Temperature-Programmed Desorption of Pyridine on Zeolites in the

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Temperature Programmed Desorption of Pyridine on Zeolites in the Presence of Liquid Solvents Nicholas Gould, and Bingjun Xu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02536 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 7, 2018

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Temperature Programmed Desorption of Pyridine on Zeolites in the Presence of Liquid Solvents

Nicholas S. Gould, Bingjun Xu* Catalysis Center for Energy Innovation Department of Chemical and Biomolecular Engineering, University of Delaware 150 Academy St., Newark, DE 19716 *Email: [email protected]

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Abstract Selecting the proper solvent is a major challenge involved in biomass conversion processes, as there is little predictive understanding of how solvent choice will affect reaction rates. To date, most attempts to understand solvent behavior are on a case by case basis, through catalytic activity testing and screening multiple solvent choices. There is a need for characterization techniques aimed toward fundamental understanding of solvent behavior in a simple and systematic fashion. In this work, the effect of solvent on the Brønsted acid site – pyridine interaction was isolated to improve understanding of a solvent’s ability to stabilize charge in non-ideal environments like zeolite micropores. Pyridine temperature programmed desorption (TPD) was applied to zeolites ZSM-5, Beta, and Y in back pressurized, flowing solvent. The desorption temperatures of pyridine from Brønsted acid sites in vacuum were compared in liquid water, acetonitrile, and n-heptane. For H/ZSM-5 (Si/Al = 12), the pyridine desorption temperature profiles increased in the order: water < acetonitrile < n-heptane ≈ vacuum. For these solvents, the TPD profiles shifted to lower temperature with increasing solvent dielectric constant, suggesting that stabilization of the proton is driving deprotonation, and that liquid phase pyridine TPD is a qualitative probe for proton stability in non-ideal, porous environments. Pyridine TPD profiles were also compared across zeolites ZSM-5, Beta, and Y in liquid acetonitrile. Compared to pyridine TPD profiles on H/ZSM-5 (Si/Al = 12) across different solvents, the TPD profiles in one solvent (acetonitrile) across different zeolite frameworks (ZSM-5, Beta, and Y) displayed only minor differences. This suggests that acetonitrile solvated the proton in a similar way in most frameworks tested.

Keywords: Temperature programmed desorption, liquid phase, FTIR, zeolites, pyridine

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1. Introduction Due to the low volatility of the feedstock, biomass upgrade reactions are typically conducted in liquid phase, in the presence of a solvent.1–4 As a result, chemical transformations occur at a solid-liquid interface, where the surface and adsorbate energetics are influenced by interaction with the solvent. Regardless of whether the solvent is directly involved in the reaction mechanism, solvation energies stabilize or destabilize adsorbates, intermediates, and transition states, transforming the reaction coordinate diagram, and resulting in modified rates and selectivities.5–9 Solvent effects are most commonly studied via catalytic activity testing, with the goal of improving a particular product yield or selectivity by screening an array of solvents, and correlating the desired product yield/selectivity to a solvent property, e.g., polarity, basicity, or a substrate’s solubility in the solvent.4,7,10 Yet, predictive understanding of the optimal solvent choice is lacking because catalytic activity testing involves too many considerations at once. Solvent choice can affect catalytic sites and the transition state, stabilize/destabilize the kinetically relevant compound(s), competitively adsorb on sites, and affect phase equilibria and chemical activities (ai = γi × Ci) in non-ideal reaction media.11,12 Predictive understanding of the optimal solvent choice is also lacking because of insufficient catalyst characterization techniques that probe the liquid-solid interface, such as probe molecule adsorption in FTIR, which is typically performed in vacuum or inert gases.11,13,14 Temperature programmed desorption (TPD) of probe molecules such as ammonia and pyridine is typically conducted in vacuum or in a flow of inert carrier gas (often in conjunction with FTIR) to characterize the strength and quantify the density of acid sites.15–17 In this work, a homemade experimental setup was designed and fabricated to conduct pyridine TPD on zeolites in the presence of flowing liquid solvents, or liquid phase TPD. Compared to catalytic activity testing,

