Fast Thermolysis/ FT-IR Spectroscopy N e w Dimensions in Combined Thermal Analysis and Spectroscopy
Thomas B. Brill Department of Chemistry University of Delaware Newark, DE 197 16
Real-time analysis of dynamically changing chemical or physicochemical processes offers the opportunity to characterize events in a way only hinted at by static analysis. Of course, many dynamic analytical techniques exist, but few allow scientists to probe the neat reacting condensed phase in situ and in near-real-time at high heating rates. The high heating rate condition is relevant for fundamental research in combustion- and explosionlike environments, but it also provides a means (by thermolysis or pyrolysis) to analyze materials routinely by studying the gas products liberated. Fast pyrolysis/FT-IR spectroscopy using filament heating was first described in 1976 by Liebman et al. (1). However, combining a thermal analysis technique with FT-IR spectroscopy generally implies TG (thermogravimetry)/FT-IR. (See Reference 2 for a review of modern thermogravimetry.) With TG/FT-IR, the mass change of the sample is monitored as a function of temperature while the evolved gases are channeled into an IR spectrometer beam (3). The relatively slow heating rate of TG (~20 °C/min) enhances thermal equilibration and facilitates quantifying the data. By imposing a faster heating rate (~170 °C/s), howev0003-2700/89/0361-897A/$01.50/0 © 1989 American Chemical Society
er, one gains a new dimension in combined thermal analysis and spectroscopy. Temperature and mass changes as well as iR active gases can still be measured, but they are measured as a function of time during the rapid heating phase. An advantage of the real-time/ fast heating approach is the detection of some relatively reactive molecules that are lost to side reactions at slower heating rates or with time delays in the detection step. Thus reaction schemes different from those occurring with slow heating can be studied by fast heating. The technique becomes even
gases at a high heating rate. The examples here come from the rocket propellent and explosives field, but the methods are applicable to any material that can be thermally decomposed. Near real-time FT-IR spectra of gases from fast-heated compounds When studying combustion and explosion phenomena, one needs to obtain thermal decomposition measurements at relevant heating rates and pressures. This information can be derived by using a fast-thermolysis cell (4-5, 8-9), shown in Figure 1. The antireflection-
INSTRUMENTATION more powerful because temperature and mass changes can be measured simultaneously. The broad area described here is fast thermolysis/FT-IR spectroscopy (4). Variations on this basic theme include temperature profiling/FT-IR spectroscopy (5), in which the temperature changes of the condensed phase are measured simultaneously with the gas evolution; fast-heat-and-hold/FT-IR spectroscopy (6), in which isothermal decomposition is studied following rapid heating to a selected temperature; and SMATCH/FT-IR (7), in which Simultaneous M-Ass and Temperature Cifange measurements are made along with the IR detection of the evolved
coated 0.5" X I " diameter ZnSe windows are held in a 3" diameter aluminum cylinder by brass end caps. The cell was designed to withstand a static pressure of 5000 psi but is used only in the 1-1000-psi range. The filament is a creased nichrome IV ribbon (2.5 X 0.6 X 0.012 cm) supported on pressuretight feedthrough insulators. Although studies were not conducted on a variety of compounds, a detailed study of a sample of a liquid gun propellant expected to be especially sensitive to catalysis revealed little dependence of the thermolysis products on the filament material {10). Typically, 1-2 mg of sample (solid, liquid, or mixture) were heated using a Foxboro
ANALYTICAL CHEMISTRY, VOL. 6 1 , NO. 15, AUGUST 1, 1989 · 897 A
INSTRUMENTATION 40 Pyrochem controller. The constant voltage-variable current aspect of the controller is useful for the tempera ture-profiling experiments discussed below. In principle, any reasonable heating rate of the sample could be achieved, but a rate < 400 °C/s was chosen because the spectral collection rate does not distinguish processes at higher heating rates. True combustion heating rates are thousands of degrees per second; thus, the heating rates of fast thermolysis/FT-IR fall between those of conventional thermal analysis and combustion. The argon gas pres sure in the cell was adjusted as desired in the 1-1000 psi range. Because of the importance of collect ing IR spectra at high temporal resolu tion, the