Coprecipitated nickel-alumina methanation catalysts

COPRECIPITATED NICKEL-ALUMINA

METHANATION CATALYSTS

E . C . Kruissink

BIBLIOTHEEK TU Delft P 1666 4358

COPRECIPITATED NICKEL-ALUMINA

METHANATION CATALYSTS

Proefschrift

ter verkrijging van

de graad van doctor in de technische wetenschappen

aan de Technische Hogeschool Delft,

op gezag van de rector magnificus Prof. Ir. B.P.Th. Veltman,

voor een commissie aangewezen

door het college van dekanen

te verdedigen op woensdag 30 september 1981 te 14.00 uur door

Edgar Charles Kruissink

scheikundig doctorandus

geboren te Utrecht

Delft University Press/19 81

C10049

Dit proefschrift is goedgekeurd door de promotor

Prof. Dr. Ir. L . L . van Reijen

This investigation was supported by the Netherlands Foundation for Chemical

Research (SON) with financial aid from the Netherlands Organization for the

Advancement of Pure Research ( Z . W . O . )

-Aan mijn ouders

Aan Cora

H i e r b i j w i l i k g r a a g a l l e n , d i e hebben b i j g e d r a g e n aan de t o t s t a n d k o m i n g van d i t p r o e f s c h r i f t , h a r t e l i j k bedanken. In h e t b i j z o n d e r w i l i k v e r m e l d e n de a a n z i e n l i j k e b i j d r a g e van de h e e r Y. Timmerman i n h e t e x p e r i m e n t e l e werk.

Een b e l a n g r i j k e b i j d r a g e b i j h e t t o t s t a n d komen van h e t i n d i t p r o e f s c h r i f t b e s c h r e v e n o n d e r z o e k werd g e l e v e r d d o o r d r . J.R.H. Ross van de u n i v e r -s i t e i t van B r a d f o r d ( G r o o t - B r i t t a n n i ë ) , op w i e n -s i n i t i a t i e f een -samenwerking-s- samenwerkings-v e r b a n d t o t s t a n d kwam, w a a r i n ook een g r o e p samenwerkings-van de K.F.A. t e Jülich (West-D u i t s l a n d ) onder l e i d i n g van d r . B. Höhlein deelnam. Een a a n t a l s t i m u l e r e n d e u i t w i s s e l i n g e n en w e r k b e s p r e k i n g e n werden m o g e l i j k gemaakt door een NATO R e s e a r c h G r a n t .

I r . H.L. P e l t en i r . H. S c h a p e r , b e i d e n t e D e l f t , ben i k z e e r e r k e n t e l i j k v o o r de d o o r hen i n h e t k a d e r van hun a f s t u d e e r w e r k g e l e v e r d e b i j d r a g e n . Van g r o o t n u t waren de a d v i e z e n van d r . i r . G. H a k v o o r t op h e t g e b i e d van de t h e r m i -sche a n a l y s e . V e r d e r z i j v e r m e l d de b i j d r a g e van i r . E.B.M. Doesburg, d i e de f l o w r e a c t o r o n t w i e r p v o o r de a c t i v i t e i t s m e t i n g e n t e D e l f t .

V e e l dank ben i k v e r s c h u l d i g d aan de h e r e n J . T e u n i s s e en N. van Westen v o o r de c h e m o s o r p t i e m e t i n g e n en de t e x t u u r b e p a l i n g e n ; aan mw. L.A. S c h o u t e n v o o r de n i t r a a t en c a r b o n a a t b e p a l i n g e n en aan de h e e r J . P . Koot v o o r de a n a l y -ses van n i k k e l , aluminium en n a t r i u m . V e r d e r heb i k d e s k u n d i g e s t e u n gehad van de d i e n s t e n b i n n e n h e t Gebouw v o o r S c h e i k u n d e , met name v a n de g l a s b l a z e r i j .

B u i t e n D e l f t g a a t m i j n b i j z o n d e r e dank u i t n a a r p r o f . i r . J.W. Geus ( A n a l y -t i s c h C h e m i s c h L a b o r a -t o r i u m , R i j k s u n i v e r s i -t e i -t U -t r e c h -t ) v o o r h e -t v e r v a a r d i g e n van de e l e c t r o n e n m i c r o s c o p i e - o p n a m e n en v o o r de l e e r z a m e d i s c u s s i e s o v e r d i t onderwerp; b o v e n d i e n n a a r d r . S. O r r van de u n i v e r s i t e i t van B r a d f o r d ( G r o o t -B r i t t a n n i ë ) v o o r a c t i v i t e i t s - en s e l e c t i v i t e i t s m e t i n g e n .

V o o r de z o r g v u l d i g e b e w e r k i n g v a n de f i g u r e n ben i k de h e e r J . van W i l l i g e n en de r e p r o g r a f i s c h e d i e n s t van h e t Gebouw v o o r S c h e i k u n d e z e e r e r k e n t e l i j k . V o o r h e t typewerk en de l a y - o u t van h e t p r o e f s c h r i f t ben i k t e n s l o t t e v e e l dank v e r s c h u l d i g d aan mw. M.J.A. W i j n e n .

C O N T E N T S

I INTRODUCTION 1

1. GENERAL INTRODUCTION 1 2. THE N.F.E. PROJECT 3

3. SURVEY 4

I I PREPARATION, CHEMICAL COMPOSITION AND STRUCTURE OF T H E CO- 7

P R E C I P I T A T E D MATERIALS

1. INTRODUCTION 7

1.1. Preparation methods 7

1.2. Composition and structure of the phases formed 8

2. EXPERIMENTAL 11

2.1. Copreoipitation 11

2.2. Copreoipitation from ammoniacal solutions 12

2.3. Sample characterization 12

3. CHEMICAL COMPOSITION AS A FUNCTION OF PREPARATION CONDITIONS 13

3.1. Samples prepared at increasing pH 13

3.2. Samples prepared at constant pH 15

3.2.1. H y d r o x y c a r b o n a t e s and - n i t r a t e s 15

3.2.2. H y d r o x y c h l o r i d e s 19 3.2.3. H y d r o x y a c e t a t e s 19

3.3. Samples prepared from ammoniacal solutions 19

3.4. Sodium content of the precipitates 20

4. STRUCTURE AS A FUNCTION OF CHEMICAL COMPOSITION 22

4.1. Indexing of X-ray diffraction pattern 22

4.2. Hydroxycarbonates 23

4.3. Hydroxynitrates 26

4.4. Solid solution of hydroxy carbonate and -nitrate 27

4.5. Hydroxychlorides 27

4.6. Hydroxyacetates 28

5. THERMAL DECOMPOSITION OF PRECIPITATES 28 6. INFRARED SPECTROSCOPY OF PRECIPITATES 30

7. DISCUSSION 34

I I I CHARACTERIZATION OF C A L C I N E D AND REDUCED C O P R E C I P I T A T E S 39

1. INTRODUCTION 39 2. EXPERIMENTAL 40 3. PHASE COMPOSITION OF THE CALCINED MATERIALS 44

I I I 3.1. Introduction 44

3.2. Results and discussion 46

4. REDUCTIBILITY OF THE CALCINED MATERIALS 49

4.1. Introduction 49

4.2. Results and discussion 51

4.3. Reduction in recirculation reactor; effect of hydrogen pressure 54

5. DISPERSION OF NICKEL OXIDE IN CALCINED MATERIALS AND OF NICKEL 56 IN REDUCED CATALYSTS AS A FUNCTION OF PREPARATION CONDITIONS

5.1. Introduction 56

5.2. Dispersion of nickel as a function of reduction procedure 56

5.3. Dispersion of nickel oxide and of nickel as a function of the 58

properties of the precipitate

5.3.1. P r e s e n t a t i o n o f d a t a 58 5.3.2. N i c k e l o x i d e c r y s t a l l i t e s i z e s 58 5.3.3. N i c k e l c r y s t a l l i t e s i z e s and n i c k e l s u r f a c e a r e a s 61 5.3.4. D i s p e r s i o n o f r e d u c e d n i c k e l i n samples o f n i c k e l 62 c o n t e n t x < 0.50 N i

