Boosted CO2 reaction with methanol to yield dimethyl carbonate over Mg-Al hydrotalcite-silica lyogels

Article abstract of DOI:10.1039/c3cc41298a

Nonimmobilized and immobilized Mg-Al hydrotalcite-like materials on silica lyogels were prepared and activated by calcination to be tested as catalysts in the direct carboxylation reaction of methanol. The HTs supported on silica lyogels showed an important improvement and high stability in the direct synthesis reaction of DMC from CO₂ and MeOH.

Full text of DOI:10.1039/c3cc41298a

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Boosted CO2 reaction with methanol to yield dimethyl
carbonate over Mg–Al hydrotalcite-silica lyogels†
Cite this: DOI: 10.1039/c3cc41298a
D. C. Stoian,ab E. Taboada,cd J. Llorca,cd E. Molins,e F. Medina*ab and
A. M. Segarra*ab
Received 19th February 2013,
Accepted 29th April 2013
DOI: 10.1039/c3cc41298a
www.rsc.org/chemcomm
Nonimmobilized and immobilized Mg–Al hydrotalcite-like materials on
silica lyogels were prepared and activated by calcination to be tested as
catalysts in the direct carboxylation reaction of methanol. The HTs
supported on silica lyogels showed an important improvement and high
stability in the direct synthesis reaction of DMC from CO2 and MeOH.
Scheme 1 Carboxylation reaction of alcohols.
Recently, there has been considerable interest in the valorization of DMC from MeOH and CO2 in a batch reactor. However, the
of CO2 by synthesizing fine chemicals such as organic carbonates. methanol conversion was very low (around 1%).5–8 Several studies
Dimethyl carbonate (DMC) is a versatile and green reagent, used have confirmed that the equilibrium of the reactions (Scheme 1) is
as a precursor for resins and fuel additives and also as a largely shifted to the left and can reach equilibrium concentrations of
methylating and carbonylating agent.1,2 DMC has been generally alkyl carbonates between 1% and 2% depending on the reaction
synthesized using the toxic and hazardous phosgene (ClCQOCl) temperature.9 Nevertheless, when CeO2–ZrO2 was used as catalyst,
as a reagent. An alternative route for synthesizing DMC via the and when the H2O was removed from the reaction system using 2,2-
oxidative carbonylation reaction of MeOH has been developed up dimethoxypropane as a dehydrated agent, the methanol conversion
to the industry scale using expensive catalysts.3 The main problem improved from 0.81% to 4.2%.10 The authors suggested that the
with this process is that the reactant mixture used (CH3OH + CO + presence of both acidic and basic sites is an important factor in
O2 + CuClcat) is highly flammable and toxic. Consequently, there is selective DMC synthesis. Ikeda et al. also suggested that the inter-
much interest in the field of CO2 chemical fixation and green mediate product, methyl carbonate, is formed by activation of CO2
chemistry in finding other active catalytic systems for the direct and MeOH on basic sites,6 whereas the methyl cation is formed by
reaction between CO2 and alcohols in order to synthesize the the MeOH activation on the acidic sites. Finally, the methyl carbonate
corresponding carbonate compound4 (Scheme 1). Carboxylation reacts with the methyl cation to form dimethyl carbonate (DMC).
m+
reaction is thus a new, safe, eco-friendly and economic alternative Hydrotalcite-like materials (HTs; [M2+nM3+m(OHÀ)2(n+m)
] -
m/x
way of valorizing CO2 for use in generating fine chemicals.
[AxÀ
]
ÁyH2O, where M2+ and M3+ are divalent and trivalent metal
Some studies have been reported on the heterogeneous carboxyl- cations, respectively, and A represents the x-valent compensating
ation reaction because of the advantages of heterogeneous catalysts anion) present both acidic–basic active sites and, consequently, they
compared to their homogeneous counterparts.2 A recently published appear to be suitable materials for use as heterogeneous catalysts in
study reported that heterogeneous catalytic materials such as CeO2, the direct carboxylation reaction. The mixed Mg–Al oxides (HTO)
SnO2 and pure and modified ZrO2 have catalyzed the direct synthesis derived from the calcination of HTs are known to exhibit both weak
acidic and weak basic properties.11 The acidic–basic properties of
a
´
Departament d’Enginyeria Quımica, Universitat Rovira i Virgili,
calcined HTs can be modulated by changing the compensating
anions, the amount and the nature of structural cations and the
calcination temperature.12–14 In addition to this, different approaches
to increase the number of active sites have been reported in the
literature.15–18 One of these consists of synthesizing supported cata-
lysts by depositing 20 nm long HT nanolayers on carbon nanofibers.
This was found to yield a tenfold increase in the activity of the HTs in
the acetone condensation reaction.19,20 The success of this approach
lies in the increased HT surface area that is made available
¨
Av. Paısos Catalans, 26, Campus Sescelades, Tarragona, Spain 43007.
