Kinetic and Infrared Spectroscopy Study of Hydrodeoxygenation of 2

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Kinetic and Infrared Spectroscopy Study of Hydrodeoxygenation of 2-Methyltetrahydrofuran on a Nickel Phosphide Catalyst at Atmospheric Pressure Phuong P. Bui, Atsushi Takagaki, Ryuji Kikuchi, and S. Ted Oyama ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02396 • Publication Date (Web): 04 Oct 2016 Downloaded from http://pubs.acs.org on October 6, 2016

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Kinetic and Infrared Spectroscopy Study of Hydrodeoxygenation of 2-Methyltetrahydrofuran on a Nickel Phosphide Catalyst at Atmospheric Pressure Phuong Bui1,2, Atsushi Takagaki1, Ryuji Kikuchi1, S. Ted Oyama*1,2 1

The University of Tokyo, Department of Chemical System Engineering, Faculty of Engineering Bldg.3, 5F #5A07, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan.

2

Virginia Tech, Department of Chemical Engineering, Suite 245 Goodwin Hall, 635 Prices Fork Road, Blacksburg, Virginia 24061, USA.

* Author to whom correspondence should be addressed. S. Ted Oyama, [email protected]

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Abstract Understanding the reactions of heteroatom cyclic compounds is essential for developing good catalysts for the upgrading of bio-oils into liquid fuels. The present study presents the reaction network of 2-methyltetrahydrofuran (2-MTHF, C5H10O), a bio-oil model compound, on silica supported nickel phosphide at 0.1 MPa and 300 oC. Contact time experiments showed that 2MTHF reacted to first form 1-pentanol and 2-pentanol, then n-pentanal, 2-pentanone, and 1- and 2-pentenes, and finally n-pentane. The observation is consistent with a reaction network in which adsorption of 2-MTHF is followed by rate-determining ring-opening steps on the more hindered side (Path I) or the more open side (Path II) to first produce adsorbed alcohols. The alcohols then transform into adsorbed aldehyde, ketone, and pentenes species which can simply desorb or react to produce the final product n-butane (decarbonylation of adsorbed n-pentanal) or n-pentane (hydrogenation of adsorbed pentenes). Kinetic modeling of the proposed reaction network gave good agreement with the experimental data and predicted that Path I intermediates would be more numerous than Path II intermediates on the surface. A series of in situ FTIR results showed further support for the mechanism with the presence the C=O and C=C bands of the adsorbed aldehyde/ketones and alkenes species. Transient experiments gave evidence for the model calculations that predicted more plentiful Path I surface species.

Keywords: hydrodeoxygenation; kinetics; FTIR spectroscopy; 2-methyltetrahydrofuran; nickel phosphide


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Introduction Although the shale oil revolution has brought ample supplies and low prices to the petroleum and chemical industries, the development of alternative energy and chemical sources remains of strategic importance for many countries [1, 2, 3,4]. Biomass is a readily available resource and its products can be integrated into current production and use facilities [5, 6, 7]. Substantial research has been carried out to investigate the thermo-chemical conversion of biomass [8,9,10], the hydroprocessing of biomass-derived oils [11, 12, 13], and the reaction mechanisms of bio-oil model compounds on multi-component catalysts such as transition metal sulfides, carbides, nitrides and phosphides [14], metal modified zeolites [15], and metals [1]. Plant based biomass contains high levels of cellulose and lignin which require effective cyclic heteroatom removal during their conversion to hydrocarbon fuels. Due to the complexity of biooils, fundamental research on the reactions of simple compounds can provide knowledge to improve catalysts. An appropriate probe molecule is 2-methyltetrahydrofuran (2-MTHF, C5H10O),

related to the platform compound furfural, with a methyl group that acts as a “marker”

during ring-opening reactions [16,17]. Prior work was carried out at 0.5 MPa, while the present study is conducted at 0.1 MPa. Comparison of hydrodeoxygenation of 2-MTHF showed activity to be in the order Ru > Pd ≥ Ni2P, but oxygenates were 85% with Ru, 70% with Pd, and only 12% with Ni2P [17]. Transition metal phosphides are a class of environmental friendly and cost effective catalysts [18], that have shown high reactivity in hydroprocessing reactions [19] such as hydrodesulfurization [20,21,22,23,24], hydrodenitrogenation [25, 26,27], and hydrodeoxygenation (HDO) [28, 29]. In HDO they have advantages over sulfides in not requiring sulfiding agents and over noble metals in price [18]. Kinetic studies combined with 3 ACS Paragon Plus Environment

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spectroscopic measurements and theoretical analysis can give insights about reaction mechanisms, and are employed in this study. Nickel phosphide is reported to be the most active phosphide for HDO of model compounds [30,31,32,33] and real feeds [34,35], and is used in this work. It is found that the HDO of 2-methyltetrahydrofuran at atmospheric pressure produces mainly pentane and some butane. Contact time measurements show that ring-opening occurred on either side of the oxygen; to form first alcohols (n-pentanols) which were then dehydrogenated to n-pentanal or 2-pentanone or dehydrated to n-pentenes. These were hydrogenated to the final product, n-pentane. Contact time measurements allowed determination of the reaction sequence and a kinetic fit was carried out which considered possible adsorbed intermediates. Measurements of adsorbed intermediates by in situ Fourier transform infrared spectroscopy give support for the suggested sequence. Methods Materials and synthesis The silica supported nickel phosphide catalyst (7.9 wt% Ni2P/SiO2) was synthesized via a phosphate precursor as reported earlier [28]. Briefly, nickel(II) nitrate (Ni(NO3)2.6H2O (Alfa Aesar, 99.6%), 337 mg, 1.16 mmol) was added to an ammonium hydrogen phosphate ((NH4)2HPO4 (Aldrich 99%), 306 mg, 2.32 mmol) aqueous solution (2 mL) at a metal to phosphorous ratio of 2:1. The resulting solution was impregnated onto 1 g of silica support (Cab-osil ® EH5 supplied by Cabot Corp) using the incipient wetness method. The mixture was dried at 120 oC for 4 h and calcined at 600 oC for 6 h to obtain a phosphate precursor. Then, a temperature-programmed reduction in a H2 stream (1 L min-1 of H2 per gram of precursor) was carried out to obtain the Ni2P/SiO2 with a loading of 7.9 wt %. The temperature was increased


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from 25 oC to 580 oC at a rate of 2 oC min-1. Finally, the catalyst was cooled to 25 oC in a He stream and was passivated in a 0.5 % O2/He stream for 2 h. Characterization General characterization techniques such as X-ray diffraction, Brunauer Emmett Teller (BET) apparent surface area, and carbon monoxide (CO) chemisorption measurements were carried out to confirm the presence of nickel phosphide and to quantify the surface metal sites on the catalyst. The procedures were the same as reported previously [28,30] and are summarized below. The X-ray diffraction (XRD) measurements were made with a PANalytical X’pert Pro power diffractometer operated at 45 kV and 40 mA, using Cu Kα monochromatized radiation (λ = 0.15418 nm). The BET apparent surface areas were obtained from nitrogen adsorption isotherms at liquid nitrogen temperature measured by a Micromeritics ASAP2010 instrument. Amounts of 0.2 g of sample were dried in vacuum at 130 oC for 4 h prior to measurements. Carbon monoxide (CO) uptake determination was used to estimate the number of active sites on the catalyst using a home-built apparatus. An amount of 0.3 g of passivated catalyst was re-reduced in H2 at 450 oC for 2 h and cooled to room temperature in He. Pulses of 2 µmol of CO in a He carrier were injected onto the sample until saturation. An on-line mass spectrometer monitored the signal of mass 28 (CO) throughout the experiment. The amount of CO needed to reach saturation was normalized with the sample weight to give the CO uptake result.

