Synthesis, characterization and catalytic activity of the pillared molecular sieve MCM-36

Microporous and Mesoporous Materials 25(1998)207–224Synthesis,characterization and catalytic activity of the pillaredmolecular sieve MCM-36Y.J.He,G.S.Nivarthy,F.Eder,K.Seshan,J.A.Lercher *Uni v ersity of Twente,Faculty of Chemical Technology,Catalytic Processes and Materials,P.O.Box 217,7500AE Enschede,The NetherlandsReceived 23January 1998;received in revised form 23June 1998;accepted 13July 1998AbstractMCM-36materials were prepared by swelling the layered MCM-22precursors with large organic molecules and then pillaring the resulting material with polymeric silica.A mesopore region with 0.25–0.3nm thickness between the microporous layers was identified.The BET surface area obtained for MCM-36was 2.5to 3times higher than that of MCM-22.The sorption characteristics of linear alkanes are similar for MCM-22and MCM-36materials,indicating that sorption is dominated by the 10-membered ring microporous channel system and that the pore structure of the MCM-22layers stays intact during the swelling and pillaring processes.Swelling and pillaring,however,decreased the concentration of Brønsted acid sites compared with the starting material.The largest fraction of strong Brønsted acid sites is located in this zeolite layer.Adsorption of 2,2,4-trimethylpentane indicates that only about 10%of the bridging hydroxyl groups are located in the mesoporous region.The superior catalytic performance of MCM-36compared with MCM-22for alkylation of isobutane with n-butene indicates that the open mesoporous structure can be successfully utilized to make acid sites of the layers accessible to large molecules.©1998Elsevier Science B.V.All rights reserved.Keywords:Layered zeolites;MCM-22;MCM-36;Mesoporous materials;Pillared zeolites;Characterization;Sorption1.Introductionother layered phases such as zirconium phosphates,silicas and metal oxides has also been explored [1].The combination of their specific pore struc-Pillared layered structures [1–3]are built of ture and catalytic properties has been exploited in inorganic layers with inorganic or organic pillars many commercial applications [5,6].Most nota-appended on both sides of the sheets.These materi-bly,however,these materials contain moderately als are potentially most attractive for catalysis,strong to weak acid sites,much weaker than the because they combine high specific surface areas strong Brønsted acid sites (bridging hydroxyl and good accessibility for larger molecules to a groups)in zeolites.large number of catalytic sites [4].Traditionally Conceptually,the high acid strength and uni-the focus was on pillared clays,but pillaring offormity of the zeolite pores o ffer an unprecedented tool to control catalytic conversion by constraints *Corresponding author.Fax:+31534894683;E-mail:j.a.lercher@ct.utwente.nl (shape selectivity).The dimensions of the micro-1387-1811/98/$–see front matter ©1998Elsevier Science B.V.All rights reserved.PII:S1387-1811(98)00210-8208Y.J.He et al./Microporous and Mesoporous Materials 25(1998)207–224pores,on the other hand,limit the maximum mine (HMI,99%,Aldrich)as organic template molecular size of reactants and products that can molecule,SiO 2(Aerosil 200,Degussa)or colloidaldi ffuse through the internal voids [7].Con-SiO 2(40wt %SiO 2,Aldrich),NaAlO 2(NAP-120,sequently,the synthesis of large-pore materials has Sumitomo Chemical Co.Ltd),NaOH (Merck)attracted significant interest.The stability of the and water.zeolite phases,however,apparently seems to limit Based on the synthesis procedure reported in the pore size to less than 0.8nm [8].Recently it [15],the synthesis parameters were varied to opti-was observed that some zeolites tend to crystallize mize the synthesis process.Two variations of this by condensation of silicate sheets,and that the procedure using di fferent silicon precursors,one condenation of such sheets occurs sometimes only from Aerosil and the other from colloidal silica,upon calcination of a crystalline precursor [9].are presented here.MCM-22was one of the first of these materials reported [10].The precursor of MCM-22is based 2.1.1.Method Ion an aluminosilicate framework with a two-Sodium aluminate (NaAlO 2)(1.1g)and NaOHdimensional sinusoidal channel system within hex-(0.7g)were dissolved in 149g of H 2O.To thisagonal sheets,accessible through 10-membered solution,9.1g of HMI and 11.1g of SiO 2werering apertures.During the calcination procedure,added subsequently under vigorous stirring for