5947
J. Phys. Chem. 1993,97, 5947-5953
Interaction of Oxomolybdenum Species with rc-A1203 and rc-A1203 Modified by Silicon. 1. The Si02/yc-A1203 System P. Sarrazin,* S. Kasztelan,*~*N. Zanier-Szydlowski,s J. P. Boonelle,+and J. Grimblot+ Laboratoire de Catalyse Het6rogPne et Homoghe, URA CNRS 402, Universite des Sciences et Technologies de Lille, 59655 Villeneuve d'Ascq Cedex, France, and Institut FranCais du P6trole. 1 et 4 Avenue de Bois-Prbau. B.P. 311, 92506 Rueil Malmaison Cedex, France Received: November 3, 1992; In Final Form: March 9, 1993
The combination of several characterization techniques, i.e., surface area and porosimetry measurements, X-ray microprobe, X-ray photoelectron spectroscopy,Fourier transform infrared spectroscopy,temperature programmed desorption of ammonia, and basic site titration, allowed us to investigate a rc-A1203 support modified by grafting varying amounts of Si using tetraethoxysilane. Three Si concentration domains have been evidenced: (1) a first domain from 0 to 1 Si atom.nm-2 characterized by a strong interaction with the alumina support, (2) a second domain from 1 to 5 - 6 Si atomnm-2 corresponding to the formation of a species weakly interacting with alumina, and (3) for a Si loading higher than 6 Si atom*nm-2a third domain where bulk Si02 was detected. It appeared that only the basic hydroxyls of yc-A1203 were reacting with the molecule used for depositing Si. Neither the neutral hydroxyls nor the acidic sites were affected, except for some enhancement in the strength of the acidic sites. Upon Si deposition, silanol groups also appeared.
-
Introduction MoSz/y-AlzO3 catalysts, associatedwith a promotor, classically Co or Ni, are largely used for eliminating S-and N-containing molecules considered as impurities in petroleum fractions by hydrotreating. As the genesis of the supported MoSz catalysts proceeds through several stages, the first one being the deposition on the support of oxomolybdenum species dissolved in the impregnating solution, it appears important to control the reactivity and acido-basicity of the surface of the support. One way to improve such catalysts is to find supports other than y A l z 0 3tocorrectlydispersethe (promoted) active phase. Indeed, MoSz/TiOz catalysts with a low Mo loading appear more active in thiophene hydrodesulfurization (HDS) than the MoSz/y-AlzO3 ones.' It has been discovered that smaller MoS2 particles are present on TiOz2which therefore enhances catalytic performances. Recently, a rather complete investigation, mainly focused on the hydrodenitrogenationreaction, has shown the attractive potentiality of a (Ni)-MoS2 active phase supported on zirconia3or on Mg and Ni aluminates4 Another field of interest is to modify the surface propertiesof the reference y A l z 0 3carrier. For example, due to the interesting behavior of TiOz, supports such as TiO2/ A1203have been prepared with high surface area and characteri~ed.~ It is well established that the hydroxyl groups of y-A1203are involved in the fixation of molybdates in solutionG9and that their number or strength can be modified by the presence of foreign elements on the alumina surface. In particular, the Si02/A1~03 system, prepared by impregnation of Si(CzHs0)d in isopropyl alcohol solution, has been found to develop a different acidic character than that of A1203as the presence of Si02 increases the acid strength but decreases the total acidity.I0 Adsorption of pyridine indicates the presence of both nonprotonic and protonic acid sites on Si02-Al203 prepared by coprecipitation. The presence of molybdenum oxide deposited on supports with a low S i 0 2 loading increases Brijnsted acidity significantly.' I It has also been shown that the presence of foreign elements, namely, Si added, for example, as tetraethoxysilane, leads to marked stabilizationagainstthe loss of surfacearea by calcination
* To whom correspondence should be addressed.
Universitd des Sciences et Technologies de Lille. Institut Franqais du Petrole.
0022-3654/93/2097-5947%04.00/0
at 1233 Kl2 or in the presence of water partial pressure as a function of time.I3 A reaction between the hydroxyl groups of alumina and the Si-containingprecursor was found to occur, and the optimum Si deposit was 3 wt %I for sintering ~tabi1ization.l~ For those reasons, a systematicinvestigation of the interactions between Mo oxo species and SiOz/A1203 prepared with different Si loadings has been undertaken despite the report that MoO3/ SiOz/A1203 catalysts exhibit poorer performances in thiophene HDS than M00~/A1203.~ In the present paper, characterization with complementary techniques and chemical titrations of the SiOz/A1203 system prepared by Si grafting using tetraethoxysilane will be reported. In the second part,I5 the MoO~/Si02/ A1203system will be examined.
