Temperature-controlled phase-transfer hydrothermal synthesis of MWW zeolites and their alkylation performances

Abstract

MWW zeolites have been synthesized with hexamethyleneimine/aniline as the structure-directing/promoting agent. As structure-promoting agent, aniline contributes to the crystallization of MWW zeolites without being trapped within zeolites. Meanwhile the temperature-controlled phase-transfer hydrothermal synthesis of MWW zeolites could be achieved because the solubility of aniline increases with a rise of temperature. For delaminated MCM-56 with intermediate nature, the conversion to MCM-49 could be avoided even when subjected to a much longer crystallization time. Partially delaminated MWW monolayers of MCM-56 possesses larger external surface area, pore volumes, better accessibility of active centers, which resulted in higher ethylene conversion than MCM-22 and MCM-49 in the liquid-phase alkylation of benzene with ethylene above 210 °C.

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1. Introduction

As typical MWW (Framework Type Code assigned by International Zeolite Association) zeolites,^1–13 MCM-22 and MCM-49 consist of three pore systems: two-dimensional, sinusoidal, 10 membered-ring (MR) intralayer channels, 12 MR interlayer supercages with 10 MR apertures and 12 MR cups on the external surface (half of 12 MR supercages). However MCM-56 is considered as the first mono-layered (delaminated) zeolite with MWW topology comprising layers stacked in a disordered manner without interlayer connectivity. Generally with more exposed 12 MR cups on the external surface, MCM-56 is not expected to contain supercages at least in its idealized form. MCM-56 could be viewed as a delaminated zeolite obtained by direct synthesis and it is the only one prepared with simple structure-directing agents (SDAs). The X-ray diffraction (XRD) patterns of as-synthesized MCM-22, i.e., MCM-22 precursor (MCM-22P), shows one of its featured peaks at 2θ 6.6°, ascribed to interlayer [002] reflection, and the c parameter is 2.7 nm, which could be envisioned as MCM-22P with layers linked by hydrogen bonded silanol pairs between neighboring surfaces in the presence of SDAs.^13,14 Upon calcination, MCM-22P undergoes a significant structure change and conversion to MCM-22C with complete 3D structure as evidenced by the [002] reflection disappearing in the corresponding XRD patterns, as for the case of as-synthesized/calcined MCM-49 (c parameter 2.5 nm).¹³ As for MCM-22C, the contraction of the c parameter by approximately 0.2 nm is attributed to the condensation of the surface Si–OH groups and formation of T–O–T interlayer bridges via thermal oxidative destruction of hydrogen bonded silanol pairs by HMI between two MWW layers. As the intermediate of MCM-49, a notable feature of as-synthesized MCM-56 comprises a collection of monolayers, which are disordered/misaligned and unbound except for possible coincidental cross linking, and showing continuous scattering in the 8–10° 2θ range. As for the monolayer MWW zeolite, the c parameter of MCM-56 is also 2.5 nm.^6,15 Therefore, every individual material of the MWW family differs in the distance between the layers and the translation in the degree of regularity/irregularity in organization of neighboring layers.^16–18 As MCM-56 could be further transformed to the MCM-49 phase, two 12 MR cups on the neighboring MWW layer are transformed to one 12 MR supercage via the formation of oxygen-bridges with 10 MR windows. Therefore, MCM-56 has more 12 MR cups with fewer 12 MR supercages in idealized conditions than MCM-49 and MCM-22C with regular connection between individual layers via oxygen-bridge bonding. Individual MCM-56 layers are considered to be a partially delaminated zeolite comprising a disordered collection of MWW monolayers and having a large portion of exposed external surface. According to published data, 12 MR cups are claimed to provide more useful architecture with unexpected performances especially in the alkylation of aromatics.^19–24

Hexamethyleneimine (HMI), piperidine, diethyl(dimethyl)ammonium hydroxide, N,N,N-trimethyl-1-adamantanammoniumhydroxide, 1,4-bis(N-methylpyrrolidinium)butane, N,N,N,N′,N′,N′-hexamethyl-1,5-pentane-diammonium, N,N,N,N′,N′,N′-hexamethyl-1,6-hexane-diammonium and other amines have been reported to be SDAs to form MWW zeolites,^{1–10,25–29} however for aluminosilicate MWW zeolites, HMI is the most efficient SDA, especially for MCM-22, MCM-49 and MCM-56 in similar conditions because of its effective structure-directing capability and low cost.^2,4–13 In our recent publications, aniline (AN) was introduced as the structure-promoting agent (SPA) to couple with HMI as the SDA to achieve temperature-controlled phase-transfer hydrothermal synthesis of MCM-22/49 zeolites.^30–32 The improved solubility of aniline with temperature in water was summarized by Stephen et al.³³ Poor solubility of aniline at room temperature leads to phase separation of the organic phase and mother-liquor before crystallization. While after crystallization, poor solubility of aniline provides a simple method to recover HMI and aniline by liquid separation. During crystallization (temperature at 145 °C, AN/SiO₂ = 0.2, HMI/SiO₂ = 0.1, H₂O/SiO₂ = 15), both HMI and aniline became completely soluble to favor the formation of MWW zeolites above 110 °C according to previous results.³⁰ With aniline only, no formation of MWW phase could be observed because of p–π conjugation which leads to low electronegativity and low alkalinity in water (pK_b = 4.78). However, lack of aniline led to insufficient crystallization with HMI/SiO₂ = 0.10. More interestingly, it seemed that aniline was not occluded into MWW zeolites. According to the above results, aniline can be regarded as an SPA, which contributed to the formation of MWW zeolites, but is not trapped within MWW zeolites, i.e., there was almost no consumption of aniline during the synthesis of MWW zeolites. To some extent, aniline as the SPA could be regarded as the “catalyst” of the whole process. In all, aniline acts as the phase-transfer medium, extractor of HMI and the SPA during the temperature-controlled phase-transfer hydrothermal synthesis of MCM-22 zeolite, which decreased HMI trapped within MWW zeolites by about 1/3.³¹ It seems that the involvement of aniline could offer a facile method to generate MWW zeolites with less economical and environmental impact.

