Metal organic frameworks as heterogeneous catalysts for the selective N-methylation of aromatic primary amines with dimethyl carbonate

Article abstract of DOI:10.1016/j.apcata.2010.01.042

Metal organic frameworks (MOFs) with aluminium, copper and iron as central metal atoms with 1,4-benzenedicarboxylic acid (aluminium) and 1,3,5-benzenetricarboxylic acid (copper and iron) as ligands are selective and active catalysts in promoting the polymethylation of aromatic amines with dimethyl carbonate (DMC). N-methylation prevails over carbamoylation even though they are competing parallel processes. The present N-methylation protocol using DMC enjoys advantages such as convenient reaction conditions, benign, reusable, cost-effective catalyst, avoids the use of additional solvent and uses a safe, green methylating agent that only produces CO₂ and methanol as by-products.

Full text of DOI:10.1016/j.apcata.2010.01.042

Applied Catalysis A: General 378 (2010) 19–25
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Metal organic frameworks as heterogeneous catalysts for the selective
N-methylation of aromatic primary amines with dimethyl carbonate
*
Amarajothi Dhakshinamoorthy, Mercedes Alvaro, Hermenegildo Garcia
Instituto Universitario de Tecnologı´a Quı´mica CSIC-UPV and Departamento de Quı´mica, Universidad Polite´cnica de Valencia, Av. De los Naranjos s/n, 46022 Valencia, Spain
A R T I C L E I N F O
A B S T R A C T
Article history:
Metal organic frameworks (MOFs) with aluminium, copper and iron as central metal atoms with 1,4-
benzenedicarboxylic acid (aluminium) and 1,3,5-benzenetricarboxylic acid (copper and iron) as ligands
are selective and active catalysts in promoting the polymethylation of aromatic amines with dimethyl
carbonate (DMC). N-methylation prevails over carbamoylation even though they are competing parallel
processes. The present N-methylation protocol using DMC enjoys advantages such as convenient
reaction conditions, benign, reusable, cost-effective catalyst, avoids the use of additional solvent and
uses a safe, green methylating agent that only produces CO2 and methanol as by-products.
ß 2010 Elsevier B.V. All rights reserved.
Received 8 December 2009
Received in revised form 22 January 2010
Accepted 28 January 2010
Available online 4 February 2010
Keywords:
Green chemistry
Heterogeneous catalysis
Dimethyl carbonate as methylating agent
Methylation of amines
Metal organic frameworks as solid catalysts
1. Introduction
Thus, while methyl iodide and dimethyl sulfate do not require
catalysts to effect the methylation of amines, in the case of DMC, no
Alkylation is a general organic transformation that is widely
used in industry for the production of a wide range of fine and bulk
chemicals [1,2]. N-methylation of aromatic amines is still a
challenging process from the catalytic point of view since aromatic
amines are considerably more reluctant than aliphatic amines to
undergo N-methylation [3]. Generally methylation reactions are
carried out with alkyl halides [4] or dimethyl sulfate [5] which are
known to be toxic, hazardous, mutagenic compounds [6] and
hence it would be convenient to find the suitable alternative
reagents. Besides toxicity, stoichiometric amounts of halides or
sulfates wastes are formed when using these conventional
methylating reactants. Recently, the need to develop more
reaction occurs at moderate temperatures in the absence of a
catalyst. Another problem that is associated with the use of DMC is
its dual behaviour as methylating and carbamoylating reagent,
which can lead to low selectivity towards the desired N-methyl
derivatives [9–11]. Thus, for instance, pyrroles when reacted with
DMC undergo simultaneous N-methylation and carbamoylation in
the presence of phosphazene as base catalysts [12]. Scheme 1
illustrates the possible dual reactivity of DMC. Therefore, highly
active and selective catalysts are needed to implement DMC and
other organic carbonates such as glycerol carbonate as reagents in
organic synthesis [13].
