Rare Earths in Noncracking Catalysts - American Chemical Society

7 Rare Earths in Noncracking Catalysts

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 29, 2016 | http://pubs.acs.org Publication Date: September 3, 1981 | doi: 10.1021/bk-1981-0164.ch007

A L A N W. PETERS and GWAN KIM W. R. Grace & Company, Davison Chemical Division, Columbia, M D 21044

Since 1962 r a r e earths have been used t o stabilize z e o l i t e c r a c k i n g c a t a l y s t s f o r the petroleum i n d u s t r y (1, 2 ) . Until recently t h i s a p p l i c a t i o n t o catalysis has been the only commercially significant one. C u r r e n t l y , however, a number o f new a p p l i c a t i o n s of potential commercial s i g n i f i c a n c e are appearing. One of the most important of these is the use o f cerium in c a t a l y s t s f o r automobile exhaust emission c o n t r o l . We will emphasize t h i s application in our review without n e g l e c t i n g other a p p l i c a t i o n s . The r a r e e a r t h oxides have a number of distinguishing properties important in catalytic a p p l i c a t i o n s . The oxides are b a s i c (3) compared t o alumina, lanthanum oxide (La 0 ) being the most b a s i c . The oxides a l s o have good thermal stability, a valuable characteristic in most industrial a p p l i c a t i o n s . Some r a r e earths i n c l u d i n g cerium, praseodymium, and terbium form non-stoichiometric oxides ( 4 ) , an important property shared by many good o x i d a tion c a t a l y s t s . These mixed valence s t a t e compounds are typically polymorphic. Cost and abundance are important p r o p e r t i e s t o be considered f o r any commercial a p p l i c a t i o n . Table I l i s t s recent cost and abundance data o f i n d i v i d u a l r a r e earths d e r i v e d from major ores. The expensive oxides are the l e a s t abundant. Of the c a t a l y t i c a l l y i n t e r e s t i n g r a r e earths forming n o n - s t o i c h i o m e t r i c o x i d e s , cerium i s by f a r the most abundant and l e a s t expensive. Important p o t e n t i a l c a t a l y t i c a p p l i c a t i o n s i n c l u d e : • Ammonia Synthesis • Hydrogenation/Dehydrogenation • Polymerization • Isomerization • Oxidation • Auto Exhaust Emission C o n t r o l • Applications of Perovskites Some o f these areas have been r e c e n t l y reviewed by Rosynek (5), p o l y m e r i z a t i o n of o l e f i n s has been reviewed by Mazzei ( 6 ) , and Minachev (7) i n a recent paper summarized some experimental r e s u l t s i n the areas of i s o m e r i z a t i o n , hydrogenation, and o x i d a t i o n . We w i l l t r y not t o overlap these recent reviews. 2

3

0097-6156/81/0164-0117$05.00/ 0 © 1981 American Chemical Society

In Industrial Applications of Rare Earth Elements; Gschneidner, Karl A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

118

RARE EARTH ELEMENTS

TABLE I

Rare Earth Oxides Abundance and Cost* 3

$/lb. Pure Oxide Ce0

Nd 0 Pr 0n 2

3

6


Sni2 03

Gd 0 Eu 0 2

3

2

3

Dy 0 Ho 0 Er 0 Tm 0 Yb 0 Lu2 0 Y 0 2

2

3

2

3

2

3

2

3

2

3 3

3

Abundance i n Ores B a s t n a e s i t e Xenotime

Monazite

7.50 7.25 18.00 32.00 32.00 55.00 700.00 350.00 45.00 120.00 45.00 1000.00 85.00 2000.00 30.00

2

La2 03

5

45 20 18 5 5 2 0.1

49 32 13 4 0.5 0.3 0.1

2.1

0.1

5 0.5 2.2 0.7 2 4 0.2 1.0 8.7 2.1 5.4 0.9 6.2 0.4 61

U Residues 4 0.8 4.1 1.0 4.5 9 0.2 1.2 11.2 2.6 5.5 0.9 4.0 0.4 51

Concentrates Cerium Lanthanum REO

0.85 1.05 0.80

(a)

M i n e r a l F a c t s and Problems, 5 t h . Ed., U.S. Bureau of Mines, 1975.

(b)

M i n e r a l s Yearbook, Volume 1, 1977, U.S. Bureau of Mines, 1980.

Performance o f the Cerium Promoted Lummus C a t a l y s t

TABLE I I

Average Reactor Temperature, °F

% NH i n Product Equilibrium Conventional 3

Ce Promoted

710

39

10.4

13.5

840

22.4

12.8

17.8

910

16.8

12.7

15.6

^250°F

V330°F

AT (T Max. - T i n l e t ) Conditions: 150 atm pressure Gas h o u r l y space v e l o c i t y = 16,000 Mole R a t i o H /N 2

2

=

^2.8


PETERS AND KIM

7.