5.4. Effect of sodium content on the dispersion of nickel oxide 63

and of nickel

5.5. Effect of calcination temperature on the dispersion of nickel 64

oxide and of nickel

5.6. Effect of reduction temperature on nickel dispersion 66

6. TEXTURE OF THE CALCINED AND REDUCED MATERIALS 70

6.1. Introduction 70

6.2. Texture data from nitrogen capillary condensation 70

6. 3. Electron microscopy 71

6.4. Comparison of crystallite sizes as determined from different 76

techniques

7. SUMMARY AND CONCLUSIONS 78

IV A C T I V I T Y AND S E L E C T I V I T Y OF C O P R E C I P I T A T E D C A T A L Y S T S FOR THE 83

PRODUCTION OF METHANE

1. INTRODUCTION 83 2. EXPERIMENTAL 84

2.1. The recirculation reactor 84

2.2. The atmospheric pressure flow reactor 85

3. GENERAL OBSERVATIONS 88 4. EFFECT OF CONDITIONS OF CATALYST PREPARATION 89

IV 4.2. Effect of anion present in the precipitate 90

4.3. Effect of nickel content 97

4.4. Effect of hydrothermal treatment 98

4.5. Effect of sodium content 100

4.6. Effect of pH of precipitation 103

4.7. Effect of calcination temperature 104

4.8. Effect of temperature and degree of reduction 107

5. RESULTS OBTAINED FROM MEASUREMENTS IN A D.S.C. APPARATUS 110 6. COMPARISON OF RESULTS FROM DIFFERENT REACTORS AND 115

COMPARISON WITH LITERATURE DATA

7. DISCUSSION OF DATA ON SELECTIVITY AND SPECIFIC ACTIVITY IN 119 RELATION TO THE MECHANISM OF THE METHANATION REACTION

7.1. Summary of experimental observations 119

7.2. Mechanism of methanation reaction 120

7.3. Effect of sodium on selectivity and specific activity 121

7.4. Effect of degree of reduction on selectivity and specific 122

activity

8. SUMMARY 123

V CONCLUSIONS AND FINAL REMARKS 125

R e f e r e n c e s 129

Summary 133

CHAPTER I . INTRODUCTION

I . l GENERAL INTRODUCTION

M e t h a n a t i o n o f c a r b o n monoxide a t low c o n c e n t r a t i o n s has been a p p l i e d i n c h e m i c a l i n d u s t r y i n t h e p u r i f i c a t i o n o f ammonia s y n t h e s i s gas f o r many y e a r s . More r e c e n t l y , m e t h a n a t i o n o f h i g h e r c o n c e n t r a t i o n s o f c a r b o n monoxide and c a r b o n d i o x i d e has g a i n e d an i n c r e a s i n g i n t e r e s t . The most i m p o r t a n t r e a s o n f o r t h i s i s found i n t h e p o t e n t i a l a p p l i c a t i o n o f m e t h a n a t i o n i n t h e p r o d u c t i o n o f s y n t h e t i c n a t u r a l gas from c o a l .

However, m e t h a n a t i o n i s a l s o i m p o r t a n t i n a p r o c e s s under development i n West G e r m a n y ^ ^ , w h i c h aims a t t h e l o n g d i s t a n c e t r a n s p o r t o f h e a t o b t a i n e d from a n u c l e a r h i g h t e m p e r a t u r e r e a c t o r . In t h i s p r o c e s s , w h i c h i s c a l l e d "NFE" ("Nukleare F e r n e n e r g i e " ) p r o c e s s , heat i s t r a n s p o r t e d by means o f a c h e m i c a l c i r c u i t . In a s o - c a l l e d EVA r e a c t o r , h e a t o b t a i n e d from a n u c l e a r r e a c t o r i s u s e d f o r t h e e n d o t h e r m i c steam r e f o r m i n g o f methane. CH„ + H „ 0 = CO + 3H„ AH = +205 k j / m o l 4 2 2 25oc A f t e r t r a n s p o r t o f t h e c o o l e d p r o d u c t gas t o t h e consumer a r e a t h e e n e r g y can be r e g a i n e d w i t h h i g h e f f i c i e n c y - i n t h e s o - c a l l e d ADAM r e a c t o r - by p e r f o r m i n g t h e r e v e r s e r e a c t i o n , t h e e x o t h e r m i c m e t h a n a t i o n r e a c t i o n CO + 3H„ = CH_ + H „ 0 AH „ = -205 k J / m o l 2 4 2 25°C F o r t h e steam r e f o r m i n g r e a c t i o n , s u i t a b l e c a t a l y s t s a r e a v a i l a b l e . In o r d e r t o be s u i t a b l e f o r t h i s p r o c e s s , however, a m e t h a n a t i o n c a t a l y s t must meet an u n u s u a l c o m b i n a t i o n o f r e q u i r e m e n t s . At f i r s t , t h e c a t a l y s t must be s u f f i c i e n t l y a c t i v e t o " i g n i t e " t h e r e a c t i o n o at r e l a t i v e l y low t e m p e r a t u r e s , f o r example 300 C. M o r e o v e r , f o r r e a s o n s o f o v e r a l l e f f i c i e n c y w h i c h w i l l be d i s c u s s e d more c l o s e l y i n t h e n e x t s e c t i o n , i t o o i s n e c e s s a r y t o o b t a i n h i g h gas t e m p e r a t u r e s o f 600 C t o 800 C a t t h e e x i t o f 1

t h e m e t h a n a t i o n r e a c t o r . Thus t h e r e q u i r e m e n t s t o be met by a s u i t a b l e m e t h a n a t i o n c a t a l y s t a r e : - h i g h a c t i v i t y a t a low t e m p e r a t u r e o f a p p r o x i m a t e l y 300°C o o - s u f f i c i e n t s t a b i l i t y i n t h e t e m p e r a t u r e r a n g e between 300 C and 600-800 C. S i n c e a t t h e b e g i n n i n g o f t h i s i n v e s t i g a t i o n no c a t a l y s t was a v a i l a b l e a t t h e m a r k e t , which comes up t o t h e s e demands, a c o o p e r a t i o n was s t a r t e d between a group a t t h e KFA ( K e r n f o r s c h u n g s a n l a g e ) a t Jülich (West Germany), a group a t t h e U n i v e r s i t y o f B r a d f o r d ( G r e a t - B r i t a i n ) and o u r g r o u p a t t h e T e c h n i c a l U n i v e r s i t y o f D e l f t . The aim o f t h i s c o o p e r a t i v e i n v e s t i g a t i o n was t o g a i n a g r e a t e r u n d e r s t a n d i n g o f t h e i n t e r r e l a t i o n s h i p s between t h e method o f c a t a l y s t p r e p a r a t i o n and t h e s t r u c t u r e , a c t i v i t y and s t a b i l i t y o f t h e c a t a l y s t w i t h t h e hope t h a t we may be a b l e t o d e s i g n a b e t t e r c a t a l y s t f o r p o s s i b l e use i n t h e ADAM r e a c t o r .

In t h e f i r s t s t a g e o f t h i s i n v e s t i g a t i o n , m a i n l y s o l i d s t a t e c h e m i c a l a s p e c t s o f c a t a l y s t p r e p a r a t i o n were s t u d i e d a t D e l f t , whereas a c t i v i t y and s e l e c t i v i t y measurements were p e r f o r m e d a t B r a d f o r d . In l a t e r y e a r s , c a t a l y t i c a c t i v i t y d e t e r m i n a t i o n s c o u l d a l s o be c a r r i e d o u t a t D e l f t . Some o f t h e c a t a l y s t s were t e s t e d i n an i n t e g r a l r e a c t o r a t t h e KFA a t Jülich.

D u r i n g t h e s e i n v e s t i g a t i o n s , o u r a t t e n t i o n has m a i n l y been c o n c e n t r a t e d on c o p r e c i p i t a t e d n i c k e l - a l u m i n a c a t a l y s t s . T h i s c h o i c e needs some comment.

A t f i r s t , when l o o k i n g f o r a m e t h a n a t i o n c a t a l y s t , t h e c h o i c e o f n i c k e l as t h e a c t i v e component seems t o be o b v i o u s .

Among n i c k e l c a t a l y s t s , an i m p o r t a n t group i s formed by t h o s e w h i c h a r e u s e d i n t h e m e t h a n a t i o n o f t r a c e s o f CO and CO^ p r e s e n t i n t h e ammonia s y n t h e s i s gas; g e n e r a l l y t h e s e a r e p r e p a r e d by i m p r e g n a t i o n . T h e s e c a t a l y s t s have a s u f f i c i e n t l y h i g h a c t i v i t y a t low t e m p e r a t u r e , b u t s i n t e r r e a d i l y a t t h e h i g h t e m p e r a t u r e s w h i c h w i l l be a t t a i n e d i n t h e f i n a l p a r t o f t h e m e t h a n a t i o n r e a c t o r . On t h e o t h e r hand steam r e f o r m i n g c a t a l y s t s a r e s t a b l e a t h i g h t e m p e r a t u r e s , b u t show i n s u f f i c i e n t a c t i v i t y a t low t e m p e r a t u r e . An i n t e r m e d i a t e p o s i t i o n i s o c c u p i e d by t h e c o p r e c i p i t a t e d n i c k e l - a l u m i n a c a t a l y s t s o f h i g h n i c k e l c o n t e n t (~ 70 wt % n i c k e l ) w h i c h a r e c u r r e n t l y u s e d i n t h e p r o d u c t i o n o f s y n t h e t i c n a t u r a l g a s (S.N.G.) from n a p h t a ^ ' ^ '4^ i n t h e s o

-c a l l e d " C a t a l y t i -c R i -c h Gas" ("C.R.G.") p r o -c e s s o f B r i t i s h Gas. I n t h e steam r e f o r m i n g s e c t i o n o f t h e p r o c e s s , t h e c a t a l y s t i s e x p o s e d t o t e m p e r a t u r e s up t o a p p r o x i m a t e l y 5 0 0 ° C ^ ^ ; t h e same c a t a l y s t i s a l s o u s e d i n t h e m e t h a n a t i o n s t a g e s o f t h e p r o c e s s , and t h u s seems p r o m i s i n g f o r t h e p u r p o s e o u t l i n e d above.

