Synthesis of Organic Pillared MFI Zeolite as Bifunctional AcidâBase

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Synthesis of Organic Pillared MFI Zeolite as Bifunctional Acid-Base Catalyst Baoyu Liu, Chaiyaporn Wattanaprayoon, Su Cheun Oh, Laleh Emdadi, and Dongxia Liu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm5033833 • Publication Date (Web): 05 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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Chemistry of Materials

Synthesis of Organic Pillared MFI Zeolite as Bifunctional Acid-Base Catalyst

Baoyu Liua,b‡, Chaiyaporn Wattanaprayoona‡, Su Cheun Oha, Laleh Emdadia and Dongxia Liua∗

a. Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, MD 20742, USA b. School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong 510640, P.R. China

*Corresponding author: Prof. Dongxia Liu Email: [email protected] Phone: (+1) 301-405-3522 Fax: (+1) 301-405-0523

∗

Corresponding author. Tel.:+1 301 405 3522; fax: +1 301 405 0523 E-mail address: [email protected] ‡ Baoyu Liu and Chaiyaporn Wattanaprayoon equally contributed to the work.

1 ACS Paragon Plus Environment


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Abstract: Organic pillared MFI zeolite has been synthesized by detemplation of diquaternary ammonium surfactant ([C22H45-N+(CH3)2-C6H12-N+(CH3)2-C6H13]Br2, C22-6-6) and intercalation of arylic silsesquioxane (1,4-bis(triethoxysilyl)benzene, BTEB) molecules between multilamellar MFI layers. The acid extraction and UV light radiation were sequentially employed for removal of C22-6-6 surfactant located not only between MFI layers but also in the zeolite micropores. The removal of C22-6-6 template by non-thermal calcination method prevents the condensation of external silanol groups of zeolitic layers stacked next to each other, which allows intercalation of BTEB molecules between the zeolitic layers and their successive reaction with the silanol groups to form the organic pillared zeolite structure. An amination post-treatment of the resultant zeolite sample introduced amino groups in the organic BTEB pillars. The acid sites from the zeolite framework aluminium and basic sites from the amino groups in BTEB pillars endow organic pillared MFI bifunctionality for cascade catalytic reactions. The synthesis of organic pillared MFI is a new addition to the structural modifications of lamellar zeolite with more open structures for processing bulky molecules. The resultant inorganic-organic hybrid zeolite structure creates new opportunities for potential applications of lamellar zeolites with tailored physicochemical properties.


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1. Introduction Crystalline microporous zeolites are recognized as three-dimensional (3D) frameworks with pore sizes of molecular dimension. The processing of bulkier molecules over 3D zeolites in the adsorption, separation and catalytic applications, in certain cases, however, is limited by the inaccessibility to Brønsted sites in zeolite micropores1-4. Vigorous research has been conducted to create zeolites with more open structures and enhanced accessibility to active sites for bulkier molecules5-15. Non-destructive post-synthesis modification, such as intercalation16, exfoliation17, and pillaring18, etc., of zeolite frameworks, with an objective of generating open zeolite structures while keeping active sites intact, is not applicable to 3D zeolites. The emergence of lamellar zeolite, or in more general case, two-dimensional (2D) zeolite precursors opens up opportunities to develop zeolites with open structures and surface exposed active sites by non-destructive post-synthesis modification methods13,

19-24

. The 2D zeolite

precursors contain stacked sheets of one-to-two unit cell or smaller thickness organized by weak forces through the interactions of organic/inorganic ions and/or molecules located in the ‘‘interlayer’’ spaces. A themed issue on layered inorganic materials in Dalton Transactions has recently summarized the state-of-the-art of synthesis, characterization, and properties of 2D zeolite materials.25 Structural modifications such as intercalation, exfoliation, and pillaring, etc. of these 2D zeolite precursors can deliver pillared, delaminated and disordered zeolite lamellae with more open structures18, 26-32. Pillared zeolites with strong acid sites, controlled acid site accessibility and high external surface areas are promising materials for processing bulky molecules in practical adsorption and/or catalysis applications. The first pillared 2D zeolite is MCM-36, which is derived from a layered precursor, MCM-22(P), reported by Mobil scientists in 1990’s33.In the synthesis, the layers of MCM-22(P)



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were intercalated by silica species, which led to silica pillared meso-/microporous MCM-36 zeolite, with micropore within the zeolitic layer and mesopores between the layers. In recent years, the layered precursor versions of other zeolite frameworks such as FER17, 34, SOD35, MFI19, and IPC36, 37 have been found. Silica pillared 2D structures built from these frameworks have been synthesized, most notably structures like ITQ-3638 (silica pillared FER) and pillared MFI zeolites31. In addition, pillaring of the 2D zeolites with compounds other than silica material has been reported. For example, alumina and magnesia-alumina have been applied for pillaring MCM-22(P), which resulted in MCM-36 with variable acid–base and ion-exchange properties as well as the dimensions/structure of the mesopores29. It should be noted that all these pillaring modifications of 2D zeolite precursors have been focused on the incorporation of inorganic pillars. Advancing the post-synthesis structural modifications of zeolite structures, pillaring of 2D zeolites with organic compounds will create opportunities for endowing these materials with new structural and functional properties. The high flexibility and specific functionality of organic pillars, together with the thermal and structural stability of inorganic zeolite frameworks, are expected to expand the 2D zeolites for a range of potential applications. The organic pillared MWW zeolite has been reported recently by Corma and co-workers30. The organic pillars, aminoaryl-bridged silsesquioxanes, were formed by two condensed silyl-acrylic groups from 1,4bis(triethoxysilyl)benzene (BTEB) molecules, which reacted with the external silanol groups of the zeolitic layers. An amination post-treatment introduced basic groups in the organic pillars, which prepared the pillared MWW with bifunctional acid-base sites for cascade reactions. A recent report by Céjka’s group39 showed that layered zeolitic organic-inorganic materials have been synthesized by pillaring IPC-1P (a precursor obtained by top-down synthesis from UTL


