Microporous and Mesoporous Materials 129 (2010) 126–135

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Microporous and Mesoporous Materials

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Synthesis and characterization of aluminium containing CIT-1and their structure–property relationship to hydrocarbon trapperformance

Thomas Mathew a,1, S.P. Elangovan a,2, Toshiyuki Yokoi b, Takashi Tatsumi b, Masaru Ogura c,Yoshihiro Kubota d, Atsushi Shimojima a, Tatsuya Okubo a,*

a Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japanb Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japanc Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japand Division of Materials Science and Chemical Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan

a r t i c l e i n f o

Article history:Received 13 May 2009Received in revised form 6 September 2009Accepted 10 September 2009Available online 16 September 2009

Keywords:ZeolitesCIT-1CON topologyHydrocarbon trapVehicle emission

1387-1811/$ - see front matter � 2009 Elsevier Inc. Adoi:10.1016/j.micromeso.2009.09.007

* Corresponding author. Tel.: +81 3 5841 7348; faxE-mail address: okubo@chemsys.t.u-tokyo.ac.jp (T

1 Present address: Toyota Central R&D Labs. Inc., Na2 Present address: Nippon Chemical Industrial Co. Lt

a b s t r a c t

The present work focuses on the synthesis of Al-CIT-1 by various post-modification procedures of B-CIT-1and their structure–property relation to hydrocarbon trap performance. Al substitution in B-CIT-1 wascarried out by three different post-modification procedures, viz., direct exchange (Al-CIT-1_DEX), inser-tion (Al-CIT-1_INS), and impregnation (Al-CIT-1_IMP), and characterized in detail by various physico-chemical techniques such as XRD, N2 adsorption, SEM, NH3-TPD, XPS, and solid state 27Al and 29Si MASNMR. The Al-CIT-1 samples were tested for their adsorption–desorption characteristics of toluene, and2,2,4-trimethylpentane (2,2,4-TMP) to understand the Al distribution in CIT-1 as well as the efficacy ofthese materials as hydrocarbon traps. The Al distribution in Al-CIT-1 plays a critical role in the adsorp-tion–desorption characteristics of toluene and 2,2,4-TMP. Al-CIT-1 with high relative proportion offramework Al and less extraframework Al (Al-CIT-1_DEX) showed maximum toluene and 2,2,4-TMPadsorption capacity. The presence of finely distributed extraframework Al2O3 species in Al-CIT-1_INSmodify the pores and causes a slow release of toluene during desorption. Al-CIT-1_IMP with less Al con-tent and acidity showed the lowest toluene desorption temperature. Large amount of surface Al specieshinder a relatively bulky molecule 2,2,4-TMP to access the available active sites of tetrahedral Al and theassociated Br/nsted sites in the zeolite, and as a result, Al-CIT-1_INS showed hardly any adsorptiontowards 2,2,4-TMP. Thus, by varying the post-modification method and treatment condition, it is possibleto tune the performance of Al incorporated CIT-1 as a useful material for hydrocarbon trap.

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1. Introduction

The rapid build-up of greenhouse gases and the associated glo-bal warming and adverse health effects urge many scientificresearchers to focus on developing new technology or formulationfor eliminating them [1]. In vehicles, for example, a three-way cat-alytic converter (TWC) has been employed for emission control.One drawback of TWC is that it needs 175 �C or slightly abovefor initiation process though its activity is appreciable during thecourse of the vehicle run. However, vehicles require at least1–2 min to reach the minimum temperature of 175 �C from enginestarts, and by this time up to 70–80% of the pollutants in a drive

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cycle will be emitted [2,3]. Therefore, it is essential to control theemission during the start of the vehicle and it can be achieved bya process which uses a hydrocarbon trap. In the hydrocarbon trapprocess, the emitted hydrocarbons during the cold start of thevehicles adsorb on a suitable porous material and then desorbsat somewhat higher temperature, preferably close to the light-offtemperature of TWC [4,5].

Adsorption capacity, desorption temperature (must be equal orhigher than the light-off temperature of TWC) and hydrothermalstability are the critical factors for a hydrocarbon trap material [6–11]. There are few reports on the use of different kinds of zeolitematerials (Beta, MOR, ZSM-5, Y, etc.) for hydrocarbon trap applica-tion [6–18]. However, they show either one of the drawbacks suchas low adsorption capacity, poor hydrothermal stability, compara-tively fast desorption of hydrocarbon under aged condition, etc. Dif-ferent kinds of zeolites and aluminophosphate-based hydrocarbontrap studies in our laboratory revealed that SSZ-33 belongs to CON

T. Mathew et al. / Microporous and Mesoporous Materials 129 (2010) 126–135 127

family excels in terms of its reasonably good adsorption capacity,relatively high desorption temperature, and hydrothermal stability[11,19–21]. The CON topology possesses a multidimensional poresystem formed by intersecting 10 R and 12 R pores [22,23]. Althoughthe 10 � 12 R combination has been reported on few zeolites, theCON topology materials (SSZ-33, SSZ-26 and CIT-1) are unique intheir pore architecture as the internal void spaces of these materialsare accessible through both 10 R (5.1 Å � 5.1 Å) and 12 R(6.4 Å � 7.0 Å) pores [23–30]. This 10 R–12 R special pore junctionnetwork allow them to accommodate reasonable amount of organicmolecules with kinetic diameters close to the pore size, where as, the10 R opened to the exterior surface make sure the slow release oftrapped molecules during the temperature rise in an adsorption–desorption process. Owing to the 10 R–12 R combination, thesematerials have properties intermediate between Beta and ZSM-5zeolites; the former shows good adsorption capacity, whereas thelatter is known for its high stability.

