DEPENDENCE OF
OF
THE
DIRECTIONS
METHYLCYCLOPENTANE
DISTRI]~UTION
OF
OF
ON ELECTRON
Pd-AI20 DENSITY
HYDROLYSIS 3 ON IN
THE
THE
MOLECULE I. I. Levitskii, Kh. and A. M. Boganov
M.
UDC 542.97:547.514.2
Minaehev,
It was shown in [1] that with i n c r e a s i n g hydrogen p r e s s u r e the f r a c t i o n of molecules of m e t h y l c y c l o pentane (MCP) entering into a r e a c t i o n of hydrogenolysis with cleavage of the bonds of the ring c l o s e s t to the methyl group and f a r t h e s t f r o m it, and with the f o r m a t i o n of n-hexane and 3-methylpentene, r e s p e c t i v e ly, increa~,les. T h e s e r e s u l t s w e r e obtained in a study of c a t a l y s t s containing platinum or compounds of other t r a n s i t i o n m e t a l s - c h r o m i u m oxide, vanadium oxide, tungsten sulfide. A c o m p a r i s o n of the e x p e r i mental and quantum c h e m i c a l data showed that the hydrogen p r e s s u r e p r o m o t e s cleavage of the bonds of the MCP ring that w e r e f o r m e d with the p a r t i c i p a t i o n of the c a r b o n a t o m s c a r r y i n g a r e l a t i v e l y low negative charge. Cn the b a s i s of this c o m p a r i s o n , it was suggested in [1] that the direction of the r e a c t i o n is d e t e r mined by the r a t i o of the c h a r g e s of the c a r b o n a t o m in the ring of MCP and the c h a r g e s of the a t o m s or ions of the t r a n s i t i o n m e t a l s . M o r e o v e r , the change in the direction of hydrogenolysis of MCP with inc r e a s i n g P~I is explained in [1] b y an intensification of the e l e c t r o n a c c e p t o r capacity of the m e t a l s contain2 ed in the investigated c a t a l y s t s , as a r e s u l t of an i n c r e a s e in the c h e m i s o r p t i o n of hydrogen. However, t h e r e a r e n u m e r o u s indications in the l i t e r a t u r e of protonation of hydrogen in palladium hydrides and in the case of a d s o r p t i o n of H 2 on Pd [2,3]. Consequently, the e l e c t r o n a c c e p t o r capacity of p a l l a d i u m does not increa~,le with the f o r m a t i o n of P d - H 2 s y s t e m s , but on the c o n t r a r y , it is weakened. T h e r e f o r e , on the b a s i s of the hypotheses of [1] we shouldhave expected that in the case of P d - c a t a l y s t s the hydrogen p r e s s u r e would change the d i r e c t i o n of the hydrogenolysis of methylcyclopentane in the opposite direction in c o m p a r i son with that indicated above. This consequence of the proposed hypothesis w a s subjected to an e x p e r i mental v e r i f i c a t i o n in this work. The influence of H 2 p r e s s u r e on the direction of hydrogenolysis of m e t h y l cyclopentane with Pd-AI203 and Pt-A1203 c a t a l y s t s was investigated. EXPERIMENTAL
METHOD
