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Operando Solid-State NMR Observation of SolventMediated Adsorption-Reaction of Carbohydrates in Zeolites Long Qi, Ricardo Alamillo, William A Elliott, Amity Andersen, David W. Hoyt, Eric Walter, Kee Sung Han, Nancy M Washton, Robert M. Rioux, James A. Dumesic, and Susannah L Scott ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01045 • Publication Date (Web): 03 Apr 2017 Downloaded from http://pubs.acs.org on April 4, 2017
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ACS Catalysis
Operando Solid-State NMR Observation of Solvent-Mediated Adsorption-Reaction of Carbohydrates in Zeolites Long Qi,†,‡ Ricardo Alamillo,§ William A. Elliott,ǁ Amity Andersen,# David W. Hoyt,# Eric D. Walter,# Kee Sung Han,# Nancy M. Washton,# Robert M. Rioux,ǁ,¶ James A. Dumesic,§ Susannah L. Scott†,‡,* †
Department of Chemistry & Biochemistry, and ‡Department of Chemical Engineering, University of California, Santa Barbara, California 93106, United States. § Department of Chemical and Biological Engineering, University of Wisconsin, Madison, WI 53706, United States. ǁ Department of Chemical Engineering and ¶Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, United States. # Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99354, United States. KEYWORDS: Interfacial Reaction, Operando Spectroscopy, Solid-state NMR, Carbohydrate Isomerization, Selective Adsorption, Solvent Non-ideality. In the liquid-phase catalytic processing of molecules using heterogeneous catalysts – an important strategy for obtaining renewable chemicals from biomass – many of the key reactions occur at solid-liquid interfaces. In particular, glucose isomerization occurs when the glucose is adsorbed in the micropores of a zeolite catalyst. Since solvent molecules are co-adsorbed, the catalytic activity depends strongly and often non-monotonically on the solvent composition. For glucose isomerization catalyzed by NaX and NaY zeolites, there is an initial steep decline when water is mixed with a small amount of the organic co-solvent γ-valerolactone (GVL), followed by a dramatic and surprising recovery as the GVL content in the mixed solvent increases. Here we elucidate the origin of this complex solvent effect using operando solid-state NMR spectroscopy. The glucopyranose tautomers immobilized in the zeolite pores were observed their transformations into fructose and mannose followed in real time. The microheterogeneity of the solvent system, manifested in a non-monotonic trend in the mixing enthalpy, influences the mobility and adsorption behavior of the carbohydrates, water and GVL, which were studied using pulsed-field gradient (PFG) NMR diffusivity measurements. At low GVL concentrations, glucose is depleted in the zeolite pores relative to the solution phase, and changes in the local structure of co-adsorbed water serve to further suppress the isomerization rate. At higher GVL concentrations, this lower intrinsic reactivity is largely compensated by strong glucose partitioning into the pores, resulting in dramatic (up to 32´) enhancements of the local sugar concentration at the solid-liquid interface.
INTRODUCTION Solid-liquid interfaces are ubiquitous in both naturally-occurring and manufactured systems, ranging from the thin water films covering aerosol particles in the Earth’s atmosphere1 to the electrolyte double layers present at electrode surfaces.2 These interfaces are also important in large-scale continuous processing of lignocellulosic biomass feedstocks using solid catalysts,3 in which a partial replacement of fuels and chemicals derived from fossil carbon may be achieved via selective depolymerization of the carbohydrate components of lignocellulose to access its energy-rich monosaccharides (glucose, fructose, xylose, etc.), and their subsequent transformation to renewable platform chemicals. With the exception of unselective high temperature gasification/pyrolysis, biomass processing is conducted with a solvent to facilitate mass transfer between the low volatility feedstock and the solid catalyst. The liquid phase is usually aqueous or semi-aqueous, to promote carbohydrate solubility. Water is also ubiquitous as a reactant (e.g., in the hydrolysis of glycosidic bonds), as a reaction product (e.g., in polyol dehydration) or as an intermediate (e.g., in sugar isomerization and
epimerization).4 Solvent effects can direct selectivity in carbohydrate transformations,5 although there is at present very little fundamental understanding of the origins of these effects at the molecular level. For example, adding a polar aprotic organic co-solvent such as tetrahydrofuran, dioxane, or biomass-derived γ-valerolactone (GVL) to an aqueous glucose solution results in major improvements in both the activity and selectivity of homogeneous acid-catalyzed isomerization of glucose or xylose, compared to the fully aqueous reacting systems.6-9 For solid catalysts, interactions between the active sites on the surface of solid catalysts with solvent and substrate molecules control catalytic activity and selectivity, and may lead to very different solvent effects relative to homogeneous catalysis. Direct and detailed evidence for these effects is needed to optimize them in the development of effective and robust catalysts for next-generation technologies using renewable feedstocks. Microporous zeolites have been widely used as solid catalysts in petroleum refineries where they adsorb volatile, hydrophobic hydrocarbons and catalyze their subsequent conversion in the absence of solvent. Zeolites can also be effective catalysts in liquid-phase biomass processing, in which
