TECHNICAL REVIEW
Part II. Oxidation of Glucose to K-Gluconate Platinum-Catalyzed Oxidation with Oxygen in Aqueous Alkaline Solutions Henk G. J. de Wilt' and Hessel S. van der Baan Department of Chemical Engineering, Eindhouen University of Technology, Eindhoven, The Netherlands
A survey of the literature concerning the noble metal catalytic oxidation of glucose has been included in a previous HESSELS. VAN DER BAANis Dean of the Department of Chemical Technology, Uniuersity of Technology, Eindhouen, The Netherlands. He obtained his doctorate at CToningen University (1946) and then worked with the Dutch Organization for Applied Scientific Research and the Royal Dutch Shell Group until 1965. I n that year he was appointed full professor at Eirulhoven. His main research interest is the catalytic oxidation of hydrocarbons and carbohydrates.
paper (de Wilt, 1972); however, kinetic descriptions concerning a complete conversion of the glucose are lacking. We have developed a kinetic model for the platinumcataryzed oxidation of D-glucose to K-gluconate with oxygen in aqueous alkaline solution. Proposed Mechanisms
Peroxide and dehydrogenation mechanisms for noble metal catalyzed liquid-phase oxidation of organic compounds can be found in the literature. From an extensive discussion of these mechanisms on the oxidation of carbohydrates by Heyns and Paulsen (19621, i t follows that the latter mechanism is the most probable. Experimental
Experiments were carried out batchwise under conditions
of controlled pH and temperature in a reactor equipped with
' Present address, Chemical Engineering Laboratory, St?te University Groningen, Scheikunde complex Paddepoel, Zernlke laan Groningen, The Netherlands. To whom correspondence should be addressed. 374 Ind.
Eng. Chem. Prod. Res. D&volop., Vel. 11, No. 4,1972
Platinum-catalyzed oxidation of glucose to K-gluconate i s studied in an oxygen-sparged batch reactor. In each experiment the reaction appears to be first order in glucose, and the rate constant i s inversely proportional to the initial concentration of glucose. A kinetic model describing these experimental observations i s developed. The influence of the initial concentrations of glucose, hydroxide ion, and oxygen in the liquid phase, the concentration of catalyst, and temperature on the reaction-rate constant are described. The average selectivity of the process i s about 90%.
Table 1. Reaction Conditions
Initial glucose concn, mmol/l. Hydroxide ion concn, mmol/l. Catalyst concn, g/l. Oxygen concn in liquid phase (corresponding gas pressure), atm Temperature, "C
1
I
25-250 0.007-7.2 0-2
0 2-20 25-65
a high-speed stirrer. A schematic representation of the reactor, a flow scheme of the reactor system, and the experimental procedure have been published elsewhere (de Wilt and Kuster, 1972). The finely divided catalyst was added to the reactor together with the water. The quantitative determination of glucose and gluconic acid by gas chromatography has been reported separately (Verhaar and de Wilt, 1969). Table I lists the parameters investigated. Platinum (10 wt %) on charcoal (Johnson Matthey, London) with a specific area of 800-1000 m2/g (BET) and a specific platinum area of about 20 m2/g [hydrogen titration method of Benson and Boudart (1965)l was employed as the catalyst. A typical model experiment, for which the experimental conditions are given below, is represented in Figure 1. T,
[OH],
Dll,
[Glo,
O C
rnmol/l.
rnrnol/l.
mrnoI/I.
0 072
0 75
100
55
1
*'- I
[catl,
gll.
1.00
I n most experiments the course of the reaction was followed in a simplified way, viz., by TAC. Table I1 lists the experiments actually performed. Influence of Oxygen Transport on Reaction Rate
time
mln.
