- Email: [email protected]
The ethane oxidative chlorination process and efficient catalyst for it
3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.
305
The ethane o x i d a t i v e c h l o r i n a t i o n process and efficient catalyst for it M.R. Flid, I.I. Kurlyandskaya, Yu.A. Treger and T.D. Guzhnovskaya Scientific Research Institute "Syntez", 2 ,Ugreshskaya str., P.O. Box 56, Moscow, 109432 Russia Formation of the mixed cement-containing systems within the range of low copper concentrations with addition of alkali metal dopants as well as catalytical properties of these systems in the ethane oxidative chlorination process have been investigated. Based on the obtained data the efficient and stable copper-cement catalyst has been worked out. This catalyst will assist in the development of a new technology of the vinyl chloride production from ethane. The basic parameters of the ethane oxychlorination process have been determined : at 623-673K, time-on-stream 3-5s and reactant ratio of C2H6: HCI: :02 = 1:2:1 the conversion of ethane is more than 90% and the total selectivity to ethylene and vinyl chloride is 85-90%.
1.1ntroduction The gas-phase catalytic process for oxidative chlorination of ethane to vinyl chloride according to overall equation C2H6 + HC1 + 02 = C2H3C1 + 2H20,
(I)
proceeds in two consecutive kinetically independent reactions: (1) the oxidation of hydrogen chloride to chlorine and (2) the chlorination of ethane. This process is promising for developing a rational technology of vinyl chloride production, because ethane utilized in it is a cheap hydrocarbon raw material [ 1,2 ]. The process is conducted at high temperatures, and ethane converts to vinyl chloride due to a combination of consecutive and parallel radical-chain and heterogeneously catalyzed reactions: oxidation, chlorination, and dehydrochlorination. The contributions of homogeneous and heterogeneous reactions to the overall rate of chlorinated hydrocarbon conversion depends on the temperature ranges at which the reaction proceeds. The process as the whole may be represented by the following schematic diagram [3]: CO + C02
C2H6 - -
t
t
C2H5CI~2H4CI2 C2H4 ~
C2H3C1
t
~2H3C13
t
~2H2C14
--~ C2H2C12 --~ C2HC13
CO + CO2
t
~2HCl5 --~ C2C14
--
C2C16 (II)
306 from which it follows that the major products of ethane oxidative chlorination are ethyl chloride, 1,2- and 1,1-dichloroethanes, 1,1,2-trichloroethane, chloroethylenes, and carbon oxides as the products of deep oxidation. At relatively low temperatures (623m723K), the reaction mixture consists mainly of chloroorganic saturated compounds [3-6]. The situation changes dramatically with raising the temperature. Figure 1 demonstrates the effect of the temperature on the oxidative chlorination of ethane over the well-known conventional salt CuC12--KC1/silica gel copper-containing catalyst. 80
1
70
.
'
~
-
"
60,
-~ 9~,
3 50
L 0
,'.o
40
2
30 "0
o
5
20 10
4
"'
0 723
773
823
Temperature,K
Figure 1. The effect of temperature on the ethane oxidative chlorination process (silica gel as the support, copper content of 6.0 wt %, potassium content of 4.0 wt %, reactant ratio C2H6 : HC1 : O2= 1 : 1 : 1, x = 3 s). 1 is the conversion of ethane; 2 is the yield of oxidation chlorination products; 3.4, and 5 are the yields of ethylene, deep oxidation products, and vinyl chloride, respectively ( x is time- on-stream ). Thus, in the presence of traditional catalytic systems, the yield of vinyl chloride to converted ethane does not exceed 35%. The total yield of vinyl chloride and ethylene ranges up to 80%. It was shown [3,5,6] for saturated compounds ethane, ethyl chloride, 1,2dichloroethane, and 1,1,1-trichloroethane that the observed conversion rates are satisfactory described by the equation r, = k,. Pi" Pci2 0"5
(1)
The observed rate constant in equation (1) in this case decreases in the order C2H6 > > C2H5C1 > C2H4C12. The activation energies for the transformations of saturated (130 kJ/mol) and unsaturated compounds (40--90 kJ/mol) differ dramatically; as a consequence, the yield of chloroalkenes increases with temperature. Oxidative chlorination of ethane gives rise to considerable amounts of carbon oxides. The overall rate of these side reactions is described by the empirical equation rco + c02 = ki. pi" Po_~"Pcl2~
(2)
Unsaturated compounds make the dominant contribution to formation of carbon oxides. Whereas the introduction of one chlorine atom into ethylene molecule results in a 7m 8-fold increase in the observed rate constant of deep oxidation, the further increase of chlorine content in molecule diminishes the oxidation of chloroalkenes.
307 It is essential that the reactions of saturated compounds exhibit zero orders with respect to both oxygen and hydrogen chloride and proceed kinetically independently of one another. For the unsaturated compounds, the conversion rates represent complex functions of the reaction mixture composition. Under the conditions when the reaction exhibits zero order with respect to hydrogen chloride, the kinetics of unsaturated compounds oxidative chlorination is described by the equation: 2ko2 po2" ki'pi ri
(3)
--
2ko2 "po2+ L-k,.p, where index i relates to unsaturated compounds. The process of ethane oxidative chlorination imposes heavy demands on the catalysts. The conventional salt supported catalysts are composed of Cu, K, Ca, Mn, Co, Fe, Mg, and other metal chlorides containing various additives; these salts are precipitated on alumina, zeolites, silica gel, and other supports. Catalytic systems that represent solid solutions of iron cations in the lattice of the o~-A1203 and a-Cr203 phases doped with cations, such as K, Ba, Ce, and Ag are also known [7]. The activity of the known catalytic systems and, especially, their selectivity to vinyl chloride are insufficient. In addition, the known catalytic systems tend to rapid deactivation because of gumming and carbonization of their surfaces. The main problem that determines the possibility for industrial utilization of the process is the creation of highly efficient, stable, and selective catalytic systems performing at relatively low temperatures. This problem was alleviated due to the development of a new generation of heterogeneous catalysts based on high-alumina cements and intended for the synthesis of chloroorganic compounds, l These catalysts fortunately combine the properties required in industry and genetically intrinsic to cements thermal stability, high mechanical strength, and basicity of the surface, which prevents its carbonization with the possibility of imparting the system special properties desired in a particular process [8]. The mechanism of the ethane oxidative chlorination process is distinguished by the fact that the catalyst accelerates primarily the reactions of hydrogen chloride oxidation and dichloroethane dehydrochlorination. This necessitates the modeling of cement catalytic system with the surface carrying active sites capable of catalyzing both reactions mentioned. The analysis of the known and our own experimental data indicated that the properties required may be offered by a copper-containing cement-based catalytic system modified with alkali metals. In this catalyst, copper-containing active sites catalyze the oxidation of hydrogen chloride, whereas the activity of the catalyst in the dehydrochlorination reaction is determined by the acid--base surface properties, which are inherent to cements with different phase compositions. The development of this catalytic system made it necessary to investigate the formation process of mixed cement systems within the range of low copper concentrations and with addition of alkali dopants and determination of the correlation between properties of the obtained catalytical systems and their activity in the ethane oxychlorination process.
