194
Ind. Eng. Chem. Prod. Res. Dev. 1980, 79, 194-197
McCaulay, D. A., US. Patent 3 819 743 (June 25, 1974). Olah. G. A.. WhRe, A. M., J . Am. Chem. SOC.,91,5801 (1969). Schriesheim, A., U S . Patent 2943 126 (June 28, 1960). Schriesheim, A., Gilbert, G. R.,U S . Patent 2 966 535 (Dec 27, 1960). Schneider, A., J. Am. Chern. SOC.,74,2553 (1952),and references cited therein. Siskin, M., J. Am. Chem. SOC.,98, 5413 (1976).
Siskin, M., Chludzinski, G. R., Hulme, R., Porcelli, J. J.. Tyler, W. E. 111, pari 9 of this series, submitted for publication in Ind. Eng. Cbem. Prod. Res. Dev.,
1980.
Received f o r review April 23, 1979 Accepted January 28, 1980
Unusual Ammoxidation Route to Trimethylpyridine Serge R. Dolhyj" and Louis J. Velenyi Technology and Planning, The Standard Oil Go. (Ohio), Warrensville Heights, Ohio 44 128
The ammoxidation and oxydehydrogenationtype catalysts such as bismuth phosphomolybdates, uranium antimonates, and iron antimonates normally attack the terminal carbon in !he a position to the G==Cdouble bond. Thus in propylene ammoxidation acrylonitrile is produced; similarly, atroponitrile is formed by the ammoxidation of a-methylstyrene. However, when we studied the ammoxidation of d-limonene we found a surprising product indicating an unusual reaction mechanism. In a conventional mechanism we would expect ammoxidation attack on two terminal carbons and potentially two nitrile groups could be formed. Another expected competitive reaction may proceed via aromatization since these catalysts also have strong oxydehydrogenation activity. However, to our surprise trimethylpyridine was predominantly formed in the gas-phase reaction. This unusual reaction may proceed by intramolecular cyclization of a transient mononitrile precursor. Several possible reaction paths will be discussed and the favored mechanism will be postulated.
Introduction Limonene is an interesting model molecule for studying various reaction mechanisms. In this work we selected this species for the study of the ammoxidation reaction. Since the discovery of the propylene ammoxidation process at Sohio in the 1950's this reaction was the subject of extensive mechanistic study. In the original Sohio ammoxidation process, which is now used commercially worldwide, propylene is oxidized in the presence of ammonia to acrylonitrile. If no ammonia is present in the feed then propylene is oxidized to acrolein. The mechanism of the oxidation reaction over bismuth molybdates and CuO was studied by Adams and Jennings (1964) and Adams et al. (1964) using deuterated propylene. They proposed initial formation of surface adsorbed allylic species after first hydrogen abstraction. This allylic intermediate underwent second hydrogen abstraction forming a proposed vinyl carbene species. Oxygen is then incorporated into the molecule to form acrolein. Callahan et al. (1970) concluded from the kinetic study that in the presence of ammonia (i.e., in the ammoxidation mode) over Bi/P/Mo catalyst, acrylonitrile is formed largely (over 90%) by a mechanism not involving acrolein intermediate but proceeds via a parallel path. Grasselli and Suresh (1972) studied the ammoxidation mechanism over U/Sb catalysts and also postulated the parallel paths of oxidation and ammoxidation. Hucknall (1974) summarized the mechanism of selective oxidations and ammoxidations in a book. At this time the predominant evidence suggests a direct path to acrylonitrile not involving acrolein intermediate. Other olefins were also reported to form substituted acrylonitrile species: e.g., isobutylene forms methacrylonitrile and a-methylstyrene is converted to atroponitrile (Mekhtiev et al., 1972; Callahan et al., 1967; Grasselli and Callahan, 1969). These reactions apparently proceed via a mechanism similar to that for propylene, involving initial attack on the terminal carbon in the a position to the C=C double bond. 0196-4321 /80/12 19-0 194$0 1.OO/O
