Zr-MOF-808@MCM-41 catalyzed phosgene-free synthesis of polyurethane precursors

Article abstract of DOI:10.1039/c8cy02235f

In this work, a catalytic method is presented for the synthesis of aromatic carbamates from aromatic amines using dimethyl carbonate instead of phosgene as a green and safe reaction process. Microcrystalline Zr-MOF-808 is reported as an active and efficient heterogeneous catalyst for the selective carbamoylation of anilines and industrially relevant aromatic diamines, under mild reaction conditions with near quantitative yields. We have accomplished the selective growth of well-dispersed Zr-MOF-808 nanocrystals within the mesoporous material MCM-41. A superior catalytic performance of the Zr-MOF-808@MCM-41 is demonstrated that together with increased stability stands out as an advantageous heterogeneous catalyst for polyurethane production. In situ FTIR studies have allowed a better understanding of the reaction pathway at the molecular level when the active MOF catalyst is present.

Full text of DOI:10.1039/c8cy02235f

Catalysis
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Zr-MOF-808@MCM-41 catalyzed phosgene-free
synthesis of polyurethane precursors†
Cite this: Catal. Sci. Technol., 2019,
9, 146
*
*
Sergio Rojas-Buzo, Pilar García-García
and Avelino Corma
In this work, a catalytic method is presented for the synthesis of aromatic carbamates from aromatic
amines using dimethyl carbonate instead of phosgene as a green and safe reaction process. Microcrystal-
line Zr-MOF-808 is reported as an active and efficient heterogeneous catalyst for the selective
carbamoylation of anilines and industrially relevant aromatic diamines, under mild reaction conditions with
near quantitative yields. We have accomplished the selective growth of well-dispersed Zr-MOF-808 nano-
crystals within the mesoporous material MCM-41. A superior catalytic performance of the Zr-MOF-
808@MCM-41 is demonstrated that together with increased stability stands out as an advantageous
heterogeneous catalyst for polyurethane production. In situ FTIR studies have allowed a better understand-
ing of the reaction pathway at the molecular level when the active MOF catalyst is present.
Received 29th October 2018,
Accepted 22nd November 2018
DOI: 10.1039/c8cy02235f
rsc.li/catalysis
(Scheme 1) stands out as an excellent, safe, clean and green
process with the generation of methanol as the only by-
Introduction
Aromatic polyurethanes are an important class of polymeric
materials with a market size of 54 billion USD in 2015, which
is anticipated to grow at a CAGR (compound annual growth
rate) of more than 7% from 2016 to 2025. At present, the
commercial method for synthesizing polyurethanes is based
on the use of phosgene B (Scheme 1). Aromatic diisocya-
nates C are raw materials in the production of polyure-
thanes and they are prepared by the direct reaction of aro-
matic diamines A with phosgene B (Scheme 1). While the
environmental and safety problems associated with phosgene
have been recognized already several decades ago and have
indeed triggered extensive research in the development of
alternative chemical procedures precluding its usage, the re-
ality is that phosgene-methods are still prevalent. New, cost
effective and environmentally friendly synthetic procedures
should be developed for the production of these commer-
cially valuable intermediates.
product in the transformation. The latter could be even fur-
ther recycled in the production of dimethyl carbonate
through a catalytic carbonylation reaction. Following this pro-
cedure, a zero-emission process can be realized, and this syn-
thetic route has gained increasing interest.
Dimethyl carbonate (DMC) is considered a good example
of a green compound due to its non-toxicity, good biodegrad-
ability and clean industrial methods of synthesis.1,2 On these
bases, DMC appears as an excellent candidate to substitute
phosgene in the bulk preparation of diisocyanates. However,
there are several aspects that limit its application: 1)
dimethyl carbonate exhibits low reactivity at moderate tem-
perature and consequently it should be used in combination
of a catalyst. 2) its dual behavior3 as methylating and car-
bamoylating reagent can lead to low selectivity towards the
desired N-carbamoylated derivative.
In the case of aromatic diisocyanates C, the alternative
synthetic strategy that appears more appealing consists in a
two-step sequence wherein aromatic diamines A are trans-
formed first to the dicarbamates D that after a thermal treat-
ment give the desired diisocyanate products C (Scheme 1). In
this regard, several methods have been reported for the syn-
thesis of carbamates from amines. Among them, the reaction
with organic carbonates such as dimethyl carbonate, DMC,
Instituto de Tecnología Química, UPV-CSIC, Universitat Politècnica de València,
Consejo Superior de Investigaciones Científicas, Avenida de los Naranjos s/n
46022, Valencia, Spain. E-mail: pgargar@itq.upv.es, acorma@itq.upv.es
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8cy02235f
Scheme 1 Synthetic route to polyurethanes involving either phosgene
or dimethyl carbonate.
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Therefore, highly active and selective catalysts are needed
to implement DMC as reagent in organic synthesis, and ex-
tensive research has been performed in this regard.
more challenging and also industrially relevant, aromatic di-
amines such as toluene diamines or 4,4′-methylene dianiline.
Probably the most active heterogeneous catalyst reported
hitherto for the carbamoylation of aromatic diamines is
based on gold-dopped nanoparticulated-ceria.30,31 This is a
highly effective and selective heterogeneous catalyst for the
carbamoylation reaction of toluene diamine (and other ani-
lines) that affords the product in 96% yield (140 °C, 7
hours).30,31 Furthermore, the solid could be reused up to 3
times without apparent change in the activity. Due to the
economic impact of these carbamate products, and the obvi-
ous advantages of phosgene-free synthetic routes, the search
for new, better-performing and heterogeneous catalysts re-
mains quite active.
