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Silica as catalyst for cyclohexanone ammoximation with molecular oxygen: a preliminary approach to the kinetic analysis
Chemical Engineering Science, printed in Great Britain.
Vol.
47. No. 9-I
I. pp. 2641-2646.
ooo9-2509#2 $5.00+0.00 01992PC~.pUlOIlPE3SLtd
1992.
SILICA
AS CATALYST FOR CYCLOHEXANONE AMMOXIMATION WITH MOLECULAR OXYGEN: A PRELIMINARY APPROACH TO THE KINETIC ANALYSIS E. PIERI, D. PINELLI and F. TRIFIRb*
Dip. Chimica Industriale e dei Materiali.
V.le de1 Risorgimento
4.40136
Bologna - ITALY
Catalytic data relative to the influence of reaction temperature, contact time and reactant concentrations are used for a preliminary approach to the kinetic analysis of cyclohexanone ammoximation with 02. A set of empirical kinetic equation is proposed, which accounts qualitatively for the kinetics in a pseudo-stationary state and for the change in catalytic behaviour with the time-on-stream. KEYWORDS Cyclohexanone Ammoximation, Silica, Molecular Oxygen, Approach to Kinetic Analysis. INTRODUCTION Cyclohexanone oxime is produced by a multi-step process from cyclohexanone and hydroxylamine (Arpe and Weissermel. 1978). A new and economically more interesting reaction was discovered in the 1980’s for its production (Armor, 1981). This new class of reaction, called ammoximation, involves reaction of cyclohexanone with ammonia and an oxidizing agent over silica based cataiysts. Much work has been done more recently by our research group to study the ammoximation of cyclohexanone to the corresponding oxime in the gas-phase with 02 over a commercial amorphous silica (AKZO F- 7) (Dreoni et al., 1990, 1991, 1992a,b,c). In previous work, the reaction network was investigated on the basis of flow-reactor experiments (Dreoni ef al., 1990, 1991) and confirmed by in situ FI-IR measurements (Dreoni et al., 1992a). On the basis of the reaction network, it was possible to identify the key step of Ihe reaction. The oxime is produced by oxidation of the adsorbed species of the intermediate corresponding imine by some oxygen species activated by the silica surface. However, other important products am also formed: i) tars and ii) other volatile heavy products (dimers and trimers of the ketone) which are formed by aldol condensation and other types of condensation of the imine and the ketone. The tars which deposit with time-on-stream on the silica surface cause a complex time evolution of the catalytic behaviour and, finally, deactivation of the catalyst. Although much information has been obtained on the selective pathway, there is still a considerable lack of knowledge regarding the parasitic pathways. In this work, the data obtained by flow-reactor experiments are use to approach the design of a kinetic analysis of the reaction network in order, not only to optimize the rate of formation of the oxime but also gain information on the reaction mechanism of formation of the other products. Therefore, this paper represents the frrst step of a more involved study aimed towards the design of a kinetic model based on a realistic surface mechanism which accounts for the phenomenology observed. EXPERIMENTAL The apparatus for the catalytic test consisted of a conventional glass micro-reactor with accumulation of the reaction products in a solvent, and gas-chromatogmphic analysis using an internal standard method. A complete description of the whole apparatus has been reported elsewhere (Dreoni et aZ., 1991). The following abbreviations wiil be used hereinafter: CH = cyclohexanone, CI-IN = cyclohexanone imine, 2641
E. F’IERI et (II.
2642
CHO = cyclohexanone oxime. OVP = other volatile products. Standard conditions for the catalytic test were the following: reactant concentration in the reaction gas CH=2.8 mol%, NH3=35 mol%. 02=10 mol%, T=22o’C. catalyst weight W=OS g, molar F=4.75E-5 moUrnin, flow rate w/F=175 g.h/mol.cH, total flow rate V=40 ml/min (contact time 3.0 s). The catalyst used in all the catalytic tests was a commercial amorphous silica by AK20 (AK20 F-7, surface area = 472 m2/g. pore volume 2.0 cm3/g) which has been shown in previous work to be the best catalyst for the reaction (Dreoni et al., 1990, 1991).
M cumulated yields (mop/,)
-o.--D___G--o-’ 0
5
10
15
20
25
time-on-stream
30
F4
35
40
(h)
RESULTS 0-f
Fig.1 - Time evolution of the catalyticbehaviour.
AND
DISCUSSION lh?&yiour
c&trc
wz&
eaA catalytic test was carried out in the standard conditions which preliminary tests showed to be optimal
-.50 40
30
20
t0
0 170
100
210
Temperature
( 71
250
Fig.2 - Influence of reaction tempemtuie on the catalytic behaviour. Same symbols as in Fig.1.
2
0
cot&t
Fig.3
-
Influence
of
timi contact
(8) time
behaviour.Same symsols as in Fig. 1.
