Low-Temperature Adsorption and Diffusion of Methanol in ZIF-8

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Low Temperature Adsorption and Diffusion of Methanol in ZIF-8 Nanoparticle Films Amber M. Mosier, Hanna Lo Williams Larson, Elizabeth R. Webster, Mia Ivos, Fangyuan Tian, and Lauren Benz Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b04455 • Publication Date (Web): 07 Mar 2016 Downloaded from http://pubs.acs.org on March 8, 2016

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Low Temperature Adsorption and Diffusion of Methanol in ZIF-8 Nanoparticle Films

Amber M. Mosier,† Hanna L. W. Larson, † Elizabeth R. Webster, † Mia Ivos,† Fangyuan Tian,‡ Lauren Benz †,* †

Department of Chemistry & Biochemistry, University of San Diego, San Diego, CA 92110, USA

‡

Department of Chemistry & Biochemistry, California State University, Long Beach, CA 90840, USA

Corresponding Author *Tel.: 1-(619)-260-4117. Fax: 1-(619)-260-2211. E-mail: [email protected].

ACS Paragon Plus Environment

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ABSTRACT. The adsorption of methanol by a ZIF-8 nanoparticle thin film was studied in situ using temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) under low temperature, low pressure conditions. Partial pore penetration was observed at 90 K, but upon increasing the exposure temperature of the film to 130 K pore penetration was significantly enhanced. Although many studies exist involving bulk powders, this is the first work to our knowledge that demonstrates the ability to control and monitor the entry of a molecule into a metal organic framework (MOF) film in situ using temperature. In this case, nanoparticle films of zeolitic imidazolate framework-8 (ZIF-8) were prepared and studied in ultra-high vacuum. The ability to control and monitor surface adsorption versus pore adsorption in situ is key to future fundamental study of MOFs, for example, in the identification of active sites in reaction mechanisms.

KEYWORDS Metal-Organic Frameworks (MOFs), Zeolitic Imidazolate Frameworks (ZIFs), ZIF-8, Methanol, Gas Adsorption, Temperature Programmed Desorption (TPD), X-ray Photoelectron Spectroscopy (XPS).


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1. Introduction Metal-organic frameworks (MOFs) have emerged as a new type of nanoporous material composed of metal ions or clusters (also known as secondary building units, SBUs) connected with organic linkers.1,2

Compared to other porous materials such as primarily inorganic

mesoporous silica3 and zeolites,4 and primarily organic nanostructured carbon5 and polymers such as polydimethysiloxane (PDMS),6 MOFs present highly controllable hybrid materials with nanopores formed by self-assembly, and high porosities ranging from approximately 2000 to 7000 m2/g.1 The high porosities, relatively large surface areas, tunable molecular structures and selective adsorption have brought considerable attention to MOFs for potential use in the fields of gas capture,7,8 sensing,9-11 separations,12-15 and catalysis.16,17 As such, MOFs stand to play an important role in the solution to numerous current challenges including the energy challenge. For example, the effective capture and conversion of CO2 into a usable energy source such as methanol or other commercially viable chemical would be a holy grail of sorts for any material,18 and in particular, MOFs have recently demonstrated promise to this end.19-23 Zeolitic imidazolate frameworks (ZIFs) are a particularly interesting subcategory of MOFs as they form with familiar zeolitic topologies since the angle of connectivity between the metal and organic components matches that of the Si-O-Si bond (145°) in aluminosilicate zeolites.24 ZIF-8 is a highly promising ZIF composed of tetrahedrally coordinated Zn ions connected by methylimidazolate linkers which assemble to form a zeolitic sodalite topology with pore diameters of 11.6 Å that can be accessed through 3.4 Å apertures.25 ZIF-8 is remarkably stable relative to other MOFs,25 and sufficiently stable in nanoparticle film form to be studied using TPD and XPS in ultra-high vacuum, which is one reason it is the focus of this work.26 In


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addition, ZIF-8 has demonstrated promising performance in separations14,15,27 as well as reactions.28-31 In previous work, we investigated the interaction of CO2 and water with supported ZIF-8 films,32 and herein we turn our attention to methanol. High CO2 uptake has been demonstrated by several studies of porous ZIF-8, while water is known to resist entering the hydrophobic pore structure unless high pressures are employed.33 The adsorption properties of methanol are more complex, falling between that of CO2 and water, as will be shown in this in situ study. An understanding of the interaction of methanol with MOFs is of broad interest for a variety of potential applications including the separation of methanol from molecular mixtures and reactions involving methanol.

