non-nitrogenous compounds while also providing some enhancement of the nitrogen compounds. The negative peak for the chlorinated hydrocarbon agrees with the results of Aue and Lakota (3) while the negative peaks for the other three compounds are due to a disturbance of the detector. I n the sulfur mode, it is possible to detect compounds such as hydrogen sulfide and carbon disulfide which give no response with a standard FID. Optimum operating conditions for the detection of hydrogen sulfide o n this Model 900 with a 5-foot X I/s-inch 0.d. stainless steel Porapak R column a t 50 “C isothermal were 240 ml/min of air and 70 ml/min of hydrogen with a helium carrier gas flow of 50 ml/min. The injector and detector block temperatures were maintained a t 100 O C . A sample of 0.0011 mole per cent hydrogen sulfide in nitrogen was detected (Figure 3) using a K 2 S 0 4AFID bored to 0.040 inch. Performance characteristics of the alkali salt filled front ferrules are in good agreement with those reported by Kar(3) W. A. Aue and S. Lakota, J . Chromatogr., 44,412 (1969).
men (4), Aue (3, and Craven ( I ) for the nitrogen mode and Dressler and Janfik (6) in the sulfur mode. The ease of construction and ruggedness of this A F I D made it practical to convert any FID into an AFID and make this technique available to more laboratories. The simple manner of construction also makes it practical t o study the effects of different salts in the operation of the AFID. ACKNOWLEDGMENT
The author wishes t o thank D. M. Schoengold for invaluable assistance.
RECEIVED for review December 16, 1971. Accepted March 1, 1972. (4) A. Karmen, Science, 7, 541 (1967). (5) W. A. Aue, C. W. Gehrke, C. D. Ruyle, D. L. Stalling, and R. C. Tindle, J. Gas Chromatogr., 5, 381 (1967). (6) M. Dressler and J. JanBk, J. Chromatogr. Sci., 7, 451 (1969).
Novel Type of Hydrogenator P. G . Simmonds and C. F. Smith Space Sciences Division,Jet Propulsion Laboratory, California Institute of Technology, Pasadena, Calif. 91103 VAPORPHASE HYDROGENATION of unsaturated organic compounds is of general interest for structure determination in organic chemistry. A convenient technique for performing analytical hydrogenation is t o place a reactor, containing a hydrogenation catalyst, in series with a gas chromatographic column (1, 2). This method of micro vapor phase hydrogenation has been demonstrated to be particularly useful for determining the olefin content of mixtures of hydrocarbons (3)and fatty acids (4-6). A minor limitation of the technique is that certain compounds, such as aldehydes, halides, and sulfides, may undergo some hydrogenolysis to form the corresponding saturated hydrocarbons. During a recent study (7) of a palladium-hydrogen separator for interfacing a gas chromatograph-mass spectrometer system, it was observed, not unexpectedly, that certain unsaturated compounds were reduced during passage through the narrow-bore palladium-silver tubing which was used to construct the separator. In general, hydrogenation was restricted to a,P-conjugated double bonds, such as in unsaturated aldehydes, ketones, nitriles, and esters. Monoolefinic compounds, such as alkenes, showed less than 5z reduction to the corresponding alkanes. However, if conditions are arranged so that hydrogen permeates from a high (1) M. Beroza, and M. N. Inscoe, in “Ancillary Techniques of Gas Chromatography,” L, S. Ettre and W. H. McFadden, Ed., Wiley-Interscience, New York, N.Y., 1969, Chap. 4, p 89. ( 2 ) M. Beroza, and R. Sarmiento, ANAL.CHEM., 38,1042 (1966). (3) R. Rowan, Jr., ibid., 33,658 (1961). (4) J. H. Dutton andT. L. Mounts, J. Cutul., 3,363 (1964). 37,641 (1965). (5) T. L. Mounts and J. H. Dutton, ANAL.CHEM., (6) H. J. Dutton, in “Advances in Tracer Methodology,” Vol. 2, S. Rothchild, Ed., Plenum Press, New York, N.Y., 1965, pp 123-34. (7) P. G. Simmonds, G. R. Shoemake, and J. E. Lovelock, ANAL. CHEM., 42,881 (1970).
