Bingham. E. C., Green, H., Amer. SOC. Test. Mater. Proc.. 19 ( I I ) , 640
(1919). Bowden, J. N., Dimitroff, E . , Amer. Chem. SOC., Div. Petrol. Chem., Prepr., 7 (4).8-45(1962). Brinkman. H. C., J. Chem. Phys.. 20, 571 (1952). Cecil, R., J. lnst. Petrof., 59 (569), 201 (1973). Eilers, H.. KolloidZ.. 97, 313 (1941). Einstein, A., Ann. Phys. (Leipzig), 34, 591 (1911). Fowkes, F. M.. "The Chemistry and Physics of Interfaces. 1," S. Ross, Ed., pp 1-12.American Chemical Society, Washington, D. C., 1965. Fowkes. F. M., "The Chemistry and Physics of Interfaces. 2,"S. Ross, Ed., pp 153-168, American Chemical Society, Washington, D. C.,
Kabel, R. H., SA€ Trans. 79 ( 3 ) , 1688 (1970). Kuhn, R. R., Amer. Chem. SOC.,Div. Petrol. Chem., Prepr., 694 (1973), Mack, C., J. Phys. Chem., 36, 2901 (1932). Michaels. A. S.,Bolger, J. C., lnd. Eng. Chem., Fundam., 1, 24 (1962a) Michaels, A. S., Bolger. J. C., lnd. Eng. Chem.. Fundam., 1, 153
(1962b). Michaels, A. S.,Bolger, J. C., lnd. Eng. Chem.. Fundam.. 3, 14 (1964). Reerink. H., lnd. €ng. Chem., Prod. Res. Develop., 12, 82 (1973). Rutgers, Ir. R., Rheol. Acta, 2 (4),305 (1962a). Rutgers. lr. R., Rheol. Acta, 2 (3), 202 (1962b). Thomas, D. G., J. Colloid Sci., 20,267 (1965).
Receiued for reuieu: M a y 10, 1974 Accepted August 30, 1974
1971. Hamaker, A. C.. Physica. 4, 1058 (1937).
Preparation and Characterization of 12-Molybdophosphoric and 12-Molybdosilicic Acids and Their Metal Salts George A. Tsigdinos Research Laboratory. Climax Molybdenum Company of Michigan, A n AMAX Subsidiary, Ann Arbor, Michigan 48105
A method for preparing sodium 12-molybdosilicate a n d t h e free 12-molybdosilicic acid is described a s
well as an improved method for preparing 12-molybdophosphoric acid. Methods for preparing salts of t h e two heteropoly acids with t h e cations manganese, cobalt, nickel, copper, lanthanum, and silver are also described. T h e acids and salts t h u s prepared were examined for their thermal behavior, solubility, and hydrolytic stability in aqueous a n d mixed solvents.
Considerable research has been devoted to the use of heteropoly compounds in catalysis (Tsigdinos, 1969). In particular, 12-molybdophosphoric and 12-molybdosilicic acids and several of their metal salts are shown to partake in such diverse catalytic processes as hydrodesulfurization (McKinley, 1957), epoxidation of olefins (Sheng and Zajacek, 1968), alkylation (Shenderova, et al., 1967; Sebulsky and Henke, 1971), preparation of saturated carbonyl compounds (British Patent, 1965), and in the direct oxidation of benzene to phenol (British Patent, 1969). Heteropoly compounds have also been found to act as flame retardants for wood (Truax, 1933, 1935; Amaro and Lipska, 1973). Of particular importance to the catalytic behavior of the compounds in question is the preparation, solubility, and solvolytic behavior in both aqueous and organic media, and the thermal stability and oxidation-reduction behavior. Although the preparation of some of these compounds has been reported in the older literature, the products obtained were often not fully characterized, nor was the pure compound obtained in all cases. A critical evaluation of preparative procedures of heteropoly compounds has appeared elsewhere (Tsigdinos, 1974). Therefore, the present work constitutes a detailed investigation of new and improved methods for preparing, in pure form, 12-molybdophosphoric and 12-molybdosilicic acids and their salts with the cations M n z f , Co2+, Nizf, C$+, La3+, and Naf . In addition, these compounds were characterized by means of chemical analysis, thermal stability studies, solubility, and solvolytic behavior in aqueous and organic media. The oxidation-reduction behavior of the 12-heteropoly acids has been reported elsewhere (Tsigdinos and Hallada, 1973). It is anticipated that this knowledge will be useful in the application of these compounds to catalytic processes.