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zeolite) with bridged silsesquioxanes or using a polyhedral oligomeric siloxane. The resultant pillared materials exhibited substantially larger surface areas and controlled porosities of the interlamellar spaces. Prior to the report of organic pillared MWW and UTL zeolites, the organic 4,4′-bis(triethoxysilyl)biphenyl (BESB) pillared RWR has been reported by Ishii et al40, 41. To the best of our knowledge, an organic pillared inorganic 2D MFI zeolite has not been reported. In the present work, we report the synthesis of the organic pillared MFI lamellar zeolite and exploration of its textural and catalytic properties for potential applications. The synthesis of 2D lamellar MFI zeolite precursor has been reported by Ryoo’s group19, using diquaternary ammonium surfactants as the templates. The removal of the surfactant templates in 2D MFI zeolite without condensation of the lamellar structure, however, is challenging, given that the templates are entrapped not only between the zeolitic layers but also in the zeolite micropores. The acid extraction process only removed 26% of surfactant templates19, mostly occluded between the zeolite layers. Here, we employed a photochemical method by exposing the samples to ultraviolet (UV) light to decompose the organic templates while keeping zeolite layers intact. The resultant 2D zeolitic layers were intercalated with BTEB molecules, which reacted with the external silanol groups of the zeolitic layers to form organic pillared MFI zeolites. An amination treatment of the BTEB pillared MFI introduced basic amino groups in the organic pillars, which prepared the sample with acid-base bifunctionality for cascade catalytic reactions.

2. Experimental 2.1. Synthesis of organic pillared MFI zeolite Preparation of multilamellar MFI zeolite. The synthesis of multilamellar MFI zeolite started from the organic surfactant [C22H45-N+(CH3)2-C6H12-N+(CH3)2-C6H13]Br2 (C22-6-6)



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preparation following a reported procedure19. The recipe, 30Na2O/1Al2O3/100SiO2/10C22-66/4000H2O/18H2SO4

was used for hydrothermal synthesis of multilamellar MFI zeolites as

described by Ryoo and coworkers19. After the synthesis, one portion of the product was dried at 343 K and heated at 823 K to produce calcined multilamellar MFI, designed as MFI-Cal. The rest of the multilamellar MFI product was dried at 343 K and kept for the uses described below. For consistence in nomenclature, the as-synthesized multilamellar MFI was defined as MFI-C226-6.

Removal of C22-6-6 from multilamellar MFI (MFI-C22-6-6) zeolite. The removal of C22-6-6 surfactant templates from MFI-C22-6-6 sample was conducted by sequential acid extraction and UV light irradiation steps. Typically, 1.0 g of MFI-C22-6-6 sample was dispersed in 50 mL of 0.05 M H2SO4 ethanol solution and refluxed at 348 K for 4 h. After the sample was collected by centrifugation and washed with ethanol, it was re-dispersed in 50 mL of 0.2 M HCl ethanol/nheptane solution (1/1, volume ratio) and refluxed at 363 K for 24 h. The sample was then centrifuged and washed with ethanol/n-heptane (1/1, volume ratio) solution. These two acid extraction steps were used to remove the templates entrapped between zeolite layers. Finally, the sample was dispersed in a 40 mL of a mixed solution (13H2O/6H2O2/Acetone/0.5NH4OH, volume ratio) and exposed to a UV light radiation generated by a medium pressure mercury lamp (184-257 nm) under magnetic stirring for 5 h. Afterwards, the sample was collected by centrifugation and washed by deionized water. An acid extraction step using HCl ethanol/nheptane solution described above was used to remove decomposed C22-6-6 species, followed by water washing and centrifugation to collect the samples. The water washing and centrifugation steps were repeated three times. Finally, the sample was dried in a convective oven at 333 K


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overnight. The as-obtained samples after acid extraction and acid extraction followed by UVirradiation were named MFI-Ext and MFI-UV, respectively. Intercalation of lamellar MFI with aryl-bridged silsesquioxane. The intercalation of BTEB molecules between the zeolitic layers was carried out as follows. 2.0 g of MFI-UV sample and 2.0 g of BTEB were added into 200 mL of dioxane in a flask. The mixture was bubbled with a flowing He gas (50 mL min-1) for 1 h. The mixture was then heated at 353 K under magnetic stirring condition for 2 days. The sample was finally collected by centrifugation and washed by dispersing in dioxane and ethanol, respectively, and dried under room temperature for 5 days. The lamellar MFI with intercalated BTEB molecules was organic pillared MFI, designed as MFI-BTEB. Incorporation of amino (-NH2) groups in organic pillared MFI zeolite. An amination process was employed to incorporate amino (-NH2) group into the MFI-BTEB sample, following the reported procedure30. Firstly, an acid solution comprised of 15.2 g of H2SO4 (98 wt%) and 3.47 g of HNO3 (65 wt%) was prepared and added dropwise to 0.5 g of MFI-BTEB sample. After the mixture was stirred mildly at room temperature for 3 days, 300 mL of deionized water was added to dilute the acid mixture. The as-obtained yellowish suspension was separated by centrifugation, washed with 300 mL DI water, and dried at 333 K overnight. Secondly, the dried sample was added to a mixed solution of 15 mL of HCl (37%) and 1.59 g of SnCl2 (98% purity) and stirred at room temperature for 3 days. 300 mL of DI water was added into the solution to dilute the acid solution. Finally, the product was collected by centrifugation and washed with DI water and ethanol, respectively, followed by drying at 333 K overnight. The sample was named as MFI-BTEB-NH2 consequently to indicate the presence of amino group in the organic pillared MFI sample.



