Langmuir 1989,5, 911-916
911
Phosphoric Acid on Oxide Carriers. 1. Characterization of Silica, Alumina, and Titania Impregnated by Phosphoric Acid Guido Busca, Gianguido Ramis, Vincenzo Lorenzelli," and Pier Francesco Rossi Istituto di Chimica, Facoltci di Ingegneria, Universitci, P.le Kennedy, 16129 Genova, Italy
Aldo La Ginestra Dipartimento di Chimica, Universitci di Roma "La Sapienza", P.le Aldo Mor0 5, 00185 Roma, Italy
Pasquale Patron0 Istituto di Metodologie Avanzate Inorganiche, CNR, Area della Ricerca, C.P. 10, 00016 Monterotondo Scalo (Roma),Italy Received September 19, 1988. I n Final Form: March 1,1989 The structure of the species arising from phosphoric acid impregnation on silica, alumina, and titania (mainly anatase) has been investigated by T G D T A analyses, X-ray diffraction, and FT-IR spectroscopy also using CO as a probe molecule. It has been concluded that the interaction between supported phosphoric acid and silica is weak, producing mainly liquidlike species, together with covalently bonded phosphate and hydrogen phosphate species. The interaction with the ionic supports alumina and titania looks much stronger, producing well-spread ionicdly bonded hydrogen phosphate and phosphate species that poison the support's basic sites. The Lewis sites of the support seem instead to be only slightly affected by the supported species.
Introduction Several industrial heterogeneous catalysts contain phosphoric acid or phosphate and pyrophosphate ions together with oxide phases. The so-called "solid phosphoric acid" (phosphoric acid impregnated on silica or kieselguhr) is a typical acidic heterogeneous catalyst widely used in industry for many years for olefin hydration to alcohols and propylene oligomeri~ation.'-~ H3P04on alumina and H3P04on titania have been characterized as "solid s~peracids".~Jj Phosphoric acid or metal phosphates are also added as dopants to several catalysts such as in the case of alumina-supported NiW and CoMo HDS and HDN catalysts.6 Formulations such as vanadium-molybdenum-phosphorus oxides supported on titania' and vanadium-phosphorus oxides on titania%gare used for maleic anhydride syntheses from benzene and butane, respectively. The role of phosphate species, either only as a support stabilizerlo or as a chemical promoter, is still not known in detail. As a part of studies of acidic catalyst surfaces, we have prepared and characterized oxide samples impregnated by phosphoric acid. The aim of the present work was to investigate the structure taken by phosphoric acid when it is supported on different oxide carriers in relatively s m d amounts. Experimental Section Supported phosphoric acid samples (denoted PS, PT, and PA, prepared by using silica, titania, and alumina as the supports, respectively) containing an amount proportional to the support surface area were prepared by a conventional impregnation technique using H3P04water solutions and preformed oxide powders followed by calcination at 723 K. The amount loaded was calculated to be similar to that needed to cover all the support surface in the case of a homogeneous dispersion (area of a PO4 unit = 24 A2). Some data on the catalysta are summarized in Table I.
The thermal behavior was investigated with a Stanton Model 781 DTA-TG-DTG thermoanalyzer (Pt crucibles, Pt-Pt/Rh
* Author to whom correspondence should be addressed. 0743-7463/89/2405-0911$01.50/0
Table I. Characteristics of the Supported HnPOl Catalysts area loading, m2/g
m2/g
(catalyst) (support) PS PA PT
106 84 50
115 90 52
wt %
PO4
support
8 6
A1203
3
TiOz
SiOz
area, structurea m2/g amorphous 133
+6 anatase y
96 49
90 %
Commercial products from Degussa (Hanau, West Germany).
thermocouples, heating rates 2-5 K/min). XRD analyses were performed on a Philips diffractometer (Cu Ka radiation). The surface areas were measured through nitrogen adsorption on a Perkin-Elmer 212 C sorptometer. The FT-IR spectra were recorded with a Nicolet MX1 instrument. Self-supporting disks were obtained by pressing the pure catalyst powders. They have been activated by evacuation at 773 K in the IR cell connected to a conventional evacuation/gas manipulation apparatus.
