Microwave treatment: A facile method for the solid state modification of potassium-promoted iron on silica Fischer-Tropsch catalysts

Article abstract of DOI:10.1039/c5ra26628a

Potassium-promoted (0-1.5 wt%) iron-silica catalysts for Fischer-Tropsch synthesis (FTS) have been modified using microwave radiation. Radiation produced few or no modifications in the bulk properties, but surface and catalytic behaviour were markedly changed in K promoted 10 wt% of Fe/SiO₂ (10Fe/SiO₂) catalysts. The effect of potassium on CO adsorption was relatively insignificant in untreated catalysts, but was large in microwave-modified catalysts. Radiation induced an increase in CH₄ formation in CO + H₂ temperature programmed surface reactions. Microwave treatment promoted CH₄ formation from graphitic carbon in these catalysts, while decreasing CH₄ formation from α- and β-carbon species, and overall favoured strong CO adsorption onto the catalyst surface. Microwave effects were catalyst particle size and treatment duration-dependent. At low alkali concentration, microwaved samples showed improved ethene selectivities, higher alpha values and lower methane and light alkene selectivities. When 0.7 wt% K was added to the 10Fe/SiO₂ catalyst, the α value increased from 0.59 to 0.66 after treatment of the sample with microwave radiation in the solid state.

Full text of DOI:10.1039/c5ra26628a

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Microwave treatment: a facile method for the solid
state modification of potassium-promoted iron on
silica Fischer–Tropsch catalysts†
Cite this: RSC Adv., 2016, 6, 22222
a
a
ab
Mbongiseni W. Dlamini, Neil J. Coville and Michael S. Scurrell*
Potassium-promoted (0–1.5 wt%) iron–silica catalysts for Fischer–Tropsch synthesis (FTS) have been
modified using microwave radiation. Radiation produced few or no modifications in the bulk properties,
but surface and catalytic behaviour were markedly changed in K promoted 10 wt% of Fe/SiO
catalysts. The effect of potassium on CO adsorption was relatively insignificant in untreated catalysts, but
was large in microwave-modified catalysts. Radiation induced an increase in CH formation in CO + H
temperature programmed surface reactions. Microwave treatment promoted CH formation from
graphitic carbon in these catalysts, while decreasing CH formation from a- and b-carbon species, and
2 2
(10Fe/SiO )
4
2
4
4
overall favoured strong CO adsorption onto the catalyst surface. Microwave effects were catalyst particle
size and treatment duration-dependent. At low alkali concentration, microwaved samples showed
improved ethene selectivities, higher alpha values and lower methane and light alkene selectivities. When
Received 13th December 2015
Accepted 25th January 2016
DOI: 10.1039/c5ra26628a
2
0.7 wt% K was added to the 10Fe/SiO catalyst, the a value increased from 0.59 to 0.66 after treatment of
www.rsc.org/advances
the sample with microwave radiation in the solid state.

ndings were that microwave treatment enhanced the surface
1
. Introduction
properties of such catalysts, as was clearly revealed by studies
The Fischer–Tropsch (FT) synthesis is a key technology for using a temperature programmed surface reaction (TPSR)
producing clean fuels and chemicals using syngas derived from between adsorbed CO and hydrogen. The microwave-induced
coal, shale gas, natural gas or biomass and is proving to be an effects are considered to be the result of localized temperature
1–3
effective substitute for liquid fossil fuel. As a consequence of gradients set up almost instantly when the solids are exposed
the current unstable crude oil prices and its nancial signi- to radiation. Some preliminary studies on supported
cance, the FT process has attracted renewed interest from potassium-promoted iron have indicated similar benecial
6
researchers both in academia and industry to improve the effects. The essential changes seen are an increase in specic
overall performance of FT catalysts. Of all the transition metals activity, a decrease in methane selectivity, an increase in chain-
which are active in FT synthesis, iron-based catalysts are growth probability, a higher olenic content of the product
preferred for synthesis gas compositions with a low H
due to their ability to produce additional H
via the water-gas evidenced by a higher selectivity to carbon dioxide. Most of
2
/CO ratio hydrocarbons and an increase in the water gas shi activity, as
2
shi (WGS) reaction. Furthermore, iron-based catalysts are these changes would normally be considered to be desirable in
cheap and readily available and are highly selective for the the context of producing synthetic liquid hydrocarbon fuels
production of short-chain olens and oxygenates, which are key from sources with relatively low hydrogen content such as coal
4
industrial chemical building blocks. Iron FT catalysts are and biomass. Indications are that the surface elemental ratios
6
typically promoted with potassium to enhance their selectivity of Fe : K : Si are altered by exposure to microwave radiation.
towards long-chain hydrocarbons.