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non-volatile carbohydrates are dissolved in a polar solvent.10 For example, aldose-ketose isomerization11 is catalyzed by zeolites with Brønsted-acidic,12 Lewis-acidic,13 or basic properties.14 Zeolites with a wide range of framework types, including MOR, BEA, and FAU, catalyze the isomerization and further reactions of aldoses and ketoses to products such as 5hydroxymethylfurfural (HMF) and levulinic acid (LA).15 The choice of solvent system can have a dramatic effect on the reactivity, selectivity, and lifetime of zeolite catalysts.16 Within the pore system of the solid catalyst, solvent molecules can enhance or impede diffusion of reactants, intermediates and products, compete for adsorption on active sites, or modify transition state energies via co-adsorption or preferential solvation.17 Non-ideal solution thermodynamics can also affect the partitioning of molecules between adsorbed and liquid phases.18 The origin and magnitude of these solvent effects can significantly alter the effectiveness of a solid catalyst in biomass conversion. Molecules confined at or near interfaces have been characterized by a variety of techniques such as X-ray absorption spectroscopy,2 grazing incidence high-energy X-ray scattering,19 sum-frequency generation vibrational spectroscopy,20 and solid-state NMR spectroscopy.21 However, observations of reacting systems are rare. Signals from the interface are often overwhelmed by those from the bulk,22 and it is challenging to obtain information at the elevated temperatures and/or pressures relevant to chemical transformations. Carbohydrates usually lack distinctive signals in their IR spectra, and the presence of large amounts of water complicates attempts to obtain information about adsorbed species using IR methods, although they have been used for lower temperature reactions involving carbonyl-containing compounds in hydrocarbon solvents.23 In contrast, solid-state NMR methods are well-suited to detecting carbohydrates and can be used in the presence of liquid water over a wide temperature range. They are also capable of probing processes involving porous materials in situ, easily differentiating adsorbed species from their more mobile counterparts present in solution via differences in their chemical shifts, peak shapes and relaxation times. In this study, we employ solid-state NMR to monitor the adsorption of glucose and its isomerization to fructose inside a faujasite zeolite. Other heterogeneous catalysts such as SnBEA zeolite are also effective, but suffer from rapid deactivation under hydrothermal reaction conditions.24,25 Recently, Resasco et al. reported higher productivity in glucose isomerization for NaX zeolites compared to Sn-Beta.26 Customized chemically-resistant NMR rotors27-29 were adapted to withstand temperatures up to 523 K and pressures up to 200 bar. Magic-angle spinning (MAS) was employed to improve the spectral resolution under reaction conditions. The spectra allow us to relate changes in reactivity to variations in glucose concentration and solvent composition at the solid-liquid interface.
RESULTS AND DISCUSSIONS Co-solvent effect on the glucose isomerization rate. The isomerization of glucose to fructose was studied using alkali metal-exchanged faujasite zeolites14 (NaX and NaY) as catalysts. Although the corresponding Brønsted acidic forms30 (HX and HY) are active catalysts as well, they also promote the subsequent conversion of the fructose to HMF and other products,31 complicating our investigation of solvent effects on the isomerization reaction. The activity of NaX
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(SiO2:Al2O3 = 2.5) for glucose isomerization was first evaluated in the presence of various GVL-water solvent mixtures at 403 K in a continuous flow, packed-bed reactor. Both external and internal mass transfer rates are fast relative to the intrinsic reaction kinetics under these conditions (see SI). Figure 1a illustrates the dramatic and non-monotonic effect of solvent composition: the presence of just 4 mol% GVL as cosolvent caused the turnover frequency (TOF) to decline by >95%, compared to the experiment in which only water was present and in contrast to the promoting effect of GVL on homogeneous acid-catalyzed carbohydrate reactions.7 Surprisingly, most of the activity was recovered when the GVL content was increased further. At 46 mol% GVL (or 81 wt% GVL, representing the maximum GVL content in which glucose is soluble at 0.1 mol L-1), the activity recovered to 62 % of its original value in pure water.
a NaX Zeolite
OH HO HO
O OH
Glucose
HOH 2C
O OH CH 2OH
OH
OH HO
Fructose
´ 12
b
Figure 1. (a) Effect of GVL on the catalytic activity of NaX zeolite for glucose isomerization (red, measured at 403 K), and on glucose adsorption in the zeolite pores (blue, measured at 298 K). The initial TOF is for fructose production and is normalized per Na+ cation. (b) Arrhenius plot of initial rates for glucose isomerization to fructose, catalyzed by NaX zeolite in 46 mol% GVL.
