Facile synthesis of mesostructured ZSM-5 zeolite with enhanced mass transport and catalytic performances

Accepted Manuscript Title: Facile synthesis of mesostructured ZSM-5 zeolite with enhanced mass transport and catalytic performances Author: Chao Li Yanqun Ren Jinsheng Gou Baoyu Liu Hongxia Xi PII: DOI: Reference:

S0169-4332(16)31922-5 http://dx.doi.org/doi:10.1016/j.apsusc.2016.09.054 APSUSC 33982

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Received date: Accepted date:

17-7-2016 13-9-2016

Please cite this article as: Chao Li, Yanqun Ren, Jinsheng Gou, Baoyu Liu, Hongxia Xi, Facile synthesis of mesostructured ZSM-5 zeolite with enhanced mass transport and catalytic performances, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.09.054 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Facile synthesis of mesostructured ZSM-5 zeolite with enhanced mass transport and catalytic performances

Chao Li a, Yanqun Ren a, Jinsheng Gou b, Baoyu Liu a, Hongxia Xi a,*

a

School of Chemistry and Chemical Engineering, South China University of Technology, 381

Wushan Road, Tianhe District, Guangzhou, China, 510641 b

College Material Science and Technology, Beijing Forestry University, Key Laboratory of

Wooden Material Science and Application, Ministry of Education, 35 Tsinghua East Road, Haidian District, Beijing, China, 100083

* Corresponding author: Hongxia Xi Email: [email protected]; Tel.: +86 20 87113501; Fax: +86 20 87113735

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Graphical abstract:

Research Highlights    

A mesostructured MFI zeolite was synthesized via dual-functional surfactant approach. Mass transport was investigated by applying zero length column technique. The catalyst exhibited excellent catalytic activity and long lifetime. Gaussian DFT was employed to study the role of surfactant in crystallization process.

Abstract A mesostructured ZSM-5 zeolite with multilamellar structure was successfully synthesized by employing a tetra-headgroup rigid bolaform quaternary ammonium surfactant. It was characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), nitrogen adsorption/desorption isotherms, amines temperature programmed desorption (amines-TPD), and computer simulation. These results indicated that the dual-functional amphiphilic surfactants play a critical role for directing the multilamellar structure 2

with high mesoporosity. The mass transport and catalytic performances of the zeolite were investigated by zero length column (ZLC) technique and aldol condensation reactions to evaluate the structure-property relationship. These results clearly indicated that the mass transport of selected molecules in hierarchical zeolite can be accelerated by introducing mesoporous structure with mesostructure with reduced diffusion length and an overall enhanced resistance against deactivation in reactions involving large molecules. Furthermore, the dual-functional surfactant approach of making hierarchical zeolite with MFI nanosheets framework would open up new opportunities for design and synthesis of hierarchical zeolites with controllable mesoporous structures.

Keywords: Mesoporous zeolite; Mass transport; Structure directing agents; Zero length column technique

1. Introduction Zeolites, a class of crystalline aluminosilicates, have been used as shape-selective catalysts and adsorbents in industrial applications including petrochemical synthesis, oil refining, and organic chemical synthesis duo to their shape-selective properties, tunable framework structures, compositions and hydrothermal stabilities [1-5]. The unique microporous structures of zeolite materials can provide shape selectivities in catalytic processes; however, the sole presence of the microporous network also imposes significant diffusion limitation for the reactions involving bulky molecules. The diffusion limitation often causes low catalyst efficiency and deactivation, which are major drawbacks in most zeolite catalyzed processes [6]. Reducing diffusion limitation, accelerating accessibilities of acid sites and improving catalyst effectiveness are critical for these zeolites catalyzed processes [7-9]. 3

In recent years, several promising strategies have been developed to solve the diffusion limitation problems, the synthesis of hierarchical zeolites with both mesoporosity and microporosity is considered to be one of the most promising methods due to the significantly reduced diffusion length. It was mainly divided into two routes: non-templating method and templating method [10]. Dealumination and desilication of pre-synthesized zeolite crystals, as a non-templating method, has a great potential for commercialization due to the relatively simple process. However, it is difficult to generate controllable mesopores using the post-synthesis methods [11-13]. On the other hand, mesoporosity can be precisely controlled by confining the crystal growth within hard templates, such as carbon nanotubes [14,15], nanofibers [16] and three dimensionally ordered mesoporous (3DOm) carbon [17], etc. Mesopores can also be directly introduced into microporous zeolite by assembling aluminosilicate species or precursors using cationic polymers as soft templates [18]. Especially, Ryoo and co-workers designed a series of diquaternary ammonium surfactants as soft templates for the fabrication of hierarchical zeolites with intracrystalline mesopores [7,19]. The resulting materials with an ordered multilamellar mesostructure wherein the mesopores are built between the single-unit-cell nanosheets of only 2 nm thickness shows enhanced activities and long life-time in catalytic reactions involving large molecules. Tsapatsis and co-workers have also reported one-step synthesis of self-pillared pentasil (SPP) zeolite composed of orthogonally connected single-unit cell lamellae [20]. Many other hierarchical zeolite materials possessing similar nanosheets structure reported recently [21-24]. Zeolites commonly used in adsorption, separation and catalysis areas duo to their unique capabilities, such as molecular sieving and shape selectivity [25]. They not only excluded bulky molecules from penetrating the micropore structure or accessing the active acid sites located in the micropores, but also imposed significant diffusion limitation to molecules that smaller than the 4