I 'Fhe catalysts based on high-alumina cements were developed in collaboration with Prof. V.I. Yakerson (Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, Russia) and Prof. E.Z. Golozman (Institute of Nitrogen Industry, Novomoskovsk, Russia).
308
2.Experimental. The catalysts were prepared by chemical mixing of high-alumina cements ( technical calcium aluminate- talyum) or cement-based supports (calcium aluminates with the developed surface area and various CaO/A1203 ratio - galyumin or galyumin C) [9] with the sources of copper and alkali metals in water--ammonia or ammonia ---carbonate media; the mixing was followed by the drying and thermal treatment of the samples obtained. A comprehensive study on the formation of cement catalytic systems was performed by X-ray diffraction, thermal analysis, electronic diffuse-reflectance spectroscopy and IRspectroscopy. Table 1 presents characteristics of some of the investigated catalytic systems. Table 1 Characteristics of copper--cement catalysts No
Sample
Support
Preparation
Phase composition
Ssoec,
conditions
without thermal with thermal m2/g treatment treatment at 673K 1 CuO-K20- Galyumin 348K water-gibbsite, C3AH6, KC1, CaCO3, CuO, 130 CaO-A1203 ammonium CaCO3, CHA, CuO, ,/-A1203,C12A7 solution KC1
2 CuO-Cs20- Galyumin 348K CaO-Al203
C
water-- CsC1, CaCO3, CuO, CsC1, CuO, ammonium C3AH6 CaCO3(calcite), solution C12A7, CaCO3 (aragonite)
15
Kinetic measurements were made at 623 - 773K using a circulatory flow installation. Reactions were studied in the fixed bed catalyst. Time - on- stream was varied within the range 1,5 - 10s at a reactant ratio of C2H6: HCI: O 2 = 1:1 +3,3, : 1 +1,4. Air was used as a source of oxygen. The grains of the catalyst were 0,25 - 0,5 mm in size. The gas was fed at a volumetric flow rate of 600 h - ~ . The catalytic systems were preactivated with a hydrogen chloride nitrogen mixture at 573-623 K. The analyses were based on the chemical methods (determination of hydrogen chloride and chlorine) and the gas chromatography.
3.Discussion and results. The stage of chemical mixing of the catalysts preparation involves the hydration of cements with forming C3AH6 (C is CaO, A is A1203, and H is H20), gibbsite, and calcium carboaluminate as well as the exchange processes with forming CaCO3 and copper hydroxoaluminate (CHA). The depth and the rate of hydration as well as the distinctions in the exchange processes are determined by the type of cement-containing agent. The stage of thermal treatment involves the formation of C12A7, ,/-alumina, and solid solution of aluminum and copper oxides, which is followed by the precipitation of the excess of highly dispersed copper oxide and by the formation of copper aluminate spinels with various degrees of disorder.
309 Thus, copper-containing phases can occur both as free oxide and as the forms bound with the matrix of the support; the concentrations of bound forms increase with temperature and with the duration of chemical mixing. The estimation of the depth of interaction revealed that not only the implantation of the Cu 2. ions into the matrix lattice with forming isolated ions is possible, but the formation of small surface clusters (CuO)• with highly covalent Cu--O bonds. The distribution of catalytically active component between the free oxide, clusters, and ions implanted into the matrix lattice depends both on the conditions of formation and on the composition of the catalytic system as well as on the type of cement-containing agent. As it was shown in [ 10], cement-containing matrix exerts a strong modifying effect on the active copper-containing sites. At equal concentrations of the active component, the activity of copper-containing sites incorporated into the copper--cement catalyst is higher than that of the supported salt catalysts. When the concentration of copper and surface concentration of copper-containing sites are decreased, specific catalytic activity of coppercontaining centers sharply increases. So, at an extended specific surface of the copper-cement catalyst, high catalytic activity to the oxidation of hydrogen chloride can be accomplished even at a low concentration of active copper-containing component provided that the latter is bound with the matrix of the support. The surface area of cement catalysts, which carries aluminum- and calcium-containing oxide fragments, exhibits pronounced acid--base properties. These properties can manifest itself as a catalytic activity to the reactions of dehydrochlorination, which proceed via the formation of donor--acceptor complexes between the substrate and acid or base sites at the catalyst surface. The existence of different calcium aluminate phases in the aluminum-calcium catalysts was proved by diffuse-reflectance IR spectroscopy. The presence of these phases is responsible for the complex structure of the catalyst surface. At the surfaces of these catalysts, calcium ions with lower coordination numbers can occur together with the ions octahedrally surrounded by oxygen anions. These ions can act as balance cations in the structure of C12A7, being responsible for the existence of specific terminal hydroxyls and Lewis acid sites bound to calcium. At the surfaces of galyumins, bridging hydroxyls exhibiting somewhat stronger acid properties are present along with terminal hydroxyl groups. The hydration of galyumin surface can supposedly be attended with the weakening of the A1--O--M bond (M = A1 and Ca) resulting in the appearance of additional strong adsorption sites[8]. The enrichment of surface layer in galyumin C with Ca )-+ ions at the increase of CaO/ AL203 ratio is essential for reducing the yield of deep oxidation products and preventing the carbonization of the surface. The data on the state of copper-containing phases and acid--base properties of