rapid-scan mode of a Nicolet 60SX FT-IR spectrometer was used in all of these studies. Typically, the in strument was operated at a rate of 10 scans/s, 2 spectra per file, and 4 c m - 1 resolution. With the beam fo cused several millimeters above the fil ament surface, the IR-active gas prod ucts from the fast heated sample can be detected in near real time. No signifi cant change in the pressure occurred from the evolved gases because of the small sample size. However, the gas composition can differ somewhat de pending on whether the sample is thin ly spread or heaped. Small samples that are thinly spread on the filament give more reproducible results. In the above experiment only a small time delay exists between thermolysis and the detection of the evolved gases because the gases need only diffuse several millimeters from the sample to reach the IR beam. The argon atmo sphere is relatively cool, so that the gas es are in their ground states based on the Ρ and R rotational branches. Fig ure 2 illustrates the good quality of the spectra that are obtained for a sample of RDX explosive. When heated at a rate dT/df m 170 °C/s from room tem perature, the initial gases from RDX are first detected at about 1.15 s be cause thermal decomposition occurs at about 200 °C. By using the IR intensi ties and effective linewidths, the ob served absorbances can be converted to the relative percent composition of the IR-active gases (4). The moderate tem perature and relatively high pressure in the cell preclude the detection of un stable radicals and molecular frag ments in the scan time available. Figure 3 shows relative gas composi tion versus time profiles for RDX. H2O, IR-inactive molecules, and any species for which the IR intensities are un known (i.e., HNCO) are not included. The importance of near real-time anal ysis of the gas products in fast-ther-
Figure 1. Basic fast thermolysis/FT-IR spectroscopy cell. The pressure ports and gauge are drawn in abstraction.
Figure 2. Selected absorption spectra of the gas products, along with a small amount of RDX aerosol, when 2 mg of RDX is heated at 170 °C/s under 15 psi N2. The times given are those following the onset of heating. Spectra were recorded at 4 c m - 1 resolution, 2 spectra per file, and 10 scans/s.
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molysis research is evident from Figure 3. NO2 is the dominant early decomposition product of RDX, but, because of secondary redox reactions, it decreases rapidly in concentration whereas NO increases. Note that NO is negligible at the onset of decomposition. As a result, if the time delay between the analysis and the onset of thermolysis were to exceed 5 s, then different and potentially incorrect conclusions could be drawn about the thermal decomposition process of RDX. The changes in the gas concentrations with time indicate secondary reactions among the gases, whereas the initial gas composition most closely reflects the thermolysis of the parent molecule. Fast-thermolysis/FT-IR data as described here are mostly the result of condensedphase chemistry. Studies of the formation of HONO from secondary nitramines, R 2 NN0 2 (R = -CH2-) illustrate an advance made possible by fast thermolysis/FTIR (11). HONO has been considered to be an important intermediate in the thermal decomposition of nitramines (12), but because of its reactivity, it was
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Figure 3. Relative percent composition versus time profile of the quantified gas products from RDX in Figure 2. Any H 20, HNCO, IR-inactive products, and RDX aerosol are not included. Note the strong time dependence of several products.
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INSTRUMENTATION previously proposed from indirect evidence (13,14). By using fast thermolysis/FT-IR, we can readily detect both cis- and trans-HONO in the IR spectrum of the gas from RDX (see the PQR pair at 700-900 cm"1 in Figure 2). However, as shown in Figure 3, HONO is transient under the conditions of the experiment. The initial concentrations most closely reflect the connection of the gas products to the composition of the parent molecule. Figure 4 shows the quantity of HONO as a percentage of the initial gas products for various nitramines (11) versus the H/NO2 ratio in the parent molecule. The general trend suggests that HONO arises from adventitious bimolecular encounters of H' and NO2'
radicals in the condensed phase (11) rather than from concerted decomposition of the 4- and 5-center intermediates (below) that may contribute in unimolecular gas-phase reactions (15).
