English

• Propylene, as one of the most important intermedi-ates in petrochemical industry, is traditionally pro-duced by the steam cracking of naphtha or ethane, as well as the fluid catalytic cracking (FCC) process. To meet the ever-increasing demand of propylene and reduce its dependence on crude oil resource, new strat-egies for propylene production from cheaper and/or renewable feedstocks have attracted an increasing interest in recent decades. Among the newly-developed technologies, methanol to olefins (MTO) and methanol to propylene (MTP) technologies have been paid much attention due to the easy availability of methanol and high selectivity to light olefins (C₂⁼~C₄⁼). Specifically, the MTP process can optimize the propylene yield over ZSM-5 catalyst due to its unique microporous structure and surface acidity. However, the propylene selectivity and catalytic lifetime need to be enhanced. Many stud-ies have shown that the physiochemical properties of ZSM - 5 zeolite such as surface acidity, pore structure, and crystal sizes, play important roles in improving pro-pylene selectivity and catalyst lifetime. For example, the acid sites in ZSM-5 zeolites are the active sites not only for methanol conversion and propylene formation, but also the undesired secondary reactions such as hydrogen transfer, cyclization and aromatization^[1]. Gen-erally, high^-1density acid sites induce the rapid deacti-vation of catalysts due to coke deposition; and the intrinsic micropores of ZSM-5 zeolites increase the dif-fusion resistance of propylene, the secondary reactions of propylene decrease its selectivity^[2-3]. Therefore, in the last decade, efforts have been devoted to the ZSM-5 zeolites: (a) synthesizing the hierarchical or nanosized ZSM - 5 to increase the diffusion of propylene^[3-4]; (b) modifying the surface acid strength and acid density to decrease the adsorption and hydrogen transfer of pro-pylene on acid sites^{[5, 7]}. For example, Liu et al.^[8] modi-fied high^-1silicon ZSM-5 with 0.1% phosphorus to reduce the acid strength of Brønsted acid sites, which increased the propylene selectivity by 10% in the MTP reaction. In our previous study, the Lewis acid sites were found to be important sites for coke formation in hydrocarbon cracking reactions^[9-11]. However, the effect of acid sites type of ZSM -5 zeolite in the methanol con-version process has not been fully considered yet. In this study, to elucidate the role of Lewis and Brønsted acid sites on the activity and selectivity in the MTP reaction, ZSM - 5 zeolites with varied acid properties were synthesized with boron and/or fluorine doping and tested in the MTP reaction.

  1.   Experimental

  1.1   Raw materials

  Ammonium fluoroborate (NH₄ BF₄, 97.00%(w/w)) was purchased from Shanghai SSS Reagent Co., Ltd. Ammonium fluoride (NH₄F), boric acid (H₃BO₃), tetra-ethyl orthosilicate (TEOS), aluminium isopropoxide, and sodium hydroxide (NaOH) with analytical purity were purchased from Sinopharm Group Chemical Reagent Co., Ltd. Tetrapropyl ammonium hydroxide (TPAOH, 25%(w/w) aqueous solution) was purchased from Sinopharm Group Chemical Reagent Co., Ltd.

  1.2   Sample preparation

  By varying the synthesis temperature during the crystallization, the nucleation and growth process should be controlled to obtain ZSM-5 zeolites with small and uniform crystals^[12]. Here, boron and/or fluo-rine doped ZSM - 5 zeolites were synthesized using a two - stage crystallization method. Appropriate amounts of TEOS, TPAOH, NaOH, and aluminum isopropoxide were mixed with deionized water and stirred at room temperature for 6 h. Then, NH₄BF₄, NH₄F, or H ₃ BO₃ was added into the above mixture as modifiers and stirred for another 12 h. The final gel compositions of synthesizing system (n_SiO2:n_Al2O3 :n_Na2O:n_TPAOH:n_H2O: n_additive) were 50:1.0:2:8:3 000:x. Here, three samples were marked as Z5 -BF1, Z5-BF2, Z5-BF3 when NH₄BF₄ was used as additive with x=3, 6, and 9, in turn; two samples were marked as Z5 - F1 and Z5 - F2 when NH₄F was used as additive with x=12 and 36, respectively; two samples were marked as Z5 - B1 and Z5-B2 when H₃BO₃ was used as additive with x=3 and 9, respectively. In comparison, the sample was marked as Z5 when x=0. The above gel was then transferred into an autoclave for pre-crystallization at 110 ℃ for 3 h and further crystallization at 170 ℃ for 48 h. Thereaf-ter, the as-synthesized solid products were filtered, washed with deionized water, dried overnight, and calcined at 550 ℃ for 4 h in air to remove the organic templates. The H - form ZSM - 5 zeolites were finally obtained after twice ion-exchanges with 1.0 mol·L^-1 of NH₄Cl solution and once calcination at 550 ℃ for 4 h in air.

