The Fischer-Tropsch reaction in the aqueous phase over rhodium catalysts: A promising route to selective synthesis and separation of oxygenates and hydrocarbons

Article abstract of DOI:10.1039/c9cc09026f

In the present communication, we uncovered the aqueous phase Fischer-Tropsch reaction over rhodium catalysts. The reaction results in the synthesis and consecutive separation of hydrocarbons and oxygenates into two phases. Use of a rhodium Schiff base complex as a precursor for catalyst preparation allows efficient control of the Rh metal nanoparticle size distribution and leads to higher alcohol selectivity.

Full text of DOI:10.1039/c9cc09026f

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DOI: 10.1039/C9CC09026F
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Fischer-Tropsch Reaction in the Aqueous Phase over Rhodium
Catalysts: A Promising Route to Selective Synthesis and
Separation of Oxygenates and Hydrocarbons
Received 00th January 20xx,
Accepted 00th January 20xx
a, b
a
a
b
Aleksandra S. Peregudova , Alan J. Barrios , Vitaly V. Ordomsky , Nataliya E. Borisova and
a*
DOI: 10.1039/x0xx00000x
Andrei Y. Khodakov
In the present paper, we uncovered aqueous phase Fischer-Tropsch iron carbide) and formation of inactive metal-support
reaction over rhodium catalysts. The reaction results in synthesis compounds such as silicate or aluminate in the catalysts
and consecutive separation of hydrocarbons and oxygenates into supported by refractory oxides. Recently, ruthenium-based
two phases. Use of the rhodium Schiff base complex as a precursor catalysts showed some activity in aqueous phase FT synthesis
for the catalyst preparation allows efficient control of the Rh metal [8]. While oxygenates were reported at very low conversion
nanoparticle size distribution and leads to higher alcohol (<1%) on Ru catalysts, the yield of oxygenates was extremely
selectivity.
low for any practical applications [9]. Ru catalysts selectively
produce relatively cheap hydrocarbons.
Fischer–Tropsch synthesis (denoted as FT) is a heterogeneously
catalysed polymerization reaction, where syngas, a mixture of
carbon monoxide (CO) and hydrogen (H ), derived from
2
biomass, organic waste, natural gas or coal, is converted into a
wide spectrum of hydrocarbons and oxygenates [1,2]. The most
common catalysts for FT synthesis are transition metals of VIII
Differently to metals of VIII group active in FT synthesis (Ru, Co,
Fe), rhodium (Rh) exhibits the highest selectivity towards the C2+
oxygenates in syngas conversion [10-13]. In previous reports,
alcohol synthesis over Rh catalysts has been conducted in
gaseous phase at temperature higher than 250°C. In present
work, we report an aqueous-phase process at much lower
temperatures, which results in much more selective
hydrogenation of carbon monoxide to oxygenates occurring
over new highly dispersed rhodium catalysts. To the best of our
knowledge, aqueous-phase FT synthesis with rhodium catalysts
has not been reported in the literature.
2
group, since they can adsorb, activate H and CO and catalyse
hydrocarbon chain growth [3]. The selectivity to specific
products is a major challenge of FT synthesis [4]. During recent
years, there has been growing interest to direct syngas
conversion to value-added chemicals, such as alcohols, esters,
aldehydes, organic acids. These syngas-derived chemicals can
be considered as sustainable alternatives to the relevant
petroleum-based products. Because of insufficient selectivity,
no commercial process exists nowadays for direct synthesis of
the C2+ oxygenates from syngas.
Conventionally, FT reaction occurs over heterogeneous
catalysts placed in gaseous or liquid organic phases. Liquid
organic phase is typically constituted by hydrocarbons
produced during FT synthesis. The effect of large amounts of
water present in gaseous or liquid phase is usually deleterious
for conventional FT catalysts based on cobalt or iron [2, 5-7].
Conducting FT synthesis in the aqueous phase results in the
oxidation, sintering of the active components (metallic cobalt,
a. CNRS, Centrale Lille, Univ. Lille, ENSCL, Univ. Artois, UMR 8181 - UCCS - Unité de
Catalyse et Chimie du Solide, F-59000 Lille, France.
b.
