Langmuir 1996, 12, 4185-4189
4185
Adsorption of PMo12 and SiMo12 on Activated Carbon in Aqueous and Acidic Media Wen-ling Chu, Xiang-guang Yang, Xing-kai Ye, and Yue Wu* Changchun Institute of Applied Chemistry, Academia Sinica, Changchun Jilin 130022, People’s Republic of China Received May 26, 1995. In Final Form: May 21, 1996X In various acidic media, such as H2SO4, HCl, H3PO4, acetic acid of 3 M in hydrogen ion concentration, and pure acetic acid, the adsorption of heteropolyacids composed of molybdenum with the Keggin structures PMo12 and SiMo12 on different activated carbons is studied. In acidic media, the adsorbed amount of heteropolyacids is much higher than that in water. By considering the relation between adsorbed amount and the acid strength of the media, as far as SiMo12 and PMo12 are concerned, there exist different trends.
Introduction Catalysis by heteropolyacids (HPAs), a new type of multifunctional catalysts, is a field of growing importance, attracting increasing attention worldwide, in which many new and exciting developments are taking place in both basic research and technology.1,2 In addition to their welldefined structure, HPAs have special properties which are of great value for catalysis, such as strong Bronsted acidity, some 100 times stronger than that of sulfuric acid when applied as a solid or in nonaqueous media,3 fairly high stability in the solid state, etc. These properties render HPAs potentially promising acid, redox, and bifunctional catalysts in homogeneous as well as heterogeneous systems. Compared with the tungsten-based compounds. HPAs composed of molybdenum related to the Keggin structure are effective multielectronic oxidation catalysts. In the last two decades, the broad utility of HPAs in oxidation catalysis has been demonstrated in a wide variety of synthetically useful selective transformations of organic substances, for example, oxidation of acrolein and methacrolein to their corresponding unsaturated acids.4 Remarkable progress has been made in the application of HPA catalysts, and new industrial processes based on HPA catalysis carried out in homogeneous phases have recently been developed and commercialized.5 The importance is self-evident from the practical viewpoint that a HPA is supported to realize heterogenization of a homogeneous reaction. Izumi and Urabe6 stated that HPAs had a very strong affinity for activated carbon. Even washed with hot water or hot methanol by means of a Soxhlet extractor, entrapped catalysts with a maximum HPA content of 7.2-13.9 wt % were obtained. It is well-known that activated carbon is the most significant adsorbent and widely applied commercial adsorbent due to the following properties: low cost, ease of handing, environmental neutrality, high affinity, and high capacity. On the basis of literature data3 and our own previous work,7 we used activated carbons * Author to whom all correspondence should be addressed. X Abstract published in Advance ACS Abstracts, July 15, 1996. (1) Misono, M. Proceedings of the 10th International Congress on CatalysissPart A, Budapest, Hungary, 19-24 July, 1992; Academiai Kiado: Budapest, 1993; pp 69-101. (2) Xu, B.; Wu, Y. Chemistry (in Chinese) 1985, 4, 34. (3) Schwegler, M. A.; Vinke, P.; van der Eijk, M.; van Bekkum, H. Appl. Catal., A 1992, 80, 41-57. (4) Pearce, R.; Patterson, W. Catalysis and Chemical Processes; Hill: London, 1981; p 279. (5) Misono, M.; Nojiri, N. Appl. Catal. 1994, 61, 1. (6) Izumi, Y.; Urabe, K. Chem. Lett. 1981, 663. (7) Chu, W. L.; Ye, X. K.; Wu, Y. Adsorpt. Sci. Technol., accepted.
S0743-7463(95)00412-4 CCC: $12.00
Table 1. General Characteristics of Activated Carbon Samples sample
surface areaa (m2/g)
C
C1 C3 C4 C5
1294.0 1245.0 933.0 957.0
98.00 84.83 89.47 81.21
H
N
Ob
0.53 0.39 0.58 0.77 11.50 0.52 0.72 7.45 0.25 0.20
ash contentc (%) 1.10 2.32 1.84 19.3
a By the N BET method. b By difference. c The ash content 2 was determined by calcining the sample at 800 °C in air with a porcelain crucible.
