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Ammoxidation of ethane
Applied Catalysis A: General 188 (1999) 211–217
Ammoxidation of ethane V. Solid-state ion exchange to prepare cobalt zeolite catalysts Y. Li1 , J.N. Armor ∗ Air Products and Chemicals, Inc., 7201 Hamilton Blvd., Allentown, PA 18195, USA Received 12 February 1999; accepted 28 May 1999
Abstract Earlier, we reported prior dealumination of Y zeolite provides much higher activity and selectivity for the ammoxidation of ethane to acetonitrile. In this manuscript, we describe the preparation of active dealuminated Y catalysts by both aqueous and solid-state ion-exchange procedures. Although it is difficult to achieve exactly the same levels of exchange by these two techniques, in general solid state exchange of cobalt ion resulted in less cobalt in the exchange sites, but higher yields of acetonitrile at lower Co/Al values. Solid-state exchange of three different zeolite topologies (ZSM-5, beta, and USY) also produced enhanced selectivity for NH3 incorporation into acetonitrile. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Ammoxidation; Solid-state ion exchange; Co-beta; Co-USY; Co-ZSM-5 zeolites
1. Introduction
C2 H6 + NH3 + 23 O2 → C2 H3 N + 3H2 O
In previous publications [1–5] we have described the selective oxidation of ethane to acetonitrile (Eq. (1)) which is facilitated by the presence of ammonia to yield acetonitrile. A wide variety of catalysts were studied, and the greatest activity and selectivity were obtained with a variety of Co ion-exchanged zeolites. The type of zeolite is very important. The most active zeolites supports share common features in topology, i.e., multidimensional channel structures with either 10 or 12 ring openings. Recently we described the importance of using dealuminated Y zeolites to considerably boost the activity of traditional Y zeolite [4,5].
During the course of our studies on Reaction (1), it became necessary to consider the preparation of large quantities of catalyst. In order to simplify the former aqueous ion-exchange procedures, we considered solid-state ion exchange of cobalt salts into the zeolite. This approach to ion exchange [6–13] is a proven, powerful technique for post synthesis modification of zeolites [6]. In our previous work, the Co-zeolite catalysts were made by multiple exchanges of a zeolite (in either NH4 + or Na+ form) with a Co2+ (acetate) salt in a aqueous solution. The exchange is limited by cation equilibrium in solution. Therefore, repeated exchange is necessary to obtain high Co2+ exchange levels. Normally, the preparation is time consuming – at least five aqueous exchanges (three exchanges for NH4 + and two for cobalt ion). In recent years, solid-state ion-exchange method has received greater attention
∗ Corresponding author. Tel.: +1-610-481-5792; fax: +1-610-481-2989 E-mail address: [email protected] (J.N. Armor) 1 Co-corresponding author. Present address: Engelhard Corporation, 101 Wood Ave., Iselin, NJ 08830-0770, USA.
0926-860X/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 9 9 ) 0 0 2 3 6 - 7
(1)
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[6,12]. Typically, a H-zeolite (or NH4 -zeolite) powder is mixed with a metal chloride salt. Ball milling of a physical mixture ensures uniform mixing. The mixture is then heated in vacuum or in an inert gas to a high temperature to allow cations to exchange. If the chloride salt of the desired cation is chosen, this process is not equilibrium-limited because HCl generated during the exchange escapes continuously from the system. One can reach a complete exchange if a stoichiometric mixture is used. This exchange method is efficient since it normally takes less than 1 day. Jentys, Lercher et al. [7] showed that solid-state exchange of cobalt ions into ZSM-5 led to formation of two new Lewis acid sites and that the solid-state exchange approach provided complete exchange. A great deal has been written on the characterization of materials produced by solid state exchange. In principle, solid-state exchange method allows one to exchange multivalent cations into zeolite sites, which is normally very difficult because of large size of a hydrated metal complex. More importantly, solid-state exchange can create some sites that are different from those obtained by aqueous exchange. We describe here Co-zeolites made by the solid-state exchange method and by the traditional aqueous exchange method and compared as catalysts for the C2 H6 ammoxidation reaction (Reaction (1)). We wondered whether a solid-state exchange approach would produce more active catalysts for the ammoxidation of ethane given in Reaction (1). We also sought to compare the performance of catalysts made by both solid state and aqueous exchange of three different zeolite topologies for the ammoxidation of ethane and studied a number of variations of solid-state ion exchange with dealuminated USY. As shown below solid-state exchange did provide better catalysts.
slurry was filtered, washed with de-ionized water, filtered, and dried at 110◦ C overnight. Procedures for solid-state exchange were developed following typical recipes given in the literature for exchanging transition metal salts (7,12). These and all other zeolites will be designated by the following terminology: Co-zeolite topology (bulk Si/Al) – % metal exchanged. (Note that for divalent cations, such as Co2+ , a Co/Al atomic ratio of 0.5 is equivalent to 100% of its theoretical exchange capacity.) In the example below, (Co-ZSM-5(16.9)-76) represents a Co-ZSM-5 with Si/Al = 16.9 at a level of 76% cobalt exchange. 2.1.1. Cobalt aqueous exchange with NH4 -ZSM-5 (Co-ZSM-5(16.9)-76) 30 g of H-ZSM-5 (PQ Corp., CBV 3020E) was washed with 1 l of water containing 1.26 g of reagent grade ammonium hydroxide for 5 h and filtered. It was then washed with a 1 M solution of NH4 NO3 (18 cc solution/g zeolite), filtered, and washed with de-ionized water. 20 g of this NH4 -ZSM-5 was exchanged with 1 l, 0.01 M cobalt acetate aqueous solution (pH = ∼6) at 70–80◦ C for 24 h. After two identical exchanges, the resulting zeolite slurry was filtered, washed with 1 l de-ionized water and then filtered again. Finally, the zeolite was dried at 110◦ C overnight. Elemental analysis showed that the Si/Al ratio was 16.9 and the Co/Al atomic ratio of this catalyst was 0.38, or 76% of the cation-exchange capacity. Extensive washing was used to assure removal of trace chloride using these water soluble cobalt salts, although there would be no tendency for anionic chloride to remain during the cation exchange process. We also observed no differences in catalyst performance (see Table 4, below) when a two-fold excess of chloride salt was used to prepare the catalyst.