Results a. Solid-state Characterization of the Fresh Catalysts. The simultaneous T G and DTA curves of the three freshly prepared catalysts performed in air are reported (1) Jones, E. K. Adu. Catal. 1966,8,219. (2)Weisand, E.; Engelhard, P. A. Bull. SOC.Chim. Fr. 1968, 1811. (3)Friedman, P.; Pinder, K. L. Ind. Eng. Chem. Process. Des. Deu. 1971,10,548. (4)Krzywicki, H.; Marczewski, M. J. Chem. Soc., Faraday Tram, 1 1980,76, 1311. (5)Cornejo, J.; Steinle, J.; Boehm, H. P. Z. Naturforsch. 1978,33b, 1238. (6)Fitz, C.W.; Rose, H. F. Ind. Eng. Chem. Prod. Res. Deu. 1983,22, 40. (7)Fiolitakis, E.; Schmid, M.; Hofmann, H.; Silveston, P. L. Can. J. Chem. Eng. 1983,61,703. (8)Honicke, D.; Griesbaum, K.; Augenstein, R.; Yang, Y. Chem.-ZngTech. 1987,59,222. (9)Haas, J.; Plog, C.; M a w , W.; Mittag, K.; Gollmer, K.-D.;Klopriea, B. Proc. IXth Int. Congr. on Catalysis, Calgary, Canada, 1988,p 1632. (10)Gishti, K.; Iannibello, A.; Marengo, S.; Morelli, G.; Tittarelli,P. Appl. Catal. 1984,12,381. Stanislaus, A.;Absi-Halabi, M.; AI-Dolama, K. Appl. Cntal. 1988,39,239.
0 1989 American Chemical Society
912 Langmuir, Vol. 5, No. 4, 1989
Busca et al. 0
0
C
-
n 0
a
n
U
c - 5 *
P
Y
10
X 0
4000 3000 3600 3400 3200 3000 2000 2600 2400 2200
r
.1 E -
wavenuaers
cm m
i .
Figure 2. FT-IR spectra (voHregion) of PS activated in vacuo a t 630 K (a), 790 K (b),890 K (c), and 1070 K (d).
0 0
273
473
673
873
1073
i273 T (lo
Figure 1. Simultaneous TG (upper curves) and DTA (lower curves) analyses of PT (dashed lines), PA (full lines), and PS (broken lines).
in Figure 1. The samples undergo consistent weight losses below 473 K, due to water desorption, roughly proportional to their surface area and, consequently, to the absolute phosphoric acid load. Successively, a slight continuous weight loss is obseved until 720 K for the PS and PT samples and until 970 K for the PA sample. This difference may be due to the stronger retention of water by the alumina support than by the other oxides. Desorption of water corresponds to endothermic peaks in the DTA curves. Their multiplicity indicates that different adsorbed water species are present. The absence of relevant weight loss above 720 K in the case of PT and PS samples would also indicate that phosphoric acid is rather strongly bonded to the supports and does not desorb nor evaporate by heating. Exothermic peaks observed very clearly for PT and, to a lower extent, for PA in the region near 1220-1270 K without any corresponding weight loss are very likely due to solid-state transformations. In the case of the PT sample, this peak corresponds to the anatase-to-rutile transformation, as confirmed by XRD data. Its position at so high a temperature agrres with the inhibiting effect of phosphate species on the anatase-to-rutile transformation already reported." XRD analyses indicate that the impregnation with phosphoric acid and the following calcination at 723 K do not modify detectably the structure of the solid. Impregnation results in a small decrease of the surface areas of silica and alumina (Table I). b. FT-IR Studies of the Surface Sites and of the Effect of Activation in Vacuo. The comparison of the IR spectra of pressed disks of the supported phosphoric acid catalysts with those of the pure supports evidences the absorptions due to the supported species. In all cases, the blackout limit, due to the bulk metal-oxygen stretching absorptions of the supports, is shifted toward higher frequencies, due to the further presence of P-0 stretching bands. In the case of PA and PT, whose supports do not present overtone bands in the region 2500-1000 cm-l, unlike silica, a complex broad overtone band of F-0 stretchings is detectable in the region 2400-2000 cm-', with a maximum near 2230 cm-l, corresponding to a fundamental absorption centered just near 1115-1130 cm-l. This agrees with the position of such a fundamental band, as
