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Chapter 16
Chemistry of Methoxynaphthalene in Supercritical Water 1
2
Johannes M.L.Penninger and Johannes M. M. Kolmschate
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1
Akzo Salt and Basic Chemicals bv., 7550 GC Hengelo, Netherlands University of Twente, Enschede, Netherlands
2
Decomposition of methoxynaphthalene in supercritical water at 390 °C occurs by proton-catalyzed hydrolysis and results in 2-naphthol and methanol as main reaction products. The rate of hydrolysis is enhanced by dissolved NaCl. The dielectric constant and the ionic strength of supercritical water was found to affect the hydrolysis rate constant according to the "secondary salt effect" rate law, which commonly describes ionic reactions in liquid solvents. In subcritical water vapor the decomposition of the ether results in a mixture of cracking products and polycondensates, which is characteristic for a radical type thermolysis.
Decomposition o f ethers i n s u p e r c r i t i c a l (SC) water i s studied as a model f o r the conversion of natural raw materials, such as coal, shale and biomass, i n t o chemicals and fuels by SC water exposure. The decomposition o f such materials i s proposed to occur by chemical action at hetero functional groups, i n p a r t i c u l a r at oxygencontaining functional groups, such as ester and ether functions. The dramatic change i n product s e l e c t i v i t y as compared to thermal p y r o l y s i s has l e d to the hypothesis that SC water interacts chemically with those materials, e.g. subbituminous coal (1). Work with pure ethers (2-4) has shown that the ether r e a c t i v i t y i s structure-dependent and that decomposition paths are affected by the density o f SC water. Reactive i n SC water are a l k y l - a l k y l e t h e r s and a l k y l - a r y l e t h e r s while aryl-arylethers, e.q. diphenylether, are i n e r t . The change observed i n product d i s t r i b u t i o n with the density has been explained as a gradual transformation from a homolytic r a d i c a l decomposition at zero or low water density to a hydrolytic decomposition at densities generally higher than the c r i t i c a l density. The same hypothesis has been forwarded f o r the decomposition o f ethanol and 1,3 dioxalane (£) and f o r d i - n butylphthalate i n SC water ( 6 ) . The current work focusses on the investigation o f the reaction paths o f a s p e c i f i c alkyl-arylether, v i z . methoxynaphthalene, i n Ο097-6156/89ΛΜ06-Ο242$06.00/0 ο 1989 American Chemical Society
Johnston and Penninger; Supercritical Fluid Science and Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
16.
PENNINGER AND K O L M S C H A T E
Chemistry ofMethoxynaphthalene 243
r e l a t i o n to the density o f SC water. The p r i n c i p a l objective i s to present sound experimental evidence f o r hydrolysis chemistry a t SC d e n s i t i e s and t o e s t a b l i s h the reaction k i n e t i c s o f such chemistry. Experimental The experiments were c a r r i e d out i n a 316 SS bomb with a volume o f 64 cm'. In each run 5.0 grams o f methoxynaphthalene were held at 390 C (T = 1.04) f o r 90 minutes i n a quantity o f water varying with e
p
3
each run. The density was thus varied from 0 gram/cm (no water) to 0.51 gram/cm ( ρ = 1.57) by addition o f 33 grams o f water. 1
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ρ
A f t e r the bomb was cooled to ambient temperature the gases formed were released i n a gasometer over water and the volume measured at barometric pressure. Subsequently the bomb was opened and i t s content dissolved i n acetone. This solution-was transfered to a 50 ml volumetric f l a s k and topped with a d d i t i o n a l acetone. The gas phase c o l l e c t e d i n the gasometer was analyzed by GC (2 m χ ν · " 5 A molsieves, i n s e r i e s with 4 m χ / · Porapak S, He, hot wire), to determine the absolute concentration o f i t s constituents. The acetone s o l u t i o n was also analyzed by GC (CP-SIL-5 c a p i l l a r y 25 m χ 0.32 mm, Ni , FID) and the concentration o f i t s constituents determined r e l a t i v e to ethylnaphthalene, added as an i n t e r n a l standard. This a n a l y t i c a l procedure allowed f o r accurate mass balancing so that material p r e v a i l i n g as polycondensates could be quantified as the difference between consumed methoxynaphthalene and the sum o f a l l products e l u t i n g from the GC column. 1
H
Results Thermal P y r o l y s i s . As a base o f reference, data are presented f o r the thermal p y r o l y s i s o f methoxynaphthalene, at a P o f zero p
