Fatty acid methyl esters into nitriles: Acid-base properties for enhanced catalysts

Article abstract of DOI:10.1016/j.jcat.2013.05.032

Fatty nitriles have lately become of interest in the frame of biofuels or for the valorization of the oil part of biomass as fine chemicals such as polymers. The production of long-chain fatty nitriles by direct reaction of esters with ammonia has however not been academically extensively studied, although several catalysts were developed and published in patents. Acid-base features are implicitly considered as leading the catalysis of this reaction, but no direct correlation was investigated with any nature or number of acidic or basic sites. The present study aims at understanding which sites are responsible of this reaction and thus how to design better catalysts. Strong acidity correlates at 300 C for ester conversion and nitrile yield, suggesting a common nature of the reaction among all kinds of catalysts. An upper strength limit, over which undesirable side-products appear, was evaluated, and the factors influencing the production of N-methyl amide were analyzed.

Full text of DOI:10.1016/j.jcat.2013.05.032

Journal of Catalysis 306 (2013) 30–37
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Fatty acid methyl esters into nitriles: Acid–base properties for enhanced
q
catalysts
a
a
b
b
c
a,
⇑
A. Mekki-Berrada , S. Bennici , J.P. Gillet , J.L. Couturier , J.L. Dubois , A. Auroux
a
IRCELYON, UMR5256 CNRS – Université Lyon1, 2 Avenue A. Einstein, 69626 Villeurbanne Cedex, France
ARKEMA, Centre de Recherche Rhône Alpes, Pierre Bénite, 69493 Cedex, France
ARKEMA, Direction Recherche & Développement, 420 Rue d’Estienne d’Orves, 92705 Colombes, France
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Fatty nitriles have lately become of interest in the frame of biofuels or for the valorization of the oil part of
biomass as fine chemicals such as polymers. The production of long-chain fatty nitriles by direct reaction
of esters with ammonia has however not been academically extensively studied, although several cata-
lysts were developed and published in patents. Acid–base features are implicitly considered as leading
the catalysis of this reaction, but no direct correlation was investigated with any nature or number of
acidic or basic sites. The present study aims at understanding which sites are responsible of this reaction
and thus how to design better catalysts. Strong acidity correlates at 300 °C for ester conversion and nitrile
yield, suggesting a common nature of the reaction among all kinds of catalysts. An upper strength limit,
over which undesirable side-products appear, was evaluated, and the factors influencing the production
of N-methyl amide were analyzed.
Received 27 March 2013
Revised 24 May 2013
Accepted 29 May 2013
Available online 3 July 2013
Keywords:
Fatty nitrile
Biomass
Microcalorimetry
Acid–base catalysis
Ó 2013 The Authors. Published by Elsevier Inc. All rights reserved.
1
. Introduction
Nitriles are platform molecules that are useful in medicine as
The nitrilation of fatty acids or esters by direct reaction with
ammonia is mainly performed via two processes: first the batch
one, where the acid reactant is in liquid phase and the nitrile re-
mains in the reactor; second the gas-phase continuous one, where
the acid is vaporized prior to a catalytic bed, through which it is
passing together with ammonia. While the gas-phase process
(few seconds contact time) consumes energy for the vaporization
of the reactant and then could lead to modification of the carbon
chain in the evaporation chamber before the catalytic bed, the
batch liquid-phase process needs few hours of reaction at high
temperature, where side-reactions are likely to happen, especially
with unsaturated chains. Thus, the gas-phase process is usually
more adapted to short carbon chains (C 6 12) and to unsaturated
carbon chains, since the contact time with the catalyst is short,
whereas the liquid-phase process is more adapted to long carbon
chains (C P 12) and especially saturated chains. A previous study
focused on the liquid-phase batch process [3], and the scope of
the present study is the transformation of shorter chain fatty esters
(C12:0) in a gas-phase continuous process in the attempt of
decreasing the working temperature lower than the state-of-the-
art. Investigations on mono-unsaturated material are under pro-
gress and will not be addressed in the present article.
well as in polymer chemistry. Besides, in the frame of renewable
energy resources, the valorization of non-edible biomass into bio-
fuels and fine chemicals has become a major field of research, and
the conversion of the oil part of biomass, that is, triglycerides, into
fatty esters and nitriles has been envisaged for biofuel production
[
1,2]. Nitriles have a high energy density which makes them attrac-
tive to be investigated as aviation fuels, even though the possibility
of NOx exhaust has to be cared about. Their use as fine chemicals
however brings the necessity of controlling the chain’s nature,
especially regarding the unsaturations. Indeed, the nature of com-
monly used catalysts and also, and perhaps mainly, the high work-
ing temperature of the nitrilation processes are the source of
isomerization and several side-reactions, such as Piria, Diels–Alder,
or peroxidation in
a position of the unsaturations (Fig. 1). Thus, in
order to produce higher added value nitriles with control over the
fatty chain, we can aim at reducing the operating temperature or
the contact time with the catalyst, which can be achieved in a
gas-phase reactor.
Catalyzed direct reaction of acids with ammonia in gas phase
for the production of nitrile was first reported in 1916 by Van Epps
and Reid [4], using alumina and thoria and operating at 500 °C for a
q
This is an open-access article distributed under the terms of the Creative
Commons Attribution-NonCommercial-No Derivative Works License, which per-
mits non-commercial use, distribution, and reproduction in any medium, provided
the original author and source are credited.
