Combined In Situ XAFS and FTIR Study of the Hydrodeoxygenation

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Combined In Situ XAFS and FTIR Study of the Hydrodeoxygenation Reaction of 2‑Methyltetrahydrofuran on Ni2P/SiO2 Ayako Iino,† Atsushi Takagaki,†,‡ Ryuji Kikuchi,† S. Ted Oyama,*,†,‡,§ and Kyoko K. Bando∥ †

Department of Chemical Systems Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan College of Chemical Engineering, Fuzhou University, Fuzhou 350116, China § Department of Chemical Engineering, Virginia Tech, Blacksburg, Virginia 24061, United States ∥ Nanosystem Research Institute, Research Group for Nanoscientific Measurements, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Downloaded via UNIV OF SOUTH DAKOTA on July 13, 2018 at 03:14:58 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

ABSTRACT: This paper deals with a study of the mechanism of hydrodeoxygenation (HDO) at 0.1 MPa and 300−350 °C of 2-methyltetrahydrofuran (2-MTHF) over a Ni2P/SiO2 catalyst. The study was conducted using in situ Fourier transform infrared (FTIR) spectroscopy to monitor the adsorbed species during reaction and in situ X-ray absorption fine structure (XAFS) to probe the oxidation state of the Ni component. The work is relevant to the upgrading of bio-oil derived from the pyrolysis of biomass. It was deduced that the initial interaction of the 2-MTHF was with OH groups on the catalyst surface, followed by adsorption on Ni sites. Ring-opening of the tetrahydrofuran led to the formation of the main products of the reaction, n-pentane by HDO and n-butane + CO by decarbonylation. The CO band appeared with a slight delay in the FTIR spectrum because the reaction to form n-pentane was favored and turned over about four times before one n-butane + CO was formed.

1. INTRODUCTION The understanding of catalytic reactions is a subject of great importance that can give fundamental knowledge about the relationship between catalyst structure and reaction rate, and can thus be used to develop new materials for difficult transformations. Great insights can be derived from the study of chemical transformations on single crystal and thin film models coupled with in situ spectroscopic techniques1−3 as well by the exploration of the structure and energetics of surfaces and adsorbates using advanced in silico calculation methods.4,5 Indeed, understanding of catalysis at the atomic level will come from “experiment and theory going hand-inhand”.6 The key to the understanding of catalytic mechanisms is the examination of catalytic surfaces in their working state, a concept first articulated in 1965 with studies of in situ adsorption.7 The relevance of using spectroscopic techniques under in situ conditions is now commonly accepted and has been discussed in recent papers,8,9 books,10,11 and reviews.12 In the present work, use is made of simultaneous in situ Fourier transform infrared spectroscopy (FTIR), to gain insight into the adsorbed intermediates, and X-ray absorption fine structure (XAFS), to garner information on the state of the catalyst site. These are carried out simultaneously under in situ conditions during an actual reaction. The combined information on adsorbed molecular species and coordination and structural changes of the metal species can give insight into the catalytic reaction mechanism.13 © XXXX American Chemical Society

The system studied is the hydrodeoxygenation (HDO) of the model compound 2-methyltetrahydrofuran, a cyclic fivemembered ether, over a supported nickel phosphide catalyst. The reaction is relevant to the upgrading of bio-oil derived from pyrolysis of lignocellulosic feedstocks.14−16 There have been considerable efforts in this area, and many model feedstocks as well as real feeds have been studied. Many catalysts have also been employed, and among them, transition metal phosphides have emerged as promising materials because of their high activity and selectivity in hydroprocessing type reactions as described in an original article,17 and subsequent studies of HDS,18,19 HDN,20 HDO21−24 and in several reviews.25−27 There have been a number of studies of the HDO of 2methyltetrahydrofuran (2-MTHF) in our group which provide a good foundation for the present research. A first study28 conducted at atmospheric pressure (0.1 MPa, 300 °C) established that the order of reactivity of 2-MTHF on various metal phosphides was Ni2P > WP > MoP > CoP > FeP, while the order of selectivity to HDO was MoP > WP > Ni2P > FeP > CoP. The high activity of Ni2P has been confirmed for several reactions19,23 and motivated its use in the present work. A second study carried out under the same conditions (0.1 Special Issue: Hans-Joachim Freund and Joachim Sauer Festschrift Received: April 5, 2018 Revised: June 18, 2018 Published: June 19, 2018 A

DOI: 10.1021/acs.jpcc.8b03246 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C MPa, 300 °C) mapped out the reaction network and established that ring-opening of the five-membered ring was the rate-determining step.29 The ether ring could open on two sides, the methyl group side or the unsubstituted side, with the unsubstituted side somewhat favored. The rate constants that described the network were physically realistic. The network is shown below (Scheme 1) and indicates the two pathways, both of which start with ring-opening.