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Figure 1: Schematic of the pyridine deprotonation and desorption process in zeolite micropores

the liquid phase TPD experiment in this work isolates a few key considerations of the solvent. Namely, the experiment probes the ability of a solvent to stabilize a pyridinium ion (a diffuse positive charge) versus a proton (a concentrated positive charge) within the zeolite framework (Figure 1, deprotonation step). However, one must take care interpreting TPD data, especially when comparing different zeolite frameworks. The typical intent of vacuum TPD experiments has been to measure the intrinsic Brønsted acid site (BAS) strength differences defined by deprotonation enthalpy (DPE). However, TPD profiles can be affected by the strength of Van der Waals contacts with pore walls,18,19 by internal or external diffusion limitations, and by the influence of re-adsorption sites.20 In the context of liquid phase TPD, the intent is to probe the role of solvent in a non-ideal environment (the zeolite pore) in the deprotonation step (Figure 1, left). This requires that the desorption step (Figure 1, right) is facile and does not affect the TPD profiles. A solvent’s relative stabilization of concentrated versus diffuse charges, such as protons versus pyridinium, is commonly responsible for solvent effects encountered in acid catalysis.5,21– 24

In Brønsted acid catalyzed reactions, the rate limiting step (RLS) often involves a protonated

transition state (TS) and an independently solvated proton and substrate (Figure 2). The degree of stabilization of the proton by the solvent (∆Hsolvation) is affected by the solvent dielectric constant, a commonly used metric of polarity in bulk, homogeneous solution. The theoretical

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understanding is that species of different charge densities can be stabilized to different extents by solvents of varying polarity.7 Thus, a proton will be significantly stabilized by a polar solvent like water, while a protonated TS (or pyridinium) will be stabilized to a lesser extent due to its lower charge density (Figure 2). Important to notice is that the relative stabilization of the proton versus the protonated TS has a substantial impact on the Figure 2: Depiction of the effect of solvation on the thermodynamic barrier for homogeneous protonation of a substrate (R), in liquid water.

activation barrier (Figure 2). An example of this is the Brønsted acid catalyzed dehydration of

xylose to furfural, where the relative ability of the solvent (either gamma valerolactone or water) to stabilize a free proton versus the protonated TS played a critical role in determining the turnover frequency (TOF).5 Note that liquid phase pyridine TPD involves similar thermodynamic considerations, i.e., the solvation of pyridinium (RH+) versus a proton (Figure 1, deprotonation step). However, in a non-ideal environment like a zeolite micropore, the proper measure of solvent polarity remains unclear, as only a small integer number of solvent molecules can effectively surround the substrate or proton. In this work, liquid phase pyridine TPD experiments were conducted to study the effect of solvent on acidity in confined phases such as zeolite micropores. On an H/ZSM-5 sample, the liquid phase pyridine TPD profiles shift to lower temperature with increasing dielectric constant (ε) of the solvents tested in this work: water, acetonitrile, n-heptane, and vacuum (a “solvent” with ε = 1). Compared to the influence of solvent, the effect of the zeolite framework in a

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common solvent revealed minimal differences in TPD profiles. The trends observed across both solvent and zeolite framework geometry suggest the thermodynamics of proton solvation in zeolite pores is reasonably similar to those in bulk, homogeneous solution (at least for the solvents employed in this work). It is important to note that the geometric/steric constraints of the zeolite pore should raise skepticism that a pyridinium ion is similarly solvated in a zeolite pore as in bulk solvent. However, the solvation of the proton has a more dramatic impact on the thermodynamics of pyridine deprotonation (Figures 1 and 2).

2. Experimental Technique Description 2.1 Materials Preparation NH4/ZSM-5 (Si/Al = 11.5), NH4/Beta (Si/Al = 12), NH4/ZSM-5 (Si/Al = 40), and H/Y (Si/Al = 15) were purchased from Zeolyst International. All zeolite samples were calcined in air at 500 °C for 10 h with a heating ramp of 1 °C/min prior to use. Pyridine, acetonitrile, ethanol, and n-heptane were obtained from Sigma Aldrich, and used without further purification.

2.2 Materials Characterization Micropore volumes of the samples were determined using N2 adsorption at -196 °C and the t-plot method and are recorded in supporting information (Table S1). Several zeolites in Table S1 were treated with solvent under an identical temperature program performed during TPD experiments with a maximum temperature of 260 °C. These samples were calcined in flowing air at 550 °C for 12 h to remove organic compounds prior to N2 adsorption experiments. Isotherms were collected on a Micromeritics ASAP 2020 instrument. Prior to the adsorption

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measurements, all samples were degassed at 300 °C for 24 h. 29Si and 27Al magic angle spinning nuclear magnetic resonance (MAS-NMR) spectra of the zeolite samples were recorded on a Bruker DSX-200 spectrometer with a Bruker 7 mm MAS probe. A 90 degree pulse of 4 microseconds was used and a strong 1H decoupling pulse was applied during signal application. Sample spinning rate was 4 kHz and the recycle delay was 30 s. The 29Si and 27Al NMR spectra are included in the supporting information (Figures S1 and S2) and are labelled with Si/Al ratios estimated from the 29Si NMR results. Powder X-ray diffraction (XRD) patterns were collected on a Bruker D8 Discover powder diffractometer with a Cu Kα source over the range 5° to 50° with a step size of 0.025° and 2 s per step. XRD patterns of all zeolite samples are presented in Supporting Information (Figure S3).