⇑
8
5% yield in acetonitrile, while no reaction was occurring when
starting from the ethyl ester. However, in 1918, Mailhe [5] per-
formed the conversion of ethyl acetate into acetonitrile with the
Corresponding author.
E-mail address: aline.auroux@ircelyon.univ-lyon1.fr (A. Auroux).
0
021-9517/$ - see front matter Ó 2013 The Authors. Published by Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2013.05.032

A. Mekki-Berrada et al. / Journal of Catalysis 306 (2013) 30–37
31
Adsorbed carboxylates : Piria ketone
Dimers, trimers
Fatty acid
2
H O
H O
2
NH
3
Fatty amide
Fatty Nitrile
Fatty ester
alcohol
N-alkyl amide
Unsaturations : Diels-Alder, oxidation,…
N,N-dialkyl amide
Fig. 1. Reaction scheme of the conversion of fatty esters into nitriles by direct reaction with ammonia, with the possible side-reaction.
same catalysts and same temperature. Then in 1931, Mitchell and
Reid [6] performed a more extensive study with silica gel at 400–
conversion is presented to reach 98% at 300 °C, with a 94% yield
of lauronitrile for about 4 g/h of reactant. Recent academic litera-
ture is quite scarcer concerning these issues; however, Bizhanov
et al. [12] reported in 1985 a kinetic study of the nitrilation at
300 °C of mixtures of aliphatic acids of 10–22 carbon chain length.
By using alumina catalyst, they obtained almost full conversion at
5
75 °C (optimum at 500 °C) on a large variety of acids and esters.
The catalyst was stable for reactions with acids, whereas with es-
ters it was rapidly fouled, probably due to the formation of alde-
hyde from the produced alcohol. Besides, such as described by
Ralston et al. in 1935–1936 [7], some ‘‘cracking’’ is occurring above
40 min contact time and observed zero order kinetics for the for-
À1
4
00 °C on the long-chain acids and esters. Ralston et al. present
mation of amide and nitrile with 123.3 mmol s
and
À1
high-temperature pyrolysis on alumina bed at 400–600 °C as a
way of producing shorter nitriles and hydrocarbons out of long-
chain compounds, preventing the formation of heavy side-prod-
ucts such as polymers or resins; however, further stages of separa-
tion and purification are then needed. Afterward, most advances
are recorded in the patent literature, such as Wortz in 1940 [8],
reporting the preparation of long-chain (C P 8) aliphatic nitriles
from acids of the same chain length and at 425–450 °C, in contra-
diction with Ralston’s earlier statements. Several metal oxide
dehydrating catalysts are presented and the ratio of ammonia to
acid is of 2.5. Above 450 °C, cracking is reported, while below
97.7 mmol s , respectively. Although the choice of esters as start-
ing reactants can be the source of fouling or pollution of the prod-
uct, their comparatively higher vapor pressure is of high interest
(about twice larger at 250 °C). Furthermore, some biomass sources
are more easily transformed into esters than into acids; thus, the
overall conversion of triglycerides into fatty nitriles can become
more interesting via esters.
2
. Experimental
Commercial zinc oxide (Sigma–Aldrich), c-alumina (DEGUSSA),
niobium oxide (STARCK), zirconia (Norpro St-Gobain), zeolite H-
4
25 °C, severe decrease in conversion is observed. Then in 1992,
Akikubo and Takaoka [9] reported the conversion at lower temper-
ature, that is, ‘‘200–400 °C,’’ of fatty acids or esters of carbon chain
length from 6 to 22. Conversion of methyl laurate at 1–3 g/h, per-
formed at 300 °C on several catalysts (mean residence time of 4–
MFI Si/Al 28 (Sud Chemie AG), faujasite HY with a 5.1:1 mol ratio
of SiO
O Sud Chemie), titanium dioxide (Degussa P25), phosphotungstic
acid on TiO , phosphated zirconia (Norpro St-Gobain), H PO /SiO
Johnson Matthey), silica–alumina Siralox 30 and Siralox 40-450
SASOL), hydrotalcites (Mg/Al = 3 [13] and Norpro St-Gobain),
2 2 3
/Al O (Alfa Aesar), tungsten oxide (Fluka), bentonite (KSF/
2
3
4
2
1
3 s), is given as an example, displaying good results for oxides
(
(
of Zr, Ta, Ga, In, Sc, Nb, Hf, Fe, Zn and Sn, and bad results with oxi-
des of Si, Mn, V and W; no results were disclosed at significantly
lower temperature. However, high acid strength is pointed out as
a source of side-reactions, and oxides of zirconium or potassium,
as well as alkali impregnated alumina, appeared to display less of
these drawbacks. The main problem with starting from an ester
is the handling of the produced alcohol, not only because it may
foul the catalyst, but also because it induces side-products. Tak-
aoka et al. reported in 1998 [10] a series of zirconia-based catalysts
in order to reduce the amount of such side-products, especially N-
methyl-amide. Multivalent metal cations (Al, Sb, Zn, Ce, V, Nb, Ta,
Cr, Mn, Fe, Co, Ni, and lanthanides) showing no strong acidity were
impregnated at around 0.4 wt% on zirconia, performing a 96% con-
version of methyl laurate at 1 g/h at 300 °C, displaying low
amounts of N-methyl-amide in the heavy fraction and of methyl-
amine in the lighter fraction (water, methanol, ammonia). Lately,
solutions to overcome the question of methylated side-products
follow the pathway of ammoxidation of alcohols, recently per-
formed at ‘‘240–290 °C’’ using both dehydrating and dehydroge-
nating catalysts mixed in a catalytic bed [11], and lauryl alcohol
hydroxyapatites (Fluidinova 1.66 reference nanoXIM.HAp402 [14]
and Ca/P = 1.66 calcined at 400 °C following Lamonier et al. proce-
dure [15]), magnesium oxide (MERCK), Cs3.0PW12
O40 (from Lefeb-
vre et al. [16]) and Cs2.5 40 (Nippon-Kokan Kabushiki
H1.5SiW12O
Kaisha), boron oxide (STREM), iron (III) oxide hydrated (Aldrich),
aluminum fluoride 55% (from Brunet et al. [17]) were used. Zinc–
indium mixed oxides and
ized in a previous article [18]. Tungstated zirconia ‘‘16%WO
was prepared by calcination of commercial tungstated zirconium
hydroxide (MEL Chemicals) and ‘‘24%WO /ZrO ’’ as well as every
M/ZrO (M = Al, Fe, Co, Zn, each at 0.4 wt%) was prepared by incip-
c
-alumina were prepared and character-
x
/ZrO ’’
2
x
2
2
ient wetness impregnation [19]. Boron-containing alumina’s prep-
aration was performed by Dubois and Fujieda [20].