2. Experimental 2.1. Preparation of catalysts For comparison metal-zeolite catalysts were prepared by the conventional aqueous cation-exchange method. Typically, an NH4 -zeolite was exchanged with a metal cation in an aqueous solution, e.g. cobalt acetate solution (pH = 6), at an 80◦ C for 24 h [2,9,14]. After two identical exchanges, the resulting zeolite
2.1.2. Co solid-state exchange of H-ZSM-5: (Co-ZSM-5(14.0)-62) 10 g of H-ZSM-5 (PQ Corp., CVB 3020) was physically mixed in a mortar and pestle with 1.1 g of CoC12 ·6H2 O. The mixture was loaded into a poly bottle with toughened, partially stabilized zirconia milling balls. The container was rolled on a jar mill for 2 h to give a pink colored powder, and the powdered material pelletized and then sieved to 8–16
Y. Li, J.N. Armor / Applied Catalysis A: General 188 (1999) 211–217
mesh material. The solid was heated in air at 2◦ C/min to 100◦ C and held for 2 h and then heated at 2◦ C/min to 500◦ C and held for 4 h before cooling to room temperature. The blue product was crushed to a fine powder (whereby it turns pink) and washed twice with de-ionized water (75◦ C for 30 min), filtered, and dried at 110◦ C in air overnight. 2.1.3. Cobalt aqueous-exchanged beta zeolite: Co-beta(12.3)-130 30 g of NH4 -beta obtained from PQ Corp. (CP814-25, Valley Forge, PA) having an Si/Al ratio of 12, were exchanged with a 2 l, 0.02 M cobalt acetate solution (pH = 6) at 70–80◦ C for 24 h. This exchange was carried out twice. After the second exchange, the resulting zeolite slurry was filtered, then washed with 1 l of de-ionized water, filtered again, and then dried at 110◦ C overnight in air. 2.1.4. Cobalt solid-state exchanged beta zeolite: Co-beta(12.1)-74 10 g of H-beta (PQ Corp., CP811-BL25) was mixed with 1.66 g of CoC12 ·6H2 O by hand using a mortar and pestle for about 10 min until it appeared to be uniformly mixed. The mixture was loaded into a poly bottle with toughened partially stabilized zirconia milling balls. The container was rolled for 2 h, and the powdered material pelletized and then sieved to produce 8–16 mesh material. The solid was heated in air at 1◦ C/min to 100◦ C and held for 2 h and then heated at 1◦ C/min to 500◦ C and held for 4 h before cooling to room temperature. The product was crushed to a fine powder and washed twice with de-ionized water (75◦ C for 30 min), filtered, and dried at 110◦ C in air overnight. 2.1.5. Cobalt aqueous exchange with ultra-stable Y (USY) (Co-Y(2.9)-112) The ultra-stable used in this preparation EZ190 was purchased from Engelhard Co. (Iselin, NJ). The USY was first exchanged with ammonium nitrate solution (1 M, 15 ml solution/g zeolite) at room temperature once to convert it to a NH4 + form. The resulting Y zeolite was then exchanged with a 0.05 M cobalt acetate solution at 80◦ C three times, and each exchange was conducted for 24 h. Then, the resulting Co-Y was filtered, washed and dried at 110◦ C overnight. The ele-
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mental analysis show the following bulk composition: Si/Al = 2.9. Co/Al = 0.55. The Co loading corresponds to 9.81% by weight. 2.1.6. Cobalt exchange with USY in solid-state mode (Co-Y(3.0)-46) 10 g USY (Engelhard Co., EZ190, Si/Al = 3.0) was mixed with 3.64 g CoC12 ·6H2 O in a mortar with grinding. Then this mixture was ball milled for 2 h. This thoroughly mixed zeolite sample was then pelletized, sieved to 8–16 mesh and loaded into a treatment tube. This packed bed was treated with a stream of He (150 cc/min) and heated up with the following temperature program. The temperature was held at 100◦ C for 2 h then at 500◦ C for 4 h, and the ramp rate was 1◦ C/min. After the solid-state exchange, the zeolite was washed twice with 1 l hot water (70◦ C). Finally, it was dried at 110◦ C overnight. This catalyst has the following elemental composition: Si/Al = 3.0 and Co/Al = 0.23. The Co loading corresponding to 5.56 wt.%. 2.1.7. Cobalt aqueous exchange with HCl treated USY (Co-Y(5.3)-90 15 g of USY (Engelhard Co., EZ190) was immersed in 300 ml, 0.37 M HCl solution with stirring for 3 h at room temperature. After the acid soak, the zeolite was washed with 1 l of de-ionized water. The resulting USY was then further exchanged with NH4 + (275 ml, 1 M ammonium nitrite solution at room temperature twice). Then 10 g of this resulting zeolite was exchanged with Co2+ (1 l, 0.036 M) at 70◦ C twice. The resulting sample was filtered, washed and dried at 110◦ C overnight. Elemental analyses of this catalyst show Si/Al = 5.3, Co/Al = 0.45. The Co loading corresponds to 6.56% by weight. 2.1.8. Cobalt solid-state exchange with calcined, HCl treated USY: (Co-Y(7.9)-94) 10 g of HCl treated USY (as above) was ground with 2.88 g of CoC12 ·6H2 O for 10 min (until it appeared uniform). The material was loaded into a poly bottle with hardened ZrO2 grinding stones, as before and rolled for 4 h. After pelletizing to 8–16 mesh, the solid was heated in air at 1◦ C/min to 100◦ C and held for 2 h and then heated at 1◦ C/min to 500◦ C and held for 8 h before cooling to room temperature. The product was crushed to a fine powder and washed twice with
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hot, de-ionized water (75◦ C for 30 min), filtered, and dried at 110◦ C in air overnight. 2.2. Process conditions and experimental apparatus The reaction runs were conducted in a micro-reactor system operating in a steady-state plug-flow mode at atmospheric pressure. The reactor was a U-shaped quartz tube with 1/4 in. o.d. at the inlet section and 3/8 in. o.d. at the outlet section. The catalyst was located in the outlet section at the center of the electrical furnace which surrounds the reactor tube. Quartz wool plugs were used to support and secure the catalyst bed. The reactor’s gas delivery system consisted of four flow channels (NH3 , a hydrocarbon, e.g. C2 H6 , O2 /He mixture and He), each controlled by an independent mass flow controller, and these channels merged and mixed before going to the reactor inlet. The concentration of NH3 was varied between 2 and 20%, that of hydrocarbon between 2 and 20% and O2 between 1 and 9%. The catalysts were first pelletized, crushed and sieved to 20/40 mesh before loading in the reactor. The total flow rate was controlled between 50 and 200 ml/min. The amount of catalyst varied from 0.05 to 0.4 g. The space velocity, therefore, varied between 7500 and 240,000 cc/h g. However, for performance comparison a typically sample weight of 0.3 g and a feed flow rate of 100 cc/min were used with the following feed composition: 10% NH3 , 10% C2 H6 and 6.5% O2 in He. Reaction temperature was controlled by a temperature programmer (Yokogawa, Model UP 40) and was varied between 350 to 500◦ C. A catalyst was routinely pretreated with flowing helium at 500◦ C for 1 h before a reaction run in order to establish a standard operating procedure allowing time for the product distributions to stablize. The reactor effluent was analyzed by two gas chromatographs in series both equipped with a thermal conductivity detector (TCD). Hydrocarbons, nitrile, CO2 , and N2 O were separated by a Porapak Q column, while N2 , O2 , and CO were separated by a molecular sieve 5A column. The reactor system was also connected to an on-line mass spectrometer, which confirmed the product identification. All routine product quantification was therefore carrier out with the GC technique. The reactor effluent was analyzed by two gas chromatographs in series both equipped with a thermal
