Effect of Clay Binder on Sorption and Catalytic Properties of
Zeolite Pellets
Raksh V.Jasra,*,†Beena Tyagi,†Yogi M.Badheka,†种绿豆
Venkateswarlu N.Choudary,‡and Thirumaleswara S.G.Bhat‡
Central Salt and Marine Chemicals Rearch Institute,Gijubhai Badheka Marg,Bhavnagar364002,India,
and Rearch Centre,Indian Petrochemicals Corporation Limited,Vadodara391346,India
Sorption of N2,O2,Ar,and CH4in zeolites X,Y,and mordenite in powder as well as pellets was
studied using gas chromatography.Heats of sorption in pellets formed using clay as a binder
incread by34-78%for N2and7-18%for CH4relative to the respective zeolite powder.Surface
acidity of zeolites upon pelletization as measured by ammonia temperature-programmed
desorption showed that high-strength acid sites completely disappear for both zeolite Y and
mordenite when bound with bentonite and attapulgite.Decreas in weaker acid sites upon
pelletization for mordenite and HY are47-52%and34%,respectively.Fourier transform infrared
spectroscopy also showed a decrea in acidity upon pelletization of zeolite powders as obrved
from the reduction in the intensity of the peaks corresponding to acidic hydroxyl groups.Influence
of the decrea in surface acidity upon pelletization was also obrved from conversion values
for alkylation of toluene with methanol in zeolite pellets.For example,reduction in total xylene
formed was by50%,87%,and100%for HY520,HY510,and HM510,respectively.The
obrvations have been interpreted in terms of solid-state migration of cations prent in clay
interlayer space to zeolite extraframework sites during pelletization.Reexchange of cations in
zeolites HY and HM with an aqueous solution of Na+/Ca2+salts shows sorption isotherms clor
to the respective pelletized samples.X-ray diffraction studies of zeolites in powders as well as
pellets confirm migration of clay cations to zeolite extraframework sites.
1.Introduction
Zeolites are synthesized as polycrystalline powders, normally in the1-10µm particle size range,and invariably need to be shaped into bodies such as granules,spheres,and extrudates prior to their u as adsorbents and catalysts in commercial reactors in order to avoid pressure drop and to have higher mechanical strength particles.1,2Shaping of the zeolite powder into pellets is commercially done using natural clays such as bentonite,attapulgite,and kaolin to the extent of15-20%of the zeolite pellet as inorganic binders.2During the process of forming of zeolite pellets,the are subjected to high-temperature(>823K)treatment to destroy the surface area and activity of the clay.The process of pellet formation of zeolite powders creates meso/macropores in a pellet,thus modifying the diffu-sion characteristics of the sorbate/reactant molecules.3,4 The clays ud as binders consist of substantial amounts of group1a and1b elements as relatively mobile nonframework cations.Clay-type binders are also re-ported to affect the catalytic characteristics of the zeo-lite by modifying the acidity of the zeolites during pelletization.5-14For example,an appreciable decrea in both the inter-and intracrystalline acidity of H-gallosilicate zeolite was reported with kaolin clay as a binder.5Zeolite mordenite bound with sodium montmo-rillonite clay sho
wed lower n-butane isomerization and high disproportionation activity compared to zeolite powder,indicating a decrea in weak acidity and an increa in high strength acid site density.6Similarly, lower conversion in n-butane hydroisomerization is reported for Pd/ZSM-5and Pd/H zeolites after pelleti-zation with bentonite clay.7,8The prence of binder has been reported to decrea the zeolite strong acid sites as a result of solid ion exchange between zeolite protons and clay sodium.8In this ca,the product lectivity was also shown to be influenced,indicating that zeolite hydrogen transfer activity,metal/acid site balance,and diffusion of products get modified during pelletization. Pelletization of dealuminated mordenite with kaolin clay was reported to cau an improvement in its coke stability becau of trapping of coke precursors by kaolin.9However,this caud a decrea in zeolite activity for methanol to gasoline conversion due to neutralization of strongest acid sites by clay cations.It has been shown from the study on the preparation parameters of powder and pelletized Pt/KL catalysts for n-hexane aromatization that preparation of the cata-lysts directly on the pellets resulted in a large fraction of Pt clusters external to the pores and likely on the binder which caud rapid deactivation of the catalysts by coke formation.10Therefore,it was propod that Pt impregnation be done on a zeolite powder prior to pellet extrusion.In some cas,binder show beneficial effects like trapping of metal poisons such as vanadium and nickel from petroleum feedstock and improvement of the catalyst activity.11,12Unlike clays,when alumina is
ud as a binder for zeolites,it has been reported13to enhance the acidity of the catalysts by forming new acid sites
*To whom correspondence should be addresd.Fax:+91-278-2567562.E-mail:s.in.
