Role of Caesium in Higher Alcohol Synthesis over Modified Copper–Cobalt Nanocomposites under Mild Conditions

Article abstract of DOI:10.1002/cctc.201700013

Cu–Co nanocomposites with and without Cs promoter on a modified lanthanum–alumina support were prepared by the combination of two unique synthesis methods. The synthesized materials were exclusively characterized by means of XRD, BET surface area determination, H₂ temperature-programmed reduction, CO₂ temperature-programmed desorption, N₂O frontal chromatography, X-ray photoelectron spectroscopy, high-resolution TEM, and elemental mapping techniques, and its catalytic properties were thoroughly investigated for the selective synthesis of higher alcohols in the CO hydrogenation reaction by using a H₂/CO ratio of 1, a temperature range of 180–300 °C and a pressure variation of 15–60 bar. Cs incorporation and a Cu/Co ratio of three was found to be a very efficient combination to obtain alcohol selectivities up to 55 %. This was explained by the fact that Cs increases the surface area of Cu particles by migrating Cu towards the surface layer and strengthens the surface interaction of Cu and Co. The further increase in higher alcohols selectivity then results from the limited formation of methane and carbon dioxide.

Full text of DOI:10.1002/cctc.201700013

Accepted Article
Title: Role of Cs in higher alcohol synthesis over modified Cu-Co
nanocomposites under mild condition
Authors: Subhasis Das, Manideepa Sengupta, and Ankur Bordoloi
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Role of Cs in higher alcohol synthesis over modified Cu-Co
nanocomposites under mild condition
Subhasis Das, Manideepa Sengupta, Ankur Bordoloi*
Abstract: Cu-Co nanocomposite with Cs promoter on modified
lanthanum alumina support has been prepared by combinations of
two unique synthesis methods. These developed materials were
exclusively characterised by means of XRD, BET, H2-TPR, CO2-TPD,
N2O frontal chromatography, XPS, HRTEM and elemental mapping
and thoroughly evaluated its catalytic properties for selective
synthesis of higher alcohols with a ratio of H2/CO = 1, over the
temperature range of 180 - 300℃ with a pressure variation of 15 – 60
bar. Cs with Cu/Co ratio of 3 is found to be a very efficient combination
to obtain alcohol selectivity up to 55%. This could be due to Cs
reduces the surface area of Cu particles by immigrating them towards
the surface layers and strengthening the surface interaction of Cu and
Co and hence, there is further increase in higher alcohols selectivity
by limiting the formation of methane and carbon dioxide.
syngas; however, a very few practical demonstrations of catalysts
are existing in either bench or pilot scale. Snamprogetti, Enichem
and Haldor Topsoe has demonstrated one modified Zn-Cr based
catalyst system by modifying HT methanol synthesis catalyst,
Lurgi & Sud chemie jointly ventured a catalyst system based on
modified Cu/Zn/Al by modifying their low temperature methanol
catalyst system, IFP France also developed a process for higher
alcohol synthesis with 25% ethanol based on Co-Cu based
catalyst system. however, all these three processes suffered from
the high amount of methanol in their product stream (more than
60%). Additionally, Dow has shown a Mo based catalytic process
with a good amount of ethanol (48%), however, this process
agonises from very high pressure [4]. Here, we can observe, there
are few processes are presented in the market, regardless,
neither of them is capable of producing sufficient amounts of
higher alcohols in an economical way. Therefore, there is ample
space to work in the search of either a new catalyst system or to
tune up the existing catalyst systems to maximise the higher
alcohol yields.
Introduction
Synthesis of higher alcohols renewed its interest in industry and
academia as a sustainable fuel option, in recent years due to
stringent environmental regulations and fluctuation in oil prices.
Cobalt-based catalysts in FT synthesis are quite interesting due
to their efficiency in long-chain product formation, low activity
towards water–gas shift reaction, and high stability. Further, Cu
based catalyst systems are well known for methanol synthesis.
As a consequence, bimetallic copper–cobalt catalysts system
would be a fair option for selective synthesis of higher alcohols[5].
Cs improvement, Alkali metals as electronic promoters were
highly explored with methanol synthesis catalyst by different
researchers to increase the yield of higher alcohols, however,
limited achievement was observed till date. Among all the alkali
promoters, 'Cs has the unique property to improve the yield of
oxygenates claimed by several literature reports by altering the
High energy density, high cetane number and blending flexibility
[1]
make higher alcohols an alternative fuel choice for vehicles
.
