Science - USA (2022-03-04)

INSIGHTS | PERSPECTIVES

978 4 MARCH 2022 • VOL 375 ISSUE 6584

GRAPHIC: K. FRANKLIN/

SCIENCE

science.org SCIENCE

By Mari S. Rosen

L

inear and branched a-olefins are an

important class of chemicals that are

used for making polyolefins, deter-

gents, plasticizers, and lubricants, as

well as other products, and are made

in quantities that exceed 3.2 million

tonnes per year ( 1 ). These a-olefins, which

have been produced on an industrial scale

for almost a century ( 2 ), are typically made

in processes that generate a broad distribu-

tion of products. More recently, on-purpose

technologies have been developed

to synthesize particular linear a-

olefins. By contrast, branched

a-olefins are produced in non-

selective processes such as fluid

catalytic cracking and oligomer-

ization of olefins with acid-based

or transition metal catalysts ( 3 ,

4 ). On page 1021 of this issue,

Dietel et al. ( 5 ) now add to the

collection of on-purpose a-olefin

technologies with their report of

a method for selectively synthe-

sizing branched a-olefins.

On-purpose technologies for

linear a-olefin synthesis first ap-

peared with the report of ethyl-

ene dimerization to make 1-butene in 1960

(see the figure) ( 6 ). Later advances include

ethylene trimerization to produce 1-hexene

in 1989 ( 7 ) and ethylene tetramerization

to produce 1-octene in 2004 ( 8 ). The im-

portance of these processes is reflected in

their subsequent commercialization, and

all three are practiced on an industrial scale

today ( 9 , 10 ).

The impressive selectivities of the tita-

nium- and chromium-based catalysts used

for selective ethylene oligomerization have

been attributed to a mechanism that uses

metallacycle intermediates ( 7 , 11 ). N otably,

the catalyst designed by Dietel et al. in-

stead makes use of more traditional transi-

tion metal–based ethylene polymerization

catalysts, which typically operate through

a Cossee-Arlman mechanism, to produce

a Schulz-Flory distribution of polymers of

different lengths ( 9 ). Polymer molecular

weight is not only defined by catalyst fea-

tures such as transition metal and ligand

choice but also by process parameters such

as ethylene concentration and reaction

temperature ( 12 ).

The system reported by Dietel et al. uses

a titanium-based catalyst with aminopyr-

idinato and imidazolidiniminato ligands

that, under typical olefin polymerization

conditions, produces a Schulz-Flory distri-

bution of low–molecular weight oligomers.

However, by selecting process conditions

that feature a very low concentration of

ethylene and a high concentration of an

a-olefin, the authors produced a branched

a-olefin trimer product that incorporates

two ethylene molecules and one a-olefin

(see the figure).

The combination of catalyst and process

conditions described by Dietel et al. results

in up to an impressive 74 mole % selectiv-

ity of the desired branched a-olefin prod-

uct. This selectivity is excellent given that,

until this report, it had been thought that a

metallacycle mechanism would be needed

to explain a selective olefin trimerization

( 7 , 11 ). Important to achieving the high

selectivity in this work are key attributes

of the catalyst: It terminates the growing

chain readily, and, as a good a-olefin in-

corporator, it has a high probability for in-

cluding any a-olefins present in the grow-

ing oligomer chain. When such a catalyst

is combined with a reaction environment

that is rich in a-olefins and lean in ethyl-

ene, the selective synthesis of a branched

a-olefin product results. The side products

of this reaction include a branched species

with an internal olefin, an a-olefin tetra-

mer product made from three ethylenes

and one a-olefin, and 1-butene. Notably,

no polymer by-products were observed

that could lead to fouling issues and that

plague many ethylene tetramerization cat-

alyst systems ( 9 ).

This excellent selectivity does not come

with any sacrifices in catalyst activity, with

turnover frequencies reported to be as

high as 2.3 × 10^6 per hour. Based on the

catalyst and product amounts in the pro-

vided examples, this rate can be converted

to a catalyst activity of 2.1 × 10^6 grams of

product per gram of titanium per hour,

which is a comparable activity to

top-performing chromium-based

ethylene trimerization and tet-

ramerization catalysts ( 8 , 13 ).

Dietel et al. also demonstrate the

versatility of the catalyst system

by successfully performing this

trimerization reaction with lin-

ear a-olefins from 1-pentene to

1-hexadecene and also with non-

aliphatic and cyclic a-olefins like

styrene and norbornene.

This work enables the selec-

tive synthesis of a broad range of

branched a-olefins and undoubt-

edly will spur additional research

in this area. Ultimately, the sys-

tem described by Dietel et al. demonstrates

the power of combining both judicious

catalyst design along with careful choice

of process conditions to make an otherwise

conventional catalyst perform useful reac-

tions. New products are likely to be devel-

oped that take advantage of the availability

of pure branched a-olefins. j

REFERENCES AND NOTES

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2. C. P. Nicholas, Appl. Catal. A Gen. 543 , 82 (2017).


3. Y. V. Kissin, Catal. Rev., Sci. Eng. 43 , 85 (2011).


4. A. Forestière, H. Olivier-Bourbigou, L. Saussine, Oil Gas
   Sci. Technol. 64 , 649 (2009).


5. T. Dietel, F. Lukas, W. P. Kretschmer, R. Kempe, Science
   375 , 1021 (2022).


6. K. Ziegler, H. Martin, US Patent 2,943,125 (1960).


7. J. R. Briggs, J. Chem. Soc. Chem. Commun. 1989 , 674
   (1989).


8. A. Bollmann et al., J. Am. Chem. Soc. 126 , 14712 (2004).


9. O. L. Sydora, Organometallics 38 , 997 (2019).


10. D. S. McGuinness, Chem. Rev. 111 , 2321 (2011).


11. T. Agapie, S. J. Schofer, J. A. Labinger, J. E. Bercaw, J. Am.
    Chem. Soc. 126 , 1304 (2004).


12. M. C. Baier, M. A. Zuideveld, S. Mecking, Angew. Chem.
    Int. Ed. 53 , 9722 (2014).


13. S. Kuhlmann et al., J. Catal. 262 , 83 (2009).

10.1126/science.abo1265

CATALYSIS

Catalyst coaxed to make branched a-olefins

Process conditions allow a titanium catalyst to add just two ethylene molecules to an a-olefin

Packaging & Specialty Plastics and Hydrocarbons, The

Dow Chemical Company, 240 Abner Jackson Parkway, Lake

Jackson, TX 77566, USA. Email: [email protected]

1-octene

2004

1-hexene

1989

1-butene

1960

Linear a-olefins

4-ethyloct-1-ene

Dietel et al. 2022

Branched a-olefins

On-purpose oligomerization

The synthesis of a-olefins usually forms a broad product distribution, but

several selective routes for linear a-olefins have been developed. Dietel et al.

now report that a titanium catalyst run under olefin-rich conditions is highly

selective for producing branched a-olefins.