The apparent activation energy in this GVL-water mixture (46:54 mol:mol) is (102 ± 5) kJ mol-1 (Figure 1b). It is comparable to a value reported using the same catalyst in water (104 kJ mol-1),14 suggesting that the mechanism of the basecatalyzed isomerization is not solvent-dependent. When the
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less basic NaY (SiO2:Al2O3 = 5.1) was used instead, a similar solvent trend was observed (Figure S1), although all activities were lower.
1 α-Glcp
2 4
3
β-Glcp
a
298 K
298 K (Figure S2). In contrast, glucose dissolved in 46 mol% GVL in contact with NaX exhibits four distinct C1 signals, Figure 2a. The two sharp signals represent highly mobile glucopyranose tautomers in the bulk solution (1: β-Glcp, 96.8 ppm; 2: α-Glcp, 92.7 ppm), while the two broad signals arise from the same tautomers confined in the zeolite pores (3: βGlcp, 96.4 ppm; 4: α-Glcp, 92.2 ppm). The larger linewidths of the latter are the result of restricted mobility and/or inhomogeneous adsorption.36 Their more ready detection in the presence of GVL compared to pure water suggests enhanced partitioning of glucose into the zeolite pores from the semiaqueous solution. The frequencies of the adsorbed molecules are, on average, slightly lower than the frequencies for their mobile counterparts in the bulk solution. Deconvolution of the NMR spectrum in Figure 2a reveals that 65% of the glucose is confined in the micropores at 298 K. Furthermore, the α-Glcp:β-Glcp tautomer ratios are the same (42:58) in both the solution and adsorbed phases at this temperature (Figure 2c).
a
110
90 100 95 13C δ (ppm)
105
298 K
80 70
α-Glcp 58
O
C2
85
O C1
C5
C5
C2 C1
C5
C5 C1
393 K
c
70
β-Glcp
c
58
60 50
b C1
b
393 K
Tautomer Distribution (%)
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49 51 42
42
40
30
30 20 10 0 Adsorbed
Mobile
Adsorbed
Mobile
Figure 2. Direct polarization MAS solid-state 13C NMR study (125.77, 5 kHz MAS) of a mixture of NaX zeolite (200 mg) with a solution of glucose-1-13C (0.100 mol L-1, 1.00 mL) in 46 mol% GVL at 298 K and 393 K. (a) Spectra recorded at 298 K (green dots indicate the locations of the 13C labels, corresponding to the observed signals). (b) First spectrum recorded shortly after heating the rotor to 393 K. (c) Relative amounts of α-Glcp and β-Glcp glucose tautomers in the adsorbed or solution phases, as determined by solid-state NMR at 298 and 393 K (error bars represent the standard deviations of 3-5 independent measurements).
In situ observation of adsorbed glucose. Solid-state NMR spectroscopy can be used to detect molecules adsorbed at solid-liquid interfaces, via changes in their dynamic behavior.32-35 The 13C MAS NMR spectrum of glucose-1-13C (0.100 mol L-1, dissolved in 1.00 mL water and mixed with 200 mg NaX) consists of two C1 signals representing the two glucopyranose tautomers which dominate in the bulk solution at
Figure 3. DFT-optimized models of glucose adsorbed in NaX: (a) α-Glcp adsorbed at 6T surface site (bottom) with Fischer projection-type view along C1-C2 bond, illustrating sterically unfavorable “eclipsed” structure of absorbed a-Glcp (top). (b) β-Glcp adsorbed at 6T surface site (bottom) with Fischer projection-type view along C1-C2 bond, illustrating sterically favorable “staggered” structure of absorbed β-Glcp (top). (c) Ab initio molecular dynamics snapshot of a single α-Glcp molecule in NaX supercage, accompanied by water molecules. Color scheme: magenta: Al; yellow; Si, red: O; white: H; gray: C; blue, Na; dashed lines, hydrogen-bonds.
At elevated temperatures (of which 373-433 K is a typical range for carbohydrate reactions in zeolites), increased mobility results in motional narrowing of the signals for adsorbed molecules. This behavior is evident in Figure 2b, which shows
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a spectrum recorded at 393 K. In addition, all four signals are displaced to slightly higher frequency (1: 97.5 ppm; 2: 93.3 ppm; 3: 96.7 ppm; 4: 93.0 ppm) relative to the values at 298 K. The signal for adsorbed β-Glcp experiences the smallest displacement, resulting in a larger difference between solution and adsorbed signals for this tautomer. In the solution phase, the α-Glcp:β-Glcp ratio increases slightly, to 49:51, while the ratio in the adsorbed phase is perturbed more and shifts in the opposite direction, to 30:70 (Figure 2c; the partitioning of glucose tautomers between the adsorbed and mobile phases is shown in Figure S3). The latter result suggests that α-Glcp experiences less stabilization on average due to adsorption as the temperature increases than does β-Glcp.
a
β-Glcp
Solution
α-Glcp Ads.