8
10
on
catalytic
F4
2643
Silica as catalyst for cyclohexanone ammoximation with molecular oxygen
formation of tars is constant up to 20 hours of time-on- stream and, then, deems to zero. The OVP yield decreases only slightly with time-ondream up to 40 h of time-on-stream when the OVP are the only products of the Eaction. The oxime yield presents a completely different behaviour. Its rate of formation increases with time after approximately IO hours and then decreases to zero more rapidly than the rate of formation of tars. Therefore, the catalytic behaviour presents two time dependent phenomena: i) an activation process whose consequence is an increase in the rate of formation of the oxime alone, and ii) a 2 6 8 following deactivation process which lowers the CHconc4. (mot%) rate of formation of oxime and, then, of tars. The Fig.4 - Influence of CH concentrationon the rates of deposition of tars could easily explain the formationof theproducts.Same symbols as in Fig. 1. deactivation process, since it may be hypothesized that they cover and destroy the sites responsible for 02 activation. However, two important question remain: what is the nature of the sites responsible for activation of oxygen and what does the rate of formation of the oxime increase with time during the first 10 h?. , rates x E+5 (mol/min.g)
- In order to optimize the rate of formation of oxime and at the same time obtain new information on the mechanism of formation of the several products, a first approach to the kinetic investigation was attempted in pseudo-stationary conditions when the best performance is achieved. The validity of this approach is limited by several factors: i) the presence of time evolution of the catalytic behaviour, ii) the gathering of a series of products in two classes (tars and OVP), iii) the fact that it is impossible to operate at very low conversion due to the presence of homogeneous reactions and, finally, iv) the presence of internal diffusion limitations indicated by some dependence of the data on the catalyst particle size. To minimize this last effect, the catalyst was loaded as powder. Nevertheless. in spite of the limils cited, the approach used in the present work is useful to address the problem and clarify many obscure points in the reaction mechanism. A more sophisticated approach would not have been useful and justified at the present state of the research. Many catalytic runs were carried out with varying reaction temperature, contact time, and reactant concentrations using a large batch of homogeneous pm-activated catalyst. In this way, interference due to
the change of catalytic behaviour with time-on-stteam was minimized. The pre-activated catalyst was prepared by running a special catalytic test on a large scale untill the maximum activity was reached. The data showing the influence of the reaction 3 rates x E+5 (mcWg.min) temperature and contact time on the product yields, reported in Fig.2 and 3. showed that the e standard conditions are those necessary to avoid _.: _:* oxime decomposition. At T>23O’C and contact ..._ _.: 0 2 ---_ time longer than 4 s a decrease in oxime .I--_ 0 b selectivity occurs and decomposition products were found (among them also COX). It should
noted from Fig.2 that the yields of CHO and tars tend to zero for low contact time and that the yield of OVP seems to be constant for contact times lower than 5s. Since the tests were carried out with decreasing catalyst be
weight, this last evidence seems to indicate that the OVP are formed at least partly in the gas phase. The residence time inside the heated zone, in fact, does not change in all the tests. On
_.:
1.5 A......
25! 1
/’
..-.
--..i
0.5 -0 I
”
_Axygen ccLo_ (moi?!)
_. 1s
Fig.5 - Influence of Oz concentrationon the rates of formationof theproducts.Same symbols as in Fig.1.
2644
E. hmu
et al.
F4
the basis of the data of Fig. 1-3, at least as a first approximation, a simple reaction scheme can be proposed with three parallel pathways for the formation of the main reaction products (CHO, tars and OVP). The data obtained in the standard conditions and various NH3,02 and CH concentrations are reported in FigA. 5 and 6. The reaction rates were calculated supposing differential conditions. The conversions obtained in the tests were ail less than 40% and were achieved by decreasing the amount of catalyst charged in the reactor. The data in Fig.4 show the following apparent orders in regard to CH concentration of the reaction rates in a power-type equation: CHOa.5, tars=O& OVP=l.O. The data in Fig.5 show that the 02 concentration has a considerable influence on the reaction. In particular, the calculated apparent reaction orders are near 1 for CHO and tars, whereas there seems to be no influence on the condensation products (OVP). The data in Fig.6 show that two different kinetic regimes might be present at different ammonia lower than 5 mol%, the rates of formation of CHO and tars depend contents. For NH3 concentrations approximately linearly on the -3. On the other hand, if the ammonia concentration is maintained higher than 5 mol%, au increase in the NH3 concentration results in a lower increase in the rate of formation of CHO, whereas the yield of tars seems no longer to depend on ammonia concentration. On the other hand, the rate of formation of the OVP seems not to depend on NH3 concentration over the entire range studied. To explain the variation in the rate of formation of CHO. it may be hypothesized that at low ammonia concentration the rate limiting step is the imine formation whereas, at high ammonia concentration, the rate determining step may become the oxidation of the imine. This hypothesis seems to be confirmed by some catalytic tests carried out 3 ,ratesx E-c5 (mol/g.min) changing 02 concentration for low NH3 content (2.5 mol%). In these conditions no dependence of the product formation rates on Po2 was evidenced_ On the basis of the considerations made above and the approximation of pseudo- stationary state conditions, the following kinetic equations can be written to describe the catalytic hehaviour in the standard conditions: XHO lYm=
= kHo
PCH
a pNH3
a PO2 / PH20
(1) 0
ktars PCH
poz
rovP=kovPPc~
with cz and Btl.
