Conversion of Methanol to Olefins over H-ZSM-5 Zeolite: Reaction

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Conversion of Methanol to Olefins over H-ZSM-5 Zeolite: Reaction Pathway is Related to the Framework Aluminum Siting Tingyu Liang, Jialing Chen, Zhangfeng Qin, Junfen Li, Pengfei Wang, Sen Wang, Guofu Wang, Mei Dong, Weibin Fan, and Jianguo Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01771 • Publication Date (Web): 23 Sep 2016 Downloaded from http://pubs.acs.org on September 23, 2016

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Conversion of Methanol to Olefins over H-ZSM-5 Zeolite: Reaction Pathway is Related to the Framework Aluminum Siting Tingyu Liang,a,b Jialing Chen,a,b Zhangfeng Qin,*,a Junfen Li,a Pengfei Wang,a Sen Wang,a,b Guofu Wang,a Mei Dong,a Weibin Fan,a and Jianguo Wang*,a a

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of

Sciences, P.O. Box 165, Taiyuan, Shanxi 030001, PR China b

*

University of the Chinese Academy of Sciences, Beijing 100049, PR China Corresponding authors. Tel.: +86-351-4046092; Fax: +86-351-4041153. E-mail address:

[email protected] (Z. Qin); [email protected] (J. Wang)

ABSTRACT: As the conversion of methanol to olefins (MTO) over a zeolite catalyst is conducted on the acid sites derived from framework aluminum (AlF), it is possible to enhance the catalytic performance by altering the siting of AlF if one knows the catalytic behavior of specified AlF located at certain sites. In this work, two series of H-ZSM-5 zeolites, viz., S-HZ-m and T-HZ-m, were synthesized with silica sol and tetraethyl orthosilicate, respectively, as the silicon source. Both series of H-ZSM-5 zeolites exhibit similar acidity, morphology, and textual properties. However, they are quite different in the AlF siting, as determined by UV-vis-DRS of Co(II) ions and 27Al MAS NMR; AlF of S-HZ-m is enriched in the sinusoidal and straight channels, whereas AlF of T-HZ-m is concentrated in the channel intersections. When they are used as the catalyst in MTO, T-HZ-m gives higher selectivity to ethene and aromatics and larger hydrogen transfer index (HTI) than S-HZ-m, 1

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whereas S-HZ-m exhibits higher selectivity to propene and higher olefins. Moreover, the 13

C/12C-methanol-switching experiments indicate that the incorporation of

12

C into

pentamethylbenzene and hexamethylbenzene is faster on T-HZ-m, whereas the scramble of 12

C for C3–C5 olefins is speedier on S-HZ-m. All these illustrate that AlF in the channel

intersections of H-ZSM-5 is probably more favorable to the propagation of aromatic-based cycle, whereas AlF in the sinusoidal and straight channels is more encouraging for the alkene-based cycle. These results help to clarify the catalytic behavior of given framework acid sites of H-ZSM-5 in MTO and then bring forward an effective approach to improve the catalytic performance by regulating the framework aluminum siting.

KEYWORDS: methanol to olefins; H-ZSM-5; framework aluminum siting; acid site; aromatic-based cycle; alkene-based cycle.

1. INTRODUCTION The conversion of methanol to olefins (MTO) over acidic zeolite catalysts, as a non-petroleum route to get light olefins, has attracted considerable attention in recent years, as methanol can be easily produced via syngas from multifarious carbon sources such as coal, natural gas, and biomass. A great deal of effort has been devoted in the past decades to elucidate the reaction mechanism and kinetics of MTO.1–10 As all the product olefins are able to participate in the reaction cycle and accelerate the reaction themselves, MTO can be considered as an autocatalytic reaction process, as illustrated by Huang and co-workers;5–9 such a methylation-cracking mechanism has proved quite effective in kinetically depicting practical MTO processes with high precision. Meanwhile, the hydrocarbon pool mechanism proposed by Dahl and Kolboe has also received wide recognition,10–13 which can well explain the formation of olefins, product distribution and retained species detected in a

2


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zeolite catalyst with special porous structure and acidity. According to the sort of hydrocarbon pool (HCP) species, the HCP mechanism can be further divided into aromatic-based and alkene-based cycles.14 In the aromatic-based cycle, polymethylbenzenes (polyMBs), serving as the main HCP species,15,16 act as a scaffold for methanol methylation and olefin elimination via the side-chain and paring routes.17–19 The alkene-based cycle, in which higher olefins serve as the active HCP species,20,21 is based on the repeated methylation and cracking of C3+ alkenes. The MTO product distribution is in general related to the actual reaction pathway; 22 – 25 over H-ZSM-5, for instance, the aromatic-based cycle produces mainly ethene, propene, and aromatics, whereas the alkene-based cycle leads predominantly to propene and higher alkenes. Moreover, it was revealed recently that the alkene-based cycle and aromatic-based cycle may proceed interdependently:14,20,25,26 higher olefins can participate in the aromatic-based cycle via cyclization and hydrogen transfer reactions, whereas olefins dealkylated from the aromatic intermediates can be recycled into the alkene-based cycle via repeated methylation and oligomerization. The actual reaction pathway for MTO is closely related to the topology of zeolite catalyst, because the molecular size and reactivity of HCP species are controlled by the confinement effect from zeolite framework.15,27,28 H-ZSM-5 zeolite possesses 2D sinusoidal channels (0.51 × 0.55 nm, a-direction) crossed with 1D straight channels (0.53 × 0.56 nm, b-direction); intersection cavities of 0.9 nm in diameter are created. Unlike H-ZSM-22 over which MTO is dominated by the alkene-based cycle in the 1D straight channels29 or H-Beta over which the aromatic-based cycle prevails in the large 3D 12-MR channels,30 H-ZSM-5 is propitious to both the aromatic-based and the alkene-based cycles because of its peculiar topology, as proved by the methanol isotopic experiment of Bjørgen and co-workers.14 Therefore,

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H-ZSM-5 is ideal for the observation of dual-cycle mechanism (aromatic-based and alkene-based cycles) in MTO. It is also possible to improve the MTO product distribution through selectively regulating the relative propagation of different reaction cycles.31,32 In general, the regulation of dual-cycle course in MTO can be achieved through three approaches: (1) Co-feeding olefins or aromatics with methanol, which may alter the relative concentrations of different HCP species. The addition of aromatics in the methanol feed can significantly suppress the formation of C3+ olefins and enhance the methylation of aromatic rings and the formation of alkanes and ethene, whereas co-feeding C3–6 olefins with methanol has minor effect on the selectivity to aromatic products due to the relatively fast methylation and cracking of C3–6 olefins under industrial conditions.32–35 (2) Adjusting the reaction conditions such as temperature and contact time, as two cycles are kinetically different. The alkene-based cycle could be enhanced and the selectivity to higher olefins can be increased by raising the reaction temperature and/or decreasing the contact time;31,36 however, high temperature may also promote the secondary reactions that lead to undesirable by-products, whereas short contact time results in a low methanol conversion and yields less hydrocarbon products. (3) Regulating the framework aluminium (AlF) siting. This should be an effective way to alter the reaction pathway, as the acid sites located at different framework sites may also be quite different in their catalytic behavior in MTO. The framework aluminium (AlF) siting can be regulated either by directly modifying the synthesis processes such as incorporation of certain heteroatoms and alteration of structure directing agents (SDAs),37–40 or by using certain post-synthesis treatment methods such as dealumination. 41 , 42 Through changing the type of organic SDAs, Liu and co-workers synthesized RTH zeolites with different AlF distributions;37 various Brönsted acid sites (BAS) different in accessibility and strength were then produced, which also performed differently

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in methanol conversion. Recently, Chen and co-workers found that incorporation of heteroatom boron was an effective measure to regulate the siting of AlF in H-MCM-22;40 incorporating a proper content of boron could concentrate the BAS in the sinusoidal channels rather than in the surface pockets and supercages, which was favorable to the alkene-based cycle and could significantly alleviate the carbonaceous deposition and improve the catalyst stability in the process of methanol to hydrocarbons (MTH). The regulation of AlF in H-ZSM-5 zeolite has also been made by many researchers. Sazama and co-workers synthesized H-ZSM-5 zeolites with the same Si/Al ratio and crystal size, but with different fractions of AlF pairs and single AlF;43 they found that AlF pairs were beneficial for 1-butene oligomerization and hydrogen transfer reaction, whereas “single” AlF was

favorable

for

1-butene

cracking.

For

the

synthesis

of

H-ZSM-5

with

tetrapropylammonium hydroxide (TPAOH) as SDA, Yokoi and co-workers found that the addition of sodium could enrich AlF in the straight and sinusoidal channels, whereas in the Na-free synthesis system, AlF was concentrated in the channel intersections; 44 the enrichment of acid sites in the channel intersections was beneficial for 3-methylpentane cracking and aromatics disproportionation which required bulky transition states. Janda and co-workers reported that high fraction of AlF located at the channel intersections of H-ZSM-5 could be obtained by increasing Al content, which was beneficial for n-butane dehydrogenation rather than its cracking.45 By using DFT calculation, Wang and co-workers confirmed that for MTO over H-ZSM-5, the aromatic-based and alkene-based cycles were different in product distribution;25 ethene and propene were produced in nearly the same probability via the aromatic-based cycle, whereas via the alkene-based cycle, propene was the dominant product. However, the calculation is only based on the acid sites located in the channel intersections of H-ZSM-5; the catalytic behavior of various acid sites located at different framework positions remains unclear. 5


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H-ZSM-5 zeolite is considered as one of the most effective catalysts for MTO, owing to its excellent catalytic performance. The acid sites in H-ZSM-5 can be distributed in all 12 T-sites; in other words, AlF can be distributed in straight channels, sinusoidal channels, and channel intersections on the basis of MFI topology. As MTO is conducted on the acid sites derived from AlF,25,40 it will be possible to enhance the performance of zeolite catalyst by altering the siting of AlF if one knows the catalytic behavior of specified AlF located at certain framework sites. In this work, the regulation of AlF siting and acid distribution in H-ZSM-5 was achieved by using different silicon sources during the hydrothermal synthesis. Various measures such as UV-vis-DRS of Co(II) ions and NMR spectra were used to differentiate the acid sites in the straight and sinusoidal channels and the channel intersections of H-ZSM-5 framework. Through both multiple-pulse and consecutive tests as well as

