Farag, H.

Weitkamp, J.

English

Weitkamp J.

Farag H.

University of Szeged

< I S O M E R I Z A T I O N O F T H E M E T H Y L N O N A N E S A N D 2 - M E T H Y L O C T A N E O N A B I F U N C T I O N A L Z E O L I T E C A T A L Y S T By J. WEITKAMP and H. FARAG Engler-Bunte-Institute, Division of Gas, Oil, and Coal, University of Karlsruhe, D-7500 Karslruhe, West-Germany 7Received 1th February, 1978) 2-Methyl-, 3-methyl-, 4-methyl-, 5-methylnonane, and 2-methyloctane were hydroisomerized on a Pt/Ca-Y-zeolite catalyst at a total pressure of 4.1 MPa and temperatures ranging from 210 to 230 °C. No hydrocracking occured under these conditions. The product distributions were deter-mined by temperature programmed capillary GLC using different stationary phases. The mechanism is discussed in terms of nonbranching (type A) and branching (type B) carbenium ion rearrangements. Unknown selectivities in the rearrangement behaviour of long chain alkyl carbenium ions are revea-led. Introduction Catalytic isomerization of alkanes is of considerable importance in petroleum refining. Commercial objectives are improvement of octane rating of light gasoline components or the production of wobutane. Another potential application is pour point lowering of heavier petroleum fractions, e.g., of jet fuel or diesel fuel [1, 2]. Whereas there is a wast number of publications dealing with product distribu-tions, kinetics and possible mechanisms of isomerization of light hydrocarbons containing up to six or seven carbon atoms, hardly any information is available con-cerning the isomerization of long chain alkanes. Such informations are desirable both for a better understanding of potential processes for pour point lowering and for a more detailed insight into the mechanisms of hydrocracking and catalytic crack-ing where cleavage is preceded by skeletal rearrangement. The existing' gap in our knowledge on isomerization of long chain alkanes is due to both the difficulties en-countered in product analysis and the fact that cracking reactions often interfere if the feed hydrocarbon contains more than six carbon atoms. Recently it has been shown [3] that long chain alkanes may be hydroisomerized in high yields if special Afunctional catalysts, e.g., of the noble metal/zeolite type are used. With the «-alkanes of 13 to 16 carbon atoms remarkable selectivities were encountered which are not observable with light «-alkanes [4j. The present paper reports on the hydroisomerization of long chain alkanes containing one methyl branching. 3 2 8 J. WEITKAMP AND H. FARAG Experimental Principally, the apparatus consisted of a high pressure saturator for loading the hydrogen vyith hydrocarbon vapor, and a downflow fixed bed reactor. Details are given elsewhere [5]. 0.45 g of the Union Carbide SK 200 catalyst, i.e., 0.5% Pt/Ca-Y-zeolite were used. Drying and activation of the catalyst were carried out in situ by purging with nitrogen and hydrogen, respectively, at 0.1 MPa and a maximum temperature of 400 °C. Purities of the feed hydrocarbons as determined by capillary GLC were as follows: 2-methylnonane 99.7%, 3-methylnonane 99.95%, 4-methylnonane 97.4%, 5-methylnonane 99.95 %, and 2-methyloctane 98.2 %. Product distributions were correc-ted under the assumption that the feed impurities underwent no reaction. All experi-ments were conducted at a hydrogen partial pressure of 4.0 MPa and a hydrocarbon partial pressure of 0.1 MPa. The required high resolution analyses of the products could be achieved by capillary GLC with squalane and polypropylene glycol as stationary phases. Length and internal diameter of the capillaries were 100 m and 0.25 mm, respectively. Starting at ambient temperature the column oven was heated with 1 °C/min. A flame ionization detector and an electronic integrator were used. As a whole the resolution of the peaks either on squalane or polypropylene glycol was good. The only major analytical problem was the separation of 3-ethyloctane from 2-methylnonane when the latter was the feed. For assignment of the peaks the experience from earlier work on hydrocracking [3, 6] was available. In addition to this, use was made of retention index values reported in the literature [7, 8]. 