ZeoIite Cata Iysts Hydrocracking and Hydroisomerization of n-Dodecane Hans
F. Schulzl and Jens H. Weitkamp
Carl Engler und Hans-Bunte-Institut f u r Lliineralol-und Kohleforschung und Institut f u r Gastechnik, Feuerungstechnik und Wasserchemie der Universitat, Karlsruhe, West Germany
With a Pt/Ca/Y-zeolite, a C12-isomerization conversion maximum of 48% is obtained, and up to a 100% cracking conversion of the C12 pure primary cracking i s achieved. Pd/Mn-H/Y-zeolite and Pd/H/Y-zeolite catalysts give poorer isomerization and show secondary cracking even at low cracking conversion level. Relative rates of individual primary and secondary cracking reactions ate given and related to the Clz-isomerizate composition showing /3-scission proceeding from tertiary carbonium ions. 2-Methylisomers are primary cracked products. Paraffin isomers with quaternary C-atoms could be produced by n-dodecane hydrocracking. The total observed branching of the cracked products i s correlated to contributions of primary and secondary isomerization.
T h e use of zeolites for heterogeneous catalysis, in particular for mineral oil processing through reactions involving carbonium ions, in catalytic cracking, hydrocracking, and hydroisomerization is of increasing importance. Acidity and hydrogenation activity of bifunctional zeolite catalysts may easily be varied by ion exchange. Active components for hydrogenation, normally noble metals such as platinum or palladium, are introduced to the pores of zeolites as ammonia complexes and reduced to their elemental forms with hydrogen at ca. 500°C. Thus, the metal is finely distributed, probably mainly atomically in the pores of the zeolite lattice (10, 16). The composition of the reaction products from hydrocracking is principally dependent on reaction temperature and the relationship of hydrogenation-dehydrogenation activity to the acidity responsible for the cracking activity of the bifunctional catalyst (1, 6, 8, 9, 11). The principal and widely accepted reaction scheme of hydrocracking (6, 8, 12) starts with the formation of olefins from paraffins a t metallic centers and the formation of carbonium ions from these olefins a t acidic centers. The ions may undergo rearrangement and splitting according to certain rules of carbonium ion behavior. Olefins resulting from carbonium ions are in turn saturated to give a mostly paraffinic product. This general reaction path of hydrocracking is by far simpler than that of catalytic cracking, which involves additional polymerization, alkylation, cyclization and aromatization, hydrogen transfer, and coke formation that rapidly deteriorate catalyst activity. Thus, hydrocracking is more accessible than catalytic cracking to kinetic investigations. Results of hydroisomerization on zeolite noble metal catalysts have been publiqhed (3, 14, 16, b l ) , however, only 'To whom correspondence should be addressed. 46 Ind. Eng.
Chem. Prod. Res. Develop., Vol. 1 1 , No. 1 , 1972
by use of low molecular paraffins u p to C7 as feed materials which may easily be converted by isomerization associated only with minor cracking. The products are analyzed without great efforts. Hydrocracking of higher molecular paraffinic hydrocarbons on highly active platinum on an amorphous AlzO3- SiOz- catalyst has been reported by Coonradt and Garwood (6) who pointed out the possibility of pure primary cracking and the high degree of isomerization of the uncracked feed. However, no detailed analyses of cracked and isomerized products were given. Literature on hydrocracking of pure hydrocarbons with zeolite catalysts is limited (2,4,21). Experimental