In this study we focused on CIT-1, which possesses the CONtopology, a pure end polymorph of the SSZ-26/33 family, for thehydrocarbon trap application. Although Al-CIT-1 has been reportedfor its superior catalytic performance towards cracking and m-xy-lene isomerization [24,25,31], exploring this material for othercommercially important reactions or application is scanty. To-wards this, Al incorporated CIT-1 is synthesized by three differentpost-modifications of B-CIT-1 and studied in detail by variousphysico-chemical and spectroscopic techniques to distinguish thecharacteristic property of each material. The above prepared Al-CIT-1 is then studied for their structure–property relation tohydrocarbon trap performance with a particular aim to elucidatethe influence of Al distribution on the adsorption–desorptioncharacteristics.

2. Experimental section

2.1. Synthesis

Syntheses of SDA and B-CIT-1 were carried out according to theprocedure reported in the literature [30]. B incorporated CIT-1 wassynthesized using the structure-directing agent N,N,N-Trimethyl-(�)-cis-myrtanylammonium (I) hydroxide (ROH). Unless otherwisementioned, B-CIT-1 for the present study was synthesized at 150 �Cwith an initial gel composition, SiO2:0.02 B2O3:0.24 ROH:0.1NaOH:60 H2O (Si/B ratio of 25), and �1–2% as-synthesized B-betaseeds (B-beta with a Si/B ratio of 15 synthesized by a dry gel processand crystallized at 150 �C was used as seeds [32]). As previously re-ported, small amounts of as-synthesized B-beta seeds as nucleationpromoters are found effective for reducing the crystallization periodof CIT-1 [30]. We observed that fully crystallized CIT-1 can be ob-tained in 22–25 days at 150 �C with high reproducibility by an auto-clave procedure if 1–2 wt.% B-beta seeds were used. In the presentstudy, CIT-1 synthesized in 25 days was used. The ICP-AES analysisof the calcined sample (calcined at 650 �C) showed a Si/B molar ratioof 26.6 indicating that a major amount of B was included in the finalcrystallized product.

B containing CIT-1 in the calcined form was used for the Al-CIT-1synthesis. The B removal and the subsequent Al incorporationwas accomplished by three different post-modification proceduresas previously adopted by several researchers for hetero-atom sub-stitution in B containing zeolites [30,31,33]. These post-modifica-tion procedures are addressed as Al-CIT-1_DEX (direct exchange),Al-CIT-1_INS (insertion), and Al-CIT-1_IMP (impregnation). In thedirect exchange method (Al-CIT-1_DEX), B was exchanged for Alby a single-step procedure using a low-temperature hydrothermaltreatment. About 0.5 g of calcined B-CIT-1, 1 g of Al(NO3)3�9H2O,and 25 g of deionized water (zeolite, Al(NO3)3�9H2O, and water

by weight ratio 1:2:50) were taken in a 100 ml polypropylene bot-tle and stirred for 5 min at room temperature. The bottle was thenclosed tightly and kept at 90 �C statically for 3 days in an oven. Thereaction mixture was filtered, washed with 50 ml of 0.01 M HClfollowed by distilled water to remove the loosely adsorbed triva-lent species (B3+ and Al3+) and chloride ions and finally dried over-night at 100 �C. The insertion method (Al-CIT-1_INS) involves atwo-step procedure, viz., the B removal from the framework byan acid treatment followed by Al incorporation under refluxcondition. To achieve this, about 1 g of calcined B-CIT-1 was firsttreated with 100 ml of 0.01 M HC1 solution (solid/liquid ratio of1 g/100 ml) at room temperature with continuous stirring forabout 24 h to remove boron and create vacant space in the zeoliteframework. The material was filtered, washed with 100 ml of0.01 M HCl followed by water and dried overnight at 100 �C. Inthe second step, Al insertion was performed under reflux conditionat 90 �C for about 12 h using HCl treated CIT-1 material in an aque-ous solution of Al(NO3)3�9H2O (CIT-1:Al(NO3)3�9H2O:H2O ra-tio = 1:2:50 by weight). The Al treated sample was washed withdistilled water and dried overnight at 100 �C. The impregnationmethod (Al-CIT-1_IMP) also involves a two-step procedure; Bwas first removed by acid treatment as described for Al-CIT-1_INS to create vacant tetrahedral sites. Al was then introducedto the vacant tetrahedral site by the usual impregnation procedure.To achieve this, 0.5 g of dried deboronated CIT-1 was mixed with50 ml 0.002 M aqueous Al(NO3)3�9H2O solution and stirred at roomtemperature for 24 h. The solution was then evaporated to drynessby heating at 60 �C in a uniformly heated oven under stirring.Thereafter, the solid material was washed with distilled waterand dried overnight at 100 �C. The Al incorporated CIT-1 materialsby all the three processes were finally calcined to 550 �C at a heat-ing rate of 2.9�/min and maintained at this temperature for 5 h.