size of micropores. Previous study showed that the “configurational diffusion” (intracrystalline diffusion) is the dominant transport process in a catalyzed reaction the size of the reactants approaches the dimension of the micropores of zeolite. Although a series of breakthroughs have demonstrated that the enhanced catalytic properties of hierarchical zeolites are attributed to the crystalline micropores and interconnected mesopores, the mechanism of this enhancement is not fully established yet [4,26,27]. Indeed, mass transport behavior of hierarchical zeolites and its influence on catalyst performance are unclear. Therefore, in order to evaluate the benefits derived from mesoporosity, it is necessary to study and characterize the mass transport behavior hierarchical zeolites and correlate it to the mesopore structure which generated by the dualfunctional surfactant approach. Herein, a mesoporous MFI zeolite with multilamellar structure was successfully synthesized by employing a tetra-headgroup rigid bolaform quaternary ammonium surfactant, which served as an organic structure directing agents (OSDAs) for simultaneously generating micropores and mesopores. The molecular structures and properties of the surfactants was studied by the density function theory (DFT) method, it provided insights into the formation mechanism of micro/mesoporous structures. The intracrystalline diffusion of cyclohexane within the hierarchical zeolites was studied using zero length column (ZLC) technique. The obtained mass transport results were used to achieve the fundamental understanding of diffusion limitations in the hierarchical zeolites, and optimizing of the catalytic properties of the novel zeolitic materials. In addition, the activity and selectivity of catalyst were evaluated in aldol condensation reactions. It was concluded that the hierarchically structured zeolites combine the advantages of both mesoporous structure and microporous zeolite framework, providing reduced diffusion length and enhanced accessibilities to the active acid sites located within micropores. 5

2. Experimental 2.1. Synthesis of surfactant The surfactant was synthesized according to a literature procedure [28], as shown in Scheme S1. Typically, 1.86 g (0.01 mol) of 4,4'-dihydroxybiphenyl (99%, Alfa-Aesar) and 1.18 g (0.021 mol) of KOH (98%, J&K) were first dissolved in 75 ml hot ethanol at 323 K (200 proof, Sigma-Aldrich), then, 18.6 g (0.05 mol) of 1,8-dibromooctane (98%, Sigma-Aldrich) was added dropwise under the protection of N2. The mixture was then refluxed and stirred under N2 at 353 K for 20 h, after cooling down, the precipitated product with the formula of CH2Br-(CH2)7-O-C6H4C6H4-O-(CH2)7CH2Br (denoted as Cbiphen-8) was filtered and washed with hot ethanol several times, and dried in a vacuum oven at 333 K overnight. Second, 6.8 g (0.01 mol) of Cbiphen-8 and 17.2 g (0.10 mol) of N, N, N', N'-tetramethyl-1,6-hexanediamine (99%, Sigma-Aldrich) were dissolved in 100 mL acetonitrile/ toluene mixture (1:1 Vol./Vol.) and refluxed under N2 at 353 K for 10 h. After cooling to room temperature, the precipitation with the formula of N(CH3)2-C6H12-N+(CH3)2-(CH2)8-OC6H4C6H4-O-(CH2)8-N+(CH3)2-C6H12-N(CH3)2·2[Br]- (denoted as Cbiphen-8-6) was then filtered, washed with diethyl ether repeatedly, and dried in a vacuum oven at 333 K overnight. Finally, 28.1 g (0.01 mol) of Cbiphen-8-6 and 3.56 g (0.02 mol) of 1-bromohexane (98%, Sigma-Aldrich) were dissolved in 30 mL of acetonitrile and refluxed under N2 at 361 K for 10 h. After cooling to room temperature, the precipitation was filtered, washed with diethyl ether repeatedly, and dried in a vacuum oven at 333 K overnight. The target product with a formula of C6H13-N+(CH3)2-C6H12N+(CH3)2-(CH2)8-O-C6H4C6H4-O-(CH2)8-N+(CH3)2-C6H12-N+(CH3)2-C6H13·4[Br]- is denoted as Cbiphen-8-6-6 (as shown in Fig. 1).

2.2. Synthesis of mesostructured ZSM-5 zeolite 6

In a typical synthesis, a homogeneous gel was first prepared by mixing tetraethylorthosilicate (TEOS, 98%, J&K), NaOH, NaAlO2 (44.7 wt % Na2O, 52 wt % Al2O3, J&K), Cbiphen-8-6-6 surfactant, ethanol and DI water with a molar ratio of 25SiO2: 3Na2O: 0.25Al2O3: 1Cbiphen-8-6-6: 100EtOH: 1000H2O. The clear synthesis gel was aged under magnetic stirring at 338 K for 12 h before transferring into a 50 mL Teflon-lined stainless steel autoclave and heated in a rotary oven (40 rpm) at 423 K for 5 days. After crystallization, the obtained product was filtered, washed with DI water, dried in an oven at 343 K overnight, and then calcined at 823 K for 6 h to remove the organic surfactants. Finally, a mesostructured ZSM-5 zeolite with multilamellar structure was obtained (which is denoted as MLMFI hereafter). Two nano-sized MFI zeolites, 100 nm MFI and 800 nm MFI, were prepared according to published synthesis procedures for comparison [29].