Experimental Section Materials. The molybdenum trioxide used was Climax Pure grade. The heteropoly acids employed were materials prepared according to procedures developed in this work. All other reagents used were Baker Analyzed grade. Preparation of Compounds. (a) 12-Molybdophosphoric Acid. One mole (144 g) of Moo3 was placed in a 2-1. flask equipped with stirring and reflux condenser, and 1400 ml of water was added. To this was then added 9.57 g 85% H 3 P 0 4 (1h~mole), and the solution was brought to boiling (30 min) and boiled for 3 hr with vigorous stirring. The green color that developed during this period was removed by the addition of a few drops of bromine water. At the end of the heating period, the yellow solution was cooled and the white insolubles remaining were filtered through Whatman No. 42 paper. The mother liquor was then concentrated to a volume of 100 ml by evaporative boiling for 3-4 hr. Upon cooling, the concentrate developed yellow crystals which were filtered and air dried (approximately 130 g yield). This crude product was purified by dissolving in 100 ml of water, filtering the small amount of fine insolubles, if present, and allowing the clear yellow solution to crystallize in the air. The large yellow crystals that formed were filtered and air dried. The yield was 106 g. The crystalline acid effloresces slowly a t room temperature; thus the amount of water of crystallization varies somewhat from sample to sample. The analysis of the product, H3[PMo12040].14H20 is given in Table I. It was ascertained in separate experiments that a volume of nearly 6 1. of water was necessary to bring all molybdenum trioxide employed into solution by the phosphoric acid during the boiling step of the preparation. (b) 12-Molybdosilicic Acid. For this preparation, 42.63 g of NazSi03-9H20 (0.150 mol), 36.3 g (0.150 mol) of NazMo04.2H20, and 237.6 g (1.65 mol) of Moo3 were placed in a 2-1. flask equipped with stirrer and condenser. TO Ind. Eng. Chem., Prod. Res. Develop.. Vol. 13, No. 4, 1974
267
Table I. Analysis of 12-Molybdophosphoric and 12-Molybdosilicic Acids
% ' P o r Si % Mo c7( HzO
M o / P or Mo/Si
1.50 55.49 12.02 lT.94:l.OO
1.49 55.49 12.02 12:1
1.35 55.13 12.65 11.95:l.OO
12 l1
1.34 55.13 12.65 12:1
ratio Analysis of the crude acid obtained without recrystallization showed Mo/P ratios ranging from 11.03:1.00 to 11.71:1.00. b Found to contain 0.033% Na.
i
c
4
a
these solids was added 600 ml of water, and the mixture was brought to a boil in 0.5 hr and the boiling was continued for 1.5 hr at which time all solids had dissolved giving a clear yellow solution containing a very small amount of gelatinous solid. The clear yellow solution (700 ml) obtained upon filtration had a pH of 3.65. This solution contained about 30% by weight of sodium 12-molybdosilicate and was used to prepare 12-molybdosilicic acid by passing through an ion-exchange column containing 900 ml of Dowex-50 (X8) 20-50 mesh resin, in the hydrogen form, with a capacity of 1.9 mequiv/ml of wet resin. The flow rate was 21 ml/min. The resin bed was 70 X 4.1 cm. After 930 ml of colorless effluent was collected, the effluent began to attain a yellow color, after which time 930 ml of bright yellow solution was collected containing the desired acid. The free 12-molybdosilicic acid can be isolated by evaporation in the air at room temperature. Alternatively, the acid can be isolated by first reducing the volume of the solution by a factor of 2.5 by evaporation a t 90°C and then allowing the concentrated solution to evaporate to dryness in the air. The yields are nearly 100%. If the solution is not allowed to proceed to dryness, the large yellow crystals of the acid thus obtained effloresce to a lower hydrate upon standing. Analysis of the solid obtained by evaporation of the solution to dryness is given in Table I, i.e., H4[SiMo12040]. 15H20. ( c ) Sodium 12-Molybdosilicate. Sodium 12-molybdosilicate was prepared in situ as already given under (b) and the solution was allowed to crystallize. The yellow crystalline solid obtained was air-dried. The analysis of the product Na4[SiM01~0~~].14.5H20 is given in Table 111. An alternate preparation of the sodium salt was carried out as follows: to 600 ml of water was added 12.0 g (0.300 mol) of NaOH, and stirring was continued until all of the solid had dissolved. This was followed by the addition of 42.63 g (0.1500 mol) of Na2SiOs.SHz0, and again stirring continued until all solids had dissolved. To this was then added 259.2 g (1.80 mol) of Moos, and with continuous stirring the solution was brought to a boil in 15 min; boiling was continued for 2 hr. The green color that developed could be removed by the addition of a few drops of bromine water. The solution was filtered after cooling, and the desired salt was obtained by evaporation either to crystals or to dryness. A typical analysis of the sodium 12-molybdosilicate thus obtained is given in Table 111. (d) Nickel 12-Molybdophosphate.One hundred twelve grams (0.05 mole) of H3[PMo12040].23H20 was dissolved in 200 ml of water and 9.76 g (0.075 g-atom of nickel) of nickel carbonate was added to the acid solution. Since no apparent evolution of gas occurred after the addition of 268
Ind. Eng. Chem., Prod. Res. Develop., Vol. 13, No. 4, 1974
MOLES OF NaOH ADDED
PER MOLE
OF SALT
M solutions Figure 1. Potentiometric titration of 100 ml 3 X of Mnz[SiMo12040]and Mns[PMo120~~]2 with 0.1000 N NaOH.