Many catalytic systems have been developed for N-alkylation of
amines using DMC including zirconia [14], Zn1ꢀxCoFe2O4 [15],
diphenylammonium triflate [16], phosphonium salts [17], onium
salts [18], aluminophosphates [19] and DBU [20]. Recently, also
transition metal containing zeolites and mesoporous MCM-41 has
been reported as heterogeneous catalysts for N-alkylation of
amines using DMC [21,22]. On the other hand, we have also studied
the N-alkylation of amines with DMC using various organocata-
lysts under mild conditions [23]. In pioneering studies, Tundo and
coworkers have extensively reported the methylation of amines
using DMC and observed good selectivity in the presence of
faujasites as heterogeneous catalysts [24–26]. Calcined Mg–Al
hydrotalcite has been used as an efficient catalyst in the selective
mono-N-methylation employing DMC as methylating agent in the
vapour phase at 275 8C [27]. Spectroscopic and theoretical
calculations using NaY zeolite indicated that DMC becomes
environmentally acceptable processes has triggered
a great
interest towards dimethyl carbonate (DMC) as safe, clean and
green methylating agent [7,8] with lower negative environmental
impact, with carbon dioxide and methanol being the reaction by-
products in the methylation [4]. The alcohol can be recycled and
used in the formation of more DMC.
One of the problems that limits the application of dialkyl
carbonates and particularly DMC, as alkylating agents is that their
use must be combined with adequate catalysts, because the
reactivity of organic carbonates at moderate temperatures is
significantly lower than that of alkyl halides or dimethyl sulfate.
* Corresponding author. Tel.: +34 96387 7807; fax: +34 96387 7809.
E-mail address: hgarcia@qim.upv.es (H. Garcia).
0926-860X/$ – see front matter ß 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.01.042

20
A. Dhakshinamoorthy et al. / Applied Catalysis A: General 378 (2010) 19–25
Scheme 1. Dual reactivity of DMC.
activated by the coordination of DMC to Na+ ions present in the
zeolites cavity, Na+ acting as Lewis acid [28,29]. Al-containing
MCM-41 can act as catalyst for the N-methylation of 2,4-
diaminotoluene with DMC [21] as well as aniline by methanol
on MCM-41 systems [30]. We anticipated that metal organic
frameworks (MOFs) containing different metal ions that can act as
Lewis acids could also promote N-methylation by DMC.
Fig. 1. Powder XRD pattern of (a) Cu3(BTC)2, (b) Al2(BDC)3 and (c) Fe(BTC).
Metal organic frameworks (MOFs) are solid materials with
infinitely extended crystal structures, which are generally con-
structed by cross-linking of polytopic organic ligands and metal ions
or clusters via coordination bonds. Depending on the coordination
geometryofthemetalandthebondingpreferenceofdonatingatoms
as well as the geometry of the bridging ligand, polymeric structures
may result in different dimensionalities with high porosity, stability
and surface area and these properties make them attractive in the
field of catalysis through their catalytic sites either integrated at
metal nodes or located on the backbones [31–35]. The reader is
referred to recent reviews on the use of MOFs as heterogeneous
catalysts for organic reactions [36,37]. In the present study, we will
show the catalytic activity of MOFs with different metals for the N-
methylation of amines with DMC. We will present the data that
indicate that the activity and the selectivity of MOFs are similar to
that previously reported for zeolytes [21].
2.3. Catalytic test
The reaction was carried out by charging 5 mL hermetic
glassware that can withstand pressures up to 20 bar with 100 mg
of the corresponding MOF, 5 equiv. of DMC and 50 mg of substrate.
