Catalysts with cerium in a membrane reactor for the removal of formaldehyde pollutant from water effluents

Article abstract of DOI:10.3390/molecules21060668

We report the synthesis of cerium oxide, cobalt oxide, mixed cerium, and cobalt oxides and a Ce-Co/Al₂O₃ membrane, which are employed as catalysts for the catalytic wet oxidation (CWO) reaction process and the removal of formaldehyde from industrial effluents. Formaldehyde is present in numerous waste streams from the chemical industry in a concentration low enough to make its recovery not economically justified but high enough to create an environmental hazard. Common biological degradation methods do not work for formaldehyde, a highly toxic but refractory, low biodegradability substance. The CWO reaction is a recent, promising alternative that also permits much lower temperature and pressure conditions than other oxidation processes, resulting in economic benefits. The CWO reaction employing Ce- and Co-containing catalysts was carried out inside a slurry batch reactor and a membrane reactor. Experimental results are reported. Next, a mixed Ce-Co oxide film was supported on an γ-alumina membrane used in a catalytic membrane reactor to compare formaldehyde removal between both types of systems. Catalytic materials with cerium and with a relatively large amount of cerium favored the transformation of formaldehyde. Cerium was present as cerianite in the catalytic materials, as indicated by X-ray diffraction patterns.

Full text of DOI:10.3390/molecules21060668

molecules
Article
Catalysts with Cerium in a Membrane Reactor for the
Removal of Formaldehyde Pollutant from
Water Effluents
Mirella Gutiérrez-Arzaluz *, Luis Noreña-Franco, Saúl Ángel-Cuevas, Violeta Mugica-Álvarez
and Miguel Torres-Rodríguez
Area of Applied Chemistry, Universidad Autónoma Metropolitana, Azcapotzalco, México City 02200, Mexico;
lnf@correo.azc.uam.mx (L.N.-F.); saul_cuevas00@hotmail.com (S.Á.-C.); vma@correo.azc.uam.mx (V.M.-Á.);
trm@correo.azc.uam.mx (M.T.-R.)
*
Correspondence: gam@correo.azc.uam.mx; Tel.: +52-55-5318-9570
Academic Editor: Raffaele Molinari
Received: 18 March 2016; Accepted: 13 May 2016; Published: 24 May 2016
Abstract: We report the synthesis of cerium oxide, cobalt oxide, mixed cerium, and cobalt oxides
and a Ce–Co/Al O membrane, which are employed as catalysts for the catalytic wet oxidation
2
3
(
CWO) reaction process and the removal of formaldehyde from industrial effluents. Formaldehyde
is present in numerous waste streams from the chemical industry in a concentration low enough
to make its recovery not economically justified but high enough to create an environmental hazard.
Common biological degradation methods do not work for formaldehyde, a highly toxic but refractory,
low biodegradability substance. The CWO reaction is a recent, promising alternative that also
permits much lower temperature and pressure conditions than other oxidation processes, resulting
in economic benefits. The CWO reaction employing Ce- and Co-containing catalysts was carried
out inside a slurry batch reactor and a membrane reactor. Experimental results are reported. Next,
a mixed Ce–Co oxide film was supported on an γ-alumina membrane used in a catalytic membrane
reactor to compare formaldehyde removal between both types of systems. Catalytic materials with
cerium and with a relatively large amount of cerium favored the transformation of formaldehyde.
Cerium was present as cerianite in the catalytic materials, as indicated by X-ray diffraction patterns.
Keywords: catalytic wet oxidation; formaldehyde; cerium oxide; cobalt oxide
1
. Introduction
The catalytic wet oxidation processes fall within the three-phase reaction categories, which are
somewhat difficult to operate and are therefore not in a state of technological readiness for commercial
application. Thus, it is widely recognized that the technology of catalytic wet air oxidation (CWAO)
needs further development to become a commercial option, economically viable, environmentally
responsible, and a way to remove the organic waste generated and increase the possibility of managing
emerging problems in the treatment of toxic wastewater.