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
pressure source into the interior of a heated palladium alloy tube containing unsaturated compounds, we have observed that quantitative reduction of both conjugated and unconjugated olefins can take place. Wahlin (8, 9) has previously described the hydrogenation of cyclohexene in a palladiumsilver tube in which the hydrogen was generated from the electrolysis of water by making the tube cathodic in a n electrochemical cell. The present paper describes the investigation of a palladium-silver tube as a catalytic reactor for the vapor phase hydrogenation of unsaturated carbon-carbon bonds in a variety of organic compounds. The device is simple to construct from a n appropriate length of palladium-silver tubing and may be used for both continuous and batch hydrogenations. As a batch hydrogenator, it may also be used as a component of the gas chromatographic system and in this respect its performance is similar to that of the packed catalyst reactor. However, it is not necessary to change the carrier gas to hydrogen, since in practice hydrogen diffuses from a n external supply into the interior of the catalytic tube where it rapidly mixes with any convenient carrier gas. Furthermore, in contrast to the packed catalyst hydrogenator, the palladium tube device does not cause hydrogenolysis of sensitive aldehyde groups. Unfortunately with the present palladiumsilver alloy, there is some reaction with both halogen- and sulfur-containing compounds; presumably due to the formation of silver halides and sulfides. However, it is possible that this difficulty may be overcome by the use of other palladium alloys which d o not react so readily with these compounds (IO). (8) H. B. Wahlin and V. 0. Naumann, J. Appl. Phys., 24,42 (1953). (9) H. B. Wahlin, U.S. Patent 2,749,293 (1956). (10) D. L. McKinley, U.S. Patent 3,359,845 (1967).
LIZ IN. SWACELOK CAP THRU DRILLED 1116 IN.
7
/
[-VALVE
r rPAUllIUM-SILVER HYDROGEN
,-HEATER
a
2.0 IN. d
r PRESSURE
/
!I[
GAUGE
GAS SAMPLE
THRU DRILLED 1116 IN. TYP
Figure 1. Construction of the palladium-silver hydrogenator
EXPERIMENTAL Apparatus. The construction of the hydrogenator is depicted in Figure l. A 125-cm length of palladium-silver 25 % tubing (Matthey Bishop Inc., Malvern, Pa.) with a n internal diameter of 0.025 cm (wall thickness 0.01 cm) forms the central component of the hydrogenator. The tubing was coiled in a 1-cm diameter helix and a n 8-cm length of 0.158cm o.d., 0.076-cm i.d. stainless steel capillary tubing was silver soldered onto each end. The helix was sealed by means of Swagelok fittings into a stainless steel chamber which could be pressurized with hydrogen from a n external supply. The hydrogenator was separately heated by tightly wrapping with a flexible heating tape. Alternatively it may be heated by mounting within the chromatographic oven or the instrument injection block. Prior to use, the palladium-silver tube was activated over a 6-hour period by drawing air through the center of the tube with the hydrogenator heated to 350 "C. The hydrogen permeation rate as a function of temperature and with an external hydrogen pressure of 50 psig was determined by sealing off one end of the hydrogenator and measuring the flow of hydrogen from the opposite end. Hydrogenation Tests. The performance characteristics of the hydrogenator were investigated using simple olefinic compounds and with the experimental arrangement shown in Figure 2. Gases were delivered from lecture bottles directly to the hydrogenator at a flow rate of 4 ml min-1 as measured by a mass flowmeter (Hastings-Raydist Co.) (Figure 2, Path a). For liquid samples helium carrier gas was saturated with vapor by passage through a small bubbler (Figure 2, Path 6). The effluent from the hydrogenator was trapped by condensation in a small glass vial immersed in liquid nitrogen. The extent of hydrogenation was determined mass spectrometrically by periodic injection of samples collected from the trap into the gas analysis system of a Quadrupole 300 mass spectrometer (Electronics Associates, Inc., Palo Alto, Calif.). Recorded spectra were then compared with the spectra of authentic standards obtained under identical conditions. In the first experiment, the hydrogenation of butene-1 was studied over the temperature range of 25-300 "C and at a flow rate of 3 ml min-' with an external hydrogen pressure of 50 psig. The percentage conversion of 1-butene to butane was determined from the relative ratios of the parent ion peaks (m]e 56 and 58) after