1.2 THE N.F.E. PROJECT

The aim o f t h e NFE p r o j e c t i s t o i n v e s t i g a t e t h e p o s s i b i l i t y o f t r a n s p o r t i n g heat by means o f a c h e m i c a l c i r c u i t . The r e a c t i o n s t o be u s e d i n s u c h a

c h e m i c a l c i r c u i t must meet a number o f r e q u i r e m e n t s ; most o f t h e s e a r e f u l f i l l e d by t h e c o m b i n a t i o n o f t h e e n d o t h e r m i c steam r e f o r m i n g r e a c t i o n and t h e e x o t h e r m i c m e t h a n a t i o n r e a c t i o n

T r a n s p o r t o f energy by means o f such a system would be much more e f f i c i e n t t h a n t r a n s p o r t i n form o f e l e c t r i c a l e n e r g y . In a d d i t i o n , i t has t o be n o t e d t h a t t h e l a r g e s t p a r t o f t h e t o t a l e n e r g y c o n s u m p t i o n i s i n form o f h e a t : f o r

(5)

example i n West Germany t h i s amounts t o o v e r 70% . The o v e r a l l e f f i c i e n c y from n u c l e a r r e a c t o r t o consumer o f a p r o c e s s , i n w h i c h h e a t i s t r a n s f o r m e d i n t o e l e c t r i c a l e n e r g y , w h i c h i s t r a n s p o r t e d and t h e n t r a n s f o r m e d back i n t o h e a t , would be o n l y 30% ; f o r a l o n g d i s t a n c e e n e r g y system as m e n t i o n e d (7) above t h i s can be as h i g h as 70% 20 °C CO, H2,C02 64 bar 1« 70 km | 20 °C 20 bar hot-water (130 °C, 8 bar) steam (530 °C,100 bar)

Fig. 1.1 (taken from ref. 1). Principle underlying nuclear long-distance energy (closed circuit system). Key: 1 high temperature reactor; 2 steam

reformer; 3 pre heater; 4 coolant fan; 5 waste heat recovery; 6 H^, CO, COg compressor; 7 methanation 8 heat exchanger; 9 CH^ compressor.

F i g . 1.1 shows a s c h e m a t i c r e p r e s e n t a t i o n o f a d e s i g n o f t h e p r o c e s s . The h e a t f o r t h e e n d o t h e r m i c steam r e f o r m i n g o f methane, w h i c h i s p e r f o r m e d a t a t e m p e r a t u r e o f 800°C, i s s u p p l i e d by t h e h e l i u m c o o l a n t , w h i c h l e a v e s t h e n u c l e a r r e a c t o r a t a t e m p e r a t u r e o f 950°C. The c o o l e d p r o d u c t gas i s t r a n s p o r t e d t o t h e consumer a r e a , where t h e gas m i x t u r e i s c o n v e r t e d back i n t o methane and steam. The h e a t l i b e r a t e d i n t h e r e a c t i o n can be u s e d t o p r o d u c e hot w a t e r f o r 3

house-warming, and steam f o r t h e p r o d u c t i o n o f e l e c t r i c i t y and f o r c h e m i c a l i n d u s t r y . To c l o s e t h e " c a r b o n c i r c u i t " , methane i s t r a n s p o r t e d back t o t h e steam r e f o r m i n g r e a c t o r .

The p r o c e s s c a n a l s o be o p e r a t e d as an open c i r c u i t : p a r t o f t h e p r o d u c e d methane can be f e d i n t o t h e n a t u r a l gas p i p e l i n e system and r e f o r m e r gas can be u t i l i z e d i n c h e m i c a l i n d u s t r y e.g. m e t h a n o l s y n t h e s i s , h y d r o g e n p r o d u c t i o n , F i s c h e r - T r o p s c h s y n t h e s i s and d i r e c t r e d u c t i o n o f i r o n o r e s . To compensate f o r t h i s l o s s o f c a r b o n c o n t a i n i n g compounds from t h e c i r c u i t , s y n t h e s i s gas o b t a i n e d from n u c l e a r c o a l g a s i f i c a t i o n can be s u p p l i e d .

In view o f t h e o v e r a l l e f f i c i e n c y o f t h e p r o c e s s , t h e gas t e m p e r a t u r e w h i c h i s a t t a i n e d i n m e t h a n a t i o n i s an i m p o r t a n t p a r a m e t e r . The c o n c e p t o f t h e u s e o f t h e NFE system r e q u i r e s a gas t e m p e r a t u r e o f 600°C o r h i g h e r t o p r o d u c e steam o f h i g h t e m p e r a t u r e and p r e s s u r e f o r t h e g e n e r a t i o n o f e l e c t r i c i t y and

(8) to p r o d u c e p r o c e s s steam o f h i g h t e m p e r a t u r e f o r c h e m i c a l i n d u s t r y . F o r t h i s r e a s o n , m e t h a n a t i o n w i l l be p e r f o r m e d a d i a b a t i c a l l y . S i n c e t h e e q u i l i b r i u m s h i f t s t o t h e c a r b o n monoxide s i d e w i t h i n c r e a s i n g t e m p e r a t u r e , no c o m p l e t e c o n v e r s i o n can be r e a c h e d a t t h e end o f t h e a d i a b a t i c m e t h a n a t o r . To o b t a i n c o m p l e t e c o n v e r s i o n , a number o f a d i a b a t i c s t a g e s w i t h s u b s e q u e n t l y l o w e r e x i t t e m p e r a t u r e s can be u s e d . However, when no s p e c i a l p r e c a u t i o n s a r e t a k e n , a v e r y h i g h t e m p e r a t u r e w i l l be r e a c h e d a t t h e e x i t o f t h e f i r s t m e t h a n a t o r : d e p e n d i n g on t h e

c o m p o s i t i o n o f t h e gas w h i c h i s o b t a i n e d from t h e steam r e f o r m i n g r e a c t o r t h i s o o

w i l l be between 780 C and 850 C. S i n c e i t seems not v e r y p r o b a b l y t h a t i n a s h o r t term a c a t a l y s t can be o b t a i n e d w h i c h i s s t a b l e i n t h e whole t e m p e r a t u r e r a n g e from 300°C t o 780°C, t h e maximum t e m p e r a t u r e a t t a i n e d i n m e t h a n a t i o n has t o be l o w e r e d . T h i s can be a c h i e v e d by r e c y c l i n g p a r t o f t h e p r o d u c t gas.

Thus t h e p i l o t p l a n t ADAM I a t t h e KFA a t Jülich has been d e s i g n e d on b a s i s o f a scheme i n v o l v i n g m e t h a n a t i o n i n t h r e e s t a g e s w i t h r e c y c l i n g o f p r o d u c t gas a f t e r t h e f i r s t s t a g e . In t h i s way t h e t e m p e r a t u r e a t t h e e x i t o f the f i r s t m e t h a n a t o r c o u l d be l i m i t e d t o 6 0 0 ° C ^8\

The o p t i m a l t e m p e r a t u r e t o be d e s i r e d a t t h e e x i t o f t h e f i r s t m e t h a n a t o r w i l l be a compromise between t h e demands o f t h e e n e r g e t i c e f f i c i e n c y o f t h e p r o c e s s , and t h e s t a b i l i t y o f t h e a v a i l a b l e c a t a l y s t s . I t has t o be n o t e d , t h a t s i m i l a r c o n s i d e r a t i o n s a p p l y a l s o i n t h e d e s i g n o f (7) a p l a n t f o r t h e p r o d u c t i o n o f SNG from c o a l 1.3 SURVEY From t h e d i s c u s s i o n i n t h e p r e c e d i n g s e c t i o n s i t f o l l o w s t h a t t h e most 4

i m p o r t a n t p a r a m e t e r s t o be i n v e s t i g a t e d f o r t h e n i c k e l - a l u m i n a m e t h a n a t i o n c a t a l y s t s a r e : o - t h e m e t h a n a t i o n a c t i v i t y a t low t e m p e r a t u r e ( = 300 C ) ; - t h e s t a b i l i t y a t t e m p e r a t u r e s o f 600°C and h i g h e r . In o r d e r t o i n v e s t i g a t e w h i c h f a c t o r s a r e g o v e r n i n g t h e r e p r o d u c i b i l i t y o f t h e p r e p a r a t i o n , and which f a c t o r s a r e most i m p o r t a n t i n d e t e r m i n i n g t h e

p r o p e r t i e s o f t h e f i n a l c a t a l y s t , much a t t e n t i o n has been p a i d t o t h e

p r e p a r a t i o n and c h a r a c t e r i z a t i o n o f t h e c a t a l y s t s . A l s o , f o r many samples, t h e o

a c t i v i t y a t low t e m p e r a t u r e (300 C) has been measured.

