Carbohydrate dehydration using porous catalysts

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Carbohydrate dehydration using porous catalysts Jacob S Kruger, Vladimiros Nikolakis and Dionisios G Vlachos Sugar dehydration is an effective way to deoxygenate biomass for the production of renewable chemicals and fuels. This chemistry typically happens using inorganic acids that impose major environmental burdens. We review the use of heterogeneous microporous and mesoporous catalysts for this chemistry and the key attributes of such materials, that is, the ratio of Brønsted to Lewis acid sites, mesoporosity, and hydrophobicity. While some of these materials, especially those combining microporosity and mesoporosity, show promising results for biomass processing in aqueous environment, there is a clear lack of fundamental understanding that severely limits their commercial use for these reactions. Potential barriers that need to be overcome for the use of heterogeneous acid catalysts are discussed. Address Catalysis Center for Energy Innovation, Department of Chemical and Biomolecular Engineering, University of Delaware, 150 Academy Street, Newark, DE 19716, USA Corresponding author: Vlachos, Dionisios G. ([email protected])

Current Opinion in Chemical Engineering 2012, 1:312–320 This review comes from a themed issue on Reaction engineering and catalysis Edited by Theodore T Tsotsis For a complete overview see the Issue and the Editorial Available online 6th July 2012

The attractiveness of shape selectivity has led to the investigation of microporous and mesoporous solid acid catalysts, such as ion-exchange resins and zeolites, for the conversion of sugars since the late 1970s. Recent research has continued with these materials, with increasing focus on the development of new mesoporous silicas. Results have indicated that porous acid catalysts have the potential to fill an interesting niche in sugar dehydration if their catalytically active sites can be stabilized to leaching and thermal regeneration. Table 1 compares some carbohydrate dehydration catalysts. Each catalyst within these general categories has a characteristic ratio of Lewis and Brønsted acidity, porosity, and set of interaction parameters with solvents and reactants (i.e. hydrophobicity). These properties have often been invoked to explain empirical trends, but a fundamental understanding is still lacking. What makes understanding materials properties challenging is that these parameters are often interconnected and their role varies with operating conditions. As a result, phenomenological models fitted to experimental data have a very limited scope. This review focuses on some of the properties of microporous and mesoporous catalysts, particularly catalyst acidity, mesoporosity, and hydrophobicity, as well as adsorption and diffusion characteristics, that offer timely opportunities for fundamental research.

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Catalyst properties

http://dx.doi.org/10.1016/j.coche.2012.06.003

Traditionally, simple sugars and polysaccharides have been hydrolyzed and dehydrated with Brønsted acid catalysts. However, some Lewis acid catalysts have also been shown to be active for carbohydrate conversion. Roma´n-Leshkov and Davis recently reviewed the role of Lewis acidity in the chemistry of aqueous carbonylcontaining molecules [11]. The authors emphasized the chemistry of sugar conversion, highlighting insights and noting several gaps in the fundamental understanding. To our knowledge, the most fundamental experiments to elucidate the roles of Brønsted and Lewis acidity in sugar dehydration were performed by Weingarten et al. [12], who compared a series of homogeneous and heterogeneous acid catalysts in aqueous-phase dehydration of xylose to furfural, shown schematically at the top of Figure 1. Their series of catalysts contained Brønstedto-Lewis ([B]/[L]) ratios ranging from zero to infinity. The authors found that catalyst activity was correlated with Lewis acidity, while selectivity to furfural was correlated with Brønsted acidity. Further, they found that both Brønsted and Lewis sites were active for converting

Introduction One of the primary challenges in the field of biomass conversion to fuels and chemicals is the controlled removal of oxygen-containing functional groups. Biomass can be deoxygenated through two routes: dehydration, yielding H2O, or decarbonylation and decarboxylation, yielding CO and CO2, respectively. Dehydration is particularly promising in the conversion of sugar derivatives of biomass (cellulose and hemicellulose, as well as other polysaccharides such as starch and inulin) because it does not reduce the number of carbon atoms in the feedstock and does not produce CO2. Reactions that are important in sugar dehydration include isomerization of aldoses and ketoses, dehydration of sugars to their furan derivatives, and rehydration of furans to acidic platform molecules, such as levulinic acid. Several recent reviews cover the catalysis and reaction engineering literature for these reactions [1–10]. Current Opinion in Chemical Engineering 2012, 1:312–320

