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copper chromite catalysts
Catalysis Communications 8 (2007) 1070–1073 www.elsevier.com/locate/catcom
A highly selective synthesis of pyrazine from ethylenediamine on copper oxide/copper chromite catalysts B. Madhavi Latha *, V. Sadasivam, B. Sivasankar Department of Chemistry, Anna University, Chennai 600 025, India Received 19 January 2006; received in revised form 31 May 2006; accepted 6 June 2006 Available online 16 June 2006
Abstract Ethylenediamine (ED) vapor on passing over a series of copper oxide/copper chromite catalysts in the temperature range of 340– 440 °C yielded pyrazine with a very high selectivity (98–100%), the reaction proceeds by intermolecular deamination and cyclization of ED to form piperazine, which undergoes subsequent dehydrogenation to form pyrazine. Reaction parameters like effect of temperature, contact time, reactant feed ratio and time on stream were found to have profound effect on the reactant conversion as well as the product selectivity. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Ethylenediamine; Pyrazine; Copper oxide/copper chromite catalysts
1. Introduction Pyrazine is the starting material for a variety of drugs and agro chemicals. Though it is of topical interest, literature on the synthesis of pyrazine is meager. In general pyrazine is prepared by the dehydrogenation of piperazine or dealkylation of methyl pyrazine. Cyclization of a mixture of ethylenediamine (ED) and ethylene glycol in the presence of metal catalysts such as Co, Ni, Fe, Al and/or Cr yields pyrazine [1]. It was demonstrated that diethylenetriamine could be converted into a mixture of piperazine and pyrazine in the presence of kaolin-alumina catalyst at 280– 400 °C via deamination and dehydrogenation [2], similar observation was made by Anderson et al. [3]. Many researchers studied the deamination reactions on various catalysts but much focus was not accorded to the deamination leading to cyclization. It was also observed that ED and its linear and cyclic oligomers transform into piperazine and triethylenediamine over H+-pentasil zeolites [4]. The present study includes highly selective synthesis of pyr-
*
Corresponding author. Tel.: +91 44 22203150; fax: +91 44 22200660. E-mail address: [email protected] (B. Madhavi Latha).
1566-7367/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.06.007
azine by intermolecular cyclization of ED involving deaminocyclization and dehydrogenation steps over copper chromite catalysts in a vapor phase reaction. 2. Experimental A series of catalysts with different copper to chromium atom ratio (Cu/Cr = 0.5–4) was synthesized. Copper nitrate (AR grade, Merck), Chromium nitrate (AR grade, Loba Chemie) and NaHCO3 (AR grade, Nice chemicals) were used as received in the preparation of the copper chromite catalysts. Ethylenediamine was purchased from Merck (with purity >99%). Deionized water was used throughout the reaction. 2.1. Synthesis and characterization Copper chromite catalysts were prepared by co-precipitation method. Sodium bicarbonate was added with stirring to a homogeneous mixture of nitrate solutions of copper and chromium. The precipitate formed was collected on a filter, washed and dried at 110 °C. The solid mass was calcined at 650 °C for 6 h. The chemical composition was determined using Atomic Absorption
B. Madhavi Latha et al. / Catalysis Communications 8 (2007) 1070–1073
1071
Scheme 1. A generalized reaction pathway for the formation of pyrazine and other products.
Spectrometer (Perkin–Elmer 373). BET surface area was measured by sorption–desorption of N2 using a single point BET surface area analyzer. The crystallinity and phase purity of all the samples were confirmed by the XRD (PANalytical X-Pert Pro) using Cu Ka ˚ ). (k = 1.5406 A 2.2. Vapor phase condensation of ethylenediamine on copper chromite The vapor phase condensation reactions were carried out in a fixed bed down flow glass reactor at atmospheric pressure in the temperature range 340–440 °C over varied amounts of the catalyst loaded in the middle of the glass reactor tube. The catalyst bed was preceded with ceramic beads to facilitate smooth feeding of the reactant. The catalyst was activated in a flow of hydrogen gas (4 mL/min) at 200 °C for 30 min prior to the reaction. The feed was prepared by mixing ED in water (30% v/v) and introduced into the reactor by a syringe feed pump at a flow rate of 2 mL/h. The liquid products were collected in ice-cold traps and analyzed by gas chromatography (Konik HRGC 4000 series) fitted with an FID detector using OV-101 column (4 m length and 1/8th in. od). All the products were identified by comparison with standard samples and confirmed by GC–MS (Clarus 500, Perkin–Elmer). The mass balance for all the reactions was >90%. Table 1 Important physico-chemical characteristics of the catalyst and the percentage selectivity to the products with the conversion of ED Catalyst (Cu/Cr)
0.5 1.0 2.0 3.0 4.0
Surface area (m2/g)
Total copper (%)
Cu as CuO (%)
ED conversion (%)
Selectivity to products (%) Pyrazine
Others
46.9 43.2 36.0 32.0 23.1
37.9 54.7 70.9 78.6 83.0
0.0 16.8 33.0 40.7 45.1
35.1 56.4 84.9 66.1 49.3
100.0 99.1 98.5 98.2 98.0
0.0 0.9 1.5 1.8 2.0
PY: pyrazine; PIP: piperazine.