A brief example of a potential application from these two

categories also involving ZIF-8 is highlighted below. A “methanol economy” is one possible intriguing alternative to our petroleum based economy. Methanol is currently produced from petroleum-derived syngas, but it can also be produced by methane oxidation or CO2 reduction, the latter of which could simultaneously mitigate CO2 levels and global warming.34 In the former process, biomass, an alternative to petroleum, can be degraded to produce syngas,35 which can then lead to methanol production by reacting syngas over Cu/ZnO/Al2O3 based catalysts.36 The use of biomass to produce fuels is a carbon-neutral process, as an equivalent amount of CO2 is taken in to grow the biomass as is produced in the combustion of the resulting alcohols.35 Once methanol is produced it can be used in fuel cells to generate electrical power,34 and can also be combined with various oils in a transesterification reaction to produce biodiesel.35


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In many of these processes, including syngas production, water is a byproduct35 and must therefore be separated from methanol (and other alcohols produced from biomass) prior to utilization. To separate alcohols from water, a selective material is necessary. To this end MOFs have been recently explored for use in the separations of water and alcohol. approaches can be employed: the use of a hydrophilic sorbent like

Two main

MOF JUC-11037 to

preferentially adsorb water, or the use of a hydrophobic sorbent with strong alcohol adsorption affinity such as ZIF-8.38 In other in-between cases such as MIL-100(Fe),39 the adsorption equilibria depend on the relative amounts of water and alcohol in the mixture. Even in the case of hydrophobic ZIF-8, care must be taken to fully understand the adsorption and diffusion dynamics which can be significantly influenced by the presence of external surface terminations, as was shown in a systematic study of ZIF-8 nanoparticle size on the adsorption of ethanol and water.40 These effects can become significant in cases of dilute alcohol/water mixtures. Since hydrophobic ZIF-8 exhibits selective adsorption for alcohols at high pressures, it has been studied extensively both computationally and experimentally for the separation of alcohols from alcohol/water mixtures.38,40,41 Previous studies show both simulated and experimental Sshaped adsorption isotherms of C1-C5 primary alcohols with increasing pressure for microcrystalline ZIF-8.27,42 The adsorption of these alcohols can be explained by a “clusterformation and cage filling” mechanism.43 It was found that at low pressures (1 kPa) and ambient temperatures, both methanol and ethanol form clusters at the C=C bonds of the methylimidazolate linkers due to weak van der Waals interactions, resulting in little adsorption in pores. With increasing pressures (~3-5 kPa, with lower uptake pressures for longer alcohols), however, the clusters grow and cage–filling occurs in the sodalite structure.38


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In addition to its ability to participate in separations involving alcohols, ZIF-8 has recently demonstrated activity in the transesterification of rapeseed oil to alkyl esters via reaction with alcohols.44 What is particularly noteworthy is that this reaction involves the framework itself rather than metal nanoparticles or other external catalyst loaded into the framework. This reactivity is surprising because the Zn2+ nodes are presumable fully coordinated in the bulk of the framework and thus not expected to be particularly active. Nevertheless, ZIF-8 exhibits considerable activity which is comparable to that of zinc aluminate, a competitor to zinc oxide.45 In the transesterification reaction, alcohols are typically activated via deprotonation by basic sites thus rendering them nucleophilic, while esters become electrophilic when activated by acidic sites. The reaction can therefore proceed in the presence of acidic and/or basic sites. The authors of this work conclude that external surface groups which include undercoordinated Zn acidic sites and possibly basic surface sites such as protonated imidazole groups are responsible for the activity, and indirect yet convincing evidence for the existence of these sites is provided. Clearly, an understanding of terminating surface groups and how they influence adsorption, separations, and reactivity is important.

