Hydrophobic modification of SAPO-34 membranes for improvement of stability under

State Key Laboratory of Materials-Oriented Chemical Engineering,College of Chemical Engineering,Jiangsu National Synergetic Innovation Center for Advanced Materials,Nanjing Tech University,Nanjing 210009,China

Keywords:SAPO-34 membrane CO2/CH4separation Hydrophobic modification Stability Moisture environment

ABSTRACT SAPO-34 zeolite membranes show high efficiency for CO2/CH4separation but suffer from the reduction of separation performance when exposed to humid atmosphere.In this work,n-dodecyltrimethoxysilane(DTMS)was used to modify the hollow fibers supported SAPO-34 membranes to increase the external surface hydrophobicity and thus sustain their performance under moisture environment.The modified membranes were fully characterized.Their separation performance was extensively investigated in both dry and wet gaseous systems and compared with the un-modified ones.The un-modified SAPO-34 membrane exhibited a high separation selectivity of 160 and CO2permeance of 1.18×10-6 mol·m-2·s-1·Pa-1 for separation of dry CO2/CH4at 298 K.However,its separation selectivity declined to 0.9 and the CO2permeance was only about 1.7×10-8 mol·m-2·s-1·Pa-1 for wet CO2/CH4at same temperature.High temperature(e.g.353 K)could reduce the effect of moisture to improve SAPO-34 separation selectivity,but further increasing temperature(e.g.373 K)led to decrease in CO2/CH4separation selectivity.A significant decrease of selectivity was observed at higher pressure drop.The modified SAPO-34 membrane showed decreased CO2permeance but increased separation selectivity for dry CO2/CH4gas mixture,and super performance for wet CO2/CH4gas mixture due to the improved hydrophobicity of membrane surface.A separation selectivity of 65 and CO2permeance of 4.73×10-8 mol·m-2·s-1·Pa-1 for wet CO2/CH4mixture can be observed at 353 K with a pressure drop of 0.4 MPa.Furthermore,the modified membrane exhibited stable separation performance during the 120-hour test for wet CO2/CH4mixture at 353 K.The hydrophobic modification paves a way for SAPO-34 membranes in real applications.

1.Introduction

Natural gas is a promising alternative fuel,being intensively used worldwide.However,the presence of undesirable contaminants and impurities,such as carbon dioxide(CO2)and water(H2O),must be removed,because they not only reduce the calorific value but also make the gas streams acidic and corrosive[1,2].Conventional CO2separation systems are suffering from intrinsic drawbacks such as significant energy consumption,installation cost,and high environmental impact.Therefore,the demand for natural gas purification encouraged the development of new CO2separation technology[3,4].

Membrane technology is one of the most attractive techniques due to its low energy consumption,and easy processing,which play an important role in making separation cost-effective[3,5].Polymeric(organic)membranes were successfully commercialized in the 1980s for naturalgas upgrading[6-9].However,polymeric membranes have some limitations and insecure behaviors at high temperatures and pressures[2,4,10].Zeolite membranes are promising alternative for CO2removal due to its unique characters of high thermal,mechanical and chemical stability[8,11-18].SAPO-34 membranes with chabazite(CHA)framework type of three-dimensional structure containing large cage(0.67 nm×1 nm)separated by small windows(0.38 nm×0.38 nm),displayed high selective CO2permeation for CO2/CH4mixture[4,19-22].To date,SAPO-34 membranes have been successfully synthesized on different supports,which showed a high separation selectivity of 30-270 for the dry CO2/CH4mixture as well as CO2permeance of 10-8-10-6mol·m-2·s-1·Pa-1as reported in the literatures[20-25].However,the separation performance of SAPO-34 membranes could decline significantly in a wet environment even at low feed pressure due to its strong hydrophilicity.Poshusta et al.[22]demonstrated that the performance of the SAPO-34 membrane was greatly influenced by vapor in the CO2/CH4mixture;CO2permeance decreased from 6.2×10-8mol·m-2·s-1·Pa-1to 4×10-9mol·m-2·-s-1·Pa-1and selectivity from 14 to 2.8 at 298 K containing 0.09 mol%water vapor in the feed.Li et al.[24]measured the effects of water vapor on CO2/CH4separations in SAPO-34 membranes.They found that vapor completely blocked the pore channels of SAPO-34 membranes,so that the flow through the membranes was mainly due to defects.Further,they also reported that permeance of CO2decreased by 37%for an equimolar feed CO2/N2at 378 K by adding 8 mol%water vapors[26].Moreover,it was confirmed that long-term vapor adsorption below 373 K can break Si--O--Al bonds and damaged the SAPO-34 membrane structure permanently[27-29].Therefore,the stability of SAPO-34 membranes under wet environment is still a challenging issue.