condensation of the OH groups of the sheets helps 30min.After this,the resulting homogeneous gel to form the MCM-22structure with a two-dimen-was loaded into a PTFE-lined stainless steel auto-sional channel system containing a strongly dis-clave and contained at 423K for 10–14days.The torted supercage.Prior to calcination the layers of solid product was recovered by filtering,washing MCM-22can be separated by swelling agents,and with water and drying in air at room temperature.the interlayer space between them can be partly The MCM-22precursor (MCM-22(P))was iden-filled with thermally stable inorganic pillars.The tified by means of X-ray di ffraction.pillars serve to preserve the interlayer separation,while the void space created after removal of the 2.1.2.Method IIswelling agent produces a mesopore system.This NaAlO 2(1.13g)and NaOH (0.54g)were dis-material is denoted as MCM-36[11,12].It con-solved in 99.33g of distilled water and 6.85g of tains mesopores having diameters between 3and HMI was added to this solution.This mixture was 3.5nm and sinusoidal,10-membered ring channels blended thoroughly with 20.77g of colloidal SiO2in the intact layers [13].The large pores have been (40wt %solids)under vigorous stirring.The result-claimed to improve catalysis for reactions produc-ing gel was introduced into a 150cm 3PTFE-lined ing bulky molecules [14].stainless steel autoclave and statically heated at In this paper,the synthesis of zeolite MCM-22423K for 5–7days.Further treatments were the and the pillared zeolite MCM-36is described.A thorough physicochemical characterization is pre-same as for Method I.sented to provide a basis for understanding the complex transformations.Alkylation of isobutane For both methods,the gel compositions were with n-butene was used to probe the influence of adjusted to Si /Al ratios of 10and 14in the final the mesoporous structure in MCM-36on the cata-materials.To remove the organic template from lytic activity and to compare with the microporous the zeolite materials,the MCM-22(P)was calcined material MCM-22.in flowing nitrogen with 8%oxygen by heating at 1.5K /min to 823K and holding at this temper-ature for 15h.2.Experimental2.2.Synthesis of MCM-362.1.Synthesis of MCM-22For the synthesis of MCM-36according to the The hydrothermal synthesis of aluminosilicate MCM-22was carried out by using hexamethylenei-method reported in [12],the wet cake (25–30%209Y.J.He et al./Microporous and Mesoporous Materials 25(1998)207–224solids)of MCM-22(P)was mixed with cetyltri-the sample during the scan.For MCM-36,the methylammonium chloride (CTMACl,25%,XRD data at low angles of 2h (less than 4°)are Aldrich)solution and tetrapropylammonium very important because of the increased c -axis hydroxide (TPAOH,Aldrich)with a relative dimension (≥5nm)due to pillaring.weight ratio of 1:4:1.2MCM-22(P)/CTMACl /Thermal analysis (thermogravimetric analysis /TPAOH.The pH of the solution was adjusted to di fferential scanning calorimetry,TGA /DSC)of 13.5and the mixture reacted in a flask at 373K the as-synthesized samples was performed on an for 68h and at room temperature for 4h under APL Thermal System (STA 625).The experiments continuous stirring.The resulting swollen material were carried out in nitrogen in the temperature was filtered and washed with a small amount of range 298–1073K with a heating rate of 10K /min.distilled water,and dried at ambient conditions to Nitrogen adsorption measurements were carried obtain swollen MCM-22.Tetraethylorthosilcate out at 77K with a Micromeretics ASAP 2400(TEOS,98%,Aldrich)was added as pillaring agent system.The samples were degassed at 573K and to the swollen material.A mixture of TEOS and 10−3Pa for 24h prior to the adsorption measure-the swollen material with a relative weight ratio of ments.The specific surface areas were calculated 5:1was heated at 363K for 25h in a nitrogen by the Brunauer–Emmet–Teller (BET )method.atmosphere under continuous stirring,and then On the basis of argon adsorption measurements,filtered and dried overnight.The resulting solid the micro-and meso-pore size distributions were was hydrolyzed in water with a 1:10ratio (wt /wt)obtained from the adsorption branch of the iso-of solid powder and water at 413K for 6h,and therm.The pore volume and the average diameter then filtered and dried at 300K overnight.During of the pores were calculated by using t -plots.hydrolysis,the pH of the mixture was controlled The morphologies and crystal sizes of the synthe-at 1.92or 8.0with 1M HNO 3.Finally,the