Application of Atomic Absorption for Determination of Metal Nanoparticles in Organic-Inorganic Nanocomposites

The Pd-catalyzed Suzuki cross-coupling reaction has been shown as an efficient method for the construction of C-C bonds and plays an important role in pharmaceutical industry and organic synthesis (Makhubela et al., 2010; Venkatesan & Santhanalakshmi, 2010; Zhao et al., 2011). Numerous Pd complexes, such as palladacycles (Mu et al., 2011) and N-heterocyclic carbine (Chanjuan et al., 2008), have been developed for use in these reactions. However, these Pd(0) or Pd(II) complexes cause difficulties in the synthesis and purification of the final product. Another class of catalysts for these reactions, namely heterogeneous catalysts, is easy to prepare and readily separated from the products (Jana et al., 2009; Tamami & Ghasemi, 2010) In catalytic applications, a uniform dispersion of nanoparticles and an effective control of particle size are usually expected. However, nanoparticles frequently aggregate to yield bulk-like materials, which greatly reduce the catalytic activity and selectivity. Therefore, they must be embedded in a matrix such as polymer or macromolecular organic ligands (Sanchez-Delgado et al., 2007; Luo & Sun, 2007). However, nanoparticle-polymer composites usually suffer from disadvantages such as absence of complete heterogeneity and high temperature annealing, which generally causes thermal degradation of organic polymers. In addition, to avoid the problems associated with metal nanoparticles such as homogeneity, recyclability and the separation of the catalyst from reaction system, some other works have focused on immobilizing metal nanoparticles on suitable support materials such as immobilization in pores of heterogeneous supports (Thomas et al., 2003; Jacquin et al., 2003), like ordered mesoporous silica. Although nanoparticle-mesoporous materials are completely heterogeneous, the hydrophilicity of these catalysts causes a reduction in the activity of such catalysts in organic reactions. Therefore, preparation of organic-inorganic hybrid catalysts with a hydrophobe-hydrophile nature is interesting. The discovery of M41S-type ordered mesoporous materials opened a new class of periodic porous solids (Huo et al. 1996). Mesoporous silica structures have been regarded as ideal


Introduction
The Pd-catalyzed Suzuki cross-coupling reaction has been shown as an efficient method for the construction of C-C bonds and plays an important role in pharmaceutical industry and organic synthesis (Makhubela et al., 2010;Venkatesan & Santhanalakshmi, 2010;Zhao et al., 2011).Numerous Pd complexes, such as palladacycles (Mu et al., 2011) and N-heterocyclic carbine (Chanjuan et al., 2008), have been developed for use in these reactions.However, these Pd(0) or Pd(II) complexes cause difficulties in the synthesis and purification of the final product.Another class of catalysts for these reactions, namely heterogeneous catalysts, is easy to prepare and readily separated from the products (Jana et al., 2009;Tamami & Ghasemi, 2010) In catalytic applications, a uniform dispersion of nanoparticles and an effective control of particle size are usually expected.However, nanoparticles frequently aggregate to yield bulk-like materials, which greatly reduce the catalytic activity and selectivity.Therefore, they must be embedded in a matrix such as polymer or macromolecular organic ligands (Sanchez-Delgado et al., 2007;Luo & Sun, 2007).However, nanoparticle-polymer composites usually suffer from disadvantages such as absence of complete heterogeneity and high temperature annealing, which generally causes thermal degradation of organic polymers.In addition, to avoid the problems associated with metal nanoparticles such as homogeneity, recyclability and the separation of the catalyst from reaction system, some other works have focused on immobilizing metal nanoparticles on suitable support materials such as immobilization in pores of heterogeneous supports (Thomas et al., 2003;Jacquin et al., 2003), like ordered mesoporous silica.Although nanoparticle-mesoporous materials are completely heterogeneous, the hydrophilicity of these catalysts causes a reduction in the activity of such catalysts in organic reactions.Therefore, preparation of organic-inorganic hybrid catalysts with a hydrophobe-hydrophile nature is interesting.The discovery of M41S-type ordered mesoporous materials opened a new class of periodic porous solids (Huo et al. 1996).Mesoporous silica structures have been regarded as ideal were placed in a round bottom flask.The mixture was heated to 80 ˚C for 5 h while being stirred under N 2 gas.Then, 0.6 mL (9.89 mmol) aqueous solution of hydrazine hydrate (80 vol.%) was added to the mixture drop by drop in 15-20 minutes.After that, the solution was stirred at 60 ˚C for 1 h.Afterwards, the solution was filtered and precipitated, and was washed sequentially with chloroform and methanol to remove excess N 2 H 4 .H 2 O and was dried in room temperature to yield palladium nanoparticle-poly(N-vinyl-2pyrrolidone)/SBA-15 composite (Pd-PVP/SBA-15).In order to specify the amount of Pd, the composite should be decomposed by perchloric acid, nitric acid, fluoric acid and hydrochloric acid.For this purpose, Pd-PVP/SBA-15 nanocomposite (0.1 g), perchloric acid (0.5 mL), nitric acid (0.5 mL) and the known amount of water were placed in a round bottom flask.The mixture was heated for 1 h to evaporate the liquids.Then, to the precipitated, hydrofluoric acid (2.5 mL) and hydrochloric acid (2.5 mL) were added and the mixture was heated again for 2 h to evaporate the liquids.In the last step, hydrochloric acid (0.5 mL) and water (5 mL) were added to the precipitated and the mixture was heated for 30-60 minutes.Then, the Pd content of the catalyst was estimated by Atomic Absorption Spectroscopy (AAS).The Pd content of the catalyst estimated by AAS was 0.56 mmol g -1 .

