Ind. Eng. Chem. Res. 1995,34, 1440-1448
1440
Evolution of Porosity and Surface Acidity in Montmorillonite Clay on Acid Activation Prakash Kumar, Raksh V. Jasra, and Thirumaleshwara S. G. Bhat* Research Centre, Zndian Petrochemicals Corporation Limited, Vadodara, 391 346, India Evolution of surface acidity and porosity in montmorillonite clay on treating with sulfuric acid has been studied. It is observed that the clay treated with 4 N sulfuric acid shows the maximum surface acidity. Acid strength distribution as measured by Benesi's technique of nonaqueous titration shows acidity range mainly between HO+4.6 and +3.3. Nitrogen adsorption-desorption hysteresis data indicate the transformation of pores from slit-shaped to spheroidal or ink bottle type as the acid concentration is increased from 1to 8 N. These observations are explained in terms of structural modifications of clay on treatment with acid.
Introduction Acid-treated montmorillonite clay finds industrial applications as catalysts, catalytic supports, and adsorbents. Treatment of clays with mineral acids is the most widely used technique to impart acidity to the clay surface. Though a few investigations on the effect of mineral acid on the chemical structure of the clay have been reported in the past (Mills et al., 1950; Thomas et al., 1952; Weaver and Pollard, 1973; Granquist and Gardner-Summer, 1959; Breen, 19911, studies depicting the correlation of surface acidity and texture with structural changes are sparse (Pesquera et al., 1992; Mendioroz et al., 1987). Acid treatment of montmorillonite has been observed (Rhodes and Brown, 1991, 1992) to enhance mesoporosity making it an effective catalytic support. Thus, it is important to understand the textural as well as surface acidity changes resulting from acid treatment of clays under different conditions. In the present study, we have investigated the changes in the surface acidity as well as the porosity of montmorillonite clay on treating with varied sulfuric acid concentration. An attempt has also been made t o correlate clay porosity and surface acidity with the structural changes it undergoes on treatment with acid.
Experimental Section Materials. Bentonite clay containing more than 90% by weight montmorillonite was obtained from the mines of Kutch region in Western India. The sample was found to have minor impurities of feldspar, quartz, and carbonates. The virgin clay was found to have a cation exchange capacity of 80.19 mequiv1100 g. Sulfuric acid used for clay treatment was of AR grade from WS S.D. Fine Chemicals, India. Methods. Fifty grams of Bentonite clay ground and sieved to 40-60 mesh particle size was refluxed with 250 mL of H2SO4 of desired concentration at 80 "C for 2 h in a round-bottom flask. Acid concentration was varied from 1 to 8 g equivb. Different acid-treated samples were prepared by varying the acid concentration from 1to 8 g equivb. The slurry was cooled in air and filtered through a G4 sintered glass crucible. The filter cake was repeatedly washed with hot distilled water until the filtrate was neutral to litmus paper. The acid-treated clay samples were designated as N1 t o N8 with the sufix indicating the acid concentration. The virgin clay was designated as NO. Chemical analysis of the clay samples was made using a Techtron 1200 automatic atomic absorption spectro0888-588519512634-1440$09.00/0
