Dual template preparation of MFI zeolites with tuning catalytic

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Materials and Interfaces

Dual template preparation of MFI zeolites with tuning catalytic properties in alkylation of mesitylene with benzyl alcohol Xin Yan, Baoyu Liu, Jiajin Huang, Ying Wu, Huiyong Chen, and Hongxia Xi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04783 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 9, 2019

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Industrial & Engineering Chemistry Research

Dual template preparation of MFI zeolites with tuning catalytic properties in alkylation of mesitylene with benzyl alcohol Xin Yana,‡, Baoyu Liub,‡,, Jiajin Huangb, Ying Wua, Huiyong Chenc, Hongxia Xia, aSchool

of Chemistry and Chemical Tecnology, South China University of Technology, Guangzhou, Guangdong 510640, P.R. China bSchool

of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, Guangdong 510006, P.R. China cSchool of Chemical Engineering, Northwest University, Xi'an, Shaanxi 710069, PR China



Corresponding author. Tel.: +86 020-39322237, Email: [email protected] Corresponding author. Tel.: +86 020-87113501, Email: [email protected] ‡ Xin Yan and Baoyu Liu equally contributed to the work. 1 

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Abstract The meso- and microporous ZSM-5 zeolites were synthesized through combination of the tetrapropylammonium hydroxide (TPAOH) and single quaternary ammonium surfactant (Cph–ph–10–6) as dual templates. The structure-directing ability of Cph–ph–10–6 and TPA+ was investigated by DFT calculation. By tuning the molar ratios of Cph–ph–10–6/TPAOH from 5/0 to 5/8, the structure-directing effects between Cph–ph– 10–6

and TPA+ could be systematically modulated, resulting in the morphology of the

mesoporous ZSM-5 zeolites changed from ultrathin nanosheets, through splint-like nanosheets, to condensed packing plates. In addition, the hierarchical ZSM-5 zeolites exhibited a greater catalytic activity in alkylation of mesitylene with benzyl alcohol compared with conventional ZSM-5 zeolite. The fine control of meso- and microporous structure of ZSM-5 zeolites by a simple one-step dual template synthesis approach allows the design of the desired catalysts for a green and sustainable future. Keywords: Dual Templates; ZSM-5; Tuning Acidity; Meso-/Microporosity; Alkylation 1. Introduction Friedel-Crafts alkylation of aromatics is a class of important reactions in the field of petrochemical and fine chemical industry1. Traditionally, these reactions were performed under homogeneous acid catalysts such as HF, H2SO4 and AlCl32, which suffer from the serious issues such as corrosion, high toxicity, and separation and recovery of the catalyst3. Thus, it is necessary to develop a novel of cleaner production in order to efficiently utilize the resources. The substitution of current 2


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homogeneous catalysts by heterogeneous solid catalysts is an effective strategy4 . Among the heterogeneous Friedel-Crafts alkylation catalysts, zeolite catalysts are extensively used in industrial process due to the intrinsic acidity, high stability, shape selectivity and easy separation from reaction mixture5, 6. However, the wide application is hindered by the limited accessibility of micropores, e.g. catalysis involved bulky molecules in Friedel-Crafts alkylation of aromatics7. To this end, hierarchical zeolites integrating the accessibility for larger molecules and the feasibility of reactions between reactants and active acid sites, is a promising candidate to overcome this issue. Numerous works have been devoted to fabricate hierarchical zeolites8-16, most of which create mesopores by post-synthesis dealumination and desilication from intrinsic frameworks17, 18. However, it offers only mediocre effects to obtain ordered structures. Alternatively, hard/soft-templating approach is a facile and beneficial strategy to prepare targeted zeolites with promising performance19-22. With few amount of ingredients, especially the quaternary ammonium compounds, the approach is able to distinctly guide the production of zeolites with desired pore size23-25. A remarkable breakthrough in preparing single-unit-cell nanosheets ZSM-5 zeolites was successfully carried out by Ryoo et al.16, who synthesized functional ZSM-5 zeolite nanosheets by employing exclusive amphiphilic structure-directing agents (SDAs) C22–6–6. Based on the material, tunable hierarchical zeolites were subsequently produced by changing the length of hydrophobic alkyl chains and the number of nitrogen atoms13,

16, 25-27.

