Relation of Catalytic Performance to the Aluminum Siting of Acidic

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Relation of Catalytic Performance to the Aluminum Siting of Acidic Zeolites in the Conversion of Methanol to Olefins, Viewed from a Comparison between ZSM-5 and ZSM-11 Sen Wang, Pengfei Wang, Zhangfeng Qin, Yan-Yan Chen, Mei Dong, Junfen Li, Kan Zhang, Ping Liu, Jianguo Wang, and Weibin Fan ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01054 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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Relation of Catalytic Performance to the Aluminum Siting of Acidic Zeolites in the Conversion of Methanol to Olefins, Viewed from a Comparison between ZSM-5 and ZSM-11 Sen Wang,a,b,† Pengfei Wang,a,† Zhangfeng Qin,a,* Yanyan Chen,a Mei Dong,a Junfen Li,a Kan Zhang,a Ping Liu,a Jianguo Wang,a Weibin Fan 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 Chinese Academy of Sciences, Beijing 100049, PR China

†

The authors equally contributed to this work

*

Corresponding authors. Tel.: +86-351-4199009; fax: +86-351-4041153. E-mail address:

[email protected] (Z. Qin); [email protected] (W. Fan)

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ABSTRACT: ZSM-5 and ZSM-11 zeolites are similar in the crystalline framework structure, acidity, morphology and textual properties but considerably different in the catalytic performance for conversion of methanol to olefins (MTO). Such an unexpected but exciting finding was extensively explored by various techniques and DFT calculations. A detailed investigation shows that it is the different Al distribution in ZSM-5 and ZSM-11 framework that causes the significant difference in MTO catalytic performance. In ZSM-5, Al atoms are enriched in the intersection, whereas in ZSM-11, the Al atoms are concentrated in the straight ten-membered ring channel. The acid sites located in the intersection enhance arene-based cycle that generates more ethene, alkanes and aromatics. Nevertheless, these hydrocarbon molecules can easily diffuse out of the zeolite channel, and hence, retarding carbonaceous materials deposition and increasing catalytic stability. However, the acid sites located in the straight channel promote the alkene-based cycle, thus preferentially generating higher olefins that could transform into aromatics and carbon precursors difficult to diffuse out of ZSM-11. The fast accumulation of coke species leads to its short catalytic lifetime. By shifting the Al atoms of ZSM-11 from the straight channel to the intersection by incorporating appropriate amounts of B or altering silica and alumina sources and adding sodium cations, its MTO catalytic performance (activity, selectivity and stability) become highly comparable to that of ZSM-5. The insights attained in this work not only help to clarify the relationship of Al siting in zeolite with its MTO catalytic performance, but also provides a cue for improving the catalytic properties of zeolites by regulating the sitings of active sites in lattice sites.

KEYWORDS: MTO; ZSM-5; ZSM-11; Al siting; regulation; periodic DFT calculation.

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1. INTRODUCTION

Hydrogenation of CO to methanol over Cu/ZnO/Al2O3 catalyst has been massively commercialized.1–5 This makes the conversion of methanol to olefins (MTO) over acidic zeolite catalysts turn into an increasingly important process alternative to naphtha cracking to produce light olefins6–8 because syngas can be expediently produced from multifarious carbon sources such as coal, natural gas and biomass. The uniform pore structure and strong acidity of zeolitic materials endow them show excellent catalytic performance in MTO.9–12

Up to date, although the catalytic performance of zeolite in MTO have been proved to be influenced by its pore structure, crystal morphology and size, and acid properties, including acid site type, strength and density, 13–17 the effect of acid site location has not been unambiguously clarified because it is still a great challenge to accurately determine the acid site or Al atom positions in its framework despite that various techniques, such as Fourier 27

transform infrared (FT-IR) spectroscopy,18,19

Al magic angle spinning nuclear magnetic

resonance (MAS-NMR) spectroscopy,20,21 diffuse reflectance ultraviolet-visible (DR UV-vis) spectroscopy, 22 , 23 extended X-ray absorption fine structure (EXAFS), 24 atom probe tomography (APT),25 and density functional theory (DFT) calculation,26 have been used (Figure S1 in the Supporting Information). In addition, the Al locations in zeolite lattice sites are firmly correlated to the types and concentrations of synthesis materials that often cause a change in the crystallinity, acidity, and crystal morphology and size of zeolites. It is more difficult for the MTO catalysts because most of them (e.g. ZSM-5) have a high Si/Al ratio (> 100). The low Al content in the framework makes spectral resolution very poor.

Generally, except for the weak Brøsted acid sites of Si-OH, the other types of acid sites all come from Al species, and in particular, the strong acid sites are generated by

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isomorphous substitution of aluminum (Al) for silicon (Si). Therefore, the positions of acid sites are firmly related to that of Al species. Thus, the catalytic performance of zeolite would be highly dependent on the Al locations because its framework generally contains different-structured (or sized) pores and/or void spaces that directly influence reaction route, e.g. of methanol to hydrocarbons (MTH).27–30 Therefore, a full understanding of the Al locations in zeolite framework is essential to tailor and enhance its catalytic performance.

In such a context, many researchers have devoted to study of this topic. Janda and co-workers found that the catalytic activity of ZSM-5 for monomolecular cracking and dehydrogenation of alkanes depended on the Al locations in its framework, which could be properly regulated by altering the Si/Al ratio in the synthesis gel.31 This alteration is also effective for adjusting the Al sitings in zeolite H-Beta. For the sample with a Si/Al ratio of 150, Al atoms are mainly located at T3 + T5 + T6 sites, whereas a decrease of the Si/Al ratio to 25 leads to Al atoms preferentially positioning at T2 and T7 sites.24 Our recent works show that the Al locations in zeolites, e.g. H-ITQ-13, H-MCM-22 and H-ZSM-5, can be orientated by incorporating other framework atoms, such as Ge and B, and changing silicon source,32–35 and as a result, the reaction routes of MTO process are appropriately regulated, which leads to a significant increase in catalytic stability and propene selectivity.

In addition, the Al sitings or acid site distributions can be altered by modifying Al sources and templating agents and/or adjusting the amount of alkali metal cations such as Na+ cations. Dedecek and co-workers reported that the type and concentration of Al source in synthesis gel determined the amount of positive TPA+ that can influence the incorporated Al amount and sitings in the framework of ZSM-5.22,36,37 Besides as a structure-directing agent, the organic template confined in the micropores of zeolites could also act as a conductor to distribute Al. Davis and co-workers found that pyrrolidine (Pyr) as a template

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for synthesis of FER results in the acid sites mainly in the 8-MR channel, whereas hexamethyleneimine (HMI) gives FER with increased amounts of acid sites in the 10-MR channel.38 Di Iorio and co-workers found that introduction of small amounts of sodium ions (Na+) during synthesis could adjust the location of Al atoms in the framework of SSZ-13 zeolite; the paired Al densities linearly increased with the Na+ content incorporated in the crystalline solids.23

ZSM-5 and ZSM-11 have similar crystalline structure although their channel structure are slightly different; ZSM-5 consists of intersectional straight and sinusoidal ten-membered ring (10-MR) channels, (5.1 × 5.5 Å, 5.3 × 5.6 Å) (Figure 1), and the intersection cavity is about 9 Å, whereas ZSM-11 has two intersectional cavities (9 Å and 11.7 Å) formed by crossing two straight 10-MR channels (5.3 × 5.4 Å, 5.3 × 5.4 Å). 39–41 Their similar crystalline feature and channel structure provide us an opportunity to investigate the effect of Al sitings on the MTO catalytic performance.

{Figure 1}

Thus, attempts are made here to synthesize these two types of zeolites with similar morphology, textual properties, and acid site density, type and strength, but different acid site distribution. It is found that the two types of zeolites show significantly different catalytic performance in MTO process due to their different framework Al distributions, as revealed by different types of spectroscopy, probe experiment and DFT calculation results. In addition, two methods are provided for effectively regulating Al positions in zeolite lattice sites. By shifting Al from straight 10-MR channel to the intersection of ZSM-11, comparable catalytic properties to that of ZSM-5 are obtained.

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2. EXPERIMENTAL SECTION

2.1. Zeolite Synthesis. ZSM-5 (MFI) and ZSM-11 (MEL) zeolites with various Si/Al molar ratios (Si/Al = 60, 120, and 240) were synthesized by the hydrothermal method.42 The tetrapropylammonium hydroxide (TPAOH), tetrabutylammonium hydroxide (TBAOH), sodium aluminate (NaAlO2), sodium hydroxide (NaOH), silica sol (JN-40), and deionized water were first mixed and stirred for 2 h at room temperature. The compositions of the obtained gels were 18TPAOH : xAl2O3 : 120SiO2 : 1440H2O : 15Na2O for ZSM-5 or 12TBAOH : xAl2O3 : 120SiO2 : 1420H2O : 8.6Na2O for ZSM-11 (x = 0.25, 0.5 and 1.0). The synthesis gel was then sealed into a Teflon-lined stainless steel autoclave and crystallized at 15 rpm and 443 K for 48 and 20 h for ZSM-5 and ZSM-11 respectively.

Boron-incorporated ZSM-5 ((B,Al)-ZSM-5) and ZSM-11 ((B,Al)-ZSM-11) with a Si/Al ratio of 240 were also hydrothermally synthesized by the method similar to that of the boron-free ZSM-5 (Al-ZSM-5) and ZSM-11 (Al-ZSM-11). Boric acid (H3BO3) was added in the synthesis gel with compositions of 36TPAOH : Al2O3 : 480SiO2 : 2880H2O : 30Na2O : 60H3BO3 for (B,Al)-ZSM-5 or 24TBAOH : Al2O3 : 480SiO2 : 2839.2H2O : 17.2Na2O : 60H3BO3 for (B,Al)-ZSM-11.

For investigating the effect of acid site distribution in zeolite framework on its catalytic performance in MTO, I-ZSM-5 and I-ZSM-11 with more acid sites in the intersection were synthesized through fine tune of synthesis materials and conditions. Aluminium nitrate (Al(NO3)3), sodium chloride (NaCl), deionized water, TPAOH and/or TBAOH were mixed and stirred until the solution became clear. To this solution was tetraethyl orthosilicate (TEOS) dropwise added, followed by continuously stirring for 12 h at room temperature. The resultant gel had a composition of 32TPAOH : Al2O3 : 80SiO2 : 2880H2O : 0.8NaCl for

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synthesis of I-ZSM-5(40) or 32TBAOH : Al2O3 : 80SiO2 : 1280H2O : 1.0NaCl for synthesis of I-ZSM-11(40).

After the crystallization ended, the autoclave was cooled down in a water bath. The solid product was separated by centrifugation and washed several times with deionized water, dried overnight at 373 K, and calcined at 833 K for 10 h. The H-form sample was obtained by repeatedly ion-exchanging of the calcined Na-type sample with 1 M NH4NO3 solution at 353 K for three times, drying at 373 K for 12 h, and calcining at 833 K for 5 h. For clarity, the H-form ZSM-5 and ZSM-11 samples with a specified Si/Al molar ratio of n (n = 60, 120, and 240) were designated as ZSM-5(n) and ZSM-11(n) respectively.