Brønsted versus Lewis acidity

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Dehydration of carbohydrates Kruger, Nikolakis and Vlachos 313

Table 1 General comparison of common sugar dehydration catalysts

Advantages

Type

Mesoporous silica

Ion-exchange resin

Zeolite

Homogeneous acid a

Disadvantages

High yields in some conditions Minimal diffusion limitations Easy separation from product Potentially tunable acidity a

Potential hydrothermal instability Potentially difficult to regenerate

Highest yields to date Minimal diffusion limitations Easy separation from product Potentially tunable acidity a

Hydrothermal instability Difficult to regenerate

Hydrothermal stability Easy to regenerate thermally Easy separation from product Potentially tunable acidity a

Potential diffusion limitations Generally low yields Potential trapping of desired products

Material cost High yields in some conditions

Difficult to separate from product No potential for shape selectivity Corrosivity

In terms of Brønsted and Lewis site concentration and strength.

Figure 1

HO

O

O

OH

O OH

H

H

OH

H

O

O

acid

extraction

H

(aq.)

(aq.)

(tol.)

Weingarten et al., water [12] Lima et al., water/toluene [16-18]

0.6

Furfural Selectivity

0.5

water water/toluene

0.4

0.3

0.2

0.1

0.0 0.0

0.2

0.4

0.6

0.8

1.0

Fraction of Bronsted Acid Sites Current Opinion in Chemical Engineering

Top: Scheme of conversion of xylose to furfural. Scheme created with Advanced Chemistry Development, Inc. (ACD/Labs) Chemsketch (ACD/Labs, Toronto, ON, Canada, URL: http://www.acdlabs.com). Bottom: Results from Weingarten et al. [12] (&) and Lima et al. [16,17,18] ($) for xylose dehydration to furfural. The line is drawn only to guide the eye. www.sciencedirect.com

Current Opinion in Chemical Engineering 2012, 1:312–320

314 Reaction engineering and catalysis

xylose to furfural and furfural to humins, but that the reaction of xylose with furfural to produce humins was primarily catalyzed by Lewis acid sites. The low furfural selectivity obtained over the H-Y zeolite despite the high fraction of Brønsted sites was attributed to irreversible adsorption and subsequent polymerization of furfural within the catalyst micropores; none of the other catalysts tested had appreciable micropore volume. Other recent studies indicate that the sugar can dehydrate to furan either directly via Brønsted acids or indirectly, whereby Lewis acids perform isomerization of the aldose to the ketose and the latter is dehydrated by the Brønsted acid. Our group has recently shown that this cascade of reactions (isomerization followed by dehydration) has a lower activation barrier and thus it is faster than the direct dehydration [13,14] (V. Choudhary et al., under review). It is also clear that multifunctional catalysts offer significant advantages over a single function catalyst but general catalyst-design principles are lacking. Similar findings hold also for the glucose to HMF conversion (the C6 sugars) [15]. Less mechanistic understanding exists for the side reactions and in order to minimize the side reactions leading to formation of humins, it is common to use a biphasic system where the furan produced is selectively extracted into an organic phase (reactive separation). Lima et al. [16,17,18] also provided insight into the roles of Brønsted and Lewis acidity in the conversion of xylose to furfural in such a biphasic water/toluene mixture. It is clear from Figure 1 that the reactive extraction significantly increases the selectivity of the process since it removes the product and minimizes the side reactions. Comparison of all these data clearly underscores that aside from the ratio of Brønsted to Lewis acid sites, the processing scheme and conditions can have a profound effect on yield, and thus [B]/[L] is simply one, but not the only, key variable. Lima et al. [18] suggested that Lewis acidity may be more efficient at converting pentoses into furfural than hexoses into HMF using Al-TUD-1, which contained strong and weak Lewis acid sites but only weak Brønsted acid sites. Holm et al. [19] also observed a more rapid conversion of C5 than C6 sugars to furans, lactic acid, and glycolaldehyde with the Lewis acid Sn-BEA zeolite in water. They also report that low temperatures favor sugar isomerization, while higher temperatures favor the formation of lactic acid and other fragmentation products with SnBEA. Additionally, pentoses displayed better carbon balances (81%) than hexoses (72%) at 43 hours reaction time [19], indicating the greater proclivity of hexoses and their derivatives to polymerize. These results indicate that the size and molecular structure of sugars can be important in their conversion to furans, but a quantitative understanding of these effects is lacking. The work of Holm et al. [19] also highlights another valuable function of Lewis acids: isomerization of aldoses Current Opinion in Chemical Engineering 2012, 1:312–320