2.3 Catalyst characterizations Powder X-ray diffraction (XRD) patterns were recorded on X'pert Powder (PANalytical) diffractometer system equipped a Cu Kα radiation (operation at 40 kV, 30 mA, λ = 0.15418 nm) with a Ni filter from a 2 θ range of 4° to 40° with a scanning step size of 0.04°. Small angle Xray scattering (SAXS) patterns were collected on a Molecular Metrology SAXS line using a Cu Kα radiation with a sample-to-detector distance of 1.48 m. Scanning electron microscopy (SEM) images were recorded on a FEI-Magellan 400 microscope equipped with a field-emission gun operated at 3.0 kV. The samples were coated with Pt by sputtering at 20 mA for 60 s using Sputter Coater 208. Transmission electron microscopy (TEM) images were taken on a JEOL 2000FX microscope operated at 100 kV. The samples were first dispersed in ethanol with sonication, and then placed on a carbon-coated copper grid followed by evaporation at ambient conditions. N2 adsorption/desorption isotherms were measured on an Autosorb®-iQ system (Quantachrome) at 77 K. Samples were outgassed at 573 K for 3 h before measurement. The Brunauer-Emmett-Teller 7

surface areas (SBET) were calculated from the adsorption branch in the relative pressure (P/P0) range of 0.05 to 0.20. Pore size distribution and cumulative adsorbed volume were calculated by using non-local density functional theory (NLDFT) adsorption model which describes N2 adsorbed onto zeolites with cylindrical pores at 77 K. The mesopore surface area and pore volume were calculated by the t-plot method. For the catalytic reactions, the zeolite catalysts were transferred into a proton form by repeating the ion-exchange in 1 M NH4NO3 solution three times followed by calcination at 823 K for 12 h. Isopropylamine temperature programmed desorption (IPA-TPD) technique was used to determine the concentration of Brønsted acid sites of catalysts [30]. The IPA-TPD was carried out on a Q500 TA instrument by following a procedure reported in literature [31,32]. Typically, 15.0 mg of catalyst was dehydrated by ramping up the temperature to 823 K with a rate of 10 K min-1 under helium atmosphere at a flow rate of 100 mL min-1. After cooling to 393 K, IPA vapor carried with helium flow was introduced and adsorbed on the sample for 10 min. After the sample was saturated with IPA, the gas flow was switched back to helium. The desorption procedure was carried out by increasing the temperature to 937 K at a ramping rate of 10 K min-1. The quantity of Brønsted acid sites is calculated based on the total weight loss between 575 K and 650 K [30]. 2,4,6-collidine (CLD, 99%, Alfa-Aesar) TPD was used to determine the acid sites on the external surface of the zeolites since CLD is too bulky to diffuse into micropore of MFI [20,33,34]. The measurement process of CLD-TPD was similar to the IPA-TPD. The IPATPD and CLD-TPD measurement profiles can be found in Fig. S3.

2.4 Mass transfer Measurement It has been proved that the slow molecular diffusions in zeolites is the causative factor for zeolite catalyst deactivation [35]. Diffusion limitation can be minimized by reducing intracrystalline diffusion path length in hierarchical zeolites with mesopores, and enhancing accessibility of acid 8

sites in zeolite channels. ZLC technique, developed by Eić and Ruthven in 1980’s [36,37], can measure the intracrystalline diffusivities of probe molecules within hierarchical materials with both micro- and mesoporosity by monitoring the desorption profile from a previously equilibrated sample [36,38]. An experimental schematic of the ZLC system used in this study is shown in Scheme S2. A gas chromatograph (GC) with a flame ionization detector (FID) was used as the main part of the system to monitor the adsorption/diffusion process. Cyclohexane (>99%, Acros Organics)/helium stream was obtained by bubbling a liquid cyclohexane reservoir at 283 K with a flow rate of 2 mL min-1 for He. This flow was mixed with another purge He flow (100 mL min-1), then fluxed to the zeolite sample. The partial pressure of cyclohexane in the main stream was maintained at about 2 Torr to make sure the experiment was carried out in the linear region of Henry’s law. Prior to the measuring, all the studied samples were pretreated at 523 K for 8 h under a He flow of 50 mL min−1 to remove physically adsorbed water and impurities. After cooling down, cyclohexane was adsorbed on the zeolite sample at 363K. After the adsorption reached equilibrium, the steam was switched to a pure He flow for desorption. The ZLC measurement was first performed on a commercial zeolite with MFI framework, CBV8014 (Zeolyst Inc.) at 363 K with a flow rate ranging from 25 to 100 mL min−1 to examine whether the desorption process is controlled by the diffusion kinetics as shown in Fig. S2. The flow rate of 50 mL min−1 was used in the experiments for measuring the mass transport properties of zeolite samples. A long time (L-T) fitting model based on ZLC technique was employed to analyze the cyclohexane desorption curves and described elsewhere in the supplementary information (detailed analysis method is available in SI). The other two nano-sized zeolites, 100 nm MFI and 800 nm MFI, are also measured on ZLC system for comparison. 9