(Table I ) . The main reaction products are naphthols and naphthalenes as a r e s u l t o f reactions a t the methoxy group; simultaneously gases such as CO and CH* and small quantities o f CO* and Hi are formed. Methylation occurs p r e f e r e n t i a l l y with naphthalene, but also naphthol and methoxynaphthalene are p a r t i a l l y methylated. From the mass balance follows however that most o f the ether i s converted i n t o non-identified polycondensates, amounting to 56 % o f consumed naphthyl groups, 66 % o f the oxygen and 44 % o f the methyl groups. P y r o l y s i s reactions proceed most commonly through r a d i c a l intermediates; as a working hypothesis i s therefore proposed the r a d i c a l mechanism as i s i l l u s t r a t e d i n Figure 1. The p y r o l y s i s i s i n i t i a t e d by thermal f i s s i o n o f the CHi-0 bond (reaction 1). The r e s u l t i n g oxynaphthyl r a d i c a l and methyl r a d i c a l abstract H-atoms from the parent forming naphthol, CH% and the methylene-naphthylether r a d i c a l (reactions 2 and 3 ) · The l a t t e r decomposes i n t o CO and Hi and a naphthylradical (reaction 4) which subsequently abstracts a Η-atom from the parent molecule forming naphthalene and regenerating the methylene-naphthylradical (reaction 5 ) ·
Johnston and Penninger; Supercritical Fluid Science and Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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SUPERCRITICAL FLUID SCIENCE A N D T E C H N O L O G Y
ArOCH,
—
ArO · + · C H
ArO · + ArOCH,
—
ArOH
•CH
—
C H + ArOfcH,
ArOÔH,
—
Ar ·
ΑΓ· + ArOCH,
— ArH + ArOÔH
• C H , , Ar · , ArO · + H
—
3
+ ArOCH,
2
A r O C H , ArO · , Ar · , · C H , 2
model experiment Figure 1.
+ ArOÔH,
+
CO + H
12» 131
t
2
2
14) IS1
CH>1
if I6KI7I » if I61.I71 «
UH5Ï UI.I5I
0.42/0.49
Radical mechanism of methoxynaphthalene
pyrolysis.
Johnston and Penninger; Supercritical Fluid Science and Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
16.
PENNINGER AND K O L M S C H A T E
Chemistry ofMethoxynaphthalene
Table I. Product d i s t r i b u t i o n by thermal p y r o l y s i s of methoxynaphthalene
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Conversion Selectivity of naphthol methylnaphthol methylnaphthalene naphthalene naphthaldehyde methylmethoxynaphthalene
28.5 %
17.3 7.1 10.7 4.1 1.9 2.2 14.4 16.1 0.35 1.4
CO CH* CO*
H
2
390 C , 90 minutes, P water = 0 r
S e l e c t i v i t y : (mol product/mol ether consumed) χ 100 Radicals may further react with molecular Hz formed i n reaction 4, thus accounting f o r the less-than-stoichiometric r a t i o o f Hz to CO found i n the experiment, o r recombine with each other (reaction 7) to form polycondensates. According to the model proposed, the r a t i o o f naphthol to CH* would be unity provided oxynaphthyl- and methylradicals are equally reactive i n reactions 2 and 3 · The experiment shows a r a t i o o f 0.49/0.46, indeed close to unity. Also the r a t i o o f CO to naphthalenes i s predicted to be unity provided the naphthyl r a d i c a l p r e f e r e n t i a l l y saturates i t s e l f by Η abstraction, over recombination with other r a d i c a l s . The experimental r a t i o o f 0.41/0.42 i s again i n agreement with t h i s model. F i n a l l y the r a t i o o f naphthalene to naphthol indicates the rates o f the propagation reactions 4, 5 r e l a t i v e to the i n i t i a t i o n reaction 1. The experimental value o f 0.42/0.49 shows that the methylene-naphthylether r a d i c a l i s formed mainly through reactions 1 and 3» the chain transfer reactions 4 and 5 are slow r e l a t i v e to i n i t i a t i o n 1 and to reactions 6, 7 otherwise naphthalene and CO would have been formed i n concentrations well over naphthol. The formation o f polycondensates l i k e l y r e s u l t s from recombinations o f large r a d i c a l fragments. The r a t i o o f "polycondensed" oxygen to polycondensed naphthyl structures with a value close to 1 may suggest leading recombinations to be those of oxynaphthyl r a d i c a l s with each other and with the parent ether. Recombinations are important because more than h a l f o f the converted parent i s recovered as polycondensed matter. Reaction Patterns i n SC Water. The reaction pattern o f methoxynaphthalene changes d r a s t i c a l l y i n SC water. As a f i r s t r e s u l t one observes an increase o f conversion when water d e n s i t i e s increase beyond the c r i t i c a l density (Figure 2). More dramatic i s
Johnston and Penninger; Supercritical Fluid Science and Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
245
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SUPERCRITICAL FLUID SCIENCE AND T E C H N O L O G Y
Figure 2. density.