2.2. Characterization

The Al substituted CIT-1 samples were characterized by variousphysico-chemical techniques such as XRD, ICP-AES, N2 adsorption,SEM, Solid state MAS NMR, ammonia TPD, and XPS. Powder X-raydiffraction patterns of all the materials were recorded on a BrukerAXS, MO3X-HF diffractometer using Cu Ka radiation with a Ni filter,in the 2h range 5–50� at a scan rate of 4�/min. The elemental analysisof the calcined samples was performed by ICP-AES (Hitachi, P-4010AES spectrometer). The BET specific surface area and pore character-istics were measured by nitrogen adsorption at�196 �C using a sur-face area analyzer (Quantachrome, Autosorb-1). Scanning electronmicroscopic (SEM) images of zeolite particles were recorded oneither Hitachi S-900 (working voltage 6 kV) or S-5200 (working volt-age 1 kV). Solid-state 11B, 27Al and 29Si MAS NMR experiments wereperformed on a JEOL ECA-400 spectrometer at 128.335, 104.229, and79.46 MHz, respectively. NH3-TPD was performed on a BEL-CATinstrument (BEL Japan Inc.) to compare the acidic behavior of thematerial. The sample was allowed for NH3 adsorption at 100 �C(5% NH3 in He) for 10 min after pre-treatment at 500 �C in a Hestream. The desorption behavior was then monitored by a thermalconductivity detector (TCD) while heating the sample from ambienttemperature to 550 �C at a ramp rate of 10�/min. X-ray photoelectronspectra (XPS) were acquired on a Quantum 2000 (Ulvac 5/) X-rayphotoelectron spectrometer using a nonmonochromatized Al Karadiation as the excitation source (1486.6 eV). Ar etching wasperformed to obtain a clean surface before the measurement. Thebinding energies of XPS results for the catalyst samples were refer-enced to O 1s in zeolite at 532.6 eV. For quantification, the intensitiesof the peaks were estimated by calculating the integral of each peakafter the baseline subtraction. The peak area ratio was employedalong with photoionization cross-section factors.

Table 1Physico-chemical properties of calcined samples of B and Al incorporated CIT-1.

Sample Chemical analysis(ICP-AES)

N2 adsorption data Amount ofacid sitesa

(lmol/g)Si/B Si/Al BET surface

area (m2/g)Microporevolume (cm3/g)

B-CIT-1 26.6 – 627 0.18 30Al-CIT-1_DEX 1 55 640 0.18 90Al-CIT-1_INS 1 37 648 0.17 40Al-CIT-1_IMP 1 173 634 0.15 20

a Obtained from NH3-TPD by considering the high temperature peak alone.

128 T. Mathew et al. / Microporous and Mesoporous Materials 129 (2010) 126–135

2.3. Hydrocarbon trap test

Hydrocarbon trap performances of the materials were tested bytemperature programmed desorption (TPD) method using tolueneand 2,2,4-trimethylpentane (2,2,4-TMP) as probe molecules. TheTPD of hydrocarbon (toluene or 2,2,4-TMP) was carried out at50–390 �C using a gas chromatograph equipped with a TCD detec-tor (Shimadzu, model 9 AM). The TPD process was performed in aU-shaped quartz tube with 4 mm i.d. using 20 mg sample eachtime. The sample was first activated at 390 �C under a He flow(30 ml/min) for 1 h before hydrocarbon injection and subsequentTPD processes. The temperature of the sample was then reducedto 50 �C and a total of 5 ll toluene or 2,2,4-TMP was introducedto the sample as 1 ll dose each time by the normal liquid injectionmethod. The material was then allowed for desorption by heatingto 390 �C at a ramp rate of 10�/min.

3. Results and discussion

B-CIT-1 with a Si/B ratio of 26.6 was used as a parent materialfor various post-modification procedures to obtain Al-CIT-1. Theincorporation of B in the framework was confirmed by 11B MASNMR analysis (Fig. S1, Supporting information). Fig. 1 comparesthe XRD patterns of B-CIT-1 and Al incorporated CIT-1 samples.The XRD patterns exhibited the typical features of CIT-1 with sharpreflections [30]. The acid treated deboronation and the post-mod-ification procedures did not significantly affect their crystallinity asevidenced from the XRD pattern. Chemical analysis by ICP-AESmethod proved different amount of Al incorporated in the zeolitematrix, presumably due to the type of post-modification proce-dures and the reaction parameters. The Si/Al ratio of Al-CIT-1 sam-ples obtained from chemical analysis is summarized in Table 1. Forimpregnation method, as we limited the quantity of Al source tomake sure minimal pore blockage, the zeolite product ended upwith less Al content as expected. Although we used excess amountof Al precursor during post-modification, both Al-CIT-1_DEX, andAl-CIT-1_INS has resulted a lower proportion of Al by consideringthe Si/B mole ratio of 26.6 in the parent borosilicate material. Thissuggests that only about half of the B sites are being exchanged forAl in the particular case of Al-CIT-1_DEX. We believe the low pro-portion of Al incorporation in the present study is not due to anartifact of our preparation method. In fact, the Al-CIT-1-basedstudies that reported in the literature also had relatively low Alcontent; whereas, SSZ-33 usually provides an almost complete ex-

Fig. 1. X-ray diffractograms of B-CIT-1 (calcined at 650 �C) and Al-CIT-1. Al-CIT-1samples calcined at 550 �C are compared.

change of B for the Al [24,25,31]. The comparatively bigger crystalsize of CIT-1 could be one of the reasons for the reduced Al incor-poration with respect to SSZ-33. Besides the diffusional constraint,the highly hydrophobic property of B-CIT-1 might play a detrimen-tal effect to the second stage Al incorporation by considering thecomparatively less favourable hydrophobic–hydrophilic interac-tion, where the zeolite material is more hydrophobic and the aque-ous Al species is hydrophilic. The hydrophobic nature of the parentborosilicate was evidenced from the TG-DTA analysis of the as-syn-thesized B-CIT-1, which showed only about 0.6 wt.% water content(Fig. S2, Supporting information). The TG-DTA of calcined sampleof B-CIT-1 that exposed to atmosphere for 5 days showed about2.3 wt.% adsorbed water in the temperature range from room tem-perature to 300 �C; however, which we found comparatively lessthan that observed for other zeolites (Fig. S2, Supporting informa-tion). The SEM images of B-CIT-1 and Al-CIT-1 samples showedboth truncated pyramidal and interposed truncated pyramidal par-ticles (more like twinned crystal) (Fig. 2). Most of them have a par-ticle size of 4.8 lm at the tapered end of the truncated pyramidalshape. However, some tiny particles are visible on the crystal sur-face of Al-CIT-1 samples, which is noticeable in the cases of acidtreated post-modified samples. The acid treatment may introducenew silanol groups due to the hydrolysis of B–O–Si bonds and thesubsequent B elimination. This facilitates the removal of looselybound silica species from the zeolite framework and deposit asextraframework species on the crystallite surface [34–36], whichis further confirmed from the SEM analysis of pure siliceous CIT-1that obtained after acid treatment followed by calcination at550 �C (Fig. 2E).