Since potassium loading affects various aspects of iron cata-
Recently, the modication of unsupported iron-based cata- lyzed Fischer–Tropsch synthesis, it is likely that microwave
5
lysts by microwave radiation was reported. The essential induced changes in the surface potassium are responsible for
the observed effects, though the exact chemistry is yet to be
revealed. In addition, we have noted that iron oxides supported
DST-NRF Centre of Excellence in Catalysis (c*change) and Molecular Sciences on silica treated with microwave radiation perform better than
a
Institute, School of Chemistry, University of the Witwatersrand, Johannesburg, South
untreated samples, in part because any (relatively inactive)
Africa. E-mail: scurrms@unisa.ac.za
b
iron-silicate phases formed in the catalysts are effectively
Department of Civil and Chemical Engineering, University of South Africa, Florida,
6
destroyed by microwave treatment. This aspect merits further
Johannesburg, South Africa
study in its own right, but is consistent with experience gained
by using microwave radiation to enhance the metallurgical
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c5ra26628a
1
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extraction of various mineral ores which are otherwise difficult were done with catalysts in the solid state. Microwave pre-
7,8
to extract.
treatment was done for 10 s at a power level of 450 W. The
Microwave applications in chemistry usually focus on effect of the microwave irradiation time was also studied. The
synthesis in the liquid-phase though some studies of radiation pre-treatment time was increased from 10 s to 20, 30, 40, 60, 300
effects seen during heterogeneous catalysis are also avail- and 600 s, while the power level was maintained at 450 W for all
9
–11
able.
both supported and unsupported iron-based FT catalysts, but
the magnitude of the effect seems to be higher for supported the K/Fe and K/Fe/SiO
Further, as mentioned above we have seen effects for experiments.
Thin samples for TEM analysis were prepared by crushing
catalysts with a mortar and pestle fol-
2
5,6
materials, for reasons which are not yet clear. In this work we lowed by dispersion in methanol using an ultra-sound bath. An
have therefore investigated supported FT catalysts based on appropriate amount of the sample was then placed onto
silica and have studied the behaviour of observed microwave a carbon coated copper grid, dried and then introduced to the
effects as a function of potassium loading up to 1.5 wt% with microscope. The samples were studied using a TECNAI G2
a constant Fe loading of 10 wt%. We have also included actual SPIRIT transmission electron microscope operating at an
catalytic studies of the microwave induced synthesis on these accelerating voltage of 120 keV.
Fischer–Tropsch catalysts. Catalysts prepared by incipient
Energy dispersive X-ray spectroscopy (EDS) EDS is an X-ray
wetness impregnation or deposition–precipitation have both technique that was used to identify the elemental composi-
been examined because there are signicant differences tion of the catalysts. When a conducting sample is bombarded
between the two and they have been found to respond differ- with a beam of electrons, X-rays are emitted. The emitted X-rays
ently to microwave treatment. In summary, our catalyst prepa- are unique for a given element and can therefore be used to
ration method is conventional in nature but we have used an identify elements present in a given sample. The EDS system
unconventional microwave radiation treatment to synthesize used in this study was integrated to the TECNAI G2 SPIRIT
the catalysts before catalyst reduction and use.
transmission electron microscopy instrument.
All the gases used in this study (UHP grade) were supplied by
AFROX (African Oxygen) Ltd.
micro-reactor was used for the catalytic reactions. All gas lines
A stainless steel xed-bed
2
. Experimental
ꢀ
2
.1 Catalyst preparation
For incipient wetness impregnation (IWI), a ferric nitrate solu-
tion obtained by dissolving Fe(NO $9H O in deionized water
aer the reactor were kept at 150 C, and a hot trap, also held at
ꢀ
1
50 C, was placed immediately aer the reactor in order to collect
wax. A second trap kept at ambient temperature was used for the
collection of the oil and water mixture. A catalyst mass of 0.5 g was
loaded to a stainless steel xed bed reactor, and the catalyst
3
)
3
2
was added dropwise to the silica support. The slurry was then
dried at 120 C for 12 h to remove moisture. Calcination was
performed at 350 C for 6.5 h using a heating rate of 10
ꢀ
ꢀ
ꢀ
ꢀ
reduction was carried out at 350 C for 18 h using hydrogen gas
C
ꢀ
ꢁ1
ꢁ
1
and a heating rate of 10 C min . Catalytic reactions were con-
min . In this study, the loading of iron was maintained at
0 wt%. The 10Fe/SiO catalyst base powder was then impreg-
ꢀ
ducted at 275 C with synthesis gas composition (H
2
: CO : N
2
¼
1
2
ꢁ1
60 : 30 : 10) at a ow rate of 20 mL min and the reactor pressure
nated with appropriate amounts of K CO solutions to produce
the desired catalyst compositions. For this study, potassium
loadings of 0.2, 0.5, 0.7, 1.0 and 1.5 wt% were desired. The
catalyst was then dried at 120 C for 12 h followed by calcination
at 350 C for 6.5 h. For brevity, these catalysts are referred to as
2
3
was raised to 1.0 MPa. Analysis of the product spectrum was done
online using two GCs equipped with a thermal conductivity
detector (TCD) and a ame ionization detector (FID), identical to
ꢀ
5
ꢀ
that used before. Mass balances were performed on carbon and
oxygen and were close to levels of 100 ꢂ 5%.
xK/10Fe/SiO
2
, where x is the nominal potassium loading (wt%).
A precipitation (DP) procedure using ferric nitrate, potassium
carbonate and urea was also used to make the catalyst. In situ 3. Results and discussion
ꢀ
urea hydrolysis was allowed to proceed for 3 h at 100 C. The
3
.1 Catalyst characterization
ꢀ
slurry was then dried at 90 C under vacuum for 40 min followed
by another drying period of 12 h at 120 C. Calcination of the
DP-synthesized samples was also done at 350 C for 6.5 h.