In this fundamental work we examined methanol

adsorption by a film comprised of nanoparticles of ZIF-8, thus maximizing the presence of external surface groups in an effort to uncover the interplay between external surface and internal pore adsorption and diffusion. Methanol was selected as a representative model alcohol with the ultimate goal of being able to monitor alcohols in situ along with other small molecules such as water and carbon dioxide in order to distinguish surface and bulk adsorption and reaction sites. The two main techniques used in this study, XPS and TPD, are well established and have both been employed extensively in single crystal studies, and to a lesser extent to zeolites and other porous materials. XPS is a well-known technique that offers chemical state information


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specific to the first few nanometers of a material, and can therefore be used to identify key surface species which often differ from that of the bulk material.46 TPD helps identify the types and relative populations of desorption states, and can also provide kinetic parameters such as desorption order and activation energy.47 In the event that a reaction occurs, active sites can be identified using TPD and/or XPS.

For example, in the McMurry coupling reaction of

benzaldehyde to form stilbene over single crystalline reduced titanium dioxide, reactive sites were identified to be mobile interstitial bulk Ti3+ sites, which was unexpected as much of the reactivity over this surface is attributed to surface oxygen vacancies.48 This reaction proceeds through an interstitial-stabilized diol intermediate that was later imaged on the surface with scanning tunneling microscopy.49 TPD and XPS have also been employed in the interrogation of porous materials such as nanotubes and zeolites. For example, a variety of binding sites for nnonane and carbon tetrachloride on carbon nanotubes could be identified using TPD, including sites inside the tubes, outside on the surface of the tubes, and to sites in between the nanotubes.50 In addition, TPD is routinely used to probe the number and strength of acidic sites in zeolites via the adsorption and desorption of ammonia.51 Despite the proven utility of TPD and XPS in other systems, very few reports exist to date in the literature on the application of these methods to metal organic frameworks. This is likely due in part to the complexity of MOFs, and also the fact that MOFs are relatively new in comparison to zeolites and single crystals. Herein and in our previous work26,32 we use XPS and TPD to probe molecular adsorption over ZIFs, in this case focusing on methanol adsorption and the ability to distinguish and control adsorption on outer surface sites from bulk sites. Such a distinction is key to understanding the unique behavior of these materials.


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2. Experimental Ultra-High Vacuum Chamber All methanol adsorption/desorption experiments were carried out in an ultra-high vacuum (UHV) chamber with a base pressure of 1x10-10 torr. The chamber has a custom directed doser comprised of a leak valve welded to a ¼ in. stainless-steel tube. The tube was positioned 0.5 cm in front of the sample during exposure to anhydrous methanol (dispensed by a Pure Solv dry system, EMD OmniSolv grade), carbon dioxide (Airgas, 99.999% purity) and water (Millipore filtered). Each gas was dosed at a specific pressure (range of 10-10 – 10-6 torr, depending on the gas) and time.

Longer exposure times led to greater uptake, with approximately linear

correlation between exposure time and uptake. An X-ray photoelectron spectrometer (XPS) with a cylindrical mirror analyzer for surface analysis (PHI 15-255G) and a quadrupole mass spectrometer for temperature-programmed desorption studies (300 amu range, Hiden Analytical, HAL 301/3F) were also employed as part of this system. Sample Preparation A ZIF-8 solution was prepared using a previously established method52 by mixing two 100 mL methanolic solutions and stirring for 1 hr. One solution contained 1.50 grams (50 mM) of zinc nitrate and the other 3.25 grams (400 mM) of 2-methylimidazole. The resulting colloidal white precipitate was separated using centrifugation (14,000 rpm for 15 minutes) then resuspended by sonication in absolute ethanol (KOPTEC, anhydrous, 200 proof) to remove the excess unreacted species. This rinsing process was performed a total of 3 times. The resulting ZIF-8 nanoparticles as reported in our previous work were 34.0+/-2.9 nm in diameter.26