(2)
(3)
10
amrE3nia c&c. (rZ?A)
so
60
Fig.6 - Influence of NH3 content on the rates of formation of the products. Same symbols as in Fig. 1.
uence of water on the catarvtrc behaviour - In order to confirm the influence of H20 concentration on the rate of formation of oxime and to obtain more information on the formation of tars and OVP. a catalytic test was carried out with the addition of water to the inlet gas stream (about 20 mol%). the other experimental conditions being the same of Fig. 1. The evolution of the catalytic behaviour is about the same as in the standard test (Fig. 1) but conversion and oxime yield are lower. The catalytic data of test are summarized in Table I (Expt.2) and the catalytic data of the standard test are also reported in Table I (Expt. 1) for comparison. The influence on conversion and on oxime yield may be easily explained if one considers that the water concentration influences the equilibrium of the reaction of formation of the irnine. It should be noted that water seems not to be involved in the chemical process of fpnnation of tars and that the kinetics of the activation and deactivation are not affected by the change in water concentration, suggesting that the activation and deactivation processes are not related to the desired reaction pathway and may be related to the process of deposition of tars.
, . . p the absence of 02, catalytic behaviour
- In order to study the importance of the catalytic reaction which rakes place in a catalytic test was carried out in the presence of only CH and NH3. The initial showed conversion of the ketone with the production of tars in high yield, other
Silica as catalyst for cyclohexano~
F4
ammoximation withmolecular oxygen
2645
Best catalytic performances of tests in Fig.1 (Expel). with N2JNHs pre-activatedcatalyst Gxpt.2) and with wateraddedto thegas stream(Expt.3).
Tab1e.k
Nr. apt 1
CH (mol%)
(mol%)
02 (mol%)
Conv. (mol%)
Y-CHO (mol%)
Y-tars (mol%)
Y-OVP (mo196)
2.8
35
10
61.9
25.2
16.5
29.2
2
2.8
35
10
51.6
16.0
16.9
18.6
3
2.8
35
10
70.1
24.3
23-2
22.6
I
condensation products and traces of the imine. Also in this case, the catalytic behaviour changed with time-on-stream and a fast decrease in the conversion and in the rate of formation of tars occurred during the first 2-3 h of reaction, after which they remained approximately constant. In these conditions, very low conversion (about 35%) was obtained and the only products found were cyclohexanone imine (15 mol%) and condensation products (the remainder). The spent catalyst was not discharged but was then tested in the standard conditions following the time evolution with time-on-stream. The catalytic data am summarized in Table I (Expt.3). The phenomenology, the conversion and the CHO and OW yields are approximately the same as in the test of Fig. 1 whereas the rate of formation of tars is higher. However, the most important fact to be noted is that the same phenomena take place but 5 h earlier than in Fig-l. this time in correspondence with the time-on-stream necessary to deposit in standard conditions the amount of tars present at the beginning of the test. This datum suggests that the activation process is really related to the total tar content in the catalyst, regardless of the type of tars or the conditions in which they am generated.
of cyciohe&eaone oxi= - The rate equation for the rate of formation of oxime can be deduced in the form of an empirical power-type equation. In the standard conditions, the rate limiting step is assumed to be the oxidation of the adsorbed imine, in equpbrium on the silica surface with the ketone, to the adsorbed oxime by some activated oxygen species 0 . These species are assumed to be generated by active sites whose concentration increases with the total tar content and to migrate and react with the adsorbed imine. Therefore, the following scheme may be suggested:
Rate own
cH* <
NH3 ____>
-Hz0
*
Cm*
___?__, CHO*
which accounts for equation (1) even though, in regard to the apparent order with respect to the CH. no data are available at present to differentiate between a possible effect of pore internal diffusion and a real saturation effect of the sites responsible for CH adsorption. However, as a first approximation, the empirical equation (1) can be used to describe correctly the experimental data. Pate o~fotmation of tars - At this stage of understanding of the mechanism of formation of the tars, it can be proposed that they arc formed by a polymerization reaction which occurs at the silica surface and involves N containing compounds (i-e. CHN and/or condensation products) in equilibrium with the ketone. The polymerization must, in some way, depend on the presence of oxygen. It can be proposed that activated oxygen species may be involved in determining the number of sites responsible for propagation of the reaction in a radical polymerization mechanism. The empirical equation (2) and the proposed mechanism would explain the high N content and the low 0 content of the tars as well as the constancy of the rate of formation of the tars (since oxygen activated species are not involved as reactant species. no dependence is hypothesized on the concentration of the sites responsible for 02 activation during the first 40 h). The decrease in tar formation after the 40th h can also be explained by the proposed mechanism since, in the end, the concentration of oxygen activated species decreases as a consequence of the
destruction of most of the sites responsible for 02 activation. n of the cxher vokxde orodtms fOVP1 - Taking into account that the OW
are a mixture
of different products, the apparent first order with respect to the ketone concentration and the fact that
