Al2O3 on vanillin

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Effect of surface hydrophobicity of Pd/Al2O3 on vanillin hydrodeoxygenation in a water/oil system Pengxiao Hao, Daniel K Schwartz, and J. Will Medlin ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03141 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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ACS Catalysis

Effect of surface hydrophobicity of Pd/Al2O3 on vanillin hydrodeoxygenation in a water/oil system Pengxiao Hao, Daniel K. Schwartz, and J. Will Medlin* Chem. Biol. Eng UCB 596, University of Colorado Boulder, Colorado 80309, United States * Corresponding author. Email: [email protected] (J.W. Medlin).

Keywords Surface

modification;

Hydrophobicity;

Pickering

emulsion;

Vanillin;

Hydrodeoxygenation

Abstract

Pickering emulsions stabilized by solid catalysts have received increasing attention as a reaction platform due to potential applications in the chemical industry. Here, we report a

ACS Paragon Plus Environment

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surface modification strategy to control catalytic performance of Pd/Al2O3 during vanillin hydrodeoxygenation (HDO) in a water/decalin emulsion environment. A series of organophosphonic acids (PAs) were applied as modifiers to control the surface hydrophobicity, thereby determining the continuous and dispersed emulsion phases. The PAs also introduced Brønsted acid sites that promoted vanillin HDO. Reaction studies showed that the yield to the HDO product p-creosol (CR) was improved from 20% to as high as 90% upon PA-modification after a reaction duration of 1 hr at 50 oC. This improvement greatly depended on the surface hydrophobicity, which influenced the selectivity to different reaction pathways and emulsion structures. Statistical analysis of the kinetic data confirmed the hydrophobic/hydrophilic effect.

1. Introduction The liquid-solid interface is of fundamental importance in liquid-phase heterogeneous catalysis, and is critical in applications such as biomass conversion and petrochemical production.1,2 Compared to gas-phase conditions, a key character of liquid-phase reactions is the use of solvents, which adds a degree of freedom to tune catalytic selectivity and activity, for instance, by controlling the solubility of reaction species3 and by varying the polarity and proticity of solvents.4 This reactive tunability is primarily due to the participation of solvents in reaction steps, such as mass transport of solutes at the solvent-catalyst interface,2 as well as solvation and adsorption of reaction species on the catalytic surface.5–7 Although a number of experimental and computational studies have


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elucidated the complex nature of solvent effects, it remains challenging to control reactivity at the liquid-catalyst interface.3,8

From economic and environmental aspects, water is a favorable solvent compared to organics and has been extensively applied in industry.9 Water is particularly attractive for biomass conversion due to its intrinsically high content in bio-oil and as a reactant in hydrolysis reactions.10,11 Because water is polar, protic, and able to form hydrogen bonds, it has been reported to alter the reaction pathway on metal and oxide surfaces in a complex manner. For instance, proton transfer through the hydrogen bond network in aqueous conditions was hypothesized to affect the adsorption of reaction intermediates to enhance Brønsted-acid-catalyzed tautomerization on metal surfaces, leading to higher activities for phenol hydrogenation and hydrodeoxygenation (HDO).12,13 In addition to being employed as a monophasic solvent, water has also been incorporated with immiscible oils to form water/oil Pickering emulsions stabilized by solid catalysts.14 One of the advantages of the emulsion platform is the potential improvement of catalytic activity for reactions involving both water- and oil-soluble species, due to the increased interfacial area between water and oil.14 In addition, through rational catalyst design, reactions in either the aqueous or organic phase can be precisely selected.15 Compared to the monophasic aqueous environment, however, the introduction of a second phase to form the water/solid/oil interface further complicates the reaction system and therefore requires more investigation to understand the interfacial chemistry.14


