FTIR Spectroscopic Studies of Silication of γ-Alumina with FCC

Engelhard Corporation, Research Center, 101 Wood AVenue, Iselin, New Jersey 08830 ... catalysts, formation of new Brönsted acid sites, and changes of...
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J. Phys. Chem. B 1999, 103, 2647-2652

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FTIR Spectroscopic Studies of Silication of γ-Alumina with FCC Catalysts via Steaming Xinsheng Liu Engelhard Corporation, Research Center, 101 Wood AVenue, Iselin, New Jersey 08830 ReceiVed: September 17, 1998; In Final Form: December 31, 1998

The framework structures, surface hydroxyls, and acidity of silicated γ-alumina with and without transitionmetal oxides were studied using FTIR techniques. The silication was made via steaming γ-alumina with FCC catalysts. The results clearly show migration of silicon species from FCC catalysts to the surface of γ-alumina, forming Bro¨nsted acid sites. The newly formed Bro¨nsted acid sites are weaker in strength compared to those in zeolites. Reactions between mobile silicon species and surface sites of γ-alumina proceed selectively and occur only on stronger Lewis acid sites. Transition-metal oxides introduced by impregnation onto γ-alumina have no effect on the site-selective silication reactions.

Introduction Silicated aluminas (chiefly in the γ-form), generally prepared by reacting aluminas with silicon alkoxides,1 are active olefin skeletal isomerization catalysts.2,3 Studies4-9 on these materials have not only shown incorporation of silicate species in/onto the surface of aluminas, generating weak Bro¨nsted acid sites, but also demonstrated improvement in thermal stability of these materials. It has been known for some time that Si(OH)4, although it is nonvolatile at ordinary temperatures and quickly polymerizes in water when heated, becomes volatile at elevated temperature and pressure in steam.10 Such volatility of silica species was also observed for FCC catalysts when they were subjected to steaming.11 However, reactivity of such volatile silicate species is still unknown, and only a phenomenon of migration of silicon species from matrix to zeolite component and reinsertion into zeolite framework has been discussed.12 It is, therefore, of paramount importance to know details about the reactivity of the volatile silicon species in helping us understand many industrial processes such as deactivation of FCC de-sulfur catalysts, formation of new Bro¨nsted acid sites, and changes of acidity of FCC catalysts during sample steaming.13 In this paper, we present results of our studies on silication of γ-aluminas via steaming with FCC catalyst microspheres using FTIR spectroscopic techniques. The questions we focused on are the following. (1) Does transportation of silicate species occur from FCC microspheres to the surface of aluminas during steaming of a mechanically mixed different particle sized γ-alumina and FCC catalyst microspheres? (2) Are there any new Bro¨nsted acid sites formed on the surface of silicated γ-aluminas? (3) If migration of silicate species from FCC catalyst microspheres to the surface of aluminas occurs during steaming, does the silication reaction proceed selectively or randomly toward the surface sites? (4) If other oxides such as transition-metal oxides are introduced, is there any preferred occupation/reactions of alumina surface sites by/with the transition-metal oxides? (5) When other oxides are present, does the silication of alumina surface still occur and follow the same reaction mechanism? To answer these questions a series of samples of γ-aluminas with and without transition-metal ions were examined. Experimental Section Samples. γ-Alumina with a surface area of 257 m2/g was prepared by calcination of Versal-250 alumina at 552 °C for