General procedure for Suzuki-Miyaura coupling reaction
The general procedure was as follows (Scheme 2): In the typical procedure for Suzuki-Miyaura coupling reaction, a mixture of iodobenzene (1 mmol), phenylboronic acid (1.5 mmol), K 2 CO 3 (5 mmol), and catalyst (0.12 g, Pd-PVP/SBA-15) in H 2 O (5 mL) was placed in a round bottom flask.The suspension was stirred at room temperature for 8 h.The progress of reaction was monitored by Thin Layer Chromatography (TLC) using n-hexane as eluent.After completion of the reaction (monitored by TLC), for the reaction work-up, the catalyst was removed from the reaction mixture by filtration, and then the reaction products were extracted with CH 2 Cl 2 (3×5 mL).The solvent was removed under reduced pressure.The crude product was purified by flash column chromatography (hexane or hexane/ethyl acetate) to afford the desired coupling product (98% isolated yield).One intense peak at about 0.9-0.95°and two weak peaks at about 1.5-1.65°and 1.7-1.9°can be indexed as ( 100), ( 110), and ( 200) reflections associated with two-dimensional hexagonal symmetry (P6mm) (Kalbasi et al., 2010).The PVP/SBA-15 and Pd-PVP/SBA-15 (2Ө = 0.7-6) samples show the same pattern indicating that the structure of the SBA-15 ( 100) is retained even after the support of the surface of the SBA-15 with PVP and Pd (Fig. 1).However, the intensity of the characteristic reflection peaks of the PVP/SBA-15 and Pd-PVP/SBA-15 (2Ө = 0.7-6) samples are found to be reduced (Fig. 1).This may be attributed to the symmetry destroyed by the hybridization of SBA-15 which is also found in the ordered mesoporous silica loading with guest matter (Kalbasi et al., 2010).In addition, composites contain much less SBA-15 due to the dilution of the siliceous material by PVP and Pd; therefore, this dilution can also account for a decrease in the peak intensity.The wide angle XRD pattern of the Pd-PVP/SBA-15 nanocomposite is shown in Fig. 1.All diffraction peaks in the XRD pattern are from the fcc Pd (Harish et al., 2009).The average grain sizes of the Pd nanoparticles in the nanocomposite were estimated from the full width at half maximum of the diffraction peaks using Scherrer equation.The average grain size of the Pd nanoparticles is about 7 nm for Pd-PVP/SBA-15.The average grain size of the Pd nanoparticles is the same as the average particle size of the Pd nanoparticles determined by the TEM observations, which indicates that the Pd nanoparticles are single crystals.