MCM-22 synthesized through temperature-controlled phase-transfer hydrothermal synthesis in the presence of aniline, was nearly identical concerning chemical composition and structure, and showed nearly identical property with respect to porosity, SiO₂/Al₂O₃ ratio, thermal behavior and similar catalytic ethylation performance, compared with that made from reference synthesis with HMI as the only SDA. The difference lies in the reduction in the HMI usage by at least 2/3 and about 1/3 decrease in HMI trapped within MCM-22 zeolites.^30,31 In this paper, we expanded the concept of SPA and temperature-controlled phase-transfer hydrothermal synthesis to MCM-49 and MCM-56, all of which could be synthesized in similar conditions with HMI as the SDA. Also, the aluminosilicate MWW zeolites were fully studied in terms of their properties, acidity and catalytic alkylation performance. As is well known, the crucial issue of delaminated MCM-56 zeolite lies in its intermediate nature and conversion to MCM-49 if not quenched at the right time. Besides the advantages of temperature-controlled phase-transfer hydrothermal synthesis, other improvements in reproducibility and stability of high quality MCM-56 have always been desired. Therefore we anticipate that the involvement of aniline could lead to significant advances for similar systems to obtain high quality MCM-56, the only prepared MWW zeolite with simple SDAs, for improvement its alkylation performance.

2. Experimental

2.1 Synthesis of MWW zeolites and catalysts

All materials (solid silica gel, fumed SiO₂, NaAlO₂, NaOH, HMI, aniline and deionized water) were used as purchased. The molar ratios of typical batch composition and hydrothermal synthesis conditions are shown in Table 1. The hydrothermal synthesis was conducted in Teflon-lined autoclaves (25 mL) with tumbling condition at 30 rpm and a stainless-steel vessel (2500 mL) with stirring speed at 400 rpm. The synthesized samples were filtered, washed with deionized water until pH = 7, and dried at 100 °C overnight. Synthesized samples were calcined at 550 °C in ambient air for 6 h in a muffle furnace to remove organics. H-type zeolites/catalysts were prepared according to the method described in our previous publications.^34–37

Table 1 Synthesis conditions of MWW zeolites

No.	Silica source	Batch composition mol/mol			Crystallization conditions^a					Product
		SiO2/Al2O3	NaOH/SiO2	H2O/SiO2	HMI/SiO2	AN/SiO2	Time/h	Speed/rpm	Reactor	XRD
a The crystallization temperature is set at 145 °C, a: 25 mL Teflon-lined autoclave in tumbling condition, b: 2500 mL stainless-steel vessel in stirring condition.b MCM-49 is the major product.
1	Solid silica gel	30	0.18	15	0.1	0.2	72	30	a	MCM-22
2	Solid silica gel	30	0.18	45	0.1	0.2	72	30	a	MCM-22
3	Solid silica gel	25	0.18	15	0.1	0.2	72	30	a	MCM-22/MCM-49^b
4	Solid silica gel	30	0.18	15	0.1	0.2	72	400	b	MCM-22
5	Solid silica gel	25	0.18	15	0.1	0.2	72	400	b	MCM-49
6	Fumed SiO2	25	0.18	45	0.1	0.2	72	30	a	MCM-22
7	Fumed SiO2	25	0.18	45	0.1	0.2	72	400	b	MCM-56
8	Fumed SiO2	25	0.18	45	0.1	0.2	168	400	b	MCM-56

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2.2 Characterization of MWW zeolites

XRD patterns of samples were collected on a D/MAX-III X-ray diffractometer (Rigaku Corporation, Japan) with filtered Cu-Kα radiation at a tube current of 35 mA and a voltage of 35 kV. The scanning range of 2θ was 5–35°.The crystal morphology was measured on a FEI Quanta scanning electron microscope (SEM). The elemental analyses of the solids were performed on an X-ray fluorescence (XRF) spectrometer MagiX (Philips). Nitrogen adsorption–desorption isotherms were recorded on a Micromeritics ASAP 2010 instrument. The samples were first outgassed under vacuum at 363 K for 1 h and at 623 K for 15 h. The total surface area was obtained by application of the BET equation using the relative pressure range of 0.05–0.16 in the nitrogen adsorption isotherm as a range of linearity (using a molecular cross-sectional area for N₂ of 0.162 nm²). The micropore volume was calculated by the t-plot method. The external surface area was calculated by the formula S_external = S_BET − S_micro.