While aliphatic amines react smoothly with DMC to form
the corresponding carbamates,4 aromatic amines find strug-
gle in this procedure owning to the inherent lower nucleophi-
licity. Several homogeneous and heterogeneous catalytic sys-
tems have been developed for the N-carbamoylation of
aromatic amines with DMC. Homogeneous catalysts are lead
compounds,5,6 zinc derivatives,7–15 YbIJOTf)3 (ref. 16) and
organocatalysts.17,18 Among these, ZnIJOAc)2 often gives the
best activity for aromatic dicarbamate synthesis.7–9,12,13,19
However, it possesses certain drawbacks. Apart from the
separation and recovery of the homogeneous catalyst, it was
found that it loses activity easily because of its transforma-
tion to ZnO by reaction with methanol13 which is the
byproduct in the procedure. To overcome these problems,
several studies have been performed dealing with immobili-
zation of such catalyst into different supports. ZnIJOAc)2
supported on activated carbon allow carbamate formation
from aniline and DMC in 78% yield, though extensive
leaching is reported resulting in a loss of activity.20 Zinc
alkylcarboxylates have been covalently grafted in several types
of silica materials such as SBA-15 (ref. 21) and in plain
SiO2.22 ZnIJOAc)2/SiO2 material has been prepared by incipient
impregnation and affords N-phenyl carbamate from aniline
and DMC in 94% yield.23 With these strategies, improved sta-
bility of ZnIJOAc)2 has been achieved and certain recyclability
of the materials was probed. As an example, for the latter
ZnIJOAc)2/SiO2 material, the authors report a drop in N-phenyl
carbamate yield to 38% after the fifth use.23 A polystyrene
supported zinc complex have been recently shown to effi-
ciently produce carbamates from anilines using DMC.24
Several other heterogeneous catalysts have been found to pro-
mote the carbamoylation of aniline at different degrees of
selectivity. Zirconia supported on SiO2 affords N-phenyl
carbamate from aniline and DMC in 80% yield (at 170 °C
during 7 hours).25 Al-SBA-15 gives the same product with
71% selectivity after 3 hours reacting at 100 °C.26 ZnO–TiO2
also allows formation of the same product from aniline in
moderate yield of 67% (170 °C, 7 hours).27 Basic zinc carbon-
ate has been used as a heterogenous catalyst in the same re-
action yielding the desired carbamoylated product in 94%
yield, and it could be further reused with a moderate de-
crease in the selectivity.10 This latter system has also allowed
to carry out the carbamoylation reaction of aniline in a fixed-
bed continuously fed reactor.28 A nanostarchfuntionalized
ionic liquid containing a cobalt chelate anion have been
successfully applied in the carbamoylation reaction of ani-
lines and aliphatic amines by the reaction with DMC.29
Curiously, all of the above heterogeneous catalysts have been
exclusively tested in the carbamoylation reaction of aniline
with DMC. While that transformation is extensively studied
and optimized in terms of reaction temperature, catalyst
loading and DMC to aniline ratio, the procedure was not
extended to other substituted anilines and neither to the
Metal–organic frameworks (MOFs) are crystalline and
three-dimensional reticular structures formed by organic
ligands coordinatively bonded to metal ions or metal ion
clusters.32–34 The high metal concentration together with
some other interesting properties such as modularity, high
surface area and porosity, organic–inorganic hybrid composi-
tion, active-site uniformity and well-defined structures make
these solid materials interesting candidates to address sev-
eral challenges relating to energy and environmental suitabil-
ity,35 such as gas separation, hydrogen storage, carbon diox-
ide capture and heterogeneous catalysis.36–40 To facilitate
applications of MOFs to key market sectors such as industrial
chemical synthesis, it is essential that the final solid catalyst
displays further characteristics such as thermal, chemical
and mechanical stability. In this regard, Zr-based MOFs fea-
ture exceptional stability (together with acid–base properties)
which makes them especially attractive candidates in hetero-
geneous catalysis.41 In addition, the versatility of Zr-based
nodes as structural elements leads to a number of high po-
rosity MOFs with diverse organic linkers, topologies and, po-
tentially, with different catalytic activities. To date, Zr6 MOFs
with 12-, 10-, 8- and 6-connected nodes have been reported.
We have here selected UiO-66 (Zr) MOF (12-connected) and
Zr-MOF-808 (6-connected) for further investigations. Recent
reports account for the higher catalytic activity observed for
Zr-MOF-808 (6-connected) compared to UiO-66 (Zr) MOF
(ideally 12-connected) in a variety of transformations such as
the Brønsted acid catalyzed regioselective ring-opening reac-
tions of epoxides,42 hydrolysis of nerve-agent simulants43 and
catalytic transfer hydrogenation of carbonyls using alcohols
as hydrogen donors.44,45 This activity difference is attributed
to the coordinatively unsaturated units existing by design in
Zr-MOF-808, together with the presence of larger apertures
that facilitate delivery of the reacting starting materials to the
interior of the MOF, thus enabling a much greater percentage
of nodes to act as catalysts.
We here show that microcrystalline Zr-MOF-808 is an
active and efficient heterogeneous catalyst for the highly se-
lective carbamoylation reaction of anilines and industrially
relevant aromatic diamines. Furthermore, we have accom-
plished the growth of the Zr-MOF in the mesoporous silica
material MCM-41. In this manner, highly dispersed and
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nano-sized crystals of the MOF are attained and the resultant
solid exhibits higher activity and stability in the aforemen-
tioned reaction.
Results and discussion
Previous studies have described the utilization of various
MOFs materials as heterogeneous catalysts in the reaction of
aromatic amines with dimethyl carbonate.46 It was reported
that Al2IJBDC)3 (BDC = 1,4-benzenedicarboxylate), Fe(BTC) and
Cu3IJBTC)2 (BTC = 1,3,5-benzenetricarboxylate) promote meth-
ylation of aromatic amines in the reaction with DMC. Al-
though there are some differences in the activity and selectiv-
ity, depending on the nature of MOFs, (poly)methylation is
the predominant pathway.