The i~ydrogenolysis of methylcyclopentane was investigated o v e r 0.3% Pt-A1203 and 0.5% Pd-AI203 c a t a l y s t s in a flow-type s y s t e m . The method of p r e p a r a t i o n of the 0.3% Pt-AI203 c a t a l y s t was d e s c r i b e d in [4]. The Pd-A1203 c a t a l y s t was produced by i m p r e g n a t i o n of powdered A1203 with a solution of PdC12 in HC1; the subsequent o p e r a t i o n s a r e d e s c r i b e d in [4]. The investigations w e r e conducted with e l e c t r o l y t i c H2, f r e e d of O 2 and H20 a c c o r d i n g to [5] (H20 content 0.002-0.006% by volume). To r e m o v e a d s o r b e d water, the c a t a l y s t s w e r e t r e a t e d with H2 f o r 10-12 h b e f o r e the e x p e r i m e n t s at 550 ~ p r e s s u r e 30 arm, and r a t e of flow 30 l i t e r s / h . Between e x p e r i m e n t s the c a t a l y s t s w e r e t r e a t e d with H2 under the s a m e conditions f o r 6 h. The e x p e r i m e n t s at r e l a t i v e l y high p r e s s u r e w e r e a l t e r n a t e d with e x p e r i m e n t s at a lower p r e s s u r e . The s e l e c t i v i t y of the c a t a l y s t s with r e s p e c t to cleavage of individual bonds of the methylcyclopentane ring was evaluated a c c o r d i n g to the r a t i o s of n-hexane f o r m e d to 2-methylpentane (NH : 2MP), n-hexane to 3 - m e t h y l pentane (NIt : 3MP), and 3 - m e t h y l p e n t a n e to 2-methylpentane (3MP : 2MP). All the e x p e r i m e n t s w e r e conducted at350 ~ In each e x p e r i m e n t 1 5 - 2 0 g of ChHgCH 3 w a s p a s s e d o v e r the catalyst. The c a t a l y s t s w e r e analyzed on a c h r o m a t o g r a p h with a f l a m e - i o n i z a t i o n d e t e c t o r and with a column filled with 15% squalane on c h r o m o s o r b P. Nitrogen w a s used as the c a r r i e r gas. The catalyzate of each e x p e r i m e n t w a s analyzed twice. In i;he f i r s t a n a l y s i s the content of r e a c t i o n products in the catalyzate w a s d e t e r m i n e d using N. D. Zelinskii Institute of Organic Chemistry, Academy of Sciences of the USSR. Translated from Izvestiya A~kademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 762-767, April, 1973. Original articie submitted :Hay 15, 1972, 9 1973 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. I0011. All rights reserved. This article cannot be reproduced for any purpose whatsoever without per~nission of the publisher. A copy of this article is available from the publisher for $15.00.
741
NH/2MP NH]3MP ... /'b 3MP/2MP
/
27b "~-~
" ....
l" /
/
!
$'~
..D
n-heptane as an internal standard (n-heptane content in the samples 3-5%). The second analysis was p e r f o r m e d with a substantially l a r g e r sample for a m o r e a c c u r a t e determination of the ratios of the hydrogenolysis products according to the a r e a s of the corresponding peaks in the c h r o m a t o g r a m s . The ratios of the r e a c t i o n products are cited below on the basis of the data of the second analysis.