†Central Salt and Marine Chemicals Rearch Institute.
‡Indian Petrochemicals Corp.Ltd.3263
黑色素细胞
Ind.Eng.Chem.Res.2003,42,3263-3272
10.1021/ie010953l CCC:$25.00©2003American Chemical Society
Published on Web06/03/2003
due to migration and inrtion of Al species in the zeolite framework.13Alumina binder was also reported to act as a sink for alkaline sodium cations,thereby improving the acidity of the zeolites.14Therefore,it is important to carefully choo the binder,also have suitable condi-tions of blending and granulation,and have information on the influence of the binder on sorption and catalytic properties of the zeolite pellets.However,despite the commercial significance of this informat
ion for develop-ing an industrial adsorbent or catalyst,there is spar literature on the influence of the binder on the catalytic and sorption properties of zeolites.The prent work was undertaken with an objective of finding the effect of bentonite and attapulgite clay binders on the sorption and catalytic properties of zeolite X,Y,and mordenite pellets.
2.Experimental Section
2.1.Materials.Zeolite NaX,both powder and pellets, were supplied by CATAD,a division of Indian Petro-chemicals Corp.Ltd.,Vadodara,India,and zeolites powders NaY90,HY510,HY520,NaM60,and HM510 were supplied by Zeocat,Montoir de bretagne,France. Three types of bentonite and attapulgite clays were ud as inorganic binders.Except for bentonite clay,B-ind, which was of local origin,other clays ud were supplied by the Clay Mineral Society,Source Clay Mineral Repository,University of Missouri,Columbia,MO. Chemical compositions of the zeolites and clays as determined by X-ray fluorescence spectroscopy are given in Tables1and2,respectively.Ultrahigh-purity(99.99%) N2,O2,Ar,CH4,and He were ud in all sorption measurements.Toluene and methanol were of analyti-cal grade with>99.5wt%purity.
2.2.Cation Exchange and Formation of Zeolite Pellets.Sodium and calcium cation exchange in zeolit
es HY510and HM510was done by taking1g of zeolite in 80mL of a1M solution of NaCl/CaCl2and refluxing at 353K for8h.A zeolite sample after the reflux was recovered by filtration and was thoroughly washed with distilled water until the abnce of Cl-in the washed solution as tested by a silver nitrate solution.The extent of cation exchange was determined by analysis for Na+ or Ca2+in the washed solution.Samples were oven dried at373K and activated at623K for8h prior to sorption measurement.
Zeolite pellets were formed by thoroughly mixing20 wt%clay binder with zeolite powder,converting the mixed powder into paste by adding distilled water,and kneading.The paste was extruded using a hand ex-truder having1.5mm diameter die.No viscosity modi-fier was added as an extrusion aid.The formed bodies were air-dried for about12h followed by oven drying at383K for6h.Thus,dried pellets were then calcined in air at873K for6h to destroy the surface area of the clay.The mechanical strength of the pellets formed after calcination was8kg.
Zeolite powders pelletized using bentonite and atta-pulgite clay supplied by Clay Mineral Society were designated with suffixes B and A,respectively,to the zeolite code.For example,zeolite powder NaY pelletized with bentonite clay was designated as NaY90B.For zeolite powder pelletized with Indian bentonite,the suffix was B-ind instead of B.
