Article Cite This: J. Phys. Chem. C 2019, 123, 15637−15647
pubs.acs.org/JPCC
Effect of Al Distribution in MFI Framework Channels on the Catalytic Performance of Ethane and Ethylene Aromatization Hua Liu,† Hui Wang,‡ Ai-Hua Xing,† and Ji-Hong Cheng*,‡ †
National Institute of Clean-and-Low-Carbon Energy, Beijing 102211, China NICE America Research, Inc., Mountain View, California 94043, United States
‡
Downloaded via UNIV FRANKFURT on July 21, 2019 at 19:03:10 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: The relationship between the formation of aromatics in catalytic ethane/ethylene aromatization and Al distribution in Mobil-type five (MFI) channel was investigated using ZSM-5 zeolites with controlled Al distribution, which were obtained by employing different organic structural directing agents (OSDAs) during the ZSM-5 synthesis step. ZSM-5 samples with framework Al mainly distributed in the MFI straight/sinusoidal channels or intersections were obtained using pentaerythritol or tetrapropylammonium hydroxide as OSDAs, respectively. Samples with Al distributed in both channel and intersection were synthesized using n-butylamine as a template. Ethane aromatization results showed that Pt-modified ZSM-5 with acid sites located in the channel intersection tended to produce more aromatic compounds and less methane than that of a catalyst with acid sites located in the straight and/or sinusoidal channels under the same conversion of ethane. Ethylene aromatization was performed to understand how the intermediates evolve within acid sites. The results indicated that acid sites located in the intersection with more space void were conducive for the geometry transformation of intermediates, resulting in a much faster hydrogen transfer reaction rate than that in the straight and/or sinusoidal channels with bulky bimolecular reaction intermediates involved. Hence, ZSM-5 with acid site mainly located in the channel intersection would be more favorable for the formation of aromatics over the other two types of catalysts in ethylene aromatization.
1. INTRODUCTION By unlocking hydrocarbons in the low-permeability shale formation, the successful application of horizontal drilling and hydraulic fracturing techniques in oil and gas production had significantly expanded the production of natural gas liquids (NGLs, a mixture of C2−C5 light alkanes) over the past decade. Among these mixtures, ethane is the largest component (∼40% of NGLs), and its average production over a month has increased from ca. 20 000 barrels in 2008 to ca. 50 000 barrels in 2018,1 which greatly cut the cost of ethane processing (e.g., its downstream product ethylene). This had created a promising prospect for converting this low-cost ethane into more valuable and easy-to-transport aromatic products, such as benzene, toluene, and xylene isomers (BTX), which are used commercially as fuel additives, raw materials for polymers, and other value-added aromatic-derived products. The development of such catalytic process for the conversion of ethane to BTX could be profitable in the near future, competing with its mainstream naphtha-reforming process, in which the cost of feedstock constitutes about 80% of the BTX production cost. Nevertheless, the ethane aromatization process over acidic zeolites is quite distinct and more difficult to accomplish than other paraffin aromatization conversions. The activation rate for alkanes over zeolite decreases dramatically with carbon © 2019 American Chemical Society
numbers. On H-ZSM-5 at 773 K, butane, for example, is 401 times more reactive than propane and ethane.2 Hence, the early interest of industrial community focused more on the aromatization of hydrocarbons with carbon numbers C ≥ 3, e.g., M2-forming process:3 alkane/olefins with C ≥ 3 on HZSM-5; Cyclar process:4 propane/butane on gallium containing ZSM-5 zeolites; aroforming:5 C5−C6 alkanes; and Zforming:6 C3−C4 alkanes. While the most relevant Cyclar process was successfully industrialized with an aromatic yield of 58−60 wt % using liquid petroleum gas (C3/C4) as the feedstock,7 a similar ethane-to-aromatic process has not yet been commercialized. Though there is still some distance to go for ethane-toaromatic industrialization, some important knowledge about the catalyst formulations for the ethane-to-aromatic process had been acquired by both academic groups and industrial entities since the 1970s.7−19 It is believed that ethane aromatization consists of the transformation of ethane into ethylene and the transformation of as-formed ethylene into aromatic hydrocarbons, and bifunctional catalyst components with metal sites for ethane dehydrogenation and acid sites for Received: April 14, 2019 Revised: May 20, 2019 Published: May 30, 2019 15637
DOI: 10.1021/acs.jpcc.9b03507 J. Phys. Chem. C 2019, 123, 15637−15647
Article
