Catalyst Screening for Oxidative Coupling of Methane Integrated in Membrane Reactors

Increased availability of methane from shale gas and stranded gas deposits in the recent years may facilitate the production of ethylene by means of potentially more competitive routes than the state-of-the-art steam cracking processes. One appealing route is the oxidative coupling of methane (OCM), which is considered in this work for the production of ethylene by means of the use of catalytic membrane reactors (CMR) based on Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) ceramic material. In a first approach, a screening of 15 formulations as catalysts for the ethylene-ethane production was conducted on CMR consisting of disk-shaped planar BSCF membranes. At 900°C, the maximum C₂ selectivity was 70%, reached with Ba_0.5Sr_0.5FeO_3−δ and La_0.5Ce_0.1Sr_0.4Co_0.8Fe_0.2O_3−δ catalysts. On the other hand, low CH₄ conversions (X_CH4) resulted in C₂ yields below 3%. Operation at 1,000°C significantly shifted X_CH4 for all the activated membranes due to the decrease in CH₄/O₂ ratios, thus obtaining C₂ yields close to 9% and productivities of ca. 1.2 ml·min⁻¹·cm⁻² with Ce_0.9Gd_0.1O_2−δ and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ impregnated with Mn-Na₂WO₄ catalysts. The performance of OCM reaction was also studied in a tubular catalytic membrane reactor. Tubular configuration improved C₂ yield by minimizing CH₄/O₂ ratios up to 1.7, obtaining a maximum of 15.6% at 900°C with a BSCF capillary membrane activated with a packed bed of 2 wt% Mn/5 wt% Na₂WO₄ on SiO₂ catalyst.

Introduction

Ethylene is the most demanded organic compound worldwide, with a yearly production of 150 Mton in 2016 (Reporters, 2016). It is probably the most important basic petrochemical since it is used for producing other essential chemicals in nowadays economy, such as polyethylene, ethylene oxide, acetaldehyde or alcohols. Currently, ethylene is mainly produced by steam cracking of hydrocarbon feedstocks (naphta, gasoil, and condensates). Amongst all the existing feedstocks, ethane cracking is the most preferred, since almost no coproducts are obtained. Steam cracking is a non-catalytic process run at very high temperatures (typically 850°C). The main drawbacks are the process complexity and the downstream processing, the high energy requirements due to the endothermic reaction, and the operation complications due to the high coke formation rate. Despite the high maturity of the technology no major advances are foreseen for overcoming these weaknesses.

In the past recent years, ethylene production is focused on the use of light alkanes such as propane, ethane, and methane, mainly due to the more economic availability from shale gas and stranded gas exploitations that facilitate cost-competitive routes for the production of light olefins. Therefore, by considering these available feedstocks, several routes are being studied and developed for the production of ethylene from ethane such as the oxidative de-hydrogenation of ethane (ODHE) (Bhasin et al., 2001; Grubert et al., 2003), and from methane such as methanol-to-olefins reaction (Keil, 1999; Chen et al., 2005), Fischer-Tropsch synthesis (Lunsford, 2000), and the Oxidative Coupling of Methane (OCM) (Keller and Bhasin, 1982; Ito and Lunsford, 1985; Amenomiya et al., 1990; Lunsford, 1995). Amongst these, the use of CH₄ for the production of valuable chemicals is preferred due to economic reasons (Mleczko and Baerns, 1995; Mleczko et al., 1996), and in particular OCM is considered one of the most attractive options as a direct route for the direct conversion of CH₄ into C₂H₄.

