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NaZSM-5 for C2H6 dehydrogenation in the presence of CO2: Conjugated effect of silanol
Accepted Manuscript Ga2O3/NaZSM-5 for C2H6 dehydrogenation in the presence of CO2: Conjugated effect of silanol Yanhu Cheng, Tianqi Lei, Changxi Miao, Weiming Hua, Yinghong Yue, Zi Gao PII:
S1387-1811(18)30226-9
DOI:
10.1016/j.micromeso.2018.04.041
Reference:
MICMAT 8896
To appear in:
Microporous and Mesoporous Materials
Received Date: 31 October 2017 Revised Date:
22 February 2018
Accepted Date: 25 April 2018
Please cite this article as: Y. Cheng, T. Lei, C. Miao, W. Hua, Y. Yue, Z. Gao, Ga2O3/NaZSM-5 for C2H6 dehydrogenation in the presence of CO2: Conjugated effect of silanol, Microporous and Mesoporous Materials (2018), doi: 10.1016/j.micromeso.2018.04.041. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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C2H6 Conversion (%)
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Paper submitted for publication in Microporous and Mesoporous
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Materials
Ga2O3/NaZSM-5 for C2H6 dehydrogenation in the presence
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of CO2: conjugated effect of silanol
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Yanhu Cheng a, Tianqi Lei a, Changxi Miao b, Weiming Hua a, Yinghong Yue a,*, Zi Gao a
a
Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative
Shanghai Research Institute of Petrochemical Technology, Shanghai 201208, PR China
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b
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Materials, Fudan University, Shanghai 200433, PR China
* Corresponding author. Tel.: +86 21 65642409; Fax: +86 21 65641740. E-mail address: [email protected] (Y. Yue)
ACCEPTED MANUSCRIPT ABSTRACT GaOx catalysts supported on Na-form ZSM-5 with different crystallite sizes were studied in the ethane dehydrogenation reaction in the presence of CO2. Submicron
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zeolite ZSM-5-S supported catalyst showed the best activity among these catalysts, with an ethane conversion of 25% and ethylene selectivity of 92%. Py-IR tests revealed that Ga/ZSM-5-S contained abundant Lewis acids and Brönsted ones, which
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can have a synergistic effect in the dehydrogenation. Silanols on the surface play an
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important role in dispersing the active GaOx species. Silanol nests can not only be helpful in dispersing active species but also act as Brönsted acid sites which facilitate the conjugated effect of Brönsted acidic sites and Ga species, thus promotes the dehydrogenation reaction.
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Keywords: Ethane dehydrogenation; Ga2O3; Acidity; NaZSM-5; Silanol nests
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1. Introduction
Light alkenes like ethylene and propylene are the versatile commodity materials for
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producing polymers, alkene oxides and other derivates. Dehydrogenation of light alkanes into their corresponding alkenes has attracted much attention recently due to the rapidly growing demand for alkene and the shortage of petroleum resources. Pt/Al2O3 used in the UOP Oleflex process and Cr2O3/Al2O3 in the Lummus Catofin process are two commercial catalysts for light alkane dehydrogenation. The widespread application of above catalysts was limited by the drawback of high cost or environmental hostile. In addition, long-term stability is also a challenge for them.
ACCEPTED MANUSCRIPT Recently, CO2 has been applied as a soft oxidant for oxidative dehydrogenation of various alkanes (e.g. ethane, propane and ethylbenzene) and methane coupling for ethylene production [1-4]. Introduction of CO2 into dehydrogenation reaction system
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can not only shift the reaction equilibrium via reverse water-gas shift reaction but also enhance the catalyst stability by facilitating the desorption of alkene produced and removing coke via Boudouard reaction. However, the two commercial catalysts above
are
not
applicable for this
Efficient
and
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environmental-benign catalysts need to be developed.
new process.
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referred
Ga2O3 is an efficient catalyst for the dehydrogenation of ethane and propane in the presence of CO2 and the yield of ethylene was reported to be doubled in the presence of CO2 than in its absence [5]. Moreover, the ethylene yield increased from 3% to 25%
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with increasing Ga2O3 surface area from about 1 to 50 m2/g [6]. Similar results were also drawn by Wu et al [7]. Therefore, supports with high surface areas are desired to improve the turnover frequency of gallium species. The catalytic performance of
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Ga2O3 based catalysts supported on different carriers has been studied and TiO2 was
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found to be superior to the other oxide supports like ZrO2, ZnO, Al2O3 and SiO2 in the dehydrogenation of light alkanes [8-9]. However, ethylene (or propylene) yield decreased remarkably with reaction time over Ga2O3/TiO2 due to quick carbon deposition. Ga2O3-loaded ZSM-5 was also found to be quite effective in activation of C-H bond, but aromatics were readily formed simultaneously, resulting in low olefin selectivity [10-15]. Bell et al revealed that increasing proximity of Brönsted acid sites (origin from Si-OH-Al) would enhance the formation of aromatic species [16]. Our
ACCEPTED MANUSCRIPT previous works showed that higher Si/Al ratio of HZSM-5 supported Ga2O3 catalysts were more efficient in higher C2H4 selectivity as well as more resistant toward deactivation [17]. The superior performance was ascribed to the decrease in the
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number of acid sites with medium to strong strength, which resulted in the suppression of the side reactions such as oligomerization, cyclization and cracking. However, the initial ethylene yield reduced with further increasing Si/Al ratio since
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that active sites are decreased due to the weakening of interaction between Ga2O3
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species and the HZSM-5 support. It is evident that acidity is quite important in the dehydrogenation with CO2 over Ga2O3-loaded catalysts. Positive role in promoting dehydrogenation and negative role in forming byproducts should be balanced by tuning carefully the acidity of the catalysts [18-20]. Proton-poor gasilicalite catalyst
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obtained by solid-state ion exchange of reduced univalent Ga cations was found to be effective in the propane dehydrocyclization reaction [21-22]. A spinel-type gallia-alumina solid solution catalyst was also reported to be effective in the propane
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dehydrogenation reaction with CO2 [23]. It seemed that high dispersion of gallium
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species which could increase the efficiency of gallium species played a crucial role in light alkane dehydrogenation reaction. Framework Al atoms of HZSM-5 was reported to be quite useful in dispersing Ga2O3 through their strong interaction with GaOx species, which would enhance the Lewis acidity of gallia species as well as their abilities in activation of C-H bond of propane [24]. Recently, Na-form ZSM-5 was found to be more helpful in dispersing Cr2O3 than HZSM-5, though it was barren of Al-OH-Si bridging hydroxyls [25]. The above results are sure to be helpful in
ACCEPTED MANUSCRIPT designing efficient Ga/MFI dehydrogenation catalysts. In the present work, Ga2O3 supported catalysts on Na-form ZSM-5 with different crystallite sizes and surface properties were prepared and characterized by SEM,
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27
Al MAS NMR,
29
Si MAS NMR and
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HRTEM, N2 adsorption, XRD, IR, ICP-OES,
Ga NMR. The acidity of the supported catalysts was characterized by Py-IR. The
catalytic performance of these catalysts for dehydrogenation of ethane into ethylene in
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the presence of CO2 was compared and discussed in relation to their acidity as well as
2. Experimental
Nanosize
ZSM-5
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2.1. Catalyst preparation
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the state of GaOx species.
zeolite
was
prepared
in
two
steps.
First,
24.4
g
tetrapropylammonium hydroxide (TPAOH, 25% aqueous solution) was added to the
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mixture of 25.0 g tetraethylorthosilicate (TEOS) and 16.8 g water, and stirred at 80 °C
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for 24 h. 0.1 g NaAlO2 in aqueous solution was then dropped into the mixture above. After stirred for another 1 h, the gel was transferred into an autoclave and crystallized for 24 h at 170 °C. The molar composition of the gel was 120SiO2: 0.6Al2O3: 30TPAOH: 1920H2O. The obtained product was then centrifuged, washed, dried at 110 °C overnight and then calcined in air at 600 °C for 6 h to remove the template. Submicron ZSM-5 zeolite was prepared following the procedures in the literature using TPAOH as the template [26]. Typically, 0.1 g NaAlO2 was dissolved in an
ACCEPTED MANUSCRIPT aqueous solution containing 9.8 g TPAOH, then 25.0 g TEOS was added into the above solution. The resultant mixture was stirred for 6 h at room temperature, followed by heating at 50 °C under stirring to evaporate ethanol formed. The molar
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composition of the clear gel was 120SiO2: 0.6Al2O3: 48TPAOH: 3600H2O. This gel was transferred into an autoclave and crystallized at 170 °C for 2 d. The obtained product was treated with the same method as nanosize ZSM-5 to remove the template.
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Microsize ZSM-5 zeolite was synthesized as follows: 43.6 g Na2SiO3•9H2O, 0.126
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g NaAlO2 and 16.0 g tetrapropylammonium bromide (TPABr) were dissolved in 65.0 g water [27]. Diluted H2SO4 aqueous solution was then added slowly under stirring until the pH value is 9.0. The molar composition of the mixture was 120SiO2: 0.6Al2O3: 48TPABr: 19200H2O. After being stirred at room temperature for 4 h, the
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gel was transferred into an autoclave and crystallized at 180 °C for 2 d. The obtained product was filtered, washed, dried at 110 °C overnight and then calcined in air at 600 °C for 6 h.