Che et al. designed the novel amphiphilic 3



surfactant, which was bridged by aromatic groups to induce nanosheets self-assembly via π-π stacking interaction25,

26, 28.

Srivastava. et al. prepared a series of

1,4-diazabicyclo[2.2.2]octane with multiquaternary ammonium functional groups, these surfactants exhibited an effective strcuture-directing ability for synthesis of ZSM-5 nanosheets, which were composed of extremely thin slice of ZSM-5 zeolite crystals (~ 10 nm along b axis)29. Compared to these methods, dual-template synthesis of hierarchical materials consumes less ingredients and synthesis time. In past decades, the dual-template strategies have been intensively developed30-33. Emdadi et al.32 has synthesized nanosheet-assembled ZSM-5 zeolites with tailoring activity in benzylation of mesitylene with benzyl alcohol by using dual templates (Gemini-type surfactant and TPAOH). The hydrophobic alkyl chains of Gemini-type surfactant effectively limit the growth of ZSM-5 nanosheets, while TPAOH plays a facile role to induce zeolite precursors self-pillaring to strengthen interlamellar structure. Similarly, hierarchical matericals based on ZSM-5 nanosheets intertwined growth were also obtained by using dual-functional surfactant (C16–6–6Br2) and the progressive addition of cetyltrimethyl ammonium bromide (CTAB), without excessive collapse after calcinations33. In addition, Srivastava. et al developed a novel of dual template mediated synthesis approach for preparation of nanocrystalline zeolites with nanosheet-like morphology, the zeolite frameworks can be modulated by tuning the type of micropore structure-directing agent34. These works highlight the outstanding performance of dual template strategy in preparing hierarchical zeolites. In this work, inspired by the structure of meso- and microporous ZSM-5 zeolite 4


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nanosheets with tailored performances, we investigated hierarchical ZSM-5 zeolites as green solid acid catalysts for Friedel–Crafts alkylation of mesitylene with benzyl alcohol, and the desired hierarchical ZSM-5 zeolite catalysts were synthesized by using a single head-group quaternary ammonium surfactant with rigid benzene rings [C6H5–C6H4–O–C10H20–N(CH3)2–C6H13][Br-] (designed as Cph–ph–10–6) as well as an addition of increasing assisted template TPAOH to tailor the hierarchical structure of zeolites. The influences of ratios of dual templates on morphology, textural parameters and acid sites over zeolites were investigated by a series of characterization techniques. In addition, the DFT calculation was carried out to unravel the complicated structure-directing action between Cph–ph–10–6 and TPAOH, which expect to understand why the Cph–ph–10–6 and TPAOH can be utilized in a concerted manner to tune the morphology of ZSM-5 zeolites. Moreover, the resultant hierarchical ZSM-5 zeolites exhibited the superior catalytic performances compared with conventional ZSM-5 zeolite in alkylation of mesitylene and benzyl alcohol due to the additional mesopores, and the activity of benzyl alcohol and selectivity of alkylated products can be systematically tailored by tuning the molar ratios between Cph–ph–10–6 and TPAOH. These observations imply that the present developed hierarchical ZSM-5 zeolites may find a cleaner production for alkylation of aromatics as the green catalysts. 2. Experimental 2.1. Synthesis of organic surfactant The bifunctional surfactant with strong π-π stacking effect derived from the 5