2.3 Characterization of Brønsted Acid Site Densities Brønsted acid site densities are listed in Table S2 and were measured via n-propylamine decomposition into propylene and ammonia using a flow reactor with an on-line Agilent 7890A gas chromatograph (GC) equipped with an HP-PLOT Q column. This was accomplished using a microreactor system similar to that described by Kresnawahjuesa et al.25 All gas lines between the location of n-propylamine introduction and the GC were heat traced with temperature maintained at or above 75 °C. Approximately 15 mg of a zeolite sample was loaded into a quartz tube in the reactor. The sample was heated to 500 °C at a rate of 10 °C/min in flowing He (100 mL/min) and held at 500 °C for 45 min. Then, the sample was cooled to 100 °C and exposed to flowing He saturated with n-propylamine for 15 min via a bubbler. The sample was subsequently heated to 200 °C and held for 90 min to desorb excess n-propylamine and ensure a 1:1 ratio of adsorbed n-propylamine to BAS. Finally, the temperature was increased to 500 °C at a rate of 8


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30 °C/min. The GC sampling loop was immersed in liquid nitrogen to collect the desorbed reaction products, which were subsequently quantified via a thermal conductivity detector (TCD).

2.4 In-situ Attenuated Total Reflection FTIR Spectroscopy Liquid phase in-situ FTIR spectroscopic studies with a 4 cm-1 spectral resolution and 0.5 cm-1 aperture were conducted in a homemade multiple reflection ATR flow cell described in detail elsewhere.26,27 A trapezoidal ZnSe (45° cut) ATR crystal provides six total internal reflections. Before depositing catalyst on the ZnSe crystal, the mirror between the ATR cell and the detector was adjusted to maintain a constant interferogram intensity of 5.0 (arbitrary units on an Agilent CARY 660 FTIR spectrometer). The catalyst layer is directly deposited on the top side of the ATR crystal by evaporation of the solvent from a catalyst slurry. The seal between the ZnSe crystal and the top plate is achieved by pressing an O-ring into a groove on the bottom surface of the top plate. The internal dimensions of the cell are 1.75 in. length × 0.5 in. width × 20 µm thickness for a total volume of 1.1 × 10-2 cm3. A piece of quartz microfiber filter paper (2.7 µm pore size, Whatman Inc.) is placed between the catalyst layer and the top plate to prevent catalyst powder from being washed out by flowing solvent. Control experiments proving no loss of catalyst during experiment, and that pyridine FTIR signal results from adsorption on internal acid sites can be found in previous work.26,27 Solvents with or without dissolved probe molecules, e.g., pyridine, are introduced to the catalyst layer via an HPLC pump (Waters 515) at a rate ranging from 0.5 to 2 mL/min.

2.5 Transmission FTIR Spectroscopy and Vacuum Phase Temperature Programmed Desorption

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Transmission Fourier transform infrared (FTIR) spectra were obtained on an Agilent Cary 660 FTIR Spectrometer equipped with an MCT detector (128 scans at a spectral resolution of 2 cm-1) with a homemade in situ transmission cell.28 A vacuum level of 0.01 mTorr in the transmission cell can be reached through a vacuum manifold, which is connected to a mechanical pump and a diffusion pump. A self-standing zeolite wafer was loaded into a custom made sample holder, followed by annealing at 450 °C under vacuum for 30 min after a heating ramp of 2.5 °C/min to completely remove adsorbed water. After cooling to 100 °C, 100 mTorr of pyridine was introduced to the transmission cell via the vacuum manifold. For vacuum phase temperature programmed desorption (TPD) measurements, the pyridine saturated zeolite wafer was heated in 20 °C increments and subsequently returned to 100 °C after each increment for quantitative comparison to the original 100 °C saturated spectrum (e.g., 100 °C → 120 °C → 100 °C → 140 °C → 100 °C …). This is necessary because the extinction coefficients of vibrational bands corresponding to adsorbed pyridine depend on the temperature of the sample. The zeolite wafer was heated at a rate of 5 °C/min to every new increment temperature and held at that temperature for 1 h.