The first works on OCM were reported in the early 80′s by Keller and Bhasin (1982). Since then, OCM has attracted much attention from both the academia and industry. A big number of developments were mainly focused on catalysts that maximize C₂H₄ yields and optimizing the reactor design for managing heat and streams (Otsuka et al., 1986; Hutchings et al., 1989; Amenomiya et al., 1990; Maitra, 1993; Mleczko et al., 1996). With regard to OCM reaction mechanism, Stansch et al. (1997) presented a kinetic model (Figure 1) where C₂H₄ formation mechanism involves a dehydrogenation step to form the radical ${\text{CH}}_3^{·}$ species, first producing C₂H₆ by ${\text{CH}}_3^{·}$ coupling (step 2) and then C₂H₄ by ethane dehydrogenation (steps 5 and 7). Since CH₄ is a very stable molecule, the CH₄ activation for the coupling requires breaking a strong C-H bond (ca. 439 kJ·mol⁻¹). Issues such as thermodynamic limitations for CH₄ conversion, low C₂ selectivity and catalysts deactivation due to coke formation, make OCM practically and commercially inviable (Karakaya et al., 2017). The techno-economical threshold for considering OCM as commercially available is set at 30% C₂ yield per single-pass (Farrell et al., 2016; Spallina et al., 2017).

FIGURE 1

Figure 1. Simplified OCM reaction mechanism presented by Stansch et al. (1997).

Catalytic Membrane Reactor (CMR) technology (Aseem and Harold, 2018) with the integration of Oxygen Transport Membranes (OTMs) (Tenelshof et al., 1995; Zeng et al., 1998; Akin and Lin, 2002; Sunarso et al., 2008) is a suitable option for overcoming these drawbacks. As can be seen in the model depicted in Figure 1, the OCM reaction performance is affected by several secondary reactions leading to undesirable CO_x formation, mainly due to an O₂ feeding excess. Therefore, one of the main aspects of OTMs for their application in OCM reaction is the dosing of O₂ in a controlled manner by tuning parameters such as temperature, pO₂ at feed side, residence time and space velocity by reactant gas stream flow variation, etc. Therefore, by adjusting conveniently the CH₄/O₂ stoichiometry the complete oxidation of CH₄ to CO_x can be avoided, while increasing C₂ selectivity. In addition, higher conversion can be reached without safety problems related to flammability limits in conventional co-feeding reactors.

Several research groups have performed OCM reactions by considering different OTM materials, geometries and catalysts for improving the reaction toward C₂ formation. In Table 1, some of the reported results are summarized. Tubular geometries are preferred for conducting OCM in CMRs, with a special emphasis in the use of catalysts that improve C₂ selectivity. Amongst all the considered cases, the best results have been obtained by Othman et al. reaching a C₂ yield of 39% on a LSCF hollow fiber activated with Bi_1.5Y_0.3Sm_0.2O_3−δ catalyst.

TABLE 1

Table 1. C₂ selectivity and yield for different OCM studies conducted on CMRs.

Oxygen transport membranes consist of gastight Mixed Ionic Electronic Conductors (MIEC) that diffuse oxygen through vacancies in the crystal lattice, and simultaneously transport electrons in the opposite direction, thus obviating the need for an external electrical short circuit. Their major advantage is an infinite oxygen selectivity, assuming no leakage through the membrane layer or the sealing, and resulting in high purity oxygen that can be directly provided to oxyfuel power plants or chemical processes. Oxygen permeation through the bulk material is governed by the Wagner equation¹, favored by increasing temperatures and chemical gradients across the membrane. On the other hand, thin membranes (below 50 μm) are strongly limited by the surface exchange kinetics of molecular oxygen.

In addition to the benefits of OTMs application in the conduction of OCM reaction, the possibility of adding catalytic layers consisting of MIEC active materials may produce an increase in the rate of surface reactions (Lobera et al., 2012b). The use of catalysts based on perovskites and fluorite compounds is of great interest due to their ionic-electronic conductive properties allowing the presence of ionic oxygen species at the materials surface. Therefore, the OCM reaction can take place all over the catalytic layer at the active sites localized on the oxygen vacancies on the surface of catalyst particles. Nano-structured materials leading to highly porous catalytic layers may improve the OCM performance by ensuring a proper gas diffusion media as well as a high specific surface area. Additionally, by properly doping with metal cations presenting different ionic radii, oxidation states and redox behavior, it is possible to improve the catalytic activity by increasing the number of oxygen vacancies and by varying the O-cation bond strength (Sunarso et al., 2008).

