Ga-Substituted Nanoscale HZSM-5 in Methanol Aromatization: The

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Kinetics, Catalysis, and Reaction Engineering

Ga-substituted nanoscale HZSM-5 in methanol aromatization: the cooperative action of the Brønsted acid and the extra-framework Ga species Chi-Ying Hsieh, Yu-Yin Chen, and Yu-Chuan Lin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00126 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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Industrial & Engineering Chemistry Research

Ga-substituted nanoscale HZSM-5 in methanol aromatization: the cooperative action of the Brønsted acid and the extra-framework Ga species Chi-Ying Hsieh, Yu-Yin Chen, and Yu-Chuan Lin* Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan *Corresponding author’s email: [email protected]

ABSTRACT. A series of nano-sized [Al]-, [Ga, Al]-, and [Ga]-HZSM-5 catalysts with a fixed Si-to-M3+ ratio (M = Al or Ga) was synthesized by a seed-induced crystallization method. In order to reveal the catalytic nature of the extra-framework Ga species, an acid treatment was applied to selectively extract Lewis acidic amorphous Ga cations of as-synthesized catalysts. A comparative evaluation of freshly prepared and acid-treated catalysts in methanol conversion to aromatics (MTA) showed the dehydrogenative nature of the extra-framework Ga species, which is essential in the enhancement of aromatics. Among tested catalysts, [Ga, Al]-HZSM-5 with a low extra-framework Ga-to-Brønsted acid ratio (0.06) was the most effective. The efficacy is due to the contact synergy of the extra-framework Ga species and the Brønsted acid, by which aromatics originated from the dual-cycle mechanism of methanol conversion can be accelerated.

KEYWORDS. aromatics; gallosilicate; methanol; nanoscale; ZSM-5

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Introduction Methanol conversion to aromatics (MTA) is propelled by the coal-based chemical industry in China. Since methanol conversion to hydrocarbons is a consecutive reaction of C-C bond formation, MTA is deemed to be a longer residence time process than that of methanol-toolefin (MTO), which focuses on C2-C5 alkenes production.1 Though still at its infancy, demonstration plants of MTA on the scale of 30 to 100 kt/a have been successfully launched, and a 1 Mt/a commercial plant is currently being designed.2 Unlike the chemistry of methanol-togasoline (MTG), which targets higher-octane gasoline products with a 35-40% weight ratio of aromatics at 350 oC, MTA aims to generate a high yield of aromatics (a 90% weight ratio) in the range of 400-500 oC.3 The differences in reaction conditions and favorable products imply that conventional catalysts used in MTG technology may not be as effective as they are used in MTA. Metal-promoted ZSM-5 zeolites are the most frequently encountered aromatization catalysts: ZSM-5 provides Brønsted acidity together with the confinement effect (also termed shape selectivity), while incorporated metal cations (such as Zn, Ga, or Ag) act as the centers of dehydrogenation. The combination thereof is effective to enhance aromatic yields through cyclodehydrogenation.4 A successful example in industrial practice is the UOP/BP Cyclar process, which employs Ga-impregnated ZSM-5 catalysts in aromatization of liquid petroleum gas.5 As for MTA, to our best knowledge, the highest aromatic yield at 80.3% by using Ag/HZSM-5 was reported by Inoue et al.6 Ag/HZSM-5 can generate aromatic yields approximately twofold higher than parent HZSM-5 can (43.0%). Nevertheless, the low thermal stability and costly price of silver hinder the industrial practice of Ag/HZSM-5 in MTA. Another key player to affect aromatic yields is the mass transport limitation of microporous network in ZSM-5. A common way to suppress the transport limitation is to


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improve mesoporosity or to decrease the crystalline size of ZSM-5 zeolites, either of which reduces the length of diffusion path and allows aromatics to move out of micropore cavities in a shortened residence time to avoid cracking. Mesoporous (or termed hierarchical) and nano-sized ZSM-5 zeolites synthesized by a constructive (such as the templating method) or a destructive method (such as dealumination and desilication) have been extensively reviewed.7-12 Gallosilicate MFI catalysts ([Ga]-ZSM-5) have been employed in aromatization of liquefied petroleum gas alkanes.13-16 Why the aromatization activity of [Ga]-ZSM-5 catalysts is great is because of the dehydrogenation nature of the incorporated Ga species, through which carbenium ions can be formed and subsequently transformed into aromatics by oligomerization and cyclization.17-18 Choudhary and Kinage19 tested [Ga]-ZSM-5 in a MTG process with a low aromatic yield of approximately 40% of hydrocarbon products at 400 oC. Recently, nano-sized [Ga]-ZSM-5 has been deployed in 1-hexene aromatization.20 An improved aromatic yield from 40% to 64% was found by replacing commercial HZSM-5 with nano-sized [Ga]-ZSM-5. Unexpectedly, limited information of applying nanocrystalline [Ga]-ZSM-5 in MTA is available. One open question of [Ga]-ZSM-5 is about the dehydrogenation nature of the extraframework (amorphous) Ga species. Hydrocarbons, particularly alkanes, are regarded as byproducts in MTA. Conversion of hydrocarbon intermediates through dehydrogenation followed by cyclization is a key step to increase aromatic yields. Various groups have asserted that the extra-framework Ga species is indispensable in the dehydrogenation of hydrocarbons, whereas some researchers claimed that the amorphous Ga species can be inert. Guisnet et al.21 proposed that the extra-framework Ga species can only be dehydrogenation-active with the help of the framework Ga species in [Ga]-ZSM-5. Faro et al.22 employed in situ X-ray absorption techniques to characterize amorphous Ga species of Ga-impregnated ZSM-5 catalysts, and they