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The above ZSM-5 zeolites were turned into Na form by three consecutive ion
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exchanges with 1 M NaNO3 aqueous solution at 80 °C for a total of 12 h. The obtained nanosize, submicrosize and microsize Na-form ZSM-5 zeolites are labelled as ZSM-5-N, ZSM-5-S and ZSM-5-L, respectively. The supported gallium oxide catalysts were prepared by impregnating an aqueous
solution of Ga(NO3)3•xH2O on ZSM-5 using an incipient wetness method. The impregnated samples were dried at 110 °C overnight and calcined in air at 650 °C for 6 h. The obtained catalysts are denoted as Ga/ZSM-5-N, Ga/ZSM-5-S and
ACCEPTED MANUSCRIPT Ga/ZSM-5-L, respectively. The loading amount of Ga2O3 in all catalysts was 5 wt %.
2.2. Catalyst characterization
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X-ray diffraction (XRD) patterns were recorded on a Persee XD-2 X-ray diffractometer using nickel-filtered Cu Kα radiation at 40 kV and 30 mA. Scanning electron microscopy (SEM) was recorded digitally on a FEI Nova NanoSem 450
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microscope operating at 40 kV and a Philips XL 30 microscope operating at 30 kV.
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High-resolution transmission electron microscopy (HRTEM) was recorded on an FEI Tecnai G2 F20 S-TWIN instrument. The BET surface areas and pore volumes of the catalysts were determined by N2 adsorption at –196 °C using a Micromeritics ASAP 2000 instrument. Bulk Si/Al ratios of the prepared ZSM-5 samples were measured by
720 equipment. 27
Al and
29
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inductively coupled plasma-optical emission spectrometry (ICP-OES) on an Agilent
Si MAS NMR measurements were performed on a Bruker DSX 27
Al MAS NMR spectra a resonance frequency of 104.3 MHz, a
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spectrometer. For
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recycle delay of 0.5 s, short 0.8 s pulses, a spectral width of 62.5 kHz and a spin rate of 12 kHz were applied, whereas for
29
Si MAS NMR spectra a resonance frequency
of 79.5 MHz, a recycle delay of 3 ms, 3.5 s pulses, a spectral width of 32.05 kHz and a spin rate of 4 kHz were applied. External Al(H2O)63+ and tetramethylsilane were used as references.
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Ga NMR was performed on the same spectrometer with a
resonance frequency of 122.1 MHz, a recycle delay of 0.1 s, a spectral width of 73.5 kHz and a spin rate of 10 kHz was applied. The chemical shifts were referenced to 1
ACCEPTED MANUSCRIPT M Ga(NO3)3 solution. DRIFTS data were recorded on an FTIR (Nicolet 6700) equipped with an MCT detector cooled by liquid N2. The DRIFTS cell was fitted with CaF2 windows and a
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heating cartridge. He gas was taken as the carrier gas and was dried by a drying tube before contacting with the samples. Samples were activated in the He atmosphere at 450 °C for 4 h, and cooled to 300 °C under flowing He (50 ml/min), and treated in
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flowing N2 (15 ml/min) for 20 min before the scanning. Spectra were collected at
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300 °C under flowing He in order to exclude the impact of water on the samples, with a resolution of 4 cm−1 and accumulation of 32 scans. Pyridine-adsorbed FT-IR (Py-IR) spectra were recorded on a Nicolet iS50 FTIR spectrometer. Self-supporting disks of the samples were set in an evacuable Pyrex IR cell and pre-activated at 550 °C for 1 h
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under a vacuum of 10-6 mbar. After cooling to 100 °C, pyridine vapor is introduced into the cell until the samples are saturated and re-evacuated before IR spectra were recorded. Further measurements were made after the sample had been evacuated at
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varied temperatures. Quantitative determination of Brönsted and Lewis acid sites was
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derived from the integrated areas of the IR band at ca. 1540 and 1450 cm−1, respectively.
2.3. Catalytic testing
Catalytic tests for ethane dehydrogenation in the presence of CO2 were performed at 650 °C in a fixed-bed flow microreactor at atmospheric pressure. The catalyst with the loading of 200 mg was pretreated at 650 °C for 2 h in nitrogen flow prior to the
ACCEPTED MANUSCRIPT reaction. The gas reactant contained 3 vol% ethane, 15 vol% carbon dioxide and balancing nitrogen. The total flow rate of the gas reactant is 30 ml/min. The hydrocarbon products were analyzed using an on-line GC equipped with a 6-m
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packed column of Porapak Q and a FID. The gas products were analyzed on-line by another GC equipped with a TCD and a carbon molecular sieve 601 column. The
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reaction data in the work were reproducible with a precision of less than 5%.
3.1. Support characterization
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3. Results and Discussion
The XRD patterns of the prepared Na-form ZSM-5 are illustrated in Fig. 1. All the
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three samples exhibit well crystallized MFI structure, with five typical characteristic reflections of 2θ = 8.0, 8.9, 23.1, 23.4, 24.0 [28]. No peaks corresponding to gallium oxides are found after Ga2O3 supported (not shown here), indicating that gallium
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species are well dispersed on these ZSM-5 supports or existing as tiny gallium oxide
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particles which are beyond XRD detection limitation. SEM and TEM images of ZSM-5 supports are presented in Fig. 2. All the samples
were well crystallized and exhibited a uniform crystallite size distribution (shown in Fig. 2 insert). The uniformity of the ZSM-5 crystals decreased with the increase of the crystallite size. The average crystallite size of the three samples is around 84 nm (ZSM-5-N), 377 nm (ZSM-5-S) and 10.3 µm (ZSM-5-L), respectively. The differences of crystallite sizes of the above samples were caused by different synthesis
ACCEPTED MANUSCRIPT parameters such as crystallization time, Si source, type of SDA, Si/SDA ratio and pH of the final gel, proved by various previous results [26, 29-30]. The composition and textural properties of the ZSM-5 supports are summarized in
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Table 1. The bulk Si/Al ratio of the crystallized ZSM-5 supports after calcination is close to that of the initial gel, showing that almost all the Al atoms are incorporated into the framework of ZSM-5 during the synthesis. The specific surface areas and
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pore volume are quite similar for ZSM-5-L and ZSM-5-S, while increase a little for
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ZSM-5-N because of small crystallite size. Specific surface areas of the ZSM-5 supports changed little after Ga2O3 supporting, indicating that the zeolitic channels were not blocked by the Ga2O3 species. 27
Al MAS NMR spectra were conducted to characterize the local coordination
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environment of aluminium atoms in ZSM-5 supports. Only one intense peak at around 55 ppm assigned to the ZSM-5 framework aluminium atoms in tetrahedral coordination appeared in the spectra (shown in Fig. 3). No discernable signal at 0 ppm,
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attributed to extra-framework aluminium atoms in octahedral coordination, was
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observed, suggesting that all the Al atoms in the gels were incorporated into the framework of MFI structure [31]. 29
Si MAS NMR spectra of the ZSM-5 supports were illustrated in Fig. 4, which can
be deconvoluted into multiple peaks by Gaussian fitting. According to previous studies, peaks with chemical shift below -110 ppm was usually assigned to siloxane Q4[(OSi)4] while peaks at around -106 ppm and -102 ppm was assigned to siloxane Q3[(AlO)Si(OSi)3] and Q3[HOSi(OSi)3], respectively [31-35]. Similarity in the
ACCEPTED MANUSCRIPT number, shifts and intensities of the peaks were observed for the three ZSM-5 samples, indicating the identical structure of these samples. However, the proportion of Q3[HOSi(OSi)3] of the samples was quite different, which decreased in the order of
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ZSM-5-N (3.8%) > ZSM-5-S (3.7%) > ZSM-5-L (2.1%), showing that amount of silanols on the surface of ZSM-5 supports changed with the crystallite sizes. ZSM-5-L has the least surface silanols, which may be due to the most regular framework and
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the largest crystalline size.
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A further study on the properties of hydroxyl groups on ZSM-5 surface was studied by the infrared spectroscopy, since silanol groups were reported to be important for the dispersion of chromium species onto ZSM-5 and SBA-15 supports as well as the dehydrogenation activities of the supported samples [36-38], and this may adapt to
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Ga2O3 based catalysts. The IR tests were conducted in a dry holder at 300 °C to avoid the influence of H2O in the atmosphere. The details in the hydroxyl stretching region (4000～3000 cm−1) are displayed in Fig. 5. Distinct peaks at ～3740 cm-1 and ～
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3500 cm-1 were presented in the ZSM-5-S spectrum. The peak at ～3740 cm-1
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involved with terminal silanol groups was usually located on the exterior of the crystal, while the broad peak around 3500 cm−1 was assigned to silanol nests [30, 37-39, 40-41]. For the ZSM-5-N sample, more intense peak at ～3740 cm-1 was observed with the absence of peak at ～3500 cm−1. Both of ～3740 cm−1 and ～ 3500 cm−1 peaks were found on ZSM-5-L support, despite of their weak intensities. It should be noted that the band at 3605 cm−1 associated with O-H vibration of strongly acidic bridging Si-OH-Al group in zeolites was not observed in any of the spectra,
ACCEPTED MANUSCRIPT indicating a thorough exchange of protons by Na cations. The relative intensities of the peak of terminal silanol decreased in the order of ZSM-5-N > ZSM-5-S > ZSM-5-L, which agreed with the literature that zeolites of larger crystal size had
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relatively smaller external surface areas and weaker band at 3740 cm−1 [39]. The intensity of peaks around 3500 cm−1 displayed a different sequence, i.e. ZSM-5-S > ZSM-5-L > ZSM-5-N, meaning that ZSM-5-S possessed the largest amount of silanol
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nests. In a word, silanol groups differ in both amount and types among the three
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ZSM-5 supports, which have no direct relationship with the zeolite particle sizes. The different synthesis parameters mentioned above probably account for this difference [26, 30, 42-45].
