Synthesis and Characterization of Polyimide-Zeolite Mixed Matrix Membrane for Biogas Purification

Transcription

1 Interntionl Journl of Bsic & Applied Sciences IJBAS-IJENS Vol:10 No:01 1 Synthesis nd Chrcteriztion of Polyimide-Zeolite Mixed Mtrix Membrne for Biogs Purifiction Budiyono #,,1, T.D. Kusworo #,2, A.F. Ismil*, 3, I.N. Widis #,4, Seno nd # Deprtment of Chemicl Engineering, University of Diponegoro, Jl. Prof. Sudhrto SH, Semrng, Indonesi 1 budhi.66@googlemil.com 2 tutukjoko@yhoo.com 4 Deprtment of Animl Agriculture, University of Diponegoro, Jl. Prof. Sudhrto SH Semrng, Indonesi 5 senojohri@yhoo co.id 6 sunrso_undip@yhoo.com * Advnced Membrne Reserch Technology Center, Fculty of Chemicl nd Nturl Resources Engineering, Universiti Teknologi Mlysi, UTM, Mlysi 3 fuzi@utm.my Abstrct Biogs hs become n ttrctive lterntive energy source due to the limittion of energy from fossil. In this study, new type of mixed mtrix membrne (MMM) consisting of polyimide-zeolite ws synthesized nd chrcterized for biogs purifiction. The MMM consists of medium concentrtion of polymer (20 %wt polyimide), 80 % N-Methyl-2-pyrrolidone (NMP) nd 25 % zeolite 4A in totl solid were prepred by dry/wet phse inversion technique. The fbricted MMM ws chrcterized using SEM, DSC, TGA nd gs permetion. Post tretment coting procedure ws lso conducted. The reserch showed tht surfce coting by 3 % silicone rubber towrd MMM PI 20 % gve the significnt effect to improve membrne selectivity. The idel selectivity for CO 2 /CH 4 seprtion incresed from 0.99 for before coting to 7.9 fter coting for PI-Zeolite MMM, respectively. The results suggest tht PI-Zeolite MMM with good post tretment procedure will increse the membrne selectivity nd permebility with more sver polymer requirement s well s energy sving due to low energy for mixing. Index Term mixed mtrix membrne, polyimide, zeolite, biogs purifiction I. INTRODUCTION Biogs hs become n ttrctive lterntive energy source due to the limittion of energy from fossil. This is due to its chrcteristics i.e. renewble source, environment friendly, clen, chep nd verstile fuel. However, the use of biogs is limited by its low qulity. Except the min component of CH 4 (55-80 vol%), biogs contins substntil mount of CO 2 (20-45 vol%) nd H 2 S (0-1 vol%) [1]. In ddition, biogses re lso frequently sturted by wter vpour. The presence of CO 2 s n incombustible gs reduces its clorific vlue nd mkes it uneconomicl to compress nd trnsport to longer distnces. The H 2 S content poses serious problems of odor, toxicity for humn nd niml helth, nd corrosion [2]. Nevertheless, if biogs is clened up sufficiently, biogs hs the sme chrcteristics s nturl gs (NG) [3]. There is lot of potentil if biogs could be mde vible s trnsport vehicle fuel like Compressed Nturl Gs (CNG). Besides the use of biogs s vehiculr fuel, smll scle or bulk biogs storge for domestic consumption is lso potentil lrge mrket. Therefore, biogs purifiction before compression t high pressure for storge in cylinders is essentil. A vriety of processes re being used for removing CO2 nd H2S from NG in petrochemicl industries. Severl bsic mechnis ms re involved to chieve selective seprtion of gs constituents. These my include physicl or chemicl bsorption, dsorption on solid surfce, membrne seprtion, cryogenic seprtion nd chemicl conversion. However, membrne seprtion processes hve emerged during the lst two decdes. This is due to the fct tht membrne seprtion processes my offer more cpitl nd energy efficiency compred to the conventionl seprtion processes [4]. In ddition, dvntges of membrne technology re its simplicity, i.e. no bsorbent, which hs to be regenerted; it cn be esily retrofitted, modulrized nd scled-up for severl pplictions [5]. Commercil membrne systems for gs seprtion were vilble since the mid-1970s, but the most importnt innovtion for lrge-scle commerciliztion of polymeric membrnes for gs seprtion ws chieved by Perme (now Air Products) in 1980 [6]. Currently, the min chllenge in ppliction of the membrne technology for gs seprtion is how to get the mterils with higher permebility nd selectivity s well s good therml stbility [7]. Generlly, better selectivity is obtined with glssy polymers. The glssy stte offers more structured sieving mtrix thn the rubbery stte s