Gallium-immobilized carbon nanotubes as solid templates for the

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Kinetics, Catalysis, and Reaction Engineering

Gallium-immobilized carbon nanotubes as solid templates for the synthesis of hierarchical Ga/ZSM-5 in methanol aromatization Yu-Yin Chen, Ching-Jung Chang, Hwei V. Lee, Joon Ching Juan, and Yu-Chuan Lin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00726 • Publication Date (Web): 19 Apr 2019 Downloaded from http://pubs.acs.org on April 19, 2019

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Industrial & Engineering Chemistry Research

Gallium-immobilized carbon nanotubes as solid templates for the synthesis of hierarchical Ga/ZSM-5 in methanol aromatization Yu-Yin Chen,a Ching-Jung Chang,a Hwei Voon Lee,b Joon Ching Juan,b Yu-Chuan Lina,* a

Department of Chemical Engineering, National Cheng Kung University Tainan 70101, Taiwan

b

Nanotechnology & Catalysis Research Centre, Institute of Postgraduate Studies University of Malaya, Kuala Lumpur 50603, Malaysia *Corresponding author’s email: [email protected]

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Hierarchical Ga/ZSM-5 was synthesized by using a combinative method of steam-assisted crystallization (SAC) and hard templating. Ga-immobilized carbon nanotubes (GaCNTs) were used as hard templates. Comparing with Ga-doped ZSM-5 catalysts made by impregnation and by sequential CNT-templating and impregnation, hierarchical Ga/ZSM-5 had a moderate mesoporosity and possibly a high concentration of (GaO)+-Brønsted acid site. The moderate mesoporosity shortens residence times of products in ZSM-5 matrix, while the high concentration of (GaO)+-Brønsted acid enhances aromatization activity. These advantages are critical to improve catalytic performance and durability of hierarchical Ga/ZSM-5 in methanol aromatization.

Keywords: aromatics; carbon nanotubes; gallium; methanol; ZSM-5 2 ACS Paragon Plus Environment

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Introduction Mesoporous MFI zeolites (hierarchical ZSM-5) have solid acidity and integrated micro- and mesoporous systems. A hierarchical ZSM-5 has benefited from reduced diffusion resistances of reactants and products.1 A longer stability is usually achieved by using a hierarchical ZSM-5 than by using a pristine ZSM-5 because side reactions (e.g., coking) evolved in micropores can be suppressed.2-3 Introducing heteroatoms such as Ag,4 Zn,5 and Ga6 in ZSM-5 is a common way to manipulate product distributions. Among metal dopants, Ga is prone to enhance aromatics yields due to its dehydrogenation nature. A Lewis acidic extra-framework Ga species in contact with a nearby Brønsted acid has been known to be aromatization-active.7-8 However, a major challenge to prepare Ga-doped ZSM-5 is the low diffusivity of hydrated Ga ions (precursors of gallyl species) in microporous channels. Therefore, immobilizing Ga cations by using conventional impregnation method frequently yields poorly dispersed Ga2O3 on the exterior surface of ZSM-5. Strategies such as isomorphous substitution of Al by Ga in ZSM-5 framework,9-11 subsequent reduction and oxidation after impregnation,12-14 chemical vapor deposition,15-16 and hierarchicalization of ZSM-5 followed by impregnation17-18 have been developed to overcome the transport limitation of Ga precursors in synthesizing highly dispersed, extra-framework Ga species in ZSM-5. The hard templating method using different types of carbon-based materials, such as carbon black, carbon fibers, and carbon nanotubes (CNTs), as sacrificial agents is flexible in designing size, shape, and connectivity of mesoporous network in ZSM-5.19 Supported metal on carbon material (such as Mo/CMK-3,20 Co/mesoporous carbon,21 Co/CNT,22 or Zn zeolitic imidazolate framework-8 (ZIF-8)23 has been used as hard templates to prepare metal-doped hierarchical ZSM-5: mesopores can be developed by a combustive removal of embedded template and carried promoter can be anchored concurrently. Sometimes, carried metal, e.g. Co, can induce different germination behaviors during zeolitization.22, 24 Surprisingly, according to a recent review of Feliczak-Guzik25 and our best knowledge, synthesizing hierarchical Ga/ZSM-5 catalyst by using Ga-immobilized CNTs as hard templates has not yet been fully explored. This study used Ga-immobilized CNTs (GaCNTs) as hard templates to synthesize hierarchical Ga/ZSM-5. Hierarchical Ga/ZSM-5 made through a 3 ACS Paragon Plus Environment