The Journal of Physical Chemistry C the oligomerization and aromatization were employed.7,15 For metal sites, Cu, Zn, Ga, Mo, and Pt are proved to be effective for dehydrogenation of ethane.7,20,21 Among these metals, Pt possesses powerful dehydrogenation ability but with unfortunate high hydrogenolysis side reaction that usually leads to a large amount of unwanted methane. Secondary metal (Sn, Ge, Ga, and Fe)-modified Pt had been extensively investigated to overcome the weakness of Pt metal sites. For acid sites, the zeolite of the MFI framework with parallel straight and sinusoidal 10-membered ring channels was employed for ethane aromatization. The two channels intersected to form a void with a large spherical space (ca. 10 A in diameter), which is different from the size of sinusoidal and straight channels that are similar to the aromatic ring size (ca. 5.5 A).22 A large amount of existing research on the catalyst formulation of ethane aromatization mainly focuses on metal modification, but little attention was reported as to how the catalytic performance of ethane aromatization was influenced by zeolite structures, especially aluminum location and its distribution in the MFI framework. It is generally admitted that neither the Al location nor acid distribution in the zeolite framework is random or controlled by simple theoretical rules but instead depends on the conditions of the zeolite synthesis.23−25 Isomorphic displacement of Al3+ in silica frameworks results in negative framework charge that is often balanced with extra framework cations such as organic structural directing agent (OSDA) that is present in the aluminosilicate zeolite synthesis process. It has been assumed that in the ZSM-5 synthesis when tetrapropylammonium hydroxide (TPA) is used as OSDA, TPA cations preferably reside at the intersection due to steric effect, which results in framework Als preferentially located at the intersection. Yokoi et al. used various alcohols to control the Al atoms in the MFI framework and showed that the Al atoms in the ZSM-5 synthesized with bulky and branched-chain alcohols were preferentially located in the narrow straight and/ or sinusoidal channels,26 and Al distribution within these two types of pores had a major influence on the methanol-to-olefin and catalytic cracking of n-hexane on MFI zeolites. However, little knowledge had been acquired about the effect of Al distribution on ethane and ethylene aromatization. This study focused on the influence of the location and distribution of Al framework atoms on the catalytic performance of ethane and ethylene aromatization. The location and distribution of framework Al in MFI zeolite were controlled by the type of OSDA in the synthesis of zeolites. Constraint index (CI), which is the ratio between the cracking rate of n-hexane and 3-methylpentane, coupled with 27Al magic angle spinning nuclear magnetic resonance (MAS NMR), was employed to determine the acid distribution in MFI zeolites. After carefully assuring that the model catalysts possess similar physicochemical properties, the prepared catalysts with different acid site distributions were introduced in ethane aromatization and further ethylene aromatization was performed to clarify the impact of the distributions of acid sites on their catalytic performances.
composition of 1.0SiO2:0.39TPA:13.5H2O. The colloidal solution was transferred to autoclave, sealed, and crystallized for 24 h at 423 K. The silicalite-1 seed was obtained by filtering and washing three times with deionized water and drying at 393 K overnight without further calcination. Zeolites with acid located in the straight/sinusoidal channels were synthesized by employing pentaerythritol (PET) as OSDA according to the previous method with minor modification.26 The synthesis gel was prepared by mixing PET with a solution containing Al(NO3)3 and silica sol (Ludox-AS 40%) in NaOH aqueous solution to give a gel with a molar composition of 1.0SiO2:xAl2O3:0.5PET:0.125Na2O:25H2O, where x = 0.033 for Si/Al2 of 30 and x = 0.025 for Si/ Al2 of 40. The above gel was stirred for 1 h before transferring to a Teflon-lined stainless steel autoclave followed by crystallization at 448 K for 24 h. The synthesized MFI zeolites were designated as “PET-MFI(30)” and “PET-MFI(40)”, respectively. Zeolites with framework Al distributed in both straight/ sinusoidal and intersection channels were synthesized using nbutylamine (NBA) as OSDA.27 In a typical synthesis, silica fume was dispersed into a solution containing NaAlO2 and NBA to obtain a gel of 1.0SiO2:xAl2O3:0.98NBA:0.08Na2O:10H2O, where x = 0.033 for Si/Al2 of 30 and x = 0.025 for Si/ Al2 of 40. The above gel was stirred for 10 h at room temperature and then placed in a Teflon-lined stainless steel autoclave followed by hydrothermal crystallization at 443 K for 5 days under the static state. The synthesized MFI zeolites were denoted as “NBA-MFI(30)” and “NBA-MFI(40)”, respectively. Zeolites with framework Al distributed in intersection channels were synthesized using TPA as OSDA. In a typical synthesis, colloidal silica was mixed with Al(NO3)3 to form a clear solution, and the rest of water and TPAOH was added to form a homogeneous suspension under stirring for 4 h to give a gel molar composition of 1.0SiO2:xAl2O3:0.5TPAOH:50H2O, where x = 0.033 for Si/Al2 of 30 and x = 0.025 for Si/Al2 of 40. The suspension was transferred to a Teflon-lined stainless steel autoclave and statically crystallized for 5 days at 448 K. The synthesized MFI zeolites were denoted as “TPA-MFI(30)” and “TPA-MFI(40)”, respectively. All of the above solid products were filtered and washed with H2O and dried at 393 K for 12 h after hydrothermal crystallization step. The products were calcined at 823 K for 10 h under air to remove organic templates. To convert to NH4+form zeolite, ion exchange process was carried out with a 1.5 M NH4NO3 solution at 353 K for 4 h twice, followed by drying overnight at 393 K. For ethane aromatization catalyst preparation, Pt was impregnated on the MFI zeolites using the incipient wetness method, and the loading of Pt was fixed at 0.05 wt %. Specifically, the catalysts were prepared by mixing a solution containing tetraammineplatinum(II) chloride with dried NH4+-MFI. After drying at 393 K for 4 h, the catalysts were calcined at 823 K for 4 h (2.5 K min−1) in a muffle furnace before the test for ethane aromatization. 2.2. Characterization of Catalysts. The X-ray diffraction (XRD) patterns of zeolites were collected on a Bruker X-ray diffractometer using Cu Kα radiation (40 kV, 40 mA) with a step size of 0.02° in the 2θ range of 5−60°. Surface morphology was observed on an FEI Nova NanoSEM 450 scanning electron microscope. Textural properties of samples were measured by the N2 adsorption−desorption method on a
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Silicalite-1 was used as seed for the zeolite synthesis below. Specifically, tetraethyl orthosilicate was mixed with tetrapropylammonium hydroxide (TPA, 25 wt %) and was stirred for 4 h until the mixture became transparent to form a colloidal solution with a molar 15638
DOI: 10.1021/acs.jpcc.9b03507 J. Phys. Chem. C 2019, 123, 15637−15647
Article
The Journal of Physical Chemistry C
Figure 1. Scanning electron microscopy (SEM) images of as-synthesized zeolites: (a) PET-MFI(30), (b) PET-MFI(40), (c) TPA-MFI(30), (d) TPA-MFI(40), (e) NBA-MFI(30), and (f) NBA-MFI(40); scale bar denotes 4 μm.
were recorded at a magnetic field of 9.4 T on a Bruker AVANCE III HD 400M (WB) spectrometer with 4 mm ZrO2 rotors. The 29Si MAS NMR spectra were obtained using a 3 μs single-pulse time with a 60 s acquisition delay, and the spectra were used for the analysis of the Si(nAl, 4-nSi) atoms. 27Al MAS NMR spectra were recorded using a 3 μs single-pulse time (90°) with a 4 s delay for quantitative analysis, and 1 M Al(NO3)3 was used for 27Al NMR shift calibration. All of the samples were recorded after equilibrating with atmospheric moisture. In situ CO dynamic chemisorption was performed on a Micromeritics 3FLEX surface characterization system, utilizing the pulse chemisorption of CO with a homemade gas loop. Before measurements, all samples were in situ reduced by hydrogen at 903 K, followed by a He purge and cooling to 298 K. CO (10%, balanced in He) was used as the titrating gas at 298 K, and the amount of adsorbed CO was measured by a thermal conductivity detector (TCD). The amount of coke deposited on the catalyst was measured by thermogravimetric analysis in air flow (DTG-60 Shimadzu) by heating ∼15 mg of the sample from room temperature to 1123 K (ramp 5 K min−1). The total coke content was estimated as the sample weight loss between 523 and 1123 K. 2.3. Catalytic Performance Evaluation. 2.3.1. Estimation of Constraint Index. Powder NH4+ zeolites (100 mg) were mixed with 1.5 g of silica sand (20−40 mesh), and the catalyst was loaded into a vertical tubular quartz reactor and
Micromeritics ASAP2460 apparatus at 77 K after the samples were degassed at 623 K for 6 h under vacuum. The amount of Si and Al was quantified by an X-ray fluorescence spectrometer (XRF), which is carried out on a Rigaku ZSX Primus II X-ray fluorescence instrument. Temperature-programmed desorption of ammonia (NH3TPD) was conducted to measure the acidity of samples on a Micromeritics AutoChem II2920/AutoChem HP2950 chemisorber. Specifically, a 200 mg sample was pretreated in He stream (25 mL min−1) at 873 K for 1 h and then cooled to 373 K. The adsorption of NH3 was performed by flushing a 10 v/v % NH3/He mixture stream for 80 min, and then the sample was purged with He for 20 min to remove the physically adsorbed NH3. The TPD profile was recorded at a heating rate of 10 K min−1 from 373 to 1073 K. Infrared spectra were collected on a Bruker VERTEX instrument in transmission mode, using a self-supporting wafer (∼20 mg cm−2) and an in situ IR transmission cell with a CaF2 window. The wafer was pretreated under vacuum (1.33 × 10−2 Pa) for 2 h at 773 K, and the background spectra of the wafer were collected after cooling the cell to 303 K. The spectra were recorded at 423 K in the scanning range of 650−4000 cm−1 with a 2 cm−1 resolution by a Hg−Cd−Te (MCT) detector with averaging 64 scans. All spectra were normalized by the intensity of the Si−O−Si overtones (2100−1750 cm−1).28 29 Si and 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra of hydrated zeolite samples 15639
