New Approaches in Coal Chemistry - American Chemical Society

11 Coal Mineral Matter Catalysis in Liquefaction B R A D L E Y C. B O C K R A T H and K A R L T. SCHROEDER

Downloaded by EAST CAROLINA UNIV on November 3, 2016 | http://pubs.acs.org Publication Date: October 26, 1981 | doi: 10.1021/bk-1981-0169.ch011

U.S. Department of Energy, Pittsburgh Energy Technology Center, P.O. Box 10940, Pittsburgh, PA 15236

It is generally appreciated that the mineral matter associated with some coals may act as a catalyst for liquefaction. A common observation is that among bituminous coals from the eastern United States, those with a relatively high mineral matter content also provide relatively high liquefaction yields. Also, addition of coal-derived mineral matter increases the liquefaction yields from those coals with low mineral matter content. The involvement of pyrite in these effects has been fairly well established. The importance of clay and/or other minerals is less well defined. The catalytic properties of coal-derived mineral matter are of fundamental importance to the practical operation of coal liquefaction processes. However, l i t t l e is known about the basic chemical reactions by which the mineral matter acts or how these reactions are related to the liquefaction of coal. Such knowledge might assist us in reaching the full potential in the use of these catalysts. Especially attractive goals include increasing the activity of the catalyst, manipulating its selectivity to provide maximum distillate yields at minimum hydrogen consumption, and determining the optimum operating conditions under which these catalysts may be employed. Inferences may be drawn from r e p o r t s on l i q u e f a c t i o n s t u d i e s regarding the types of chemical r e a c t i o n s that might be c a t a l y z e d by m i n e r a l matter. For example, h y d r o g é n a t i o n a c t i v i t y has been i n d i c a t e d by an increased r a t e of hydrogen uptake by creosote o i l i n the presence of p y r i t e (I) and the a c c e l e r a t e d disappearance of major aromatic compounds such as naphthalene ( 2 ) . The v i s c o s i t y of l i q u i d products from c o a l has been observed to decrease with a d d i t i o n of p y r i t e 0,4), which i m p l i e s removal of p o l a r f u n c t i o n a l groups and7or r e d u c t i o n i n molecular weight. A f t e r h y d r o g é n a t i o n with p y r i t e p r e s e n t , the s u l f u r content of creosote o i l was found to be somewhat lower than when hydrogenated without p y r i t e (I). Conf i r m a t i o n that p y r i t e i s indeed connected with catalytic a c t i v i t y has been made by s t u d i e s with model compounds. Hydro-

This chapter not subject to U.S. copyright. Published 1981 American Chemical Society

Blaustein et al.; New Approaches in Coal Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