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3.2. State of Ga species
Ga MAS NMR chemical shift is known to reflect the local environment of Ga
species dispersed on the supports, which is quite helpful in understanding the nature 71
Ga MAS NMR spectra of the three supported catalysts were
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of active sites.
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presented in Fig. 6. Each spectrum shows the presence of three or four broad and overlapped resonances, with support-dependent intensities. For Ga/ZSM-5-S sample, the main peaks are centred at around 105 and 155 ppm, with two smaller peaks at about -60 and 35 ppm. Based on previous
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Ga NMR measurements of four- and
six-coordinate gallium and the correlation between
27
Al and
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Ga documented by
Bradley and Massiot, chemical shift above 100 ppm can be assigned to four-coordinate gallium species, below 0 ppm to six-coordinate gallium species, and
ACCEPTED MANUSCRIPT the peak in the middle to five-coordinate ones [46-49], which was affirmed by many other reports on
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Ga NMR spectra [50-52]. Similar results are obtained for
Ga/ZSM-5-L though the peak at 35 ppm is much weaker. However, there are only
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three peaks for Ga/ZSM-5-N without any presence of five-coordinate species. The relative quantification of gallium species was made by deconvolution. It can be concluded that the proportion of six-coordinate gallium species has the sequence of
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Ga/ZSM-5-L > Ga/ZSM-5-N > Ga/ZSM-5-S, while the proportion of five-coordinate
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ones decrease in the order of Ga/ZSM-5-S > Ga/ZSM-5-L > Ga/ZSM-5-N. Since six-coordinate gallium species are often involving with Ga2O3 particles, the above results indicate that the dispersion of GaOx species became worse in the order of ZSM-5-S, ZSM-5-N and ZSM-5-L.
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3.3. Acidity measurements
Surface acidity of the ZSM-5 based catalysts before and after GaOx supporting
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were investigated by Py-IR and the infrared spectra of absorbed pyridine at increasing
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temperatures were recorded. Characteristic bands of pyridine coordinatively bonded to Lewis acid sites at ca. 1450 cm−1, as well as the bands at ca. 1540 cm−1 assigned to protonation of pyridine by Brönsted acid sites, are observed on all the catalysts. The amount of Brönsted and Lewis acid sites derived from the integrated areas of the band was shown in Fig.7. Few Brönsted and Lewis acid sites were detected on all the ZSM-5 supports due to the replacement of H+ by Na+ as well as the absence of extra-framework aluminum species. Both of the Brönsted and Lewis acid sites
ACCEPTED MANUSCRIPT increased a lot after the introduction of GaOx species, indicating that the Ga species make a crucial contribution to the acidity of the catalysts. The amount of total Brönsted acid sites (data degassed at 150 °C) has the order of Ga/ZSM-5-S >>
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Ga/ZSM-5-L > Ga/ZSM-5-N while the amount of strong Brönsted acid sites (data degassed at 300 °C) has the order of Ga/ZSM-5-S > Ga/ZSM-5-L >> Ga/ZSM-5-N, indicating Ga/ZSM-5-S contains the largest amount of Brönsted acid sites as well as
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the highest strong Brönsted acid, Furthermore, despite of similar amount of Brönsted
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acid sites, Ga/ZSM-5-N possesses mainly weak ones while Ga/ZSM-5-L contains mainly strong ones. The situation is different in the case of Lewis acid sites. Either the amount of total Lewis acid sites (data degassed at 150 °C) or that of strong Lewis acid sites (data degassed at 300 °C) has the same sequence of Ga/ZSM-5-S >
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Ga/ZSM-5-N > Ga/ZSM-5-L. Again, Ga/ZSM-5-S contains the highest ones.
3.4. Catalytic performance
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Dehydrogenation of ethane over three Ga/ZSM-5 catalysts in the presence of CO2
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was studied at 650 °C and the results are given in Table 2 and Fig.8. Intra-particle diffusion limitation can be excluded according to the Weisz-Prater analysis (shown in the supporting information) [53]. The major product formed in the reaction is ethylene, and the minor products are methane and propylene. Aromatics were also formed over Ga/ZSM-5-S, though the selectivity was very low. The addition of CO2 has little effect on the activity. However, the selectivity as well as the stability is improved. This is probably because CO2 can promote desorption of C2H4 and inhibit its further
ACCEPTED MANUSCRIPT reaction; on the other hand, CO2 can be helpful in removing coke via Boudouard reaction [6]. Compared with the performance of Ga/HZSM-5 under the same reaction condition, Ga/ZSM-5-S showed a comparable activity and a higher selectivity of the
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targeted product C2H4 [17]. It should be noted that Ga/ZSM-5-S exhibits much higher activity as compared with Ga/ZSM-5-N and Ga/ZSM-5-L. The initial activity of Ga/ZSM-5-S is ca. 7.7 times as that of Ga/ZSM-5-N and ca. 11.5 times as that of
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Ga/ZSM-5-L. Ga/ZSM-5-S also shows a much higher steady activity than that of the
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other two catalysts despite of its initial deactivation. This is quite interesting since no such big difference is found either in composition or textural properties of these catalysts. Detailed investigation should be done.
Dehydrogenation of light alkane over Ga2O3-based catalysts has been suggested to
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proceed through a heterolytic dissociation reaction pathway [54], different from the redox mechanism over reducible metal oxide catalysts such as chromium and iron oxides. Ethane is thought to be first heterolytically dissociated on gallium oxide
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forming gallium hydride and alkoxide species (M can be Ga or Si atoms):
Gax+ O 2-
My+ + C 2H 6
H-
C 2H 5+
Gax+ O2-
My+
(1)
The alkoxides would decompose further to form the dehydrogenation products:
(2)
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H-
H+
Ga x+
O2-
My+
Gax+
O 2-
My+ + H2
(3)
Reaction (2) is the rate determined step in the ethylene formation. When both
exchange with H+ through surface migration reaction:
Ga x+
C 2H5+ O2-
My+ + H+ Z-
H-
H+
Ga x+
O2-
Ethylene is then resulted from the equilibrium:
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H+Z + C 2H4
C2H5+Z -
My+ + C2H5+Z
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H-
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Ga2O3 and H+ are present on the catalyst, the ethyl carbenium ion on Ga2O3 can
(4)
(5)
The conjugated effect of gallium oxide and proton is to replace the slow step (2) by the fast equilibrium (4) and (5).
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The above reaction mechanism gives a good explanation for the outstanding activity over Ga/ZSM-5-S catalyst since both Brönsted and Lewis acid sites are abundant on its surface, which can promote the dehydrogenation reaction through fast
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equilibrium (4) and (5). The amount of Lewis acid sites is also high on Ga/ZSM-5-N,
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indicating the enough active sites for dissociative adsorption of ethane (reaction 1). However, the rate determined step (reaction 2) cannot be replaced by the fast equilibrium (4) and (5) because of the lack of Brönsted acid sites (especially strong ones) on the surface, resulting in much lower activity for ethane dehydrogenation. On the contrary, the lowest activity over Ga/ZSM-5-L can be attributed to the lowest amount of Lewis acid sites since ethane can only be activated on these sites. From the above results it can be concluded that both Brönsted and Lewis acid sites on
ACCEPTED MANUSCRIPT Ga2O3-based catalysts play an important role in the dehydrogenation, although the latter plays a decisive one. It was reported that acid sites on Ga2O3 were the Lewis acid ones formed via
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abstraction of OH groups from gallium cations in tetrahedral positions [55]. And the dispersed 4-fold coordinated GaOx species were believed to be the active sites for Ga2O3 based catalysts in light alkane dehydrogenation [23, 56]. Considering that few
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Lewis acid sites exist in ZSM-5 support, the high amount of the Lewis acid sites in
This can be confirmed by
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our present work may come from better dispersion of Ga2O3 on ZSM-5 surface [24]. Ga NMR results shown in Fig. 6. The proportion of
six-coordinate gallium involving with Ga2O3 particles decreases in the sequence of Ga/ZSM-5-L > Ga/ZSM-5-N > Ga/ZSM-5-S, meaning dispersion of Ga2O3 decreases
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in an opposite order. This can also be confirmed by the results of Lewis acid sites obtained by Py-IR, since better dispersion of Ga2O3 results in higher amount of Lewis acid sites. Our previous results showed that lower Si/Al ratio of HZSM-5 support
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benefited in stronger interaction between the Ga2O3 and ZSM-5, leading to better
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Ga2O3 dispersion and more acid sites [17]. Ga/Al ratio was also reported to have an important effect on the dispersion of gallium oxide on HZSM-5 [24]. Since Si/Al and Ga/Al ratio of the three catalysts are similar here, factors other than Si/Al or Ga/Al ratio may account for the differences of Ga2O3 dispersion. In our previous work, silanol groups are found to play an important role in the dispersion of CrOx species on NaZSM-5 [38]. Terminal and nest silanol groups are both responsible for the introduction of chromates onto ZSM-5 supports, whereas the
ACCEPTED MANUSCRIPT latter is more efficient and helpful. Similar phenomenon may occur during dispersing Ga2O3 over NaZSM-5 supports. Well dispersion of Ga2O3 can be obtained on ZSM-5-N and ZSM-5-S with abundance of silanol groups, while poor dispersion on
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ZSM-5-L which was lack of silanol groups. Better dispersion obtained on ZSM-S than on ZSM-N is due to the higher amount of silanol nests. It is quite reasonable since silanol groups, especially nest silanol groups might interact with GaOx which offer
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help in the dispersion of Ga2O3 species. This can be confirmed by the change of IR
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spectra of the samples after gallium oxide supported shown in Fig. 5, whose intensities of silanol band reduced distinctly.
