Effect of the Si/Al ratio in Ga/mesoporous HZSM-5 on the production of benzene, toluene, and xylene via coaromatization of methane and propane

Min Yeong Gim a, Changyeol Song a, Yong Hyun Lim a, Kwan-Young Lee b and Do Heui Kim *a
aSchool of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, South Korea. E-mail: dohkim@snu.ac.kr
bDepartment of Chemical and Biological Engineering, Korea University, 145 Annam-ro, Sungbuk-ku, Seoul 02841, South Korea

Received 13th August 2019 , Accepted 3rd October 2019

First published on 4th October 2019


In this work, a series of GaOy supported on mesoporous HZSM-5 (GaOy/meso-XHZSM-5; Si/Al (X), X = 10, 20, 30, and 40) catalysts with different Si/Al molar ratios were prepared for use in the coaromatization of methane and propane. The coaromatization of methane and propane was conducted at 550 °C with a molar ratio of 10[thin space (1/6-em)]:[thin space (1/6-em)]1. Gallium oxide (2 wt%) supported on mesoporous HZSM-5 with a Si/Al molar ratio of 20 showed the highest catalytic performance, with an initial BTX yield of 14.2% and a final BTX yield of 13.8% after 6 h of reaction. The catalytic performances of the catalysts showed volcano-shaped trends depending on the Si/Al molar ratio. All of the catalysts showed a slight decrease in activity with time, although the degree of deactivation increased with increasing Si/Al ratio. The poisoning of acid sites by coke deposition is suggested to be the main reason for deactivation. The effects of the Si/Al molar ratio on the physicochemical properties and catalytic performance of the catalysts were investigated. As the molar ratio of Si/Al increased, the BET surface area and pore volume of the catalysts increased, while their total acidity decreased. Hence, it is concluded that both the acid properties and textural properties of the catalysts play a crucial role in determining the catalytic performance in the coaromatization of methane and propane. In summary, GaOy/meso-20HZSM-5 with optimum physicochemical properties demonstrated the best catalytic performance in the coaromatization of methane and propane.


1. Introduction

As environmental issues have recently come to attract worldwide attention, many ways to lower dependence on petroleum and to use other sustainable and low-priced energy resources have been proposed.1 One such proposal for efficient energy utilization involves the use of methane as an energy resource and raw material. Methane, which is an abundant resource as a main component of shale gas and natural gas, is considered to be the best available feedstock to replace petroleum.2 However, since methane has mostly been used to produce heat and electricity through combustion, valuable chemical production from methane has been extremely low. Hence, many studies have attempted to convert methane into high-value products.3–7 Currently, indirect processes such as multi-step syngas-based processes are used to synthesize long-chain hydrocarbons or methanol from methane.8–10 Indirect processes have several drawbacks, such as low efficiency of the multi-step reactions and a large amount of energy consumption. To overcome these disadvantages, direct processes such as methane dehydroaromatization (MDA) and oxidative coupling of methane (OCM) have been widely studied.11–14 In particular, the non-oxidative MDA reaction has attracted more attention because of the increasing demand for benzene, toluene, and xylene (BTX).

The MDA reaction is an endothermic reaction that proceeds at high temperature (>700 °C) due to the difficulty associated with the activation of the C–H bond in the CH4 molecule.15 The reaction is also vulnerable to carbon deposition due to the fact that it is operated under reducing conditions at high temperature.15,16 It is unattractive in terms of energy consumption or reaction efficiency. Therefore, light paraffin was introduced as a co-feedstock in order to overcome the thermodynamic limitation. It has been reported that image file: c9cy01619h-t1.tif for the formation of benzene by the direct conversion of methane (6CH4 → C6H6 + 9H2) is 532.2 kJ mol−1, while that for coaromatization of methane and n-pentane (CH4 + C5H12 → C6H6 + 5H2) is 304.2 kJ mol−1. Furthermore, image file: c9cy01619h-t2.tif for the production of toluene by the coaromatization of methane and propane (CH4 + 2C3H8 → C6H5CH3 + 6H2) is 332.6 kJ mol−1.17 Based on the thermodynamic parameters, the activation of the C–H bond of methane is easier in the presence of light hydrocarbon. Previous studies have reported that the heat of reaction of methane aromatization decreases with the addition of light hydrocarbon (both alkanes and alkenes in the feed).17–19 In other words, methane activation with a co-reactant is more advantageous for an efficient reaction. In this respect, MDA with co-reactants (coaromatization) has attracted much attention as an efficient process for BTX production from methane.

The most widely studied catalysts for coaromatization are transition metal oxide catalysts supported on zeolite.20–22 A reaction mechanism of coaromatization involving dual-active sites has been widely proposed where metal sites activate reactants to form intermediates such as C2H4, C3H6, carbenium ions and radicals, and the acid sites in the pores of zeolite promote the oligomerization and cyclization of intermediates to BTX.1,17,23–25 From a thermodynamic point of view, reaction intermediates can be more easily converted from propane than methane.26 As a result, since additional reaction intermediates supplied from propane can react with methane to form activated feedstock, it has been shown that methane conversion can be increased when propane is included in the coaromatization reaction.27

