Designing a Capsule Catalyst and Its Application for
Direct Synthesis of Middle Isoparaffins
Jingjiang He,†Yoshiharu Yoneyama,†Bolian Xu,‡Norikazu Nishiyama,§and
Noritatsu Tsubaki*,†
Department of Applied Chemistry,School of Engineering,Toyama University,Gofuku 3190,Toyama 930-8555,Japan,Department of Chemistry,Nanjing University,Nanjing 210093,China,and Division of Chemical Engineering,Graduate School of Engineering Science,Osaka
University,Osaka 560-8531,Japan
Received November 12,2004.In Final Form:January 12,2005
A catalyst in the form of a capsule catalyst was prepared by coating HZSM5membrane on a preshaped Co/SiO 2catalyst pellet.The capsule catalyst with HZSM5membrane exhibited excellent selectivity for light hydrocarbon synthesis,especially for isoparaffin synthesis from syngas (CO +H 2).Long-chain hydrocarbon formation was totally suppressed by the zeolite membrane.The modificatio
n of membrane and core catalyst significantly improved the catalytic properties of these new kinds of capsule catalysts.
Introduction
The Fischer -Tropsch synthesis (FTS)reaction,CO +H 2)hydrocarbons +H 2O,was found by Fischer and Tropsch in 1925.1It can produce liquid fuels such as gasoline and diesel oil from coal or natural gas.As more and more natural gas resources are found in the world presently,the FTS process becomes more promising.2Furthermore,as the technique of producing syngas (CO +H 2)from biomass is being developed,synthetic liquid fuel can be obtained from biomass via FTS.3The FTS products are almost normal hydrocarbons,either olefins or paraffins,and the product selectivity follows the ASF distribution.The advantages of FTS hydrocarbons are their high n -paraffin content,high cetane number as diesel fuel,and sulfur-free,aroma-free,nitrogen-free properties,especially on cobalt-based catalysts.4-7The production of hydrocarbons rich in isoparaffins,alkylates,has gained much attention because of its high octane numbers if used as synthetic gasoline.Several groups have tried to make isoparaffins by utilizing FTS catalyst,which is metal dispersed on acidic zeolite or other acidic supports.8But these catalysts performed with very low conversion due to a rather low reduction degree.We have recently reported that when a mechanical mixture of zeolite and normal FTS catalyst,
Co/SiO 2,was used in a single-or dual-step FTS reaction,the formation of short-chain isoparaffins was enhanced while the formation of longer hydrocarbons was suppressed.9
Zeolite is a special material with unique pores and channels.The varied molecular diffusion rate in these pores and the shape selectivity,as well as acidic properties,make it widely used.Many studies on preparing zeolite membrane and its application for separation have already been reported.10On the other hand,zeolite is also a good hydrocracking/hydroisomerization catalyst due to its acidic properties.11In the present work,zeolite membrane was tailor-made coated on the FTS catalyst pellet,Co/SiO 2,and we call it a capsule catalyst.In the reaction,feed gas,CO +H 2,diffused through zeolite membrane and arrived at the FTS catalyst.Then the hydrocarbons formed there and desorbed.When the hydrocarbons diffused into the zeolite membrane,all of them,in the form of normal hydrocarbons,could enter zeolite channels and must be cracked and isomerized by acidic sites inside zeolite channels.A schematic image is shown in Figure 1.For long-chain hydrocarbons,their low diffusion rate in zeolite membrane makes them stay in the membrane layer longer,having a higher possibility of isomerization and cracking reaction inside the membrane.Furthermore,compared to conventional membrane reactors,the catalyst designed above has larger membrane area per unit reactor volume.This kind of capsule catalyst is of great advantage in practical application,because membranes with large area and without pinholes or cracks are very difficult to
*To whom correspondence should be addressed.E-mail:ama-u.ac.jp.
†Toyama University.‡Nanjing University.§Osaka University.(1)Olive,H.G.;Olive,S.The Chemistry of the Catalyzed Hydrogena-tion of Carbon Monoxide ;Springer-Verlag:Tokyo,1984;p 144.
(2)Wegrzyn,J.E.;Mahajan,D.;Gurevich,M.Catal.Today 1999,50,97.