In wet air oxidation (WAO) processes, organic pollutants dissolved in water are degraded, either
partially through an oxidizing agent in biodegradable products or mineralized into harmless inorganic
compounds, such as CO , H O, and inorganic salts, which remain in the liquid phase [1].
2
2
Compared to the WAO process, the presence of a catalyst in CWAO has lower energy
requirements [ ]; in addition, it has the advantage of reaching a greater oxidation conversion.
Consequently, less severe conditions can be used to reduce the chemical oxygen demand because in
this process, organic compounds are oxidized to inorganic usable by-products, such as CO and H O.
2
2
2
Wet air oxidation [3] represents an alternative technology to treat water streams with a low
concentration of toxic organic compounds. However, the absence of a catalyst implies that high
Molecules 2016, 21, 668; doi:10.3390/molecules21060668
www.mdpi.com/journal/molecules

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temperatures and oxygen pressures are required [
4
]. However, the use of catalysts could diminish
temperature and air pressure requirements, resulting in a more efficient process of abating organic
agents. There is considerable potential for this catalytic wet air oxidation process to ultimately destroy
organic pollutants in industrial effluents [1].
Nevertheless, despite this great effort in catalyst formulations, several problems can remain in
this process [
process that is easily limited by the transfer of the gaseous reactant. Relatively few innovations have
been published concerning the CWAO of organic agents in trickle bed reactors [ ] to improve these
1], for example, obtaining efficient contact among gas, liquid, and solid phases in a
5–7
gas/liquid/solid contact performances. Moreover, in reactors with high liquid-to-catalyst volumetric
ratios (such as slurry and bubble column fixed-bed reactors), carbonaceous deposition on the surface
of catalyst particles is enhanced because oxidative coupling reactions are favored in the bulk liquid
phase [8].
One method of improving the gas/liquid/solid contact could be to use contactor type catalytic
membrane reactors (CMR) [9–11] as interfacial contactors with an organic compound solution and air
(
or oxygen) separately fed from both sides of the catalytic membrane. The gas overpressure can shift
the gas-liquid interface location into the membrane wall, which is closer to the catalytic zone, thus
realizing several advantages: the oxygen concentration in contact with the catalyst layer is maximized,
the desorption of pollutants into the gaseous phase is favored, the catalyst exposure to leaching is
reduced, substantial flexibility for operative conditions is available and scale-up issues are facilitated.
Conversely, the proper location of the gas-liquid interphase must be ensured to take advantage of the
previously cited benefits.
Formaldehyde (HCHO) is an important environmental pollutant because of its adverse health
effects [5] and has been chosen as a model pollutant for the wet catalytic oxidation reaction.
Among the several metal oxides with catalytic properties, we selected CeO because of its high oxygen
2
storage/transport capacity (OSC) and particular redox properties [12]. The incorporation of other
oxides, such as cobalt oxide, into the ceria lattice enhances redox properties, providing high thermal
stability. The Ce–Co mixed oxides are more resistant to poisoning phenomena and are generally
less active than supported precious metals for oxidizing VOC streams [13]. Considerable research
efforts have focused on catalyst preparation, thus obtaining Ce–Co mixed oxides with a large surface
area. Among the synthesis methods, the co-precipitation procedure from the corresponding salts has
found a recurrent application attributable to the high oxygen storage capacity achieved by the final
material [14
,15]. The addition of an appropriate amount of CeO could increase the surface area of
2
2+
3+
Co O and improve the redox ability [16] of Co /Co . This work was intended to evaluate materials’
3
4
catalytic properties based on Ce and Co oxides in two arrangements: (1) in bulk; and (2) supported
on a mesoporous alumina membrane. Conventional bulk catalysts have pores with a single entrance,
whereas a porous catalytic membranes have interconnected pores with open ends towards two faces.
The catalytic evaluation was performed both in a slurry-type batch reactor (the bulk catalyst) and in a
membrane reactor (the supported catalyst).
