Thermal Cracking of the Isomeric Hexanes with ... - ACS Publications


Product distributions from cracking of n-hexane, 2-methylpentane, 3-methylpentane7 2 ... The modified product distribution from the modestly accelerat...
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M. L. Poutsma and S. 8.Schaffer

158

Thermal Cracking of the Isomeric Hexanes with That Catalyzed by Potassium Ion Exchanged Y Zeolite’ Marvin L. Poutsma” and Sherry R. Schaffer Union Carbide Research Institute, Tarrytown Technical Center, Tarrytown, New York 70597 (Received September 5, 1972)

f’ublicaii’oncosts assisted by Union Carbide Corporation

Product distributions from cracking of n-hexane, 2-methylpentane, 3-methylpentane7 2,2-dimethylbutane, and 2,3-dimethylbutane over potassium ion exchanged Y zeolite (KY) a t 500” and 1 atm pressure are compared to those from thermal cracking at the same conditions. The thermal products are qualitaI ively fully consistent with the Rice-Kossiakoff radical chain decomposition mechanism. Quantitative consideration of product yields allows calculation of the relative reactivities of tertiary (t),secondary (s), and primary (p) hydrogen atoms in the hydrogen abstraction step from each substrate. The ratios of hydrogen abstraction from substrate to fl scission(s) for the various intermediate alkyl radicals are also calculated At the reaction conditions, these two processes are competitive in rate for n-propyl, isopropyl, and tert-butyl radical while hydrogen abstraction predominates for ethyl radical and fl scission predominates for 1- and 2-butyl radicals. The modified product distribution from the modestly accelerated cracking over KY can be accommodated within the radical mechanism with two modifications: (1) there is a six-tenfold greater tendency toward hydrogen abstraction compared with fl scission for n-propyl, isopropyl, and tert-butyl radicals, and (2) olefinic products undergo double bond shifts, but not skeletal isomerization, subsequent to cracking, presumably because of weakly acidic defect sites. The possibility that effect I is related largely to an effectively greater substrate concentration on KY compared with the gas phase with resultant enhancement of bimolecular hydrogen abstraction over unimolecular 0 scission is considered.

Cracking of various classes of hydrocarbons over polyvalent ion-exchanged and/or proton-containing X and Y zeolites (crystalline aluminosilicates) has been extensively studied,l,2 and assignment of carbonium ion character3 to the organic intermediates involved is widely accepted. Less attention has been paid to the hydrocarbon conversion properties of the monovalent alkali metal ion containing zeolites because they are generally much less active as catalysts 1 , 2 For cracking of n-paraffins at 500”, an activity order CaX 3 NaX > amorphous silica-alumina was r e p ~ r t e d Whereas .~ n-hexane was converted by CaX to a “carbonium ion” product mixture characterized by highly isomerized olefins, paraffins of carbon number greater than 3, low yields of C1 and C2 compared to C3 and Cq products, and extensive skeletal isomerization of both substrate and products, NaX gave a mixture comparatively richer in C1 and C2 products, poorer in C4 and Cg paraffins, and free from skeletal isomerization (although mixtures; of double bond isomers of olefins were obtained).4 Sim lar observations were reported5 for the Y zeolite series; cracking of n-hexane a t 450” was somewhat more rapid over NaU than over silica-alumina; however, introduction of Ca2+ ions led to sharply increased activity and formation of a ‘‘carbonium ion” product mixture beyond 60% @ai’+.on exchange. Cracking of 2,3-dimethylbutane over NaX and sulfur-treated NaX zeolite has been compared6 with thermal cracking and with cracking over Y zeolite. Tn all thme cases,4-6 the authors assigned a “radical” mechanism to the cracking over the Na forms of the zeolites, alttough direct experimental comparison of activity and product composition was made with thermal cracking, for which a radical chain mechanism is well established,7 only in the last case.6 In fact, although the product analyse3 were not complete, examination of the data reported shows txlat they do not conform directly to The Journal of Physical Chemistry, Vol. 77, No. 2, 1973

those expected for either the radical’ or carbonium ion mechanisms. We report herein a direct comparison of the products from thermal cracking of the five isomeric hexanes (C6H14) with those from cracking over potassium ion exchanged Y zeolite (KY). The products over KY can indeed be rationalized remarkably well with a radical chain cracking mechanism but only if certain specific modifications of the normal thermal reactions mediated by the zeolite are postulated.

