Polish J. of Environ. Stud. Vol. 15, No. 6A (2006), 165-168
Catalytic Decomposition of Mtbe over Carbon Catalysts G. S. Szymański Faculty of Chemistry, Nicholas Copernicus University, Gagarina 7, 87-100 Toruń, Poland, Fax: (056) 654 24 77, e-mail:
[email protected]
Abstract Decomposition of methyl fcrt-butyl ether (MTBE) in the gas phase was studied using carbon catalysts with chemically modified surface. Carbon samples with different surface chemical properties were obtained from commercial activated carbon D43/1 (CarboTech, Essen, Germany) by oxidation in liquid phase with various oxidants. The catalytic tests were performed in a flow reactor at a temperature range of 423 - 523 K. Isobutene and methanol are the only products of the methyl /ert-butyl ether (MTBE) decomposition. The generation of surface acidic oxides enhances the catalytic activity whereas their removal (by HTT) diminishes it. However, the activity is controlled not only by the number and strength of acidic groups, but also by their accessibility. K e y w o r d s ; activated carbon, MTBE, decomposition, catalysis
Introduction Treatment of MTBE-contaminated water is focused on five technologies: air stripping, sorption with granular activated carbon, biofiltration, chemical oxidation (with ozone, ozone/hydrogen peroxide or ultraviolet/hydrogen peroxide), and separation using hydrophobic hollow fiber membranes [1]. Air stripping and the hydrophobic hollow fiber membranes are the most cost-effective options, but it is necessary to consider the additional treatment of MTBE vapors in the gas phase, using either adsorption or catalytic destruction. One alternative is to use gas phase catalytic decomposition with or without oxygen. To increase the rate of the reaction, the gas-phase is usually heated before decomposition, requiring the use of an external fuel source since the MTBE vapors are usually too dilute to sustain a high temperature in the catalytic reactor. Lower treatment costs can be achieved if the catalytic decomposition can be conducted at relatively low temperatures. However, only heteropolyacids and ion-exchange resins are catalytically active at such temperatures. But they have some drawbacks. They strongly adsorb polar substances, like water and alcohols, the presence of whidM^Gongly retards the reaction [2].
Granular activated carbon can be used not only as an adsorbent or catalyst support but also as true catalyst. It was reported earlier that suitably oxidized activated carbon effectively promotes dehydration of aliphatic alcohols C2-C4 at relatively low temperatures due to the presence of acidic sites of moderate strength [3]. In addition, MTBE demonstrates low affinity towards granular activated carbon. Thus, it seems reasonable to study catalytic behavior of modified activated carbons in the MTBE decomposition reaction, too.
Experimental Commercial granulated activated carbon D43/1 (Dl), obtained from Carbo-Tech GmbH (Essen, Germany), was used in the present investigations. Portion of this carbon was deashed with cone. HF and HC1 (D). Other portions of the D43/1 carbon were oxidized in liquid phase using (i) 65% HNO3 (353 K, 3 h), (ii) Hummer's reagent (338 K, 3 or 1 h) and (iii) (NH4)2S20g (353 K, 12 h). All oxidized samples were washed with redistilled water untill the rest of oxidant and low molecular products of oxidation were removed. Subsequently, the samples were dried at room temperature and then desorbed in a stream of helium at 498 K for 3 h to remove, unsoluble in
166
Szymański
water, low molecular products of oxidation. The samples are designated as D 10x1498, D 10x2498, D 10x3498 and D 10x4498, respectively. In additon, a part of the carbon oxidized with nitric acid was desorbed at 523, 548 and 573 K. (D10xl523, D10xl548 and D10xl573 samples, respectively). All the prepared samples were then stored in ambient air.
C1s
1?
8000
£
6000
Ii!
296
294
292
290
288
286
284
282
280
BE[eV] D10x1498
(J. .S'.
C 1s
The porous structure of the carbon samples was elucidated from low-temperature (T = 77 K) nitrogen adsorption data (ASAP 2010 analyzer, Micromeritics, Atlanta, USA| by applying the D-A equation and by mercury porosimetry (Carlo Erba 1500, Milan, Italy) [4]. The acid-base properties of the modified carbon surface were estimated by acid-base titrations according to the method of Boehm [5]. The surface chemical composition of the samples was determined by the X-ray photoelectron spectroscopy (a VSW spectrometer). More details on carbon characterization are given elsewhere [4, 6], The decomposition of MTBE was investigated in a fixedbed flow-type microreactor with a micropulse technique. The product analysis was performed by on-line gas chromatography. The catalytic tests were conducted in the temperature range of 423 -523 K.