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(8)Chen,Y.W.;H.T.Tang;Goodwin,J.G.,Jr.J.Catal .1983,83,415.Nijs,H.H.;Jacobs,P.A.J.Catal .1980,66,401.Jacobs,P.A.Catalysis by Zeolites ;Imelik,B.,Naccache,C.,Vedrine,J.C.,Eds.;Elsevier:Amsterdam,1980;p 293.
(9)Li,X.;Asami,K.;Luo,M.;Michiki,K.;Tsubaki,N.;Fujimoto,K.Catal.Today 2003,84,59.
(10)de Vos,R.M.;Verweij,H.Science 1998,279,1710.Nishiyama,N.;Miyamoto,M.;Egashira,Y.;Ueyama,K.Chem.Commun.2001,18,1746.Lai,Z.;Bonlla,G.;Diaz,I.;Nery,J.G.;Sujaoti,K.;Amat,M.A.;Kokkoli,E.;Terasaki,O.;Thompson,R.W.;Tsapatsis,M.D.;Vlachos,G.Science 2003,300,456.
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reaction diffusionFigure 1.A schematic image of the capsule catalyst role in the FTS reaction.
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prepare in most cases.12This new kind of capsule catalyst is expected to have wide applications,if the combination of core catalyst and membrane catalyst is varied according to the target reaction.
Experimental Section
The conventional FTS catalyst was prepared by incipient-wetness impregnation of an aqueous solution of Co(NO 3)2‚6H 2O and two kinds of silica support (Cariact Q-10,Fuji Silysia Co.;specific surface area,323m 2‚g -1;pore volume,1.03mL ‚g -1;pore diameter,10nm)whose pellet size was 0.85-1.7and 0.38-0.50mm,respectively.The catalyst precursors were dried in air at 393K for 12h and then calcined in air from room temperature to 673K with a ramping rate of 2K ‚min -1and kept at 673K for 2h.After calcination,the catalysts were cooled to room temperature in nitrogen.
In zeolite membrane synthesis,distilled water and ethanol (Wako Pure Chemical Industries Ltd.,99.5%)were used for solutions.The templet was TPAOH (tetrapropylammonium hydroxide solution,
Wako Pure Chemical Industries Ltd).Al and Si sources were Al(NO 3)3‚9H 2O (99.5%)and TEOS (tetraethyl ortho silicate;Wako Pure Chemical Industries Ltd.),respectively.TEOS/TPAOH/H 2O/EtOH/Al(NO 3)3)1:0.25:60:4:0.025.First,TEOS,10%TPAOH water solution,ethanol,and water were added in a 100mL Teflon tank.Then Al(NO 3)3‚9H 2O was added to the mixture solution carefully and stirred at 333K for 2h until the lucid sol was formed.Continuously,the normal FTS catalyst was added in the sol,and the capped tank was put in hydrothermal synthesis equipment (DRM-420DA,Hiro Co.,Japan),heated to 453K,and run at 10rpm with various duration times for crystallization.In this process,the zeolite membrane was coated on the surface of FTS catalyst pellets.Then,the coated catalyst was separated from the synthesis solution and dried at 393K for 12h,followed by calcination at 773K for 5h where the temperature rising rate was 1K/min from 393to 773K.Thus,a capsule catalyst was obtained.The pure zeolite was
synthesized by the same method without adding the Co/SiO 2catalyst,as a blank experiment.
The morphology and surface component analysis of the Co/SiO 2catalyst and the prepared capsule catalysts were investi-gated with a scanning electron microscope equipped with an EDX attachment (JEOL JSM-6360LV,15-20kV,1.0nA).The catalyst was precoated with Pt before characterization.
The FTS reaction was conducted under pressurized conditions,1.0MPa,533K,by using a flow-type fixed reactor.Before reaction,the catalyst was reduced in flowing hydrogen at 80mL/min at 673K for 10h and at last cooled to 353K in nitrogen.The catalyst amount was 0.5g on the Co/SiO 2base,and the H 2/CO ratio of the feed gas was 2.During the reaction,effluent gas released from the reactor was analyzed by on-line gas chromatography using an active charcoal column equipped with a thermal conductivity detector (TCD).The hydrocarbons were also ana-lyzed on-line using a capillary column (J&W Scientific GS-Alumina,30m)equipped with a hydrogen flame ionization detector (FID).A trap with concentrated sulfuric acid was attached to the system for adsorbing the olefins.The olefin hydrocarbons were calculated from difference of FID peaks after the olefins were absorbed by the concentrated sulfuric acid.
