Acetone alkylation with ethanol over multifunctional catalysts by a borrowing hydrogen strategy

Article abstract of DOI:10.1039/c5ra17889d

Step by step alkylation of acetone (A) with ethanol (E) in a ratio of 1: 2 was investigated. A fixed bed flow-through reactor system was used at a total pressure of 21 bar and in the temperature range of 150-350 °C in inert He or a reducing H₂ medium. Following the hydrogen borrowing methodology, two types of catalysts were prepared; using neutral activated carbon (AC) and alkaline hydrotalcite (HT) supports, namely 5 wt% Pd/AC in the presence of alkaline additives (10, 20 and 30 wt% KOH or 20% K₃PO₄); 9 wt% Cu/HT and 5 wt% Pd/HT. The catalysts were activated in a H₂ flow at 350 °C. Different yields of mono- or dialkylated ketones were observed. In a hydrogen medium over the same catalyst systems the ketone products could be reduced to alcohols. In this study the Pd/HT catalyst seems to be the most promising for fuel production based on biomass fermentation.

Full text of DOI:10.1039/c5ra17889d

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Acetone alkylation with ethanol over
multifunctional catalysts by a borrowing hydrogen
strategy
Cite this: RSC Adv., 2015, 5, 99502
*
Gy. Onyestyak, Gy. Novodarszki, R. Barthos, Sz. Klebert, A´. Farkas Wellisch
´
´
´
and A. Pilbath
´
Step by step alkylation of acetone (A) with ethanol (E) in a ratio of 1 : 2 was investigated. A fixed bed flow-
through reactor system was used at a total pressure of 21 bar and in the temperature range of 150–350 ꢀC in
inert He or a reducing H2 medium. Following the hydrogen borrowing methodology, two types of catalysts
were prepared; using neutral activated carbon (AC) and alkaline hydrotalcite (HT) supports, namely 5 wt%
Pd/AC in the presence of alkaline additives (10, 20 and 30 wt% KOH or 20% K3PO4); 9 wt% Cu/HT and
5 wt% Pd/HT. The catalysts were activated in a H2 flow at 350 ꢀC. Different yields of mono- or
dialkylated ketones were observed. In a hydrogen medium over the same catalyst systems the ketone
products could be reduced to alcohols. In this study the Pd/HT catalyst seems to be the most promising
for fuel production based on biomass fermentation.
Received 3rd September 2015
Accepted 13th November 2015
DOI: 10.1039/c5ra17889d
www.rsc.org/advances
toluene C5–C11 or longer-chain ketones were formed with
different yields depending on composition of the reaction
Introduction
mixture and the applied catalysts. Ketones may be deoxygen-
ated to paraffins, the components of fuel (in this cited work only
partial reduction to alcohols was presented over a Pt-catalyst).
Carbon supported palladium using in various forms was
found to be outstanding compared with other metals (Ir, Ru,
Rh, Pt, Ni). Different bases were added in some molar equiva-
lent to the reaction mixture. K3PO4 as alkaline additive seemed
to be the most efficient.5 Although the well-reviewed borrowing
hydrogen methodology6,7 is suggested applying metal catalysts,
type and amount of the used bases catalysing aldol reaction step
are also important factors. In the shown experiments high, at
least equivalent amount of bases are used.
Mimicking real aqueous ABE fermentation product G. Xu
and co-workers8 showed direct a-alkylation of ketones with
alcohols in a small autoclave using water as solvent instead of
toluene5 over various Pd/C catalysts. In this study equivalents of
different bases (LiOH, NaOH, KOH or K3PO4) to the amount of
ketone were also added to the reaction mixture in the batch
system. Q. Xu and co-workers9 conceive that transition-metal-
catalysed a-alkylation of ketones with alcohols still have draw-
backs, consequently they prefer the “catalyst-free” dehydrative
a-alkylation studying only various aromatic reactants. However,
high amount of bases (NaOH or KOH) are still applied in the
studied alkylation reactions.
