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Journal of Molecular Structure 882 (2008) 24–29

A joint experimental–theoretical study on trimethylphosphineadsorption on the Lewis acidic sites present in TS-1 zeolite

Gang Yang a,b,*, Jianqin Zhuang c, Ding Ma a, Xijie Lan a, Lijun Zhou b,Xianchun Liu a, Xiuwen Han a, Xinhe Bao a

a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR Chinab Key Laboratory of Forest Plant Ecology, Northeast Forestry University, Ministry of Education, Harbin 150040, PR China

c Center for Drug Discovery and Departments of Pharmaceutical Sciences and Chemistry and Chemical Biology, Northeastern University, Boston, USA

Received 24 May 2007; received in revised form 2 September 2007; accepted 9 September 2007Available online 18 September 2007

Abstract

31P MAS NMR desorption experiments and density functional calculations were combined to study the interactions between TS-1zeolite and probe molecules. Two types of Lewis acidic sites at �34.2 and �32.0 ppm in the 31P MAS NMR spectra were identified,which correspond to the Ti(OSiO3)4 and (OSiO3)3Ti(OH) species with the Ti atoms at the T12 sites. Trimethylphosphine (TMP) waschemisorbed on the Lewis acidic sites with the adsorption energies calculated to be �6.2 and �19.6 kJ mol�1, respectively. On all theother T sites, TMP was physisorbed with slightly positive adsorption energies, in the range 0.82–5.92 kJ mol�1. The two types of Lewisacidic sites are responsible for the catalytic reaction of TMP oxidation, with their catalytic activities explained by structural and Mullikencharge analyses. It was also found that further hydrolysis of Si–O–Ti links to form (SiO)2Ti(OH)2 or (SiO)Ti(OH)3 species were notobserved in TS-1 zeolite.� 2007 Elsevier B.V. All rights reserved.

Keywords: Density functional; Catalysis; Lewis acid; MAS NMR; Zeolite

1. Introduction

Since its discovery in 1983 [1], titanium silicalite (TS-1)zeolite has attracted more and more attention in selectiveoxidation processes, which can be attributed to the highreactivity and mild conditions catalyzed by TS-1 zeolite[2,3]. Usually, the active sites in TS-1 zeolite were createdunder the presence of oxidative hydrogen peroxide(H2O2). A lot of experimental and theoretical works havefocused on this research topics, resolving the local struc-tures of active sites and reaction mechanisms of epoxida-

0022-2860/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.molstruc.2007.09.005

* Corresponding authors. Address: State Key Laboratory of Catalysis,Dalian Institute of Chemical Physics, Chinese Academy of Sciences,Dalian 116023, PR China. Tel.: +86 0411 84379528; fax: +86 041184694447.

E-mail addresses: theobiochem@gmail.com (G. Yang), xhbao@dicp.ac.cn (X. Bao).

tion and radical processes [4–12]. The active sites containa bidental coordinated peroxide complex of g2-TiOaObH[4,5,9,12], which catalyze the epoxidation reactions throughthe Oa insertion into the C@C double bonds of the olefinmolecules [5,12].

Recently, Corma et al. [13,14] found that the Lewisacidic centers in TS-1 zeolite can produce catalytic effectsas well. Using ab initio Hartree–Fock and density func-tional theory calculations, they [15] found that Ti-Beta zeo-lite shows higher Lewis acidity than TS-1 zeolite , and thateven for one specific Ti-incorporated zeolite, the Lewisacidity may change from one T site to another T site. In situ31P MAS NMR experiments were previously performed byus to study the in situ oxidation reaction of trimethylphos-phine (TMP) to form trimethylphosphine oxide (TMPO),concluding that this reaction was catalyzed and acceleratedby two types of Lewis acidic sites corresponding to the 31PMAS NMR peaks �34.2 and �32.0 ppm, respectively [10].

G. Yang et al. / Journal of Molecular Structure 882 (2008) 24–29 25

Till now, no descriptions of structural models were pro-vided in terms of geometric environments and chemicalreactivities of the two Lewis acidic sites, let along theirdynamic behaviors during catalytic reactions. With theaid of in situ 31P MAS NMR technique and ab initio den-sity functional methods, the questions posed above wereattempted to be tackled in the present work. We hope thatan in-depth understanding on Lewis acidities will beachieved through our efforts.