Improving the surficial hydrophobicity is an effective way to reduce the interaction between water and membrane surface to improve their stability and separation performance.Generally,there are three kinds of post-treatment methods to modify zeolite surface:chemical vapor deposition(CVD),chemical liquid deposition(CLD)and molecular layer deposition(MLD)[30-32].Among them,the chemical liquid deposition(CLD)method is a simple and practical approach to improve the hydrophobicity of zeolites without damaging the structure.Sayari et al.[33]demonstrated that grafting organo-silane on the surface of zeolite altered their hydrophilic nature to hydrophobic without damaging the zeolite structure.Study of Zapata et al.[31]has shown that hydrophobic modification of zeolite membranes by silane showed high performances for recovery of bio-oil from water.Kuwahara et al.[34]improved hydrophobicity of high-silica zeolite membranes by grafting triethoxyfluorosilane(TEFS)for ethanol/water separation.

Previously,we have successfully prepared high quality of SAPO-34 membrane on a 4-channel hollow fiber(α-Al2O3)for CO2/CH4mixtures[25].In consideration of large-scale application,we wondered if the surface grafting could effectively improve the stability of the SAPO-34 membrane in wet CO2/CH4separation system.In this study,ndodecyltrimethoxysilane(DTMS),a long-chain,highly reactive,and hydrophobic organo-silane,was used for grafting on SAPO-34 membranes to decrease the competitive adsorption of water vapor and improve the wet CO2/CH4mixture separation performance.To the best our knowledge,there is still no research about the modification of hollow fiber supported SAPO-34 membrane to enhance its hydrophobicity.The modified membranes were fully investigated in both dry and wet gaseous systems.

2.Experimental

2.1.Preparation of SAPO-34 membranes

The SAPO-34 membranes were grown hydrothermally on the external surface of home-made 4-channel hollow fiber(α-Al2O3,4CHF)substrates by secondary growth method.Ball-milled seeds with an average size of～0.3 μm were coated on the substrate surface before hydrothermal synthesis.For a typical synthesis,H3PO4(85% aqueous solution by weight),Al(i-C3H7O)3(98% by mass)and DI(deionized water,H2O)were stirred for 3 h to form a homogenous solution.Next colloidal silica(40%aqueous suspension by mass)was then added in the mixture and stirred for 1 h.The tetraethylammonium hydroxide(TEAOH,35%aqueous solution by mass)was added in the mixture and stirred again for 1 h.Finally,the second template dipropylamine,(DPA,99%aqueous solution by mass)was added to the solution then aged 48 h at 298 K with stirring.All the chemicals were purchased from Sigma-Aldrich.The synthesis gel has a composition of 1.0 Al2O3:1.0 P2O5:0.45 SiO2:1.2 TEAOH:1.6 DPA:100 H2O in molar ratio.The synthesis gel was then transferred to an autoclave,wherein a seeded substrate was vertically placed for hydrothermal crystallization.After crystallization for 18 h at 453 K,the membrane was washed with DI and dried overnight at 373 K.The templates were then removed from the prepared membranes by calcination in air at 673 K for 10 h with a heating and cooling rate of 0.7 K·min-1.