13

C/12C-methanol-switching

experiment, the catalytic behavior of specified acid sites in MTO was investigated on the basis of the dual-cycle mechanism. A correlation between the dual-cycle reaction pathways for MTO and the Brönsted acid distribution among the straight and sinusoidal channels and intersections in H-ZSM-5 was then established. The insight shown in this work provides a better understanding of the dual-cycle mechanism in MTO, which should be of great benefit to designing better catalysts for MTO by controlling the AlF siting.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Two series of nano-scale H-ZSM-5 zeolites, viz., S-HZ-m and T-HZ-m, were hydrothermally synthesized with silica sol and tetraethyl orthosilicate (TEOS), respectively, as the silicon source; m represents here the targeted Si/Al molar ratio. NaAlO2, tetrapropylammonium hydroxide (TPAOH), NaOH, and H2O were used to get the

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synthesis gel. For S-HZ-40, the molar composition of synthesis gel was SiO2 : 0.011Al2O3 : 0.009Na2O : 0.15TPAOH : 30H2O; 0.2 wt.% silicalite-1 seeds were added to guarantee a uniform crystal size. For T-HZ-40, the molar composition of synthesis gel was SiO2 : 0.011Al2O3 : 0.02Na2O : 0.15TPAOH : 19.2H2O. The synthesis gel was then transferred into a teflon-lined stainless steel autoclave, crystallized at 135–170 °C for 48–72 h, followed by centrifuging, rinsing, drying overnight at 110 °C, and calcining at 560 °C for 5 h, to obtain Na-ZSM-5 (S-NaZ-40 or T-NaZ-40). H-form H-ZSM-5 (S-HZ-40 or T-HZ-40) was obtained through ion-exchanging the directly calcined samples twice with NH4NO3 aqueous solution (1 M) at 80 °C for 5 h, which was then calcined at 540 °C for 6 h in air. Similarly, the nano-scale S-HZ-20/80/160 and T-HZ-20/80/160 with a Si/Al molar ratio of 20/80/160 were also synthesized by following the same procedures. Co-ZSM-5 (S-CoZ-m or T-CoZ-m) was prepared following the procedures described by Dědecěk and co-workers,45–47 which should be devoid of isolated and bridging cobalt oxides. Briefly, H-ZSM-5 was ion-exchanged back to Na-ZSM-5 with aqueous NaNO3 solution (1 M) at 80 °C for 7 h, the Na-ZSM-5 sample obtained here was then ion-exchanged with aqueous Co(NO3)2 solution (0.05 M) at 80 °C for 7 h. After the ion-exchanges, Co-ZSM-5 sample was then rinsed in distilled water for three times and dried in air. The exchange procedure was repeated for three times to get over-exchanged Co-ZSM-5. 2.2. Catalyst Characterization. The actual atomic composition of ZSM-5 zeolites was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Autoscan16, TJA). X-ray powder diffraction (XRD) patterns were collected on a Rigaku MiniFlex II desktop X-ray diffractometer with monochromatic Cu Kα radiation (154.06 pm, 30 kV, and 15 mA) in the range of 2θ from 5° to 40° with a scan speed of 4 min−1. Taking S-HZ-80 as a 7


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reference (assuming that it had a crystallinity of 100%), the relative crystallinity of other zeolite samples was estimated by comparing their diffraction peak areas around 2θ of 23° with that of the reference sample. Nitrogen adsorption/desorption isotherms were measured at −195.8 °C on a TriStar II 3020 gas adsorption analyzer of Micromeritics. Prior to the measurement, the zeolite sample (80–120 mesh) was degassed under high vacuum at 300 °C for 8 h. The total surface area was calculated from the adsorption branch in the range of relative pressure from 0.05 to 0.25 by Brunauer–Emmett–Teller (BET) method, whereas the total pore volume was estimated at a nitrogen relative pressure of 0.99. The micropore volume and external surface area were calculated from the isotherms by t-Plot method; the micropore surface area was obtained from the difference between the total surface area and external surface area. The scanning electron microscopy (SEM) images to characterize the surface morphology of zeolite sample were taken on a field emission scanning electron microscope (FESEM, JSM 7001-F, JEOL, Japan). The particle size distribution was estimated through counting all the crystal particles according to their diameter in the selected area of SEM images. Temperature-programmed desorption of NH3 (NH3-TPD) was performed on an AutoChem II 2920 chemisorption analyzer of Micromeritics. Approximately 100 mg of zeolite sample was first pretreated at 550 °C for 2 h in an argon stream (30 mL min−1) and then cooled to 120 °C. Saturated adsorption of NH3 on the zeolite sample was achieved by introducing gaseous NH3 (5 vol.% in argon, 30 mL min−1) into the sample tube for 30 min. After that, the physically adsorbed NH3 was removed by flushing the sample tube with the argon flow (30 mL min−1) at 120 °C for 2 h. To get the NH3-TPD profile, the zeolite sample was then heated up from 120 to 550 °C at a ramp of 10 °C min−1; the amount of NH3 released during heating for desorption was recorded by a thermal conductivity detector 8


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(TCD). The quantities of weak and strong acid sites were determined by the amounts of ammonia desorbed at 120–250 and 250–550 °C, respectively, through integrating the NH3-TPD profile in each temperature interval. Fourier transform infrared (FT-IR) spectra were measured on a Tensor 27 FT-IR spectrometer of Bruker. To get the FT-IR spectra in the OH stretching vibration region and those for pyridine adsorption (Py-IR), the zeolite samples were directly pressed into thin wafers, which were mounted into a vacuum cell. Prior to the measurement, the sample cell was evacuated to 10−2 Pa at 450 °C for 2 h; the IR spectra were then recorded at room temperature and 150 °C. Pyridine vapor was introduced into the sample cell at room temperature for 1 h; after evacuation for 1 h, the spectra were then recorded at 150, 250, and 350 °C. The concentrations of Brönsted and Lewis acid sites were calculated according to the procedures reported by Madeira and co-workers.48 The content of external Brönsted acid sites was calculated from the FT-IR spectra of 2,6-di-tert-butylpyridine (DTBPy) adsorption at 150 °C, as reported by Corma, Góra-Marek and co-workers.49,50 The ultraviolet-visible diffuse reflectance spectra (UV-vis-DRS) were collected on a Cary 5000 UV-vis-NIR spectrophotometer of Agilent equipped with a diffuse reflectance attachment with a polytetrafluoroethylene integrating sphere. Before each measurement, the hydrated Co-ZSM-5 (S-CoZ-m and T-CoZ-m) samples were transformed to their dehydrated form under 10−1 Pa and 400 °C for 7 h; the dehydrated samples were then transferred into a sample cell in a glove box (O2 < 0.1 ppm; H2O < 0.1 ppm). Operated at a scan speed of 10 nm s–1, a step length of 1 nm and a slim width of 5 nm, the UV-vis-DRS were collected in a differential mode referenced to their parent H-ZSM-5 zeolites. In the framework of H-ZSM-5, Al atoms can exist in the forms of Al pairs and single Al, as reported by Dědecěk and co-workers.47 As Al atoms were separated by (Si–O)n groups and could be expressed as

9


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[Al–O–(Si–O)n–Al]; the Al pairs were then defined as two Al atoms separated by more than two (Si-O) groups (n ≥ 2). Two Al atoms in pairs could locate in one framework ring or different framework rings, because in either case two Al atoms could be balanced by hexaaquocomplexes of Co(II) cations. On the other hand, the single Al atoms were defined as one Al atom siting in a 5- or 6-membered ring which could not be coordinated by Co(II) cation. The concentration of single Al and Al pairs can be calculated by [single Al] = [Altotal] − 2 × [Comax]

(1)

[Al pairs] = 2 × [Comax]

(2)

where [Altotal] and [Comax] were Al content and Co content in Co-ZSM-5, respectively, both were determined by the ICP-AES. Magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra of 27Al and 29Si were collected on an Avance III 600 MHz Wide Bore spectrometer of Bruker operating at a magnetic field of 14.2 T in a single pulse sequence. The chemical shifts for

27

Al and

29

Si

MAS NMR spectra were referenced to the aqueous solution of Al(NO3)3 and tetramethylsilane (TMS), respectively. The

27

Al MAS NMR spectra were acquired at a

spinning rate of 13 kHz with a π/12 pulse width of 1.2 µs and a recycle delay of 1 s; the number of scans used was 8000. The

29

Si MAS NMR spectra were obtained at a spinning

rate of 5 kHz with a π/2 pulse width of 6 µs and a recycle delay of 20 s; the number of scans used was 2500. The 27Al and 29Si MAS NMR spectra were further deconvoluted by Voigt or Gaussian functions. From the 29Si MAS NMR deconvolution results (the intensity of various Si species such as ISi(0Al) and ISi(1Al)),51 the Si/AlF molar ratio was then estimated by the equation Si/AlF = (ISi(0Al) + ISi(1Al)) / 0.25ISi(1Al)

(3) 10


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2.3. Catalytic Test. Four kinds of tests were conducted to investigate the catalytic performance of H-ZSM-5 zeolites in MTO and to reveal the catalytic behavior of specified acid sites. 2.3.1. Multiple Pulse Experiment. The catalyst sample, which was pressurized into wafers and crushed and sieved to 80–120 mesh in advance, was loaded between quartz sand layers in a U-shaped quartz tube with an inner diameter of 6 mm. As schematically depicted in Figure S1 (Supporting Information), the quartz tube reactor was settled in a furnace with a type-K thermocouple buried in it. The valve and tube system were shielded with heating band and kept warm during the reaction test. To evaluate the product selectivity at similar methanol conversion (about 14%), the quantity of catalyst samples loaded in the pulse reactor was adjusted to make them all have the same amount of BAS;42,52 the catalyst samples were also diluted with quartz sand to get the same volume for the catalyst layer. For example, the quantities of S-HZ-40 and T-HZ-40 loaded in the reactor were about 36 and 54 mg, respectively. Meanwhile, as the impurities in the zeolite catalyst, carrier gas, and methanol might also interfere with the formation of initial HCP species,53 elaborate precautions were taken before the multiple pulse experiment. The zeolite catalyst was rigorously calcined in air flow at 550 °C for 3 h before switching to argon flow. The argon flow, calibrated to have a contact time of 77 ms, was passed through deoxygenation tube and molecular sieve traps (13X and 5A) before carrying the 13

C-methanol (>99%, Cambridge Isotope Laboratories, Inc) or

12

C-methanol (HPLC grade,

Fisher Scientific) pulse to the catalyst bed. Methanol was injected with a syringe (1 µL, SGE); the quantity of methanol per pulse was equivalent to twice the number of BAS in each catalyst.54,55 With each pulse separated by a predetermined flushing time, a total of 11–19 pulses were injected at a mild temperature of 280 °C. After the multiple pulse experiment,