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Gold-coated silicon wafers (50±5 nm Au on 500±30 µm p-type Si with orientation, Ted Pella, Inc., cut into 1 x 1 cm2 squares) were cleaned with acetone (BDH, ACS grade) and dried under nitrogen (dry house N2). After cleaning, the wafer pieces were dip-coated using a commercial dip-coater (Chemat Technology, Inc., 180 mm/min withdrawal speed, 10 seconds still time) in the purified solution of ZIF-8 in ethanol, heated to 130 °C for 5 minutes on a hot plate, dipped in ethanol, dipped a second time in the ZIF-8 solution, dried with nitrogen gas, and heated again to 130 °C for 5 minutes to produce “2-cycle” films. The parameters for dip-coating were determined from earlier work demonstrating the successful preparation of ZIF-8 films using this method.26,53 The ZIF-8-coated Au film was then mounted onto an equally sized tantalum back plate using thin tantalum wires (0.25 mm, 99.9+% purity). The sample mount was in contact with a liquid nitrogen reservoir, and a type-K thermocouple was glued into a small slit cut on the side of the Ta plate with pre-cured UHV-compatible ceramic glue (Ceramabond 503) such that it also contacted the wafer. The sample was cooled to approximately 90 K and controllably heated to 365 K using a tungsten wire positioned approximately 2 mm behind the sample. Characterization X-ray photoemission data were collected using a Mg Kα X-ray source (hν= 1253.6 eV, with a maximum resolution of 1.1 eV) at 90 K. Prior to the experiments, binding energies were calibrated using a half-copper (2p3/2, 932.4 eV) half-gold (4f7/2, 83.8 eV) substrate. Data analysis was performed using CasaXPS software (CasaXPS Version 2.3.14), and peaks were


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fitted with a range of 2.5-3.5 full width at half max (FWHM). Photoelectrons were collected at a 45o take-off angle, and an estimated escape depth of approximately 4 nm based on these parameters and the density of ZIF-8.54 During the temperature-programmed desorption (TPD) studies, the sample was heated at a rate of 2 K/s while positioned ~1 cm in front of the mass spectrometer’s ionization source. A small aperture (4 mm) to the mass spectrometer was used to ensure that all ions collected were indeed from the ZIF-8 film rather than sample holder parts, which were of minimal area.

3. Results and Discussion 3.1 Methanol adsorption at 90 K Methanol (MeOH) desorption from a ZIF-8 nanoporous film was first monitored using TPD following adsorption at 90 K. By comparing with previous studies of methanol desorption from a pristine single crystalline TiO2 (110) surface (3.2 x 1014 methanol molecules corresponding to a monolayer on a 1 x 1 cm2 TiO2 (110) surface55, data and additional details in supporting information, Figure S1), the desorption of methanol from ZIF-8 nanoparticle films can be quantified. As presented in Figure 1, the desorption spectra of methanol from a 2-cycle ZIF-8 film reveal a feature at approximately 170 K at the lowest exposure, which gradually shifts to 165 K with increasing exposure.

Above this exposure, a peak emerges at 160 K with

overlapping leading edges, suggesting zeroth order, multilayer-like desorption.56 This multilayer desorption temperature is consistent with a control bare Au wafer using the same sample holder,


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although slightly broader (Figure S2), and similar to that reported in the literature (approximately 20 K higher, likely due to the fact that we use an as-installed Au-coated Si wafer with different thermal properties as compared to a sputter-cleaned Au(111) single crystal, but both features do not saturate with increasing exposure).57 However, we will demonstrate below that this feature is not solely due to the accumulation of a traditional surface multilayer. At larger exposures (Figure S3) this feature grows and continues to exhibit zeroth order behavior, shifting gradually to higher temperature, but no additional features were observed.