2.6 Liquid Phase Temperature Programmed Desorption The experimental design used for pyridine TPD from zeolites in solvent is provided in Figure 3. The solvent to be studied was introduced to the system via a Waters 515 HPLC pump at a rate of 1.0 mL/min. All lines between the HPLC pump and the back pressure regulator (BPR) were wrapped in heating tape and well insulated. The catalyst bed was situated inside ¼ inch stainless steel tubing in an up-flow geometry and was supported by quartz wool, 1 mm quartz beads, quartz microfiber filter paper (2.7 µm pore size, Whatman Inc.), and a 1/8 inch 10


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thermocouple in the geometry shown in Figure 3 (inset). Prior to loading the catalyst bed, the zeolite was pelletized (20 – 40 µm), loaded into the transmission FTIR set up described in section 2.5, annealed at 450 °C under vacuum for 30 min after a heating ramp of 2.5 °C/min to completely remove

adsorbed

water,

and

exposed to pyridine vapor for 2 Figure 3: Experimental design for pyridine TPD in flowing solvent.

hour at 150 °C to saturate the acid sites without condensation of liquid pyridine (b.p. ≈ 115 °C). The exhaust from the catalyst bed passed through a Swagelok BPR with a 2000 psi maximum pressure rating, followed by 1/16 inch tubing to cool the effluent while reducing the dead volume. The TPD experiment was run at room temperature for three hours (three samples taken, 60 mL each), followed by three hours at 100 °C (three samples taken) to remove molecularly adsorbed pyridine, and then one hour at every subsequent 20 °C temperature increase (e.g., 20 °C → 20 °C → 20 °C → 100 °C → 100 °C → 100 °C → 120 °C → 140 °C → 160 °C → …). One effluent sample was collected per hour, after every increase in temperature (60 mL volume). The pressure was maintained at a value roughly 20 % greater than the solvent’s vapor pressure, estimated using Antoine’s equation at all temperatures. Pyridine quantification analysis was performed using an Agilent 7890A gas chromatograph (GC) equipped with a Volamine column.

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3. Results and discussion At temperatures below the solvent boiling point, pyridine selectively desorbs from molecularly adsorbed sites (MAS) in flowing solvent. ATR-FTIR spectra of adsorbed pyridine on zeolites in flowing solvent can differentiate pyridine on Brønsted acid sites (BAS, pyridinium, “B” in Figure 4) from pyridine that is molecularly adsorbed (“M” in Figure 4). The spectra in Figure 4 were obtained by feeding a dilute pyridine in solvent solution to saturate all acid sites, followed by switching to pure solvent to attempt to remove adsorbed pyridine (full spectra can be found in Figures S4-S5). Adsorbed pyridine extinction coefficients for both BAS and MAS were determined in a previous work.26 After 1 hour of flowing pure solvent at 20 °C, the majority of molecularly adsorbed pyridine ( ̴ 1450 cm-1) was removed from H/ZSM-5 (Si/Al = 12) in water and acetonitrile (Figure 4). The elevated temperature spectra were acquired after 1 h of flowing pure solvent near each solvent’s respective boiling point, followed by returning the ATR-FTIR to 20 °C to ensure identical extinction coefficients between 20 °C and elevated temperature spectra.

After

elevated

temperature

treatment in pure solvent, pyridine is also removed from MAS in n-heptane. Further, Figure 4: ATR-FTIR spectra of pyridine adsorbed on comparison H/ZSM-5 (Si/Al = 12) after flowing pure solvent for 1 h at 20 °C or elevated temperatures below the respective solvent boiling point. All spectra collected at 20 °C after the listed temperature treatment. ACS Paragon Plus Environment

of the BAS band intensities

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across the two temperatures shows that pyridine does not significantly desorb from BAS in the 20 – 100 °C range. The same conclusion was reached for zeolites H/Beta (Si/Al = 12), H/ZSM-5 (Si/Al = 36), and H/Y (Si/Al = 12), where flowing acetonitrile was able to remove molecularly adsorbed pyridine, but could not remove pyridinium below 75 °C (Figure S6). Liquid phase pyridine TPD profiles over H/ZSM-5 (Si/Al = 12) shift to lower desorption temperatures with increasing solvent dielectric constant (Figure 5). The liquid phase pyridine TPD experiments were run using temperature segments held at every 20 °C interval for one hour rather than using a continuous temperature ramp (details in section 2.6). The “segment method” mitigated the complications of dead volume post catalyst bed ( ̴ 8 mL), as

continuous

ramp

conditions

would cause difficulty associating a desorption temperature with the eluting pyridine concentration. To estimate the effect of dead volume post catalyst bed, a 1 mL “plug” of ethanol was introduced to the liquid phase TPD system in flowing water (1 mL/min), and samples were taken from the exhaust in 5 minute segments and analyzed via GC. Approximately 85 % of the ethanol eluted from the system within 10 Figure 5: Liquid phase pyridine TPD profiles from H/ZSM-5 (Si/Al = 12) in A): water, B): acetonitrile, and C): n-heptane. D): Pyridine TPD from H/ZSM-5 (Si/Al = 12) in vacuum using transmission FTIR. ACS Paragon Plus Environment

