S. Bougacha Ghorbel et al. / Applied Catalysis A: General 493 (2015) 142–148
143
adsorption [5]. Calcinating the hydrotalcite-like compound also
increased its phosphate adsorption capacity. This is attributed to
the following physicochemical properties of the calcined hydro-
talcite: (i) the memory effect of the hydrotalcite layers, which
allow phosphates to be incorporated as an interlayer anion, (ii) the
greater surface area, which allows higher surface adsorption of the
phosphates, and (iii) the elimination of the interlayer carbonate,
which is the major obstacle to the incorporation of the phosphates
as an interlayer anion [6]. Recently, Parida et al. have prepared
new materials based on molybdophosphoric and tungstophos-
phoric acid intercalated in Zr/Al-hydrotalcite-like compounds by
co-precipitation using an indirect method based on the terephthalic
acid intercalated Zr/Al hydrotalcite as precursor [7].
The synthesis of dimethyl carbonate (DMC) by carboxylation
reaction is a very interesting catalytic process since DMC is a
safe, non-corrosive, and environmentally acceptable alternative to
methylating and carbonylating agents [8]. Nowadays, industrial
scale DMC synthesis is based on the oxidative carbonylation of
MeOH by means of phosgene-free technologies. However, the reac-
tant mixture used in this process is highly flammable and toxic;
source of economic and environmental benefits for the industry
and the society.
2. Experimental
2.1. Catalyst preparation
All chemicals used for synthesis were purchased from Fluka or
Merck.
The hydrotalcite-like materials were obtained by the co-
precipitation method, which involved either the simultaneous
addition of de-carbonated MgCl ·6H O and AlCl ·9H O solutions
2
2
3
2
(
Table 1, entries 1–6); or Mg(NO ) ·6H O and Al(NO ) ·9H O
3
2
2
3
3
2
(Table 1, entries 7–9) in a molar ratio of Al/(Al + Mg) = 0.25. The pH
was controlled by the simultaneous addition of 2 M NaOH solu-
tion. Both solutions were mixed under vigorous stirring. A 0.1 M
Na HPO4 solution was added during the synthesis to incorporate
2
the phosphates species in the interlayer space of the HTlc materials.
The reactants were added under inert atmosphere to prevent car-
bonate species from entering the interlayer space of the HTlcs. After
the precipitation, the resulting gel was maintained under a vigor-
consequently, DMC synthesis using CO represents an eco-friendly
2
and economic alternative. Moreover, DMC has been proposed for
use as an octane booster in gasoline and as an additive to diesel fuel
to decrease particulate matter emissions [9].
The advantages of heterogeneous catalysts over their homo-
geneous counterparts mean they are preferred by the chemical
industry [9]. Recently, it has been reported that DMC can be synthe-
◦
ous agitation at 80 C for 18 h. The precipitated solid was filtered
and washed several times with decarbonated water to remove the
◦
excess ions and dried at 100 C to yield the different materials. The
experimental conditions for the synthesis of the different samples
are presented in Table 1.
sized selectively and directly from CH OH and CO2 in the presence
3
of CeO2 [10] and ZrO2 [11] as heterogeneous catalysts in a batch
reactor with a conversion of around 1%. The results of the catalyst
characterization suggest that the acid–base bifunction is an impor-
tant factor for a selective DMC synthesis. In this context, the LDH
materials, especially Mg(Al)O mixed oxides, are potential candi-
dates for use as successful heterogeneous catalysts in this reaction.
2.2. Catalyst characterization
The crystallographic characterization of the different samples
was performed by X-ray powder diffraction (XRD). Diffraction pat-
terns of intensity versus two theta (2ꢀ) were recorded with a
Phillips XRD 1050/70 X-ray diffractometer using Cu K␣ radiation
(ꢁ = 1.5418 A˚ ). Mg, Al, Cl and P analyses were performed by induc-
Developing
a more active direct heterogeneous reaction
between CO2 and the alcohols MeOH and EtOH in order to syn-
thesize the carbonic acid diesters dimethyl carbonate or diethyl
carbonate (DEC) (Eq. (1)) is a very attractive prospect in terms of
CO2 chemical fixation and green chemistry, and may also help to
further the substitution of phosgene for CO2 [12]. Therefore, it is
tive coupled plasma (ICP) using a Perkin Elmer 3300 instrument.
The carbon analysis was performed using a Bernard Calcimeter. The
presence of nitrate ions was determined by ionic chromatography
of the NE-ENIS 10304-1-1995 type.
FTIR spectra of the samples were recorded in a Perkin Elmer
spectrum BX, averaging 100 scans with a nominal resolution of
worthwhile exploring new ways to valorize CO by generating fine
2
−
1
chemicals such as carbonates.
4 cm . The Raman spectra were measured using a Renishaw
Raman imaging microscope system 2000, with laser excitation
2
ROH + CO ꢀ (RO) CO + H O (R = Me, Et)
The keys for achieving this aim are the design and development
(1)
2
2
2
−1
at 785 nm in the range between 100 and 4000 cm
(resolution
−
1
27
31
2 cm ). Al NMR and P NMR spectra were recorded at ambient
temperature on Varian Mercury V-400 spectrometer using 7 mm
CPMAS probes. The resonance frequencies were 162 and 149 MHz
of an efficient process with a success catalyst. For the catalyst, the
challenge remains to develop easy, environmentally friendly and
versatile methods for the synthesis of layered double hydrotal-
cites with tuneable morphology, textures, chemical compositions
and desirable acid and basic properties. Furthermore, obtaining
an in-depth characterization and understanding of the properties
of phosphate intercalated HTlcs is critical for their practical use
as catalysts and it will help to avoid the trial-and-error practice.
On the other hand, the process design should facilitate displacing
equilibrium to DMC.
In this context, the current study describes the synthesis by co-
precipitation of hierarchical Mg–Al HTlcs containing phosphates.
The solids were characterized by NH3 TPD, elemental analysis and
Raman, IR and NMR spectroscopies. Furthermore, to the best of
our knowledgement, the present work also describes for the first
time the use of hydrotalcite-like materials as heterogeneous cata-
lyst in the carboxylation reaction of methanol using a continuous
reactor. The preliminary results obtained in this work have slightly
improved with respect to the described in the literature. In addition,
they are in the direction of designing more-efficient heterogeneous
catalysts and reaction systems for CO2 valorization as a potential
31
27
for P and Al, respectively.
Specific surface areas of the samples were determined by
◦
nitrogen adsorption at −196 C using a Micromeritics ASAP 2000
◦
equipment. Samples were previously degassed in situ at 120 C
under vacuum for 4 h. The surface area of the catalysts was calcu-
lated using the Brunauer–Emmet–Teller (BET) method over a p/p0
range where linear relationship was maintained.
Morphological features of the HTlcs samples were studied using
a scanning electron microscope (SEM, model: JEOL model JSM-
6400). For the preparation of the samples for SEM, the powdered
materials were first spread on sample buttons using conducting
cement and then sputtered with gold. The acidity measurements
were determined by the temperature programmed desorption
(TPD) of NH , on a Thermo Finnigan TPDRO 1100 equipped with
3
a TCD detector. Typically, ca. 0.200 g of sample were pretreated
◦
with Ar at 80 C during 1 h and then cooled to room temperature
and treated NH3 flow (5% NH3 in He). The desorption of NH3 were
◦
measured heating the sample from room temperature to 700 C
◦
and at heating rate of 10 C/min in He flow (20 ml/min). The water