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Hydrogenation of ethyne over an ion-exchanged copper on silica catalyst
Applied Catalysis, 58 (1990) 209-217
209
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
H y d r o g e n a t i o n of E t h y n e over an I o n - E x c h a n g e d Copper on Silica Catalyst M.R. STAMMBACH, D.J. THOMAS, D.L. TRIMM* and M.S. WAINWRIGHT
School of Chemical Engineering and Industrial Chemistry, The University o[ New South Wales, P.O. Box 1, Kensington, N.S. W. 2033 (Australia) (Received 26 June 1989, revised manuscript received 12 October 1989)
ABSTRACT The selective hydrogenation of ethyne (acetylene) has been studied over an ion-exchanged copper on silica catalyst in a tubular flow reactor. Three different regimes could be detected as time on line increased. Initially, the catalytic activity was high and ethene, ethane and foulant were produced. At longer times, ethane production ceased and ethene production changed as the amount of foulant on the catalyst increased. Finally a near steady-state catalyst was produced in which ethyne was converted to ethene and foulant. The kinetics of the reactions have been measured and the results explained in terms of chemical reaction rate control of the production of ethene, ethane and foulant on a fresh catalyst. As the amount of foulant builds up, ethene production is controlled by the diffusion of hydrogen in the foulant film but foulant production is still controlled by chemical kinetics. Finally both ethene and foulant are controlled by the diffusion of hydrogen in the foulant film.
INTRODUCTION
The production of olefins by steam cracking is an important industrial process. Light naphtha or ethane is pyrolysed in the presence of steam to produce light olefins and a range of by-products amongst which are ca. 370 of several C2, C3 and C4 alkynes. These are particularly unwelcome in that they poison many of the catalysts used for the down-stream processing of the olefins [ 1,2 ]. Alkynes may be removed from the olefin stream by hydrogenation. The main requirement is to remove alkynes, with selective hydrogenation to olefins rather than complete hydrogenation to alkanes or formation of polymeric green oil being preferred. Supported palladium catalysts are used widely [3-5 ], but it is necessary sometimes partially to poison the catalyst in order to avoid overhydrogenation. Supported copper is used selectively to hydrogenate C4 alkynes (1-butyne and 1-buty-3-ene) [2,3] and it has been recommended as a selective hydrogenation catalyst for alkyne removal in general [2,3]. Previous studies have focused on the use of copper-based catalysts for the 0166-9834/90/$03.50
© 1990 Elsevier Science Publishers B.V.
210
selective hydrogenation of C3 and C4 alkynes [2,3,6 ]. As part of a more detailed study of the role of copper in the reaction, the present investigation has concentrated on the selective hydrogenation of ethyne on copper supported on silica. This catalyst has been reported to show the highest selectivity to ethene as a result of hydrogenation [ 7 ]. EXPERIMENTAL
Preparation of the Cu/SiO2 has been described previously [8]. The copper content was obtained by X-ray fluorescence analysis. Copper surface area was found to be 4.1 +0.4 m 2 g-1 by nitrous oxide decomposition at 363 K by the reactive frontal chromatography method [9]. Total surface area was determined by nitrogen adsorption (BET) as 235 m 2 g-1 (fresh catalyst) and 178 m 2 g-1 (spent catalyst: 200 min on line). Ethyne hydrogenation was studied using a conventional flow apparatus. Hydrogen, nitrogen and a mixture of ethyne in nitrogen (4.64 vol.-% ) were fed through mass flow controllers (Brooks model 5878) into a temperature controlled ( + 1 K) reactor containing up to 1 g of catalyst. The exit gases were injected into a gas chromatograph were they were separated on a 1/8 inch Poropak N column (2 m length, 303 K, 20 ml N2 min -1 ) and detected using a flame ionisation detector. This system allowed the separation of ethyne, ethene and ethane. In some cases, the exit gases were taken through a trap containing chloroform, in order to obtain samples of "green oil" for analysis. Analysis involved GC/MS, the gases being separated on an OV-1 column (33 mX0.5 mm: temperature programmed from 353 to 548 K at 5 K min -1) and analysed in an AEI MS12 mass spectrometer (70 eV ionisation voltage: 8000 V accelerating voltage). Calcined catalyst [8] was crushed and sieved (420-246 pm) and placed in the reactor between glass wool plugs. The reactor was mounted in a stirred air oven and the catalyst reduced in a stream of 20 ml min -1 (273 K, 101.3 kPa) hydrogen plus 20 ml min-1 nitrogen, the temperature being ramped at 10 K m i n - 1from ambient to 523 K. After at least one hour at 523 K, the temperature was reduced to the desired value and, after adjusting flows, the reacting gases were introduced into the system. Reaction conditions were chosen on the basis of a full 2 × 2 design, with catalyst weight varying between 1 and 0.2 g, ethyne between 2.4 and 0.4 vol.-% and hydrogen between 25 and 5 vol.-%. Temperatures were controlled at 403 K and 373 K. Total flow was always maintained at 200 ml min-1 and the hydrogen-toethyne ratio was generally kept at 10.8 vol/vol. The total pressure was 105 kPa and the pressure drop across the bed was generally less than 8 kPa. A mass balance for C2 gases across the empty reactor and the bypass showed an error of less than 0.5% with a maximal base line drift of 2% over 6 h.
