Synthesis of Nanosized SAPO-34 via an Azeotrope Evaporation and

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Synthesis of nanosized SAPO-34 via an azeotrope evaporation and dry gel conversion route and its catalytic performance in chloromethane conversion Jingwei Zheng, Dongliang Jin, Zhiting Liu, Kake Zhu, Xinggui Zhou, and Wei-Kang Yuan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03326 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 23, 2017

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Industrial & Engineering Chemistry Research

Synthesis of nanosized SAPO-34 via an azeotrope evaporation and dry gel conversion route and its catalytic performance in chloromethane conversion Jingwei Zhenga, Dongliang Jina, Zhiting Liua, b*, Kake Zhua, Xinggui Zhoua and Weikang Yuana a

State Key Laboratory of Chemical Engineering, East China University of Science and

Technology, 130 Meilong road, Shanghai 200237, P. R. China b

School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, P.

R. China *Corresponding Author: Email: [email protected]

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Abstract: Nanosized SAPO-34 has been successfully synthesized with a facile and low-cost

method

involving

an

azeotrope

evaporation

and

steam-assisted

crystallization route without either expensive poregens or home-made starting materials. The as-synthesized nanosized SAPO-34 shows the ultra-small grain sizes and an impressively high total pore volume confirmed by XRD, nitrogen physisorption, SEM, and TEM techniques. The formation process and the pivotal role of supersaturation in the synthesis of nanosized SAPO-34 have been revealed by a series of control experiments, by which a non-classic oriented attachment mechanism under high supersaturation ratio conditions is proposed. When used as the catalyst for the conversion of chloromethane to olefins, the nanosized SAPO-34 exhibits comparable acidity to the conventional SAPO-34, but a double catalytic lifetime and a slightly lower selectivity of light olefins. Keywords:

SAPO-34,

nanosized,

azeotrope

evaporation,

supersaturation,

chloromethane conversion

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1. Introduction Zeolites and zeotype materials are a family of crystalline materials with well-defined micropores and tunable acidity. As their pore size is within the range of molecular scale, they can selectively accommodate molecules by the discrimination of size and shape. Therefore, zeolites and zeotype materials have been widely used as adsorbents, ion-exchangers, and catalysts.1 In the past decades, SAPO-34 has drawn intensive attention from research communities in academia and industries owing to its excellent catalytic performance in the methanol-to-olefin (MTO) process. The selectivity toward light olefins can exceed 85% due to its unique channel structures and mild acidity.2 However, the rapid deactivation and the resulting frequent regeneration weaken the overall economy of MTO process.2 Thus, it is essential to elongate the catalytic lifetime of SAPO-34 for its application. Currently, it has been recognized that either downsizing to nanometer scale or generating hierarchical porous structures is an effective way to alleviate the coking and deactivation of SAPO-34 catalysts caused by sluggish diffusion.3-5 Usually, the hierarchical porous structures can be constructed by using expensive sacrificial poregens or home-made starting materials (e.g., purpose-designed organosilanes and carbon nanotubes), along with tedious procedures.5-11 Moreover, the use of poregens results in not only a high cost but also the decreased acidity or crystallinity of SAPO-34.7, 9 So far, few strategies have been established to synthesize nanosized or hierarchical SAPO-34 in the absence of additives or special starting materials. Bein et al. synthesized SAPO-34 with an average particle size of about 300 nm in a colloidal 3 ACS Paragon Plus Environment


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solution, and further reduced their size to 100 nm via microwave heating.8 Yu et al. succeeded in producing SAPO-34 nanosheets with the thickness of 20 nm without additives by replacing tetraethyl orthosilicate (TEOS) with colloidal silica as silica source during microwave-assisted synthesis.4 Nevertheless, careful handling was required for the preparation of colloidal solutions to avoid forming dense gel. Furthermore, the microwave-assisted techniques often suffer from low yields.12 Dry gel conversion (DGC), an alternative route to the conventional hydrothermal crystallization, features high yields and simplicity. When it was applied to synthesize SAPO-34, the samples with a board size distribution of 94-565 nm and with an average

size

of

about

75

nm

were

obtained

using

morpholine

and

tetraethylammonium hydroxide (TEAOH) as the structure directing agent (SDA), respectively.13, 14 It is noteworthy that TEAOH was not normally employed as a SDA during the DGC process, because its hydrophilic nature is detrimental to the formation of dry gel. For instance, a sticky mass rather than dry powder was obtained in zeolite beta synthesis when TEAOH was used as the SDA.15,