For investigating the Al locations in the framework with DR UV-vis spectroscopy, the H-form ZSM-5 and ZSM-11 were transformed into Co-type samples by reversely ion-exchanging with 1.0 M NaCl and subsequently 0.05 M Co(NO3)2 solutions at 353 K for respective three times (each time of 6 h for Na-exchange and of 12 h for Co-exchange) under stirring conditions.

For comparison, the H-form ZSM-5(60) and ZSM-11(60) were dealuminated with 1.0 M HNO3 solution (liquid (L)/solid (S) = 30 (mL/g)) at 353 K for 8 h under stirring conditions. The attained samples (designated as D-ZSM-5(60) and D-ZSM-11(60)) were separated by centrifugation, washed with deionized water for several times, dried at 373 K for 12 h, and calcined at 833 K for 5 h.43

2.2. Catalyst Characterization. The purity and crystallinity of zeolite samples were examined by X-ray diffraction (XRD) conducted on a Rigaku MiniFlex II desktop diffractometer with Cu Kα radiation (30 kV, 15 mA) at a scanning rate of 4 min−1 in the 2θ range of 5–50°. The relative crystallinity of ZSM-5(120) and ZSM-5(240) was estimated by 7


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comparing the intensity of their diffraction peaks at 2θ of 23.0°, 23.4°and 24.0°for (0 5 1), (5 0 1) and (0 3 3) crystal faces, respectively, with that of ZSM-5(60) which crystallinity was assumed to be 100%. Simialrly, the relative crystallinities of ZSM-11(120) and ZSM-11(240) were obtained by comparing the intensity of their diffraction peaks at 2θ of 23.0°and 24.0° with that of ZSM-11(60), which was considered to have a crystallinity of 100%.

Field-emission scanning electron microscopy (FE-SEM) images were taken on a JEOL JSM-7001F instrument and used to determine the crystalize size and morphology of zeolites. Scanning transmission electron microscopy (STEM) images were acquired on a G2F20 transmission electron microscope (FEI) operated at 200 kV using a high-angle annular dark field (HAADF) detector. The elemental distribution of boron and aluminum in (B,Al)-ZSM-5 and (B,Al)-ZSM-11 was detected by the energy-dispersive spectroscopy (EDS) with a line scanning model. The attainable energy resolution of the EDX detector is 130 eV.

The Si/Al and B/Al molar ratios were determined by a Thermo inductively coupled plasma atomic emission spectrometer (ICP-AES, iCAP6300). The surface areas and pore volumes of zeolite samples were measured by N2 sorption on a Micrometritics TriStar II 3020 instrument at 77 K. Prior to the measurement, the samples were dehydrated at 573 K 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 the Brunauer–Emmett–Teller (BET) method, and the pore volume was calculated from the desorption isotherm by the t-Plot method.

The strength and quantity of acid sites in zeolite samples were analyzed by temperature-programmed desorption of NH3 (NH3-TPD) on a Micrometritics AutoChem II 2920 apparatus. 0.1 g Sample was loaded into a U-type tube and pretreated at 823 K for 0.5 h in He flow. Then, the sample was cooled to 393 K and allowed to fully adsorb ammonia

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that was pulse injected. After being swept with He, the temperature was ramped from 393 to 823 K at a rate of 10 K min−1, and the desorbed ammonia was monitored by a thermal conductvity detector (TCD).

The amounts of Brønsted and Lewis acid sites were determined by pyridine-adsorption Fourier transform infrared (Py-IR) spectroscopy (Bruker Tensor 27). The self-supporting sample wafer was first pretreated at 673 K for 2 h in vacuum (10−2 Pa). Then, it was cooled to room temperature to adsorb pyridine for 30 min. This is followed by degasing for 80 min under vacuum to remove physically adsorbed pyridine. Finally, the spectra was recorded at 423, 523 and 623 K after respective evacuation for 1 h. The concentrations of Brønsted and Lewis acid sites (c, μmol g−1) were estimated by the follwoing equation by integrating the vibration bands at 1540 and 1450 cm−1, respectively.44

c

A S  m

where A is the intensity of the vibration band, S is the sample wafer surface area (1.33 cm2), ε is the molar extinction coefficient (1.13 and 1.28 cm2 μmol−1 for Brønsted and Lewis acid sites, respectively), and m is the sample mass (g).

The acid quantity on the external surface was measured by FT-IR spectroscopy with 2,4-dimethylquinoline (2,4-DMQ) adsorbed on the zeolite sample.45 The zeolite wafer was pretreated at 673 K for 2 h in vacuum and cooled to room temperature. The 2,4-DMQ was solubilized in CH2Cl2 (20–40 μmol in 1 mL solvent), and the solution was dropped on the surface of pretreated zeolites. After adsorption for 30 min, the solvent was evacuated in vacuum for 40 min (10−2 Pa, 373 K). The temperature was then increased to 473 K, and the IR spectra were collected after 20 min. The acid concentration on the external surface was

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calculated by integrating the vibration band at 1647 cm−1. The framework IR spectra was also measuered in the region from 1600 to 400 cm−1 by using the conventional KBr pellet method. The thermogravmetric analysis (TGA) of deactivated zeolite samples was conducted on a Rigaku Thermo Plus Evo TG 8120 thermogravimetric analyzer. 10 mg Sample was heated in air (30 mL min−1) by raising the temperature to 1073 K at a rate of 10 K min−1.

The

11

B,

27

Al and

29

Si solid state magic angle spinning (MAS) NMR spectra were

recorded on a 600-MHz Bruker Avance Spectrometer equipped with a 4.0-mm probe operating at a magnetic field of 14.2 T. The

11

B MAS NMR spectra were recorded at a

spinning rate of 10 kHz by accumulating 4000 scans with probe background signal removed by using Hahn-echo with 90° (20.5 μs) and 180° (41 μs) soft pulses and employing an rf field strength of ν1 = 12.2 kHz and a rotor-synchronized echo delay of 5 rotor period (469 μs). The 27Al and 29Si 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 by scanning 10000 times, and a spinning rate of 5 kHz with a π/2 pulse width of 8 μs and a recycle delay of 20 s by scanning 11

1400 times, respectively. The chemical shifts for

B,

27

Al and

29

Si were calibrated by

referring H3BO3 (19.6 ppm), Al(NO3)3 (0 ppm) and TMS (0 ppm), respectively. The broad peak between 45 and 65 ppm in the 27Al MAS NMR spectra was deconvolved by using the mixed Gaussian-Lorentzian equation.35,46 For the

27

Al multiple quantum (MQ) MAS NMR

spectroscopy, the MQ excitation pulse, the MQ-1Q conversion pulse and the delay time were 5.0 μs, 1.6 μs and 0.5 s, respectively. The second-order quadrupolar effect parameter (SQ) and isotropic chemical shifts (δiso) were obtained by the following equations:

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SQ 2 = δiso - δF2  k , δiso =

3 4 I ( I + 1) - 3 17 10   106  δF1 +  δF2 , k = 10 [4 I (2 I - 1)ν0]2 27 27

where I (5/2) is the spin quantum number of 27Al and v0 is Larmor frequency.47

The DR UV-vis spectra were measured on an Agilent Cary 5000 UV-vis-NIR spectrophotometer equipped with a poly(tetrafluoroethylene) integrating sphere. The DR UV-vis spectra of Co2+-exchanged ZSM-5 and ZSM-11 were collected at room temperature after dehydration at 773 K for 5 h under high vacuum conditions ( T2 > T8 > T3 > other sites. This reveals that Al atoms are inclined to insert in the intersectional T sites (T9 and T2+T3). With respect to the ZSM-5 having a Si/Al ratio of 191, although two unit cells contain one Al atom, Al atoms also preferably occupy at the intersection T sites (T9 and T2+T3) (Figure 3b and Table 3). This is supported by the results obtained by Redondo, Grau-Crespo and Muraoka.26,62,64

{Figure 3 & Table 3}

In contrast, for ZSM-11 with a Si/Al ratio of 95, the most probable position for locating Al is T1 (Figure 3c), subsequently followed by T2, T7, T3/T5, and other sites. This holds true for the sample having a Si/Al ratio of 191 (Figure 3d), showing that Al atoms are preferably located at the T sites in straight 10-MR channel for ZSM-11 regardless of the

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Si/Al ratio in the framework.

One may say that the substitution energy and occupation probability of Al at different T sites only account for the intrinsic thermodynamic stability, while the distribution of acid sites is also strongly influenced by kinetic factors in gelling and crystallization processes. Therefore, both ZSM-5 and ZSM-11 were thoroughly investigated with

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Al and

29

Si

MQ/MAS NMR spectroscopy and DR UV-vis spectroscopy of Co2+-exchanged samples.

Al Siting Evaluated by

27Al

MQ/MAS NMR. Solid-state NMR spectroscopy has been

proved to be a powerful technique for determining Al states in zeolite.24,65,66 Figure S6 in the Supporting Information shows that all the ZSM-5 and ZSM-11 samples exhibit a sharp resonance peak at 45–65 ppm and a very small peak at 0–10 ppm in the 1D 27Al MAS NMR spectra, confirming that most of Al atoms are incorporated in the framework. On the basis of the isotropic chemical shifts (δiso) and the second-order quadrupolar effect parameter (SQ) (Table S2 in the Supporting Information) calculated from the isotropic protection (F1) and the observed dimension (F2) that are obtained from the four cross sections in the 2-dimentional (2D) 27Al MQ/MAS NMR spectra (Figure 4(a−d)), the resonance peak at 45–65 ppm in the 1D

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Al MAS NMR spectra of ZSM-5 and ZSM-11 with Si/Al ratios of

120 and 240 was deconvolved into four components at 45–50, 50–57, 57–60, and 60–65 ppm (Figure 5a), in agreement with that done by Vjunov, Liu, Dedecek, and co-workers.24,66,67 The appearance of the four characteristic peaks is due to the presence of twelve distinct T sites, which can be estimated by the DFT calculation (Table 4).20,24,54,68 The resonance peaks at 45–50 and 50–57 ppm are related to the Al located at T1+T2+T3 and T9 sites, respectively, whereas those at 57–60 and 60–65 ppm correspond to Al positioned at the T4+T10 and T8+T12 sites, respectively. Obviously, the peaks at 45–50 and 50–57 ppm (75.1 %) are much more intense than those at 57–60 and 60–65 ppm (24.9 %), revealing that

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more Al atoms are located in the intersection cavity of ZSM-5 (T9 and T1+T2+T3). This finding is supported by the

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Al triple quantum magic angle spinning (3Q/MAS) NMR and

DFT calculation results that a major part of Al is located in the intersection, whereas small amounts of Al are incorporated in the straight and/or the sinusoidal channels.37,69

{Figure 4, Figure 5, Table 4, & Table 5}

Figure 5b shows that Al atoms in the ZSM-5 with Si/Al ratio of 240 are also mainly located in the intersection, as confirmed by the area of the peaks at 45–50 and 50–57 ppm is 2.25 times as large as that of the peaks at 57–60 and 60–65 ppm. Periodic DFT calculation results show that the probabilities of Al located at relevant T sites are similar to the relative area ratios of the four resonance peaks at 45–50, 50–57, 57–60, and 60–65 ppm (Table 5), confirming that the distribution of Al atoms in ZSM-5 framework at certain specific positions is energetically more stable than a random distribution.

