Regulation of Framework Aluminum Siting and Acid Distribution in H

Research Article pubs.acs.org/acscatalysis

Regulation of Framework Aluminum Siting and Acid Distribution in H‑MCM-22 by Boron Incorporation and Its Effect on the Catalytic Performance in Methanol to Hydrocarbons Jialing Chen,†,‡ Tingyu Liang,†,‡ Junfen Li,† Sen Wang,†,‡ Zhangfeng Qin,*,† Pengfei Wang,† Lizhi Huang,†,‡ Weibin Fan,† and Jianguo Wang*,† †

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, Shanxi 030001, P.R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P.R. China S Supporting Information *

ABSTRACT: As the process of methanol to hydrocarbons (MTH) is catalyzed by acid sites, the regulation of framework aluminum siting and acid distribution in a zeolite catalyst to enhance its performance in MTH is an important and challenging task. In this work, the regulation of framework aluminum siting in H-MCM-22 was achieved through boron incorporation; the relation between catalytic performance and acid distribution was investigated. The results illustrate that the distribution of framework aluminum and Brönsted acid sites among three types of pores in HMCM-22 can be regulated through adjusting the content of boron incorporated during synthesis, due to the competitive occupancy of various framework T sites between boron and aluminum, whereas the textural properties and overall acid types and amounts are less influenced by boron incorporation. Incorporating a proper content of boron can concentrate the Brönsted acid sites in the sinusoidal channels rather than in the surface pockets and supercages. The acid sites located in the surface pockets and supercages are prone to carbonaceous deposition, whereas those acid sites in the sinusoidal channels are crucial for MTH in the steady reaction stage. Moreover, the acid sites in the sinusoidal channels are favorable to the olefin-based cycle that produces preferentially higher olefins. As a result, the incorporation of proper content of boron delivers the H-MCM-22 zeolite much greater stability and higher selectivity to higher olefins such as propene and butene in MTH than previously reported. These results help to clarify the relation between the catalytic performance of HMCM-22 in MTH and its acid distribution and then bring forward an effective approach to develop better MTH catalysts by regulating the acid distribution. KEYWORDS: methanol to hydrocarbons, H-MCM-22, aluminum siting, boron incorporation, acid distribution, catalytic stability × 0.55 nm) windows, and pockets on the external surface (0.71 nm in diameter and 0.70 nm in height). Previous theoretical calculations demonstrated that these three types of pores were quite different in their catalytic action on MTH because of their large difference in the pore size and shape, which influenced the space confinement and electrostatic stabilization effects from the framework.12 At the initial stage, propene could be effectively produced through both the aromatic-based and the olefin-based cycles in the supercages, whereas in the sinusoidal channels, propene was formed primarily via the olefin-based cycle. Meanwhile, the surface pockets were probably detrimental to MTH due to the facile formation of coke precursors, as certain large intermediates were easily formed in the pockets but were difficult to decompose due to the lack of electrostatic stabilization effects from the zeolite framework.

1. INTRODUCTION The conversion of methanol to hydrocarbons (MTH), as a nonpetroleum route to get valuable chemicals from multifarious carbon resources such as coal, natural gas, and biomass, has attracted considerable attention in recent years.1−4 A variety of molecular sieves have been explored as potential catalysts, among which H-MCM-22 zeolite, with a unique framework structure, exhibits excellent catalytic performance in MTH;5−11 for instance, it has a relatively long lifetime and gives a high ratio of propylene to ethylene (P/E). However, when used as a catalyst in MTH, H-MCM-22 is still prone to carbonaceous deposition and suffers from rapid deactivation.7,8,11 Because MTH is catalyzed by acid sites, the performance of a zeolite catalyst should be closely related to its acid distribution in the zeolite framework. As shown in Figure 1, H-MCM-22 has three types of pores:5 two-dimensional sinusoidal channels with elliptical 10-membered ring cross sections (0.41 × 0.51 nm), cylindrical supercages (0.71 nm in diameter and 1.82 nm in height) that are accessible through 10-membered ring (0.40 © XXXX American Chemical Society

Received: December 15, 2015 Revised: February 23, 2016

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DOI: 10.1021/acscatal.5b02862 ACS Catal. 2016, 6, 2299−2313

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siting of AlF in ZSM-5 zeolites, which brought about a distinct change in the chemical shift of 4-coordinated aluminum species in the 27Al MAS NMR spectra of ZSM-5.25,29 H-MCM-22 zeolite is a potential catalyst for MTH, and its three types of pores are quite different in their catalytic action. It is natural to consider that the catalytic performance of HMCM-22 in MTH can be greatly enhanced through regulating the AlF siting and acid distribution by selectively concentrating or diminishing the acid sites in a certain type of pores. However, it is still a quite challenging task, because the detailed mechanism of boron incorporation and the influence of boron on AlF siting and consequent catalytic behavior of H-MCM-22 remain rather obscure. In this work, the regulation of framework aluminum (AlF) siting and acid distribution in H-MCM-22 was achieved through boron incorporation. The competitive occupancy of various framework T sites between boron and aluminum was investigated; the relation between the catalytic performance of H-MCM-22 in MTH and its Brönsted acid distribution among three types of pores was then clarified. The insights shown in this work should be of great benefit to the development of better MTH catalysts and reaction processes. Figure 1. Framework of the H-MCM-22 zeolite with eight different T sites and three types of pores: supercages, surface pockets, and sinusoidal channels.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The boron-incorporated MCM-22 zeolites were hydrothermally synthesized according to the previous studies,30,31 from a mixture of silica sol, sodium meta-aluminate, boric acid, hexamethylenimine (HMI), sodium hydroxide, and deionized water, with a modified molar composition of SiO 2 : 0.017Al 2 O 3 : nH 3 BO 3 : 0.8HMI: 0.1NaOH: 20H2O (n = 0, 0.017, 0.034, 0.1, 0.2, and 1). The synthesis gels were crystallized in a Teflon-lined stainless steel autoclave under rotation at 150 °C for 5 days. The solid products were filtered, washed, and dried overnight at 100 °C; the resultant products are denoted as as-synthesized (as-syn) MCM-22 samples. After that, the as-synthesized samples were further calcined at 560 °C for 10 h in air, to obtain the directly calcined MCM-22 zeolites. H-form MCM-22 or H-MCM-22 (MCM-22 in hydrogen form) was obtained through ionexchanging 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. The MCM-22 zeolites obtained in this work are labeled as xB-Al-M22 (in H-form, if not specially indicated), where x (x = 0, 0.5, 1, 3, 6, and 30) represents the atomic ratio of boron to aluminum in the synthesis gel. To exclude the effect of weak acidity introduced by boron on MTH, a borosilicate sample in hydrogen form (H-B-MWW) was also synthesized by using the same procedures as described above, with fume silica as the silicon source; the molar composition of the synthesis gel was SiO2:H3BO3:0.8HMI:0.1NaOH:20H2O and the crystallization was performed at 170 °C for 7 day.32 After that, it was subjected to the same calcination and NH4NO3 ion-exchange procedures as those for the xB-Al-M22 zeolites. To differentiate the acid sites in three types of pores in the H-form xB-Al-M22 catalysts, the catalyst samples in which a portion of acid sites was selectively poisoned or passivated were also prepared. m-Xylene poisoning was used to precoke the acid sites in the supercages, as described previously;33,34 the resultant catalyst samples are denoted as xB-Al-M22-mx. Similarly, through dropwisely adding the solution of 2,4dimethylquinoline (2,4-DMQ) in dichloromethane into the xB-

The difference of various types of pores in their catalytic performance, especially in their resistance to carbonaceous deposition which is related to the long-term stability in MTH, has also been demonstrated in practice. For instance, it is wellknown that the deactivation of cage-based H-SAPO-34 is much faster than that of the channel-based H-ZSM-5 in MTH. Min, Wang and co-workers found that for MTH over H-MCM-22, the supercages were rapidly deactivated by coking at the first reaction stage.7,11 As the external surface acid sites are prone to the deposition of coke species that may block the sinusoidal channels and lead to catalyst deactivation, it is able to enhance the selectivity to propene and long-term stability in MTH through selective removal of surface acid sites with oxalic acid.11,13 In general, the regulation of zeolite acid distribution to improve its catalytic performance can be achieved through two approaches: (1) various postsynthesis treatment processes such as dealumination,14,15 hierarchical porosity creation,16 and surface passivation;17,18 (2) modification of synthesis conditions such as adjusting the organic structure-directing agent (OSDA) and the silicon and aluminum sources.19−21 As the postsynthesis treatment often has some drawbacks such as the decrease of hydrothermal stability, narrowing of the poremouth, and loss of activity,22,23 direct controlling aluminum siting in the zeolite framework through adjusting the synthesis conditions is then probably more effective in the regulation of acid distribution. However, it is often difficult to get appropriate OSDAs and aluminum and silicon sources to effectively control the siting of framework aluminum (AlF). Fortunately, heteroatom incorporation is an effective method to adjust the acid distribution, owing to the competitive occupancy of various framework T sites between the heteroatom and aluminum.24,25 It was demonstrated that boron incorporation could markedly improve the catalytic performances of zeolites in MTH.26−28 Furthermore, boron incorporation could effectively control the 2300