4-Propylheptane was purchased as a reference substance from Roth, Karslruhe and its structure ascertained by mass spectrometry. Its retention index reported in [8] for Apiezon L was found to be approximately valid for polypropylene glycol while that given in [7] did not agree with the experimental value. Hydrocarbons with a carbon number exceeding that of the feed were never observed. Undesired hydrocracking reactions were absent or negligible under the conditions applied. The highest degree of cracking conversion was 0.3%. In this case the distribution of the cracked products showed all features of an ionic hydrocrack-ing rather than those of hydrogenolysis over platinum [3]. Results 50 100 W / F [g CAIAL'rS: n I moll I5i Fig. I. Hydroisomerization of the methylnonanes on Pt/Ca-Y-zeolite (7=210 °C) ISOMERIZATION OF THE METHYLNONANES 329 Table I Distributions of the Products (mole- %) Formed from the Methylnonanes at Low Degrees of Conversion X (T= 210 °CJ Feed 2-ME-Nonane 3-ME-Nonane 4-ME-Nonane 5-ME-Nonane r g catalyst-h 1 41 53 74 51 ' L mol feed J Xl%] 2.4 3.4 10.7 4,9 2-ME-Nonane 27.3 0.8 0.4 3-ME-Nonane ' 77.5 — 27.4 9.3 4-ME-Nonane 6.9 36,6 — 52.7 5-ME-Nonane 0.9 5.1 31.8 — 3-ET-Octane (1) 13.7 14.5 6.2 4-ET-Octane 0.5 8.4 20.5 28.0 4-PR-Heptane 0 0.1 1.6 2.4 n-Decane + DM-Octanes 14.2 8.8 3.4 1.0 100.0 100.0 100.0 100.0 (1) Not determined Table 11 Distributions of the Products (mole- %) Formed from the Methylnonanes at Medium Degrees of Conversion X (T= 230 °C) Feed Xl%] 2-ME-Nonane 127 31 3-ME-Nonane 168 39 4-Me-Nonane 95 38 5-ME-Nonane 91 37 2-ME-Nonane 26.2 v 2.9 1.6 3-ME-Nonane 55.0 — 30.9 13.1 4-ME-Nonane 14.0 34.6 — 46.7 5-ME-Nonane 4.5 9.1 26.8 — 3-ET-Octane (1) 10.0 12.8 7.3 4-ET-Octane 4.1 9.2 17.8 23.0 ' 4-PR-Heptane 0.1 0.6 1.8 2.1 n-Decane+DM-Octanes 22.3. 10.3 7.0 6.2 100.0 100.0 100.0 100.0 «-Decane 9 13 15 18 2,3-DM-Octane 13 6 15 13 2,4-DM-Octane 17 10 12 15 2,5-DM-Octane + 3,5-DM-Octane 24 26 28 24 2,6-DM-Octane 14 8' 3 3 2,7-DM-Octane .9 6 4 3 3,4-DM-Octanes (2) 3 7 5 .7 3,6-DM-Octanes (2> 4 13 5 5 4,5-DM-Octanes (2) 1 . 2 5 3 Others <3> 6 9 8 9 -100 100 100 100 <!) Not determined. (2) Both diastereomers. (3) Essentially 4 unidentified peaks; possibly including 2,2-DM- and 3,3-DM-octane. 3 3 0 J. WEITKAMP AND H. FARAG In Fig. 1 the degree of conversion in hydroisomerization of the methylnonanes at 210 °C is plotted versus the time factor W/F. The actual degree of conversion for 2-methylnonane is somewhat higher than that represented by the dashed line since 3-ethyloctane formed could not be determined. Approximate rates of reaction at 210°C for 2-ME-, 3-ME-, 4-ME-, and 5-ME-nonane are 0.50, 0.65, 1.0, and 1.5 mmole feed/g catalyst • h. The distributions of the products formed from the methylnonanes in the runs with the lowest degrees of conversion are represented in Table I. Product distributions obtained from the same feed hydrocarbons at higher degrees of conversion ranging from 30 to 40% are listed in Table II. In this case the distribution of the dimethylocta-nes is also given. The distribution of the isomers obtained in a typical experiment with 2-methyloctane is shown in Table III. Table III Distribution of the Products (mole- %) Formed from 2-Methyloctane (T= 220 °C; fV/F=110 g-h/mol; X=9.4%) 3-ME-Octane 72.4 n-Nonane 8 4-ME-Octane 10.5 2,2-DM-Heptane . 5 3-ET-Octane 2.3 2,3-DM-Heptane 20 4-ET-Octane -i- 3,4-DM-Heptanes«1'. 