Supercritical Fluids - American Chemical Societypubs.acs.org/doi/pdf/10.1021/bk-1987-0329.ch007Similar10- 1 4 and by hom...
0 downloads
158 Views
975KB Size
Chapter 7
Heterolysis and Homolysis in Supercritical Water Michael Jerry Antal, Jr., Andrew Brittain, Carlos DeAlmeida, Sundaresh Ramayya, and Jiben C. Roy
Downloaded by NORTH CAROLINA STATE UNIV on January 11, 2013 | http://pubs.acs.org Publication Date: December 16, 1987 | doi: 10.1021/bk-1987-0329.ch007
1
Department of Mechanical Engineering, University of Hawaii, Honolulu, HI 96822
The reaction chemistry of simple organic molecules in supercritical (SC) water can be described by heterolytic (ionic) mechanisms when the ion product Kw of the SC water exceeds 10 - 1 4 and by homolytic (free radical) mechanisms when Kw«10-14. For example, in SC water with Kw>10-14 ethanol undergoes rapid dehydration to ethylene in the presence of dilute Arrhenius acids, such as 0.01M sulfuric acid and 1.0M acetic acid. Similarly, 1,3 dioxolane undergoes very rapid and selective hydration in SC water, producing ethylene glycol and formaldehyde without catalysts. In SC methanol the decomposition of 1,3 dioxolane yields 2 methoxyethanol, illustrating the role of the solvent medium in the heterolytic reaction mechanism. Under conditions where Kw«10-14 the dehydration of ethanol to ethylene is not catalyzed by Arrhenius acids. Instead, the decomposition products include a variety of hydrocarbons and carbon oxides. Efficient thermochemical processes underlie the conversion of crude oil and natural gas liquids to higher value chemicals and fuels. Unfortunately, attempts to develop similar conversion processes for biomass feedstocks (such as wood chips and bagasse) have been frustrated by the non-specificity of high temperature pyrolysis reactions involving biopolymer substrates (1,2). For example, the pyrolysis of bagasse yields a liquid mixture of carbohydrate sirups and phenolic tars, a gas composed primarily of carbon oxides and hydrogen, and a solid charcoal. Variations of the conventional engineering parameters (temperature, heating rate, residence time and pressure) do not provide a good control over the complex set of concurrent and consecutive pyrolysis reactions. Thus it has not been possible to engineer the pyrolysis reactions to produce a few high value products from biomass materials. Recent advances in materials technology, high pressure pumps and other high performance liquid chromatography (HPLC) equipment, have created new opportunities (3) for fundamental studies of 'Current address: Gonoshathaya Pharmaceuticals Limited, P.O. Nayarhat via Dhamrai, Dhaka, Bangladesh 0097-6156/87/0329-0077$06.00/0 © 1987 American Chemical Society
In Supercritical Fluids; Squires, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
78
SUPERCRITICAL FLUIDS
c h e m i c a l r e a c t i o n s i n s o l v e n t s at v e r y h i g h p r e s s u r e s (>30 MPa) and t e m p e r a t u r e s ( > 4 0 0 ° C). For s u f f i c i e n t l y d i l u t e s o l u t e - s o l v e n t m i x t u r e s at p r e s s u r e s P>P , no l i q u i d - v a p o r phase t r a n s i t i o n o c c u r s as t h e m i x t u r e i s h e a t e d . F u r t h e r m o r e , when t h e t e m p e r a t u r e Τ i s near t h e s o l v e n t ' s c r i t i c a l t e m p e r a t u r e and P>P many s o l v e n t s m a n i f e s t e x t r a o r d i n a r y p r o p e r t i e s (4,5). T h e s e unusual p r o p e r t i e s o f f e r o p p o r t u n i t i e s f o r the c o n t r o l o f c h e m i c a l r e a c t i o n s i n v o l v i n g b i o p o l y m e r s and o t h e r s u b s t r a t e s . As d i s c u s s e d i n the f o l l o w i n g s e c t i o n s , when t h e d e n s i t y o f s u p e r c r i t i c a l (SC) water exceeds 0.4 g / c m the f l u i d r e t a i n s i t s i o n i c p r o p e r t i e s (high d i e l e c t r i c c o n s t a n t and i o n p r o d u c t ) - even at h i g h t e m p e r a t u r e s . These p r o p e r t i e s p r o v i d e new o p p o r t u n i t i e s t o c a t a l y z e a v a r i e t y o f h e t e r o l y t i c bond c l e a v a g e s w i t h a h i g h degree o f s p e c i f i c i t y . Examples d i s c u s s e d i n t h i s paper i n c l u d e the d e h y d r a t i o n o f e t h a n o l t o e t h y l e n e and the h y d r a t i o n o f 1,3 d i o x o l a n e t o g l y c o l and f o r m a l d e h y d e . In each o f t h e s e examples t h e s p e c i f i c i t y o f t h e h e t e r o l y t i c bond c l e a v a g e i s h i g h ; whereas t h e c o n v e n t i o n a l , h i g h e r t e m p e r a t u r e , f r e e r a d i c a l r e a c t i o n s o f f e r lower y i e l d s o f the d e s i r e d p r o d u c t s . In t h e case o f e t h a n o l d e h y d r a t i o n , f i n d i n g s r e p o r t e d here may have e x c i t i n g i m p l i c a t i o n s f o r the p r o d u c t i o n o f e t h y l e n e from e t h a n o l . C
C
Downloaded by NORTH CAROLINA STATE UNIV on January 11, 2013 | http://pubs.acs.org Publication Date: December 16, 1987 | doi: 10.1021/bk-1987-0329.ch007
3
Prior
Work
I n t e r e s t i n t h e use o f SC s o l v e n t s as a r e a c t i o n media i s founded upon r e c e n t advances i n our u n d e r s t a n d i n g o f t h e i r unique thermop h y s i c a l and c h e m i c a l p r o p e r t i e s . Worthy o f s p e c i a l note are t h o s e t h e r m o p h y s i c a l p r o p e r t i e s (6) which can be m a n i p u l a t e d as parameters t o s e l e c t i v e l y d i r e c t the p r o g r e s s o f d e s i r a b l e c h e m i c a l r e a c t i o n s . These p r o p e r t i e s i n c l u d e the s o l v e n t ' s d i e l e c t r i c c o n s t a n t (7), ion p r o d u c t (8,9), e l e c t r o l y t e s o l v e n t power (10,11), t r a n s p o r t p r o p e r t i e s " [ v i s c o s i t y (12), d i f f u s i o n c o e f f i c i e n t s (13) and i o n m o b i l i t i e s (14)], hydrogen bonding c h a r a c t e r i s t i c s (T5), and s o l u t e s o l v e n t "enhancement f a c t o r s " (6). A l l t h e s e p r o p e r t i e s are s t r o n g l y i n f l u e n c e d by t h e s o l v e n t ' s d e n s i t y Ρ i n t h e s u p e r c r i t i c a l state. For e x a m p l e , SC water w i t h ρ =0.47 g / c m at 4 0 0 ° C (Ρ = 35. MPa) e n j o y s a d i e l e c t r i c c o n s t a n t o f about 10 (comparable t o a po^ar o r g a n i c V i q u i d under normal c o n d i t i o n s ) , an i o n p r o d u c t o f 7x10"* (vs 10" at room c o n d i t i o n s ) and a dynamic v i s c o s i t y u =0.57 m i l l i p o i s e (vs 10 at room c o n d i t i o n s ) . Under t h e s e c o n d i t i o n s SC water behaves as a w a t e r - l i k e f l u i d w i t h s t r o n g e l e c t r o l y t i c s o l v e n t power, h i g h d i f f u s i o n c o e f f i c i e n t s and i o n m o b i l i t i e s , and c o n s i d e r a b l e hydrogen bonding. These p r o p e r t i e s f a v o r c h e m i c a l r e a c t i o n s i n v o l v i n g h e t e r o l y t i c ( i o n i c ) bond c l e a v a g e s which can be c a t a l y z e d by t h e p r e s e n c e o f a c i d s o r bases. Dramatic changes o c c u r when the t e m p e r a t u r e o f t h e SC water i s r a i s e d t o 5 0 0 ° C at c o n s t a n t p r e s s u r e (P=0.144 g / c m ) . Decreases i n t h e d i e l e c t r i c c o n s t a n t t o a v a l u e o f 2 and i o n p r o d u c t t o 2.1 χ 1 0 " ^ cause the f l u i d t o l o s e i t s w a t e r - l i k e c h a r a c t e r i s t i c s and behave as a h i g h t e m p e r a t u r e gas. Under t h e s e c o n d i t i o n s h o m o l y t i c ( f r e e r a d i c a l ) bond c l e a v a g e s are e x p e c t e d t o dominate t h e reaction chemistry. Thus by u s i n g t h e e n g i n e e r i n g parameters o f 3
3
3
υ
In Supercritical Fluids; Squires, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
7.