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number of scans used was 384; the empty rotor background signal was subtracted from the spectrum of the catalyst sample. Prior to the 1H MAS NMR measurement, the H-MCM-22 sample was dehydrated at 450 °C and a pressure below 10−2 Pa for 20 h; after that, the sample was sealed and kept in glass tubes before being filled into the MAS NMR rotor in a glovebox purged with dry nitrogen gas. Quantitative 1H MAS NMR measurement was performed by comparing the signal area of the zeolite sample under study with that of adamantine as an external standard, as reported by Su and co-workers.35 The 1H MAS NMR spectra were deconvoluted with Gaussian− Lorentzian line shapes. The 27Al 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, and the number of scans used was 10000. The 29Si 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, and the number of scans used was 1400. The percentages of Al species in the framework (AlF) and extraframework (AlEF) were calculated from the integration of peaks at 56 and 0 ppm, respectively, in the 27Al MAS NMR spectra. The broad peak around 56 ppm in 27Al MAS NMR spectra was deconvoluted into three peaks at 50, 56, and 61 ppm (represented as Al(50), Al(56), and Al(61), respectively) by Voigt functions, as proposed by Kennedy, Vjunov, and co-workers.36,37 The 29Si MAS NMR spectra were deconvoluted by Lorentzian functions. Fourier transform infrared (FT-IR) spectra of the zeolite framework were measured on a Bruker Tensor 27 FT-IR spectrometer. To get the infrared spectra in the framework region (1600−400 cm−1), each zeolite sample was pressed into a self-supported wafer by using the KBr pelleting method. 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 (250 °C for the Py-IR measurement); the IR spectra were then recorded at room temperature. Pyridine vapor was introduced into the sample cell at room temperature for 1 h; the spectra were then recorded after evacuation at 150 °C for 1 h. The concentrations of Brönsted and Lewis acid sites in xB-Al-M22, xB-Al-M22-mx and xB-Al-M22-mx-DMQ were calculated according to the procedures reported by Madeira and co-workers.38 The overall Brönsted and Lewis acid concentrations (Btotal and Ltotal, respectively) were calculated from the Py-IR spectra of xB-AlM22, whereas the acid concentrations in the sinusoidal channels (Bsin and Lsin, respectively) were determined from the Py-IR spectra of xB-Al-M22-mx-DMQ. Subsequently, the Brönsted and Lewis acid concentrations in the supercages (Bsup and Lsup, respectively) were estimated from the differences in the acid concentrations between xB-Al-M22 and xB-Al-M22mx, whereas the acid concentrations in the surface pockets (Bpoc and Lpoc, respectively) were obtained from the differences between xB-Al-M22-mx and xB-Al-M22-mx-DMQ.34 Thermogravimetric (TG) analysis was conducted in flowing air (30 mL min−1) on a Rigaku Thermo plus Evo TG 8120 instrument at a heating rate of 10 °C min−1, to characterize the elimination of OSDAs upon calcination and the carbonaceous species deposited during the MTH reaction. 2.3. Catalytic Test. The reactions of methanol to hydrocarbons (MTH) were performed in a continuous flow fixed-bed reactor with an inner diameter of 10 mm. The catalyst sample was pressurized into wafers and then crushed and sieved

Al-M22-mx wafers, the catalyst samples, which were believed to have only acid sites accessible in the sinusoidal channels, were obtained and denoted as xB-Al-M22-mx-DMQ.34 2.2. Catalyst Characterization. The actual atomic composition of as-synthesized xB-Al-M22 zeolites and those in hydrogen form were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Autoscan16, TJA). Nitrogen adsorption/desorption isotherms were measured at −195.8 °C on a TriStar II 3020 gas adsorption analyzer. Prior to the measurement, the zeolite sample 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; the micropore volume and external surface area were calculated from the isotherms by t-Plot method. Total pore volume was estimated at a nitrogen relative pressure of 0.99, and micropore surface area was obtained from the difference between the total surface area and external surface area. X-ray powder diffraction (XRD) patterns were collected on a Rigaku MiniFlex II desktop X-ray diffractometer with monochromated Cu Kα radiation (154.06 pm, 30 kV, and 15 mA). Taking the 0B-Al-M22 zeolite as a reference (assuming that it had a crystallinity of 100%), the relative crystallinity of other zeolite samples was estimated by comparing their diffraction peak intensity at 2θ of 26° for the (310) crystal face with that of the reference sample. 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). Temperature-programmed desorption of NH3 (NH3-TPD) was performed on a Micromeritics AutoChem II 2920 chemisorption analyzer. 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 measured by a thermal conductivity detector (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. Magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra of 11B, 1H, 27Al, and 29Si were collected on a Bruker Avance III 600 MHz Wide Bore spectrometer operating at a magnetic field of 14.2 T. The chemical shifts for 11B and 27 Al MAS NMR spectra were referenced to the aqueous solution of H3BO3 (19.6 ppm) and Al(NO3)3 (0 ppm), respectively. The chemical shifts for 1H and 29Si MAS NMR spectra were referenced to tetramethylsilane (TMS). 11B MAS NMR spectra were recorded at a spinning rate of 10 kHz with 3200 scans by using Hahn-echo with 90° (20.5 μs) and 180° (41 μs) soft pulses, employing an rf field strength of v1 = 12.2 kHz and rotor-synchronized echo delay of 5 rotor period (469 μs) to remove the 11B probe background signal. 1H MAS NMR spectra were acquired at a spinning rate of 13 kHz with a π/2 pulse width of 3.05 μs and a recycle delay of 5 s, and the 2301


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Table 1. Chemical Composition and Textural Properties of the Boron-Incorporated MCM-22 Zeolites with Different Boron Contents Si/Ala atomic ratio

surface areac (m2 g−1)

Si/Ba atomic ratio b

pore volumec (cm3 g−1)

zeolite

as-syn

H-form

as-syn

H-form

crystallinity (%)

total

micro

total

micro

0B-Al-M22 0.5B-Al-M22 1B-Al-M22 3B-Al-M22 6B-Al-M22 30B-Al-M22

27 28 28 27 27 28

27 29 28 29 29 29

∞ 182 133 35 24 16

∞ 484 300 189 145 174

100 120 113 94 118 103

429 458 463 486 459 456

287 352 344 331 328 310

0.460 0.383 0.419 0.479 0.453 0.496

0.132 0.172 0.166 0.154 0.157 0.146

a

Si/Al and Si/B ratios of the as-synthesized (as-syn) xB-Al-M22 zeolites and those in hydrogen form (H-form) obtained through ion-exchange were measured by ICP-AES. bRelative crystallinity of H-form xB-Al-M22 zeolites was estimated by comparing the peak area of each zeolite sample for the (310) crystal face at 2θ of 26° with that of 0B-Al-M22 as a reference. cSurface area and pore volume of H-form xB-Al-M22 zeolites were determined by nitrogen physisorption.

to 20−40 mesh before use. In a typical run, 0.95 g of zeolite catalyst was loaded and pretreated at 550 °C for 2 h in a nitrogen flow (40 mL min−1). The reaction was then carried out at 450 °C and atmospheric pressure; methanol was pumped into the reactor with a liquid weight-hourly space velocity (WHSV) of 2 h−1, and nitrogen was used as a diluting gas (40 mL min−1). The gas and liquid products were separated with a cold trap. The gaseous products were online analyzed by an Agilent 7890A gas chromatograph equipped with one thermal conductivity detector (TCD), two flame ionization detectors (FID), 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 gas chromatograph 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 the third Agilent 7890A gas chromatograph equipped with a TCD, a FID and a capillary column (Agilent 19091N-136, 60 m × 250 μm × 0.25 μm). It should be noted that dimethyl ether (DME) was considered as unconverted reactant when calculating the conversion of methanol.