Conversion of methoxynaphthalene vs. pure water
Johnston and Penninger; Supercritical Fluid Science and Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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Chemistry ofMethoxynaphthalene 247
the change i n product d i s t r i b u t i o n with the density. This i s i l l u s t r a t e d i n Figures 3 and 4 as s e l e c t i v i t y diagrams. Increase o f the water density p r e f e r e n t i a l l y favors the formation o f 2-naphthol while such products as (methy1)naphthalenes,methylmethoxynaphthaiene and naphthaldehyde are gradually depressed. Figure' 3 also shows the inverse e f f e c t o f water density on naphthyl-polycondensation reactions; only 6 % o f consumed naphthyl structure p r e v a i l s as poly condensates at the highest water density, t h i s f i g u r e i s 56 % at zero density. The methoxy group i s converted i n t o CO, CH* and methyl groups at s u b c r i t i c a l d e n s i t i e s , i n addition to small amounts o f CO2 and aldehyd functional group (naphthaldehyde). Small concentrations o f hydrogen were found over the e n t i r e density range i n q u a n t i t i e s approximately equimolar to CO2. At SC d e n s i t i e s the methoxy group i s p r e f e r e n t i a l l y converted i n t o methanol, a t the cost o f mainly CO but also CH*. Figure 4 i l l u s t r a t e s c l e a r l y that approximately only 50 % o f consumed methoxy groups are recovered while the remainder i s incorporated i n polycondensates. Quite c l e a r l y the water density has no e f f e c t on t h i s s e l e c t i v i t y pattern and t h i s i s i n s t r i k i n g contrast to the polycondensation of naphthyl f u n c t i o n a l groups. Organic oxygen, recovered i n i d e n t i f i e d products, increases from 34 % at thermal p y r o l y s i s conditions to over 100% at SC d e n s i t i e s . (Figure 5) The l a t t e r means that products contain more organic oxygen than could be provided f o r by consumed methoxynaphthalene. This i l l u s t r a t e s also the occurence o f hydrolysis reactions at SC d e n s i t i e s . The reaction paths changing with the water density consequently are the r e s u l t o f a r a d i c a l type p y r o l y s i s transforming gradually to hydrolysis as leading mechanism. The product d i s t r i b u t i o n a t h y d r o l y t i c conditions i s remarkably simple, i n contrast to p y r o l y s i s conditions, with an almost complete suppression o f aromatic condensation. I t follows therefore that SC hydrolysis provides a clean-cut mechanism f o r conversion o f t h i s ether. Mechanism and K i n e t i c s o f SC Water Hydrolysis. Hydrolysis i n l i q u i d water i s known to proceed through i o n i c intermediates; as such the rate i s affected by the i o n i c strength o f the solvent. On the other hand the observation that a reaction i s affected by solvent i o n i c strength, e.g. as the r e s u l t o f dissolved i n e r t s a l t s , i s general proof o f an i o n i c mechanism. This test was also applied to SC water hydrolysis. From Figure 6 i t follows that the conversion rate o f methoxynaphthalene i s p o s i t i v e l y affected by already small concentrations of NaCl dissolved i n the SC water; the higher the water density the stronger the e f f e c t s and the l a r g e r the contribution o f hydrolysis to t o t a l conversion. Product s e l e c t i v i t y i s also markedly affected, as i s i l l u s t r a t e d with Figures 7 and 8 by addition o f only 1.01 % wt NaCl to SC water. Hydrolysis i s known to be catalyzed by protons i n s o l u t i o n . This again i s also found to be true i n SC water. Table I I shows that addition o f HC1 i n 68 ppm to SC water enhanced the rate constant by one order-of-magnitude; on the other hand addition o f NaOH, hence OH anions, i n a s i m i l a r concentration d i d not measurably a f f e c t the rate.
