Research paper
Porous materials obtained by acid treatment processing followed by
pillaring of montmorillonite clays
Lindiane Bieseki
a, Helen Treichel
b, Antonio S. Araujo
c, Sibele Berenice Castellã Pergher
a,⁎
aProgram in Science and Materials Engineering, Federal University, Rio Grande do Norte, Laboratory of Molecular Sieves— LABPEMOL, 59078-970, RN, Brazil bFederal University of Fronteira Sul, 99700-000 Erechim, RS, Brazil
c
Institute of Chemistry, Federal University of Rio Grande do Norte, 59078-970 Natal, RN, Brazil
a b s t r a c t
a r t i c l e i n f o
Article history:
Received 30 November 2012
Received in revised form 27 August 2013 Accepted 30 August 2013 Available online xxxx Keywords: Montmorillonite clay Acid treatment Pillaring
Montmorillonite clay was treated with hydrochloric acid and subsequently pillared with aluminum polyoxocations. The acid-treated samples were evaluated for the removal of structural elements (Al, Fe and Mg) and for conservation of the lamellar organization. The more severe the treatment, the greater the specific area obtained. Despite the loss of structural organization, all samples were pillared, with a displacement of peak (001) to 2θ lower angles. The pillaring of all acid-treated samples promoted an increase in the specific area of these materials. Pillared samples previously treated at 50 °C with HCl concentrations of 2 and 4 mol L−1underwent an increase in their specific areas of approximately 45%. Pillared samples treated with 4 mol L−1of HCl at 80 °C underwent the lowest percent increase in surface area, approximately 10%, compared to the untreated samples. All samples possessed greater uniformity in pore size. Acid treatment prior to pillaring promoted an increase in the number of acid sites of moderate strength compared to pillared natural clay.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Acid activation in clay minerals promotes an increase in the catalytic activity of isomerization reactions (Flessner et al., 2001; Rodrigues et al., 2006; Silva and Garla, 1999; Volzone et al., 2001) and benzene alkyl-ation (Sabu et al., 1999).Pushpaletha et al. (2005)compared the cata-lytic activity of clays treated with sulfuric acid and hydrochloric acid and observed that sulfuric acid was more effective for acid activation.
Acid treatment in commercial-type montmorillonite K10 and KSF is performed using H2SO4, and for KSF, the treatment is milder at room
temperature. In a study comparing these clays (Cseri et al., 1995), it was observed by pyridine adsorption analysis that KSF clay possessed higher amount of Brönsted acid sites and a lower number of Lewis sites compared to K10 clay. In addition to an increase in the acidity of materials as determined by the presence of Lewis and Brönsted acidic sites (Adams and McCabe, 2006), following acid treatment, there is also an increase in porosity, which favors reactions for catalytic applica-tions. One of the most common applications of acid-treated clays is the bleaching of oils (Christidis et al., 1997). For this reason, studies related to the acid activation of clay minerals usually involve the preparation of adsorbents from natural clay minerals produced in different regions to bleach oil (Nguetnkam et al., 2011; Noyan et al., 2007).
When acid treatment is applied to clay mineral samples, different cations, such as iron, calcium, magnesium and sodium, are removed depending on the treatment, and in some cases, the structure may be destroyed by excessive removal of Al3+(Christidis et al., 1997; Kumar et al., 1995; Rodrigues et al., 2006; Woumfo et al., 2007). Two other characteristics of clays influenced by acid treatment are the swelling capacity and the cation exchange capacity (CEC).Önal (2007)observed that the CEC decreases with increasing H2SO4concentration used for
activation and that the swelling capacity decreases up to 20% compared with that of untreated clay mineral.
