Studies on the Nature of Catalysis: Suppression of the Catalytic

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Studies on the Nature of Catalysis: Suppression of the Catalytic Activity of Leached Pd by Supported Pd Particles during the Heck Reaction Lin Huang,* Pui Kwan Wong, Jozel Tan, Thiam Peng Ang, and Zhan Wang Institute of Chemical and Engineering Sciences, Agency for Science, Technology and Research, 1 Pesek Road, Jurong Island, Singapore 627833, Singapore ReceiVed: December 18, 2008; ReVised Manuscript ReceiVed: March 30, 2009

In the Heck coupling of bromobenzene and styrene over conventionally prepared Pd particles supported on SiO2 at 135 °C under Ar with Na2CO3 as the base, the catalysis of 2-4 ppm of leached Pd in solution is considerably affected by the supported Pd particles. Although such low concentration leached Pd alone is very active for the Heck reaction, its catalytic activity is suppressed in the presence of the supported Pd particles. Despite this, the variation of TON with leached Pd concentration follows the same trend of a homogeneous Pd catalyst. Homogeneous catalysis in nature is suggested based on the fact that the solid-liquid reaction rate is dependent on the leached Pd concentration rather than the supported Pd loading. The change of homogeneous catalysis relating to the variation of active Pd fraction in the presence and absence of the supported Pd particles is discussed. 1. Introduction Of several commonly used C-C coupling reactions such as the Heck, Suzuki, Stille, and Sonogashira reactions, the most attractive is perhaps the Heck reaction because of a high selectivity for producing fine chemicals including pharmaceuticals in a single operation.1–5 All forms of Pd can be used as precatalysts to carry out simpler Heck reactions (e.g., activation of aryl iodides and bromides). The catalysis by ligand-free supported Pd metal has been attracting great interest in the area of C-C coupling.6–8 Much work has been reported on the application and performance of supported Pd metal catalysts such as Pd0/C, Pd0/oxide, Pd0/zeolite, Pd0/MCM-41, and Pd0/ SBA-15 in a variety of Heck reactions.8–15 It is generally admitted that Pd leaching into the solution from the support occurs inevitably when Pd particles supported on solid are used as precatalysts for the Heck reaction and that in many cases ppm or ppb levels of Pd are sufficient to give high product yields.5,9,10 Therefore, a common concern about the nature of catalysis for the Heck reaction arises where it has been questioned whether the catalysis is truly heterogeneous over Pd particles supported on solid, or whether leached Pd in solution is also active for the Heck reaction. If leached Pd in solution is active, then to what extent does it contribute to the overall catalysis in a mixed catalyst system comprising Pd particles supported on solid and leached Pd in solution? The most common chemical methods for distinguishing homogeneous catalysis from heterogeneous catalysis for the Heck reaction over Pd supported on solid involve catalyst poisoning, filtration (or split test), 3-phase test, and correlation between reaction rate and Pd concentration in solution.5,9,10 By the first three approaches, the catalytic reaction system is subjected to particular control while the reaction proceeds, and the nature of catalysis is identified in terms of the activity of the liquid phase or/and of the solid phase.15–19 For instance, a 3-phase test used a resin to which the substrate was grafted to determine solely the activity of leached Pd in solution irrespective of Al2O3-supported * To whom correspondence should be addressed. E-mail: huang_lin@ ices.a-star.edu.sg.