The experiments with Pt-AL203 catalyst were conducted with one sample (weight 5.0 g, d r y volume 7 ml, size of tablets 4 • 4 ram). The influence of the H2 p r e s E 6' ~ I l I i I ~ I sure on the direction of the hydrogenolysis of m e t h y l c y c l o 10 30 50 7 0 9p, atm pentane was investigated in two s e r i e s of experiments, "in Fig. 1. Influence of p r e s s u r e on the ratio each of which the partial p r e s s u r e of CsHgCH 3 (1.2 or 2.2 of the hydrogenolysis products of methylarm) and the time of contact (4.4 or 8.0 sec) were less cyclopentane when the r e a c t i o n was conductconstant. Thus, the total p r e s s u r e was changed by changed over a 0.3% P t - A l z O 3 catalyst. P a r t i a l ing p u . The r e s u l t s obtained are cited in Figs. 1 and 2. p r e s s u r e of methyleyclopentane and time With increasing p r e s s u r e f r o m 10 to 40 atm, the ratios of contact: I) 1.2 atm, 4.4 sec; 2) 2.2 atm, NH : 2M1a, NH:3MP, and 3MP : 2MP increased c o r r e s p o n d 8.0 sec. Here and henceforth: a) N H : 2 M P ; ingly f r o m 1 to 13-15 f r o m 2-3 to 20-24, and f r o m 0.4 to b) N H : 3 M P ; c) 3MP :2MP. 0.6-0.7. A subsequent i n c r e a s e in the p r e s s u r e f r o m 40 to 80 atm was accompanied by a c e r t a i n d e c r e a s e in the values of NH : 2M1a and NH : 3MP. Then the dependence of the ratios of the r e a c t i o n products on the degree of the r e a c t i o n was investigated. F o r this purpose two s e r i e s of experiments were conducted at 50 and 80 atm. In each s e r i e s of experiments the partial p r e s s u r e s of H2 and CsHgCH3 remained constant, and only t h e i r r a t e s of delivery, and, consequently, the time of contact were varied. The r e s u l t s of experiments at 50 arm and a partial p r e s s u r e of methylcyclopentane of 5.7 atm a r e plotted in Fig. 3. F r o m Fig. 3 it follows that increasing the conversion f r o m 3-4 to 30% had no effect on the ratios of the r e a c t i o n products, and only at a degree of hydrogenolysis > 40% was the c e r t a i n d e c r e a s e in the NH: 2MP and NH: 3MP ratio observed. Analogous data were also obtained at 80 atm (partial p r e s s u r e of methylcyclopentane 8.7 arm). In these experiments an i n c r e a s e in the content of hexanes in the catalyzates f r o m 6-7 to 42-48% led to a change in the NH: 2MP r a t i o only f r o m 14 to 12-13, NH : 3M1a f r o m 21 to 17-19, and 3MP : 2MP f r o m 0.65
E
C6H~,~,q~
#0 NH/MP
30
NH/3MP
2O -~^~ 3MP/2MP
2.0-
a ~
g
~
o -~"
i
~ t 12 *
117 ff
Io
30
5o
Fig. 2
7D p, atrn
" o
o
o
C
I
I
I
i
I0
2o
?g
q8 E G6 ~1~, %
Fig. 3
Fig. 2. Influence of p r e s s u r e on the yield of hexanes in the h y d r o genolysis of methylcyclopentane over 0.3% Pt-A1203 catalyst. P a r tial p r e s s u r e of methylcyclopentane and time of contact: 1) 1.2 atm, 4.4 sec; 2) 2.2 atm, 8.0 sec. Fig. 3. Ratios of the products of hydrogenolysis of methylcyclopentane at various d e g r e e s of the r e a c t i o n on a 0.3% Pt-A1203 catalyst. Total p r e s s u r e 50 arm, partial p r e s s u r e of methylcyclopentane 5.7 atm.
742
TABLE 1. Hydrogenolysis of Methylcyclopentane Over a 0.5% P d A1203 C a t a l y s t ( T e m p e r a t u r e 350 ~ partial p r e s s u r e of m e t h y l e y c l o pentane 1.2 aim) 5