photometer. Si02 content was determined gravimetrically. The samples were brought to solution by first digesting the clay with a mixture of 1:l sulfuric acid and 48% hydrofluoric acid in a platinum crucible followed by dissolving the residue with 1:l warm hydrochloric acid. Surface acidity of the clay samples was determined by the Benesi technique of nonaqueous titration with n-butyl amine (Benesi, 1957; Tanabe, 1981). Prior to titration, clay was dried in an oven at 120 "C under vacuum for 6 h and stored in a desiccator. Thus activated clay was quickly suspended in benzene (dried with molecular sieve 4A) in a glass vial fitted with a screw cap. As n-butylamine titration is sensitive to moisture, precautions were taken to prevent moisture pickup at every stage. Indicator solutions in benzene were added to this suspension which was shaken for 24 h to attain equilibrium. The suspension was titrated with n-butylamine in benzene with various Hammett indicators by successive titration method at ambient temperature. Surface acidity of the clay samples was also determined by pyridine desorption technique. In this method, pyridine vapors were adsorbed on the clay sample in a Mcbain-Bakr gravimetric balance equipped with a sensitive (30 cdg-l) quartz spring (Thermal Syndicate, UK). A cathetometer with a precision of kO.001 cm was used for measuring the spring extension. Vapor pressure of pyridine was measured using digital pressure transducer (MKS Instruments Inc., USA). Clay sample was activated a t 120 "C for 6 h under vacuum Torr) prior t o adsorption. After adsorption, samples were evacuated for 30 min at the desired temperature and the pyridine retained was measured. The pH of the aqueous clay suspensions (10 w t %) was measured with a digital pH meter at 28 "C. Acidity of the clays was also determined by volumetric titration. In this method, 0.5 g of the clay, previously dried at 120 "C for 6 h, was taken in a conical flask to which 15 mL of 0.1 N NaOH was added. After stirring the flask for 10 min, the clay was titrated with 0.1 N H2S04 acid using phenolphthalein indicator. Acidity was then determined as milliequivalents of NaOH used per 100 g of clay. Surface area and pore size distribution of the samples were determined by adsorption-desorption of nitrogen at 77 K using Sorptomatic 1900 (Carlo Erba instruments, Italy). Surface area was calculated using BET isotherm. Pore size distribution was determined from nitrogen desorption data at plpo = 0.3 and above, using
0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995 1441 Table 1. Chemical Analysis (% by weight) of the Virgin and Other Acid-Treated Clay Samplesa compound Si02 A203
Fez03 CaO MgO Na2O
KzO ZnO volatile
NO
N1
N2
N3
N4
N5
N6
N7
N8
30.83 16.86 (54.68) 27.12 1.25 1.72 8.12 0.55 0.02 13.53
38.59 16.56 (42.91) 20.43 0.29 1.69 1.15 0.52 0.03 20.73
40.66 16.40 (40.33) 19.89 0.12 1.78 1.02 0.50 0.02 20.11
50.52 11.00 (21.77) 14.29 0.27 1.48 0.99 0.51 0.02 21.42
52.15 10.19 (19.53) 13.93 0.36 1.05 1.65 0.41 0.02 20.24
60.82 6.87 (11.29) 12.45 0.21 0.71 0.55 0.27 0.01 18.10
62.70 6.97 (11.11) 8.85 0.25 0.97 0.64 0.26 0.00 19.37
67.5 4.44 (6.58) 7.12 0.17 0.74 0.35 0.18 0.00 19.50
70.19 4.8 (6.8) 5.8 0.24 0.68 0.42 0.21
The values given in parentheses are the alumina percentages normalized against Si02 which stays about the same in all samples.
Table 2. Cation Exchange Capacity (CEC) and Surface Acidity Data on Various Acid-Treated Clay Samples clay
CEC, mequiv/100 g
25
NO N1 N2 N3 N4 N5 N6 N7 N8
80.2 76.8 77.0 65.8 67.2 42.2 34.2 25.6 23.1
2.30 2.32 1.89 2.75 2.60 2.00 1.74 1.57 1.47
a
pyridine retained, mmol/g, at temp, "C 50 115 150 1.84 1.90 1.40 1.76 1.75 1.33 0.88 0.90 0.95
0.61 1.00 0.75 0.76 1.25 0.35 0.34 0.20 0.20
0.12 0.51 0.31 0.38 0.87 0.19 0.19 0.04 0.03
200
PH
acidity: mequiv/100 g
0.03 0.02 0.10 0.15 0.44 0.05 0.07 0.00 0.00
9.70 3.67 3.23 2.58 2.27 3.03 3.30 3.11 2.79
45.5 136.6 134.0 131.7 145.5 113.9 110.9 95.1 87.1
NaOH titration.
Sorptomatic software based on the Kelvin equation and the method of Barret, Joyner, and Halenda (Gregg and Sing, 1982). Cation exchange capacity (CEO was measured (Grim, 1968)by the NH4+ ion uptake method using ammonium acetate solution. X-ray diffractograms were obtained using Philli s PW1710 equipment, using Cu Ka line (2 = 1.5406 ) for virgin as well as acid-treated samples. Infrared spectra were recorded using KBr pellets on a Bruckers IFS 113V Model FT IR spectrometer.