2
. Results and Discussion
2
.1. Bulk Catalyst Characterization
The X-ray diffraction patterns of the mixed oxides Ce–Co with 1:1, 2:1, and 1:2 molar ratios are
presented in Figure 1, along with all of the samples exhibiting the cerianite structure (CeO ) with
2
characteristic peaks at the (111), (200), (220), (311), (222), and (400) planes [17
exhibit corresponding peaks for the cobalt oxide [19,20].
,18]; these patterns do not
Apparently, the effect of cobalt oxide is to reduce the intensity of the peaks, probably either
because of the comparatively small size of the cobalt oxide particles or because some of the cobalt
oxide is incorporated into the cerianite framework [17,18]. The diffraction pattern was not affected by

Molecules 2016, 21, 668
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the relative molar ratio, and the same structure was observed for all three Ce–Co oxides molar ratios
1:1, 1:2, and 2:1).
(
Cerianite Pattern
Pure oxide Ce
fluorita JCDS 34-0394
Ce-Co 2:1
Ce-Co 1:2
Ce-Co 1:1
Ce
26
28
30
-theta
32
34
2
Ce-Co 2:1
Ce-Co 1:2
Ce-Co 1:1
10
20
30
40
-theta
50
60
70
80
2
Figure 1. XRD patterns of the mixed Ce–Co oxides: 1:1, 1:2, and 2:1.
Employing Scherrer´s equation, we discovered that the particle size changed after the
incorporation of the cobalt oxide into the cerium oxide. The pure cerium oxide had a 372 Å particle
size, whereas the particle size of the 1:1 Ce–Co mixed oxide was 365.2 Å. For the 2:1 Ce–Co mixed
oxide, the particle size has increased slightly to 378.5 Å, and for the 1:2 Ce–Co mixed oxide, the particle
˝
size increased to 850 Å. Furthermore, taking the (111) diffraction peak at 28.5 of the XRD pattern of the
JCDS 34-0394 fluorite as reference and considering the XRD pattern of pure cerium oxide, we observed
˝
a slight displacement of the peak position (at 28.8 ) that corresponded to a parameter lattice of 5.37 Å
˝
for the 1:1 Ce–Co mixed oxide; this displacement was further increased (29 ) with a parameter lattice
of 5.33 Å for the 1:2 Ce–Co sample. The 2:1 Ce–Co sample did not exhibit a diffraction peak shift, had
a larger crystal size, and had a parameter lattice of 5.41 Å, which is the same as that of pure oxide.
Figure 2a shows the FTIR spectra of the cerium and cobalt pure oxides. We observed the
´
1
characteristic absorption band of cobalt at approximately 500 cm , which is associated with the
vibrational modes of the Co O spinel phase [21]. Figure 2b shows the FTIR spectra of the Ce and
3
4
´
1
Co mixed oxides, which exhibit small absorption bands at approximately 1316 and 1492 cm that
correspond to inorganic carboxylates (Ce–CoO) [20 22]. We should note that in the FTIR spectra of
,
the three different molar ratios of the Ce and Co mixed oxides, there was no evidence of the cobalt
spinel phase.
Figures 3–5 show the micrographs of the cerium and cobalt mixed oxides, and we can observe that
by varying the Ce/Co molar ratio (1:1, 1:2, and 2:1, respectively), larger and more irregularly shaped
particles were obtained. Compared to the 1:1 Ce–Co sample (Figure 3), the sample with the high
cerium content had larger and sharper particles (Figure 4). The sample with the high cobalt content
(
Figure 5) had a sort of sponge morphology formed by an inter-crossing laminate. Therefore, we can
infer that increasing the Co content promotes both a change in morphology and larger particle clusters.
SEM images correlate to XRD results, showing that higher Co content increases the particle size.