interaction of rigid benzene rings was synthesized according to a literature procedure26. Typically, first 0.01 mol of 4-phenylphenol and 0.1 mol of 1,10-dibromodecane (97%, J&K) were dissolved in 100 ml of ethanol involving 0.6 g of KOH. The precipitate [C6H5–C6H4–O–C10H20][Br] (simply named Cph–ph–10–0) was obtained by sequential filtration and washing with diethyl ether after the mixture refluxed and stirred at 85 oC for 20 h under the atmosphere of N2. Then the precipitate Cph–ph–10–0 was dried in vacuum environment at 50 oC for 10 h. Second, 0.01 mol of the Cph–ph–10–0 and 0.01 mol of N,N-dimethylhexylamine were dissolved in 80 ml of acetonitrile, as well as refluxed and stirred at 95 oC for 20 h. The final product [C6H5– C6H4–O–C10H20–N(CH3)2–C6H13][Br-] specified Cph–ph–10–6 was filtered and washed with diethyl ether at room temperature, dried in vacuum environment at 50 oC for a whole night. The assisted template, tetrapropylammonium hydroxide (TPAOH, 40% of water), was purchased from J&K. 2.2. Synthesis of mesostructured ZSM-5 zeolites In a typical synthesis recipe (Table 1), a homogeneous mixture, NaOH, tetraethyl orthosilicate (TEOS, 98%, J&K), NaAlO2 (44.7 wt% Na2O, 52 wt% Al2O3, J&K), CPh–Ph–10–6, tetrapropylammonium hydroxide (TPAOH, 40% of water, J&K), and H2O with corresponding ratio of 25: 100: 2: 5: x: 4000 (x = 0, 1, 2, 4, 8), was continuously heated and stirred at 60 oC for 10 h. Then, the resultant gel was shifted into the Teflon-lined stainless steel autoclave and placed into autoclave to further crystallize with tumbling at 40 rpm and 150 oC for 5 days. After crystallization, the zeolite samples were filtered, washed with de-ionized water, dried in air, and calcined at 550 6


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oC

for 6 h to remove the template. The ultimate products, mesostructured ZSM-5

zeolites were named ZSM-5/0, ZSM-5/1, ZSM-5/2, ZSM-5/4 and ZSM-5/8, following the increased molar ratio of CPh–Ph–10–6/TPAOH, respectively. In addition, the conventional ZSM-5 zeolite (CZSM-5) was purchased from Nankai University Catalyst Co.,Ltd. 2.3. Ion-exchange of mesostructured ZSM-5 zeolites After removal of dual template at high temperature, ZSM-5/x samples were ion-exchanged three times by employing 1 M NH4NO3 solution at 80 oC for 10 h. Then, these processed samples were washed and filtered with deionized water and dried at 120 oC for 10 h, and then calcined under 550 oC for 5 h. The aim of ion-exchange was to obtain H+ form mesostructured zeolites. Finally, the H+ form samples (denoted as HZSM-5/x) were employed to examine their catalytic properties in liquid phase benzylation of mesitylene and benzyl alcohol. 2.4. Characterization Powder XRD patterns were analyzed by a Bruker D8 ADVANCE diffractometer with a CuKα radiation (operation at 40 kV, 40 mA, λ = 0.15418 nm). Scanning electron microscope (SEM) analysis on SU8220 was operated at 15 kV. Transmission electron microscope (TEM) analysis was performed on JEM-2010, operated at 200 kV. N2 adsorption-desorption isotherms were measured with ASAP 2020 at -196 oC after pretreating at 250 oC for over 6 h. The apparent surface area of samples was counted by the Brunauer-Emmett-Teller (BET) equation and the pore size distribution was calculated from the adsorption branch of the isotherm using the non-local Density 7



Functional Theory (NLDFT) method. Thermogravimetric analysis was performed by TG-209 from room temperature to 800 oC with the ratio of 5 oC /min under air atmosphere. Infrared spectroscopy was recorded on Bruker Vector 33 in a range of 4000 cm-1–400 cm-1. The IR-spectroscopy of pyridine adsorbed on zeolites was employed to determine the varieties, intensities, and quantities of acid sites on surface of zeolites. Before the IR measurements, each sample was degassed at 500 oC for 2 h, and then pyridine was adsorbed on the degassed sample for 1 h at ambient temperature. These samples were desorbed at 250 oC for the IR test corresponding with 2 cm-1 resolution and 40 times scanning in range of 4000 cm-1–400 cm-1. The molar extinction coefficient ε = 1.88 cm μmol-1 at υ = 1545 cm-1 and ε = 1.42 cm μmol-1 at υ = 1455 cm-1 were employed to calculate the concentrations of Brønsted acid sites and Lewis acid sites of zeolites, respectively. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used to measure Si and Al contents in the framework of samples. 2.5. Computer calculation The DFT (density functional theory) was employed to calculate the molecular properties of CPh–Ph–10–6 surfactant and TPA+, which was performed on a Gaussian 03 program under B3LPY level with 6-31G*. More detailed information involving the calculation could refer to our previous literatures35, 36. Here, the present research didn't discuss it. 2.6. Catalytic reaction The liquid-phase alkylation of mesitylene and benzyl alcohol was performed in 8


