Heterogeneous catalysts testing

Wilkin, O.M., Maitlis, P.M., Haynes, A. and Turner, M.L. (2003). Mid-IR spectroscopy for rapid on-line analysis in heterogeneous catalyst testing. Catal. Today, 81, 309-317. 

In the presence of copper perchlorate, the heterogenized catalyst was successfully used for the test Diels-Alder reaction and proved to be similarly active and enantioselective than its homogeneous counterpart. It was further-... 

In the case of functionalized olefins, heterogeneous catalysts have usually not performed well [84,85]. Methyl oleate is the typical test substrate the following order of stability towards the ester functional group for the different SOM catalysts has been observed (based on the number of turnovers) Re W > Mo, which is similar to what has been described for related homogeneous systems. In the specific case of Re, 900 TON can be reached, while TON for other Re-based heterogeneous catalysts do not exceed 200 [79,84]. 

Mercury is a classical test to identify heterogeneous catalysts (bulk metal or colloids) due to its ability to poison metal(O) heterogeneous catalysts by formation of amalgam or adsorption on the metal surface [23]. If the catalytic activity remains unaffected when mercury is present, this fact represents an evidence for a homogeneous catalyst. But mercury can induce side reactions [23c] and also react with some molecular complexes [23c,24]. Consequently, the results obtained with mercury are not enough to conclude about the catalyst nature. From a practical point of view, it is important to use a large excess of Hg(0) with respect to the catalyst to favour the contact with it. 

This test was successfully applied for the hydrosilylat-ion of trimethyl(vinyl)silane by triethoxysilane catalysed by Rh or Pt colloids. The addition of mercury to catalytic mixture led to catalytic activity loss, consistent with a heterogeneous catalyst [12,14]. 

Senkan, S., Krantz, K., Ozturk, S. et al. (1999) High-throughput testing of heterogeneous catalyst libraries using array microreactors and mass spectrometry. Angew. Chem. Int. Ed., 38, 2794. 

The addition of dopants is found to have beneficial effects. However, they are not restricted only to transition metals. The hydrogenation of acrylic acid can be promoted significantly by the addition of neodymium ions onto the palladium particles [142], The selective transformation of 3,4-dichloronitrobenzene to the corresponding aniline has been selected to test pre-prepared Pt hydrosols as heterogeneous catalyst precursors (see Figure 3.9) [143],... 

Keeping in mind the industrial usage of the heterogeneous catalysts, the leaching and recycle ability of all polymer-supported catalysts has also been tested. All these catalysts are stable and do not leach during the catalytic reactions. 

Table 3. In order to test the hypothesis that heterogeneous catalyst is better than the homogeneous one, beeause the former allows improving the control over iron ions during the Fenton ehemistry, several consecutive degradations were performed with a fixed amount of eatalyst. Figure 19 shows the time needed for four consecutive batch degradations of 100 ml 50 mM Na2S04 (pH 2.8) + 0.06 mM RB5 + 10.7 mM H2O2 (twice of the stoiehiometrie amount required) + 0.1 g eatalyst Cl. At the end of each discoloration the solution was allowed to settle down into the reaetor and the solution was withdrawn carefully. Then, a fresh solution (100 ml 50 mM Na2S04 + 0.06 mM RB5 + 10.7 mM H2O2) was reloaded over the previous 0.1 g eatalyst Cl to start a new RB5 discoloration. The process was repeated three times.

In the present work, some promising results obtained with this kind of asymmetric heterogeneous catalyst, based on silica-supported Ni, Rh and Pt, chemically modified with chiral organotin compounds, are presented. The systems were tested in the enantioselective hydrogenation of ethyl pymvate, acetophenone and 3,4-dimethoxyacetophenone. The stabiUty of these catalysts was also studied to check if they could be reused. 

Ln = Sc, Y, La), and have been tested as heterogeneous catalyst in the Diels-Alder cyclization of terH-methoxy-3-trimethylsilyloxy-l,3-butadiene with benzaldehyde (Danishefsky transformation of 1,3-dienes), and proved active [51]. The surface yttrium bis-dionate has also been synthesized directly from the molecular precursor [Y( Bu-COCHCO-"C3F7)3] by reaction with MCM-41 surface silanols [51]. 

As the Beckmann rearrangement is believed to be a typical acid-catalysed reaction, many researchers have reported the relationship between the vapour phase reaction catalysis and the acidity of the catalysts tested on non-zeolitic catalysts - i2s- i3i. 318-334 and on zeolitic catalysts Another interesting point for the heterogeneous gas-phase Beckmann rearrangement is the location of the reaction on the catalyst and different studies have been published ° . The outer surface of the catalyst particle seems to be the most probable place for the Beckmann rearrangement supported by the traces of reagents, and notable amounts of by-products found only in the outer layers of the zeolite crystal. Development of new and more efficient catalysts have also been reported " . ... 

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