combinative method of steam-assisted crystallization (SAC) and hard templating showed better performances of methanol aromatization (MTA) than those of its counterparts made by impregnation and by consecutive hierarchicalization-impregnation. The enhanced MTA performance of hierarchical Ga/ZSM-5 can be attributed to an improved mesoporosity and an enhanced permeability of Ga species in ZSM-5 cavities. An improved mesoporosity allows cracking of aromatics to be suppressed, while an enhanced permeability of Ga species may lead to an increased concentration of aromatization-active (GaO)+-Brønsted site.7, 26 Both factors are important to elevate aromatics yield and to improve deactivation resistance of hierarchical Ga/ZSM-5 in MTA. Experimental Catalyst synthesis The SAC technique27-28 was used to synthesize ZSM-5 based catalysts. In short, proper amounts of aluminum nitrate and tetrapropylammonium hydroxide (TPAOH, Alfa Aesar, 40% aqueous solution) were completely dissolved in deionized water, following by adding tetraethylorthosilicate (TEOS, Acros 98%) dropwise into the solution to form a transparent mixture. The molar composition of the mixture was 1.5 Al: 60 Si: 16 TPAOH: 5400 H2O. After vigorous agitation for 24 h, the yielded sol was dried in an oven at 80 oC for 20 h. The resultant powder was collected and placed into a 4 mL PTFE-made cup. The cup was then transfer into a 100-mL autoclave, which was prefilled with 4 mL deionized water. The autoclave was then sealed and moved to an oven to be heated at 175 oC for 24 h. The resultant solid was washed with deionized water several times until the pH value of the filtrate was at approximately 7. The paste was dried at 60 oC overnight, and calcined at 600 oC for 20 h to obtain HZSM-5 (denoted as HZ). Single-walled CNTs were purchased from Golden Innovation Business Company (Taiwan). The diameter of CNTs is approximately 10-20 nm with the length of approximately 10-20 μm. Prior to be used, CNTs were treated in 68% nitric acid solution (a ratio of 0.3 g CNTs/25mL acid solution) at room temperature for 15 h under mild stirring. The nitric acid treatment was employed to remove the catalyst used in CNTs synthesis and to enhance amorphization (increasing surface carboxyls and hydroxyls) of CNTs to facilitate subsequent Ga immobilization.29-30 4 ACS Paragon Plus Environment

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Ga(NiO3)3*6H2O was used as the precursor to prepare 5.0 wt% Ga/CNTs by using incipient wetness method. Approximately 0.5 M of Ga(NiO3)3 solution was impregnated on 1 gram of acid-treated CNTs. The residual paste was air-dried and then treated in a N2 stream (100 mL/min) at 350 oC for 3 h to obtain GaCNTs. Acid-treated CNTs and GaCNTs were used as solid templates to synthesize hierarchical ZSM-5 (CNT-HZ) and hierarchical Ga/ZSM-5 (GaCNT-HZ), respectively, using the aforementioned SAC method. Approximately 0.52 g of CNTs or GaCNTs was added into the mixture of aluminum nitrate, TEOS, TPAOH, and H2O in the middle (the 12th h) of the agitation time (24 h). Implanted carbon templates were removed after subjecting to a calcination step at 600 oC for 20 h. An equivalent of 1.8 wt% Ga loading was used to prepare GaCNT-HZ. For comparison, 1.8 wt% Ga supported on HZ (Ga/HZ) and on CNT-HZ (Ga/CNT-HZ) were prepared by incipient wetness method. Characterization The N2 adsorption-desorption isotherm was measured by using a ASAP 2020 Plus (Micromeritics). The Brunauer-Emmett-Teller (BET) method in the relative pressure range of 0.01 to 0.1 was used to estimate the equivalent surface area (SBET).31 The t-plot method was used to access microporosities of HZ and Ga/HZ. Because of a possible underestimation of microporosity of hierarchical samples by using the t-plot method, an abacus for t-plot analysis correction32 was used to estimate microporosites of CNT-HZ, Ga/CNT-HZ and GaCNT-HZ. Powder X-ray diffraction (XRD) analysis was performed on a diffractometer (Rigaku D/Max-IIB) at 40 kV and 40 mA with Ni-filtered Cu Kα radiation. Synchrotron small-angle X-ray scattering (SAXS) patterns were collected in transmission mode with the samples packed in lithium borate glass capillaries (0.3 mm diameter and 0.01 mm thickness) at beamline 01C2 of Taiwan Light Source. An inductively coupled plasma-atomic emission spectrometry (ICP-AES, Kontron S-35) was used to quantify the compositions of Si, Al, and Ga. The morphology of tested catalyst was observed by a cold field emission scanning electron microscopy (SEM, Hitachi SU8010) and high-resolution transmission electron microscopy (HR-TEM, JEOL JEM-2010). The 27Al and 29Si magic angle spinning nuclear magnetic resonance (MAS NMR, Bruker Avance 400) spectra were obtained at the spinning speeds of 10 and 8 kHz in the magnetic field of 5 ACS Paragon Plus Environment


