S. Kamal and S. Mahajani
Applied Catalysis A, General 608 (2020) 117841
Table 1
Different products using acetaldehyde as a reactant in acid catalysed reactions.
S.
Product of interest
Catalyst
Reaction Temperature
Information on Side products
Ref.
No.
Acylal formation
1,1-ethylenediol
dimethacrylate
Prins reaction
1
FeCl
3
273ꢀ 287 K
39 % product yield
[20]
2
2,4,4,6 and 2,4,6 alkyl-diol
Sulfuric acid
283ꢀ 313 K, 283ꢀ 285 K
9ꢀ 11% and 9% of acetaldehyde
consumed as side product
[21]
[22]
3
Trans-2-alkyl-4-
Heteropoly acid phosphomolybdic acid
298 K
79 % yield for main product while rest
go for side reaction
hydroxypiperidines
Mannich reaction
4
β-amino aldehydes
Proline
273 K
The product yield varies in the range of
[23]
4
0–58 %
5
(S)-4-nitro-3-phenylbutanal
Proline
298 K
75 % yield
[24]
Acetalization
6
Dialkylacetal
Cation exchange resins
Cation exchange resins
278ꢀ 323 K
313ꢀ 358 K
Not reported
Not reported
[25,26,
2
7,28]
7
Glycerol ethyl acetal
[29]
Self-Aldol condensation reaction
8
9
Crotonaldehyde
Crotonaldehyde
Sulphuric acid [5], Amino acids [11]
Metal oxides (Mg, Zr)
295ꢀ 298 K
Not reported
[5,11]
[9]
403 K
Crotonaldehyde selectivity is in range
of 83ꢀ 87%.
1
1
0
1
Crotonaldehyde
Crotonaldehyde
Metal (Mg, Zr, Ti, Hf, Ta) Oxides [7], Impregnated
alkali/alkaline earth metal [10]
Heteropolyacid
523ꢀ 673 K [7], 593 K
Crotonaldehyde selectivity is nearly 94
% [7], 88 % [10]
[7,10]
[8]
[10]
298 K
Not reported
Cross Aldol condensation reaction
1
1
2
3
3-methylpent-3-en-2-one
MPO)
3-methylpent-3-en-2-one
Cation exchange resin, Clay supported solid catalyst
Sulfuric acid, Hydro chloric acid
343ꢀ 423 K
MPO yield 50ꢀ 80%
[30,31]
(
278ꢀ 343 K [31,32];
65 %-77.7 % [31,32]; 90% [33]
[31,32,
33]
3
23ꢀ 343 K [33]
carrier made of alkali metals or alkaline earth metals is reported for
self-aldol condensation reaction of acetaldehyde. Yield obtained by this
catalyst was nearly 88 % [10]. Use of amino acids such as glycine,
alanine, serine, arginine, and proline at room temperature (295ꢀ 297 K)
predicting the side products in several acid catalysed reactions in which
acetaldehyde is used as one of the reactants. Based on the information
available in literature [3], a reaction scheme showing different possible
products formed due to alcohol condensation, repeated aldol conden-
sation and subsequent dehydration of aldols is shown in scheme 1 .
Scheme 2 shows the mechanism for formation of aldol that involves
reaction between oxonium ion as electrophile and enol form of acetal-
dehyde as nucleophile. It also shows the mechanism of dehydration and
subsequent formation of some representative aldols by repeated aldol
condensation reactions.
2 2 4
in water and aqueous salt solutions of NaCl, CaCl , Na SO4, and MgSO
for the production of crotonaldehyde are also reported in the literature.
Overall reaction is found to be first order with respect to acetaldehyde
concentration at lower concentration of amino acid while at higher
amino acid concentration, reaction follows second order kinetics [11].
However, there is no discussion on the oligomerization reaction in any
of these studies reported in literature.
Cation exchange resins are promising solid acid catalysts and as seen
in Table 1, they are used for many industrial reactions such as cross aldol
condensation, acetalization reaction involving acetaldehyde [18,19].
The objective of the present work is thus to develop the reaction scheme
and the overall kinetics when acetaldehyde is contacted with acidic
catalyst like cation exchange resin under mild conditions (<373 K). The
molecular weight distribution of the condensation products is compared
with the predictions of Flory’s statistical method. The effect of different
parameters such as solvent selection, temperature, catalyst loading is
studied in the absence of internal and external mass transfer resistances.
Finally, a suitable kinetic model is proposed to explain the experimental
data. This model can be conveniently used in a broader reaction scheme
wherein, oligomerization takes place as a parallel side reaction.
It is mentioned in the literature that acetaldehyde undergoes addi-
tion polymerization in the presence of acidic catalyst [3]. A viscous tarry
mass is formed by oligomerization of aldehydes. The oligomers are
formed by condensation of anywhere between 3–7 molecules of alde-
hyde. The molecular weight of polymers formed by acetaldehyde poly-
merization is in the range 320–340, which is also equivalent to the
molecular weight of polymers formed by crotonaldehyde polymeriza-
tion. This led to a conclusion that irrespective of the parent reactant, be
it acetaldehyde or crotonaldehyde, oligomers formed are the same [12].
In another study, Yamamoto et al. have reported a bimodal molecular
weight distribution of the oligomers with lighter ones in the range of 230
±
30, and the heavier ones over 800 ± 100 [13]. To the best of our
knowledge, barring few studies [3,4], there is not much information
available on the structure of high-boilers. It is also reported in the case of
aldolization reaction that the formation of oligomers is independent of
the main reaction and may be considered as a parallel side reaction [14].
Few studies investigate kinetics of acetaldehyde self-aldol conden-
sation however, oligomerization reaction is not taken into account in the
reaction scheme and the kinetic parameters are estimated with the
assumption that crotonaldehyde is the only product [11,15–17]. The
importance of side products formed by oligomerization or any other
routes has been ignored, and that the kinetics is inadequate while
identifying the right reaction conditions and the reactor design. Hence,
it is necessary to investigate all the reactions leading to condensation
products and the associated kinetics for the given catalyst in detail. This
would not only help in crotonaldehyde production, but also in
2. Experimental section
2.1. Chemicals and catalyst
The chemicals used for the experiments were acetaldehyde (98 %
pure, Harmony Organics, Pune, India) and toluene (99.5 % pure, from
Merck, Mumbai, India). The purity was verified by gas chromatography.
The catalyst, dry Amberlyst-15 (Rohm and Hass, France), was obtained
from S.D. Fine-chem Ltd., Mumbai, India. Catalyst was first dried in the
oven for 6ꢀ 7 hours at 373 K in vacuum (0.6 bar gauge). Amberlyst-15 is
a macro reticular polystyrene-based cation exchange resin with an ex-
change capacity of 4.7 meq/g. Its average particle size is 600
μm and it
2
has an active surface area of 50 m /g.
2