Lauric acid methyl ester (98+%, Sigma–Aldrich) was used as
reactant in this study. Reference material for GC–FID and GC–MS
was dodecanenitrile (99%, Sigma–Aldrich), dodecanamide (>98%,
TCI), and dodecanoic acid (98%, Alfa Aesar).
The specific surface area (BET: Brunauer–Emmet–Teller) was
determined by nitrogen adsorption at 77 K on an ASAP 2020

3
2
A. Mekki-Berrada et al. / Journal of Catalysis 306 (2013) 30–37
(
Micromeritics). Adsorption microcalorimetry coupled with volu-
performed on a 30-m (ELITE-WAX ETR 30 m  0.25 mm  0.5
lm)
metry was used in order to evaluate the acid–base features of
catalysts in matters of amount and strength [21]. After pretreat-
ment of the catalyst at 300 °C under vacuum overnight, the
calorimetric cell is inserted inside a C80 microcalorimeter from
capillary column. The temperature program starts at 70 °C and in-
À1
creases until 240 °C with a 10 °C min rate, then stays 10 min at
À1
240 °C, while helium is flushed at a 2 ml min flow rate.
Setaram, set at 80 °C, while accurate dosing of adsorption of NH
or SO is performed. Cycles of adsorption–desorption–readsorption
3
3. Results and discussion
2
provide measurement of the irreversible volume of adsorption by
difference of the adsorbed and readsorbed volumes at 27 Pa
3.1. Ammoniation: an acid–base reaction
equilibrium pressure, this is an estimation of the number of chemi-
Converting esters into nitriles consists of a series of acid–base
reactions, and both academic and patent literature focused their
attention on searching catalysts with acid–base features. High re-
dox features are also more susceptible of enhancing side-reactions,
thus reported catalysts are mostly supported metal oxides actually.
Catalysts with amphoteric character such as zinc, gallium, indium,
zirconium, or aluminum oxides gave better results than catalysts
À2
sorption sites (
l
mol m ), while the successive thermograms
(
(
integrated amount in J) and corresponding adsorbed quantities
À1
in
l
mol g ) provide measurement of the strength of adsorption
À1
sites (kJ mol ). Adsorption of ammonia probes the acidic sites,
while adsorption of sulfur dioxide probes the basic sites. The
number of strong acid sites corresponds to adsorption energies of
À1
ammonia higher than 120 kJ mol
.
2 5 3
such as the basic MgO or the acidic V O or WO in the present pro-
The experiments have been carried out on a lab-scale gas-phase
continuous process consisting of a vertical downstream stainless
steel microreactor provided by controlled flow rates of reactants
and connected downstream with a series of condensers in order
to collect heavy products together on one side and methanol,
water, ammoniated water, methylamine, dimethyl-ether if any
on the other side. The microreactor consists of two connected
capacities: upstream side is the evaporation chamber where liquid
ester is dipped inside the heated volume and blown downwards by
cess. Besides, it was also pointed out that too strong acid sites were
the source of side-reactions toward polymers or resins [22]. Early
steps of the transformation include the production of methanol,
which can be transformed into formaldehyde, which can be per-
formed by transition metal oxide catalysts such as titania or sup-
ported vanadia [23], and foul the catalyst or produce
methylamine [6] which is a problem for effluent treatment. Meth-
anol can also be activated by the catalyst and induce methylation
of amides; thus, redox features have to be carefully observed.
Although acid–base features seem to be the key to efficient nitrila-
tion of acids or esters, no correlation between measured amounts
or nature of these sites and any step of this transformation was al-
ready investigated, to our knowledge, most probably because the
several steps do not necessarily get catalyzed by the same features.
Adsorption microcalorimetry appears as a perfectly dedicated
technique to investigate these acid–base features, and the purpose
of this article is to observe which nature and strength of acidic, ba-
sic, or both features can correlate with the rate-determining step of
the present reaction, whatever the elements composing the cata-
lyst. Acid–base features of tested catalysts are presented in Table 1.
a controlled nitrogen flow (Brooks mass flow meter, range 3–
À1
3
0 ml min ) and downstream is the catalytic bed of 1.60 ml vol-
ume on a stainless steel frit disc (Interchrom, pore size 2 lm).