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One strategy to control water-involved reactions is to adjust the surface wettability as a means to tune the adsorption of water on the solid surface. This hydrophobic/hydrophilic effect has been reported to be critical to solid-acid-catalyzed reactions such as dehydration, esterification, and alkylation.16–19 Solid acid catalysts such as zeolites and γAl2O3 are prone to deactivation in hydrothermal environments due to structural changes.2 In this context, hydrophobilizing the surface can enhance the structural stability by reducing the penetration of condensed water.17–20 For example, Van Cleve et. al. showed that alkylphosphonate modification effectively suppressed the phase transition of γ-Al2O3 to boehmite during hydrothermal treatments.20 This resistance to degradation positively correlated with the alkyl chain length of the phosphonate modifier; a similar chain length effect was also observed for zeolite silylation.18,19 In addition to improving the structural stability of solid acid catalysts, a hydrophobic surface has also been hypothesized to enhance the surface acidity in aqueous environments by protecting acid sites against water solvation, which aided acid-catalyzed conversions.16,21–23

The current study investigates the effect of Pd/Al2O3 catalyst hydrophobicity on the activity for hydrodeoxygenation (HDO) in a low-temperature water/oil biphasic environment. Although the surface hydrophobicity is a key surface property that has been reported to affect the catalytic activity and selectivity, an extensive understanding of the hydrophobic effect is still lacking.24 In addition, it has been difficult to separate the effects of hydrophobicity from other confounding factors, since most strategies previously applied to tune surface hydrophobicity also modified other surface properties, such as the support material and the particle size of catalysts.25,26 Here, we have used


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organophosphonic acids (PAs) with various alkyl chain lengths and functionalities as surface modifiers to adjust the hydrophobicity of a commercial Pd/Al2O3 catalyst, thereby largely maintaining the intrinsic properties of the support and metal particles. The PAmodification itself does cause a change in catalyst properties, since Brønsted acid sites are introduced;27–29 the application of PA-modifiers has been reported to significantly improve the HDO activity in both gas- and liquid-phase conditions.27,28,30 The Brønsted acidity has been found to be primarily determined by the electron-withdrawing ability of the functional group adjacent to the phosphorus atom or the formation of intramolecular hydrogen bonds.27,28 Therefore, the hydrocarbon chain length and the ethylene glycol chain that vary the surface hydrophobicity are hypothesized to have little effect to the surface acidity, as confirmed by liquid-phase reactions in ethanol (see Figure S1 in the supporting information); the effect of acidity and the hydrophobic effect on the catalytic activity can thus to a large extent be separated.

In this study, we used vanillin, a direct product of biomass pyrolysis and a model compound for bio-oil upgrading, as the probe reactant.15,31,32 We report a hydrophobic/hydrophilic effect on the activity of biphasic vanillin HDO to selectively produce p-creosol (CR) on PA-coated Pd/Al2O3, where a CR yield of as high as 90% was obtained when highly hydrophobic ligands were used. Kinetic analysis indicated that the hydrophobic effect resulted in promotion of a direct deoxygenation pathway that was less favorable on uncoated or more hydrophilically coated catalysts.

2. Experimental


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2.1. Materials. The 5 wt % Pd/Al2O3, standard grade aluminum oxide (activated, neutral, Brockmann I, 155 m2/g), vanillin (99%), vanillyl alcohol (>=98%), 2-methoxy-4-methylphenol (pcreosol, >=98%), styrene (>=99%), decahydronaphthalene (decalin, mixture of cis + trans,

>=99%),

1,2-dichloroethane

(99.8%),

propylphosphonic

acid

(95%),

butylphosphonic acid (>=88%), and hexylphosphonic acid (95%), were purchased from Sigma-Aldrich. Methylphosphonic acid (98%, Alfa Aesar), decylphosphonic acid (98%, Alfa Aesar), octadecylphosphonic acid (97%, Fisher Scientific), hydroxyl-terminated polyethylene

glycol

(PEG)

phosphonic

Acid

(PO(OH)2(CH2)3(OCH2CH2)10OH,

SPECIFIC POLYMERS), methanol (>=99.8%, VWR), ethanol (100%, Decon Laboratories), HPLC-grade water (>=99.9%, Fisher Scientific), and HPLC-grade tetrahydrofuran (>=99.9%, Fisher Scientific) were purchased from suppliers as specifically indicated. All chemicals were used as received without further treatments. Prepurified nitrogen and ultra-high purity H2 were obtained from Airgas.