Contact time studies


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The reactivity studies were carried out at 0.1 MPa and 300 oC in a tubular quartz reactor of diameter 13 mm following the parameters listed in Table 1. The catalyst was packed with quartz chips of the same size to a fixed bed with a volume of 1.5 cm3 in the reactor. Prior to the tests, the supported nickel phosphide was activated in H2 with the temperature raised at 5 oC min1

from 25 oC to 450 oC and held at this level for 2 h. The reactant was fed to the catalyst bed by

bubbling a H2 stream through a saturator containing a solution of 95 volume % of 2MTHF and 5 volume % of heptane as an internal standard. The saturator was kept at 0 oC to ensure a stable feed concentration. The feed composition was obtained using the Antoine equation and Raoult’s law for liquid mixtures [36]. At 0 oC the vapor pressures were 26.5 mmHg for 2-MTHF and 11.2 mmHg for heptane. At atmospheric pressure, the gas-phase concentration of the reactant 2MTHF in H2 was 3.3 mol %, that of n-heptane was 0.1 mol%, and hydrogen 96.6 mol %. The contact time based on active sites was defined as 𝑊𝑊𝑁𝑁𝐶𝐶𝐶𝐶

𝜏𝜏 = 𝐹𝐹

(1)

𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀

where W is the weight of catalyst (g), NCO is the CO uptake (µmol g-1), and F2MTHF is the molar flow rate of 2-MTHF (µmol s-1). For a test with an amount of 0.1g of catalyst that has a CO uptake of 50 µmol g-1 in a flow of 596 cm3 min-1 (NTP) containing 3.3 mol% of 2-MTHF the contact time would be 𝜏𝜏 =

0.1𝑔𝑔×50×10−6 𝑚𝑚𝑚𝑚𝑚𝑚 𝑔𝑔−1 ×60𝑠𝑠 𝑚𝑚𝑚𝑚𝑚𝑚−1 596𝑐𝑐𝑐𝑐3 𝑚𝑚𝑚𝑚𝑚𝑚−1 ×1.5𝑚𝑚𝑚𝑚𝑚𝑚 𝑐𝑐𝑐𝑐−3

= 0.37𝑠𝑠. The contact time was varied

by varying the amount of catalyst and/or the flow rate of the carrier gas through the saturator

kept at 0 oC so as to keep the reactant concentration the same in the feed (Table 1). Notice that the order of measurements was randomized in order to avoid systematic errors and to verify


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catalyst stability during the experiments. The products were monitored by an online gas chromatograph (SRI 8610B) equipped with an HP-1 100m x 0.25mm capillary column and a flame ionization detector. The reactants and products were identified by comparing their retention times with those of commercial standards and were confirmed by gas chromatography – mass spectrometry (GC-MS) (Hewlett – Packard, 5890-5927A). The turnover frequency (TOF) was calculated from

𝐹𝐹

𝑋𝑋

𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 𝑇𝑇𝑇𝑇𝑇𝑇 = 𝑊𝑊𝑁𝑁 100 𝐶𝐶𝐶𝐶

(2)

where F2MTHF, W, and NCO are the same as defined in the contact time equation, and X is the total conversion (%) of 2-MTHF. For example at a contact time of 20 s and conversion of 8.3% the TOF is 0.0042 s-1.

Table 1 Parameters for contact time study Contact time s 0.37 0.44 0.57 0.92 1.4 2.7 5.4 11 20 47 107

Total flow rate cm3 min-1 596 495 383 236 160 82 40 20 50 21 9.2

Reactant flow rate µmol s-1 14 11 8.8 5.4 3.7 1.9 0.92 0.45 1.1 0.5 0.2

Catalyst weight g 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.45 0.45 0.45


Order of measurement 4 3 5 2 6 1 7 8 9 11 10

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A mathematical model was developed for the proposed reaction network using kinetic rate law equations (Supporting Information). Gaseous concentrations of 2-MTHF [A], 2methylfuran [MF], 1-pentanol [OL1], n-pentanal [ALD], 1-pentene [EN1], 2-pentanol [OL2], 2pentanone [ONE], 2-pentene [EN2], n-butane [BUT], and n-pentane [PAN] have units of µmol cm-3. The corresponding surface species for vacant adsorption sites [*], 2-MTHF [A*], 1pentanol [OL1*], n-pentanal [ALD*], 1-pentene [EN1*], 2-pentanol [OL2*], 2-pentanone [ONE*], and 2-pentene [EN2*] have units of µmol cm-2. The adsorption rate constants are denoted as kA, kOL1, kOL2, kALD, kONE, kEN1, kEN2, and kMF (cm3 µmol-1 s-1), the desorption rate constants are k-A, k-OL1, k-OL2, k-ALD, k-ONE, k-EN1, k-EN2, k-MF, kBUT, k1PAN, and k2PAN (s-1), and the surface reaction rate constants are k1, k2, k1a, k-1a, k1b, k2a, k-2a, and k2b (s-1). Rate equations of gaseous species and surface species were solved simultaneously using an open-source programming software, Python 2.7. The rate constants were optimized by a sequential leastsquares programming method, provided by SciPy package. Details are in the Supporting Information.

