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Template-free Synthesis and Catalytic Applications of Microporous and Hierarchical ZSM-5 Zeolites from Natural Aluminosilicate Minerals Yuanyuan Yue, Liliang Gu, Yanni Zhou, Haiyan Liu, Pei Yuan, Haibo Zhu, Zhengshuai Bai, and Xiaojun Bao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02531 • Publication Date (Web): 23 Aug 2017 Downloaded from http://pubs.acs.org on August 29, 2017

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Industrial & Engineering Chemistry Research

Template-free Synthesis and Catalytic Applications of Microporous and Hierarchical ZSM-5 Zeolites from Natural Aluminosilicate Minerals Yuanyuan Yue,† Liliang Gu,† Yanni Zhou,‡ Haiyan Liu,‡ Pei Yuan,*,† Haibo Zhu,† Zhengshuai Bai§ and Xiaojun Bao*,†,§

†

National Engineering Research Center of Chemical Fertilizer Catalyst, College of Chemical

Engineering, Fuzhou University, Fuzhou 350002, P. R. China. ‡

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249,

P. R. China. §

State Key Laboratory of Photocatalysis on Energy & Environment, Fuzhou University, Fuzhou

350116, P. R. China.

1 ACS Paragon Plus Environment


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ABSTRACT: Organic templates play a crucial role in zeolites synthesis, but their use causes serious pollutant emissions and increases product costs. Herein, we report a template-free approach to synthesize both microporous and hierarchical ZSM-5 zeolites from natural aluminosilicate mineral, which is achieved by (1) utilizing the mother liquid to construct mesopores without using any mesoscale template and (2) reusing the Si-rich alkali liquor and ion-exchanging solution to improve the atom economy without any alkali/acid liquor discharges. The results show that the resultant zeolites own more open pore channels and less strong acid sites compared with a conventional ZSM-5 zeolite and demonstrate superior catalytic activity in n-octene hydroisomerization. Significantly, compared with the traditional synthesis process, our approach can greatly reduce the material and energy consumptions and pollutant discharges while significantly increase the raw material utilization efficiency, endowing itself an economic, eco-sustainability and industrially feasible process.

KEYWORDS: ZSM-5 zeolite, Template-free synthesis, Hierarchical pore structure, Mother liquid, Hydroisomerization.


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1. INTRODUCTION Nowadays energy and environment have become the most important global issues for the sustainable development of chemical and petrochemical industries, to which zeolites and zeolitic materials are considered to play important roles in addressing.1-3 This is especially true for catalytic conversion processes, because the involvement of zeolites or zeolitic materials with high activity and selectivity can substantially increase the atomic economy of the process by enhancing the yield of the target product.4-7 Paradoxically, the current production of zeolites and zeolitic materials themselves is not green, which makes their based manufacturing processes not green from the source and therefore constitutes a great challenge to the sustainability of the zeolite production industry. This is because: (1) most commercially available microporous zeolites and nearly all micro-mesoporous materials with hierarchical pore structures are obtained via the so-called hydrothermal synthesis technique, which usually involves the use of organic templates (also known as structure-directing agents, SDAs) that must be removed from the assynthesized products via decomposition by calcination at high temperature, incurring pollutant emission to the environment and additional energy consumption;8-17 (2) the most commonly used raw materials for synthesizing zeolites are inorganic silicon- and/or aluminum-containing chemicals (such as sodium silicate and sodium aluminate), whose production processes from natural silicate/aluminosilicate minerals are comprised of complicated reaction and separation steps and therefore are associated with huge energy consumption and waste discharges;18, 19 and (3) the as-synthesized zeolites are in Na-form, thus have no catalytically active acid sites and 3 ACS Paragon Plus Environment


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therefore need to be converted to H-form via repeated ion-exchanges using NH4+ aqueous solution and calcinations at high temperature, also giving rise to additional energy consumption and pollutant emissions.6 Recently, an organotemplate-free synthesis approach based on a seeddirected technique was developed to synthesize aluminosilicate zeolites.6, 10, 20-22 However, this methodology normally uses the relatively expensive silicon- and aluminum-containing chemicals, and the resultant zeolites are purely microporous rather than hierarchically micromesoporous. To construct hierarchical pore structure without using any mesoporogen template, several groups have employed the so-called top-down post-synthetic modification of microporous zeolites,12, 17, 23-26 nevertheless, additional alkaline or acidic liquors are still needed. More recently, we have successfully explored natural aluminosilicate minerals that have abundant reserves in the earth as cheap starting materials to synthesize hierarchical ZSM-5 and FeZSM-5 zeolites,18, 19 unfortunately, these works did not avoid the involvement of microporous template. To the best of our knowledge, it seems that although extensive efforts have been made during the last few decades,6,