samplesized specimens were examined by scanning was heated at 723K for 3h in nitrogen and at electron microscopy (SEM,JEOL JSM-35CT )812K for 6h in air (heating rate of 2K /min).and transmision electron microscopy (TEM,Philips CM 30/TWIN).2.3.Ion exchangeTemperature-programmed desorption (TPD)of ammonia was used for determining the acidity of The calcined zeolite powders were ion-the samples using a Balzers QMG 420mass spec-exchanged with an excess of 1N NH 4NO 3aqueoustrometer as detector for the desorbing gases.About solution (liquid-to-solid ratio of 10cm 3/g)and 50mg of the H +-form zeolite sample was placed stirred continuously at room temperature for 8h.in a quartz U tube.The sample was degassed by The pH of the mixture was adjusted to 7with evacuating to 10−3mbar,heating at a rate of NH 4OH.This procedure was repeated twice.After10K /min to 773K and maintaining that temper-filtering and drying,the resulting NH +4forms ofature for 60min.After cooling to room temper-MCM-22and MCM-36were calcined at 773K ature,the sample was equilibrated with 10mbar for 5h in air to form H-MCM-22and H-MCM-36.of ammonia.Then,the system was evacuated to 10−3mbar for 60min and heated to 873K at a rate of 10K /min.During this temperature ramp 2.4.Characterizationmass spectra were collected at 5K intervals to determine the rate of ammonia desorption.Powder X-ray di ffraction (XRD)patterns were The concentration of acid sites was determined collected to estimate crystallinities and the struc-by ammonia adsorption /desorption in a modified tural types of the synthesized material on a Philips Setaram TGA /DSC 111instrument.The system PW3050(X’Pert-MPD)di ffractometer with was evacuated (p <10−6mbar)at 373K for 12h.Cu K aradiation.Data were collected in the 2hAmmonia at 3mbar was equilibrated with approx-range from 0.5to 50°with a step size of 0.02°and a step time of 10s under continuous rotation of imately 15mg of activated sample.The amount of210Y.J.He et al./Microporous and Mesoporous Materials25(1998)207–224ammonia irreversibly bound to the Brønsted acid sidering the differences in the localized adsorption sites at373K was used to calculate the concen-at the acid sites and delocalized sorption in the tration of acid sites.Subsequently,the samples pores.The concentrations of acid sites used in the were heated in vacuum with a temperaturecalculations were taken from ammonia adsorption increment of5K/min to673K.The desorbing data.The equilibrium constants and the directly ammonia was detected with a Balzars QME-125measured enthalpies were used to calculate the mass spectrometer.Gibbs free enthalpies and the entropies of sorption, Infrared(IR)spectra of the zeolite samples wererespectively.recorded with a Bruker IFS-88FTIR spectrometerin the transmission mode(typical resolution of 2.5.Catalytic tests4cm−1)in situ during activation,adsorption andTPD.For the IR spectroscopic measurements,the Isobutane/2-butene alkylation experiments were materials were pressed into thin,self-supporting carried in the slurry phase,in a50cm3Hastelloy wafers and placed into a vacuum chamber(base C-276autoclave(Autoclave Engineers Co.,pro-pressure of10−7mbar)equipped with IR-transpar-duct code:CRAHC05ZH16A)operated in the ent windows.For activation,the zeolite samples continuous mode under well-stirred conditions. were heated in vacuum(p<10−6mbar)to773K The stirrer was driven by a magnetically coupled with a heating rate of10K/min and maintained motor with a stirring speed of3500rev/min.The at that temperature for60min.The adsorption of autoclave was heated by a tight-fitting furnace.In pyridine or trimethylpentane was carried out at each case,catalytic materials were pre-dried ex situ room temperature at a pressure of10−2to at423K overnight and in situ for2h at473K. 10−3mbar.The desorption measurements were The experimental procedure consisted offirstfilling carried out in vacuum(p<10−6mbar)under heat-the autoclave with isobutane.After this,a definite ing from room temperature to853K.The decreaseflow rate of a mixture of isobutane and cis-2-butene in intensity of the OH stretching vibration band was admitted into the reactor maintained at348K. corresponding to the Brønsted acid sites was used Products of the reaction were analyzed on-line by to determine the coverage of the acid sites with a gas chromatograph(Hewlett Packard5890)equ-sorbed molecules.ipped with a FID detector and a30m long DB-1 Alkane sorption measurements were also per-column(J&W Scientific)with helium as the formed with the modified Setaram