2.5 Catalytic Reaction The liquid phase aldol condensations of benzaldehyde with glycol or n-butyl alcohol were carried out at 351 K under N2 atmosphere. Typically, 10.6 g (0.1 mol) of benzaldehyde (ACS reagent, ≥99.0%, Aldrich) was added to a 150 mL three-necked flask containing 29.6 g (0.4 mol) of n-butyl alcohol (98%, Aldrich), meanwhile, 0.3 g of MFI samples, corresponding to 1.4 ×10-4 mol of Brønsted acid sites was added, then the flask was sealed and placed in an oil bath at 351 K with stirring and refluxing. Reaction time was defined as the moment when the catalyst was added. Samples were collected periodically, filtered with a filter (~0.04 mm from Alfa-Aesar) and analyzed on a Waters HPLC system (pump, Model 600; manual injection, Model Rheodyne 7725i; CAPCELL PAK C18 column, 250 mm × 4.6 mm i.d., 5 mm particle size; Model 2487 UV absorbance detector). Acetonitrile was used as a mobile phase at a flow rate of 1.0 ml min-1 through the column. The UV detection wavelength was set at 257.1 nm. The injection volume was 20 µL.

2.6 Computational Study Surfactant geometry optimization was undertaken using the density functional theory (DFT), which provided a very useful tool for understanding molecular properties and for describing the behavior of atoms in molecules [39], with a hybrid functional B3LYP level theory using a 6-31G* basis set with Gaussian 03 program. This basis set provided accurate geometry and electronic properties for a wide range of organic compounds [40]. Besides, molecular electrostatic potential (MEP) is calculated at the same level of theory B3LYP/6-31G* with the optimized structure [41].

3. Results and discussion 3.1. Characterization 10

Fig. 2 shows low-angle and wide-angle powder X-ray diffraction (XRD) patterns of the MLMFI sample before calcination. There are three well-resolved Bragg diffraction peaks indexed as (1), (2) and (3) exhibiting in low-angle range, whereas the peaks in the wide-angle range are the characteristics of a crystalline MFI zeolite. These three peaks at low-angle range can be assigned to the first-, second- and third-order diffractions from the multilamellar structure of the MLMFI zeolites [42]. The first-order reflection is observed at 2θ=1.606°, the corresponding d spacing calculated from the Bragg’s law is d(1) = 5.5 nm. Therefore, the interspace distance between the two nanosheets (3.5 nm) can be calculated by subtracting the thickness of MFI nanosheets (2.0 nm) [43] from the d spacing (5.5 nm) which is the thickness of the surfactant micelle located between the two adjacent MFI nanosheets, and this is in good agreement with the scanning electron microscopy observations as shown in Fig. 3c. The other two d-spacing values are d(2)=2.9 nm and d(3)=1.8 nm, respectively. The ratio of d(1): d(2): d(3) is about 3.05: 1.61 : 1, close to the ideal ratio of 3: 2: 1 expected for lamellar morphology [44]. The diffraction peaks of the MLMFI in the wideangle range (Fig. 2b) show good agreement with nano-sized MFI samples and standard XRD pattern of zeolite with MFI framework. The (h0l) reflections are well-resolved due to the zeolite exhibits an ultrathin morphology along the a-c planes while the long-range order along the b-axis is lost. The results indicate that the tetra-headgroup bolaform quaternary ammonium surfactants with tail groups play a dual role in the generation of multilamellar stacking of MFI nanosheets. The hydrophilic quaternary ammonium head groups can form electrovalent bonds with silica and alumina sources in the solution for synthesis of MFI framework in the microscale, while the hydrophobic long tails form a bulky hydrophobic barrier in the micelle, which confine the growth of zeolite crystals within the ammonium region along the b-axis of the nanosheets, as reported by

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Ryoo et al. [7] The wide-angle XRD patterns of 100 nm MFI and 800 nm MFI are collected and shown in Fig. 2b, both are exhibiting the characteristics of a standard crystalline MFI [45]. The particle morphology of MLMFI was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The SEM images obtained for the assynthesized MLMFI sample are shown in Fig. 3a and 3b. The sample synthesized in the presence of Cbiphen-8-6-6 template possesses a stacked plate-like morphology with an intergrowth structure. Such a multilamellar nanosheets morphology is similar to the ultrathin MFI zeolite that was synthesized using asymmetric diquaternary ammonium surfactants [7], in which plate-like zeolite crystals exist a highly three-dimensional intergrown morphology. Each nanosheet is composed of three pentasil sheets, which are periodically separated by the surfactant micelle. However, although the as-synthesized MLMFI exhibits a highly ordered multilamellar structure, the structural order disappeared upon calcination (as shown in Fig. 3d). It can be explained by the interlayer condensation after removal of the OSDAs. The SAXS pattern of MLMFI indicates that the thickness of the surfactant layer from Cbiphen-8-6-6 (3.5 nm) is slightly smaller than that of C22-66