Figure 1. Course of reaction as function of time
-
gluconic acid can be affected by four unwanted consecutive and parallel reactions, viz.: noncatalytic
Glucose Glucose
-
catalytic
Gluconic acid
products XI
products X2
noncatalytio
Gluconic acid
catalytw
-+
products X 3
(c)
products X4
(d)
Reaction (a): The noncatalytic oxidation of glucose has been described elsewhere (de Wilt and Kuster, 1972). About 2.5 mmol acidic products are formed per mmol converted glucose, with an oxygen uptake of 1.25 mmol. Thus, for this reaction TAC, = TOC, = 2.5. The rate of this side reaction is slow under the normal experimental conditions of the catalytic main reaction. The formation of fructose through the enolization of glucose was negligible by the gas chromatographic technique referred to previously (Verhaar and de Wilt, 1969). Reaction (b): According to the literature (de Wilt, 1972), glucosaccharic acid can be obtained from the platinumcatalyzed oxidation of glucose.
In general, the overall reaction sequence may be broken down into three steps: transport of the reactants to the catalyst surface; their adsorption, reaction, and desorption; and transport of products away from the catalyst surface. According to Nagy et al. (1966), it is probable that if any transport limitation of the reaction occurs, it would be the osygen transfer from the gaseous to the liquid phase and the oxygen diffusion rate that are responsible. This transport problem can be prevented by vigorously agitating the system to provide a sufficiently large gas-liquid contact area. From a number of experiments a t 55°C a t stirring rates above 1500 rpm, the reaction rate became independent of the stirring rate. At 2000 rpm the oxygen concentration in the liquid phase always remained above 75% of the saturation concentration. Moreover, we obtained a straight line for our Arrhenius plot of the reaction rate vs. reaction temperature yielding an overall activation energy of 18.5 kcal/mol. Both facts indicate that oxygen transfer could not be rate determining.
For this reaction the theoretical values of TOC, and TAC, are 3 and 2, respectively, and the ratio TOCITAC = 1.5. Reaction (c) : Gluconic acid can be reacted further to yield arabinose by means of the Ruff degradation mechanism (Stanek et al., 1963).
Selectivity
CHzOH. (CHOH)4.COOH
The selectivity of the catalytic oxidation of glucose to
CHzOH. (CHOH)4.CHO
+ 3/202+ Pt COOH. (CHOH)4. COOH
+ 1/20, +
CHzOH. (CHOH)S.CHO
+ CO2 + H20
Ind. Eng. Chern. Prod. Res. Develop., Vol. 1 1 , No. 4, 1972
375
Table II. Experiments Performed Initial concn,
Catalyst,
[Glo
[cat1
Hydroxide ion, [OH]
1
100 25-50-75-100 125-150-200-250 100 100
1 1 0 . 1 - 1 . 8 (Figure 3) 1
0.00727 . 2 (Figure 4) 0.072 0.072 0.072
Oxygen concn 1 in liquid, 0
r, o c 55
0.75
55
0.75
55
0.75 Oxygen pressure 1 atm 1 atm 1 atm 1 atm = 0 . 7 5 mmol/l. 1 atm
25-35-45 55-60-65
1 1.8 1.8 1.8
100 100 100 100
0.72 0.72 0.72 0.072
35 35 55 35
conic acid (d) can also be neglected since its reaction rate appears to be slow, and the ratio (dTOC/dt)/(dTAC/dt)did not increase noticeably during the experiments. The influence of reactions (a) and (b) on the selectivity can be expressed as follows: When we define the normalized initial concentration of glucose [GI0 as 1.00, the fraction of glucose noncatalytically oxidized as x, and the fraction of glucose catalytically oxidized to glucosaccharic acid as y, it can be derived that a t the end of the reaction: T.4Ce = 1 1.5 2: y, and TOC, = 1 1.5 x 2 y. From the substitution of the results of the model experiment (Figure l), it follows that 2: 0.02 and y = 0.07. In this experiment the selectivity, S,, defined as lOO(1 x - y), is 91%. This value is in good agreement with an analytically determined selectivity of about 90% a t the end of the reaction. The value x = 0.02 agrees with electrophoretically determined concentrations of arabinonic acid and formic acid a t the end of the catalytic reaction.
+
+
+
+
f:
Kinetic Model
0.1
0
10 -
20
30
40
50
60
70
80
90 100 time m i n .
