Preparation of Terephthalic Acid Using Paraldehyde Promotor Kenji Nakaoka,* Yoshio Miyama, Seikichi Matsuhisa, and Shigeru Wakamatsu Chemicals Research Laboratories, Toray Industries, Inc., Kamakura, Kanagawa, J a p a n
A new process for the oxidation of p-xylene to terephthalic acid i s developed commercially. Paraldehyde i s used as a promotor in acetic acid solution together with cobalt catalyst. Compared with the known promotors of the reaction (acetaldehyde and methyl ethyl ketone), paraldehyde is more effective and gives higher yield of acetic acid. Bis( 1 -acetoxyethyl) ether i s isolated and identified as an intermediate in the oxidation of paraldehyde. The moderate oxidation condition provides sufficiently pure terephthalic acid (over 99% purity) in very high yield (over 97%).
Liquid-phase air osidation of p-xylene (PX) to t,erephtlialic acid (TPA) is the most important route for the production of raw materials of polyester fibers and films, t'he demand of which is st'ill growiiig world wide a t a rate over 10yo/year. The manufacture of poly(ethy1ene terephthalate) has been mainly based on the ester-interchange reaction of dimethyl tereplit'lialate ( D l I T ) with ethylene glycol followed by polycondensation of bis(2-hydrosyethyl) terephthalate obtained. Recently, the direct esterification of purified TI'A is gradually replacing t,he D l I T route, focusing more interest, on t,he production of high-purity T P 1 by the air osidatioii of PX. Liquid-phase air oxidation of PX using metal cat,alysts alone gives p-toluic acid (PTh) as a main product instead of desired TPA. Three methods have been successfully del-eloped to commercial processes for TPA production, using cocatalysts or promotors along with metal catalysts. These are the Mid-Century process, using bromine compounds as cocatalyst's (Saffer and Barker, 1958), Eastman process, using acetaldehyde (AA) as promotor (Thompson and Neelp, 1966), and hlobil process, using met'hyl ebhyl ketone (hlEK) as promotor (Ardis, et al., 1962). The fourth remarkable commercial process using paraldehyde (PA) as promotor has been established by Toray Industries, Inc. (1966), which has been operat'ing a 25,000-ton/ year TPX plant a t the Nagoya Plant since March 1971 and is preparing to expand the capacity to around 95,000 tons/ year by 1974. This paper describes the results of experiments carried out for developing the Toray TPA process, together with the outline of its commercial process. Experimental Section
Materials. Paraldehyde (PA) (reagent grade) was distilled before use. Impurities were acetaldehyde (AA) (0.09%), water (O.l), crotonaldehyde (0.58), acetic acid (AcOH) (0.31), and aldol (0.52). Cobaltic acetate was prepared by oxidizing cobaltous acetate with oxygen and PA a t 100' in acetic acid solution. p Toluic acid was obtained by hydrolyzing rectified methyl p-toluate. Other materials were reagent grade. General Procedure of Batchwise Oxidation. Batchu ise oxidation reactions were carried out in a 100-nil shaking type stainless steel autoclave. After reactants were placed 150 Ind. Eng. Chem. Prod. Res. Develop., Vol. 12, No. 2, 1973
and oxygen was charged into the autoclave, it was heated with shaking in a temperature-regulated bath. Aromatic acids separated from the reaction mixture by adding excess water were filtered and dried. General Procedure of Continuous Oxidation. The reactant solution was pumped into a 4-1. stainless steel autoclave fitted with a stirrer, temperature regulator, air inlet a t the bottom, condenser a t the top, and outlet for products. T P A produced in the reactor was continuously taken out as acetic acid slurry and separated by filtration. Analysis of Reaction Froducts. Gaseous and liquid products were analyzed by gas chromatography with a Shimazu Model GC-3AH instrument. Solid products were mostly analyzed by acid-base titration. Based on the yield of solid and acid value (AV; mg of KOH/g) determined, yield of T P h and selectivity to TPA from PTA were calculated according to the equations y-ield of TPA
(yo)= yield of solid (g)
X
AV - 412.1 X 676.4 - 412.1
PTd used (g) selectivity to TPA
x
136 166
- x 100
(yo)= yield of TPA (70) X PTA used (g)
-
675.4 - AV PTd used (g) - yield of solid (g) X __ 675.4 - 412.1 where 675.4 is the AV of TPA, 412.1 is the AV of PTA, 166 is the molecular weight of TPX, and 136 is the molecular weight of PTA. Isolation and Identification of meso- and dl-Bis(1acetoxyethyl) Ether. A solution of P A (5 ml) and (.AcO),Co.4H10 (0.1 g) in acetic acid (10 ml) was heated a t 110' under the oxygen pressure of 20 a t m for 15 min and then was rapidly cooled to room temperature. The combined reaction mixture from 22 runs was condensed under vacuum t o remove acetic acid. Benzene was added into t h e residual solution, which was washed with water to remove cobalt ion and then dehydrated with MgSO1. The distillation of the benzene solution gave 25.1 g of colorless liquid fraction boiling a t 62-67' (3 mm). Anal. Calcd for CsHlaOr: C, 50.52; H , 7.42; mol wt 190.2, Found: C, 50.71; H, 7.27; mol wt 193.