For the coaromatization of methane and propane, various transition metal oxides have been investigated, including Cr, Ni, Mo, Zn, and Ga oxides.23,28–31 In particular, Ga and Zn oxide catalysts have been mainly used in the coaromatization reaction. It has been reported that both Ga and Zn species are suitable for the activation of methane and propane.24,32 However, Zn oxide catalysts supported on zeolite lack catalytic stability due to their rapid deactivation.33 Therefore, Ga-based zeolites are considered to be more favorable for coaromatization. Among the various types of zeolites, HZSM-5 is an MFI-structured microporous crystalline zeolite that is widely used in aromatization reactions due to its unique shape selectivity. The main weakness of HZSM-5 is the small sizes of its channels (<0.8 nm) and cavities (typically <1.5 nm). These characteristic structures cause a diffusion limitation of reactants and products in catalytic reaction systems.34–37 In order to overcome these drawbacks, previous studies have used mesoporous zeolite in the coaromatization reaction.38,39 The initial BTX yield of Ga oxide supported on mesoporous HZSM-5 (GaOy/meso-HZSM-5, 11.2%) has been shown to be higher than that of Ga oxide supported on microporous HZSM-5 (GaOy/micro-HZSM-5, 8.9%). After 6 h of the reaction, the BTX yields of GaOy/meso-HZSM-5 and GaOy/micro-HZSM-5 were measured to be 10.5% and 6.9%, respectively. As a result, the introduction of mesoporous HZSM-5 was proven to enhance the catalytic reaction system by decreasing the diffusion resistance and increasing the accessibility of channels. It is known that acid properties can be changed as a function of the molar ratio of Si/Al in HZSM-5.40 According to the coaromatization reaction mechanism, acid properties are crucial factors in determining catalytic performance. In this respect, a study dealing with the optimization of the Si/Al ratio on the catalytic performance would be valuable.

Therefore, in this study, mesoporous HZSM-5 (meso-HZSM-5) zeolites with various Si/Al molar ratios were synthesized by applying the hard-templating method, which uses a carbon template (BP-2000, Cabot Corp.) to introduce mesoporosity into conventional HZSM-5 under hydrothermal treatment. A series of meso-XHZSM-5 with different Si/Al molar ratios (X) was prepared by varying the compositions of Si and Al. A wetness impregnation method was used to prepare Ga oxide supported on meso-XHZSM-5 (GaOy/meso-XHZSM-5) catalysts. The catalysts were then applied to the coaromatization of methane and propane for the production of BTX. The textural and morphological properties of the catalysts were investigated by N2 adsorption–desorption and SEM analyses, respectively. Then, XRD, 27Al NMR, 29Si NMR, H2-TPR, and NH3-TPD analyses were conducted to further elucidate the physicochemical properties of the catalysts.

2. Experimental

Materials

The raw materials used for mesoporous HZSM-5 synthesis were as follows: sodium hydroxide (NaOH, 98.0%, Samchun Chem.), tetraethyl orthosilicate (TEOS, 98.0%, reagent grade, Sigma Aldrich) as a silicon source, sodium aluminate (NaAlO2, Na2O 35.0%, Al2O3 52.0%, Junsei) as an aluminum source, tetrapropylammonium bromide (TPABr, 98% Sigma-Aldrich) as a structure directing agent, a carbon template (BP-2000, Cabot Corp.) which has an average particle size of 12 nm for mesopore formation, ammonium nitrate (NH4NO3, 99.0%, Junsei), and deionized water. Gallium(III) nitrate hydrate (99.0%, Sigma-Aldrich) was used as a gallium precursor.

Preparation of catalysts

A series of mesoporous HZSM-5 (meso-HZSM-5) with varying Si/Al molar ratios was prepared using the hard-templating method under hydrothermal conditions.38 A Ga oxide supported on mesoporous HZSM-5 (GaOy/meso-HZSM-5) catalyst was then synthesized using a wetness impregnation method. The typical synthesis methods of GaOy/meso-HZSM-5 are described in the following: NaOH (1.9 g) was dissolved in de-ionized water (162 ml) at room temperature to achieve an appropriate pH value. Next, appropriate amounts of NaAlO2 and TPABr (2.04 g) were added into the prepared solution under vigorous stirring for 1 h. Then, TEOS (57.3 ml) was added into the solution, and the mixtures were continuously stirred at room temperature for 3 h. After stirring for 3 h, a calculated amount of carbon template (1 wt% with respect to the Si and Al sources) was introduced into each solution. The solutions were then vigorously stirred for an additional 3 h. For the crystallization process under hydrothermal conditions, the prepared mixtures were transferred into a Teflon-lined autoclave, which was then operated under autogenous pressure conditions at 160 °C for 72 h. Following hydrothermal synthesis, the resulting solid mixtures were filtered and washed with de-ionized water (1 L) three times. The filtered solid mixtures were dried at 105 °C in an oven overnight, and then calcined under flowing air conditions at 550 °C for 10 h to remove impurities and the carbon template. In this procedure, the mesoporous structure in HZSM-5 is developed at the site where carbon particles were combusted. The resulting products were ion-exchanged with 1 M NH4NO3 solution at 130 °C for 6 h three times. The NH4+-form ion-exchanged catalysts were finally calcined at 550 °C for 5 h to synthesize H+-form mesoporous ZSM-5. The mesoporous HZSM-5 catalysts with varying Si/Al molar ratios are denoted as meso-XHZSM-5 (X = 10, 20, 30, and 40), with X representing the Si/Al molar ratio.