conductivity detector (TCD). Hydrocarbons, nitrile, CO2 , and N2 O were separated by a Porapak Q column, while N2 , O2 , and CO were separated by a molecular sieve 5A column. The reactor system was also connected to an on-line mass spectrometer, which confirmed the product identification. All routine product quantification was therefore carrier out with the GC technique. The conversion and selectivities are defined as follows P i ni i yP (1) Conversion of C2 H6 , X = yE nE + i yi ni Selectivity of product Pi (carbon basis), y i ni Si = P i yi ni Yield of product Pi (carbon basis), Yi =XSi
(2) (3)
where yi and yE are the mole fractions of product Pi and C2 H6 , respectively; ni and nE are the number of carbon atoms in each molecule of product Pi and C2 H6 , respectively, and all the terms were evaluated for the exit stream. Selectivity of product Pj (nitrogen basis), yj nj P Sj = yA nA + j yj nj where yj and yA are the mole fractions of nitrogen containing product Pj , and that of ammonia, respectively; nj and nA are the number of nitrogen atoms in each molecule of product Pj and ammonia, respectively. The major products of C2 H6 ammoxidation reaction over Co-zeolite catalysts are C2 H3 N, C2 H4 , CO2 and N2 . Other by-products also produced were very small quantities of CO, N2 O, C3 H5 N (propionitrile), and HCN, and these by-products are insignificant compared to the major products. However, for the conversion and selectivity calculations, all products and by-products were included. 3. Results and discussion 3.1. Solid-state exchange of Co2+ into H-zeolites Table 1 compares the Co-ZSM-5 catalysts made from aqueous exchange and solid-state exchange. At
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Table 1 Co-ZSM-5a for C2 H6 ammoxidationb : solid-state exchange versus aqueous exchange Exch. method
Aq. S–S
C2 H6 conv. (%)
27.6 32.1
Selectivity based on C2 H6 (%) C2 H3 N
C 2 H4
CO2
54.7 56.7
31.8 29.4
7.6 7.3
NH3 sel. to C2 H3 N (%)
C2 H3 N yield (%)
22.5 25.9
15.1 18.2
a Aqueous exchange: (Co-ZSM-5 (16.9)-76); Solid-state exchange: (Co-ZSM-5(14.0)-62) Note, both the catalysts were made from the same precursor zeolite (CBV3020). b Feed: 10% C H , 10% NH , 6.5% O ; F = 100 cc/min; 0.2 g cat, T = 475◦ C. 2 6 3 2
Table 2 Co-Betaa for C2 H6 ammoxidationb : solid-state exchange versus aqueous exchange Catalysta
Aq. Aq. Aq. S–S S–S S–S a b
Temp. (◦ C)
425 450 475 425 450 475
C2 H6 conv. (%)
26.2 31.2 34.3 22.8 30.9 38.4
Selectivity based on C2 H6 (%) C2 H 3 N
C2 H 4
CO2
65.5 57.8 49.9 65.8 60.0 56.2
14.8 22.3 29.9 10.2 17.0 22.8
12.6 11.8 11.1 16.5 15.0 12.7
NH3 sel. to C2 H3 N (%)
C2 H3 N yield (%)
22.5 27.8 24.8 34.3 33.7 33.9
17.1 18.3 17.1 15.0 18.6 21.6
Aqueous exchanged Co-beta(12.3)-130; Solid-state exchange Co-beta(12.1)-74. Feed: 10% C2 H6 , 10% NH3 , and 6.5% O2 balanced by He; F = 100 cc/min, w = 0.3 g.
475◦ C, the solid-state method yielded a slightly higher C2 H6 conversion and NH3 selectivity, and therefore a higher nitrile yield (18% for solid state and 15% for aqueous method). For Co-beta catalysts (see Table 2), solid-state method produced a much larger enhancement in NH3 selectivity (by an average of 10% points). It has been pointed out previously that not only is hydrocarbon selectivity important, but so is NH3 selectivity, since NH3 is a valuable reagent and its oxidation to N2 is detrimental for this reaction. On the other hand, only a slightly higher CO2 selectivity was also observed for the solid-state exchange Co-beta. Thus we see that the solid state exchange technique does impact the activity of the catalyst in a number of important ways. Solid-state exchanged Co-USY (made from Engelhard’s EZ-190) is compared with its aqueous exchanged counterpart at three temperatures (Table 3). Moderate increase can be seen across the temperature range for NH3 selectivity and C2 H6 conversion. The resulting enhancement in nitrile yield is quite significant with correspondingly higher selectivity for acetonitrile and less CO2 . The solid-state exchanged Co-USY was washed de-ionized water three times after the exchange to
assure no residual CoCl2 salt remained in the zeolite. If the preparation is not washed the unexchanged CoC12 would be converted to cobalt oxides under reaction conditions. We tested one unwashed Co-USY (made by the solid-state exchange method), the results were very similar to those normally observed over a good Co-ZSM-5 catalyst (32% C2 H6 conversion, 57% C2 H3 N selectivity (based on ethane), 26% NH3 selectivity at 475◦ C with 0.2 g catalyst).One potential factor that may affect the degree of exchange is the Co/Al ratio in the mixture. We made two preparations with different Co chloride to Al-zeolite ratios (0.5 and 1.0), and their catalytic performance is compared in Table 4. No appreciable difference was found. The compositions of these two catalysts are also very similar, suggesting that the exchange level reached the maximum level with respect to the number of exchangeable sites, and additional amounts of cobalt salt did not change the degree of exchange. Any excess CoCl2 was washed out, because the preparation was washed with hot water after the exchange. Essentially, similar activity was obtained regardless of their initial Co/Al ratios used in the mixture. Solid-state exchange was also conducted at 550◦ C, and we did not find any significant
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Table 3 Co-USYa for C2 H6 ammoxidationb : solid-state exchange versus aqueous exchange Exch. method
Aq. Aql Aql S–S S–S S–S a b
Temp. (◦ C)
425 450 475 425 450 475
C2 H6 conv. (%)
6.8 11.1 16.8 10.6 15.9 24.1
Selectivity based on C2 H6 (%) C2 H 3 N
C2 H 6
CO2
61.8 60.6 52.8 65.2 65.4 58.1
14.2 19.2 27.4 14.0 19.5 26.8
20.5 14.7 14.0 15.9 10.8 7.6
NH3 sel. to C2 H3 N (%)
C2 H3 N yield (%)
25.7 28.9 29.3 32.9 33.1 32.3
4.2 6.7 8.9 6.9 10.4 14.0
Aqueous exchanged Co-USY: Co-Y(2.9)-112; Solid-state exchange Co-USY: Co-Y(3.0)-46. Feed: 10% C2 H6 , 10% NH3 , and 6.5% 02 balanced by He; F = 100 cc/min, 0.3 g. catalyst.
