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Isotopic tracer studies of the conversion of alcohol mixtures with a high silica ZSM-5 catalyst
Fuel Processing Technology, 33 (1993) 1 12 Elsevier Science Publishers B.V., Amsterdam
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Isotopic tracer studies of the conversion of alcohol mixtures with a high silica ZSM-5 catalyst L i - M i n T a u a n d B u r t r o n H. D a v i s *
Center for Applied Energy Research, 3572 Iron Works Pike, Lexington, K Y 40511 (USA) (Received May 29th, 1992; accepted in revised form November 9th, 1992)
Abstract
A mixture of methanol and propanol (2:1 mole ratio) has been converted to C2 C4 gases and gasoline range hydrocarbons with a low (60:1) and high (800:1) Si/A1 H-ZSM-5type zeolite catalyst. In one case the methanol was labeled with 14C and the propanol was unlabeled; in the other instance the labeling was reversed so that propanol was labeled. The data are consistent with the major fraction of the C,++1 hydrocarbons (n = 3 or greater) being derived from the alkylation of the C + hydrocarbons with a C~ species derived from methanol. An alkene oligomerization/cracking reaction pathway is superimposed upon the alkylation reaction pathway and this redistributes the ~4C isotope to produce C4, C5 and C7 carbon number groupings which have a relative activity corresponding to a near equilibration of the carbon isotopes.
INTRODUCTION T h e s e l e c t i v e c o n v e r s i o n of m e t h a n o l to g a s o l i n e u s i n g a ZSM-5 c a t a l y s t w a s a n i m p o r t a n t d i s c o v e r y [1]. T h e m e c h a n i s m of t h e m e t h a n o l - t o - g a s o l i n e ( M T G ) r e a c t i o n is s t i l l w i d e l y d e b a t e d i n s p i t e of n u m e r o u s i n v e s t i g a t i o n s . O n e of t h e a p p r o a c h e s to i a n d e r s t a n d i n g t h e m e c h a n i s m h a s e m p l o y e d a n a n a l y s i s of t h e e t h e n e p r o d u c e d w h e n 13C l a b e l e d m e t h a n o l or a n a l k e n e w a s a d d e d to t h e r e a c t a n t [2, 3]. I f e t h e n e w a s f o r m e d o n l y f r o m m e t h a n o l , t h e e t h e n e f o r m e d w h e n 13CH3OH w a s fed t o g e t h e r w i t h a n u n l a b e l e d a l k e n e s h o u l d be d o u b l y l a b e l e d (13C=13C). T h e e t h e n e f o r m e d b y r e a c t i o n s inv o l v i n g b o t h m e t h a n o l a n d t h e u n l a b e l e d a l k e n e w o u l d be e i t h e r s i n g l y l a b e l e d (13C=12C) or u n l a b e l e d (12C=12C). T h u s , a m e a s u r e of t h e (13 C = 12C)/( 13C = ~3C) r a t i o w o u l d p r o v i d e a n i n d i c a t i o n of t h e e x t e n t of e t h e n e f o r m e d d i r e c t l y f r o m t h e l a b e l e d m e t h a n o l . T h e r a t i o of t h e s e t w o i s o t o p i c e t h e n e s , (13 C = 12C)/(13 C _- 13C) ' is v e r y n e a r to t h e o n e e x p e c t e d for a s t a t i s t i c a l d i s t r i b u t i o n ; e v e n t h e c o m p o s i t i o n of all t h r e e i s o t o p i c a l l y l a b e l e d e t h e n e s is close to a s t a t i s t i c a l d i s t r i b u t i o n . E t h e n e is t h e l a s t c a r b o n n u m b e r g r o u p to a t t a i n i s o t o p e e q u i l i b r a t i o n [4]. T h e r e f o r e , t h e i s o t o p i c t r a c e r s t u d i e s to d a t e
* To whom correspondence should be addressed.
0378-3820/93/$06.00
~; 1993 Elsevier Science Publishers B.V.
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L.-M. Tau and B.H. Davis/Fuel Processing Technol. 33 (1993) 1 12
have not provided data t h a t is a p p r o p r i a t e for defining the r e a c t i o n p a t h w a y s in the M T G reaction. One of the problems e n c o u n t e r e d with this i n t e r m e d i a t e pore zeolite is a kinetic disguise because the e q u i l i b r a t i n g r e a c t i o n s within a zeolite crystal is more rapid t h a n the diffusion of p r i m a r y products to the e x t e r i o r of the particle [5, 6]. M a n y of the early studies h a v e been carried out with low Si/A1 ( ~ 60) ratio. Since each f r a m e w o r k A1 provides an acid site, the low Si/A1 zeolite c a t a l y s t has an acid site density t h a t n e a r l y ensures t h a t kinetic disguise will d o m i n a t e the results of the study. Our earlier studies with alcohol mixtures was carried out using a low Si/A1 r a t i o ZSM-5 sample and the isotopic distribution indicated t h a t the C~ products had a t t a i n e d a statistical isotope distribution [4, 7]. A silicate sample obtained from U n i o n Carbide has a m u c h h i g h e r Si/A1 (~800) r a t i o t h a n the one we previously utilized for isotopic t r a c e r studies. This material thus has a s t r u c t u r e e q u i v a l e n t to the low Si/A1 ZSM-5 sample but has a m u c h lower acid site density. It was t h e r e f o r e of i n t e r e s t to learn w h a t the isotope distribution within the c a r b o n n u m b e r products from the c o n v e r s i o n of alcohol mixtures would be for this high Si/A1 material.
EXPERIMENTAL Catalysts
Two catalysts were used in this study. A low Si/A1 ratio of a b o u t 60 was utilized as a sample r e p r e s e n t a t i v e of a ZSM-5 type c a t a l y s t with a high acid site density; this sample was provided by Mobil Oil. The m a t e r i a l r e p r e s e n t a t ive of a low acid site density c a t a l y s t was a sample of Silicalite S-115 with a Si/A1 ratio of a b o u t 800 and was provided by U n i o n Carbide.
Reagents
Unlabeled r e a g e n t grade alcohols and r e s e a r c h grade gases were used without f u r t h e r purification. Radioactive alcohols were p u r c h a s e d from New E n g l a n d N u c l e a r or were synthesized in-house. The chemical p u r i t y of the labeled alcohol was g r e a t e r t h a n 98%; the r a d i o a c t i v e p u r i t y was also in this range.
Procedure
The r e a c t i o n system and a n a l y t i c a l t e c h n i q u e s have been described previously [4, 7 9]. Briefly, the c a t a l y s t (1 g zeolite diluted with l g of low area alpha-alumina) is held at the midsection of a plug-flow reactor; glass beads above the c a t a l y s t bed serve as a p r e h e a t e r section. The c a t a l y s t was calcined in situ in flowing air at 530 °C for at least 4 h o u r s prior to its use, and t h e n
L.-M. Tau and B.H. Davis/Fuel Processing Technol. 33 (1993) 1 12
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flushed with n i t r o g e n as the c a t a l y s t t e m p e r a t u r e was d e c r e a s e d to the reaction t e m p e r a t u r e . T h e r e a c t i o n t e m p e r a t u r e was 300 °C and the p r e s s u r e was 1 atm. Liquid p r o d u c t s w e r e added using a s y r i n g e pump. U n l a b e l e d gases were added t h r o u g h a mass-flow r e g u l a t o r . T h e c a t a l y s t was c o n t a c t e d w i t h the r e a c t a n t s for 10 15 m i n u t e s to e s t a b l i s h a " s t e a d y - s t a t e " condition; the products formed d u r i n g this period w e r e discarded. P r o d u c t s w e r e t h e n collected d u r i n g the n e x t 30 40 m i n u t e s of r e a c t i o n . Gas and liquid p r o d u c t s were a n a l y z e d using an a p p r o p r i a t e gas c h r o m a t o g r a p h i c (GC) column; the effluent from the GC passed t h r o u g h a gas p r o p o r t i o n a l c o u n t e r to o b t a i n a m e a s u r e of the r e l a t i v e r a d i o a c t i v i t y of the effluent peaks.
