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Oxidative dehydrogenation of glycolic acid to glyoxylic acid over Fe-P-O catalyst
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.
527
O x i d a t i v e d e h y d r o g e n a t i o n of glycolic acid to g l y o x y l i c acid over Fe-P-O catalyst M. Ai a and K. Ohdan b aDepartment of Applied Chemistry and Biotechnology, Niigata Institute of Technology, 1719 Fujihashi, Kashiwazaki 945-11, Japan bUbe Laboratory, UBE Industies Ltd., 1978-5 Kogushi, Ube 755, Japan Glyoxylic acid was found to be produced by a vapor-phase oxidative dehydrogenation of glycolic acid over iron phosphate catalysts with a P/Fe atomic ratio of 1.2. The best results were obtained with iron phosphates freshly calcined at 400 to 450~ Reduced iron phosphates showed a markedly lower activity. The optimum reaction temperature was about 240~ The selectivity to glyoxylic acid was 74 tool% up to the glycolic acid conversion of about 70%; the highest yield of glyoxylic acid was 56.5 mol% at the conversion of 80 %.
1. INTRODUCTION Glyoxylic acid is a raw material for various chemicals. It is generally produced by enzymatic or nitric acid oxidation of glyoxal, or electrolytic reduction of oxalic acid. It is also known that alkyl esters of glyoxylic acid are obtained by a vaporphase oxidation of corresponding alkyl esters of glycolic acid [1]. The yield of ester reached 69 mol% at the conversion of 94%. HOCH2-COOR + 0.5 02
9 OHC-COOR + H20
However, it seemed still difficult to obtain glyoxylic acid directly from glycolic acid in a vapor-phase process. In the preceding studies [2-4], it was found that iron phosphate catalysts show a high selectivity in the oxidative dehydrogenation of compounds in which the carbon atom at the a-position of an electron-attracting group (X) such as -COOH, -CHO, or CN, is tertiary; for example, isobutyric acid, isobutyraldehyde, and isobutyronitrile, but that they are inactive for oxygen insertion reactions. R-CH2-CHR'-X + 0.5 02
9 R-CH=CR'-X + H20
528 It was also found recently that iron phosphate catalysts show a high selectivity both in the oxidative dehydrogenation of lactic acid to pyruvic acid and in the oxidative decarboxy-condensation of pyruvic acid to citraconic anhydride [5-7]. CH3-CH(OH)-COOH + 0.5 02 ; CH3-CO-COOH + H20 2 CH3-CO-COOH + 0.502 ; C5H403 + CO2 + 2 H 2 0 These findings led us to study the catalytic performance of iron phosphate in the oxidative dehydrogenation of glycolic acid to glyoxylic acid. HOCH2-COOH + 0.5 02
, OHC-COOH + H20
2. EXPERIMENTAL
2.1. Catalysts The catalysts used in this study were iron phosphates with a P / F e atomic ratio of 1.2, prepared according to the procedures described in the previous studies [2,4,8]. On the other hand, a V-P oxide catalyst with a P / V atomic ratio of 1.06 consisting of vanadyl pyrophosphate [(VO)2P207] was prepared according to a patent [9]. H3PMo12040 and HsPMol0V 2040 catalysts were prepared by supporting molybdophosphoric acid and molybdovanadophosphoric acid, respectively, on an equal weight of natural pumice between 10 to 20 in mesh size [10].