Results Thermal cracking of n-hexane (NH), 2-methylpentane (2MP), 3-methylpentane (3MP), 2,2-dimethylbutane (22DMB), and 2,3-dimethylbutane (23DMB) was carried out a t 500” in a cylindrical quartz flow reactor loosely packed with quartz wool at a superficial contact time of ca. 24 sec which was sufficient to achieve ca. 2% cracking. Product compositions were determined by glpc analysis of the entire gaseous reactor effluent by use of heated transfer lines and sampling valves to avoid any complications of preliminary gas-liquid fractionation. Conversion levels

(3) (4) , . (5) (6)

(7)

Presented in preliminary form at the Second International Conference on Molecular Sieve Zeolites. Worchester, Mass., 1970; J. A. Rabo and M. L. Poutsma, Advan. Chem. Ser., No. 102,284 (1971). P. 8. Venuto and P. S. Landis, Advan. Catal., 18, 259 (1968); J. Turkevich, Cafal. Rev., 1, 1 (1967). 8. S. Greensfelder, H. H. Voge, and G. M. Good, Ind. Eng. Chem., 41, 2573 (1949); C. L. Thomas, ibid., 41, 2564 (1949). V. J. Frilette. P. B. Weisz, and R. L. Golden. J. Catal., 1, 301 ( I 9623. S. E. Tungand E. Mclninch,J. Catal., 10, 166 (1968). 2, Dudzik and R . J. Cvetanovic, Repr., Int. Congr. Catal., 4th, 3, 1082 (1968). (a) D. A. Leathard and J. H. Purnell, Annu. Rev. Phys. Chem.. 21, 197 (1970); (b) A. Kossiakoff and F. 0. Rice, J. Amer. Chem. Soc., 65, 590 (1943): F. 0. Rice and K. K. Rice, “The Aliphatic Free Radicals,” Johns Hopkins Press, Baltimore, Md.. 1935, Chapter VII.

Thermal Cracking of the Isomeric Hexanes (per cent crackling) were calculated by comparing the sum of the total observed products ( C 1 - c ~ )with unconverted c6 substrate on a weight basis. Cracking over KY zeolite pellets, preaccivated a t 550” m vacuo, was carried out in the same reactor under the same experimental conditions. Conversion lwels calculated as described above are reasonably accurate since no isomeric c6 hydrocarbons were produced and residue formation on the catalyst was negligible. Results are listed in Table I; each entry is an average of at least two analyses taken after the system had reached steady-state behavior. More extensive data for NW over KU shown in Table I1 indicate the degree of reproducibility of the data. The effect of lower conversion level, achievcd by use of shorter contact time, on the products from the NW-KY system is shown in Table 111.

iscussion

159

paraffins >C4), but a second partitioning point is involved if more than one type of 0C-C bond is available. Olefin-forming 0 scissions then proceed in sequence until some small radical R.” is reached for which p scission is no longer overwhelmingly predominant compared with hydrogen abstraction from RH. Here the third partitioning point arises: the ratio of bimolecular hydrogen abstraction (rate = k~~ ’. “[RH][R.”]) to mimolecular @ scission (rate = kpR ‘ ”[R.”]). Whereas the quantity (k~[RHl/ks)R”‘ is s. A "best" set of a, values can be calculated by a least-squares procedure12 to minimize the sum of the squares of the residuals T k = ( Z q = l s a q a q ) - ( X , ) by setting all dS/aa, = 0 where S = I ? k = l n T k 2 . However, this procedure seemed somewhat unrealistic for cases where some XI, values are numerically small compared to the rest since the experimental error in X k , measured by glpc analysis, i s surely not an absolute number but must be somewhat proportional to the numerical value of x k . Therefore, the least-,squares procedure was actually carried out with Y k = ( & ? q = l S a q a q ) W k X k where the weighting factor W k was arbit,rarily chosen such that W k 2 x k was approximately constant for all measurements x k . Note that the extent to which the Za, so determined approaches 100 gives some feel for the internal consistency of the d a t a and the model. The calculated a, values can then be used to quantify the partitioning points in Schemes I-V. The procedure is illustrated for the case of NH (see Scheme I). The amounts of each of the seven major prod(12) D. H. Menzel, "Fundamental Formulas of Physics," Vel. 1, Dover Publications, New York, N. Y., 1960, p 80.

Thermal Cracking of the Isomeric Hexanes

22DMBa

23DMBa _____l_.l__l_-CalcdC KYh CalcdC

3.8 76.8

_ I _

Calcd"

KYb

GalcdC

Thermalb

1.2e 33.2 53.5 35.0 20.7

30.3 46.5 35.3 19.1

1.6e 19.2 47.8 37.6 24.3

17.2 41.1 38.2 22.2

24.4 27.9 11.5 42.4 57.8 19.2

18.4 26.1 10.9 41.5 63.8 16.2

40.5 0.2

41.5

3.5 68.9

0.4 46.7 20.5

0.5 37.8

34.2

2MPa

Thermalb

Ile 13.1 68.7

I61

1-

39.0 32.0

CalcdC

KYb

2e

10.3

0.5

CalcdC

[email protected] 4.3 21.7

6.5 43.5

44.2 36.4

3.7 20.9 6.3 4'1.8 47.2 33.4

0.9 45.4 5.1

49.4 5.1

36.6 21.3

39.4 21.0

1.0

0.8 67.9

41.3

actor loosely filled with quartz wool: KY conversion.