2646
E. ha~l er al.
F4
their rate of formation does not depend on ammonia concentration seems to indicate that i) their formation might occur at least partly in the gas phase by condensation teactions such as aldol condensation reactions catalyzed by ammonia. ii) they are in equilibrium with the ketone. Indeed, a certain decrease in the yield of the OVP as conversion increases is observed. On the basis of this hypothesis, the simple equation (3) that can efficiently describe the experimental data, can be proposed for the formation of the entite group of OVP. , . * . the kznetrcevolvnc behavrour. In previous work (Dreoni et al., 1991). the deactivation was easily correlated to the deposition of the tars which were supposed initially to destroy the sites responsible for 02 activation (15-4Oh) and, then, to destroy or make not accessible the surface silanol B@nsted sites responsible for chemical adsorption of CH and for its transformation into CHN. On the other hand, the test with the catalyst pre-activated in a NH*2 atmosphere (Expt. 2 in Tab.1) showed that the rate of formation of oxime is related to the total tar content. Moreover, the data in Fig. 1 show that the extrapolated rate of formation of CHO at t=O h is about zero. These observations suggest that the tars can also play a fundamental role in the activation process and in the selective oxidation pathway. The set of equations (l)-(3) describes the kinetic behaviour of the reacting system in the pseudo-stationary state corresponding to the neighbourhood of maximum activity. A further step in the analysis would requite the description of time evolution of the catalytic behaviour. The data pResented in the present paper indicate that the complex behaviour is related to the change of 0 concentration with time-on-stream. At this stage of understanding of the activation and deactivation processes, only an empirical approach can be proposed. In particular, it can be assumed that the concentration of the sites responsible for 02 activation are function of tar content in the catalyst (Ct&. In this case, the rate constant KCHO would actually depend on Ctarsaccording to an empirical equation. Work is currently in progress to define the form of the equation. The dependence of Ctarson time-on-stream can be obtained from integration of equation (2) or, alternatively, from experimental measurements. CONCLUSIONS The data in Fig.1 show that a simple kinetic analysis of the reaction network which does not take into account the influence of the time-on-stream parameter would not be meaningful In fact, two time dependent phenomena are present: i) activation of the catalyst (from 0 to 10 h) and ii) deactivation (from 15 h to the end of the catalyst life). On the other hand, this tentative approach to the analysis of the kinetics in the pseudo- stationary state adds important information on the reaction mechanisms and has allowed the formulation of a surface model. This model was then used to write tentative preliminary kinetic equations describing the dependence of the rates of formation of the products on the reactant and product concentrations in the pseudo-stationary state and their time evolution. The set of equations so obtained should be used in future work to verify the correctness of the model proposed by fitting of the experimental data and to estimate the kinetic parameters for the process. ACKNOWLEDGMENT. The financial support from CNR - “Progetto finalizzato - CHlMlCA
FINE 2” is gratefully acknowledged.
REFERENCES Arpe, H.J. and Weissermel, K., Industrial Organic Chemistry, Springer Verlag, 1978, pp. 222-231. Armor, J.N., 1981, J. Catal., 7O,p.72. Dreoni, D.P., Pinelli, D. and Trifirb, F., in Proceedings, “12 Simposio Ibero Americano de Catalise”, Rio de Janeiro 1990, ~01.2, p. 305. Dreoni, D.P., Pinelli, D. and Trifirb, F., 1991, J. Mol. Catal., 69, p. 171. Busca, G., Dreoni, D.P., Pinelli, D. and Trifirb, F. and Lorenzelli, V., 1992, .I. Mol. Catal., 71, p.101. Dreoni, D.P., Pinelli, D. and Trifirh, F., 1992, in “New Development in Setective Oxidation by Heterogeneous Catalysis ‘0 book series ‘Srudies in Surface Science and Catalysis”, Elsevier Science Pub., Amsterdam, ~01.72, p. 109. Giamello, E., Pedulli, G., Pinelli, D., Trifirb, F. and Vaccari, A., 1992, CataJ. Lett., in pub.