-
Experimental Section Sample Preparation. Grafting of silicium on a yc-A1203 reference support (surface area 252 mz.gl, pore volume 0.56 cm3.g-I) in the form of extrudates of 1.2-mm diameter, which have been previously calcined at 500 OC, has been performed with tetraethoxysilanewhich is reacting with the surface hydroxyl groups of alumina. The grafted species like Si(OCzHs),,( n S 3) can be hydrolyzed by water to form Si(OH), with parallel formation of ethanol which is easily eliminated by drying and calcination. The amount of Si deposited on the prepared solids has been titrated by X-ray fluorescence or atomic absorptionspectroscopy depending on the loading. In Table I are reported the loadings of the set of prepared SiOz/A1203 solids using the following nomenclature conventions for the samples: x with x being the number of Si atom*nm-2of alumina surface. This definition arises from the concept of monolayer-like dispersion but does not necessarily imply that Si adopts that conformation. Sample Characterization. Surface area and pore volume determination have been obtained with an automatic sorptometer (GIRA QSORB-IFP License) using the BET equation. The Si radial profiles of the extrudates of grafted alumina have been determined by an X-ray ,microprobeof Castaing type. For this analvsis the samdes. as extrudates. were cut at different lengths and ihen fixed by a'resin. Fourier-Transform Infrared Spectroscopy (FI'IR). The infrared spectra have been recorded with an interferometer 0 1993 American Chemical Society
5948 The Journal of Physical Chemistry, Vol. 97, No. 22, 1993
TABLE I: Prepared Samples Noted as x Si/A1203 where x Represents the Number of Si Atoms Deposited per nnr2 of Alumina Surface sample
Si02 (wt %)
sample
0.05 Si/A1203 0.20 Si/A1203 0.36 Si/A1203 0.45 Si/A1203 0.60Si/A1203 0.80 Si/A1203 0.86 Si/A1203
0.12 0.49 0.90 1.11 1.50 1.98 2.12
2.70 Si/A1203
5.00 Si/AI203 5.70 Si/A1203 6.40 Si/A1203 9.00 Si/A1203 12.40 Si/A1203
si02 (wt
%y
Sarrazin et al.
290 -
270
-
12.50 13.80
260
-
.
.
280 -
6.40 11.20
.
18.40
23.70
DIGILAB FTS 15Eequipped with a cell permitting the treatment of the samples at 10-6 Torr up to 550 OC. The aim is to detect the hydroxyl groups present at or in these samples. Their wavenumbers occur between 3 100 and 4000 cm-I. A preliminary study16 has shown that a pretreatment a t typically 450 OC overnight allows one to (almost) completely eliminate the contribution of the very broad band due to residual water present in the sample. The spectra have been obtained after 300 accumulations with a resolution of f 4 cm-I. Each sample (20 mg) was pressed at 1780 bar to make a thin self-supported pellet of 16-mm diameter. The recorded spectra have been decomposed into four Gaussian peaks for pure alumina and into five Gaussian peaks for the Si02/ A1203 samples with a procedure which has been described in detail elsewhere.16 X-ray Photoelectron Spectroscopy (XPS). The XPS spectra have been recorded with an AEI ES 200B spectrometer equipped with an A1 X-ray source (hv = 1486.6 eV) working with a power of 300 W. The grounded samples were pressed on an indium foil mounted on the sample probe. Typically, during the XPS examination, the vacuum in the analysis chamber was l e 8Torr. The C Is, 0 Is, A1 2p, and Si 2s levels have been systematically recorded for all the samples. The Si 2p level was not exploited because of overlap with A1 Auger peaks. The binding energies (BE) reported in this work were calculated by reference to the A1 2p peak a t 74.8 eV or to the 0 1s peak at 531.5 eV when the intensities of each peak were integrated and noted, for example, Z Si 2s for Si 2s level. Ammonia Thermdesorption. The experimental sequences of an NH3 TPD experiment were the following: (a) 0.5 g of samples were pretreated from 25 to 350 "C (heating rate = 5 OC-min-I) under a flow of He. (b) After complete elimination of residual water at 350 OC, the sample was maintained at 150OC and isolated and then NH3 was admitted for 20 min. (c) Excess NH3 and physisorbed NH3 were eliminated under a flow of helium overnight. (d) NH3 thermodesorption was carried out, starting at 150 O C and going up to 700 OC with a heating rate of 5 OC-min-1 under helium. The quantity of desorbed NH3 was measured by a gas chromatograph equipped with a catharometer. When desorption isstartedat 150 OC, theamount ofreleasedammoniaisdetermined and can be expressed as a partial pressure. The evolution up to 650 O C allows one, by integration, to deduce the total amount of adsorbed ammonia. Chemical Titration of Strong Basic Sites. This method, described by Vit et al.," uses the adsorption of benzoic acid, a weak acid, in methanol solutions on the catalyst surface. Then adsorbed benzoic acid is displaced by a stronger acid, Le., acetic acid, and the released benzoic acid in solution is titrated by UV spectrometry. To improve the efficiency of the adsorption, the bed of catalyst was fluidized in the acidic solutionis and the following conditions were adopted: (a) Impregnation of 200 mg of catalyst by a 0.015 M solution of benzoic acid for 2 h. (b) Removal of benzoic acid contained in the pores of the solid by elution with 30 mL of methanol a t a flow rate of 2 mlamin-'. (c) Desorption of benzoic acid with