It is surprising that quite a lot of Brönsted acid sites are found on Ga/ZSM-5-S since the protons of ZSM-5 support are thoroughly exchange by Na cations, proved
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by the absence of 3610 cm−1 peaks on IR spectra and the absence of 1540 cm-1 bands of Py-IR spectra on ZSM-5-S supports. Meanwhile, no peak at about 3610-3625 cm−1 was found on the Ga/ZSM-5 catalysts, indicating that gallium atoms were not
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incorporated into the framework of ZSM-5 and the bridging hydroxyl Si-OH-Ga
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could not be the origin of the Brönsted acid sites [57,58]. Functional groups other than Si-OH-Al and Si-OH-Ga should be involving with these Brönsted acid sites. From IR measurements, it could be seen that Ga/ZSM-5-S with Brönsted acid sites had an extra broad peak at around 3500 cm−1 associated with nest silanol groups of ZSM-5-S support while this peak was not found on Ga/ZSM-5-N which is barren of Brönsted acid sites. The similar phenomenon was found for Ga/ZSM-5-L, where weak peak at 3500 cm−1 was observed and a few Brönsted acid sites were detected. Thus, it is
ACCEPTED MANUSCRIPT reasonable to deduce that instead of bridging hydroxyl Si-OH-Al on HZSM-5 based catalysts, nest silanol groups may act as Brönsted acid sites in ethane dehydrogenation reaction. Similar results were reported that nest silanol groups showed stronger acid
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strength than terminal Si-OH but weaker than that of Si-OH-Al and could work as Brönsted acid sites in esterification of HMF and Beckmann rearrangement reactions [59,60]. The intensity of nest silanol groups decreased obviously after Ga2O3 was
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introduced (see Fig. 5), indicating some GaOx species were anchored to the surface
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via reaction with nest silanol groups. The remaining nest silanol groups have a strong interaction with these GaOx species, resulting in a distortion of the traditional tetrahedral GaO4 species. This is the reason why 5-fold GaO5 species at chemical shift around 35 ppm could only be detected on Ga/ZSM-5-S and Ga/ZSM-5-L. Chemical 71
Ga NMR was also reported to be related to the
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shift of ca. 35 ppm in the
extra-lattice gallium species GaO(OH), which is quite similar with our present case [56,61,62]. This did not happen on the Ga/ZSM-5-N because of the absence of silanol
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nests. The proximity of Brönsted and Lewis acid sites was reported to enhance their
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acid strength [63], which also explained why the Brönsted acid strength of Ga/ZSM-5-S and Ga/ZSM-5-L was stronger and can have a conjugated effect with GaOx species in the ethane dehydrogenation.
4. Conclusions
Na-form ZSM-5 with different crystallite sizes were synthesized and taken as supports to prepare Ga/ZSM-5 catalysts for the dehydrogenation of ethane in the
ACCEPTED MANUSCRIPT presence of CO2. No obvious relationship was found between the dispersion of Ga species and zeolite particle sizes. Surface silanol groups on the surface play a decisive role in the dispersion of Ga species. Silanol nest is more efficient and helpful than
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terminal silanol. Submicron ZSM-5-S supported gallium catalyst exhibited the best catalytic performance. The excellent catalytic behaviour comes from the improved dispersion of GaOx species on the surface of ZSM-5-S, resulting in a larger number of
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Lewis acid sites for the dissociative adsorption of ethane. Abundant Brönsted acid
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sites were also found on Ga/ZSM-5-S, which facilitated the conjugated effect of protons and gallium oxide and promoted the dehydrogenation reaction. These Brönsted acids come from the nest silanol groups whose acid strength are enhanced by the interaction with GaOx species. Elucidating the role of silanol nests in
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Acknowledgements
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Ga/ZSM-5 will offer help in designing new bifunctional catalysts.
This work was supported by the National Key R&D Program of China
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(2017YFB0602204), the National Natural Science Foundation of China (91645201 and 21273043) and the Science & Technology Commission of Shanghai Municipality (13DZ2275200).
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ACCEPTED MANUSCRIPT Table 1 Textural properties of ZSM-5 supports samples
Si/Ala
Si/Alb
SBET
SBETc
Vtotal
Vmicro
m2/g
m2/g
ml/g
ml/g
100
126
396
388
0.31
0.22
ZSM-5-S
100
119
346
335
0.27
0.16
ZSM-5-L
100
106
341
329
0.26
0.23
a
from the synthesis gel from ICP c data obtained after Ga2O3 supported
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ZSM-5-N
ACCEPTED MANUSCRIPT Table 2 Reaction data for ZSM-5 supported catalysts in the DHE in the presence of CO2 a Samples
Con. b
Sel. (%) b
Yield b
TOF b,c
CH4
C 2H 4
C 3H 6
Aromatics
%
h−1
Ga/ZSM-5-N
3.3(2.4)
9.1(12.0)
90.9(88.0)
0.0(0.0)
0.0(0.0)
3.0(2.1)
0.7(0.5)
Ga/ZSM-5-S
25.3(14.6)
6.1(5.4)
91.7(92.0)
1.3(1.2)
0.8(1.4)
23.2(13.4)
5.3(3.1)
Ga/ZSM-5-L
2.2(2.0)
1.8(1.9)
97.5(98.0)
0.0(0.0)
0.0(0.0)
2.1(2.0)
0.5(0.4)
Ga/ZSM-5-Sd
24.9(12.2)
10.0(6.7)
84.2(92.7)
1.8(0.9)
4.0(0.0)
21.0(11.3)
5.2(2.6)
a
o
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%
Reaction conditions: 200 mg catalyst, 650 C, 3% C2H6/15% CO2 in N2, flow rate 30 ml/min. The values out and in the brackets are obtained at 10 min and 6 h, respectively. c Turnover frequency (TOF) was defined for the (reacted C2H6 molecules) / (Ga atoms × time) d The data was obtained in the absence of CO2 (3% C2H6 in N2)
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Figure Captions
Fig. 1. XRD patterns of various ZSM-5 samples.
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a) ZSM-5-N, b) ZSM-5-S, c) ZSM-5-L
Fig. 2. SEM (a,b,c), TEM images (d,e) and particle size distribution (insert) of
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a,d) ZSM-5-N, b,e) ZSM-5-S, c) ZSM-5-L
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ZSM-5.
Fig. 3. 27Al MAS NMR spectra of ZSM-5 samples. a) ZSM-5-N, b) ZSM-5-S, c) ZSM-5-L
Fig. 4. 29Si MAS NMR spectra of ZSM-5 supports.
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a) ZSM-5-N, b) ZSM-5-S, c) ZSM-5- L
Fig. 5. IR spectra of ZSM-5 supports and the corresponding supported catalysts.
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Ga/ZSM-5-L
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a) ZSM-5-N, b) ZSM-5-S, c) ZSM-5-L, a’) Ga/ZSM-5-N, b’) Ga/ZSM-5-S, c’)
Fig. 6. 71Ga NMR spectra of ZSM-5 supported catalysts. a) Ga/ZSM-5-N, b) Ga/ZSM-5-S, c) Ga/ZSM-5-L
Fig. 7. The number of A) BrÖnsted acid sites and B) Lewis acid sites on ZSM-5 and ZSM-5 supported catalysts. a) Ga/ZSM-5-N, b) Ga/ZSM-5-L, c) Ga/ZSM-5-S, a’) ZSM-5-N, b’) ZSM-5-L, c’)
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Fig. 8. C2H6 conversion A) and C2H4 selectivity B) as a function of time on stream for
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a) Ga/ZSM-5-N, b) Ga/ZSM-5-S, c) Ga/ZSM-5-L
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ZSM-5 supported catalysts.
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Intensity (a.u.)
Figure 1
b
a 20
30 2Theta (°)
40
50
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c
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20
40
60 80 100 Particle size (nm)
377.1±67.0
b
200
120
1 µm d
10.3±4.3
c
300 400 500 600 Particle size (nm)
2 µm
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200 nm
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e
5
500 nm
10 15 Particle size (µm)
20
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84.1±12.5
a
10 µm
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Intensity (a.u.)
Figure 3
c
a -50 0 50 100 27 Al Chemical Shift (ppm)
150
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-115 -110 -105 -100 29 Si Chemical Shift (ppm)
-95
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-101.7
-115 -110 -105 -100 29 Si Chemical Shift (ppm)
-95
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Intensity (a.u.)
b
-104.8
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-105.0 -102.5
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Intensity (a.u.)
c
-120
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-120
-105.9 -102.6
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Intensity (a.u.)
a
-115 -110 -105 -100 29 Si Chemical Shift (ppm)
-95
ACCEPTED MANUSCRIPT Figure 5
3740
3500
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b' a' c b a 3600 3200 Wavenumber (cm-1)
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Intensity (a.u.)
c'
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Figure 6
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c' b' a' 200 250 Temperature (°C)
c a
300
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b
200 250 Temperature (°C)
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c' a' b' 150
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b a
150
Amounts of L acid sites (a.u.)
A
c
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Amounts of B acid sites (a.u.)