the carbonate, the mixture was heated slowly to 65°C a t which point evolution of carbon dioxide began to occur. The reaction mixture was then stirred for 0.5 hr with cooling and filtered. The green mother liquor had a pH of 1.10. A greenish-yellow solid was obtained upon evaporation of the solution to dryness in the open. The analysis of the product Ni3[PMo12040]2.34H20 (yield 110 g) is given in Table 11. (e) Manganese 12-Molybdosilicate. In 200 mi of water was dissolved 74.71 g (0.03569 mol) of H4[SiMo12040]. 15Hz0, and to this solution was then added 8.77 g (0.07138 g-atom Mn) of manganous carbonate. Upon stirring, evolution of carbon dioxide occurred, and the solution was heated to 70°C to dissolve all carbonate. The hot solution was filtered and the yellow-red mother liquor evaporated to dryness in the open to yield red-yellow crystals of composition Mn2[SiM01204o].22HzO. The analysis of the solid is given in Table 111. All other metal heteropolymolybdates were prepared employing similar procedures to those used for the nickel and manganese salts. Characterization of Compounds. (a) Analytical Procedures. The heteropoly compounds were analyzed as follows. Molybdenum was determined gravimetrically by the a-benzoin oxime procedure; phosphorus was determined by precipitation as magnesium ammonium phosphate and weighing as magnesium pyrophosphate; silicon was determined gravimetrically as silica. All other elements were determined by atomic absorption. Water of hydration was determined by ignition a t 500°C. The analyses of the phosphorus compounds are shown in Table 11; those of silicon in Table III. (b) Measurements of pH. The potentiometric titration of the salts Mn2[SiMo12040] and Mn3[PMo12040]2 with 0.100 N NaOH was performed employing glass and saturated calomel electrodes and a Beckman Research pH meter Model 1019. The manganese 12-molybdophosphate was titrated both in water and in 40% dioxane solution (with 0.100 N NaOH in 40% dioxane). The titration curves are shown in Figure 1. The pH readings were repeated until a constant value was reached. The pH of 2% solutions of the salts of the [PMo12040]-~ and [SiM012040]-~anions were measured and are tabulated in Table IV. ( c ) Determination of Density of Salts. The density of the heteropoly salts prepared was determined pycnometrically at 25°C using toluene as the displacement liquid. The densities thus found are given in Table V. (d) Solubility of Compounds. The solubility of the heteropoly compounds prepared was determined in water, methanol, acetone, dimethylformamide (DMF), dimethyl
Table 11. Analysis of Metal 12-Molybdophosphates Found
Calculated
% Metal
% Metal cation
'% P
% Mo
% HzO
cation
%P
% Mo
R H2O
3.49 4.00 4.02 6.46 4.36 14.51
1.30 1.36 1.35 1.40 1.44 1.44
49.96 51.36 51.13 53.34 53.62 49.89
16.02 13.83 13.85 8.21 10.69
3.62 3.97 3.99 6.49 4.44
1.36 1.40 1.39 1.44 1.44
50.63 51.94 51.94 53.77 53.62
16.23 13.81 13.80 8.40 10.69
...
...
...
...
...
Analysis obtained indicates some decomposition. Thus the Ag/P is 2.84:l.OO;the Mo/P is 11.18:l.OO.
