In-Exchanged CHA Zeolites for Selective Dehydrogenation of Ethane: Characterization and Effect of Zeolite Framework Type

Abstract

In this study, the characterization of In-exchanged CHA zeolite (In-CHA (SiO₂/Al₂O₃ = 22.3)) was conducted by in-situ X-ray diffraction (XRD) and ammonia temperature-programmed desorption (NH₃-TPD). We also prepared other In-exchanged zeolites with different zeolite structures (In-MFI (SiO₂/Al₂O₃ = 22.3), In-MOR (SiO₂/Al₂O₃ = 20), and In-BEA (SiO₂/Al₂O₃ = 25)) and different SiO₂/Al₂O₃ ratios (In-CHA(Al-rich) (SiO₂/Al₂O₃ = 13.7)). Their catalytic activities in nonoxidative ethane dehydrogenation were compared. Among the tested catalysts, In-CHA(Al-rich) provided the highest conversion. From kinetic experiments and in-situ Fourier transform infrared (FTIR) spectroscopy, [InH₂]⁺ ions are formed regardless of SiO₂/Al₂O₃ ratio, serving as the active sites.

1. Introduction

The valorization of ethane is highly demanded in chemical industry owing to increasing availability of inexpensive ethane from shale gas [1,2]. The most frequently used method for this purpose is the steam cracking to obtain ethylene which is an important feedstock for production of various fundamental chemicals, including polyethylene, styrene, ethylene oxide, and acetic acid. However, this method requires high reaction temperature (1073–1173 K) to achieve sufficient ethylene yields [3]. Moreover, the rapid cooling of the outlet gas is necessary for suppression of ethylene polymerization. The steam cracking of ethane is an energy-intensive process. Catalytic dehydrogenation of ethane to ethylene is a promising alternative to decrease the reaction temperature and to conserve energy consumption [4,5,6]. Various catalyst systems using platinum group metal-based alloys [7,8,9,10], Cr [11,12,13], and Ga [14,15], have been developed. However, the reported systems often suffer from low ethylene selectivity and catalysts deactivation due to coke formation. In addition, the applicability of those processes under industrially meaningful conditions, i.e., high ethane concentration regime, has rarely been investigated [11]. From the practical viewpoint, the development of selective and durable catalyst systems that are workable even under the high concentration conditions is highly demanded.

Zeolites are porous aluminosilicates that comprise tetrahedral SiO₄ and AlO₄ units. To compensate for the cation deficiency derived from the isomorphic substitution of Si⁴⁺ with Al³⁺, cationic species such as protons are present on Al sites within pores [16]. These cations are exchangeable with other metal cations through ion-exchange methods. Proton-type and metal-exchanged zeolites show potential as heterogeneous catalysts for exhaust gas purification and chemical syntheses [17,18,19,20,21,22,23,24,25,26]. Numerous different zeolites consisting of 8–12-membered rings with different crystal structures, including CHA (8-membered rings), MFI (10-membered rings), BEA and MOR (12-membered rings), have been developed. The different activities and selectivities of zeolites are closely related to their different pore sizes and crystal structures [27,28,29,30,31,32]. The SiO₂/Al₂O₃ ratio also affects catalytic properties because the configuration of Al sites and surface properties strongly depend on the ratio [33,34,35,36,37]. Thus, investigating the effects of framework structure and SiO₂/Al₂O₃ ratio in zeolites can help control their catalytic properties, allowing the design of more effective catalysts.

Group 13 metal-exchanged zeolites have attracted much attention as catalysts for the transformation of light alkanes. Ga-exchanged MFI zeolites (Ga-MFI) have been most widely studied since the 1990’s [38,39,40,41]. Ono and coworkers prepared Ga-MFI using liquid-phase ion-exchange and then investigated the mechanism of the dehydrocyclodimerization (DHCD) of propane to aromatics [42]. Improving the activity of Ga-MFI by loading larger amounts of Ga species through reductive solid-state ion-exchange (RSSIE) was investigated [43]. Several cationic Ga species, such as Ga⁺, [GaH]²⁺, and [GaH₂]⁺ ions, have been reported as catalytically active species for the dehydrogenation of propane [44,45,46]. Recently, Bell and coworkers reported that [GaH]²⁺ rather than [GaH₂]⁺ ions are active species using a combination of kinetic, spectroscopic, and theoretical studies [47], whereas Lewis-Brønsted acid pairs (Ga⁺ and H⁺) have been proposed as active sites by the group of Lercher based on their investigation of the Ga/Al effect and acidity characterization along with theoretical investigation [48]. Various Ga-exchanged zeolites including Ga-MFI have been applied to the transformation of other light alkanes to aromatic compounds, the dehydrogenation of light alkanes, and reactions of methane with light alkenes to produce higher hydrocarbons [49,50,51].