2.1. Reaction procedures The reaction was carried out with a continuous-flow system at atmospheric pressure. The reactor was made of a stainless steel tube, 50 cm long and 1.8 cm I.D., mounted vertically and immersed in a lead bath. The catalyst was placed near the bottom of the reactor and porcelain cylinders, 3 m m long and 1.5 m m I.D./3.0 m m O.D., were placed both under and above the catalyst bed. Air or a mixture of nitrogen and oxygen was fed in from the top of the reactor; an aqueous solution containing 100 g of glycolic acid in I 000 ml was introduced into the preheating section of the reactor with a syringe pump. Unless indicated otherwise, the feed rates of glycolic acid, oxygen, water, and nitrogen were 12.26, 10.0, 480, and 500 m m o l / h , respectively, and the reaction temperature was 240~ The amount of catalyst used was 10 g; the contact time was about 2.5 s. The extent of reaction was varied by changing the amount of catalyst used from 0.7 to 20 g, while fixing the feed rates. The effluent gas from the reactor was led successively into four chilled scrubbers to recover the water soluble compounds. The products were analyzed by GC and LC. The definitions of contact time, conversion, yield, and selectivity were the same as those in a previous study [11]. 3. RESULTS 3.1. Performance of metal phosphate catalysts Since the reaction is an "acid to acid type" transformation, it is predictable that the effective catalysts should be acidic [12]. The V-P oxide and heteropoly
529 compound catalysts, which show a good performance in oxidation producing acidic compounds such as maleic anhydride and methacrylic acid, were tested for the oxidative dehydrogenation of glycolic acid together with an iron phosphate catalyst. The results are summarized in Table 1. It is clear that the iron phosphate catalyst is much more effective than the V-P oxide and heteropoly compound catalysts. Table 1 Performance of metal phosphate catalysts Catalyst Temp Fe-P
V-P Mo-P Mo-V-P
02 feed
Conv
oc
mmol/h
%
GXAD
HCHO
Yield (mol%) HCCK)H
COx
mol%
S
240 240 240 240 265 240 250 240 270
11.3 12.5 16.2 7.5 7.5 11.0 11.0 10.0 10.0
53.0 59.0 70.4 54.4 93.7 66.0 83.7 70.5 86.9
40.0 45.0 53.8 15.1 16.0 16.6 15.5 19.5 24.4
3.9 4.0 4.1 10.0 30.0 9.1 10.6 2.5 11.2
0.7 0.9 1.6 2.0 4.5 10.0 10.9 5.7 11.0'
7.4 6.7 10.7 15.2 45.1 32.4 38.0 15.2 42.1
77 76 76 28 18 25 19 27 28
Conv: conversion of glycolic acid, GXAD: glyoxylic acid, S: selectivity to glyoxylic acid, Fe-P: iron phosphate, V-P: vanadyl pyrophosphate, Mo-P: H3PMo12040, Mo-V-P: H5PMol0V 2040.
3.2. Effect of calcination temperature on the catalytic performance It was found in previous studies [13,14] that when the temperature of calcination of freshly prepared iron phiosphate is raised, the surface area decreases markedly and the structure changes clearly, that is, amorphous phase FePO4 is transformed into tridymite type FePO4 at 400 to 500~ and the tridymite type FePO4 is then transformed into quartz type FePO4 at a temperature above 500 to 550~ The effects of the calcination temperature on the catalytic performance were studied. The results are summarized in Table 2. When the temperature of calcination was low; 300~ the catalyst consisted of amorphous FePO4. It showed a high activity for consumption of glycolic acid, but the main products were COx and formaldehyde; the selectivity to glyoxylic acid was low. On the other hand, when the temperature of calcination was high; 500~ the catalyst consisted of a mixture of tridymite and quartz FePO4. The catalyst showed a low activity, though the selectivity to glyoxylic acid was not bad. As a result, it is concluded that the optimum temperature of calcination of iron phosphate is in the range of 400 to 450~
530 Table 2 Effect of calcination temperature on the catalytic performance of iron phosphate Calcin Temp ~ 300 400
450
500
Reaction Temp c.t. ~ s 240 0.6 230 1.2 240 1.2 240 2.5 240 4.0 240 1.2 240 2.5 240 3.0 240 4.0 240 5.0 240 2.5 240 5.0
GAD Conv % 42.0 63.5 44.5 66.0 84.0 36.3 62.0 69.5 76.4 85.0 11.5 45.0
GXAD 13.8 9.2 31.0 46.2 55.8 26.8 45.0 51.2 55.0 56.3 6.7 31.8
Yield (mol%) HCHO HCOOH 8.2 14.3 3.2 6.0 6.0 2.1 4.0 4.0 5.4 5.4 1.8 3.2
1.6 2.3 0.7 3.0 5.5 0.8 1.9 2.9 3.2 6.3 0.0 0.5
COx 14.4 26.0 4.6 12.4 17.3 2.8 9.3 10.1 13.6 18.6 0.8 4.8
Select GXAD tool% 33 14 70 70 67 74 73 74 72 67 58 70
Calci: calcination of iron phosphate, Select: selectivity, c.t.- contact time, Cony" conversion, GAD: glycolic acid, GXAD: glyoxylic acid. 3.3. Effect of reduction of iron phosphate on the catalytic performance It is known from a previous study [13] that fresh iron phosphate catalysts consising of FePO4 (Fe3+), in which amorphous, tridymite type, and quartz type are included, are reduced to Fe2P207 (Fe 2+) via an intermediate compound consisting of Fe3(P207)2 (Fe233+) 9 In the oxidative dehydrogenation of lactic acid to pyruvic acid, the catalysts consisting of Fe3(P2C)7)2 phase show a higher selectivity than the FePO4 and Fe2P207 catalysts [5]. While in the oxidative dehydrogenation of isobutyric acid to methacrylic acid, the catalytic performance; activity and selectivity, were almost independent of the variation in both the structure and the degree of reduction [15]. These findings led us to check the effect of reduction of iron phosphate on the catalytic performance. The reaction was performed using three catalysts; an iron phosphate freshly calcined at 450~ the one which was partially reduced, and the one which was fully reduced by hydrogen. The results are summarized in Table 3. The partially and fully reduced iron phosphates are clearly lower in both the catalytic activity and selectivity than the iron phosphate consisting of tridymite FePO4.