The acidity of H-type zeolites was measured by temperature programmed desorption (TPD) and Fourier transform infrared spectrometry (FTIR), using ammonia (NH₃) and pyridine/2,4,6-trimethylpyridine as probe molecules, respectively. NH₃-TPD was carried out on an Autochem II 2920 unit equipped with a thermal conductivity detector. For FTIR spectroscopy of adsorbed probe molecules (pyridine and 2,4,6-trimethylpyridine), all samples were pressed into self-supporting wafers and measured in transmission mode in an FTS3O00 FTIR spectrometer by 64 scans with a resolution of 4 cm⁻¹. Prior to the measurements, each sample was dehydrated under vacuum (10⁻³ Pa) at 350 °C for 1 h, and then cooled to 50 °C for pyridine adsorption. The IR spectra of the samples before pyridine/2,4,6-trimethylpyridine adsorption were recorded at different temperatures (200 and 350 °C), and after adsorbing pyridine for 10 s, the samples were purged under vacuum (10⁻³ Pa) to higher temperature at a heating rate of 10 °C min⁻¹. Then the IR spectra of pyridine on samples were recorded at different temperatures (200 and 350 °C), while IR spectra of pyridine was collected at 200 °C. All the spectra given in this work were differential spectra. The number of Brønsted and Lewis acid sites was calculated according to adsorbed pyridine IR peak area, and the number of acid sites by 2,4,6-trimethylpyridine was calculated based upon adsorbed 2,4,6-trimethylpyridine IR peak height at 200 °C.

²⁹Si/²⁷Al MAS NMR experiments were performed on a Bruker AVANCE III 500WB/600WB spectrometer at a resonance frequency of 99.3/156.4 MHz using a 7/4 mm double-resonance MAS probe with a recycle delay of 4/1 s, respectively. The magic-angle spinning rate was 5/13 kHz in all experiments, and a typical length of 1.8/0.4 μs was adopted for ²⁹Si/²⁷Al resonances, respectively. The chemical shift of ²⁹Si/²⁷Al was referenced to tetramethylsilane (TMS)/1 M aqueous Al(NO₃)₃ respectively. ¹³C CP/MAS NMR experiments were performed on a Bruker AVANCE III 600WB spectrometer at a resonance frequency of 150.9 MHz using a 4 mm double-resonance MAS probe at a sample spinning rate of 7.5 kHz. The chemical shift of ¹³C was determined using a solid external reference, hexamethylbenzene (HMB). ¹³C CP/MAS NMR spectra were recorded by using a recycle delay of 5 s and a contact time of 2 ms. The liquid ¹³C NMR experiments were performed on a Varian INOVA 500 spectrometer at a resonance frequency of 125.6 MHz using a 5 mm double-resonance probe. The chemical shift of ¹³C was referenced to tetramethylsilane. ¹³C NMR spectra were recorded by small-flip angle technique using a pulse length of 3.2 ms (π/6) and a recycle delay of 4 s.

2.3 Liquid-phase alkylation of benzene with ethylene

The liquid-phase alkylation of benzene with ethylene was carried out in a stainless-steel fixed bed reactor. The catalyst (8 mL) was placed in the center of the reactor and purged with nitrogen. Benzene was pumped into reactor to fill the bed fully under reaction pressure, and then the temperature was raised up to 200 °C before ethylene was introduced. The alkylation conditions were as follows: pressure of 3.5 MPa, weight hourly space velocity (WHSV) of benzene of 3 h⁻¹, and molar ratio of benzene/ethylene 12/1, 200–260 °C. At each temperature the reaction was given at least 15 h to reach a steady-state condition before samples were collected for analysis. The reaction products were analyzed by an Agilent 6890 gas chromatograph (GC) equipped with a flame ionization detector and a capillary column. It is well known that the alkylation of benzene with ethylene is a consecutive reaction, and the reaction products were made up of EB, para-diethylbenzene (p-DEB), ortho-diethylbenzene (o-DEB), meta-diethylbenzene (m-DEB), triethylbenzenes (TEB), other by-products (diphenylethane) in our work. The ethylene conversion, EB selectivity, DEB selectivity and DEB selectivity distribution were calculated based upon formulas as follows:

H = x_EB/M_EB + 2x_DEB/M_DEB + 3x_TEB/M_TEB + x_{diphenylethane}/M_{diphenylethane};

x_DEB = x_p-DEB + x_o-DEB + x_m-DEB;

M: molar mass (g mol⁻¹); x: mass percentage concentration (wt%) from GC analysis; H: molar number percentage concentration (mol%) of ethyl;

Ethylene conversion (%): C_ethylene = H/(H + x_ethylene/M_ethylene) × 100% (1)

