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Methacrylic acid synthesis
Applied Catalysis A: General, 88 (1992) 163-177 Elsevier Science Publishers B.V., Amsterdam
163
APCAT A2314
Methacrylic acid synthesis I. Condensation of propionic acid with formaldehyde over alkali metal cation on silica catalysts O.H. Bailey, R.A. Montag and J.S. Yoo Amoco Research Center, P.O. Box 3011, NaperviUe, IL 60566 (USA) (Received 13 January, 1992, revised manuscript received 26 May, 1992)
Abstract
Alkali metal cations supported on silica are effective base catalysts for the production of methacrylic acid via the condensation of propionic acid with formaldehyde. Activity and selectivity are dependent on the choice of alkali metal cation and increase in the order Li < Na < K < Cs. Activity also increases with increasing cation loading. A model for the dependence of product selectivity on cation loading is proposed. Keywords: alkali metal cation, condensation, formaldehyde, propionic acid, silica.
INTRODUCTION
Methyl methacrylate (MMA) is an important industrial monomer used in a wide range of polymerization products, including sheeting for signs and building materials, surface coatings, and molding resins. The predominant commercial technology for MMA manufacture is the Acetone Cyanohydrin Process, which uses as raw materials acetone, hydrogen cyanide, methanol, and sulfuric acid. Technical and economic drawbacks of this process, including HCN handling, raw material cost, and ammonium bisulfate waste, have spurred a considerable industrial research effort to develop alternate routes to MMA [1-6]. Much of this research has focused on condensation reactions of propionald e h y d e , p r o p i o n i c acid, o r m e t h y l p r o p i o n a t e w i t h f o r m a l d e h y d e o r its dim e t h y l acetal, m e t h y l a l [ 7 - 1 5 ] . O n e d r a w b a c k a s s o c i a t e d w i t h t h i s t y p e o f p r o c e s s h a s b e e n t h e l i m i t e d single p a s s y i e l d n e c e s s i t a t i n g e x t e n s i v e r e c y c l i n g Correspondence to: Dr. 0.H. Bailey, Amoco Research Center, P.O. Box 3011, Naperville, IL 60566, USA. Tel. ( + 1-708)4205405, fax. ( + 1-708)4204678.
0926-3373/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
164
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
of reactants. Because methylal decomposes under reaction conditions to yield formaldehyde and methanol, the condensation product obtained with this reagent always consists of a mixture of methyl methacrylate and methacrylic acid. This further complicates downstream processing including product separation and recycle. On the other hand, formaldehyde is generally supplied as 35-45% solutions (formalin), the remainder being water and methanol. Water has been shown to limit reaction [ 16 ] as well as contribute to catalyst deactivation. A process has been developed to effectively remove methanol and dry formalin solutions prior to the condensation reaction, and to recycle unreacted propionic acid following separation from product methacrylic acid [17]. MMA suitable for commercial application can be prepared by esterification of this methacrylic acid [18]. In order to take advantage of this type of downstream processing, active and selective catalysts for the condensation of propionic acid with formaldehyde are required. Silica supported alkali metal cation catalysts are known to be effective base catalysts for a variety of aldol-type condensation reactions [7,19,20]. Effects of alkali metal cation loading on catalyst activity have also been reported [ 19-21 ]. In this paper, the choice of alkali metal cation and loading are evaluated for their effect on the activity and selectivity of silica supported catalysts for the vapor phase synthesis of methacrylic acid (MA) from propionic acid (PA) and formaldehyde (FA). EXPERIMENTAL Catalyst preparation
Each of the alkali metal cation on silica catalysts was prepared by the gelation of a solution of the alkali metal carbonate and a low sodium colloidal silica supplied by Nalco (1034-A). After the pH of the solution was adjusted to 7 with nitric acid, gelation was initiated by addition of concentrated ammonium nitrate solution. Because of the limited solubility of Li2CO3, LiOH-H20 was used for the preparation of the lithium catalyst. The hydrogel was dried in a microwave oven, crushed and sieved to 20-40 mesh, and calcined in air for 8 h at 540 ° C. Cesium content was measured by atomic absorption. Other alkali metal contents were measured using inductively coupled plasma. Surface areas and porosities were determined usiilg the B E T / N 2 desorption technique. All infrared spectra were obtained on a Nicolet 6000C FT-IR. Pressed pellet samples were loaded in a heated cell for thermal and vacuum treatment as well as scanning. Temperature-programmed desorption data were collected on 80 mg-samples
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
165
calcined in dry air for 1 h at 550 ° C. After purging with helium, samples were dosed with helium gas saturated with propionic acid for 30 min. The desorption temperature rate was 10 °C per min.
Catalyst evaluations Reactor feed was prepared by dissolving trioxane in propionic acid in a 3 : 2 molar ratio (PA: FA). For some experiments this solution was diluted with 25 wt.-% heptane, an inert diluent. Space velocities [weight hourly space velocity (WHSV): g feed/g catalyst h] are reported on a total weight basis (including heptane). Reactions were carried out in 1/2 in. O.D. quartz reactors equipped with 1/8 in. thermowells. Heating was accomplished with triple-zone furnaces with the temperature of the first zone (packed with quartz wool) maintained at a m i n i m u m of 400 ° C to ensure complete cracking of trioxane to formaldehyde before entering the catalyst bed (typically 1.5-2.5 g catalyst). Nitrogen was also fed to the reactor at a rate of 6 sccm. Reactor effluent was collected by sparging through a volume of isopropanol equal to the anticipated volume of the liquid product. All components except formaldehyde were quantified by gas chromatography analysis. Results are reported based on the reaction of propionic acid. Conversion: moles propionic acid reacted (PAin-PAout) per mol propionic acid fed; selectivity: mols of product produced per mol propionic acid reacted (normalized to 100% based on reaction stoichiometry). RESULTS
In order to assess differences in initial catalytic performance attributable to the nature of the alkali metal component, silica-based catalysts were prepared with equimolar loadings of lithium, sodium, potassium and cesium. Actual TABLE 1 Physical properties of alkali metal cation on silica catalysts Catalyst
Suppo~ Li Na K Cs
Loading
Na (ppm)
wt.-%
( m m o l / l O 0 g)
0.20 0.65 0.97 3.8
29 28 25 29
570 1040 1110 1140
Surface area (m2/g)
Pore volume (cruZ/g)
Initial
Final
Initial
Final
118 154 102 112
0.56 0.58 0.92 0.57 0.56
0.57 0.67 0.55 0.58
~ 165 133 263 132 127
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
166 40-
+
35
//i
+
I "~ 30
-
I
330 3,0 s5o ~6o 3~o 35o 3~o ,oo Temperature (C)
Fig. 1. Effect of alkali metal cation on catalyst activity. (WHSV = 4.5, 25% heptane in feed) 3.8% Cs (/x ), 0.97% K ( + ), 0.65% Na ( O ) , 0.20% Li ( V ) .