2.3 2,4-DM-Heptane 23 n-Nonane + DM-Deptanes 12.5 2,5-DM-Heptana 28 inn n 2,6-DM-Heptane 12 J u u . u 3,3-DM-Heptane 2 3,5-DM-Heptanes(1) 2 4,4-DM-Heptane 0.4 100 (1) Both diastereomers. Discussion It is widely accepted that the isomerization of alkanes over Afunctional catalysts proceeds via carbenium ions. A possible reaction sequence is: metal acid Alkane I -Alkene I . Carbenium ion I tl Alkane II • Alkene II Carbenium ion II. This scheme implies that a single product may be formed as, e.g., in n-pentane//--pentane isomerization. If, however," the reaction starts from a long chain alkane there are several of many possible products. The measurable distribution of the product alkanes then reveals the selectivities of carbenium ion rearrangement pro-vided that the latter is the rate controlling step of the overall reaction. This requires that carbenium ion generation and desorption from the acidic sites are relatively rapid steps. Earlier work on hydroisomerization and hydrocracking led to the conlu-sion that this is the case on the SK 200 catalyst [3—6]. Starting from branched carbenium ions two types of rearrangement reactions may be discerned [9]. In the so-called type A rearrangements the degree of chain ISOMERIZATION OF THE METHYLNONANES 331 branching in the carbon skeleton is not changed whereas in type B rearrangements a ramification is created or vanished. Type A rearrangements are considered to proceed via a classical sequence of hydride and alkyl shifts. Type B rearrangements of light alkanes were shown to be best interpreted in terms of protonated cyclopro-pane intermediates [10, 11]. 'X s/v^v/x i î 'X VN/ 'v/X X/X/V^s/ ! î ' 1 Î 1 j i 1 11 . / S / X x / x / x ^ ® \ y X/X/^/ V n / n / n / \y 'N /N /V x / x / \ A / Y . v x Y // y ^ y . Fig. 2. Reaction séquences in type A rearrangements of the tertiary carbenium ion derived from 4-methylno-nane. In Fig. 2 the type A mechanism is applied, as an example, to the tertiary carbe-nium ion derived from 4-methylnonane. Applying the same procedure to all positional isomers results in the following prediction of primary products (relative statistical probabilities other than unity are given in brackets): 2-ME-nonane — 3-ME-nonane (2X) 3-ME-nonane — 2-ME-nonane, 3-ET-octane, 4-ME-nonane 4-ME-nonane — 3-ME-nonane, 3-ET-octane, 5-ME-nonane, 4-ET-octane 5-ME-nonane - 4-ME-nonane (2X), 4-ET-octane (2X) The actual product distributions at low degrees of conversion (Table I) essentially confirm the assumed mechanism. In particular, the methyl shift along the main chain is clearly found to be a stepwise reaction and the ethyloctanes are formed as predicted. As expected 4-propylheptane occurs as a secondary product of 4-ethyloctane. A more subtle consideration, however, reveals two systematic deviations from the simple statistic prediction of primary products: A methyl shift is always faster than a compet-ing shift of a bulkier alkyl group. As a consequence, e.g., the rate of formation of 4-methylnonane from 5-methylnonane is ca. twice that of 4-ethyloctane. Furthermore, competing methyl shifts do not proceed at equal rates. The preferred methyl shift is the one towards the center of the main chain. E.g., 3-methylnonane yields 4-methyl-nonane with a higher rate than 2-methylnonane. At higher degrees of conversion the selectivities are still visible though less pronounced due to secondary reactions, cf. Table II. At Z=38% 4-methylnonane yields somewhat more 3-methylnonane than 5-methylnonane. This is due to the higher thermodynamic equilibrium concentration of 3-methylnonane. The behaviour of 3 3 2 J. WEITKAMP AND H. FARAG 2-methyloctane (Table III) in type A rearrangement is quite analogous to 2-methyl-nonane. Possible type B rearrangements of carbenium ions derived from 2-methyloctane represented in Fig. 3. For the sake of brevity the protonated cyclopropane intermedia-tes are written in a face-protonated form although corner- or edge-protonated entities appear to be more likely [10, 12]. If the branching is assumed to start from the most stable ion, i.e., the tertiary 2-methyl-(2)octyl-cation then 2,2-DM- and 2,3-DM-hep-tane are expected as the only primary products. (II) I- Hw ( I ) (III) I H ® /Ny^sZi/ ( I V ) ( V ) 2 .2-DM - H E P Ï A N E 2 . 3 - D M - H E P T A N E 2 . 3 - D M - H E P T A N E 2 . 4 - D M - H E P T A N E 2 . 2 - D M - H E P T A N E 2 . 3 - D M - H E P T A N E - 2 , 4 - D M - H E P T A N E 2 . 5 - D M - H E P T A N E 2 . 3 - D M - H E P T A N E 2 . 