The nitrogen adsorption analysis of Al-CIT-1 exhibited a micro-pore volume (t-plot) in the range 0.15–0.18 cm3/g, which is veryclose to the value reported for CIT-1 [30]. The pore volume andthe surface area of B-CIT-1 and Al-CIT-1 samples are listed in Table1. No large difference in the surface area was observed betweenB-CIT-1 and Al incorporated samples. However, a decrease of porevolume was apparent on the acid treated Al-CIT-1 samples; mostlydue to the partial pore block by the amorphous silica particle depo-sition and the relatively more extraframework Al2O3 species [36].In the case of Al-CIT-1_DEX, as the B removal and the Al incorpora-tion happens by a simultaneous process, formation of new silanolgroups and the subsequent amorphization of the zeolite frame-work is expected to be less. The N2 adsorption analysis result ofAl-CIT-1_DEX is a direct corroboration to the above assumption.B-CIT-1 and Al-CIT-1_DEX, for example, exhibited same microporevolume.

27Al MAS NMR spectra of Al-CIT-1 samples, as shown in Fig. 3,provide valuable information on the coordination environment ofAl and the relative contribution of different Al species with respectto the preparation method. The strong signal around 55 ppm isattributed to Al in the zeolitic framework at tetrahedral coordina-tion (AlTd), and the lines around 0 ppm are due to extraframeworkaluminium species (AlOh). The small peak between 20 and 50 ppm

Fig. 2. SEM picture of B-CIT-1 after calcination at 650 �C (A) and Al-CIT-1 and HCl treated pure siliceous CIT-1 samples after calcination at 550 �C: Al-CIT-1_DEX (B), Al-CIT-1_INS (C), Al-CIT-1_IMP (D), and siliceous CIT-1 (E). Note that SEM image of B-CIT-1 was acquired from Hitachi S-5200 (working voltage 1 kV). For all other samples, SEMimage was obtained from S-900 (working voltage 6 kV).

T. Mathew et al. / Microporous and Mesoporous Materials 129 (2010) 126–135 129

Fig. 3. 27Al MAS NMR spectra of Al-CIT-1 samples obtained after calcination at550 �C.

Fig. 4. 29Si MAS NMR spectra of Al-CIT-1 samples obtained after calcination at550 �C.

Fig. 5. Temperature programmed desorption (TPD) profiles of NH3 on B and Alincorporated CIT-1 samples.

130 T. Mathew et al. / Microporous and Mesoporous Materials 129 (2010) 126–135

has been assigned to pentacoordinated or distorted tetrahedrallycoordinated Al atoms in extraframework species [37–39]. A roughestimation based on the peak intensity indicates that the relativeproportion of AlTd to AlOh species is more on Al-CIT-1_DEX in com-parison with Al-CIT-1_INS and Al-CIT-1_IMP. Fig. 4 compares the29Si MAS NMR spectra of various Al-CIT-1 samples. The broad peakaround �100 ppm is due to HOSi-(OSi)3 (Q3) and OHSi-(OSi)2(OAl)(Q3) species [30,40–42]. The high intensity peak at �104 to�118 ppm on deconvolution shows at least three components,which corresponds to Si atoms located at crystallographicallynon-equivalent Si(4Si) (Q4) and Si-(OSi)3(OAl) (Q4) sites. Sevencrystallographically different Si atoms have been reported in apurely silicious CIT-1 [31]. The 29Si BD NMR spectra of CIT-1 heatedwith acetic acid showed five identifiable Q4 silicon maxima at�109.35, �111.15, �111.95, �112.95 and �116.2 ppm [42]. Thereis no much variation in their peak pattern for Al-CIT-1_INS and Al-CIT-1_IMP. The ratio between the Q3 and Q4 species indicated thatthe relative contribution of Q3 species is less with Al-CIT-1_DEX incomparison with Al-CIT-1_INS and Al-CIT-1_IMP. The higher pro-portion of defect sites in Al-CIT-1_INS and Al-CIT-1_IMP is presum-ably due to the pre-acid treatment.

Although the 29Si NMR analysis of purely siliceous CIT-1 thatobtained after acid treatment was not performed, we expect ahigher proportion of Q3 species on siliceous CIT-1 with respect tothe acid treated Al-CIT-1 as previously noted for CON topologymaterial [42]. Because of the presence of relatively more reactivedefect sites in acid treated samples and as the Al precursor hasbeen introduced from the solution phase via a second stage post-treatment method, it is possible that at least some amount ofAl3+ could be precipitated on the zeolite surface sites via a surfacereaction rather than framework insertion; obviously the extent ofwhich depends on the relative concentration of H+/OH� ion atthe silica–solution interface. The precipitated extraframework Alspecies that strongly chemisorbed on the zeolite crystallite surfaceis, however, not very easy to remove by washing with water likethe case with simply adsorbed Al3+ ion species. The use of com-plexing agent such as acetylacetone and/or excess hot water withpersistent washing may be helpful for reducing the extraframe-work Al [43]. Thus, it is apparent that in the latter cases the newsilanol groups that formed during the acid treatment were not nec-essarily healed by the second step Al incorporation. Though it ispossible to calculate the Si/Al ratio by 29Si NMR, in the present casethe Si/Al ratio may underestimate the actual Si/Al ratio as the de-fect sites (Si(OH)x groups) present in the zeolite framework hasbeen not well resolved into a separate peak to distinguish fromthe Al coordinated Si species. The evidence of Q3 species suggeststhat none of the post-modification procedure in the present studywas sufficient to heal the defect sites completely.

