40 Volume 53, Number 1, 1999 APPLIED SPECTROSCOPY0003-7028 / 99 / 5301-0040$2.00 / 0

q 1999 Society for Applied Spectroscopy

FT-IR Spectroscopic Study on Basicity of Alumina withAdsorption of Boric Acid Trimethyl Ester

JIANKE LIU, PINLIANG YING, QIN XIN, and CAN LI *State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110,

Dalian 116023, China

Surface basicity of alumina and modi® ed alumina has been studied

by FT-IR spectroscopy using boric acid trimethyl ester (BATE) as

the probe molecule. The surface basic sites on alumina can beprobed by the adsorption of BATE and coadsorption of BATE and

CO 2. BATE is found to interact with both types of basic sites on

alumina: basic hydroxyls and surface oxygen anions. A general in-teraction model of BATE with basic sites on metal oxide is pro-

posed.

Index Headings: FT-IR spectroscopy; Adsorption; Surface basicity;

Boric acid trimethyl ester (BATE); Carbon dioxide; Alumina.

INTRODUCTION

g -Alumina is widely used as a catalyst or the supportof catalysts, and its surface acid/base property plays animportant role in its practical applications. Many earlierstudies have been focused on the surface acidity of alu-mina. CO and pyridine were the frequently used IRprobes for its acidity characterization.1±5 However, lesswork has been devoted to characterizing the basicity ofalumina. CO2 adsorption has been used to study the ba-sicity of alumina,6,7 and the intensity of the IR band at1230 cm 2 1 due to bicarbonate species has been taken asa measure of the number of basic OH groups. It is be-lieved that Lewis basic sites on alumina can be producedafter degassing at high temperatures,8 but CO2 adsorptionis unsuitable for the titration of surface basic oxygen an-ions because the neighboring metal cations (Lewis acidsites) could also participate in the adsorption process; be-sides, CO 2 cannot adsorb on the weak Lewis basic sites.In our previous work, boric acid trimethyl ester (BATE)was found to be a suitable probe molecule to characterizethe basic sites of oxide catalysts. 9 In this investigation,we use in situ FT-IR spectroscopy and the probe mole-cules, BATE and CO 2, to probe the surface basicity of g -alumina. BATE adsorption of g -alumina modi® ed by thealkali metal cation and F 2 anion was also to identify dif-ferent surface basic sites.

EXPERIMENTAL

g -Al2O3 with a BET surface area of 230 m 2 /g and atotal pore volume of 0.60 cm3/g was obtained from RhoÃne-Poulenc. F 2 /Al2O3 sample was prepared from g -Al2O3 ex-changed with NH4F (0.1 M) at 298 K and 333 K for 3days, respectively, and then washed thoroughly with ex-cess deionized water in order to remove the residual F 2

ions. Na2O/Al2O3 with a loading of Na 1 from 5 to 10%(mol) was obtained by impregnating g -Al2O3 with an

Received 13 April 1998; accepted 8 September 1998.* Author to whom correspondence should be sent.

aqueous solution of NaOAc. These samples were pressedinto self-supporting wafers, each with a weight of ; 20mg, for IR studies.

First, a sample wafer was outgassed at 773 K; then itwas cooled down to room temperature for adsorption.Boric acid trimethyl ester was introduced into an IR cell,and IR spectra were recorded on a double-beam FT-IRspectrometer (Perkin-Elmer 1800) equipped with a liquidnitrogen-cooling HgCdTe (MCT) detector, with fourscans at a resolution of 4 cm 2 1. All absorbance spectrawere obtained by subtracting corresponding backgroundspectra at room temperature.

RESULTS AND DISCUSSION

The IR spectrum of boric acid trimethyl ester in thegas phase is comprised of several bands at 2966, 2860,1490, 1360, and 1030 cm 2 1. The IR bands at 1490, 1360,and 1030 cm 2 1 can be assigned to the deformation modeof methyl, stretching vibration of B±O, and stretchingvibration of C±O, respectively, and the IR bands at 2966and 2866 cm 2 1 can be assigned to the C±H stretchingvibrations of methyl.