The depletion of fossil fuels has prompted interest in producing
alternatives that are compatible with petroleum liquids. Tech-
nologies on various biomass platforms involve combinations of
mechanical, thermal, chemical and biochemical processes.1–3
Instead of the less selective thermochemical method, favour-
able microbiological destruction processes are preferred.4
Commercial production of ethanol as a fuel blend has already
been well recognized worldwide. Compared to ethanol, butanol
has about 33% higher energy content per mass and is more
compatible with the existing petroleum infrastructure. Nowa-
days, the anaerobe bacteria of genus Clostridium attract again
great interest to produce acetone, butanol and ethanol (ABE
mixture) in a 2.3 : 3.7 : 1.0 molar ratio from sugars, carbohy-
drates, lignocelluloses, etc. for use as renewable alternative
transportation fuels.5 Although co-production of acetone lowers
the yield of alcohol biofuels, but its presence give an important
potential to obtain longer carbon chains via alkylation with
alcohol content of the mixture. Nucleophilic a-carbons of
acetone can form C–C bond with electrophilic a-carbon of the
alcohols, resulting in longer chain length organic compounds
than the original fermentation products from two-carbon, three
carbon and four-carbon precursors.
First Anbarasan and co-workers5 proposed the alkylation
route to convert ABE fermentation product into fuel. Using
stirred pressurized tubes, small batch systems at 110 ꢀC in
The signicant basicity and intercalated copper ions in Mg,
Al hydrotalcite supported copper sample showed multifunc-
tional activity in various catalytic transformations of alcohols.10
Cu–hydrotalcite was found to be as an efficient and cheap
catalyst for a-alkylation of various aromatic compounds such as
Institute of Materials and Environmental Chemistry, Research Centre for Natural
Sciences, Hungarian Academy of Sciences, H-1519 Budapest, P. O. Box 286,
Hungary. E-mail: onyestyak.gyorgy@ttk.mta.hu
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ketones, b-alkylation of alcohols, alkylation of amines and containing 5 wt% palladium instead of copper was also
amides.11
prepared for comparison impregnating HT with tetraamine–
The cited literature shows as yet only investigations in small palladium(II)nitrate solution applying incipient wetness
batch reactor systems5,8–11 which have several disadvantages and method. These samples were rst dried in air slowly heating up
those tests give information with limited details. Broad spec- to 110 ꢀC and were named 9Cu/HT and 5Pd/HT. The HT
trums of catalysts were already tested, but the acting catalysts samples were calcined at 520 ꢀC for 4 h to obtain Mg–Al mixed
were not at all characterized. A lot of different reaction mixtures oxides (AHT). Usually the metal catalysts are formed in ow
were studied in ref. 5, but at presented low yields the compar- of H2 gas before testing the catalytic performance in
ison of the results are vague. Thus the results of the literature dehydrogenation/hydrogenation reactions however, in the cited
data are hard to be compared signicantly with our new results. studies10,11 the samples were used without reducing the copper
Processing of ABE mixture – which can be one important way species. In other important cited ref. 5 and 8 information
of biomass utilization in high volume – needs an effective couldn't be found about the state of metallic components. In
continuous procedure. For this purpose our rst aim in this those studies investigations are carried out in small batch
study was to move from batch to ow-through system. reactor systems in liquid phase below 200 ꢀC. Contrary we used
Furthermore to avoid application of alkalies as additives, the ow-through system in vapour phase at higher reaction
use of alkaline support (hydrotalcite) instead of the practically temperatures where the applied reactants surely can reduce the
inert carbon was aimed.
precursors of Cu or Pd. Consequently, in general the catalysts
In this study working in vapour phase through xed beds a- were pre-treated equally in situ in hydrogen ow in the reactor at
alkylation using acetone and ethanol, as model reactants was 350 ꢀC and 21 bar for 1 h in order to obtain active metallic
investigated in more details than in the literature shown. In our surface.
work it should be taken in consideration that third component
of ABE mixture, butanol was not applied as reactant in order to
Methods
follow and understand better the reaction mechanism.
Nitrogen physisorption measurements were carried out at
However, it can inevitably form as Guerbet side product in
ꢀ
ꢁ196 C using Quantachrome Autosorb 1C instrument. Before
a self-aldol reaction of ethanol. Our ndings are more general
the adsorption analysis samples were outgassed under high
than processing of ABE mixture, because alkylation of ketones
ꢀ
vacuum at 200 C for 3 h.
with alcohols has great importance independently on resources
The catalytic alkylation of acetone (A) (99.5%, Reanal) with
of the reactants. The suggested method is using less hydrogen
in the production of longer carbon chain hydrocarbons as fuel
compounds through longer ketone formation. The formation of
one molecule of C7–C11 product is a result of two dehydration
using inert helium or reducing hydrogen streams. The reaction
steps. Three oxygen atoms per one longer alkane molecule can
be removed, using two hydrogen molecules only for the reduc-
tion of longer dialkylated ketones.