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the reaction zone of quartz tube was quickly quenched in liquid nitrogen. For each methanol pulse, the gaseous products were either analyzed online by a mass spectrometer (MS, Oministar) for quick qualifying or switched to a gas mixer (to minimize effluents inletting discrimination). The effluents in the gas mixer were then on-line analyzed by a Shimadzu 2010plus gas chromatograph (GC) equipped with two flame ionization detectors (FIDs) and two capillary columns (CP-LowOx, 12.5 m × 0.53 mm × 10 µm; PLOT-Al2O3 KCl, 50 m × 0.53 mm × 15 µm). The carbon-based product selectivity (si) and methanol conversion (x) were then calculated by

si = ni ci

(∑ n c − c i i

MeOH

x = (1 − (cMeOH + 2cDME )

− 2cDME )× 100%

(4)

∑ n c )× 100%

(5)

i i

where ci represented the molar concentration of product i with ni carbon atoms in the effluents, including unreacted methanol (MeOH) and dimethyl ether (DME). It should be noted that when calculating methanol conversion, the retained products in the catalyst bed were not counted because of their relatively smaller quantity in comparison with the effluent products.20 After quenching, the retained products in the zeolite catalyst bed were dissolved in 20 wt.% HF aqueous solution and subsequently extracted with dichloromethane; they were then analyzed with hexachloroethane as an internal standard (I.S.) on a Shimadzu GCMS-QP2010 gas chromatogram-mass spectrometer (GC-MS) equipped with Rtx-5MS capillary column (60 m × 0.25 mm × 0.25 µm).56 The total ion chromatograms were normalized to the I.S. and then qualified according to NIST08 mass spectra library. 2.3.2. 13C/12C-Methanol-Switching Experiment. To determine the reactivity of different 12


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intermediates or products in the dual-cycle reaction system,

13


experiment was conducted in the pulse reactor under the same conditions as described previously. 15 pulses of 13C-methanol were first injected to the catalyst bed to construct the 13

C-hydrocarbon pool species. After that, four successive 12C-methanol pulses were injected;

the effluents of each 12C-methanol pulse was collected in a gasbag and analyzed immediately on the GC-MS with an Al2O3 capillary column (30 m × 0.32 mm × 20 µm). The retained products in the zeolite catalyst (in four parallel experiments) were also collected after quenching and analyzed on the GC-MS using the same procedures as mentioned above. The 12

C content in both effluent and retained products was calculated following the same

procedures as described by Iglesia and co-workers;57 to diminish experimental errors, each measurement was repeated at least twice. 2.3.3. Consecutive Methanol Conversion Experiment. To discern the retained aromatic species from those in the effluent products, the consecutive methanol conversion experiment was conducted on the same pulse reactor. 50 mg S-HZ-40 catalyst was loaded and pretreated at 550 °C for 2 h in air flow (40 mL min−1) before switching to argon flow. The reaction was then carried out at 325 °C and atmospheric pressure. Methanol was pumped into the reactor with a liquid volume flow rate of 0.04 mL h−1 and diluted with argon (40 mL min−1); the methanol weight hourly space velocity (WHSV) was 2 h−1. After reaction for 20 min, the effluents were passed directly through a sample vial (preloaded with dichloromethane as solvent and hexachloroethane as internal standard) for 2 min, through which C6+ aliphatics and aromatics in gas phase could be dissolved and collected. After that, the reaction was quickly quenched by liquid nitrogen. The effluent products were then analyzed on the GC-MS equipped with Rtx-5MS capillary column and the retained products trapped in catalyst were processed by the same procedures as mentioned above.

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2.3.4. Catalytic Tests in Fixed-bed Reactor. The performance of zeolite catalyst in MTO was evaluated in a continuous flow fixed-bed reactor with an inner diameter of 6 mm. About 150 mg catalyst sample (80–120 mesh) was loaded. The catalyst sample was first pretreated at 550 °C for 2 h in nitrogen flow (40 mL min−1). The MTO reaction was then carried out at 450 °C and atmospheric pressure, by pumping methanol into the reactor with a liquid volume flow rate of 0.8 mL h−1, viz., with a methanol WHSV of 4.23 h−1; nitrogen was used as a diluting gas (14.5 mL min−1). The gas and liquid products were separated with a cold trap. The gaseous products were on-line analyzed by an Agilent 7890A GC equipped with a TCD, two FIDs, and two capillary columns (J&W 127-7031, 30 m × 530 µm × 0.25 µm; Agilent 19095P-S25, 50 m × 530 µm × 15 µm). The liquid oil phase was analyzed by another Agilent 7890A GC equipped with a FID and a capillary column (Agilent 19091S-001, 50 m × 200 µm × 0.5 µm). The liquid aqueous phase, including mainly water, methanol, and oxygenates, was analyzed by another Agilent 7890A GC equipped with a TCD, a FID, and a capillary column (Agilent 19091N-136, 60 m × 250 µm × 0.25 µm). It was noteworthy that dimethyl ether here was also considered as unconverted reactant when calculating the conversion of methanol.

3. RESULTS AND DISCUSSION 3.1. Textural and Structural Properties. As shown in Figure 1, all the synthesized H-ZSM-5 zeolite samples have typical MFI topology, with high crystallinity and negligible impurity phase; taking S-HZ-80 as a reference, they exhibit a relative crystallinity of 78–100%, as given in Table 1. In comparison with the standard spectrum of MFI zeolite, the broadening of typical peaks observed in the synthesized samples is probably ascribed to their nano-sized crystallites. Single peak at 24.4° is observed for the synthesized samples,

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suggesting that they belong to typical orthorhombic phase.51,58 {Figure 1, Table 1, & Figure 2} All the synthesized zeolite samples exhibit similar morphology and uniform particle size distribution, as illustrated by their SEM images in Figure 2 and Figure S2 (Supporting Information). They have an average particle size of 235–310 nm, as shown in Figure S3 (Supporting Information). The particle size of each zeolite sample is distributed over a span of less than 100 nm and the difference in the average particle size between S- and T-series zeolites is less than 40 nm. In comparison with larger particles, the nano-sized particles shown here may promote the diffusion of effluent products, which is beneficial to the minimization of secondary reactions in MTO.59 As also given in Table 1, the nitrogen physisorption measurement illustrates that two series of H-ZSM-5 zeolites, viz., S-HZ-m and T-HZ-m, have similar surface area and pore volume. S-HZ-m and T-HZ-m also exhibit similar Si/Al ratios, as determined by ICP-AES; moreover, the deconvolution of

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Si MAS NMR spectra suggests that the Si/AlF ratios of

S-HZ-20/40 and T-HZ-20/40 match closely with their Si/Al ratios. As a whole, two series of H-ZSM-5 zeolites (S-HZ-m and T-HZ-m) exhibit similar morphology, crystal size, textual properties, and Si/AlF ratio; the difference between two series of H-ZSM-5 zeolites in the catalytic performance in MTO should then be irrelevant to their morphology and textual properties. 3.2. Acidity of H-ZSM-5 Zeolites. As given in Table 2, the NH3-TPD results illustrate that two series of H-ZSM-5 zeolites have almost the same amount of weak acid sites; however, S-HZ-m (especially for S-HZ-20 and S-HZ-40) has more strong acid sites than T-HZ-m. The Py-IR results suggest that two series of H-ZSM-5 zeolites have similar amount 15


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of Lewis acid sites, whereas S-HZ-20 and S-HZ-40 have much more Brönsted acid sites (BAS) than T-HZ-20 and T-HZ-40. However, the difference between S-HZ-80/160 and T-HZ-80/160 in the amounts of strong acid or Brönsted acid sites becomes less distinct. {Table 2} By following the similar procedures as reported by Blasco, Lin and co-workers,60,61 the acid distribution across the strength was estimated from the quantities of BAS determined by the Py-IR spectra at different temperatures, as given in Table 2. It illustrates that two series of H-ZSM-5 zeolites, viz., S-HZ-m and T-HZ-m, exhibit similar distribution across the acid strength (weak, medium and strong); for example, the difference in the relative fraction of medium BAS observed between S-HZ-40 and T-HZ-40 is less than 10%. In addition, the relative fraction of external BAS measured by the DTBPy-IR spectra is almost the same for all these H-ZSM-5 zeolites, only 4–7% of total BAS (Table 2). As a result, it is reasonable to assume that the effect of external BAS on MTO is equivalent for two series of H-ZSM-5 zeolites. It was reported that the Lewis acid sites could promote the cracking reactions;62 the similarity in the Lewis acidity between two series of H-ZSM-5 zeolites may also suggest that they perform similarly in catalyzing the cracking reaction. The Brönsted acid sites (BAS) are in general derived from framework aluminum (AlF). As proposed by Ong and co-workers,63 AlF includes (1) those could form BAS which are detectable by Py-IR and (2) those could not form BAS (invisible in the 27Al MAS NMR spectrum). T-HZ-20/40 prepared from TEOS has much less BAS than S-HZ-20/40 from silica sol, which may be ascribed to that the former holds a higher content of AlF species that cannot form BAS, considering the fact that both S-HZ-20/40 and T-HZ-20/40 have similar Si/Al ratio and Si/AlF ratio.

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Moreover, as given in Table 2, the total amount of acid sites in S-HZ-40 (0.56 mmol g−1) even exceeds its Al content (0.45 mmol g−1, as measured by ICP-AES), suggesting that the detectable acid sites in S-HZ-40 by NH3-TPD are not merely formed by aluminum in zeolite. The weak acid peak in the NH3-TPD profiles may also come from the desorption of NH3 from weakly acidic silanol groups, and/or the release of NH3 that is hydrogen-bounded to NH4+ formed on Brönsted acid sites;64 these factors, especially the latter one, may lead to the overestimation of acid amounts in S-HZ-40. On the contrary, S-HZ-20 and T-HZ-20 with a low Si/Al ratio display lower overall acid amount (0.58 and 0.45 mmol g−1, respectively) than those estimated from their Al content (0.70 and 0.90 mmol/g, respectively, as measured by ICP-AES), which is probably attributed to the relatively high content of extra-framework Al in the low Si/Al ratio H-ZSM-5.65 FT-IR spectra in the OH stretching vibration region of two series of H-ZSM-5 zeolites, which were normalized with the Si-O-Si overtone bands in the region of 1400–1700 cm−1,66 are also displayed in Figure 3 and Figure S4 (Supporting Information). The bands at 3744, 3728, 3611, and 3480 cm−1 are assigned to free external silanols, internal silanols, bridging hydroxyl, and silanols nests, respectively. The weak signals at 3650–3670 cm−1 suggest that the extra-framework Al–OH species are negligible, as they were easily dehydroxylated.67 The extra band at 3690 cm−1 in S-HZ-80 can also be assigned to internal silanols.68 The deviation between S-HZ-m and T-HZ-m with a lower Si/Al ratio (m = 20, 40) in the intensity of bridging hydroxyl is more distinct than that between those with a higher Si/Al ratio (m = 80, 160), which corresponds well with their acid amounts determined by Py-IR and NH3-TPD (Table 2). In addition, S-HZ-m exhibits more nest silanols (represented by a broad peak at 3480 cm−1) and internal silanols than T-HZ-m. All these suggest that S-HZ-m has apparently more internal framework defects and hydroxyl nests than T-HZ-m.