210t 'I 200' 0
1
'
2
I
I
3
4
'
'
I
I
I
I
'
I
'
I
5 6 7 8 9 101112131415 Si loadlng (atom.nm2)
Figure 1. Specific surface area variation of the alumina support versus the amount of grafted Si (in Si atom.nm-2).
Mean pore dlameter (A)
Pore volume (cm3.g-1 x 100)
- 70 -
60
-
50
- 40 - 30
*OI I 10
" 0
1
2
3
4
5 6 7 0 9 1011 1 2 1 3 1 4 1 ; Si loadlng (atom.nm-2)
Figure 2. Variation of (a, left axis) the total pore volume (in cm3& X 102)and (b,right axis) mean pore diameter (in A) of the alumina support as a function of the amount of grafted Si (in Si atom.nm-2).
300 mL of acetic acid (0.05 M) with a flow rate of 5 mL.min-I. (d) Titration of released benzoic acid by UV spectrometry at 229 nm.
Results Textural Characterization. The addition of Si to yc-A1203 greatly modifies the surface area, the pore volume, and the mean pore diameter of the resulting solids. In Figure 1 the evolution of the surface area as a function of the amount of grafted Si (in Si atomnm-2) is reported. The surface area decreases from 252 to -220 m2.g-I upon addition of 1 Si atomnm-2. Then, the surface area progressively increases and reaches an asymptotic value of -270 m2& for Si loadings higher than 6 Si atomnm-2. Simultaneously, addition of Si modifies the pore volume and the mean pore diameter (Figure 2). Three domains can be detected: (i) from 0 to 1 Si atomnm-2 where a sharp decrease of the mean pore diameter occurs (the decrease of the pore volume is less visiblein thisdomain), (ii) from 1 to -6 Siatom.nm-* thedecrease of both the pore volume and the pore diameter is very small, and (iii) for Si loadings higher than 6 Si atomnm-2 the decrease of both parameters is more pronounced. The general evolution, the decrease of both parameters, appears normal as the grafted silicium species occupy a part of the pore volume of the host alumina. The sharp decrease of pore volume and pore diameter in the first part (Figure 2) has to be related to the sharp decrease of the surface area (Figure 1). On the other hand (Figure 3), a microporosity is developing for Si amounts higher than -5 Si atom.nm-2. This is probably due to the formation of bulk Si02 which develops its own porosity. This is
-
The Journal of Physical Chemistry, Vol. 97, No. 22, 1993 5949
The SiOz/yc-Al2O3System
analyze all the specimens, even the highest Si-loaded supports, for which no opacity limit of the self-supported pellets has been detected. This is not the case for the MoO3/SiO2/Al203 samples highly loaded with M o . ' ~The typical spectral evolution upon Si deposition has been reported in Figure 6. On yc-A1203,one generally observes five bands, better resolved at higher wavenumbers in the 3600-3800-~m-~domain. KnBzinger and RatnasamyZ5have identified these bands as representing hydroxyl groups, and a basicity scale has been proposed, the highest wavenumber bands corresponding to the most basic hydroxyl (3795 and 3770 cm-I). At first, addition of Si affects the relative intensity of these bands. For the samples with 0.80 Si atom-nm-2, or more, another band at 3740 cm-I assigned to silanol groupsZ6 can also be detected. Its detection at lower Si loading was not possible due to the presence of the very intense band of y,-A1203 at 3730 cm-I assigned to neutral hydroxyls. The IR spectra have been decomposed by a computer procedure16 into five Gaussian bands. It is therefore possible to plot the relative contributions of each of these bands as a function .__ ._ -.-.-.-.-.-.