DOI: 10.1021/acs.jpcc.9b03507 J. Phys. Chem. C 2019, 123, 15637−15647
Article
The Journal of Physical Chemistry C Table 1. Physicochemical Properties of the H-Type Samples textural properties samples
Si/Al2a
Si/Al2b
acid amount (mmol NH3 g−1)c
PET-MFI(30) NBA-MFI(30) TPA-MFI(30) PET-MFI(40) NBA-MFI(40) TPA-MFI(40)
29.1 27.0 30.8 38.9 37.1 39.2
31.4 31.9 31.2 42.8 41.3 42.7
1.19 1.20 1.17 0.95 0.89 0.93
Al concentration (mmol g−1)
SBET,total (m2 g−1)d
Sext (m2 g−1)d
Smicro (m2 g−1)e
Vtotal (cm3 g−1)d
Vmicro (cm3 g−1)e
1.08 1.16 1.02 0.82 0.86 0.80
346.5 365.0 350.3 358.6 353.5 360.6
52.9 68.8 57.1 54.1 50.2 60.3
293.6 296.2 293.2 304.5 303.3 300.3
0.21 0.24 0.22 0.22 0.21 0.23
0.12 0.12 0.12 0.13 0.12 0.13
a e
Determined by XRF. bCalculated from 29Si MAS NMR in Figures 3 and S4. cDetermined by NH3 TPD. dMeasured by N2 adsorption at 77 K. Calculated by the t-plot method.
activated in flowing He at 823 K for 1 h to convert to H-ZSM5. After cooling to 623 K, the catalyst bed was first injected with 50 μL 2,4-dimethylquinoline to poison the external acid site29 and then passed over by a feed mixture obtained by bubbling He through a saturator containing n-hexane and 3methylpentane (45:55 mol/mol) at 313 K. The helium flow was adjusted to achieve 15−20% of the C6 paraffin conversion. The reactor effluent was analyzed by gas chromatography (Shimadzu GC-2014 equipped with a one flame ionization detector (FID) and a PLOT-Q capillary column). The constraint index (CI) value was calculated by the following equation constraint index =
and the evaluation test began by introducing a gas mixture of 50 v/v % ethylene and 50 v/v % N2 under the same temperature. The calculations of ethylene conversion, product selectivity, and yield were similar to eqs 2−4 by replacing ethane with ethylene. All reaction effluents were drained to online gas chromatography (Shimadzu GC-2014 equipped with two TCD/one FID detectors and four valve/six column system) through a heating line of 473 K to avoid any condensation of reactants.
3. RESULTS AND DISCUSSION 3.1. Synthesis and Physicochemical Properties. To exclusively investigate the effect of framework Al on its catalytic activity, it was important to rule out the effect of particle size and morphology, assuring that model samples are in similar size and morphology. PET-MFI crystallized at a fast rate under the synthetic condition and can easily form a relatively large crystal structure; hence, to match the morphology of PET-MFI sample, TPA-MFI and NBA-MFI samples were crystallized in a static state mode with a longer time. As shown in Figure 1, all of the zeolite aggregate sizes were in the range of ∼4 μm and consisted of a cubic or spherical crystal, and increasing the Al content did not significantly change the morphology of the zeolites. All of the samples exhibited a similar patterning of N2 adsorption and desorption isotherms for microporous materials with an H4-shaped hysteresis loop at 0.45 < p/po < 0.90 (Figure S1), which is caused by the capillary condensation during adsorption due to the tensile strength effect. Similar textual porosities including micropore volume (ca. 0.11−0.13 cm3 g−1) and surface area (Sext and Smicro) were observed among these samples (Table 1). XRD patterns of all zeolites exhibited characteristic MFI structure with a high crystallinity (Figure S2). The elemental compositions of the zeolites as determined by XRF are listed in Table 1. The product Si/Al2 ratios are slightly lower than that of the corresponding starting gel, indicating that not all of the Si species were recovered in the crystallization process. The NH3-TPD curves of the PETMFI, NBA-MFI, and TPA-MFI zeolites with different Si/Al2 are shown in Figure S3, and the amounts of NH3 desorbed are listed in Table 1. All of the zeolites exhibited two main types of NH3 desorption in the spectrum: the low-temperature desorption peaks (ca. 463 K) related to weak acid sites and the high-temperature peaks (ca. 723 K) that are attributed to strong acid sites. The total acid amounts of different samples determined by NH3-TPD were slightly larger than the theoretical Al concentration calculated by Si/Al2 ratio; this was due to additional weak acid sites originated from extracrystalline oxide species that weakly absorbed NH3.
