5282
J . Phys. Chem. 1990, 94, 5282-5285
of the isotherm at relatively high concentration, M/Mo = 1.4 mol/mol. This is attributed primarily to M2+ at high concentrations being able to compete for the basic surface O H groups. Acknowledgment. We thank Dr. M. P. van Dijk and Mr. C.
T. J. Mensch for the EXAFS measurements, and Mr. R. de Ruiter for the data reported in Figure 5. Registry No. A H M , 12027-67-7; A120,, 1344-28-1; Mo. 7439-98-7: A I O O H , 24623-77-6.
Chemistry of Phosphomolybdate Adsorption on Alumina Surfaces. 2. The Molybdate/Phosphated Alumina and Phosphomolybdate/Alumina Systems J. A. R. van Veen,* P. A . J. M. Hendriks, R. R. AndrCa, E. J. G. M. Romers, and A. E. Wilson Koninklijke/Shell- Laboralorium, Amsterdam (Shell Research B. V.), Badhuisweg 3, 1031 CM Amsterdam, The Netherlands (Received: February 21, 1989: I n Final Form: December 7 , 1989)
The adsorption of ammonium heptamolybdate (AHM) on phosphated alumina and of PzM05023~and PMo,,0m3- on y-A1,03 has been studied by using FTIR, Raman, and 31Psolid-state NMR spectroscopies, and TPR. Adsorption of phosphate prior to contacting of y-Al,03 with AHM leads to a deactivation of the alumina surface still remaining. Phosphate reacts with y-Al,O, to form an AIPO4-type surface phase, which itself is capable of adsorbing molybdate, with the formation of an irreducible surface molybdophosphate, its adsorption capacity being lower, however, than that of the original alumina. The primary phosphomolybdate adsorption reaction on alumina is shown to be the same as that observed in the case of AHM, viz., a reaction with the basic surface OH groups, leading to the decomposition of the adsorbing species. Thiophene hydrodesulfurization (HDS) activity data of variously prepared Mo(P)/A1203 catalysts are in agreement with the idea that HDS activity increases with increasing Mo reducibility.
Introduction In the previous paper,' it has been shown that the adsorption of poly(oxomolybdates) on yA1203(and a - A I 0 0 H ) involves two processes: in the first instance, we have a reaction with the basic surface hydroxyl groups, leading to depolymerization of the sorbing poly(oxomolybdate) ion and adsorption of (part of) the resulting monomolybdate ions, eventually followed by physisorption on the coordinatively unsaturated (cus) AI3+sites. In the present paper, we discuss the mechanism of the well-known interfering effect of phosphate on molybdate ad~orption,~J and determine whether the same surface sites, basic O H groups and cus AI3+ sites, are involved in the adsorption of phosphomolybdates, as expected on the ground that they are base-sensitive polyanions just as heptaand ~ctamolybdate.~ Since a plethora of phosphomolybdate species exists in solutions containing both phosphate and molybdate at pH 5 7,4-7 and since it is known that phosphate interacts more strongly with y-A1203than m ~ l y b d a t e , it~ ~was ~ ~decided * to study in the first instance the adsorption of AHM on alumina samples which had previously been exposed to phosphate ((NH4)H2P04), rather than the coadsorption of phosphate and molybdate. The interfering effect of phosphate on molybdate adsorption could simply mean that the former species interacts with the same alumina surface sites as the latter, but more strongly. In current adsorption models, indeed, phosphate is pictured as adsorbing either covalently or electrostatically on protonated (and, therefore, On the other hand, originally basic9) surface hydroxyl (I! van Veen, J. A. R.; Hendriks, P. A. J. M.; Romers, E. J. G. M.; Andrea, R. R. J . Phys. Chem., preceding paper in this issue. (2) Parfitt, G. D. Pure Appl. Chem. 1976, 48, 415. (3) Gishti, K.; Iannibello, A.; Marengo, S . ; Morelli, G.;Tittarelli, P. Appl. Catal. 1984, 12, 381. (4) Pope, M. T. Heteropoly and Isopoly Oxometallates; Springer: Berlin, 1983. ( 5 ) Souchay, P. folyanionr and Polycationr; Gauthier-Villars: Paris, 1963. (6) Van Veen, J. A. R.; Sudmeijer, 0.;Emeis, C. A,; de Wit, H. J . Chem. SOC.,Dalton Trans. 1986, 1825. (7) Pettersson, L.; Andersson, I.; Ohman, L.Inorg. Chem. 1986, 25, 4726. (8) Hsu, P. H. Minerals in Soil Enuironments; Soil Science Society of America, Inc.: Madison, WI, 1977; Chapter 4. (9) Van Veen, J. A. R. J. Colloid Interface Sci. 1988, 121, 214. ( I O ) Muijadi, D.; Posner. A . M.; Quirk, J. P. J . SoilSci. 1966, 17, 212.