Table 1. Effect of Promotors. Amt, ml
Promotor
Table 111. Effect of Cobalt Saltsa
Acid Yield of volue, mg Yield of solid, g of KOH/g TPA,
Selectivity to TPA,
70 76
None 1.77 409.9 Methyl ethyl ketone 0 . 3 2 . 1 3 549.0 4 5 . 5 0 . 3 2 . 1 4 571.0 5 2 . 8 Paraldehyde 0 . 3 1 . 9 9 517.4 3 2 . 5 Benzaldehyde p-Tolualdeh yde 0 . 3 1 . 8 0 515.1 28.8 Chloroacetone 0 . 3 1.87 471.7 1 7 . 3 1 . 0 1.72 429.5 Acetaldehyde 4.6 1 . 0 1.79 413.8 Ethanol Paraformaldehyde 1 . 8 0 413.4 1g Acetone 1 . 0 1 . 8 3 413.6 Methanol 1 . 0 1 . 8 4 413.3 1 . 0 1 . 8 1 417.5 Ethyl acetoacetate Ethylene glycol 1 . 0 1 . 8 4 414.1 Dioxane 1 . 0 1 . 8 4 414.8 Malonic acid 1 . 0 1 . 8 5 421.9 1 . 0 1 . 8 6 422.4 Acetic anhydride 1 . 0 1 . 8 5 415.8 Ethyl acetate 1 . 0 1 . 8 6 417.5 2-Nitropropane 0 . 5 1.85 414.3 Chloral 1 . 0 1.82 411.8 30y0 glyoxal 1 . 7 8 413.0 tert-Butyl alcohol 1g Acrolein 1 . 0 1 . 7 1 413.8 Methyl Cellosolve 0 . 5 1 . 8 1 412.5 a Conditions: PTA, 2 g; AcOH, 15 g ; (AcO)zCo, HzO, 0.8 ml; 0 2 , 20 atm; 130"; 1 hr.
92.8 92.0 80.5 73.6 62.6 23.4
Yield of solid, g
Cobalt salt
-
PX
Yield of solid, g
Yield of TPA, %
15 wt$
\;;!:;mo'5wt%
0.2
0.4 g ;
-
0.1 0.08
-
0.04 0.06
Catalyst
Yield of Selectivity TPA, % to TPA, %
Cobalt acetate 2.11 570.0 51.5 90.5 Cobalt formate 1.98 516.0 32.2 80.5 Cobalt carbonate 2.06 549.7 43.8 87.0 Cobalt naphthenate 1,84 459.8 13,8 55.3 Cobalt oxalate 1.78 425.9 Cobalt phosphate 1.78 414.7 Cobalt chloride 1.78 444.1 Cobalt sulfate 1.81 417.6 a Conditions: PTA, 2 g; AcOH, 15 g; paraldehyde, 0.3 g; HZO, 0.3 g; cobalt salt, 0.4 g; 0 2 , 20 atm; 130"; 2 hr.
