Selective Catalyst for Formaldehyde Removal

ABSTRACT: This work delineates a concept of wettability-selective catalyst, which is realized by functionalizing metal oxide catalyst within zeolite s...
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Hydrophobic Zeolite Containing Titania Particles as Wettability-Selective Catalyst for Formaldehyde Removal Zhu Jin, Liang Wang, Qingxun Hu, Ling Zhang, Shaodan Xu, Xue Dong, Xinhua Gao, Runyuan Ma, Xiangju Meng, and Feng-Shou Xiao ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00732 • Publication Date (Web): 03 May 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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ACS Catalysis

Hydrophobic Zeolite Containing Titania Particles as WettabilitySelective Catalyst for Formaldehyde Removal Zhu Jin,a Liang Wang,a* Qingxun Hu,e Ling Zhang,a Shaodan Xu,b Xue Dong,c Xinhua Gao,d Runyuan Ma,a Xiangju Meng,a and Feng-Shou Xiao a* a

Key Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University, Hangzhou 310028, China. b College of Materials & Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, China. c

Department of Chemistry and Biochemistry, Texas Tech University, Texas 79409, United States.

d

State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, Ningxia University, Yinchuan 750021, China. e Lanzhou Petrochemical Research Center, Petrochemical Research Institute, PetroChina Company Limited, Lanzhou 730060, PR China. ABSTRACT: This work delineates a concept of wettability-selective catalyst, which is realized by functionalizing metal oxide catalyst within zeolite sheath with controllable wettability. We synthesized such zeolite sheaths by anchoring organic groups (e.g. CH3) into faujasite framework during the crystallization, achieving hydrophobic micropores for selective capture of organic compounds but hindering water diffusion. Further fixation of TiO2 particles within these zeolite crystals resulted in core-shell catalysts denoted as TiO2@HP-zeolite, which combined the functions of both wettability selectivity for the zeolite sheath and photocatalytic activity for the TiO2. Owing to the synergism of these features, TiO2@HP-zeolite displayed superior performances in complete removal of wet formaldehyde in a long-period continuous test under irradiation of sunlight frequencies, outperforming the conventional catalysts with poor water tolerance.

KEYWORDS. Heterogeneous catalysis; Zeolite; Core-shell structure; Formaldehyde; Wettability INTRODUCTION: The metal oxide-catalyzed transformation of organic molecules has dominated the technology of chemical production, fuel upgrading, biomass conversion, solar energy harvest, and pollutant removal.1-3 The performance of many catalysts is strongly influenced by the water species, which might cover on the catalyst surfaces to deactivate the active sites or lead to side reactions by over-reaction with the products.4-11 To overcome these issues, a wettability-selective catalyst, which is designed by combining the metal oxide catalyst with an ordered nanoporous sheath for selectively adsorbing organic molecules but hindering the water diffusion, is expected. A promising material agreed with the expected sheath is zeolite with rich microporosity and rigid framework. Eliminating the Al sites for all-silica and defect-free zeolites with hydrophobicity might hinder the water diffusion, but the synthesis is high cost and environmentally unfriendly because of the utilization of F-media (e.g. NaF, NH4F and HF).12-14 Pioneering work done using a postsilylation method to functionalize the organic groups on the zeolite external surface set the standard as a hydrophobic zeolite.9,15 However, this method disables to construct a hydrophobic framework but just a hydrophobic surface. Also, the silylation might block the orifice.16 Therefore, the functionalization of organic groups uniformly on the zeolite framework by silylation is expected but still challengeable because of the limitation of pore sizes in zeolites for accommodation of functionalized groups.17 Apart from the demand of hydrophobic zeolite sheath, the immediate need for a wettability-selective catalyst has spurred the research interest into the bifunctional materials with the combination of hydrophobic zeolite and catalytically active metal oxides. Preliminary methods like physically mixing metal oxides with