RESULTS B e c a u s e the d a t a from the t r a c e r studies in complex r e a c t i o n p a t h w a y s are f r e q u e n t l y difficult to define by a n a l y t i c a l equations, we consider in the following d e s c r i p t i o n the e x t r e m e cases. T h e r e l a t i v e r a d i o a c t i v i t y / m o l e v e r s u s carbon n u m b e r c u r v e s for four possible r e a c t i o n p a t h w a y s are i l l u s t r a t e d in Fig. 1. F o r a p a t h w a y w i t h labeled m e t h a n o l t h a t forms h y d r o c a r b o n s only from C1 and with the r e l a t i v e r a d i o a c t i v i t y per mole of m e t h a n o l t a k e n as C1 = 1, a line w i t h a slope of I is o b t a i n e d (14C1 only, Fig. 1). If the p a t h w a y i n v o l v e s only the alkylation of the unlabeled C3 species derived from 1-propanol with a 14C~ species derived from m e t h a n o l , the r a d i o a c t i v i t y per mole defines a line w i t h a slope of 1 but with the r e l a t i v e a c t i v i t y of the C4 g r o u p i n g b e i n g I (14C~ a l k y l a t i o n , Fig. 1) and the C1 C3 p r o d u c t s h a v i n g no
j /
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03 .O m '10 03 nO
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1
2
3
4
5
6
7
Carbon Number
Fig. 1. Schematic of the relative radioactivity expected for four reaction pathways for producing hydrocarbons from: (a)I~CH3OH only, (b) 14CH3OH alkylation of C3H6, (c) CH3OH alkylation of ~C3H 6, and (d) equilibration of the 1~C isotope with the unlabeled carbons of methanol and propene.
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L.-M. Tau and B.H. Davis~Fuel Processing Technol. 33 (1993) 1 12
radioactivity. F o r a p a t h w a y involving only a l k y l a t i o n of 14C3 labeled species derived from ~4C labeled l-propanol by an unlabeled C~ species, a line parallel to the X-axis with the relative activity of 1 for all c a r b o n n u m b e r groupings of C ; is obtained ( a l k y l a t i o n of ~4C3, Fig. 1). If all c a r b o n n u m b e r groupings result from the e q u i l i b r a t i o n of the isotopic species with the c a r b o n of the o t h e r compound, the activity of each c a r b o n will become 0.2, and the line defining the activity of the c a r b o n n u m b e r groupings has a slope of 0.2 (equilibration, Fig. 1).
Low Si/Al ZSM-5
It is observed t h a t the C3 C6 products from the c o n v e r s i o n of the propanolm e t h a n o l m i x t u r e with the low Si/A1 c a t a l y s t are essentially the ones expected for a chemical equilibrium distribution (Table 1). Thus, with the ZSM-5 (Si/A1 _= 60) catalyst, the products had essentially a statistical isotope distribution (Fig. 2) and an equilibrium chemical composition. The data in Fig. 2 are typical of our results using the h i g h e r acid site density H-ZSM-5 material (see ref. [4] for details). The results in Fig. 2 are for the c o n v e r s i o n of m e t h a n o l and propanol (2 moles m e t h a n o l / m o l e propanol): in one r u n the labeled molecule was 14CH3OH and in the o t h e r r u n it was [1-14C] l-propanol. Similar results were obtained for both runs. The r a d i o a c t i v i t y per mole of the C~ products increased with c a r b o n n u m b e r so t h a t the activity per c a r b o n atom was c o n s t a n t for all c a r b o n n u m b e r compounds. These d a t a indicate t h a t the 14C initially present in e i t h e r of the two alcohols equilibrated with the carbon(s) added in the o t h e r alcohol for all of the C~ products. Similar results for isotope distribution were obtained for products produced with the Si/A1 ~ 60 zeolite for t e m p e r a t u r e s of 180, 200 and 300 cC; the major difference noted for this t e m p e r a t u r e r a n g e was t h a t the a m o u n t of a r o m a t i c s in the h y d r o c a r b o n fraction increased with increasing t e m p e r a t u r e [4]. However, alkanes, alkenes and a r o m a t i c s showed a similar p a t t e r n for the r e l a t i v e radioisotope activity with c a r b o n n u m b e r for the entire t e m p e r a t u r e range. Using this ZSM-5 c a t a l y s t with a crystal size in the 2 5 gm range, we h a v e only
TABLE 1 Product distribution from the conversion of alcohol mixtures with ZSM-5 type catalysts at LHSV=9.7 cm3/h, 300 °C and 1 atm Reactant
PrOH/MeOH PrOH/MeOH PrOH Equilibrium distribution
Catalyst
HZSM-5 $115 $115
Product (wt.%) C3
C4
C5
C6
18.1 71.6 82.7 15.5
36 14.4 4.25 37.8
30.6 6.2 5.7 34
15.3 7.8 7.4 12.4
L.-M. Tau and B.H. Davis/Fuel Processing Technol. 33 (1993) 1 12
O
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3.21 2.8
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2.4 2.0 1.6
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5
1.2 o 0.8
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0.4
AROMATIC --> I M~OH larOH I C3
I
I
I
C4
Cs
C6
I
C7
I
I
Ce
C7
I
Cs
Carbon Number (or Compound)
Fig. 2. Relative radioactivity per mole in the products from the conversion of ~CH3OH and propanol (Q) or CH3OH and I~C labeled 1-propanol (m) with an H-ZSM-5 catalyst at 300'C (activity of the products was compared to that of methanol (©) or 1-propanol (_), respectively, which were taken as 1.0). been able to obtain products in which the 14C has equilibrated among the carbons added in the labeled and the unlabeled reactants (equilibration line, Fig. 1) [4].
Silicalite catalysts
Mixtures of methanol and l-propanol (2 moles methanol per mole propanol) were converted with the silicalite catalyst at 300 °C and 9.7 cm3/h flow rate. Under these conditions methanol conversion, not including that which formed dimethyl ether, was only about 30%. l-Propanol, on the other hand, underwent complete conversion; neither l-propanol nor the ethers methyl-l-propyl nor di-l-propyl were detected in the reaction products. For a lower temperature (e.g., 180~'C) reaction condition, the alcohol mixture did contain the mixed ether (methyl-l-propyl ether) but did not contain the ether of only the higher alcohol (di-l-propyl ether). Methane was present in such a low concentration th at it could barely be detected by GC. Propene was the major hydrocarbon product. The hydrocarbon products were estimated, from identification of the components of the GC trace, to be approximately 85 90% alkenes. The C4 C6 carbon number product distributions are presented in Table 1 and the C2-C9 product distributions, including or omitting C3 products, are shown in Fig. 3. The relative molar radioactivities of the products from the conversion of l-propanol and ~4CH3OH (2 mole methanol per mole l-propanol) are shown in Fig. 4. The C4 product grouping has a relative radioactivity that is about 0.87 higher than the C3 grouping, indicating that alkylation is responsible for the formation of 87% of the C4 product group. The relative molar radioactivity of the C3 and C4 products is believed to provide a reliable measure of the contribution of C~ alkylation to the total 14C isotope distribution. This belief is also supported by a comparison of the relative molar radioactivity in the
L.-M. Tau and B.H. Davis~Fuel Processing Technol. 33 (1993) 1 12
6 80 70 60 5O ,,C
"6
40 30 2O 10 0
i
C2
C3
C4
C5
C6
C7
C8
C9
C8
C9
Carbon Number 50
40
3o
'~
2o
lo
C4
05
Carbon
C6
C7
Number
Fig. 3. The product distribution for the conversion of methanol and 14C labeled l-propanol at 300C with the high Si/A1 H-ZSM-5 catalyst including (top) and excluding (bottom) the C3 product grouping.
