Anal. Chem. 1989, 6 1 , 1815-1821
Eckels of Ames Laboratory for his advice and assistance. LITERATURE CITED (1) Grifflths, Peter R. Appl. Spechosc. 1072, 26, 73-76. (2) Jones, Roger W.; McClelland, John F. Anal. Chem. 1989, 67, 650-656. ... ....
(3) Donaldson, D. J.; Leone, Stephen R. Chem. W s . Lett. 1986, 132, 240-246. (4) Fletcher, T. Rick; Leone, Stephen R. J . Chem. Phys. 1988, 88, 4720-4731.
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(5) Erk, S.; Keller, A.; PoRz, H. Phys. 2. 1937, 38, 394-402.
RECEIVED for review March 10,1989. Accepted May 12,1989. This work was funded by the Center for Advanced Technology Development (formerly the Center for New Industrial Materials) which is Operated for the u.s*Department Of Commerce by Iowa State University under Grant No. ITA 87-02.
Element Selective Detection after Supercritical Fluid Chromatography Using a Radio Frequency Plasma Detector R. J. Skelton, Jr.,l P. B. Farnsworth,* K. E. Markides, and M. L. Lee* Department of Chemistry, Brigham Young University, Provo, Utah 84602
A radlo frequency plasma was evaluated as an element selective detector for caplliary supercrltlcal fluid chromatography. Atomic emission from S and CI was detected at 921.3 and 837.6 nm, respectively. The analytical performance of the detector was evaluated by monitoring its response to components of several mlxtures Introduced chromatographically, and to test solutes Introduced by uslng an exponented dllutlon flask. Minimal spectral Interferences were found for the CO, and N,O doped plasmas. Detectlon limits and sensltlvltles were dependent on the mass flow of CO, into the detector. The detection iimlts ranged from 50 to 300 pg/s.
INTRODUCTION Element selective detectors, such as the flame photometric and thermionic ionization detectors, have been important to the development of gas chromatography as an analytical technique. These detectors are known to be very selective and relatively easy to operate on a routine basis. In addition, the thermionic ionization detector is among the most sensitive detectors used for chromatographic detection. Although these detectors are well established, they are not without shortcomings. The investigation of element selective detectors has continued, with much attention given to the use of atomic emission as a basis for detection. Detection based on atomic emission offers many potential advantages. First, atomic emission lines are narrow and often intense, affording high selectivity and sensitivity. In addition, the atomic-emission-based detectors are tunable. A single detector can be used for several elements, instead of only one or two as with conventional selective detectors. Also, all emission lines are present in the plasma simultaneously, and either single or multichannel detection is possible. The most common device based on this principle is the microwave induced plasma detector (MIP), which was first reported by McCormack et al. ( I ) and Bache and Lisk ( 2 ) . The number of workers using MIPS is significant, and many excellent applications have been described (3-6). Other detectors based on plasmas have also been investigated. These include direct current discharge (7), glow discharge (8,9), and inductively
* Authors to whom correspondence should be addressed. *Current address: Shell Development Co., P.O. Box 1380, Houston, TX 77251. 0003-2700/89/0361-1815$01.50/0
coupled plasmas (10-15). Rice et al. have worked extensively with afterglow detectors (16),and they have recently reported some encouraging results from a helium afterglow detector based on a primary discharge generated by a radio frequency (rf) power source (17). More recently, we reported the use of a helium radio frequency generated plasma as a gas chromatographic detector (18). This detector is based on an rf discharge formed directly between a pointed metal electrode and a ground, and it has shown promise as a simple, tunable multielement selective detector for gas chromatography. This detector differs from that described by Rice et al. (17) in that emission is observed from the interelectrode region through the wall of a quartz tube. No attempt is made to divide the discharge into a primary discharge and afterglow. Small quantities of oxygen are doped into the plasma to create an ideal environment for sample decomposition and atomic emission. Intense emission lines for the nonmetals of chromatographic interest exist in this portion of the spectrum,and background emission from the plasma arising from molecular species is much less intense than in the UV region. Because of this spectral simplicity, low-resolution light sorting can be used. The combination of a doped helium plasma and the use of the near-infrared emission allows for a simple, low-cost detector. Because of the good operational characteristics of this plasma for gas chromatographic detection, its use as a detector for supercritical fluid chromatography (SFC) was investigated. Other element selective detectors, such as the thermionic ionization and the flame photometric detectors, have been employed with SFC (19,20),but their use has not yet become popular because of problems of detector incompatibility with the SFC mobile phases. Recently, the use of surface-wavesustained microwave induced plasma system was evaluated for the detection of sulfur-containing compounds after SFC (21,22). In like manner, this paper is intended as an evaluation of the potential use of the radio frequency plasma detector with SFC, including its inherent advantages and limitations. EXPERIMENTAL SECTION Supercritical Fluid Chromatography. The supercritical fluid chromatograph consisted of a Hewlett-Packard 5890 gas chromatographic oven (Hewlett-Packard, Avondale, PA) equipped with a flame ionization detector. A varian 8500 syringe pump (Varian Associates, Palo Alto, CA) controlled by an Apple IIe computer was used to deliver the supercritical fluid. The COz and N20 used were of SFC grade (Scott Specialty Gases, Plum0 1989 American Chemical Soclety
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1. schematk dlagam oi Um radio frequency plasma detector
cell.