TEM
The morphologies of SBA-15 (Fig. 2a), Pd-PVP/SBA-15 nanocomposite (Fig. 2b) and the distribution of Pd nanoparticles in the Pd-PVP/SBA-15 (Fig. 2c) were studied by the TEM observations.The typical TEM micrographs (Fig. 2) clearly show that the Pd-PVP/SBA-15 nanocomposite have a hexagonal pore array structure.In the side view micrographs of the nanocomposite (Fig. 2b), the dark spots are the Pd nanoparticles.It can be seen in Fig. 2 that most of the Pd nanoparticles are distributed within the channels of the mesoporous silica.The particle sizes of the Pd nanoparticles in Pd-PVP/SBA-15 are between 3 and 5 nm (Fig. 2c). of PVP/SBA-15 (Fig. 3b), the new band at 1666 cm -1 is corresponds to the carbonyl bond of PVP (Iwamoto et al., 2009).Moreover, the presence of peaks at around 2800-3000 cm -1 corresponds to the aliphatic C-H stretching in PVP/SBA-15 (Fig. 3b).The appearance of the above bands showed that PVP has been attached to the surface of SBA-15 and the PVP/SBA-15 has been obtained.As shown in Pd-PVP/SBA-15 spectrum (Fig. 3c), the band around 1666 cm -1 which corresponds to carbonyl bond of PVP, is shifted to lower wave numbers (1639 cm -1 ) (red shift).Moreover, the peak intensity of the carbonyl bond in the spectrum of Pd-PVP/SBA-15 is lower than that of PVP/SBA-15.This may be due to the interaction between the Pd nanoparticles and C=O groups.This means that the double bond CO stretches become weak by coordinating to Pd nanoparticles.Thus, it is confirmed that PVP molecules exist on the surface of the Pd nanoparticles, and coordinate to the Pd nanoparticles (Metin et al., 2008;Hirai et al.,1985).type hysteresis loops indicative of mesoporous materials with one-dimensional cylindrical channels (Kalbasi et al., 2010) and narrow pore size distributions.The BET surface areas, the BJH pore diameters, and the pore volumes for the SBA-15, PVP-SBA-15 and Pd-PVP/SBA-15 nanocomposite are summarized in Table 1.A specific surface area of 1430 m 2 /g, a pore volume of 1.9 cm 3 /g, and a pore diameter of 9.9 nm are obtained from the isotherm of SBA-15.After hybridization with PVP through in situ polymerization, PVP/SBA-15 exhibits a smaller specific area, pore size and pore volume in comparison with those of pure SBA-15, which might be due to the presence of polymer on the surface of the SBA-15 (Table 1 and Fig. 4).However, there is a noticeable increase in pore diameter for Pd-PVP/SBA-15, and the pore volume of Pd-PVP/SBA-15 is smaller than that of PVP/SBA-15.It might be due to the incorporation of Pd nanoparticles into the pores of PVP/SBA-15 composite (Chytil et al., 2005).
According to the results, Pd-PVP/SBA-15 still has a mesoporous form with reasonable surface area and it is suitable to act as a catalyst.
Sample BET surface area (m 2 g -1 ) V P (cm 3 g -1 ) a BJH pore diameter (nm)  The mass loss at temperature <100 °C (around 6%, w/w) is attributed to desorption of water present in the surfaces of the SBA-15 (Fig. 5a).The TGA curves of PVP show a small mass loss (around 7.5%, w/w) in the temperature range 50-150 °C, which is apparently associated with adsorbed water (Fig. 5b).At temperatures above 200 °C, PVP shows one main stage of degradation.The mass loss for PVP in the second step is equal to 80% (w/w) which corresponds to the effective degradation of the polymer (Fig. 5b).Thermo analysis of PVP/SBA-15 shows two steps of mass loss (Fig. 5c).The first step (around 3%, w/w) that occurs at temperature <150 °C is related to desorption of water.The second step (around 9%, w/w) which appeared at 220 °C is attributed to degradation of the polymer, and the degradation ended at 400 °C (F i g .5 c ) .B y c o mparing the PVP and PVP/SBA-15 curves, one can find that PVP/SBA-15 has higher thermal stability and slower degradation rate than PVP (Fig. 5b,c).Therefore, after hybridization, the thermal stability is enhanced and this is very important for the catalyst application.However, for Pd-PVP/SBA-15 sample, two separate weight loss steps are seen (Fig. 5d).The first step (around 5%, w/w) appearing at temperature <100 °C corresponds to the loss of water.The second weight loss (about 200-500 °C) amounts around 6% (w/w) is related to the degradation of the polymer.Obviously, the hybrid Pd-PVP/SBA-15 shows higher thermal stability than PVP/SBA-15.It may be attributed to the presence of Pd nanoparticles in the composite structure.Therefore, it is very important for the catalyst application that the thermal stability was enhanced greatly after hybridization.

Uv-Vis
Fig. 6 displays the result of UV-Vis spectra of Pd-PVP/SBA-15.The UV-Vis spectra of Pd(OAc) 2 which reveal a peak at 400 nm refer to the existence of Pd(II) (Ahmadian et al.,2007).As mentioned in the experimental section, and according to scheme 1, Pd nanoparticle-PVP/SBA-15 was prepared by adding hydrazine hydrate to the Pd (II)-PVP/SBA-15.However, as can be seen in Fig. 6, there isn't any peak at 400 nm in the UV-Vis www.intechopen.comspectra of Pd-PVP/SBA-15, which indicates complete reduction of Pd(II) to Pd nanoparticles.