C6 and C~ fractions. If 14C labeled methanol added to the C 6 compounds having a relative molar radioactivity of about 0.65 as indicated in Fig. 4, the C7 fraction should have a relative radioactivity per mole of 1.65. The experimentally determined molar radioactivity for the C7 fraction is about 1.2 so that the activities of the C~, and C7 carbon number fractions are consistent with about 55% C1 alkylation. The molar radioactivity of the C4, C5 and C7 products define a reasonably good straight line with a slope of about 0.1 (relative molar radioactivity of methanol = 1). The radioactivity per mole of the C3 products is very low (0.01 or less) and that of the C6 products is much lower than expected for the relative activity of the C4, C5 and C~ products. The low molar radioactivity for the C6 products confirms that dimerization of propene is a significant reaction under these reaction conditions. The per- mole radioactivity of the products is constrained by three limits. If the higher carbon number compounds arise only from propene oligomerization-cracking reactions, a line defined by the molar radioactivity with increasing carbon number would be the positive X-axis with a slope of zero. For another extreme, the 14C distribution could correspond to complete isotope equilibration. For carbon isotope equilibration, 2 moles of
L.-M. Tau and B.H. Davis/Fuel Processing Technol. 33 (1993) 1 12
7
1.4 0
*d
1.2 1.0
0
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0.6 n"
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J
0.4
0.2
/
3
4
I
5
I
6
I
7
Carbon Number
Fig. 4. Relative radioactivity per mole in the products from the conversion of '4CH30H and 1-propanol (molar ratio methanol/propanol = 2) at 300 ~C with a silicalite S-115 catalyst.
methanol, with a relative per-mole radioactivity for methanol of 1, and 1 mole of propanol, with 3 moles of unlabeled carbon, will become equivalent so that each mole of carbon will have a relative radioactivity of 2/5 =0.4. Thus, for carbon equilibration, the relative radioactivities per mole with increasing carbon number will define a straight line with a slope of 0.4. The upper limit defined by forming labeled products only from I~CH3OH is not considered; for this to occur the relative radioactivity per mole of the C3 products must be three times that of methanol, and the data in Fig. 4 are clearly inconsistent with this. Likewise, the upper limit of C1 alkylation of unlabeled propene to give a slope of 1 was not considered for the C4, C5 and C7 products. The extent of alkylation by C1 for the first two limits is then the ratio of the slope, 0.133, of the line defined by the C~, C5 and C7 products and the slope (0.4) of the line expected for isotope equilibration; i.e., 0.133/0.4 = 0.33 showing that 33% of the products result from alkylation by 14C labeled C1 species. This conclusion is, at first glance, not consistent with the relative activities of C3 and C4. The C4 product grouping has a relative molar radioactivity that is about 0.87 higher th an the C3 grouping, indicating that alkylation is responsible for the formation of at least 87% of the C4 product. However, the slope for isotope equilibration was based upon complete conversion of the methanol, whereas the actual conversion was only about 30%. If we take this adjusted value for the contribution of methanol (2 x 0.3 = 0.6), the radioactivity per mole of carbon at isotopic equilibrium becomes 0.6/3.6=0.167 rat her than 0.4. Repeating the
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L.-M. Tau and B.H. Davis~Fuel Processing Technol. 33 (1993) 1-12
above calculation on the basis of 30% methanol conversion increases the extent of C1 incorporation to 80%. The actual value will fall, therefore, between 33 and 80% since the data in Fig. 3 indicate the incorporation of the C3 fraction has not attained the chemical equilibrium distribution. In order to utilize the slope to define exactly the extent of the C1 incorporation one would have to obtain a series of lines at various methanol and C3 conversion levels or to utilize another reactor design such as a continuous stirred tank reactor. The relative molar radioactivities of the products from the conversion of [l-14C]-l-propanol and methanol (propanol:methanol mole ratio 1:2) are shown in Fig. 5. It is apparent that the C3 and C~ products have a higher molar radioactivity than they would if they fit the line defined by the C4, C5 and C7 products. The radioactivity per mole of the C2 products is lower t han anticipated from the line defined by the C~, C5 and C7 products. There may be questions concerning whether the deviation of the molar radioactivity data for the C2, C3 and C6 products deviate from the line because of the reaction pathways or whether it is due to experimental uncertainty. The deviation of the data points for the oligomerization-cracking of propene from the line as shown in Figs. 2 and 6 are representative of the experimental u n c e r t a i n t y for our experiments. Without question, the deviation of the C2, C3 and C6 molar radioactivity from the line is due to the reaction mechanism, and not to experimental errors. The relative radioactivities on a molar basis (relative to propanol -- 3.0) of the C3 and C4 carbon number groupings (Fig. 5) are 2.99 and 3.50, respectively. If all of the C4 had been formed by alkylation of C3 by the unlabeled C~ species, the activity of the C~ would have been the same as C3 (2.99); if the C4 had
0
6.0
> o O "O 0~ n" O ~>
4.5
3.0
n" 1.5
ol
2
7
Carbon Number
Fig. 5. Relative radioactivity per mole in the products from the conversion of [1-14C]l-propanol and methanol (molar ratio methanol/propanol = 2 at 300 °C with a silicalite S-115 catalyst).
L.-M. Tau and B.H. Davis/Fuel Processing Technol. 33 (1993) 1 12
9
o
"6 ~;
6.0
.~_ ~
4.5
0 r,"
3.C
0 :.~
1.5
0.0
Carbon Number
Fig. 6. Relative radioactivity per mole in the products of the conversion of [2-14C]-propene (m) at 300~C with a silicalite S-115 catalyst.
been derived only from C3 the activity would have been 3.99 (4/3 x C3 activity). Thus, the f r a c t i o n a l increase c o r r e s p o n d i n g to a l k y l a t i o n is ( 3 . 9 9 - 3.50)/(3.99- 2.99) = 0.49. This indicates t h a t 49% of the C4 was formed by a l k y l a t i n g a C3 species derived from propanol with a C1 species derived from m e t h a n o l . A similar c a l c u l a t i o n based on the m o l a r r a d i o a c t i v i t y of the C6 and C7 c a r b o n n u m b e r groupings indicate t h a t 51% of the C7 is formed by alkylation of C6 by an u n l a b e l e d C1 species. In this experiment, the r e l a t i v e m o l a r r a d i o a c t i v i t y of the products must fall b e t w e e n two limits. The upper limit will be defined by a s t r a i g h t line with a slope t h a t depends upon the r e l a t i v e molar r a d i o a c t i v i t y assigned to the labeled propene; when propene has a r e l a t i v e molar r a d i o a c t i v i t y of 3.0 as s h o w n in Fig. 4 the slope is expected to be 1.0. This will o c c u r when the 14C label is r e d i s t r i b u t e d only by oligomerization and c r a c k i n g as was the case for the c o n v e r s i o n of 14C labeled propene alone (Fig. 6). A good m e a s u r e of the e x p e r i m e n t a l e r r o r is shown in Figs. 2 and 6 where the data provide a good fit to a s t r a i g h t line for all of the c a r b o n n u m b e r groupings. The d a t a in Figs. 2 and 6 indicate t h a t the deviations of the molar r a d i o a c t i v i t y for the C3 and C6 products from the s t r a i g h t line in Fig. 4 are real. The lower limit will be defined by e i t h e r a line with slope of 0 or 0.2. The zero slope will apply w h e n all of the C~ products result from the addition of u n l a b e l e d C1 species to the laC labeled C3 species ( a l k y l a t i o n of 14C3, Fig. 1). If t h e r e is complete isotope equilibration, the slope will be 0.2 since 2 moles of m e t h a n o l provide two u n l a b e l e d carbons while one mole of [1-14C]-1-propanol provide, on average, two u n l a b e l e d carbons and one labeled c a r b o n (equilibration, Fig. 1). The r e l a t i v e a c t i v i t y / c a r b o n will thus be 0.2 so t h a t each increase in c a r b o n n u m b e r will result in adding a r e l a t i v e activity of 0.2 to provide a line with a slope of 0.2. The slope of the line defined by the C4, C5 and C7 products is 1.16. The s c h e m a t i c shown in F i g u r e 1 as well as the two c o n v e r s i o n p a t h w a y s discussed above does not provide a p a t h w a y t h a t would produce a slope g r e a t e r t h a n 1.0. The r e a s o n for the slope being g r e a t e r t h a n 1.0 in this p a r t i c u l a r case
10
L.-M. Tau and B.H. Davis~Fuel Processing Technol. 33 (1993) 1 12
is the activity of the propene in the products has been assigned a relative activity of 3.0 rather than the reactant, l-propanol, which has undergone complete conversion and could, therefore, not be used. As shown in Fig. 3 the lower carbon compounds (C3 and C4) are the dominant components of the product; if the comparison is made on a mole basis the dominance of the C3 and C4 products is even greater. Therefore, alkylation by the unlabeled C~ fragment occurs to decrease the label of the lower carbon number products to a greater extent than the higher carbon compounds; the C6 products are initially derived predominantly by oligomerization of labeled propene formed from 14C labeled propanol. Furthermore, in a plug flow reactor the initial C6 formed by oligimerization will be derived from a C3 that has a higher amount of 14C per mole than the C6 formed at the exit of the reactor; this will cause the C6 to have a higher amount of 14C per mole than the C4 product. Ethene is of special interest. The product distribution for the run with unlabeled methanol and ~4C labeled propanol is shown in Fig. 3. Neglecting the C3 products, ethene accounts for only 11 mol% (4.6 wt.%) of the C2-C9 products while the C4 products account for 44 mol% of these products. The Cs and C~ fractions appear high compared to the C7 fraction; however, the greater Cs and C9 fractions are a result of the aromatics that are formed. In the run with t4C labeled methanol the amount of 14C labeled ethene is higher than the level of label expected for carbon equivalency (Fig. 4). On the other hand, if ethene was formed only from ~4C labeled methanol the relative molar radioactivity of ethene should have been 2 rather than the 1.05 that is observed; this indicates that about 53% of the ethene is formed directly from C~ species derived from 14C labeled methanol. This estimate is actually high since ethene formed from oligomerization and cracking will also contain 14C. Assuming that the radioactivity per mole of ethene formed during the oligomerization-cracking conversions fits the line defined by the C4 and C5 products, it is calculated that only 30% of the ethene is formed directly from two C1 species. The ethene formed during the conversion of unlabeled methanol and ~4C labeled propanol has a lower radioactivity/mole than expected if it arose only from oligomerization and cracking reactions. Taking the deviation of the radioactivity/mole of the line defined by the C4, C5 and C~ products (0.41) divided by the radioactivity per mole required for C2 to fall on this line (1.16) indicates that 35% of the ethene is formed directly from methanol. Thus the results of the two runs are consistent and indicate that at about 30% methanol conversion to hydrocarbons in the presence of propanol (and propene), only about 30% of the ethene is formed directly from two C~ species derived from methanol.