Solution Fructose Tautomers Ads.
*
100
98
96
94 92 δ (ppm)
66 65 64 63
13C
b
13
Figure 4. Direct polarization MAS solid-state C NMR study (125.77 MHz, 5 kHz MAS) of a mixture of NaX zeolite (200 mg) with a solution of glucose-1-13C (0.100 mol L-1, 1.00 mL) in 46 mol% GVL at 393 K. (a) Time-resolved operando spectra, showing glucose conversion to fructose (* denotes mannose side-product). (b) Kinetic profiles (points) for glucose adsorbed in the zeolite, glucose present in the solution phase, total fructose, and total mannose. The lines are curve fits to a bi-exponential rate equation (see Eq 1-2), where k1,obs and k2,obs are the pseudo-first-order rate constants for fructose production and degradation, respectively.
The relative stability of the adsorbed glucose tautomers was investigated by DFT calculations, which included water solvent effects via the COSMO implicit solvent model. The 6T ring site (Figure S4) is a common supercage wall motif (there
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are four per supercage) that is nearly commensurate with α/βGlcp (Figure 3). At this site, the adsorption of β-Glcp via Hbonding of the OH groups located on C1 and C2 is stronger than that of α-Glcp (Table S1). This is likely due to more favorable steric interactions with the hydroxyl substituents on C1 and C2, as the Fischer projection-type views of adsorbed α-Glcp and β-Glcp in Figures 3a and 3b show. Operando measurement of interfacial reaction kinetics. Glucose isomerization proceeded when the temperature of the NMR rotor described in the above experiment was maintained at 393 K for several hours. The reaction progress was monitored by recording a time series of direct polarization solidstate NMR spectra, shown in Figure 4a. Monotonic decreases in concentration were observed separately for adsorbed glucose in the zeolite, and for mobile glucose in the solution phase. At ca. 65 ppm, several fructose signals (one for each of the five cyclic tautomers of fructose-1-13C) are visible even in the first recorded spectrum, acquired 10 min after heating commenced. The total fructose yield passed through a maximum of 36% after 4 h, then began to decrease due to known processes such as aldol condensation37 and retro-aldol reactions.38 A small extent of epimerization is evident in the appearance of signals at 94.9 and 94.5 ppm for two mannopyranose tautomers (overall yield < 5% at 393 K). The NMR measurements reveal that adsorbed glucose in the catalyst pores and mobile glucose present in the solution phase (i.e., not directly interacting with the catalyst) are converted to fructose at the same rate. This observation implies that the two distinct glucose populations exchange rapidly on the timescale of the reaction, and that the rate is not limited by mass transfer between the adsorbed and mobile phases under MAS conditions. Ab initio molecular dynamics (AIMD) simulations confirm that cyclic glucopyranoses diffuse readily into the supercages of NaX through the 12T aperture without prior ring-opening, Figure 3c (see also Movies S1 and S2). Quantitative analysis of these NMR spectra confirmed that the adsorbed:solution ratio of glucose molecules remained constant throughout the 6 h duration of the experiment, as did the α-Glcp:β-Glcp tautomer ratios in both the adsorbed and solution phases (Figure S5). The kinetic profiles, extracted from the total integrated area for each set of carbohydrate signals as a function of time, are consistent with a faster glucose isomerization step followed by slower degradation processes, Figure 4b. This bi-exponential kinetic behavior is particularly apparent in the fructose kinetic profile. The evolution of glucose and fructose are described by Eq 1 and 2,39 respectively. 𝐶"#$,&'() = 𝑎𝑒 -./,012 3 + (𝑏 − 𝑎)𝑒 -.9,012 3 𝐶:(;$,&'() = 𝑎[ 1 − 𝑒 -./,012 3 + 𝑒 -.,0129 3 − 1]
(1) (2)
where a and b are adjustable parameters which reflect the change in normalized concentration for each kinetic phase. Pseudo-first-order rate constants were extracted by curvefitting the appropriate rate equation to each dataset, Table 1, entry 1. Based on the fructose formation rate constant (k1,obs), the initial turnover frequency in the NMR rotor is (0.079 ± 0.007) h-1 at 393 K (see SI), similar to that measured in the flow reactor (0.066 h-1 at the same temperature). When the experiment was repeated, the same initial TOF for fructose formation was obtained, (0.072 ± 0.010 h-1), within the uncertainty of the measurement. The kinetics were also recorded at 403 K, and the increase in the observed fructose formation