Molecules 2016, 21, 668
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100
Ce
Co
100
Ce-Co 1:1
Ce-Co 2:1
8
6
4
2
0
0
0
8
6
4
0
0
0
Ce-Co 1:2
0
4
20
4000
000
3500
3000
2500
2000
1500
1000
500
3500
3000
2500
2000
1500
1000
500
−
1
−1
Wavenumber (cm )
Wavenumber (cm )
(
a)
(b)
Figure 2. FTIR spectra of the (a) pure oxides: Ce and Co; (b) mixed oxides: Ce–Co 1:1, Ce–Co 1:2,
Ce–Co 2:1.
Figure 3. SEM micrograph of mixed oxide: Ce–Co 1:1.
Figure 4. SEM micrograph of mixed oxide: Ce–Co 2:1.

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Figure 5. SEM micrograph of mixed oxide: Ce–Co 1:2.
2
.2. Catalytic Membrane Characterization
We will describe the XRD results of the Ce–Co/Al O membrane both as a reference and for
2
3
comparison with the membrane that was used for the HCOH CWO reaction. Figure 6 shows the XRD
pattern, presenting the Al O characteristic peaks of the corundum phase with a rhombohedral crystal
2
3
cell [23]. The alumina peaks were slightly shifted towards smaller diffraction angles. We could not
see either the cobalt oxide or the cerium oxide XRD signals, indicating that the mixed oxide film is
relatively thin and that when using conventional 2
film generally produces a weak signal from the film and an intense signal from the substrate, in this
case alpha alumina [24 25]. In addition, the diffraction peaks of alpha alumina support were slightly
θ scanning methods for supported film, very thin
,
displaced towards lower angles relative to the pattern; we associate this displacement with the stiffness
and geometry of the support.
Ce-Co/Al O Membrane
2
3
Al O pattern
2
3
Ce pattern
1
0
20
30
40
50
60
2
-Theta
Figure 6. XRD pattern of the Ce–Co/Al O membrane.
2
3
Figure 7 shows the Ce–Co/Al O membrane micrographs. We present a representative section of
2
3
the inner side surface (Figure 7a), noticing a relatively uniform surface with no fractures or pinhole.

Molecules 2016, 21, 668
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Figure 7b shows the cross-section image with three of the four layers of the alumina support and the
top Co-Ce film, featuring a considerably uniform thickness of approximately 1 µm.
(
a)
(b)
Figure 7. SEM micrographs of Ce–Co/Al O membrane, (
a) surface morphology, and (b) cross-section.
2
3
Figure 8 describes the elemental analysis obtained with the SEM-EDS technique, verifying the
presence of cobalt and cerium in the film with the alumina support. We also notice a higher percentage
cobalt content with respect to cerium content. The elemental analysis indicates a high level of oxygen
and aluminium belonging to the alumina support. Other elements are impurities.
Figure 8. SEM/EDS spectra of the Ce–Co/Al O membrane.
2
3
Considering the SEM results, the Co and Ce mixed oxides impregnation method was effective for
the growth of a uniform film of homogeneous thickness. For this method, sequential 2 M precursor
solutions for each oxide were used, beginning with cobalt and followed by cerium, and a precipitation
step was included for each oxide. A 15-cm-long catalytic membrane for the HCOH CWO reaction was
prepared in this manner.
2
.3. Catalytic Wet Oxidation Reaction
2
.3.1. Commercial Autoclave Slurry Batch-Type Reactor
Figure 9 shows the conversion time compared to the reaction time of the HCOH CWO reaction
that employed three different Ce–Co powder mixed oxides with 1:1, 2:1, and 1:2 Ce–Co molar ratios.