----=---, of the amount of grafted Si (Figure 7). For reasons of convenience 10 100 1000 10000 100000 and to minimizeuncertainties in the decomposition,the two bands Pore diameter ( A ) Figure 3. Variation of the alumina support pore size distribution as a at 3795 and 3770 cm-I have not been separated. It is clear in function of the Si loading (in Si a t o " r 2 ) . Figure 7 that the acidic (3695 cm-I) and neutral hydroxyls (3730 cm-I) are not at all affected by deposition of Si. Whereas the also reflected by a higher surface area in this third domain (- 270 basic hydroxyls (3795 and 3770 cm-I) are strongly affected in m2.g-I). the 0-1 Si atomnm-2 domain. Nevertheless, further increase of Si Localization and Its Chemical State. In Figure 4 are the Si loading does not allow one to completely eliminate the presented the typical Si radial concentration profiles of the Si/ bands assigned to basic hydroxyls. A1203extrudates obtained by the X-ray microprobe for six samples In Figure 8 the evolution of the band assigned to the silanol of the studied series. Only for the 0.2 Si/A1203sample has a groups (3740 cm-I) as a function of the Si loading is also reported typical U profile been obtained, which shows that Si is preferand a curve has been temptatively drawn, showing a shape similar entially located at the external layers of the extrudates. This U to the curve of the XPS I Si 2s/I A1 2p ratio evolution. type profile is not observed for samples with higher Si loadings. The chemical titration of strong basic sites should complete It can therefore be concluded that, using tetraethoxysilane, Si is the IR characterizations as most of the basic hydroxyls of ycwell distributed in the yc-A1203grains with the exception of the A1203 are involved in grafting tetraethoxysilane. In Figure 9 the lowest loaded sample. evolution of the basic site density determined by chemical titration XPS may also be informative in determining the Si location as a function of the Si loading is reported. A sharp decrease at a microscopic level as well as on its chemical state. In Figure occurs up to 1 Si atomnm-2 followed by a slow decrease. For 5 the evolution of the Si 2s binding energy (BE) and of the intensity the highest Si loading sample, the remaining basic sites correspond ratio I Si 2s/I A1 2p as a function of the Si loading is reported, to -0.1 site.nm-2, whereas on the fresh alumina, the density was respectively. -0.5 site.nm-2. So 80% of the basic sites have disappeared by Once again three domains can be distinguished. In the low the grafting procedure. The strong parallel between the evolution loading domain (up to 1 Si atomnm-2), a sharp increase of Si of the basic site density reported in Figure 9 and the relative IR BE from 151.9 to 153.4 eV is observed. In parallel, the I Si 2s/I contribution of the basic hydroxyl groups (Figure 7), namely, for A1 2p ratio increases linearly. From 1 to 5 Si atomnm-2,both loading up to 1 Si atomnm-2,is revealed by the linear correlation parameters exhibit no large variations. For loadings higher than reported in Figure 10. Then it is concluded that the chemical 5 Si atomnm-2, the Si 2s BE tends to reach that observed for bulk titration allowed us to really count all the basic hydroxyls Si02 whereas the i Si 2slIA12p ratio increases sharply, possibly corresponding to the 3770- and 3795-cm-I bands. due to the complete coverage of the alumina surface and growth The temperature programmed desorption of ammonia also of SiO2-like tridimensional crystallites. allows one to control the evolution of the acidic properties of the The linear evolution of the I Si 2s/I A1 2p ratio between 0 and set of Si/A1203 samples. The curves of Figure 11 also allow one 5 Si atomnm-2 with a bceak at 1 Si atomnm-2 is typical of a to precisely determine the temperature of the maximum of monolayer dispersion of the grafted Si species at low Si loadings. desorption TMAX and that corresponding to the end of desorption Such curves have already been observed for m ~ l y b d a t e , l ~ ~ ~T ~E N DTypical . results are reported in Table 11. Within the or vanadium species22on yc-A1203. It therefore experimental precision, the overall acidity of the samples does appears that the Si grafting procedure (which is detailed in the not change with the amount of grafted Si. This observation is Experimental Section) is successful in dispersing Si on yc-A1203 consistent with the stability of the contribution (relative) of the only up to -1 Si atomnm-2 well. hydroxyl group assigned to acidic sites (Figure 7, band at 3695 The lowest Si 2s BE, Le., 151.9 eV, found in the domain where cm-I) and with the fact that NH3 titrated essentially Bronsted S i is well dispersed, is 2.5 eV lower than that found in bulk S O z . acidity. Nevertheless, both TMAX and T E Nslightly D increase upon Such a large shift has already been observed on zeolites exchanged Si grafting, which implies an increase of the strength of the acidic with calcium23or with sodium.24 This large shift can be interpreted sites. by a decrease in net positive charge of the first grafted silicon species, which implies that the bonds with the alumina support Discussion are highly polarized. Si Grafting on the ~ ~ - A 1 2 Surface. 03 During the preparation Surface Hydroxyls and Acido-Basic Properties. FTIR spectra of the samples by a grafting procedure, it is assumed that in the 3400-4000-~m-~domain are informative about the tetraethoxysilane reacts with the surface OH groups of yc-Alz03. evolutions of the surface hydroxyl groups of y,-A1203 upon Si The grafted species then undergo hydrolysis, and after a more grafting. With this series of samples, it has been possible to ~