Figure 1. TPD spectra of methanol desorption from a 2-cycle ZIF-8 film To further study the interaction of methanol with the ZIF-8 film, we also employed XPS. Since XPS is a surface sensitive technique which can detect the elemental composition and chemical environment of the outer ~4 nm of the surface (see above), while TPD collects signal from the entire film upon desorption, the two can be used together to monitor the adsorption of


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methanol both in and on the film.32 For example, in a previous study, we were able to use XPS and TPD to determine the uptake of CO2 by 2- and 4-cycle ZIF-8 nanoparticle films.32 In that case, CO2 entered the pores of ZIF-8 initially, rather than adsorbing to the outer particle surfaces, and was only detected at the surface of ZIF-8 by XPS once the pores were filled to near capacity. Methanol differs from CO2 in that it has the potential ability to hydrogen bond to outer surface groups, and it also has a larger kinetic diameter (3.7 vs. 3.3 Å, respectively).58,59 XPS can also be used to determine any changes to the surface groups upon adsorption and/or reaction. In our previous studies, using XPS we found that clean ZIF-8 films are terminated not only by methylimidazolates, but terminations also include carbonates, hydroxide and/or water groups, and undercoordinated Zn ions as well as a small amount of protonated imidazole nitrogen.26 In the O 1s region of the “clean” sample which shows the non-native surface terminations of ZIF-8 (bottom traces of Figure 2), the dominating signal fit with a red trace is due to the presence of carbonate oxygen atoms due to Zn-coordinated carbonate groups. Oxygen coordinated directly to Zn is a smaller component of the total O1s signal shown in green, while the magenta trace is attributable to Zn-coordinated hydroxyls and/or water molecules. After adsorption of 1.1 x 1015 molecules of methanol at 90 K, (corresponding to 3.4 ML on a single-crystal TiO2 surface55), the peak in the O 1s region shown in Figure 2a broadens considerably due to the presence of methanol in the surface region. The broadening is caused by the appearance of a new component shown in blue at approximately 533.6 eV, attributable to the –OH moiety of methanol.60 With increasing exposure to methanol, the intensity of the –OH feature in the O 1s region increases. By comparison, in earlier work on H2O on a ZIF-8 film, an exposure of only 1.4 x 1014 molecules of water at 90 K (corresponding to 0.4 ML on a single-crystal TiO2 surface55) produced a corresponding O 1s signal roughly 4 times greater than that observed here.32 In that


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case it was clear that water was building up nearly exclusively on the surface. The underlying substrate peaks in the region remained unchanged in relative intensity and position upon and following exposure to MeOH. In the C 1s region, the main peak on the “clean” surface can be attributed to 2methylimidazolate groups, and is shown in yellow. Flanking smaller brown and red traces have been previously attributed to adventitious carbon and carbonate carbon, respectively.26 Upon exposure to methanol, a peak related to the methanol carbon appears at 286.3 eV.60 This peak also grows with increasing exposure, and the oxygen to carbon atomic ratio for the methanol peaks was calculated to be 1.1:1, close to the expected 1:1 C:O stoichiometry. The underlying substrate peaks in the carbon region remained nearly unchanged following methanol adsorption/desorption, however, a small amount of additional adventitious carbon was observed following the larger exposure. Our results suggest methanol’s adsorption behavior is between that of water and carbon dioxide, which were studied previously. Carbon dioxide, which enters the pore structure under these conditions, required exposures roughly 2-3 orders of magnitude larger, and the corresponding desorption (~1017 molecules) was similarly larger compared to that of methanol and water, respectively, before corresponding peaks were clearly visible in XPS.32 For further comparative purposes we did a select number of TPD and XPS experiments involving butanol, which exhibited stronger interactions, yet overall behaved similarly to methanol. The details of these experiments can be found in the supporting information.


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Figure 2. XPS spectra of a 2-cycle ZIF-8 film before and after the adsorption of 1.11015 (XPS threshold amount) and 1.51015 molecules of methanol in a) C 1s and b) O 1s regions. Molecular values were calculated following subsequent desorption by TPD, after which the XPS spectra reverted back to the clean state (bottom). 3.2 Methanol adsorption on ZIF-8 at 130 K In order to study the adsorption behavior of methanol further, we explored the possibility of using temperature to control methanol’s access to the cage space in our system.