A series of Ga supported on meso-XHZSM-5 catalysts were synthesized using an impregnation method. A calculated amount (2 wt% with respect to meso-XHZSM-5) of Ga precursor (Ga(NO3)3) was dissolved in de-ionized water (10 ml). Then, for wetness impregnation, meso-XHZSM-5 (1 g) was added into the prepared solution, and the temperature was maintained at 85 °C until the de-ionized water completely evaporated. Afterwards, the resulting products were dried at 105 °C in a heating oven overnight, and calcined at 550 °C for 4 h. The samples of Ga supported on meso-XHZSM-5 catalysts are denoted as GaOy/meso-XHZSM-5.

X-ray diffraction (XRD)

In order to examine the crystalline structures of the catalysts, X-ray diffraction (XRD) measurement (Rigaku, D-MAX2500-PC) was carried out under the conditions of 50 kV and 100 mA with Cu Kα radiation (λ = 1.54056 Å).

N2 adsorption–desorption

N2 adsorption–desorption experiments (BELSORP-mini II, BEL Japan) were conducted to examine the textural properties of the catalysts. All of the catalysts were degassed at 150 °C for 4 h under vacuum conditions to eliminate physically adsorbed species prior to nitrogen adsorption. The Brunauer–Emmett–Teller (BET) equation was used to calculate the surface areas of the catalysts. The t-plot and BJH (Barrett–Joyner–Halenda) methods were used to measure the micropore and mesopore volumes, respectively.

Magic angle spinning nuclear magnetic resonance (MAS NMR)

27Al MAS NMR analysis was conducted in a solid state 500 MHz NMR system (Bruker, Bruker AVANCE II). The chemical shifts of the Al signals were referenced to Al(NO3)3, and a single pulse was used to obtain the 27Al MAS NMR spectra. 29Si MAS NMR analysis was also conducted using the same instrument for 27Al MAS NMR with a 4 mm MAS probe (Bruker, Bruker AVANCE II).

Field emission scanning electron microscopy (FE-SEM)-energy dispersive X-ray spectroscopy (EDX)

FE-SEM analysis was performed with EDX mapping to investigate the morphologies of the samples and to confirm the distribution of elements (Ga, Si, Al, and O) in the catalysts, respectively. SEM-EDX mapping with an EDS system (Thermo Fisher) was performed using a Merlin compact operating at 15 kV.

Inductively coupled plasma atomic emission spectroscopy (ICP-AES)

The chemical compositions of the catalysts were determined through ICP-AES (Shimadzu, ICP-1000IV) and EDX-mapping.

H2-Temperature programmed reduction (TPR)

The reducibility of gallium species in the catalysts was investigated by H2-TPR analysis using a BEL-CAT II (BEL Japan Inc.) apparatus. H2 consumption was recorded with a thermal conductivity detector (TCD). To begin, 0.03 g of each catalyst was charged into a quartz TPR cell. Prior to the analysis, the catalysts were pretreated under Ar flow conditions (30 ml min−1) at 450 °C for 1 h to eliminate any impurities. Following the pretreatment, the samples were cooled down to 50 °C, and the temperature was increased from 50 °C to 900 °C at a ramping rate of 5 °C min−1 while flowing 5% H2/Ar at 30 ml min−1.

X-ray photoelectron spectroscopy (XPS)

XPS analysis of the catalysts was performed with a K-alpha (Thermo Scientific Inc., U.K.) in order to investigate the state of Ga species in the catalysts. The binding energy values were corrected for charging effects by referencing to the adventitious C 1s line at 284.5 eV. The instrument uses a 400 μm-diameter beam and a focused monochromatic Al Kα X-ray (1486.6 eV) source operated at 36 W.

NH3-Temperature programmed desorption (TPD)

The acid properties of the catalysts were investigated by NH3-TPD measurement. First, 0.03 g of each catalyst was charged into a quartz U-shaped TPD cell. For pretreatment, the temperature of the cell was maintained at 200 °C for 2 h under a He flow of 50 ml min−1. Afterwards, the catalyst was cooled to 50 °C and saturated with NH3 (50 ml min−1) at 50 °C for 30 min. After NH3 saturation, the physisorbed species were eliminated at 100 °C for 1 h under the same flow conditions as pretreatment. After cooling down the catalyst, the amount of NH3 desorbed was monitored using a TCD within the temperature range of 50–550 °C at a ramping rate of 5 °C min−1 under a flow of He (30 ml min−1).

Pyridine Fourier transform infrared spectroscopy (pyridine FT-IR)

The distribution of acid sites (Brønsted acid site and Lewis acid site) of the catalysts was investigated by FT-IR spectroscopy (Thermo Fisher Scientific, Nicolet 6700) equipped with an MCT detector and a diffuse reflectance accessory with ZnSe windows, using pyridine as a probe. Prior to the measurement, 10 mg of each catalyst was mixed with 100 mg of potassium bromide (KBr, Sigma-Aldrich). The solid mixture was charged into the accessory and pretreated at 400 °C for 2 h with a stream of N2 (100 ml min−1). After cooling the catalyst to 50 °C, pyridine vapor was injected under a flow of N2 (30 ml min−1) until the acid sites were saturated with pyridine. Physisorbed pyridine was removed by evacuating the catalyst sample at 200 °C for 1 h. After cooling the catalyst sample, the temperature was increased from room temperature to 150 °C at a heating rate of 10 °C min−1 under a flow of N2 (30 ml min−1). IR spectra were recorded at 150 °C in the spectral range of 1650–1400 cm−1 using a KBr background.

