917
Langmuir 1989,5, 917-923
Phosphoric Acid on Oxide Carriers. 2. Surface Acidity and Reactivity toward Olefins Gianguido Ramis, Pier Francesco Rossi, Guido Busca, and Vincenzo Lorenzelli* 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 A. Moro 5, 00185 Roma, Italy
Pasquale Patrono 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 surface acidity of catalysts prepared by impregnation of silica, alumina, and titania with phosphoric acid (called PS, PA, and PT, respectively) has been studied by FT-IRspectroscopy of adsorbed n-butylamine, ammonia, pyridine, tetrahydrofuran, and acetonitrileand by adsorption microcalorimetry using n-butylamine as the probe. The adsorption of olefins such as 1-butene and isobutene has also been studied by FT-IR. Moreover, the activity in the catalytic conversion of 1-butene has been investigated. Both Lewis and Bransted acid sites are observed on PT and PA, while only Bransted sites are detected on PS. The Bransted acid strength of PS is the strongest, as deduced by FT-IR, while the strongest total acidity is detected on PT by microcalorimetry. The strongest activity of PT in polymerizing olefins in the IR cell has been related to the tendency to form carbonaceous deposits mainly on this catalyst, as well as with its higher catalytic activity in skeletal isomerization, cracking, and hydrogenation of 1-butene.
Introduction A number of industrial processes involve reactions carried out on solid acids as heterogeneous catalysts or catalyst components. One of the problems when these materials are used as catalysts for organic reactions is their deactivation by coking.’S2 However, it has also been shown that carbonaceous materials might be useful in some cases to modify the catalytic behavior of the solid, inducing dehydrogenating and oxidizing abilitie~.~ An acid catalyst largely used in industry for many years is the so-called “solid phosphoric acid”, constituted by a siliceous material, generally kieselguhr, impregnated by large amounts of phosphoric acid.4 It is used for olefin oligomerization5and hydration to the corresponding alcohol.6 It has also been reported that other oxides, such as alumina and titania, increase enormously their surface acid strength when doped with phosphoric acid and become “ s u p e r a ~ i d s ” .This ~ ~ ~ phenomenon is similar to that reported for iron, titanium, and zirconium oxides, when doped with sulfuric acid?JO In order to better understand the generation of surface acidity and the behavior of the corresponding sites on inorganic solids, we have characterized the surface acid activity of three oxides, silica, alumina, and titania, when impregnated with phosphoric acid. This work follows a previous one where the surface structure of these solids has been investigated.’l Both chemisorption techniques, (1) Naccache, C. In Deactivation and poisoning of catalysts; Oudar, J., Wise, H., Eds.; Marcel Dekker: New York, 1985; p 185. (2) Barbier. J. In Catalvst deactivation IK Delmon. B.. Froment, J. F., Eds.; Elsevier: Amsteidam, 1987; p 1. (3) Ekhigoya, E.;Sano, H.; Tanaka, M. Proc. 8th I n t . Congr. Catalysis; Verlag Chemie: Berlin, 1984; Vol. V, p 623. (4) Weisang, E.;Engelhard, P. A. Bull. SOC.Chim. Fr. 1968, 1811. (5) Jones, E. K. Adu. Catal. 1956, 8, 219. (6).Weissermel,K.; Arpe, H. J. Industrial Organic Chemistry; Verlag Chemie: Berlin, 1978. (7) Krzywicki, A.; Marczewski, M. J . Chem. Soc., Faraday Trans. 1 1980, 76, 1311. (8) Cornejo, J.; Steinle, J.; Boehm, H. P. Z.Naturforsch. 1978, 33E, 1238. (9) Jin, T.; Yamaguchi, T.; Tanabe, K. J. Phys. Chem. 1986,90,4794. (10) Rojadhyaksha, R. A.; Chaudhari, D. D. Ind. Eng. Chem. Prod. Res. Deu. 1987, 26,1743.
0743-7463/89/2405-0917$01.50/0
Table I. Bases Utilized for Characterization of Surface Acidity base n-butylamine ammonia pyridine
THF acetonitrile
PK, 10.9 9.25 5.21 -2.02 -10.4
such as IR spectroscopy and adsorption microcalorimetry, and a test catalytic reaction have been used in order to have a picture as complete as possible of the acidic character of these solids.