B
Results and Discussion Chemical and Structural Changes. Chemical composition of the different clay samples following acid treatment is given in Table 1. Normalizing against the amount of Si02 which stays about the same in all samples, the data indicate that for samples N1 and N2 substantial amounts of Na+, Ca2+,and Fe3+are leached out on treating with acid. On the other hand, there is marginal decrease in the aluminum and magnesium content in N1 and N2 samples. However, by treating the clay with an acid of higher normality (i.e., samples N3 to NB), there is a progressive decrease in the aluminum, magnesium, and ferric content of the clay. The decrease is very sharp in the case of samples N3 and N4. Cation exchange capacity (CEC) data given in Table 2 show that there is negligible decrease in CEC for N1 and N2. However, clays N3 and N4 show a decrease of around 18%,after which there is a progressive decrease in CEC values for clays treated with higher acid concentration. CEC predominantly results from the substitution of A13+ions by lower valent ions like Mg2+ in the octahedral sheet. The negligible change in CEC for samples N1 and N2 indicates that tetrahedral and octahedral sheets of the clay structure are only marginally affected up to acid concentrations of 2N. The decrease in CEC in N3 and N4 samples indicates that the cations in the tetrahedral or octahedral layer are readily attacked by acid solutions of higher concentrations.
Infrared spectra of various clay samples (Figure 1) shows that the shoulder (885 cm-l) corresponding to -OH deformation linked to Fe3+and A13+in octahedral sheet (Van-Olphen and Fripiat, 1979) decreases from samples N1 to N3 and is very weak in case of other clays. Similarly, the shoulder at 936 cm-l corresponding to -OH deformation linked to 2A13+ decreases up to sample N3 and increases for N4. There is a progressive decrease in the intensity of the band at 1115 cm-' corresponding to Al-OH vibration. These data show that the acid attack on the octahedral sheet is greater at higher acid concentration. The IR bands at 1032 and 532 cm-l corresponding to Si-0-Si stretching and Si-0 deformation show that the tetrahedral sheet of the clay is relatively stable and is affected only at higher acid concentrations. For example, the band at 532 cm-l becomes a very weak shoulder in N7 and N8 clay samples. The bands at 468 and 804 cm-l due to free silica show (Van-Olphen and Fripiat, 1979) increasing intensity with increasing acid concentration for samples higher than 3 N. Infrared spectral data show that sulfuric acid affects first the octahedral sheet followed by the tetrahedral sheet. These observations are corroborated by X-ray diffraction data as shown in Figure 2. The line intensities of almost all reflections decrease without any change in 28 values for clay sample N2. The intensity of line 28 = 6" corresponding to the basal spacing (001) of laminar clay decreases and starts broadening from sample N3 onward. This indicates that delamination of clay structure starts at acid concentration higher than 2N. A strong line at 26 of 28" correspondingto silica is also observed for samples aRer N3 and is very prominent for N5 clay. The clay samples prepared with very high acid concentration (8 N) were X-ray amorphous. The structure of montmorillonite clay comprises (Grim, 1968)two tetrahedral silicon layers surrounding a central octahedral aluminum layer. Substitution of Mg2+for AI3+in the octahedral layer and A13+for Si4+ in the tetrahedral layer result in the development of a negative charge in the silicate layers which is normally neutralized by the hydrated cations present between the
1442 Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995
I
I
I
1
I
I500
500
3 500
2500
I
2560
3 m
I
1500
I
50(
WVEMJMBER (cm-1)
WAVENUMBER (C m-I
Figure 1. Infrared spectra for various clay samples.
5N
3.0
20
40
60
29 Figure 2. X-ray diffraction data for various clay samples.
80
30
20
40 28
60
1
Ind. Eng. Chem. Res., Vol. 34,No. 4,1995 1443
60)
ON
.IN
A2N
03N
A
4N
X5N
0
6N
A 7 N m8N a
m
m 0
40
do
xmb
20
m
f
A
GO
40
b
0
0
0
"0
b o
A
80
P/PO*1W Figure 3. Pyridine adsorption data a t ambient temperature for different clay samples.