The flask that contained the reactants and catalyst was sealed and
then deeply introduced into a preheated oil bath and stirred
magnetically. After the required time, the reaction mixture was
diluted with diethyl ether (15 mL) and filtered. A known amount of
decane as external standard was added before analyzing the
products by GC and GC–MS. The reusability of Al2(BDC)3 was tested
for the methylation of aniline. At the end of the reaction, the
mixture was filtered, washed with diethyl ether (5 mL), dried at
70 8C for 2 h and Al2(BDC)3 was reused directly without further
purification for a subsequent run with fresh aniline. The same
Al2(BDC)3 sample was used for four consecutive runs with some
loss in activity. Powder XRD patterns of fresh and four-times used
Al2(BDC)3 were recorded in a Philips X’Pert diffractometer using
2. Experimental
2.1. General remarks
the CuK radiation (l
1.5418 nm) at a scan rate of 0.28 minꢀ1. IR
a
All reagents and starting materials were obtained commercially
from Aldrich and used without any further purification. The
percentage conversion, purity and relative yields of the final
products were determined using Hewlett-Packard 5890 series II
gas chromatograph with FID detector and high purity helium as
carrier gas. The products were identified by comparing them with
authentic samples and using a GC-MS Hewlett-Packard 6890 series.
measurements were carried out by KBr pellet method.
3. Results and discussion
Three different types of MOFs have been tested for their
catalytic activity in N-methylation of 2,4-diaminotoluene (DAT)
using DMC as green methylating agent. Among the three MOFs,
two contain transition metals and the third one has aluminium as
the coordinating metal. In the three cases the organic ligands are
benzenecarboxylic acids. The crystal structure of Al2(BDC)3 is
constructed from corner sharing octahedral Al3+ ions linked by BDC
2.2. Catalyst characterization
The MOFs used in the present work are commercially available
from Aldrich. Al2(BDC)3 is constituted by Al3+ as a nodal metal
atom coordinated to 1,4-benzenedicarboxylic acid (BDC) as a
ligand. This is supplied by Sigma–Aldrich with the sample code as
Basolite A100 with BET surface area of 1500 m2 gꢀ1 and
corresponds to MIL-53 (MIL stands for Materiaux Institute
Lavoisier). The structure of Fe(BTC) is constituted by Fe3+ as a
central metal atom coordinated to 1,3,5-benzenetricarboxylic acid
(BTC) as a ligand. This is commercialized by Sigma–Aldrich with
the sample code as Basolite F300 with BET surface area of
1600 m2 gꢀ1 corresponding to MIL-100. Cu3(BTC)2 is constituted
with Cu2+ as a central metal atom with 1,3,5-benzenetricarboxylic
acid (BTC) as a ligand. This is also supplied by Sigma–Aldrich with
the sample code as Basolite C300 with BET surface area of
2100 m2 gꢀ1 corresponding to commercial HKUST (HKUST stands
for Hong Kong University of Science and Technology) were used as
received. Figs. 1 and 2 show the XRD and IR spectra of the MOFs
used in the present work.
˚
ligands. Al2(BDC)3 has the pore dimension of 8.5 A with good
thermal stability [38]. The structure of Cu3(BTC)2 is constituted by
a cluster of two metal atoms with paddle wheel shape that has a
square planar coordination [39]. The BTC ligand acts as a trigonal
planar ligand connecting the diatomic metal clusters [39] which
defines nano cages of about 2.5 nm with windows of 0.8 nm.
Similarly, the structure of Fe(BTC) is constituted by the trimers of
m3-O linked by the
benzene-1,3,5-tricarboxylate moieties in such a way that this leads
to two types of mesoporous cages of free apertures of 25 and 29 A,
accessible through microporous windows of 5.5 and 8.6 A [40].
iron octahedra sharing a common vertex
˚
˚
3.1. Polymethylation of DAT using MOFs as solid catalysts
DAT was selected as aromatic amine, because it contains two
amine groups, offering possibility to discuss regioselectivity of
monomethylation and polyalkylation in addition to N-carbamoy-

A. Dhakshinamoorthy et al. / Applied Catalysis A: General 378 (2010) 19–25
21
Fig. 2. IR spectra of the various MOF catalysts used in the present work.
lation. Blank controls in the absence of catalyst at 170 8C and 1:30
DAT to DMC ratio at 24 h yielded 21% DAT conversion mainly to the
para mono methyl isomer.