The evaporation chamber and the catalytic bed are separated by
a stainless steel grid, enhancing the evaporation surface for the
possibly remaining liquid ester. Ammonia flow is controlled
À1
(
Brooks mass flow meter, range 4–40 ml min ) and delivered di-
rectly at the top of the catalytic bed, via a vertical tube through
the evaporation chamber, ensuring that first contact between both
reactants happens in the catalytic bed or at its surface, and pre-
venting possible amide condensation above the bed. The microre-
actor is disposed inside a vertical furnace, ensuring a control over
the catalytic bed’s temperature (stability within 1 °C at 300 °C)
and on the evaporation chamber’s performance. Contrary to exam-
ples of the patent literature which evaporate the ester beforehand,
liquid ester is here delivered to the evaporation chamber and con-
trolled by a peristaltic pump (Gilson Minipuls 3), and the heating
provided to the evaporation chamber (>2.5 W) and the flow
parameters ensure full evaporation of the ester (<1 W in the pres-
ent tests). The tubing for the peristaltic pump (0.50 mm inner
3.2. Stability toward harsh conditions
Experimental conditions are especially harsh and thus catalysts
have to sustain high temperature and high partial pressure of
ammonia, water, and methanol. For example, oxides of zinc and
to a lesser extent of indium were observed to be leached by carbox-
ylates and ammonia [3,24], vanadium oxide can also be leached by
water and acids, and titania was observed to be leached in the
present conditions (pink color appearing in the condensate). Het-
eropoly acids are also quite unstable under high pressure of ammo-
nia, for ammonium cations can proceed to exchanges inside their
structure [25]. In the case of cesium heteropoly tungstate, cesium
cations may be replaced by ammonium and therefore change the
acid–base properties and lead to leaching of cesium salts out of
the catalytic bed. Boron-containing aluminas are also good acidic
catalysts that are however also sensitive to water partial pressures
[20]. Microcalorimetric study and chemical analysis of used 5%B/
À1
diameter) was chosen in order to provide 0.5–5 g h of methyl
laurate and calibrated on the peristaltic pump. The contact time
is defined here as the ratio of the catalytic bed’s volume (ml) by
À1
the combined flow rates (ml s ). Moreover, the mass and compo-
sition of the outflowing heavy gases were monitored in order to
ensure the mass balance of the setup. The product stream is
brought to a first condenser thermostated at 150 °C, condensing
heavy products such as ester, acid, amide or nitrile in a graduated
vat, and then, the remaining flow is brought to a second condenser
thermostated by industrial water at 12–15 °C and to a dry ice trap,
both condensing water, methanol, ammoniated water, and other
products if any. Every sampling corresponds to the accumulated
condensation since the previous sampling, and sampling rate is
about twice per hour. The samples taken were then analyzed by
GC–FID (Perkin–Elmer Clarus 500), with on-column injection onto
2 3
Al O catalyst was performed and it could be observed that no bor-
on was leached out of the catalyst’s surface but that some coking
took place (about 0.5 wt% of carbon) and that the overall acidity
was reduced by about 10% of all strengths, most probably due to
covering of sites by coke.
Besides, the size of the fatty compounds can hinder their access
to a part of the active sites; this problem can be encountered with
some micro- and mesoporous catalysts. The presence of microp-
ores could generate selective adsorption of methanol, ammonia
or water in places inaccessible to fatty molecules and could act
as a source of methylation of amides or other side-reactions, by
a
5 m pre-column connected to
a
30 m (DB-WAX
3
0 m  0.53 mm  50
l
m) capillary column. The temperature pro-
gram started at 100 °C and increased until 210 °C with
a
À1
1
0 °C min rate and then stayed 5 min at 210 °C, while helium
was flushed with a 12 psig inlet pressure. GC–MS analysis was also

A. Mekki-Berrada et al. / Journal of Catalysis 306 (2013) 30–37
33
Table 1
Surface properties, ester conversion, and nitrile yield for catalysts tested for 4.4 s contact time at 300 °C.
a
Acidic sitesc
Basic sitesc Results at 300 °C
N bas. sites Conversion Nitrile
yield
Label Catalyst
Surface
Pore
Volume
(ml)
Mass
(g)
Literature
2
b
(
m /g)
size
nm)
NMA4
(mol%)
N ac. sites
N strong ac.