3. Results and discussion The important physico-chemical characteristics of the catalyst along with the conversion of ED and selectivity to the product were presented in Table 1. Pyrazine was found to be the major product (98% and greater) as identified by GC and GC–MS. Negligible amounts of piperazine and other products such as alkyl pyrazines (methyl pyrazine and dimethyl pyrazine), and low boilers (ethyl amine and methyl amine) were also identified by GC–MS. A general reaction pathway for the synthesis of pyrazine is shown in Scheme 1. The atom ratio of copper to chromium seems to play a major role in the conversion of ED. Further studies were continued with the catalyst Cu/Cr = 2, since it showed maximum conversion and selectivity. Two molecules of ED condense to form piperazine by releasing two molecules of ammonia, which immediately dehydrogenates to form pyrazine. The decomposition products of ED interact with piperazine and pyrazine to form other alkyl pyrazines [5]. However the percentage of these products is not significant. The formation of pyrazine as the major component suggests the high activity of the copper based catalysts to effect spontaneous dehydrogenation of piperazine [6]. In addition, attainment of aromaticity may also be a major driving force to promote dehydrogenation of piperazine to pyrazine. 3.1. Effect of temperature The intermolecular cyclization of ED over CuO/ CuCr2O4 (Cu/Cr = 2) was studied in the temperature range of 340–440 °C, in steps of 20 °C increment with a flow rate of 2 mL/h over 200 mg of the catalyst. The percentage conversion of ED and selectivity to pyrazine are presented in Table 2. The percentage conversion increased with increase in temperature from 340 to 400 °C and then decreased. The increase in conversion with the raise in temperature from 340 to 400 °C suggests that the reaction is energy demanding. The competitive chemisorption of steam and the
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B. Madhavi Latha et al. / Catalysis Communications 8 (2007) 1070–1073
Table 2 Effect of temperature on synthesis of pyrazine over CuO/CuCr2O4 catalyst
suggests that there is a change in the nature of the active sites responsible for dehydrogenation.
Temperature (°C)
ED conversion (%)
Selectivity to products (%) Py
MP
Others
3.3. Effect of feed
340 360 380 400 420 440
68.2 73.6 83.4 84.9 80.3 77.1
94.1 97.5 98.5 96.5 94.6 94.9
0.0 0.3 0.6 1.2 1.4 0.1
5.9 2.2 0.9 2.3 4.0 5.0
PY: Pyrazine; PIP: Piperazine. Reaction conditions: pressure = atmospheric; contact time = 0.1 h; feed ratio = 30%ED in water (v/v).