2. EXPERIMENTAL SECTION The catalyst used for the present studies was a Ni2P/SiO2 sample of weight loading 7.9 wt % (Ni amount 1.156 mmol/gsupport) prepared by a phosphate reduction method reported earlier.31 Briefly, a silica support was impregnated by incipient wetness with a solution of Ni(NO3)2·6H2O and (NH4)2HPO4 in distilled water using amounts to give a ratio of Ni/P of 1/2. Early work34 had shown that the use of a stoichiometric ratio of Ni/P of 2/1 gave rise to P-deficient Ni12P5 and that excess P was needed in the preparation of fully phosphided Ni2P. This was confirmed by others.35,36 After calcination at 500 °C, the solid was pelletized and sieved to a size of 650−1180 μm. Finally, the material was reduced in H2 to 590 °C with a heating rate of 3 °C/min and maintained at that temperature for 2 h. The final material was passivated in a 0.5 mol % O2/He mixture for 4 h. A reference metallic Ni sample with the same molar Ni content was prepared in a similar manner but without the phosphorus to give 6.4 wt % Ni/SiO2. The phase composition of the materials was determined from X-ray diffraction (XRD) patterns obtained with an XRD diffractometer (Rigaku) using a Cu Kα monochromatized beam generated at 40 kV and 100 mA. The number of surface metal atoms was estimated by pulse adsorption of CO in a He carrier at room temperature using 0.1−0.3 g quantities of catalysts reduced in H2 (100 mL/min) at 450 °C for 1 h. The CO signal was monitored by a thermal conductivity detector (TCD). BET surface areas were obtained on samples dried at 120 °C and evacuated for 1 h, before measurement using a Micromeritics ASAP 2010 instrument. The combined in situ XAFS and FTIR measurements were carried out at beamline BL9C of the Photon Factory at the Institute of Materials Structure Science, High-Energy Accelerator Research Organization (KEK-IMSS-PF), in Tsukuba, Japan. A cross-shaped cell with controlled atmosphere was used for the combined in situ experiments which permitted an infrared red beam and X-ray beam to be passed perpendicular to each other. A similar cell was described in a previous publication,37 and a schematic diagram is shown in Figure 1, which shows the two arms of the device. The ends of the arms for the X-ray beam were provided with seals made of thin polyimide films, which were gastight and inert, and did not attenuate the X-rays. The ends of the arms for the IR beam

Scheme 1. Depiction of the Reaction Sequence of 2-MTHF Reaction at 0.1 MPa (Adapted from ref 29)a

a

The circles indicate CH2 groups, and they are more numerous in the lower branch (red) than in the upper branch (blue). The species in brackets are very reactive and appear only at very low concentrations.

Ancillary studies on 2-MTHF were conducted at a moderate pressure (0.5 MPa) where again Ni2P showed high activity. A third study of the reaction at this pressure (0.5 MPa, 300 °C) found most of the same steps observed at lower pressure, and provided direct evidence for the presence of the expected reactive intermediates by FTIR.30 A fourth study under the same conditions (0.5 MPa, 300 °C) reported high HDO selectivity for Ni2P (88%) and low selectivity for Pd (30%) and Ru (15%), establishing Ni2P as one of the most effective HDO catalysts.31 The kinetics of the overall reaction was found to be described by a Langmuir−Hinshelwood reaction with the ratedetermining step being the reaction between adsorbed 2MTHF and a hydrogen atom. This was in agreement with the network analysis. A fifth study of the 2-MTHF reaction under the same conditions (0.5 MPa, 300 °C) utilized FeNi alloys to probe the site requirements for the 2-MTHF reaction.32 It was found that the turnover frequency based on Ni atoms titrated by CO chemisorption did not change with Fe substitution, demonstrating that the reaction was structure-insensitive and involved at most one or two surface Ni atoms.33 The selectivity, however, was affected by Fe proximity to Ni and indicated that the steps after the rds were structure-sensitive. The present study follows on to these previous studies on the HDO of 2-MTHF on Ni2P/SiO2 but returns to the lower pressure (0.1 MPa, 300−350 °C). Simultaneous in situ FTIR and XAFS measurements are carried to better understand the relationship between adsorbed intermediates and active sites. The objectives are to track the state of the Ni during reaction, to shed more light on the activation of the 2-MTHF reactant, and to confirm the competition between the two ring-opening steps.