Figure 7
300
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10 a c 0
100
1
2 3 4 5 Time on stream (h)
c b a
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80 60 40 1
2 3 4 5 Time on stream (h)
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C2H4 Selectivity (%)
A
b
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C2H6 Conversion (%)
Figure 8
6
ACCEPTED MANUSCRIPT Highlights Submicron NaZSM-5 supported Ga catalyst showed high dehydrogenation activity
Surface silanol on ZSM-5 is crucial for Ga2O3 dispersion
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Excellent activity comes from its abundant Lewis acids and Brönsted ones
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Silanol nests can serve as Brönsted acid sites promoting the dehydrogenation
S1387-1811(18)30226-9
DOI:
10.1016/j.micromeso.2018.04.041
Reference:
MICMAT 8896
To appear in:
Microporous and Mesoporous Materials
Received Date: 31 October 2017 Revised Date:
22 February 2018
Accepted Date: 25 April 2018
Please cite this article as: Y. Cheng, T. Lei, C. Miao, W. Hua, Y. Yue, Z. Gao, Ga2O3/NaZSM-5 for C2H6 dehydrogenation in the presence of CO2: Conjugated effect of silanol, Microporous and Mesoporous Materials (2018), doi: 10.1016/j.micromeso.2018.04.041. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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20
10 a c
0
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2 3 4 5 Time on stream (h)
6
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b
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Paper submitted for publication in Microporous and Mesoporous
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Materials
Ga2O3/NaZSM-5 for C2H6 dehydrogenation in the presence
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of CO2: conjugated effect of silanol
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Yanhu Cheng a, Tianqi Lei a, Changxi Miao b, Weiming Hua a, Yinghong Yue a,*, Zi Gao a
a
Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative
Shanghai Research Institute of Petrochemical Technology, Shanghai 201208, PR China
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b
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Materials, Fudan University, Shanghai 200433, PR China
* Corresponding author. Tel.: +86 21 65642409; Fax: +86 21 65641740. E-mail address: [email protected] (Y. Yue)
ACCEPTED MANUSCRIPT ABSTRACT GaOx catalysts supported on Na-form ZSM-5 with different crystallite sizes were studied in the ethane dehydrogenation reaction in the presence of CO2. Submicron
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zeolite ZSM-5-S supported catalyst showed the best activity among these catalysts, with an ethane conversion of 25% and ethylene selectivity of 92%. Py-IR tests revealed that Ga/ZSM-5-S contained abundant Lewis acids and Brönsted ones, which
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can have a synergistic effect in the dehydrogenation. Silanols on the surface play an
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important role in dispersing the active GaOx species. Silanol nests can not only be helpful in dispersing active species but also act as Brönsted acid sites which facilitate the conjugated effect of Brönsted acidic sites and Ga species, thus promotes the dehydrogenation reaction.
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Keywords: Ethane dehydrogenation; Ga2O3; Acidity; NaZSM-5; Silanol nests
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1. Introduction
Light alkenes like ethylene and propylene are the versatile commodity materials for
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producing polymers, alkene oxides and other derivates. Dehydrogenation of light alkanes into their corresponding alkenes has attracted much attention recently due to the rapidly growing demand for alkene and the shortage of petroleum resources. Pt/Al2O3 used in the UOP Oleflex process and Cr2O3/Al2O3 in the Lummus Catofin process are two commercial catalysts for light alkane dehydrogenation. The widespread application of above catalysts was limited by the drawback of high cost or environmental hostile. In addition, long-term stability is also a challenge for them.
ACCEPTED MANUSCRIPT Recently, CO2 has been applied as a soft oxidant for oxidative dehydrogenation of various alkanes (e.g. ethane, propane and ethylbenzene) and methane coupling for ethylene production [1-4]. Introduction of CO2 into dehydrogenation reaction system
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can not only shift the reaction equilibrium via reverse water-gas shift reaction but also enhance the catalyst stability by facilitating the desorption of alkene produced and removing coke via Boudouard reaction. However, the two commercial catalysts above
are
not
applicable for this
Efficient
and
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environmental-benign catalysts need to be developed.
new process.
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referred
Ga2O3 is an efficient catalyst for the dehydrogenation of ethane and propane in the presence of CO2 and the yield of ethylene was reported to be doubled in the presence of CO2 than in its absence [5]. Moreover, the ethylene yield increased from 3% to 25%
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with increasing Ga2O3 surface area from about 1 to 50 m2/g [6]. Similar results were also drawn by Wu et al [7]. Therefore, supports with high surface areas are desired to improve the turnover frequency of gallium species. The catalytic performance of
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Ga2O3 based catalysts supported on different carriers has been studied and TiO2 was
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found to be superior to the other oxide supports like ZrO2, ZnO, Al2O3 and SiO2 in the dehydrogenation of light alkanes [8-9]. However, ethylene (or propylene) yield decreased remarkably with reaction time over Ga2O3/TiO2 due to quick carbon deposition. Ga2O3-loaded ZSM-5 was also found to be quite effective in activation of C-H bond, but aromatics were readily formed simultaneously, resulting in low olefin selectivity [10-15]. Bell et al revealed that increasing proximity of Brönsted acid sites (origin from Si-OH-Al) would enhance the formation of aromatic species [16]. Our
ACCEPTED MANUSCRIPT previous works showed that higher Si/Al ratio of HZSM-5 supported Ga2O3 catalysts were more efficient in higher C2H4 selectivity as well as more resistant toward deactivation [17]. The superior performance was ascribed to the decrease in the
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number of acid sites with medium to strong strength, which resulted in the suppression of the side reactions such as oligomerization, cyclization and cracking. However, the initial ethylene yield reduced with further increasing Si/Al ratio since
SC
that active sites are decreased due to the weakening of interaction between Ga2O3
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species and the HZSM-5 support. It is evident that acidity is quite important in the dehydrogenation with CO2 over Ga2O3-loaded catalysts. Positive role in promoting dehydrogenation and negative role in forming byproducts should be balanced by tuning carefully the acidity of the catalysts [18-20]. Proton-poor gasilicalite catalyst
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obtained by solid-state ion exchange of reduced univalent Ga cations was found to be effective in the propane dehydrocyclization reaction [21-22]. A spinel-type gallia-alumina solid solution catalyst was also reported to be effective in the propane
EP
dehydrogenation reaction with CO2 [23]. It seemed that high dispersion of gallium
AC C
species which could increase the efficiency of gallium species played a crucial role in light alkane dehydrogenation reaction. Framework Al atoms of HZSM-5 was reported to be quite useful in dispersing Ga2O3 through their strong interaction with GaOx species, which would enhance the Lewis acidity of gallia species as well as their abilities in activation of C-H bond of propane [24]. Recently, Na-form ZSM-5 was found to be more helpful in dispersing Cr2O3 than HZSM-5, though it was barren of Al-OH-Si bridging hydroxyls [25]. The above results are sure to be helpful in
ACCEPTED MANUSCRIPT designing efficient Ga/MFI dehydrogenation catalysts. In the present work, Ga2O3 supported catalysts on Na-form ZSM-5 with different crystallite sizes and surface properties were prepared and characterized by SEM,
71
27
Al MAS NMR,
29
Si MAS NMR and
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HRTEM, N2 adsorption, XRD, IR, ICP-OES,
Ga NMR. The acidity of the supported catalysts was characterized by Py-IR. The
catalytic performance of these catalysts for dehydrogenation of ethane into ethylene in
SC
the presence of CO2 was compared and discussed in relation to their acidity as well as
2. Experimental
Nanosize
ZSM-5
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2.1. Catalyst preparation
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the state of GaOx species.
zeolite
was
prepared
in
two
steps.
First,
24.4
g
tetrapropylammonium hydroxide (TPAOH, 25% aqueous solution) was added to the
EP
mixture of 25.0 g tetraethylorthosilicate (TEOS) and 16.8 g water, and stirred at 80 °C
AC C
for 24 h. 0.1 g NaAlO2 in aqueous solution was then dropped into the mixture above. After stirred for another 1 h, the gel was transferred into an autoclave and crystallized for 24 h at 170 °C. The molar composition of the gel was 120SiO2: 0.6Al2O3: 30TPAOH: 1920H2O. The obtained product was then centrifuged, washed, dried at 110 °C overnight and then calcined in air at 600 °C for 6 h to remove the template. Submicron ZSM-5 zeolite was prepared following the procedures in the literature using TPAOH as the template [26]. Typically, 0.1 g NaAlO2 was dissolved in an
ACCEPTED MANUSCRIPT aqueous solution containing 9.8 g TPAOH, then 25.0 g TEOS was added into the above solution. The resultant mixture was stirred for 6 h at room temperature, followed by heating at 50 °C under stirring to evaporate ethanol formed. The molar
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composition of the clear gel was 120SiO2: 0.6Al2O3: 48TPAOH: 3600H2O. This gel was transferred into an autoclave and crystallized at 170 °C for 2 d. The obtained product was treated with the same method as nanosize ZSM-5 to remove the template.
SC
Microsize ZSM-5 zeolite was synthesized as follows: 43.6 g Na2SiO3•9H2O, 0.126
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g NaAlO2 and 16.0 g tetrapropylammonium bromide (TPABr) were dissolved in 65.0 g water [27]. Diluted H2SO4 aqueous solution was then added slowly under stirring until the pH value is 9.0. The molar composition of the mixture was 120SiO2: 0.6Al2O3: 48TPABr: 19200H2O. After being stirred at room temperature for 4 h, the
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gel was transferred into an autoclave and crystallized at 180 °C for 2 d. The obtained product was filtered, washed, dried at 110 °C overnight and then calcined in air at 600 °C for 6 h.