10, 17-28

none of strategies proposed so far can provide a

comprehensive solution to the aforementioned problems. Herein we present a sustainable approach to synthesize microporous and hierarchical ZSM-5 zeolites. This methodology starts from natural aluminosilicate minerals rather than chemicals, excludes the use of any organic template, and does not discharge any alkali or acid liquor (the experimental details are described in Experimental section). As illustrated in Figure 1, our strategy mainly includes the following three parts: 4 ACS Paragon Plus Environment

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(i) Synthesis of microporous Na-zeolite: using an activated aluminum-rich mineral, i.e., submolten salt (SMS) depolymerized kaolin, as the only aluminum source and a partial silicon source, a Na-form zeolite is first synthesized via a template-free hydrothermal crystallization route assisted with seeds of a commercial NaZSM-5 zeolite (with a SiO2/Al2O3 molar ratio of 37.7), and the filtration of the resulting suspension yields a mother liquid that is a Si-rich alkaline liquor and a solid product that is further washed thoroughly with deionized water to obtain a microporous Na-zeolite. (ii) Generation of hierarchical structure in the Na-form microporous zeolite and transformation of the hierarchical zeolites from Na-form to H-form: the microporous zeolite obtained in step (i) is treated with the mother liquid to generate hierarchical pore structure, and the resulting hierarchical Na-form zeolite is ion-exchanged with an acid solution to obtain a hierarchical Hform zeolite (Figure 1). Here we must point out that the microporous Na-form zeolite obtained in step (i) can be also directly transformed to a microporous H-form zeolite without experiencing the mother liquor treatment step. (iii) Reuse of the mother liquid: the Si-rich alkali liquor produced obtained in step (ii) is neutralized with the ion-exchanged acid liquor, and the generated insoluble solid precipitate (H3SiO3) was recycled to the crystallization step as a makeup silicon source.

2. EXPERIMENTAL SECTION



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2.1. Materials. The natural kaolin containing 50.5% SiO2 and 44.6% Al2O3 was acquired from China Kaolin Clay Co., Ltd. (P. R. China). Water glass (containing 27.6% SiO2 and 8.9% Na2O) was purchased from Beijing Red Star Metal Chemical Building Materials Co., Ltd. (P. R. China). Sulfuric acid and sodium hydroxide were obtained from Sinopharm Chemical Reagent Co., Ltd. (P. R. China). The reference ZSM-5 zeolite was purchased from the Catalyst Plant of Nankai University (P. R. China). 2.2. Synthesis of microporous and hierarchical ZSM-5 zeolites. A typical procedure for the synthesis of microporous ZSM-5 zeolite using kaolin as the starting material without using any organic template is as follows. The activated kaolin, which was obtained via the SMS treatment we previously reported,8, vigorous

stirring

with

18, 19

the

was mixed with distilled water, water glass and seeds under final

molar

composition

of

the

resulting

mixture

at

Al2O3:SiO2:Na2O:H2O = 1:40:5.6:1600 and seeds:SiO2 (mass ratio) = 1%; then a H2SO4 solution was used to adjust the pH value of the synthesis system. After being aged at 70 oC for 4 h, the resulting mixture was transferred into a Teflon-lined stainless-steel autoclave and hydrothermally crystallized at 170 oC for 48 h. The resultant solid product was recovered by filtration, washing with distilled water and drying at 120 oC overnight, obtaining a microporous ZSM-5 zeolite. The mother liquid was collected and reused for generating mesopores in the synthesized microporous ZSM-5 zeolite. The hierarchical ZSM-5 zeolite was prepared by putting a certain amount of the synthesized microporous ZSM-5 zeolite into the mother liquid, stirring the slurry at 65 oC for 2 h, cooling the slurry to room temperature, filtrating the slurry, washing the filter 6 ACS Paragon Plus Environment

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cake obtained with deionized water, and drying the cake at 120 oC in air overnight. The alkali liquid was collected for reuse. The synthesized microporous and hierarchical NaZSM-5 zeolites were converted to HZSM-5 by successive ion-exchange with 0.1 mol/L HCl solution at 70 oC for 4 h and calcination at 520 o