C o n c e r n i n g t h e s t a b i l i t y , o n l y some p r e l i m i n a r y r e s u l t s were o b t a i n e d on t h e r e s i s t a n c e o f t h e c a t a l y s t s a g a i n s t s i n t e r i n g o f t h e n i c k e l c r y s t a l l i t e s ; r e f e r e n c e t o t h e s e r e s u l t s w i l l be f o u n d a t t h e end o f c h a p t e r V. A s e r i o u s p r o b l e m was p o s e d by t h e m e c h a n i c a l s t r e n g t h o f t h e c a t a l y s t p e l l e t s : i t t u r n e d out t o be v e r y d i f f i c u l t t o p r e p a r e p e l l e t s o f s u f f i c i e n t s t r e n g t h , t o be s u i t a b l e f o r a t e s t i n one o f t h e r e a c t o r s a t t h e KFA. To c i r c u m v e n t t h e p r o b l e m o f p r e p a r i n g s t r o n g p e l l e t s o f c o p r e c i p i t a t e d m a t e r i a l , a p a r a l l e l i n v e s t i g a t i o n was s t a r t e d , i n w h i c h n i c k e l - a l u m i n a c a t a l y s t s were p r e p a r e d by d e p o s i t i o n - p r e c i p i t a t i o n o f n i c k e l on m e c h a n i c a l l y s t r o n g and s t a b l e a l u m i n a s u p p o r t s . T h e s e i n v e s t i g a t i o n s , p e r f o r m e d by H. S c h a p e r , a r e o u t s i d e t h e s c o p e o f t h i s t h e s i s . C h a p t e r I I i s d e a l i n g w i t h v a r i o u s a s p e c t s o f t h e f i r s t s t a g e o f t h e p r e p a r a t i o n o f t h e c a t a l y s t s , t h e c o p r e c i p i t a t i o n . S p e c i a l a t t e n t i o n i s g i v e n t o t h e r e l a t i o n between p r e c i p i t a t i o n c o n d i t i o n s on t h e one hand, and c h e m i c a l c o m p o s i t i o n and s t r u c t u r e o f t h e p r e c i p i t a t e s on t h e o t h e r hand. C h a r a c t e r i z a -t i o n me-thods u s e d a r e X - r a y d i f f r a c -t i o n , c h e m i c a l a n a l y s i s , -t h e r m a l -t e c h n i q u e s (TG-DTA) and i n f r a r e d s p e c t r o s c o p y . In c h a p t e r I I I t h e c a l c i n a t i o n and s u b s e q u e n t r e d u c t i o n o f t h e p r e c i p i t a t e s i s d i s c u s s e d . S u b j e c t s t o be t r e a t e d a r e t h e phase c o m p o s i t i o n and r e d u c i b i l i t y o f t h e c a l c i n e d m a t e r i a l s , t h e d i s p e r s i o n o f n i c k e l o x i d e i n t h e c a l c i n e d m a t e r i a l s and o f n i c k e l i n t h e f i n a l r e d u c e d c a t a l y s t , whereas a l s o some d a t a on t h e t e x t u r e w i l l be p r e s e n t e d . C h a p t e r IV d e a l s w i t h t h e m e t h a n a t i o n a c t i v i t y a t low t e m p e r a t u r e : t h e e f f e c t o f v a r i o u s p a r a m e t e r s i s d i s c u s s e d . The r e s u l t s a r e compared w i t h t h o s e r e p o r t e d i n l i t e r a t u r e . F i n a l l y c h a p t e r V g i v e s a r e v i e w o f t h e most i m p o r t a n t r e s u l t s . 5

C h a p t e r I I . PREPARATION, CHEMICAL COMPOSITION AND STRUCTURE OF T H E

C O P R E C I P I T A T E D MATERIALS

I I . 1 INTRODUCTION

II.1.1 Preparation methods

In t h e l i t e r a t u r e , i n c l u d i n g t h e p a t e n t - l i t e r a t u r e , v a r i o u s r e c i p e s f o r t h e c o p r e c i p i t a t i o n o f n i c k e l and a l u m i n i u m h y d r o x i d e s a r e d e s c r i b e d . In t h e s e , a l a r g e number o f p a r a m e t e r s i s i n v o l v e d : t e m p e r a t u r e , c o n c e n t r a t i o n s , pH o f p r e c i p i t a t i o n , c o m p o s i t i o n o f t h e b a s i c s o l u t i o n added d u r i n g t h e p r e c i p i t a t i o n , s p e c i e s o f m e t a l s a l t s used, method o f a g e i n g and so on. V a r y i n g any o f t h e s e p a r a m e t e r s may have an e f f e c t on t h e c a t a l y t i c p r o p e r t i e s o f t h e f i n a l p r o d u c t . T h e r e f o r e i t i s i m p o r t a n t t o p e r f o r m t h e p r e c i p i t a t i o n p r o c e s s i n a w e l l

-d e f i n e -d an-d c o n t r o l l e -d way. In t h e p r e s e n t i n v e s t i g a t i o n , s p e c i a l a t t e n t i o n was g i v e n t o t h e way i n w h i c h pH o f p r e c i p i t a t i o n and c o m p o s i t i o n o f p r e c i p i t a n t i n f l u e n c e t h e a n i o n c o m p o s i t i o n and t h e sodium c o n t e n t o f t h e p r e c i p i t a t e s . R e g a r d i n g t h e pH o f p r e c i p i t a t i o n , we can d i s t i n g u i s h between p r o c e d u r e s i n w h i c h t h e pH i s kept a t a c o n s t a n t v a l u e d u r i n g t h e p r o c e s s , and t h o s e i n w h i c h t h e pH v a l u e i s i n c r e a s i n g o r d e c r e a s i n g . In most c a s e s t h e i n i t i a l s o l u t i o n , c o n t a i n i n g t h e m e t a l s a l t s , has a low pH. A s i m p l e p r e c i p i t a t i o n p r o c e d u r e c o n s i s t s o f t h e a d d i t i o n o f a b a s e t o t h i s s o l u t i o n u n t i l t h e pH r e a c h e s a v a l u e o f about s e v e n ^ ' . In a n o t h e r common method a s o l u t i o n o f t h e m e t a l s a l t s and a b a s i c s o l u t i o n a r e added s i m u l t a n e o u s l y t o a v e s s e l , t h e r a t e s o f a d d i t i o n o f t h e s o l u t i o n s b e i n g a d j u s t e d i n s u c h a way as t o m a i n t a i n t h e pH a t a c o n s t a n t , (2) v a l u e (3) A t o t a l l y d i f f e r e n t a p p r o a c h has been d e s c r i b e d by M e r l i n e t a l . . N i c k e l and a l u m i n i u m n i t r a t e s a r e d i s s o l v e d i n an ammoniacal aqueous s o l u t i o n ;

p r e c i p i t a t i o n o c c u r s as t h e pH i s d e c r e a s i n g when t h e ammonia i s removed by steam d i s t i l l a t i o n .

In t h e p r e s e n t work, most samples were p r e p a r e d by c o p r e c i p i t a t i o n a t a c o n s t a n t pH. A c o m p a r i s o n w i l l be made between t h e r e s u l t s o b t a i n e d i n t h i s way and t h o s e o b t a i n e d by t h e a d d i t i o n o f a s o l u t i o n o f sodium c a r b o n a t e t o a

s o l u t i o n o f t h e n i t r a t e s . A l s o some a t t e n t i o n was p a i d t o t h e p r e c i p i t a t i o n (3)

from ammoniacal s o l u t i o n a c c o r d i n g t o t h e p a p e r by M e r l i n e t a l . . The r e l a t i o n between c h e m i c a l c o m p o s i t i o n and p r e p a r a t i o n c o n d i t i o n s w i l l be d i s c u s s e d i n s e c t i o n I I . 3 . Then s e c t i o n s I I . 4 - I I . 6 w i l l d e a l w i t h t h e s t r u c t u r e and f u r t h e r c h a r a c t e r i z a t i o n o f t h e p r o d u c t s formed. F i r s t o f a l l t h e s e c o n d p a r t o f t h e I n t r o d u c t i o n w i l l g i v e a r e v i e w o f l i t e r a t u r e d a t a on c h e m i c a l c o m p o s i t i o n and s t r u c t u r e o f t h e c o p r e c i p i t a t e d m a t e r i a l s .

II.1.2 Composition and structure of the phases formed

A l l t h e p r o c e d u r e s f o r t h e c o p r e c i p i t a t i o n o f n i c k e l and a l u m i n i u m h y d r o x i d e s d e s c r i b e d i n t h e f o r m e r s e c t i o n r e s u l t i n t h e p r e c i p i t a t i o n o f compounds w h i c h c r y s t a l l i z e i n a s o - c a l l e d d o u b l e - l a y e r s t r u c t u r e , r a t h e r t h a n i n t h e f o r m a t i o n o f t h e s e p a r a t e h y d r o x i d e s . The f i r s t t o d e s c r i b e some s y n t h e t i c compounds o f t h i s t y p e , a l s o f o r o t h e r c o m b i n a t i o n s o f d i v a l e n t and t r i v a l e n t m e t a l s , was (4) F e i t k n e c h t . He s u g g e s t e d a s t r u c t u r e c o n s i s t i n g o f b r u c i t e (MgCOH)^) t y p e l a y e r s , i n w h i c h t h e d i v a l e n t m e t a l i s p r e s e n t on o c t a h e d r a l p o s i t i o n s , i n t e r -c h a n g i n g w i t h i n t e r l a y e r s o f t r i v a l e n t m e t a l h y d r o x i d e . (5) Some y e a r s l a t e r , L o n g u e t - E s c a r d p r e p a r e d a s e r i e s o f s o - c a l l e d n i c k e l h y d r o a l u m i n a t e s by a d d i t i o n o f sodium c a r b o n a t e t o a s o l u t i o n o f n i c k e l and a l u m i n i u m n i t r a t e s . The X - r a y d a t a g i v e n f o r t h e s e compounds a r e s i m i l a r t o (4) t h o s e r e p o r t e d by F e i t k n e c h t . L o n g u e t - E s c a r d i n t e r p r e t e d t h e X - r a y p a t t e r n s a l s o by assuming a l a y e r e d s t r u c t u r e , b u t i n c o n t r a s t t o F e i t k n e c h t she p r o p o s e d n i c k e l and a l u m i n i u m t o be s t a t i s t i c a l l y d i s t r i b u t e d about o c t a h e d r a l p o s i t i o n s w i t h i n t h e b r u c i t e l a y e r . F u r t h e r a t t e n t i o n was g i v e n t o t h i s t y p e o f compounds by m i n e r a l o g i s t s . (4) F e i t k n e c h t a l r e a d y m e n t i o n e d t h e s i m i l a r i t y o f t h e X - r a y p a t t e r n o f t h e d o u b l e - l a y e r compounds t o t h a t o f t h e m i n e r a l p y r o a u r i t e , a magnesium i r o n h y d r o x y c a r b o n a t e . T h i s b e l o n g s t o a l a r g e g r o u p o f m i n e r a l s o f a p p r o x i m a t e c o m p o s i t i o n M*IM*I I(0H),_C0_.4H_0 i n w h i c h M1 1 i s among o t h e r s Mg, Zn o r N i and b £, l b o Z I I I

i n w h i c h t h e most i m p o r t a n t r e p r e s e n t a t i v e s o f M a r e A l , Fe and C r . Two m o d i f i c a t i o n s a r e known, d i f f e r i n g i n t h e s t a c k i n g s e q u e n c e o f t h e l a y e r s , namely a rhombohedral and a h e x a g o n a l one. P y r o a u r i t e i s r h o m b o h e d r a l ; t h e h e x a g o n a l e q u i v a l e n t i s named s j o g r e n i t e . Reviews about t h e s e m i n e r a l s have

lm (8) member o f t h i s group has a l s o been f o u n d i n n a t u r e and i s named t a k o v i t e