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{Figure 3} 3.3. Aluminum Siting Observed by UV-vis-DRS of Co(II) Ion. As the signal around 31000 cm−1 for cobalt oxides is not detected in the Co(II) UV-vis-DRS, the content of Co in Co-ZSM-5 determined by ICP-AES is then ascribed to Al pairs (Equation 2). Meanwhile, minor difference is observed between Co-ZSM-5 and their parent H-ZSM-5 zeolites in the cyclohexane adsorption isotherms (Figure S5 of the Supporting Information), confirming that the formation of cobalt oxides is negligible in the actual ion-exchange process of Co(II) cations and the cyclohexane diffusion is hardly affected by the introduction of Co species in Co-ZSM-5. The contents of various aluminum species in the Co-ZSM-5 zeolites are given in Table 3, which illustrates that the majority of AlF exists in the form of Al pairs (52–77%). The ratio of (2Co + Na)/Al are close to the fraction of Al pairs that can be ion-exchanged with Co2+, suggesting that the sodium content in Co-ZSM-5 is negligible and the Al pairs are mainly balanced by Co(II) cations. As the Al atoms in the ZSM-5 zeolites exist mainly in the form of Al pairs which are coordinated with Co(II) cations, it is then possible to estimate the distribution of AlF among various framework T-sites through identifying the state of Co(II) by analyzing the UV-vis-DRS spectra of Co-ZSM-5. It is noteworthy that the (2Co + Na)/Al ratio is below one, due to the partial exchange of proton with Na+ during the preparation of Co-ZSM-5 samples.45–47 Py-IR was further used to determine the residual Brönsted acid amount in Co-ZSM-5, to validate the actual content of Al pairs. As pyridine was sufficient to neutralize both the protonic sites and Co(II) cations, the Py-IR spectra of Co-ZSM-5 displayed the bands of pyridinium ions and Co-bonded pyridine at 1545 and 1450 cm−1, respectively.69 As given in Table 3, the content of Al pairs calculated from Py-IR (45–62%) corresponds well with the content of Al pairs (2Co/Al) determined by ICP-AES.

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{Table 3 & Figure 4} The UV-vis-DRS spectra of Co(II) cations in Co-ZSM-5 are then deconvoluted by using Gaussian function,46 as shown in Figure 4 and Table 3. Seven bands are obtained, which can be sorted to three types of Co(II) ions coordinated with different Al T-sites: α-type Co(II) ions in the straight channel is linked with a single band at 15100 cm−1; β-type Co(II) ions in the channel intersections are characterized by four bands at 16000, 17150, 18600, and 21200 cm−1; and γ-type Co(II) ions in the sinusoidal channels are related to the bands at 20100 and 22000 cm−1. Figure 4 illustrates that S-HZ-40 and T-HZ-40 are obviously different in the distribution of AlF among various framework T-sites, though in both cases a majority of AlF may be located at channel intersections. For S-HZ-40, as given in Table 3, more AlF is distributed in the straight and sinusoidal channels (36%), in comparison with that for T-HZ-40 (33%); on the contrary, T-HZ-40 holds more Al T-sites distributed in the channel intersections (67%) than S-HZ-40 (64%). Similar results are also obtained for the S-HZ-20/80 and T-HZ-20/80 zeolites with different Si/Al ratios, as illustrated by the deconvolution of Co(II) UV-vis-DRS for S-CoZ-20/80 and T-CoZ-20/80 (Figure S6, the Supporting Information). As also given in Table 3, S-HZ-20 and S-HZ-80 hold more AlF in the straight and sinusoidal channels (31% and 49%, respectively) than T-HZ-20 and T-HZ-80 (27% and 40%, respectively), whereas T-HZ-20 and T-HZ-80 have more AlF sited at the channel intersections (73% and 60%, respectively) than S-HZ-20 and S-HZ-80 (69% and 51%, respectively). All these clearly illustrate that S-HZ-m zeolites synthesized with silica sol have more AlF sited in the straight and sinusoidal channels, whereas T-HZ-m zeolites synthesized with TEOS have more AlF sited in the channel intersections. 3.4. Aluminum Siting Observed by 27Al and 29Si MAS NMR. The 27Al and 29Si MAS 19


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NMR spectra of S-HZ-40 and T-HZ-40 are also measured to clarify their difference in the AlF distribution among various T-sites, as shown in Figures 5 and 6. The NMR chemical shifts of various T-sites could be theoretically calculated on the basis of average T–O–T angles;70,71 the chemical shifts of 12 T-sites in the orthorhombic H-ZSM-5, which are proximal to the actual spectra, are displayed in Figures 5c and 6c. As revealed by Dedecek and co-workers,72,73 the relations between the NMR chemical shift and the local geometry of AlO4− tetrahedron are quite intricate; it is infeasible to accurately assign AlF to various T-sites in H-ZSM-5 simply on the basis of NMR chemical shifts. {Figure 5 & Figure 6} An indirect method based on the transition-state selectivity can be used to discern different kinds of T-sites in H-ZSM-5, considering their difference in the space confinement and electrostatic stabilization effect on the probe molecules. For hexane cracking on H-ZSM-5, Yokoi and co-workers found that more AlF siting in small spherical spaces (straight and sinusoidal channels) was beneficial to the cracking of C6 paraffins.44 Through deconvoluting

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Al MAS NMR spectra into five peaks at 52, 53, 54, 56 and 58 ppm, they

suggested that the peak at 54 ppm (Al(54)) was related to the acid sites in the channel intersections with lower cracking activity, whereas the peak at 56 ppm (Al(56)) could correspond to the acid sites in the straight and sinusoidal channels with higher cracking activity. In a similar way, the

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Al MAS NMR spectra of S-HZ-40 and T-HZ-40 are

deconvoluted, as shown in Figures 5a and 5b; meanwhile, the deconvolution of

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Al MAS

NMR spectra for S-HZ-20/80/160 and T-HZ-20/80/160 is shown in Figure S7 of the Supporting Information and the deconvolution results are given in Table 3. Obviously, S-HZ-40 exhibits a higher fraction of the peak at 56 ppm (41%) than T-HZ-40 (12%), suggesting that S-HZ-40 has more AlF distributed in the straight and 20


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sinusoidal channels, as shown in Figure 5 and Table 3; meanwhile, T-HZ-40 shows a higher fraction of the peak at 54 ppm (48%) than S-HZ-40 (18%), meaning that for T-HZ-40 more AlF atoms are sited in the channel intersections. For S-HZ-80 and T-HZ-80 with a higher Si/Al ratio, similar phenomena are observed; Al(56) for S-HZ-80 (51%) is higher than that for T-HZ-80 (26%), whereas T-HZ-80 has a higher Al(54) (36%) than S-HZ-80 (12%). Such a result is well consistent with that obtained from the UV-vis-DRS spectra of Co-ZSM-5. It is noteworthy that the UV-vis-DRS of Co(II) cations in Co-ZSM-5 only reflect the siting trend of Al pairs, though Al pairs are the major form of AlF (52–77%, Table 3); hence, using Co(II) UV-vis-DRS to determine AlF siting implies a reasonable assumption that single Al and Al pairs are similar in their distribution in the zeolite framework. However, the 27Al MAS NMR spectra consider the siting of all AlF including those cannot be balanced by Co(II). For two series of ZSM-5 zeolites with different Si/Al ratios, a correlation of the Al(54)/Al(56) ratio estimated from 27Al MAS NMR spectra with the β/(α+γ) ratio from Co(II) UV-vis-DRS is illustrated in Figure 7. In general, the ratio of Al(54)/Al(56) is directly proportional to the value of β/(α+γ), though both of them increase with the decrease of the Si/Al ratio for H-ZSM-5 from 80 to 20.45 Because of the multiple controlling factors during zeolite synthesis and the complexity for characterizing Al siting, it is difficult to accurately site and assign AlF to specified T-sites in H-ZSM-5 at present. However, the linear correlation illustrated in Figure 7 does suggest that both

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Al MAS NMR and Co(II)

UV-vis-DRS spectra can provide valuable and also consistent information for the siting of AlF in H-ZSM-5. {Figure 7} The 29Si MAS NMR spectra are also helpful in considering the AlF distribution, as shown in Figure 6. No signals are observed in chemical shift higher than −100 ppm, indicating that 21


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there are no Si(2Al) sites in both S-HZ-40 and T-HZ-40. Three Q4 bands at −112.4, −113.8 and −115.8 ppm for S-HZ-40 are clearly declined and broadened, in comparison with those for T-HZ-40, which is attributed to the structure distortions induced by framework defects.74 This is in line with the fact that S-HZ-40 has more nest silanols and higher ratio of internal to external silanols, in comparison with T-HZ-40 (Figure 3). During the hydrothermal synthesis, Si and Al compete for different T-sites in the zeolite framework, which was influenced by both thermodynamical and kinetic factors;40 as a result, the difference between S-HZ-40 and T-HZ-40 in the relative percentages of three Q4 bands in Figure 6 may also reflect the variance in the possibility for AlF siting at different T-sites. However, it is difficult to exactly differentiate various Al T-sites by the chemical shifts in 29Si MAS NMR spectra. Similar results can also be observed in the 29Si MAS NMR spectra for S-HZ-20/80/160 and T-HZ-20/80/160 zeolites with different Si/Al ratios, as shown in Figure S8 of the Supporting Information. All these illustrate that the distribution of AlF among various framework T-sites in H-ZSM-5 can be regulated by using different silicon sources; for S-HZ-m obtained from silica sol, AlF is relatively enriched in the straight and sinusoidal channels, whereas for T-HZ-m, AlF is concentrated in the channel intersections. Above results suggest that silicon source has a distinct influence on the AlF siting in H-ZSM-5, as the alteration of Si source may also significantly change the nucleation and crystallization environment and kinetics.75 As reported by Mintova and co-workers, the nucleation and crystallization rates depended on the dissolution of the silica precursors;76,77 the fragile silicate intermediates released during the dissolution of silicon source were influenced by the nature of the silica source used. The silicon atoms in the gels originating from monomeric TEOS were probably more highly networked than that obtained from silica sol, which in turn had an effect on the incorporation of aluminum during the hydrothermal