A 1.0 wt% K loading was maintained for the DP-prepared
catalysts, while the iron loading was kept at 10 wt%. These
ꢀ
The elemental compositions of the samples were determined by
XRF and are presented in Table S1.† As inferred from the
tabulated data, the calculated and analyzed weight loadings
were similar. Table S1† also summarizes the textural properties
of the various catalysts prior to and aer exposure to microwave
radiation (10 s). The BET surface areas of all the silica-supported
ꢀ
catalysts are denoted as 1.0K/10Fe/SiO -DP.
2
2
ꢁ1
catalysts are in the range of 220–284 m
higher K loadings having lower BET surface areas. A study of Dry
g , samples with
2
.2 Catalyst characterization, MW treatment and testing
The samples were characterized using various techniques as and Oosthuizen over a fused iron catalyst gave similar obser-
5
described in our previous publication. Prior to microwave vations which were ascribed to the fact that potassium improves
treatment, about 0.5 g catalyst mass was evenly distributed as agglomeration of the FeOOH precursor and could further
12
a thin layer in a microwave-transparent Petri dish. A Defy enlarge the crystallite size of a-Fe O on calcination. The
2
3
(900 W, 2.45 GHz) domestic microwave was used to pre-treat the morphology and particle size of the crystallites was analyzed
calcined catalysts. All microwave pre-treatment experiments using TEM and from the images it was noted that the iron
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ꢀ
crystallites were well dispersed on the silica support (Fig. 1). The pre-reduced at 350 C for 6 h with H
2
. The adsorbed species
average particle size for the non-microwaved iron particles was were subsequently hydrogenated with H at a ow rate of 20 mL
2
ꢁ
1
in the range 40–60 nm for the various loadings of potassium. min to give hydrocarbons. In the measured proles (Fig. S2†)
Microwave pre-treatment did not alter the morphology or the the FID and the QMS signals are similar and indicate that
particle sizes of the catalysts.
methane is the main hydrocarbon produced. Higher hydrocar-
2
-TPR bons (ethane, propane, etc.) were not detected during these
The calcined catalysts were further characterized using H
prior to microwave pre-treatment (Fig. S1, see also Table S2†). experiments. Two methane signals are observed, suggesting
While the reduction of unsupported iron catalysts occurs in two that there are two kinds of CO adsorption sites on the surface of
simple steps (a-Fe O to Fe O , followed by Fe O to a-Fe), a more the catalyst. The observed peaks are broad, suggesting that
2 3 3 4 3 4
complex behaviour is exhibited by silica supported samples. At several overlapping peaks coexist. The peaks have been sub-
a low promoter loading (0.2 wt% K) the prole shows four reduc- jected to deconvolution using Gaussian curve tting
ꢀ
tion peaks. The small peak at 310 C could be related to a minor procedures.
process of reduction of small particles of Fe
silica surface i.e. at temperatures <390 C. This peak increases in ated catalysts with varying loadings of potassium (0 to 1.5 wt%
intensity and also shis to higher temperatures as the loading of K) reveal that methane was the main hydrocarbon detected
2
O
3
occurring on the
Fig. S3a† shows TPSR proles of the non-microwave irradi-
ꢀ
2
potassium is increased in the samples. Also, at potassium loadings during the experiments. Data for the 0.2K/10Fe/SiO sample is
of 0.7, 1.0 and 1.5 wt% K the peak gradually separated into two shown as an example in Fig. 1a. The CH
distinct peaks. The second reduction peak for the 0.2K/10Fe/SiO
catalyst occurs at 425 C and it represents the reduction of bulk are common in the proles. However, the peaks shi to higher
4
proles are similar in
shape i.e. two sets of peaks appearing at ꢃ300 and at ꢃ500 C
ꢀ
2
ꢀ
Fe O to Fe O . Corresponding peaks in the other samples at temperatures as the loading of potassium is increased
2
3
3 4
higher promoter contents have been highlighted using a dotted (Fig. S3a†). It is known that K
2
O on the surface of an iron
line (Fig. S1†). Clearly the peak shis to higher temperatures with catalyst increases the Fe–C bond strength for CO bonded to
1
6–18
an increase in the potassium loading since potassium inhibits the Fe,
leading to a shi in the desorption process to higher
13
ꢀ
19
reducibility of iron. The reduction of Fe O to Fe occurs at 577 C temperatures with K loading. Graf and Muhler reached the
for the 0.2K/10Fe/SiO sample and is labelled as Peak 3. This peak same conclusion on the inuence of the potassium promoter
3 4
2
does not shi on addition of potassium. Peak 4 occurs between aer performing CO temperature programmed desorption
ꢀ
6
57 and 663 C for the various samples as tabulated in Table S2.† (TPD) experiments. The peaks recorded in this work are broad
This peak represents the reduction of iron(II) silicate to a-Fe. This and indicate the existence of several overlapping peaks that
peak is broad and it indicates that large amounts of iron silicates coexist. The total areas under these peaks were obtained by
were present in the samples. The large amounts of iron silicates integration and were used together with methane calibration
are attributed to a strong interaction between the iron species and data to quantify the CH
4
evolved during the TPSR experiments.
