Lewis and Brönsted acidic sites in.pdf

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Journal of Molecular Catalysis A: Chemical363364 (2012) 371 379

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Journal of Molecular Catalysis A: Chemical

j o u rn a l h o me p ag e : www.e l sev i e r. co m/ l o ca t e / mo l ca t a

Lewis and Brnsted acidic sites in M4+-doped zeolites (M=Ti,Zr, Ge,Sn, Pb) aswell as interactions with probe molecules: ADFTstudyGang Yanga, c, , LijunZhoua , Xiuwen Han ba Engineering Research Center of Forest Bio-preparation, Ministry of Education, Northeast Forestry University, Harbin 150040, Chinab State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Chinac Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, NL-5600 MB Eindhoven, The Netherlands

a r t i c l e i n f o

Article history:Received 23 January 2012Received in revised form 8 July 2012Accepted 12 July 2012Available online 21 July 2012

Keywords:Brnsted acidityLewis acidityDensity functional calculationsAdsorption energyProbe molecules

a b s t r a c t

Tetravalent-ion (M4+)-doped zeolites show excellent performances for a variety of catalytic processes,including the focusing biomass conversions. In this work, density functional calculations were per-formed to probe the Lewis and Brnsted acidities of various M4+-doped zeolites as well as to studyinteractions with probe molecules. The Lewis and Brnsted acidities increase in the orders of Silicalite-1 Ge< Ti


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372 G. Yang et al. / Journal of Molecular CatalysisA: Chemical 363364 (2012) 371379

Scheme 1. TheLewis acidity (LA) and Brnsted acidity (BA) created by doping theM 4+ ion in Silicalite-1.

remains largely unknown in contrast to trivalent ions (M 3+). TheBrnsted acidities of zeolites with incorporation of Ti 4+, Zr4+, Ge4+,Sn4+ and Pb4+ ions were calculated and compared with each other.These Brnsted acidicsites were then to interact with ve differentprobe molecules (pyridine, trimethyphosphine, water, ammoniaand acetone). To aid the understanding of the Brnsted acidity of zeolites with incorporation of tetravalent ions (M 4+), parallel cal-culationswere performedon twoM 3+ ions (B andAl). Thebasicitiesof the ve probe molecules change within a wide range and thuspresent a dynamic image of acidbase interactions.

2. Computational

In agreement with the previous studies [2729] , the clustermodels containing 17-T sites were chosen to represent the localstructures of MFI-type zeolite ( Fig. 1). The doped tetravalent ions(M4+)occupiedoneoftheT7site,whichweresupportedbytheneu-trondiffraction experimentsof TS-1zeolite [15,16] and partiallybythe calculations [33,3739] . The boundary Si atoms were saturatedwith H atoms with the Si H distances of 1.500 A and kept at theircrystallographic coordinates.

All the calculations were performed using B3LYP densityfunctional [40,41], Gaussian03 program suite [42]. The standard6-31G(d) basis set was applied to the elements with atomicnumbers below 21. As to Ti, Ge, Sn, Pb and Zr, their coreelectrons were represented by LANL2DZ effective core poten-tial (ECP) [43,44] and valence electrons by LANL2DZ basis set,respectively [17,2729] . The combined basis sets are denotedas B1 (default). For the adsorption of NH 3 on the Lewis acidicsites (Scheme 1a), the Wiberg bond indices (bond orders)were derived from the natural bond orbital (NBO) program

[45], helping us to judge the strengths of the Lewis acidity[46].The computationalmethods and cluster models werevalidated:

(1) larger basis set (B2). In B2, the LanL2TZ(f) and LanL2DZ(d,p)ECP basis sets [44,47,48] were used for the Ti, Zr and Ge, Sn,Pb atoms, respectively, whereas the 6-311G(d,p) basis set for theother atoms, (2) larger cluster model models (37-T) with com-plete straight channels ( Fig.2).The two-layerONIOMmethodology(ONIOM)[49,50] was used for the calculations. The [(HO) 3SiO]4Mfragment and probe molecules were dened as the high-levelregion and treated with the B3LYP/B1 method. The rest asthe low-level region was treated at B3LYP/3-21G level [30,51].The boundary Si H distances were set to 1.500 A. The atomsof the low-level region were retained at their crystallographic

coordinates.

3. Results and discussion

3.1. The Lewis acidity

The Lewis acidic sites in tetravalent-ion (M 4+)-doped zeolitesare shown in Scheme 1a and Fig. 1a, where the M 4+ ions are

Fig. 1. Thelocalstructures of theLewis acidity (LA) inthe M 4+-dopedzeoliteas well

as of the Lewis acidic site with adsorption ofNH 3.