2.2. Catalyst preparation and characterization. The phosphonate modification of the Pd/Al2O3 and Al2O3 was performed by incubating the catalyst in a 10 mM phosphonic acid solution in tetrahydrofuran (THF) with vigorous stirring for 16 hr at room temperature. The total amount of phosphonic acid was controlled at 5-10 times of the amount required to form a monolayer on the support surface. The mixture was then centrifuged to remove the THF supernatant, followed by annealing at 120 oC in ambient air for 6 hr to obtain the chemisorbed phosphonate-


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modified catalyst. The catalyst was then rinsed with THF four times to remove any physisorbed phosphonic acid, followed by drying in air overnight.

The PA-modified catalysts were further characterized by inductively coupled plasma mass spectrometry (ICP-MS) to determine the phosphorus content. Because previous studies had indicated phosphonate adsorption on Pd surface, we measured the site blocking following PA-modification by determining catalytic activity for styrene (STY) hydrogenation in ethanol, hypothesizing that the activity for this reaction would depend chiefly on the number of surface Pd sites due to the structure-insensitivity of this reaction.33,34

To measure the wettability of the PA-modified catalysts in water and decalin, the Washburn capillary rise method was used as a conventional technique for powders.35 For convenience of the measurement, the white Al2O3 powder was used as an alternative support rather than the black Pd/Al2O3 powder to obtain better visibility of the capillary rise; we assumed a similar wettability trend upon PA-modification on the two substrates. For the measurement, the powders were first manually packed in capillary tubes (inner diameter of 1.5 – 1.8 mm) with a sample length of 5 cm to ensure packing precision,36 followed by centrifuging at 4000 rpm for 5 min to obtain reproducible packings.36 For methylphosphonic acid (MPA) measurement in water, because of the slow capillary rise, a sample length of 1 cm was employed. The as-prepared sample tube was immersed in water or decalin (3 cm below the surface of solvent); the penetration time was measured at 5 mm rise intervals (1 mm intervals for the MPA-coating in water). The water/decalin


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contact angles were then calculated by the Washburn equation. The error bars were calculated by measurements of separately loaded samples. A detailed calculation is shown in the supporting information.

The Pd catalysts modified with different PA modifiers were further used to prepare emulsions, the phase behavior of which was visually observed. Each emulsion was prepared by combining 8.5 mL water, 8.5 mL decalin, and 10 mg native or PA-modified Pd/Al2O3 (the same ratio as the reaction mixture), followed by mixing at room temperature at 5000 rpm with a 20 mm shaft rotor/stator (Virtishear Mechanical Homogenizer, the VirTis Co.) for 2 min. The mixture was then immediately transferred to a 20 mL glass vial and photographed.

2.3. Catalytic reactions, yield, and rate calculations. Vanillin hydrogenation/hydrodeoxygenation (HDO) reactions were performed in a 100 mL liquid-phase semi-batch reactor at 50 oC in 250 psi H2 for 1 hr. A stirring rate of 1200 rpm was used to maintain the reaction mixture as an emulsion. Preliminary reaction tests were conducted using water/decalin mixtures of different volume ratios for both the native and C10PA-coated catalysts (Figure S3). While the trends in rate were similar for the coated and uncoated catalysts, the creosol yield was highest in both cases for a 1:1 solvent mixture; this mixture was therefore used in the subsequent studies. The reaction solution included 17 mL water and 17 mL decalin (solvents), 267 mg (0.05 M) vanillin (reactant), 20 mg supported Pd catalysts (PdTotal:Vanillin = 1:180 mol/mol), 1 mL methanol (internal standard for aqueous phase for gas chromatography analysis), and 1


