Matrix-isolation study of the thermal decomposition of pyridine

Matrix-isolation study of the thermal decomposition of pyridine. Vernon R. Morris, Subhash C. Bhatia, Arthur W. Stelson, and John H. Hall Jr. Energy F...
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Energy & Fuels 1991,5, 126-133

126

Matrix-Isolation Study of the Thermal Decomposition of Pyridine Vernon R. Morris,t**Subhash C. Bhatia,*ft Arthur W. Stelson,t and John H. Hall, Jr.tJ Dolphus E. Milligan Science Research Institute, Atlanta University Center, Inc., Atlanta, Georgia 30310, and School of Geophysical Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332 Received February 20,1990. Revised Manuscript Received August 13, 1990 The pyrolysis and thermal oxidation of pyridine have been studied in the temperature range of 700-900 "C using matrix-isolation infrared spectroscopy. The matrices employed in the studies were argon, nitrogen, and various mixtures of N2:02(41,8:1, and 161). Our results indicate the formation

of bipyridine and cyanomethylene as major products in pure argon and nitrogen matrices. Carbon monoxide, carbon dioxide, nitric oxide, and HOCN are observed as products in the N$02 matrices. Isotopic studies using lSO2were used to confirm the identity of the observed products. Reaction mechanisms are proposed for the formation of observed products.

Introduction Coal-derived liquids (CDL's) are complex mixtures of aromatic hydrocarbons containing significant amounts of oxygen, sulfur, and nitrogen. Model compounds provide a simplified means of studying the chemical processes that occur when CDL's undergo combustion and pyrolysis. The majority of the nitrogen found in CDL's is in the form of heterocyclic aromatic compounds.' Pyridine is a very suitable model compound, and studies of the pyrolysis of pyridine can be useful for approximating the thermal behavior of compounds present in CDL's. Previous studies2of the thermal decomposition of pyridine (600-900 "C) have reported the formation of atomic hydrogen and bipyridines at temperatures between 600and 650 "C. Hydrogen cyanide has been observed a t temperatures above 650 "C. The complete cleavage of the pyridine ring was observed a t 900 0C.2 Other investigators'? have reported the formation of HCN, isomeric forms of bipyridine, H2,CH,, olefins, and traces of acetylene. In the temperature range 825-850 "C, the formation of quinoline, benzonitrile, benzene, acetonitrile, and acrylonitrile have been r e p ~ r t e d . The ~ quantity of nitriles present in the observed products is dependent on the temperature at which the thermal decomposition of pyridine is performed. A study on the oxidative pyrolysis of pyridine and benzonitrile found the products to be primarily HCN and NH3 at lower temperatures (-720 "C), while N2,N20,and NO2 are observed at higher temperatures (-760 "C)., Production yields of NO, (NO and NO2) and N 2 0 are maximum at temperatures of 900 and 770 "C, respectively! The most recent investigations4on the pyrolysis of pyridine were performed in a liquid phase using a stirred-flow reactor. Quantification problems arose in these studies because a brown tarlike byprodud was consistently observed along the inside walls of the reactor and exit 1ine.l~~ This study was undertaken to identify the products of the pyrolysis and suggest possible mechanisms for the thermal decomposition of nitrogen-containing molecules in coal-derived liquids. The matrix-isolation method5s6was used to isolate transient species, and infrared spectroscopy was employed to identify products. By performing these 'Atlanta University Center, Inc. *Georgia Institute of Technology.

experiments at high dilutions, we are able to separate the combustion chemistry from transport processes.' In addition, matrix isolation is the only technique that allows one to isolate and characterize reactive intermediates.

Experimental Procedure Pyridine (Fisher Scientific, 99%)was purified via trap-btrap distillation. At a pressure of 12.89 mmHg, pyridine was trapped in a 2-L flask. The excess pyridine was then pumped out and an appropriate amount of matrix material was introduced to make the dilute mixtures. The pyrolysis of pyridine took place in a quartz tube (25-cm length, inner diameter of 4.5 cm) inserted through a Thermolyne 21100 tube furnace connected between the cryotip and the gas line (Figure 1). A dispersive cool air flow was maintained at the quartz tube/cryotip junction in order to offset conductive heat processes. In this study, nitrogen, argon, and various ratios of Nz:02(UHP Matheson) gases were employed. The pyrolysis temperatures and matrices used in each experiment are listed in Table I. The mixtures (at ambient and high temperature) were deposited on a precooled CsI window (10 K). The CsI window was maintained at 10 K by an Air Products helium refrigerator and monitored by a Chromel-Alumel thermocouple. The infrared spectra of pure argon, nitrogen, and oxygen show the presence of water (- 10 absorbance units (AU)) and carbon dioxide ( - 5 AU). No absorption due to carbon monoxide and methane were observed. Typical deposition times were on the order of 2 h. During deposition,the furnace was held at constant temperature until 2.33 mmol of mixture was deposited. The quartz tube was cleaned with dilute hydrofluoric acid solution before each experiment in order to minimize the possibility of wall reactions. All spectra were recorded with a Beckman Model IR-4250X dispersive infrared spectrometer. Results and Discussion Pyrolysis in Argon and Nitrogen. The observed spectra (Table 11) of pyridine in argon and nitrogen matrices deposited from room temperature (25 "C) samples (1)Houser, T. J.; McCarville, M. E.; Biftu, T. Int. J. Chem. Kinet. 1980, 12, 555. (2) Ruhemann, S. Braunkohle 1929,28, 749. (3) Hurd, C. D.; Simon, J. I. J. Am. Chem. SOC.1962,84,4519. (4) Axworthy, A. E.; Dayan, V. H. Coo. Rep. Announce. Index 1977, 477 (115), 15: (5) Vibratronal Spectroscopy of Trapped Species; Hallam, H. E., Ed.; Wiley: London, 1973; Chapter 5. (6) Formation and Trapping of Free Radicals; Bass, A. M., Broida, H. P., Eds.; Academic Press: New York, 1960; Chapter 4. (7) Gardiner, W. C.; Olson, D. B. Annu. Reu. Phys. Chem. 1980,31, 377.