sheets. Consequently, clay will have two types of cations, viz., exchangeable extralattice cations like Na+ and Ca2+and octahedral A13+,Mg2+,and Fe3+cations. Our data show that the majority of the exchangeable cations (Na+, K+, and Ca2+) get leached out when treated with moderately strong acid. Chemical analysis shows that a part of Fe3+was present outside the lattice. There seems to be little effect of the dilute acid (below 2 N), if any, on the octahedral sheet of the clay. Acid with concentration range of 3-4 N has the maximum effect on octahedral sheet. X-ray diffraction and infrared spectra show an increase in free silica when clay is treated with acid solutions of higher concentration, i.e., above 5 N. In a study of montmorillonite with hydrochloric acid Pesquera et al. (1992) have observed that free silica generated at higher acid concentration acts as a barrier and protects the octahedral sheet from acid attack. As a result, Mg2+and A13+are leached out less readily at higher acid concentrations (8 N) and clay sample after 8 N hydrochloric acid treatment is akin to the one obtained after 3 N acid treatment. However, we have not observed such passivation effect. Surface Acidity. Total acidity of various clays has been determined by pH measurements on aqueous clay suspensions, sodium hydroxide titrations, n-butylamine titrations, and pyridine desorption. pH measurements on aqueous clay suspensions at room temperature (Table 2) shows that acidity increases from N1 to N 4 after which it shows a decreasing trend. Acidity in clays arises from H+ ions occupying exchange sites on the surface or by dissociation of the water hydrating the exchangeable metal cations as [M(H20),lnf
-
+
[M(OH) (H20)x-lln-1 H+
Pyridine is a weakly basic molecule and is adsorbed on the clay surface through acid-base interactions. Pyridine is adsorbed on the Bronsted acid sites generated by the exchange of interlamellar cations with protons. It is also partly adsorbed on the Lewis acid sites of Mg2+ and A13+present in octahedral sheet. Pyridine adsorption and retention a t different temperatures are given in Figure 3 and Table 2. While comparing the acidity, pyridine retained only above 115 "C (i.e., its boiling
Table 3. Acidity of Different Clays As Determined by n-Butylamine Titration Using Hammett Indicators n-butylamine titer value, mequiv/g clay
methyl red (Ho = +4.8)
bromocresol green (Ho = $4.6)
p-diaminoazobenzene (Ho= +3.3)
N1 N2 N3 N4 N5 N6 N7 N8
0.52 0.49 0.51 0.60 0.52 0.44 0.35 0.31
0.13 0.12 0.11 0.14 0.12 0.05 0.03 0.03
0.28 0.26 0.41 0.46 0.40 0.37 0.20 0.19
point) was attributed to acidity of the clay surface. These data also support our earlier conclusion that when clay is treated with 1N sulfuric acid substantial surface acidity is generated and the highest acidity is generated by a 4 N sulfuric acid treatment. Pyridine molecules also interact through hydrogen bonding t o form pyridinium ions. However, such pyridine molecules are not likely to be retained above 150 "C. Thus, the value of pyridine retained (Table 2) a t 150 "C gives the amount of protonated pyridine which in turn indicates the number of protons accessible to pyridine, which has a lower KB than butylamine or sodium hydroxide. Acid strength distribution was determined using the following Hammett indicators: methyl red (Ho = f4.81, bromocresol green (Ho = +4.6),p-diaminoazobenzene (Ho= +3.3),2-amino-5-azotoluene (Ho = +2.0),crystal violet (Ho= +OB), and anthraquinone (Ho = -8.2). However, the color change was observed only for the first three indicators, showing thereby that clay possesses acidity with Ho < 3.3. Table 3 shows that the distribution of acidity lies between Ho +4.6 and +3.3 reflecting milder acid sites on the clay surface. These results also show that the acidity increases with acid concentration up to sample N 4 after which it shows a decreasing trend. Similar results were observed by sodium hydroxide titration as seen in Table 2. Furthermore, it is seen from Table 3 that acid sites with strength HO+3.3 and 4.8increase and acid sites with HO+4.6 remain more or less constant on the clay surface up t o N4. Beyond sample N4,acid sites corresponding to all the strengths decrease with increasing acid
+
1444 Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995 Table 4. Textural Parameters for Virgin and Acid-Treated Clay Samples clay SBET, m2/g
NO N1 N2 N3 N4 N5 N6 N7 N8
38.03 138.36 189.84 326.27 347.74 370.41 370.36 297.49 377.10
CBET VPO,cm3/g 135.86 148.22 222.54 132.63 116.63 200.03 129.69 75.58 48.48
0.101 0.201 0.231 0.331 0.383 0.489 0.566 0.582 0.805
St-PiOt,m2/g
Su-piot, m2/g