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rate constant is consistent with the known activation energy. At lower and higher temperatures, the time scale of the reaction made it difficult to measure rate constants accurately using the operando NMR technique. In 4 mol% GVL as solvent at 393 K, both k1,obs and k2,obs are an order of magnitude lower compared to their values in 46 mol% GVL, Table 1. This result confirms that the dramatic difference in rates for glucose isomerization to fructose observed in the flow reactor (Figure 1) is also observed under the batch conditions of the solidstate NMR rotor. Table 1. Pseudo-first-order rate constants for glucose isomerization, followed by glucose/fructose degradationa k1,obs (h-1)a
k2,obs (h-1)b
393
wLarmor (13C, MHz) 125.77
0.666(61)
0.104(27)
2
393
125.77
0.606(87)
0.401(56)
d
393
125.77
0.0412(24)
0.014(17)
4
403
75.43
0.640(136)
0.270(52)
5
403
125.77
1.033(133)
0.247(34)
Exp#
T (K)
1 3
(486 m2/g), presumably due to the greater solubility of the oligomers in GVL. When the spent catalyst was flushed with aqueous acetone (50 v%) after 8 h at 403 K in the flow reactor with a reaction solvent containing 46 mol% GVL, the original activity was largely recovered. When the reaction solvent was pure water, catalyst fouling was irreversible: the activity decline was not reversed by attempted catalyst regeneration with acetone.
a
a
Measured using DP-MAS solid-state 13C NMR, for a mixture of NaX zeolite (200 mg) and a solution of glucose (0.100 mol L1 , 1.00 mL) in 46 mol% GVL, except where noted. Values in parentheses represent uncertainties from non-linear regression. b Curvefit using Eq 1 with three adjustable parameters (a, b, and k2,obs). c Curvefit using Eq 2, with two adjustable parameters (a, and k1,obs). d Solvent contains only 4 mol% GVL.
The similarity in rates observed in the conventional flow reactor and in the NMR rotor suggest that the chemical processes are not affected by the fast rotation. The maximum centrifugal force at the internal wall surface of the rotor is estimated to be ca. 13 bar at a spinning rate of 5 kHz (see Experimental section). The isothermal compressibility of GVL is likely to resemble that of similar organic solvents such as THF;40 for both THF and water, the volume change is £ 0.1% at 20 °C, and is not strongly temperature-dependent. The activation volume for isomerization is expected to be small and not affected. The effect of the pressure on crystalline microporous materials is also minimal; their mechanical strengths are sufficient to withstand higher pressures in demanding industrial applications.41 Even the much less stable ordered mesoporous silica SBA-15 remains structurally unchanged up to 160 bar.42 The Si/Al and Na/Al ratios of the zeolites, as well as their powder XRD diffraction patterns (Figure S6), remained unchanged after 8 h at 403 K in a batch reactor, demonstrating the stability of the zeolite framework under the reaction conditions. Side-reactions that form oligomers are the likely cause of the slow decrease in fructose concentration (k2,obs) shown in Figure 4b; their accumulation can block zeolite pores. Longer-term catalyst performance was assessed during consecutive 8 h runs in a packed-bed reactor (Figure S7). Similar declines in activity were observed for reaction solvents containing 0 or 46 mol% GVL during the first 500 min onstream. Consistent with this finding, spent catalysts recovered from three solvent mixtures (0, 4, and 46 mol% GVL) after 8 h in a batch reactor at 403 K had lower B.E.T. surface areas (determined by N2 physisorption, Figure S6) than the fresh catalyst (647 m2/g). However, catalysts used with GVL as cosolvent retained more internal surface area (e.g., 580 m2/g for 46 mol% GVL) than the material recovered from water alone
b c
Figure 5. Adsorption of carbohydrates in faujasite zeolites at 298 K, based on concentrations determined by HPLC: (a) Relative changes in liquid-phase glucose concentrations in GVL-water mixtures (1.00 mL, 0.100 mol L-1 glucose) due to adsorption by NaX, NaY and HY zeolites (200 mg); (b) Calculated glucose occupancy of the zeolite supercages due to adsorption by NaX; (c) Relative changes in liquid-phase concentrations in GVL-water mixtures (1.00 mL of 0.100 mol L-1glucose or fructose, or an equimolar mixture thereof) due to adsorption by NaX (200 mg).
Effect of solvent composition on glucose partitioning. To investigate the extent to which the co-solvent effect on catalytic activity is a consequence of selective glucose adsorption, fresh zeolite samples (200 mg NaX) were mixed with glucose solutions (1.00 mL, 0.100 mol L-1) in various GVL-water mixtures at room temperature. After separation of the solids by centrifugation, the remaining glucose concentrations in the supernatant liquids were quantified by solution-state 1H NMR.