9.4 T. The 27Al spectra were recorded using a 3-μs single-pulse time (90°) with a 4-s acquisition delay. For the 29Si spectra, a 3-μs single-pulse time and a 60-s delay were used. The NMR spectra were normalized to the weight of catalyst. The X-ray photoelectron spectroscopy (XPS) spectrum was recorded by using an AXIS Ultra DLD Kratos spectrometer equipped with a monochromatized aluminum source with a wavelength at 1486.6 eV. The C 1s binding energy of adventitious carbon at 285.0 eV was used to correct the energy shift. The tests of temperature-programmed reduction of H2 (H2-TPR), temperature-programmed desorption of NH3 (NH3-TPD), isopropylamine (IPA)-TPD, and temperature-programmed surface reaction (TPSR) were performed by using a chemisorption analyzer (AutoChem-II, Micromeritics) and the signals were recorded by a thermal conductivity detector or a quadrupole mass gas analysis system (ThermoStar GSD 320 T, Pfeiffer Vacuum). Detailed pretreatments and operating conditions of temperature-programmed analyses were reported elsewhere.17-18 Commercial HZSM-5 (Zeolyst CBV 8014, Si/Al = 40), possessing a total acidity of 420 μmol/g33 and a concentration of Brønsted acid of 335 μmol/g,34 was used as the standard to estimate acidity of each catalyst. For TPSR analysis, a 0.1 g of sample was consumed per trial and a 10 oC/min heating rate in a 54.7% methanol/He stream (50 ml/min) was used, corresponding to a contact time of 0.04 min*kgcat/mol. Fourier-transform infrared (IR) spectra of dehydrated and pyridine adsorption (Py-IR) samples were recorded using a Thermo Scientific Nicolet iS50 spectrometer and an in situ quartz cell. The dehydrated sample was treated at 400 °C for 3 h under a vacuum of 1.2 × 10-3 Pa. The dehydrated sample was then cooled to 150 °C and exposed to pyridine vapor. Excess pyridine was removed at 350 °C under a vacuum of 1.2 × 10-3 Pa for 1 h, and the spectra were recorded at 350 °C.8 Activity evaluation A continuous flow fixed-bed reactor connected to an on-line gas chromatography (GC) was used as the activity evaluation system. The contact time was set at 0.42 kgcat*min/mol (WHSV = 4.6 h-1) and reaction temperatures were set at 450 oC and 500 oC. Detailed reactor setup, operating procedures, pretreatments and reaction conditions, and calculations of conversion of reactants (methanol and dimethyl ether (DME)) and yields of products can be found elsewhere.17-18


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Results and discussion Table 1 presents the elemental composition of each catalyst estimated by ICP analysis. The bulk Si/Al ratios ranged from 36 to 41. The Ga loadings were in the range of 1.6 wt% to 2.2 wt%. All these numbers are close to the designated values of Si/Al ratio (40) and the Ga loading (1.8 wt%). Figure 1 shows the SEM images of tested catalysts. All samples showed sponge-like external surface with aggregated particles of approximately 1 μm. Aggregated particles suggests possibly assembled nanounits of SAC-synthesized HZ catalysts.35 Figure 1 also exhibits the TEM images. Several dark spots in the range of 4 to 10 nm were ascribed to Ga2O3 particles17 of Ga/HZ and Ga/CNT-HZ. In contrast, no Ga2O3 clusters appeared on GaCNT-HZ. Figure 2 displays the XRD patterns of tested catalysts. All catalysts had characteristic responses of MFI-type zeolite. A semiquantitative analysis of relative crystallinity was conducted by using the peak intensity of (5 0 1) diffraction plane of each catalyst using commercial HZSM-5 (Zeolyst CBV 8014, not shown) as the reference. The relative crystallinity showed a descending trend following the order as: HZ (87%) > Ga/HZ (83%) > CNT-HZ (77%) > Ga/CNT-HZ (70%) > GaCNT-HZ (68%). This descending trend implied enhanced amorphization causing by CNT-templating and Ga-incorporating. The diffractions of Ga2O3 were undetectable, implying a finely dispersed Ga species in Ga/HZ, Ga/CNT-HZ, and GaCNT-HZ. Moreover, no peak shift was observed for the (5 0 1) plane of MFI zeolite at 2θ = 23.4o, suggesting Ga species were in extra-framework positions.8 Table 2 lists the porosities and Figure 3 shows the N2 adsorption-desorption isotherms and estimated pore size distribution (PSD) using the Barrett-Joyner-Halenda (BJH) method. The values of SBET of HZ (489 m2/g) and CNT-HZ (491 m2/g) were slightly higher than those of Ga/HZ (451 m2/g), Ga/CNT-HZ (447 m2/g), and GaCNT-HZ (455 m2/g), implying Ga immobilization can deteriorate surface area partially. The surface areas of micropore (Smicro) of HZ and Ga/HZ were greater than 300 m2/g, while CNT-HZ, Ga/CNT-HZ, and GaCNT-HZ were in the range of 268 m2/g to 295 m2/g. The surface areas of mesopores (Smeso) of CNT-HZ, Ga/CNT-HZ, and GaCNT-HZ seemed to be compensated by their lower Smicro values, showing greater Smeso values (160-201 m2/g) than those of HZ (141 m2/g) and Ga/HZ (137 m2/g). The lower values of Smicro of 7 ACS Paragon Plus Environment