In contrast to that by Ga-exchanged zeolites, studies on dehydrogenative transformation of light alkanes by In-exchanged zeolites have been rarely reported. Their preparation by RSSIE between In₂O₃ and proton-type zeolites has been studied by several groups [52]. The reaction is described in Equation (1).

$${\mathrm{In}}_2{\mathrm{O}}_3+2{\mathrm{H}}^{+}{\mathrm{Z}}^{-}+2{\mathrm{H}}_2 \to 2{\mathrm{In}}^{+}{\mathrm{Z}}^{-}+3{\mathrm{H}}_2\mathrm{O}$$

(1)

where Z⁻ denotes a zeolite anion. In the context of catalysis, Hert and coworkers prepared In-exchanged MFI (In-MFI) by RSSIE and compared its catalytic activity for propane DHCD with that of Ga-MFI [53,54]. They reported that In-MFI exhibits poor durability due to the reduction of active In⁺ cations to In metal during the reaction. The reaction of methane with ethylene/benzene was investigated by the group of Baba using In-MFI, for which the activation of methane at In⁺ cations coordinated to zeolite anions was proposed [55]. Recently, our research group has reported that In-exchanged CHA zeolite (In-CHA) catalyzes the selective dehydrogenation of ethane and exhibits high durability [56]. A combination of experimental and theoretical studies revealed that [InH₂]⁺ ions, which are formed in situ through the reaction of In⁺ cations with H₂, serve as catalytically active sites. However, the detailed investigation including effects of zeolite framework and SiO₂/Al₂O₃ on ethane dehydrogenation were not conducted.

As a continuation of our previous study, we herein report the detailed characterization of In-CHA by in-situ X-ray diffraction (XRD) and ammonia temperature-programmed desorption (NH₃-TPD). We also investigated the effect of zeolite framework and SiO₂/Al₂O₃ to achieve higher conversion compared to In-CHA. In-MFI, In-MOR, and In-BEA exhibit inferior activities compared to that of In-CHA, whereas a higher conversion value was achieved using In-CHA with a higher Al content (In-CHA(Al-rich)). Kinetic study and in-situ Fourier transform infrared (FTIR) spectroscopy indicated that [InH₂]⁺ ions are formed in CHA zeolites regardless of SiO₂/Al₂O₃ ratio, and that they serve as catalytically active sites for the dehydrogenation of ethane.

2. Results

2.1. XRD

The RSSIE reactions of In₂O₃ and zeolites under H₂ at high temperature have been investigated by H₂ temperature-programmed reduction (H₂-TPR) and/or thermogravimetric analysis in several previous studies [52,54,57]. However, in-situ characterization of In₂O₃ and zeolites has not been reported yet. We have reported the preparation of In-CHA through RSSIE of CHA-supported In₂O₃ obtained by impregnation (In₂O₃/CHA), revealing the formation of highly dispersed In species by spectroscopic and microscopic analysis [58]. In this study, in-situ XRD was conducted for RSSIE of In₂O₃/CHA under 5% H₂/N₂ flow. Figure 1a shows a series of XRD patterns obtained at different temperatures. The characteristic peak at 2θ = 30.6° derived from In₂O₃ (222) [59] and the typical diffraction pattern of CHA zeolites [60] were observed at 303 and 473 K, respectively. The intensity of the peak derived from In₂O₃ was slightly decreased at 573 K and the peak was no longer observed at 773 K (Figure 1b). After cooling to 303 K, no peaks derived from In₂O₃ were observed. The sharp peaks in the zeolite diffraction patterns at 2θ = 9.6° and 20.8° decreased in intensity above 573 K due to temperature-induced disordering and then returned to their initial intensities upon cooling from 773 to 303 K [61]. These observations indicate the occurrence of RSSIE reaction at temperatures from 573 to 773 K, which is consistent with the results of H₂-TPR measurement in our previous study [56]. A series of In-exchanged zeolites including In-MFI, In-MOR, In-BEA, and In-CHA(Al-rich) were also prepared through similar RRSIE reactions of In₂O₃ supported on the corresponding zeolites. The XRD measurements showed that the peaks assignable to In₂O₃ (222), (400), and (440) disappeared after RSSIE in each case (Figure 2), indicating the high dispersion of In species.