Experimental Section The preparation and composition of the catalysts have been reported in a previous paper.” The notations PS (8% PO4 by weight, 106 m2/g), PT (3% PO4, 50 m2/g),and PA (6% PO4, 84 m2/g)refer to silica, titania, and alumina, respectively, impregnated by phosphoric acid. The FT-IR spectra were recorded by using a Nicolet MX1 instrument. Self-supportingdisks 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. Microcalorimetric measurements were carried out at room temperature with a Tian-Calvet heat-flow calorimeter, equipped with a Setaram NV724 amplifier nanovoltmeter and Servotrace Sefram recorder. Conventional evacuation/gas manipulation ramps and Pyrex cells were used to perform adsorption from the gas phase. The powders were pretreated into the calorimetric cell under high vacuum at 723 K for 3 h. Catalytic experiments were performed in a flux microreactor at 700 K and GHSV = 0.41 V, VL1h-l. All sampleswere preheated at 720 K for 24 h in oxygen flow. The products obtained after different working times were determined by gas chromatography using a Carlo Erba Model 4300 instrument and spherosil XOB075/10% squalane column. n-Butylamine,pyridine, tetrahydrofuran, and acetonitrile were analytical grade products from Carlo Erba (Milano,Italy). Ammonia was taken from commercial cylinders from Baker (Phil(11) Busca, G.;Ramis, G.; Lorenzelli, V.; Rossi, P. F.; La Ginestra, A.; Patrono, P.Langmuir, preceding paper in this issue.
0 1989 American Chemical Society
Ramis et al.
918 Langmuir, Vol. 5, No. 4, 1989
+
Table 11. Wavenumbers of the wcN,wcc Beer Fermi Resonance Doublet of Liquid and Chemisorbed Acetonitrile state wavenumber, cm" liquid ads TiOz ads PT ads A120, ads PA
m
u C a
n
L
0 W
n
0
y I 4000
I
I
3200
I I 2400
wavenumberr
I
I 1600
cm -L
Figure 1. FT-IR spectra of ammonia adsorbed on PS, PT, and PA catalystsactivated at 630 K. These spectra, as those of several other figures, are obtained by subtracting the spectrum of the activated catalysts from those of the catalysts after adsorption. The spectra present inverse bands (in this case uOH bands of the free hydroxy groups in the region 3800-3600 cm-') due to surface species that are perturbed or disappear upon adsorption. V,bands due to NH4+species; 0 , bands due to chemisorbed NH3.
lipsbourgh, NJ), and olefin gases were from SI0 (Milano,Italy).
Results a. Characterization of the Surface Acidity by FTIR Spectroscopy. A set of bases of different strengths has been employed in order to have a detailed characterization of the acid strength of the surface Bronsted and Lewis sites. These bases and their literature pK, values are summarized in Table I. Lewis Acidity. When adsorption of the strongest bases, n-butylamine, ammonia, and pyridine, is carried out, the formation of molecular species chemisorbed as such on surface electron-withdrawing centers (Lewis sites), together with the protonated species arising from reaction with Brosted sites (as will be discussed below), is evident on both P A and PT surfaces, while only protonated species are observed on PS. This is shown, for example, in Figure 1, where the spectra of ammonia adsorbed at room temperature and evacuated shortly at the same temperature on the three catalysts all activated at 630 K are compared. The bands marked with circles, near 3010,2920,2745 (NH stretchings), 1680, and 1450 cm-' (symmetric and asymmetric NH4 deformation modes), present in all three cases but very evident mainly in the case of PS, are due to NH4+ cations." Instead, the sharper NH stretching bands in the region 3400-3100 cm-' together with the asymmetric and symmetric NH3 deformations observed at 1605 and (12) Ramis, G.; Busca, G.; Lorenzelli, V.; R o d , P. F.; Bensitel, M.; Saur, 0.;Lavalley, J. C. Proc. 9th Int. Congr. Catalysis, Calgary, 1988; p 1874. (13) Tsyganenko, A. A.; Pozdnyakov, D. V.; Filimonov, V. N. J . Mol. Struct. 1975, 29, 299.