Pd particles since the resin-grafted substrate is inaccessible to the supported Pd particles.16 This determination led to a conclusion on whether leached Pd in solution is responsibly active for a Heck reaction or whether a Heck reaction proceeds via a homogeneous catalyst. By the last approach, homogeneous or heterogeneous catalysis is judged in terms of whether the reaction rate correlates with the content of leached Pd in solution during a Heck reaction over Pd metal supported on solid.20–24 The advantage of these existing methods is that they can provide efficient evidence for the identity of the true catalyst by comparing the activity of the liquid phase or the reaction profile with the concentration of leached Pd in solution. However, if the concentration of active species in the liquid phase is subject to the coexisting solid phase (as shown later), these methods do not permit us to establish properly the relationship between reaction rate and concentration of liquid active species. In that case, they either cannot apply or have limitations in identifying the nature of catalysis. In fact, the concentration of leached active Pd may vary as a function of the presence and absence of the solid phase, thus leading to the variation of reaction rate in the liquid phase. The observed reaction rate in the liquid phase cannot be simply correlated with the total concentration of leached Pd. Introducing a poison for a solid Pd catalyst may alter the influence of the solid phase on the leached soluble Pd and thus the reaction rate in the liquid phase, which may deviate from the actual situation. Although a 3-phase test can authentically reflect the status of an actual catalyst system under reaction conditions regardless of the concentration of leached active Pd, its use may be much restricted. This is because the decrease of active Pd concentration in the liquid phase probably caused by the presence of a solid Pd catalyst will result in a low catalytic response (low reaction rate) in a 3-phase test. Such a test may offer insignificant evidence for the occurrence of heterogeneous catalysis, especially in harder reactions such as the Heck reactions of aryl chlorides and aryl bromides. On the other hand, the use of catalyst poisoning can hardly ensure that the selective poisons for a homogeneous active species or a heterogeneous active site in a mixed catalyst system work successfully under certain circumstances.25

10.1021/jp811188f CCC: $40.75  2009 American Chemical Society Published on Web 05/15/2009

Studies on the Nature of Catalysis SCHEME 1: Heck Coupling of PhBr and Styrene over Pd0/SiO2

Here we report a case of Heck chemistry using conventionally prepared Pd0/SiO2 as a heterogeneous precatalyst (Scheme 1) where the action of leached Pd in solution is affected by the supported Pd particles. The catalytic activity of leached soluble Pd is seriously suppressed in the presence of the supported Pd particles. The catalysis by leached soluble Pd coexisting is assumed to be negligible in terms of the comparison of reaction kinetic behaviors over different catalyst systems including Pd0/ SiO2, leached Pd, Pd(acac)2, and (Pd(acac)2 + Pd0/SiO2). Nonetheless, the mechanism of homogeneous catalysis by leached Pd is still proposed over Pd0/SiO2 by a study of dependences of reaction rate on Pd loading and on leached Pd concentration. For the first time, this work reveals the effect of supported metal on the catalytic activity of soluble metal in solid-liquid systems. 2. Experimental Section Silica gel (SiO2, Merck grade 10184) having a BET surface area of 366 m2/g, palladium acetylacetonate (Pd(acac)2, 99%), bromobenzene (PhBr, 99%), styrene (>99%), anhydrous sodium carbonate (Na2CO3, g99.5%), anhydrous N,N-dimethylacetamide (DMA, 99.8%), and anhydrous toluene (99.8%) were purchased from Sigma-Aldrich. DMA and toluene were dried and stored over dehydrated 5A molecular sieves before use. The gases H2 and Ar had a purity of 99.999%. In the preparation of a conventional heterogeneous precatalyst Pd0/SiO2, SiO2 (1.0 g) was first dehydrated at 400 °C in flowing air for 8 h before being impregnated with a toluene (5 mL) solution of Pd(acac)2 (0.029 g) at room temperature under Ar. The impregnated system was then stirred under Ar for 2 h followed by evacuation of the solvent to give dry Pd(acac)2/ SiO2. The latter was reduced in flowing H2 at 400 °C for 2 h to produce Pd0/SiO2 containing 1.1% Pd. The Heck coupling of PhBr and styrene was conducted at 135 °C under Ar. To a three-necked flask were introduced 10 mmol of PhBr, 15 mmol of styrene, 5 mmol of Na2CO3, a certain amount of Pd0/SiO2 or/and Pd(acac)2, and 10 mL of DMA under Ar. After the mixture had been stirred at room temperature for 10 min under Ar, the flask was placed in a preheated oil bath with vigorous stirring (500 rpm). The reaction solutions were sampled at the reaction temperature and atmosphere by using 0.45 µm Whatman syringe filters through a septum. The reactants and products in the samples were analyzed by gas chromatography on a Perkin-Elmer Clarus 500 gas chromatograph with a J&W DB-1 capillary column (30 m × 0.320 mm × 1.00 µm) and a flame ionization detector. trans-Stilbene was formed as the dominant coupling product in all cases, giving a selectivity of 91-93% throughout the Heck reaction. In the case with Pd0/SiO2, the solid was filtered off from the reaction mixture after the Heck reaction had ceased and washed with DMA, acetone, deionized water, and acetone. The recovered Pd/SiO2 was dried at 110 °C overnight prior to elemental analysis. The Pd contents in the solution and Pd/SiO2 samples were determined by the inductively coupled plasma (ICP) technique on a Varian Vista-MPX CCD simultaneous ICP-OES spectrograph.