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2.2. Materials characterization Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku Rotaflex Diffractometer using CuKα radiation (λ = 1. 5418 Å). N2 adsorption–desorption isotherms were measured with an Autosorb-iQ analyzer (Quantachrome Instruments) at 77 K. The samples were outgassed for 12 h at 423 K before the isotherm measurement. Scanning electron microscopy (SEM) images were recorded with a Hitachi SU-70. Transmission electron microscopy (TEM) was performed with a JEM 2100 LaB6 electron microscope operated at 200 kV. Infrared spectra of samples were recorded using a Fourier transform infrared (FT-IR) spectrometer (Nicolet Magna-IR 560) with a resolution of 1 cm-1. The thermal stability and the organic composition behavior of samples were examined on a Thermogravimetric analysis (TGA) 2950, TA Instruments. In the measurement, the sample was heated to 960 K at a ramp rate of 5 K min-1 under an air flow of 30 mL min-1. Solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra were recorded on a Bruker Avance AV III 400 spectrometer. The 1H to 13C cross-polarization (CP) spectra were acquired by using a 90° pulse for 1H of 4.2 µs, a contact time of 1 ms, and a recycle of 2 s. The 29Si Bloch decay (BD) spectra were acquired at 79.5 MHz with a 7-mm Bruker BL-7 probe using pulses of 1.1 µs corresponding to a flip angle of 1/8 π radians, and a recycle delay of 90 s. A simple-pulse with spinal64 high-power decoupling program was used. 13C and 29Si were referred to adamantine and tetramethylsilane, respectively. 2.3. Cascade reactions over organic pillared MFI zeolite The MFI-BTEB-NH2 sample was activated by dispersing in an aqueous NH4Cl/NH4OH (0.1 wt %, pH ≈ 10-11) buffer solution at the room temperature for 4 h before the catalytic reactions. The sample was recovered by centrifugation and washed with ethanol, and then dried


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in an oven at 473 K overnight. For comparison purpose, MFI-cal sample was also tested in the catalytic reactions. Before the reaction, it was activated by ion-exchange with 1 M NH4NO3 solution at 363 K for 3 times, and then converted to the H+-form after calcination in a flowing air at 823 K for 6 h. The acetal hydrolysis-Knoevenagel condensation cascade reaction was chosen as a model reaction involving two chemical reaction steps that requires acid and base sites simultaneously to investigate the catalytic properties of the organic pillared MFI zeolite. Typically, a reaction mixture containing malononitrile (1.36 mmol), acetonitrile (18.78 mL) and DI water (7.5 µL) with 0.2 g of catalyst were added into the flask in sequence. After the mixture was bubbled with flowing helium for 0.5 h, the flask was immersed into an oil bath preheated at 353 K. The reaction mixture was maintained for 0.5 h at the required reaction temperature and stirring conditions and then 0.20 mL of benzaldehyde dimethyl acetal (1.36 mmol) was added. This moment of addition was taken as the initial reaction time. The liquid sample was withdrawn periodically and analyzed on a Gas Chromatograph (Agilent, 7890A) equipped with a flame ionization detector and a methyl-siloxane capillary column (HP-1, 50.0 m x 320 µm x 0.52 µm) to analyze the reaction products.

3. Results and discussion 3.1 Removal of C22-6-6 template and intercalation of BTEB molecules. The preparation of organic pillared MFI zeolite with layered mutillamellar MFI precursor involves successive removal of C22-6-6 surfactant template and intercalation of BTEB molecules from and into the zeolitic layers. It has been proposed that C22-6-6 surfactant aligned along the straight micropores of the MFI framework, which functioned as interlamellar support and a structure-directing agent,



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respectively, in the crystallization of multilamellar MFI12, 31. The calcination method can remove the surfactant templates completely, but it leads to condensation of the lamellar structures that cannot be reversely expanded for the intercalation of organic molecules. The removal of the surfactant templates by non-calcination method such as acid extraction only removes surfactant molecules located in the surfactant micelle as 'dummy' fillers31. The occupation of surfactant molecules in the zeolite micropores limits the application of the as-produced zeolite materials for practical applications. Critical methods are needed to remove the non-extractable surfactant molecules entrapped in the zeolite micropores while keep the lamellar structure intact for the intercalation of organic molecules. Photochemical method using short-wavelength UV radiation in an ozone environment at room temperature conditions together with the acid extraction have been used for the removal of organic structure-directing agents in the multilamellar MFI-C22-6-6 samples. A prerequisite acid extraction process before UV irradiation removed the C22-6-6 templates entrapped between the zeolitic layers. The UV irradiation smashed the surfactant template located in the MFI frameworks. Subsequently, another acid extraction step removed the most of fragments of surfactant, the residual surfactant or fragments embedded in the space between layers. The intercalation of organic BTEB molecules into the multilamellar MFI zeolites was conducted after the surfactant molecules have been removed by the UV irradiation. The effective removal of the surfactant templates and intercalation of organic molecules from and into the multilamellar MFI zeolite was tracked by XRD patterns shown in Figure 1. The reflections in the wide-angle XRD patterns (2θ ~ 5-50º) of the samples in the synthesis steps resemble the characteristic of crystalline MFI42, which indicates that MFI zeolite structure was preserved in the organic pillared MFI synthesis process. The low-angle XRD patterns (2θ ~ 1.25º) of these samples are remarkably different. Two reflection peaks (2θ ~ 1.4°and 4.2°) are


