EFFECT OF THE REACTION CONDITIONS ON AL-PILLARED MONTMORILLONITE SUPPORTED COBALT-BASED CATALYSTS FOR
FISCHER TROPSCH SYNTHESIS
N. AHMADa,b*, S.T. HUSSAINb, B. MUHAMMADa, T. MAHMOODb, Z. ALIb,d, N. ALIc, S.M. ABBASb, R. HUSSAINb, S.M. ASLAMe
a Department of Chemistry, Hazara University, Mansehra, Pakistan.
b National Centre for Physics, QAU campus, Islamabad, Pakistan.
c Department of Physics, University of the Punjab, Lahore, Pakistan.
d CEES, University of the Punjab, Lahore, Pakistan.
e Department of Physics, KUST, Kohat, Pakistan.
Fischer-Tropsch (FT) synthesis in fixed bed micro reactor (high pressure reaction cell) was studied over different wt% Co supported Na-montmorillonite (NaMMT) and Al-pillared montmorillonite (AlMMT) catalysts. The synthesized catalysts were characterized by XRD, TPR, NH3-TPD, TGA, BET and SEM techniques. FT reaction was carried out at temperature 225, 260 and 275 oC and pressure of 1, 5, and 10 bar. Co supported NaMMT catalysts showed very lower CO-conversion along with increase methane selectivity while AlMMT supported Co catalysts gave higher CO-conversion and lower methane selectivity. With increase in reaction temperature from 225 oC to 275 oC methane selectivity and CO-conversion increased and the selectivity towards C5+ hydrocarbons decreased. Decrease in methane selectivity while increase in C5+ hydrocarbons and CO- conversion was observed when the pressure was increased to 5 and 10 bar.
(Received January 8, 2013; Accepted February 8, 2013)
Keywords: Pillared montmorillonite, Aluminium, Cobalt, FT Synthesis, Catalysis, Coal gasification
1. Introduction
Due to the increasing energy demand and rapidly depleting resources of petroleum, huge reservoir of natural gas, coal and biomass can be utilized as an alternative to crude oil for the synthesis of sulfur, aromatics and nitrogen free ultra clean fuel along with value added fine chemicals via Fischer-Tropsch (FT) synthesis [1,2]. FT synthesis reaction has attracted worldwide attention to convert syngas (CO and H2) via gasification of coal, natural gas and biomass to liquid hydrocarbons by the use of different catalysts [3,4].
The most widely used catalysts for the FT synthesis include different forms of Ni, Fe, Ru and Co metals [5]. The use of these catalysts is limited due to the excessive CH4 production in case of Ni and high price of Ru, leaving only Co and Fe as feasible catalysts in FT synthesis [6]. Due to the lower extent of deactivation, water gas shift activity and selectivity towards linear hydrocarbons, Co is preferred over Fe-based catalysts in academic as well as on industrial scale [7].
The choice of support plays a significant role in FT synthesis, the activity and product selectivity of catalysts is markedly affected by the nature of support [8]. A variety of supports such as SiO2, Al2O3 SBA-15, MCM 48 have been used to improve the dispersion of Co hence its catalytic activity [9-13].
*Corresponding author: [email protected], [email protected]
In FT synthesis, Zeolites have been used as a support to synthesize hydrocarbons of specific molecular weights and to change the FT catalyst products distribution. The use of montmorillonite (MMT) as a support for iron based FT catalyst and for the catalysts used in hydrocarking of petroleum have been extensively studied [14]. But the main problem associated with the MMT supported FT catalysts is the production of large amount of methane and carbon dioxide, which are thought to be the most unwanted products in FT synthesis and must be reduced to the minimum possible levels. The production of large amount of methane and carbon dioxide was attributed to hindered reduction of metal oxide to metallic state by sodium montmorillonite (NaMMT) [9]. Beside this, the presence of alkali and alkaline earth metals specially Na showed marked negative effect on activity of FT catalysts due to the inhibition of metal oxide reduction [2]. Sodium ions and calcium ions present between the layers of minerals clay MMT can balance the net negative charge on the lattice produced by the substitution of Si4+ with Al3+ and Al3+ with Mg2+ due to which the MMT governs the property to grip guest molecules between its layers.