shdsville, PA). The columns employed were either purchased from Lea Scientific (Salt Lake City, UT)or made in our laboratory hy pmcedures described in previous publications (23).Capillaries were 50 pm in internal diameter and were mated with film thicknesses between 0.25 and 0.5 wm. The stationmy phases used are listed appropriately in the text. Injection into the capillary column was achieved hy using a VICI (Houston, TX) valve with a 0.2-pL sample loop. Either split or delayed split (24)techniques were used for sample introduction The solute mas on the column was in the range of 4C-100 ng per aolute for the applications shown in this publication. Radio Frequency Plasma Detector. The configuration for the radio frequency plasma detector (RPD) was very similar to the one reported for gas chromatography. Figure 1 shows a schematic diagram of the detector used in this work. The radio frequency power source (EN1 Power Systems, Rochester, NY) was connected to the stainless steel electrode hy a high-voltage cable. This cable was heavily insulated to prevent arcing to any instrumentation. The ground from the power supply was connected to the mounting block for the detector. The stainless steel electrode was held in place above the quartz discharge tube by a ceramic cell body machined from Macor (Coming Glass Workss, Corning, NY), which also acts as an electrical insulator. The quartz discharge tube had an internal diameter of 1mm. This was held in place by a graphitized vespel ferrule seated in the detector base. High-purity helium (99.9999%. Scott Specialty Gases, Plumsteadsville, PA) entered from the side of this base and passed up through the quartz tube. Flow regulation was accomplished by a bellows type needle valve placed between a high-purity gas regulator and the detector base. This helium passed up and around the frit restrictor, which was placed ahout 1cm into the plasma. The helium flow rate waa optimized mound 65 mL/min for GC experiments and around 100 mL/min for SFC. The dopant gases used in GC were unnecessary for the SFC experiment. The plasma power source was tuned to minimize reflected power at a frequency around 330 KHz. Power levels employed ranged between 60 and 100 W, with the higher power levels suited better for the SFC experiments. A slot window in the side of the Macor body allowed for observation of the atomic emission through the quartz tube. A 0.32-m-focal-length monochromator (Model HR-320, Instruments S.A,, Metuchen, NJ) equipped with a 1200 gmove/mm
grating Was mounted on an x,y translating stage. The monodvomator,was equipped with continuously variable entrance and exit slits. Most experiments utilized slit widths of 0.5 mm, which maximized optical throughput without seriously affecting selectivity. The spectral bandpass was approximately 1.3 nm. The optics were held in an anodized aluminum tube mounted directly on the entrance slit housing of the monochromator. The opt& consisted of a pair of 12.7-mm-diameter achromatic lenses (f = 50.8 mm, Newport Corp., Fountain Valley, CA) and a 12.7-mmdiameter UV cutoff filter (595-nm cutoff, Oriel Corp., Stratford, CT). The translating stage allowed for easily controlled viewing of the discharge. Detection of the near-infrared radiation was accomplished with a red-sensitive photomultiplier tube (R2658, Hammamatsu, Middlesex. NJ). This InGaAs tube had a red reaponse extending to 1010 nm and waa operated at approximately loo0 V. The current generated hy the photomultiplier was fed into an electrometer (Keithley Model 602, Cleveland, OH). The output was filtered with a low-pass fdter and recorded with a strip chart recorder (Houston Instruments, Houston, TX). Figure 2 illustrates the complete SFC-RPD system. Exponential Dilution. Exponential dilution experiments were done with a mass flow