0887-062419 112505-0126$02.50/0 0 1991 American Chemical Society

Thermal Decomposition of Pyridine

Energy & Fuels, Vol. 5, No. 1, 1991 127

Cyrogenic Cell pyridinelAir

1y-+ 1

c.1 Yindow f o r Spectral O b w v a t i o n

Cold CsI Window

I u

/

t o Vacuum manifold

T

Furnace

Figure 1. Experimental setup. Table I. Experimental Conditions and Products Observed deposn rate, mixture ratio mmol/h temp, "C species obsd Py/Ar (1:lOO) 25 Pv,PvH,O,

Py/Ar (1:lOO)

1.52

860

Py/Np (1:50)

25

Py/N2(1:50)

700

Py/Np (1:50)

1.9

860

Py/N,:O, (1:lOO)(4:l)

1.8

890

1.59

890

Py/Np:180p (1:50)(4:l)

1.3

890

Py/N2:l8O2 (1:lOO)(8:l)

1.05

890

are essentially the same as the matrix-isolation work of Castellucci et a1.8 The absorptions due to pyridine! water? water dimer, pyridine/H20 complex,1° rotational bands for water," carbon dioxide,l2 and COZ/H20complex13 are listed in Table 11. (8) Castellucci, E.; Sbana, G.; Verderame, F. D. J. Chem. Phys. 1969, 51, 3762. (9) Catalano, E.; Milligan, D. E. J. Chem. Phys. 1959, 30, 45. (10) Wolff, H.; Hagedom,W.; Mathais, D.; Rethel, R.; Millermann, E.; Leidner, L. J. Phys. Chem. 1978,82, 2404. (11) Redington, R. L.; Milligan, D. E. J. Chem. Phys. 1962,37,2162. (12) Fredin, L.; Nelander, B.; Ribbegard, G. J. Mol. Spectrosc. 1974, 53, 410. (13) Fredin, L.; Nelander, B.; Ribbegard, G. Chem. Scr. 1975, 7, 11.

The observed spectrum of pyridine (Table 11) in the nitrogen matrix is in agreement with that of Caatellucci et al.8 No evidence for pyridine pair association is observed in the matrix-isolated spectra of pyridine in Nz and Ar m a t r i ~ e s . ~The J ~ observed absorptions due to HzO, HzO dimers,13-16and carbon dioxide12 are shown in Table 11. Argon Matrix. The observed absorptions due to the pyrolysis of pyridine in the argon matrix are listed in Table 111. Broad absorptions are observed in the 3100-3600-cm-' region and are tentatively assigned to a pyridine/H20 complex. An oxygen-bonded complex, if attached at the nitrogen, would be expected to induce red shifting of OH stretch and also produce weak HO-N and N.-0 stretching modes.16J7 No evidence for this is observed in our spectra. If this interaction is occurring at the hydrogen positions, one would expect to see both red shifting and perhaps a splitting of OH stretching modes due to the influence of the pyridyl hydrogen on the O-H bond. A corresponding splitting of the CH degenerate stretching modes may also be expected as a result of such an interaction. No changes in the spectra are observed in the region (3000-3100 cm-9 in which these interactions will be manifested. A slight blue shifting of the absorption due to the HO-H bending mode at 1627 cm-' is observed. Note that this shift would result from the effect of hydrogen-bondinginteractions at the nitrogen position of the pyridine ring." The absorptions at 3446,3384, and 3360 cm-' are in a region typical of N-H stretch. This supports the assignment of these absorptions to an H-bonded pyridine/H20 c ~ m p l e x . ~ ~ J ~ J ~ A benzoyl chloride test was performed on the liquid pyridine sample in order to determine the presence of significant pyridine/H20 complex. The addition of benzoyl chloride to pyridine resulted in immediate precipitation of benzoic anhydride. This fact indicates the presence of significant amounts of water complexed to pyridine in solution, and is also observed in the infrared spectra of the pyridine/argon mixture.18 Absorptions at 1460,1002, and 611 cm-I are assigned to 2,2'-bipyridine.lg The spectrum of bipyridine exhibits significant overlap with the spectrum of pyridine. The (14) Taddei, G.; Castellucci, E.; Verderame,F. D. J. Chem. Phys. 1970, 53, 2407. (15) Tursi, A. J.; Nixon, E. R. J. Chem. Phys. 1970,52, 1521. (16) Bellamy, L. J. The Infra-red Spectra of Complex Molecules, 2nd ed.; Wiley: New York, 1964; Chapters 15 and 17. (17) Bellamy, L. J. Advances in Infra-red Group Frequencies; Methuen: Suffolk, Great Britain, 1969; Chapter 8. (18) Noller, C. R. The Chemistry of Organic Compounds, 3rd ed.; W . B. Saunders: Philadelphia, PA, 1965; Chapter 31. (19) Neto, N.; Muniz-Miranda, M.; Angeloni, L.; Castellucci, E. Spectrochim. Acta 1983, 39A, 97.

Morris et al.

