Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 4, 1978
331
tracted by pyridine, was only 1.8 w t YO.This suggests that less than one-third of total V and Ni in the feed is porphyrin type. Otherwise, porphyrins would have to undergo extensive changes on the surface resulting in nitrogen removal to account for the low nitrogen content. There is an almost total lack of information on the behavior of vanadium and nickel porphyrins and other metal compounds under conditions of catalytic hydrotreatment. Thus one cannot escape speculation as to their contribution to coke. To clarify this problem, extensive model compound studies are needed. This can be accomplished by blending sulfur-, nitrogen-, and oxygencontaining model compounds with pure metals-containing compounds in a convenient solvent. Here the effect of the metals on sulfur, nitrogen, and oxygen removal, as well as other catalytic reactions, can be followed. These experiments might be extended to no-metals-containing crudes and residuums; i.e., metals-containing crudes could be simulated by doping these feeds with vanadium and nickel compounds. In view of increasing needs for processing high metallic feeds, such information would be of great value. Literature Cited
According to Constantinides and Arich (1964), most V can be removed by precipitation of asphaltenes. This suggests that V content increases relative to Ni in the direction of heavy asphaltene fractions. This is supported by a number of experimental results on deasphalting of crudes and residuums published by Riediger (1976). In his work the V/Ni ratios in the "asphalite" precipitated by normal alkanes were always higher than that in the de-tarred oil. In addition to this, the V/Ni ratios increased when the de-tarring process was conducted on low "asphaltite" yield; i.e., only the heaviest asphaltene fractions were precipitated. In scientific literature, little attention has been paid to Fe. Present results show that the amount of Fe relative to V and Ni is small. During catalytic hydrotreatment, the Fe is most likely converted to sulfides. These are known to be catalytically active. Therefore, the contribution of Fe to poisoning of catalyst surface is less significant. A portion of the metals in crudes is assumed to be porphyrin type. According to Costantinides and Arich (1964). this represents about one-third of the metals present. The molecular weight of the porphyrins is seven to ten times that of the metal's atomic weight. This would indicate that, after the pyridine or quinoline extractions, the amount of the carbonaceous material left on the pellet surface is nearly equal to the amount of porphyrins containing this amount of metals (Table IV). Assuming four nitrogens per one metal atom in porphyrins, the amount of nitrogen in the coke deposit after the pyridine extraction which may originate from the porphyrins is almost 4.7 wt 5%. However, the amount of nitrogen in the deposit, determined after the catalyst pellets were ex-
Beuther, H., Schmidt, B. K., "Proceedings, Sixth World Petroleum Congress", Sect. 111. 297. 1964. Costanthide6 G., Arich, G., "Proceedings, Sixth World Petroleum Congress", Sect. V. D 65. 1964. Khulbe, C. P.: Ruden, B. B., Denis, J. M., Merrill, W. H., "A Pilot Plant Investition of Thermal Hydrocracking of Athabasca Bitumen. 2. Effect of Recycle of Heavy Oil on Product Quallty", CANMET Report 77-20, 1976. Mochida, I., Inone, S., Maedor, K., Takeshita, K., Carbon, 15 (1). 9 (1977). Riediger, B., Int. Chern. Eng., 16 (2),203 (1976). Ternan, M., Packwood, R. H., Parsons, B. I.. "Proceedings, Fitth North American Meeting, Catalysis Society", p 11, 1977.
Received for review February 7, 1978 Accepted June 16, 1978
Reaction Mechanism of Methylcyclopentane Ring Opening over a Bifunctional Pt-AI,O, Catalyst Erhard G. Chrlstoffel' and Karl-H. Robschlager Lehrstuhl fur Technische Chemie, Ruhr-Universitat Bochum, 4630 Bochum 1, West Germany
Ring opening of methylcyclopentane and 1,2-dimethyIcyclopentane over "fresh" and over partially aged Pt-AI,03 catalyst was investigated in a microcatalytic fixed bed reactor by use of the pulse method. The reactions proceed by a platinum catalyzed mechanism over fresh catalyst yielding ring opening products from nonselective ring cleavage and by an acid-catalyzed route over partially aged catalyst yielding n-hexane and n-heptane as main products.