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The above ZSM-5 zeolites were turned into Na form by three consecutive ion
AC C
exchanges with 1 M NaNO3 aqueous solution at 80 °C for a total of 12 h. The obtained nanosize, submicrosize and microsize Na-form ZSM-5 zeolites are labelled as ZSM-5-N, ZSM-5-S and ZSM-5-L, respectively. The supported gallium oxide catalysts were prepared by impregnating an aqueous
solution of Ga(NO3)3•xH2O on ZSM-5 using an incipient wetness method. The impregnated samples were dried at 110 °C overnight and calcined in air at 650 °C for 6 h. The obtained catalysts are denoted as Ga/ZSM-5-N, Ga/ZSM-5-S and
ACCEPTED MANUSCRIPT Ga/ZSM-5-L, respectively. The loading amount of Ga2O3 in all catalysts was 5 wt %.
2.2. Catalyst characterization
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X-ray diffraction (XRD) patterns were recorded on a Persee XD-2 X-ray diffractometer using nickel-filtered Cu Kα radiation at 40 kV and 30 mA. Scanning electron microscopy (SEM) was recorded digitally on a FEI Nova NanoSem 450
SC
microscope operating at 40 kV and a Philips XL 30 microscope operating at 30 kV.
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High-resolution transmission electron microscopy (HRTEM) was recorded on an FEI Tecnai G2 F20 S-TWIN instrument. The BET surface areas and pore volumes of the catalysts were determined by N2 adsorption at –196 °C using a Micromeritics ASAP 2000 instrument. Bulk Si/Al ratios of the prepared ZSM-5 samples were measured by
720 equipment. 27
Al and
29
TE D
inductively coupled plasma-optical emission spectrometry (ICP-OES) on an Agilent
Si MAS NMR measurements were performed on a Bruker DSX 27
Al MAS NMR spectra a resonance frequency of 104.3 MHz, a
EP
spectrometer. For
AC C
recycle delay of 0.5 s, short 0.8 s pulses, a spectral width of 62.5 kHz and a spin rate of 12 kHz were applied, whereas for
29
Si MAS NMR spectra a resonance frequency
of 79.5 MHz, a recycle delay of 3 ms, 3.5 s pulses, a spectral width of 32.05 kHz and a spin rate of 4 kHz were applied. External Al(H2O)63+ and tetramethylsilane were used as references.
71
Ga NMR was performed on the same spectrometer with a
resonance frequency of 122.1 MHz, a recycle delay of 0.1 s, a spectral width of 73.5 kHz and a spin rate of 10 kHz was applied. The chemical shifts were referenced to 1
ACCEPTED MANUSCRIPT M Ga(NO3)3 solution. DRIFTS data were recorded on an FTIR (Nicolet 6700) equipped with an MCT detector cooled by liquid N2. The DRIFTS cell was fitted with CaF2 windows and a
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heating cartridge. He gas was taken as the carrier gas and was dried by a drying tube before contacting with the samples. Samples were activated in the He atmosphere at 450 °C for 4 h, and cooled to 300 °C under flowing He (50 ml/min), and treated in
SC
flowing N2 (15 ml/min) for 20 min before the scanning. Spectra were collected at
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300 °C under flowing He in order to exclude the impact of water on the samples, with a resolution of 4 cm−1 and accumulation of 32 scans. Pyridine-adsorbed FT-IR (Py-IR) spectra were recorded on a Nicolet iS50 FTIR spectrometer. Self-supporting disks of the samples were set in an evacuable Pyrex IR cell and pre-activated at 550 °C for 1 h
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under a vacuum of 10-6 mbar. After cooling to 100 °C, pyridine vapor is introduced into the cell until the samples are saturated and re-evacuated before IR spectra were recorded. Further measurements were made after the sample had been evacuated at
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varied temperatures. Quantitative determination of Brönsted and Lewis acid sites was
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derived from the integrated areas of the IR band at ca. 1540 and 1450 cm−1, respectively.
2.3. Catalytic testing
Catalytic tests for ethane dehydrogenation in the presence of CO2 were performed at 650 °C in a fixed-bed flow microreactor at atmospheric pressure. The catalyst with the loading of 200 mg was pretreated at 650 °C for 2 h in nitrogen flow prior to the
ACCEPTED MANUSCRIPT reaction. The gas reactant contained 3 vol% ethane, 15 vol% carbon dioxide and balancing nitrogen. The total flow rate of the gas reactant is 30 ml/min. The hydrocarbon products were analyzed using an on-line GC equipped with a 6-m
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packed column of Porapak Q and a FID. The gas products were analyzed on-line by another GC equipped with a TCD and a carbon molecular sieve 601 column. The
SC
reaction data in the work were reproducible with a precision of less than 5%.
3.1. Support characterization
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3. Results and Discussion
The XRD patterns of the prepared Na-form ZSM-5 are illustrated in Fig. 1. All the
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three samples exhibit well crystallized MFI structure, with five typical characteristic reflections of 2θ = 8.0, 8.9, 23.1, 23.4, 24.0 [28]. No peaks corresponding to gallium oxides are found after Ga2O3 supported (not shown here), indicating that gallium
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species are well dispersed on these ZSM-5 supports or existing as tiny gallium oxide
AC C
particles which are beyond XRD detection limitation. SEM and TEM images of ZSM-5 supports are presented in Fig. 2. All the samples
were well crystallized and exhibited a uniform crystallite size distribution (shown in Fig. 2 insert). The uniformity of the ZSM-5 crystals decreased with the increase of the crystallite size. The average crystallite size of the three samples is around 84 nm (ZSM-5-N), 377 nm (ZSM-5-S) and 10.3 µm (ZSM-5-L), respectively. The differences of crystallite sizes of the above samples were caused by different synthesis
ACCEPTED MANUSCRIPT parameters such as crystallization time, Si source, type of SDA, Si/SDA ratio and pH of the final gel, proved by various previous results [26, 29-30]. The composition and textural properties of the ZSM-5 supports are summarized in
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Table 1. The bulk Si/Al ratio of the crystallized ZSM-5 supports after calcination is close to that of the initial gel, showing that almost all the Al atoms are incorporated into the framework of ZSM-5 during the synthesis. The specific surface areas and
SC
pore volume are quite similar for ZSM-5-L and ZSM-5-S, while increase a little for
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ZSM-5-N because of small crystallite size. Specific surface areas of the ZSM-5 supports changed little after Ga2O3 supporting, indicating that the zeolitic channels were not blocked by the Ga2O3 species. 27
Al MAS NMR spectra were conducted to characterize the local coordination
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environment of aluminium atoms in ZSM-5 supports. Only one intense peak at around 55 ppm assigned to the ZSM-5 framework aluminium atoms in tetrahedral coordination appeared in the spectra (shown in Fig. 3). No discernable signal at 0 ppm,
EP
attributed to extra-framework aluminium atoms in octahedral coordination, was
AC C
observed, suggesting that all the Al atoms in the gels were incorporated into the framework of MFI structure [31]. 29
Si MAS NMR spectra of the ZSM-5 supports were illustrated in Fig. 4, which can
be deconvoluted into multiple peaks by Gaussian fitting. According to previous studies, peaks with chemical shift below -110 ppm was usually assigned to siloxane Q4[(OSi)4] while peaks at around -106 ppm and -102 ppm was assigned to siloxane Q3[(AlO)Si(OSi)3] and Q3[HOSi(OSi)3], respectively [31-35]. Similarity in the
ACCEPTED MANUSCRIPT number, shifts and intensities of the peaks were observed for the three ZSM-5 samples, indicating the identical structure of these samples. However, the proportion of Q3[HOSi(OSi)3] of the samples was quite different, which decreased in the order of
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ZSM-5-N (3.8%) > ZSM-5-S (3.7%) > ZSM-5-L (2.1%), showing that amount of silanols on the surface of ZSM-5 supports changed with the crystallite sizes. ZSM-5-L has the least surface silanols, which may be due to the most regular framework and
SC
the largest crystalline size.
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A further study on the properties of hydroxyl groups on ZSM-5 surface was studied by the infrared spectroscopy, since silanol groups were reported to be important for the dispersion of chromium species onto ZSM-5 and SBA-15 supports as well as the dehydrogenation activities of the supported samples [36-38], and this may adapt to
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Ga2O3 based catalysts. The IR tests were conducted in a dry holder at 300 °C to avoid the influence of H2O in the atmosphere. The details in the hydroxyl stretching region (4000～3000 cm−1) are displayed in Fig. 5. Distinct peaks at ～3740 cm-1 and ～
EP
3500 cm-1 were presented in the ZSM-5-S spectrum. The peak at ～3740 cm-1
AC C
involved with terminal silanol groups was usually located on the exterior of the crystal, while the broad peak around 3500 cm−1 was assigned to silanol nests [30, 37-39, 40-41]. For the ZSM-5-N sample, more intense peak at ～3740 cm-1 was observed with the absence of peak at ～3500 cm−1. Both of ～3740 cm−1 and ～ 3500 cm−1 peaks were found on ZSM-5-L support, despite of their weak intensities. It should be noted that the band at 3605 cm−1 associated with O-H vibration of strongly acidic bridging Si-OH-Al group in zeolites was not observed in any of the spectra,
ACCEPTED MANUSCRIPT indicating a thorough exchange of protons by Na cations. The relative intensities of the peak of terminal silanol decreased in the order of ZSM-5-N > ZSM-5-S > ZSM-5-L, which agreed with the literature that zeolites of larger crystal size had
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relatively smaller external surface areas and weaker band at 3740 cm−1 [39]. The intensity of peaks around 3500 cm−1 displayed a different sequence, i.e. ZSM-5-S > ZSM-5-L > ZSM-5-N, meaning that ZSM-5-S possessed the largest amount of silanol
SC
nests. In a word, silanol groups differ in both amount and types among the three
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ZSM-5 supports, which have no direct relationship with the zeolite particle sizes. The different synthesis parameters mentioned above probably account for this difference [26, 30, 42-45].