Table 4 Ammoxidationa over solid-state exchanged Co-USY: effect of exchange condition Co/Al mix ratio Exch. cond. Temp. (◦ C) C2 H6 conv. (%) Selectivity based on C2 H6 (%)
0.5b
500◦ C,
1c
500◦ C, 8 h
0.5d
550◦ C, 4 h
8h
425 450 475 425 450 475 425 450 475
10.6 15.9 24.1 12.9 18.8 26.6 10.6 15.9 24.1
C2 H 3 N
C2 H6
CO2
65.2 65.4 58.1 65.7 61.0 51.4 65.2 65.4 58.1
14.0 19.5 26.8 18.4 27.1 35.4 14.0 19.5 26.8
15.9 10.8 7.6 12.2 8.5 6.9 15.9 10.8 7.6
NH3 sel. to C2 H3 N (%) C2 H3 N yield (%)
32.9 33.1 32.3 32.3 30.7 29.1 32.9 33.1 32.3
6.9 10.4 14.0 8.5 11.5 13.7 6.9 10.4 14.0
a
Feed: 10% C2 H6 , 10% NH3 , and 6.5% 02 balanced by He; F = 100 cc/min, 0.3 g catalyst. Cobalt chloride/Al-zeolite ratio = 0.5; Co-Y(3.0)-44; h = hours. c As prepared in b above but with Co/Al=1.0, Co-Y(3.0)-56. d As prepared in b above but heated to 550◦ C, Co-Y(3.0)-44. b
difference (see Table 4) in catalyst composition and performance. In our previous manuscript [5], we reported that HCl post treatment of USY enhanced the C2 H3 N yield over CoUSY. Since the solid-state exchange method, in general, results in a better catalyst compared to the aqueous exchange method, we wondered whether we could further enhance the HCl post treated USY by incorporating solid-state ion exchange into the catalyst synthesis procedure. We prepared Co-(HCl treated) USY by solid-state exchange method. (Since cobalt levels are difficult to control precisely for HCl treated zeolites, these two catalysts contain different cobalt loadings: 6.6 wt.% for the aqueous method and 5.0 wt.% for the solid-state method). Table 5 compares the catalytic results of these two methods to make Co-(HCl treated) USY. There is no substantial difference between there two catalysts in C2 H6
conversion and nitrile yield. However, the aqueous exchanged catalyst had a slightly higher NH3 selectivity and the solid-state exchanged catalyst resulted in less CO2 . In general, cobalt ion solid state exchange of an HCl treated USY zeolite did not offer much, if any improvement over the aqueous ion exchange approach. In our previous manuscript [5], we used FTIR to observe and characterize structural and chemical changes in preparing USY catalysts. Unlike the aqueous exchanged Co-USY [5], the solid-state preparation reduced the intensity of all three acidic OH bands above 3500 cm−1 in the same proportion, suggesting that the Co2+ cations occupy all exchangeable sites. We suspect, this is due to the high temperature used for the preparation and the absence of any metal-hydrate complex. As observed with the aqueous exchanged Co-USY-5, after the solid-state
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Table 5 Co-(HCl treated) USYa for C2 H6 ammoxidationb : solid-state versus aqueous exchange Exch. method
Aq. Aq. Aq. S–S S–S S–S a b
Temp. (◦ C)
425 450 475 425 450 475
C2 H6 conv. (%)
10.6 14.6 20.5 10.0 15.1 20.1
Selectivity based on C2 H6 (%) C2 H 3 N
C2 H 6
CO2
71.5 59.2 56.0 69.7 65.3 59.8
11.6 17.6 25.9 16.6 22.6 30.2
14.1 17.8 12.6 11.2 9.4 7.8
NH3 sel. to C2 H3 N (%)
C2 H3 N yield (%)
45.2 39.2 38.1 33.2 33.0 31.8
7.6 8.6 11.5 7.0 9.9 12.0
Aqueous exchange catalyst: Co-Y(5.3)-90; Solid-exchanged catalyst: Co-Y(7.9)-94. Feed: 10% C2 H6 , 10% NH3 , and 6.5% O2 ; F = 100 cc/min, 0.3 g catalyst, T = 475◦ C.
exchange, the 3605 cm−1 band seemed to develop a slight shoulder at a higher frequency. In summary, solid-state ion exchange procedures generally produced a slightly improved catalyst. Aqueous ion exchange can be difficult to control, often resulting in apparent over-exchange. In our work with beta and USY above, we had substantially less cobalt ion present upon solid-state exchange, yet the activity was improved over the aqueous preparation. The fact that we did not need full exchange of the cobalt is somewhat of a surprise. Perhaps, since the reaction has been shown [2,3] to occur at the cobalt sites exchanged within the zeolite, this suggests that the solid state preparative procedure results in a more effective distribution of cobalt ions in the parent zeolite; hence full exchange of all the sites might not be required.
4. Conclusions Solid-state exchange of USY with CoCl2 results an improved catalyst for the ammoxidation reaction, while providing a much simpler means of preparing the catalysts. Solid-state exchange at either 500 or 550◦ C produced about the same level of activity
for the ammoxidation of ethane. Improvements in the selective use of NH3 for the production of acetonitrile and/or the absolute yield of acetonitrile were obtained by solid-state exchange of cobalt chloride into acidic ZSM-5, beta, and USY zeolites. References [1] [2] [3] [4] [5] [6] [7]
[8] [9]
[10] [11] [12] [13] [14]
Y. Li., J.N. Armor, Chem. Comm. 2013 (1997). Y. Li, J.N. Armor, J. Catal. 173 (1998) 511. Y. Li, J.N. Armor, J. Catal. 176 (1998) 495. Y. Li, J.N. Armor, U.S. Patent 5,756,802 (1998). Y. Li, J.N. Armor, Appl. Catal. A, 183 (1999) 107. H. Karge, Stud. Surf. Sci. 105 (1997) 1901. A. Jentys, A. Lugstein, O. El Dusouqui, H. Vinek, in:M. Absi-Halabi, et al. (Eds.), Catalysts in Petroleum Refining and Petrochemical Industries 1995, Elsevier, 1996, 525 pp. .A. Kucherov, A. Slinkin, Zeolites 7 (1987) 43. J.N. Armor, in: Y. Izumi, H. Arai, M. Iwamoto (Eds.), Science and Technology in Catalysis 1994, Kodansha, Tokyo,1995, Stud. Surf. Sci. 92 51–62. V. Kanazirev, G. Price, J. Mol. Catal. A 96 (1995) 145. J. Weitkamp, S. Ernst, T. Bock, T. Krommings, A. Kiss, P. Kleinschmit, US Patent 5,434,114 (1995). A. Kucherov, A. Slinkin, J. Mol. Catal. 90 (1994) 323. H. Karge, B. Wichterlova, H. Beyer, J. Chem. Soc., Faraday Trans. 88 (1992) 1345. Y. Li, J.N. Armor, Appl. Catal. B. Environmental 2 (1993) 239–256.