1.5 h. Metal-containing γ-aluminas (metal ions ) Zn2+, Cu2+, Ni2+) were prepared by incipient wetness impregnation of γ-aluminas with corresponding solutions of metal nitrates. After low-temperature oven drying, the samples were calcined at 450 °C for 3 h to remove nitrates. The final obtained samples contain, as oxides, 10 wt % Zn2+, and 1.25 wt % Cu2+ or 1.25 wt % Ni2+. The metal nitrates were all purchased from Aldrich. The FCC catalyst used was Precision (an in-situ FCC catalyst, Engelhard Corporation) containing 1 wt % rare earth. Steamed Sample Preparation. First, the FCC catalyst having particle sizes greater than 70 µm was blended in a 70:30 ratio with the calcined Versal 250 alumina (with or without metal ions) having particle sizes smaller than 53 µm. Then, the blended particles were steamed under 90% humidity at 788 °C for 4 h. Finally, the alumina particles were screened out from the FCC catalyst particles and the portion of alumina with particle sizes 45-38 µm was collected for the study. The surface areas of the steamed aluminas are ∼100 m2/g. Instruments and Procedures for Acidity Measurements. Diffuse reflectance Fourier transform infrared (DRIFTS) spectra were recorded on a Perkin-Elmer Paragon PC1000 spectrometer equipped with a MCT detector and a Spectra-Tech diffuse reflectance high-temperature chamber with KBr windows allowing gases such as N2 to flow through. Samples were ground into a fine powder with an agate mortar and transferred evenly into an aluminum sample cup. The samples were first dehydrated at 450 °C for 1 h under flowing dry N2 (40 mL/min), and then cooled to room temperature. Pyridine vapor (partial pressure 18 mmHg) was introduced in a N2 stream for about one minute (∼4 × 10-5 moles of pyridine), then shut off. The system was equilibrated at room temperature for 1 h under flowing N2, allowing removal of physisorbed pyridine. Samples were then heated to 180 °C under flowing N2 and maintained at this temperature for 1 h, then cooled to room temperature. Finally, the procedure was repeated at 400 °C. Single-beam spectra were collected at room temperature after heating using an automation program. For the single-beam background spectrum, a fine ground KBr powder was used. The ratios of the sample spectra against the background spectrum were made, and the spectra were converted to Kubelka-Munk spectra14 (defined by (1transmittance)2/(2×transmittance) in ref 14. Note that transmittance under diffuse reflectance mode is actually equal to reflectance) after baseline and offset corrections. Difference spectra were obtained by subtracting the Kubelka-Munk sample spectra without pyridine from those containing pyridine. Due

10.1021/jp9837636 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/19/1999

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Figure 1. Transmittance FTIR spectra of FCC-silicated γ-alumina (solid line) and steamed γ-alumina (dotted line).

to interactions of adsorbed pyridine with the surface, bands generated by adsorption are positive, while bands “removed” by adsorption are negative. Measurements of Framework Vibrations. Framework vibrations of the samples were measured using the KBr wafer technique. Results and Discussion Before giving the results, it is pertinent to describe briefly the structure and surface of γ-aluminas. The structure of γ-aluminas is built by AlO6 octahedra and AlO4 tetrahedra, and is slightly tetragonally distorted from a face-centered cubic lattice structure.15 The surface of γ-aluminas is covered by hydroxyls, and depending on conditions, various types of surface hydroxyls can be differentiated by FTIR spectroscopy and other techniques.16-26 The hydroxyls observed from IR studies16-21 correspond to OH groups attached to one, two and three aluminum atoms with different coordination numbers (4 or 6 coordinated). Dehydroxylation of alumina surface generates more types of hydroxyls due to changes of coordination of aluminum atoms (3, 4, 5, or 6 coordinated) and results in formation of Lewis acid sites with different acid strength. Very recently, we have found21 that the Lewis acid sites formed on γ-alumina surface have neighboring hydroxyls and that a specific arrangement of surface hydroxyls and Lewis acid sites exists. The weak Lewis acid sites are surrounded by bridged OH groups, the medium-strong Lewis acid sites have triply bridged OH as neighbors, and the strong Lewis acid sites have isolated OH groups nearby. Due to this kind of arrangement of the surface sites, interactions of adsorbed guest molecules with the surface OH groups are not determined by the acidity of the OH groups (weak acidic in nature), but rather by their neighboring Lewis acid sites. Adsorbed molecules such as pyridine on γ-alumina simultaneously interact with Lewis acid sites and their neighboring hydroxyls. If temperature is not high enough to desorb the molecules adsorbed on the Lewis acid sites, the disturbance of the molecules toward OH groups will remain. Such surface structures of OH groups and Lewis acid sites on γ-aluminas determine the chemistry occurred on alumina surfaces. Steaming of γ-Alumina with FCC Catalysts. Figure 1 shows transmittance FTIR spectra of FCC-silicated γ-alumina together with a steamed γ-alumina. Compared to the steamed γ-alumina, the spectrum of the FCC-silicated γ-alumina has an extra band around 1029 cm-1. The position of the band matches O-Si-O or Si-O-Si(Al) vibrations of silica, silicates, or aluminosilicates,27,28 and suggests that incorporation of silicate