Moreover, the presence of additional oxygen in the molecule
during combustion, which drastically reduces the smoke emission
from the engine and improved ignition quality enhanced the
attraction towards higher alcohols as a renewable fuel option.
There are also several other advantages of using higher alcohols
and diesel blend in vehicles, such as high miscibility and stability
of the blend deprived of using co-solvent and emulsifying agents,
[2]
less knocking property, less corrosive and safer to handle it
.
normal reaction pathways[4a]
.
The application of higher alcohols as fuels was discouraged
earlier due to limited production from non-renewable feedstock
and dilemma regarding food vs fuel choice. Nevertheless, several
biotechnological breakthroughs in the last decade for the
production of higher alcohols creates a promising future for higher
Here, we are reporting a modified Co-Cu based nanocomposite
catalyst system, for the synthesis of higher alcohols under mild
conditions. Here, we have modified the high surface area alumina
with lanthanum, to make the catalyst surface little basic to control
the methane formation one hand and on the other, to restrict the
formation of hardly reducible cobalt aluminate species. The
surface of lanthanum aluminate is decorated with cobalt–copper
nanoparticles by using a facile organic matrix decomposition
method and further modified with Cs as an electronic promoter to
study the behaviour of Cs in presence of Co-Cu nanoparticles
towards the synthesis of higher alcohols.
alcohols in the automotive sector [3]
.
Biomass is an important renewable energy resource because of
its flexibility of sources and waste management. Since higher
alcohols are being considered as a potential alternative synthetic
fuel for automobiles or as a potential source of hydrogen for fuel
cells as it can produce from biomass. Higher alcohol can be
produced catalytically from biomass by generating syngas
(CO+H2) either by gasification or reforming. A lot of literature
reports are available for the synthesis of higher alcohols from
[a]
Subhasis Das, Manideepa Sengupta, Ankur Bordoloi*
Nano Catalysis area, Refinery Technology Division,
CSIR-Indian Institute of Petroleum,
Dehradun-248005, India
*Corresponding Author. ankurb@iip.res.in
Tel.: +91 135 2525898;
Fax: +91 135 2660202
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Results and Discussion
3.1: Catalyst characterization:
3.1.1: Surface area and composition analysis:
The bulk composition and structural parameters including surface
area (SBET) and total pore volume (Vtot) of the calcined catalysts
and catalyst support were analysed by ICP-AES and N2
physisorption analysis displayed in Table 1. According to ICP-
AES result, the Cu/Co ratio is very close to the theoretical value,
indicating the complete precipitation of the precursor salts. The
isotherms of nitrogen adsorption and desorption for these newly
synthesised materials are found to be type IV isotherms with H1
hysteresis loop and c apillary condensation step at a relative
pressure above 0.45, which are characteristic of mesoporous
materials (shown in Fig. 1.a). The SBET and Vtot for the lanthanum
planes, confirms the presence of cobalt oxide (JCPDS card no:
78-0431). The 0Co20Cu catalyst shows characteristics peaks at
around 3.3°, 50.4°, and 74.1° corresponding to (111), (200)
and (220) planes of metallic copper (JCPDS no: 04-0836).
However, in Cu-Co catalysts with increasing copper content a set
of characteristics peak at around 51.8° and 60.6°
corresponding to (111), and (200) planes confirms the presence
of metallic cobalt in the catalysts system (JCPDS card no: 15-
0806). It is noteworthy that, with increasing copper content in
CuCo catalysts, metallic cobalt and copper phase predominates
over cobalt oxide phase, plays an important role towards higher
alcohol synthesis.[7]. After caesium promotion in CuCo system, no
significant peak corresponding to caesium was observed.
Although with caesium promotion the peak intensity of metallic
copper and cobalt increases, however in 3Cs15Cu5Co both
copper and cobalt peak intensity found to be decreased which
could be due to some sort of site blockage by an excess of
caesium[8], with the appearance of small cobalt oxide phase.
3.1.3: H2- Temperature-programmed reduction analysis:
The reducibility of the fresh catalysts has been assessed by H2-
TPR, and the corresponding reduction profiles are represented in
Fig.1. d. The cobalt containing sample 20Co0Cu, characterised
by two predominant broad peaks at 450℃ and 750℃ are
attributed to the stepwise reduction of Co3O4 to CoO and CoO to
metallic Co. The copper containing sample 0Co20Cu exhibit a
sharp peak at 225℃, attributed to the reduction of CuO to metallic
Cu. In CuCo samples, however, with increasing copper content,
the broad cobalt reduction peak gradually shifts towards the lower
temperature region, keeping the copper reduction peak constant.