The present work is focused on the development of catalytic membrane reactors (CMR) for the production of C₂H₄ through the OCM reaction. For that aim, a screening of 15 catalyst formulations was performed on Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) planar membranes, following the CH₄ conversion, C₂₊ selectivity, C₂₊ yield, and C₂₊ production. The catalyst study was mainly focused on the use of different MIEC catalysts and by considering state-of-the-art OCM catalysts. Furthermore, a study on the influence of reactor geometry on OCM performance was conducted by considering a BSCF capillary activated with a Mn-Na₂WO₄ on SiO₂ catalytic packed bed.

Experimental

Dense Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ planar membranes of 15 mm diameter and 0.8 mm thickness were prepared by uniaxial pressing at 150 kN, and subsequently sintered in air for 5 h at 1,100°C. BSCF powder was provided by Fraunhofer Institute for Ceramic Technologies and Systems (IKTS, Germany). The catalysts for the activation of membranes can be grouped in (i) MIEC perovskites, (ii) lanthanide doped cerias, (iii) MgO, and (iv) LaSr/CaO. The catalysts based on MIEC perovskites consisted of ABO_3−δ formulations, obtained by doping A- and B- sites according to A_0.6Sr_0.4Co_0.5Fe_0.5O_3−δ and Ba_0.6Sr_0.4BO_3−δ formulations where A = La, Pr, Ba, Ce, Sm, and Nd and B = Fe and Co. MIEC catalysts powders were synthesized by Pechini method. For each catalyst, metal precursor nitrates were mixed in distilled water in order to obtain a clear solution. After complete dissolution, citric acid was added as a chelating agent, and ethylene glycol was added to polymerize with the chelating agent and produce an organometallic polymer (in a molar ratio 1:2:4 with respect to nitrates solution, citric acid, and ethylene glycol, respectively). This complexation was followed by dehydration at low temperature (up to 270°C) and finally, thermal decomposition of the precursors at 600°C formed the desired structural phases. Regarding the fluorite compounds, two different catalyst based on the system Ce_1−xLn_xO_2−δ (x = 0.1; Ln = Tb, and Gd) were prepared by co-precipitation method. This technique consists of the dissolution of commercial lanthanide nitrates mixture in distilled water at 50°C. (NH₄)₂CO₃ solution in a 1:1.5 molar ratio was dropped to achieve the total precipitation of the mixed cations. The resulting precursor powder was dried at 100°C after filtration and rinsing. MgO powders were supplied by Sigma-Aldrich. LaSr/CaO catalyst was prepared by co-precipitation method being the nominal composition 10 wt% La, 20 wt% Sr on CaO.

Catalytic activation of planar membranes was conducted by screen printing catalyst porous layers on one of the sides of the membrane, which will be exposed to the reaction. The screen printing ink consists of the ceramic powder, an organic binder (ethylcellulose) and a plasticizer (terpineol), mixed and subsequently refined in a three-roll mill. The ink is printed on the membrane surface through a 9 mm diameter mesh. Finally, the coated membranes were calcined in air at T = 1,010–1,050°C for 2 h.

The OCM experimental setup is illustrated in Figure 2. Gold O-ring gaskets were used for membranes sealing. Synthetic air (200 ml·min⁻¹) was fed into the feed side while a dilution of 10% methane in argon (100 ml·min⁻¹) was used as the sweep gas on the reaction side.

FIGURE 2

Figure 2. Schematic representation of the CMR used for planar membranes tests.