active sites occurring at the surface of mixed cement systems, which were presented above, enable us to conclude that these catalysts can be employed in the oxidative chlorination of ethane. It ~is known that the chlorination of ethane with chlorine formed in the oxidation of hydrogen chloride proceeds by a heterogeneous--homogeneous mechanism [3]. This is why the efficiency of cement catalysts was studied separately by the examples of Deacon reaction and dichloroethane dehydrochlorination reaction. It was found that for galyumin-based cement system, the variation of copper content within 8--25% (in terms of CuO) virtually does not affect the rate of chlorine formation. For the oxidation of HC1, the rate constant is 1.2.10 -3 mol HC1/g cat.h. This value is comparable with the rate constant of HC1 oxidation in the presence of copper-containing salt catalysts. The
310 introduction of potassium chloride into a copper--cement system results in a 1.5-fold rise of the rate constant for the HC1 oxidation. Thus, the activity of copper--cement catalysts in Deacon reaction is comparable with that of commonly used salt catalysts. Systematic investigations on the performance of cement-containing catalytic systems with various chemical and phase compositions in the reaction of 1,2-dichloroethane dehydrochlorination with forming vinyl chloride C2H4CI 2 -4
C2H3C1 +
HC1
(III)
revealed that the catalytic activity of these catalysts in the process under consideration is high. At the constant composition of the reaction mixture, the maximum reaction rate was accomplished with using a cement system whose specific surface is 130 m2/g. Thus, at 623K and time-on-stream of 3.8 s, the reaction rate was 0.42--0.46 mol of vinyl chloride per litre.hour. This value is more than two times higher than the reaction rate accomplished with using a well-known supported salt catalyst CsC1--SiO2. It was also shown that the presence of copper in cement catalytic systems does not affect the activity of the catalyst in the dehydrochlorination reaction (see Fig. 2).
70 60 50 "6 ,..
40
0
P,
~
~ L
30
C 0
0
20
0
[
523
.
.
.
.
.
r
. . . . . . . . . . . . . . . . . . . . . . .
573
' ....
623
" ........
673
Temperature,K
Fig. 2. The conversion of 1,2-dichloroethane in the dehydrochlorination reaction at various catalysts as a function of temperature, z = 8 s; 1- galyumin (Sspec = 130 m2/g), 2- galyumin with a dopant of copper (8 wt % in terms of CuO); 3- CsC1/silica gel. Thus, cement-containing systems provide the conversion of dichloroethane to be increased to more than 70% even at 673K. An important positive factor is that vinyl chloride molecule is stable at this temperature. At 673K, the side reaction of vinyl chloride dehydrochlorination with forming acetylene proceeds slowly, acetylene does not form, and the reaction is not complicated by the formation of a number of by-products, for example, of perchloroethylene. Thus, the above-made supposition about bifunctional character of copper--cement catalytic systems was confirmed in the investigations of their activity in the above-mentioned reactions.
31l The oxidative chlorination of ethane as a whole was studied by using of the cementcontaining catalysts with a specific surface of 130 m2/g (sample 1) and 15 m2/g (sample 2). Copper concentration was kept constant and equal to 8 wt % in terms of CuO (see Table 1). It was found during the investigations that when the temperature was raised from 623 to 773 K, the conversion of ethane somewhat increased, and sample 1 exhibited better activity in comparison with that of sample 2. At the moderate temperatures (623--673K), an extended specific surface of sample 1 was favorable for increasing the yield of target unsaturated compounds: ethylene and vinyl chloride. The further temperature increase led to a decrease in the process selectivity because of a noticeable increase in the yield of deep oxidation products, CO• The effect is more pronounced for sample 1 (see Fig. 3). 100 F
...........................................................................................................................................................................................................................
.__
...,,-
90
-,--I'"
.
.
.
.
I-'-
""
""
-----4
80 7O
..... -1 b
......._-
I
-6
.~
__,a
6o 5O
t-
40
o
(..) 30 20 ~C
1~ L ,
0 ,"F:
623
-.
-
'"'-
. w
.
.
.
.
.
.~C
_ .
._~
--T
723
673
773
Temperature,K
Fig. 3. The conversion of ethane and the yields of reaction products for catalysts 1 and 2 as functions of temperature. Time-on-stream of 3 s; 1; the reactant ratio of C2H6 : HC1 : 02 - 1 : 2 : 1;-- - catalyst i ; catalyst 2; a is the conversion of ethane, b is the total yield of ethylene and vinyl chloride to converted ethane, c is the yield of deep oxidation products COx. The dependences shown in Fig. 3 reveal that employing a catalyst with a larger specific surface area with rising temperature would, probably, lead to the deep oxidation of vinyl chloride and, to a lesser extent, of ethylene, resulting in a decrease in the total yield of ethylene and vinyl chloride. A certain increase in the overall yield of CO• products, which was observed for catalyst 2, is accompanied with an increase in the total yield of ethylene and vinyl chloride. This suggests that saturated chlorinated h y d r o c a r b o n s - ethyl chloride and 1,2-dichloroethane m are oxidized predominantly and that the rate of oxidation is lower rate compared to that of the dehydrochlorination of these compounds. Thus, the decrease in specific surface of the catalyst involves a noticeable drop of the yield of deep oxidation products, whereas the yields of vinyl chloride and ethylene remain high. We see little reason in the further cut of the specific surface, because the rate of catalytic dehydrochlorination therewith decreases.