211 Some of th e used catalysts were examined by t h e r m o g r a v i m e t r i c analysis (TGA, D u p o n t 99 Series T h e r m a l Analyser) in air or nitrogen. T h e temperature was r amp ed from 323 K to 1123 K at 5 K / m i n and held at 1123 K for 30 min. For convenience, the t e r m foulant is used for all products which contain more t h a n two carbons. Results are expressed in terms of conversion x = p r o d u c t o u t / e t h y n e in. Conversion for C2 gases refers to e t h y n e plus e t h e n e plus e t h a n e conversion. Conversion to foulant was defined as (1 - conversion to C2 gases ). RESULTS A preliminary run was carried out with the silica support in t he reactor to show t h a t no conversion of e t h y n e could be observed in t he absence of copper u n d er the reaction conditions. A run was also carried out using ca. 3% et hyne
10
0
k
J
04r gi
~1 ZoneII
0
40
I
Zone Ill
ao
120
"---"*"-~
160
200
240
Time on line (min)
Fig. 1. Conversion of ethyne to various products vs. time on line. Reaction conditions: T = 403 K, wt. of catalyst = 0.4 g, flow-rate= 200 ml min- 1 (STP ), ethyne = 2.32 vol.-%, hydrogen= 25 vol.%. (F]): C2H2total conversion ( A ) C2H4total conversion, ( + ) foulant, (x) C2Hs. TABLE1 Reaction orders for hydrogenation and for deactivation Reaction/Zone
order
I
C2H2--, C2H4
in H2 in C2H 2
1.2±0.2 -0.5±0.5
C2H2--*foulant
in H 2 in C2H2
1.0±0.1 0.1±0.1
C2H2-~C 2 H4 C2H2-~foulant
deactivation deactivation
1 -1
II
2 1
III
>5 >5
212 TABLE 2 Apparent activation energies for hydrogenation and for deactivation Reaction
Energy (kJ/mol)
C2H2-, C2H4 C2H2--~foulant
Ereaction
C2H2--~foulant
Edeactivation
Ereaction
Zone I
II
III
72.9±10.7 29.6± 5.8
63.1±3.6 38.7±1.6
62.5±5.3 45.0±3.2
20.4±2.2
14.6±2.6
1.00
.'•_•
% " . % % % %
o.gs-
A
I
.>
',,,? ----..
I
,wb=#
0.90-
n-
0.85 300
500
76o T (Z)
96o
,'oo
Fig. 2. Temperature-programmed heating of q used catalyst in nitrogen (A) and in air (B).
in ethene rather than nitrogen. No significant difference in the product spectrum could be observed, probably because of the preferred adsorption of ethyne on the catalyst [2,3,6 ]. Typical results obtained in the presence of catalyst are summarised in Fig. 1. The amount of ethyne leaving the reactor was initially small and dropped rapidly (zone I, ca. 0-17 rain on line; Fig. 1 ). After ca. 17 min on line, ethyne increased steadily (zone II, ca. 17-80 rain time on line) until emergent ethyne approached a steady value (zone III, time on line longer than 80 rain). Foulant and ethene paralleled the reverse of these changes (Fig. 1 ), while ethane was mainly produced during the first part of the run. The results summarised in Fig. 1 were shown not to be affected by, for example, flow switching, since such disturbances occurred in much shorter times. The kinetics of ethyne hydrogenation were studied over a steady-state catalyst (Fig. 1, Zone III, time on line greater than 120 min). The reaction to ethene was found to be first order ( _ 0.1 ) with respect to hydrogen and nega-
213
0.5
!