Pillaring agent such as hydroxy-metal cations can be used for the separation or support of MMT lattice layers by replacing the alkali metals and alkaline earth metals cations which upon calcinations results into the formation of large surface area microporous material. To obtain high surface area and pore volume, different metal oxides e.g. oxide of Al, Ti, Zr, and Cr have been used to replace the sodium ion present in the interlayer of MMT clay [15].
It is reported by different investigators that increase in acidic properties as well as texture modification of clay can be achieved when the clay is exchanged with different cations. MMT clays shows efficient catalytic activity towards liquid phase organic synthesis [14]. The pillaring of NaMMT with Al enhances the surface area of support MMT. It also increases the pore volume and pore diameter due to the contribution of Al to replace the Na+ present in the interlayer of MMT [2].
The use of Co catalysts for lower temperature FT synthesis has attracted worldwide attention. But after a long run of reaction the catalytic activity of cobalt-based FT catalysts become lowered which was attributed to the formation of higher molecular weight hydrocarbons (waxes) responsible for the blocking of active sites of catalysts.
The aim of present study was to convert these waxes during FT synthesis to lower hydrocarbons by the use of MMT (acidic support) supported Co–based FT catalysts. So far the application of pillared MMT catalysts is mainly concerned with the hydrocracking of petroleum products in refineries and there are very limited reports on the application of Co supported MMT catalysts in FT synthesis. The effect of reaction temperature, pressure and reactant gas flow is not reported yet.
2. Experimental 2.1 Materials
NaMMT, aluminium chloride (AlCl3.6H2O) and cobalt nitrate (Co(NO3)2.6H2O) from Sigma Aldrich were of analytical grade and were used without any further purification. NaMMT was selected as a starting material to prepare the AlMMT.
2.2 Preparation of AlMMT
In the typical synthesis procedure, 0.5 M NaOH solution was slowly added to 0.25 M solution of AlCl3.6H2O with OH/Al ratio of 2/1 with continuous stirring. The above suspension was aged at room temperature for 24 h. In the second step, above pillaring solution was added (5 mmol Al/g of clay) to the suspension of 2wt% MMT and stirred for three hour at 100 oC. The resultant slurry was centrifuged at 5000 rpm, washed several times with deionized water in order to remove excess sodium and chlorine, dried at 100 oC overnight, grinded and finally calcined at 400 oC for 5 h [16,17].
2.3 Preparation of 10, 15, 20, 25wt% Co/AlMMT
Wet impregnation method was adopted to synthesize AlMMT catalyst containing 10,15, 20 and 25wt% Co. To prepare 5g of catalyst 4.5, 4.25, 4 and 3.75g of calcined pillared clay was added to 0.1 M solution of Co(NO3)2.6H2O (2.46, 3.69, 4.92 and 6.15g) respectively with constant stirring. The resultant suspension was evaporated at 40 oC by rotary evaporator until the dry mass is obtained, washed several time with distilled water, dried overnight at 100 oC and finally calcined at 400 oC for 5 h.
2.4 Characterization of Prepared catalyst
X-ray diffraction (XRD) of sample was carried out using Scintag XDS 2000 diffractometer. Coulter SA 3100 BET surface area and pore volume analyzer using liquid nitrogen at a temperature of (78 K) were used for BET studies. H2-TPR and NH3-TPD profile was recorded On a TPDRO/1100 Series, Thermo Electron Corporation, Italy. Mettler TGA/SDTA 851e was used for Thermo gravimetric analysis (TGA) of all catalysts. JEOL JSM 6490-A equipped with Energy Dispersive X-ray Spectrometer (EDX) were used for the morphological and compositional characterization of catalysts.
2.5 Catalyst evaluations
As seen in Fig.1 the equipment was made of a stainless steel cell with two windows through which the spectrometer beam can pass. It was connected to a temperature programmer to heat the inside and the windows of the cell. There was a mass flow controller connected to three gas cylinders containing synthesis gas, hydrogen and Argon. The Gas Chromatography was placed at the outlet of the cell and was alimented by helium, air and hydrogen shown in Fig.1.
Fig. 1 Schemtic diagram showing the positioning of equipment and flow of gases.