For the present work, we have selected several Zr and Hf-
based MOFs that have been prepared following recently
reported procedures.42,47 UiO-66(Zr) and UiO-66(Hf) with the
nominal chemical formula [M6O4IJOH)4IJBDC)12] were obtained
using a solvothermal synthetic method by heating solutions
containing ZrCl4 or HfCl4, respectively, and terephthalic acid
(1,4-benzenedicarboxylic acid, BDCH2) in dimethyl formamide
employing formic acid as a modulating agent.47 UiO-66-
NH2IJHf) features the 2-aminoterephthalate as organic linker
and was prepared using the same procedure except that
2-aminoterephthalic acid was used instead of terephthalic
acid. In defect-free UiO-66 frameworks, a 12-coordination en-
vironment is presented in the Zr6 or Hf6 metal nodes. Zr6-
Based MOFs with lower connectivity such as NU-1000
(8-connected) or MOF-808 (6-connected) have demonstrated
superior catalytic activities in several reactions when com-
pared to the UiO-66 (12-connected) materials.42–44 We hypoth-
esized that the coordinatively unsaturated centers in MOF-
808 materials together with the larger apertures (calculated
pore diameters of 4.8 and 18.4 Å versus 8 and 11 Å in UiO-66)
could be beneficial for attaining selectivity in the catalytic
carbamoylation reaction, whose transition state requires
larger space than the competitive methylation reaction when
DMC reacts with aromatic amines. Consequently, Zr-MOF-
808 and Hf-MOF-808, with the nominal chemical formula
[M6O4IJOH)4IJBTC)2IJHCOO)6] were also prepared within this
work by means of the corresponding solvothermal process
using 1,3,5-benzenetricarboxylic acid (BTCH3) as the organic
linker.42 The MOF materials prepared within this work have
been characterized (ESI†) and the obtained data correlate
well with those reported previously. However, in our hands,
in the case of MOF-808 materials (Fig. 1), we notice that the
N2 sorption isotherms at 77 K showed a mild hysteresis be-
tween adsorption and desorption branches (Fig. 2a and S16
in ESI†) indicating hierarchical micro/mesoporous textures.
That fact was not reported previously for this type of solids,48
although several reports account on this point in UiO-66 type
materials.49 Interestingly, the pore size distribution calcu-
lated using the Barrett–Joyner–Halenda (BJH) model suggests
the existence of mesopores with a size of 3.8 nm for both Zr-
MOF-808 and Hf-MOF-808 (Fig. 2b and S17 in ESI,† respec-
Fig.
1
Zr-Based secondary building units combined with 1,3,5-
benzenetricarboxylic acid as organic linkers to form Zr-MOF-808. Atom
color scheme: C, black or grey; O, red; Zr coordinated polyhedral, blue.
Green spheres fill small tetrahedral cages. Figure reprinted with
permission from ref. 51. Copyright 2014 American Chemical Society.
tively). The pore size distribution in the microporous range
could be estimated by the Ar adsorption isotherm (Fig. S23,
ESI†) measured for Zr-MOF-808 and using the Hörvath–
Kawazoe model that evidences a microporous size of 16.4 Å
(Fig. 2c), further demonstrating the hierarchical porosity of
this material. Unfortunately, the smaller pore in the range of
4.8 Å calculated for Zr-MOF-808 (ref. 48) could not be
detected by using this particular technique. The observed hi-
erarchical porosity of MOF-808 is shown here for the first
time, as far as we know, and could be a result of missing
linkers and clusters in the ideal MOF-808 framework. This
unique feature could be beneficial for heterogeneous cataly-
sis because of the dense catalytic sites packed in micropores
and facile mass transfer through mesopores, as has been
suggested in other MOF systems.50
Octahedral crystals of Hf-MOF-808 with a size between
500 and 1200 nm were observed by field emission scanning
electron microscopy (FESEM) as it is shown in Fig. 3(a and b).
The above prepared Zr- and Hf-based MOFs have been
tested in the reaction of o-toluidine (as a model aromatic
amine) with dimethyl carbonate as carbamoylating agent and
solvent. Initial experiments (see ESI†) allowed us to narrow
down experimental conditions to carry out the transforma-
tion. The reaction was followed over time by using gas chro-
matography and dodecane as internal standard. Fig. 4 shows
the kinetic results when the reaction was performed at 120
°C and using 30 mol% loading (based on the metal) of the
corresponding MOF catalyst with a 12 : 1 DMC : o-toluidine
molar ratio. In this transformation, several products could be
expected (Fig. 4). The desired carbamoylation product 1 was
obtained with different selectivity depending on the MOF cat-
alyst employed. N-Methyl-o-toluidine, N,N-dimethyl-o-toluidine
and N-methyl-N-carbamate-o-toluidine were also formed in
some cases as a consequence of the dual behavior of DMC.
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Fig. 2 (a) N2 adsorption (+) and desorption (○) isotherm of Zr-MOF-808 sample. (b) Pore size distribution of Zr-MOF-808 calculated by BJH
model. (c) Pore size distribution of Zr-MOF-808 calculated by the Hörvath–Kawazoe model.
No other byproducts were detected by gas chromatography.