A higher temperature post-treatment may be effective for pro-ducing Al-CIT-1 with less extraframework Al by consuming moredefect sites [42]. Jones et al., in a recent study, reported the synthe-sis of defect free hydrophobic zeolites; as an example they demon-strated if calcined SSZ-33 was treated with an equal amount ofAl(NO3)3�9H2O in aqueous acetic acid at 160 �C for 6 days by anautoclave procedure, a large portion of the defect silanols can behealed with very less extraframework Al [42]. Although the AlTd/AlOh ratio of the 160 �C treated Al-SSZ-33 was higher in comparisonwith another sample of Al-SSZ-33 that they prepared by a similarprocedure of Al-CIT-1_DEX in the present study [31], the quantityof Al in both cases were almost same. Probably, the high tempera-ture autoclave post-treatment also results in dealumination andthereby quantitatively less Al in the final product though the incor-porated Al are intact to the framework.

The NH3-TPD profile of Al substituted CIT-1 shows two well re-solved broad desorption peaks between 100 and 400 �C (Fig. 5).The main desorption peak at 100–225 �C shown by Al incorporated

samples is due to physisorbed NH3. The relatively low intensebroad peak at 230–400 �C is characteristic of chemisorbed NH3

and it comprises acid sites from medium to high acid strength. Incontrast, for B-CIT-1, the major contribution of acidity is due tophysisorption and the high temperature desorption peak appearedlike a tail as a continuation of the main peak with very low inten-sity. In this case, desorption of NH3 is completed before 275 �C

Table 2Toluene and 2,2,4-TMP adsorption–desorption characteristics for various Al-CIT-1samples.

Sample Amount oftolueneadsorbed(mmol/g)

Amount of2,2,4-TMPadsorbed(mmol/g)

Toluene desorption characteristicsa

Peakmaximum(�C)

Desorption endtemperature(�C)

Al-CIT-1_DEX 0.60 0.37 134 (126) 223 (188)Al-CIT-1_INS 0.47 0.06 145 (136) 226 (215)Al-CIT-1_IMP 0.58 0.17 125 (125) 195 (195)

a The data for the corresponding hydrothermally treated sample is given in theparentheses.

T. Mathew et al. / Microporous and Mesoporous Materials 129 (2010) 126–135 131

indicating that B-CIT-1 has low acid strength. The relative acidityvalue obtained from the NH3-TPD measurement for CIT-1 is in-cluded in Table 1. The amount of acid sites was determined fromthe total amount of desorbed ammonia until the long tailingdesorption appeared by considering the high temperature peakalone. From the NH3-TPD, it is clear that the total acidity dependedon the amount of Al in the framework rather than the total Al con-tent. The smaller concentration of acid sites in Al-CIT-1_INS(40 lmol/g), for example, is due to the lower amount of tetrahedralAl as evidenced from the 27Al MAS NMR spectra. Al-CIT-1_DEXwith high relative proportion of tetrahedral Al had more acid sites(90 lmol/g). Al-CIT-1_IMP with less Al content, as expected, exhib-ited the lowest relative acidity. It is noteworthy that the NH3

desorption peak for Al incorporated CIT-1 continued to higher tem-perature, resulting a tail-like TPD spectrum. Such a tailing behaviorof the NH3 desorption was characteristic of Beta zeolite, and notcommon for ZSM-5 [44,45]. Therefore, the NH3-TPD pattern of Alincorporated CIT-1 is expected to behave more like 12 R zeolites(e.g., Beta) rather than 10 R zeolites (e.g., ZSM-5). However, thetailing behavior in the present case is not as prominent like Betaindicating that the material shows an intermediate behavior be-tween Beta and ZSM-5.

The Al incorporated CIT-1 samples were tested for their efficacytowards hydrocarbon trap. Fig. 6 shows the temperature pro-grammed desorption profile (TPD) of toluene on Al-CIT-1 obtainedby three different methods. The toluene adsorption–desorptioncharacteristics of Al-CIT-1 are included in Table 2. From Fig. 6and Table 2, it is evidenced that Al-CIT-1_INS desorbs at a highertemperature of 225 �C with the highest Tmax (145 �C) in compari-son with other samples. The desorption end temperature (Tend)and the desorption peak maximum (Tmax) for different Al-CIT-1samples follows the order Al-CIT-1_INS > Al-CIT-1_DEX > Al-CIT-1_IMP. On the other hand, Al-CIT-1_DEX exhibited a relativelyhigher capacity to adsorb toluene and the adsorption capacity fol-lows the order Al-CIT-1_DEX > Al-CIT-1_IMP > Al-CIT-1_INS. Thetoluene TPD profiles of B-CIT-1 and pure siliceous CIT-1 (obtainedafter deboronation by HCl treatment followed by calcinations at550 �C) are also included in Fig. 6 to compare the hydrocarbon trapbehavior of these two materials with Al-CIT-1. The amount of tol-uene adsorbed on B-CIT-1, and pure siliceous CIT-1 were 0.90 and0.51 mmol/g, respectively; evidently the adsorption capacity fortoluene is higher on B-CIT-1 as compared to Al-CIT-1. This maybe ascribed to the fact that being a directly synthesized material,majority of the pores in B-CIT-1 is accessible for toluene adsorp-

Fig. 6. Temperature programmed desorption (TPD) profiles of toluene on B-CIT-1,pure siliceous CIT-1, and Al-CIT-1 samples.

tion. However, both B-CIT-1, and non aluminous CIT-1 resulted incomparatively low desorption end temperature (194 and 187.5 �C,respectively) with respect to Al-CIT-1; the low acid strength of theformer two materials are responsible for this. On Al incorporation,the acid strength increases and as a result desorption occurs at ahigher temperature.