Figure 2. 11B Hahn-echo MAS NMR spectra (after subtraction of the probe background and the number of scans used was 3200) of the assynthesized boron-incorporated MCM-22 zeolites with different boron contents: (a) 0.5B-Al-M22; (b) 1B-Al-M22; (c) 3B-Al-M22; (d) 6BAl-M22; and (e) 30B-Al-M22.

xB-Al-M22 zeolites are completely incorporated into the MWW framework. Moreover, the peak intensity in the shift range of −5 to 0 ppm increases gradually with the increase of boron content. For 3B-Al-M22, 6B-Al-M22, and 30B-Al-M22, three kinds of tetrahedrally coordinated boron species centered at −2.2, −3.5 and −4.2 ppm are observed, whereas for 0.5B-AlM22 and 1B-Al-M22, with a lower content of boron, the resonance signal at −2.2 ppm is probably submerged by the noise and becomes invisible due to the low boron content. On the other hand, the relative intensities of three resonance peaks vary with the boron content, which may also be related to the sequence for boron incorporation among various T sites in the MWW framework, which is further discussed later in detail in Section 4.1. For the directly calcined xB-Al-M22 zeolites, as shown in Figure S1 (Supporting Information), the intensity of 11B Hahnecho MAS NMR peaks in the shift range of −5 to 0 ppm (tetrahedrally coordinated boron species) is decreased after calcination, whereas new resonance peaks emerge at 0−20 ppm. As reported by Wiper, Zones, and co-workers,41,42 these newly emerged resonance peaks can be attributed to the framework and extra-framework trigonally coordinated boron species with apparent second-order quadrupolar line-shape characteristics, due to the deboronation upon calcination. The trigonally coordinated framework boron species should be accompanied by silanol groups (Si−OH) if they are not located on the external zeolite surface.25 Moreover, the 11B MAS NMR spectra for the xB-Al-M22 zeolites in hydrogen form are quite blurred (not shown here) due to their low boron content, as the ion-exchange process for obtaining H-form zeolites may provoke a further deboronation.30

3. RESULTS 3.1. Textural and Structural Properties. Table 1 gives the chemical composition and textural properties of the boronincorporated MCM-22 zeolites (xB-Al-M22) with different boron contents. The Si/Al ratios of all the as-synthesized MCM-22 zeolites and those in hydrogen form (H-form) are close to the designated value (ca. 30) in the synthesis gel. However, the Si/B ratios of the as-synthesized xB-Al-M22 samples are much higher than those of the synthesis gels, suggesting that the incorporation of boron into the MWW framework is more difficult than that of aluminum. After the calcination and ion-exchange processes, the Si/B ratios in the H-form xB-Al-M22 zeolites are further increased, indicating that these processes are accompanied by deboronation due to the instability of boron in MWW framework.30 To reveal the status of boron incorporated in MCM-22, the 11 B Hahn-echo MAS NMR spectra of as-synthesized boronincorporated zeolites are shown in Figure 2. All the assynthesized xB-Al-M22 zeolites display resonances in the shift range of −5 to 0 ppm, which are attributed to tetrahedrally coordinated framework boron species.39 The extra-framework boron species in the range of 10 to 17 ppm are not observed,40 suggesting that the boron species present in the as-synthesized 2302


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ACS Catalysis The FT-IR spectra in the framework region of H-form xB-AlM22 zeolites are shown in Figure 3. In comparison with the

After calcination, as displayed in Figure S2 (Supporting Information), the as-synthesized xB-Al-M22 zeolites (lamellar 2D precursor) are transformed into 3D MWW framework, reflected by the shift of (002) peak from 6.4° to 7.1° which is merged with the (100) peak, due to the formation of 10-MR structures between the lamellar layers along c-axis.32 In addition, a slight shift of the (310) peak to smaller angle in 6B-Al-M22 and 30B-Al-M22 is also observed, which can be attributed to the lattice relaxation and expansion as well as the formation of defect sites, as also observed by Ruiter and coworkers, 45 due to the framework deboronation upon calcination (as proved by 11B Hahn-echo MAS NMR spectra in Figure S1 (Supporting Information)). Moreover, after the ion-exchange process, the diffraction peaks of H-form xB-AlM22 zeolites shift further to a smaller angle, especially in the samples with high boron content, which indicates that the ionexchange process is also accompanied by lattice expansion due to the deboronation. Similar phenomena are also observed on the XRD patterns for aluminum-free borosilicate MWW zeolites (Figure S3 (Supporting Information)), suggesting that the lattice contraction from boron incorporation can be compensated to a certain extent by subsequent lattice expansion from deboronation upon calcination and ionexchange. On all accounts, however, the crystal lattice difference among various xB-Al-M22 zeolites is relatively trivial. The SEM images of H-form and as-synthesized xB-Al-M22 zeolites are shown in Figure 5 and Figure S4 (Supporting

Figure 3. FT-IR spectra in the framework region of the boronincorporated H-MCM-22 zeolites with different boron contents: (a) 0B-Al-M22; (b) 0.5B-Al-M22; (c) 1B-Al-M22; (d) 3B-Al-M22; (e) 6B-Al-M22; and (f) 30B-Al-M22.

boron-free 0B-Al-M22, new bands at 1390 and 940 cm−1 are observed in other five boron-incorporated xB-Al-M22 zeolites. The weak peak at 1390 cm−1 band can be assigned to the trigonally coordinated framework boron species, whereas the peak around 940 cm−1 is characteristic of the stretching vibration of Si−O−B band and can be assigned to the tetrahedrally coordinated boron species.43,44 Moreover, these two bands become more intense with the increase of boron content, further confirming the effective incorporation of boron into the MWW framework. As shown in Figure 4I and Figure S2 (Supporting Information), all the as-synthesized, directly calcined and H-

Figure 4. XRD patterns of the boron-incorporated H-MCM-22 zeolites with different boron contents: (a) 0B-Al-M22; (b) 0.5B-AlM22; (c) 1B-Al-M22; (d) 3B-Al-M22; (e) 6B-Al-M22; and (f) 30B-AlM22.

form xB-Al-M22 zeolites exhibit typical diffraction peaks for the MWW framework structure, indicating that pure-phase MCM22 zeolites are attained in this work. Moreover, with the increase of boron content, the diffraction peaks of xB-Al-M22 zeolites are gradually shifted toward wider angles, in comparison with that of the boron-free 0B-Al-M22. The 2θ section for the most intensive diffraction peak of (310) crystal face is magnified in Figure 4II; from 0B-Al-M22 to 30B-AlM22, this peak shifts from 25.9° to 26.2°. The shift of the diffraction peaks toward wider angles should be attributed to the lattice contraction due to boron incorporation, as B3+ ion is smaller than Al3+ and Si4+ ions.43 These phenomena clearly indicate that boron is incorporated into the MWW framework, in line with the 11B Hahn-echo MAS NMR and FT-IR results (Figures 2 and 3).

Figure 5. SEM images of the boron-incorporated H-MCM-22 zeolites with different boron contents: (a) 0B-Al-M22; (b) 0.5B-Al-M22; (c) 1B-Al-M22; (d) 3B-Al-M22; (e) 6B-Al-M22; and (f) 30B-Al-M22.

Information), respectively. All the xB-Al-M22 zeolites are in disk-like shape of typical MWW-type with a similar particle size of around 1 μm in diameter and 50−100 nm in thickness, further illustrating that the deboronation upon calcination and ion-exchange has little disturbance to the textural properties of MCM-22 crystals. The results of nitrogen physisorption on the directly calcined and H-form xB-Al-M22 zeolites are also given in Table S1 (Supporting Information) and Table 1, respectively. Although the H-form xB-Al-M22 zeolites have similar surface areas as the 2303


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density and the acid distribution across the acid strength for the H-from xB-Al-M22 zeolites. Unlike NH3-TPD, Py-IR is able to differentiate the Brönsted and Lewis acid sites. Moreover, the acid sites in a specific type of pores can be measured by selectively enshrouding those in other types of pores. As described by Laforge and co-workers, m-xylene transformation for a sufficient time could selectively and completely deactivate the acid sites in the supercages of HMCM-22;47−49 however, the acid sites in the sinusoidal channels and external pockets remained mostly intact. As a result, m-xylene transformation is taken as a classic precoking reaction to selectively poison the supercages of H-MCM22.33,50,51 On the other hand, 2,4-dimethylquinoline (2,4DMQ) cannot penetrate into the inner micropore system of HMCM-22 and hence is used to selectively poison the acid sites in the external surface pockets of H-MCM-22.33,34,48,52 Through a combination of m-xylene transformation and 2,4DMQ poisoning, the distribution of Brönsted and Lewis acid sites in the sinusoidal channels, supercages, and surface pockets of boron-incorporated H-MCM-22 zeolites can then be determined by Py-IR. As shown in Figure 7 and Table 2, xB-Al-M22 zeolites with high boron contents (3B-, 6B-, and 30B-Al-M22) exhibit slightly higher density of Lewis acid sites due to the remaining boron species, whereas all the H-form xB-Al-M22 zeolites are similar in the overall density of Brönsted acid sites. It suggests that boron incorporation has little influence on the overall density of Brönsted acid sites. However, the Lewis and the Brönsted acid sites are quite different in their distributions among three types of pores. Most of Lewis acid sites are located in the supercages (ca. 70%) and all the xB-Al-M22 zeolites have a similar distribution of Lewis acid sites. For H-MCM-49 zeolite (MWW framework), Liu and co-workers also found that 73% of Lewis acid sites were located in the supercages,34 coincident with current results. On the contrary, the distribution of Brönsted acid sites among three types of pores is obviously related to the content of boron incorporated into MCM-22 during synthesis. For the boron-free 0B-Al-M22, the sinusoidal channels, supercages, and surface pockets hold 33%, 52%, and 15% of the Brönsted acid sites, respectively; the ratio of Bsin/(Bpoc + Bsup), viz., the amount of Brönsted acid sites in the sinusoidal channels divided by the sum in the supercages and the surface pockets, is 0.49. With the increase of boron content from 0B-Al-M22 to 3B-Al-M22, the Brönsted acid sites in the supercages are suppressed, whereas those in the sinusoidal channels are greatly