ethanol (E) (99.7%, Reanal) mixed in 1 : 2 molar ratio was
studied in a high-pressure xed bed ow-through reactor12 at
21 bar total pressure in the temperature range of 150–350 ꢀC
was allowed to run one hour at each condition to attain steady
state. The effluent during the second hour was collected, dep-
ressurized and cooled to room temperature. The liquid product
mixture at ambient conditions was analysed by gas chromato-
graph using a GC-MS (Shimadzu QP2010 SE) capable to identify
Experimental
Materials
products formed in low concentration, equipped with a ZB-WAX
plus capillary column. The gaseous reactor effluent was ana-
lysed for detection of CO2, CO, CH4 and light hydrocarbons
using an on-line gas chromatograph (HP 5890) with thermal
conductivity detector (TCD) on Carboxen 1006 PLOT capillary
column.
The conversion of the two component reaction mixture
cannot be well dened, due to the complex reaction network
shown later. Both of the reactants are transformed to such
a kind of by-products, which can take part further in the main
alkylation reaction network. The main alkylated products
(ketones and alcohols) yields are used for characterizing the
activity and selectivity of the applied catalysts.
Ref. 5 and 8 did not give specication about the used “5%
palladium on carbon” catalysts. In this study, a commercial
pelletized activated carbon (AC) (cylinders with 0.8 mm diam-
eter and 2–4 mm length, Norit ROX 0.8 EXTRA, specic area:
1150 m2 gꢁ1) as inert support was applied in order to compare
the results obtained over the newly suggested hydrotalcite
support.10,11 The AC rst was dried at 110 ꢀC, then impregnated
with KOH (Fluka AG) or K3PO4 (Aldrich) solutions using incip-
ient wetness method and dried again at 110 ꢀC. For 1 g support
0.1 g, 0.2 g or 0.3 g bases were added resulting in plus 10, 20 or
30 m% loadings. Finally 5 m% palladium is also loaded using
tetraamine–palladium(II)nitrate solution (STREM Chemicals).
Preparation and physico-chemical properties of Cu–Mg, Al
hydrotalcite (HT) was described in details by Dixit and co-
workers.10 Documentation was carefully followed obtaining
catalysts with identical features. Incipient wetness impregna-
Results and discussion
Adsorption properties of supports and supported catalysts
tion of HT was carried out with aqueous solution of copper Since the structure of support can strongly affect the formation,
acetate (Aldrich). Only discrepancy from the ref. 10 is the higher location and accessibility of the catalytically active components,
copper loading used: 9 wt% instead of 3–3.5 wt%. Catalyst checking of the overall catalyst texture is essential – including
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ꢀ
both the support and the active species, changes in the course range of 270–500 C structure change was attributed to loss of
of the different preparation steps. For instance the diffusion hydroxyls of brucite layer and interlayer anions. The mean
resistance of the material may change aer each step and the mesopore diameter in all the samples is approx. the same
consequently altered mass transport may result in signicantly (16 nm). Presence of metal, copper or palladium particles do not
altered yields and selectivities. In Fig. 1 adsorption properties of inuence the texture similarly to the impregnated bases and/or
activated carbon (AC) and Mg–Al hydrotalcite (HT) supported palladium on AC. Since only slight decrease can be observed of
catalysts are compared. Different textural features of the applied the adsorbed amount of nitrogen for different loadings thus
supports are reected by the nitrogen adsorption and desorp- catalytic inuence of textural changes can be also negligible.
tion isotherms.
Differences in texture for different supports (AC or HT) are
AC support, which is practically inert under conditions of the evident however in our case these hardly inuence the pathway
investigated reactions, has a well-dened pore structure. Shape of alkylation reaction or the product distribution as it can be
of isotherms (typical type I indicating a microporous adsorbent seen hereunder. The role of catalytically active species, quantity
with slight mesoporosity) reects highly microporous materials and quality of alkaline and metallic active site centres seems to
with high specic surface area (>1000 m2 gꢁ1) which is char- be dominant compared to the support texture.
acteristic for activated carbons containing less slit-like pores
than 1 nm between carbon sheets. Appearance of hysteresis
loop indicates mesopores (mean pore diameter (BJH) is 4 nm) of
low diffusional resistances evolved in the course of pellet
formation process. Presence of impregnated bases and/or
palladium on AC only hardly decrease the adsorbed amount
of nitrogen related to AC content (in Fig. 1 only the parent
carbon and the 30KOH/AC samples are shown). This observa-
tion means that alkaline salts (and loaded palladium metal
also) can form larger crystals than the entrance of micropores in
the mesopores. However, the mesoporous volumes, area of
hysteresis loops are decreased signicantly. Since active
components, bases and the metal are located in the larger pores
consequently the catalytic reactions take place in the
mesopores.
