Enhanced Hydrothermal Stability of Î³-Al2O3 Catalyst Supports with

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Enhanced Hydrothermal Stability of #-Al2O3 Catalyst Supports with Alkyl Phosphonate Coatings Tim Van Cleve, Devon Underhill, Mariana Veiga Rodrigues, Carsten Sievers, and J. Will Medlin Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00465 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 13, 2018

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Enhanced Hydrothermal Stability of γ-Al2O3 Catalyst Supports with Alkyl Phosphonate Coatings Tim Van Clevea, Devon Underhilla, Mariana Veiga Rodriguesb,c, Carsten Sieversb, J. Will Medlina,* a

Department of Chemical and Biological Engineering, University of Colorado Boulder, JSCBB

D125, 3415 Colorado Ave., Boulder, CO 80303, USA b

School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta,

GA 30332, USA c

Instituto de Química de Araraquara (UNESP), Rua Prof. Francisco Degni 55, 14800-900

Araraquara, SP, Brazil * corresponding author KEYWORDS: Self Assembled Monolayers, Phosphonic Acids, Hydrothermal Stability, Catalyst ABSTRACT In this study, monolayers formed from organophosphonic acids were employed to stabilize porous γ-Al2O3, both as a single component and as a support for Pt nanoparticle catalysts, during exposure to hydrothermal conditions. To provide a baseline, structural changes of uncoated γAl2O3 catalysts under model aqueous phase reforming conditions (liquid water at 200 °C and autogenic pressure) were examined over the course of 20 h. These changes were characterized by

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X-ray diffraction, NMR spectroscopy, N2 physiosorption, and IR spectroscopy. It was demonstrated that γ-alumina was rapidly converted into a hydrated boehmite (AlOOH) phase with significantly decreased surface area. Deposition of alkyl phosphonate groups on γ-alumina drastically inhibited the formation of boehmite, thereby maintaining its high specific surface area over 20 h of treatment.

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Al MAS NMR spectra demonstrated that hydrothermal stability

increased with alkyl tail length despite lower P coverages. Although the inhibition of boehmite formation by the phosphonic acids was attributed primarily to the formation of Al2O3-POx bonds, it was found that use of longer-chain octadecylphosphonic acids led to the most pronounced effect. Phosphonate coatings on Pt/γ-Al2O3 improved stability without adversely affecting the rate of a model reaction, catalytic hydrogenation of 1-hexene. 1. INTRODUCTION Gamma-phase alumina (γ-Al2O3) is a common catalyst support because of its high abundance, mechanical and chemical stability in many reaction environments, and high surface area. However, γ-Al2O3 is unstable in the presence of the high-temperature liquid water and highpressure steam commonly used in aqueous phase reforming. Under these conditions, the support undergoes a phase transition to hydrated boehmite (AlO(OH)) accompanied with a significant loss in the support’s surface area as the pore structure deteriorates.1,2 Supported metal particles can sinter or be encapsulated as a result of the phase transition.3 Consequently, catalytic performance is greatly diminished as active metal clusters become less accessible or detach from support. To prevent catalyst degradation, hydrophobic coatings can be deposited onto the supports for increased hydrothermal stability. Recently, Datye et al. has shown that a carbon overlayers improve the stability of aqueous phase reforming catalysts on mesoporous silica.4 While Al2O3