CNT-HZ, Ga/CNT-HZ, and GaCNT-HZ suggests that amorphization was enhanced by embedded CNTs.36 The higher Smeso values of CNT-templated catalysts implied the presence of newly developed mesopores. CNT-templated catalysts had higher total pore volumes (Vtotal, 0.46-0.69 cm3/g) compared to that of non-templated counterparts (0.38 cm3/g). Since the volumes of Vmicro were similar (0.12-0.14 m3/g), the improved Vtotal values can be attributed to the greater mesoporosities of CNT-templated catalysts (0.33-0.56 m3/g). The PSD plots showed two responses at 3-4 nm and 10-40 nm regions of CNT-templated catalysts. The former is attributed to the artifacts of forced closure of sorption hysteresis;37 the latter, confined intercrystalline mesopores by burning engulfed CNTs at the calcination step.38 Lower volume of mesopore (Vmeso) was found for GaCNT-HZ (0.33 cm3/g) than those of CNT-HZ (0.56 cm3/g) and Ga/CNT-HZ (0.51 cm3/g). The decreased value of Vmeso of GaCNT-HZ is possibly due to an increased extent of entangled GaCNT after subjecting to a thermal treatment in Ga immobilization. The TEM images of fresh CNT and GaCNT (see Figure S1 of Supporting Information) further supported this claim: fresh CNTs were loosely dispersed while some GaCNTs were agglomerated. It is also worth mentioning that Ga2O3 clusters were mainly located at the external surface of GaCNTs and deposited carbonaceous impurities were absent (see Figure S1). This indicated that the nitric acid treatment used in this study improved the extent of surface amorphization of CNTs, but did not open the ends of CNTs or damage the walls. Figure S2 of the Supporting Information displays the SEM and TEM images of uncalcined CNT-HZ and GaCNT-HZ. Partially encapsulated CNTs in the nanocrystals and entangled CNTs clusters separating from nanocrystals were observed. Hence, CNTs can act as mesopore-filling agents to create confined space and can be transient inhibitors to suppress zeolitization into large crystals.39 These two factors were attributed to the improved mesoporosities of CNT-templated catalysts. In addition, the SAXS pattern of each catalyst (see Figure S3 of the Supporting Information) had no response in the 2θ range of 1o-5o, suggesting the CNT-induced mesoporous network was amorphous. Figure 4 shows the 27Al and 29Si NMR spectra. The resonance of framework tetrahedral Al (Al(Td), 54 ppm) and the resonance of non-framework octahedral Al (Al(Oh), 0 ppm) can be seen.40 The stronger Al(Td) response of HZ than those of the 8 ACS Paragon Plus Environment

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other catalysts underlines that CNT-templating and Ga impregnation do enhance amorphization. Ga/CNT-HZ and GaCNT-HZ seemed to have more non-framework Al species than the other catalysts did because of their pronounced shoulders of Al(Oh) in 0-10 ppm,41 in line with their lower crystallinities. The resonances of Si(0Al) at −112.0 ppm and Si(1Al) at −105.0 ppm42 were identical for all samples, suggesting the effect of deshielding of Si nuclei caused by the replacement of Al with larger Ga cations43 were absent. That is, doped Ga species were located in the non-framework positions. The absence of framework Ga species is also supported by the IR spectra of dehydrated samples, shown in Figure S4. The vibrations of bridging OH group of Si(OH)Al at 3610 cm-1 and the vibrations of free and internal Si-OH and Ga-OH at 3726 cm-1 were identified; however, the characteristic band of Si(OH)Ga-bridged Brønsted acids9 at 3617 cm-1 could not be seen. Therefore, the presence of framework Ga species was excluded. Figure S5 shows the profiles of NH3-TPD and Table 3 lists the estimated overall acid concentration. Noted that the low-temperature peak of NH3-TPD is caused by physically adsorbed NH3, and the overall acid concentrations were estimated by using the response of high-temperature peak.44 The overall acid concentrations were in the range of 188 μmol/g to 264 μmol/g. The overall acid sites can be discriminated into Brønsted acids (BAS) and Lewis acids (LAS) by using the results of IPA-TPD. An IPA molecule can be decomposed into an ammonia and a propylene over a BAS via the Hofmann degradation sequence.45 Hence, the concentration of BAS can be estimated by using the desorbed amounts of ammonia (m/e = 17)46-47 (see Table 3 and Figure S6 of Supporting Information). It is worth mentioning that the concentrations of overall acidity and BAS of tested catalysts were lower than those of commercial HZSM-5, possibly due to considerable amounts of amorphous alumina and silica existed in tested catalysts using the SAC method.48 Moreover, the parent HZ had a higher BAS concentration than those of other catalysts, indicating hierarchicalization and Ga impregnation can further demolish the protons of framework oxygens. The concentration of LAS can then be calculated by subtracting the concentration of the BAS from the overall concentrations of acids, listed in Table 3. Table 3 also presents the ratios of BAS-to-LAS (B/L). Generally, Ga-impregnated catalysts had higher LAS and lower B/L ratios than those of un-promoted counterparts. The enhanced LAS concentration 9 ACS Paragon Plus Environment