Another process that increases the porosity, accessibility, and acidity is the pillaring of clay, which consists of the separation of layers by the insertion of inorganic compounds (Keggin ions) that act as pillars. The insertion of pillars increases the specific surface area of these materials in addition to an observed increase in Lewis acidity (Pergher et al., 2005). Pillared materials are thermally stable and possess micro- and/ or mesoporosity (Schoonheydt et al., 1999). These materials can be used in adsorption processes, including the application in adsorption fungicides such as thiabendazole (RocaJalil et al., 2013) and the removal of dyes such as tartrazine azo dye from water using the catalytic action of Al–Fe-pillared clays (Banković et al., 2012); they can also be used as a support for catalysts, as observed byRomero-Pérez et al. (2012) in studies involving sulfided ruthenium supported on pillared clays in hydrodesulfurization (HDS) reactions.
This study performed the pillaring of montmorillonite clays treated with hydrochloric acid at concentrations of 2, 3 and 4 mol L−1and at temperatures of 50, 65 and 80 °C to determine the effect of these treat-ments on the porosity and acidity of the materials obtained. Acid
⁎ Corresponding author.
E-mail addresses:lindiane.bieseki@gmail.com(L. Bieseki),helentreichel@gmail.com
(H. Treichel),araujo.ufrn@gmail.com(A.S. Araujo),sibelepergher@gmail.com
(S.B.C. Pergher).
0169-1317/$– see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.clay.2013.08.044
Contents lists available atScienceDirect
Applied Clay Science
treatment was carried out according to a 22statistical design with three
center points. The pillaring of materials was performed by ex-situ meth-odology with aluminum polyoxocations. This study presents the results from N2adsorption analysis, X-ray diffractometry, infrared spectroscopy
and acidity by n-butylamine desorption followed by thermogravimetry. 2. Experimental
2.1. Materials
The clay mineral used in this work was calcium-montmorillonite which was provided by Colorminas Colorificio — SA. This clay mineral was prepared as follows: it was dried at 90 °C, milled and passed through a sieve of mesh 45. This sample contains quartz as an impurity. Its cation exchange capacity (CEC) is 155 meq/100 g, which was measured by saturating the clay with a solution of sodium acetate and subsequently displacingfive times with ammonium acetate.
2.2. Acid treatment
The clay minerals used in this study were treated with hydrochloric acid using a 22factorial statistical design with three central points. This plan was developed using the parameters given inTable 1. The factor levels used were +1 (high), 0 (midpoint) and−1 (low). The clay mineral:acid ratio was 1 g to 30 mL of solution. All treated samples were washed until pH 5 was reached and were dried at 100 °C over-night. From the natural clay mineral, seven acid-treated clay mineral samples were obtained. Three samples were produced by repeating the center point.
2.3. Pillaring of treated clays
Thefive acid-treated clay mineral samples were pillared according to the following procedure: 1.5 g of treated clay was dispersed in 150 mL of H2O with stirring for 2 h. Then, 300 mL of pillaring agent
was added with 16.6 meq Al/g. The pillaring agent was prepared by the slow addition of 200 mL of 0.25 mol L−1 sodium hydroxide to 100 mL of 0.25 mol L−1aluminum chloride and was aged for 6 days. The clay mineral/pillaring agent mixture was stirred for 2 h, after which it wasfiltered and dried at 100 °C for 12 h.
2.4. Calcination of pillared samples
The samples were calcined at 450 °C for 3 h using the following protocol: heating from room temperature to 150 °C at a heating rate of 5 °C min−1for 30 min, heating from 150 °C to 450 °C at a heating rate of 5 °C min−1, and maintaining a temperature of 450 °C for 3 h. The treated samples were labeled PxMy, where P indicates that the samples were pillared, x is the acid concentration used in acid treatment and y is the temperature used for each treatment. For example, the sam-ple treated with 2 mol L−1of HCl at 50 °C was labeled 2M— 50, and after pillaring, it was labeled P2M— 50; after calcination, it was labeled P2M— 50C.
