A Novel Glass Fiber-Supported Platinum Catalyst

CHINESE JOURNAL OF CATALYSIS Volume 28, Issue 11, November 2007 Online English edition of the Chinese language journal

Cite this article as: Chin J Catal, 2007, 28(11): 947–952.

Received date: 2007-03-09. * Corresponding author. Tel/Fax: +86-571-87953712; E-mail: [email protected] Foundation item: Supported by the National Natural Science Foundation of China (50473036). Copyright © 2007, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.

RESEARCH PAPER

A Novel Glass Fiber-Supported Platinum Catalyst for Self-healing Polymer Composites: Structure and Reactivity

YANG Haitang1, FANG Zhengping1,2,*, FU Xiaoyun1, TONG Lifang1,2

1 Institute of Polymer Composites, Zhejiang University, Hangzhou 310027, Zhejiang, China 2 Key Laboratory of Macromolecular Synthesis and Functionalization of Ministry of Education, Zhejiang University, Hangzhou 310027,Zhejiang, China

Abstract: A platinum-based catalyst supported on glass fiber grafted with 1,3,5,7-tetramethyl-1,3,5,7-tetravinyl cyclotetrasiloxane (D4Vi) was prepared and evaluated for the hydrosilylation reaction of styrene and methyldiethoxysilane. The silanization of the glass fi-ber was carried out with methyldichlorosilane. D4Vi was then anchored on the modified fiber with Si–H bonds through a hydrosilylation reaction mediated by a Pt(0)-D4Vi complex solution catalyst. Fourier transform infrared spectroscopy, field emission scanning electron microscopy, X-ray energy dispersion spectroscopy, and thermogravimetric data showed that D4Vi was covalently bonded onto the fiber surface. The very high density of D4Vi grafting confers a strong hydrophobic character to the modified fiber surface. Fairly good cata-lytic activity for hydrosilylation between styrene and methyldiethoxysilane was observed on this catalyst.

Key Words: self-healing composite; glass fiber; platinum; supported catalyst; styrene; methyldiethoxysilane; hydrosilylation

Many metal complexes are known to be catalysts for the hydrosilylation reactions [1,2], but the discovery by Speier et al. [3] that hexachloroplatinic acid is a very active catalyst even under ambient conditions has led to Pt complexes be-coming the catalyst of choice for these reactions. Pt-based catalysts are not only useful when alkyl and alkoxysilanes are employed, but they are also not deactivated by chlorosilanes. This versatility and their remarkable turnover frequency ex-plain the dominance of these catalysts in industrial processes [4].

Carrier-supported metal complex catalysts are used more and more widely for their high efficiency. Transition metal complexes can be immobilized on inorganic substrates [5] (such as carbon black [6,7], silica [8,9], -Al2O3, and carbon nanotubes [10]) or organic polymers [5,11] (such as polysty-rene, polyamides, and polyphenylsilanes) and are commonly anchored through some functional linkages. Generally, the catalytic activities of all types of immobilized complexes (in-cluding Pt, Rh, Ni, and Pd) are comparable to that of their homogenous analogs. Sometimes an increase in selectivity or

activity can be noted in the heterogenized state [12]. These properties are useful in meeting the demands of the self-healing polymer composite systems designed by this group [13,14].

Based on previous results, the authors designed an “im-plant” system aimed at developing new self-healing polymer composites [14], which was discovered by Dry et al. [15–17] and further developed by Brown et al. [18–25]. In this de-signed system, healing is accomplished by incorporating a microencapsulated healing agent dispersed in a matrix and a supported catalyst on particulate or fiber fillers. The catalyst is a supported platinum catalyst, which can catalyze the hy-drosilylation reaction quickly in ambient atmosphere. The healing agent can be any kind of organosilane or organosi-loxane that contains Si–H bonds and Si–vinyl bonds. A de-veloping crack in the polymer would rupture the embedded microcapsules to release the healing agent into the plane of the crack through capillary action. Hydrosilylation of the healing agent is triggered by contact with the catalyst sup-ported on the fillers (particles or fibers), and the product of