can be attributed to impregnated Ga species.49 Figure 5 shows the IR spectra of pyridine-adsorbed catalysts. The band in 1440-1470 cm-1 was assigned to coordinately adsorbed pyridine on LAS; in 1520-1560 cm-1, the band of protonated pyridinium ion on BAS. A close inspection of the response of LAS-adsorbed pyridinium ion showed an ascending order as: HZ (1453.1 cm-1) = CNT-HZ (1453.1 cm-1) < Ga/HZ (1454.1 cm-1) = Ga/CNT-HZ (1454.1 cm-1) < GaCNT-HZ (1455.0 cm-1). This ascending order suggests the tendency of increased strength of LAS of respective catalysts. It is worth mentioning that strong Lewis acidic (GaO)+ species are aromatization-active.7, 50 The signals of protonated pyridinium ion on BAS were at approximately 1542 cm-1 for all catalysts. The close band positions of protonated pyridinium ion suggested similar acidities of BAS of tested catalysts. Figure 6 shows the H2-TPR profiles of Ga-containing catalysts. The profiles can be deconvoluted into three responses at approximately 500 oC, 700 oC, and 800 oC, corresponding to the reductions of small Ga2O3 clusters, (GaO)+ ions, and large Ga2O3 clusters.14, 47, 51 A (GaO)+ ion is an oxo Ga3+ species located in the cavities of ZSM-5 at the cation position.16, 52 The relative composition of the reduction of small Ga2O3 clusters was similar (21%-25%) for each Ga-containing catalyst. The relative composition of (GaO)+ response increased in the order as follows: Ga/HZ (26%) < Ga/CNT-HZ (49%) < GaCNT-HZ (75%). Moreover, GaCNT-HZ had negligible reduction response of large Ga2O3 particles, whereas the reduction responses of large Ga2O3 accounted for 30% of Ga/CNT-HZ and for 53% of Ga/HZ. The absence of reduction response of large Ga2O3 clusters of GaCNT-HZ indicated that using GaCNTs as the carrier of Ga is more effective to facilitate Ga migration into the inner cavities to form (GaO)+ than by using impregnation is. To validate the H2-TPR results, Ga 2p XPS spectra were recorded and Py-IR measurements of Ga-containing catalysts treated by a 3-cycled reduction-oxidation (RO)53 were conducted. The binding energy of Ga 2p peak (see Figure S7) showed an ascending trend following the order as: Ga/HZ (1118.2 eV) < Ga/CNT-HZ (1118.4 eV) < GaCNT-HZ (1118.8 eV). The higher binding energy of Ga 2p peak indicated the stronger interaction between Ga species and ZSM-5 support.54 Since (GaO)+ has a stronger interaction with ZSM-5 than that of Ga2O3,55 the highest binding energy of Ga 2p response of GaCNT-HZ implied that it had the most abundant (GaO)+ among 10 ACS Paragon Plus Environment