2. Experimental section

With a home-made device [9], the TS- zeolite sampleswere heated at 673 K for 20 hs (below 10�2 Pa). At roomtemperature, the adsorption was performed with exposureto saturated TMP vapor for 30 min (from Acros Organics).After being evacuated for 30 min and removing the physi-sorbed TMP on the zeolite surfaces, the samples were filledin situ into the NMR rotor without exposure to air. All theNMR measurements were carried out on Bruker DRX-400spectrometer with a BBO MAS probe-head, using 4 mmZrO2 rotors. 31P MAS NMR spectra with high-power pro-ton decoupling were recorded at 161.9 MHz, using a 2.0 lspulse, a 2 s repetition time and 2048 scans. The chemicalshifts were referenced to 85% H3PO4.

3. Calculational details

As displayed in Fig. 1a, the cluster models of TS-1 zeo-lite contain 17 T sites, which are much larger than the usu-ally adopted 5 T ones [16–18]. In order to retain the localstructures of TS-1 zeolites, the boundary Si atoms were sat-urated with H atoms and fixed in their Cartesian coordi-nates. When the O atoms are situated between twoterminal SiHx groups (x = 1, 2 or 3), their Cartesian coor-dinates will also be fixed. According to neutron power dif-fraction results [6,7], there are seven T sites; i.e., T3, T6, T7,T8, T10, T11 and T12, are possible Ti locations in TS-1

Fig. 1. Configurations of TMP chemisorbed on the two

zeolite whereas the other T sites are excluded from Ti occu-pations. Combined with the preliminary theoretical results[19] obtained under DMOL3 program [20], the present the-oretical work will focus on two T sites; i.e., T7 and T12.

Density functional theory (DFT) calculations were per-formed under Gaussian98 program [22]. Hybrid B3LYPexchange correlation functional [23,24] was employed, forit provides good description of reaction profiles, particu-larly geometries and reaction heats. The commonly used6-31G* basis was applied to all the elements except Ti.To describe the element of Ti, the core electrons were rep-resented by LANL2DZ effective core potential (ECP), withthe valence electrons by LANL2DZ basis as treated in ourprevious work [9,11]. Frequency calculations were per-formed on the TMP/TS-1 adsorption systems, ensuringthat these structures are located at the energy minima onpotential energy surface (PES). To distinguish the twoLewis acids, 31P NMR tensors were computed using gaugeindependent atomic orbitals (GIAO) and density func-tional theory [25,26].

The interacting strengths between TS-1 zeolite andadsorbed TMP molecules can be estimated by adsorptionenergy (Ead) as shown by the equation:

Ead ¼ EðTS-1=TMPÞ � EðTS� 1Þ � EðTMPÞ ð1Þwhere E(TS-1/TMP) represents the energy of TS-1 clusteradsorbed with TMP, whereas E(TS-1) and E(TMP) thoseof isolated TS-1 cluster and TMP, respectively.

4. Results and discussion

4.1. 31P MAS NMR desorption spectra

The samples of TS-1 zeolite were prepared and charac-terized as reported previously [9,10]. The Si/Ti ratio equals41.2 and only three elements of Si, Ti and O are present inour TS-1 samples. Moreover, fine crystallinity of MFI typewas obtained.

types of Lewis acidic sites present in TS-1 zeolite.

Fig. 2. In situ 31P MAS NMR desorption spectra of TMP adsorption onTS-1 zeolite Fresh adsorption at room temperature (a); Desorption for30 min at 333 K (b), 353 K (c), 373 K (d), 393 K (e), 413 K (f),respectively.

26 G. Yang et al. / Journal of Molecular Structure 882 (2008) 24–29

Fig. 2 shows the 31P MAS desorption spectra of TMPadsorbed on TS-1 samples. The peaks at �59.8 and�4.8 ppm were assigned to the physisorbed TMP and ionic[(CH3)3P-H]+ species, respectively [9,10]. As the desorptiontemperature increases, the peak to TMP physisorptionquickly descends and vanishes at ca. 373 K, implying theweakest interaction between physisorbed TMP and TS-1zeolite; however, the peak to the ionic [(CH3)3P-H]+ speciesremains almost invariable, which can be caused by twopossible reasons: (a) Very strong mutual interaction existsbetween the ionic species and TS-1 zeolite. The desorptiontemperature was raised up to 413 K, with only slightchanges observed for this peak. Accordingly, this presump-tion is untenable; (b) The 31P MAS NMR chemical shift ofthe ionic species doesn’t have much relationship with thecarriers. It can be seen that in the [(CH3)3P-H]+ ion, theP atom is deeply buried among the three methyl groupsand the hydrogen atom, making the foreign molecules arenot easily accessible to the P atom. Therefore, the 31PMAS NMR chemical shift of the ionic species is insensitiveto the environments. This presumption is feasible and sup-ported by previous 31P NMR experimental data on differ-ent carriers [9,27]. Besides, another two resonancesappeared at �34.2 and �32.0 ppm in the 31P MAS NMRspectra (Fig. 2) and were attributed to the Lewis acidic sites[10]. These two types of Lewis acids were designated as TiLewis Acid I (abbreviated as TiLAI) and Ti Lewis Acid II(abbreviated as TiLAII), respectively. It was found that the