37.74 162.44 216.58 382.11 386.75 464.10 541.45 454.97 418.26
36.70 153.06 170.05 348.75 349.99 370.23 382.57 325.25 436.72
concentration. The pyridine desorption data (Table 2) also suggest similar acid strength distribution as it is known (Jasra and Bhat, 1985) that acid sites stronger than Ho I+3 retain pyridine well above 300 "C. Most of the samples excepting N4 do not retain significant adsorbed pyridine as seen in Table 2. These data clearly establish that sample N4 shows the highest acidity. When clay samples are treated with moderate acid, Bronsted acid sites are generated by the exchange of interlamellar cations with proton. In case of clay samples N3 and N4, delamellation occurs and the octahedral Mg2+and A13+ cations are attacked. It has been suggested (Mendioroz et al., 1987)that on treating the clay with high acid concentration one of the pair of octahedrally coordinated aluminum atoms along with two hydroxyl groups is leached out. As a result, the remaining aluminum atom acquires tetrahedral coordination with the four remaining oxygen atoms. This aluminum, which is negatively charged, becomes protonated. The maximum acidity observed for clay sample N4 is also in tune with this explanation. Consequently, until 50% of the aluminum is removed, there is a generation of tetrahedral aluminum sites as well as progressive increase in the acidity of the clay. However, further acid attack removes the tetrahedrally coordinated aluminum atom as well leads to a decrease in the acidity. This explanation agrees well with our observation that acidity of the clay is highest when the aluminum is removed to the extent of 40-60% (Tables 1 and 3). IR data show that acid attack on the tetrahedral layer begins at the same acid concentration and substantial free silica is observed in sample N5 and beyond. The maximum acidity observed for N4 sample may also be accompanied by disaggregation and delamellation of octahedral sheet which is reflected in its very high surface area value. Evolution of Pore Structure. Nitrogen adsorption-desorption isotherms a t 77 K of clay samples are given in Figure 4 and show the presence of desorption hysteresis, type H3 for samples NO to N3 and type H2 for samples N5 to N7. Sample N4 appears to exhibit a transition from H3 to H2 type hysteresis. For clays N5 to N7, there is a steep rise in the slope of adsorption isotherm in theplpo region of 0.4-0.5 reflecting capillary condensation in the mesoporous region. The size and shape of the hysteresis loop indicate the evolution and transformation of slit-shaped pores to ink bottle type pores on treating with acid of higher concentration. Table 4 shows that this change is also associated with an increase in the surface area. The changes in the texture of clay surface with acid treatment are further elucidated by treating nitrogen adsorption-desorption data with other techniques, viz., t-plots (Gregg and Sing, 1982; Lecloux and Pirard, 19791, a-plot (Gregg and Sing, 19821, andf-plot (Gregg, 1975). t-Plot is the curve obtained by plotting the amount adsorbed against t, the statistical thickness of
film, rather than against plpo. The change of independent variable from plpo to t is made by reference to the standard t-curve. In the present case, the untreated clay, which is practically nonporous, is taken as the reference adsorbent for obtaining the t-plots. This sample has a linear t-plot passing through the origin as expected by its nonporous character. t-Plots for different clays are given in Figure 5 . t-Plots of samples N1 and N2 are nearly linear indicating monomolecular layer adsorption. The remaining clay samples show increasingly upward deviation from linearity suggesting capillary condensation and hence the presence of mesopores. The deviation is more prominent for samples N5 onward. The values of surface area determined from t-plots are also given in Table 4 and show values higher than BET surface areas. Use of reduced plots defined as a, = (u/us)ref where u s is the amount adsorbed by the reference sample at plpo = s has also been proposed (Gregg and Sing, 1982)for texture analysis. It is further assumed that at plpo = 0.4 monolayer coverage is complete when a, = 1;micropore filling occurs at plpo = 0.4 and capillary condensation takes place at plpo > 0.4. By plotting asagainst volume of adsorbed nitrogen as a function of a, at different plpo values, information about porous structure is obtained. a,plots for different clays (Figure 6) shows positive deviations from linearity reflecting the formation of mesoporosity from samples N3 onward. Samples N1 and N2 also possess some microporosity as seen from the positive intercept of the a, plots. Surface area is calculated using