The product distributions from conversions of 1,2-dimethyIcyclopentane over partially aged catalyst are not consistent with those expected from /%cleavage of cyclic carbenium ions and may be explained by a direct ring opening mechanism. At higher reaction temperatures the primarily formed ring opening products undergo secondary reactions such as isomerization and cracking.
Introduction The reactions of alkylcyclopentanes play an important part in catalytic reforming of hydrocarbons. On the one hand, alkylcyclopentanes are compounds of the reforming plant feedstocks, where they are dehydroisomerized into benzene or alkylbenzenes; on the other hand, they are intermediate products in the dehydrocyclization of paraffins yielding aromatics. At reaction conditions usually applied in reforming plants, cracking reactions may take 00 19-789Ol7811217-0331$01 .OO/O
place. These may proceed over the metal component of the catalyst, over the acidic carrier or, by a bifunctional mechanism, over both catalytic active sites. While the mechanism of the hydrogenolysis of C-C bonds in cyclic hydrocarbons over low and high dispersed noble metal catalysts is well understood (Corolleur et al. 1972; Anderson 1973), there is disagreement on the mechanism of bond rupture of C-C bonds in alkylcyclopentanes over acidic metal oxides. Donnis (1976) explains the experi0 1978 American Chemical Society
332 Ind. Eng. Chern. Prod. Res. Dev., Vol. 17, No. 4, 1978
mental result of n-hexane being the main ring opening product of the acid-catalyzed cracking of methylcyclopentane by means of a carbenium ion mechanism, while Brandenberger (1976) postulates a direct ring opening of a protonated methylcyclopentane ring structure, based on the argument that the stability of cyclic carbenium ions is much higher than that of acyclic ones. The aim of the present study was therefore to gain further knowledge of the acid-catalyzed ring opening of C5 rings by investigating the product distributions from the reactions of several alkylcyclopentanes. Experimental Section (a) Materials. Pt-A1203 catalyst (0.35 wt %) was provided from Kalichemie Engelhard. The pure grade reactants methylcyclopentane and methylcyclopentene (Fluka AG) and cis-1,2-dimethylcyclopentane(ICN Pharmaceuticals) were further purified by drying over molecular sieves. The carrier gas hydrogen was dried as well to a content of less than 0.5 ppm of HzO. (b) Apparatus and Operation Conditions. The experiments were carried out in an isothermally operated pulse microreactor of 0.6 cm i.d. By evaluating the moments of reactant peaks at the reactor entrance and exit, suitable operating conditions such as catalyst particle diameter, carrier gas velocity, pulse width, and length of the catalytic bed were selected. For the reported experiments the following reaction conditions were chosen: particle diameter, 0.1-0.3 mm; carrier gas velocity, 100 mL/min (STP); input pulses of 0.1-0.5 p L of reactant; 100 mg of catalyst. The products were analyzed by gas chromatography, with 50-m OV 101 and squalane capillary columns. Constant catalyst activity was checked by repeated conversion measurements at standard conditions. The reactions were performed over a catalyst only being heated up to the reaction temperature chosen, in the following named “fresh” catalyst, and over a catalyst, thermally pretreated a t 530 “C for 12 h and partially deactivated at this temperature by means of the reaction of ten 5-pL pulses of methylcyclopentane. The relative reproducibility of successive measurements over the same catalyst charge and over various charges of thermally pretreated catalyst was 5% of the actual conversion. Over various charges of “fresh” catalyst maximum deviations of 50% were found, while the selectivity of product formation remained nearly unchanged. The reported data represent mean values obtained from at least ten different catalyst charges. Results and Discussion Over highly dispersed platinum a nonselective or statistical cleavage of alkylcyclopentane C-C bonds takes place (Anderson, 1973; Smith et al., 1971). From this the expected product distributions for the conversion of methylcyclopentane should be methylcyclopentane + H2 2 / 5 n-hexane + ‘ I 53-methylpentane + 2/5 2-methylpentane
-
-
and for the conversion of 1,2-dimethylcyclopentane 1,2-dimethylcyclopentane+ H2 ‘I5n-heptane + 2 / 5 3-methylhexane + 2 / 5 2,3-dimethylpentane The formation of n-hexane as the main product of acidcatalyzed ring opening of methylcyclopentane (Smith et al., 1971; Christoffel et al., 1975) may be discussed in terms of carbenium ion mechanism (Donnis, 1976; Whitmore, 1948; Greensfelder et al., 1949) in the following manner.