  1.3   Characterization

  The crystalline phases were measured by X - ray diffraction (XRD) on a Bruker D8 Advance Diffractom-eter, using a Cu Kα radiation (λ=0.154 06 nm) operat-ing at 40 kV and 30 mA, and the scanning angle of 5°~ 50°. The surface area and pore volume of zeolites were derived from the nitrogen sorption curves on a Micro-metrics Tristar 3000 analyzer. Solid - state magic angle rotation nuclear magnetic resonance (MAS NMR) spec-tra were obtained from a Bruker Advance Ⅲ HD 600 MHz instrument. The ²⁹Si NMR spectra were used to analyze the framework silica - to - alumina ratios (n_SiO2/n _Al2O3) of zeolites. Acid sites of all samples were mea-sured on a Thermo Nicolet iS5 Fourier transform infra-red (FT - IR) spectrometer, using pyridine as a probe molecule. Acid strength distributions were measured by NH₃ temperature-programmed desorption (NH₃-TPD) on a Quanta chrome ChemStar^TM instru-ment. About 100 mg of samples were activated in flow-ing Ar at 350 ℃ for 1 h, then cooled to room tempera-ture and exposed to flow of 6.5% (V/V) of NH₃ /Ar gas. Physically adsorbed NH₃ was removed in Ar flow at 100 ℃ for 2 h before collecting data at temperature ramped up to 650 ℃ at a rate of 10 ℃·min^-1 in helium flow of 30 mL·min^-1. Field emission scanning electron microscopy (SEM) images were collected by a Quanta 250 instrument with an acceleration voltage of 15 kV.

  1.4   Evaluation of MTP performance

  The methanol to propylene reaction tests were per-formed in a fixed bed of a quartz tubular reactor with inner diameter of 6 mm. ZSM-5 catalysts with particle sizes of 40~60 mesh were prepared by pressing, crush^-1ing and sieving process. 100 mg of catalyst was placed in the constant temperature zone of the reactor and pre-treated at 550 ℃ for 1 h under Ar gas before the reac-tion. Methanol was injected into the reactor by a con-stant flux pump using a 50 mL·min^-1 of Ar as carrier gas. The reaction temperature was set to 450 ℃ and the weight hourly space velocity (WHSV) of methanol was 4.0 h^-1. The reaction products were analyzed by an on-line gas chromatograph (GC-2014C, Shimadzu GC), which is equipped with a TG-BONG Q column, a ther-mal conductivity detector (TCD) and a flame ionization detector (FID). The maximum absolute error (< 4%) was measured by triplicate experiments. The methanol con-version was defined as usual: X_MeOH=(m_{MeOH, in}-m_{MeOH, out})/m_{MeOH, in}, while selectivity to i product was defined as fol-lows: S_i=m_i/Σm_i, where m_i is the mass of compound i, and Σm_i is the total mass of all products.

  2.   Results and discussion

  The X-ray diffraction (XRD) pattern in Fig. 1 shows the characteristic peaks of MFI structure of ZSM-5 zeolite, corresponding to the (101), (020), (501), (151) and (303) crystal faces, respectively (PDF No.01-085-1208) ^[13-14]. It shows that all samples exhibited a typical MFI phase and no peaks of crystallized impuri-ty appeared. Fig. 1b shows the shifts of two diffraction peaks to higher 2θ angles with increasing the NH₄BF₄ usages, indicative of the increase of framework SiO₂/ Al₂O₃ ratios (n_SiO2/n_Al2O3). The diffraction peak shift of H₃BO₃ modified samples such as Z5-B2 was more obvi-ous than NH₄F modified ones, indicating that H₃BO₃ does more to increase the framework SiO₂/Al₂O₃ ratio. It might be attributed to the substitution of Al atoms by B atoms, since an increased B content was detected with increasing the boron usages^[15-16]. The relative crys-tallinities (RC) were calculated by comparing the peak areas of the characteristic peaks at 22°~25° and listed in Table 1. It shows that the crystallinity of ZSM-5 zeo-lites changed a little with heteroatom doping, and the relative crystallinity values of ZSM-5 zeolites decreased with increasing the NH₄BF₄ usages.

  Figure 1

  

  Figure 1.  XRD patterns of as-synthesized ZSM-5 zeolites

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  Table 1

  

  Table 1.  Physiochemical properties of as-synthesized ZSM-5 zeolites

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  Sample	SiO2/Al2O3ratio (NMR)a	SiO2/Al2O3ratio(ICP)b	B content/%(w/w)b	F content/%(w/w)b	RC/%c
  Z5	50	52	—	—	98
  Z5-BF1	54	55	0.024	1.85	95
  Z5-BF2	57	55	0.061	2.68	93
  Z5-BF3	62	56	0.094	4.05	86
  Z5-F1	53	55	—	1.93	100
  Z5-F2	56	56	—	4.11	98
  Z5-B1	56	57	0.070	—	96
  Z5-B2	63	58	0.097	—	99
  a Derived from the 29Si NMR spectra; b Determined by ICP method; c Relative crystallinity(RC) was derived from the XRD pattern.