Johnston and Penninger; Supercritical Fluid Science and Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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SUPERCRITICAL FLUID SCIENCE A N D T E C H N O L O G Y
Johnston and Penninger; Supercritical Fluid Science and Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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PENNINGER AND K O L M S C H A T E
Chemistry ofMethoxynaphthalene 249
ο 0 -) 0
, 0.1
, Q2
. 03
, r-J OA 0.5 3
gram.cm Figure 5·
Balance o f organic oxygen (pure water experiments)
Johnston and Penninger; Supercritical Fluid Science and Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
250
SUPERCRITICAL FLUID SCIENCE A N D T E C H N O L O G Y
1.0-
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2-NAPHTHOL 0.5NAPHTHALENES ME-NAPHTHOL NAPHTHALDE HYO
0.1
0.2
0.3
OA
0.5
gram.cm *
γ .
F i g u r e 7. C - b a l a n c e o f consumed n a p h t h y l groups i n 1.01 aqueous N a C l
% wt
τ
1.0-
POLY CONDENSATES
0.5 H
0.1
METHANOL
0.2
03
OA
0.5
gram.cm F i g u r e 8. C - b a l a n c e o f consumed methoxy g r o u p s i n 1.01 aqueous N a C l
wt %
Johnston and Penninger; Supercritical Fluid Science and Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
16.
Chemistry ofMethoxynaphthalene 251
PENNINGER AND K O L M S C H A T E
Table I I . E f f e c t s o f dissolved acid and a l k a l i on methoxy naphthalene conversion i n SC water
11 50.2 χ 10-»
91.7
390 C, 90 minutes,
1
5.67 x 10-»
38.3-42.0 40.8
None NaOH, 94 ppm* HC1, 68 ppm* Downloaded by UNIV OF NEW ENGLAND on February 10, 2017 | http://pubs.acs.org Publication Date: August 29, 1989 | doi: 10.1021/bk-1989-0406.ch016
F i r s t - o r d e r k**, min-
Conversion %
Additive
= 0.45 gram/cm
1
* wt ppm added to water. ** from ether conversion a f t e r 90 minutes a t reaction temperature; autoclave heating/cooling had n e g l i g i b l e influence on conversion. Conclusively the decomposition o f methoxynaphthalene i n SC water i s a proton-catalyzed i o n i c reaction, a l l i n agreement with proton-catalyzed hydrolysis. The mechanism proposed i s i l l u s t r a t e d i n Figure 9· The rate-determining step i s assumed to be the decomposition o f the protonated ether, e i t h e r i n a unimolecular A - l mechanism o r i n a bimolecular A-2 mechanism where water i s involved i n the formation o f the t r a n s i t i o n state complex. The e s s e n t i a l difference between both mechanisms i s that A-2 regenerates a protium ion i n a c y c l i c process by proton transfer, while A - l requires a new i o n i z a t i o n o f water f o r every mole o f ether converted. Which o f both mechanisms dominates remains uncertain a t t h i s stage. The k i n e t i c rate expression f o r hydrolysis i s derived by assuming steady-state f o r a l l reaction intermediates. Assuming further that the rate o f hydrolysis i s f i r s t - o r d e r i n ether, the following equations are obtained: f o r Mechanism
A-l: η
= ki [Ε]
(1)
and f o r Mechanism
A-2:
= ki [E]
(2)
with
kt . k i . { | § ^ . Ρ
and
kt » kze [Hi0*].P
η
(3)
ν
(4)
w
The term [ H i 0 ] i n Equations 3 and 4 follows as +
[H,0 ] - V . » / Y
(5)
+
±
from the i o n i c d i s s o c i a t i o n constant o f water, which i s defined (2) +
as
K
with
[ H , 0 ] » [OH"]
w
= [H,0 ][0H ] T T +
+
Johnston and Penninger; Supercritical Fluid Science and Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
(6) (7)
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SUPERCRITICAL FLUID SCIENCE A N D T E C H N O L O G Y
2H 0*5 Η,0®ΟΗ
θ
2
EH®
..H
Δ=1
-^naphthol •
ΟΓΟΤ '"'CH,
C CHf Hf • 0Η
Θ
CH,0H
1 + A-2
EH®+
H 0^ 2
0,, ;CH,
Η / CH ; 0 Θ H 3
N
H — 0*
\
Η
+H0 2
CH OH • H 0 Figure 9· water
f
Hydrolysis mechanism of methoxynaphthalene i n SC
Johnston and Penninger; Supercritical Fluid Science and Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
16.