Besides the nature of MOF, other reaction parameters that have
been considered are the DAT to DMC ratio (1:30, 1:20, 1:10 and
1:5; see Table 1), the reaction temperature (from 90 to 170 8C) and
catalyst loading (from 50 to 100 mg). The results obtained using
different DMC-to-DAT molar ratios at different temperatures are
included in Table 1. The reaction mixtures contain a distribution of
N-mono (mainly para with some ortho), N-di (mostly para-, para-)
and minor amounts of N,N,N-trimethylated (mostly para-, para-,
ortho-) derivatives of DAT, together with variable proportions of
compound 5 arising from simultaneous N-methylation and N-
carbamoylation. Scheme 2 shows the various products observed
for the reaction between DAT and DMC. Concerning the catalytic
behaviour of MOFs, it is appropriate to comment that transition
Table 1
Reaction between DAT and DMC using MOFs as catalyst under various reaction conditions.a The products formed are compounds 1–4 with minor amounts of compound 5
(Scar).
Al2(BDC)3
T (8C)
DAT Con (%)b
Cu3(BTC)2
Fe(BTC)
S1+2
S3
S4
Scar
DAT Con (%)b
S1+2
S3
S4
Scar
DAT Con (%)b
S1+2
S3
S4
Scar
DAT/DMC 1:30
170
150
115
90
88
60
26
5
51
76
34
18
8
15
6
4
–
2
–
95
64
55
7
56
87
22
4
22
9
36
–
93
–
48
–
26
–
26
–
23
–
90
2
89
3
8
–
–
–
–
–
–
100
–
–
100
–
–
–
4
100
–
–
–
DAT/DMC 1:20
170
150
115
90
85
80
42
3
46
43
40
42
12
–
14
15
3
2
2
1
–
94
77
51
14
52
48
23
38
4
25
14
6
23
2
90
82
34
5
33
41
40
48
15
–
27
11
–
4
–
–
–
85
90
2
85
100
–
100
–
–
–
100
–
DAT/DMC 1:10
170
150
115
90
91
83
20
–
31
47
93
–
42
42
7
27
11
–
1
2
–
–
99
83
38
–
63
43
87
–
20
41
13
–
17
16
–
38
4
96
83
31
–
43
37
84
–
27
46
16
–
30
17
–
9
–
–
–
–
–
–
–
–
–
DAT/DMC 1:5
170
150
115
90
99,89c, 88d
38
44
–
40
43
–
22
13
–
1
1
–
–
97
93
–
52
36
–
24
38
–
24
26
–
19
8
95
90
–
37
34
–
30
42
–
33
24
–
6
1
–
–
82
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
a
b
c
DAT (50 mg), catalyst (100 mg), 8 h.
Determined by GC. % Selectivity of methylation is always 100%.
With 75 mg of catalyst.
d
With 50 mg of catalyst.

22
A. Dhakshinamoorthy et al. / Applied Catalysis A: General 378 (2010) 19–25
Scheme 2. Products formed in a reaction between DAT and DMC.
metal exchanged zeolites exhibit similar tendency to promote a
polymethylation of DAT by DMC but generally zeolites do not
promote N-carbamoylation [21].
irrespective of DMC molar ratio. Based on related precedents in
the literature about the influence of the reaction temperature in
the range of 80–150 8C on the selectivity towards carbamoylation,
it could be expected that lower temperatures should favour
carbamoylation [26]. However this influence of the temperature
was not observed and higher percentages towards carbamoylation
are for Cu3(BTC)2 and Fe(BTC) only at high temperatures.
Generally, all the three MOFs accelerate the rate of the reaction
of DAT at higher temperatures. To be more specific, Al2(BDC)3 gives
almost exclusively methylated products and the other two MOFs
with transition metal gives methylated as well as carbamoylated
product 5. None of the conditions studied with three MOFs is able
to give carbamoylated product with high selectivity for a reaction
To compare the selectivity of different MOFs, we have
distinguished the products arising exclusively from N-methylation
(1–4) from compound 5 that is the only one containing a N-
carbamoyl group. In Table 1, the selectivity towards carbamoyla-
tion (Scar) refers to the selectivity towards compound 5.