and ketoses. Generally, Brønsted acids are not efficient catalysts for aldose isomerization [20,21], although the efficacy may be a function of reaction conditions [22]. Recently, however, Roma´n-Leshkov et al. provided conclusive evidence via NMR that Lewis acidity can catalyze sugar isomerization [23]. In particular, the Lewis acid zeolite Sn-BEA is effective for catalyzing the isomerization of a series of C5 and C6 sugars [13, 15,19,23,24,25] by a mechanism similar to enzymatic catalysts [26]. Coupling sugar isomerization over Sn-BEA with aqueousphase dehydration using a homogeneous HCl catalyst in a biphasic system has resulted in high yields of furans [15,24]. Sn-BEA is a more efficient isomerization catalyst than other tetravalent-substituted analogs (e.g. Ti-BEA), but the difference in activity has not been quantified in terms of the catalyst properties [15]. Corma et al. [27] and Boronat et al. [28] showed that for some, but not all, reactions, activity is correlated with Lewis acid strength, but to our knowledge, no such trend has been established regarding these materials for sugar isomerization. Because Sn-BEA is a particularly promising catalyst, studies that could clarify the role of water in carbohydrate isomerization with Sn-BEA, and safer (i.e. without the use of hydrofluoric acid) and more efficient methods (i.e. incubation less than 21 days) for the synthesis of Sn-BEA will be valuable. Finally, Takagaki et al. [20] noted that very strong Brønsted acidity (e.g. employing Nafion NR50 resin instead of Amberlyst-15) could promote side reactions, and thus, for a given set of reaction conditions, there is an optimal acid strength to maximize furan yields. However, in Weingarten’s work [12], Nafion SAC-13 gave a lower xylose conversion than Amberlyst-70, but performed similarly in terms of furfural selectivity at a given conversion. The discrepancy could be due to the different solvents and reaction temperatures (N,N-dimethyl formamide at 1008C for Takagaki, water at 1608C for Weingarten), highlighting the importance of reaction conditions on determining the effects of catalyst properties. Pore size distribution

The relative pore sizes of potential catalysts and molecular dimensions of the molecules of interest are an important consideration in choosing a catalyst. Solids are generally classified as microporous if they contain pores of <20 A˚ and mesoporous if they contain pores between 20 A˚ and 500 A˚[29]. Solids with pore diameters >500 A˚ are considered macroporous [29]. Table 2 shows pore dimensions of some of the porous catalysts discussed in this manuscript, while Figure 2 includes molecular dimensions of sugars and their derivatives for comparison. It can be seen that the sizes of relevant molecules are similar to pore sizes of microporous materials, implying that internal diffusion is an important consideration for these catalysts. www.sciencedirect.com

Dehydration of carbohydrates Kruger, Nikolakis and Vlachos 315

Table 2 Comparison of dimensions for representative sugar dehydration catalysts

Material

Structure

Dimensions (A˚)

1D channels

3.2  4.3 2.4  4.8

3D channels

5.3  5.6 5.1  5.1

2D channels

6.5  7.0 2.6  5.7

3D channels

6.4  7.6 5.5  5.5

H-Y (FAU [4])

3D channels cavities

7.4 11.8

SAPO 11b [16] SAPO 11a [16]

1D channels 1D channels

6.4  6.4 6.4  6.4

Al-Montmorillonite [37] Cr-Montmorillonite [37] Fe-Montmorillonite [37] H-Montmorillonite [37]

Pillared Pillared Pillared Pillared

10.8 12.0 14.9 17.2

MCM-20 [38] MCM-41 [38] SBA-15 [54,57]

1D channels 1D channels 1D channels

27.4 32.8 45–300

BEATUD [17] TUD-1 [17]

3D channels 3D channels

45.0 70.0

Nafion NR50 a Nafion SAC-13 a Amberlyst-15 a Amberlyst-70 a

N/A 3D channels 3D channels 3D channels

N/A >100 300 220

Nu-6(2) [56] H-ZSM5 (MFI) [4] H-MOR [4] H-BEA [4]

a

Layer Layer Layer Layer

" microporous # mesoporous

Information provided by manufacturer.

