VOC Removal:Comparison of MCM-41with
Hydrophobic Zeolites and Activated Carbon
X.S.Zhao,Q.Ma,and G.Q.(Max)Lu*
Department of Chemical Engineering,University of Queensland,St Lucia,
QLD4072,Australia
Received May5,1998.Revid Manuscript Received July21,1998
The recently discovered mesoporous molecular sieve MCM-41was tested as an adsorbent for VOC removal.Its adsorption/desorption properties were evaluated and compared with other hydrophobic zeolites(silicalite-1and zeolite Y)and a commercial activated carbon,BPL.The adsorption isotherms of some typical VOCs(benzene,carbon tetrachloride,and n-hexane)on MCM-41are of type IV according to the IUPAC classification,drastically different from the other microporous adsorbents,indicating that VOCs,in the gas pha,have to be at high partial pressures in order to make the most of the new mesoporous material as an adsorbent for VOC removal.However,a proper modification of the pore openings of MCM-41can change the isotherm types from type IV to type I wi
thout remarkable loss of the accessible pore volumes and,therefore, significantly enhance the adsorption performance at low partial pressures.Adsorption isotherms of water on the adsorbents are all of type V,demonstrating that they posss a similar hydrophobicity.Desorption of VOCs from MCM-41could be achieved at lower temperatures(50-60°C),while this had to be conducted at higher temperatures(100-120°C)for microporous adsorbents,zeolites,and activated carbons.
Introduction不假思索反义词
One of the most formidable challenges pod by the increasingly stringent regulations on air pollution in many countries is the arch for efficient and economical control strategies for volatile organic compounds(VOCs). The most currently applicable technology for VOC control is adsorption on activated carbon with sub-quent solvent recovery or incineration.1,2However,it has been recognized that an activated carbon prents veral disadvantages,3such as fire risk,pore clog(due to polymerization of some VOCs catalyzed by ashes prent on activated carbon surfaces),hygroscopicity, and some problems associated with regeneration,etc. Hence,much effort has been focud at finding alterna-tive adsorbents.Hydrophobic zeolite adsorbents have been proven to be an advancement in VOC adsorption/ paration technology.4By employing a hydrophobic zeolite or molecular sieve,the problems associated with activated carbon adsorbents co
uld be overcome.
In1992,a novel mesoporous molecular sieve family, M41S,was discovered by scientists at the Mobil Corp.5 MCM-41,one member of M41S family,posss a high surface area and a large pore volume with highly ordered hexagonally packed cylindrical pores.Most importantly,both the surface chemistry and pore open-ings of MCM-41can be tailored by postsynthesis to meet a given requirement.6,7A number of applications of this novel material have been suggested in catalysis,8ad-sorption,9and other relevant areas.10In our group,we have propod to develop MCM-41-bad adsorbents/ catalysts for VOC emission control.
In a practical VOC removal process,an ideal adsor-bent is expected to have(1)a large amount of reversible adsorption capacity(large accessible pore volume),(2) no catalytic activity,(3)a hydrophobic property,(4)high thermal and hydrothermal stability,and(5)an easy regeneration property.An adsorption isotherm can provide much information concerning the saturated adsorption capacity,the surface property,and the capillary condensation point(if exists)of an adsorbent. On the other hand,a desorption curve can tell one the regeneration temperature,reversible or irreversible adsorption,and the desorption enthalpy and entropy of an adsorbent.In the prent paper,the adsorption-desorption properties of MCM-41were evaluated and compared with some of the other hy
drophobic zeolites, i.e.,dealuminated zeolite Y and silicalite-1,and a commercial activated carbon BPL in terms of the adsorption isotherm and temperature-programmed de-sorption(TPD)curve.
Experimental Section
Adsorbent Samples.The MCM-41sample was prepared using dodecyltrimethylammonium bromide as the template.
*To whom correspondence should be addresd.Fax:61-7-
33654199.E-mail:maxlu@cheque.uq.edu.au.
(1)Rubby,E.N.;Carroll,L.A.Chem.Eng.Prog.1993,28-35.