EB selectivity (%): S_EB = x_EB/(x_EB + x_DEB + x_TEB + x_{diphenylethane}) × 100% (2)

DEB selectivity (%): S_DEB = x_DEB/(x_EB + x_DEB + x_TEB + x_{diphenylethane}) × 100% (3)

TEB selectivity (%): S_TEB = x_TEB/(x_EB + x_DEB + x_TEB + x_{diphenylethane}) × 100% (4)

3. Results and discussion

3.1 Synthesis of MWW zeolites

MCM-22 (Table 1 sample 4), MCM-49 (Table 1 sample 5) and MCM-56 (Table 1 sample 7 and 8) zeolites are characterized by XRD analysis (Fig. 1). MCM-22P showed the typical [002] diffraction peak at ca. 6.6° 2θ due to the layer stacking of the MWW sheets with HMI trapped into zeolites by forming hydrogen-bonded silanol pairs between two MWW layers.^14,22 After calcination HMI could be completely removed, which generated a 3D MWW structure via interlayer dehydroxylation and condensation. Another region between 26 and 29° could also distinguish MCM-22P from MCM-49 and MCM-22C. In this region, MCM-22P possessed only one broad peak centered at 26.22° with a smooth, sloping tail visible on the high 2θ side. For MCM-49 and MCM-22C this feature was replaced by three sharp peaks, occurring in the approximate region 26.5–29.0°. As for as-synthesized MCM-56 (Table 1 samples 7 and 8), there was a relatively sharp peak at 7.1° 2θ accompanied by a typical broad band range in 8–10° 2θ. However, there was slight separation at 8–10° 2θ for H-MCM-56 and the intensity of other peaks became slightly stronger, indicative of partial ordering.

Fig. 1 XRD patterns of MCM-22 (a, 72 h), MCM-49 (b, 72 h), MCM-56 (c, 72 h) and MCM-56 (d, 168 h) zeolites before and after calcination.

Generally, even a slight change in material sources or in synthetic conditions often led to a mixture of MCM-22/49 or MCM-56/49, especially when the synthesis were conducted with SiO₂/Al₂O₃ feeding ratio between 25 and 30, the borderline of MCM-22/49. Compared with our previous work,³⁰ it also happened that MCM-22 (Table 1 sample 6) and MCM-56 (Table 1 sample 7 and 8) were synthesized in the same feeding composition with fumed SiO₂ as silica source, the only difference being that the autoclave (25 mL/2.5 L) was under tumbling/stirring condition at 30/400 rpm respectively, similarly for sample 3 (mixture of MCM-22 and MCM-49) and 5 (MCM-49) with solid silica gel as the silica source in Table 1. Based upon our previous results,³⁰ the SPA aniline favored the formation of MCM-49 due to lower content of HMI. Similarly, the results might disclose that the hydrothermal synthesis conditions (silica source, reactor and so on) influenced the crystal phase of final product; the better mixing of raw materials (fumed SiO₂) during hydrothermal synthesis showed the tendency towards the formation of MCM-56 with partially delaminated and a disordered collection of MWW monolayers. MCM-56 could be easily transformed into MCM-49 with regular 3D MWW structure with longer heating time and can be regarded as the intermediate of MCM-49 during the synthesis.⁸ However, there was no other competing phase during MCM-56 synthesis even when extending the crystallization time from 72 to 168 h (Table 1 sample 8, and Fig. 1d) in the HMI/AN system with high H₂O/SiO₂ ratio without transformation to MCM-49 phase, indicating that this strategy can be exploited and modified to obtain high quality MCM-56 with improved stability. With aniline as the SPA, MCM-56 could be obtained with HMI/SiO₂ at 0.10, i.e. only 1/3 or less HMI usage compared to previously.^19–24 The above results might provide a facile and reproducible method to generate stable MCM-56 with less economical and environmental impact due to the properties of the temperature-controlled phase-transfer hydrothermal synthesis.

It has been proven by ¹³C CP/MAS NMR that there were three resonances centered at about 27, 49 and 57 ppm, which are ascribed to C2/C3, C1 (intralayer) and C1 (interlayer) of HMI for as-made MWW zeolites synthesized in the HMI only system.¹³ However, ¹³C CP/MAS NMR spectra of as-synthesized MCM-22, MCM-49 and MCM-56 via temperature-controlled phase-transfer hydrothermal synthesis clearly showed two resonances of organics trapped within MWW zeolites. One of the resonances, centered at about 27 ppm, was ascribed to overlapped C2 and C3 resonances of HMI, and the other located at 49 ppm, was related to the C1 resonance of HMI (Fig. 2a). Similar to our previous results,³⁰ remarkably decreased C1 resonance at 57 ppm supported the promoting effect of aniline as the SPA during the hydrothermal synthesis. However no resonances of aniline were detected on the as-synthesized MWW zeolites, which drove us to consider aniline as simply a “catalyst” in the temperature-controlled phase-transfer hydrothermal synthesis. In order to verify this, as-synthesized MWW zeolites were treated by HF for a liquid ¹³C NMR test (Fig. 2b), the results of which clearly proved that there was no aniline trapped within the as-synthesized MWW zeolites. The promoting effects on the crystallization of aniline without being trapped within MWW zeolites strongly support the role as an SPA during the temperature-controlled phase-transfer hydrothermal synthesis.