2.2.Modification of SAPO-34 membranes

The prepared SAPO-34 membranes were further functionalized with DTMS to increase the hydrophobicity of membrane surface.The silanization between--OH groups of the SAPO-34 membrane surface and DTMS was conducted based on literature[35].The DTMS was first mixed with n-octane solvent with a final mass ratio of 0.4%,1.2%,2%,and 2.9%.The prepared DTMS/n-octane 25-gram solution was poured into a Teflon-lined autoclave,where SAPO-34 membranes with both ends sealed were placed.Silanization was then performed under autogenous pressure at 383 K for 6 h.After silanization,the outer surface of the membranes was washed with ethanol 3 times to remove the residual chemicals before being dried in the oven at 393 K for 12 h.

2.3.Characterizations

The morphologies of un-modified and modified membranes were observed by field emission scanning electron microscopy(FESEM,S-4800,Hitachi)at an acceleration voltage of 5-10 kV.The water contact angle was carried out using a goniometer(Rame-Hart model 200)for hydrophobicity of un-modified and modified SAPO-34 membranes.N2adsorption/desorption isotherm was used to measure the Brunauer Emmett Teller(BET)surface area and pore volume by using a Micromeritics ASAP 2460 apparatus at 77 K.The samples were degassed at 423 K overnight prior to BET analysis.Adsorption isotherm for CO2and CH4were measured at 298 K.Fourier transform infrared spectroscopy(FTIR)analysis of SAPO-34 membranes was carried out on a spectro-photometer(Thermo,Nicolet 6700)in the range of 500-4000 cm-1.The energy dispersive X-ray spectroscopy(EDS)data were acquired on(Bruker,Quantax-200)at 20 kV of un-modified and modified crystals.The crystal phases of un-modified and modified membranes were detected by Xray diffraction(XRD,Mini-Flex 600,Rigaku)using CuKαradiation source with a 2θ scan range of 5°-50°and a step size of 0.05°.

2.4.Gas permeation

Fig.1 shows a schematic diagram of CO2/CH4mixture gas separation set-up.SAPO-34 membranes with an operative length of 7 cm were mounted on a stainless-steel module using rubber gaskets(O-ring)for sealing at each end.The membrane module was situated in an oven with a thermocouple for monitoring temperature.An equimolar binary mixture of CO2/CH4with approximately 800 ml·min-1flow rate was fed into the membrane and a helium(He)flow(200 ml·min-1)was used as a sweep gas for separation measurement.The feed pressure was managed to be between 0.1 and 0.6 MPa(gauge pressure)by a back-pressure controller,and the permeate side maintained at the atmospheric pressure(101.325 kPa).The gas flows were measured through a bubble flow meter with soap film.A gas chromatography apparatus(GC-2014,Shimadzu)with thermal conductivity detector(TCD),was used to measure the gas composition.For wet gas separation,the dry CO2/CH4mixture feed was saturated through a humidifier and addition of water content was controlled with the assistance of feed pressure and humidifier temperature[22,36].The content of water vapor in CO2/CH4gas was around 1.5 mol%.For the analysis of composition adsorption column of anhydrous calcium chloride was used for the removal of water vapor prior to GC analysis.All gas separation measurements were repeated 3-5 times,for accuracy at steady state.The gas permeances of component(i)were calculated by using Eq.(1).

where Piis the permeance of component i(mol·m-2·s-1·Pa-1),Jiis the permeation flux of component i(mol·m-2·s-1),Δpln(i)is a log-mean trans-membrane pressure drop of component i(Pa).Pf,Ppand Prare partial pressures for component i in feed,permeate and retentate sides,respectively.The separation selectivity(αij)for component(i)over component(j)was calculated by using Eq.(3).

Fig.1.Schematic diagram of the experimental apparatus for CO2/CH4mixture gas separation.

where Piand Pjare the gas permeances(mol·m-2·s-1·Pa-1),of components(i)and(j),respectively.

3.Results and Discussion

3.1.Characterizations of DTMS/SAPO-34

Fig.2.FTIR spectra of un-modified SAPO-34 and DTMS modified SAPO-34 particles.