1.0 0.8
Table II. Effect of Metal Salts. Acid value, mg of KOH/g
Acid value, m g of KOH/g
Selectivity toTPA,
76
Cobalt acetate 2.14 608.5 65.4 9 0 . 0 Nickel acetate 1.47 491.9 1 8 . 3 3 7 . 5 Manganese acetate 1.54 411.6 Chromium nitrate 1.63 390.7 Aluminum acetate 1 . 4 0 413.3 Zinc acetate 1 . 2 4 418.2 Ammonium molybdate 1 . 7 5 415.7 Copper acetate 1 . 5 9 424.9 Lead acetate 1 . 4 1 422.3 Mercuric acetate 1 . 6 3 417.3 Iron(1I) acetate 1.72 423.6 Stannic chloride 1 . 7 4 394.9 Conditions: PTA, 2 g; AcOH, 7.5 g; paraldehyde, 0.3 g ; 0 2 , 20 atm; 130"; 2 hr; catalyst, 0.3 g. Q
Since the gas chromatograph of the fraction gave two separate peaks of close retention time, it was separated with preparative gas chromatography into the two fractions, which were identified as meso- and dl-bis( 1-acetoxyethyl) ether (DXEE) , respectively, by nmr and ir analyses. Results and Discussion
Oxidation Reaction. PX or P T X in acetic acid solution is oxidized by air to T P A using cobalt catalyst and suitable promotors. The type of t h e promotors has a profound effect in this reaction. As shown in Table I, PAkis one of the most effective promotors among many organic compounds which are expected to form peroxidic species in t h e system . PA has never been used as a promotor in liquid-phase air oxidation reactions arid is accordingly a novel promotor for the TPA process.
0.02
-
I ,,A,, A&
, 2
3
4
6
810
1520
Figure 1 . Effect of PA in t h e continuous oxidation of PX
Tables I1 and 111 indicate that only cobalt salts (especially acetate) have the outstanding catalytic effect' in this process. Coexistence of other metals such as Mn, Cu, arid Cr with cobalt does not improve the reaction but rather inhibits the oxidation to TP-1. The amount, of Prl added to the system is critical for the progress of the oxidation to TI'.\. Figure 1 shows that the fraction of unreacted matter in the oxidation products from PX in the continuous process is reduced according to approximately the second power of the amount of P A added. The oxidation reaction is also profoundly affected by the oxygen pressure and temperat'ure of the system as indicated in Figures 2 and 3. The reaction is accelerated with the increase of oxygen pressure and the optimum temperature is around 120-140". Reaction Mechanism. T h e scheme of this reaction is assumed to be such chain steps a s shown in Figure 4. en to form peroxidic compounds which promote the oxidation of cobaltous ion into cobaltic ion (eq 1). Cobaltic ion then abstracts hydrogen from methylbenzenes arid is reduced to the cobaltous state (eq 2 ) . This scheme is supported by the fact that the use of cobaltic ion in place of cobaltous ion can reduce the induction period of the reacbion and promote the oxidation of PT.1 even under the absence of P A and that approximately a half of cobalt Ind. Eng. Chem. Prod. Res. Develop., Vol. 12, No. 2, 1973
15 1
Figure 4. Reaction scheme Oxygen Pressure (atm) 70
k
30 ' I
2 4
36
90 Time (min) $0
i20
Figure 5. Effect of initial composition Time (min)
( ) : for O2 1 atm
Figure 2. Effect of oxygen pressure
Time (min)
Figure 6. Effect of initial composition
-
" 0
100
Table IV. initial Composition.
120 140 160 Temperature ( " C )
-Concn,
No.