312 The results obtained circumstantially testify that the dehydrochlorination and oxidation reactions proceed at different active sites. It is likely that the oxidation of chlorinated hydrocarbons proceeds at the copper-containing sites. This agrees with the data we obtained in the oxidative chlorination of ethylene [ 11 ]. Taking into account the fact that the value of specific surface is a crucial factor in the choice of catalyst, the further investigations we conducted with using catalyst 2. Both the time-on-stream and the reactant ratio are important chemical engineering parameters affecting the characteristics of the process. It was found that the increase in the time-on-stream at T = 673K can improve both the conversion of ethane and the yield of ethylene. The total yield of chloroorganic products therewith decreases, but the concentration of vinyl chloride passes through a maximum. We also observed an increase in the yield of deep oxidation products COx (see Table 2). Table 2 The effect of time-on-stream on the oxidative chlorination of ethane Catalyst- 8 wt % CuO/cement; T = 673K; reactant ratio C2H6 : HC1 : O2 = 1 : 2 : 1. No.
~, s
Reactant conversion, % C2H6
HC1
O2
Yields scaled to converted ethane, % C2H4C12
C2H3C1
C2H4
COx
1
1.5
80.6
36.0
91.2
25.0
34.1
34.6
2.6
2
3.2
87.5
31.7
89.5
19.4
36.8
40.1
3.2
3
5.6
89.1
30.4
88.6
11.3
38.2
43.5
4.0
4
7.9
90.9
30.0
87.9
10.1
35.6
46.8
6.5
5
10.0
92.7
29.1
86.0
8.2
33.0
47.6
9.7
We can suppose on the strength of the data listed in Table 2 that at the short times-onstream, the major contribution to the formation of deep oxidation products is made by saturated chlorinated hydrocarbons: 1,2 dichloroethane and ethyl chloride. On increasing timeon-stream to more than 6 s, we observed a sharp increase in the yield of deep oxidation products together with the decrease in the yield of vinyl chloride. It is likely that at the longer times-on-stream, the rate of deep oxidation of vinyl chloride would increase and become higher than the rate of dichloroethane dehydrochlorination. Taking into account this fact, we believe that the optimum time-on-stream assuring the best total yield of ethylene and vinyl chloride would be 3--5 s. It was shown in the investigations that the ratio of initial reactants also essentially affects the process. It was found that the excess of hydrogen chloride is favorable for improving the selectivity of the process with reducing the yield of deep oxidation products. At 673K and the reactant ratio of C2H6 : HC1 = 1 : 1, the yield of COx ranges from 6 to 7%; at the reactant ratio of C2H6 : HC1 = 1 : 2, the corresponding yield is 3-----4% (see Table 2). A positive factor is that the carbonization of the catalyst therewith decreases. On the other hand, the increase in the excess of HC1 to ethane up to 3 : 1 involves the decrease in the yield of unsaturated hydrocarbons due to the inhibition of the dehydrochlorination of 1,2dichloroethane and ethyl chloride with hydrogen chloride. The excess of oxygen increases the conversion of ethane mainly due to its oxidation: the yield of carbon oxides increases by 1.8-2 times. Thus, the optimum reactant ratio to provide the best yields of the target products is C2H6 : HC1 : O2 = 1 : 2 : 1.
313 Perfect stability of copper-containing cement catalysts in the oxidative chlorination of ethane was confirmed by their performance for 1500 hours without any decrease in the catalytic activity.
4.Conclusions The results obtained substantiate that the utilization of copper---cement catalysts offers promise for the synthesis of vinyl chloride from ethane at law temperatures in a single step. The proposed efficient and stable copper-cement catalyst will assist in the development of a new technology for the production of vinyl chloride from ethane. This technology is lowwaste and balanced in raw materials with meeting modem requirements of ecological safety. It would be appropriate to conduct the process of vinyl chloride production from ethane, hydrogen chloride, and oxygen in a fixed bed of copper---cement catalyst modified with alkali metals, for example, at 623--673K, time-on-stream of 3--5 s, and reactant ratio of C2H6 : HC1 : 02 - 1 : 2 : 1. Under these conditions, the conversion of ethane is more than 90%, and the total selectivity to ethylene and vinyl chloride is 85-90% at the yield of deep oxidation products COx no more than 3--4%.
REFERENCES
1. Yu.A. Treger, V.N. Rozanov, M.R. Flid, L.M.Kartaschov, Usp. Khim., 57,No 4(1988) 577 2. H.Rigel, H.D.Schindler, M.C.Sze. Chem.Engng.Progr.,.69, Nol0, (1973) 89 3. E.I. Gel'perin, Yu.M. Bakshi, A.K.Avetisov, A.I. Gel'bschtein, Kinet. Katal., 19, No 6 (1978) 527. 4. A.J.Magistro, P.P.Nicholas, R.T.Carrol, J.Organ. Chem., 34 (1969) 271 5. E.I. Gel'perin, Yu.M. Bakshi, A.K.Avetisov, A.I. Gel'bschtein, Kinet. Katal., 20, No 1 (1979) 129. 6. E.I. Gel'perin, Yu.M. Bakshi, A.K.Avetisov, A.I. Gel'bschtein, Kinet. Katal., 24, No 3 (1983) 633. 7. M.M. Mallikarjunan and S. Zahed Hussain, J. Sci., Ind. Res., 42 (1983) 209. 8. V.I. Yakerson, E.Z. Golosman. React. Kinet. Catal. Let., 55, No2 (1995) 455 9. V.I. Yakerson, E.Z. Golosman. Scientific Bases for the Preparation of Heterogeneous Catalysts. VI Intern. Symp.Preprint. 3 Poster Session II Louvain-la-Neuve (Belgium), (1994) 105 10. I.I. Kurlyandskaya, I.G. Solomonik, E.D.Glazunova, E.A.Boevskaya, Yu.M.Bakshi, E.Z. Golosman,V.I. Yakerson, Khim. Prom-st, Moscow, No. 6 (1996) 368. 11. M.R. Flid, I.I. Kurlyandskaya, I.G. Solomonik, M.V.Babotina, Khim. Prom-st, Moscow, No. 6 (1996) 364.