Fig.2 shows the FI-IR spectra of un-modified and modified crystal samples.The strong vibration of un-modified SAPO-34 in the range of 3680 cm-1to 2820 cm-1are related to--OH stretching vibration,while 1280-980 cm-1are associated with the internal tetrahedra asymmetrical stretch of Al--O or Si--O groups,and the 630 cm-1band is related to the--(CH2)3--vibrations[37].The FT-IR spectra of modified SAPO-34 samples the upper-most peak(Al--O or Si--O)was stretched after silane(DTMS)grafting,this change is caused by the addition of new bonds of(O--Si--O)network.Moreover,the intensity of double ring vibrations increases within the range 780-630 cm-1due to the addition of--(CH2)3--group.Meanwhile,the modified DTMS/SAPO-34 crystal samples showed large decrease of band from 3680 cm-1to 2820 cm-1for the hydroxyl groups,indicating that the hydroxyl groups of the SAPO-34 have been modified chemically by grafting with DTMS.The shift in positions could be attributed to the chemical modification of the SAPO-34 with the DTMS.

Fig.3 shows the DTMS/SAPO-34 modified membranes and unmodified membranes had similar crystal morphologies.The surface of the DTMS/SAPO-34 membrane consisted of many small dots which was attributed to a silanized DTMS attachment on the membrane surface(Fig.3c).The membrane SAPO-34 layer was about 7-8 μm observed by cross-sectional image.No modification layer was observable on zeolite layer from the cross-sectional view(Fig.3d),indicating that the coated DTMS layer was considerably thin.

Fig.4 shows the surface water contact angle of the SAPO-34 membranes before and after DTMS modification.As can be seen,the water contact angle of the un-modified SAPO-34 membrane is extremely small 30°due to the hydrophilic character of the SAPO-34 caused by the surface hydroxyl functional groups.Modification of the SAPO-34 membrane with 0.4 wt%DTMS increased the water contact angle up to 141°.It suggested that the hydrophobic groups were successfully grafted onto the membrane surface after the liquid phase silanization reaction.However,the water contact angle on SAPO-34 membranes did not change much when the DTMS amount further increased from 0.4 wt%to 2.9 wt%.This could be due to saturation of bonding between the--OH groups of the SAPO-34 and the hydrolyzable groups of the DTMS.A similar observation was also reported by Ahmad et al.[38]and Junaidi et al.[37].Even though there is little to no change in contact angle with increasing DTMS amount.On the other hand,when the DTMS grafting amount is increased,the carbon content increased which was investigated by energy dispersive x-ray spectroscopy(EDS)as shown in Fig.5.

Fig.3.SEM images of SAPO-34 membranes:(a)un-modified membrane surface;(b)un-modified membrane cross-section;(c)modified membrane surface;(d)modified membrane cross-section.

Fig.4.Water contact angles measurement for un-modified SAPO-34 and modified SAPO-34 membranes.

To better understand the DTMS modification,we examined BET surface areas of SAPO-34 particles before and after modification.As shown in Table 1 the SAPO-34 sample before modification showed a BET surface area of 503.3 m2·g-1,which is comparable to the literature[37].A gradual decline in surface area for the modified samples could be observed as DTMS content increased.Typically,the sample modified with 2.9 wt%DTMS displayed dramatically drop in the BET surface area of 334.2 m2·g-1.These results indicated that the silane group attachment with the--OH group of SAPO-34 crystals had a significant influence on the pore structure and BET surface area[37,38].To further confirm pore volume,we plotted N2adsorption isotherms for the samples,which was a typical type I isotherm.The calculated micropore volume decreased from 0.263 cm3·g-1to 0.173 cm3·g-1as DTMS percentage increased(Table 1).Fig.6 shows CO2and CH4adsorption isotherms at 298 K for SAPO-34 particles before and after modification with 2 wt%DTMS.As can be seen,both samples exhibited preferential selective adsorption of CO2.However,the sample had less adsorbed amounts of CO2and CH4after modification.The reason for decreased adsorption amount after DTMS grafting was that a certain amount of pores were tapered when the bonds were formed.Similarly,the BET surface area of the DTMS-modified crystal was lower than the original one.