Coaromatization of methane and propane

In order to test the effect of the Si/Al ratio on the catalyst activity, coaromatization of methane and propane was performed in a continuous flow fixed-bed quartz reactor under atmospheric pressure at 550 °C. To begin, 0.2 g of each catalyst was loaded into the reactor. The reactant flow consisted of CH4[thin space (1/6-em)]:[thin space (1/6-em)]C3H8[thin space (1/6-em)]:[thin space (1/6-em)]N2 = 2[thin space (1/6-em)]:[thin space (1/6-em)]0.2[thin space (1/6-em)]:[thin space (1/6-em)]3 with a total GHSV of 3900 ml h−1 gcat. The outlet line temperature was maintained at 250 °C to prevent condensation of the product. The product stream was periodically collected and analyzed using a gas chromatograph (YL-6100, Younglin) equipped with a GS-gaspro capillary column used as a flame ionization detector (FID) and a Carboxen-1000 packed column used as a thermal conductivity detector (TCD). The catalytic performance was evaluated using the following equations. The conversion of reactants, the selectivity for BTX and the BTX yield were calculated on the basis of carbon balance. yi represents the stoichiometric coefficient of BTX (B = 6, T = 7, X = 8).
 
image file: c9cy01619h-t3.tif(1)
 
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image file: c9cy01619h-t5.tif(3)
 
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3. Results and discussion

3.1. Catalyst characterization

X-ray diffraction (XRD) measurements were conducted to examine the crystalline structures of the GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts as shown in Fig. 1. All of the catalysts exhibited explicit crystalline structures of HZSM-5, which has five distinctive diffraction peaks at approximately 23.2°, 23.4°, 23.8°, 24.1°, and 24.5°, corresponding to the [501], [051], [151], [303], and [133] crystal planes, respectively. Also, two strong HZSM-5 characteristic peaks at around 7.9° and 8.8°, corresponding to [011] and [200], were identified.41 Additional phases corresponding to impurities and the amorphous structure of silica were not identified.42 This implies that meso-HZSM-5 structures were successfully synthesized, and the formation of the HZSM-5 crystalline structure was not disrupted by the introduction of the carbon template. Interestingly, significant XRD peaks for Ga species were not identified in all of the catalysts, indicating that Ga species were finely dispersed on the meso-HZSM-5.
image file: c9cy01619h-f1.tif
Fig. 1 X-ray diffraction of GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts.

ASTM standard D5758-01 was used to analyze the effect of the Si/Al ratio on the extent of HZSM-5 crystallization.43 The relative crystallinity of the GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts is presented in Table 1. It should be noted that the crystallinity of the catalysts was increased with increasing Si/Al molar ratio. This suggests that the Si/Al molar ratio affects the degree of crystallization of HZSM-5.

Table 1 Textural properties of GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts
Catalyst Relative crystallinitya (%) Surface areab (m2 g−1) Pore volume (cm3 g−1)
Microporec Mesopored
a Determined by ASTM standard method D5758-01. b Calculated by the BET (Brunauer–Emmett–Teller) equation. c Determined by the t-plot method. d BJH (Barrett–Joyner–Halenda) desorption pore volume.
GaOy/meso-10HZSM-5 67.5 319.2 0.142 0.105
GaOy/meso-20HZSM-5 70.3 356.6 0.170 0.121
GaOy/meso-30HZSM-5 91.3 380.9 0.179 0.146
GaOy/meso-40HZSM-5 100 398.3 0.195 0.153


To investigate the textural properties of the GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts, nitrogen adsorption–desorption experiments were conducted. As shown in Fig. 2, type IV isotherms with H3 to H4 type hysteresis loops were exhibited in the catalysts. At relative pressures above 0.45, an apparent hysteresis loop was developed, indicating that the mesoporous structure was well-formed in all of the catalysts. The results showed that the hysteresis loop was more well-developed as the Si/Al ratio increased. The detailed textural properties of the GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts are summarized in Table 1. The BET surface area, micropore volume, and mesopore volume increased with increasing Si/Al molar ratio. This means that the textural properties of the catalysts, such as the BET surface area, micropore volume, and mesopore volume, were enhanced as the Si/Al molar ratio increased.


image file: c9cy01619h-f2.tif
Fig. 2 Nitrogen adsorption–desorption isotherms of GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts.

27Al MAS NMR analyses were conducted to examine the surrounding environments of the Al atoms in the GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts, as shown in Fig. 3. The detailed Al compositions of the GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts are summarized in Table 2. Fig. 3 shows that tetra-coordinated framework Al and hexa-coordinated extra-framework Al were detected at around 54 and 0 ppm, respectively.44 As shown in the 27Al MAS NMR spectrum of GaOy/meso-10HZSM-5, an additional peak appeared at 30 ppm, which originates from the presence of several different Al species such as distorted tetrahedral Al, penta-coordinated Al, or the overlap of both distorted tetra- and penta-coordinated Al species.45 It is suggested that in the case of the GaOy/meso-10HZSM-5 catalyst, a slight collapse of the HZSM-5 framework occurred as a result of dealumination or instability of the framework during the crystallization step. It should be noted, as shown in Table 2, that the ratios of framework aluminum (Alf) to total aluminum (Altot) in the catalysts increased with increasing Si/Al molar ratio. This means that an appropriate ratio of Si/Al was important for the formation of stable structures of the mesoporous HZSM-5 framework.


