9 Early Stages of Coal Carbonization: Evidence for
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Isomerization Reactions S. K. CHAKRABARTTY and N. BERKOWITZ Fuel Sciences Division, Alberta Research Council, Edmonton, Alberta, Canada
At temperatures between
350° and 425°C, the molecular struc-
tures that characterize coal are rapidly, and very obviously, transformed into more stable carbon configurations through loss of ''volatile matter" (as tar and gas). But little is known about possible configurational changes at lower temperatures. Thermograms of coal (1) indicate specific heat effects as endo- and exotherms from
200°C
up, but because enthalpy changes recorded
in this manner include sensible heats as well as heats of reaction, it is difficult to assess the nature of the chemical processes which produce the thermograms. On the other hand, low-temperature chemical changes, if such did in fact occur, should be reflected in the "reactivity" of heat-treated coal - and, in particular in its response to oxidation; and if oxidation could be performed so as to yield identifiable products, it should be possible to detect the major configurational changes in the distribution of oxidation products. It was previously shown (2) that subbituminous coals and derivatized bituminous coals react readily with sodium hypochlorite, and that they are thereby completely converted into water-soluble materials. The principal products are carbon dioxide and carboxylic acids. For a typical bituminous coal, 18 per cent of total carbon survive oxidation as single benzene ring 6 per cent as 2, 3 or 4 condensed aromatic (and/or heteroaromatic) rings, 7 per cent as methyl or methylene and 20 per cent as carboxyl groups. Acetic, propionic, succinic, glutaric, adipic, benzene- and toluene-carboxy1ic acids constitute the simple products of oxidations. In all, over 90 per cent of carbon in coal could be accounted for from the product yields. Although there are some uncertainties about the mechanism of coal-oxidation by hypochlorite, it is reasonable to expect that, under constant reaction conditions, the same mechanism would hold for variously heat-treated coal samples. Accordingly we thought it pertinent to determine whether this technique could be used to monitor low-temperature changes in coal-carbonization. The present paper reports the results of such an
0-8412-0427-6/78/47-071-131$05.00/0 ©
1978 American Chemical Society
Larsen; Organic Chemistry of Coal ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
132
ORGANIC CHEMISTRY OF COAL
exploratory
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Experimental
study. and R e s u l t s
For t h e p u r p o s e s o f t h i s i n v e s t i g a t i o n , two c o a l s - a W e s t e r n A l b e r t a l v b c o a l w i t h C = 30%, a n d a K e n t u c k y hvb c o a l w i t h C = 85% - w e r e u s e d . Ten g s a m p l e s o f t h e s e c o a l s , e a c h s i z e d t o -60 +115 mesh, were preheated i n h e l i u m f o r 2 h r a t t h e d e s i r e d temperature, c o o l e d , and then s t o r e d u n d e r p u r e He u n t i l r e q u i r e d . No s i g n i f i c a n t w e i g h t l o s s e s o r changes i n e l e m e n t a l c o m p o s i t i o n s w e r e o b s e r v e d w i t h e i t h e r c o a l up t o 375°C, b u t 5-10% w e i g h t l o s s e s , and s l i g h t (0.5-1.2%) i n c r e a s e s i n c a r b o n c o n t e n t s , w i t h c o r responding r e d u c t i o n s i n oxygen were noted a f t e r p r e h e a t i n g a t 390°-400°C. For t h e o x i d a t i o n e x p e r i m e n t s , 2 g (preheated) samples were f i r s t " a c t i v a t e d " by r e a c t i o n w i t h n i t r o n i u m - t e t r a f l u o r o b o a t e i n a c e t o n i t r i l e , a n d t h e r e a f t e r t r e a t e d w i t h 125 ml o f an aq 1.6N s o d i u m h y p o c h l o r i t e s o l u t i o n a t 60°C. The pH o f t h e r e a c t i o n m i x t u r e was m a i n t a i n e d a t 12 by a d d i n g NaOH p e l l e t s a t r e g u l a r invervals. When r e a c t i o n was c o m p l e t e , t h e m i x t u r e was a c i d i f i e d ; i n s o l u b l e m a t t e r was f i l t e r e d o f f ; and s o l u b l e c a r b o x y l i c a c i d s were e x t r a c t e d w i t h e t h e r . The r e s i d u a l s o l u t i o n was f r e e d o f w a t e r by l o w - p r e s s u r e d i s t i l l a t i o n a t 40°C, and s o l i d m a t e r i a l l e f t b e h i n d was e x t r a c t e d w i t h a n h y d r o u s m e t h a n o l . The e t h e r - a n d m e t h a n o l - e x t r a c t s w e r e t h e n c o m b i n e d , c o n v e r t e d t o m e t h y l e s t e r s by r e a c t i o n w i t h d i a z o m e t h a n e , and s e p a r a t e d by g e l p e r m e a t i o n c h r o m a t o g r a p h y on a (Water A s s o c i a t e s ' ) P o r a g e l column i n t o two f r a c t i o n s w i t h m o l e c u l a r w e i g h t s 600 r e s p e c t i v e l y ( f r a c t i o n s A and B ) . Fraction Β (mol w t