Chemistry ofMethoxynaphthalene 253
PENNINGER AND K O L M S C H A T E
and
(8)
Τ = Τ
The l a t t e r represents the a c t i v i t y c o e f f i c i e n t s o f HiO ions i n s o l u t i o n .
+
resp. OH
M u l t i p l i c a t i o n o f [HiO*] by P^ i s required f o r dimensional consistency; [HiO ] i n Equation 6 has the dimension o f grammole/kg while a l l other concentrations terms are expressed i n grammole/1. By s u b s t i t u t i o n o f Equations 5, 6, 7 and 8 i n Equations 3 and 4 follows Downloaded by UNIV OF NEW ENGLAND on February 10, 2017 | http://pubs.acs.org Publication Date: August 29, 1989 | doi: 10.1021/bk-1989-0406.ch016
+
log ki • log k i t + log P
w
+
0.5 l o g
K
w
- log Ύ
±
and l o g ki • l o g kt» + l o g P
w
•
0.5 l o g
K
w
- log T
±
The a c t i v i t y c o e f f i c i e n t Y
±
- l o g [HiO]
(9)
(10)
i s determined by the i o n i c strength
and the d i e l e c t r i c constant o f the solvent, i n f i r s t approximation according to the Debye-Huckel l i m i t i n g law f o r d i l u t e solutions,
where
(11)
e
In Τ
- - Z * « (u) «» ±
= charge o f ion i U
= i o n i c strength o f the s o l u t i o n which i s defined as
(12)
0.5 Σ C Z * ±
with
±
s concentration o f ion i ,
grammole/1
The summation i s taken over a l l ions present i n the s o l u t i o n . The parameter α i s defined as
N, e and k are independent o f solvent properties and temperatures; ε however i s a function o f both solvent density and temperature. By s u b s t i t u t i o n o f Equations 11, 12 and 13 i n equations 9 and 10 i t follows that ε*·» l o g k i , !
= ε*·* l o g k * i
9
| 2
+ C* ( μ ) ·
(14)
5
with log k*i
+ 0.5 l o g Κ
= (log ki β + l o g Ρ
w
(15)
- l o g [HiO])
w
or l o g k* = (log k*o 2
+ log P
w
+
0.5 l o g
(16)
K ) w
Equation 15 represents the A - l mechanism and Equation 16 the A-2 mechanism. Equation 14 correlates the rate constant ki 2 f o r ether hydrolysis i n SC water with solvent properties ε, u and and 1
predicts a l i n e a r increase with the square-root o f the i o n i c strength o f the SC s o l u t i o n . This was tested with rate data obtained at water d e n s i t i e s o f resp. 0 . 2 5 , 0.35 and 0.45 gram/cm . The pure water values o f ε and at each density and a reaction temperature 3
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SUPERCRITICAL FLUID SCIENCE A N D T E C H N O L O G Y
of 390 C were obtained from the l i t e r a t u r e (7. 8) and substituted i n Equation 14. These values are l i s t e d i n Table I I I . e
Table I I I . Values f o r ε and K
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density,
gram/cm*
(8)
ε
0.25 0.35
o f SC water at 390 C
w
K
3-59 5-89 8.20
0Λ5
(I)
w
8.91
χ
9.91
χ io-
10-
1 1 l f
3.34 χ 10- * 1
The i o n i c strength o f the SC solution was varied by dissolving. NaCl i n concentrations nearly up to saturation (9)« The i o n i c strength +
was c a l c u l a t e d from the s o l u t i o n concentrations o f Hi0 , OH Na
+
t
+
and CI ions. The Na and C l ~ concentrations were calculated from l i t e r a t u r e data on NaCl i o n i z a t i o n i n SC water at the p r e v a i l i n g d e n s i t i e s (10) ; the HiO* and 0H~~ concentration followed from again at p r e v a i l i n g d e n s i t i e s (7). The contributions o f the l a t t e r two ions to t o t a l i o n i c strength i s n e g l i g i b l e as s a l t ion concentrations are two orders o f magnitude larger. Assuming that p y r o l y s i s s e l e c t i v i t y i s not affected by SC water, rate constants o f p y r o l y s i s at various SC water d e n s i t i e s were calculated with naphthalenes as p i l o t compounds and the d r y experiment as the reference pyrolysis rate. The rate constant o f hydrolysis k i , ζ subsequently followed as the difference o f t o t a l ether conversion rate constant and p y r o l y s i s rate constants. I t was found that, i n agreement with Equation 14, a l i n e a r r e l a t i o n e x i s t s between the rate constant ki i and ιι · at each density, and that k*^! i s density dependent. The values o f k*i z followed from the l o g ki 1 v s . u · data by l i n e a r extrapolation to zero i o n i c strength at each s i n g l e density (Table IV). M