Fig. 2 ¹³C CP/MAS NMR spectra of as-synthesized MWW zeolites (a) and liquid ¹³C NMR spectra of as-synthesized MWW zeolites decomposed by HF (b).

3.2 Physicochemical properties of H-MWW zeolites

Fig. 3a and b show typical morphology of H-MCM-22 and H-MCM-49 with rose-like shape of about several μm. However, H-MCM-56 appeared as some overlapped plate-like crystals of less than 20 nm in thickness (Fig. 3c), which was totally different from the morphology of H-MCM-22 and H-MCM-49. H-MCM-56 with very thin plate-like crystals aggregated at high stacking disorder, is supposed to be a partially delaminated and disordered collection of MWW monolayers with larger outer surface area exposing more 12 MR cups on the external surface and more acid sites accessed by reactants/probe molecules.

Fig. 3 SEM image of H-MCM-22 (a), H-MCM-49 (b) and H-MCM-56 (c) zeolites.

The nitrogen (N₂) isotherms of H-type zeolites are presented in Fig. 4. Obviously, the N₂ adsorption amounts of H-MCM-56 were higher than H-MCM-22 and H-MCM-49, which was related to the special morphology of stacking disorder for H-MCM-56 layer accumulation. H-MCM-22, H-MCM-49 and H-MCM-56 show the behavior of typical microporous zeolites, which was consistent with the reported results.^20,38 However, the clear hysteresis for H-MCM-56 was observed at high relative pressures (p/p₀ = 0.7–0.98), unlike the case of H-MCM-22 and H-MCM-49, which disclosed that the presence of a larger mesoporosity of H-MCM-56 originated from a disordered collection of partially delaminated MWW monolayers.

Fig. 4 Nitrogen isotherms of H-MCM-22, H-MCM-49 and H-MCM-56 zeolites.

Table 2 presents the BET analysis of H-type zeolites. As anticipated, H-MCM-56 showed much lower micro surface area (298 m² g⁻¹) and micro volumes (0.14 cm³ g⁻¹) than H-MCM-22 and H-MCM-49, however, H-MCM-56 (145 m² g⁻¹) showed almost as twice the external surface area as those of H-MCM-22 (69 m² g⁻¹) and H-MCM-49 (75 m² g⁻¹). The reason for this lies in the preservation of 10 MR intralayer channels and more formation of 12 MR cups due to partial delamination and a disordered collection of MWW monolayers, which led to the formation of the clear hysteresis in the N₂ isotherms and obviously enhanced the total pore volume (0.79 cm³ g⁻¹) compared to H-MCM-22 (0.52 cm³ g⁻¹) and H-MCM-56 (0.57 cm³ g⁻¹). As stated in the introduction, 12 MR cups have been proven to be more beneficial for excellent performances especially in the alkylation of aromatics.^38–41 It is expected that the larger external surface area means that more 12 MR cups are available for acid sites and active centers accessed by probe molecules/reactants, which would be beneficial for catalytic performances in the liquid-phase alkylation of benzene with ethylene.

Table 2 Textural properties of H-type zeolites^a

No.	XRD	SiO2/Al2O3	SBET/m^2 g^−1	Smicro/m^2 g^−1	Sexternal/m^2 g^−1	Vmicro/cm^3 g^−1	Vtotal/cm^3 g^−1
a SBET were calculated by BET method, micropore volume and surface area were calculated by t-plot method, Sexternal = SBET − Smicro.
4	H-MCM-22	25	428	359	69	0.17	0.52
5	H-MCM-49	23	465	390	75	0.19	0.57
7	H-MCM-56	24	443	298	145	0.14	0.79

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²⁹Si/²⁷Al MAS NMR spectra of as-synthesized and H-type zeolites are given in Fig. 5, which shows the poorly separated resonances for MWW zeolites ²⁹Si/²⁷Al. Because of the same MWW topology structure and the chemical shift of the framework Si and Al depending strongly on the geometric environment of each Si and Al atom, there was no significant difference in the resonances of Si(0Al) and Si(1Al) species from −100 to −119 ppm (²⁹Si MAS NMR spectra) and from 49 to 61 ppm (²⁷Al MAS NMR spectra) among three MWW zeolites.^13,38 However, the signal peak at −100 ppm of H-MCM-56 seemed slightly larger than those of MCM-22 and MCM-49. Regardless of the difference in feeding SiO₂/Al₂O₃ ratio, the final MWW zeolites had similar bulk SiO₂/Al₂O₃ ratio, which indicated similar Al₂O₃ content. Since increase in resonances at −100 ppm of H-MCM-56 would be originated from Si–OH rather than (SiO)₃Si(OHAl), MCM-56 with partially delaminated and a disordered collection of MWW monolayers, led to more framework Si defect formation. There were no resonances of extra-framework Al for any as-synthesized samples; after calcination, the framework Al became distinguishable due to slight differences in the connection of MWW layers, and dealumination usually caused the formation of extra-framework Al at about 0 ppm, as reported previously.^13,42–46

Fig. 5 ²⁹Si/²⁷Al MAS NMR spectra of as-synthesized samples and H-type zeolites.