image file: c9cy01619h-f3.tif
Fig. 3 27Al MAS NMR spectra of GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts.
Table 2 Chemical composition of GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts
Catalyst Composition (Si/Al molar ratio) Gallium loading (wt%) Alf/Altota
29Si-NMR EDX ICP-AES EDX
a Alf/Altot values determined from 27Al MAS NMR.
GaOy/meso-10HZSM-5 14.0 11.3 1.7 2.5 0.62
GaOy/meso-20HZSM-5 22.8 25.4 1.8 1.8 0.89
GaOy/meso-30HZSM-5 29.4 32.9 2.1 1.7 0.92
GaOy/meso-40HZSM-5 36.9 37.6 1.9 1.9 0.94


The morphological features of the GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts were investigated by SEM, as shown in Fig. 4. Cubic-shaped crystals could be seen in the SEM images of all the catalysts. The average crystal sizes were also measured by SEM. The crystal sizes of the GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts were measured in the range of 4.0–6.0 μm. Note that the completeness of the cubic shape of the crystals was enhanced as the ratio of Si/Al in the catalyst increased. Therefore, based on the XRD, BET, 27Al MAS NMR, and SEM results, it is summarized that the physical properties of the GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts were improved with increasing Si/Al ratio.


image file: c9cy01619h-f4.tif
Fig. 4 SEM images of the prepared catalysts with different Si/Al molar ratios. (a) GaOy/meso-10HZSM-5, (b) GaOy/meso-20HZSM-5, (c) GaOy/meso-30HZSM-5, and (d) GaOy/meso-40HZSM-5.

As shown in Fig. 5, EDX-mapping images of the GaOy/meso-20HZSM-5 catalyst with distributions of Si, Al, O, and Ga species were investigated. Each element was clearly distinguishable in these EDX mapping images. It could also be seen that Ga species were well distributed throughout the catalyst without noticeable aggregation. This implies that the catalyst was successfully prepared as attempted in this work.


image file: c9cy01619h-f5.tif
Fig. 5 SEM-EDX mapping images of the GaOy/meso-20HZSM-5 catalyst.

As summarized in Table 2, the Si/Al ratios of the catalysts were calculated by the deconvolution of the 29Si MAS NMR and EDX results, while the gallium loading on the catalysts was also investigated through the ICP-AES and EDX results. Despite the fact that the Si/Al ratio results between 29Si MAS NMR and EDX-mapping were slightly different, as were the gallium loadings between ICP-AES and EDX-mapping, the Si/Al molar ratios and gallium loadings of all the prepared catalysts were consistent with the designed theoretical values.

The reducibility of each of the GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts was analyzed by H2 temperature-programmed reduction (H2-TPR), as presented in Fig. 6. In general, the characteristic peaks for Ga2O3 are shown as two distinctive peaks in the H2-TPR profiles. The low temperature peak near 550 °C and the high temperature peak near 800 °C are assigned to the reduction of small Ga2O3 particles and segregated bulk Ga2O3, respectively.46 GaOy/meso-XHZSM-5 (X = 20, 30, and 40) catalysts have two consecutive H2 consumption peaks that originate from the reduction of specific Ga species, while the GaOy/meso-10HZSM-5 catalyst shows an additional peak at a high temperature of 800 °C. The first peak (peak I) at around 520–570 °C arises from the reduction of Ga2O3(III) to Ga2O(I), while the second peak (peak II) at around 650 °C can be assigned to the reduction of GaO+(III) ions to Ga+(I).46 The third peak (peak III) that is shown only in the TPR profile of GaOy/meso-10HZSM-5 can be assigned to the reduction of segregated bulk Ga2O3. It was reported that the reduction temperature of large size Ga2O3 species could be higher than that of small size species, even when they are in the same oxidation states, because the reduction temperature of Ga species depends on both the size and the oxidation state.47 It might be understood that the structural instability of the meso-10HZSM-5 catalyst affected the formation of the bulk phase of Ga2O3. Previous studies have shown that GaO+ is generated during the preparation process because of the strong interaction between dispersed gallium species and zeolite.48 At the end of the impregnation step, Ga(OH)xy+ species interact with HZSM-5. Some Ga(OH)xy+ can be changed into GaO+ by the following reaction after calcination:49

 
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Fig. 6 H2-TPR profiles of GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts.

In the presence of H2, the highly dispersed Ga2O3 in HZSM-5 can be reduced to produce mobile Ga2O. It can then interact with the Brønsted acid sites of HZSM-5 to form Ga+ ions.48

 
Ga2O3 + 2H2 → Ga2O + 2H2O(7)
 
Ga2O + 2Hz+Z → 2Ga+Z + H2O(8)
where Z is the negative charge of ZSM-5 and Hz is the proton (H+) from the Brønsted acid sites of HZSM-5.

It has been reported that the aromatization process is affected by two types of Ga species: Ga2O generated by the reduction of Ga2O3 and GaO+. Due to its high mobility, Ga2O can strongly interact with the Brønsted site of HZSM-5 to form Ga+, which acts as a Lewis acid site. GaO+ serves as a Lewis acid site without further reduction.46,48 Ga species can be reduced by the hydrogen produced from the dehydrogenation of methane and propane during the activation step.