Noncracking

119

Catalysts

Ammonia Synthesis

Ertl ten:

C a t a l y t i c ammonia s y n t h e s i s has been r e c e n t l y reviewed by (8) and by Emmett (9). The c a t a l y t i c r e a c t i o n s can be w r i t -

N (g) 2

^±

N (ad) 2

N ( a d ) JZ± 2N(ad) 2

H (g)

^±

N(ad)

+


2

2H(ad) 3H(ad)

NH (ad) « ±

3

NH (g)

3

The

NH (ad)

3

step: N ( a d ) Z ± 2N(ad) 2

i s r a t e determining, although a t h i g h conversions the removal o f NH from the c a t a l y s t s u r f a c e may be important. A t y p i c a l NH s y n t h e s i s c a t a l y s t (10) contains i r o n oxide p l u s 1% K 0 , 1-2% A 1 0 , and may c o n t a i n VL% CaO on the s u r f a c e . A f t e r f u s i o n and r e d u c t i o n the s u r f a c e i s l a r g e l y m e t a l l i c i r o n p l u s reduced promoters concentrated on the s u r f a c e (8). Sze and Wang (11) have shown that a c a t a l y s t washed w i t h C e ( N 0 ) and subsequently reduced i s much more a c t i v e than the c o n v e n t i o n a l c a t a l y s t , Table I I . Mischmetal s a l t s may be s u b s t i t u t e d f o r the c e r i um s a l t . Since i n d u s t r i a l c a t a l y s t s can be very s e n s i t i v e t o p r e t r e a t ment, the source of the a c t i v i t y improvement i s u n c l e a r . F o r example, washing even i n the absence o f cerium may have some c a t a l y t i c effect. 3

3

2

2

3

3

3

Hydrogenation In an i n t e r e s t i n g s e r i e s of experiments Van Mai and co-workers (12, 13, 14) have found that i f r a r e earths a r e combined w i t h t r a n s i t i o n metals a t h i g h temperatures, the a l l o y w i l l absorb l a r g e amounts of hydrogen as hydrides under m i l d c o n d i t i o n s ; 1 atm. H and room temperature. Some examples o f these compounds and t h e i r hydrides are: 2


120

RARE EARTH ELEMENTS

Compound

Hydrides

Ce Ni

CesNiHa.^

LaNi

LaNiH .i

3

LaNi YFe

3

LaNi5He

5

YFe H^. 3

3


YFe

8

YFezfti* • 2

C a t a l y t i c a p p l i c a t i o n s of these m a t e r i a l s to hydrogenation (15), methanation (16) and ammonia s y n t h e s i s (17) have been des c r i b e d and some i n f o r m a t i o n concerning the s t r u c t u r e s of these m a t e r i a l s i s a v a i l a b l e (18). By themselves, r a r e e a r t h s are l e s s a c t i v e than the convent i o n a l nickel/molybdenum/cobalt, tungsten combinations, the Raney N i a l l o y s , or the noble metal c a t a l y s t s . Polymerization A r e c e n t l y developed c l a s s of compounds c a l l e d "Super S l u r p e r s " based on s t a r c h - p o l y a c r y l o n i t r i l e copolymers are a b l e t o absorb as much as 500 to 1000 times t h e i r weight of water, dependi n g on the p u r i t y of the water (19, 20, 21). The formation of these copolymers i s c a t a l y z e d by C e ^ ^ i o n . These polymers were developed by the U.S. Department of A g r i c u l t u r e and have p o t e n t i a l a g r i c u l t u r a l uses as water storage a d d i t i v e s as w e l l as obvious consumer and i n d u s t r i a l a p p l i c a t i o n s . Oxidation A redox mechanism i n v o l v i n g l a t t i c e oxygen o r i g i n a l l y p r o posed i n 1954 by Mars and Van K r e v e l e n (22) f o r hydrocarbon o x i d a t i o n over V2O5 can be a p p l i e d to a v a r i e t y of c a t a l y t i c o x i d a t i o n r e a c t i o n s (23). The f o l l o w i n g i l l u s t r a t e s a l a t t i c e redox mechanism f o r CO o x i d a t i o n : 1.

CO A d s o r p t i o n C0(g)

2.

CO(ad)

CO O x i d a t i o n by L a t t i c e Oxide, C a t a l y s t Reduction 2

[oj "

+

CO (ad)

Rare Earths in Noncracking Catalysts - American Chemical Society

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