2.5. Acidity measurement
Acidity measurements were performed by adsorption of n-butylamine vapor, followed by desorption monitored in a thermobalance. Desorption was performed in a thermogravimetric analyzer from TA Instrument model TGA-50H Shimadzu under nitrogen flowing at 25 mL min−1 and a heating ramp of 10 °C min−1 to determine the amount of base desorbed at each temperature range. Prior to adsorption, the calcined samples were submitted to activation at 400 °C for 2 h under an atmosphere of N2. Then, the temperature was lowered to 95 °C, and
the vapors of n-butylamine were passed through the sample for 1 h. Finally, the samples were again submitted to N2flow at 95 °C, in order
to remove physically adsorbed n-butylamine. The number of molecules of n-butylamine adsorbed at each acid site was calculated from Eq.(1), and the number of acid sites per mass of sample was calculated from Eq.(2).
nðn‐butylamineÞ¼ mðn‐butylamineÞ=MMðn‐butylamineÞ ð1Þ
N¼ nðn‐butylamineÞ=m catalystð Þ ð2Þ
where:
n(n-butylamine) number of n-butylamine molecules;
m(n-butylamine) mass of n-butylamine adsorbed;
MM(n-butylamine) molecular mass of n-butylamine (73.14 g mol−1);
N number of acidic sites;
m(catalyst) mass of the n-butylamine-free catalyst.
2.6. Characterization of materials
X-Ray diffraction analysis was performed in a Diffraktometer — Model D5000 (Siemens) using a nickelfilter, Cu-Kα radiation (λ = 1.54 Å), a scanning interval of 2θ = 2° to 65° and a scanning step size of 0.02°. For atomic absorption analysis, an atomic absorption spectrom-eter Varian— AA model was used. The samples were analyzed by infra-red spectroscopy (DRIFTS) in FT-IR BRUKER alpha-p equipment without prior treatment. Specific area measurements were performed by nitro-gen adsorption using QuantaChrome Nova 2200E equipment. Before adsorption, the samples were pretreated at 300 °C for 3 h under vacuum.
3. Results and discussion 3.1. Acid treatment
The removal of iron and other elements after the acid treatments was evaluated as a function of the applied temperature and acid concen-tration. The quantification of removed Fe, Al and Mg was performed by atomic absorption spectrometry. The values obtained are shown in
Table 2.
Based on the data obtained, treatment using the Statistical 6.0 soft-ware was performed. The values given in the Supplementary data
Table 1
Parameters of the 22
factorial design with three central points. The reaction time wasfixed at 6 h, and the clay:HCl ratio was 1 g:30 mL.
Experiment Temperature (°C) Concentration (mol L−1)
1 −1 (50) −1 (2) 2 −1 (50) +1 (4) 3 +1 (80) −1 (2) 4 +1 (80) +1 (4) 5 0 (65) 0 (3) 6 0 (65) 0 (3) 7 0 (65) 0 (3)
* Temperature and concentration for each factor are in parentheses.
Table 2
Values in mg of cations removed per g of clay, according to a 22
factorial experimental design with three central points.
Experiment Concentration and temperature Fe2+ Al3+
Mg2+
Central point 3 mol L−1— 65 °C 7.70 12.55 7.58
Central point 3 mol L−1— 65 °C 7.58 12.26 7.53
Central point 3 mol L−1— 65 °C 7.52 13.55 7.57
Exp 1 2 mol L−1— 50 °C 2.57 6.12 5.96
Exp 2 4 mol L−1— 50 °C 5.00 6.87 6.27
Exp 3 2 mol L−1— 80 °C 9.19 9.50 8.72
Tables S1, S2 and S3 demonstrate the influence of temperature and concentration on the removal of Fe2+, Mg3+and Al3+. Concentration
and temperature had a positive effect on the removal of these cations. Thus, increasing the temperature and concentration promoted greater cation removal from the treated clay mineral. The interaction of these two factors appeared to be more relevant for the removal of Al3+,
followed by the removal of Mg2+and Fe2+. Comparing the effect of
temperature on the removal of iron and aluminum, it was observed that the effect was similar for both of them, whereas concentration had a greater influence on the removal of aluminum compared with iron for this design at 80 °C.