calibration of the mass spectrometer with standard mixtures of the two gases. The reduction of a variety of olefinic compounds was studied in a second series of experiments in which the hydrogenator was maintained at a temperature of 250 "C and with a n external hydrogen pressure of 50 psig. In a third series of experiments, the hydrogenator was incorporated in the flow path of the gas chromatograph. The chromatographic column was a 152-m by 0.05-cm i d . SCOT capillary column coated with diethylene glycol succinate (Perkin-Elmer Corp., Norwalk, Conn.) and operated isothermally at a temperature of 180 "C. Since the hydrogenator was mounted directly in the chromatographic oven, its temperature of operation was also 180 "C. The external hydrogen pressure was 50 psig. Aliquots (0.2
LIQUID NITROGEN TRAP
Figure 2. Experimental arrangement for hydrogenation of gases (path a) and liquids (path 6) ~
Table I. Hydrogenation of 1-Butene as a Function of Temperature and Hydrogen Permeation Rate Temperature, Percentage Hydrogen permeation "C butane yield, rate, ml/min-' 25 8 0.1" 50 82 2.52 100 98 16.6 150 100 31.4 200 100 43.2 250 100 48.0 300 100 53.2 Actual permeation rate could be +50% a Only approximate. of this measured rate.
z
PI) of a standard mixture of fatty acid methyl esters (Applied Science Corp., State College, Pa.) diluted with acetone to a concentration of 10 mg/ml were then injected into the chromatograph. The resulting chromatograms were compared with chromatograms of the same mixture with the hydrogenator removed from the system. RESULTS AND DISCUSSION The rate at which hydrogen permeates through the wall of the palladium-alloy tube is governed by such factors as temperature, concentration gradient across the wall, the catalytic efficiency of the intermediate reaction steps on the internal and external surfaces and on the transport of reactants and products up to and away from these surfaces (11). Since the hydrogenator illustrated in Figure 1 has a catalytic surface area of only 9.8 cm2,it is obviously important that it be highly efficient and that an adequate permeation rate be maintained for the stoichiometric uptake of hydrogen Table I demonstrates the efficiency of hydrogenation of 1butene as a function of temperature. The hydrogen permeation rate is also included in the table; it is immediately clear that quantitative reduction of butene does not occur until hydrogen is substantially in excess. However, there is a significant reduction of butene even at room temperature with only a very low hydrogen permeation rate. This is most probably explained by the rapid scavenging of hydrogen from the catalytic surface by the butene. Since the surface hydrogen is rapidly replaced with fresh hydrogen diffusing from the metal lattice, the absolute hydrogen permeation rate with a flow of olefin may be greater than the rate measured in the absence of olefin. (1 1) F. A. Lewis, "The Palladium-Hydrogen System," Academic Press, New York, N.Y., 1967. ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, J U L Y 1972
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Table 11. Types of Compounds Reduced by PalladiumSilver Hydrogenator Compound Product 1-Butene Butane 1,3-Butadiene Butane Methyl methacrylate Methyl isobutyrate a-Methacrylonitrile 2-Methyl butanenitrile Acetylene Ethane Acrolein Propionaldehyde Methyl ethyl ketone Methyl vinyl ketone Ditertiary butyl ethane Ditertiary butyl ethylene The types of compound which have been hydrogenated and their products are listed in Table 11. Careful analysis of the mass spectral data showed no trace of starting material for any of the compounds tested. Hydrogenation of both olefinic and acetylinic bonds is therefore quantitative, a t least for the compounds tested and under the experimental conditions described above. Other unsaturated centers within the molecule are unaffected by passage through the hydrogenator as there was no reduction of either nitriles to amines, or aldehydes to alcohols. Even a sterically hindered olefin such as ditertiary butyl ethylene was completely hydrogenated. This suggests that despite its modest surface area, the palladiumsilver hydrogenator is extremely efficient. Compounds which might be sensitive to hydrogenolysis, such as acrolein, underwent only simple reduction of the olefinic bond to yield the corresponding saturated aldehyde. A loss of catalytic activity was experienced only once during the present investigation when methyl methacrylate was accidentally condensed in the palladium-silver tube and some