2.2. NH₃-TPD

The acidity profiles of In-CHA samples with different In/Al values (In/Al = 0.2, 0.4, and 0.8) were investigated by NH₃-TPD. In each case, the catalyst was exposed to NH₃/He at 473 K followed by purging with He. Thereafter, the temperature was increased from 473 to 973 K under He flow while the desorbed NH₃ (m/z = 16) was monitored. The TPD profiles of proton-type CHA (H-CHA) and a series of In-CHA samples are shown in Figure 3. The profile of H-CHA showed a wide peak around 700–800 K due to NH₃ desorbed from Brønsted acid sites. The intensity of the peak decreased with the increase of In/Al. This trend is similar to that observed in the NH₃ adsorption experiment [56]. In addition, a desorption peak around 600 K appeared upon In loading and its intensity increased with In/Al ratio. This peak might be due to NH₃ weakly adsorbed on In⁺ cations [62]. The results of NH₃-TPD are consistent with the occurrence of RSSIE reactions as described in Equation (1).

2.3. Effect of Zeolite Host upon Ethane Dehydrogenation Catalyzed by In-Exchanged Zeolites

We previously found that In-CHA with SiO₂/Al₂O₃ = 22.3 and In/Al = 0.8 exhibits high selectivity for dehydrogenation of ethane to ethylene and carbon balance at 933 K. In this work, In-MFI, In-MOR, and In-BEA having similar SiO₂/Al₂O₃ ratios (22.3, 20.0, and 25.0, respectively) and In-CHA(Al-rich) (SiO₂/Al₂O₃ = 13.7) were prepared using the same In/Al value of 0.8 and compared with In-CHA in ethane dehydrogenation (

Table 1. Conversion, selectivity, and carbon balance values for 30 min of ethane dehydrogenation at 933 K using several In-exchanged zeolites.

Entry	Catalyst	SiO2/Al2O3	In/Al	Conv. [%] a	Sel. [%] a	Carbon Balance [%] a
1	In-MFI	22.3	0.8	12.9	99.2	99
2	In-MOR	20.0	0.8	14.9	97.6	99
3	In-BEA	25.0	0.8	13.5	97.5	94
4	In-CHA(Al-rich)	13.7	0.8	37.9	96.6	98
5	In-CHA b	22.3	0.8	25.9	96.1	99

Reaction conditions: 0.1 g of catalyst, 10% C₂H₆/He (10 mL/min). [^a] The determination was performed using a gas chromatography (GC). [^b] The data were previously reported in our paper [56].

). In-MFI, In-MOR, and In-BEA showed lower conversions of 12.9%, 14.9%, and 13.5%, respectively, than that for In-CHA (25.9%) (entries 1–3 vs. entry 5) despite their larger zeolite pores (5.6 Å × 5.3 Å, 7.0 Å × 6.5 Å, and 7.6 Å × 7.3 Å, respectively) compared to that of CHA (3.8 Å × 3.6 Å). In contrast, In-CHA(Al-rich) exhibited a superior conversion (37.9%) while maintaining the high selectivity of 96.1% (entry 5). For all the In-exchanged zeolites, high selectivity for ethylene (>96%) and carbon balance (>93%) were observed while main byproduct was methane.

The ethane dehydrogenation was conducted under severe reaction condition (80% C₂H₆/N₂, 993 K) using 0.1 g of In-CHA(Al-rich) (Figure 4). The conversion and selectivity values at 1 h were 32.7% and 90.7%, respectively. Although the conversion value decreased to 30.9% during the 40 h reaction, good selectivity was maintained. This catalytic performance is comparable with those of other reported catalyst systems for nonoxidative ethane dehydrogenation [4]. The catalyst was reusable after regeneration treatment (50% O₂/N₂ for 60 min and 10% H₂/N₂ for 30 min at 993 K) without significant loss of activity or selectivity (32.5% conversion and 90.8% selectivity at 1 h) (Figure 4). The diffraction pattern derived from CHA framework was maintained after the reaction, as confirmed by XRD measurement (Figure 2), which is consistent with the high reusability of In-CHA(Al-rich).