  The ²⁹Si NMR spectra in Fig. 2 exhibits a strong resonance at about -113 (chemical shift, the same below), which corresponds to the Si(OSi)₄ species in the zeolites framework. A weak shoulder resonance at about -106 was ascribed to the Si(OSi)₃(AlO) species. A weak resonance at about -102 was ascribed to the (SiO)₃SiOH species. No resonance was observed below -100, indicating that there were no other silicate spe-cies^[17-18]. The framework SiO₂/Al₂O₃ ratios were calcu-lated from the deconvoluted profile using a Gauss-ian-Lorentzian mixed function and listed in Table 1. It shows that the variation of framework SiO₂/Al₂O₃ ratios was in consistent with that derived from XRD pattern.The doping of B and/or F increased the framework SiO₂/Al₂O₃ ratios compared with Z5 sample. The ²⁷Al NMR spectra of four selected ZSM-5 zeolites in Fig. 3a exhibits two distinct resonances at 52 and -3, corre-sponding to a tetra-coordinated framework and a hexa-coordinated extra-framework of the Al species, respectively. No shoulder resonance of a penta-coordinated Al species was observed at approximately 35. The results show that combining F and B for Z5-BF2 remarkably decreases the percentage of the extraframe-work Al species, but the percentages of extraframework Al species with B and F alone changed small.

  Figure 2

  

  Figure 2.  ²⁹Si NMR spectra of as-synthesized ZSM-5 zeolites with revolved resonances

  No.1~3 correspond to the Si(OSi)₄ species, No.4 is attributed to the Si(OSi)₃(AlO) species, and No.5 is related to (SiO)₃SiOH species

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  Figure 3

  

  Figure 3.  ²⁷Al NMR spectra(a), ¹¹B NMR spectra (b), and ¹⁹F NMR spectra (c) of selected as-synthesized ZSM-5 zeolites

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  The ¹¹B NMR spectra in Fig. 3b displays two main peaks at -3.9 and 13.8. The former resonance is attrib-uted to the tetrahedral coordinated framework boron, and the latter broad band is assigned to the trigonal coordinated extraframework boron species^{[15, 19]}. The small shoulder between these two bands is ascribed to the second- order quadrupole broadening of the trigonal coordinated framework boron, which is easily derived from the dehydration of tetrahedral coordinated frame-work boron species^[16]. This result confirms that the majority of B species is incorporated into the frame-work of zeolite. The ¹⁹F NMR spectra in Fig. 3c shows that there are five peaks for zeolites Z5-BF2 and Z5-F1. The peaks from -66 to -87 are attributed to F^-anions located in the cages of silica zeolites^[20-21]. The peak at around - 123 is assigned to the presence of the F^- ion in zeolite channels as counter ion of balanced cations such as H⁺, Na⁺ or NH₄^{+ [22-23]}. The peaks at -141 and -142 are associated with the extraframework Al species such as AlF₆^3-. The peaks at -158 and - 161 are related to Si - F groups which are caused by the replacement of hydroxyl group in the silanols by fluo-rine atom, or a terminal fluorine atom at the surface, indicative of the fluorination in the form of (SiO₃)Si - F groups^{[9, 23]}. The peaks at -177 and -180 are assigned to the partially hydrated AlF₃ species^[24-25]. The NMR data indicate that the boron and fluorine exist in the form of framework incorporation and chemical bonding with extraframework Al species.

  Fig. 4 shows the N₂ sorption isotherms and pore size distributions of the ZSM-5 zeolites. The isotherms are identified as type Ⅳ, which is characteristic of micropore and mesopore mixed structure of ZSM-5 materials. The Brunauer-Emmet-Teller (BET) specific surface areas of the samples were calculated from N₂ isotherms and listed in Table 2. It shows that with increasing the NH₄BF₄ usages, microporous surface areas of ZSM-5 zeolites increased, but their mesopo-rous surface areas decreased obviously. As a result, the total surface areas gradually decreased, from 322 m²· g^-1 for Z5 to 272 m² ·g^-1 for Z5-BF3. The B and F dop-ing contributes to the formation of microspore but impaired the mesopore, being in consistent with that of H₃ BO₃ and NH₄F modified samples. The same tenden-cy has also been observed for the pore volumes, i. e., the micropore volumes of ZSM-5 zeolites containing B and F were higher than that of Z5. The pore size distri-bution obtained from the desorption branch indicates the reduction of mesoporous structure for B and F doped samples, particularly for the F doped samples.