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claimed that the isolated Ga+ ion coordinated to a framework oxygen atom (Si-O-Al) is aromatization-active. Hutchings et al.23 and our groups24-25 have reported the dehydrogenation nature of Ga2O3 and of the amorphous Ga species in Ga-supported ZSM-5 in MTA; however, the aromatization nature of the extra-framework Ga species of [Ga]-ZSM-5 in MTA is scarce. This study intends to investigate the role of the extra-framework Ga species of nano-sized aluminogallosilicate MFI catalysts in MTA. With a fixed Si-to-M3+ (M = Ga and/or Al) ratio of 40, the Ga/Al ratios from zero to infinite were used to synthesize nano-sized [Ga, Al]-ZSM-5 catalysts. The outcome reveals a proper composition of a Brønsted acid and the amorphous Ga species (Lewis acids) for aromatization, and the synergy of these two acids was explored. In this regard, a proper extra-framework Ga-to-Brønsted acid ratio should be considered in designing the [Ga, Al]-ZSM-5 catalyst in MTA.

Experimental Section Materials Nanocrystalline aluminosilicate, aluminogallosilicate, and gallosilicate MFI catalysts were synthesized by a seed-induced crystallization method.26 The seeding gel was prepared by mixing colloidal silica (Aldrich), tetrapropylammonium hydroxide (Acros), and deionized water with a molar ratio of 1.0: 0.4: 19.6 at 30 oC for 2 h. The mixture was then transferred into a hydrothermal bomb, treated at 100 oC for 24 h, and cooled down to room temperature to prepare the seeding gel.27 Sodium aluminate (Strem Chemicals) and gallium nitrate hydrate (Alfa Aesar) were used as aluminum and gallium sources. The mixture with a molar composition of 100 SiO2 : x Al2O3 : y Ga2O3 : 8 Na2O : 2500 H2O (x = 1.25, y = 0; x= 0.83, y = 0.42; x = 0.63, y = 0.63; x = 0.42, y = 0.83; x = 0, y = 1.25) was prepared and blended with the seeding gel (5 wt% with


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respect to the mixture) under vigorous stirring for 2 h. The resultant solution was then poured into a hydrothermal bomb for crystallization at 180 oC for 30 h. The yielded paste was collected by filtration, washed in deionized water for 3 times, and dried at 80 oC for 12 h. The dried powder was then calcined at 550 oC for 3 h in an air stream (50 mL/min). The calcined sample was transformed into the proton-form via the ion-exchange method.28 Hereafter, the nano-sized [Al]-HZSM-5, [Ga, Al]-HZSM-5 (Ga/Al = 1/2, 1/1, and 2/1) and [Ga]-HZSM-5 were denoted as HnAl, HnGa1Al2, HnGa1Al1, HnGa2Al1, and HnGa, respectively. Commercial ZSM-5 was obtained from Zeolyst (CBV 8014). In order to clarify the influence of the extra-framework Ga species in MTA, an acid treatment for freshly-prepared catalysts was employed to selectively remove the amorphous Ga species.29 Approximately 1 g of the sample was immersed in 20 mL of the 0.2 M HCl solution for 1 h at 60 oC. The remaining mixture was filtered, washed with deionized water for 3 times, dried at 80 oC for 12 h, and transformed into the proton form following the method mentioned above. Acid-washed samples were denoted as HnAl-a, HGa1Al2-a, HnGa1Al1-a, HnGa2Al1-a, and HnGa-a, respectively.

Catalyst characterizations Powder X-ray diffraction (XRD) measurements were conducted by a Rigaku D/Max-IIB diffractometer using Cu Kα radiation with a scanning size of 0.0167o in a 2θ range of 5o to 50o. Surface morphology and the particle size were observed by a cold field emission scanning electron microscope (SEM, Hitachi SU8010) and a high-resolution transmission electron microscopy (HR-TEM, JEOL JEM-2010). The N2 adsorption-desorption isotherms and microporosity measured by the t-plot method were all performed by an automated N2



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physisorption analyzer (Micromeritics ASAP 2010 Plus). The 27Al and 29Si magic angle spinning nuclear magnetic resonance (MAS NMR, Bruker Avance 400) spectra were obtained at the spinning speeds of 10 kHz and 8 kHz in the magnetic field of 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. 1 M of [Al(H2O)6]3+ and tetramethylsilane were used to correct the 27Al and 29Si chemical shifts. The NMR spectra were all normalized to the sample weight. The composition of Si, Al, and Ga were quantified by the inductively coupled plasma-atomic emission spectrometry (ICP-AES, Kontron S-35). Temperature-programmed desorption of ammonia (NH3-TPD), isopropylamine (IPA-TPD), and methanol were performed on a chemisorption analyzer (Micromeritics, AutoChem II). NH3-TPD was recorded using a thermal conductivity detector (TCD), while IPA- and methanol-TPD were monitored by a quadrupole mass gas analysis system (ThermoStar GSD 320 T, Pfeiffer Vacuum). Detailed pretreatments and operating conditions of temperature-programmed analyses can be found in recent studies.24-25 The Fourier transform infrared spectroscopy (FT-IR) of pyridine adsorption was conducted using a Thermo Scientific Nicolet iS50 spectrometer and an in situ quartz cell (transmission mode, Dalian Xuanyu Technology). The sample was dehydrated at 400 oC for 3 h under a vacuum of 1.2*10-3 Pa. The dehydrated sample was then cooled to 150 oC and exposed to pyridine vapor. Excess pyridine was extracted at 400 oC under a vacuum of 1.2*10-3 Pa for 1 h, and the FTIR spectra were recorded at 400 oC.

