3. Acid Catalysis As stated already in chapter 2, the hydrolysis of pentosan to pentose and the dehydration of pentose to furfural are both catalyzed by acids. It is, therefore, appropriate to give a brief summary of important features of acid catalysis.
3.1. The Temperature Dependence of Acidity In the second half of the nineteenth century, Svante Arrhenius (1859-1927) found the rate of acid-catalyzed reactions to be proportional to the hydrogen ion concentration. Although it turned out later that this is only a special case of a more general proton transfer concept, the hydrogen ion concentration remains an important aspect in acid catalysis. In general laboratory practice, acid catalysis is commonly carried out at only slightly elevated temperature. Under such conditions, strong mineral acids such as hydrochloric acid and sulfuric acid are usually considered as "completely dissociated". This in itself is erroneous, and, worse yet, at any given acid concentration the hydrogen ion concentration diminishes with increasing temperature, and the extent of this phenomenon differs from one acid to another. The reason for this lies in the fact that the dielectric constant of water, responsible for the dissociation of the acids, diminishes strongly with increasing temperature [5] as shown in Figure 3. For hydrochloric acid, sulfuric acid, and phosphoric acid, the resulting decrease in acidity [6] is illustrated in Figure 4. The same trend is seen in the temperature dependence of the dissociation constant of acetic acid [7] shown in Figure 5, and of the dissociation constant for the second dissociation step of sulfuric acid [8] shown in Figure 6. For a kinetic appraisal of furfural processes, which are universally carried out at temperatures in excess of 150 ~
the decrease of acidity with increasing temperature brings
about a major problem as in all kinetic studies made for furfural to date, for obvious reasons of convenience, it has been customary to formulate the reaction rate as being proportional to the initial hydrogen ion concentration measured before the reaction, at room temperature, although in reality, at the high reaction temperatures of interest, the acidities are quite different. In view of the temperature dependence of the acidity being different for different acids, this means that when a kinetic formulation with the initial hydrogen ion concentration is derived from reaction experiments with hydrochloric acid, this formulation cannot be
,oo[ ( 80
4O
20
100
200 TEMPERA TURE, ~
3 0
Figure 3. The Dielectric Constant of Water as a Function of Temperature.
100 0.1 N HCl
80-
6O
U4
~o e~
2O
----.e
~
.1 N H3Po ~
i
I
100 20O TEMPERA TURE, ~
Figure 4. The Hydrogen Ion Concentration of Various Acids as a Function of Temperature.
10
100
I0
"o...
s
v.,..
0.1
0.001
0
200 TEMPERATURE, ~C
300
Figure 5. The Dissociation Constant of Acetic Acid as a Function of Temperature.
lJ
10/+ L~ ,,..M
...., 103
(:b
~g
100
~o H S O 4g ~
5
H + . S OS~
100
150
Z00
TEMPERA TURE, ~
Figure 6. The Dissociation Constant for the Second Dissociation Step of Sulfuric Acid as a Function of Temperature.
11
applied to sulfuric acid, and vice versa, and it is fundamentally objectionable to pack the temperature dependence of the reaction rate into a single term (the "exponential factor" containing the activation energy) when in reality there are two different and opposing effects of increasing temperature, one being due to the given decrease of acidity, and the other being due to the growing energy of the molecules.
3.2. The Proton Transfer Concept The claim of Arrhenius that the rate of acid-catalyzed reactions is proportional to the hydrogen ion concentration was soon found to require amendments as catalytic effects were discovered where the hydrogen ion concentration was negligible. In view of this predicament, T. M. Lowry [9] created a generalized proton transfer theory. For the most simple case of a mere rearrangement (isomerization) of a molecule, this theory can be outlined as follows: Any acid-catalyzed reaction consists of three steps: (a) An addition of a proton to the molecule to be converted. (b) A rearrangement of the molecule activated (destabilized) by the added proton. (c) A withdrawal of the added proton to yield a neutral product molecule. The species adding the proton is called "proton donator", and the species withdrawing the proton is called "proton acceptor". Against this background, in Lowry's words, the overall catalytic process can be seen as if a voltage is applied to the molecule to be converted (addition of a proton at one point of the molecule, and withdrawal of a proton at some other point of the molecule), so that an electric charge (an electron deficiency) is pulled through the molecule. Very instructively, Lowry speaks of "proton sources" and "proton sinks", thereby underlining the important fact that acid catalysis requires two agents ("terminals" in Lowry's words), whereas the concept of Arrhenius gave the erroneous impression that only one agent is involved (the hydrogen ion). An illustration of Lowry's concept of acid catalysis is given in Figure 7, where S is the molecule to be converted (rearranged) to T. The proton donators cited as examples are the oxonium ion H30 +, an undissociated acid molecule HA, and water as a special case of HA, while the proton acceptors cited as examples are water (transformed to H30+), and the acetate ion (transformed to acetic acid). Contrary to the concept of Arrhenius, Lowry's concept can explain why water as a proton donator and acetate ions as proton acceptors represent a power-
12
DONATOR REACTIONS
S . N30§
l ]
SH §
TH*
SH+ §N20
S +HA --~-SH + +A-
_] ACCEPTOR REACTIOIVSTH*+ H2 0 --~ T . H3 O+
TH§ CH3 CO0- --,-T +CH3COOH
S +H20 "-'~SH§ OHFigure 7. Lowry's "Voltage Model" of Acid Catalysis.
ful catalytic system even when the hydrogen ion concentration is insignificant. With acetate ions voraciously "sucking up" protons, this system has a high "catalytic voltage". Consequemly, at high temperature (180 ~
furfural can be effectively produced with water as the
"catalyst", even when the carboxylic acids liberated from the raw material are neutralized by an excess of calcium carbonate [ 10]. Lowry's ideas extended the notion of an acid to that of a substance capable of acting as a proton donator, so that, in his terminology, even pure water is an acid. On the other hand, the notion of a base was extended to that of a substance capable of acting as a proton acceptor, so that, in Lowry's terminology, water is a base as well. Hence, water turns out to be an amphoteric substance of central importance for catalytic processes. When the catalysis is supported not only by hydrogen ions but also by other speccies Xi, the reaction rate is expressed as r = k0 [H +] + kl
[XI] a
+k2
[X2] b + .......
which is commonly referred to as the equation of "general acid catalysis". For kl, k2,... = 0, this relationship degenerates to r = k0 [H +] known as the equation of "specific acid catalysis". Thus, "specific acid catalysis", representing the ancient finding of Arrhenius, is merely an approximation of "general acid catalysis", sometimes a fairly good approximation, but a totally unacceptable approximation in other cases. In furfural technology, rate equations based on the assumption of "specific acid catalysis" are sufficiently accurate when use is made of high hydrogen ion concentrations pro-
13
duced by strong mineral acids such as H2804, but such formulations fail completely when the only "catalyst" used is water.
References [5] E. U. Franck, Z. physik. Chemic, Neue Folge, 8 (1956) 107-126. [6] A. A. Noyes, A. C. Melcher, H. C. Cooper, and G. W. Eastman, Z. physik. Chemie 70 (1909) 335-377. [7] A. A. Noyes, Y. Kato, and R. B. Sosman, Z. physik. Chemic 73 (1910) 1-24. [8] E. U. Franck, D. Hartmann, and F. Hensel, Discuss. Faraday Soc. 39 (1965) 200-206. [9] T. M. Lowry, J. Chem. Soc. 1927, 2554-2567. [ 10] S. I. Aronovsky and R. A. Gortner, Ind. Eng. Chem. 22 (1930) 264-274.