For ZSM-11 with a Si/Al ratio of 120 (Figure 5c), the 27Al MAS NMR spectrum in the range of 45–65 ppm can also be deconvolved into four characteristic resonance peaks at 45–50, 50–57, 57–60, and 60–65 ppm. The resonance peaks at 45–50, 57–60, and 60–65 ppm are related to Al atoms situated at T3, T4+T6 and T5 sites in the intersection, respectively, whereas the peak at 50–57 ppm corresponds to Al atoms located at T1+T2 sites of the straight 10-MR channel. Obviously, the peak at 50–57 ppm is much more intense than the peaks at 45–50, 57–60 and 60–65 ppm (71.2% vs. 28.8%), indicating that more Al atoms are located in the straight channel for ZSM-11. This is also true for the ZSM-11 with Si/Al ratio of 240 (Figure 5d). Table 5 shows that the relative intensity of the four resonance peaks is comparable to the probability of Al sited at relevant T sites, as estimated by periodic DFT calculation.

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Al Siting Evaluated by 29Si MAS NMR. The 29Si MAS NMR spectrum of ZSM-5 shows four Q4 (Si(0Al)) peaks around −110, −112.5, −115, and −117.5 ppm (Figure 6(a, b)), which can be roughly assigned to Si located at T6+T12, T9+T11, T8+T10, and T1–T5 sites, respectively, 70 in terms of the DFT calculation results (Table 4). For ZSM-5(120) and ZSM-5(240), the area of the peak at −112.5 ppm is smaller than that at −115 ppm (Table S3 in the Supporting Information), indicative of smaller number of Si atoms in (T9+T11) sites than in (T8+T10) sites. This is consistent with the 27Al MAS NMR spectroscopy result that the peak at 50–57 ppm in (corresponding to Al at T9 site) is more intense than the peak at 57–65 ppm (Al at T8+T10 sites) (Table 5).33

{Figure 6} For ZSM-11, the three Q4 (Si(0Al)) peaks in the range of −110 to −115, −115 to −117, and −117 to −120 ppm are assigned to T1+T2+T7, T3+T4+T5, and T6 sites, respectively (Figure 6(c, d) and Table 4). It is clear that the area of the peak at −110 to −115 ppm is smaller than that at −115 to −117, and −117 to −120 ppm for ZSM-11(120) and ZSM-11(240) (Table S3 in the Supporting Information), being consistent with the 27Al MAS NMR spectra that show the peak at 50–57 ppm (Al at T1+T2 sites) is more intense than that at 57–65 ppm (Al at T3–T6 sites) (Table 5). The

27

Al and

29

Si MAS NMR spectroscopy results both

confirm that Al atoms are inclined to distribute in the straight channel for ZSM-11, while in the intersection for ZSM-5.

Al Siting Evaluated by DR UV-vis Spectroscopy. The distribution of Al in zeolite framework was further investigated by DR UV-vis spectroscopy.22 Co2+-type ZSM-5 and ZSM-11 show several broad bands in the DR UV-vis spectra (Figure 7(a, b)), indicating that Co2+ ions are coordinated to Al atoms at different positions. With respect to Co2+-ZSM-5, the

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band at 15100 cm−1 corresponds to Co2+ ions in the straight channel (α), while those at 16000, 17150, 18600, and 21200 cm−1 are attributed to Co2+ ions in the intersection (β). As for the bands at 20100 and 22000 cm−1, they are due to the Co2+ ions coordinated to framework oxygen at the opening of “boat-shaped” sites (γ).19,22 The framework structure of ZSM-11 is similar to that of ZSM-5 except that the sinusoidal channel of ZSM-5 is changed to the other straight channel of ZSM-11. Therefore, it is rational to assume that β-type Co2+ ions are located at the intersection, while the other types of Co2+ ions are distributed in the straight channels for ZSM-11.71

Table 6 shows that about 62.2−73.9% of the Al atoms in ZSM-5 are located in the intersection, as estimated by the amount of β-type Co2+ ions. In contrast, only 34.0−40.6% of Al atoms in ZSM-11 is positioned in the intersection, whereas the other 59.4−64.0% is located in the straight channel.

{Figure 7 & Table 6}

In summary, the synthesized ZSM-5 and ZSM-11 with the same Si/Al ratio are considerably different in the framework Al siting although they are similar in the crystal size, surface area, pore volume, and the acid site distribution across the type and the strength; most of Al atoms are concentrated in the intersection for ZSM-5, whereas in the straight channel for ZSM-11.

4.3. Catalytic Performance of H-ZSM-5 and H-ZSM-11 in MTO. Figure 8, Figure 9 and Table 7 show the methanol conversion and product selectivity obtained over H-ZSM-5 and H-ZSM-11 with different Si/Al ratios in MTO process. The catalytic lifetime is estimated by the time when the methanol conversion decreases to 90%. H-ZSM-11(60) shows a catalytic lifetime of 84 h, being nearly twice that of H-ZSM-5(60) (44 h). However, 22


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when the Si/Al ratio was increased to 120, the catalytic life time of H-ZSM-5 (128 h) is slightly longer than that of H-ZSM-11 (100 h). In particular, a further increase in the Si/Al ratio to 240 causes H-ZSM-5 have a catalytic lifetime of 225 h, which is about three times of that of H-ZSM-11 (80 h). Such a change of the MTO catalytic performances of H-ZSM-11 and H-ZSM-5 with their framework Si/Al ratio is also observed by Dyballa and co-workers.72 Table 7 also shows the turnover numbers (TONs), as determined by the accumulated number of methanol molecules converted per Brønsted acid site before deactivation,73,74 of different samples for more accurately estimating their catalytic activity. Clearly, H-ZSM-11(60) gives a TON as high as 1.98 times of that of H-ZSM-5(60). In contrast, the TON of H-ZSM-11(120) is lower than that of H-ZSM-5(120), and the difference is more significant for H-ZSM-11(240) and H-ZSM-5(240). The increase of the TON with the Si/Al ratio is probably due to the stronger Brønsted acid site strength of zeolite with higher Si/Al ratio.

{Figure 8, Figure 9, & Table 7}

In addition, these two types of zeolites also show quite different product selectivity. ZSM-5(60) exhibits lower selectivity to propene (25–32%) and butene (12–15%) than ZSM-11(60) with the propene and butene selectivity of 30–40% and 15–20% respectively. Moreover, more C1–C5 alkanes (17–35%) and aromatics (BTX, 6.5–9.6%) are produced on ZSM-5(60) than on ZSM-11(60) (10–22% for C1–C5 alkanes and 3.6–8.8% for aromatics). Increase of Si/Al ratio leads to an increase in the selectivity to propene and butene for both the zeolites at the expense of alkanes, aromatics and ethene. When having the same Si/Al ratio, ZSM-5 shows higher selectivity to ethene but lower selectivity to propene and butene than ZSM-11.

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As we know, aromatic cycle generates almost equal amounts of ethene and propene, whereas alkene cycle produces much more propene than ethene.75–77 Thus, the relative contribution of the alkene cycle to the aromatic cycle could be estimated by the ratio of ([propene] − [ethene])/[ethene], viz., (P−E)/E. As is clearly shown in Table 7 and Figure S7 in the Supporting Information, ZSM-11 gives a much higher (P−E)/E ratio than ZSM-5 with the same Si/Al ratio, suggesting that the alkene cycle is more significantly enhanced over ZSM-11 in comparison with that over ZSM-5. Recently, Bhan and co-workers reported that the relative contribution of the alkene cycle to the aromatic cycle in the initial MTO reaction stage can be described by the ratio of (2-methylbutane + 2-methyl-2-butene) yield to ethene yield (2MB/E).78 A comparison of these two measures shows that both of them indeed indicate a higher relative contribution of the alkene cycle to the aromatic cycle in H-ZSM-11 than in H-ZSM-5 (Figure S7(c) and (d) in the Supporting Information) despite that the difference in the relative contribution estimated by these two measures are different. Another parameter for judging the relative contribution of the aromatic cycle to the alkene cycle is hydrogen transfer index (HTI) of various zeolites in MTO process. Table 7 and Figure S7 in the Supporting Information obviously show that C4-HTI and C5-HTI of ZSM-5 are always higher than those of ZSM-11, implying that the hydrogen transfer reactions are much more intense on ZSM-5 than on ZSM-11. Thus, more saturated hydrocarbons such as alkanes are formed on ZSM-5 with concomitant generation of more hydrogen-deficient aromatics.

4.4. Relation of Catalytic Performance to the Al Siting. The above results show that ZSM-5 and ZSM-11 are considerably different in the MTO catalytic performance, including the catalytic stability and product selectivity, even when they have similar Si/Al ratio, crystal size, and acid site type and strength. In addition, ZSM-5 and ZSM-11 have the nearly same crystalline structure with similar pore openings except that the framework of ZSM-5 are

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intersected by one 10-MR sinusoidal channel and one 10-MR straight channel, whereas that of ZSM-11 consists of two intersectional 10-MR straight channels. Therefore, their remarkably different catalytic performance in MTO process should be mainly ascribed to the different Al locations or acid site distributions in the framework.

Relation of Catalytic Performance to the Acid Sites on External Surface. The larger amounts of external surface acid sites on ZSM-5(60) and ZSM-11(60) accounts for their lower catalytic stability (Table 2 and Figure 8),32,79,80 as the formed aromatic intermediates can rapidly evolve into coke species due to lack of proper geometric confinement.32,81,82 Nordvang and co-workers reported that the polyaromatic coke species rapidly formed on the external surface of H-ZSM-5 with high Al content hinder the diffusion of reactants and products into and out of the zeolite, and as a consequence, leading to its deactivation.83 This is demonstrated by the theoretical calculation results that some large intermediates could be easily formed on the external surface, but they are difficult to be decomposed owing to lack of space confinement and acid cracking of zeolite framework.32,81,82

Figure 10 shows that the amounts of coke deposited on ZSM-5(60), ZSM-5(120), and ZSM-5(240) are 21.3%, 15.3%, and 12.1%, with the coking rate of 0.48, 0.12, and 0.05 h−1, respectively, whereas the deposited coke amounts and the coking rates on ZSM-11(60), ZSM-11(120), and ZSM-11(240) are 17.1% and 0.20 h−1, 14.7% and 0.15 h−1, and 9.9% and 0.12 h−1, respectively. This clearly shows that the samples with a low Si/Al ratio (60) generate more coke species and exhibit a higher coking rate for both zeolite structures.14 It is unexpected that the deactivation of ZSM-11(60) is slower than that of ZSM-5(60), while ZSM-5(240) exhibits much higher catalytic stability than ZSM-11(240) owing to its far slower coking rate and higher tolerance to coke deposition.