Vol.
47. No. 9-I
I. pp. 2641-2646.
ooo9-2509#2 $5.00+0.00 01992PC~.pUlOIlPE3SLtd
1992.
SILICA
AS CATALYST FOR CYCLOHEXANONE AMMOXIMATION WITH MOLECULAR OXYGEN: A PRELIMINARY APPROACH TO THE KINETIC ANALYSIS E. PIERI, D. PINELLI and F. TRIFIRb*
Dip. Chimica Industriale e dei Materiali.
V.le de1 Risorgimento
4.40136
Bologna - ITALY
Catalytic data relative to the influence of reaction temperature, contact time and reactant concentrations are used for a preliminary approach to the kinetic analysis of cyclohexanone ammoximation with 02. A set of empirical kinetic equation is proposed, which accounts qualitatively for the kinetics in a pseudo-stationary state and for the change in catalytic behaviour with the time-on-stream. KEYWORDS Cyclohexanone Ammoximation, Silica, Molecular Oxygen, Approach to Kinetic Analysis. INTRODUCTION Cyclohexanone oxime is produced by a multi-step process from cyclohexanone and hydroxylamine (Arpe and Weissermel. 1978). A new and economically more interesting reaction was discovered in the 1980’s for its production (Armor, 1981). This new class of reaction, called ammoximation, involves reaction of cyclohexanone with ammonia and an oxidizing agent over silica based cataiysts. Much work has been done more recently by our research group to study the ammoximation of cyclohexanone to the corresponding oxime in the gas-phase with 02 over a commercial amorphous silica (AKZO F- 7) (Dreoni et al., 1990, 1991, 1992a,b,c). In previous work, the reaction network was investigated on the basis of flow-reactor experiments (Dreoni ef al., 1990, 1991) and confirmed by in situ FI-IR measurements (Dreoni et al., 1992a). On the basis of the reaction network, it was possible to identify the key step of Ihe reaction. The oxime is produced by oxidation of the adsorbed species of the intermediate corresponding imine by some oxygen species activated by the silica surface. However, other important products am also formed: i) tars and ii) other volatile heavy products (dimers and trimers of the ketone) which are formed by aldol condensation and other types of condensation of the imine and the ketone. The tars which deposit with time-on-stream on the silica surface cause a complex time evolution of the catalytic behaviour and, finally, deactivation of the catalyst. Although much information has been obtained on the selective pathway, there is still a considerable lack of knowledge regarding the parasitic pathways. In this work, the data obtained by flow-reactor experiments are use to approach the design of a kinetic analysis of the reaction network in order, not only to optimize the rate of formation of the oxime but also gain information on the reaction mechanism of formation of the other products. Therefore, this paper represents the frrst step of a more involved study aimed towards the design of a kinetic model based on a realistic surface mechanism which accounts for the phenomenology observed. EXPERIMENTAL The apparatus for the catalytic test consisted of a conventional glass micro-reactor with accumulation of the reaction products in a solvent, and gas-chromatogmphic analysis using an internal standard method. A complete description of the whole apparatus has been reported elsewhere (Dreoni et aZ., 1991). The following abbreviations wiil be used hereinafter: CH = cyclohexanone, CI-IN = cyclohexanone imine, 2641
E. F’IERI et (II.
2642
CHO = cyclohexanone oxime. OVP = other volatile products. Standard conditions for the catalytic test were the following: reactant concentration in the reaction gas CH=2.8 mol%, NH3=35 mol%. 02=10 mol%, T=22o’C. catalyst weight W=OS g, molar F=4.75E-5 moUrnin, flow rate w/F=175 g.h/mol.cH, total flow rate V=40 ml/min (contact time 3.0 s). The catalyst used in all the catalytic tests was a commercial amorphous silica by AK20 (AK20 F-7, surface area = 472 m2/g. pore volume 2.0 cm3/g) which has been shown in previous work to be the best catalyst for the reaction (Dreoni et al., 1990, 1991).
M cumulated yields (mop/,)
-o.--D___G--o-’ 0
5
10
15
20
25
time-on-stream
30
F4
35
40
(h)
RESULTS 0-f
Fig.1 - Time evolution of the catalyticbehaviour.
AND
DISCUSSION lh?&yiour
c&trc
wz&
eaA catalytic test was carried out in the standard conditions which preliminary tests showed to be optimal
-.50 40
30
20
t0
0 170
100
210
Temperature
( 71
250
Fig.2 - Influence of reaction tempemtuie on the catalytic behaviour. Same symbols as in Fig.1.
2
0
cot&t
Fig.3
-
Influence
of
timi contact
(8) time
behaviour.Same symsols as in Fig. 1.