Catalytic activity
In this section, we firstly investigated the corresponding parameters for the Suzuki crosscoupling reaction.It includes different solvents and bases for the room-temperature Suzuki reaction, of iodobenzene, and phenylboronic acid in the presence of 0.12 g (Pd-PVP/SBA-15) catalyst.The molar ratio of iodobenzene to phenylboronic acid was set at 1:1.5 for the Suzuki cross-coupling reaction.Solvent plays a crucial role in the rate and the product distribution of Suzuki-Miyaura coupling reactions.Since water is known to increase the activity of the Suzuki-Miyaura catalyst (Bedford et al., 2003), two kinds of solvents were used: H 2 O or MeOH/H 2 O (3:1 v/v).In recent years, a large number of studies have been devoted by academic and industrial research groups to the development of environmentally friendly processes.In this context, the use of water as a reaction medium in transition metalcatalyzed processes has merited increasing attention and is currently one of the most important targets of sustainable chemistry (Anastas et al., 2000).Water, an inexpensive, readily available, non-in flammable, non toxic solvent, provides remarkable advantages over common organic solvents both from an economic and an environmental point of view.
The experimental results show that the time the reaction is completed is rarely shorter in the case of using MeOH/H 2 O (3:1 v/v) as a solvent, but neat H 2 O was chosen because of the advantages of using water that were mentioned above.
We then examined the effect of bases for the Suzuki reaction.The inorganic bases including K 2 CO 3 and Na 3 PO 4 afforded high yields of 70-98%, as shown in Table 2.However, the organic base NEt 3 gave a lower yield of 30% as shown in Table 2, entry 3. Thus, K 2 CO 3 was selected as the base and H 2 O as the solvent.
Table 3. Suzuki-Miyaura reaction of aromatic aryl halides and phenylboronic acid catalyzed by Pd-PVP/SBA-15 a Using our optimized reaction conditions, we selected a series of aryl iodides, some aryl bromides and aryl chloride in the Suzuki reaction.All reactions were performed using 1:1.5 stoichiometric ratios of aryl halides and phenylboronic acid in air.The results are summarized in Table 3.Reactions were carried out in water at different times.In some cases, coupling reactions with aryl iodide (entry 3), aryl bromide (entry 4) and aryl chloride (entry 7) required higher temperature (95 °C) in order to obtain excellent yields.The results are listed in Table 3.It is well known that activation of C-Cl bond is much more difficult than C-Br and C-I bonds, and in general requires harsher reaction conditions in a heterogeneous catalysis system (Yin & Liebscher, 2007;Martin & Buchwald, 2008).Thus, the catalyst afforded average to excellent yields of biaryl products even at room temperature.In the literature, only a few catalysts are known for affecting the Suzuki-Miyaura cross-coupling reactions under mild conditions (Littke et al., 2000;Cuia et al., 2007;Marion et al., 2006, Navarro et al., 2006;Fairlamb et al., 2004).
The catalyst reuse and stability were checked using Suzuki reaction of iodobenzene with phenylboronic acid at room temperature in present water as solvent.The catalyst was separated from the reaction mixture after each experiment by simple filtration, washed with diethylether and acetone and dried carefully before using it in the subsequent run.The results showed that this catalyst could be reused without any modification, 5 times.It should be mentioned that there was rarely low catalyst leaching (4.5%) (Pd content of the catalyst was determined by AAS after each cycle) during the reaction and the catalyst exhibited high stability until 5 recycles (Table 4).

Conclusion
In this chapter, we demonstrated a facile palladium nanoparticle-PVP preparation inside modified mesoporous silica and the utilization of this nanocomposite as a new heterogeneous organic hybrid catalyst system.The catalytic activity of this catalyst was excellent for Suzuki-Miyaura cross-coupling reaction of aryl chloride, bromide and iodides at room temperature under aerobic conditions.Further, easy catalyst recovery and reasonable recycling (at least 5 times) efficiency of the catalyst made it an ideal system for coupling reactions in the aqueous phase.For determination of the amount of remained Pd on the catalyst structure, atomic absorption spectroscopy was used.There are various methods such as XRD, BET, TEM, FT-IR, UV-Vis and etc. for characterization of solid catalysts.Estimation of the stability of the catalysts is one the important factors for the performance of the catalysts.The stability of the catalysts is directly related to the amounts leaching of the active species of the catalysts.AAS is the best technique for determination of the metal leaching in the catalysts containing metal nanoparticles.

Table 2 .
Effect of different bases on Suzuki-Miyaura reaction a

Table 4 .
The catalyst reusability for the Suzuki-Miyaura reaction a