-
-
Sarrazin et al.
5950 The Journal of Physical Chemistry, Vol. 97,No. 22, 1993
100,s wt%Si
I
50t
Or
2b
660
750
ldoo
1:
io
t 300
600
Boo
1200
1500
100-
JCI
. 50
100lOdwt%Si
-
Diameter (pm) Figure 4. X-ray microprobe radial profiles of Si in the grafted Si02/A1203 samples: (a) 0.20 Si/A1203, (b) 0.36 Si/AI2O3,(c) 0.60 Si/A1203, (d) 0.86 Si/A1203, (e) 2.70 Si/AI2O3,and (f) 5.00 Si/A1203.
or less complete dehydration, the so-called SiOz/A1203 samples are obtained. The schematic reaction schemes proposed in refs 27 and 28 are reported in Figure 12, reactions 12.1 and 12.2. Reactions 12.1 and 12.2 involve respectively one or two surface hydroxyls which are consumed by one molecule of Si(OC2Hs)d. Reaction 12.3 implies that acidic Lewis sites of rc-A1203 are involved in grafting. In addition to these three reactions, it has also been proposed27that the surface ethoxysilanemay react with other surface ethoxysilanes to developa bi- and/or tridimensional structure (reaction 12.4). From the set of experimental data presented above, it appears that the Si grafting significantly affects the texture and surface reactivity of the yc-A1203support. By carefully comparing the results, there appear three domains of silicium loading. In thefirst domain (0 < Si < 1 Si atomnm-2), one observes a sharp surface area decrease (Figure l), texture modifications (Figure 2), a monolayer-likesiliciumrepartition (Figure 5b) with a chemical state of Si quite different of that found in Si02 (Figure Sa) and a net decrease of the density of basic surface sites (Figure 9) which are associated with the IR bands of the hydroxyls at 3795 and 3770 cm-I (Figure 7). The chemical reactions involved for that domain could be either (1 2.1) or (1 2.2) as they correspond toa monolayer-like coverage. The presence of a silica monolayer on alumina has already been discussed30andis presented as being beneficial for preventing alumina sintering at high temperature.14 Reaction 12.3 cannot be involved as a pair of acid and basic sites is needed and only basic sites have been found reacting in this work (Figure 7). From the basic site titration, it appears (Figure
9) that 1 Si atom reacts with -0.3 basic OH. Therefore, the reaction scheme described in reaction 12.5 appears better adapted for describing the grafting reaction in that first domain. It is easily conceivable that such species, located in the pore structure of the support, have an influence on the mean pore diameter (Figure 2) and hence on the pore volume and the surface area. The strong chemical shift of the Si 2s BE observed (Figure 5) for very small Si loadings reflects the bond formation between alumina bridges and Si through Si-0-AI. So probably at the beginning of grafting, the charge transfer from A1 to Si is high (species corresponding to reaction 12. l), but as species described in reaction 12.5 are progressively found, the positive charge is delocalized on all the Si atoms and the BE tends to that found for bulkSi02. This situation is perhaps comparable to thedeposit of various amounts of Mo oxo species on alumina.31 At low Mo loading, corresponding to isolated tetrahedral Mo species, the Mo 3d5p BE is 232.5 eV. For higher Mo loadings, poly(oxomolybdates) are on the support and the corresponding BE is 233.2 eV like in bulk MoO3. Theseconddomain detected correspondsto Si loadings between 1 and -5-6 atomnm-2. In that domain, the previous parameters are not modified much (Figures 2, 5, 7, and 9) except for the surface area (Figure 1). This domain probably corresponds to the formation of bi/tridimensional structures like those described in reaction 12.4. As this kind of reaction does not necessarily involve surface OH groups, the decrease in the amount of basic OH (Figures 7 and 9) is very weak. The thirddomain at the highest Si loading (>6 Si atom.nm-2)
The Journal of Physical Chemistry, Vol. 97, No. 22, 1993 5951
The Si02/y,-A1203 System 154.5
si02 154 153.5153 -
*
(a)
r-.