Fig. 4a showcases yield to the carbamate product 1. It
was found that all the MOF catalysts tested promote the reac-
tion of o-toluidine and DMC, while varied selectivity was
attained to the desired carbamate product 1 (Fig. 4b). It can
be seen that UiO-66 materials are not selective in the trans-
formation giving similar amounts of methylated and
carbamoylated products. No significant activity difference
was observed when different node metal, Hf or Zr, was
employed, and neither when employing UiO-66-NH2IJHf) with
the 2-aminoterephthalate linker. Interestingly, the procedure
was found highly selective when the catalyst employed was
Zr-MOF-808 that affords the carbamate product 1 in 93%
yield at full conversion of the starting material with only a
minor amount of methylated products formed. This superior
selectivity could be related to the lower connectivity in Zr6
clusters that together with the larger apertures found in
MOF-808 catalyst versus UiO would favor the carbamoylation
transition state. The related Hf-MOF-808 showed slightly
lower selectivity (85% for the carbamate product 1). That
Fig. 4 Hafnium and zirconium catalysts for the reaction of o-toluidine
with DMC. (a) Time-yield plots of carbamate product 1. (b) Time-selec-
Fig. 3 Field emission scanning electron microscopy (FESEM) images
of (a) and (b) Hf-MOF-808 microcrystals. (c) and (d) Zr-MOF-
808@MCM-41. Scale bars are included in each micrograph.
tivity plots of carbamate product 1. Reaction conditions: 0.5 mmol
o-toluidine, catalyst (30 mol% in metal), dodecane (35 mg) as internal
a
standard, DMC (0.5 mL), T_ = 120 °C.
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could be explained by the higher Brønsted acidity42 exhibited
by the μ3-OH groups of Hf clusters in Hf-MOF-808 compared
to those in Zr-MOF-808, which would lead to methylated
products in higher percentage with the former.
shuts down the reaction. The results indicate that no active
species were leached to the reaction media from the solid cat-
alyst and that Zr-MOF-808 acts as a true heterogeneous cata-
lyst (Fig. 5). Stability of the catalyst in the carbamoylation re-
action was next evaluated. With this intention, the catalyst
was recovered after the reaction of o-toluidine was completed.
It was then washed (with EtOAc) and dried before it was sub-
mitted to a second reaction cycle with a fresh solution. It was
found that activity of the solid catalyst decreases significantly
and the yield of the carbamate product 1 drops to 14% in the
second run (Fig. 5b). Analysis of the powder XRD pattern of
Zr-MOF-808 after utilization shows that crystallinity of the
material is almost completely lost (Fig. S33, ESI†), which
could explain the strong decrease of activity. The low stability
of the material under the reaction conditions could explain
the requirement of the relatively high catalyst loading (30
mol%) to achieve good conversion to the desired carbamate
product.
In view of the poor stability of the catalyst Zr-MOF-808 un-
der the reaction conditions, a new direction was explored.
We considered the increasing awareness, that the typical
MOF microcrystals obtained directly from the traditional
solvothermal process are not necessarily the best configura-
tion for the final application. Hence, the latest trends focus
on the design of sophisticated hybrid materials based on
MOFs as active functional species. Thus, MOFs are combined
with different supports in order to integrate the physico-
chemical characteristics of both materials while avoiding
their weaknesses as single components such as mechanical,
thermal and chemical stability. Moreover, it could be
expected that the support can add supplementary synergistic
properties that might arise from the interactions in the
resulting composite material. To this respect, MOFs confine-
ment in porous materials has been demonstrated as an effi-
cient strategy to enhance the dispersion and stability of
MOFs,53,54 and mesoporous silica and zeolites appeared as
ideal hosts due to their well-defined channels, large porosity
and good thermal/chemical stability. We have carried out
here the selective growth of MOF nanocrystals within meso-
porous pure silica material MCM-41 (see ESI† for experimen-
tal details). To achieve that, both precursor solutions (ZrCl4
and ligand salt) were sequentially incorporated within pure
silica MCM-41 via incipient wetness impregnation, including
an intermediate step for the acidification of the organic li-
gand salt to the acid form via the gas phase (using hydrogen
chloride). Each step is followed by a heating treatment at 50
°C under vacuum to evacuate the water from the previous im-
pregnation step. After that, the resulting dry solid was ex-
posed to the specific MOF synthesis conditions in the pres-
ence of dimethyl formamide and formic acid. Finally, the
product was washed with distilled water, dimethyl formamide
and acetone.
The reaction was also tested with the homogeneous cata-
lysts ZrCl4 and HfCl4, which showed a much lower activity
and selectivity than the MOFs tested above due to the prefer-
ential formation of the methylated products with the former
(Fig. 4). The MOF component, 1,3,5-benzenetricarboxylic acid
was also tested which resulted not active for the transforma-
tion (data not shown). The combination of ZrCl4 and 1,3,5-
benzenetricarboxylic acid as a catalytic system showed similar
behavior as ZrCl4 on its own. Microcrystalline (5 μm, Sigma-
Aldrich) and nanocrystalline (<100 nm, Sigma-Aldrich) ZrO2
catalysts were also tested in the transformation (data not
shown) and they were both found not active, yielding a small
amount of methylated product along with unreacted starting
material, under otherwise identical conditions. Hence, good
results were attained with Zr-MOF-808 that was found an ef-
fective and selective catalyst for the carbamoylation reaction
of o-toluidine with DMC under mild reaction conditions.
The procedure was next extended to a variety of aromatic
amines (Table 1). As an example, aniline can be efficiently
converted to produce methyl phenylcarbamate 2 that was iso-
lated in high yield of 92% (Table 1, entry 2). The presence of
different substituents in the aromatic ring was also studied.
For example, electron-donating groups such as o-methyl or
p-methoxy didn't affect the efficiency in the transformation,
and the carbamate product was isolated in 93 and 92%, re-
spectively (Table 1, entries 1 and 3). The presence of electron
withdrawing groups such as chloride (Table 1, entry 4) or an
ester group (Table 1, entry 5) required longer reaction time,
but the carbamate products were also formed with high selec-
tivity. The procedure was also tested with industrially relevant
aromatic diamines such as 4,4′-diaminodiphenylmethane
(Table 1, entry 6) and 2,4-diaminotoluene (Table 1, entry 7).