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{Figure 10 & Figure 11}

The synthesized H-ZSM-5(60) and H-ZSM-11(60) samples were further treated with proper concentration of HNO3 for selectively dissolving the aluminum species on the external surface and at the pore mouth of zeolites.43,84 XRD and N2 sorption results confirm that the HNO3 treatment does not have a great effect on the crystallinity, surface area and pore volume (Figure S8 and Table S4 in the Supporting Information), but, as expected, considerably decreases the external surface acid site quantities from 29 and 18 μmol g−1 to 10 and 6 μmol g−1 for H-ZSM-5(60) and H-ZSM-11(60), respectively. The decreased Brønsted acid amounts on the external surface and at the pore mouth, as detected by 2,4-DMQ-IR, are only slightly smaller than those in the ZSM-5 (23 vs. 19 μmol g−1) and the ZSM-11 (17 vs. 12 μmol g−1), as measured by Py-IR, respectively, evidencing that the HNO3 treatment mainly removed Al atoms on the external surface.

Figure 11 shows that the catalytic lifetimes of H-ZSM-5(60) and H-ZSM-11(60) are prolonged from 44 and 84 h to 71 and 132 h, respectively, after the HNO3 treatment, indicating that the abundant external surface acid sites on H-ZSM-5(60) and H-ZSM-11(60) zeolites play a severely detrimental role to their MTO catalytic stability probably due to lack of proper space confinement for growth of coke precursors. The increase of ZSM-5 coking resistance after selective removal of external surface Al species was also observed by Inagaki and co-workers.43

Relation of Catalytic Performance to the Al Location at Certain T Sites. For ZSM-5 and ZSM-11 with a high Si/Al ratio (120 and 240), the acid sites are predominantly located in the interior of zeolite framework, but their catalytic performance are still remarkably different. This suggests that such a difference in the catalytic performance should be caused

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by the Al locations in lattice sites.

As is shown above, Al atoms are enriched in the intersection in ZSM-5, and hence, promoting the formation of methylbenzene (MB) and polymethylbenzenes (polyMBs) that can be comfily accommodated in the intersection cavity without suffering serious geometric restriction to transition state during the side-chain elimination of light olefins. These polyMBs and generated alkenes and alkanes can diffuse out of zeolites through both the sinusoidal and the straight 10-MR channels. In contrast, in ZSM-11, majority of Al atoms are concentrated in the straight 10-MR channels. Thus, the formed MB and polyMBs mainly diffuse out through the channel that they are formed. In particular, the much higher acid site density of H-ZSM-11 in the 10-MR channels than that of H-ZSM-5 makes MB and polyMBs more rapidly evolve into more bulky aromatic species or coke precursors during the diffusion process. Therefore, H-ZSM-5 shows lower coking rate, and thus, higher catalytic stability than H-ZSM-11 except for the sample with low Si/Al ratio. This is supported by the findings that the MTO reaction over zeolite catalysts is a space-demanding process,85,86 and the blockage of zeolite pores by intermediate species is the dominant reason for the rapid deactivation of 10-MR ZSM-22.

The above results show that the coking rate and tolerance depend on both acid site density and acid site distribution in zeolite framework. Although the coking precursors formed in H-ZSM-5 easily diffuse out of zeolite channels, the large cavity of the intersection allows further condensation of the coking precursors, and the higher the acid density is, the faster the condensing rate is. With respect to H-ZSM-11, the aromatics generated in the 10-MR straight channel are hardly transformed into coke species because of the steric confinement despite that the high acid site density accelerates the coking process. Thus, it is the combinative role of acid site distribution and acid site density that causes ZSM-5(120)

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exhibit slightly lower coking rate and higher coke deposition amount, and hence, slightly longer catalytic lifetime than ZSM-11(120).

The product distribution of MTO is also related to the detailed Al siting or acid site distribution in zeolite framework. Compared to ZSM-11, ZSM-5 shows higher selectivity to ethene, alkanes and aromatics, larger HTI, and lower (P−E)/E ratio, as the acid sites located in the intersection is more favorable to the aromatic cycle propagation. In contrast, ZSM-11 produces more propene and butene, and exhibits a lower HTI and a higher (P−E)/E ratio (Table 7) as the Al atoms in the straight channel more promotes the alkene cycle.

4.5. Nature of Al Siting That Influences the Catalytic Performance. Transient MTO Reaction Investigated by In Situ Spectroscopy. Time-dependent DR UV-vis and DRIFTS spectroscopy could monitor the hydrocarbon pool species formed over zeolite catalysts during the MTO reaction. 87 , 88 Figure 12 displays the in situ DR UV-vis spectra of H-ZSM-5(120) and H-ZSM-11(120) for catalyzing MTO reaction at 573 K. It is clear that the bands centered at 220, 320, and 350 nm, attributed to dienes (e.g. cyclopenta- and cyclohexadienes), monoenylic carbenium ions (e.g. cyclopentenyl and cyclohexenyl cations), and oligomethylbenzenes, respectively, quickly appear in the spectrum of ZSM-5(120) (Figure 12a).89–94 When the reaction was carried out for 4 min, the band (around 420 nm) assigned to polyMBs and naphthalenic species was observed too, and its intensity gradually increased with the reaction time. Compared with ZSM-5(120), ZSM-11(120) exhibits a much longer induction period; the bands at 220, 320, 350, and 420 nm were only detected after reaction of 11 min (Figure 12b). The lower intensity of these characteristic bands implies that hydrocarbon pool species accumulate very slowly in the framework of ZSM-11. Another noteworthy difference is that two broad bands at about 470 and 520 nm, ascribed to polyaromatic species, appeared in the spectrum of ZSM-5(120) at 20 min and intensified

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with the reaction time, while they are difficult to be resolved within 20 min in the spectrum of ZSM-11(120). Similar phenomenon was also observed for ZSM-5(240) and ZSM-11(240) (Figure S9 in the Supporting Information). This further confirms that most of Al atoms are located in the intersection in ZSM-5, whereas in the straight channel in ZSM-11, as the much larger void space of the intersection than that of the straight channel allows to form bulkier molecules such as fused-ring aromatics.

{Figure 12 & Figure 13}

Figures 13 and Figure S10 in the Supporting Information show that three bands at 2870, 2920, and 3010 cm−1 were present in the DRIFT spectra of ZSM-5(120) and ZSM-5(240) after reaction of 5 min at 573 K. These three bands are assigned to the C-H stretching vibration of CH2, CH3 groups and aromatics.95–98 In addition, the bands at 1200 and 1320 cm−1 characteristic of aliphatic C-H bending vibrations97,98 were detected at the reaction time of 10 min. The appearance of negative bands at 3590 and 3720 cm−1 in the hydroxyl stretching region results from the interaction of the hydrocarbon pool species with the Brønsted acid sites and silanol groups in zeolites.99 For ZSM-11(120) and ZSM-11(240), however, the bands at 2870, 2920 and 3010 cm−1 were not distinct after reaction of 5 min, and the signals at 1200 and 1320 cm−1 were detected only after reaction for 30 min, further revealing that the amount of formed hydrocarbon pool species in ZSM-11 is smaller than that in ZSM-5.

Figure S11 in the Supporting Information depicts the GC-MS charts of the CH2Cl2-extracted organic species from ZSM-5(120), ZSM-5(240), ZSM-11(120) and ZSM-11(240) after reaction for 60 min in the DRIFTS tests. Obviously, polyMBs (mono- to hexa-methylbenzene) and some naphthalenic species are the main species retained in the two

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zeolites, and more bulky polyMBs (tetramethylbenzene to hexamethylbenzene) are generated in ZSM-5 than in ZSM-11, verifying that the formation of aromatic hydrocarbon pool species is more favorable in ZSM-5 due to the location of more Al species in the intersection.

p-Xylene Isomerization Probe Reaction. p-Xylene isomerization is a typical model reaction for probing the effect of local environment and shape selectivity of active acid sites63, 100 , 101 . Large reaction space can effectively accelerate p-xylene conversion and enhance bulkier isomer, i.e. m-xylene, production. Thus, p-xylene isomerization over various ZSM-5 and ZSM-11 zeolites was carried out, and the time-dependent IR spectra were used to determine the product formation rate and the relative fractions of three xylene isomers inside the pores.

It was found that H-ZSM-5 and H-ZSM-11 zeolites both show an intense band around 1510 cm−1 (p-xylene) and two weak bands at 1500 (o-xylene) and 1605 cm–1 (m-xylene) after introducing p-xylene into the cell at 673 K for 1 min (Figures S12 and S13 in the Supporting Information),101,102 and the band for m-xylene gradually increases with the reaction time. Regardless of this, they are quite different in the distribution of xylene isomers with the time on stream (Figure 14a). H-ZSM-5(120) gives more m-xylene than H-ZSM-11(120) (21–45% vs. 17–40%), and the difference is more evident (30–47% vs. 20–38%) when the Si/Al ratio is increased to 240 as a result of higher production rate of m-xylene (Figure 14b). The apparent activation energy for m-xylene formation over H-ZSM-5 was calculated in terms of the Arrhenius plot (Figure 14c) to be 16.6 kJ mol–1 for H-ZSM-5(120) and 17.1 kJ mol–1 for H-ZSM-5(240) in contrast to 20.7 kJ mol–1 for H-ZSM-11(120) and 19.3 kJ mol–1 for H-ZSM-11(240), showing that the formation of m-xylene is more favorable over H-ZSM-5 than over H-ZSM-11. Their similar crystalline

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framework, pore openings, crystal size and acid site density suggest that their different reaction behavior in p-xylene isomerization should be related to their Al sitings in the framework. The acid sites located in the intersection with larger cavity more considerably promote the isomerization of p-xylene to bulkier isomer, viz., m-xylene.63,100,101 Decrease of acid site density would facilitate the diffusion of m-xylene out of zeolite framework by alleviating coking process, thus, leading to formation of more m-xylene over H-ZSM-5(240) and H-ZSM-11(240) than H-ZSM-5(120) and H-ZSM-11(120), and observation of more significant difference between the former two samples than the latter two.

{Figure 14}

4.6. Regulation of Al Siting in the Zeolite Framework by Incorporation of Boron. Incorporation of heteroatoms in synthesis gel has been proved be an effective method for controlling acid site distribution.33,35 Boron (B) added in the synthesis gel could compete with Al for substituting Si at certain T sites during the gelling and crystallization processes,33,103 and hence, could adjust the Al siting in zeolite framework and influence the catalytic performance. Thus, boron-incorporated ZSM-5 and ZSM-11, viz., (B,Al)-ZSM-5 and (B,Al)-ZSM-11, were synthesized to selectively regulate the Al sitings in ZSM-5 and ZSM-11 framework.