Fourier transform infrared (FTIR) spectroscopic experiments The spectroscopic studies were carried out using a Digilab Excalibur Series FTS 3000 spectrometer. A detachable reaction cell was positioned parallel so that the infrared beam passed through the catalyst wafer prior to reaching a liquid N2 cooled mercury-cadmium-telluride detector. The cell had an outlet and inlet for gas flows and a thermocouple connected to a temperature controller to monitor and control the sample temperature. Potassium bromide windows were used at the ends of the cell and the calcium bromide rods were placed in the path


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of the beam line to minimize gas-phase interference. The catalyst sample was made of 32.5 mg of finely ground Ni2P/SiO2 pressed into a self-supporting wafer (13 mm in diameter). The catalyst was pretreated in H2 (100 cm3 (NTP) min-1) at 450 oC for 2 h prior to experiments. Background spectra were taken as the catalyst cooled down in either H2 or He (100 cm3 (NTP) min-1). All spectra are shown with subtraction of the background contribution. In the temperature-variation adsorption experiments, the catalyst sample was dosed with 0.32 mol% 2MTHF/He (100 cm3 (NTP) min-1) at 100oC and 0.1 MPa until saturation occurred. Then, the sample was purged with carrier gas for 300 s to remove the excess 2-MTHF. The adsorption data were recorded at 25 oC interval as temperature was increased from 100 oC to 325 oC. Results The silica-supported nickel phosphide catalyst used in this study was characterized by Xray powder diffraction (XRD), Brunauer-Emmett-Teller (BET) surface area determinations, and CO uptake measurements. The XRD pattern confirmed the sole presence of the nickel phosphide phase with signature peaks at 54.2o, 40.8o, 44.6o, and 47.3o, and the line-broadening gave an estimated crystallite size of 12 nm. The BET apparent surface area was 177 m2 g-1 and the CO uptake was 50 µmol g-1. The HDO of 2-MTHF on this catalyst at 0.1 MPa and 300 oC and a high contact time of 107 s gave a conversion of 33% and turnover frequency (TOF) of 0.0031 s-1 and produced npentane with 70 % selectivity, n-butane with 20 % selectivity, and 2-pentanone with 10% selectivity. The TOF was invariant with contact time because the conversion was linear with τ in the range of experimentation.


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For a sequential reaction, intermediate products are observed at low contact times, while final products appear at high contact times. The conversion and product selectivities during the contact time study of 2-MTHF on Ni2P/SiO2 at 0.1 MPa and 300 oC are presented in Figure 1. The raw data are presented in Table S2 of the Supporting Information. The data were obtained in a randomized order (Table 1) with two catalyst samples from the same batch, and the linearity of the conversion is proof of catalyst stability. If deactivation had occurred later points would show lower conversion, and deviations from a line would have been observed. These indicate that the reaction proceeds by ring-opening on the hindered methyl-substituted side denoted as Path I, or the accessible unsubstituted side, referred to as Path II. The plots describe the total conversion and three types of products: a) intermediates containing primary functional groups such as 1-pentanol, n-pentanal, and 1-pentene assigned as Path I products, b) intermediates containing secondary functional groups such as 2-pentanol, 2-pentanone, and 2-pentenes attributed as Path II products, and c) final products from both paths. For Path I the selectivities for 1-pentanol and n-pentanal started high (11% and 8% respectively at 0.4 s) then dropped sharply at low contact time, with n-pentanal following 1-pentanol closely, while the selectivity for 1-pentene went through a small maximum (2% at 1.4s). These trends suggest that 1-pentanol is an initial product and n-pentanal and 1-pentene are intermediate products. For Path II the selectivity for 2-pentanol started high (21% at 0.4 s) then dropped sharply, a similar trend to that of 1-pentanol, while the selectivities for 2-pentanone and 2-pentenes both went through a maximum (20% at 11 s and 16% at 0.5 s, respectively) then decreased. These trends indicate that 2-pentanol is also an initial product and 2-pentanone and 2-pentenes are intermediate products. Unlike n-pentanal, the selectivity to 2-pentanone remained high (> 10 %) compared to those of the other intermediate products (2-pentanol < 2 % and 2-pentenes < 5 %) through most


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of the contact time period (> 20 s). This pattern of 2-pentanone selectivity implies that its rate of consumption is slow. The selectivities for n-pentane and n-butane increased steadily with contact time, clearly showing a final product behavior.

a)

10

Path I 1-Pentanol n-Pentanal 1-Pentene

5

Selectivity / %

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0 b)

20

Path II 2-Pentanone 2-Pentene 2-Pentanol

10 0 60 40 20 0

c)

0

n-Pentane n-Butane 2-Methylfuran Conversion

20 40 60 80 100 120

Contact Time / s Figure 1. Product selectivity as a function of contact time in the hydrodeoxygenation of 2MTHF on Ni2P/SiO2, 0.1 MPa, 300 oC, 3.3 mol% 2-MTHF in H2. Continuous lines are simulation results using rate constants from the simple scheme (Scheme 1) and points are experimental data. Contact time results indicate a general reaction route as in Scheme 1. The simple sequence involves gas phase species only where 2-MTHF reacts to form alcohols as initial species, aldehyde, ketone, and pentenes as intermediate compounds, and butane and pentane as final products. This follows the results of the contact time measurements described above.


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kB n-Pentanal O

2

Butane

kPAL 1

O

2-MTHF k-MF 0.0004

PATH I k1 1-Pentanol OH 0.0017 0.0019 k2 PATH II

kMF 0.05

kP1 1.1

1-Pentene

OH

2-Pentanol k-PONE kPONE 0.02 6

6.5 kP2

kP1P 17 1.3

2-Pentene

Pentane

kP2P

O

O

2-Pentanone 2-Methylfuran

Scheme 1. Simple gas phase reaction network for 2-MTHF on Ni2P/SiO2 at 0.1 MPa and 300 oC. The simulated rate constants are reported in Scheme 1 and the fitting results are shown in Figure 1. Calculated data follow similar trends to the experimental data, though not closely, especially for 1-pentene and 2-pentanone. The regression coefficient was R2 = 0.75. Although Scheme 1 accounts for all observed products and gives a good fit to the contact time data (Figure 1), it is unrealistic in that it does not consider adsorbed intermediates. As an alternative, a detailed reaction network involving surface species is proposed (Scheme 2). The network depicted here represents the simplest realistic description of the observed gaseous species with each assigned a corresponding surface intermediate that is capable of desorbing or reacting. The sequence starts with the adsorption of 2-MTHF on an active site through the oxygen atom which activates the adjacent C-O bonds. Ring-opening reactions follow by scission of these bonds to produce adsorbed alcohols. The adsorbed alcohols can simply 12 ACS Paragon Plus Environment

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desorb or serve as precursors for further transformations into adsorbed aldehyde/ketone species and adsorbed alkenes. The adsorbed aldehyde can desorb as n-pentanal and the adsorbed ketone can desorb as 2-pentanone. The aldehyde may also go through a decarbonylation to form nbutane. The final product n-pentane is a result of hydrogenation of the adsorbed alkenes. Key facilitators for these surface reactions include vacant active sites and surface hydrogen. The order of product formation follows that obtained for the contact time measurements.

