PYROLYSIS STUDY OF HYDRIDE-FUNCTIONALISED SOL-GEL SILICA

Transcription

1 PYRLYSIS STUDY F YDRIDE-FUNCTINALISED SL-GEL SILICA Renzo Campostrini, Adriana curelli, Giovanni Carturan, Marco Ischia

2 X(R) 3 alkoxides are precursors of 2 -based xerogels and advanced materials. -X functional groups, not hydrolyzable, do not hamper the formation of -- networks and -X bonds are maintained during the gelling process. This fact determines a wide variability of xerogel features: porosity, surface area, hardness, wettability.

3 Advantages of supports prepared by the sol-gel method high specific surface area and porosity control shape variability possible use as coating of inert materials specific surface reactivity for anchoring of metallic species chemical stability and inertness

4 Disadvantages of (R) 4 shrinkage above 400 C sudden decrease of the specific surface area and porosity To avoid these disadvantages (R) 4-y X y X=R,, F Release at about 400 C of X 4 species with increase of specific surface area and porosity by mass loss.

5 In the perspective to obtain metallic dispersion in a silica matrix, (R) 3 alkoxide precursor presents further advantages. (1) - reduction can be exploited for in situ metallic ion reduction. (2) 4 release at high temperature favours surface area maintenance and exposition of metallic clusters.

6 Characteristics (C 3 ) 3 presents a simple molecular structure and an exalted reactivity of -C 3 hydrolysis and condensation, due to low steric hindrance of the methoxy group and polarity of - bond.

7 Solution behavior of (C 3 ) 3 1 -NMR spectrum of the commercial silicon alkoxide is registered in TF-d 8. gnal pattern indicates the predominant presence (80%) of (C 3 ) 3. The ratio of the hydrogen integrated signals /(C 3 ) 3 is This product is unstable; aging affords oligomeric species, still holding -.

8 Reactivity of (C 3 ) 3 Tetrahydrofuran-d 8 silicon alkoxide solutions are prepared at -80 C with different volumes of D M Cl solution. Spectra are recorded starting from -80 C to 25 C, monitoring the solutions on time up to 2 days aging. 1 -NMR spectra are independent of /TFd 8 /D 2 /Cl molar ratio and indicate that -C 3 hydrolysis is completed.

9 1 -NMR spectra 3.52 ppm -(C 3 ) ppm C 3 D NMR spectrum of commercial (C 3 ) 3 From the NMR data: a very fast hydrolysis at -80 C a progressive increase of the molecular complexity as the temperature is increased at 25 C. NMR spectra after hydrolysis at -80 C (up), and at 25 C (down).

10 Preparation of -gel Transparent colourless granular solid

11 Characteristics of -gel Surface area m 2 g -1 Average pore diameter nm Pore volume cm 3 g -1 Density g cm -3 Sample 80 C 72(±2) 3.0(±0.1) (± ) 1.59(±0.02) Sample 500 C 360(±2) 3.5(±0.1) (± ) - Sample 1000 C 190(±2) 3.1(±0.1) (± ) -

12 IR results decrease of the - signal shift of the - signals to higher frequencies (increased cross-linking network)

13 Pyrolysis process TG curve, recorded at 10 C/min, shows a continuous mass loss in the range C with a total intensity of 7.6%. This loss can be subdivided in 3 events: a first step in the C 1.6% a second step in the C 4.8% a latter step in the C 1.2%

14 TG/TIC curve TIC curve is characterized by two peaks: a small peak centred at 170 C an intense asymmetric band at 420 C.

15 TG/GC/MS analysis Gas chromatographic elutions of the gas phase released at 170 and 420 C during the pyrolysis of the crude-gel.

16 Ion currents Thermogravimetric mass spectrometric coupled analysis of -crude-gel. Ion currents of selected ions used to monitor specific evolved species during pyrolysis process.

17 Thermal behaviour of -crude-gel 2 Reaction 1: water released by silanol groups condensation. 2 Reaction 2: silane released by /- bond exachange. C C Reaction 3: methoxysilane release by -/- bond exchange. Reaction 4: disiloxane release by -/- exchange.

18 Chemical composition of gas mixture Chemical species j (molecular mass) 2 (18.02) Dihydrofuran (70.09) 4 (32.12) 3 C 3 (62.14) 3 3 (78.22) (124.32) Molar precentages Moles Nominal formula to describe chemical composition of -crude-gel: () (C 3 ) 0.002

19 Conclusions The gelling of (C 3 ) 3 mixtures in tetrahydrofuran under acid catalysis affords at 80 C a solid gel preserving 44% of original - bond. The peculiar reactivity of the hydride-methoxysilane mixture is considered to account for the low porosity and high density of the derived gel.