305
The ethane o x i d a t i v e c h l o r i n a t i o n process and efficient catalyst for it M.R. Flid, I.I. Kurlyandskaya, Yu.A. Treger and T.D. Guzhnovskaya Scientific Research Institute "Syntez", 2 ,Ugreshskaya str., P.O. Box 56, Moscow, 109432 Russia Formation of the mixed cement-containing systems within the range of low copper concentrations with addition of alkali metal dopants as well as catalytical properties of these systems in the ethane oxidative chlorination process have been investigated. Based on the obtained data the efficient and stable copper-cement catalyst has been worked out. This catalyst will assist in the development of a new technology of the vinyl chloride production from ethane. The basic parameters of the ethane oxychlorination process have been determined : at 623-673K, time-on-stream 3-5s and reactant ratio of C2H6: HCI: :02 = 1:2:1 the conversion of ethane is more than 90% and the total selectivity to ethylene and vinyl chloride is 85-90%.
1.1ntroduction The gas-phase catalytic process for oxidative chlorination of ethane to vinyl chloride according to overall equation C2H6 + HC1 + 02 = C2H3C1 + 2H20,
(I)
proceeds in two consecutive kinetically independent reactions: (1) the oxidation of hydrogen chloride to chlorine and (2) the chlorination of ethane. This process is promising for developing a rational technology of vinyl chloride production, because ethane utilized in it is a cheap hydrocarbon raw material [ 1,2 ]. The process is conducted at high temperatures, and ethane converts to vinyl chloride due to a combination of consecutive and parallel radical-chain and heterogeneously catalyzed reactions: oxidation, chlorination, and dehydrochlorination. The contributions of homogeneous and heterogeneous reactions to the overall rate of chlorinated hydrocarbon conversion depends on the temperature ranges at which the reaction proceeds. The process as the whole may be represented by the following schematic diagram [3]: CO + C02
C2H6 - -
t
t
C2H5CI~2H4CI2 C2H4 ~
C2H3C1
t
~2H3C13
t
~2H2C14
--~ C2H2C12 --~ C2HC13
CO + CO2
t
~2HCl5 --~ C2C14
--
C2C16 (II)
306 from which it follows that the major products of ethane oxidative chlorination are ethyl chloride, 1,2- and 1,1-dichloroethanes, 1,1,2-trichloroethane, chloroethylenes, and carbon oxides as the products of deep oxidation. At relatively low temperatures (623m723K), the reaction mixture consists mainly of chloroorganic saturated compounds [3-6]. The situation changes dramatically with raising the temperature. Figure 1 demonstrates the effect of the temperature on the oxidative chlorination of ethane over the well-known conventional salt CuC12--KC1/silica gel copper-containing catalyst. 80
1
70
.
'
~
-
"
60,
-~ 9~,
3 50
L 0
,'.o
40
2
30 "0
o
5
20 10
4
"'
0 723
773
823
Temperature,K
Figure 1. The effect of temperature on the ethane oxidative chlorination process (silica gel as the support, copper content of 6.0 wt %, potassium content of 4.0 wt %, reactant ratio C2H6 : HC1 : O2= 1 : 1 : 1, x = 3 s). 1 is the conversion of ethane; 2 is the yield of oxidation chlorination products; 3.4, and 5 are the yields of ethylene, deep oxidation products, and vinyl chloride, respectively ( x is time- on-stream ). Thus, in the presence of traditional catalytic systems, the yield of vinyl chloride to converted ethane does not exceed 35%. The total yield of vinyl chloride and ethylene ranges up to 80%. It was shown [3,5,6] for saturated compounds ethane, ethyl chloride, 1,2dichloroethane, and 1,1,1-trichloroethane that the observed conversion rates are satisfactory described by the equation r, = k,. Pi" Pci2 0"5
(1)
The observed rate constant in equation (1) in this case decreases in the order C2H6 > > C2H5C1 > C2H4C12. The activation energies for the transformations of saturated (130 kJ/mol) and unsaturated compounds (40--90 kJ/mol) differ dramatically; as a consequence, the yield of chloroalkenes increases with temperature. Oxidative chlorination of ethane gives rise to considerable amounts of carbon oxides. The overall rate of these side reactions is described by the empirical equation rco + c02 = ki. pi" Po_~"Pcl2~
(2)
Unsaturated compounds make the dominant contribution to formation of carbon oxides. Whereas the introduction of one chlorine atom into ethylene molecule results in a 7m 8-fold increase in the observed rate constant of deep oxidation, the further increase of chlorine content in molecule diminishes the oxidation of chloroalkenes.
307 It is essential that the reactions of saturated compounds exhibit zero orders with respect to both oxygen and hydrogen chloride and proceed kinetically independently of one another. For the unsaturated compounds, the conversion rates represent complex functions of the reaction mixture composition. Under the conditions when the reaction exhibits zero order with respect to hydrogen chloride, the kinetics of unsaturated compounds oxidative chlorination is described by the equation: 2ko2 po2" ki'pi ri
(3)
--
2ko2 "po2+ L-k,.p, where index i relates to unsaturated compounds. The process of ethane oxidative chlorination imposes heavy demands on the catalysts. The conventional salt supported catalysts are composed of Cu, K, Ca, Mn, Co, Fe, Mg, and other metal chlorides containing various additives; these salts are precipitated on alumina, zeolites, silica gel, and other supports. Catalytic systems that represent solid solutions of iron cations in the lattice of the o~-A1203 and a-Cr203 phases doped with cations, such as K, Ba, Ce, and Ag are also known [7]. The activity of the known catalytic systems and, especially, their selectivity to vinyl chloride are insufficient. In addition, the known catalytic systems tend to rapid deactivation because of gumming and carbonization of their surfaces. The main problem that determines the possibility for industrial utilization of the process is the creation of highly efficient, stable, and selective catalytic systems performing at relatively low temperatures. This problem was alleviated due to the development of a new generation of heterogeneous catalysts based on high-alumina cements and intended for the synthesis of chloroorganic compounds, l These catalysts fortunately combine the properties required in industry and genetically intrinsic to cements thermal stability, high mechanical strength, and basicity of the surface, which prevents its carbonization with the possibility of imparting the system special properties desired in a particular process [8]. The mechanism of the ethane oxidative chlorination process is distinguished by the fact that the catalyst accelerates primarily the reactions of hydrogen chloride oxidation and dichloroethane dehydrochlorination. This necessitates the modeling of cement catalytic system with the surface carrying active sites capable of catalyzing both reactions mentioned. The analysis of the known and our own experimental data indicated that the properties required may be offered by a copper-containing cement-based catalytic system modified with alkali metals. In this catalyst, copper-containing active sites catalyze the oxidation of hydrogen chloride, whereas the activity of the catalyst in the dehydrochlorination reaction is determined by the acid--base surface properties, which are inherent to cements with different phase compositions. The development of this catalytic system made it necessary to investigate the formation process of mixed cement systems within the range of low copper concentrations and with addition of alkali dopants and determination of the correlation between properties of the obtained catalytical systems and their activity in the ethane oxychlorination process.