Fig.5.Carbon content of the SAPO-34 zeolite with different DTMS concentrations.

Table 1 BET surface area and t-Plot micropore volume for SAPO-34 particles with different DTMS contents

Fig.6.Adsorption isotherms for CO2and CH4on un-modified SAPO-34 powders and modified(2 wt%DTMS/SAPO-34)powders at 298 K.

3.2.Effect of grafting DTMS amount on separation performance

Fig.7 shows CO2/CH4separation results of SAPO-34 membranes modified with different DTMS amounts.The tests were carried out using a 50:50 CO2/CH4mixture as a feed with a pressure drop of 0.1 MPa under both dry and wet conditions at 298 K.In the dry environment,the separation selectivity for the membranes increased gradually with DTMS content from 0.4 wt% to 2.9 wt%.However,CO2and CH4permeances decreased as the DTMS increased.When the DTMS was 0.4 wt%,the CO2and CH4permeances decreased by 63%and 65%respectively.As the DTMS increased to 2.9 wt%the CO2and CH4permeances decreased by 77%and 83%respectively.It indicates that the grafting of the SAPO-34 pores hindered components for permeation to some extent resulting in the reduced gas permeances.The increase of separation selectivity could be due to that some intercrystalline pores were mended by DTMS modification.This type of non-zeolitic pores had a primary effect on the permeance of CH4,since large molecule(e.g.＞0.38)diffusion is restricted by zeolitic(SAPO)pore channels.The CH4permeance reduced with increasing DTMS obviously,which led to an increased CO2/CH4separation selectivity.

Fig.7.Effect of DTMS amount on gas separation performance of SAPO-34 membranes for CO2/CH4mixture at 298 K and a pressure drop of 0.1 MPa.

In the case of wet gas separation with 1.5 mol%water vapor content in the feed mixture,the un-modified membrane showed a low separation selectivity of 0.9 for CO2/CH4with CO2permeances of 1.7×10-8mol·m-2·s-1·Pa-1at 298 K.This was mainly due to strong adsorption of water in the zeolitic pores,which prevented CO2from entering into zeolite channels[22,24].The surface modification could influence the water affinity.As shown in Fig.7(c):the separation selectivity of the wet CO2/CH4increased to 6 with CO2permeance of 5.55×10-9mol·m-2·s-1·Pa-1when 1.2 wt%DTMS was used for membrane modification.As the DTMS amount increased up to 2 wt.%,the membrane displays a CO2/CH4separation selectivity of 17 with a CO2permeance of 4.8×10-9mol·m-2·s-1·Pa-1.As DTMS further increased to 2.9 wt%the membrane had higher loss of CO2permeance than CH4permeance and a little lower CO2/CH4separation selectivity of 15 with CO2permeance of 3.1×10-9mol·m-2·s-1·Pa-1which might be due to more obvious effect on selective pore entrance at higher DTMS concentration.As discussed above,the DTMS had a significant effect on the BET surface area and pore structure of the SAPO-34 zeolite.Meanwhile,the modification increased the hydrophobicity of membrane surface and prevented water adsorbing in the zeolitic pores.This would allow more CO2molecules to pass through separation channels.Thus,although the gas permeation decreased,the separation selectivity increased.These above results demonstrated that SAPO-34 membranes have good potential for upgrading natural gas,only when dry CO2/CH4mixture was adopted.In wet CO2/CH4mixture the SAPO-34 membrane showed low separation selectivity of CO2.However,in a commercial application,the existence of water is unavoidable in natural gas.So,it is highly desired to improve the hydrophobic property to improve the gas separating under a wet environment.As discussed above 2 wt% DTMS/SAPO-34 membrane exhibits good hydrophobicity and good separation performance under wet environment.Thus,we made a further evaluation of the modified membranes in the following sections.