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synthesis.78 The mechanism for Al siting during synthesis and the reason for the effect of Si source on the Al distributions are a very important subject that deserves a comprehensive investigation; nevertheless, it is beyond the main purpose of this work. 3.5. Retained Aromatics Trapped in the H-ZSM-5 Channels. As mentioned above, MFI framework topology of H-ZSM-5 consists of the straight and sinusoidal channels and the channel intersections. During MTO reaction, certain aromatic HCP species can be flushed out of the straight and sinusoidal channel mouth as effluent products, whereas others retain in the MFI channels as retained products. These two kinds of aromatic HCP species may be different in their catalytic effect on MTO, as they interact with the acid sites located at different framework positions. After consecutively conducting the MTO reaction over S-HZ-40 at 325 °C for 20 min, both the effluent and the retained products were analyzed by GC-MS, as shown in Figure 8. The effluent products consist of mainly toluene, xylenes, trimethylbenzenes (triMBs), and 1,2,4,5-tetramethylbenzene (tetraMB), whereas the retained products include mainly 1,2,3,5-tetraMB, 1,2,4,5-tetraMB, pentamethylbenzene (pentaMB), and hexamethylbenzene (hexaMB). 1,2,4,5-tetraMB is probably the largest species that can diffuse out of the MFI channel mouth, whereas 1,2,3,5-tetraMB is the smallest retained product which is unable to diffuse out of channels, consistent with the results of Bjørgen and co-workers.14 Meanwhile, higher polyMBs may probably retain in the channel intersections with less space restrictions rather than in the straight and sinusoidal channels.25 {Figure 8} As the consecutive MTO reaction is always accompanied by a series of secondary reactions, multiple pulse reaction experiment is necessary to investigate the intrinsic catalytic

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behavior.36 For the multiple pulse experiment, there is no consecutive supplement for the reactant methanol during pulse interval and the catalyst bed is flushed with inert gas; therefore, the majority of lower MBs formed in previous pulse can be flushed out of pore mouth and then exist in the effluent products, whereas the retained species, mainly higher MBs formed in previous pulses, turn to be the main HCP species that interfere in the dual-cycle MTO reactions of next pulse. 3.6. Catalytic Performance of Two Series of H-ZSM-5 in MTO, Multiple Pulse Experiment. To investigate the impact of AlF distribution on MTO, the catalytic performance of S-HZ-40 and T-HZ-40 were comparatively evaluated by the multiple pulse experiment. As mentioned above, the amounts of two zeolites were adjusted to make them have the same number of Brönsted acid sites (BAS) for MTO in the multiple pulse tests.42 The methanol conversion and product distribution are determined for each pulse, as displayed in Figure 9. The adsorption of methanol and formation of C-C bond can readily occur even for the first pulse,79,80 though there may be some uncertainties in determining the conversion and product distribution due to the adsorption of reactants and products on the clean reactor and pipelines. {Figure 9 & Table 4} Great differences in MTO reaction behavior are observed between S-HZ-40 and T-HZ-40, as shown in Figure 9. S-HZ-40 exhibits a much higher initial methanol conversion than T-HZ-40; the methanol conversion over S-HZ-40 reaches 13.6% even at pulse 3, whereas over T-HZ-40, the methanol conversion scrambles slowly from 3.1% at pulse 1 to 13.0% at pulse 11. However, after about 10 pulses, the MTO reaction enter the steady stage and both S-HZ-40 and T-HZ-40 then give a similar methanol conversion of about 14%. Moreover, S-HZ-40 and T-HZ-40 are also quite different in the product distribution in both the initial 24


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and steady stages. Over S-HZ-40, more C3–5 olefins and C6+ aliphatics are produced, whereas over T-HZ-40, more C3–5 alkanes, ethene and effluent aromatics are formed. For MTO in the steady stage, as given in Table 4, the average selectivities to C3–5 olefins and C6+ aliphatics over S-HZ-40 are 70.2% and 5.8%, respectively, which are much higher than those (43.7% and 3.8%, receptively) over T-HZ-40, although two catalysts have the similar methanol conversion (about 14.0%). Meanwhile, the average selectivities to ethene, C1–5 alkanes and effluent aromatics over T-HZ-40 are 15.3%, 34.2% and 3.1%, respectively, which are much higher than those (3.7%, 18.8% and 1.5%, respectively) over S-HZ-40. The great difference between S-HZ-40 and T-HZ-40 in their catalytic performance in MTO can be well explained by their difference in the AlF distribution. As mentioned above, AlF of S-HZ-m is enriched in the sinusoidal and straight channels, whereas AlF of T-HZ-m is concentrated in the channel intersections. When they are used as the catalyst in MTO, AlF in the channel intersections of H-ZSM-5 is probably more favorable to the propagation of aromatic-based cycle, whereas AlF in the sinusoidal and straight channels is more encouraging for the alkene-based cycle. For MTO over S-HZ-40, which is dominated by the alkene-based cycle, more C3–5 olefins and C6+ aliphatics are produced, whereas over T-HZ-40, in which the aromatic-based cycle is probably the principal reaction pathway for MTO, more C3–5 alkanes, ethene and effluent aromatics are then formed. The hydrogen transfer index (HTI) and the selectivity ratio of ethene to (2-methylbutane + 2-methyl-2-butene) (C2=/2-mb) are further compared to investigate the relative propagation of the alkene-based and aromatic-based cycles, as given in Table 4. Hydrogen transfer reaction serves as a connection between the aromatic-based and alkene-based cycles; the extent of hydrogen transfer reactions could be described by HTI, the ratio of alkanes to (alkenes + alkanes) in the products, as proposed by Tsang and coworkers.81 The C5-HTI

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based on C5 species of T-HZ-40 (0.68) is much higher than that of S-HZ-40 (0.43), suggesting that the propagation of aromatic-based cycles for MTO over T-HZ-40 is more prominent than that over S-HZ-40. For the hydrogen transfer reactions, the detection of saturated species such as C1–5 alkanes was generally accompanied with hydrogen-deficient species such as aromatics;36,82 therefore, more aromatic species are retained in T-HZ-40. Moreover, T-HZ-40 also exhibits a higher C2=/2-mb ratio (1.40) than S-HZ-40 (0.37); as proposed by Ilias and coworkers,31 larger C2=/2-mb index also represents a relatively higher fraction of aromatic-based cycle. By using the in-situ

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C NMR, Dai and co-workers observed that for MTO over

SAPO-34, the intermediates of alkene-based cycle (polymethylcyclopentenyl and polymethylcyclohexenyl cations) were formed before the detection of methylbenzenes (HCP species of the aromatic-based cycle).21 By DFT calculation, Wang and co-workers found that for MTO over H-ZSM-5, the initial methylation steps in the alkene-based cycle had much lower apparent free energy barrier and lower overall free energy height than those in the aromatic-based cycle.25 As a result, the lower barrier alkene-based cycle can be quickly established on S-HZ-40, leading to a higher initial methanol conversion (it reaches 13.6% even at pulse 3, as shown in Figure 9) and shorter time to reach the steady stage. On the contrary, a relatively higher barrier aromatic-based cycle on T-HZ-40 leads to the slow formation of HCP species; as MTO over T-HS-40 is probably dominated by the aromatic-based cycle, it has a lower initial methanol conversion and needs longer time to come into the steady stage. Similar phenomena are also observed over the S-HZ-20/80/160 and T-HZ-20/80/160 zeolites with different Si/Al ratios, as shown in Figures S9–S11 of the Supporting Information and summarized in Table 4. Two series of H-ZSM-5 zeolites, viz.,

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S-HZ-20/80/160 and T-HZ-20/80/160, are also quite different in the MTO product distribution even at a similar methanol conversion (about 13%); the selectivity to C3–5 olefins and C6+ aliphatics over S-HZ-20/80/160 are much higher than that over T-HZ-20/80/160, whereas the selectivity to C1–5 alkanes and effluent aromatics over T-HZ-20/80/160 are much higher than that over S-HZ-20/80/160, as given in Table 4. T-HZ-20/80/160 exhibits much higher C2=/2-mb and C5-HTI than corresponding S-HZ-20/80/160, also indicating that the propagation of aromatic-based cycle is more prominent for MTO over T-HZ-m with acid sites concentrated on the channel intersections, whereas the alkene-based cycle may take priority for MTO over S-HZ-m with acid sites enriched in the straight and sinusoidal channels. These results are further supported by the analysis of the retained products in S-HZ-40 and T-HZ-40, which were collected by quenching the MTO reaction after 11 methanol pulses at 280 °C, as described in Section 2.3.1. It should be noted that all the retained products were dissolvable in dichloromethane and detectable by GC-MS; no graphite-like coke is observed during extraction, probably due to the relatively short reaction time and low reaction temperature. As shown in Figure 10, in both S-HZ-40 and T-HZ-40, the content of lower methylbenzenes (MBs) is negligible in comparison with that of higher MBs, as lower MBs have a much higher diffusion rate and can be easily flushed out of H-ZSM-5 pores.14 It was proved recently that 1,2,4-triMB and/or 1,2,4,5-tetraMB as HCP species were highly active over H-ZSM-5 under current conditions;14,22,83 the low concentration of triMB and tetraMB detected here may also be ascribed to their high reactivity. On the contrary, massive pentaMB and hexaMB are retained in both S-HZ-40 and T-HZ-40, as they are less reactive and also very difficult to diffuse out of the channel mouth.14,83 {Figure 10}

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The products extracted from S-HZ-40 is in yellow color, in comparison with the pink color of the products extracted from T-HZ-40, suggesting that the retained products in S-HZ-40 and T-HZ-40 are also some different in the detailed compositions. More aromatics (mainly pentaMB and hexaMB) are retained in T-HZ-40 than that in S-HZ-40, in accord with the higher selectivity to effluent aromatics over T-HZ-40. Similar phenomena are observed by comparing the main retained products of S-HZ-20/80 with that of T-HZ-20/80, as shown in Figure S12 of the Supporting Information. Current results illustrate that two series of H-ZSM-5 zeolites, viz., T-HZ-m and S-HZ-m, exhibit similar methanol conversion if they are provided with the same Brönsted acid quantity, as methanol conversion was intrinsically catalyzed by the Brönsted acid sites.5 However, the product distribution is significantly influenced by the location of acid sites or the siting of AlF, as the reaction pathway for MTO is related to the actual location of acid sites; AlF in the channel intersections of H-ZSM-5 is probably more favorable to the propagation of the aromatic-based cycle, whereas AlF in the sinusoidal and straight channels is more encouraging for the alkene-based cycle. 3.7.