Figure 1. Schematic diagram of the cell for the combined in situ XAFS and FTIR measurements. B


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450 °C at 5 °C/min to remove organic compounds on the catalyst surface and to regenerate the catalyst. The catalyst regeneration was continued for 1 h, and after the regeneration step, the temperature was decreased to the next reaction temperature. This procedure was repeated five times to conduct the HDO reactions at five different temperatures. After the experiments in H2 at the five temperatures, the carrier flow of H2 for the 2-MTHF was replaced with He at 150, 250, and 350 °C. Before the experiments in He, the catalyst was regeneratd in H2 at the same flow rate of 50 mL/min used before. All of these steps are summarized in Figure 2. The XAFS and FTIR spectra were obtained as described above.

were equipped with KBr windows, which were transparent to infrared radiation. In addition, carefully sized CaF2 rods were placed within the arms to reduce the dead volume and drastically reduce the amount of gas in contact with the IR beam to minimize signal interference. The CaF2 passed infrared radiation, and also were more resistant to moisture than the KBr. The dead volume of the cell is estimated to be 38 cm3. The sample consisted of a disk of diameter of 15 mm and thickness of 0.15 mm made from 35 mg of ground powder of a passivated Ni2P/SiO2 catalyst. The disk was placed at the point of intersection between two cylinders of the cross-shaped cell so as to be at 45° from both the X-ray beam and IR beam. Gas was introduced into the cell from all four edges, and was vented from the center of the cell to have good contact of the reactant with the catalyst. The reactant 2-methyltetrahydrofuran (2-MTHF) was sent to the cell by a bubbler connected to a H2 or He gas line. The conditions of reaction were 0.1 MPa and 300−350 °C. For the XAFS measurements, X-rays generated in a synchrotron operated at 2.5 GeV and 300 mA were monochromatized by a Si(111) double crystal and focused on the sample using a bent conical mirror. The silica monochromator rotated from 14.6321 to 12.2743° to monochromatize the X-rays in the energy range from 7827 to 9300 eV. For in situ XAFS measurements, the quick XAFS (QXAFS) technique was used, using scans of the Ni−K edge consisting of 4042 steps and taking 30 s for one spectrum. It took another 30 s for the monochromator to return to the initial position, so spectra were taken every 60 s continuously. The XAFS spectra were obtained in transmittance mode, and the intensity of the initial X-ray beam (I0) and that of the transmitted beam (I) were measured by ionization chambers filled with 100% N2 and 25% Ar in N2, respectively. Before starting all measurements, the X-rays were calibrated using a Ni foil. The obtained XAFS data were analyzed by commercial software, REX2000, using theoretical standards calculated by FEFF 8 after smoothing and interpolation. The in situ FTIR spectra were collected by a JASCO VIR9500 instrument with a MCT detector. The IR beam was brought to the cell through a flexible optical fiber made of ZnSe. Spectra were taken with 4 cm−1 resolution, and 100 scans were summed up and averaged to get all spectra. It took 67 s to get one spectrum, and the collection of the IR spectra was continuously conducted after starting measurements. The SiO2 support adsorbed strongly below 1350 cm−1, so IR spectra could not be obtained below this wavenumber. Experimentally, the absorbance was calculated using an IR spectrum with inert gas as a background. Before starting the HDO reactions, the sample disk of passivated Ni2P/SiO2 was reduced in a 25 mL/min flow of H2. All flow rates reported here are under normal conditions. The temperature was raised to 450 °C at 5 °C/min, and the temperature was kept at 450 °C for 2 h. The temperature was increased to 500 °C and kept for another 2 h to make sure that catalyst was reduced completely. After this pretreatment step, first, in situ measurements were made at 150, 250, 200, 300, and 350 °C in H2. Then, first, a gas mixture of 2.6 mol % 2methyltetrahydrofuran (2-MTHF) in H2 at a flow rate of 50 mL/min and a pressure of 0.1 MPa was introduced into the cell. The reactant gas mixture was flowed for around 60 min, and after that, the gas was switched to pure H2 at a flow rate of 50 mL/min for 30 min and the temperature was increased to

Figure 2. Experimental procedure showing the temperature and gases used for the combined XAFS and FTIR measurements at 0.1 MPa.

Steady-state reactivity measurements were made with 300 mg of Ni2P/SiO2 or 200 mg of Ni/SiO2 loaded in a vertical tubular flow reactor using 2.6 mol % 2-MTHF in H2 at a total flow rate of 140 mL/min at 0.1 MPa. Again, flow rates are reported under normal conditions. The contact time is calculated to be τ=

(0.5 MPa)(0.300 g)(55 μmol g −1)(1.5 mL min−1/ μmol s−1) (140 mL min−1)(0.1 MPa)(0.026)

= 34 s

Analysis of the products was with gas chromatography as reported previously.31 The reported turnover frequency (TOF) values were extrapolated to the same conditions (2.6 mol % 2-MTHF, 0.1 MPa) as the simultaneous FTIR and XAFS measurements. TOF (s−1) =

Conversion × Flow rate of 2 − MTHF (μmol s−1) Catalyst weight (g) × Quantity of CO uptake sites (μmol/g)

The lack of mass transfer limitations was ascertained by calculation of the Weisz−Prater criterion using the following equation:38 C WP =

′ −rA(obs) ρc R2 DeCAs

, where the catalyst particle radius

is R = 0.1 cm, the apparent density of the Ni2P/SiO2 is ρc = 0.3 g cm−3, and the effective diffusivity is 0.1 cm2 s−1. The reactant concentration at the catalyst surface CAs calculated at 300 °C was 2.3 × 10−6 mol cm−3. At 300 °C, the observed rate was −r′A(obs) = 1.0 × 10−6 mol−1 g−1 s−1 and CWP was 0.013. At 350 °C, the observed rate was −rA(obs) ′ = 3.6 × 10−6 mol−1 g−1 s−1 and CWP was 0.047. Both values are below the criterion requirement39 of CWP < 0.3, and no mass transfer limitations are expected in these studies.

