5373
J . Phys. Chem. 1986, 90, 5373-5377
Thermal Reactions of Cyclic Ethers at High Temperatures. 3. Pyrolysis of Furan behind Reflected Shocks Assa Lifshitz,* Menashe Bidani, and Shimon Bidani Department of Physical Chemistry, The Hebrew University, Jerusalem 91 904, Israel (Received: March 13, 1986)
The thermal decompositionof furan was studied behind reflected shocks in a single shock tube, over the temperature range 1050-1460 K, at total gas densities of approximately 3 X mol/cm3. Methylacetylene and carbon monoxide are the major reaction products and are formed by the reaction furan CH3C*H CO (l), with a rate constant k l = 1015.25*0.5 exp(-(77.5 f 2.5) X 103/Rq s-l. A second initiation reaction produces acetylene and ketene according to the reaction furan CH*H + CH2=€0 (2), with a rate constant k2 = 1014.7M.5 exp(-(77.5 2.5) X 103/Rq 8. The rate constant obtained for the overall decomposition of furan in the temperature range 106C-1260 K is = 10'5.43M.45 exp(-(78.3 i 2.0) X 103/Rg s-'. The overall pyrolysis rate measured in this investigation is about 8 times lower than the rate extrapolated from an estimated value suggested for the low-pressure pyrolysis. Additional reaction products which appear in the pyrolysis are CH,=C=CH,, C4H6,C2H4, CH4, C4H4,C4H2, and C&6. They appear in noticable quantities at high temperatures and are probably secondary products.
-
-
Introduction We have recently published an investigation describing the thermal decomposition of tetrahydrofuran behind reflected shocks, in a single-pulse shock tube.' A wide spectrum of products was analyzed in this pyrolysis and a mechanism for their formation was suggested. In an effort to elucidate the pyrolysis pattern of other fivemember ring ethers, we have carried out a detailed investigation of the pyrolysis of furan behind reflected shocks. As in the case of tetrahydrofuran, very little effort has been devoted in the past to the study of the thermal reactions of furan. The only investigation that we are aware of is a recent study by Grela, Amorebieta, and Colussizwho studied the very low pressure pyrolysis (VLPP) of furan, 2-methylfuran, and 2,5-dimethylfuran over the temperature range 1050-1270 K. The reactant molecules were heated in a steady flow reactor and the product analysis was done by an on-line mass spectrometry. The overall pyrolysis was determined by the decay of the parent ion intensity at m l z 68 (F), 82 (MF), and 96 (DMF). From the measured low-pressure unimolecular rate constants and by using various assumptions and predictions, the authors suggested the following unimolecular high-pressure rate constant for these molecules: kF
= 1015'6eXp(-73.5 x 1O3/RT)
S-'
kMF
= 1015.3exp(-74.2 x 103/RT) s-I
k2MF
=
exp(-74.1 x i03/RT) sF1
In the VLPP study, only the overall decomposition was reported. Neither the rates of formation of the individual reaction products nor their general distribution was presented and discussed. In this article we report the product distribution in furan pyrolysis over the temperature range 1050-1450 K, as well as the mechanism and the rate of formation of several reaction products.
Experimental Section Apparatus. The pyrolysis was studied behind reflected shocks in a pressurized driver 52-mm-i.d. single-pulse shock tube. The tube and the mode of its operation have been reported in a previous and will be described here only very briefly. The tube had a 4-m driven section, a variable driver section up to a ( 1 ) Lifshitz, A.; Bidani, M.; Bidani, S. J . Phys. Chem. 1986, 90, 3422. (2) Grela, M. A.; Amorebieta, V. T.; Colussi, A. J. J . Phys. Chem. 1985, 89, 38. (3) Lifshitz, A,; Bidani, M.; Carroll, H. F. J. Chem. Phys. 1983, 79, 2742.