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G. Yang et al. / Journal of Molecular CatalysisA: Chemical 363364 (2012) 371379 373

Fig. 2. The Lewis acidic site of the M4+-doped zeolite as well as the structure withadsorptionof NH3, which arecalculatedby thetwo-layerONIOMmethodology. Thehigh-level regionis displayedin ball andstickwhereas thelow-level regionin tube,respectively.

coordinated to four O atoms. The deviations of [MO 4] from regulartetrahedron canbe characterized bythemean squaredeviation ( )[30,52] dened,

= 1

6

6

i= 1

( i )2 (1)

In Eq. (1), i represents the ith O M O angle, and is the aver-age of the six O M O angles. The values of Ge, Ti, Pb, Snand Zr are calculated to be 3.12 , 3.70 , 4.28 , 3.57 and 5.87 respectively, indicating the different deviation degrees from regu-lar tetrahedron. Compared with Silicalite-1 ( =2.81 ), the dopingof tetravalent ions (M 4+) enlarges thedeviations from regular tetra-hedron, probably due to the radius increase by ion doping. The

M O distances are listed in Table 1 and show elongations by eachdoping case. The average M O distances increase in the orderGe(1.708 A)


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Table 1The (average) M O distances, deviations of [MO4] from regulartetrahedron( ) and Mulliken/NPA charges of the Lewis acidic sites in tetravalent-ion (M 4+) doped zeolitesand Silicalite-1, beforeand after adsorption of NH 3.a

M O1 M O2 M O3 M O4 M O b b q(M)c q(NH3)c

Si4+ Bare 1.605 1.616 1.618 1.609 1.612 (1.607, 1.613) 2.81 (2.78, 2.35)NH3 1.604 1.625 1.622 1.609 1.615 (1.610, 1.617) 4.19 (4.17, 4.61)

Ge4+ Bare 1.700 1.715 1.715 1.702 1.708 (1.709, 1.710) 3.12 (3.37, 2.64) 1.917 (2.569)NH3 1.716 1.764 1.742 1.723 1.736 (1.733, 1.745) 11.06(11.61,13.05) 1.921 (2.536) 0.129 (0.126)

Ti4+ Bare 1.758 1.783 1.785 1.771 1.774 (1.771, 1.776) 3.70 (4.00, 3.40) 1.054 (1.712)

NH3 1.783 1.810 1.813 1.784 1.797 (1.793, 1.803) 11.39(12.03,13.23) 1.070 (1.594) 0.177 (0.163)Pb4+ Bare 1.905 1.926 1.918 1.899 1.912 (1.912) 4.28 (4.89) 1.765 (2.529)NH3 1.927 1.950 1.941 1.917 1.934 (1.933) 13.51(14.80) 1.823 (2.533) 0.155 (0.141)

Sn4+ Bare 1.844 1.862 1.858 1.842 1.851 (1.856, 1.853) 3.57 (4.00, 3.77) 2.032 (2.714)NH3 1.867 1.896 1.886 1.863 1.878 (1.883, 1.885) 12.65(13.92,14.48) 2.053 (2.708) 0.143 (0.127)

Zr4+ Bare 1.915 1.949 1.949 1.927 1.935 (1.921, 1.935) 5.87 (6.55, 5.83) 1.309 (2.324)NH3 1.947 1.959 1.971 1.939 1.954 (1.940, 1.960) 12.72(13.36,14.94) 1.341 (2.272) 0.167 (0.110)

a Units of distances, and charges arein angstrom and |e| , respectively.b The average distances in parentheses are calculated at the B3LY/B2 level, 17-T cluster model and at the ONIOM(B3LY/B1:B3LYP/3-21G) level, 37-T cluster model

(underlined), respectively.c The NPA charges aregiven in parentheses.

Table 1 shows that the Mulliken/NPA charges of the M 4+ ionsdo not decrease but increase, which seems to contradict the con-ception of Lewis acidity wherein the empty d orbitals of the M 4+

ions should accept electrons from the incoming molecules. In fact,the Mulliken/NPA charge analyses reveal that electron transfersare conducted from the NH 3 molecules toward the zeolitic sys-tems; that is, the M 4+ ions accept electrons but disperse themrapidly around, which is supported by the delocalization of theLUMOs inFig. 3. Accordingly, the Lewis acidities of the M 4+-dopedzeolites should be more accurately described as the local sitesaround the M 4+ ions rather than merely the M 4+ ions. The adsorp-tion energies, LUMO energies and are global descriptors andmore suitable for predicting Lewis acidity than the fukui func-tions ( f M+) that are based solely on the charges of the M 4+ atoms[63]. In addition, the adsorption energies instead of the LUMOenergies and correctly predict that Sn has higher Lewis acid-ity than Ti [61,62,64] . Hence, the adsorption energies are superiorin characterizing the Lewis acidities of zeolites, which increase asSilicalite-1 Ge


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Fig. 3. The LUMO diagrams of Silicalite-1and theM4+-doped zeolites.