3. RESULTS AND DISCUSSION 3.1. Catalyst Pretreatment Step. The Ni2P/SiO2 sample had a BET surface area of 214 m2 g−1 and a CO uptake of 55 C


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The Journal of Physical Chemistry C μmol g−1. The Ni/SiO2 sample had a BET surface area of 284 m2 g−1 and a CO uptake of 25 μmol g−1. Basic characterization data are summarized in Figure 3.

shows a decrease in intensity at 8349 eV (the white line) in going from the passivated to the reduced state. The intensity of the white line is very sensitive to the oxidation state,44 and this will be employed in the current investigation to monitor adsorption on the Ni centers. The maximum change in absorbance in going from the passivated state to the reduced state was ΔμT = 0.38. Figure 5 shows the Fourier-transform spectrum of X-ray absorption data for Ni2P/SiO2 after the catalyst pretreatment

Figure 3. Routine characterization of the Ni2P/SiO2 and Ni/SiO2 catalysts. (a) Temperature-programmed reduction profiles. (b) X-ray diffraction patterns of the prepared passivated samples. (c) Transmission electron microscopy image of Ni2P/SiO2. (d) Transmission electron microscopy image of Ni/SiO2.

Figure 5. Fourier-transform EXAFS spectrum (solid line) of Ni2P/ SiO2 in H2 atmosphere after reduction at 450 °C and calculated Ni− Ni and Ni−P fits (dotted and dashed lines).

step. A broad signal can be observed, which can be deconvoluted into two curves that show a Ni−P distance at 0.22 nm and a Ni−Ni distance at 0.25 nm. The parameters are summarized in Table 1. The results were consistent with previously reported Ni2P catalysts and bulk Ni2P,34,43 so the generation of Ni2P phase was confirmed.

The basic characterization work revisits results already reported in the past by others, and is presented here briefly just to establish that well-defined materials were used in this study. The temperature-programmed reduction profiles (Figure 3a) show that the reduction of the phosphate precursor to form Ni2P occurs close to 600 °C, while that of Ni is at much lower temperature, close to 300 °C. This is in accordance with previous work.34,40−43 The X-ray diffraction patterns (Figure 3b) demonstrate that phase pure materials were obtained. The transmission electron microscopy images show that synthesis produced supported nanoparticles. These were of size 4−10 nm for Ni2P/SiO2 (Figure 3c) and 20−40 nm for Ni/SiO2 (Figure 3d). There is a size difference between the materials, but past work has shown that the HDO of 2-MTHF is structure-insensitive on Ni2P (32) and the comparison of selectivity is what is important here. Figure 4 shows changes in the X-ray absorption near-edge spectroscopy (XANES) curve of the Ni2P/SiO2 catalyst during the pretreatment step. The absorption signal, denoted as μT,

Table 1. Curve Fitting Results of Reduced Ni2P/SiO2 (Figure 5) scattering

N

R (nm)

ΔE (eV)

σ (nm)

R factor (%)

Ni−P Ni−Ni

1.7 3.6

0.22 0.25

−7.1 −1.7

0.0082 0.010

0.33 0.33

3.2. Steady-State Reactivity Results. The results of 2MTHF hydrodeoxygenation in a packed-bed reactor at 0.1 MPa are shown below. Results are presented for Ni2P/SiO2 (Figure 6a) and, for comparison, for a reference Ni/SiO2 (Figure 6b). Conversion of 2-MTHF on the Ni2P/SiO2 in the temperature range 250−300 °C was moderate, with a maximum conversion under 50%. In comparison, the Ni/SiO2 catalyst had high conversion close to 100%. For the present Ni2P/SiO2 (BET 214 μmol g−1, CO uptake 55 μmol g−1), the reactivity studies were conducted at 0.1 MPa with a contact time of 34 s, using 2.6 mol % 2-MTHF. The contact time was calculated from the flow rate (140 mL/min), reactant concentration (2.6 mol %), weight of catalyst (0.300 g), CO uptake (55 μmol g−1), and pressure (0.5 MPa) (Experimental Section). The resulting TOFs were 0.015 s−1 (conversion 6%) at 250 °C, 0.076 s−1 (conversion 13%) at 300 °C, and 0.18 s−1 (conversion 44%) at 350 °C. At 150 and 200 °C, there was no activity to convert 2-MTHF. For a similar Ni2P/SiO2 catalyst (BET area 177 m2 g−1, CO uptake 50 μmol g−1) at 0.1 MPa with the same (extrapolated) contact time of