Heterolysis and Homolysis in Supercritical Water
ANTAL ET AL.
79
t e m p e r a t u r e and p r e s s u r e one can d r a m a t i c a l l y change t h e c h e m i c a l p r o p e r t i e s o f t h e s o l v e n t ( d i e l e c t r i c c o n s t a n t and i o n product) t o f a v o r h e t e r o l y t i c o r h o m o l y t i c bond c l e a v a g e s . T h i s paper emphasizes t h e m a n i p u l a t i o n o f t h e s e parameters as a means f o r engineering the r e a c t i o n chemistry of biopolymer m a t e r i a l s .
Downloaded by NORTH CAROLINA STATE UNIV on January 11, 2013 | http://pubs.acs.org Publication Date: December 16, 1987 | doi: 10.1021/bk-1987-0329.ch007
Apparatus
and E x p e r i m e n t a l
Procedures
F i g u r e 1 i s a s c h e m a t i c o f the SC f l o w r e a c t o r used i n t h i s work. P r i o r t o the i n i t i a t i o n o f f l o w , t h e system i s brought up t o p r e s s u r e by an a i r compressor. A f t e r w a r d s , an HPLC pump f o r c e s a pure s o l v e n t i n t o t h e r e a c t a n t a c c u m u l a t o r at a measured r a t e o f f l o w . T h i s f l o w d i s p l a c e s the s o l v e n t / s o l u t e r e a c t a n t m i x t u r e out o f t h e a c c u m u l a t o r , through the r e a c t o r and a 10 p o r t v a l v e dual l o o p s a m p l i n g system, and i n t o t h e p r o d u c t a c c u m u l a t o r . The f l o w o f p r o d u c t s i n t o t h e second a c c u m u l a t o r d i s p l a c e s a i r through a b a c k p r e s s u r e r e g u l a t o r and i n t o a water d i s p l a c e m e n t a p p a r a t u s , which measures the r a t e o f a i r f l o w at ambient c o n d i t i o n s . The r e a c t a n t f l o w i s r a p i d l y heated t o r e a c t i o n t e m p e r a t u r e by the e n t r y heat g u a r d , and m a i n t a i n e d at i s o t h e r m a l c o n d i t i o n s by a Transtemp I n f r a r e d f u r n a c e and an e x i t heat guard. Samples c a p t u r e d i n 5.54 ml sample l o o p s are r e l e a s e d i n t o s e a l e d , e v a c u a t e d t e s t tubes f o r q u a n t i t a t i v e a n a l y s i s by GC, GC-MS, and HPLC i n s t r u m e n t s w i t h i n the laboratory. The o u t e r annul us o f t h e r e a c t o r i s a 4.6 mm ID H a s t e l l o y C-276 t u b e , and the i n n e r a n n u l u s i s a 3.2 mm 0D s i n t e r e d a l u m i n a t u b e , g i v i n g the r e a c t o r an e f f e c t i v e h y d r a u l i c d i a m e t e r o f 1.4 mm. The a l u m i n a tube accommodates a m o v a b l e t y p e Κ t h e r m o c o u p l e a l o n g t h e r e a c t o r ' s a x i s , which p r o v i d e s f o r the measurement o f a x i a l temperature gradients along the r e a c t o r ' s f u n c t i o n a l l e n g t h . R a d i a l t e m p e r a t u r e g r a d i e n t s are measured as d i f f e r e n c e s between t h e c e n t e r l i n e t e m p e r a t u r e and t e m p e r a t u r e s measured at 10 f i x e d p o s i t i o n s along the outer w a l l of the r e a c t o r using type Κ thermocouples. The l o c a t i o n o f the m o v a b l e t h e r m o c o u p l e w i t h i n the r e a c t o r i s measured e l e c t r o n i c a l l y t o w i t h i n 0.01 mm by a TRAK d i g i t a l p o s i t i o n r e a d out system. The e n t i r e r e a c t o r and s a m p l i n g system i s housed i n a " b u l l e t p r o o f " e n c l o s u r e which can be purged o f a i r (oxygen) d u r i n g s t u d i e s i n v o l v i n g f l a m m a b l e s o l v e n t s (such as methanol). The f o l l o w i n g r e p r e s e n t a t i v e n o n d i m e n s i o n a l numbers c h a r a c t e r i z e the r e a c t o r o p e r a t i n g at 4 0 0 ° C : Re = 420, Pr = 1.86, Sc = 0 . 