3.3.Effect of temperature

Fig.8.Separation results of SAPO-34 and 2 wt%DTMS/SAPO-34 membranes as a function of temperature for dry CO2/CH4mixture at a pressure drop of 0.1 MPa:(a)gas permeances;(b)CO2/CH4separation selectivity.

Fig.8 shows the effect of temperature on separation performance of un-modified membrane(SAPO-34)and modified membrane(2 wt%DTMS/SAPO-34).The tests were carried out using a 50:50 CO2/CH4mixture in a dry environment as a function of temperature under a pressure drop of 0.1 MPa.The un-modified membrane showed a separation selectivity of 160 and CO2permeance of～1.18×10-6mol·m-2·s-1·Pa-1for CO2/CH4at 298 K.The CO2permeance was decreased by 52%as the temperature increased to 373 K,which is mainly due to reduced CO2adsorption in zeolite pores at a higher temperature.However,the CH4permeance was almost dissociated with the temperature.As a result,CO2/CH4mixture separation selectivity declined from 160 to 63 when the temperature improved from 298 K to 373 K.

The modified membrane showed a similar tendency to the unmodified membrane for separation of the dry CO2/CH4gas mixture.As the temperature was improved from 298 K to 373 K,the CO2permeance just had an 11%decline from 2.48×10-7mol·m-2·s-1·Pa-1.It is interesting to find that the decrement of CO2permeance was really slower compared with the un-modified membrane.It may be attributed to the fact that the zeolite adsorption capability decreased after modification.Thus the amount of CO2adsorbed was not so sensitive to the temperature.However,the CH4permeance was increased slightly with the temperature after modification.Thus,the CO2/CH4mixture separation selectivity decreased from 181 to 65 when the temperature increased to 373 K.

Fig.9.Separation results of SAPO-34 and 2 wt%DTMS/SAPO-34 membranes as a function of temperature for wet CO2/CH4mixture and a pressure drop of 0.1 MPa:(a)gas permeances;(b)CO2/CH4separation selectivity.

Fig.9 shows the wet CO2/CH4mixture separation performance of unmodified and modified membranes(2 wt%DTMS/SAPO-34).The feed gas was a 50:50 CO2/CH4mixture(800 ml·min-1)with 1.5 mol%water vapor under a pressure drop of 0.1 MPa.As we mentioned above,the unmodified membrane showed low permeances for CO2at 298 K due to water adsorption.Meanwhile,almost no CO2/CH4separation selectivity was found at this temperature.The CO2permeance increased from 1.7×10-8mol·m-2·s-1·Pa-1to 2.58×10-7mol·m-2·s-1·Pa-1as the temperature increased from 298 K to 353 K.This was attributed to reduced water adsorption in the SAPO-34 pores at elevated temperature,which allowed more CO2molecules to permeate through membrane.The permeance tended to be stable when the temperature further increased to 373 K.Meanwhile,the CO2/CH4separation selectivity increased to 60 and then decreased to 53.It was demonstrated that high temperature detained the effect of water vapor and improved SAPO-34 separation selectivity.However,with further improvement from 353 K to 373 K,the CO2adsorption amount in the zeolite channels decreased,which resulted in reduced CO2permeance as well as separation selectivity.

Compared with the un-modified membrane,the modified membrane showed an improved separation selectivity for wet CO2/CH4at lowtemperature range from 298 K to 313 K due to strong hydrophobicity.As shown in Fig.9,the CO2permeance of 4.8×10-9mol·m-2·s-1·Pa-1and CO2/CH4selectivity of 17 could be achieved at 298 K.As the temperature increased,the CO2permeance increased,which resulted in an increase of separation selectivity.However,at the temperature larger than 353 K,the separation selectivity for the modified membrane was close to that for the un-modified membrane.The result suggested that the water adsorption effect could be low for the un-modified membrane at a higher temperature for the feed water partial pressure of 3.17 kPa.However,in practical applications,high feed pressures are required,and the moisture effect could be amplified significantly.