As it can be seen in Table 1, high DAT conversion to various
products [polymethylation as well as carbamoylation (compound
5)] is achieved for reaction temperatures in the range of 150–
170 8C for the series of the catalysts studied. The percentage
conversion is very poor at temperatures 115 8C or below
Fig. 3. Conversion of DAT or selectivity for methylation or carbamoylation for the reaction of DAT with DMC as a function of time in the presence of (a) Al2(BDC)3, (b) Cu3(BTC)2
and (c) Fe(BTC). (*) DAT conversion; (&) methylation selectivity; (~) methylcarbamoylation selectivity. Reaction conditions: DAT (50 mg), DMC (1.1 mL), catalyst (100 mg),
8 h, 170 8C.

A. Dhakshinamoorthy et al. / Applied Catalysis A: General 378 (2010) 19–25
23
between DAT and DMC. In this context, Cu3(BTC)2 is the catalyst for
which higher percentage of carbamoylated products (up to 36%
selectivity) at 170 8C with excess DMC. The observed product
distribution and particularly formation of compound 5 indicates
that DAT reacts with DMC in the presence of MOFs giving initially
mono N-methylated products mainly the para isomer as reported
[21]. Then, methylation increases the nucleophilicity of the N atom
and then trimethyl amine reacts with different selectivity than
DAT and carbamoylation is observed but only for the trimethylated
DAT derivative. Thus, according to our kinetics, N-methylation
precedes always to carbamoylation which should be considered a
consecutive reaction and hence selectivity of methylated products
is always 100% as it can be seen from Fig. 3. This figure shows the
time conversion plot and product selectivity for the reaction of DAT
and DMC in the presence of the three MOF catalysts. Fe(BTC)
exhibits similar activity as Cu3(BTC)2 but the carbamoylated
product 5 is formed with somewhat lower yield than for Cu3(BTC)2.
From the point of view of selectivity towards N-methylation,
Al2(BDC)3 shows the highest selectivity. The variations in the
percentage of carbamoylation should reflect the influence of the
structure of MOF (cation type and cavity size) on the activity of
the catalyst. In zeolites and mesoporous aluminosilicates, it has
been found that large pore sizes are necessary to observe the
formation of carbamoylation and it has been attributed to the
larger size of the transition state leading to carbamoylation
compared to methylation. In this regard, it is remarkable that also
for MOFs carbamoylation is only observed for catalyst with higher
porosity (BTC ligands). Also, it has been reported that the softness
and hardness of the Lewis acid sites influences the ratio between
methylation/carbamoylation [7]. This could explain why
Cu3(BTC)2 having softer metal ion exhibits higher tendency to
form carbamoylated products. For Al2(BDC)3, the effect of catalyst
loading is tested at 170 8C with 5 equiv. of DMC (see Table 1) and it
is noticed that DAT conversion gradually decreases from 100 to
50 mg.
Scheme 3. Methylation of primary aromatic and aliphatic amines with DMC in the
presence of Al2(BDC)3.
reaction time progress the formed mono is converted to di N-
methyl and N-methyl-N-carbamoyl products. Ortho substitution
with respect to the amino group makes difficult dimethylation due
to steric hindrance. For this reason, 2,4-dichloroaniline and 2,4-
dimethylaniline yield the corresponding mono N-methyl deriva-
tive with high selectivity without any carbamoylation (entries 6
and 7, Table 2).
Likewise, heteroatom substituted anilines are subjected to N-
methylation and carbamoylation products are completely sup-
pressed (entries 8 and 9, Table 2). Here, O- and S-methylation is
only observed after N-methylation at long reaction times. This is in
agreement with the relative nucleophilicity of the N, O and S atoms
under the reaction conditions. Hydrogen bonding influences the
course of the reaction by determining the product distribution and
ultimately favouring polymethylation. Hydrogen bonding where
the amino group is the hydrogen donor increases the nucleophi-
licity of the N atom by increasing its partial negative charge. Thus,
o-phenylenediamine is also subjected to polymethylation without
carbamoylation. Bis-(4-aminophenyl)methane is also subjected to
methylation and again here polymethylation is a major product.