Dicarbamates 6 and 7 are direct precursors of aromatic
diisocianates, which in turn, are used in the bulk production
of several polyurethane materials. Both products 6 and 7 are
obtained here in good isolated yield with high selectivity,
although higher reaction temperature was required to allow
an efficient transformation with 20 mol% catalyst load. It
should be noted that dicarbamate 6 could also be obtained
by reaction of carbamate 2 with formaldehyde in the pres-
ence of an acidic catalyst such as zeolite Hβ.52 The kinetic
profiles for the reaction of 2,4-diaminotoluene with DMC
could be found in the ESI† when using different catalyst
loading. It was observed that at 120 °C with 30 mol% cata-
lyst, only the para-carbamate product was mainly formed.
When the temperature was raised to 150 °C, dicarbamate
product 7 was obtained and a catalyst loading of 20 mol%
was appropriate to afford the product in 91% yield after 10
hours reaction time.
With this approach, two key advantages have been
achieved for the implementation of Zr-MOF-808 as heteroge-
neous catalyst in the carbamoylation reaction of industrially
relevant aromatic diamines, as will be shown below. In the
We have also examined the heterogeneity and recyclability
of Zr-MOF-808. A hot filtration test of the solid catalyst was
performed after 1 hour reacting at 120 °C which completely
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Table 1 Carbamoylation reaction of several aromatic amines with DMC in the presence of Zr-MOF-808 and Zr-MOF-808@MCM-41
Catalyst
Zr-MOF-808a (20 or 30 mol%)
Zr-MOF-808@MCM-41b (2.5 or 5.0 mol%)
Entry
1
Carbamate
T
a_ (°C)
Time (h)
20
Conv. (%)
98
Yieldc (%)
93
Ta_ (°C)
Time (h)
3
Yieldc (%)
120
160
85
1
2
2
3
120
120
16
17
99
98
92
92
160
160
2
89
90
2.5
3
4
5
120
120
24
48
98
90
90
80
160
160
5.2
5
84
89
4
5
6
7
150
150
19
10
90
99
88d
91d
160
160
3
3
95e
93e
6
7
a
b
Reaction conditions for catalyst Zr-MOF-808: 0.5 mmol of amine, Zr-MOF-808 (30 mol% in Zr), DMC (0.5 mL). Reaction conditions for
c
catalyst Zr-MOF-808@MCM-41: 0.5 mmol of amine, Zr-MOF-808@MCM-41 (2.5 mol% in Zr), DMC (0.5 mL). Isolated yield after column chro-
matography. d Zr-MOF-808 (20 mol% in metal) was used, DMC (1 mL). Zr-MOF-808@MCM-41 (5 mol% in Zr) was used.
e
first place, the concentration of catalytic active sites at the
external crystal surface is maximized by reducing the crys-
tals size down to the nanometer domain by confinement
within the mesoporous platform. Secondly, the host protects
and confers additional stability to the highly active catalytic
MOF.
The Zr-MOF-808@MCM-41 here prepared was character-
ized and the XRD pattern shows the peaks corresponding to
a typical MCM-41 material (Fig. S25, ESI†). It is observed one
intense peak of (100) and two weak diffraction peaks of (110)
and (200) indicating that the mesopore hexagonal order of
the material was preserved after MOF crystallization. How-
ever, diffraction peaks corresponding to Zr-MOF-808 were not
observed. It should be notice that diffraction peaks of
Zr-MOF-808 are indeed observed when a physical mixture is
prepared containing the two solids, i.e., the plain MCM-41
silica and microcrystalline Zr-MOF-808 (prepared separately
by the traditional solvothermal process) with analogous Zr
weight% than that measured in Zr-MOF-808@MCM-41 (3.2 Zr
wt%). This phenomenon could be attributed to the small
crystal size in Zr-MOF-808@MCM-41 that together with the
high dispersion exceeds the detection limit of the XRD
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Fig. 5 (a) Time–yield plots for the N-carbamoylation of o-toluidine
with Zr-MOF-808 (black line) and removing the catalyst after 1 h (red
line), and (b) catalyst reusability. Reaction conditions: 0.5 mmol amine,
catalyst (30 mol%), dodecane (35 mg) as internal standard, DMC (0.5
mL), T = 120 °C.
apparatus. Field emission scanning electron microscopy
(FESEM) images show the presence of the two phases in Zr-
MOF-808@MCM-41 (Fig. 3c and d). MOF crystals with a size
around 20–40 nm are observed in the outer surface of the sil-
ica material. Elemental mapping evidences the zirconium
content of the crystals observed. Other characterization tech-
niques were used to provide evidence on the formation of the
MOF on the mesoporous material. On the one hand, the UV/
vis spectra of Zr-MOF-808@MCM-41 show a main peak
around 242 nm, which is attributed to the absorption of Zr–O
clusters in this MOF, while the related band in, for instance,
ZrO2 nanopowder appeared at 229 nm (Fig. S26, ESI†) and
those of ZrCl4 are also observed at lower wavelengths. On the
other hand, the FTIR spectra of Zr-MOF-808@MCM-41 shows
several characteristic bands of Zr-MOF-808 such as those at
1380 and 1450 cm−1 and also bands at 1580, 1663 and 1693
cm−1 (Fig. 6). Additionally, FTIR showed the disappearance of
key vibrational band corresponding to carbonyl stretching
vibration ν(CO) of the carboxylic acid at 1720 cm−1, which
confirmed the formation of the corresponding MOF.
Fig.
7
(a) Time–yield plots for the N-carbamoylation of 2,4-
diaminetoluene with Zr-MOF-808@MCM-41. (b) Catalyst reusability.