Ratio of hydrogeno~products
Contact in catalyzate, %
B c~ c,.1r
o
t0 10 t0 10 10 i0 50 50 50 50 50
4 6 8 tt 12 t3 5 7 9 10 14
3,7 3,8 0,98 11,3 t,4 5,1 21,7 2t,4 10,8 11,4 22,5
4,0 3,9 0,8 t0+4 0,9 4,2 4,6 4,3 t,4 1,5 4,3
0,2t 0,23 0,06 0,98 0,15 0,65 3,1 3,5 1,t i,3 5,0
D-J~ ~
0,t7 0,t8 0,04 0,71 0,11 0,46 2,t 2,4 0,91 0,97 3,4
~oo a=
1,5 4,t 5,t 4,2 4,6 1,4 1,t 3,0 1,2 t3,0 5,3 t,4 t,7 0,94 0,32 6,2 2,4 0,90 26,9 4,0 0,85 27,3 3,6 0,71 t2,8 4,2 1,5 13,7 t,8 0,18 30,9 1,5 -
-
17,t t5,2 t6,2 10,9 9,2 7,5 6,9 6,3 8,6 7,9 4,6
21,! t9,8 21,0 15,0 t2,8 t0,7 9,9 8,9 tt,8 t0,8 6,8
0,81 0,76 0,77 0,72 0,72 0,70 0,70 0,70 0,73 0,73 0,67
to 0.71-0.73.* All the experiments w i t h a Pd-AI203 catalyst were conducted at a constant partial p r e s s u r e of methylcyclopentane (1.2 aim) with two samples of the catalyst. A f r e s h l y p r e p a r e d 0.5% Pd-AI203 c a t a lyst showed no activity even at 37 5~ however, after it was treated with H 2 at 550 ~ the degree of hydr ogenolys is of methylcye Lopentane at the s a m e r a t e of delivery and at 3 50 ~reached 40%. Table i pres eats the r e s u l t s of the f i r s t s e r i e s of experiments with 0.5% Pd-A1203 (weight 5.0 g, d r y v o l u m e 7 ml, size of tablets 4• 4 m m ) . WhentheH 2 p r e s s u r e was increased f r o m 8.8 to 48.8 atm, the values of all t h r e e ratios of the reaction products d e c r e a s e d .
* In [6] the influence of the H2 p r e s s u r e on the direction of hydrogenolysis of methylcyclopentane over the same catalyst - 0.3% Pt-A1203 - was investigated. In this case an i n c r e a s e in the ratios o f t h e h y d r o g e n o l y sis products with i n c r e a s i n g p a r t i a l p r e s s u r e of H2 f r o m 10 to 50 aim was also observed. However, a subsequent i n c r e a s e in p u did not affect the values of these ratios (in contrast to the r e s u l t s of this work), xi 2 which prolapted us to investigate the dependence of the ratios of the reaction products on the d e g r e e of the r e a c t i o n m~d to d e t e r m i n e the r o l e of s e c o n d a r y reactions of i s o m e r i z a t i o n of the hexanes formed.
CH, %
NH/2MP NH/3MP /b-,
3MP/2MP 15
Z5
la~ \ \ \
~..\
10
z %",C"
"~
-
45 I,o
N1~,% z ,,-<+J~-_ ~0,5 I 8 g
t
I
I
I
I
2
#
5
&
IO
1
/2
Expt. Fig. 4
8
q
i
2
I
t
#
t
t
5
r
i
8
o
1
. - - .
,
~
t
I0
J-_.u_J
/2 Expt.
Fig. 5
Fig. 4. Influence of p r e s s u r e and duration of work of a 0.5% Pd-A1203 catalyst on the ratio of hydrogenolysis products of methylcyclopentane. P a r t i a l p r e s s u r e of methylcyclopentane 1.2 aim, time of contact 2.0 sec. Total p r e s s u r e , aim: 1) 10; 2) 5). Fig. 5. Influence of p r e s s u r e and duration of work of a 0.5% Pd-A1203 catalyst on the yields of hexanes (solid curves) and eyclohexane (CH) (dotted curves) in the hydrogenolysis of CsH~CH3. P a r t i a l p r e s s u r e of methyleyclopentane 1.2 aim, time of contact 2.0 sec. Total p r e s s u r e , aim: 1) 10; 2) 50.