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three-necked flask equipped with a reflux condensation with stirring at 100 oC. Typically, first 4.8 g (40 mmol) of excess mesitylene and 100 mg of proton form HZSM-5/x catalyst were added to the flash with 0.5 g n-dodecane primarily holding 30 min. Then, it was marked at the initial reaction time when 0.433 g (4 mmol) of benzyl alcohol was added immediately. Liquid samples were withdrawn at regular intervals and analyzed by Gas Chromatographic (Agilent 7890A) equipped with a polyorganosiloxane capillary column (DB-264, 30.0×320 μm, 1.80 Micron-20 to 260C) connected to a flame ionization detector (FID). 3. Results and discussion 3.1 Characterization of mesostructured ZSM-5 zeolites The high-angle region of XRD patterns (Fig.1) clearly showed that the characteristic peaks of as-synthesized ZSM-5/x zeolites were in extreme accordance with conventional ZSM-5, revealing that the high-crystallization ZSM-5/x zeolites were successfully prepared by dual templates with Cph–ph–10–6/TPAOH. All ZSM-5/x samples exhibited distinct intensity of diffraction peaks with changing x values. Especially, ZSM-5/2 and ZSM-5/4 with adequate content of TPAOH showed the highest peak attributed to well-defined crystallization, and the corresponding crystallinity of resultant zeolites were listed in Table S1. These results indicated that the single-head quaternary ammonium surfactants could induce nucleation of ZSM-5 zeolites, and the additional TPAOH templates never destroyed the framework of zeolites. The present results suggested that the additional TPAOH can efficiently promote zeolite precursors to self-grow between layers, which reserved spatial 9



mesoporous system to inhibit the partial collapses as removing organic templates at higher temperature32, 33. SEM images (Fig. 2) showed the straightforward appearance of samples prepared by various molar ratios of Cph–ph–10–6/TPAOH. The ZSM-5/0 sample (Fig. 2a) was composed of disordered flake-like nanosheets, similar to the results reported by Che et al.32. The TEM image of ZSM-5/0 zeolite (Fig. 2b and Fig. S1) showed the inter framework structure fabricated by ultrathin nanosheets assembling. It indicated that Cph–ph–10–6 as a dual-functional quaternary ammonium surfactant played a role on the growth of hierarchical zeolites with uniform phase and well-developed mesoporosity, analogical to the multilamellar ZSM-5 zeolites26. The Cph–ph–10–6 was composed of a biphenyl group (C6H5–C6H4–O–) and a single quaternary ammonium head group connected by a C10 alkyl chain. The biphenyl group served as a space-packing agent to limit excessive zeolite crystal growth, resulting in the formation of interlayer mesopores for ZSM-5 zeolites. The ZSM-5/1 sample turned thicker and intergrowed along one direction as viewed (Fig. 2c), corresponding to the TEM result as shown in Fig. 2d. The ZSM-5/2 sample showed a special surface morphology (Fig. 2e), splint-like nanosheets regularly stacked to construct bulky particles (Fig. 2f) due to the interaction between hydrophobic alkyl chains of template Cph–ph–10–6 and TPAOH. With further adding TPAOH component into synthesis recipes, the ZSM-5/4 and ZSM-5/8 zeolites ultimately assembled into homogeneous bulky particles, as shown in Fig. 2g-j. These results extremely confirmed that TPAOH as an effective structure-assisted agent can induce zeolites to generate three 10


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dimensional crystalline frameworks, just accordant with Liu et al. reported experimental data 32. N2 adsorption-desorption isotherms and NLDFT (nonlocal density functional theory) pore size distributions of HZSM-5/x samples were robust evidence for the coexistence of meso- and microporositiy, as shown in Fig. 3. Meanwhile, the specific pore properties of samples were listed in Table 2. As the Fig. 3a shown, the HZSM-5/x zeolites exhibited the representative type-IV isotherms with H3 type hysteresis loop due to capillary condensation at the relative pressure extent about 0.42 to 1.0 corresponding to desirable mesoporous volumes32,

37.

