Oxidation of organic solvent vapours on Pt-catalysts contaminated with paint components

Atmospheric Environmew Vol. 13. pp. 335-339. Pergamon Press Ltd. 1979. Printed inGreatBribin.

OXIDATION OF ORGANIC SOLVENT VAPOURS ON Pt-CATALYSTS CONTAMINATED WITH PAINT COMPONENTS KAZUKI

NATSUKAWA

KENJI

and

YASUDA

Chemistry Division, Osaka Prefectural Industrial Research Institute, Enokojima, Nishi-Ku, Osaka 550, Japan (First received 19 June 1978 ond inJinalform

12 September 1978)

Abstract - Oxidation of solvent vapours on the several poisoned platinum-catalysts was studied with the differential thermal analyser and the catalytic combustion apparatus on the laboratory scale. Differential thermal analysis (DTA) revealed that two patterns existed among the DTA curves for the combustion on the contaminated catalysts. The one pattern showed a shift ofexothermic peak to higher temperature than on the fresh catalyst, on which most of the solvents gave DTA curves with simple exothermic peaks. The other pattern showed rather complicated DTA curves on some other catalysts, which were contaminated mainly with heavy metals. The oxidation of commonly used solvents on contaminated catalysts in the combustion apparatus often yielded a considerable amount of partial oxidation products, i.e. carbonyl compounds, which are sometimes more toxic than the original solvent compounds

INTRODUCTION

Because of its low cost and high effect, catalytic combustion has been widely adopted for the air

pollution control, especially for the treatment of industrial exhaust gases containing organic compounds. The method is, however, still occasionally applied to some gases such as exhausts from paint booths, since the heavy metal contaminants in the gases rapidly poison the catalysts. A number of studies have been presented on the poisoning of automobile exhaust control catalysts. Sakai and Enokida (1976), Kummer (1975), Suzuki (1977), and Fishel et al. (1974) have clarified the mechanisms of deactivation of catalyst on the poisoning by sulfur and lead and on the exposure to high temperature in the case of the oxidation of lower hydrocarbons and carbon monoxide. But few papers have been published on toxic actions of the heavy metal contaminants on platinum-catalysts in the case of the catalytic combustion of the exhaust gas from paint booths. This study was, therefore, made for a survey of illeffects of catalytic poisons on the application of catalytic combustion to the exhaust gas from paint factories. Toluene, n-hexane, n-propyl alcohol, ethylacetate, and methylethylketone were chosen as organic solvents because of the wide use in the field of painting, and zinc, copper, iron, titanium, chromium, and lead were chosen as contaminants of the catalyst because of the high frequency of existence in some paints. Zinc, copper, and iron were found in the order of a few per cent in some poisoned platinum-catalysts which had

operated in air pollution control treatment (Inagaki, 1974). Lead is the well-known catalytic poison, but it was also claimed that a small amount of the metal accelerates the catalytic combustion of automobile exhaust gas (Stephens, 1969). Hydrochloric acid was also added to the list ofcontaminants which is produced by the oxidation of chlorinated organic solvents and sometimes used as the regenerating reagent of the lead-contaminated catalyst (Univ. Oil Prod. Co., 1964). been

EXPERIMENTAL

(1) Catalyst and reagent The catalyst used in this study was from Japan Engelhard Industry (DASH-220, Platinum-content 1.8 + 0.1 g I-‘, 4-6mm$, Support y-Al,03). Hydrochloric acid and metal salts; zinc nitrate, copper sulfate, ferrous ammonium sulfate, chromium nitrate, lead nitrate, and titanium trichloride were of reagent grade. (2)

Preparation

of contaminated

catalyst

The catalyst was dipped in an aqueous solution containing contaminant in molar ratios of 1, lo,50 and 100 to platinum and the solution was condensed together with the catalyst on the water bath. The catalyst with the condensate was dried at 100°C for 1 h and then heated at 400°C for 4 h. (3) Differential thermal anfzlysis Gallagher et 01. (1976) and Wedding and Ferrauto (1974) investigated the activity of catalysts with DTA. The ap paratus was so designed that the sample gas containing lower hydrocarbons and carbon monoxide might pass through the furnace in a constant flow. The authors, however, employed the following procedure. One particle (15-20 mg) of the catalyst and a similar amount of a-Al,O, were put in a sample pan and a reference pan, 335

336

KAIIIKI NATXIKAWA and KENJI YASLW~

outlet of the apparatus was introduced mto the methanolic solution of 2,4_dinitrophenylhydrazine. The hydrazone derivatives precipitated wereisolated and identified with NMR. IR, and TLC by comparing with the hydrazones of acetaldehyde. propionaldehyde, and acetone. which are expected to be produced on the partial oxidation of sample solvents. i.e. ethylacetate or ethylalcohol. n-propyl alcohol, and i-pr<>pyl alcohol.