C for 4 h. The acidic liquid was neutralized with the previously obtained alkali liquid and then

the thus-obtained solid was recycled as silicon source. 2.3. Preparation of catalysts. To prepare model hydroisomerization catalysts, the protonic ZSM-5 zeolites were first extruded with pseudo-boehmite (30 wt%) as the binder into sticks of 1.5 mm in diameter and ~3 mm in length. Then, the sticks were dried at 120 oC overnight, calcined at 520 oC for 5 h, and impregnated with aqueous solutions of ammonium molybdate and nickel nitrate successively. Finally, the resulting solids were dried at 120 oC for 4 h and calcined at 480 oC for 4 h to obtain oxidic catalysts. The MoO3 and NiO loadings of the catalysts were 3 and 1 wt%, respectively. The catalysts prepared from the reference ZSM-5 zeolite and the two synthesized (microporous and hierarchical) ZSM-5 zeolites were denoted as CAT-Ref-Z, CATMic-Z and CAT-Hie-Z, respectively. 2.4. Characterizations. The chemical compositions of the kaolin and ZSM-5 samples were determined by X-ray fluorescence (XRF) conducted on a Bruker S4 Explorer instrument. The phase structure of the samples was determined by X-ray powder diffraction (XRD) on a PANalytical X’pert Pro diffractometer with CuKα radiation and operated at 40 kV and 40 mA in the 2θ range of 5-50°. The relative crystallinity values of the synthesized ZSM-5 samples were 7 ACS Paragon Plus Environment


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determined by comparing the areas of their characteristic peaks (2θ = 22.5-25o) with those of the reference ZSM-5 zeolite which was considered to be 100% in crystallinity. The skeleton structure of the samples was obtained on a Nicolet Magna-IR 560 ESP Fourier transform infrared (FT-IR) spectrometer (USA) and each spectrum was recorded at a resolution of 1 cm-1. The morphology and size of the samples were observed by field-emission scanning electron microscopy (FESEM) on a Hitachi-S4800 equipment. The transmission electron microscopy (TEM) images were taken using an FEI Tecnai F20 (200 kV) high-resolution transmission electron microscope with the sample mounted on a C-flat TEM grid. The textural properties of the samples were studied by nitrogen adsorption-desorption measurements at -196 °C on an ASAP 2020M Micromeritics instrument (USA). The specific surface areas were calculated by the Brunauer-Emmett-Teller (BET) method, the specific micropore surface areas and pore volumes were obtained by the t-plot method. The BJH pore size model was employed to obtain the mesopore size distribution using the adsorption branch of the isotherm. The acidity of the samples was measured by temperature programmed desorption of ammonia (NH3-TPD) on an AutoChem 2920 apparatus equipped with a thermal conductivity detector (TCD). The nature of acid sites was determined by pyridine-adsorbed IR (Py-IR) spectroscopy on a Nicolet 5700 spectrometer. The

27

Al and

29

Si magic angle spinning nuclear magnetic

resonance (MAS NMR) spectroscopy characterizations were performed on a Bruker DSX 500 MHz spectrometer with a spinning rate of 14 kHz and a π/8 pulse length of 1 ms. The framework 8 ACS Paragon Plus Environment

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29

SiO2/Al2O3 molar ratios of the samples were calculated from the

Si MAS NMR spectra using

the following equation: 4

2∑ ISi(nAl) Framework SiO 2 /Al2 O3 ratio=

n =0 4

∑ 0.25n[ I

]

Si(nAl)

n =0

where n = 0, 1, 2, 3 and 4.

2.5. Catalytic tests. Hydroisomerization of n-octene as a model reaction was conducted in a 10 mL continuous flow fixed-bed microreactor. The presulfurization of the catalysts was firstly carried out at 320 oC for 2 h, with a CS2 (3 wt%)/cyclohexane mixture as vulcanizer and hydrogen as carrier gas, a total pressure of 2.8 MPa, a hydrogen-to-hydrocarbon volume ratio of 200, a weight hourly space velocity (WHSV) of 2 h-1. Subsequently, n-octene was fed into the reactor by a microscale pump at a given flowrate, and the hydroisomerization reaction was carried out at a temperature of 300 oC, a total pressure of 1.5 MPa, a hydrogen-to-hydrocarbon volume ratio of 300, and a WHSV of 1.5 h-1. At a constant time interval, the product was collected and analyzed with a GC-950 gas chromatograph.