The FT experiments were performed using a sample of catalyst lightly pressed, so as to obtain a disk of 13 mm of diameter with a defined weight; the disk was put into the cell. To remove air from the cell, the experiment begun by flushing Ar at 50 ml/min for 20 min. Once the catalyst had been reduced at 400 °C during 1 h in H2, the FTS reaction was carried out at 225, 260, 275 °C at 1,5 and 10 bar pressure and feed flow of synthesis gas during 2-30 h.The synthesis gas was composed of carbon monoxide and hydrogen with a ratio of CO/ H2 (1:2). The products were analyzed on-line by a gas chromatograph (Hewlett-Packard, Model 6890) equipped with flame ionization detector.
3. Results and discussion
3.1 Textural and Structural properties
AlMMT, 10, 15, 20 and 25wt% Co/AlMMT was synthesized and their XRD patterns are given in Fig. 2 and 3.
The naturally occurring MMT was pillared with Al by replacing the small cations such as Ca2+, K+ and Na+ present between the electronegative silicate layer of MMT having two dimensional structure by ion exchanged method. The sizes of the interlayer cations give the gallery height of MMT, hence any change in them will surly change the gallery height and d001 basal plane [17].
Fig.2 shows the XRD pattern of NaMMT having (001) diffraction peak with a basal spacing of 12.06 Å. Most of the authors agree that the thickness of the host layer of MMT is 9.3 Å [17]. So, the gallery height was found to be 3.3 Å calculated by subtracting the host layer thickness from the obtained basal spacing of the NaMMT.
By observing the XRD spectrum of AIMMT, as shown in Fig.2(b) we find that effect of Al addition has shifted the peak to a comparatively lower theta value and consequently, result an increase of d spacing from 12.06 to 18.7 Å and the gallery height from 3.3 to 9.4 Å. This causes the expansion of the clay interplanar distance with Al pillar, similar to the Keggin type ionic structure [Al13O4(OH)24(H2O)12]7+ as reported in refrence [16]. The XRD spectra of AlMMT give diffraction planes at 8o, 19.7o, 35o and 61.8o which are characteristic of two dimensional layers structure of clay. The intensity of the 001 peak was also decreased which can be attributed to the intercalation of Al into the MMT clay interlayer’s. Moreover, no other structural changes were observed in the XRD spectra of both the samples as rest of the peak positions remained unchanged.
Fig.3 shows the XRD pattern of Al MMT catalysts loaded with cobalt. All the catalysts show 001 diffraction peaks and basal reflection of AlMMT with a small decrease in their intensity which corresponds to the slight decrease in layered structure of MMT.
Cobalt loaded catalysts giving cubic cobalt oxides (Co3O4) diffraction at 19.5o, 32o, 37o, 45o and 60o (JCPDS 65-3103) indicates a uniform dispersion of pure Co3O4 on pillared MMT.
Fig.2 XRD pattern of (a)NaMMT and (b)AlMMT.
Fig.3 XRD pattern of the 10, 15, 20, 25wt% Co/AlMMT.
Table: 1 Textural property of the prepared catalysts.
Sample BET (m2/g) Pore diameter(Å) Pore volume(cm3/g) NaMMT
AlMMT
10wt% Co/AlMMT 15wt% Co/AlMMT 20wt% Co/AlMMT 25wt% CO/AlMMT
39.24 254.00 238.00 235.00 235.00 229.68
4.81 18.31 15.67 15.35 15.75 15.32
0.08 0.20 0.18 0.19 0.18 0.19
Table 1 show that NaMMT having lower surface area of 39.243 m2/g while AlMMT having larger surface area of 254 m2/g. The larger surface area of AlMMT is due to the ability of Al to replace the Na+ present in MMT results an increase in pore volume responsible for N2
adsorption [14]. Small decrease in BET surface area for Co-loaded catalysts seen from the table can be attributed to the presence of metal particle at pore opening and on the surface of the pores.
The average pore diameter and pore size calculated for all of the sample were constant therefore the pore blocking of Co can be neglected [18]. Enhanced FT activity can be expected from the AlMMT supported Co catalysts due to its high surface area and pore diameter larger than the interlayer distance of MMT similar to the previous study [17]. The adsorption isotherm (Fig.4) for NaMMT shows the non porous nature of NaMMT and the type IV isotherm for Co/Al-MMT confirms its microporous nature. After calcinations at 400 oC the same type isotherm obtained for the all the sample with slight change in pore diameter and pore volume confirmed that the narrow slit shaped porous structure of MMT did not change after calcinations.