This phenomenon was easily explained by H2 spill over effect.
The easily reducible copper oxides in CuCo catalyst adsorbs
hydrogen dissociative and facilitate the reduction of the
neighbouring cobalt oxide. In 5Co15Cu the copper reduction
profile and first cobalt reduction profile (i.e., Co3O4 to CoO)
overlaps at 225℃ and the second step reduction of cobalt occurs
at only ~400℃. This is a significant achievement in CuCo
catalysts, where both the metals reduced at around 450℃, which
alumina support are 231 m2 g−1 and 0.78 cm3 g−1, respectively,
while that of Cu-Co catalyst samples reduced to those of
lanthanum alumina support, which could be due to pore blockage
by cobalt and copper nanoparticles on the support. The variation
of the derivative of pore volume with pore diameter shown in Fig.
1.b., obtained by analysing the fresh samples and catalyst support
also clearly indicates the decreasing trend with increasing metal
loading. There are no significant changes in the pore structure,
however, marginal changes in the surface area have been
observed with (1-3 wt%)Cs loadings. Furthermore, these porosity
measurement results were concomitant with HRTEM analysis
and play a vital role towards higher alcohol synthesis, due to the
well dispersion of active metal nanoparticles upon this
mesoporous supports [6]
.
3.1.2: X-ray Diffraction analysis:
The XRD patterns of the prepared catalysts are shown in Fig 1.c.
The higher metal distribution with very small crystallite sizes
renders the degree of crystallinity, hence XRD analysis was
carried out with the reduced samples (after treating 450℃ for 6h
with 10% H2 balance Argon). The 20Co0Cu catalyst shows
characteristics peaks at 36.5°, 42.3°,61.5° and 73.6°
corresponding to the reflection from (111), (200), (220), (311)
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is assumed to be a key factor towards the higher activity of
5Co15Cu catalyst in the applied reaction condition, where copper
adsorb CO associatively and cobalt adsorb CO dissociatively to
produce higher alcohols. This fact was further confirmed from the
reduced XRD pattern of fresh catalysts, where, the peak
corresponding to cobalt oxide decreases constantly with the
increase of copper content in the sample.
and 795.9 eV with satellites, which is the characteristic of Co3O4
[9]. The binding energy of Cu2p3/2 peak is around 933 eV with
shakeup peaks 940-945 e.V., is confirmed as characteristics for
the presence of Cu2+[9]. The addition of caesium shifts the both
copper and cobalt binding energy towards the higher region,
confirms increasing in the interaction of the metal with the support.
3.1.5: CO2- Temperature Programmed Desorption analysis:
The surface basicity of the prepared catalysts was investigated
with CO2- Temperature Programmed Desorption technique. Fig.
2.c shows the CO2-tpd profile, and the deconvolution graphs of
the distribution of strong, medium and weak basic sites are
provided in supporting info. The amount of CO2 desorbed was
divided into three sites, weak basic sites (100- 200℃), medium
basic sites (200-400℃) and strong basic sites (400- 700℃). The
addition of copper or cobalt in Cu-Co catalyst generally decreases
surface basicity with respect to individual copper or cobalt-based
catalysts. The addition of caesium does not affect too much to the
overall basic strength of the catalysts, however, slightly increase
In presence of caesium, the reduction peaks were slightly shifted
towards the higher temperature region. The retardation in the
reduction of CuO is due to the closer interaction between metallic
phases with the promoter [18, 19]. In 1Cs5Co-15Cu catalyst the
broad peak at 235℃ is owing to reduction of CuO to Cu and
Co3O4 to CoO, with a shoulder peak at 400℃ for the reduction of
CoO to Co. However, with the increase in caesium loading, in
2Cs5Co-15Cu and 3Cs5Co-15Cu, these peaks again shifted to
strong basic sites.
Generally, surface basicity decreases
methane selectivity with enhancing higher alcohols selectivity. In
Cu-Co catalysts the desorption of CO2 at low-temperature region
indicates the presence of weak and medium basic sites,
conversely, in caesium promoted catalysts presence of strong
CO2 desorption peak indicates the formation of surface
carbonates, which enhance the higher alcohol selectivity.