Table 1Distances, Mulliken charges and adsorption energies for TMP/TS-1 zeolite sy

T3 site T6 site T7 site

Ti–P/�A0

5.120 8.601 7.954 8.336 5.611

<Ti–O> /�A0

1.788 1.789 1.788 1.789 1.787 1.789 1.800 1.797q(Ti)/|e| 1.147 1.146 1.150 1.143 1.153 1.142 1.113 1.113q(P)/|e| 0.326 0.324 0.327 0.324 0.327Ead/kJ mol�1 2.39 0.82 1.19 0.47 5.82

a As to each T site, the data before TMP adsorption are in the first column

31P chemical shift is more negative on TiLAI than onTiLAII by 2.2 ppm. These two peaks gradually decreasewith the increase of the desorption temperature or time,and the one of TiLAI (at �34.2 ppm) drops more quicklythough slightly than that of TiLAII (at �32.0 ppm), sug-gesting the stronger mutual interaction between TMPand TS-1 zeolite in the latter.

4.2. Density functional calculations on the Ti7 site

The preliminary theoretical results [19] obtained byDMOL3 program revealed that TMP is physisorbed onthe Ti7 site in TS-1 zeolite with slightly positive adsorptionenergies (Table 1). It implies that this site does not showLewis acidity toward TMP whereas does towards otherprobe molecules such as H2O or NH3 [8,19]. The furthergeometry-optimization under the default theoreticalmethod obtained a Ti7-P distance at 5.591A, with thestructure confirmed by frequency analysis at the stationarypoint. Similar to the results of DMOL3 program [19], theadsorption energy was also calculated to be slightly posi-tive with the exact value of 2.91 k mol�1. Other probe mol-ecules along with TMP were chosen to further theunderstanding of Ti7 site. From the orbital analysis itwas found that whether physisorbed or chemisorbed, theLowest Unoccupied Molecular Orbital (LUMO) and High-est Occupied Molecular Orbital (HOMO) are mainly local-ized on the TS-1 clusters and adsorbed probe molecules,respectively, with TMP illustrated in Fig. 3. Accordingly,it is feasible to adopt the HOMO energies of probe mole-cules to characterize their Lewis basicities and the LUMOenergies of TS-1 clusters to characterize their Lewis acidi-ties as suggested by Sastre et al. [15]. The energy gapsbetween the two orbitals can be used to predict how theTS-1 zeolite interacts with probe molecules, see theequation below

ELUMO�HOMO ¼ ELUMOðTS-1Þ � EHOMOðProbe moleculeÞð2Þ

where ELUMO(TS-1) and EHOMO(Probe molecule) refer tothe LUMO and HOMO energies of the isolated TS-1clusters and probe molecules, respectively.

Fig. 4 provides the ELUMO-HOMO data of several probemolecules. The ELUMO-HOMO value of NH3 or PO(CH3)3

(i.e., TMPO) is smaller than that of NF3 or POCl3, respec-tively. For molecules of similar structures, smaller energy

stems obtained under DMOL3 program [19]a

T8 site T10 site T11 site T12 site

5.280 7.514 5.311 3.036 5.441

1.800 1.800 1.814 1.815 1.809 1.809 1.802 1.825 1.8021.216 1.217 1.153 1.153 1.176 1.179 1.114 1.011 1.113

0.330 0.327 0.345 0.443 0.3335.48 1.19 5.92 �3.31 3.87

.

Fig. 3. HOMO and LUMO sketches for TMP adsorbed on TS-1 clusters.

0.14

0.16

0.18

0.20

0.22

0.24

0.26

0.28

EL

UM

O-H

OM

O /a

.u.

On T7 site On T12 site

P(CH3)

3NH

3PO(CH

3)

3PCl

3POCl

3NF

3

Probe molecule

Fig. 4. Prediction of adsorption behaviors using the energetic gapsbetween LUMO of TS-1 clusters and HOMO of probe molecules.