Results and Discussion
The morphology of the Co/SiO 2FTS catalyst and the obtained capsule catalysts is shown in Figure 2.The image of a capsule catalyst in Figure 2B suggested that the HZSM5crystal formed on the surface of the Co/SiO 2catalyst pellet,while no crystals were observed on the Co/SiO 2pellet surface as in Figure 2A.The EDS plane analysis results confirmed the formation of zeolite mem-brane on the Co/SiO 2pellet because there was Al X-ray signal on the capsule catalysts but not on the Co/SiO 2cata
lyst as shown in Figure 2C,D.Furthermore,no cobalt was detected on the capsule catalyst,indicating the zeolite membrane was perfect.Figure 3shows a cross-sectional view of the prepared capsule catalyst pellet.The zeolite membrane can be clearly distinguished from the cobalt
(12)Nishiyama,N.;Ichioka,K.;Park,D.H.;Egashira,Y.;Ueyama,K.;Gora,L.;Zhu,W.;Kapteijn,F.;Moulijn,J.A.Ind.Eng.Chem.Res.2004,43,
1211.
Figure 2.External surface of the Co/SiO 2pellet and the capsule catalyst pellet and the EDS analysis result:(A)Co/SiO 2;(B)2-Co/SiO 2-zeolite;(C)Co/SiO 2;(D)2-Co/SiO 2-zeolite.
1700Langmuir,Vol.21,No.5,2005Letters
catalyst supported on silica because of the quite different morphology.The thickness of the zeolite of the membrane was measured as about 10µm.The intensity of Si K R and Al K R X-ray signals from an EDX line scan is also shown in Figure 3.The intensity of Al K R X-rays increased dra-matically at the zeolite membrane layer,while at the cen-ter of the catalyst,the Al K R X-ray intensity was near to zero.The Si signal intensity changed according to its con-tent in the Co/SiO 2FTS catalyst and HZSM5.The SiO 2/Al 2O 3ratio of the membrane of the capsule catalyst was 48.
The catalytic properties of the catalysts were studied with a high-pressure flow-type fixed bed reactor.Co/SiO 2catalyst only and its mechanical mixture with zeolite (20wt %)were operated in the same conditions as a comparison.All the capsule catalysts gave similar CO conversion,which was slightly lower than mixture catalyst or normal FTS catalyst Co/SiO 2,probably because the zeolite membrane slowed the diffusion rate of CO and H 2as shown in Table 1.Also,methane selectivities incr
eased when the membrane was coated and increased when the zeolite coating amount increased.The low diffusion efficiency of CO and H 2led to a high H 2/CO ratio in the interior part of the catalyst pellet,which might increase methane selectivity,13because H 2diffuses more quickly than CO,especially inside small pores or channels.The CO 2in FTS reaction is mainly from WGS (water gas shift)reaction,and its selectivity hardly changed.
The hydrocarbon distributions of conventional FT catalyst Co/SiO 2and the mixture catalyst (Co/SiO 2+zeolite)are compared in Figure 4A,B.The mixture catalyst gives a narrower distribution than the Co/SiO 2catalyst,because the waxy product migrating from the conventional FTS catalyst to the surface of zeolite catalysts was subject to secondary isomerization and hydrocracking,forming lighter hydrocarbons containing isoparaffins.The se-quential isomerization and hydrocracking reaction on zeolite reduced the selectivity of long-chain paraffins and remarkably enhanced the selectivity of light isoparaffins.Different from the random behavior in the mechanical mixture catalyst,all products,in the form of straight chains,from Co/SiO 2core FTS catalyst must enter zeolite channels and diffuse through the zeolite membrane in capsule catalysts,which ensures that all waxy products receive secondary reactions inside zeolite membrane.The hydrocarbon diffusion rate in zeolite membrane
depends
Figure 3.A cross-sectional image of one capsule catalyst (2-Co/SiO 2-zeolite)and the intensity of Si K R and Al K R X-ray signals from the EDX line scan indicated in the
image.