3.4. Performance of fresh (non-reduced) iron phosphate catalyst In order to clarify in more detail the characteristic features of the oxidative dehydrogenation of glycolic acid to glyoxylic acid, the study was performed using a iron phosphatre catalyst freshly calcinbed at 450~
531 Table 3 Effect of oxidation states and structure on the catalytic p e r f o r m a n c e Catalyst structure
Reaction c.t. Temp s ~
[Fresh] FePO4 tridymite
2.5 2.5 4.0
Fe3(P207)2
Fe2P207
Yield (mol%) HCHO HCOOH
GXAD
240 250 240
62.0 77.0 76.4
45.0 54.2 55.0
4.0 6.1 5.4
1.9 3.9 3.2
9.3 15.8 13.6
73 70 72
2.5 2.5 4.0 4.0
240 250 240 250
33.0 47.5 48.3 72.6
25.0 32.7 32.3 45.0
3.3 4.4 4.8 7.8
1.0 1.4 0.9 3.8
3.5 6.9 6.3 18.2
75 68 67 62
2.5 2.5
240 260
35.2 70.0
25.0 40.0
3.7 11.0
0.7 2.1
5.0 17.6
71 57
COx
A b b r e v i a t i o n s are the s a m e as those in Tables 1 a n d 2
3.4.1. Product distribution 60
50
~.
40
3o
20 o
10
Select GXAD mol%
GAD Conv ~
|/I
f
0 L/ 0
9 -" e ~~ , 20 30 40 50 60 10 C o n v e r s i o n of glycolic acid / %
Figure 1. P r o d u c t distribution
, 70
HCQOH 80
90
532 ~-
601 300
~ 50 40
~- 40 30
240 ~C
280oc
~ 30
0~
"~ 20 ~ 10
" i
0
0
"
", 0.2
,
,
,
,
,
0.4 0.6 0.8 1.0 1.2 Contact time / s
Figure 2. Effect of temperature on the rate
1020
, . . . . 40 50 60 70 80 9 0 Conversion of glycolic acid / % ,
30
Figure 3. Effect of temperature on the yield of glyoxylic acid
The reaction was performed under the fixed reaction conditions described in the Experimental section, while changing the contact time from 0.3 to 5.0 s. The main products detected were glyoxylic acid, formaldehyde, formic acid, and carbon oxides. The yields of these products are plotted as a function of the conversion of glycolic acid in Figure 1. The slope of lines from the origin indicate the selectivities to each product. The selectivity to glyoxylic acid remains unchanged at about 74 mol% with an increase in the conversion of glycolic acid up to about 70%. With a further incrase in the conversion, the yield of glyoxylic acid levels off; the m a x i m u m yield is 56.5 tool% at the conversion of 80%. Glyoxylic acid is relatively stable under the reaction conditions used. The side reactions are the formation of formaldehyde and formic acid, that is, C-C bond fission by oxygen insertion. 3.4.2.
Effect
of reaction
temperature
The reaction was performed by changing the reaction temperature from 200 to 300~ and the contact time from 0.3 to 5 s, while fixing the feed rates as described in the Experimental section. Figure 2 shows the conversion of glycolic acid as a function of the temperature. The results indicate that the rate of reaction becomes about double as the temperature is raised by 20~ The yield of glyoxylic acid obtained at different temperatures are plotted as a function of the conversion of glycolic acid in Figure 3. As the temperature is raised, the selectivity to glyoxylic acid decreases, especially at higher conversion regions. However, it should be noted that vaporization of glycolic acid becomes difficult at tempertures below 230~ because the boiling point of glycolic acid is high. It is therefore concluded that the optimum temperature is about 240~ under the reaction conditions used.