Figure 2. Semilogarithmic plot of conversion vs. time
Owing to this consecutive reaction, TOC can exceed the value 1.00, but TAC is not affected because a t the actual hydroxyl ion concentration, carbon dioxide is a monovalent acid. We found experimentally that a t 55°C and [OH] = 0.7 mmol/l., starting from a gluconic acid concentration [Galo of 100 mmol/l., this reaction proceeds slowly. The initial rate d(Ga)/ d t , as determined by TOC, is lower than 0.001/hr. Reaction (d) : We also investigated the catalytic oxidation of gluconic acid. At [Galo = 100 mmol/l. with [OH] = 0.7 mmol/l. and [cat] = 1 g/l. an initial rate, d(Ga)dt of about O.Ol/hr a t 55OC was obtained a t which the ratio TOCITAC was about 2.0 to 2.5. This ratio can be explained by the coincidence of the above-mentioned reaction (c) and the oxidation of gluconic acid to glucosaccharic acid, for which TOCITAC = 2.0.
H20_ G . H ~ O
G*H20
+G.HzO+
G.H20+ -+ Ga+
+ Hz+ i+
i, adsorbed on the catalyst (Pt) surface
Ga+
HP
+
[b1
[c]
+Ga
[dI
+ o1 HzO
[e1
Reaction [a] is considered to proceed instantaneously and completely owing to the large excess of water. Reaction [e] is supposed to be rapid, so that reaction [e] can be considered irreversible. The part of the active catalyst surface area occupied by Hz+is negligibly small. As a result of these considerations, the above system can be simplified to a nondissociative monomolecular reaction onthe catalyst surface: G . HzO e Ga, which can be generalized to: G.H20+ + Ga+ kl
(1)
I;-1
The influence of the degradation of gluconic acid (c) on the selectivity can be neglected. The catalytic oxidation of gluInd. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No.
G
G + a Z G +
Discussion and Conclusions
376
From the dehydrogenation mechanism, the following kinetic model for the platinum-catalyzed oxidation of glucose to gluconic acid was developed:
4, 1972
G + -% Ga+
(2)
__ kr
Ga+
Ga
k-3
+u
(3)
According to the theory of Langmuir and Hinshelwood, we assumed that the heat of adsorption of any compound on the catalyst surface is independent of the degree of occupation of the active sites and also that G or Ga can be adsorbed on the same sites. Consequently, the following rate equations can be derived: 9-11
d[G+] rz=----
at -
d[Ga+]
=
ky9g. [ u ]
(5)
Figure 3. Reaction-rate constant as function of catalyst concentration
which means the adsorption energies of glucose and gluconic acid on the catalyst sites are almost equal, or second, that The accumulation of G and/or Ga on the platinum surface can be neglected. Consequently, a t any moment t > 0, a steadystate condition is attained, so r1 = rz = r3. Elimination of eo and Oea from the above equations leads to d[GI -_ _dt
+
( k l k ~ klk3) [GI
+
kikzka[G][u] (klk3 k2kJ [Gal
+
+ (k--lk3 +kZk3)
(7)
kl,
k-3
- and -
ki
k3
are small, which means both G and Ga are weakly adsorbed on the catalyst sites. From a series of experiments in which the initial glucose concentration [GI0 was varied, CY is proportional to ~ / [ G ] o . This observation excludes the second possibility and allows simplification of Equation 11 to
This equation becomes considerably simplified when the oxidation step (Equation 5 ) is assumed to be rate determining.