In comparison, the

conventional ZSM-5 zeolite showed type-I isotherms owing to the sole presence of micropores. The pore distributions of CZSM-5 and HZSM-5/x samples (Fig. 3b) were derived from the adsorption branch by NLDFT method. The textural properties of these samples were listed in Table 2. It was noted that micropore sizes of HZSM-5/x were equally centered at ~0.56 nm and mesopore sizes of HZSM-5/1 to HZSM-5/8 were approximately occupied at 2.0–3.0 nm. It was worth mentioning that HZSM-5/0 sample synthesized by dual-functional template Cph–ph–10–6 had inferior BET surface area (229 m2/g), which was different from the data reported by Che et al. (395 m2/g)26. In addition, HZSM-5/0 sample exhibited a broad pore size distribution due to the disordered assembly of nanosheets compared with other samples. However, the micropore volumes of all HZSM-5/x zeolites were lower than CZSM-5 owing to the limitation growth by meso-templates. Moreover, The Sext/SBET of HZSM-5/x zeolites 11



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showed similar volcano-shaped trend in Table 2. These results revealed that TPAOH was an effective assisted template to systematically tailor textural properties of hierarchical ZSM-5 zeolites. Table 3 gave the Si/Al ratios of HZSM-5/x and CZSM-5 obtained from the analysis of ICP-AES. Besides, as a comparison, the theoretical Si/Al ratios were also provided based on the data shown in Table 1. It was observed that the theoretical Si/Al ratios were consistent with experimental data, indicating that silica and aluminum species were successfully incorporated into the framework of zeolites. Moreover, the IR spectrum of pyridine adsorbed on protonated sample was a familiar method to evaluate the type and quantity of acid sites. The characteristic absorption bands of acid sites were obtained after completely removing weakly- and physically-desorbed pyridine at 250 oC. It was worth noting that the charge interaction between H+ and pyridine in the form of “Al‒OH + : B ⇌ Al‒O‒ + H: B+” (B = pyridine), which was characteristic of the Brønsted acid sites at ~1545 cm-138,

39.

Meanwhile, the characteristic absorption band of Lewis acid sites was around 1455 cm-1 that was attributed to the interaction between pyridine and unsaturated coordinated Al ions38, 39. As can be seen from Fig. 4, all the investigated samples were in the coexistence of two typical adsorption bands belonging to Brønsted and Lewis acid sites. Apparently, the concentration of Brønsted acid sites recorded in Table 3 was higher than Lewis acid sites. For HZSM-5/0, the concentration of Brønsted acid sites (0.06 mmol/g) was lower than the average level (0.1012 mmol/g) due to the lowest Si/Al ratio. Meanwhile, the concentration of Brønsted acid sites of HZSM-5/x 12


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zeolites showed a plausible volcano-like trend (HZSM-5/0 < HZSM-5/1 > HZSM-5/2 > HZSM-5/4 > HZSM-5/8) with increasing TPAOH, indicating that the varying TPAOH ratios in the dual template synthesis recipe resulted in tunable acidity in the obtained mesoporous ZSM-5 zeolites. In addition, NH3-TPD (NH3 temperature-programmed desorption) curves of synthesized zeolites were further carried out to determine acidity of samples (Fig. S2). It was observed that all the samples exhibited a low-temperature peak at about 210 oC and a high-temperature broad peak at ~ 420 oC, which was attributed to the weak acid sites and strong acid sites, respectively. The

curve shape of synthesized HZSM-5/x samples was similar

to that of CZSM-5,

indicating that the acidic strength of synthesized zeolite

samples was similar to that of conversional ZSM-5 zeolite40, which was important for catalytic application. Fig. 5 exhibited the FTIR spectra of the calcined ZSM-5/x samples. All of samples showed the vibrational bands at about 450 cm-1 and 554 cm-1, which was attributed to the bending vibration of Si‒O‒Si bond and the five- and six-rings of Si‒O‒Si or Si‒O‒Al on behalf of ZSM-5 type zeolite, respectively41, 42. Fig. 6 gave the thermogravimetric curves of as-synthesized ZSM-5/x zeolites obtained with Cph– ph–10–6/TPAOH

molar ratios from 5/0 to 5/8. As can be seen from Fig. 6, the weight

loss below 100 oC was attributed to physically adsorbed water in zeolites. A slight weight loss in the range about 100 oC to 250 oC was assigned to the evaporation of intercrystalline water within framework of zeolites43,

44.