β
M
β
y
#
9
1
1
Table IV. Values f o r k*i,z i n r e l a t i o n to SC water density
#
log k i ζ
1
density, gram/cm
f
0.25 0.35 0.45
- 3.15 - 2.73 - 2.65
By s u b s t i t u t i o n o f these values of k * i t i n Equation 14 a l l rate data a t the d i f f e r e n t NaCl concentrations and solvent d e n s i t i e s could be correlated with μ·· by a s i n g l e straight l i n e . This i s i l l u s t r a t e d i n Figure 10. With the same data values f o r ki Q and kie were f
8
Johnston and Penninger; Supercritical Fluid Science and Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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PENNINGER AND K O L M S C H A T E
Chemistry ofMethoxynaphthalene
30H
Figure 10. C o r r e l a t i o n of Equation 14 © P - 0.25 gram/cm ; NaCl: 0.106-1.012 % wt 1
W
0P &P
= 0.35 gram/cm ; NaCl: 0.106-1.013 % wt 1
w
W
= 0.45 gram/cm*; NaCl: 0.0095-1.013 % wt
Johnston and Penninger; Supercritical Fluid Science and Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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SUPERCRTTICAL FLUID SCIENCE AND
TECHNOLOGY
calculated from Equations 14, 15t 16 by s u b s t i t u t i o n of the appropriate values f o r p^, K^, [HiO], C* (slope i n Figure 10) and U. A p l o t of these values, according to D
L N
K
T
T
) " d Ρ
= -
RT
Σ ν Β V
(17) K
f )
i s i l l u s t r a t e d i n Figure 11. The slope of the l i n e s i s equal to the right-hand term of Equation 17. With the A - l mechanism Σ equals zero; wt* follows therefore d i r e c t l y from the slope value as 2377 cm /grammole. With the A-2 mechanism Σ equals minus 1 and the
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1
solvent compressibility 3 therefore enters the c a l c u l a t i o n . This r e s u l t s i n values f o r AV^ from 3300 cm*/grammole at 2k MPa down to 2900 cm*/grammole at 31 MPa. The p o s i t i v e sign of AV^ indicates a decrease of the rate constant with the pressure; the magnitude of approximately 3000 cm*/grammole i s about two orders larger than values t r a d i t i o n a l l y obtained i n l i q u i d systems. An explanation f o r the p o s i t i v e sign i s not immediately available but i t suggests that the t r a n s i t i o n i s accompanied by expulsion of water molecules, which coordinate strongly i n the s u p e r c r i t i c a l range with solute molecules. Rate constant of P y r o l y s i s I t i s assumed that p y r o l y s i s k i n e t i c s have a f i r s t - o r d e r ether dependence and that p y r o l y s i s reactions are complementary to hydrolysis reactions. The f i r s t - o r d e r rate constant k follows now p
from
k t
with k
fc
(18)
= k + k. ρ η
as the apparent f i r s t - o r d e r rate constant f o r t o t a l ether
conversion and k^ as the f i r s t - o r d e r rate constant f o r hydrolysis. (Equations 1, 2 ) . The values found are plotted vs u - i n Figure 12. The scatter of the data i s inherent to the c a l c u l a t i o n of k as the Ρ small difference of two r e l a t i v e l y large numbers. Nevertheless the graph indicates k to be independent of solvent i o n i c strength and also of reaction pressure, an observation to be expected for p y r o l y s i s with radical-type, non-ionic reaction intermediate. Conclusions e
1
p
The decomposition of methoxynaphthalene occurs by two p a r a l l e l mechanisms; hydrolysis predominates at SC water density while thermal p y r o l y s i s i s dominant at zero and s u b c r i t i c a l water d e n s i t i e s . The hydrolysis i s proven to be a proton catalyzed mechanism and i s p o s i t i v e l y affected by dissolved NaCl, a l l i n agreement with the secondary s a l t e f f e c t rate law. This rate law has t r a d i t i o n a l l y been applied to l i q u i d media but the current work has proven that the same rate law applies also f o r SC water. The d i e l e c t r i c constant changes s i g n i f i c a n t l y within the SC range of densities; i t s e f f e c t on the rate constant has now also been proven to be i n agreement with correlations proposed i n the
Johnston and Penninger; Supercritical Fluid Science and Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
16.