ꢀ
silica. A small peak at ꢃ750 C is also observed. It could be The total amounts of methane evolved from the various cata-
2 3
assigned to the reduction of the K CO phase, in agreement with lysts are listed in Table 1 as a function of the potassium content.
its growth with the increase of the potassium content and the It can be seen that the CH produced from all the samples is in
4
1
4
ꢁ1
analogous assignment of this peak by Stobbe et al. and Gugliel- the range 31–32 mmol g , indicating that the potassium
15
minotti and co-workers. Microwave treatment did not introduce loading does not have a signicant impact on the amount of
any signicant changes in the reduction proles of the silica methane produced from these catalysts. This implies that
supported catalysts.
surface iron crystallites are not affected by the potassium ions.
The data of Table 1 also shows the reproducibility of the results
obtained from the TPSR technique. It is noted that even though
3.2 Temperature programmed surface reactions (TPSR)
potassium did not affect the total CH evolved, it did change the
4
studies
surface carbon species from which CH4 is produced. For
example adsorbed atomic carbon (C ) formation decreased
3.2.1 Non-microwave irradiated samples. In the TPSR
a
monotonously as the loading of potassium was increased. The
rate of reactivity of the C entities obviously has implications for
FTS catalysis.
study, CO was adsorbed on K/Fe/SiO catalysts (1 h) that were
2
3.2.2 The effect of microwave treatment. Solid-state modi-

cation of the catalysts using microwave radiation was carried
out using a power level of 450 W. Prior to the pre-treatment,
catalysts were evenly dispersed in a microwave-transparent
Petri dish. Microwave pre-treatment was investigated as a func-
tion of the loading of potassium in the catalysts (Fig. 2b and
S3b†). From these plots it can be noted that the proles differ
signicantly from those measured without microwave treat-
ment (Fig. 2a and S3a†). The results suggest that there might be
four different distinguishable sites or states involving adsorbed
CO. These different iron sites can be linked to four different
2 2
Fig. 1 TEM images of 0.2K/10Fe/SiO and 0.7K/10Fe/SiO .
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4 2 2
Table 1 Peak parameters of individual carbon species and total CH peak areas from the TPSR profiles of 0.2K/10Fe/SiO , 0.5K/10Fe/SiO , 0.7K/
1
0Fe/SiO
2
, 1.0K/10Fe/SiO
2
2
and 1.5K/10Fe/SiO -MW catalysts prior and after microwave pretreatment
a
Peak parameters
ꢁ
1
Catalyst
Carbidic a
Amorphous b
Carbide g
Graphitic d
4
CH total area (mmol g )
0
0
0
0
0
0
1
1
1
1
.2K/10Fe/SiO
.2K/10Fe/SiO
.5K/10Fe/SiO
.5K/10Fe/SiO
.7K/10Fe/SiO
.7K/10Fe/SiO
.0K/10Fe/SiO
.0K/10Fe/SiO
.5K/10Fe/SiO
.5K/10Fe/SiO
2
2
2
2
2
2
2
2
2
2
18.9/288
14.5/289
12.7/296
12.7/354
14.1/314
3.8/288
9.4/328
1.6/280
7.0/304
3.6/297
35.5/348
24.2/344
35.4/357
13.8/413
39.4/381
32.1/359
55.7/386
26.2/368
32.1/360
16.1/358
16.7/493
23.9/511
29.6/468
46.2/509
35.6/507
31.3/500
26.5/512
27.2/510
36.1/439
8.0/426
28.9/528
37.4/563
22.3/594
27.3/586
10.9/601
32.8/559
8.4/594
45.0/553
24.8/540
72.3/541
32
36
32
40
31
40
32
53
31
51
-MW
-MW
-MW
-MW
-MW
a
ꢀ
Peak parameters are presented as: deconvoluted peak area (%)/peak position ( C).
enhanced from the g- and d-carbon species. As a result the g- and
ꢁ1
the d-species contribute a combined 61% of the 36 mmol g
produced from the 0.2K/10Fe/SiO -MW catalyst. Upon increasing
2
the loading of potassium from 0.2 to 0.7 wt%, a signicant change
in the TPSR prole for the microwave modied catalyst was noted.
The methanation plots obtained from 0.7K/10Fe/SiO
2
and the
0
.7K/10Fe/SiO -MW catalysts are displayed in Fig. S5a and
2
b† respectively and peak parameters for the individual carbon
species are listed in Table 1. The microwaved catalyst displays an
overall increase in the rate of methanation of 23% relative to the
non-microwaved catalyst. This increase in methane production is
attributed to the microwave effect as all other parameters were
kept constant. Fig. S5b† shows that the change in the prole is due
ꢀ
to CH
4
being produced from d-carbon species at 559 C.
Fig. S6 and S7† compares TPSR proles for the 1.0K/10Fe/
SiO and the 1.5K/10Fe/SiO catalysts before and aer micro-
wave radiation (Table 1). The TPSR proles for the modied
catalysts (1.0K/10Fe/SiO -MW and 1.5K/10Fe/SiO -MW) clearly
2
2
2
Fig. 2 TPSR profiles of 0.2K/10Fe/SiO (unmodified) and 0.2K/10Fe/
SiO -MW (modified using microwave radiation) catalysts displaying the
2
2
2
evolution of methane as a function of temperature. Data on the indi- differ from those recorded for the unmodied catalysts. Both
ꢀ
vidual contributions of the carbon species are tabulated in Table 1.
these catalysts display a large signal at ca. 540 C which is due to
CH being formed from carbide and graphitic carbon. These
4
signals account for a combined 72 and 80% of the total
Fe–C interactions and the four C types are assigned as a, b, g hydrocarbons produced from the 1.0K/10Fe/SiO -MW and 1.5K/
2
and d-carbons in this work (Table 1). These carbon species are 10Fe/SiO -MW samples, respectively. This is to be compared
2
suggested to be due to adsorbed atomic carbon, amorphous with the 35 and 61% combined contributions from carbide and
surface methylene chains or lms, bulk iron carbide, and graphite-like carbons recorded from the non-microwaved cata-
2
0,21
graphitic carbon.
to the total hydrocarbon content were assessed.