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d e s u l f u r i z a t i o n of thiophene (5) and benzothiophene (6) has been reported, but the p y r i t e c a t a l y s t s were found to be much l e s s a c t i v e than commercial CoMo c a t a l y s t s . In one case ( 6 ) , p y r i t e was found to promote hydrogénation of benzothiophene but was i n e f f e c t i v e at a c t u a l e x t r a c t i o n of s u l f u r from the hydrogenated product, dihydrobenzοthiophene. The low temperature ash of mineral matter from Western Kentucky c o a l was reported to isomerize 1-butene to trans-2-butene ( 5 ) . P y r i t e s from mineral deposits have been shown to catalyze the dehydrogenation of t e t r a l i n * (7). Residues from coal l i q u e f a c t i o n processes and c r y s t a l l i n e p y r i t e mineral were shown to c a t a l y z e the conversion of isopropanol to acetone and propene ( 8 ) . Although mineral matter may provide a c a t a l y t i c surface f o r v a r i o u s r e a c t i o n s d u r i n g the l i q u e f a c t i o n o f c o a l , i t i s a l s o p o s s i b l e that a l a r g e number of f r e e r a d i c a l r e a c t i o n s are i n i t i a t e d by thermolysis of the organic components i n c o a l . Any study of c a t a l y t i c a c t i v i t y must separate e f f e c t s caused by the former from those caused by the l a t t e r . A s i z e a b l e p o r t i o n of the work d e s c r i b e d below i s devoted to e s t a b l i s h i n g that separation. Experimental Materials. T e t r a l i n , purchased commercially, was passed over a c t i v a t e d alumina and stored under argon b e f o r e use. In order to c o r r e c t l y i d e n t i f y the isomer of methylindan found i n the product mixtures, a u t h e n t i c 1- and 2-methyl ind an s were p r e pared from the corresponding indanones by r e a c t i o n with methylmagnesium i o d i d e , dehydration, and subsequent hydrogénation over Pd on asbestos. M i n e r a l matter was a D e i s t e r t a b l e concentrate from Robena mine c o a l . I t contained 68% p y r i t e and l e s s than 4% organic m a t e r i a l . The remainder was l a r g e l y c l a y . In one case, a handpicked sample taken from a p y r i t e nodule found i n a P i t t s b u r g h seam c o a l was used. The m i c r o c r y s t a l s were crushed and sieved to 325 χ 400 mesh. X-ray d i f f r a c t i o n a n a l y s i s i n d i c a t e d the only major component was p y r i t e , with a trace of marcasite a l s o present. A f t e r h e a t i n g i n t e t r a l i n at 450°C f o r 15 min., the Xray diffraction p a t t e r n s of the recovered m i c r o c r y s t a l s i n d i c a t e d conversion was complete to p y r r h o t i t e 1C. The c o a l was hvB, Homestead Mine, Kentucky, ground to pass 200 mesh. Ash and p y r i t e contents were 16.8% and 4.9%, r e s p e c t i v e l y . The asphaltene was a homogenized mixture of samples i s o l a t e d from l i q u i d products d e r i v e d from P i t t s b u r g h seam, hvA c o a l . Its

ash content was 0.98. Most of the analyses were done with 10 χ 1/8" stainless steel columns packed with Carbowax 20M on Chromosorb-WAW. Some were done with a 50M fused silica capillary column coated with SP-2100. A HewlettPackard 5840* gas chromatograph equipped with autosampler was used to provide replicate analyses. Relatively short times for f u l l chromatographic analyses were achieved with the capillary column. When i t was used in conjunction with the autosampler and computerized data reduction, analysis of each unknown and standard solution could be replicated at least 3 and usually 46 times within a reasonable length of time. Use of many standard solutions and many replicate analyses allows a fairly high degree of precision to be obtained. We estimate typical precision of the determination of an unknown to be jf 2% of its value.


0

1

Results and Discussion The reactions of tetralin in the presence of coal were investigated to determine the extent of conversion along various pathways in the absence of further added catalysts. As may be seen from Figure 1, the yield of products generated under typical liquefaction conditions (450°C, 30 minutes) increases with the amount of coal added. Three products arise from tetralin: naphthalene, n-butylbenzene, and 1-methylindan. Positive identification of the latter isomer was made by comparison of gas chromatographic retention times with those of authentic samples of 1and 2-methylindan prepared by independent synthesis.