Figure 4.FT synthesis product distribution on (A)Co/SiO 2,(B)Co/SiO 2+zeolite-MX,(C)2-Co/SiO 2-zeolite,and (D)1-Co/SiO 2-zeolite-S catalyst;H 2/CO )2;1.0MPa;W/F )10g ‚h/mol based on Co/SiO 2;533K.
Table 1.FT Reaction Properties of Capsule Catalysts a
sample
CO conversion (%)
CH 4selectivity (%)
CO 2selectivity (%)
isoparaffin/n -paraffin
zeolite coating amount (%)Co/SiO 2
98.415.710.60Co/SiO 2-zeolite-MX b 93.616.98.00.4920.0mixed 1-Co/SiO 2-zeolite c 83.622.79.950.3711.52-Co/SiO 2-zeolite 85.531.310.20.7317.27-Co/SiO 2-zeolite 86.137.47.0  1.8824.31-Co/SiO 2-zeolite-S d 91.524.310.4  1.2129.12-Co/SiO 2-zeolite e
80.1
29.9
6.5
0.51
17.2
a
Reaction conditions:533K,1.0MPa,W/F )10g ‚h ‚mol -1,H 2/CO )2.b MX in the name of sample means the physical mixture of Co/SiO 2catalyst and zeolite whose zeolite additive is 20%.c Numbers in sample names represent crystallization time in days in zeolite synthesis.d S in the sample name means the catalyst was prepared from small Co/SiO 2
pellets (0.38-0.50mm),while the others were prepared from large pellets (0.85-1.7mm).e W/F )5g ‚h ‚mol -1and other reaction conditions were the same.
Letters Langmuir,Vol.21,No.5,20051701
on chain length.Consequently,long-chain compounds stayed in the zeolite membrane longer,which caused all the long-chain hydrocarbons to crack and isomerize.All capsule catalysts gave a very sharp hydrocarbon distribu-tion that ended at C9-C10,while there were still some C13-C20hydrocarbons in the products of the mechanical mixture catalyst.Figure4C,D shows the hydrocarbon distribution for two typical capsule catalysts.It is sug-gested that the covering membrane had an excellent selectivity for short-chain hydrocarbons,inhibiting the long-chain hydrocarbon completely.In addition,the capsule catalyst produced much isoparaffins and olefins. The yield of isoparaffin depended on the zeolite membrane content,which is also listed in Table1.With the increase of zeolite membrane content,the ratio of isoparaffin to n-paraffin(>C3)increased.With a similar zeolite content, the capsule catalyst(2-Co/SiO2-zeolite)produced rather more isoparaffins than the mixture catalyst.These findings indicate that zeolite membrane had higher secondary reaction efficiency than the mechanically mixed zeolite because the capsule catalyst avoided the random occurrence of the secondary reactions of FTS hydrocarbons in the mechanical mixture catalyst.
Also compared with the2-Co/SiO2-zeolite catalyst in Table1,when contact time W/F changed from10to5 g‚h‚mol-1,CO conversion decreased slightly due to the faster flow rate.Methane selectivity decreased and the ratio of isoparaffin to normal paraffin became lower,indicating the residence time of
normal paraffins from the core FTS catalyst,inside zeolite membrane,was too short to receive enough isomerization and hydrocracking, for this consecutive reaction regime.CO2selectivity was down because the amount of water formed was reduced, related to the decreased CO conversion.
Moreover,the capsule catalyst prepared from small FTS silica support pellets which crystallized in24h(1-Co/ SiO2-zeolite-S)showed a higher isoparaffin/n-paraffin ratio than the catalyst prepared from large silica pellets in48 h crystallization(2-Co/SiO2-zeolite)but a lower methane selectivity.It was implied that the pellet size of the core catalyst had a strong effect on the capsule catalyst properties.Modifying the membrane catalyst and the core catalyst or optimizing the reaction conditions might enhance the catalytic activity and selectivity of capsule catalysts.Further studies are needed soon. Furthermore,the typical chemical process can be represented as
Building catalyst Cat1as a membrane on the surface of catalyst Cat2pellets can be applied to prepare a lot of capsule catalysts.This process can realize the combination of these two sequential reactions coupled with the in situ reaction-separation effect.These new kinds of capsule catalysts can be applied to many fields of chemical processes.
LA047217H
(13)Madon,R.J.;Iglesia,E.J.Catal.1994,149,428.A98
Cat1
B98
Cat2
C
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