533 ~60 m
C
40
-----O'-
~ t~
U
~ ~,,,i
;>
O U
@ =
o;> 30 -
S
Feed of glycolic acid - 12.2 retool/h
olO
o
4:0
~
30
,- 20
=
E 5o
0
t
t3
I
L
I
I
//,
o 20
i
5 10 15 20 25 100 Oxygen feed rate / m m o l / h
Figure 4. Effect of oxygen on the rate
10
~, ~ a
Oxygen feed rate o 10 m m o l / h
-
a
a~ I
25 m m o l / h
9 100 m m o l o / h I
I
I
l
l
I
20 30 40 50 60 70 80 Conversion of glycolic acid /% Figure 5. Effect of oxygen on the yield of glyoxlic acid
3.4.3. Effect of oxygen feed rate The reaction was performed by changing the feed rate of oxygen from 1.5 to 100 r e t o o l / h and the a m o u n t of catalyst used, while fixing the temperature and the feed rates of glycolic acid, water and nitrogen as described in the Experimental section. The conversions of glycolic acid obtained at a short contact time of 0.62 s (amount of catalyst used = 2.5 g) are plotted as a function of the feed rate of oxygen in Figure 4. The rate of reaction increases with an increase in the feed rate of oxygen, but the oxgen dependency is very low; far from first order dependency. Figure 5 shows the yields of glyoxylic acid obtained with different oxygen feed rates and at different contact times as a function of the conversion of gylcolic acid. W h e n the c o n v e r s i o n is not high; less than 60 %, the selectivity to glyoxylic acid is almost independent of the oxygen feed rate. However, with a further increase in the conversion, the selectivity decreases with an increase in the feed rate of oxygen. It is therefore concluded that the o p t i m u m oxygen feed rate is in the range of 10 to 25 m m o l / h under the reaction conditions used. 4. DISCUSSION Three reactions of glycolic acid may take place in parallel as follows: HOCH2-COOH + 0.5 02 > OHC-COOH + H20 HOCH2-COOH + 02 > HCHO + CO2 + H 2 0 HOCH2-COOH + 02 > HCOOH + CO + H 2 0 Mo and V based oxides and phosphates possess both acidic and redox functions. Therefore, they show a good performance as catalysts in many partial oxidations for producing especially acidic compounds [12]. However, they possess
534 double bond oxygen species, that is, M = O species, which promote the oxygen insertion reactions as well as the dehydrogenation. As a result, in the reaction of glycolic acid, they promote the C-C bond fission by oxygen insertion. On the other hand, iron phosphate possesses both acidic and redox functions as like Mo and V oxides and phosphates. However, it possesses no double bonded oxygen species unlike the Mo and V compounds. Therefore, its function to promote oxygen insertion processes is very weak. As a result, in the reaction of glycolic acid, the formation of formaldehyde and formic acid is suppressed, to a certain extent, and a relatively good performance for glyoxylic acid is obtained. As is seen in Figures 4 and 5, use of a low oxygen feed rate seems to be beneficial. However, it should be noted that under the reaction conditions described in the Experimental section, the catalytic activity was stable enough during the first 1 or 2 days, but after then the activity decreased slowly. The deactivated catalyst was found to be reduced during the reaction. As shown in Table 3, the catalytic activity for the reaction of glycolic acid is markedly decreased by the reduction of iron ions. As is seen in Figure 3, use of a low temperature of 240~ is beneficial to the selectivity to glyoxylic acid. The iron phosphate catalyst shows a relatively high oxidation activity even at a low temperature of 240~ This suggests that the redox cycles on the surface takes place rapidly. However, the rate of re-oxidation of reduced bulk iron phosphate should be very slow at a low temperture of 240~ It is therefore necessary to avoid the reduction of bulk in order to keep the activity. It is also noted that the deactivated catalyst was fully regenerated by heattreatment at about 450~ in stream of air. REFERENCES
1. P.R. Anantaneni and T.P. Li, US Patent No. 4 900 864 (1990). 2. M. Ai, E. Muneyama, A. Kunishige, and K. Ohdan, Bull. Chem. Soc. Jpn., 67 (1994) 551. 3. M. Ai, E. Muneyama, A. Kunishige, and K. Ohdan, Catal. Lett., 24 (1994) 362. 4. E. Muneyama, A. Kunishige, K. Ohdan, and M. Ai, J. Mol. Catal., 89 (1994) 371. 5. M. Ai and K. Ohdan, Chem. Lett., (1995) 405. 6. M. Ai and K. Ohdan, Chem. Lett., (1996) 247' 7. M. Ai and K. Ohdan, Stud. Surf. Sci. Catal., 101 (1996) 201. 8. E. Muneyama, A. Kunishige, K. Ohdan, and M. Ai, Appl. Catal., A. 116 (1994) 165. 9. M. Katsumoto and D.M. Marquis, US Patent No. 4 132 670 (1979). 10. M. Ai, J. Catal., 116 (1989) 23. 11. M. Ai, E. Muneyama, A. Kunishige, and K. Ohdan, Appl. Catal., A. 109 (1994) 135. 12. M. Ai, Proceedings, 7th International Congress on Catalysis, Tokyo, 1980, Kodansha, Tokyo - Elsevier, Amsterdam, 1981, p. 1060. 13. E. Muneyama, A. Kunishige, K. Ohdan, and M. Ai, J. Catal., 158 (1996) 378. 14. M. Ai, E. Muneyama, A. Kunishige, and K. Ohdan, J. Catal., 144 (1993) 632. 15. M. Ai, E. Muneyama, A. Kunishige, and K. Ohdan, Catal. Lett., 24 (1994) 355.