16

This seems to exert an

adverse effect on the control of crystal sizes, since the high water content will lead to the crystal growth induced by the fusion of crystal grains. Despite these efforts, a highly reliable, low-cost, and scalable approach is still urgently demanded for the fabrication of hierarchical or nanosized SAPO-34. SAPO-34 is also catalytically active in the conversion of chloromethane to olefins besides MTO.17 This process is supposed to be more economic than MTO with respect to methane activation, since methane can be converted into chloromethane 4 ACS Paragon Plus Environment

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through direct oxidative chlorination, thereby bypassing the costly steam reforming and methanol synthesis process.18 The catalytic conversion of chloromethane shares the similar hydrocarbon pool mechanism and product distribution to the MTO reaction over SAPO-34 catalysts but requires more strong acid sites for the lower activity of chloromethane compared with methanol.19-21 Worse still, SAPO-34 catalysts suffer from more severe deactivation by coke deposition in chloromethane conversion than MTO.17 Therefore, a desirable SAPO-34 catalyst for the conversion of chloromethane to olefins should have both improved diffusion property and sufficient strong acid sites. In this work, ethanol instead of water was used as the solvent, thus permitting the removal of almost all solvents to form dry gel. The low solvent content in the dry gel offers a high supersaturation ratio, which is conducive to the creation of a large number of tiny nuclei.22 As a result, SAPO-34 nanocrystals (50-90 nm) will be produced during the simple DGC process. With the aid of steam-assisted crystallization (SAC), in which vapor source water was separated from other starting materials,23 the translucent dry gel was converted into nanosized SAPO-34. The crystallization process was monitored by a combination of characterization techniques including X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric Analysis (TGA), and scanning electron microscopy (SEM) to shed light on the formation mechanism. The pivotal role of supersaturation in the preparation of nanosized SAPO-34 will be demonstrated through a series of control experiments. The structural, textural, and acidic properties of nanosized SAPO-34 5 ACS Paragon Plus Environment


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will be unveiled by XRD, SEM, transmission electron microscopy (TEM), nitrogen (N2) physorption, and ammonia temperature programmed desorption (NH3-TPD). Finally, the catalytic performance of nanosized SAPO-34 in chloromethane conversion was evaluated and compared with a conventionally derived counterpart. 2. Experimental 2.1 Synthesis of nanosized SAPO-34 and conventional SAPO-34 The reagents include aluminum isopropoxide (AIP, > 98 wt %, Shanghai Lingfeng Chemical Reagent Co. Ltd.), pseudoboehmite (75 wt % Al2O3, Aluminum Corporation of China), phosphoric acid (85 wt %, Shanghai Lingfeng Chemical Reagent Co. Ltd.), tetraethyl orthosilicate (TEOS, 28 wt % SiO2, Shanghai Lingfeng Chemical Reagent Co. Ltd), fumed silica (99%+ SiO2), tetraethylammonium hydroxide (TEAOH, 35 wt %, Shanghai Cainorise Chemicals Co., Ltd), and ethanol (99.7%, Shanghai Titan Scientific Co., Ltd). These chemicals were used as provided without further purifications. Nanosized SAPO-34 was derived from dry gel prepared by solvent evaporation. The

molar

composition

of

the

initial

dry

gel

was

1.0Al2O3:1.0P2O5:0.6SiO2:2.0TEAOH. In a typical run, 4.08 g AIP was added into 100 mL ethanol followed by sonicating for 30 mins to afford a homogenous suspension. 1.29 g TEOS was added to the suspension and stirred for 10 mins. 8.40 g TEAOH and 2.40 g phosphoric acid (H3PO4) were diluted with 20 mL ethanol, respectively, and then added to the above suspension sequentially. After continuous stirring for 4 hrs, the formed suspension in a beaker was transferred to a 90 oC oil 6 ACS Paragon Plus Environment