BSCF membranes with tubular geometry were acquired from Fraunhofer IKTS. BSCF capillaries were manufactured by plastic extrusion as described in Schulz et al. (2012), obtaining tubes with a length of 220 mm, an outer diameter of 3.25 mm, and an inner diameter of 2.55 mm, resulting in a wall thickness of 0.35 mm. The capillaries were dead-ended (sealed at one tube end). The capillary capping was made by joining a flat small disk by reactive air brazing (Erskine et al., 2002). Catalytic activation of BSCF capillary was done with a packed-bed consisting of 255 mg of Mn-Na₂WO₄/SiO₂ catalyst (d_p = 0.4–0.6 mm) and SiC in a relation 50% v/v. The experimental set-up used for conducting the tests on BSCF capillaries is depicted in Figure 3. The capillary exposed area was ~3.15 cm², corresponding to a capillary length of 3 cm. The contact of the rest of the capillary with the reactant gases was avoided by placing a quartz tube–acting as liner- above the packed-bed all along the capillary. This quartz tube presents an inner diameter of 3.75 mm, being sufficient for wrapping completely the BSCF capillary. Top and bottom inlets of the tube were blocked, thus avoiding the reactant gases to be in contact with the membrane. This was done mainly for performing the OCM tests in the 3 cm-long isothermal zone of the used furnace, thus ensuring a constant temperature in all the reaction media. Same gas stream compositions and flow rates were used as for the tests using planar membranes.

FIGURE 3

Figure 3. Simplified diagram of the lab-scale reactor for the conduction of capillary membrane studies.

Complete analysis of gases at both sides of the membrane was performed by gas chromatography (micro-GC Varian CP-4900 equipped with Molsieve5A, Pora-Plot-Q glass capillary, and CP-Sil modules). The flow and composition of the gas streams were individually controlled. Membrane gas-leak-free conditions were confirmed by continuously monitoring the N₂ concentration in the gas stream exiting the catalyst chamber. An acceptable sealing was achieved when the ratio between the oxygen flow leak and the oxygen flux was lower than 1%. The temperature was measured by a thermocouple attached to the membrane (reaction side). A proportional–integral–derivative (PID) controller maintained temperature variations within 2°C of the set point. For evaluating the reaction performance, parameters such as methane conversion (X_CH4), C₂₊-hydrocarbons selectivity (S_C2+) and yield (Y_C2+), and ethylene productivity are determined from GC compounds analysis. The equations for the determination of these parameters are herein presented:

$\begin{array}{rc}{X}_{CH4}=\frac{{\displaystyle {\sum }_i}{n}_i·F_i^{out}}{F_{CH4}^{out}-{\displaystyle {\sum }_i}{n}_i·F_i^{out}}·100& (1)\end{array}$

$\begin{array}{rc}{S}_{C2+}=\frac{n_{C2+}·F_{C2}^{out}}{{\displaystyle {\sum }_i}{n}_i·F_i^{out}}·100& (2)\end{array}$

$\begin{array}{rc}{Y}_{C2+}=0.01·X_{CH4}·S_{C2}& (3)\end{array}$

$\begin{array}{rc}{C}_{2+}productivity=\frac{Y_{C2+}·Q_{CH4}^{in}}{A_{effective}}& (4)\end{array}$

Where i includes all the species with carbon atoms in the products gas stream, F is the flow rate of the species expressed in mol·min⁻¹, n_i is the number of carbon atoms of component i, $Q_{CH4}^{in}$ is the initial CH₄ flow rate (in ml·min⁻¹) and A_effective is the effective membrane area for OCM reaction.

Results and Discussion

In a first approach, the OCM reaction was studied on BSCF disk-shaped planar membranes with thicknesses of 0.8 mm. As previously observed (Lobera et al., 2011, 2012a,b, 2017), surface catalytic activation of oxygen membranes produces a performance improvement of the chemical reactions, especially ethylene production. This is mainly due to the presence of active elements towards CH₄ activation. Therefore, by considering catalyst particles with high O²⁻ mobility and redox properties, products selectivity and yield can be significantly increased. Furthermore, the presence of porous structures increases the specific surface area, enlarging the number of active sites and improving gas flow dynamics for a better gas diffusion. Then, the activation of a BSCF membrane with several known active catalysts was considered (Lobera et al., 2011, 2012a,b, 2017). On Table 2, all the tested membranes are listed indicating the specifications of the catalytic layers.

TABLE 2

Table 2. Catalysts for the activation of BSCF membranes.