log(fraction of n‐hexane remaining) log(fraction of 3‐methylpentane remaining) (1)
2.3.2. Ethane Aromatization. A vertical tubular quartz reactor with an inner diameter of 10 mm was used for the evaluation of catalysts under atmospheric pressure. Prior to ethane aromatization test, catalysts were crushed and sieved to collect 1.00 g pelletized catalysts (20−40 mesh). The thermocouple was placed outside the reactor wall next to the catalyst bed to avoid any unwanted side reactions catalyzed by a thermocouple. For ethane aromatization test, all Pt/ZSM-5MFI catalysts were pretreated in situ under the flow of N2 (99.999%, 50 STP mL min−1) at 773 K for 1 h at a heating rate of 5 K min−1. After the pretreatment, H2 (99.99%, 50 STP mL min−1) was introduced to reduce the catalyst for 30 min under the same temperature before catalytic measurements. The ethane aromatization evaluation began by switching to inlet gas mixture containing 85 v/v % of ethane (99.999%, BAIF gas Inc.) and 15 v/v % of N2 used as the internal standard for reactivity. The following equations were used to calculate ethane conversion, product selectivity, and yield ethane conversion (%) ethane(in, vol % STP) − ethane(out, vol % STP) × =
N2(in, STP) N2(out, STP)
ethane(in, vol % STP)
(2)
× 100
product selectivity (%) =
carbon moles of products × 100 carbon moles of ethane converted
(3)
yield (%) = ethane conversion × product selectivity
(4)
NH4+-type
2.3.3. Ethylene Aromatization. Dried zeolite (50 mg) was diluted with 1.50 g of quartz sand (20−40 mesh) to ensure a suitable bed height. The catalyst was pretreated in situ by passing a flow of 50 mL min−1 of N2 at 773 K for 1 h, 15640
DOI: 10.1021/acs.jpcc.9b03507 J. Phys. Chem. C 2019, 123, 15637−15647
Article
The Journal of Physical Chemistry C
for all zeolites was observed, indicating that these samples possessed a similar amount of external silanols, which was in agreement with their similar morphology and large crystal size. Note that unlike zeolites with Si/Al2 of 40, there were some small bands centered at 3647 cm−1 for zeolites with Si/Al2 of 30, which indicated that trace extra-framework aluminum species existed within samples of Si/Al2 30. The absence of silanol nest bands (3550−3300 cm−1) on all zeolite samples implies that all samples can be considered as pristine materials.31 3.2. Al Distribution in the MFI Framework. 3.2.1. 29Si and 27Al MAS NMR Measurements. 29Si MAS NMR spectra were performed to investigate the SiO4 environments of the type Qm{nAl}, where m denotes the number of bridging oxygen atoms and n the number of connected Al tetrahedral. As shown in Figure 3a, a broad line centered at −110 to −117 ppm can be attributed to Q4{0Al},32 and two component signals of −112 and −116 ppm are assigned to symmetric and slightly asymmetric Q4 silicon. The signals at ca. −106 ppm in the fitting curve of 29Si MAS NMR correspond to Q4{1Al} of Si connecting to one Al tetrahedron in the framework structure. The signal centered at −100 ppm referred to tertiary silicon atoms (0Al)Si*(OSi)3 in silanol groups located on the surface of ZSM-5.33 The Q3 signal was almost the same among these samples, indicating a similar external surface among samples investigated, which were in accordance with Brunauer−Emmett−Teller (BET) and SEM results. The Si/ Al2 ratio can be calculated from the 29Si MAS NMR results according to the previously reported protocol,33 and the calculated Si/Al2 results are listed in Table 1. The calculated Si/Al2 for different samples were close to the values obtained from XRF data. No significant differences were observed in the 29 Si MAS NMR signals, probably because the Al content of the highly siliceous zeolites is so low that it does not affect the 29Si NMR spectrum.34 Figures 4a and S5 show the 27Al MAS NMR for hydrated NH4-type zeolites. All of the samples featured a strong 4coordinate framework Al peak centered at ∼56 ppm and no peak at ∼0 ppm, indicating that all Al atoms were inserted into
The normalized FTIR spectra in the OH-stretching region (3800−3400 cm−1) of the zeolites are depicted in Figure 2.