A l s o t h e name e a r d l e y i t e h a s been p r o p o s e d b u t t h i s name i s r e j e c t e d now^'^"'. The s t r u c t u r e s o f s e v e r a l m i n e r a l s o f t h i s group were f i r s t d e t e r m i n e d i n t h e s i n g l e - c r y s t a l i n v e s t i g a t i o n s o f A l l m a n n ^1 1^ and o f T a y l o r ^1 2^ , who

i n v e s t i g a t e d among o t h e r s h y d r o t a l c i t e ( c a t i o n s Mg and A l ) and p y r o a u r i t e ( c a t i o n s Mg and F e ) . They f o u n d t h a t t h e s t r u c t u r e ( s e e F i g . I I . 1 ) c o n s i s t s o f 8

Fig. II.1. (110) projection of the idealized crystal structure of

pyroaurite-like compounds.

I rhombohedral modification: c

q

parameter amounts to three times the "layer

(pyroaurite) spacing".

II hexagonal modification : c

q

parameter amounts to two times the "layer

(sjogrenite) spacing".

O • metal ions.

O : hydroxy I ions.

® : interlayer ions e.g. oxygen from carbonate groups,

nitrate groups or water molecules.

The blackened symbols represent ions at equal distances above and below the

(110) plane. The open symbols represent ions within the (110) plane.

b r u c i t e - l i k e l a y e r s , i n agreement w i t h the i d e a o f F e i t k n e c h t , but t h e y f o u n d t h i s l a y e r t o c o n t a i n b o t h d i v a l e n t and t r i v a l e n t c a t i o n s on o c t a h e d r a l s i t e s . However, i n i n v e s t i g a t i o n s o f s y n t h e t i c s a m p l e s , t h e r e has been d i s a g r e e m e n t as

(4) t o t h e p o s i t i o n s o f d i v a l e n t and t r i v a l e n t c a t i o n s . A f t e r F e i t k n e c h t , a l s o Ross and Kodama^'''^^ and B r a d s h a w ^ ^ f a v o u r e d a model i n which t h e A l i o n s occupy a p o s i t i o n somewhere between t h e b r u c i t e l a y e r s . We a r e t h e r e f o r e c o n c e r n e d i n t h i s work w i t h t h e p o s i t i o n s o f t h e c a t i o n s i n t h e s t r u c t u r e o f t h e p r e c i p i t a t e .

A n o t h e r p o i n t o f i n t e r e s t c o n c e r n s t h e range o f MI I/ M1 1 1 r a t i o s f o r w h i c h

t h e s e compounds e x i s t . F o r n e a r l y a l l the m i n e r a l s o f t h e p y r o a u r i t e group t h i s

( 6 7 ) (15) r a t i o i s t h r e e ' . C o n s i d e r i n g t h e s y n t h e t i c samples Brown and G a s t u c h e

f o u n d i n t h e i r work on t h e Mg-Al system t h a t two compounds o c c u r h a v i n g r a t i o s (16 )

(Mg/Al) o f two and o f f i v e r e s p e c t i v e l y . A c c o r d i n g t o L o n g u e t - E s c a r d the N i / A l r a t i o o f t h e n i c k e l h y d r o a l u m i n a t e s r a n g e s between one and s e v e n and a

(17) h a l f . V e r y r e c e n t l y , B r i n d l e y and Kikkawa r e p o r t e d t h e s e l i m i t s t o be a p p r o x i m a t e l y two and f o u r f o r n i c k e l - a l u m i n i u m h y d r o x y c a r b o n a t e s . The p o s s i b i l i t y o f t h e i n c o r p o r a t i o n o f d i f f e r e n t s p e c i e s o f a n i o n s i s i n t e r e s t i n g , e s p e c i a l l y b e c a u s e , as w i l l be shown i n l a t e r c h a p t e r s , t h e a n i o n p r e s e n t i n t h e p r e c i p i t a t e i n f l u e n c e s t h e p r o p e r t i e s o f t h e c a t a l y s t w h i c h i s (4) (5) u l t i m a t e l y p r e p a r e d from i t . F e i t k n e c h t and L o n g u e t - E s c a r d assumed t h a t no o t h e r a n i o n s t h a n h y d r o x y l i o n s are p r e s e n t i n t h e s e compounds. A c c o r d i n g t o o u r p r e s e n t knowledge t h i s a s s u m p t i o n i s n o t c o r r e c t . Most o f t h e s y n t h e t i c samples and m i n e r a l s d e s c r i b e d l a t e r on c o n t a i n c a r b o n a t e i o n s i n t h e i n t e r -(6 7) l a y e r ' . A l s o some a n a l o g u e s o f t h e m i n e r a l s o f t h e p y r o a u r i t e group a r e (18) known c o n t a i n i n g c h l o r i d e , n i t r a t e , h y d r o x i d e o r s u l p h a t e a n i o n s ( s e e B i s h ). (19) Roy e t a l . s y n t h e s i z e d a n i t r a t e a n a l o g u e o f h y d r o t a l c i t e w h i l e , more r e c e n t l y , M i y a t a ^2 0^ p r e p a r e d t h e M g - A l - N O „ , M g - A l - C l , Mg-Al-C10., N i - A l - C l and (3) Z n A l C l a n a l o g u e s . M e r l i n e t a l . c l a i m e d t h a t a v e r y p u r e n i c k e l h y d r o -a l u m i n -a t e , c o n t -a i n i n g o n l y h y d r o x y l i o n s -as -a n i o n , c o u l d be p r e p -a r e d from (18) ammoniacal s o l u t i o n . B i s h c a r r i e d out an i o n - e x c h a n g e s t u d y o f n a t u r a l l y o c c u r r i n g t a k o v i t e and f o u n d t h a t t h e c a r b o n a t e i o n may be r e p l a c e d by s e v e r a l o t h e r a n i o n s ; h i s p a p e r , however, does not i n c l u d e a n a l y t i c a l d a t a .

In t h i s c h a p t e r , a s y s t e m a t i c i n v e s t i g a t i o n i s r e p o r t e d o f t h e p r e p a r a t i o n , c o m p o s i t i o n and s t r u c t u r e o f s y n t h e t i c N i A l h y d r o x y c a r b o n a t e s and h y d r o x y -n i t r a t e s . A l s o some d a t a about h y d r o x y c h l o r i d e s a-nd h y d r o x y a c e t a t e s w i l l be p r e s e n t e d . At f i r s t t h e c h e m i c a l c o m p o s i t i o n o f t h e hydroxycompounds w i l l be d i s c u s s e d as a f u n c t i o n o f t h e c o n d i t i o n s o f t h e c o p r e c i p i t a t i o n , where pH and c h o i c e o f p r e c i p i t a n t a r e t h e i m p o r t a n t v a r i a b l e s . In s u b s e q u e n t s e c t i o n s t h e r e s u l t s o f X - r a y d i f f r a c t i o n (XRD) s t u d i e s w i l l be g i v e n i n s u p p o r t f o r a model o f t h e s t r u c t u r e o f t h e p r e c i p i t a t e s and t h e r e s u l t s o f i n v e s t i g a t i o n s o f t h e 10

t h e r m a l d e c o m p o s i t i o n and 1R measurements w i l l be g i v e n t o c o n f i r m t h e model. New d a t a a r e p r e s e n t e d r e g a r d i n g t h e r e g i o n s o f s t a b i l i t y w i t h r e s p e c t t o c a t i o n r a t i o , t h e dependence o f c e l l p a r a m e t e r s on t h i s r a t i o , t h e e f f e c t o f t h e a n i o n on t h e l a y e r d i s t a n c e and t h e t h e r m a l b e h a v i o u r o f t h e samples. I t w i l l t u r n o u t , t h a t t h e f i n d i n g s o f t h i s c h a p t e r a i d i n u n d e r s t a n d i n g d e t a i l s o f t h e p r e p a r a t i o n r o u t i n e e s t a b l i s h e d i n an e m p i r i c a l way f o r t h e c a t a l y s t s under c o n s i d e r a t i o n . I I . EXPERIMENTAL