Catalytic evaluation MTA was conducted at 400, 450, and 500 oC in a continuous fixed bed system under atmospheric pressure.30 Approximately 0.2 g of a tested catalyst (177 to 400 μm pellet size) was


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sandwiched by quartz wool in the center of a reactor and was activated in an air flow (50 mL/min) at 500 oC for 1 h before the test. Methanol was continuously fed into the system at a rate of 0.01 mL/min and was vaporized at 150 oC in an upstream preheater. N2 served as the internal standard and the carrier gas (60 mL/min). The contact time of methanol was at 0.49 min * kgcat/mol. The exhaust was analyzed by an on-line parallel-dual-column gas chromatograph (GC, SRI 8610C) with a 60 m MXT-1 capillary column and a HayeSep D packed column. Methanol and its dehydrated product, dimethyl ether (DME), were considered the reactants. Detailed calculations of conversion and product yields can be found elsewhere.30

Results and discussion Figure 1 displays XRD patterns. The MFI characteristic responses were identified for each catalyst. Using commercial HZSM-5 as the reference (not shown), a semi-quantitative analysis of relative crystallinity was performed by using the peak area in the range of 2θ = 2225°. The crystallinity declined as Ga content increased, following the order as: HnAl (89%) > HnGa1Al2 (86%) > HnGa1Al1 (82%) > HnGa2Al1 (80%) > HnGa (79%). The lower crystallinity implied that each catalyst has smaller crystalline size than commercial HZSM-5. In addition, the declining order of crystallinity with increasing Ga content suggests enhanced amorphization caused by Ga incorporation. However, no diffraction of Ga2O3 could be identified. Through a close inspection of planes (5 0 1) and (3 0 3) we could see slight downward shifts from 2θ = 23.2o to 23.0o and from 23.9o to 23.7o, respectively, with increasing Ga concentration (shown in Figure 1(b)). According to Bragg’s law, this low-angle shift is attributed to the replacement of smaller Al cations by larger Ga cations in the crystalline structure, resulting in increases of dspacing between lattice planes. The undetectable Ga2O3 signals and the low-angle shift of the



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diffraction pattern of Ga-containing catalysts suggest that Ga cations were either well dispersed in extra-framework positions or incorporated into the ZSM-5 scaffold. Figure 2 shows the SEM and TEM images of tested catalysts. Aggregated microspheres consisting of cubic crystals in a range of approximately 50-150 nm were identified for all catalysts with no substantial difference caused by partial or complete Ga substitution. The TEM images also displayed small crystal size (mostly less than 100 nm). Moreover, for Ga-substituted catalysts, no darker contrast was observed in the region close to the exterior surface, implying the existence of the finely dispersed extra-framework Ga species. Figure 3 shows the N2 isotherms and Table 1 lists the textural properties of tested catalysts. A type IV isotherm with a wide hysteresis loop (type H4) in the range of P/P0 = 0.45 to 0.95 could be specified for each catalyst. This adsorption-desorption behavior indicates a hybrid texture composed of both meso- and micropores. Note that the Brunauer-Emmett-Teller (BET) method is not applicable for the estimation of the surface area of ZSM-5 due to the limitation of small pore size, which prevents multilayer adsorption and limits the uptake of adsorbate.31 The micropore surface areas estimated using the t-pot method and pore volumes of the tested samples were close, ranging from 228 to 267 m2/g and from 0.22 to 0.24 cm3/g, respectively. HnAl has a slightly higher micropore surface area than Ga-substituted catalysts. This should be correlated to the greater crystallinity (less amorphization) of HnAl. The crystallinity of HnAl is greater than its Ga-replaced counterparts, and thus it contains a higher surface area of micropores. All samples had mesopore volumes in the range of 0.12 to 0.15 cm3/g. This similar mesoporosity can be attributed to the intercrystalline void space caused by the agglomeration of nano-sized particles.


