Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 619-623
819
Molybdate Solutions for Catalyst Preparation. Stability, Adsorption Properties, and Characterization George A. Tslgdinos,' Helen Y. Chen, and B. J. Streusand Research Laboratory, Climax Molybdenum Company of Michigan, A SubsMary of AMAX, Inc., Ann Arbor, Michigan 48105
Because of the complexity of molybdate solutions often used for catalyst preparation, studies have been carried out to ascertain their solution properties and adsorption behavior as a function of pH, concentration, time, temperature, and the presence of oxalate ligands. It has been shown that the solubility and solution stability of ammonium isopolymolybdatesvary as a function of the concentration of molybdenum, the pH of solutjon, and the NH3/Mo03molar ratio of the solution. The presence of hydrogen peroxide markedly increases both the solution stabili and solubilii of the isopolymolybdate anions. Extensive adsorption studies of such solutions onto y-Al,O:, and silica-aluminas were carried out employing (NH4)2M0207, (NH4)BMo7024.4H20 along with their peroxy species, and H2[Mo03C204].2H20and its two ammonium salts. The adsorption behavior of these anionic species is dependent upon the nature and concentration of the complex and upon the time of contact. Spectral characterization has been used to elucidate the interactions occurring in the supported species in the solvated and dried forms.
Introduction Although solutions containing molybdate anions are extensively used in the impregnation of catalyst carriers, relatively little has been published on the solubility and stability of solutions containing such complex molybdate anions. Such information is important in the preparation and manufacture of molybdenum-containing catalysts. Work published to date on complex molybdates has been of a more general nature and has dealt primarily with the structure and properties of such compounds. For example, the structure and properties of isopoly compounds of molybdenum have been reviewed by Tytko and Glemser (1976) and by Tsigdinos and Hallada (1969). Reviews on peroxy compounds of molybdenum (Tsigdinos, 1973) and heteropoly compounds of molybdenum have also been published (Tsigdinos, 1978). The adsorption (Ashley and Mitchell, 1969) and ion-exchange (Iannibello et al., 1979) behavior of molybdate ions with y-alumina have been reported; the present work includes the investigation of this behavior as a function of time and concentration. Sonneman and Mars (1973) have studied the amount of Mo adsorbed onto y-alumina as a function of pH under constant flow conditions. They concluded that the Mo loading was a function of the pH of the solution. This study was carried out using solution concentrations sufficiently low that only the MOO:- ion could be present. Therefore, any concentration effect on the species present in solution would be negated. These results cannot be extrapolated to more concentrated solutions of ammonium molybdates since under these conditions the solutions are unstable. Consequently, the present study has centered on studying the solubility and stability of various ammonium molybdates and other anionic species containing molybdenum as well as the adsorption/ion exchange behavior of these onto various catalyst supports. The interaction of these anions with the supports also constitutes part of the investigation. Information of this kind is directly related to catalyst preparation. In the future, extension of this work will include the investigation of bimetallic molybdenum-containing solutions. Experimental Section Ammonium dimolybdate (ADM), (NH,J2M0207, and ammonium heptamolybdate (AHM), (NH&MqO,dH,O, 0196-432 1/81/1220-0619$01.25/0
were produced by Climax Molybdenum Co. Their peroxy species were prepared in solution by the addition of hydrogen peroxide to water followed by the addition of the solid, in this sequence. The solutions contained 100 mL of H20, 25 mL of 30% H202,and 230 g of AHM; or 200 mL of H20, 36 mL of 30% H202,and 230 g of ADM. The aquaoxalatomolybdic acid (molybdenum oxalate), Hz[Mo03C204].2H20,used was prepared by adding 75.5 g of Moo3 to a hot solution of oxalic acid (55 g in 435 mL of H20)and refluxing for 2 h. The solution was then filtered. Vacuum distillation was then employed to concentrate the resulting solution to near dryness and the product was collected after further air drying. The mono- and diammonium salts of molybdenum oxalate were prepared in solution by the addition of a solution of ADM to a solution of oxalic acid containing the stoichiometric quantities of reagents. The order of addition is important in this case. The quantities used in preparing the monoand diammonium salts of molybdenum oxalate solutions at room temperature in order to obtain the optimum concentration were 32.6 g of ADM, 25.3 g of oxalic acid with a final volume of 300 mL. Concentrated NH40H (13.3 mL) was added to the monoammonium salt solution to produce the