II.2.1 Copreoipitation

A l l t h e c h e m i c a l s u s e d f o r t h e p r e p a r a t i o n s d e s c r i b e d i n t h i s t h e s i s were o f P r o a n a l y s i s q u a l i t y . The c o p r e c i p i t a t i o n was c a r r i e d o u t u s i n g two s o l u t i o n s ,

i

one c o n t a i n i n g n i c k e l and a l u m i n i u m n i t r a t e s ( t o t a l m e t a l c o n c e n t r a t i o n o f 0.9 -3

mol dm ) , t h e o t h e r c o n t a i n i n g t h e e q u i v a l e n t amount o f Na CO, and/or NaOH. A

3 2 J

volume o f 150 cm o f d i s t i l l e d w a t e r was p l a c e d i n a b e a k e r e q u i p p e d w i t h s t i r r e r , thermometer and pH e l e c t r o d e s ( P h i l i p s CAH-11NS), t h e pH b e i n g measured w i t h a P h i l i p s d i g i t a l i o n a c t i v i t y meter (PW414/01). Volumes o f 175

3

cm o f e a c h s o l u t i o n were added s i m u l t a n e o u s l y t o t h e b e a k e r , t h e r a t e o f a d d i t i o n b e i n g a d j u s t e d so t h a t t h e pH was k e p t c o n s t a n t a t a p r e d e t e r m i n e d v a l u e ; t h e s o l u t i o n s were m a i n t a i n e d t h r o u g h o u t a t a t e m p e r a t u r e o f 80°C and t h e a d d i t i o n was c a r r i e d o u t o v e r a p e r i o d o f about 30 min. In t h o s e c a s e s where i t was d e s i r e d t o p r e p a r e l a r g e r b a t c h e s , t h e s e f i g u r e s were m u l t i p l i e d by a c o n s t a n t f a c t o r . One c a r b o n a t e - f r e e sample was p r e c i p i t a t e d under a n i t r o g e n atmosphere u s i n g d e c a r b o n a t e d s o l u t i o n s and f o r t h i s a s i x - n e c k e d round-bottomed f l a s k was u s e d w h i c h was p r o v i d e d w i t h a s t i r r e r , e l e c t r o d e , thermometer, gas i n l e t and o u t l e t t h r o u g h w h i c h n i t r o g e n was c o n t i n u o u s l y b u b b l e d . A number o f p r e p a r a t i o n s were a l s o c a r r i e d out by a d d i n g t h e s o l u t i o n o f N a2C 03 t o t h e s o l u t i o n o f n i t r a t e s ( r i s i n g pH method), t h e t e m p e r a t u r e b e i n g

m a i n t a i n e d a t 95°C; i t was found t h a t t h e e n d - r e s u l t was a m a t e r i a l s i m i l a r t o t h a t p r e p a r e d by t h e " C o n s t a n t pH method" a t a pH e q u i v a l e n t t o t h e f i n a l pH o f t h e p r e p a r a t i o n .

In many c a s e s , t h e s u s p e n s i o n was d i v i d e d i n two a f t e r p r e c i p i t a t i o n , p a r t b e i n g f i l t e r e d on a g l a s s f i l t e r , t h o r o u g h l y washed w i t h d i s t i l l e d w a t e r o f a t e m p e r a t u r e o f 90100°C and d r i e d o v e r n i g h t a t 80°C; t h e r e m a i n d e r was h y d r o -t h e r m a l l y -t r e a -t e d i n a -t e f l o n -t u b e i n an a u -t o c l a v e a -t 150°C o r 275°C f o r -two d a y s . Some samples, a f t e r washing and d r y i n g , were h y d r o t h e r m a l l y t r e a t e d i n a 11

g o l d t u b e . The tube was s e a l e d a f t e r t h e a d d i t i o n o f a few d r o p s o f d i s t i l l e d o

w a t e r t o t h e d r y powder and was k e p t a t 300 C a t a p r e s s u r e o f a p p r o x i m a t e l y g

10 Pa f o r two d a y s . In t h e f o l l o w i n g t h e n i c k e l c o n t e n t s w i l l be e x p r e s s e d as mole f r a c t i o n x = N i / ( N i + A l ) .

II. 2. 2 Coprecipitation from ammoniacal solutions

A n o t h e r s e r i e s o f samples was p r e p a r e d s t a r t i n g from an ammoniacal s o l u t i o n _3

(1.7 mol dm ) c o n t a i n i n g n i c k e l and a l u m i n i u m n i t r a t e s a t low c o n c e n t r a t i o n s

—3 -3 (3) (8.0 x 10 mol dm o f each) u s i n g t h e p r o c e d u r e d e s c r i b e d by M e r l i n e t a l .

and a l s o t h r e e v a r i a n t s o f i t . S e v e r a l samples were a l s o made w i t h a h i g h e r

-3 -3 p r o p o r t i o n o f n i c k e l x = 0 . 7 5 u s i n g a s o l u t i o n c o n t a i n i n g 1 2 . 0 x 1 0 mol dm

-3 1 -3

o f NiNOg and 4 . 0 x 1 0 mol dm o f A 1 ( N 03)3. P r e c i p i t a t i o n i s a c h i e v e d by

r e m o v i n g t h e ammonia from t h e s o l u t i o n by h e a t i n g , t h i s c a u s i n g t h e pH t o d e c r e a s e from i t s i n i t i a l v a l u e o f about 11.6 t o 6.5. The r e m o v a l o f ammonia was a c h i e v e d i n f o u r ways: (a) by steam d i s t i l l a t i o n as i n t h e o r i g i n a l r e c i p e ; (b) i n an open b e a k e r a t 90°C; ( c ) a t 60°C and a t a p r e s s u r e o f 3.6 kPa u s i n g a r o t a r y f i l m e v a p o r a t o r ; and (d) i n t h e r o u n d - b o t t o m e d f l a s k u s e d above f o r p r e p a r a t i o n s i n n i t r o g e n , o t h e t e m p e r a t u r e a g a i n b e i n g 90 C.

The samples p r e p a r e d by t h e s e methods were f i l t e r e d and washed i n t h e same way as w i t h t h e c o p r e c i p i t a t e d s a m p l e s .

II. 2. 3 Sample Characterization

The samples were c h a r a c t e r i z e d by X - r a y d i f f r a c t i o n , c h e m i c a l a n a l y s i s , t h e r m a l a n a l y s i s and i n f r a r e d s p e c t r o s c o p y .

X - r a y measurements were c a r r i e d o u t u s i n g a G u i n i e r - D e W o l f f camera o r a G u i n i e r - L e n n é H i g h T e m p e r a t u r e Camera, b o t h m a n u f a c t u r e d by E n r a f - N o n i u s

( D e l f t ) . Many samples were a l s o s t u d i e d w i t h t h e a i d o f a P h i l i p s powder d i f f r a c t o m e t e r . Cu Ka r a d i a t i o n was used i n a l l cases.

Some o f t h e samples were a n a l y s e d by c h e m i c a l methods f o r N i , A l , C0g and/ o r NOg. The n i c k e l c o n t e n t was d e t e r m i n e d e i t h e r by a t o m i c a b s o r p t i o n o r g r a v i m e t r i c a l l y u s i n g d i m e t h y l g l i o x i m e ; a l u m i n i u m was d e t e r m i n e d u s i n g a t o m i c a b s o r p t i o n , CO^ by d i s s o l v i n g t h e p r e c i p i t a t e i n p h o s p h o r i c a c i d and t i t r a t i n g t h e C 02 e v o l v e d , and t h e n i t r a t e by r e d u c t i o n t o NHg w h i c h was t h e n t i t r a t e d

(Devarda method).

Weight l o s s e s up t o about 950°C were measured by means o f a s y m m e t r i c a l

t h e r m o b a l a n c e , c o n s t r u c t e d i n o u r l a b o r a t o r y by G. H a k v o o r t , f u r n i s h e d w i t h a C . J . E l e c t r o n i c s m i c r o b a l a n c e . About 20 t o 25 mg sample was h e a t e d a t a r a t e o f 6°C/min. i n a f l o w o f h i g h p u r i t y n i t r o g e n (1.8 dm3h1) . To g a i n more

i n f o r m a t i o n about t h e d e c o m p o s i t i o n , some samples were s t u d i e d by s i m u l t a n e o u s TG/DTA/EGA. To t h i s end a TG/DTA a p p a r a t u s from S a r t o r i u s - W e r k e GmbH and a T o p a t r o n B a p p a r a t u s from L e y b o l d - H e r a e u s were used. Sample w e i g h t and h e a t i n g r a t e were t h e same as i n t h e TG e x p e r i m e n t s .

IR s p e c t r a were r e c o r d e d by a Beckman s p e c t r o p h o t o m e t e r IR 4210. About 2 mg sample was powdered and mixed w i t h 300 mg KBr and p r e s s e d i n t o a d i s k .

I I . 3 CHEMICAL COMPOSITION AS A FUNCTION OF PREPARATION CONDITIONS

II.3.1 Samples prepared at increasing pH

In t h i s p r e p a r a t i o n method, sodium c a r b o n a t e i s added t o a s o l u t i o n o f n i c k e l and a l u m i n i u m n i t r a t e s , i n most c a s e s up t o a pH v a l u e o f about s e v e n .

