i t i a l l y removal was attempted by c a l c i n a t i o n i n a i r or i n oxygen but the s o d a l i t e became black, with the e v o l u t i o n of large amounts of v o l a t i l e s , and i t remained black even a f t e r being heated to 900°C f o r s e v e r a l days. The formation of a r e s i s t a n t black product on c a l c i n a t i o n i s i n contrast to another r e p o r t ^ ) that c a l c i n a t i o n of EG-SS i n a i r at 500°C y i e l d e d a white product. Thermogravimetric a n a l y s i s showed that the majority of the EG was l o s t but a s i g n i f i c a n t carbonaceous residue remained. The X-ray d i f f r a c t i o n pattern of the black s i l i c a s o d a l i t e showed no change except that the peaks became s i g n i f i c a n t l y broader. This suggests the generation of some degree of s t r u c t u r a l d isorder or the

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Figure 4. Scanning electron micrograph of EG-SS prepared from the SiO :NaOH:EG system.

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generation of defects a r i s i n g from the i n t e r n a l forces generated by the decomposition products of the EG, which w i l l have a greater volume than that of the parent molecule. Water generated during the decomposition may also promote some s t r u c t u r a l breakdown.

A d e t a i l e d i n v e s t i g a t i o n of the v o l a t i l e decomposition products products was c a r r i e d out using TD/MS. The r e s u l t s of heating a sample of EG-SS at 10°C/minute are shown i n Figure 5. At ca. 400°C there was rapi d e v o l u t i o n of large q u a n t i t i e s of water, carbon monoxide, carbon dioxide, and hydrogen. The black carbonaceous product c l e a r l y s t i l l contained oxygen as carbon oxides s t i l l continued to be evolved as the temperature was increased, peaking again between 600°C and 700°C. Carbon monoxide production continued to r i s e from ca. 800°C (not shown here), p o s s i b l y due to the reduction of the S 1 O 2 framework by the i n t i m a t e l y associated carbonaceous m a t e r i a l . The carbonaceous product a l s o dehydrogenated at the higher temperatures, the y i e l d of hydrogen peaking between 600°C and 700°C.

The lack of success i n removing the remaining carbonaceous m a t e r i a l by ox i d a t i o n i n a i r i s not s u r p r i s i n g i n view of the diameter of the 6-ring opening (ca. 2.3 A) and c r o s s - s e c t i o n a l diameter of the oxygen molecule (2.8 A). However, there have been studies (10,11) of the encapsulation i n z e o l i t e s of molecules which, under normal conditions of temperature and pressure, could not

encapsulated i n the s o d a l i t e cage of zeolite-Α and that argon and krypton (diameters 3.8 and 3.9 A r e s p e c t i v e l y ) can be encapsulated i n s o d a l i t e . ^ ^

A sample of EG-SS was pyrolysed i n vacuo at a heating rate of ca. 2°C/minute from 300°C to 700°C. The r e s u l t a n t black product was then treated with high pressure oxygen f o r 15 minutes at ca. 400°C. A f t e r t h i s treatment, the decapsulated products evolved on heating were analysed using TD/MS. A f t e r the f i r s t treatment, only carbon dioxi d e and a small amount of oxygen were evolved with no i n d i c a t i o n of carbon monoxide. Repeating the high pressure oxygen procedure produced a sample which evolved both oxygen and carbon dioxide i n s i m i l a r q u a n t i t i e s (Figure 6). A f t e r 5 c y c l e s , a sample was produced which evolved mainly oxygen and which was a l i g h t grey i n colo u r .

A f t e r p y r o l y s i s , i t appeared that most of the s o d a l i t e cages s t i l l contained some carbonaceous m a t e r i a l . On treatment with oxygen at high temperatures, some degree of oxygen encapsulation occurred with the oxygen r e a c t i n g with the carbon to give carbon d i o x i d e . As shown i n Figure 6, t h i s décapsulâtes from 400°C. On repeating the c y c l e , the same r e a c t i o n occurred but t h i s time there were more i n i t i a l l y empty s o d a l i t e cages which now became occupied with oxygen only and thus on decapsulation both oxygen and carbon dioxi d e evolved. The r a t i o of oxygen to carbon dioxide increased with each c y c l e as the carbonaceous ma t e r i a l was removed as carbon d i o x i d e . Water was not observed as any water formed from hydrogen associated with the carbonaceous residue would be desorbed at 400°C or lower during the high pressure oxygen treatment as the water molecule r e a d i l y passes through the 6-ring opening. The colour of the m a t e r i a l darkened s l i g h t l y during decapsulation. This was probably due to the gas-saturated s o d a l i t e having a higher

penetrate the framework. that oxygen can be

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Figure 6. Ion signal versus temperature for pyrolysed EG-SS containing encapsulated 0« and CCL.

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15. BIBBY ET AL. Nonaqueous Synthesis of Silica Sodalite 219

r e f r a c t i v e index than that of the decapsulated m a t e r i a l . The l i g h t s c a t t e r i n g power of the saturated s o d a l i t e would, therefore, be higher and as a r e s u l t the powdered sample appeared l i g h t e r i n colour.

Since carbon dioxide décapsulâtes from 400°C, an extended treatment i n high pressure oxygen at that temperature should be e f f e c t i v e i n removing the carbonaceous residue i n one step. A pyrolysed sample of s i l i c a s o d a l i t e was heated at 400°C i n high pressure oxygen f o r 64 hours. The gases subsequently decapsulated contained predominantly oxygen with a small amount of carbon d i o x i d e . This experiment confirms that during the high pressure treatment carbon dioxide can desorb and be replaced by oxygen. However, the sample was s t i l l black a f t e r 64 hours and s u b s t a n t i a l l y longer times - or p o s s i b l y higher temperatures would be required f o r a l l of the carbon to be o x i d i s e d and f o r the carbon dioxide to be desorbed and replaced with oxygen.

Caution has to be used i n the high pressure treatment. As the s t r u c t u r e becomes depleted of carbon i t tends to c o l l a p s e to an amorphous s o l i d i f temperatures and pressures are increased too r a p i d l y .

Conclusion

The c r y s t a l l i s a t i o n f i e l d s f o r synthesis of s i l i c a s o d a l i t e from two non-aqueous systems have been determined. The s i l i c a s o d a l i t e synthesised i s i n a form containing ethylene g l y c o l i n the s o d a l i t e cages. I t has a l s o been shown po s s i b l e to remove the ethylene g l y c o l from the s o d a l i t e cages using a combination of p y r o l y s i s followed by repeated cycles of high temperature, high pressure o x i d a t i o n .

Acknowledgments

We thank N e i l Milestone f o r h e l p f u l and c o n s t r u c t i v e comments.

References

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2. Bibby, D.M. and Dale, M.P. Nature 1985, 317, 157. 3. Richardson, J.W. Jr., Pluth, J.J., Smith, J.V., Dytrych, W.J.

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RECEIVED December 22, 1988

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