.-§■ II c
i £
6000 -
Results and Discussion 296
294
292
290
288
286
284
282
280
BE [eV) 5500
01s
D
c=o -
5000
4500 ■
& IA C
1 '^ \ i
4000 -
\
\ \\
II
i v
//
II 11 11
S £
C-OH H,0.0,
3500
/ / / /
3000 •
\ V
\ \
x\ \ y \\ / \
2500 •
2000 ■ 540
538
536
534
532
530
528
526
BE[eVJ 7000 -
D10x1498
0 1s
c=o -
6000
/
y\
\
1 i \ 1 i \
5000
li
4000
/'
',-■• i
/
3000
—wg
/ i.. /
C-OH H,0.0,
i
\ \
\f\
A \ \
i \\
\
\\
2000 ■ 542
540
538
536
534
532
530
528
526
BEIeVT^ai
Fig. 1. Cis and Ols XP spectra of the Dand DIOxl498 carbons
The estimated pore volume of the carbons indicates that they exhibit a well-developed porous structure (Table I). Oxidation of the activated carbon with liquid oxidants changes its porous structure slightly only. As for other carbon materials, some decrease in the pore volume after the oxidation was observed. This is a consequence of fixation of a part of surface oxygen complexes at the entrance of the pores, which increases their constriction (in other words, not all the micropores are accessible to N2 molecules), or of widening of the microporosity [4]. The results of acid-base titrations (Table I) show that the unmodified carbon and its oxidized forms demonstrate mainly acidic properties. The higher the oxygen content, the higher the acidity. The application of several oxidants with different reactivity leads to generation of various amounts of surface oxides with different acid-base properties. This is also confirmed by the XPS measurements. According to the XPS results (Fig.I) and previous XPS and FTIR data [6], various oxygen groups, like carbonyl, hydroxyl, carboxylic and their derivates, such as lactones and carboxylic anhydrides, are present on the surface of the oxidized D43/1 carbon samples. The higher surface oxygen content, the lower the C-OH/C=0 ratio. Among the oxidized samples, the carbon treated with nitric acid (D 10x1498) exhibits the highest acidity while the deashed sample (D) demonstrates the lowest acidity, due to the presence of mainly weak acidic groups (Table 1). The catalytic results show that isobutene and mcthanol arc the only products of the methyl 'er/-butyl ether (MTBE) decomposition. All catalysts are active in the MTBE decomposition in the investigated range of temperature. Even the carbon sample with the lowest surface acidity (D) demon-
Catalytic Decomposition ofMtbe...
167
80 -
MTBE
70 -
0
•
8
O
•
■
P'60o
C
.2
a
■ ■
A
50 -
•
CO
u o> > 40§ O
D
o
■
30 -
•
•
0
A
20 -
■
□ %
■ A
D A
D D10x1498 D 10x2498 D 10x3498 D 10x4498
e
10 420
1—
1
1
430
440
i
460
450
470
Temperature [K] 80
i
MTBE O 70
■ O
•^
60
c O '55
50
■ • O
□
C O
o
•
0
40
■ •
1
• o
■
30 H
D
D
20 420
D10x1498 D10x1523 D10x1548 D10x1573
1
1
i
430
440
450
Temperature [K] Fig. 2. Catalytic activity of the modified carbons
3,2
MTBE, XPS 80
3,0
▼
T
▼
60 H
T
2,8
♦i
r 2,6
D
2,4 9
♦ i
O
h2,2 £
40-
O
>
c o o
2,0
▼ 20 H
▲
♦
▲
T D
9
^
1,8
443 K 453 K 463 K -C-OH/>C=0
10
1,6 1,4
11
12
13
14
Oxygen content [at%]
Fig. 3. The effect of oxygen content on the catalytic activity
1,2
O
168
Szymański G. S.
strates reasonable catalytic activity (Fig. 2). This indicates that weak acidic groups play also the role of active sites in the MTBE decomposition. The carbons treated with cone, nitric acid and then partially desorbed at 523 K (D 10x1523) are most active. The increase in the catalytic activity, after partial desorption of surface oxides, suggests that there exists an optimum acidic oxygen group content. This conclusion is supported by the XPS results (Fig. 3). It seems that, as in the case of fór/-butanol dehydration over oxidized carbon [4], steric effects play an important role in this process. Thus, the activity is controlled not only by the number and strength of acidic groups, but also by their accessibility. When their concentration is too high, the activity decreases due to steric hindrances. In addition, the reasonable catalytic activity of the samples containing only weak surface acidic oxides indicates that these groups play also the role of active sites in this process.
Conclusions The deashed activated carbon D43/1 promotes effectively the catalytic decomposition of MTBE at a temperature range of 423 - 523 K. Isobutene and methanol are the only products. The generation of additional surface acidic oxides enhances the catalytic activity further. However, the activity is controlled not only by the number and strength of acidic groups, but also by their accessibility.
References 1.
2.
3.
4.
5.
6.
MITANI M. M., KELLER A. A., GOLDEN S.J., HATFIELD R., CHEETHAM A.K. Low temperature catalytic decomposition and oxidation of MTBE. Appl. Catal. B: Environ. 34, 87, 2001. USHKUBO T., WADA K. Preparation, characterization, and catalytic activities of silica-supported tantalum oxide for the vapor phase decomposition of methyl /