TG-DSC111carrier gas.system(see[16]for details).Approximately15mgof H-MCM-22or H-MCM-36were charged intothe quartz sample holder of the balance,heated invacuum(p<10−6mbar)with a temperature 3.Results and discussionincrement of10K/min to673K and held at thistemperature for1h to remove water.After cooling 3.1.Crystallizationto323K,the alkanes were discontinuously dosedinto the closed system and equilibrated with the The molar compositions of the gels are compiled surface.The equilibration was confirmed byin Table1.The crystallization temperature was observing the heatflow and the changes in weight.adjusted at423K according to the results of The experiments were carried out in the pressureRavishankar et al.[17].At higher temperatures range from10−3to13mbar.The pressure was(T>423K),impurity phases(especially MFI)may recorded with a Baratron pressure transducer(typebe formed after about2days[17].At lower 122A).The adsorption isotherms of all hydro-temperatures,the crystallization rate of MCM-22 carbons were determined gravimetrically with awas found to be very slow.simultaneous determination of the differential As seen from Table1,the use of colloidal silica(Method II)led to well-crystallized MCM-22with sorption enthalpies.The isotherms werefitted to asum of Langmuir-type adsorption isotherms con-a Si/Al ratio ing Aerosil200211 Y.J.He et al./Microporous and Mesoporous Materials25(1998)207–224Table1Gel compositions and synthesis conditions of zeolite MCM-22Sample Method Gel composition Temp.(K)Time(days)CrystallinitySiO2/Al2O3Na/SiO2R*/SiO2H2O/SiO2I-1I27.920.160.3341.7242314fullI-2I27.380.170.544.9242314fullI-3I200.20.544.9242312fullI-4I200.20.544.9242310fullI-5I200.20.544.924237partII-1II200.20.544.924235fullR*:Organic template.as silica source(Method I)did not lead to a fully calcined materials).As seen in Fig.1,the bands of crystalline product even after7days(sample I-5).MCM-22(P)are broad and some of them overlap, This suggests that the choice of silica has a strong while after calcination MCM-22generally has influence on the rate of crystallization,the more sharp peaks.The difference between the XRD soluble source leading to a faster nucleation rate patterns of calcined MCM-22and MCM-22(P)is and crystal growth.After calcination of the fully due to the creation of ordered linkages between crystallized precusor,thefinal MCM-22was the sheets during the calcination procedure.The formed.For the calcined sample I-4(Method I)MCM-22precursor is a two-dimensional layer and sample II-1(Method II),41.0and41.1wt%with a hexagonal pore structure(see Fig.2(a)and Si,4.03and3.77wt%Al,and1.74and2.11wt%2(b))[10].The condensation of OH groups during Na were measured by X-rayfluorescence(XRF),calcination creates linkages in the c-direction, respectively.In Fig.1typical XRD patterns of resulting in a three-dimensional system(see as-synthesized MCM-22(P)and calcined MCM-22Fig.2(c))[10,20]and obvious changes in the d-are pared with the results in values corresponding to planes002,111,200,201, [18,19],the coincidence of the XRD patterns202,212,300,301,302and312.The characteristic identifies the products as a highly crystalline mate-002reflection of MCM-22(P)at the d-spacing ofrial(Table1)and pure MCM-22(precursor and13.3A˚(2h~6.6°,Cu Ka )disappears because thispeak becomes sharp and overlaps with the100peak at2h~7.2°after calcination.The displace-ment of the002peak to higher2h and its sharpnessindicate that the unit-cell c-parameter decreasesand ordering becomes more regular.This phenom-enon has been identified also by Lawton et al.[20].The other distinct difference between theprecursor and calcined samples is in the2h range12–25°,where the pattern of calcined MCM-22ischaracterized by sharp and separated peaks.Following the current models of Leonowicz et al.[10],calcination connects the hexagonal sheets byirreversible condensation of OH groups to formthe second pore system(see Fig.2).The supercages(inner height 1.82nm)are accessible through Fig.1.Powder X-ray diffraction pattern of as-synthesized and10-membered ring windows with a free diameter calcined MCM-22:(a)MCM-22precursor,(b)calcinedMCM-22.of0.71nm[10].As the condensation is irreversible,212Y.J.He et al./Microporous and Mesoporous Materials25(1998)207–224Fig.2.Schematic representation of the MCM-22layer structure[10,20]:(a)a hexagonal sheet as seen along the c-axis,(b)pore structure with10-membered ring windows,(c)calcined MCM-22.213Y.J.He et al./Microporous and Mesoporous Materials 25(1998)207–224the MCM-22precursor should be