Liu

Figure 2. Diffuse reflectance FTIR spectra (DRIFTS) of silicated γ-alumina (solid line) and steamed alumina (dotted line) in the hydroxyl spectral region.

species into the surface of alumina occurs. The reactions of silicate species with the surface of alumina can also be visualized from the decrease of the relative intensity of the alumina bands around 840 cm-1. The silicate species incorporated are more likely to be present in the form of orthosilicates, as proposed by Finocchio et al.4 for organosilane-silicated aluminas. Later examination of surface hydroxyls of the silicated γ-aluminas supports this understanding. Due to the characteristics of the in-situ FCC catalyst (good attrition property and zeolite particles grown on the wall of the caves in the microspheres), it is unlikely to occur that the zeolite particles are broken down and migrate out from the microspheres during steaming. The absence of the band around 450 cm-1 in the spectrum (see the mark in Figure 1) supports the view of absence of residual zeolite in the silicated γ-alumina samples, and suggests that all silicate species observed are solely from silication of the γ-alumina surface. Microprobe and XPS analyses of the silicated γ-alumina samples show about 3 wt % of silica incorporation. Figure 2 shows diffuse reflectance FTIR spectra (DRIFTS) of the silicated γ-alumina and steamed alumina in the hydroxyl spectral region. The spectrum of the silicated γ-alumina exhibits sharp bands at 3741 and 3728 cm-1 and a broad band around 3569 cm-1. The sharp band at 3741 cm-1 is due to surface SiOH groups, while the band at 3729 cm-1 is to Al-OH groups. The broad one is associated with acidic Si-OH-Al groups superimposed by hydrogen-bonded hydroxyls. Comparison with the spectrum of steamed γ-alumina shows that Al-OH hydroxyls having strong and medium-strong Lewis acid sites as neighbors are selectively removed by the incorporated silicate species (see disappearance of the bands at 3756 and 3688 cm-1). Disappearance of these two types of Al-OH hydroxyls suggests that silication of the alumina occurs preferentially on stronger Lewis acid sites. This understanding is further supported by studies of silication of γ-alumina containing transition-metal oxides (see below). Pyridine adsorption followed by IR measurements provides information about surface acidity and the types of hydroxyls which are disturbed by pyridine. Figure 3 gives difference spectra in the hydroxyl region of silicated γ-aluminas dehydrated at 450 °C for 1 h under flowing N2, followed by adsorption of pyridine at room temperature and desorption at 180 and 400 °C. Due to disturbance of adsorbed pyridine to hydroxyls, difference spectra give negative bands representing “removal” of hydroxyls by adsorbed pyridine. Desorption removes the adsorbed pyridine and recovers the hydroxyl bands, as a consequence, the negative band intensity decreases. From Figure 3, it is seen that except for Si-OH band at ∼3740 cm-1, two

Silication of γ-Alumina with FCC Catalysts via Steaming

J. Phys. Chem. B, Vol. 103, No. 14, 1999 2649 TABLE 1: IR Band Intensity of Pyridine Adsorbed on Bro1 nsted (1547 cm-1) and Lewis (∼1452 cm-1) Acid Sites sample V250 V250/FCC V250-Zn V250-Zn/FCC V250-Zn-Cu V250-Zn-Cu/FCC V250-Zn-Ni V250-Zn-Ni/FCC a

Figure 3. Difference spectra in the hydroxyl region of silicated γ-alumina dehydrated at 450 °C for 1 h under flowing N2 followed by adsorption of pyridine at room temperature (solid line) and desorption at 180 °C (dashed line) and 400 °C (dotted line).