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mL 1,2-dichloroethane (internal standard for organic phase). STY hydrogenation reactions were performed in the same reactor at room temperature, with a lower H2 pressure of 35 psi and a lower catalyst loading (10 mg Pd catalysts, Pdtotal:STY = 1:930 mol/mol) to obtain a moderate reaction rate. Six 1 mL liquid samples were taken from the reactor during the 1 hr reaction period for vanillin hydrogenation/HDO. For STY hydrogenation, a total of 6 samples were taken at 30 s intervals in the first 2 min of the reaction to calculate initial rates. With vigorous stirring, the solid catalyst was expected to be uniformly dispersed in the reaction mixture; the ratio of the catalyst to the reaction solution was thus held constant within the reactor by simultaneously sampling liquid and catalyst, and subsequently filtering out the catalyst using syringe filters (0.22 µm, nylon). For biphasic vanillin hydrogenation/HDO, two successively connected filters were required in order to sufficiently break the emulsion, followed by separating the aqueous and organic layers using a pipette. The liquid samples were analyzed by an Agilent 7890A gas chromatography with a flame ionization detector, using an Agilent HP-5 capillary column.

The concentration of each reactant and product in the reaction solution was determined from the peak area obtained from GC analysis using the corresponding response factor, which was measured using a series of standard solutions. The concentrations of the standard solutions were comparable to those in reaction conditions. To analyze biphasic vanillin hydrogenation/HDO, the standard solutions were prepared in water and decalin individually. The product yields (the ratio of products to initial reactant) and rate calculations were both based on a combined concentration in both phases; the rate


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calculation using a single combined concentration instead of the individual concentration in the two phases was confirmed to be valid by the constant partition coefficient during the reaction course of 1 hr at a high stirring rate (1200 rpm) and a constant reaction temperature (Figure S4). However, due to the extremely low solubility of vanillyl alcohol (VA) in decalin, its component in the organic phase was neglected. Error bars were calculated based on replicate reactions.

2.4. Kinetic fitting and Cluster Analysis. The production of CR from vanillin was hypothesized to undergo 1) direct deoxygenation (DDO) and 2) a stepwise hydrogenation-hydrodeoxygenation (HDO) with VA as the intermediate product (Scheme 1). First-order kinetics with respect to vanillin or VA was used as an approximate model for the reaction steps involved on all native and PA-modified Pd catalysts. The HDO rate constant kHDO was obtained first through a firstorder fitting of VA HDO to CR; the obtained value was applied to subsequent analysis of vanillin hydrogenation/HDO. For the reaction starting from vanillin, concentrations of vanillin, VA, and CR as a function of time were solved analytically according to firstorder kinetics, followed by fitting using the NonlinearModelFit function in Wolfram Mathematica to obtain the hydrogenation rate constant kH and direct deoxygenation rate constant kDDO. As shown in the Supporting Information and Figure S5, the assumption of first-order kinetics appeared to be accurate in most cases, particularly at shorter times and at conversions 90

>90

>90

Decalin

0

60±7

73±4

67±3

57±3

82±2

a) The details of the calculation are provided in the Supporting Information. b) The uncoated γ-Al2O3 had a high surface free energy due to its large surface area, resulting in apparently complete wetting by water and decalin. Thus, while the contact angles on unmodified γ-Al2O3 was difficult to measure quantitatively, a value of 0 o in both water and decalin was assumed for the convenience of calculation.

According to previous studies, the degree of surface hydrophobicity of the particles stabilizing a Pickering emulsion may affect the phase of emulsion.39,40 Therefore, we observed the emulsions containing Pd catalysts modified with different PAs. As shown in Figure 1, while the native Pd catalyst was dispersed in the aqueous phase, PA modification resulted in a reversed dispersion of Pd/Al2O3 particles in the oil phase; the C1PA-coating enhanced the formation of an oil-in-water emulsion as suggested by the white-grey color shown below, possibly due to its amphiphilic nature.


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Figure 1. Photographs of water/oil emulsion formed with PA-coated Pd/Al2O3 catalysts.