DISCUSSION The current results permit us to better define the reaction pathways for alcohol conversion with the ZSM-5 catalyst. Whereas the sample with a low Si/A1 ratio led to nearly a statistical 14C distribution in the hydrocarbon
L.-M. Tau and B.H. Davis/Fuel Processing Technol. 33 (1993) 1 12
11
c a r b o n n u m b e r groupings, this was not the case with the h i g h e r Si/A1 zeolite catalyst. In the present study we have grouped the h y d r o c a r b o n products a c c o r d i n g to c a r b o n number. We believe t h a t this is e n t i r e l y justified since, in those c a r b o n n u m b e r groups for C2 C5 where we get s e p a r a t i o n of n e a r l y all a l k e n e and a l k a n e isomers, all compounds within a c a r b o n n u m b e r group has essentially the same specific m o l a r radioactivity. The p r o p o r t i o n a l c o u n t e r can only be utilized in series with a packed GC column with the isotope levels utilized in this study. A C a r b o w a x 20M GC c o l u m n permitted s e p a r a t i o n s to show t h a t individual compounds in the a r o m a t i c s also had the same specific molar r a d i o a c t i v i t y as the c o r r e s p o n d i n g carbon n u m b e r grouping of aliphatic compounds. In addition it appeared that, where s e p a r a t i o n was possible, each c o m p o u n d had the same specific activity within a class in the C3 h y d r o c a r b o n c a r b o n n u m b e r fractions. The experimental data are plotted in Fig. 7 t o g e t h e r with the line representing isotopic c a r b o n equilibration. It is n e c e s s a r y to make the r e l a t i v e molar r a d i o a c t i v i t y of one of the c a r b o n n u m b e r grouping the same since absolute m o l a r r a d i o a c t i v i t i e s were not obtained; for this plot the r a d i o a c t i v i t i e s per mole of the C4 groupings are equated. The d a t a for the C4, C5 and C~ groupings are a p p r o a c h i n g the line for c a r b o n isotope equilibration. Because the isotope is present in the C1 species in one r u n and the C3 species in the o t h e r case, the isotope d i s t r i b u t i o n in the two runs will differ even t h o u g h the chemical composition by c a r b o n n u m b e r groupings may (and should) be the same. Considering the d a t a for the two runs with m e t h a n o l and propanol with the high Si/A1 ratio c a t a l y s t and the slope of the line defined by the C4, C5 and C~ groupings, it appears t h a t scrambling c o n t r i b u t e s slightly more to the
/ / /
0
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,
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2
3
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Fig. 7. Comparison of the ~4C distributions from the conversion of methanol (©) and propanol (•). The relative radioactivities of the C4 carbon number grouping set equal for the two runs. The solid line defines the data expected for carbon equilibration.
12
L.-M. Tau and B.H. Davis~Fuel Processing Technol. 33 (1993) 1 12
14C d i s t r i b u t i o n t h a n C 1 a l k y l a t i o n for the C4, Cs and C7 c a r b o n n u m b e r groupings. F o r the c o n v e r s i o n of u n l a b e l e d p r o p a n o l and 14C labeled m e t h a n o l , the d a t a for the r e l a t i v e a c t i v i t y in the C3 and C4 f r a c t i o n s as well as t h a t in the C6 and C~ f r a c t i o n s i n d i c a t e t h a t C~ a l k y l a t i o n s are r e s p o n s i b l e for the formation of 87 and 55%, r e s p e c t i v e l y , of the C4 and C7 fractions. Similarly, for the c o n v e r s i o n of 14C labeled p r o p e n e and u n l a b e l e d m e t h a n o l , C1 a l k y l a t i o n a c c o u n t s for 49 and 51% of the C4 and C7 c a r b o n n u m b e r groupings, respectively. T h e e t h e n e labeling c l e a r l y d e m o n s t r a t e t h a t e t h e n e is a p r i m a r y p r o d u c t d u r i n g the c o n v e r s i o n of m e t h a n o l in the p r e s e n c e of significant c o n c e n t r a tions of C~ alkenes. U n d e r the c o n v e r s i o n c o n d i t i o n s utilized in this study only a b o u t 30% of the e t h e n e r e s u l t e d d i r e c t l y from the c o m b i n a t i o n of two C~ species derived from m e t h a n o l . H o w e v e r , the f r a c t i o n of m e t h a n o l being c o n v e r t e d directly to e t h e n e is small c o m p a r e d to the t o t a l c o n v e r s i o n of methanol. The d a t a also show t h a t the use of the h i g h Si/A1 ZSM-5 zeolite with a l o w e r acid site d e n s i t y allows d a t a to be o b t a i n e d t h a t r e p r e s e n t the k i n e t i c r e g i o n a l t h o u g h t h e r e a p p e a r s to be some " k i n e t i c disguise" due to s e c o n d a r y reactions o c c u r r i n g in the c h a n n e l s of the zeolite crystals. The d a t a show t h a t the r e a c t i o n p a t h w a y for h y d r o c a r b o n s y n t h e s i s from m e t h a n o l in the p r e s e n c e of a p p r e c i a b l e C~ a l k e n e s is p r e d o m i n a n t l y one t h a t i n v o l v e s CI a l k y l a t i o n of C~ alkenes, and not t h r o u g h the f o r m a t i o n of Cz c a r b o n n u m b e r species from m e t h a n o l . S u p e r i m p o s e d u p o n the C~ alkylation p a t h w a y is one t h a t i n v o l v e s o l i g o m e r i z a t i o n and c r a c k i n g r e a c t i o n s t h a t serve to r e d i s t r i b u t e isotope label and to m o v e the c a r b o n n u m b e r g r o u p i n g t o w a r d the t h e r m o d y n a m i c e q u i l i b r i u m v a l u e and s t a t i s t i c a l isotope distribution. The r e l a t i v e f r a c t i o n of e a c h c a r b o n n u m b e r g r o u p i n g t h a t arises for o l i g o m e r i z a t i o n and c r a c k i n g r e a c t i o n s will depend u p o n the degree of methanol c o n v e r s i o n and the mole f r a c t i o n s of a l k e n e s of all of the c a r b o n n u m b e r groupings.
REFERENCES 1 2 3 4 5 6
Chang, C.D. and Silvestri, A.J., 1977. J. Catal., 47: 249. Dessau, R.M. and LaPierre, R.B., 1982. J. Catal., 78: 136. Mole, T., 1983. J. Catal., 84: 423. Tau, L.M., Fort, A.W., Bao, S.Q. and Davis, B.H., 1990. Fuel Processing Technol., 26: 209. Olson, D.H. and Haag, W.O., 1984. ACS Symp. Ser., 248: 275. Haag, W.O.. 1984. In: B.L. Shapiro (Ed.), Heterogeneous Catalysis. Texas A&M University Press, College Station, TX, pp. 95 120. 7 Tau, L.M., Fort, A.W. and Davis, B.H., 1986. In: Y. Murakami, A. Iijima and J.J. Ward (Eds.) Proc. 7th Int. Zeolite Conference. Tokyo, p. 899. 8 Bao, S.Q., Tau, L.M. and Davis, B.H., 1988. J. Catal., 111: 436. 9 Tau, L.M., Bao, S.Q. and Davis, B.H., 1988 J. Catal., 114: 190.