I 'Fhe catalysts based on high-alumina cements were developed in collaboration with Prof. V.I. Yakerson (Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, Russia) and Prof. E.Z. Golozman (Institute of Nitrogen Industry, Novomoskovsk, Russia).
308
2.Experimental. The catalysts were prepared by chemical mixing of high-alumina cements ( technical calcium aluminate- talyum) or cement-based supports (calcium aluminates with the developed surface area and various CaO/A1203 ratio - galyumin or galyumin C) [9] with the sources of copper and alkali metals in water--ammonia or ammonia ---carbonate media; the mixing was followed by the drying and thermal treatment of the samples obtained. A comprehensive study on the formation of cement catalytic systems was performed by X-ray diffraction, thermal analysis, electronic diffuse-reflectance spectroscopy and IRspectroscopy. Table 1 presents characteristics of some of the investigated catalytic systems. Table 1 Characteristics of copper--cement catalysts No
Sample
Support
Preparation
Phase composition
Ssoec,
conditions
without thermal with thermal m2/g treatment treatment at 673K 1 CuO-K20- Galyumin 348K water-gibbsite, C3AH6, KC1, CaCO3, CuO, 130 CaO-A1203 ammonium CaCO3, CHA, CuO, ,/-A1203,C12A7 solution KC1
2 CuO-Cs20- Galyumin 348K CaO-Al203
C
water-- CsC1, CaCO3, CuO, CsC1, CuO, ammonium C3AH6 CaCO3(calcite), solution C12A7, CaCO3 (aragonite)
15
Kinetic measurements were made at 623 - 773K using a circulatory flow installation. Reactions were studied in the fixed bed catalyst. Time - on- stream was varied within the range 1,5 - 10s at a reactant ratio of C2H6: HCI: O 2 = 1:1 +3,3, : 1 +1,4. Air was used as a source of oxygen. The grains of the catalyst were 0,25 - 0,5 mm in size. The gas was fed at a volumetric flow rate of 600 h - ~ . The catalytic systems were preactivated with a hydrogen chloride nitrogen mixture at 573-623 K. The analyses were based on the chemical methods (determination of hydrogen chloride and chlorine) and the gas chromatography.
3.Discussion and results. The stage of chemical mixing of the catalysts preparation involves the hydration of cements with forming C3AH6 (C is CaO, A is A1203, and H is H20), gibbsite, and calcium carboaluminate as well as the exchange processes with forming CaCO3 and copper hydroxoaluminate (CHA). The depth and the rate of hydration as well as the distinctions in the exchange processes are determined by the type of cement-containing agent. The stage of thermal treatment involves the formation of C12A7, ,/-alumina, and solid solution of aluminum and copper oxides, which is followed by the precipitation of the excess of highly dispersed copper oxide and by the formation of copper aluminate spinels with various degrees of disorder.
309 Thus, copper-containing phases can occur both as free oxide and as the forms bound with the matrix of the support; the concentrations of bound forms increase with temperature and with the duration of chemical mixing. The estimation of the depth of interaction revealed that not only the implantation of the Cu 2. ions into the matrix lattice with forming isolated ions is possible, but the formation of small surface clusters (CuO)• with highly covalent Cu--O bonds. The distribution of catalytically active component between the free oxide, clusters, and ions implanted into the matrix lattice depends both on the conditions of formation and on the composition of the catalytic system as well as on the type of cement-containing agent. As it was shown in [ 10], cement-containing matrix exerts a strong modifying effect on the active copper-containing sites. At equal concentrations of the active component, the activity of copper-containing sites incorporated into the copper--cement catalyst is higher than that of the supported salt catalysts. When the concentration of copper and surface concentration of copper-containing sites are decreased, specific catalytic activity of coppercontaining centers sharply increases. So, at an extended specific surface of the copper-cement catalyst, high catalytic activity to the oxidation of hydrogen chloride can be accomplished even at a low concentration of active copper-containing component provided that the latter is bound with the matrix of the support. The surface area of cement catalysts, which carries aluminum- and calcium-containing oxide fragments, exhibits pronounced acid--base properties. These properties can manifest itself as a catalytic activity to the reactions of dehydrochlorination, which proceed via the formation of donor--acceptor complexes between the substrate and acid or base sites at the catalyst surface. The existence of different calcium aluminate phases in the aluminum-calcium catalysts was proved by diffuse-reflectance IR spectroscopy. The presence of these phases is responsible for the complex structure of the catalyst surface. At the surfaces of these catalysts, calcium ions with lower coordination numbers can occur together with the ions octahedrally surrounded by oxygen anions. These ions can act as balance cations in the structure of C12A7, being responsible for the existence of specific terminal hydroxyls and Lewis acid sites bound to calcium. At the surfaces of galyumins, bridging hydroxyls exhibiting somewhat stronger acid properties are present along with terminal hydroxyl groups. The hydration of galyumin surface can supposedly be attended with the weakening of the A1--O--M bond (M = A1 and Ca) resulting in the appearance of additional strong adsorption sites[8]. The enrichment of surface layer in galyumin C with Ca )-+ ions at the increase of CaO/ AL203 ratio is essential for reducing the yield of deep oxidation products and preventing the carbonization of the surface. The data on the state of copper-containing phases and acid--base properties of