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Direct conversion of methane to aromatics in a catalytic co-ionic membrane reactor

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Science  05 Aug 2016:
Vol. 353, Issue 6299, pp. 563-566
DOI: 10.1126/science.aag0274

Membranes to make benzene from methane

Methane gas is expensive to ship. It is usually converted into carbon monoxide and hydrogen and then liquefied. This is economically feasible only on very large scales. Hence, methane produced in small amounts at remote locations is either burned or not extracted. A promising alternative is conversion to benzene and hydrogen with molybdenumzeolite catalysts. Unfortunately, these catalysts deactivate because of carbon buildup; plus, hydrogen has to be removed to drive the reaction forward. Morejudo et al. address both of these problems with a solid-state BaZrO3 membrane reactor that electrochemically removes hydrogen and supplies oxygen to suppress carbon buildup.

Science, this issue p. 563

Abstract

Nonoxidative methane dehydroaromatization (MDA: 6CH4 ↔ C6H6 + 9H2) using shape-selective Mo/zeolite catalysts is a key technology for exploitation of stranded natural gas reserves by direct conversion into transportable liquids. However, this reaction faces two major issues: The one-pass conversion is limited by thermodynamics, and the catalyst deactivates quickly through kinetically favored formation of coke. We show that integration of an electrochemical BaZrO3-based membrane exhibiting both proton and oxide ion conductivity into an MDA reactor gives rise to high aromatic yields and improved catalyst stability. These effects originate from the simultaneous extraction of hydrogen and distributed injection of oxide ions along the reactor length. Further, we demonstrate that the electrochemical co-ionic membrane reactor enables high carbon efficiencies (up to 80%) that improve the technoeconomic process viability.

Natural gas constitutes a large and relatively clean fraction of fossil hydrocarbon resources, but the high capital cost of multistage industrial conversion via synthesis gas (i.e., syngas, a mixture of H2 and CO) leaves much of it stranded. Nonoxidative methane (CH4) dehydroaromatization (MDA) is a promising catalytic route that directly converts natural gas into valued petrochemicals such as benzene. The MDA reaction is conventionally run at ~700°C in the presence of bifunctional catalysts comprising carbided molybdenum nanoclusters dispersed in acidic shape-selective zeolites such as ZSM-5 and MCM-22 (1). The process suffers from two major hurdles that challenge its further development and industrial implementation: The per-pass conversion is limited by thermodynamics, and the catalyst activity rapidly drops with time on stream because of the accumulation of polyaromatic-type coke on the external zeolite surface that impedes the access to internal active sites (2, 3).

Attempts to overcome thermodynamic limitations by selective removal of the coproduct H2 from the reactor using, for instance, Pd-type (4) or ceramic (La5.5W0.6Mo0.4O11.25-δ) (5) membranes were limited by enhanced coke formation that accelerated catalyst decay. Strategies based on fine-tuning the zeolite acidity and porosity and cofeeding small amounts of CO2, CO, H2, and H2O with CH4 were applied to stabilize the catalyst by restraining the production of coke, but with limited success (2, 6, 7).

Recently, a direct nonoxidative CH4 conversion path on single-iron sites embedded in a silica matrix with negligible coke formation and high stability has been reported (8). However, this reaction requires very harsh conditions (950°C) and produces ethylene (rather than liquids) as the major product, with selectivity of ~55%.

Here we present an approach to circumvent the current limitations of the MDA reaction by integrating an ion-conducting membrane into the reactor. We report an innovative catalytic membrane reactor (CMR) for intensification of the MDA process that resulted in high and prolonged aromatic yields. In addition, a high-purity H2 stream is produced during CMR operation. The CMR is driven by a tailored co-ionic membrane that enables fast and accurate simultaneous control of H2 extraction and injection of oxygen species along the catalyst bed (Fig. 1A). The concerted action of both functions leads to marked gains in aromatics yield and catalyst stability and, consequently, in the viability of MDA technology.

Fig. 1 Current-controlled co-ionic membrane reactor.

(A) CH4 is converted to benzene and hydrogen via a Mo/zeolite catalyst. H2 is transported as protons to the sweep side. Oxide ions are transported to the reaction medium to react with H2 and form steam as an intermediate before reacting with coke to form CO and H2. (B) Scanning electron microscopy image of the membrane electrode assembly (focused ion beam section). Cathode porosity formed upon reduction of NiO can be observed beneath the dense electrolyte. (C) Percentage of H2 extracted and O2 injected versus current density at 700°C. The anode is swept with a 10/90 mixture of H2/CH4 and the cathode with a 3/5/92 mixture of H2O/H2/Ar.