743
However, these r a t i o s Ms o d e c r e a s e d duringthe work of the catalyst, evidently as a result of saturation of Pd with hydrogen and a change inthe phase composition of the P d - H s y s t e m (see, for example, [2,3]). T h e r e f o r e a s e c ond s eries of experiments was conducted (weight of catalyst 2 . 0 g, dry volume 3 ml), in which the duration of all t h e t r e a t m e n t s o f t h e c a t a l y s t w i t h h y d r o g e n w a s reduced (to 2 h), as was the p r e s s u r e of the t r e a t m e n t s (to 10 atm), to slow down phase transitions inthe P d - H s y s t e m . (In experiments with another sample of the catalyst it was established that lowering the t e m p e r a t u r e of the t r e a t m e n t f r o m 550 to 350 ~ leads to a rapid but r e v e r s i b l e deactivation of the eatalyst.) All the experiments of the second s e r i e s were conducted not only at a constant partial p r e s s u r e of MCP, but also at a constant time of contact (2 sec). In each e x p e r i ment 15.0 g of MCP was p a s s e d through the catalyst in a period of ~80 min. The results of the experiments of the second s e r i e s a r e cited in Figs. 4 and 5. In this s e r i e s of experiments also, all three ratios of the hydrogenolysis products of MCP d e c r e a s e d both with increasing hydrogen p r e s s u r e and during the work of the catalyst (see Fig. 4, experiments 1-6). Subsequently (experiments 7-12) these ratios were equalized and were unchanged f r o m experiment to experiment, but they continued to d e c r e a s e somewhat with the p r e s sure (for example, the NH : 2MP ratio in experiments 8, 10, and 12 at 10 arm was an average of 3.13 -~ 0.11, while in experiments 7, 9, and 11 at 50 atm it was 2.73 • 0.18). DISCUSSION
OF
THE
EXPERIMENTAL
RESULTS
It was noted e a r l i e r that in the experiments w i t h a Pt-A1203 catalyst when the p r e s s u r e was increased f r o m 10 to 40 atm, the NH :2MP, NH :3MP, and 3MP : 2MP ratios increased sharply. However, a s u b s e quent i n c r e a s e in the p r e s s u r e f r o m 40 to 80 arm led to a c e r t a i n d e c r e a s e in the NH : 2 MP and NH : 3MP ratios. It might have been a s s u m e d that the observed d e c r e a s e in these ratios was due to an i n c r e a s e in the rate of i s o m e r i z a t i o n of the hexanes f o r m e d with increasing p r e s s u r e . (Let us note that in our e x p e r i ments, with d e c r e a s i n g values of the N H : 2 M P and N H : 3 M P ratios, they approach their equilibrium values, which a r e 0.61 at 327 ~ and 0.72 at 427 ~ for NH : 2MP, and 1.16 and 1.31 for NH :3MP r e s p e c t i v e l y . ) In this case, in experiments at a p r e s s u r e of 40-80 atm, with increasing degree of the reaction, and, consequently, increasing concentration of the hexanes formed, the rate of their i s o m e r i z a t i o n should also have increased, while the ratios of the hydrogenolysis products should have d e c r e a s e d . However, it follows f r o m Fig. 3 that in experiments at 50 arm, an i n c r e a s e in the conversion f r o m 3 to 30% had no effect on the values of HN : 2MP and NH : 3MP, and only at a degree of hydrogenolysis > 40% did these ratios d e c r e a s e somewhat. Analogous r e s u l t s were also obtained at 80 atm. Consequently, the observed d e c r e a s e in the ratios N H : 2 M P and N H : 3 M P was not due to s e c o n d a r y reactions of i s o m e r i z a t i o n of the hexanes formed, but was associated with a change in the p r o p e r t i e s of the catalyst. According to [7], hydrogen chemisorbed on platinum can exist in two f o r m s : M + - H - a n d M - - H 2 +. I t e a n b e assumed that when the surface is covered with chemisorbed hydrogen, the ratio o f these two f o r m s changes - the fraction of H2+ i n c r e a s e s . P