Fourth generation catalysts

Important objectives of the later prodnction methods were to control the size and shape of the catalyst particles during precipitation of the magnesium chloride and to improve stabihty. Catalysts with better-controlled size and shape were based on the reaction of a precipitated magnesium chloride with titanium tetrachloride in a high-boiling-point hydrocarbon diluent at 80°C, with di-isobutyl phthalate added as an internal electron donor. After separation, the sohd formed was reacted with more titaninm tetrachloride at 120°C, before being washed and dried. The catalyst contained between 2-3% titanium and the phthalates used were hmited to C4-C8 esters to avoid potential problems with colloid formation. The catalysts prodnced with phthalates as the internal donor had mnch higher snrface area and pore volume than when ethyl berrzoate was nsed. This method provided more active arrd stereospecifrc catalysts when used with the same triethyl aluminum co-calalyst and an external electron donor such as phenyl triethoxy silane. 

It was pointed out by Tait that the rapid initial increase in the rate of polymerization, with both the spherical magnesinm chloride and magnesium ethoxide catalyst, decayed rapidly in the same way as ball-milled magnesiiun chloride catalysts. The loss of activity was presnmably dne to the small particles or fragments becoming blocked with polymer. 

Particles of catalysts for polypropylene, and the corresponding polypropylene particles can be examined using a scaiming electron microscopy to show that the polymer particle is about 20 times as big as the original catalyst. Porous 

TABLE 8.4. Third- and Fourth-Generation Ziegler-Natta Cataly sts (1977/1983). 

TiCU/MgClj/EB/AlEtj/EA Ethyl benzoate (EB) complexes MgCb- Ethyl anisate (EA) or p-methyl toluate added with cocatalyst. 300 92 

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Efficient and high performance with fourth-generation catalysts ... 

Some of the most relevant fourth-generation catalysts are shown in Figure 1.4. The high steric demand of the ligands, in addition to their particular electronic properties, makes these new-generation catalysts highly selective and efficient for the amination reactions. 

Figure 1.4 Fourth-generation catalysts for the amination of aryl halides.

Fourth generation catalysts produce 25,000 pounds of product per pound of catalyst. This saves money not only for the catalyst, it also means there is so little catalyst left in the polymer that it remains as such in a dechirated state. The atactic content is also so low that it need not be removed. The product is in the form of spherical particles and does not require post-reactor extrusion. 

FIG. 3 (a) How the fourth generation catalyst changes a polypropylene plant, (b) Yields obtained after 2 hr. pol5unerization time, 50°C and 5 atm. with different cocatalytic systems. 

Kulshreshtha and Talapatra 1955 Initial work on stereospedfic polymerization 1960 Commercial solution and slurry processes 1967 Commercial gas phase process 1970-75 Second generation TiCIs-AIEtaCI catalysts 1975-80 Third generation supported cataiysts 1980-85 Super-active third generation catalysts 1990-2000 Fourth generation catalysts ... 

Hartwig JF (2008) Evolution of a fourth generation catalyst for the amination and thioetherification of aryl halides. Acc Chem Res 41(11) 1534-1544... 

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