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Figure 4 shows the 27Al NMR resonances. The sharp response of framework tetrahedral Al (IV) at 54 ppm and an insignificant signal of amorphous octahedral Al (VI) at 0 ppm were observed.32 The intensity of Al(IV) resonance decreased as the following sequence: HnAl > HnGa1Al2 > HnGa1Al1 > HaGa2Al1 > HnGa. This sequence underlines that Al cations in the framework were substituted by increasing Ga content. Figure 5 displays the 29Si MAS NMR spectra. The resonance can be resolved into responses of Si(0M) at -114.0 ppm and Si(1M) at -107.0 ppm (M = Al or Ga).33 Slight chemical shifts of Si(0M) from -114.0 to -113.5 ppm and of Si(1M) from -107.0 to -105.7 ppm were observed as the content of incorporated Ga increased. The downfield shift can be attributed to the deshielding of Si nuclei caused by the replacement of Al with larger Ga cations.34 Table 2 presents the overall composition estimated by the ICP-AES analysis. The overall compositions of bulk Si/M3+ of as-synthesized catalysts ranged from 31 to 36, and the range was close to their designated value (40). The Ga/Al ratios increased from 0.56 (HnGa1Al2) to 0.98 (HnGa1Al1) and then to 1.96 (HnGa2Al1), and the increasing order of Ga/Al ratio of aluminogallosilicate MFI catalysts were in line with the respective Ga-to-Al ratios. Acidity of each catalyst was quantitatively analyzed by NH3-TPD, IPA-TPD, and pyridine IR. NH3-TPD profiles (see Figure S1 of Supporting Information) revealed the overall acid concentration, containing both Brønsted and Lewis sites. IPA-TPD profiles (see Figure S2 of Supporting Information) were used to estimate the Brønsted acid concentration. The decomposition of an IPA molecule follows the Hofmann degradation sequence,35 yielding one molecule of ammonia and one molecule of propylene over a Brønsted acid site. Hence, the concentration of the Brønsted acid is equal to the desorbed amounts of ammonia (m/e = 17).36-37



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By subtracting the concentration of the Brønsted acid from overall concentrations of acids, the Lewis acid concentration can be obtained. To provide more information about Brønsted and Lewis acidities, pyridine IR spectra were also collected, shown in Figure 6. The pyridine IR spectra displayed three bands at approximately 1457, 1490, and 1545 cm-1, corresponding to adsorption of pyridine on Lewis sites, on both Lewis and Brønsted sites, and on Brønsted sites (vibration of pyridinium ion), respectively.38-39 Note that each pyridine IR spectrum was collected at 400 oC to quantitatively analyze strong Lewis acid sites, which are deemed to be active in aromatization.40 The response of pyridinium ion showed a downward shift from 1545 cm-1 (HnAl) to 1542 cm-1 (HnGa1Al2, HnGa1Al1, HnGa2Al1, and HnGa). The red-shift indicates a decrease of Brønsted acidity by partial or total replacement of Al by Ga.41-42 A close inspection at approximately 1457 cm-1 (see Figure 6 (b)) can find that the responses of HnGa1Al2, HnGa1Al1, HnGa2Al1, and HnGa are slightly blue-shifted, and shoulders in the range of 1460-1465 cm-1 become more pronounced than that of HnAl. Moreover, the amplitude is growing with increased Ga content. This indicates an increase of the concentration of Lewis acid site with the extent of Ga substitution.20 The concentrations of Brønsted and Lewis sites can then be estimated by the equations proposed by Emeis.43

C ( B )  IMEC ( B ) 1  IA( B ) 

1

C ( L)  IMEC ( L)  IA( L) 

 R2

(1)

W

 R2

(2)

W

where C (B) and C (L) are concentrations of Brønsted and Lewis sites (μmol/g catalyst); IMEC (B) and IMEC (L) are integrated molar extinction coefficients of Brønsted (1.67 cm/μmol) and Lewis (2.22 cm/μmol) sites; IA (B) and IA (L) are integrated absorbance bands of Brønsted


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(integration region of 1515-1565 cm-1) and Lewis (integration region of 1440-1470 cm-1) sites; R is the radius of catalyst wafer (cm), and W is the catalyst weight (g). Table 3 lists the concentrations of overall acids, Brønsted acid, Lewis acid, and the ratio of Brønsted and Lewis acid sites (B/L). The concentrations estimated from pyridine IR were much lower than those obtained from the profiles of NH3- and IPA-TPD. Noting that the kinetic diameters of pyridine, NH3, and IPA are approximately at 0.55 nm,44 0.26 nm,45 and 0.52 nm,46 respectively. Therefore, some internal acids are inaccessible by using pyridine as the probe due to its lower diffusivity in MFI microchannel than those of NH3 and IPA.18 Moreover, the basicity of ammonia (pKa = 9.3) is stronger than pyridine (pKa = 5.2), allowing some ammonia to be adsorbed on weak acid sites which cannot adsorb pyridine.40 However, even using different methods for acidity evaluation, similar trends could be found: concentrations of overall acid and Brønsted site declined monotonically while concentrations of Lewis acid were in a similar range as Ga content of tested catalysts increased. Since the molecular structure of the Brønsted acid is a proton on a Si-O-M3+ bridge in the MFI framework, the decreasing concentration of the Brønsted acid implied the decreasing content of the Si-O-M3+ bridging structure, in agreement with the previous assumption that Ga is less prone to be incorporated into the ZSM-5 scaffold than Al is. Lewis acids are amorphous Al and Ga species. The decrease of overall concentration of acid with increasing Ga substitution suggests the extent of amorphization (amorphous M3+) increased. The similar Lewis acid concentration (increasing Lewis acid percentage of overall acid) are in line with the decreasing crystallinity observed by XRD. Table 3 also includes the ratio of Brønsted and Lewis acid sites (B/L). Table 4 presents the initial product selectivity (averaged results during the first two hours of the experiment) of as-synthesized catalysts at 400, 450, and 500 oC. The time course of