Characteristic transformations in inert atmosphere over 5Pd/
AC
Tables 1–3 are designed in identical structure to demonstrate
signicant differences in formation of main products over
various catalysts and using inert (helium) or reactive (hydrogen)
carrier gases, respectively. In helium the mono-alkylated
(2-pentanone, 2-heptanone) and the bi-alkylated-ketones
(4-heptanone, 4-nonanone) are the main products formed
with desired good selectivity over AC supported Pd catalysts.
Table 1 shown the results obtained on catalyst loaded with
different alkaline materials (KOH and K3PO4) and with KOH
applied in various concentrations. The inuence of reaction
temperature is also reected in Table 1.
Without palladium loading very low catalyst activities can be
observed (not shown in Table 1). The “catalyst-free” (in presence
only of alkaline material, i.e. over alkaline monofunctional
catalyst) dehydrative a-alkylation9 cannot be suggested in ow-
through system. Without bases (K3PO4 or KOH) over mono-
functional Pd/C catalyst only low alkylation activity (also not
shown in Table 1) can be observed similarly to one component
Pd-free catalyst, but the Pd/C catalyst is very active in producing
of fragmented molecules. In absence of hydrogen high
concentration of the undesired carbon dioxide, methane and
carbon monoxide gases can be produced by splitting the C–C
bonds of the reactants.
Hydrotalcite (HT) isotherms are of the form well known in
the literature as type IV with H2 hysteresis loop. These are
practically overlapping applying different loadings (Cu or Pd
and precursꢀors of them) howeverꢀdifferent calcination temper-
atures (200 C – Cu/HT and 520 C – Cu/AHT) inuencing the
HT structure, result in signicantly different isotherms and
consequently different BET surface areas, 178 and 274 m2 gꢁ1
respectively. Decomposition bellow 250 C was due to removal
of physisorbed water and interlayer water molecules. In the
,
ꢀ
The signicant production of 2-heptanone and 4-nonanone
is interesting which testify that butanol can form as a Gourbet
by-product, although it cannot be directly detected using
helium. Consequently numerous variations of potential ketones
can be detected in very different concentration alike as real ABE
mixture has been studied.
Characteristic transformations in reducing atmosphere on
5Pd/AC
Alcohols didn't appear applying helium carrier gas. Those were
signicantly formed only in hydrogen (see in Table 2). Over Pd
catalysts ketones can be reduced only to alcohols. Alkanes
preferable for fuels can be detected only under severe reaction
conditions, at high temperatures where numerous useless by-
products can be also formed. The alkaline loaded Pd/C cata-
lyst is highly efficient in the desired alkylation reactions
Fig. 1 Adsorption isotherms of nitrogen obtained at ꢁ196 ꢀC on
parent carbon (AC) and the 30KOH/AC samples (AC + KOH) as well as
on copper loaded hydrotalcite precursor calcined at 200 ꢀC (Cu–HT)
and 520 ꢀC (Cu–AHT).
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ꢁ1
Table 1 Yields (wt%) of typical main products obtained in helium stream at different alkaline loadings on 5Pd/AC catalysts (WHSV ¼ 1 h gcat gAE
ptot ¼ 21 bar; A/E ¼ 1 : 2)
;
Catalyst
5Pd,10KOH/AC
5Pd,20KOH/AC
5Pd,30KOH/AC
5Pd,30KOH/AC
Reac. temp. ꢀ
C
200
250
3000
200
250
300
200
250
300
200
250
300
Isopropyl alc.
Butanol
1.3
—
2.2
—
2.4
—
1.2
—
1.6
—
1.8
—
1.4
—
1.3
—
1.4
—
—
—
0.9
—
2.0
—
Ethyl acetate
2-Pentanone
2-Pentanol
4-Heptanone
2-Heptanone
4-Heptanol
4-Nonanone
CO
—
16.1
—
3.5
—
—
0.4
—
—
—
22.0
—
17.9
1.8
—
1.5
2.0
3.4
3.8
—
24.0
—
26.3
2.2
—
9.9
2.2
3.3
3.4
—
16.3
—
5.1
0.2
—
0.4
—
—
21.9
—
17.5
2.1
—
3.6
0.9
1.9
2.1
—
24.4
—
25.9
2.9
—
9.1
1.1
2.2
2.3
—
16.4
—
7.9
0.5
—
0.3
—
—
21.8
—
17.1
2.7
—
6.4
0.5
0.9
1.5
—
24.6
—
25.6
4.0
—
9.9
0.7
1.2
1.9
—
0.7
—
0.8
—
—
0.2
0.3
—
—
18.3
—
4.6
1.1
—
0.6
1.8
1.7
0.3
—
24.9
—
14.3
1.7
—
1.8
4.1
4.2
1.7
CH4
CO2
—
—
—
—
—
—
Table 2 Yields (wt%) of main products produced in hydrogen flow
(WHSV ¼ 1 h gcat gAEꢁ1; ptot ¼ 21 bar; A/E ¼ 1 : 2)
pressure was applied. Any alkenes or unsaturated ketones could
not be detected, thus the used carbon support seems to be inert.