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exhibits superior stability compared to SiO2, prolonged exposure at these conditions shows significant restructuring occurs, reducing available surface area.1,3,5,6 The incorporation of dopants such as Ba, Sr, La, Gd, Sn, SiO2, and PO4 into Al2O3 structure have been shown to dramatically improve stability compared to undoped alumina.2,6–9 Similarly, the adsorption of highly oxygenated hydrocarbons such as glycerol or sorbitol can prevent water molecules from interacting with Lewis acid sites on Al2O3, thereby improving its stability.10–12 Ideally, such chemical modifications could selectively adhere to the vulnerable support without adversely affecting pore structure or interfering with active catalytic sites. Specifically, coating catalysts with phosphonic acids (PA) has been a promising approach, as these capping agents selectively bind to the oxide support and form robust bonds with the surface.13–16 Furthermore, the physical properties of catalysts can be further tuned by changing the phosphonate’s tail group, which impacts both monolayer assembly and surface hydrophobicity, which may inhibit the rate of hydrolytic decay. In this report, we show that the deposition of alkyl phosphonic acids on alumina and on Pt/Al2O3 catalysts drastically improved their hydrothermal stability. The enhanced retention of both metal and support surface areas resulted from suppressing boehmite formation, which was more successful with longer alkyl tail groups. The beneficial effects of phosphonate head (POx-) and tail (-R) groups were compared by measuring surface areas following various oxidative and hydrothermal (HT) treatments. Ultimately, we determined that the long-chain modifier octyldecyl phosphonate provided the best stability against restructuring to boehmite. 2. EXPERIMENTAL SECTION 2.1. Synthesis. Phosphonate coatings were deposited on Al2O3 (Alfa Aesar 45035, 99.997% γphase, 3 micron) and Pt/Al2O3 (Aldrich 205974, 5%wt Pt) using a modified synthesis originally


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developed for deposition on TiO2 and ZrO2 powders.16 In general, 400 mg of powder was added to 150 mL of 10 mM solution of phosphonic acid (PA, Alfa Aesar, ≥98% purity) dissolved in tetrahydrofuran (THF, Fisher, HPLC grade). The solution was mixed under magnetic stirring for at least 15 hours before recovering the PA-coated powder by centrifugation. After decanting excess solvent, powder was heated at 120 °C for 6 hours in air. Afterwards, the cooled powder was rinsed three times with THF to remove unbound PA. Samples were left overnight in a fume hood to evaporate residual solvent prior to characterization and hydrothermal treatment. 2.2. Hydrothermal Treatment. For hydrothermal treatment, 75-80 mg of sample was loaded along with 5 mL of distilled water into custom built pressure vessels that consist of stainless steel tube capped with Swagelok fittings. These tubes were loaded into a preheated oven at 200 °C and left for the desired treatment duration (2.5-20 hours) after which tubes were removed and allowed to cool. The resulting HT-treated samples were collected by filtration with DI water, recovered and dried in a heating oven (80-100 °C) before characterization. 2.3. Characterization. N2 physisorption measurements were performed on a Micrometrics ChemiSorb 2720 instrument. Prior to measurement, the samples were heated at 200 °C for 1 hour in 30% N2/He mixture to remove residual moisture. The specific surface area was calculated using the BET method normalized by sample mass.17 Samples were characterized using diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy using a Thermo Nicolet 6700 FTIR with a reflective mirror for background measurements. A resolution of 4 cm−1 was used. Powder X-ray diffraction patterns were obtained using an Inel CPS 120 powder X-ray diffraction (PXRD) system with a monochromated Cu Kα radiation source. Phosphonate loading was determined by ion coupled plasma atomic emission spectroscopy of oxidized samples. 27Al MAS NMR measurements were performed on a Bruker DSC 400 spectrometer. The samples were


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packed into a 4 mm zirconia rotor and spun at 12 kHz. The resonance frequency for

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Al was

104.2 MHz. A π/12 pulse was used for excitation, and the recycling delay was 250 ms. For each spectrum, a minimum of 2400 scans were accumulated. Solid Al(NO3)3 was used as a reference compound (δ = −0.543 ppm). To calculate the boehmite fraction, the normalized