released NH3 with the reactant molecules on the active sites of the catalyst could be overcome at the temperature close to 400 °C. The conversion of ED below 340 °C was virtually zero. This is due to the chemisorption of water on the active sites of the catalyst. The decrease in conversion above 400 °C might be due to partial blocking of the active sites by coke formation. The percentage selectivity to the desired product increased from 94.1% at 340 °C to 98.5% at 380 °C and gradually decreased to 94.9% at 440 °C. The lower selectivity at low temperatures may be due to lack of sufficient energy for the cyclization to take place. And at high temperatures formation of high molecular weight alkyl pyrazines which are very difficult to desorb from the catalyst surface may be blocking the active sites thereby reducing the selectivity to pyrazine. 3.2. Effect of contact time The effect of contact time on conversion/selectivity was studied and the results are presented in Table 3. The conversion slightly increased and then decreased with increase in contact time. The decrease in ED conversion at high contact time is due to the formation of multiple alkyl pyrazines, resulting from the reaction between formed pyrazine with the decomposed reactant molecules [4]. The probability for the formation of these products is less at low contact time. The high selectivity to pyrazine even at low contact time indicates the rapid deaminocyclization as well as dehydrogenating ability of the copper catalysts. However, the presence of piperazine at high contact time Table 3 Effect of contact time on synthesis of pyrazine over CuO/CuCr2O4 catalyst Weight of catalyst (g)
W/F (h)
0.1 0.2 0.3 0.4 0.5
0.05 0.1 0.15 0.2 0.25
ED conversion (wt%)
Selectivity to products (%) PY
PIP
Others
82.7 84.9 81.7 66.3 61.8
99.2 98.5 96.8 96.2 93.1
0 0 0.9 2.2 2.3
0.8 1.5 2.3 1.6 4.6
PY: pyrazine; PIP: piperazine. Reaction conditions: pressure = atmospheric; flow rate = 2 mL/h; feed ratio = 30% ED in water (v/v); temperature = 380 °C.
In order to study the effect of water on catalytic activity and selectivity of products, 10%, 30% and 50% (v/v) of ED in water was examined and the results are presented in Table 4. The conversion decreases with increase in the percentage of ED in the feed mixture. Higher conversion with the feed of 10% ED is due to complete adsorption of ED on the active sites, leaving very less ED content in the vapor phase. Also the amount of ED passing through the catalyst bed at a particular time was also less for the feed with low ED content and the product pyrazine could be easily desorbed from the catalyst surface favoring further cyclization. This shows that steam has the expected tendency to reduce coke formation [7]. With an increase in ED content in the feed, in addition to pyrazine, products with high molecular weight can also be formed. However, the latter cannot be easily desorbed from the catalyst surface and hence are prone to form a layer that completely masks the active sites of the catalyst. As a result, the conversion decreases with high ED content in the feed. The selectivity to pyrazine increased with decrease in ED content in the feed. The increase in selectivity could be attributed to the higher volumes of water present in the feed with low ED content. Steam helps to desorb the basic product molecules thus leaving free active sites on the catalyst surface facilitating further reaction [8]. But at higher ED content in the feed, blocking of the active sites by strongly adsorbed product molecules may inhibit the dehydrogenation of piperazine to pyrazine, thus leading to lower selectivity of pyrazine. The presence of piperazine for the feed with high ED content supports the observation. Though the conversion of ED in 30% feed is slightly less compared to the feed with 10%, on account of similar selectivity and considering the reaction time, feed with 30% ED content was fixed to be optimum for the present study. 3.4. Effect of time on stream The effect of time on stream on conversion/selectivity was studied for 7 h at 380 °C and the results are shown in Table 5. The conversion of ED decreased from 85.2% to 42.3% by the end of 7 h. The selectivity to pyrazine gradTable 4 Optimization of feed ratio on synthesis of pyrazine over CuO/CuCr2O4 catalyst Feed (%)
ED conversion (%)
Selectivity to products (%) PY
PIP
Others
10 30 50
92.3 84.9 55.7
100 98.5 84.6
0.0 0.0 4.7
0.0 1.5 10.7
PY: pyrazine; PIP: piperazine. Reaction conditions: pressure = atmospheric; temperature = 380 °C, contact time = 0.1 h.
B. Madhavi Latha et al. / Catalysis Communications 8 (2007) 1070–1073 Table 5 Effect of time on stream on synthesis of pyrazine over CuO/CuCr2O4 catalyst Time (h)
1 2 3 4 5 6 7
ED conversion (%)
85.2 77.8 73.9 66.4 57.7 50.1 42.3
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to Thermo Gravimetric Analysis (TGA). The reactivation process invariably returned the catalyst to its initial activity.
