Species observed C
cz
CH CN
co+
H Kr N2
Table 111. Analysis of Argon Research grade argon, Welding grade argon, ppm‘ PPmnb 2.9 13.8 6.4 47.1 0.9 10.0 6.1 16.1 -
Figure 3. Variation of palladium separator efficiency with different external gases. Hydrogen flow, 2 cm*/min
zu 5
(a) External gas flow rate, 50 cm3/min (b) External gas flow rate, 1liter/min
40
3.1
TEMPIRATURE, *C
flow rates of hydrogen with the test separator unheated, and then observing both the integrated hydrogen signal recorded by the mass spectrometer and the ion source pressures. A calibration curve plotted from these data permitted easy determination of exit flow rates of hydrogen as the temperature of the separator was changed by ohmic heating. Percentage efficiency was then calculated as follows : Hzflow in - Hz flow out x 100 Pd separator efficiency = HZflow in CATALYTIC REACTION STUDIES T o test the separator for catalytic effects, 5-121 vapor samples of various compounds were injected individually through injection port B shown in Figure 1. The separator was operated at a temperature of 250 "C and with a hydrogen input flow of 2 cma/min; the external gas was air at a flow rate of 50 cma/min at STP. In addition, mixtures known to be resolved by the gas chromatographic column were injected Diu port A . The mass spectrum of each compound was recorded after passage through the separator and compared with the mass spectrum of the pure compound admitted separately into the mass spectrometer through a conventional gold leak. When catalytic change was observed, an estimate of the percentage conversion was calculated by calibration of the mass spectrometer with known mixtures of product and starting compound. RESULTS AND DISCUSSION The observation that hydrogen of the highest possible purity is obtained by permeation of the gas through palladium-silver alloys (9, IO) clearly implies that these alloys are for all practical purposes impervious to other gases. Any loss of sample in the palladium tube is principally due to adsorption or reaction in contrast to other separator types where both adsorption and effusion contribute to a loss of sample. Hence the ratio of sample out to sample in of the palladium separator will approach 100 %, since losses by adsorption are likely to be minimal. The device is to an extent its own indicator of adsorption in that hydrogen removal requires a clean surface. The separator is also unique in the sense that it is possible to remove all of the carrier gas; under these conditions the enrichment factor (11) expressed as : Hz GC Enrichment factor = - X QGC HZMS (9) L. R.Rubin, Englehard Ind., Tech. Bull., 2, 1115 (1961). (10) G. Cohn and H. E. Straschil, Amer. Chem. SOC.,Diu. Petrol. Chem. Prepr. 8 (4), B43-B48 (1963). (11) M. A. Grayson and C. J. Wolf, ANAL. CHEM., 42, 426 (1970).
where QMs = amount of eluate entering MS QGO = amount of eluate leaving G C Hs MS = flow of carrier entering MS HzG O = flow of carrier leaving G C may approach infinity. It is therefore apparent that the enrichment factor calculated by the definition given above is a variable quantity which depends on the chromatographic conditions. The impermeability of the palladium device to sample components and the potential for 100% efficiency means that the performance of the palladium separator cannot be rigorously calculated in terms used for other types of separator. Figure 2 shows the efficiency of the test separator as a function of separator temperature for different hydrogen flow rates. Air was the external gas at a flow rate of 50 cm3/min. Hydrogen flowing at rates of up to 5 cm3/min at STP is completely removed by the separator at a temperature of 425 "C or greater. At a more practical operating temperature of 250 "C an efficiency of 99 can still be maintained for a hydrogen input flow of 2 cm3/min. This efficiency corresponds to an ion source pressure of 5 X lo-' torr (uncorrected). This relatively low column flow of 2 cm3/min was of special interest for the Mars experiment, since the general constraints of the mission do not permit carrier gas flow rates in excess of this value. However, it should be clearly recognized that if the separator was made larger and there was a more generous air supply, or if pure oxygen were used, flow rates as high as 200 cm3/min could be removed. The variation of palladium separator efficiency with other external gases at two different flow rates is presented in Figure 3. In Figure 3b, with oxygen as the external gas at a flow of approximately 1 liter/min, maximum hydrogen removal is maintained at temperatures as low as 150 "C. Decreasing the external flow of oxygen to 50 cm3/min as in Figure 3u lowers the efficiency by 4 % at the same temperature. Gases which are not reduced at the palladium surface, such as argon, nitrogen, and carbon dioxide, are less effective than oxygen at all flow rates. However, at flow rates in excess of 500 cm3/ min and at temperatures > 200 "C, these gases appear to be as effective as air for the rapid removal of newly diffused hydrogen. This may in part be due to traces of oxygen in these gases, although care was taken to exclude air from leaking into the system. At the higher flow rates, there is an approximately 15% decrease in the surface temperature of the palladium tube. No attempt was made to correct for this effect since a 15% decline in the separator temperature at 250 'C with a 2 cm3/min. input flow of hydrogen represents only a 3 % decrease in the overall efficiency. ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
883
ioo
2 p.