preferably left and MCM-36materials are shown in Fig.4.MCM-22shows platelets approximately 0.5–1m m as a wet cake for preparing MCM-36.in length and 0.05–0.1m m in thickness.Some While several polymeric inorganic oxides (e.g.,larger particles are formed by the aggregation of SiO 2,Al 2O 3,ZrO 2,TiO 2)can be used as pillaringthese platelets.In contrast,MCM-36shows larger agents,the size and structure of the pores created agglomerated crystallites with diameters varying depend critically upon the nature of the pillaring from 0.5to 3m m,while the distinct platelet struc-material.Here,only pillaring by silica will be ture has disappeared completely (see Fig.4(b)).discussed.Based on the sample I-4(MCM(P)),We attribute this to the TEOS creating silica pillars MCM-36was produced by swelling and subse-between layers of swollen MCM-22,resulting in quent pillaring processes.For this sample 42.2wt %at least a doubling of the platelet thickness.Si and 3.75wt %Al were measured by pared with the MCM-22sample,MCM-36 3.2.Thermal stabilitycontains about 2.5wt %pillars of polymeric silica between the layers.The XRD pattern of the The TGA /DSC patterns are shown in Fig.5for MCM-36sample is shown in pared as-synthesized MCM-22and MCM-36.The weight with the pattern of MCM-22(P)in Fig.1,the loss at T <473K is due to the desorption of watercharacteristic 002plane reflection at 2h ~6.6°dis-appears upon pillaring.Instead,an intense low-angle peak appears at 2h between 1and 2°corre-sponding to a d -spacing larger than 5nm,which represents the new c -parameter of the unit cell.This d -value includes the c -parameter of the unit cell of MCM-22and the spacing distance between the layers of MCM-36.The distance between two layers in MCM-36is calculated by subtracting the thickness of the layer (c -parameter of MCM-22,2.51nm [10]).The values for the samples obtained suggest an average interlayer distance of 2.5nm.All peaks observed correspond perfectly to those (a)(b)Fig.4.Scanning electron micrographs of MCM-22(a)and of the MCM-36material reported by Roth et al.MCM-36(b).[13].Scanning electron micrographs of the MCM-22Fig.5.Thermogravimetric analysis and di fferential scanning calorimetry patterns for MCM-22(———)and MCM-36Fig.3.Powder X-ray di ffraction pattern of MCM-36.(···).214Y.J.He et al./Microporous and Mesoporous Materials 25(1998)207–224from the materials.Above this temperature,the only in the interlayer space,interacts weakly with the zeolite layers and is removed at a relatively weight loss is associated with loss of the organic template molecules and the swelling agent incorpo-low temperature.rated into the materials.Above 473K,the mass loss of the samples in nitrogen occurs in two major 3.3.Pore structure of MCM-36steps corresponding to two endothermic peaks in the DSC curves,the steps being below and above Fig.6shows typical N 2adsorption /desorptionisotherms of MCM-22and MCM-36samples.The 743K.For the sample of MCM-22,the weight loss amounts to a total of 15.5wt %in steps of 7.5isotherms show a H4-type hysteresis loop accord-ing to the classifiation of IUPAC or a B-type and 8.0wt %.The weight loss for MCM-36is markedly higher than that of MCM-22and hysteresis loop (de Boer [22]).This type of hystere-sis loop is usually related to slit-shaped pores or amounts to a total of 30wt %in steps of 20and 10wt %.to platelet particles.However,this figure also shows clearly the di fference between the isotherms In MCM-22,the organic content arises from the template incorporated in the zeolite cavities.As of MCM-36and MCM-22.At a relative pressure (p /p 0)of 0.1,78%of the sorption capacity forindicated by Ravishankar et al.[21],in an inert atmosphere the template (hexamethyleneimine)MCM-22is already used because of the large driving force to adsorb in the zeolite micropores.decomposes into small fragments such as CH 4,C 2H 4,C 3H 6and NH 3below 743K.We attribute For MCM-36,the marked increase of the adsorp-tion capacity up to p /p 0=0.4is evidence for capil-the weight loss in the first step (473–743K)to the removal of hydrocarbon fragments.It should be lary condensation in mesopores,indicating the presence of a significant mesopore volume.Note mentioned that a certain amount of weight loss in this stage also comes from the removal of H 2Othat mesoporous solids with a clear hysteresis loop of type H4at higher p /p 0are expected to be layereddue to the condensation of OH groups.Some carbon-rich species remain in the zeolite and materials [8,23].Compared with the BET surface area of 386±15m 