  Figure 4

  

  Figure 4.  N₂ sorption isotherms (a) and pore size distributions (b) of as-synthesized ZSM-5 zeolites

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  Table 2

  

  Table 2.  Textural properties of as-synthesized ZSM-5 zeolites

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  Sample	Specific surface area / (m2·g-1)				Pore volume / (cm3·g-1)
  	SBETa	Smicro	Sext		Vtotalb	Vmicroc	Vmeso
  Z5	322	154	168		0.212	0.080	0.132
  Z5-BF1	308	177	131		0.213	0.095	0.118
  Z5-BF2	300	185	115		0.195	0.094	0.101
  Z5-BF3	272	198	74		0.219	0.102	0.117
  Z5-F1	316	234	82		0.202	0.121	0.081
  Z5-F2	322	237	85		0.215	0.123	0.092
  Z5-B1	314	232	82		0.204	0.120	0.084
  Z5-B2	309	243	66		0.187	0.126	0.061
  a Calculated by BET method; b Calculated by t-plot method; c Pore volume at p/p0=0.985.

  FT-IR analysis was carried out to characterize the acid amounts of ZSM- 5 zeolites. As highlighted in Fig. 5a, it is generally recognized that the bands at 1 447 and 1 546 cm^-1 are ascribed to the pyridine mole-cules coordinated to Lewis acid sites and pyridinium ions formed by protonation of pyridine on Brønsted acid sites, respectively^{[10, 26-27]}. The band at 1 490 cm^-1 is assigned to pyridine associated with both the two kinds of acid sites. The band at 1 597 cm^-1 is ascribed to the hydrogen bonded pyridine. The acid amount was calcu-lated using a semi-quantitative method proposed by Emeis et al^[28]. The results in Table 3 show that with the increase of NH₄BF₄ usages, the amounts of Brønsted acid sites of Z5 - BFx obviously increased; in contrast, the amounts of Lewis acid sites remarkably decreased. It is caused by the doping of boron and fluorine atoms, which could effectively increase the amount of Brønsted acid sites but decrease the amount of Lewis acid sites, in view of the results of H₃BO₃ and NH ₄F used alone.

  Figure 5

  

  Figure 5.  FT-IR spectra of pyridine absorbed by as-synthesized ZSM-5 zeolites

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  Table 3

  

  Table 3.  Surface acidity of as-synthesized ZSM-5 zeolites

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  Items	Acid amount/(mmol·g-1)a				Relative amount of acid sites / %b
  	Total	Brønsted	Lewis		Total	Weak	Strong
  Z5	192	96	96		82	38	62
  Z5-BF1	129	110	19		75	30	70
  Z5-BF2	145	128	17		77	19	81
  Z5-BF3	231	214	17		88	27	73
  Z5-F1	253	183	70		97	21	79
  Z5-F2	225	200	25		96	28	72
  Z5-B1	192	175	17		90	23	77
  Z5-B2	268	201	67		100	28	72
  a Derived from pyridine adsorbed IR spectra; b Obtained from NH3-TPD curves.

  The FTIR spectra of OH stretching region in Fig. 5b is used to characterize the effect of B and F dop-ing on OH groups. The band at 3 742 cm^-1 is ascribed to the OH stretching vibration (n_OH) of free silanols which is thought to be located at the external surface of zeolite, namely SiOHs^{[26, 29]}. It shows that combing B and F increases the amount of external surface SiOHs which is indicative of the external surface dealumina-tion, being in consistent with the results of ²⁷Al NMR analysis in Fig. 3. The bands at 3 691 and 3 604 cm^-1 are assigned to the vibration of extraframework Al-OH and bridging Si -OH-Al groups, respectively^[30-31]. Obvi-ously, B and/or F doping could decrease the amounts of extraframework Al-OH and bridging Si -OH-Al species. Those species play important roles in constituting the acid sites, especially Brønsted acid sites, which is con-sistent with the variation tendency of Brønsted acid amounts with increasing the heteroatom doping (Table 3).

  NH₃ - TPD tests were also performed to character-ize the acid strength distributions of ZSM - 5 zeolites. As shown in Fig. 6, all samples exhibited two resolved desorption peaks: the low - temperature around 250 ℃ and high - temperature peak higher than 300 ℃, corre-sponding to the weak and strong acid sites, respectively. It is obvious that the peak of strong acid sites on B and/ or F doped samples shifted to higher temperatures com-pared to Z5. The relative amounts of acid sites derived from the resolved overlapped curves are listed in Table 3. It can be seen that the weak acid sites of B and/or F doped samples decreased compared with Z5 sample, which is consistent with the change of Lewis acid amount; however, the change of strong acid amount is in keeping with the Brønsted acid amount (Table 3). The decrease of Lewis acid sites is probably attributed to the isomorphous substitution of Al atom by B atom or the bond combination of B and Al atoms; in contrast, the strong electronegativity of F atom reduces the elec-tron cloud on Si-OH-Al group, enhancing the strength of Brønsted acid sites^{[9-10, 32]}.