Nit. yield
(mol%)
(
(
lmol/g)
sites (
lmol/g)
(lmol/g)
(mol%)
0
(
mol%)
X
Glass beads
blank)
–
1.5
–
0.0
0.0
0.0
0
0.0
(
Acidic solids
A
B
C
D
E
F
24%WO
Nb
Phosphated ZrO
x
/ZrO
2
112.05
106.5
130.3
266
1.5
1.5
1.5
1.4
1.45
1.5
1.0892 190.5
1.2322 170.4
1.5997 254.1
0.9455 345.8
1.0088 454.1
0.7220 450.0
142.0
64.3
90.9
123.2
177.0
100.0
11.2
10.7
27.0
–
40.0
–
98
98
98
82
71
70
96
97
95
77
66
53
1.0
1.1
2.1
3.8
2.6
7.7
98.1A
2
O
5
5.6
2
3%B/Al
5%B/Al
2
O
O
3
9.0
8.1
0.5
2
3
239
Faujasite HY Si/Al 730
.1
5
G
H
I
J
K
L
M
N
O
P
Bentonite
HPA/TiO2
H-MFI Si/Al 28
186.6
33.3
–
44.5
91.4
–
3.5
–
50
6.0
1.5
1.5
1.5
1.55
1.5
1.1
1.5
1.45
1.5
1.6
0.8680 380.0
1.5572 96.6
60.0
33.0
–
38.9
–
–
–
–
5.0
–
–
–
–
6.5
–
–
–
–
–
64
63
44
43
28
13
8
7
5
0
54
61
34
42
26
8
7
5
1
0
5.8
0.8
3.7
0.8
1.1
1.5
0.0
0.2
0.1
0.0
0.9117
–
NbOPO
Phosphated SiO
4
0.7240 106.8
2
1.2684
1.9540
–
–
Cs2.5H
1.5SiW12
O
40
WO
Cs3.0PW12
AlF 55%
3
2.3000 12.8
1.6675
0.9090 175.0
1.4272
58.7A
O
40
–
3
B
2
O
3
–
–
–
Amphoteric solids
a
b
c
d
e
f
g
h
i
Al/ZrO
Zn/ZrO
16%WO
TiO
Siralox 30
7%WO /ZrO
-Al
Fe/ZrO
ZrO
2
52.6
57.5
88
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.0981 94.7
1.0574 103.5
1.2378 234.0
0.1480 282.0
0.7410 297.6
1.5680 192.1
0.8655 170.2
0.9675 110.4
1.8129 80.0
55.7
64.9
122.0
120.0
147.2
74.9
69.9
74.3
45.0
191.5
205.3
147.0
56.4
90.0
140.0
–
98
98
98
98
98
97
96
96
95
96
96
97
96
96
94
93
90
95
0.6
0.2
0.6
0.8
2.6
0.7
0.6
0.4
0.1
2
x
/ZrO
2
50
11.6
7.1
6.9
98.5A
2
496
87.3
196
73.6
53
x
2
c
2
O
3
(BASF)
96.5B
96.2C
2
220.8
196.1
A
2
98.2
C
9
6.7
A
j
k
l
Fe
Co/ZrO
Siralox 40-450
-Al [18]
2
O
3
141
68.3
600
420
1.5
1.5
1.5
0.85
1.8386 140
1.0891 61.5
0.5534 282.0
0.5930 350
30
400
95
93
89
73
94
91
83
59
0.6
0.3
3.1
1.0
98.1
2
29.1
149.1
60
107.2
90.0
540
6.9
9.6
m
c
2
O
3
96.5B
A
9
8.9
n
o
p
q
ZnInOx (3:1)
ZnInOx (19:1)
ZnInOx (9:1)
41
37
57
115
1
0.7500 68.1
0.8320 81.0
0.3500 98.0
0.1300 197.8
49.3
43.7
56.4
84.3
176.3
155.4
241.1
165.6
62
42
39
24
62
40
37
9
0.3
0.0
0.2
2.5
1.5
1.1
1.5
c
(
-Al
DEGUSSA)
ZnO
2
O
3
96.5B
A
98.9
r
5
1.5
1.1188 20.5
8.6
22.3
12
12
0.2
98.5A
Basic solids
a
b
Hydroxyapatite
Hydroxyapatite
.66
99.8
137.4
1.5
1.55
0.6550 121.8
0.7770 144.3
0.0
9.3
210.0
320.0
37
30
28
10
1.2
2.0
1
c
Hydrotalcite Mg/
Al = 3
120
1.5
1.7215 57.2
0.0
587.0
5
0
2.5
d
e
MgO
Hydrotalcite
50
80
1.5
1.5
1.0700 1.0
1.0639 18.4
0.0
0.0
200.0
265.6
4
3
2
2
0.0
0.2
a
b
c
BET surface area.
Mean pore size as calculated by the BJH method on mesopores as 4 V/A.
Measured by adsorption calorimetry of NH (acid-) and SO (basic sites).
Patent JP 04-208260 (about 4–13 s mean residence time).
Patent JP 04-283549 (13 s).
3
2
A
B
C
Patent JP 10-195035 (5 s).
creating spots with high concentration of these molecules. Con-
densation of fatty molecules inside the pores could also enhance
side-reactions, since in liquid phase, distances between molecules
are importantly decreased and since concentrations of heavier
molecules are considerably higher than in the gas phase. Using
the Barrett–Joyner–Hallenda method (with nitrogen at
À195.8 °C) on several mesoporous catalysts presented in Table 1,
the smallest measured mean pore size is 5 nm. It can be evaluated
from the Kelvin equation that the partial pressure at which pore
condensation occurs in a 5 nm pore at 300 °C is about 0.4 bar for
lauramide (which stands as the most susceptible of condensing
among main molecules). Since the maximum reachable pressure
for fatty compounds is capped at 0.13 bar, this is the feed concen-
tration, no pore condensation should occur inside mesopores at
300 °C.
3.3. Diffusion limitations on silica–alumina and niobium oxide
In order to compare catalysts with each other, the mean resi-
dence time was kept similar and physical limitations have been

3
4
A. Mekki-Berrada et al. / Journal of Catalysis 306 (2013) 30–37
avoided. Diffusion limitations were evaluated at temperatures
lower than 300 °C for the purpose of posterior investigations: at
2
50 °C on a Siralox 30 silica–alumina and at 200 °C on niobium
oxide. In the range of gas velocity and particle size where no diffu-
sion limitation occurs at 200 and 250 °C, there should also be no
limitations at higher temperature, that is, 300 °C here. Both cata-
lyst/temperature couples were chosen for their intermediate con-
version ranges (that is, 20–60 mol%) for a 4.4-s contact time,
since the variations are easier to evaluate. On Siralox 30, external
À1
diffusion limitations were not detected below at least 7.5 mm s
linear velocity, whereas internal diffusion limitations could be ob-
served for particle size higher than about 200 lm. On niobium
oxide, external diffusion limitations could be observed above
À1
6
.8 mm s linear velocity, and internal diffusion limitation also
for particle size higher than about 150 lm. Patent literature does
not disclose particle size parameters; however by the choice of lin-
ear velocity (see Table 2), it is implicitly stated that no external dif-
À1
fusion limitation occurs until 22.4 mm s
.