diammonium salt. The catalyst supports used in this study were y-Alz03 (Catapal SB, SA 200 m2/g) and 75% sio2-25% 7-AlZO3 (W. R. Grace Co., Lot 980-25, SA 300 m2/g). The silica-alumina was ground and sieved at -140 mesh to obtain particle sizes comparable to that of the Catapal. All support materials were calcined at 500 "C for 2 h in air prior to use. The adsorption/ion exchange experiments were carried out by slurrying 25 g of the support material with 100 mL of the molybdenum-containing solutions for fiied periods of time. The adsorption of ADM and AHM solutions were also studied as a function of concentration. The contact time varied from 5 minutes to 2 h. Concentrations used for ADM and AHM solutions were 0.1 M, 0.5 M, 1.0 M, and 2.2 M in Mo. At the end of the adsorption/exchange period, the slurry was filtered, washed three times with 5 mL of deionized water, and the filtrate plus washings were collected for analysis. From the molybdenum concentrations in the solutions before and after the adsorption/exchange experiment, the molybdenum depletion was found
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@ 1981 American Chemical Society
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of such solutions, i.e., appearance with time of turbidity or precipitate, decreases with decreasing NH3/Mo03 ratio. The solubility behavior of ammonium orthomolybdate, (NH4)2M004,shown in Figure 1, is based on solutions prepared by the in situ addition of ammonium hydroxide to ammonium dimolybdate. While this solution is quite stable, solid ammonium orthomolybdate is not so stable. This salt, which is isolated by the addition of excess ammonia to an ADM solution, decomposes to insoluble ammonium polymolybdates at about 40 "C. The solubility data shown in Figure 1 should be considered only approximate because of the possible loss of ammonia, since the experiments were not carried out in a closed system. The solubility found in this work is approximately 65 g (NH4)2M004dissolved in 100 mL of water at 25 "C, which is consistent with a value of 74.1 g of salt dissolved in 100 mL of 0.01% NH3 solution at 25 "C as found by Karov (1971). An increase in temperature to 50 "C does not result in an appreciable increase in the solubility. Ammonium orthomolybdate, however, is nearly as soluble (20.8% Mo) as ammonium heptamolybdate (21.5% Mo) at 25 "C; although as the ammonia concentration of the saturated solution increases, the solubility decreases appreciably to 3.17% Mo in 28% NH3 solution (Karov, 1971). The stability time at various temperatures of such solutions could not be ascertained since ammonia losses could not be prevented during the heating period (about 2 h). However, these solutions remained clear for the duration of the experiment. Saturated solutions of ammonium dimolybdate are stable for hours when maintained at the temperature at specified in Figure 1. The solubility of (NH4)2M0207 25 "C was found to be 43 g of salt/100 mL of water (17% Mo); however, stirring of the solution for at least 1 h is required to bring all of the salt into solution, whereas saturated solutions of ammonium heptamolybdate are obtained readily within minutes. This observation has been attributed to the breaking of Mo-0 bonds, thus slowing dissolution. Ammonium dimolybdate consists of a chain structure (Armour et al., 1975),whereas the heptamolybdate anion dissolves in water without rupture of the structure. Saturated solutions prepared in situ from ADM and Moo3 that contain NH3/Mo03 ratios of 0.9/1 show decreasing stability, as expected, especially at the higher temperatures. For the ratio of 0.9/1, at 40 "C the solution remained clear upon standing overnight; at 50 "C the solution remained clear for 5 h; thereafter a turbidity developed that increased with time; but at 65 "C the solution remained clear for only 45 min, thereafter becoming turbid. Solutions containing NH3/Mo03ratios of 0.87/ 1remained clear at 40 "C for 4 h, thereafter becoming turbid. At 50 "C, the solution remained clear for 2 h, and at 65 "C the solution remained clear for only 20 min. Finally, solutions of ammonium heptamolybdate (NH3/Mo03ratio 0.857/1) possess an even greater solubility than those described earlier (21.5% Mo); however, at 25 "C the stability of these solutions is decreased considerably. At this temperature the solution is stable for long periods, but it has been reported (Glemser et al., 1970) that after several weeks such solutions deposit the less soluble &ammonium octamolybdate, (NH4)4M~S024(0H)2.2H20,which crystallizes out via hydrolysis of the parent salt. At 40 " C , the solution is stable for 1 h, thereafter developing increasing turbidity. At 65 " C , an ammonium heptamolybdate solution never gets clear, and when held at this temperature for 30 min, it forms a nonpourable gel. An exception to the influence of the
" 0
20
30
40
50
60
70
TEMPERATdPi { C i
Figure 1. Solubility of ammonium molybdates as a function of NH3/Mo03ratio and temperature.