corresponding directly calcined samples, the H-form samples in general have larger pore volume. The trigonal boron species retained in the directly calcined xB-Al-M22 zeolites (Figure S1 (Supporting Information)) may partially block the pore system of zeolites; however, these boron species can be removed in the subsequent ion-exchange process by slurrying in water or mild acid solutions,46 leading to the increase of Si/B ratio and retrieval of pore volume for the H-form xB-Al-M22 zeolites. Moreover, the increment of pore volume upon the ionexchange process increases from 0.02 to 0.206 cm3 g−1 with the increase of boron content from 0B-Al-M22 to 30B-Al-M22. In any case, as given in Table 1, all the resultant H-form xB-AlM22 zeolites have a similar surface area and pore volume. As a whole, the content of boron remained in the H-form xBAl-M22 zeolites (Si/B atomic ratio = 145−484) is much lower than the aluminum content (Si/Al atomic ratio = 28). Table 1 also illustrates that all the H-form xB-Al-M22 zeolites exhibit similar crystallinity as well as similar textural properties. The trivial alteration in the crystal lattice and crystallinity as well as in the textual properties upon boron incorporation during synthesis and deboronation during calcination and ionexchange should have little influence on their catalytic performances. 3.2. Acidity of Boron Incorporated H-MCM-22. As shown in Figure 6, all the H-from xB-Al-M22 zeolites are quite

Figure 6. NH3-TPD profiles of the boron-incorporated H-MCM-22 zeolites with different boron contents: (a) 0B-Al-M22; (b) 0.5B-AlM22; (c) 1B-Al-M22; (d) 3B-Al-M22; (e) 6B-Al-M22; and (f) 30B-AlM22.

similar in their NH3-TPD profiles; two desorption peaks appear at around 200 and 360 °C, which are assigned to weak and strong acid sites, respectively. The quantitative NH3-TPD results given in Table 2 also demonstrate that boron incorporation has a negligible influence on the overall acid

Table 2. Overall Acidic Properties and Hydroxyl Concentration of the Boron-Incorporated H-MCM-22 Zeolites with Different Boron Contents acidity by NH3-TPDa (μmol g−1)

acidity by Py-IRb (μmol g−1)

OH concentration by NMRc (μmol g−1)

zeolite

total

weak

strong

total

Lewis

Brönsted

nSiOHAl

nAlOH

nSiOH


618 631 634 600 587 544

284 262 270 273 259 240

334 369 364 327 328 304

425 391 436 436 444 429

164 131 157 186 191 201

261 260 279 250 253 228

320 328 345 337 372 388

243 212 234 256 337 415

332 207 288 350 385 457

a

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. bThe quantities of Brönsted and Lewis acid sites determined by Py-IR were calculated from the Py-IR spectra of xB-Al-M22 zeolites by following the procedures reported by Madeira and co-workers.38 cOH concentration were determined by 1H MAS NMR; nSiOHAl, and nAlOH were derived from the fitted peaks at 3.9 and 2.6 ppm, respectively, whereas nSiOH was from the sum of fitted peaks at 1.8 and 2.0 ppm. 2304


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The regulation of acidic properties of H-MCM-22 through boron incorporation is further proved by the 1H MAS NMR spectra, as shown in Figure 8. The signals at 1.8 and 2.0 ppm

Figure 8. 1H MAS NMR spectra (after subtraction of the probe background and the number of scans used was 384) of the boronincorporated H-MCM-22 zeolites with different boron contents: (a) 0B-Al-M22; (b) 0.5B-Al-M22; (c) 1B-Al-M22; (d) 3B-Al-M22; (e) 6B-Al-M22; and (f) 30B-Al-M22.

Figure 7. Distribution of the Brönsted (I) and Lewis (II) acid sites in the sinusoidal channels (sin), supercages (sup) and surface pockets (poc) of the boron-incorporated H-MCM-22 zeolites with different boron contents, determined by m-xylene transformation and 2,4-DMQ poisoning and Py-IR techniques. The ratio of Bsin/(Bpoc+Bsup) (or Lsin/ (Lpoc+Lsup)) is the amount of the Brönsted (or Lewis) acid sites in the sinusoidal channels divided by the sum in the supercages and the surface pockets.

can be ascribed to the external and internal silanol groups (Si− OH) at the framework defects, respectively,55 whereas the weak broad band at 2.6 ppm and the peak at 3.9 ppm are generally assigned to the hydroxyl groups bonded to extra-framework aluminum species (AlEF−OH) and the bridging hydroxyl groups (Si(OH)Al) in zeolites, respectively.56,57 The hump at about 4.7 ppm is attributed to the residual water molecules survived from the dehydration treatment, as described by Wiper and co-workers with 2D 1H−1H DQ-SQ spectrum.41 However, the assignment of the weak signal at 8.7 ppm, which was also observed by Wang and co-workers in H-MCM-22,9 is somewhat controversial. Ma and co-workers assigned it to aluminum hexaaqua complexes (Al(H2O)63+),57 whereas Hunger and co-workers attributed it to hydrogen bonded internal silanol groups.58 As shown in Figure S5 (Supporting Information), the aluminum-free borosilicate H-B-MWW displays an intense peak at 2.0 ppm (indicating the presence of large amount of internal silanol groups) but has no signal at 8.7 ppm in the 1H MAS NMR spectrum. Therefore, it is reasonable here to assign the signal at 8.7 ppm to the aluminum hexaaqua complexes. Besides the signals at 1.8 and 2.0 ppm, a sharp peak at 2.3 ppm, a shoulder at 2.6 ppm, and a broad band at 3.3 ppm are also observed for the H-B-MWW borosilicate, which should be assigned to different protons close to boron in zeolites.41 The concentrations of different hydroxyl groups in the xB-AlM22 zeolites are also given in Table 2, through deconvolution of the 1H MAS NMR spectra (Figure S7, Supporting Information). With the increase of boron content, the concentrations of Si(OH)Al groups (nSiOHAl, acting as the Brönsted acid sites) and extra-framework AlEF−OH groups (nAlOH) first keep almost constant from 0B-Al-M22 to 3B-AlM22 and then increase significantly from 3B-Al-M22 to 30B-AlM22. Because all the xB-Al-M22 zeolites are similar in Si/Al ratio (Table 1) and extra-framework aluminum content (AlEF species, as shown in the next section), as mentioned above, the unusually large values of nSiOHAl and nAlOH in the xB-Al-M22 zeolites with high boron content (6B-Al-M22 and 30B-Al-M22) may be attributed to the protons close to the residual boron. Meanwhile, the concentration of silanol groups at the framework defects (nSiOH) is related to the boron content; it decreases first from 332 μmol g−1 for 0B-Al-M22 to 207 μmol

increased. Especially for 3B-Al-M22, the Brönsted acid sites in the sinusoidal channels outnumber those in the supercages and surface pockets; the ratio of Bsin/(Bpoc + Bsup) turns out to be 1.10. However, with a further increase of boron content from 3B-Al-M22 to 30B-Al-M22, the majority of Brönsted acid sites are relocated into the supercages and surface pockets from the sinusoidal channels; the ratio of Bsin/(Bpoc + Bsup) for 30B-AlM22 is decreased back to 0.37. All these results illustrate that boron incorporation can effectively regulate the distribution of Brönsted acid sites among three types of pores in H-MCM-22; meanwhile, it has little influence on the distribution of Lewis acid sites. A proper content of boron is able to concentrate Brönsted acid sites in the sinusoidal channels. As MTH is catalyzed by the Brönsted acid sites,1,53,54 the catalytic performance of xB-Al-M22 can then be modulated accordingly through regulating the acid distribution by boron incorporation. It should be mentioned that the amount of acid sites in the surface pockets (Bpoc and Lpoc) may be overestimated in this work, because 2,4-DMQ used to enshroud acid sites in the surface pockets may also poison some acid sites in the pore mouths.52 Fortunately, the acid sites in the surface pockets only account for a relatively small fraction of total acid sites in the xB-Al-M22 zeolites, as illustrated in Figure 7, even when the acid sites in the pore mouths which can adsorb 2,4-DMQ are counted. Moreover, the Brönsted acid sites in the pore mouths which can come into contact with bulky 2,4-DMQ molecules should have a similar deactivation behavior with those in the surface pockets for the MTH reaction; therefore, it may also be reasonable to count this part of the Brönsted acid sites in the pore mouths into those in the surface pockets. As a result, the overestimation of acid sites in the surface pockets may have little effect on the ratio of Bsin/(Bpoc + Bsup), which represents the distribution of Brönsted acid sites among three types of pores in H-MCM-22 relating to its catalytic performance in MTH. 2305


