Flame Ionization Detection of Carbon Oxides and Hydrocarbon Oxygenates Meredith B. Colket, David W. Naegeli,' Frederick L. Dryer, and lrvin Glassman Department of Aerospace and Mechanical Sciences, Princeton University, Princeton, N.J. 08540
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The catalytic conversion-flame ionization technique developed to detect trace quantities of the carbon oxides by gas chromatography has been improved and demonstrated to be a very powerful trace analysis technique for several other pollutants. The use of ruthenium or thorium oxide-nickel catalysts significantly reduce the operating temperature necessary for carbon dioxide methanation; thus possible side reactions that may interfere with catalyst life or quantitative conversion are suppressed. The catalyst-detector technique is a facile and precise ,method for detecting trace quantities of hydrocarbon oxygenates such as formaldehyde. Furthermore, alkanation of unsaturates also results in increased flame ionization response for these compounds. Extensive use of the flame ionization detector (FID) in organic pollutant analysis is a direct result of its exceptional stability, its linear dynamic range, and its excellent response characteristics for most hydrocarbons. However, the FID's observed insensitivity to the carbon oxides and many of the hydrocarbon oxygenates has generally required the use of detectors with less desirable operating characteristics or the use of techniques other than gas chromatography (gc). However, in recent years, several investigators (Bufalini et al., 1972; Porter and Volman, 1962; and Shwenk et al., 1961) have demonstrated a novel technique which permits detection of carbon oxides a t the parts-per-million level by flame ionization. After separation by the gas chromatograph, the eluting carbon oxides were hydrogenated over a pure nickel catalyst, and the resulting methane was subsequently detected by an FID. Though the method is quantitative for both CO and COz, the nickel catalyst temperature necessary for conversion of both carbon oxides (>340°C) must be considered a problem (Williams et al.. 1972). Above 340°C, the surface reaction:
2co
c + CO?
may be significant (Hightower and White, 1928), and the conversion to methane may be less than quantitative. Even if conversion remains nearly complete, in time the number of active sites and therefore the catalyst efficiency will be reduced by the carbon deposition. Thus one must expect pure nickel catalysts to have limited lifetimes. Studies in this laboratory have confirmed earlier work (Medsforth, 1923), which suggested that added promoters, particularly thorium oxide, can produce the equivalent catalyst activity of pure nickel a t lower temperatures. However, a survey of the literature on Fischer-Tropsch synthesis (Pichler, 1952; McKee, 1967; and Randhava et al., 1969) suggested to us that catalyst detector systems might be most improved by employing ruthenium as the catalyst material. Ruthenium has long been recognized as the most effective catalyst for methanation of the carbon oxides, particularly COz (Bond, 1962). However, its largescale industrial use has generally been considered prohibitively expensive. Surprisingly, the literature reveals no attempts to es'To whom correspondence should be addressed
tablish alkanation as a technique to improve the FID response of unsaturates and hydrocarbon oxygenates. This is a particularly intriguing prospect for oxygenates such as HCHO which produce no FID response themselves. Although HCHO has been successfully analyzed by gas chromatography, its detection limit (ca. 100 ppm) has been established by that of thermal conductivity detection. Furthermore, helium ionization detection has remained impossible because of the high column temperatures necessary for HCHO analysis and the extreme sensitivity of this detector to column bleed. Thus, much more tedious wet chemical methods continue to be employed for analysis of HCHO at concentrations generally found in pollution studies. Thus, the following investigation was conducted to evaluate the relative performance of nickel, promoted nickel, and ruthenium catalysts for methanation of the carbon oxides, and to demonstrate the application of the alkanation-FID detection technique to several hydrocarbon oxygenates including HCHO.