YANG Haitang et al. / Chinese Journal of Catalysis, 2007, 28(11): 947–952

hydrosilylation bonds the faces of the crack. Thus, the me-chanical properties of the composite material are recovered to some extent. This system possesses a number of important merits over the previous self-healing methodology, including: (1) the mechanical properties are not compromised by the addition of the catalyst because the catalyst is supported on the surface of the filler and the healing reaction occurs in the interface between the filler and the matrix; (2) this system is more readily adapted for the fiber/particle reinforced plastics, which is more common in the industry; (3) the amount of catalyst required is decreased while maintaining healing effi-ciency.

In this paper, the authors report on the preparation and structure of a novel glass fiber-supported platinum catalyst that can meet the demand of self-healing polymer composites. Its merits include: (1) it is effective enough for the hydrosily-lation reaction at room temperature; (2) the catalyst acts as both a reactant and a catalyst, which enhances the interaction between the silane from the healing reaction and matrix.

1 Experimental

1.1 Preparation of the platinum-based catalyst supported on glass fiber grafted with D4Vi

The glass fiber (GF) was obtained from Hangzhou Saint-Gobain Vetrotex Fiber Glass Co. Ltd., China, and treated with Piranha solution prior to use. The glass fiber (2 g) was placed in toluene (50 ml) containing methyldichlorosilane (5 ml, Xinan Chemical and Industrial Group Co. Ltd., China) under a nitrogen atmosphere. The mixture was stirred at reflux temperature for 48 h. The glass fiber was taken away from the mixture after cooling. The silanized glass fiber was then washed three times with an excess amount of aqueous ace-tone. The silanized glass fiber was obtained after drying under vacuum.

A mixture of sodium bicarbonate (0.10 g), H2PtCl6·H2O(0.20 g, Shanghai Chemical Reagent Factory, China), 1,3,5,7-tetravinyl-1,3,5,7-tetramethyl cyclotetrasiloxane (D4Vi, 0.40 g, Shanghai Jiancheng Industrial and Trade Co. Ltd., China), and ethanol (30 ml) were stirred at 70ºC for 24 h. The mixture was then purged with nitrogen to remove the volatiles, followed by the addition of 20 ml of D4Vi. A yel-low liquid, the Pt(0)-D4Vi complex, was obtained after the mixture was cooled to room temperature and filtered.

One gram of silanized GF was added to a mixture of 0.1 ml of Pt(0)-D4Vi complex solution and 10 ml of D4Vi. After stirring at 60ºC for 12 h, the solution was decanted from the mixture, and the solid was washed five times with an excess amount of aqueous ethanol. It was then dried under vacuum (70ºC, 26 kPa) to yield the desired glass fiber-supported Pt complex catalyst (denoted GF-Pt).

1.2 Hydrosilylation of styrene and methyldiethoxysilane

Methyldiethoxysilane was prepared by an alcoholysis reac-tion of methyldichlorosilane. Methyldichlorosilane (0.25 mol, 26.0 ml) was added to a three-necked flask containing xylene (100 ml) equipped with a condenser/N2 inlet and mechanical stirrer, which was airproof. Anhydrous alcohol (0.45 mol, 26.2 ml) was charged into the flask in 30 min. Methyldieth-oxysilane was purified by fractional distillation under vacuum after reaction at 30ºC for 6 h and characterized by FTIR and 1H NMR. IR (neat. cm 1): 3467, 2969, 2929, 2909, 2167, 1407, 1262, 1102, 1046, 962, 891, 848, 807, 768; 1H-NMR (CDCl3): 4.58 (SiH), 3.80 (SiOCH2CH3), 1.24 (SiOCH2CH3),0.20 (SiCH3).

To a solution of methyldiethoxysilane (0.402 g, 0.003 mol) and styrene (0.312 g, 0.003 mol, Shanghai Chemical Reagent Factory, China), catalyst GF-Pt (0.02 g) was added at 25ºC, and the moment of this addition was marked as the beginning of the reaction. The disappearance of starting materials and formation of products were recorded by gas chromatography (GC).