13

C/12C-Methanol-Switching Experiment. As mentioned above, the accumulation

of aromatic species in the zeolite pore system may also mean that they are easily formed (with lower barrier) but difficult to be consumed by either transforming to other products such as lighter aromatics and light olefins (with higher barrier) or escaping from the pore structure (with serious diffusion obstacle). As a result, the reactivity of aromatic HCP species should also be well considered, as the formation of ethene and propene via the aromatic-based cycle is realized mainly through the dealkylation of aromatic intermediates.86 Therefore,

13

C/12C-methanol-switching experiment is applied to investigate the activity of

various MTO intermediate products in the dual-cycle mechanism.84,85

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For the

13

C/12C-methanol-switching experiment, 15 pulses of

13

C-methanol were first

injected to construct the 13C-labeled hydrocarbon pool, followed by 4 pluses of 12C-methanol (pulses 1–4 in Figure 11); all the pulse reactions of 12C-methanol for MTO were compared in the steady state with a similar methanol conversion (ca. 14%). The contents of 12C detected in the main effluent olefins (ethene to hexene) and in the main retained aromatics (pentaMB and hexaMB) for MTO over S-HZ-40 and T-HZ-40 at 280 °C are shown in Figure 11. {Figure 11} The

13

C-labeled higher MBs trapped in channels during the earlier 15 pulses of

13

C-methanol serve as the origin of 13C atoms in the products for latter four successive pluses

of 12C-methanol, whereas the 12C atoms in these products stem from the later 12C-methanol pulses. With the increase of pulse number, the

13

C atoms of higher MBs can be

re-equilibrated into olefins via the side-chain and/or paring routes,86,87 which lowers the 12C content in the effluent olefins and results in a slow

12

C scrambling speed for the olefin

products. The 12C atoms of higher olefins formed from 12C-methanol can also be transferred into aromatics via cyclization and hydrogen transfer reactions, which then enhances the 12C content in the retained aromatics and results in a fast 12C scrambling speed for the aromatic products. Therefore, higher 12C content in MBs suggests a more active aromatic-based cycle for MTO, whereas higher

12

C content in olefins suggests a more vivacious alkene-based

cycle. For the dual-cycle mechanism over H-ZSM-5, ethene and aromatics are representative products of the aromatic-based cycle, whereas higher olefins are typical products of the alkene-based cycle.14,25 Hence, a comparison between the incorporation speed of 12C in the olefin and aromatic products can give certain important clues about the actual reaction pathway for MTO over S-HZ-m and T-HZ-m. For S-HZ-40, as shown in Figure 11, the 12C contents in olefins higher than propene are 29


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overlapped, increasing from 87% to 99%, with the increase of

Page 30 of 63

12

C-methanol pulse number

from 1 to 4. For T-HZ-40, hexene contains the highest fraction of 12C (from 93% to 100%), followed by pentene (80% to 98%), butene and propene (74% to 96%). The total

12

C

contents in C3–5 olefins over T-HZ-40 is evidently lower (with a lower scrambling speed) than that over S-HZ-40, indicating that the olefin methylation and cracking reactions over T-HZ-40 is inferior to those over S-HZ-40. Meanwhile, with the increase of pulse number, for T-HZ-40, the 12C contents in pentaMB and hexaMB increase from 7.6% and 2.6% to 69.0% and 16.1%, respectively, whereas for S-HZ-40, they are only increased from 4.4% and 1.4% to 11.7% and 5.3%, respectively. Obviously, the incorporation of 12C into the aromatic products or HCP species for MTO over T-HZ-40 is much faster than that over S-HZ-40, also indicating that the propagation of aromatic-based cycle over T-HZ-40 is superior to that over S-HZ-40. On the other hand, the scrambling speed of

12

C atoms in ethene for S-HZ-40 (from

37.1% for pulse 1 to 84.2% for pulse 4) is close to that for T-HZ-40 (42.3% to 85.7%); in both cases, the 12C content in ethene is much lower than that in higher (C3+) olefins. Such a result is consistent with the observation that ethene originated primarily from lower MBs in the aromatic-based cycle.14,23,25,88,89 During the multiple pulse experiment, lower MBs (both 13

C-labeled and

12

C-labeled ones) were quickly converted to higher MBs or flushed out of

zeolite channels; in either case, the content of 13C-labled lower MBs (which are accumulated from previous

13

C-methanol pulses) retained in both S-HZ-40 and T-HZ-40 may be rather

low. As a result, the formation of ethene during the following 12C-methanol pluses is hardly perturbed by the 13C-labled lower MBs accumulated from previous 13C-methanol pulses. Similar results can also be obtained in the

13

C/12C-methanol-switching experiments

conducted on S-HZ-80 and T-HZ-80 (Figure S13, the Supporting Information), further 30


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proving the observation that for MTO, S-HZ-m prefers the olefin-based cycle, whereas T-HZ-m favors the aromatic-based cycle. 3.8. Catalytic Performance of Two Series of H-ZSM-5 in the Fixed-bed Reactor. The product distribution and methanol conversion as a function of time on stream for MTO over S-HZ-40 and T-HZ-40 at 450 °C in a continuous flow fixed-bed reactor are comparatively displayed in Figure 12 and the related results for T-HZ-80 and S-HZ-80 are shown in Figure S14 of the Supporting Information. In general, S-HZ-m exhibits much longer lifetime than T-HZ-m. S-HZ-m produces more propene and higher olefins, whereas T-HZ-m (especially T-HZ-80) forms more aromatic products. The difference between S-HZ-40 and T-HZ-40 in their selectivity to aromatics is not so distinct, which may be attributed to the complete conversion of methanol (100%) over H-ZSM-5 with abundant acid sites that allow sequential reactions to the direction of equilibrium.55 Such a result is in general consistent with the observation from multi-pulse experiments: the aromatic-based cycle is preferential over T-HZ-m with framework aluminum enriched in the channel intersections, whereas the alkene-based route is superior over S-HZ-m with framework aluminum concentrated in the sinusoidal and straight channels. {Figure 12} 3.9. Relation between Acid Distribution and Catalytic Behavior in Dual-cycle. It was generally accepted that the performance of a zeolite catalyst in MTO is related to its topology, textual properties, and acidity.87 Current results indicate that two series of H-ZSM-5 zeolites, viz., S-HZ-m and T-HZ-m, have similar morphology, crystal size, and textural properties; similar methanol conversions are also obtained over the zeolites with the same quantity of Brönsted acid sites in the steady state. However, S-HZ-m and T-HZ-m are quite different in the MTO reaction pathways on the basis of dual-cycle mechanism, as revealed by their 31


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differences in the product distribution as well as in the

12

C scrambling speed during the

13

C/12C-methanol-switching tests. Although S-HZ-m may have more internal silanols than

T-HZ-m, the difference between S-HZ-m and T-HZ-m in the product distribution cannot be explained by the silanols. It was reported that the silanols or framework defects could promote

the

hydrogen

transfer

reactions

and

aggravate

coking

and

catalyst

deactivation,67,68,90 which is however contrary to the above-mentioned results. Moreover, S-HZ-40 even exhibits lower HTI (Table 4) and much longer catalyst lifetime than T-HZ-40 (Figure 12). On the other hand, abundant evidence such as UV-vis-DRS and NMR spectra indicates that S-HZ-m and T-HZ-m are quite different in the AlF distribution: for T-HZ-m, AlF is concentrated in the channel intersections, whereas for S-HZ-m, AlF is enriched in the sinusoidal and straight channels. It is then reasonable to consider that the catalytic performance of H-ZSM-5 in MTO is strongly related to the framework aluminum siting. As also reported by Chen and co-workers,40 the enrichment of BAS in larger cavity, such as surface pockets and supercages, could promote the aromatic-based cycle and result in massive coke deposition, whereas the concentration of BAS in the sinusoidal channels was beneficial for the olefin-based cycle and a prolonged catalyst lifetime. In this work, a correlation between the AlF distribution and the MTO reaction pathway can be established on the basis of dual-cycle mechanism: (1) For S-HZ-m synthesized with silica sol as the silicon source, AlF is enriched in the sinusoidal and straight channels, which is beneficial to the alkene-based cycle; (2) For T-HZ-m synthesized with TEOS as the silicon source, AlF is concentrated in the channel intersections, which is favorable for the propagation of aromatic-based cycle.

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As a result, T-HZ-m gives higher selectivity to ethene and larger HTI and relatively shorter catalyst lifetime than S-HZ-m. Moreover, the experiments indicate that the incorporation of

13


12

C into pentaMB and hexaMB is faster on

T-HZ-m, whereas the scrambling of 12C in higher olefins is speedier on S-HZ-m. This could be further interpreted by transition-state shape selectivity. For MTO over a series of SAPO zeolites (SAPO-35, SAPO-34, and DNL-6) with different cave sizes, Li and co-workers found that the size of HCP intermediates varied with the cave size due to the confinement effect.15 The identity of HCP species and their relative concentration are then related to the environment of active acid sites, which is dependent on the actual location of AlF siting. Chu and co-workers revealed that even a 0.03 nm difference in the channel size between H-ZSM-12 and H-ZSM-22 made a dramatic discrepancy in their transition-state selectivity associated with the aromatic-based cycle. 91 On the contrary, the formation of alkene intermediates may be less affected by the space constraint in comparison with the aromatic intermediates.92 Because of the larger diameter of the channel intersections than the straight and sinusoidal channels, the acid sites in the channel intersections should be prone to establish bulky aromatic HCP species, whereas the formation of aromatic HCP species was suppressed in the straight and sinusoidal channels. Therefore, the olefin-based cycle can be enhanced if relatively more acid sites are distributed in the straight and sinusoidal channels (S-HZ-m), whereas more acid sites in the channel intersections can promote the aromatic-based cycle (T-HZ-m).