The weight loss at higher

temperature range of 250‒550 oC was ascribed reasonably to the decomposition of 13



organic templates. However, the ZSM-5/x samples gave the different weight loss for decomposition of organic templates with changing Cph–ph–10–6/TPAOH molar ratios. With the increasing of TPA+, the the weight loss of samples decreased in the order of ZSM-5/0 > ZSM-5/2 > ZSM-5/4 > ZSM-5/8. One of the reasonable explanation was that TPA+ acted as a very effective SDA to generate bulk ZSM-5 crystals rapidly, overwhelming the zeolite structure-directing functions of Cph–ph–10–645. Thus, increasing TPAOH value in dual templates decreased the amount of Cph–ph–10–6 incorporated into the framework of zeolites, which resulted in the lower weight loss for synthesized samples. However, ZSM-5/1 was out of this trend, which exhibited the largest weight loss due to the appropriate TPA+ in synthesis gel that can induce formation of the strong cooperative interaction between TPA+ and Cph–ph–10–6, leading to a larger pore volume (0.25 cm3/g)34, 46. To further investigate the synergetic interaction between Cph–ph–10–6 and TPA+, the LUMO (the lowest unoccupied molecular orbital) and MEP (molecular electrostatic potential map) of Cph–ph–10–6 and TPA+ were calculated based on the DFT method, as shown in Fig. 7. For both Cph–ph–10–6 and TPA+, the distributions of LUMO were mainly centered at ammonium groups, and the positive area also focused on the ammonium groups derived from MEP analysis, which meant that ammonium groups were the most susceptible area for nucleophilic attacks with anionic aluminosilicate species47. In addition, the ELUMO value of Cph–ph–10–6 (-0.15707 eV) was similar to that of TPA+ (-0.15007 eV), indicating that they had comparable ability to accept electrons48. However, the value of E (E = EHOMO – ELUMO) for Cph–ph–10–6 (-0.19061 14


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eV) was larger than that of TPA+ (-0.40663 eV), which suggested that the TPA+ was more easily participated in crystallization process than Cph–ph–10–6. In other words, the structure-directing ability of TPA+ was stronger compared with Cph–ph–10–6. These analyses were consistent with recent Ryoo et al.’ experimental results that the TPA+ guided the formation of ZSM-5 zeolite as a very effective SDA, overwhelming the zeolite structure-directing functions of quaternary ammonium-type surfactant45. The present research revealed the different structure-directing ability of Cph–ph–10–6 and TPA+ in a mixture, and the morphology of ZSM-5 zeolites can be modulated through deliberately tuning the molar ratios between Cph–ph–10–6 and TPA+. 3.2 Catalytic performance The catalytic properties of HZSM-5/x samples were evaluated in liquid phase alkylation of mesitylene with benzyl alcohol49. As shown in Fig. 8, this reaction mainly produced dibenzyl ether (DBE) and 2-benzyl-1,3,5-trimethylbenzene (BTMB) in parallel reaction pathways. The BTMB is target product, which is an important for production of pharmaceutical intermediates and fine chemicals46. The conversion of benzyl alcohol over all HZSM-5/x samples was much higher than that of conventional ZSM-5 zeolite as shown in Fig. 9a, indicating that presence of mesopores in HZSM-5/x samples enhanced the accessibility of reactants50. However, the conversion of benzyl alcohol over HZSM-5/x samples followed the order: HZSM-5/8 < HZSM-5/4 ≈ HZSM-5/2 < HZSM-5/1 < HZSM-5/0. These observations did not show a linear dependence with textural properties of the zeolite catalysts, which was different previous report that the catalytic activity was consistent with the textural 15