Fig. 4 N2 Adsorption desorption Isotherm of Na MMT, AlMMT and Co/Al-MMT catalysts.
3.2. Acidic Property
NH3-TPD was used to evaluate the acidic properties of the prepared samples (Fig.5). It is reported that pillared clay poses both Lewis and Bronsted acidic site [19]. The NH3-TPD profile showed that the samples posses both weaker acidic sites desorption peak at 150 oC and stronger lewis acidic sites desorption at 600 oC. The weak bronsted acidic sites gives the lower temperature NH3 desorption peak while the high temperature desorption of NH3 occurs due to the strong lewis acidic sites [20]. The Co loading shows little effect on the acidity of pillared MMT by increasing the intensity of bronsted acid peak hence improving the bronsted acidity. The overall acidic property of the Co loaded sample slightly decreases, may be as a results of Co deposition on pore opening and blockage of some pore responsible for NH3 desorption and hence decreases the density of Lewis acidic sites of pillared clay similar result have already been reported for vanadium loaded pillared clay [19].
Fig. 5 NH3-TPD profile of Samples.
The SE flake li have n
small p sample and 37 to the that th respect attribu of pilla
3.3 SEM S To study th EM microgr ike morphol no effect on t
3.4 Reduc The H2 –T peak around e after calcin 79 oC is due t previously r he two steps
tively. The d uted to enhan
ared montmo Studies
he surface m raph for NaM
ogy which in the MMT lay
Fig.
ction behavio PR profile o d 210 oC for
nations, whil to the reduct reported resu reduction o decrease in r nced dispersio
orillonite as r
Fig
morphology o MMT and CO
nfer that the yered structur
6 SEM Imag or
f 10%, 15 % the sample le the first a tion of Co3O ults [14]. The
f cobalt oxid reduction tem on of cobalt results of Al-
g. 7 H2-TPR p
of our prepar O/AlMMT a calcinations re.
ges of (a)NaM
% and 20% an is due to the and second r O4 to CoO and
e TPR profil des occurs a mperature of particles on -pillaring.
profile of the p
red sample S are given in temperature
MMT(b) Co/AlM
nd 25% Co/A e decomposi eduction pea d CoO to me le of AlMM at lower temp
f AlMMT su the surface o
prepared samp
SEM charact Fig.6a and b , pillaring of
MMT
AlMMT are ition of Co(N ak for 20% C etallic Co res
T supported perature of 2 upported cob of support an
ples.
terization wa b showed th f Al and Co l
given in Fig No3)2 remain Co/NaMMT spectively is Co samples 268 oC and balt samples nd high surfa
as used.
he same loading
g.7. The n in the at 278 similar s shows 365 oC can be ace area
3.5 Thermogravimetric analysis (TGA)
TGA of 20wt% Co/NaMMT and different wt% Co/AlMMT has been carried out (Fig.8).
This is clearly seen from TGA data that all samples exhibit continuous weight lose behavior till 800 oC. The weight lose upto 160 oC can be attributed to the removal of surface adsorbed water while the weight lose between 160 oC to 500 oC is attributed to the interlayer water and due to the dehydroxylation of the clay. Interestingly, as the wt% of Co increased, the weight loses around 300 oC also increased. The continuous weight lose from 600 oC till 800 oC was attributable to the removal of hydroxide groups as a result of dehydroxylation of pillar and clay structure, causing collapse in MMT layer structure [19-23]. These results also show an agreement with previously reported results for thermal behavior of vanadia-loaded pillared clays [21].
Fig. 8 TGA Curve of the samples.
4. FT catalytic activity test
4.1 Catalytic activity of 20wt% Co/NaMMT
The catalytic performance of 20wt% Co/NaMMT in terms of selectivity, CO-conversion and time on stream is presented in Fig. 9 (a) with increasing time on steam (TOS), the catalysts show increased CH4 selectivity and very small selectivity for the remaining hydrocarbons. The catalytic stability of Co/NaMMT was studied for 10 h and we initially observed a CO-conversion of around 10% but it went on decreasing till it reached to less than 5% in 10 h TOS.