3. RESULTS AND DISCUSSION 3.1. Structure and pore properties. The phase structure of the synthesized microporous and hierarchical samples was determined by XRD and FTIR spectroscopy, and the results are shown in Figure 2. The XRD patterns of the synthesized microporous and hierarchical samples in Figure 2A are identical to the diffractogram of the reference ZSM-5 zeolite, all of which exhibit



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the characteristic diffraction peaks attributed to MFI structure with no other unidentified peaks.29 This confirms the high purity and crystallinity of the two synthesized samples (Table 1). Additionally, the FTIR spectra in Figure 2B show the bands at 1219, 1080, 790, 542 and 450 cm1

attributed to external asymmetric stretching, internal asymmetric stretching, external symmetric

stretching, double ring vibrations and T-O bending, respectively, further proving that the two synthesized samples are pure-phase ZSM-5 zeolites.30, 31 Figure 3A and 3B are the representative FESEM and high-resolution TEM images of the microporous ZSM-5 zeolite, showing that the zeolite is present in the form of flat-prism like particles with smooth surface and uniform size (ca. 2 µm). Figure 3B also shows that the lattice fringes run completely through each of the crystals, being consistent with the XRD results and thus further demonstrating the high crystallinity. Differently, Figure 3C show that the hierarchical sample is present in the form of flat-prism particles with rough surface on which there appear abundant pinholes, and accordingly many intra-crystalline cavities are observed in the hierarchical ZSM-5 zeolite, as seen from the TEM image in Figure 3D. It is considered that the unique morphology of the hierarchical ZSM-5 zeolite different from that of microporous ZSM-5 zeolite is related to the post-synthetic desilication treatment using the mother liquid as the base source. The pore structure of the different ZSM-5 zeolites was characterized by nitrogen adsorptiondesorption measurements and the results are summarized in Figure 4 and Table 1. Figure 4 shows that the isotherms of the reference sample and the synthesized microporous sample belong 10 ACS Paragon Plus Environment

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to the type I isotherm with a plateau at higher relative pressure (P/P0) but without any obvious hysteresis loop, in comply with the microporous nature of the samples; differently, the isotherm of the hierarchical sample has a distinct H4-type hysteresis loop at P/P0 ranging from 0.4 to 1.0, indicating the coexistence of micropores and mesopores in the sample. The pore size distribution estimated by the Barrett-Joyner-Halenda (BJH) method reveals the mesopores in the hierarchical ZSM-5 zeolite are distributed between 4 and 10 nm, as seen in inset of Figure 4. The slightly higher specific surface area (SBET) and pore volume (VTotal) of the microporous ZSM-5 zeolite compared to the reference one (Table 1) indicates the higher crystallinity of the former, also in good agreement with the XRD results. It is worth to note that the hierarchical ZSM-5 zeolite has substantially increased mesopore specific surface area (Smeso) and volume (Vmeso) and therefore the highest SBET and VTotal among the three samples, attributed to its abundant intracrystal mesopores. The above characterization results lead to the conclusion that the sample synthesized without the further treatment with the mother liquid is a typical microporous ZSM-5 zeolite with MFI topological structure, while the sample synthesized with the subsequent treatment with the mother liquid is a hierarchical ZSM-5 zeolite with bimodal meso-microporous structure.

3.2. Importance of the filtration and washing steps. In our approach, the filtration of the synthesis suspension and the washing of the obtained solid product are the crucial steps for constructing hierarchical pore structure. Without the filtration and washing steps, even the synthesis suspension was treated under 65 oC for 12 h, no mesopores were generated in the 11 ACS Paragon Plus Environment


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resultant zeolite (Figure S1). This can be explained by the fact that, in the synthesis system, the zeolite, i.e., the crystallization product, and the mother liquid are in an equilibrium state, so their further contact after crystallization can hardly break this state. After being filtrated and washed with water, the zeolite synthesized becomes neutral, but the mother liquid whose properties are listed in Table 2 remains strongly basic. Therefore, when the neutral zeolite and the basic mother liquid get contact with each other again, partial silicon species in the zeolite are dissolved into the basic mother liquid (Table 2), i.e., desilication takes place in the zeolite and results in the formation of hierarchical pore structure (Figure 4).