o s s i b l y , under our experiments, when the p r e s s u r e was increased f r o m 40 to 80 atm there was a weakening of the electron a c c e p t o r p r o p e r t i e s of platinum as a r e s u l t of chemisorption of hydrogen in the f o r m of H2. T h e r e f o r e , in these experiments with increasing p r e s s u r e the ratios NH : 2 MP and NH : 3MP d e c r e a s e d , i. e . , the relative yields of n-hexane d e c r e a s e d . In experiments with a Pd-AI203 catalyst, increasing the p r e s s u r e f r o m 10 to 50 atm was accompanied by a d e c r e a s e in all three ratios of the hydrogenolysis products of methylcyclopentane (see Fig. 4). It was noted above that the d e c r e a s e in the ratios NH : 2MP and NH : 3MP might be associated with an i n c r e a s e in the rate of i s o m e r i z a t i o n of the hexanes formed with increasing p r e s s u r e . The values of the ratios of the reaction products d e c r e a s e not only with the p r e s s u r e , but also during the work of the catalyst. T h e r e f o r e , we were unable to determine their dependence on the d e g r e e of the reaction (as was investigated with P t A1203). However, other facts a r e evidence of the absence of a distorting influence of s e c o n d a r y reactions of i s o m e r i z a t i o n of hexanes on their distribution. The yields of cyclohexane in experiments at a constant time of contact not only did not i n c r e a s e with the p r e s s u r e , but somewhat d e c r e a s e d (see Fig. 5). Consequently, under the conditions studied, p r e s s u r e inhibits the r e a c t i o n of s t r u c t u r a l i s o m e r i z a t i o n of hydrocarbons, and the observed d e c r e a s e in the ratios of hexanes formed when the p r e s s u r e was r a i s e d f r o m 10 to 15 arm was associated with a change in the p r o p e r t i e s of the Pd-A1203 catalyst. Evidently the observed d e c r e a s e in the ratios of the hydrogenolysis products in experiments with Pt-A1203 when the p r e s s u r e was raised f r o m 40 to 80 arm and that in the experiments with Pd-A1203when the p r e s s u r e was r a i s e d f r o m 10 to 50 atm, were due to a single cause - a shift of the electron density f r o m the adsorbed hydrogen to the metal and an intensification of the electron donor p r o p e r t i e s of the latter with increasing pH 2 within the indicated limits.
744
CONCLUSIONS 1. Raising the pressure from 10 to 40-50 atm leads to opposite changes in the direction of hydrogenolysis of methylcyclopentane, depending on the catalyst: in experiments with an aluminoplatinum catalyst, the ratios of n-hexane to 2-methylpentane formed, n-hexane to 3-methylpentane, and 3-methylpentane to 2-methylpentane increase, while in the case of an aluminopalladium catalyst these ratios decrease. 2. The results obtained agree with the earlier hypothesis of a dependence of the direction of the hydrogeno[ysis reaction on the charges of the carbon atoms and charges of the metal atoms contained in the cataly~,~t.
LITERATURE 1.
2. 3. 4. 5. 6. 7.
CITED
A. ~. Gyul'maliev, I. I. Levitskii, Kh. M. Minachev, and I. V. Stankevieh, Izv. Akad. Nauk SSSR, Ser. Khim., 2475 (1972). K. Maekey, Hydrogen Compounds of Metals [Russian translation], "Mir" (1968). D. V. Sokol'skii and A. M. Sokol'skaya, Metal Catalysts of Hydrogenation [in Russian], "Nauka," Alm~-Ata (1970). I. I. Levitskii, G. M. Gonikberg, Kh. M. Minachev, and D. A. Kondrat'ev, Izv. Akad. Nauk SSSR, Otd. Khim., 1169 (1962). I. I. Levitgkii, Kh. M. Minachev, V. I. Bogomolov, and E. A. Udal'tsova, Neftekhimiya, 7, 230 (1967). V. V. Voronin, I. I. Levitskii, and Kh. M. Minachev, Izv. Akad. Nauk SSSR, Ser. K h i m . , 832 (1969). N. ~r. K a v t a r a d z e , in: M e c h a n i s m of the Interaction of Metals with Gases [in Russian], "Nauka" (196,1), p. 36.
745