Pt 1.2-dirnethylcyclopentane
1.2-dirnethylcyclopentene
IH A
,
I
Figure 1. Proposed carbenium ion mechanism for acid-catalyzedring opening.
The possible carbenium ions with ,methylcyclopentane structure are I
I
I
+I
By P-cleavage of these carbenium ions intermediates with 2-methylpentane, 3-methylpentane, and n-hexane structure are formed. With the two assumptions that the rate-determining step of the consecutive reaction is the bond rupture of the carbenium ion and that P-cleavages by which secondary and tertiary are converted into primary carbenium ions are energetically unfavored, only two possible transformations of the above carbenium ions remain by which a secondary ion is formed from a secondary one and a primary ion from an initially primary one, respectively. The resulting carbenium ions are of n-hexane structure. The conversion of 1,2-dimethylcyclopentane via a carbenium ion mechanism yields 3methylhexane and n-heptane as main products, as can be shown by similar considerations (Figure 1). The energetically favored routes for ring cleavage are steps Ia, Ib, IIIa, and IV, by which primary carbenium ions are formed from initially primary ones. In contrast to the carbenium ion mechanism, Brandenberger et al. (1976) postulate a ring opening mechanism by which in the transition state a protonated methylcyclopentane ring structure is formed, which is stabilized by alkyl substitution. In reversion to the direct C5 ring closure this ring structure undergoes direct ring opening yielding n-hexane. That means by both mechanisms the formation of n-hexane as main product of the acid catalyzed ring rupture of methylcyclopentane may be explained. Further information on the operating mechanism can be obtained from the conversion of 1,2dimethylcyclopentane. A P-cleavage of 1,2-dimethylcyclopentane yields 3-methylhexane and n-heptane, whereas a direct ring opening mainly yields n-heptane. Ring Opening over F r e s h Catalyst Table I shows the product distributions of the conversion of methylcyclopentane, methylcyclopentene, and 1,2-dimethylcyclopentaneover “fresh” catalyst. The mole ratios of the primary ring opening product obtained from the conversions at 400 “C (Table I) are n-hexane:2methylpentane:3-methylpentane = 1.9:2:1 and n-heptane:3-methylhexane:2,3-dimethylpentane = 1:1.6:1.4. They are consistent with product distributions anticipated from unselective C-C bond rupture over platinum, similar results being reported by Maire et al. (1965). From this it is obvious that with “fresh” catalyst the main fraction of ring opening products is formed over platinum. In addition to ring rupture yielding hexanes, a considerable cleavage of the methyl groups of the alkylcyclopentanes