3.2 Reaction studies of PA-modified Pd/Al2O3 catalysts for biphasic vanillin HDO Liquid phase vanillin hydrogenation/HDO (Scheme 1) was performed in a semi-batch liquid phase reactor at 323 K under a constant H2 pressure of 250 psi. A stirring rate of 1200 rpm was used to maintain the reaction mixture in an emulsion phase. The formation of an emulsion could be visually observed by the in-situ sampling of the reaction mixture and was further confirmed by the inefficient breaking of the emulsion through a one-time filtration. Figure 2 shows the characteristic reaction profile of the Pd/Al2O3 catalyst before and after PA-modification. Similar to previous observations in ethanol, the native catalyst exhibited a high activity for hydrogenation but a slow deoxygenation to CR, leading to a CR yield of 20% after 1 hr. The PA-coated catalysts, however, behaved significantly differently in the emulsion compared to in ethanol (Figure S1). For instance, PEGPA- and C1PA-coatings comparably promoted the CR yield in ethanol, which was attributed to the increased Brønsted acidity associated caused by PA modification;30 however, in a water/decalin emulsion, the CR yield dropped from ~40% to ~20% for PEGPA, but increased from 25% to ~50% for C1PA after 30 min (Figure S1). The


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improvement in CR yield was even higher for the PA-coatings with longer chain lengths (Figure S1). These varying performances of PA-coatings in the two solvent systems suggested a possible solvent effect in the reaction, which could be influenced by the PA tail structure.

As shown in Figure 2, in the water/decalin emulsion, compared to the PEGPA-coated catalysts, the C18PA-modification increased the CR yield from 20% to ~80% with a similar number of active surface sites as measured by STY hydrogenation rates (Table S4). Similar improvements were also observed using other relatively hydrophobic modifiers with various tail lengths, C3PA, C4PA, C6PA, and C10PA (Table S3 and Figure S9), suggesting that HDO was promoted by hydrophobic PA-modifiers in the biphasic environment. As shown in Figure 2, C1PA had a significantly higher CR yield than PEGPA. However, this difference was attributed to a larger effective surface area for C1PA (measured by STY hydrogenation, Table S4); the two coatings in fact led to similar rate constants as discussed in section 3.3.


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ACS Catalysis

CR 1.0

Normalized Concentration

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

VA

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Vanillin

a)

b)

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2 Pd

PEGPA 0.0

0.0 1.0

c)

d)

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2 C18PA

C1PA

0.0

0.0 0

10

20

30

40

50

60 0

10

20

30

40

50

60

Time (min)

Figure 2. Reaction profile of vanillin hydrogenation/HDO on a) the native Pd/Al2O3, b) PEGPA-coated Pd/Al2O3, c) C1PA-coated Pd/Al2O3, and d) C18PA-coated Pd/Al2O3. Reaction conditions: 0.05 M vanillin, 20 mg Pd catalyst (PdTotal:Vanillin=1:180 mol/mol), 17 mL water and 17 mL decalin (solvents), 1 mL methanol and 1 mL dichloroethane (internal standards), 323 K, 250 psi H2, and stirring at 1200 rpm.

Despite the similarly high yields to CR for the C1PA and C18PA coatings, a careful observation of the reaction profile revealed different kinetics on the two surfaces. As shown in Figure 2c, the C1PA-coated surface performed similarly to the native Pd during the initial stage of the reaction, exhibiting a high hydrogenation activity to reach a VA yield of 80% during the first 5 min. The HDO enhancement by the C1PA modification was therefore primarily due to an increased rate of conversion from VA to CR. On the


16

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other hand, the promotion by the C18PA modification was largely attributed to the fast CR production at the initial stage of the reaction. Compared to C1PA, C18PA in fact decreased the rate of VA HDO, indicated by the slower decrease of VA concentration as shown in Figure 2c and 2d. In order to characterize the different reaction kinetics, the initial rates of CR and VA production were determined for all uncoated and PA-coated catalysts (Table S3) and were used to generate the initial selectivity to CR in vanillin hydrogenation/HDO. As shown in Figure 3, interestingly, the initial CR selectivity positively correlated with the tail length of the PA modifier. This correlation was confirmed by a positive slope generated by a weighted linear fitting, with a confidence level higher than 99%. More details on the statistical analysis are found in the Supporting Information.