The

11

B MAS NMR spectra of the as-synthesized (B,Al)-ZSM-5 and (B,Al)-ZSM-11

show only one intense peak around –3.6 ppm, verifying that all the B species in the samples have been incorporated in the framework (Figure S14 in the Supporting Information).104,105 This is also substantiated by appearance of a band at 920–960 cm−1 in the FT-IR spectra, which is attributed to the stretching vibration of Si-O-B band (Figure S15 in the Supporting Information).106 Table S5 in the Supporting Information shows that (B,Al)-ZSM-5 and

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(B,Al)-ZSM-11 have similar Si/Al and B/Al ratios. XRD and N2 sorption results (Figure S16 and Table S5 in the Supporting Information) confirm that incorporation of boron in ZSM-5 and ZSM-11 has a small effect on their crystallinity and textural properties. The NH3-TPD results reveal that the amount of strong acid sites is hardly affected by the incorporation of B in the framework despite of an increase in weak acid sites, which is supported by the results obtained by Yang, Yaripour, and co-workers.107,108

Table 3 shows the thermodynamic stability of B sited at different T sites in ZSM-5 and ZSM-11, as evaluated by periodic DFT calculation. The similar substitution energies of B and Al for Si in ZSM-5 suggests B and Al atoms competitively insert in the framework, as found by Chen, Qiao, Zhu and co-workers.33,103,109 Figure 15(a, b) shows that both B and Al atoms are located at T9 site in ZSM-5. Thus, addition of abundant B in the synthesis gel will push part of Al to other sites such as T2+T3. However, with respect to ZSM-11, both B and Al atoms are inclined to locate at T1+T2 sites (Figure 15(c, d)), and hence, introduction of B will impel part of Al from T1+T2 to T3+T4 sites.

{Figure 15}

The periodic DFT calculation results are supported by the

27

Al MAS NMR results

(Figure S17 and Table S6 in the Supporting Information); incorporation of B in ZSM-5 increases the intensity of the peak at 45–50 ppm (T1+T2+T3) at the expense of that at 50–57 ppm (T9). Nevertheless, Al atoms are still preferentially located in the intersection for ZSM-5. Notably, when B is incorporated in ZSM-11 framework, some Al atoms located in the straight channels (50–57 ppm, T1+T2) move to the intersectional T sites (45–50 and 57–60 ppm, T3+T4+T5). This can be further demonstrated by the elemental distribution of B and Al in (B,Al)-ZSM-5 and (B,Al)-ZSM-11. The energy-dispersive spectroscopy (EDS)

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with the line scanning model based on the STEM-HAADF66 reveals that the variation of elemental line distribution of B is highly consistent with that of Al in (B,Al)-ZSM-5 (Figure 16), whereas it is nearly opposite in (B,Al)-ZSM-11. This reveals that B and Al atoms are sited in a similar structural unit in (B,Al)-ZSM-5, while they distribute in different positions in (B,Al)-ZSM-11. As a result, the catalytic lifetime of H-ZSM-11 was dramatically increased from 80 h (Al-ZSM-11(240)) to 250 h (B,Al)-ZSM-11(240) after incorporation of B while that of H-ZSM-5 just slightly increased (225 h for Al-ZSM-5(240) vs. 280 h for (B,Al)-ZSM-5(240)) (Figure 17). It needs to point out that the increase in weak acid sites after incorporation of B has little effect on the MTO catalytic performance as they are not active sites.33 The migration of certain Al atoms from the straight channel to the intersection is conducive to the diffusion of intermediate and product species, consequently slowing down the coking process, and enhancing the catalytic stability.

{Figure 16, & Figure 17}

The effect of incorporation of B on the Al siting and catalytic performance is dependent on the content of B added in the synthesis gel33 because adjustment of the B content can accurately regulate the Al locations. ZSM-11 framework contains 16 T1 sites every unit cell. Incorporation of small amounts of B atoms may only cause some Al atoms migrate from T1 to other T1 sites.39 Thus, the catalytic lifetime of (B,Al)-ZSM-11(240) can only be greatly elevated when the B/Al ratio is higher than 3.0 (Figure S18 in the Supporting Information).

To further illustrate that the significant difference in catalytic properties of H-ZSM-5 and H-ZSM-11 in MTO is primarily due to their different framework acid site distributions, I-ZSM-5(40) and I-ZSM-11(40) zeolites with similar Si/Al ratio (35.6 vs. 33.6) and acid site distributions were successfully synthesized by finely tuning silicon and aluminum sources

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and adding proper amounts of Na+ ions in the synthesis gels.

The XRD measurements confirm that the as-synthesized I-ZSM-5(40) and I-ZSM-11(40) zeolites are highly crystalline single-phase MFI and MEL respectively (Figure S19 in the Supporting Information). Table S7 demonstrates that I-ZSM-5(40) and I-ZSM-11(40) are very similar in the Si/Al ratio, surface area and pore volume. The Py-IR results evidence that the difference in the quantity of both Brønsted and Lewis acid sites between I-ZSM-5(40) and I-ZSM-11(40) is insignificant too. The

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Al MAS NMR spectroscopy result of

I-ZSM-5(40) shows that the intensity of the peaks at 45–50 and 50–57 ppm (66.9%) is much higher than that at 57–65 ppm (33.1 %) (Figure 18a and Table S8 in the Supporting Information). This holds true for the I-ZSM-11(40) (68.0% vs. 32.0%), indicating that they contain nearly the same amount of Al atoms in the intersection. The DR UV-vis spectroscopy of their Co2+-exchanged analogues further confirms this point. It reveals that more than 62% of the Al atoms are located in the intersection, as estimated in terms of the amount of β-type Co2+ ions (Figure 18b and Table S8 in the Supporting Information). Since the intersection cavity of ZSM-11 (9 Å and 11.7 Å) is just slightly larger than that of ZSM-5 (9 Å),39–41 the two samples are very suitable for studying the effect of acid site distribution on the catalytic performance.

Figure 19 displays the methanol conversion, product selectivity and HTI obtained over I-ZSM-5(40) and I-ZSM-11(40) zeolites. Clearly, the two samples show very similar catalytic stability. In addition, they are also highly comparable in the selectivity of ethene, propene, butene, C1-C5 alkanes and aromatics as well as C4-HTI and C5-HTI. These results undoubtedly prove that the significant difference in the MTO catalytic performance between H-ZSM-5 and H-ZSM-11 is mainly ascribed to their different acid site distributions in framework, rather than to the slight difference in the pore and cavity sizes.

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{Figure 18, & Figure 19}

In summary, the catalytic performance of zeolite in MTO is closely related to its acid site or Al positions in framework, as the local environment of acid sites has a great effect on the type and activity of hydrocarbon pool species, and subsequently on the actual reaction route and coking behavior. The acid sites are preferably distributed in the intersection in H-ZSM-5, and hence, promoting the formation of higher polyMBs and enhancing the aromatic cycle that produces more ethene, alkanes and aromatics. Nonetheless, no serious constraint occurs to the formation of the transition state from these polyMBs and subsequent elimination of side chains to light olefins. Thus, the products can easily diffuse out of the zeolite channel, consequently greatly retarding the carbonaceous materials deposition and increasing the catalytic stability of H-ZSM-5 with high Si/Al ratio. In contrast, Al atoms are concentrated in the straight 10-MR channel in H-ZSM-11. Hence, the alkene cycle plays a major role, resulting in generation of more propene and butene. Nevertheless, the diffusion of formed polyMBs species in the straight channel is difficult due to the space confinement. Thus, they are further transformed into bulkier aromatic species and accelerate the deactivation of H-ZSM-11. Regulation of Al sitings in ZSM-11 framework by finely tuning synthesis parameters and by selectively removing external surface Al species is effective to improve its MTO catalytic performance.

5. CONCLUSION

Two series of H-ZSM-5 and H-ZSM-11 zeolites with similar acidity, morphology, and textural properties but different acid site distributions have been synthesized. MTO test shows that these two types of zeolites exhibit significantly different catalytic performance. Various spectroscopy, p-xylene isomerization and DFT calculation results reveal that it is due

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to their different Al locations in zeolite framework; in H-ZSM-5 Al atoms preferably occupy the intersectional T sites, whereas in H-ZSM-11 they are mainly sited in the straight 10-MR channel. The acid sites located in the intersection enhance the aromatic cycle that produces more ethene, alkanes and aromatics. Nonetheless, these generated hydrocarbon species could easily diffuse out of the zeolite channel, and hence, alleviating the deposition of carbonaceous materials and increasing the catalytic stability of H-ZSM-5 with high Si/Al ratio. In contrast, the acid sites positioned in the straight 10-MR channel preferentially produce propene and butene as a result of promoting the alkene cycle. However, the formed coke precursors are difficult to diffuse out of the zeolite, but apt to transform into deposited carbonaceous materials due to the geometric limit, therefore, leading to the decrease in the catalytic durability of ZSM-11. Selective removal of external surface acid sites by acid treatment and oriented regulation of Al sitings in the intersection of ZSM-11 by incorporating boron or alterating silica and aluminum sources and adding sodium cations can significantly prolong its catalytic lifetime in MTO. This work not only provides direct and solid evidences for the strong effect of Al sitings or acid site distribution on the catalytic performance, but also offers two effective methods for oriented control of Al locations in zeolite lattice sites for the purpose of developing high-performance MTO catalyst.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.catal.xxxxxxx.

Characterization of HNO3-treated and boron-incorporated ZSM-5 and ZSM-11 as well as

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I-ZSM-5 and I-ZSM-11 zeolites; periodic models used to calculate the substitution energy and probability of Al or B for Si at different T sites of ZSM-5 and ZSM-11 frameworks; SEM images and

27

Al solid-state MAS NMR spectra of ZSM-5 and ZSM-11 with different

Si/Al ratios; dependence of the hydride transfer index on the time on stream; the in situ DR UV-vis and DRIFTS spectra of H-ZSM-5(240) and H-ZSM-11(240) in MTO; GC-MS charts of organic species retained in the zeolites after catalyzing MTO for 60 min in the DRIFTS tests;

time-resolved

IR spectra

of

p-xylene isomerization over

H-ZSM-5(240), H-ZSM-11(120) and H-ZSM-11(240); and

11

H-ZSM-5(120),

B MAS NMR spectra of

(B,Al)-ZSM-5 and (B,Al)-ZSM-11.

AUTHOR INFORMATION

Corresponding Authors *Z. Qin: e-mail, [email protected]. *W. Fan: Tel.: +86-351-4199009; fax: +86-351-4041153; e-mail, [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

The authors are grateful to the financial supports of the National Natural Science Foundation of China (21573270, U1510104, 21773281), Natural Science Foundation of Shanxi Province of China (2015021003), the Innovative Talent Program of Shanxi Province (201605D211001), CAS/SAFEA International Partnership Program for Creative Research Teams, and Youth Innovation Promotion Association, CAS (2016161). The calculations are performed on the Computer Network Information Center of Chinese Academy of Sciences 37


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and National Supercomputer Centers in Lvliang of China.

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ACS Catalysis

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ACS Catalysis

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Incorporation on the Structure, Products Selectivities and Lifetime of H-ZSM-5 Nanocatalyst Designed for Application in Methanol-to-Olefins (MTO) Reaction. Micropor. Mesopor. Mater. 2015, 203, 41–53.

(109) Zhu, Q. J.; Kondo, J. N.; Yokoi, T.; Setoyama, T.; Yamaguchi, M.; Takewaki, T.; Domen,

K.;

Tatsumi,

T.

The

Influence

of

Acidities

of

Boron-

and

Aluminium-containing MFI Zeolites on Co-reaction of Methanol and Ethene. Phys. Chem. Chem. Phys. 2011, 13, 14598–14605.