J. Phys. Chem. C, Vol. 113, No. 23, 2009 10121 The procedures for the determination of Pd contents are as follows. For Pd in solution, a certain volume of filtrate collected from the reaction mixture was first subjected to evaporation of the organic components in a glass vessel by heating under vacuum. Then a certain volume of aqua regia was added to the obtained sample. For Pd on the support, a certain volume of aqua regia was added to a certain amount of solid sample directly. After 10 min of gentle boiling, the system was diluted with a certain volume of deionized water followed by filtration with a Whatman syringe filter. The clear aqueous solution thus prepared was analyzed by ICP. Analysis of every sample was performed twice regularly. The microscopic images of Pd particles in the solution and Pd/SiO2 samples were observed by transmission electron microscopy (TEM) on a JEOL TecnaiG2 microscope. The Pd particle size distributions were determined by counting the sizes of approximately 100 particles on several images taken from different places. The evolution of Pd(acac)2 in solution during the Heck reaction was followed by infrared spectroscopy (IR) on a BIO-RAD spectrophotometer. 3. Results and Discussion 3.1. Effect of Pd0/SiO2 on the Catalysis of Leached Soluble Pd. Figure 1 shows the Heck reaction and Pd leaching into the solution profiles at 135 °C under Ar with regards to Pd0/SiO2 with 0.14 mol % of Pd relative to PhBr, using Na2CO3 as the base. First, a typical kinetic curve over Pd0/SiO2 was obtained, on which a PhBr conversion of 19.2% was noticed after 10 h of reaction (Figure 1a). Concurrently, the relevant curve of Pd leaching into the solution from Pd0/SiO2 was determined by ICP in the same system, on which the values between 1.8 and 4.3 ppm were recorded as the reaction proceeded from 10 min to 10 h (Figure 1b). After 10 h of reaction, a small amount of metal black was observed and 74% of Pd was retained on the support according to the ICP analysis of Pd/SiO2. This experiment indicated that the Heck reaction took place over Pd0/SiO2 with a low PhBr conversion together with the production of small amounts of soluble Pd and Pd black in the system. Second, the reaction solutions were drawn off through syringe filters at the reaction temperature and atmosphere, when the reactions proceeded up to 1, 5, and 10 h respectively in three separate experiments. Each filtrate was transferred into an empty flask under Ar. For the purpose of examining the catalytic activity of leached Pd in solution for the Heck reaction, to each filtrate was added an extra amount of Na2CO3 to maintain the initial Na2CO3:PhBr molar ratio of 1:2. The reaction was quickly initiated in each filtrate system under equivalent operating conditions. As a consequence, the reaction kinetics with the three filtrates no longer followed the reaction profile with Pd0/SiO2 but showed sharply increased PhBr conversions with time, as seen in Figure 1c-e. The activity of Filtrate (1 h) led to an increase in conversion from 6.3 to 62% after 4 h. The activity magnitude of the filtrates varied in the order of Filtrate (1 h) > filtrate (5 h) > Filtrate (10 h). Namely, the activity of leached Pd itself decreased with reaction time over Pd0/SiO2. The appreciably high activity of the filtrates compared to that of Pd0/SiO2 significantly implies that the leached Pd at 2.5-4.3 ppm is considerably active itself for the Heck coupling of PhBr and styrene under operating conditions in the absence of Pd0/ SiO2. It was meanwhile noted in ICP that the concentrations of leached Pd in the filtrates remained substantially unchanged during the Heck reaction in the absence of Pd0/SiO2. Third, another filtrate (1 h) obtained in a separate experiment was transferred into a flask containing 0.15 g of SiO2 pretreated at