Experimental Techniques A Hewlett Packard model 7624 gas chromatograph, specially instrumented for limited volume gas analysis, was employed in these experiments. A more complete description of this system has been published (Dryer, 1972). A catalyst oven was added between the gc column exit and the inlet to the FID. The detector hydrogen supply was moved upstream of the catalyst oven, and signal output from the FID was integrated on a Hewlett Packard model 337OA electronic integrator. Multicomponent samples were separated by temperature programming a 7-ft Porapak $-7 ft Porapak R series column constructed of 0.080 in. i.d. stainless steel tubing and 80/100-mesh packing material. Ultrapurity helium was used as the carrier gas. Gas samples were injected into the gc using a heated gas sampling valve (110°C) with a li2-c~sampling loop. Sample injection pressure was maintained a t 38 & 0.1 cm Hg. Overall system calibration was performed using a primary standard mixture (Matheson Corp.) containing CO (2%), CH4 (1701, COS ( 2 7 ~ ) ,C Z H ~ (17'01, CzHs (l%), and C3Hs (1%) in nitrogen. Dilution of this calibration mixture and samples of other compounds (CHBOH. CzHsOH, and CH3CHO) were obtained by partial pressure mixing of the component in nitrogen. If any interference between eluting species was suspected, single-component samples in nitrogen were prepared. Accuracy of the partial pressure technique was periodically checked against the primary standard measurements for CH4, and results always agreed to within 1%. Formaldehyde gas was obtained by thermal decomposition of paraformaldehyde. Samples with approximate amounts of HCHO were prepared in nitrogen and heated prior to injection to prevent polymerization. An accurate measurement of the HCHO concentration was obtained by first oxidizing a portion of the HCHO/Nz sample to COz with hot CuO and then detecting the COZ with the catalyst-FID system. Formaldehyde samples were also prepared in a vessel heated to 180°C to prevent polymerization during both sample preparation and injection. The two methods of sample preparation agreed to within 5%. Volume 8,Number 1 , January 1974 43
Catalgst The catalyst oven consisted of a solid, 1-in. diameter stainless steel or copper cylinder 3 in. long. A U-shaped passage, 0.125 in. in diameter and 5 in. long, was machined longitudinally in the cylinder, and the catalyst material, coated on or suspended in 100/120-mesh Corning glass beads, was packed into the passage. The body was heated electrically and was surrounded with sufficient insulation to reduce thermal gradients. Catalyst temperature was measured to k2"C with an iron-constantan thermocouple embedded in the catalyst body. Nickel (5%) and nickel (5%)-thorium oxide (1%) promoted Ldtalysts were prepared from an aqueous slurry of Ni(N03)2-6Hz0, Th(N03)4,4HzO, and 100/120-mesh acid-washed Corning glass beads. Uniform coating of the beads was assured by removing the solvent with a rotating evaporator. NiO and T h o z were obtained by oxidizing the coated beads in situ a t 500°C for 4 hr. Reduction of the NiO appeared to be complete after about 12 hr of hydrogenation a t 350°C. Ruthenium catalysts were prepared from a slurry of and 100/120-mesh acid-washed R u ( N H ~ ) ~ ( O H(1 ) Cgram), ~ Corning glass beads (15 grams) in water (100 ml). A small amount of NaOH was added to the slurry, and the mixture was agitated at 90°C for 2-3 hr to hydrolyze the R u ( N H ~ ) ~ ( O Hto ) CRuOz. ~ The insoluble RuOz and glass beads were vacuum filtered from the alkaline solution, washed with distilled water, and dried a t 150°C for 1 hr. The resulting material was broken up with mortar and pestle to achieve a uniform dispersion of the RuOz powder and the glass beads. After the catalyst oven was packed, active ruthenium was produced by hydrogenation at 300°C for 12 hr. Catalysts Performance The pure nickel, promoted-nickel, and ruthenium catalysts were compared in terms of efficiency of carbon oxide hydrogenation at several different catalyst temperatures. The same helium plus hydrogen flow rate through the catalyst oven was maintained for all of the experiments to eliminate residence time effects from the comparison. The primary standard calibration mixture mentioned earlier was used in these experiments, and the percent conversion of the carbon oxides was determined using ethane as an internal standard. The results of these experiments (Figure 1) clearly showed ruthenium to be the most active of the catalysts studied. While the temperature for complete conversion of
-CC,
---co
I 250
300
I 350
CATALYST TEMPERATURE ('CI
Figure 1. Comparison of temperature required for conversion of COz over ruthenium, promoted nickel, and pure nickel catalysts 44
Environmental Science & Technology
1
0
1
5 10 I5 T I M (minl (a 1
0
5 10 TIME (mini
IS
(bl
Figure 2. Effect of catalyst temperature on peak shape (a) 300°C and (b) 240°C. Column temperature program -2O"C, hold four min.. program to 45-30°C/min. He carrier, 30 cc/min H P
COa over nickel was considerably lowered by the thorium oxide promoter, the temperature for complete conversion of COz over ruthenium was nearly 75OC below that of pure nickel. At this low temperature, the possible significance of the reaction
2co
=
c + CO?
is expected to be considerably reduced since this reaction should have a relatively high activation energy. Thus catalyst deterioration from carbon formation would not be possible. It should be noted that the conversion efficiencies for CO and COz over ruthenium are nearly the same. This observation is in great contrast to the efficiencies evident for the nickel-based catalysts. Thus, it may be that the mechanisms of hydrogenation over ruthenium and nickel catalysts are significantly different. The ruthenium catalyst results shown in Figure 1 could be obtained most easily when the catalyst oven was made of copper. With the stainless steel oven, similar results were less reproducible (omitted in figure). It appears that copper may play some role in facilitating the catalyst conditioning process-i.e., the reduction of the initial ruthenium dioxide. Similar effects have been noted in conditioning cobalt, nickel, and iron catalysts (Anderson, 1956); however, sizable amounts of copper shorten the lifetime of these catalysts by enhancing sintering of the reduced metal. Y o such sintering effect was noted here with the ruthenium catalysts. This result was expected since the catalyst was exposed only to the copper walls of the oven. Additional experiments to quantify the effect of copper have not been performed. Concerning longevity, nickel catalysts, operating a t temperatures necessary for full conversion ( -340"C), deteriorate after periods as short as two weeks. In contrast, the ruthenium catalyst has operated for more than 500 sample analyses over a period of six months. These samples were of combustion products requiring hydrogenation of about 2% carbon-containing species. Water present in these combustion samples had no effect on catalyst performance, and no signs of catalyst deterioration have yet been observed. The effect of the catalyst temperature on peak broadening and tailing of the FID chromatograph output is shown in Figure 2. The figure displays the FID output a t two catalyst temperatures with the injected sample composed of 0.2% CO and COz, and 0.1% CH4, C&, and CzHs. At temperatures near 300"C, the catalyst had no effect on the peak shapes of CO and COz. Even a t 240°C, where peak broadening and tailing became apparent, the catalyst was useful; the conversion of COz was about 97% and the peaks were still resolved. However, below 240"C, peak tailing began to cause significant deterioration in resolution and proper peak integrations. The results in Table I show that the complete conversion of CO and COz was independent of concentration over the range 0.02-270. The linearity of the system was
Table I. Conversion of Compounds to Alkanes coo
CO?