4.9

5.6

36.2

40.9

40.1 1.6

41.8

8.5

1 .i 9.2

2.3 8.4

111.7

4.5

4.9

1.6 1.7

3.4

24.8

= reactor filled with K+-exchanged

Y zeolite; reactor volume

= 20 cc; feed rate =

6.14 cc of hexanelhr.

Cal-

SCHEME I: Det:ompo!jition Network for n-Hexane

+

c/c,c/c,c/c

4

c=c

c,ck/c,c/c

-----t

c=c-c

c=c

+

c--c

c/c\c-% c=c

+

c.

c/c,c/E

----t

I

+

+

RK

+

c=c-c-c

c-c-

+

c=c-c-c-c

c.

a, = 4.3 u2 = 17.6 u3 = 39.3

a , = 31.4

= 44.0 a 3 = 10.9 a2

a 5 = 7.2 Z U= ~ 99.8

Thermal

a6 = 22.7 a 7 = 7.8 a8 = 35.1

i

a, = 14.2 a,, = 22.3 a q = 102.1

c +

R.

R

+

c

t- R*

+

It

KY

Thermal

a, = 6.3

c-c

RH.

-RA

+

c-c

RH

R*

4

RH

k--

a4 = 32.3 a5 = 6.5 E a , = 100.0

KY (a7

+

= 19.0 as) =z 45.2 U6

a, = 8.5 a10=

Za,

28.1

= 100.8

The Journalof Physical Chemistry, Voi. 77, No. 2, 1973

M. L. Poutsrna and S.R. Schaffer

162 SCHEME Ill: Clecomposition Network for 2,3-Dimethylbutane all

* -I

+

a12

Thermal a,,= 13.1 u14 = 67.9 (zi2 = 20.5 Z a p = 102.3

C=C-C

+ C-6-CI

+ XI. ’;;;;” H, +

%- C=C-C

R.

MY ( a l 3 1- al,) = 68.9

a,, = 3.5

zap = 104.4

= 32.0

= 0.8

ucts (Table I) can be expressed in terms of al-ab

.XC = a2 + as X C = C= 2al f

a2

X - C = a1 + a4

+

X(;=c-c = a2 + a3 X C - C - c = a3 .xc=c-c--c -- Q X(ic:=c-c-c-c = a5 The sum of the squares of the residuals, with chosen weighting factors, then is S =: (a2 + as - X C ) +~ (2a1 a2 X C = C ) ~1.5N(al a4 .- X C - C )+~ (a2 + a3 - X C = C - C ) ~ 6(a3 - X C - - C - C ) ~ 2(a4 - X C = C - C - C )f~ 8(a5 Xc=c-c-c-c)2. Setting SSlSa, = 0 and inserting the experimental X i , values gives the set of linear equations 5.5a1 -t 2nz + 1 . 5 ~ 4 = 169.55

+

+

+

+

2r:zl 4-3az -!- a3

-0.2 1.5121 a2

-b l a 3

+ a5

gen before cleavage. If one then assumes that the amount of 3-hexyl radical formed is “normal” for secondary C-H bonds and equates the amount of 2-hexyl radical formed by abstraction to this value and the excess to rearrangement, then the amount of 1-hexyl radical formed by (54.9 - 38.6) = 22.6 and k s - H / k p - H = abstraction is 6.3 2.6, well in line with expectation; this value represents the composite behavior of an R.” composition of 51% methyl radicals, 38% ethyl radicals, and 11% rz-propyl radicals. The postulation then is that 28% of the 1-hexyl radicals formed cleave to ethylene and 1-butyl radical while 72% rearrange to 2-hexyl radical. After statistical correction, the relative reactivities of the various hydrogens in 3MP toward abstraction can be exmessed as

+

3.3

33

I

= 162.5

c

= 119.9

0.7

+ 3 . 5 ~ 4 = 119.15 + 9a5 = 108.7

The solution. is shown in Scheme I under the heading thermal. The other four substrates were treated similarly to give the parameters shown in Schemes 11-V. Because of certain redundancies in the observation equations, certain a4 values could only be determined as parts of sums. Calculated product compositions based on these parameters are compared wZth those observed in Table I. The value!s of al-n5 indicate that 1-, 2 - , and 3-hexyl radicals are formed by hydrogen abstraction from NH in the ratio of 6.3:54.9:38.6 for an averaged k s - H / k p - H = 11.1 after statistical correction for numbers of hydrogens. This value seenis much too high compared with the value of ea. 3 estimated by Ri.ce.7b The apparent anomaly disappears, however, if one postulates a partial conversion of 1- to %hexyl. radicals by the well-documented 1,5 hydro-