Ammoxidation of ethane V. Solid-state ion exchange to prepare cobalt zeolite catalysts Y. Li1 , J.N. Armor ∗ Air Products and Chemicals, Inc., 7201 Hamilton Blvd., Allentown, PA 18195, USA Received 12 February 1999; accepted 28 May 1999
Abstract Earlier, we reported prior dealumination of Y zeolite provides much higher activity and selectivity for the ammoxidation of ethane to acetonitrile. In this manuscript, we describe the preparation of active dealuminated Y catalysts by both aqueous and solid-state ion-exchange procedures. Although it is difficult to achieve exactly the same levels of exchange by these two techniques, in general solid state exchange of cobalt ion resulted in less cobalt in the exchange sites, but higher yields of acetonitrile at lower Co/Al values. Solid-state exchange of three different zeolite topologies (ZSM-5, beta, and USY) also produced enhanced selectivity for NH3 incorporation into acetonitrile. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Ammoxidation; Solid-state ion exchange; Co-beta; Co-USY; Co-ZSM-5 zeolites
1. Introduction
C2 H6 + NH3 + 23 O2 → C2 H3 N + 3H2 O
In previous publications [1–5] we have described the selective oxidation of ethane to acetonitrile (Eq. (1)) which is facilitated by the presence of ammonia to yield acetonitrile. A wide variety of catalysts were studied, and the greatest activity and selectivity were obtained with a variety of Co ion-exchanged zeolites. The type of zeolite is very important. The most active zeolites supports share common features in topology, i.e., multidimensional channel structures with either 10 or 12 ring openings. Recently we described the importance of using dealuminated Y zeolites to considerably boost the activity of traditional Y zeolite [4,5].
During the course of our studies on Reaction (1), it became necessary to consider the preparation of large quantities of catalyst. In order to simplify the former aqueous ion-exchange procedures, we considered solid-state ion exchange of cobalt salts into the zeolite. This approach to ion exchange [6–13] is a proven, powerful technique for post synthesis modification of zeolites [6]. In our previous work, the Co-zeolite catalysts were made by multiple exchanges of a zeolite (in either NH4 + or Na+ form) with a Co2+ (acetate) salt in a aqueous solution. The exchange is limited by cation equilibrium in solution. Therefore, repeated exchange is necessary to obtain high Co2+ exchange levels. Normally, the preparation is time consuming – at least five aqueous exchanges (three exchanges for NH4 + and two for cobalt ion). In recent years, solid-state ion-exchange method has received greater attention
∗ Corresponding author. Tel.: +1-610-481-5792; fax: +1-610-481-2989 E-mail address: [email protected] (J.N. Armor) 1 Co-corresponding author. Present address: Engelhard Corporation, 101 Wood Ave., Iselin, NJ 08830-0770, USA.
0926-860X/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 9 9 ) 0 0 2 3 6 - 7
(1)
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[6,12]. Typically, a H-zeolite (or NH4 -zeolite) powder is mixed with a metal chloride salt. Ball milling of a physical mixture ensures uniform mixing. The mixture is then heated in vacuum or in an inert gas to a high temperature to allow cations to exchange. If the chloride salt of the desired cation is chosen, this process is not equilibrium-limited because HCl generated during the exchange escapes continuously from the system. One can reach a complete exchange if a stoichiometric mixture is used. This exchange method is efficient since it normally takes less than 1 day. Jentys, Lercher et al. [7] showed that solid-state exchange of cobalt ions into ZSM-5 led to formation of two new Lewis acid sites and that the solid-state exchange approach provided complete exchange. A great deal has been written on the characterization of materials produced by solid state exchange. In principle, solid-state exchange method allows one to exchange multivalent cations into zeolite sites, which is normally very difficult because of large size of a hydrated metal complex. More importantly, solid-state exchange can create some sites that are different from those obtained by aqueous exchange. We describe here Co-zeolites made by the solid-state exchange method and by the traditional aqueous exchange method and compared as catalysts for the C2 H6 ammoxidation reaction (Reaction (1)). We wondered whether a solid-state exchange approach would produce more active catalysts for the ammoxidation of ethane given in Reaction (1). We also sought to compare the performance of catalysts made by both solid state and aqueous exchange of three different zeolite topologies for the ammoxidation of ethane and studied a number of variations of solid-state ion exchange with dealuminated USY. As shown below solid-state exchange did provide better catalysts.
slurry was filtered, washed with de-ionized water, filtered, and dried at 110◦ C overnight. Procedures for solid-state exchange were developed following typical recipes given in the literature for exchanging transition metal salts (7,12). These and all other zeolites will be designated by the following terminology: Co-zeolite topology (bulk Si/Al) – % metal exchanged. (Note that for divalent cations, such as Co2+ , a Co/Al atomic ratio of 0.5 is equivalent to 100% of its theoretical exchange capacity.) In the example below, (Co-ZSM-5(16.9)-76) represents a Co-ZSM-5 with Si/Al = 16.9 at a level of 76% cobalt exchange. 2.1.1. Cobalt aqueous exchange with NH4 -ZSM-5 (Co-ZSM-5(16.9)-76) 30 g of H-ZSM-5 (PQ Corp., CBV 3020E) was washed with 1 l of water containing 1.26 g of reagent grade ammonium hydroxide for 5 h and filtered. It was then washed with a 1 M solution of NH4 NO3 (18 cc solution/g zeolite), filtered, and washed with de-ionized water. 20 g of this NH4 -ZSM-5 was exchanged with 1 l, 0.01 M cobalt acetate aqueous solution (pH = ∼6) at 70–80◦ C for 24 h. After two identical exchanges, the resulting zeolite slurry was filtered, washed with 1 l de-ionized water and then filtered again. Finally, the zeolite was dried at 110◦ C overnight. Elemental analysis showed that the Si/Al ratio was 16.9 and the Co/Al atomic ratio of this catalyst was 0.38, or 76% of the cation-exchange capacity. Extensive washing was used to assure removal of trace chloride using these water soluble cobalt salts, although there would be no tendency for anionic chloride to remain during the cation exchange process. We also observed no differences in catalyst performance (see Table 4, below) when a two-fold excess of chloride salt was used to prepare the catalyst.