Fast thermolysis/FT-IR thus far also has made key contributions to uncovering relationships between the parent molecular structure and decomposition
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Figure 4. Plot of the initial relative percent concentration of HONO versus the H/NO2 ratio in the parent molecule for a series of nitramine compounds. DMHDNA = N, N'-dimethyl-W, N'-dinitro-1,6-hexanediamine; MBNA = W-methyl-W-nitro-1-butanamine; frans-TNAD = fraf?s-decahydro-1,3,5,7-tetranitropyrimido[5.4-d]pyrimidine; DNCP = 1,3-dinitroimidazolidine; DNP = W, W'-dinitropiperidine; TNSD = 1,3,7,9-tetranitro-1,3,7,9-tetraazaspiro[4.5]decane; TNSU = 2,4,8,10-tetranitro-2,4,8,10-tetraazaspiro[5.5]undecane; TNDBN = 1,3,5,7-tetranitro-3,7-diazabicyclo[3.3.1]nonane; DMEDNA = N, W'-dimethyl-W, W'-dinitro-1,2-ethanediamine; c/s-TNAD = c/s-(±)-decahydro-1,3,5,7-tetranitropyrimido[5.4-i/]pyrimidine;HNDZ = 1,3,3,5,7,7-hexanitro-1,5-diazacyclooctane; DNNC = 1,3,5,5-tetranitrohexahydropyrimidine; RDX = hexahydro-1,3,5-trinitro-1,3,5-triazine; TNAZ = 1,3,3-trinitroazetidine; HMX = octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine.
900 A · ANALYTICAL CHEMISTRY, VOL. 61, NO. 15, AUGUST 1, 1989
INSTRUMENTATION gases such as HONO, N 0 2 (16), CH 2 0 (17,18), and NO (11). These gases are important in determining the ignition and flame chemistry as well as the haz ards of the parent compounds at ele vated temperature. Temperature profiling/FT-IR spectroscopy In the fast-thermolysis/FT-IR method discussed above, the final temperature and heating rate of the filament were established by spot-welding a type J thermocouple to the filament and re cording its output on a digital oscillo
scope. Subsequently it was found that by positioning a type Ε thermocouple on the underside of the filament oppo site the sample and leaving it in place during the thermolysis experiment, the endothermic and exothermic events of the condensed phase were tracked si multaneously with the detection of the gas products. This led to the tempera ture profiling/FT-IR technique (5). Development of this technique was straightforward because the heating of the filament is under constant voltagevariable current control. Figure 5 shows a block diagram of
Figure 5. Block diagram of the circuit used for real-time filament temperature mea surements in temperature profiling/FT-IR spectroscopy.
Figure 6. Filament temperature trace without sample (reference), the filament trace with 2 mg of ethylenediammonium dinitrate (EDD) spread on it (sample), and the dif ference thermal trace (sample trace minus reference trace). The initial heating rate is about 70 °C/s, and the atmosphere is 15 psi Ar.
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the circuit used for this temperatureprofiling experiment. The 60-Hz noise on the filament was removed by a lowpass filter, and the signal was amplified by about 100 X with a differential am plifier. The analog output was pro cessed through a Metrabyte DAS-16 A/D converter to an IBM-PC. The "take data" cycle of the interferometer triggered the heater so that there is a direct correlation between the time, temperature, and interferogram. Four hundred data points were collected in the 10-s temperature measurement. Figure 6 illustrates the type of re sults now available; the sample under study is ethylenediammonium dini trate (EDD). A reference thermal trace (filament with no sample present) and a sample thermal trace (filament with 2 mg of EDD thinly spread on the cen ter portion) are shown superimposed on the difference trace (sample trace minus reference trace). Most of the endotherms and exotherms are evident in the sample thermal trace, but the dif ference trace clearly shows an endotherm preceding an exotherm in the 300-330 °C range. The temperature signature in Figure 6 will be explained below for EDD, but first let us consider several sources of endotherms and exotherms in the fast heating conditions used. Melting, sub limation or evaporation, evolution of decomposition gases, and endothermic chemical reactions in the condensed phase are common origins of endo therms. Exothermic chemistry in the condensed phase and filament "catch u p " are sources of exotherms. Exother mic gas-phase chemistry makes a negli gible contribution to the filament tem perature in this experiment. Filament catch up results from the fact that the portion of the filament in contact with the sample and thermocouple can have a lower temperature than regions away from the sample. This is because the sample may leave the filament by en dothermic decomposition off-gassing, evaporation, or sublimation. Toward the end of this process heat can flow rapidly from the hotter regions of the filament toward the cooler thermocou ple area, resulting in a rapid tempera ture rise. Therefore, an apparent ex otherm is sensed that may not be con nected to an exothermic chemical event in the sample. A true sample exotherm is a tem perature rise that drives the sample thermal trace above that of the refer ence trace. An apparent exotherm that leaves the filament temperature below the reference trace may or may not be attributable to exothermic chemistry. In this case, a chemical event cannot be readily distinguished from catch up by