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ACS Catalysis

Table 1. Chemical Compositions and Texture Properties of H-ZSM-5 and H-ZSM-11 Zeolites Synthesized with Synthesis Gels Having Different Si/Al Ratios.

zeolite

Si/Al ratio a crystallinity surface area (m2 g−1) c (%) b Total

Micro

pore volume (cm3 g−1) c Total

Micro

H-ZSM-11(60)

58

100

357

256

0.23

0.12

H-ZSM-5(60)

69

100

383

262

0.26

0.12

H-ZSM-11(120)

121

102

353

244

0.24

0.11

H-ZSM-5(120)

134

99

392

269

0.26

0.12

H-ZSM-11(240)

213

101

343

247

0.23

0.12

H-ZSM-5(240)

224

95

394

295

0.26

0.14

a

The Si/Al ratio of the synthesized zeolites was measured by ICP-AES. For the sample

denotation (e.g. ZSM-5(60)), the numeral in the parenthesis (e.g. 60) is the designated value of Si/Al ratio in the synthesis gel.

b

The relative crystallinity of ZSM-5 obtained by XRD was estimated by comparing their

peak intensity for the (0 5 1), (5 0 1) and (0 3 3) crystal faces at 2θ of 23°, 23.4°and 24° with that of the ZSM-5(60), wheras the relative crystallinity of ZSM-11 was estimated by comparing their peak intensity for the (5 0 2) and (3 0 3) crystal faces at 2θ of 23°and 24° with that of the ZSM-11(60).

c

Surface area and pore volume were determined on the basis of nitrogen physisorption data

by using BET method and t-plot method, respectively.

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Page 56 of 85

Table 2. Acid Properties of H-ZSM-5 and H-ZSM-11 Zeolites with Different Si/Al Ratios.

zeolite

acidity by NH3-TPD a (μmol g−1)

acidity by Py-IR b (μmol g−1)

weak

strong

H-ZSM-11(60)

115

249

104

37

18

H-ZSM-5(60)

100

197

101

37

29

H-ZSM-11(120)

33

117

42

17

n.d.

H-ZSM-5(120)

33

97

35

13

n.d.

H-ZSM-11(240)

38

54

17

8

n.d.

H-ZSM-5(240)

44

52

18

12

n.d.

a

Brønsted

acidity by DMQ-IR c (μmol g−1)

Lewis

external

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

quantities of ammonia desorbed at 400−500 and 550−700 K, respectively.

b

The quantities of Brønsted and Lewis acid sites were determined by Py-IR as the amounts

of pyridine (Py) adsorbed on the zeolites at 423 K.

c

The quantity of external surface acid sites was determined by 2,4-DMQ-IR as the amount

of 2,4-dimethylquinoline (2,4-DMQ) adsorbed at 473 K; for ZSM-5 and ZSM-11 with a high Si/Al ratio (120 and 240), the amount of external surface acid sites is too low to detect.

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ACS Catalysis

Table 3. Multiplicity of T sites, Substitution Energy (ESub.), and Occupation Probability of Aluminum or Boron located at Different T Sites in the Frameworks of ZSM-5 and ZSM-11 Zeolites.

T site

Multiplicity

ESub. ( kJ mol–1) a

Probability (%) b

Si/Al=95

Si/Al=191

Si/B=191

Si/Al=95

Si/Al=191

Si/B=191

ZSM-5 T1

8

24.1

31.2

28.3

0.1

0.0

0.0

T2

8

2.3

15.2

9.3

29.3

1.7

7.4

T3

8

12.1

15.6

40.4

2.1

1.3

0.0

T4

8

22.2

34.5

45.1

0.1

0.0

0.0

T5

8

42.5

47.1

29.5

0.0

0.0

0.0

T6

8

46.4

48.6

34.1

0.0

0.0

0.0

T7

8

24.3

28.6

28.5

0.1

0.0

0.0

T8

8

5.5

35.8

47.0

12.5

0.0

0.0

T9

8

0.0

0.0

0.0

55.5

96.5

90.4

T10

8

26.4

24.0

14.1

0.0

0.1

2.0

T11

8

19.3

20.2

21.9

0.3

0.4

0.2

T12

8

39.3

56.5

55.5

0.0

0.0

0.0

T1

16

0.0

0.0

0.0

71.4

92.8

86.3

T2

16

5.9

12.7

7.9

14.3

2.9

10.2

T3

16

10.2

19.0

21.2

4.4

0.6

0.3

T4

16

23.9

15.0

12.5

0.1

1.6

2.9

T5

16

10.7

13.9

21.4

3.9

2.1

0.3

T6

8

36.2

65.7

63.2

0.0

0.0

0.0

T7

8

6.6

40.9

33.6

5.9

0.0

0.0

ZSM-11

a

Relative substitution energy of aluminum or boron for Si at different T sites, as calculated

with the T site having the lowest substitution energy as zero.

b

The probability calculated in terms of the relative substitution energy of aluminum or boron

for Si located at different T sites.

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Page 58 of 85

Table 4. Calculated Absolute Chemical Shielding Tensors (δabs, ppm) and Relative Chemical Shift (δrel, ppm) for MAS NMR of

27

Al and

29

Si Located at Different T Sites in the

Framework of ZSM-5 and ZSM-11 Zeolites. T Site

δabs (ppm) 27

δrel (ppm) a

Al NMR

29

27

Si NMR

Al NMR

29

Si NMR

ZSM-5 T1

537.5

497.3

49.4

–121.2

T2

537.5

497.9

49.4

–121.8

T3

537.9

502.2

49.0

–126.1

T4

529.6

497.8

57.3

–121.7

T5

528.4

495.8

58.6

–119.7

T6

536.3

484.8

50.6

–108.7

T7

529.9

471.9

57.0

–95.8

T8

523.6

490.4

63.3

–114.4

T9

532.9

487.8

54.0

–111.7

T10

529.2

492.1

57.7

–116.0

T11

544.8

488.5

42.1

–112.4

T12

525.3

484.6

61.6

–108.5

T1

532.2

485.2

54.7

–109.1

T2

530.4

486.6

56.5

–110.5

T3

540.3

491.7

46.6

–115.6

T4

529.8

491.4

57.1

–115.3

T5

521.7

490.5

65.2

–114.4

T6

529.9

499.6

57.0

–123.5

T7

537.8

484.9

49.1

–108.8

ZSM-11

a

The relative chemical shift of

27

Al MAS NMR is obtained by the difference between the

calculated chemical shift of Al(H2O)63+ (586.9 ppm) and the absolute chemical shielding tensors of different T sites, viz., δrel = 586.9 − δabs, whereas the relative chemical shift of 29Si MAS NMR is the difference between the calculated chemical shift of Si(CH3)4 (376.1 ppm) and the absolute chemical shielding tensors of different T sites, viz., δrel = 376.1 − δabs.

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ACS Catalysis

Table 5. Proportion of Various Peaks Obtained by Curve Fitting of the

27

Al MAS NMR

Spectra of H-ZSM-5 and H-ZSM-11 Zeolites.

zeolite

AlF (%) AlEF (%) proportion of various peaks (%) b a

H-ZSM-5(120)

a

60–65 ppm

57–60 ppm

50–57 ppm

45–50 ppm

95.1

4.9

4.9 (12.5)

20.0 (0.1)

64.6 (55.5) 10.5 (31.5)

H-ZSM-11(120) 95.5

4.5

1.6 (3.9)

18.8 (0.1)

71.2 (85.7) 8.4 (4.4)

H-ZSM-5(240)

95.2

4.8

6.7 (0.0)

24.1 (0.1)

65.6 (96.5) 3.6 (3.0)

H-ZSM-11(240) 95.5

4.5

5.0 (2.1)

4.7 (1.6)

82.0 (95.7) 8.3 (0.6)

a

AlF and AlEF represent the percentages of aluminum atoms in the framework and

extra-framework, respectively, obtained by dividing the intensity of the signals at 45–65 and 0 ppm by the total intensity of the signals at 0 plus 45–65 ppm, respectively.

b

The proportions of various peaks were obtained by dividing their intensity by the total

intensity of the signals in the range of 45–65 ppm. The data in the parentheses represent the probability of aluminum siting at relevant T sites obtained by DFT calculation. For ZSM-5, the resonance peaks at 45–50 and 50–57 ppm are related to Al located at the T1+T2+T3 and T9 sites of the intersection cavity, respectively, whereas the peaks at 57–60 and 60–65 ppm correspond to Al settled at the T4+T10 and T8+T12 sites of the straight and/or sinusoidal channel, respectively. For ZSM-11, the resonance peaks at 45–50, 57–60, and 60–65 ppm are related to Al atoms situated at T3, T4+T6 and T5 sites in the intersection, respectively, whereas the peak at 50–57 ppm corresponds to Al atoms located at T1+T2 sites of the straight channel.

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Page 60 of 85

Table 6. Distributions of Different Types of Al Species in ZSM-5 and ZSM-11 with Different Si/Al Ratios, as Measured by DR UV-vis Spectroscopy after being Exchanged with Co2+ Ions.

Al distribution (%) c Zeolite

Si/Al ratio a

Alclose (%) b

Alsingle (%) b (α+γ)-type

β-type

ZSM-5(120)

121

59.3

41.7

37.8

62.2

ZSM-11(120)

134

65.9

34.2

59.4

40.6

ZSM-5(240)

213

47.6

52.4

26.1

73.9

ZSM-11(240)

224

41.3

58.7

64.0

34.0

a

The Si/Al ratio of the Co2+-exchanged zeolites was measured by ICP-AES.

b

The fractions of Alclose and Alsingle were calculated by the equations of [Alclose]=2×[Comax]

and [Alsingle]=[Altotal]-2×[Comax], respectively. [Altotal] and [Comax] are the contents of Al and Co atoms in the Co2+-exchanged zeolites, as measured by the ICP-AES technique. c

The distribution of various types (α, β, and γ) of Co2+ ions was determined by the

deconvoloving the DR UV-vis spectra of Co2+-exchanged ZSM-5 and ZSM-11 zeolites.

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ACS Catalysis

Table 7. Catalytic Results of H-ZSM-5 and H-ZSM-11 with Different Si/Al Ratios for MTO at Steady Reaction Stage.