3.3. 29Si and 27Al MAS NMR spectra. The 29Si MAS NMR spectra of the samples in their Hforms are shown in Figure 5A. Three peaks in each spectrum can be well resolved by deconvolution using three Gaussian lines. The two peaks at -115.4 and -112.6 ppm can be assigned to Si(OSi)4 units, i.e., Si(0Al); and the other broad signal at -106.7 ppm at the lower field can be assigned to Si(OAl)(OSi)3 units, i.e., Si(1Al).32 The framework SiO2/Al2O3 ratios of the microporous and hierarchical samples calculated from the MAS NMR spectra based on the integral area of Si(0Al) and Si(1Al) described in Experimental section are 30.8 and 26.9 (Table 1), respectively, evidencing that the occurrence of desilication during the treatment of the microporous sample with the mother liquid. The structural configuration and location of Al atoms in the ZSM-5 zeolites were investigated by 27Al MAS NMR and the results are shown in Figure 5B. In the spectra, the dominant peak at 55 ppm corresponds to tetrahedrally coordinated framework Al (AlIV), while the weaker peak at 12 ACS Paragon Plus Environment

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0 ppm represents octahedrally coordinated AlVI.33, 34 The relative distributions of the two kinds of Al species obtained by deconvoluting the corresponding spectra are also given in Figure 5B. It can be seen that treatment of the microporous ZSM-5 zeolite with the mother liquid results in the increased AlVI signal accompanied with the decreased AlIV signal. This indicates that after desilication, some Si-O bonds in the zeolite framework are destructed and the original Si-O-Al bonds are transformed to some separate aluminum-rich phases (such as segregated alumina phase or amorphous silica-alumina phase). The Al species in such separate aluminum-rich phase are Lewis (L) acid sites in nature.

3.4. Acidity properties. The acidity properties of the reference zeolite and the synthesized microporous and hierarchical ZSM-5 zeolites were measured by NH3-TPD and Py-IR spectroscopy. From the NH3-TPD results given in Figure 6 and Table S1, we can observe that all of the samples show two desorption peaks centered at 190-220 and 380-430 oC, corresponding to weak and strong acid sites, respectively.35 Nevertheless, the acid strength and amount of each sample are distinctly different. Specifically, the acid strength of both weak and strong acid sites of the different samples is in the order of the reference zeolite > the synthesized microporous zeolite > the synthesized hierarchical zeolite, whereas the acid amount of weak and strong acid sites is in a different order: the synthesized microporous sample has the highest amount of weak acid sites, the reference zeolite owns the highest amount of strong acid sites, and the hierarchical zeolite simultaneously possesses the lowest amounts of weak acid sites, strong acid sites and total acid sites. These data are in accordance with the previous report in literature

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36

. It can be 13


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seen that the treatment of the synthesized microporous ZSM-5 zeolite with the mother liquid led to the hierarchical ZSM-5 zeolite with dramatically decreased total acid amount. Figure 7 shows the FTIR spectra of adsorbed pyridine after evacuation at 200 and 350 oC, respectively, and Table 3 gives the quantitative results. The IR bands at 1540 and 1450 cm-1 after pyridine adsorption are assigned to B and L acid sites, respectively. Clearly, the two zeolites we have synthesized have significantly lower amounts of B acid sites but higher amount of L ones than the reference ZSM-5 zeolite, which is associated with the concomitance of desilication and framework dealumination during the alkali-treatment (Table 2 and Figure 5B).33, 36 The results show that the treatment of the synthesized microporous ZSM-5 zeolite with the mother liquid significantly decreased the amount of Brönsted (B) acid sites but dramatically increased the amount of L acid sites.

3.5. Catalytic tests. The above results show that, compared with the two microporous ZSM-5 zeolites, the hierarchical ZSM-5 zeolite we have obtained contains significantly enhanced mesoporosity, dramatically increased external surface area, and greatly decreased acidity, which endow itself as a potential catalytic material for catalyzing the hydroisomerization reactions of hydrocarbons to produce multi-branched isomers.37-40 To test this inference, three NiMo/HZSM5-γ-Al2O3 catalysts, denoted as CAT-Ref-Z, CAT-Mic-Z and CAT-Hie-Z, respectively, were prepared by using the reference ZSM-5 zeolite and the synthesized microporous and hierarchical ZSM-5 zeolites blended with an appropriate amount of γ-Al2O3 as supports. The preparation procedure had been documented elsewhere.41 The catalytic performance of the three catalysts 14 ACS Paragon Plus Environment

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were test using the hydroisomerization of n-octene as a model reaction, and the results are summarized in Figure 8. Under the experimental conditions that are typical in industrial practice, all of the three catalysts gave a 100% conversion ratio of n-octene, but the product distributions are distinctively different: the selectivities to di-branched and mono-branched C8 products follow the order of CAT-Hie-Z > CAT-Mic-Z > CAT-Ref-Z, while the selectivities to cracking products (

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