Simultaneously, increase in the CO2 and CH4 selectivity of catalysts was observed which increased to more than 40% during this TOS. These gases are the most unwanted products in FT synthesis and must be reduced to the minimum possible levels. The low FT activity of the Co/NaMMT observed in this case can be attributed to their lower surface area, larger particle size of Co over NaMMT and hindrance of CoO reduction to Co by NaMMT as reported in reference [24]. From this study, it was found that first two had no significant effect on the CO-conversion but the lower FT activity and higher selectivity toward CH4 was mainly due the incomplete reduction of cobalt oxides. Beside this, the presence of alkali and alkaline earth metals specially Na showed marked negative effect on activity of FT catalysts due to the inhibition of CoO reduction [14].
The FT products obtained at a TOS of 10 h for this catalyst mainly include the C2-C4 and lesser amount of C5-C7 hydrocarbons. From the above discussions, it can be concluded that Co/NaMMT shows very poor catalytic activity in FT synthesis.
to 254 replace The in MMT, 26, 27 decrea conver reache Co cat has a p Wang selectiv hydroc long c hydroc
Fig. 9 CO-co
4.2 Cataly The pillari 4 m2/g. It also
e the Na+ in nter layer cati , Al-pillared , 28 and 27.2 ases with TO rsion was se es a minimum
talysts was h positive imp et al [14]. T vity of C2-C carbons C21
chain hydroc carbons was
nversion and
ytic activity ing of NaMM
o increases t the MMT in ion plays a v MMT suppo 2% for 10, 1 OS and reac en for 20wt%
m of 11% af igher than N pact on the C The FT produ C12 hydrocar at a TOS of carbons at t found maxim
methane selec (b Different
of Al pillare MT with Al e the pore volu nterlayer [14 vital role in t orted catalyst 15, 20 and 25 ches a minim
% Co/AlMM fter 30 h of T NaMMT supp CO-conversio ucts obtained rbons and d f 2- 30 h. Th the acidic si mum at a TO
ctivity verses wt% Co/AlMM
ed MMT (D enhances the ume and por 4] and hence the CO-conv ts show high 5wt% Co/Al mum of 10 MT catalysts
TOS. The CO ported cataly
on in FT syn d (Table 2) o decreased se
his decrease ite of MMT OS of 8 h.
Time on strea MT catalysts.
ifferent wt%
e surface area re diameter d the catalytic version.As co her CO-conv
lMMT cataly .5% after 30
which was O-conversion ysts, indicatin
nthesis was over Co/AlM
lectivity tow in C21 selec T [25-29]. T
am for (a) 20%
% Co/AlMM a of support due to the co c activity of C ompared to C
ersion. The C ysts respectiv
0 h of TOS 28 %, decre n in case of ng that the A alike to the MMT catalyst
wards higher tivity is due The selectivi
% Co/NaMMT
MT)
MMT from ontribution o Co/MMT ca Co supported CO-conversi vely (Fig.9b) S. The highe eases with TO
AlMMT sup Al-pillaring o
results obtai st showed im r molecular e to the crack
ity towards T,
39.243 of Al to atalysts.
d on Na ion was ) which er CO- OS and pported f MMT ined by mproved weight king of C2-C12
20w 20w 10w 15w 25w
investi conver and 27 respect attribu perform to 275 an agre
reactio respect C5 hyd molecu with in increas
Catalysts
wt% Co/NaM wt% Co/AlMM wt% Co/AlMM wt% Co/AlMM wt% Co/AlMM
4.3 Effect The effect igated (Fig.1 rsion with m 75 oC the CO tively along uted to the mance of sta
oC produces eement with
Fig. 10 Effec
4.4 Effect Reaction p on carried ou tively (Fig.1 drocarbons d ular weight h ncreasing pr se in partial p
Table
CO-co
MMT MT MT MT MT
2 2 2 2
of reaction t of reactio 0 a). The rea methane selec O-conversion with decreas high hydrog andard cobal s too much m
the results r
ct of reaction
of reaction pressure larg ut at a pressu 0b). The sel decreases wit hydrocarbon ressure as re pressure of
2 Results of D
onversion
10.2 28.3 26.5 27.0 27.2
temperatur n temperatu action carried ctivity of 27.