2292, (2320) 2302, 2324, 2330, 2335 (2305),
2254 2274 2292 2300 2310 (2272)
1245 cm-' for PT and at 1618 and 1300 cm-' for PA (marked with triangles) are ascribed to coordinated ammonia.12 These last bands are not present at all on PS. On PT and PS, a third species is also well evident, characterized by a sharp and strong v m band at 3400 cm-' and a weaker one near 3495 cm-'. This species is very likely also responsible for a weak scissoring mode at 1550 cm-': this species is identified as a P-NH2 amide group.12J4 From the spectra, it is not clear whether this species is also formed on PA. In any case, the presence of Lewis sites on PA and PT, as well as their absence on PS, corresponds to what is foreseen on the basis of the behavior of the respective supports. It is known that in the absence of very strong vacuum pretreatments, Lewis sites are absent also on pure silica, unlike pure alumina and titania. It may also be remarked that the comparison of the intensities of the bands due to coordinated and protonated species of ammonia (Figure l ) , as well as also of pyridine and n-butylamine, assuming comparable extinction coefficients for the corresponding species on the different surfaces, indicates that the ratio between Bronsted and Lewis sites is much higher on PT than on PA. When weaker bases are adsorbed, coordinated species are observed on PT and PA, together with species Hbonded on surface OH groups. Acetonitrile appears to be the most sensitive probe to measure the strength of Lewis sites. In fact, the position of the doublet due to the Fermi resonance of v C N with the vcc + ~ c Hcombination mode is shifted up and the intensity ratio of its components varied with respect to the free molecule when it interacts with electron-withdrawing center^.'^.'^ The wavenumbers observed for the components of this doublet on the free molecule and on the species chemisorbed on the Lewis sites of PT, PA, and the pure supports are summarized in Table 11. From the shifts, we deduce that, according to the results arising from CO adsorption,"J3 the Lewis centers of titania and alumina are slightly perturbed by the presence of phosphate species and that the strength of the most abundant ones is slightly enhanced. The spectra of chemisorbed pyridine also confirm that Lewis sites are only slightly perturbed, if at all.173'8 Br$nsted Acidity. At least three kinds of interactions between bases and OH Brcansted sites may be distinguished spectroscopically in our conditions. The strongest interaction results in the formation of the protonated form of the base, while the weakest one results in the formation of a H bond with the consequent perturbation of the VOH band, shifted downward and broadened. In an intermediate case, when highly polarized very short and strong H bonds are formed, continuous absorptions with several broad maxima and minima are observable, as the result of the Fermi resonance of the VOH band, very strongly (14) Low, M. J. D.; Ramamurthy, P. J. Phys. Chem. 1968, 72,3161. (15) Knozineer. H.: Krietenbrink, H. J. Chem. Soc., Faraday Trans. i 1976, 72, 2421. (16) Fernandez Bertran. J.: La Serna. B.: Doerffel. K.; Dathe, K.; Kabish, G. J. Mol. Struct. '1982, 95, 1. (17) Busca, G.; Saussey, H.; Saur, 0.; Lavalley, J. C.; Lorenzelli, V. A p p l . Catal. 1985, 14, 245. (18) Morterra, C.; Chiorino, A.; Ghiotti, G.; Garrone, E. J.Chem. Soc., Faraday Trans. 1 1979, 75, 271.
Langmuir, Vol. 5, No 2, 1989 919
Phosphoric Acid on Oxide Carriers
LA,, 4000 3600
, , , , , , , , 3200 2800
2400
2000
,I
1600
- .
wavenumberr cm !. Figure 2. FT-IR spectra upon pyridine adsorption on PS, PA, and PT and evacuation at rmm temperature (see caption to Figure 1).