128 Energy & Fuels, Vol. 5, No. 1, 1991 Table 11. Observed Absorptions Due to Pyridine in Argon and Nitrogen Matricesa v(PvlAr)b

v(PY/N,) 3725 s

assignment

3709 s 3693 m 3674 w 3631 m 3620 w 3544 w 3519 w 3399 vw

3077 vw 3031 vw 3019 vw 3000 vw 2344 m 1691 w 1652 w 1627 w 1607 vs 1592 m 1587 w 1573 vw 1492 w 1481 w 1474 vw 1446 w 1388 w

1076 w 1039 w 1028 w 992 vw 771 m 748 vw 716 w 665 w 654 vw

3392 m 3364 m 3223 w 3090 m 3074 vw 3062 w 3043 m 3027 m 3007 m 2996 vw 2349 s

1636 w 1621 w 1603 vs 1597 w 1587 s 1579 w 1572 w 1487 m 1475 vw 1444 vs 1436 vw 1389 vw 1348 vw 1222 m 1208 vw 1151 m 1074 m 1035 s 1025 vw 1006 w 994 m 768 m 748 s 707 m 668 w 664 w

a Room temperature deposition. Py = pyridine. Major source of water is pyridine.

relative intensities of pyridine fundamentals observed in the room temperature deposition (Table 11) were compared to the observed intensities in the pyrolysis spectrum. The ~ 2 5 ,ug, and ~ 1 fundamentals 4 of pyridine exhibit optical density changes of -29, -70, and -95% relative to the v16 fundamental. The intensity of the 748-, 1033-, and 1446cm-1 absorptions also decreased relative to the 706-cm-' absorption band. Our observed patterns of relative intensity changes agree with the reported relative intensity of 2,2'-bipyridine absorption peak^.'^*^^ A sharp, but weak absorption, is observed at 2143 cm-' and is assigned to C0.21 Since no oxygen-containing species (except O2in Ar) are introduced in this experiment, the observed presence of carbon monoxide may not due to the pyrolysis of pyridine. The formation of CO is at(20) Muniz-Miranda, M.; Castellucci, E.-Angeloni, L.; Fbana, S. Spectrochtm. Acta 1983, 39A, 107. (21) Ewing, E.; Pimentel, G. J . Chem. Phys. 1961, 35, 925.

Table 111. Observed Absorptions Due to Products of Pyrolysis of Pyridine in Argon and Nitrogen Matrices v(Py/Ar) a t 860 "C 2964 w 2143 w d 2125 vw shn 1733 w d 1716 w d

@Y/Nz) a t 700 OC a t 860 "C 2961 w 2849 vw 2147 w 2136 vs 2128w 1733 m d 1560 w sh

1561 w 1459 w

1364 w

1361 vw 1344 w w 1332 w w 1305 w 1265 w 1172m 1095 vw 1056 w w 1001 m 949 w

1460 w 1371 w d 1332 w sh 1306 w 1264 w d 1171 m

1002 w 947 w 933 w 913 w 611 w

1003 m

618 w

823 w 738 w sh 611 s

assignment unassigned unassigned (CO)2

co

CO HCCN Py (comb. band) 2,2'-bpy 2,2'-bpy unassigned unassigned unassigned unassigned CHI 2,2'-bpy HCCN unassigned unassigned 2,2'-bpy unassigned unassigned unassigned unassigned bpy/Py 2,2'-bpy

"sh = shoulder. d = doublet.

tributed to the presence of O2 and methane as impurities (110 ppm) in argon.22 The absorption at 1306 cm-' is assigned to methane.24.25 Several weak to medium intensity absorptions observed in the spectrum of the pyrolysis of pyridine in argon remain to be assigned. Absorptions due to products of pyrolysis are observed at 2964, 1733,1332,1171,947,933, and 913 em-'. The absorptions in the 1720-1745-cm-' spectral region are generally due to keto-type (RC-0) stretching m o d e ~ . ' ~ J ' ,Recently, ~~ Jacox26reported the vibrational spectrum of matrix-isolated H&N, which exhibits a C-N stretching mode at 1725 cm-'. Due to intensity arguments, the product absorptions in the 1720-1745-cm-' region cannot be assigned to H2CN. Acrylonitrile, or a similar radical product, is expected to be formed on pyridine ring cleavage. However, the reported absorptions for these species do not agree with the observed spectra.27 The absorption centered at 1733 cm-' and the absorption at 1171 cm-' are assigned to HCCN in agreement with previous matrix-isolation work.28 We feel that the more plausible explanation for the remaining absorption at 1727 5 em-') cm-' is combination band of ug (992 cm-') + ~ 2 (745 or u26. (704 cm-') + ug (992 cm-I) absorptions. The absorptions at 2964, 947, 933, and 913 are unassigned. Nitrogen Matrix. Pyrolysis of pyridine was also performed at temperatures of 700 and 860 "C using nitrogen as the matrix material. In the spectrum of the 700 "C (22) (a) Matheson Cas Data Book; Matheson Co. Inc.: Whitby, Canada, 1966; p 397. (b) Jaiyesihnmi, M. A,; Bhatia, s. C.; Hall, J. H., Jr. Presented at the Central Regional Meeting of ACS, Bowling Green, OH, June 1986. (23) Randall, H. M.; Fowler, R. G.; Fuson, N.; Dangl, J. R. Infrared Determination o/ Organic Structures; 1st ed.; Van Nostrand: Princeton, NJ, 1956; Chapter 2. (24) Comeford, J. J.; Gould, J. H. J . Mol. Spectrosc. 1960, 5, 474. (25) Cabana, A.; Savitsky, G. B.; Hornig, D. F. J. Chem. Phys. 1968, 39, 2942. (26) Jacox, M. E. J . Phys. Chem. 1987,91,6595. (27) George, W. 0.;Hirani, P. K.; Maddams, W. F.; Williams, D. A. J . Mol. Struct. 1986, 141, 227. (28) Dendramis, A.; Harrison, J. F.; Leroi, G. E. Ber Bunsen-Ges. Phys. Chem. 1978,82, 7.