Therefore, the oxidation of glucose to gluconic acid can be described kinetically by
A plot of In [G]/[G]O vs. time (Figure 2) gives a straight line for the conversion range of 10-90yo, which implies that over that range the kinetics can be described as first order in glucose, Le.: _ _ d[G1 _ =
a[G]
(13)
A standard overall reaction-rate constant has been defined as: ko CY a t [cat] [ u ] = 1 g/l. and [GIo = 100 mmol/l., in which ko is dependent of [OH], [Ol], T , and the type of catalyst. Both a and ko can be determined from semilogarithmic plots of the conversion vs. time (Figure 2).
dt in which
Influence of Some Reaction Conditions (de Wilt, 1969)
Ly=-
1
kl + -[GI + +[Gal k-1
= constant
(10)
k3
By the approximation that the selectivity is 1 0 0 ~ o(as opposed to the 90% actually found), Equation 10 can be transformed with the substitution of [Gal = [G]o- [GI into
The observed constancy of a over the range from [GI [GI0 to [GI = 0.1 [Gloimplies that
_ -- -
kl k-i
IC-3
k3
=
0.9
The influence of the catalyst concentration on the reactionrate parameter CY is given in Figure 3. ilt a given ratio [G]o/ [cat], an absolute quantity of catalyst appears to be ineffective. A selectivity, S,, of about 90% a t the end of the reaction becomes almost independent of the catalyst concentration above 0.8 g/l. As already stated, the reaction-rate parameter is inversely proportional to the initial glucose concentration a t a constant effective catalyst concentration. The selectivity, S,, slightly increases a t decreasing [GIo owing to a relative increase of a poisoning effect by the initial glucose. The influence of the hydroxide ion concentration on IC0 and S, is represented in Figure 4.The loss of selectivity a t higher [OH] is caused by the increasing influence of a consecutive oxidation of gluconic acid to glucosaccharic acid and a noncatalytic oxidative degradation of glucose. From the influence of the temperature (25-65OC) on ko, an overall activation energy of 18.5 kcal/mol has been found. The selectivity increases slightly a t decreasing temperature. Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No.
4, 1972 377
I 0.09
0 0.03
0~
.
0
6
1 8
I
I
9
10
0.072
0.72
~
I 11 1.2
pHi5S0C1 [Of!] mmOl/l
Figure 4. Reaction-rate constant as function of hydroxide ion Concentration
We investigated the influence of the oxygen concentration in the liquid phase by a series of experiments with various oxygen pressures from 0.2 to 20 atm in a stainless steel reactor. 4 scarcely decreasing tendency of S, and a weak increase of k were observed at higher oxygen pressures. This small effect is in agreement with our assumption that the degree of occupation of active sites by hydrogen is relatively small. Nomenclature
OH
0, G
378
= = =
hydroxide ion dissolved oxygen in the liquid phase glucose
Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No.
Ga = gluconic acid T = temperature, “C TAC = total alkali consumption relative to [GIo, mmol OH-/mmol G TOC = total oxygen consumption relative to [GIo, mmol 0 2 X 2/mmol G S = selectivity, ([Gal/[G10 - [GI). 100, % k = reaction-rate constant ko = overall reaction-rate constant, min-1 t = time, min [iJ = concentration of compound i, mmol/l. [ i ] ~ = concentration of i a t t = 0, mmol/l. [cat] = concentration of catalyst, g/l. i. = value of i a t end of the reaction u = active catalyst site e = degree of occupation of the present number of active catalyst sites literature Cited
Benson, J. E., Boudart, M., J . Catal., 4,704 (1965). De Wilt, €1. G. J., Ind. Eng. Chem. Prod. Res. Develop., 11 (4),370 (1972). De Wilt, H. G. J., PhD thesis, University of Technology, Eindhoven, The Netherlands, 1969. De Wilt, H. G. J., Kuster, B. F. A I . , Carbohyd. Res., 19, 5 (1471 j - _ . - \,. Heyns, K., Paulsen, H., Advan. Carbohyd. Chem., 17, 169 (1962). Nagy, F., Petho, A,, Moger, D., J . Catal., 5 , 348 (1966). Stanek, J., Cerny, M., Kocourek, J., Pacak, J., “The Monosaccharides,” p 114, Academic Press, Xew York, N.Y., 1963. Verhaar, L. A. Th., de Wilt, H. G. J., J . Chromatoyr., 41, 168 (1969). RECEIVED for review August 27, 1971 ACCEPTEDSeptember 14, 1972
4, 1972