71
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3.2. State of Ga species
Ga MAS NMR chemical shift is known to reflect the local environment of Ga
species dispersed on the supports, which is quite helpful in understanding the nature 71
Ga MAS NMR spectra of the three supported catalysts were
EP
of active sites.
AC C
presented in Fig. 6. Each spectrum shows the presence of three or four broad and overlapped resonances, with support-dependent intensities. For Ga/ZSM-5-S sample, the main peaks are centred at around 105 and 155 ppm, with two smaller peaks at about -60 and 35 ppm. Based on previous
71
Ga NMR measurements of four- and
six-coordinate gallium and the correlation between
27
Al and
71
Ga documented by
Bradley and Massiot, chemical shift above 100 ppm can be assigned to four-coordinate gallium species, below 0 ppm to six-coordinate gallium species, and
ACCEPTED MANUSCRIPT the peak in the middle to five-coordinate ones [46-49], which was affirmed by many other reports on
71
Ga NMR spectra [50-52]. Similar results are obtained for
Ga/ZSM-5-L though the peak at 35 ppm is much weaker. However, there are only
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three peaks for Ga/ZSM-5-N without any presence of five-coordinate species. The relative quantification of gallium species was made by deconvolution. It can be concluded that the proportion of six-coordinate gallium species has the sequence of
SC
Ga/ZSM-5-L > Ga/ZSM-5-N > Ga/ZSM-5-S, while the proportion of five-coordinate
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ones decrease in the order of Ga/ZSM-5-S > Ga/ZSM-5-L > Ga/ZSM-5-N. Since six-coordinate gallium species are often involving with Ga2O3 particles, the above results indicate that the dispersion of GaOx species became worse in the order of ZSM-5-S, ZSM-5-N and ZSM-5-L.
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3.3. Acidity measurements
Surface acidity of the ZSM-5 based catalysts before and after GaOx supporting
EP
were investigated by Py-IR and the infrared spectra of absorbed pyridine at increasing
AC C
temperatures were recorded. Characteristic bands of pyridine coordinatively bonded to Lewis acid sites at ca. 1450 cm−1, as well as the bands at ca. 1540 cm−1 assigned to protonation of pyridine by Brönsted acid sites, are observed on all the catalysts. The amount of Brönsted and Lewis acid sites derived from the integrated areas of the band was shown in Fig.7. Few Brönsted and Lewis acid sites were detected on all the ZSM-5 supports due to the replacement of H+ by Na+ as well as the absence of extra-framework aluminum species. Both of the Brönsted and Lewis acid sites
ACCEPTED MANUSCRIPT increased a lot after the introduction of GaOx species, indicating that the Ga species make a crucial contribution to the acidity of the catalysts. The amount of total Brönsted acid sites (data degassed at 150 °C) has the order of Ga/ZSM-5-S >>
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Ga/ZSM-5-L > Ga/ZSM-5-N while the amount of strong Brönsted acid sites (data degassed at 300 °C) has the order of Ga/ZSM-5-S > Ga/ZSM-5-L >> Ga/ZSM-5-N, indicating Ga/ZSM-5-S contains the largest amount of Brönsted acid sites as well as
SC
the highest strong Brönsted acid, Furthermore, despite of similar amount of Brönsted
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acid sites, Ga/ZSM-5-N possesses mainly weak ones while Ga/ZSM-5-L contains mainly strong ones. The situation is different in the case of Lewis acid sites. Either the amount of total Lewis acid sites (data degassed at 150 °C) or that of strong Lewis acid sites (data degassed at 300 °C) has the same sequence of Ga/ZSM-5-S >
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Ga/ZSM-5-N > Ga/ZSM-5-L. Again, Ga/ZSM-5-S contains the highest ones.
3.4. Catalytic performance
EP
Dehydrogenation of ethane over three Ga/ZSM-5 catalysts in the presence of CO2
AC C
was studied at 650 °C and the results are given in Table 2 and Fig.8. Intra-particle diffusion limitation can be excluded according to the Weisz-Prater analysis (shown in the supporting information) [53]. The major product formed in the reaction is ethylene, and the minor products are methane and propylene. Aromatics were also formed over Ga/ZSM-5-S, though the selectivity was very low. The addition of CO2 has little effect on the activity. However, the selectivity as well as the stability is improved. This is probably because CO2 can promote desorption of C2H4 and inhibit its further
ACCEPTED MANUSCRIPT reaction; on the other hand, CO2 can be helpful in removing coke via Boudouard reaction [6]. Compared with the performance of Ga/HZSM-5 under the same reaction condition, Ga/ZSM-5-S showed a comparable activity and a higher selectivity of the
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targeted product C2H4 [17]. It should be noted that Ga/ZSM-5-S exhibits much higher activity as compared with Ga/ZSM-5-N and Ga/ZSM-5-L. The initial activity of Ga/ZSM-5-S is ca. 7.7 times as that of Ga/ZSM-5-N and ca. 11.5 times as that of
SC
Ga/ZSM-5-L. Ga/ZSM-5-S also shows a much higher steady activity than that of the
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other two catalysts despite of its initial deactivation. This is quite interesting since no such big difference is found either in composition or textural properties of these catalysts. Detailed investigation should be done.
Dehydrogenation of light alkane over Ga2O3-based catalysts has been suggested to
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proceed through a heterolytic dissociation reaction pathway [54], different from the redox mechanism over reducible metal oxide catalysts such as chromium and iron oxides. Ethane is thought to be first heterolytically dissociated on gallium oxide
AC C
EP
forming gallium hydride and alkoxide species (M can be Ga or Si atoms):
Gax+ O 2-
My+ + C 2H 6
H-
C 2H 5+
Gax+ O2-
My+
(1)
The alkoxides would decompose further to form the dehydrogenation products:
(2)
ACCEPTED MANUSCRIPT
H-
H+
Ga x+
O2-
My+
Gax+
O 2-
My+ + H2
(3)
Reaction (2) is the rate determined step in the ethylene formation. When both
exchange with H+ through surface migration reaction:
Ga x+
C 2H5+ O2-
My+ + H+ Z-
H-
H+
Ga x+
O2-
Ethylene is then resulted from the equilibrium:
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H+Z + C 2H4
C2H5+Z -
My+ + C2H5+Z
SC
H-
RI PT
Ga2O3 and H+ are present on the catalyst, the ethyl carbenium ion on Ga2O3 can
(4)
(5)
The conjugated effect of gallium oxide and proton is to replace the slow step (2) by the fast equilibrium (4) and (5).
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The above reaction mechanism gives a good explanation for the outstanding activity over Ga/ZSM-5-S catalyst since both Brönsted and Lewis acid sites are abundant on its surface, which can promote the dehydrogenation reaction through fast
EP
equilibrium (4) and (5). The amount of Lewis acid sites is also high on Ga/ZSM-5-N,
AC C
indicating the enough active sites for dissociative adsorption of ethane (reaction 1). However, the rate determined step (reaction 2) cannot be replaced by the fast equilibrium (4) and (5) because of the lack of Brönsted acid sites (especially strong ones) on the surface, resulting in much lower activity for ethane dehydrogenation. On the contrary, the lowest activity over Ga/ZSM-5-L can be attributed to the lowest amount of Lewis acid sites since ethane can only be activated on these sites. From the above results it can be concluded that both Brönsted and Lewis acid sites on
ACCEPTED MANUSCRIPT Ga2O3-based catalysts play an important role in the dehydrogenation, although the latter plays a decisive one. It was reported that acid sites on Ga2O3 were the Lewis acid ones formed via
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abstraction of OH groups from gallium cations in tetrahedral positions [55]. And the dispersed 4-fold coordinated GaOx species were believed to be the active sites for Ga2O3 based catalysts in light alkane dehydrogenation [23, 56]. Considering that few
SC
Lewis acid sites exist in ZSM-5 support, the high amount of the Lewis acid sites in
This can be confirmed by
71
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our present work may come from better dispersion of Ga2O3 on ZSM-5 surface [24]. Ga NMR results shown in Fig. 6. The proportion of
six-coordinate gallium involving with Ga2O3 particles decreases in the sequence of Ga/ZSM-5-L > Ga/ZSM-5-N > Ga/ZSM-5-S, meaning dispersion of Ga2O3 decreases
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in an opposite order. This can also be confirmed by the results of Lewis acid sites obtained by Py-IR, since better dispersion of Ga2O3 results in higher amount of Lewis acid sites. Our previous results showed that lower Si/Al ratio of HZSM-5 support
EP
benefited in stronger interaction between the Ga2O3 and ZSM-5, leading to better
AC C
Ga2O3 dispersion and more acid sites [17]. Ga/Al ratio was also reported to have an important effect on the dispersion of gallium oxide on HZSM-5 [24]. Since Si/Al and Ga/Al ratio of the three catalysts are similar here, factors other than Si/Al or Ga/Al ratio may account for the differences of Ga2O3 dispersion. In our previous work, silanol groups are found to play an important role in the dispersion of CrOx species on NaZSM-5 [38]. Terminal and nest silanol groups are both responsible for the introduction of chromates onto ZSM-5 supports, whereas the
ACCEPTED MANUSCRIPT latter is more efficient and helpful. Similar phenomenon may occur during dispersing Ga2O3 over NaZSM-5 supports. Well dispersion of Ga2O3 can be obtained on ZSM-5-N and ZSM-5-S with abundance of silanol groups, while poor dispersion on
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ZSM-5-L which was lack of silanol groups. Better dispersion obtained on ZSM-S than on ZSM-N is due to the higher amount of silanol nests. It is quite reasonable since silanol groups, especially nest silanol groups might interact with GaOx which offer
SC
help in the dispersion of Ga2O3 species. This can be confirmed by the change of IR
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spectra of the samples after gallium oxide supported shown in Fig. 5, whose intensities of silanol band reduced distinctly.