well s higher lod bering properties llowing for high pressure drop cross the membrne. Typicl glssy polymers used in membrnes for commercil gs seprtion re polysulphone, cellulose cette nd polyimide [8-9]. Among them, romtic polyimide is well known hving excellent properties, e.g. high therml stbility, chemicl resistnce, mechnicl strength, nd low dielectric constnt [4]. Due to high glss trnsition temperture nd low solubility of polymer, it is possible to pply polyimide in wide rnge of tempertures nd pressures. Moreover, the polymer is chrcterized by high selectivity nd permebility to the gses in comprison to the others. According to Hrsimowicz et l. [10] polyimide hve permebility higher thn 400 Brrers for CO2 nd their CO2/CH4 selectivity is higher thn 15. Morever, ccording to Ho et l. [11], polyimide mde from 6FDA-HAB hs CO2/CH4 selectivity up to 60. However, PI polymer still hs some limittion i.e

2 Interntionl Journl of Bsic & Applied Sciences IJBAS-IJENS Vol:10 No:01 2 reltively expensive nd high viscosity cuse the high energy requirement for mixing during membrne preprtion. Kusworo et l. [12] hs mde n effort to decrese polymer cost of polyimide nd increse processbility by mixing PI with the cheper polymer (polyether sulphone, PES) with rtio 20:80 by 25 wt% of totl polymer in N- Methyl-2-pyrrolidone (NMP). These composite membrnes offer reltively low cost of polymer besides esy to cst the polymer solution so it will reduce the mixing power. However, Kusworo t l [12] is still use polymer in 25 % concentrtion. In effort to decrese the polymer requirement, in this study, the MMM consists of medium concentrtion of polymer (20 wt% PI) were synthesized nd chrcterized for biogs purifiction. The effect of coting on the seprtion performnce of fbricted membrne ws lso exmined. II. EXPERIMENTAL A. Mteril selection The selection of polymers nd fillers in this reserch is to obtin MMM hving high permebility nd selectivity of CO 2 /CH 4. The polyimide polymer is selected due to it s chrcteristics tht polyimide polymer hs romtic rings nd functionl groups of big volume, which ct like moleculr sieves, s mteril for membrnes used for gs seprtion is resonble. Due to high glss trnsition temperture nd low solubility of polymer, it is possible to pply polyimide in wide rnge of tempertures nd pressures. Moreover, the polymer is chrcterized by high selectivity nd permebility to the gses in comprison to the others. According to Ho et l. [11], polyimide mde from 6FDA -HAB hs CO 2 /CH 4 selectivity up to 60. Polyimide polymers used in this reserch ws mtrimide resin. Mtrimide ws obtined from Alf Aesr Johnson Mtthey Compny USA. The mtrimide polymers were dried overnight in vcuum oven t 120 C before used for dope preprtion. N-methyl-pyrrolidinone (NMP) ws from Merck. The chemicl structure of the polyimide is shown in Fig. 1. Zeolite moleculr sieve ws selected due to their high selectivity nd comptibility towrds polr compounds, such s H 2 S [12]. The zeolite 4A ws purchsed from Aldrich nd the prticle size ws 2 m. In order to remove the dsorbed wter vpour or other orgnic vpors, ll zeolite prticles were dehydrted t 300 o C for 3 h before use. Fig. 1. Chemicl structure of polyimide B. Fbriction of symmetric flt sheet MMM In this study, the polymer solution consists of 20 wt% PI polymer 80% NMP nd 25 wt % zeolite in the totl of solid. The homogeneous polyimide were prepred ccording to the following procedure; the inorgnic moleculr sieve prticles were dispersed into the solvent nd stirred for 24 h followed by the ddition of desired mount of polyimide. The solution ws gitted with stirrer t lest 24 h to ensure complete dissolution of the polymer. Before csting, the homogeneously prepred solution ws degssed under vcuums for 3 h. Flt sheet membrne ws prepred ccording to the dry/wet phse inversion technique. The solution ws poured onto cler, flt nd smooth glss plte tht ws plced on the trolley. Stinless steel support csting knife ws used to spred the solution to uniform thickness by pneumtic force. Membrnes were cst t different sher rtes by vrying the speed of the trolley (csting speed). The glss plte with the membrne film ws then immersed into the cogulnt bth or wter bth. During this process, solvent exchnge occurred nd solidified the membrne film to