2.3.Sorption Measurements.Sorption of N2,O2, Ar,and CH4was measured both in powder and zeolite pellets formed using bentonite and attapulgite clay using a gas chromatograph(GC)method described in detail elwhere.15-17This method is limited to the low-pressure region of the isotherm and hence yields values of heats of adsorption in the Henry region only.A Varian Vista6000GC fitted with a gas sampling valve and thermal conductivity detector was ud for measure-ments.The zeolite sample was crushed and sieved to get a180-250-µm-sized fraction,which was packed carefully using a mechanical vibrator in a500-mm-long stainless steel column of6mm outer diameter and4 mm inner diameter.The linear velocity through the bed was8cm/s.The effect of the pul volume was carried out and found to be in a linear range,showing that the system operated in the Henry region.The effect of the pul volume on the retention time was studied to arrive at the pul volume of0.5mL ud in measurements. With the total volume of the system and packed bed volume being8.1and6.3mL,respectively,the total dead volume works out to be4.0mL,assuming that35% of the packed volume is void.The sample was activated by heating the column at673K under a hydrogen flow (60mL/min)for4h.The column temperature was then lowered to303K and the carrier gas flow decread to 30mL/min.A pul(0.5mL)of the gas mixture consisting of0.5vol%of one of the sorbate gas, namely,N2,O2,Ar,or CH4,and2vol%of He in H2 was injected into the column,and the retention times were measured.This procedure was repeated at differ-ent temperatures in the range
of303-373K.The retention times were corrected by subtracting the He retention time from tho of sorbate gas.Reproduc-ibility of the measurements of the retention time was done for many systems,and the values of the retention times were preci within(0.01min.Corrected reten-tion times were ud to calculate the net retention volume,V N,and specific retention volume,V m,using eqs 1and2.where j is given by j)(3/2){[(p i/p o)2-1]/[(p i/
Table1.Chemical Composition of Zeolite Samples Studied
constituent NaX NaY90HY510HY520NaM60HM510 SiO2(%)48.366.492.195.880.192.1 Al2O3(%)33.920.97.8 4.112.47.8 Na2O(%)17.812.70.10.147.50.04 Si/Al overall 1.2 2.710.020.0 5.510.0 Si/Al framework 1.2 2.710.020.0 5.510.0 unit cell(Å)25.1124.6224.3324.28
Table2.Chemical Composition(wt%)of Clays Ud a
开学的英文constituent Indian
bentonite,
Swy-1
Clay Mineral Society
attapulgite,
Pfl-1
Clay Mineral Society
SiO244.062.960.9
换奶粉Al2O316.019.610.4
Fe2O312.5 3.35 2.98
TiO2 1.00.900.49
MnO0.0060.058
MgO 2.4 3.0510.2
CaO 2.6 1.68 1.98
Na2O 2.2 1.530.058
K2O0.20.530.80
LOI18.0 6.0610.31
a Oxides such as FeO and P2O3with<1wt%concentration not included.
V N )Ft
R
j/(1-p
w
/p
o
)T/T
通用作文
R
(1)
V
m
)V
N
/w
s
(2)
3264Ind.Eng.Chem.Res.,Vol.42,No.14,2003
p o)3-1]}and F is the carrier gas flow rate measured using a soap-film meter,t R is the retention time,p i and p o are the column inlet and outlet pressures,p w is water vapor pressure at room temperature T R,T is the column temperature,and w s is the total weight of the adsorbent in the colu
mn.
Initial heats of sorption,∆H0,were calculated from specific retention volumes using eq3where R is the gas constant and T is the GC column temperature in kelvin. The uncertainty in the values of V m and∆H0as calculated using the method of propagation of errors from the known errors in the experimental parameters were(0.8and(1.8%,respectively.
N2sorption isotherms at303K for heat-treated/ cation-reexchanged clays and zeolites were measred using a Micromeritics volumetric adsorption tup, model ASAP2010.
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2.4.Acidity Measurement by Ammonia Temper-ature-Programmed Desorption(TPD).TPD of am-monia was carried out employing a microreactor(10cm length and4mm diameter)fitted with a gas sampler and GC with a thermal conductivity detector.He was ud as a carrier gas.About0.05g of the zeolite sample loaded in the microreactor was activated at813K for1 h under a He flow(30mL/min)before cooling to398K. Ammonia gas(0.75mL)was injected in puls until the zeolite sample was saturated.The unadsorbed ammonia was determined from the respon of the thermal conductivity detector of GC.This was followed by TPD of ammonia by heating the sample from398to873K at the heating rate of10K/min.The sample was kept at the final temperature for30min.
2.5.Diffu Reflectance Infrared Fourier Trans-form(DRIFT)Spectroscopy.Diffu reflectance in-frared spectra were recorded on a Perkin-Elmer Spec-trum GX spectrophotometer having“The Selector”DRIFT accessory(Graby Specac P/N19900ries) attached with Environmental Chamber(Graby Specac P/N19930ries)and Automatic Temperature Control-ler(P/N20130).About120mg of a powder zeolite sample was taken in the microcup of Environmental Chamber equipped with a ZnSe window.IR spectra (400-4000cm-1)were recorded first at room tempera-ture and then at temperatures from383to623K with He(40mL/min)as a carrier gas.The zeolite sample was held at the temperature of measurements for20min before recording the spectra.Background spectra was recorded with KBr.For both background and sample spectra measurements,100scans were co-added at a resolution of4cm-1using a standard mid-IR DTGS detector.