It is interesting that the reactant acetone in hydrogen ow can
be fully reduced to isopropyl alcohol already below 200 ꢀC over
the tested Pd-catalysts. Also the alkylation is materialized
despite of the signicant consumption of the acetone in the
isopropyl alcohol formation.
The ABE mixture can become a compatible fuel aer less or
deeper transformations. The simplest method is if the reactive
acetone content is eliminated. A present study was aimed to
develop an improved Clostridum acetobutylicum strain with
enhanced alcohol fuel production capability.13 Contrary, our
nding is more efficient that in hydrogen atmosphere, over Pd
catalysts acetone content of ABE mixture can be completely
reduced to isopropyl-alcohol below 200 ꢀC, which method
seems to be simpler than the efforts shown in ref. 13.
Catalyst
5Pd,20KOH/AC
5Pd,20K3PO4/AC
Reac. temp. ꢀC
300
350
300
350
Isopropyl alc.
Butanol
11.3
3.2
—
11.6
10.6
12.8
1.2
1.2
1.4
1.3
1.5
5.4
2.1
—
15.0
3.1
—
9.1
1.2
—
Ethyl acetate
2-Pentanone
2-Pentanol
4-Heptanone
2-Heptanone
4-Heptanol
4-Nonanone
CO
17.9
9.9
18.9
2.6
3.9
9.1
3.0
3.1
1.6
10.6
9.4
7.6
0.5
2.7
1.1
1.8
2.6
2.8
19.5
5.5
18.4
2.0
2.7
3.6
6.7
5.6
2.3
CH4
CO2
0.6
Using hydrogen, signicant butanol formation can be
already observed. In situ formed butanol results in mimicking
of ABE mixture. Gaseous, cracked by-products are formed in
less quantity by applying hydrogen stream over the applied
“multifunctional” catalysts (dehydrogenation + aldol reaction +
dehydration + hydrogenation), which is the primal advantage of
H2 use together with alcohol production from ketones. Neces-
sarily elevated total pressure also decreases production of
fragmented compounds which can result in more molecules in
the reactor space. Finally, the use of inert carrier gas can be
preferred to attain higher yields of alkylates since there is no
signicant loss of reactive acetone in H2 medium due to
reduction to isopropyl alcohol. A second reactor (hydroge-
nating) can be suggested for production of alkanes from
ketones obtained in the rst stage.
The yield of alkylated products is increasing at higher
temperature, but gaseous by-products are lowered at lower
temperatures.
The efficiencies of the studied Pd/C catalysts (5Pd,20K3PO4/
AC; 5Pd,10KOH/AC; 5Pd,20KOH/AC; 5Pd,30KOH/AC) applied in
different medium (He and H2) is compared in Tables 1 and 2.
The nature of different bases (e.g. KOH or K3PO4) seems to be
important in line with the batch results.5,8 However under
Table 3 Yields (wt%) of main products obtained over hydrotalcite
based catalysts (WHSV ¼ 1 h gcat gAEꢁ1; ptot ¼ 21 bar; A/E ¼ 1 : 2)
9Cu/AHT + 9Cu/AHT + 5Pd/AHT + 5Pd/AHT +
Catalyst
He
H2
He
H2
Reac. temp. ꢀ
C
300 350 300 350 250 300 250 300
Isopropyl alcohol 6.3
5.6
2.5
20.5 14.3 2.9
5.3
—
—
12.5 8.5
Butanol
4.3
3.4
3.2
2.9
1.1
7.0
—
—
—
—
Ethyl acetate
2-Pentanone
2-Pentanol
4-Heptanone
2-Heptanone
4-Heptanol
4-Nonanone
CO
11.6 3.3
10.6 20.5 5.9
—
1.3
1.1
—
—
—
15.6 20.7 5.7
8.3
11.4
16.3
—
—
11.6 18.4
—
—
8.6
6.9
—
4.5
3.2
—
—
—
3.9
0.4
—
7.6
0.6
—
0.3
1.3
—
0.3
3.2
—
5.2
—
0.2
0.1
—
11.8
0.9
2.8
2.0
0.3
—
—
—
—
0.2
0.1
3.3
1.3
0.3
8.1
0.7
0.8
2.2
2.4
2.9
5.1
2.2
1.3
—
9.0
5.1
—
CH4
CO2
however total deoxygenation could not be reached similarly to
the results of P. Anbarasan and co-workers.5 In order to increase
the hydrogenation reaction rate higher than atmospheric
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conditions of this continuous process K3PO4 as alkaline addi-
tive was proven to be less efficient than KOH similarly than in
ref. 8. Using KOH more dialkylated products can be obtained
reecting its higher activity in the aldol reaction. The concen-
tration of alkaline component (KOH) on the catalyst surface
seems to be less important regarding to main product yields,
however signicantly less yields of undesirable gaseous by-
products can be observed with increasing KOH content.