complete membrne structure. To ensure ll of the solvent in the membrne structure is removed, membrnes were immersed in n queous bth for 1 dy, followed by immersion in methnol for 4 h nd irdried for 24 h t room temperture. The mixed mtrix membrnes were finlly dried in oven t 120 C for 4 h to remove ll the residul solvents. C. Post-tretment procedure The membrne sheets were coted with highly permeble elstomeric silicone polymer (Sylgrd 184 Dow Corning). The membrne coting ws done fter the uncoted membrnes were tested. The intention of coting is to fill ny surfce pinholes or defects on membrne surfce. Membrnes were submerged in the 3% w/w solution of silicone in n-hexne for 24 hours nd subsequently plced in oven for 3 dys t 120 o C to llow curing before permetion testing. D. Membrne chrcteriztion A Supr 35 VP Field Emission Scnning Electron Microscopy (FESEM) ws used to observe the membrne structure nd to determine the dimension of the fibers. Membrne smples were frctured in liquid nitrogen. The membrnes were mounted on n luminum disk with double surfce tpe nd then the smple holder ws plced nd evcuted in sputter-coter with gold. The chnges in the chemicl structure cused by silne tretment were identified using Fourie r trnsform infrred spectroscopy (FTIR). The IR bsorption spectr were obtined t room temperture in rnge of 4000 to 500 cm -1 with spectrl resolution of 8 cm -1 nd verged over 16 scns. The glss trnsition temperture of ech cst film ws determined using differentil scnning clorimetry (Mettler Toledo DSC 822e). A smll piece of membrne or pure polymer smple ws first stored under vcuum t 100 o C for 24 hours to remove dsorbed wter; then weighed nd plced into luminium DSC pns. The scnning rnge ws o C with scnning rte of 10 o C min -1 in the first DSC cycle to remove therml history nd then cooled from 320 to 25 o C t the rte of 10 o C min -1 ; finlly the second cycle ws crried out with the sme procedure. E. Module fbriction nd gs permetion experiment Membrnes formed were exposed to two different gses CH 4 nd CO 2 nd were mesured its permebilities. The permetion test involved the use of gs permetion cell in which the membrne ws plced on sintered metl plte nd pressurized t the feed side. Gs permetion rtes were mesured by constnt pressure system using sop bubble flow meter. Fig. 2 illustrtes the gs permetion cell set up. The cross-membrne pressure difference ws mintined 1 br. Pressure normlized gs permetion flux or permence for gs i, (P/l) i, in (GPU), ws clculted s follows:

3 Interntionl Journl of Bsic & Applied Sciences IJBAS-IJENS Vol:10 No:01 3 P l i Qi Ap where Q i is the volumetric flow rte of gs i, (1) p is pressure difference cross the membrne (cmhg), A is membrne ffective surfce re (cm 2 ) nd l is membrne skin thickness (cm). The idel seprtion fctor cn be clculted by using eqution below: i / j ( P / l) ( P / l) i j i / j (2) hollow fiber membrne re shown in Fig Fig. 3-d illustrtes the prtil cross-sectionl re of representtive net polyimide membrne with uncoted nd coted. As cn be seen from the SEM microgrphs, of ll membrnes exhibited the porous structure. The skin lyer of net PI membrne becme incresingly denser when membrne ws subjected silicone coting s shown in Fig. 3c-d. Fig. 3 lso suggests tht the net PI membrne clerly revels the presence of some mcro voids under the skin lyer. Moreover, both net membrnes coted nd uncoted hve substructure reltively thick nd hve pore sizes suitble for Knudsen diffusion, the determining step of the overll selectivity is the Knudsen diffusion occurred in the substructure. Additionl silicone coting only sels defects of the outermost skin, thus the overll fiber selectivity is not improved. Fig. 2. Gs permetion test cell For biogs seprtion testing, in this study we were used gs smple of CO 2 nd CH 4 to represent the biogs product. The gs permetion properties for ech flt sheet polyimidezeolite mixed mtrix membrne were mesured by using vrible pressure constnt volume method. b III. RESULTS AND DISCUSSION A. Morphology of symmetric PI-Zeolite mixed mtrix membrnes An understnding of how preprtion conditions led to different morphologies is vitl for generting mixed mtrix membrnes with desirble trnsport properties. The FESEM nlysis ws performed in order to investigte the effect of