0022-3654/90/2094-5282$02.50/0
it has been reported that phosphate eliminates the strongly acidic sites of alumina3.I2and that treatment of ZSM-5 zeolites with, e.g., H3P04 or (NH4)2 HPO,, decreases the concentration of strong Bransted ~ i t e s , ' which ~ J ~ suggests that phosphate primarily reacts with acidic hydroxyls. Our own results concerning (NH4)H2P04 adsorption on AI2O3-Aindicate that in fact both basic and acidic O H groups are involved. The adsorption of some phosphomolybdates has been recently described in the 1iterat~re.l~In that paper, 12-molybdophosphate, PMO,~O~;-,has been reported to adsorb intact on an alumina. This would be very surprising, if true, since PMolz is not a very stable compound6 and should be at least as susceptible to depolymerization by surface hydroxyl groups as heptamolybdate. To ascertain whether the various adsorbed phases encountered in this study differ at all in their catalytic properties, we have measured their specific activity in the hydrodesulfurization of thiophene. Experimental Section To study the alumina-phosphate interaction, 2-g samples of freshly calcined y-A1203(denoted A1,03-A in the previous paper') were contacted with aqueous (NH4)H2P04solutions (100 mL), and the mixtures were shaken occasionally. Standard adsorption time was 1 day. The P uptake was determined by measuring the P content of the solid after drying at 120 O C in vacuo with X-ray fluorescence. An A1PO4/AI20, sample was prepared by contacting y-A120, with 1 M H3P04 for 2 days. Adsorption of ammonium heptamolybdate (AHM) on phosphated AI203 was effected as ( I I ) Mikami, N.; Sasaki, M.; Hachiya, K.; Astumian, R. D.; Ikeda, T.; Yasunaga, T. J. Phys. Chem. 1983, 87, 1454 and references therein. (12) Stanislaus, A.; Absi-Halabi, M.; AI-Dolama, K. Appl. Catal. 1988, 39, 239. ( I 3) Lercher, J. A.; Rumplmayr, G.; N o h , H. Acta Phys. Chem. 1985, 31, 71. (14) Ashton, A. G.; Dwyer, J.; Elliott, I.
S . ; Fitch, F. R.; Qin, G.; Greenwood, M.; Speakman, J. Proceedings of the 6th International Zeolite Conference Butterworths: London, 1984; p 704. (IS) Cheng, W.-C.; Luthra, N. P. J. Catal. 1988. 109, 163. (16) Van Veen, J. A. R.;Jonkers, G.; Hesselink, W. H. J . Chem. SOC., Faraday Trans. 1 1989, 85, 389.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 13, 1990 5283
Phosphomolybdate Adsorption on Alumina
ABS0RBANCEla.u.
ABSORBANCE/a.u. P-OH
Mo(ads.)/mg.g-’
Mo(ads.)/mg. g-’
1
20
0
3800
3600
3400
3800
u , cm-’
Figure 1. FTIR spectra (OH stretch) of (a) Al20,-A, (b) H2POC/A1203 [PI = 1.8 wt%, and (c) as (b), but after A H M adsorption. Spectra taken after 30 min evacuation at 525 OC. Inset: adsorption isotherms of ammonium heptamolybdate (AHM) on (A)A1203-A,and ( 0 )A1203-A after adsorption of H2P04. Spectrum c corresponds to the open circle.
described in the previous paper.’ For the adsorption of phosphomolybdates we followed similar procedures, where solutions of P2M050236-were prepared by dissolving (NH4)2M004and (NH4)H2P04in the molar ratio 5:2, and acidifying the resulting solution with concentrated HNO, to pH = 2; solutions of PMoi20a3- were prepared from phosphomolybdic acid obtained from Merck. The FTIR, Raman, and TPR techniques have been described before.’ 3iPsolid-state N M R spectra were recorded on a CXP-300 machine, applying high-power decoupling and magic-angle spinning (,lP at 121.46 MHz). Chemical shifts are negative towards higher field and are referenced to external 85% H3P04.