In o r d e r t o g a i n more i n s i g h t i n t o t h e p r e c i p i t a t i o n p r o c e s s , t h e pH has been measured as a f u n c t i o n o f t h e amount o f sodium c a r b o n a t e added, f o r two s o l u t i o n s o f mole f r a c t i o n x.T = 0.75 and 0.50 r e s p e c t i v e l y . The r e s u l t i n g

N i

c u r v e s A and B r e s p e c t i v e l y , a r e shown i n F i g . I I . 2 . I n i t i a l l y t h e pH i n c r e a s e s o n l y s l o w l y , up t o a v a l u e o f about 2.5. Upon f u r t h e r a d d i t i o n o f c a r b o n a t e a s h o r t , r a t h e r s t e e p i n c r e a s e i s o b s e r v e d , l e a d i n g t o a s e c o n d l e v e l . In c u r v e A t h i s p l a t f o r m , w h i c h seems t o be d i v i d e d i n two l e v e l s , e n d s a t a pH v a l u e o f about 5, and then the pH i n c r e a s e s f u r t h e r up to a v a l u e o f approximately 7.5, whereas c u r v e B e x h i b i t s a more g r a d u a l t r a n s i t i o n between t h e n e a r l y h o r i

-z o n t a l p a r t and t h e s t e e p e r p a r t .

The l e v e l s p r e s e n t i n t h e g r a p h s must c o r r e s p o n d t o s t a g e s o f t h e p r o c e s s (22)

when OH i o n s a r e consumed. Vermeulen e t a l . , i n a s t u d y o f t h e homogeneous p r e c i p i t a t i o n o f A l ( 0 H ) g , a l s o o b s e r v e d a l e v e l i n t h e pH v e r s u s t i m e c u r v e s l i g h t l y above a pH v a l u e o f two, f o r measurements a t 90°C. A c c o r d i n g t o t h e s e a u t h o r s , t h e h y d r o x y l i o n s a r e bound t o t h e a l u m i n i u m , w i t h o u t a p r e c i p i t a t e o r even a c o l l o i d a l s o l u t i o n b e i n g formed d u r i n g t h i s s t a g e o f t h e a d d i t i o n . In t h e e x p e r i m e n t a l d e s i g n u s e d by us f o r t h e measurements g i v e n i n F i g . I I . 2 , u n f o r t u n a t e l y v i s u a l o b s e r v a t i o n s were d i f f i c u l t t o make. However, i n t h e p r e p a r a t i o n o f a l a r g e r b a t c h o f p r e c i p i t a t e , w h i c h y i e l d e d a s i m i l a r t i t r a t i o n c u r v e , w h i t e f l o c c u l a t e s were a l r e a d y o b s e r v e d a t a pH o f 1.5. The w h i t e s o l i d t u r n e d out t o be X - r a y amorphous, but p r o b a b l y c o n s i s t s o f A l ( 0 H ) g . A l s o , i n many s i m i l a r e x p e r i m e n t s w h i t e f l o c c u l a t e s were o b s e r v e d a t a pH v a l u e o f about

1.5-2. The o b s e r v a t i o n i n o u r e x p e r i m e n t s o f a p r e c i p i t a t e a t s u c h a low pH (22) v a l u e , whereas t h i s was n o t f o u n d by V e r m e u l e n , c a n be e x p l a i n e d by t h e f a c t t h a t we d i d n o t p r e c i p i t a t e homogeneously. As a c o n s e q u e n c e i n o u r c a s e l o c a l s u p e r s a t u r a t i o n s c a n o c c u r , when a d r o p o f b a s e e n t e r s t h e s o l u t i o n . T h i s can g i v e r i s e t o t h e f o r m a t i o n o f a p r e c i p i t a t e , w h i c h i s v e r y d i f f i c u l t t o r e -d i s s o l v e .

Fig. II.2. pH as a function of the amount of Ha^CO^ added. Temperature 95°C. Curve A: i n i t i a l solution 0.62 M Ni(NO^) 0.21 M Al(N02)3. Curve B: i n i t i a l solution 0.41 M NiiNO^)gJ

0.41 M AI(N0J„. Added solution: 2.0 M Na0CO,.

In t h e e x p e r i m e n t m e n t i o n e d above i t was a l s o o b s e r v e d t h a t t h e s e c o n d p l a t f o r m c o r r e s p o n d s t o t h e f o r m a t i o n o f c o n s i d e r a b l e amounts o f p r e c i p i t a t e , s t a r t i n g a t a pH o f about 3.6.X-ray p h o t o g r a p h s o f samples t a k e n a t pH = 3.6 and pH = 4.6 show a l r e a d y t h e c h a r a c t e r i s t i c p a t t e r n o f t h e n i c k e l a l u m i n i u m

hydroxycompounds. The n a t u r e o f t h e p h a s e s formed i n t h i s pH r e g i o n has been i n v e s t i g a t e d f u r t h e r by p r e p a r i n g some samples a t a c o n s t a n t pH v a l u e o f 5, t o be d i s c u s s e d i n t h e n e x t s e c t i o n . From t h e r e s u l t s i t w i l l be shown t h a t a t s u c h low pH v a l u e s h y d r o x y n i t r a t e s a r e formed. The a n i o n c o m p o s i t i o n o f t h e p r o d u c t s w h i c h r e s u l t e d from t h e t i t r a t i o n s i s g i v e n i n T a b l e I I . 1 . Sample A_~ c o r r e s p o n d s t o t i t r a t i o n c u r v e A o f F i g . I I . 2 , and sample A t o c u r v e B. A p p a r e n t l y most o f t h e n i t r a t e has been ex-changed f o r c a r b o n a t e d u r i n g t h e l a s t s t a g e o f t h e a d d i t i o n .

Table II.1. Carbonate and nitrate contents of some samples prepared at

in-creasing pH.

Sample X . i n N l T e m p e r a t u r e pH at t h e end C° 3 N 03 s o l u t i o n (°C) o f a d d i t i o n (wt %) (wt %) A2 6 0.75 95 7.4 6.5 2 8 A2 7 0.50 9 5 6.9 5.7 3 7 A4 0.72 95 ~7 n o t 2 6 measured B3 0.37 9 5 = 5

"

16 4 F i n a l l y a l s o t h e r e s u l t s a r e g i v e n f o r samples A^ and B^, p r e p a r e d a t t h e U n i v e r s i t y o f B r a d f o r d (U.K.) by S. O r r , f o l l o w i n g t h e same method. Remarkable i s t h e v e r y h i g h n i t r a t e c o n t e n t o f t h e sample p r e p a r e d up t o a pH o f about 5.

II.3.2 Samples prepared at constant pH

I I . 3 . 2 . 1 H y d r o x y c a r b o n a t e s and - n i t r a t e s

We p r e s e n t i n t h i s s e c t i o n t h e r e s u l t s o f c h e m i c a l a n a l y s e s , w h i c h e n a b l e us t o d e f i n e c l e a r l y t h e c o n d i t i o n s u n d e r w h i c h h y d r o x y c a r b o n a t e s , h y d r o x y -n i t r a t e s o r compou-nds o f i -n t e r m e d i a t e c o m p o s i t i o -n a r e formed.

T a b l e I I . 2 g i v e s t h e a n a l y t i c a l r e s u l t s f o r most o f t h e samples u s e d i n t h e s e i n v e s t i g a t i o n s . Not aj.1 t h e p r e p a r a t i o n s have been a n a l y s e d , and f o r example some X - r a y r e s u l t s w i l l be d i s c u s s e d l a t e r on f o r samples w h i c h a r e not i n c l u d e d i n t h e T a b l e . In T a b l e I I . 2 samples w i t h p r e f i x A a r e t h o s e shown t o i n c l u d e p r e d o m i n a n t l y c a r b o n a t e a n i o n s and t h o s e w i t h p r e f i x B i n c l u d e p r e -d o m i n a n t l y n i t r a t e i o n s . T h r e e o f t h e s e , samples B ^ , Bl ln and Bi 2n> c o n t a i n

Table II.2. Chemical analytical data of samples prepared at constant pH.

Sample XN i i n pH o f p r e - P r e c i p i t a t i n g C h e m i c a l a n a l y s i s (wt %) T o t a l w e i g h t l o s s i n i t i a l c i p i t a t i o n a g e n t on t h e r m a l de-s o l u t i o n C03 N 03 N i A l N i / A l XN i Na c o m p o s i t i o n A1 3 0 75 10 N a O H / N a2C 03 ( a ) 7 1 n.d.<b> 38.8 6 7 2.66 0.73 0 37 36.2 A1 8h 0 75 10 N a O H / N a2C 03 ( a )7 5 0 85 41.2 7 0 2.70 0.73 0 17 33.7 A3 0 75 7 N a2C 03 7 3 2 3 40. 9 6 0 3.13 0.76 0 07 _n.d. A1 9h 0 67 10 N a 0 H / N a „ C 0 _( a ) 7 9 <0 2 37.0 8 6 1.98 0.66 n d. 33.2 A5 0 50 10 N a 0 H / N a2C 03 ( a ) 8 8 0 35 n.d. n d. 0 07 40. 2 A6 0 50 7 N a2C 03 8 6 0 88 n.d. n d. 0 05 n.d. A6h 0 50 7 N a2C 03 8 4 <0 02 32. 5 14 2 1.05 0.51 0 13 _n.d. A2 3 0 33 7 N a2C 03 6 1 1 9 19.1 17 4 0. 51 0.34 <0 01 _n.d. A7 0 25 10 N a 0 H / N a _ C 0 _( a )5 2 0 0 17.5 21 6 0.37 0.27 <0 05 _n.d. Bi o 0 75 10 NaOH 0 76 10 2 n.d. n d. n d. n.d. Bl 0 75 6 NaOH 1 5 13 8 n.d. n d . 0 01 n.d. V 0 75 6 NaOH 0 85 14 5 39.6 6 4 2 .87 0.74 0 09 36.1 B1 2h 0 67 6 N a O H / N a2C 03 ( a ) 3 8 10 7 n.d. n d. n d. _n.d. Bl l 0 67 6 N a2C 03 5 0 7 4 n.d. n d. n d. _n.d. Bl lh 0 67 6 N a2C 03 4 7 7 9 37.9 8 1 2.15 0.68 n d. _n.d. V 0 67 5 N a 0 H / N a2C 03 ( a ) 0 24 18 3 34. 2 8 4 1.88 0.65 n d. 40.6 B2 0 50 7 NaOH 1 4 13 8 n.d. n d. 0 02 n.d. B5 0 50 5 N a2C 03 1 3 15 9 n.d. n d. 0 01 n.d.