RESLLTS AND DISC’C’SSION (1) Diflhmtial

-j_ Fig. I. DTA apparatus: (1) the fresh or contaminated catalyst in sample pan; (2) a-Al,O, in reference pan; (3) alumel wire; (4) chrome1 wire ; (5)chrome1 plate : (6) furnace: (7) lid; (8) aluminum jar.

respectively, on a dambcl type detector in Fig. 1. Immediately after dropping 1 or 2 ~1 of solvent onto a-AIZO,, the furnace was covered with a lid, and heated at a programmed rate of 20°C min- ‘. The deviation of the temperature of the catalyst from the reference was recorded on the DTA chart showing any reaction of the solvent vapour, such as vaporization from a-Al,O,, diffusion in the cell and adsorption on the catalyst, and oxidation.

The gas containing a solvent in a constant concentration was introduced into the combustion chamber in a constant flow, and the temperature of the chamber was controlled and recorded automatically. Gas samples were collected with a gas syringe from rubber plugs before and behind the catalytic reactor and subjected to gas chromatography. (5) Identification

of partial

ouitiatiorl

product\

The gas chromatograms showed minor peaks assumed to be corresponding to partial oxidation products in some cases. The apparatus was operated in the optimum conditions for the production of such compounds. and then the gas from the

:“\I

thermal unulysis

When the extent of poisoning of the catalyst was less than equal molar ratio (contaminant/platinum), the DTA curves similar to those for the fresh catalyst were obtained for the contaminated catalyst in each case of the five solvents. But, as the amount of the contaminant increased, the curves became more or less different from those for the fresh catalyst. The effect of the contamination appeared in the DTA curves was tabulated in Table 1. Typical examples of the DTA curves are shown in Fig. 2, in which those for the oxidations of toluene on the fresh. chromium-coated, and hydrochloric acid-treated catalysts are indicated by solid, dotted, and solid and dotted lines, respectively. The peaks which appeared at a lower temperature than 1OO~C on the curves indicate only the vaporization of the solvents from the reference pan and are of little importance for the present discussion, hence it is suggested that the peaks which first appeared after these peaks be called “first peak”. The first peak at 221°C in the solid line showed the exothermic reaction of toluene vapour on the fresh catalyst. Since the gas chromatography revealed the absence of toluene in the cell after the end point of the peak at 221’C, the peak can be assumed to correspond to the perfect oxidation of the solvent on the fresh catalyst. The DTA curves for the oxidation of the solvents on the contaminated catalyst were grouped into two classes. The contamination by hydrochloric

5%

*cc

Temperature, Fig. 2. DTA curves for the oxidation

400

“C of toluene

on Catalysts

337

Oxidation of organic solvent vapours on Pt-catalysts Table 1. The furnace temperatures at the top of exothermic peaks by the catalytic oxidations of solvents DTA peak temperature (“C)

Contaminant Molar* ratio Nonet HCI

CU

Fe Ti Zn Cr Pb

0

10 50 100 100 100 100 100 50 100 50 100

CH,-C,H,

n-C,H 14

221 249 258 308 264 255 258 253 249,366 181,253,349 291 327

187 213 256 243 228 212 236 (190) 282 180,245 223 293 307

CH$OC,Hs 159 190 207 232 190,241 197 223 180 153,312 140,191,297 225,278 215,307

C2HSOCOCH, 165, (267): 195 188 256 250 233 212 181,308 157,319 193,330 185,282,337 223,311

n-&H,OH 163 185 205 244 248 198,230 259 170 141,181,280 126,189,263 180,277 220,300

* Molar ratio = contaminant/Pt. t The fresh catalyst. t The broad peak.

acid, titanium, and lead chromate yielded the curves with single peak shifted to the higher temperatures, as is shown by the solid and dotted lines in Fig. 2. This is probably due to the decrease of active site on the catalyst. The DTA curves of another type, which have two or more peaks as shown by the dotted line in Fig. 2, were resulted from the contamination with chromium and lead. In these cases, the first peaks often appeared even at lower temperature than those expected for fresh catalyst, and hence another reaction prior to the perfect oxidation is considered to take place easily on these catalysts. The metal salts added, except copper sulfate which remained as it was, were considered to change to the corresponding oxides under the condition of the contamination employed. The oxides of chromium and lead are known as the catalyst in the petroleum industry for obtaining various oxygencontaining compounds from hydrocarbons. It still seems risky, however, to assign simply one peak to one oxidation stage, because carbonyl compounds, the main partial oxidation products of the solvents, appeared in the cell at lower temperatures than the beginning of the first peaks. The oxides apparently play an important role on this abnormal combustion, but they could not give similar DTA curves to those for the contaminated catalyst by themselves. The complicated curves will result from a joint action of the oxides and platinum metal. Toluene and n-hexane were comparatively less affected by the contamination by these oxides, and yielded the DTA curves with single peaks in many cases. n-propyl alcohol and ethyl acetate were, on the contrary, apt to produce complicated curves. (2) Catalytic combustion on the laboratory scale The catalytic combustion of five solvents was carried out using three catalysts, poisoned by hydro-

