1-hexene Phillips process

Polymerizations that use supported chromium (Phillips) catalysts are conducted predominantly in slurry processes (though a small portion employs the gas phase process, see below). The historical development of the Phillips process has been expertly reviewed by Hogan (5, 6) and McDaniel (7-9). The slurry process originally developed by Phillips Petroleum (now Chevron Phillips) has been called the "particle form loop slurry process" and the "slurry loop reactor process" for production of HDPE and LLDPE (10). Hexene-1 is most often used as comonomer for LLDPE in the Phillips process. A simplified process flow diagram for the Phillips loop-slurry reactor process is shown in Figure 7.3 and key operating features are summarized in Table 7.4. 

The LLDPE product represents the outcome of a method developed to produce a low-density polyethylene but by using the more moderate, and therefore less costly, conditions employed by the processes used to produce high-density polyethylene. It is not, strictly speaking, a polyethylene since it is not a homopolymer of ethylene. An LLDPE is actually a copolymer of ethylene, which includes traces of 1-octene (Dow and Du Pont), 1-hexene (Phillips), or 1-butene (Union Carbide). This results in a polymer that has entirely short branches, and these are more uniformly spaced along the backbone than in LDPE. The spacing of the branches obtained in these cases can be closely controlled by the proportion of a-olefin to ethylene used in the feed, and the lengths by the choice of the a-olefin comonomer. Properties intermediate to those of low- and high-density polyethylene are obtained for the product (Table 23.2). 

Poly(1-butene co ethylene co 1-hexene) n A copolymer produced commercially by the Phillips process and containing up to 5% butene-1. It is similar to high density polyethylene but has a slightly lower density and better resistance to creep. 

Another, highly selective oligomerisation reaction of ethene should be mentioned here, namely the trimerisation of ethene to give 1-hexene. Worldwide it is produced in a 0.5 Mt/y quantity and used as a comonomer for ethene polymerisation. The largest producer is BP with 40 % market share utilizing the Amoco process, formerly the Albemarle (Ethyl Corporation) process. About 25 % is made by Sasol in South Africa where it is distilled from the broad mixture of hydrocarbons obtained via the Fischer-Tropsch process, the conversion of syn-gas to fuel. The third important process has been developed by Phillips. 

Phillips Petroleum discloses a process for the trimerization of ethylene to 1-hexene [13]. According to patents, the process employs a complex catalyst system comprising 2,5-dimethylpyrrole, triethylaluminum, and diethylaluminum chloride in combination with a chromium(III) salt in the presence of a solvent. The purity of 1-hexene in the hexene fraction is reported higher than 99%. The main byproducts are decenes and polymer. 

In 1968, new copolymers were introduced containing 1-hexene instead of 1-butene. This change provided improved physical properties of polymers made with Cr/silica catalysts. A new process to produce LLDPE was announced by Phillips in 1969 [22,23]. Polymers with densities as low as 0.925 g mL-1 were produced in a modified PF process with chromium-containing catalysts. Nevertheless, polymers with densities <0.93 g mL 1 were not common among Phillips licensees because of the tendency of low-density polymer to swell, and this swelling limited reactor output. 

This product distribution has been the basis of the Phillips commercial 1-hexene process discovered in 1989 [674]. Clearly, there is another mechanism involved in addition to the traditional growth reaction. It has been suggested that a metallacyclic intermediate may be involved, as in Scheme 43. The key to the selective formation of 1-hexene lies in the relative stability toward intramolecular hydride transfer of the chromacyclopentane ring relative to that of the chromacycloheptane ring, consistent with the known behavior of other metallacycles [675-678]. 

Copolymerization with a-olefins over a Phillips catalyst is a key method for controlling the density and microstmctures of the polyethylene products in industrial processes. Table 5 also listed the energy barriers for the primary 1,2-insertion of 1-butene and 1-hexene, and the subsequent chain transfer by p-H elimination for all the three kinds of Ti-modified models. The calculated energy barriers showed that Ti-modification could also promote the activity for ethylene copolymerization with a-olefins. The energy differences between comonomer insertion and chain transfer can lead to a conclusion on the effect of Ti-modification on the distribution of the inserted comonomers in polyethylene chains. As listed in Table 5, the difference between energy barriers for chain propagation and for chain transfer decreased for model sites 4g, 12g, and 15g. Therefore, it was reasonable to conclude that Ti-modified catalyst was likely to make low MW polyethylene with much less comonomer insertion because the inserted comonomer mainly led to a chain transfer reaction and left the inserted comonomer at the chain end. As a result, the increased chain termination by comonomer resulted in less SCBs in the low MW fraction and higher density of the polyethylene product for the Ti-modified Phillips catalyst. 

These linear elastomers are produced by coordination polymerization using a Phillips or Z-N catalyst at low P and T. Here belongs Mxsten XLDPE from Eastman Chem. and Attane ULDPE from Dow. The first metallocene-catalyzed VLDPE was a hexene copolymer with p = 0.912 g mL made in the UNIPOL gas-phase process with Z-N catalyst and introduced by ExxonMobil as Exceed metallocene VLDPE. The resin has outstanding sealing properties (hot tack and seal strength) compared with ZN-VLDPE. The solution polymerization in a hydrocarbon usually is carried out in a continuously stirred tank reactor (CSTR), at r = 160-300 °C and P = 2.5-10 MPa with the residence time of 1-5 min [Dow in 1992 and UCC in... 

The selective trimerization and tetramerization of ethylene to form 1-hexene and 1-octene has become an important process to generate monomers for the synthesis of LLDPE. 1-Hexene was detected as a byproduct in the polymerization of ethylene catalyzed by homogeneous chromium complexes. Chromium complexes have now been identified that catalyze this oligomerization to form 1-hexene with remarkably high selectivity. Phillips patented the combination of 2,5-dimethylpyrrole, triethylalu-minum, and diethylaluminum chloride with a chromium(lll) salt, and researchers at Union Carbide patented a catalyst generated from chromium(lll) 2-ethylhexanoate and hydrolyzed triisobutylaluminum. - Researchers at BP have described a particularly active and selective catalyst based on a chromium(lll) precursor, a bis(diphosphino)amine ligand, (o-MeO-C H ),PN(CH,)P(o-MeOC H ) and methylaluminoxane (MAO). " ... 

For example, prior to the discovery of this new single-site catalyst type, commercial grades of polyethylene were primarily manufactured over the compositional range of 0-4 mol% of comonomer (1-butene, 1-hexene or 1 -octene) that provided ethylene copolymers over the density range of 0.915-0.970 g/cc. Commercial catalysts were primarily the Cr-based Phillips-type of catalyst or a Ti-based Ziegler catalyst with the xmderstand-ing that both types of catalyst consisted of many different types of active sites. Each type of active site produced a different composition of polyethylene (different molecular weight and branching content) which resulted in a final polyethylene material with a complex molecular structure. These multi-site catalysts limited the composition of the polyethylene that was commercially available due to both process and product constraints imposed by such catalysts. 

Chevron Phillips Chemical Co., LP LPE process from Phillips Petroleum Co., isobutane slurry, loop reactor, very high activity proprietary catalysts comonomers butene-1 hexene-1, 1,4 methyl-1 pentene, and octene-1, no waxes and other by-products, minimum environmental emissions 82 reactor lines, 34% of worldwide capacity slurry-loop reactor. LPE homo- and co-polymers (density 920-970 kg/m ) for films, blow moulding, injection moulding, rotomoulding, pipes, sheets and thermoforming, and wire and cables. 

---