2.5

2.0

1.5

rDDO/rH

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

1.0

0.5

0.0 0

2

4

6

8

10

12

14

16

18

Tail length Figure 3. Correlation between the initial CR selectivity to the tail length of the PA modifier.


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As indicated above, the promotion of CR production in a biphasic environment by the PA-modification depended on 1) the hydrophobicity and 2) the tail length of the PA modifier. In order to further probe the kinetics of vanillin hydrogenation/HDO, reactions starting from VA were performed, corresponding to the second HDO step in Scheme 1 after the initial hydrogenation of vanillin. As shown in Figure 4 and Table S3, PEGPA modification had a stronger effect on the CR yield (nearly doubling it compared to the uncoated catalyst) when VA was used as a reactant, suggesting that the promoting effects of the PA ligands may be sensitive to the reactant. Initial rates of CR production from VA HDO (rHDO , Scheme 1) were calculated and further compared with those in vanillin deoxygenation (rDDO ). As shown in Figure 5, PA-coatings affected the two reactions differently. For the uncoated, PEGPA-, and short-alkyl-chain PA-coated catalysts, the initial activity for VA HDO was equal to or higher than creosol production from vanillin, suggesting that on these surfaces, vanillin primarily underwent a stepwise hydrogenation/HDO reaction pathway. Meanwhile, the PA-coatings with longer tail lengths resulted in a faster vanillin deoxygenation than VA HDO, which was most significant for C18PA, indicating that additional active sites or reaction pathways for CR production were promoted, hypothetically via the DDO pathway.


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HO

Normalized Concentration of CR

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6

HDO OH

O

k HDO

HO O CR

VA

Pd PEGPA C1PA C18PA

0.4

0.2

0.0 0

10

20

30

40

50

60

Time (min) Figure 4. Reaction profiles of CR production from VA HDO on the native Pd/Al2O3, PEGPA-coated Pd/Al2O3, C1PA-coated Pd/Al2O3, and C18PA-coated Pd/Al2O3. Reaction conditions: 0.05 M VA, 20 mg Pd catalyst (PdTotal:VA=1:180 mol/mol), 17 mL water and 17 mL decalin (solvents), 1 mL methanol and 1 mL dichloroethane (internal standards), 323 K, 250 psi H2, and stirring at 1200 rpm. The reaction profile for CR production from VA HDO on C3PA-coated Pd/Al2O3, C4PA-coated Pd/Al2O3, C6PA-coated Pd/Al2O3, and C10PA-coated Pd/Al2O3 are included in Figure S11.


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ACS Catalysis

rDDO rHDO rH

1.0

100

0.8

75

0.6 50 0.4 Hydrophilic PA

0.2 0.0

Hydrophobic PA

25 0

Yield to CR after 1 hr (%)

125

1.2 Initial Rates (s−1 mol/molSurfPd)

C 44P A C 66P PAA CC 1100P PAA C C18 18 P PA A

PE G PA

C 11P A C 33P PAA

-0.2

Pd

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 5. Effect of PA-modifiers on the initial rates of hydrogenation, HDO, and DDO, as well as the yield to CR (starting from vanillin) after a reaction course of 1 hr. Reaction conditions: 0.05 M vanillin or VA, 20mg Pd catalyst (PdTotal:Vanillin=1:180 mol/mol), 17 mL water and 17 mL decalin (solvents), 1 mL methanol and 1 mL dichloroethane (internal standards), 323 K, 250 psi H2, and stirring at 1200 rpm. Initial rates were measured during the reaction course of the first 5 min. Specifically, rHDO was obtained from VA HDO; rH and rDDO was the rate of VA and CR production in vanillin HDO, respectively. The as-measured initial rates were further normalized according to the number of available surface Pd sites that were calculated from the rate of STY hydrogenation in ethanol (Table S4). The same set of data with standard deviations is included in Table S3.