Figure 1 shows the IR spectra of BATE adsorbed atroom-temperature on well-outgassed Al2O3. Three dis-tinct bands appear at 1490, 1410, and 1285 cm 2 1 whenBATE is introduced into the IR cell at room temperature(Fig. 1a). The bands at 2960 and 2857 cm 2 1 related toC±H vibrations are similar to those of BATE in the vaporphase. A small shoulder band at 1467 cm 2 1, together withsome weak bands around 1050 cm 2 1, is due to methoxidespecies10,11 that may be produced from the dissociatedadsorption of BATE on some strong basic sites. The num-ber of strong basic sites involved in the dissociated ad-sorption of BATE seems to be a small portion of thesurface basic sites, because the bands at either 1467 or1050 cm 2 1 are relatively much weaker. The appearanceof the reverse bands at 3740, 3730, and 3687 cm 2 1 in-dicates that both tetrahedral Al±OH and octahedral Al±OH are perturbed by the adsorbed BATE. The bands at1410 and 1285 cm 2 1 are assigned to the adsorbed BATEon surface basic sites of g -alumina because BATE is atypical Lewis acid. Various surface basic sites such ashydroxyls and oxygen anions are all possible candidatesites for BATE adsorption. The following experimentswere performed to further clarify the interaction of BATEwith different basic sites on the g -alumina surface.

The basic hydroxyl groups on oxide surface can beselectively exchanged by NH 4F at room temperature.12 IRspectra of adsorbed BATE on F-exchanged alumina areshown in Fig. 1b and 1c. Compared with the spectrumof BATE adsorbed on alumina alone, the reverse band at

APPLIED SPECTROSCOPY 41

FIG. 1. IR spectra of adsorbed BATE on (a) Al2O3; (b) Al2O3 ex-changed with NH 4F at 298 K; (c) Al2O 3 exchanged with NH 4F at 333K; (d ) 5% Na2O/Al2O3; (e) 10% Na2O/Al2O3. (All the spectra includingthe background are difference spectra between the spectra before andafter the adsorption of probe molecule.)

FIG. 3. IR spectra of CO 2 adsorbed on (a) Al2O3; (b) 1 Torr BATE;(c) difference spectrum of b and a. (All the spectra including the back-ground are difference spectra between the spectra before and after theadsorption of probe molecule.)

FIG. 4. IR spectra of BATE adsorbed on (a) Al2O3; (b) 20 Torr (1 Torr

5 133.3 N m 2 2) CO2 and then outgassed at room temperature for 15min; (c) difference spectrum of b and a. (All the spectra including thebackground are difference spectra between the spectra before and afterthe adsorption of probe molecule.)

FIG. 2. Interaction of BATE with different surface sites of a metaloxide, where R 5 CH3 and M 5 metal cation. (I ) Weak adsorption; theplanner structure is retained. (II ) Strong adsorption; the planner struc-ture is distorted. (III ) Interacted with basic hydroxyl. (IV ) Interactedwith basic site and acidic hydroxyl via hydrogen bonding.

3740 cm 2 1 disappears when Al2O3 is exchanged withNH4F at room temperature (Fig. 1b), and the band around3730 cm 2 1 becomes much weaker while the band around3690 cm 2 1 remains unchanged. For g -alumina exchangedwith NH4F at 333 K (Fig. 1c), only the reverse band at3693 cm 2 1 is observed. This result is in accordance withthe results reported by Berteau et al.13 showing that sur-face basic hydroxyls were suppressed after F 2 exchange.In the B±O vibration region, the strong bands at 1410and 1285 cm 2 1 have no apparent change after basic hy-

droxyls are exchanged with F 2 ions, so these two bandscan be assigned to BATE interacting mainly with surfaceoxygen anions or some acidic hydroxyls.

The addition of sodium oxide can suppress and neu-tralize the surface acidic hydroxyl groups on g -Al2O3, asreported by Shen et al.14 Figure 1d shows IR spectra ofBATE adsorbed on Na2O/Al2O3. In the vibrational regionof surface hydroxyl, the reverse band at 3685 cm 2 1 dueto acidic hydroxyls attenuates when Al 2O3 doped with 5%Na2O (Fig. 1d) is used, and basic hydroxyls with IRbands at 3752 and 3739 cm 2 1 are perturbed too. For high-

42 Volume 53, Number 1, 1999

er loading of Na2O (10%) only one weak band appearsat 3739 cm 2 1 (Fig. 1e). It is obvious that the addition ofNa2O dramatically suppresses the surface hydroxyls ofalumina. The acidic hydroxyls disappear for loadings ofNa2O up to 10%. As with the IR spectrum of BATEadsorbed on alumina exchanged with NH 4F, IR bands at1410 and 1285 cm 2 1 in the B±O vibrational region re-main unchanged after the alumina surface is modi® ed byNa2O. The only difference is that the intensity of theshoulder band at 1467 cm 2 1 increases with the loadingamount of Na2O. This observation suggests that the sur-face basic oxygen anions are the main active sites forBATE adsorption.

