Ester 123 Heterogeneous catalyst

There have been many approaches published to "immobilize" a homogeneous catalyst. These heterogeneous catalysts must have better activity, selectivity, lifetime or some other property to warrant their use in an industrial chemical process. This talk will draw upon the author s experiences at DuPont and Air Products with Nation , Nafion on carbon, anionic attached [Rh(CO)2l2] to Reillex-425, carbons as catalysts for the hydrolysis of esters, heterogeneous catalyst to activate dimethyl ether as a methyl transfer reagent, and monoliths as hydrogenation catalysts. These individual experiences will show the complexities of a simple problem - "immobilization". 

Fatty acid methyl ester Heterogeneous catalyst Homogeneous catalyst T ransesterifi cation Triglyceride Vegetable oils... 

Hydrogenolysis Process. Patty alcohols are produced by hydrogenolysis of methyl esters or fatty acids ia the presence of a heterogeneous catalyst at 20,700—31,000 kPa (3000—4500 psi) and 250—300°C ia conversions of 90—98%. A higher conversion can be achieved using more rigorous reaction conditions, but it is accompanied by a significant amount of hydrocarbon production. 

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]. 

Roche carries out asymmetric hydrogenation of a p-keto-ester for a pancreatic lipase inhibitor using their Ru (II) BIHEHP catalyst. For scaling up, Roche decided to use a heterogeneous catalyst, modified Ni /L-tartaric acid with NaBr, since this was economically more attractive. 

Jacobsen proposed a related heterogeneous catalyst in which the complex is bound via an ester linkage to commercial hydroxymethylpolystyrene beads. Such catalysts are of value not only in large-scale processes, but also in the combinatorial automated synthesis of libraries of new compounds.180... 

The first example of a heterogeneous catalyst able to metathesize olefinic esters was recently reported (92). The combination of Re207/ (CH3)4Sn at an olefin/Re/Sn molar ratio of 219/6/1 converted methyl 4-pentenoate at 50°C in 51% conversion to ethylene and the corresponding dimethyl ester of 4-octene-l,8-dioic acid. This reaction exhibited a high degree of selectivity (>99%), and in the absence of (CH3)4Sn the rhenium catalyst was inactive (90). 

Although in general functionally-substituted olefins are unreactive, the disproportionation of acrylonitrile with propylene and of certain unsaturated fatty acid esters have been recently reported (8, 9). The disproportionation of acetylenes has been observed over heterogeneous catalysts (10). 

Finally, a titanium(IV) pillared clay (Ti-PILC) catalyst has been prepared.71 In the presence of tartaric acid esters as chiral ligands Ti-PILC is an effective, heterogeneous catalyst for the asymmetric epoxidation of allylic alcohols. Enantioselectivities were comparable to those observed in the homogeneous system - and reactions could be carried out at concentrations up to 2M with a simple work-up via filtration of the catalyst. 

A large number of heterogeneous catalysts have been tested under screening conditions (reaction parameters 60 °C, linoleic acid ethyl ester at an LHSV of 30 L/h, and a fixed carbon dioxide and hydrogen flow) to identify a suitable fixed-bed catalyst. We investigated a number of catalyst parameters such as palladium and platinum as precious metal (both in the form of supported metal and as immobilized metal complex catalysts), precious-metal content, precious-metal distribution (egg shell vs. uniform distribution), catalyst particle size, and different supports (activated carbon, alumina, Deloxan , silica, and titania). We found that Deloxan-supported precious-metal catalysts are at least two times more active than traditional supported precious-metal fixed-bed catalysts at a comparable particle size and precious-metal content. Experimental results are shown in Table 14.1 for supported palladium catalysts. The Deloxan-supported catalysts also led to superior linoleate selectivity and a lower cis/trans isomerization rate was found. The explanation for the superior behavior of Deloxan-supported precious-metal catalysts can be found in their unique chemical and physical properties—for example, high pore volume and specific surface area in combination with a meso- and macro-pore-size distribution, which is especially attractive for catalytic reactions (Wieland and Panster, 1995). The majority of our work has therefore focused on Deloxan-supported precious-metal catalysts. 

In 1925, Fischer and Tropsch developed a process for producing a mixture of saturated and unsaturated hydrocarbons, and oxygenated products such as alcohols and esters by the reaction of synthesis gas (a mixture of CO and H2) using heterogeneous catalysts of Fe and Co (eq. 1.1) [1],... 

These esters can be used as fragrances with harmonious fruit odor or as intermediates for the syntheses of herbicides (e.g. triazinones) and of L-amino acids. Also in this reaction, the weakly acidic boron pentasil zeolite and even better the Cs-doped boron pentasil zeolite are the most favored catalysts and are superior to other heterogeneous catalyst systems (Fig. 15.2). 

---