active sites occurring at the surface of mixed cement systems, which were presented above, enable us to conclude that these catalysts can be employed in the oxidative chlorination of ethane. It ~is known that the chlorination of ethane with chlorine formed in the oxidation of hydrogen chloride proceeds by a heterogeneous--homogeneous mechanism [3]. This is why the efficiency of cement catalysts was studied separately by the examples of Deacon reaction and dichloroethane dehydrochlorination reaction. It was found that for galyumin-based cement system, the variation of copper content within 8--25% (in terms of CuO) virtually does not affect the rate of chlorine formation. For the oxidation of HC1, the rate constant is 1.2.10 -3 mol HC1/g cat.h. This value is comparable with the rate constant of HC1 oxidation in the presence of copper-containing salt catalysts. The
310 introduction of potassium chloride into a copper--cement system results in a 1.5-fold rise of the rate constant for the HC1 oxidation. Thus, the activity of copper--cement catalysts in Deacon reaction is comparable with that of commonly used salt catalysts. Systematic investigations on the performance of cement-containing catalytic systems with various chemical and phase compositions in the reaction of 1,2-dichloroethane dehydrochlorination with forming vinyl chloride C2H4CI 2 -4
C2H3C1 +
HC1
(III)
revealed that the catalytic activity of these catalysts in the process under consideration is high. At the constant composition of the reaction mixture, the maximum reaction rate was accomplished with using a cement system whose specific surface is 130 m2/g. Thus, at 623K and time-on-stream of 3.8 s, the reaction rate was 0.42--0.46 mol of vinyl chloride per litre.hour. This value is more than two times higher than the reaction rate accomplished with using a well-known supported salt catalyst CsC1--SiO2. It was also shown that the presence of copper in cement catalytic systems does not affect the activity of the catalyst in the dehydrochlorination reaction (see Fig. 2).
70 60 50 "6 ,..
40
0
P,
~
~ L
30
C 0
0
20
0
[
523
.
.
.
.
.
r
. . . . . . . . . . . . . . . . . . . . . . .
573
' ....
623
" ........
673
Temperature,K
Fig. 2. The conversion of 1,2-dichloroethane in the dehydrochlorination reaction at various catalysts as a function of temperature, z = 8 s; 1- galyumin (Sspec = 130 m2/g), 2- galyumin with a dopant of copper (8 wt % in terms of CuO); 3- CsC1/silica gel. Thus, cement-containing systems provide the conversion of dichloroethane to be increased to more than 70% even at 673K. An important positive factor is that vinyl chloride molecule is stable at this temperature. At 673K, the side reaction of vinyl chloride dehydrochlorination with forming acetylene proceeds slowly, acetylene does not form, and the reaction is not complicated by the formation of a number of by-products, for example, of perchloroethylene. Thus, the above-made supposition about bifunctional character of copper--cement catalytic systems was confirmed in the investigations of their activity in the above-mentioned reactions.
31l The oxidative chlorination of ethane as a whole was studied by using of the cementcontaining catalysts with a specific surface of 130 m2/g (sample 1) and 15 m2/g (sample 2). Copper concentration was kept constant and equal to 8 wt % in terms of CuO (see Table 1). It was found during the investigations that when the temperature was raised from 623 to 773 K, the conversion of ethane somewhat increased, and sample 1 exhibited better activity in comparison with that of sample 2. At the moderate temperatures (623--673K), an extended specific surface of sample 1 was favorable for increasing the yield of target unsaturated compounds: ethylene and vinyl chloride. The further temperature increase led to a decrease in the process selectivity because of a noticeable increase in the yield of deep oxidation products, CO• The effect is more pronounced for sample 1 (see Fig. 3). 100 F
...........................................................................................................................................................................................................................
.__
...,,-
90
-,--I'"
.
.
.
.
I-'-
""
""
-----4
80 7O
..... -1 b
......._-
I
-6
.~
__,a
6o 5O
t-
40
o
(..) 30 20 ~C
1~ L ,
0 ,"F:
623
-.
-
'"'-
. w
.
.
.
.
.
.~C
_ .
._~
--T
723
673
773
Temperature,K
Fig. 3. The conversion of ethane and the yields of reaction products for catalysts 1 and 2 as functions of temperature. Time-on-stream of 3 s; 1; the reactant ratio of C2H6 : HC1 : 02 - 1 : 2 : 1;-- - catalyst i ; catalyst 2; a is the conversion of ethane, b is the total yield of ethylene and vinyl chloride to converted ethane, c is the yield of deep oxidation products COx. The dependences shown in Fig. 3 reveal that employing a catalyst with a larger specific surface area with rising temperature would, probably, lead to the deep oxidation of vinyl chloride and, to a lesser extent, of ethylene, resulting in a decrease in the total yield of ethylene and vinyl chloride. A certain increase in the overall yield of CO• products, which was observed for catalyst 2, is accompanied with an increase in the total yield of ethylene and vinyl chloride. This suggests that saturated chlorinated h y d r o c a r b o n s - ethyl chloride and 1,2-dichloroethane m are oxidized predominantly and that the rate of oxidation is lower rate compared to that of the dehydrochlorination of these compounds. Thus, the decrease in specific surface of the catalyst involves a noticeable drop of the yield of deep oxidation products, whereas the yields of vinyl chloride and ethylene remain high. We see little reason in the further cut of the specific surface, because the rate of catalytic dehydrochlorination therewith decreases.