E-mail: anamaria.segarra@urv.cat; Fax: +34 977 559 651; Tel: +34 977 559 621
b EMaS-Research Center on Engineering of Materials and Micro/NanoSystems,
U. Rovira i Virgili, Marcel-li Domingo, Tarragona, Spain 43007
c
`
`
`
Institut de Tecniques Energetiques, Universitat Politecnica de Catalunya,
Diagonal 647, ed. ETSEIB, Barcelona, Spain 08028
d
e
`
Centre for Research in NanoEngineering (CRnE-UPC), Universitat Politecnica de
Catalunya, Pasqual i Vila 1-15, Barcelona, Spain 08028
`
Institut de Ciencia de Materials de Barcelona (ICMAB-CSIC), Campus UAB,
Bellaterra, Spain 08193
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cc41298a to reactants when supported onto the carbon nanofibers.
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Table 1 HT-catalyzed carboxylation of methanol in a continuous reactora
Similarly, silica lyogels (LGs) (solid hydrophilic materials obtained
after drying silica aquagels by lyophilisation)21 are also appropriate
materials for use as catalyst supports. They present exceptional
surface area values (up to 1000 m2 gÀ1), which make them suitable
for use as matrices to highly dispersed small amounts of other
materials, in our case, HTs. Here we report, for the first time, the use
of HTs supported on LGs (LG-HTO) as heterogeneous catalysts in
the carboxylation reaction of methanol using a continuous reactor.
The pure silica and the composite silica lyogels were obtained
through sol–gel chemistry and drying of the wet gels by lyophiliza-
tion. To obtain the initial sol, tetraethoxysilane (Si(OEt)4, TEOS),
water, ethanol and 32% ammonia were mixed in a 1 : 4 : 12.25 : 0.016
molar ratio. The sol needed 17 min to gellify. The gels were aged in
Residence
Conversion
Entry Catalyst time (ml minÀ1) T (K) to DMCb,c (%) Selectivityb,c (%)
0
1
2
3
4
5
6
7
8
9
10
LG
0.047
0.047
0.047
0.047
403
403
423
448
363 14.1
383 15.0
403 15.9
423
403 13.2
403 12.8
0
0
HTO
HTO
HTO
LG-HTO 0.047
LG-HTO 0.047
LG-HTO 0.047
LG-HTO 0.047
LG-HTO 0.024
LG-HTOd 0.024
LG-HTO 0.024
1.8
2.5
1.0
99.9
40.0
5.0
99.9
99.9
99.9
58.0
77.6
79.0
45.0
8.4
423
8.5
a
Standard conditions: continuous system formed by a tubular quartz
ethanol for three days. In order to lyophilize the gels, the ethanol packed-bed reactor (10 mm I.D.) and a furnace equipped with a temperature
controller. The MeOH at 273 K was bubbled by using He as a carrier gas with
was previously exchanged with water. The resulting aqua gels were
immersed in liquid nitrogen to freeze the water and afterwards
a flow rate of 2.5 or 5 ml minÀ1 and introduced into the pre-heated reactor
with 250 mg of fresh catalyst. The molar ratio of CO2/CH3OH was 25. b The
lyophilized at room temperature under high vacuum (50 mbar) for %C and %S of the reactor effluent were analyzed on-line using a Shimadzu
GC 17A gas chromatograph equipped with a capillary column (HP-Plot 1909
24 hours. The pure silica lyogel sample was labeled as LG. The
1P-Q04). c The results given correspond to the time on stream of 140 min.
d Reactivated LG-HTO by simple calcination at 723 K.
hydrotalcite-silica composite was synthesized following the previous
protocol, but by substituting the ethanol for an ethanolic solution of
HTs with a Mg/Al molar ratio of 3/1 (1.4 g HT per 100 ml ethanol).