Molecules 2016, 21, 668
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Conversion maintains an ascending linear trend from the first minute of the reaction through 90 min;
afterwards, for the 1:1 and 1:2 Ce–Co samples, conversion becomes relatively constant, whereas the
2:1 Ce–Co sample continues to show an increase in conversion, achieving an approximately 35%
HCOH conversion. Overall, the conversion obtained was smaller than 40%, but we should consider
the relatively high initial formaldehyde content (1000 ppm). The oxidation of HCOH leads to various
secondary products, such as oxalic acid, formic acid, and acetic acid, depending on the degree of
oxidation; moreover, we should note that no secondary products were detected by gas chromatography
of the liquid stream. Other authors have reported the production of low molecular weight carboxylic
acids and, more importantly, formic acid, a highly refractive compound, when carrying out the
HCOH CWO reaction [26
,27]. In previous work [28], which was also during formaldehyde removal,
the presence of carboxylic acids as secondary reaction products using a Pt/Al O powder catalyst
2
3
was observed.
5
4
3
2
1
0
0
0
0
0
0
Ce-Co 2:1
Ce-Co 1:1
Ce-Co 1:2
5
0
100
150
200
250
Time (min)
Figure 9. CWO of HCOH with mixed oxides: Ce–Co 1:1, Ce–Co 1:2, Ce–Co 2:1.
According to the HCOH conversion results, the 2:1 Ce–Co sample exhibited higher activity
than the 1:2 Ce–Co sample, indicating that a higher cerium content promotes HCOH oxidation
because of the high capacity of Ce for oxygen storage, as reported in the literature [29].
´
3
´1
(L min) initial reaction rate for the 1:1 Ce–Co catalyst and a
We measured a 5.75
ˆ
10 mol
¨
¨
´
3
´1
6.08
ˆ
10 mol
¨
(L
¨
min) initial reaction speed for the 2:1 Ce–Co catalyst, whereas the 1:2 Ce–Co
catalyst had an initial reaction speed of 4.00
low as the sample with high cerium content. The catalyst with the higher Ce content, the 2:1 Ce–Co
exhibiting the higher catalytic activity, had a small particle size.
A similar HCOH conversion of 38.1%, after a 3 h reaction at 200 C and 15 bar of oxygen pressure
with a prolonged induction step, was reported by Silva et al. [ 26] for the oxidation reaction with no
´
3
´1
(L min) , which is approximately twice as
ˆ
10 mol
¨
¨
˝
5,
catalyst (WO). Our conversion rate was likely affected by the high HCOH concentration (1000 ppm),
considering that Tang et al. [30] report a reduction in conversion when the HCOH initial concentration
increased from 100 to 580 ppm.
2
.3.2. Catalytic Membrane Reactor, Contactor type
Figure 10 shows the catalytic evaluation of the Ce–Co mixed oxide/alumina membrane reactor
after 240 min reaction, achieving a 79% total conversion. At the beginning of the reaction, conversion
followed an increasing trend, with an almost linear behavior for the first 60 min. The measured initial
´
3
´1
(L min) . Subsequently, conversion increased at a slower pace.
reaction speed was 16.5
ˆ
10 mol
¨
¨

Molecules 2016, 21, 668
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As with powder catalysts, we found no secondary products in the liquid phase (as determined by gas
chromatography), and the reaction products were those corresponding to HCOH mineralization.
Figure 10 shows the conversion rate for the two catalytic materials: the bulk Ce–Co mixed oxide
(
(
employing a slurry batch reactor) and the Ce–Co mixed oxide supported on mesoporous alumina
as part of a membrane reactor). We attempted to maintain the same reaction conditions: 100 C
˝
temperature, 250 mL reaction volume, 1000 ppm initial HCOH concentration, and 5 bar oxygen
´
min O for the membrane reactor
2
1
pressure. However, we used a continuous gas phase feed of 40 mL
¨
but a discontinuous one for the slurry batch reactor; the catalyst mass to the reaction volume ratio
´
1
´1
L for the
was also not the same, at 1 g
¨
L
for the conventional slurry batch reactor and 0.10 g
¨
membrane reactor, similar to previous work [11]. The high conversion obtained with the catalytic
membrane reactor can be explained by the efficient contact between the liquid phase and the gas
phase, specifically at the membrane’s active surface, via a pressure differential between the two phases.