The electrolyte of the membrane is based on acceptor-doped BaZrO3, which takes up protons from steam and exhibits high proton (H+) and minor oxide ion (O2–) conductivity at elevated temperatures (9). Applications of protonic conductivity have shown promising results (1012), but, as shown here, the co-ionic transport property of the material—more specifically, the conduction ratio of protons and oxide ions—allowed the successful implementation into the MDA process. The tubular membrane consists of a dense 25-μm-thick BaZr0.7Ce0.2Y0.1O3–δ (BZCY72) electrolyte film on a porous BZCY72-Ni support, which also acted as a cathode (11). The metallic Ni had sufficient catalytic activity for H2 evolution and reduction of steam (Fig. 1B). A Cu-based anode was applied on the electrolyte film facing the catalyst; it activates the electrochemical oxidation of H2 into protons while preventing secondary conversion of hydrocarbons into coke, as typically reported for Ni- or Pt-based electrodes (13). As the current density was increased, both hydrogen extraction and oxygen injection increased proportionally, so that the amount of O2 injected was ~0.3% that of extracted H2 (Fig. 1C).

Figure 2 shows the results of MDA experiments, comparing our CMR with a fixed-bed reactor (FBR) under otherwise similar conditions using 6Mo/MCM-22 as a catalyst. The catalyst behavior in the FBR is fully representative of the state of the art at standard MDA conditions: The aromatics yield initially increased during the induction period, reached a maximum of ~10%, and rapidly fell as the reaction progressed. In contrast, by applying an electrical current to the CMR (“on” mode), the aromatics yield continued to increase beyond the induction period and attained a maximum of ~12%, after which the catalyst activity started to decline (Fig. 2A). An almost-instant catalytic response (for conversion, see fig. S2) (14) to on-off switching, as well as to changes in the intensity of the imposed electrical current, allowed us to accurately tune the catalytic performance of our CMR system. Interestingly, the yield enhancement observed upon current application occurred while maintaining the characteristic high selectivity to aromatics [particularly to benzene, >85% on a coke-free basis (Fig. 2B)] of the shape-selective 6Mo/MCM-22 catalyst. However, CMR operation produced some CO, albeit in relatively low selectivity (see below). The most notable result in Fig. 2A is the improved stability of the catalyst in the CMR, with an average decay rate about one order of magnitude lower than that observed in the conventional FBR. Although the aromatics yield decreased to ~1.5% in the FBR after 45 hours of reaction, it remained as high as ~9% in the CMR, translating into a twofold increase in the cumulative yield (Fig. 2C). The high stability exhibited by the catalyst in the CMR arose from a decreased tendency to form coke, which became more evident at longer reaction times (Fig. 2C).

Fig. 2 FBR and co-ionic CMR performance in MDA using a 6Mo/MCM-22 catalyst.

(A) Aromatics yield versus time. Gray-shaded areas indicate when hydrogen is extracted. (B) CH4 conversion and selectivity to main products after 5 hours (FBR) and 9 hours (CMR). (C) Coke deposition in 6Mo/MCM-22 and cumulative aromatics production in grams per gram of catalyst. Reaction conditions: 710°C, 1500 ml g–1 hour–1, 1 bar, and current density of 40 mA cm–2.

Thermodynamic calculations predict that in situ H2 extraction increases CH4 conversion and shifts selectivity toward heavier aromatics (and, ultimately, coke) at the cost of benzene and C2 hydrocarbons (fig. S3) (14), as experimentally demonstrated with H2 permselective membranes (15, 16). Although thermodynamics thus accounts for the increase in CH4 conversion, the high benzene selectivity and improved catalyst stability during the galvanic operation in our CMR cannot be anticipated by considering merely those effects related to H2 extraction.

The BZCY72 membrane enables the concomitant transport of oxide ions toward the reaction medium, where they rapidly oxidize the produced H2 to steam at the electrode (17). Thus, we investigated the isolated effect of steam on the performance of the 6Mo/MCM-22 catalyst in the FBR by cofeeding 0.25 to 0.9 mole % steam together with CH4, corresponding to steam concentrations within the range achieved by the O2 injection in our CMR. Whereas the observed decrease in both conversion and aromatics selectivity (fig. S4) (14) is thermodynamically consistent (18), postreaction characterization of the spent catalysts by thermogravimetry and temperature-programmed oxidation analyses showed that the improved stability achieved in the CMR could be ascribed to the inhibition of coke formation by the in situ–generated steam (fig. S5) (14). The steam-promoted coke suppression during MDA has also been reported for oxygen-permeable membrane reactors (19) and probably occurs by a mechanism that involves scavenging of reactive carbon from the catalyst surface via steam reforming (20), which accounts for the observed formation of CO (Fig. 2B). The superior stability achieved in our CMR compared with the FBR experiment with an equivalent steam concentration (0.25 mole %) indicates that the controlled and distributed oxygen injection is more effective in improving the catalyst stability than the external addition of steam.