Figure 4. Changes in the XANES spectra of Ni2P/SiO2 during the pretreatment step in H2 at 25 mL/min. D


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Figure 7. Steady-state FTIR spectra of the 2-MTHF reaction as a function of temperature. Conditions: 2.6 mol % 2-MTHF in a flow of He or H2 (50 mL/min) at 0.1 MPa. Figure 6. Comparison of reactivity at 0.1 MPa. (a) Conversion and selectivity of Ni2P/SiO2. (b) Conversion and selectivity of Ni/SiO2.

clearly in previous work.29,30 The peaks in the range 3000− 2800 cm−1 are due to C−H stretching vibrations; specifically the peak at 2980 cm−1 is due to CH3 groups, while the peak at 2880 cm−1 is primarily due to CH groups. There is a band in between at around 2930 cm−1 due to CH2 groups,30 but it is not clearly visible at the resolution of this experiment. The band at 1460 cm−1 is due to the bending vibration of −CH3 groups. In addition, a peak appears at 2058 cm−1 only at 300 and 350 °C in H2, and is assigned to linearly adsorbed CO from a decarbonylation step. The peak is larger at 300 °C than at 350 °C, and is hardly observed at lower temperatures or in He. As mentioned at the beginning of this section, previous work with higher acquisition times showed significant enhancement of the CH2 signal versus the CH3 signal in H2 relative to He.30 Those results could only be slightly discerned in the wide scan spectra (Figure 7) but were clearly demonstrated in experiments conducted at 300 °C as a function of time (Figure 8). The measurements were concentrated on the region of C−H stretches.

34 s using 3.3 mol % 2-MTHF, the TOF was 0.031 s−1 (conversion 17%) at 300 °C and produced n-pentane with 52% selectivity, n-butane with 19% selectivity, and 2pentanone with 19% selectivity.29 Thus, the results at 300 °C are reasonably similar. 3.3. FTIR Studies. Before discussing the in situ FTIR results, it will help in the understanding to briefly discuss what is known about the reaction steps from previous FTIR work29,30 and the reaction network (Scheme 1). The scheme shows two branches; the top branch results from ring-opening on the least substituted side of 2-MTHF, and the kinetics shows that it is the preferred pathway. The bottom branch results from ring-opening on the more substituted side, and the reactions are slower along this pathway. The pathways can be distinguished by FTIR because the intermediates in the bottom pathway have more CH2 bonds than those in the top pathway. Indeed, the previous FTIR work showed that the proportion of CH2 bonds grew with reaction temperature and gave evidence for the proposed reaction network. The present study addresses the time dependence of the C−H FTIR signals, and also examines the formation of CO by decarbonylation. Figure 7 shows the FTIR spectra at steady state (60 min after the start of the reaction) at different temperatures in He (top) and H2 (bottom). Compared to previous work,29,30 there is a lot of noise in the spectra because they were taken in quick scan mode simultaneously with XAFS measurements to be presented in the next section. The negative peak at 3730 cm−1 is due to the consumption of SiO−H, and the broad peak centered at 3200 cm−1 shows the presence of hydrogen bonded hydroxyl groups.45 The weak feature at 3690 cm−1 is due to consumption of PO−H groups,46 but it cannot be seen well because the noise level is high due to the low acquisition times. The negative feature at 3730 cm−1 is due to the interaction of adsorbed compounds with the OH on the SiO2 surface, and its intensity should be stronger (more negative) at lower temperatures like 150 °C in H2 or in He because the compounds desorb at higher temperatures. The high noise level in this region obscure the trends, which are barely discernible. The FTIR results in this region were shown more

Figure 8. FTIR spectra in H2 and He for the 2-MTHF reaction on Ni2P/SiO2. The conditions were the same as those in Figure 7: 2.6 mol % 2-MTHF in a flow of H2 or He (50 mL/min) at 0.1 MPa.

To compare the amount of adsorbed organic compounds on the catalyst surface, the peak areas of the C−H stretching vibration region from 3000 to 2800 cm−1 were measured (Figure 9). The peak area calculated from the spectrum at steady state at the lowest temperature of 150 °C in H2 was defined as a coverage θ of 1, a reasonable assignment as the E


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became dominant, with these relating to the conversion of 2MTHF. Calculations of the relative CO signal based on the peak area of the stretching vibration of C−O at 2058 cm−1 were also conducted for H2 at 300 and 350 °C as a function of time. The peak area at steady state for the lower temperature of 300 °C was set to a relative CO signal of 1, and the results are shown in Figure 10. The CO signal at 350 °C reached a stable value

Figure 10. Relative FTIR CO signal calculated from the peak area at 2058 cm−1 after reaction started with 2.6 mol % 2-MTHF in 50 mL/ min H2 at 300 and 350 °C and 0.1 MPa.