The H2-TPR results of the GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts are summarized in Table 3. As the molar ratio of Si/Al increased, the ratio of Ga2O3 increased, while that of GaO+ decreased. These results can be explained by the acid properties of the catalysts, because the Brønsted acid sites that could be exchanged with GaO+ decreased with increasing Si/Al ratio.46,50,51

Table 3 H2-TPR results of GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts
Catalyst H2 uptake (mmol g−1) Total H2 uptake (mmol g−1)
Ga2O3 GaO+ Segregated bulk Ga2O3
GaOy/meso-10HZSM-5 0.081 0.065 0.030 0.176
GaOy/meso-20HZSM-5 0.110 0.064 0.174
GaOy/meso-30HZSM-5 0.129 0.057 0.186
GaOy/meso-40HZSM-5 0.138 0.052 0.190


XPS analyses were carried out to confirm the interaction between Ga species and HZSM-5 in the GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts. Fig. S1 shows the XPS spectra for the Ga 2p level of the GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts. The Ga 2p3/2 binding energy value for all the catalysts (1118.1–1118.8 eV) was higher than that of Ga2O3 (1117.9 eV), indicating a strong interaction between Ga species and HZSM-5.46,50 This is attributed to GaO+ that interacts with the framework of zeolite by exchanging with the acidic proton (Brønsted acid site).46,50 As the Si/Al molar ratio increased above 20, the peak was drastically shifted toward a lower binding energy. As mentioned earlier in H2-TPR, this is explained by the decrease of GaO+ with increasing Si/Al molar ratio, while Ga2O3 increased.

The acid properties of the GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts were investigated through NH3-TPD measurement, and the results are shown in Fig. 7. The amount of NH3 desorbed from NH3-TPD is summarized in Table 4. Three NH3 desorption peaks of all the catalysts appeared at around 190 °C, 310 °C, and 400 °C, which were respectively assigned to the NH3 desorption from weak, medium, and strong acid sites. The weak acid site is posed by desorbed NH3 from the non-exchangeable cationic site of the HZSM-5 framework.41 The medium acid site mainly originates from the interaction of Ga species with the acidic protons of the HZSM-5 framework, which serves as a strong Lewis acid site.52 The strong acid site (Brønsted–Lowry acid site) arises from the NH3 desorbed from the exchangeable protonic site of the HZSM-5 framework.52 It was observed that the GaOy/meso-10HZSM-5 catalyst retained the largest amount of medium acid sites. According to Fig. 3, a substantial amount of Al was found to be extra-framework Al, which can function as a Lewis acid site.53 Therefore, more medium acid sites were created in this catalyst than in the other catalysts. The peak temperature attributed to the medium acid site increased gradually with increasing Si/Al molar ratio. This implies that the medium acid strength became stronger with the increase in Si/Al ratio. As the ratio of Si/Al increased, the total acidity and the amounts of both medium sites and strong sites decreased. The medium and strong acid sites are known to be more important than the weak acid site in the coaromatization reaction, because they are mainly used to produce aromatic compounds according to the reaction mechanism.10,20,24 Since the weak acid site is generally attributed to NH3 adsorbed on non-framework Al or Si–OH in the structural defects, it does not directly affect the activity of the reaction.54,55 It has been reported that the dehydrogenation of hydrocarbon reactants occurs on the Lewis acid sites, which is generated by Ga species.24 The Brønsted–Lowry acid sites play an essential role in oligomerizing the reaction intermediates produced by the activation of reactants. Therefore, it is suggested that both acid sites are important for enhancing the BTX yield of the coaromatization reaction, and the amount of acid sites is a crucial factor for the catalytic activity.


image file: c9cy01619h-f7.tif
Fig. 7 NH3-TPD profiles of GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts.
Table 4 Acid properties of GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts
Catalyst Acidity (μmol NH3 per gcat)
Weak site (weak acid) Medium site (strong Lewis acid) Strong site (Brønsted acid) Total acidity
GaOy/meso-10HZSM-5 526.2 (46.3%) 310.2 (27.3%) 300.0 (26.4%) 1136.4
GaOy/meso-20HZSM-5 456.7 (47.4%) 245.7 (25.5%) 261.1 (27.1%) 963.5
GaOy/meso-30HZSM-5 348.9 (45.7%) 190.9 (25.0%) 223.7 (29.3%) 763.5
GaOy/meso-40HZSM-5 238.3 (46.0%) 125.4 (24.2%) 154.4 (29.8%) 518.0


In order to identify the different types of acid sites (Brønsted and Lewis acid sites) of GaOy/meso-XHZSM-5 (X = 10, 20, 30 and 40) catalysts, pyridine FT-IR spectroscopy was conducted. The pyridine FT-IR spectra of GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts are shown in Fig. S2. The GaOy/meso-XHZSM-5 catalysts showed IR bands ascribed to hydrogen bonded pyridine (1440 and 1595 cm−1), Lewis acid bound pyridine (1450 and 1580 cm−1), and Brønsted acid bound pyridinium ions (1490 and 1545 cm−1).56 The band at 1490 cm−1 was attributed to both Brønsted and Lewis acid sites.56 The ratio of Brønsted and Lewis acid sites (B/L ratios) of each catalyst was calculated from the IR band area at 1545 and 1450 cm−1.56 The integrated molar extinction coefficients of the Brønsted and Lewis acid sites were chosen to be 1.67 cm μmol−1 and 2.22 cm μmol−1, respectively.57

The B/L ratios of GaOy/meso-XHZSM-5 (X = 10, 20, 30 and 40) catalysts are summarized in Table S1. As the Si/Al molar ratio increased, the B/L ratio of the catalysts slightly increased. According to NH3-TPD and H2-TPR results, the total acidity and the amount of GaO+ which plays the role of a Lewis acid site decreased with increasing Si/Al ratio. It is suggested that the site where Ga species act as Lewis acid sites is reduced with increasing Si/Al ratio. Therefore, the B/L ratio of the catalysts increased, because the decrease of Lewis acid sites was more dominant than the decrease of Brønsted acid sites, as the Si/Al molar ratio increased.