2. Experimental 2.1. Preparation of catalysts For comparison metal-zeolite catalysts were prepared by the conventional aqueous cation-exchange method. Typically, an NH4 -zeolite was exchanged with a metal cation in an aqueous solution, e.g. cobalt acetate solution (pH = 6), at an 80◦ C for 24 h [2,9,14]. After two identical exchanges, the resulting zeolite
2.1.2. Co solid-state exchange of H-ZSM-5: (Co-ZSM-5(14.0)-62) 10 g of H-ZSM-5 (PQ Corp., CVB 3020) was physically mixed in a mortar and pestle with 1.1 g of CoC12 ·6H2 O. The mixture was loaded into a poly bottle with toughened, partially stabilized zirconia milling balls. The container was rolled on a jar mill for 2 h to give a pink colored powder, and the powdered material pelletized and then sieved to 8–16
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mesh material. The solid was heated in air at 2◦ C/min to 100◦ C and held for 2 h and then heated at 2◦ C/min to 500◦ C and held for 4 h before cooling to room temperature. The blue product was crushed to a fine powder (whereby it turns pink) and washed twice with de-ionized water (75◦ C for 30 min), filtered, and dried at 110◦ C in air overnight. 2.1.3. Cobalt aqueous-exchanged beta zeolite: Co-beta(12.3)-130 30 g of NH4 -beta obtained from PQ Corp. (CP814-25, Valley Forge, PA) having an Si/Al ratio of 12, were exchanged with a 2 l, 0.02 M cobalt acetate solution (pH = 6) at 70–80◦ C for 24 h. This exchange was carried out twice. After the second exchange, the resulting zeolite slurry was filtered, then washed with 1 l of de-ionized water, filtered again, and then dried at 110◦ C overnight in air. 2.1.4. Cobalt solid-state exchanged beta zeolite: Co-beta(12.1)-74 10 g of H-beta (PQ Corp., CP811-BL25) was mixed with 1.66 g of CoC12 ·6H2 O by hand using a mortar and pestle for about 10 min until it appeared to be uniformly mixed. The mixture was loaded into a poly bottle with toughened partially stabilized zirconia milling balls. The container was rolled for 2 h, and the powdered material pelletized and then sieved to produce 8–16 mesh material. The solid was heated in air at 1◦ C/min to 100◦ C and held for 2 h and then heated at 1◦ C/min to 500◦ C and held for 4 h before cooling to room temperature. The product was crushed to a fine powder and washed twice with de-ionized water (75◦ C for 30 min), filtered, and dried at 110◦ C in air overnight. 2.1.5. Cobalt aqueous exchange with ultra-stable Y (USY) (Co-Y(2.9)-112) The ultra-stable used in this preparation EZ190 was purchased from Engelhard Co. (Iselin, NJ). The USY was first exchanged with ammonium nitrate solution (1 M, 15 ml solution/g zeolite) at room temperature once to convert it to a NH4 + form. The resulting Y zeolite was then exchanged with a 0.05 M cobalt acetate solution at 80◦ C three times, and each exchange was conducted for 24 h. Then, the resulting Co-Y was filtered, washed and dried at 110◦ C overnight. The ele-
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mental analysis show the following bulk composition: Si/Al = 2.9. Co/Al = 0.55. The Co loading corresponds to 9.81% by weight. 2.1.6. Cobalt exchange with USY in solid-state mode (Co-Y(3.0)-46) 10 g USY (Engelhard Co., EZ190, Si/Al = 3.0) was mixed with 3.64 g CoC12 ·6H2 O in a mortar with grinding. Then this mixture was ball milled for 2 h. This thoroughly mixed zeolite sample was then pelletized, sieved to 8–16 mesh and loaded into a treatment tube. This packed bed was treated with a stream of He (150 cc/min) and heated up with the following temperature program. The temperature was held at 100◦ C for 2 h then at 500◦ C for 4 h, and the ramp rate was 1◦ C/min. After the solid-state exchange, the zeolite was washed twice with 1 l hot water (70◦ C). Finally, it was dried at 110◦ C overnight. This catalyst has the following elemental composition: Si/Al = 3.0 and Co/Al = 0.23. The Co loading corresponding to 5.56 wt.%. 2.1.7. Cobalt aqueous exchange with HCl treated USY (Co-Y(5.3)-90 15 g of USY (Engelhard Co., EZ190) was immersed in 300 ml, 0.37 M HCl solution with stirring for 3 h at room temperature. After the acid soak, the zeolite was washed with 1 l of de-ionized water. The resulting USY was then further exchanged with NH4 + (275 ml, 1 M ammonium nitrite solution at room temperature twice). Then 10 g of this resulting zeolite was exchanged with Co2+ (1 l, 0.036 M) at 70◦ C twice. The resulting sample was filtered, washed and dried at 110◦ C overnight. Elemental analyses of this catalyst show Si/Al = 5.3, Co/Al = 0.45. The Co loading corresponds to 6.56% by weight. 2.1.8. Cobalt solid-state exchange with calcined, HCl treated USY: (Co-Y(7.9)-94) 10 g of HCl treated USY (as above) was ground with 2.88 g of CoC12 ·6H2 O for 10 min (until it appeared uniform). The material was loaded into a poly bottle with hardened ZrO2 grinding stones, as before and rolled for 4 h. After pelletizing to 8–16 mesh, the solid was heated in air at 1◦ C/min to 100◦ C and held for 2 h and then heated at 1◦ C/min to 500◦ C and held for 8 h before cooling to room temperature. The product was crushed to a fine powder and washed twice with
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hot, de-ionized water (75◦ C for 30 min), filtered, and dried at 110◦ C in air overnight. 2.2. Process conditions and experimental apparatus The reaction runs were conducted in a micro-reactor system operating in a steady-state plug-flow mode at atmospheric pressure. The reactor was a U-shaped quartz tube with 1/4 in. o.d. at the inlet section and 3/8 in. o.d. at the outlet section. The catalyst was located in the outlet section at the center of the electrical furnace which surrounds the reactor tube. Quartz wool plugs were used to support and secure the catalyst bed. The reactor’s gas delivery system consisted of four flow channels (NH3 , a hydrocarbon, e.g. C2 H6 , O2 /He mixture and He), each controlled by an independent mass flow controller, and these channels merged and mixed before going to the reactor inlet. The concentration of NH3 was varied between 2 and 20%, that of hydrocarbon between 2 and 20% and O2 between 1 and 9%. The catalysts were first pelletized, crushed and sieved to 20/40 mesh before loading in the reactor. The total flow rate was controlled between 50 and 200 ml/min. The amount of catalyst varied from 0.05 to 0.4 g. The space velocity, therefore, varied between 7500 and 240,000 cc/h g. However, for performance comparison a typically sample weight of 0.3 g and a feed flow rate of 100 cc/min were used with the following feed composition: 10% NH3 , 10% C2 H6 and 6.5% O2 in He. Reaction temperature was controlled by a temperature programmer (Yokogawa, Model UP 40) and was varied between 350 to 500◦ C. A catalyst was routinely pretreated with flowing helium at 500◦ C for 1 h before a reaction run in order to establish a standard operating procedure allowing time for the product distributions to stablize. The reactor effluent was analyzed by two gas chromatographs in series both equipped with a thermal conductivity detector (TCD). Hydrocarbons, nitrile, CO2 , and N2 O were separated by a Porapak Q column, while N2 , O2 , and CO were separated by a molecular sieve 5A column. The reactor system was also connected to an on-line mass spectrometer, which confirmed the product identification. All routine product quantification was therefore carrier out with the GC technique. The reactor effluent was analyzed by two gas chromatographs in series both equipped with a thermal