From the data in Table 1 and Fig. 2 it is evident that which shows a 66% increase in CH relative to the 1.0K/10Fe/
The contributions of these carbon species lysts with the same composition. The total methane evolved
ꢁ1
when using the 1.0K/10Fe/SiO -MW catalyst was 53 mmol g
,
2
4
ꢁ
1
microwave pre-treatment induces a small increase in the overall SiO catalyst which produced 32 mmol g . An increase of 64%
2
CH evolved. For example, the total area for the CH peaks in was induced by microwave pre-treatment for the catalysts with
4
4
ꢁ1
the 0.2K/10Fe/SiO
2
ꢁ
-MW catalyst corresponds to 36 mmol g
,
1.5 wt% potassium. Methanation of the a- and b-carbon species
1
4
whereas 32 mmol g CH (i.e. 13% increase) was evolved from was found to decrease aer modication of both the 1.0 and
the untreated catalyst. An analysis of the contributions from the 1.5 wt% K samples, hence the decline in their percentage
various carbon species shows that the a- and the b-carbon contribution to the total CH evolved.
4
ꢀ
species appearing at 289 and 344 C, respectively, decreases
A comparison of the total CH content produced in both the
4
aer microwave pre-treatment. However, Fig. 2 and Table 1 microwaved and non-microwaved catalysts during TPSR exper-
show that CH formation in the microwaved sample is greatly iments is summarized in Fig. 3 as a function of the potassium
4
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loading. Clearly the increases seen were induced by microwave demonstrated in the table, the peaks from the microwaved
pre-treatment since all other variables were kept constant. From samples show a marginal shi to lower temperatures and
this plot it can be seen that the microwave effect increases with illustrate that the irradiation time does not affect the reduction
the loading of potassium and attains an optimum value at behaviour of the catalyst. Further, proles measured aer 40 s
1.0 wt% potassium loading for which case an increase of 66% in of microwave pre-treatment were similar to those recorded aer
the total hydrocarbon content results. At low K loadings, 20 s of treatment.
microwave induced effects were seen to promote methanation
TPSR measurements were done to investigate the effect of the
from mainly g- and d-carbon species, while involvement of a- MW irradiation duration. It was found that the proles followed
and b-carbon decreased. At high K loading, methanation from the general pattern observed in Fig. 2 with the display of two broad
the a-, b- and g-species was seen to decline aer microwave peaks before deconvolution. Data calculated from the peak areas
pretreatment. In contrast, CH produced from graphitic carbon are tabulated in Table 2. The total methane production increased
4
increases upon MW exposure. Unmodied catalysts display the as the microwave irradiation time was increased from 0 s (i.e. no
opposite behaviour since methanation is generally favoured pre-treatment) to 10 min. To identify the origin of the increases in
from a- and b-carbon species.
methane production aer microwave pre-treatment, a plot of the
from the two broad peaks is shown in Fig. 4. As seen in
The general increase in microwave-induced effects as CH
4
a function of potassium can be explained by considering the preceding sections, the low temperature broad peak accounts
known effects of potassium in Fischer–Tropsch synthesis. With mainly for methane produced from a- and b-carbon species, while
ensembles of iron atoms it is difficult to cleave the C–O bond. the high temperature broad peak accounts for methane evolved
The addition of K enhances bond-breaking. The effect of from g- and d-carbon species. As illustrated by this gure, the
microwave pre-treatment on the catalysts is believed to result in broad low temperature peak area is fairly constant throughout the
6
the migration of potassium ions to the surface of the catalyst.
irradiation period. It is not affected by the irradiation time. In
Thus surface iron atoms in a microwave modied catalyst are contrast, the high temperature peaks displays signicant increases
surrounded by potassium ions and become “electron rich”. The as the irradiation time is increased, especially between 40 and 120
C–O bond on such iron atoms is easily cleaved and hydrocar- s of MW irradiation.
bons are more readily produced. It is well known that surface
The most probable effects of microwave radiation are due to the
basicity can promote CO adsorption and suppress H2 development of local high temperature spots. The silica support
2
2
adsorption.