(2)Stenzel,M.H.Chem.Eng.Prog.1993,36-43.
(3)Fajula,F.;Plee D.Stud.Surf.Sci.Catal.1994,85,633-651.
(4)Blocki,S.W.Environ.Prog.1993,12,226-230.
脖子上长痤疮越字成语(5)Kresge,C.T.;Leonowicz,M.E.;Roth,W.J.;Vartuli,J.C.;Beck, J.S.Nature1992,359,710-712.
(6)Zhao,X.S.;Lu,G.Q.J.Phys.Chem.B1998,102,1556-1561.
(7)Zhao,X.S.;Lu,G.Q.Nature1998,manuscript in preparation.
(8)Corma,A.Chem.Rev.1997,97,2373-2419.
(9)Beck,J.S.;Vartuli,J.C.Curr.Opin.Solid State Mater.Sci. 1996,1,76-87.
(10)Zhao,X.S.;Lu,G.Q.;Millar,G.J.Ind.Eng.Chem.Res.1996, 35,2075-2082.
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Energy&Fuels1998,12,1051-1054
10.1021/ef980113s CCC:$15.00©1998American Chemical Society
Published on Web09/04/1998
The detailed synthesis procedures can be found elwhere.11Hydrophobic zeolite Y (Si/Al )300)was obtained by steaming and acid treatments of the parent NaY (Si/Al )2.45,from CU Chemie Uetikon AG,The Netherlands)according to the methods described by Ward 12and Fleisch et al.13Silicalite-1(
pure-silica ZSM-5)was synthesized using tetrabutylammo-nium bromide as the template at 175°C for 24h.All zeolites and molecular sieves were calcined at 550°C for 6h before u.The commercial activated carbon,BPL,was obtained from Calgon without further treatment.
Characterization .X-ray diffraction (XRD)patterns were recorded on a PW 1840diffractometer with Co K R radiation.Nitrogen adsorption measurements were conducted at liquid nitrogen temperature using a NOVA 1200analyzer (Quan-tachrome).Samples were outgasd at 553K overnight before measurement.Surface areas were calculated using the BET model.Total pore volumes were estimated at a relative pressure of 0.95,assuming full surface saturation of nitrogen.Pore size distributions were obtained using the BJH model.Adsorption isotherms of three typical ,benzene,carbon tetrachloride,and n -hexane,and water vapor were measured using a gravimetric technique using a quartz spring balance (Wilmad Glass).The nsitivity of the weight mea-surement is 0.01mg.Before measurement,a sample was outgasd at 473K overnight.The residual pressure within the adsorption chapter was below 5×10-4Torr.The vapor pressure for an adsorptive was measured using the capacitance manometer (MKS Baratron 122B)with a nsitivity of 0.1Torr.Lower vapor pressures were also monitored using a silicon oil manometer,which were in excellent agreement with that measured by the transducer.The adsorption equilibrium for VOCs coul
d be reached within 20min,while the adsorption equilibrium for water was normally achieved within 60min.Desorption of the organics was evaluated by temperature-programmed desoprtion (TPD)on a thermogravimetric ana-lyzer (TGA)(Shimadzu).A sample was first dried at 150°C overnight before being rapidly transferred into a desiccator containing the specific VOCs.Adsorption was conducted at room temperature (22°C)and ambient pressure for 48-1200h until no weight gain was obrved by an analytical balance.Then a VOC-loaded sample was rapidly loaded into the TGA sample holder,and TPD was started in a pure helium atmosphere with a flow rate of 25mL/min.The desorption curves were plotted as the first derivative TG (DTG).