0./, 0z o
E 0.2 E 0.1 0
0
~.0
80 120 Time on line (min)
Fig. 3. Build up of foulant vs. time on line. Run conditions: Run T cat.wt. (K) (g) A B C C
373 373 373 373
1 0.5 1.00 0.5
160
200
flow-rate STP (ml min - 1) 200 200 200 200
ethyne (vol.-%)
H2 (vol.-%)
2.32 2.32 0.464 0.464
25 25 5 5
tive one half order ( + 0 . 5 ) with respect to ethyne (Table 1). The apparent activation order was found to be 63 + 5 kJ/mol (Table 2 ). The ratio of selectivities of ethene to foulant production in zone III was 0.133 + 0.010 at 373 K and 0.081 + 0.005 at 403 K. "Green oil" emerging from the reactor during a hydrogenation run was analysed to show the presence of benzene, ethylbenzene and C6 to Clo even numbered olefins and aromatic compounds. At least some of these can be expected to condense on the catalyst at reaction temperature and to volatilise as the temperature is raised (Fig. 2 ). The used catalyst exhibited a strong unpleasant odour on removal and the particles were stuck together. TGA of spent catalyst was carried out in nitrogen and in air (Fig. 2). Even in nitrogen the catalyst slowly lost weight on heating from 300 to 1100 K. Partial pyrolysis during heating may also occur and the small amount of residual material left on the catalyst after pyrolysis to 1100 K could be carbon produced during the hydrogenation or carbon from pyrolysis of foulant (Fig. 2). In air, a significant increase in loss of weight occurred at about 600 K {Fig. 2), and this was associated with oxidation of the deposit. The weight change resulting from the oxidation of copper was calculated to be negligible in terms of the weight loss by foulant oxidation. The kinetics of hydrogenation were then measured where possible over all three zones (Tables 1 and 2) as were the kinetics of deactivation. The latter
214
were analysed using the approach of Levenspiel [ 10 ] as developed by Mardaleishvili and Rapaport [11 ]. It is clear that the catalyst is deactivating rapidly, apparently as a result of the formation of foulant. The amount of foulant produced as a function of time on line was calculated by integrating the rate curves: typical results are shown in Fig. 3. Comparison of these integral amounts of foulant per catalyst weight with results obtained from TGA (Fig. 2) showed that ca. 95% of all foulant species produced remain on the catalyst under reaction conditions. Deposition of foulant on the catalyst does reduce surface area (from 235 m 2 g-1 to 178 m 2 g-~) but the size of the decrease is more pertinent to surface coating rather than massive pore closure. Burn off of foulant returned the surface area to a higher value (196 m 2 g-~). The initial activity was nearly the same after burn off and the reduced catalyst behaved in the same way as the fresh material. DISCUSSION
The supported copper catalyst is seen to be active and selective for the hydrogenation of ethyne to ethene, but to deactivate readily (Fig. 1 ). This deactivation is found to be due to the accumulation of foulant on the surface of the catalyst. A final near-steady-state catalyst is produced in which ca. 38% of ethyne is converted to 5% ethene and 33% foulant (Fig. 1 ). The exact nature of the foulant is open to question, but 80% of the deposits may be volatilised by heating to 1100 K (Fig. 2). The so-called "green oil" emerging from the reactor was shown to contain benzene, ethylbenzene and C6 to Clo even numbered olefins and aromatic compounds. At the temperature of reaction, only higher hydrocarbons could be expected to remain in the reactor but a clear boiling point cut off cannot be given. The catalyst is active in the initial stage of the reaction (zone I, Fig. 1 ) but is less selective in that ethane is produced. Two causes could contribute to the increasing selectivity with time. Highly active sites could be eliminated by accumulation of foulant and similar results have been reported for other systems (ethyne over palladium [12,13] ). Alternatively, preadsorbed hydrogen remaining from the reduction could be responsible for unselective hydrogenation; this is unlikely as a result of the fact that the amount of ethane produced is more than the amount of (weakly [14] ) adsorbed hydrogen on copper. It is convenient next to consider the near-steady-state behaviour of the catalyst (zone III, Fig. 1), since this facilitates an explanation of catalyst behaviour in zone II. In zone III (Fig. 1) the hydrogenation of ethyne to ethene occurs at a low rate and the accumulation of foulant is also slow. Due to the foulant build up, a carbonaceous film must be on the surface. If all the foulant were permanently deposited evenly all over the catalyst, it is possible to calculate the thickness
215 of the film that would be present. Taking an average value of 120 min on line (Fig. 3), the mass of foulant per mass of catalyst is seen to vary between ca. 0.075 and 0.425. Assuming the foulant is a highly unsaturated liquid with an assumed density of ca. 0.8 g/ml at 373 K, this corresponds to a film thickness of between 0.4 n m and 2.3 n m over the entire catalyst. These thicknesses would be some 50 to 60 times greater if the foulant were concentrated on the copper where it is produced. The actual coverage on the copper will most probably be between these extremes. With these amounts of foulant on the surface, it seemed possible that the kinetics of the reactions could be controlled by diffusion through the foulant film. Representing diffusion by Fick's Law
N= -D~
(1)
where N, the flux of gas, is related to the concentration gradient over the film thickness (x) by the diffusion coefficient, D. It would be expected, for mass transfer control, that rate (conversion achieved at given residence time) would be inversely related to film thickness (expressed as mass of foulant/mass of catalyst). As seen from Fig. 4, this is indeed found to be the case over a wide range of foulant loadings. For ethene production, diffusion control was rate controlling for zones II and III (longer t h a n ca. 17 rain on line) while for foulant production, diffusion control occurred in zone III (longer than ca. 80 min on line). In agreement with this suggestion, both the production of ethene and of foulant were found to be first order in hydrogen in the respective regimes (Table 1 ). Similar orders have been reported by Taghavi et al. [7 ], but it seems 1.0
1 ÷I
÷
°6!
l I
®
O.Z,
0
+/
ZoneII
I
O.2 0
ZoneI
1 I
0
[-------_.