3.1.6. CO Temperature programmed surface reaction:
The progress of the reaction has been monitored by using CO
temperature programmed surface reaction. In CO hydrogenation
reaction over Cu-Co catalysts system basically occurs through
the following steps, adsorption, CO dissociation then
235℃ and 425℃ to 265℃ and 475℃. It is interesting to observe
that with the increase of caesium content up to certain level (2%)
the surface enrichment of Cu-Co occurs, which is reflected in the
peak intensity of respective XPS profile. An excess of caesium in
3Cs5Co-15Cu increases the active metal particle sizes and
hinders their reduction. This is a well concomitant with the XPS
data obtained and the TEM image of 3Cs5Co15Cu (given in
supporting info).
hydrogenation to form CHx species.
CO+ H2 -> CH4
Now being the thermodynamically more stable and favourable
CH4 production is very rapid, therefore the production of methane
was associated with the ability of Cu-Co catalyst to adsorb,
dissociate and hydrogenate of CO molecule. The TPSR profile of
the various catalysts shown in Fig 2.d. the first peak at ~200℃ is
due to hydrogenation of more active carbon species, derives from
CO dissociation. The next peak is due to hydrogenation of less
active carbon species, that requires relatively high temperature
for hydrogenation. Since cobalt in the catalyst surface was more
favourable for hydrogenation; therefore, in Fig 2.d. it is clearly
observed that with decreasing cobalt content the CO dissociation
get decreases. Now, since higher alcohol selectivity is achieved
. 3.1.4: X-ray photoelectron spectroscopy analysis:
To gain some insight regarding oxidation state of metal, surface
composition and metal support interaction X-ray photoelectron
spectroscopy has been performed. XPS results of Co 2p and Cu
2p regions for the fresh catalysts are shown in Fig. 2. a and b.
The binding energy (BE) of Co 2p can be seen around 780.59 eV
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by moderate hydrogenation ability[10], further modification for
better alcohol selectivity has been observed with 5Co15Cu
catalyst. The addition of caesium enhances the amount of
chemisorbed CO on the surface of the catalyst and hence, the
ability of CO dissociation get increases in the presence of
caesium. However, the addition of excess caesium could
suppress the CO adsorption ability and decrease catalyst
the reaction owing to several parallel side reactions, including
Fisher Tropsch synthesis, alcohol synthesis, homologation and
water-gas shift reaction[13]
.
3.2.1: Effect of reaction temperature and pressure:
The catalytic activity of the catalysts for the selective synthesis of
higher alcohols has been tested in the temperature range of 180-
300℃ with varying pressure 15 – 60 bar and the result is depicted
in Fig. 5.a.
activity[11]
.
Fig. 5.a. clearly picturise that with any condition keeping the
pressure constant the CO conversion increases with the increase
in temperature, which is attributed to the stronger ability of the
dissociative adsorption of CO at higher temperature region,
3.1.7: HRTEM and Elemental Mapping:
produces mainly methane and carbon dioxide. With the decrease
in pressure, the CO conversion decreases and higher alcohol
selectivity get increases. However, the intrinsic conversion (i.e.,
conversion with the accessible metal surface area from N2O-
Refractive frontal chromatography) decreases with increasing Cu
content in the sample, shown in Fig. 5.b. The higher intrinsic
conversion value of 20Co0Cu catalyst owing to its higher
hydrogenation activity, confirmed from higher methane and
hydrocarbon selectivity (shown in Fig 5.c) with very low alcohol
selectivity (shown in Fig 5.d) could be due to increasing the
copper particle sizes. Within hydrocarbons, methane is always a
major product, nevertheless, significantly suppresses with the
increase of copper content in Cu-Co based catalysts and the
catalytic properties shift from FT products to higher alcohols.
Henceforward, alcohol selectivity is found to be increased with the
increase of copper content, with a simultaneous decrease in
hydrocarbon formation. The majority of methanol in alcohol
product mixture is due to the favourable formation of methanol on
copper sites. However, with increasing copper content the higher
alcohol selectivity (mainly ethanol) also increases and reaches at
maximum (14.6% ethanol selectivity) with 5Co15Cu catalyst.
The catalytic performance results at 220℃ at 15 bar pressure are
presented in Table 2. The selectivity to higher alcohols increase
from 12% in 20Co0Cu to 19.16% in 5Co15Cu and then again start
The morphology of 5Co15Cu and 2Cs5Co15Cu are shown in Fig.