Fig. 2. Volumic amounts of acid sites, strong and medium strength acid sites as a
function of the methyl laurate conversion for the three aluminas labeled g, m, and q
in Table 1.
3
.4. Correlations between acid–base properties and conversions
Several catalysts were proposed for this reaction in patents,
and Table 2 reports the experimental conditions described by re-
cent patent literature concerning conversion of lauric acid or
methyl laurate. Several different conditions were used, but the
ammonia to ester ratio was always at a value of about 4. Results
reported by Akikubo and Takoaka in 1992 [9] correspond to con-
tact times that can be evaluated at about 4–13 s, which is in the
same order of magnitude as the results presented in this article.
Apart from zinc oxide, some aluminas and tungsten oxide, these
results are in agreement with the ones presented here (see
Table 1). However, their experimental conditions concerning the
high volume catalytic bed (11 l) are significantly different: it is
filled at about 0.5% of the total volume, thus in the form of a very
thin disc, and with a significantly slower gas linear velocity of
from catalysts in liquid phase for which the first step can occur
without catalyst [3,26]. It can be globally observed in Table 1 that
basic catalysts are quite inefficient, while acidic and amphoteric
catalysts perform mostly well.
Different results can be obtained for a same nature of catalyst:
the three
c-alumina catalysts display significantly different effi-
ciencies in this test. The volumic amounts of acid, strong, and med-
ium strength acid sites are plotted in Fig. 2 for three aluminas
(labels g, m, and q in Table 1) as a function of their ester conver-
sion. The alumina (m) with the highest BET surface area does not
display the best conversion, nor is it the total volumic density of
acid sites that determines the best efficiency among these alumin-
as, but more probably the amount of high or medium strength acid
sites.
À1
À1
0
.3 mm s (present article: 6.8 mm s ). Results from 1998 [10]
correspond to contact time and gas velocity similar to the present
experiments and zirconia-based catalysts perform here as well as
in this patent.
Considering the whole list of catalysts, the volumic amount of
strong acid sites (adsorption strength of ammonia higher than
À1
Table 1 reports the ester conversion, the nitrile and N-methyl
amide yields, as well as the acid–base features measured by
adsorption microcalorimetry. Glass beads of 1 mm diameter were
used as a reference in order to evaluate the reaction without catal-
ysis in same conditions, and it can be observed that no ester is con-
verted at 300 °C without catalyst for a 4.4-s contact time. Then, for
gas-phase ester conversion into nitrile, the catalyst has to play on
both aminolysis and dehydration, contrary to what is usually asked
120 kJ mol ) was found to display good correlation with both es-
ter conversion and nitrile yield, as can be observed in Fig. 3. A qua-
si-linear dependency of conversion and nitrile yield appears for the
À1
volumic amount of strong acid sites below 50
lmol ml
, and
catalyst
then, maximum conversion is reached for the present conditions
À1
of 300 °C working temperature and ester flow rate of 2
lmol s
À1
(this is 1.5 g h ). Once made sure that no diffusion limitations oc-
cur, if these sites are responsible of the rate-determining step, then
Table 2
Parameters of the recent patent literature.
Patent
JP 04-208260
JP 04-208260
JP 04-283549
JP 10-195035
This article
Publication year
Temperature
Mass cata. (g)
Reactant (lauric)
Tested catalysts
Vol. reactor (ml)
1992
300 °C
1
Ester
Metal oxides
23.6
0.785
1–2
438
1992
300 °C
138
1992
300 °C
1998
300 °C
1
Ester
Impregnated ZrO
2
2.4
0.785
0.80
500
300 °C
0.1–2
Ester
Metal oxides
1.6
0.503
1.50
600
Ester
Acid
Metal oxides
11309.4
314.150
30–60
22,400
50
0.9619
0.2333
4.1
Alkali treated
124.7
3.464
100
22,200
50
0.9533
0.2333
4.1
c
2
-Al O
3
2
Cross section (cm )
Actual catalyst volume (ml)
Flow rates (ml/h) NH
g/h) ester
Flow rates (mol/h) NH
mol/h) ester
Ratio NH /ester
3
(
1
1
1.51
3
0.0188
0.0047
4.0
0.0215
0.0047
4.6
0.0258
0.0070
3.7
(
3
MRT full bed (s)
Actual MRT (s)
Linear velocity (mm s
153.2
6.6–13.2
1.96
1204.0
3.8–7.7
0.30
16.0
12.9
22.44
13.8
4.7
2.18
4.7
4.4
6.77
À1
)

A. Mekki-Berrada et al. / Journal of Catalysis 306 (2013) 30–37
35
the turnover frequency corresponding to these conditions can be
3.5. N-methyl amide formation
À2 À1
evaluated at about 3 Â 10
s .