L 0~.
08
i 1.2
i 16
2.0
mcatlm f (glg)
Fig. 4. Conversion of ethyne to foulant and ethene as a function of mass of foulant per mass of catalyst. ( + ) foulant, (~) C2H4;T=403 K, flow-rate= 200 ml rain- 1 (STP), wt. of catalyst = 0.4 g, ethyne = 2.32 vol.-%, hydrogen= 25 vol.-%.
216
unlikely that their suggested Rideal-Eley mechanism can be correct in the presence of so much foulant. If the diffusion of hydrogen is rate-controlling, the small negative orders in ethyne (Table 1 ) are somewhat surprising. These orders, as measured within a single run by changing the pressure of ethyne, were always well defined. However, significant variation was found between runs, accounting for the large errors in the measurement (Table 1). The best explanation of the small negative orders and the large errors would seem to lie in the effect of ethyne on film thickness. An increase in ethyne pressure would be expected to increase foulant production and, as a result, film thickness. Hydrogen then has to diffuse further and, as a result, ethene production becomes slower. The variation between runs then reflects the differences in film thickness when the apparent orders of reaction were measured. It is possible to estimate the onset of diffusion control by comparing predicted and observed rates. Diffusion of hydrogen would be expected to be rate limiting at high foulant levels from the observed reaction order of unity. Back extrapolation of the diffusion controlled rate (Fig. 4) to one theoretical monolayer shows that the predicted diffusion controlled rate is much greater than the observed rate confirming, as expected, that the rate of the chemical reaction is rate controlling when the foulant deposit is low. As the thickness of deposit grows, the diffusion of hydrogen decreases to the point where the predicted diffusion rate equals the observed rate (ca. 17 rain on line for ethene production and 80 min for foulant). The difference in these values reflects the fact that diffusion only controls rate when it is slower than the rate of the chemical reaction. Since the production of foulant is faster than that of ethene, it takes a greater thickness of film (longer time on line ) for this to occur in the former case. It would appear to be possible to estimate the diffusion coefficient of hydrogen in the foulant film, but this is complicated by the fact that the concentration gradient across the film also depends on the (unknown) solubility of the gas in the foulant. Estimates of diffusion/solubility show that ethyne, as expected, is ca. 10 times more concentrated in the film than hydrogen [ 15 ]. Returning to zone II, it is seen that the production of ethene is controlled by the rate of diffusion of hydrogen but that the production of foulant is controlled by the chemical kinetics. Under these circumstances, the increase in thickness of the film, the rate of diffusion of hydrogen and the rate of production of ethene are all dictated by the chemical kinetics of foulant production. Orders of reaction could not be measured, as a result of the rapid changes in activity occurring during zone II (Fig. 1 ), but the apparent activation energies observed are in agreement with this finding (Table 2). Values are high for a hydrogen diffusion controlled reaction, but it must be remembered that the result also reflects hydrogen solubility in the foulant. Thus it would seem that the production of ethene and of foulant on a fresh
217
supported copper catalyst is controlled by the kinetics of the chemical reaction. The fresh surface rapidly becomes covered in a layer of foulant and, when this builds up to an estimated 0.4 nm thickness, the rate of diffusion of hydrogen in the foulant film becomes rate determining. Foulant production, on the other hand, is faster than ethene production and does not become controlled by hydrogen diffusion until about an estimated 1.8 nm of foulant have built up. After this point, a near-steady-state catalyst is produced which converts ca. 38% ethyne to 5% ethene and 33% foulant. ACKNOWLEDGEMENT
The support of the Australian Research Council is gratefully acknowledged.