IR spectra of BATE adsorbed on Na2O/Al2O3 corrob-orate the above speculation that the strong bands at 1410and 1285 cm 2 1 in the B±O vibration region are attributedto BATE that has interacted mainly with surface oxygenanions (Lewis base). These coordinatively unsaturatedoxygen anions serve as Lewis basic sites, 7 and the co-ordination of the surface oxygen anions to the boron atomof BATE distorts the planar con® guration of the adsorbedBATE molecule. As a result, the IR band at 1360 cm 2 1

is split into two bands at 1410 and 1285 cm 2 1 becausethe symmetry of the molecule is lowered. According tothe work of Shen et al.,14 a low loading of potassiumoxide on alumina is not effective enough to generate thestrong basic sites; even the number of strong basic sitesdoes not increase very much with a loading of 5% K2O.Thus the Na2O loading in the range of 5±10% may neu-tralize strong acid sites on g -Al2O3, but is not enough togenerate new strong basic sites; accordingly the splittingbands in the B±O vibration region are almost the samefor the different samples. The interaction of BATE withthe basic sites on alumina surface can be schematicallydescribed, as shown in Fig. 2. It is believed that the in-teraction model is common to the BATE adsorption onmost metal oxides.

To differentiate the basic sites on g -alumina, we per-formed coadsorption of BATE and CO2 at room temper-ature. IR spectra of adsorbed BATE and CO 2 are dis-played in Figs. 3 and 4. When CO 2 is introduced, IRbands due to bicarbonate species 6 appear at 1653, 1438,and 1227 cm 2 1 (Fig. 3a). The appearance of the reverseband at 3772 cm 2 1 due to the most reactive and basichydroxyl on g -Al2O3

15 indicates that the basic hydroxylis perturbed. Subsequently, with the addition of BATE,the band at 1653 cm 2 1 of adsorbed CO2 apparently atten-uates; however, IR bands of adsorbed BATE similar tothat of adsorbed BATE alone are observed, suggestingthat the BATE is a stronger Lewis acid than CO 2.

Several weak bands can be observed at 1627, 1476,and 1372 cm 2 1 (Fig. 4c) when CO2 is introduced onto g -Al2O3 with preadsorbed BATE (Fig. 4b), but these bandsare absent for CO2 adsorption alone. Lamotte et al. re-ported similar bands for the coadsorption of CO2 andmethanol on g -alumina.16 They assigned these bands tomethyl carbonate-like species produced from CO 2 react-ing with surface methoxide. BATE may slightly disso-ciate on the Al2O3 surface even at room temperature, pro-ducing the methoxy-like species. The strong Lewis basicsites on the Al2O3 surface that are derived after evacua-

tion at high temperatures are responsible for the disso-ciated adsorption of BATE. The bands at 1442 and 1327cm 2 1 together with the broad band in the hydroxyl regioncan be attributed to surface bicarbonate species.

From the IR results of coadsorption of BATE and CO2

on Al2O3 it can be seen that there is a remarkable differ-ence between BATE and CO 2 as the probe molecule forcharacterizing the basicity. CO 2 interacts mainly with sur-face basic hydroxyls, producing bicarbonate species. Theinteraction between CO 2 and surface oxygen anions can-not be discerned clearly. The fact that adsorbed CO 2 onbasic hydroxyls can be replaced by the subsequent ad-mission of BATE (Fig. 3b) indicates that BATE interactswith basic hydroxyl more strongly than CO2 does. More-over, surface hydroxyls with weak basicity are also per-turbed to a certain extent by the BATE adsorption. Allthese results can be due to the stronger Lewis acidity ofBATE. In terms of the concept of Lewis acid and base,oxygen anions of surface hydroxyls are a Lewis base, soBATE can interact with these sites. In contrast to CO 2,BATE can probe both surface oxygen anions, i.e., Lewisbase sites and basic hydroxyls, and is capable of char-acterizing surface basic sites with strong, medium, andweak basicity.

CONCLUSION

In summary, with the use of FT-IR spectroscopy withthe adsorption of boric acid trimethyl ester, both surfaceoxygen anions and surface basic hydroxyls on aluminasurface can be probed. BATE can interact with basic sitesof strong, medium, and even weak basicity on alumina,while CO2 interacts mainly with strong basic surface hy-droxyls.

ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundationof China for Distinguished Young Scholars (No. 29625305).

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