SaS= 2.87u/aS where u is the amount adsorbed on a particular sample. Gregg (Gregg, 1975) has proposed a simple method for comparing the isotherms of closely related samples whereby deviations of the adsorption isotherms of the test sample from that of the reference sample are shown by calculating f, which is a ratio of the ordinate of different points in the two isotherms corresponding to respectiveplpo values. Figure 7 gives fversusplpo plots called f-plots which show three distinct groups. The first one, which is almost parallel t o the x-axis (up to plpo = 0.4) indicates that the morphological feature of the virgin clay is still preserved wherein a predominantly mono-multilayer adsorption process is involved. The second group of f-plots showing increasing positive deviation denotes the capillary condensation. In the case of N3 and N4, the f-plots are less steep indicating capillary condensation in slit-shaped pores whereas for N5 to N7 (Figure 7) the pores are predominantly spheroidal or ink bottle shaped in nature. The third group of plots show negative deviation at high plpo = 0.8 signifylng the loss of the asymptotic character of the isotherms. The pore size distribution obtained from nitrogen desorption isotherm in the range plpo = 0.3-0.995 as given in Figure 8 gives further insight into the textural changes occurring on acid treatment. The vir 'n clay possesses little porosity in the range of 18-20x radii, but following 1-4 N acid treatment not on1.y is there an increase in the volumes of these pores, but there is also a generation of substantial extent of finer pores of radii < lOA. However, as the method of Barret et al. (Gregg and Sing, 1982)is based on the Kelvin equation, pore size distribution for pores less than 10 A should be taken as indicative only. Acid concentration beyond 5-6 N causes distinct enhancement of mesoporosity and
Ind. Eng. Chem. Res., Vol. 34,No.4,1995 1445
1
A 3001 2 50
-
ON
*IN
A 2N
o
3N
A4N
2 00 vl 0
>
150
50 n
0
Ok
0.2
0.8
0.6
1
P / Po
I
0
I
I
I
1
0 .G
0.2
0.8
0.6
1
PIP0
c
5
0
0' 0
1
0.2
0
1
1
0.6
a4
PIP0 Figure 4. Nitrogen adsorption isotherms a t 77 K for different clay samples.
2
I
0.8
1446 Ind. Eng. Chem. Res., Vol. 34,No. 4,1995
t Figure 5. t-Plots for different clay samples.
0.5
I
1
1
1.5
1
2
1
1
2.5
3
M Figure 6. cis plots for different clay samples.
the pore radii increases from 20 to 50 A. Recently, a somewhat similar observation (Rhodes and Brown, 1993) has been reported on treatment of sodium montmorillonite with 30% sulfuric acid for 15 min, but faiIed to notice the evolution of finer pore structure. This, probably, was due to the destruction of the lamellar structure within clay platelet due to high acid concentration used by Rhodes and Brown. Their observation is somewhat akin t o ours in the case of N6 sample. Treatment with dilute sulfuric acid (1-2 N) dissolves the basic Na+ and Ca2+ions occupying the interlayer space as well as extraframework mineral impurities present in the void between the clay platelets. Treatment with acid (3-4 N) concentration results in the broadening of slits or voids between the layers due to depleted cation concentration and interlamellar attraction as well as destruction of octahedral layer. Although there are indications of acid attack on the tetrahedral silica layer in the initial stages, also the significant amount of free silica is generated only at higher concentration as seen in X-ray and IR data. This free
silica produces spheroidalfink bottle type pores by depositing at some pore openings. The above observations are consistent with the chemical analysis on the one hand and porosity and acidity data on the other. Disaggregation of the individual clay particles and losses in interparticulate voids of the natural clay are also observed for samples N3 and N4 for which f-plots show a downward deviation starting from plpo = 0.4.
Conclusion When montmorillonite clay is treated with sulfuric acid, the surface area and porosity of the clay are greatly enhanced. It has been shown by X-ray, IR, and surface acidity measurements, that at moderate concentration (2N) of the acid, leaching and exchange of the matter in the interIameIlar space with protons generates the surface area and acidity. With higher acid concentration, removal ofA13+ and Mg2+ ions from octahedral sites in the framework leads to enhanced pore volume and acidity. At still higher acid concentration ('4 N),
Ind. Eng. Chem. Res., Vol. 34,No. 4, 1995 1447
20
15
;10 \ I
>
5
0
0.2
0.CI
0.6
0.8
P /PO
Figure 7. f-Plots for different clay samples.