REFERENCES 1 L.F. Albright (Ed.), Pyrolysis: Theory and Industrial Practice, Academic Press, New York, 1983. 2 F.W. Billmeyer Jr., Textbook of Polymer Science, 2nd Ed., Wiley/Toppan, 1971. 3 R.B.Nesbitt, Feedstock Update, Proc. 19th Annual Meeting of Int. Inst. of Synthetic Rubber Producers, April, 1978. 4 H.C. Hudson, U.S. Patent 4 551 443 (1985). 5 H. Mueller, K. Deller, G. Vollheim and W. Kuehn, Chem. Ing. Tech., 59 (1987) 645. 6 C.L. Thomas, Catalytic Processes and Proven Catalysts, Academic Press, New York, 1970. 7 M.B. Taghavi, G. Pajonk and S.J. Teichner, Bull. Soc. Chim. Fr., 7-9 (1978) 302. 8 M.A.Kohler, J.C. Lee, D.L. Trimm, N.W. Cant and M.S. Wainwright, Appl. Catal., 31 (1987) 309. 9 G.C. Chinchen, C.M. Hay, H.D. Vandervell and K.C. Waugh, J. Catal., 103 (1987) 79. 10 O. Levenspiel, J. Catal., 25 (1972) 265. 11 R.E. Mardaleishvili and Y.I. Rapaport, J. Catal., 95 (1985) 447. 12 A. Sarkany, L. Guczi and A.H. Weiss, Appl. Catal., 10 (1984) 369. 13 A. Sarkany, A.H. Weiss, T. Szilagyi, P. Sandor and L. Guczi, Appl. Catal., 12 (1984) 373. 14 G.C.Bond, Heterogeneous Catalysis: Principles and Applications, Clarendon Press, Oxford, 1974, p. 22. 15 R.H. Perry and C.H. Chilton (Eds.), Chemical Engineer's Handbook, 5th Ed., McGraw-Hill Kogakusha, Tokyo, 1974.
209
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
H y d r o g e n a t i o n of E t h y n e over an I o n - E x c h a n g e d Copper on Silica Catalyst M.R. STAMMBACH, D.J. THOMAS, D.L. TRIMM* and M.S. WAINWRIGHT
School of Chemical Engineering and Industrial Chemistry, The University o[ New South Wales, P.O. Box 1, Kensington, N.S. W. 2033 (Australia) (Received 26 June 1989, revised manuscript received 12 October 1989)
ABSTRACT The selective hydrogenation of ethyne (acetylene) has been studied over an ion-exchanged copper on silica catalyst in a tubular flow reactor. Three different regimes could be detected as time on line increased. Initially, the catalytic activity was high and ethene, ethane and foulant were produced. At longer times, ethane production ceased and ethene production changed as the amount of foulant on the catalyst increased. Finally a near steady-state catalyst was produced in which ethyne was converted to ethene and foulant. The kinetics of the reactions have been measured and the results explained in terms of chemical reaction rate control of the production of ethene, ethane and foulant on a fresh catalyst. As the amount of foulant builds up, ethene production is controlled by the diffusion of hydrogen in the foulant film but foulant production is still controlled by chemical kinetics. Finally both ethene and foulant are controlled by the diffusion of hydrogen in the foulant film.
INTRODUCTION
The production of olefins by steam cracking is an important industrial process. Light naphtha or ethane is pyrolysed in the presence of steam to produce light olefins and a range of by-products amongst which are ca. 370 of several C2, C3 and C4 alkynes. These are particularly unwelcome in that they poison many of the catalysts used for the down-stream processing of the olefins [ 1,2 ]. Alkynes may be removed from the olefin stream by hydrogenation. The main requirement is to remove alkynes, with selective hydrogenation to olefins rather than complete hydrogenation to alkanes or formation of polymeric green oil being preferred. Supported palladium catalysts are used widely [3-5 ], but it is necessary sometimes partially to poison the catalyst in order to avoid overhydrogenation. Supported copper is used selectively to hydrogenate C4 alkynes (1-butyne and 1-buty-3-ene) [2,3] and it has been recommended as a selective hydrogenation catalyst for alkyne removal in general [2,3]. Previous studies have focused on the use of copper-based catalysts for the 0166-9834/90/$03.50
© 1990 Elsevier Science Publishers B.V.
210
selective hydrogenation of C3 and C4 alkynes [2,3,6 ]. As part of a more detailed study of the role of copper in the reaction, the present investigation has concentrated on the selective hydrogenation of ethyne on copper supported on silica. This catalyst has been reported to show the highest selectivity to ethene as a result of hydrogenation [ 7 ]. EXPERIMENTAL
Preparation of the Cu/SiO2 has been described previously [8]. The copper content was obtained by X-ray fluorescence analysis. Copper surface area was found to be 4.1 +0.4 m 2 g-1 by nitrous oxide decomposition at 363 K by the reactive frontal chromatography method [9]. Total surface area was determined by nitrogen adsorption (BET) as 235 m 2 g-1 (fresh catalyst) and 178 m 2 g-1 (spent catalyst: 200 min on line). Ethyne hydrogenation was studied using a conventional flow apparatus. Hydrogen, nitrogen and a mixture of ethyne in nitrogen (4.64 vol.-% ) were fed through mass flow controllers (Brooks model 5878) into a temperature controlled ( + 1 K) reactor containing up to 1 g of catalyst. The exit gases were injected into a gas chromatograph were they were separated on a 1/8 inch Poropak N column (2 m length, 303 K, 20 ml N2 min -1 ) and detected using a flame ionisation detector. This system allowed the separation of ethyne, ethene and ethane. In some cases, the exit gases were taken through a trap containing chloroform, in order to obtain samples of "green oil" for analysis. Analysis involved GC/MS, the gases being separated on an OV-1 column (33 mX0.5 mm: temperature programmed from 353 to 548 K at 5 K min -1) and analysed in an AEI MS12 mass spectrometer (70 eV ionisation voltage: 8000 V accelerating voltage). Calcined catalyst [8] was crushed and sieved (420-246 pm) and placed in the reactor between glass wool plugs. The reactor was mounted in a stirred air oven and the catalyst reduced in a stream of 20 ml min -1 (273 K, 101.3 kPa) hydrogen plus 20 ml min-1 nitrogen, the temperature being ramped at 10 K m i n - 1from ambient to 523 K. After at least one hour at 523 K, the temperature was reduced to the desired value and, after adjusting flows, the reacting gases were introduced into the system. Reaction conditions were chosen on the basis of a full 2 × 2 design, with catalyst weight varying between 1 and 0.2 g, ethyne between 2.4 and 0.4 vol.-% and hydrogen between 25 and 5 vol.-%. Temperatures were controlled at 403 K and 373 K. Total flow was always maintained at 200 ml min-1 and the hydrogen-toethyne ratio was generally kept at 10.8 vol/vol. The total pressure was 105 kPa and the pressure drop across the bed was generally less than 8 kPa. A mass balance for C2 gases across the empty reactor and the bypass showed an error of less than 0.5% with a maximal base line drift of 2% over 6 h.