8
10
on
catalytic
F4
2643
Silica as catalyst for cyclohexanone ammoximation with molecular oxygen
formation of tars is constant up to 20 hours of time-on- stream and, then, deems to zero. The OVP yield decreases only slightly with time-ondream up to 40 h of time-on-stream when the OVP are the only products of the Eaction. The oxime yield presents a completely different behaviour. Its rate of formation increases with time after approximately IO hours and then decreases to zero more rapidly than the rate of formation of tars. Therefore, the catalytic behaviour presents two time dependent phenomena: i) an activation process whose consequence is an increase in the rate of formation of the oxime alone, and ii) a 2 6 8 following deactivation process which lowers the CHconc4. (mot%) rate of formation of oxime and, then, of tars. The Fig.4 - Influence of CH concentrationon the rates of deposition of tars could easily explain the formationof theproducts.Same symbols as in Fig. 1. deactivation process, since it may be hypothesized that they cover and destroy the sites responsible for 02 activation. However, two important question remain: what is the nature of the sites responsible for activation of oxygen and what does the rate of formation of the oxime increase with time during the first 10 h?. , rates x E+5 (mol/min.g)
- In order to optimize the rate of formation of oxime and at the same time obtain new information on the mechanism of formation of the several products, a first approach to the kinetic investigation was attempted in pseudo-stationary conditions when the best performance is achieved. The validity of this approach is limited by several factors: i) the presence of time evolution of the catalytic behaviour, ii) the gathering of a series of products in two classes (tars and OVP), iii) the fact that it is impossible to operate at very low conversion due to the presence of homogeneous reactions and, finally, iv) the presence of internal diffusion limitations indicated by some dependence of the data on the catalyst particle size. To minimize this last effect, the catalyst was loaded as powder. Nevertheless. in spite of the limils cited, the approach used in the present work is useful to address the problem and clarify many obscure points in the reaction mechanism. A more sophisticated approach would not have been useful and justified at the present state of the research. Many catalytic runs were carried out with varying reaction temperature, contact time, and reactant concentrations using a large batch of homogeneous pm-activated catalyst. In this way, interference due to
the change of catalytic behaviour with time-on-stteam was minimized. The pre-activated catalyst was prepared by running a special catalytic test on a large scale untill the maximum activity was reached. The data showing the influence of the reaction 3 rates x E+5 (mcWg.min) temperature and contact time on the product yields, reported in Fig.2 and 3. showed that the e standard conditions are those necessary to avoid _.: _:* oxime decomposition. At T>23O’C and contact ..._ _.: 0 2 ---_ time longer than 4 s a decrease in oxime .I--_ 0 b selectivity occurs and decomposition products were found (among them also COX). It should
noted from Fig.2 that the yields of CHO and tars tend to zero for low contact time and that the yield of OVP seems to be constant for contact times lower than 5s. Since the tests were carried out with decreasing catalyst be
weight, this last evidence seems to indicate that the OVP are formed at least partly in the gas phase. The residence time inside the heated zone, in fact, does not change in all the tests. On
_.:
1.5 A......
25! 1
/’
..-.
--..i
0.5 -0 I
”
_Axygen ccLo_ (moi?!)
_. 1s
Fig.5 - Influence of Oz concentrationon the rates of formationof theproducts.Same symbols as in Fig.1.
2644
E. hmu
et al.
F4
the basis of the data of Fig. 1-3, at least as a first approximation, a simple reaction scheme can be proposed with three parallel pathways for the formation of the main reaction products (CHO, tars and OVP). The data obtained in the standard conditions and various NH3,02 and CH concentrations are reported in FigA. 5 and 6. The reaction rates were calculated supposing differential conditions. The conversions obtained in the tests were ail less than 40% and were achieved by decreasing the amount of catalyst charged in the reactor. The data in Fig.4 show the following apparent orders in regard to CH concentration of the reaction rates in a power-type equation: CHOa.5, tars=O& OVP=l.O. The data in Fig.5 show that the 02 concentration has a considerable influence on the reaction. In particular, the calculated apparent reaction orders are near 1 for CHO and tars, whereas there seems to be no influence on the condensation products (OVP). The data in Fig.6 show that two different kinetic regimes might be present at different ammonia lower than 5 mol%, the rates of formation of CHO and tars depend contents. For NH3 concentrations approximately linearly on the -3. On the other hand, if the ammonia concentration is maintained higher than 5 mol%, au increase in the NH3 concentration results in a lower increase in the rate of formation of CHO, whereas the yield of tars seems no longer to depend on ammonia concentration. On the other hand, the rate of formation of the OVP seems not to depend on NH3 concentration over the entire range studied. To explain the variation in the rate of formation of CHO. it may be hypothesized that at low ammonia concentration the rate limiting step is the imine formation whereas, at high ammonia concentration, the rate determining step may become the oxidation of the imine. This hypothesis seems to be confirmed by some catalytic tests carried out 3 ,ratesx E-c5 (mol/g.min) changing 02 concentration for low NH3 content (2.5 mol%). In these conditions no dependence of the product formation rates on Po2 was evidenced_ On the basis of the considerations made above and the approximation of pseudo- stationary state conditions, the following kinetic equations can be written to describe the catalytic hehaviour in the standard conditions: XHO lYm=
= kHo
PCH
a pNH3
a PO2 / PH20
(1) 0
ktars PCH
poz
rovP=kovPPc~
with cz and Btl.