Department of Chemistry, Moscow State University, 119992 Moscow, Russia
Electronic Supplementary Information (ESI) available: [Experimental details, TPR,
XRD and TEM data]. See DOI: 10.1039/x0xx00000x
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DOI: 10.1039/C9CC09026F
2
Table 1. CO hydrogenation over Rh-based catalysts in liquid aqueous phase. Reaction conditions: 0.3 g cat. for Rh-SiO (10 % Rh) and 0.15
g for 20%Rh/SiO
90%)
2
2
and 0.03 g for Rh-NPs), H O, T = 200°C, p (CO) = 10 bar, p (H
2
) = 20 bar, t = 24 h, stirring rate = 500 rpm, carbon balance
>
CO
CO
selectivity,
%
2
Carbon selectivity, %
Catalyst
T, oC
conversion, %
(CO
2
free)
CH
4
2
C -C
4
5
C -C
7
1 4
C -C oxyg.
Schiff – Rh - SiO
Schiff – Rh - SiO
2
200
195
86
72
36
30
14.4
9.4
41.6
50.3
7.6
5.6
31.5
25.9
2
Rh - SiO
Rh - SiO
0%Rh-SiO
2
Schiff - Rh
Rh - NPs
2
200
72
62
33.6
35.8
8.9
13.9
2
180
200
200
200
46
60
0
34
38
-
28.2
29.3
-
31.1
38.8
-
19.2
10.9
-
9.0
12.1
-
2
71
36
17.0
58.4
12.7
2.2
Conducting FT synthesis over rhodium catalysts in aqueous reaction was conducted at 180-200°C with 500 rpm stirring.
phase in a batch reactor represents major advantages Importantly, the reaction temperature in the aqueous phase FT
compared to the conventional gas-phase FT synthesis. First, it synthesis over rhodium catalysts are much lower (180-200°C)
allows decreasing the reaction temperature and thus, compared to the conventional gas phase operation mode
enhancement of the reaction selectivity to long-chain alcohols. (>250°C).
Water has major advantages compared to the organic solvent The catalytic results obtained for supported rhodium catalysts
for liquid phase FT synthesis. Among different metallic catalysts prepared using impregnation from the rhodium Schiff complex,
for syngas conversion, rhodium has the highest selectivity to rhodium trichloride and over non-supported rhodium
oxygenates. Light alcohols such as methanol, ethanol, ethanol nanoparticles are shown in Table 1. A Schiff-base complex
and propanol have higher solubility in water. Furthermore, (ligand N,N’-propanebis(3-phormyl-5-tert-butyl-salicylaldimine
carrying out the reaction in aqueous phase allows combined [14], Fig. S2, ESI) was used only as a precursor to obtain highly
selective synthesis and in-situ separation of hydrocarbons and dispersed rhodium metallic particles supported on silica.
oxygenates into two phases: light hydrocarbons are collected in Among a plurality of macrocyclic metal complexes [15-18],
the gaseous phase, while oxygenates are mostly present in the Schiff bases are broadly considered as stable and able to form
liquid aqueous phase. Conducting a catalytic reaction in complexes with vast variety of metal ions due to their chelating
combination with the separation of oxygenates in the aqueous properties [19, 20-22]. The TGA data suggest (Fig. S3, ESI) that
phases represents the major advantage of aqueous phase FT
synthesis compared to the FT synthesis in organic liquid phase.
The reaction is favoured by higher rhodium dispersion, which
can be obtained in the catalysts via decomposition of the Schiff
metal complexes. Further details about the synthesis of Rh/SiO
catalysts by impregnation, Rh non-supported nanoparticles, Rh
complex and Rh-complex supported on SiO catalyst
2
2
,
characterization and catalytic tests are available in Electronic
Supplementary Information (ESI).
The aqueous phase carbon monoxide hydrogenation
experiments were performed with the Rh catalysts dispersed
into deionized water in a stainless steel autoclave. Prior to
reaction, the freshly prepared catalysts were activated at 170°C
with 2.0 MPa of hydrogen for 2 h under vigorous stirring. The
Temperature Programmed Reduction (TPR) profiles for the
rhodium catalysts are shown in Fig. S1, ESI. FT synthesis occurs
over Rh metallic phase [10-13]. That was the reason why the
TPR data were used to optimize the catalyst activation
temperature in hydrogen. The silica supported Rh catalysts
exhibit a single broad reduction peak at about 90 – 95 °C, which
catalyst calcination at 400°C for 4 h results in decomposition of
the Rh Schiff complex. The complex itself without any
decomposition was not active in the CO hydrogenation.