Thermal Decomposition of Pyridine experiment we observed absorptions at 3528,3511,3451, 2136,1560,1364,1264,1003, and 618 cm-' that can be due to products. Absorptionsdue to matrix-isolated water and (H,O), aggregate are listed in Table III.13-15 A new absorption was observed at 3451 cm-l, and new features also appear as high- and low-frequency shoulders on the 3372-cm-' absorption. The absorptions at 3511, 3451, and 3372 cm-' are assigned to the H-bonded pyridine/HzO complex.'O These new absorptions are in the region that is characteristic of N-H ~tretching.'~J'However, intensity arguments preclude the assignment of these absorptions to NH3, NHz, or NH molecule^.^^^^^ This suggests that the pyridine/H,O complex is formed by water attaching to the nitrogen of the pyridine ring. The observed pattern is similar to what was observed in the argon pyrolysis spectrum. Thus, these absorptions are assigned to pyridine/HzO complex.'" Through intensity arguments and comparison with the argon spectra, the absorptions at 1560,1264,1003, and 618 cm-' are attributed to 2,2'-bipyridine.19vmThe weak absorption observed at 1364 cm-' is unassigned. The presence of 2,2'-bipyridine suggests that ring cleavage is dominated by C-H bond breakage and radical-radical combination. Our observations agree with the previous reports that pyridine does not significantly decompose at 700 0C.1-4 The observed spectrum in the 860 "C pyrolysis of pyridine experiment is similar to the previous spectrum (700 "C) with several additional absorptions (Table 111). The 3713-, 3624-, and 3217-cm-' absorptions are assigned to (H,O),, and the 3682-, 3434-, and 3352-cm-' absorptions to (H,0),.13-15 The absorptions at 3488, 3383, and 3147 cm-' are assigned to a pyridine/H20 complex.'O Two new absorptions at 3713 and 3682 cm-' are observed. These new absorptions are similar in shape and intensity to the absorptions at 3725 and 3693 cm-' which were observed in the spectrum of the room temperature deposition of pyridine/N, matrix sample. A broad absorption is observed between 3600 and 3100 cm-'. This absorption cannot be attributed to ice formation because of the sharpness of other matrix-isolated water bands throughout the spectrum and the absence of prominent ice absorptions in the spectrum. We observe a doublet centered at 3368 cm-' and a sharper medium absorption at 3217 cm-'. In comparison to the ambient temperature spectrum, we observe an extremely weak absorption a t 3544 cm-l. The absorptions at 3488,3434, and 3124 cm-' exhibit new shoulder formation. The weak 3124-cm-' absorption is close to reported frequencies for matrixisolated NH.30 The 3434-cm-' absorption is close to the u3 stretching mode of NHQ.10The presence of NH and NH3 was excluded by intensity arguments, and these absorptions were assigned to a pyridine/HzO ~omplex.~'The absorptions at 3383 and 3147 cm-' shift from ambient temperature deposition values by -9 and -5 cm-', respectively. All absorptions are observed in the range of intermolecular NH ~ i b r a t i o n s . ' ~ Thus, J ~ ~ ~a good possibility exists that both polymeric formation of H20 and water complexes involving pyridine and pyrolytic products are responsible for the broad absorptions in this region. The observed absorptions for 2,2'-bipyridine and methane are assigned according to previous spectra (Table III).16J9* New absorptions are observed at 3027 and 3054 cm-' with relative optical densities at 63 and 300% greater (29)Ribbegard, G. Chem. Phys. 1975,8,185. (30) (a) Pimentel, G. C.; Bulanin, M. 0.; Van Thiel, M. J. Chem. Phys. 1962,36,500.(b) Milligan, D. E.; Jacox, M. E. J. Chem. Phys. 1965,43,

4487. . .

(31)Milligan, D. E.; Jacox, M. E. J. Chem. Phys. 1964, 41, 2838.

Energy & Fuels, Vol. 5, No. 1, 1991 129 than the nearest corresponding absorptions in the room temperature spectrum. The relative intensity profile of this spectrum differs from both of the previous spectra of pyridine in nitrogen. The optical densities of the CH stretching fundamental absorptions exhibit a uniform tendency to decrease while the lower wavenumber fundamentals exhibit intensity increases. The observed absorptions at 2961,2849,1361,1344,1332,1095,1056,949, and 823 cm-' are unassigned. Several of these absorptions were also observed in the argon experiment. The intensity patterns of all features were similar and exhibited little or no correlation with each other. Of the ten unassigned absorptions, five appeared within 1 1 5 cm-l and two others within f25 cm-' of reported features in the spectrum of matrix-isolated pyridine. Thus, the observed behavior of the pyridine absorptions is indicative of a perturbing influence by other absorbing molecules as mentioned earlier. The 1600-17OO-cm~'absorption region had a broad band at 1640 cm-'. This band is near established frequencies of matrix-isolated (H20),.14J5 The blue shift of this band may be due to the influence of the pyridine/H20 complex. By attributing the red shift of O-H stretching frequencies to increased ?r-character of the oxygen bond toward the hydrogen atom in the water molecule, this increase in the frequency of the corresponding bending modes is justified.lB The increased intensity of this absorption (1640 cm-') is due to the increased water content in the sample. The 860 "C pyridine/nitrogen pyrolysis matrix was annealed to 30 K and recooled. The subsequent spectra showed an anticipated broadening of absorptions in the 3600-3000-cm-' region indicating the formation of H 2 0 aggregates. Several sharp, well matrix-isolated absorptions did persist at 3685, 3080, 3060, 3037, and 3004 cm-'. A sharp, medium absorption is observed at 2900 cm-'. This may be due to reaction of intermediates since it did not grow with continued warming. We observe a broad absorption extending from 1720 cm-' to nearly 1630 cm-'. This absorption overlaps with the region in which absorptions at 1684,1680,1676, and 1640 cm-' are observed. We observe a weak absorption at 1615 cm-' and a triplet structure centered at 1585 cm-'. These two bands are attributed to the presence of pyridine and pyridyl type aggregates. The intensity of the 1603-cm-' fundamental absorption of pyridine decreases by 44%. Other absorptions of monomeric pyridine at 1485 and 1440 cm-I exhibit much smaller intensity changes. The absorption at 1170 cm-' broadened and decreased in intensity. Broadening of the 1170-cm-' absorption is an indication that HCCN either aggregates or is converted to a saturated species. This possibility is enhanced by the increased ability of H atom migration through the more flexible matrix at elevated temperatures. The slight red shifting of the C-N stretch is in agreement with this observation. The presence of HCCN and methane indicates that ring cleavage has occurred to a significant degree in the hightemperature (5" I860 "C) experiments. The primary products of ring cleavage can undergo gas-phase reaction with pyridine to form an assortment of substituted products. The presence of the substituted pyridines could be partially masked by the overlapping spectra. However, the observed splittings and intensity patterns of the spectra would be indicative of the presence of structurally similar species. In the process of matrix isolation, products of ring cleavage can be trapped in the same matrix cavity as other products or with pyridine. The spatial orientations of the species in the common host site may lead to bonding interactions, aggregation of the radical fragments, or perturbative interaction^.^ Aggregation and/or perturbative