G. Yang et al. / Journal of Molecular Structure 882 (2008) 24–29 27

gaps suggest stronger mutual interactions and thus thechemisorption processes are favored. As confirmed by the-oretical results, NH3 and PO(CH3)3 are chemisorbedwhereas NF3 and POCl3 physisorbed on TS-1 clusters.TMP is an exception, however, whose energy gap is muchsmaller than that of the chemisorbed PCl3. The large size ofTMP molecule can be the reason for TMP physisorptionon the Ti7 site, since it is chemisorbed on the Ti12 site (vide

post) where the energy gap is very close to that of the Ti7site (Fig. 4). It should be noted here that the interactionsbetween TS-1 and probe molecules are much more complexthan expected; i.e., the predication from the energy gapmay fail, and therefore a thorough investigation is underway.

4.3. Density functional calculations on T12 site

T3, T6 and T12 sites are located in the intersections ofMFI-type zeolite, and the TMP molecules can approachthese T sites in two directions. Accordingly, there are twoTi-P distances for these T sites (Table 1). As to the Ti12 siteadsorbed with TMP, the two Ti12-P distances under thedefault theoretical methods are equal to 2.978 and5.092 A, respectively. The former is characteristic of chemi-sorption with the Ti12 atom showing Lewis acidity to TMPwhereas the latter is a physisorption process. Their adsorp-tion energies according to Eq. (1) were calculated at �6.2and 2.9 kJ mol�1, respectively. During the chemisorptionprocess, the TMP molecule and TS-1 cluster were severelydisturbed whereas on the contrary during the physisorptionprocess. As to the chemisorbed structure, the average Ti–Obonds were elongated by approximately 0.023 A, similar tothe situations of TS-1 clusters adsorbed with NH3 or H2O[8,19]. The symmetry of TiO4 tetrahedron was alsodestroyed, which can be deduced from the sum of

Table 2Parameters of TMP adsorbed on TS-1 clusters as well as of isolated TMPand TMPO molecules

TMP Physisorptionat Ti7 site

Physisorptionat Ti12 site

TiLAI TiLAII TMPO

P–C/�A0

1.868 1.864 1.862 1.852 1.847 1.8351.868 1.866 1.864 1.854 1.851 1.8351.868 1.867 1.866 1.854 1.854 1.835

\CPC/o 99.3 99.1 99.5 100.8 102.5 104.399.3 99.5 99.6 101.9 102.6 104.399.4 99.9 100.2 102.7 102.8 104.3

q(P)/|e| 0.406 0.427 0.438 0.556 0.542 0.884

28 G. Yang et al. / Journal of Molecular Structure 882 (2008) 24–29

\O1TiO2, \O1TiO3 and \O2TiO3 angles (Fig. 1a). Thesum in TS-1 cluster amounts to 327.7� characteristic ofTd symmetry whereas is enlarged to 350.9� after TMPchemisorption, i.e., the Ti12 atom nearly falls within theO1O2O3 plane. Meanwhile, the adsorbed TMP moleculewas distorted, whose \CPC angles are no longer equivalentas in the isolated state (Table 2).

Through the hydrolysis of Ti–O–Si linkages, the defectsite of [Ti(OSi)3OH] was formed in TS-1 zeolite [4,28],see Fig. 1b. TMP was chemisorbed on this defect site, withthe Ti12-P distance optimized at 2.822 A and slightlyshorter than that in the perfect site (Fig. 1a). As expected,the adsorption energy was calculated to be �19.6 kJ mol�1

and larger than that in the perfect site, suggesting a stron-ger mutual interaction.

The 31P NMR tensors were computed to distinguish thetwo Lewis acidic sites present in 31P MAS NMR spectra(Fig. 2). The 31P chemical shift in the perfect site was com-puted to be more negative by 1.5 ppm than that of defectsite. Accordingly, the 31P MAS NMR experimental peaksat �34.2 and �32.0 ppm; i.e., TiLAI and TiLAII [10] canbe ascribed to the perfect and defect sites, respectively(Fig. 1). The computational difference of the chemicalshifts of these two Lewis acids is very close to that of the31P MAS NMR results. Moreover, the calculated adsorp-tion energy is larger in TiLAII than in TiLAI, agreeing wellwith the slower decrease of the 31P MAS NMR peak in theformer as discussed above. However, both of the calculatedadsorption energies are very small, which implies thatadsorbed TMP molecules are not tightly bound on thetwo types of Ti Lewis acidic sites. It is consistent with the31P MAS NMR desorption experimental results that thepeaks to the two Lewis acids disappeared as the desorptiontemperature goes up. There are only two peaks around�33.0 ppm in 31P MAS NMR spectra which were attrib-uted to TiLAI and TiLAII (Fig. 2), and therefore the fur-ther hydrolysis of Si–O–Ti links to form (SiO)2Ti(OH)2

or (SiO)Ti(OH)3 species will probably not proceed in TS-1 zeolite, at least under detection.