Selectivity to products (%) PY
PIP
Others
97.9 98.6 93.7 92.5 91.1 90.3 83.7
0 0.3 2.8 4.4 5.2 6.5 8.8
2.1 1.1 3.5 3.1 3.7 3.2 7.5
PY: pyrazine; PIP: piperazine. Reaction conditions: pressure = atmospheric; temperature = 380 °C, contact time = 0.1 h.
ually decreased from 97.9% to 90.3% by the end of 6 h, and a sharp decrease to 83.7% was observed by 7th h on stream. Interestingly the percentage of piperazine the intermediate product, which was not observed at any of the temperatures, increased with increase in time, suggesting the gradual loss of the active sites responsible for dehydrogenation. The selectivity to a mixture of other products also increased with the increase in time on stream. The decrease in conversion can be attributed to the deactivation of the copper catalyst by coke formation from the organics in the feed [9]. The catalyst could be regenerated by burning off the carbonaceous deposits in dry flowing air at 250 °C for 2 h which was confirmed by observing the evolution of CO2 by passing the vent gas into barium hydroxide solution. Normally the percentage of coke formation is very less on this catalyst, which was calculated to be 5.3% by subjecting the spent catalyst
4. Conclusion 1. The deaminocyclization of ED followed by dehydrogenation over copper chromite catalysts resulted in the formation of pyrazine with greater selectivity. 2. Increase of water content in the reactant feed reduced formation of other products and facilitated better conversion as well as selectivity. 3. The activity of the catalyst could be easily regained by burning off the carbonaceous deposits in flowing air.
References [1] K.K. Tokai Denka Kogyo, J.P. Tokyo, German Patent P27 22 307.0.44, 1978. [2] E.C. Herrick, US Patent 2,937,176, 1960. [3] A. Anderson, S. Jurels, M.V. Shimanskaya, Chem. Abst. 67 (1967) 100107S. [4] W.T. Reichle, J. Catal. 14 (1993) 556. [5] R. Anand, B.S. Rao, Catal. Commun. 3 (2002) 479. [6] M. Subrahmanyam, S.J. Kulkarni, B. Srinivas, React. Kinet. Catal. Lett. 49 (1993) 455. [7] N. Srinivas, D. Venu Gopal, B. Srinivas, S.J. Kulkarni, M. Subrahmanyam, Micro. Meso. Mater. 51 (2002) 43. [8] S.J. Kulkarni, M. Subrahmanyam, A.V. Rama Rao, Ind. J. Chem. 32A (1993) 28. [9] M. Subrahmanyam, S.J. Kulkarni, A.V. Rama Rao, Ind. J. Chem. Tech. 2 (1995) 237.
A highly selective synthesis of pyrazine from ethylenediamine on copper oxide/copper chromite catalysts B. Madhavi Latha *, V. Sadasivam, B. Sivasankar Department of Chemistry, Anna University, Chennai 600 025, India Received 19 January 2006; received in revised form 31 May 2006; accepted 6 June 2006 Available online 16 June 2006
Abstract Ethylenediamine (ED) vapor on passing over a series of copper oxide/copper chromite catalysts in the temperature range of 340– 440 °C yielded pyrazine with a very high selectivity (98–100%), the reaction proceeds by intermolecular deamination and cyclization of ED to form piperazine, which undergoes subsequent dehydrogenation to form pyrazine. Reaction parameters like effect of temperature, contact time, reactant feed ratio and time on stream were found to have profound effect on the reactant conversion as well as the product selectivity. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Ethylenediamine; Pyrazine; Copper oxide/copper chromite catalysts
1. Introduction Pyrazine is the starting material for a variety of drugs and agro chemicals. Though it is of topical interest, literature on the synthesis of pyrazine is meager. In general pyrazine is prepared by the dehydrogenation of piperazine or dealkylation of methyl pyrazine. Cyclization of a mixture of ethylenediamine (ED) and ethylene glycol in the presence of metal catalysts such as Co, Ni, Fe, Al and/or Cr yields pyrazine [1]. It was demonstrated that diethylenetriamine could be converted into a mixture of piperazine and pyrazine in the presence of kaolin-alumina catalyst at 280– 400 °C via deamination and dehydrogenation [2], similar observation was made by Anderson et al. [3]. Many researchers studied the deamination reactions on various catalysts but much focus was not accorded to the deamination leading to cyclization. It was also observed that ED and its linear and cyclic oligomers transform into piperazine and triethylenediamine over H+-pentasil zeolites [4]. The present study includes highly selective synthesis of pyr-
*
Corresponding author. Tel.: +91 44 22203150; fax: +91 44 22200660. E-mail address: [email protected] (B. Madhavi Latha).