and the Mo loading as weight percent as Mo was calculated. This analysis of molybdenum was carried out by atomic absorption or photometrically. The loadings are precise to &5% of the value reported. Oxalate was determined by permanganate titration. The characterization of supported molybdenum species was accomplished using a Beckman DK2A spectrophotometer with the diffuse reflectance mode in the W region from 230 to 360 nm. Spectra were recorded with the samples as an aqueous slurry and as a dried powder. Spectra of the slurry were measured in a cell built for this purpose. A block of high-density polyethylene was hollowed and fitted with a quartz window. After reaching equilibrium, the slurry was pipetted into the cell and allowed to settle. The excess supernatant liquid was then removed, and the process was repeated until the window area was filled. The reference cell was filled with 7-A1203 in a similar manner. Powder samples were prepared by filtering of the slurry after it reached equilibrium and drying in a desiccator over calcium sulfate. Spectra of the dried sample were recorded by placing the sample in a recessed aluminum plate. The powder was held in place by a quartz cover slip.
Results and Discussion Ammonium Molybdates. Although the preparation and structure of several ammonium molybdates have been described in the literature (Tytko and Glemser, 1976), published data on the solubility and solution stability of the various ammonium molybdates are lacking. Such information is valuable in the preparation of catalysts. For example, although the solubility of several such salts increases with rising temperature, the solution stability decreases, at times making such solutions unsuitable for catalyst preparation. The solubility and stability of solutions of ammonium molybdates containing NH3/Mo03 molar ratios of 2/ 1, 1/1,0.9/1,0.87/1,0.857/1 and 0.5/1 were examined as a function of time and temperature and are presented in Figure 1. In general, with the exception of ammonium octamolybdate (NH3/Mn03ratio 0.5/ l),the solubility of ammonium molybdate at a given temperature increases with decreasing NH3/Mo03ratio. Meanwhile, the stability
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 4, 1981 621 6.4
I (WJ
t d
I
I
I
I
I
I
I
FRESH SOLUTION SOLUTION AFTER ONE DAY
I
,
I
I
I
,
1
1
1
I
1
A
280
I
I
I
I
I
I
I
I
I
8
10
12
14
16
18
20
J
(NHrI2M0207
I
1 2
BY WEIGHT
(NHr)2M0207
09
(NHQ)~Mo~OZ~.~H~O
Figure 2. pH of ADM and AHM solutions as a function of concentration.
NH3/Mo03 ratio on the solubility of ammonium molybdates is shown by ammonium octamolybdate, (NH4)4Mo8oB-4H20,(NH3/Mo03ratio 0.5/1). The solubility of (NH4)4M~8026.4H20 is between 3 and 5 g/100 mL of water from 25 to 50 "C. The hydrolytic instability of polymolybdate ions has been well established (Baes and Mesmer, 1976). A typical equilibrium for such hydrolysis behavior (Sasaki and Sillen, 1968) of the heptamolybdate anion is represented by the equation 7M0042- + 8H+ is M07O24~+ 4H20
2o
t 2
4
6
22
24
ml 300 H 2 0 2 PEQ 100 111 WATER
Figure 3. Solubility of ADM and AHM in dilute hydrogen peroxide solutions.