1
Isotopic tracer studies of the conversion of alcohol mixtures with a high silica ZSM-5 catalyst L i - M i n T a u a n d B u r t r o n H. D a v i s *
Center for Applied Energy Research, 3572 Iron Works Pike, Lexington, K Y 40511 (USA) (Received May 29th, 1992; accepted in revised form November 9th, 1992)
Abstract
A mixture of methanol and propanol (2:1 mole ratio) has been converted to C2 C4 gases and gasoline range hydrocarbons with a low (60:1) and high (800:1) Si/A1 H-ZSM-5type zeolite catalyst. In one case the methanol was labeled with 14C and the propanol was unlabeled; in the other instance the labeling was reversed so that propanol was labeled. The data are consistent with the major fraction of the C,++1 hydrocarbons (n = 3 or greater) being derived from the alkylation of the C + hydrocarbons with a C~ species derived from methanol. An alkene oligomerization/cracking reaction pathway is superimposed upon the alkylation reaction pathway and this redistributes the ~4C isotope to produce C4, C5 and C7 carbon number groupings which have a relative activity corresponding to a near equilibration of the carbon isotopes.
INTRODUCTION T h e s e l e c t i v e c o n v e r s i o n of m e t h a n o l to g a s o l i n e u s i n g a ZSM-5 c a t a l y s t w a s a n i m p o r t a n t d i s c o v e r y [1]. T h e m e c h a n i s m of t h e m e t h a n o l - t o - g a s o l i n e ( M T G ) r e a c t i o n is s t i l l w i d e l y d e b a t e d i n s p i t e of n u m e r o u s i n v e s t i g a t i o n s . O n e of t h e a p p r o a c h e s to i a n d e r s t a n d i n g t h e m e c h a n i s m h a s e m p l o y e d a n a n a l y s i s of t h e e t h e n e p r o d u c e d w h e n 13C l a b e l e d m e t h a n o l or a n a l k e n e w a s a d d e d to t h e r e a c t a n t [2, 3]. I f e t h e n e w a s f o r m e d o n l y f r o m m e t h a n o l , t h e e t h e n e f o r m e d w h e n 13CH3OH w a s fed t o g e t h e r w i t h a n u n l a b e l e d a l k e n e s h o u l d be d o u b l y l a b e l e d (13C=13C). T h e e t h e n e f o r m e d b y r e a c t i o n s inv o l v i n g b o t h m e t h a n o l a n d t h e u n l a b e l e d a l k e n e w o u l d be e i t h e r s i n g l y l a b e l e d (13C=12C) or u n l a b e l e d (12C=12C). T h u s , a m e a s u r e of t h e (13 C = 12C)/( 13C = ~3C) r a t i o w o u l d p r o v i d e a n i n d i c a t i o n of t h e e x t e n t of e t h e n e f o r m e d d i r e c t l y f r o m t h e l a b e l e d m e t h a n o l . T h e r a t i o of t h e s e t w o i s o t o p i c e t h e n e s , (13 C = 12C)/(13 C _- 13C) ' is v e r y n e a r to t h e o n e e x p e c t e d for a s t a t i s t i c a l d i s t r i b u t i o n ; e v e n t h e c o m p o s i t i o n of all t h r e e i s o t o p i c a l l y l a b e l e d e t h e n e s is close to a s t a t i s t i c a l d i s t r i b u t i o n . E t h e n e is t h e l a s t c a r b o n n u m b e r g r o u p to a t t a i n i s o t o p e e q u i l i b r a t i o n [4]. T h e r e f o r e , t h e i s o t o p i c t r a c e r s t u d i e s to d a t e
* To whom correspondence should be addressed.
0378-3820/93/$06.00
~; 1993 Elsevier Science Publishers B.V.
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L.-M. Tau and B.H. Davis/Fuel Processing Technol. 33 (1993) 1 12
have not provided data t h a t is a p p r o p r i a t e for defining the r e a c t i o n p a t h w a y s in the M T G reaction. One of the problems e n c o u n t e r e d with this i n t e r m e d i a t e pore zeolite is a kinetic disguise because the e q u i l i b r a t i n g r e a c t i o n s within a zeolite crystal is more rapid t h a n the diffusion of p r i m a r y products to the e x t e r i o r of the particle [5, 6]. M a n y of the early studies h a v e been carried out with low Si/A1 ( ~ 60) ratio. Since each f r a m e w o r k A1 provides an acid site, the low Si/A1 zeolite c a t a l y s t has an acid site density t h a t n e a r l y ensures t h a t kinetic disguise will d o m i n a t e the results of the study. Our earlier studies with alcohol mixtures was carried out using a low Si/A1 r a t i o ZSM-5 sample and the isotopic distribution indicated t h a t the C~ products had a t t a i n e d a statistical isotope distribution [4, 7]. A silicate sample obtained from U n i o n Carbide has a m u c h h i g h e r Si/A1 (~800) r a t i o t h a n the one we previously utilized for isotopic t r a c e r studies. This material thus has a s t r u c t u r e e q u i v a l e n t to the low Si/A1 ZSM-5 sample but has a m u c h lower acid site density. It was t h e r e f o r e of i n t e r e s t to learn w h a t the isotope distribution within the c a r b o n n u m b e r products from the c o n v e r s i o n of alcohol mixtures would be for this high Si/A1 material.
EXPERIMENTAL Catalysts
Two catalysts were used in this study. A low Si/A1 ratio of a b o u t 60 was utilized as a sample r e p r e s e n t a t i v e of a ZSM-5 type c a t a l y s t with a high acid site density; this sample was provided by Mobil Oil. The m a t e r i a l r e p r e s e n t a t ive of a low acid site density c a t a l y s t was a sample of Silicalite S-115 with a Si/A1 ratio of a b o u t 800 and was provided by U n i o n Carbide.
Reagents
Unlabeled r e a g e n t grade alcohols and r e s e a r c h grade gases were used without f u r t h e r purification. Radioactive alcohols were p u r c h a s e d from New E n g l a n d N u c l e a r or were synthesized in-house. The chemical p u r i t y of the labeled alcohol was g r e a t e r t h a n 98%; the r a d i o a c t i v e p u r i t y was also in this range.
Procedure
The r e a c t i o n system and a n a l y t i c a l t e c h n i q u e s have been described previously [4, 7 9]. Briefly, the c a t a l y s t (1 g zeolite diluted with l g of low area alpha-alumina) is held at the midsection of a plug-flow reactor; glass beads above the c a t a l y s t bed serve as a p r e h e a t e r section. The c a t a l y s t was calcined in situ in flowing air at 530 °C for at least 4 h o u r s prior to its use, and t h e n
L.-M. Tau and B.H. Davis/Fuel Processing Technol. 33 (1993) 1 12
3
flushed with n i t r o g e n as the c a t a l y s t t e m p e r a t u r e was d e c r e a s e d to the reaction t e m p e r a t u r e . T h e r e a c t i o n t e m p e r a t u r e was 300 °C and the p r e s s u r e was 1 atm. Liquid p r o d u c t s w e r e added using a s y r i n g e pump. U n l a b e l e d gases were added t h r o u g h a mass-flow r e g u l a t o r . T h e c a t a l y s t was c o n t a c t e d w i t h the r e a c t a n t s for 10 15 m i n u t e s to e s t a b l i s h a " s t e a d y - s t a t e " condition; the products formed d u r i n g this period w e r e discarded. P r o d u c t s w e r e t h e n collected d u r i n g the n e x t 30 40 m i n u t e s of r e a c t i o n . Gas and liquid p r o d u c t s were a n a l y z e d using an a p p r o p r i a t e gas c h r o m a t o g r a p h i c (GC) column; the effluent from the GC passed t h r o u g h a gas p r o p o r t i o n a l c o u n t e r to o b t a i n a m e a s u r e of the r e l a t i v e r a d i o a c t i v i t y of the effluent peaks.