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conversion of each sample at 500 oC was also reported (see Figure S3 of Supporting Information). Each catalyst displayed a more than 36-h stability at a level of complete conversion of methanol and DME, indicating that the initial catalytic results are representative with negligible coke formation. Methane increasing with elevating temperature indicates an enhanced extent of hydrogen transfer (HT) reaction.47 Increased C2 products, including ethane and ethylene, suggests the enhanced HT of two olefin molecules and the cracking of higher olefins and aromatics, respectively, with increasing temperature. Decreasing products of C4=, C4, and C5-C7 can be explained by the enhanced cracking and the improved dehydrogenation of alkanes.24-25 It is worth mentioning that the C4 hydrogen transfer index (C4 HTI),48 calculated by C4 divided by the sum of C4= and C4, decreased from 400 to 500 oC for all Ga-containing catalysts. Since HT was promoted with increased temperature,47, 49 the decreasing C4 HTI implies the conversion of higher alkanes to alkenes, which is proceeded mainly through dehydrogenation.24-25 The sum of propylene and propane (C3 products) should increase with elevating temperature due to the growing extent of cracking47 and HT.3 Decreasing C3 products, particularly for a 35% decrease of HnGa1Al2 from 400 oC (11.3%) to 500 oC (7.4%), indicates that HnGa1Al2 is highly active in the conversion of C3 products. An outstandingly high aromatic yield (79.5%) was obtained by using HnGa1Al2 at 500 oC, a 41% increase from 56.6% at 400 oC. Other Ga containing catalysts had lower fractional increases of aromatics (less than 30%), generating approximately 60% yields of aromatics at 500 oC. A detailed inspection of aromatics distribution can find that excluding ethylbenzene (EB), all aromatic products, including benzene (B), toluene (T), xylenes (o-, m-, and p-X), C9 aromatics (e.g., trimethylbenzene), and C10 aromatics (e.g., durene) increased with elevating temperature. The low EB selectivity implied that the surface alkylation of B with ethyl species50 is not possible. The increases of remaining


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aromatics suggests that p-X isomerization to o- and m-X and xylenes alkylation/dealkylation to B, T, C9, and C10 aromatics were promoted with increasing temperature.3 As for HnAl, a 10% decrease of aromatics with significant increases of C2 (143.0%) and C3 (53%) products from 400 to 500 oC implies the cracking was enhanced. To verify the influence of the extra-framework Ga species in MTA, acid-treated catalysts were tested under the same reaction conditions. Table 5 presents the initial product selectivity and Figure S3 of Supporting information shows the durability at 500 oC of each acid-leached catalyst. Noting that the physicochemical properties of acid-treated catalysts were nearly intact, as their porosities, XRD patterns, NMR resonances, and NH3- and IPA-TPD profiles were similar compared to their freshly prepared counterparts (see Figure S4 to Figure S9 and Table S9 of Supporting Information). The durability tests also revealed similar trends of conversion of acid-washed catalysts and their pristine counterparts. This indicates that the mild acid treatment had negligible influence on porosity, crystalline structures, and Brønsted acidity.51 The major changes after acid treatment are in Ga content, as the ICP analysis results (see Table S1 of Supporting Information) showed slight increases of overall Si/Ga ratios and decreases of Ga/Al ratios of acid-leached Ga-containing catalysts compared to their untreated counterparts. The acidity estimated by using pyridine IR spectra (see Figure S10 and Table S2 of Supporting Information) also displayed similar concentrations of Brønsted acid of acid-leached catalysts and their untreated counterparts. However, concentrations of Lewis acid decreased substantially for all post-treated Ga-containing catalysts (in the range of 10-12 μmmol/g) compared to their pristine counterparts (in the range of 11-31 μmmol/g). This underlines that the acid treatment extracts mainly amorphous Ga species, i.e., Lewis acid sites. Therefore, the amount of extracted Ga by the acid treatment was assumed to be equal to the amount of extra-framework Ga species,



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and Table 3 presents the extra-framework Ga-to-Brønsted acid ratios of Ga-containing catalysts. The extra-framework Ga-to-Brønsted acid ratio increased as Ga content increased, following the order as: HnAl (0) < HnGa1Al2 (0.06) < HnGa1Al1 (0.08) < HnGa2Al1 (0.19) < HnGa (0.22). Decreases of methane and aromatics together with increases of C2-C7 hydrocarbon products were observed at the same reaction temperature for acid-treated Ga-containing catalysts, compared with their untreated counterparts. At 500 oC, there are more fractional changes in product distribution, especially for HnGa1Al2-a, as methane decreased from 2.3% to 0.7% (a 70% decrease) and aromatics decreased from 79.5% to 31.4% (a 61% decrease) with more than 100% increases of products of C2-C7 were observed, compared to HnGa1Al2. Little variation of product distribution was discovered when using HnAl-a compared to HnAl. This outcome again highlighted that the presence of the extra-framework Ga species does enhance the yields of aromatic in MTA. To further validate the aromatization activity of the extra-framework Ga species, methanol-TPD of fresh and acid-treated samples were performed. Figure 7 shows the desorption profiles of hydrogen (m/e = 2) and xylenes (m/e = 91) that evolved from chemisorbed methanol. Desorbed H2 had drastically different patterns: the H2 signal was undetectable when HnAl was used, while a spike combined with a multi-hump response was observed in the hydrogen desorption profile of each Ga-containing catalyst. The strong H2 signal underlines the dehydrogenation nature of incorporated Ga cations. Among Ga-replaced catalysts, HnGa1Al2 possessed a relative stronger desorption response at 469 oC. The dehydrogenation activity is the key for a high aromatic yield, and the desorption response of xylenes is consistent with the following claim: the maximum desorption rate (Tmax) of xylenes was located in the region of 300 to 350 oC for Ga-substituted catalysts, while the Tmax of xylenes of HnAl was at 375 oC. For