211 Some of th e used catalysts were examined by t h e r m o g r a v i m e t r i c analysis (TGA, D u p o n t 99 Series T h e r m a l Analyser) in air or nitrogen. T h e temperature was r amp ed from 323 K to 1123 K at 5 K / m i n and held at 1123 K for 30 min. For convenience, the t e r m foulant is used for all products which contain more t h a n two carbons. Results are expressed in terms of conversion x = p r o d u c t o u t / e t h y n e in. Conversion for C2 gases refers to e t h y n e plus e t h e n e plus e t h a n e conversion. Conversion to foulant was defined as (1 - conversion to C2 gases ). RESULTS A preliminary run was carried out with the silica support in t he reactor to show t h a t no conversion of e t h y n e could be observed in t he absence of copper u n d er the reaction conditions. A run was also carried out using ca. 3% et hyne
10
0
k
J
04r gi
~1 ZoneII
0
40
I
Zone Ill
ao
120
"---"*"-~
160
200
240
Time on line (min)
Fig. 1. Conversion of ethyne to various products vs. time on line. Reaction conditions: T = 403 K, wt. of catalyst = 0.4 g, flow-rate= 200 ml min- 1 (STP ), ethyne = 2.32 vol.-%, hydrogen= 25 vol.%. (F]): C2H2total conversion ( A ) C2H4total conversion, ( + ) foulant, (x) C2Hs. TABLE1 Reaction orders for hydrogenation and for deactivation Reaction/Zone
order
I
C2H2--, C2H4
in H2 in C2H 2
1.2±0.2 -0.5±0.5
C2H2--*foulant
in H 2 in C2H2
1.0±0.1 0.1±0.1
C2H2-~C 2 H4 C2H2-~foulant
deactivation deactivation
1 -1
II
2 1
III
>5 >5
212 TABLE 2 Apparent activation energies for hydrogenation and for deactivation Reaction
Energy (kJ/mol)
C2H2-, C2H4 C2H2--~foulant
Ereaction
C2H2--~foulant
Edeactivation
Ereaction
Zone I
II
III
72.9±10.7 29.6± 5.8
63.1±3.6 38.7±1.6
62.5±5.3 45.0±3.2
20.4±2.2
14.6±2.6
1.00
.'•_•
% " . % % % %
o.gs-
A
I
.>
',,,? ----..
I
,wb=#
0.90-
n-
0.85 300
500
76o T (Z)
96o
,'oo
Fig. 2. Temperature-programmed heating of q used catalyst in nitrogen (A) and in air (B).
in ethene rather than nitrogen. No significant difference in the product spectrum could be observed, probably because of the preferred adsorption of ethyne on the catalyst [2,3,6 ]. Typical results obtained in the presence of catalyst are summarised in Fig. 1. The amount of ethyne leaving the reactor was initially small and dropped rapidly (zone I, ca. 0-17 rain on line; Fig. 1 ). After ca. 17 min on line, ethyne increased steadily (zone II, ca. 17-80 rain time on line) until emergent ethyne approached a steady value (zone III, time on line longer than 80 rain). Foulant and ethene paralleled the reverse of these changes (Fig. 1 ), while ethane was mainly produced during the first part of the run. The results summarised in Fig. 1 were shown not to be affected by, for example, flow switching, since such disturbances occurred in much shorter times. The kinetics of ethyne hydrogenation were studied over a steady-state catalyst (Fig. 1, Zone III, time on line greater than 120 min). The reaction to ethene was found to be first order ( _ 0.1 ) with respect to hydrogen and nega-
213
0.5
!