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;
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;
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,
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has been directly observed in the case of phosphated zirconia12 and a1~mina.l~ Phosphoric Acid on Silica. The spectra of the PS catalyst in the vOH region, after activation a t different temperatures, are reported in Figure 2. Two sharp bands are evident, a t 3743 and 3662 cm-l, certainly due to nonH-bonded hydroxy groups. According to previous data on pure silica1*and phosphate cata1ysts,l5J6they are assigned to silanol and hydrogen phosphate surface species, respectively. Moreover, a very broad band centered near 3000 cm-' is well evident after activation at 630 K, certainly due to H-bonded hydroxy groups. Also this band, not present on pure silica, i s certainly due to supported phosphoric acid species. However, the observation of the shape of the band a t 3662 cm-' indicates that it is composed of two absorptions, a sharper one and a broader one, centered very near each other. This becomes very evident (12)Ramis, G.;Busca, G.; Lorenzelli, V.; Rossi, P. F.; Bensitel, M.; S a w , 0.; Lavalley, J. C. Proc. IXth Int. Congr. on Catalysis, Calgary, Canada, 1988, p 1874. (13) Mennour, A.; Ecolivet, C.; Cornet, D.; Hemidy, J. F.; Lavalley, J. C.; Mariette, L.; Engelhard, P. Mater. Chem. Phys. 1988,19, 301. (14)Ghiotti, G.;Gamone, E.; Morterra, C.; Boccuzzi, F. J.Phys. Chem. 1979, 83, 2863. (15) Busca, G.; Centi, G.; Trifir6, F.; Lorenzelli, V. J. Phys. Chem. 1986, 90, 1337.
(11)Criado, J.; Real, C. J. Chem. SOC.,Faraday Trans. I 1983, 79, 2765.
(16)Busca, G.;Lorenzelli, V.; La Ginestra, A.; Galli, P.; Patrono, P. J . Chem. Soc., Faraday Trans. 1 1987,83, 853.
Langmuir, Vol. 5, No. 4, 1989 913
Phosphoric Acid on Oxide Carriers when the catalyst is put into contact with a basic molecule from the gas phase (Figure 3b). In these conditions, in fact, both sharp components disappear while broad bands at much lower frequencies are formed as a result of H bondings (as will be discussed in a successive paper.)17 A relatively broad band centered at 3670 cm-' remains intact and well evident. This band is also present on pure silicals and is due to weakly H-bonded internal SiOH groups, not available as sites for adsorption of gas-phase molecules. Evacuation at 790 K (Figure 2b) causes mainly the disappearance of the broad band near 3000 cm-l, which still leaves a broad adsorption now centered near 3200 cm-', much weaker. Further evacuation at 890 K (Figure 2c) causes the decrease of the absorption in the region 3750-3500 cm-' but its increase below 3500 cm-'. The subtraction spectrum shows that this treatment mainly affects the broad "internal" component at 3670 cm-'. This result may be interpreted as evidence of the condensation of H-bonded internal silanol groups, observed in this temperature range also on pure silica, producing water that rehydrates the supported phosphate species, restoring in part the band near 3000 cm-'. Heat treatment at 1070 K under evacuation (Figure 2d) causes the further decrease of all bands, including the two sharp components a t 3745 and 3660 cm-l, already progressively weakened during the previous treatments. However, it must be remarked that the relative intensity of the two sharp components, due to free SiOH and POH groups, respectively, is significantly changed upon evacuation at high temperatures: the ratio I(SiOH)/I(POH) of these bands (measured on the ratios of the spectra obtained before and after the adsorption of a base, so involving only the surface free groups available for H bondings) is progressively lowered from 1.25 for activation at 630 K to 0.69 for activation at 1073 K. This