H

representative of an R.” composition of 58% methyl radicals and 42% ethyl radicals. For 23DMB, k t - H / k p - H = 11.8 representative of an R.” composition of 13% hydrogen atoms, 67% methyl radicals, and 20% isopropyl radicals. For 22DMB, the calculated pattern is 0.25

C

I 1.7 c-c-c-c

0.25

1.0

I

C 0.25

involving 30% hydrogen atoms, 46% methyl radicals, 19% ethyl radicals, and 5% tert-butyl radicals. The fourfold difference in reactivity between the two kinds of primary H seems surprising although the role of steric effects on radical abstraction reactions at high temperatures is not well studied. The inability to break up the sums (azo a23) and (a21 f 2z5) in anything but an arbitrary manner does not allow an assignment of k t - H / k s - H / k p - H for 2MP. Taking all the values together, one concludes that, while the inequality k t - H > k s - H > k p - H is generally valid, as of course expected, the simplification of assigning a numerical value to each class of C-H bonds independent of, the

+

(13) C. Walling in “Molecular Rearrangements,” Vol. 1, P. deMayo, Ed., Interscience, New Y o r k , N. Y . , 1963, Chapter 7. The Journal of Physical Chemistry, Vol. 77, No. 2, 1973

I63

Thermal Cracking of the Isomeric Hexanes SCHEME IV: Decomposilion Network for 2,2-Dimethylbutane

C

C -+

C=C

+

C-C.

I 3 CI

C=C

+

/C

Ha

Hz

+

R.

\c C

&

+

C-h

RH

I

Re

C

c=c /c + c-c.

C

I C-C-C-.-C I

..

c=c

C

I

RH

/c-c

+ c. 4RH c

uI6= 5.1 a,? = 19.1

uI8 = 5.6 U I 9 = 40.9 Z a , = 101.0

local molecular environment of the C-H bond and independent of the identity of the attacking radical is at best a gross one. The ratio a4/ae = 4.4 expresses the relative tendencies of 3-hexyl radical t o cleave to form 1-butene and ethyl radical compared with forming 3 -pentene and methyl radical; not surprisingly, the "more stable" ethyl radical is lost preferentially. Based on reported values1* for Aff1,298, s298, Cp.30o, and CP,s00 for 1-butene, 1-pentene, ethyl radical, and methyl radical, the calculated equilibrium constant between these pairs of products at 500" is K = 16. "I"c.

& C=C-C-C

4-

R.

4- R.

KY

Thermal

a15= 30.3

c-C

C'

C

C=C-C--C-C

+

C'

+ c-c.

Two other competitive B scissions involve a similar situation, i.e., loss of a primary ethyl us. methyl radical with both olefinic products having the same substitution pattern at the double bond. For 3-methyl-2-pentyl radical, formation of 2-butene plus ethyl radical is preferred over 2-pentene plus, methyl radical by a ratio of a8/a9 = 2.5, while for 5!,2-dimethyl-l-butyl radical, formation of 2methylpropene plus ethyl radical is favored over 2methyl-1-butene plus methyl radical by a ratio of a17/a18 = 3.4. In the case of 2-methyl-3-pentyl radical, methyl radical is lost in each p scission pathway but the more stable olefin 2.pentene is preferred over 3-methyi-1-butene by a ratio of 1 ~ 2 6 / a 2 ? -- 1.9; the equilibrium constant between these olefins at 500" calculated from thermochemical properties14 is 2.4. Finally, for 2,3-dimethyl-l-butyl radical, the competition is between a secondary isopropyl radical and methyl radical. and the competition (all + a12)/ a13: = 42 is as expected strongly in favor of the considerably "more stable" former radical. In summary, the kinetic cornpetition between fi scissions follows thermodynamic control in each case.

a I 5= 17.2

(al8 -t u19) = 41.1 Z u , = 101.5

U t @=

21.0 = 22.2

Finally, the derived a4 values can be used to evaluate

(k~[RHl/ks)R." values for various radicals; [RH] is effectively constant (PRH= 1 atm) subject to the consideration of differences owing to different distributions and reactivities of tertiary, secondary, and primary hydrogens. Consider first the series of linear primary radicals. For 1-hexyl and 1-butyl radicals, (k~[IiH]/.ll~)" is clearly