CH4
HCHOa
CHnOH
A
B
A
B
A
B
A
B
A
1.98 1.12 0.237 0.0535 0.0107
1.94 1.12 0.230 0.0528 0.0114
1.99 0.635 0.176 0.0448 0.0187
2.00 0.638 0.177 0.0460 0.0191
1.01 0.326 0.0916 0.0234 0.00935
1.01 0.322 0.0893 0.0228 0.0095
2.058 0.508 0.156 0.0432 0.0103
2.06 0.498 0.158 0.0451 0.0116
0.821
CzHsOH
B
A
0.787
0.635
CHaCHO
B
0.632 C?Hi
_______-
A
B
A
B
0.842 0.853
0.848 0.84
1.18
1.15
Analysis with c o l u m n s bypassed. A. Actual molar percent. 6.Molar percent detected a s s u m i n g c o r n p l e t e conversion t o associated alkane. Experimental Precision: 1%(molar) sample + 2% relative; 0.01% (molar) s a m p l e i 5% relative. Catalyst temperature = 290°C
0
checked by measuring the response to known concentrations of CHI.
Conversion of Unsaturated and Oxygenated Hydrocarbons The effect the catalyst might have on unsaturated or oxygenated hydrocarbons-e.g., CHaOH, C ~ H B O H , CH3CH0, CzHz-was also investigated. Compared to the alkanes, these compounds produce a smaller FID response. For example, the FID responses (per carbon atom) for CH3CHO and CzH2 are, respectfully, 60% and 65% of that for C2Hs. Therefore, if the compound is converted to an alkane, an increased response should result. Experimental results (Table I) showed that these compounds were fully hydrogenated. A distinct advantage of the catalyst-FID technique is that all of these compounds produced the same FTD response (per carbon atom). Thus, it was not necessary to calibrate separately for each of them. A special effort was made to extend the methanization technique to HCHO, which ordinarily has no FID response. Some preliminary experiments produced reasonably good results, but the apparent conversion of HCHO to CH4 was only about 80% and was essentially independent of HCHO concentration. If this observed discrepancy were due to incomplete conversion, one would expect that the response should vary with catalyst temperature. However, over the catalyst temperature range, 250-350°C, the response was unchanged. A remaining possibility was that HCHO was somehow being lost in the analysis system. A column bypass was installed to check this hypothesis. The response was then complete, and results are listed in Table I. Again, with the column bypass, the pure nickel and promoted-nickel catalysts, a t their respective operating temperatures (340°C and 300"C), converted HCHO quantitatively. Although we are unable fully to explain the experimental results, we believe that quantitative conversion of HCHO via catalytic hydrogenation can be an accurate and sensitive tool for HCHO analysis. In practice, apparent losses in the column complicate the problem in that a separate calibration is necessary for HCHO. However, as indicated above, percent losses in our columns (20%) were nearly independent of HCHO concentration over the concentration range examined (0.1-1.0%). Thus, under these conditions, this experimentally observed linear response obviates the need for a complete calibration curve.
Catals s t Kinetics Randhava et al. (1969) have shown that CO methanization over a ruthenium catalyst obeys pseudo-first-order kinetics-Le.,
when the concentration of Hz is large compared to that of CO. This fact is very important in that even a poorly
Table II. Partial Conversion of COsOver Nickel Catalyst Conc., molar %
Conc. detected, molar %
Converted, %
1.11 0.292 0.1165
0.83 0.22 0.0844
75.5 75.4 72.5
operating catalyst might still be useful. That is, if the catalyst temperature and the residence time of the sample in the catalyst are constant, the percent conversion of CO to CHI should be independent of concentration. Thus, the response of catalyst-detector systems would remain linear. An experiment was proposed to test the linearity of the nickel and ruthenium systems by operating them a t low temperatures where hydrogenation would be incomplete. For the ruthenium catalyst, these low temperatures (