The gelification time was 10 minutes. The final composite lyogel ratio of 3 : 1 activated by calcination at 723 K (HTO) as catalysts in a
was labelled LG-HT. The nominal weight per cent of the hydrotalcite continuous reactor at moderate temperatures (403 K). These results
in the final material was 4.4 wt%. After drying, the sample was show twice the catalytic activity than the previously reported cata-
activated by calcination at 723 K for 5 h to obtain the mixed Mg–Al lysts, and total selectivity to the DMC. No dimethyl ether (DME) was
oxide composite catalyst, labelled LG-HTO. X-Ray diffractograms of obtained. The acidic–basic properties of the HTO22 may play an
the LG-HT and LG-HTO samples showed only one broad peak at important role in improving the activity. However, the selectivity to
around 221 of 2y, which is characteristic of amorphous silica. No DMC decreases noticeably when the temperature increases, and the
diffraction peaks corresponding either to crystalline HT or to the main product obtained is DME (entries 1–3). In order to increase the
mixed oxide phase were observed. This is an expected result since activity and the selectivity of the process, the HTO was immobilized
HT is nanoparticulated and represents a low wt% of the sample. on the silica lyogel. The very high surface area of the silica lyogel may
Nitrogen adsorption–desorption isotherms show a very high surface modulate the acidic–basic properties of the HTO by increasing the
area, 636 m2 gÀ1, and a high pore volume, 1.2 cm3 gÀ1, in the case accessibility of the reagents to the active sites due to the physical–
of the LG calcined at 723 K (LGc). Pore size diameters (roughly, chemical properties of the lyogel. The direct carboxylation reaction
2 o + o 20 nm) are in the mesopore range. A very small number catalyzed with the LG-HTO composite showed a high activity (14.1%
of macropores (+ > 50 nm) are also present in the pure silica conversion to DMC) with almost total selectivity working at a mild
lyogel. Therefore, the sample is mainly mesoporous. Furthermore, temperature (363 K) (entry 4). When the temperature was increased,
the composite sample also has a high surface area, 646 m2 gÀ1, and the conversion to DMC increased slightly whilst maintaining the
exhibits a similar pore volume, 1.5 cm3 gÀ1. Its pore size diameters total selectivity (entries 5 and 6). However, when the temperature
are mostly smaller than 24 nm and, in this case, macropores are not reached 423 K, the conversion to DMC and the selectivity decreased
present (see Fig. SM1, ESI†). Thus, this sample is also mesoporous. significantly at the expense of DME formation (entry 7). The
It should also be highlighted that the composition of LG with HT carboxylation reaction was also performed under the same condi-
does not decrease the LG-HTO high surface area, and does not tions in the presence of the LG and no activity was observed. As
significantly alter the pore size distribution in the mesoporous demonstrated, the dispersion of the HTs in LGs strongly increases
region, which is an essential characteristic for a catalyst support. the catalytic activity of the hydrotalcite-derived material, which is
TEM images provide us with structural information. HTs are layered compared with pure HTO catalysts, the conversion of methanol to
materials with sheet-like particles ranging from 23 to 140 nm in DMC rises to 8-fold at 403 K, with total selectivity towards DMC.
length. Sample LGc shows the typical structure of LGs: a tri- Higher activity is observed when LG-HTO is used instead of HTO,
dimensional network of spherical particles (between 3 and 9 nm and this increase in activity is even greater (15 times) than when
in diameter) with an open-pore structure. The LG-HTO bears both other published catalysts are used.5–8 This could be because HTO on
materials. The LG network has the same microscopic structure, with the LG increases the accessibility to the active sites and also because
spherical particles of a few nm in diameter. The mixed oxides are of the hydrophilicity of the LG (0.013 mol H2O per g LG). Removing
still sheet-like materials, although the shape that can be observed in water from the reaction medium shifts the equilibrium of the
our images has changed compared to the HT particles due to the carboxylation reaction towards the formation of DMC. However, at
calcination process (see Fig. SM2, ESI†).
423 K efficient absorption of water from the MeOH–CO2 mixture
Table 1 shows the preliminary catalytic results of the carboxyl- becomes more difficult and this could affect the conversion to DMC
ation reaction of methanol obtained using HTs with a Mg/Al molar (entry 7).23 Furthermore, the increase in the residence time decreases
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as a side product could mostly be due to the thermal decomposition
of the DMC formed in the carboxylation reaction. To study the
catalytic stability of the system based on LG-HTO, we monitored the
conversion to DMC over 200 min at different temperatures (Fig. 2).
Regardless of the temperature, the catalytic system was quite
stable and showed resistance to the deactivation. The deactiva-
tion of the catalyst was observed at longer periods of time on
stream than 480 min. The catalytic system was easily reactivated
by simple calcination (Table 1, entry 9). In the case of the process
performed at 363 K, an induction time was necessary to activate
the reagents and to reach a conversion to DMC of about 13%.
In conclusion, the results show that the combination of a
catalyst with more accessible acidic–basic sites and an appro-
priate way of removing the H2O can provide a new and highly
efficient process for selective DMC production from CO2.
In this context, LG-HTO is a further advance in the design of
more-efficient heterogeneous catalysts and reaction systems for
valorizing CO2 as a potential source of economic and environ-
mental benefits for the industry and society.
Fig. 1 Thermal stability of the DMC and MeOH in the presence of HTO at
tr = 0.024 ml minÀ1 with a total flow of 5 ml minÀ1: (a) thermal decomposition of
DMC; (b) MeOH intermolecular dehydration.
The authors acknowledge the financial support received from the
Spanish Government’s Ministry of Science and Technology (project
CTQ2006-08196, CSD2007-00041 and CTQ2009-12520). D.C.S and
E.T. are grateful to the URV and UPC for the grant, respectively. F.M.
and J.L. are grateful to the ICREA Academia program.
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