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46117-AC5
Nanofiber Catalyst Supports and Solution-Based Processes for Deposition of Catalytic Metals and Metal Oxides

Wayne E. Jones, State University of New York at Binghamton

The adsorption of stabilized metal nanoparticles on metal oxide nanostructured materials confers profound prospects to catalysis in organic synthesis.^1,2 These include; ease of recovery and recyclability of these catalysts from reaction medium, large surface area to volume ratio, as well as stability at high temperatures that are fundamental in production and refining of petrochemicals.^3,4 The bottom-up approach and solution-based fabrication processes provide low cost catalysts with a low catalyst loading, good selectivity, and enhanced reactivity under mild conditions. This is attributed to novel properties accrued to nanomaterials relative to bulk materials due to quantum level interactions.⁵ In phase one of the PRF progress, we report a simple template based approach via electrospinning of fabrication of titanium dioxide (TiO₂) nanostructures followed by solution based impregnation and reduction of palladium nanoparticles onto TiO₂ nanostructured supports. These catalysts (Pd-TiO₂) were applied in Heck C-C coupling reactions to determine the conversion rate, selectivity and stability of the catalysts.

Electrospinning provides an inexpensive, straightforward route to the fabrication of high surface area fiber membranes.⁶ This process entails the application of an electric field to a polymer solution or composite leading to generation of fibers on nanometer scale, and was exploited in fabricating nanofibers and nanotubes. Nanotubes were synthesized by a fiber-based template process, where electrospun fibers served as a scaffold upon which a metal oxide precursor was deposited via sol gel process whereas nanofibers were fabricated via electrospinning a composite mixture of titanium isopropoxide (TiP)/polymethylmethacrylate (PMMA) as illustrated by the following schemes.^7,8

Pd-TiO2

Pd deposition

� TiO2 nanofibers

Thermolysis

PMMA/TiP composite fibers

PMMA/TiP(1:2) in CHCl3/DMF(1:1)

1 KV/cm

Electrospinning

Hydrolyzed in air

����������� aa Scheme 1: Fabrication of metal oxide nanofibers

c

b

a

d

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(a)Scanning Electron Microscopy (SEM) image of electrospun composite PMMA/TiP (b) TiO₂ nanofibers after pyrolysis at 400�C, with diameters 150�50 nm (d) Transmission Electron Microscopy image TiO₂ nanofibers (150�50 nm)� (d) Pd nanoparticles on TiO₂ nanofibers with diameters range 6-10 nm.

The morphology of fabricated composite fibers was smooth with dimensions ranging between 150�50 nm in diameter. Pyrolysis temperature, 400�C, was based upon thermogravimetric analysis of the degradation profile of PMMA. FTIR and Powder X-ray diffraction confirmed complete removal of organics leaving behind an anatase crystalline phase of titania. Pd nanoparticles were uniformly distributed with 5% loading and diameters ranging between 6-10 nanometers over the TiO₂ support.

Metal oxide nanotube

Pyrolysis

Coaxial fiber

(1) Sn+2/Pd+2

(2) Sol-gel Coating

Polymer fiber

Nanoparticle impregnation

Scheme 2: Fabrication of metal oxide nanotubes by templating

Catalyst supported on nanotube

a

b

c

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SEM images (a) Electrospun polylactide, PLA, template with diameters 200�50 nm (b) PLA-TiO₂ coaxial fibers after sol gel coating with diameters 250�100 nm (c) TiO₂ after pyrolysis at 400 �C, diameters 200�50 nm and wall thickness ~100 nm.

b

a

� �

d

c

� �� ��

�(a)Thermogravimetric analysis, TGA, illustrating the thermal decomposition of polylactide polymer (b) Infrared spectrum before and after calcinations indicating complete degradation of PLA polymer (c) EDS indicating TiO₂ formation (d) Powder X-ray diffraction indicating anatase crystalline phase of TiO₂ nanotube.

Pd-TiO₂ catalysts fabricated were tested for Heck C-C coupling of Iodobenzene with scheme (3) styrene and scheme (4) n-butylacrylate.^9,10 The reactions were carried out in an air and comparisons were made with (a) Pd/C in an inert atmosphere, (b) Pd(OAc)₂ as unsupported powders.

Scheme 4: C-C coupling of iodobenzene with n-butylacrylate

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From the GC, Pd-TiO₂ showed higher conversion of reactants to products compared to Pd(OAc)₂ complex, and comparable rates to those given by Pd/C (the commercially obtained catalyst) even though reaction with Pd-TiO₂ was manipulated under air atmosphere. Pd-TiO₂ catalyst exhibited high activity and selectivity for the desired product even at 0 minute reaction time, with yields comparable to Pd/C. The yield of stilbene was constant and increased exponentially indicating stability of the product and catalyst up to 200 minute reaction time. The Pd-TiO₂ catalysts recorded selectivity values of up to 100% and high yields of up to 83% as calculated from GC relative areas. In addition Pd-TiO₂ was effectively eliminated from the final products by filtration as compared to unsupported Pd(OAc)₂ and Pd/C catalysts. Similar trends were observed for C-C coupling of iodobenzene with n-butyl-acrylate.

Work in progress is geared towards mechanistic studies to understand the nature of very active metal species in solution.� Similarly, related work is underway on fabrication and characterization of gold nanoparticles and other metal oxide nanostructured supports such as ZnO and ZrO₂ and their application to Heck and Suzuki C-C coupling reactions.

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