Figure 2. Normalized infrared spectra of OH-stretching region on Htype PET-MFI, NBA-MFI, and TPA-MFI zeolites with Si/Al2 of 30 (a) and Si/Al2 of 40 (b).
Three main bands can be assigned to bridged hydroxyl groups (Si(OH)Al, 3610 cm−1), and external and internal silanols (3745 and 3726 cm−1).30 A similar intensity of external silanols
Figure 3. 29Si MAS NMR spectra of (a) NH4-type ZSM-5 zeolites with Si/Al2 of 30 and its corresponding curve fittings of (b) PET-MFI(30), (c) NBA-MFI(30), and (d) TPA-MFI(30). 15641
DOI: 10.1021/acs.jpcc.9b03507 J. Phys. Chem. C 2019, 123, 15637−15647
Article
The Journal of Physical Chemistry C
Figure 4. 27Al MAS NMR spectra of (a) NH4-type ZSM-5 zeolites with Si/Al2 of 30 and its corresponding curve fittings of (b) PET-MFI(30), (c) NBA-MFI(30), and (d) TPA-MFI(30).
Table 2. Relative Area Obtained from the Curve Fitting of 27Al MAS NMR Spectra of NH4-Type ZSM-5 Zeolites chemical shift relative area (%) sample
58 ppm
56 ppm
54 ppm
53 ppm
52 ppm
54 ppm/56 ppm
PET-MFI(30) NBA-MFI(30) TPA-MFI(30) PET-MFI(40) NBA-MFI(40) TPA-MFI(40)
37.2 39.0 25.1 35.3 33.7 33.4
33.7 34.8 35.1 26.8 31.7 18.9
12.4 13.2 27.4 12.1 14.8 15.1
1.1 1.1 8.2 7.8 10.8 13.7
15.4 11.8 4.0 18.0 8.9 18.9
0.37 0.38 0.78 0.45 0.47 0.80
can be used to indicate the acid distribution over different pore type within the MFI framework with the exclusion of the influence of external acid sites of zeolites. To avoid the influences of the external acid sites on the CI value and make sure all of the cracking reactions occurred inside the micropores of ZSM-5, 2,4-DMQ was used to selectively poison the external acid sites before CI testing in the cracking reactions of C6 paraffins.38 Moreover, the amount of acid sites estimated from NH3-TPD with the similar Si/Al2 molar ratio was almost identical among these samples. All of the above combinations assured that the difference in the CI experiments can be realistically attributed to the distribution of Al atoms in the framework. CI values of PET-MFI, NBA-MFI, and TPA-MFI samples are demonstrated in Figure 5. Significant differences in CI values were observed among samples synthesized with different templates. The CI values for the sample with a Si/Al2 ratio of 30 synthesized with PET and NBA were estimated to be 10.8 and 6.8, respectively, which were remarkably higher than that of TPA-MFI(2.0) despite their identical structures and similar acidic properties. This indicated that the number of acid sites located at the intersection of PET-MFI and NBA-MFI is noticeably lower compared with that of the TPA-MFI sample. Due to its bulkiness, OSDA typically occupies the intersection during the zeolite crystallization process. The neutrality of PET and NBA molecules would mean that the channel intersection would be less likely to contain framework Al, which will create a negative charge that needs to be balanced with additional
the MFI framework in a tetrahedral form. Figure 4b,c shows the enlarged tetrahedrally coordinated Al peak ranging from 65 to 45 ppm. Simulation of the spectra yielded five resonances at around 52, 53, 54, 56, and 58 ppm, and their relative percentage area is listed in Table 2. Significant differences in the proportions between TPA-MFI and the other two zeolites are observed. The relative area ratio of 54−56 ppm increased in the sequence of PET-MFI, NBA-MFI, and TPA-MFI. It is believed that the peak at 54 ppm was related to the framework Al in the channel intersections, whereas the peak at 56 ppm was assigned to the framework Al in the straight and/or sinusoidal channels.35 Although it is difficult to accurately determine the framework Al position over 12 distinct T-sites using NMR chemical shifts, it is a useful tool to get some information about the distribution of framework Al over 12 distinct T-sites. For PET-MFI and NBA-MFI samples prepared using neutral OSDA, the ratio of 54/56 ppm was lower than that of the TPA-MFI sample. This suggests that the framework Al of PET-MFI and NBA-MFI samples was mainly in the straight and/or sinusoidal channels, while for the TPA-MFI sample, the framework Al atoms resided within the intersection channel to a large extent. 3.2.2. Constraint Index. CI is determined by measuring the relative cracking rate ratio of n-hexane to 3-methylpentane, and the value is quite sensitive to the pore diameter.36 PET-MFI, NBA-MFI, and TPA-MFI samples possess the same MFI topology with the size of the 10-ring micropores (5.1−5.6 A) inside the three-dimensional channel structure.37 Hence, CI 15642