Results and Discussion
Sample Characterization.Shown in Table 1are the characterization results of the various adsorbents.XRD results indicate that both the MCM-41and sili-calite-1samples prepared in this study are highly qualified,as well as the dealuminated zeolite Y.The strong (100)diffraction together with the other three diffraction peaks at higher angles in the XRD pattern
demonstrate a good MCM-41sample.The silicalite-1sample shows a XRD pattern similar with that of ZSM-5,characteristic of a MFI pore structure.Nitrogen adsorption measurements further confirmed th
e XRD results.All samples had reasonable pore volumes and surface areas.It can also be en that the BET surface area of the activated carbon adsorbent is similar to that of MCM-41,whereas its pore volume is much lower than that of MCM-41.The pore volumes of hydrophobic zeolite Y and silicalite-1are far lower compared to MCM-41.It is,therefore,expected that MCM-41would be a promising adsorbent for VOC removal.In addition,MCM-41is a mesoporous material that is capable of encapsulating larger organic molecules with little dif-fusion resistance.
儿子订婚祝福语Adsorption Isotherm .An adsorption isotherm can provide information on the adsorption capacity,hydro-phobicity/hydrophilicity,and the capillary condensation point (if prent)of an adsorbent.Figures 1-3show the adsorption isotherms of benzene,carbon tetrachlo-ride,and n -hexane over the various adsorbents at 22°C,respectively.For the MCM-41sample,adsorption isotherms of the organics are all typical type IV iso-therms according to IUPAC classification.A mono-layer -multilayer adsorption occurred before capillary condensation.The P /P 0points where capillary conden-sation occurred are centered at 0.15-0.2for all three organics.For both the hydrophobic zeolites and acti-vated carbon adsorbents,the adsorption isotherms are of a Langmuir type.The adsorption capacities of benzene and n -hexane on the adsorbents at lower concentration levels followed the quence of activated carbon >hydrohpobic zeolite Y >silicalite-1>MCM-41,while
at higher concentration levels,the quence changed to MCM-41>activated carbon >hydrophobic zeolite Y >silicalite-1.The saturated adsorption amounts of the adsorbents are in good agreement with
(11)Zhao,X.S.;Lu,G.Q.;Millar,G.J.;Li,X.S.Catal.Lett .1996,38,33-37.
(12)Ward,J.W.J.Catal .1970,18,348-351.
(13)Fleish,T.H.;Meyers,B.L.;Ray,G.J.;Hall,J.B.;Marshall,C.L.J.Catal .1986,99,117.
(14)Richards,R.E.;Rees,L.V.C.Zeolites 1986,6,17.
Table 1.Characterization Results of the Four Adsorbents
lo的名词
sample
XRD BET surface area (m 2/g)
total pore volume (mL/g)
BJH pore size distribution (PSD)pore configuration Si-MCM-41
d 100)32.3Å10600.87very narrow PSD centered at 22.5Å1-D cylindrical pore,hexagonally packed hydrophobic zeolit
e Y FAU 6920.31very narrow PSD centered at 7.4Å3-D cage-like pore silicalite-1
MFI 3710.21very narrow PSD centered at 5.5Å3-D channel-like pore activated carbon BPL
N/A
923
航海地图0.46浙江师范大学录取分数线
bidisperd PSD 8.5Å,14.1Å
防水涂料施工方法
Slit-shaped mainly,
containing
micropores
Figure 1.Adsorption isotherms of benzene over the various adsorbents.
1052Energy &Fuels,Vol.12,No.6,1998
Zhao et al.
the results of nitrogen adsorption,demonstrating that all the pores are highly accessible to the organics except the pores of silicalite-1,which adsorbed little carbon tetrachloride.This is becau carbon tetrachloride could not penetrate into the internal pores of silicalite-1due to the smaller pores of silicalite-1compared to the kinetic diameter of carbon tetrachloride (ca.6.0Å).The results indicate that MCM-41does have large accessible internal pore volumes which can be filled at higher relative pressures compared to the microporous adsorbents.However,the type IV isotherm behavior of MCM-41requires VOCs,in the gas pha,to be at a high partial pressure in order to fully fill the accessible pores of MCM-41.This is not the situation with most industrial applications where VOCs are normally prent at low partial pressures.However,by properly tailoring the pore openings of MCM-41,7the adsorption isotherm type can be modified from type IV to type I without much loss of the accessible pore volumes (e below).Shown in Figure 4are the adsorption isotherms of water va
por on the adsorbents.We were interested in measuring water adsorption becau the VOC stream contains a large amount of water.As expected,dealu-minated zeolite Y and silicalite-1are very hydrophobic without much loading of water.The surface hydropho-
bicity of MCM-41is similar to that of activated carbon,reflected by their adsorption amounts at lower relative pressures.However,clo inspection of the adsorption isotherms reveals that the P /P 0where capillary con-densation occurred is larger for MCM-41(0.55-0.60)than for the activated carbon PBL (0.40-0.45)(Figure 4).This difference is of significant importance since VOCs are generally prent in VOC streams with a relative humidity of around 50%.