polymerization presumably occurred. Full catalytic activity was restored by heating the hydrogenator in air for 6 hours at a temperature of 400 “C. However, it must be remembered that sulfur and certain halogen compounds can cause poisoning of the palladium-silver catalyst (12). Previous experience ( 7 ) has indicated that injections of low concentrations of sulfides or halides cause temporary poisoning of the catalytic surface. The mechanism apparently involves the initial formation of a metal sulfide or halide which momentarily blocks the diffusion of hydrogen. For gas chromatographic components the duration of this effect is generally no longer than twice the peak width as the sulfur or halogen is removed by formation of hydrogen sulfide o r hydrogen halide. The present palladium-silver hydrogenator is obviously unsuited for the reduction of halogen or sulfurcontaining compounds. However, McKinley has observed that alloys of palladium-gold (40 %) are not extensively poisoned by continuous exposure to hydrogen containing up to 20 ppm of hydrogen sulfide, although there was a decrease in the hydrogen diffusion rate. The maximum hydrogen diffusion rate and presumably catalytic activity could be immediately restored by switching to pure hydrogen (10). The lifetime of the palladium-silver catalytic tube will therefore be governed by the rate of contamination and poisoning. However, in one experiment in which the hydrogenator was operated continuously for a period of 10 hours with a 1butene feed rate of 3 ml min-l, there was n o apparent decrease in catalytic efficiency. During this time 1.8 liters of butene was quantitatively hydrogenated to butane. The efficiency of the hydrogenator as a precolumn reactor for the reduction of unsaturated compounds in the gas chromatographic stream is demonstrated in Figure 3. This figure (12) A. S. Darling, “Symposium on Less Common Means of Separation,” Institution of Chemical Engineers, 1964, p 103.
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
b
a
3LVENT
’18
C I
I
0
1 1
1
I
,
I
2
4
6
8
0
T I M , min
ci I
2
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8
Figure 3. Reduction of fatty acid methyl ester mixture by palladium-silver hydrogenator; (a) hydrogenated mixture; (b) standard mixture of fatty acid methyl esters before hydrogenation compares the analysis of a standard mixture containing both saturated and unsaturated fatty acid methyl esters with the hydrogenator placed between the instrument injection block and the column (a) and without a hydrogenator (b). Clearly fatty acid methyl esters with one (oleic), two (linoleic), and three double bonds (linolenic) are quantitatively reduced to methyl sterate. This was confirmed by calculating the ratios of C18 C18:14-Ciaz2 c 1 8 : & 6 and C 1 8 / C 1 6 from their peak areas. These values were 13.17 and 13.3, respectively. These preliminary experiments were designed to demonstrate the potential of the hydrogenator for the continuous gas phase reduction of olefins and also its practical use in gas chromatography. No attempt has been made to optimize the dimensions of the tube for a particular hydrogenation. The size of the catalytic palladium-silver tube may be conveniently scaled up or down depending on the feed rate of hydrogenatable material and other variables such as temperature, and the hydrogen Ap across the tube. The apparatus should be useful for the hydrogenation of gas chromatographic components a t the submicrogram level, particularly where there is insufficient material for isolation. Mass spectrometric analysis of reduced products is also possible by arranging a second palladium tube in series with the hydrogenator. This second palladium alloy tube when heated in air would serve as a hydrogen separator in which hydrogen would selectively permeate through the tubing wall to combine with oxygen at the exterior surface (7, 13). Although palladium-silver was the only alloy used in the present investigation, other palladium alloys, particularly those containing ruthenium or rhodium also diffuse hydrogen readily and may offer a means of varying both the properties and activity of the catalytic surface.
+
+
RECEIVED for review October 14, 1971. Accepted March 7, 1972. This paper presents the results of one phase of research carried out a t the Jet Propulsion Laboratory, California Institute of Technology, under Contract No. NAS 7-100, sponsored by the National Aeronautics and Space Administration. (13) J. E. Lovelock, K. W. Charlton, and P. G. Simmonds, ANAL. CHEM., 41,1048 (1969).