evidences that free POH groups are more stable than SiOH groups with respect to condensation by heating in vacuo. Upon contact of the activated sample with a base such as tetrahydrofuran, both SiOH and POH free groups are totally involved in H bonding, their bands being completely disappeared (Figure 3b). Successive evacuation a t room temperature (Figure 3c-d) causes the progressive slow desorption of the adsorbate, with the corresponding reappearance of the bands of the free hydroxy groups. We may observe that the band of free SiOH groups is restored almost completely, although at its first appearance the maximum is observed at 3747 cm-' and progressively shifts to 3743 cm-l. Instead, after 2 h of evacuation (Figure 3d) the band of free POH groups is still extremely weak. Moreover, this weak band is now centered a t 3670 cm-', against the value of 3663 cm-' observed for the catalyst before adsorption and of 3660 cm-I observed for the catalyst activated at 1073 K. We deduce from these data that (i) POH groups are relevantly more acidic than SiOH groups and that (ii) POH groups are not all identical, being constituted by a distribution of different sites characterized by VOH values shifting from 3670 to 3660 cm-', corresponding to an increasing acid strength and thermal stability. The lowering of t~~ corresponding to an increasing acid strength agrees with what may be foreseen on theoretical bases. Another relevant modification of the spectrum upon heating under vacuum is evident in the lowest part of the available spectral range (Figure 4). In this region im(17) Ramis, G.; Rossi,P. F.; Busca, G.; Lorenzelli, V.; La Ginestra, A.; Patrono, P. Langmuir, in press. (18)Kondo, S.; Yamagouchi, H.; Kajiyama, Y.; Ishikawa, T. J . Chem. Soc., Faraday Trans. 1 1984, 80, 2033.
1500
1400
1500
1400
wavenumberr
1300
CI
-'.
Figure 4. Left side: FT-IR spectra of PS (wregion) activated at 630 K (a),890 K (b), and 1070 K (c). Right side: subtraction spectra b/a (b) and c/a (c). a U
a
n 0
m
n
4000 3800 3600 3400 3200 3000 2800 2600 wavenumberr
cm",
Figure 5. FT-IR spectra (vOH region) of PA activated in vacuo at 630 K (a), 790 K (b), 890 K ( c ) ,and 1070 K (d). mediately above the cutoff limit near 1300 cm-', due to the S i 4 stretching vibrations, an absorption is already present after calcination a t 630 K and grows further upon evacuation a t 790 and 890 K. In the first step, it seems constituted by two bands, poorly resolved, placed at 1400 and 1360 cm-' (shoulder). Further heat treatment in vacuo at 1073 K causes this absorption to split clearly into two components a t 1400 and 1340 cm-'. These bands may be assigned to the stretching frequencies of P=O double bonds. These frequencies are even higher than those of vpo of liquid phosphoric acid esters. These bands are perturbed by adsorption of molecules such as water or bases, showing that these species are placed on the surface. It seems then probable that heat treatment, besides dehydration of the surface species, also causes a reaction of the support surface with the supported phase, producing surface phosphoric acid esters such as (SiO),P=O(OH) and/or (SiO),P=O species. Phosphoric Acid on Alumina. The spectra in the voH region of PA after calcination at different temperatures are reported in Figure 5. Sharp bands due to free surface OH groups are well distinguishable at 3790 and 3677 cm-' with a shoulder at 3730 cm-l, while broader absorptions are observed at 3580, 3490, and near 3300 cm-' when the sample is activated at 630 K (Figure 5a). Evacuation at 790 K (Figure 5b) almost completely destroys the broad bands, assigned to H-bonded hydroxyls. Further evacuation at increasing temperatures up to 1073 K reduces all vOH bands (Figure 5c,d). The three bands of free OH groups have final positions at 3796, 3737, and 3677 cm-',
Busca et al.