Figure 9. Transient surface coverage in oxygenates from the FTIR areas of the C−H stretching vibrations at 0.1 MPa. The peak area at steady state at 150 °C in H2 was set to a coverage of θ = 1. (a) Adsorption due to reaction of 2.6 mol % 2-MTHF in a flow of H2 at 50 mL/min. (b) Desorption in a flow of H2 at 50 sccm. (c) Adsorption of 2.6 mol % 2-MTHF in He at 50 sccm. (d) Desorption in He at 50 mL/min.

relatively quickly 5 min after the reaction started with a value half of that at 300 °C. At 300 °C, the CO coverage increased relatively slowly taking 30 min to reach a stable value. The CO coverage at 350 °C was less than that at 300 °C because the desorption of CO was faster at the higher temperature. The coverage θ shown in Figure 9 includes the adsorption of all of the adsorbed organic species, assuming similar extinction coefficients for the species, but does not include CO. From the CO peak intensity at 300 °C in Figure 10 compared to that at room temperature for a similar sample,51 it is estimated that the CO coverage is 1/60th of a monolayer at its maximum, so small in comparison to the oxygenates. The oxygenate coverage from previous work at 300 °C and 0.5 MPa was estimated to be about θ = 0.5 using a Langmuir−Hinshelwood model,30 and the results here (Figure 9), corrected for the pressure difference, are consistent with this. The CO signal observed in this study is a result of the HDO reaction of 2MTHF, part of which involves a decarbonylation reaction of npentanal. 3.4. In Situ X-ray Absorption Fine-Structure Measurements. To analyze the quick XAFS data, first the obtained raw measurements were smoothed and interpolated and then they were analyzed by the software REX. All of the data were normalized by the absorption edge using the same energy value of 8390.6 eV. To track the oxidation number of the nickel species in the catalyst, the intensity of the absorbance at 8349 eV was tracked during and after the reactions. The energy 8349 eV occurs right after the Ni K-edge in the X-ray absorption near-edge structure (XANES) and as shown in Figure 11a marks the position of the white line, a feature of high intensity, that is highly sensitive to the state of oxidation of the Ni. The absorbance is denoted by μT, with μ commonly used to denote XANES data47 and T indicating transmission mode. In the oxidized state (red), the prominent feature is due to transitions from filled 2p states to vacant 3d states. In the reduced state (blue), the 3d states are filled and there is little intensity in the position. Figure 11b shows two XANES spectra at 300 °C in H2, one right before starting the reaction (purple)

greatest adsorption will occur at the lowest temperature. The coverage under different conditions was derived from this set value. To compare the adsorption and desorption processes, the peak areas for 12 min right after the reaction was started and for 5 min after the reaction was stopped were measured at approximately 1 min intervals. The starting or stopping of the reaction was carried out by switching the flow of 2-MTHF on or off using a four-way valve. Transient plots of θ versus time are shown in Figure 9. Figure 9a shows the results after starting the reactions in H2, Figure 9b shows the results after stopping the reaction in H2, while Figure 9c and d show the corresponding results obtained in He. Overall, whether in H2 or in He, the coverage θ at the lowest temperature of 150 °C was much larger than that at higher temperatures. Comparing the coverages in H2 and in He, at 150 °C, it was found that the coverage at steady state was higher in H2. This is probably because in H2 the initially adsorbed 2-MTHF is transformed to other surface species which occupy surface sites. Only at the highest temperature of 350 °C was the coverage higher in He than in H2. This might be because in H2 absorbed organic compounds are removed from the catalyst surface by reaction, while in He the absorbed species do not react. For the case of desorption at 350 °C in H2, the coverage became close to zero immediately after the flow of 2-MTHF was stopped. For the other temperatures and for He, a small amount of adsorbed 2-MTHF persisted on the surface. These overall results suggest that, at lower temperatures like 150 and 200 °C, the adsorbed organic compounds, most of them likely to be 2-MTHF,30 interact with the catalyst surface mainly by hydrogen bonds via the oxygen atom to SiO−H or PO−H. These weakly held species desorbed as the temperature rose, and at higher temperatures, other adsorption modes F


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exposed to the 2-MTHF reactant. The respective changes in absorbance (ΔμT) were small, 0.0050, 0.0052, and 0.0053. At the end of the experiments when the 2-MTHF was cut off but the H2 flow continued, the μT signal decreased but did not return to the initial value. The data is consistent with the adsorption of some of the 2-MTHF on Ni atoms, which increases the Ni oxidation state. After the 2-MTHF reactant is stopped, the μT signal does not return to the original low level because some strongly adsorbed 2-MTHF remains bonded to Ni centers. Considering the corresponding XAFS data in He at low temperatures of 150 and 250 °C, a slightly higher μT signal than that in H2 is observed (Figure 12a and c, crosses) with exposure to the 2-MTHF reactant. The signal is higher in He because the 2-MTHF does not react in the inert gas, whereas in H2 the 2-MTHF can transform to other species with less electron-withdrawing character or can actually desorb. The adsorption process is depicted below (Figure 13).