Pore-Opening Modification .Figure 5shows the adsorption isotherms of benzene over MCM-41before and after pore modification,together with the isotherms of the activated carbon and hydrophobic zeolite Y for comparison.It is en that after modification of the pore openings of MCM-41,the adsorption capacity of benzene at lower partial pressure was significantly enhanced without too much loss of the pore volumes.The adsorp-tion capacity of the modified MCM-41adsorbent is still higher than that of the microporous adsorbents,acti-vated carbon,and hydrophobic zeolite Y.The saturated adsorption amount of benzene was decread by about 25%after modification due to the partial loss of the pore volumes during modification.The adsorption type changed from type IV to typ
e I as a result of the reduction of the pore openings of the modified
MCM-
Figure 2.Adsorption isotherms of carbon tetrachloride over the various
adsorbents.
Figure 3.Adsorption isotherms of n -hexane over the various
adsorbents.
Figure 4.Adsorption isotherms of water over the various
adsorbents.
Figure 5.Adsorption isotherms of benzene over MCM-41samples before and after modification.
VOC Removal Energy &Fuels,Vol.12,No.6,19981053
41.More information concerning the modified MCM-41adsorbent is being obtained and will be reported elwhere.7
TPD Study .Figures 6-8show the DTG curves of benzene,carbon tetrachloride,and n -hexane from the four adsorbents.It can be en that the maximum mass loss rates of all the organics from MCM-41occurred at about 60°C.In addition,the desorption peaks are very sharp,demonstrating that the desorption of organics from MCM-41was very fast.However,the maximum mass loss rates from both the hydrophobic zeolites and activated carbon adsorbent were all found to occur above 100°C.Also,the desorption peaks are broad,indicating that desorption of VOCs from the microporous adsor-bents are slow.Two desorption peaks could be resolved on the microporous adsorbents by decreasing the heat-
ing rates from 10/min to 2°C/min,while this was not the ca for MCM-41.The results demonstrate
that VOCs are easier to be desorbed from the mesoporous adsorbent,MCM-41,than from the microporous adsor-bents,hydrophobic zeolites,and activated carbons.This is very important from the consideration of energy saving since the VOC-loaded adsorbents are normally regenerated by hot steam in order to recover the solvent.
Conclusion
It is concluded that hydrophobic MCM-41is a poten-tial adsorbent for the removal of VOCs prent in high concentrations and high humidity streams.Proper modification of the pore openings of MCM-41can significantly enhance the adsorption performance with-out a remarkable loss of the accessible pores.Hydro-phobic zeolites are candidate adsorbents in removing VOCs in low concentrations with high humidity.A composite adsorbent compod of hydrophobic mi-croporous zeolites and mesoporous molecular sieves,such as hydrophobic zeolite Y plus MCM-41,may be expected to be promising in VOC removal becau it could handle VOCs in a larger spectrum of concentra-tions with high humidity.At the moment,activated carbon adsorbents will still be the first choice for u in air-flow treatment systems becau they are cost-effective materials,although some of the above-men-tioned disadvantages are frequently encountered in practice.However,MCM-41will be a particularly efficient and competitive adsorbent for VOC recovery at high concentrations or for applic
ations where low-temperature waste heat is available for regeneration.
EF980113S
Figure 6.TPD profiles of benzene from the various adsor-
bents.
Figure 7.TPD profiles of carbon tetrachloride from the various
adsorbents.
Figure 8.TPD profiles of n -hexane from the various adsor-bents.
1054Energy &Fuels,Vol.12,No.6,1998Zhao et al.