Morris et al.

130 Energy & Fuels, Vol. 5, No. 1, 1991 Table IV. Observed Absorptions Due to Products of Pyrolysis oP Pyridine in NZ:OzMatrices at 890 OC v(NZ:02=4:1) 3102 w 2975 w 2940 w 2285 vw 2155 w sh"

u(Nz:Oz=8:l) 3102 w 2936 w 2289 w 2264 w 2151 w sh 2143 vs

2135 m d 2111 w 2095 w 2074 vw 2051 vvw 1880 m 1733 m 1726 s 1718 s 1502 w 1463 w sh 1374 m d 1361 w sh 1340 w d 1310 m 1265 w 1239 m 1173 m 1102 w 1004 m 957 w 938 w 920 w 840 w 826 w 720 w

1726 w sh 1718 m 1460 w sh 1374 m d

1267 w 1240 w 1102 w 1005 m 962 w 920 w 726 w 614 m

assignment unassigned unassigned unassigned HOCN HNCO (C0)P (C0)P

co

unassigned HCN unassigned CN

NO HCCN Py (comb. band) Py (comb. band) unassigned 2,2'-bpy unassigned unassigned unassigned CHI 2,2'-bpy unassigned HCCN HOCN 2,2'-bpy NH3 unassigned unassigned unassigned unassigned HCN 2,2'-bpy

"sh = shoulder. d = doublet.

phenomena may explain the several unassigned absorptions in the matrix-isolated spectra of the pyrolysis of pyridine in both argon and in nitrogen matrices. All of these absorptions are observed in spectral regions associated with methylene stretching and bending vibrations. The presence of unassigned features and their subsequent disappearance from the spectrum on annealing is evidence for their assignment to transient species. Thus, we feel that some of the unassigned absorptions are due to primary products of pyrolysis or unstable reaction intermediates. Thermal Oxidation. The thermal oxidation of pyridine was carried out in mixed matrices of nitrogen and oxygen. The nitrogen to oxygen ratios used were 8:l and 4:l. Unique product absorptions are observed at 3102,2936, and 962 cm-' 2289,2264,2095,2071,2051,1880,1240,1102, in the spectrum of the thermal oxidation of pyridine in the N2:02(81)matrix, hereafter referred to as TOS (81). The absorptions due to products and their assignments are listed in Table IV. The broad band extending over the 3100-3600-cm-' region increases in relative intensity in this spectrum. The absence of both broad features in the low-wavenumber region and parallel development of (H,O), absorptions indicate that this feature is not solely due to matrix-isolated water or ice. Matrix trapping of intermediates, NH, NH,, or NH3 in the same lattice site as water could be responsible for the observed absorption. Previous assignment of these absorptions was to water complexation of pyridine. This may result in active molecules with dimensions exceeding that of the host sites and, thus, the broad absorption bands in the NH stretching regi0n.~+~2 The relative intensities of the CO, (CO),, and CO, absorptions exhibit dramatic increases. These are the pri-

mary products of thermal oxidation. The 62% optical density increase in the CO, absorption at 2350 cm-l relative to the background spectrum (pyridine/& at 25 "C)is an indication of CO, formation due to pyridine decomposition. The medium-intensity absorption observed in the TOS (81)at 1880 cm-' is assigned to NO.% Previous gas-phase infrared studies have identified the NO absorption at 1875 cm-1a33,34