The alkaline loaded catalysts seem to be stable during the
long experimental time and sustain a constant activity. Thus
vapour phase alcohol reactant or water product cannot remove
alkaline salts from the support.
Applying reductive carrier gas in the reactor (i.e. hydrogen
ow), lower gaseous by-product yields can be obtained (see
Table 2). However it seems to be not too characteristics in line
with the lower yields of desired products.
In this work the reaction temperature is much higher than in
batch studies and gas/solid interactions are studied resulting in
higher reaction rates. In our ow-through reactor system the
productivity seems to be higher at least with more than one
magnitude of order.
Qualitatively the product distributions demonstrate good
analogy with cited references, reecting identical reaction
mechanism in the reacting zones, in the course of solid–liquid
or solid–gas catalytic surface chemistry. However the quantita-
tive comparison is very complicated because of signicant
difference of the studied catalytic systems and mainly of poor
data shown in the cited papers.5,8
Scheme 1 A basic approach to the transition-metal catalysed reaction
network for alkylation of acetone with alcohols on the base of
detected products in significant concentration.
Reaction network of acetone alkylation with ethanol
Based on products shown in Tables 1 and 2 and further
components detectable by GC-MS a complex reaction scheme
can be recognized. The main variation of the alkylation reac-
tions are presented in Scheme 1, but several products are
formed in low concentration as well. The alcohols, the ethanol
reactant and the in situ formed butanol are dehydrogenated by
the palladium catalyst, generating the reactive aldehyde and
hydrogen. At this stage the aldehydes undergo either a self-aldol
reaction to form Guerbet product precursor, or an aldol reaction
with acetone. Under the conditions employed here, condensa-
tion with acetone seems to be favoured because acetone is
present at much higher concentrations than the transient
product aldehydes. By changing inert carrier gas to hydrogen
ow the initial acetone–alcohol ratio signicantly decreases due
to the transformation of a great part of acetone to isopropyl
alcohol, which results in more signicant production of butanol
as by-product. The longer, mono-alkylated ketones produced in
increased concentration have concentration-proportionated
atmosphere) isopropyl alcohol can necessarily form from
acetone resulting in branched products also (e.g. 4-methyl-2-
pentanone, 2-methyl-4-heptanone, etc.). Naturally longer
ketones formed with butanol or isopropyl alcohol can be also
reduced to alcohols in signicant, but lower concentrations.
The visualization of these compounds in the scheme would
make the scheme very diffuse.
Summarizing, the studied alkylation reactions involves
a complex sequence of many reactions. This route includes four
different types of reactions: I dehydrogenation, II aldolization,
III dehydration and IV hydrogenation. The most commonly
accepted path involves the aldolization reaction as the C–C
forming step.