meter (Sierra Instruments, Camel Valley, CA) and a gas handling syringe. The dilution flask was constructed from a glass sample bulb purchased from Chrompack (Bridgewater, NJ). Dead volume was minimized by use of 320rm-i.d. fused silica tubing to transfer the gas to the detector. The effluent from the exponential dilution flask was added to the plasma support gas through a tee connection. Mobile phase waa introduced into the plasma through a capillary chromatographic column configured as described above. A digital oscilloscope (Nicolet Instruments, Madison, WI) was used to collect and process the data. Further details of this experiment are given later. Reagents. Supracide and Chlorpyrifos were obtained from Ciba Geigy, Greensboro, NC. DDT, a-BHC, BBHC, and chlordane were obtained from the Pesticides Repwitory, Perrine, FL. Carbofwan was obtained from FMC Corp.,Middleport, NY. Spectroscopic grade CSz (Fisher, Fairlawn, NJ) and CHZCI2(EM Science, Cherry Hill, NJ) were used in the exponential dilution experiments.
RESULTS AND DISCUSSION The mobile phases commonly used in SFC are molecular fluids. Because the plasma does not differentiate between molecular species, energy from the active species in the plasma is used for decomposition and excitation of both the mobile phase and the d y t e . In addition, when passive restrictors such as frits are used, the masa flow rate of the mobile phase into the detector increases as the supercritical fluid is density-programmed. These complications are the primary reasons for incompatibility of SFC with traditional element selective detectors. T o be useful, the detector must be insensitive to the increasing amounts of fluid entering the system. Because the RPD is bhsed on atomic emission, the detector would not be expected to respond to a fluid like COPunlesscarbon or oxygen were being monitored. Plasma sources, however, usually exhibit emission arising from molecular species such as CO, C2, and CN. In our case, emission in the near-infrared portion of the spectrum is utilized for element selective detection, and CO and C,emissions, which lie primarily in the W and visible regions, are not a serious problem. Emission from the CN molecule is a potential problem in the near-infrared, however, since several bands exist in this spectral region. In the case of COP,the large amount of oxygen in the plasma limits the formation of CN because CO is somewhat more stable. The formation of CN in significant amounts should not occur because only trace amounts of nitrogen exist in a clean system. The presence of COz in the system gives a green appemance to the otherwise orange-pink color of the pure helium plasma. The presence of NzO in the system gives a purple hue to the plasma. These colors, from the green Cz band the blue Nz+
ANALYTICAL CHEMISTRY. VOL. 61. NO. 17, SEVTEMBER 1, 1989 ___4
Fgure 2. Schematic diagram of the SFC-RFU system.
m
E a
mo
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om
WAVQENQTH 0 Fgvn 9. Spectrum of the near-Infrared region of the helium radio frequency plasma wntalning CO,.
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am
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Fgum 4. Spectrum of the near-infrared region of the helium radio frequency plasma wntainlng N@.
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 17, SEPTEMBER 1, 1989
2
B YOCNHCH,
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i 0.3 0.3
a4
OA
I
I 30
I
0
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0.55
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Figure 5. Supercritical fluid chromatograms of a standard pesticide mixture. Conditions: 2.5 m X 50 pm i.d. biphenyl capillary column (0.5-pm film thickness); CO:, at 100 OC. Key: (A) FID; (B) RPD (837.6 nm for chlorine): (C) RPD (921.3 nm for sulfur).