Reaction conditions: 0.5 mmol amine, catalyst (5 mol%), dodecane (35
mg) as internal standard, DMC (0.5 mL), T = 160 °C.
pacity and pore size for Zr-MOF-808@MCM-41 compared to
the starting solid MCM-41. Unfortunately, Ar physisorption
experiment did not reveal micropores around 16.4 Å corre-
sponding to the Zr-MOF-808. We believe that the low MOF
loading in the silica material (3.2 Zr wt% as determined by
ICP analysis) does not allow the pore size distribution estima-
tion using the Hörvath–Kawazoe model. The 13C CP/MAS
NMR spectrum of Zr-MOF-808@MCM-41 (Fig. S29, ESI†)
shows the peaks corresponding to the organic linker, 1,3,5-
benzenetricarboxylate and all the carbon atoms were unequiv-
ocally assigned and the signals appeared at the same ppm
as in Zr-MOF-808 (broad band at 132–135 ppm corresponding
to aromatic carbons and the peak at 169 ppm corresponding
to the carboxylate carbon). Further confirmation of MOF
formation is attained by the observation that the peak at 130
ppm corresponding to the Cipso in 1,3,5-benzenetricarboxylic
acid is not observed for Zr-MOF-808 and neither for Zr-
MOF-808@MCM-41 for which the Cipso of 1,3,5-
benzenetricarboxylate falls in the peak at 132–135 ppm. The
corresponding peak of formate ligand at 167 ppm was also
detected for Zr-MOF-808@MCM-41 (Fig. S29, ESI†). XPS exper-
iment additionally reveals MOF formation in the host with Zr
(3d) bands at 180.8 and 182.9 for Zr-MOF-808 that appeared
similarly in Zr-MOF-808@MCM-41 (Fig. S30 and S31, ESI†).
Finally, the obtained Zr-MOF-808@MCM-41 material was
employed as catalyst for the carbamoylation reaction of aro-
matic diamines such as 2,4-diaminetoluene (Fig. 7). Remark-
ably, a low catalyst loading (5 mol%) was enough to promote
efficiently the transformation to the desired dicarbamylated
The specific surface area of the resultant material was
measured by N2 physisorption experiment (Fig. S28, ESI†). As
expected, it was observed a decrement in N2 adsorption ca-
Fig. 6 FTIR spectra.
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product 7. Fig. 7a shows the kinetic profile of the transforma-
tion wherein the highest yield of 88% of dicarbamate 7 was
achieved after 2.5 hours reacting at 160 °C, as determined by
GC using dodecane as internal standard. Reported catalysts
for this same transformation of 2,4-diaminetoluene to
dicarbamate 7 are ZnIJOAc)2·2H2O (ref. 8) and gold-dopped
nanoparticulated-ceria30 that give the desired product 7 in 98
and 96% yield, respectively. While those both exhibit slightly
higher selectivity than Zr-MOF-808@MCM-41 (93% isolated
yield, see entry 7 in Table 1), the heterogeneous character of
the material here reported offers additional advantages versus
homogeneous catalyst ZnIJOAc)2·2H2O. When comparing to
gold-dopped ceria,30 the solid presented herein offers an al-
ternative non-precious metal based solid catalyst with very
high selectivity in the transformation.
For comparative purposes, ZrO2 nanopowder (<100 nm,
Sigma-Aldrich) was tested in the transformation and no activ-
ity was register even after extended time of 8 hours (ESI†).
The solid support, pure silica MCM-41, was also assayed in
the same reaction and it was shown that it basically pro-
motes (poly)methylation reaction although at a slower pace
(ESI†), in the sense that when Zr-MOF is present the high
activity of the MOF precludes extensive methylation reac-
tion. Comparison was also performed with a traditional
We have carried out an in situ FTIR spectroscopy study to
gain insight of what is occurring at the molecular level on
the surface of the catalytically active MOF crystals. This
spectroscopic investigation takes advantage of the fact that
DMC and aniline (as a model aromatic amine) are both liq-
uids, and they can be easily dosed from the gas phase onto
the MCM-41 materials that, upon diffusion inside the pores,
the IR spectrum of the wafer containing DMC and DMC + an-
iline can be easily recorded in situ. The available frequency
range for the IR study on these MCM-41 materials (1900–
1200 cm−1) only allows observation of the ν(CO) mode and
the νasymm.(CO2) of the carbonyl bond in DMC (1810–1680
cm−1 and 1320–1235 cm−1, respectively) and the δ(CH3)
modes of the methyl groups (1400–1500 cm−1). The bands as-
sociated with, νsymm.(CO2), νsymm.(O–CH3), and νasymm.(O–
CH3), which would be of high diagnostic utility, cannot be
observed because they fall within the framework region.
In the first place, we have evaluated interaction of DMC
with the active sites in the solid materials with the aim to de-
termine structural perturbations induced in DMC upon inter-
action and, therefore, elucidate activation pathways that
would account for the attained selectivity.
Fig. 8 shows the FTIR spectra of bare DMC (Fig. 8a), DMC
adsorbed on MCM-41 solid (Fig. 8b) and DMC adsorbed on
Zr-MOF-808 (Fig. 8c). Difference FTIR spectrum is shown in
the latter case due to the framework signals that otherwise
preclude observation of the vibrational modes assigned to
DMC when adsorbed to this material. From the observation
of these FTIR spectra we can infer that the interaction of
DMC with these solids MCM-41 and Zr-MOF-808 induces a
relevant perturbation of the molecule which influences the vi-
brational properties and hence, the force constants of bonds
within the molecule. It will be shown that such interaction of
DMC with the Zr sites in the MOF would explain why Zr-
MOF-808 is so effective in promoting the carboxymethylation
activity of DMC.
DMC possesses two distinct nucleophilic sites as has been
determined by analysis of the molecular electrostatic poten-
tial.57 One nucleophilic site is located on the oxygen of the
carbonyl group and the second one corresponds to the oxy-
gen atoms of the O–C–O moiety. On this bases three different
DMC adducts (Fig. 9) could be formed when interacting with
the zirconium sites (or with silanol groups in the case of
MCM-41).