Comparing the values of Fe and Al that were removed in experi-ments 3 and 4 (seeTable 2), the removal of aluminum in experiment 4 was much higher than that recorded for experiment 3, ranging from 9.5 to 20.17 mg g−1of clay mineral. For iron, this difference was smaller, ranging from 9.19 to 12.9 mg g−1.
3.2. Physicochemical characterization
Both samples treated with hydrochloric acid and natural samples were analyzed by IR spectroscopy for the purpose of comparison.
Fig. 1shows the infrared spectra for the samples studied.
The bands at 3405 and 1635 cm−1correspond to adsorbed water (Christidis et al., 1997). Acid treatment and drying processes removed water from the structure, and as a result, there were less intense bands, and the band at 3405 cm−1disappeared. The presence of characteristic bands of dioctahedral smectites has been observed both in natural smec-tite and in samples treated with acid (regions of 3629 cm−1— symmet-ric stretching Al\OH\Al and at approximately 920 cm−1 due to
Al\OH\Al vibrations) (Christidis et al., 1997; Pushpaletha et al., 2005). The isomorphous substitution of Al by Mg may be associated with the presence of a vibration band at 835 cm−1due to Al\Mg\OH linking, but the presence of a band at 875 cm−1related to this vibration (Al\Fe\OH) has not been observed (Tyagi et al., 2006).
Due to the removal of aluminum in samples treated with acid, these bands are less intense, in addition to the band at 1116 cm−1, which is related to Al\OH vibration (Kumar et al., 1995). Bands related to the presence of Mg with isomorphic substitution (835 cm−1) lose intensity upon acid treatment, which was also observed byTyagi et al. (2006)in the treatment of montmorillonite clay with H2SO4. The decrease in
intensity occurs gradually as the aggressiveness of the acid treatment increases. In the 4M— 80 samples, the band at 1116 cm−1was no
longer observed. Thus, the acid treatment preferentially attacks the octahedral layers of the clay mineral.
There is a decrease in the intensities of the bands at 997 cm−1 (Si\O\Si stretching) and at 518 cm−1(Si\O deformation) of treated
samples compared to natural samples, thus indicating that the tetrahe-dral structure is somewhat affected by acid treatment (Kumar et al., 1995). These bands show little difference when compared with treated samples, and the 4M— 80 samples treated with HCl possessed less intense bands in both regions.
Fig. 2shows the X-ray diffraction analysis of treated samples and their basal spacing values. All treated samples contained montmorillon-ite and quartz, for the sample 2M— 50 the presence of beidelite is also observed (seeFig. 2). The X-ray diffraction results show that even when lower acid concentrations were used, the crystallinity of the material decreased with higher temperatures during treatment. In the study by
Okada et al. (2006), the X-ray diffraction analysis of montmorillonite samples treated with H2SO4showed a decrease in peak (001), as was
the case for samples in the current study, which exhibited structural dis-organization due to the excessive removal of Al, Fe and Mg from the structure, as seen inTable 1.
All samples, except for that treated at 3 mol L−1at 65 °C, exhibited a decrease in the basal spacing. This decrease in basal spacing is due to the cation exchange of Ca2+by H+in the form of H
3O+. The highest basal
spacing value that was obtained for the 3M— 65 samples may be attrib-uted to re-hydration of the material.Önal (2007)observed a trend of reduction of the swelling capacity of up to 20%, after activation with H2SO4, varying the ratio bentonite/acid at the temperature of 97 °C. It
is interesting that, during the acid treatment, the samples do not lose their pronounced swelling capacity, which is necessary to expand the lamellae to insert the Al pillars.
4000 3500 3000 2500 2000 1500 1000 500 Transmittance wave number (cm-1) 3629 3405 1635 1116 997 518 Clay 2 M - 50 4 M - 50 3 M - 65 2 M - 80 4 M - 80 920 835 793
Fig. 1. Infrared spectra for natural clay samples and for those treated with HCl at different concentrations and temperatures.