3.3 Acidity of H-type zeolites

The three H-type zeolites have slightly different SiO₂/Al₂O₃ ratios at 25 (H-MCM-22), 23 (H-MCM-49) and 24 (H-MCM-56). Generally speaking, the acid sites determined by base probes are influenced by the size of selected probes. In this section, base probes with different molecule sizes were selected to detect the acid sites of the three H-MWW zeolites. NH₃ was selected because of little diffusion restriction into MWW pore systems. Pyridine (kinetic diameter 0.58 nm) with identical kinetic diameter as benzene (kinetic diameter 0.58 nm) was selected to reveal reactant selectivity. As is well known, the size of benzene is smaller than that of 2,4,6-trimethylpyridine (kinetic diameter 0.74 nm). Similarly, the size of transition states in the EB formation process could be envisaged as elongated benzene structures, and therefore are much smaller than 2,4,6-trimethylpyridine, especially in comparison to the opening in the 12 MP cups. Based upon previous results, 2,4,6-trimethylpyridine was selected to determine the acid sites located in the 12 MR cups on the external surface of MWW zeolites, which were postulated to be main active centers for liquid-phase alkylation of benzene with ethylene.^31,47–49

Fig. 6 shows the NH₃-TPD curves of the H-type zeolites. The H-MWW zeolites exhibited two desorption peaks in the temperature range 200–250 °C and 380–420 °C, ascribed to NH₃ desorbed from weak and strong acid sites, respectively. The amounts of acid sites for H-MCM-22 and H-MCM-56 zeolites were similar to each other, but for H-MCM-49, there were higher amounts of both weak and strong acid sites than for H-MCM-22 and H-MCM-56. Py-FTIR results of the three H-MWW zeolites are presented in Table 3. The number of Lewis acid sites were 288 μmol g⁻¹ for H-MCM-22, 213 μmol g⁻¹ for H-MCM-49, and 312 μmol g⁻¹ for H-MCM-56 at 200 °C, 119 μmol g⁻¹ for H-MCM-22, 182 μmol g⁻¹ for H-MCM-49, and 260 μmol g⁻¹ for H-MCM-56 at 350 °C; the number of Brønsted acid sites were 178 μmol g⁻¹ for H-MCM-22, 352 μmol g⁻¹ for H-MCM-49, and 167 μmol g⁻¹ for H-MCM-56 at 200 °C, 152 μmol g⁻¹ for H-MCM-22, 316 μmol g⁻¹ for H-MCM-49, and 134 μmol g⁻¹ for H-MCM-56 at 350 °C. It is clearly seen that there were more Lewis acid sites over H-MCM-22 and H-MCM-56, although the number of total acid sites was similar to each other. However, H-MCM-49 showed much more Brønsted acid sites and less Lewis acid sites, which resulted in a much larger B/L ratio. The difference in acid sites originated from their difference in structure.

Fig. 6 NH₃-TPD curves of H-MCM-22, H-MCM-49 and H-MCM-56 zeolites.

Table 3 Acid properties of H-type zeolites

No.	XRD	200 °C			350 °C
		Lewis acid/μmol g^−1	Brønsted acid μmol g^−1	B/L	Lewis acid/μmol g^−1	Brønsted acid/μmol g^−1	B/L
4	H-MCM-22	288	178	0.62	119	152	1.28
5	H-MCM-49	213	352	1.65	182	316	1.73
7	H-MCM-56	312	167	0.54	260	134	0.52

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Brønsted acid sites are usually regarded as active centers for alkylation and more Brønsted acid sites would be beneficial for the improvement of zeolite-based catalysts’ activity. However in some cases, Lewis acid sites also contribute to catalytic performances either via synergistic effect with Brønsted acid sites or via direct conversion of reactants. In early studies, AlCl₃ was proved to be very active for the liquid-phase alkylation of benzene with ethylene. According to our previous data,^32,36,37 we are anticipating that the H-MCM-49 and H-MCM-56 with higher total amount of acid sites shows a superior alkylation performance.