The absorptions at 2289 and 1102 cm-' are assigned to HNCO and the 2264-cm-' absorption is assigned to HOCN in agreement with the previous matrix-isolation The absorptions at 2095 and 726 cm-' are assigned to HCN.36 The most intense reported absorption for HCN is at 3296 cm-' and is partially obscured by the broad band centered at 3300 cm-', but the relative intensities of the remaining absorptions are in good agreement with literature value^.^^^^^ The sharp, but weak absorption at 2051 cm-' is assigned to CN in good agreement with the matrix-isolation work of Milligan and J a ~ o x . ~ ~ We did not observe any correlation between the relative intensities of the unassigned absorptions at 2936 and 1374 cm-'. This suggests that these absorptions are arising from different species. The absorptions at 1374 and 920 cm-' in the TOS (8:l)are observed in the argon pyrolysis spectrum at 1371 and 913 cm-' with identical relative intensities. This observation suggests that these absorptions are due to the same species. Thermal oxidation of pyridine was also performed with a 4:l (N,:O,) matrix comparable to the mixing ratio of nitrogen to oxygen in ambient air. The vibrational assignments are listed in Table IV. The broad band extending over the high-wavenumber region (3100-3600cm-') persisted in this spectrum. A doublet centered at 2135 cm-' is assigned to CO. The absorption at 2155 cm-' is tentatively assigned to (CO),. These absorptions are of much weaker relative intensity than observed in the TOS (8:l). No absorption due to NO (- 1880 cm-') was observed in the TOS (4:1).32The absence of NO absorption in the 4:l experiment is contrary to what was expected (based on the observation of NO in the TOS (8:l)). Though prompt mechanism^,^^ which are favored by low temperatures, short residence times, and rich mixture conditions, may be responsible in the conversion of N, and 0, to NO, the inability of the system to stabilize comparableamounts of NO at higher oxygen concentrations may suggest that different reaction mechanisms dominate at the different N2:02ratios. This possibility can only be clarified by further varying N2:02ratios under identical conditions. The absorptions at 1726,1374,and 1173 cm-' of the TOS (4:l)all exhibit large relative intensity increases relative to the corresponding absorptions in the other spectra, indicating enhanced concentrations. Each of these absorptions has been observed in previous experiments in which impurity was the only source of oxygen. Thus, all of these absorptions are considered to be arising from anoxic species. The normalized and relative optical densities of eight product frequencies unique to the TOS are shown in Table V. The frequencies listed in Table V represent that average of the frequencies observed in the TOS. The ob(32) Tevault, D. E. Plasma Chem. Plasma Process. 1985,5, 391. (33) Guillory, W. A.; Hunter, C. E., Jr. J. Chem. Phys. 1969,50,3516. (34) Laane, J.; Olsen, J. R. Proc. Inorg. Chem. 1980,27, 465. (35) Jacox, M. E.; Milligan, D. E. J. Chem. Phys. 1964,40, 9. (36) King, C. M.; Nixon, E. R. J. Chem. Phys. 1968, 48,4. (37) Milligan, D. E.; Jacox, M. E. J. Chem. Phys. 1964,47, 278. (38) Miller, J. A.; Fisk, G. A. Chem. Eng. News 1987, 67 (37), 22.

Thermal Decomposition of Pyridine Table V. Optical Densities of Product AbsorDtionso Y,

cm-'

3104 2936 2287 2265 1878 1240 1102 960

thermal oxidation spectra 16:l 81 4:l 0.037 0.194 0.034b (1.82) (9.70) (1.70)' 0.037 0.019 0.018 (1.85) (0.95) (0.90) 0.045 0.013 0.013 (2.25) (0.65) (0.65) 0.00 0.030 0.013 (1.50) (0.65) 0.00 0.087 0.013 (4.35) (0.65) 0.161 0.000 0.036 (8.05) (1.80) 0.000 0.060 0.125 (3.00) (6.25) 0.000 0.040 0.039 (2.00) (1.92)

Listed absorptions represent the average of the observed values in the three thermal oxidation spectra. *Optical density of absorption a t the given frequency. Relative optical density calculated with respect to the 1570-cm-' absorption.

served frequencies are within f 4 cm-' from the average values reported in Table V. There is a general increase in relative optical density with increasing oxygen mixing ratio for six of the eight absorptions listed in Table V. Two of the eight absorption bands exhibit increased absorption intensity with increased oxygen mixing ratio, and yet neither is assigned to an oxidized product. The remaining absorptions with average absorption values of 3104,2287, 2265, 1878, 1102, and 962 cm-' all exhibit maximal absorption intensities in the TOS (81).The absorptions that are assigned to oxygen-containing species, HNCO (2287 cm-'), HOCN (2265 cm-'), and NO (1878 cm-'), have maximum absorbance in the TOS (8:l). Since equal amounts of mixture were deposited in all experiments, the increased amounts of oxidized products is not a result of variation in amounts of the mixture deposited. The observed increase in concentration suggests that certain oxidation products may form as a function of deposition rate as well as oxygen content within the system. Deposition rate is known to influence both concentration and type of free radical isolated for a given experimenL6 In order to determine what factor played the larger role, the TOS (81) experiment was performed with a slower deposition rate than that of the TOS (4:l) (Table I). Similar results were obtained for the TOS (4:l) performed at the slower deposition rate. Thus, we find that deposition rate is not the controlling factor but the oxygen mixing ratio influences the observed products in our experiments. A separate thermal oxidation experiment was performed employing a mixing ratio of N2:02(16:l) and deposition rate of 1.56 mmol/h. Intense CO absorptions at 2134 cm-' and much weaker absorptions due to NO (1876 cm-l), HNCO (2286 cm-'), and HOCN (2266 cm-') were obs e r ~ e d . ~Another ~ $ ~ experiment was performed with an h 0 2(81)matrix. In this TOS, we observed the presence of CO, HNCO, NH3, and HCN. NO was not observed in this experiment. This suggests that the source of nitrogen for the NO production in our experiment is the matrix nitrogen rather than the nitrogen from the pyridine molecule. This also suggests that HNCO, HCN, and NH3 were formed from the nitrogen present in pyridine. Our results suggest that the oxygen content of the mixture controls the chemistry of the system to some extent, but the N2:02ratio 8:l behaved as a critical N2:02 ratio at which the oxidized products of pyridine decomposition have maximum yield.