3 after 6h reduction at 450℃ and the corresponding distribution of
the metal particles in the catalysts are examined through
elemental mapping, shown in Fig. 4. The 15Cu-5Co catalysts
show copper particles size ranging from 30-50 nm, while in
2Cs15Cu-5Co, the copper particle size is ~10 nm. Which clearly
indicates that the Cs is not only enhanced the surface basicity
(from CO2 TPD results) but also helps to reduce the size of the
copper nanoparticles. The coexistence of both the cobalt and
copper particles side by side are confirmed from elemental
mapping and Co is found to be in very dispersed form rather than
copper (Fig 4). Earlier reports predicted that for better higher
alcohol selectivity Co- Cu should be in side by side to work CO
dissociation and CO association mechanism simultaneously[7, 12]
.
Although from TEM image it is observed that particle size of
2Cs5Co15Cu are smaller then that of 5Co15Cu, at reduced
condition 2Cs5Co15Cu forms metal cluster, that helps associative
CO adsorption and selective synthesis of higher alcohols.
3.2: Catalytic evaluation for the specific synthesis of higher
alcohols:
The catalytic performance of higher alcohol synthesis from
syngas is shown in Fig 5 and 6, indicating the complex nature of
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to decrease and found to be 13.4% in case of 0Co20Cu catalyst
with 4000 GHSV at same syngas ratio. The observed increase in
higher alcohol selectivity over 5Co15Cu catalyst is due to the
uniform distribution and coexistence of both the Cu-Co active
phases. This can significantly increase the synergistic effect
between copper and cobalt particles in the catalyst system by
performing CO association and dissociation mechanism
simultaneously for better yield of higher alcohols [5]. The support
lanthanum alumina may also play a substantial role in improving
in higher alcohol selectivity by promoting the formation of Co2C
species and optimises the formation of Co2+ to Co0, in which Co2C
favours the formation of higher alcohols[14]. Therefore, the
selectivity of higher alcohols in the case of 20Co0Cu catalyst
found to be quite appreciable around 12%. It is interesting to note
that water is also produced during CO hydrogenation, with the
alcohols, and it reacts with CO, through a water gas shift (WGS)
reaction and produces CO2.
3.2.2: Effect of Caesium doping:
decreases rapidly with the further increase in Cs loading. The
corresponding C1+ hydrocarbon selectivity (shown in Fig 6.c) and
ethanol and C2+ alcohol selectivity (shown in Fig. 6.d) also passes
through a peak point at 2% Cs loading and then drops with the
increase in caesium loading. Interestingly, the reverse trend has
been observed in the case of methane, methanol and CO2
formation.
Alkali metals are well known in CO hydrogenation due to their
unique ability of increase alcohol selectivity by suppressing
methanation and alcohol dehydration. The presence of alkali
metals neutralises the surface acidity, which reduces several
undesired reactions and promotes molecular CO adsorption. The
higher alcohol selectivity over the alkali metals increases with
decreasing electronegativity [15]. Among the alkaline metals
promoters, caesium highly facilitates C2+ alcohol selectivity.
The selectivity of main by-products is shown in Fig.6.c. The
undesired lower alkanes are well suppressed with the caesium
modification in the catalyst system in comparision with without
caesium incorporated Cu-Co catalysts. With the increase in
caesium loading, methane selectivity starts to decreases at first
and passes through a minimum at 2% caesium loading and again
it starts to increase. The opposite trend has been observed with
C1+ hydrocarbons, with the increase in caesium loading
C1+hydrocarbons (mainly ethane and acetylene) begins to
increase and passes through a maximum at 2% caesium loading,
and then began to decreases. The hydrocarbon formation could
be attributed to two different routes 1) direct of dissociation of CO
followed by hydrogenation and 2)dehydrogenation of alcohols on
the acidic surface followed by hydrogenation[13]. Moreover, it is
also anticipated that alkane formation largely dependent on acidic
surfaces, and this sites were neutralised or block by the alkali
metal promoters, which increases the higher alcohol selectivity[16]
.
To observe the effect of caesium promotion, stability test was
done with the three caesium promoted catalysts. The CO
conversion and the selectivity of alcohols substantially remain
stable up to 100h time on stream investigation (shown in Fig.6.b.).