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ACS Catalysis g−1 for 0.5B-Al-M22, and then increases gradually to 457 μmol g−1 for 30B-Al-M22. As reported by Xu and co-workers, the framework boron species in MCM-22 might generate Si−O−− B sites to compensate the surplus HMI+, which could inhibit the formation of Si−O− defect sites and then cause a decrease of silanol groups for 0.5B-Al-M22 at the framework defects.30 However, with the increase of boron content, serious deboronation occurs in the subsequent calcination and ionexchange processes (Table 1 and Figure S1 (Supporting Information)), which may form massive Si−O− defect sites, resulting in an increase of the signal intensity at 1.8 and 2.0 ppm in the 1H MAS NMR spectra. Such a result is also supported by FT-IR spectra in the OH stretching vibrations region, as shown in Figure S6 (Supporting Information). All the xB-Al-M22 zeolites exhibit two major bands at 3622 and 3746 cm−1, which are assigned to Brönsted acid sites and silanol groups, respectively. The signals at ca. 3670 cm−1 representing the extra-framework Al−OH species are negligible, as they were easily dehydroxylated.59 The intensity of the band at 3622 cm−1 (representing the Brönsted acid sites) is almost unchanged for all the xB-Al-M22 zeolites, whereas the intensity of the band at 3746 cm−1 (representing the silanol groups) changes with boron content following the same trend as the resonance signals at 1.8 and 2.0 ppm in the 1 H MAS NMR spectra. In addition, the concentration of Brö nsted acid sites calculated from 1H MAS NMR spectra is slightly higher than those determined by Py-IR and NH3-TPD (Table 2), which may be ascribed to the high sensitivity of 1H MAS NMR that can detect all the hydrogen groups in H-MCM-22 zeolites, including some buried hydrogen groups inaccessible for probe molecules such as pyridine and NH3. Although 1H MAS NMR is effective to differentiate various hydroxyl groups, it is still quite difficult to classify the Brönsted acid sites according to their locations. 3.3. Aluminum Siting Determined by 27Al MAS NMR. As the Brönsted acid sites in a zeolite are primarily derived from the framework aluminum (AlF) species,60 27Al MAS NMR spectra were used to investigate the aluminum siting and acid distribution. As displayed in Figure 9, the intense band around

the observation that all the xB-Al-M22 zeolites have a similar overall acid concentration, as determined by NH3-TPD, 1H MAS NMR, and Py-IR (Table 2). On the other hand, Figure 9 also clearly illustrates that the broad peak around 56 ppm is moved gradually toward lower chemical shift with the increase of boron content from 0B-AlM22 to 30B-Al-M22, which was also observed by Zhu, Qiao, and co-workers in boron isomorphously framework-substituted ZSM-5.25,29 Such a movement in the 27Al MAS NMR spectra should be related to the variances of AlF siting in the zeolite framework. It is obvious that the broad band around 56 ppm consists of several peaks; according to the P6/mmm hexagonal structure, Kennedy and co-workers resolved it into three overlapped signals at 50, 56, and 61 ppm, which were attributed to the AlF species located in T6+T7, T1+T3+T4+T5+T8, and T2 sites, respectively.36 In a similar way, the broad band around 56 ppm here is deconvoluted, as given in Table 3 and Figure S8 (Supporting Information). Obviously, the relative fractions of the resonance peaks at 50, 56, and 61 ppm (Al(50), Al(56), and Al(61), respectively) for all the xB-Al-M22 zeolites deviate from the theoretical value of 33:50:17,36,62 indicating that the distribution of AlF species here is not a random event. Moreover, the distribution of AlF species is related to the content of boron incorporated into MCM-22; the boron-incorporated xB-Al-M22 (x = 0.5−30) zeolites have a much lower Al(61) content than the boron-free 0B-Al-M22. Especially, 3B-Al-M22 exhibits a minimal content of Al(50) and a maximal content of Al(56). The framework structure of H-MCM-22 shows that T2 sites are located in the supercages and/or surface pockets (Figure 1), considering that a pocket is a half supercage. In addition, Mériaudeau and co-workers found that the peak at 50 ppm disappeared after selective surface dealumination of MCM-22 zeolite and then attributed Al(50) to AlF species located in the external surface.63 Al(56) species are contributed by T1, T3, T4, T5, and T8 sites; T1 and T3 are located in the supercages and/or surface pockets,64 whereas T4 is buried in framework which is inaccessible to the basic probe molecules and should be excluded from acidity analysis.65 T5 in the sinusoidal channels is unstable and unfavorable for siting Al; T8 is located in the sinusoidal channels connected to the bottom of the supercages, with thermodynamically stable bridging hydroxyls pointing to the 10-MR sinusoidal channels.64,66,67 As displayed in Table 3 and Figure 7, the ratio of Al(56)/ (Al(50)+Al(61)) changes with the boron content in a similar way as that of Bsin/(Bsup+Bpoc). Therefore, the peaks for Al(50) and Al(61) species are probably ascribed to AlF species located in the supercages and surface pockets, which contribute to the Brönsted acid sites in the supercages and surface pockets (Bsup+Bpoc). On the other hand, Al(56) can be related to the Brönsted acid sites in the sinusoidal channels (Bsin) for two matters: (1) Al(56) peaks in the 27Al MAS NMR spectra are probably generated from AlF species located at T8 sites rather than in T1, T3, T4 and T5 sites; (2) all these T sites may contribute to the Al(56) peaks, whereas only AlF species located in T8 sites can be effectively regulated by adjusting the content of boron incorporated into MCM-22. It is noticed that 3B-AlM22 has a maximum of 2.50 for Al(56)/(Al(50)+Al(61)), which is higher than the value of 1.10 for Bsin/(Bsup+Bpoc). Therefore, the second explanation is probably more reasonable. Zhou and co-workers predicated that the Brönsted acid sites in H-MCM22 were preferably located at T2, T3, and T8 sites,64 supporting

Figure 9. 27Al MAS NMR spectra (the number of scans used was 10 000) of the boron-incorporated H-MCM-22 zeolites with different boron contents: (a) 0B-Al-M22; (b) 0.5B-Al-M22; (c) 1B-Al-M22; (d) 3B-Al-M22; (e) 6B-Al-M22; and (f) 30B-Al-M22.

56 ppm and the weak peak at 0 ppm can be assigned to 4coordinated framework (AlF) and 6-coordinated extra-framework aluminum (AlEF) species, respectively.61 As also given in Table 3, all the xB-Al-M22 zeolites have a similar concentration of total aluminum species (Altotal, 556−600 μmol g−1) and a similar fraction of AlF species (83−85%), which are in line with 2306


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Table 3. Concentration of Various Aluminum Species and Their Distribution in the Boron-Incorporated H-MCM-22 Zeolites Determined by 27Al MAS NMR distribution of AlF speciesc (%) −1

a

zeolite

Altotal (μmol g )

AlF/AlEF


600 556 571 556 556 560

84/16 83/17 85/15 84/16 85/15 84/16

b

Al(50)

Al(56)

Al(61)

Al(56)/(Al(50)+Al(61))d

24 26 26 23 34 43

63 67 68 71 60 51

14 7 6 6 6 5

1.67 2.05 2.13 2.50 1.51 1.05

a

Altotal is the concentration of total Al species measured by ICP-AES. bAlF/AlEF is the ratio of framework AlF to extra-framework AlEF species, where AlF and AlEF were calculated from the integration of peaks at 56 and 0 ppm, respectively. cAl(50), Al(56) and Al(61) are the fractions of three AlF species in the total AlF species resolved at 50, 56, and 61 ppm, respectively. dAl(56)/(Al(50)+Al(61)) is the ratio of Al(56) amount to the sum of Al(50) and Al(61).