The micropore volume and framework Al (which are balancedby exchangeable cation: in this case, H+) are supposed to influencethe adsorption capacity and Tend, respectively. Owing to the highestmicropore volume and framework Al (AlTd), Al-CIT-1_DEX exhib-ited maximum uptake for toluene compared to the other Al-CIT-1 samples. Contrarily, the highest Tend observed on Al-CIT-1_INSwas quite unexpected by considering its lower relative proportionof tetrahedral Al and acidity with respect to Al-CIT-1_DEX. Thehighest Tend observed for Al-CIT-1_INS illustrates that the distribu-tion of Al species should play a decisive role in the desorptionbehavior. The 27Al MAS NMR spectra of this sample showed a goodamount of Al in the extraframework position and it mostly existsas finely distributed species on the zeolite surface rather than asa separate bulk crystalline phase. The finely distributed extra-framework Al2O3 species modify the pore mouth that opens tothe exterior surface of the zeolite. It appears to impart certain dif-fusion constraint during the desorption process and causes desorp-tion to occur at a higher temperature. It should be noted thatamong the Al incorporated CIT-1 samples, Al-CIT-1_INS with thehighest toluene desorption temperature resulted in the lowestadsorption capacity (Table 2). Thus, it is likely that there couldbe diffusional constraint during the adsorption as well, which inturn reduces the adsorption capacity. Al-CIT-1_IMP with less Alcontent showed the lowest desorption temperature (Tend) and Tmax

among various Al-CIT-1 samples in our case. It is worth mentioningthat SSZ-33 has been identified as a promising material as hydro-carbon trap until now, adsorbs ca. 1.5 times larger amount of tol-uene than CIT-1 used in this study although the nSi/nAl of CIT-1and SSZ-33 that used in our previous study was different [11,20].Besides the compositional difference between these two ana-logues, the smaller crystal size with relatively high pore volume(0.20–0.21 cc/g), and the stacking fault in SSZ-33 are the mostprobable reasons for this material to show the enhanced hydrocar-bon adsorption capacity.

XPS was performed mainly to have a clear picture on the surfacedistribution of the Al species on Al-CIT-1. In order to have a preciseview about the peak profile of the components, the O 1s, Si 2p, andAl 2p core levels of the Al-CIT-1 samples are displayed in Fig. 7. Thebinding energy (BE) of the Al 2p core levels of various Al-CIT-1samples appears at 74.9 ± 1 eV, which is higher than the most oftenreported value of 73–74.5 eV for different kinds of zeolites [46].The Si 2p photoemission line of Al-CIT-1_DEX, however, showedan increased BE value at 104 eV, which is about 0.5 eV higher thanthe other two Al incorporated samples. The reasons for the Si 2p

132 T. Mathew et al. / Microporous and Mesoporous Materials 129 (2010) 126–135

and Al 2p BE difference in zeolite is controversial [46–48]. How-ever, it is obvious that the presence of more than one coordinationmode of atomic components in Al-CIT-1 may lead to an unequalcharge distribution around Si and Al and thereby a shift in the BE[46–48]. The acid treated Al-CIT-1 samples that showed relativelyhigh proportion of Q3 species and amorphous silica deposition onthe zeolite crystal surface surely make a difference in the Si 2pBE shift to a lower level. The surface concentration was estimatedusing the relations between peak intensity, photoelectron cross-section and kinetic energy, as reported in the literature [49]. Forthis calculation, we used the core level photo ionization cross-sec-tions from the literature [50]. The surface atomic concentration asAl/Si ratio (obtained from XPS analysis) for various Al-CIT-1 sam-ples is compared to the corresponding bulk ratio (obtained fromchemical analysis) in Fig. 7d. With respect to the bulk, a higherAl/Si surface atomic ratio was observed for all the cases of post-modifications. However, as evidenced in Fig. 7d, among allAl-CIT-1 samples, Al-CIT-1_INS exhibited a distinct enrichment ofsurface Al. In this particular case, the Al/Si ratio on the surface isapproximately 1.9 times higher than in the bulk. The non-randomdistribution of boron in the parent borosilicate influences the modeof Al incorporation and their surface/bulk distribution during thepost-modification [51]. The observed heterogeneity between thebulk and the surface can be explained based on the fact that Alincorporation in CIT-1 is achieved through a second step post-modification procedure; during this process Al precursors havemore contact with the zeolite crystallite surface. However, as wepreviously mentioned, the acid treated samples with more numberof reactive Q3 species may have tremendous impact on the inho-mogeneous distribution of Al and therefore the variation in thesurface composition that obtained by XPS in our case was not sur-prising. Although, the X-rays used in XPS analysis is capable of pe-netrating few cages into the crystal, it gives informationconcerning the only first few layers of the sample [31]. In the pres-ent study, as we observed a more heterogeneous surface (with re-spect to the bulk) irrespective of the preparation method, XPS may

Fig. 7. XPS analysis of various Al-CIT-1 samples. Photoemission spectra of O 1s, Si 2p andatomic ratio for both bulk (from chemical analysis by ICP method) and surface (from XP

not be a valid tool to explain the diffusion limitations for the Al ex-change in our case.