8 6 , P e = 3 5 8 , Peu = 776 and Da = 0.4. Figure 2 displays a t y p i c a l temperature p r o f i l e of the r e a c t o r during o p e r a t i o n . Because t h e t h e r m a l d i f f u s i v i t y o f SC water i s c o m p a r a b l e t o t h a t o f many h i g h q u a l i t y i n s u l a t i o n m a t e r i a l s , g r o s s r a d i a l t e m p e r a t u r e g r a d i e n t s can e a s i l y e x i s t i n a f l o w r e a c t o r . As shown i n F i g u r e 2, r a d i a l t e m p e r a t u r e g r a d i e n t s w i t h i n t h e a n n u l a r f l o w r e a c t o r are negligible. A computer program, which a c c u r a t e l y a c c o u n t s f o r t h e e f f e c t s o f t h e v a r i o u s f l u i d ( s o l v e n t , s o l v e n t and s o l u t e , a i r ) c o m p r e s s i b i l i t i e s on f l o w measurements, c a l c u l a t e s mass and e l e m e n t a l b a l a n c e s f o r each e x p e r i m e n t . A t y p i c a l experiment e v i d e n c e s mass and e l e m e n t a l b a l a n c e s o f 1.00+0.05. m
In Supercritical Fluids; Squires, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
In Supercritical Fluids; Squires, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987. 2
F i g u r e 1. S u p e r c r i t i c a l f l o w r e a c t o r . Key: (1) M e t t l e r b a l a n c e ; (2) f l a s k w i t h H 0 ( f i l t e r e d and d e a e r a t e d ) ; (3) HPLC pump; (4) bypass (three-way) v a l v e ; (5) f e e d c y l i n d e r ; (6) weather b a l l o o n w i t h f e e d s o l u t i o n ; (7) probe thermocouple (type K ) ; (8) c e r a m i c a n n u l u s ; (9) H a s t e l l o y C-276 tube (10) e n t r a n c e c o o l i n g j a c k e t ; (11) e n t r a n c e h e a t e r ; (12) f u r n a c e c o i l s ; (13) q u a r t z g o l d - p l a t e d IR m i r r o r ; (14) window (no c o i l s ) ; (15) guard h e a t e r ; (16) o u t l e t c o o l i n g j a c k e t ; (17) t e n - p o r t d u a l l o o p sampling v a l u e ; (18) p r o d u c t a c c u m u l a t o r ; (19) a i r compressor; (20) b a c k - p r e s s u r e r e g u l a t o r ; and (21) o u t f l o w m e a s u r i n g assembly.
Downloaded by NORTH CAROLINA STATE UNIV on January 11, 2013 | http://pubs.acs.org Publication Date: December 16, 1987 | doi: 10.1021/bk-1987-0329.ch007
In Supercritical Fluids; Squires, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
Figure 2.
10
12 (inches)
14
16
Typical wall (Δ Δ ) and centerline (0 9) temperature p r o f i l e s of the s u p e r c r i t i c a l flow reactor.
A x i a l distance
Ο
Downloaded by NORTH CAROLINA STATE UNIV on January 11, 2013 | http://pubs.acs.org Publication Date: December 16, 1987 | doi: 10.1021/bk-1987-0329.ch007
20
22
I I ί
SUPERCRITICAL FLUIDS
82
Downloaded by NORTH CAROLINA STATE UNIV on January 11, 2013 | http://pubs.acs.org Publication Date: December 16, 1987 | doi: 10.1021/bk-1987-0329.ch007
Results R e s u l t s o f experiments probing the dehydration chemistry of ethanol i n SC w a t e r (P = 34.5 MPa) a r e s u m m a r i z e d i n T a b l e 1. If e q u i l i b r i u m were e s t a b l i s h e d , the c o n v e r s i o n o f e t h a n o l t o e t h y l e n e at 4 0 0 ° C and 34.5 MPa would be 74%. U n f o r t u n a t e l y , t h e u n c a t a l y z e d r e a c t i o n i s v e r y s l o w and l i t t l e e t h y l e n e i s formed. M o r e o v e r , many A r r h e n i u s a c i d c a t a l y s t s e i t h e r decompose o r r e a c t w i t h the e t h a n o l at t h e s e h i g h t e m p e r a t u r e s , f o r m i n g unwanted b y p r o d u c t s . For e x a m p l e , n i t r i c a c i d c o m p l e t e l y decomposes and l e a d s t o the f o r m a t i o n o f CO2, CO and o t h e r b y p r o d u c t s . Formic a c i d a l s o decomposes i n SC water under t h e s e c o n d i t i o n s , and does not c a t a l y z e the d e h y d r a t i o n r e a c t i o n . At h i g h c o n c e n t r a t i o n s (0.1M) s u l f u r i c a c i d s u f f e r s some d e c o m p o s i t i o n and r e a c t s w i t h t h e e t h a n o l , as w e l l as c a t a l y z i n g t h e d e h