A 2.2
M
0 1.0 II
0 0.511 0 O.IM
The value of the equilibrium constant, log K , is 57.7 in 3 M NaC104 at 25 "C. Although acidification of MOO^^- is known to yield progressively condensed polymolybdates, it should be realized that the solubility and stability of such species is dependent on the cation, Na+, K+, NH4+,R4N+, etc. (Tytko and Glemser, 1976; Tsigdinos and Hallada, 5 10 20 30 60 120 1969). Acidification of concentrated solutions of ammoTIME I n n ) nium molybdates is different from that of dilute solutions Figure 4. Effect of Mo concentration on Mo loading onto 7-aluof the same species. For example, saturated solutions of mina. ammonium heptamolybdate at 25 "C (pH 5.5) immediately yield a precipitate with little acidification prior to reaching rated AHM solution yields insolubles. Thus, the hydrogen the pH (5.1) of 1% solutions of such salt. Thus, solutions peroxide route may allow for the preparation of stable of polymolybdate ions can be considered buffered and solutions containing various metals, since the peroxy AHM containing metastable species. Figure 2 demonstrates the and ADM species behave differently from their nonperoxy small pH range over which solutions of ADM and AHM analogues, a fact very important in catalyst preparation. are stable as a function of concentration. Adsorption. The adsorption onto y-alumina of Mo Both the stability and solubility of ADM and AHM can species from aqueous solutions of isopolymolybdates be increased considerably by the formation of their peroxy probably occurs through an ion-exchange process. Prespecies. It has been reported (Hansson and Lindqvist, sumably the surface hydroxyl groups are protonated in the 1949; Tsigdinos, 1973) that AHM solutions, when treated acidic medium leaving a positively charged site on the with hydrogen peroxide, yield lemon-yellow peroxy hepy-alumina surface that invites anionic adsorption. This tamolybdates of composition (NH4)6M07024[O]2-x.6H20, mechanism has been proposed by D'Aniello (1981) for the where x is c0.5. Efforts to obtain these compounds richer ion-exchange process which occurs in the absence of any in oxygen failed. The solubility of the peroxy species of direct chemical reaction between complex and alumina. ADM and AHM are shown in Figure 3. The solubility The adsorption behavior of four isopolymolybdate soluof the ADM has been increased from 17% Mo contained tions with y-alumina has been studied here. Some of the in solution to 28% Mo at 25 "C in the presence of hydrogen adsorption properties of molybdenum oxalate have also peroxide. Saturated solutions of AHM/H202 at 25 "C been studied. contain 37% Mo whereas solutions of AHM without hyIn Table I is shown a summary of the maximum loadings drogen peroxide contain only 21 % Mo. Unlike solutions attained by adsorption of solutions of AHM and ADM not containing hydrogen peroxide, the peroxy solutions of onto y-alumina, with and without the presence of H20z ADM and AHM are stable upon boiling and can be readily in the solution. Also included are the pH values of the acidified without the formation of insolubles. For example, solution before the absorption begins and after 2 h, when a saturated AHM/H202solution (pH 5.6) can be acidified the equilibrium is reached. The relative loading of AHM with concentrated HN03 to a pH of nearly 2.5 before onto y-alumina as a function of slurrying time is shown yielding insoluble polymolybdates, whereas, as already in Figure 4. The system reaches equilibrium very rapidly stated, addition of even a small amount of acid to a satuat the lower concentrations. Similar studies were under-
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Table I. Parameters and Results of Adsorption Studies of Mo on r-Alumina solution AHM
ADM AHM-H,O,
ADM-H,O,
a After 2 h slurry time.
concn (as Mo)
initial pH
equiliba PH
0.1 0.5 1.0 2.2 0.1 0.5 1.0 0.1 0.5 1.0 2.2 0.1 0.5 1.0 2.2
5.2 5.4 5.4 5.4 5.5 5.9 5.9 5.2 5.6 5.4 5.5 5.5 6.0 5.9 7.1
7.0 6.6 6.3 6.1 7.1 6.7 6.8 7.0 6.5 6.4 6.3 7.3 6.6 6.4 7.2
% Mo loadinga) at equilib
% Moa depletion at equilib
2 11 18 18 2 12 17 2 9 14 12 2 8 14 14
62 68 68 28 65 66 58 63 61 50 22 68 62 42 23
Calculated t o correspond t o catalyst at 500 "C.