4. CONCLUSIONS Two series of H-ZSM-5 zeolites, viz., S-HZ-m and T-HZ-m, were synthesized with silica sol and TEOS, respectively, as the silicon source. Their morphology, textual properties, acidity, framework aluminum (AlF) siting as well as catalytic performance in MTO were 33


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well characterized. The results illustrated that the AlF siting and Brönsted acid distribution in H-ZSM-5 can be regulated by using proper silicon source, which have a significant influence on the reaction pathway on the basis of dual-cycle mechanism and subsequently determine the catalytic performance of H-ZSM-5 in MTO. Although S-HZ-m and T-HZ-m exhibit similar acidity, morphology, and textual properties, they are quite different in the AlF siting, as determined by UV-vis-DRS of Co(II) ions and

27

Al MAS NMR. For S-HZ-m synthesized from silica sol, AlF is enriched in the

sinusoidal and straight channels, whereas for T-HZ-m synthesized from TEOS, AlF is mainly located in the channel intersections. The catalytic performance of H-ZSM-5 in MTO is strongly related to the AlF siting. T-HZ-m gives higher selectivity to ethene and aromatics and larger hydrogen transfer index (HTI), whereas S-HZ-m exhibits higher selectivity to propene and higher olefins. Moreover, the

13

C/12C-methanol-switching experiments indicate that the incorporation of

12

C into

pentaMB and hexaMB is faster on T-HZ-m, whereas the scramble of 12C for C3–C5 olefins is speedier on S-HZ-m. All these illustrate that the identity and relative concentration of HCP species as well as the concomitant reaction pathway are related to the actual location of AlF siting, which has a significant effect on the catalytic environment of active acid sites. AlF in the channel intersections of H-ZSM-5 is more favorable to the propagation of aromatic-based cycle, whereas AlF in the sinusoidal and straight channels is more encouraging for the alkene-based cycle. These results help to clarify the catalytic behavior of given framework acid sites of H-ZSM-5 in MTO and then bring forward an effective approach to improve the catalytic performance by regulating the framework aluminum siting.

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ASSOCIATED CONTENT Supporting Information Available: Schematic diagram of the multiple pulse reaction system for methanol to olefins (MTO); isotherms of cyclohexane vapor adsorption; more SEM images, particle size distribution diagrams, more Co(II) UV-vis-DRS spectra, more FT-IR spectra, more

27

Al and

29

Si MAS NMR spectra; more graphs for the product

distribution and methanol conversion for MTO during the multi-pulse experiments, more results of 13C/12C-methanol-switching experiments; more graphs of product distribution and methanol conversion for MTO in a continuous flow fixed-bed reactor, for S-HZ-m and T-HZ-m with different Si/Al ratios (which were not supplied in the main manuscript). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *Z. Qin: tel, +86-351-4046092; fax, +86-351-4041153; e-mail, [email protected]. *J. Wang: e-mail, [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We are grateful for the financial supports of the National Natural Science Foundation of China (21273264, 21273263, 21227002, 21573270, 21603257, U1510104), Natural Science Foundation of Shanxi Province of China (2013021007-3, 2015021003), Youth Innovation Promotion Association CAS (No. 2016161), and the CAS/SAFEA International Partnership

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Program for Creative Research Teams. We also want to thank Dr. Xiaoning Guo of the Institute of Coal Chemistry, Chinese Academy of Sciences, for his kind help in the UV-vis-DRS analysis.

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Page 44 of 63

Table 1. Textual Properties and Chemical Composition of the H-ZSM-5 Zeolites Synthesized with Different Silicon Sources

zeolite

Si/Al a

Si/AlF b

surface area c (m2 g−1)

pore volume c (cm3 g−1)

total

micro

total

micro

crystallinity d (%)

S-HZ-20

23

26

374

249

0.39

0.11

92

T-HZ-20

17

21

352

271

0.29

0.12

78

S-HZ-40

36

35

355

248

0.30

0.11

86

T-HZ-40

34

37

380

296

0.29

0.12

84

S-HZ-80

88

--

374

283

0.21

0.13

100

T-HZ-80

83

--

357

291

0.20

0.12

95

S-HZ-160

161

--

366

295

0.24

0.12

98

T-HZ-160

145

--

351

283

0.25

0.12

88

a

The Si/Al molar ratios were measured by ICP-AES.

b

The Si/AlF molar ratios (AlF, framework Al) were obtained from 29Si MAS NMR spectra;

the Si/AlF values for S-HZ-80, T-HZ-80, S-HZ-160, and T-HZ-160 were unavailable because of their weak Si(1Al) bands. c

The surface area and pore volume were determined by nitrogen physisorption.

d

The relative crystallinity determined by XRD was estimated by comparing the diffraction

peak area of a zeolite sample around 2θ of 23° with that of the reference sample (S-HZ-80, assuming that it had a crystallinity of 100%).

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ACS Catalysis

Table 2. Acidic Properties of the H-ZSM-5 Zeolites Synthesized with Different Silicon Sources

zeolite

acidity by NH3-TPD a (mmol g−1)

acidity by Py-IR b (mmol g−1)

weak

medium

external BAS d (%)

total

weak

strong

total

Lewis

S-HZ-20

0.58

0.31

0.27

0.43

0.17

0.26

16

16

68

7

T-HZ-20

0.45

0.25

0.20

0.27

0.14

0.13

17

17

66

5

S-HZ-40

0.56

0.22

0.34

0.32

0.07

0.25

15

9

76

5

T-HZ-40

0.42

0.19

0.23

0.22

0.08

0.14

13

19

68

5

S-HZ-80

0.24

0.09

0.15

0.13

0.03

0.10

4

20

76

4

T-HZ-80

0.22

0.09

0.13

0.11

0.03

0.08

9

13

78

5

S-HZ-160

0.10

0.02

0.08

0.04

0.01

0.03

16

4

80

--

T-HZ-160

0.10

0.03

0.07

0.04

0.01

0.03

15

6

79

--

a

Brönsted

BAS distribution c (%) strong

The quantities of weak and strong acid sites determined by NH3-TPD were measured by the

amounts of ammonia desorbed at 120–250 and 250–550 °C, respectively. b

The quantities of Brönsted and Lewis acid sites were calculated from the Py-IR spectra.

c

The acid strength distribution was estimated from the quantities of Brönsted acid sites

(BAS) determined by Py-IR at different temperatures; the difference in BAS between 150 and 250 °C, the difference in BAS between 250 and 350 °C, and remained BAS at 350 °C correspond to weak, medium, and strong acid sites, respectively. d

The content of external Brönsted acid sites was calculated from the DTBPy-IR spectra.

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Table 3. Composition of the Co-ZSM-5 Zeolites and AlF Distribution Determined from 27Al MAS NMR Spectra

zeolite

(2Co+Na)/Al c Al pairs distribution (%) Al content by NMR (%)

Al content (%) single a 2Co/Al a H+loss b

αd

βd

γd

Al(54) e

Al(56) e

S-CoZ-20

48

52

54

0.56

11

69

20

22

14

T-CoZ-20

23

77

62

0.78

21

73

6

43

7

S-CoZ-40

30

70

57

0.77

15

64

21

18

41

T-CoZ-40

35

65

61

0.70

23

67

10

48

12

S-CoZ-80

46

54

45

0.67

37

51

12

12

51

T-CoZ-80

47

53

52

0.61

21

60

19

36

26

S-CoZ-160

--

--

--

--

--

--

--

11

44

T-CoZ-160

--

--

--

--

--

--

--

13

34

a

The contents of single Al and Al pairs (2Co/Al) were calculated by Equations (1) and (2),

respectively, from the ICP-AES analysis of Co-ZSM-5. b

H+loss is the loss of Brönsted acid sites (BAS) in Co-ZSM-5 in comparison with H-ZSM-5,

which was calculated as H+loss = 1 − BAS(Co-Z)/BAS(H-Z), by titrating the residual acid amount of Co-ZSM-5 using Py-IR. c

The ratio of (2Co + Na)/Al was determined by ICP-AES.

d

The distribution of Al pairs among different types (α, β, and γ) was determined by the

deconvolution of Co(II) UV-vis-DRS of Co-ZSM-5. e

The contents of Al(54) and Al(56) peaks were determined by deconvolution of the 27Al MAS

NMR spectra of H-ZSM-5.

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ACS Catalysis

Table 4. A Comparison of Two Series of H-ZSM-5 Zeolites in the Product Distribution for MTO in the Steady Stage during the Multiple Pulse Experiment

Zeolite

Product distribution a (%) Methanol a conversion (%) CH4 C2–50 C2= C3= C4=

C5-HTI b C2=/2-mb c C5=

C6+

aromatics

4.5

8.5

2.1

0.55

0.71

6.6

2.2

5.2

2.8

0.84

1.34

47.7 16.8

5.7

5.8

1.5

0.43

0.37

8.1

2.4

3.8

3.1

0.68

1.40

9.0

48.9 19.1

7.3

10.6

1.6

0.15

1.05

15.1 11.8

45.2 14.0

4.5

7.3

1.7

0.50

1.33

S-HZ-20

14.9

1.1

23.3

T-HZ-20

14.4

1.7

39.4 18.1 24.0

S-HZ-40

14.3

0.7

18.1

T-HZ-40

13.0

1.9

32.3 15.3 33.2

S-HZ-80

13.1

0.2

T-HZ-80

13.3

0.4

S-HZ-160

14.2

0.5

0.5

1.2

43.9 25.5

9.9

16.4

2.0

0.02

0.11

T-HZ-160

13.4

1.4

10.0

3.7

49.5 16.1

5.7

11.2

2.3

0.35

0.42

a

3.2

7.2

3.7

41.2 12.1

The product distribution and methanol conversion are estimated from pulse 11 in the steady

stage during the multi-pulse experiments for MTO under 280 °C; C2–50 is alkanes of ethane to pentane; C2=, C3=, C4=, and C5= represent ethene, propene, butene, and pentene, respectively; C6+ means aliphatics higher than pentane/pentene, whereas aromatics collected are mainly toluene, xylene, trimethylbenzenes, and tetramethylbenzenes in gas phase. b

C5-HTI represents the C5 hydrogen transfer index, which is calculated from the selectivity

to pentane (C50) and pentene (C5=) by s(C50)/(s(C50) + s(C5=)) in the products. c

C2=/2-mb is the ratio of selectivity for ethene to that for 2-methylbutane and

2-methyl-2-butene, viz., s(C2=)/s(2-mb).