W H E A T O N a single thermocouple measurement. Detailed quantitation of the thermal trace, as is done in differential scanning calorimetry and differential thermal analysis measurements, is difficult because of the complexity of the heat transfer phenomena at these high heating rates. Nevertheless, much qualitative information is produced, and we are attempting to move toward more quantitation. Returning to the description of the thermal decomposition of EDD, we see that Figure 7 shows the quantified gas products from EDD superimposed on the difference thermal trace (19). This permits chemical and physical events to be attached to the temperature deflections. The initially negative slope of the thermal trace results from the additional heat capacity of the filament with the sample present. The melting endotherm occurs at about 175-180 °C. At about 200 °C, melting is complete (not isothermal because of the rapid heating rate) and the liquid phase continues to heat to about 275 °C without evidence of decomposition off-gassing. : At 275 °C, the first gas products are detected. These are HN03(g) formed by proton transfer and desorption, and a small quantity of NCMg), probably from thermal decomposition of HNO3. NH3(g) then appears, perhaps from C-N bond heterolysis. The lag in the thermal trace shows that this stage of decomposition is, as expected, overall endothermic. Howev-
er, above 330 °C, CO2 from backbone oxidation and the more reduced nitrogen oxide products, NO and N2O, can be detected. These products are created by exothermic reactions in the condensed phase, as evidenced by the increased heating rate of the filament. Thus the combination of real-time temperature record and near real-time observation of the gas products helps map the overall reaction sequence during the fast thermal decomposition of a complex material. Pressure as a variable
Apart from its practical value for suppressing sublimation and evaporation of the sample when needed, pressure is a useful research variable. Because the initial pressure in the cell can be set as desired, we have found that performing thermolysis with the initial pressure as the major variable gives additional insight (20). Pressure differences affect gas diffusion rates; that is, the decomposition gases are forced to remain in contact with the condensed phase for different lengths of time. At lower pressure the most reactive gases (NO2, HONO) usually are detected in high relative concentrations because they are able to diffuse away from the reaction zone. When the applied pressure is increased, these gases remain in the reaction zone longer and react to the extent that products having intermediate stability dominate (NO, HCN). At the highest pressures studied, the most
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WHEATON Figure 7. Difference thermal trace superimposed on the quantified gas products from EDD. H20, NH4NO3 aerosol, and any IR-inactive gases are excluded.
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INSTRUMENTATION thermally stable products dominate (CO,C0 2 and, undoubtedly, N 2 ). We do not believe that a change in the decom position mechanism is implied by these results, but simply that the order of reactivity of the nitrogen compounds ( N 0 2 > N O > N 2 ) and carbon com pounds (CH 2 0>CO,C0 2 ) in this envi ronment is reflected in the length of time the gases remain in the reaction zone. The thermal trace can also display pressure dependence. If the position of the exotherm is insensitive to pressure, it suggests that the exotherm is driv en by condensed-phase reactions with
little participation of the gas phase (21 ) . Conversely, a significant tempera ture shift of the exotherm with pres sure, as occurs with nitrate salts (19), implies that heterogeneous gas-phasecondensed-phase chemistry is impor tant. Isothermal decomposition studies following rapid heating The uniformity and efficiency of heat transfer to the sample is enhanced and the complication of filament catch up is reduced by employing a much smaller filament (5.0 X 1.2 X 0.02 mm), smaller sample mass (200-300 μg), and a higher
Figure 8. Temperature profiles of 200 μg of HMX showing the times to exotherm as a function of the sample temperature using the fast-heat-and-hold filament design.