Previously, we investigated the active species ([InH₂]⁺, [InH]²⁺, or In⁺ cations) for ethane dehydrogenation catalyzed by In-CHA based on kinetic and in-situ spectroscopic studies with transition state (TS) calculations [56]. The TS calculations revealed that the calculated activation enthalpies (ΔH^⧧) for C−H activation of ethane on In hydrides ([InH₂]⁺ or [InH]²⁺ ions) are ca. 200–240 kJ/mol, whereas that on In⁺ cations is much higher (ca. 400 kJ/mol). The experimental ΔH^⧧ value for ethane dehydrogenation based on Eyring plot was determined as 236 kJ/mol, suggesting that the catalytically active sites are In hydrides. To obtain insight into the effect of the zeolite, the turnover frequency (TOF) values normalized to the total amount of In species were compared (Figure 5a). The TOF value for In-CHA(Al-rich) was similar to that for In-CHA and was more than five-time higher than those for other In-zeolites. Eyring plots for ethane dehydrogenation were also investigated. The obtained plots are shown in Figure 5b with estimated apparent ΔH^⧧ values. The ΔH^⧧ of In-CHA(Al-rich) was determined to be 258 kJ/mol, which is similar to that for In-CHA, while In-MFI, In-MOR, and In-BEA showed much higher ΔH^⧧ values (364, 300, and 322 kJ/mol, respectively). Comparison of TOF and ΔH^⧧ values among In-zeolites indicates that (1) In hydrides ([InH₂]⁺ or [InH]²⁺ ions) are possible active sites for In-CHA(Al-rich) rather than In⁺ cations and that (2) the active species and/or reaction mechanism are different between In-CHA zeolites and other In-exchanged zeolites.

Furthermore, in-situ FTIR spectroscopy was conducted for In-CHA(Al-rich) to determine possible In hydrides in In-CHA(Al-rich). After H₂ treatment at 773 K, the band derived from the stretching vibration of In−H (ν(In−H)) appeared around 1700 cm⁻¹ (Figure 6a). This band can be deconvoluted into two peaks assignable to ν(In−H) (1777 and 1715 cm⁻¹) with other peaks (1661 and 1610 cm⁻¹) (Figure 6b). Based on our previous vibration analysis by density functional theory calculations [56], the highest peak at 1715 cm⁻¹ is derived from the symmetric ν(In−H) of [InH₂]⁺ ions, while the smaller peak at 1777 cm⁻¹ is assignable to the asymmetric ν(In−H) of [InH₂]⁺ ions or the ν(In−H) of [InH]²⁺ ions. These results indicate that the main In hydrides are [InH₂]⁺ rather than [InH]²⁺ in In-CHA(Al-rich). The other peaks might be derived from different-type adsorbed water molecules [63,64]. From the above combined results, [InH₂]⁺ ions are likely formed in In-CHA zeolites regardless of SiO₂/Al₂O₃ ratio and are likely to be the active sites.

3. Materials and Methods

3.1. Catalyst Preparation

The preparation of In-CHA was conducted according to our previous study [58]. In₂O₃ immobilized on CHA zeolite (In₂O₃/CHA) was obtained from In(NO₃)₃·nH₂O (purchased from Kanto Chemical Co., Inc., Japan) and the NH₄⁺-type CHA zeolite (Tosoh, SiO₂/Al₂O₃ = 22.3) through impregnation and calcination under air. Next, the RSSIE of In₂O₃/CHA was carried out under H₂ flow at 773 K to obtain In-CHA. Other In-exchanged zeolites (In-MFI, In-MOR, In-BEA, and In-CHA(Al-rich)) were obtained in a similar manner from MFI (Tosoh, SiO₂/Al₂O₃ = 22.3, HSZ-820NHA), MOR (Tosoh, SiO₂/Al₂O₃ = 20, JRC-Z-HM20), BEA (SiO₂/Al₂O₃ = 25, JRC-Z-HΒ25, provided by the Catalysis Society of Japan, Japan), and CHA (Tosoh, SiO₂/Al₂O₃ = 13.7). The In/Al ratio was estimated from the amounts of In(NO₃)₃·nH₂O. H-CHA was prepared by calcination of NH₄⁺-type CHA at 873 K under air for 1 h.