(4.1 nm), direct evidence for this is provided by the TEM image shown in Fig. 3c. It indicates

that we can precisely control the thickness between two nanolayers by selecting OSDAs with different tail length [28] . SEM images of 100 nm MFI and 800 nm MFI samples are shown in Fig. S1. The particle size is rationally controlled by varying the mole ratio of water in the initial composition. The textural properties of the calcined MLMFI, 100 nm MFI and 800 nm MFI were characterized by N2 adsorption-desorption isotherms to determine the Brunauer-Emmett-Teller specific surface area (SBET) and corresponding pore size distribution. As shown in Fig. 4a, MLMFI exhibits a type-IV N2 adsorption isotherm, showing capillary condensation around the relative 12

pressure range between 0.5 and 1.0, which clearly indicates the presence of mesoporous structures [46]. 100 nm MFI and 800 nm MFI show less pronounced hysteresis loops which means the two nano-sized samples have much less mesopores than MLMFI. Fig. 4b shows the cumulative pore volumes and pore size distributions of three MFI samples calculated by the nonlocal density functional theory (NLDFT) assuming a cylindrical pore model. A sharp increasing in the cumulative pore volume of MLMFI in the pore width range of 3~10 nm, suggesting that the calcination results in a partial condensation of MFI zeolite nanosheets. These findings clearly confirm the co-existence of micropores and mesopores in MLMFI, which is in good consistent with the XRD, SEM and TEM characterization results. In addition, texture properties of these three samples are listed in Table 1, the SBET of MLMFI is 519 m2 g-1, and the micropore and mesopore volumes are 0.122 cm3 g-1 and 0.154 cm3 g-1, respectively. It is clear that the mesopore volume of MLMFI is much higher than 100 nm MFI (0.032 cm3 g-1) and 800 nm MFI (0.019 cm3 g-1). The hierarchically structured zeolite with large mesopore volume can provide them with a large portion of external active acid sites and easy access to the active acid sites in micropores (as listed in Table 3), which are of great important to enhancement of catalytic performance [18,47].

3.2. Mass transport Despite many efforts have been made in fabrication, characterization, and catalytic activity evaluation of hierarchical zeolites, the structure-property relationship of such catalysts has not been fully understood, which is mainly due to the complicated mass transport mechanism of hierarchical porous materials [35]. The mass transport properties of MLMFI were evaluated using the ZLC system to investigate the pore structure and related mass-transport properties of the zeolites with different mesoporosities. Meanwhile, effective diffusivities in the zeolites are 13

determined for directly testify to the superior catalytic performance of MLMFI in the conversion of aldol condensation of benzaldehyde. Cyclohexane is selected as a probe molecule because it is one important component of crude oil, the kinetics diameter of cyclohexane (~6.0 Å) is similar to the channel openings of ZSM-5 (5.1 × 5.6 Å), allowing accurate assessment of mass transport properties of hierarchical zeolite with MFI framework structure. The representative ZLC desorption curves of cyclohexane on the MFI zeolite samples with different mesoporosities are fitted by the L-T fitting model (Eq. S5) and shown in Fig. 5. All measurements are recorded at 323 K - 363K allowing for adequate temporal resolution of the desorption curves. There are linear regions exist in the experimental curves with slopes at long time as shown in Fig. 5a-c. The linear regions become steeper trended with the increase of temperatures. It indicates that the diffusion rates of cyclohexane in different samples increase with temperature, as expected from the L-T fitting model. Furthermore, the desorption curves at the same temperature (363 K) are gathered in Fig. 5d, indicating the desorption rate of cyclohexane in MLMFI (2 nm nanosheets of domain size) is faster than that of two nano-sized zeolite samples with primary particle size of about 100 nm and 800 nm, respectively. This data provides qualitative evidence that introducing mesoporosity in MFI zeolite crystals indeed enhances mass transfer of large molecules like cyclohexane. Diffusion parameters extracted from the experimental desorption curves of the three samples by using L-T fitting model are summarized in Table S1. All the values of L obtained from the theoretical fitting are greater than 10, indicating that the desorption is kinetically controlled, and that analysis of the diffusion pattern from ZLC desorption curves can be accomplished according to the theoretical criteria [48]. In Table S1, it shows that the effective diffusional time constants (Deff/R2) are in the order of 10-5-10-3 s-1 for all the samples, which suggests that the whole diffusion 14