conductivity detector (TCD). Hydrocarbons, nitrile, CO2 , and N2 O were separated by a Porapak Q column, while N2 , O2 , and CO were separated by a molecular sieve 5A column. The reactor system was also connected to an on-line mass spectrometer, which confirmed the product identification. All routine product quantification was therefore carrier out with the GC technique. The conversion and selectivities are defined as follows P i ni i yP (1) Conversion of C2 H6 , X = yE nE + i yi ni Selectivity of product Pi (carbon basis), y i ni Si = P i yi ni Yield of product Pi (carbon basis), Yi =XSi
(2) (3)
where yi and yE are the mole fractions of product Pi and C2 H6 , respectively; ni and nE are the number of carbon atoms in each molecule of product Pi and C2 H6 , respectively, and all the terms were evaluated for the exit stream. Selectivity of product Pj (nitrogen basis), yj nj P Sj = yA nA + j yj nj where yj and yA are the mole fractions of nitrogen containing product Pj , and that of ammonia, respectively; nj and nA are the number of nitrogen atoms in each molecule of product Pj and ammonia, respectively. The major products of C2 H6 ammoxidation reaction over Co-zeolite catalysts are C2 H3 N, C2 H4 , CO2 and N2 . Other by-products also produced were very small quantities of CO, N2 O, C3 H5 N (propionitrile), and HCN, and these by-products are insignificant compared to the major products. However, for the conversion and selectivity calculations, all products and by-products were included. 3. Results and discussion 3.1. Solid-state exchange of Co2+ into H-zeolites Table 1 compares the Co-ZSM-5 catalysts made from aqueous exchange and solid-state exchange. At
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Table 1 Co-ZSM-5a for C2 H6 ammoxidationb : solid-state exchange versus aqueous exchange Exch. method
Aq. S–S
C2 H6 conv. (%)
27.6 32.1
Selectivity based on C2 H6 (%) C2 H3 N
C 2 H4
CO2
54.7 56.7
31.8 29.4
7.6 7.3
NH3 sel. to C2 H3 N (%)
C2 H3 N yield (%)
22.5 25.9
15.1 18.2
a Aqueous exchange: (Co-ZSM-5 (16.9)-76); Solid-state exchange: (Co-ZSM-5(14.0)-62) Note, both the catalysts were made from the same precursor zeolite (CBV3020). b Feed: 10% C H , 10% NH , 6.5% O ; F = 100 cc/min; 0.2 g cat, T = 475◦ C. 2 6 3 2
Table 2 Co-Betaa for C2 H6 ammoxidationb : solid-state exchange versus aqueous exchange Catalysta
Aq. Aq. Aq. S–S S–S S–S a b
Temp. (◦ C)
425 450 475 425 450 475
C2 H6 conv. (%)
26.2 31.2 34.3 22.8 30.9 38.4
Selectivity based on C2 H6 (%) C2 H 3 N
C2 H 4
CO2
65.5 57.8 49.9 65.8 60.0 56.2
14.8 22.3 29.9 10.2 17.0 22.8
12.6 11.8 11.1 16.5 15.0 12.7
NH3 sel. to C2 H3 N (%)
C2 H3 N yield (%)
22.5 27.8 24.8 34.3 33.7 33.9
17.1 18.3 17.1 15.0 18.6 21.6
Aqueous exchanged Co-beta(12.3)-130; Solid-state exchange Co-beta(12.1)-74. Feed: 10% C2 H6 , 10% NH3 , and 6.5% O2 balanced by He; F = 100 cc/min, w = 0.3 g.
475◦ C, the solid-state method yielded a slightly higher C2 H6 conversion and NH3 selectivity, and therefore a higher nitrile yield (18% for solid state and 15% for aqueous method). For Co-beta catalysts (see Table 2), solid-state method produced a much larger enhancement in NH3 selectivity (by an average of 10% points). It has been pointed out previously that not only is hydrocarbon selectivity important, but so is NH3 selectivity, since NH3 is a valuable reagent and its oxidation to N2 is detrimental for this reaction. On the other hand, only a slightly higher CO2 selectivity was also observed for the solid-state exchange Co-beta. Thus we see that the solid state exchange technique does impact the activity of the catalyst in a number of important ways. Solid-state exchanged Co-USY (made from Engelhard’s EZ-190) is compared with its aqueous exchanged counterpart at three temperatures (Table 3). Moderate increase can be seen across the temperature range for NH3 selectivity and C2 H6 conversion. The resulting enhancement in nitrile yield is quite significant with correspondingly higher selectivity for acetonitrile and less CO2 . The solid-state exchanged Co-USY was washed de-ionized water three times after the exchange to
assure no residual CoCl2 salt remained in the zeolite. If the preparation is not washed the unexchanged CoC12 would be converted to cobalt oxides under reaction conditions. We tested one unwashed Co-USY (made by the solid-state exchange method), the results were very similar to those normally observed over a good Co-ZSM-5 catalyst (32% C2 H6 conversion, 57% C2 H3 N selectivity (based on ethane), 26% NH3 selectivity at 475◦ C with 0.2 g catalyst).One potential factor that may affect the degree of exchange is the Co/Al ratio in the mixture. We made two preparations with different Co chloride to Al-zeolite ratios (0.5 and 1.0), and their catalytic performance is compared in Table 4. No appreciable difference was found. The compositions of these two catalysts are also very similar, suggesting that the exchange level reached the maximum level with respect to the number of exchangeable sites, and additional amounts of cobalt salt did not change the degree of exchange. Any excess CoCl2 was washed out, because the preparation was washed with hot water after the exchange. Essentially, similar activity was obtained regardless of their initial Co/Al ratios used in the mixture. Solid-state exchange was also conducted at 550◦ C, and we did not find any significant
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Table 3 Co-USYa for C2 H6 ammoxidationb : solid-state exchange versus aqueous exchange Exch. method
Aq. Aql Aql S–S S–S S–S a b
Temp. (◦ C)
425 450 475 425 450 475
C2 H6 conv. (%)
6.8 11.1 16.8 10.6 15.9 24.1
Selectivity based on C2 H6 (%) C2 H 3 N
C2 H 6
CO2
61.8 60.6 52.8 65.2 65.4 58.1
14.2 19.2 27.4 14.0 19.5 26.8
20.5 14.7 14.0 15.9 10.8 7.6
NH3 sel. to C2 H3 N (%)
C2 H3 N yield (%)
25.7 28.9 29.3 32.9 33.1 32.3
4.2 6.7 8.9 6.9 10.4 14.0
Aqueous exchanged Co-USY: Co-Y(2.9)-112; Solid-state exchange Co-USY: Co-Y(3.0)-46. Feed: 10% C2 H6 , 10% NH3 , and 6.5% 02 balanced by He; F = 100 cc/min, 0.3 g. catalyst.