It is difficult to establish a trend in selectivity with pore size, in part because it is difficult to decouple the role of pore size from other variables, including pore structure. For example, Agirrezabal-Telleria et al. [30] attributed a maximum in furfural selectivity from xylose dehydration to a balance between enhanced diffusion and enhanced side reactions as pore size increased, but were unable to decouple pore size effects from variation in micropore–mesopore structure and catalyst stability. Figure 3 shows furan selectivity as a function of pore size for a variety of catalysts under representative reaction conditions at 40–60% conversion. Plotting selectivity for other conversion ranges gives similarly scattered data. Additionally, previous studies about the effect of catalyst mesoporosity on humin selectivity in carbohydrate dehydration are contradictory [31–33]. Mesoporosity may increase humin formation in aqueous systems by increasing the concentration of hydronium ions within the mesopores, and hence H3O+-catalyzed side and secondary reactions [31]. A more open framework may also allow for oligomers of sugars and furans to form within the pores, which would not be possible in micropores [31,34,35]. On the other hand, mesoporosity may enhance diffusion of furan products away from www.sciencedirect.com

active sites, where secondary reactions, such as coke formation, can occur. In addition to pore size, pore structure is also an important consideration. For example, Weingarten et al. [12] attributed the high rate of humin formation due to furfural polymerization to the large cavities of the H-FAU-Y catalyst. Jow et al. [36] noted not only the formation of insoluble residue in the dehydration of pure fructose over a FAU-Y zeolite, but also that the pore structure of the zeolite trapped HMF and increased the yield of levulinic acid. In both cases, the coupling of large cages and smaller pores in the Y-zeolite framework may have allowed humins to form, but in one case may have also allowed higher yields of a valuable furan derivative. The most direct assessment of the effect of pore size was done by Lourvanij and Rorrer [32,37,38] who proposed that pore sizes larger than 10 A˚ enhanced sugar diffusion into the catalyst, but that pore diameters larger than 17 A˚ were necessary to facilitate diffusion of HMF away from catalyst active sites. Lima et al. [17], Dias et al. [33,39– 41], and Shi et al. [42] also compared mesoporous and microporous silicas impregnated with a variety of solid Current Opinion in Chemical Engineering 2012, 1:312–320

316 Reaction engineering and catalysis

Figure 2

Medium Pore

Large Pore

Formic Acid

Levulinic Acid

Small Pore

HMF

Furfural

Acids

2

3

4

5

6

Fructose Sugars

H-Y

SAPO-11 H-BEA H-MOR

Nu-6(2)

H-ZSM5

Xylose

Glucose

Furans

7

8

9

Diameter, Å Current Opinion in Chemical Engineering

Comparison of catalyst pore dimensions from Table 2 and molecular dimensions. Molecular dimensions are from the literature [38,58], except for xylose, which was calculated using ACD Labs ChemSketch (ACD/Labs, Toronto, ON, Canada, URL: http://www.acdlabs.com).

acids in xylose dehydration, and found that large diameter mesopores seem to generally enhance furfural yields. It is important to note that in every case the trend cannot be completely isolated from other variables, such as catalyst structure, reactivity, stability and in some cases hydrophobicity (and hence partition function between reacting and extracting phase). Still, it seems that enhanced diffusion of products tends to be more important than potentially higher hydronium ion concentrations within the mesopores. In nonaqueous systems, it may be more important to facilitate diffusion of water, rather than furans, away from the active sites to suppress side reactions such as hydrolysis or rehydration [43], but this concept needs more systematic investigation.