The contact between the liquid and gas phases occurs at a 1
µm-thick section, enabling very small mass
transference limitations. Such mass transfer limitations are considerable in a slurry type reactor [1].
1
00
Catalytic Membrane
Bulk Catalyst
Ce-Co/Al O
2
3
80
60
40
20
0
Ce-Co 2:1
0
50
100
150
200
250
Time (min)
Figure 10. CWO of HCOH, comparison of catalytic membrane (Ce–Co/Al O ) and bulk catalyst
2
3
(Ce–Co 2:1).
The mineralization of HCOH was corroborated for the two types of reactors after the CWO
reaction with the FTIR analysis of the CaCO precipitate. The band’s characteristic of calcite
3
´
1
transmittance was identified at 713 and 877 cm as vibrations of C-O in plane and out of plane,
´
1
respectively [31,32]; the band at approximately 1400 cm corresponded to the bending C=O [33].
Furthermore, the measured pH of the formaldehyde solution before the CWO reaction was 6.3,
and after reaction with the mixed oxide catalyst powder the first two reactions (Ce–Co 1: 1 and Ce–Co
1
5
: 2), the pH slightly decreased (to approximately 5.4). However, reactions with Ce–Co 2:1 had a pH of
.0, and those with the membrane showed an important decrease to a pH of 4.5. The decreased pH
in the membrane case probably occurred because of the dissolution of the CO obtained by HCOH
2
mineralization, which was dissolved in the solution, resulting in a more acidic pH.
3
. Discussion
Even if the amount of catalyst employed in the membrane reactor was approximately 10 times
lower than that of the slurry batch reactor, the reaction speed (and thus the reaction kinetics) obtained
with the membrane reactor was approximately three times faster (with the 2:1 Ce–Co catalyst).

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The conversion curve trend for the membrane reactor shows that the contactor-type configuration
exhibits a higher conversion compared to the conversion slurry batch reactor throughout the reaction
time; the higher slope registered for CMR suggests a greater local concentration of active oxygen in
the membrane film, showing similar reaction times that produce an increase in the highest reaction
rate of oxidation by increasing local oxygen concentration. As mentioned in Section 2.3.1, no side
products were identified for CMR compared to secondary products in the batch reactor, suggesting
higher selectivity for CMR towards mineralization products.
The results can be related to several factors, including a higher local oxygen concentration
inside the pores and a better distribution of the active sites. We should also consider the liquid-solid
volumetric ratio, which is also related to the formation of carbonous deposits [1,34]. Slurry-type batch
reactors have a high liquid-catalyst volumetric ratio that favors the formation of polymer species in
the liquid phase through either addition reactions or condensation reactions of intermediate species.
With small liquid-catalyst volumetric ratios (such as in trickle-bed reactors and membrane reactors),
the liquid phase is specifically in contact with the solid active layer and therefore propagation reactions
in the liquid phase are suppressed, producing an almost quantitative transformation of formaldehyde
into CO ; fact that is corroborated by the formation of calcium carbonate, formed by the capture of
2
CO and pH more acidic.
2
The advantages of the membrane reactor arise out of the gas-liquid contact in the solid catalytic
thin mesoporous layer formed by a nanometre-size open-pore system, whereas the bulk solid
catalyst is formed mainly by a closed-pore system that is much more prone to blocking by deposits.
In summary, the highest conversion of formaldehyde (Figure 10) shows that the membrane reactor-type
contactor arises out of the gas-liquid contact at the solid catalytic thin mesoporous layer formed by a
nanometre-size open-pore system. The membrane reactor has the additional advantage of not needing
the separation of the reaction products from the catalyst after the reaction, as required by conventional
batch reactors.