914 Langmuir, Vol. 5, No. 4 , 1989 1
U
c
n 0
I
m
n
I I I
I
I I \
4000 3800 3600 3400 3200 3000 2800 2600
wavenumbers
cm
-f
.
Figure 6. FT-IR spectra (uOH region) of PT activated in vacuo at 630 K (a) and 790 K (b).
with a slight shift up of the two higher frequency ones. The location on the surface of the free hydroxyls responsible for these bands is confirmed by their complete disappearance upon adsorption of bases, even weak as tetrahydrofuran, with the corresponding appearance of broad bands due to H-bonded hydroxyls. During successive evacuation at room temperature, a progressive desorption of the weak base is observed, restoring progressively the vOH bands of free hydroxy groups. The comparison of the spectra of the pure alumina support after evacuation, reported and discussed in ref 19, with that of PA clearly indicates that the sharp band a t 3677 cm-' is due to POH groups, while those at 3790 and 3730 cm-' correspond to bands also present on pure alumina. It seems remarkable that on the pure support activated in the same conditions a further band is observed, at 3770 cm-', not observed on PA, as well as a fourth one a t 3690 cm-', possibly also lacking on PA. In this case, unlike the case of PS, the band of POH groups does not undergo very significant shifts (3680-3677 cm-'1. As previously observed for PS, also in the PA spectrum a band is formed upon evacuation at high temperature in the low-frequency region. However, in this case its frequency is much lower, 1265 cm-'. Also, this band falls in a region characteristic of vpg bands. Phosphoric Acid on Titania. The spectrum of PT after activation a t 630 K shows only one strong band in the region 3680-3650 cm-', with a much smaller component near 3700 cm-' (Figure 6a). Only a weak very broad absorption without definite maxima is observed in the region below 3500 cm-', and it disappears by evacuation a t 790 K (Figure 6b). The main band is much larger (half-height width ca. 35 cm-') that those of POH groups in the spectra of PS and PA (half-height width ca. 20 cm-') and appears complex after evacuation at 630 K. The main maximum is at 3667 cm-', but shoulders appear near 3655 and 3650 cm-'. Evacuation at 790 K causes a partial sharpening of the band, with disappearance of the two low-frequency shoulders. However, the main maximum shifts slightly downward and is now observed at 3664 cm-'. Adsorption of T H F on the sample activated a t 630 K causes again the complete disappearance of these bands while evacuation restores substantially the main maximum at 3667 cm-l, more than the shoulders. The comparison with the spectra of the pure TiOz support, discussed in ref (19) Lavalley, J. C.; Benaissa, M.; Busca, G.; Lorenzelli, V. Appl. Catal. 1986, 24, 249.
4000
3600
3200
2800
2400
2000
wavenumbers
1600
1200
cm -.L.
Figure 7. FT-IR spectra of PT activated at 790 K (a) and then put into contact with water vapor at room temperature and evacuated at room temperature for 10 min (b) and 2 h (c) and at 390 K for 10 min (d).
4000 3600 3200 2800 2400 2000 1600 1200 wavenumbers
cmri
Figure 8. FT-IR spectra of PS activated at 790 K (a) and then put into contact with water vapor at room temperature and evacuated at room temperature (b); ( c ) subtraction b/a.