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minutes of reaching the catalyst bed, and 100 % eluted after 15 minutes (Figure S7). Temperature segments were held for one hour to significantly exceed the typical diffusion time (10-15 min). A lack of significant high-end tailing (after the segment with maximum desorption) in the TPD profiles in Figures 5A and 5B also suggests that back diffusion of pyridine was insignificant. In Figure 5, the amount of desorbing pyridine was normalized by the weight of catalyst used in experiments for comparison to BAS densities measured via n-propylamine decomposition (Table S2). The percentage of pyridine that desorbed from BAS in the liquid phase TPD experiments was compared to the total BAS densities measured via n-propylamine decomposition (Figure 5). In acetonitrile and water, these values typically varied from 80 to 90 % of the propylamine decomposition estimate of the BAS density, while heptane was significantly lower (≈ 20 %). In acetonitrile and water, the minor disparity in the two BAS counts (10 – 20%) could come from a combination of a minor amount of pyridine desorption from BAS at low temperature (≤ 100 °C) and/or a minimal amount of catalyst getting flushed out the exhaust of the TPD set up prior to pyridine desorption. In the case of acetonitrile (Figure 5B), the spent catalyst from the liquid phase TPD experiment was examined in transmission FTIR in vacuum, and no detectable pyridine was observed (Figure 6), suggesting that all pyridine desorbed after the 260 °C segment in acetonitrile. Note that in heptane, the large disparity between the detected pyridine and the propylamine decomposition BAS density values suggests that the pyridine desorbing in the 240 – 280 °C range is only the leading edge of the liquid phase TPD profile. The liquid phase TPD setup could not access temperatures above 280 °C, due to convective heat losses inside the reactor, which varied slightly across solvents, as well as safety concerns at these conditions. This was further confirmed by examining the spent catalyst in transmission FTIR in vacuum, where the remaining adsorbed pyridine was detected on BAS

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(1547 cm-1, Figure 6). The spent catalyst from heptane TPD also featured the combination band (1490 cm-1) which results from pyridine adsorption on BAS or MAS, and a 1470 cm-1 band resulting from residual heptane in the pores. The dielectric constant (ε) of the bulk solvent is listed alongside each respective solvent TPD profile in Figure 5. Qualitatively, the desorption temperature of pyridine from Figure 6: Transmission FTIR spectra of spent H/ZSM-5 (Si/Al = 12) samples in vacuum after the TPD experiments in Figure 5 in acetonitrile and heptane.

BAS

decreases

with

the

increasing

dielectric constant of the solvent from heptane, to acetonitrile, to water. Liquid phase TPD experiments were also conducted in ethanol (ε ≈ 24), which resulted in a desorption profile similar to that of water (Figure S8). However, ethanol dehydrated to diethyl ether during the TPD experiment, and the water produced from reaction likely had an effect on the result. For example, ~10% conversion of ethanol to diethyl ether was observed on H/ZSM-5 (Si/Al = 12) at 200 °C. Further, the composition of the solvent is temperature dependent in this case because the rate of ethanol dehydration rises with temperature, increasing the concentration of water and diethyl ether. Gas chromatograms of the liquid phase TPD profiles in this work reveal insignificant side product formation for water, acetonitrile, and heptane, making these solvents ideal candidates for fundamental studies (Figures S9-S14). The pyridine TPD profile from BAS was also conducted in transmission FTIR in vacuum in a quantitative manner (Figure 5D, see methods section 2.5), and compared to the

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liquid phase TPD profiles in Figures 5A-C. Of particular interest is the similar pyridine desorption profiles, both qualitatively and quantitatively, in heptane compared to vacuum (Figure 5C-D). One can view vacuum as a solvent with a dielectric constant of 1, and thus a poor ability to stabilize charge. The similar pyridine TPD profiles suggest that heptane is roughly equivalent to vacuum in terms of its ability to stabilize charge. For the solvents tested in this work, the bulk solvent dielectric constant qualitatively predicted the properties of solvation even in a non-ideal, confined environment like the zeolite micropores. The dependence of the liquid phase pyridine TPD profiles on solvent identity is indicative of a late transition state in liquid phase TPD experiments. Liquid phase TPD experiments could probe two kinetic barriers (Figure 1). The first barrier is the deprotonation of pyridinium to pyridine and a proton, while the second is the removal of pyridine from the micropores into the external solution, where it is collected and analyzed. The ATR-FTIR spectra in Figure 4 show that molecularly adsorbed pyridine is easily removed at low temperature in flowing solvent, while pyridine on BAS are less affected. This confirms that the desorption of pyridine

from

zeolite

pores is facile, and that deprotonation

of

pyridinium

is

responsible for the TPD profiles in Figure 5.