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Figure 1. Profiles of (a) Heck coupling of PhBr and styrene over Pd0/SiO2 at 135 °C; (b) Pd leaching into solution from Pd0/SiO2 for (a); (c) Heck coupling of PhBr and styrene over Pd0/SiO2 for 1 h then over the isolated leached Pd in solution at 135 °C; (d) Heck coupling of PhBr and styrene over Pd0/SiO2 for 5 h then over the isolated leached Pd in solution at 135 °C; (e) Heck coupling of PhBr and styrene over the isolated leached Pd in solution at 135 °C following (a); and (f) Heck coupling of PhBr and styrene over Pd0/SiO2 for 1 h then over the isolated leached Pd in solution plus SiO2 (1.5 g) at 135 °C.

Figure 2. Heck coupling profiles of PhBr and styrene at 135 °C under Ar (a) over Pd0/SiO2; (b) over Pd0/SiO2 for 1 h, then over the isolated leached Pd in solution for 1.5 h, then over (leached Pd + Pd0/SiO2) for 3 h, and finally over the isolated leached Pd in solution.

400 °C and an additional amount of Na2CO3 to keep the initial Na2CO3:PhBr molar ratio of 1:2. The reaction was run in this system under equivalent operating conditions to check the influence of SiO2 on the catalysis of leached Pd. The resulting reaction profile showed a sharper increase in PhBr conversion with reaction time (Figure 1f) than that in the absence of SiO2 (Figure 1c). On the basis of the activity test of Filtrate (1 h), the reaction solution was drawn off from the reaction mixture with Filtrate (1 h) in another separate experiment through a syringe filter at the reaction temperature and atmosphere at the point where the conversion reached 33.3% as shown in Figure 2b, and transferred back into the initial flask having Pd0/SiO2 and Na2CO3 not consumed under Ar. As a result, it was noted that the reaction slowed down upon initiation with Pd0/SiO2. The conversion no longer ascended rapidly with reaction time but reached a plateau value after 2.3 h. The reaction kinetics with this mixed catalyst system resembled progressively that with Pd0/SiO2. This further indicates that the reaction kinetics over the mixed catalyst system relies on the catalysis of Pd0/SiO2. When the reaction proceeded up to a conversion of 47.6%, the filtrate of the reaction mixture was transferred into the previous flask again. The resulting reaction kinetics continued to follow the reaction profile with Filtrate (1 h) in the absence of Pd0/

SiO2 under the reaction conditions, which shows the restoration of high catalytic activity of leached homogeneous Pd. In the above experiments, the catalysis that the leached Pd displayed is affected by the supported Pd particles coexisting in the mixed catalyst system, namely, the ideal activity of leached homogeneous Pd cannot occur in the presence of the supported Pd particles. Since the catalysis of homogeneous Pd is enhanced in the presence of SiO2 (Figure 1f), it is clear that the unique and strong detrimental factor arises from the supported Pd particles coexisting in the mixed catalyst system. Evidently, the isolated leached Pd can remain much more active than the mixed catalyst system throughout 10 h of the reaction (when leached Pd is separated from Pd0/SiO2 at different reaction stages), and the reaction kinetics with leached homogeneous Pd can vary regularly in the presence and absence of Pd0/SiO2 (Figure 2b). Figures 3 and 4 illustrate the TEM micrographs and Pd particle size distributions of Pd0/SiO2 and leached Pd in solution respectively during the Heck reaction. As a whole, the sizes of the supported Pd particles increased as the Heck reaction proceeded. After 17 h of reaction, the percentage of Pd particles below 10 nm fell from 51.3% to 18.2% and a higher proportion of Pd particles above 20 nm (18.0%) appeared on SiO2. The size of the largest Pd particles reached 30 nm, which occupied

Studies on the Nature of Catalysis

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Figure 3. TEM images and Pd particle size distributions of Pd0/SiO2 (a) before and (b) after 17 h of Heck coupling of PhBr and styrene at 135 °C.