Fig. 7 shows the FT-IR spectra of H-MCM-22, H-MCM-49 and H-MCM-56 zeolites before/after 2,4,6-trimethylpyridine adsorption. There were similar bands for all three H-type zeolites: the bands in the range 3740–3750 cm⁻¹ were assigned to terminal Si–OH groups on the surface of zeolites while the bands in the range 3605–3665 cm⁻¹ were ascribed to acidic bridged hydroxyls, which were located on the different inner pores (10 MR channels and 12 MR supercages via 10 MR windows) of H-type zeolites. Clearly, there was no significant disappearance of bands at 3605–3665 cm⁻¹ after 2,4,6-trimethylpyridine adsorption, demonstrating that the acidic bridged hydroxyls inside the pores (10 MR channels and 12 MR supercages via 10 MR windows) corresponding to the 3605–3665 cm⁻¹ bands could not be approached by 2,4,6-trimethylpyridine due to its high steric bulk.⁵⁰ According to previous results, 2,4,6-trimethylpyridine could only detect acid sites residing in 12 MR cups on the external surface of MWW zeolites.^47–49 Furthermore, adsorbed species then led to two new obvious bands at 1650 and 1633 cm⁻¹, assigned to 2,4,6-trimethylpyridine adsorbed on Brønsted and Lewis acid sites on the external surface 12 MR cups, respectively.⁵⁰ The amount of 2,4,6-trimethylpyridine was calculated by the height of adsorption band (1633 cm⁻¹) to determine the accessibility of acid sites, 23.0 μmol g⁻¹ for H-MCM-22, 34.9 μmol g⁻¹ for H-MCM-49 and 39.5 μmol g⁻¹ for H-MCM-56. Also, some reports have claimed that active centers useful for EB synthesis were mainly located in 12 MR cups on the outer surface of MWW zeolites, which do not restrict the diffusion of reactant/product toward/from the active centers.^47–49 Therefore it is strongly emphasized that H-MCM-56 with partially delaminated and a disordered collection of MWW monolayers and larger external surface area presented more 12 MR cups and the better accessibility of active centers, which would contribute to the higher catalytic performance.

Fig. 7 2,4,6-Trimethylpyridine spectra of H-type zeolites (a: H-MCM-22; b: H-MCM-49; c: H-MCM-56).

3.4 Alkylation performance over H-MWW catalysts

As a typical consecutive reaction, liquid-phase alkylation of benzene with ethylene (Scheme 1) has been extensively studied on both laboratory and industrial scale. Usually the ethylene conversion is almost 100% to ensure that no ethylene enters the trans-alkylation reactor in an industrial unit. If there was unconverted ethylene, fast deactivation will happen on ethylation and trans-alkylation catalysts. Also a benzene/ethylene ratio of 12, which is similar to that of a single ethylation stage with multi-ethylene feeding points, is enough to guarantee that all ethylene could dissolve in the benzene. Additionally, a benzene/ethylene ratio at 12 ensures pseudo-first-order reaction due to large excess of benzene over ethylene. High conversion of ethylene leads to dramatically decreased ethylene concentration, thus requiring higher activity to convert ethylene at much lower concentration, i.e. small variation in ethylene conversion and EB selectivity should be obtained.

Scheme 1 Reaction scheme of liquid-phase alkylation of benzene with ethylene.

The H-MCM-22 catalyst showed a stepwise increase of ethylene conversion from 89.8% at 200 °C to 96.6% at 260 °C in Fig. 8a, indicating that diffusion limitations may be one of the key factors to the activity of H-MCM-22. H-MCM-49 exhibited a higher ethylene conversion than H-MCM-22, from 96.7% at 200 °C to 99.2% at 260 °C. Ethylene conversion was not completely achieved up to 100% over H-MCM-22 catalyst even at a temperature of 260 °C, or even H-MCM-49 with higher activity. The lower alkylation activity over H-MCM-22 and H-MCM-49 catalysts might be related to their rose-like shape, smaller external surface and poor accessibility of active centers (less 12 MR cups exposed on the external surface) by 2,4,6-trimethylpyridine. The less the amount of 2,4,6-trimethylpyridine adsorption, the lower the ethylene conversion over H-MCM-22. For H-MCM-22 and H-MCM-49 with complete 3D structure we have three different pore systems. Although there were abundant active centers inside the 10 MR channels, they seemed to contribute little to EB formation. Both benzene (kinetic diameter 0.58 nm) and ethylene (kinetic diameter 0.39 nm) could penetrate the 10 MR channels, however they contributed little to the formation of EB due to transition-state selectivity. As for 12 MR supercages with 10 MR windows, reactants could penetrate 12 MR supercages through 10 MR windows, and 12 MR supercages were large enough to form the transition-state intermediate of EB, but there is some diffusion restriction on reactants/products. It is proposed that poor diffusion of reactants into 12 MR supercages through 10 MR windows was the key reason to the stepwise increase of ethylene conversion over H-MCM-22 and H-MCM-49.

Fig. 8 Ethylene conversion (%, a), EB selectivity (%, b), DEB selectivity (%, c) and TEB selectivity (%, d) over H-type catalysts (liquid-phase alkylation conditions: 8 mL catalysts, T = 200 °C to 260 °C, p = 3.5 MPa, benzene WHSV⁻¹ = 3.0 h⁻¹, benzene/ethylene molar ratio = 12.0).