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Examining the deprotonation step in detail (Figure 7), the pyridinium ion must overcome an activation barrier through a TS that either resembles a pyridinium ion (early TS) or a pyridine molecule separated from the proton (late TS). The significant differences in desorption temperature across solvents (Figure 5A-C), suggest that the solvent has an impact on the activation barrier (Ea). Note that the activation barrier for pyridine deprotonation is dependent on the relative energies of the initial state (Figure 7, left) and the transition state. If the TS resembles the initial state (early TS), the solvent would stabilize the initial state and the transition state similarly, such that ∆Hsolvation, IS ≈ ∆Hsolvation, ETS, regardless of the identity of the solvent. As a result, the solvent would have no effect on the activation barrier, making Ea,

ETS

solvent

independent (Figure 7). The late transition state, however, resembling pyridine separated from the proton, would result in a solvent dependent activation barrier (Ea, LTS). Typically, increasingly polar solvents are more effective at stabilizing charge, and the stabilizing effect is more dramatic for increasingly charge dense species, e.g., the proton over pyridinium. In this case, a high dielectric constant (polar) solvent like water would stabilize the proton to a greater extent than pyridinium, and lower the barrier for deprotonation because ∆Hsolvation,

LTS

is greater than

∆Hsolvation, IS (Figure 7). A low polarity solvent like heptane would not stabilize either ion well, and would result in a barrier similar to that of the vacuum TPD experiment, in agreement with Figure 5. Thus, the significant disparities between pyridine TPD profiles across solvents in Figure 5 suggests that the TS is either intermediate or likely late in structure, resembling more a solvated proton and pyridine than a solvated pyridinium ion. It is worth noting that the qualitative correlation of liquid TPD profiles with the dielectric constant suggests that the proton might be solvated similarly in the non-ideal zeolite pore as in bulk solvent (where dielectric constants apply more rigorously). It is likely that nearly a full solvation sphere of water or

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acetonitrile could surround the proton in the zeolite pore. This is consistent with extensive work on water cluster formation in zeolites in gas phase and vacuum, where only two water molecules are needed to transfer the Al-T site proton to the water cluster.29 The number of water molecules per Al-T site can vary, but is typically in the three to five range near water saturation pressure at room temperature for H/ZSM-5 and H/MOR zeolites (the value can vary depending on zeolite framework and Al-T site location).29,30 In liquid phase, the number of water molecules per T-site is likely greater than or equal to that at saturated water vapor pressure, and the effect of high temperature may be an increasingly delocalized water cluster. However, the approximation that this protonated water cluster is similar to that in bulk solvent may not be relevant to solvation of larger, more diffuse charges (like pyridinium), where a single solvation shell cannot form in the confinement of zeolite pores. Compared to the disparities in liquid phase pyridine TPD profiles among solvents, the disparity across zeolite frameworks is minimal. Acetonitrile was chosen to compare TPD profiles across frameworks because water was shown to degrade H/Y and H/Beta zeolites at high temperature (260 °C) based on measured micropore volumes of the spent samples. The stability of zeolite ZSM-5 and the instability of zeolite Y in high temperature liquid water (200 °C) has been previously documented,31 which is the main reason to use zeolite ZSM-5 to compare TPD profiles across different solvents (Figure 5). At the highest temperature examined in TPD trials (260 °C), liquid acetonitrile had little effect on zeolite micropore volumes (< 5 % loss, Table S1), X-Ray Diffraction patterns (Figure S3), and 27Al NMR spectra (Figure S2) of the four zeolites in Figure 8. In acetonitrile, the liquid phase pyridine TPD profiles across zeolite frameworks were relatively similar, with the maximum density of pyidine desorbing during the 240 °C segment for three of the four samples (Figure 8A-C). Only the H/Beta sample exhibited a distinctive shift in

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profile from the other zeolites, with a maximum desorbing pyridine density occuring at 200 – 220 °C. To gain further insight, the TPD profiles in liquid acetonitrile were compared

to

TPD

profiles

in

vacuum (Figure 9) and quantitative estimates

of

BAS

and

MAS

densities in vacuum (Table 1). Only the leading edge of pyridine desorption profiles can be quantitatively compared in TPD experiments

in

vacuum.