1.3 Catalyst characterization

1H NMR spectra were recorded on an Avance DMX 500 instrument (Bruker, Germany) at 25ºC and referenced to TMS. GC–MS data were obtained on a Trace GC 2000/Trace MS (ThermoQuest, USA) equipped with an HP-5MS capillary column and an electron impact ionizer (70 keV). The Fourier transform infrared spectroscopy (FTIR) study was carried out at room temperature using a Bruker Vector 22 FTIR instru-ment with a resolution of 2 cm 1. The spectra obtained were analyzed in the range 4000–400 cm 1. Thermogravimetric (TG) experiments were carried out on an SDT Q600 TG in-strument (TA, USA), and the measurements began from 50 to 800ºC at a heating rate of 10ºC/min. The scanning electron microscopy (SEM) and X-ray energy dispersion spectroscopy (EDX) data of the samples were acquired using a SIRION-100 field emission scanning electron microscope (FEI, Holland) with an EDX spectrometer.

2 Results and discussion

2.1 Structure of the glass fiber-supported Pt complex catalyst

FTIR spectroscopy of the parent glass fiber gave the lattice and O–H vibrations at approximately 1635 and 3442 cm 1,respectively (Fig. 1(1)). After silanization of the glass fiber with methyldichlorosilane, the bands corresponding to Si–H (2165 cm 1) and Si–C (1406 cm 1) vibration modes are also observed (Fig. 1(2)). The significant decrease in the O–H


band intensity at 3442 cm 1 provided evidence of the grafting of methyldichlorosilane. Finally, the FTIR spectrum of GF-Pt (Fig. 1(3)) showed a single peak at 1091 cm 1, which is the distinct characterization of the vibration mode of the D4 ring, instead of a broad band at 1400–1000 cm 1 that corresponds to the vibration of the Si–O–Si chain. Bands at 2980–2890, 1650, and 1450 cm 1 appeared in the FTIR spectrum of GF-Pt, in contrast to that of the parent glass fiber and the silanized glass fiber. These peaks can be assigned to the –CH2–,–CH=CH2 (stretching vibration), and –CH=CH2 (scissor vi-bration) bands, indicating the presence of the D4Vi ring on the surface of GF-Pt. Furthermore, the significant decrease in the intensity of the Si–H band revealed that most of the Si–H bonds on the surface of the glass fiber have reacted with D4Vi. On the basis of the above results, it can be deduced that D4Vi molecules were covalently bonded to the glass fiber.

Fig. 1 FT-IR spectra of the glass fiber (GF) (1), silanized glass fiber (2), and GF-Pt catalyst (3)

In addition, the strength of the bond between D4Vi and the glass fiber was measured using TG experiments (Fig. 2). For the parent glass fiber, there were two weight loss peaks below 100ºC and at approximately 300ºC corresponding to the re-lease of physisorbed H2O (ca. 3.5%) and dehydroxylation, respectively (Fig. 2(1)). The release of H2O (ca. 1%) was less upon the silanization with methyldichlorosilane (Fig. 2(2)). Additional dehydroxylation and the loss of alkoxysilane fragments occurred with this material above 200ºC. Finally, the very low weight loss (0.2%) at low temperature for the GF-Pt provided evidence of the high hydrophobicity of the surface (Fig. 2(3)). On the other hand, the weight loss occur-ring at 450ºC very likely corresponded to the release of D4Vi after Si–C bond cleavage.

Besides, a very clear shift from a hydrophilic surface (glass fiber) to a hydrophobic surface was observed in the surface treated experiment. Thus, the structure of the bonding be-

tween the glass fiber and D4Vi can be proposed as shown in Scheme 1. There was also evidence that the content of plati-num in GF-Pt was 1.18 at% from the EDX result (Table 1), which represented the composition of the surface.

Further evidence was provided by SEM photos (Fig. 3). A uniform coating was observed after the treatment with me-thyldichlorosilane, whereas the parent glass fiber showed a clear and slick surface. There was a thicker coating on the surface after being further treatment with the D4Vi-Pt com-plex.