chloric acid, chromium, and lead, which showed characteristic patterns in the DTA curves, and fresh catalyst. The relationship between the amount of hydrochloric acid added and the removal ratio of the solvent in Fig. 3 is accepted; increasing the amount of catalytic poison results in a decreasing of activity of catalyser. This seems to be in conflict with the report by Yamanaka (1975), who pointed out that the catalyser kept its activity even on exposure to the gas containing 400ppm of hydrogen chloride at 4OO’C. This difference may be due to the presence of water in the preparation of the contaminated catalyst. Accordingly, the operation using the hydrochloric-acidpoisoned catalyst needs higher temperature for the same effect as the fresh catalyst. Such a condition, however, is generally unadvisable for economic reasons and the probable risk of further deactivation of catalyst which may be caused from an alteration in the surface of platinum catalyst by exposure at high temperature. Miyazaki (1972) observed an alteration in crystalline orientation and Dalla (1976) reported the growth of platinum particle at the elevated temperature. As shown in Fig. 3, the addition of chromium and lead compounds to the catalyst gave the interesting result that the removal ratio of the solvents was seemingly improved. The removal ratio in Fig. 3 was calculated only from the solvent peaks in the gas chromatograms and the minor peaks, which indicated the presence of partial oxidation products, were not included in the calculation. Accordingly, it is tempting to deduce that this improvement is due to the production of partial oxidation compounds in many cases. But the results obtained are not enough to reject entirely the other consideration viz. that the contamination by these metals increases the oxidation activity of the catalyst. The minor peaks on the gas chromatograms are

338

KAZUKI NATSUKAWA and KENII YAS~JIJA

n - Hexane

Space

velocity

HCI/Pt A

molar 0 IO

240 pprn 12000 h ratio

F

Space Cr/Pt l

Temperature

of

catalyst,

veloaty 12000 molar ratto 0 100

hr

OC

Fig. 3. Removal ratio of solvents in the catalytic combustion experiments

Table 2. The temperature for the maximum yield of partial oxidation products and their relative retention times on the gas chromatograms Solvent

Contaminant

Temperature (“C)

RR,*

Yield (‘:,)t

C,H,OCOCH,

HCI Pb Cr

200 180 120

0.6 1 0.64 0.62

n-C,H,OH

HCI Pb Cr HCI Pb Cr

140 130 120

0.44 0.44 0.44

6.5 4.6 1.5 14.4 14.3 16.3

I 60

0.72

1.2

C2H,COCH,

* Relative retention time based on solvent. t The area ratio of outlet peak of partial oxidation product to inlet solvent peak on the gas chromatograms obtained by the column packed with Shimalite 60/80 mesh coated with lo”,, polyethylene

glycol 6000.

tabulated in Table 2. The peaks for the contaminated catalysts have the same RRt value for each solvent and were assigned to the corresponding carbonyl compounds : experiment 5). Judging from the production of same carbonyl compound, a similar reaction is expected to occur independently of the contaminants which gave the different DTA curves and removal ratios. Further investigation seems necessary for clarifying the mechanism of the catalytic poisoning.

were distinct from those for a fresh catalyst. Substantial partial oxidation compounds were produced on the contaminated catalysts in some cases. Acknowledgements The authors wish to thank Miss lkuko Satake for identification of partial oxidation products and Mr. Takuji Yamashita (Department of Applied Chemistry, Faculty of Engineering, Kinki Univ.) for his help in carrying out these experiments.

CONCLUSION REFERENCES The DTA analysis was found to be a useful method to test the activity of contaminated catalysts for the oxidation of solvents. The DTA curves for the catalysts

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8, 260-267.

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339

automotive oxidation catalysis. Jidosha Gijutsu (Japan) 30,740-746.

Stephens R. E. (1969) Lead-Promotive Platinum Catalysts for the Oxidation of Oletinic and Aromatic Hydrocarbons U.S. Patent 3, 425, 792. Suzuki M. (1977) About automotive emission control technology. Kogai (Japan) 12,90-102. Universal Oil Product Co. (1964) Reactivation of leadcontaminated oxidation catalysts. Brit. Patent 948, 946. Wedding B. and Farrauto R. J. (1974) Rapid evaluation of automotive exhaust oxidation catalysts with a differential scanning calorimeter. Ind. Engng Chem. 13(l), 45-47. Yamanaka T. (1975) Use ofcatalysts for air pollution control. Kagaku Kogyo (Japan)

26, 1036-1040.