Results and Discussion I . Adsorption of Phosphate on y A 1 2 0 3 . At low P loadings, Le., [PI 5 0.1 mmol.gi (0.3 wtW), adsorption of (NH4)H2P04 appears to take place by a ligand-exchange mechanism involving the basic hydroxyl groups: S-OH + H2PO4S-H2P04 + OH(1)
-
(where S denotes surface) as follows from (i) a large pH shift in the adsorption solution from about 4.5 to about 9.3, and (ii) the only change in the O H stretch region of the IR spectrum of the alumina being, apart, of course, from a new band at 3680 cm-’ due to ZP-OH, a decreased absorbance of the high-frequency band commensurate with the amount of P adsorbed.” Whether the stoichiometry of the adsorbed phase is really [H2P04] is uncertain. It could equally well be [NH,HPO,] since adsorption of HP0,2- at similarly low loadings leads to P/A1203 materials, which, after drying, give rise to the same ,‘P solid-state N M R spectrum: a single resonance at 6 = -6 ppm. At higher P loadings, however, it is not so much the band due to the basic O H groups that is affected but those due to the neutral and acidic, Le., bridged,18ones, cf. Figure 1, The precise sorption mechanism remains unclear at present; what we can establish is the following: (i) Cus AI3+sites do not appear to be involved, since far more phosphate is taken up than could be accommodated on such sites
3600
2 4 6 C’,,M/mmot. L-’
3400 v , cm-’
Figure 2. FTIR spectra of (a) 4.5 wt% P/A120, ex H2P04- adsorption, (b) 7.2 wt% P/A1,03 ex H3P04 adsorption. Inset: A H M adsorption isotherms on the latter sample.
(assuming a P/Al3+ stoichiometry of GI), and since pretreatment of the A1203with acetylacetone to block (most of) the cus AI3+ sitesi6 hardly influences the H2P0, adsorption level. (ii) There is stoichiometric coadsorption of the cation (which we established using NaH,PO,). (iii) The surface area starts to decrease upon adsorption of more than -0.1 mmo1.g’ P, as is ~ e l l - k n o w n , ’ ~indicating J~ a surface restructuring. In fact, an AIPO,-type phase8 appears to be formed, as the IR spectrum of high-loading samples show two bands at 3795 (A10-H) and 3680 (PO-H) cm-I, characteristic of (high surface area) A1P0420 (see Figure 2; the ,lP solid-state N M R spectra of such an AIPO, sample2‘ and of calcined P/AI20, samples show a single resonance at -29 ppm). As there are many aluminum orthophosphates,22the precise structure of the adsorbed AlPO, phase cannot be given. It should be realized that, although the surface area reduction already takes place in the adsorption/drying step, full AlPO, formation occurs only upon calcinating the sample: 31Psolid-state NMR spectra of dried P/A1203 samples show a single resonance with 6 increasing from about -10 ppm to about -18 ppm with increasing P loading (and, therefore, decreasing final pH in the adsorption solution). This shifting line position probably reflects a gradual change in adsorbate structure, but definite assignments cannot as yet be made. Thus, it would appear that phosphate does not adsorb electrostatically on protonated surface hydroxyl groups, contrary to the conclusion reached in ref 1 1. On the other hand, its adsorption also does not to any large extent involve the sites responsible for AHM adsorption, basic O H groups, and cus AI3+ sites but instead the neutral and acidic O H groups. 2. AHM Adsorption on Phosphated Alumina. A H M adsorption isotherms, at neutral and acidic pH’s, on 7.2 wt% P/ A1203,prepared by contacting AI2O3-Awith I M HSPO,, giving effectively A1PO4/AI2O3, are shown in the inset of Figure 2. AlPO, is reported to contain cus A13+sites,20and we have verified their existence in our A1P04/A1203sample through adsorption of acacH (cf. ref 16). Formation of Al(acac), hardly occurred and the IR absorbance of the bands due to the Al(acac) surface (19) Fitz, C. W.; Rase, H. F. Ind. Eng. Chem., Prod. Res. Deo. 1983, 22,
40. (17) On the identification of the high-frequency band as due to the basic OH groups, see the previous paper’ and the references cited therein. (18) Knozinger, H.; Ratnasamy, P. Catal. Reu.-Sci. Eng. 1978, 17, 31.