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Al NMR

spectra were fitted as a linear combination of the spectra of pure boehmite and pure alumina. The phosphorus and platinum contents were measured by inductively coupled plasma−optical emission spectrometry using a modified protocol developed by Farrell, Matthes, and Mackie.18 All samples were oxidized at 400 °C prior to acid digestion to yield more reliable results especially for long alkyl phosphonate coatings. Samples were dissolved in a 7:3:4 mixture of hydrochloric acid, hydrofluoric acid, and nitric acid then digested at 95 °C for 2 h. After cooling, the mixture was diluted with 1.5 % (g·g−1) boric acid solution and reheated to 95 °C for about 15 min. Samples were analyzed with an ARL 3410+ inductively coupled optical emission spectrometer (ICP-OES). 2.4. Hydrogenation Studies. Catalyst performance was evaluated in a tubular packed bed flow reactor at atmospheric pressure. Helium was bubbled through the 1-hexene at room temperature and then mixed with hydrogen before reaching the catalyst bed. Reaction conditions for 1-hexene were: temperature: 60 °C, pressure: 1 atm; gas-phase mole fraction: YH2 = 10 %, Yhexene = 6 %; gas flow rates: 120 ml min-1; mcat =1.5 mg. The reactor effluent was analyzed online using an Agilent Technologies 7890A gas chromatograph equipped with an Agilent HP-5 capillary column and a flame ionization detector. Selectivity and conversion are reported at steady state. Hydrogenation rates (hexane formation) were normalized by catalyst mass and reported as an average of sample triplicates. 3. RESULTS AND DISCUSSION


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3.1. Stability of Al2O3@PA samples. Consistent with previous studies, we found that γ-Al2O3 was unstable in prolonged exposure to high-temperature water.3 Figures 1a and 1b compare the powder XRD patterns and DRIFT spectra of untreated and hydrothermally treated alumina exposed to 200 °C saturated steam for 20 hours. In Figure 1a, the disappearance of characteristic γ-Al2O3 peaks (2Θ = 39°, 45°, and 69°) and emergence of boehmite peaks (2Θ = 14°, 28°, 38°, and 49°) suggested this hydrothermal treatment was sufficient for a bulk phase transition from alumina to boehmite

19,20

. Similarly, the comparison of DRIFT spectra shown in Figure 1b

indicates boehmite formation by appearance of sharper peaks at 3090 and 3300cm-1 corresponding to O-H stretches in boehmite 20. These vibrations persist after an extended drying at 120°C and 200°C indicating that these features were not caused by residual moisture from the hydrothermal treatment (see Supplementary Figure S1). a)

b)

20 h 20 h Untreated

Untreated

Figure 1. a) X-ray diffraction patterns and b) DRIFTS spectra of γ-Al2O3 before and after hydrothermal treatment at 200°C for 20 h. Following the deposition of different alkyl phosphonic acids, the hydrothermal stability of alumina dramatically improved. We tested a series of alkyl chain lengths, including methyl-, nbutyl-, n-decyl, and n-octyldecyl phosphonic acids, referred to here as C1PA, C4PA, C10PA, and


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C18PA, respectively. Figure 2a shows the powder XRD patterns for uncoated, C1PA- and C18PAcoated alumina exposed to 20 h HT treatment. It is clear that both phosphonate coatings inhibited bulk boehmite formation. Figure 2b compares DRIFT spectra of hydrothermally treated alumina samples. The presence of C-H stretching peaks in C4, C10, and C18 samples around 2900 cm-1 indicate the presence of alkyl tails of these phosphonates following HT treatment.21,22 The C-H vibrations on C1PA are quite weak compared to O-H stretches of alumina and boehmite, but presence of sharp feature around 1400 cm-1 suggests the presence of P-CH3 before and after hydrothermal treatment. Additionally, the growth of 3090 and 3300 cm-1 vibrations suggests some boehmite formation occurred, although it seems suppressed on materials modified by longer alkyl phosphonate groups. DRIFT spectra from untreated PA-coated alumina do not exhibit these features (Supplementary Figure S2). a)

b)

Al2O3

20 h Al2O3

C1 PA 20 h C1 PA

C4 PA C10 PA

20 h C18 PA

C18 PA

Figure 2. a) X-ray diffraction patterns and b) DRIFT spectra of various γ-Al2O3 samples following 20 h hydrothermal treatment. Further insight into the organization of alkyl phosphonic acid SAMs can be garnered by examining the C-H vibrations in DRIFT spectra. Supplementary Figure S3 compares DRIFT spectra of C4 PA, C10 PA, and C18 PA coated Al2O3, which have been baseline corrected. Here,


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we focus on the C10 and C18 PA coatings due to the greater signal-to-noise afforded by those longer-chain coatings.