Chemistry ofMethoxynaphthalene
PENNINGER A N D K O L M S C H A T E
^10
/ 6.0 A
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Θ
JA
\ ι /
5.0 \
A
Δ
·
« 1 25
».
\&
. . , 30
,
PRESSURE MPa
Figure 11. C o r r e l a t i o n o f Equation 17 Symbol legend: Figure 10
Figure 12. C o r r e l a t i o n o f p y r o l y s i s rate constant with i o n i c strength and solvent density Symbol legend: Figure 10
past f o r l i q u i d solvents. Water, at SC d e n s i t i e s , therefore behaves as an expanded l i q u i d and supports hydrolysis reactions o f components which do not react a t s u b c r i t i c a l temperatures. Legend o f Symbols r rate o f h y d r o l y s i s , grammole/min ki,t hydrolysis rate constant k*ijt
hydrolysis rate constant a t zero i o n i c strength
T
activity coefficient of Hi0
+
+
(Equation 14)
i n solution
Johnston and Penninger; Supercritical Fluid Science and Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
257
258
SUPERCRITICAL FLUID SCIENCE A N D T E C H N O L O G Y
t_
a c t i v i t y c o e f f i c i e n t of 0H~~ i n s o l u t i o n charge of i o n i
u
i o n i c strength of s o l u t i o n
C*
p r o p o r t i o n a l i t y constant i n Equation 14; equals Z*e (2t!N) » /(kT) s o l u t i o n concentration of i o n i , grammole/1 Avogadro Number, 6.022 χ 10* grammolecharge of electron, 1.6022 χ 10- * C Boltzmann constant, 1.380 χ 1 0 J/K d i e l e c t r i c constant of solvent temperature of s o l u t i o n , Κ s o l u t i o n concentration of ether, grammole/1 s o l u t i o n concentration of water, grammole/1
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3
Ν e k ε Τ [Ε] [HiO]
B
B
lee
3
1
1
2S
[ H J 0 ] molality of protium ion, grammole/kg Ρ s t a t i c pressure of reaction system. Pa +
AV^ R
1
a c t i v a t i o n volume, cm /grammole gas constant, 8.3134 Pa m /mol Κ 3
stoichiometric c o e f f i c i e n t of reactant i involved i n the formation of 1 grammole of t r a n s i t i o n complex Β
isothermal compressibility of reaction mixture = - ^ dV/dP
V Ρ
reaction volume, 1 density of SC water, kg/1
Literature Cited 1. Penninger, J.M.L.; In Supercritical Fluid Technology; Penninger, J.M.L; Radosz, M . ; McHugh, M.A.; Krukonis, V.J., Eds.; Elsevier Science Publ.: Amsterdam, 1985; p 309. 2. Lawson, J.R.; Klein, M.T.; Ind.Eng.Chem.Fundam. 1985, 24, 203. 3. Townsend, S.H.; Klein, M.T.; Fuel 1985, 64, 635. 4. Townsend, S.H.; Abraham, M.A.; Hupper, G.L.; Klein, M.T.; Paspek, S.C.; Ind.Eng.Chem.Res. 1988, 27, 143. 5. Antal, M . J . ; B r i t t a i n , J., DeAlmeida, C.; Ramayya, S.; Roy, J.C.; In Supercritical Fluids; Squires, T.G.; Paulaitis, M.E.; Eds.; ACS Symp. Ser. 329, ACS Washington, DC, 1987 6. Penninger, J.M.L.; Fuel 1988, 67, 490. 7. Marshall, W.L.; Franck, Ε.; J.Phys.Chem.Ref.Data 1981, 10, 295. 8. Fogo, J.K.; Benson, S.W.; Copeland, C.S.; J.Chem.Physics 1954, 22, 209. 9. Baierlein, H . ; Ph D Thesis Erlangen-Nürnberg 1983 10. Fogo, J.K.; Benson, S.W.; Copeland, C.S.; J.Chem. Physics 1954, 22, 212 RECEIVED May 23, 1989
Johnston and Penninger; Supercritical Fluid Science and Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