- 14OOC
/ . '
reactant log
PTA 20wt5, P A 4 w t % ( A C O ) ~ C O 1.4wt46
3.6wt5 rest AcOH O2 5 atm
H20
Time (min)
(
1:
f o r 100°C
Figure 3. Effect of temperature
ion always exists as cobaltic ion during the progress of the oxidation (Table IV and Figures 5 and 6). However, the hydrogen abstraction by active free radicals from PA (eq 3) may also proceed in this reaction, since the use of a small amount of PA in addition to cobaltic ion can accelerate the reaction, even though the cobaltic ion concentration is kept a t an almost constant level (Table IV and Figures 5 and 6). 152 lnd. Eng. Chem. Prod. Res. Develop., Vol. 12, No. 2, 1973
g-ion Total Co
X 1 o-~--. cos+
0 12 8.4 12 6.6 12 40 20.8 6.6 E 12 Conditions: AcOH, 10 ml; PTA, 1 g.
A B C D
[COS+]/
[total Co],
%
Amt of PA, g
0 70 55 52 55
0.10 0.02 0 0 0.10
The ratio of cobaltic ion concentration to total cobalt ion concentration is another important controlling factor for the reaction. The oxidation is retarded with the decrease of the ratio even under the constant cobaltic ion concentration (Table V). Since cobaltic ion is the principal reagent for hydrogen abstraction, it is quite important to keep the concentration of cobaltic ion in the system as high as possible. The rate of the reduction of cobaltic ion in acetic acid solution can be well described as a n irreversible first-order reaction. The rate constants in anhydrous and hydrous acetic acid solutions are given in Table VI. The exceedingly high reduction rate of cobaltic ion above 130" correlates with the difficulty in oxidizing methylbenzenes by this process above 140".
Table V. Effect of the Ratio of Cobaltic Ion to Total Cobalt lonu -Concn, g-ion CO~+
X
Co3+/[total Co],
%
co2+
8.4 3.6 8.4 15.6 8.4 27.6 8.4 43.7 UConditions: PA, 0.02 g ; PTA, atm; 100"; 2hr.
Yield of TPA,
%
73.6 73.0 61.0 42.7 1 g; AcOH, 10 ml; 02,10 70.0 35.5 23.3 16.1
Table VIII. Effect of Promotors in the Continuous Oxidation of PX. of products from PX, mol % FBAb PTA p-TAC
-Amt TPA
Promotor
PX
90.9 2 4 6.7 0 0 17.7 0.3 0 78.3 3.7 MEK 72.8 4.1 21.5 0.5 1.2 a Conditions: PX, 20 parts; promotor, 6.7 parts; (AcO)2Co, 0.75 part; HzO, 1 part; AcOH, 73.25 parts; 120"; air, 15 atm; 3 hr. b p-Formylbenzoic acid. p-Tolualdehyde. PA AA
Table VI. Reduction Rate of Cobaltic Ion in Acetic Acid Solution , - _ ~ _
Temp, OC
In AcOH"
90 100 110 120 130 a E,,, = 42.3 kcal /mol.
0.00806 0.0242 0.374 1.27 4.90 kcal/mol.
Rate const. hr-' 7 In 5% In 10% H20-95% AcOHb H20-90% AcOHC
0.0307 0.0887 0.238 0.726 2.74 Eaot= 33.4 kcal/mol.
CHS- C -OH
0.0443 0.0968 0.300 0.866 2.29 Eaot= 27.8
, CHI
.
$ *
COz *CHJ-CH~-CCOH
iC t CHz -CH3 I
j
cd,
ZCHICOOH
MEK
Table VII. Oxidation Products from Promotors. CHC
YO---
Amt of products from promotor, mol CO COz AA MACb PrAC
I -
Promotor
Figure
7.Decomposition scheme
for AA a n d MEK
AcOH
PA
0 . 1 5 0 . 4 5 4 . 5 0 0.02 0 . 7 7 0 91.19 4.49 0 . 5 5 12.58 0 . 1 7 6 . 7 3 0 75.48 0.10 1 . 0 3 15.70 0 1 . 5 3 4.92 76.72 a Conditions: promotor, 1.25 nil; AcOH, 5 ml; (Ac0)~Co. 4H20, 0.05 g ; H@, 0.3 ml; 02,10 atm; 120"; 3 hr. *Methyl acetate. Propionic acid.