It is surprising that quite a lot of Brönsted acid sites are found on Ga/ZSM-5-S since the protons of ZSM-5 support are thoroughly exchange by Na cations, proved
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by the absence of 3610 cm−1 peaks on IR spectra and the absence of 1540 cm-1 bands of Py-IR spectra on ZSM-5-S supports. Meanwhile, no peak at about 3610-3625 cm−1 was found on the Ga/ZSM-5 catalysts, indicating that gallium atoms were not
EP
incorporated into the framework of ZSM-5 and the bridging hydroxyl Si-OH-Ga
AC C
could not be the origin of the Brönsted acid sites [57,58]. Functional groups other than Si-OH-Al and Si-OH-Ga should be involving with these Brönsted acid sites. From IR measurements, it could be seen that Ga/ZSM-5-S with Brönsted acid sites had an extra broad peak at around 3500 cm−1 associated with nest silanol groups of ZSM-5-S support while this peak was not found on Ga/ZSM-5-N which is barren of Brönsted acid sites. The similar phenomenon was found for Ga/ZSM-5-L, where weak peak at 3500 cm−1 was observed and a few Brönsted acid sites were detected. Thus, it is
ACCEPTED MANUSCRIPT reasonable to deduce that instead of bridging hydroxyl Si-OH-Al on HZSM-5 based catalysts, nest silanol groups may act as Brönsted acid sites in ethane dehydrogenation reaction. Similar results were reported that nest silanol groups showed stronger acid
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strength than terminal Si-OH but weaker than that of Si-OH-Al and could work as Brönsted acid sites in esterification of HMF and Beckmann rearrangement reactions [59,60]. The intensity of nest silanol groups decreased obviously after Ga2O3 was
SC
introduced (see Fig. 5), indicating some GaOx species were anchored to the surface
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via reaction with nest silanol groups. The remaining nest silanol groups have a strong interaction with these GaOx species, resulting in a distortion of the traditional tetrahedral GaO4 species. This is the reason why 5-fold GaO5 species at chemical shift around 35 ppm could only be detected on Ga/ZSM-5-S and Ga/ZSM-5-L. Chemical 71
Ga NMR was also reported to be related to the
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shift of ca. 35 ppm in the
extra-lattice gallium species GaO(OH), which is quite similar with our present case [56,61,62]. This did not happen on the Ga/ZSM-5-N because of the absence of silanol
EP
nests. The proximity of Brönsted and Lewis acid sites was reported to enhance their
AC C
acid strength [63], which also explained why the Brönsted acid strength of Ga/ZSM-5-S and Ga/ZSM-5-L was stronger and can have a conjugated effect with GaOx species in the ethane dehydrogenation.
4. Conclusions
Na-form ZSM-5 with different crystallite sizes were synthesized and taken as supports to prepare Ga/ZSM-5 catalysts for the dehydrogenation of ethane in the
ACCEPTED MANUSCRIPT presence of CO2. No obvious relationship was found between the dispersion of Ga species and zeolite particle sizes. Surface silanol groups on the surface play a decisive role in the dispersion of Ga species. Silanol nest is more efficient and helpful than
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terminal silanol. Submicron ZSM-5-S supported gallium catalyst exhibited the best catalytic performance. The excellent catalytic behaviour comes from the improved dispersion of GaOx species on the surface of ZSM-5-S, resulting in a larger number of
SC
Lewis acid sites for the dissociative adsorption of ethane. Abundant Brönsted acid
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sites were also found on Ga/ZSM-5-S, which facilitated the conjugated effect of protons and gallium oxide and promoted the dehydrogenation reaction. These Brönsted acids come from the nest silanol groups whose acid strength are enhanced by the interaction with GaOx species. Elucidating the role of silanol nests in
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Acknowledgements
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Ga/ZSM-5 will offer help in designing new bifunctional catalysts.
This work was supported by the National Key R&D Program of China
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(2017YFB0602204), the National Natural Science Foundation of China (91645201 and 21273043) and the Science & Technology Commission of Shanghai Municipality (13DZ2275200).
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ACCEPTED MANUSCRIPT Table 1 Textural properties of ZSM-5 supports samples
Si/Ala
Si/Alb
SBET
SBETc
Vtotal
Vmicro
m2/g
m2/g
ml/g
ml/g
100
126
396
388
0.31
0.22
ZSM-5-S
100
119
346
335
0.27
0.16
ZSM-5-L
100
106
341
329
0.26
0.23
a
from the synthesis gel from ICP c data obtained after Ga2O3 supported
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ZSM-5-N
ACCEPTED MANUSCRIPT Table 2 Reaction data for ZSM-5 supported catalysts in the DHE in the presence of CO2 a Samples
Con. b
Sel. (%) b
Yield b
TOF b,c
CH4
C 2H 4
C 3H 6
Aromatics
%
h−1
Ga/ZSM-5-N
3.3(2.4)
9.1(12.0)
90.9(88.0)
0.0(0.0)
0.0(0.0)
3.0(2.1)
0.7(0.5)
Ga/ZSM-5-S
25.3(14.6)
6.1(5.4)
91.7(92.0)
1.3(1.2)
0.8(1.4)
23.2(13.4)
5.3(3.1)
Ga/ZSM-5-L
2.2(2.0)
1.8(1.9)
97.5(98.0)
0.0(0.0)
0.0(0.0)
2.1(2.0)
0.5(0.4)
Ga/ZSM-5-Sd
24.9(12.2)
10.0(6.7)
84.2(92.7)
1.8(0.9)
4.0(0.0)
21.0(11.3)
5.2(2.6)
a
o
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Reaction conditions: 200 mg catalyst, 650 C, 3% C2H6/15% CO2 in N2, flow rate 30 ml/min. The values out and in the brackets are obtained at 10 min and 6 h, respectively. c Turnover frequency (TOF) was defined for the (reacted C2H6 molecules) / (Ga atoms × time) d The data was obtained in the absence of CO2 (3% C2H6 in N2)
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Figure Captions
Fig. 1. XRD patterns of various ZSM-5 samples.
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a) ZSM-5-N, b) ZSM-5-S, c) ZSM-5-L
Fig. 2. SEM (a,b,c), TEM images (d,e) and particle size distribution (insert) of
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a,d) ZSM-5-N, b,e) ZSM-5-S, c) ZSM-5-L
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ZSM-5.
Fig. 3. 27Al MAS NMR spectra of ZSM-5 samples. a) ZSM-5-N, b) ZSM-5-S, c) ZSM-5-L
Fig. 4. 29Si MAS NMR spectra of ZSM-5 supports.
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Fig. 5. IR spectra of ZSM-5 supports and the corresponding supported catalysts.
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a) ZSM-5-N, b) ZSM-5-S, c) ZSM-5-L, a’) Ga/ZSM-5-N, b’) Ga/ZSM-5-S, c’)
Fig. 6. 71Ga NMR spectra of ZSM-5 supported catalysts. a) Ga/ZSM-5-N, b) Ga/ZSM-5-S, c) Ga/ZSM-5-L
Fig. 7. The number of A) BrÖnsted acid sites and B) Lewis acid sites on ZSM-5 and ZSM-5 supported catalysts. a) Ga/ZSM-5-N, b) Ga/ZSM-5-L, c) Ga/ZSM-5-S, a’) ZSM-5-N, b’) ZSM-5-L, c’)
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Fig. 8. C2H6 conversion A) and C2H4 selectivity B) as a function of time on stream for
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a) Ga/ZSM-5-N, b) Ga/ZSM-5-S, c) Ga/ZSM-5-L
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ZSM-5 supported catalysts.
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Intensity (a.u.)
Figure 1
b
a 20
30 2Theta (°)
40
50
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20
40
60 80 100 Particle size (nm)
377.1±67.0
b
200
120
1 µm d
10.3±4.3
c
300 400 500 600 Particle size (nm)
2 µm
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500 nm
10 15 Particle size (µm)
20
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84.1±12.5
a
10 µm
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Intensity (a.u.)
Figure 3
c
a -50 0 50 100 27 Al Chemical Shift (ppm)
150
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-115 -110 -105 -100 29 Si Chemical Shift (ppm)
-95
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c
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Intensity (a.u.)
a
-115 -110 -105 -100 29 Si Chemical Shift (ppm)
-95
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3740
3500
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c'
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c' b' a' 200 250 Temperature (°C)
c a
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Amounts of L acid sites (a.u.)
A
c
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Amounts of B acid sites (a.u.)
Figure 7
300
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100
1
2 3 4 5 Time on stream (h)
c b a
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C2H4 Selectivity (%)
A
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C2H6 Conversion (%)
Figure 8
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ACCEPTED MANUSCRIPT Highlights Submicron NaZSM-5 supported Ga catalyst showed high dehydrogenation activity
Surface silanol on ZSM-5 is crucial for Ga2O3 dispersion
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Excellent activity comes from its abundant Lewis acids and Brönsted ones
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Silanol nests can serve as Brönsted acid sites promoting the dehydrogenation