Figure 4. Diffuse reflectance spectra in the pyridine absorption spectral region of silicated γ-alumina dehydrated at 450 °C for 1 h under flowing N2 followed by adsorption of pyridine at room temperature (solid line) and desorption at 180 °C (dashed line) and 400 °C (dotted line).

bands at 3613 and 3526 cm-1 are present in the spectra. These two bands can be attributed to newly formed acidic Si-OHAl groups. The broad spectral feature of these bands implies that the newly formed acidic Si-OH-Al groups on the surface experience a wide range of local environments. Similar spectral features were also observed for amorphous alumina-silica catalysts.29 In some cases, the bands are so broad that the OH groups are “IR-invisible”.30 Desorption at different temperatures (see Figure 3) shows a fast recovery of the 3613 cm-1 band, compared to the 3526 cm-1 band. This suggests that acid strength of the 3613 cm-1 sites is weaker than that of the 3526 cm-1 sites. Based on these results, the bands can be further assigned to the Si-OH-Al groups formed from reactions of silicate species with Lewis acid sites having different acid strength: the 3526 cm-1 band is associated with the strong Lewis acid sites, while the 3613 cm-1 band is with the medium-strong Lewis acid sites. Examination of pyridine spectral region confirms the assignment made for the two Si-OH-Al bands (Figure 4). A band at ∼1540 cm-1 which represents pyridinium ions is observed from the spectra. The formation of pyridinium ions in the sample indicates that the newly formed Si-OH-Al groups are indeed acidic, and the acidity of them is strong enough to donate their H+ to the adsorbed pyridine. However, in comparison with H+form zeolites and the HY component of the FCC catalyst, the acidity of the newly formed Si-OH-Al groups is weaker. At 400 °C (see Figures 3 and 4), a complete removal of pyridinium ions (Figure 4) and a simultaneous recovery of hydroxyls (Figure

I (1547 cm-1)

I (∼1452 cm-1) 1.33 0.72 1.45 1.20 2.13 1.78 1.48 1.22

0.10 0.03 0.02 0.04

∆Ia 0.61 0.25 0.35 0.26

-1

Intensity difference for Lewis (∼1452 cm ) acid sites.

3) occur for the silicated aluminas, while for zeolites, only partial removal of pyridinium ions is possible. Measurements of Lewis acidity of the silicated aluminas provide evidence supporting the understanding that silication of alumina occurs on the Lewis acid sites. The acidity data for the steamed γ-alumina and silicated γ-alumina given in Table 1 show a decrease of Lewis acidity of the silicated γ-alumina. The disturbance of Si-OH groups by adsorbed pyridine is interesting. In the literature,31 this disturbance is generally attributed to hydrogen bonding between Si-OH and physisorbed pyridine, because it is well-known that Si-OH groups are nonacidic in nature. However, recent studies13,29 have shown that the disturbance of Si-OH groups by adsorbed pyridine may also be due to close proximity of these Si-OH groups to Lewis acid sites. Pyridine adsorbed on the Lewis acid sites can simultaneously interact with their neighboring Si-OH groups. The disturbance of the Si-OH groups is therefore determined by the interactions of pyridine with the Lewis acid sites, but not by hydrogen-bonding of pyridine with the Si-OH groups. The presence of the disturbance after 180 and 400 °C desorption demonstrates this. For the silicated aluminas, a similar phenomenon was observed. Therefore, a similar mechanism for the disturbance of Si-OH groups should follow. The SiOH groups are located close to Lewis acid sites, and the pyridine adsorbing on the Lewis acid simultaneously interacts with these SiOH groups. The above-mentioned results have demonstrated that silication of alumina occurs when steaming a mixture of alumina and FCC catalyst microspheres. The silicate species migrate out from the FCC catalyst miscrospheres to the alumina surface. The silicate species selectively react with the stronger Lewis acid sites of alumina, forming Si-OH-Al hydroxyls. The Si-OH-Al hydroxyls are acidic enough to donate H+ to the adsorbed pyridine, but their acid strength is not as strong as those present in FCC catalysts and H+-zeolites. γ-Alumina Containing Transition-Metal Oxides. Figure 5 shows transmittance FTIR spectra of framework vibrations of calcined γ-alumina and steamed Zn2+-γ-alumina. The spectrum of the calcined γ-alumina (the dotted one in Figure 5) exhibits bands at 795 and 597 cm-1 as well as a shoulder around 934 cm-1. Introduction of Zn2+ (10 wt %) onto the surface of γ-alumina generates a new band at 691 cm-1 and shifts the alumina bands. The band at 795 cm-1 is shifted to 815 cm-1, while that at 597 cm-1 is to 560 cm-1. The shoulder around 934 cm-1 is also shifted and appears around 906 cm-1. From the spectral changes it is clear that the new band at 691 cm-1 belongs to Zn2+-associated species. Comparison with the spectrum of ZnO given in the literature32 excludes the presence of ZnO as a separate phase in the sample. It is therefore concluded that Zn2+ is incorporated into alumina as surface species. The other samples containing Zn2+/Cu2+ and Zn2+/ Ni2+ gave similar IR spectra to the Zn2+-γ-alumina (spectra