will tend to be microwave transparent but iron oxide, and espe-
cially magnetite, is a good microwave absorber. We believe that
solid-state modications, albeit at a local level, may well result
from local temperature excursions. We have seen that microwave
3
.3 Effect of the MW treatment time
The microwave pre-treatment effect reported thus far has been effects are dependent on the power levels used, the duration of
5
9
accomplished by exposing the catalyst to microwave radiation irradiation and the shape of the sample. We have also seen that
that is at 450 W power level for only 10 s. To investigate the Fe : K surface atomic ratios in potassium–iron–silica catalysts can
6
effect of longer duration times, temperature programmed be signicantly modied by microwave action The magnetite/
reduction (TPR) studies were performed with the 1.0 wt% K haematite ratio in the initial sample is likely to determine the
sample since it was shown to be optimal for the microwave overall microwave absorption realized. The water content of the
effect (Fig. 3). Reduction proles of iron catalysts that were sample may also play a role, but none of these factors have yet been
irradiated for varying periods of time (10–40 s) with microwaves systematically examined in our work. Some, and perhaps all of the
did not change signicantly (Fig. S8, Table S3†). As effects seen using microwave radiation, might be seen by using
conventional heating, though again, this has not been examined in
any critical way to date. What we do propose is that microwave
Table 2 Total methane produced by the TPSR experiments using the
1
2 2
.0K/10Fe/SiO (IWI) and 1.0K/10Fe/SiO -DP (DP) catalysts that were
treated with microwave radiation for varying periods
ꢁ
1
Total CH
IWI
4
evolved (mmol g
)
Microwave pre-
treatment time
DP
0
1
2
3
4
1
s
32
53
50
59
57
188
194
232
41
46
—
—
—
49
0 s
0 s
0 s
0 s
20 s
Fig. 3 Comparison of methane formed in microwaved and non- 300 s
microwaved samples as a function of potassium loading. All the 600 s
catalysts are supported on silica and contain 10 wt% Fe.
88
117
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Fig. 4 Variation in the methane content produced by the low and high
Fig. 5 Total CH
4
produced in the TPSR by the 1.0K/10Fe/SiO
2
-DP and
temperature TPSR peaks as a function of the microwave irradiation
the 1.0K/10Fe/SiO
2
catalysts that were irradiated for various periods.
2
time. Data shown is for the 1.0K/10Fe/SiO catalyst.
catalyst surface. Fig. 6 shows the methane proles resulting
from TPSR experiments performed in which a methanator was
used for the 0.7K/10Fe/SiO and 1.0K/10Fe/SiO catalysts before
2 2
and aer microwave treatment. The proles depict several CH4
irradiation is a simple and convenient method for modifying
solids catalysts and that the action is clearly demonstrated in
a modication of surface properties, including elemental ratios
6
and chemisorptive properties.
peaks as a result of the hydrogenation of pre-adsorbed CO. The
ꢀ
low T peaks occur at 116 and 216 C while a broad peak was
3.4 The effect of particle size on microwave treatment
ꢀ
observed at T > 500 C. Both low temperature peaks are attrib-
The microwave pre-treatment effect on catalysts with different uted to weak CO chemisorption in the molecular state. The peak
ꢀ
crystallite sizes was examined by preparing catalysts of the same at 116 C is due to catalytically active sites that are not inu-
ꢀ
composition using two techniques; deposition–precipitation enced by the potassium promoter, while the signal at 211 C is
(
DP) and incipient wetness impregnation (IWI). Iron particles in due to sites that are inuenced by K. A similar classication has
a catalyst prepared using deposition–precipitation generally also been used by Pour et al. when copper was used to promote
2
5,26
have much smaller particle sizes than those prepared using the an Fe catalyst.
CH signals produced from strongly chem-
4
23,24
incipient wetness impregnation technique.
this nding, our 1.0K/10Fe/SiO
particle size in the range 10–25 nm, whereas the catalyst
prepared by incipient wetness impregnation (1.0K/10Fe/SiO
had an average particle size in the range 40–60 nm. A plot of the the total CH
total CH
evolved for the two types of catalysts is shown in Fig. 5. Examination of the peak areas shows that Peak 1 and Peak 2
Consistent with isorbed CO shis to higher temperatures when a methanator is
ꢀ
-DP catalysts had an average used (see peak at T > 510 C).
2
Analysis of data for the 0.7K/10Fe/SiO
MW catalysts (Fig. 6a) showed that there is a slight increase in
produced (6%) from the pre-treated sample.
2 2
and 0.7K/10Fe/SiO -
2
)
4
4
As expected, the catalyst prepared using the deposition– decreased slightly upon microwave treatment while Peak 3
precipitation technique produced more methane relative to its increased (Table S4†). Since Peak 3 signal is attributed to
incipient wetness impregnation counterpart for the unmodied strongly (dissociatively) chemisorbed CO, it is clear that
catalysts (i.e. at 0 s treatment) (Table 2).
This is because catalysts prepared using the DP procedure CO. Fig. 6b shows methane proles for 1.0K/10Fe/SiO
have small crystallites and are therefore highly active. As the 1.0K/10Fe/SiO -MW done via the methanator (data in Table
pre-treatment time was increased to 10 s, 2 min, 5 min and then S4†). The microwave effect at this potassium loading was more
microwave pre-treatment favours the strong chemisorption of
2
and
2
1
0 min, the total methane produced is seen to increase for both pronounced. Peaks due to weakly chemisorbed CO (116 and
ꢀ
samples. The DP-prepared catalyst showed lower TPSR activity 216 C) resulted in 2142 and 1303 units of CH
for the
4
relative to the catalyst prepared by the IWI method. This shows non-microwaved sample, whereas the microwaved sample had
that microwave pre-treatment is favoured for larger particle 789 and 1716 peak area units, respectively. However, Peak 3
sizes than for smaller ones. As seen in the preceding sections, resulted in a larger peak area of 5794 units in the microwaved
MW pre-treatment also favours CH
d-carbon species in these samples.
4
formation from g- and sample relative to its non-microwaved counterpart (2750 units).