preprtion process such s the effect of medium polymer concentrtion on the morphology of mixed mtrix membrnes nd the reltion with the seprtion performnce of these membrnes. As suggested by previous studies [13-14] tht the min problems fced during fbriction of mixed mtrix membrne were surfce effects, ggregtion nd poor polymer-sieve contct. Koros nd co-workers [13] observed tht mixed mtrix membrne with zeolite nd glssy polymer possessed poor polymer-sieve contct. It ws found tht the mixed mtrix membrne contined the unselective voids of bout m round the sieves. Moreover, the interfce between polymer mtrix nd inorgnic filler plys n importnt role to determine the performnce of fbricted mixed mtrix membrnes. Therefore, in this study, the fesibility of fbriction mixed mtrix membrne t medium polymer concentrtion ws investigted. Hence, the chrcteriztion nd comprison of polyimide-zeolite for uncoted nd coted would be further discussed. FESEM chrcteriztion ws crried out on the fbricted membrne in order to determine the qulittive nlysis of polyimide-zeolite mixed mtrix membrne. The FESEM microgrphs of the polyethersulfone zeolite mixed mtrix c

4 Interntionl Journl of Bsic & Applied Sciences IJBAS-IJENS Vol:10 No:01 4 Fig. 3. SEM picture of symmetric uncoted net polyimide membrne t the: () uncoted membrne cross section; nd (b) outer surfce imge lyer (c) coted membrne t cross section nd (d) t outer surfce imge lyer The FESEM microgrphs of the polyimide zeolite mixed mtrix membrne t medium polymer concentrtion re shown in Fig Fig. 4-5 presents the prtil crosssectionl re of representtive PI-zeolite mixed mtrix membrne with nd without silicone coting. voids voids d Fig. 4-c clerly revels the presence of some gglomertion of zeolite nd voids in the interfce of zeolite nd polymer mtrix. As suggested by previous study tht during fbriction of polyimide-zeolite membrne, one fctor plys gret importnce is prticle gglomertion due to sedimenttion or migrtion to the surfce [13-15]. Due to the totlly different physicl properties nd difference in density between zeolite nd polymers, precipittion of zeolite my occur during the MMM preprtion, resulting in formtion of inhomogeneous zeolite nd polymer phses in the filled membrne. The gglomertion of zeolites will cuse the pinholes tht cnnot be reched by polymer segments, forming s non-selective defects in the MMM. On the medium concentrtion of polymer ws generlly the zeolite prticle do not hve enough time to precipitte; while the solvent ws rpidly evported to form membrne. Moreover, the zeolite prticle will tend to form gglomertion. The effect of silicone coting on the morphology of PIzeolite mixed mtrix membrne ws depicted in the Fig. 5. The FESEM microgrphs revel tht the mixed mtrix membrne with silicone coting hs denser on the outer skin lyer compred to uncoted membrne. Furthermore becuse of the silicone coting effect the pcked chins in the polymer mtrix denser nd the pcked structure in the outer skin lyer provide high degree of size nd shpe discrimintion between the gs molecules. voids No void b b c Fig. 4. SEM picture of symmetric uncoted polyimide-zeolite mixed mtrix membrne t the: (, b) cross section nd (c) outer surfce imge lyer Fig. 5. SEM picture of symmetric coted polyimide-zeolite mixed mtrix membrne t the: () cross section nd (b) outer surfce imge lyer Fig. 5-b depict tht the interfcil gps between polymer host nd zeolite cn be reduced by post tretment membrne with silicone rubber coting. Moreover, the smooth surfce of PI-zeolite membrne in the Fig. 5b indictes tht the zeolite prticle dheres well to the polymer mtrix. Foremost, by referring to Fig. 4-5, the differences in dhesion between PI-zeolite with post tretment using