527
O x i d a t i v e d e h y d r o g e n a t i o n of glycolic acid to g l y o x y l i c acid over Fe-P-O catalyst M. Ai a and K. Ohdan b aDepartment of Applied Chemistry and Biotechnology, Niigata Institute of Technology, 1719 Fujihashi, Kashiwazaki 945-11, Japan bUbe Laboratory, UBE Industies Ltd., 1978-5 Kogushi, Ube 755, Japan Glyoxylic acid was found to be produced by a vapor-phase oxidative dehydrogenation of glycolic acid over iron phosphate catalysts with a P/Fe atomic ratio of 1.2. The best results were obtained with iron phosphates freshly calcined at 400 to 450~ Reduced iron phosphates showed a markedly lower activity. The optimum reaction temperature was about 240~ The selectivity to glyoxylic acid was 74 tool% up to the glycolic acid conversion of about 70%; the highest yield of glyoxylic acid was 56.5 mol% at the conversion of 80 %.
1. INTRODUCTION Glyoxylic acid is a raw material for various chemicals. It is generally produced by enzymatic or nitric acid oxidation of glyoxal, or electrolytic reduction of oxalic acid. It is also known that alkyl esters of glyoxylic acid are obtained by a vaporphase oxidation of corresponding alkyl esters of glycolic acid [1]. The yield of ester reached 69 mol% at the conversion of 94%. HOCH2-COOR + 0.5 02
9 OHC-COOR + H20
However, it seemed still difficult to obtain glyoxylic acid directly from glycolic acid in a vapor-phase process. In the preceding studies [2-4], it was found that iron phosphate catalysts show a high selectivity in the oxidative dehydrogenation of compounds in which the carbon atom at the a-position of an electron-attracting group (X) such as -COOH, -CHO, or CN, is tertiary; for example, isobutyric acid, isobutyraldehyde, and isobutyronitrile, but that they are inactive for oxygen insertion reactions. R-CH2-CHR'-X + 0.5 02
9 R-CH=CR'-X + H20
528 It was also found recently that iron phosphate catalysts show a high selectivity both in the oxidative dehydrogenation of lactic acid to pyruvic acid and in the oxidative decarboxy-condensation of pyruvic acid to citraconic anhydride [5-7]. CH3-CH(OH)-COOH + 0.5 02 ; CH3-CO-COOH + H20 2 CH3-CO-COOH + 0.502 ; C5H403 + CO2 + 2 H 2 0 These findings led us to study the catalytic performance of iron phosphate in the oxidative dehydrogenation of glycolic acid to glyoxylic acid. HOCH2-COOH + 0.5 02
, OHC-COOH + H20
2. EXPERIMENTAL
2.1. Catalysts The catalysts used in this study were iron phosphates with a P / F e atomic ratio of 1.2, prepared according to the procedures described in the previous studies [2,4,8]. On the other hand, a V-P oxide catalyst with a P / V atomic ratio of 1.06 consisting of vanadyl pyrophosphate [(VO)2P207] was prepared according to a patent [9]. H3PMo12040 and HsPMol0V 2040 catalysts were prepared by supporting molybdophosphoric acid and molybdovanadophosphoric acid, respectively, on an equal weight of natural pumice between 10 to 20 in mesh size [10].