10 20 30 40 50 60 d 060 = 1,51Å B d 001 = 13,22Å Intensity 2 theta d 001 = 15,12Å d 001 = 12,62Å d 001 = 12,91Å d 001 = 15,48Å d 001 = 12,67Å Clay mineral 2M-50 4M-50 3M-65 2M-80 4M-80 M M M Q Q M
Fig. 2. X-Ray diffraction analysis of clay samples treated with HCl. Phases identified: (M) montmorillonite, (Q) quartz and (B) beidellite.
10 100 1000 0,000 0,001 0,002 0,003 0,004 0,005 0,006 0,007 0,008 0,009 ~ 40Å clay 2M-50 4M-50 2M-80 4M-80 3M-65 Pore Volume (cc/g/Å) Pore diameter (Å) 50 - 60Å
Fig. 3shows the pore size distribution curve for materials obtained after acid treatment.
As seen inFig. 3, acid treatment leads to an increase in pore size as a function of increased temperature and concentration of the acid used, as demonstrated by the pore size distribution curves. The average pore size for natural clay mineral is approximately 4 nm. For the 2M— 80 and 4M— 80 samples, the pore size ranges from 4 to 10 nm, which leads to an increase in the specific area of these materials, as well as increased mesoporosity.
3.3. Pillaring of acid-treated samples
Pillared samples were analyzed before and after calcination. The X-ray diffraction results inFig. 4show the comparison between treated and pillared samples prior to calcination and natural clay sample.
The diffractograms show the displacement of peak d001which
indi-cates the intercalation of Al13oligomers between lamellae. Samples
treated at 80 °C with concentrations of 2 and 4 mol L−1 were also pillared and showed even less lamellar organization. In samples treated at a higher temperature (80 °C), the pillaring process caused no signi fi-cant changes other than increasing the basal spacing. Even with mini-mal structural organization, the basal spacing values were close to those of samples that received a milder acid treatment.
Pillared samples possessed higher basal spacing values compared to untreated samples and natural clay (15.12 Å). The highest basal spacing value was observed for the 2M— 50 sample, and the lowest value was observed for 4M— 50 leached clay. It is evident from the proximity of
the observed values that if the material maintains a small portion of its structural organization (lamellar), it can be pillared.Table 3shows the values of the specific area, micropore area, pore volume, micropore volume and basal spacing of samples.
A decrease in the basal spacing values of pillared samples was observed after calcination; this is normal due to the Keggin ion concen-tration. The BET area increased with increasing treatment severity (increasing acid concentration and temperature). A significant increase in the micropore volume was observed for samples treated with acid and pillared. This increase was due to the insertion of pillars in the interlayer region, which organized the structure.
InFig. 5, which shows the pore distribution curve, there is greater uniformity in pore size in the samples treated only with acid.
The pore uniformity led to a decrease in pore volume (BJH) in calcined pillared samples compared to the treated samples, as shown inTable 3. It was also found that the difference in BET specific areas for treated clays and pillared samples decreased as the acid treatment became more severe. The largest difference in area was obtained for samples treated at lower temperatures. It can be inferred that samples treated at lower temperatures have more organized structures, which facilitate the entry of pillars, and those samples exhibited marked increases in the BET specific area compared to the untreated samples.
The observed difference between the BET and micropore areas (see
Table 3) indicates that the pillaring process was effective for treated clay minerals. The insertion of pillars causes an increase on microporos-ity, as can be observed by the increase in the values of At-plotof the
pillared samples when compared to the samples treated with acid
10 20 30 40 50 Q Q M M d 001 = 18,01Å d 001 = 18,38Å d 001 = 18,01Å d 001 = 17,79Å d 001 = 19,02Å Intensidade 2 theta Clay P2M -50 P4M-50 P3M-65 P2M-80 P4M-80 d 001 = 15,12Å M M
Fig. 4. X-Ray diffraction results of natural and pillared acid-treated clays. Phases identified: (M) montmorillonite and (Q) quartz.
Table 3
Textural parameters for acid-treated samples and pillared acid-treated samples.