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acid-treated samples, dwindled desorption responses of H2 and xylenes were found for all Gacontaining catalysts. This again proves that the extra-framework Ga species is essential in promoting aromatic yields through dehydrogenation. Nevertheless, at the current stage, it is not possible to rule out the dehydrogenation nature of the Ga cation in the framework position since desorbed H2 signal can still be seen in the hydrogen desorption profile of each acid-leached, Gasubstituted catalyst. Figure 8 shows the correlation between the B/L ratio (estimated by pyridine IR spectra), the extra-framework Ga/B ratio, and the yields of aromatic at 400 oC by using fresh catalysts. The yields of aromatic at 400 oC were presented due to lower extent of side reaction (e.g., cracking) involved. At a relatively higher B/L ratio (3.15), the highest yield of aromatic (56.6%) could be achieved at 400 oC. This is in contrast to earlier findings of conversion of hydrocarbons, which showed that the lower the B/L ratio is, the better the yields of aromatic can be obtained. For example, Al-Yassir et al.52-53 reported aluminogallosilicate MFI zeolites with low B/L ratios (less than 2) can produce high aromatics (approximately 55%) in propane conversion. Su et al.20 tested a series of aluminogallosilicate MFI catalysts with B/L ratios ranging from 6.7 to 4.4 in 1hexene aromatization. At the lowest B/L ratio (4.4), the highest aromatics could be generated. Recently, a monotonically increasing trend of aromatics with a decreasing B/L ratio of Cd/HZSM-5 was reported in MTA.54 It is claimed that the reason why metal-incorporated ZSM5 with a low B/L ratio has a better aromatization performance is due to the combinational effect of enhanced dehydrogenation by Lewis acidity of metal ions and suppressed cracking by reduced Brønsted acidity of proton sites. However, this is not the case herein. One possible explanation for a high B/L ratio (i.e., a low extra-framework Ga/B ratio) catalyst to have better MTA performances is the bifunctional catalysis of the extra-framework



Page 16 of 40

Ga and the Brønsted acid site, as observed by other research groups in ethylene55 and propane5657

aromatization. A low extra-framework Ga/B ratio (0.06; or a high B/L ratio at 3.15 estimated

by pyridine IR spectra) was discovered to be appropriate in this work. Presumably, a high concentration of the Brønsted acid is needed to promote dehydration of methanol and DME and then to facilitate subsequent oligomerization.58-59 Extra-framework Ga species are located in the exchangeable positions of proton, thereby consuming a little portion of Brønsted acid sites.57 However, the exchanged Ga species can dehydrogenate the intermediates of alkene and alkane formed by neighboring Brønsted acid sites, accelerating cyclization for the formation of aromatics.60 Figure 9 shows the fragments of ethylene and C3+ hydrocarbons of methanol-TPD. The fragmentation of hydrocarbons usually yielded ions with the same mass peak, and herein m/e = 27 and m/e = 43 were selected to represent ethylene and C3+ hydrocarbons, respectively. Responses of ethylene and C3+ hydrocarbons were highly overlapped, indicating the coprocessing of arene and alkene cycles.59 The Tmax of ethylene and C3+ hydrocarbons increased in the order as follows: HnGa1Al2 (292 oC) < HnGa1Al1 (298 oC) < HnGa2Al1 (305 oC) ≈ HnAl (306 oC) ≈ HnGa (308 oC), and this increasing order was in agreement with the decreasing trend of the aromatic yields at 400 oC: HnGa1Al2 (56.6%) > HnGa1Al1 (54.6%) > HnGa2Al1 (47.4%) ≈ HnAl (48.3%) ≈ HnGa (47.8%). Evidently, the dual-cycle mechanism, which is the origin of aromatics in MTA, is substantially enhanced by HnGa1Al2, which is deemed to have a proper extra-framework Ga/B ratio.

Conclusions


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Through a head-to-head comparison of as-synthesized and acid-treated nanoscale [Ga, Al]-HZSM-5 catalysts, the extra-framework Ga species is proved to be essential in promoting aromatic yields in MTA. A nano-sized [Ga, Al]-ZSM-5 catalyst with a relatively low extraframework Ga-to-Brønsted acid ratio (0.06) was discovered to be the most effective. This advantage in aromatization is mainly correlated to a contact synergy of the Brønsted acid and the extra-framework Ga species: the former promotes dehydration, oligomerization, and cyclization; the latter, dehydrogenation. The bifunctional catalysis nature of nanoscale aluminogallosilicate MFI catalysts is proved to be able to facilitate the dual-cycle mechanism in methanol conversion to aromatics.