All the catalysts were subjected to the same conditions of temperature and pO₂, necessary to seal the membrane and be used as a CMR for OCM, so the final microstructure depends on the sintering behavior of each catalyst material under these preparation and sealing conditions. Figure 4, shows SEM images of some of the activated BSCF membranes. As expected, different layer morphologies are obtained depending on the deposited material, which will also play a role on the catalytic performance of each porous layer. A highly porous structure of 15 μm is obtained when depositing a BSCF layer and subsequently sintering at 1,010°C (Figure 4b). NdSCF, SmSCF, and LaCeSCF catalysts (Figures 4c,e,f) present a layer thickness in the range of 12–15 μm and also lower porosity degree than BSCF layer. LaSr/CaO activation (Figure 4d) layer, on the other hand, resulted in negligible porosity.

FIGURE 4

Figure 4. SEM images of different BSCF-coated membranes: (a) bare, (b) BSCF initially sintered at 1,010°C, (c) NdSCF, (d) LaSr/CaO, (e) SmSCF, and (f) LaCeSCF.

The activated BSCF membranes were tested at 900°C. Gas feed rate was 200 ml·min⁻¹ of synthetic air (21% O₂) and, as sweep gas, a reactant gas stream consisting of 100 ml·min⁻¹ of 10% CH₄ in argon was employed in the reaction chamber. Under these conditions, the O₂ resulting from membranes permeation was in the range of 1.16–1.68 ml·min⁻¹·cm⁻², corresponding therefore to CH₄/O₂ ratios of 7.5–11. The obtained results are shown in Figure 5, where CH₄ conversion, selectivity toward C₂₊ production, C₂₊ yield, and C₂₊ productivity are presented for each catalyst. The best results are obtained with the BSF_1010 activated membrane, reaching nearly 70% of C₂₊ selectivity, two fold the C₂₊ selectivity obtained with a non-activated BSCF membrane. Similar results were also observed in an ODHE catalytic study conducted on BSCF membranes where, despite the lower reducibility of BSF, higher C₂₊ selectivity was obtained (Lobera et al., 2012a). This can be related to a more suitable capability of BSF for conducting the ${\text{CH}}_3^{·}$ coupling regardless its lower redox properties leading to a lower CH₄ abstraction and subsequently, to a lower methane conversion. LaCeSCF and BaLaPrSCF activated membranes also present higher C₂₊ selectivity than BSCF (S_C2+ in the range of 27–34%), with values of 64.7 and 58.2%, respectively. This improvement is ascribed to the incorporation of cations with mixed valence (Ce and Pr) in the A lattice position, that results in the modification in the redox behavior. The latter seems to alleviate the acidity increase resulting from the doping with cations presenting higher oxidation states (Zhou et al., 2008) that would lead to secondary reactions and to a loss in C₂₊. This gain in catalyst basicity is observed for SmSCF, which yields better C₂₊ selectivity than BSCF, being above 55%, whereas a more basic NdSCF performs similarly to BSCF. With regard to the lanthanide-doped cerias (CeTbO and CeGdO) the better performance is attributed to the combination of high oxygen-ion mobility and their better red-ox properties (Balaguer et al., 2011). The performance of LaSr/CaO catalyst regarding S_C2+ is lower than the observed in the literature (Wang et al., 2005; Olivier et al., 2009) due to the lower surface specific area that the catalytic layer presents in this work (Figure 4d), resulting in a lesser number of accessible active sites, as well as a hindrance for the gas diffusion on the catalytic media. The low selectivity (13–15%) observed for the membranes activated with MgO interlayer is expected from to the low activity of MgO in the formation of CH₃· radicals (Pak et al., 1998) and the observed deactivation of MgO catalysts with OCM reaction time (Wang et al., 1995; Pak et al., 1998). It is also significant the low C₂₊ selectivity observed for the membrane activated with Mn-Na₂WO₄ on SiO₂ catalyst. Only a S_C2+ of 30% is obtained, when previous studies show C₂₊ selectivity of up to 80% (Farrell et al., 2016). The reason for such a low result can stem from the reaction between SiO₂ and BSCF leading to the formation of a SiO₂ layer on membrane surface and to the partial blocking of O₂ permeation (Thaler et al., 2016). Moreover, SiO₂ sintering and transition to α-Cristobalite from amorphous silica (catalyst support) also results in a loss of selectivity (Palermo et al., 1998; Asadi et al., 2012). It is worthy to mention that BSCF membrane activation with a BSCF porous layer does not entail a performance improvement in all the systems. Some of them present lower C₂₊ selectivity as compared with the bare membrane, which can be related with a gas diffusion hindrance resulting from the layer deposition, thus preventing CH₄ to reach the active sites or due to a fast recombination of surface O²⁻ into gaseous O₂, which would react with CH₄ causing combustion, and making these catalysts not useful for the system in a first approach. Concerning CH₄ conversion (Figure 5A) the obtained results are in the range of 2–8%, with MgO and MgO_Mn-Na₂WO₄ yielding the highest methane conversion rates, in spite of their low C₂₊ selectivity. Moreover, the use of 2 wt% Mn/5 wt% Na₂WO₄ on SiO₂ as catalyst coated on top of the 3 BSCF_1010 layer rises X_CH4 from 2 to 7%, while maintaining S_C2+. This important increase in conversion may arise from the high activity of Mn-Na₂WO₄ on SiO₂, in spite of the low C₂₊ production. Ceria based catalysts also present high conversion rates. Activation with CeTbO and CeGdO results in X_CH4 of 5.3 and 6.5%, being higher than most of the tested perovskite catalysts. Again, the better redox properties and the higher ionic conductivity favor the conduction of OCM reaction.