20, indicates that the strong band at 3667-3664 cm-' is due to POH groups, while the small one near 3705 cm-' may be due to residual surface hydroxy groups of the support. As for the shoulder near 3655 and 3650 cm-', they also may arise from free TiOH groups that are accordingly present in this region.20 It is possible that their perturbation by bases is due to a secondary effect. As will be discussed in a successive paper, Lewis acid sites are also present on the PT sample. It is frequent that coordination of bases on Lewis sites perturbs near surface OH group^.'^ Also, on PT a upo band is observable and grows during heating under vacuum, and it is placed at 1340 cm-'. c. Sample Rehydration. Figure 7 shows the overall spectra of PT activated at 790 K and, successively, exposed to water vapor, and again evacuated. Water adsorbs, producing essentially molecular chemisorbed species. This is deduced by the features of the spectrum: in fact there are, well evident,,the two stretching bands nearly 3400 and 3000 cm-' (overlapping), as well as the sharp scissoring mode near 1620 cm-'. Also a t lower coverages, Le., after evacuation at 400 K, the features, less intense, are placed at the same frequencies. A very similar picture is obtained from similar experiments using PA. In the case of PS (Figure 8)) a much more complex spectrum is observed, constituted by an almost diffuse absorption with three very broad maxima near 2940,2300, (20) Busca, G.; Saussey, H.; Saur, 0.;Lavalley, J. C.; Lorenzelli, V. Appl. Catal. 1985, 14, 245.
Phosphoric Acid on Oxide Carriers
Langmuir, Vol. 5, No. 4, 1989 915 Scheme I
1-
h
I
\
J O H =3745 c m - l
I
I
VOH = 3660-3670
o
"
I
*OH = 3000 cm-l
VPO = 1 4 0 0 ClIl-l
VPO = 1340 cm-I
YPO = 1 3 4 0 cm-l
V O H = 3667 cm-l -:
covalent bond;
+:
CO and H20 adsorption site
coordination bond:
and 1660 cm-'. This kind of spectrum is typical of species involved in short and strong H bonds. The complexity of the very broad absorption is due to the Fermi resonances of the fundamental OH stretching band, much broadened and lowered in frequency, with the first overtones of the corresponding in-plane and out-of-plane deformations both shifted upward by this interaction.21 A spectrum very similar to that shown in Figure 8, showing three broad components, has been reported for several systems as, for example, the solid-state spectrum of CsHzP0Z2and also the spectrum of phosphoric acid in water solution.23 So, it seems evident that hydration of the PS sample previously calcined at 790 K forms a phase containing strongly H-bonded hydrogen phosphate or phosphoric acid species and water. The behavior of PS seems then very different from the behavior of PA and PT from this point of view. d. Adsorption of Carbon Oxides. The spectra of carbon monoxide adsorbed on the PT sample treated at three different temperatures are reported in Figure 9. On the sample treated a t 473 K, a single band is observed, centered a t high coverages near 2193 cm-' but shifting slightly upward at the lowest coverages (up to 2197 cm-'). On samples activated a t higher temperatures, this band is always present, only slightly shifted upward, but a second band is also detected and grows with increasing pretreatment temperature. This band is placed near 2120 cm-' and is much stronger on these samples than on pure titania. It has been assigned, according to its stronger resistance to evacuation treatments, to CO interacting with reduced Ti3+centers. Only at very low coverages are traces observed of a third band near 2205 cm-'; it is instead well evident on the pure support.20 This is the only relevant perturbation of the cation sites of titania arising from surface phosphate species. As for alumina, the spectra of CO adsorbed under different pressures are substantially the same as reported for pure alumina samples in comparable condition^,'^^^^ showing that the nature of cation sites and their relative density are roughly the same on alumina and on PA. The main results concerning the adsorption of C 0 2 on PT and PA have already been reported.12 It has been found that coordinated molecular species are formed on both surfaces, as the result of the interaction with Lewis (21) Claydon, M. F.; Sheppard, N. Chem. Commun. 1969, 1431. (22) Novak, A. Bull. SOC.Chim. Fr. 1982, 1-330. (23) Chapman, A. C.; Thinvell, L.E. Spectrochim. Acta 1964,20,937. (24) Zecchina, A.; Escalona Platero, E.; Otero Arean, C. J. Catal. 1987, 107, 244.
. e . :
H-bond:
n coordinative U
unsaturation
D 0
5
n L
0 Io
n
2250
2150
wavenumberr
2050
cm-1.