0022-3654/86/2090-5373$01.50/0
+
*
maximum of 2.7 m, and a 36-L dump tank. The latter was introduced in order to prevent reflection of transmitted shocks. The low- and the high-pressure sections were separated by Mylar polyester film of various thickness. The tube was pumped down to approximately Torr and had a leak or degassing rate of - 5 X Torr/min. Incident shock velocities were measured with two high-frequency pressure , to - f 1 5 transducers with an accuracy of --f2 ~ scorresponding K. Cooling rates were (0.5-1.0) X lo6 K/s, defining a reaction dwell time of -1.8 ms with an accuracy of - f 5 % . The reflected shock parameters were calculated from the measured incident shock velocities by using the three conservation equations and the ideal gas equation of state. The molar enthalpies of furan were taken from a report by Wilhoit et aL4 Materials and Analysis. A reaction mixture containing 1% furan in argon was prepared and stored at high pressure in a stainless steel cylinder. This served as a stock mixture for all the experiments. The cylinder and the gas-handling manifold were Torr before the preparation of the mixture. pumped down to The materials used in this investigation were obtained from the following sources. Furan was obtained from Fluka A.G. It was of purum grade, listed as better than 99% pure. The argon and the helium were obtained from the Matheson Gas Co. and were listed as 99.9995% and 99.999%, respectively. Shocked samples were taken from the end block of the driven section and were analyzed on a Packard 800 series gas chromatograph using a flame ionization detector. The following two analyses were performed on each postshock sample. 1. A 2-m Porapak N column initially at 35 OC was gradually elevated to 150 OC and was used for obtaining the general analysis of the products. A complete analysis lasted a little over 1 h, starting with methane (0.8 min) and ending with benzene (68 min). A few analyses which were run for additional 2 h at 150 OC did not reveal additional reaction products. The carrier gas in these analyses was argon. A typical gas chromatogram taken at the high temperature end of the study is shown in Figure 1. 2. A 2-m molecular seive SA column operated at room temperature was used to separate carbon monoxide from methane. It provided the ratio [CO]/[CH4]. From this ratio, together with the results of the Porapak N analysis, the concentration of the carbon monoxide was evaluated. The gases after being eluted from the column passed through a Chrompak methanyzer in order to reduce the carbon monoxide to methane for FID detection. The
-
(4) Kudchadker, A. P.; Kudchadker, S. A,; Wilhoit, R. C. Key Chemical Data Boos, Furan Dihydrofuran Tetrahydrofuran; Texas A&M University: 1978.
0 1986 American Chemical Society
5374 The Journal of Physical Chemistry, Vol. 90, No. 21, 1986
Lifshitz et al.
TABLE I: Details of the Experimental Conditions and Results of Representative Tests 104c5,
shock no.
T5,K
mol/cm3
% CH,
20 5 8 9 3
1165 1225 1260 1300 1370
0.292 0.305 0.310 0.321 0.321
0.02 0.03 0.07 0.18 0.97
RETENTION
% CzH4 0.03 0.07 0.15 0.23 0.50
% C2Hz
% allene
% propyne
% C4H6
0.34 1.35 2.61 4.23 9.81
0.32 0.99 2.07 6.48
0.84 4.77 9.34 14.23 23.04
0.04 0.20 0.33 0.47 0.87
% C4H4
%furan
% CO
0.03 0.06 0.20
95.92 87.73 78.80 65.60 29.73
2.81 5.53 7.64 13.01 27.68
TIME ( m i n l
Figure 1. A typical gas chromatogram of a postshock mixture of 1%
furan in argon. The numbers on the peaks are attenuations in powers of 2.
carrier gas in this analysis was composed of 50% argon and 50% hydrogen. The identification of the reaction products was based on their retention time in the gas chromatograph but was assisted by GC-MS analyses which were carried out on a V.G. ZAB-2F mass spectrometer. The relative sensitivities of the reaction products to the flame ionization detector were determined from standard mixtures. They were roughly proportional to the number of carbon atoms in the molecule. In the carbon monoxide analyses, a standard mixture of C H 4 and CO was periodically tested to ensure a complete conversion of the latter to methane. The areas under the peaks in the gas chromatograph were integrated by a Spectra Physics Model SP-4100 computing integrator and were processes on line by an Apple IIe computer. Evaluation of the Product Concentration. The evaluation of the concentrations of the reaction products from their peak area was done in the following manner:' 1. The concentration of furan behind the reflected shock prior to decomposition, C5(furan)o,is given by where pI is the pressure in the tube prior to shock heating, %(furan) is the percentage of furan in the original mixture, p s / p l is the compression of the sample behind the reflected shock, and TI is room temperature. 2. The concentration of furan behind the reflected shock prior to decomposition in terms of its peak area, A(furan),, is given by A(furan)o = A(furan),
+ 0.252N(pri) A(pri),/S(pri)
(11)
where A(furan), is the peak area of furan in the shocked sample, A(pri), is the peak area of a product i in the shocked sample, S(pri) is the sensitivity of a product i relative to furan, and N(pri) is the number of carbon atoms in a product i. 3. The concentration of a product i in the shocked sample is given by where A(furan)o is calculated by eq 11.