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Adsorption by NaX from solvents containing 0 or 4 mol% GVL resulted in the glucose concentration in the solution phase increasing slightly (Figure 5a). A higher concentration in the solution phase indicates preferential adsorption of solvent molecules by the zeolite, relative to the adsorption of glucose. However, when the GVL content of the solution rose further, the uptake of both water and glucose by the zeolite increased (Tables S2 and S3), indicating that both components prefer the zeolite pores over the GVL-rich liquid phase. For the highest GVL concentration, water partitioning into the zeolite pores resulted in a 5-fold enrichment in its concentration there relative to the remaining hydrophobic solution phase (whose GVL content had climbed to 53 mol%). Glucose was also adsorbed preferentially, its concentration in solution decreasing by up to 64% (i.e., to 0.036 mol L-1). For the highest GVL concentration, the resulting glucose concentration in the zeolite pores reached 1.16 mol L-1, representing a 32-fold enrichment relative to the remaining solution phase (and a 12fold enhancement relative to the initial solution concentration, Figure 1). Since just one glucose molecule can be accommodated in a single zeolite supercage, a majority (56 %) of the supercages are occupied by glucose at this concentration (Figure 5b). Similar trends were observed when the amount of zeolite was varied (Tables S4 and S5). The adsorption of fructose was also evaluated, Figure 5c. Weak adsorption leads to a 6% decrease in the liquid phase concentration of fructose when water is the solvent, while much stronger adsorption results in a 64% decrease in the liquid phase concentration when the solvent contains 46 mol% GVL. The results were similar when glucose and fructose were dissolved in the same solution (resembling the equilibrium reaction mixture). Thus adsorbed glucose and fructose do not compete with each other under the reaction conditions. Measurements made using in situ solid-state NMR confirmed these glucose distributions between the adsorbed and mobile phases at 298 K, as well as at 403 K (Figure S8). Computational predictions indicate a more energetically favorable interaction of glucose with the zeolite compared to GVL (Figure S3 and Table S1). The difference is presumably due to glucose hydrogen-bonding to the basic oxygen atoms of the zeolite framework.43 Nevertheless, both glucose and GVL are adsorbed by NaX in comparable amounts in these systems (Tables S3 and S5), due to the much higher solution concentration of GVL. NaY shows similar behavior but adsorbs less glucose (Figure 5a), consistent with its lower Al content and therefore weaker framework basicity. Changing the extraframework cation from NaY to HY reduces both the basicity and the glucose adsorption capacity further. Likewise, glucose adsorption in different alkali metal-exchanged X zeolites rises with increasing framework basicity,44 in the order LiX < NaX < KX (Figure S9). The changing concentration of glucose adsorbed in NaX tracks the increase in catalytic activity closely (Figure 1). Therefore, the rate acceleration as the GVL content of the solvent rises from 4 to 46 mol% can be explained principally by a local concentration effect (Figure S10). Physicochemical origin of the co-solvent effect. At 298 K, the initial mixing of GVL with water is slightly exothermic, with a minimum in the mixing enthalpy at ca. 3 mol% GVL (Figure 6a). The slightly negative excess volume (Figure 6b) suggests that GVL solvation in such solutions causes little overall change to (or possibly even enhances) the hydrogen-bonding structure of water.45 However, as the GVL content increases, mixing becomes
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endothermic. While GVL-rich solutions remain macroscopically homogeneous, they are likely to be heterogeneous on the microscale. The tendency for segregated microdomain formation results from the preferential association of water molecules with each other via hydrogen-bonding, and of GVL molecules with each other by dipole-dipole interactions. Similar behaviors are exhibited by aqueous mixtures containing organic co-solvents such as acetonitrile,46 dimethylsulfoxide,47 and tetrahydrofuran.48
a
b
Figure 6. (a) Excess molar enthalpy of mixing (HE) and (b) excess molar volume (VE), for GVL-water solutions at 298 K.
Figure 7. Relative diffusion coefficients for glucose, GVL, and water in aqueous glucose solutions containing 0, 4, and 46 mol% GVL at 298 K (* represents the diffusion coefficient of pure GVL, reported for comparison).