I\
I
I
1
301
1
1
20
lot
I
O
I
100
I 200
I
300
1 I
400
PRESSURE, torr
Figure 4. Efficiency of palladium separator in a reduced pressure environment
The effect of operating the separator a t reduced pressures is illustrated in Figure 4. Efficiencies of hydrogen removal of 95 or better can be achieved provided the separator housing pressure is maintained below 5 torr. This mode of operation is generally less efficient since a t the mass spectrometer end of the separator the pressure may be in the lo-' torr range. Obviously external pressures less than 5 torr are only effective for pumping the high pressure gas chromatographic end of the separator; pure oxygen would be required to pump the residual hydrogen at low pressure (12). In several experiments the separator has been operated successfully without any housing, simply by suspending the device between the gas chromatography and mass spectrometer and open t o the laboratory air. It was, however, more convenient to enclose the separator in a glass tube, through which air was passed a t a constant rate, thereby avoiding any sudden transients in the laboratory air flow. Since the passage of hydrogen through the palladium-silver alloy is related to the temperature and catalytic activity of the surface, it was expected that some compounds were catalytically hydrogenated during passage through the separator. Fortunately, a t a practical operating temperature of 250 "C, those catalytic changes which take place are specific and essentially complete. Representative members of most classes of organic compound which pass through the separator unchanged are listed in Table I. It is interesting to note that potentially sensitive functional groups, such as aldehydes and nitriles, d o not undergo reduction under the conditions used. I n general, catalytic reduction occurs in compounds with conjugated unsaturation such as dienes, a,P-unsaturated aldehydes, ketones, and nitriles. Although certain compounds pass through the separator unchanged, they may cause poisoning of the palladium surface. Sulfur containing gases, e.g., hydrogen sulfide, sulfur dioxide, and mercaptans, as well as iodine compounds are poisoning agents. Concentrations of less than 500 ppm of these sulfur containing gases in the carrier stream cause temporary poisoning of the palladium surface, allowing some hydrogen to break through into the mass spectrometer. The (12) J. R. Young, Rev. Sci.Instrum.,34,374(1963). 884
ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
Table I. Compounds Unaltered by Passage through Palladium-Silver Separator. Hydrocarbons Esters Hexane Ethyl acetate 1-Hexene Methyl laurate 2-Hexene Nitrogen compounds Cyclohexane Methyl amine Cyclohexene Benzonitrile Benzene Phenyl acetonitrile 2-Methyl-2-Hexene Pyrrole Pyridine Alcohols Ethanenitrile n-Propyl alcohol Phenol Sulfur compounds Benzyl alcohol Ethyl mercaptanb Cyclohexanol Thiophene Ketones Methyl ethyl ketone Halogen compounds Cyclohexanone Butyl chloride Acetophenone Chlorobenzene Furan Perfluoropentane 2,4-Pentanedione Ethyl iodideb Gases Carbon dioxide Carbon monoxide Methane Argon Nitrogen Aldehydes Nitric oxide Propionaldehyde Nitrogen dioxide Furfural Nitrous oxide Benzaldehyde Hydrogen sulfide* Sulfur dioxideb a Calculated efficiency of the palladium separator for these tests was 99.0%. These compounds caused temporary hydrogen breakthrough and loss of efficiency due to poisoning effects. Ethers Diethyl ether Tetrahydrofuran Anisole
~~
~
Table 11. Percentage Conversion of Compounds Reduced5 Starting compounds Product Acrolein Propionaldehyde 96.7 Acrylonitrile Propanenitrile 96.1 Methyl acrylate Methyl propanoate 97.5 Methyl vinyl ketone Methyl ethyl ketone 92.1 Isoprene 2-Methyl-1-butene 93.0 2,4-Hexadiene Hexene 97.2 1,5-Hexadiene Hexene 87.2 Ethyl benzene 95.0 Styrene Acetylene Ethylene polymers >90.0 Water >99.0 Oxygen Palladium separator temperature was 250 "C. Hydrogen input flow was 2 cm3/min.