zeolite or loading metal species on the zeolite crystals were explored,18-23 but neither exhibited satisfactory efficacy. These limitations have necessitated the exploration of new bifunctional materials with maximized synergistic effects between two individuals. The maximized cover of metal oxides with zeolite sheath is regarded to be an indispensable principle for developing ideal wettability-selective catalysts. Herein, on the basis of the aforementioned design, we reported a strategy to achieve the ideal wettability-selective catalyst by fixing metal oxides inside of hydrophobic zeolite crystals. We made the hydrophobic zeolites (HP-zeolite) by functionalizing organic groups (e.g. -CH3, -Ph, -CF3) on the framework of faujasite during the crystallization process. This strategy gives rise to a wettability-selective adsorbent to capture organic molecules with completely avoiding the water. Considering TiO2 is one of the most widely investigated oxide catalysts, a core-shell catalyst of TiO2@HP-zeolite was developed by fixing TiO2 particles within the hydrophobic zeolite crystals (Scheme 1), where TiO2 degrades the enriched formaldehyde in HP-zeolite crystals under irradiation. We employ the removal of wet formaldehyde as a model reaction, because formaldehyde is a dominant indoor air pollutant and known to cause headache, pneumonia, and even cancer.24-27 In a long-period test with a continuous formaldehyde flow, the TiO2@HP-zeolite can completely remove formaldehyde under sunlight irradation without water adsorption, outperforming the catalysts of pure TiO2, TiO2 fixed in conventional zeolties (TiO2@zeolite), and TiO2 physilically mixed with HP-zeolite (TiO2/HP-zeolite). The extraordinary performances of TiO2@HPzeolite are attributed to the fixed TiO2 with hydrophilic zeolite sheath, which facilitates the selective capture and enrichment of

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formaldehyde in the hydrophobic zeolite micropores, followed by the photodegradation over TiO2. The HP-zeolite was synthesized from methyl group modified SiO2, which was obtained from controllable hydrolysis of tetraethylorthosilicate (TEOS) and dimethyldiethoxylsilane in a mixed liquor containing aqueous ammonia, water, and ethanol at room temperature. By employing the aforementioned methyl group modified SiO2 as silica precursor in an aluminosilicate gel (8.0SiO2/1.0 Al2O3/4.14 Na2O/133 H2O) for hydrothermal crystallization of Y zeolite the methyl group can be connected to the zeolite framework (see Supporting Information for more synthesis details), namely Y-Me-x, where x% is the molar ratio of methyl modified silica to the total amount of silica. In addition, the organic groups on the faujasite zeolites can be adjusted to -Ph and -CF3 (Y-Ph and Y-CF3) by employing the corresponding organosilane precursors. Furthermore, when P25 TiO2 was capsulated in the organosilane-modified silica-based gel for synthesis of faujasite zeolite, it was obtained the TiO2 particles fixed inside of the faujasite crystals, designated as TiO2@Y-CH3-x. The successful synthesis of faujasite zeolites was confirmed by XRD and SEM characterizations (Figures S1-S6, Table S1). Conventional Y

Y-Me-x

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still gives strong F signals with similar distribution to the contour of the zeolite crystal (Figures S12C and D). These results confirm that the organic groups are indeed uniformly distributed in the crystal. For the -CF3 modified Y zeolite by post-silylation method (Y-CF3-post-silyl, Figures S13-15), the F signal is very strong on the fresh sample, but very weak after the tomographic treatment (Figure S16), demonstrating that the -CF3 groups indeed exist on the external surface of zeolite crystals, which is further confirmed by the F1s XPS characterization (Figure S17). (A)

Y-Me-15

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Figure 1. (A) 29Si and (B) 13C NMR spectra of various samples.