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 4, 1978 333 Table I. Product Distributions for the Conversions of Methylcyclopentane, Methylcyclopentene, and cis-1.2-Dimethvlcvclo~entaneover "Fresh" Catalvst at 3 50 and 400 "C"
~ ~ ' ~ mol %
methylcyclopentane at product
350 "C
400 "C
0.07 0.03
0.71 0.15
0.30 0.09
0.28 0.11
1.81 0.92 1.72 88.97 2.89 2.78 0.05
0.24 0.10 0.23 98.50 0.33 0.20 0.01
methane C,-C, hydrocarbons I:dimethylbutanes 2-methylpentane 3-methylpentane n-hexane methylcyclopentane c methylcyclopentenes benzene cyclohexane + cyclohexene n-heptane
0.27 98.83 0.38 0.03
350 "C
1,2-dimethylcyclopentane at 350 "C 400 "C
1
- 1% 0.21 0.41
3-methy lhexane
2-methylhexane 2,3-dimethylpentane trans-l,2-dimethylcyclopentane cis-l&?-dimethylcyclopentane 1,3-dimethylcyclopentane Cdimethylcyclopentenes + C, isomers toluene
i
p%
0.19 75.28 20.94 0.09 1.14 0.74
1.07 1.76 0.13 1.55 46.70 16.00 3.53 21.54 5.72
1.1: 2.2: 1
1: 1.6: 1.4
mol ratio n-hexane: 3-methylpentane: 2-methylpentane n-heptane : 3-methylhexane: 2,3-dimethylpentane
" Reaction conditions:
2.5: 1: 2.5
1.9: 2: 1
2.3: 1: 2.4
pulse width: 0.2 ML;catalyst: 115 mg RD 150 C; carrier gas velocity: H, 100 mL/min.
Table 11. Product Distributions for the Conversions of Methylcyclopentane, Methylcyclopentene, and cis-1,2-Dimethylcyclopentaneover Partially Aged Catalyst at 400 and 480 a C mol %
product methane C,-C, hydrocarbons c dimethylbutanes 2-methylpentane 3-methylpentane n-hexane methylcyclopentane I: meth ylc yclopentenes benzene cyclohexane + cyclohexene n-heptane
methylcyclopentane at
methylcyclopentene 1,2-dimethylcyclopentane at at
400°C
480°C
400°C
0.04 0.04
1.47 1.32 0.21 1.06 0.84 1.85 69.35 7.20 15.93 0.81
0.11 0.07 0.40 94.31 2.51 2.10 0.44
480°C
400°C
0.13 0.09 0.41 93.72 2.64 2.36 0.49
2.28 1.70 2.76 43.04 10.88 32.90 0.98
I
I
0.70 0.08
3-meth ylhexane
2-methylhexane 2,3-dimethylpentane trans-1,2-dimethylcyclopentane cis-l,2-dimethylcyclopentane 1,3-dimethylcyclopentane L: dimethylcyclopentenes + C, isomers toluene
480°C
7
0.09 0.07
0.03 15.67 67.30 6.45 6.12 1.65
2.21 0.89 0.15 0.98 3.14 18.04 2.87 4.71 62.01
mol ratio n-hexane: 3-methylpentane: 2-methylpentane n-heptane : 3-methylhexane: 2,3-dimethylpentane
"
5.7: 1: 1.6
2.2: 1: 1.3
4.5: 1: 1.4
1.6: 1: 1.3
23.3: 2.5: 1: 2.7: 1 1.1 Reaction conditions: pulse width: 0.2 M L ;catalyst: 1 1 5 mg RD 150 C; carrier gas velocity: H, 100 mL/min.