Samples treated with HCl Samples after pillaring and calcination
Vtotal (cm3 /g) VBJH (cm3 /g) Vt-plot (cm3 /g) At-plot (m2 /g) ABET (m2 /g) *Vtotal (cm3 /g) *VBJH (cm3 /g) *Vt-plot (cm3 /g) *At-plot (m2 /g) *ABET (m2 /g) *d001 Å 2M— 50 0.085 0.052 0.017 34 96 0.135 0.045 0.071 138 192 17.1 4M— 50 0.118 0.076 0.014 29 114 0.148 0.053 0.071 139 205 16.1 3M— 65 0.149 0.093 0.024 49 150 0.159 0.069 0.063 121 206 16.4 2M— 80 0.181 0.177 0.012 35 176 0.187 0.088 0.064 124 233 16.7 4M— 80 0.260 0.196 0.007 14 216 0.247 0.159 0.033 65 244 16.2
* Pillared clay minerals after acid treatment.
VBJH— Pore volume by BJH cumulative adsorption method.
Vtotal— Total pore volume for pores smaller than 193.82 Å at p/p0 = 0.95. Vt-plot— Micropore volume by t-plot method.
At-plot— Micropore area by t-plot method. ABET— Specific area by BET method. d001— Basal spacing. 10 100 1000 0,000 0,001 0,002 0,003 0,004 0,005 0,006 0,007 0,008 clay P2M - 50C P4M - 50C P2M - 80C P4M - 80C P3M - 65C Pore volume (cc/g/Å) Pore diameter (Å) ~ 35 Å
only. In all samples, after pillaring the area related to microporosity (At-plot) increased by around 60–79%. In relation to values of specific
area, a more significant increase in area was observed for the 2M — 50 and 4M— 50 samples, which had an increase of over 50% in their specific area.
3.4. Acid properties
n-Butylamine adsorption assays were performed in 4 pillared acid-treated clay samples, in a pillared natural clay sample (obtained
according to methodology of Pergher et al., 2011) and in an acid-treated clay without pillaring (3M— 65). The n-butylamine desorption was followed by thermogravimetric analysis, which provided informa-tion on the density and strength of acid sites present in the material.
Fig. 6shows the thermogravimetric analysis results of 3 samples prior to and after adsorption with n-butylamine.
In all samples with n-butylamine, an initial loss over a temperature range from 30 to 100 °C was observed. This loss is related to adsorbed water; thus, the subsequent mass loss events were attributed to the presence of weak (II), moderate (III) and strong (IV) acid sites.Table 4
a
b
c
d
f
e
Fig. 6. Thermogravimetric curves of the following samples: a) pillared clay, b) pillared clay adsorbed with n-butylamine, c) P3M— 65C, d) P3M — 65C n-butylamine, e) 3M — 65 and f) 3M— 65 n-butylamine.
shows the percentage losses for events II, III and IV. The values for the density and the relative strength of acidic centers of the three adsorbed samples are listed inTable 5. Samples without n-butlyamine were also analyzed. They showed weight losses in the range 479–706 °C, due to dehydroxylation of the clay. This was used to correct the weight losses of the samples loaded with n-butylamine.
Natural pillared clay mineral had a higher total acidity value com-pared to all of the other samples adsorbed with n-butylamine. Compar-ing the 3M— 65 and P3M — 65C samples, it was observed that the pillared samples possessed more acid sites that are weak and strong.
Pergher et al. (1999)presented a study on the synthesis of montmoril-lonite clay minerals with aluminum polyoxocations. The authors found that Lewis acidity is predominant in the samples that are produced and that it increases upon calcination. The decrease in the Brönsted acidity of acid-treated pillared clays was shown to be greater than the increase in their Lewis acidity upon calcination (Mokoya and Jones, 1995). In samples treated at lower temperatures and concentrations, the pillaring process promoted an increase in the specific area; thus, the acidity of these materials may be related to the greater amount of pillars.