For the present investigation the following three catalysts were used: 0.5% Pt/Ca/Y-zeolite (SK 200), 0.5% Pd/MnH/Y-zeolite (SK 110), 0.5% Pd/H/Y-zeolite (SK loo), all supplied from Union Carbide International Corp., Linde Division. The reaction conditions were molar ratio Hl/ndodecane = 20, LHSV = 1.0 h-l, and pressure = 40 atm. The reaction temperature was varied in the range from 250400°C according to catalyst activity and required degree of conversion. The high analytical sensitivity permitted quantitative results even a t low conversion levels. Purity of ndodecane feed material was controlled by gas chromatography. The flow type fixed bed reactor was designed for quantitative experiments a t small throughputs of 1-5 grams of hydrocarbon feed per hour (Figure 1). The problem of continuously feeding a t a constant rate-e.g., only 1 gram n-dodecane per hour to the reactor-was solved with a special saturator. The n-dodecane in an amount of 50 grams was impregnated on inactivated kieselguhr (100/100 g/g). This material was held at a n appropriate temperature in t h e saturator where the stream of hydrogen passed the packing. The equilibrium vapor pressure was attained even in about 1 mm of bed
height. Thus, the hydrocarbon concentration in the feed gas is independent of flow, and almost all of a saturator filling can be used without changes in feed rate. For long duration tests a second saturator in a parallel feed line is used. For quantitative recovery of the hydrocarbons from the product stream, two traps a t liquid air temperature were used, the second filled with activated carbon for quantitative adsorption of methane. Condensate and adsorbate were separated in fractions of liquid and gaseous hydrocarbons whose quantities were accurately determined a t room temperature. T h e gaseous hydrocarbons with compounds u p t o CS were analyzed with a reoplex modified Ak1203gas chromatographic column b y applying a temperature program u p to 190OC. The liquid hydrocarbons were separated with 100-mm film capillaries containing polypropylene glycol and squalane as liquid phases. Temperature programmed capillary gas chromatography of complex hydrocarbon mixtures Ca-Cle was reported in earlier publications (18, 19). Quantitat'ive values for individual hydrocarbons could be derived from the chromatogram for almost all compounds produced by cracking and in the (212-isomerizate fraction for methylundecanes and ethyldecanes. Multiple branched Clzhydrocarbons were calculated cumulatively. The electronically int'egrated values of peak areas from the different chromatograms and the values of fraction quantities were fed to a digital computer for total evaluation of each experiment. Results and Discussion
Cracking a n d Isomerization Conversion. At a sufficient high hydrogenat,ion activity of the catalyst and corresponding high hydrogenation a n d dehydrogenation rat'es, a ready exchange of chemisorbed CI2-carbonium ions with Clz-olefins from t,he gas phase takes place. P a r t of the CI:-carbonium ions desorbs from the catalyst surface after rearrangement (but before C-C cleavage) to give branched olefins which, in turn, are hydrogenated t o branched Clzparaffins. At low hydrogenation activity of the catalyst, in general, only reactions in the sequence Clz-paraffin C12olefin Clz-carbonium ion cracked products < Clz are possible, and only low concentrations of branched feed hydrocarbons can be detected in the reaction product. Figure 2 shows the cracking and isomerization conversion