S H Al O C TiO2 i Scheme 1. Models of conventional Y, Y-Me-x, and TiO2@Y-Mex samples. Figure 1A displays 29Si NMR spectra of conventional Y and Y-Me-15 samples, exhibiting very similar signals in the region of -57~-110 ppm, which are assigned to the Si sites coordinated with Si and Al species (Table S2).28,29 Interestingly, compared with the conventional Y zeolite, the Y-Me-15 gives an additional 29Si NMR signal at -24 ppm, assigned to Si coordinated with the carbon species. In addition, 13C NMR spectrum of Y-Me-15 shows a signal at 0.9 ppm (Figure 1B) related to methyl groups. Further characterization of the Y-Me samples by FTIR spectroscopy gives the bands at 1261 and 2960 cm-1 (Figure S7), which are assigned to the Si-C and C-H bonds, respectively. All these data demonstrate the presence of methyl groups on the Y-Me-15. Moreover, after fixation of the TiO2 particles, the TiO2@Y-Me still shows the characteristic bands of methyl group. Also, the FTIR spectra of Y-Ph and Y-CF3 exhibit the characteristic bands of -Ph (1450 and 2850 cm-1) group, confirming the successful functionalization of organic groups on the faujasite zeolites (Figures S7 and S8). In order to study the distribution of organic groups on the zeolite crystals, we performed the electronic microscopy of Y-CF3-15 (Figures S9-S11) as a model to detect the F distribution. As shown in the elemental maps of a randomly selected Y-CF3-15 crystal (Figures S12A and B), the F distribution is in good agreement with the zeolite crystal, demonstrating the uniform dispersion. A tomography of Y-CF3-15 crystal provided a section view of the crystal, which avoids the -CF3 groups distributing on the external surface of zeolite crystal. Interestingly, the EDS analysis

For the organosilane-functionalized zeolite framework, the open microporous channels are well maintained as confirmed by their N2 sorption isotherms. For example, the Y-Me-15 still gives high surface area (437 m2/g) and pore volume (0.2 cm3/g, Table S1 and Figure S4). Figure S18 shows contact angles of waterdroplet on the surface of various samples to evaluate their wettability. The conventional Y zeolite gives the water-droplet contact angle (CA) of 27.4°, demonstrating the hydrophilicity. Introducing the methyl or phenyl groups into the zeolite framework remarkably enhanced the CA values, giving the CA of 113.2°over Y-Me-15 and 106.6°over Y-Ph-15 samples, confirming the hydrophobicity (Figures S18 and S19). On the basis of the aforementioned data, a hydrophobic zeolite framework was constructed, as well, the block of the orifice could be effectively avoided. Figure 2A shows water adsorption capacity over various Y zeolites in a stationary adsorption at room temperature for 1 h. The conventional Y zeolite efficiently adsorbed 89 mg/g of water owing to the hydrophilic surface, while the hydrophobic zeolite framework remarkably hindered the water adsorption (e.g. 14 mg/g over Y-Me-15). Figure 2B gives the dependences of zeolite crystallinities on time in the hydrothermal treatment of Y and YMe-15 zeolites at 200 °C. The crystallinity of Y zeolite significantly decreased with the hydrothermal treatment time increases, owing to the cleavage of the Si-O-Si/Si-O-Al bonds, while the crystallinity of Y-Me-15 zeolite is almost unchanged within errors (350 nm). The green arrows highlighted the injection of water. Furthermore, we measured the adsorption efficacy of conventional Y and Y-Me-15 in one-pass tests in a fixed bed reactor (a quartz tube with fixed catalyst in a black box to keep away from