(20) Peri, J. B. Discuss. Faraday SOC.1971, 52, 5 5 . (21) Kearby, K. K. Acres DeuxiPme Congr. In?. Catal. 1961, 2567. (22) Kniep, R. Angew. Chem. 1986, 98, 520.
5284
The Journal of Physical Chemistry, Vol. 94. No. 13, 1990
van Veen et al. REDUCTION RATE/a.u.
species was much less than in the case of comparable acacH/A120, samples, so we may conclude that the AIPO4/AI20, material contains fewer cus AI3+ sites than the original alumina. Nevertheless, since basic O H groups are hardly present, and since AHM adsorption on A1PO4/AI2O3is not, under near-neutral conditions, accompanied by a pH shift,z3we supposed at first that we were dealing here with a simple physisorption of AHM (or AOM = ammonium octamolybdate, the main species at pH = 2) on cus AI3+sites. However, TPR of samples calcined at 250 OC revealed that the adsorbed phase is totally irreducible (up to 700 "C)! Since adsorbed AHM and AOM are quite reducible,' their presence here can be ruled out. From our study of phosphomolybdates in solution,6we know that, apart from MOO^^-, there exist two species which are (electrochemically) irreducible: P2Mo@236-, and a PMo, or PMo, species whose composition is not established; we opt here for the formulation PMo70277-(cf. Discussion in ref 6). In both cases we can write an adsorption equation which does not involvc Fi" or OH":
300 I
T, OC 7 00
500 I
1
i
1
b
I
i
1 00
60
140
TIME, min OH
'0.
Figure 3. TPR profiles of (a) 2.5 wt % Mo/AI2O3:P, sample corresponding to the open circle in Figure 1, and (b) 5.5 wt % Mo/AIPO4.
or 2P,-OH
+ M o ~ O ~ (P,-0)2M~50152~ ~ + 2 M 0 0 , ~ -+ HzO
A1,0,.
-+
0 UNTSIa .u ,
(3)
We do not know whether adsorbed phosphate carries two hydroxyl groups or only one (although the latter is perhaps more likely), and the experimentally determined ratio of loss in PO-H IR absorbance to Mo adsorbed, assuming a P dispersion of 100%(two measurements), is halfway that expected for eqs 2 and 3. The high-frequency Raman band of the adsorbed phase, on the other hand, at 940 cm-I, is closer to that observed for P2M05(935 cm-I) than for PMo7(?) (950 The AHM adsorption isotherm on 1.8 wt% P/AI2O3,prepared via adsorption of (NH4)H2P04,is shown in the inset of Figure 1. It is seen that less Mo adsorbs compared with the unphosphated alumina. The IR spectrum of Mo/A1203:P, (Figure I ) , taken, it should be remembered, after evacuation at 525 OC, shows a loss in absorbance of all types of hydroxyl groups present. As TPR of the sample calcined at 250 O C again shows that the adsorbed phase is irreducible, we conclude that the primary adsorption reaction is with the POH groups according to, probably, eq 3. That is, although some of the original AHM adsorption sites, e.g., the basic (alumina) O H groups, are still available, the presence of phosphate prevents them from interacting effectively with AHM in the adsorption/drying step (for sterical reasons?); in other words, phosphate effectively deactivates3 the remaining alumina patches. Calcination at 500 "C leads to the decomposition of the adsorbed P-Mo species, in the 1.8 wt% P/A1203case all the way down to monomolybdate, no doubt due to the low Mo loading and the relatively high concentration of AI-OH sites remaining, while in the A1PO4/AI2O3case a relatively highly aggregated Mo-ox species is generated, as is evident from the respective TPR profiles, Figure 3 (cf. the TPR profiles of the various Mo/AI2O3 samples ex AHM adsorption, discussed previously'). (NH4)6P2.Mo5023 adsorbed on A1PO4/AI2O3,on the other hand, remains irreducible even after calcination at 500 O C . I n conclusion, phosphate interferes with molybdate adsorption on alumina not so much by direct blocking of molybdate adsorption sites, as by reacting with the support to form an AIPO,-type phase, thereby deactivating the remaining alumina surface. AIPO4/AI20, does itself adsorb AHM via a reaction with POH groups, but this leads to Mo loadings lower than those obtained with the original alumina under comparable conditions. 3. Phosphomolybdate Adsorption. The adsorption of at pH = 2 leads, in agreement with a previous report,Is to its decomposition: if it adsorbs molecularly, e.g., electrostatically
-5.7 /
1000
900
800
700
600 v , cm-'
Figure 4. Raman and 31Psolid-state NMR spectra of H3PMo,20,-impregnated y-Al,03.