Among the samples treated at a higher severity, the P2M— 80C and P4M— 80C samples exhibited the highest acidity values, which may have been related to the acid treatment applied.Kumar et al. (1995)found that materials such as montmorillonite K10 possess a higher Lewis acidity than Brönsted acidity. As the samples 2M— 80 and 4M— 80 are treated in the more severe conditions, pillaring is not so effective due to disorganization of the clay mineral layers. How-ever, these pillared samples present similar acidity to natural pillared clay because of the contribution from acid treatment applied.
All pillared acid-treated clays that were analyzed possessed a greater number of acid sites of moderate strength than weak and strong acid sites, compared to natural pillared clay minerals.
4. Conclusions
From the data obtained, it was concluded that acid treatment promotes disorganization in the clay mineral structure. The more severe the temperature and concentration used, the less maintained the lamel-lar structure organization. The temperature has a greater effect on the removal of structural cations than the acid concentration. From the X-ray data, it was determined that the decrease in the peak intensity of d001may be related to the disorganization of structural layers, and
the commitment of octahedral sheets was supported by infrared analy-sis with the decrease of bands that were indicative of the presence of structural Mg and Al species. The pore size distribution in the material was less uniform. It was also possible to insert Al pillars into the interlayer space in materials with less structural organization following acid treatment. The basal spacing values and the area increased for all samples. It was observed that for some samples, such as the P4M— 80C sample, the increase in area was not significant. All samples exhibited increased pore uniformity. In those samples whose acid treat-ment was more aggressive, the pillaring process contributed to the existence of regions with microporosity (layers separated by pillars) and of regions of meso- and macroporosity due to the organization of lamellae or due to particle disordering. There may also be regions of col-lapsed lamellae. Acidity measurements indicated that the pillared clays had higher acidity than the acid-treated pillared clays, but the latter possessed more acid sites of moderate strength. The total number of acid sites present in pillared acid-treated clays may be related to the presence of pillars in the material as well as to the Lewis acidity, which is promoted by the aggressiveness of the acid treatment that is applied prior to the pillaring process.
Acknowledgments
The authors acknowledge ANP, CNPq, CAPES and FINEP/RECAT for theirfinancial support.
Appendix A. Supplementary data
Supplementary data to this article can be found online athttp://dx. doi.org/10.1016/j.clay.2013.08.044.
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Table 4
Temperature ranges and mass losses obtained from TG/DTG curves.
Samples Temperature range (°C) Mass loss (%)
II III IV II III IV Pilc-n 109–277 308–505 509–777 3.25 2.59 3.76 Pill 522–664 1.83 P2M— 50C-n 121–274 302–486 525–787 2.33 2.82 2.60 P2M— 50C 542–694 1.56 P3M— 65C-n 110–261 281–467 504–764 2.78 2.41 3.19 P3M— 65C 479–689 2.02 P2M— 80C-n 118–263 302–493 508–785 2.69 2.94 3.39 P2M— 80C 526–700 2.02 P4M— 80C-n 94–273 306–472 481–688 3.37 2.74 2.60 P4M— 80C 531–676 1.53 a 3M— 65-n 122–273 295–472 507–693 2.68 2.88 2.68 a 3M— 65 504–706 2.36 a
Non-pillared clay sample treated with acid.
Table 5
Density and strength of acidic centers on samples obtained by n-butylamine desorption. Samples Acidic centers (mmol/gcat)
II (Weak) III (Moderate) IV (Strong) Total M/f M/F Pilc-n 0.462 0.368 0.270 1.101 0.796 1.363 P2M— 50C-n 0.339 0.410 0.143 0.891 1.209 2.867 P3M— 65C-n 0.406 0.352 0.151 0.909 0.887 2.331 P2M— 80C-n 0.399 0.436 0.191 1.020 1.093 2.282 P4M— 80C-n 0.477 0.389 0.138 1.004 0.815 2.819 a 3M— 65-n 0.390 0.418 0.032 0.840 1.072 13.062
F = Strong, M = Moderate and f = Weak. a
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