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shown for MFI-C22-6-6 sample in Figure 1a, which are characteristic (0k0) bands of lamellar MFI materials at 6.25 nm and 2.07 nm, respectively, consistent with the lamellar MFI zeolite reported by Ryoo and co-workers31. Taking into account that multilamellar MFI has 1.5 unit cell thickness (~3.4 nm) along the b-axis direction, the interlayer spacing of MFI-C22-6-6 is ~2.85 nm. After the UV irradiation and acid extraction treatments, the first-order reflection of MFI-UV sample (Figure 1b) shifted to 2θ = 2.01º, indicating that the interlayer space between MFI nanosheets shrank to 0.83 nm due to the effective removal of C22-6-6 surfactant molecules. The interlayer spacing in MFI-UV sample may result from the fact that some residual C22-6-6 molecules (~3wt% from TGA measurement below) exist in the sample, although the rest of (~40wt%) surfactant molecules have been removed by repetitive UV-radiation and acid extraction treatments. In addition, the interlayer interaction by hydrogen bonds formed between surface silanol groups on adjacent layers helps to retain the lamellar zeolitic structure. It has been reported that IPC-1P layers derived from zeolite UTL has a relatively large concentration of surface silanols, which account for ~ 80% of interlayer interactions.43

4.23

(a) 4.66

(b) (c)

m m n n

Intensity (a.u.)

6.25 nm

m n

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4.45

(d) (e)

1 2 3 4 55

10

15 20 25 2θ (degree)

30

35

40

45

50

Figure 1. XRD patterns of (a) MFI-C22-6-6, (b) MFI-UV, (c) MFI-BTEB, (d) MFI-BTEB-NH2 and (e) MFI-Cal samples. 11 ACS Paragon Plus Environment


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The intercalation of BTEB molecules into the gallery of lamellar MFI increases the interlayer spacing to 1.26 nm, as indicated by the reflection at 1.88º in the low-angle XRD pattern of MFI-BTEB (Figure 1c). The BTEB molecules could interact with the external silanol group of MFI zeolite and form covalent bonds that hold the lamellar structure and expand the layer space. The amination treatment of BTEB components in MFI-BTEB decreased the interlayer space slightly to 1.05 nm, as shown in the XRD pattern of MFI-BTEB-NH2 (Figure 1d). It should be noted that the interlayer space of MFI-BTEB-NH2 is closed to the molecular length of single silyl-BTEB molecule (1.02 nm) acting as pillars. For comparison, the XRD pattern of the calcined multilamellar MFI sample (Figure 1e) is also represented in Figure 1. The very weak broad reflection peak in the low-angle region indicates that calcination leads to collapse of ordered lamellar structure and irregular distortion of zeolite layers19. The XRD data confirm that the processing steps for preparation of organic pillared MFI zeolites preserve the lamellar zeolite structure. The existing space between the MFI layers may bring new textural features for the organic pillared MFI zeolite. N2 adsorption-desorption isotherm was measured on the sample after each processing step to examine the changes in the textural properties of the organic pillared MFI. Figure 2a, 2b and Table 1 shows isotherms and the corresponding surface areas and pore volumes for MFI-C226-6,

MFI-UV, MFI-BTEB, and MFI-BTEB-NH2, respectively. The as-synthesized MFI-C22-6-6

sample exhibited a very low N2 uptake, and low BET surface area, micropore and total pore volumes. The presence of abundant C22-6-6 surfactant apparently occupies most of the pores and channels of the MFI-C22-6-6 sample. After UV irradiation and acid extraction treatments, the MFI-UV sample exhibited much higher sorption capacity compared to MFI-C22-6-6. The abrupt


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400

(a)

3 -1 Adsorbed amount (cm g )

MFI-C22-6-6 MFI-UV

300

MFI-BTEB

3

-1

Adsorbed amount (cm g , STP)

400

MFI-BTEB-NH2 MFI-Ext

200

100

0

0.0

0.2

0.4 0.6 0.8 Relative pressure (P/P0)

0.06

MFI-Ext

100

0.04

0.02

0.00 0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

1E-3

0.01

0.1

1

Relative pressure (P/P0)

(d)

0.08

MFI-Ext

MFI-UV MFI-BTEB MFI-BTEB-NH2

0 1E-4

MFI-C22-6-6 MFI-UV MFI-BTEB MFI-BTEB-NH2

MFI-C22-6-6

200

1.0

dV/dD (cm3 g-1 nm-1)

3 -1 -1 dV/dD (cm g nm )

(c)

(b)