Acknowledgment We are thankful to IPCL management for according permission to publish this work. We wish to express our sincere thanks to Dr. N. V. Choudary for meanin@ discussions during this work. This is IPCL Communication No. 272.
Literature Cited
25-
3N
-
LN
25
8N
Pore Radius f &
Figure 8. Pore size distribution determined from nitrogen adsorption-desorption data.
the clay structure tends to be destroyed. Surface acidity and the porosity evolved have been explained in terms of change in the chemical structure of the clay on treatment with acid.
Benesi, H. A. Acidity of catalyst surfaces: 11Amine titration using Hammett indicators. J . Phys. Chem. 1957,61, 970. Breen, C. Thermogravmetric study of the desorption of cyclohexylamine and pyridine from a n acid treated Wyoming bentonite. Clay Mineral. 1991,26, 473. W. T. Granquist, W. T.; Gardner-Summer, G. Acid dissolution of Texas Bentonite. Clays Clay Miner. 1959,6 , 292. Gregg, S. J. A simple method for comparing the shapes of closely related adsorption isotherms. J . Chem. Soc., Chem. Commun. 1975,699. Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area, and Porosity; Academic Press: London, 1982. Grim, R. E. Clays (Uses). InKirk Othmer Encyclopedia of Chemical Technology; Wiley: New York, 1968; Vol. 6, p 207. Grim, R. E. Clay Mineralogy, McGraw Hill: New York, 1968. Jasra, R. V.; Bhat, S. G. T. Adsorption and catalytic conversion of olefins over acid treated clays-A thermal desorption mass spectrometric study. In Proceeding of Seventh National Symposium in Catalysis; Prasada Rao, T. S. R., Ed.; Wiley Eastern: New Delhi, 1985; p 423. Lecloux, A,; Pirard, J . P. The importance of standard isotherms in the analysis of adsorption isotherms for determining the porous texture of solids. J . Colloid Interface Sci. 1979,7,269. Mendioroz, S.; Pajares, J.; Bentino, I.; Pesquera, C.; Gozalez, F.; Blanco, C. Texture evolution of Montmorillonite under progressive acid treatment: Change from H3 to H2 type of hysteresis. Langmuir 1987,3,676. Mills, G. A.; Holmes, J.; Cornelius, E. B. Acid activation of some Bentonite clays. Phys. Colloid Chem. 1950,54, 1170. Pesquera, C.; Gozalez, F.; Bentino, I.; Blanco, C.; Mendioroz, S.; Pajares, J. Passivation of a Montmorillonite bt the Silica created in acid activation. J . Mater. Chem. 1992,2,907. Rhodes, C. N.; Brown, D. R. Structural characterization and optimisation of acid-treated Montmorillonite and high porosity Silica supports for ZnClz alkylation catalysts. J . Chem. Soc., Faraday Trans. 1992,88, 2269. Rhodes, C. N.; Brown, D. R. Surface properties and porosities of Silica and acid-treated Montmorillonite catalysts supports: Influence on activities of supported ZnClz alkylation catalyst. J. Chem. Soc., Faraday Trans. 1993,89, 1387.
1448 Ind. Eng. Chem. Res., Vol. 34,No. 4,1995 Rhodes, C. N.; Franks, M.; Parkes, G. M. B.; Brown, D. R. The effect of acid treatment on the activity of clay supports for ZnCl2 alkylation catalyst. J. Chem. Soc., Chem. Commun. 1991,804. Tanabe, K. Solid acid and base catalysts. In Catalysis-Science and Technology; Anderson, J. R. Boudart, M., Eds.; Springer-Verlag: New York, 1981; Chapter 5, p 231. Thomas, C. L.; Hickey, J.; Stecker, G. Chemistry of clay cracking catalysists. Ind. Eng. Chem. 1950,42, 866. Van-Olphen, H.; Fripiat, J. J. Data Handbook for Clay Materials and other Non-Metallic Materials; Pergamon Press: New York, 1979.
Weaver, C. E.; Pollard, L. D. Developments in sedimentology. The chemistry of clay minerals; Elsevier: New York, 1973; Vol. 15, p 214.
Received for review August 1, 1994 Revised manuscript received December 29, 1994 Accepted J a n u a r y 6 , 1995@ IE9404691 @
Abstract published in Advance ACS Abstracts, March 1,
1995.