3.2. Catalytic performance of GaOy/meso-XHZSM-5 catalysts

The catalytic performances of the GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts in the coaromatization of methane and propane are shown in Fig. 8. The propane conversions of the catalysts increased as the Si/Al molar ratio increased from 10 to 20, while they decreased as the Si/Al molar ratio increased from 20 to 40. The propane conversion is a significant factor in the reaction. In general, propane is decomposed into methane and ethylene. In particular, ethylene directly participates in the oligomerization process on the B acid site of the catalyst. Propane can also promote the activation of methane to reaction intermediates.17 Therefore, a decrease in propane conversion lowers the conversion to BTX. As shown in Fig. 8, the methane conversion can be affected by the amount of methane produced from propane. The methane conversions of all the catalysts were increased in the final stage of the reaction when compared to the initial stage of the reaction. This is attributed to the fact that the amount of propane converted to methane was reduced as the propane conversion decreased. As a result, the methane conversion can be increased with decreasing methane production from propane.
image file: c9cy01619h-f8.tif
Fig. 8 Catalytic performance of GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts. (a) Conversion of methane and propane and (b) BTX yield.

It is noteworthy that the catalytic performance of GaOy/meso-10HZSM-5 was quite excellent despite its substantially low Alf composition when compared to the other catalysts. This is because additional Lewis acid sites arising from extra-framework Al may enhance the catalytic performance of the catalyst. It should be noted that the BTX yield showed a volcano-shaped trend depending on the molar ratio of Si/Al. Among the catalysts tested, the GaOy/meso-20HZSM-5 catalyst exhibited the best catalytic performance in terms of the BTX yield. Detailed initial and final conversions of the reactants and yields for the products are summarized in Tables 5 and 6, respectively. By-products such as C2–C5 compounds, C10H8, and CO2 were produced along with the desired products (BTX). It is suggested that CO2 can be generated by oxygen of some Ga oxide species which are not reduced during the reaction. It should be noted that the main by-products were C2 species such as ethane and ethylene (3.3–3.6% in the initial yield) and propylene (1.6–3% in the initial yield). The sums of the yields for CO2, C4 series, C5 series, and C10H8 were measured to be 0.7–1.3% in the initial yield.

Table 5 Catalytic performance of GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts in the BTX production by coaromatization of methane and propane
Catalyst X m (%) X p (%) Yieldc (%)
CO2 C2H6 C2H4 C3H6 C4 C5 BTX C10H8
C6H6 C7H8 C8H10
a X m is the initial conversion of methane. b X p is the initial conversion of propane. c Yield is the initial yield for BTX and by-products.
GaOy/meso-10HZSM-5 4.9 75.3 0.7 0.8 2.8 2.4 0.1 0.5 7.9 5.1 0.4 <0.1
GaOy/meso-20HZSM-5 7.8 87.9 0.4 1.5 2.1 1.6 <0.1 0.1 8.5 5.3 0.4 <0.1
GaOy/meso-30HZSM-5 7.4 67.4 0.5 1.0 2.3 2.6 0.1 0.2 5.9 4.6 0.3 <0.1
GaOy/meso-40HZSM-5 7.4 61.0 0.6 0.8 2.5 3.0 0.1 0.2 5.4 4.3 0.2 <0.1


Table 6 Catalytic performance of GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts after 6 h of reaction in the BTX production by coaromatization of methane and propane
Catalyst

image file: c9cy01619h-t9.tif

(%)

image file: c9cy01619h-t10.tif

(%)
Yieldc (%)
CO2 C2H6 C2H4 C3H6 C4 C5 BTX C10H8
C6H6 C7H8 C8H10
a image file: c9cy01619h-t11.tif is the final conversion of methane after 6 h of reaction. b image file: c9cy01619h-t12.tif is the final conversion of propane after 6 h of reaction. c Yield is the final yield for BTX and by-products after 6 h of reaction.
GaOy/meso-10HZSM-5 6.4 69.9 0.6 0.8 2.9 2.9 0.1 0.5 7.4 4.9 0.4 <0.1
GaOy/meso-20HZSM-5 8.5 81.9 0.4 1.4 2.3 2.0 <0.1 0.1 8.0 5.4 0.4 <0.1
GaOy/meso-30HZSM-5 11.0 59.9 0.4 1.0 2.2 2.9 0.1 0.2 5.0 4.1 0.3 <0.1
GaOy/meso-40HZSM-5 13.2 44.1 0.5 0.6 2.2 3.6 0.1 0.3 3.5 3.2 0.2 <0.1