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Liu

Figure 5. Transmittance FTIR spectra of framework vibrations of calcined γ-alumina (dotted line) and steamed Zn2+-γ-alumina (solid line).

Figure 6. Diffuse reflectance spectra in the hydroxyl region of the samples containing transition-metal ions and starting γ-alumina. Solid line, starting γ-alumina; dashed line, Zn2+-γ-alumina; dotted line, Zn2+/ Cu2+-γ-alumina; dashed and dotted line, Zn2+/Ni2+-γ-alumina.

not shown), indicating that a further introduction of Cu2+ and Ni2+ ions (1.25 wt %) does not disturb the surface as significantly as the Zn2+ does. UV-vis spectroscopic studies of the Zn2+/Cu2+- and Zn2+/Ni2+-γ-alumina samples show an intense band at 234 nm and a broad band above 650 nm for the Cu2+-containing sample, and bands at 361, 588, and 629 nm for the Ni2+-containing sample. From the spectral features of these samples, it is known that Cu2+ and Ni2+ are present in the isolated ion form, and octahedrally coordinated for Cu2+, and octahedrally and tetrahedrally coordinated for Ni2+.33-34 Figure 6 shows diffuse reflectance IR spectra in the hydroxyl region of the samples containing these transition-metal ions together with the spectrum of starting γ-alumina. The spectrum of the alumina has bands at 3763, 3729, 3676, and 3563 cm-1, corresponding to isolated, bridged, triply bridged, and hydrogenbonded Al-OH hydroxyls, respectively.15-20 Introduction of the transition-metal ions to the alumina uniformly decreases the band intensity of all types of hydroxyls, indicating that transition-metal ions do not interact specifically with any type of hydroxyls. Subtle changes observed from the spectra of the samples due to transition-metal ions are the resolution and the splitting of the isolated Al-OH band around 3763 cm-1 (two bands are observed at 3768 and 3757 cm-1). The transitionmetal ions play a “dilution” role in the samples. Similar to γ-aluminas (see Figure 1), steaming of the transition-metal-containing γ-alumina samples with FCC catalyst microspheres introduces silicate species onto the surface. The IR spectra of framework vibrations also show a new band around 1030 cm-1 (Figure 7), as seen for the samples without transition-

Figure 7. Transmittance FTIR spectra of framework vibrations of the samples containing transition-metal ions. (A) Zn2+-γ-alumina; (B) Zn2+/Cu2+-γ-alumina; (C) Zn2+/Cu2+-γ-alumina. Solid line, silicated sample; dotted line, starting sample.

metal ions. The silication occurs for all samples, regardless of the nature of the transition-metal ions. Comparing the spectra of the samples steamed with and without FCC catalyst, it is seen that the relative band intensity at 818 cm-1 is decreased after being steamed with the FCC catalyst. Effects of transition-metal ions on silication of the γ-alumina surface were examined by comparing the samples with the silicated alumina. The band at ∼560 cm-1 of alumina was taken to be an internal reference owing to its insensitivity toward the treatment. The intensity ratios of the silicate band to the alumina band for all samples containing transition-metal ions were compared with that of alumina. With transition-metal ions present, only 69, 64, and 75% of silication were achieved in the samples containing Zn2+, Cu2+/Zn2+, and Ni2+/Zn2+ respectively. The lower uptakes (∼25-30% lower) of silicate species by the transition-metal-ion-containing samples suggest that ∼25-30% of the alumina surface is covered by the transition-metal ions which exclude silicate species. To know whether the silication occurs similarly as seen in the case of pure alumina, the hydroxyl spectral region was examined. Figure 8 shows DRIFTS spectra of the samples after

Silication of γ-Alumina with FCC Catalysts via Steaming

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Figure 8. DRIFTS spectra of the samples after dehydration at 450 °C for 1 h under flowing N2. Solid line, starting γ-alumina; dashed line, Zn2+-γ-alumina; dotted line, Zn2+/Cu2+-γ-alumina; dashed and dotted line, Zn2+/Ni2+-γ-alumina.