percent coverage of the filament by the sample (6). The center one-third of the filament was covered by a thin layer of sample about 0.2 mm thick. Upon melting, the sample layer becomes much thinner, which further enhances the efficiency of heat flow. The smaller filament is also more responsive to the heater circuit. This modification per mits rapid heating to a specified tem perature and then holding at that tem perature. As before, the IR spectrum of the gas phase is monitored simulta neously with the temperature of the condensed phase. The experiments were termed fastheat-and-hold/FT-IR and can be used to simulate time-to-explosion tests of explosives (6). Hence, fast thermolysis/ FT-IR can be used to investigate the explosion hazard of materials directly. Engineering tests, such as that de scribed by Henkin (22) and the onedimensional time to explosion (23), measure the time to explosion as a function of the sample temperature. A similar measurement, the time to ex otherm, is made in the fast-heat-andhold experiment (6) for the explosive HMX (Figure 8). An apparent activa tion energy can be calculated from these data. In addition, the IR spectra of the gas products are obtained simultaneously, making the cell a spectroscopically in strumented thermal explosion test de vice. For instance, thermal decomposi tion gases almost always appear in ad vance of the exotherm, indicating that autocatalysis occurs to achieve the ex otherm. For HMX, N 2 0 and N 0 2 are the first detected gases and their ap pearance precedes the appearance of C H 2 0 . The formation of N 2 0 and CH 2 0 from HMX has frequently been considered to be a coupled process (24, 25). The sequence that we observe sug gests that their formation is not neces sarily coupled. Also, only a small differ ence exists in the gas-phase concentra tion profiles for all of the thermal traces in Figure 8, implying that there is no change in the thermal decomposi tion mechanisms over this range of temperatures. SMATCH/FT-IR spectroscopy
Figure 9. Sketch of the sample holder and cell region for SMATCH/FT-IR spectros copy. 904 A · ANALYTICAL CHEMISTRY, VOL. 61, NO. 15, AUGUST 1, 1989
If more than one decomposition step occurs upon rapid heating, then there is a question about the amount of sam ple involved in each step. To address this question we recently developed the SMATCH/FT-IR technique, which, as the name implies, permits the mass and temperature change to be measured at high heating rates (7). Figure 9 shows the essential features of the sample holder for SMATCH/ FT-IR. The sample is coated onto a
INSTRUMENTATION
Figure 10. Plot of the simultaneously measured mass change, sample and reference filament temperature change, and evolved gases from nitrocellulose (13.4% N). These data were measured simultaneously and in real time at the initial heating rate of about 100 °C/s.
stainless steel end tip attached to a quartz capillary tube. The tube is set into 130-Hz motion by the exciter pis ton. Heating of the metal end tip is effected by a 1-kW rf power source. The measurement of the mass change is based on the change in the vibration al frequency of the capillary tube as measured by the output of a phototran sistor that senses the motion. The tem perature change is measured by a type Ε thermocouple attached to the metal end tip. The gas products are evolved into a cell and detected by the IR spec trometer beam focused about 1 cm above the metal end tip. Although this experiment is more complicated than the other fast-thermolysis/FT-IR tech niques described above, it gives even more detailed information about the fast-thermolysis process. Figure 10 shows a result from the first study using SMATCH/FT-IR. It is a composite plot of the mass change, temperature change, and gas products from essentially fully nitrated nitrocel lulose (13.4% N). Conclusions On the research front, fast-thermoly sis/FT-IR spectroscopy gives consider able new mechanistic insight into the physicochemical processes that occur in materials undergoing rapid heating. The pressure and the composition of the atmosphere can be set as desired to gain an additional variable. The condi tions of the experiment relate to fire and other combustion situations and