3.6% of the whole supported Pd particles. In the meantime, smaller Pd clusters (e3 nm) began to appear in solution at the initial reaction stage (Figure 4a), which was further indicative of the occurrence of Pd leaching into the solution from Pd0/ SiO2 upon initiation of the Heck reaction. In the presence of Pd0/SiO2, the sizes of soluble Pd clusters increased significantly with reaction time: The percentage of soluble Pd clusters above 3 nm was 25.3% after 1 h and went up to 44.8% after 4 h. The largest soluble Pd clusters were observed at 7 nm in very small proportions. In the absence of Pd0/SiO2, in contrast, the sizes of soluble Pd clusters preformed changed little after 1 h of reaction in view of parts a and d of Figure 4: Soluble Pd clusters below 4 nm did not grow globally and the proportion of the largest soluble Pd clusters at 4 nm was negligible. These results may be explained by assuming that SiO2supported Pd particles leach Pd into the solution on the one hand, and promote active Pd in solution to aggregate on the other hand. Such a dual role has been observed with carbonsupported Pd particles.22,26 In our case, the leaching role of the supported Pd particles is clearly proven through studies of the reaction solutions by ICP and TEM. In the absence of Pd0/SiO2, leached soluble Pd exhibits a high catalytic activity. The observed stabilities in Pd concentration and in Pd cluster size in solution during the Heck reaction suggest that active leached Pd itself is stable at low concentrations without further transformation into larger Pd clusters in solution as the Heck reaction proceeds. On the basis of the Pd leaching into the

solution profile of Pd0/SiO2, the concentration of leached Pd remains relatively stable in a short reaction interval, which seems buffered by the supported Pd particles. Leached Pd in solution is composed of molecular Pd species being catalytically active and Pd clusters catalytically inactive,27,28 both of which maintain a chemical equilibrium in solution within a reaction time. The marked increases in the sizes of both the supported Pd particles and soluble Pd clusters with reaction time suggest that the supported Pd particles play a promoting role in the aggregation of leached Pd to form Pd particles redeposited on Pd0/SiO2. In the presence of the supported Pd particles possibly as an aggregation nucleus, molecular Pd species may aggregate more efficiently than Pd clusters, which results in a smaller fraction of molecular Pd species with the growth of Pd particles in solution. As the reaction proceeds, this fraction becomes smaller and smaller so that the contribution of leached Pd to the catalysis may be negligible (the reaction is slower and slower) since the total concentration of leached Pd in solution is considered nearly constant. The fact that following the Heck reaction over Pd0/ SiO2, Filtrate (10 h) is less active than Filtrate (5 h) that is less active than Filtrate (1 h) supports this. The promoting role vanishes once the supported Pd particles are removed, which is embodied in the fact that the sizes of soluble Pd clusters hardly increase with reaction time (Figure 4d). Naturally, the limited growth of soluble Pd clusters hardly leads to the formation of larger Pd particles and Pd black even after a long reaction time, agreeing with the Pd ICP result in the absence of Pd0/SiO2. In

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Figure 4. TEM images and Pd particle size distributions of the reaction solutions after the Heck coupling of PhBr and styrene over Pd0/SiO2 at 135 °C for 12 min (a), 1 h (b), and 4 h (c), and (d) after the Heck coupling of PhBr and styrene over the isolated leached Pd solution at 135 °C for 1 h following (a).



Figure 5. Heck coupling profiles of PhBr and styrene at 135 °C over (a) Pd0/SiO2, (b) Pd(acac)2 (0.5 ppm Pd), (c) Pd(acac)2 (5.5 ppm Pd), (d) Pd(acac)2 (118 ppm Pd), (e) (Pd(acac)2 (0.5 ppm Pd) + Pd0/SiO2), (f) (Pd(acac)2 (5.5 ppm Pd) + Pd0/SiO2), and (g) (Pd(acac)2 (118 ppm Pd) + Pd0/SiO2).