Results and Discussion Presentation of the Experimental Results. A series of experiments covering the temperature range 1050-1450 K at total gas densities of approximately 3 X l V 5 mol/cm3 were run. Details
T, K Figure 2. Product distribution of postshock mixtures of 1% furan in argon shown over the temperature range covered in this investigation.
of five representative tests are given in Table I. The table shows the temperature behind the reflected shock T,, the overall density behind the reflected shock C, in units of mol/cm3, and the percent of the various reaction products as obtained in the postshock analysis (not including the argon). The concentration of furan behind the reflected shock prior to decomposition (C5(furan)o) is given by the percent of furan in the original mixture times C,. Figure 2 shows the product distribution obtained in postshock samples of 1% furan in argon over the temperature range covered in this investigation. The percent of a given product out of the total shown in the figure corresponds to its mole fraction, 1 0 0 C i / z C i , irrespective of the number of its carbon atoms. (Molecular hydrogen is not included.) The rate of formation of the various products defined as rate(Pri) = [Pri],/t represents an average rate over a period of time r (the reaction dwell time). At the low temperature end of the study where the reaction is in its very early stages, the rate as expressed by the right-hand side of eq IV is a good substitute for dC(pri),/dt. At higher temperatures it can be a very crude average since some of the products are produced and decompose at the same time. As has been done in a previous study,' rate constants and their temperature dependence (E(pri)] are calculated from the lower section of the temperature range, before bending of the Arrhenius curves begins to occur. In Figures 3-9 such curves for seven reaction products are shown, where the logarithm of [prdu~t]~/[furan], divided by the reaction dwell time ( 1 ) are plotted against the reciprocal temperature. The points on the curves resemble first-order rate constants (kf,, order), although some of the products are not formed in a unimolecular process. It is, however, a good way to present the experimental data from which Arrhenius temperature de-
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 5375
Thermal Reactions of Furan
T ,K 1500
T, K
1300
1400
1500
I
I
" -I
I
I 8 I / T x lo4
I
I
I
7
1300
1400
3
I
Figure 6. A plot of log {[CH,],/[f~ran]~J/t against the reciprocal temperature. Methane is obtained from the decomposition of methylacetylene. T, K
IIOO
I
I
9
V T x lo4 K - '
1200
1300
I
8
7
K-l
T, K
1400
I
-2
9
Figure 3. A plot of log ([propyne]/[f~ran]~)/t against the reciprocal temperature. The slope of the line in the low-temperature section of the figure gives an Arrhenius temperature dependence of 77.5 kcal/mol. 1500
1200
I
1500
I
!
1400
1300
0
0
I
I
I
8
7
I
ai
I -2
9
Figure 4. A plot of log ([CzHz],/[furan]oJ/ragainst the reciprocal temperature. The slope of the Arrhenius curve at the low-temperature section of the figure gives a temperature dependence of 77.5 kcal/mol. T, K
1200
1300
1400
8
7
I / T x lo4 K-'
1500
\o
Figure 7. A plot of log ([C,H,],/[furan]$/r
against the reciprocal temperature. A possible path for the formation of ethylene is 2CH2=C=CI, CzH4 + C4H4.
-
T, K
1500
1300
1400 I
I
I
\
-2
I
I
7
8
9
I / T x lo4 K - '
1100
\
I
?
1200
I
\\
9
I / T x IO4 K-'
Figure 5. A plot of log ([allene],/[f~ran]~l/t against the reciprocal temperature. The high-temperature dependence of 105 kcal/mol contains
a contribution from the production of propyne and its isomerization to allene. pendencies and preexponential factors can be calculated. They can at a later stage serve as a basis for computer modeling of the
Figure 8. A plot of log ([C4H,],/[furan]O]/tagainst the reciprocal tem-
perature. overall reaction. It should be added that these temperature dependencies and preexponential factors do not necessarily represent activation energies or A factors of elementary steps, only in specific
5376 The Journal of Physical Chemistry, Vol. 90, No. 21, 1986
Lifshitz et al.