channels originate preferably from the AlF species located at T8 sites. Through controlling the probability for AlF species siting at T8 sites by adjusting boron content in the synthesis gel, the density of Brönsted acid sites in the sinusoidal channels of xBAl-M22 zeolites can then be regulated. 3.5. Catalytic Performance of H-B-MWW in MTH. To exclude the effect of weak acidity introduced by boron on the MTH reaction, the catalytic performance of aluminum-free borosilicate H-B-MWW was measured. As illustrated in Table S2 and Figure S3 (Supporting Information), H-B-MWW obtained in this work is a typical MWW-type crystallite in disk-like shape with a particle size of ca. 2−3 μm in diameter and 100−200 nm in thickness. It exhibits a Si/B atomic ratio of 59 as measured by ICP-AES and a very weak acidity as determined by NH3-TPD (about 30 μmol g−1 NH3 desorbed at 160 °C). The catalytic test shows that H-B-MWW is almost inactive in MTH at given temperature (450 °C); the conversion of methanol and selectivity to C2+ hydrocarbons are lower than 15% and 10%, respectively. Such a result confirms the fact that the weak acidity introduced by boron has little influence on the catalytic activity of zeolites in methanol conversion.72−74 The poor activity of H-B-MWW in MTH should be mainly attributed to its weak acidity introduced by boron in the framework, though the larger crystal size of H-B-MWW than that of xB-Al-M22 zeolites may also have a certain negative effect on its catalytic performance.75 By using DFT calculations, Wang and co-workers revealed that for the key reaction steps in both the aromatic-based and the olefin-based cycles of MTH, borosilicate MFI zeolites exhibited much higher free energy barriers than the aluminosilicate MFI zeolites.76 Actually, previous studies have clearly illustrated that the borosilicate zeolites were almost inactive in MTO regardless of their topology structure and particle size.72−74,77 Moreover, the lowest Si/B atomic ratio in this work is 145, observed for 6B-Al-M22, suggesting that the boron content in xB-Al-M22 zeolites is much lower than that in H-B-MWW (with a Si/B atomic ratio of 59). Meanwhile, the boron content in the H-form xB-Al-M22 zeolites is also much lower than the aluminum content, as mentioned above. Therefore, it is quite reasonable to deduce that the effect of residual boron itself in the xB-Al-M22 zeolites on their catalytic performance in MTH is negligible; the influence of boron incorporation on the MTH behavior over xB-Al-M22 zeolites should be primarily ascribed to the regulation of aluminum siting and acid distribution among three types of pores. 3.6. Catalytic Performance of xB-Al-M22 in MTH. The conversions of methanol over xB-Al-M22 zeolites with different

the speculation that the Brönsted acid sites in the sinusoidal channels originate preferably from the AlF species at T8 sites. 3.4. 29Si MAS NMR Spectra. As shown in Figure 10, five 4 Q (Si(OSi)4) peaks at −105.4, −111.3, −113.4, −116.1, and

Figure 10. 29Si MAS NMR spectra (the number of scans used was 1400) of the boron-incorporated H-MCM-22 zeolites with different boron contents: (a) 0B-Al-M22; (b) 0.5B-Al-M22; (c) 1B-Al-M22; (d) 3B-Al-M22; (e) 6B-Al-M22; and (f) 30B-Al-M22.

−119.9 ppm appear in the 29Si MAS NMR spectra of xB-AlM22, corresponding to the T2, T1+T3+T4+T5, T8, T7, and T6 sites, respectively,68−70 whereas a weak peak at −98.8 ppm is attributed to Si(1Al) and Si(OH) (OSi)3 (Q3).57,71 As also shown in Figures S9 and S10 (Supporting Information), all the xB-Al-M22 zeolites exhibit a similar intensity for the peaks at −98.8 ppm with a relative area less than 3%, coincident with their similar Si/Al atomic ratios (ca. 28). However, the relative areas of five Q4 peaks change slightly with the increase of boron content, suggesting that the location of silicon atoms as the primary component in framework is hardly influenced by boron incorporation. Among the xB-Al-M22 zeolites with different boron contents, 3B-Al-M22 exhibits the lowest relative peak areas at −113.4 ppm (T8) and −111.3 ppm (T1+T3+T4+T5) but the highest relative peak area at −119.9 ppm (T6) in the 29Si MAS NMR spectra. On the other hand, the 27Al MAS NMR results given in Table 3 show that 3B-Al-M22 has the highest Al(56) peak area for T1+T3+T4+T5+T8 but the lowest Al(50) peak area for T6+T7. Taking these facts into account, it can then be acquainted that the siting of aluminum atoms in the xB-Al-M22 framework T sites is competitive with the siting of silicon atoms during synthesis, which is influenced greatly by the content of boron incorporated in the synthesis gel. The obvious variances in the peak intensity at −113.4 ppm for T8 with the boron content in the 29Si MAS NMR spectra further encourage the speculation that the Brönsted acid sites in the sinusoidal 2307


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4. DISCUSSION 4.1. Regulation of Framework Aluminum Siting by Boron Incorporation. It is widely accepted that the distribution of aluminum atoms in the zeolite framework is not a random event but controlled by certain rules depending on the synthesis conditions.20 During synthesis, the organic structure-directing agents (OSDAs) and cations such as Na+ are located near the framework aluminum (AlF) and framework boron (BF) species to balance the negative charges introduced by Al3+ and B3+ in the framework T sites;19,78,79 the location of OSDAs and Na+ ions may provide some useful information on the siting of AlF species. Because of the low concentration of Na+ ions, most of the framework negative charges are balanced by the protonated hexamethylenimine (HMI) molecules, as given in Table S3 (Supporting Information); therefore, the elimination of OSDAs in the as-synthesized zeolites by TG analysis may provide certain evidence to understand the siting of AlF species. As shown in Figure 12I, three ranges of weight loss are observed for the as-synthesized xB-Al-M22 zeolites in the TG

boron contents are plotted as a function of time on stream, as shown in Figure 11; meanwhile, the product distributions and

Figure 11. Conversion of methanol into hydrocarbons as a function of time on stream over (a) 0B-Al-M22; (b) 0.5B-Al-M22; (c) 1B-AlM22; (d) 3B-Al-M22; (e) 6B-Al-M22; and (f) 30B-Al-M22. The reactions were conducted at 450 °C, with a methanol WHSV of 2 h−1.

catalytic lifetime are summarized in Table 4. All the xB-Al-M22 zeolites exhibit an initial methanol conversion of near 100%; however, they are quite different in the long term stability. From 0B-Al-M22 to 3B-Al-M22, the catalytic lifetime is increased from 52 to 102 h with the increase of boron content, whereas a further increase of boron content (from 3B-Al-M22 to 30B-Al-M22) leads to a dramatic decline of catalytic lifetime to 32 h (Table 4). On the other hand, the coking rate and C4 hydrogen transfer index (C4−HTI) change in an opposite way to the catalytic lifetime. Especially, 3B-Al-M22 exhibits the longest catalytic lifetime with the lowest C4−HTI and coking rate. These results suggest that the carbonaceous deposition is the primary cause of catalyst deactivation for MTH over the xB-Al-M22 zeolites, which can be alleviated by incorporating proper content of boron during synthesis. Similarly, Yang and co-workers also observed that the incorporation of proper content of boron could significantly enhance the catalytic stability of ZSM-5 catalyst in MTH.26 Moderate differences in the product distribution are also observed for MTH over the xB-Al-M22 zeolites with different boron contents. As given in Table 4, 3B-Al-M22 exhibits the highest selectivity to propene and butene and the lowest selectivity to C1−50, ethene and aromatics; a lower or higher boron content is unfavorable for the formation of propene and butene.

Figure 12. TG (I) and DTG (II) profiles of the as-synthesized boronincorporated MCM-22 zeolites with different boron contents: (a) 0BAl-M22; (b) 0.5B-Al-M22; (c) 1B-Al-M22; (d) 3B-Al-M22; (e) 6B-AlM22; and (f) 30B-Al-M22.

Table 4. Catalytic Performance of the Boron-Incorporated H-MCM-22 Zeolites with Different Boron Contents in MTHa product distributionb (%) b

zeolite

methanol conversion (%)

C1−50


99.9 99.2 99.1 99.8 99.7 99.2

10.6 9.6 9.3 8.1 9.9 11.1

C2=

C3=

C4=

C5=

aromatics

others

C4-HTIc

lifetimed (h)

coking ratee (h−1)

5.8 6.7 6.6 5.2 6.0 5.0

40.4 38.5 41.1 44.2 39.5 35.9

17.7 17.7 18.4 19.5 17.2 15.4

10.5 14.3 12.3 12.1 11.9 12.5

10.3 6.0 7.3 6.1 9.5 13.1

4.7 7.0 5.0 4.8 5.8 6.8

0.14 0.11 0.10 0.09 0.13 0.16

52 62 72 102 55 32

0.46 0.38 0.39 0.30 0.38 0.48

The reaction was carried out at 450 °C and atmosphere pressure, with a methanol WHSV of 2 h−1; the feed was diluted by 66.7 vol % nitrogen. The methanol conversion and product distribution are provided at the half lifetime and calculated on the basis of carbon atoms; C1−50 represents alkanes with 1−5 carbon atoms, whereas C2=−C5= represent ethene to pentenes. cC4−HTI represents the hydrogen transfer index, which is calculated from the selectivity of butane (C40) and butene (C4=) by n(C40)/(n(C40) + n(C4=)) in the products. dLifetime is defined as the time on stream when the conversion of methanol drops to 90%. eCoking rate was determined by TG analysis, which is given as the weight percentage of coke deposited in the spent catalyst divided by the lifetime. a b