Ai MEK
K a t e r accelerates the reduction of cobaltic ion under 100" though it retards the reduction above 110". Oxidation Products from Promotors. T h e product distribution from PA is compared with those from AA and hIEK iii Table VII. It is obvious t'hat, anioiig the three promotors, PA gives the highest yield of acetic acid which is the most valuable and most desirable product. Considerable amounts of AA and MEK burn to COz. The loiver acetic acid yield from A=\ could be ascribed to facile decarboxylatioii or decarbonylatioii of the intermediate acetoxyl or acetyl radicals, and some fission between the methyl and keto groups of AIEK, instead of between the methylenc and keto groups, would lead to the appreciable formation of propiouic acid and COz, instead of 2 mol of acetic acid, from hlEK (Figure 7). Comparison of Promoting Effects. The comparison of promoting effect' of promotors f o r batchwise oxidation of I T A was already give11 in Table I. Easily oxidizable AA is ineffective in the batchwise operation, since i t is almost completely coiisunied before the reactants reach the opt'im u m temperature for t h e oxidation reaction. Table VI11 compares the effects of PA, Ai, and MEK in the continuous oxidation of PX. It is clearly demonstrated that PA, again iii this continuous reaction, is the most effective promotor, giving the one-path TPA yield of over 90%. 1)ifficultly oxidizable AIEK is the least effective in this continuous operatioii under 15 a t m pressure of air. This could be the reason that the commercial process promoted by MEK uses comliressed pure 0x1-geii, instead of air, as the oxidizing
k2
Figure 8. Routes for PA oxidation agent (Bryant, et al., 1971). ;\loreover, a special recovery system for unreacted N E K might be indispensable with this promotor. The distinct advantage of PA over AA in the promoting effect might be ascribed a t least partly to the inertness of the former for the reduction of cobaltic ion. AA is a typical reducing agent as shown in the reaction with Ag+ and Cu+ and does reduce C o 3 + even at room temperature. Furthermore since PA is more stable and less volatile than AA, i t is oxidized more slowly and more effectively as a promotor in the TPA process. Oxidation Scheme of PA. It is quite interesting to investigate t h e oxidation mechanism of PA, in comparison with 41-1,in this system. Two probable routes are presented in Figure 8. According t o route A, 1 mol of P A depolymerizes to 3 mol of AA before reacting with oxygen; t h u s the function of PA will be essentially equal t o t h a t of AA on this route. P A can exert a promoting effect in a unique way, if it takes route 13. The studies on the depolymerization and oxidation reaction of PA gave the following results. (1) Equilibrium between PA and A.4 is inclined almost perfectly to the latter in acetic acid solution at 120'. Therefore, the depolymerization reaction of 1 mol of PA into 3 mol of A h can be regarded as a n irreversible reaction under the typical conditions for PX oxidation. (2) Rate of PA consumption in the system is remarkably increased with the increase of oxygen pressure (Table IX). (3) Intermediates in the direct oxidation of PA (route B) were successfully isolated and identified as meso- and dl-bisInd. Eng. Chem. Prod. Res. Develop., Vol. 12, No. 2, 1 9 7 3
153
REACT03
Table IX. Effect of Oxygen Pressure on the Rate of PA Consumptiona
'1
SEPARATOB
gz:!LN(f
SEPARATOR
DRYER
OFF GAS
02 pressure, k, hr-l
atm
0.3 2 0.8 5 4.5 aconditions: AcOH, 5 ml; PA, 1.25 ml; (AcO)zCo.4HzO, 0.05 g; HzO, 0.3 ml; 100'. 0
PA
HEAE
I n I
AcOH L CATALYST
J DISTILLATION COLVMA
C H3
Figure 12. Outline of Toray TPA process DAEE
Table X. Analysis of Toray Technical Grade TPA
Figure 9. Estimated steps f r o m PA to DAEE PA
'tk:
HEAE
I
DPlki
3 AA
I
1
AcOH
. E:
DAEE
k3
4
J
AcOH
AcOH
Figure 10. Scheme of PA oxidation. k l = 0.3 hr-' (100"); k z = 4.5 hr-i (100"); kS = 9 hr-i (120"); k4 = 2 hr-' (1 20")
Y
Q
1
C k
Q
O? A A * CH3-F-O-C-CH3
!?
ifI'Klon
H E A E L H O - C - 0 -C-O-C-CH,
---tAcOH
+
CH3CH-0-C -CH3
dH3
0 CH3$-O-?-CH3
Ox
OH
I.