T, K
1500
1400
T, K
1200
I300
I100
1500
1400
I
O: -2
7
9
8 I/T x
-I
1300
I
I
1200
I
I
I
I
8
9
I/TX
-
+
TABLE II: Preexponential Factors ( A ) and Arrhenius Temperature Dependencies ( E ) for the First-Order Rate Constant of Formation of Various Reaction Products molecule A , s-' E , kcal/mol CH4 14.52 85.2 C2H4 10.98 63.4 C2H2 14.70 77.5 CH 2=C=CH 2 18.10 105 CH,C=CH 15.25 77.5 C4H6 12.80 71.7 C4H4 13.63 83.7 furan" 15.43 78.3
"The parameters for furan correspond to its decomposition rate constant.
I
7
to4 K - I
Figure 9. A plot of log {[C4H6],/[furan],)/fagainst the reciprocal temperature. The Arrhenius curve has a slope of 72 kcal/mol in the lowtemperature section of the figure. A possible path for the formation of C4H, is the recombination CH2C=CH CH, C4H6.
1100
I
io4 K - I
Figure 10. A plot of log k,,,, against the reciprocal temperature for the
overall decomposition of furan. At the low-temperature section it gives k,,,,, = 1015.48 exp(-78.3 X lO'/RT) s-l.
' Y\
IO0
I
I I
-,
u
v) 0,
-
\\ \
IO
7
s I
urn
I
N
=N
cases. They are experimentally determined quantities and they only represent a way to summarize the experimental results in a quantitative manner. Values of 4.576 d log knrstorder/d(1/T) defined as E(pr,) and preexponential factors calculated from Figures 3-9 are summarized in Table 11. The Pyrolysis Mechanism. a. The Initiation Steps. In trying to elucidate the pyrolysis mechanism one should first identify the initiation steps and then look for products that are formed by secondary reactions. The major products of the pyrolysis are methylacetylene and carbon monoxide which appear in roughly equal quantities (Table I, Figure 2). It is therefore reasonable to assume that both these products are formed simultaneously by a unimolecular opening of the furan ring:
0-
CHaCGCH
+
CO
(1)
This channel was suggested also in the VLPP study as the major decomposition channel.* Figure 3 shows the Arrhenius curve for reaction 1 where the rate constants were calculated from the relation k , = ([CH,C=CH],/
[furanIO)/t
(VI
At low temperatures where the depletion of the reactant is still minimal, these rate constants can be considered as true rate constants. The value obtained for k , is 1015.25*0.5 exp(-(77.5 f 2.5) x 1 0 3 / ~ qs-1. In Figure 10 the Arrhenius curve for the overall decomposition of furan is shown, where the rate constants ktotalwere calculated from the relation
ktotalt= -In ([furan],/[furan],)
(VI)
v
01
0.01 I
I
7
I
I
I
IO4 / T,
I
9
8
I
10
OK-'
Figure 11. A plot of the ratio [C2H2],/[propyne],against the reciprocal temperature. The open circles represent the ratio obtained in this study and the solid line represents the ratio obtained in the study of the pyrolysis of propyne. The two lines are entirely different from one another and do not represent similar reaction channels.