Al substituted CIT-1 samples were further evaluated for theirhydrocarbon trap performance by studying the adsorption–desorption characteristics of 2,2,4-trimethyl pentane (2,2,4-TMP).There are two main reasons for choosing this molecule as an adsor-bate (i) 2,2,4-TMP is one of the major aliphatic compounds thatemitted during the vehicle start and (ii) the proximity betweenits molecular diameter (6.8 Å) and the 12 R channel width in CIT-1 (5.9 � 7 Å and 6.4 � 7 Å) makes 2,2,4-TMP a useful probe mole-cule for understanding the Al distribution and the subsequent poremodification with respect to the preparation method. Fig. 8 showsthe TPD profile of 2,2,4-TMP on Al-CIT-1 obtained by three differ-ent preparation methods. Unlike the toluene TPD profile, thedesorption profile of 2,2,4-TMP exhibited a dramatic variation be-tween each method of preparation. Al-CIT-1_DEX covers a largepeak area with the highest desorption temperature indicating thatan appreciable amount of 2,2,4-TMP was adsorbed (0.37 mmol/g).On the contrary, both Al-CIT-1_INS and Al-CIT-1_IMP showed a de-crease in the adsorption amount (0.06 and 0.17 mmol/g, respec-tively). Al-CIT-1_INS with the highest Al content showed themost unexpected result; more specifically in this case, a negligibleamount of 2,2,4-TMP was adsorbed (0.06 mmol/g).

Fig. 9 shows a schematic illustration of the Al distribution andthe adsorption–desorption behavior of toluene and 2,2,4-TMPthrough 10 R and 12 R channels (in the case of 2,2,4-TMP, only poremouth adsorption is considered and diffusion through the channelsis not shown). CIT-1 projections along [0 1 0] and [0 0 1] directionscorresponds to 10 R and 12 R pores, respectively, are included inthe figure. As illustrated in the figure, framework Al and the asso-ciated Br/nsted acid sites is the strong adsorption sites for bothtoluene and 2,2,4-TMP (note that the interaction between 2,2,4-TMP and these sites are portrayed by dotted lines, but it is notshown for toluene to view the scheme more simple). 2,2,4-TMPadsorption depicted by dotted line indicates strong chemical inter-action with AlTd and the associated Br/nsted acid sites and on such

Al 2p core levels from Al-CIT-1_DEX, Al-CIT-1_INS and Al-CIT-1_IMP are shown. Al/SiS) are compared in the figure for each sample.

Fig. 8. Temperature programmed desorption (TPD) profiles of 2,2,4-trimethylpentane (2,2,4-TMP) on Al-CIT-1 samples.

T. Mathew et al. / Microporous and Mesoporous Materials 129 (2010) 126–135 133

sites cracking of 2,2,4-TMP is possible [21]. On the other hand,those without any dotted line is more like physisorbed 2,2,4-TMPat the accessible pores. Therefore, Al-CIT-1_DEX with higheramount of AlTd and less pore blocking showed maximum tolueneand 2,2,4-TMP adsorption [21]. Contrarily, the relative proportionof AlTd on both Al-CIT-1_INS and Al-CIT-1_IMP was less withrespect to Al-CIT-1_DEX. Among all Al incorporated CIT-1, Al-CIT-1_INS showed the highest amount of extraframework Al. The sur-face modification by extraframework species play a vital role in theadsorption–desorption processes. The finely distributed extra-framework Al species in Al-CIT-1_INS (as evidenced from XPS)causes a diffusion constraint during the adsorption–desorptionprocesses and as a result toluene desorption occur at a relativelyhigh temperature. Few zeolitic pores in Al-CIT-1_INS may not beaccessible for toluene entering into the channel and hence rela-tively less toluene adsorption. Additionally, the large amount ofsurface Al species in Al-CIT-1_INS is a hindrance for a relatively

Fig. 9. A schematic illustration of Al distribution (depicted as framework and extraframe(b) on Al-CIT-1_DEX (1a and 1b), Al-CIT-1_INS (2a and 2b), and Al-CIT-1_IMP (3a and 3shown by dotted lines. CIT-1 projections along [0 1 0] and [0 0 1] directions correspond

bulky molecule like 2,2,4-TMP to access the available active sitesand as a result hardly any 2,2,4-TMP adsorption was observed(0.06 mmol/g). In other words, the zeolite crystallite surface inAl-CIT-1_INS literally acts as an inert surface for 2,2,4-TMP adsorp-tion. Al-CIT-1_IMP with very low Al content has fewer number ofactive sites and its adsorption capacity for 2,2,4-TMP (0.17 mmol/g), as expected, was smaller than that of Al-CIT-1_DEX. However,even with less Al content, Al-CIT-1_IMP showed an appreciableamount of 2,2,4-TMP adsorption with respect to Al-CIT-1_INS, sug-gesting that the available active sites in the former case are acces-sible for 2,2,4-TMP adsorption. In other words, the absoluteamount of extraframework Al species in Al-CIT-1_IMP is not en-ough in quantity to block many pores that are accessible for2,2,4-TMP adsorption. Our results are in accordance with the re-cent reports by Ribeiro Carrot et al. [36]. They studied the adsorp-tion of N2, n-pentane and iso-octane on dealuminated Beta zeolitesand observed that dealumination causes an enhancement in thezeolite external surface area and consequently less 2,2,4-TMPadsorption. Highly dealuminated samples showed hardly anyadsorption towards 2,2,4-TMP though their adsorption capacityfor smaller molecules like N2 and n-pentane are appreciable [36].

To check the stability of these materials, TPD of toluene was car-ried out on hydrothermally treated samples. The hydrothermal(HT) treatment of the sample (�100 mg) was performed in a cera-mic tube mounted horizontally in an electrically heated furnacewith a temperature controller. Hydrothermal treatment wasaccomplished by heating the sample at 800 �C for a period of 5 h,while an air stream saturated with a water partial pressure of19.9 kPa (water temperature at 60 �C) was passed at a flow rateof 25 ml/min over the catalyst. Structural integrity of the hydro-thermally treated samples was ensured by XRD, SEM, and surfacearea. No extra phases were observed in the XRD pattern indicatingthat the material is both thermally and hydrothermally stable un-der our experimental conditions (not shown).