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Ga-containing catalysts. The RO treatment allows Ga2O3 to re-disperse and to penetrate into the intracrystalline space, forming (GaO)+ with a stronger Lewis acidity than that of Ga2O3.53 Figure S8 exhibits the Py-IR analysis of each RO-treated catalyst. A noticeable growth of intensity of LAS response was observed for RO-treated Ga/HZ or for Ga/CNT-HZ, whereas a slight decrease of LAS intensity of RO-treated GaCNT-HZ was found. Furthermore, a blue-shifted LAS response from 1454.1 cm-1 to 1455.2 cm-1 was observed for the RO-treated Ga/HZ and from 1454.1 cm-1 to 1455.0 cm-1 for that of Ga/CNT-HZ. The peak position of LAS response was nearly identical for GaCNT-HZ before and after the RO treatment. The increased amplitudes and blue-shifted signals of Ga/HZ and Ga/CNT-HZ suggested Ga2O3 particles were disassembled and then re-dispersed to form (GaO)+ in micropores during the RO treatment. However, re-dispersion of Ga species was insignificant for RO-treated GaCNT-HZ since most of Ga species were already in the form of (GaO)+. Figure 7 displays the MS fragments of H2 (m/e = 2), C2H4 (m/e = 27), C3+ hydrocarbons (m/e = 41), and xylene (m/e = 91) of TPSR measurements. These compounds were produced simultaneously at approximately 350 oC, in agreement with the co-processing mechanism of alkene- and arene-cycles of methanol conversion in ZSM-5.56 For Ga-containing catalysts, the signals of H2 and xylene increased coherently (xylene-to-H2 response ratio is at approximately 0.2-0.3) with increasing temperature, underlining the correlation between aromatization and dehydrogenation (i.e., cyclodehydrogenation) activities. Among them, stronger H2 and xylene amplitudes were obtained by using GaCNT-HZ than by using Ga/CNT-HZ and Ga/HZ, indicating the higher cyclodehydrogenation activity of GaCNT-HZ. In contrast, the signals of H2 and xylene of CNT-HZ and HZ were trivial. Non-framework Ga species, either inside or outside of micropores, have been noted to have dehydrogenation activity.57 Herein, the TPSR of physical mixture of β-Ga2O3 and CNT-HZ (equivalent to a 2:98 weight ratio) was performed, shown in Figure S9. Relatively weak responses of H2 and xylene were detected comparing to those of Ga-containing catalysts. Moreover, signals of H2 and xylene did not grow coherently: desorbed xylene was observed at ~355 oC while the onset temperature of H2 was at ~438 oC. The weak responses and irrelevant desorption trends of xylene and 11 ACS Paragon Plus Environment


H2 implied that aromatization and dehydrogenation proceed independently over physically mixed β-Ga2O3 and CNT-HZ. In other words, the contact synergy of doped Ga species and Brønsted acid in micropores is important to promote cyclodehydrogenation activity.58-59 Another difference between Ga-containing and Ga-free catalysts is the evolved C2H4 signals. For Ga-catalysts, the C2H4 signal firstly increased, reaching a plateau at 400-450 oC, and then decreased at above 450 oC; for Ga-free catalysts, their C2H4 signals grew steadily. Different trends of evolved C2H4 implied that C2H4 may be converted at above 450 oC by using Ga-containing catalysts. Table 4 listed the catalytic results of MTA at 450 oC and 500 oC. Methanol and DME were completely consumed (100% conversion) for all trials. C3 (propene and propane), C4-C7, and aromatics are major products with trace C1 (methane) and C2 products (ethylene and ethane) for all catalysts. In general, C1, C2, and aromatics increased while C3 and C4-C7 decreased with increased temperature. Methane and ethane are hydrogen-transfer products60-61 and ethylene is originated from dealkylation of methylated aromatics in the micropores;56, 60 these species may be bystanders in MTA;8, 17-18 C3 and C4-C7 are mainly generated from the alkene cycle,60-61 and are precursors of aromatics through cyclodehydrogenation.57, 62 Aromatics yields of HZ and CNT-HZ were all less than 50%. Higher aromatics were obtained by using Ga-containing catalysts. The aromatics yield of Ga-containing catalyst at 500 oC is increased in the order as follows: Ga/HZ (58.8%) < Ga/CNT-HZ (61.2%) < GaCNT-HZ (72.6%). This highlighted that aromatization activity depends heavily on the preparation methods, and GaCNT-HZ is the most effective in MTA. However, the values of fraction of benzene, toluene, xylenes (BTX) in aromatics of HZ (0.84) and Ga/HZ (0.83) were higher than those of CNT-HZ (0.79), Ga/CNT-HZ (0.81), and GaCNT-HZ (0.77) at 500 oC. The lower BTX fractions of CNT-templated catalysts can be attributed to improved mesoporosity and increased amorphization: the former facilitates larger aromatics to diffuse out of micropores1 while the latter enhances isomerization and alkylation of xylenes by acids on external surfaces.41, 61 Figure 8 shows the fractional changes in selectivities of C2, C3-C7, and aromatics of CNT-HZ, Ga/HZ, Ga/CNT-HZ, and GaCNT-HZ using the respective product selectivities of HZ at 500 oC as the basis. A marginal decrease of C2 (~1%) together with a 60% increase of aromatics and 35% decrease of C3-C7 was found by using 12 ACS Paragon Plus Environment