2.1. Reaction procedures The reaction was carried out with a continuous-flow system at atmospheric pressure. The reactor was made of a stainless steel tube, 50 cm long and 1.8 cm I.D., mounted vertically and immersed in a lead bath. The catalyst was placed near the bottom of the reactor and porcelain cylinders, 3 m m long and 1.5 m m I.D./3.0 m m O.D., were placed both under and above the catalyst bed. Air or a mixture of nitrogen and oxygen was fed in from the top of the reactor; an aqueous solution containing 100 g of glycolic acid in I 000 ml was introduced into the preheating section of the reactor with a syringe pump. Unless indicated otherwise, the feed rates of glycolic acid, oxygen, water, and nitrogen were 12.26, 10.0, 480, and 500 m m o l / h , respectively, and the reaction temperature was 240~ The amount of catalyst used was 10 g; the contact time was about 2.5 s. The extent of reaction was varied by changing the amount of catalyst used from 0.7 to 20 g, while fixing the feed rates. The effluent gas from the reactor was led successively into four chilled scrubbers to recover the water soluble compounds. The products were analyzed by GC and LC. The definitions of contact time, conversion, yield, and selectivity were the same as those in a previous study [11]. 3. RESULTS 3.1. Performance of metal phosphate catalysts Since the reaction is an "acid to acid type" transformation, it is predictable that the effective catalysts should be acidic [12]. The V-P oxide and heteropoly
529 compound catalysts, which show a good performance in oxidation producing acidic compounds such as maleic anhydride and methacrylic acid, were tested for the oxidative dehydrogenation of glycolic acid together with an iron phosphate catalyst. The results are summarized in Table 1. It is clear that the iron phosphate catalyst is much more effective than the V-P oxide and heteropoly compound catalysts. Table 1 Performance of metal phosphate catalysts Catalyst Temp Fe-P
V-P Mo-P Mo-V-P
02 feed
Conv
oc
mmol/h
%
GXAD
HCHO
Yield (mol%) HCCK)H
COx
mol%
S
240 240 240 240 265 240 250 240 270
11.3 12.5 16.2 7.5 7.5 11.0 11.0 10.0 10.0
53.0 59.0 70.4 54.4 93.7 66.0 83.7 70.5 86.9
40.0 45.0 53.8 15.1 16.0 16.6 15.5 19.5 24.4
3.9 4.0 4.1 10.0 30.0 9.1 10.6 2.5 11.2
0.7 0.9 1.6 2.0 4.5 10.0 10.9 5.7 11.0'
7.4 6.7 10.7 15.2 45.1 32.4 38.0 15.2 42.1
77 76 76 28 18 25 19 27 28
Conv: conversion of glycolic acid, GXAD: glyoxylic acid, S: selectivity to glyoxylic acid, Fe-P: iron phosphate, V-P: vanadyl pyrophosphate, Mo-P: H3PMo12040, Mo-V-P: H5PMol0V 2040.
3.2. Effect of calcination temperature on the catalytic performance It was found in previous studies [13,14] that when the temperature of calcination of freshly prepared iron phiosphate is raised, the surface area decreases markedly and the structure changes clearly, that is, amorphous phase FePO4 is transformed into tridymite type FePO4 at 400 to 500~ and the tridymite type FePO4 is then transformed into quartz type FePO4 at a temperature above 500 to 550~ The effects of the calcination temperature on the catalytic performance were studied. The results are summarized in Table 2. When the temperature of calcination was low; 300~ the catalyst consisted of amorphous FePO4. It showed a high activity for consumption of glycolic acid, but the main products were COx and formaldehyde; the selectivity to glyoxylic acid was low. On the other hand, when the temperature of calcination was high; 500~ the catalyst consisted of a mixture of tridymite and quartz FePO4. The catalyst showed a low activity, though the selectivity to glyoxylic acid was not bad. As a result, it is concluded that the optimum temperature of calcination of iron phosphate is in the range of 400 to 450~
530 Table 2 Effect of calcination temperature on the catalytic performance of iron phosphate Calcin Temp ~ 300 400
450
500
Reaction Temp c.t. ~ s 240 0.6 230 1.2 240 1.2 240 2.5 240 4.0 240 1.2 240 2.5 240 3.0 240 4.0 240 5.0 240 2.5 240 5.0
GAD Conv % 42.0 63.5 44.5 66.0 84.0 36.3 62.0 69.5 76.4 85.0 11.5 45.0
GXAD 13.8 9.2 31.0 46.2 55.8 26.8 45.0 51.2 55.0 56.3 6.7 31.8
Yield (mol%) HCHO HCOOH 8.2 14.3 3.2 6.0 6.0 2.1 4.0 4.0 5.4 5.4 1.8 3.2
1.6 2.3 0.7 3.0 5.5 0.8 1.9 2.9 3.2 6.3 0.0 0.5
COx 14.4 26.0 4.6 12.4 17.3 2.8 9.3 10.1 13.6 18.6 0.8 4.8
Select GXAD tool% 33 14 70 70 67 74 73 74 72 67 58 70
Calci: calcination of iron phosphate, Select: selectivity, c.t.- contact time, Cony" conversion, GAD: glycolic acid, GXAD: glyoxylic acid. 3.3. Effect of reduction of iron phosphate on the catalytic performance It is known from a previous study [13] that fresh iron phosphate catalysts consising of FePO4 (Fe3+), in which amorphous, tridymite type, and quartz type are included, are reduced to Fe2P207 (Fe 2+) via an intermediate compound consisting of Fe3(P207)2 (Fe233+) 9 In the oxidative dehydrogenation of lactic acid to pyruvic acid, the catalysts consisting of Fe3(P2C)7)2 phase show a higher selectivity than the FePO4 and Fe2P207 catalysts [5]. While in the oxidative dehydrogenation of isobutyric acid to methacrylic acid, the catalytic performance; activity and selectivity, were almost independent of the variation in both the structure and the degree of reduction [15]. These findings led us to check the effect of reduction of iron phosphate on the catalytic performance. The reaction was performed using three catalysts; an iron phosphate freshly calcined at 450~ the one which was partially reduced, and the one which was fully reduced by hydrogen. The results are summarized in Table 3. The partially and fully reduced iron phosphates are clearly lower in both the catalytic activity and selectivity than the iron phosphate consisting of tridymite FePO4.