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batch to evaporate the azeotrope of ethanol and water, after which, translucent dry gel was obtained. The dry gel was grounded by mortar and pestle into fine powder before crystallization. The crystallization was conducted with the SAC method. The dry gel was charged in a Teflon beaker, which was then placed in a Teflon lined stainless steel autoclave. 0.50 g water was poured outside the Teflon beaker in the autoclave as a steam source. The autoclave was heated to 200 oC in a convection oven for 72 hrs (crystallization time). The product was separated by centrifugation, and washed with water repeatedly for three times. The washed samples were dried at 80 oC for 6 hrs and calcined at 550 oC for 6 hrs. The as-prepared product was considered as a standard sample, which is donated as S2.0-0.5. To understand the crystallization process, the above procedure was exactly followed but quenched at various crystallization time intervals. The as-synthesized samples were named according to the crystallization time, for example, S2.0-1h means the sample was steamed for 1 hr for crystallization and the number before the dash (i.e., 2.0) stands for the TEAOH content in the recipe. To clarify the role of supersaturation in the synthesis of nanosized SAPO-34, a series of samples were synthesized by the same procedure as that set forth above but using varied contents of steam source and TEAOH. They are donated as S2.0-0.5, S2.0-3.0, S2.0-10.0, S1.0-0.5, and S0.5-0.5. The number before the dash stands for the TEAOH content in molar composition and the number after the dash represents the amount of steam source during crystallization. A conventional SAPO-34 sample was synthesized as a control counterpart via a 7 ACS Paragon Plus Environment


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hydrothermal method. This sample was prepared with the similar molar composition with the standard sample of 1.0Al2O3:1.0P2O5:0.6SiO2:2.0TEAOH:50H2O. In a typical synthesis process, 2.19 g pseudoboehmite was dissolved in a solution containing 4.20 g H2O and 3.46 g H3PO4, forming solution I. 0.54 g fumed silica was dissolved into a 12.64 g aqueous solution of TEAOH, forming solution II. Then solution II was dropwise added into solution I, forming a gel. The gel was aged at room temperature for 4 hrs before transferred into a Teflon lined stainless steel autoclave. The crystallization was conducted at 200 oC for 72 hrs under static condition, followed by the same wash and calcination procedure as described above. This sample was designated as S2.0-C. The molar compositions of dry gel precursors and synthesis conditions for all the samples are listed in Table 1. 2.2 Characterization XRD patterns were recorded on a Rigaku D/MAX 2550VB/PC diffractometer with Cu Kα radiation (λ=1.5418 Å) operating at 40 kV and 100 mA. SEM and TEM measurements were performance on NOVA Nano SEM 450 and JEM-2011, respectively. N2 physisorption was conducted on a Micromeritics ASAP 2020 HD analyzer at -196 oC. The samples were degassed at 300 oC under vacuum for 6 hrs before measurement. FTIR spectra were obtained with a Nicolet 6700 instrument. 1 mg sample was well mixed with 99 mg KBr powder to prepare self-support pellets used for FTIR measurement. Thermogravimetric analysis (TGA) was carried out on a Perkin-Elmer Pyris 1 TG analyzer at a heating rate of 10 oC/min from room temperature to 800 °C in air. Elemental composition was analyzed with a XRF-1800 8 ACS Paragon Plus Environment

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sequential X-ray fluorescence (XRF) spectrometer. NH3-TPD profiles were obtained from a ChemiSorb 2720 instruments (Micromeritics, USA). 0.10 g sample was pretreated in a He flow of 30 mL/min at 500 oC for 2 hrs before cooled to 100 oC for ammonium adsorption. The data were collected from 100 oC to 800 oC at a ramp of 10 o

C/min after 2 hrs flushing to remove physical adsorption ammonium. Solid-state

NMR measurements were performed on the Bruker Avance III-500 MHz spectrometer operated at a magnetic field strength of 11.7 T.