85
bt 79 ..i-
.-I-
o 76"-6 {/3 7370
© 2+8 51 5~4 ~7 Conversion (%) I
25
40
Fig. 2. Effect of alkali metal cation on methacrylic acid selectivity. (WHSV = 4.5, 25% heptane in feed) 3.8% Cs ( A ), 0.97% K ( + ), 0.65% Na ( © ) , 0.20% Li ( V ) .
loadings ranged from 25 to 29 mmol/100 g catalyst. Each catalyst was found to contain approximately 1100 ppm sodium. This sodium corresponds to less than 5 mmol Na/100 g catalyst and is considered to have had no significant effect on the outcome of these evaluations. The important physical properties of the catalysts are summarized in Table 1. A comparison of the activities of the equimolar silica-based catalysts is presented in Fig. 1. Propionic acid conversion is plotted as a function of reaction temperature for each catalyst. Conversion level increases linearly with reaction temperature across the range of study. For a given conversion level, activ-
167
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
ity increases (the required reaction temperature decreases) with descending order within the Periodic Table ( C s > K > > N a > > Li). Fig. 2 provides a comparison of the selectivities of equimolar silica-based catalysts for methacrylic acid synthesis. Selectivity to methacrylic acid is plotted as a function of propionic acid conversion level. Conversion level was varied by changing reaction temperature while maintaining a constant space velocity. Regardless of cation, methacrylic acid selectivity decreases as propionic acid conversion increases. At higher conversion levels the selectivity to methacrylic acid varies considerably with alkali metal cation, and like activity, increases with descending order within the Periodic Table. Table 2 provides a comparison of by-product selectivities as a function of alkali metal cation at the 30% propionic acid conversion level. Although increased selectivities to 3-pentanone (3-P), light gases, and heavies are correlated with increased reaction temperatures, selectivities to by-products 2,5-dimethyl-2-cyclopentene-l-one ( D M C P ) , 2,4,4-trimethylbutyrolactone ( T M B ) , and ethyl isopropenyl ketone ( E I K ) show no such relationship. Selectivity to D M C P is particularly important because it has been shown to accelerate the polymerization of methacrylic acid making processing of reactor effluent more difficult [22]. Because cesium on silica catalysts afforded the highest activities and selectivities for methacrylic acid synthesis, a series of catalysts differing only in cesium content were prepared to evaluate the effect of cation loading on catalyst performance. Cesium levels ranged from 0.83 to 17.5% by weight (6-132 mmol/100 g catalyst). Table 3 summarizes the physical properties of this set of catalysts. The effect of cesium loading on catalyst activity is illustrated in Fig. 3. As before, propionic acid conversion level is plotted as a function of reaction temTABLE2 Comparison of catalyst selectivities at 30% propionic acid conversion WHSV = 4.5; 25% heptane in feed; mol-% selectivities Catalyst
T (°C)
MA
DMCP a
TMBb
3-Pe
EIK~
Otherse
Cs K Na Li
339 344 372 395
81 79 77 71
2.4 2.1 2.2 3.0
1.8 1.9 1.8 2.5
1.4 1.6 2.3 3.6
0.5 0.5 0.7 1.8
12.9 14.9 16.0 18.1
aDMCP, 2,5-dimethyl-2-cyclopenten-1-one. bTMB, 2,4,4-trimethylbutyrolactone. c3-P, 3-pentanone. JEIK, ethyl isopropenyl ketone. eOthers, primarily light gases and heavies.
O.H. Bailey et al. /Appl. Catal. A 88 (1992) 163-177
168 TABLE 3
Physical properties of cesium on silica catalysts Cs Loading wt.-%
mmol/lO0 g
0.83 2.0 3.9 9.2 17.5
6 15 29 69 132
Surface area (m2/g)
Pore volume (cma/g)
149 146 134 100 64
0.58 0.57 0.59 0.57 0.72
45
k
40-
v
35c-
O "~ > C 0 (D
30
25
15
280
[
i
i
t
i
300
520
540
360
380
400
Temperafure (C) Fig. 3. Effect of cesium loading on catalyst activity. (WHSV = 1.5) 0.83% Cs ( a ), 2.0% Cs ( + ), 3.9% Cs ( 0 ) , 9.2% Cs (~7), 17.5% Cs ( [ ] ) .
100
-
0 90-
80
> 0ID
70
60
%
5O
18
24
3i0
36
42
Conversion (%) Fig. 4. Effect of cesium loading on methacrylic acid selectivity. ( W H S V = 1.5) 0.83% Cs ( A ) , 2.0% Cs ( + ), 3.9% Cs ( O ) , 9.2% Cs ( V ) , 17.5% Cs ( [ ] ) .
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
169
perature for each of the catalysts. As cesium loading is increased from 0.83 to 3.9%, methacrylic acid synthesis activity increases significantly. Conversion at 340°C (WHSV=I.5, 3:2 PA:FA) increases from 16 to 42%. However, as cesium loading is increased to levels greater than 4%, little increase in activity is observed under these reaction conditions. The relationship between cesium loading and catalyst selectivity is more complicated. In Fig. 4 selectivity to methacrylic acid is plotted as a function of conversion level for each of the catalysts. Again, conversion level was varied by changing reaction temperature at a constant space velocity. For all cesium loadings selectivity decreases as conversion level increases. However, as is shown more clearly in Fig. 5, a plot of selectivity at 30% conversion as a function of cesium loading, methacrylic acid selectivity reaches a maximum for loadings between 4 and 9% cesium. As expected from the decrease in selectivity to methacrylic acid with increasing propionic acid conversion level, the sum of the selectivities to major byproducts increases with increasing conversion level. Fig. 6 shows the dependence of DMCP selectivity on conversion level, as well as the effect of cesium loading on that selectivity. Although selectivity to methacrylic acid reaches a maximum at intermediate cesium loadings (4-9%), selectivity to DMCP undergoes a modest decrease even upon increasing cesium loading from 9.2 to 17.5%. Table 4 shows the effect of cesium loading on by-product selectivities at 30% propionic acid conversion. Selectivities to 3-P and EIK decrease steadily as cesium loading increases, even though reaction temperature goes through a minimum. Selectivities to DMCP and TMB reach a maximum at a cesium loading near 2%.
~.~
90-
o9 > E O (.9
80-
O
@
I1) 0"1
70-
60
0
i 5
i I0
i 15
2~0
Cs Loading (%) Fig. 5. Effect of cesium loading on methacrylic acid selectivity (WHSV= 1.5).
170
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
///+
~
.-
-~
~
.?_ 0
0
20
215 310 315 Conversion (%)
40
Fig. 6. Effect of cesium loading on DMCP selectivity. (WHSV= 1.5) 0.83% Cs (A), 2.0% Cs (+), 3.9% Cs ( 0 ) , 9.2% Cs (V), 17.5% Cs (r-q).
TABLE 4 Effect of cesium loading on catalyst selectivity at 30% propionic acid conversion WHSV= 1.5; mol-% selectivities. Terms as in Table 2 Cs (%)
T (°C)
MA
DMCP
TMB
3-P
EIK
Others
0.83 2.0 3.9 9.2 17.5
364 339 310 314 317
83 83 88 86 74
4.0 5.1 2.9 0.9 0.4
2.2 2.4 1.1 0.6 0.7
2.5 1.6 0.8 0.5 0.4
1.4 1.0 0.5 0.3 0.1
6.9 6.9 6.7 11.7 24.4
DISCUSSION T h e results p r e s e n t e d in t h e p r e v i o u s section clearly show t h a t alkali m e t a l and c a t i o n loading significantly affect c a t a l y s t activity for t h e c o n d e n s a t i o n of f o r m a l d e h y d e with p r o p i o n i c acid. T h i s t y p e of effect has b e e n o b s e r v e d in closely r e l a t e d reaction s y s t e m s a n d has b e e n a t t r i b u t e d to differences in t h e n u m b e r of basic sites a n d t h e b a s i c i t y o f e a c h site [ 7]. A r e d u c t i o n in activity as cation loading is i n c r e a s e d above a " s a t u r a t i o n " p o i n t has also b e e n r e p o r t e d and a t t r i b u t e d to a loss of silica s u p p o r t surface area at h i g h e r c a t i o n loadings [191.