Over all studied rhodium catalysts, hydrocarbons (in the C
range), oxygenates and CO were detected as reaction
products. The selectivity to oxygenates varied between 2 and
1%. Interestingly, the highest selectivity to oxygenates (>30%)
1 7
-C
2
2 3
corresponds to the reduction of Rh O to metallic rhodium. The
reduction temperature of 170°C seems therefore to be
sufficient to reduce all Rh. The carbon monoxide hydrogenation
3
was found on the catalyst prepared by decomposition of the
2
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Table 2. Selectivities of carbon monoxide hydrogenation to hydrocarbons in gaseous phase and oxygenates present in aqueous liquid
phase (CO
2
excluded)
Gas phase selectivity, %
Liquid phase selectivity, %
Catalysta
Methaneb
Hydrocarbonsb
Oxygenatesb
(Alcohols)
2+
(C )
C
1
C
2
C
3
C
4
Schiff – Rh - SiO
Rh - SiO
0%Rh-SiO
2
14.4
36.6
29.3
49.2
44.7
49.7
2.1
2.2
1.8
11.6
4.1
5.6
3.4
1.4
12.2
4.0
2
2
2
4.0
4.8
Schiff - Rh
Rh - Nps
-
-
-
-
-
-
17.0
71.1
Ʃ2.2
) = 20 bar, t = 24 h, stirring rate = 500 rpm.
aReaction conditions: 0.3 g cat. (0.03 g for Rh-Nps), H
O, T = 200 C, p (CO) = 10 bar, p (H
o
2
2
rhodium Schiff base complex (Table 1). The CO
due to the water gas shift (WGS) reaction occurring under these [1, 2, 4]. The chain growth probability varied between 0.68 and
conditions: CO+H O=CO +H . After the 24 h reaction time, the 0.72. Interestingly, alcohol distribution does not follow the ASF
2
production was the hydrocarbon chain length distribution follows the ASF law
2
2
2
CO conversion was between 46 and 86%. The methane statistics (Table 2). Probably, aldehyde dimerization can
selectivity varied from 14 to 33 %. Among hydrocarbons, light contribute to the alcohol synthesis over Rh.
paraffins are mostly produced. Further information about
hydrocarbon composition is available in Table S1, ESI
Fig. 1. ASF plots for different rhodium catalysts
Fig. 2. TEM micrographs and Rh particle size distributions: (a)
Schiff-Rh/SiO catalysts and (b) Rh/SiO
2
2
.
Though the rhodium catalysts were located in the aqueous The Rh-based catalysts were characterized by X-ray diffraction
liquid phase, the products of syngas conversion were present in (XRD). The XRD patterns (Fig. S4, ESI) show broad peaks at 2θ=
both liquid and gaseous phases in the reactor. In the aqueous 20-30°, which are attributed to the silica amorphous phase. No
phase FT synthesis over Rh catalysts elaborated in this work, XRD peaks assigned to rhodium species were detected. This is
efficient separation of hydrocarbons and alcohols was readily indicative of smaller rhodium particle size and suggests that
rhodium is highly dispersed in the silica supported catalysts.
2 2
The TEM images of Schiff-Rh/SiO , Rh/SiO catalysts, Rh
40
35
30
25
20
15
10
5
0
Mean 2.1 nm
nanoparticles and particle size distributions are shown in Figs. 2
and S5, ESI. The XRD results are consistent with the TEM data.
The Rh nanoparticles with the sizes between 1 and 5 nm were
detected. This is consistent with high rhodium dispersion on all
prepared rhodium catalysts observed by XRD (Fig. S4, ESI). The
rhodium average d3,2 particle size was 2.1 and 3.1 nm for Schiff-
0
,1-0,5 0,6-1,0 1,1-1,5 1,6-2,0 2,1-2,5 2,6-3,0 3,1-3,5 3,6-4,0
Particle size (nm)
Rh/SiO
supported Rh nanoparticles. Increase in Rh content in the
Rh/SiO catalyst from 10 to 20 wt. % results in formation of
2 2
and Rh/SiO catalysts respectively and 3.2 nm for non-
3
2
2
1
1
0
5
0
5
0
5
0
Mean 2.2 nm
2
agglomerates in addition to Rh individual nanoparticles. Note
that the catalyst based on the organometallic complex Schiff-
Rh/SiO
distribution compared to the conventional Rh/SiO
Catalyst stability of the catalyst toward deactivation is an
important issue. The stability of Schiff–Rh-SiO catalyst in
2
exhibits smaller Rh nanoparticle size and narrower
2
catalysts.