process in each pore system is controlled by intracrystalline diffusion rather than Knudsendiffusion dominating in mesopores [49]. Furthermore, Deff/R2 of all measured samples follow the order of 800 nm MFI < 100 nm MFI < MLMFI ≈ SPP MFI at the same temperatures (as shown in Fig. 6a), indicating the introduced mesoporosity in zeolite truly accelerates the diffusion rate. Effective diffusivities (Deff) were calculated by using characteristic diffusion length (R) of each sample (listed in Table S1). Arrhenius plots in Fig. 6b shows the temperature dependence of Deff plotting with ln(Deff) versus time. It shows that Deff, ML and Deff, SPP is about 3-5 orders of magnitude lower than Deff, 100nm and Deff, 800nm, indicating the mass transfer happened in hierarchical zeolite is not only restricted by the intrinsic micropore framework, but also by the surface barriers [50,51]. A “surface barrier” is a general term for any resistance to mass transfer at or near the surface of a zeolite pore, and it is accounted for up to 60% of overall mass transfer in nanoparticles. The possible origin of the surface barriers have been extensively discussed and probed by experimental and computational methods [52,53]. Reitmeir et al. proposed that the surface barrier is mainly caused by pore blockage on the surface which leads to longer diffusion path, and such a resistance is prevalent and even dominant in zeolite with particle size in nano-scale [54]. In particular, this might be true for MLMFI and SPP MFI due to the multilamellar structure with only 2 nm thickness of nanosheets. The observation indicates that the mass transport in or near the surface layer might be crucial for the overall mass transfer behavior of molecules in hierarchical zeolites. It suggests that although the overall mass transport properties of MLMFI is improved compared to nano-sized MFI, the improvement is not as much as we expected. Optimization of the mass transport on the external surface and within the micropores surface structure is essential for the novel porous materials.

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As all the zeolite samples studied here are identical in the respect of microporous system, the activation energy for diffusion (Ea) should be constant regardless of their morphology or mesoporosity if diffusion in micropores are the dominant diffusion mechanism [50]. Ea, estimated from the slopes of ln(Deff) versus 1000/T curves in Fig. 6b, are listed in Table S1, and these values are consistent with literature data shown in Table S2. From the results above, the presence of mesopores leads to an obvious enhancement of the overall mass transport of cyclohexane in hierarchically structured zeolite crystals, which can be attributed to the reduced diffusion length, high accessibility to micropores by introducing of mesopores. While the overall mass transport is also limited by surface barriers, a second mechanistically unclear mass transfer limitation.

3.3. Catalytic activity The aldol condensation of benzaldehyde reaction is chosen as the probe reaction because it involves bulky reactant molecules and can be catalyzed by Brønsted acid sites. As can be seen from Fig. 7a and Table 2, the conversions of benzaldehyde in aldol condensations with glycol in three catalysts are similar at the beginning in 0.5 h. As expected, the MLMFI catalyst exhibits higher benzaldehyde conversion (94.8 %) after 8 h than other two nano-sized catalysts (48.2% for 100 nm MFI and 35.1% for 800 nm MFI, respectively). Fig. 7b shows the conversion of benzaldehyde in n-butyl alcohol, from which we can see that MLMFI still shows a considerable high conversion of benzaldehyde (86.7 %) after 5 h. However, the conversion efficiencies of benzaldehyde on 100 nm MFI and 800 nm MFI drop clearly, this means 100 nm MFI and 800 nm MFI are inevitably suffering from deactivation by pores blockage of coke than MLMFI. It is proved that hierarchically structured MLMFI exhibit a much more remarkable advantage in aldol

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condensations of benzaldehyde with n-butyl alcohol compared those with much less mesoporosities, such as 100 nm MFI and 800 nm MFI. The Brønsted acid sites distribution was also characterized by IPA-TPD and CLD-TPD (TPD profiles are shown in Fig. S3). Table 3 shows the distribution of Brønsted acid sites of the zeolite catalysts. These three catalysts have the similar Si/Al ratio, yet different fB,ext values, fB,ext stands for the ratio of Brønsted acid sites on the external surface to the total Brønsted acid sites. As expected, there is a large portion of Brønsted acid sites on the external surface of MLMFI which possess less diffusion length and higher accessibility to the bulky reactants in reactions. During the reaction, the catalyst deactivation occurred due to the formation and built-up of coke in the micropores [55]. Owing to shortened diffusion length, reactants can more easily reach the active sites, and products can move out from the micropores more efficiently. Ryoo et al. have reported that coke forms mostly on the external surface of MFI nanosheets, rather than inside the micropores [7]. The formation of coke on external surface shows much less of a barrier to reactants trying to diffuse into the framework of the zeolites than the accumulation of coke inside pores, and that this explains the longer catalytic lifetime of hierarchical zeolites, such as MLMFI [56]. These data provide strong evidences that the mesostructured zeolites, with high ratio of Brønsted acid sites on surface, can significantly facilitate the catalytic activities resulted from the shorter diffusion length and high accessibility to acid sites in micropores when bulky molecules were involved.

3.4 Computational study A proposed structure model for the MLMFI combined with the surfactants is illustrated in Fig. 8. The bolaform amphiphilic surfactant molecular is mainly composed of two bi-quaternary ammonium groups as the hydrophilic heads and a long alkyl chain which centered with a biphenyl 17