Similarly, good correlation is observed in Fig. 4 with the volu-
mic amount of medium strength acid sites (ammonia adsorption’s
In Fig. 5 are plotted the yields of N-methyl amide as a function
of the mean pore diameter calculated by the BJH method on the sil-
ica–aluminas, a bentonite, niobium oxide, boron-containing alum-
inas, alumina (DEGUSSA), the HY faujasite, the 7% tungstated
zirconia, and titania. Results on faujasite and ZSM-5 discard any
problem of spatial constraint, and then, the globally decreasing
yield with increasing pore size could stem from some degree of
condensation inside the pores. Kelvin equation was used to evalu-
ate the lauramide saturation pressure for small pore size at 300 °C
(Fig. 5), and it appears that condensation could only take place in
pores smaller than 1.7 nm in the present case (ester partial pres-
sure of 0.125 bar). Then, condensation should only happen in
micropores and N-methylation might be enhanced in condensed
medium. A second hypothesis concerning the origin of N-methyl
amide is the reaction of amide with ester. In both scenarios, amide
accumulation necessarily improves N-methylation; however, there
is no direct monitoring of amide inside the catalytic bed, and then,
the amount of amide analyzed at the end of the bed is probably
underestimating its accumulation. No clear correlation of N-meth-
À1
strength between 120 and 140 kJ mol ). This fits better with bor-
on-containing aluminas (high amount of strong acidity and aver-
age nitrile yield) and literature indicates how too strong acidity
can be the source of side-reactions (such as coking) that can foul
the active surface and block the access to reactants. Thus, a popu-
lation of too strong acid sites could be discarded, by deciding a
3
maximum strength (measured by NH adsorption calorimetry)
above which sites are fouled or useless to the catalysis.
Catalytic test results do not lead to any correlation with basic
sites, and correlation with the total number of acid sites raises sev-
eral exceptions, such as hydroxyapatite, aluminum fluoride and
hydrotalcite, which display high amount of acid sites but bad con-
version nitrile yield; besides, faujasite Y ratio 5.1, bentonite, and a
c-alumina perform only slightly above 50 mol%, while displaying
very high amounts of acid sites. This is the reason why a better cor-
relation was sought with only a fraction of the total number of acid
sites.
À1
Fig. 3. Conversion of ester and nitrile yield (mol%) as a function of the volumic density of strong acid sites (
l
mol ml ). Labels are reported in Table 1 and colors are referring
to acidic (red), basic (blue), and amphoteric (black) solids. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this
article.)
À1
Fig. 4. Conversion of ester and nitrile yield (mol%) as a function of the volumic density of medium acid sites (
l
mol ml ). Labels are reported in Table 1 and colors are
referring to acidic (red), basic (blue), and amphoteric (black) solids. (For interpretation of the references to color in this figure legend, the reader is referred to the web version
of this article.)

3
6
A. Mekki-Berrada et al. / Journal of Catalysis 306 (2013) 30–37
anol at 80 °C and the metal cation is thought to assist in the nucle-
ophilic attack of ammonia, possibly like a Lewis acid, while imide is
proposed as an intermediate [29]. This imide, or N,N-diamide,
could be the product of reactions between two amide molecules
and could react on the catalyst surface into a nitrile and a by-prod-
uct; however, no scenario related to this hypothesis could be sup-
ported by the present tests. Both Lewis and Brønsted acid features
exist on the tested catalysts, but some are mainly Lewis (most
impregnated zirconias do not display any infrared signature of
Brønsted acidity) while other possess more Brønsted sites (nio-
bium oxide [30],
2 3
a-Fe O , tungstated zirconias [19]). Concerning
the conditions of the present catalytic test and the relevancy of Le-
wis or Brønsted sites, it can be pointed out that at 300 °C, under
flow of ammonia and produced stream of water in stoichiometric
amount, the catalyst surface’s medium and strong sites should be
covered by ammonia or water derived molecules, and then only
the weak sites, that is, most probably not the Lewis sites, would
be accessible. However, in case of niobium oxide, it was observed
that Lewis acid sites were still accessible and catalytically active
when immerged in water at 120 °C [31,32], which are conditions
less favorable to the stability of Lewis sites than in the present arti-
cle; thus, if we can generalize to other presently studied catalysts,
it can be assumed that some or all of these sites are still catalyti-
cally active here.
Fig. 5. N-methyl lauramide yield as a function of the mean pore size calculated by
the BJH method (black squares) and the lauramide saturation pressure in the pores,
as calculated by the Kelvin equation (curve).
ylation could be observed with lauramide or with lauric acid con-
tent, which would be a product of amide N-methylation by the es-
ter, was only detected in very small amounts by GC–FID and GC–
MS.
The amounts of N-methyl-lauramide at 300 °C are reported in
Table 1. The HY faujasite, H-MFI, bentonite, high surface silica alu-
mina, hydrotalcite, one alumina (DEGUSSA), and boron-containing
aluminas are the catalysts displaying the highest yields of N-
methyl amide. Impregnated zirconias, niobium oxide and phos-
phate, heteropolyacids, and hydroxyapatites also display about
If Lewis sites are still active to promote the nucleophilic attack
of ammonia by pulling on the methoxy, we still need to know the
whereabouts of ammonia. Guimon et al. discussed how at high
temperatures ammonia could dissociatively adsorb onto metal
oxide surface and interact with Lewis acid sites and surface oxy-
gens to convert them into adsorbed –NH
2
and a Brønsted acid site
[
33]. This appears however conflictual with earlier statements
1
mol% of it. Some global tendency can be observed between the
about the necessity of Lewis sites. Moreover, there is no certainty
about whether ammonia is attacking the nucleophilic carbon of
the ester function as an ammonium (adsorbed on a Brønsted site)
introduced surface of catalyst (BET surface area multiplied by the
introduced mass) and the yield of N-methyl amide, which lets us
assume that the N-methylation happens on the surface. Methyla-
tion of amides needs an ‘‘activated’’ form of methyl (usually methyl
halogenide), which can be here a methoxy adsorbed on the cata-
lyst. Alkali-exchanged Y faujasites (pore size about 0.75 nm) and
ZSM-5 (pore size about 0.55 nm) were tested by Fu and Ono [27]
for the N-methylation of aniline with dimethyl carbonate, and it
was observed that spatial constraints (not the case here) and both
base features and weakly acidic sites were crucially enhancing the
efficiency of this reaction. Both HY faujasite and ZSM-5 (H-MFI)
tested here display almost no mesoporosity but high microporos-
ity. On the present list of catalysts, no clear correlation appeared
with any acid–base features.