0./, 0z o
E 0.2 E 0.1 0
0
~.0
80 120 Time on line (min)
Fig. 3. Build up of foulant vs. time on line. Run conditions: Run T cat.wt. (K) (g) A B C C
373 373 373 373
1 0.5 1.00 0.5
160
200
flow-rate STP (ml min - 1) 200 200 200 200
ethyne (vol.-%)
H2 (vol.-%)
2.32 2.32 0.464 0.464
25 25 5 5
tive one half order ( + 0 . 5 ) with respect to ethyne (Table 1). The apparent activation order was found to be 63 + 5 kJ/mol (Table 2 ). The ratio of selectivities of ethene to foulant production in zone III was 0.133 + 0.010 at 373 K and 0.081 + 0.005 at 403 K. "Green oil" emerging from the reactor during a hydrogenation run was analysed to show the presence of benzene, ethylbenzene and C6 to Clo even numbered olefins and aromatic compounds. At least some of these can be expected to condense on the catalyst at reaction temperature and to volatilise as the temperature is raised (Fig. 2 ). The used catalyst exhibited a strong unpleasant odour on removal and the particles were stuck together. TGA of spent catalyst was carried out in nitrogen and in air (Fig. 2). Even in nitrogen the catalyst slowly lost weight on heating from 300 to 1100 K. Partial pyrolysis during heating may also occur and the small amount of residual material left on the catalyst after pyrolysis to 1100 K could be carbon produced during the hydrogenation or carbon from pyrolysis of foulant (Fig. 2). In air, a significant increase in loss of weight occurred at about 600 K {Fig. 2), and this was associated with oxidation of the deposit. The weight change resulting from the oxidation of copper was calculated to be negligible in terms of the weight loss by foulant oxidation. The kinetics of hydrogenation were then measured where possible over all three zones (Tables 1 and 2) as were the kinetics of deactivation. The latter
214
were analysed using the approach of Levenspiel [ 10 ] as developed by Mardaleishvili and Rapaport [11 ]. It is clear that the catalyst is deactivating rapidly, apparently as a result of the formation of foulant. The amount of foulant produced as a function of time on line was calculated by integrating the rate curves: typical results are shown in Fig. 3. Comparison of these integral amounts of foulant per catalyst weight with results obtained from TGA (Fig. 2) showed that ca. 95% of all foulant species produced remain on the catalyst under reaction conditions. Deposition of foulant on the catalyst does reduce surface area (from 235 m 2 g-1 to 178 m 2 g-~) but the size of the decrease is more pertinent to surface coating rather than massive pore closure. Burn off of foulant returned the surface area to a higher value (196 m 2 g-~). The initial activity was nearly the same after burn off and the reduced catalyst behaved in the same way as the fresh material. DISCUSSION
The supported copper catalyst is seen to be active and selective for the hydrogenation of ethyne to ethene, but to deactivate readily (Fig. 1 ). This deactivation is found to be due to the accumulation of foulant on the surface of the catalyst. A final near-steady-state catalyst is produced in which ca. 38% of ethyne is converted to 5% ethene and 33% foulant (Fig. 1 ). The exact nature of the foulant is open to question, but 80% of the deposits may be volatilised by heating to 1100 K (Fig. 2). The so-called "green oil" emerging from the reactor was shown to contain benzene, ethylbenzene and C6 to Clo even numbered olefins and aromatic compounds. At the temperature of reaction, only higher hydrocarbons could be expected to remain in the reactor but a clear boiling point cut off cannot be given. The catalyst is active in the initial stage of the reaction (zone I, Fig. 1 ) but is less selective in that ethane is produced. Two causes could contribute to the increasing selectivity with time. Highly active sites could be eliminated by accumulation of foulant and similar results have been reported for other systems (ethyne over palladium [12,13] ). Alternatively, preadsorbed hydrogen remaining from the reduction could be responsible for unselective hydrogenation; this is unlikely as a result of the fact that the amount of ethane produced is more than the amount of (weakly [14] ) adsorbed hydrogen on copper. It is convenient next to consider the near-steady-state behaviour of the catalyst (zone III, Fig. 1), since this facilitates an explanation of catalyst behaviour in zone II. In zone III (Fig. 1) the hydrogenation of ethyne to ethene occurs at a low rate and the accumulation of foulant is also slow. Due to the foulant build up, a carbonaceous film must be on the surface. If all the foulant were permanently deposited evenly all over the catalyst, it is possible to calculate the thickness
215 of the film that would be present. Taking an average value of 120 min on line (Fig. 3), the mass of foulant per mass of catalyst is seen to vary between ca. 0.075 and 0.425. Assuming the foulant is a highly unsaturated liquid with an assumed density of ca. 0.8 g/ml at 373 K, this corresponds to a film thickness of between 0.4 n m and 2.3 n m over the entire catalyst. These thicknesses would be some 50 to 60 times greater if the foulant were concentrated on the copper where it is produced. The actual coverage on the copper will most probably be between these extremes. With these amounts of foulant on the surface, it seemed possible that the kinetics of the reactions could be controlled by diffusion through the foulant film. Representing diffusion by Fick's Law
N= -D~
(1)
where N, the flux of gas, is related to the concentration gradient over the film thickness (x) by the diffusion coefficient, D. It would be expected, for mass transfer control, that rate (conversion achieved at given residence time) would be inversely related to film thickness (expressed as mass of foulant/mass of catalyst). As seen from Fig. 4, this is indeed found to be the case over a wide range of foulant loadings. For ethene production, diffusion control was rate controlling for zones II and III (longer t h a n ca. 17 rain on line) while for foulant production, diffusion control occurred in zone III (longer than ca. 80 min on line). In agreement with this suggestion, both the production of ethene and of foulant were found to be first order in hydrogen in the respective regimes (Table 1 ). Similar orders have been reported by Taghavi et al. [7 ], but it seems 1.0
1 ÷I
÷
°6!