This data conrms earlier ndings that microwave pre-
treatment promotes the formation of strong CO chemisorp-
tion sites while suppressing weak CO chemisorption sites on
iron catalysts. This indicates that the MW effect increases the
3.5 Methanation studies
Methanation studies were performed to study the microwave
binding of CO to iron. Microwave pre-treatment resulted in
pre-treatment effect on the CO chemisorption strength at the
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Fig. 7 A comparison of CO conversions for microwaved and non-
microwaved samples obtained at steady state conditions.
rate, the FT reaction rate and the activity. The activity decreases
ꢁ1
ꢁ1
from 21.4 to 3.96 mmol s
g
Fe
as the potassium content is
increased from 0.2 to 1.5 wt%. Selectivity towards different
hydrocarbon fractions was studied using an online GC tted
with an FID and they are reported as percentages. Selectivity
towards methane was observed to decline as the promoter
content increased. The 1.5K/10Fe/SiO catalyst had the highest
2
C –C selectivity (44.9 wt%), followed by the 0.7K/10Fe/SiO2
5
8
catalyst (28.0%). This indicates that as the K/Fe ratio increases,
the proportion of methane in the products decreases and the
fraction of C2+ products increases.
Fig. 6 TPSR profiles recorded from methanation studies using the (a)
Addition of the alkali metal ions is also seen to increase
0
2 2
.7K/10Fe/SiO and (b) 1.0K/10Fe/SiO catalysts.
ethylene selectivity in the C
2
hydrocarbons. The increase is from
7.7 to 62%. The observed trends were expected and they are
consistent with the tendency of the alkali metal ions to decrease
the hydrogenation ability of a catalyst and thus accelerate chain
growth probability and the olen selectivity, possibly as a result
of the increased catalyst basicity. The chain growth probability
was seen to increase from 0.55 to 0.79 as the loading of potas-
sium was increased.
4
a 34% overall increase in the total CH evolved at a 1% K
loading, illustrating that microwave modication of the cata-
lysts does affect the production of hydrocarbons and the effect
is dependent on the loading of potassium.
3
.6 Fischer–Tropsch catalysis
The prepared samples were subsequently microwaved to
modify their catalytic properties, average CO conversions
measured at steady-state for the microwaved and non-
microwaved samples were compared (Fig. 7). The CO conver-
sions of the catalysts declined from 30.5 to 14.6% as the
promoter content increased from 0.2 to 1.5 wt%. It is evident
that both microwaved and non-microwaved samples display
a similar trend: as the K/Fe ratio increases; the activity of the
catalyst declines (Fig. S9 and S10†). It was observed, however,
The effect of potassium ions on FT activity was evaluated at
ꢀ
275 C in a stainless steel xed-bed reactor. Carbon monoxide
(CO) conversions with respect to time on stream (TOS) for the
0
.2K/10Fe/SiO , 0.7K/10Fe/SiO and 1.5K/10Fe/SiO catalysts are
2
2
2
presented in Fig. 7. The steady-state CO conversions were found
to decline with an increase in the promoter content, with
averages of 24.6, 17.8 and 12.4% for catalysts with K loadings of
0.2, 0.7 and 1.5 wt%, respectively (Fig. 7). The effect of potas-
that the 0.2K/10Fe/SiO -MW sample gave a signicantly higher
2
sium on FT synthesis activity has been investigated by several
researchers,
passes through a maximum,
as a function of the presence of K.
Table 3 summarizes the steady state performance of the
catalysts in the FT synthesis. From the table it is evident that
increasing the promoter loading decreases the CO consumption
CO conversion (30.5%) relative to its non-microwaved counter-
part (24.6%) (Fig. 7).
Data comparing the steady state performances of the
microwave pre-treated and the untreated samples are displayed
in Table 3. Addition of the promoter to the microwaved samples
lowered the selectivity towards methane and the very light
27–30
31
and the activity was reported to increase,
22,32
had little effect or suppressed it
22,33
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Table 3 Effect of potassium content and microwave pre-treatment (10 s) on catalyst activity and selectivity
Catalyst
0.2K/SiO
2
0.2K/SiO
2
-MW
0.7K/SiO
2
0.7K/SiO
2
-MW
1.5K/SiO
2
1.5K/SiO
2
-MW
CO conversion (%)
24.6
30.5
17.8
17.7
12.4
14.6
ꢁ1
ꢁ7
7
ꢁ7
ꢁ7
ꢁ7
ꢁ7
ꢁ7
ꢁ7
ꢁ7
ꢁ7
ꢁ7
ꢁ8
ꢁ8
ꢁ8
ꢁ7
ꢁ8
ꢁ7
Rate CO (mol s
)
ꢁ5.34 ꢄ 10
ꢁ3.71 ꢄ 10
ꢁ3.84 ꢄ 10
ꢁ3.50 ꢄ 10
ꢁ9.90 ꢄ 10
ꢁ2.13 ꢄ 10
ꢁ
ꢁ
Rate CO
Rate FT
Activity (mmol s
2
(WGS)
2.08 ꢄ 10
3.26 ꢄ 10
21.4
1.16 ꢄ 10
1.25 ꢄ 10
2.59 ꢄ 10
15.4
1.16 ꢄ 10
2.33 ꢄ 10
7.57 ꢄ 10
3.96
7.34 ꢄ 10
7
2.55 ꢄ 10
2.34 ꢄ 10
1.39 ꢄ 10
ꢁ
1
Feꢁ1
g )
14.8
0.56
16.8
14.0
0.66
41.5
8.50
0.77
55.6
a
0.55
7.69
0.59
39.3
0.79
61.9
C
2
olen %
Selectivity
C
C
C
1
2
5
28.2
38.6
18.2
8.55
26.2
39.9
18.9
4.77
14.6
42.4
28.0
5.11
11.4
38.3
35.3
4.74
3.8
12.4
35.8
36.7
3.09
–C
–C
4
36.2
44.9
1.27
8
CO
2
hydrocarbons (C
hydrocarbons (C
2
–C
–C
4
), while the selectivity towards the heavier signicant inuence on CO conversion was noted. However, the
8
) was enhanced. Analysis of the microwave data conrmed that microwave induced effects are generated
5
effect on samples of the same composition indicates that immediately aer a short radiation period. This duration effect
selectivity towards methane is suppressed in the microwave did however reduce the CH selectivity. Also the alpha value
4
modied catalysts at low potassium loadings (0.2 and 0.7 wt%). dropped from 0.79 to 0.63% as the irradiation period was
Microwave pre-treatment appears to enhance methane selec- increased to 10 min. Although the percentage CO conversions
tivity at high potassium loadings. The modied samples also are similar, the rate of CO consumption increased from 9.90 ꢄ
ꢁ
8
ꢁ7
ꢁ1
displayed improved chain growth probabilities at low alkali 10 to 2.25 ꢄ 10 mol s as the irradiation time increased.