1566-7367/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.06.007
azine by intermolecular cyclization of ED involving deaminocyclization and dehydrogenation steps over copper chromite catalysts in a vapor phase reaction. 2. Experimental A series of catalysts with different copper to chromium atom ratio (Cu/Cr = 0.5–4) was synthesized. Copper nitrate (AR grade, Merck), Chromium nitrate (AR grade, Loba Chemie) and NaHCO3 (AR grade, Nice chemicals) were used as received in the preparation of the copper chromite catalysts. Ethylenediamine was purchased from Merck (with purity >99%). Deionized water was used throughout the reaction. 2.1. Synthesis and characterization Copper chromite catalysts were prepared by co-precipitation method. Sodium bicarbonate was added with stirring to a homogeneous mixture of nitrate solutions of copper and chromium. The precipitate formed was collected on a filter, washed and dried at 110 °C. The solid mass was calcined at 650 °C for 6 h. The chemical composition was determined using Atomic Absorption
B. Madhavi Latha et al. / Catalysis Communications 8 (2007) 1070–1073
1071
Scheme 1. A generalized reaction pathway for the formation of pyrazine and other products.
Spectrometer (Perkin–Elmer 373). BET surface area was measured by sorption–desorption of N2 using a single point BET surface area analyzer. The crystallinity and phase purity of all the samples were confirmed by the XRD (PANalytical X-Pert Pro) using Cu Ka ˚ ). (k = 1.5406 A 2.2. Vapor phase condensation of ethylenediamine on copper chromite The vapor phase condensation reactions were carried out in a fixed bed down flow glass reactor at atmospheric pressure in the temperature range 340–440 °C over varied amounts of the catalyst loaded in the middle of the glass reactor tube. The catalyst bed was preceded with ceramic beads to facilitate smooth feeding of the reactant. The catalyst was activated in a flow of hydrogen gas (4 mL/min) at 200 °C for 30 min prior to the reaction. The feed was prepared by mixing ED in water (30% v/v) and introduced into the reactor by a syringe feed pump at a flow rate of 2 mL/h. The liquid products were collected in ice-cold traps and analyzed by gas chromatography (Konik HRGC 4000 series) fitted with an FID detector using OV-101 column (4 m length and 1/8th in. od). All the products were identified by comparison with standard samples and confirmed by GC–MS (Clarus 500, Perkin–Elmer). The mass balance for all the reactions was >90%. Table 1 Important physico-chemical characteristics of the catalyst and the percentage selectivity to the products with the conversion of ED Catalyst (Cu/Cr)
0.5 1.0 2.0 3.0 4.0
Surface area (m2/g)
Total copper (%)
Cu as CuO (%)
ED conversion (%)
Selectivity to products (%) Pyrazine
Others
46.9 43.2 36.0 32.0 23.1
37.9 54.7 70.9 78.6 83.0
0.0 16.8 33.0 40.7 45.1
35.1 56.4 84.9 66.1 49.3
100.0 99.1 98.5 98.2 98.0
0.0 0.9 1.5 1.8 2.0
PY: pyrazine; PIP: piperazine.
3. Results and discussion The important physico-chemical characteristics of the catalyst along with the conversion of ED and selectivity to the product were presented in Table 1. Pyrazine was found to be the major product (98% and greater) as identified by GC and GC–MS. Negligible amounts of piperazine and other products such as alkyl pyrazines (methyl pyrazine and dimethyl pyrazine), and low boilers (ethyl amine and methyl amine) were also identified by GC–MS. A general reaction pathway for the synthesis of pyrazine is shown in Scheme 1. The atom ratio of copper to chromium seems to play a major role in the conversion of ED. Further studies were continued with the catalyst Cu/Cr = 2, since it showed maximum conversion and selectivity. Two molecules of ED condense to form piperazine by releasing two molecules of ammonia, which immediately dehydrogenates to form pyrazine. The decomposition products of ED interact with piperazine and pyrazine to form other alkyl pyrazines [5]. However the percentage of these products is not significant. The formation of pyrazine as the major component suggests the high activity of the copper based catalysts to effect spontaneous dehydrogenation of piperazine [6]. In addition, attainment of aromaticity may also be a major driving force to promote dehydrogenation of piperazine to pyrazine. 3.1. Effect of temperature The intermolecular cyclization of ED over CuO/ CuCr2O4 (Cu/Cr = 2) was studied in the temperature range of 340–440 °C, in steps of 20 °C increment with a flow rate of 2 mL/h over 200 mg of the catalyst. The percentage conversion of ED and selectivity to pyrazine are presented in Table 2. The percentage conversion increased with increase in temperature from 340 to 400 °C and then decreased. The increase in conversion with the raise in temperature from 340 to 400 °C suggests that the reaction is energy demanding. The competitive chemisorption of steam and the
1072
B. Madhavi Latha et al. / Catalysis Communications 8 (2007) 1070–1073
Table 2 Effect of temperature on synthesis of pyrazine over CuO/CuCr2O4 catalyst
suggests that there is a change in the nature of the active sites responsible for dehydrogenation.