Figure 9. FT-IR spectra of CO adsorbed at room temperature on PT activated at 473 K (a), 630 K (b), and 790 K (c).
acid sites, while carbonates or bicarbonates, thought to be formed on basic sites, are not observed a t all on PT; on PA, small amounts of bicarbonates are detectable. As expected on the basis of the behavior of silica, the adsorption of CO and COz on PS is negligible a t room temperature, according to the absence of both Lewis and basic sites.
Discussion The above results give several indications of the structure of the phosphoric acid species supported on the oxide carriers under study. Assignments of the main IR features discussed above to surface structures are reported in Scheme I for PS and PT. The last is representative also of the situation on PA. A common feature is the detection of a strong and relatively sharp band due to free POH groups, of broad bands in the region 2400-2000 cm-', mainly due to the first overtones of the P-0 vibrations, and of sharper bands in the region 1450-1300 cm-' due to P=O stretchings. Additional bands typical of strong H bonds are also observed on silica. A sharp band in the region 3680-3650 cm-' is typical of surface POH groups also on bulk phosphates, pyrophosphates, hydrogen phosphates, and hydroxyphosphates, such as AlP04,25BP0,,26 (VO)2P20,,15the pyrophosphates (25) Peri, J. B. Discuss. Faraday SOC.1971, 52, 55. (26) Haber, J.; Szybalska, V. Discuss. Faraday SOC.1981, 72, 263.
916 Langmuir, Vol. 5, No. 4, 1989
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and hydrogen phosphates of quadrivalent Ti, Zr, Sn, and support surface. These species are characterized by POH as well as Calo(P04)6(OH)2.28It seems difficult to groups, free on the activated surface, with a uOH frequency correlate the exact frequency observed with some property slightly higher than that measured on PS, available to H of the particular compound of cation involved and with bonds. It has been shown,12and it will be discussed further the nature of the phosphate species (ortho or pyro and else~here,’~ that these groups on P A and PT are definitely their coordination state). However, in many of these cases weaker as Brernsted acids than those observed on PS. We a weaker band is also observed a t lower frequencies, tenmay propose for this species a structure like that reported tatively assigned to uOH of P(OH)2g r o ~ p s . ~ ~InJthe I, which is the same taken by hydrogen phos~ p ~ ~ in~ Scheme ~ phate species in a number of salts.31 By heat treatment present cases, we have not observed similar bands. in vacuo this species may lose its proton, which would give Concerning H3P04-Si02,our experimental results are consistent with those reported by Low and Ramamurthy.2B water together with an OH group from the support, producing an ionically bonded orthophosphate species charThe detection of the unperturbed band of surface silanol acterized by a up+ frequency much lower than that of groups (still very intense) in spite of the large amount of phosphoric acid loaded indicates that the silica surface, covalently bonded phosphate species observed on silica, whose chemistry is dominated by the behavior of surface due to the resonance of the negative charge and the consilanols, is largely exposed on our sample. sequent lowering of the P-0 bond order. The supported phosphoric acid sites on PS are evident According to this model and to its acidic nature, H3P04 through the detection of several bands. The sharp band would interact with basic sites on ionic oxides that become, shifting from 3670 to 3660 cm-’ under different conditions consequently, poisoned. This agrees with the lack of and perturbed upon adsorption of bases points to the formation of carbonates and bicarbonates from COPadsorption. A partial poisoning of basic sites on H3P04presence of surface free POH groups. The bands observed at 1340 and 1400 cm-’, growing upon heat treatment in treated alumina was already cited by K r ~ y w i c k i . ~The ~ stronger interaction of titania and alumina toward H3P04 vacuo, and perturbed by adsorption of water indicate the with respect to silica may be then related to their basicity3 presence of surface species