3.3. Heterogeneity of the reaction
3.2. Al2(BDC)3 as solid catalyst for the methylation of aromatic amines
To verify whether the observed catalysis is derived from the
presence of Al2(BDC)3 or from any possible dissolved aluminium
species leached from the solid to the solution, N-methylation of
aniline was carried out with 20 equiv. of DMC and 100 mg of
Al2(BDC)3 at 170 8C. The catalyst was filtered in hot from the
reaction mixture at approximately 33% conversion of aniline. After
removal of the catalyst, the solution in the absence of solid was
again stirred. After 8 h, the percentage of aniline conversion was
increased to 38%. This marginal increase in the conversion of
aniline in the absence of catalyst may be attributed to the non-
catalytic process taking place at the reaction temperature. Also the
absence of dimethyl terephthalate as determined by GC analysis of
the solution, as opposed to the case of aliphatic amines (see the
corresponding section below), indicates the absence of free ligand.
These data are compatible with the assumption that the catalysis is
truly heterogeneous.
After optimizing the reaction conditions with DAT, the present
protocol was extended to other aromatic amines with various
substituents as well as aliphatic amines (Scheme 3) with 5 equiv. of
DMC and Al2(BDC)3 as catalyst. The results obtained are presented
in Table 2. In related precedents in the literature that have used
DMC as a methylating agent, an even larger excess of DMC (74-fold
molar excess, with respect to amine) has been used [17,24].
The reaction of aniline with DMC in the presence of Al2(BDC)3
yields 89% conversion with 96% selectivity of methylated products.
Selectivity towards dimethylation increases with the reaction
time. In the case of aniline carbamoylation occurs in a minor
extent. The catalytic activity of Al2(BDC)3 was also determined in
the presence of an excess of aniline (500 mg) with 2 equiv. of DMC.
After 24 h, 60% of aniline was methylated selectively without any
carbamoylation and the mono- and dimethylaniline formed with
66% and 34% selectivity, respectively.
3.4. Reusability of Al2(BDC)3 as heterogeneous catalyst
In a similar fashion, substituted anilines are also subjected to N-
methylation. Good conversions are obtained in most of the cases
except for nitro substituted aniline. The strong electron with-
drawing effect of the nitro group decreases the reactivity of this
aniline. Also, carbamoylated products are absent except with a
small amount (4%) for p-toludine and anisidine (Entries 4 and 5 in
Table 2). These results are in line with the previous observations on
DAT that increasing nucleophilicity on the N atom favours N-
carbamoylation [23]. In order to gain information on the reaction
pathway an aliquot of the sample for the reaction of p-toludine was
subjected to GC analysis after 30 min and it clearly indicates that
the initial product is the mono N-methylated derivative and as the
Reusability of Al2(BDC)3 as heterogeneous catalyst for the
reaction of aniline with DMC was also investigated under
optimized conditions with 5 equiv. of DMC at 170 8C. After the
reaction time, the solid was collected, washed with diethyl ether,
and dried at 70 8C, and used for another consecutive run without
further treatment. The catalyst exhibited a gradual decrease in the
activity upon reuse, aniline conversion of the four consecutive
reactions being 89, 88, 87 and 81% with minor variations in the
product distributions towards lower polymethylations. Compari-
son of the powder XRD patterns of the fresh and four-times reused
Al2(BDC)3 is provided in Fig. 4. It can be seen from Fig. 4(a) and (b)

24
A. Dhakshinamoorthy et al. / Applied Catalysis A: General 378 (2010) 19–25
Table 2
Selective N-methylation of various anilines with Al2(BDC)3 using DMCa.