4.4. Density functional calculations on silicate-1

Clusters of the same size (Fig. 1a) were extracted fromsilicalite-1 and geometry-optimized under the same condi-

tions. It was found that even if the TMP molecules wereprearranged as chemisorbed, the slicalite-1 clusters andthe adsorbed TMP molecules gradually become apart asthe optimization processes go on. The final Si7-P andSi12-P distances are equal to 5.581 and 4.207 A, respec-tively. Analogous to the physisorption processes in TS-1zeolite, the geometric parameters of sililcate-1 and TMPremain almost unchanged before and after TMPadsorption. It agrees finely with the 15N-enriched NMRexperimental results that only physisorption of 2-buta-none[15N]oxime takes place on silicalite-1 [29]. TS-1 zeolitediffers from silicate-1 in that it has the incorporated transi-tion metal ions of Ti(IV), which have vacant d orbitals thatcan accept lone-pair electrons from probe molecule such asTMP. As a result, TS-1 zeolite shows Lewis acidities whilesilicalite-1 fails to show Lewis acidity since the central Siatoms have been covalence-saturated.

4.5. Relationship between TMP oxidation reaction and

Lewis acidities

Whether chemisorbed on TiLAI or on TiLAII, the phe-nomena of charge transfer were observed between TMPand TS-1 clusters. The Mulliken charges on the P atomsincrease in the order of isolated TMP < Physisorp-tion < TiLAII < TiLAI < isolated TMPO (Table 2). Thelarger the Mulliken charge, the closer its chemical stateto that in the product of TMPO, and naturally the easierto be oxidized. Accordingly, the sequence of TMP oxida-tion in TS-1 zeolite is chemisorbed on TiLAI, chemisorbedon TiLAII, and then physisorbed on other T sites. Such asequence is perfectly consistent with the in situ 31P MASNMR experimental results [10]. Compared with isolatedor physisorbed TMP molecules, the P–C bonds and CPCangles of the TMP molecules chemisorbed on TiLAI

and TiLAII are much more close to those of TMPO(Table 2). As a result, these two types of Lewis acids acti-vate the reactants and thus catalyze the reactions such asTMP oxidation as reported previously [10].

5. Conclusions

The joint theoretical-experimental studies performed inthe present work can arrive to the conclusions summarizedbelow:

1. 31P MAS NMR desorption experiments and densityfunctional calculations well elucidated the sequence ofTMP desorption: physisorbed with slightly positiveadsorption energies, chemisorbed on TiLAI with�6.2 kJ mol�1 adsorption energy, and then chemisorbedon TiLAI with �19.6 kJ mol�1 adsorption energy. Thecarriers exert little influence on the ionic [(CH3)3P-H]+

species.2. To TS-1 zeolite, the TMP molecules are physisorbed on

the Ti sites except Ti12 site, suggesting that these T sitedo not show Lewis acidities towards TMP whereas on

G. Yang et al. / Journal of Molecular Structure 882 (2008) 24–29 29

the contrary towards other probe molecules (e.g., NH3

and H2O). The large molecular size of TMP may bethe reason for the physisorption.

3. The structures of two types of Lewis acidic sites presentin TS-1 zeolite were resolved by theoretical calculations,and their corresponding peaks in 31P MAS NMR spec-tra were identified using computational chemical shiftsand adsorption energies. By comparing the results of sil-icalite-1, it was understood that TS-1 zeolite showsLewis acidities due to the presence of vacant d orbitalsof the incorporated Ti atoms.

4. After the formation of defect site (SiO)3Ti(OH), furtherhydrolysis of Si–O–Ti links to form (SiO)2Ti(OH)2 or(SiO)Ti(OH)3 species will not likely to proceed in TS-1zeolite.

5. The two types of Lewis acids are responsible for the cat-alytic oxidation of TMP into TMPO, and the experi-mental catalytic effects were well explained usingstructural and Mulliken charge analysis.

Acknowledgements

Financial support was provided by the National NaturalScience Foundation and the Ministry of Science and Tech-nology of PR China (2003CB615806).

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