light, Scheme S2, see Supporting Information for details) with continuous flow of formaldehyde (90 ppm in air, Figures 3A-C), where the formaldehyde concentration in the emission gas was detected. As shown in Figure 3B, the formaldehyde was undetectable in the beginning of the tests because of the complete adsorption of formaldehyde over both samples. When the tests lasted for 120 min, the formaldehyde appeared in the emission gas over Y zeolite, and the value reached 90 ppm after 200 min, demonstrating the saturated adsorption. Interestingly, the complete removal of formaldehyde lasted for 150 min over Y-Me-15, and the saturated adsorption occurred until 480 min. These data confirm the superior efficiency of Y-Me-15 than the conventional Y zeolite in capture of formaldehyde. When water was introduced into the feed gas during the tests, as shown in Figure 3C, the capture of formaldehyde over Y zeolite was immediately switched off, because water was more favourable to diffuse into the hydrophilic Y zeolite micropores in the competition with formaldehyde. Interestingly, water in the feed gas does not influence the formaldehyde removal over YMe-15, giving very similar adsorption curves to that in the waterabsent test. These data demonstrate the unusual adsorption selectivity, which is potentially important for the practical applications to the selective adsorption of volatile organic compounds in wet atmosphere. By fixation of the P25 TiO2 inside of the Y-Me-15 (TiO2@YMe-15, Figure S24), the selective capture and photocatalytic degradation were effectively combined. After saturated capture of formaldehyde in the dark, the TiO2@Y-Me-15 can be fully regenerated by simple irradiation within sunlight frequencies (xenon lamp with a filter of λ>350 nm, 252 W) in flowing air at room temperature, giving unchanged formaldehyde adsorption capacity in the 2nd run (Figures S25 and S26). Even after 6 recycles in the cascade capture and irradiation, the decrease in efficiency for removal of volatile organic compounds is undetectable, confirming the good recyclability (Figures S27-S29). Notably, when the formaldehyde (90 ppm in air) was flowed through the TiO2@Y-Me-15 in the quartz tube reactor under irradiation (xenon lamp with a filter of λ>350 nm, 252 W) in a continuous test (see Supporting Information for details about the reactor), the complete removal of formaldehyde was achieved in a long period of 80 h, giving CO2 in the emission gas. Even if gaseous water was injected into the reaction system, the complete removal of formaldehyde is still achieved because the hydrophobic zeolite sheath hindered the water diffusion into the catalyst. In contrast, when the test was performed over the TiO2 or conventional Y zeolite fixed TiO2 (TiO2@Y) catalysts, the catalysts were deactivated immediately when gaseous water was injected, because water was adsorbed into the zeolite or covered on the TiO2, which hindered the capture and degradation of formaldehyde. In addition, a physical mixture of TiO2 and Y-Me-15 (weight ratio of TiO2/zeolite at 1/2) shows enhanced water tolerance compared to the TiO2 and TiO2@Y, but it is still difficult to achieve complete formaldehyde removal with water in the feed gas. As shown in Figure 3D, injection of water caused an increase of formaldehyde concentration in the emission gas over the physically mixed catalyst, demonstrating the unsatisfactory water tolerance. These data confirm the superior efficacy of TiO2@Y-Me-15 in removal of wet formaldehyde, outperforming the physical mixture of TiO2 and Y-Me-15, TiO2@Y, and pure TiO2, which is reasonably attributed to the fixation of TiO2 within the HP-zeolite crystal with the core-shell structure to maximize the synergistic effect of wettability-selective sheath and photocatalytically active TiO2 particles. In summary, we reported a new concept of wettability-selective catalyst, which was realized by fixation of TiO2 particles within the zeolite sheath with controllable wettability. Such materials combined the functions of wettability selectivity on zeolite sheath

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and photocatalytic activity on TiO2. On the basis of such features, the hydrophobic zeolite fixed TiO2 (TiO2@HP-zeolite) displayed superior performances in complete removal of wet formaldehyde in a long-period continuous test under irradiation (λ>350 nm). In this design, the key success is to construct a core-shell structure to form a water-tolerance catalyst, which maximizes the synergy between the zeolite adsorbent and metal oxide catalyst, exhibiting superior performances to outperform the catalysts with poor water tolerance. The concept of wettability-selective catalysts may open an avenue for developing highly efficient heterogeneous catalysts for removal of volatile organic compounds in the future.

ASSOCIATED CONTENT Supporting Information Experiment details, XPS, XRD, N2 sorption, TG and more adsorption data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Email: [email protected] (L.W.) Email: [email protected] (F.S.X.)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by the National Key Research and Development Program of China (2017YFC0211101), National Natural Science Foundation of China (91645105, 91634201, and U1462202), Natural Science Foundation of Zhejiang Province (LR18B030002), and Foundation of State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (Grant No.2017-K12).

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ACS Catalysis

HCHO HCHO HCHO

zeolite air

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