Energy & Fuels, Vol.5, NO. 1, 1991 131 Table VI. Observed Absorptions Due to Products of Pyrolysis of Pyridine in N2:*802Matrices at 890 O C ~ ( N , : ~ ~ 0 , = 4 : 1 ) v(N,:'180,=8:1) 3646 w sha 3501 w sh 3507 w sh 2971 vw 2975 w 2939 w 2935 w 2916 w 2330 w sh 2329 vs 2313 vs 2286 w 2266 w 2246 w 2141 w 2147 s sh 2135 vs 2095 w 2087 vs 2071 w 2040 w 1871 w 1876 m 1826 w 1731 m sh 1732 w d 1720 w sh 1716 m 1466 w 1460 w sh 1373 m d 1368 w 1368 w 1361 w sh 1360 w 1338 w d 1340 w 1308 w 1307 vw 1265 w 1268 w 1240 w 1173 m 1174 w 1098 w 961 w 920 w 826 w 659 vw 657 vw

assignment

HJ80 C'*O* unassigned unassigned unassigned C'60'80 C'*02 HOCN HNCO C'80* (COh

co

HCN C'80 unassigned C'~O'*O NO N180 HCCN Py comb. 2,2'-bpy unassigned unassigned unassigned unassigned CH, 2,2'-bpy unassigned HCCN HOCN NH3 unassigned unassigned C'802

"sh = shoulder. d = doublet.

Isotopic Experiments The thermal oxidation of pyridine was repeated employing matrices with mixing ratios of N2:02( 8 1 and 41) enriched with isotopic oxygen 1 8 0 2 (99.2%). The absorptions previously assigned to 2,2'-bipyridine, methane, HCCN, HCN, NO, HNCO, HOCN, and CO are observed and their absorption frequencies are listed in Table VI. Isotopic water is observed in the 8:l spectrum at 3646 cm-1.39 This indicates that water is formed as a byproduct of the thermal oxidation of pyridine and explains the increased intensities of water absorptions in TOS. Isotopic carbon dioxide is observed in both spectra. The absorptions are much more intense in the labeled TOS (8:l). Absorptions at 3507, 2246, and 657 cm-I in the labeled TOS (8:1), and 3501 and 659 cm-' in the labeled TOS (41) are assigned to Cl8OPB Absorptions at 2329 and 2040 cm-' and 2330 cm-' are assigned to C'60180.B Isotopic carbon monoxide, P O , is observed at 2087 cm-' in the 8:l ~ p e c t r a . ~Nitric ' oxide (NO) is observed-in both of the isotopically enriched TOS, but N180 (1826 cm-') is only observed in the 8:l (N2:1802)mixture.32 We also observe a very weak absorption at 1368 cm-I and a doublet centered at 1373 cm-'. This doublet (1373 cm-') is observed in both of the unlabeled N2:02(4:l and 8:l) spectra. The 1368-cm-' absorption may be due to site effects rather than due to a new product. At this time neither absorption has been definitively assigned. Another product absorption is observed at 2916 cm-I in the labeled TOS (4:l). This absorption is in the methylene (39) Fraley, P. E.; Rao, K. N.; Jones, L. H. J. Mol. Spectrosc. 1969, 29, 312. (40) Chackerian, C., Jr.; Eggers, D. F., Jr. J. Chem. Phys. 1965,48737. (41)Eggers, D. F., Jr.; Berney, C. V. J. Chem. Phys. 1964, 40, 990.

Morris et al.

132 Energy & Fuels, Vol. 5, No. 1, 1991

stretch region which could be indicative of the presence of an unsaturated product of ring cleavage. The observed frequencies due to products and their assignments are listed in Table VI. We did not observe any absorptions due to the isotopic Cl80 stretch of H180CN or HCN180.35p41From isotopic shifts of carboxyl absorptions in other species (XCO), one would expect the magnitude of such a shift for HOCN or HNCO to be approximately 30 cm-1.16J7The absence of observable shift (at &30 cm-' from 1098 cm-I) does not preclude the existence of either species because the intensity of the C-N stretching absorption is expected to be of much greater intensity than the CO stretching absorption. The C-N stretching absorption is not expected to shift as a result of this substitution. As expected, no isotopic shifts for the 1731- and 1716cm-' absorptions are observed. The possibility of H2C0 or other ketones contributing to the absorptions observed in the 1700-1745-cm-' spectral region can thus be ruled out.

Proposed Reaction Schemes HCCN has been identified as a major product in all of the pyridine pyrolyses and thermal oxidations performed at temperatures above 700 "C. PRDDO (partial retention of diatomic differential overlap) calculations were performed on the pyridine molecule to investigate bond ord e r ~ From . ~ ~these ~ ~calculations we observe typical bond order values of 1.44 for CN and CC bonds but only 0.97 for CH bonds.43 The lowest bond orders were observed for the CH bonds adjacent to the nitrogen atom. Bondorder calculations indicate the C-H bond breakage is the energetically favorable first step in pyridine thermal decomposition. Hydrogen abstraction has been observed by other investigators at 650 0C.233 In our experiment, the observation of 2,2'-bipyridine as the major product in the 700 "C pyrolysis experiment supports the assumption that the hydrogen abstraction step dominates at lower temperatures. Assuming that CH bond cleavage is the first step in the thermal decomposition of pyridine C5H5N+ heat C5H4N+ H (1)

+ - +

The subsequent primary products of ring cleavage are C5H4N heat

C3H3N+ C2H

(24

CHN C4H3 (2b) Previous kinetic studies' have noted a preference of the primary products of pyridine ring cleavage to consist of four and two ring-atom fragments over other combinations. If two CH bonds in pyridine cleave before the ring, then the expected products after ring cleavage are C5H3N+ heat C3H2N C2H (34

- +

+

CN C4H3 (3b) Gas-phase reaction with hydrogen atoms and matrix tunneling could result in the generation of C3H2Nand C2H2from the radical products of reactions 2a and 3. No absorptions due to C,H2N, C2H, or C2H2are observed, while CHI is observed. Methane is present as an impurity in argon at less than 10 ppm, and no methane absorptions were observed in the background spectra. Thus, spectroscopic evidence indicates that these fragments readily form methane in our experimental system. In the presence of (42) Halgren, T.; Lipscomb, W. J . Chem. Phys. 1973,58, 1569. (43) Unpublished results.