171
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
Infrared spectroscopy has been used extensively to study and quantify hydroxyl groups on silica surfaces [23 ]. The infrared spectra of cesium on silica catalysts show that the intensity of the 3748 cm-1 band of isolated silanols decreases with increasing cesium loading [26]. This observation is consistent with a general model of the catalyst (prior to introduction of feed) as an alkali ion-exchanged surface. As the proportion of surface hydroxyls are exchanged, the basicity of the catalyst is expected to increase. However, the number of surface hydroxyl groups on typical silica gels is of the order of 4-5 per nm 2 [23]. As shown in Table 1, except for sodium, the different alkali cations had a similar effect on catalyst surface area. They each enhanced surface area loss. Surface area also decreased as cation loading increased (Table 3). Thus, for higher cesium loadings the amount of cation exceeds the exchange capacity of the silica support indicating that a portion of the cesium salt must exist as physically adsorbed CsOH or Cs20. The basis of any base-catalyzed aldol condensation is the enhancement of the acidity of the a-proton. For aldehydes and ketones, the a-proton is the most acidic site in the entire molecule. In contrast, the proton of the acid functionality of the carboxylic acid is far more acidic than any other proton in that type of molecule (reaction 1 ). It is only by reversibly chemisorbing the acid molecule in a form which eliminates this highly acidic position that the ~proton can become the most acidic site facilitating formation of a nucleophilic carbanion at the a-position with high selectivity (reaction 2).
R
H
O
i
i
- C-
C-
BO 0
- H
I
........
R
........
0
I
~-
0
I
H
R-
H
O---
l
- C-
C-
0 e
(1)
+ HB
i
H
BO
H
I
C. . . . . . . . .
R- - ~ -
O
I
-C- - O - - -
+ Hs
(2)
H
Reactions of carboxylic acids with silica surfaces have been studied by a number of techniques. The general reaction is commonly written as (reaction 3 ). R OH
sI i
0
I
I
+ RC-OH
c=o ........ . . . . . . .
I
0
+ ,%0
(3)
172
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
Infrared spectroscopic studies have provided strong evidence for reaction 3 even on highly pure Cab-O-Sil powder [24]. A strong absorption band at 1745 cm-1 was assigned to the carbonyl stretch of the surface-bound silyl ester. Previous work had suggested that "impurities" such as alkali metal cations were instrumental in the chemisorption of carboxylic acid on hydroxylated silica surfaces [25 ]. Following treatment of a cesium cation on silica catalyst ( 2.5 % cesium ) with excess propionic acid at room temperature, two bands in the carbonyl stretching region of the infrared spectrum were observed suggesting the presence of physically adsorbed propionic acid (1721 c m - 1) and cesium propionate (1567 cm-1). A shoulder at 1682 cm -1 was present on the 1721 cm -1 absorption band. After raising the sample temperature to 200 °C and evacuating the sample cell, the band associated with physically adsorbed propionic acid was not observed; however, a weak absorbance, presumably that of a surface-bound silyl ester (1740 cm-1), was detected along with the carboxylate absorbance which had shifted slightly to 1577 c m - 1. After increasing the sample temperature to 300°C, only the carboxylate absorbance was intense enough to be observed [26 ]. These results suggest that under typical reaction conditions (275-375°C) much of the cesium exchanged silica is converted to cesium propionate and surface silanols. Under a high partial pressure ofpropionic acid, surface-bound silyl esters should readily and reversibly form at these silanol sites. Temperature-programmed desorption (TPD) experiments carried out using the silica support and silica impregnated with equimolar loadings of cesium, sodium and lithium suggest that propionic acid adsorption is affected by the nature of the cation. The T P D response for each catalyst consisted of two peaks, a low temperature peak in which physisorbed propionic acid is desorbed, and a high temperature peak in which a mixture of propionic acid and 3-pen~anone is released. As shown in Fig. 7, the integrated detector response (relative) associated with the high temperature peak increased in the following order: support (28) < L i (53) < N a (61) < C s (100). The temperature of the maximum response for the high temperature peak increased in the following order: Cs ( 3 4 8 ° C ) < N a ( 3 5 6 ° C ) < L i ( 3 7 0 ° C ) < s u p p o r t (426°C). These trends are consistent with the observed increases in activity and selectivity for these alkali-based catalysts, although they run counter to the expected T P D result where the strongest base (cesium) would exhibit the highest acid desorption temperature. The low temperature peak maxima corresponding to desorption of physisorbed propionic acid show the expected behavior: support (145 ° C) < Li (153 ° C) < Na (157 ° C) < Cs (165 ° C). This suggests that the high temperature T P D peak is associated with the desorption of chemisorbed propionic acid (decomposition of surface silyl ester ), and that alkali metal cations influence the ease of that desorption.
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
173
9 8 ~ >6
~ 2
~o -2 600
I
1200
I
I
1800 2400 Seconds
I
3000
3600
Fig. 7. T P D of propionic acid. Support ( .... ), Li ( - - ) ,
N a ( - - ) , Cs ( . . . .
).