DOI: 10.1021/acs.jpcc.9b03507 J. Phys. Chem. C 2019, 123, 15637−15647
Article
The Journal of Physical Chemistry C
ZSM-5(40) samples would be mainly due to the differences of Al framework distribution. Conversions of ethane at a constant weight hourly space velocity (WHSV = 1.1 g gcat−1 h−1, 903 K) over these samples are shown in Figure 6. The conversion of ethane over time of stream (TOS) (Figure 6a) curve showed a drastically different catalytic performance over these three types of catalysts. Pt/ TPA-MFI demonstrated the most stable catalytic performance with an ethane conversion loss of only 32.0% even within 300 min of TOS (3.1 wt % coke), while Pt/NBA-MFI and Pt/PETMFI showed much faster conversion losses with 68.5% (215 min of TOS, 8.5 wt % coke) and 73.0% (133 min of TOS, 13.7 wt % coke) of its initial catalytic conversion, respectively. Figure 6b,d,f,h,j shows the changes in the product yield with TOS. The intermediate yields of ethylene, C3, and C4 showed a similar trend among the three catalysts; Pt/TPA-MFI catalyst with aluminum distributed in the channel intersection demonstrated a much lower yield of C3 and C4−C5 than that of the other two catalysts, indicating that Pt/TPA-MFI converted the intermediate species to the terminal products more efficiently than Pt/NBA-MFI and Pt/PET-MFI catalysts did. For the yield of BTX and methane, which were the terminal products of ethane aromatization, Pt/TPA-MFI showed higher yield than the other two catalysts, and Pt/ PET-MFI decayed at a faster rate than that of Pt/NBA-MFI (Figure 6b). Pt/TPA-MFI catalyst featured a significantly different characteristic compared with the other two catalysts in terms of the corresponding conversion with a selectivity curve (Figure 6c,e). Even though Pt/TPA-MFI catalyst showed more methane formation, it is largely due to its high conversion over time. At a given ethane conversion, methane selectivity was actually lower for Pt/TPA-MFI than that of Pt/ NBA-MFI and Pt/PET-MFI (Figure 6e). For BTX selectivity with ethane conversion, the curve of Pt/TPA-MFI sits above that of Pt/NBA-MFI and Pt/PET-MFI. The results above suggested that Pt/TPA-MFI was more favored to produce an aromatic compound and less tendency to methane formation over Pt/NBA-MFI and Pt/PET-MFI under the same conversion of ethane. It is believed that the dehydrogenation of ethane is followed by a complex of oligomerization, cyclization, and aromatization reactions of the as-formed ethylene, which are accompanied by cracking and isomerization of long-chain intermediates in ethane aromatization.42 When the protonic acid is distributed at the intersection void, less steric constraint of the transitionstate geometry of C6+ intermediates will be imposed compared to the situation in the straight and sinusoidal counterparts, and this will have a significant effect on the overall activity and selectivity for ethane aromatization. To understand how the intermediates evolve within the acid sites, ethylene aromatization was performed to eliminate the complexity of dehydrogenation sites for a better understanding of the effect of the aluminum distribution on the transformation of intermediates. 3.3.2. Ethylene Aromatization. Table 3 shows the product distributions of ethylene aromatization at 773 K. Ethylene conversion reached 93−99% at the space velocity of 15 000 h−1 (50 vol % ethylene) with more than 30% of the products being BTX, which were formed by the cyclization of olefinic intermediates followed by hydrogen transfer accompanied by the formation of alkane.43,44 High proportions of nondimeric species in the product such as propylene (4−20% selectivity), propane (25−34% selectivity), and C4−C5 (16−26% selectivity) species were observed, indicating a high cracking rate of
Figure 5. Constraint index values for H-type zeolites with different Si/Al2.