takes place, yielding methane and cyclopentane, which could be detected in all product samples. The concentration of methane exceeds by far the sum of Cz-C6 hy-
drocarbon concentrations. This may be explained by a polymerization reaction, which the cyclopentene formed by methyl cleavage undergoes. Up to reaction tempera-
334
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 4, 1978
tures of 400 “C nearly the same product distributions are obtained from methylcyclopentane and methylcyclopentene conversions. At reaction temperatures above 450 “ C the selectivity toward the dehydroisomerization products benzene and toluene increases appreciably, whereas the fraction of benzene formed from methylcyclopentene is significantly higher than that formed from methylcyclopentane. This is valid for conversions over partially aged catalyst too. Ring Opening over Partially Aged Catalyst From the product distributions given in Table I1 it can be observed that the conversions over thermally pretreated and partially aged catalyst are lower than those over “fresh” catalyst. The mole ratio of the primary ring opening products of methylcyclopentane reaction at 400 “C amounts to n-hexane:2-methylpentane:3-methylpentane = 6.3:1.6:1, a product distribution which corresponds to acid-catalyzed ring rupture. This implies that the noble metal component has been partially deactivated during thermal pretreatment, although the activity of platinum is still high enough to equilibrate the methylcyclopentane-methylcyclopentene reaction. At a reaction temperature of 480 “C secondary cracking increases and also the isomerization of the primary ring opening products yielding larger amounts of 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane7and 2,3-dimethylbutane, which could only be detected in very small amounts at a reaction temperature of 400 “ C . The mole ratio n-hexane:2-methylpentane:3-methylpentaneof 2.2:1.3:1 is removed in the direction of the equilibrium concentrations. An inhibition of methylcyclopentane ring opening rate by methylcyclopentene as reported by Brandenberger et al. (1976) could not be found. A t a reaction temperature of 400 “Canalogous product distributions were obtained from the conversions of methylcyclopentane and mixtures of methylcyclopentane and methylcyclopentene. This is attributed to the fact that under the reaction conditions applied in this study, equilibrium concentrations between methylcyclopentane and the methylcyclopentenes were always adjusted. As over “fresh” catalyst, an appreciable
cleavage of the methyl group from the C5 ring takes place in addition to ring rupture. Ring opening of 1,2-dimethylcyclopentane yields selectively n-heptane at a reaction temperature of 400 “C. 2,3-Dimethylpentane and 3-methylhexane are only formed in very small amounts, the mole ratios of the three products being 23:1:2.7 (Table 11). This experimental result is not consistent with product distributions expected from a carbenium ion mechanism and supports a direct ring-opening mechanism as postulated by Brandenberger (1976). Over “fresh” catalyst an extensive cis-trans isomerization of the cis-1,2-dimethylcyclopentane takes place, which is largely suppressed over partially aged catalyst. A t higher reaction temperatures the primarily formed ring opening products undergo isomerization, by which the mole ratios are shifted toward lower values. The occurence of isomerization of the primary ring opening products can be observed from the formation of, for instance, 2-methylhexane7a product that is not formed at lower reaction temperatures. From the experimental results discussed, it is obvious that under operating conditions applied in this study, acid-catalyzed cleavage of alkylcyclopentanes proceeds via direct ring opening yielding n-hexane from methylcyclopentane and n-heptane from 1,2-dimethylcyclopentaneconversions. At higher reaction temperatures the primary ring opening products undergo secondary reactions such as isomerization and cracking. In addition to ring rupture an appreciable cleavage of the methyl group from alkylcyclopentanes occurs over “fresh” and over partially aged catalyst. Literature Cited Anderson, J. R., Adv. Catal., 23, 1 (1973). Brandenberger, S.G.,Callender, W. L., Meerbott,W. K.. J. Catal., 42, 282 (1976). Corolleur, C., Tomanova, D., Gault, G. G. J., J. Cafal., 24, 401 (1972). Christoffel, E., Fetting, F., Vierrath, H., J. Cafal., 40, 349 (1975). Donnis, B. B., Ind. fng. Chem. Prod. Res. Dev., 15, 254 (1976). Greensfelder. 8.S.,Voge, H. H., Good. G. M., I&. fng. Chem., 41, 2573 (1949). Maire, G.,Plouidy, G., Prudhomme. J. C., Gault, F. G., J . Catal., 4, 556 (1965). Smith, R . L., Naro, P. A., Silvestri, J. A., J. Cafal., 20, 359 (1971). Whitmore. F. C., Chem. fng. News. 26, 668 (1948).
Received for review February 22, 1978 Accepted September 5 , 1978