5 Interntionl Journl of Bsic & Applied Sciences IJBAS-IJENS Vol:10 No:01 5 silicone rubber coting cn be successfully distinguished. The post tretment using silicone rubber cn sel the voids between zeolite nd polymer host. B. Therml Grvity Anlysis Therml stbility is one of the most importnt properties of polymer nnocomposites membrne. Decomposition temperture, vry differently depending on the interction of polymer mtrix nd inorgnic filler. This chrcteriztion is conducted to study the reltionship between the therml decomposition tempertures with the weight loss of the composition. From the TGA curve, lthough the onset therml decomposing temperture is the sme but the weight losses of the composition re differing ccording to its content. The weight loss of the composition is shown in Tble I. T ABLE I EFFECT OF SILICONE RUBBER ON THE THERMAL STABILITY OF PI-ZEOLITE MIXED MATRIX MEMBRANE Onset Therml Decomposing Temperture t 550 o C Weight before Weight Composition Decomposing, mg loss, mg % weight loss Net PI Uncoted PI-zeolite Coted PI-zeolite The sme therml decomposing temperture is used to identify the rte of ech composition to diffuse s lredy shown in Tble I. The silicone rubber coting hs gret effect on the onset decomposing temperture. Introducing silicone coting to the PI mtrix cn increse the initil decomposing temperture of net mtrix more or less. This cn be seen when the incresing of the composition of mino functionl, the weight losses of the composition retrded. It is proven when silicone coting subjected to PI-zeolite mixed mtrix membrne gives the smllest percent of we ight loss mong others. In this condition, the compositions become stble which ws ffected from the strong interction between PI mtrix nd zeolite, so the diffusion of smll molecules ws retrded lthough under the high temperture. The influence of silicone rubber on the glss trnsition temperture of PI-zeolite mixed mtrix membrnes re tbulted in Tble II. The T g of the mixed mtrix membrnes incresed bout o C with the ddition silicone rubber on the mixed mtrix membrne s depicted in Tble II. This phenomenon indictes tht the surfce modifiction of mixed mtrix membrne using silicone rubber coting ffects the mechnicl properties. The increse of T g might be due to the zeolite restricts the movement of the polymer chins by the ddition of silicone rubber, therefore the interction of zeolite prticles nd polymer ws incresed. However, Li et l [15] observed tht the min fctors cusing the increse of T g with n increse of zeolite loding is due to fir interfcil interction between polymer nd zeolite. As presented in Tble II, it cn be concluded tht the silicone rubber coting could induce the dherence between polymer mtrix nd zeolite prticles. T ABLE II EFFECT OF SILICONE RUBBER ON THE GLASS TRANSITION TEMPERATURE OF PI-ZEOLITE MIXED MATRIX MEMBRANE Membrne T g ( o C) Net PI Uncoted PI-zeolite 320 coted PI-zeolite C. Gs Seprtion Performnce of PI-zeolite mixed mtrix membrne The effect of modifiction on gs seprtion performnce is importnt prmeter in membrne modifiction for gs seprtion. Thus, this testing ws crried out in order to study the membrne effectiveness due to the effect of coting on polyimide-zeolite surfce. The membrne effectiveness in the gs seprtion performnce ws determined by the membrne permebility nd selectivity for tested biogs purifiction. In this study, we were used gs smple of CO 2 nd CH 4 to represent the biogs product. The gs permetion properties for ech flt sheet polyimide-zeolite mixed mtrix membrne were mesured by using vrible pressure constnt volume method. The permebility nd selectivity for tested gses CO 2 /CH 4 obtined re presented in Tble III. Tble III summrizes the permebility nd selectivity dt of uncoted PI, coted PI, uncoted PI-zeolite nd coted PIzeolite. Generlly, the ide to put inorgnic filler into orgnic polymer is to enhnce gs permebility of polymer nnocomposites membrnes due to the disturbed polymer chin pcking by the nnofillers [16]. Therefo re, the well dispersed nd good dherence of polyimide-zeolite will be effectively incresed the gs permebility due to more effectively insert between polymer chins of the mtrix. As shown in Tble III, the permebility of uncoted PI nd uncoted PI-zeolite membrne for ll gses were very high. These results indicted tht these membrnes hve big porosity or mny voids were formed on the uncoted PIzeolite mixed mtrix membrne s shown in the Fig. 4. However, the selectivity vlues for uncoted net PI nd uncoted PI-zeolite t 20 wt% polymer concentrtion for ll gses were very low. These might be n indiction of the Knudsen diffusion phenomen resulted in the membrne due to the presence of severe voids between zeolite prticles nd gglomertion s shown in Fig. 3-b nd Fig. 4. Generlly, Knudsen-diffusion controls the permetion of gs through porous membrne nd the selectivity for binry