Path I

Path II

[ALD]

*+

*+ O

k-ALD

kALD

277

32

kBUT 24.9

*+ [BUT]

[A]

[OL1]

*+

HO

k-OL1

kOL1

2067

5.02

k1a +O

1207

* [ALD*]

0.16

k-1a

[EN1]

kEN1 1.6

kONE

0.0046 2023

5.01

1778

0.2

k2a

44

O

+

* [A*]

* [OL1*]

O H

+

886

k-2a

* [OL2*]

k-MF

kMF

10

0.1

216

O+

* [ONE*]

k2b 1467

*+

k-EN1 836

k-ONE

k2

21

637

O

kOL2

k1 O

OH

k-OL2

kA

0.65

[ONE]

*+

*+

O

k-A

k1b

*+

[OL2]

k-EN2 2093

O

[MF]

*k

k2PAN

[EN1*] 1PAN 1056

1703

* [EN2*]

kEN2

+* [EN2]

1.5

[PAN]

Scheme 2. Reaction pathways of 2-MTHF on Ni2P/SiO2 at 0.1 MPa, 300 oC. The units of the adsorption rate constants (kA, kOL1, kOL2, kALD, kONE, kEN1, kEN2, kMF) are cm3 µmol-1 s-1. The units of all other rate constants including the desorption (k-A, k-OL1, k-OL2, k-ALD, k-ONE, k-EN1, k-EN2, k-MF, and kBUT) and surface reactions (k1, k2, k1a, k-1a, k1b, k2a, k-2a, and k2b) are s-1. Bold arrows indicate adsorption/desorption, thin arrows indicate surface reactions. 13 ACS Paragon Plus Environment

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Details of the scheme are presented in the Supporting Information as Scheme S1. These include the differential equations, units, and the calculations of the surface area.

a)

10

Path I 1-Pentanol n-Pentanal 1-Pentene

5

Selectivity / %

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 b)

20

Path II 2-Pentanone 2-Pentene 2-Pentanol

10 0 60

c) n-Pentane n-Butane 2-Methylfuran Conversion

40 20 0

0

20 40 60 80 100 120

Contact time / s Figure 2. Fits for the detailed reaction network (Scheme 2) of 2-MTHF on N2P/SiO2 at 0.1 MPa, 300 oC, 3.3 mol% 2-MTHF in H2. Continuous lines are simulation results and points are experimental data.

The reaction scheme is a network of first-order reactions that only considers the organic species. This is because hydrogen is used in excess (~97%) and is not expected to be limiting. An earlier study [17] reports the kinetics for the overall reaction, and there was a hydrogen dependency found that became significant at high hydrogen pressures.


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The simulation results for product selectivities and yields (continuous curves) show a close match to the experimental data (Figure 2) with a regression coefficient of R2 = 0.91. The optimized values for rate constants are noted in the reaction network. A kinetic model allows estimation of surface species concentrations (Figure 3). The results show that adsorbed 2-MTHF (A*) and n-pentanal (ALD*) are the two most abundant species on the catalyst surface at levels of coverage 10-9 µmol cm-2. Adsorbed 2-MTHF was twice as high as adsorbed aldehyde, a precursor for both n-pentanal and n-butane, and the adsorbed aldehyde was at least five times higher than all other surface species. It should be noted that the rate constants of the adsorption steps range from 0.0046 – 32 cm3µmol-1s-1. Converting to molecular units (6.02 x 1017 atoms µmol-1) and using a standard surface atomic density (1015 atoms cm-2) gives 7 x 10-6 – 5x10-2 cm s-1, which obeys the criterion for the range of LA < 104 cm s-1 expected for an adsorption process [37] with rate constant k = LAexp(-E/RT). Here L= site density (cm-2) and A = pre-exponential factor for a first-order adsorption process (cm3s-1). Using LA = 104 cm s-1 activation energies of E = 120-70 kJ mol-1 can be calculated, which are reasonable.


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A* ALD*

1E-9

ONE*

1E-10 OL1* EN1* OL2* EN2*

1E-11 0

20

40 60 80 100 120 Contact time / s

Figure 3. Simulated concentrations of surface species. Continuous lines are Path I intermediate products and dashed lines are Path II intermediate products. A* > ALD* >> ONE* > OL1* > EN1* > OL2* > EN2*. Overall, Path I intermediates have higher concentration than the corresponding Path II intermediates. A series of infrared spectroscopy measurements were carried out to identify the adsorbed species on the catalyst surface at various temperatures. All spectra shown include background subtraction of the spectra obtained with the same sample without adsorbate at the same temperature. Figure 4 shows infrared spectra of adsorbed 2-MTHF on Ni2P/SiO2 in hydrogen at 0.1 MPa, from 103 oC to 325 oC. In the high wavenumber region, two negative peaks are attributed to the consumption of hydroxyl groups associated with the support (SiO-H at 3745 cm1

) and phosphorus (PO-H at 3667 cm-1) from the catalyst [16, 38, 39, 40]. The intensity of these

peaks is negative because the hydroxyl groups are consumed by interaction with the 2-MTHF reactant. As expected, these peaks recover at higher temperature as the 2-MTHF desorbs. A set of features at 2978, 2939, and 2885 cm-1 corresponds to aliphatic C-H stretching of vibrations, which can be predominantly assigned to CH3-, CH2-, and CH- groups, respectively. These can arise from various adsorbed species including the alcohols, aldehyde, ketone, alkanes, and the


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ACS Catalysis

reactant itself [30,38,39,40]. A peak is observed for surface phosphine species (P-H stretching at 2450 cm-1) [41]. In the low wavenumber region, a peak appears at 200 oC and reaches a maximum at 250 oC which belongs to linearly adsorbed CO species (2066 cm-1), a decarbonylation product [16,42]. At lower wavenumbers, a broad region contains peaks assigned to a C=O stretch from adsorbed aldehyde and ketone species (1693 cm-1) and to a C=C stretch from adsorbed alkenes (1634 cm-1) [41]. Two peaks (1464 and 1385 cm-1) were also observed over pure silica where no reaction took place. The first peak has a small left shoulder and a right shoulder due to the CH2 scissoring vibration (1450 cm-1) of the non-aromatic ring of 2-MTHF. The second peak is attributed to C-H bending (1385 cm-1) of the methyl group of 2-MTHF. Overall, the spectra of Ni2P/SiO2 in H2 at atmospheric pressure indicate clearly the presence of adsorbed intermediate species.


ACS Catalysis

C=O C=C

2939 2885

2978

0.1

1464 1385 0.1 1634 1693

103oC 125oC

3745

Si-OH

150oC

2460

3667

Absorbance

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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P-OH

175oC 201oC 226oC 251oC 275oC 300ooC 325 C

2066

4000

3500

3000

2500

2000

1500

1000

-1

Wavenumber / cm

Figure 4. FTIR spectra of 2-MTHF adsorption on a 35 mg wafer of Ni2P/SiO2 in 3.3 mol% of 2-MTHF in H2 as a function of temperature at 0.1 MPa.