perturbed and shifted down, with the harmonics of the in-plane and out-of-plane MOH deformation vibrations.lg With the strongest bases we used, n-butylamine and ammonia, protonated species are observed in the three cases due to the complete proton transfer. The spectra of the protonated species are well distinguishable from those of the bases chemisorbed as such.12p20 Also in the case of pyridine, protonated species are present in the three cases; however, in the three cases, strong H bonds are also observed, according to the formation of very broad continuous absorptions as described above (Figure 2). The position of the minima and maxima and also their multiplicity vary significantly in the case of PS with respect to PA and PT. The shape of these absorptions, whose features are shifted down in the case of PS with respect to PT and PA, clearly points to the presence of stronger H bonds on PS than on the other two catalysts. To have confirmation of this different behavior, the interaction with the weaker base tetrahydrofuran (THF) has been investigated. In Figure 3, the spectra of THF upon contact with the three catalyst surfaces are compared. In any case, no evidence has been found of protonation of THF. In the case of PS, two spectra are reported. The full line spectrum is obtained by subtracting the spectrum of activated PS from that recorded after contact with THF gas and subsequent evacuation at room temperature for 2 h. This spectrum represents the modifications of the PS spectrum arising from the presence of THF adsorbed on the strongest acidic OH groups, of the POH type, characterized by a sharp band at 3663 cm-' in the activated sample." Under these conditions, in fact, the band of free SiOH groups (3745 cm-') is almost completely restored in the unratioed spectrum and, consequently, appears negative but very weak in the ratio, while the band of free POH groups, still involved in H bonds, appears negative and strong in the ratioed spectrum. The broken line spectrum is obtained by subtracting the spectrum of the sample evacuated for 2 h from that of the sample briefly evacu(19) Novak, A. Bull. SOC.Chim. Fr. 1982, 1-330. Claydon, M. F.; Sheppard, N. Chem. Commun. 1969, 1431. (20) Ramis, G.; Busca, G. J. Mol. Struct. 1989, 193, 93.
41
3000
3
2000
wavenunbere
1000
cm'd.
Figure 3. FT-IR spectra upon THF adsorption on PS, PT, and PA and evacuation at room temperature (see caption to Figure 1). Broken line corresponds to the effect of THF adsorbed on
SiOH groups of PS (see text).
ated, both after THF adsorption. So it mainly refers to THF adsorbed on the poorly acidic SiOH groups, whose band appears strong and negative. It is evident that THF on SiOH groups of PS forms weakly H-bonded species, characterized by "localized" yoHnear 3080 cm-' (shift 660 cm-'), similar to THF on pure silica.21 Instead, the interaction with POH groups (full line) is much stronger, and causes the continuous absorption described above. The same features are observed on PT and PA but are shifted markedly toward higher frequencies. This would indicate that the H-bonding interaction of THF with surface POH groups is stronger on PS than on PT and PA. These data agree with those arising from the interaction with acetonitrile, presented elsewhere,12which forms weak H bonds: also in this case, the stronger shift down of the uOH bands shows that the interaction with POHs of PS is definitely stronger than the interaction with POHs of PA and PT. So our data clearly indicate that free POH groups on PS are definitely stronger Bransted acids than those on PA and PT. b. Characterization of the Surface Acidity by Volumetric Adsorption and Microcalorimetry. In Figure 4, the volumetric adsorption isotherms of n-butylamine on the three catalysts as well as on the pure supports after the same activation procedures are reported. For the phosphated catalysts, the total adsorbed amount per unit area follows the order PT > PA > PS. It is interesting to note that treatment with phosphoric acid decreases the adsorption site density of silica while it increases that of titania. As for alumina, the total number of adsorbed molecules per unit area is almost the same at saturation, with and without phosphates, although the (21) Unpublished results.
920 Langmuir, Vol. 5, No. 4, 1989
Ramis et al.
5 r4 \ c (
s
4
3
2
1
0
2
0
8
6
4
2
0
6
4
8
2
0
e
6
4
P (torr)
P (torr)
P (torr)
Figure 4. Volumetric adsorption isotherms of n-butylamineon PS, PA, and PT (full lines, full circles) and on the correspondingpure supports (broken lines, open circles). r(
400
R
\ 3 Y
E
-
300
PS
.*
PA
!i
0
U
200
100
0
0
1
2
3
4
S
1
1
~ R O ~ / R
2
3
4
0
1
2
3
4
5
).IR01/B12
Figure 5. Differential adsorption heats of n-butylamine on PS, PA, and PT (full lines, full circles) and on the corresponding pure supports (broken lines, open circles). shape of the isotherm is significantly varied. The total adsorbed amount observed corresponds in these cases to 33-48 A2 per molecule, according to the area of this molecule calculated from the van der Waals diameter (23.5 A2).
