Iron Sulfide Catalysis in Coal Liquefaction - American Chemical Society

29 Iron Sulfide Catalysis in Coal Liquefaction P. A. Montano, Y. C. Lee, A. Yeye-Odu, and C. H . Chien

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 1, 2016 | http://pubs.acs.org Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch029

Department of Physics, West Virginia University, Morgantown, WV 26506-6023

A diverse number of techniques has been employed to characterize the iron sulfides under coal liquefaction conditions. It is observed that the stoichiometry of the iron sulfides is determined by the H S partial pressure and that greater activity is observed when the surface is rich in vacancies. A l l the iron sulfides show great affinity toward oxygen-containing compounds. This activity was demonstrated both with low rank coals as well as model compounds. It is proposed that pyrrhotites can act as catalysts for the cleavage of oxygen bonds in coal. 2

The d i r e c t l i q u e f a c t i o n of c o a l i s a p r o c e s s t h a t i n v o l v e s the i n t e r a c t i o n between c o a l , hydrogen, s o l v e n t , and c a t a l y s t s . M i n e r a l matter has been known t o enhance the c o n v e r s i o n of c o a l t o l i q u i d p r o d u c t s ( 1 , 2 , 3 ) . A d d i t i o n of p y r i t e , p y r r h o t i t e , and l i q u e f a c t i o n r e s i d u e s W t o c o a l has been shown t o a f f e c t the c o a l c o n v e r s i o n y i e l d s and the v i s c o s i t y of the p r o d u c t s ( 5 ) . Of a l l the m i n e r a l s p r e s e n t i n c o a l , p y r i t e (and m a r c a s i t e ) are the most important f o r coal u t i l i z a t i o n , e s p e c i a l l y i n d i r e c t coal l i q u e f a c t i o n (1,5). However, one has t o remember t h a t under c o a l l i q u e f a c t i o n conditions p y r i t e r a p i d l y transforms to a nonstoichiometric i r o n s u l f i d e Fe S(0 and Fe«Sg a f t e r r e a c t i o n w i t h CO. I t has been observed t h a t CO undergoes a d i s p r o p o r t i o n a t i o n r e a c t i o n on F e , 2C0 + C + C0p, w i t h the f o r m a t i o n o f s u r f a c e o x i d e due t o the d i s s o c i a t i o n o f CO (18.). The s u r f a c e o x i d e i s r a p i d l y removed by H . The s u r f a c e s o f the s u l f i d e s do not show any e v i d e n c e i n t h e Auger and EEL s p e c t r a o f Q

p

7

f t

2

Vorres; Mineral Matter and Ash in Coal ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

Iron Sulfide

Catalysis in Coal

Liquefaction


MONTANO ET AL.


420

MINERAL MATTER AND ASH IN COAL

reactions with NH , CHn, and C H . However, they interact strongly with molecular oxygen forming a surface oxide. Figure 3 shows the surface oxide formed on pyrite after interaction with oxygen; for comparison purposes the EEL spectrum for a-Fe^O^ i s also shown. The present measurements indicate a high r e a c t i v i t y of the s u l f i d e surfaces toward oxygen-containing compounds and very l i t t l e toward l i g h t hydrocarbons and ammonia.


3

2

4

CEMS Measurements of the Surface Interaction of^Naphthoquinone with Iron and Iron S u l f i d e Surfaces. A high purity Fe f o i l was used for these measurements; such a f o i l was necessary in order to record r a p i d l y a Mossbauer spectrum (less than one-half hour). The sample was placed in the holder inside the reactor and H was flown for 2 hours at 350 C to reduce the surface and clean o f f any residual contamination. Figure 4a shows the Mossbauer spectrum at room temperature inside the reactor after cleaning. We studied the hydrogénation of naphthoquinone by introducing about 20 mg of the compound and flowing hydrogen at about 0.5 cc/sec. The temperature of the reaction was 305 C and 405°C and the time of reaction was about one-half hour. After reaction the CEM spectrum was taken inside the reactor. No evidence of the formation of any known oxide was detected (Figure 4b). The same experiment was repeated using a sulfided gample (produced from the Fe f o i l by flowing H^/H^sO.I at 350 C). The spectrum for such a sample i s shown in Figure 5a before reaction. After reaction with naphthoquinone we see clear evidence of the formation of Fe 0^ on the surface (Figure 5b). Magnetite i s formed at the expense of the iron s u l f i d e . This observation i s in very good agreement with our e a r l i e r iji s i t u Mossbauer work ( 1_4), where a less surface-sensitive technique was used for the measurements. We interpret these results as evidence of a greater r e a c t i v i t y of the iron s u l f i d e surfaces toward oxygencontaining organic molecules than the pure metal. I t i s noted that magnetite can be e a s i l y removed by further flow of H /H S. The magnetite layer i s formed in the f i r s t few surface layers of the iron s u l f i d e (see Figure 5). This i s suggestive of a possible c a t a l y t i c role of the iron sulfides in the cleavage of oxygen bonds. 2

2

2

Interaction of Iron Sulfides with Low Rank Coal. In s i t u measurements were performed on low rank coals, one North Dakota l i g n i t e and one Australian Victorian Morwell c o a l . These two coals are characterized by their high oxygen content (as high as 24% for the Australian c o a l ) . Since the Australian coal i s very low i n mineral matter, a small amount of F e C l was added ( 1_6). In t h i s case the amount of sulfur present i s small and the p a r t i a l pressure of oxygen i s vegv^ high during reaction. Two charge states of iron are observed, Fe and Fe . Cassidy and co-workers explain the catal y t i c a c t i v i t y of iron in t h i s coal as related to a redox mechanism. Figure 6 shows the Mossbauer spectra after reaction at 300 C and at 440 C. The spectra were taken inside the reactor and never exposed to a i r . The i n s i t u Mossbauer measurements at 300 C show a similar spectrum to that shown in Figure 6a. At 440 C the spectrum i s c h a r a c t e r i s t i c of FeJD^. A similar result i s obtained for the North Dakota l i g n i t e . Both coals are characterized for the 2


Iron Sulfide

MONTANO ET AL.

Catalysis in Coal

Liquefaction

F e S / 0


2

TO 4 0

2

•V

7.0

57.8

o-Fe2Û3

23Ό

20

60

F i g u r e 3. EEL s p e c t r a o f F e S and o f a - F e 0 ^ ( b ) .

2

after

eV

r e a c t i o n with

oxygen

(a)

2

60%

-5 57 F i g u r e M. CEM s p e c t r a o f Fe f o i l (a) and a f t e r w i t h napthoquinone ( b ) . X - a x i s i s i n mm/sec. c

reaction


422

MINERAL MATTER AND ASH IN COAL


s

I

s

s

s

JL 0

I

=5

s

I

.

5

57 F i g u r e 5. CEM s p e c t r a o f (a) s u l f i d e d Fe f o i l , s: i n d i c a t e s the s u l f i d e s u r f a c e ; (b) A f t e r r e a c t i o n w i t h naphthoquinone m: m a g n e t i c , bottom spectrum i s t h e s u r f a c e l a y e r s a f t e r r e a c t i o n . X - a x i s i s mm/sec. 100.0 ,

Fe

cn cn t=i

12 +

330 C

97.0 E-

Iron Sulfide Catalysis in Coal Liquefaction - American Chemical Society

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