properties of zeolites51. A plausible explanation was that the etherification reaction of benzyl alcohol can occur on both internal and external active sites of zeolites52, Thus, the fraction of external surface of investigated zeolites was not a crucial factor to determine the activity of benzyl alcohol. In addition, the selectivity of desired BTMB for all HZSM-5/x samples versus reaction time was shown in Fig. 9b. It was noted that the selectivity of BTMB was almost zero for HZSM-5/2, HZSM-5/4 and HZSM-5/8 samples in the initial stage of reaction, which was attributed to the condensed structure of zeolites as shown in Fig. 2 owing to the excess amount of TPAOH in synthesis recipe. Compared to Cph–ph–10–6, the structure-directing action of TPA+ was too fast for the growth of the zeolite framework, inducing the formation of bulk ZSM-5 zeolites. However, the molecular size of mesitylene was almost ~0.87 nm45 that was larger than the pore size of ZSM-5 zeolite (~0.56 nm). Thus, the condensed structure of HZSM-5/2, HZSM-5/4 and HZSM-5/8 samples was unfavorable for alkylation of mesitylene, resulting in the low selectivity of BTMB. With the time increasing, dibenzyl ether gradually participated in the alkylation reaction acted as the alkylating agent50, which leaded to an increasing selectivity of BTMB for all HZSM-5/x samples after reaction 4 h. In order to further investigate the correlation between catalytic properties and hierarchical structure of obtained zeolites. Fig. 9c showed conversion of benzyl alcohol and selectivity of BTMB versus relative external surface area of HZSM-5/x samples. It was worth noting that selectivity of BTMB exhibited a plausible volcano trend with increasing the Sext/SBET of HZSM-5/x catalysts, suggesting that the 16


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selectivity of target alkylated product can be systematically tailored by tuning the molar ratios of Cph–ph–10–6/TPAOH. However, the conversion of benzyl alcohol decreased with increasing the Sext/SBET of HZSM-5/x samples. This phenomenon indicated that the acidity of zeolites was another key factor to determine activity of benzyl alcohol3. Subsequently, the conversion of benzyl alcohol and Brønsted/Lewis acid site ratio versus HZSM-5/x samples gave a similar trend as shown in Fig. 9d, which revealed that the acidity of zeolites severely influenced the activity of benzyl alcohol. It was well known that the sole Brønsted acid sites were benefit for activation of benzyl alcohol to produce the dibenzyl ether, while the additional Lewis acid sites can effectively inhibit the intermolecular dehydration of benzyl alcohol3. The synergy between Brønsted and Lewis acid sites can enhance the catalytic performances of alkylation between mesitylene and benzyl alcohol49. From this view, the well-designed hierarchical ZSM-5 zeolites had great potential can be used in Friedel– Crafts alkylation of aromatics with benzyl alcohol as the green solid acid catalysts, which met the requirements of cleaner production. 4. Conclusion In summary, the hierarchical ZSM-5 zeolites were synthesized in a one step by combing a single quaternary ammonium surfactant (Cph–ph–10–6) with TPAOH. DFT calculation showed that the TPA+ had higher structure-directing ability for ZSM-5 zeolite compared with Cph–ph–10–6, but they possessed the comparable ability to accept the electrons, these results verified that the Cph–ph–10–6 and TPA+ can be used in a concerted way to tailor the morphology. Using this principle, the textural parameters 17



of hierarchical ZSM-5 zeolites were systematically modulated by tuning the molar ratios of Cph–ph–10–6/TPAOH from 5/0 to 5/8. As a consequence, the HZSM-5/x samples exhibited a tunable catalytic performance in alkylation of mesitylene with benzyl alcohol. Moreover, the HZSM-5/1 showed the best catalytic performance in alkylation of mesitylene with benzyl alcohol in present research. These insights observed in this work have important implication for potential industrial application, as well as catalysts design, a balance between the acidity and meso- and microporous of zeolites can be achieved by the dual template synthesis strategy to obtain the desired catalytic properties. This also indicates that the hierarchical ZSM-5 zeolites with tunable properties will definitely promote clean production technology in the field of Friedel-Crafts alkylation as next step green catalysts. Supporting Information TEM images of ZSM-5/0 samples; NH3 temperature-programmed desorption curves of synthesized ZSM-5/x samples and commercial ZSM-5 zeolite; the relative crystallinity of ZSM-5/x zeolites. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 21808040, 21436005, 21506066 and 21576094), the Science and Technology Program of Guangzhou, China (201804010172), and the National High Technology Research and Development Program of China (No. 2013AA065005), SRFDP (No.20130172110012) were gratefully acknowledged. References 18


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Figures

Figure. 1 High-angle XRD patterns of ZSM-5/x and conventional ZSM-5, x was changed from 0 to 8.

22


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Figure. 2 SEM images (left) and TEM images (right) of ZSM-5 zeolites obtained with Cph–ph–10–6/TPAOH molar ratio of (a, b) 5/0, (c, d) 5/1, (e, f) 5/2, (g, h) 5/4, (i, j) 5/8, respectively. 23



Figure. 3 N2 adsorption-desorption isotherms (a) and NLDFT pore size distributions (b) of ion-exchanged samples HZSM-5/x (x = 0, 1, 2, 4, 8) and conventional ZSM-5 zeolites.