cations in a confined space. Hence, Na+ cations are located at the straight and/or sinusoidal channels during the crystallization, resulting in the preferential sitting of Al atoms in the MFI framework as opposed to the channel intersections.26,35,39 It should be noted that although PET and NBA molecules are both neutral OSDA for MFI zeolite, the smaller size of NBA allows it to be more flexible when occupying the intersection so that smaller cations such as Na+ can still squeeze into the intersection to balance the framework charge if necessary. As a result, NBA-MFI zeolites exhibited noticeably lower CI values than those of PET-MFI, indicative of more framework Al residing in the intersection. As for the TPA-MFI samples, the introduced TPA cations can only be located at the intersections during the crystallization resulting in a local positively charged intersection. Therefore, more Al atoms would be located in the channel intersections, leading to a decreased CI value. Note that CI values for different Si/Al2 ratio samples with same templates were quite similar; this indicates that the amount of Al inserting into the framework would not influence the trend of aluminum distribution within the similar synthesis environment. 3.3. Catalytic Performance. 3.3.1. Ethane Aromatization. Unlike the aromatization of more reactive propane and higher alkanes, ethane conversion requires efficient catalytic dehydrogenation components. Despite the drawback of undesirable hydrogenolysis reaction leading to CH4 product, Pt was such an effective dehydrogenation component for the ethane aromatization. So, we selected Pt/H-ZSM-5 with different types of MFI zeolites to test the ethane aromatization performance. Pt (0.05 wt %) was introduced onto NH4-ZSM-5 by employing incipient wetness impregnation followed by oxidative pretreatment. During these treatments, Pt migration and redispersion often occur within the channel system during the catalyst preparation step, especially in the oxidation treatment of Pt/ZSM-5.40 Pt species can either move into deeper micropore from the outer zeolite layer to form highly dispersed Pt (cluster and/or single Pt atom) or migrate out of deep micropores to form PtO cluster on the outer zeolite layer under the pretreatment and reaction conditions,41 and this would likely produce a catalyst with similar Pt distribution. Moreover, Table S1 shows that all Pt/PEI-MFI(40), Pt/NBAMFI(40), and Pt/TPA-MFI(40) samples posses high CO adsorption capacity with similar Pt dispersion exceeding 90%. Based on the above discussion and the Pt dispersion data, it is reasonable to believe that the catalytic differences of Pt/H15643
DOI: 10.1021/acs.jpcc.9b03507 J. Phys. Chem. C 2019, 123, 15637−15647
Article
The Journal of Physical Chemistry C
Figure 6. Conversion and product yield with time on stream (TOS), and product selectivities with the conversion of ethane aromatization over Pt/ PEI-MFI(40), Pt/NBA-MFI(40), and Pt/TPA-MFI(40). Reaction conditions: WHSV = 1.1 g gcat−1 h−1, 85 vol % ethane + 15 vol % N2, T = 903 K and P = 1 atm.
Table 3. Catalytic Activity and Selectivity of H-ZSM-5 in the Ethylene Aromatization Reaction at 773 Ka selectivity (%) zeolites
conversion (%)
C1
C2
C3=
C3
C4−C5
BTX
B
T
X
naphtha
B/BTX
T/BTX
X/BTX
coke (%)
PET-MFI(40) NBA-MFI(40) TPA-MFI(40) PET-MFI(30) NBA-MFI(30) TPA-MFI(30)
93.5 94.8 95.9 93.8 94.8 96.8
0.8 0.8 1.4 0.9 1.0 1.5
2.5 2.6 2.7 2.9 2.9 3.3
9.5 7.5 5.1 9.1 7.4 4.0
25.5 27.1 29.9 26.3 28.6 33.7
24.7 25.7 19.0 23.7 22.8 16.7
33.8 33.6 39.1 33.1 34.5 37.9
5.4 5.5 7.5 5.6 6.0 7.9
14.7 14.5 17.6 14.4 15.1 17.5
13.6 13.6 14.1 13.1 13.3 12.6
0.6 0.4 0.4 0.9 0.5 0.4
0.16 0.16 0.19 0.17 0.17 0.21
0.44 0.43 0.45 0.44 0.44 0.46
0.40 0.40 0.36 0.40 0.39 0.33
2.2 1.9 0.8 3.1 2.1 1.4
Reaction conditions: TOS = 10 min, WHSV = 8.7 g gcat−1 h−1, 50 vol % ethylene + 50 vol % N2, T = 773 K and P = 1 atm.
a
oligomers that were formed via isomerization after oligomerization.45 Since there was no hydrogenation site on H-ZSM-5, methane and ethane formations mainly result from hydrogen transfer of primary methyl- and ethyl-carbenium cations that
are cracking products of oligomer intermediates. However, their formation among all samples was very low (