(2)
(3)
10
amrE3nia c&c. (rZ?A)
so
60
Fig.6 - Influence of NH3 content on the rates of formation of the products. Same symbols as in Fig. 1.
uence of water on the catarvtrc behaviour - In order to confirm the influence of H20 concentration on the rate of formation of oxime and to obtain more information on the formation of tars and OVP. a catalytic test was carried out with the addition of water to the inlet gas stream (about 20 mol%). the other experimental conditions being the same of Fig. 1. The evolution of the catalytic behaviour is about the same as in the standard test (Fig. 1) but conversion and oxime yield are lower. The catalytic data of test are summarized in Table I (Expt.2) and the catalytic data of the standard test are also reported in Table I (Expt. 1) for comparison. The influence on conversion and on oxime yield may be easily explained if one considers that the water concentration influences the equilibrium of the reaction of formation of the irnine. It should be noted that water seems not to be involved in the chemical process of fpnnation of tars and that the kinetics of the activation and deactivation are not affected by the change in water concentration, suggesting that the activation and deactivation processes are not related to the desired reaction pathway and may be related to the process of deposition of tars.
, . . p the absence of 02, catalytic behaviour
- In order to study the importance of the catalytic reaction which rakes place in a catalytic test was carried out in the presence of only CH and NH3. The initial showed conversion of the ketone with the production of tars in high yield, other
Silica as catalyst for cyclohexano~
F4
ammoximation withmolecular oxygen
2645
Best catalytic performances of tests in Fig.1 (Expel). with N2JNHs pre-activatedcatalyst Gxpt.2) and with wateraddedto thegas stream(Expt.3).
Tab1e.k
Nr. apt 1
CH (mol%)
(mol%)
02 (mol%)
Conv. (mol%)
Y-CHO (mol%)
Y-tars (mol%)
Y-OVP (mo196)
2.8
35
10
61.9
25.2
16.5
29.2
2
2.8
35
10
51.6
16.0
16.9
18.6
3
2.8
35
10
70.1
24.3
23-2
22.6
I
condensation products and traces of the imine. Also in this case, the catalytic behaviour changed with time-on-stream and a fast decrease in the conversion and in the rate of formation of tars occurred during the first 2-3 h of reaction, after which they remained approximately constant. In these conditions, very low conversion (about 35%) was obtained and the only products found were cyclohexanone imine (15 mol%) and condensation products (the remainder). The spent catalyst was not discharged but was then tested in the standard conditions following the time evolution with time-on-stream. The catalytic data am summarized in Table I (Expt.3). The phenomenology, the conversion and the CHO and OW yields are approximately the same as in the test of Fig. 1 whereas the rate of formation of tars is higher. However, the most important fact to be noted is that the same phenomena take place but 5 h earlier than in Fig-l. this time in correspondence with the time-on-stream necessary to deposit in standard conditions the amount of tars present at the beginning of the test. This datum suggests that the activation process is really related to the total tar content in the catalyst, regardless of the type of tars or the conditions in which they am generated.
of cyciohe&eaone oxi= - The rate equation for the rate of formation of oxime can be deduced in the form of an empirical power-type equation. In the standard conditions, the rate limiting step is assumed to be the oxidation of the adsorbed imine, in equpbrium on the silica surface with the ketone, to the adsorbed oxime by some activated oxygen species 0 . These species are assumed to be generated by active sites whose concentration increases with the total tar content and to migrate and react with the adsorbed imine. Therefore, the following scheme may be suggested:
Rate own
cH* <
NH3 ____>
-Hz0
*
Cm*
___?__, CHO*
which accounts for equation (1) even though, in regard to the apparent order with respect to the CH. no data are available at present to differentiate between a possible effect of pore internal diffusion and a real saturation effect of the sites responsible for CH adsorption. However, as a first approximation, the empirical equation (1) can be used to describe correctly the experimental data. Pate o~fotmation of tars - At this stage of understanding of the mechanism of formation of the tars, it can be proposed that they arc formed by a polymerization reaction which occurs at the silica surface and involves N containing compounds (i-e. CHN and/or condensation products) in equilibrium with the ketone. The polymerization must, in some way, depend on the presence of oxygen. It can be proposed that activated oxygen species may be involved in determining the number of sites responsible for propagation of the reaction in a radical polymerization mechanism. The empirical equation (2) and the proposed mechanism would explain the high N content and the low 0 content of the tars as well as the constancy of the rate of formation of the tars (since oxygen activated species are not involved as reactant species. no dependence is hypothesized on the concentration of the sites responsible for 02 activation during the first 40 h). The decrease in tar formation after the 40th h can also be explained by the proposed mechanism since, in the end, the concentration of oxygen activated species decreases as a consequence of the
destruction of most of the sites responsible for 02 activation. n of the cxher vokxde orodtms fOVP1 - Taking into account that the OW
are a mixture
of different products, the apparent first order with respect to the ketone concentration and the fact that