Figure. 4 The IR spectrum pyridine adsorbed on protonated HZSM-5/x and conventional ZSM-5.

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Figure. 5 FTIR spectra of HZSM-5/x zeolites obtained with Cph–ph–10–6/TPAOH molar ratios from 5/0 to 5/8.

Figure. 6 TGA curves of ZSM-5/x zeolites as-synthesized with Cph–ph–10–6/TPAOH molar ratios from 5/0 to 5/8.

25



Figure. 7 Molecular orbital surfaces, energy levels and molecular electrostatic potential map of Cph-ph-10-6+ and TPA+.

Figure. 8 Friedel-Crafts alkylation of mesitylene with benzyl alcohol.

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Figure. 9 Conversions of benzyl alcohol in benzylation with mesitylene (a) and selectivity (α = Cc/2CE) determination for mesitylene alkylation reaction [CE is dibenzyl

ether

concentration,

Cc

is

concentration

of

1,3,5‒trimethyl‒2‒benzylbenzene] (b) over HZSM-5/x and conventional ZSM-5 zeolites respectively. (c) Conversions of benzyl alcohol and selectivity of alkylation reaction versus relative external surface area of HZSM-5/x and conventional ZSM-5 zeolites, (d) Conversions of benzyl alcohol and Brønsted/ Lewis acid site ratio versus HZSM-5/x and conventional ZSM-5 zeolites.

Table 1 Molar ratios of compositions in synthesis gel of hierarchical ZSM-5 zeolites. Samples

Molar ratios NaOH

SiO2

NaAlO2

CPh–Ph–10–6

TPAOH

H2O

ZSM-5/0

25

100

2

5

0

4000

ZSM-5/1

25

100

2

5

1

4000

ZSM-5/2

25

100

2

5

2

4000

ZSM-5/4

25

100

2

5

4

4000

ZSM-5/8

25

100

2

5

8

4000

Table 2 Textural properties of hierarchical HZSM-5/x 27



Page 28 of 29

and conventional ZSM-5 zeolites. Sample

aV

3 micro(cm /g)

bV

3 meso(cm /g)

cV

3 total(cm /g）

dS

2 ext(m /g)

eS

2 BET(m /g)

Sext/SBET

CZSM-5

0.110

0.06

0.170

96

339

0.283

HZSM-5/0

0.066

0.17

0.238

62

229

0.271

HZSM-5/1

0.090

0.16

0.250

173

375

0.461

HZSM-5/2

0.080

0.16

0.240

198

374

0.529

HZSM-5/4

0.067

0.12

0.187

225

380

0.592

HZSM-5/8

0.070

0.14

0.210

209

364

0.574

aDetermined dExternal

from t -plot method; bVmeso = Vtotal – Vmic; cTotal pore volumes is obtained at P/P0 = 0.95; surface area Sext = SBET ‒ Smic; eTotal surface area by Brunauer-Emmett-Teller (BET) method.

Table 3 The Si/Al ratio and acidity of HZSM-5/x and conventional ZSM-5 catalysts. Sample

Si/Ala

Si/Alb

Total Al sitesc

B-sitesd (mmol/g)

L-sitesd (mmol/g)

(mmol/g) CZSM-5

50

49.4

0.370

0.131

0.080

HZSM-5/0

50

45.2

0.630

0.060

0.023

HZSM-5/1

50

54.5

0.267

0.140

0.019

HZSM-5/2

50

55.9

0.350

0.121

0.017

28


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HZSM-5/4

50

66.7

0.299

0.116

0.016

HZSM-5/8

50

53.8

0.365

0.069

0.020

aThe

theoretical Si/Al ratios of as-synthesized HZSM-5/x samples and conventional ZSM-5 zeolites calculated by

molar ratio in the synthesis recipe; bThe actual Si/Al ratios of as-synthesized ZSM-5/x samples and conventional ZSM-5 zeolites calculated by ICP-AES; cThe content of framework and non-framework Al sites determined by ICP-AES; dCalculated from IR spectra of adsorbed pyridine.

For Table of Contents Only

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Dual template preparation of MFI zeolites with tuning catalytic

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