In T P D experiments, the catalysts were exposed to propionic acid well below reaction temperature. Infrared studies have shown that cesium propionate readily forms under these conditions [26]. Formation of the related alkali metal propionates is also expected. There is some concern that the T P D high temperature peak maxima may be attributable to decomposition of the propionate salts rather than indicative of differences in chemisorption. However, the melting point and onset of decomposition of the alkali metal propionates are reported to increase in the order Na < Cs < Li < Rb < K and Li < Cs < Rb < K < Na, respectively [27 ]. Both of these orders are inconsistent with T P D and catalytic performance results. T P D experiments where cesium loading was varied were also carried out. The total response of the high temperature peak and the peak maxima were found to increase with increasing cesium loading. Because cesium propionate is known to form and decompose under these conditions, these results may not be indicative of increased chemisorption with increasing cation loading. Additional evidence for the importance of the chemisorbed species and an important role of the cation in influencing activity and selectivity was obtained from two control experiments. Cesium propionate deposited on a low surface area hydroxyl-free surface (quartz wool) exhibited some activity for methacrylic acid synthesis although selectivity was very low (22% propionic acid conversion and 30% methacrylic acid selectivity at 390 ° C). The silica support in the absence of added cesium exhibited significantly higher activity and selectivity for methacrylic acid synthesis (31% propionic acid conversion and 63% methacrylic acid selectivity at 390 °C ), although it should be remembered that this support did contain trace levels of sodium. To appreciate how alkali metal cation on ~ilica catalysts might influence byproduct selectivities requires an understanding of the chemical relationship between propionic acid, formaldehyde, methacrylic acid, and each of the byproducts. The elucidation of this network will be the subject of another paper
174
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
[28], however, the following reactions indicate that by using different combinations of condensation and decarboxylation reactions, each of the major byproducts can be accounted for. PA÷FA -, M A + H 2 0 P A + P A -~ 3 - P ÷ C O 2 + H 2 0 3P ÷ FA -, EIK ÷ H20 PA + MA ~ EIK ÷ CO2 ÷ H20 F A + P A + M A -, T M B + C O 2 + H 2 0 M A + M A --~ TMB+CO2 M A + M A --, D M C P + C O 2 + H 2 0 Because by-product DMCP is obtained by sequential reaction of product MA, it is not surprising that selectivity to DMCP increases with increasing PA conversion level as shown in Fig. 6. Although now shown, selectivity to TMB shows a similar response to PA conversion level. At a PA conversion level of 30%, there is only a small difference in the selectivity to MA for cesium, potassium and sodium catalysts. (Table 2.) As a result, selectivities to DMCP and TMB would not be expected to vary significantly with cation. This is exactly what was observed. On the other hand, the MA selectivity for the lithium catalyst was significantly lower than that of the other catalysts. At the same time, selectivities to DMCP and TMB were significantly enhanced. This is consistent with MA selectivity loss as a result of sequential reaction. Selectivity to 3-P at the 30% PA conversion level correlated with increased reaction temperature. Because 3-P is derived directly from PA, no direct relationship between 3-P and MA selectivities would be expected. EIK can be derived from either 3-P or MA. Arguments could be made that EIK selectivity might track either temperature and 3-P selectivity, or MA selectivity. The results presented here are inconclusive. Overall the effect of alkali metal cation on product selectivity can largely be explained by the effect of cation on catalyst activity. As catalyst activity increases, lower reaction temperatures are required for a given level of conversion. As a result, the rates of competitive side reactions are reduced and selectivity to methacrylic acid increases. The effect of alkali metal cation on catalyst activity is consistent with an increase in the base strength of the active sites as proposed for other systems [7,19]; however, TPD and infrared evidence suggests an effect of cation on chemisorption and silyl ester stability. The effect of cation loading on product selectivity (Table 4) is not as easily explained. As cesium loading increases from 0 to 4%, catalyst activity and se-
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
175
lectivity to MA increase. At cesium loadings greater than 4%, catalyst activity levels out but selectivity to MA at the 30% PA conversion level drops significantly. While the selectivity to MA increases as cesium loading increases from 0 to 4%, selectivities to D M C P and T M B first increase, then decrease. The initial increase in DMCP and T M B selectivities with increasing cation loading could be related to the sequential nature of their synthesis from product MA, however, the concurrent decline in DMCP and T M B selectivities and increase in MA selectivity with an increase in cesium level from 2% to 4% suggests that a more involved explanation is required. Additional support comes from the progressive decrease in 3-P and EIK selectivities with increasing cesium loading, even though reaction temperature does not continue to decrease. Based on the results presented here it is proposed that prior to reaction with formaldehyde propionic acid first chemisorbs on the silica surface resulting in the formation of a surface-bond silyl ester. Alkali metal cations affect the temperature at which this chemisorption readily occurs. The proximity of these silyl esters relative to each other affects the selectivity of methacrylic acid synthesis. Scheme 1 summarizes these interactions. At low cation loadings, the probability of forming silyl esters at sites close enough to interact is high favoring formation of 3-P (interaction of two propionate esters), DMCP (two methacrylate esters), and T M B (two methacrylate esters). As cation loading is increased, the excess propionate salt tends to separate the reactive silyl esters favoring selective reaction with gas phase formaldehyde. At the same time higher cation loadings reduce the activity of the catalyst by reducing the number of sites where intermediate silyl esters can form, and ultimately by contributing to silica degradation. The decrease in methacrylic acid selectivity at high cation loadings is not explained by this scheme but presumably is related to an increased rate of monomolecular decarboxylation of the silyl ester. CONCLUSIONS Alkali metal cations supported on silica are effective base catalysts for the production of methacrylic acid via the condensation of propionic acid and formaldehyde. Infrared evidence indicates that silica surfaces exchanged with alkali metal cations are capable of chemisorbing propionic acid yielding surface-bound silyl propionate esters and metal propionate salts. T P D experiments suggest that the alkali metal cation influences the temperature at which desorption of the ester occurs (Cs < Na < Li < support). For silica catalysts of equimolar cation loading, activity and selectiVity to methacrylic acid show this same trend, Cs > K > Na > Li. Catalyst activity increases with increasing cation loading until the silica surface becomes "saturated" and potential chemisorption sites are blocked or destroyed. Methacrylic acid selectivity reaches a maximum at intermediate cation loadings where interaction of adjacent silyl esters is minimized.
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
176
o~ OCs
OH
0
I
I
SI
I
SI I CsPA I/II
PA
o~ sI I /I/I
CsPA
o=~ c~.
.F,A
o
sI I II/I
PA A, -H20
-+,,,
o sI I /111
CsPA
~,o:~o~ o~o~ OH
O,
0
OH
0
9
I
l
t
l
.l
o "sl'o/S/.o/S~.o/St.o~S~.o/S,,.o Low
o~
CsPA ~ OH T
Cation Loading
o~
CsPA ~ OH T
o~
CsPA OH Y
o~l.o~l.o~l.o~i.o~i.o~l-o High Cation Loading
O
,f'-•,
® H20 0 Cs
.¢.o~ Scheme 1.
v
.¢-o~
OOCs~ O H
-3-p .~.o..S~.. -CO 2
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
177
ACKNOWLEDGEMENTS
Infrared studies were carried out by J.A. Kaduk and A.G. Nerheim. TPD studies were performed by B.L. Meyers. We would also like to acknowledge the contributions of all the other members of Amoco's Methyl Methacrylate Group, especially G.P. Hagen and L.D. Lillwitz.