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3.3 Kinetic Fitting and Cluster Analysis We hypothesized that two routes were likely to occur in vanillin hydrogenation/HDO to produce CR: 1) a one-step DDO of the C=O bond, and 2) a stepwise hydrogenationhydrogenolysis process with VA as the intermediate product (Eqn 1 – 3). kDDO

Vanillin CR (1) kH

Vanillin VA (2) kHDO

VA CR (3) In order to simplify the reaction model, first-order kinetics was applied as an approximate model for all three steps. Rate constants were calculated through the kinetic fitting of vanillin and VA HDO reactions for the reaction network defined by equations (1)-(3). As shown in Table 2, compared to the native catalyst, the hydrogenation activity was improved by C1PA-modification, while it was decreased by PA-coatings with larger alkyl tail lengths. The hydrodeoxygenation rate constant kHDO, as indicated previously by VA HDO activity, was promoted similarly by all PA-coatings independent of the tail structure. The direct deoxygenation rate constant kDDO, on the other hand, was promoted by alkyl PA-coatings with longer tails. Comparing with kHDO and kDDO, interestingly, kDDO was the dominant reaction pathway for CR production on the C3-C18-coated surfaces.

Table 2. Rate constants of vanillin hydrogenation/HDO on uncoated and PA-coated surfacesa Entry

Modifier

kH

kHDO


kDDO

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1

None

0.29±0.09

0.004±0.002

0.010±0.005

2

PEG-PA

0.27±0.07

0.013±0.003

0.014±0.009

3

C1PA

0.36±0.12

0.016±0.005

0.03±0.02

4

C3PA

0.21±0.06

0.015±0.004

0.08±0.03

5

C4PA

0.18±0.04

0.029±0.006

0.09±0.02

6

C6PA

0.17±0.04

0.035±0.008

0.11±0.03

7

C10PA

0.17±0.05

0.027±0.006

0.12±0.04

8

C18PA

0.07±0.02

0.021±0.004

0.08±0.02

a) All rate constants were normalized with the availability of surface Pd sites that was measured by STY hydrogenation in ethanol. The concentration of reaction species was normalized previous to kinetic fitting. Details of the kinetic fitting and the corresponding R-squared values are found in Supporting Information.

Having observed the different promotion effects among various PA-coatings, we employed a data-driven approach to determine whether the PA-coatings could be categorized into groups to reveal a hydrophilic/hydrophobic effect. Here, cluster analysis (using a Gaussian mixture model as described above) was applied as an unbiased strategy to statistically classify the catalysts based on their reaction kinetics, which were determined in the same way for all surfaces. In principle, a categorization using the three rate constants (kH, kHDO, and kDDO) as the descriptors for each catalyst could be performed. However, given the fact that only eight distinct experimental conditions (i.e., catalysts) were employed, it was beneficial to reduce the effective number of variables to two that described the yield to CR at a long reaction duration. This was justified by the observation that the long-time kinetics best captured the distinctive behavior of the different catalysts. To do this, we simplified the solution to the time-dependent coupled rate equations in the limit of small vanillin concentration, which was generally obtained


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after only 30 min (a conversion of higher than 97% could be reached for all PA-coatings except for C18PA; the conversion on the C18PA-coated catalyst was 94%). Specifically, the expression for the time-dependent CR concentration was solved based on Eqn 1-3 assuming first-order kinetics for each step. As described in more detail in the Supporting Information, this expression comprised two parameters kHDO and kH/(kH+kDDO-kHDO) in the long-time (i.e. low vanillin concentration) limit. Interestingly, as shown in Figure 6, cluster analysis using a Gaussian mixture model naturally divided the coatings into two groups: one containing the native, PEGPA-, and C1PA-coatings, and the other group the C3-C18 PA-coatings. It should be noted that there was some dependence of the clustering on the assumption that STY hydrogenation provided an accurate measurement of the density of active sites. When the rate constants were not normalized based on STY hydrogenation rates (Figure S8), a clear difference in clustering between hydrophobic and hydrophilic ligands was still observed, but with C1 and C18 PAs representing separate clusters with their own behavior. The overall approach confirmed a significant difference between the PA-coatings based on an unbiased analysis of rate constants alone, suggesting that a hydrophobic effect was very likely to occur in the PA-promotion of vanillin HDO in a biphasic environment.