Zeolite

Conv. (%) a

TON b

product selectivity (%) a C2=

C3=

C4=

C1–5

HTI c BTX

C4-HTI

C5-HTI

(P−E)/E ratio d

lifetime (h) e

H-ZSM-11(60)

99.9

4.96×104

14.1

31.6

15.7

20.9

6.4

0.35

0.48

1.2

84

H-ZSM-5(60)

99.9

2.51×104

15.2

27.4

13.1

27.1

8.0

0.49

0.65

0.8

44

H-ZSM-11(120)

99.8

1.44×105

10.4

41.7

24.5

8.8

3.0

0.12

0.25

3.0

100

H-ZSM-5(120)

99.9

2.16×105

12.7

38.4

21.5

13.2

4.0

0.22

0.37

2.0

128

H-ZSM-11(240)

99.8

2.86×105

8.5

40.8

26.8

6.5

3.5

0.07

0.16

3.8

80

H-ZSM-5(240)

99.7

7.49×105

10.6

37.4

26.0

7.7

4.3

0.09

0.19

2.6

225

a

The MTO catalytic tests were carried out at 723 K, atmosphere pressure, and methanol

WHSV of 1.9 h−1; methanol conversion and product distribution were determined at 10 h on stream; C2=, C3=, and C4= are the ethene, propene, and butene, respectively, C1–5 means methane to pentane, while BTX represents the sum of benzene, toluene and xylene. b

Turn-Over Number (TON) is the accumulated number of methanol molecules converted

per Brønsted acid site, as obtained TON = [ FMeOH 



t90

0

( XMeOH, t.dt)] / nH + , where FMeOH is

the flow rate of methanol, XMeOH,t is the transient methanol conversion at time on stream of t, t90 is the reaction time when the methanol conversion is decreased to 90%, and nH+ is the quantities of Brønsted acid sites determined by Py-IR after evacuation at 423 K. c

The C4-HTI and C5-HTI are the butane and pentane hydrogen transfer indexes, obtained as

C4-HTI = S(C40)/(S(C40) + S(C4=)) and C5-HTI = S(C50)/(S(C50) + S(C5=)), respectively, where S(C40), S(C50), S(C4=), and S(C5=) are the selectivities to butane, pentane, butene, and pentene, respectively. d

(P−E)/E is the ([propene]selectivity-[ethene]selectivity)/[ethene]selectivity ratio.

e

Lifetime is defined as the reaction time when the methanol conversion is decreased to 90%.

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Figure Captions

Figure 1. Topological structures of ZSM-5 (a) and ZSM-11 (b) zeolites with labeled pore sizes of corresponding channels.

Figure 2. XRD patterns of as-synthesized ZSM-5 and ZSM-11 zeolites with synthesis gels having different Si/Al ratios.

Figure 3. Relative substitution energies of Al for Si and occupation probability of Al at various T sites in the periodic-ZSM-5 and ZSM-11 models with different Si/Al ratios: (a) H-ZSM-5(95), (b) H-ZSM-5(191), (c) H-ZSM-11(95), and (d) H-ZSM-11(191).

Figure 4. The 2D 27Al MQ/MAS NMR spectra of (a) H-ZSM-5(120), (b) H-ZSM-5(240), (c) H-ZSM-11(120) and (d) H-ZSM-11(240). The four cross sections of the isotropic protection (F1) and the observed dimension (F2) in the 27Al MQ/MAS NMR spectra correspond to four isotropic chemical shifts (δiso) calculated by the equation of δiso = (17δF1 +10δF2)/27.

Figure 5. Deconvolution of the

27

Al MAS NMR spectra of H-ZSM-5 and H-ZSM-11

zeolites with different Si/Al ratios: (a) H-ZSM-5(120), (b) H-ZSM-5(240), (c) H-ZSM-11(120), and (d) H-ZSM-11(240). The experimental spectra are in black line and the fitted spectra are shown in red dashed line. The assignment of Al atoms at different T sites is based on the chemical shifts estimated by DFT computation results.

Figure 6. Deconvolution of the 29Si MAS NMR spectra of H-ZSM-5 and H-ZSM-11 zeolites with different Si/Al ratios: (a) H-ZSM-5(120), (b) H-ZSM-5(240), (c) H-ZSM-11(120), and (d) H-ZSM-11(240). The experimental spectra are in black line and the fitted spectra are shown in red dashed line. The assignment of Si atoms at different T sites is based on the chemical shifts estimated by DFT computation results. 62


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ACS Catalysis

Figure 7. DR UV-vis spectra of Co2+-exchanged ZSM-5 with Si/Al ratios of 120 (a1) and 240 (a2) and ZSM-11 with Si/Al ratios of 120 (b1) and 240 (b2). The black and red dashed lines represent the experimental and the fitted spectra respectively.

Figure 8. Conversion of methanol along with the time on stream for MTO over H-ZSM-5 and H-ZSM-11 zeolites with different Si/Al ratios at atmospheric pressure, 723 K, and WHSV of 1.9 h−1. Figure 9. Product selectivity to ethene, propene, butene, C1–C5 alkanes, and BTX aromatics as a function of time on steam for conversion of methanol over ZSM-5 and ZSM-11 zeolites with different Si/Al ratios at atmospheric pressure, 723 K, and WHSV of 1.9 h−1: (a1) H-ZSM-5(60), (b1) H-ZSM-11(60), (a2) H-ZSM-5(120), (b2) H-ZSM-11(120), (a3) H-ZSM-5(240), and (b3) H-ZSM-11(240).

Figure 10. TGA curves of deactivated H-ZSM-5 (a) and H-ZSM-11 (b) zeolites with different Si/Al ratios in MTO. The weight loss percentage (at temperature > 700 K) and the coking rate (h−1, as obtained by dividing the coke amount deposited on 1 g catalyst by the catalyst lifetime) are labeled.

Figure 11. Conversion of methanol along with the time on stream for MTO over H-ZSM-5(60) (a1), H-ZSM-11(60) (b1), and dealuminized H-ZSM-5(60) (D-ZSM-5, a2) and H-ZSM-11(60) (D-ZSM-11, b2) with HNO3 (reaction conditions: atmospheric pressure, 723 K, and WHSV = 1.9 h−1).

Figure 12. Time dependent DR UV-vis curves for MTO over (a) H-ZSM-5(120) and (b) H-ZSM-11(120) at 573 K. Methanol was continusously introduced into the in situ reaction chamber with an Ar flow through a saturator and the spectra were collected at a fixed

63


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

interval of 3 min. From the bottom to the up, the green dotted lines represent the initial spectra (0 min), the red dashed lines correspond to the spectra in the induction period (0–3 min for H-ZSM-5(120); 0–12 min for H-ZSM-11(120)), whereas the blue solid lines to the spectra in the steady stage (6–30 min for H-ZSM-5(120); 15–30 min for H-ZSM-11(120)).

Figure 13. Time dependent DRIFTS curves for MTO over (a) H-ZSM-5(120) and (b) H-ZSM-11(120) at 573 K. Methanol was continusously introduced into the in situ reaction chamber with an Ar flow through a saturator from the bottom to the up. The spectra were collected at a fixed interval of 5 min, from 0 to 60 min after introducing methanol into the chamber.

Figure 14. Relative fraction of m-xylene (a) (obtained with the peak area of m-xylene divided by the total peak areas of p-, o-, and m-xylene in the situ IR spectra), the formation rate of m-xylene (b) (defined as the mole of formed m-xylene on 1 mole of acid sites per second) along with the reaction time over the ZSM-5 and ZSM-11 zeolites with different Si/Al ratios at 673 K, and Arrhenius plot (c) for the conversion of p-xylene into m-xylene at different temperature (473, 573 and 673 K).

Figure 15. Relative substitution energies of Al for Si and occupation probabilities of Al and B at different T sites in (a) Al-ZSM-5(191), (b) B-ZSM-5(191), (c) Al-ZSM-11(191), and (d) B-ZSM-11(191), as calculated with periodic models.

Figure 16. STEM-HAADF images (a1, b1) and elemental distributions of boron and aluminum, detected by energy-dispersive spectroscopy (EDS) with the line scanning model (a2, b2), of H-(B,Al)-ZSM-5 and H-(B,Al)-ZSM-11.

Figure 17. Conversion of methanol along with the reaction time obtained over

64


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ACS Catalysis

H-ZSM-5(240)

(a1),

H-ZSM-11(240)

(b1),

H-(B,Al)-ZSM-5(240)

(a2)

and

H-(B,Al)-ZSM-11(240) (b2) zeolites in MTO (reaction conditions: atmospheric pressure, 723 K, and WHSV = 1.9 h−1).

Figure 18. Deconvolution of the 27Al MAS NMR (a1, b1) and DR UV-vis (a2, b2) spectra of I-ZSM-5(40) and I-ZSM-11(40) (the black and red dashed lines represent the experimental and fitted spectra respectively).

Figure 19. Methanol conversions and product selectivities at different reaction time, and HTI calculated on the basis of C4 and C5 species at half-lifetime over I-ZSM-5(40) and I-ZSM-11(40) at 723 K, atmospheric pressure, and WHSV of 3.8 h−1.

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Page 66 of 85

Figure 1.

(b)

(a)

Figure 1. Topological structures of ZSM-5 (a) and ZSM-11 (b) zeolites with labeled pore sizes of corresponding channels.

66


Page 67 of 85

Figure 2.

ZSM-11(60) ZSM-11(120)

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

ZSM-11(240) ZSM-5(60) ZSM-5(120) ZSM-5(240) 5

10

15

20

25

30

35

40

45

50

2

Figure 2. XRD patterns of as-synthesized ZSM-5 and ZSM-11 zeolites with synthesis gels having different Si/Al ratios.

67


ACS Catalysis

Figure 3.

80

(a) ZSM-5(95)

60

40

40 20

20

0 80

0 100

(b) ZSM-5(191)

80

60

60

40

40

20

20

0

Occupation probability (%)

Substitution energy (kJ mol-1)

60

0 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10T11T12

(c) ZSM-11(95)

80 60

40

40 20

20

0 80

0 100

(d) ZSM-11(191)

80

60

60

40

40

20

20

0


60


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 68 of 85

0 T1

T2

T3

T4

T5

T6

T7

Figure 3. Relative substitution energies of Al for Si and occupation probability of Al at various T sites in the periodic-ZSM-5 and ZSM-11 models with different Si/Al ratios: (a) H-ZSM-5(95), (b) H-ZSM-5(191), (c) H-ZSM-11(95), and (d) H-ZSM-11(191).

68


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ACS Catalysis

Figure 4.

Figure 4. The 2D 27Al MQ/MAS NMR spectra of (a) H-ZSM-5(120), (b) H-ZSM-5(240), (c) H-ZSM-11(120) and (d) H-ZSM-11(240). The four cross sections of the isotropic protection (F1) and the observed dimension (F2) in the 27Al MQ/MAS NMR spectra correspond to four isotropic chemical shifts (δiso) calculated by the equation of δiso = (17δF1 +10δF2)/27.

69


ACS Catalysis

Figure 5.

(a) ZSM-5(120) T9 T4+T10

T1+T2+T3

T8+T12

(b) ZSM-5(240)

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 70 of 85

(c) ZSM-11(120) T1+T2 T4+T6 T3

T5

(d) ZSM-11(240)

65

60

55

50

45

Chemical shift (ppm)

Figure 5. Deconvolution of the

27

Al MAS NMR spectra of H-ZSM-5 and H-ZSM-11

zeolites with different Si/Al ratios: (a) H-ZSM-5(120), (b) H-ZSM-5(240), (c) H-ZSM-11(120), and (d) H-ZSM-11(240). The experimental spectra are in black line and the fitted spectra are shown in red dashed line. The assignment of Al atoms at different T sites is based on the chemical shifts estimated by DFT computation results.

70


Page 71 of 85

Figure 6.

(a) ZSM-5(120)

T8+T10

T9+T11 T1~T5

T6+T12 Q [Si1Al)] 3

(b) ZSM-5(240)

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

(c) ZSM-11(120) T3+T4+T5 T1+T2+T7 T6

3

Q [Si1Al)]

(d) ZSM-11(240)

-108

-110

-112

-114

-116

-118

Chemical shift (ppm)

Figure 6. Deconvolution of the 29Si MAS NMR spectra of H-ZSM-5 and H-ZSM-11 zeolites with different Si/Al ratios: (a) H-ZSM-5(120), (b) H-ZSM-5(240), (c) H-ZSM-11(120), and (d) H-ZSM-11(240). The experimental spectra are in black line and the fitted spectra are shown in red dashed line. The assignment of Si atoms at different T sites is based on the chemical shifts estimated by DFT computation results.