H-MCM-56 showed higher ethylene conversion from 95.7% to 99.9% (from 200 to 260 °C) with the latter value higher than those of H-MCM-22 and H-MCM-49. According to the above characterized results for H-MCM-56, partially delaminated and a disordered arrangement of mono MWW layers could offer larger external surface areas, more acid sites and more opportunities for reactants to access the active centers. As presented in Table 2, the external surface area and volume of H-MCM-56 was clearly larger relative to the other H-MWWs, 145 m² g⁻¹ and 0.79 cm³ g⁻¹, which would offer more active centers accessed by reactants with less diffusion restriction. Although there was varying differences in the total acid sites for H-MCM-22, H-MCM-49 and H-MCM-56 by NH₃-TPD and Py-FTIR results, more acid sites accessed by 2,4,6-trimethylpyridine were observed for H-MCM-56 (39.5 μmol g⁻¹) than H-MCM-22 (23.0 μmol g⁻¹) and H-MCM-49 (34.9 μmol g⁻¹). All these factors contributed to the improved alkylation performance for the H-MCM-56 catalyst. Additionally, there was also a stepwise increase of ethylene conversion from 200 to 230 °C, indicating that there were also diffusion limits on ethylation of benzene but that this was less restricted than that over H-MCM-22 and H-MCM-49. On the other hand, this result suggested that H-MCM-56 without 12 MR supercages, through with 10 MR windows in idealized conditions had more 12 MR cups on the external surface. Similarly to H-MCM-22, no ethylation of benzene occurred in 10 MR channels of H-MCM-56 due to restriction of transitional state selectivity. Therefore the key of improving the ethylene conversion lies in how to tailor the diffusion restriction of 12 MR supercages through 10 MR windows to form more 12 MR cups exposed on the external surface of MWW zeolites, which led to a stepwise increase of ethylene conversion. According to above results, synthesis of mono-layered (delaminated) MCM-56 zeolite could increase the catalytic activity in liquid-phase alkylation of benzene with ethylene over MWW catalysts.

Low ethylene conversion over H-MCM-22 usually has two potential consequences because of consecutive reaction with intermediate product (EB) as the target product. One was that corresponding EB selectivity was highest from 96.5% at 200 °C to 95.3% at 260 °C (Fig. 8b), the other, meant less DEBs and TEBs produced over H-MCM-22 in Fig. 8c and d, especially no TEB formation at temperature range from 200 to 220 °C. Although improved ethylene conversion was observed over H-MCM-49, the EB selectivity decreased from 95.6% at 200 °C to 95.0% at 260 °C. For H-MCM-56, with the best activity, this showed the lowest EB selectivity from 95.1% at 200 °C to 94.1% at 260 °C and higher DEB and TEB formation than for H-MCM-22 and H-MCM-49, evincing the higher activity and poorer EB selectivity over H-MCM-56. As a consecutive reaction of benzene with ethylene, generally improved ethylene conversion, this is accompanied by decreased EB selectivity.^51,52 Therefore, we must carefully consider the trade-off between ethylene conversion and EB selectivity because of the intermediate EB as the target product. In our future work, we will focus on tailoring the crystallinity and morphology of H-MCM-56 to improve the regular aggregation of MWW layers with partially delaminated and disordered packing of mono MWW layers, which might be beneficial for the improvement of EB selectivity. The most effective achievement for tailoring MWW zeolites would enhance the ethylene conversion reaching up to 100% without any loss of EB selectivity, or achieve the enhancement of both the ethylene conversion and EB selectivity to break out the “see-saw” in the consecutive reaction.

4. Conclusion

MCM-22, MCM-49 and MCM-56 were synthesized with HMI as the structure-directing agent and aniline as the structure-promoting agent. As structure-promoting agent, aniline obviously deceased the usage of HMI compared to conventional HMI route. Meanwhile it contributed to the formation of MWW zeolites without being trapped within MWW zeolites. In other words, aniline was useful during crystallization, however it was not consumed during the temperature-controlled phase-transfer synthesis. With appropriate recycling the non-consumption of aniline could be achieved. The key lies in the increasing solubility of aniline with temperature which facilitates the temperature-controlled phase-transfer hydrothermal synthesis, and most of HMI and aniline could be recovered by phase separation after crystallization. As for MCM-56, the first mono-layered (delaminated) zeolite obtained by direct synthesis, it remains the only one prepared with simple templates. The problem of MCM-56 synthesis lies in its intermediate nature and conversion to MCM-49 if not quenched at the correct time. In this contribution, MCM-56 with partially delaminated and disordered MWW monolayers was synthesized in HMI/AN, fumed SiO₂ as silica source and high H₂O/SiO₂ system. There was no other competing phase in this MCM-56 synthesis process. The most important advance is that the conversion from MCM-56 to MCM-49 could be avoided even subject to much longer crystallization time. Hence we have provided a facile and reproducible method to generate MCM-56 with reduced economical and environmental impact. Because of a partially delaminated and disordered collection of MWW monolayers, MCM-56 showed larger external surface area (145 m² g⁻¹) and pore volume (0.79 cm³ g⁻¹), which would have less diffusion restriction of reactants to enhance its catalytic activity. The acidity characterization results by different probe molecules indicate that MCM-56 had better accessibility of acid sites by 2,4,6-trimethylpyridine, that is in accordance with the fact that H-MCM-56 possessed more 12 MR cups exposed on the external surface, resulting in higher ethylene conversion in liquid-phase alkylation of benzene with ethylene.

Acknowledgements

This work was supported by the National Basic Research Program of China (973 Program, No. 2012CB224805). Special thanks to the Department of Analysis in Research Institute of Petroleum Processing, Sinopec.

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