31

P MAS NMR

experiment was conducted at 202.3 MHz with spinning rate of 14 kHz. The chemical shifts were referenced to 85 wt% H3PO4. 2.3 Catalytic test Chloromethane conversion was carried out at atmospheric pressure in a quartz fix-bed reactor. The weight hourly space velocity (WHSV) for the test was 2.4 h-1. The catalyst (40-60 mesh) was pretreated at 500 °C in N2 flow of 30 mL/min for 1 hr and then cooled to 450 °C for reaction. Chloromethane and N2 were co-feed with a molar ratio of 3.5:1.0. The products were analyzed by an on-line gas chromatography (GC 9860, Shanghai Qiyang information technology Co. Ltd.) equipping a flame ionization detector (FID) and PLOT Al2O3 capillary column (30 m×0.32 mm). The catalysts were considered to be completely deactivated when the conversion of chloromethane was less than 20%. 3. Results and discussion 3.1 Crystallinity, textural properties, and chemical composition of nanosized SAPO-34 9 ACS Paragon Plus Environment


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The XRD pattern of the standard sample (S2.0-0.5) is shown in Figure 1a. It matches well with the standard powder diffraction lines (JCPDS No. 47-0617) without detectable impurities or amorphous materials. The weak intensity of diffraction peaks suggests that S2.0-0.5 is composed of small primary crystal domains.16 The

31

P MAS NMR spectrum (Figure 1b) shows only one shark peak at

-30 ppm along with two weak spinning sidebands, indicating that all P atoms are incorporated into frameworks.24 These results unravel that the modified preparation has a trivial effect on the crystalline structure. Figure 2 presents the representative SEM and TEM images of S2.0-0.5. In the low magnification SEM image (Figure 2a), numerous small coarse particles (50-90 nm) were observed together with the existence of several relatively big crystals with a typical rhombohedral morphology. A high magnification image (Figure 2b) reveals that those small particles are the aggregation of many rhombohedral crystal blocks, showing a sphere-like morphology with rough external surfaces. Some steps were observed at the edges of the big rhombohedral crystals, implying that they are most likely formed via the alignment or ripening of primary crystals. The TEM images in Figure 2c-f further confirm that the alignment of several neighboring rhombohedral crystal domains on most particles, suggesting a crystallographic alignment of building blocks in their growth history. Noteworthily, the building blocks in the present work are rhombohedral rather than spherical reported in literature, suggesting their high crystallinity and few defects.25 The void space and surface roughness can be intuitively seen, which are created by the alignment of primary crystals. Some big 10 ACS Paragon Plus Environment

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rhombohedral crystals were observed as well, in line with the SEM observation. Furthermore, the alignment structures have a single-crystal-like feature, as manifested by the selected area electron diffraction (SAED) pattern in the inset of Figure 2c. A twin SAED pattern results from the mismatched alignment of building blocks, which is a typical phenomenon for mesocrystal synthesis.26 The irregular shape, complex internal structures, and single-crystal-like diffraction behavior give a strong indication of a non-classic crystal growth mechanism.27 Overall, crystalline nanosized SAPO-34 with ultra-small crystal sizes has been successfully prepared with the present synthesis method by taking advantage of the azeotrope evaporation and SAC process. Texture parameters of the as-prepared SAPO-34 were evaluated by N2 physorption. In Figure 3, the isotherm of S2.0-0.5 is a hybrid of type I and type IV with an obvious hysteresis loop. The type I isotherm at low relative pressure (P/Po < 0.1) is attributed to the filling of micropores. The N2 uptake shows a striking increase at high relative pressure (P/Po > 0.9), which can be ascribed to capillarity condensation in mesopores and/or macropores. The gentle slope of isotherm in the middle region of relative pressure (0.1

Synthesis of Nanosized SAPO-34 via an Azeotrope Evaporation and

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