Figure 6. Heck coupling profiles of PhBr and styrene at 135 °C over (a) Pd(acac)2 (0.5 ppm Pd), (b) (Pd(acac)2 (0.5 ppm Pd) + SiO2), (c) Pd(acac)2 (3.1 ppm Pd), (d) (Pd(acac)2 (3.1 ppm Pd) + SiO2), (e) Pd(acac)2 (18.4 ppm Pd), and (f) (Pd(acac)2 (18.4 ppm Pd) + SiO2.

that case, the concentration of molecular Pd species may increase by erosion of Pd clusters or by equilibrium shift between molecular Pd species and Pd clusters. Hence the reactions become faster with the isolated leached Pd. The reversible decrease and increase in the fraction of molecular Pd species in solution caused by the presence and absence of the supported Pd particles are consistent with the observed reaction rate changes as a function of the presence and absence of the supported Pd particles (Figure 2b). The TEM observations also support this assumption. 3.2. Effect of Pd0/SiO2 on the Catalysis of Pd(acac)2Derived Soluble Pd. Aggregation of active Pd in the leached Pd solution with the supported Pd particles implies that the catalysis to which leached homogeneous Pd copresent with Pd0/ SiO2 contributes is suppressed. To support this with further evidence, we studied and compared the Heck reaction kinetics over Pd(acac)2 and over (Pd(acac)2 + Pd0/SiO2 (0.14 mol % Pd)) as shown in Figure 5. Under similar reaction conditions, a Pd compound has proven to mainly derive into soluble Pd0 species among which molecular Pd0 is a very active species.27,28 Our IR observations showed that Pd(acac)2 was fully decomposed regardless of the presence of Pd0/SiO2 after 5 min of reaction under the reaction conditions. Thus soluble Pd0 species are believed to form from Pd(acac)2 at the initial stage of reaction as well, which include molecular Pd0 and Pd clusters. The former is first formed by reduction of Pd(acac)2 in the reaction media4,11 and subsequently aggregates to form the latter.

In principle, the structure of the active Pd and its transformation process to Pd clusters should not differ between Pd(acac)2 and leached Pd from Pd0/SiO2. We hence chose Pd(acac)2 as a precursor of the active Pd for studying the effect of Pd0/SiO2 on the aggregation and catalysis of the active Pd. In striking contrast with Pd0/SiO2, the use of Pd(acac)2 enabled the Heck reaction to proceed promptly. The conversion reached 77.5% within 6 h over Pd(acac)2 with 0.5 ppm of Pd and 84.7% within 2 h over Pd(acac)2 with 5.5 ppm of Pd. However, mixing Pd(acac)2 and Pd0/SiO2 led to a tremendous decrease in reaction rate compared to that over Pd(acac)2. The observed kinetic curves deviated greatly from those over Pd(acac)2. The conversion was only 34% after 5 h over (Pd(acac)2 (118 ppm Pd) + Pd0/SiO2). The lower the Pd(acac)2 concentration, the more the kinetic pattern with (Pd(acac)2 + Pd0/SiO2) deviates from that with Pd(acac)2 while it resembles that with Pd0/SiO2. As in the case of leached homogeneous Pd, the presence of 0.15 g of SiO2 alone under equivalent conditions did result in obvious enhancement in activity of Pd(acac)2derived homogeneous catalysts as shown in Figure 6. This confirms that the Pd(acac)2-derived homogeneous catalysts are drastically deactivated by the supported Pd particles with (Pd(acac)2 + Pd0/SiO2). Similarly, filtrates containing soluble Pd0 that were collected from the reaction mixtures with (Pd(acac)2 + Pd0/SiO2) exhibited a higher activity than the mixed catalyst systems to a different extent as seen in Figure 7. With respect to the activity enhancement of homogeneous

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Figure 7. Heck coupling profiles of PhBr and styrene at 135 °C (a) over Pd0/SiO2, (b) over (Pd(acac)2 (10.7 ppm Pd) + Pd0/SiO2) for 5 h then over the isolated soluble Pd, (c) over (Pd(acac)2 (118 ppm Pd) + Pd0/SiO2) for 30 min then over the isolated soluble Pd, and (d) over (Pd(acac)2 (118 ppm Pd) + Pd0/SiO2) for 5 h then over the isolated soluble Pd.

Figure 8. Profiles of Pd concentrations in solution during the Heck coupling of PhBr and styrene at 135 °C over (a) (Pd(acac)2 (18.4 ppm Pd) and (b) (Pd(acac)2 (18.4 ppm Pd) + SiO2).