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ACS Catalysis and DTG profiles: (1) release of H2O below 260 °C; (2) elimination of HMI OSDA from the interlayer supercages of MWW framework at low temperature (LT, 260−500 °C); (3) further elimination of HMI OSDA from the intralayer sinusoidal channels at high temperature (HT, 500−700 °C).80,81 Figure 12 and Table S3 (Supporting Information) show that both the LT and HT weight losses of the assynthesized xB-Al-M22 zeolites are increased with the increase of boron content, suggesting that more boron is incorporated into the interlayer supercages and intralayer sinusoidal channels of MWW framework, which needs more HMI to balance the framework negative charges introduced by BF species. It is noteworthy that the deboronation during TG measurement should also take place, with endothermic effect, as the framework boron species in the as-synthesized xB-Al-M22 zeolites can be transformed into framework and extraframework trigonal boron species upon calcination, due to the breaking of B−O bonds.46 However, the endothermic effect of deboronation is negligible in comparison with the elimination of massive HMI molecules occupied in the zeolite channels and between the 2D MWW layers; the highest boron content in the as-synthesized 30B-Al-M22 is 0.53 wt %, as measured by ICP-AES, whereas the lowest HMI content in the boron-free 0B-Al-M22 is 13.72 wt %. Meanwhile, boron oxide is difficult to eliminate during the TG measurement in this work, which was conducted far below the boiling point of boron oxide. As a result, it is difficult at present to assign the weight loss due to deboronation in the TG and DTG profiles. With the increase of boron content in xB-Al-M22, the LT weight-loss peak is shifted from 460 to 370 °C, whereas the HT weight-loss peak is moved from 600 to 650 °C (Figure 12II). As the protonated HMI molecules for balancing the negative framework charges are strongly bound to the framework structure, it is expected that they are decomposed at higher temperature than the H-bonded or physically adsorbed base.82 Moreover, OSDAs can also direct the siting of AlF species, as various framework T sites are different in their interaction strength with the OSDAs.78,79,83 It is then reasonable to assume that the framework T sites in the sinusoidal channels have stronger interaction with the protonated HMI molecules than those in the supercages. As both boron and aluminum are trivalent in the framework T sites, boron may compete with aluminum for siting at specific T sites and push aluminum to other T sites during the synthesis process.25 All the xB-Al-M22 zeolites have a similar aluminum content (ca. 2.6 atom per unit cell), whereas the boron content is changed from 0 for 0B-AlM22 to 4.50 atom per unit cell for 30B-Al-M22 (Table S3 (Supporting Information)). The variation in boron content may influence the competitive siting between boron and aluminum at various framework sites and then alter the distribution of Brönsted acid sites among various types of pores. Previous calculation results illustrated that the substitution of aluminum atom in eight T sites of MCM-22 took place in the order of T4 ≈ T2 > T6 ≈ T8 ≈ T3 > T7 > T5 > T1,67 whereas the substitution of boron atom was in the sequence of T2 ≈ T1 > T3 ≈ T8 ≈ T5 > T7 ≈ T6 > T4 (Table S4 (Supporting Information)). As a result, the competitive siting between aluminum and boron at T2, T8, and T3 sites rather than at T4 and T6 sites may occur during the synthesis of MCM-22 when boron is incorporated in the synthesis gel containing aluminum. Proper content of boron in the synthesis gel may promote the competition of boron for T2 site from aluminum and then

push the later to T4, T6, T8, and T3 sites, leading to the concentration of Brönsted acid sites in the sinusoidal channels (Figure 7). It is also supported by the enervation of Al(61) peak and the reinforcement of Al(50) and Al(56) peaks in the 27Al MAS NMR spectra through incorporating a proper content of boron (Figure 9 and Table 3); the gradual increase of Brönsted acid sites in the sinusoidal channels from 0B-Al-M22 to 3B-AlM22 is probably caused by the increase of AlF species at T8 sites. A further increase of incorporated boron content may also promote the siting of boron atoms at T3, T8, and T5 sites, which will push aluminum atoms to T6 and/or T7 sites and hence leads to a decrease of Brönsted acid sites in the sinusoidal channels from 3B-Al-M22 to 30B-Al-M22, coincident with the decrease of Al(56) peak intensity and increase of Al(50) peak intensity in the 27Al MAS NMR spectra (Figure 9). It should be pointed out that the siting of aluminum in the zeolite framework, especially in the presence of competitive components, is a very complicated event controlled by both thermodynamic and kinetic factors. Although it is impractical at the moment to give a detailed depiction about the siting of aluminum in MCM-22 in competition with boron during synthesis, current evidence clearly illustrates that the incorporation of a proper content of boron is effective to regulate the siting of aluminum and the distribution of acid sites among three types of pores in H-MCM-22. 4.2. Relation between Acid Distribution and Catalytic Performance. The catalytic test results demonstrate that the performance of xB-Al-M22 zeolites in MTH, especially the long-term stability, is closely related to the content of boron incorporated in the framework. However, as illustrated in previous sections, the residual boron itself in xB-Al-M22 zeolites has little influence on their catalytic performance in MTH. Sazama, Barbera, and co-workers proposed that the framework defects in zeolites might interact with hydrocarbons, which could inhibit the product diffusion, enhance hydrogen transfer reactions and then accelerate the catalyst deactivation in MTH.59,84 In this work, as the catalytic performance of xBAl-M22 zeolites in MTH is independent of the concentration of silanol groups (Tables 2 and 4), the framework defects derived from boron incorporation and subsequent deboronation should also have little influence on their catalytic performance. As a result, the catalytic performance of xB-Al-M22 zeolites in MTH should be mainly determined by their framework structure and acidity.2,85 In fact, all the xB-Al-M22 zeolites employed here are quite different in the distribution of AlF species or Brönsted acid sites, in spite of the similarity in their morphology, textural properties, and overall acid quantity. As shown in Figure 7 and Tables 3 and 4, 3B-Al-M22 zeolite, which exhibits the highest Brönsted acid density in the sinusoidal channels and the most intense Al(56) peak in the 27Al MAS NMR spectra, also has the longest catalytic lifetime in MTH. As the Brönsted acid sites are catalytically responsible for the MTH reaction,1,53,54 it is reasonable to assume that the distribution of Brönsted acid sites among three types of pores in H-MCM-22 has a crucial influence on the long-term stability in MTH. As a matter of fact, an excellent linear correlation is observed between the Bsin/(Bpoc+Bsup) ratio (or the Al(56)/(Al(50)+Al(61)) ratio) in the boron-incorporated H-MCM-22 zeolites and their catalytic lifetime in MTH, as shown in Figure 13, taking the uncertainties for the Py-IR measurement and 27Al MAS NMR spectra deconvolution into account. This result also confirms the previous assignments that the framework Al(56) species 2309


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formation in the transformation of n-heptane,89,93 xylene,47−49 and methylcyclohexane.51 In summary, all the boron-incorporated H-MCM-22 zeolites in this work are similar in their overall Brönsted acid amounts but quite different in the distribution of Brönsted acid sites among three types of pores; the concentration of Brönsted acid sites in the sinusoidal channels and the diminishing of Brönsted acid sites in the surface pockets and supercages by incorporating proper content of boron is of great benefit to enhancing the long-term catalytic stability of H-MCM-22 in MTH by alleviating the coke deposition. 4.3. About the Product Distribution for MTH over xBAl-M22. In general, the product distribution in MTH over a zeolite catalyst is related to the product shape selectivity and transition-state shape selectivity.60 As all the xB-Al-M22 zeolites here have an identical framework structure and similar textural properties, their difference in the MTH product distribution (Table 4) should be irrelevant to the product shape selectivity. Liu and co-workers proved that the transition-state shape selectivity controlled the molecular size and reactivity of confined species, which determined the product selectivity in MTH.94,95 According to the dual-cycle mechanism proposed by Svelle and co-workers,96,97 both the aromatic-based and olefinbased cycles contributed to MTH; the type of intermediates participated in each catalytic cycle, which determined the product selectivity, was then controlled by the steric limitation.98 Ilias and co-workers found that the propagation of olefin-based cycle from methylation to cracking reactions was favorable for the formation of higher olefins (mainly C3−5 olefins).99,100 In contrast, the aromatic-based cycle was mainly propagated from hydrogen transfer and aromatic methylation to olefin elimination, resulting in high selectivity to lower olefins, aromatics and alkanes. Previous theoretical calculations suggested that both the aromatic-based and olefin-based cycles were feasible in the supercages, whereas the MTH reaction in the sinusoidal channels was suppressed due to the space confinement.12 However, those results may only apply to the initial period of MTH over H-MCM-22. In the steady stage of MTH, the selectivity to C3−5 olefins is gradually increased, whereas the selectivity to ethene and aromatics is decreased with the time on stream, as shown in Figure 14. Similar results were also observed by Lacarriere, Wang and co-workers.8,11 Moreover, among all the xB-Al-M22 zeolites, 3B-Al-M22 with the highest Bsin/(Bpoc+Bsup)) ratio, that is, with the Brönsted acid sites concentrated in the sinusoidal channels, also exhibits the

