Nickel-
and alkali-earth-modified LTA based zeolites catalyze the
dimerization of 1-butene in the absence of Brønsted acid sites.
The catalyst reaches over 95% selectivity to n-octenes
and methylheptenes. The ratio of these two dimers is markedly influenced
by the parallel isomerization of 1-butene to 2-butene, shifting the
methylheptene/octene ratio from 0.7 to 1.4 as the conversion increases
to 35%. At this conversion, the thermodynamic equilibrium of 90% cis- and trans-2-butenes is reached. Conversion
of 2-butene results in methylheptene and dimethylhexene with rates
that are 1 order of magnitude lower than those with 1-butene. The
catalyst is deactivated rapidly by strongly adsorbed products in the
presence of 2-butene. The presence of π-allyl-bound butene and
Ni-alkyl intermediates was observed by IR spectroscopy, suggesting
both to be reaction intermediates in isomerization and dimerization.
Product distribution and apparent activation barriers suggest 1-butene
dimerization to occur via a 1′-adsorption of the first butene
molecule and a subsequent 1′- or 2′-insertion of the
second butene to form octene and methylheptene, respectively. The
reaction order of 2 for 1-butene and its high surface coverage suggest
that the rate-determining step involves two weakly adsorbed butene
molecules in addition to the more strongly held butene.
The presence of alkali (Na+ or Li+) or alkali‐earth (Ca2+ or Mg2+) cations adjusting the acid‐base properties on amorphous silica‐alumina influences markedly the catalytic properties of supported Ni for 1‐butene dimerization. The low concentration of Brønsted acid sites on these catalysts reduces the double bond isomerization of butene and inhibits the formation of dimethylhexene as primary product. While the alkali and alkali‐earth cations act as weak Lewis acid sites, only Ni2+ sites are catalytically active for dimerization of 1‐butene. n‐Octene and methylheptene are formed selectively as primary products; dimethylhexene is a secondary product. The open environment of the Ni2+ sites does not induce different reaction pathways compared to Ni2+ in the pores of zeolites.
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