Figure 11. XANES spectra (a) before and after reduction in H2 and (b) before reaction and 60 min after the start of reaction at 300 °C in H2. The sample was used right after reduction.

and one 60 min after the start of the reaction (orange). The two spectra appear almost identical, but magnification of the scale indicates that there are differences in the white line region, with the spectrum after the start of reaction showing higher intensity, indicating a higher oxidation state of the Ni. This type of small difference is common in XANES analysis37,48 and is measured in experiments to be described subsequently. Figure 12 shows the results of the tracking of the absorbance at 8349 eV at 150, 200, 250, 300, and 350 °C with 2.6 mol %

Figure 13. Schematic of adsorption of 2-MTHF on a Ni2P/SiO2 surface. All organic species are visible by FTIR, while only Ni species interacting with adsorbates are observable by XAFS.

It should be recalled that the FTIR showed different results. At low temperature (blue symbols in Figure 8), the coverage in H2 was higher than that in He, while at high temperatures (red symbols in Figure 8) the coverage in H2 was lower than that in He. This is because FTIR probes all species, while XAFS detects only Ni atoms that are altered in oxidation state because of adsorption. Not all of the 2-MTHF is adsorbed on the Ni, as some 2-MTHF can interact with −OH groups on the silica support or −OH groups associated with surface P. At low temperature in H2, some of the adsorbed 2-MTHF is transformed to other species without desorbing, so the FTIR signal is high. At high temperature in H2, these other species react and desorb so the FTIR signal comes down. Considering next the XAFS data in H2 at the high temperatures of 300 and 350 °C, a large increase in the absorbance (μT) can be seen (Figure 12d and e, square sympols) as the 2-MTHF reactant was introduced. The absorbance signal increases rapidly at first and then levels off, especially for the measurements at 350 °C. The respective changes in absorbance (ΔμT) were 0.017, and 0.018, which were substantially larger than the values of around 0.0050 for the lower temperatures. The data in He at the high temperature of 350 °C show a large increase that persists through the investigated period, likely indicating that species adsorbed on other sites were moving on to Ni sites. The signal with He was higher than that with H2 because reductive desorption does not occur in inert gas, while reductive elimination can occur in the H2 atmosphere.

Figure 12. XAFS absorbance at 8349 eV versus time. Colored plots show the changes in H2 and purple curves in He. (a) 150 °C, (b) 200 °C, (c) 250 °C, (d) 300 °C, (e) 350 °C. The concentration of 2MTHF was 2.6 mol % in 50 mL/min of H2 or He, as indicated in Figure 2.

2-MTHF in H2 and selected temperatures in He at 0.1 MPa. Data in H2 are shown by colored squares ranging from blue to red as the temperature is increased. Data in He are shown by purple crosses. Considering first the XAFS data in H2 at low temperatures of 150, 200, and 250 °C, a small increase in the absorbance (μT) can be seen (Figure 12a−c, square sympols) as the catalyst was G


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absorbance curves show a minor increase (ΔμT = 0.0052− 0.0053) and the curve at 250 °C shows a slight delay of about 8 min before rising, but this could be experimental error due to the low signal levels. At these temperatures, 2-MTHF probably adsorbed quickly on the catalyst surface by weak physisorption or hydrogen bonding to surface −OH groups (Figure 13) on the SiO2 support or the Ni2P. Some of the species may have transferred to Ni centers, causing the slight increase in the absorbance. There was no signal due to CO at these low temperatures, consistent with the lack of reaction. Figure 15 shows a superposition of the absorbance signal at 8349 eV (Figure 12), the coverage of the organic compounds

The activity trends are consistent with the increase of the absorbance, which indicates that the nickel is slightly positively charged during reaction. The μT signal increases rapidly at first (Figure 12) because 2-MTHF is adsorbing. The signal generally levels off, indicating that Ni reaches a stable state as the reaction reaches steady state. 3.5. Reaction Steps. In situ FTIR measurements can give information on the structure and bonding of adsorbed organic compounds,49 and in situ XAFS experiments can provide knowledge on the metal species during reactions like oxidation state, coordination number, and distance between atoms.37,48 If those techniques are applied at the same time, the combined information on adsorbed molecular species and coordination and structure changes of the metal species can give insight on the catalytic reaction mechanism.12,13,50,51 In the previous sections (3.2 and 3.3), the results of the in situ FTIR and XAFS measurements were explained separately. Actually, those data were taken simultaneously, and in this section, the HDO reaction mechanism on Ni2P/SiO2 will be discussed in more detail by combining those results. Briefly, from the in situ FTIR data, the coverage of organic compounds (Figure 8) and the signal from adsorbed CO (Figure 9) were calculated, and from the in situ XAFS data, the increase of the oxidation state during adsorption and reaction was monitored. Figure 14 shows a superposition of data at 200 and 250 °C for the coverage of organic compounds from the peak area of

Figure 15. Superposition at high temperature of absorbance data at 8349 eV and the coverage of the organic compounds and the signal of CO on the catalyst surface during the reactions with 2.6 mol % 2MTHF in H2 at 0.1 MPa and at (a) 300 °C and (b) 350 °C. Exposure to 2-MTHF was stopped at the time shown by the the gray vertical line.