1
152.5-' 152 151.5' 0
I
1
'
2
"
3
4
3
4
: j
"
I
'
"
'
'
'
I
I
I
I
I
'
5 6 7 8 9 101112131415 SI loading (atom.nm2)
ISi2s/lA1211
0.3 0.2 0.1
0
0
1
2
4
I
6 7 8 9 10 11 12 13 14 ii loading (8tom.nm-2)
Figure 5. XPS variation of (a) the XPS Si 2s binding energy (eV) and (b) the intensity ratio I Si 2s/I AI 2p versus S i loading (in Si atomnm-2).
corresponds to the formation of bulk SiOz which develops a microporoustexture (Figure 3). As this Si02materialcompletely covers the A1203support, the ZSi 2slZA12p intensity ratio rapidly increases (Figure Sb). The increase of the surface area of the samples in both domains 2 and 3 probably corresponds to formation of 5 3 0 2 , as the limit between domains 2 and 3 is far less defined than the limit between domains 1 and 2. Surface Hydroxyls and Acido-Basic hoperties. The surface acidity of transition aluminas, namely, that of yc-A1203,and hence the nature of the surface hydroxyls receive considerable attention25932 as it is commonly believed that the catalytic properties and the ability to disperse well additional elements (i.e., molybdate, tungstate, vanadate, chromate, ...) on its surface are closely related to the density, nature, and strengths of the surface acidic/basic sites.*%' 5,22,25~33-36 Recently, the acidic properties of the Si02/A1203 system prepared by impregnation of Si(OC2H5)4, was investigatedlO by several procedures such as adsorption-desorption of ammonia, 1-butanoldecomposition,and I-butene isomerizationwhich allows one to differentiate the strengths of the acidic sites. It was shown that addition of Si increased the acid strength while the total acidity decreased. On the other hand, a monolayer of silica on yc-A1203 obtained by chemical vapor deposition of silicon methoxide Si(OCHj)430possesses Brcinsted acid sites assigned to the silanol groups. These sites are not strong enough to activate cumene cracking. The results reported in this paper indicate an enhancement of the strength of acidic hydroxyls (see Table 11) upon deposition of Si, but the total acidity titrated by temperature programmed desorption of ammonia (table 11) and the relative proportion of acidic hydroxyls (Figure 7) are not modified. A similar conclusion arises with the neutral hydroxyls (3730 cm-1, Figure 7). Si deposition on y,-A1203 by grafting only affects the basic hydroxyls (Figures 7 and 9) by eliminating 80% of them even after deposition of high Si loadings (higher than 6 Si
100
I.--1
90 -
.3795
¶noem1
3790 om1
80 70 -
60 5O.b. *-
40
. .
*.
y
30'20
-
10
e4r,
0
I
I
I
I
I
1
4
atomnm-2). As these basic hydroxyls are also involved in the reaction with oxomolybdates during the impregnation step,ls a competition should occur between Si and Mo anions, a phenomenon that will be investigated in the second part of this work.i5 At last, in agreement with Niwa et al.30 the deposition of Si generates specifichydroxyl groups of the silanol type, the amount ofwhichisincreasingwiththeSiloading(Figure8). Theproposed curve in Figure 8 presents a similar shape to the curve of Figure Sb. Even though the number of experimental points is reduced, such an evolution of silanol groups should be consistent with the above discussion. At low Si loading each Si entity generates
5952
The Journal of Physical Chemistry, Vol. 97, No. 22, 1993
Sarrazin et al.