as adsorbents to study the adsorption of HPAs based on molybdenum, e.g. SiMo12 and PMo12, in various media. It will be shown that the surface structure of activated carbons as well as properties of the medium and HPA itself has a profound bearing on the adsorption strength and amount of HPAs (SiMo12 and PMo12). Experimental Section Materials. Four different activated carbons used in the present study are commercially available: coconut shell carbon (C1), hawnut and hickory nut pit carbons (C3 and C4), and coal based carbon (C5). 12-Molybdophosphoric acid and 12-molybdosilicic acid (denoted as PMo12 and SiMo12 in the text, respectively) were prepared according to the principle reported by Tsigdinos.8 By means of elementary analysis, infrared spectroscopy (IR), and thermogravimetric-differential thermal analysis (TG-DTA), it was confirmed that the samples possessed the Keggin structure. Measurement Methods. The activated carbon sample (0.2 g) was added into 5 mL of a HPA solution whose concentration was given. The mixture was kept at room temperature (15 °C) for 3 h with constant shaking. After the resultant product was centrifuged, the supernatant was diluted quantitatively with distilled water. The concentration of HPA in the diluted supernatant was measured using a SPECODE UV-vis spectrophotometer (Carl Zeiss, Germany) at 240-270 nm. The immobilized amount of HPA was expressed as milligrams of HPA per gram of support.
Results and Discussion Surface Properties of Activated Carbons. It is wellknown that the surface properties of the activated carbon, such as its structure, pore distribution, and especially surface acid-base properties, can be affected by the difference of initial material and preparation conditions. For four initial activated carbons chosen as adsorbent in the present study and their physicochemical properties are listed in Table 1. (8) Tsigdinos, G. A. Ind. Eng. Chem. Prod. Res. Dev. 1974, 13 (4), 267-274.
© 1996 American Chemical Society
4186 Langmuir, Vol. 12, No. 17, 1996
Chu et al.
Figure 1. pH titration curves for the activated carbons.
Figure 4. Adsorption isotherms of SiMo12 on C1 from acidic media: H2SO4 (O); HCl (b); H8PO4 (2). The dashed line is adsorption from aqueous solution (0). Figure 2. NH3-TPD curves of four activated carbons.
Figure 3. Pore size distribution curves of activated carbons.
It could be noted that the content of oxygen on C1 and C5 is very close, and the result of pH titration (Figure 1) proved that C1 and C5, compared to C3 and C4, could be regarded as weakly basic carbons. However, referring to C3 and C4, the same kind of initial material (pit of different nuts) results in similar surface structure and their surfaces give a weak acidity. This was particularly true for the results obtained from Figure 2, which strongly suggest that C3 and C4 are weakly acidic due to their obvious NH3-desorption peak. Whereas, for C1 and C5, no NH3adsorption peak appeared at all. The difference of relatively weak basic carbon (C1 and C5) and weak acidic carbon (C3 and C4) can also be proved further by their pore distributions (see Figure 3). The pore size distribution based on the desorption branch reveals that the maximum pore distribution of both C1 and C5 is below 17 Å; whereas, there are two pore sizes in the C4 sample: i.e. a bimodal distribution with an average pore diameter of about 17 Å and another one distributed about 40 Å. Adsorption of HPAs on Weakly Basic Carbons (C1 and C5). The results showed that the acidity of the medium has a profound influence on the adsorption of Mo
Figure 5. Adsorption isotherms of SiMo12 on C5 from acidic media: H2SO4 (O); HCl (b); H8PO4 (2). The dashed line is adsorption from aqueous solution (0).
HPAs on weakly basic activated carbons. It indicated that the adsorption amount of SiMo12 and PMo12, in acidic media, is much higher than that in aqueous solution. The adsorption amount of both SiMo12 and PMo12 in acidic media is much higher than that in pure H2O, which means that the acidity of the medium has a positive influence on the adsorption of HPAs in corresponding systems. Nevertheless, we found that the form of the adsorption isotherms (cf. Figures 4 and 5) is analogous to that in pure H2O. All the adsorption isotherms on C1 and C5 in various media are dominantly classical Type-I curves, obeying the Freundlich equation
ln X ) ln k + 1/n ln C where k and 1/n are constants related to the energy of adsorption. And corresponding constants from these Freundlich plots are given in Table 2 (for SiMo12) and Table 3 (for PMo12). This may be ascribed to the uniform surface pore distributions of both C1 and C5, whose average pore diameters are 17 Å, and the relatively weak basicity of C1 and C5.
Adsorption of PMo12 and SiMo12 on Activated Carbon
Langmuir, Vol. 12, No. 17, 1996 4187
Table 2. Freundlich Parameters of SiMo12 on C1 and C5 C1 mediuma H2SO4 HCl H3PO4 a
C5
pH value
k
1/n
k
1/n
0.10 0.51 0.88
101.9 134.7 137.2
0.293 0.302 0.187
115.7 138.4 158.7
0.402 0.378 0.363
3 M in hydrogen ion.
Table 3. Freundlich Parameters of PMo12 on C1 and C5 C1 mediuma H2SO4 HCl H3PO4 a