-
-+
-+
0
C EACTOR
N -DODECANESATURATOR
''
G
H
Figure 1 . Scheme of small scale reaction apparatus for hydrocracking (A) Active carbon plus molecular sieve tube for Hz-purification; (B) molten salt bath for saturator temperature control; (C) n-dodecane on kieselguhr; (D) heated connection tube; (E) preheating zone; (F) 2 ml of catalyst, 0.20.3 mm; ( G ) liquid air cooling trap; (H) activated carbon trop at liquid air temperature
as a function of temperature for different hydrocracking catalysts. The additional curves for Co-Uo-&0s-SiO2, sulfided CO-ILIO-A~~O~-S~OZ, and Co-hIo-Al2O~ were obtained with n-hexadecane under similar conditions (23, 20). The various catalysts are active in different regions of reaction temperature-e.g., Pt/Ca/Y-zeolite below 300°C and CO-?\.IO-AIZOSat about 450OC. But the principal shape of the curves is similar in most cases where there is a sufficient hydrogenation activity related to cracking activity. The conversion b y isomerization increases with temperature and then goes through a maximum when increasingly isomerized Clz-molecules are cracked. The different behavior of the sulfided Co-Mo-AlzO~SiO& catalyst with its relatively low hydrogenation activity as related to its high acidity shows the transition in reactions to normal catalytic cracking with a low maximum of feed hydrocarbon isomerization conversion of only ca. 5%. On Pt/Ca/Y-zeolite the maximum of isomerization conversion of 48% is found a t a cracking conversion of only 17% a t
P t / C O / Y -ZEOLITE A Pd/Mn-H/Y-ZEOLITE
Figure 2. Hydrocracking and hydroisomerization conversion Dependence on reaction temperature for different bifunctional catalysts
Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 1 , 1 9 7 2
47
Table I. Molar Distribution of Cracked Products with Pt/Ca/-, Pd/Mn-H/-, Catalysts at Different Reaction Temperatures
and Pd/H/Y-Zeolite
Moles of cracked products per 100 moles of C12 cracked Catalyst Temp, OC
. ~ ~ _ PdlMn-H/Y-zeolite _
Pt/Ca/Y-zeolite
265
285
275
300
350
250
Conversion, yo Cracking 56 5 5 17 99.5 100 Clz-isomerization 34 0.47 . . . 5 35 48 Methane ... . . . . . . 0.1 0.5 ... Ethane .. ... , . . 0.1 0.6 0.1 Propane 6 . 7 6 . 7 7 . 0 9 . 0 48.5 11.7 13utaiies 29.7 30.4 31.8 38.9 101.2 48.0 Pentanes 42 3 41.9 41.9 46.3 66.4 48.9 Hexanes 43.5 43.5 42.9 44.2 43.3 43.8 Heptanes 42.3 41.2 41.0 40.3 8 . 1 36.7 Octanes ... 23.0 29.5 30.7 30.6 25.9 Konanes 2.7 6.3 6.0 5.9 3.4 . . . . . . . . . ... ... ... ... Decanes . . . . . . . . . . . . . ... Undecanes Total moles 200 200 201 208 269 215 a Inrluding 0.6 mole aromatics. * Including 0.8 mole aromatics.
CL2--
-
+
-
+
1 ' 2-*
c.4
+ Cs
cs + c7
e4 + cs C6 + c7 C6 + c 6 c 3
+ C4 + cs
29.6 42.3
30.5 41.5
31.2 41 4 21.4 ...
31.2 41.4
18.6 33.7 22.1 16.6
30.4 41.7
15.3 32.3 22.2 20.8
275
300
12 18
50 31
. . . . . .
0.2 14.8 52.6 50.6 44.0 34.7 20.8 2.1
0.2 17.5 55.0 51.6 44.2 33.7 18.6 2.1
. . . . . . . . . . . .
220
223
48
Ind. Eng. Chem. Prod. Res. Develop., Vol. 11, No. 1 , 1972
Pd/H/Y-zeolite
350
100
100
...
...
0.1 0.3 19.6 57.3 53.2 44.2 32.9 16.9 1.6 ... ...