to protonated surface O H groups, it would be irreducible (vide supra), but TPR indicates the presence of highly reducible species, H2/Mo being 0.42 at 1.7 wt% Mo and 0.65 at 5.2 wt % M O . ~ ~ Since H2/Mo for adsorbed AHM is 0.44 and for adsorbed AOM 0.72 (cf. ref I ) , we take it that the basic O H groups are involved in the P2M05adsorption reaction, depolymerizing it to HM at low Mo loadings, but only to O M at higher ones. It has to be admitted, however, that T,,, after calcination at 500 "C, with 450 and 490 "C, is somewhat on the high side, the expected values being 435 and 450 "C, respectively-this is perhaps caused by the presence of phosphate. ~~~~~~
(23) During adsorption at pH = 2, pH rises slightly. which could simply be due to some support dissolution
in
(24) The H,/Mo ratio the AHM/A120, case
IS
independent of the calcination temperature, as
The Journal of Physical Chemistry, Vol. 94, No. 13, 1990 5285
Phosphomolybdate Adsorption on Alumina Mo1adr.llmo. g-'
COUNTS1a.u.
THIOPHENE CONVERSION/% TO
REDUCTION RATE1a.u
30
:oL
10.861
'-c
20
CS /,' . 40
a 10 751
20
I-'
1MM
800
Y.
300
7w
500
cm-'
T. C '
Figure 5. ( A ) H 3 P M o 1 2 0 ~ / A 1 2 0 3 -adsorption A isotherms. The solid symbol corresponds to the impregnated sample. Numbers in parentheses are pH after adsorption. (B) Raman spectra of samples a and d. (C) TPR profiles of samples a and c.
Let us now turn to the adsorption of molybdophosphoric acid. An AI2O3-Asample, dry-impregnated with an aqueous solution of PMo12to achieve the same surface loading as Cheng and LuthraIs obtained, 0.30 mg of Mo m-2, was studied as such with 31P N M R (solid state) and, after drying at 90 OC, with Raman spectroscopy. The results of these two measurements are reported in Figure 4. The N M R spectrum shows two main resonances, one at 6 = +2.2 ppm, the other at 6 = -5.7 ppm. If the chemical shifts in the solid state are anywhere near those in solution, the former resonance should be ascribed to adsorbed P2Mo5, while the latter is due to adsorbed HP042-,as determined in a separate experiment. Hence, the adsorption reaction can be formally written as H3PM012040 + 3 0 H i
+
-
+
PM012040~-+ 3H20
(4)
1 4 P M 0 ~ ~ 0 ~1140H; ~~7P2M050236- + 19M0702,6- + 57H20 (5)
+
7P2M0~022- 160H;
+
14HP042-
+ 5M070246- + H2O (6)
(where OH; denotes a basic surface hydroxyl group) followed by adsorption of P2M050236, HPO,", and MqO24+. This scheme is in accordance with the Raman result, P2M05 having its main band at 935 cm-I and H M at 950 cm-l. Now, we know the number of basic hydroxyl groups, viz., -0.8 mmol-g-', and we know the amount of PMoI2added, so that we can calculate to what extent reactions 4-6 should proceed. The result is that reactions 4 and 5 should be complete and that about 55% of the P2M05 formed in reaction 5 should have been converted to H P 0 2 and M070246 via reaction 6; this is quite consistent with the NMR line intensities observed (Figure 4). In view of the fact that the depolymerization of PMolzinvolves many intermediate stages, i.e., in solution one has, ignoring some minor specie~,62~ PMoI2-.PMo9 -.PMo,, P2Mo5 MOT Mo, one would not expect depolymerization to go very far when contacting an alumina with excess PMoI2. The results of a few pertinent experiments are collected in Figure 5. The Raman band at 963 