where [furan], is the concentration of furan left after a dwell time t and [furanlo is the initial furan concentration. In view of the fact that reaction 1 is the major reaction channel and is much faster than all the others put together, the rate parameters for the overall decomposition are not much different from those of reaction 1 as long as the conversion does not exceed several percent. The rate parameters obtained for the t,otal furan disappearance are ktotal= 1015.43*0.45 exp(-(78.3 f 2.0) X 103/RT)S-I. It is interesting to compare the value of ktotalobtained in this investigation to the one obtained by Colussi et aLz at low temperatures using the VLP conditions. From a measured low pressure rate constant of kVLp= 1.0 X 1O1O exp(-49.8 X 103/RT) s-l, by using several assumptions, the authors suggested a value of k,,, = 1015.6 exp(-73.5 X lo3) s-' for the high pressure unimolecular rate constant. This preexponential factor is very similar to the one obtained in this investigation but the activation energy
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 5377
Thermal Reactions of Furan is smaller by -5 kcal/mol, generating at 1200 K about a factor of 8 difference in the value of k. The values of k,,, obtained in this investigation are lower than the extrapolated ones indicating some error in the estimate of k,,,,, from the low pressure studies but alltogether the agreement is quite good. b. Production of Acetylene. The question that arises is whether acetylene, the third major product of the pyrolysis, is formed directly from furan according to the reaction
0-
CHsCH
+
CH2=CO
SCHEME I
0-
+
CH3C=CH
CO
(1)
(2)
or whether it is formed by further decomposition of methylacetylene. The latter is known to produce acetylene as one of its high temperature pyrolysis product^.^ In order to decide which reaction channel is operative we examined the ratio [C2HJ1/ [CH3C=LH], in this study to the ratio [C2H2lr/[CH3-CHIo in the study of the pyrolysis of methyla~etylene.~ The results are shown in Figure 11. The open circles represent the ratios obtained in this study and the solid line represents the ratios obtained in the investigation of methylacetylene pyrolysis. Both are plotted against the reciprocal temperature. It can be seen that, at least up to 1300 K, the ratios in this study are approximately 0.3 and are almost temperature independent. On the other hand, the ratios in the methylacetylene pyrolysis are very small at the low temperatures and are highly temperature dependent. These two lines are entirely different from one another and do not represent similar reaction channels. One may therefore conclude that the acetylene obtained from furan pyrolysis arises from reaction 2 and not from the pyrolysis of C H 3 m H and is therefore a primary product. If indeed acetylene is formed by reaction 2 then ketene must also be present in the postshock mixture. It was not discovered both in the G C and the GC-MS analyses probably because of its high reactivity toward water absored on the walls of the shock tube, the bulb, or the injection system. In several runs we collected the postshock samples in glass bulbs containing several Torr of methyl alcohol. Similar to its reactivity with water ketene is known to react instantaneously with methyl alcohol to form methyl acetate at room temperature. We analyzed these bulbs in the GC-MS for methyl acetate and found, just after the furan peak, a G C signal at 57-min retention time having a mass spectrum of peaks a t m / z 43 (CH3CO+), 74 (CH3COOCH3+),59 (CH3COO+),42 (CH20+),and 29 (HCO') with intensity ratios typical of methyl acetate. We do not have a quantitative measure of the latter but its presence in the postshock mixture is proven without any doubt. This finding and the one regarding the ratio [C2H2]/[CH3C=--CH] clearly show that acetylene is a primary product. Its rate constant k2 as obtained from Figure 4 is given by kz = (14.7 f 0.5) exp(-(77.5 f 2.5) x 1 0 3 / ~ qS-1. c. Production of Secondary Products. The secondary product of the highest concentration is allene. Methylacetylene is known (5) Lifshitz, A,; Frenklach, M.; Burcat, A. J . Phys. Chem. 1976,80,2453.
(111
CH3* C2H2
- CH3.
C4H4
C2He CgHg
(12)
(13)
(schematic presentat ion)
to isomerize to allene in a unimolecular reaction, under experimental conditions similar to the ones employed in this investigatione6 At the low temperature end of this study the ratio [allene]/ [methylacetylene] is very small; at higher temperatures when the isomerization is faster it reaches the equilibrium value of -0.3. The production of allene has a very high temperature dependence (Table 11) resulting from the temperature dependence of the formation of methylacetylene and its isomerization to allene. Both methylacetylene and allene produce in their pyrolysis a wide spectrum of products, similar to the ones observed in this investigation. Their production mechanism was discussed in a previous publication describing the pyrolysis of allene and methylacetylene behind reflected shock^.^ On the basis of the findings in this investigation and the aforementioned study,s the main reactions suggested for furan pyrolysis are shown in Scheme I.
Acknowledgment. This work was supported by a grant from the US.-Israel Binational Science Foundation under grant agreement 84-00161. Dr. Wing Tsang was the American cooperative investigator for this grant. Registry No. Furan, 110-00-9. ( 6 ) Lifshitz, A.; Frmklach, M.; Burcat, A. J . Phys. Chem. 1975, 79, 1148.