explosions. The small sample size per mits studies to be performed safely and with limited research materials. The sample can be a mixture, a pure materi al, a solid, or a liquid. On the quality control and routine analysis front, the opportunities are ex cellent because samples can be studied with reasonably rapid turnover. Once established on an authentic sample, the gas product distribution is fre quently characteristic and distinguish able among various materials. Because these techniques are applicable to any material that can be thermalized, the outlook for new insight into many im portant materials and situations is in deed bright. Doctoral students Richard J. Karpowicz, Yoshio Oyumi, Stephen F. Palopoli, James T. Cronin, Thomas P. Russell, Peter J. Brush, and Jangkang Chen, and postdoctoral student Mark D. Timken took this field from dream to reality. The Air Force Office of Scientific Research (AFOSR-80-0258, AFOSR-85-0353, AFOSR-87-0033), the Army Re search Office (DAAG29-84-K-0198), the Air Force Armament Laboratory (F08635-87-C-0130), and Morton-Thiokol, Inc., supported chemistry pro grams during which most of the methods de scribed here were developed. References (1) Liebman, S. Α.; Ahlstrom, D. H.; Grif fiths, P. R. Appl. Spectrosc. 1976,30,355. (2) Earnest, C. M. Anal. Chem. 1984, 56, 1471 A. (3) Lephardt, J. O. Appl. Spectrosc. Rev. 1982,18, 265. (4) Oyumi, Y.; Brill, T. B. Combust. Flame 1985,62, 213. (5) Cronin, J. T.; Brill, T. B. Appl. Spec trosc. 1987,41,1147.
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(6) Brill, T. B.; Brush, P. J. Proc. Symp. (Int.) Detonation, 9th; Portland, OR, in press. (7) Timken, M. D.; Chen, J-K.; Brill, T. B., unpublished results. (8) Karpowicz, R. J. Ph.D. Dissertation, University of Delaware, 1984. (9) Cronin, J. T.; Brill, T. B. Appl. Spec trosc, in press. (10) Cronin, J. T.; Brill, T. B. Combust. Flame 1988, 73, 81. (11) Brill, T. B.; Oyumi, Y. J. Phys. Chem. 1986, 90,6848. (12) Melius,CF.J.Phys. 1987,48,C4-341. (13) Bulusu, S.; Axenrod, T.; Milne, G.W.A. Org. Mass. Spectrom. 1970,3,13. (14) Farber, M.; Srivastava, R. D. Chem. Phys. Lett. 1979,64, 307. (15) Shaw, R; Walker, F. E. J. Phys. Chem. 1977 81 2572. (16) ΒΓΜ,'Τ. B.l Oyumi, Y. J. Phys. Chem. 1986,90,1970. (17) Oyumi, Y.; Brill, Τ. Β.; Rheingold, A. L. J. Phys. Chem. 1986,90, 2526. (18) Oyumi, Y.; Brill, Τ. Β. Prop. Explos. Pytech. 1988,13, 69. (19) Russell, T. P.; Brill, T. B. Combust. Flame 1989, 76, 393. (20) Oyumi, Y.; Brill, T. B. Combust. Flame 1987,68, 209. (21) Brill, T. B.; Subramanian, R. Combust. Flame, in press. (22) Henkin, H.; McGill, R. Indust. Eng. Chem. 1952,44,1391. (23) McGuire, R. R.; Tarver, C. M., Proc. Symp. (Int.) Detonation, 7th; U.S. Naval Academy: Annapolis, MD; 1981, 56. (24) Schroeder, M. A. Chemical Propulsion Information Agency Publication 308, 1979,//, 17. (25) Schroeder, M. A. Chemical Propulsion Information Agency Publication 347, 1981,//, 395.
Thomas B. Brill is professor of chemis try at the University of Delaware, where he has been a member of the faculty since 1970. He received his B.S. degree from the University of Mon tana in 1966 and his Ph.D. from the University of Minnesota in 1970. His research interests include the devel opment ofIR and Raman spectroscopy methods to investigate chemistry at high heating rates and elevated pres sure conditions, structure/property/ decomposition relationships, thermal explosions, combustion, supercritical water, the synthesis of energetic mate rials, and the Arbuzov reaction. He is the 1989 "Spectroscopist-of-theYear" of the Society for Applied Spec troscopy, Delaware Valley Section.