cm-I indicates the adsorbed phase to be PMo9 or PMo,, or a mixture thereof6 (bands at -700 cm-l are weak and nondiscriminatory), while the high H2/Mo ratios (TPR) are in agreement with the presence of relatively highly aggregated species (PMo12has its Raman band at 995 cm-' and is not therefore present as such). We have not quantitatively determined the r d e of cus AI3+sites in the PMo,, adsorption process. However, on account of the adsorption levels obtained, which are higher than would be expected on the basis of a PMo12-basic OH reaction alone, it seems likely that the cus AI3+sites are involved. As we have previously shown that Mo02(acac), adsorption involves both basic O H groups and cus AI3+ sites, the above implies that adsorption of Moo2(acac)2should be strongly reduced on a PMoI2/Al2O3sample such as curve d in Figure 5A. This is indeed found to be the case. 4. Thiophene Hydrodesulfurization Activity of Some Mol AI2O3Catalysrs. From the relevant literature, one would expect -+
- -
t
C,
1
I
/
i i
i
/
I
/
I
i Ir I
I
I lo
/p
OV 0
t
1 5
1 10
I
15 Mo LOADING/%w
Figure 6. Thiophene HDS activity vs Mo loading of various Mo/Al2O3 adsorbed monomolybdate, ex AHM adsorption at pH cv catalysts: (0) 6; (0)adsorbed HM, ex AHM adsorption at pH LV 6; (0)adsorbed monomolybdate, ex AHM adsorption at pH = 2; (m) adsorbed HM/OM, ex AHM adsorption at pH = 2; (A) samples ex-PMolz adsorption; (a) catalyst containing, apart from adsorbed Mo phases, a surface precipitate.
that the more reducible the adsorbed Mo phase is, the higher its activity in the HDS of thiophene25v26will be. That is, specific HDS activities should increase in the order monomolybdate < H M < OM < PMo9/PMoll. From Figure 6 it can be seen that this is indeed the case, although the differences are not large. On the other hand, the P2Mo5 adsorbed on AI2O3:Psample, which showed zero reducibility, also had virtually zero activity in the thiophene test. It is also noted that the precipitate does not lead to an active MoS phase, in contrast to what can be deduced from the data of Siri et aI.27
Conclusions 1. Phosphate does not interfere with molybdate adsorption by reacting with the adsorption sites of the latter. It reacts with bridging hydroxyls to form an A1P04-like phase, which by an as yet nonelucidated mechanism deactivates the remaining alumina surface. AHM can adsorb on the AIP04-like phase, with the formation of a surface phosphomolybdate, here tentatively identified as a P2Mo5 species, but the Mo adsorption level is lower than that of the original A1203. 2. As in the case of H M and OM, P2M050236 and PMoI20& interact with the basic surface O H groups of alumina, which leads to their decomposition. No attempt was made to quantify the role of cus AI3+sites in phosphomolybdate adsorption, but, in view of the adsorption levels observed, it seems likely that they are involved. Registry No. AHM, 12027-67-7; Al2O3, 1344-28-1; P2M050236-, 54723-82-9; PMo,,O.,,,~-, 12379-13-4; Mo, 7439-98-7; P, 7723-14-0; thiophene, 1 10-02-1. (25) Houalla, M.; Kibby, C. L.; Petrakis, L.; Hercules, D. M . J . Card. 1983, 83, 50.
(26) Thomas, R.; van Oers, E. M.; de Beer, V. H. J.: Medema, J.; Moulijn,
J. A. J . Card. 1982, 76, 241. (27) Siri, G. J.; Morales, M. I.: Blanco, M. N.; Thomas, H. J . Appl. Cam/. 1985, 19, 49.
