(611e) Gasoline-Selective Fischer-Tropsch Synthesis with Hierarchical ZSM-5 Coated Monolith Catalysts

Energy independence
is important for economic stability and security. Despite significant incentives
and developments for the decrease of crude oil imports, the U.S. still imports
about 24% of its oil (2015 EIA data) [1]. Moreover, the
infrastructure of the U.S. transportation sector imposes requirements for high-octane,
gasoline-range hydrocarbons. Gasoline consumption accounts for about 50% of the
total U.S. oil consumption [2].
Fischer-Tropsch Synthesis (FTS) is a well-known process for the conversion of (U.S.
abundant) natural and shale gas to hydrocarbons [3,4]. However, FTS
is historically conceived as a diesel-producing process, with long-chain
hydrocarbon products favored due to the polymerization nature of the reaction
network [3,5,6]. Also, FTS
produces mainly linear olefins and paraffins [7]; and therefore,
low-octane gasoline. These limitations can be overcome by the addition of an
acidic co-catalyst to enhance cracking, oligomerization, isomerization and
aromatization. In-situ upgrading of primary FTS products is possible by
use of bifunctional or hybrid catalysts comprising the FTS active phase and an
acid catalyst. Acid catalysts enhance oligomerization, cyclization,
aromatization, cracking, and isomerization reactions and provide shape
selectivity towards gasoline-range hydrocarbons. In this work, multilayered structured
catalysts were synthesized, composed of a supported FTS active phase (Co catalyst),
and coated with a layer of ZSM-5 catalyst. Both layers were formed on the
internal surface of a monolith support. The structure of these multilayered
catalysts is conceptually presented in Figure 1. The synthesized multilayered catalysts
enable: a) relaxation of heat and mass transfer limitations; b) high activity
with low water-gas shift potential; c) control of product size through manipulation
of the catalyst outer film thickness; and d) enhanced selectivity to
gasoline-range hydrocarbons. Early work has shown excellent selectivity and
performance of these catalysts. The bi-functional, multi-layered catalysts with
Co dispersed on an Al₂O₃ layer over the monolith and
coated with ZSM-5 doubled the selectivity to gasoline range compounds, without
affecting the overall conversion of the process.

Figure 1: Bi-functional, multi-layered,
structured catalyst.

The production
of gasoline-range hydrocarbons depends on the extent of cracking and
isomerization reactions, which in turn depend on the ZSM-5 to intermediate
hydrocarbons ratio in the local neighborhoods of the catalyst acid sites.
Increasing the ZSM-5 loading (layer thickness) can possibly improve the
gasoline selectivity of the process. However, increase of the ZSM-5 membrane
thickness also contributes to diffusion limitations. In essence, a thick layer
of ZSM-5 has the potential to enhance selectivity, but negatively impacts
conversion because the process becomes diffusion-limited. For that reason, we
explored mesoporous ZSM-5 as an acidic catalyst layer capable of relaxing
diffusion limitations. Hierarchical zeolitic materials have demonstrated
improved mass transfer properties at high catalytic activity for a number of
applications [8,9]. Inclusion of
mesopores in the ZSM-5 structure eases transport, specifically of molecules in
the C5-C12 range [10]. Thus, it can
enable higher loadings of ZSM-5 on the FTS monolith catalysts without affecting
conversion and diffusional characteristics; i.e. the transport of syngas to the
catalyst and hydrocarbons out of it. We will present the synthesis methods,
process configuration and results of the effect of hierarchical ZSM-5 catalytic
films, as selective and reactive membranes for FTS. The hierarchical ZSM-5 was prepared
using a finely controlled desilication technique, with mesopore diameters between
100 and 300 Å. They were then coated over FTS monoliths, using wash-coating
methods and were tested for their reactivity, selectivity and stability as
gasoline-producing FTS catalysts. The effect of pressure, temperature and gas
residence time will be presented. The effect of thickness and the improvement
of diffusion tolerances of the mesoporous catalysts will be presented and
explained on the basis of materials characterization results. FTS with these
catalysts maintains CO conversion, but may also shift selectivity from
diesel-range compounds to high-octane gasoline.        

Acknowledgment

Support from the
ACS PRF 53648-DNI5 is gratefully acknowledged.

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