REFERENCES 1 R.G. Denney, Methacrylic Acid Methacrylic Esters, Process Economics Program, Report No. 11, Stanford Research Institute, Menlo Park, CA ( 1966 ). 2 D.C. Thomas, Methacrylic Acid Methacrylic Esters, Process Economics Program, Report No. 11A, Stanford Research Institute, Menlo Park, CA (1974). 3 R.H. Schwaar and M.M. Koorse, Methacrylic Acid Methacrylic Esters, Process Economics Program, Report No. 11B, Stanford Research Institute, Menlo Park, CA (1980). 4 R.H. Schwaar, E. Matsunaga, S. Kikuchi and W. Nishimoto, Methacrylic Acid and Esters, Process Economics Program, Report No. 11C, Stanford Research Institute, Menlo Park, CA (1984). 5 Chem Systems, Process Evaluation Research Planning Petrochemical Reports. 90-10: Methacrylic Acid/Methacrylates, Tarrytown, NY (1991). 6 R.H. Schwaar, Methacrylic Acid and Esters, Process Economics Program, Report No. l l D , Stanford Research Institute, Menlo Park, CA (1992). 7 S. Malinowski, H. Jedrzejewska, S. Basinski and S. Benbenek, Chim. Ind., 85 (1961) 885. 8 J.F. Vitcha and V.A. Sims, US Patent 3 089 902 (1963). 9 C.N. Wolf and J.F. McMahon, US Patent 3 440 276 (1969). 10 A.J.C. Pearson, US Patent 3 840 588 (1974). 11 F.W. Schlaefer, US Patent 3 933 888 (1976). 12 W. Gaenzler, K. Kabs and G. Schroeder, US Patent 4 147 718 (1979). 13 J.-Y. Ryu, US Patent 4 430 252 (1984). 14 G.P. Hagen, US Patent 4 631 264 (1986). 15 P.T. Wierzchowski and L.W. Zatorski, Catal. Lett, 9 (1991) 411. 16 A. Mamoru, Appl. Catal., 63 (1990) 365. 17 M.O. Baleiko and E.F. Rader, US Patent 4 599 144 (1986). 18 S.J. Pietsch and H.M. Naim, US Patent 4 748 268 (1988). 19 S. Malinowski and S. Basinski, J. Catal., 2 (1963) 203. 20 A.W. Klaassen and C.G. Hill, Jr., J. Catal., 69 (1984) 299. 21 Z. Dudzik and Z.M. George, J. Catal., 63 (1980) 72. 22 R.A. Montag and S.T. McKenna, US Patent 4 801 571 (1989). 23 R.K. Iler, The Chemistry of Silica, Wiley, New York, 1979, pp. 633-642. 24 R.P. Young, Can. J. Chem., 47 (1969) 2237. 25 M.L. Hair, Infrared Spectroscopy in Surface Chemistry, Marcel Dekker, New York, 1967, p. 167. 26 A.G. Nerheim, personalcommunication 27 T. Meisel and Z. Halmos, in H.G. Wiedeman (Editor), Thermal Behavior of some Organic Compounds with Ionic Character, Thermal Analysis, Vol. 3, Proceedings Third ICTA, Davos, Bickhaeuser Verlag, Basel, 1971, p. 43. 28 L.D. Lillwitz, in preparation.
163
APCAT A2314
Methacrylic acid synthesis I. Condensation of propionic acid with formaldehyde over alkali metal cation on silica catalysts O.H. Bailey, R.A. Montag and J.S. Yoo Amoco Research Center, P.O. Box 3011, NaperviUe, IL 60566 (USA) (Received 13 January, 1992, revised manuscript received 26 May, 1992)
Abstract
Alkali metal cations supported on silica are effective base catalysts for the production of methacrylic acid via the condensation of propionic acid with formaldehyde. Activity and selectivity are dependent on the choice of alkali metal cation and increase in the order Li < Na < K < Cs. Activity also increases with increasing cation loading. A model for the dependence of product selectivity on cation loading is proposed. Keywords: alkali metal cation, condensation, formaldehyde, propionic acid, silica.
INTRODUCTION
Methyl methacrylate (MMA) is an important industrial monomer used in a wide range of polymerization products, including sheeting for signs and building materials, surface coatings, and molding resins. The predominant commercial technology for MMA manufacture is the Acetone Cyanohydrin Process, which uses as raw materials acetone, hydrogen cyanide, methanol, and sulfuric acid. Technical and economic drawbacks of this process, including HCN handling, raw material cost, and ammonium bisulfate waste, have spurred a considerable industrial research effort to develop alternate routes to MMA [1-6]. Much of this research has focused on condensation reactions of propionald e h y d e , p r o p i o n i c acid, o r m e t h y l p r o p i o n a t e w i t h f o r m a l d e h y d e o r its dim e t h y l acetal, m e t h y l a l [ 7 - 1 5 ] . O n e d r a w b a c k a s s o c i a t e d w i t h t h i s t y p e o f p r o c e s s h a s b e e n t h e l i m i t e d single p a s s y i e l d n e c e s s i t a t i n g e x t e n s i v e r e c y c l i n g Correspondence to: Dr. 0.H. Bailey, Amoco Research Center, P.O. Box 3011, Naperville, IL 60566, USA. Tel. ( + 1-708)4205405, fax. ( + 1-708)4204678.
0926-3373/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
164
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
of reactants. Because methylal decomposes under reaction conditions to yield formaldehyde and methanol, the condensation product obtained with this reagent always consists of a mixture of methyl methacrylate and methacrylic acid. This further complicates downstream processing including product separation and recycle. On the other hand, formaldehyde is generally supplied as 35-45% solutions (formalin), the remainder being water and methanol. Water has been shown to limit reaction [ 16 ] as well as contribute to catalyst deactivation. A process has been developed to effectively remove methanol and dry formalin solutions prior to the condensation reaction, and to recycle unreacted propionic acid following separation from product methacrylic acid [17]. MMA suitable for commercial application can be prepared by esterification of this methacrylic acid [18]. In order to take advantage of this type of downstream processing, active and selective catalysts for the condensation of propionic acid with formaldehyde are required. Silica supported alkali metal cation catalysts are known to be effective base catalysts for a variety of aldol-type condensation reactions [7,19,20]. Effects of alkali metal cation loading on catalyst activity have also been reported [ 19-21 ]. In this paper, the choice of alkali metal cation and loading are evaluated for their effect on the activity and selectivity of silica supported catalysts for the vapor phase synthesis of methacrylic acid (MA) from propionic acid (PA) and formaldehyde (FA). EXPERIMENTAL Catalyst preparation
Each of the alkali metal cation on silica catalysts was prepared by the gelation of a solution of the alkali metal carbonate and a low sodium colloidal silica supplied by Nalco (1034-A). After the pH of the solution was adjusted to 7 with nitric acid, gelation was initiated by addition of concentrated ammonium nitrate solution. Because of the limited solubility of Li2CO3, LiOH-H20 was used for the preparation of the lithium catalyst. The hydrogel was dried in a microwave oven, crushed and sieved to 20-40 mesh, and calcined in air for 8 h at 540 ° C. Cesium content was measured by atomic absorption. Other alkali metal contents were measured using inductively coupled plasma. Surface areas and porosities were determined usiilg the B E T / N 2 desorption technique. All infrared spectra were obtained on a Nicolet 6000C FT-IR. Pressed pellet samples were loaded in a heated cell for thermal and vacuum treatment as well as scanning. Temperature-programmed desorption data were collected on 80 mg-samples
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
165
calcined in dry air for 1 h at 550 ° C. After purging with helium, samples were dosed with helium gas saturated with propionic acid for 30 min. The desorption temperature rate was 10 °C per min.
Catalyst evaluations Reactor feed was prepared by dissolving trioxane in propionic acid in a 3 : 2 molar ratio (PA: FA). For some experiments this solution was diluted with 25 wt.-% heptane, an inert diluent. Space velocities [weight hourly space velocity (WHSV): g feed/g catalyst h] are reported on a total weight basis (including heptane). Reactions were carried out in 1/2 in. O.D. quartz reactors equipped with 1/8 in. thermowells. Heating was accomplished with triple-zone furnaces with the temperature of the first zone (packed with quartz wool) maintained at a m i n i m u m of 400 ° C to ensure complete cracking of trioxane to formaldehyde before entering the catalyst bed (typically 1.5-2.5 g catalyst). Nitrogen was also fed to the reactor at a rate of 6 sccm. Reactor effluent was collected by sparging through a volume of isopropanol equal to the anticipated volume of the liquid product. All components except formaldehyde were quantified by gas chromatography analysis. Results are reported based on the reaction of propionic acid. Conversion: moles propionic acid reacted (PAin-PAout) per mol propionic acid fed; selectivity: mols of product produced per mol propionic acid reacted (normalized to 100% based on reaction stoichiometry). RESULTS
In order to assess differences in initial catalytic performance attributable to the nature of the alkali metal component, silica-based catalysts were prepared with equimolar loadings of lithium, sodium, potassium and cesium. Actual TABLE 1 Physical properties of alkali metal cation on silica catalysts Catalyst
Suppo~ Li Na K Cs
Loading
Na (ppm)
wt.-%
( m m o l / l O 0 g)
0.20 0.65 0.97 3.8
29 28 25 29
570 1040 1110 1140
Surface area (m2/g)
Pore volume (cruZ/g)
Initial
Final
Initial
Final
118 154 102 112
0.56 0.58 0.92 0.57 0.56
0.57 0.67 0.55 0.58
~ 165 133 263 132 127
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
166 40-
+
35
//i
+
I "~ 30
-
I
330 3,0 s5o ~6o 3~o 35o 3~o ,oo Temperature (C)
Fig. 1. Effect of alkali metal cation on catalyst activity. (WHSV = 4.5, 25% heptane in feed) 3.8% Cs (/x ), 0.97% K ( + ), 0.65% Na ( O ) , 0.20% Li ( V ) .