z
+
duration of this effect is generally no longer than twice the peak width. Higher concentrations cause progressive loss of separation efficiency, and repeated injections at a concentration of 5000 ppm may render the separator inactive. Activity could often be restored by heating the separator t o 500 OC for several hours. Other alloys of palladium have been reported to be less susceptible to poisoning by sulfur compounds, but are generally less efficient than palladium-silver for the diffusion of hydrogen (13). Table I1 lists the test compounds catalytically changed by the separator and the percentage yield of each product. 1,5-Hexadiene, unlike most of the altered compounds, is unconjugated, although it is probable that catalytic isomerization precedes hydrogenation. The effect of temperature (13) D. L. McKinley, U. S. Patent 3350845 (1967).
TEMPERATURE,
O C
Figure 5. Effect of temperature on catalytic conversion of styrene to ethyl benzene
on the catalytic reaction was studied for styrene, a typical reactive compound. The results are presented in Figure 5. The extent of conversion to ethyl benzene is clearly temperature dependent, and goes through a maximum between 225275 "C. At temperatures above 300 "C all of the hydrogen in the carrier stream is being removed, which tends to establish molecular flow conditions within the narrow bore of the palladium tube. The sample, therefore, experiences an undesirably long exposure time to the catalytic surface. Furthermore, in the now hydrogen-depleted atmosphere, dehydrogenation reactions can take place. For example, cyclohexanol at 350 "C gave a mixture of 40 % cyclohexanone, 10 % phenol, and 50 % unchanged cyclohexanol. These adverse effects may be eliminated by maintaining a very small flow of hydrogen through the separator to ensure that the sample is swept into the mass spectrometer. Alternatively
it might be possible to use a separator of variable bore in which the finest bore is at the high pressure end and the widest bore is at the entry to the ion source, so that the mass spectrometer can more effectively pump the sample. While many of the catalytic changes which occur with the palladium separator are of casual chemical interest, it is also worth considering their elimination or at least their limitation to a predictable and an acceptable level. Normal silanization procedures were ineffectual in preventing catalysis. Coating of the interior surface of the palladium tube with silicone stationary phases, such as SE 30 did reduce the catalytic effects, but the permeability to hydrogen was simultaneously reduced to an unacceptable level. A considerable effort is presently in progress to overcome this potential drawback of the palladium separator. It was also possible to achieve efficient hydrogen removal at temperatures as low as 120 "C where catalytic change is unlikely to be a problem. This further improvement occurs when the palladium tube is operated as the anode of an electrolytic cell. An advanced form of the separator in which the hydrogen is generated and removed in the same electrolytic cell will be the subject of a subsequent paper. The palladium separator made from a short length of tubing heated in air is a simple and rugged construction. Its advantages of near 100% efficiency, small volume, and a quantitative delivery of most sample substances more than offset the small loss of qualitative resolution due to catalytic change. ACKNOWLEDGMENT
We thank C. F. Smith and R. H. Duncan for expert technical assistance. RECEIVED for review February 13,1970. Accepted April 23, 1970. This paper presents the results of one phase of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, under Contract No. NAS 7-100, sponsored by the National Aeronautics and Space Administration.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
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