  Figure 6

  

  Figure 6.  NH₃-TPD profiles of as-synthesized ZSM-5 zeolites

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  SEM images in Fig. 7 show that all samples pos-sessed relatively uniform particles. The pre-crystalliza-tion at low temperature contributes to the formation of crystal nucleus and further crystallization at high tem-perature are beneficial to the synthesis of small sized ZSM-5 zeolite with good uniformity in a short time. Sample Z5 had well dispersed spherical shape parti-cles ca. 2 μm. With increasing the NH₄BF₄ usages, the particle sizes of Z5- BFx decreased gradually. Z5- BF1 had cubic and round cornered particles with diameter of ca. 500 nm, while Z5-BF3 had the smallest particles with diameter of approximately 200 nm. Compared to NH₄BF₄, H₃BO₃ and NH₄F used alone had the reverse effect on the particle size of ZSM - 5 zeolites. At low usage of modifiers, the particle sizes of Z5-B1 and Z5-F1 were smaller than sample Z5, while they were much larger than Z5 at higher usage. Notably, high concentra-tion of fluorine alone could not only accelerate the crys-tallization rate of zeolite but also etch the crystal sur-face (Fig. 7g), as previously reported^[33-34]. In contrast, boron alone made the ZSM - 5 particle surface coarser than Z5 (Figs. 7h and 7i).

  Figure 7

  

  Figure 7.  SEM images of as-synthesized ZSM-5 zeolites

  (a, b) Z-5; (c) Z5-BF1; (d) Z5-BF2; (e) Z5-BF3; (f) Z5-F1; (g) Z5-F2; (h) Z5-B1; (i) Z5-B2

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  TEM images of typical Z5 and Z5-BF2 samples is shown in Fig. 8. Both samples possessed relatively uni-form crystals with smooth surface, being in consistent with that measured by SEM images. Some cavities are observed on the border and inside the crystals, indicat-ing that the mesoporous structures detected in the N₂ sorption isotherms are mainly intra-crystal meso-pores.

  Figure 8

  

  Figure 8.  TEM images of Z5 (a, b) and Z5-BF2 (c, d)

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  The products distributions of all ZSM - 5 catalysts in MTP reaction are listed in Table 4. All ZSM- 5 cata-lysts exhibited nearly full methanol conversion during the initial reaction period, indicative of their high ini-tial activity. By defining 85% of methanol conversion as the point of catalyst deactivation, the boron and/or fluorine doped catalysts showed longer catalytic life-time than Z5. With the increases of NH ₄BF₄ usages, the lifetimes of Z5-BF x increased first and then decreased. Z5-BF2 possessed the longest lifetime of 23 h. By con-trast, catalysts with boron used alone had better perfor-mance in improving the catalyst lifetime compared with that used fluorine alone. As for product selectivity, the Z5 - BF2 had the highest propylene selectivity up to 41.5%. In contrast, the propylene selectivity on cata-lysts with fluorine alone decreased greatly compared with Z5 catalyst.

  Table 4

  

  Table 4.  MTP reaction results on H-form ZSM-5 catalysts with boron and/or fluorine doping^a

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  Sample	Conv.	Average selectivity/%b									P/Ec	Lifetimed
  		CH4	C2H4	C2H6	C3H6	C3H8	C4H8	C4H10	C5+	C2=~C4=
  Z5	97.4	2.4	12.4	0.1	32.4	2.4	11.3	7.6	31.4	56.1	2.6	7
  Z5-BF1	100	0.9	12.2	0.1	36.0	2.6	11.2	9.0	28.0	59.4	3.0	15
  Z5-BF2	100	0.3	9.1	0.0	41.5	1.3	13.0	10.3	24.5	63.6	4.6	23
  Z5-BF3	99.7	3.7	15.0	0.2	31.6	4.4	11.7	8.3	25.1	58.3	2.1	14
  Z5-F1	99.6	1.5	8.0	0.2	25.5	3.4	7.3	5.3	48.8	40.8	2.6	8
  Z5-F2	99.5	2.1	9.8	0.1	27.3	2.7	7.4	5.5	45.1	44.5	2.8	7
  Z5-B1	100	1.1	10.3	0.2	29.9	4.4	8.7	7.4	38.0	48.9	2.9	20
  Z5-B2	100	2.2	12.1	0.2	31.2	4.0	9.6	8.3	32.4	52.9	2.6	16
  a Reaction conditions: WHSV of methanol=4 h-1, 0.1 MPa, catalyst dosage=0.10 g; b Arithmetic average of the selectivity of the products with-1in several times of sampling analysis before the catalyst deactivation; c nC3H6/nC2H4; d 85% of methanol conversion as the point of catalyst deactiva-tion.