4
. Materials and Methods
The preparation of pure Ce and Co oxides and Ce–Co mixed oxide-based catalysts was carried
out by the co-precipitation method through controlled additions of the precursor solution containing
˝
an excess basic agent (CO(NH ) ) followed by a calcination stage at 350 C, as reported in previous
2
2
works [35]. Briefly, to obtain pure Ce and Co oxides and Ce–Co mixed oxides, 2 M solutions of
Co(NO ) and Ce(NO ) were employed as precursors, as provided by Sigma-Aldrich Química,
3
2
3 3
S.L. (Toluca, Mexico). Urea was used as precipitating agent. Ce–Co mixed oxides were prepared
with three different molar ratios: 1:1, 2:1, and 1:2. After precipitation, all of the catalytic materials
˝
were washed with deionized water, dried for 16 h at ambient temperature and calcined at 350 C for
˝
´1
2
h under an N atmosphere at a heating rate of 3 C min . According to Chen and Isa [36], urea
¨
2
˝
decomposes into volatile by-products at 190 C.
We will also describe the preparation steps of the inorganic Ce–Co/
films were prepared on the inner surface of an alumina tubular support (Pall Exekia, Bazet, France)
with 7 and 10 mm i.d. and o.d., respectively, with an average pore size of 5 nm and a thickness of 10
on the top layer ( -Al O ); the remainder of the support is composed of alpha alumina. The length of
γ
-Al O membrane. Ce–Co
2 3
µm
γ
2
3
the permeation section was 120 mm, and both ends of the tubular support were coated with a glass
sealant. The alumina supports were first cleaned ultrasonically in acetone and rinsed in deionized
˝
γ-alumina support was heated at 110 C for 24 h and covered with
water. Prior to film growth, the
Teflon tape (the outer side) to promote film growth only on the inner side. The commercial support
˝
was impregnated with a 2 M Co(NO) 6H O precursor dissolution for 10 min at 95 C, then with
3
2
˝
a 4 M urea solution for 1 h at 95 C, then with a 2 M Ce(NO) 6H O precursor solution for 10 min
3
2
˝
˝
at 95 C and finally with a 4 M urea solution for 1 h at 95 C. Next, the tube was washed using
˝
˝
deionized water and dried for 12 h at 120 C; further calcinations were performed at 350 C for 3 h in a
laboratory-air atmosphere.

Molecules 2016, 21, 668
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The samples were characterized using various techniques such as powder X-ray diffraction (XRD),
on a Philips X-pert diffractometer (Nottingham, UK) using a Cu K radiation source with a goniometer
α
˝
˝
of a 2-theta angle range between 4 and 80 operating at 40 kV and 25 mA. Elemental analysis and
surface topographic features were observed through scanning electron microscopy (SEM/EDS) (FEI
Company, Eindhoven, The Netherlands) employing a Philips XL30 ESEM microscope allowing both
high vacuum and ambient operating conditions and a LEO 440 microscope with secondary electrons
and EDS detectors for Fourier-transform infrared spectroscopy (FTIR) (Varian Inc., Palo Alto, CA, USA)
´
1
with a Varian Excalibur 3600 instrument having a 4000 to 400 cm wavenumber range for which the
samples were finely ground with an agate mortar.
The performance of the catalytic systems was studied during the catalytic wet formaldehyde
oxidation (CWO) reaction at ppm concentrations. The trials were conducted inside a conventional
stainless steel “batch” reactor (commercial autoclave Parr Reactor, Parr Instrument Company, Moline,
IL, USA) for the liquid phase, using pure oxygen (99.6% purity, Praxair México, Mexico City, Mexico)
˝
as an oxidizing agent at a 100 C reaction temperature. The initial formaldehyde concentration was
´
1
1
000 ppm. The ratio of catalyst mass to reaction volume was 1 g
¨
L
; testing was conducted at
oxidizing agent pressure, namely, 5 bar. The catalytic HCHO oxidation test was studied for 4 h.
The reaction progress was monitored by sampling the liquid phase at different times and analyzed by
gas chromatography using an HP 5890 series II brand Plus chromatograph with a flame ionization
©
detector (FID) and GC ChemStation software (Agilent Technologies, Inc., Santa Clara, CA, USA),
along with an HP brand PLOT Q capillary column, 30 m long and 0.32 mm in diameter, suitable for
separating organic acids and formaldehyde of low molecular weight.