Temperature (°C)
ED conversion (%)
Selectivity to products (%) Py
MP
Others
3.3. Effect of feed
340 360 380 400 420 440
68.2 73.6 83.4 84.9 80.3 77.1
94.1 97.5 98.5 96.5 94.6 94.9
0.0 0.3 0.6 1.2 1.4 0.1
5.9 2.2 0.9 2.3 4.0 5.0
PY: Pyrazine; PIP: Piperazine. Reaction conditions: pressure = atmospheric; contact time = 0.1 h; feed ratio = 30%ED in water (v/v).
released NH3 with the reactant molecules on the active sites of the catalyst could be overcome at the temperature close to 400 °C. The conversion of ED below 340 °C was virtually zero. This is due to the chemisorption of water on the active sites of the catalyst. The decrease in conversion above 400 °C might be due to partial blocking of the active sites by coke formation. The percentage selectivity to the desired product increased from 94.1% at 340 °C to 98.5% at 380 °C and gradually decreased to 94.9% at 440 °C. The lower selectivity at low temperatures may be due to lack of sufficient energy for the cyclization to take place. And at high temperatures formation of high molecular weight alkyl pyrazines which are very difficult to desorb from the catalyst surface may be blocking the active sites thereby reducing the selectivity to pyrazine. 3.2. Effect of contact time The effect of contact time on conversion/selectivity was studied and the results are presented in Table 3. The conversion slightly increased and then decreased with increase in contact time. The decrease in ED conversion at high contact time is due to the formation of multiple alkyl pyrazines, resulting from the reaction between formed pyrazine with the decomposed reactant molecules [4]. The probability for the formation of these products is less at low contact time. The high selectivity to pyrazine even at low contact time indicates the rapid deaminocyclization as well as dehydrogenating ability of the copper catalysts. However, the presence of piperazine at high contact time Table 3 Effect of contact time on synthesis of pyrazine over CuO/CuCr2O4 catalyst Weight of catalyst (g)
W/F (h)
0.1 0.2 0.3 0.4 0.5
0.05 0.1 0.15 0.2 0.25
ED conversion (wt%)
Selectivity to products (%) PY
PIP
Others
82.7 84.9 81.7 66.3 61.8
99.2 98.5 96.8 96.2 93.1
0 0 0.9 2.2 2.3
0.8 1.5 2.3 1.6 4.6
PY: pyrazine; PIP: piperazine. Reaction conditions: pressure = atmospheric; flow rate = 2 mL/h; feed ratio = 30% ED in water (v/v); temperature = 380 °C.