85
bt 79 ..i-
.-I-
o 76"-6 {/3 7370
© 2+8 51 5~4 ~7 Conversion (%) I
25
40
Fig. 2. Effect of alkali metal cation on methacrylic acid selectivity. (WHSV = 4.5, 25% heptane in feed) 3.8% Cs ( A ), 0.97% K ( + ), 0.65% Na ( © ) , 0.20% Li ( V ) .
loadings ranged from 25 to 29 mmol/100 g catalyst. Each catalyst was found to contain approximately 1100 ppm sodium. This sodium corresponds to less than 5 mmol Na/100 g catalyst and is considered to have had no significant effect on the outcome of these evaluations. The important physical properties of the catalysts are summarized in Table 1. A comparison of the activities of the equimolar silica-based catalysts is presented in Fig. 1. Propionic acid conversion is plotted as a function of reaction temperature for each catalyst. Conversion level increases linearly with reaction temperature across the range of study. For a given conversion level, activ-
167
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
ity increases (the required reaction temperature decreases) with descending order within the Periodic Table ( C s > K > > N a > > Li). Fig. 2 provides a comparison of the selectivities of equimolar silica-based catalysts for methacrylic acid synthesis. Selectivity to methacrylic acid is plotted as a function of propionic acid conversion level. Conversion level was varied by changing reaction temperature while maintaining a constant space velocity. Regardless of cation, methacrylic acid selectivity decreases as propionic acid conversion increases. At higher conversion levels the selectivity to methacrylic acid varies considerably with alkali metal cation, and like activity, increases with descending order within the Periodic Table. Table 2 provides a comparison of by-product selectivities as a function of alkali metal cation at the 30% propionic acid conversion level. Although increased selectivities to 3-pentanone (3-P), light gases, and heavies are correlated with increased reaction temperatures, selectivities to by-products 2,5-dimethyl-2-cyclopentene-l-one ( D M C P ) , 2,4,4-trimethylbutyrolactone ( T M B ) , and ethyl isopropenyl ketone ( E I K ) show no such relationship. Selectivity to D M C P is particularly important because it has been shown to accelerate the polymerization of methacrylic acid making processing of reactor effluent more difficult [22]. Because cesium on silica catalysts afforded the highest activities and selectivities for methacrylic acid synthesis, a series of catalysts differing only in cesium content were prepared to evaluate the effect of cation loading on catalyst performance. Cesium levels ranged from 0.83 to 17.5% by weight (6-132 mmol/100 g catalyst). Table 3 summarizes the physical properties of this set of catalysts. The effect of cesium loading on catalyst activity is illustrated in Fig. 3. As before, propionic acid conversion level is plotted as a function of reaction temTABLE2 Comparison of catalyst selectivities at 30% propionic acid conversion WHSV = 4.5; 25% heptane in feed; mol-% selectivities Catalyst
T (°C)
MA
DMCP a
TMBb
3-Pe
EIK~
Otherse
Cs K Na Li
339 344 372 395
81 79 77 71
2.4 2.1 2.2 3.0
1.8 1.9 1.8 2.5
1.4 1.6 2.3 3.6
0.5 0.5 0.7 1.8
12.9 14.9 16.0 18.1
aDMCP, 2,5-dimethyl-2-cyclopenten-1-one. bTMB, 2,4,4-trimethylbutyrolactone. c3-P, 3-pentanone. JEIK, ethyl isopropenyl ketone. eOthers, primarily light gases and heavies.
O.H. Bailey et al. /Appl. Catal. A 88 (1992) 163-177
168 TABLE 3
Physical properties of cesium on silica catalysts Cs Loading wt.-%
mmol/lO0 g
0.83 2.0 3.9 9.2 17.5
6 15 29 69 132
Surface area (m2/g)
Pore volume (cma/g)
149 146 134 100 64
0.58 0.57 0.59 0.57 0.72
45
k
40-
v
35c-
O "~ > C 0 (D
30
25
15
280
[
i
i
t
i
300
520
540
360
380
400
Temperafure (C) Fig. 3. Effect of cesium loading on catalyst activity. (WHSV = 1.5) 0.83% Cs ( a ), 2.0% Cs ( + ), 3.9% Cs ( 0 ) , 9.2% Cs (~7), 17.5% Cs ( [ ] ) .
100
-
0 90-
80
> 0ID
70
60
%
5O
18
24
3i0
36
42
Conversion (%) Fig. 4. Effect of cesium loading on methacrylic acid selectivity. ( W H S V = 1.5) 0.83% Cs ( A ) , 2.0% Cs ( + ), 3.9% Cs ( O ) , 9.2% Cs ( V ) , 17.5% Cs ( [ ] ) .
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
169
perature for each of the catalysts. As cesium loading is increased from 0.83 to 3.9%, methacrylic acid synthesis activity increases significantly. Conversion at 340°C (WHSV=I.5, 3:2 PA:FA) increases from 16 to 42%. However, as cesium loading is increased to levels greater than 4%, little increase in activity is observed under these reaction conditions. The relationship between cesium loading and catalyst selectivity is more complicated. In Fig. 4 selectivity to methacrylic acid is plotted as a function of conversion level for each of the catalysts. Again, conversion level was varied by changing reaction temperature at a constant space velocity. For all cesium loadings selectivity decreases as conversion level increases. However, as is shown more clearly in Fig. 5, a plot of selectivity at 30% conversion as a function of cesium loading, methacrylic acid selectivity reaches a maximum for loadings between 4 and 9% cesium. As expected from the decrease in selectivity to methacrylic acid with increasing propionic acid conversion level, the sum of the selectivities to major byproducts increases with increasing conversion level. Fig. 6 shows the dependence of DMCP selectivity on conversion level, as well as the effect of cesium loading on that selectivity. Although selectivity to methacrylic acid reaches a maximum at intermediate cesium loadings (4-9%), selectivity to DMCP undergoes a modest decrease even upon increasing cesium loading from 9.2 to 17.5%. Table 4 shows the effect of cesium loading on by-product selectivities at 30% propionic acid conversion. Selectivities to 3-P and EIK decrease steadily as cesium loading increases, even though reaction temperature goes through a minimum. Selectivities to DMCP and TMB reach a maximum at a cesium loading near 2%.