The spectra of Al2O3@C10PA and Al2O3@C18PA did not change

significantly following hydrothermal treatment. Additionally, the asymmetric CH2 stretches around 2920 cm-1 indicate both SAMs retained their well-organized structure following HT treatment. In fact, all CH2 and CH3 peaks observed on Al2O3@C18PA matched the corresponding vibrational modes of ODPA on single crystal Al2O3.13 3.2. Effects of HT Treatment on Surface Area. To better understand how the alkyl tail length affects the kinetics of boehmite formation on porous γ-Al2O3, nitrogen physisorption and

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Al

MAS NMR measurements were conducted on samples following various HT exposure times (2.5-20 h). Figure 3 shows BET specific surface area (normalized by sample mass) for Al2O3 and C1 PA-, C4 PA-, C10 PA-, and C18 PA-coated Al2O3 as a function of hydrothermal treatment time. For Al2O3, the measured surface area initially increased during the first few hours before drastically decreasing after 5 hours. Similar behavior has been previously reported and has been explained by considering boehmite formation as a surface-mediated transition.3 According to the previous reports, boehmite formation initially roughens the surface without destroying the bulk structure, which helps maintain the pore structure. After sufficient boehmite formation, the pore structure collapses, and dense crystalline bulk boehmite domains are formed.6,7,23 In the current work, the deposition of phosphonate coatings was found to decrease the initial specific surface area by 35-50% relative to pure alumina; however, incorporation of PAs by Al2O3 only accounts for less than a 10% increase in sample mass estimated from elemental analysis (see SI). This observation suggests that larger alkyl tails may have blocked smaller pores. Meanwhile, samples with shorter alkyl tails followed a similar trend to pure Al2O3 where initially the treatment increased the surface area before beginning to have a detrimental impact. The behavior on C18


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samples was unique in that it took over 10 hours of treatment before the surface area began to increase.

Figure 3. Changes in surface area of uncoated and coated samples relative to treatment time as measured by N2 physisorption.


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Figure 4. 27Al NMR spectra of uncoated, C1 PA, C4 PA, and C10 PA coated Al2O3 as a function of hydrothermal treatment time. 3.3. Quantification of Boehmite Formation by 27Al MAS NMR. Figure 4 compares 27Al MAS NMR spectra of Al2O3 samples following hydrothermal treatment. Before hydrothermal treatment, the spectra of all samples contained two peaks at chemical shifts of 65 and 10 ppm corresponding to tetrahedrally and octahedrally coordinated

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Al found in γ-phase Al2O3,

respectively.3,24 Consistent with previous results, the growth of the peak at 10 ppm at the expense of the one at 65 ppm during the hydrothermal treatment of Al2O3 illustrates the phase transition to boehmite, which only contains octahedrally coordinated Al.3,12 This transformation was essentially complete after only 2.5 hours of treatment. Unlike uncoated Al2O3 samples, all PAcoated Al2O3 samples exhibited both peaks even after hydrothermal treatment indicating the sustained presence of Al2O3. However, the peak of octahedrally coordinated Al continued to grow with longer treatment times. Additionally, samples with longer alkyl tails exhibited slower growth of the peak at 10 ppm, consistent with slower boehmite formation.