H O CH3-t-O-t-CHs
dH
Figure 1 1 . Steps f r o m HEAE to AcOH (1-acetoxyethyl) ether (DXEE). ?;early 50% yield of D.1EE from PA could be attained under suitable conditions. Based on these results, it is concluded that the contribution of route A is less than 10% and that practically all of PA takes route B and reacts directly with oxygen to exert the unique effect as a novel promotor. The estimated steps from PA to the intermediate D.1EE are given in Figure 9. According to the preceding investigations, the entire scheme of PA oxidation is estimated as shown in Figure 10. 1-Hydroxyethyl 1'-acetoxyethyl ether (HEAE), which is supposedly formed as a forerunner or hydrolysis product of DAEE, has not been identified, probably due to its unstable structure. The first-order rate constants (kl-k4) under typical conditions for PX oxidation are determined as given in the caption to Figure 10. 154 Ind.
Eng. Chem. Prod. Res. Develop., Vol. 12, No.
Analysis
Assay as TPA p-Toluic acid p-Formylbenzoic acid Isophthalic acid Cobalt
2, 1973
Value, wt
%
Over 99 0.05-0.10 0.20-0.50 0.010-0.020 0.003-0.010
Based on these rate constants, i t is considered that the main route for the oxidation of PA would be PA --t HEAE AcOH. The elemental steps from HEAE to AcOH are estimated as indicated in Figure 11. Process Description. The outline of the Toray T P A process is described in Figure 12. PX, promotor, and catalyst solution are pumped into a bubbling tower type reactor, which has a n air inlet a t the bottom. The oxidation is carried out continuously in one step at 110-140° under a pressure u p to 30 a t m . The T P A produced in the reactor is taken out as a n AcOH slurry, from which T P A is separated in a decanter. The filtrate containing the catalyst, oxidation intermediates, solvent, and by-product water is dehydrated by distillation before recycling to t h e reactor. A portion of AcOH is separated from t h e solution, since AcOH is also a by-product when PA is used as promotor. The crude TPA is washed with hot hcOH to recover contained catalyst and to remove impurities. Drying gives "technical grade TPA" product, which is over 99% pure, the principal impurities being PTA, p-formylbenxoic acid, isophthalic acid, and cobalt salts (Table X). The amount of impurities is varied depending on the reaction and washing conditions. The advantageous points of the Toray TPA process over the conventional bromine-catalyzed process could be reviewed as follo\F s. (1) Exclusion of bromine compounds from the catalytic system allows the use of stainless steel for the process installation, instead of espensive titanium. ( 2 ) Moderate oxidation conditions result in the higher TP.1 yield from PX (over 9 i % ) . (3) Simpler catalyst systems and milder reaction conditions reduce the formation of by-products, especially the inclusion of colored impurities into TPh. -f
(4) The oxidation mother liquor is recycled without any special purification, leading to the negligible consumption of the catalyst. Acknowledgment
The authors wish t o thank R . Yokouchi, K. Hagihara, T. Kato, Y. Takehisa, and 31. Sat0 for their advice and encouragement. Also thanks are due h l . Shoji, K. Ozaki, and H. Yoshihara for their cooperation.