C5
pH value
k
1/n
k
1/n
0.10 0.45 0.73
240.8 219.2 139.8
0.307 0.390 0.432
97.3 184.5 126.9
0.565 0.377 0.432
3 M in hydrogen ion.
By the comparison of the data shown in Tables 2 and 3 we found that the adsorption character of SiMo12 and PMo12 on the weakly basic carbons C1 and C5 is distinctive to some extent due to the difference in heteroatom, although the adsorption isotherms of PMo12 and SiMo12 are very analogous with each other. As may be seen from the data tabulated, contrary to that of PMo12 (Table 3) and SiW12,7 the Freundlich parameters of adsorption isotherms of SiMo12 (Table 2) are directly proportional to the pH value; that is, the higher the pH value of the medium, the higher the ln k value and the adsorption strength. It could be concluded that the acidic properties of the HPA itself are responsible for the adsorption. It is well-known that HPAs in general are strong acids with higher acid strengths. A heteropolyanion (HPAn) is analogous to anions such as SO42-, PO43-, and AsO43-, etc. and also to inorganic oxygen-containing anions. The reaction between a HPAn and a supposed hydroxyl on a support may proceed according to the twostep ligand exchange mechanism9 as follows:
M-OH + H+(aq) f MOH2+(s) M-OH2+ + [HPAn]-1 f M[HPAn](s) + H2O(l)
Figure 6. Adsorption isotherms of SiMo12 on C3 from acidic media: H2SO4 (O); HCl (b); H8PO4 (2). The dashed line is adsorption from aqueous solution (0).
(1) (2)
The protonation step (eq 1) is thought to render the surface hydroxyl more exchangeable. This mechanism could be used to elucidate our experimental results well. According to results reported by Izumi, the acid strength of crystalline HPAs decreased in the series10 H3PW12O40 > H3PMo12O40 > H4SiW12O40 > H4SiMo12O40 (H4GeMo12O40), which is identical with that in solutions. So we can thus suggest that the weaker the acidity of the HPA (e.g. SiMo12), the stronger the affinity of a strong acidic medium to the heteropolyanion. This effect would make SiMo12 difficult to be adsorbed on activated carbon. If PMo12 and SiW127 are taken into account, the picture is thus changing. Because the acidities of PMo12 and SiW12 are stronger, relatively speaking, the weaker affinity of a strong acidic medium, for example, H2SO4 or HCl, for the hetropolyanion would result in PMo12 or SiW12 being easily adsorbed on the surface of activated carbon. It is because the stronger acidities of HPAs and the medium are both beneficial to the protonation and exchange reations. According to the opinion stated above and the change in the adsorption amount of SiMo12 from various acidic media, it can be said that the higher the acidity of the medium, the stronger the affinity for weaker HPAs (9) Anderson, M. A.; Rubin, A. J., Eds. Adsorption of Inorganics at Solid-Liquid Interfaces; Ann Arbor Science Publishers Inc.: Woburn, MA, 1981. (10) Izumi, Y.; Mastuo, K.; Urabe, K. J. Mol. Catal. 1983, 18, 299.
Figure 7. Adsorption isotherms of SiMo12 on C4 from acidic media: H2SO4 (O); HCl (b); H8PO4 (2). The dashed line is adsorption from aqueous solution (0).
(i.e. SiMo12) (that is, H2SO4 > HCl > H3PO4 . CH3COOH) and the less the adsorption amount on activated carbon. But in the case of a stronger HPA, i.e. PMo12 or SiW12, the variation is just the contrary. Adsorption of HPAs on Weakly Acidic Carbons (C3 and C4). Adsorption isotherms of SiMo12 on the weakly acidic carbons C3 and C4 (see Figures 6 and 7) are different from those on relatively weak basic carbons. The isotherms are S-shaped and do not fit the Freundlich equation. The most suitable explanation is that HPAassociation probably occurs on the surface of weakly acidic activated carbons. As a result, the obstruction of micropores leads to a decrease in the adsorbed amount of HPA. In addition, an inflection on the curve is observed at low concentration. According to the general principle,11 adsorption of a HPA in different solvents is restricted by three factors: The first is the affinity of solvent molecules for the HPA, the second is the affinity of the polar surface of the adsorbent for the HPA, and the third is the obstruction of the pore structure to solventized HPA molecules. Adsorption isotherms in this case indicate that (11) Shevereba, I. V.; Boit, A. V.; Serjanuna, N. V. Zh. Fiz. Khim. 1993, 67 (8), 1654.
4188 Langmuir, Vol. 12, No. 17, 1996
Chu et al.