0
-0,5 0,6-1,0 1,1-1,5 1,6-2,0 2,1-2,5 2,6-3,0 3,1-3,5 3,6-5,0
Particle size (nm)
2
achieved after cooling the reactor to ambient temperature. The
compositions of gaseous and liquid phase are given in Table 2.
The gaseous phase contained methane,
hydrocarbons and carbon dioxide, while oxygenates, typically
the C -C alcohols were detected in the liquid phase.
The Anderson–Schulz–Flory (ASF) product distribution for the
produced hydrocarbons is shown in Fig. 1. The nearly linear
n n
trend of Ln (W /n) versus n (where W is the weight fraction of
carbon monoxide hydrogenation has been tested in several
cycles in the batch reactor at 195°C. Fig. S6, ESI displays the CO
conversion, selectivity to methane and oxygenates of the
Schiff–Rh-SiO catalyst after 3 cycles. The CO conversion does
2
not change significantly, while the selectivity to oxygenates
decreases and the methane selectivity increases. They exhibit
2
C -C
7
light
1
4
2
the values similar to those observed over Rh-SiO with larger
rhodium particles (Table 1). Note that XRD patterns of the
a specific hydrocarbon, n is the carbon number) suggests that
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catalyst after the catalytic tests indicate some rhodium sintering selective production of oxygenate compounds VfireowmArticsleyOnnglianes
Fig. S7, ESI).This suggests that higher selectivity to oxygenates (with the selectivity higher than 30%) anDdOcI:a1n0.b10e39sc/Ca9leCdC0u9p02fo6Fr
requires high rhodium dispersion available in the fresh Schiff– the industrial conditions.
Rh-SiO catalyst. Increase in the Rh particle size because of
sintering in the spent catalysts results therefore, in lower Conflicts of interest
(
2
selectivity to oxygenates.
There are no conflicts to declare.
The Turnover Frequencies (TOF) for CO hydrogenation over
rhodium catalysts calculated from Rh particle sizes and reaction
rates are shown in Table 3. The catalytic results do not show any
Acknowledgements
The authors are grateful to Olivier Gardoll, Laurence Burylo and
Joelle Thuriot for help with TPR, XRD and XRF measurements. A.
noticeable effect of catalytic preparation and supporting by SiO
2
on TOF. Indeed, similar values of TOF from 1.1-1.7 s- were S. P. is grateful to the French Ministry for Foreign Affairs for the
observed on all the catalysts. Though oxygenates and opportunity to perform her PhD studies in the UCCS (Vernadskii
hydrocarbons are produced in aqueous liquid phase over all fellowship). Chevreul Institute (FR 2638), Ministère de
catalysts, higher oxygenate selectivity is observed over the l’Enseignement Supérieur, de la Recherche et de l’Innovation,
1
catalyst prepared using the Schiff complex, which exhibits
higher Rh dispersion and narrower distribution of Rh particles.
Our experiments therefore suggest structure sensitivity of
alcohol synthesis from syngas over the Rh supported catalysts.
Higher alcohol selectivity is observed over smaller rhodium
particles. Synthesis of higher alcohols from syngas is a complex
reaction and involves numerous elementary steps such as CO
adsorption and possible dissociation, chain growth with can
Hauts-de-France Region and FEDER are acknowledged for
supporting and funding partially this work. The authors
acknowledge financial support of the French National Research
Agency (NANO4-FUT project, Ref. ANR-16-CE06-0013).
Notes and references
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in the selective synthesis of oxygenates. Higher yields of
oxygenates were obtained. At the same time, hydrocarbons and
oxygenates are clearly separated in the reactor. Hydrocarbons
are preferentially localized in the gaseous phase, while
oxygenates are mostly dissolved in the liquid aqueous phase.
Use of rhodium Schiff base complexes as catalyst precursor
resulted in the catalysts with higher metal dispersion and
enhanced catalytic performance in direct synthesis of
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