  Methanol conversion and selectivity to light ole-fins (C₂⁼, C₃⁼ and C₄⁼) as a function of time on stream (TOS) over three selected catalysts are displayed in Fig. 9. It shows that the selectivity of light olefins (C₂⁼~C₄⁼) on ZSM-5 catalysts was high at the initial period, and then decreased to a stable stage with slight fluctua-tion. Of all catalysts, Z5-BF2 possessed the highest average selectivity to propylene and light olefins, up to 41.5% and 63.6%, respectively. In the methanol con - version on zeolites, aromatic-based cycle and olefin - based cycle simultaneously occur, and ethylene mainly comes from the former while propylene and other heavi-er olefins mainly come from the latter^[35]. Taking Z5 and Z5- BF2 for examples, decreasing acid site density and the amount of Lewis acid sites could reduce the rates of aromatization and hydrogen transfer, resulting in lower ethylene selectivity but higher propylene selectivity for Z5 - BF2. In contrast, the higher ethylene selectivity of Z5-BF3 catalyst is attributed to its improved amount of Brønsted acid sites, compared to Z5 catalyst. Similarly, high acid site density and high amount of Lewis acid site could increase the selectivity of C₅+ products (mainly aromatic species), due to the aromatization reactions^{[18, 36]}.

  Figure 9

  

  Figure 9.  Conversion and product selectivity for the MTP reaction vs time-on-stream (TOS) over catalysts: (a) Z5; (b) Z5-BF2; (c) Z5-B1

  Condition: WHSV of methanol=4.0 h^-1, 0.1 MPa, catalyst dosage=0.10 g

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  From the foregoing, boron alone could improve the lifetime of ZSM-5 catalyst while fluorine alone had limited effect on prolonging the catalyst lifetime. The combination of boron and fluorine on Z5- BFx catalysts improved both catalyst lifetime and selectivity of light olefins, propylene in particular. Generally, the de-activation of MTP catalysts is caused by the accumula-tion of coke species on the surface of ZSM - 5 zeolites, which cover the surface acid sites or plug pores and suppress the access of reactants to acid sites^{[7, 37-38]}. How-ever, it seems that mesopore did not do much in reduc-ing the coke deactivation, considering that the de-crease of mesoporous volumes of Z5-BFx did not affect their lifetimes compared with Z5 catalyst.

  The crystal size is important in improving propyl-ene selectivity and catalyst lifetime^[39-42]. Generally, nanosized ZSM - 5 zeolites have been reported to show superior catalytic stability in the MTP reaction due to containing short diffusion paths. Despite prolonging the catalytic lifetime for nanosized ZSM-5 zeolites in most cases, their exposed external surface acid sites are more likely to absorb light olefins and coke precursors and contribute little to improving propylene selectivity because of their weakened shape selectivity. In this study, with increasing the NH₄BF₄ usages, the particle sizes of Z5-BF x decreased gradually. However, propyl-ene selectivity and catalyst lifetime did not monotoni-cally increase or decrease with the variation of crystal sizes. The superior catalytic performance of Z5 -BF2 more reasonably results from the optimization of acid sites in view of the significant changes of surface acidi-ty compared to Z5 in Table 3.

  By contrast, acid sites, especially the strong acid sites, are thought to be the predominant reason for coke deposition^{[38, 43-46]}. The strong adsorption of light olefins on strong acid sites (the Brønsted acid sites in this work) and further polymerization with other light ole-fins should be responsible for the catalyst deactiva-tion^{[2, 39]}. In view of the acid properties of catalysts in Table 3, the decrease of Lewis acid amount of Z5-BF1 and Z5 - BF2 obviously improved the catalyst lifetimes compared with Z5 catalyst; while the sharp increase of Brønsted acid amount of Z5 - BF3 reduced the catalyst lifetime compared with Z5-BF2. Therefore, Lewis acid sites and strong acid sites, especially strong Brønsted acid sites, are unfavorable due to the fast coking deacti-vation, as previously reported^{[11, 47-48]}. The amount of strong acid sites on ZSM-5 catalyst should be con-trolled to a relatively low level.

  Some references related with boron or fluorine modified ZSM- 5 and their catalytic performances in the MTP reactions are listed in Table 5. In the previous studies, most of the B or F modified ZSM - 5 catalysts possessed higher propylene selectivity and longer cata-lyst lifetimes than unmodified ZSM-5 catalysts. As shown in Table 5, all catalysts had relatively high pro-pylene selectivity more than 40%, however, their cata-lyst lifetimes differed widely. The big differences of cat-alyst lifetimes are caused by many factors such as the acidity, pore structure, and crystal sizes. It is noted that the reaction conditions such as catalyst loading amount, water addition into methanol feed, and even inner diameter of the reactor are also important factor influencing the results. In our two studies^{[36, 49]}, the com-mercial MTP catalysts which was tested under two different reaction conditions exhibited totally different catalytic lifetimes (25 h vs > 400 h). In this study, we used the prepared ZSM - 5 catalyst in the same reactor with that used in our previous studies^{[3, 36]}. 23 h of cata-lyst lifetime was comparable to that of the commercial MTP catalyst. More importantly, we believe that it is more meaningful to compare the catalytic performances of catalysts in the same reactor and same reaction conditions.