Atroponitrile is relatively unstable and dimerizes easily to form a cyclic species. The dimerization does not involve cyclization through N; apparently the nitrile species after it is formed is resistant to cyclization (Newey and Erickson, 1950). Method Recently we extended our ammoxidation studies to cyclic substituted olefins. We used d-limonene as a model reactant. The ammoxidation of d-limonene was conducted in vapor phase and liquid phase environments. A fixed bed microreactor shown schematically in Figure 1 was used for vapor-phase study and a glass reflux reactor was applied for liquid-phase experiments. The liquid products were analyzed by GC techniques and confirmed by a combined GC/MS method. The tailgas was analyzed using a separate GC for CO, C02,NP, and O2 components. The material balances were calculated on a carbon basis. The catalyst systems used in our study constituted bismuth phosphomolybdates, uranium antimonates, and iron antimonates. These systems were described in earlier ammoxidation work in relation to propylene/acrylonitrile reactions. All were act,ive and selective in producing acrylonitrile in high yields (Callahan et al., 1970; Grasselli and Suresh, 1972). Results and Discussion The d-limonene molecule has two terminal carbons ad-
d-LIMONENE
jacent to the C=C double bond. One constitutes a sub0 1980
American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 2, 1980
195
Table I. Results of Ammoxidation of d-Limonene
C.T.,a catalyst type
a
s
Bi P molybdate 2 2 Bi P molybdate Fe antimonate 2 Fe antimonate 2 2 U antimonate U antimonate 2 2 U antimonate C.T. contact time (seconds).
conversions, %C
reaction temp, " C
isoprene
cymene
3 50 375 4 00 420 3 50 400 425
6.2 11.2 8.5 9.5 6.4 8.4 10.7
4.2 14.2 2. I 1.3 1.6 1.3 0.4
SAGE SYRINGE PUMP (d-LIMONENE FEED)
I
TMP 21.5 31.9 13.3 23.3
C,,H,, 22.6 29.2 20.0 20.1 3.9 9.1 9.9
10.1 8.4 6.9
CO
+ CO,
unreacted
41.0 5.8 52.0 40.5 76.7 70.1 66.2
1.5 3.0 3.5 5.3 1.3 2.8 5.9
Chart I
H3C/$CH3
F I X E D BED MICROREACTOR WITH HEATED BLOCK
CYMENE
r
CH3
I
,
SURFACE SPECIES TRILE PRECURSOR)
AIR
,'R==
/I1
* TO T A I L GAS G i C
\
REACTION CONDITIONS: TEMP. C.T. HC/A I R/NH3
250-425 "C 2 SEC. = I /20/4
Figure 1. Fixed-bed microreactor.
stituted propylene. The other is attached to the cyclic double bond. If the classical ammoxidation mechanism were operative in this case we would expect conversion to two nitrile species with the predominance of the side chain species. Another expected competitive reaction may proceed via aromatization, since those catalysts have also demonstrated strong oxydehydrogenation activity. However, to our surprise the product contained predominantly trimethylpyridine species. A typical product distribution for the gas-phase reaction with different catalyst types is summarized in Table I. This process was patented by Sohio (Dolhyj and Velenyi, 1979). The presence of side reaction products, cymene, substituted a-methylstyrene, isoprene, CO, and COz, can be explained by known paths involving dehydrogenation or cracking and total oxidation of fragments. The mechanism of cymene formation is similar to other known schemes involving conjugated cyclic systems which may involve initial H transfer from the cyclic ring to the side chain. An example of such well known paths is ethylbenzene formation from vinylcyclohexene. However, the forni ation of trimethylpyridine may involve a novel mechanism which is proposed in Chart I. In this mechanism we eiivision first the double-bond migration from the side chain into the cyclic ring. This is simply accomplished by H ti-ansfer from the ring into isopropyl group. The resulting intermediate p-methylisopropyl hexadiene (a-terpenene) was identified by NMR at substantial concentration levels in our reaction products. A t this point the reaction branches off along two paths. One path involves continuation of oxydehydrogenation resulting in complete aromatizrttion to cymene, as indicated earlier.
,&
1
'CH3 2.3.6
CH3 TRIMETHYLPYRIOINE
Chart I1
Q-
6" 5& Po 0 HoQ Oon @ h
QHO
d-LIMONENE
PULEGONE
I-p-MENTHEN-9-.I
DIHYDROCARVONE
4
CARVONE
QH
DIPENTENE O X I D E
d -TERP INEOL
A.
O = C -