band, are indicative of significant concentrations of these species. Fortunately they produce little emission in the near-infrared region. Figures 3 and 4 show spectra of the near-infrared portions of the spectra for systems containing COPand N20, respectively. The atmospheric pressure flow rate for both gases was approximately 1 mL/min. Major features of the spectra are labeled, with the emission lines for the major nonmetals of chromatographic interest labeled in the locations where they would occur. It can be seen that for many elements, these fluids do not create major background interferences. Although these spectra indicate that the potential exists for element selective detection with these fluids as mobile phases, other possible problems exist. As discussed earlier, the energetic species in the plasma attack the mobile phase molecules as readily as the analyte. If too much of the mobile phase is in the plasma, the plasma will be quenched and sensitivity will be lost. In an effort to reduce this problem, 50-pm-i.d. capillary columns of 2-3 m in length and mobile phase flow rates near the optimum linear velocity were used to carry out the separations. These columns afforded good separations due to the efficiencies obtained at the low linear velocities. The short lengths of the columns kept the total analysis times within reason. This combination allowed the use of only slightly higher than normal plasma support gas flows of 100 mL/min (compared with GC). This is important, since sensitivity drops as the support gas flow increases beyond approximately 50 mL/min. The 100 mL/min helium flow allowed reasonable sensitivity while maintaining a robust plasma, even when the mobile phase was programmed to high densities. The frit restrictors used to maintain flow control from the capillary columns did not show any signs of degradation during these experiments, although the polyimide coating on the columns was stripped from the 1-cm length inserted into the plasma. The atmospheric pressure flow rate of mobile phase varied from 0.8 to 3 mL/min during a density program from 0.3 to 0.7 g/mL. Figure 5 shows three chromatograms of a pesticide test mixture. The first chromatogram (A) is a flame ionization detector (FID) trace of the separation. The second (B) was generated by using the RPD, monitoring 837.6 nm for chlorine selective detection. The third chromatogram (C) was from the RPD by monitoring 921.3 nm for sulfur selective detection. It can be seen that the selectivity obtained is excellent. In
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Figure 6. Supercritiil fluid chromatogram of chlordane and impurities. Conditions: 2.5 m X 50 Fm i.d. biphenyl capillary column (0.5-pm film thickness): C02 at 100 OC; chlorine detection (837.6 nm).
the case of chlorine, even the large amount of solvent resulted in a negative response. The base line in each case drifted somewhat as a function of density programming. Slight adjustments to the support gas flow allowed this drift to be minimized. Also, the drift was reproducible for given parameters so that base-line compensation, such as that used with UV detection after SFC, could be used. Figure 6 is a chromatogram of chlordane and its related impurities. The chlordane used to make the standard was of technical grade, and many isomers and degradation products of the compound exist. The detection of these compounds with chlorine selectivity (notice the negative solvent response) suggests that they contain chlorine. The polychlorinated nature of chlordane would seem to support this result. Figure 7 shows the selective detection of DDT in a milk extract containing primarily triglycerides. The presence of
ANALYTICAL CHEMISTRY, VOL. 61, NO. 17, SEPTEMBER 1, 1989
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20 40 60 min min 0 Figure 7. Supercritical fluid chromatogram of a milk extract containing DDT. Conditions: 2.5 m X 50 pm i.d. biphenyl capillary column (0.5-pm film thickness); COP at 100 OC. Key: (A) FID; (B) chlorine detection (837.6 nm).