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C PA 6

C PA

C PA

4

10

C PA

C PA

18

1

C PA 3

PEGPA Uncoated

Figure 6. Cluster analysis for different PA-coatings using a Gaussian mixture model, revealing that the different coatings could be categorized into two groups with a statistical significance. A similar analysis using rate constants without normalization by surface Pd sites was also conducted and shown in Figure S8.

4. Discussion Here we have observed a hydrophobic/hydrophilic effect on the activity of PAmodified Pd/Al2O3 catalysts for vanillin HDO in a water/decalin emulsion environment. This hydrophobic effect was similarly observed in a single aqueous phase, where a longer tail of the PA-coating resulted in a higher CR yield (Table S6). The formation of an emulsion further enhanced the CR yield, agreeing with previous studies (Figure S3).41,42 It should be noted, however, that the different coatings also generated different emulsions (Figure 1). As shown in the figure, while the uncoated Pd/Al2O3 was dispersed in the


24

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ACS Catalysis

aqueous phase, most PA-modified catalysts, including the relatively hydrophilic PEGPAcoated Pd/Al2O3, were dispersed in the oil phase. A special case was with the C1PAcoating, which possibly generated the most stable emulsion by visual observation. None of the PA-coatings tested here were hydrophilic enough to result in a catalyst dispersion in the aqueous phase; further studies are required to determine how the change of emulsion phase would affect the catalytic activity for PA-modified catalysts.43

We have hypothesized that vanillin underwent both HDO and DDO to produce CR (Scheme 1). It should be noted here that DDO mechanisms have been mostly reported for phenolic hydroxyl groups.44 However, the calculated rate constants in the current study indicated a surprisingly dominant DDO activity over HDO on hydrophobic surfaces. Aijaz et. al. previously reported the DDO pathway for vanillin conversion to CR on a Pd catalyst immobilized in a metal–organic framework (MIL-101).45 While VA was observed as a product on the Pd/MIL-101 catalyst, where the Pd particles were dispersed on the outer surface of MIL-101 without pore restriction, the production of VA was minimal on the Pd particles immobilized in the cavities of MIL-101 across the entire conversion range, indicating the DDO pathway dominated. The authors associated the DDO mechanism with the steric hindrance of the encapsulated Pd particles by the MIL101 framework, which was proposed to contribute to a strong adsorbate-surface interaction that led to the DDO pathway. Here, the proposed DDO mechanism on the PAcoated catalysts might be ascribed to a similar steric hindrance: the longer tail of the PAmodifier resulted in a higher initial CR selectivity (Fig. 3), which can be primarily attributed to DDO.


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Page 26 of 34

5. Conclusion In the current study, we proposed that in the water/decalin emulsion, vanillin underwent 1) hydrogenation-hydrodeoxygenation (HDO) and 2) apparent direct deoxygenation (DDO) on the PA-coated Pd/Al2O3 to produce p-creosol (CR). The activity of vanillin DDO was improved preferably by hydrophobic PA-modifications, resulting in the highest CR yield of ~90% at 50 oC after a reaction course of 1 hr. Reaction studies indicated that this enhancement by a hydrophobic surface was due to a more selective DDO pathway, while the hydrogenation-HDO pathway was preferred on hydrophilic surfaces. This hydrophobic/hydrophilic effect was further confirmed to be statistically significant by cluster analysis. While more studies are required to fully understand the hydrophobic/hydrophilic effect, the current contribution demonstrated that the liquid-solid interface is a key factor to tune the deoxygenation activity in a waterinvolved reaction system.

Supporting Information Additional reaction data, characterization, and statistical analysis as described in the manuscript.

Acknowledgement This work was supported by the Department of Energy, Office of Science, Basic Energy Sciences Program, Chemical Sciences, Geosciences, and Biosciences Division


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[Grant No. DE-SC0005239]. The authors would also like to acknowledge Dr. Lea L. Sorret (University of Colorado Boulder) for assistance with emulsion preparation, and Dr. Nathaniel Nelson (University of Colorado Boulder) for assistance with cluster analysis and useful discussions.

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TOC Graphic


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Al2O3 on vanillin

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