The

vacuum pyridine TPD profiles were Figure 8: Pyridine TPD profiles in liquid acetonitrile from A): H/ZSM-5 (Si/Al = 12), B): H/ZSM-5 (Si/Al = 36), C): H/Y (Si/Al = 12), and D): H/Beta (Si/Al = 12).

conducted with transmission FTIR spectroscopy

in

a

quantitative

manner. The method involved saturating the zeolite sample with pyridine at 100 °C in vacuum, and returning the sample to 100 °C after each temperature increase for quantification (see section 2.5). This approach avoids the need to estimate extinction coefficients at all temperatures. The adsorbed pyridine concentrations on BAS and MAS were compared up to 300 °C, as higher temperatures resulted in the appearance of a 1462 cm-1 peak indicative of pyridine cracking products (spectra in Figures S15 – S18).32,33 At 300 °C, the fraction of surviving adsorbed pyridinium ions increases in the order H/Beta (Si/Al = 12) ≈ H/Y (Si/Al = 12) < H/ZSM-5 (Si/Al

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Figure 9: TPD profiles of pyridine adsorbed on H/ZSM-5 (Si/Al = 12, red), H/ZSM-5 (Si/Al = 36, green), H/Beta (Si/Al = 12, violet) and H/Y (Si/Al = 12, blue) in vacuum from BAS (left) and MAS (right). The value of 1.0 represents the amount of pyridine on BAS or MAS at equilibrium at 100 °C.

= 12) < H/ZSM-5 (Si/Al = 36). The analysis of pyridine adsorption on MAS is non-trivial, as pyridine may adsorb on a variety of sites without protonation.32,34,35 These sites include purely siliceous re gions, with a band at roughly 1440 cm-1, and on MAS originating from aluminum in various defect forms, including extraframework (EF) aluminum, which results in a higher wavenumber band at or above 1450 cm-1.32,34,35 Adsorption on siliceous sites was significantly weaker than aluminum containing sites, and was fully removed from all zeolite samples by 200 °C (Figures S15 – S18). In the 200 - 300 °C range, pyridine is strongly adsorbed to the MAS of H/Beta (12) and H/ZSM-5 (36), as evidenced by the almost complete lack of desorption from these sites (Figure 9). However, H/Y (12) and H/ZSM-5 (12) exhibit a gradual decrease of pyridine desorption from MAS in this range, concurrent with the decrease of pyridinium. Quantitative estimates of pyridine adsorbed on aluminum based MAS in vacuum at 200 °C were made using the BAS:MAS ratio of FTIR extinction coefficients (EC) in Emeis’ work,36 and estimates of BAS densities measured via n-propylamine decomposition (Table 1).25 The MAS

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estimates show that the H/Beta (12) sample has a significant concentration of aluminum form MAS compared to the other three samples.

Zeolite Framework (Si/Al via 29Si NMR)

BAS Density (µmol/gcat)

Pyridine on MAS at 200 °C in Vacuum (µmol/gcat)

H/Beta (12)

690

360

lower desorption temperature

H/Y (12)

370

100

TPD

H/ZSM-5 (12)

990

60

acetonitrile on H/Beta remains

H/ZSM-5 (36)

280

20

While the reason for the

profile

liquid

unclear, it is possible that extraframework

Table 1: Quantitative estimates of the amount of pyridine adsorbed on aluminum form MAS sites in vacuum at 200 °C. Values were estimated via BAS densities measured via n-propylamine decomposition and 36 BAS:MAS peak area ratios in vacuum (ECs from Emeis ).

in

hydroxyl

groups have an influence. From

transmission

FTIR

spectra of the four zeolites studied in the TPD experiments, H/Beta is the only zeolite sample with visible Al-OH groups not resulting from framework BAS (3781 and 3663 cm-1, Figure 10).37 The zeolite Y and ZSM-5 (Si/Al = 12) samples only exhibit external silanols ( ̴ 3740 cm-1) and framework BAS (3628 and 3564 cm-1 for Y, 3609 cm-1 for ZSM-5).35,38 It is possible that the non-framework Al-OH on H/Beta participate in the deprotonation of pyridine, by solvating the proton via hydrogen bonding in a way similar to water. As water exhibits a lower desorption temperature TPD profile than acetonitrile (Figure 5), the hydroxyl groups might stabilize the proton more effectively than acetonitrile does, leading to a lower TPD profile. This is consistent with the vacuum and liquid TPD results for the H/ZSM-5 (Si/Al = 36) sample (Figures 8-9). In vacuum, this H/ZSM-5 sample exhibited the lowest amount of pyridine desorption from BAS below 300 °C but exhibited comparable TPD profiles to the H/Y and H/ZSM-5 (12) samples in liquid acetonitrile. This might suggest that the presence of solvent had a more dramatic impact