As the data in Tables 1 and 3 show, the M O dis-tances are increased due to the formation of Brnsted acidicsites. The two M O3 and M O5 bonds ( Fig. 4), which arepotentially related with the acidic protons (H +), are obviouslylonger than the other three (M O1, M O2, M O4). Espe-cially the M O3 distances are 0.3530.520 A larger than theaverage values of the other three; however, the M O3 dis-tances are dramatically reduced in the deprotonated states(BA ), where the protons are deprived by the incoming probe

molecules.

The Brnsted acidity strengths of the M 4+- and M3+-dopedzeolites can be evaluated with the aid of proton afnity (PA)[5,710,27,66,67],

PA = E (BA ) E (BA) (8)

where E (BA) and E (BA ) represent the energies of the Brnstedacidic sites (BA) and corresponding deprotonated states (BA ),respectively.

The proton afnities (PA) of the various M 4+- and M3+-

doped zeolites decrease in the order Silicalite-1 (324.9) Ti


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Table 3The M O distances,coordinationnumbers ofthe doped ions( nM), Mulliken charges (q) in theBrnstedacidic(BA) anddeprotonated (BA ) statesas well as protonafnities(PA) and formation energies of theBrnsted acidic sites ( F BA) forthe M 4+-, M3+-doped MFI-type zeolites and Silicalite-1. a

M O b M O3 M O5 nM qH3 qO3H3 F BA PA

Si4+ BA 1.623 3.511 1.628 4.43 0.466 0.279 20.2 324.9BA 1.713 1.757 1.719

Ti4+ BA 1.801 2.272 1.854 4.69 0.478 0.283 30.9 307.4BA 1.857 1.899 1.904

Ge4+ BA 1.742 2.362 1.765 4.63 0.464 0.299 18.9 306.4

BA

1.805 1.846 1.811B3+ BA 1.369 2.195 0.450 0.277 303.6BA 1.462 1.467

Pb4+ BA 1.952 2.367 1.980 4.71 0.482 0.307 42.9 300.9BA 1.995 2.019 2.031

Zr4+ BA 1.964 2.363 2.014 4.74 0.485 0.296 52.7 300.2BA 2.016 2.056 2.050

Sn4+ BA 1.891 2.244 1.922 4.73 0.481 0.331 40.2 299.2BA 1.935 1.968 1.956

Fe3+ BA 1.786 2.019 0.506 0.273 292.8BA 1.823 1.832

Al3+ BA 1.694 1.884 0.491 0.242 288.2BA 1.726 1.735

a Units of distances charges and energies arein angstrom, |e| and kcal mol 1, respectively.b The average distances ofM O1, M O2andM O4.

(307.4)> Ge (306.4)> B (303.6) >Pb (300.9) Zr (300.2) Sn(299.2)> Fe (292.8)> Al (288.2), units in kcalmol 1 (Table 3and Fig. 5). The acidic sites with higher PAs are more dif-cult to donate protons and thus have lower Brnstedacidities. Accordingly, the acidities increase as Silicalite-1 Ti4+ < Ge4+ < B3+ pyridine >acetone >water[68]. For comparisons, two M3+ ions B and Al are studied as well.The results are summarized in Tables 4 and 5 . Some adsorptionstructures are illustrated in Figs. 68. In the covalent structures(Figs. 7a and 8), the acidic protons remain on the zeolites anddo not transfer to probe molecules. Instead, the acidic protons

have been transferred to probe molecules in the ionic structures(Figs. 6 and 7b).The formations of covalent and/or ionic structures are the

proton-competing results between the probe molecules and Brn-sted acidic sites. If the probe molecule has strong enough basicity,the acidic proton will be denitely on it; that is, even if startingfrom the covalent mode, the proton will gradually transfer towardto theprobemolecule(PM)andresult in thePMH + andBA species.

Fig. 5. The ranks of Brnsted acidities and Mulliken charges for the various M 4+-

and M3+

-doped zeolites as well as Silicalite-1.