The curves of the corresponding differential adsorption heats are reported in Figure 5. The comparison of the curves measured on silica and PS, where weak and strong Bransted sites, respectively, are observable by IR spectroscopy, evidences that the number of strong Brernsted sites of PS (arising from surface phosphate species and corresponding to adsorption heats definitely higher than those measured on pure silica) is of the order of 1pmol/m2. The heat evoluted by the interaction of n-butylamine with these sites, producing protonated species,miz is of the order of 320-200 kJ/mol. Considering that about 6.7 kmol of phosphoric acid has been loaded per meter squared of the silica surface means 8.4 pmol/m2 of catalyst, even considering only one acidic proton per phosphoric acid molecule; this result shows that the interaction of phosphoric acid with silica lowers strongly the number of active protons carried by the supported acid and available to the gas-solid interaction. In the case of pure alumina, a very small number of strongly acidic sites, whose interaction with n-butylamine produces heats up to near 320 kJ/mol, are present in these conditions. However, it has been shown22that, according (22)Rossi, P.F.;Busca, G.; Ramis, G.; Oliveri, G. XIX Congres JCAT, Lille, 1988, com. C22.
to the known behavior of alumina, in these conditions only a part of the very strong Lewis sites of this material is free. Evacuation at 873 K makes available much stronger sites (adsorption heats 430-300 kJ/mo1).22 The treatments with phosphoric acid do not modify substantially the evoluted heat. These results may be intepreted on the basis of the facts that the interaction of butylamine with Brernsted sites of PS and with Lewis sites of pure alumina produces similar adsorption heats. It seems that the interaction of phosphoric acid with alumina substitutes a relatively small number of Lewis sites with Brernsted sites, whose interaction with n-butylamine has a similar heat evolution. In the case of titania, instead, the number of active sites is relevantly increased by the presence of phosphoric acid, while the evoluted heats are very slightly increased at low coverages. So in this case the Bransted sites arising from supporting phosphoric acid would add in part to the Lewis sites arising from the pure support. The present volumetric and microcalorimetric data would indicate that the total acidity of the catalysts, both in terms of average strength and of amount of acid sites, including both Lewis and Brernsted sites, follows the order PT > PA 2 PS. However, if only the medium-strong sites are taken into account, those responsible for heat evolution higher than 200 kJ/mol, their numbers follow the trend PT 1 PS > PA. According with IR data, it seems that in the case of PA the surface acidic behavior is dominated by the support Lewis sites, with the presence of a particularly small amount of Bransted sites arising from surface phosphoric acid species.
Langmuir, Vol. 5, No. 4, 1989 921
Phosphoric Acid on Oxide Carriers
catalyst
Table 111. Analysis of the Outflowing Gases in the Catalytic Conversion of 1-Butene (mol W ) n-butenes isobutene butane isobutane Cl+C2+C3 conversion’ time, h
PS
1 2 3 1 2 3
PT PA
1
2 3
85.67 89.86 92.79 65.26 79.62 84.81 93.80 94.39 94.87
13.14 9.25 6.54 14.58 9.91 7.35 5.80 4.95 4.40
0.22 0.24 0.22 11.90 7.10 5.60 0.12 0.33 0.40
0.24 0.11 0.08 4.18 1.03 1.10 0.08 0.10 0.10
0.73 0.54 0.37 4.08 1.54 1.14 0.20 0.23 0.23
14.33 10.14 7.21 34.74 20.38 15.19 6.20 5.61 5.13
’Conversion defined as the amount percent of products different from n-butenes. Scheme I. Network of 1-Butene Transformations’
i
I
100 I
N
1
\ v
~
u
.cI
70
-< Polvn
Carbon
1, double-bond isomerization; 2, skeletal isomerization;3, oligomerization;4, hydrogenation;5, dehydrocyclization;6, cracking.