Table 111. Analysis of Metal 12-Molybdosilicates ~~
Found
Calculated
% Metal Salt
-
Mnz [SiMoizOio1 22 H 2 0 Co, [ S i M 0 ~ ~ 0 1H20 ~~]*2 Ni2[SiMoiz040]*21Hz0 Cu2[SiMo~z0401~20. 5Hz0 La, [SiMo1204 1, -7 1H 2 0 Na,[ SiMoi20,,].14. 5H20" Na, [ SiMo,,O, 1* 15H20 Ag,[ SiMol2O,, 1'9. 5 H z 0 Q
% Metal
cation
%Si
'%Mo
% H20
cation
R Si
% Mo
%H,O
4.71 4.95 5.17 5.44 7.31 4.47 3.50 17.66
1.17 1.14 1.17 1.14 1.12 1.26 1.27 1.12
48.16 48.90 49.23 48.61 46.69 52.97 52.80 46.67
16.96 16.33 16.41 16.03 17.53 12.07' 12.33' 7.04
4.72 5.09 5.07 5.49 7.62 4.23 4.20 17.81
1.21 1.21 1.21 1.21 1.15 1.29 1.287 1.16
49.51 49.73 49.74 49.72 47.37 53.00 52.78 47.54
17.03 16.33 16.33 15.94 17.53 12.01 12.38 7.60
Prepared from NazMoOd, NazSi03.9Hz0, and MOOS. b From NaOH, NazSi03.9HzO and Moos. Based on molybdenum.
Table IV. p H of 2% Aqueous Solutions of Salts
Table V. Density of Salts -
Compound
pH of 2% solution
Compound
1.84 2.05 2.01 2.11 3.77 3.68 3.90 3.72 3.16 sulfoxide (DMSO) and toluene (Table VI). In several of these solvents, it was difficult to ascertain the point of saturation since thick sirups were obtained. Thus the data shown in Table VI are only approximate. (e) Thermal Stability Studies. Thermal stability studies on the heteropoly compounds prepared were conducted by thermogravimetric analysis (tga), differential thermal analysis (dta), and by heating a t fixed temperatures followed by solubility determinations of the heated materials to ascertain decomposition. All dta and tga experiments were performed under nitrogen a t a heating rate of 5"C/ min. A Harrop Precision Furnace Co. .unit was used for the d t a and tga experiments. The heating a t fixed temperatures was performed in air in a muffle furnace. To ascertain decomposition due to heating, the heated sample (2-3 g) was placed in 50 ml of water and stirred for 0.5 hr. The thermal behavior of the 12-molybdosilicates is given in Table VII; that of the 12-molybdophosphates in Table VIII. The dta and tga curves for 12-molybdophosphoric
Density, g/mt
3.673 2.913 3.144 3.134 3.078 2.979 3.080 3.053 3.009 acid, 12-molybdosilicic acid, and nickel 12-molybdosilicate are given in Figures 2, 3, and 4, respectively. Results a n d Discussion Preparation of Compounds. Failure to recognize the hydrolytic instability of 12-molybdophosphoric acid and the formation of heteropoly species containing atomic ratios lower than 12 to 1 has led to the development of incorrect preparative procedures and misleading physicochemical results carried out on impure compounds. Thus, the preparation of the free 12-molybdophosphoric acid and its salts was undertaken with caution. The acid in question can be prepared (Wu, 1920) as a yellow crystalline solid by ether extraction of acidified solutions of sodium molybdate and sodium phosphate. A procedure for preparing 12-molybdophosphoric acid, uia a non-ether route, by boiling molybdenum trioxide and phosphoric acid (Linz, 1942; Killefer and Linz, 1952) has been found in this work to yield only a crude product containing Mo/P Ind. Eng. Chem., Prod. Res.
Develop., Vol. 13, No. 4 , 1974
269
Table VI. Solubility Data on Heteropoly Compoundsa
Compound
Solubility in water, g/1OO ml of solvent
Solubility in methanol, g/IOO ml of solvent
Solubility in acetone, g/IOO ml of solvent
93.2 (27) 85 (27) 35 (30) 100 (25) 64 (23) 158 (23) 63.5 (24) 67 (24) -320 (20)
>40 (25) 186 (25) 0.6 (30) 14 (25) -500 (20) 40 (18) 75 (23) 24.5 (24) 3.5 (24) l o 0 (27) 113 (26.5) Insoluble 9.2 (25) -400 (23) 13 (13) 6 (23) 3 (24) 2 (24) Insoluble
...
...
*..
...
0
Solubility in dimethylformamide, g/IOO ml of solvent
Solubility in dimethyl sulfoxide g/1OO m l of solvent
Solubility in toluene at 2 5 T , g/IOO m l of solvent
... 56 (28) 33.5 (30) 70 (28) 100 ( 2 2 ) 31 (23.5)