300

0.08


0.06

MFI-Ext 0.04

0.02

0.00

Pore width (nm)

0

5

10

15

20

25

Pore width (nm)

0.08

(e) dV/dD (cm3 g-1 nm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



0.06

MFI-Ext 0.04

0.02

0.00

0

5

10

15

20

Pore width (nm)

Figure 2. (a) N2 adsorption–desorption isotherms in linear scale, (b) N2 adsorption–desorption isotherms in log scale, (c) HK micropore size distributions, (d) NLDFT pore size distributions and (e) BJH pore size distributions of MFI-C22-6-6, MFI-UV, MFI-BTEB, MFI-BTEB-NH2 and MFI-Ext zeolite samples, respectively. sorption below the relative pressure p/p0 ~ 0.1 is due to the large driving force to adsorb in the zeolite micropores, similar to that of N2 sorption in traditional microporous MFI zeolite. The hysteresis loop above p/p0 = 0.5 corresponds to capillary condensation in mesopores of the materials. The increase in micro- and mesopore volume of MFI-UV sample demonstrates the 13 ACS Paragon Plus Environment


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effective removal of surfactant template from the multilamellar MFI by using the non-calcination method. On the contrary, the acid extraction cannot achieve the complete removal of template from both micropores and interlayer galleries of MFI zeolites, as shown by the isotherm of MFIExt in Figure 2a. The intercalation of BTEB molecules into the MFI-UV sample resulted in a decrease in sorption capacity. MFI-BTEB sample exhibited lower sorption at both p/p0 ~ 0.1 and p/p0 ~ 0.5, suggesting that the BTEB molecules occupies partial micropore and mesopores. Thus, the decrease in specific surface area and pore volume of MFI-BTEB was observed in Table 1. The organic pillared MFI with amino groups (MFI-BTEB-NH2), however, showed a similar sorption in the micropore to that of MFI-BTEB, but a higher sorption in mesopores. The strong acids employed in the amination process may remove the physically trapped BTEB molecules in the interlayer space of MFI zeolite. The N2 isotherm data also suggests that the as-obtained MFIBTEB-NH2 has dual meso-/microporous pore systems. Table 1. Textual properties of the zeolite samples along the synthesis steps. SBET a (m2 g-1)

Smic b (m2 g-1)

Sext d (m2 g-1)

Vtot c (cm3 g-1)

Vmic b (cm3 g-1)

Vext e (cm3 g-1)

MFI-C22-6-6

31

0

31

0.18

0.00

0.18

MFI-UV

353

185

168

0.54

0.08

0.46

MFI-BTEB

265

165

100

0.47

0.08

0.39

MFI-BTEB-NH2

272

113

159

0.53

0.07

0.46

Zeolite

120 0 120 0.38 0.00 0.38 MFI-Ext b Surface area determined by Brunauer-Emmett-Teller (BET) method; Determined from t-plot method; c Total pore volumes obtained at P/P0 = 0.95; d Sext = SBET ‒ Smic; e Vext = Vtotal − Vmic.

a

The pore size distributions in Figure 2c and 2d further underscore the porosity changes with processing steps in the synthesis of organic pillared MFI samples. MFI-UV, MFI-BTEB and MFI-BTEB-NH2 exhibit a broad Horvath–Kawazoe (HK) micropore size distribution of ~0.56 nm (Figure 2c), which is attributed to micropores associated with 10-membered rings of


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ZSM-5 zeolite43. For the MFI-C22-6-6 and MFI-Ext samples, the microporosity is significantly reduced in comparison with MFI-UV, MFI-BTEB and MFI-BTEB-NH2 samples because the embedded C22-6-6 surfactant prohibits the access of guest molecules into the micropores of the zeolite. Additionally, MFI-UV has the strongest peak among the samples collected along the synthesis steps, which confirms that the UV irradiation method was effective in removing the C22-6-6 surfactant template occluded in the zeolite micropore framework. The nonlocal density functional theory (NLDFT) pore size distributions of these samples (Figure 2d) determined from the adsorption branch of N2 isotherms using cylindrical-type pore and silica absorbent model demonstrate that MFI-UV and MFI-BTEB-NH2 samples have much higher mesoporosity than MFI-C22-6-6 and MFI-Ext samples, which is caused by the absence of C22-6-6 template molecules from UV irradiation removal method. The intercalation of BTEB and incorporation of amino groups onto BTEB molecules in the synthesis steps did not collapse or clog the zeolitic layers, and thus mesoporosity was maintained in the MFI-BTEB-NH2 sample. The mesopore size distributions of the zeolite samples were further determined from the adsorption branches of N2 isotherms using Barrett–Joyner–Halenda (BJH) method, and the results were shown in Figure 2e. A similar trend in mesoporostiy with processing steps to that obtained from the NLDFT analyses was observed for these zeolite materials. The repetitive acid extraction and UV-light radiation removed C22-6-6 surfactant located between MFI layers. The mesopore was retained in the processing steps of intercalation of BTEB molecules between the zeolitic layers and amination of BTEB to obtain the organic pillared MFI-BTEB-NH2 sample. The slight discrepancy between pore size distributions in Figure 2d and 2e might be caused by the different calculation models in NLDFT and BJH methods and more complicated surface chemistry in the organic-inorganic hybrid materials compared to inorganic nanoporous materials.



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3.2 Bridging information of BTEB between MFI layers.