c. Catalytic Activity i n 1-Butene Conversion. The
analysis of the outflowing gas after contact of 1-butenewith catalysts in the conditions reported above is summarized in Table 111. The experimental conditions chosen, with so high a reaction temperature, are related to our interest to detect skeletal isomerization and transformation of 0 1 e f i n s . ~ Four ~~~~ types of reactions are observable: double-bond isomerization, producing cis- and tram-1-butene; skeletal isomerization, producing isobutene; cracking, producing short-chain hydrocarbons such as methane, ethane, ethylene, propane, and propylene; and hydrogenation, producing butane and isobutane. The probable reaction network, deduced from the data discussed below, is reported in Scheme I. The relative amount of the three linear butene isomers is in all cases the same and approaches the equilibrium value. We may then deduce that the double-bond isomerization reaction is at equilibrium in our conditions on all three catalysts. So, neglecting the double-bond isomerization, the dependence from time of the percent amount of product different from linear butenes, we will call “conversion”, is reported in Figure 6. PT is much more active than PS and PA. It remains, although it undergoes significant deactivation after 3 h. We must remark that for the experiments concerning the different catalysts the gas hourly space velocity was kept equal. These results were obtained by using different catalyst weights, in order to obtain the same exposed areas (1g of PT, 0.64 g of PA, 0.46 g of PS). The conversion scale PT > PS > PA seems governed essentially by intrinsic activity properties of the three catalyst surfaces. However, it must also be remarked that all catalysts progressively deactivate in our working conditions. After 3 h, the activity scale is the same, although the deactivation, measured as the ratio of conversion after 1h versus (23) Raghavan, N. S.;Daraiswamy, L. K. J. Catal. 1977, 48, 21. (24) Bathia, T. K.; Phillips, M. J. J. CataE. 1988, 110, 150.
0 ’
1
0
I
2
3 time
h
Figure 6. Conversion to products different from n-butenes (full symbols) and selectivity to isobutene (open symbols) in 1-butene conversion over PS (squares), PA (circles), and PT (triangles).
the value after 3 h, also follows the same scale (2.3 for PT, 1.99 for PS, 1.2 for PA). Also, the product distribution (shown in detail in Table 111) is remarkably different between the three catalysts. The selectivity to isobutene (Figure 6) is very high and rather stable on PS, high but decreasing on PA, and much lower and increasing with time on PT. However, this small value, due to the formation of bigger amounts of cracking products and of butane and isobutane mainly on this catalyst, corresponds to an absolute amount of isobutene formed that is still higher on PT than on PS and PA. To measure the “skeletal isomerization activity” of these catalysts, isobutane (formed almost only on PT) must be added to isobutene. This evidences that PT has a much higher “skeletal isomerization activity” than PS and PA. The amount of C1 C2 + C3 products is also much higher on PT than on PS and PA, showing that PT has also the highest ”cracking activity”. Also significant is the detection of the hydrogenation product normal butane (detected on all catalysts, although much more on PT) and isobutane (only detected on PT). As for their origin, we must suppose they arise from the hydrogenation of normal butane and isobutene at the expense of carbonaceous species, which act as hydrogen sources. The formation of these species would then be correlated to the deactivation of these catalysts by coking. d. FT-IR Study of Adsorption of Olefins. In Figure 7a, the spectrum ( Y C H region) of the species adsorbed on PT preactivated at 673 K after contact with 1-butene gas (150 Torr) at room temperature is reported. In the presence of the gas (whose spectrum has been subtracted), the band at 3078 cm-‘, due to the asymmetric stretching of the vinylic methylene group, as well as the sharp com-
+
Ramis et al.
922 Langgmuir, Vol. 5, No. 4, 1989 0
F.
0
D 0
Y
c
015
4
D
I K
i?