y d r a t i o n r e a c t i o n . However, at lower c o n c e n t r a t i o n s , t h e a c i d e x e r t s a s t r o n g and s e l e c t i v e c a t a l y t i c i n f l u e n c e on t h e d e h y d r a t i o n o f e t h a n o l . As shown i n T a b l e 1, i n the p r e s e n c e o f 0.01 M s u l f u r i c a c i d , e t h a n o l undergoes 21% d e c o m p o s i t i o n i n 13s a t 4 0 0 ° C , y i e l d i n g 82% e t h y l e n e . U n l i k e f o r m i c a c i d , a c e t i c a c i d i s a b s o l u t e l y s t a b l e i n SC water t o 5 0 0 ° C at 34.5 MPa. At 4 0 0 ° C t h e p r e s e n c e o f a c e t i c a c i d (1.0M) t r i p l e s t h e r a t e o f f o r m a t i o n o f e t h y l e n e from e t h a n o l ; however, the f o r m a t i o n o f t h e l i q u i d b y p r o d u c t e t h y l a c e t a t e a c c o u n t s f o r 46% o f t h e e t h a n o l which r e a c t e d . N o r m a l l y the p r e s e n c e o f a s t r o n g a c i d (such as H S 0 O i s r e q u i r e d t o c a t a l y z e t h e f o r m a t i o n o f the a c e t a t e from e t h a n o l and a c e t i c a c i d . C o n s e q u e n t l y , the f o r m a t i o n o f t h e a c e t a t e ( a l o n g w i t h d i e t h y l e t h e r as a m i n o r , 1 0 ~ \ As mentioned e a r l i e r , at 5 0 0 ° C and 34.5 MPa s u p e r c r i t i c a l water has a s m a l l d i e l e c t r i c c o n s t a n t , a v e r y low ion p r o d u c t , and behaves as a h i g h t e m p e r a t u r e gas. These p r o p e r t i e s would be expected to minimize the r o l e of h e t e r o l y s i s in the dehydration chemistry. As shown i n T a b l e 1, t h e c o n v e r s i o n o f e t h a n o l t o e t h y l e n e a t 5 0 0 ° C i s s m a l l , even i n t h e p r e s e n c e o f 0.01M s u l f u r i c acid catalyst. The appearance o f t h e b y p r o d u c t s CO, C 0 , CH^ and C 2 H 6 p o i n t s to the onset of n o n s e l e c t i v e , f r e e r a d i c a l r e a c t i o n s in t h e d e c o m p o s i t i o n c h e m i s t r y , as would be e x p e c t e d i n t h e h i g h t e m p e r a t u r e gas phase t h e r m o l y s i s o f e t h a n o l . S i m i l a r e x p e r i m e n t s i n v o l v i n g t h e h y d r a t i o n o f 1,3 d i o x o l a n e i n water (see F i g u r e 3) at 350° C and 34.5 MPa e v i d e n c e d t h e c o m p l e t e d e c o m p o s i t i o n o f t h e r e a c t a n t and produced a y i e l d o f 99.8% e t h y l e n e g l y c o l and f o r m a l d e h y d e (formed i n e q u i m o l a r amounts) w i t h a r e s i d e n c e time o f 100s. An experiment i n v o l v i n g 1,3 d i o x o l a n e i n SC methanol at 4 5 0 ° C and 13.8 MPa formed the e x p e c t e d p r o d u c t 2m e t h o x y e t h a n o l , s u b s t a n t i a t i n g t h e r o l e o f t h e s o l v e n t medium i n t h e r e a c t i o n c h e m i s t r y as i n d i c a t e d i n F i g u r e 3. The e x t r a o r d i n a r y s p e c i f i c i t y and r a t e o f h y d r a t i o n r e a c t i o n r e n d e r e d the use o f c a t a l y s t s unnecessary. These r e s u l t s c o n t r a s t w i t h e a r l i e r f i n d i n g s (18) c o n c e r n i n g the h i g h t e m p e r a t u r e ( > 6 0 0 ° C), f r e e r a d i c a l d e c o m p o s i t i o n o f 1,3 d i o x o l a n e i n steam at 0.1 MPa, where a wide v a r i e t y o f gaseous p r o d u c t s ( i n c l u d i n g H2, CO, CO2, CHi», C H n and C H ) were o b s e r v e d and no s p e c i f i c i t y i n t h e r e a c t i o n c h e m i s t r y c o u l d be r e a l i z e d . 2
11
W
2
2
2
6
In Supercritical Fluids; Squires, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
In Supercritical Fluids; Squires, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
3
4
4
0.01
-
500
Ά
low background c o n c e n t r a t i o n o f H 2
reactant
34.5
31.7
is
17
2.0
9.5
12
21
2.6
40
25
12
1.2
0.2
25
0.2
0.1
0.0
1.1
3.0
0.1
57
0.0
0.0
2
14
0.0
0.0
42.