2/g for MCM-22,a higher desorb at the higher temperature corresponding to the second step above 743K.These indicate that specific surface area of 915±32m 2/g was obtained for MCM-36.part of the template or its fragment is more strongly adsorbed and held inside the zeolite pore.To analyze the adsorption isotherms further,the external surface areas were estimated by using t -In this case,the template molecules are present in two types of location:inside the 10-membered ring plots [24,25]which allowed us to estimate the average pore size and an equivalent microporous channels of the layer structure and outside the layers.The interaction of the template with the volume (V micro )or mesoporous volume (V meso).Inlattice in 10-membered ring channels is reported to be stronger than outside these channels [21].Thus,the removal of fragments from the templatein the zeolite pores of the intralayers occurs at a relatively higher temperature.With MCM-36,the weight loss in the first step (473–743K)is much higher than that for MCM-22.This is due to the removal of the organic swelling agent.In contrast,the mass loss (~10%)above 743K is slightly larger than that (~8%)for MCM-22.At higher temperature,the similarity between the weight losses of MCM-36and MCM-22indicates the desorption of HMI or its fragments from the intralayer channels.It also Fig.6.N 2adsorption isotherms of MCM-22(a)and MCM-36(b).suggests that the organic swelling agent is present215Y.J.He et al./Microporous and Mesoporous Materials 25(1998)207–224particular,the equation of Harkins-Jura was used:and MCM-36(see Fig.8).For MCM-22,the graph shows porosity only in the micropore region.The t (A˚)=[13.99/(0.034−log p /p 0)]0.5.The pore vol-umes (V meso and V micro)were obtained from themesoporous distribution performed with a cylin-drical or slit-shaped pore model shows an intense ordinate intercept of the straight line through thepoints in the range <6A˚and 6–9A ˚for the maximum at 2.5–3.0nm in MCM-36(see Fig.8)in the cases of both argon and nitrogen adsorption micropores and mesopores,respectively,and con-verted to liquid volumes (density conversion factor measurements.These results are in good agreement with the results of XRD and the t -method.The cm 3liquid /cm 3STP gas =15.47×10−4).Based on the adsorption branch of the isotherm and using narrow shape of pore size distribution indicates that a homogeneous structure is formed in the the Harkins-Jura equation as in the procedure given by Roberts [26],t -plots of both MCM-22interlayer of MCM-36.The mesopores are formed in MCM-36by two and MCM-36were calculated and are shown in Fig.7.The pore volumes were obtained from the processes during the calcination procedure.One is that the polymeric silicon hydroxide is decomposed intercept (y -axis)of the line through the points inthe range <6A˚and 6–9A ˚for the micropore and to form long polymeric silica chains between two zeolitic layers,spacing them apart.The other is mesopore volumes.For MCM-22,micropores are dominant with a pore volume of 0.12cm 3/g.For that the organic swelling molecules are removed simultaneously from these spaces to leave voids.MCM-36,mesopores are dominant with a pore volume of 0.492cm 3/g.The total surface area A schematic representation of MCM-36is depicted in Fig.9.(S t )is 878m 2/g obtained from the slope of the line through the points in the range <6A˚.The external In the MCM-36phase,the polymeric silica as pillars is formed during the hydrolysis and conden-surface area,S ext,in the larger mesopores is esti-mated at about S ext =63m 2/g based on the secondsation of silicates from tetraethylorthosilicate.The hydrolysis reaction replaces an alkoxy group (OR)line through the points in the range 6–9A˚.From a tubular model as 4V meso /(S t −S ext ),the averagewith a hydroxyl (OH)group.Subsequent conden-sation reactions involving the silanol groups pro-mesopore size of 24A ˚was obtained.In order to assess the microporosity of MCM-36duce siloxane bonds (Si–O–Si)and alcohol or water as byproducts,leading initially to oligomeric quantitatively,argon adsorption was carried out.The sample was dried at 523K and 10−5mbar for and polymeric structures.Depending on the condi-12h prior to adsorption.A micropore volume of 0.14cm 3/g is present in MCM-36,and is similar to that in MCM-22.The micropore diameter distri-bution shows a maximum at 5.5A˚,according to the Horvath–Kawazoe method,for both MCM-22Fig.8.Pore size distribution (Horvath–Kawazoe)plots for Fig.7.The t -plots of MCM-22and MCM-36.MCM-22and MCM-36.。