Figure 9. Adsorption isotherms of SiMo12 on activated carbons in pure acetic acid. Figure 8. Adsorption isotherms of SiMo12 on activated carbons in 3 M acetic acid.
the acidity of the solvent would exert a significant effect on these factors, especially the first and second. Thus the affinity of the solvent molecules for the HPA increases gradually as the concentration of HPA increases. At last, this affinity can exceed the affinity of the surface of the activated carbon for the HPA. On the other hand, it has been stated that C4 has two maximum pore size distributions, that is, one below 17 Å and the other at about 40 Å. A mesopore with size 40 Å can provide an unblocked path for the spread of the HPA. After these mesopores are occupied by HPA molecules, it is relatively difficult for other HPAs to enter micropores with sizes less than 17 Å. Thus the adsorption amount slightly decreases in the range of certain concentrations. For the reasons above, inflection is observed and isotherms appear S-shaped. For C3, a similar surface structure derived from the same kind of initial material as C4 (pit of different nuts) is likely to be the main reason for analogous isotherms. In view of PMo12 on C3 and C4, the isotherms are the same as that of SiMo12, which illustrates that they have identical adsorption character and affecting factors. As may be seen from the relevant results above, in various acidic media, it has been further proved that adsorption of HPAs is dependent on the acid-base properties of the adsorbent surface. In addition, there is a marked difference in pore size distribution between shell or coal-based carbon and pit carbons. For weakly basic carbon C1 or C5, a uniform pore size distribution is observed, which results in type-I curve isotherms, whereas in the case of weakly acidic carbon C3 or C4, isotherms are S-shaped, which is caused by two different pore size distributions of the activated carbons or weak surface acidity. Adsorption of HPAs from Acetic Acid Medium. Interestingly, in organic acidic media, isotherms of HPAs based on molybdenum are very exceptional. These isotherms are given in Figures 8 and 9 for SiMo12 and Figures 10 and 11 for PMo12. As shown above, regardless of the surface properties of carbons, weakly basic (C1 and C5) or weakly acidic (C3 and C4), the amount of HPAs adsorbed is strikingly increased and the isotherms are linear. This kind of isotherm can be produced by a proportional increase in the adsorption amount with the concentration. It is wellknown that the solution chemistry of molybdenum is very
Figure 10. Adsorption isotherms of PMo12 on activated carbons in 3 M acetic acid.
complex; this means that the different media, pH values and molybdenum concentrations have obvious effects on the existing state of molybdenum.12 Owing to the hydrolytic instability of high-valent molybdenum in the medium, when the heteroatom of the HPA is changed, its hydrolytic stability decreases in the order Si > Ti > Ge > P > As. For example, the heteropolyanion [SiMo12O40]4-would degrade as the pH value of the solution changed to produce a heteropolyanion with a defect or even different molybdate ions,13 as shown from the following formula: pH ) 1.0-2.5
[SiMo12O40]4- 98 pH ) 4.2-5.4
[SiMo11O39]8- 98 MoO42It could be said that only the degraded product, instead of the heteropolyanion, would be adsorbed. So this (12) Ardon, M.; Peinick, A. J. Less-Common Met. 1977, 54, 233. (13) Highfield, J. G.; et al. Proceedings of the 8th International Congress on Catalysis Berlin (West), Vol. V, 2-6 July, 1984; Verlag Chemie and Schon & Wetzel GmbH: Frankfurt am Main, 1984; p 611.
Adsorption of PMo12 and SiMo12 on Activated Carbon
Langmuir, Vol. 12, No. 17, 1996 4189
Owing to the decrease in size of the molecule adsorbed, the micropore of the activated carbon could lose its obstruction action and solventized HPA molecules not only could adsorb in the mesopores of the activated carbon but also could diffuse into the micropore. Especially in pure acetic acid medium, the amount adsorbed has almost no selectivity to activated carbon; that is, the adsorption amounts of SiMo12 on various original activated carbons are almost the same. Conclusion
Figure 11. Adsorption isotherms of PMo12 on activated carbons in pure acetic acid.
indicated further that the acidic properties of the medium have important effects on the adsorption of HPAs on activated carbons. We have measured from experimental data that, in pure acetic acid medium (pH value of SiMo12 is approximately 4.5), it can be expect that a considerable amount of [SiMo12O40]4- has been degraded to MoO42-.
From the results given above, apparently a combination of the surface acid-base properties of activated carbons, the acidic character of the medium as well as the chemical properties of the HPA itself including acidity and stability determined the amount of HPA-Mo adsorbed. For SiMo12, whose acidity is weaker, it is known that the amount of SiMo12 adsorbed is directly related to the pH value of the system due to the stronger affinity of a strongly acidic medium for a heteropolyanion, which can lead to the heteropolyanion being difficult to adsorb on the surface of the activated carbon. However, an inverse correlation has been observed between the acid strength of the system and the adsorption amount of the stronger HPAs, i.e. SiW12 and PMo12. LA950412Z