Figure 9. Diffuse reflectance spectra in the pyridine absorption spectral region of silicated γ-alumina containing Zn2+/Cu2+ transition-metal ions. Room temperature (solid line); desorbed at 180 °C (dashed line); at 400 °C (dotted line).

dehydration at 450 °C for 1 h under flowing N2. The spectrum of Zn2+-alumina (solid line spectrum) shows bands at 3763, 3750, 3729, 3676, and 3563 cm-1. As mentioned before, the band at 3763 cm-1 is associated with isolated Al-OH groups, that at 3729 cm-1 with bridged Al2)OH groups, and the band at 3676 cm-1 is due to triply bridged Al3≡OH groups. The band around 3563 cm-1 is associated with hydrogen bonded OH groups. Steaming with the FCC catalyst shows the following spectral changes: (1) the isolated Al-OH bands disappeared; (2) the triply bridged (Al)3tOH band was significantly depressed, (3) a new band around 3740 cm-1 appeared, and (4) the hydrogen-bonding bands at 3560 cm-1 were broadened. The new 3740 cm-1 band is assigned to Si-OH groups according to its position. The removal or decrease of the isolated Al-OH and triply bridged (Al)3tOH groups indicates that silication still occurs selectively on the more acidic Lewis acid sites of alumina. The remaining of the bridged Al2)OH groups after silication again confirms this. The “removal” of the Al-OH groups, which are close to the strong and medium Lewis acid sites, by silicate species proceeds through hydrogen bonding, but not through direct reactions of silicate species with them. A direct experimental evidence for this is the broadening of the hydrogen-bonding bands observed in the spectra (see the bands around 3500 cm-1 in Figure 8). Another argument for this is that, if a direct condensation reaction occurred between alumina surface OH and silicate species, the reaction would follow the acidity sequence of the Al-OH groups and would not selectively react with the most acidic and the most basic Al-OH groups on the surface. Compared to the steamed alumina/FCC sample, the samples containing transition-metal ions (see Table 1) show low Bro¨nsted acidity (Figure 9). The low Bro¨nsted acidity could be caused by two factors: (1) decrease in degree of silication due to coverage of the surface by transition-metal ions, and (2) charge balancing by transition-metal ions. The contribution from the first can be known from the effects of transition-metal ions on silication discussed before (∼25%), while that of the second can be estimated from the difference between the theoretically calculated and the actual measured. From the estimation it is clear that the decrease of the Bro¨nsted acidity is mainly due to the presence of transition-metal ions as charge balancing cations to the Al-O--Si linkages. The remaining Bro¨nsted acidity observed (see the band around 1540 cm-1 in Figure 9) is probably due to the protons generated only from hydrolysis of the cations. Most of them could be removed at 180 °C desorption. The Ni2+/Zn2+-containing sample shows slightly