Pd catalysts in the presence of SiO2 alone, the cause seems complicated. From Figure 6, it appears that the extent of enhancement tends to decrease with increasing Pd concentration. In the case with 18.4 ppm of Pd, the activity was slightly increased by the addition of SiO2. During the Heck reaction, similar profiles of soluble Pd concentrations were observed in the presence and absence of SiO2 (Figure 8). At the end of the reaction, it was found that only 3% of Pd was deposited onto SiO2 from the solution. The obtained Pd deposited on SiO2 displayed an obvious activity under equivalent reaction conditions despite a very low Pd loading (Figure 9). The results indicate that the copresence of SiO2 with soluble Pd derived from Pd(acac)2 does not affect the behavior of soluble Pd in the Heck reaction (as also shown below) while it also breeds an extra catalytic source at the same time. Undoubtedly, the Pd deposited on SiO2 resulting from Pd(acac)2 during the Heck reaction contributes to the catalysis in the meantime and enhances the activity of the system. The higher the initial soluble Pd concentration, the greater the mean size of the resulting Pd particles deposited on SiO2, which would result in less enhancement in activity. Figure 10 shows the profiles of soluble Pd concentrations with Pd(acac)2 and with (Pd(acac)2 + Pd0/SiO2) during the Heck reaction. Their comparison interestingly shows that the mixed catalyst systems can preserve a high soluble Pd concentration whereas the homogeneous catalyst systems cannot. With Pd(acac)2 alone, the Pd concentrations in solution decreased with

reaction time. The solutions became nearly colorless and metal black appeared remarkably at the end of reaction. This suggests that molecular Pd species in solution aggregate readily to form Pd black under the reaction conditions in the absence of Pd0/ SiO2, consistent with what has been reported for the Heck coupling of iodobenzene and methyl acrylate with a series of Pd inorganic compounds.29 With (Pd(acac)2 + Pd0/SiO2), the Pd concentrations in solution fell at the initial reaction stage and subsequently ascended with reaction time. The solution colors deepened with reaction time and turned deep brown after 1 h of reaction. A small amount of metal black was observed at the end of reaction. At the same time, the Pd content on Pd/ SiO2 was noticed to exceed the initial Pd loading of Pd0/SiO2 throughout the Heck reaction in the case with (Pd(acac)2 (118 ppm Pd) + Pd0/SiO2). On the other hand, it is noteworthy that the Pd leaching into the solution from Pd0/SiO2 does not surpass 4 ppm during 5 h of reaction in the absence of Pd(acac)2 (Figure 1). Therefore, we reason in the case with (Pd(acac)2 + Pd0/ SiO2) that while molecular Pd species in solution derived from Pd(acac)2 aggregate to give mainly Pd particles deposited on Pd0/SiO2 and partially Pd black during the Heck reaction, Pd0/ SiO2 leaches readily a portion of the supported Pd into the solution in a concerted way. Note that for Pd0/support, the Pd deposition occurs preferentially on the surface of supported Pd particles, as demonstrated previously.26 The catalytic contribution of a trace amount of Pd deposited on the bare surface of SiO2, if any, cannot be seen under such conditions. The results



Figure 9. Heck coupling profile of PhBr and styrene at 135 °C over Pd deposited on SiO2 obtained after the reaction in Figure 6f.

Figure 10. Profiles of Pd concentrations in solution during the Heck coupling of PhBr and styrene at 135 °C over (a) (Pd(acac)2 (118 ppm Pd), (b) (Pd(acac)2 (118 ppm Pd) + Pd0/SiO2), (c) (Pd(acac)2 (10.7 ppm Pd), and (d) (Pd(acac)2 (10.7 ppm Pd) + Pd0/SiO2).

are consistent with the comparative TEM observations shown in Figure 11 on the reaction solutions with Pd(acac)2 (16.3 ppm Pd) and with (Pd(acac)2 (16.3 ppm Pd) + Pd0/SiO2) during the Heck reaction. With Pd(acac)2 alone, the distribution of Pd particle sizes in solution covered a wider range until 9 nm, being centered at roughly 5 nm. The percentage of smaller Pd clusters (

Studies on the Nature of Catalysis: Suppression of the Catalytic

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