FIGURE 5

Figure 5. (A) CH₄ conversion and selectivity to C₂₊-hydrocarbon and (B) yield to C₂₊-hydrocarbon for different planar BSCF membrane reactors at 900°C. 10% of CH₄, Q_(CH4+Ar) = 100 ml·min⁻¹, Q_Air = 200 ml·min⁻¹ (pO₂ = 0.21 bar).

The highest C₂₊ yields at 900°C (Figure 5B) range among 2.5–3% and correspond to CeTbO, CeGdO, LaCeSCF, and LaSr/CaO catalysts. On the other hand, maximum C₂₊ productivities correspond to LaCeSCF and CeTbO activated membranes. Their C₂₊ productivities of 0.37 ml_C2+·min⁻¹·cm⁻² represent nearly a four-fold improvement with respect to the non-activated membrane.

Further OCM tests were performed at 1,000°C maintaining the same feed and reactant gas flow rates as in the previous tests conducted at 900°C. Nevertheless, and due to the increase in temperature, the O₂ permeation of the membranes also increased (up to 2.2–2.6 ml·min⁻¹·cm⁻²). This led to lower CH₄/O₂ ratios in the range of 4.8–5.6, and subsequently, to a significant increase in CH₄ conversion for all the catalysts, with a maximum X_CH4 of 18.5% for the 3 BSCF_1010 Mn-Na₂WO₄ activated membrane (Figure 6A). The increase in temperature and the subsequent reduction in CH₄/O₂ ratios also allowed an improvement in C₂₊ selectivity (74%) for most of the catalysts, presenting values above 50%. This gain in X_CH4 and S_C2+ can also be ascribed to the higher generation of ${\text{CH}}_3^{·}$ when operating at higher temperatures (Xu and Thomson, 1997). Again, the highest C₂₊ selectivity is reached with BSF_1010 catalytic layer. As can be seen in Figure 6B, operation at 1,000°C produces an important shift in C₂₊ yield and productivity, with a peak yield of nearly 9% when considering CeTbO activation. C₂₊ yields >8.7% are also obtained with CeGdO and 3 BSCF_1010 Mn-Na₂WO₄ catalysts. A maximum of C₂₊ productivity of 1.14 ml_C2+·min⁻¹·cm⁻² is obtained with CeTbO. According to the 30% yield target established for considering OCM reaction as techno-economically viable, the obtained results are far below this threshold, despite the good C₂₊ selectivities obtained for some of the catalysts.