All the catalysts demonstrated slight decreases in activity with time. During 6 h of the reaction, the BTX yields of all the catalysts decreased. The decreases in the propane conversion likely led to the decreased BTX yields of all the catalysts since propane acts as a key factor for reactant activation at low reaction temperature. The degree of deactivation is defined as the difference between the initial BTX yield and the final BTX yield, Δ(YBTX,initialYBTX,final). The Δ(YBTX,initialYBTX,final) values of GaOy/meso-10HZSM-5, GaOy/meso-20HZSM-5, GaOy/meso-30HZSM-5, and GaOy/meso-40HZSM-5 were measured to be 0.7%, 0.4%, 1.7%, and 3.0%, respectively. In general, the degree of deactivation of each catalyst increased with increasing Si/Al molar ratio, which can be explained by the acid properties of the catalysts. According to the acid properties listed in Table 4, since the total acidity decreased with increasing Si/Al molar ratio, a catalyst with a small amount of acid sites could be easily affected by coke deposition. In other words, as the reaction proceeded, access to the acid sites of the catalyst with a small amount of acid sites could be easily blocked by coke deposition, since the absolute amount of acid sites is small. The textural properties and coke deposition of the GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts after 6 h of reaction are summarized in Table 7. Pore blockage was observed in all the catalysts, and the amount of coke deposited during 6 h of reaction was similar. Thus, the catalyst with a small amount of acid sites (a high Si/Al ratio) could be easily poisoned by a similar amount of coke deposited, leading to an increased degree of deactivation.

Table 7 Textural properties and coke deposition of GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts after 6 h of reaction
Catalyst Surface areaa (m2 g−1) Pore volume (cm3 g−1) Carbon depositiond (mg carbon per g catalyst)
Microporeb Mesoporec
a Calculated by the BET (Brunauer–Emmett–Teller) equation. b Determined by the t-plot method. c BJH (Barrett–Joyner–Halenda) desorption pore volume. d Calculated by elemental analysis.
GaOy/meso-10HZSM-5 294.6 0.117 0.095 5.34
GaOy/meso-20HZSM-5 322.1 0.131 0.110 7.36
GaOy/meso-30HZSM-5 337.2 0.135 0.136 6.79
GaOy/meso-40HZSM-5 354.4 0.150 0.144 7.51


The correlation between the acid properties, textural properties, and initial BTX yields of the GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts is presented in Fig. 9. It should be noted that medium and strong acidity decreased with increasing Si/Al molar ratio, while the total pore volume increased. Based on these results, an optimum point exists between these two characteristics. The catalytic performance showed a volcano-shaped trend because the textural properties and acid properties have opposing tendencies. It is considered that the textural properties are dominant at Si/Al molar ratios below 20, while the acid properties become more important at Si/Al molar ratios exceeding 30.


image file: c9cy01619h-f9.tif
Fig. 9 Correlation between medium and strong acidity, pore volume and initial BTX yield of GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts.

GaOy/meso-XHZSM-5 act as bi-functional catalysts, since two active sites are responsible for the coaromatization reaction. The activation of reactants to intermediates takes place on Ga sites, which serve as strong Lewis acid sites (medium acid site in Fig. 7). The strong Lewis sites dehydrogenate methane and propane by the polarization of C–H bonds to convert reactants to intermediates.52,58 Reaction intermediates undergo oligomerization and cyclization at the Brønsted–Lowry acid sites (strong acid site in Fig. 7) of the catalysts, producing BTX.59,60 The acid properties of the catalysts substantially affect the catalytic performance since each step of the reaction is driven by the acid sites.1,49 In addition, the textural properties of the catalysts also have a major influence on the catalytic performance.38,42 The relative crystallinity and stability of the HZSM-5 structures were enhanced with increasing Si/Al molar ratio. In particular, when comparing between GaOy/meso-10HZSM-5 and GaOy/meso-20HZSM-5, the catalytic performance of the GaOy/meso-20HZSM-5 catalyst was more improved than that of GaOy/meso-10HZSM-5. The textural properties of the catalysts, such as the relative crystallinity, BET surface area, pore volume, and Alf/Altot, increased with increasing Si/Al ratio, while the acid properties of the catalyst decreased. Therefore, it should be concluded that both the acid properties and textural properties of the catalysts play an essential role in determining the catalytic performance of the coaromatization reaction. Among the catalysts, the GaOy/meso-20HZSM-5 catalyst was determined to have the optimum point of the two opposing effects, showing the best catalytic performance in the coaromatization of methane and propane.

Conclusions

In this study, a series of GaOy/meso-XHZSM-5 (X = 10, 20, 30, and 40) catalysts with different Si/Al molar ratios were prepared for the coaromatization of methane and propane. The effects of the Si/Al molar ratio on the physicochemical properties and catalytic performances of the catalysts were then investigated. All of the catalysts retained the well-developed crystalline structure of HZSM-5 and mesopores. As the molar ratio of Si/Al increased, the BET surface area and pore volume of the catalysts increased, while their medium and strong acidity decreased. The H2-TPR analysis results revealed that the composition of active Ga species changed with the Si/Al molar ratio. In the coaromatization reaction, the initial BTX yield turned out to have a volcano-shaped trend depending on the Si/Al molar ratio. All of the catalysts demonstrated slight deactivation during the reaction, while the degree of deactivation of each catalyst generally increased with increasing Si/Al molar ratio. This was attributed to the acid properties of the catalysts. Access to the acid sites of the catalyst with a small amount of acid sites could be easily blocked by coke deposition since the absolute amount of acid sites is small. It should be noted that medium and strong acidity decreased with increasing Si/Al molar ratio, while the total pore volume increased. Therefore, it can be concluded that the GaOy/meso-20HZSM-5 catalyst showed the best coaromatization performance, which was attributed to the optimum point of the two opposing effects.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the C1 Gas Refinery Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT & Future Planning (2016M3D3A1A01913252).

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Footnote

Electronic supplementary information (ESI) available: Fig S1 and S2 and Table S1. See DOI: 10.1039/c9cy01619h

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