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Table 4Distances and interaction energies ( E in) forthe ionic structures dueto adsorption of probe molecules on the Brnsted acidic sites in M 4+-andM3+-doped zeolites. a

Ti4+ Ge4+ B3+ Pb4+ Zr4+ Sn4+ Al3+

NH3O3 H3 1.526 1.557 1.604 1.622 1.584 1.602 2.494N H3 1.092 1.083 1.078 1.072 1.080 1.075 1.037E in 22.2 24.5 25.4 28.4 26.2 30.0 36.2

Pyridine

O3 H3 1.492 1.545 1.616 1.557 1.523 1.560 1.640N H3 1.094 1.079 1.065 1.070 1.084 1.074 1.060E in 12.2 13.8 19.2 18.4 16.7 19.7 29.0

TMPO3 H3 1.702 1.751 2.235 1.725 1.745 1.733 1.996P H3 1.432 1.421 1.398 1.425 1.426 1.424 1.408E in 2.7 3.3 5.7 9.8 7.9 10.5 20.0

a Units of distances and energies in angstrom and kcal mol 1, respectively.

It is applicable to NH 3 where only the ionic structure hasformedineach doping case ( Table 4 and Fig. 6). Owing to the lower basicity,TMPcan result in both covalent and ionic structures; that is, nei-ther of the probe molecules and Brnsted acidic sites can depriveprotons from the other sides ( Tables 4 and 5 ). As the basicities of probe molecules continue to decrease, the deprotonated Brnsted

acidic sites (BA ) are strong enough to seize the protons from theprotonated probe molecules (PMH +), causing the absence of theionic modes. It is applicable to acetone andwater, wherein only thecovalentstructureexists ( Table5 and Fig.7) [52,69,70].Forpyridine(Py) adsorption, stronger Brnsted acidic sites result in only ionicstructures, and weaker Brnsted acidic sites cause co-existence of

Fig. 6. Theoptimized structures of NH 3 adsorption on theBrnstedacidicsite (BA)

of zeolites with the doping of M4+

(a) and M3+

(b)ions.

Table 5Distancesand interactionenergies ( E in) forthe covalentstructures dueto adsorptionof probe molecules on theBrnsted acidic sites in M 4+-andM3+-doped zeolites. a

Ti4+ Ge4+ B3+ Pb4+ Zr4+ Sn4+ Al3+

PyridineO3 H3 1.031 1.040 1.055N H3 1.644 1.615 1.561E in 12.0 11.4 14.6

TMP

O3 H3 1.006 1.003 0.990 1.024 1.020 1.026 1.044P H3 2.242 2.271 2.431 2.127 2.146 2.128 2.102E in 5.6 4.5 5.9 7.2 8.1 7.4 11.8

AcetoneO3 H3 1.006 1.005 1.003 1.015 1.011 1.108 1.024O H3 1.642 1.658 1.681 1.599 1.617 1.589 1.583E in 10.0 9.6 10.2 12.3 12.7 12.7 15.1

H2OO3 H3 1.014 1.020 1.020 1.023 1.018 1.028 1.046O H3 1.608 1.591 1.612 1.594 1.607 1.569 1.511E in 18.3 20.5 18.7 22.5 22.6 23.3 22.8

a Units of distances and energies in angstrom and kcal mol 1, respectively.

the ionic and covalent structures, see Ti, Ge and Zr in Fig. 7 andTables 4 and 5 . The strengths of the Brnsted acidities are thus dif-fered. Such differences can also be observed from the geometricanalyses and interaction energies ( E in). In the ionic structures, theH3 protons form H-bonds with the zeolite-O3 atoms. Generally,the stronger the Brnsted acidity, the longer the O3 H3 H-bond,and the shorter the N H3 bond ( Table 4). In the covalent struc-tures, the H3 atoms form H-bonds with probe molecules, and the

Fig. 7. The optimized structures of acetone adsorption on the Brnsted acidic site

(BA) of zeolites with the doping of M4+

(a) and M3+

(b)ions.


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Fig. 8. The covalent and ionic structures due to pyridine (Py) adsorption on theBrnsted acidic site (BA) in the M4+-doped zeolites (M= Ti, Ge, Zr).

strongerBrnsted acidity generallyleadsto theshorter O H3 bond(Table 5).

The interaction energies between theprobemolecules (PM) andBrnsted acidic sites (BA) can be calculated,

E in = E (Product) E (BA) E (PM) (9)

where E (Product) is the energy of the optimized structure, inthe covalent or ionic form. TMP, often used to probe the zeo-lite acidities [17,27,7173] , is the only one that in all the dopingcases, the covalent and ionic modes will co-exist. The energy

differences between the ionic and covalent structures ( E diff )decrease in the order Silicalite-1 (22.4) Ti (2.9)> Ge (1.2)> Zr(0.2) =B (0.2)>Pb ( 2.6)> Sn ( 3.1)> Al ( 8.2), units in kcalmol 1(Fig. 9). It suggests that the Brnsted acidity increases as Silicalite-1 Ti


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Lewis and Brönsted acidic sites in.pdf