226
400
00 ... 2.9 4.6 76.0 05.8 64.6 35.1 0.4
0.4 1.1 42.9 89.9 62.0 45.6 16.2 1.4 . . . 0.1 . . . ... ... ... ... 260 289
275
300
325
350
16 30 49 63 2.9 3.0 3.0 2.2 ... ... ... 0.1 0 . 2 0.2 0 . 2 0 . 4 18.2 20.9 22.1 27.3 55.6 56.8 57.8 63.8 52.5 52.2 53.9 5 5 . 1 43.7 43.1 44.3 44.4 34.0 33.4 3 2 . 3 28.1 17.2 16.9 1 5 . 3 12.6. 2 . 5 2 . 3 2 . 1 2.5* ... ... ... ... ... ... 224 226 228 234
molar distributions of t h e cracked products on t h e basis of 100 moles of n-dodecane cracked. O n Pt/Ca/Y-zeolite t h e sum of moles of cracked products is approximately equal t o 200 over almost t h e total region of cracking conversion from 0- loo%, showing pure primary cracking. There is a sufficient supply of C12-olefins by fast dehydrogenation for immediate displacement of lower carbonium ions from the acidic sites to prevent their secondary cracking. The distribution of cracked products is symmetrical to C g (half the C12 feed molecule) with approximately equal values are about 2/3 of the for Cg, (26, and (27. Values for CS and former, and values for Cs and CBare relatively small. C1, Cz and Cln,CI1products are materially absent. With Pd/Mn-H/Y-zeolite, even a t a low conversion level secondary cracking occurs and ranges from 15-26% with increasing primary cracking conversion from 0 to ea. 100%. The molar distribution of cracked products is correspondingly asymmetrical with higher values for Cs,C4, and C3 than for C7, CS,and CQ,respectively. At higher temperatures than those necessary for 100% of cracking conversion, the sum of moles increases rapidly, and a sharp maximum a t Cq is obtained. Probability of Cracking Reactions. From the molar distribution of cracked products, the relative probabilities of distinct cracking reactions can be derived. This is particularly true for t h e exclusive n-dodecane primary cracking on the Pt/Ca/Y-zeolite catalyst. I n Table I1 this selectivity of dodecane cracking to CB CS, C4 CS, C6 C7, and C6 C6is reported. The low values of cleavage to CB CScould be related to energetic effects since the C3 fragment cannot form a tertiary ion. But a remarkably low value of ea. 30Y0 for the reaction to C4 CScannot easily be explained because both fragments can form energetically favored tertiary carbonium ions. Isobutane is a common product of ionic hydrocarbon cracking. A slight increase in the probability of this reaction with temperature is observed. This selectivity will be discussed later in coiinectioii with isomerization reactions which inseparably accompany ioiiic cracking. The probabilities of overall cracking reactions on Pd/MnH/Y-zeolite and Pd/H/Y-zeolite can also be calculated from
+
275OC. Complete cracking of the Clz-feed is attained already a t 300°C. The conversion of n-Clz b y isomerization a t low temperatures (250-265°C) on Pt/Ca/Y-zeolite is 7-10 times greater than that by cracking, showing a much greater rate for carbonium ion rearrangement than for carbonium ion cleavage. With Pd/Mn-H/Y-zeolite the lower hydrogenation-dehydrogenation activity of the catalyst is limiting for isomerization conversion, its maximum being only 30%. At low temperatures conversion b y isomerization is slightly higher than that of cracking. Distribution of Cracked Products. Table I gives the
325
+
+
+
+
+
the molar distributions of the cracked products and are listed in Table I11 along with those for primary cracking on Pt/Ca/Y-zeolite for a 50% cracking conversion level. The different values are obtained as follows: Reaction to Cg C3 as moles of Cg, to Cs Cq as moles of CS, to C? C5 as moles of C7, to Cg cg as half the moles of C6, to C3 C4 Cg as the mean value of the differences of moles C3 C9 and C5 - C7, and to C4 Cq C4 as l / 3 of the difference between (C4 - Cs) and (C3 - Cg)/2 (C5 - C7)/2. For extensive secondary cracking at higher temperatures, the negligence of t'he reaction c12to e6 C3 c3 is not tenable. I n t'he case of secondary cracking, the reaction to C3 Cq C s is obviously favored to C4 C4 -tC4. h'ote the minor secondary cracking of (26 as is seeii from almost the same values for C6 in all three distributions. Thorough examination of all experimental data showed that, as a good approximation, averaging the values of primary cracking observed with Pt/Ca/Y-zeolite (Table 11) yields the probabilities of primary cracking reactions on Pd/hln-H/Y-zeolite and Pd/H/Y-zeolite. On this basis the probabilities of the single secondary cracking reactions can be calculated and are given for the Pd/Mn-H/Y-zeolite a t 5070 conversion in Table IV. The contributious of the primary cracking products as sources for secondary cracking follow the order C,S (11.8) > C I (8.0) > cs (4.3) > C6 ( CS (39%) > C7 (19%) > c6 (