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Figure 5. Boehmite fraction of Al2O3, C1 PA, C4 PA, C10 PA, and C18 PA capped Al2O3 as a function of hydrothermal treatment time based on 27Al MAS NMR spectra. Figure 5 shows the fraction of boehmite in hydrothermally treated alumina samples as a function of treatment duration (see details in section 2.3 and Ref 3). Compared to uncoated Al2O3, which rapidly transitioned to boehmite following initial hydrothermal treatment, treated PA-coated Al2O3 exhibited both slower rates of boehmite formation and much less boehmite after 20 h HT treatment. Specifically, boehmite fractions of 52 %, 25 %, 11%, and 0 % were observed on C1 PA, C4PA, C10 PA, and C18 PA coated Al2O3 after 20 h treatment. 3.4. Effect of tail length on PA loading. Despite this obvious trend, it is not clear whether increased hydrothermal stability could be attributed to the hydrophobic properties of the alkyl tails, or whether variations in the density of phosphonate groups bound to the surface could account for the observed changes. Elemental analysis (ICP-OES) was performed on oxidized Al2O3 catalysts to determine both PA uptake following the deposition procedure as well as the retention of P within the sample following hydrothermal treatment (Table S1). As expected, there was < 0.03 wt% P observed on uncoated Al2O3 samples. After PA deposition, C1PA, C4PA, C10PA, and C18PA coated samples exhibit P loadings of 1.52, 1.35, 1.17, and 0.98 wt%,


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respectively. Interestingly, higher PA uptake was observed on coatings with shorter alkyl tails, with C1 PA having 50 % higher loading compared to C18 PA. We note that the longer alkyl tails experience greater steric hindrance, affecting the density of surface and pore packing.25 Hydrothermal treatments appeared to slightly decrease P loading over 20 h, but these decreases were not statistically significant from untreated samples. These results suggest that while different alkyl phosphonate coatings have different initial loadings, their coverage does not significantly change over the duration of treatment. These differences in PA loading cannot alone explain the increased stability of longer alkyl tail coated samples. 3.5. Oxidation of PA tail groups. To further evaluate the role of alkyl tail length in suppressing boehmite formation, hydrothermal treatment was performed on C18 PA-coated Al2O3 that underwent an oxidative treatment to remove the hydrophobic alkyl tail. Figure 6a shows DRIFT spectra of these samples before and after oxidation at 400 °C in air for 6 hours. The absence of C-H and C-C vibrations indicate that the oxidative treatment was successful in removing the octadecyl tail. Figure 6b compares BET surface areas of both untreated Al2O3@C18PA and oxidized Al2O3@C18PA at different hydrothermal treatment durations. Oxidized Al2O3@C18PA gave higher surface areas compared to Al2O3@C18PA across all treatment times. In fact, the initial specific surface area of oxidized Al2O3@C18PA (36.1m2/g) was comparable to untreated Al2O3 (40.8m2/g) and greater than all PA-coated samples (12-27m2/g). Since PA only accounts for less 10% of catalyst mass (based on ICP results), the 2-3 fold increase in specific surface area is likely due to decreased pore blockage by C18 tails following the oxidative treatment. Like C18 PA-coated alumina, the oxidized Al2O3@C18PA maintained their specific surface area following hydrothermal treatment. This result suggests the strong interaction between the phosphonate head group and alumina surface was largely responsible for increased hydrothermal stability of


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coated samples. However, it has been shown that alkyl tail groups facilitate self-assembly of monolayers on catalytic surfaces. Despite slightly lower P coverage, longer alkyl chains provided greater stability compared to shorter chains such as C1 PA. One possible explanation for this observation is that the high coverage of the C1 PA leads to decreased surface coordination on average compared to more sparsely packed C10 or C18 PA samples. 31P NMR spectra of HT treated C1 PA and C18 PA Al2O3 clearly showed an upfield shift in peaks consistent with a better shielding of P center, indicative of better surface coordination (Supplementary Figure S8). While it is difficult to directly assign such peaks to specific bonding modes, a shift of 5-10 ppm is consistent with an additional surface bond between the P and metal oxide.14,26–29 After exposure to hydrothermal conditions, shifts in 31P spectra suggest changes in surface coordination without evidence of bulk aluminum phosphonate formation, which would produce peaks around -30 ppm.29 a)

b)

C18 PA

C18 PA ox

Figure 6. a) DRIFT spectra of untreated and oxidized Al2O3@C18PA samples prior to hydrothermal treatment. b) Changes in surface area of untreated and oxidized Al2O3@C18PA relative to treatment time as measured by N2 physisorption. Error bars correspond to standard deviation of multiple measurements on a single sample.