The 1Cs5Co15Cu catalyst shows ~16% CO conversion with
22.5% methanol being the predominant product in alcohols with a
higher quantity of undesired methane and carbon dioxide.
2Cs5Co15Cu catalyst shows ~ 15% CO conversion with ethanol
being the predominant product among all alcohols. The selectivity
to undesired methane and CO2 decreases with increasing C1+
To reduce the selectivity of the main by-products i.e., CO2, CH4
and higher hydrocarbons caesium has been doped in the
lanthanum alumina support. Cu-Co catalyst with a different
amount of caesium and their promotional effect is depicted in Fig.
6. The catalysts with different Cs loading are tested at 15 bar
pressure 220℃ temperature with a same syngas ratio of H2: CO
in syngas with a space velocity of 4000 mL g-1 h-1. From Fig.
6.a.and 6.b. it could be seen that CO conversion increases with
Cs loading and reach at the maximum with 2% of Cs, however, it
hydrocarbon selectivity.
In 3Cs5Co15Cu again methanol
becomes again the predominant product with higher methane and
CO2 selectivity. The alcohol distribution over all the prepared
catalysts is recalculated by o; Anderson-Schulz-Flory (ASF) rule
and the plot between ln(Xn) against carbon number (n) is drawn
in Fig. 7. A close distance between cobalt and copper sites on
catalyst surface has steered the formation of hydrocarbons into
primary alcohols by insertion of undissociated CO molecule
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absorbed on copper site. Deviation from the ASF distribution was
observed upon the addition of caesium on Co-Cu samples. Based
on the characterization results, the addition alkali metal caesium
was beneficial for higher alcohol synthesis was further confirmed.
temperature. The obtained gels were dried under vacuum and finally
calcined for 6 h at 700℃ applying a heating rate of 1℃ min−1
.
2.1.2. Cu-Co nanoparticle deposition upon the catalyst surface:
The metal (Cu-Co) nanoparticles were deposited on the support surface
through modified procedure reported by Matzapetakis et al.[17] Requisite
amount of cobalt nitrate and copper nitrate salts were dissolved individually
in an aqueous solution of citric acid monohydrate under continuous stirring
and allowed to stir for overnight at 60℃. Then the solution was nearly dried
in a rotary evaporator and finally adjust the pH (~8) with aqueous ammonia
solution, Later, both the solutions were mixed and catalyst support was
added to it and stir for overnight at room temperature. The ultimate
nanocomposites solutions were dried and calcined at 450℃ for 5h applying
a heating rate of 1℃ min−1.
Conclusions
Cs promoted and un-promoted mesoporous lanthanum aluminate
supports was synthesised by using
a templet assisted
solvothermal method. A series of Co-Cu catalyst with varied
compositions has been prepared on mesoporous supports by a
unique complex precipitation method. Furthermore, these newly
developed materials have been explored for CO hydrogenations
and lead us to the following conclusions.
a) CO conversions observed to be increased with the increase
in catalyst bed temperature, with methane and carbon
dioxide as a predominant product. However, the alcohol
selectivity found to be higher at lower temperatures. It is
because, at a higher temperature, water gas shift (WGS)
reaction is thermodynamically favoured over Cu particles
and followed by surface carbon hydrogenation, which
practically favours the formation of more methane and
carbon dioxide.
b) It has been found that high pressure favours the CO
conversion with a higher amount of C1+ hydrocarbons as
major products. Nevertheless, higher pressure does not help
much to get better ethanol yield. This observation leaves us
in a state of the conclusion that low temperature and low
pressure favours the high selectivity towards alcohols in this
particular reported catalyst system.
c) Among the several prepared copper-cobalt combinations,
5% cobalt 15% copper composition on lanthanum alumina is
observed to be the best performed catalyst system towards
alcohol selectivity up to ~34%.
For sake of simplicity the prepared catalysts are designated as zCs-xCu-
yCo, where x, y are the respective weight percentage of Cu and Co
deposited upon z weight percentage of caesium incorporated modified-
lanthanum alumina surface.
Catalyst characterization:
The nature of the metal loaded catalysts was examined by N2 adsorption-
desorption isotherms at -180 °C (Micromeritics Tristar 3000) using the BET
equation. Before the N2 physisorption measurement, all samples were
degassed by evacuating at 300 oC for 3h to remove physically adsorbed
impurities. The total pore volume (Vp) was derived from the adsorbed N2
volume at a relative pressure of approximately 0.99, and the Barrett-
Joyner-Halenda (BJH) method was used to calculate the pore size
distributions according to the desorption branch of the isotherms
measured.