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CNT-HZ, suggesting mesopores do suppress cracking of aromatics to light olefins.1 Fractional increases of aromatics and decreases of C3-C7 were all higher for Ga-containing catalysts than those of CNT-HZ. Among tested catalysts, GaCNT-HZ has the highest aromatization activity since the highest fractional increase of aromatics (1.33) and the highest decrease of C3-C7 (67%) were achieved. Interestingly, C2 decreased for all Ga-doped catalysts significantly (33%-37%). The decreased C2 implied that not only aromatics cracking can be suppressed by improved mesoporosity, but also C2 conversion can be promoted by incorporate Ga species.17, 63 Comparing with mesoporosity and doped Ga, acidity seemed to play a trivial role in affecting C2 distribution because no correlation could be found. Figure 9 shows the time-on-stream test of each catalysts at 500 oC. The conversion fell with prolonged time, and was recorded until more than 20% of the reactants were unconsumed. Hence, the lifetime was defined as the time at which conversion decreased to be lower than 80%. For un-promoted catalysts, the lifetime of CNT-HZ (28 h) is higher than that of HZ (18 h); for Ga-promoted catalysts, the lifetimes of Ga/CNT-HZ (16 h) and GaCNT-HZ (15 h) are both higher than that of Ga/HZ (12 h). Apparently, the developed mesopores showed positive influence on the durability of either un-doped or Ga-doped catalyst due to the improved diffusivities of reactants and products.1 In sum, amorphous mesoporous structure and likely a high concentration of (GaO)+-Brønsted acid are vital to promote aromatization activity and durability in MTA. Among tested catalysts, hierarchical Ga/ZSM-5 prepared by a combinative method of steam-assisted crystallization (SAC) and GaCNT-templating is the most effective catalyst in MTA. How an improved permeability of Ga in ZSM-5 micropores by using GaCNTs as templates can be achieved is currently under investigation. Conclusions Ga-impregnated HZSM-5, Ga-impregnated mesoporous HZSM-5, and hierarchical Ga/ZSM-5 were synthesized by impregnation, consecutive hierarchicalization-impregnation, and combinative steam-assisted crystallization and hard templating methods, respectively. Among them, hierarchical Ga/ZSM-5 is discovered to be the most promising: not only mesoporosity but also (GaO)+-Brønsted 13 ACS Paragon Plus Environment


sites in micropores could be improved. Developed mesopores can alleviate cracking of aromatics, while (GaO)+ species in micropores are more active than that of Ga2O3 particles in cyclodehydrogenation. These two advantages are attributed to the enhancements of aromatics yield and deactivation resistance of hierarchical Ga/ZSM-5 in MTA. Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Information includes the TEM images of CNTs and GaCNTs; SEM and TEM images of uncalcined CNT-embedded precursors; SAXS patterns; IR spectra of dehydrates samples; NH3-TPD profiles; IPA-TPD profiles; XPS spectra of the Ga 2p level; Py-IR spectra of Ga-containing catalysts and their counterparts subjected to the RO treatment; TPSR profiles of physically mixed β-Ga2O3 and CNT-HZ. Corresponding Author *Tel +886 6 2757575 ext. 62668. Fax +886 2344496. Email: [email protected] Notes The authors declare no competing financial interest. Acknowledgements This study was supported by the Ministry of Science and Technology (Projects 106-2221-E-006-188-MY3 and 107-2218-E-155-001) and Taiwan’s Deep Decarbonization Pathways toward a Sustainable Society (Project 107-0210-02-19-01). The authors also appreciate Dr. Tai-Wei Tzeng and Prof. Po-Wen Chung (Institute of Chemistry, Academia Sinica) for SAXS analysis. References (1) Bjørgen, M.; Joensen, F.; Spangsberg Holm, M.; Olsbye, U.; Lillerud, K.-P.; Svelle, S., Methanol to gasoline over zeolite H-ZSM-5: Improved catalyst performance by treatment with NaOH. Appl. Catal., A 2008, 345, 43-50. (2) Pérez-Ramírez, J.; Christensen, C. H.; Egeblad, K.; Christensen, C. H.; Groen, J. C., Hierarchical zeolites: enhanced utilisation of microporous crystals in catalysis by 14 ACS Paragon Plus Environment

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Table 1. Composition of tested catalysts estimated by ICP-AES. Si/Al

Si/Ga

HZ

37

-

-

CNT-HZ

41

-

-

Ga/HZ

36

42

2.2

Ga/CNT-HZ

41

50

1.6

GaCNT-HZ

39

59

1.7


Ga(wt%)

Page 21 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Textural properties of tested catalysts SBET

Smicroa

Smeso

Vtotal

Vmicroa

Vmesob

(m2/g)

(m2/g)

(m2/g)

(cm3/g)

(cm3/g)

(cm3/g)