group as the hydrophobic tail. The two bi-quaternary ammonium head groups are embedding in two straight channels in two confronting MFI nanosheets along the b-crystal axis. The hydrophobic long chain tail groups containing two benzene rings self-assemble to form a stable layered micelle as an interspace between the two monolayers. The thickness of the interspace is calculated out to be 3.5 nm consisting with the TEM observation result discussed above. Fig. 9 shows the optimized structure and molecular orbital surfaces of the surfactant calculated at the B3LYP/6-31G* basis set with Gaussian 03 program. The distributions of LUMO (the lowest unoccupied molecular orbital) was mainly located on the inner ammonium group, and the LUMO+1 (the lower unoccupied molecular orbital) also gave the same result, which indicated that the inner ammonium group was more functional to direct the synthesis of zeolite. A similar conclusion was drawn by Park et al. [57] who investigated the hierarchically structure-directing effect of multi-quaternary ammonium surfactants for the generation of MFI zeolite nanosheets. They corroborated that one ammonium group was failure to generate a zeolite framework, while surfactants containing an inner ammonium group (at least two ammoniums) can direct the zeolite structure. This illustrates that the surfactants acted as an effective bifunctional SDA within both micro-/mesopore length scales. First, hydrophobic long chain tail group containing two benzene rings is supposed to be the effective driving forces in the formation of stable multilayer assemblies of the surfactants [58]. Also, the – stacking interactions is proposed to be an effective noncovalent intermolecular force which can promote the self-assembly processes of the surfactant micelle into a multilamellar structure [7]; Because of the prevention to the growths of zeolitic microporous crystals beyond the ammonium regions by the hydrophobic tails, aluminum and silica species can only crystallized around the two bi-quaternary ammonium ions, and emerged into straight channels surrounded by crystalline MFI walls along the b-axis as shown in Fig. 8. As a 18

result, a stable and highly-ordered multilamellar structure is constructed by the self-assembly process of the amphiphilic surfactants. Finally, a well-ordered mesostructured zeolite with singlecrystalline MFI nanosheets is obtained. Molecular electrostatic potential (MEP) of the bolaform surfactant was further investigated to understand the electronic density of the molecule. The MEP of bolaform surfactant was shown in Fig. 10, it can be seen that the positive regions are mainly over ammonium groups, thus, it can be proposed that the anionic aluminosilicate species can interact with the cationic surfactant by electrostatic interactions, in which the aluminosilicate species may play a nucleophile role to attack the ammonium groups position. These analyses elucidate that the bolaform surfactant could participate in the zeolite synthesis as an effective structure directing agent in the interaction with the zeolite precursor.

4. Conclusions A hierarchical MFI zeolite with multilamellar structure was successfully synthesized by using a dual-functional amphiphilic ammonium surfactant as pore-generating agent. Characterization results indicated that it possessed higher surface area, larger mesopore volume, larger portion of active sites on external surface, as well as less diffusion limitation and longer catalytic lifetime, which enhances the diffusion rate and accessibility. It will be a highly comprehensive strategy to fabricate micro-/mesoporous zeolites with diverse frameworks, such as MTW, BEA and FAU. Moreover, the computational study analysis result could not only provide insights into the molecular properties of surfactant that dominates the formation process of micro/mesoporous zeolites, but also serve as a complement to experiments and efficient theoretic guidance for synthesis technologies of other hierarchically structured porous materials. Mass transfer behavior of cyclohexane in mesostructured MLMFI zeolite was studied using ZLC technique, it indicated 19

that mesostructured zeolite with thin nanosheets morphology exhibited enhanced mass transport performance compared to large crystal MFI particles, providing with enhanced catalytic performance in those transformations of relatively large molecules. A secondary surface limitation was existed requiring for further improvement in rational design and characterization of hierarchical zeolites for catalysis, adsorption and separation fields in the future.

Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 20936001 and No. 21176084), the National High Technology Research and Development Program of China (No. 2013AA065005). The authors would like to thank Prof. Wei Fan (University of Massachusetts, Amherst) for thoughtful discussion.

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version.

20

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Accepted Manuscript Title: Facile synthesis of mesostructured ZSM-5 zeolite with enhanced mass transport and catalytic performances Author: Chao Li Yanqun Ren Jinsheng Gou Baoyu Liu Hongxia Xi PII: DOI: Reference:

S0169-4332(16)31922-5 http://dx.doi.org/doi:10.1016/j.apsusc.2016.09.054 APSUSC 33982

To appear in:

APSUSC

Received date: Accepted date:

17-7-2016 13-9-2016

Please cite this article as: Chao Li, Yanqun Ren, Jinsheng Gou, Baoyu Liu, Hongxia Xi, Facile synthesis of mesostructured ZSM-5 zeolite with enhanced mass transport and catalytic performances, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.09.054 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

(1)

(2)

(b) (503)

(501) (303)

(301) (202) (400)

(200)

(101)

Intensity (a.u.)

Intensity (a.u.)

(a)

MLMFI

100 nm MFI

800 nm MFI

(3)

Standard MFI

0

1

2

3

4

5

10

20

30

40

2 ()

2 ()

Fig. 2. (a) Low-angle powder X-ray diffraction patterns of as-synthesized MLMFI, (b) wide-angle XRD patterns of different MFI zeolite samples with a standard XRD pattern of MFI framework from IZA database for comparison.

26

Fig. 3. (a), (b) SEM images of as-synthesized MLMFI of different magnifications. TEM images of MLMFI before (c) and after (d) calcination.