containing very strong phosphoryl bonds, similar to those of phosphoric acid triesters. and results in a much lower tendency to condense and to form H bonds. A similar effect is also possibly responsible The adsorption of water resulting in formation only on this catalyst of absorptions typical of very strong and short H for the different behavior of these supports with respect to supported vanadia, molybdena, and t ~ n g s t a , %which -~~ bonds evidences the large availability, at least on the have the character of acid anhydrides. However, while in catalyst preactivated at 630 and 790 K, of phosphoric acid or hydrogen phosphate species in a liquidlike form. In the case of V, Mo, and W catalysts such a stronger interaction favors the dispersion that has an activating effect summary, our data seem to indicate that the interaction of phosphoric acid and silica is weak at low temperatures, on the catalysts, in the case of H3P04 it is possible that the strong acidic nature of phosphoric acid is retained and leaving the silica surface largely exposed. Liquidlike phosphoric acid is mainly supported on this silica surface, perhaps enhanced more than on ionic oxides on silica. The strong interaction of phosphate species with ionic although isolated hydrogen phosphate species possibly supports may also be related to the use of H3P04and of bonded to silica by two ester bonds, like (SiO)2P(OH)=0, metal phosphates as dopants of supported catalysts such are also present. At higher temperatures, probably the as Co-Mo-A1,03 for HDN and HDS reactions6 and the reaction between the support and the supported phase titania-supported ones for selective o ~ i d a t i o n . ~Solid-~ starts, and a phase like silica orthophosphate is formed on state chemistry effects, as the inhibition of the y-to cythe surface, exposing very strong P=O bonds. These reAl,03 transformationlo and of the anatase-to-rutile sults are consistent with data reported in the iiterature?O transformation,” with a consequent retainment of the showing that amorphous silica may react with phosphoric surface area, may possibly be justified also by these surface acid at 673 K producing the compound Si3(P04)4,although interactions. Moreover, the interaction of phosphate in our conditions the extent of this reaction would be too species with the surface is very likely to be competitive small to be detectable. with respect to that of Moo3, W03, and V205and could In the cases of H3P04-Ti02 and H3P04-A1203,instead, favor a better dispersion of such active surface phases. the relevant perturbation of the surface OH groups of the support (which in the case of alumina appears to be seRegistry No. H3P04,7664-38-2;TiOz, 13463-67-7;CO, 630lective, involving only some of the different OH groups), 08-0. the persistence of Lewis sites, and the almost total poisoning of the basic sites point to a “selective” interaction (31) Troup, J. M.; Clearfield, A. Inorg. Chem. 1977, 16, 3311. of phosphoric acid with the support surface. The absence (32) Krzywicki, A. Sci. Rep. Technical University Warsaw,Ser. Chim. 1980. 23. of H-bonded phosphate species even after hydration may (33) Rossi, P.F.;Busca, G.; Lorenzelli, V.; Lion, M.; Lavalley, J. C. J. be interpreted as an evidence of the formation of Catal. 1988, 109, 378. “localized” monomeric species bonded ionically to the (34) Haber, H.; Kozlowska, A.; Kozlowski, R. J . Catal. 1986,102,52. (27) Ramis, G.; Busca, G.; Lorenzelli, V.; La Ginestra, A.; Galli, P.; Massucci, M. A. J. Chem. SOC.,Dalton Trans. 1988, 881. (28) Cant, N. W.; Bett, J. A. S.; Wilson, G. R.; Hall, W. K. Spectrochim. Acta 1971, 27A, 425. (29) Low, M. J. D.; Ramamurthy, P. J . Phys. Chem. 1986, 72, 3161. (30) Durand, B.; Lenzi, M.; Boull6, A. Bull. SOC.Chim. Fr. 1972,442.
(35) Ramis, G.; Busca, G.; Lorenzelli,V. Appl. Catal. 1987,32, 305; Z . Phys. Chem. 1987,153, 189. (36) Busca, G. Langmuir 1986,2,577; Mater. Chem. Phys. 1988,19,
, 101. E”
(37) Soled, S.;Murrell, L. M.; Wachs, I. E.; McVicker, G. B.; Sherman, L. G.; Chan, S.; Dispenziere, N. C.; Baker, R. T. K. In Solid State Chemistry in Catalysis; Grasselli, R. K.; Bradzil, J. F., Eds.; American Chemical Society: Washington, DC; p 165.