Relative proportion of the SiOH groups (%)
NH3 partial pressure (torr) 7.5
2 5 1 5
2.5
0
1
2
3 4 5 6 7 8 Si loading (atom.nm-2)
9
0
10
Fiprel. EvolutionofthesilanolIR bandat 3740cm-I (relative proportion in percent) as a function of the Si loading (in Si atom.nm-*).
Temperature ("C)
Figure 11. Temperature programmed desorption of ammonia on alumina and SiO*/A1203 samples.
Basic sites denstty (site.nm-2)
H&2°\
OH
OH
I AI-
I A1
~
2
3
4 5 SI i d n g ( a " q
6
7
8
0
o b
+H2O
+ A1 - A I + 2 CzH50H 9 H5C20 I 0 I 0 A ' !,
0
1
HO ~ ,OH5 S i
2
dsi\o I I
+ Si(OC2Hr)a
o\ AI< -o\,A1 / o+Si(OC2H5)4 9 -0
~
HO I 0I, 0 A$AI
+H20
O ,
+
'A1
d
(12.2)
I I Al- AI +2 C2HsOH
O, +4 C2H5OH (12.3)
10 (12.4)
Figure9. Evolution of the basic site density (site number"t) measured by chemical titration as a function of the Si loading (in Si a t o " r 2 ) . Basic sites density (site.nm-2) 0.6 I
1 OH I AI
I
+ 3 Si(OC2Hs14
9
AI
+ 2 HJCZOC~H + ~I CiHrOH
Figure 12. Various possibilities for tetraethoxysilane interaction with alumina surface hydroxyls.
TABLE II: Data Obtained from the TPD Of NH? 1 ) ~ ~ 3 Represents the Total Amount of Desorbed Ammomum between 150 O C and the End of Desorption (TEND)
5 10 15 Rolativr proportion of basic hydroxyls (%)
20
Figure 10. Correlation between the basicsitedensity (chemical titration) and the relative proportion of basic hydroxyls (3795 and 3770 cm-I) determined by FTIR.
several silanol groups (2 or 3) while at high Si loading a bulk-like Si02 structure is necessarily poorer in silanol groups per Si atom.
Conclusion Textural and acido-basicity properties of yc-A1203have been modified by deposition of Si through a grafting procedure using tetraethoxysilane. It has been shown that this compound interacts mainly with the basic hydroxyls of alumina although they have not completely disappeared even after deposition of high Si loading. However, surface acidity is not greatly modified by deposition of Si. For the series of Si/A1203 samples prepared, three domains can be considered for the Si surface repartition:
sample
nNH3 (mol*103%-')
A1203 2.7 Si/A1203 5.0 Si/A1203 6.4 Si/A1203
0.38 0.39 0.37 0.36
TMAX
("c)
280 340 340 340
TEND ("c)
600 614 610 630
(a) In domain 1 from 0 to 1 Si atomnm-2,the grafting procedure, involving specifically basic hydroxyls, is successful and a strong interaction (modification of the Si electronic properties) between Si and alumina is observed. (b) In domain 2 from 1 to 5-6 Si atomnm-2, there is a formation of a bi/tridimensional silica phase on alumina with a weaker interaction than that observed in the first domain. (c) In domain 3 for silica loadings higher than 6 Si atom.nm-2: growth of Si02 occurs, which develops its own porosity and has no detectable electronic modification.
-
References and Notes (1) Ng, K. Y.S.;Gulari, E. J . Catal. 1985, 95, 33. (2) Ramirez, J.; Fuentes, S.;Diaz, G.; Vrinat, M.; Breysse, M.; Lacroix, M. Appl. Catal. 1989,52, 211.