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Figure 7. Effect of hydrothermal treatment on 1-hexene hydrogenation rates on uncoated, C18PA-coated, and oxidized C18PA-coated catalysts. Error bars correspond to standard deviation of 3 independent experiments. 3.6. 1-Hexene Hydrogenation on Pt/Al2O3@C18PA. Phosphonate coatings clearly improved the hydrothermal stability of γ-Al2O3, and we hypothesized that these beneficial effects could also stabilize metal supported catalysts such as Pt/Al2O3. While noble metals themselves are stable in hydrothermal conditions, metal surface areas can decrease as the Al2O3 support deteriorates, leading to metal particle growth, detachment, or encapsulation.1,4,12 Hydrogenation of 1-hexene was used as a probe reaction to assess Pt surface area on 5wt% Pt/Al2O3 following various treatments and depositions. Figure 7 shows the 1-hexene hydrogenation rates (normalized by untreated Pt/Al2O3) on uncoated Pt/Al2O3, Pt/Al2O3@C18PA, and oxidized Pt/Al2O3@C18PA before and after 20 h hydrothermal treatment at 200 °C. Hydrothermal treatment of Pt/Al2O3 significantly decreased rates resulting from Pt surface area loss. Conversely, hydrothermal treatment did not significantly impact hydrogenation on both Pt/Al2O3@C18PA and oxidized Pt/Al2O3@C18PA samples although the oxidative treatment did


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reduce hydrogenation rates on Pt/Al2O3@C18PA by roughly 40%. Lower hydrogenation rates on Pt/Al2O3 after oxidation (in Figure S10) suggests the aggressive treatment may have reduced Pt dispersion through particle growth induced by heating. Importantly, Pt/Al2O3@C18PA also exhibits nearly identical performance to uncoated Pt/Al2O3 suggesting little to no Pt site blockage following C18PA deposition. The apparent absence of Pt site blocking is consistent with earlier reports for phosphonic acid coatings.30 4. CONCLUSIONS In summary, a phosphonate deposition procedure was shown to increase the hydrothermal stability of porous γ-Al2O3 by suppressing the phase transition to boehmite. Slower degradation rates were observed on phosphonates with longer alkyl tails. Hydrothermal stability was also maintained following tail removal by oxidation, suggesting the density and coordination of POx head groups is an important criterion for stability. Finally, these beneficial effects were demonstrated on Pt/Al2O3 where active metal surface area was maintained following hydrothermal conditioning without experiencing decreased performance by Pt site blockage. The extension of PA coatings to higher surface area oxides, like aerogels or zeolites, could expand their current capabilities in industrial applications, and warrants ongoing investigation. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge at DOI: 10.1021/ DRIFT and 27Al NMR spectra of as-synthesized and hydrothermal treated Al2O3 samples. Relative surface area changes over course of hydrothermal treatment. DRIFT spectra of Pt/Al2O3 catalysts. Calculation and comparison of P coverage based on ICP results.

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P

NMR of Al2O3@C1PA and Al2O3@C18PA.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (WM) Author Contributions TVC and WM devised and developed the project. TVC and DU carried out the sample preparation, testing, and characterization. MVR and CS performed and interpreted NMR experiemnts. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests ACKNOWLEDGMENTS The authors would like to thank Dr. Fred Luiszer for his assistance with ICP-OES measurements and Sarah Dischinger for her assistance with powder X-ray diffraction. This work was supported by the Department of Energy, Office of Science, Basic Energy Sciences Program, Chemical Sciences, Geosciences and the Biosciences Division, under Grant No. DE-FG02-10-ER16206. REFERENCES 1.

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Enhanced Hydrothermal Stability of Î³-Al2O3 Catalyst Supports with

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