218
1.8
3.5
2.3 0.0
15
0.0
15
2
4
2.9
7.8
0.0
0.8
0.0
0.91
1.00
0.98
0.93
0.96
0.98
Carbon Balance
a reactant.
(%)*
0.0
Gaseous P r o d u c t Y i e l d H CO CH C0
82
^2^6 1.6
4
i n SC Water
17
2
C H
Dehydration
o b s e r v e d , even i n blank e x p e r i m e n t s w i t h pure water as
consumed)
8.5
66
52
31.7
400
500
48
31.7
400
0.01
1.0
13
34.5
400
49
(1M)
Conver sion^)
Ethanol
Res. Time (s)
0.01
* Y i e l d = (mole p r o d u c t ) / ( m o l e
2
H S0
-
CH3COOH
HN0
2
H S0
Press. (MPa)
31.7
-
-
(°C)
Temp.
400
Catalyst Conc.(M)
Catalyst
TABLE I.
Downloaded by NORTH CAROLINA STATE UNIV on January 11, 2013 | http://pubs.acs.org Publication Date: December 16, 1987 | doi: 10.1021/bk-1987-0329.ch007
Downloaded by NORTH CAROLINA STATE UNIV on January 11, 2013 | http://pubs.acs.org Publication Date: December 16, 1987 | doi: 10.1021/bk-1987-0329.ch007
84
SUPERCRITICAL FLUIDS
F i g u r e 3. H e t e r o l y t i c f o r m a t i o n o f g l y c o l and formaldehyde from 1,3 d i o x o l a n e i n SCW ( i n c o l l a b o r a t i o n w i t h M. J o n e s ) .
Discussion H e t e r o l y t i c r e a c t i o n s can be d i s t i n g u i s h e d from h o m o l y t i c ( f r e e r a d i c a l ) r e a c t i o n s by t h e f o l l o w i n g c r i t e r i a : 1) h e t e r o l y t i c r e a c t i o n s u s u a l l y can be c a t a l y z e d by A r r h e n i u s a c i d s and bases. 2) h e t e r o l y t i c r e a c t i o n s u s u a l l y a r e i n f l u e n c e d by t h e n a t u r e ( p o l a r vs n o n - p o l a r ) o f t h e s o l v e n t medium. 3) h e t e r o l y t i c r e a c t i o n s are often q u i t e s p e c i f i c . 4) h o m o l y t i c r e a c t i o n s a r e not i n f l u e n c e d by A r r h e n i u s a c i d o r base c a t a l y s t s . 5) because o f t e r m i n a t i o n s t e p s , h o m o l y t i c r e a c t i o n s a r e often non-specific. F i n d i n g s r e p o r t e d i n t h i s paper i n c l u d e : 1) t h e c a t a l y s i s o f e t h a n o l d e h y d r a t i o n by two A r r h e n i u s a c i d s i n SC water a t 400°C where K >10~ 2) t h e i n f l u e n c e o f t h e s o l v e n t (water vs methanol) on t h e p r o d u c t s and r a t e s o f t h e u n c a t a l y z e d d e c o m p o s i t i o n o f 1,3 dioxolane. 3) t h e e x t r a o r d i n a r y s p e c i f i c i t y o f t h e s e two r e a c t i o n s when K >10- . 4) t h e i n e f f e c t i v e n e s s o f H2SCK as a c a t a l y s t f o r e t h a n o l d e h y d r a t i o n i n SC water at 500°C where K « 1 0 " . 5) the l o s s of s p e c i f i c i t y of the ethanol denydration r e a c t i o n at 500°C where K « 1 0 " . These f i n d i n g s cause us t o c o n c l u d e t h a t aqueous phase c h e m i c a l r e a c t i o n s , u s u a l l y o b s e r v e d at much l o w e r t e m p e r a t u r e s , can be conducted i n SC water p r o v i d i n g K > 1 0 " . When ^ « Η Γ * SC water behaves as a h i g h t e m p e r a t u r e gas and f r e e r a d i c a l r e a c t i o n s predominate. The f o r m a t i o n o f the e t h y l a c e t a t e e s t e r from e t h a n o l and a c e t i c a c i d a l s o p o i n t s t o t h e r o l e o f c a r b o c a t i o n c h e m i s t r y i n SC 1 1 1
w
l h
w
1 1 +
w
1 1 +
w
11+
1
w
In Supercritical Fluids; Squires, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