l I
®
O.Z,
0
+/
ZoneII
I
O.2 0
ZoneI
1 I
0
[-------_.
L 0~.
08
i 1.2
i 16
2.0
mcatlm f (glg)
Fig. 4. Conversion of ethyne to foulant and ethene as a function of mass of foulant per mass of catalyst. ( + ) foulant, (~) C2H4;T=403 K, flow-rate= 200 ml rain- 1 (STP), wt. of catalyst = 0.4 g, ethyne = 2.32 vol.-%, hydrogen= 25 vol.-%.
216
unlikely that their suggested Rideal-Eley mechanism can be correct in the presence of so much foulant. If the diffusion of hydrogen is rate-controlling, the small negative orders in ethyne (Table 1 ) are somewhat surprising. These orders, as measured within a single run by changing the pressure of ethyne, were always well defined. However, significant variation was found between runs, accounting for the large errors in the measurement (Table 1). The best explanation of the small negative orders and the large errors would seem to lie in the effect of ethyne on film thickness. An increase in ethyne pressure would be expected to increase foulant production and, as a result, film thickness. Hydrogen then has to diffuse further and, as a result, ethene production becomes slower. The variation between runs then reflects the differences in film thickness when the apparent orders of reaction were measured. It is possible to estimate the onset of diffusion control by comparing predicted and observed rates. Diffusion of hydrogen would be expected to be rate limiting at high foulant levels from the observed reaction order of unity. Back extrapolation of the diffusion controlled rate (Fig. 4) to one theoretical monolayer shows that the predicted diffusion controlled rate is much greater than the observed rate confirming, as expected, that the rate of the chemical reaction is rate controlling when the foulant deposit is low. As the thickness of deposit grows, the diffusion of hydrogen decreases to the point where the predicted diffusion rate equals the observed rate (ca. 17 rain on line for ethene production and 80 min for foulant). The difference in these values reflects the fact that diffusion only controls rate when it is slower than the rate of the chemical reaction. Since the production of foulant is faster than that of ethene, it takes a greater thickness of film (longer time on line ) for this to occur in the former case. It would appear to be possible to estimate the diffusion coefficient of hydrogen in the foulant film, but this is complicated by the fact that the concentration gradient across the film also depends on the (unknown) solubility of the gas in the foulant. Estimates of diffusion/solubility show that ethyne, as expected, is ca. 10 times more concentrated in the film than hydrogen [ 15 ]. Returning to zone II, it is seen that the production of ethene is controlled by the rate of diffusion of hydrogen but that the production of foulant is controlled by the chemical kinetics. Under these circumstances, the increase in thickness of the film, the rate of diffusion of hydrogen and the rate of production of ethene are all dictated by the chemical kinetics of foulant production. Orders of reaction could not be measured, as a result of the rapid changes in activity occurring during zone II (Fig. 1 ), but the apparent activation energies observed are in agreement with this finding (Table 2). Values are high for a hydrogen diffusion controlled reaction, but it must be remembered that the result also reflects hydrogen solubility in the foulant. Thus it would seem that the production of ethene and of foulant on a fresh
217
supported copper catalyst is controlled by the kinetics of the chemical reaction. The fresh surface rapidly becomes covered in a layer of foulant and, when this builds up to an estimated 0.4 nm thickness, the rate of diffusion of hydrogen in the foulant film becomes rate determining. Foulant production, on the other hand, is faster than ethene production and does not become controlled by hydrogen diffusion until about an estimated 1.8 nm of foulant have built up. After this point, a near-steady-state catalyst is produced which converts ca. 38% ethyne to 5% ethene and 33% foulant. ACKNOWLEDGEMENT
The support of the Australian Research Council is gratefully acknowledged.
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