Figure 13. Relationships between the Bsin/(Bpoc+Bsup) ratio measured by Py-IR in the boron-incorporated H-MCM-22 zeolites and their catalytic lifetime in MTH (a, asterisks) and the Al(56)/(Al(50)+Al(61)) ratio determined by 27Al MAS NMR (b, circles).

contribute to Brönsted acid sites in the sinusoidal channels, whereas the framework Al(50) and Al(61) species to Brönsted acid sites in the supercages and surface pockets. Figure 13 clearly illustrates that an enhancement of Al(56) species in the H-MCM-22 framework or an enrichment of Brönsted acid sites in the sinusoidal channels can effectively enhance the long-term catalytic stability of H-MCM-22 in MTH. It is widely accepted that the deactivation of zeolite catalysts in MTH is related to the carbonaceous deposition.86 3B-Al-M22 zeolite with the highest Bsin/(Bpoc+Bsup) and Al(56)/ (Al(50)+Al(61)) ratios also exhibits the lowest coking rate, suggesting that the Brönsted acid sites in the sinusoidal channels have a high resistance to carbonaceous deposition during MTH. It was demonstrated that three types of pores in H-MCM-22 are quite different in their catalytic action on MTH because of the large difference in the pore size and shape;12 the deactivation behaviors of three types of pores should be discussed separately due to their independent and nonintersecting characteristics. The active sites in the surface pockets may suffer from rapid deactivation due to the coke species formed from certain large intermediates that are easily formed but are difficult to decompose.12 Although the acid sites in the supercages may be highly active for MTH in the initial reaction stage, they are also easily deactivated due to the presence of trap cages (large cages with small apertures), lack of steric hindrance and strong acid strength;66,87,88 coke precursors are easily formed in the supercages through successive hydrogen transfer reactions but cannot efficiently diffuse out of the narrow pore opening. If a sufficient number of acid sites are present in the surface pockets and/or supercages close to the edges of crystallites, the continuous growth of coke species may overflow to the outer surfaces and block the adjacent 10-MR sinusoidal channels, hence causing the complete deactivation of zeolite catalysts.89,90 In contrast, the acid sites in the sinusoidal channels are more resistant to deactivation. Similar to the observation for MTH over H-ZSM-5, the narrow 10-MR sinusoidal channels in HMCM-22 can inhibit the formation of bulky secondary products;87,91 most product molecules can diffuse out of the sinusoidal channels owing to the quasi-identical sizes of channel apertures and intersections with two-dimensional diffusion passages.49 Moreover, the relatively weak acid sites in the sinusoidal channels may also suppress the hydrogen transfer reactions.92 Guisnet and co-workers also proved that the sinusoidal channels in H-MCM-22 were highly resistant to coke

Figure 14. Methanol conversion (a) and product selectivity (to ethene (b), propene (c), butene (d), pentene (e), and aromatics (f)) as a function of time on stream for MTH over 3B-Al-M22 zeolite at 450 °C with a methanol WHSV of 2 h−1. 2310


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ACS Catalysis highest selectivity to C3= and C4= and the lowest selectivity to C1−50, C2= and aromatics (Table 4). Such an event is consistent with the recent calculation result that the olefin-based cycle dominates the MTH reaction pathways in the sinusoidal channels, which forms preferentially higher olefin products such as propene and butene rather than ethene and aromatics.12,76 All these results suggest that the olefin-based cycle in the sinusoidal channels may play an important role in the steady MTH reaction stage. The acid sites in the surface pockets and supercages, which are favorable for the aromatic-based cycle and the formation of lower olefins, are easily covered by coke species in the initial MTH reaction period. As a result, the MTH reaction in the steady stage over H-MCM-22 may be mainly catalyzed by the Brönsted acid sites in the sinusoidal channels, in which the olefin-based cycle dominates the MTH reaction, forming more higher olefins (mainly C3−5 olefins).11,101 The incorporation of proper content of boron in H-MCM-22 is able to concentrate the Brönsted acid sites in the sinusoidal channels and to reduce the acid sites in the surface pockets and supercages, which can promote the formation of higher olefins (especially propene and butane) via the olefinbased cycle besides markedly improving the long-term stability in MTH.

results help to clarify the relation between the catalytic performance of H-MCM-22 in MTH and its acid distribution and then bring forward an effective approach to develop better MTH catalysts by regulating the acid distribution.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b02862. N2 physisorption results of the directly calcined xB-AlM22 zeolites; Chemical composition, textural property, morphology and catalytic performance in MTH of the aluminum-free borosilicate zeolite (H-B-MWW); Chemical composition and thermogravimetric analysis results of the as-synthesized xB-Al-M22 zeolites; Calculated substitution energies and relative substitution energies of various B−O−Si species in [B]-MCM-22; 11B Hahnecho MAS NMR spectra of the directly calcined xB-AlM22 zeolites; XRD patterns of the as-synthesized, directly calcined, and H-form xB-Al-M22 zeolites; XRD, SEM, NH3-TPD, and N2 physisorption characterizations of the aluminum-free borosilicate zeolite (BMWW); SEM images of the as-synthesized xB-Al-M22 zeolites; 1H MAS NMR spectrum of the aluminum-free borosilicate H-B-MWW zeolite; FT-IR spectra in the OH stretching vibrations region of the H-form xB-Al-M22 zeolites; Deconvolution of the 1H MAS NMR spectra, 27 Al MAS NMR spectra and 29Si MAS NMR spectra of the H-form xB-Al-M22 zeolites with different boron contents (PDF)

5. CONCLUSIONS The regulation of framework aluminum siting and Brönsted acid distribution among three types of pores in H-MCM-22 was achieved through the incorporation of boron; the relation between the catalytic performance and acid distribution was investigated. The results demonstrate that the distribution of framework aluminum and Brönsted acid sites among three types of pores in H-MCM-22 can be regulated through adjusting the content of boron incorporated during synthesis, due to the competitive occupancy of various framework T sites between boron and aluminum, whereas the textural properties and overall acid types and amounts are less influenced by boron incorporation. Incorporating a proper content of boron, for instance, with a B/ Al atomic ratio of 3 in the synthesis gel, can concentrate the Brönsted acid sites in the sinusoidal channels rather than in the surface pockets and supercages, whereas excessive boron will push the acid sites back to the surface pockets and supercages. The Brönsted acid sites in the surface pockets and supercages are prone to carbonaceous deposition and pore blockage, whereas those in the sinusoidal channels are crucial for MTH in the steady reaction stage. Excellent linear correlation is observed between the Bsin/(Bpoc+Bsup) ratio (or the Al(56)/ (Al(50)+Al(61)) ratio) in the boron-incorporated H-MCM-22 zeolites and their catalytic lifetime in MTH. The active acid sites in the surface pockets and supercages, which are favorable for the aromatic-based cycle and the formation of ethene and aromatics, are easily covered by cokes in the initial period of MTH reactions; as a result, the remaining active acid sites in the sinusoidal channels may dominate the MTH reaction in the steady stage, which favor the olefin-based cycle and the formation of higher olefins (mainly C3−5 olefins). Current results illustrate that the incorporation of proper content of boron in H-MCM-22 is effective to reduce the Brönsted acid sites in the external pockets and supercages and meanwhile concentrate the acid sites in the sinusoidal channels, which gives the boron incorporated H-MCM-22 zeolite much greater long-term stability and higher selectivity to higher olefins such as propene and butene in MTH than ever. These

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for J.W.: [email protected]. Tel.: +86-351-4046092. Fax: +86-351-4041153. *E-mail for Z.Q.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial supports of the National Natural Science Foundation of China (21273264, 21273263, 21227002, 21573270, U1510104), Natural Science Foundation of Shanxi Province of China (2013021007-3, 2015021003), and the CAS/SAFEA International Partnership Program for Creative Research Teams. We also want to thank Prof. Dr. F. Deng, Dr. J. Xu, and Dr. A. Zheng in Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences and Dr. S. Xu in Dalian Institute of Chemical Physics, Chinese Academy of Sciences, for their kind help in the NMR measurement, as well as Dr. K. Liu in PetroChina Petrochemical Research Institute, for his kind help in acidity analysis.

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Regulation of Framework Aluminum Siting and Acid Distribution in H

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