0
20
40
50
triglycerides in the sample makes difficult the analysis of this mixture by gas chromatography under normal conditions. It should be noted here that the linear velocity used with the SFC-RPD experiments was less than that used with the FID. In practice, the lower flows were more compatible with the detector, and the corresponding frit was left installed in each detector during the duration of the experiment. Therefore, even though the samples are the same, retention times cannot be compared directly. Figure 8 show a chromatogram of four chlorine-containing pesticides eluted by using NzO as the mobile phase. The chromatogram here shows a normal profile with a large solvent response, since methylene chloride was used to dissolve the sample. Base-line drift in this case was still acceptable. Although solute detection with NzO in the plasma was successful, the selectivity is probably poorer than would be obtained with COS because of interference at the analytical wavelengths from CN bands. No base-line compensation was employed for any of the chromatograms shown. Selectivity measurements were not made for the elements studied here. In practice, only solvent interferences were observed. This is consistent with selectivity measurements observed with gas chromatography (18). The main limitation to the system as described is probably sensitivity. Although the use of 50-pm4.d. columns limits of the amount of COz entering the detector, the sample capacity is still good (50-100 ng per component). One advantage of a plasma emission detector is that the response of the detector to the amount of element present has little to do with the molecular structure of the analyte. However, because a passive restrictor was used in these experiments, it is likely that the sensitivity of the detector changed as a function of retention time in densityprogrammed runs due to the increased flow of mobile phase. Exponential Dilution. In order to characterize detector performance, exponential dilution experiments were carried out to measure sensitivity, linearity, and detection limits as a function of C 0 2 density (25,26). The apparatus used in the experiment is shown in Figure 9. When a known quantity of compound was introduced into the dilution flask, the amount of that compound entering the detector could be accurately calculated on the basis of the flow rate through the flask and the volume of the flask. A mass flow meter was used
clt(cl
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3
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CI
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20
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Figure 8. Supercritical fluid chromatogram of a pesticide standard mixture. Conditions: 1.5 m X 50 pm i.d. biphenyl capillary column (0.5-pm film thickness); N,O at 100 OC; chlorine selective detection (837.6 nm).
to measure the flow rate of helium into the flask. The volume of the flask was measured exactly by weight, using a liquid of known density. The compound containing the element of choice was first diluted in helium by using a 100-mL sample bulb so that the weight and concentration could be accurately known. The gas handling syringe was then used to transfer the gas into the dilution flask. The concentration as a function of time could be derived from the equation
C, = Co exp(-Ft/V)
(1)
where Ct is concentration at time t , Co is the initial concen-
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 17. SEPTEMBER 1, 1989
TIME (SECONDS)
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2000
3000
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Schematic diagram of the exponential dilution system.
Table 1. Detection Limits and Relative Sensitivities of SFC-RPD a8 a Function of Density element
100 atm
sulfur chlorine
59.6 116
sulfur chlorine
200 atm
300 atm
Detection Limits' 61.2 114 148 152
Relative Sensitivity 67.5 64.8 54.9 32.6 23.3 19.6
greatly depends on the flow of COP into the detector. Sensitivities, however, are good for these elements when COz is used as the mobile phase. Certainly there are same restrictions of use for the detector operating in its present form. Constant mass flow restriction would be a great asset to this technique. Further work, including the varying of electrode geometry to optimize this unit for SFC, must still be carried out to determine the ultimate utility of the RPD coupled to SFC.