or as –NH
physisorbed NH
2
(adsorbed dissociatively on a Lewis site) or even as a
molecule.
3
Variation of ammonia partial pressure from 0.25 to 0.625 bar on
zinc oxide (label ‘‘r,’’ almost no amide accumulation) and hydroxy-
apatite (label ‘‘b,’’ high amide accumulation) at 300 °C, keeping
everything else constant (ester flow rate, total flow rate, overpres-
sure), was observed to keep the ester conversion and nitrile yield
almost unchanged. The global reaction order for the ammonia par-
tial pressure is evaluated for zinc oxide at about 0.15 or below, and
for hydroxyapatite at about 0.10 or below. This could mean that in
the standard conditions (0.5 bar partial pressure), ammonia is not
limiting either ester conversion or amide dehydration. As a conse-
quence, the reactive form of ammonia is probably an adsorbed
form at the surface of the catalyst, and the equilibrium between
gas-phase ammonia and this reactive form is not limiting the
kinetics.
3.6. Rate-determining step: Brønsted or Lewis acid catalysis
About the nature of the rate-determining step, it can be ob-
served that at 300 °C, on most catalysts the amide accumulation
is rather low, which gives credit to assuming that the conversion
of ester into amide is the slowest part, rather than the dehydration
of amide. Moreover, the dehydration reaction can only happen
after the ester conversion into amide, which implies a shorter time
of contact of amide with the catalyst, and though at 300 °C it does
not lead to much accumulation. Aminolysis of ester can consist in
one concerted mechanism or in two steps through a hemiaminal
intermediate. A DFT study of the conversion of methyl ester into
amide catalyzed by a base suggested that the two-step scenario
was the most energetically favorable, and the first step of nucleo-
philic attack by ammonia appears as the rate-limiting step. The
addition of a molecule of ammonia or methanol to this system
was also calculated to reduce the energy barriers on both steps
3.7. Dehydration of amides
In the scenario where ester aminolysis is the rate-determining
step, correlation with the nitrile yield brings no information on
which feature influences amide dehydration. Studies in liquid (di-
luted) phase can be found in the literature. Enthaler et al. have
been reporting zinc and iron homogeneous catalyzed dehydration
of amides in toluene, providing 3.5 stoichiometric amounts of sily-
lation, where zinc (or iron) acts as a Lewis acid helping with the
silylation on the nitrogen atom [34]. A study by Furuya et al. on
3
dehydration of amides using perrhenic acid (ReO (OH)) at 1 mol%
in toluene or mesitylene proposes a pathway via a six-membered
cyclic transition state [35,36], thus also with a Lewis acid site. In
the present case, there is no evidence that the dehydration does
not occur out of the catalyst surface or proximity; however, it
[
28]. Amidation of esters assisted by salts (Mg(OCH
3 2 2
) , CaCl ,
NaOCH ) in stoichiometric amount was also investigated in meth-
3

A. Mekki-Berrada et al. / Journal of Catalysis 306 (2013) 30–37
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should be energetically more favorable to perform it in the vicinity
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[
[
[
[
[
[
1
4
. Conclusion
Several catalysts were tested for the ammoniation–dehydration
of lauric acid methyl ester into nitrile in a gas-phase continuous
downstream process, at 300 °C, with an ester flow rate of
À1
1
.5 g h and a mean residence time of 4.4 s. Catalysts tested in
[
[
the patent literature were discussed and compared to the present
results. A reference experiment carried out with glass beads
showed that no ester was converted in the present conditions
without a catalyst. It was observed that the rate-determining step
in this reaction, that is, ester conversion, was controlled by the vol-
umic density of medium strength acid sites (ammonia adsorption
energy between 120 and 140 kJ mol ) and was most probably
assignable to the attack of the nucleophilic carbon by an adsorbed
form of ammonia. This would correspond to a turn-over frequency
(
1
[
À1
[
[
À2
of about 3 Â 10 . No correlation with basicity was observed, and
1
0.1002/9783527645329.ch18.
furthermore, basic catalysts displayed poor efficiency. The dehy-
dration of amide occurs at the surface of the catalysts and is helped
by the presence of labile protons in the form of ammonium. The
formation of N-methyl lauramide as a side-product occurs mainly
for high surface catalysts with a porous system with mean pore
size lower than 10 nm. It is related with amide accumulation and
could be due to reaction between amide and ester.
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3
The authors are thankful to the scientific services of IRCELYON.
The research leading to these results has received funding from the
European Union Seventh Framework Programme (FP7/2007–2013)
under grant agreement no. 241718 EuroBioRef.
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