In order to study the effect of water on catalytic activity and selectivity of products, 10%, 30% and 50% (v/v) of ED in water was examined and the results are presented in Table 4. The conversion decreases with increase in the percentage of ED in the feed mixture. Higher conversion with the feed of 10% ED is due to complete adsorption of ED on the active sites, leaving very less ED content in the vapor phase. Also the amount of ED passing through the catalyst bed at a particular time was also less for the feed with low ED content and the product pyrazine could be easily desorbed from the catalyst surface favoring further cyclization. This shows that steam has the expected tendency to reduce coke formation [7]. With an increase in ED content in the feed, in addition to pyrazine, products with high molecular weight can also be formed. However, the latter cannot be easily desorbed from the catalyst surface and hence are prone to form a layer that completely masks the active sites of the catalyst. As a result, the conversion decreases with high ED content in the feed. The selectivity to pyrazine increased with decrease in ED content in the feed. The increase in selectivity could be attributed to the higher volumes of water present in the feed with low ED content. Steam helps to desorb the basic product molecules thus leaving free active sites on the catalyst surface facilitating further reaction [8]. But at higher ED content in the feed, blocking of the active sites by strongly adsorbed product molecules may inhibit the dehydrogenation of piperazine to pyrazine, thus leading to lower selectivity of pyrazine. The presence of piperazine for the feed with high ED content supports the observation. Though the conversion of ED in 30% feed is slightly less compared to the feed with 10%, on account of similar selectivity and considering the reaction time, feed with 30% ED content was fixed to be optimum for the present study. 3.4. Effect of time on stream The effect of time on stream on conversion/selectivity was studied for 7 h at 380 °C and the results are shown in Table 5. The conversion of ED decreased from 85.2% to 42.3% by the end of 7 h. The selectivity to pyrazine gradTable 4 Optimization of feed ratio on synthesis of pyrazine over CuO/CuCr2O4 catalyst Feed (%)
ED conversion (%)
Selectivity to products (%) PY
PIP
Others
10 30 50
92.3 84.9 55.7
100 98.5 84.6
0.0 0.0 4.7
0.0 1.5 10.7
PY: pyrazine; PIP: piperazine. Reaction conditions: pressure = atmospheric; temperature = 380 °C, contact time = 0.1 h.
B. Madhavi Latha et al. / Catalysis Communications 8 (2007) 1070–1073 Table 5 Effect of time on stream on synthesis of pyrazine over CuO/CuCr2O4 catalyst Time (h)
1 2 3 4 5 6 7
ED conversion (%)
85.2 77.8 73.9 66.4 57.7 50.1 42.3
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to Thermo Gravimetric Analysis (TGA). The reactivation process invariably returned the catalyst to its initial activity.
Selectivity to products (%) PY
PIP
Others
97.9 98.6 93.7 92.5 91.1 90.3 83.7
0 0.3 2.8 4.4 5.2 6.5 8.8
2.1 1.1 3.5 3.1 3.7 3.2 7.5
PY: pyrazine; PIP: piperazine. Reaction conditions: pressure = atmospheric; temperature = 380 °C, contact time = 0.1 h.
ually decreased from 97.9% to 90.3% by the end of 6 h, and a sharp decrease to 83.7% was observed by 7th h on stream. Interestingly the percentage of piperazine the intermediate product, which was not observed at any of the temperatures, increased with increase in time, suggesting the gradual loss of the active sites responsible for dehydrogenation. The selectivity to a mixture of other products also increased with the increase in time on stream. The decrease in conversion can be attributed to the deactivation of the copper catalyst by coke formation from the organics in the feed [9]. The catalyst could be regenerated by burning off the carbonaceous deposits in dry flowing air at 250 °C for 2 h which was confirmed by observing the evolution of CO2 by passing the vent gas into barium hydroxide solution. Normally the percentage of coke formation is very less on this catalyst, which was calculated to be 5.3% by subjecting the spent catalyst
4. Conclusion 1. The deaminocyclization of ED followed by dehydrogenation over copper chromite catalysts resulted in the formation of pyrazine with greater selectivity. 2. Increase of water content in the reactant feed reduced formation of other products and facilitated better conversion as well as selectivity. 3. The activity of the catalyst could be easily regained by burning off the carbonaceous deposits in flowing air.
References [1] K.K. Tokai Denka Kogyo, J.P. Tokyo, German Patent P27 22 307.0.44, 1978. [2] E.C. Herrick, US Patent 2,937,176, 1960. [3] A. Anderson, S. Jurels, M.V. Shimanskaya, Chem. Abst. 67 (1967) 100107S. [4] W.T. Reichle, J. Catal. 14 (1993) 556. [5] R. Anand, B.S. Rao, Catal. Commun. 3 (2002) 479. [6] M. Subrahmanyam, S.J. Kulkarni, B. Srinivas, React. Kinet. Catal. Lett. 49 (1993) 455. [7] N. Srinivas, D. Venu Gopal, B. Srinivas, S.J. Kulkarni, M. Subrahmanyam, Micro. Meso. Mater. 51 (2002) 43. [8] S.J. Kulkarni, M. Subrahmanyam, A.V. Rama Rao, Ind. J. Chem. 32A (1993) 28. [9] M. Subrahmanyam, S.J. Kulkarni, A.V. Rama Rao, Ind. J. Chem. Tech. 2 (1995) 237.