400 atm 178 260 42.6 15.7
Registry NO. DDT, 50-29-3;co,, 124.389; N,O, 10024-97-2; chlordane, 12789-03-6.
#Detection limits eiven in oels. tration, and F is the flow through the flask. Relative sensitivity and linearity could then be determined from a plot of
In (xt) = In (Co) + In (S) - F t / V
Flgure I O . Results from exponential dilution: (A) exponential dilution ewes fw sulha determinations at differem pressures and (6) a derived linear plot (from eq 2) for IOOatm curve showing the detection limits and uowr limits of lineark. The atmsoherk oressure flow rates of C02r&ed from appoximiteiy 1 mUmin at I00 abn to appoxmateiy 3 m L h n at 400 atm
(2)
where xt is the signal a t time t and S is the relative sensitivity. Such a plot would be a straight line over the linear range of the detector. The detection limit could he calculated by deriving the concentration that gave a signal twice the standard deviation of the backmound. These expressionswere experimentally generated exponential dilution curve, which maps detector response as a function of time. Comparison of the theoretical curve with the experimentally observed one and calculations of the response of the detector as a function of time yielded the desired information. The detector was evaluated by using methylene chloride and carbon disulfide to determine the response for CI and S, respectively. The experiments were repeated a t increasing densities to determine the effect of increasing amountsof COS on detector response. Figure 10 shows exponential dilutiion curves for four sulfur determinations. Also included is a derived linear log plot on which the upper limits of linearity and the detection limits are indicated. The sensitivity data obtained from these experiments are given in Table I. It can be seen that the relative sensitivity (S in eq 2) of the detector
LITERATURE CITED (1) McCormack. A. J.; Tong. S. C.; M e . W. D. Anal. WWm. 1985. 37, 1470. (2) BBche, C. A.; Llsk. 0. J. Anal. Chem. 1985. 37, 1477. (3) Camahan. J. W.; Mullban. K. J.; C a w . J. A. Anal. chm.Acta 1981, 130. 227. 14) haas. D. L.: Caruso. J. A. Anal &m. 1985, 57. 846. (5) Ceibm. C. S.; Wuck. J. A. SpMroCrwn .Acta 198'3. 388. 387. 16) . . K%Wei. 2.: Q l n ~ V u .0.: GUDChUen. W.: WeIIu. Y. SmchccMn. Acta 1985.40B': 349. (7) Evans. G. L. Anal. chem.1988. 40, 1142. (8) Tamklns. 8. A.; Feldman, C. Anal. Chbn. Acta 1980. 119, 283. (9) Treybig. D. S.: Ellebrachl, S. R. Anal. Chem. 1980. 52, 1033. ( I O ) Windsar. D. L.; Demon. M. 8. Anal. Chem. 1979, 51. 1116. (11) Somer. 0.; Ohls. K. Fres&'Z. Anal. chem. 1979. 295. 337. (12) Brown, R. M.: Northway. S. J.: Fly, R. C. Anal. Chem. 1981, 53, 934. (13) Eckholf. M. A.: McCarthy. J. P.; Caruro. J. A. Anal. chem. 1982. 54. 165. Wokick. K. A.: Miller. D. C.: Sellskar, C. J.; Frkke. F. L. A@. Spsctrosc. 1985. 39. 930. Fujimoto. C.; Yoshida. H.; Jhno. K.; J . MkrmWl. Sep.1989, 1 . 19. Rice. 0. W.; Richard, J. J.; Dsilva. A. P.; Fasel. V. A. Anal. CNm. Acta 1982. 142, 47. Rice. 0. W.; DSilva. A. P.; F a s d , V. A. Specbahm. Acta 1985, 408. 1573. Skekon. R. J.. Jr.; MBrkldes. K. E.; Famsworth. P. 6.; Yang. F. J.; Lee. M. L. hWC CC. J . h7gh Resolut. Chomatogu. chmmatogr. Commun. 1988. 11.75. West. W. R.; Lee. M. L. HSC CC. J . h7gh R s W . m m g . Wromalogr. Mmmun. 1988, 9 . 181. Mark&. K. E.: Lee, E. D.: Wick. R.; Lee. M. L. Anal. chem. 1988, 58. 740. , Galante. L. J.; Selby. M.; Lufhlr. D. R.; Henle, G. M.: NoMmy. M. Anal. Chem. 1988. 60. 1370.
Anal. Chem. 1989, 6 1 , 1821-1825 (22) Luffer, D. R.; Galante, L. J.; DavM. P. A,; Novotny, M.; Hieftje, G. M. Anal. Chem. 1988, 60, 1365. (23) Woolley, C . L.; Tarbet, B. J.; MarkMes, K. E.; Bradshaw, J. S.; Bartle, K. D.; Lee, M. L. HRC CC , J . H@h. Resoluf Chromatcgr . Chroma tcgr. Commun. 1988, 1 1 , 113. (24) Lee, M. L.; Xu, B.; Huang, E. C.; Djordjevic, N. M.; Tuominen, J. P.; Chang, H.G. K.; Markldes, K. E. J . Mlcrocol. Sep. 1989, 1 , 7. (25) Beenaker, C. I . M. Specfrochlm. Acfa 1977. 328, 173.
.
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(26) van D a b , J. P. J.; de Lezenne Coulander, P. A,; de Galan, L. Anal. Chim. Acfa 1977, 9 4 , 1.
-
REXEXVED for review October 21,1988. Accepted May 15,1989. This work was funded by the State of Utah, Centers of Excellence Program.