7. ANTAL ET AL.
Heterolysis and Homolysis in Supercritical Water
85
water when K > 1 0 " . In a d d i t i o n , t h i s r e a c t i o n s u g g e s t s t h e p o s s i b l e r o l e o f SC water as a c a t a l y t i c medium (due t o the " h i g h " c o n c e n t r a t i o n s o f H+ and 0H-) at s u f f i c i e n t l y h i g h t e m p e r a t u r e s and pressures. The c a t a l y t i c d e h y d r a t i o n o f e t h a n o l t o e t h y l e n e i n SC water may be c o m m e r c i a l l y i m p o r t a n t (16). A l t h o u g h h i g h q u a l i t y commercial a l u m i n a c a t a l y s t s e x i s t f o r t h e v a p o r phase d e h y d r a t i o n o f e t h a n o l , t h e commercial p r o c e s s e s r e q u i r e t h e e t h a n o l f e e d s t o c k t o be r e l a t i v e l y f r e e o f water. Hence t h e e t h a n o l must be d i s t i l l e d from t h e e t h a n o l - w a t e r m i x t u r e which i s t h e p r o d u c t o f f e r m e n t a t i o n processes. By a v o i d i n g t h i s d i s t i l l a t i o n s t e p , and s e c u r i n g phase s e p a r a t i o n o f t h e e t h y l e n e p r o d u c t from t h e e t h a n o l - w a t e r r e a c t a n t , SC d e h y d r a t i o n o f e t h a n o l c o u l d e n j o y a d v a n t a g e s o v e r e x i s t i n g commercial t e c h n o l o g i e s . Because o f the p o t e n t i a l commercial s i g n i f i c a n c e o f t h i s work, we a r e p r e s e n t l y d e v e l o p i n g k i n e t i c e x p r e s s i o n s f o r t h e r a t e o f e t h y l e n e f o r m a t i o n i n the SC water e n v i r o n m e n t . We are a l s o measuring t h e r a t e o f e t h a n o l d e h y d r a t i o n i n t h e v i c i n i t y o f t h e c r i t i c a l p o i n t o f water t o d e t e r m i n e i f the p r o p e r t i e s o f the f l u i d near t h e c r i t i c a l p o i n t have any i n f l u e n c e on t h e r e a c t i o n r a t e . In the near f u t u r e we p l a n t o b e g i n s t u d i e s o f the r e a c t i o n c h e m i s t r y o f g l u c o s e and r e l a t e d model compounds ( l e v u l i n i c a c i d ) i n SC water. 11+
Downloaded by NORTH CAROLINA STATE UNIV on January 11, 2013 | http://pubs.acs.org Publication Date: December 16, 1987 | doi: 10.1021/bk-1987-0329.ch007
w
Conclusions The s i g n i f i c a n c e o f t h i s work i s i t s i d e n t i f i c a t i o n o f SC water as a medium which s u p p o r t s and enhances aqueous phase c h e m i s t r y o r d i n a r i l y o b s e r v e d at much lower t e m p e r a t u r e s . Fundamental s t u d i e s o f t h e r e a c t i o n c h e m i s t r y o f b i o p o l y m e r r e l a t e d model compounds d e s c r i b e d i n t h i s paper o f f e r i n s i g h t s i n t o the d e t a i l s o f r e a c t i o n mechanisms, and f a c i l i t a t e t h e c h o i c e o f r e a c t i o n c o n d i t i o n s which enhance t h e y i e l d s o f v a l u a b l e p r o d u c t s . Chemical r e a c t i o n e n g i n e e r i n g i n s u p e r c r i t i c a l s o l v e n t s , based on t h e a b i l i t y t o choose between h e t e r o l y t i c and h o m o l y t i c r e a c t i o n mechanisms w i t h f o r e k n o w l e d g e o f r e s u