3.4.Effect of pressure

Fig.10.Separation results of(a)SAPO-34 membrane and(b)2 wt% DTMS/SAPO-34 membrane as a function of trans-membrane pressure drop for dry CO2/CH4mixture at 298 K.

Fig.10 shows the effect of feed pressure on separation performance of un-modified membrane and modified membrane(2 wt.% DTMS/SAPO-34)for the dry CO2/CH4mixture.The feed was an equimolar CO2/CH4mixture at 298 K,and the pressure drop was increased from 0.1 MPa to 0.5 MPa.It was found that CO2permeances for both membranes decreased with the increase of pressure drop.Meanwhile,CH4permeances slightly increased with the pressure drop.As a result,CO2/CH4separation selectivity of both membranes decreased with the pressure.This was because of CO2adsorption in SAPO-34 pores approached to saturation at high feed pressure,which would reduce CO2permeance[23].However,CH4preferentially permeated through defects(non-zeolitic pores)that followed Knudsen diffusion and viscous flow[39].The modified membrane had relatively higher separation selectivity than the un-modified membrane indicating partially patching of defects by DTMS coating.

Fig.11.Separation results of(a)SAPO-34 membrane and(b)2 wt% DTMS/SAPO-34 membrane as a function of trans-membrane pressure drop for wet CO2/CH4mixture at 353 K.

Fig.11 shows the effect of feed pressure on separation performance of the un-modified membrane and modified membrane(2 wt% DTMS/SAPO-34)for the wet CO2/CH4mixture.The experiments were carried out using a 50:50 CO2/CH4mixture containing 1.5 mol%water vapor at 353 K and the pressure drop was increased from 0.1 MPa to 0.55 MPa.Since the un-modified membrane did not have separation performance in a wet system at room temperature,we compared the separation performance at the higher separation temperature of 353 K.CO2permeance through the un-modified membrane continuously decreased with the increase of pressure drop,and about 48% of decrement in the CO2permeance was observed when the pressure drop increased from 0.1 MPa to 0.4 MPa.The decrement was apparently more significant than that in the dry gas environment.Although the un-modified membrane exhibited a high separation selectivity of 61 at 0.1 MPa,a significant decrease was found by 54%at higher pressure drop with a wet CO2/CH4selectivity of 28 at 0.55 MPa.The results indicated that the high feed pressure increased water partial pressure and more water was adsorbed into zeolitic pores,which caused the decline of separation performance for CO2/CH4.However,for the modified membrane,a slight increase in CO2permeance with the feed pressure was even observed.As a result,CO2/CH4separation selectivity increased slightly between 60 and 65 during the tested pressure,it had the highest separation selectivity of 65 with a CO2permeance of 4.73×10-8mol·m-2·s-1·Pa-1at 0.4 MPa.The results suggested that modification changed the competitive adsorption between CO2and H2O.Therefore,CO2permeance was increased due to decreasing moisture adsorption following the increase in feed pressure.For CH4,we also observed that the permeance over the un-modified membrane decreased at high pressure,while that of the modified membrane improved slightly as the pressure increased.In practical applications,the purification of natural gas is generally operated under high feed pressure.The modified membrane has potential to overcome deficiencies such as water adverse effects at high pressure compared to the un-modified membrane.

3.5.Membrane stability

Fig.12.Separation stability of(a)SAPO-34 membrane and(b)2 wt% DTMS/SAPO-34 membrane for wet CO2/CH4mixture at 353 K and a pressure drop of 0.1 MPa.