3.4. Performance of fresh (non-reduced) iron phosphate catalyst In order to clarify in more detail the characteristic features of the oxidative dehydrogenation of glycolic acid to glyoxylic acid, the study was performed using a iron phosphatre catalyst freshly calcinbed at 450~
531 Table 3 Effect of oxidation states and structure on the catalytic p e r f o r m a n c e Catalyst structure
Reaction c.t. Temp s ~
[Fresh] FePO4 tridymite
2.5 2.5 4.0
Fe3(P207)2
Fe2P207
Yield (mol%) HCHO HCOOH
GXAD
240 250 240
62.0 77.0 76.4
45.0 54.2 55.0
4.0 6.1 5.4
1.9 3.9 3.2
9.3 15.8 13.6
73 70 72
2.5 2.5 4.0 4.0
240 250 240 250
33.0 47.5 48.3 72.6
25.0 32.7 32.3 45.0
3.3 4.4 4.8 7.8
1.0 1.4 0.9 3.8
3.5 6.9 6.3 18.2
75 68 67 62
2.5 2.5
240 260
35.2 70.0
25.0 40.0
3.7 11.0
0.7 2.1
5.0 17.6
71 57
COx
A b b r e v i a t i o n s are the s a m e as those in Tables 1 a n d 2
3.4.1. Product distribution 60
50
~.
40
3o
20 o
10
Select GXAD mol%
GAD Conv ~
|/I
f
0 L/ 0
9 -" e ~~ , 20 30 40 50 60 10 C o n v e r s i o n of glycolic acid / %
Figure 1. P r o d u c t distribution
, 70
HCQOH 80
90
532 ~-
601 300
~ 50 40
~- 40 30
240 ~C
280oc
~ 30
0~
"~ 20 ~ 10
" i
0
0
"
", 0.2
,
,
,
,
,
0.4 0.6 0.8 1.0 1.2 Contact time / s
Figure 2. Effect of temperature on the rate
1020
, . . . . 40 50 60 70 80 9 0 Conversion of glycolic acid / % ,
30
Figure 3. Effect of temperature on the yield of glyoxylic acid
The reaction was performed under the fixed reaction conditions described in the Experimental section, while changing the contact time from 0.3 to 5.0 s. The main products detected were glyoxylic acid, formaldehyde, formic acid, and carbon oxides. The yields of these products are plotted as a function of the conversion of glycolic acid in Figure 1. The slope of lines from the origin indicate the selectivities to each product. The selectivity to glyoxylic acid remains unchanged at about 74 mol% with an increase in the conversion of glycolic acid up to about 70%. With a further incrase in the conversion, the yield of glyoxylic acid levels off; the m a x i m u m yield is 56.5 tool% at the conversion of 80%. Glyoxylic acid is relatively stable under the reaction conditions used. The side reactions are the formation of formaldehyde and formic acid, that is, C-C bond fission by oxygen insertion. 3.4.2.
Effect
of reaction
temperature
The reaction was performed by changing the reaction temperature from 200 to 300~ and the contact time from 0.3 to 5 s, while fixing the feed rates as described in the Experimental section. Figure 2 shows the conversion of glycolic acid as a function of the temperature. The results indicate that the rate of reaction becomes about double as the temperature is raised by 20~ The yield of glyoxylic acid obtained at different temperatures are plotted as a function of the conversion of glycolic acid in Figure 3. As the temperature is raised, the selectivity to glyoxylic acid decreases, especially at higher conversion regions. However, it should be noted that vaporization of glycolic acid becomes difficult at tempertures below 230~ because the boiling point of glycolic acid is high. It is therefore concluded that the optimum temperature is about 240~ under the reaction conditions used.