Entry
Substrate
Conversionb (%)
Selectivityb (%)
Methylation
Mono
Carbamoylationc
Di
Poly
1
2
89
34
66
62
34
–
–
3
–
60d
80
70
29
–
1
3
4
53 (9)e
87 (100)e
37 (81)e
10
–
–
–
90 (37)e
59 (14)e
4 (5)
5
6
93
91
23
64
73
34
–
–
4
2
7
39
97
3
–
–
–
–
8
94f
5
79
14
38
9
85f
51
–
10
57
24
–
9
65
86
–
–
11
100
14
a
Reaction conditions: substrate (50 mg); DMC (5 equiv.); Al2(BDC)3 (100 mg); 170 8C; 8 h.
Determined by GC. %Selectivity of methylation ¼ ðmono þ di þ poly=%conversion of substrateÞ; % Selectivity of carbomoylation = carbamoylated/% conversion of
P
b
substrate.
c
Only carbamoylated.
d
e
f
Aniline (0.5 mL); DMC (2 equiv.); Al2(BDC)3 (100 mg); 170 8C; 24 h.
Value in parenthesis corresponds to a reaction for 30 min.
O- and S-methylation also occurs in long term.

A. Dhakshinamoorthy et al. / Applied Catalysis A: General 378 (2010) 19–25
25
DMC. We have found that Al2(BDC)3 is more selective towards the
methylation while the other two MOFs form also products arising
carbamoylation. The present protocol enjoys many advantages
over other catalytic system like cheap and cost-effective catalyst,
use of DMC as methylating agent, avoids the use of conventional
organic solvents. In addition, we have found that Al2(BDC)3 can be
reused with some minor and gradual activity decrease. Therefore,
this can be considered as a green protocol for selective N-
methylation. In contrast, MOFs are not suitable catalyst for more
basic aliphatic amines that causes the partial destruction of MOF
with the liberation of dimethyl terephthalate.
Acknowledgement
Financial support by the Spanish DGI (CTQ06-6857 and
CTQ2007-67805/PPQ) and EU (Macademia) is gratefully acknowl-
edged.
Fig. 4. Powder XRD of (a) fresh Al2(BDC)3, (b) four-times reused Al2(BDC)3 and (c)
Al2(BDC)3 after the reaction with n-hexylamine.
that the diffraction pattern of Al2(BDC)3 changes somewhat during
its use as catalyst with some variation in the intensities of some
peaks. These data indicate that the crystal structure of Al2(BDC)3 is
undergoing partial damage upon its reuse as heterogeneous
catalyst.
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3.5. Al2(BDC)3 as heterogeneous catalyst for the methylation of
aliphatic amines
We are interested to see if MOFs can also act as catalysts for the
DMC reaction with aliphatic amines and the corresponding
product distribution. To explore this reactivity two aliphatic
amines were reacted with DMC in the presence of Al2(BDC)3 as
heterogeneous catalyst (Scheme 3). As expected in view of the
precedents on the reactivity of aliphatic amines with DMC, in case
of cyclohexyl amine as well as n-hexylamine methylation was
accompanied with significant amount of carbamoylated products.
However in the case of aliphatic amines, considerable amount of
dimethyl terephthalate was determined as by-product. This
indicates that Al2(BDC)3 is not totally stable under the reaction
conditions and partial destruction of MOFs is gradually taking
place during the course of reaction (see Fig. 4). This structural
damage of Al2(BDC)3 in the reaction with aliphatic amines could
reflect a strong interaction of the Al3+ ions of the MOF with
aliphatic amines replacing terephthalate as ligand. This produces
the liberation of terephthalate from the solid that becomes
methylated under the reaction conditions. The reason behind
the instability of MOF is most probably the considerably higher
basicity of aliphatic amines compared to aromatic amines.
Nevertheless, in spite of the formation of detectable amount of
dimethyl terephthalate powder XRD shows that the crystal
structure of Al2(BDC)3 is still maintained in a large extent after
its use as catalyst in the DMC methylation of n-hexylamine since
otherwise amorphous material or complete dissolution of the solid
should have been observed.
4. Conclusions
The present results show that MOFs can be used as heteroge-
neous catalysts for the transformation of aromatic amines to their
corresponding methylated amines using DMC. Although there are
some differences in the activity and selectivity, depending on the
nature of MOFs, all the three MOFs promote polymethylation with
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