l t s , h o l d s much promise as a new means t o improve our u t i l i z a t i o n o f t h e v a s t b i o p o l y m e r r e s o u r c e . Acknowledgments T h i s r e s e a r c h was s u p p o r t e d by t h e N a t i o n a l S c i e n c e F o u n d a t i o n under g r a n t CPE 8304381, the Department o f P l a n n i n g and Economic Development o f the S t a t e o f H a w a i i , and t h e C o r a l _ I n d u s t r i e s Endowment. The a u t h o r s thank Dr. M a r i a Burka (NSF), Kent K e i t h and Dr. Tak Y o s h i h a r a (DPED), and D a v i d Chalmers ( C o r a l I n d u s t r i e s ) f o r t h e i r i n t e r e s t i n t h i s work. The a u t h o r s a l s o thank W i l l i a m Mok f o r h i s c o n t i n u i n g a s s i s t a n c e w i t h t h e f l o w r e a c t o r , Dr. A l i T a b a t a b a i e R a i s s i , and P r o f e s s o r M a i t l a n d Jones ( P r i n c e t o n ) f o r many stimulating discussions. The comments o f the r e v i e w e r s were appreciated.
Literature Cited 1.
Antal, M.J. "Biomass Pyrolysis. A Review of the Literature Part 1: Carbohydrate Pyrolysis" in Advances in Solar Energy, (Eds. K.W. Boer and J.A. Duffie) Vol. 1, (American Solar Energy Society, Boulder, CO) 1983.
In Supercritical Fluids; Squires, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
86
SUPERCRITICAL FLUIDS
Downloaded by NORTH CAROLINA STATE UNIV on January 11, 2013 | http://pubs.acs.org Publication Date: December 16, 1987 | doi: 10.1021/bk-1987-0329.ch007
2.
Antal, M.J. "Biomass Pyrolysis. A Review of the Literature Part 2: Lignocellulose Pyrolysis" in Advances in Solar Energy, 2, (Plenum Press, N.Y.) 1985. 3. Paulaitis, M.E., Penninger, J.M.L., Gray, R.D, Jr. and Davidson, P. "Chemical Engineering at Supercritical Fluid Conditions" Ann Arbor Science, Ann Arbor, 1983. 4. Franck, E.U. "Supercritical Water" Endeavor, 1968,27, 55-59. 5. Marshall, W.L. "Water at High Temperatures and Pressures" Chemistry, 1975, 48, 6-12. 6. Franck, E.U. "Thermophysical Properties of Supercritical Fluids with Special Consideration of Aqueous Systems", Fluid Phase Equilib. 1983, 10(2-3), 211-22. 7. Franck, E.U. "Water and Aqueous Solutions at High Pressures and Temperatures". 8. Marshall, WL . and Franck, E.U.,J. Phys. Chem. Ref. Data, 1981, 10, 295-304. 9. Quist, A.S., Marshall, W.L., and Jolley, H.R. J. Phys. Chem., 1965, 69, 2726-2735. 10. Marshall, W.L., Rev. Pure. Appl. Chem., 1968, 18, 167-186. 11. Quist, A.S., J. Phys. Chem., 1970, 74, 3396-3402. 12. Bruges, E.A. and Gibson, M.R., J. Mech. Eng. Sci., 1969, 11, 189-205. 13. Lamb, W.J., Hoffman, G.A., Jonas, J., J. Chem. Phys., 1981, 74(12), 687580. 14. Franck, E.U., Hartmann, D., and Hensel, F., J. Phys. Chem., 1968, 72, 200-206. 15. Gorbatyi, Y.E., Probl. Fix.-Khim. Petrol., (Eds. Zharikov, V.A., Fonarev, V.I., Korikovskii, S.P.) 1979, 2, 14-24. 16. Luchi, N.R. and Trindade, S.C., Hydrocarbon Proc., 1982, 179183. 17. Knozinger, H. The Chemistry of the Hydroxyl Group, Part 2, (Ed. S. Patai), Interscience, London 1971. 18. Cutler, Α., Ph.D. thesis, Princeton University, Princeton, 1984. RECEIVED October 7, 1986
In Supercritical Fluids; Squires, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
