UNIVERSITA DEGLI STUDI DI NAPOLI FEDERICO II SYNTHESIS VIA SOL-GEL OF Nb 2 O 5 SiO 2 CATALYSTS FOR EPOXIDATION REACTIONS E. Marenna a, E. Santacesaria b, P. Pernice a, M. Di Serio b, A. Aronne a a Dipartimento di Ingegneria dei Materiali e della Produzione b Dipartimento di Chimica II WORKSHOP NAZIONALE AICIng Messina, Facoltà di Ingegneria, 12-14 Settembre 2007 1
New heterogeneous catalysts 1-2 Nb 2 O 5 SiO 2 Oxidation of methanol Catalytic applications Oligomerization of olefins Oxidative dehydrogenation of alkane Epoxidation of olefins 1 Somma, F.; Canton, P.; Strukul, G. J. Catal. 229 (2005) 490. 2 Tanabe, K. Catal. Today 78 (2003) 65. 2
Epoxidation of olefins with H 2 O 2 H 2 O 2 oxidizes olefins with the generation of water as the only by-product Heterogeneous and stable catalysts active in epoxidation of olefins with H 2 O 2 TS-1 (titanium silicalite) Mixed oxides based on silica and containing transition metal oxides (Zr, W, Ti, Nb) 3
Recent works in the application of niobium-based materials in epoxidation reactions Nb 2 O 5 -MCM41 low activity 3 Niobium peroxo compounds good selectivity, low activity 4 Aerogels Nb 2 O 5 -SiO 2 high T and high P treatment 5 3 Nowak I., Ziolek M., Micropor. Mesopor. Mater. 78 (2005) 281 4 Passoni L. C., Siddiqui M. R. H., Steiner A., Korhevnikov I. V., J. Mol. Catal. A 153 (2000) 103 5 Somma F., Strukul G., Catal. Lett. 107 (2006) 73 4
We have obtained homogeneous niobia-silica catalysts via sol-gel synthesis at room temperature which are active and stable in epoxidation reaction 5
Sol-Gel chemistry High dispersion of the active phase in the matrix on molecular scale (high activity and selectivity) Advantages in heterogeneous catalysis Chemical interactions of active phase with matrix (stability to leaching) Low temperature of process Control of porosity (high surface areas) 6
Chemical composition of catalysts x Nb 2 O 5 (100-x) SiO 2 x = 2.5 (2.5Nb), 5 (5Nb), 10 (10Nb), 20 (20Nb) 7
Synthesis of catalysts 6 Nb 2 Cl 10 (solid) + 2x C 2 H 5 OH (solvent) = 2NbCl 5-x (OC 2 H 5 ) x (solution) + 2x HCl (gas) NbCl 5 : EtOH 1 : 6 Dry box - room temperature Fluxed with dry air NbOCl 2-5 and NbCl - 6 in aqueous saturated HCl solution Nb(OH) 2 Cl - 4 and Nb(OH) 2 Cl 3 in more diluted solutions stirring stirring TEOS : EtOH 1 : 4 H 2 O / HCl / EtOH b presence of oxo-hydroxo-chloro complexes TEOS : H 2 O : HCl 1 : 4 : 0.01 sol Sample (Nb 2 O 5 ) 0.2 (SiO 2 ) 0.8 (20Nb): (a) wet gel; (b) dried gel suitable amount of water Transparent wet gel a Gel treated 3h at 400 C = CATALYST 6 Aronne A., Marenna E., Califano V., Fanelli E., Pernice P., Trifuoggi M., Vergara A., J. Sol-Gel Sci. Technol. 43 (2007) 193. 8
Termogravimetry/differential thermal analysis XRD spectra evaporation of solvents 100 10Nb eso 10Nb DTA, _TGA atm: air; h.r.= 10 C/min 98 96 94 92 400 C 3h 110 C 90 88 10 20 30 40 50 60 86 2 Θ evacuation of products of pyrolysis 200 400 600 800 Temperatura ( C) 84 No Nb 2 O 5 segregation it is well dispersed in matrix 9
Textural and surface properties Adsorbed N 2 (cm 3 /g) 200 150 100 50 2.5Nb 5Nb 10Nb N 2 adsorption-desorption isotherms at 77K elaboration with Dubinin method Sample 400 C 3h Surface area (Dubinin method) m 2 /g Micropore volume cm 3 /g 0 0,0 0,2 0,4 0,6 0,8 1,0 SiO 2 333 - P/P 2.5Nb 519 0.185 The surface area and pore volume of catalysts decreases with increasing of Nb 2 O 5 molar fraction in the catalyst composition 5Nb 10Nb 20Nb 266 235 6.7 0.094 0.083-10
Textural and surface properties Pore size distribution 0,008 dv/dr (cm 3 /g)/angstrom 0,007 0,006 0,005 0,004 0,003 0,002 2,5 Nb 5 Nb 10 Nb 0,001 0,000 10 100 1000 Diameter (Angstrom) The 2.5 Nb shows the greater fraction of micropores 11
Textural and surface properties TPD-NH 3 of SiO 2 and SiNb catalysts Sample (400 C for 3h) Adsorbed NH 3 (mmol g -1 ) (ads. NH 3 )/ (surf. a.) (µmol/m 2 ) 1.2x10-4 1.0x10-4 2.5Nb Weak acid sites (~140 C) SiO 2 0.20 0.60 8.0x10-5 5Nb 2.5Nb 1.2 2.31 mol NH 3 g -1 min -1 6.0x10-5 4.0x10-5 2.0x10-5 10Nb SiO 2 Stronger acid sites (~350 C) 5Nb 10Nb 1.2 1.1 4.52 4.68 0.0-2.0x10-5 100 200 300 400 500 600 Temperature ( C) Increasing niobium content Increasing the concentration of stronger sites on 5Nb and 10Nb 12
Catalytic Tests Epoxidation reaction of cyclooctene with hydrogen peroxide H 2 O 2 o + H 2 O Catalyst = 600 mg Methanol (solvent) = 30 ml Cyclooctene = 40 mmol H 2 O 2 (35wt.% aqueous solution) = 40mmol Temperature = 70 C Catalyst SiO 2 2.5Nb 5Nb Epoxide Conversion % - 39.2 27.8 Reaction time = 300 min 10Nb 11.7 13
Catalytic Tests Catalyst recycling: stable activity after four cycles Absence of Nb in reaction mixtures after separation of the catalyst Evidence of the heterogeneous character of these catalysts 14
Conclusions The materials obtained via Sol-Gel: are homogeneous and show high dispersion of niobium in the matrix keep the nominal composition have high specific surface areas are acid solids show stability and good activity in the epoxidation reaction 15
Thanks for Your attention! 16
Textural and surface properties of SiNb catalysts (4/4) Brönsted acid sites Lewis acid sites 3,0 5Nb 10Nb medium strong weak Absorbance 2.5Nb 2,0 20Nb 1,0 3500 3000 2500 2000 1500 Wavenumbers (cm -1 ) FT-IR spectra of adsorbed acetonitrile vapour at room temperature. 17