10
PT 0 Y
0)
f .1 0
a
3100 3000 2900 2800 2700 2600
wavenumbers
-
a,
~~
273
473
673
a73
1073
cm-F,
Figure 7. FT-IR spectra of I-butene adsorbed at room temperature on PT in the presence of the gas at 150 Torr (a, gas spectrum subtracted)and after evacuation at room temperature (b) and of isobutene adsorbed under the same conditions and evacuated at room temperature (c). ponents in the region between 3000 and 2850 cm-' is coincident to the features of species molecularly adsorbed on Ti cations on pure titania.25 However, after short evacuation a t room temperature (Figure 9b), which causes the desorption of this species as on pure titania,25 ucH bands at 2985 (shoulder), 2963,2945 (shoulder),and 2875 cm-' are still observed. They are due to saturated hydrocarbon entities relatively strongly adsorbed and correspond well to those of 1-butene oligomers. Analogous oligomers are also observed upon adsorption of isobutene (Figure 9b), having slightly different features: we conclude that in these conditions oligomerization of 1-butene is faster than its skeletal isomerization. The same oligomerization activity with respect to 1-butene is not observed on PA and PS activated at the same temperature, where only extremely weakly H-bonded species on surface OH groups are detected. However, in both cases, but particularly on PA, the oligomerizing activity appears to some extent when samples are evacuated at higher temperatures. As we have reported elsewhere,26unlike 1-butene, the more reactive olefin isobutene is polymerized relevantly on the three phosphoric acid containing catalysts, although it seems that the chain produced is the longest on PT. It is reasonable to correlate the highest polymerization activity of PT detected in our FT-IR experiments to its highest catalytic activity in the conversion and cracking of 1-butene, reported previously. e. Evidence for the Role of Carbonaceous Deposits on Deactivation of Phosphoric Acid Containing Catalysts. To have information on the reasons inducing deactivation in 1-butene conversion, the spent catalysts (all become gray-black after catalytic tests) have been analyzed by TG-DTA (Figure 8). An exothermic peak is observed for PT near 700 K corresponding to a relevant weight loss (2.8%). Similar smaller peaks with corresponding smaller weight losses (ca. 0.7%) are observed at slightly higher temperatures in the cases of PS and PA. These effects, not observed on the pure catalysts,'l are (25) Busca, G.; Ramis, G.; Lorenzelli, V.; Janin, A.; Lavalley, J. C. Spectroehim. Acta 1987, 43A, 489. (26)Busca, G.; Ramis, G.; Lorenzelli, V. J. Chem. SOC., Faraday Trans. 1 1989, 85, 137.
1273 T
(KI
Figure 8. Simultaneous TG (upper curves) and DTA (lower curves) curves of PS, PA, and PT catalysts after catalytic runs in 1-butene conversion.
0
U
C 9
n L
0
a
n a
4000 3500 3000 2500 2000 1500 io00
wwenu*err
cm-g
Figure 9. FT-IR spectrum of the adsorbed carbonaceous species arising from 1-buteneadsorption on PT at 670 K (see caption to Figure 1). ascribed to the combustion of strongly adsorbed species arising from the catalytic reaction. Above 870 K, no other weight losses are observed, confirming that the samples, which become white, no longer contain adsorbed species. Accordingly, spent catalysts calcined at 820 K are white and show the catalytic activity completely restored. These data indicate that deactivation is due to the formation of carbonaceous materials, formed in greater amounts on PT than on PS and PA. To have a confirmation of this, we have studied by FT-IR spectroscopy the interaction of 1-butene at 630 K on the three catalysts. Accordingly, all samples became black or gray. However, only in the case of PT are the carbonaceous materials well detectable (Figure 9). This spectrum is similar to those reported for carbonaceous materials detected on deactivated acid catalysts such as acid zeolites and silica-alum i n a . ' ~ ~Further ~ contact of the catalyst with 1-butene does not correspond to formation of any adsorbed species, confirming the deactivation of the surface sites. Calcining at 820 K causes the complete burning of the carbonaceous material, with the formation of gas-phase COz,and restores the adsorption activity and the whiteness of the catalyst. (27) Einseinbach, D.; Gallei, J. J . Catal. 1979, 56, 377.