FIGURE 6

Figure 6. (A) CH₄ conversion and selectivity to C₂₊-hydrocarbon and (B) yield to C₂₊-hydrocarbon for different planar BSCF membrane reactors at 1,000°C. 10% of CH₄, Q_(CH4+Ar) = 100 ml·min⁻¹, Q_Air = 200 ml·min⁻¹ (pO₂ = 0.21 bar).

In a previous study, Zeng and Lin observed a relation between CMR configuration and OCM reaction performance, reporting an improvement in C₂₊ yield from 11 to 17% by increasing membrane surface area to reactor volume ratio (Zeng and Lin, 2001). Indeed, issues such as reactor design/configuration and inadequate reactant gas distribution can prevent the proper access of CH₄ molecules to the active sites, thus limiting methane conversion rates. Therefore, OCM studies on BSCF tubular membranes reactor were also considered. Tubular designs provide a distributed feed of O₂ along the reactor length and a higher surface reaction area, leading to higher CH₄ conversion rates per membrane unit length, and increasing C₂₊ yields. A catalytic packed-bed consisting of 50% vol. 2% Mn, 5% Na₂WO₄ on SiO₂ and SiC was prepared for the activation of a BSCF capillary. OCM tests were carried out at 900°C, 200 ml·min⁻¹ of synthetic air feed, and using a stream of 10% CH₄ in argon as reactant gas. The reactant gas flow rates varied from 50 to 600 ml·min⁻¹, corresponding to CH₄/O₂ ratios of 1 and 2.7, respectively. As a result, X_CH4 progressively decreased (Figure 7) when increasing reactant flow rate, thus evidencing the effect of the higher CH₄/O₂ ratio. Nevertheless, the lower residence time resulting from the increase in reactant gas flow rate is also expected to affect negatively CH₄ reaction. Differently, C₂₊ selectivity progressively improves with increasing reactant gas flow rate, with a maximum of ca. 45% at 600 ml·min⁻¹. The highest C₂₊ yield is achieved at 100 ml·min⁻¹ with a value of 15.6% at a CH₄ conversion of 54% and a C₂₊ selectivity of 29%. Concerning C₂₊ productivity, 0.9 ml·min¹·cm⁻² are obtained with a flow rate of 400 ml·min⁻¹. On the other hand, low space velocities involve higher S_CO2, due to a higher CH₄ oxidation toward CO₂ at higher residence times. As the main interest of this reaction is the production of C₂₊, then the most suitable conditions would be set at low space velocities, with flow rates in the range of 50–200 ml·min⁻¹.

FIGURE 7

Figure 7. Effect of the variation of reactant gas flow rate on X_CH4 and S_C2+, and on Y_C2+ and C₂₊H₄ productivity (Inset). Test conducted at 900°C with 200 ml·min⁻¹ synthetic air feeding and 10% CH₄ in Ar.

Finally, Figure 8 summarizes and compares the results obtained for both planar and tubular BSCF CMRs considered in the present work, represented by the S_C2+ as a function of the converted CH₄. These results were obtained at 900°C and reactant gas flow rates in the range of 100–300 ml·min⁻¹. Under such conditions, planar membranes show CH₄/O₂ ratios in the range of 7.5–11, whereas tubular configuration presents much lower ratios of 1.7–2.4. Indeed, significant shift in X_CH4 is visible when comparing planar and tubular CMRs activated with Mn-Na₂WO₄/SiO₂ catalyst due to the higher availability of O₂. While S_C2+ of 30–40% was obtained for both cases, CH₄ conversion increased from 4 to 7% up to 60% when conducting OCM in the tubular reactor. This is due to the higher membrane surface area to reactor volume ratio of tubular configurations that allows the dosing of higher amounts of O₂. This eventually improves OCM performance by boosting CH₄ conversion, achieving C₂₊ yields of 15.6%. The best results regarding selectivity toward C₂₊ production correspond to planar membranes activated with BSF, LaCeSCF, and SmSCF, presenting selectivity of ca. 65–70%. Nevertheless, the low X_CH4 rates lead to poor C₂₊ yields -ranging from 0.5 to 2.5%, which are very low for practical consideration. The better performance observed with the tubular CMR is mainly ascribed to the lower CH₄/O₂ ratios used during the tests, which results from the increase of membrane surface area with respect to reactor volume. Therefore, for a given reactor geometry, the increase of this parameter and the selection of operation conditions resulting in higher CH₄/O₂ ratios can lead to the achievement of higher C₂₊ yields. For our reactor, the high conversions of nearly 60% obtained with the tubular CMR combined with the high C₂₊ selectivity of catalysts like LaCeSCF or SmSCF set a promising scenario for reaching the considered C₂₊ yield target of 30% if these catalysts are used for the activation of BSCF capillary membranes.