3.2. XRD

In-situ XRD measurements during RSSIE of In₂O₃/CHA was conducted using a Cu Kα radiation source (Rigaku Ultima IV, Rigaku Corporation, Japan). XRD patterns were obtained while heating In₂O₃/CHA from 303 to 773 K at 20 K/min under 5% H₂/N₂ flow (100 mL/min). For ex-situ XRD measurements, a Rigaku MiniFlex II/AP diffractometer with Cu Kα radiation (Rigaku Corporation, Japan) was used.

3.3. NH₃-TPD

NH₃-TPD experiments were investigated using a BELCAT (MicrotracBEL, Japan). The In-CHA with different In/Al ratio (0.2, 0.4, and 0.8) was prepared in situ under 5% H₂/Ar at 773 K and then treated with 10% NH₃/He at 473 K followed by He purge. After these treatments, the sample was heated from 473 to 973 K under He flow while the desorbed NH₃ (m/z = 16) was monitored by a mass spectrometer (BELMass (MicrotracBEL, Japan)).

3.4. Catalytic Tests

Non-oxidative dehydrogenation of ethane was performed in a fixed-bed continuous flow system at 933 K. The catalyst was prepared in situ by treatment of a 0.1 g of the In₂O₃ immobilized on the corresponding zeolite under 10% H₂/He flow (50 mL/min) at 773 K followed by He purge before the reaction. Thereafter, the catalyst was heated to 933 K, and 10% C₂H₆/He (10 mL/min) was introduced. For determination of conversions, yields, and selectivities, gas chromatography (GC) analysis was conducted using Shimadzu GC-14B (Shimadzu Corporation, Japan) combined with a SHINCARBON ST or a Unipack S column (Shinwa Chemical Industry Ltd., Japan). For dehydrogenation under a high concentration of ethane, the in-situ preparation of In-CHA(Al-rich) was carried out under 80% H₂/N₂ (10 mL/min) at 993 K for 30 min, and the reaction was investigated at the same temperature under 10 mL/min of 80% C₂H₆/N₂. The GC analysis was performed using an Agilent 490 micro-GC with a thermal conductivity detector. The conversion, selectivity, carbon balance, and TOF were calculated as follows.

$$\mathrm{Conv}.[\%]=\frac{{\left[ethane\right]}_{init}-\left[ethane\right]}{{\left[ethane\right]}_{init}}\times 100$$

$$\mathrm{Selec}.[\%]=\frac{\left[ethylene\right]}{\left[ethylene\right]+\left[methane\right]}\times 100$$

$$\mathrm{Carbon}\mathrm{balance}[\%]=\frac{\left(\left[ethane\right]+\left[ethylene\right]+\left[methane\right]\right)}{{\left[ethane\right]}_{init}}\times 100$$

$$\mathrm{TOF}\left[{\mathrm{s}}^{-1}\right]=\frac{Formation rate of ethane \left[\mathrm{mmol}·{\mathrm{s}}^{-1}\right]}{Total In amount \left[\mathrm{mmol}\right]}$$

3.5. In-Situ FTIR Spectroscopy

FTIR spectroscopy study was conducted using a home-made in-situ cell and FT/IR-4100 (JASCO, Japan) with a mercury cadmium telluride detector. Prior to measurement, In-CHA was prepared in situ by H₂ treatment of a self-supported disk of In₂O₃/CHA (ca. 40 mg) at 773 K and was then kept under vacuum at 773 K for 2 h. After taking a background spectrum below 153 K cooling the cell using liquid N₂, the sample was treated with H₂ flow at 773 K for 2 h. Then, the temperature was decreased to below 153 K again and the FTIR spectra were recorded without exposure to air.

4. Conclusions

In this work, a detailed characterization of In-CHA and the effect of zeolite on ethane dehydrogenation were described. In-situ XRD revealed that the RSSIE of In₂O₃/CHA occurs without decomposition of the zeolite framework. NH₃-TPD measurement showed that the amount of acid sites decreases with the increase of In loading amount. In the catalytic dehydrogenation of ethane, In-CHA(Al-rich) exhibits the highest conversion value among the In-exchanged zeolites, including In-MFI, In-MOR, and In-BEA. The combined results of kinetic experiments and in-situ FTIR spectroscopy suggest that [InH₂]⁺ ions are plausible active In-hydrides for In-CHA(Al-rich) and that the active In species and/or reaction mechanism for other In-exchanged zeolites are different from those of In-CHA. This study indicates that the small pores in CHA zeolites play an important role in their unique catalytic activity. We are now investigating the effect of CHA pores on the formation of In hydrides and the reaction mechanism of ethane dehydrogenation.