Solvent micro-heterogeneity can affect the strength of hydrogen-bonding between water molecules. Indeed, in the 1H
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NMR spectrum of 46 mol% GVL-water, the 1H signal of water exhibits a shift to lower frequency compared to either pure water or water in 4 mol% GVL (Figure S11),49 suggesting weaker hydrogen-bonding. Pulsed-field gradient (PFG) NMR diffusion measurements performed on these solutions reflect the relative interaction strengths between molecules.50 According to Stokes-Einstein theory, the rate of molecular diffusion is inversely proportional to the strength of the interaction between the molecule being observed and other components in the solution. The diffusion coefficients of both glucose and water, DGlc and Dwater, decrease in the presence of added GVL (Figure S13), consistent with their confinement in microdomains. Interestingly, DGVL/Dwater increases with increasing GVL content, while DGlc/Dwater remains virtually unchanged (Figure 7). The latter result suggests that water and glucose are present in the same microdomains, and therefore implies that glucose is preferentially solvated by water.
Figure 8. Wetting exotherms, measured for solutions of various GVL-water ratios (20.0 mL) combined with NaX (20 mg) at 298 K.
Wetting exotherms were measured for NaX in contact with various GVL-water mixtures. The maximum heat release occurs at just 4 mol% GVL, Figure 8. This behavior is consistent
with co-adsorption of GVL in the zeolite from the dilute bulk solution due to the relative low amount of GVL-GVL interactions. Interactions between adsorbed water molecules and the zeolite are likely enhanced in the presence of co-adsorbed GVL.2 Also at this solvent composition, a minimum in glucose adsorption by NaX occurs (Figure 5a), presumably because glucose is better solvated in the water-rich solution phase than in the GVL-containing solid phase. At higher total GVL contents, zeolite wetting by the solvent mixture releases less heat. Solvation of glucose in the increasingly hydrophobic solution phase becomes less favorable relative to the more hydrophilic zeolite phase, where the sugar is now concentrated. The important result is that glucose is effectively excluded from the pores at low GVL-water ratios, even though it is preferentially adsorbed by the zeolite at high GVL-water ratios. Although the concentration of adsorbed glucose decreases 2.5-fold when GVL is introduced (at 4 mol%) as co-solvent, the decline does not fully account for the dramatic loss (ca. 20-fold) in the rate of catalytic isomerization (Figure 1 and Figure S10). We infer that changes in the local solvent composition and structure inside the zeolite must also play a role. Water molecules participate directly in many transition states for carbohydrate reactions, including mutarotations51 and isomerizations.52 In solution, the presence of hydrophobic molecules resembling GVL perturbs the structure of adjacent water molecules53 and restricts their ability to reorient.54 A similar effect has been reported for water molecules solvating the hydrophobic residues of ambiphilic polymers55 and proteins.56 Enhancement of such effects at the solid-liquid interface are expected inside a porous catalyst,19 and would be manifested in a smaller rate coefficient for glucose isomerization in the presence of GVL. Since the observed rate is the product of the rate coefficient and the concentration, the rate suppression is reversed at high GVL concentrations because of the large increase in local glucose concentration.
CONCLUSIONS The origin of the dramatic and non-monotonic effect of organic co-solvents on glucose conversion in microporous faujasite zeolites is a marked difference in composition between the internal reaction environment and the bulk solution, Scheme 1.
Scheme 1. Qualitative illustration of selective adsorption of organic co-solvent (GVL) and glucose molecules in the supercages of NaX zeolite, for liquid phases varying in their GVL content: 0 (blue), 4 (green), to 46 (yellow) mol%.
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The presence of a small amount of an organic co-solvent (e.g., 4 mol% GVL) suppresses glucose adsorption. As the organic content increases (up to 46 mol% GVL), there is significant enrichment of both water and glucose in the adsorbed phase. The latter process is a heterogeneous analog of the salting-out process previously reported in acid-catalyzed carbohydrate dehydration.37 In that system, a combination of polar and non-polar solvents created a macroscopically biphasic system in which both carbohydrates and hydrophilic catalysts (e.g., molecular or solid inorganic acids) associate preferentially in the polar phase. A distinctive micro-environment created inside the zeolite pores results from selective solvent adsorption. Glucose solvation in the remaining solution phase is limited by the enhanced hydrophobicity, and the resulting destabilization leads to enrichment of the interfacial region in glucose. This phenomenon presumably also occurs for meso-/macro-porous solids and at flat surfaces.20 The ability of high temperature/pressure solidstate NMR to provide real-time, molecular-level characterization of surface compositions, in combination with catalyst assessment, computational simulations, and solution calorimetry, can elucidate dynamic molecular processes at solid-liquid interfaces, and will undoubtedly potentially provide similar insight in other multi-phasic reacting systems.