Literature Cited
Ardis, A. E., Nasti, F. L., Vaitekunas, A. A. (to Olin Mathieson Chemical Corp.), U.S. Patent 3,036,122 (May 22, 1962). Bryant, H. S., Duval, C. A., ?.lchIakin, L. E., Savoca, J. I., Chem. Eng. Progr., 67 (9), 69 (1971). Saffer, A., Barker, R. S. (to Mid-Century Corp.), U. S. Patent 2,833,816 (May 6, 1958). Thompson, B., Neely, S. D. (to Eastman Kodak Co.), U. S. Patent 3,240,803 (March 15, 1966). Toray Industries, Inc., British Patent 1,043,426 (Sept 21, 1966). RECEIVED for review November 7, 1972 ACCEPTEDJanuary 29, 1973
Promises for Ultrasonic Waves on Activity of Silica Gel and Some Supported Catalysts Ramaswami Ranganathan, lndresh Mathur, Narendra N. Bakhshi," and Joseph F. Mathews Department of Chemistry and Chemical Engineering, University of Saskatchewan, Saskatoon, Sash., 871V O W O , Canada The effect of various types of irradiation, such as @-particles, neutrons, and y-rays on the activity of catalysts (Coekelbergs et al., 1962; Mikovsky and Weisz, 1962; Taylor, 1965; Taylor, 1968; Weisz and Swegler, 1955) is reported. In these studies the catalytic activity increased significantly with the radiation treatment. I n all cases the commercially available catalyst was irradiated. Usually, fairly large dosages of irradiation were required to produce significant changes in the catalytic activity (Llikovsky and Weisz, 1962). Recently, another type of irradiation, ultrasonics, was used to increase the activity of a catalyst (Li et al., 1964; Ranganathan et al., 1971). The ultrasonic treatment (insonation) was carried out during the manufacture of the catalyst as opposed to the irradiation of the commercially available catalyst. The information reported in the literature is as yet too scant to afford a clear picture of the subject. The situation is made complex by the several variables in the case of irradiation with ultrasonics-the frequency, intensity, duration of insonation, and the atmosphere over the catalyst system during irradiation. The literature on insonation of catalysts can be divided in three categories: (I) insonation of the catalyst during the preparation stage; (11) insonation of the already prepared catalyst; and (111) insonation of the reaction mixture (including the catalyst). Two of these systems have been discussed in detail in one of our earlier papers (Ranganathan et al., 1971). A brief introduction is in order. Case I: lnsonation of Catalyst during Preparation Stage
The catalyst is insonated during the preparation stage. As the ultrasonics produce cavitation and mixing a t the microlevel, insonation affects the catalyst particle size, crystallite size, pore size, and the pore-size distribution (Kapustin, 1962; Li et al., 1964; Slaczka, 1964). Case II: lnsonation of Already Prepared Catalyst
The insonation is carried out while the catalyst particles are suspended in a liquid medium. This method is useful in re-
generating spent catalysts (Graves et al., 1966; llertes, 1962). It appears that ultrasonically produced cavitation helps to remove the deposits on the spent catalyst and so to regenerate its activity. This is an example of the cavitation and micromixing acting a t various points on the catalyst surface (both inner and outer). Case 111: lnsonation of Reaction Mixture Including Catalyst
This procedure was not considered in detail in our earlier publication. It is now expanded to include the recent developments. The reaction mixture containing both the catalyst and the reactants is insonated. The insonation done in this manner could affect both the catalyst and the reactants. This is a much more complex case (compared t o Cases I and 11) and may have commercial possibilities. Kiener and Young (1958) studied the effect of stationary sound tvaves on the decomposition of formic acid and of ammonia and the hydrogenation of ethylene with nickel filament as catalyst. The reaction was carried out in the filament temperature range of 12O-19O0C. With this type of ultrasonic treatment, the decomposition rate of formic acid increased by about 50% and that of ammonia by about 1570, whereas hydrogenation of ethylene remained unaffected. The increase in reaction rate was attributed to an increase in mass transfer owing to insonation. Kikolaev and Askadskii (1958) used this method in studying the effect of insonation (20 kHz) on the decomposition rate of 0 . l X hydrogen peroxide with silica gel catalyst. The increase in the rate of decomposition was attributed to breaking of the silica gel particles. The sound waves did not change the concentration of hydrogen peroxide without the catalyst. Greguss and Greguss (1960) catalyzed the decomposition of hydrogen peroxide with manganese dioxide gels and suspensions (at 25OC) and insonated the reaction mixture a t 875 kHz. There was a faster initial evolution of oxygen compared with no insonation, but the time required t o complete the decomposition was the same in both cases. The initial increase in rate was attributed to the lesser thickness of the adsorption Ind. Eng. Chem. Prod. Res. Develop., Vol. 12, No. 2, 1973
155