loadings, while a slight decrease in the alpha value is observed This is in line with the considerably higher CO
on the sample with a high potassium loading. For simplicity the from the samples that were irradiated for 10 s and 10 min. The
olen content was analyzed by measuring the C olen WGS reaction was also enhanced. Ethylene selectivity was also
proportion relative to the total C fraction in the products. enhanced by increasing the duration of the pre-treatment
Potassium addition clearly favours the formation of olens for period.
both microwaved and non-microwaved samples. Reasons for It was observed that certain features such as the ethylene and
CO2 selectivities were enhanced by microwave pre-treatment
2
levels produced
2
2
32–34
this observation are well documented in the literature.
Comparing catalysts of similar compositions shows that (10 s) at low alkali ion concentration. These features were sup-
microwave pre-treatment enhances olen formation for the pressed by high alkali ion concentration on the surface of the
samples with 0.2 and the 0.7 wt% potassium. Ethylene content metal if pre-treatment was performed for only 10 s. Pre-
increased from 7.69 to 16.8 wt% for the 0.2K/10Fe/SiO
2
-MW treatment improved both the FT reaction rate and the activity.
catalyst, while for the 0.7K/10Fe/SiO -MW catalyst increased It is therefore believed that microwave pre-treatment can
2
from 39.3 to 41.5 wt%. The increase in the olen content is enhance FT performance at all potassium loadings, but
attributed to the effect of microwave pre-treatment, since all
other variables were kept constant. The ethylene fraction
declined from 61.9 to 55.6 wt% for the 1.5K/10Fe/SiO -MW, Table 4 The effect of the microwave irradiation time on FT activity
2
suggesting that microwave modication only enhances olen and selectivity
formation when low alkali metal loadings (0.2 to 0.7 wt%) are
used, otherwise olen formation is suppressed. Using iron
catalysts instead of cobalt catalysts in FTS promotes the water
1
2 2
.5K/SiO -MW 1.5K/SiO -MW
Catalyst
1.5K/SiO
2
(10 seconds)
(10 minutes)
gas shi (WGS) reaction and CO selectivity is normally used as CO conversion (%) 12.4
14.6
12.4
2
ꢁ1
ꢁ8
8
ꢁ7
ꢁ8
ꢁ7
ꢁ7
ꢁ8
ꢁ7
a measure of the WGS activity. Selectivity towards CO2 was Rate CO (mol s
)
ꢁ9.90 ꢄ 10
ꢁ2.13 ꢄ 10
ꢁ2.25 ꢄ 10
ꢁ
ꢁ
Rate CO
Rate FT
Activity
2
(WGS)
2.33 ꢄ 10
7.57 ꢄ 10
3.96
7.34 ꢄ 10
5.46 ꢄ 10
noted to decline relatively at low promoter loadings while it was
enhanced in the sample with the highest potassium content. A
decline in the WGS reaction is undesirable since it results in
excess water in the reactor and this can lead to catalyst deacti-
vation, particularly for medium to high conversions with iron
8
1.39 ꢄ 10
1.70 ꢄ 10
8.50
8.99
ꢁ1 Feꢁ1
)
(
mmol s
g
a
0.79
61.9
0.77
55.6
0.63
67.0
2
C olen %
35
catalysts. The 1.5K/10Fe/SiO
largest improvement in activity (3.96 to 8.5 mmol s gFe
which was induced by microwave modication of the catalyst.
2
-MW catalyst displayed the
ꢁ
1
ꢁ1
Selectivity
)
C
C
1
3.8
12.4
35.8
36.7
3.09
9.33
39.3
36.3
2.25
2
–C
4
36.2
44.9
1.27
FT performance data measured when the irradiation time C5–C8
was increased from 10 s to 10 min is displayed in Table 4. No CO
2
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different K loadings require different irradiation times. The
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