~.~
90-
o9 > E O (.9
80-
O
@
I1) 0"1
70-
60
0
i 5
i I0
i 15
2~0
Cs Loading (%) Fig. 5. Effect of cesium loading on methacrylic acid selectivity (WHSV= 1.5).
170
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
///+
~
.-
-~
~
.?_ 0
0
20
215 310 315 Conversion (%)
40
Fig. 6. Effect of cesium loading on DMCP selectivity. (WHSV= 1.5) 0.83% Cs (A), 2.0% Cs (+), 3.9% Cs ( 0 ) , 9.2% Cs (V), 17.5% Cs (r-q).
TABLE 4 Effect of cesium loading on catalyst selectivity at 30% propionic acid conversion WHSV= 1.5; mol-% selectivities. Terms as in Table 2 Cs (%)
T (°C)
MA
DMCP
TMB
3-P
EIK
Others
0.83 2.0 3.9 9.2 17.5
364 339 310 314 317
83 83 88 86 74
4.0 5.1 2.9 0.9 0.4
2.2 2.4 1.1 0.6 0.7
2.5 1.6 0.8 0.5 0.4
1.4 1.0 0.5 0.3 0.1
6.9 6.9 6.7 11.7 24.4
DISCUSSION T h e results p r e s e n t e d in t h e p r e v i o u s section clearly show t h a t alkali m e t a l and c a t i o n loading significantly affect c a t a l y s t activity for t h e c o n d e n s a t i o n of f o r m a l d e h y d e with p r o p i o n i c acid. T h i s t y p e of effect has b e e n o b s e r v e d in closely r e l a t e d reaction s y s t e m s a n d has b e e n a t t r i b u t e d to differences in t h e n u m b e r of basic sites a n d t h e b a s i c i t y o f e a c h site [ 7]. A r e d u c t i o n in activity as cation loading is i n c r e a s e d above a " s a t u r a t i o n " p o i n t has also b e e n r e p o r t e d and a t t r i b u t e d to a loss of silica s u p p o r t surface area at h i g h e r c a t i o n loadings [191.
171
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
Infrared spectroscopy has been used extensively to study and quantify hydroxyl groups on silica surfaces [23 ]. The infrared spectra of cesium on silica catalysts show that the intensity of the 3748 cm-1 band of isolated silanols decreases with increasing cesium loading [26]. This observation is consistent with a general model of the catalyst (prior to introduction of feed) as an alkali ion-exchanged surface. As the proportion of surface hydroxyls are exchanged, the basicity of the catalyst is expected to increase. However, the number of surface hydroxyl groups on typical silica gels is of the order of 4-5 per nm 2 [23]. As shown in Table 1, except for sodium, the different alkali cations had a similar effect on catalyst surface area. They each enhanced surface area loss. Surface area also decreased as cation loading increased (Table 3). Thus, for higher cesium loadings the amount of cation exceeds the exchange capacity of the silica support indicating that a portion of the cesium salt must exist as physically adsorbed CsOH or Cs20. The basis of any base-catalyzed aldol condensation is the enhancement of the acidity of the a-proton. For aldehydes and ketones, the a-proton is the most acidic site in the entire molecule. In contrast, the proton of the acid functionality of the carboxylic acid is far more acidic than any other proton in that type of molecule (reaction 1 ). It is only by reversibly chemisorbing the acid molecule in a form which eliminates this highly acidic position that the ~proton can become the most acidic site facilitating formation of a nucleophilic carbanion at the a-position with high selectivity (reaction 2).
R
H
O
i
i
- C-
C-
BO 0
- H
I
........
R
........
0
I
~-
0
I
H
R-
H
O---
l
- C-
C-
0 e
(1)
+ HB
i
H
BO
H
I
C. . . . . . . . .
R- - ~ -
O
I
-C- - O - - -
+ Hs
(2)
H
Reactions of carboxylic acids with silica surfaces have been studied by a number of techniques. The general reaction is commonly written as (reaction 3 ). R OH
sI i
0
I
I
+ RC-OH
c=o ........ . . . . . . .
I
0
+ ,%0
(3)
172
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
Infrared spectroscopic studies have provided strong evidence for reaction 3 even on highly pure Cab-O-Sil powder [24]. A strong absorption band at 1745 cm-1 was assigned to the carbonyl stretch of the surface-bound silyl ester. Previous work had suggested that "impurities" such as alkali metal cations were instrumental in the chemisorption of carboxylic acid on hydroxylated silica surfaces [25 ]. Following treatment of a cesium cation on silica catalyst ( 2.5 % cesium ) with excess propionic acid at room temperature, two bands in the carbonyl stretching region of the infrared spectrum were observed suggesting the presence of physically adsorbed propionic acid (1721 c m - 1) and cesium propionate (1567 cm-1). A shoulder at 1682 cm -1 was present on the 1721 cm -1 absorption band. After raising the sample temperature to 200 °C and evacuating the sample cell, the band associated with physically adsorbed propionic acid was not observed; however, a weak absorbance, presumably that of a surface-bound silyl ester (1740 cm-1), was detected along with the carboxylate absorbance which had shifted slightly to 1577 c m - 1. After increasing the sample temperature to 300°C, only the carboxylate absorbance was intense enough to be observed [26 ]. These results suggest that under typical reaction conditions (275-375°C) much of the cesium exchanged silica is converted to cesium propionate and surface silanols. Under a high partial pressure ofpropionic acid, surface-bound silyl esters should readily and reversibly form at these silanol sites. Temperature-programmed desorption (TPD) experiments carried out using the silica support and silica impregnated with equimolar loadings of cesium, sodium and lithium suggest that propionic acid adsorption is affected by the nature of the cation. The T P D response for each catalyst consisted of two peaks, a low temperature peak in which physisorbed propionic acid is desorbed, and a high temperature peak in which a mixture of propionic acid and 3-pen~anone is released. As shown in Fig. 7, the integrated detector response (relative) associated with the high temperature peak increased in the following order: support (28) < L i (53) < N a (61) < C s (100). The temperature of the maximum response for the high temperature peak increased in the following order: Cs ( 3 4 8 ° C ) < N a ( 3 5 6 ° C ) < L i ( 3 7 0 ° C ) < s u p p o r t (426°C). These trends are consistent with the observed increases in activity and selectivity for these alkali-based catalysts, although they run counter to the expected T P D result where the strongest base (cesium) would exhibit the highest acid desorption temperature. The low temperature peak maxima corresponding to desorption of physisorbed propionic acid show the expected behavior: support (145 ° C) < Li (153 ° C) < Na (157 ° C) < Cs (165 ° C). This suggests that the high temperature T P D peak is associated with the desorption of chemisorbed propionic acid (decomposition of surface silyl ester ), and that alkali metal cations influence the ease of that desorption.
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
173
9 8 ~ >6
~ 2
~o -2 600
I
1200
I
I
1800 2400 Seconds
I
3000
3600
Fig. 7. T P D of propionic acid. Support ( .... ), Li ( - - ) ,
N a ( - - ) , Cs ( . . . .
).
