Ind. Eng. Chem. Res. 1991, 30, 2012-2013
2012
RESEARCH NOTES Spectrophotometric Determination of Platinum in Cordierite-Supported Platinum-Tin Dioxide Catalysts The spectrophotometric determination of platinum, using the stannous chloride complex in hydrochloric acidic medium, has been adapted for the case of a platinum-tin dioxide catalyst. The decomposition of the catalysts is achieved by fusion with potassium hydroxide followed by acid digestion in aqua regia. The dissolved tin(1V) does not interfere with the measurement of absorbance in the strong acidic medium. Introduction Maziekien et al. (1960) have described a simple procedure for the spectrophotometrical determination of platinum in Pt-A1203 reforming catalysts. The method makes use of the colored platinum-stannous chloride complex formed in hydrochloric acidic medium, the absorbance being determined at a wavelength of 403 nm. In the present case the method is adapted for a Pt-SnO, oxidation catalyst, to be used in the low-temperature oxidation of carbon monoxide in C02lasers and life rescue devices (Kolts, 1988; Wright and Sampson, 1983). For that purpose the catalyst is supported by a cordierite monolith. Therefore it has to be decomposed by adequate means and the interference of tin (IV) has to be checked. It has been claimed that the complex is unaffected by the presence of stannic tin at sufficiently high hydrochloric concentration (Milner and Shipman, 1955;Snell and Snell, 1959);however, details concerning the adaptation of the method to the analysis of Pt-Sn02 on a cordierite support have not been published. Experimental Section The impregnation-type catalyst contains up to 2 wt % platinum relative to tin dioxide. The support is prepared by loading cordierite monoliths with Sn02. The final support contains about 25 wt 9% Sn02,and its BET surface is of the order of 120 m2 per g of SnOz. The support is impregnated with a solution of [Pt(NH,),](OH),, dried at 80 "C, and calcined in air at 350 "C. The platinum standard is prepared by dissolution of platinum wire in aqua regia as described by Maziekien et al. or by dilution of commercially available platinum(1V) salt solutions of known concentration. The stannous chloride solution is prepared by dissolving 28 g of SnCl2.2H20in 50 cm3of concentrated hydrochloric acid and diluting to 100 cm3. The stannic chloride solution is prepared by dissolving 69.9 g SnC14.5H20in 100 cm3 of water. The absorbance is measured by use of a Philips SP 6 500 UV spectrophotometer. The samples are prepared following the description of Maziekien et al. (1960). A typical reference sample in a 100-cm3flask contains the desired quantity of platinum, e.g., 10 cm3of the standard solution, the necessary amount of stannic chloride solution to attain a platinum-tin(1V) ratio of 1:500, and 3 cm3of stannous chloride solution. The content of concentrated hydrochloric acid is always adjusted to 35 vol % (35 cm3 in a 100-cm3flask).
