(727a) Selective F-T Synthesis for the Production of Middle Distillates

Introduction

Two categories of Co-based FTS catalysts are being developed for the
production of heavy hydrocarbons and middle distillates, respectively. Several
methods including sol-gel method, confinement by mesopores, or localized by
polymer were adopted to prepare different Co catalysts. By tuning both pore
structure and support surface, the Co-catalysts with adjustable conversion of
CO and controllable distribution of products were developed with lower CH₄
selectivity than 5%. Besides, the correlation of the catalyst structure with
the FT performance is being well established.   Research Development of Co-based F-T Catalysts

Co Catalyst
Preparation. Co catalysts are usually prepared by the impregnation
methods while the co-precipitation and melting methods are preferred for Fe
catalyst. Although these
techniques have great practical simplicity, their drawbacks are the low maximum
loadings and the sometimes-unsatisfactory distribution and low dispersion of
the active phase in the ultimate catalysts. The homogeneous
deposition-precipitation (HDP) has been developed for the preparation of highly
loaded and highly dispersion oxide-supported metal catalyst. In this case, a
solvated metal precursor is deposited exclusively onto the surface of a
suspended support by the slow and homogeneous introduction of a precipitating
agent, which then avoid nucleation of solid precursor compound in the bulk
solution.

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Surface
organic modified Co-catalysts.
CH₃-modified SiO₂ (CH₃-SiO₂) (CH₃)₂-modified
SiO₂ ((CH₃)₂-SiO₂), (CH₃)₃-modified
SiO₂ ((CH₃)₃-SiO₂), NH₂-modified
SiO₂ (NH₂-SiO₂) and NH₂(CH₂)₂NH-modified
SiO₂ (en-SiO₂) were prepared through the surface reaction
between organotrialkoxysilane and Si-OH groups on SiO₂ aerogel.

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Polymer
localized Co-catalysts. The Co/SiO₂
catalysts were synthesized by a sol-gel process from 3-aminopropyltriethoxysilane (APTS) as aminopropyl
introducer for choice addition on certain condition, polymethylhydrosiloxane
(PMHS) as methyl introducer and TEOS as main silica source. The catalysts were
named as Si_pA-R_NHB, in which A meant the molar ratio of Si from PMHS to Si from
other silica source; B meant the molar ratio of aminopropyl groups in APTS
to Co²⁺ ions.

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Mesopore confined Co-catalysts. Hollow mesoporous silica sphere (HMSS) was
prepared according to literature ¹.The supported cobalt catalyst
containing 30 wt% Co was prepared by the "two-solvent" technique which reported
by this literature ². The catalyst was eroded to confirm the
particle size of Co₃O_4.   Kinetics

The active sites of Co/SiO₂
catalysts for FT synthesis are discriminated kinetically. Several Co/SiO₂
catalysts with well-defined structure were prepared. By sol-gel route, silica
of various porosity, but of similar chemical property were realized. In TPR
profiles, there were four peaks after deconvolution. The relative intensity of
those peaks changed as the silica pore size increased, with the low-temperature
peaks (547, 588K) enlarging at expense of high temperature peaks. Quantitative
analysis suggests that the extent of reduction increased gradually. Those
catalysts were evaluated in FT synthesis under conditions of 2.0MPa, 1500GHSV
and H₂/CO =2. The CH₄ production was insensitive to the
reducibility whereas the C₂⁺ yield responded to
reducibility regularly. This meant that the CH₄ formation depended
on the total cobalt, no matter how it was reduced. In contrast, the C₂⁺
products (featured with carbon chain growth) only formed on reduced Co. On
light of the above results, the F-T synthesis over cobalt was simplified as two
reactions with the same feedstock. The product of reaction 1 was CH₄
and that of 2 was C₂+. These two reactions were independent and was
first order to H₂ partial pressure. The pre-exponential factors and
active energies were derived according to the parallel reactions mechanism. The
difference of kinetic constants between two reactions was remarkable. The
coincidence of C₂⁺ reaction parameter might confirmed the
uniform of active sites °ª metallic Co.   Catalytic
performance

1. Performance
of surface organic modified Co-catalysts

(CH₃-SiO₂), (CH₃)₂-SiO₂
and (CH₃)₃-SiO₂ reduced the surface silanol
(Si-OH) concentration of SiO₂ support, suppressed the interaction between
cobalt and silica, enhanced the reducibility of the supported cobalt species,
and thus increased the catalytic activity of Co catalysts for FT synthesis (see
Table 1). However, coordination compounds wereformed between
NH₂-SiO₂ and Co²⁺ cations, and thus the
interaction between cobalt and silica was enhanced, the reducibility of Co
catalysts for FT synthesis was decreased. Because chelated compounds were
formed between en-SiO₂ and Co²⁺ cations the supported
cobalt catalyst showed the worst performance in FT synthesis (see Table 1).

Table 1. FTS performance of Surface organic modified
catalysts

2. Performance of Polymer localized Co-catalysts

Catalyst Si_p1.8-R_NH0 performed obviously higher
catalytic activity in FT synthesis than Si_p1.8-R_NH0.6 (see Table
2). And hydrocarbons obtained by Si_p1.8-R_NH0 catalysis were
mainly low valuable C1-C4 gas, while high valuable heavy wax was main product
from the FT reaction catalyzed by Si_p1.8-R_NH0.6. The large influence
on selectivity might result from the variety of organic groups, because of
small difference in other characterization.

Table 2. FTS performance of polymer localized
Co-catalysts

3. Performance of mesopore confined Co-catalysts

The Co₃O₄ particle clusters were 100°«200 nm, but XRD illuminated the particle
size of Co₃O₄ was 10°«20 nm. Therefore the Co₃O₄ particles cluster
was composed by smaller mono-dispersed Co₃O₄
nano-particles, these nano-particles were 10°«20 nm. Actually, these mono-dispersed Co₃O₄
nano-particles were loaded equably in the pore channels of the HMMS and were
divided by the pore walls with each other. From Table 3 it can be seen
that at reaction temperature 210 °C the catalyst had good
performance and hydrocarbon distribution which concentrated on C₅°«C₁₈ in FT synthesis, methane
selectivity was only 4.8 wt%, C₅+ selectivity reached up to 93.6
wt%.

Table 3. FTS performance of mesopore
confined Co-catalysts

Outlook

In summary, the selective synthesis of
hydrocarbon could be more controllably carried out via appreciate surface
modification or confinement to supported Co-catalysts. This may provide a
potential solution for adjusting the products distribution

Acknowledgment

This work was supported by the Natural
Science Foundation of China (Contract Nos. 20590361 and 20303026) and State Key
Foundation Program for Development and Research of China (Contract No.
2005CB221402).