Table 11. Adsorption of the Mo-Oxalatelr- Alumina System Mo-Oxalate/T-Alumina System loading at equilib of solution
support
Moa
C,O,b
H,C,O, H,C,O, AHM
AHM/r-A1,0, ( l 3 % M 0 ) MoO,/r-A1,0, (14% Mo) C,O,/r-Al,O, (3% C,O,)
3% 6% 10%
3% 2% 3%
Calculated to correspond t o catalyst at 500 "C. Catalyst was air-dried at room temperature.
taken with the ADM/ y-alumina, ADM-Hz02/y-alumina, and AHM-HzOz/y-alumina systems. The curves obtained from these slurries are quite similar to the one shown and are, therefore, not presented here. Two factors support the presence of the ion-exchange mechanism in this adsorption process. An increase in pH occurs as the adsorption takes place. This arises from the removal of H+ ions from the solution during the ion-exchange process, as described by D'Aniello (1981). In addition, an adsorption experiment was carried out using a 1 M (in Mo) ADM solution with a support consisting of 25% alumina with 75% silica. The maximum loading at equilibrium in this system was 4 % (as Mo) as compared to 18% when using y-alumina in the support. Since the silica portion of the support would not be subject to an ion-exchange process, the low loading, as compared to that possible with y-alumina, lends further support to this mechanism. It is interesting to note that the loadings of Mo obtained from AHM and ADM solutions are the same, within experimental error. This supports observations by Knozinger and Jeziorowski (1978) that only the MOO:- species exists on the y-alumina surface before calcination regardless of the initial anion used. Presumably, this is due to localized pH effects at the surface. Another interesting factor is that in the adsorption experiments using the more dilute solutions, depletion of Mo from the solution is relatively constant. Using the solutions of higher concentration, a lower percentage of Mo is depleted from solution. This could indicate the existence of an equilibrium between the solution and surface up to a point of maximum loading. In Tables I1 and I11 are shown the results of the adsorption of the molybdenum oxalately-alumina system. It appears, from the pH change upon adsorption, that the ion-exchange mechanism also applies both in the case of oxalate and molybdenum oxalate. Both Moo3 and molybdate are leached from alumina by oxalic acid; however,
Table 111. Adsorption of Mo and Oxalate on 7-Alumina as a Function of pH loading at equilib of a [0.5 MI soln
initial final % % pH pH Mo C,O,
0.9 4.4 (NH4)2C204 6.4 H,[MoO,C,O,] (1.0M) 0.6 (NH,)H[MoO,C,O,] 1.8 (NH,)Z[M~O~CZO,] 4.7 H2C?.04
(NH,)HC,O,
1.3 -8.4 -8.6 -1.1 3.7 2.7 5.7 6.0 5.0
6.4 3.0 2.4 3.3 2.9 2.0
ratio b Mo: C,O,
---1:l
1.8:1 2.4:l
a After 2 h slurry time; Mo loading calculated to correspond to catalyst at 500 "C; oxalate was determined for catalyst air-dried at room temperature. Refers t o supported catalyst.
Table IV. ,A
(nm) for AHM on r-Alumina
approx loadingsa ( 2 h slurry)
soln concn (as Mo) slurry
MOO, Na,Mo04.2H,0 18% 11% 5% 3.5% 2% 0.2%
(pure solid) (pure solid)
a As
Mo.
1M 0.5 M 0.25M 0.175M 0.1 M 0.01 M
295 292 292 285 273 260
dried
calcine
294 262 286 280 278 237, 265 23 5 23 5
298 285 283 282 278
Reference materials.