In T P D experiments, the catalysts were exposed to propionic acid well below reaction temperature. Infrared studies have shown that cesium propionate readily forms under these conditions [26]. Formation of the related alkali metal propionates is also expected. There is some concern that the T P D high temperature peak maxima may be attributable to decomposition of the propionate salts rather than indicative of differences in chemisorption. However, the melting point and onset of decomposition of the alkali metal propionates are reported to increase in the order Na < Cs < Li < Rb < K and Li < Cs < Rb < K < Na, respectively [27 ]. Both of these orders are inconsistent with T P D and catalytic performance results. T P D experiments where cesium loading was varied were also carried out. The total response of the high temperature peak and the peak maxima were found to increase with increasing cesium loading. Because cesium propionate is known to form and decompose under these conditions, these results may not be indicative of increased chemisorption with increasing cation loading. Additional evidence for the importance of the chemisorbed species and an important role of the cation in influencing activity and selectivity was obtained from two control experiments. Cesium propionate deposited on a low surface area hydroxyl-free surface (quartz wool) exhibited some activity for methacrylic acid synthesis although selectivity was very low (22% propionic acid conversion and 30% methacrylic acid selectivity at 390 ° C). The silica support in the absence of added cesium exhibited significantly higher activity and selectivity for methacrylic acid synthesis (31% propionic acid conversion and 63% methacrylic acid selectivity at 390 °C ), although it should be remembered that this support did contain trace levels of sodium. To appreciate how alkali metal cation on ~ilica catalysts might influence byproduct selectivities requires an understanding of the chemical relationship between propionic acid, formaldehyde, methacrylic acid, and each of the byproducts. The elucidation of this network will be the subject of another paper
174
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
[28], however, the following reactions indicate that by using different combinations of condensation and decarboxylation reactions, each of the major byproducts can be accounted for. PA÷FA -, M A + H 2 0 P A + P A -~ 3 - P ÷ C O 2 + H 2 0 3P ÷ FA -, EIK ÷ H20 PA + MA ~ EIK ÷ CO2 ÷ H20 F A + P A + M A -, T M B + C O 2 + H 2 0 M A + M A --~ TMB+CO2 M A + M A --, D M C P + C O 2 + H 2 0 Because by-product DMCP is obtained by sequential reaction of product MA, it is not surprising that selectivity to DMCP increases with increasing PA conversion level as shown in Fig. 6. Although now shown, selectivity to TMB shows a similar response to PA conversion level. At a PA conversion level of 30%, there is only a small difference in the selectivity to MA for cesium, potassium and sodium catalysts. (Table 2.) As a result, selectivities to DMCP and TMB would not be expected to vary significantly with cation. This is exactly what was observed. On the other hand, the MA selectivity for the lithium catalyst was significantly lower than that of the other catalysts. At the same time, selectivities to DMCP and TMB were significantly enhanced. This is consistent with MA selectivity loss as a result of sequential reaction. Selectivity to 3-P at the 30% PA conversion level correlated with increased reaction temperature. Because 3-P is derived directly from PA, no direct relationship between 3-P and MA selectivities would be expected. EIK can be derived from either 3-P or MA. Arguments could be made that EIK selectivity might track either temperature and 3-P selectivity, or MA selectivity. The results presented here are inconclusive. Overall the effect of alkali metal cation on product selectivity can largely be explained by the effect of cation on catalyst activity. As catalyst activity increases, lower reaction temperatures are required for a given level of conversion. As a result, the rates of competitive side reactions are reduced and selectivity to methacrylic acid increases. The effect of alkali metal cation on catalyst activity is consistent with an increase in the base strength of the active sites as proposed for other systems [7,19]; however, TPD and infrared evidence suggests an effect of cation on chemisorption and silyl ester stability. The effect of cation loading on product selectivity (Table 4) is not as easily explained. As cesium loading increases from 0 to 4%, catalyst activity and se-
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
175
lectivity to MA increase. At cesium loadings greater than 4%, catalyst activity levels out but selectivity to MA at the 30% PA conversion level drops significantly. While the selectivity to MA increases as cesium loading increases from 0 to 4%, selectivities to D M C P and T M B first increase, then decrease. The initial increase in DMCP and T M B selectivities with increasing cation loading could be related to the sequential nature of their synthesis from product MA, however, the concurrent decline in DMCP and T M B selectivities and increase in MA selectivity with an increase in cesium level from 2% to 4% suggests that a more involved explanation is required. Additional support comes from the progressive decrease in 3-P and EIK selectivities with increasing cesium loading, even though reaction temperature does not continue to decrease. Based on the results presented here it is proposed that prior to reaction with formaldehyde propionic acid first chemisorbs on the silica surface resulting in the formation of a surface-bond silyl ester. Alkali metal cations affect the temperature at which this chemisorption readily occurs. The proximity of these silyl esters relative to each other affects the selectivity of methacrylic acid synthesis. Scheme 1 summarizes these interactions. At low cation loadings, the probability of forming silyl esters at sites close enough to interact is high favoring formation of 3-P (interaction of two propionate esters), DMCP (two methacrylate esters), and T M B (two methacrylate esters). As cation loading is increased, the excess propionate salt tends to separate the reactive silyl esters favoring selective reaction with gas phase formaldehyde. At the same time higher cation loadings reduce the activity of the catalyst by reducing the number of sites where intermediate silyl esters can form, and ultimately by contributing to silica degradation. The decrease in methacrylic acid selectivity at high cation loadings is not explained by this scheme but presumably is related to an increased rate of monomolecular decarboxylation of the silyl ester. CONCLUSIONS Alkali metal cations supported on silica are effective base catalysts for the production of methacrylic acid via the condensation of propionic acid and formaldehyde. Infrared evidence indicates that silica surfaces exchanged with alkali metal cations are capable of chemisorbing propionic acid yielding surface-bound silyl propionate esters and metal propionate salts. T P D experiments suggest that the alkali metal cation influences the temperature at which desorption of the ester occurs (Cs < Na < Li < support). For silica catalysts of equimolar cation loading, activity and selectiVity to methacrylic acid show this same trend, Cs > K > Na > Li. Catalyst activity increases with increasing cation loading until the silica surface becomes "saturated" and potential chemisorption sites are blocked or destroyed. Methacrylic acid selectivity reaches a maximum at intermediate cation loadings where interaction of adjacent silyl esters is minimized.
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
176
o~ OCs
OH
0
I
I
SI
I
SI I CsPA I/II
PA
o~ sI I /I/I
CsPA
o=~ c~.
.F,A
o
sI I II/I
PA A, -H20
-+,,,
o sI I /111
CsPA
~,o:~o~ o~o~ OH
O,
0
OH
0
9
I
l
t
l
.l
o "sl'o/S/.o/S~.o/St.o~S~.o/S,,.o Low
o~
CsPA ~ OH T
Cation Loading
o~
CsPA ~ OH T
o~
CsPA OH Y
o~l.o~l.o~l.o~i.o~i.o~l-o High Cation Loading
O
,f'-•,
® H20 0 Cs
.¢.o~ Scheme 1.
v
.¢-o~
OOCs~ O H
-3-p .~.o..S~.. -CO 2
O.H. Bailey et al./Appl. Catal. A 88 (1992) 163-177
177
ACKNOWLEDGEMENTS
Infrared studies were carried out by J.A. Kaduk and A.G. Nerheim. TPD studies were performed by B.L. Meyers. We would also like to acknowledge the contributions of all the other members of Amoco's Methyl Methacrylate Group, especially G.P. Hagen and L.D. Lillwitz.
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