Page 18 of 40

m (b) 2θ = Figure 1. XRD pattterns of testted catalystss in the rangges from (a) 2θ = 5o to 50o and from 22o to 25o


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Figure 2. SEM and TEM imagees of HnAl (a an F nd f), HnGa1Al2 (b and g), HnG Ga1Al1 (c and h)), HnGa2Al1 (d, and i), and HnG Ga (e and j)

ACS Paragon Plus Environment

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Figure 33. N2 adsorp ption-desorp ption isotheerms of testeed catalysts


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Figure 4. 27Al MAS NMR speectra of testeed catalysts



Page 22 of 40

Figure 5. 29Si MAS S NMR specctra of testeed catalysts


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Figure 6. Pyridine IR spectra oof freshly prepared cataalysts at 4000 oC; B = Brønsted acidd sites and L = Lew wis acid sites.



Page 24 of 40

Figure 7. MS profiiles of hydroogen (m/e = 2) and xyleenes (m/e = 91) of methhanol-TPD


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Figure 8. Comparisson of the B/L B ratio (esstimated by pyridine IR R spectra), thhe extra-fram mework Ga/B raatio, and aromatic yieldss at 400 oC over tested catalysts. T The amount oof extra-fram mework Ga was assumed to o be equal too the amounnt of Ga extrracted by thee acid treatm ment.



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Figure 9. Profiles of C2H4 (m//e = 27) andd C3+ hydroccarbons (m/ee = 43) in methanol-TP PD


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Table 1. Textural properties of tested catalysts Smicroa

Vtotal

Vmicroa

Vmesob

(m2/g)

(cm3/g)

(cm3/g)

(cm3/g)

HnAl

267

0.23

0.11

0.12

HnGa1Al2

237

0.23

0.10

0.13

HnGa1Al1

228

0.22

0.09

0.13

HnGa2Al1

233

0.24

0.10

0.14

HnGa

228

0.24

0.09

0.15

a

Estimated by the t-plot method

b

Calculated by Vtotal-Vmicro



Page 28 of 40

Table 2. Bulk compositions of tested catalystsa Si/(Al+Ga)

a

(Si/Al)

(Si/Ga)

(Ga/Al)

HnAl

31

31

∞

0

HnGa1Al2

33

51

92

0.56

HnGa1Al1

32

63

64

0.98

HnGa2Al1

34

101

51

1.96

HnGa

36

∞

36

∞

Bulk composition was estimated by ICP-AES.


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Table 3. Acidity of tested catalystsa Overall acid concentration (μmol/g)

Brønsted acid concentration (μmmol/g)

Lewis acid concentration (μmmol/g)

Extra Ga/B ratio

B/L ratio

catalyst

from NH3-TPD

from pyridine IR

from IPA-TPD

from pyridine IR

from NH3and IPATPD

from pyridine IR

from NH3and IPATPD

from pyridine IR

from ICPAES and IPA-TPD

HnAl

352

139

221

66

131

11

1.69

6.00

0

HnGa1Al2

351

132

216

63

135

20

1.60

3.15

0.06

HnGa1Al1

323

127

191

57

132

21

1.44

2.71

0.08

HnGa2Al1

280

113

144

47

136

26

1.06

1.81

0.19

HnGa

234

102

112

42

122

31

0.92

1.35

0.22

a

B/L ratio means the ratio of Brønsted to Lewis acid sites. NH3- and IPA-TPD mean the temperature-programmed desorption of ammonia and isopropylamine, respectively. Pyridine IR means the infrared spectrum of pyridine adsorption at 400 oC. Extra Ga/B ratio means the extra-framework Ga-to-Brønsted acid ratio. ICP-AES means inductively coupled plasma-atomic emission spectrometry.


29

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

Page 30 of 40

Table 4. Initial product selectivity of MTA of fresh catalysts at 400, 450, and 500 oC with a contact time of 0.49 kgcat*min/mola Catalyst C1 C10 ∑ C4 HTI C2 C3 C 4= C4 C5-7 B T EB p- + o-X C9 T o Arom Arom Arom m-X ( C)