Not

surprisingly, it has been shown previously in a comparative study of ZIF-90 and ZIF-8 using a combination of MD simulations and NMR that increasing temperature will increase the diffusion of methanol in ZIF-8,27 therefore it may be possible to enhance diffusion into the pores by increasing the methanol exposure temperature. In fact, there are a number of reports regarding the high diffusivity of several gases at various temperatures in ZIF-8.61-64 Additionally, in a model study of a different MOF, HKUST-1, it was shown that non-native surface groups present


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can actually block access to the pores, significantly slowing the uptake of cyclohexane in that case into the pore structure as compared to pristine films which were never exposed to air and thus never had the chance to build a “surface barrier” layer.65 Another recent paper on methane adsorption notes a similar barrier acting in ZIF-8 particles.66 We therefore investigated the adsorption of methanol on ZIF-8 nanoparticle films at 130 K, the maximum temperature before which desorption rates are appreciable (as determined by TPD, Figure 1). The TPD spectra of methanol desorption from ZIF-8 films following adsorption at 130 K is shown in Figure 3, and reveals three main features, labeled β1, α1 and α2. The first feature to populate, α1, appears at 170 K, similar to exposure at 90 K. The inset in the figure shows this in more detail, and a very broad shoulder is observed at around 220 K as well. Upon increasing the exposure amount, this peak grows and shifts to 180 K, and a small sharp peak appears at approximately 160 K (desorption of 4.1x1015 molecules), labelled β1. Finally, at the highest exposures investigated, a broad but well-defined peak, α2, appears at 260 K, shifting significantly to 275 K. The peak at 160 K, the lowest desorption temperature observed here, is similar to that observed both on Au and at higher exposures at 90 K, therefore we assign this to multilayer methanol, which in this case is likely a true multilayer at the external nanoparticle surfaces. Interestingly, this peak remains relatively small following exposures at 130 K, contrary to that observed at 90 K. We attribute α1 and α2 to desorption states associated with the nanoparticle film: α1 likely arising from desorption from within the ZIF-8 nanoparticle pores, and α2 arising from desorption from between particles.

The α1 peak shows an overlapping leading edge and increasing peak

temperature with increasing exposure, thus zeroth order behavior56 consistent with the formation of clusters of MeOH within the pores, since the main interactions within these clusters are between the MeOH molecules themselves, which break as methanol diffuses out of the pores and


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desorbs. The broader α2 peak with smaller area and greater shift in desorption temperature with increasing exposure likely correlates to desorption from between particle space, and is consistent with a similar peak observed in previous experiments of carbon dioxide on ZIF-8 films.32 The interparticle space is expected to have higher density, lower overall surface area, and likely more complex diffusion dynamics due to the presence of non-native surface groups such as carbonates and hydroxyl/water groups,65 thus the broader peak and higher desorption temperature.

Figure 3. TPD spectra of methanol desorption from ZIF-8 films following adsorption at 130 K. The TPD spectra corresponding to smaller methanol exposure amounts are shown in the inset.