Figure 3

80

Furan Selectivity (%)

70 60 50 40 Glucose, Water, 150°C Xylose, W/T, 160°C Xylose, W/T, 170°C

30 20

Hydrophobicity

10 0 1

10

100

Pore Diameter (Å) Current Opinion in Chemical Engineering

Comparison of catalyst pore dimensions and furan selectivity. W/ T = Water/Toluene biphasic system. Xylose, W/T, 1708C from Lima et al. [16,59] and Moreau et al. [60]. Xylose, W/T, 1608C from Dias et al. [53] and Lima et al. [16]. Glucose, Water, 1508C from Lourvanij and Rorrer [37,38] Current Opinion in Chemical Engineering 2012, 1:312–320

Interactions of the solid catalyst with the solvent(s), reactants, and products are also important. In particular, catalyst hydrophobicity can affect reactivity and selectivity by influencing adsorption of each component of the reacting system, especially water. Furan yields are lower in aqueous solvent, possibly due to interactions of water and sugar or furan molecules at the catalyst active sites. Altering the solvent composition by either adding a soluble organic component [44–46] or insoluble extracting phase [15,33,47,48] is advantageous. Alternatively, www.sciencedirect.com

Dehydration of carbohydrates Kruger, Nikolakis and Vlachos 317

nonaqueous systems can be considered. It is well known that high-boiling solvents, such as DMSO, ionic liquids, and N,N-dimethylformamide, can give better selectivity than single-phase aqueous systems, depending on the catalyst, but have issues with competitive adsorption, complicate separation of the furan, and may carry environmental burdens. Similarly, difficulties of product separation from homogeneous acid catalysts can be partly overcome with a biphasic system, but homogeneous catalysts are disfavored because of safety, corrosion, and potential environmental concerns. Kim et al. [49] performed a thorough study of xylose dehydration with zeolites of varying Si/Al ratios (and hence hydrophobicity) in water, dimethyl sulfoxide (DMSO), and water/toluene solvents. The authors found that for a given pore size and structure, xylose conversion generally decreases while furfural selectivity generally increases with increasing Si/Al, regardless of solvent [49]. For other catalysts in biphasic systems, catalyst hydrophobicity must be carefully considered, as it can influence partitioning of the catalyst between the aqueous and organic phases [33]. Catalyst, reagent, and product partitioning can also be influenced by altering the ionic strength of the aqueous phase [15,50]. It is clear that hydrophobicity is a key variable in sugar and other biomass conversion in microporous materials, but our understanding is rather sketchy, undercutting the opportunity to select suitable materials and maximize yield. It generally appears that more hydrophilic catalysts are beneficial for carbohydrate adsorption and furan desorption, but that use of a hydrophilic catalyst is not enough to achieve high furan yields in an aqueousonly system. Thus, designing catalysts for sugar dehydration will be aided by quantification of catalyst hydrophobicity (e.g. analogous to Lecomte et al. [51]), a property which must be considered in concert with the solvent.

compare selectivity at a given conversion across catalysts at multiple temperatures. A compilation of temperature effects among 17 catalysts shows that 14 give higher furan yields as temperature increases [16,30,32,33,35,50], while three find a peak yield within their investigated temperature range [30,42,46]. Of the 14 catalysts that give increasing yields with temperature, seven also give increasing selectivity with temperature [30,33,50]; two give decreasing selectivity with increasing temperature [16], three find a peak in selectivity [30,33,35], and the remaining two do not give a clear selectivity trend [30,32]. Therefore, although higher temperatures commonly result in higher furan yields, the trend is not universal. As a result, more systematic studies and comprehensive models are needed to understand the effect of temperature.

Kinetic models Several kinetic models have been developed for homogeneous sugar dehydration [1–10]. Much less work has been done on the mechanism of sugar dehydration using microporous and mesoporous solid acid catalysts, and none of the models accounts for both homogeneous and heterogeneous chemistry contributions. Still, the models that have been developed are instructive as to what experimental data could improve future efforts.

In addition to solvent choice, temperature and pressure are also important parameters in carbohydrate dehydration. Shimizu et al. [43] report that in DMSO solvent, mild evacuation (P = 0.97 atm) of the reaction mixture to remove water reduced yields of unidentified products. More severe evacuation (P = 0.20 atm) increased selectivity to unknown products and the authors noted that the reaction mixture turned black.