RESULTS B e c a u s e the d a t a from the t r a c e r studies in complex r e a c t i o n p a t h w a y s are f r e q u e n t l y difficult to define by a n a l y t i c a l equations, we consider in the following d e s c r i p t i o n the e x t r e m e cases. T h e r e l a t i v e r a d i o a c t i v i t y / m o l e v e r s u s carbon n u m b e r c u r v e s for four possible r e a c t i o n p a t h w a y s are i l l u s t r a t e d in Fig. 1. F o r a p a t h w a y w i t h labeled m e t h a n o l t h a t forms h y d r o c a r b o n s only from C1 and with the r e l a t i v e r a d i o a c t i v i t y per mole of m e t h a n o l t a k e n as C1 = 1, a line w i t h a slope of I is o b t a i n e d (14C1 only, Fig. 1). If the p a t h w a y i n v o l v e s only the alkylation of the unlabeled C3 species derived from 1-propanol with a 14C~ species derived from m e t h a n o l , the r a d i o a c t i v i t y per mole defines a line w i t h a slope of 1 but with the r e l a t i v e a c t i v i t y of the C4 g r o u p i n g b e i n g I (14C~ a l k y l a t i o n , Fig. 1) and the C1 C3 p r o d u c t s h a v i n g no
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1
2
3
4
5
6
7
Carbon Number
Fig. 1. Schematic of the relative radioactivity expected for four reaction pathways for producing hydrocarbons from: (a)I~CH3OH only, (b) 14CH3OH alkylation of C3H6, (c) CH3OH alkylation of ~C3H 6, and (d) equilibration of the 1~C isotope with the unlabeled carbons of methanol and propene.
4
L.-M. Tau and B.H. Davis~Fuel Processing Technol. 33 (1993) 1 12
radioactivity. F o r a p a t h w a y involving only a l k y l a t i o n of 14C3 labeled species derived from ~4C labeled l-propanol by an unlabeled C~ species, a line parallel to the X-axis with the relative activity of 1 for all c a r b o n n u m b e r groupings of C ; is obtained ( a l k y l a t i o n of ~4C3, Fig. 1). If all c a r b o n n u m b e r groupings result from the e q u i l i b r a t i o n of the isotopic species with the c a r b o n of the o t h e r compound, the activity of each c a r b o n will become 0.2, and the line defining the activity of the c a r b o n n u m b e r groupings has a slope of 0.2 (equilibration, Fig. 1).
Low Si/Al ZSM-5
It is observed t h a t the C3 C6 products from the c o n v e r s i o n of the propanolm e t h a n o l m i x t u r e with the low Si/A1 c a t a l y s t are essentially the ones expected for a chemical equilibrium distribution (Table 1). Thus, with the ZSM-5 (Si/A1 _= 60) catalyst, the products had essentially a statistical isotope distribution (Fig. 2) and an equilibrium chemical composition. The data in Fig. 2 are typical of our results using the h i g h e r acid site density H-ZSM-5 material (see ref. [4] for details). The results in Fig. 2 are for the c o n v e r s i o n of m e t h a n o l and propanol (2 moles m e t h a n o l / m o l e propanol): in one r u n the labeled molecule was 14CH3OH and in the o t h e r r u n it was [1-14C] l-propanol. Similar results were obtained for both runs. The r a d i o a c t i v i t y per mole of the C~ products increased with c a r b o n n u m b e r so t h a t the activity per c a r b o n atom was c o n s t a n t for all c a r b o n n u m b e r compounds. These d a t a indicate t h a t the 14C initially present in e i t h e r of the two alcohols equilibrated with the carbon(s) added in the o t h e r alcohol for all of the C~ products. Similar results for isotope distribution were obtained for products produced with the Si/A1 ~ 60 zeolite for t e m p e r a t u r e s of 180, 200 and 300 cC; the major difference noted for this t e m p e r a t u r e r a n g e was t h a t the a m o u n t of a r o m a t i c s in the h y d r o c a r b o n fraction increased with increasing t e m p e r a t u r e [4]. However, alkanes, alkenes and a r o m a t i c s showed a similar p a t t e r n for the r e l a t i v e radioisotope activity with c a r b o n n u m b e r for the entire t e m p e r a t u r e range. Using this ZSM-5 c a t a l y s t with a crystal size in the 2 5 gm range, we h a v e only
TABLE 1 Product distribution from the conversion of alcohol mixtures with ZSM-5 type catalysts at LHSV=9.7 cm3/h, 300 °C and 1 atm Reactant
PrOH/MeOH PrOH/MeOH PrOH Equilibrium distribution
Catalyst
HZSM-5 $115 $115
Product (wt.%) C3
C4
C5
C6
18.1 71.6 82.7 15.5
36 14.4 4.25 37.8
30.6 6.2 5.7 34
15.3 7.8 7.4 12.4
L.-M. Tau and B.H. Davis/Fuel Processing Technol. 33 (1993) 1 12
O
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3.21 2.8
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2.4 2.0 1.6
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1.2 o 0.8
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0.4
AROMATIC --> I M~OH larOH I C3
I
I
I
C4
Cs
C6
I
C7
I
I
Ce
C7
I
Cs
Carbon Number (or Compound)
Fig. 2. Relative radioactivity per mole in the products from the conversion of ~CH3OH and propanol (Q) or CH3OH and I~C labeled 1-propanol (m) with an H-ZSM-5 catalyst at 300'C (activity of the products was compared to that of methanol (©) or 1-propanol (_), respectively, which were taken as 1.0). been able to obtain products in which the 14C has equilibrated among the carbons added in the labeled and the unlabeled reactants (equilibration line, Fig. 1) [4].
Silicalite catalysts
Mixtures of methanol and l-propanol (2 moles methanol per mole propanol) were converted with the silicalite catalyst at 300 °C and 9.7 cm3/h flow rate. Under these conditions methanol conversion, not including that which formed dimethyl ether, was only about 30%. l-Propanol, on the other hand, underwent complete conversion; neither l-propanol nor the ethers methyl-l-propyl nor di-l-propyl were detected in the reaction products. For a lower temperature (e.g., 180~'C) reaction condition, the alcohol mixture did contain the mixed ether (methyl-l-propyl ether) but did not contain the ether of only the higher alcohol (di-l-propyl ether). Methane was present in such a low concentration th at it could barely be detected by GC. Propene was the major hydrocarbon product. The hydrocarbon products were estimated, from identification of the components of the GC trace, to be approximately 85 90% alkenes. The C4 C6 carbon number product distributions are presented in Table 1 and the C2-C9 product distributions, including or omitting C3 products, are shown in Fig. 3. The relative molar radioactivities of the products from the conversion of l-propanol and ~4CH3OH (2 mole methanol per mole l-propanol) are shown in Fig. 4. The C4 product grouping has a relative radioactivity that is about 0.87 higher than the C3 grouping, indicating that alkylation is responsible for the formation of 87% of the C4 product group. The relative molar radioactivity of the C3 and C4 products is believed to provide a reliable measure of the contribution of C~ alkylation to the total 14C isotope distribution. This belief is also supported by a comparison of the relative molar radioactivity in the
L.-M. Tau and B.H. Davis~Fuel Processing Technol. 33 (1993) 1 12
6 80 70 60 5O ,,C
"6
40 30 2O 10 0
i
C2
C3
C4
C5
C6
C7
C8
C9
C8
C9
Carbon Number 50
40
3o
'~
2o
lo
C4
05
Carbon
C6
C7
Number
Fig. 3. The product distribution for the conversion of methanol and 14C labeled l-propanol at 300C with the high Si/A1 H-ZSM-5 catalyst including (top) and excluding (bottom) the C3 product grouping.
C6 and C~ fractions. If 14C labeled methanol added to the C 6 compounds having a relative molar radioactivity of about 0.65 as indicated in Fig. 4, the C7 fraction should have a relative radioactivity per mole of 1.65. The experimentally determined molar radioactivity for the C7 fraction is about 1.2 so that the activities of the C~, and C7 carbon number fractions are consistent with about 55% C1 alkylation. The molar radioactivity of the C4, C5 and C7 products define a reasonably good straight line with a slope of about 0.1 (relative molar radioactivity of methanol = 1). The radioactivity per mole of the C3 products is very low (0.01 or less) and that of the C6 products is much lower than expected for the relative activity of the C4, C5 and C~ products. The low molar radioactivity for the C6 products confirms that dimerization of propene is a significant reaction under these reaction conditions. The per- mole radioactivity of the products is constrained by three limits. If the higher carbon number compounds arise only from propene oligomerization-cracking reactions, a line defined by the molar radioactivity with increasing carbon number would be the positive X-axis with a slope of zero. For another extreme, the 14C distribution could correspond to complete isotope equilibration. For carbon isotope equilibration, 2 moles of
L.-M. Tau and B.H. Davis/Fuel Processing Technol. 33 (1993) 1 12
7
1.4 0
*d
1.2 1.0
0
0.8 n"
/
Ol
0.6 n"
~ " /
J
0.4
0.2
/
3
4
I
5
I
6
I
7
Carbon Number
Fig. 4. Relative radioactivity per mole in the products from the conversion of '4CH30H and 1-propanol (molar ratio methanol/propanol = 2) at 300 ~C with a silicalite S-115 catalyst.