gs in Knudsen-diffusion is given by eqution s follow M M 1/ 2 A o (3) B where M A nd M B re the moleculr weight of component A nd B, respectively. Eqution 3 indicted tht Knudsendiffusion does not offer ttrctive seprtion fctors, especilly for gses of comprble moleculr weight. The previous study of Vu et l. [13] suggested tht gglomertion of moleculr sieve prticles usully ws cused by low viscosity of mixed mtrix s lurries. As supported by the FESEM in Fig. 4, the gs trnsport could occur through the submicron gps between the polymer mtrix wll nd the zeolite prticles. As the gs trnsport through those unselective voids, it hd been ssumed to be the Knudsen

6 Interntionl Journl of Bsic & Applied Sciences IJBAS-IJENS Vol:10 No:01 6 diffusion behviour, the permebility of CH 4 nd CO 2 becme lrger due to gs flowing through the interfce between zeolite prticles nd polymer mtrix. Moreover, the unselective voids would be functioned s pinholes tht llow ll gses molecules pss rpidly without ny selectivity. Thus, the permebility of ll gses is incresed thus reducing the gs selectivity. Hence, the selectivity of the resulting membrne ws lower thn tht of the net polymer. In this study, we investigted the effect of simple post tretment to sel the unselective void on the PI-zeolite membrne using silicone rubber. Gs seprtion performnce of coted membrne ws lso influenced by therml curing time. In this study the coted membrne fibers ws dried in vcuum oven t temperture C for 72 hours. Gs seprtion performnce of this fbricted membrne ws improved nd comprison between coted nd uncoted membrne ws mde. Tble III shows the comprison of permebility nd selectivity for tested biogs purifiction between coted nd uncoted PI-zeolite membrne. TABLE III BIOGAS PURIFICATION PERFORMANCE OF PI-ZEOLITE MIXED MATRIX MEMBRANE Single gs permence Selectivity Membrne (GPU) CO 2 CH 4 CO 2 /CH 4 Uncoted net PI Coted net PI Uncoted PI-zeolite From the tbulted dt obtined, it cn be observed tht the PI-zeolite mixed mtrix membrne fbricted with medium polymer concentrtion coted with silicon rubber yield low permebility nd increse the selectivity in the seprtion of CO 2 /CH 4 compred to the uncoted membrnes. The increse of selectivity from 0.99 to 7.99 might be cused by suppression of Knudsen diffusion mechnis m using the silicone rubber coting. This is probbly due to the defects t the outermost skin lyer hve been seled by the silicone coting. The surfce structure of the membrne ws improved by reducing the defects on the membrne surfce, hence resulting in increse selectivity nd low permebility. Therefore, the gs trnsport mechnism tht dominted this coted membrnes re combintion of moleculr sieving nd solution diffusion. This phenomenon indicted tht the voids generted by the unfvourble interction between polymer nd zeolite cn be reduced using silicone rubber coting. The PI-zeolite mixed mtrix membrnes hve been potentilly pplied on the biogs purifiction REFERENCES [1] Kpdi, S.S., V.K. Vijy, S.K. Rjesh, R. Prsd., Biogs scrubbing, compression nd storge: perspective nd prospectus in Indin context, Renew. Energy 30 (2005) [2] Truong, L.V.A., N. Abtzoglou., A H 2S rective dsorption process for the purifiction of biogs prior to its use s bioenergy vector, Biomss nd Bioenergy, 29 (2005) [3] Esteves, I.A.A.C., M.S.S. Lopes, P.M.C. Nunes, J.P.B. Mot Adsorption of nturl gs nd biogs components on ctivted crbon, Seprtion nd Purifiction Technology 62 (2008) [4] Lin, W.H., nd T.S.Chung.. 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Mter., 17 (2005) [17] Vu, D.Q., Koros, W.J. nd Miller, S.JMixed Mtrix Membrnes Using Crbon Moleculr Sieves I. Preprtion nd Experimentl Results, Journl of Membrne Science, 211 (2003): IV. CONCLUSION In this study, mixed mtrix memb rne polyimide - zeolite ws fbricted for biogs purifiction. Bsed on the experimentl results nd nlysis, the following conclusions cn be mde. The FESEM for the cross-sectionl nd surfce re imges of mixed mtrix membrne films indicted tht the simple post tretment such s silicone rubber coting cn be used to reduce the interfce voids between inorgnic prticles with polymer host in the fbriction of mixed mtrix membrne. The coted PI-zeolite mixed mtrix membrnes hd incresed the CO 2 /CH 4 selectivity nd reduce permebility of methne gs.