Nanoparticulate Silica-Clay Solid Acid with Exceptional Thermal Stability
N.J. Venkatesha, H.R. Prakruthi, B.S. Jai Prakash, Y.S. Bhat
Chemistry Research Centre, Bangalore Institute of Technology, K.R. Road, Bangalore 560 004, India
E-mail-venkatesha.312@gmail.com, prakruthihr.16@gmail.com, jprak27@yahoo.com, bhatys@yahoo.com
Introduction: One of the main drawbacks of smectite clays for their utility as catalysts is
their thermal instability. Modification by introducing hydroxyoligomers to form oxide pillars
offers thermal stability but the oxide pillars formed show only Lewis acidity. Modification by
treatment with moderately strong organic acids such as EDTA, citric acid and tartaric acid is
known to remove structural Al, Fe and Mg ions from the octahedral layer of the clay resulting
in micropores. Such generated micropores show both Lewis and Bronsted acidity and can
bring about typical organic transformations that do not take place on the clay surface.
Extensive removal of structural elements using complexing agents can totally alter the clay
surface. In the present study, vigorous treatment with silicic acid (SA) under microwave
irradiation on montmorillonite clay samples and its effect on the clay properties is
investigated.
Experimental Clay samples used in the present study have been described elsewhere [1,2].
One of the samples was procured from BHEL, Bengaluru, India designated as American
white bentonite. The second sample used was supplied by Ashapura Chemicals and was
designated as Indian Montmorillonite. The clay samples were converted to Na form before
use. The samples were characterized by XRD and FTIR. The parent materials had a CEC of
0. 9 and 0.83 for White and Indian montmorillonite samples respectively. The Na exchangeclay samples were treated with salt solution of silicic acid (SA) of different concentrations
and PHs. The mixture was subjected to microwave irradiation for varying time intervals. The
treatment was optimized to get maximum amount of Fe, Al and Mg from the clay structure
into the solution as analyzed by ICPOES. The treated samples were centrifuged, washed with
deionized water several times till the conductivity of the washings was the same as that of
deionized water. The samples were dried at 110 0C for two hours and finely ground. They
were characterized by TEM, PXRD, TGA, and ICPOES for loss of structural Fe, Al and Mg;
surface area measurements for pore size distribution, pore volumes and BJH curves; TPDNH3 for acidity, and pyridine-FTIR for Lewis and Bronsted acidities. The catalytic activity
of the treated samples were tested for some model acid catalyzed reactions.
Results and Discussion: Partial results are shown in Figs. 1- 3. Fig.1 shows TEM images at
high magnifications (10 and 5 nm) of the treated clay sample which clearly indicates
fragmentation of the clay particles having nano dimensions.
Fig.1 TEM images of SA treated montmorillonite clay samples at high magnifications
The XRD patterns reveal the structural integrity of the clay with a slight expansion in the caxis probably due to the presence of complex silicic acid species in the structure.
Considerable enhancement in the surface area and mesopore volume was observed which did
not change with heat treatment up to 800 0C. Steep nitrogen adsorption-desorption isotherms
with narrow hysteresis at very high p/p0 values (Fig.2) indicated homogeneous pore size
distribution of the mesopores.
350
Volume Adsorbed @ STP (cc/g)
300
250
200
150
100
50
0
0.0
0.2
0.4
P/Po
0.6
0.8
1.0
Fig. 2 Nitrogen adsorption-desorption isotherm of SA treated montmorillonite clay catalyst
Total acidity was measured by NH3-TPD (Fig.3) which showed existence of strong acid
centers beyond 700 0C. Bronsted and Lewis acidities were determined by pyridine adsorbed
by FTIR method (3). The samples showed enhanced Bronsted acidities upon treatment with
SA which showed very little change even after thermal treatment beyond 800 0C. The
samples heated up to 800 0C did not show any variation in the catalytic activities for vapour
phase cumene cracking at 500 0C, glycerol to glycerol acetal and alkylation of phenol which
confirmed the presence of Bronsted acidity up to and beyond 800 0C . This is very unusual
for smectite clays which normally show a collapse in the structure and loss of acidity and
catalytic activity beyond 250 0C.
Peak Analysis
Chi^2=-SS=--
Adj. R-Square=--
Degree of Freedom=% ([PeakFit1]FitPeaks1,@WL,RegStats.C1.DOF)
450
400
350
TCD signal (mV)
300
250
200
150
100
50
0
0
200
400
600
Temperature (oC)
800
Fitting Results
Fig.3 Deconvoluted NH3-TPD plot of SA treated montmorillonite clay
Peak Index
1.
2.
3.
Peak Type
Gaussian
Gaussian
Gaussian
Area Intg
19961.78184
68378.29522
75129.2287
FW HM
108.81191
194.40856
182.39184
Max Height
173.26994
330.46995
390.44864
Conclusion: Treatment with silicic acid was found to remove considerable quantities of Al
(about 60 %) from montmorillonite clay structure resulting in nanostructured material. The
amount of Si increased by about 4% which possibly plays an important role in binding the
clay layers together thus offering exceptional thermal stability and preservation of Bronsted
acid sites for treated clay samples heated beyond 700 0C. Unique characteristics of these
treated clays envisage several engineering applications.
References
[1] S. Selvaraj, B.V. Mohan, K.N. Krishna, B.S. Jai Prakash, Appl Clay Scie. 10 (1996) 439.
[2] N.J. Venkatesha, B.M. Chandrasekhar, B.S. Jaiprakash, Y.S. Bhat, J. Mol. Catal. A. 392 (2014) 181.
[3] C. Ravindra Reddy, Y.S. Bhat, G. Nagendrappa, B.S. Jai Prakash, Catal. Today, 141 (2009) 157.