  Table 5

  

  Table 5.  Comparison of MTP performance on B or F modified ZSM-5 catalysts

  下载: 导出CSV

  Catalyst	Reaction condition (0.1 MPa)	Yield of C3H6/%(w/w)	Lifetime / h	Ref.
  B-Al-ZSM-5	0.5 g catalyst, WHSV=3 h-1, nH2O:nCH3OH=1:1, 470 ℃	48	< 1	[16]
  B-Al-ZSM-5	1.0 g catalyst, WHSV=1.8 h-1, nH2O:nCH3OH=3:1, 460 ℃	~40	700	[50]
  AlCl3/B-ZSM-5	1.0 g catalyst, WHSV=1.8 h-1, nH2O:nCH3OH=3:1, 500 ℃	~45	40	[51]
  [B]-ZSM-5	4.0 g catalyst, WHSV=0.9 h-1, nH2O:nCH3OH=1:1, 480 ℃	47.6	1 280	[52]
  silicalite-1@B-ZSM-5/SS-fiber	0.4 g catalyst, WHSV=10 h-1, nH2O:nCH3OH=1:1, 450 ℃	46.4	60	[53]
  ZSM-5-F	1.0 g catalyst, WHSV=3 h-1, nH2O:nCH3OH=1:1, 500 ℃	40~45	305	[54]
  F-ZSM-5	1.0 g catalyst, WHSV=3 h-1, nH2O:nCH3OH=1:1, 470 ℃	< 45	224	[55]
  B-F-ZSM-5	0.1 g catalyst, WHSV=4.0 h-1, CH3OH in Ar, 470 ℃	41.5	23	This work

  3.   Conclusions

  A series of ZSM-5 catalysts were successfully syn-thesized in the presence of boron and/or fluorine con-taining reagents. The structural and morphological characterization results reveal that the appropriate amount of NH₄BF₄ increases the micropores but reduces the Lewis acid sites without obvious increase of Brønsted acid sites. Compared to the conventional Z5 catalyst, Z5-BF2 exhibits a superior catalytic performance in the methanol to propylene reaction with higher propyl-ene selectivity and longer lifetime. Moreover, reducing Lewis acid sites and controlling the acid strength of Brønsted acid sites is expected to be suitable for improving the performance of MTP reaction.

  Acknowledgements: This work was financially supported by the Fundamental Research Funds for the Central Universities (Grant No.2018QNB04).

  
  
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• 

  Figure 1  XRD patterns of as-synthesized ZSM-5 zeolites

  下载: 全尺寸图片 幻灯片
  

  Figure 2  ²⁹Si NMR spectra of as-synthesized ZSM-5 zeolites with revolved resonances

  No.1~3 correspond to the Si(OSi)₄ species, No.4 is attributed to the Si(OSi)₃(AlO) species, and No.5 is related to (SiO)₃SiOH species

  下载: 全尺寸图片 幻灯片
  

  Figure 3  ²⁷Al NMR spectra(a), ¹¹B NMR spectra (b), and ¹⁹F NMR spectra (c) of selected as-synthesized ZSM-5 zeolites

  下载: 全尺寸图片 幻灯片
  

  Figure 4  N₂ sorption isotherms (a) and pore size distributions (b) of as-synthesized ZSM-5 zeolites

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  Figure 5  FT-IR spectra of pyridine absorbed by as-synthesized ZSM-5 zeolites

  下载: 全尺寸图片 幻灯片
  

  Figure 6  NH₃-TPD profiles of as-synthesized ZSM-5 zeolites

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  Figure 7  SEM images of as-synthesized ZSM-5 zeolites

  (a, b) Z-5; (c) Z5-BF1; (d) Z5-BF2; (e) Z5-BF3; (f) Z5-F1; (g) Z5-F2; (h) Z5-B1; (i) Z5-B2

  下载: 全尺寸图片 幻灯片
  

  Figure 8  TEM images of Z5 (a, b) and Z5-BF2 (c, d)

  下载: 全尺寸图片 幻灯片
  

  Figure 9  Conversion and product selectivity for the MTP reaction vs time-on-stream (TOS) over catalysts: (a) Z5; (b) Z5-BF2; (c) Z5-B1

  Condition: WHSV of methanol=4.0 h^-1, 0.1 MPa, catalyst dosage=0.10 g

  下载: 全尺寸图片 幻灯片

  Table 1.  Physiochemical properties of as-synthesized ZSM-5 zeolites

  Sample	SiO2/Al2O3ratio (NMR)a	SiO2/Al2O3ratio(ICP)b	B content/%(w/w)b	F content/%(w/w)b	RC/%c
  Z5	50	52	—	—	98
  Z5-BF1	54	55	0.024	1.85	95
  Z5-BF2	57	55	0.061	2.68	93
  Z5-BF3	62	56	0.094	4.05	86
  Z5-F1	53	55	—	1.93	100
  Z5-F2	56	56	—	4.11	98
  Z5-B1	56	57	0.070	—	96
  Z5-B2	63	58	0.097	—	99
  a Derived from the 29Si NMR spectra; b Determined by ICP method; c Relative crystallinity(RC) was derived from the XRD pattern.