533 ~60 m
C
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~ t~
U
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Feed of glycolic acid - 12.2 retool/h
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o
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~
30
,- 20
=
E 5o
0
t
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L
I
I
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i
5 10 15 20 25 100 Oxygen feed rate / m m o l / h
Figure 4. Effect of oxygen on the rate
10
~, ~ a
Oxygen feed rate o 10 m m o l / h
-
a
a~ I
25 m m o l / h
9 100 m m o l o / h I
I
I
l
l
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20 30 40 50 60 70 80 Conversion of glycolic acid /% Figure 5. Effect of oxygen on the yield of glyoxlic acid
3.4.3. Effect of oxygen feed rate The reaction was performed by changing the feed rate of oxygen from 1.5 to 100 r e t o o l / h and the a m o u n t of catalyst used, while fixing the temperature and the feed rates of glycolic acid, water and nitrogen as described in the Experimental section. The conversions of glycolic acid obtained at a short contact time of 0.62 s (amount of catalyst used = 2.5 g) are plotted as a function of the feed rate of oxygen in Figure 4. The rate of reaction increases with an increase in the feed rate of oxygen, but the oxgen dependency is very low; far from first order dependency. Figure 5 shows the yields of glyoxylic acid obtained with different oxygen feed rates and at different contact times as a function of the conversion of gylcolic acid. W h e n the c o n v e r s i o n is not high; less than 60 %, the selectivity to glyoxylic acid is almost independent of the oxygen feed rate. However, with a further increase in the conversion, the selectivity decreases with an increase in the feed rate of oxygen. It is therefore concluded that the o p t i m u m oxygen feed rate is in the range of 10 to 25 m m o l / h under the reaction conditions used. 4. DISCUSSION Three reactions of glycolic acid may take place in parallel as follows: HOCH2-COOH + 0.5 02 > OHC-COOH + H20 HOCH2-COOH + 02 > HCHO + CO2 + H 2 0 HOCH2-COOH + 02 > HCOOH + CO + H 2 0 Mo and V based oxides and phosphates possess both acidic and redox functions. Therefore, they show a good performance as catalysts in many partial oxidations for producing especially acidic compounds [12]. However, they possess
534 double bond oxygen species, that is, M = O species, which promote the oxygen insertion reactions as well as the dehydrogenation. As a result, in the reaction of glycolic acid, they promote the C-C bond fission by oxygen insertion. On the other hand, iron phosphate possesses both acidic and redox functions as like Mo and V oxides and phosphates. However, it possesses no double bonded oxygen species unlike the Mo and V compounds. Therefore, its function to promote oxygen insertion processes is very weak. As a result, in the reaction of glycolic acid, the formation of formaldehyde and formic acid is suppressed, to a certain extent, and a relatively good performance for glyoxylic acid is obtained. As is seen in Figures 4 and 5, use of a low oxygen feed rate seems to be beneficial. However, it should be noted that under the reaction conditions described in the Experimental section, the catalytic activity was stable enough during the first 1 or 2 days, but after then the activity decreased slowly. The deactivated catalyst was found to be reduced during the reaction. As shown in Table 3, the catalytic activity for the reaction of glycolic acid is markedly decreased by the reduction of iron ions. As is seen in Figure 3, use of a low temperature of 240~ is beneficial to the selectivity to glyoxylic acid. The iron phosphate catalyst shows a relatively high oxidation activity even at a low temperature of 240~ This suggests that the redox cycles on the surface takes place rapidly. However, the rate of re-oxidation of reduced bulk iron phosphate should be very slow at a low temperture of 240~ It is therefore necessary to avoid the reduction of bulk in order to keep the activity. It is also noted that the deactivated catalyst was fully regenerated by heattreatment at about 450~ in stream of air. REFERENCES
1. P.R. Anantaneni and T.P. Li, US Patent No. 4 900 864 (1990). 2. M. Ai, E. Muneyama, A. Kunishige, and K. Ohdan, Bull. Chem. Soc. Jpn., 67 (1994) 551. 3. M. Ai, E. Muneyama, A. Kunishige, and K. Ohdan, Catal. Lett., 24 (1994) 362. 4. E. Muneyama, A. Kunishige, K. Ohdan, and M. Ai, J. Mol. Catal., 89 (1994) 371. 5. M. Ai and K. Ohdan, Chem. Lett., (1995) 405. 6. M. Ai and K. Ohdan, Chem. Lett., (1996) 247' 7. M. Ai and K. Ohdan, Stud. Surf. Sci. Catal., 101 (1996) 201. 8. E. Muneyama, A. Kunishige, K. Ohdan, and M. Ai, Appl. Catal., A. 116 (1994) 165. 9. M. Katsumoto and D.M. Marquis, US Patent No. 4 132 670 (1979). 10. M. Ai, J. Catal., 116 (1989) 23. 11. M. Ai, E. Muneyama, A. Kunishige, and K. Ohdan, Appl. Catal., A. 109 (1994) 135. 12. M. Ai, Proceedings, 7th International Congress on Catalysis, Tokyo, 1980, Kodansha, Tokyo - Elsevier, Amsterdam, 1981, p. 1060. 13. E. Muneyama, A. Kunishige, K. Ohdan, and M. Ai, J. Catal., 158 (1996) 378. 14. M. Ai, E. Muneyama, A. Kunishige, and K. Ohdan, J. Catal., 144 (1993) 632. 15. M. Ai, E. Muneyama, A. Kunishige, and K. Ohdan, Catal. Lett., 24 (1994) 355.