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The d e c a l i n s recovered i n the product were present i n the t e t r a l i n before r e a c t i o n as minor i m p u r i t i e s . The c i s - and trans-isomers were f u l l y r e s o l v e d i n our chromatograms and p o s i t i v e l y i d e n t i f i e d by s p i k i n g experiments with authentic samples. M a t e r i a l balance c a l c u l a t i o n s i n d i c a t e that the t o t a l amount of d e c a l i n s was e s s e n t i a l l y unchanged over the course of the r e a c t i o n . Hooper e t a l . (10) have a l s o reported that the amount of d e c a l i n i n t e t r a l i n d i d not change when i t was heated to 450°C with c o a l . However, we do observe that the r a t i o of the t r a n s - to c i s - i s o m e r i s g r e a t l y changed and seems to approach an e q u i l i b r i u m v a l u e a f t e r r e a c t i o n at the highest coal concentration. A l l of these r e a c t i o n s presumably a r i s e through f r e e r a d i c a l mechanisms. The a b s t r a c t i o n of hydrogen from t e t r a l i n by c o a l to produce naphthalene i s of course expected. What sets t h i s r e a c t i o n apart from the r e s t i s the l i n e a r dependence of naphthalene y i e l d on c o a l c o n c e n t r a t i o n . From the slope of the y i e l d curve, we c a l c u l a t e that hydrogen was removed from t e t r a l i n i n the amount of 2.5 wt. percent of the added c o a l . This i s a reasonable amount o f hydrogen to be t r a n s f e r r e d to c o a l under l i q u e f a c t i o n c o n d i t i o n s . However, we note a recent r e p o r t on the decomposition of 1,2-diphenylethane i n t e t r a l i n i n which Benjamin s t a t e s that over twice the amount of naphthalene r e q u i r e d i n the formation of toluene was produced (11). Presumably, the excess appears as molecular hydrogen. In the present case, we cannot r u l e out the p o s s i b i l i t y that some f r a c t i o n of the naphthalene was produced by a similiar mechanism. The rearrangement of t e t r a l i n to 1-methylindan i s the most prominent r e a c t i o n at c o a l c o n c e n t r a t i o n s l e s s than about 14 wt. percent c o a l . As may be seen by the i n t e r c e p t , a s i g n i f i cant amount of rearrangement took p l a c e even i n the absence of coal. There i s now good evidence that rearrangement a r i s e s p r i m a r i l y and perhaps e x c l u s i v e l y from the 2 - t e t r a l y l r a d i c a l (12,13). I t has been demonstrated that t h i s rearrangement i s to some extent r e v e r s i b l e (12) and that degradation of 1-methylindan to lower molecular weight products a l s o occurs. These r e a c t i o n s may account f o r the n o n - l i n e a r form of the y i e l d curve i n F i g u r e 1. In c o n t r a s t , i t has been shown (12,15) that under l i q u e f a c t i o n c o n d i t i o n s s i m i l i a r to those used here, t e t r a l i n i s not formed from naphthalene, the only one of these products with a l i n e a r y i e l d curve. The y i e l d of n-butylbenzene a l s o increases with c o a l conc e n t r a t i o n , although the r e l a t i v e amounts are much smaller than the y i e l d s of e i t h e r naphthalene or 1-methylindan. The r e l a t i v e amount of the c r a c k i n g of a l i p h a t i c groups from t e t r a l i n and 1-methylindan has been observed to increase i n the presence of v i t r i n i t e (12). I t has been suggested that the opening of the a l i p h a t i c r i n g of t e t r a l i n may be accounted f o r by the i n t e r v e n t i o n of hydrogen atoms (10,16). Although the * D e c a l i n i s a r e g i s t e r e d trademark of E.I.DuPont

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B O C K R A T H A N D SCHROEDER

Coal

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autoclaves were not pressured with hydrogen gas in our experiments, hydrogen atoms might still a r i s e from the i n t e r a c t i o n of f r e e r a d i c a l s with minor amounts of hydrogen gas generated during the course of the r e a c t i o n s or by β-scission of hydrogen atoms from s u i t a b l e r a d i c a l s . Vernon (17) has demonstrated that such a mechanism can account f o r the c r a c k i n g of otherwise thermally s t a b l e compounds. The i s o m e r i z a t i o n of the d e c a l i n s i s most e a s i l y explained as a f r e e r a d i c a l r e a c t i o n i n i t i a t e d by hydrogen a b s t r a c t i o n at one of the t e r t i a r y carbons. I n v e r s i o n at the f r e e r a d i c a l center followed by hydrogen a b s t r a c t i o n would y i e l d the other isomer of d e c a l i n . S u f f i c i e n t r e p e t i t i o n of the process would b r i n g about e q u i l i b r i u m between the c i s and trans forms. Figures 2 and 3 c o n t a i n y i e l d curves f o r naphthalene and 1-methylindan as a f u n c t i o n of r e a c t i o n time f o r t e t r a l i n and t e t r a l i n plus c o a l , p y r i t e , or asphaltene. The asphaltene was a homogenized mixture of s e v e r a l samples i s o l a t e d from c o a l l i q u e f a c t i o n products during other work i n our laboratory ( 9 ) . This asphaltene sample contained e s s e n t i a l l y a n e g l i g i b l e ash content (

New Approaches in Coal Chemistry - American Chemical Society

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