400

450

500

HnAl

0.2

4.7

12.8

5.5

19.0

9.5

0.4

2.1

0.7

9.9

3.3

29.3

2.7

48.3

0.78

HnGa1Al2

0.4

4.1

11.3

3.7

15.6

8.3

0.3

1.7

0.2

9.6

3.3

39.2

2.3

56.6

0.81

HnGa1Al1

0.4

4.2

12.1

4.5

15.4

8.8

0.5

2.7

0.8

11.2

3.8

33.3

2.3

54.6

0.77

HnGa2Al1

0.3

4.3

14.8

5.9

18.3

9.0

0.4

2.5

0.7

9.5

3.4

29.0

1.9

47.4

0.76

HnGa

0.4

3.4

17.6

6.0

15.5

9.3

0.4

1.9

0.3

7.6

2.6

33.0

2.0

47.8

0.72

HnAl

0.6

7.6

16.7

4.9

17.4

7.2

0.6

2.3

0.2

10.3

3.5

26.2

2.5

45.6

0.78

HnGa1Al2

1.0

5.5

11.0

2.5

10.6

3.6

0.6

2.6

0.2

12.5

4.2

42.9

2.8

65.8

0.81

HnGa1Al1

0.9

5.8

16.0

4.4

13.0

5.1

0.7

3.5

0.6

12.6

4.2

30.9

2.3

54.8

0.75

HnGa2Al1

0.5

6.5

17.6

4.7

14.3

5.1

0.8

4.3

0.6

11.7

4.1

28.1

1.7

51.3

0.75

HnGa

0.5

4.7

16.4

4.6

13.5

7.4

0.6

3.0

0.7

10.4

3.6

32.7

1.9

52.9

0.75

HnAl

0.8

11.3 20.2

4.2

15.0

5.2

0.6

1.7

0.1

10.0

3.4

25.6

1.9

43.3

0.78

HnGa1Al2

2.3

6.2

7.4

1.1

3.0

0.5

2.0

5.9

0.1

17.1

5.8

43.9

4.7

79.5

0.73

HnGa1Al1

1.5

8.8

16.8

3.4

8.9

1.8

1.9

6.3

0.2

14.4

4.9

28.9

2.2

58.8

0.72

HnGa2Al1

1.1

8.4

16.3

3.2

9.0

1.9

1.9

6.2

0.2

14.2

4.9

30.3

2.4

60.1

0.74

HnGa

0.8

6.5

16.6

3.7

8.7

2.8

1.4

5.4

0.5

13.6

4.7

33.0

2.3

60.9

0.70

C1 = 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; C4 HTI = C4 hydrogen transfer index a


30

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Table 5. Initial product selectivity of MTA of acid-treated catalysts at 400, 450, and 500 oC with a contact time of 0.49 kgcat*min/mola Catalyst C1 C10 ∑ C2 C3 C4= C4 C5-7 B T EB p- + o-X C9 T Arom Arom Arom m-X (oC)

400

450

500

C4 HTI

HnAl-a

0.2

4.4

11.5

5.0

19.1

9.1

0.6

2.3

0.7

10.4

3.5

30.5

2.7

50.7

0.79

HnGa1Al2-a

0.3

5.0

16.2

8.0

19.2

10.9

0.7

2.1

0.7

7.5

2.5

24.8

2.1

40.4

0.71

HnGa1Al1-a

0.3

5.3

18.2

8.0

18.9

9.8

0.6

1.9

0.4

6.8

2.3

25.2

2.3

39.5

0.7.0

HnGa2Al1-a

0.3

4.8

17.2

7.9

19.5

10.7

0.6

1.9

0.4

7.1

2.3

25.4

1.9

39.6

0.71

HnGa-a

0.3

3.5

19.9

9.4

19.3

10.8

0.6

1.1

0.3

5.1

1.8

26.3

1.6

36.8

0.67

HnAl-a

0.4

7.9

15.9

5.0

17.1

6.1

0.7

2.8

0.5

11.7

4.0

25.6

2.3

47.6

0.77

HnGa1Al2-a

0.4

9.8

25.2

8.5

16.6

7.2

0.8

2.5

0.5

7.6

2.5

16.9

1.5

32.3

0.66

HnGa1Al1-a

0.4

9.6

25.7

7.9

16.4

6.0

0.6

2.5

0.3

7.4

2.5

19.1

1.6

34.0

0.67

HnGa2Al1-a

0.4

8.5

23.7

7.7

16.8

6.8

0.7

2.7

0.3

7.8

2.7

20.4

1.5

36.1

0.69

HnGa-a

0.4

6.1

27.1

9.0

16.0

7.2

0.5

2.1

0.3

6.5

2.2

21.3

1.3

34.2

0.64

HnAl-a

0.9

11.7

19.5

4.6

13.5

3.9

0.8

2.8

0.2

11.3

3.9

24.8

2.1

45.9

0.75

HnGa1Al2-a

0.7

15.2

27.7

7.5

13.7

3.8

0.8

3.2

0.3

8.5

2.9

14.7

1.0

31.4

0.65

HnGa1Al1-a

0.9

14.1

29.3

6.8

12.2

3.3

0.9

3.5

0.1

8.7

2.9

16.4

0.9

33.4

0.64

HnGa2Al1-a

0.7

14.4

27.2

7.2

14.1

3.9

0.8

3.6

0.2

8.3

2.9

15.5

1.2

32.5

0.66

HnGa-a

0.8

9.6

26.9

7.1

11.9

3.6

0.8

4.1

0.3

8.9

3.2

21.4

1.4

40.1

0.63

C1 = 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; C4 HTI = C4 hydrogen transfer index a


31


Page 32 of 40

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Information includes the profiles of NH3-TPD, IPA-TPD, catalyst durability, and desorbed C2H4 and C3+ hydrocarbons of methanol-TPD of freshly prepared catalysts; XRD, porosity, MAS NMR, ICP-AES analysis, and IPA-TPD, and pyridine IR spectra of acid-treated catalysts. AUTHOR INFORMATION Corresponding Author *Tel +886 6 2757575 ext. 62668. Fax +886 2344496. Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This study was supported by Taiwan's Deep Decarbonization Pathways toward a Sustainable Society (Project 106-0210-02-11-05), the Ministry of Science and Technology (Projects 1062221-E-006-188-MY3 and 106-2218-E-155-005), and the Ministry of Economic Affairs (Project H354DP2120). REFERENCES (1)

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For Tabble of Contennts Only


Ga-Substituted Nanoscale HZSM-5 in Methanol Aromatization: The

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