higher Bro¨nsted acidity, which is consistent with its slightly higher silicon content. Silication Mechanism. From the results given above, it is clear that silication of alumina surfaces occurs upon steaming a mixture of γ-alumina and FCC catalyst. The silica source for silication must come from the FCC catalyst. The observation proves further what has been reported in the literature.10-12 Under steaming conditions, the silica component in the FCC microspheres becomes volatile. The partial pressure of silica in the gas phase has been estimated to be within a few ppm range, depending on temperature and pressure.11 The existing state of silica in the gas phase has been found to be monomeric and probably present in the form of Si(OH)4.10 Such a silica species is the reagent for silication of the alumina surfaces. γ-Alumina surface contains hydroxyls and Lewis acid sites. Pyridine adsorption followed by IR measurements, and other techniques15-25 has demonstrated the presence of them. However, under steaming conditions, whether Lewis acid sites are still present or not is unknown. It is not possible to measure acidity of the surface in situ using probe molecules, because the temperature used for steaming is too high, 788 °C. Therefore, the information can only be obtained from indirect ways such as chemical reactions.35 To answer this question, we carried out two experiments in this study. The first was an examination of alumina surface hydroxyls before and after steaming. The results show that the alumina surface is dehydroxylated under steaming condition, and cooling the sample to room temperature does not reinstate the surface. The surface of the steamed sample is similar to that of a high-temperature calcined one (Figure 10). Although the measurements given in Figure 10 are not in situ, the results obtained indeed suggest that dehydroxylation has occurred and Lewis acid sites therefore must be present under the steaming conditions. The second experiment was a thermal gravimetric analysis of alumina at 450° and 780 °C in the absence and presence of water vapor (90% room-temperature humidity). The idea was to see weight change at these temperatures under dry and wet conditions. The results show that there is very little weight gain, ∼1% for 450 °C and ∼0% for 780 °C when the sample atmosphere was switched from dry to wet. This observation therefore suggests that at temperatures well above the water desorption temperature, surface dehydroxylation is governed by temperature, although water vapor is present in the system. The observed silication reaction itself on the surface of alumina could also be considered to be direct evidence for the presence of Lewis acid sites under steaming conditions.

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Liu in charge balancing for the newly formed Si-O--Al sites; (5) Due to the nature of the reactions, degree of silication of the alumina surface is limited by the number of strong and mediumstrong Lewis acid sites present on the surface. Acknowledgment. The author thanks Dr. Glen Dodwell and Ms. Michele Frymoyer for providing the samples, and Ms. Gail Hodge for the TGA measurements. Dr. Mingting Xu is thanked for his useful discussion. References and Notes

Figure 10. Diffuse reflectance spectra in the hydroxyl spectral region of γ-alumina calcined at 700 °C and steamed at 788 °C.

After the volatile silicate species get on to the surface of alumina, they are seeking reaction sites on the surface. The orthosilicate selectively reacts with the strong and mediumstrong Lewis acid sites on the surface, forming Al-OH-Si linkages. Once the orthosilicate occupies the Lewis acid site, its other OH groups may interact with the neighboring Al-OH groups, forming hydrogen bonding. The hydrogen-bonding reactions lead to “removal” of the Al-OH bands at 3763 and 3676 cm-1, as observed in the IR spectra. The hydrogen bonds so formed give absorption bands in the hydrogen-bonding spectral region. The presence of Si-OH groups (the 3740 cm-1 band) suggests that not all the Si-OH groups in the orthosilicate are consumed in the formation of Al-OH-Si linkages and hydrogen bonding. Some of them are still present in an isolated Si-OH form. Because of the nature of the reactions of orthosilicate with surface Lewis acid sites, a limit would be expected for silication of alumina surface due to the finite number of strong and medium-strong Lewis acid sites on the surface. Indeed, chemical analysis of the silicated samples shows a saturation phenomenon. approximately 3 wt % silicon is the maximum which can be achieved on the silicated aluminas under present steaming conditions. The presence of a limit proves the selective reaction nature of silication of alumina, and also the existing state of silicate species, i.e., the orthosilicate. Conclusions The studies of framework vibrations, surface hydroxyls, and pyridine acidity of the silicated aluminas and aluminas containing transition-metal ions obtained by steaming with FCC catalyst microspheres give the following results: (1) Steaming γ-alumina with FCC catalyst microspheres leads to silication of the surface; (2) Orthosilicate species interact with strong and medium-strong Lewis acid sites, forming Al-OH-Si Bro¨nsted acid sites; (3) The silicate species simultaneously interact with their neighboring isolated Al-OH and triply bridged Al3-OH groups, forming hydrogen bonding; (4) Transition metal ions such as Zn2+, Cu2+, and Ni2+ do not preferentially interact with the surface OH groups on alumina but cover the surface. Due to the coating of the surface by these transition-metal ions, the degree of silication is decreased, indicating that silication cannot occur on the transition-metal phases. The transition-metal ions play a role

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