methanol, with a relative per-mole radioactivity for methanol of 1, and 1 mole of propanol, with 3 moles of unlabeled carbon, will become equivalent so that each mole of carbon will have a relative radioactivity of 2/5 =0.4. Thus, for carbon equilibration, the relative radioactivities per mole with increasing carbon number will define a straight line with a slope of 0.4. The upper limit defined by forming labeled products only from I~CH3OH is not considered; for this to occur the relative radioactivity per mole of the C3 products must be three times that of methanol, and the data in Fig. 4 are clearly inconsistent with this. Likewise, the upper limit of C1 alkylation of unlabeled propene to give a slope of 1 was not considered for the C4, C5 and C7 products. The extent of alkylation by C1 for the first two limits is then the ratio of the slope, 0.133, of the line defined by the C~, C5 and C7 products and the slope (0.4) of the line expected for isotope equilibration; i.e., 0.133/0.4 = 0.33 showing that 33% of the products result from alkylation by 14C labeled C1 species. This conclusion is, at first glance, not consistent with the relative activities of C3 and C4. The C4 product grouping has a relative molar radioactivity that is about 0.87 higher th an the C3 grouping, indicating that alkylation is responsible for the formation of at least 87% of the C4 product. However, the slope for isotope equilibration was based upon complete conversion of the methanol, whereas the actual conversion was only about 30%. If we take this adjusted value for the contribution of methanol (2 x 0.3 = 0.6), the radioactivity per mole of carbon at isotopic equilibrium becomes 0.6/3.6=0.167 rat her than 0.4. Repeating the
8
L.-M. Tau and B.H. Davis~Fuel Processing Technol. 33 (1993) 1-12
above calculation on the basis of 30% methanol conversion increases the extent of C1 incorporation to 80%. The actual value will fall, therefore, between 33 and 80% since the data in Fig. 3 indicate the incorporation of the C3 fraction has not attained the chemical equilibrium distribution. In order to utilize the slope to define exactly the extent of the C1 incorporation one would have to obtain a series of lines at various methanol and C3 conversion levels or to utilize another reactor design such as a continuous stirred tank reactor. The relative molar radioactivities of the products from the conversion of [l-14C]-l-propanol and methanol (propanol:methanol mole ratio 1:2) are shown in Fig. 5. It is apparent that the C3 and C~ products have a higher molar radioactivity than they would if they fit the line defined by the C4, C5 and C7 products. The radioactivity per mole of the C2 products is lower t han anticipated from the line defined by the C~, C5 and C7 products. There may be questions concerning whether the deviation of the molar radioactivity data for the C2, C3 and C6 products deviate from the line because of the reaction pathways or whether it is due to experimental uncertainty. The deviation of the data points for the oligomerization-cracking of propene from the line as shown in Figs. 2 and 6 are representative of the experimental u n c e r t a i n t y for our experiments. Without question, the deviation of the C2, C3 and C6 molar radioactivity from the line is due to the reaction mechanism, and not to experimental errors. The relative radioactivities on a molar basis (relative to propanol -- 3.0) of the C3 and C4 carbon number groupings (Fig. 5) are 2.99 and 3.50, respectively. If all of the C4 had been formed by alkylation of C3 by the unlabeled C~ species, the activity of the C~ would have been the same as C3 (2.99); if the C4 had
0
6.0
> o O "O 0~ n" O ~>
4.5
3.0
n" 1.5
ol
2
7
Carbon Number
Fig. 5. Relative radioactivity per mole in the products from the conversion of [1-14C]l-propanol and methanol (molar ratio methanol/propanol = 2 at 300 °C with a silicalite S-115 catalyst).
L.-M. Tau and B.H. Davis/Fuel Processing Technol. 33 (1993) 1 12
9
o
"6 ~;
6.0
.~_ ~
4.5
0 r,"
3.C
0 :.~
1.5
0.0
Carbon Number
Fig. 6. Relative radioactivity per mole in the products of the conversion of [2-14C]-propene (m) at 300~C with a silicalite S-115 catalyst.
been derived only from C3 the activity would have been 3.99 (4/3 x C3 activity). Thus, the f r a c t i o n a l increase c o r r e s p o n d i n g to a l k y l a t i o n is ( 3 . 9 9 - 3.50)/(3.99- 2.99) = 0.49. This indicates t h a t 49% of the C4 was formed by a l k y l a t i n g a C3 species derived from propanol with a C1 species derived from m e t h a n o l . A similar c a l c u l a t i o n based on the m o l a r r a d i o a c t i v i t y of the C6 and C7 c a r b o n n u m b e r groupings indicate t h a t 51% of the C7 is formed by alkylation of C6 by an u n l a b e l e d C1 species. In this experiment, the r e l a t i v e m o l a r r a d i o a c t i v i t y of the products must fall b e t w e e n two limits. The upper limit will be defined by a s t r a i g h t line with a slope t h a t depends upon the r e l a t i v e molar r a d i o a c t i v i t y assigned to the labeled propene; when propene has a r e l a t i v e molar r a d i o a c t i v i t y of 3.0 as s h o w n in Fig. 4 the slope is expected to be 1.0. This will o c c u r when the 14C label is r e d i s t r i b u t e d only by oligomerization and c r a c k i n g as was the case for the c o n v e r s i o n of 14C labeled propene alone (Fig. 6). A good m e a s u r e of the e x p e r i m e n t a l e r r o r is shown in Figs. 2 and 6 where the data provide a good fit to a s t r a i g h t line for all of the c a r b o n n u m b e r groupings. The d a t a in Figs. 2 and 6 indicate t h a t the deviations of the molar r a d i o a c t i v i t y for the C3 and C6 products from the s t r a i g h t line in Fig. 4 are real. The lower limit will be defined by e i t h e r a line with slope of 0 or 0.2. The zero slope will apply w h e n all of the C~ products result from the addition of u n l a b e l e d C1 species to the laC labeled C3 species ( a l k y l a t i o n of 14C3, Fig. 1). If t h e r e is complete isotope equilibration, the slope will be 0.2 since 2 moles of m e t h a n o l provide two u n l a b e l e d carbons while one mole of [1-14C]-1-propanol provide, on average, two u n l a b e l e d carbons and one labeled c a r b o n (equilibration, Fig. 1). The r e l a t i v e a c t i v i t y / c a r b o n will thus be 0.2 so t h a t each increase in c a r b o n n u m b e r will result in adding a r e l a t i v e activity of 0.2 to provide a line with a slope of 0.2. The slope of the line defined by the C4, C5 and C7 products is 1.16. The s c h e m a t i c shown in F i g u r e 1 as well as the two c o n v e r s i o n p a t h w a y s discussed above does not provide a p a t h w a y t h a t would produce a slope g r e a t e r t h a n 1.0. The r e a s o n for the slope being g r e a t e r t h a n 1.0 in this p a r t i c u l a r case
10
L.-M. Tau and B.H. Davis~Fuel Processing Technol. 33 (1993) 1 12
is the activity of the propene in the products has been assigned a relative activity of 3.0 rather than the reactant, l-propanol, which has undergone complete conversion and could, therefore, not be used. As shown in Fig. 3 the lower carbon compounds (C3 and C4) are the dominant components of the product; if the comparison is made on a mole basis the dominance of the C3 and C4 products is even greater. Therefore, alkylation by the unlabeled C~ fragment occurs to decrease the label of the lower carbon number products to a greater extent than the higher carbon compounds; the C6 products are initially derived predominantly by oligomerization of labeled propene formed from 14C labeled propanol. Furthermore, in a plug flow reactor the initial C6 formed by oligimerization will be derived from a C3 that has a higher amount of 14C per mole than the C6 formed at the exit of the reactor; this will cause the C6 to have a higher amount of 14C per mole than the C4 product. Ethene is of special interest. The product distribution for the run with unlabeled methanol and ~4C labeled propanol is shown in Fig. 3. Neglecting the C3 products, ethene accounts for only 11 mol% (4.6 wt.%) of the C2-C9 products while the C4 products account for 44 mol% of these products. The Cs and C~ fractions appear high compared to the C7 fraction; however, the greater Cs and C9 fractions are a result of the aromatics that are formed. In the run with t4C labeled methanol the amount of 14C labeled ethene is higher than the level of label expected for carbon equivalency (Fig. 4). On the other hand, if ethene was formed only from ~4C labeled methanol the relative molar radioactivity of ethene should have been 2 rather than the 1.05 that is observed; this indicates that about 53% of the ethene is formed directly from C~ species derived from 14C labeled methanol. This estimate is actually high since ethene formed from oligomerization and cracking will also contain 14C. Assuming that the radioactivity per mole of ethene formed during the oligomerization-cracking conversions fits the line defined by the C4 and C5 products, it is calculated that only 30% of the ethene is formed directly from two C1 species. The ethene formed during the conversion of unlabeled methanol and ~4C labeled propanol has a lower radioactivity/mole than expected if it arose only from oligomerization and cracking reactions. Taking the deviation of the radioactivity/mole of the line defined by the C4, C5 and C~ products (0.41) divided by the radioactivity per mole required for C2 to fall on this line (1.16) indicates that 35% of the ethene is formed directly from methanol. Thus the results of the two runs are consistent and indicate that at about 30% methanol conversion to hydrocarbons in the presence of propanol (and propene), only about 30% of the ethene is formed directly from two C~ species derived from methanol.