The actual composition of the catalysts was determined by inductively
coupled plasma atomic emission spectroscopy (ICP-AES). To prepare the
solution for elemental analysis, 2 mL of concentrated nitric acid was used
to dissolve 40 mg of catalyst sample, followed by adding 2 mL of 30%
H2O2; the solution was then diluted to 1000 mL with deionized water.
d) Caesium, the electronic promoter appears to be played an
excellent role in this applied catalyst system, which is found
to be two folded i) it reduces the copper particle sizes in the
catalyst system, and ii) it also improves the reducibility of
Co3O4 to Coo. This clearly reflects in the catalytic results by
improving the total alcohol yield up to ~55%.
The specific surface area of copper on the catalysts was measured by the
dissociate chemisorption of nitrous oxide (N2O-RFC). The experiments
were performed in a quartz microreactor at 80℃ to avoid subsurface
oxidation, prior to each experiment, the catalysts were reduced in a mixed
gas of 5 vol% H2 in helium at 250℃. Subsequently, the reactor was purged
for
1 h with He and cooled down to ambient temperature. The
measurement was started by switching from pure He to a mixed gas of 5
vol.% N2O in He in such a way that surface Cu atoms were completely
oxidised according to the reaction 2Cu(s) + N2O (g) = Cu2O(s) + N2 (g).
The effluent gas was monitored using a GC. The amount of nitrogen
formed was quantified by gas chromatography technique. The Cu surface
area was calculated assuming a molar stoichiometry of Cus/N2=2 and a
value of 1.47×1019 copper atoms per square meter. The error in the Cu
Experimental Section
2.1. Catalyst preparation:
surface area data was estimated to be less than 10%[18]
.
All chemicals in this synthesis process were used as obtained from Sigma-
Aldrich Ltd. Anhydrous ethanol(>99%) from Fisher Scientific and double
distilled water were used as a solvent during the synthesis.
X-ray diffraction (XRD) patterns were recorded on a Proto advance powder
X-ray diffractometer, using Cu 퐾훼 radiation with scanning angle (2휃 )
ranging from 10 ° to 80 ° . The identification of different phases was
performed by comparing the patterns with JCPDS standard cards.
2.1.1. Synthesis of catalyst support:
2.0g of the triblock copolymer (Pluronic P123), 0.70g of lanthanum nitrate,
hexahydrate was dissolved in 50mL of ethanol and the other solution has
been papered with 3g of aluminium isopropoxide, 3mL of nitric acid and
50mL of ethanol. Later, the two solutions were combined to make the final
solution and allowed to stir for 5h at room temperature. For the Cs modified
support, the required amount of caesium nitrate was added to the
aforementioned final solution and stirred for additional 4h at room
Temperature-programmed reduction (TPR) experiments were carried out
in a fixed-bed reactor. Prior to the TPR measurements, samples were
pretreated at 300 °C for 0.5 h in flowing Ar (50 mL/min) to remove any
moisture and adsorbed impurities. After cooling the reactor to room
temperature, the reduction gas of 4.2% H2/N2 at a flow rate of 30 mL/min
was introduced. The temperature of the reactor was raised linearly to
900 °C at a rate of 10 °C/min by a temperature controller. The total
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adsorption for 0.5 h; afterward, the H2/N2 mixture was swept again, and
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thermal conductivity detector (TCD).
Acknowledgements
S.D. sincerely acknowledges University Grants Commission, New
Delhi, India for fellowship. A.B acknowledge CSIR India for the
OLP-0913 project (synthesis of oxygenates from bio-syngas by
using nanocomposite catalyst). The Director, CSIR-IIP, also
gratefully acknowledged for his encouragement and Analytical
science division for their support.
Keywords: Bio-syngas, Cu-Co Nanoparticle reducibility,
Surface basicity, Alcohol selectivity, Role of promoter.
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Subhasis Das,
Caesium promoted
and unpromoted
Copper cobalt
catalyst has been
tested in CO
hydrogenation for
selective higher
alcohol synthesis.
Caesium being an
electronic promoter
enhanced higher
alcohol selectivity to
55% at mild reaction
condition.
Manideepa Sengupta,
Ankur Bordoloi *.
Page No. 1-8
Cs promoted higher
alcohol synthesis
over modified Cu-
Co nanocomposites
under mild
condition
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