zeolite confined nano-titania catalyst nailed down

Zeolite Confined Nano-Titania Catalyst Nailed Down

Jian-jie Liang*1 , George Fitzgerald1, Istvan Halasz2

1Accelrys Inc. 2PQ Corporation

• Some highly active, hydrophilic, MFI structured titanium silicate catalysts are characterized by an intense UV absorption centered in the 213-233 nm range.

• The assignment of this band to a specific molecular structure has remained unclear.

• Time-Dependent Density Functional Theory (TD-DFT) to decipher the structural origin of this intermediate UV band.

Prologue

TS-1 Structure

• Si94Ti2O192

• Tendency of ordering by neutron diffraction (Lamberti, et. al., 2001)

• Hydrophobic

• Characteristic UV band @ 211 nm

180 200 220 240 260 280 300 320 340

Inte

nsity [

arb

itra

ry u

nits]

Wavelength [nm]

211 221

315

265

Isolated Tetrahedral Ti4+ Octahedral Ti4+

Hydrophilic Ti-MFI

• Hydrophilic

• More active and selective, even without

applying organic solvents

• Characteristic UV band @ 221 nm

180 200 220 240 260 280 300 320 340

Inte

nsity [

arb

itra

ry u

nits]

Wavelength [nm]

211 221

315

265

Isolated Tetrahedral Ti4+ Octahedral Ti4+

TD-DFT

Translate structural hypothesis to UV spectra

Time-dependent density functional theory

(TD-DFT) in DMol3

Delley, J. Phys.: Condens. Matter 22 (2010) 384208

Necessary to extend traditional ground-state DFT method to model phenomena involving excited states, such as in optical spectra.

Dealing with Large systems-QMMM

TD-DFT is Time-Consuming

− Typically, each excited state costs the

same as the ground-state SCF

− To be computationally tractable, only the

most chemically relevant portion of a

structure can be treated quantum

mechanically (QM-region)

− The rest of the chemical environment will

be treated using atomistic (classical)

modeling (MM-region)

QM-region

MM-region

Computational Details

• TDDFT limited to molecules

• QM/MM optimization of geometry followed by DMol3 TDDFT of QM region

– ~100 atoms in QM region

– QM region radially incremented until convergence in UV property

• MM region handled through GULP/Dreiding

• QM region by mechanical embedding

Configurations for full treatment

Computed UV spectra

180 200 220 240 260 280 300 320 340

Inte

nsity [

arb

itra

ry u

nits]

Wavelength [nm]

211 221

315

265

Isolated Tetrahedral Ti4+ Octahedral Ti4+

180 200 220 240 260 280 300 320 340

Inte

nsity [

arb

itra

ry u

nits]

Wavelength [nm]

211 221

315

265

Isolated Tetrahedral Ti4+ Octahedral Ti4+

Other configurations considered

Other configurations considered

Conclusions

• TDDFT calculations can yield UV spectra that agree well with experimental spectra on solid phases such as zeolite materials.

• Combined computational and experimental studies of the target MFI-structured titanium silicates indicate that a model containing tetrahedral Ti(OH)4 nanoparticles, not covalently bound to the framework, accounts for the enhanced catalytic activity of the material.

Experimental UV spectra

180 220 260 300 340 380

1 2 3

4 5

Wavelength [nm]

Ku

bel

ka-M

un

k A

bso

rban

ce [

arb

itra

ryu

nit

s]

202 221211

310

237

270

1,2: “good” TS-1

catalysts.

5: Typical ill-

synthesized material

with low catalytic

activity and

selectivity.

3,4: Transitions

between the good

and bad TS-1

products, containing

increasing amounts

of octahedral, non-

framework,

TixOy(OH)z

oligomers

UV spectra calculated for selected clusters

The Ti(OH)4 cluster (3rd structure from left above) gave a spectrum blue-shifted by ~

50 nm compared to when in the zeolite environment