This approach has been demonstrated to be able to de-
scribe the electrostatic interaction between DMC molecule
and Na+ ions in the supercages of NaY zeolite.57 In that
study, it was concluded that when the interaction occurs
through the oxygen in the carbonyl moiety (related structures
I and II in Fig. 9), the calculations predict an elongation (and
therefore weakening) of the O–CH3 bonds, a fact which
would lead to the methylation reaction. While in adduct of
type III, calculations predict an elongation/weakening of the
C–OCH3 bond, a fact that would facilitate delivery of the
carbonylmethoxy group which hence justifies the catalytic ac-
tivity in the carbamoylation reaction. The theoretical
ZrO2@MCM-41 material that was obtained following
a
reported procedure55 by wet impregnation of MCM-41 with
ZrOCl2·8H2O and subsequent calcination in air at 500 °C for
2 hours. It was observed that the presence of ZrO2 nano-
crystals in the MCM-41 matrix further accelerates the meth-
ylation reaction and the para-carbamate product 8 was also
detected although in low yield of 5% and after extended
time of 8 hours with nearly completed conversion of the
starting material (ESI†). In the case of [Zr]MCM-41 in which
the metal ions are incorporated in the framework replacing
some silicon atoms, it has been reported previously that
N-carbamoylated products are formed along with methylated
products in similar ratio and therefore no selectivity was
achieved in the transformation.56
The heterogeneity and stability of Zr-MOF-808@MCM-41
was next evaluated. The hot filtration test showed the hetero-
geneous character of the catalyst employed (Fig. S34, ESI†).
More importantly, the catalyst could be recovered and reused
at least four times without apparent change in the reaction
efficiency and even slightly higher yields were attained in the
subsequent three runs (Fig. 7b). A slight activity decrease
was observed from run number five. That was attributed to
deposition of organic components in the solid that may
block accessibility to the MOF active sites, since characteri-
zation of the reutilized material exhibited a substantial de-
crease in the BET surface area (from 777 m2 g−1 in as-
synthesized Zr-MOF-808@MCM-41 to 464 m2 g−1 in the
reutilized material) together with a higher organic content
as determined by elemental analysis. However, XRD pattern
showed similar to the starting material which entails sup-
ports stability under the reaction conditions by the preserva-
tion of the mesopores ordering.
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Fig. 8 (a) FTIR spectrum of bare DMC, (b) FTIR spectra of MCM-41 and MCM-41 after adsorption of DMC and (c) difference FTIR spectra of Zr-
MOF-808 and Zr-MOF-808 after adsorption of DMC.
calculations performed in the case of NaY zeolite where
complemented with experimental results studying the modifi-
cation of the IR properties of DMC induced by the interaction
with the zeolite.57 It was concluded that for adducts I and II,
the ν(CO) and νasymm.(CO2) are red- and blue-shifted, respec-
tively, with respect to isolated DMC. Indeed, that fact was ob-
served here in our case with MCM-41 (Fig. 8a and b) wherein
ν(CO) shifts from 1780 and 1768 cm−1 (Fig. 8a) to 1753, 1727
and 1688 cm−1 (Fig. 8b) when interacting with the MCM-41
surface. Additionally, a blue-shift from 1294 cm−1 to 1310
cm−1 is observed for the νasymm.(CO2) of DMC. The analogy
could be stablished with the NaY zeolite study, and we could
conclude that DMC interacts with the silanol groups in the
surface of MCM-41 via structures of type I and II that are
suggested to lead to methylation reaction, which was indeed,
the observed experimental results obtained in the case of
plain siliceous material MCM-41. On the other hand, the IR
properties of adduct III in zeolite NaY display an opposite
behavior, with the ν(CO) blue-shifting and νasymm.(CO2) red-
shifting with respect to isolated DMC. In the case of Zr-MOF-
808, it was observed a blue-shift of ν(CO) from 1780 and 1768
cm−1 (Fig. 8a) to 1808 cm−1 (Fig. 8c and a) red-shift of
νasymm.(CO2) from 1294 cm−1 to 1252 cm−1. These data there-
fore suggest the presence of type III adducts upon interaction
of DMC with the Lewis acidic sites in the MOF material in
analogy to those results observed for DMC and NaY zeolite.57
The presence of adducts III when using Zr-MOF-808 would
account for the observed carbamoylation reaction attained by
this catalyst due to the weakening of the C–OCH3 bonds. Ad-
ducts of type I and II can not be ruled out for Zr-MOF-808
since a red-shift of ν(CO) to 1719 cm−1 was also detected for
this material (Fig. 8c). Hence, it seems reasonable to con-
clude that the three adduct structures (Fig. 9) could be pres-
ent in the MOF case upon interaction of DMC with the active
sites. Being type III structures the less stable,57 that would re-
act at higher rate than I and II allowing the carbamoylation
reaction as the predominant pathway.
Further FTIR experiments were conducted. The results
given in Fig. 10a show the signals corresponding to DMC
(around 1700–1800 cm−1 and 1400–1500 cm−1) upon adsorp-
tion on MCM-41. It is observed that upon desorption by ap-
plying vacuum both groups of signals experiment a decrease
in their intensity to the same extent, and DMC is completely
desorbed when further heated at 60 °C. On the contrary, Zr-
MOF-808@MCM-41 (Fig. 10b) shows different intensity in
both group of signals upon DMC desorption already at room
temperature. A substantial decrease of the signal at 1750
cm−1 (corresponding to the carbonyl group in DMC) is
Fig. 10 (a) FTIR spectra of MCM-41 before and after adsorption of
DMC at room temperature, and subsequent DMC desorption at in-
creasing temperatures (60 and 120 °C). (b) FTIR spectra of Zr-MOF-
808@MCM-41 before and after adsorption of DMC at room tempera-
ture, and subsequent DMC desorption at increasing temperatures (60
and 120 °C). (c) FTIR spectra of MCM-41 before and after adsorption of
aniline and co-adsorption of DMC at room temperature, and subse-
quent temperature raise (60 and 120 °C). (d) FTIR spectra of Zr-MOF-
808@MCM-41 before and after adsorption of aniline and co-
adsorption of DMC at room temperature, and subsequent temperature
raise (60 and 120 °C).