DISCUSSION The current results permit us to better define the reaction pathways for alcohol conversion with the ZSM-5 catalyst. Whereas the sample with a low Si/A1 ratio led to nearly a statistical 14C distribution in the hydrocarbon
L.-M. Tau and B.H. Davis/Fuel Processing Technol. 33 (1993) 1 12
11
c a r b o n n u m b e r groupings, this was not the case with the h i g h e r Si/A1 zeolite catalyst. In the present study we have grouped the h y d r o c a r b o n products a c c o r d i n g to c a r b o n number. We believe t h a t this is e n t i r e l y justified since, in those c a r b o n n u m b e r groups for C2 C5 where we get s e p a r a t i o n of n e a r l y all a l k e n e and a l k a n e isomers, all compounds within a c a r b o n n u m b e r group has essentially the same specific m o l a r radioactivity. The p r o p o r t i o n a l c o u n t e r can only be utilized in series with a packed GC column with the isotope levels utilized in this study. A C a r b o w a x 20M GC c o l u m n permitted s e p a r a t i o n s to show t h a t individual compounds in the a r o m a t i c s also had the same specific molar r a d i o a c t i v i t y as the c o r r e s p o n d i n g carbon n u m b e r grouping of aliphatic compounds. In addition it appeared that, where s e p a r a t i o n was possible, each c o m p o u n d had the same specific activity within a class in the C3 h y d r o c a r b o n c a r b o n n u m b e r fractions. The experimental data are plotted in Fig. 7 t o g e t h e r with the line representing isotopic c a r b o n equilibration. It is n e c e s s a r y to make the r e l a t i v e molar r a d i o a c t i v i t y of one of the c a r b o n n u m b e r grouping the same since absolute m o l a r r a d i o a c t i v i t i e s were not obtained; for this plot the r a d i o a c t i v i t i e s per mole of the C4 groupings are equated. The d a t a for the C4, C5 and C~ groupings are a p p r o a c h i n g the line for c a r b o n isotope equilibration. Because the isotope is present in the C1 species in one r u n and the C3 species in the o t h e r case, the isotope d i s t r i b u t i o n in the two runs will differ even t h o u g h the chemical composition by c a r b o n n u m b e r groupings may (and should) be the same. Considering the d a t a for the two runs with m e t h a n o l and propanol with the high Si/A1 ratio c a t a l y s t and the slope of the line defined by the C4, C5 and C~ groupings, it appears t h a t scrambling c o n t r i b u t e s slightly more to the
/ / /
0
/ / •
/
/
.>
/
/
j-
"0 t~ nO
n" .- J - y ~
•
,
,
~
1
2
3
~
~'
~
-~
Carbon Number
Fig. 7. Comparison of the ~4C distributions from the conversion of methanol (©) and propanol (•). The relative radioactivities of the C4 carbon number grouping set equal for the two runs. The solid line defines the data expected for carbon equilibration.
12
L.-M. Tau and B.H. Davis~Fuel Processing Technol. 33 (1993) 1 12
14C d i s t r i b u t i o n t h a n C 1 a l k y l a t i o n for the C4, Cs and C7 c a r b o n n u m b e r groupings. F o r the c o n v e r s i o n of u n l a b e l e d p r o p a n o l and 14C labeled m e t h a n o l , the d a t a for the r e l a t i v e a c t i v i t y in the C3 and C4 f r a c t i o n s as well as t h a t in the C6 and C~ f r a c t i o n s i n d i c a t e t h a t C~ a l k y l a t i o n s are r e s p o n s i b l e for the formation of 87 and 55%, r e s p e c t i v e l y , of the C4 and C7 fractions. Similarly, for the c o n v e r s i o n of 14C labeled p r o p e n e and u n l a b e l e d m e t h a n o l , C1 a l k y l a t i o n a c c o u n t s for 49 and 51% of the C4 and C7 c a r b o n n u m b e r groupings, respectively. T h e e t h e n e labeling c l e a r l y d e m o n s t r a t e t h a t e t h e n e is a p r i m a r y p r o d u c t d u r i n g the c o n v e r s i o n of m e t h a n o l in the p r e s e n c e of significant c o n c e n t r a tions of C~ alkenes. U n d e r the c o n v e r s i o n c o n d i t i o n s utilized in this study only a b o u t 30% of the e t h e n e r e s u l t e d d i r e c t l y from the c o m b i n a t i o n of two C~ species derived from m e t h a n o l . H o w e v e r , the f r a c t i o n of m e t h a n o l being c o n v e r t e d directly to e t h e n e is small c o m p a r e d to the t o t a l c o n v e r s i o n of methanol. The d a t a also show t h a t the use of the h i g h Si/A1 ZSM-5 zeolite with a l o w e r acid site d e n s i t y allows d a t a to be o b t a i n e d t h a t r e p r e s e n t the k i n e t i c r e g i o n a l t h o u g h t h e r e a p p e a r s to be some " k i n e t i c disguise" due to s e c o n d a r y reactions o c c u r r i n g in the c h a n n e l s of the zeolite crystals. The d a t a show t h a t the r e a c t i o n p a t h w a y for h y d r o c a r b o n s y n t h e s i s from m e t h a n o l in the p r e s e n c e of a p p r e c i a b l e C~ a l k e n e s is p r e d o m i n a n t l y one t h a t i n v o l v e s CI a l k y l a t i o n of C~ alkenes, and not t h r o u g h the f o r m a t i o n of Cz c a r b o n n u m b e r species from m e t h a n o l . S u p e r i m p o s e d u p o n the C~ alkylation p a t h w a y is one t h a t i n v o l v e s o l i g o m e r i z a t i o n and c r a c k i n g r e a c t i o n s t h a t serve to r e d i s t r i b u t e isotope label and to m o v e the c a r b o n n u m b e r g r o u p i n g t o w a r d the t h e r m o d y n a m i c e q u i l i b r i u m v a l u e and s t a t i s t i c a l isotope distribution. The r e l a t i v e f r a c t i o n of e a c h c a r b o n n u m b e r g r o u p i n g t h a t arises for o l i g o m e r i z a t i o n and c r a c k i n g r e a c t i o n s will depend u p o n the degree of methanol c o n v e r s i o n and the mole f r a c t i o n s of a l k e n e s of all of the c a r b o n n u m b e r groupings.
REFERENCES 1 2 3 4 5 6
Chang, C.D. and Silvestri, A.J., 1977. J. Catal., 47: 249. Dessau, R.M. and LaPierre, R.B., 1982. J. Catal., 78: 136. Mole, T., 1983. J. Catal., 84: 423. Tau, L.M., Fort, A.W., Bao, S.Q. and Davis, B.H., 1990. Fuel Processing Technol., 26: 209. Olson, D.H. and Haag, W.O., 1984. ACS Symp. Ser., 248: 275. Haag, W.O.. 1984. In: B.L. Shapiro (Ed.), Heterogeneous Catalysis. Texas A&M University Press, College Station, TX, pp. 95 120. 7 Tau, L.M., Fort, A.W. and Davis, B.H., 1986. In: Y. Murakami, A. Iijima and J.J. Ward (Eds.) Proc. 7th Int. Zeolite Conference. Tokyo, p. 899. 8 Bao, S.Q., Tau, L.M. and Davis, B.H., 1988. J. Catal., 111: 436. 9 Tau, L.M., Bao, S.Q. and Davis, B.H., 1988 J. Catal., 114: 190.