Alkalinity of the multifunctional catalyst given by the support
chance to react instead of acetone resulting in dialkylated One important aspect of this catalytic process is the use of
products. additional alkalis in high amount to efficiently stimulate the
Subsequent dehydration of aldol product yields a,b-unsatu- aldol reaction step. Availability of base particles in mixed batch
rated ketones which undergo metal-catalysed hydrogenation systems containing liquid phase contacted with solid or in ow-
with the hydrogen generated in the rst step. Completing the through reactors working with vapour/solid interactions are
same cycle with monoalkylated products attains the expected severely undened. For example only a small part of the
double alkylation. The appearance of metal-adsorbed hydrogen impregnated KOH in the inert activated carbon support can
aer the ethanol dehydrogenating step (even in inert work because practically equal product distributions were
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obtained on 5Pd,10KOH/AC, 5Pd,20KOH/AC and 5Pd,30KOH/ product formation (beside butanol or ethyl-acetate production) is
AC catalyst samples (see Table 1). The use of an originally also inuenced by support change for hydrotalcite base: isopropyl
basic support such as MgO or its mixed oxides instead of alcohol formation was already signicantly increased in absence of
a neutral or slightly acidic carbon support can be supposed to reductive atmosphere, in helium stream (compare Tables 1 and 3),
be imposing. Different hydrotalcites and their derived mixed too. Hydrogen borrowing metals, copper or palladium, producing
oxides have received a great attention in recent years as an hydrogen in the reaction step of alcohol transformation to acet-
appropriate support for copper being highly active for catalytic aldehyde, at lower concentration of unsaturated aldol reaction
transformations of alcohols due to synergistic effect between intermediers has more chance to reduce the reactant acetone to
the basicity of the support and the fast spill over of Cu nano- isopropyl alcohol in signicant extent.
particles.10 The scope of Cu–hydrotalcite was shown as an effi-
Copper and palladium above highly different efficiency in
cient catalyst for a-alkylation of ketones, b-alkylation of dehydrogenating/hydrogenating reactions has a very important
alcohols, alkylation of amines and amides.11 Catalytic activity of dissimilarity considering affinities to hydrogen. In the case of
such supported copper catalysts in alcohol activation by catalysts that include copper, dihydrogen can be released from
borrowing hydrogen was shown in numerous examples but it the surface aer the dehydrogenation reaction, and the phase
was investigated only with aromatic alcohols as reactants.11
dihydrogen and aldehyde can be in equilibrium with alcohol.
The main aim of this work was to avoid use of alkalies as Contrary palladium can absorb in its crystal structure high
catalyst additives using an originally basic support instead of amount of hydrogen. This otherness can play important role in
the practically inert carbon. We applied well dened and thor- the reaction mechanism resulting in different product distri-
oughly characterized Mg–Al hydrotalcite and Mg–Al mixed bution as reected in Table 3 and Fig. 2 and 3.
oxide supported copper catalysts which seemed to be efficient in
other reactions demonstrated by M. Dixit et al.10,11
Table 3 demonstrates evidently applicability of hydrotalcite
based catalysts. However under the same reaction conditions
these samples show signicantly lower activities than studied
KOH loaded carbon supported forms (compared with Tables 1
and 2). As it turned out, differences can be found using different
alkalines (see in Table 1 or ref. 5 and 8). Mg–Al hydrotalcite has
also different basicity as well as different accessibility of basic
sites. Along with lower conversion the product distributions
contrast also strikingly with former samples which is not too
surprising having signicant structural and chemical differences.
Dehydrogenating/hydrogenating steps can be rate-
determining in the reaction mechanism of alkylation. As is
well-known copper has drastically lower activity than palla-
dium. This difference is strikingly reected in Table 3 showing
different catalytic properties of the metals. Beyond that the
different basicity which can be a crucial effect, differences can
be observed better at the less active Cu-catalysts.
Most eye-catching effect is appearance of a new product,
ethyl acetate in high concentration parallel with detectable
acetaldehyde intermedier in measurable amount (not shown in
Table 3). Although high basicity of Mg–Al hydrotalcite sup-
ported copper samples was stated in ref. 10, in fact concentra-
tions of acidic and basic sites were shown to be
commensurable. Less basicity can result in lower alkylation rate
along aldol reaction of ketones. Under mild reaction conditions
the low alkylation rate results in higher acetaldehyde coverage
on the basic surface giving more chance for rival ethyl acetate
formation in the Tishchenko reaction by coupling of two acet-
aldehyde molecules. Over similar catalytic systems ethyl acetate
formation has been oen found to be signicant at various
reaction conditions.14 The open literature provides a wide
variety of possible explanations without conclusive evidence.15
Also the presence of more acetaldehyde can be correlated with
higher butanol production, which needs also acetaldehyde
precursor. The rate of alcohol coupling is proportional to the
ported catalysts in helium as a function of space time. The reaction
aldehyde concentration. What is more, the third pathway of by- was carried out at 21 bar total pressure and 300 ꢀC.
Fig. 2 Yield of significant products obtained over hydrotalcite sup-
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However Fig. 2 and Table 3 show that, in contrast to ref. 11,
hydrotalcite supported copper catalyst is not advantageous for
industrial processing of the ABE mixture. In their paper11 Dixit
et al. suggests that this copper catalyst has great potential for
the catalytic C- and N-alkylation of aromatic compounds.