The Si02/yC-A1203System (3) Duchet, J. C.;Tilliette, M. J.;Cornet, D.;Vivier,L.;Perot,G.;Bekakra, L.; Moreau, C.; Szabo, G. Catal. Today 1991,10, 579. (4) Duchet, J. C.; Gnofam, N.; Lemberton, J. L.; Perot, G.; Bekakra, L.; Moreau, C.; Joffre, J.; Kasztelan, S.; Grimblot, J. Catal. Today 1991,10,593. (5) Zhaobin, W.; Qin, X.;Xiexian, G.;Sham, E. L.; Grange, P.; Delmon, B. Appl. Catal. 1990,63,305. (6) Schuit, G. C. A.; Gates, B. C. AIChEJ. 1973,14, 417. (7) Massoth, F. E. Adu. Caral. 1978,27, 265. (8) Hall, W. K. Proceedings of the Fourth International Conference on the Chemistry and Uses of Molybdenum; Barry, H.F., Mitchell, P. C. H., Eds.; Climax Molybdenum Company: Ann Arbor, MI, 1982,224. (9) Kasztelan, S.; Payen, E.; Toulhoat, H.; Grimblot, J.; Bonnelle, J. P. Polyhedron 1986,5,157. (10) Yori, J. C.; Luy, J. C.; Parera, J. M. Appl. Catal. 1988,41, 1. (1 11 Valvon. J.: Henker. M.: Wendlandt. K. P. React. Kiner. Catal. Leu. (12) Murrel, L. L.; Dispenziere, N. C. J . Catal. 1988,1 1 1 , 450. (13) Johnson, M. F. L. J . Caral. 1990,123, 245. (14) Beguin, B.; Garbowski, E.; Primet, M. J . Catal. 1991,127, 595. (15) Sairazin, P.; Kasztelan, S.; Payen, E.; Bonnelle, J. P.; Grimblot, J. J . Phys. Chem., following paper in this issue. (16) Sarrazin, P. Doctoral Thesis, Universitt des Sciences et Technologies de Lille, France, 1989. (17)Vit, Z.; Vala, J.; Malek, J. Appl. Catal. 1983,7, 159. (18) Kasztelan, S.;Grimblot, J.; Bonnelle, J. P.; Payen, E.; Toulhoat, H.; Jacquin, Y. Appl. Catal. 1983,7, 91. (19) Grimblot, J.; Bonnelle, J. P.; Beaufils, J. P. J . Electron Spectrosc. 1976,8, 437.
The Journal of Physical Chemistry, Vol. 97, No. 22, 1993 5953 (20) Dufresne, P.; Payen, E.;Grimblot, J.; Bonnelle, J. P. J . Phys. Chem. 1981,85,2344. (21).Ouafi, D.; Mauge, F.;Lavalley, J. C.; Payen, E.;Kasztelan, S.;Houari, M.; Grimblot, J.; Bonnelle, J. P. Caral. Today 1988,4, 23. (22) Meunier, G.;Mocaer, B.; Kasztelan, S.; Le Coustumer, L. R.; Grimblot, J.; Bonnelle, J. P. Appl. Catal. 1986,21, 329. (23) Barr, T. L. Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy; Briggs, D., Seah, M. P., Eds.; John Wiley & Sons, Ltd.: Chichester, 1983;p 283. (24) Winiecki, A. M.;Suib, S.L.; Occelli, M. L. Langmuir 1988,4,512. (25) Knbzinger, H.; Ratnasamy, P. Catal. Reu. Sei. Eng. 1978,17, 31. (26) Zaki, M. I.; Knbzinger, H. Mater. Chem. Phys. 1987,17, 201. (27) Alexander, J. D.; Gent, A. N.; Henriksen, P. N. J . Chem. Phys. 1985, 83,5981. (28) Gates, B. C. AIChE Annual Meeting, San Francisco, 1984. (29) Nilsen, B. P.;Onuferko, J. H.;Gates, B.C. Ind. Eng. Chem. Fundam. 1986,25,337. (30)Niwa, M.; Katada, N.; Murakami, Y. J . Phys. Chem. 1990,94,6441. (31)Nag, N. K. J . Phys. Chem. 1987,91, 2324. (32) Kijenski, J.; Baiker, A. Catal. Today 1989,5, 1. (33) Peri, J. B.; Hannan, R. B. J . Phys. Chem. 1960,64, 1526. (34)Peri, J. B. J . Phys. Chem. 1965,69,220. (35)Tanabe, K. Solid Acids and Bases; Academic Press: New York, 1970. (36) Davis,J. A.; Hem, J. D. TheEnuironmental Chemistry ofAluminium; Sposito, G., Ed.; CRC Press. Inc.: Boca Raton, FL, 1985;Chapter 7, p 185.
