E. Marenna a, E. Santacesaria b, P. Pernice a, M. Di Serio b, A. Aronne a

Transcription

1 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, Settembre

2 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) Tanabe, K. Catal. Today 78 (2003) 65. 2

3 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

4 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) Passoni L. C., Siddiqui M. R. H., Steiner A., Korhevnikov I. V., J. Mol. Catal. A 153 (2000) Somma F., Strukul G., Catal. Lett. 107 (2006) 73 4

5 We have obtained homogeneous niobia-silica catalysts via sol-gel synthesis at room temperature which are active and stable in epoxidation reaction 5

6 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

7 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

8 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)

9 Termogravimetry/differential thermal analysis XRD spectra evaporation of solvents Nb eso 10Nb DTA, _TGA atm: air; h.r.= 10 C/min C 3h 110 C Θ evacuation of products of pyrolysis Temperatura ( C) 84 No Nb 2 O 5 segregation it is well dispersed in matrix 9

10 Textural and surface properties Adsorbed N 2 (cm 3 /g) Nb 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 P/P 2.5Nb 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

11 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, Diameter (Angstrom) The 2.5 Nb shows the greater fraction of micropores 11

12 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 ) 1.2x x Nb Weak acid sites (~140 C) SiO x10-5 5Nb 2.5Nb mol NH 3 g -1 min x x x Nb SiO 2 Stronger acid sites (~350 C) 5Nb 10Nb x Temperature ( C) Increasing niobium content Increasing the concentration of stronger sites on 5Nb and 10Nb 12

13 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 % Reaction time = 300 min 10Nb

14 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

15 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

16 Thanks for Your attention! 16

17 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, Wavenumbers (cm -1 ) FT-IR spectra of adsorbed acetonitrile vapour at room temperature. 17