The CWO reaction with a membrane reactor has been described in our previous work [11].
The membrane reactor was composed of a stainless steel cylinder jacket containing the catalytic
membrane. This setup was divided into two chambers; the outer one was used to feed the oxygen and
the inner chamber was used to feed the liquid phase with formaldehyde. The reactor configuration
used is known as a contactor. The typical conditions for catalytic tests were as follows: 5 bar oxygen
pressure and a 0.5 bar differential pressure between the gas and liquid phases. The membrane reactor
results presented here have been obtained by working discontinuously during the liquid phase fed to
´
1
the internal side in which the catalytic layer is located (250 mL recirculated at 20 mL
¨
min by a HPLC
min ). The gas phase was bubbled into a
Ca(OH) 0.5 M solution throughout the reaction time; the qualitative formation of CO was determined
´
1
pump) and continuously for the gas phase (40 mL O2
¨
2
2
at the end time, and the formation of a precipitate was found. This precipitate was characterized by
FTIR with an accessory of ATR (attenuated total reflectance).
5
. Conclusions
5
.1. Catalysts
According to the XRD results, the pure cerium oxide and the mixed cobalt-cerium oxides exhibited
the characteristic cerianite pattern, with a FCC unit cell. The pure cobalt oxide exhibited a structure
corresponding to the spinel phase.
A film of cobalt-cerium mixed oxides was deposited onto a
According to SEM images, we attained a uniform, homogeneous film which was crack-free and
had approximately 1 m thickness. EDS elemental analysis confirmed the cobalt and cerium content
on the Al O support, without cluster formation.
γ-Al O commercial support.
2 3
µ
2
3
5
.2. Formaldehyde CWO
Regarding the cobalt and cerium oxide bulk catalysts, the highest conversion for the HCOH
CWO reaction was obtained with the 2:1 Ce–Co material. In general terms, a relatively high cerium
content enhances the reaction conversion. No secondary products were detected in the liquid phase
and therefore, all conversion was oriented towards the mineralization products (CO and H O).
2
2

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The highest conversion (of more than 70%) for the CWO reaction of HCOH was obtained with a
catalytic membrane of the Co-Ce oxides deposited on the alumina support after a 240 min reaction at
˝
00 C and 5 bar O . Therefore, the membrane reactor was more efficient than the conventional batch
2
1
reactor, where the catalyst is in suspension form.
We found that for the membrane reactor-type contactor in which the liquid-catalyst volume ratio
is low, propagation reactions or secondary reactions in the liquid phase are eliminated, which results
in a nearly quantitative conversion of formaldehyde to CO2.
Coupling the catalytic wet oxidation with the membrane reactor is a promising alternative
for the efficient removal of formaldehyde pollutant from industrial residual water, such as in
“
tailor-made” procedures.
Acknowledgments: Mirella Gutiérrez gratefully acknowledges the CONACyT’s (Mexico) Basic Science project
for its financial support of this research.
Author Contributions: Mirella Gutiérrez-Arzaluz, Violeta Mugica-Álvarez, and Miguel Torres-Rodríguez
conceived of and designed the experiments and analyzed the data, Saúl Ángel-Cuevas performed the experiments,
and Luis Noreña-Franco wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations
The following abbreviations are used in this manuscript:
CWO
CWAO
WAO
CMR
OSC
Catalytic Wet Oxidation
Catalytic Wet Air Oxidation
Wet Air Oxidation
Catalytic Membrane Reactors
Oxygen Storage/Transport Capacity
X-ray Diffraction
XRD
SEM/EDS
FTIR
Scanning Electron Microscopy/Energy Dispersive Spectroscopy
Fourier-Transform Infrared Spectroscopy
Gas Chromatography
GC
FID
Flame Ionization Detector
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Sample Availability: Membrane samples are not available and there are powder sample availability.
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2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access
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CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
(