The EDX results in Table 1 are not very accurate for calcu-lating the percentages of the elements on the surface because hydrogen cannot be detected by EDX, but they do reflect the trends of the elements. On the basis of the above results, the change of the surface structure shown in Scheme 1 is pro-posed.

2.2 Reactivity of the glass fiber-supported Pt complex catalyst

The hydrosilylation reaction between methyldiethoxysilane (Me(EtO)2SiH) and styrene was used to determine the relative activity of this new catalyst. Fig. 4 shows the conversion ver-sus time for the process of hydrosilylation. It is obvious that

Fig. 2 TG profiles in N2 of the glass fiber (1), silanized glass fiber (2), and GF-Pt catalyst (3)

Table 1 The results of EDX analysis of the samples

Content (%) Sample

Si O Mg Al Ca C Pt

Glass fiber 24.67 57.90 2.72 7.49 7.22 — — Silanizedglass fiber

18.64 36.42 1.45 3.89 4.16 35.44 —

GF-Pt 4.79 25.88 0.01 0.53 0.31 67.30 1.18GF-silane 4.31 16.19 0.21 0.49 0.48 78.24 0.08


this catalyst has fairly good catalytic activity for this reaction. The conversion was over 95% after reaction at 25ºC for 1 h.

2.3 Potential for use in the self-healing polymer compos-ites

To meet the special demand of self-healing polymer com-posites, a novel catalyst that is different from a conventional

hydrosilylation catalyst has to be designed. One way to de-velop a new heterogeneous catalyst from an effective ho-mogenous catalyst is to support the latter on high surface area solids such as graphite, Al2O3, SiO2, zeolite, and so on. The type of the support material and the surface modifier fre-quently plays a crucial role in the performance of the resulting supported catalyst. Basically, the support has to be thermally and chemically stable during the reaction and has accessible

Scheme 1 The changes of the glass fiber surface during the treatment process

Fig. 3 SEM images of the glass fiber (a), silanized glass fiber (b), GF-Pt (c), and GF-silane (d)


and well dispersed active sites. Moreover, the surface modi-fier should provide the support surface with structures re-quired by applications. In the earlier work [8,26], the authors developed an effective SiO2-supported Pt catalyst for hy-drosilylation, where SiO2 is modified by vinyltriethoxysilane.

Here, glass fiber was selected as the support, considering that it is widely used in polymer composites and it is stable during the process. Methyldichlorosilane and D4Vi act as modifiers sequentially to promote a better dispersion of the active sites and good reactivity. The above analysis of the results show that GF-Pt has the structure as shown in Scheme 1, where the D4Vi ring provides more free vinyl to the com-plex with Pt than the vinyltriethoxysilane modifier. Thus, the D4Vi ring is dispersed more symmetrical on the surface of the glass fiber (as shown in Fig. 3), which confirmed that GF-Pt has a better dispersion of active sites.

After the hydrosilylation reaction was complete, the cata-lyst was removed from the reaction mixture and washed three times with aqueous ethanol. The appearance of its surface (Fig. 3(d)) showed that some products of the hydrosilylation reaction are bonded to the glass fiber, indicating the existence of unreacted Si–CH=CH2 on the surface of GF-Pt. This makes this supported catalyst very promising for use in self-healing composites because of the strong bond between the healing agent and fiber.

3 Conclusions

The prepared glass fiber-supported platinum catalyst has high reactivity for the hydrosilylation reaction between sty-rene and methyldiethoxysilane. D4Vi is covalently bonded

onto the glass fiber surface through hydrosilylation with si-lanized glass fiber. The surface structure and composition suggest that this fiber-supported platinum catalyst has good potential for use in self-healing polymer composites.

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Fig. 4 Conversion versus time in the hydrosilylation of Me(EtO)2- SiH with styrene over the GF-Pt catalyst

(1) Catalyst-free, (2) With catalyst

A Novel Glass Fiber-Supported Platinum Catalyst

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