3.3 TPD/XPS of methanol, water, and CO2 adsorption on ZIF-8 at 90 K and 130 K


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Due to the complexity of the TPD spectra at 130 K, we further explored the increased accessibility of methanol to the pore space of ZIF-8 by carefully comparing TPD and XPS data following exposure to MeOH at 90 K and 130 K. Since XPS probes the first few nanometers of the surface region, while TPD monitors total desorption from both the surface region and inner pores, the combined use of XPS and TPD over a broader range of exposures gives insight into the tendency of a species to enter the film porosity or remain on the surface. Figure 4 compares the integrated TPD peak area to the corresponding integrated O 1s peak area in XPS with varying exposure amount and exposure temperature. If molecules accumulate primarily at the surface, both XPS and TPD signals will be collected, with the surface signal in XPS growing quickly, as shown by the bottom arrow in the figure, eventually saturating as the photoelectron escape depth is reached. The data shown is before this saturation point is reached. On the other hand, if molecules tend to enter the pore structure, the XPS signal will remain small while the TPD signal grows rapidly, as shown by the top arrow. If a molecule simultaneously populates both the external surface and internal porosity, the XPS signal will grow more slowly, with the slope between the two extreme cases and indicative of the relative amounts of surface accumulation and pore penetration. As a set of comparison points, since we previously found that water does not enter the pore structure of ZIF-8 under these conditions, while carbon dioxide readily enters the pores for a 4-cycle film, we collected and correlated XPS and TPD areas of these two molecules as well on the 2-cycle films prepared here (open black squares and open blue circles, shown in Figure 4). Since ZIF-8 is hydrophobic, and water is expected to reside mainly at the surface, the XPS signal increases rapidly even at very low exposures and correspondingly small desorption amounts, therefore the slope is relatively small in the TPD/XPS correlation. On the other hand, for CO2 the opposite is observed: following a short initially flat region likely due to


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some CO2 accumulation at bare Au regions (some underlying Au is exposed on the 2-cycle ZIF-8 sample), because CO2 can access the pore structure, the XPS signal saturates while the TPD signal continues to build as most of this desorption comes from CO2 beneath the outermost surface residing in the bulk pores (note that the data has been corrected for the fact that CO2 will produce a related XPS O 1s signal roughly twice as large as that of H2O due to the additional oxygen atom, and the TPD data is scaled down by a factor of 4 for ease of viewing on the scale shown). We compared adsorption of MeOH at 90 K (open red triangles, Figure 4) to these two molecules (also at 90 K), and noticed that the slope of the plot is flat initially, and then increases, falling between that of water and CO2. This behavior is likely indicative of initial nucleation at the surface followed by simultaneous nucleation at the external surface and in the pores, which was not immediately clear from the observed single TPD peak. In TPD following adsorption at 90 K (Figure 1), the initial peak at 170 K can be attributed to adsorption at external ZIF-8 surface groups due to hydrogen bonding with these groups. At higher exposures the peak shifts to 160 K, which is likely due to nucleation in the pores (governed by weaker van der Waals forces) as well as multilayers on the surface. It’s likely that the energies of desorption are similar and therefore difficult to distinguish with TPD alone, though as mentioned above the width of the 160 K peak is slightly broader than that observed on pure Au (S2) and also broader than previously observed pure surface multilayer peaks of other small molecules on dense surfaces using this experimental set-up and identical heating ramp parameters.67 Finally, we dosed MeOH at 130 K, then collected XPS at 90 K. We cooled the sample to 90 K following exposure to 130 K to minimize any potential differences in the XPS data caused by differences in diffusion. The slope of the correlation plot at 130 K (filled red triangles, Figure 4) is greater than that at 90 K, suggesting a greater degree of pore penetration at higher exposure temperature,


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likely caused by enhanced diffusion into the pores upon exposure. Note that this phenomenon is distinct from “gate-opening” which has been shown to occur with other molecules under higher pressure conditions. Recent calculations show that “gate-opening” does not occur in the case of methanol adsorption.43 We performed this same experiment with water and noted that the two lines were much closer together, so in the case of water the higher temperature was not sufficient to cause water to enter the porosity of ZIF-8.

Figure 4. Correlation of XPS O1s peak area and number of molecules desorbed for CO2 (open black squares), methanol (open red triangles), and H2O (open blue circles) adsorbed at 90K on a 2-cycle ZIF-8 thin film, and methanol (filled red triangles) and water (filled blue circles) adsorbed at 130 K. The number of molecules is calculated from corresponding TPD spectra. The measured O 1s XPS signal for CO2 was divided by a factor of two as a stoichiometric correction, and the TPD signal was divided by a factor of four for visual scaling purposes.