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Page 22 of 27

on the TPD profile of H/ZSM-5 (36) than on H/Y and H/ZSM-5 (12). Similar to the case of H/Beta, the H/ZSM-5 sample contains defect hydroxyl groups (Figure 10). The transmission FTIR spectrum of H/ZSM-5 (36) contains a shoulder at 3730 cm-1 corresponding to internal Si-OH and a broad band around 3500 cm-1 attributed to Si-OH resulting from framework defects and nests.37 It is likely that these extraframework and defect Figure 10: Hydroxyl region transmission FTIR spectra of the four zeolites in Figures 8-9 at 100 °C in vacuum before introduction of pyridine.

hydroxyls

do

not

play

a

significant role in vacuum TPD because of the localization of the pyridinium ion

near the negative framework charge. However, in liquid phase the pyridinium may have extended mobility due to the shielding of the two ions (framework and pyridinium). This is evident from the TPD results across solvent, which likely would have displayed similar profiles if the TS involved a simple transfer of the proton from pyridine to the framework anion (Figure 5). It follows that the deprotonation of a more mobile, delocalized pyridinium ion would occur via the solvent that can stabilize the proton best. In the case of H/Beta and H/ZSM-5 (36), this may be a hydroxyl nest.

4. Conclusions

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An experimental setup was designed to perform pyridine TPD experiments in the presence of pressurized liquid to understand how solvents behave in non-ideal environments like zeolite micropores. The use of ATR-FTIR allowed for a clear separation of temperature ranges where pyridine almost exclusively desorbs from MAS below the solvent boiling point (< 100 °C). For an H/ZSM-5 (Si/Al = 12) sample, liquid phase pyridine TPD profiles in water, acetonitrile, and n-heptane were compared to the traditional vacuum technique. The desorption temperature decreased with increasing dielectric constant of the solvent, in line with thermodynamic theory about the ability of polar solvents to stabilize dense versus diffuse charges. The vacuum TPD profile displayed strong agreement with the liquid phase TPD in nheptane, suggesting this low dielectric constant alkane (ε ≈ 2) provided almost no charge stabilization compared to vacuum. The disparities in liquid phase TPD profiles across solvents was indicative of a late transition state for pyridinium deprotonation. For the solvents tested in this work, the correlation between liquid phase pyridine desorption temperature and the bulk solvent dielectric constant suggested that the ability of solvents to stabilize the proton in the confined zeolite pore was reasonably similar to that of bulk solution. Compared to the TPD profiles observed on H/ZSM-5 across solvents, the relative similarity of TPD profiles across zeolite frameworks in acetonitrile suggested that the solvent choice played a more significant thermodynamic role for pyridinium deprotonation than the influence of the zeolite framework interactions. Minor differences between pyridine TPD experiments conducted in vacuum versus liquid acetonitrile were observed. The two samples that were most affected by the influence of solvent happened to contain extra-framework hydroxyl groups. These hydroxyl groups originated from aluminum sources in the case of H/Beta and from silanol defects and nests in H/ZSM-5 (Si/Al = 36). It was hypothesized that the hydroxyl

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Page 24 of 27

groups might be more effective at stabilizing the proton than acetonitrile molecules in the pores, and cause the downshift in desorption temperature from vacuum to liquid phase pyridine TPD experiments.

Acknowledgements We acknowledge support from 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.

Supporting Information Table S1: Micropore volume of zeolites before and after liquid TPD experiments. Figures S1-S2: 29

Si and 27Al NMR spectra of all zeolites before and after liquid TPD. Figure S3: XRD spectra

before and after liquid TPD. Table S2: BAS densities measured via propylamine decomposition. Figures S4-S5: Full ATR-FTIR spectra from 700 cm-1 to 4000 cm-1. Figure S6: ATR-FTIR spectra purging pyridine in acetonitrile. Figure S7: Elution profile of a plug of solvent in TPD set up. Figure S8: Pyridine TPD profile from H/ZSM-5 (11) in ethanol. Figures S9-S14: Gas chromatograms of pyridine eluting from liquid TPD experiments. Figures S15-S18: Transmission FTIR spectra of pyridine TPD in vacuum.

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For Table of Contents Only

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Temperature-Programmed Desorption of Pyridine on Zeolites in the

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