EXPERIMENTAL METHODS Chemicals. D-(+)-Glucose (Sigma-Aldrich, ACS reagent) was used in isomerization reactions carried out in both batch and packed-bed reactors. Gamma-valerolactone (GVL) was purchased from Sigma-Aldrich (ReagentPlus, 99%) or Shenzhen Nexconn Pharmatechs Ltd. (99%). For solid-state NMR studies, D-glucose-1-13C (99.4% purity, 99 atom% 13C) was purchased from Omicron Biochemicals. Lithium chloride (99.99%) and potassium chloride (99.99%) were purchased from Sigma-Aldrich. Deuterium oxide (99.9 atom% D) was acquired from Cambridge Isotope Laboratories. Dimethylsulfoxide-d6 (99.9 atom% D) was purchased from Sigma-Aldrich. HPLC-grade water was purchased from Fisher Chemical. Samples of NaX zeolite (SiO2:Al2O3 = 2.5), with unit-cell formula Na86[Al86Si106O384]), were purchased from both SigmaAldrich and Strem Chemicals. Mg (≤ 1%) was detected by ICPAES, SEM-EDX, and TEM analysis as a minor component of the material supplied by Sigma-Aldrich, presumably due to the presence of binder. However, the measured isomerization activities and adsorption behaviors were similar using NaX from either source. K+-exchanged and Li+-exchanged X zeolites were prepared by heating NaX (2.0 g) in 40 mL LiCl or KCl solution (1.0 mol L-1) at 353 K for 2 h. Solids were recovered by centrifugation. After repeating the ion-exchange process three times, each zeolite was washed with water until no chloride ion was detected in the supernatant via the AgNO3 test, then calcined at 773 K for 5 h in a flow of 10% O2 in N2. The extent of cation exchange was determined by ICP-AES to be 79 and 92% for LiX and KX, respectively. NaY and HY zeolites (SiO2:Al2O3 = 5.1) were purchased from Alfa Aesar, and were calcined at 773 K for 5 h in flowing 10% O2 in N2 before use. Flow reactor studies. Glucose isomerization kinetics were assessed in a 1/4-inch o.d. stainless steel tube used as a flow reactor. A fixed-bed, up-flow configuration was achieved by packing the catalyst powder between two plugs of quartz wool. The reactor temperature was maintained using aluminum heat transfer blocks wrapped in heating tape and ceramic insulating blanket. Temperature measurements were made using a K-type
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thermocouple (Omega). Thermal control was provided by a variable transformer connected to a PID controller (Love Controls Series 16A). Liquid was fed to the reactor at 0.04 mL min-1 from a graduated cylinder using a high-performance liquid chromatography (HPLC) pump. The total pressure in the reactor was maintained at 27 bar using He and a back-pressure regulator. Liquid products were separated from non-condensable gases using a gas-liquid separator at room temperature for quantitative analysis by HPLC. Concentrations of species present in the reactor effluent were quantified using a Waters e2695 HPLC system equipped with a 2998 PDA UV detector (320 nm) and a 2414 refractive index detector. Products were separated using an Aminex HPX-87H column (Biorad) at 353 K using 0.005 mol L-1 H2SO4 as the mobile phase at a flow rate of 0.6 mL min-1. Glucose and fructose were monitored using the refractive index detector. Glucose conversion to fructose is defined as moles of fructose produced per mole of glucose fed. The first datapoint was obtained by analyzing the first 3-4 mL of reactor effluent collected. Subsequently, the steady-state reaction rate was measured at time intervals ranging from 1 to 2.5 h, by analyzing the amount of fructose produced between each sampling point. To obtain TOFs, rates were normalized per Na+ ion present in the zeolite. The activation energy was obtained from the Arrhenius plot in Figure 1b, using initial rates (based on extrapolating observed rates to time = 0) at 383, 393, 403, and 413 K. Operando solid-state 13C MAS NMR spectroscopy. 13C MAS NMR experiments were performed on an Agilent-Varian VNMRS NMR spectrometer equipped with an 11.7 T magnet, operating at 125.7747 MHz for the 13C channel and 500.1822 MHz for 1H decoupling, and using a 5 mm home-built MAS double resonance HX probe with a custom Pd–coated coil for increased sample magnetic homogeneity. 140 mg samples were loaded into 5 mm ZrO2 rotors customized for high-pressure experiments,27-29 and spun at 5 kHz. The maximum pressure P at the internal surface of the rotor was estimated using Eq 3,60 where the density r of the catalyst slurry is approx. 1.15 g mL1 at 298 K: 𝑃 = 2𝜋 B 𝜌𝑓 B 𝑟 B (3) Since the internal radius r of the rotor is 1.5 mm, and the spinning rate f is 5 kHz, the pressure is ca. 13 bar. The ramp from room temperature to the desired reaction temperature usually required ca. 10 min. In 13C direct polarization experiments, a 35 kHz 1H decoupling field was employed with an acquisition time of 300 ms. The 13C spectral width was 50 kHz, and 15,008 data points were acquired per transient, using a relaxation delay of 30 s to ensure quantitative analysis. At room temperature, T1 is ca. 1 s for mobile and