Additionally, the x-ray absorption near-edge structure analysis at the Mo K-edge and x-ray photoelectron spectroscopy Mo 3d spectral signals (fig. S5) (14) did not reveal appreciable changes in Mo speciation during CMR operation, with respect to the FBR fed with pure CH4. Conversely, a higher average Mo oxidation state is inferred for the catalyst used in the FBR experiment cofed with 0.9 mole % steam, which might imply a certain loss of active molybdenum carbide species by reoxidation (20). The crystalline structure of the zeolite host remained almost intact upon contact with the in situ–generated steam under MDA conditions (fig. S5) (14). Thus, the distributed O2 injection allowed by the BZCY72 membrane effectively suppressed coke production while preserving the structural integrity of the zeolite and active Mo-carbide sites.

In situ extraction of H2 with the CMR should shift the equilibrium of aromatics formation, which would have major consequences for process industrialization. In Fig. 3A, the experimentally obtained yield of aromatics is plotted as a function of the magnitude of both H2 extracted and equivalent O2 injected. High H2 extraction rates (>60%) with respect to the H2 produced in the MDA reaction were achieved by using the electrochemical cell reactor. By increasing the magnitude of the imposed co-ionic current, the aromatic yield rose and surpassed the theoretical equilibrium yield (12.3%) at H2 extraction rates exceeding 50%. As expected from the coke-suppression mechanism operating in the CMR, CO formation was negligible when no current was imposed and rose parallel to the aromatics yield with increasing co-ionic currents (Fig. 3A). These results show that the galvanic current drove the transport of atomic oxide ions. This oxide ion supplies reduced coke production and decreased the catalyst degradation rate by a factor of 6 with respect to the FBR, even at low extraction rates (18%), and then continued to decrease smoothly at higher currents (Fig. 3B).

Fig. 3 Effect of co-ionic membrane reactor.

(A) Aromatics and CO yield as a function of H2 extracted and O2 injected at 700°C and 1 bar. (B) Deactivation rate constant, assuming first-order catalyst decay, as a function of H2 extracted and O2 injected.

To assess the practical implications of the described CMR, we performed process simulations using Aspen tools (21). Figure 4A schematizes a complete gas-to-liquid process based on our MDA reactor architecture and includes recycling of the reactant CH4 stream. In this process, the critical parameter for maximizing the per-pass conversion is the H2 concentration in the recycle loop. By including a methanation stage, CO and H2 are converted to CH4 and steam, generating a typical H2 concentration of 5% at the reactor inlet (fig. S7) (14). Experimental CMR results under recycle operating conditions (5% H2 cofeed) resulted in aromatics yields of about 6.5%, with near-zero degradation rates (Fig. 4B). In Fig. 4C, the process performance metrics for different extraction rates (50 to 80%) are compared with (i) a plant based on a conventional MDA reactor, implementing downstream gas fractioning with polymeric membranes (FBR-PolyM) (22), and (ii) a plant based on a CMR with Pd membranes (Pd-CMR) (4). Carbon efficiency is superior for our CMR system, improving steeply with increasing extraction rates. At rates exceeding 80%, the carbon efficiency is similar to that exhibited by large and optimized Fischer-Tropsch (FT) plants (23). The difference between both processes relies on plant size and complexity. Although the traditional FT process requires multiple steps, including syngas production, the MDA co-ionic CMR produces aromatics directly. This feature allows for modularity and flexibility to adapt to the size of the natural gas field, in contrast to FT plants that become uneconomic at small to medium scales (1 to 10 metric tons hour–1).

Fig. 4 Synloop process for MDA using co-ionic CMR.

(A) Process flow diagram. (B) Aromatics yield versus time on stream at the following conditions: 700°C, 3000 ml g–1 hour–1, 3 bar, and 5% H2 cofeeding. The gray-shaded area indicates when H2 is extracted. (C) Carbon efficiency of a synloop process using co-ionic CMR for different H2 extractions (50 to 80%) compared with plants based on Pd-CMR (4) and FBR with external H2 removal (polymer membrane) (20).

Supplementary Materials

www.sciencemag.org/content/353/6299/563/suppl/DC1

Materials and Methods

Figs. S1 to S9

Tables S1 and S2

References (2427)

References and Notes

  1. Materials and methods are detailed in the supplementary materials on Science Online.
  2. Chemical process simulation and optimization were performed using ASPEN PLUS v8.0 software.
  3. Acknowledgments: This work was supported by the Research Council of Norway (grants 195912, 210418, 210765, and 219194) and the Spanish government (grants SEV-2012-0267 and ENE2014-57651). We thank the ALBA Synchrotron Light Laboratory for beam time provision. C.K. and P.K.V. have applied for a patent based on this work (PCT/EP2014/071697). Experimental data are available online at ftp://itqrepositorio.itq.upv.es/pub/.
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