Phosphoric Acid on Oxide Carriers
Discussion In the first part of this work, we have concluded that the state of phosphoric acid supported on silica is significantly different than on alumina and titania.” The data reported here show that these three catalysts all have significant acid nature. However, different sites are present. The PS catalyst has a pure Broernsted acidic nature, and its free POH groups are the strongest as the Brernsted acid strength is concerned. This may be correlated with the strongest covalency of the Si-0-P bridges with respect to the A1-0-P and Ti-0-P ones, according to previous data concerning the surface acidity of metal pyrophosphates.28 Nevertheless, it may be noted that, from our spectroscopic data, it seems that interaction with bases only involves free POHs, assumed to arise from “isolated” covalently bonded hydrogen phosphate species, while H-bonded POHs in the form of liquidlike H3P04, abundant on PS,” look unactive. It seems reasonable to think that in contact with relevant adsorbate pressures, such as in conditions similar, for example, to those of the industrial catalytic olefin hydration and oligomerization,5s6 the monomers may dissolve in this liquidlike phase and react with these protons. Both PT and PA have the dual Brernsted and Lewis acid character. The present data seem to indicate that the Lewis acidity of these materials reflects that of the corresponding supports, only poorly perturbed. Microcalorimetric data indicate that at the same activation temperatures alumina and PA have a small number of very strong Lewis sites, but PT and titania have the most abundant sites, stronger than alumina and PA. Moreover, on PT a much higher number of Brernsted sites (respective to the Lewis ones) are present than on PA. This is deduced by the intensity ratios of the bands of the protonated and chemisorbed surface species formed by base adsorption. Nevertheless, it is also important to note that the nature of the Lewis sites and of their interaction with r-bondcontaining molecules, such as olefins, is different on titania than on alumina, as well as on their phosphatized forms. Ti cations exposed on titania-based catalysts display in fact a particular ability to form a-bonded specie^^^,^^ that is much more pronounced than that of Al cations on alumina and doped aluminas, probably related to the availability of empty d-type orbitals of Ti centers. This particular chemisorption activity for olefins by Ti centers may be correlated to the stronger oligomerization activity detected at room temperature in the IR cell, as well as to the stronger tendency to form carbonaceous materials at higher temperatures shown by PT than by PS and PA. In fact, oligomerization needs the presence of Brernsted sites (it is not observed on the pure supports in absence of acid dopants) but seems clearly favored by the presence of coordinatively unsaturated cations. This has been shown in the case of PA, where stronger pretreatments make free stronger Lewis centers and favor oligom(28) Ramis, G.; Busca, G.; Lorenzelli, V.; La Ginestra, A.; Galli, P.; Massucci, M. A. J. Chem. Soc., Dalton Trans. 1988, 881.
Langmuir, Vol. 5, No. 4, 1989 923 erization activity. However, this is mainly the case of titania-based catalysts, probably just due to the abovementioned ability of the Ti cations to coordinate the monomer, so favoring its insertion into the growing chain. It seems not surprising that where polymers and high molecular weight comounds are formed in greater amounts the catalyst also has a more pronounced tendency, at higher temperatures, to cover itself by carbonaceous residues. The catalytic data, showing a higher activity of PT in 1-butene conversion and, in particular, in skeletal isomerization, hydrogenation,and cracking, may be interpreted on the basis of the considerations summarized above. In fact, it seems reasonable to propose that cracking of 1butene, observed at 670 K, may be the result not only of the breaking and rearrangement of the sec-butylcarbenium ion but also of its oligomerization products, whose formation is more pronounced on PT (Scheme I). Also, the formation of C4 paraffins is very likely to involve hydrogenation of the corresponding olefins at the expense of high molecular weight adsorbed species that evolute toward carbon by dehydrocyclization reactions. Skeletal isomerization is also observed to occur to a greater extent on PT than on PS, whose Brernsted acidity seems to be stronger on the basis of IR data. This would disagree with the general opinion that isomerization of olefins producing isoolefins is due to a purely Brernsted acid catalyzed mechanism. However, the mechanism of isomerization of the secondary n-butyl carbocation to the tertiary isobutyl carbocation is very complex29and may also involve steps where other sites are involved. For example, Krywicki30 showed that basic sites are also involved in cracking reactions on similar catalysts. Our results suggest that the catalytic activity of these catalysts in 1-butene conversion, including skeletal isomerization cannot be related only to the strength of Brernsted sites but would also reflect the particular activity of Ti cations and possibly the role of basic sites. Summarizing, these results provide evidences for the cooperative role of Brernsted sites with particular types of Lewis sites in the acid catalysis and in the activity to form surface carbons. It seems probable that the availability of coordinatively unsaturated transition-metal cations on the surface of Bransted acidic solids favors these reactions, providing the possibility of having surfaces whose activity involves both surface sites and adsorbed species. The present results also provide evidence for a particular activity of phosphoric acid treated titania, with respect to analogous solids produced by impregnation of silica and alumina by phosphoric acid. Registry No. THF, 109-99-9; H3P04, 7664-38-2; TiOz, 13463-67-7;n-butylamine, 109-73-9; ammonia, 7664-41-7; pyridine, 110-86-1; acetonitrile, 75-05-8; 1-butene, 106-98-9; isobutene, 115-11-7.
(29) Wojciechowski, B. W.; Corma, A. Catalytic Cracking; Marcel Dekker: New York, 1986; p 20. (30)Krzywicki, A. Sci. Rep. Technical Unio. Warsaw, Ser. Chim. 1980, 23.