Table 2 Ratio of CH2 to CH3 groups for the surface species formed by different pathways Path I surface species ENE1*

ALD*

Reactant OL1*

A*

Path II surface species OL2*

ONE*

ENE2*

`

*

2:1

+O

H O

*

*

3:1

+

4:1

O

+

* 3:1

O H

O+

*

*

+

1:1


1:1

*

1:2

Page 19 of 34

Figure 5 shows the results of measurements of adsorbed 2-MTHF in the C-H stretch region at different temperatures for the silica support in H2 and the Ni2P/SiO2 catalyst in He and H2. Bands for CH3, CH2 and CH vibrations are observed in all three cases at 2978, 2939, and 2880 cm-1, respectively, but there are differences in intensities. The SiO2 support (Fig. 5a) shows low intensity (note expanded absorbance scale), but the Ni2P/SiO2 samples show higher intensity. The Ni2P/SiO2 sample in He (Fig. 5b) shows similarity to the SiO2 sample, while the Ni2P/SiO2 sample in H2 (Fig. 5c) shows higher intensity for the middle 2939 cm-1 band. This will be explained in the discussion section.

a) SiO2 support in H2 b) Ni2P/SiO2 in He c) Ni2P/SiO2 in H2 2978

0.05

2978

0.02

2939

2978

0.02

2939

2939

175 oC

2880

2880

Absorbance

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

2880

175 oC 200 oC 225 oC 250 oC 275 oC 300 oC 325 oC

200 oC

250 oC 300 oC 3200

2917 2849 3000

28003200

3000

28003200

3000

2800

-1

Wavenumber / cm

Figure 5. Aliphatic C-H stretching region as a function of temperature for the adsorption of 3.3 mol% of 2-MTHF at 0.1 MPa on a) SiO2 in H2, b) Ni2P/SiO2 in He, and c) Ni2P/SiO2 in H2.


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Discussion The HDO of methylfuran derivatives is of interest because these intermediates are formed in large concentrations. The amounts are substantial and these derivatives are referred to as platform chemicals [43,44]. Recent studies report the direct one-pot conversion of sugars to 2,5dimethylfuran and 2,5-dimethyltetrahydrofuran [45]. The large-scale production of hydroxymethylfurfural is feasible [46], and its conversion to 2,5-dimethylfuran [47] and to alkyl levulinates [48] have been recently reported. The Ni2P catalyst is outstanding in producing desired alkanes in the HDO of the biomass model compound 2-methyltetrahydrofuran. As already mentioned in the introduction, the Ni2P samples studied here have much higher selectivity for HDO than Ru or Pd [17]. Contact time results showed that the initial products were pentanols, intermediate products were pentanal/pentanone and pentenes, and final products were n-pentane and n-butane, consistent with a preliminary study [28]. The linear dependence of the total conversion on contact time indicates a stable catalyst and no product inhibition. Calculations of the Weisz-Prater criterion (CWP) indicate that there are no mass transfer limitations over the entire range of measurements.

(CWP =

′ −𝑟𝑟𝐴𝐴(𝑜𝑜𝑜𝑜𝑜𝑜) 𝜌𝜌𝑐𝑐 𝑅𝑅 2

𝐷𝐷𝑒𝑒 𝐶𝐶𝐴𝐴𝐴𝐴

)

(3)

Details are provided in the Supplemental Information in which it is shown that CWP < 0.01 for all points. This obeys the criterion CWP < 0.3 for lack of internal mass transport limitations. The reaction network reported here (at 0.1 MPa) is different from the one reported earlier for the same catalytic system at a higher pressure (0.5 MPa) [16,17]. Although both had alkanes as final products, at 0.1 MPa alcohols were formed first and aldehyde / ketone species followed, 20 ACS Paragon Plus Environment

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whereas at 0.5 MPa the aldehyde and ketone species were formed first and alcohols followed. The difference may be due to the hydrogen rich environment on the catalyst surface at high pressure which led to the preferred production of pentanols as the first step in the sequence. Although temperature is known to affect reaction sequences greatly because different steps have different activation energies, pressure has not been considered to affect reaction mechanisms substantially, and rate expressions have been used over broad pressure ranges. The findings here demonstrate that pressure can have an effect on reaction pathway and, hence, mechanism. The current study provides an in-depth analysis of the reaction network taking into account possible surface species and surface reactions. Modeling of the proposed reaction network gave a good fit to the experimental data. Other simpler and more complex models were explored, but the fits were inferior. The rate-constants were the smallest for the ring-opening steps, k1 and k2, (Scheme 2) indicating that they were the rate-determining steps. The rate constant for C-O bond scission on the unsubstituted side (Path II, k2 = 44 s-1) was twice as large as on the substituted side (Path I, k1 = 21 s-1). The possible ring-opening steps involve an SN1 type nucleophilic attack for Path I and an SN2 type displacement for Path II (Scheme 3). In the SN1 route the carbenium ion intermediate is secondary so only moderately stable, and the SN2 route is preferred.


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H+

Low pressure (0.1 MPa) route SN1 O+ O

*

SN2

Page 22 of 34

O

+ H

* *

*

O+

O

H

* *

O+ H

O

(Path I)

* +

H

*

H O+

*

(Path II) Preferred

Scheme 3. Proposed ring-opening mechanisms for the ring-opening of 2-MTHF on Ni2P/SiO2 at 0.1 MPa, 300 oC, the preferred route (with higher rate constant) is noted with bold arrows.

Following ring-opening and formation of the adsorbed alcohol species, the main reaction paths produce the olefins (Scheme 2 downward arrows) followed by the final product pentane. The rate constants for Path II are higher than those for Path I, reflecting the ease of formation of the secondary alkene (the secondary hydroxyl group is a better leaving group than the primary hydroxyl group), and the greater reactivity of the more substituted alkene. This also accounts for the lower selectivity observed for 1-pentene experimentally. Again, active sites and surface hydrogen are involved in the reactions, but they are not accounted for in the kinetics, as the lack of product inhibition suggests low coverages and H2 is in excess. The side products in the sequence are the aldehyde and the ketone, which are produced by the dehydrogenation of the corresponding adsorbed alcohol species (Figure 2). Transformation of the adsorbed 1-pentanol to adsorbed aldehyde is faster than of the adsorbed 2pentanol to adsorbed 2-pentanone, reflected by a larger value of k1a than that of k2a. Here, hydrogen elimination from the unhindered side (Path I) is easier than the hindered one (Path II) which results in faster formation rate of the adsorbed aldehyde than the adsorbed ketone. The