To evaluate the stability of the un-modified membrane,the test was carried out for an equimolar CO2/CH4mixture with a pressure drop of 0.1 MPa at 353 K.After 10 h,moisture content(1.5 mol%)was introduced in the feed mixture(Fig.12a).As discussed above,the CO2permeance and CO2/CH4separation selectivity decreased drastically when humidified feed was supplied,indicating that water vapor adsorbed in SAPO-34 pores.After 4-5 h membrane showed almost stable separation performance until 90 h with the decrease of about 7%.However,after that a continuous decrease in CO2permeance and CO2/CH4selectivity was observed for the un-modified membrane.The un-modified membrane was then dried in a vacuum oven for 6 h at 393 K and tested again in CO2/CH4mixture without moisture.It was observed that the SAPO-34 membrane could not recover to the previous separation performance.The results suggested the SAPO-34 membrane could have been damaged after operated in a humidified system.Similar phenomenon was reported by Poshusta et al.[22].In their work,the SAPO-34 membrane was exposed to vapor for about 20 h and the separation performance was not recoverable by heat treatment.Briend et al.[28]suggested that adsorption of vapor permanently degraded the SAPO-34 structure.

To verify the stability of the modified membrane(2 wt% DTMS/SAPO-34)in wet mixture,the separation performance of the modified membrane was tested under the same conditions.As shown in Fig.12b according to the results,CO2permeance decreased from 2.29×10-7mol·m-2·s-1·Pa-1to 4.73×10-8mol·m-2·s-1·Pa-1as exposed in the humidified system.The results indicated that initial drop in CO2permeance might be due to vapor suppression effect on CO2permeation through the modified membrane.After that,the CO2and CH4permeances of the membranes were almost constant during a 120-h test.The CO2/CH4separation selectivity also remains about 57 in the wet gas condition.After the membrane had tested in wet CO2/CH4mixture,the membrane was dried in a vacuum oven at 393 K for 6 h.Then,the modified membrane was tested again in a dry gas stream.The CO2permeance increased from 4.46×10-8mol·m-2·s-1·Pa-1to 2.23×10-7mol·m-2·s-1·Pa-1and then remained almost constant.The dry CO2/CH4mixture separation selectivity for the modified membrane was around 67 during the whole test.The stable CO2permeance and CO2/CH4mixture selectivity in the wet gaseous system proposed that the DTMS/SAPO-34 membrane owns high stability compared to the un-modified SAPO-34 membranes.

XRD results of un-modified and modified membranes before and after stability test were shown in Fig.13.We noticed that the intensity of characteristic peaks at 2θ=9.4° and 20.5° of un-modified membranes decreased extremely after stability test.But for modified,the characteristic peaks were unaffected.These results indicated that hydrophobic modification had a significant effect on wet stability of SAPO-34 framework.SEM analysis was also applied for determining the stability of un-modified and modified membranes after long-term test shown in Fig.14.We observed that the un-modified membrane zeolite crystal surface(Fig.14a)was degraded after long-term test as compared with the fresh membrane(Fig.3a).However,no significant change was observed for the modified membrane(Fig.14b).The degradation of zeolite surface in the un-modified membrane might be mainly due to attack on Si--O--Al bonds by water vapor.Therefore,CO2/CH4separation performance of the un-modified membrane decreased.Improvement of anti-wetting property is vital in wet CO2/CH4separation for practical application.

Fig.13.XRD patterns of un-modified and modified SAPO-34 membranes before and after stability test for 120 h.

Fig.14.SEM images of used membrane after stability test:(a)surface view of un-modified membrane;(b)surface view of modified membrane.

4.Conclusions

In summary,the DTMS modified SAPO-34 membrane was developed with anti-wetting ability by covalently bonding molecules of DTMS with increasing hydrophobic property.The grafting also minimized the intercrystalline pores or cracks of SAPO-34 membranes.For the un-modified membrane,the separation selectivity declined when water existed in the system,and the effect of moisture on the separation performance was more obvious with high feed pressure.The modified membrane(2 wt%DTMS/SAPO-34)exhibited an increased separation selectivity for the dry CO2/CH4mixture due to the reduced intercrystalline pores.More importantly,the modified membrane showed a significant improvement in separation selectivity in the wet environment due to the anti-wetting ability of membrane surface.Also,the modified SAPO-34 membrane showed excellent stability during the 120-h test in the wet system.The surface grafting on the SAPO-34 membrane provides a solution to improve membrane stability in humidified gas streams,which is quite useful in natural gas purification.