O2and heat unsaturated hydrocarbons are known to react to form CO and The formation of isotopic H20, CO, and C02 in the 1802-enrichedTOS indicates that complete oxidation of unsaturated hydrocarbons has occurred in our system. The other major thermal decomposition product observed in nearly all spectra is 2,2'-bipyridine. The 2,2'bipyridine can be formed due to simple recombination of the pyridine radicals after the initial hydrogen abstraction in step 1. As expected, the intensities of the bipyridine absorptions are independent of the HCCN absorptions. However, the relative intensities of the absorptions due to other products (HCN, HOCN, NH,, CO, and C02) exhibit an inverse relationship to the HCCN absorptions. This fact suggests that these species are resulting from the subsequent thermal decomposition of HCCN. A possible reaction scheme based on the observed products in our experiments is H 2 C - C H = C = N heat CH2 + HCCN (4) CH + CN (5) HCCN + heat CN + heat + O2 NO + CO (6) This mechanism suggests the pathway for the formation of four of the six species observed and their dependence on HCCN production. The CH and CH2 radicals are subsequently converted to CH,, CO, and COP Reactions 2-5 can lead to observed anoxic cyanide containing products (HCCN, HCN, CN). Unsaturated hydrocarbons are also observed in our experiments. The observed intensities of the products resulting from reactions 2a-3a indicate that these are the dominant reaction paths for the decomposition of pyridine. Reactions 2a-5 suggest the formation of C2and C4 unsaturated hydrocarbons. These unsaturated hydrocarbon fragments are converted to CHI, CO, and COB,but may also react with pyridine molecules to form substituted pyridines. The intense CO and C02absorptions in the TOS imply that oxidation for the hydrocarbon fragments is occurring. HOCN and HNCO formation is due to the oxidation of HCN molecule^.^' At high temperatures the reaction H + 0 2 OH + 0 (7) is a well-known reaction in the combustion chemistry of aerobic system^.^" The formation of OH and 0 may lead to formation of HNCO and HOCN by the following reactions: OH + HCN HOCN + H (84 OH + HCN HNCO + H (8b) 0 + HCN NCO + H (9) (10) NCO + H2 HNCO + H HOCN + heat HNCO (rearrangement) (11) HNCO + heat NH + CO (12) Reaction 12 represents a pathway for the formation of CO and NH and to subsequent generation of NH3 in our system. The production and complexation of NH radicals may provide an alternate explanation of the broad absorptions in the high-wavenumber region (31W3600 cm-').

-

+

-

--

- 4

Conclusions The pyrolysis and thermal oxidation of pyridine in argon, nitrogen, and mixed nitrogen/oxygen matrices has (44) Ewing, G. E.; Pimentel, G. C. J . Chem. Phys. 1961,35, 925. (45) Warnatz, J. Symp. (Irtt.) Combust. 1984, 20th, 845.

Energy &Fuels 1991,5, 133-138 been investigated by matrix isolation. The major products of the pyrolysis experimentsare HCCN and 2,2'-bipyridine. Minor observed products are CHI and CO. The major products of the thermal oxidation of pyridine are HCCN, CO, and C02. The minor products of pyridine oxidative thermal decomposition are CH,, 2,2'-bipyridine, HOCN, HNCO, HCN, NO, N H , and H20. Evidence of significant NH production exists due to persistent broad absorptions in the NH stretching mode region (3100-3600 cm-') in all spectra. The strength of the CO and NH absorptions relative to the CH4 and NO absorptions indicate that the HCN HNCO NH + CO

- -

133

- -

pathway dominates over the alternative p a t h HCN HCNO CHI + NO The source of nitrogen in the NO appears to be the N2 of the matrix material rather than the pyridine. Pyridine nitrogen was primarily converted to HCN, HCCN, and NH,.

Acknowledgment. This work is supported by DOE Fossil Energy program, Grant DE-FG22-86PC90513C. Registry No. CO, 630-08-0; C02, 124-38-9;N20,10102-43-9; pyridine, 110-86-1; bipyridine, 37275-48-2; cyanomethylene, 2612-62-6; cyanic acid, 420-05-3.

Kinetics of Oxygen Chemisorption on Microporous Carbons J. K. Floess,* K.-J. Lee, and S. A. Oleksyt Department of Chemical Engineering, University of Illinois at Chicago, P.O.Box 4348, Chicago, Illinois 60680 Received April 16, 1990. Revised Manuscript Received August 6, 1990

Oxygen chemisorption rates on a microporous char have been measured at temperatures of 400-550

K and at oxygen partial pressures of 0.21-1 atm by use of a thermogravimetric analyzer. The experimental data were analyzed in terms of a distributed activation energy site model, and the model can account for most of the experimental observations made in the present study. Activation energies in the range of 70-120 kJ/mol for oxygen chemisorption were calculated by use of this model. The substantial energy barrier, at the upper end of the distribution, to oxygen chemisorption on a microporous carbon was not expected. On the basis of this result, it is postulated that the rate-controlling step for the low-temperaturecarbon-oxygen reaction may be oxygen adsorption on these high activation energy sites.

Introduction Studies of the reaction of oxygen with solid carbon have received much attention over the past several years, and although certain features of the reaction are believed to be well understood, there remains considerable uncertainty about the mechanism and the rate-controlling step of the reaction. The reaction of oxygen with carbon is believed to involve dissociative adsorption of oxygen on the carbon to form surfaceoxygen complexes and the subsequent desorption of the surface oxides to carbon monoxide and dioxide, which are both presumably primary reaction products. Elementary steps for the reaction have been proposed by a number of workers and are summarized by Marsh.' Although various intermediate steps have been proposed for this reaction, a kinetic model that is consistent with experimental observations has not yet been developed from proposed mechanisms. Recent studies of the low-temperature (