71


ACS Catalysis

Figure 7.

-type -type -type

(b1) ZSM-11(120)

Intensity (a.u.)

(a1) ZSM-5(120)

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 72 of 85

(a2) ZSM-5(240)

14000 16000 18000 20000 22000


(b2) ZSM-11(240)

14000 16000 18000 20000 22000

-1

Wavenumber (cm-1)

Wavenumber (cm )

Figure 7. DR UV-vis spectra of Co2+-exchanged ZSM-5 with Si/Al ratios of 120 (a1) and 240 (a2) and ZSM-11 with Si/Al ratios of 120 (b1) and 240 (b2). The black and red dashed lines represent the experimental and the fitted spectra respectively.

72


Page 73 of 85

Figure 8.

100

Methanol conversion (%)

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

95 ZSM-5(60) ZSM-11(60) ZSM-5(120) ZSM-11(120) ZSM-5(240) ZSM-11(240)

90

85 0

50

100

150

200

250

Time on stream (h)

Figure 8. Conversion of methanol along with the time on stream for MTO over H-ZSM-5 and H-ZSM-11 zeolites with different Si/Al ratios at atmospheric pressure, 723 K, and WHSV of 1.9 h−1.

73


ACS Catalysis

Figure 9.

(a1) ZSM-5(60)

(a2) ZSM-5(120)

(a3) ZSM-5(240)

(b2) ZSM-11(120)

(b3) ZSM-11(240)

40 30

propene butene ethene alkanes BTX

20

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

Page 74 of 85

10 0

(b1) ZSM-11(60) 40 30

propene butene ethene alkanes BTX

20 10 0

0

20

40

60

80

0

20

40

60

80

100 120

0

50

100

150

200

Time on stream (h)

Figure 9. Product selectivity to ethene, propene, butene, C1–C5 alkanes, and BTX aromatics as a function of time on steam for conversion of methanol over ZSM-5 and ZSM-11 zeolites with different Si/Al ratios at atmospheric pressure, 723 K, and WHSV of 1.9 h−1: (a1) H-ZSM-5(60), (b1) H-ZSM-11(60), (a2) H-ZSM-5(120), (b2) H-ZSM-11(120), (a3) H-ZSM-5(240), and (b3) H-ZSM-11(240).

74


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Figure 10.

100

a

95 90

Relative weight (%)

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

85

ZSM-5(240) ZSM-5(120) ZSM-5(60)

80

12.1%, 0.05 h

-1

15.3%, 0.12 h

-1

21.3%, 0.48 h

-1

100

b

95 9.9%, 0.12 h

90 ZSM-11(240) ZSM-11(120) ZSM-11(60)

85 80 400

800

600

-1

14.7%, 0.15 h

-1

17.1%, 0.20 h

-1

1000

Temperature (K)

Figure 10. TGA curves of deactivated H-ZSM-5 (a) and H-ZSM-11 (b) zeolites with different Si/Al ratios in MTO. The weight loss percentage (at temperature > 700 K) and the coking rate (h−1, as obtained by dividing the coke amount deposited on 1 g catalyst by the catalyst lifetime) are labeled.

75


ACS Catalysis

Figure 11.

100


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 76 of 85

98 96 94

b2

b1

a2 a1

92

(a1) ZSM-5 (a2) D-ZSM-5

90 0

20

40

60

(b1) ZSM-11 (b2) D-ZSM-11 80

100

120

140

Time on stream (h)

Figure 11. Conversion of methanol along with the time on stream for MTO over H-ZSM-5(60) (a1), H-ZSM-11(60) (b1), and dealuminized H-ZSM-5(60) (D-ZSM-5, a2) and H-ZSM-11(60) (D-ZSM-11, b2) with HNO3 (reaction conditions: atmospheric pressure, 723 K, and WHSV = 1.9 h−1).

76


Page 77 of 85

Figure 12.

(a) ZSM-5(120) 0.15

0.10

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

0.05

0.00 0.15

(b) ZSM-11(120)

0.10

0.05

0.00 200

300

400

500

600

Wave length (nm)

Figure 12. Time dependent DR UV-vis curves for MTO over (a) H-ZSM-5(120) and (b) H-ZSM-11(120) at 573 K. Methanol was continusously introduced into the in situ reaction chamber with an Ar flow through a saturator and the spectra were collected at a fixed interval of 3 min. From the bottom to the up, the green dotted lines represent the initial spectra (0 min), the red dashed lines correspond to the spectra in the induction period (0–3 min for H-ZSM-5(120); 0–12 min for H-ZSM-11(120)), whereas the blue solid lines to the spectra in the steady stage (6–30 min for H-ZSM-5(120); 15–30 min for H-ZSM-11(120)).

77


ACS Catalysis

Figure 13.

-1

0.6

(a) ZSM-5(120) 1320 cm-1

-1

-1

3720 cm -1 3590 cm

3010 cm

0.4 0.3

1190 cm

-1

2920 cm

0.5

Kubelka-Munk (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 78 of 85

-1

2870 cm

0.2 0.1 0.0 0.6

(b) ZSM-11(120)

0.5 0.4 0.3

-1

2920 cm

-1

1320 cm

-1

3750 cm

0.2 0.1 0.0 4000

3500

3000

2500

2000

1500

1000

-1

Wave number (cm )

Figure 13. Time dependent DRIFTS curves for MTO over (a) H-ZSM-5(120) and (b) H-ZSM-11(120) at 573 K. Methanol was continusously introduced into the in situ reaction chamber with an Ar flow through a saturator from the bottom to the up. The spectra were collected at a fixed interval of 5 min, from 0 to 60 min after introducing methanol into the chamber.

78


Page 79 of 85

Figure 14.

50 45 40 35 30 25 20 15

4.0

(a)

(b)

3.5 3.0

Formation rate ofm-xylene (s-1)

Fraction of m-xylene (%)

2.5 2.0

ZSM-5(120) ZSM-11(120)

ZSM-5(120) ZSM-11(120)

1.5 1.0 7.5

50 45 40 35 30 25 20

6.0 4.5

ZSM-5(240) ZSM-11(240) 0

20

40

60

3.0

80

-5.5

ZSM-5(240) ZSM-11(240) 0

20

40

60

80

Time on Stream (min)

Time on Stream (min) (c)

-6.0

ln (TOF of m-xylene)

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

-6.5 -7.0 -7.5 -8.0 -8.5 0.0014

ZSM-5(120) ZSM-5(240) ZSM-11(120) ZSM-11(240)

0.0016

0.0018

0.0020

0.0022

-1

1/T (K )

Figure 14. Relative fraction of m-xylene (a) (obtained with the peak area of m-xylene divided by the total peak areas of p-, o-, and m-xylene in the situ IR spectra), the formation rate of m-xylene (b) (defined as the mole of formed m-xylene on 1 mole of acid sites per second) along with the reaction time over the ZSM-5 and ZSM-11 zeolites with different Si/Al ratios at 673 K, and Arrhenius plot (c) for the conversion of p-xylene into m-xylene at different temperature (473, 573 and 673 K). 79


ACS Catalysis

Figure 15.

100

(a) Al-ZSM-5(191)

80

60

60

40

40

20

20

0

0 100

80

(b) B-ZSM-5(191)

80

60

60

40

40

20

20

0



80

0 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10T11T12

(c) Al-ZSM-11(191)

100 80

60

60

40

40

20

20

0

0 100

80

(d) B-ZSM-11(191)

80

60

60

40

40

20

20

0

0 T1

T2

T3

T4

T5

T6


80


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 80 of 85

T7

Figure 15. Relative substitution energies of Al for Si and occupation probabilities of Al and B at different T sites in (a) Al-ZSM-5(191), (b) B-ZSM-5(191), (c) Al-ZSM-11(191), and (d) B-ZSM-11(191), as calculated with periodic models.

80


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ACS Catalysis

Figure 16.

Al K-edge contour B K-edge contour

(a2)

5.0x10-7

1.0x10-6

1.5x10-6

2.0x10-6

2.5x10-6

Al K-edge contour B K-edge contour

(b2)

3.0x10-6

2.0x10-7

4.0x10-7

Line scan (m)

6.0x10-7

8.0x10-7

1.0x10-6

Line scan (m)

Figure 16. STEM-HAADF images (a1, b1) and elemental distributions of boron and aluminum, detected by energy-dispersive spectroscopy (EDS) with the line scanning model (a2, b2), of H-(B,Al)-ZSM-5 and H-(B,Al)-ZSM-11.

81


ACS Catalysis

Figure 17.

100


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 82 of 85

98 96 a2

a1

94 92

b1 b2

90

(a1) Al-ZSM-5 (a2) (B,Al)-ZSM-5 (b1) Al-ZSM-11 (b2) (B,Al)-ZSM-11

88 86 84 0

50

100

150

200

250

300

Time on stream (h)

Figure 17. Conversion of methanol along with the reaction time obtained over H-ZSM-5(240)

(a1),

H-ZSM-11(240)

(b1),

H-(B,Al)-ZSM-5(240)

(a2)

and

H-(B,Al)-ZSM-11(240) (b2) zeolites in MTO (reaction conditions: atmospheric pressure, 723 K, and WHSV = 1.9 h−1).

82


Page 83 of 85

Figure 18.

(b1) I-ZSM-11

Intensity (a.u.)

Intensity (a.u.)

(a1) I-ZSM-5

65

60

55

50

45

65

Chemical Shifts (ppm) α-type β-type γ-type

55

50

45

(b2) I-ZSM-11


Intensity (a.u.)

(a2) I-ZSM-5

60

Chemical Shifts (ppm)

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

14000 15000 16000 17000 18000 19000 20000 21000 22000

15000 16000 17000 18000 19000 20000 21000 22000 23000

-1

Wavenumber (cm-1)

Wavenumber (cm )

Figure 18. Deconvolution of the 27Al MAS NMR (a1, b1) and DR UV-vis (a2, b2) spectra of I-ZSM-5(40) and I-ZSM-11(40) (the black and red dashed lines represent the experimental and fitted spectra respectively).

83


ACS Catalysis

Figure 19.

100

100

98

I-ZSM-5 I-ZSM-11

I-ZSM-5 I-ZSM-11

C5-HTI C4-HTI

96

60

Alkane

94 92 Ethene

2

40

Propene

4

6

20

Aromatic

Butene 90

80

8

10

12

14

Product Selectivity & HTI (%)

Methanol Conversion (%)

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 84 of 85

0 16

Time on Stream (h)

Figure 19. Methanol conversions and product selectivities at different reaction time, and HTI calculated on the basis of C4 and C5 species at half-lifetime over I-ZSM-5(40) and I-ZSM-11(40) at 723 K, atmospheric pressure, and WHSV of 3.8 h−1.

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ACS Catalysis

Table of Contents Graphic

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Relation of Catalytic Performance to the Aluminum Siting of Acidic

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