Fig. 10 shows the effect of hydrothermal treatment on the HC(toluene) trap performance. The hydrothermally treated Al-CIT-1_DEX sample exhibited a noticeable decrease in the Tend and Tmax

work species) and the adsorption–desorption behavior of toluene (a) and 2,2,4-TMPb). The strong interaction between Br/nsted acid sites and 2,2,4-TMP molecule is

s to 10 R and 12 R pores, respectively, are shown in the figure.

134 T. Mathew et al. / Microporous and Mesoporous Materials 129 (2010) 126–135

(Table 2). This is due to the partial removal of framework Al andthe subsequent silanol condensation to amorphous silica, as evi-denced from the SEM, 27Al MAS NMR (Fig. S3, Supporting informa-tion), and N2 adsorption analysis of the hydrothermally treatedAl-CIT-1_DEX sample. In contrast to the fresh sample (Fig. 2b),the SEM picture of the aged Al-CIT-1_DEX sample exhibited amor-phous deposits on the crystallite surface (Fig. 11). The N2 adsorp-tion analysis of the same sample indicated a decrease of BETsurface area and micropore volume with respect to the fresh sam-ple. The BET surface area, and micropore volume of hydrothermallytreated Al-CIT-1_DEX were 622 m2/g and 0.16 cm3/g, respectively.In contrast, the activity of Al-CIT-1_INS is appreciable even afterthe hydrothermal treatment. Although Tmax is slightly reduced,the aged Al-CIT-1_INS still maintains a high desorption tempera-ture. As expected, due to the small amount of Al incorporation,Al-CIT-1_IMP showed same pattern with similar peak maximumand desorption end temperature before and after HT treatment.

Fig. 11. SEM picture of Al-CIT-1_DEX sample hydrothermally treated at 800 �C. SEMimage was obtained from S-900 (working voltage 6 kV).

Fig. 10. Temperature programmed desorption (TPD) profiles of toluene on hydro-thermally treated Al-CIT-1 samples. The samples hydrothermally treated at 800 �Care compared.

4. Conclusions

The present work deals with the synthesis and characterizationof Al-CIT-1 and its structure–property relation to hydrocarbontrap. B containing CIT-1 was used as a parent material for synthe-sizing Al-CIT-1 and B substitution by Al was accomplished by threedifferent post-modification procedures, viz., direct exchange (Al-CIT-1_DEX), insertion (Al-CIT-1_INS), and impregnation (Al-CIT-1_IMP). The evidence of Q3 species in the 29Si NMR spectra ofAl-CIT-1 samples indicates that the post-modification proceduresthat we adopted in this study were not sufficient to heal all thedefect sites present in the material. It was found that Al was incor-porated in the framework for all the cases of post-synthesisprocedure and the extent of Al incorporation depended on thepost-modification method. Al-CIT-1 prepared by direct exchangemethod (Al-CIT-1_DEX) has maximum Al in the tetrahedral posi-tion. The relative acidity of Al-CIT-1 samples by NH3-TPD indicatedthat the total acidity was independent of the concentration of Al,but dependent on the amount of AlTd. From the XPS analysis, itwas evidencedthat the surface to bulk Al/Si ratio is more irrespec-tive of the preparation method and the surface Al content ismaximum with Al-CIT-1_INS.

The Al-CIT-1 samples were evaluated for their adsorption–desorption characteristics of toluene and 2,2,4-trimethylpentane(2,2,4-TMP) to understand the Al distribution in Al-CIT-1 as wellas these materials efficacy towards hydrocarbon trap. The tolueneadsorption–desorption characteristics of fresh Al-CIT-1 samples re-vealed that Al-CIT-1_DEX adsorbs relatively more toluene, whereas, Al-CIT-1_INS desorbs at higher temperature. The difference inthe toluene adsorption–desorption characteristics between variousAl-CIT-1 samples demonstrates that the distribution of Al speciesplays a decisive role in the adsorption capacity and desorptionbehavior. The presence of finely distributed extraframework Al2O3

species in Al-CIT-1_INS modify the pores and causes a slow releaseof toluene during the desorption. Al-CIT-1_IMP with less Al contentand acidity showed the lowest toluene desorption temperature.

The TPD of 2,2,4-TMP on various Al-CIT-1 samples revealed adramatic difference in the adsorption amount between each meth-od of preparation. Al-CIT-1_DEX resulted in maximum 2,2,4-TMPadsorption, where as, Al-CIT-1_INS with maximum Al contentexhibited a negligible amount of 2,2,4-TMP adsorption. Al-CIT-1_IMP with the lowest Al content showed an appreciable amountof 2,2,4-TMP adsorption in comparison with Al-CIT-1_INS. Thelarge amount of surface Al species in Al-CIT-1_INS hinders a rela-tively bulky molecule like 2,2,4-TMP to access the available activesites and hardly any 2,2,4-TMP adsorption was observed.

The material stability for hydrocarbon trap application waschecked on hydrothermally treated samples and an appreciabledifference in the activity was observed between these samples.Al-CIT-1_DEX resulted a noticeable decrease in the desorptiontemperature and peak maxima on hydrothermal treatment. Al-CIT-1_INS exhibited good HC performance even after the hydro-thermal treatment, which exemplifies the stability of the material.Hardly any activity difference was observed between its fresh andhydrothermally treated samples for Al-CIT-1_IMP.

Acknowledgment

T.M. acknowledges Japan Society for the Promotion of Sciences(JSPS), Japan for a fellowship and the financial support.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.micromeso.2009.09.007.

T. Mathew et al. / Microporous and Mesoporous Materials 129 (2010) 126–135 135

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