n and methan se in C5+ hyd genation act lt thorium ca methane and t eported earli
temperature (
pressure on gely effect ure of 1, 5 a lectivity of C th increase in ns as a result
eported earli hydrogen an
Different catal
CO Selectivit 2.3 2.5 2.0 1.9 1.8
re on catalyt ure on catal d out at a tem .2%. When t ne selectivity drocarbons se tivity of cat atalysts [30].
therefore it i ier [31].
(a) and reacti
n catalytic ac the activity and 10 bar s C6+ hydrocarb
n pressure. H t of chain gr ier [32]. The nd the increas
lysts for FT sy
O2
ty
C1 (wt%
40.
27.2 26.
26.
28.
tic activity o lytic activity mperature of the reaction t y were also in
electivity. Th talysts at hi . Increase in is not benefic
ion pressure (b
ctivity of cat and selecti showed CO- bons increas High pressur rowth probab
e increase in se selectivity
ynthesis
%)
C2-C12
(wt%) 1
2 0 8 3
12.5 18.2 19.2 20.0 19.0
of catalysts.
y of 20wt%
f 225 oC give temperature ncreased to 3 his effect of
gh temperat reaction tem cial in Co ba
(b) on FT cata
talysts.
ivity of FT -conversion o ed and that o re favors the bility which n CO-conver y of the C6+ h
C13-C2
(wt%) 5
2 2 0 0
8.2 11.2 11.7 12.
13.
% Co/AlMM es almost 28 is increased 32, 35 and 3 the catalysts ture similar mperature fro ased FT synth
alytic activity.
catalysts. T of 28, 32 an of methane a production normally inc rsion is due hydrocarbons
20 C21 (wt%) 2
2 7 1 1
37.
41.
41.
39.
37.
MT was
% CO- d to 260 7, 40%
s can be to the om 225 hesis as
The FT nd 33%
and C2- of high creased e to the
s is due )
4 4 1 0 7
to the increase CO partial pressure while the reaction rate in such type of reaction is proportional to the hydrogen partial pressure as reported previously [33].
5. Conclusions
The ability of MMT as a support for FT catalysts was successfully improved by pillaring with Al. The inhabitation of CoO reduction and increased methane selectivity of MMT in FT reaction caused by the presence of Na in MMT was overcome by pillaring with Al. Increase in the surface area from 39.243 to 254 m2/g along with increase in pore volume and pore diameter may be explained due to the contribution of Al to replace the Na+ in the MMT results into its pore opening and production of new pores responsible for N2 adsorbtion [14].
The aptitude of Co catalysts for FT synthesis was improved by supporting it on AlMMT.
AlMMT supported Co catalysts showed higher CO-conversion and greater selectivity towards C2- C12 hydrocarbons and deceases selectivity towards C20+ hydrocarbons as a result of hydrocracking reaction taking place at the acidic sites of montmorillonite. With increase in reaction temperature methane selectivity and CO-conversion increased and the selectivity towards C5+ hydrocarbons decreased due to the high hydrogenation activity of catalysts at high temperature. Decrease in methane selectivity while increase in C5+hydrocarbons and CO-conversion was observed when the pressure was increased to 5 and 10 bar.
Acknowledgements
The author would like to thank Higher Education Commission of Pakistan for the financial support of this work under Indigenous-5000 PhD Fellowship program.
References
[1] H. Schulz, Appl.Catal .,A. 186 3 (1999).
[2] J.J.H.M. Font Freide, T.D. Gamlin, C. Graham, J.R. Hensman, B. Nay,C. Sharp, Top.Catal.
26, 3 (2003).
[3] M.E. Dry, Catal. Today. 71 227 (2002).
[4] A.Y. Khodakov, W. Chu,P. Fongarland, Chem. Rev. 107 1692 (2007).
[5] H. Wang, Y. Yang, J. Xu, H. Wang, M. Ding,Y. Li, J. Mol. Catal. A: Chem 326 29 (2010).