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Several scenarios could exist upon

intercalation of organic BTEB molecules into the multillamelar MFI zeolites. First of all, the condensation reaction of silyl-arylic groups from BTEB with two opposing silanol groups located on each of the two MFI layers results in each BTEB molecule binds to two MFI layers, and thus the intercalated BTEB molecules function as pillars running along the b-axis of the lamellar MFI zeolite. Secondly, the BTEB molecules are probably reacted with each other along a-c direction of the zeolite, condensed to form pillars to fill the interlayer spaces. Furthermore, the condensation reaction among BTEB molecules might lead to the dimer or oligomer formation on the exterior of the zeolite particles which blocks the entrance to the porosity of the MFI materials. The low-angle XRD pattern in Figure 1 has indicated the interlayer distance of MFI-BTEB-NH2 is ~ 1.05 nm, which is close to the length of the BTEB molecule, suggesting that only one BTEB molecule bridges the opposing silanol groups on two MFI layers along the b-axis direction. The presence of meso- and microporosities (shown by N2 isotherms in Figure 2) suggests condensation of BTEB molecules in the gallery of zeolitic layers along a-c direction or on the exterior of zeolite particles was insignificant. To quantify the bridging BTEB molecules between the MFI layers, TEM and TGA measurements were additionally conducted on the resultant MFI-based samples. Figure 3 shows TEM images of the MFI-C22-6-6 and MFI-BTEB-NH2 samples. The MFIC22-6-6 sample (Figure 3a) exhibits an ordered arrangement of lamellar MFI nanosheets. The lattice fringe can be clearly seen in the high-resolution TEM (HR-TEM) image of the sample shown in Figure 3b. The multilamellar stacking of MFI nanosheets is composed of three pentasil sheets that are related to the one and half unit-cell dimension along the b-axis direction. The thickness of the single MFI zeolite nanosheet is 3.4 nm and the distance between neighbor MFI


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layers is ~ 2.8 nm, in agreement with the interlayer distance analyzed from the XRD data. MFIBTEB-NH2 in Figure 3c exhibits an ordered multilamellar structure. The HR-TEM image in Figure 3d indicates the incorporation of organic molecules into the layer space did not modify the structure of MFI zeolite framework. The distance between neighboring MFI layers, however, was shortened to 1.01 nm consistent with the XRD analysis. The TEM observation further suggests that only one BTEB molecule was possibility able to be included along the b-axis direction of the MFI zeolites, different from the BTEB dimer formed in organic pillared MWW zeolite reported by Corma and coworkers30.

Figure 3. TEM images of (a, b) MFI-C22-6-6 and (c, d) MFI-BTEB-NH2 samples.



The number of BTEB molecules bridged in the gallery of the organic pillared MFI sample was analyzed by TGA measurement. Figure 4 shows the TGA curves of MFI-BTEB together with MFI-C22-6-6, MFI-Ext, and MFI-UV samples to track the systematic change of the organic components in the lamellar MFI zeolite. The MFI-UV sample (Figure 4a) has two weight losses: the first one below 423 K attributed to desorption of physically adsorbed water and the second one in the temperature range of 673‒960 K assigned to the combustion of the residual organic surfactant template in the zeolite framework. The calculation shows that the remaining organic component in MFI-UV sample is ~3 wt%. In comparison with ~41 wt% (Figure 4d) and ~16 wt%

100

(a)

90 Weight (%)

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(b) (c)

80 70

(d)

60 320

480

640 800 Temperature (K)

960

Figure 4. TGA curves of (a) MFI-UV, (b) MFI-BTEB (c) MFI-Ext and (d) MFI-C22-6-6 samples. (Figure 4c) C22-6-6 templates in the MFI-C22-6-6 and MFI-Ext samples, respectively, the TGA data indicate that the UV-radiation plus acid extraction is an effective approach to remove C22-6-6 template functioned as structure directing agent in the micropores of multilamellar MFI zeolite. The TGA curve of MFI-BTEB sample (Figure 4b) also exhibited weight losses: the one up to 433 K is attributed to desorption of physically adsorbed water, and following one in the temperature range of 433-960 K can be assigned to the decomposition of residual C22-6-6 and 18 ACS Paragon Plus Environment

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bridged BTEB molecules in the sample. Deduction of physically adsorbed water and residual C22-6-6 contents from the weight losses, the organic BTEB content in the MFI-BTEB sample is ~11 wt%, which is equivalent to ~5 BTEB molecules in each unit cell of the MFI zeolite. Considering the thickness of lamellar MFI is 1.5 unit cells along b-axis direction44, the number of BTEB molecules bridging two MFI zeolitic layers is equivalent to ~7. Corma et al.30 has reported that the BTEB pillared MWW contains 2 BTEB dimers bridging the opposite layers in each unit cell of MWW zeolite. The XRD, TEM and TGA analyses suggest that 7 BTEB monomers bridge the zeolitic layers in lamellar MFI zeolite. The different bridging scenario between lamellar MWW and MFI zeolites might result from different surface environment of the zeolitic layers in these two lamellar zeolites. In general, MWW zeolite has more hydrophobic nature compared to MFI, given that the number of silanol groups (-OH) per unit cell of a single-unit-cell thick MWW nanosheet is 2 while 8 silanol groups in a 1.5 unit-cell thick MFI nanosheet. Therefore, the density of silanol groups on zeolitic single-unit-cell thick layers of MWW is 4 times less than that of MFI zeolite. The higher number of –OH groups in MFI might lead to the fact that the condensation reactions preferentially occur between BTEB and silanol groups on the opposite zeolitic layers, and it is unlikely to form BTEB dimers as that in BTEB pillared MWW zeolite. Solid state NMR was employed to investigate the local bonding environment in the synthesized organic pillared zeolite samples. Figure 5 shows the spectra of 1H MAS NMR, 13C CP/MAS NMR, 29Si SP/MAS NMR and 29Si BD/MAS NMR measurements, respectively, on the synthesized MFI-C22-6-6 and MFI-BTEB samples. Additionally, 29Si SP/MAS NMR spectrum of MFI-UV was measured for comparison purpose, as shown in Figure 5c. 1H MAS NMR (Figure 5a) and 13C CP/MAS NMR (Figure 5b) spectra both illustrate the effective removal of C22-6-6 by