oxalate is not leached from alumina by an AHM solution. In addition, the maximum loadings possible using [ M O O ~ C ~ Ospecies ~ ] ~ - is much less than is possible using AHM and ADM solutions and their peroxy counterparts. As can be seen from Table 111, the molybdenum oxalate in the dihydrogen form presumably adsorbs intact on y-alumina even after 2 h of contact time. However, as the replacement of hydrogen ion by ammonium ion occurs in the complex, the mole ratio of Mo to CzO4 in the supported form increases indicating degradation of the anion. This can be attributed to increasing pH near the surface of the alumina. This does not occur when the dihydrogen form of the complex anion is used as the protons neutralize the hydroxyls from the alumina. Electronic Spectra. In order to determine the nature of the species adsorbed on y-alumina during the preparation described here, and of the change in their structure with loadings, a study of their electronic spectra was undertaken. Spectra were obtained from samples in the
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 4, 1981 623
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surface. Species of this nature have been shown by Cheng and Schrader (1979), and Cheng (1981). Wang and Hall (1980) have found a pH dependence for Mo loadings on y-alumina using fairly dilute solutions of AHM. They concluded from studies using Raman spectroscopy that catalysts prepared at low pH values may contain polymolybdate clusters on the surface. The aggregated Mo species found at higher loadings in the present study could also be of this type. Upon calcination, the Mo forms a polymeric aggregate, according to Knozinger and Jeziorowski (1978). This would be in a more formal form than that referred to in the last paragraph. This would also account for the less distinct inflection point from the spectra of the calcined form which we studied (Figure 5). Another possibility is that Mo only exists on the surface as Moo3. Conclusions Presumably, the adsorption of molybdates onto y-alumina occurs via an ion exchange mechanism. The solution equilibria 7M002-
+ 8H+ F? M07022- + 4Hz0
(1)
(and its ADM counterpart) are affected by the surface equilibrium, namely A1-OH
+ H+ e Al-OHZ+
(2)
which causes a localized pH effect on the surface, and hence in the solution. Consequently, the protonation of the surface hydroxyl results in the shift of equilibrium 1 to the left. Since the Al-0 bond in Al-OH2+is quite labile, it makes interaction with the molybdate species favorable. In this study we have taken into consideration both the solution and surface chemistry of molybdates. Work in this area is continuing. Acknowledgment We are indebted to our analytical staff for the analyses and to F. W. Moore, W. W. Swanson, and T. R. Weber for helpful discussions. Literature Cited Armour, A. W.; Drew, M. G. B.; Mitchell, P. C. H. J. Chem. SOC., Adton Trans. 1975, 1493. Ashley, J. H.; Mitchell, P. C. H. J. Chem. SOC. A 1969, 2732. Baes, D. F.; Mesmer, R. E. “The Hydrolysis of Catlons”; Wiiey: New York, . . 1976; pp 253-260. Cheng, C. P. Ph.D. Thesls, University of Delaware, Newark, Delaware, 1981. Cheng, C. P.; Schrader, G. L. J . Catal. 1979, 60. 276. DAnieiio, M. J., Jr. J. Catal. 1981, 69, 9. Giemser, 0.; Wagner, G.; Krebs, B. Angew. Chem. Int. Ed. En@. 1970, 9 , 639. Hansson, A.; Lindqvist, I.Acta Chem. Scand. 1949, 3, 1430. Iannibelio, A.; Marengo, S.; Trifiro, E.; Villa, P. L. In “Preparation of Hetere geneous Catalysts”; Delmon, B.; Grange, P.; Jacobs, P.; Ponceiet, G., Ed., Eisevier Scientific Publishing Co.: Amsterdam, 1979; pp 65-74. Karov, 2. G. Russ. J. Inorg. Chem. 1971, 16, 1651. Knozinger, H.; Jeziorowski, H. J . Phys. Chem. 1978, 82, 2002. Sasakl, Y.; Siilen, L. G. Ark. Kemi 1968, 29, 253. Sonneman, J.; Mars, P. J . Catal. 1973, 37, 209. Tsigdinos, G. A. “Peroxy Compounds of Molybdenum”; Climax Molybdenum Co. Bulletin Cdb-18, June 1973. Tsigdinos, G. A. “Topics in Current Chemistry”; Springer-Verlag: Heidelberg, 1978; VOi. 76, pp 1-64. Tsigdinos, G. A.; Haliada, C. J. ”Isopoly Compounds of Molybdenum, Tung sten and Vanadium”; Climax Molybdenum Co. Bulletln Cdb-14, Feb 1969. Tvtko. K-H.: Glemser. 0. Adv. Inoru. Chem. Radiochem. 1976. 19, ’ 239-315.’ Wang, L.;Hall, W. K. J . Catal. 1960, 66, 251.
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Received for reuiew May 1, 1981 Accepted July 29, 1981