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aldehyde stays longer on the catalyst surface probably due to its unhindered strong π bond interaction with the metallic center compared to the ketone. The interaction of the aldehyde on the active sites allows its decarbonylation into butane, while the ketone species simply desorbs as 2-pentanone. The analysis allows calculations of surface concentrations of the suggested surface intermediates. Adsorbed 2-MTHF, the reactant, is the most abundant surface intermediate as expected because its ring opening is the rate determining step in both Path I and Path II (Fig. 3). The aldehyde species, the precursor for n-pentanal and n-butane, is the second most abundant, about half of that of the adsorbed 2-MTHF. This reflects the slow desorption and fast readsorption of the produced n-pentanal as well as the slow decarbonylation to n-butane. Path I intermediates, adsorbed 1-pentanol and 1-pentene, are about one-and-a-half-orders-of-magnitude lower in surface concentration than the adsorbed 2-MTHF. Path II intermediates, adsorbed 2pentanone, 2-pentanol, and 2-pentenes, are about two-orders-of-magnitude lower in concentration than the adsorbed 2-MTHF (Fig. 3). It should be noted that Path I and Path II intermediates can be distinguished from their differences in the number of CH2 groups (Table 2). This will be used later on in the interpretation of the FTIR results (Fig. 5). To summarize, the decomposition of 2-MTHF on Ni2P/SiO2 occurs by two possible pathways, depending on where the ring is opened. Opening on the more hindered side (path I) produces adsorbed 1-pentanol as the initial intermediate and opening on the more open side (Path II) forms adsorbed 2-pentanol. Path I is mainly responsible for production of n-butane while Path II is mostly involved for the formation of 2-pentanone and n-pentane.


ACS Catalysis

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FTIR measurements were carried out by adsorbing 2-MTHF, flushing the catalyst surface, and increasing the temperature in steps (Figure 4). The two negative peaks at 3745 and 3667 cm-1 are attributed to the consumption of the hydroxyl groups associated with the support and phosphorus from the catalyst due to interaction with 2-MTHF. Their negative intensity decreases as temperature increases, and eventually the peaks turn positive indicating a recovery of the corresponding hydroxyl species. The silanol groups from the silica support are not acidic enough to protonate 2-MTHF. It has been shown that the silanol groups were inactive even towards a strongly basic compound such as 2-methylpiperidine [49]. In comparison, the PO-H group is more reactive. A previous FTIR analysis of P2O5/SiO2 showed that the PO-H vibration at 3660 cm-1 was close to that of a Ni2P/SiO2 catalyst [49]. It was proposed that the PO-H groups at 3667 cm-1 were associated with the Ni2P particles and the oxygen in these hydroxyl groups probably came from phosphoric-acid like species. It is likely that surface acidity has a role in the reaction mechanism. The recovery of the hydroxyl groups with increasing reactivity (increasing temperature) provides evidence for the proposed surface reactions where hydroxyl species are produced from the deoxygenation steps. The aliphatic C-H stretching signals are the strongest bands, and merit individual discussion. The C-H stretch signals can be assigned primarily to CH3- (2978 cm-1), CH2- (2939 cm-1), and CH- (2885 cm-1) groups and are due to the 2-MTHF reactant, as well as to adsorbed alcohols, aldehyde, ketone, alkanes. The peaks are shown as a function of temperature for adsorption of 2-MTHF on SiO2 in a H2 stream and on Ni2P/SiO2 in He flow (Fig. 5b) and in H2 flow (Fig. 5c). These peaks decrease with increasing temperature as expected. For the SiO2 substrate, the signals can be taken to be due to adsorbed 2-MTHF, because no reaction occurs on SiO2 at these conditions. As the temperature is raised the bands simply decrease in intensity. 24 ACS Paragon Plus Environment

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ACS Catalysis

However, on the Ni2P/SiO2 clear occurrence of surface reactions is indicated by the uneven disappearance of CH3 groups (2978 cm-1) compared to CH2 groups (2939 cm-1). As will be discussed using the results of Table 2, this can be analyzed to give information about the surface species. It will be shown that the adsorption profile in this C-H region is dominated by species from Path I reactions on active nickel phosphide catalyst (Figure 3). A peak at 2450 cm-1 originates from P-H stretching of phosphine species [41]. Unlike the trend observed in the PO-H stretching (3667 cm-1) which continues rising even as all species desorb, the P-H peak (2450 cm-1) diminishes along with the desorption of surface species, suggesting hydrogen transfer activity between the adsorbed species and the catalyst phosphine group. A peak at 2066 cm-1 appears at 200 oC and reaches a maximum at 250 oC. This peak belongs to linearly adsorbed CO species, a decarbonylation product. The peak position matches that of a Ni0- CO complex [50,51], indicating that the decarbonylation reaction involves Ni metal sites, a result that agrees with previous studies [16,39,52]. A central region contains peaks at 1693 and 1634 cm-1 in which the former is assigned to a C=O stretch from adsorbed aldehyde and ketone species and the latter is assigned to a C=C stretch from adsorbed alkenes [41]. The C=O band appears at 1693 cm-1 for pentanal and 2pentanone. The accompanying alkene C-H stretching vibrations, which usually appear at 30153100 cm-1 and overlaps with the alkane C-H stretching bands, do not contribute to the signal because the concentrations of the olefins are much smaller than those of the alkyl group containing species. Simultaneous appearance of C=O and C=C features is consistent with their formation from the same intermediate species (possibly the alcoholic species) (k1a-k1b or k2a-k2b


ACS Catalysis

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in Scheme 2). The intensity of the νC=O vibration is larger than that of the νC=C band initially and both decline with temperature. Note that the disappearance of νC=O (1693 cm-1) is followed by the appearance of νC-O (2066 cm-1), suggesting decarbonylation of the aldehyde species to form adsorbed CO. Two peaks at 1464 and 1385 cm-1 are due to the alkane C-H bending of 2-MTHF (Figure 4). The peak at 1464 cm-1 has a small left shoulder at 1488 cm-1 and a right shoulder at 1450 cm-1 due to the CH2 scissoring vibration of the non-aromatic ring of 2-MTHF. Cyclization is known to decrease the frequency of the CH2 bending vibrations while having little effect on C-H stretching vibrations for cyclic hydrocarbons with little ring strain [41]. The peak at 1385 cm-1 is attributed to C-H bending of the methyl group of 2-MTHF. On pure silica (Supporting Information), the same region showed a doublet (1385 and 1360 cm-1) which is a result of the interaction between the in-phase and out-of-phase C-H bending of the two methyl and methylene groups attached to the secondary carbon atom of 2-MTHF. The differences in the presence of the active nickelphosphide phase provide insight about the adsorption mode of 2-MTHF on the catalyst. If 2MTHF were absorbed in an upright η1 (O) mode on an active site, a doublet should be present as seen on pure silica. So, the loss of the peak at 1360 cm-1 on Ni2P/SiO2 suggests that 2-MTHF may have adsorbed onto the catalyst in a parallel η5 conformation in which the methyl or methylene group attached to the secondary carbon atom was immobilized, resulting in the loss of the 1360 cm-1 peak. As this phenomenon was only observed on Ni2P/SiO2, the bonding must have occurred on the Ni2P active phase or at the catalyst/support boundary. Overall, the spectra of adsorbed 2-MTHF as a function of temperature on Ni2P/SiO2 in H2 at atmospheric pressure indicate clearly transformation of the adsorbed 2-MTHF. The presence