27

400

0.5

Cumulative pore volume (cm3g-1)

MLMFI 100 nm MFI 800 nm MFI

300

200

100

0 0.0

0.2 0.4 0.6 0.8 Relative pressure (P/P0)

1.0

0.4

(b)

MLMFI 100 nm MFI 800 nm MFI

0.4

0.3

0.3 0.2 0.2 0.1

0.1 0.0

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

N2 volume adsorbed(cm3g-1)

(a)

0.0 0

5

10 Pore width (nm)

15

20

Fig. 4. (a) N2 adsorption-desorption isotherms and, (b) the corresponding NLDFT cumulative pore volume and pore size distributions of calcined MLMFI, 100 nm MFI and 800 nm MFI.

28

100

100

(a)

323 K 343 K 363 K

323 K 343 K 363 K

10-1 C/C0

C/C0

10-1

(b)

10-2

10-2

10-3

10-3

10-4

10-4 0

1000

2000

3000

4000

0

5000

1000

time (sec.)

2000 3000 time (sec.)

100

100

(c)

10-1

C/C0

10-1

5000

800 nm MFI 100 nm MFI MLMFI

(d)

323 K 343 K 363 K

4000

10-2

C/C0

10-2

10-3

10-3

10-4

10-4 0

1000

2000

3000

4000

5000

0

1000

2000

3000

4000

5000

time (sec.)

time (sec.)

Fig. 5. Experimental (symbols) and fitted (solid lines) ZLC desorption curves for cyclohexane in (a) 800 nm MFI, (b) 100 nm MFI, and (c) MLMFI at different temperatures. (d) Comparison of ZLC desorption curves in different MFI crystals at 363 K.

29

10-2

(a)

10-14 Deff (cm2 s-1)

Deff/R2(s-1)

MLMFI 100 nm MFI 800 nm MFI SPP MFI

10-13

10-3

10

10-12

(b)

MLMFI 100 nm MFI 800 nm MFI SPP MFI

-4

10-15 10-16 10-17 10-18 10-19

10-5 2.4

2.6

2.8 3.0 3.2 1000/T (K-1)

3.4

3.6

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

1000/T (K-1)

Fig. 6. (a) Diffusional time constants (Deff/R2) as a function of temperature, (b) Arrhenius plots show the temperature dependence of effective diffusivities (Deff) of different samples. (The open symbol corresponds to the data reported previously by Chang et al. [1].)

30

100

(b)

80 60 40 20

MLMFI 100 nm MFI 800 nm MFI

0 0

100

200 300 Reaction time (min)

400

100

Conversion of benzaldehyde in n-butyl alcohol (%)

Conversion of benzaldehyde in glycol (%)

(a)

500

MLMFI 100 nm MFI 800 nm MFI

80 60 40 20 0 0

100

200 300 Reaction time (min)

400

500

Fig. 7. Catalytic activities of catalysts in aldol condensations of benzaldehyde with (a) glycol and (b) n-butyl alcohol at 351 K.

31

Fig. 8. Proposed structure for the MFI multilamellar structure, as employed in the simulation.

32

Fig. 9. Optimized structure (a) and Molecular orbital surfaces (b,c) of the bolaform surfactant (atom legend: white = H; gray = C; blue = N; red = oxygen).

33

Fig. 10. Molecular electrostatic potential map of the bolaform surfactant.

Reference: [1] C.-C. Chang, A.R. Teixeira, C. Li, P.J. Dauenhauer, W. Fan, Enhanced Molecular Transport in Hierarchical Silicalite-1, Langmuir 29 (2013) 13943-13950.

34

Table 1. Texture properties of the studied zeolite samples. Samples

SBET a (m2 g-1)

Sext b (m2 g-1)

Vmicro b (cm3 g-1)

Vmeso c (cm3 g-1)

MLMFI

519

221

0.122

0.154

100 nm MFI

383

208

0.125

0.032

800 nm MFI

369

120

0.123

0.019

a

The BET surface area, SBET, was calculated in a relative pressure range of 0.05 to 0.3.

b

The external surface area, Sext, and micropore volumes, Vmicro, were estimated from the t-plot method.

c

The mesopore volume, Vmeso, was obtained by Vtotal (P/P0=0.95)-Vmicro.

Table 2. Conversions (%) of benzaldehyde in glycol and n-butyl alcohol after 8 h over different zeolite catalysts. Reactions

MLMFI

100 nm MFI

800 nm MFI

94.8

48.2

35.1

86.7

39.6

23.4

Table 3. The distribution of Brønsted acid sites of zeolite catalysts.

Catalysts

Si/Al ratio (Theoretical)

Brønsted Acid sites concentration (mmol g-1) Total

a

Total

fB,ext d

b

(Theoretical)

(Actual)

External surface c

(Actual)

MLMFI

30

0.538

0.533

0.154

28.9%

100 nm MFI

30

0.538

0.529

0.038

7.18%

800 nm MFI

30

0.538

0.546

0.026

4.76%

a

The number was calculated based on Si/Al ratio in the synthesis gel mole composition.

b

The number was determined by IPA-TPD.

c

The number was determined by CLD-TPD.

d

The fraction of external Brønsted acid sites calculated by (number of Brønsted acid sites from CLD-TPD profile /

number of Brønsted acid sites from IPA-TPD profile).

35

.