Fig. 9 Possible DMC adducts formed upon interaction with the solid
catalyst.
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observed while those signals at 1490, 1460 and 1433 cm−1
(corresponding to the methyl group in DMC) are still preva-
lent, which would indicate an easier desorption (and poten-
tially higher reactivity) of the carbonate than the methyl spe-
cies when the MOF crystals are present in the MCM-41
matrix. Furthermore, in this case, the signals corresponding
to the methyl group are still observed upon heating to 60 °C
while that of the carbonyl group (at 1750 cm−1) has
completely disappeared.
brid material Zr-MOF-808@SiO2 following the same proce-
dure as for MCM-41. In this way, comparable catalytic activity
was measured for Zr-MOF-808@SiO2 (1.9 wt% Zr) compared
to Zr-MOF-808@MCM-41 (3.2 wt% Zr) (see ESI†). While this
study mainly focuses on MCM-41 host, these results further
evidence the possibility of using alternative silica materials
for the preparation of related catalytically active hybrid MOF
materials.
In a different set of experiments, aniline (1606 cm−1) was
introduced into the IR cell and subsequently DMC was co-
adsorbed and the wafer was then heated at 60 °C and 120 °C
(Fig. 10c and d). No apparent change was observed by FTIR
when co-adsorbing aniline and DMC on MCM-41 and heating
the sample up to 120 °C, at least in the timeframe used for
the FTIR experiment (Fig. 10c), which reveals the poor activity
of the host. In the case of Zr-MOF-808@MCM-41 (Fig. 10d),
after co-adsorption of aniline and DMC and heating up to 60
°C, a substantial decrease in the band at 1750 cm−1 is already
observed (while prevalence of the methyl bands around
1400–1500 cm−1) which completely disappears at 120 °C (that
was not the case with plain MCM-41 in Fig. 10c). Meanwhile,
on Zr-MOF-808@MCM-41 it is observed an increment in the
band at 1662 cm−1, which can be attributed to the carbonyl
group of the newly formed carbamate product. Further incre-
ment of the latter band is observed when heating at 120 °C
which would indicate further product formation at that tem-
perature. Taken together these results, they nicely explain the
high efficacy of MOF-doped mesoporous silica in promoting
the carbamoylation reaction and the preference of that prod-
uct formation versus methylation when using DMC and aro-
matic amines.
Conclusions
We have shown that Zr-MOF-808 is an active and highly selec-
tive catalyst for the reaction of aromatic diamines with DMC
to produce dicarbamate products required for the synthesis
of aromatic polyurethanes. Furthermore, we have demon-
strated the superior catalytic performance of the Zr-MOF-
808@MCM-41 and Zr-MOF-808@SiO2 with respect to the bulk
microcrystalline Zr-MOF-808 solid. The increased catalytic ac-
tivity and mechanical stability of this material allows for mul-
tiple reuses of the catalyst without significant leaching of the
active sites. This process opens a route for polyurethane pro-
duction that avoids the use of phosgene through the reactant
DMC without generating any by-product except recyclable
methanol. In situ FTIR studies allow a better understanding
of the reaction pathway at the molecular level when the
highly active MOF catalyst is present. Furthermore, the solid
materials herein developed, Zr-MOF-808@MCM-41 and Zr-
MOF-808@SiO2 are potential highly active and stable catalyst
for several other organic transformations wherein microcrys-
talline Zr-MOF-808 has proved successful, further expanding
the scope and possibilities of the solids herein reported.
Conflicts of interest
Since Zr-MOF-808@MCM-41 exhibited remarkable activity
in the carbamoylation reaction of 2,4-diaminetoluene using
DMC and additionally, high stability of the catalyst was
probed by continuous reutilization, the method was extended
to a variety of aromatic amines (Table 1). Analogously, 4,4′-
diaminodiphenylmethane gave dicarbamate 6 in excellent
yield of 95% (Table 1, entry 6). Several aromatic monoamines
were also transformed efficiently to the monocarbamates. In
these cases, a low loading of 2.5 mol% was sufficient to
achieve high conversion of the carbamate products. The ob-
served reactivity trend was similar as that one attained with
microcrystalline Zr-MOF-808 and the presence of electro-
withdrawing groups in the aromatic ring required extended
reaction time (Table 1, entries 4 and 5) to achieve full conver-
sion as compared to aromatic amines with electron-donating
groups (Table 1, entries 1 and 3). In any case, the carbamate
products were obtained with high yields and high selectiv-
ities. An aliphatic amine such as butylamine was found to re-
act smoothly with DMC to form the corresponding carbamate
at 100 °C. The presence of the hybrid catalyst, in this particu-
lar case of aliphatic amine, did not provide additional rate or
selectivity enhancement. On another matters, since the final
goal is the large-scale application of this hybrid catalyst, a
regular silica was also utilized for the preparation of the hy-
There are no conflicts to declare.
Acknowledgements
This work was funded by the European Union through the
European Research Council (grant ERC-AdG-2014-671093,
SynCatMatch) and by the Spanish government through the
Severo Ochoa program (SEV-2016-0683). S. R.-B. acknowledges
a PhD fellowship from the Generalitat Valenciana. The Electron
Microscopy Service of the Universitat Politècnica de Valènciais
acknowledged for their help in sample characterization.
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