[6] S. Li, S. Krishnamoorthy, A. Li, G.D. Meitzner,E. Iglesia, J. Catal. 206 202 (2002).
[7] A. Dinse, M. Aigner, M. Ulbrich, G.R. Johnson,A.T. Bell, J. Catal. 288 104 (2012).
[8] S.T. Hussain, M. Mazhar,M.A. Nadeem, J. Nat. Gas Chem. 18 187 (2009).
[9] H. Li, S. Wang, F. Ling,J. Li, J. Mol. Catal. A: Chem. 244 33 (2006).
[10] J. Panpranot, J.G. Goodwin Jr,A. Sayari, J. Catal. 211 530 (2002).
[11] G. Jacobs, T.K. Das, Y. Zhang, J. Li, G. Racoillet,B.H. Davis, Appl.Catal . A. 233 263 (2002).
[12] H. Xiong, Y. Zhang, K. Liew,J. Li, J. Mol. Catal. A: Chem. 295 68 (2008).
[13] G. Prieto, A. Martínez, R. Murciano,M.A. Arribas, Appl.Catal., A. 367 146 (2009).
[14] G.-W. Wang, Q.-Q. Hao, Z.-T. Liu,Z.-W. Liu, Appl.Catal., A. 405 45 (2011).
[15] R. Issaadi, F. Garin, C.E. Chitour,G. Maire, Appl.Catal .,A. 207 323 (2001).
[16] Martı, amp, x, M.J. nez-Ortiz, G. Fetter, Domı, J.M. nguez, J.A. Melo-Banda, R. Ramos-Gómez, Microporous and Mesoporous Mater. 58 73 (2003).
[17] Y. Liu, K. Murata, K. Okabe, M. Inaba, I. Takahara, T. Hanaoka,K. Sakanishi, Top. Catal.
52, 597 (2009).
[18] H. Su, S. Zeng, H. Dong, Y. Du, Y. Zhang,R. Hu, Appl. Clay Sci. 46 325 (2009).
[19] K.V. Bineesh, D.-K. Kim, M.-I.L. Kim,D.-W. Park, Appl. Clay Sci. 53 204 (2011).
[20] K.V. Bineesh, S.-Y. Kim, B.R. Jermy,D.-W. Park, J. Ind.Eng. Chem. 15 207 (2009).
[21] K.V. Bineesh, D.-K. Kim, D.-W. Kim, H.-J. Cho,D.-W. Park, Energy Environ. Sci . 3, 302 (2010).
[22] P. Cañizares, J.L. Valverde, M.R. Sun Kou,C.B. Molina, Microporous and Mesoporous Mater. 29 267 (1999).
[23] K.V. Bineesh, D.-K. Kim, H.-J. Cho,D.-W. Park, J. Ind.Eng. Chem. 16 593 (2010).
[24] J. Thiessen, A. Rose, J. Meyer, A. Jess,D. Curulla-Ferré, Microporous and Mesoporous Mater.164 199 (2012)
[25] S.-H. Kang, J.-H. Ryu, J.-H. Kim, P.S. Sai Prasad, J.W. Bae, J.-Y. Cheon,K.-W. Jun, Catal.
Lett. 141 1464 (2011).
[26] C. Ngamcharussrivichai, A. Imyim, X. Li,K. Fujimoto, Ind.Eng. res. 46 6883 (2007).
[27] A.K. Dalai,B.H. Davis, Appl.Catal,. A: 348 1 (2008).
[28] J. Bao, J. He, Y. Zhang, Y. Yoneyama,N. Tsubaki, Angewandte Chemie International Edition 47, 353 (2008).
[29] Z.W. Liu, X.H. Li, K. Asami,K. Fujimoto, Catal. Today. 104 41 (2005).
[30] S. Colley, R.G. Copperthwaite, G.J. Hutchings,M. Van der Riet, Ind.Eng. res.
27, 1339 (1988).
[31] D. Das, G. Ravichandran,D.K. Chakrabarty, Appl.Catal,. A: 131 335 (1995).
[32] Z. Yan, Z. Wang, D.B. Bukur,D.W. Goodman, j.Catal. 268 196 (2009).
[33] H. Schulz,M. Claeys, Appl.Catal,. A: 186 91 (1999).