  下载: 导出CSV

  Table 2.  Textural properties of as-synthesized ZSM-5 zeolites

  Sample	Specific surface area / (m2·g-1)				Pore volume / (cm3·g-1)
  	SBETa	Smicro	Sext		Vtotalb	Vmicroc	Vmeso
  Z5	322	154	168		0.212	0.080	0.132
  Z5-BF1	308	177	131		0.213	0.095	0.118
  Z5-BF2	300	185	115		0.195	0.094	0.101
  Z5-BF3	272	198	74		0.219	0.102	0.117
  Z5-F1	316	234	82		0.202	0.121	0.081
  Z5-F2	322	237	85		0.215	0.123	0.092
  Z5-B1	314	232	82		0.204	0.120	0.084
  Z5-B2	309	243	66		0.187	0.126	0.061
  a Calculated by BET method; b Calculated by t-plot method; c Pore volume at p/p0=0.985.

  下载: 导出CSV

  Table 3.  Surface acidity of as-synthesized ZSM-5 zeolites

  Items	Acid amount/(mmol·g-1)a				Relative amount of acid sites / %b
  	Total	Brønsted	Lewis		Total	Weak	Strong
  Z5	192	96	96		82	38	62
  Z5-BF1	129	110	19		75	30	70
  Z5-BF2	145	128	17		77	19	81
  Z5-BF3	231	214	17		88	27	73
  Z5-F1	253	183	70		97	21	79
  Z5-F2	225	200	25		96	28	72
  Z5-B1	192	175	17		90	23	77
  Z5-B2	268	201	67		100	28	72
  a Derived from pyridine adsorbed IR spectra; b Obtained from NH3-TPD curves.

  下载: 导出CSV

  Table 4.  MTP reaction results on H-form ZSM-5 catalysts with boron and/or fluorine doping^a

  Sample	Conv.	Average selectivity/%b									P/Ec	Lifetimed
  		CH4	C2H4	C2H6	C3H6	C3H8	C4H8	C4H10	C5+	C2=~C4=
  Z5	97.4	2.4	12.4	0.1	32.4	2.4	11.3	7.6	31.4	56.1	2.6	7
  Z5-BF1	100	0.9	12.2	0.1	36.0	2.6	11.2	9.0	28.0	59.4	3.0	15
  Z5-BF2	100	0.3	9.1	0.0	41.5	1.3	13.0	10.3	24.5	63.6	4.6	23
  Z5-BF3	99.7	3.7	15.0	0.2	31.6	4.4	11.7	8.3	25.1	58.3	2.1	14
  Z5-F1	99.6	1.5	8.0	0.2	25.5	3.4	7.3	5.3	48.8	40.8	2.6	8
  Z5-F2	99.5	2.1	9.8	0.1	27.3	2.7	7.4	5.5	45.1	44.5	2.8	7
  Z5-B1	100	1.1	10.3	0.2	29.9	4.4	8.7	7.4	38.0	48.9	2.9	20
  Z5-B2	100	2.2	12.1	0.2	31.2	4.0	9.6	8.3	32.4	52.9	2.6	16
  a Reaction conditions: WHSV of methanol=4 h-1, 0.1 MPa, catalyst dosage=0.10 g; b Arithmetic average of the selectivity of the products with-1in several times of sampling analysis before the catalyst deactivation; c nC3H6/nC2H4; d 85% of methanol conversion as the point of catalyst deactiva-tion.

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  Table 5.  Comparison of MTP performance on B or F modified ZSM-5 catalysts

  Catalyst	Reaction condition (0.1 MPa)	Yield of C3H6/%(w/w)	Lifetime / h	Ref.
  B-Al-ZSM-5	0.5 g catalyst, WHSV=3 h-1, nH2O:nCH3OH=1:1, 470 ℃	48	< 1	[16]
  B-Al-ZSM-5	1.0 g catalyst, WHSV=1.8 h-1, nH2O:nCH3OH=3:1, 460 ℃	~40	700	[50]
  AlCl3/B-ZSM-5	1.0 g catalyst, WHSV=1.8 h-1, nH2O:nCH3OH=3:1, 500 ℃	~45	40	[51]
  [B]-ZSM-5	4.0 g catalyst, WHSV=0.9 h-1, nH2O:nCH3OH=1:1, 480 ℃	47.6	1 280	[52]
  silicalite-1@B-ZSM-5/SS-fiber	0.4 g catalyst, WHSV=10 h-1, nH2O:nCH3OH=1:1, 450 ℃	46.4	60	[53]
  ZSM-5-F	1.0 g catalyst, WHSV=3 h-1, nH2O:nCH3OH=1:1, 500 ℃	40~45	305	[54]
  F-ZSM-5	1.0 g catalyst, WHSV=3 h-1, nH2O:nCH3OH=1:1, 470 ℃	< 45	224	[55]
  B-F-ZSM-5	0.1 g catalyst, WHSV=4.0 h-1, CH3OH in Ar, 470 ℃	41.5	23	This work

  下载: 导出CSV
•