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Captions Table 1. Textual Properties and Chemical Composition of the H-ZSM-5 Zeolites Synthesized with Different Silicon Sources Table 2. Acidic Properties of the H-ZSM-5 Zeolites Synthesized with Different Silicon Sources Table 3. Composition of the Co-ZSM-5 Zeolites and AlF Distribution Determined from 27Al MAS NMR Spectra Table 4. A Comparison of Two Series of H-ZSM-5 Zeolites in the Product Distribution for MTO in the Steady Stage during the Multiple Pulse Experiment

Figure 1. XRD patterns of (a) S-HZ-20, (b) T-HZ-20, (c) S-HZ-40, (d) T-HZ-40, (e) S-HZ-80, (f) T-HZ-80, (g) S-HZ-160, and (h) T-HZ-160. Figure 2. SEM images of (a) S-HZ-40, (b) T-HZ-40, (c) S-HZ-80, and (d) T-HZ-80. Figure 3. FT-IR spectra in the OH stretching vibration region of various H-ZSM-5 zeolites (normalized with the Si-O-Si overtone bands in the region of 1400–1700 cm−1): (a) S-HZ-40, (b) T-HZ-40, (c) S-HZ-80, and (d) T-HZ-80. Figure 4. UV-vis-DRS spectra of Co(II) ions for (a) S-CoZ-40 and (b) T-CoZ-40. The original spectra (black) and simulated spectra (red) are vertically shifted for clarity. The percentages of α-, β-, and γ-type Al pairs are also listed. Figure 5.

27

Al MAS NMR spectra of (a) S-HZ-40, (b) T-HZ-40, and (c) the calculated

spectrum of 12 T-sites for the orthorhombic H-ZSM-5. The original spectra (black) and 48


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ACS Catalysis

simulated spectra (red) are vertically shifted for clarity. Figure 6.

29

Si MAS NMR spectra of (a) S-HZ-40, (b) T-HZ-40, and (c) the calculated

spectrum of 12 T-sites belong to orthorhombic H-ZSM-5. The original spectra (black) and simulated spectra (red) are vertically shifted for clarity. Figure 7. Relation between the β/(α+γ) and Al(54)/Al(56) ratios; β/(α+γ) and Al(54)/Al(56) ratios are determined from deconvolution results of UV-vis-DRS spectra and

27

Al MAS NMR

spectra, respectively. Figure 8. GC-MS chromatograms of (a) the effluent products and (b) the retained products after consecutively conducting the MTO reaction over S-HZ-40 at 325 °C for 20 min with a methanol WHSV of 2 h−1. Figure 9. Product distribution (bars) and methanol conversion (asterisk lines) for MTO over (a) S-HZ-40 and (b) T-HZ-40 under 280 °C within 19 pulses during the multi-pulse experiments. The data for pulses 12–18 are omitted for clarity due to their similarity. Aromatics include mainly toluene, xylenes, trimethylbenzenes, and tetramethylbenzenes in gas phase; Cn and Cn= represent alkanes and alkenes with n carbon atoms, respectively. Figure 10. GC-MS chromatograms of the retained products in (a) S-HZ-40 and (b) T-HZ-40 after 11 pluses of methanol for MTO at 280 °C. The spectra are normalized to I.S. intensity. Figure 11.

12

C content (%) in the effluent olefins (ethene to hexene) and in the retained

aromatics (pentaMB and hexaMB) for MTO over (a) S-HZ-40 and (b) T-HZ-40 at 280 °C. Four pulses of 12C-methanol (pulses 1–4 as labeled in abscissa) were injected after 15 pulses of 13C-methanol to build the 13C hydrocarbon pools. Figure 12. Product distribution and methanol conversion as a function of time on stream for

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ACS Catalysis

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

MTO over (a) S-HZ-40 and (b) T-HZ-40 at 450 °C, with a methanol WHSV of 4.23 h–1; the reactions are performed in a continuous flow fixed-bed reactor. Cn and Cn= represent alkanes and alkenes with n carbon atoms, respectively.

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Page 51 of 63

Figure 1.

24.4

o

h g f

Intensity (a.u.)

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

ACS Catalysis

e d c b a 5

10

15

20

25

30

35

40

2θ

Figure 1. XRD patterns of (a) S-HZ-20, (b) T-HZ-20, (c) S-HZ-40, (d) T-HZ-40, (e) S-HZ-80, (f) T-HZ-80, (g) S-HZ-160, and (h) T-HZ-160.

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ACS Catalysis

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

Figure 2.

Figure 2. SEM images of (a) S-HZ-40, (b) T-HZ-40, (c) S-HZ-80, and (d) T-HZ-80.

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Page 53 of 63

Figure 3.

3611 3650 3690

3744 3728

3480

Absorbance (a.u.)

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

ACS Catalysis

a b c d 3800

3700

3600 3500 3400 -1 Wavenumber (cm )

3300

Figure 3. FT-IR spectra in the OH stretching vibration region of various H-ZSM-5 zeolites (normalized with the Si-O-Si overtone bands in the region of 1400–1700 cm−1): (a) S-HZ-40, (b) T-HZ-40, (c) S-HZ-80, and (d) T-HZ-80.

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ACS Catalysis

Figure 4.

a

Intensity (a.u.)

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

Page 54 of 63

b

Original;

Simulated α (15%) β (64%) γ (21%)

Original;

Simulated α (23%) β (67%) γ (10%)

14000 16000 18000 20000 22000 24000 -1 Wavenumber (cm )

Figure 4. UV-vis-DRS spectra of Co(II) ions for (a) S-CoZ-40 and (b) T-CoZ-40. The original spectra (black) and simulated spectra (red) are vertically shifted for clarity. The percentages of α-, β-, and γ-type Al pairs are also listed.

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Page 55 of 63

Figure 5.

a

IV

II

Original Simulated

III I

Intensity (a.u.)

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

ACS Catalysis

V

b

c 8 3 4 11 2,5 6 10 7 12 1

45

Figure 5.

27

50 55 60 Chemical shift (ppm)

9

65

Al MAS NMR spectra of (a) S-HZ-40, (b) T-HZ-40, and (c) the calculated

spectrum of 12 T-sites for the orthorhombic H-ZSM-5. The original spectra (black) and simulated spectra (red) are vertically shifted for clarity.

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ACS Catalysis

Figure 6.

a

Intensity (a.u.)

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

Page 56 of 63

P3

Original Simulated

2,11 8 4

-118

29

P1

b

c

Figure 6.

P2

3

1,10,12 5 6 7

9

-116 -114 -112 Chemical shift (ppm)

-110

Si MAS NMR spectra of (a) S-HZ-40, (b) T-HZ-40, and (c) the calculated

spectrum of 12 T-sites belong to orthorhombic H-ZSM-5. The original spectra (black) and simulated spectra (red) are vertically shifted for clarity.

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Figure 7.

6

Al(54)/Al(56)

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

ACS Catalysis

T-Z-20

4

T-Z-40

2 T-Z-80 0

S-Z-80 1.2

S-Z-20 S-Z-40 1.6

2.0 β/(α+γ)

2.4

Figure 7. Relation between the β/(α+γ) and Al(54)/Al(56) ratios; β/(α+γ) and Al(54)/Al(56) ratios are determined from deconvolution results of UV-vis-DRS spectra and spectra, respectively.

57


27

Al MAS NMR

ACS Catalysis

Figure 8.

a Intensity (a.u.)

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

Page 58 of 63

b

10

I.S.

I.S.

20 30 40 Retention time (min)

50

Figure 8. GC-MS chromatograms of (a) the effluent products and (b) the retained products after consecutively conducting the MTO reaction over S-HZ-40 at 325 °C for 20 min with a methanol WHSV of 2 h−1.

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Figure 9.

100 a

aromatics C6+ aliphatics

16

80

C3-5 alkanes pentene

14

butene

10

40

12

8 propene

6

20

100 b

ethene methane

4

aromatics C6+ aliphatics

16

C3-5 alkanes

12

pentene butene

10

80 60 40

14

Methanol conversion (%)

60

Product selectivity (%)

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

ACS Catalysis

8 propene

6

20 ethene methane

0

3

6 9 12 Pulse number

15

4 18

Figure 9. Product distribution (bars) and methanol conversion (asterisk lines) for MTO over (a) S-HZ-40 and (b) T-HZ-40 under 280 °C within 19 pulses during the multi-pulse experiments. The data for pulses 12–18 are omitted for clarity due to their similarity. Aromatics include mainly toluene, xylenes, trimethylbenzenes, and tetramethylbenzenes in gas phase; Cn and Cn= represent alkanes and alkenes with n carbon atoms, respectively.

59


ACS Catalysis

Figure 10.

a

pentaMB

hexaMB

50

tetraMB

20 30 40 Retention time (min)

triMB

xylenes

I. S.

benzene toluene aliphatics

Intensity (a.u.)

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

Page 60 of 63

b

10

Figure 10. GC-MS chromatograms of the retained products in (a) S-HZ-40 and (b) T-HZ-40 after 11 pluses of methanol for MTO at 280 °C. The spectra are normalized to I.S. intensity.

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Figure 11.

100

a butene

pentene hexene

b hexene

propene

e ten butene n pe propene

80 C content (%)

ethene ethene

60

40

pentaMB

12

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

ACS Catalysis

20 pentaMB

0

hexaMB

1

Figure 11.

12

2 3 4 1 2 12 Pulse No. of C-methanol

hexaMB

3

4

C content (%) in the effluent olefins (ethene to hexene) and in the retained

aromatics (pentaMB and hexaMB) for MTO over (a) S-HZ-40 and (b) T-HZ-40 at 280 °C. Four pulses of 12C-methanol (pulses 1–4 as labeled in abscissa) were injected after 15 pulses of 13C-methanol to build the 13C hydrocarbon pools.

61


ACS Catalysis

Figure 12.

100

40

80

30

60

20

40

10

20 b

50 CH4

40

100 80

C2-5 alkanes

30

C2 ;

20

C4 ;

10

Others Aromatics 0

10

=

C3

=

=

C5

=

20 30 40 Time on stream (h)

60

Methanol conversion (%)

a

50

Product selectivity (%)

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

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40 20

50

Figure 12. Product distribution and methanol conversion as a function of time on stream for MTO over (a) S-HZ-40 and (b) T-HZ-40 at 450 °C, with a methanol WHSV of 4.23 h–1; the reactions are performed in a continuous flow fixed-bed reactor. Cn and Cn= represent alkanes and alkenes with n carbon atoms, respectively.

62


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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

ACS Catalysis

Table of Contents Graphic

The catalytic performance of H-ZSM-5 zeolites in methanol to olefins is strongly related to the framework aluminum siting, which can be regulated by synthesis with different silicon sources. For S-HZ-m synthesized with silica sol, AlF is enriched in the sinusoidal and straight channels, which is beneficial to the alkene-based cycle; for T-HZ-m synthesized with tetraethyl orthosilicate, AlF is concentrated in the channel intersections, which is favorable for the propagation of aromatic-based cycle.

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Conversion of Methanol to Olefins over H-ZSM-5 Zeolite: Reaction

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