By contrast with the alumina-supported catalyst investigated by Maziekien, the present catalyst cannot be dissolved in acidic medium alone. It has been reported in the literature that calcined tin dioxide (cassiterite) is not dissolved by strong acids (Gmelin, 1972). Furthermore Pt02which may be formed during the catalyst preparation is insoluble in aqua regia (Gmelin, 1939). On the other hand, the dissolution of tin dioxide by fused KHS04 is claimed to be incomplete (Brunck and Holtze, 1932; Gmelin, 1972), and platinum should only partially dissolve by that treatment, commonly used to separate rhodium from the platinum-group metals. In addition, both platinum and tin dioxide are contained in the pores of the cordierite support. In fact, attempts to use KHS04 fusion of the powdered catalyst samples have been unsuccessful. Thus the total catalyst has to be dissolved by basic decomposition, and eventually the following procedure has been adopted: About 1 g of the catalyst, finely powdered in a mortar, is fused with 10 g of KOH in a porcelain dish for 30 min at 410 "C. The cooled melt is neutralized with 30 cm3 of concentrated HCl and the flocculent, intensively yellow precipitate is transferred into a beaker. The platinum remaining in the dish is dissolved by heating in 10 cm3 of aqua regia, and the solution is equally transferred into the beaker. This treatment is repeated three times, leaving the dish completely clean. The content of the beaker is reduced to about 100 cm3 by heating to remove the nitric acid. Twice 30 cm3of concentrated HCl is added followed by concentration to 100 cm3. Evaporation to dryness to destroy the remaining HNOB(Ayres and Meyer, 1951) has not been found necessary. The contents of the beaker are now centrifuged,and the yellow solution is transferred into a calibrated 500-cm3flask. The remaining precipitate is digested several times with stannous chloride solution until it becomes white. The liquid solutions are all gathered in the flask, which is finally filled by concentrated HC1 up to the calibration mark. Results The interference of Sn(1V) proved to be negligible, up to an excess of 500:l relative to platinum. The differences between samples with or without addition of SnCL solution are less than the reproducibility of the measurement (less than 0.2%) in the concentration range from 1.5 to 7.5 mg of Pt/L. Due to the strong acidity of the solutions the Pt-Sn(I1) complex proved very stable even in the presence of the high excess of Sn(1V) quoted. After a week the deviation
0888-588519112630-2012$02.50/0 0 1991 American Chemical Society
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Znd. Eng. Chem. Res. 1991,30, 2013-2014
from the platinum value measured originally was less than 1%. However, due to the somewhat tedious treatment a systematic error remained. The mean platinum content of six different samples, each loaded by incipient wetness with 10 mg of Pt/g of catalyst, had been determined to 9.69 f 0.03 mg. If in kinetic measurements with Pt-Sn02 catalysts the observed reaction rate is, as usual, reported to the platinum weight of the catalyst sample, the use of the spectrophotometrically determined value of the platinum content will introduce an additional error of about 3%. Nevertheless, considering the limited precision of kinetic measurements which is rarely better than lo%, the analytical method described above is sufficiently accurate to characterize the platinum content of cordierite-supported Pt/SnO, oxidation catalysts. Acknowledgment U. Stenger has meticulously performed the chemical preparations and analysis.
Brunck, 0.;Holtze, R.Die Anwendung der Abatronschmelze in der analytischen Chemie. Angeru. Chem. 1932,45,331-334. Cmelins Handbuch der anorganische Chemie; Verlag Chemie: Berlin, 1939; System No. 68,Platin, Teil C 1, p 46. Gmelins Handbuch der anorganische Chemie, 8. Auflage; Verlag Chemie: Berlin, 1972;System No. 46,Zinn, Teil C 1, pp 105,112. Kolts, J. H.Oxidation of Carbon Monoxide and Cetalyst Composition Therefor. Eur.Pat. Appl. 88116581.5,1988,11 pp. Maziekien, I.; Ermanis, L.; Walsh, T. J. Assay Procedure for Platinum in Reforming Catalysts. Anal. Chem. 1960,32,645. Milner, 0. I.; Shipman, G. F. Calorimetric Determination of Platinum with Stannous Chloride. Anal. Chem. 1955,77,1476. Snell, F. D.;Snell, C. T. In Colorimetric Methods of Analysis; van Nostrand: Princeton, 1959; Vol. IIa, p 429. Wright, C. J.; Sampson, C. F. Tin Oxide Sola in Catalyst Preparation. Brit. Pat. Appl. P 8300554, 1983.
Hans-GClnther Lintz Znstitut fur Chemische Verfahrenstechnik Universitdt (TH)Karkrruhe, Postfach 6980 Kaiserstrasse 12,0-7500 Karlsruhe 1, Germany
Literature Cited Ayres, G. H.; Meyer, A. S. Spectrophotometric Study of the Platinum(1V)-Tin(I1) Chloride System. Anal. Chem. 1951,23,299.