Table 4 Ammoxidationa over solid-state exchanged Co-USY: effect of exchange condition Co/Al mix ratio Exch. cond. Temp. (◦ C) C2 H6 conv. (%) Selectivity based on C2 H6 (%)
0.5b
500◦ C,
1c
500◦ C, 8 h
0.5d
550◦ C, 4 h
8h
425 450 475 425 450 475 425 450 475
10.6 15.9 24.1 12.9 18.8 26.6 10.6 15.9 24.1
C2 H 3 N
C2 H6
CO2
65.2 65.4 58.1 65.7 61.0 51.4 65.2 65.4 58.1
14.0 19.5 26.8 18.4 27.1 35.4 14.0 19.5 26.8
15.9 10.8 7.6 12.2 8.5 6.9 15.9 10.8 7.6
NH3 sel. to C2 H3 N (%) C2 H3 N yield (%)
32.9 33.1 32.3 32.3 30.7 29.1 32.9 33.1 32.3
6.9 10.4 14.0 8.5 11.5 13.7 6.9 10.4 14.0
a
Feed: 10% C2 H6 , 10% NH3 , and 6.5% 02 balanced by He; F = 100 cc/min, 0.3 g catalyst. Cobalt chloride/Al-zeolite ratio = 0.5; Co-Y(3.0)-44; h = hours. c As prepared in b above but with Co/Al=1.0, Co-Y(3.0)-56. d As prepared in b above but heated to 550◦ C, Co-Y(3.0)-44. b
difference (see Table 4) in catalyst composition and performance. In our previous manuscript [5], we reported that HCl post treatment of USY enhanced the C2 H3 N yield over CoUSY. Since the solid-state exchange method, in general, results in a better catalyst compared to the aqueous exchange method, we wondered whether we could further enhance the HCl post treated USY by incorporating solid-state ion exchange into the catalyst synthesis procedure. We prepared Co-(HCl treated) USY by solid-state exchange method. (Since cobalt levels are difficult to control precisely for HCl treated zeolites, these two catalysts contain different cobalt loadings: 6.6 wt.% for the aqueous method and 5.0 wt.% for the solid-state method). Table 5 compares the catalytic results of these two methods to make Co-(HCl treated) USY. There is no substantial difference between there two catalysts in C2 H6
conversion and nitrile yield. However, the aqueous exchanged catalyst had a slightly higher NH3 selectivity and the solid-state exchanged catalyst resulted in less CO2 . In general, cobalt ion solid state exchange of an HCl treated USY zeolite did not offer much, if any improvement over the aqueous ion exchange approach. In our previous manuscript [5], we used FTIR to observe and characterize structural and chemical changes in preparing USY catalysts. Unlike the aqueous exchanged Co-USY [5], the solid-state preparation reduced the intensity of all three acidic OH bands above 3500 cm−1 in the same proportion, suggesting that the Co2+ cations occupy all exchangeable sites. We suspect, this is due to the high temperature used for the preparation and the absence of any metal-hydrate complex. As observed with the aqueous exchanged Co-USY-5, after the solid-state
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Table 5 Co-(HCl treated) USYa for C2 H6 ammoxidationb : solid-state versus aqueous exchange Exch. method
Aq. Aq. Aq. S–S S–S S–S a b
Temp. (◦ C)
425 450 475 425 450 475
C2 H6 conv. (%)
10.6 14.6 20.5 10.0 15.1 20.1
Selectivity based on C2 H6 (%) C2 H 3 N
C2 H 6
CO2
71.5 59.2 56.0 69.7 65.3 59.8
11.6 17.6 25.9 16.6 22.6 30.2
14.1 17.8 12.6 11.2 9.4 7.8
NH3 sel. to C2 H3 N (%)
C2 H3 N yield (%)
45.2 39.2 38.1 33.2 33.0 31.8
7.6 8.6 11.5 7.0 9.9 12.0
Aqueous exchange catalyst: Co-Y(5.3)-90; Solid-exchanged catalyst: Co-Y(7.9)-94. Feed: 10% C2 H6 , 10% NH3 , and 6.5% O2 ; F = 100 cc/min, 0.3 g catalyst, T = 475◦ C.
exchange, the 3605 cm−1 band seemed to develop a slight shoulder at a higher frequency. In summary, solid-state ion exchange procedures generally produced a slightly improved catalyst. Aqueous ion exchange can be difficult to control, often resulting in apparent over-exchange. In our work with beta and USY above, we had substantially less cobalt ion present upon solid-state exchange, yet the activity was improved over the aqueous preparation. The fact that we did not need full exchange of the cobalt is somewhat of a surprise. Perhaps, since the reaction has been shown [2,3] to occur at the cobalt sites exchanged within the zeolite, this suggests that the solid state preparative procedure results in a more effective distribution of cobalt ions in the parent zeolite; hence full exchange of all the sites might not be required.
4. Conclusions Solid-state exchange of USY with CoCl2 results an improved catalyst for the ammoxidation reaction, while providing a much simpler means of preparing the catalysts. Solid-state exchange at either 500 or 550◦ C produced about the same level of activity
for the ammoxidation of ethane. Improvements in the selective use of NH3 for the production of acetonitrile and/or the absolute yield of acetonitrile were obtained by solid-state exchange of cobalt chloride into acidic ZSM-5, beta, and USY zeolites. References [1] [2] [3] [4] [5] [6] [7]
[8] [9]
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