the UV irradiation and acid extraction treatment from and incorporation of BTEB molecules into the lamellar MFI zeolite, consistent with the conclusions made from the XRD, N2 isotherm, FTIR, and TGA analyses.

29

Si SP/MAS NMR spectra (Figure 5c) show that MFI-C22-6-6 and

MFI-UV have two well resolved peaks (at -113, -101 ppm, respectively), which correspond to crystallographically nonequivalent Q4 tetrahedral sites (Qn stands for X4-nSi[OSi]n)45-48 and Q3 sites arising from the silanol groups on the zeolite surface. For MFI-BTEB-NH2 sample, the spectrum shows Q4 and Q3 peaks and the additional three peaks in the range of ‒60 to ‒80 ppm, assigned to T1(C‒Si(OH)2(OSi)), T2(C‒Si(OH)(OSi)2) and T3(C‒Si(OSi)3) following the report

E

(a)

(b)

G

Intensity (a.u.)

E

F F FF F

E

F

G G

E E E

F

C22-6-6 F B

D A B C

Intensity (a.u.)

E E

C22-6-6

G

F

MFI-BTEB

D C B A

C B C

G *

E,F *

MFI-BTEB

C D

D B

A B C

A

A

20

D

C

D

10

0

-10

200

160

120

T2 T3

MFI-BTEB MFI-UV

-40

3

Q

MFI-BTEB

Q3 Q2

MFI-C22-6-6

-60

0

Q4

Intensity (a.u.)

3

Q

-30

40

(d)

Q4

1

80

MFI-C22-6-6

δ (ppm)

(c)

T

BA

MFI-C22-6-6

δ (ppm)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-90

-120

-150

0

δ (ppm)

-50

-100

MFI-C22-6-6

-150

δ (ppm)

Figure 5. Solid-state (a) 1H MAS NMR, (b) 13C CP/MAS NMR, (c) 29Si SP/MAS NMR and (d) 29 Si BD/MAS NMR spectra of zeolite samples.


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by Corma and coworkers30. A slight difference between the spectra of MFI-BTEB and MFI-UV samples is that the Q3 peak is slightly weaker in the spectrum of MFI-BTEB, indicating the presence of a lower concentration of Q3 species, which can be due to the covalent bonding of incorporated BTEB with silanol group on zeolite layers. 29Si BD/MAS NMR spectra of MFI-C226-6 and

MFI-BTEB-NH2 (Figure 5d) further confirm the lower number of Q3 species in the MFI-

BTEB sample, which is caused by the condensation reactions between the incorporated BTEB and the surface silanol groups on adjacent layers in the lamellar MFI zeolite.

3.3 Catalytic reaction over organic pillared MFI.

As discussed above, the resultant MFI-

BTEB-NH2 zeolites contain micropores in the zeolitic layer and mesopores between the layers. In addition, the zeolite framework aluminium and amino groups in BTEB pillars endow organic pillared MFI bifunctional acidic-basic sites in one single material. All these structural and functional properties might enable organic pillared MFI as an ideal catalyst for cascade reactions. As a demonstration of such application, the acid and basic sites in MFI-BTEB-NH2 sample were examined and their catalytic performances were explored in a cascade reaction. The presence of basic –NH2 sites in MFI-BTEB-NH2 were confirmed by the FTIR spectra shown in Figure 6a. Two absorbance bands centered at 1535 and 1630 cm-1, respectively, which can be assigned to the bending vibrations of N-H groups (δN-H) in the amino groups in BTEB30, are observed in MFI-BTEB-NH2 sample. In contrast, such vibration bands were not detected in the spectra of MFI-BTEB (Figure 6b) and MFI-Cal (Figure 6c), suggesting that the basic sites (– NH2 group) were introduced by the amination step in the catalyst synthesis process. Figure 6 also shows that the MFI-BTEB and MFI-BTEB-NH2 samples exhibit two absorbance bands at 2924 and 2853 cm-1, which are due to the symmetric stretch vibration of C‒H in the organic BTEB



linkers between MFI layers49. The bands at 1465 and 1384 cm-1 are ascribed to the stretching vibration of C=C in the benzene. The presence of amino and benzene groups in MFI-BTEB-NH2 confirms that the amino groups have been successfully grafted on the BTEB pillars located in the interlayer space of the MFI zeolite.

δ(−NH2)

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(a) (b) (c)

4000 3600 3200 2800 2400 2000 1600 -1 Wavenumber (cm )

Figure 6. FTIR spectra of (a) MFI-BTEB-NH2, (b) MFI-BTEB and (c) MFI-Cal samples.