Received for review January 17, 1991 Accepted May 24,1991
Transformation of Heavy Hydrocarbons into Benzene, Toluene, and Xylenes over Modified ZSM-5 Catalysts The transformation of heavy hydrocarbons (bp 623-673 K) over ZSM-5 catalyst modified with Zn (ZnZSM-5) and the combination of Zn, Nil and S elements (NiSZSM-5) was studied. The catalyst modified with the combination of Zn, Ni, and S elements had a better result. At the experimental conditions, the aromatics yield was 28.8 wt ?& and the concentration of aromatics in the gasoline fraction was 67.7 wt % for a reaction time of 9 h.
BTX compunds, namely, benzene, toluene, and xylenes, are consumed in large quantity as high-octane blending components. They are also an important source of petrochemicals. Most petroleum-derived aromatics are obtained by Pt reforming of nathphas. In the reforming, low-octane nathphas, boiling between about 338 and 473 K, are converted into high-octane gasolines containing a high concentration of aromatics. However, current Pt reforming processes are incapable of transformation of the heavy hydrocarbons, boiling above 473 K, which are often used in catalytic cracking processes to produce gasolines containing a little aromatics. Effective processes to convert the heavy hydrocarbons into aromatics would improve the availability of high-octane gasolines and increase the source of BTX as petrochemicals. The new technique of the synthesis of aromatics was reviewed by Hao (1987).' Aromatization of light hydrocarbons has been extensively studied. Chen and Yan (1986) reported a new process to produce aromatics (M2 forming) from light FCC (fluid catalytic cracking) gasoline and virgin nathphas.2 Wu et al. (1981) studied aromatization of C4hydrocarbons? Kitagawa et al. (1986) and Ono et al. (1987) researched the transformation of propane and but-1-ene into aromatics, r e s p e c t i ~ e l y .The ~ ~ ~formation of aromatics from methanol was reported by Pop et al. ( 1 9 W and Ono et al. (1988).' The aromatization of gasolines was investigated by Shen (1987)8 and Yang (1987).B Hu (1989) reported the aromatization of crude oil.l0 These studies were based on the ZSM-5 zeolite catalysts. The investigation of transformation of heavy hydrocarbons (bp 623-673 K) into aromatics over modified 0888-5885/91/2630-2013$02.50/0
ZSM-5 zeolite catalysts is presented in this paper. The zeolite sample with Si02/A1203= 53 was synthesized with n-butylamine. Ita structure was confirmed to be that of ZSM-5 zeolite by X-ray diffraction. The zeolite was converted into the ammonium form (NH4ZSM-5)by exchanging with an ammonium nitrate solution and calcined at 813 K to form HZSM-5. ZnZSM-5 was made by treating HZSM-5 with a zinc nitrate solution and had a zinc content of about 5 % by weight. It was p r o p e d that incorporation of zinc cations into ZSM-5 zeolite could increase the yield of aromatic hydrocarbons greatly!J' NiSZSM-5 was prepared by modification of ZnZSM-5 with a nickel nitrate solution and then with an ammonium sulfide solution. The reactions were carried out with a continuous-flow reactor at atmospheric pressure. The catalyst was packed in a reactor made of a stainless-steel tube (8-mm i.d.1 placed in a vertical furnace. The NiSZSM-5 catalyst was heated under a hydrogen stream at 773 K for 2 h. By this treatment, nickel cations were expected to be partly reduced into nickel metal atoms. ZnZSM-5 was not treated with hydrogen. Reaction conditions were temperature 773 K, weight hourly space velocity (WHSV) 3.5 h-l, and reactant (g):stream (g):hydrogen or nitrogen (mol) 1:l:O.W. The effluent was collected with an ice trap. The gasoline fraction was distilled, and ita aromatics composition was analyzed by a gas chromatograph equipped with a thermal conductivity detector. The reactant was a heavy distillate oil. Its properties are shown in Table I. The reaction over ZnZSM-5 was carried out with nitrogen as a carrier gas, and the results are given in Table 0 1991 American Chemical Society