2.6.Toluene Alkylation.Acid-catalyzed toluene alkylation with methanol was studied as a reaction to obrve the effect of a binder on the catalytic activity of the pellets.In a typical reaction,zeolite sample(0.779
g),activated at673K for4h in a muffle furnace,toluene
(9.20g),and methanol(6.39g)were loaded into the autoclave(Paar-4842;100mL capacity),and the reac-tion was carried out at473K at an autogenously created pressure for4h.Following the completion of the reaction,the catalyst was parated from the reaction mixture.The liquid products were analyzed using a GC (Shimadzu GC-148)having a OV-1column using a flame ionization detector and N2(30mL/min)as a carrier gas.GC calibration was done using a liquid mixture with a known composition made with pure xylene isomers.
2.7.X-ray Diffraction.X-ray diffraction of the zeolite powders and pellets using Philips X′pert MPD system and Cu K R(λ)0.154nm)radiation was taken in the 2θrange from5to65°.X-ray diffraction of the H form of zeolites Y and mordenite was also taken after exchanging zeolites with Na+or Ca2+ions by treating zeolite powders with a1M aqueous solution of NaCl/ CaCl2at80°C for8h.Cation exchange was done repeatedly to ensure100%exchange,as determined by analyzing the starting and filtrate solutions for Na+/ Ca2+.
3.Results and Discussion
3.1.Sorption Properties.Specific Retention Vol-umes.The specific retention volume(V m)is linearly related to the Henry constant,which reflects the loading on the surface of the adsorbent at infinite dil
ution. Therefore,V m reprents the sorbate-sorbent interac-tions.The data for N2,O2,Ar,and CH4at different temperatures are shown for zeolite Y in Figure1.In the ca of both zeolites Y and mordenite,specific volume values are higher for sodium-exchanged(Na form)zeolites compared to proton-exchanged(H form). For example,NaY90has the highest V m values for the sorbate studies at all temperatures compared to HY510 and HY520.Similarly,among mordenite samples,NaM showed higher V m values particularly for N2(more than 4times)compared to HM510.This probably is due to electrostatic interactions of the sorbate molecules with sodium cations prent in Na-exchanged zeolites.
It was obrved from the data that zeolite samples in H form upon pelletization show a marked increa in V m values for all of the sorbates compared to zeolite powder.For example,the increa in V m for HY520and HY510was133and58%,respectively,upon pelletiza-tion.Similarly,HM510shows incread V m for all of the sorbates upon pelletization.However,the increa was higher for N2.On the other hand,it was obrved that zeolite powders in Na form show either a decrea or a smaller increa in V m values after pelletization. For example,NaY90showed an increa of only38% in V m.NaM60showed a decrea in V m values upon pelletization with bentonite and attapulgite.Zeolite NaX powder after binding with both Indian and Swy-1clay shows a decrea in the specific retention
volumes for all of the sorbates,though the decrea is substantial for CH4and N2.The obrvations clearly indicate enhanced sorbate-sorbent interactions upon pelletiza-tion for H-form zeolites.
Heats of Sorption.Heats of sorption,∆H0,at zero coverage determined from specific retention volumes are given in Table3.The obrvation that distinctly emerges from the data is the marked increa in∆H0for N2 and CH4during pelletization of proton-exchanged(H form)zeolite powder using clay as a binder.The increa in∆H0values during pelletization follows the order N2 .CH
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>O
2∼Ar.For example,∆H0(N2)in zeolite HY powders HY510and HY520incread from12.6kJ/mol to19.6and22.3kJ/mol,respectively,when pelletized with bentonite clay.Similarly,in the ca of HM510, which is an H form of zeolite mordenite powder,∆H0(N2),incread from18.9kJ/mol to25.3and28.2 kJ/mol when converted to pellets using bentonite (HM510B)and attapulgite(HM510A)clay,respectively.
-∆H
0)R d ln(V
m
/T)/d(1/T)(3)
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∆H 0(N 2)values in pelletized mordenite samples cor-relate well with the reported heats of adsorption of N 2on Na mordenite.18,19The corresponding increa in heats of sorption in the ca of CH 4is lower (12-17%only)upon pelletization of the zeolite powder,as en from Table 3.It is also obrved from the data that zeolite powders having sodium-exchangeable cations (Na form)show higher heats of sorption compared to zeolites in H form.The obrvations show that proton-exchanged (H form)zeolite powders have enhanced sorbate -sorbent interactions upon pelletization with clays.Furthermore,Na form zeolite powders have
Figure 1.Variation of the specific retention volumes [ln(V m /T )]for N 2(a),O 2(b),Ar (c),and CH 4(d)at different temperatures (1/T )on different zeolite samples.
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