Figure 7a shows the 29Si single pulse (SP) NMR spectra of the meso-/microporous organic pillared MFI. Two well resolved peaks (at -110, -99 ppm) can be seen in the

29

Si SP NMR

spectrum of the MFI-BTEB-NH2, which correspond to crystallographically nonequivalent Q4 tetrahedral sites and Q3 sites in the zeolite material. Figure 7a also shows T1(C‒Si(OH)2(OSi)), T2(C‒Si(OH)(OSi)2) and T3(C‒Si(OSi)3) peaks in the range of ‒60 to ‒80 ppm. The existence of T-type Si‒C bond confirms that the organic species have been successfully retained in the amination treatment of MFI-BTEB to obtain the acid-base bifunctional catalyst materials.


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Q4

Intensity

(a)

Q3 T1

-30

T2

-60

T3

-90 δ (ppm)

-120

-150

(b)

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


240

180

120

60 δ (ppm)

0

-60

-120

Figure 7. (a) 29Si SP/MAS NMR and (b) 27Al MAS NMR spectra of MFI-BTEB-NH2. Figure 7b shows the

27

Al MAS NMR spectra of the organic pillared MFI-BTEB-NH2 zeolite

sample. The peak at 53 ppm is due to the tetrahedrally coordinated framework aluminum, while the peak around 0 ppm is due to an octahedral coordination typical of non-framework Al. No obvious peak owing to other aluminum species can be observed, which demonstrates that Al atoms are incorporated in the framework on tetrahedral sites with the Si atoms, establishing Al‒



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O‒Si linkages. The tetra-coordinated Al sites result in the Brønsted acid sites by forming the Si‒ OH+‒Al band via ion exchange50. The FTIR and NMR data analyses indicate that the bifunctional hybrid organic-inorganic material has been obtained through combining Brønsted acid in the inorganic zeolitic layers with basic groups in the organic pillars between the zeolitic layers. The cascade reaction of benzaldehyde dimethylacetal and malonononitrile involving an acetal hydrolysis followed by a Knoevenagel condensation51-53 was employed as a probe reaction to examine the catalytic performances of bifunctional organic-inorganic layered hybrid material. Specifically, benzaldehyde dimethyl acetal was hydrolyzed to produce benzaldehyde which reacted with malononitrile to give benzylidene malononitrile. The first reaction step requires acid sites which can be supplied by the framework Al‒O(H)‒Si sites in the MFI layers, while the subsequent condensation step involves the basic sites from the amine groups in the organic pillars of the MFI-BTEB-NH2 materials. Figure 8a shows the conversion of benzaldehyde dimethyl acetal and formation of the benzaldehyde intermediate that is reacted through the consecutive Knoevenagel condensation with malononitrile to give the final benzylidene malononitrile product. In contrast, the MFI-Cal (Figure 8b) that contains acid sites only catalysed the first step reaction in the reaction cascade. The current study is only an illustration for the catalytic application of the organic pillared MFI-BTEB zeolite. The resultant inorganic-organic hybrid lamellar zeolite structures with tailored physicochemical properties can be potentially used for many other potential adsorption or catalytic applications.


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(a)

100

Molar fraction (%)

80

Benzaldehyde Dimethylacetal Benzaldehyde Benzyilidene Malononitrile

60 40 20 0

0

2

4

6 8 Times (h)

10

100

Molar fraction (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


12

(b)

80 60 40 Benzaldehyde Dimethylacetal Benzaldehyde Benzylidene Malononitrile

20 0

0

2

4

6 Time (h)

8

10

12

Figure 8. Results of the catalytic activity for cascade reaction over (a) MFI-BETB-NH2 and (b) MFI-Cal. Molar fractions of benzaldehyde dimethylacetal, benzaldehyde, and benzylidene malononitrile are shown, respectively.

4. Conclusions As a new addition to the synthesis of organic pillared lamellar zeolite with more open structures for processing bulky molecules, an organic pillared lamellar MFI zeolite has been



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synthesized. The UV-light irradiation and acid extraction have been used to successfully remove surfactant template entrapped in both interlayer spaces and micropores of zeolites. The sequential intercalation of organic BTEB molecules resulted in their condensation reaction with external silanol groups on MFI layers to form organic pillars to stabilize the pillared lamellar structure. The bridging information analysis shows that BTEB pillared MFI only allows one BTEB monomer between the interlayers, different from organic pillared MWW or RWR zeolites. An amination post-treatment of the resultant samples introduced amino basic groups into the BTEB pillars. A cascade reaction that requires for sequential functions of acid and base sites indicated the successful preparation of the organic pillared MFI zeolite. The removal of surfactant templates by non-calcination method and synthesis of organic pillared MFI is a new addition to the structural modifications, by intercalation and pillaring, of lamellar zeolite with more open structures.

Acknowledgements The authors gratefully acknowledge the support from the ACS-Petroleum Research Fund (ACS-PRF) and National Science Foundation (NSF-CBET 1264599). We acknowledge the support of the Maryland NanoCenter and its NispLab. The NispLab is supported in part by the NSF as a MRSEC Shared Experimental Facility. The authors thank for Professor Xuefeng Guo in Nanjing University for his generous help with solid state NMR analysis of the zeolite samples. B.Y. Liu thanks the China Scholarship Council (CSC, 201306150076) for a fellowship to support his 2-years stay at UMD.


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