Combination of basic hydrotalcite support with palladium
metal (which was found to be most active and selective on
carbon support5) gives a promising catalyst for the selective
alkylation of acetone, a key process for converting the ABE
mixture into fuels. Beside good alkylation selectivity, the initial
reaction rates are also much higher over palladium than on
copper, as Fig. 2A and B demonstrate.
It is well-known that palladium is an effective
dehydrogenation/hydrogenation catalyst. This property is re-
ected in the different yields in Fig. 2 and 3, which show them
when the alkylation was carried out in helium and hydrogen
ow, respectively.
The observed differences demonstrate the competition of
coexistent four reaction routes (three side- and one main reac-
tion path) resulting in the consecutive alkylation reaction chain
illustrated in Scheme 1.
On the Cu/AHT catalyst every side reactions (ethyl acetate,
butanol and isopropyl alcohol formations) are signicant
compared to the aimed alkylation process. For example, the
ꢁ1
slope from 0 h gcat gAE to the rst measured yield values in
Fig. 2 and 3 is much steeper on the Pd catalyst than that on the
Cu catalyst, all but one, indicating that the initial rate of ethyl
acetate formation through the acetaldehyde intermediate is
faster than that through the direct acetone alkylation. High
consumption of reactant ethanol for two different and
competing side reactions hinders the formation of desired
alkylates. In hydrogen ow the consumption of acetone in the
third side reaction (reduction to isopropyl alcohol) is dominant
(Fig. 3A), which might also slow down the alkylation. In
contrast, Pd/AHT is more selective for the aimed alkylation
reaction.
Fig. 3 Yield of significant products obtained over hydrotalcite sup-
ported catalysts in hydrogen as a function of space time. The reaction
was carried out at 21 bar total pressure and 300 ꢀC.
Since the hydrotalcite supported Cu- and Pd-catalysts
contain the self-same alkaline support the different activity
and the salient diverse selectivity can be evidently connected
with different properties of the applied metals. However the
reaction network is too complicated (see a simplied summary
in Scheme 1) future studies are needed to trace signicant
points determining the selectivity. The importance of the well-
known extraordinary hydrogen absorbing property of palla-
dium can be signicant in the hydrogenation of unsaturated
ketone intermediates formed aer the aldolization and dehy-
dration reaction steps. This highly concentrated hydrogen is
directly accessible on the palladium surface for the hydroge-
nation process.
Although over the less active copper catalyst formation of
ethyl acetate by-product is signicantly restricted working in
hydrogen medium however dialkylation is remained negligible.
Contrarily, palladium/hydrotalcite catalyst is highly efficient
alkylating composite equally in helium or hydrogen. It is
important that activity and selectivity is highly sensitive for
increasing of space time giving a great potential for the pro-
cessing of ABE mixture.
The mentioned three undesired side reaction pathway can
be restrict under more severe reaction conditions, as such
effect of work at higher reaction temperature can be observed
in Table 3. Beyond the reaction temperature control, variation
of the space time results in change of conversion and selec-
tivity in a wide range (see Fig. 2 and 3). These results reect the
complexity of the reaction network, especially the progression
of consecutive reaction chains. Determination of optimal
reaction conditions for the requested products needs
compromise. Enhancement of the reaction temperature
results in higher yields of the desired alkylates but with higher
increase of gaseous by-products. Using longer catalyst bed or
feeding less reactant the yield for desired products are
increasing with slower increase of gaseous by-products
resulting in better selectivity.
Fig. 2 demonstrates that over hydrotalcite catalysts in helium
stream the yields of side products (isopropyl alcohol, butanol or
ethyl acetate) slowly decreases or hardly increases with
enhanced space time in a wide range resulting in signicantly
improving selectivity for desired products, alkylates.
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In a complicated reaction network of mono- or bi-alkylation of
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ates, and products form a complex liquid. Some of these
components only form valuable fuel ingredients when partially
or totally hydrogenated. We tested metal catalysts with great
potential for C-alkylation of acetone with alcohols. We show
that the selectivity and overall yield on palladium catalysts is
superior compared to a novel copper catalyst. To avoid
complications with alkaline additives, which have been previ-
ously employed, we used a basic catalyst support, hydrotalcite.
We show that various mono- and dialkylated ketones formed
with high yields when helium carrier gas was applied. These
products were reduced out in hydrogen ow over our Pd/
hydrotalcite catalyst. These results promise a new, efficient,
continuous, catalytic processing option for ABE mixtures, ob-
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´
´
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Thanks are due to the National Development Agency (Grant No.
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