FIGURE 8

Figure 8. C₂₊ hydrocarbon selectivity in dependence of methane conversion for different activated BSCF membranes. Test conditions: 900°C, synthetic air feeding (200 ml·min⁻¹), 10% CH₄ in argon as reactant gas (100–300 ml·min⁻¹).

Conclusions

A study on oxidative coupling of methane for the production of ethylene was conducted considering CMR based on BSCF. In a first approach, a catalytic screening in CMR for OCM was conducted on disk-shaped planar BSCF membranes. The catalytic activation of membranes with 15 different catalysts allowed the identification of the most active compounds on these temperature conditions for the maximization of C₂₊ production. The highest C₂₊ selectivity at 900°C was 70%, reached with BSF and LaCeSCF catalysts. Despite the good selectivity, the low CH₄ conversions resulted in C₂₊ yields below 3% for this CMR configuration. Operation at higher temperatures (e.g., 1,000°C) produced a significant improvement in X_CH4 for all the activated membranes due to the decrease in CH₄/O₂ ratios, thus obtaining much higher C₂₊ yields of up to nearly 9% and productivities of ca. 1.2 ml·min⁻¹·cm⁻² with CeGdO and 3 BSCF_1010 Mn-Na₂WO₄ catalysts. Conduction of OCM reaction on a catalytic membrane reactor with tubular geometry permitted the achievement of CH₄/O₂ ratios of 1.7–2.5 that improved C₂₊ yield, with a maximum of 15.6% at 900°C for a BSCF capillary activated with 2 wt% Mn/5 wt% Na₂WO₄ on SiO₂ catalyst. This improvement is ascribed to the combined effect of the catalyst high activity toward CH₄ conversion and the higher membrane surface area available for the conduction of OCM reaction and favorable geometry, resulting in lower CH₄/O₂. Therefore, the selection of the most active catalysts under CMR conditions, the operation with low CH₄/O₂ ratios, and the addressing of actions for the improvement of CMR design would allow achieving higher C₂₊, approaching techno-economic targets.

Author Contributions

JS: project manager; JS, ML, MB, and JG-F: manuscript writing and results interpretation; ML, MB, and JG-F: experiments.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

Financial support by the Spanish Government (ENE2014-57651 and SEV-2012-0267 grants) and by the EU through FP7 GREEN-CC Project (GA 608524), is gratefully acknowledged. The authors want also acknowledge the Electron Microscopy Service from the Universitat Politècnica de València for their support in the SEM analysis performed in this work.

Footnotes

1. ^ $J\left(O_2\right)=\frac{RT}{16F^{2}L}{\displaystyle {\int }_{p{O}_2{}^{\text{'}}}^{p{O}_2{}^{"}}{\sigma }_{amb}dlnp{O}_2}$

where J(O₂) is the oxygen permeation, R is the gas constant, T is the temperature, F is the Faraday′s constant, L is the membrane thickness, pO₂” and pO₂' stand for the O₂ partial pressures at feed and permeate sides, respectively, and σ_amb is the ambipolar conductivity of the membrane material.

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