Moreau et al. [31] calculated rate constants and activation energies for fructose dehydration to HMF and for HMF consumption in a biphasic system over mordenite catalysts, but mainly to rule out external mass transfer limitations. Lourvanij and Rorrer [38] constructed a 13parameter model for the dehydration of aqueous glucose to HMF, considering adsorption/desorption of all species and isomerization of glucose to fructose to be equilibrated. The authors also assumed the reaction to be isothermal and the acid site concentration to remain unchanged (no deactivation), though the reactions forming coke were the main source of uncertainty in the model. The strength of the model is the variety of catalysts employed, and the authors show that their scheme can qualitatively capture most conversion and selectivity trends for catalysts ranging from zeolites to mesoporous silicas. O’Neill et al. [35] proposed an analogous model for the dehydration of aqueous xylose to furfural over a H-ZSM5 catalyst.

The optimal reaction temperature is an interplay between the reactant and product adsorption strengths, the relative kinetics of the desired dehydration reactions, undesired polymerization and fragmentation reactions, and possibly catalyst stability. More specifically, as temperature increases, adsorption typically decreases while both homogeneous and heterogeneous reaction rates typically increase, but the contributions of each to the product distribution are a strong function of temperature. Most work with microporous and mesoporous catalysts thus far does not report full kinetic data, so it is difficult to

The majority of models lack intrinsic parameters, such as diffusivities and adsorption, and site specificity, such as the active acid site, and relate observed rates to bulk concentrations. In limited cases where such models were developed, parameters were fitted to a handful of experimental data without development of the individual submodels of diffusion and adsorption. As a result, our ability to predict rather than explain is severely limited. As an example, O’Neill’s model [35] does not consider diffusional limitations, while both Lourvanij and Rorrer [38] and Moreau et al. [31] consider the Weisz modulus to

Effects of temperature and pressure

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318 Reaction engineering and catalysis

demonstrate that internal diffusion limitations are not significant. Moreau et al. [31] calculate a fructose diffusivity from their reaction rate data, while Lourvanij and Rorrer [38] cite a literature value [52] that is roughly two orders of magnitude lower than the value of Moreau et al. [31]. Thus, if Lourvanij and Rorrer’s value is correct, Moreau et al. may have been limited by internal diffusion. This discrepancy points out the need for reliable and systematic diffusivity measurements over a broad range of parameters, particularly solvent and temperature. Adsorption and deactivation may also play an important role in determining kinetic parameters. In particular, consideration of carbonaceous deposits on acid sites is necessary, as evidenced by catalyst color changes, TGA data, or other means. Thus, choosing a catalyst that can be easily regenerated (e.g. by calcination) is an important consideration. Competitive adsorption of products and reactants will also likely affect reaction rates, and thus multicomponent adsorption isotherms in multiple solvents and catalysts should be valuable.

Conclusions and outlook Microporous and mesoporous catalysts show interesting results in the conversion of sugars to valuable platform chemicals, such as furans and levulinic acid. One important barrier for the rational design of catalysts is the lack of understanding of the reaction mechanisms, interactions between catalysts, solvents, reactants, and products, and diffusion–adsorption effects. Despite this, initial efforts in rational catalyst design can be credited to the Valente group [17,33,39–41,53] and Crisci et al. [54], who synthesized a variety of microporous and mesoporous catalysts for sugar dehydration. Mesoporous materials exhibit higher selectivity to furan products relative to microporous catalysts, but are often less active than their microporous counterparts. Furthermore, in terms of multiscale catalyst design, materials that incorporate both microporosity and mesoporosity into a well-defined structure appear to outperform catalysts that contain only one or the other, and catalysts should be constructed from materials that are able to withstand calcination at high temperature to remove the inevitable carbonaceous deposits. Techniques to synthesize hierarchically ordered and structured microporous–mesoporous materials have been developed [55], and the concept could be extended from mesoporous silica to composite structures incorporating other ordered mesoporous materials, such as ZrO2 or carbon. Systematic studies of pore size and pore morphology, and combinations of both components, are clearly necessary. Optimal levels of Brønsted acidity, Lewis acidity, and hydrophobicity of a catalyst will likely depend on the solvent, which, despite the number of solvent combinations already employed to date, is not a trivial decision. The choice can be aided by the development of kinetic models, which must incorporate both homogeneous and heterogeneous chemistry, as well as Current Opinion in Chemical Engineering 2012, 1:312–320

reliable expressions that quantitatively capture diffusion and adsorption, to give an adequate description of the system.

Acknowledgement The work was financially supported by the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001004.

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