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4. Conclusions In this work, methanol adsorption by ZIF-8 nanoparticle films was followed in situ under low pressure, low temperature conditions using a combination of XPS and TPD. At 90 K, methanol adsorption occurred both at the surface and in the porosity of the ZIF-8 nanoparticles. This was determined by comparison to CO2, which was found to penetrate the pores nearly exclusively, and water, which resides primarily at surface sites. Methanol’s behavior was between that of CO2 and water in that it is capable of interacting with outer surface groups through hydrogen bonding, while also interacting with the porosity of ZIF-8 through weaker van der Waals forces. Additionally, we found that this pore penetration process of methanol can be kinetically enhanced by increasing the exposure temperature.

This work lays a foundation for future

fundamental study of molecular adsorption and reaction in metal organic frameworks. ASSOCIATED CONTENT Supporting Information: Calibration and control TPD studies of methanol on a TiO2 (110) and methanol on the supporting gold surface, as well as additional TPD and XPS spectra related to Figures 1 and 2 are included in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This material is based upon work supported by the National Science Foundation under grant number DMR 1255326.

Additional financial support was provided by the Henry Luce

Foundation’s Clare Boothe Luce Program and the Arnold and Mabel Beckman Foundation. REFERENCES (1) Furukawa, H.; Cordova, K. E.; O'Keeffe, M.; Yaghi, O. M. The Chemistry and Applications of Metal-Organic Frameworks. Science 2013, 341, 123044. (2) Stock, N.; Biswas, S. Synthesis of Metal-Organic Frameworks (MOFs): Routes to Various MOF Topologies, Morphologies, and Composites. Chem. Rev. 2012, 112, 933-969. (3) ALOthman, Z. A. A Review: Fundamental Aspects of Silicate Mesoporous Materials. Materials 2012, 5, 2874-2902. (4) Li, J. Y.; Corma, A.; Yu, J. H. Synthesis of New Zeolite Structures. Chem. Soc. Rev. 2015, 44, 7112-7127. (5) Fang, B.; Kim, J. H.; Kim, M. S.; Yu, J. S. Hierarchical Nanostructured Carbons with MesoMacroporosity: Design, Characterization, and Applications. Acc. Chem. Res. 2013, 46, 13971406. (6) Guo, J. X.; Zhang, G. J.; Wu, W.; Ji, S. L.; Qin, Z. P.; Liu, Z. Z. Dynamically Formed Inner Skin Hollow Fiber Polydimethylsiloxane/Polysulfone Composite Membrane for Alcohol Permselective Pervaporation. Chem. Eng. J. 2010, 158, 558-565. (7) Banerjee, R.; Phan, A.; Wang, B.; Knobler, C.; Furukawa, H.; O'Keeffe, M.; Yaghi, O. M. High-Throughput Synthesis of Zeolitic Imidazolate Frameworks and Application to CO2 Capture. Science 2008, 319, 939-943. (8) Millward, A. R.; Yaghi, O. M. Metal-Organic Frameworks with Exceptionally High Capacity for Storage of Carbon Dioxide at Room Temperature. J. Am. Chem. Soc. 2005, 127, 17998-17999. (9) Allendorf, M.; Betard, A.; Fischer, R. A., Deposition of Thin Films for Sensor Applications. In Metal-organic frameworks: applications from catalysis to gas storage, Farrusseng, D., Ed. Wiley-VCH Verlag GmbH & Co.: KGaA, Weinheim, Germany, 2011. (10) Kreno, L. E.; Leong, K.; Farha, O. K.; Allendorf, M.; Van Duyne, R. P.; Hupp, J. T. MetalOrganic Framework Materials as Chemical Sensors. Chem. Rev. 2012, 112, 1105-1125. (11) Wales, D. J.; Grand, J.; Ting, V. P.; Burke, R. D.; Edler, K. J.; Bowen, C. R.; Mintova, S.; Burrows, A. D. Gas Sensing using Porous Materials for Automotive Applications. Chem. Soc. Rev. 2015, 44, 4290-4321.


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Low-Temperature Adsorption and Diffusion of Methanol in ZIF-8

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