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ACS Catalysis

of aldehyde and ketone species in the low temperature range followed by the CO species in the high temperature range indicates that the ring-opening has lower activation barrier energy than the decarbonylation step and/or that the decarbonylation step follows the ring-opening steps. Simultaneous appearance of C=O and C=C supports the proposed mechanism where aldehyde / ketone and alkenes are formed from a common intermediate alcohol. The exact assignment of all bands to adsorbed species is difficult because of considerable overlaps in structural features, like C-C, C-O, and C-H bonds. However, the bands at 2939 cm-1 due to CH3 groups and 2978 cm-1 due to CH2 groups can be used to indicate a preferred reaction pathway of 2-MTHF (Fig. 5). Recall that two pathways were proposed earlier, Path I generates primary-carbon substituted species (1-pentanol, n-pentanal, and 1-pentene) and Path II generates secondary-carbon substituted species (2-pentanol, 2-pentanone, and 2-pentene). Close examination shows that the surface species for each pathway can be differentiated according to the ratio of CH2 to CH3 groups present (Table 2). For Path I the ratio is substantially larger than 1 (2:1 to 4:1) and for Path II the ratio is 1 or less than 1 (1:2 to 1:1). Thus, if the intermediates species from path I dominate, the intensity of the CH2 band (2939 cm-1) should be stronger than that of the CH3 band (2978 cm-1). Experiments were conducted monitoring the adsorbed species after dosing with 2-MTHF as temperature was progressively raised for the silica support in H2 and for the Ni2P/SiO2 in He and H2 (Fig. 5). The crucial C-H region is highlighted. The spectra on the SiO2 support are due to weakly adsorbed 2-MTHF, and it can be seen that by 250 oC, the signal is greatly diminished, indicating desorption of the species. The spectra on the Ni2P/SiO2 in He show similarities to that on the SiO2 with peaks at 2978 cm-1 for CH3, 2939 cm-1 for CH2, and 2880 cm-1 for CH groups, all of which diminish with increasing temperature. Comparison to gas-phase spectra indicates 27 ACS Paragon Plus Environment

ACS Catalysis

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that the features are due to molecularly adsorbed 2-MTHF. However, for the Ni2P catalyst the decrease in intensity is not as severe, with the adsorbed 2-MTHF remaining up to 325 oC. The spectra on the Ni2P/SiO2 catalyst in H2 also show a decreased in IR peak intensities. However, close examination reveals a substantial difference. The middle peak at 2939 cm-1 is much more intense than the other two peaks compared to the sample treated in He. As the adsorbed species from Path I contain a higher CH2:CH3 ratio than the ones from Path II (Table 2), the slow decrease of CH2 peak with regard to CH3 peak (Figure 5) indicates that the Path I species are more abundant on the catalyst surface. Further evidence is shown by the presence of the aldehyde C-H stretch at 2849 cm-1 at high temperatures in both H2 (325 oC) and He (250 oC – 325 oC). This spectroscopic evidence gives strong support for the reaction network scheme analysis presented in Scheme 2. Although in principle difficult because of the similarity of the adsorbed intermediates, the focus on the CH2:CH3 ratio allows distinguishing between the possible pathways and comparison between model predictions and experimental results. Conclusions The reactions of 2-methyltetrahydrofuran were studied on supported nickel phosphide at 0.1MPa and 300 oC using a combination of reactivity tests, kinetic modelling, and infrared spectroscopy. The steps involved adsorption of the reactant 2-MTHF followed by rate determining ring-opening on either side of 2-MTHF to produce adsorbed 1-pentanol and 2pentanol as initial species. The surface alcohols could simply desorb or act as precursors for further transformation into adsorbed aldehyde, ketone, and alkene species as adsorbed intermediates. These could then produce n-pentanal, 2-pentanone, and pentenes. The aldehyde


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ACS Catalysis

could also undergo decarbonylation to n-butane and the alkene could go through hydrogenation to n-pentane as final products. Agreement between modeling results and experimental evidence for the proposed surface species by infrared spectroscopy further supports the proposed reaction network. Supporting information Supporting information is available. I. Weisz-Prater criterion for internal diffusion II. Reaction scheme equations and details III. Table of conversions and selectivities Acknowledgments This work was supported by the US Department of Energy, Office of Basic Energy Sciences, through Grant DE-FG02-963414669.


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References

1

Gürbüz, E. I.; Hibbitts, D. D.; Iglesia, E. J. Am. Chem. Soc. 2015, 137, 11984-11995.

2

Deuss, P. J.; Scott, M.; Tran, F.; Westwood, N. J.; de Vries, J. G.; Barta, K. J. Am. Chem. Soc. 2015, 137, 7456-7467.

3

Foo, G. S.; Rogers, A. K.; Yung, M. M.; Sievers, C. ACS Catal. 2016, 6, 1292–1307.

4

McManus, I.; Daly, H.; Thompson, J.M.; Connor, H.; Hardacre, C.; Wilkinson, S.K.; Sedaie Bonab, N.; ten Dam, J.; Simmons, M.J.H.; Stitt, E.H.; D’Agostino, C.; McGregor, J.; Gladden, L.F.; Delgado, J.J. J. Catal. 2015, 330, 344-353.

5

De Jong, W. In Biomass as a sustainable Energy Source for the Future; De Jong, W., Van Ommen, J. R., Eds.; John Wiley & Sons, Inc.: Hoboken, NJ, 2014; pp 469-502.

6

Evans, A.; Strezov, V.; Evans, T. J. In Biomass processing technologies; Strezov, V., Evans, T. J., Eds.; CRC Press: Boca Raton, FL, 2014, pp 357-376.

7

Yang, S.-T.; Yu, M. In Bioprocessing technologies in biorefinery for sustainable production of fuels, chemicals, and polymers; Yang, S.-T.; El-Ensashy, H. A., Thongchul, N., Eds.; John Wiley & Sons, Inc.: Hoboken, NJ, 2013, pp 1-26.

8

Sharma, A.; Pareek, V.; Zhang, D. Renewable Sustainable Energy Rev. 2015, 50, 10811096.

9

Agblevor, F.; Mante, O.D.; Oyama, S.T.; McClung, R. Biomass & Bioenergy 2012, 45, 130-137.