( a )T h e q u a n t i t y o f Na C0_ was c h o s e n i n such a way t h a t the r a t i o _{n.d. — not d e t e r m i n e d .}

a l s o a s u b s t a n t i a l amount o f c a r b o n a t e i o n s and r e p r e s e n t s o l i d s o l u t i o n s between h y d r o x y c a r b o n a t e s and - n i t r a t e s . The l e t t e r h i n t h e sample number i n d i c a t e s t h a t t h e sample was h y d r o t h e r m a l l y t r e a t e d . T a b l e I I . 2 g i v e s t h e mole f r a c t i o n o f n i c k e l x i n t h e n i t r a t e s o l u t i o n from w h i c h t h e p r e c i p i t a t i o n

N i

was c a r r i e d o u t , t h e pH o f p r e c i p i t a t i o n and t h e p r e c i p i t a t i n g agent u s e d . In a d d i t i o n t o t h e a n i o n a n a l y s e s , t h e N i and A l c o n t e n t s a r e g i v e n , from w h i c h a r e c a l c u l a t e d t h e a c t u a l v a l u e s o f x^. a c h i e v e d ; t h e c o r r e s p o n d i n g N i / A l r a t i o s a r e a l s o g i v e n f o r t h e p u r p o s e o f d i s c u s s i o n o f t h e c h e m i c a l f o r m u l a e g i v e n below. I t a p p e a r s t h a t i n c o m p e t i t i o n w i t h n i t r a t e i o n s c a r b o n a t e i o n s a r e p r e f e r e n t i a l l y i n c o r p o r a t e d . F o r example, f o r t h e p r e p a r a t i o n s a t pH = 10, t h e t o t a l amount o f n i t r a t e added t o t h e r e a c t i o n v e s s e l i s 16-18 t i m e s t h a t o f c a r b o n a t e , b u t m a i n l y c a r b o n a t e i s f o u n d i n t h e r e s u l t i n g p r e c i p i t a t e . G e n e r a l l y , as l o n g as c a r b o n a t e i s p r e s e n t i n t h e p r e c i p i t a n t and a t pH v a l u e s > 7, h y d r o x y c a r b o n a t e s a r e formed. I t f o l l o w s from T a b l e 11.3 i n w h i c h some d a t a o f T a b l e 11.2 a r e g r o u p e d t o g e t h e r , t h a t , when sodium c a r b o n a t e i s used as p r e c i p i t a n t , t h e c a r b o n a t e c o n t e n t o f t h e p r e c i p i t a t e d e c r e a s e s s t r o n g l y when t h e pH o f p r e c i p i t a t i o n i s l o w e r e d t o v a l u e s below s e v e n .

Table II. 3. Anion composition of some samples prepared at constant pH.

S amp1e x „ . i n i n i t i a l N i pH o f p r e - p r e c i p i t a t i n g C° 3 N 03 s o l u t i o n c i p i t a t i o n agent (wt %) (wt %) A6 0. 50 7 N a2C° 3 8.6 0.88 Bl l 0.66 6 N a2C° 3 5.0 7.4 B5 0. 50 5 Na2C03 1.26 15.9 A1 9h 0.66 10 N a 0 H / N a2C 03 7.9 <0.2 B1 2h 0.66 6 N a 0 H / N a2C 03 3.8 10.7 B9h 0.66 5 NaOH/Na2C03 0.24 18.3

Compare f o r example samples Ag, and B&, p r e c i p i t a t e d a t pH = 7, 6 and 5

r e s p e c t i v e l y . The f i r s t i s e s s e n t i a l l y a h y d r o x y c a r b o n a t e , t h e second has an i n t e r m e d i a t e c o m p o s i t i o n whereas t h e l a t t e r i s a h y d r o x y n i t r a t e . The f a c t t h a t upon p r e c i p i t a t i o n a t pH = 5 u s i n g N a2C 03 a h y d r o x y n i t r a t e i s formed, has added

i n t h e i n t e r p r e t a t i o n o f t h e t i t r a t i o n c u r v e s d e s c r i b e d i n t h e p r e c e d i n g

s e c t i o n (p. 1 4 ) . The e f f e c t o f pH c a n a l s o be seen from a c o m p a r i s o n o f samples

Ai r ,h> Bi .-.h and B„h, w h i c h have a l l t h r e e an x „ . o f 0.66, and were p r e c i p i t a t e d

19 12 9 N i by a m i x t u r e o f NaOH and N a2C 03 o f t h e same c o m p o s i t i o n . A g a i n , a t pH = 5 a

v e r y p u r e h y d r o x y n i t r a t e i s o b t a i n e d .

The e x p l a n a t i o n f o r t h e e f f e c t o f pH f o l l o w s from t h e c a r b o n a t e e q u i l i b r i a :

H2C 03 t H+ + HC0~ [1]

HCO^

t

H+ + C 03 [2]

which have v a l u e s o f pK^ = 6.4 and p K2 = 10.3. As a c o n s e q u e n c e t h e c a r b o n a t e

i o n c o n c e n t r a t i o n i n s o l u t i o n i s d r a s t i c a l l y l o w e r e d a t l o w e r pH v a l u e s . Some more h y d r o x y n i t r a t e s were p r e p a r e d i n a d d i t i o n t o t h o s e a l r e a d y m e n t i o n e d above, u s i n g o n l y NaOH as p r e c i p i t a n t . From our d a t a p r e s e n t e d i n

T a b l e I I . 2 f o r samples , B^h and B2 i t i s seen t h a t , when t h e pH i s kept

c o n s t a n t at a v a l u e j£ 7, r e a s o n a b l y p u r e h y d r o x y n i t r a t e s can be o b t a i n e d w i t h o u t t a k i n g s p e c i a l p r e c a u t i o n s t o a v o i d a t m o s p h e r i c CO . T h i s i s

(18)

i n t e r e s t i n g as i t i s known ( s e e f o r example B i s h ) t h a t h y d r o x y n i t r a t e s a r e d i f f i c u l t t o p r e p a r e i n a r e a s o n a b l y p u r e form.

F i n a l l y , a l s o an attempt was made t o p r e p a r e a c a r b o n a t e - f r e e sample a t pH = 10, i n o r d e r t o i n v e s t i g a t e whether a t h i g h e r pH i n t h e a b s e n c e o f

c a r b o n a t e i o n s s t i l l m a i n l y n i t r a t e i s i n c o r p o r a t e d , o r p o s s i b l y a l s o h y d r o x y l i o n s can be p r e s e n t i n t h e i n t e r l a y e r . To t h i s end, d e c a r b o n a t e d s o l u t i o n s were used and, b e c a u s e i t i s t o be e x p e c t e d t h a t a t h i g h e r pH more C 02 w i l l be t a k e n

up from t h e atmosphere, t h e p r e c i p i t a t i o n was c a r r i e d out u n d e r a n i t r o g e n atmosphere as d e s c r i b e d i n t h e e x p e r i m e n t a l s e c t i o n . The r e s u l t s f o r t h e sample p r e p a r e d i n t h i s way, sample B , show t h a t i n d e e d o n l y a r e l a t i v e l y s m a l l amount o f c a r b o n a t e i s i n c o r p o r a t e d . However, i t i s a l s o seen t h a t t h e n i t r a t e c o n t e n t , and t h e sum o f t h e n i t r a t e c o n t e n t p l u s t w i c e t h e c a r b o n a t e c o n t e n t , i s r a t h e r low as compared t o t h a t o f sample B., and h w h i c h have t h e same x

1 1 N i T h i s s u g g e s t s t h a t a t pH = 10 p a r t o f t h e n i t r a t e i o n s can be r e p l a c e d by h y d r o x y l i o n s . From t h e a n a l y t i c a l d a t a i t i s f o u n d t h a t t h e h y d r o x y c a r b o n a t e samples A h 18 and A h c o r r e s p o n d a p p r o x i m a t e l y t o t h e f o r m u l a e N i „ A l „ ( 0 H )1- C 0 „ . 5 H . 0 and l y b

A

l o 3 2

N i A l (OH) _C0„ .3Ho0 r e s p e c t i v e l y . The h y d r o x y n i t r a t e s B., h and B h

^ A LZ.D J Q Z 1 9

c o r r e s p o n d t o N igA i2( O H )l g( N 03)2. 3 H20 and N i4A l2( O H ) ( N 03)2. 3 H20 r e s p e c t i v e l y .

I t i s not u s e f u l t o g i v e t h e c o m p o s i t i o n f o r t h e h y d r o t h e r m a l l y t r e a t e d sample o f N i / A l r a t i o o f one, b e c a u s e t h i s sample c o n t a i n s a s e p a r a t e a l u m i n a phase, as w i l l be d i s c u s s e d l a t e r .