Spin Dynamics in the Analysis of Carbonaceous Deposits on Zeolite Catalysts by Carbon- 13 Nuclear Magnetic Resonance with Cross Polarization and Magic-Angle Spinning Benny R. Richardson and James F. Haw* Department of Chemistry, Texas A&M University, College Station, Texas 77843
The potentlal of solid-state 13C nuclear magnetlc resonance spectroscopy with cross poladzatton and magbangle splnnlng (CPIMAS) for the characterization of the carbonaceous deposits that form on zeolite catalysts during hydrocarbon processing Is explored. Partlcular attentlon Is given to the extent to which the results can be regarded as quantitative. The samples consldered In thls lnvestlgatlon were prepared by reactlon of butadiene on catalyst pellets containing zeolite HY In a flow reactor at temperatures between 150 and 600 OC. The NMR studies performed Included relaxation measurements to characterlze the spin dynamics relevant to quantltatlon and a variable-temperature 13C CP/MAS experlment. Comparlson of carbon spin counting results wlth carbon content from combustlon analysis revealed that atthough 78 % of the carbon was detected by NMR for the sample obtalned from the 150 OC reactor run, slgnlflcantly less carbon was detected for samples coked In the reactor at higher temperatures. Thls resuit correlated with the observation of organlc free radicals In the samples, but It could also be due In part to lneff lclent cross polarlratlon In hydrogen-deflclent regions.
INTRODUCTION Aluminosilicate catalysts such as zeolites are used in a number of important industrial processes including the cracking of fuel oil to yield gasoline-range products, hydrocarbon synthesis from methanol, and a number of isomerization and disproportionation reactions (1-3). A limiting factor in aJl of these processes is the formation of carbonaceous deposits (termed coke), which eventually deactivates the catalyst and necessitates regeneration (4-6).It has been stated that coke formation is one of the least understood phenomena in catalytic cracking (7). The chemical structure of coke deposits and the mechanisms by which they form have been, therefore, of considerable interest. As a result of the low solubility of coke deposits, especially those formed a t higher temperatures, most efforts to characterize the deposits have focused upon the coked catalyst particles themselves, without a prior attempt to separate the carbonaceous material from the inorganic catalyst and/or binder, although degradative methods involving either acid
* Author
to whom correspondence should be addressed. 0003-2700/89/0361-182 1$0 1.50/0
(8,9) or base (6)digestion of the catalyst matrix have also been proposed. Techniques previously applied to the characterization of coked catalyst samples have included elemental analysis (IO),electron microscopy and X-ray diffraction (11), and IR spectroscopy (12,13). There have also been several previous studies that have reported 13C CP/MAS spectra of carbonaceous residues on zeolites. For example, Derouane and co-workers have studied the residues formed in zeolites H-ZSM-5 and mordenite during the reactions of I3C-enriched methanol or 13C-enriched ethylene (14). The entrapped reaction products observed in that study were predominantly low molecular weight alkanes and simple alkylaromatics such as ethylbenzene and were therefore not properly termed carbonaceous deposits. Carlton and co-workers published 13C CP/MAS spectra of coked ZSM-5 samples that had each been subjected to one of several reactivation procedures (15). Weitkamp and Maixner studied the residues formed at relatively low temperatures by isobutanejbutene alkylation on a NaNH,Y zeolite (16). That study reported an increase in aromatics as the reaction temperature was increased from 80 to 314 OC. In none of those studies was there an investigation of the optimum conditions for the study of coke deposits on zeolites by 13CCP/MAS NMR, nor was there an investigation of the more complex deposits that are known to form on acidic Y zeolites at elevated temperatures (7, 8). At first glance, the application of I3C CP/MAS NMR to the characterization of coke deposits on oxide catalysts might appear to be a straightforward task. The experience of workers familiar with the application of CPjMAS NMR to coals (17, 18), lignins (19),and other complex carbonaceous materials (20-23), however, suggests that analogous studies of coke deposits be approached cautiously, especially if quantitative results are important. The cross polarization experiment is prone to errors in quantitation, especially for samples that are hydrogen deficient. Furthermore, the presence of paramagnetic sites can complicate efforts at quantitation by severely broadening resonances due to carbons in the vicinity of radical sites or by adversely affecting relaxation phenomena central to cross polarization dynamics. In this contribution, we have taken one catalyst (zeolite HY) and coked it with a single feed (butadiene) a t six different temperatures under otherwise identical conditions. We have then performed the detailed measurements necessary to evaluate the reliability of 13C CPjMAS NMR for these samples. We find that the 13C CPjMAS spectral intensities de0 1989 American Chemical Society