HZ

489

348

141

0.38

0.14

0.24

CNT-HZ

491

290

201

0.69

0.13

0.56

Ga/HZ

451

314

137

0.38

0.13

0.25

Ga/CNT-HZ

447

268

179

0.63

0.12

0.51

GaCNT-HZ

455

295

165

0.46

0.13

0.33

a

Estimated by the t-plot method

b

Calculated by Vtotal-Vmicro



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Table 3. Acidity of tested catalysts Overall acid

BAS

LAS

B/L ratio estimated

concentration

concentration

concentration

by NH3- and

(μmol/g)

(μmol/g)

(μmol/g)

IPA-TPD

HZ

204

100

104

0.96

CNT-HZ

188

78

110

0.71

Ga/HZ

204

75

129

0.58

Ga/CNT-HZ

202

67

135

0.50

GaCNT-HZ

264

78

186

0.42

Catalyst


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Table 4. Initial product selectivity of MTA of fresh catalysts at 450 oC or 500 oC with a contact time of 0.42 kgcat*min/mola Catalyst

T

C1

C2

C3

C4=

C4

C5-7

B

T

EB

CNT-HZ Ga/HZ Ga/CNT-HZ GaCNT-HZ 　 aC

1

o-X

m-X

(oC) HZ

p- +

C9

C10

∑

BTX/

Arom

Arom

Arom

Arom

450

0.6

5.9

27.5

13.3

12.1

16.1

6.1

3.2

0.6

7.3

2.7

4.0

0.6

24.5

0.81

500

1.9

8.2

25.9

11.6

9.9

11.4

4.0

5.4

0.6

11.8

4.2

4.5

0.6

31.1

0.84

450

1.2

6.1

20.0

9.7

10.5

17.5

4.5

6.5

0.9

12.1

3.8

6.3

0.9

35.0

0.79

500

3.7

8.1

17.1

5.9

6.1

9.3

4.7

8.6

1.2

18.4

6.2

9.3

1.4

49.8

0.79

450

0.9

5.7

15.6

6.5

9.9

17.4

3.0

5.7

0.9

20.8

4.5

8.3

0.8

44.0

0.79

500

1.9

5.5

12.1

4.7

6.6

10.4

3.1

10.1

1.3

27.7

6.6

9.5

0.5

58.8

0.83

450

0.9

5.1

16.0

7.1

8.0

13.0

3.9

8.4

1.3

21.3

5.2

9.0

0.8

49.9

0.80

500

2.7

5.2

12.4

4.4

5.5

8.8

3.8

9.7

1.8

26.7

7.3

10.5

1.2

61.0

0.81

450

1.1

5.7

12.3

4.9

6.3

9.6

2.6

12.4

1.5

22.9

7.9

10.8

2.0

60.1

0.79

500

2.9

5.3

8.4

2.7

3.4

4.7

3.5

12.2

1.7

29.7

9.1

14.3

2.1

72.6

0.77

= methane; C2 = ethene and ethane; C3 = propene and propane; C4= = butenes, C4 = n-, and i-butanes; C5-7 = C5-7 aliphatic compounds; B =

benzene; T = toluene; EB = ethylbenzene; p- + m-X = p- + m-xylenes; o-X = o-xylene; C9 Arom = trimethylbenzene and ethyl toluene; C10 Arom = durene; ∑ Arom = summation of selectivities of aromatics



(a)

Page 24 of 34

(f)

500nm

100nm

(b)

(g)

500nm

100nm

(c)

(h)

500nm

200nm


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(d)

(i)

500nm

(e)

100nm

(j)

100nm

500nm

Figure 1. SEM and TEM images of HZ (a and f), CNT-HZ (b and g), Ga/HZ (c and h), Ga/CNT-HZ (d and i), and GaCNT-HZ (e and j). The red circles indicate Ga2O3 clusters.



Figure 2. XRD patterns of tested catalysts


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Figure 3. Nitrogen adsorption-desorption isotherms and pore size distribution of tested catalysts.



Figure 4. 27Al and 29Si MAS NMR spectra of tested catalysts


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Figure 5. Pyridine adsorbed IR spectra of tested catalysts.



Figure 6. H2-TPR profiles of Ga-containing catalysts. The red-dashed line indicates the reduction of small Ga2O3 clusters; the green-dashed line, the reduction of (GaO)+; the blue-dashed line, the reduction of large Ga2O3 clusters.


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Figure 7. The MS fragments of H2 (m/e = 2), xylene (m/e = 91), C2H4 (m/e = 27), and C3+ hydrocarbons (m/e = 41) of TPSR spectra of tested catalysts.



Figure 8. Fractional changes in product selectivities by using the catalytic results of HZ as the basis at 500 oC.


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Figure 9. The time course of the conversion of methanol and DME at 500 oC with WHSV = 4.6 h-1.



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Gallium-immobilized carbon nanotubes as solid templates for the

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