4 - O M - H E P T A N E 2 . 5 - D M - H E P T A N E •2,6-DM - H E P T A N E 2 . 4 - D M - H E P T A N E 2 . 5 - D M - H E P T A N E 2 . 5 - D M - H E P T A N E 2 . 6 - D M - H E P T A N E Fig. 3. Possible pathways for type B rearrangement of carbenium ions derived from 2-methyloctane Table III unambiguously shows that such a mechanism is not consistent with the experimental result. If, on the other hand, branching is assumed-to start from the mixture of secondary carbenium ions and to be governed by mere statistics, then the expected distribution is: n-nonane: 2.2-DM-heptane: 2.3-DM-heptane : 2.4-DM-heptane : 2.5-DM-heptane : 2.6-DM-heptane : 100/15« 6.5% 100/15« 6.5% 300/15=20% 400/15 «26.5% 400/15 «26.5% 200/15« 13% which is in rather good agreement with the actual distribution shown in Table III. In particular, the relatively low rates of formation of /i-nonane, 2,2-DM-heptane, and 2,6-DM-heptane are correctly predicted. Application of the same formalism to the methylnonanes is less straightforward. One reason for this is the incomplete analysis of individual dimethyloctanes. It is ISOMERIZATION OF THE METHYLNONANES 333 interesting to note, however, that 2-methylnonane is the most reactive isomer for type B rearrangement while it is the least reactive for type A rearrangement, cf. Tables I and II. Additional systematic investigations are required to arrive at a full understanding of the selectivities encountered in ionic rearrangement of long chain alkanes containing one methyl branching. Conclusions Hydroisomerization of higher alkanes on properly balanced bifunctional cata-lysts is a powerful tool for studying the details of rearrangement of long chain alkyl carbenium ions. Both the non-branching and the branching rearrangements exhibit distinct features which are not observable with C6 or lower molecules. In a sense the latter are special cases which do not contain enough carbon atoms to reveal the detailed information on the most favorable rearrangement routes. A cknowledgemen t The authors thank PROF. D R . K. HEDDEN, Head of the Division of Gas, Oil, and Coal of the Engler-Bunte-Institute, who generously supported this work. H. FARAG acknowledges a post-doctoral scholarship granted by International Seminar for Research and Education in Chemical Engineering and Physical Chemistry at the University of Karlsruhe. Abbreviations DM=dimethyl-, ET=ethyl-, ME = methyl-, PR=propyl-. References [1] Orkin, B. A.: Ind. Eng. Chem. Prod. Res. Develop. 8, 154 (1969). [2] Gibson, J. W„ G. M. Good, G. Holzmann: Ind. Eng. Chem. 52, 113 (1960). [3] Weitkamp, J.: Hydrocracking and Hydrotreating, Am. Chem. Soc. Symp. Ser., Vol. 20, J. W. Ward, Edit., p. 1, Am. Chem. Soc., Washington, D. C., 1975. [4] Weitkamp, J., H. Farag: Erdöl, Kohle, Erdgas, Petrochem., Complementary Volume, p. 276, Industrieverlag von Hernhaussen, Leinfelden-Echterdingen, 1976. [5] Weitkamp, J., K. Hedden: Chemie-Ing.-Techn. 47, 537 (1975). [6] Weitkamp, J.: Erdöl, Kohle, Erdgas, Petrochem., Complementary Volume, p. 24, Industrie-verlag von Hernhaussen, Leinfelden-Echterdingen, 1977. [7] Matukuma, A.: Gas Chromatography, C. L. A. Harbourn, Edit., p. 55, The Institute of Pet-roleum, London, 1969. _ [8] Rappoport, S., T. Gäumann: Helv. Chirn. Acta 56, 1145 (1973). [9] Poutsma, M. L.: Zeolite Chemistry and Catalysis. J. A. Rabo, Edit., p. 437, Am. Chem. Soc., Washington, D. C., 1976. [10] Brouwer, D. M., H. Hogeveen: Progr. Phys. Org. Chem., A Streitwieser, Jr., R. W. Taft, Edit., Vol. 9, p. 179, Interscience Publ., New York, 1972. [11] Chevalier, F., M. Guisnet, R. Maurel: Proc. 6th Int. Congr. Catal., G. C. Bond, P. B. Wells, F. C. Tompkins, Edit., Vol. 1, p. 478, The Chem. Soc., London, 1977. ' [12] Olah, G. A.: Angew. Chem. 85, 183 (1973). 

Isomerization of the methylnonanes and 2-methyloctane on a bifunctional zeolyte catalyst

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Isomerization of the methylnonanes and 2-methyloctane on a bifunctional zeolyte catalyst

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