10

Mante, O.D.; Agblevor, F.; Oyama, S.T.; McClung, R. Bioresource Technology 2012, 111, 482–490.

11

Saidi, M.; Samimi, F.; Karimipourfard, D.; Nimmanwudipong, T.; Gates, B. C.; Rahimpour, M. R. Energy Environ. Sci.2014, 7, 103-129.

12

Tran, N.; Uemura, Y.; Chowdhury, S.; Ramli, A. Appl. Mech. Mater.2014, 625, 255-258.

13

Alsbou, E.; Helleur, R. J. Anal. Appl. Pyrolysis 2013, 101, 222-231.

14

Ruddy, D. A.; Schaidle, J. A.; Ferrell III, J. R.; Wang, J.; Moens, L.; Hensley, J. E. Green Chem. 2014, 16, 454-490.

15

Rezaei, P. S.; Shafaghat, H.; Daud, W. M. A. W. Appl. Catal., A 2014, 469, 490-511.

16

Cho, A.; Kim, H.; Iino, A.; Takagaki, A.; Oyama, S. T. J. Catal. 2014, 318, 151-161. 30 ACS Paragon Plus Environment

Page 30 of 34

Page 31 of 34

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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17

Iino, A.; Cho, A.; Takagaki, A.; Kikuchi, R.; Oyama, S. T. J. Catal. 2014, 311, 17-27.

18

Prins, R.; Bussell, M. E. Catal. Lett. 2012, 142, 1413-1436.

19

Oyama, S. T.; Gott, T.; Zhao, H.; Lee, Y-K. Catal. Today 2009, 143, 94-107.

20

Bai, J.; Li, X.; Wang, A.; Prins, R.; Wang, Y. J. Catal. 2013, 300, 197-200.

21

Rodriguez, J. A.; Kim, J-Y; Hanson, J. C. J. Phys. Chem., B 2003, 107, 6276-6285.

22

Zhao, H.; Oyama, S.T.; Freund, H.-J.; Włodarczyk, R.; Sierka, M. Appl. Catal. B: Env. 2015, 164, 204-216.

23

Guan, Q.; Cheng, G.; Li, R.; Li, W. J. Catal. 2012, 299, 1-9.

24

Yun, G.-N.; Lee, Y.-K. Appl. Catal. B: Env. 2014, 150-151, 647-655.

25

Oyama, S. T. J. Catal. 2003, 216, 343-352.

26

Zuzaniuk, V.; Prins, R. J. Catal. 2003, 219, 85-96.

27

Abu, I.I.; Smith, K.J. Appl. Catal. A: Gen. 2007, 328, 58-67.

28

Bui, P.; Cecilia, J. A.; Oyama, S. T.; Takagaki, A.; Infantes-Molina, A.; Zhao, H.; Li, D.; Rodriguez-Castellon, E.; Lopez, A.J. J. Catal. 2012, 294, 184-198.

29

Infantes-Molina, A.; Gralberg, E.; Cecilia, J. A.; Finocchio, E.; Rodríguez-Castellón, E. Catal. Sci. Technol. 2015, 5, 3403-3415.

30

D. Li, P. Bui, H. Y. Zhao, S. T. Oyama, T. Dou, Z. H. Shen, J. Catal. 2012, 290, 1-12.

31

Whiffen, V. M. L.; Smith, K. J. Top. Catal. 2012, 55, 981-990.

32

Bui, P.; Callow, B.; Nozaki, K.; Takagaki, A.; Oyama, S. T. ACS Catal. 2016, 6, 4549 – 4558.

33

Oyama, S.T.; Onkawa, T.; Takagaki, A.; Kikuchi, R.; Hosokai, S.; Suzuki, Y.; Bando, K.K. Top. Catal. 2015, 58, 201-210.

34

Koike, N.; Hosokai, S.; Takagaki, A.; Nishimura, S.; Kikuchi, R.; Ebitani, K.; Suzuki, Y.; Oyama, S.T. J. Catal. 2016, 333, 115-126.

35

Liu, Y.; Yao, L.; Xin, H.; Wang, G.; Li, D. Hu, C., Appl. Catal. B: Env. 2015, 174–175, 504–514.

36

Yaws, C. L.; Narasimhan, P. K.; Gabbula, C.; Eds. Antoine Equation and Coefficients for Organic Compounds. In Yaws’ Handbook of Antoine Coefﬁcients for Vapor Pressure (2nd Electronic Edition) [Online]; Knovel, 2009.

37

Vannice, M.A., Kinetics of Catalytic Reactions, Springer Science, New York 2005, p. 136.

38

Legrand, A. P.; Hommel, H.; Tuel, A. Adv. Colloid Interface Sci.1990, 33, 91-330. 31 ACS Paragon Plus Environment

ACS Catalysis

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

39

Lee, Y. -K.; Oyama, S. T. J. Catal. 2006, 239, 376-389.

40

Oyama, S. T.; Gott, T.; Asakura, K.; Takakusagi, S.; Miyazaki, K.; Koike, Y.; Bando, K. K. J. Catal. 2009, 268, 209-222.

41

Silverstein, R. M.; Webster, F. X. In Spectrometric Identification of Organic Compounds; Wiley: New York, 1998, pp 108-109.

42

Cho, A.; Shin, J.; Takagaki, A.; Kikuchi, R.; Oyama, S, T. Top. Catal. 2012, 55, 969-980.

43

F.M.A. Geilen, B. Engendahl, A. Harwardt, W. Marquardt, J. Klankermayer, W. Leitner, Angew. Chem. Int. Ed. 2010, 49,

44

Y. Roman-Leshkov, C.J. Barrett, Z.Y. Liu, J.A. Dumesic, Nature 2007, 447, 982–985.

45

C. Li, H. Cai, B. Zhang, W. Li, G. Pei, T. Dai, A. Wang, T. Zhang, Chin. J. Catal. 2015, 36, 1638-1646.

46

Y. Zhang, J. Zhang, D. Su, J. Energy Chem. 2015, 24, 548-551.

47

B. Saha, C.M. Bohn, M.M. Abu-Omar, ChemSusChem 2014, 7, 3095-101.

48

Z. Zhang, K. Dong, Z. K. Zhao, ChemSusChem 2011, 4, 112 – 118.

49

Oyama, S. T.; Lee, Y. –K. J. Phys. Chem., B 2005, 109, 2109-2119.

50

Peri, J. B. J. Catal. 1984, 86, 84-94.

51

Peri, J. B. Discuss. Faraday Soc.1966, 41, 121-134.

52

Bowker, R. H.; Smith, M. C.; Pease, M. L.; Slenkamp, K. M.; Kovarik, L.; Bussell, M. E. ACS Catal. 2011, 1, 917-922.


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Kinetic and Infrared Spectroscopy Study of Hydrodeoxygenation of 2

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