2646
E. ha~l er al.
F4
their rate of formation does not depend on ammonia concentration seems to indicate that i) their formation might occur at least partly in the gas phase by condensation teactions such as aldol condensation reactions catalyzed by ammonia. ii) they are in equilibrium with the ketone. Indeed, a certain decrease in the yield of the OVP as conversion increases is observed. On the basis of this hypothesis, the simple equation (3) that can efficiently describe the experimental data, can be proposed for the formation of the entite group of OVP. , . * . the kznetrcevolvnc behavrour. In previous work (Dreoni et al., 1991). the deactivation was easily correlated to the deposition of the tars which were supposed initially to destroy the sites responsible for 02 activation (15-4Oh) and, then, to destroy or make not accessible the surface silanol B@nsted sites responsible for chemical adsorption of CH and for its transformation into CHN. On the other hand, the test with the catalyst pre-activated in a NH*2 atmosphere (Expt. 2 in Tab.1) showed that the rate of formation of oxime is related to the total tar content. Moreover, the data in Fig. 1 show that the extrapolated rate of formation of CHO at t=O h is about zero. These observations suggest that the tars can also play a fundamental role in the activation process and in the selective oxidation pathway. The set of equations (l)-(3) describes the kinetic behaviour of the reacting system in the pseudo-stationary state corresponding to the neighbourhood of maximum activity. A further step in the analysis would requite the description of time evolution of the catalytic behaviour. The data pResented in the present paper indicate that the complex behaviour is related to the change of 0 concentration with time-on-stream. At this stage of understanding of the activation and deactivation processes, only an empirical approach can be proposed. In particular, it can be assumed that the concentration of the sites responsible for 02 activation are function of tar content in the catalyst (Ct&. In this case, the rate constant KCHO would actually depend on Ctarsaccording to an empirical equation. Work is currently in progress to define the form of the equation. The dependence of Ctarson time-on-stream can be obtained from integration of equation (2) or, alternatively, from experimental measurements. CONCLUSIONS The data in Fig.1 show that a simple kinetic analysis of the reaction network which does not take into account the influence of the time-on-stream parameter would not be meaningful In fact, two time dependent phenomena are present: i) activation of the catalyst (from 0 to 10 h) and ii) deactivation (from 15 h to the end of the catalyst life). On the other hand, this tentative approach to the analysis of the kinetics in the pseudo- stationary state adds important information on the reaction mechanisms and has allowed the formulation of a surface model. This model was then used to write tentative preliminary kinetic equations describing the dependence of the rates of formation of the products on the reactant and product concentrations in the pseudo-stationary state and their time evolution. The set of equations so obtained should be used in future work to verify the correctness of the model proposed by fitting of the experimental data and to estimate the kinetic parameters for the process. ACKNOWLEDGMENT. The financial support from CNR - “Progetto finalizzato - CHlMlCA
FINE 2” is gratefully acknowledged.
REFERENCES Arpe, H.J. and Weissermel, K., Industrial Organic Chemistry, Springer Verlag, 1978, pp. 222-231. Armor, J.N., 1981, J. Catal., 7O,p.72. Dreoni, D.P., Pinelli, D. and Trifirb, F., in Proceedings, “12 Simposio Ibero Americano de Catalise”, Rio de Janeiro 1990, ~01.2, p. 305. Dreoni, D.P., Pinelli, D. and Trifirb, F., 1991, J. Mol. Catal., 69, p. 171. Busca, G., Dreoni, D.P., Pinelli, D. and Trifirb, F. and Lorenzelli, V., 1992, .I. Mol. Catal., 71, p.101. Dreoni, D.P., Pinelli, D. and Trifirh, F., 1992, in “New Development in Setective Oxidation by Heterogeneous Catalysis ‘0 book series ‘Srudies in Surface Science and Catalysis”, Elsevier Science Pub., Amsterdam, ~01.72, p. 109. Giamello, E., Pedulli, G., Pinelli, D., Trifirb, F. and Vaccari, A., 1992, CataJ. Lett., in pub.