312 The results obtained circumstantially testify that the dehydrochlorination and oxidation reactions proceed at different active sites. It is likely that the oxidation of chlorinated hydrocarbons proceeds at the copper-containing sites. This agrees with the data we obtained in the oxidative chlorination of ethylene [ 11 ]. Taking into account the fact that the value of specific surface is a crucial factor in the choice of catalyst, the further investigations we conducted with using catalyst 2. Both the time-on-stream and the reactant ratio are important chemical engineering parameters affecting the characteristics of the process. It was found that the increase in the time-on-stream at T = 673K can improve both the conversion of ethane and the yield of ethylene. The total yield of chloroorganic products therewith decreases, but the concentration of vinyl chloride passes through a maximum. We also observed an increase in the yield of deep oxidation products COx (see Table 2). Table 2 The effect of time-on-stream on the oxidative chlorination of ethane Catalyst- 8 wt % CuO/cement; T = 673K; reactant ratio C2H6 : HC1 : O2 = 1 : 2 : 1. No.
~, s
Reactant conversion, % C2H6
HC1
O2
Yields scaled to converted ethane, % C2H4C12
C2H3C1
C2H4
COx
1
1.5
80.6
36.0
91.2
25.0
34.1
34.6
2.6
2
3.2
87.5
31.7
89.5
19.4
36.8
40.1
3.2
3
5.6
89.1
30.4
88.6
11.3
38.2
43.5
4.0
4
7.9
90.9
30.0
87.9
10.1
35.6
46.8
6.5
5
10.0
92.7
29.1
86.0
8.2
33.0
47.6
9.7
We can suppose on the strength of the data listed in Table 2 that at the short times-onstream, the major contribution to the formation of deep oxidation products is made by saturated chlorinated hydrocarbons: 1,2 dichloroethane and ethyl chloride. On increasing timeon-stream to more than 6 s, we observed a sharp increase in the yield of deep oxidation products together with the decrease in the yield of vinyl chloride. It is likely that at the longer times-on-stream, the rate of deep oxidation of vinyl chloride would increase and become higher than the rate of dichloroethane dehydrochlorination. Taking into account this fact, we believe that the optimum time-on-stream assuring the best total yield of ethylene and vinyl chloride would be 3--5 s. It was shown in the investigations that the ratio of initial reactants also essentially affects the process. It was found that the excess of hydrogen chloride is favorable for improving the selectivity of the process with reducing the yield of deep oxidation products. At 673K and the reactant ratio of C2H6 : HC1 = 1 : 1, the yield of COx ranges from 6 to 7%; at the reactant ratio of C2H6 : HC1 = 1 : 2, the corresponding yield is 3-----4% (see Table 2). A positive factor is that the carbonization of the catalyst therewith decreases. On the other hand, the increase in the excess of HC1 to ethane up to 3 : 1 involves the decrease in the yield of unsaturated hydrocarbons due to the inhibition of the dehydrochlorination of 1,2dichloroethane and ethyl chloride with hydrogen chloride. The excess of oxygen increases the conversion of ethane mainly due to its oxidation: the yield of carbon oxides increases by 1.8-2 times. Thus, the optimum reactant ratio to provide the best yields of the target products is C2H6 : HC1 : O2 = 1 : 2 : 1.
313 Perfect stability of copper-containing cement catalysts in the oxidative chlorination of ethane was confirmed by their performance for 1500 hours without any decrease in the catalytic activity.
4.Conclusions The results obtained substantiate that the utilization of copper---cement catalysts offers promise for the synthesis of vinyl chloride from ethane at law temperatures in a single step. The proposed efficient and stable copper-cement catalyst will assist in the development of a new technology for the production of vinyl chloride from ethane. This technology is lowwaste and balanced in raw materials with meeting modem requirements of ecological safety. It would be appropriate to conduct the process of vinyl chloride production from ethane, hydrogen chloride, and oxygen in a fixed bed of copper---cement catalyst modified with alkali metals, for example, at 623--673K, time-on-stream of 3--5 s, and reactant ratio of C2H6 : HC1 : 02 - 1 : 2 : 1. Under these conditions, the conversion of ethane is more than 90%, and the total selectivity to ethylene and vinyl chloride is 85-90% at the yield of deep oxidation products COx no more than 3--4%.
REFERENCES
1. Yu.A. Treger, V.N. Rozanov, M.R. Flid, L.M.Kartaschov, Usp. Khim., 57,No 4(1988) 577 2. H.Rigel, H.D.Schindler, M.C.Sze. Chem.Engng.Progr.,.69, Nol0, (1973) 89 3. E.I. Gel'perin, Yu.M. Bakshi, A.K.Avetisov, A.I. Gel'bschtein, Kinet. Katal., 19, No 6 (1978) 527. 4. A.J.Magistro, P.P.Nicholas, R.T.Carrol, J.Organ. Chem., 34 (1969) 271 5. E.I. Gel'perin, Yu.M. Bakshi, A.K.Avetisov, A.I. Gel'bschtein, Kinet. Katal., 20, No 1 (1979) 129. 6. E.I. Gel'perin, Yu.M. Bakshi, A.K.Avetisov, A.I. Gel'bschtein, Kinet. Katal., 24, No 3 (1983) 633. 7. M.M. Mallikarjunan and S. Zahed Hussain, J. Sci., Ind. Res., 42 (1983) 209. 8. V.I. Yakerson, E.Z. Golosman. React. Kinet. Catal. Let., 55, No2 (1995) 455 9. V.I. Yakerson, E.Z. Golosman. Scientific Bases for the Preparation of Heterogeneous Catalysts. VI Intern. Symp.Preprint. 3 Poster Session II Louvain-la-Neuve (Belgium), (1994) 105 10. I.I. Kurlyandskaya, I.G. Solomonik, E.D.Glazunova, E.A.Boevskaya, Yu.M.Bakshi, E.Z. Golosman,V.I. Yakerson, Khim. Prom-st, Moscow, No. 6 (1996) 368. 11. M.R. Flid, I.I. Kurlyandskaya, I.G. Solomonik, M.V.Babotina, Khim. Prom-st, Moscow, No. 6 (1996) 364.