(Figure 8), and the CO signal (Figure 10) calculated from the peak areas of the FTIR spectra at 300 and 350 °C. As mentioned previously, these temperatures are high enough for reaction of 2-MTHF to occur, and there are similarities and differences from the results in Figure 14 at lower temperature. The most similar aspect was that the coverages in the organic compounds (θorg) rose quickly and reached plateaus. However, as expected for higher temperatures, the plateaus were lower, about θ = 0.5−0.6 for 300 °C and about θ = 0.3 for 350 °C, compared to about θ = 0.6−0.7 at 200−250 °C. The notable differences were the much larger increases in the absorption coefficients (ΔμT = 0.017−0.018), indicating chemisorption on Ni sites, and the presence of the FTIR band for CO, indicating reaction. It should be noted that the maximum absorption coefficient ΔμT = 0.38 corresponded to the difference between the passivated state and the fully reduced form of Ni2P (Figure 5), and therefore, the ΔμT values obtained during reaction at high temperature are only about 4% of this amount. These low values can be due to the coverage in oxygenates being about θ

Figure 14. Superposition at low temperature of absorbance data at 8349 eV and coverage of the organic compounds during adsorption with 2.6 mol % 2-MTHF in H2 at (a) 200 °C and (b) 250 °C. Exposure to 2-MTHF was stopped at the time shown by the gray vertical line.

the C−H stretching region (Figure 8) and the absorbance at 8349 eV. At these temperatures, there is almost no reaction and the results (Figure 14) at 200 and 250 °C show similar behavior, with a large, rapid increase of coverage of organic compounds, but only slight changes of the absorbance at 8349 eV. The coverage curves (θorg) rise rapidly in the initial 2 min and then level off at coverages of about 0.6−0.7. The H


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Figure 16. Suggested reaction sequence for the HDO of 2-methyltetrahydrofuran on Ni2P/SiO2 at 300−350 °C and 0.1 MPa. The small arrows indicate electron motions in the bond rearrangements. (a) Left (blue): Main sequence that produces the major product n-pentane. (b) Right (red): Sequence that leads to formation of n-butane and CO as well as more n-pentane.

FTIR demonstrates that the reaction starts by the adsorption of 2-MTHF on OH groups, XAFS indicates that this is followed by an increase in the oxidation state of nickel, and FTIR shows that subsequently formation of carbon monoxide occurs. This sequence of events gives insight into how the reaction proceeds at the steady state, and a possible set of steps is shown in Figure 16. The reaction scheme in Figure 16 is simplified so that it leaves out a number of steps like the initial interaction of 2MTHF with OH groups or the rearrangement of an alkoxide to an alkyl. There are two cycles that begin with 2-MTHF and H2 adsorption steps shown in the central purple rectangle. The main cycle (Figure 16a) is intended to show how the majority product n-pentane is formed, and the side reaction (Figure 16b), how the secondary products, n-butane and CO, are produced from a common intermediate, the 1-pentoxide, which can react in different ways. Figure 17 shows the possible ring-opening steps of adsorbed 2-MTHF. A hydride species can carry out an SN1 type basic attack on a substituted carbon atom (Figure 17a) to cause ring-

= 0.5 (not all on Ni) and nickel being only partially positively charged to Niδ+. There are also significant differences between the two higher temperatures. At 300 °C, the μT value starts to rise with a slight delay and reaches the beginning of a plateau after 20 min. At 350 °C, the μT value rises immediately and reaches a plateau after 10 min. The faster response at 350 °C is expected because of the faster kinetics at higher temperature. Since the μT value is indicative of an oxidation state increase of Ni, these results and those of the FTIR measurements indicate that adsorbed species are first formed on hydroxyl groups and that these subsequently transfer over to the Ni centers. At 350 °C, the transfer is fast. The observation that the μT values keep increasing slightly in both cases indicates that the formation of adsorbed species on the Ni centers is a process that keeps on going beyond the initial fast period. Also, the slightly higher μT value at 350 °C indicates that there are more chemisorbed species at the higher temperature, even though the overall coverage decreases, as shown from the decrease in the FTIR band intensities (Figure 8). This is because the FTIR signal included physisorbed species which are weakly adsorbed. Of course, after the 2-MTHF feed was cut off, the μT value and the coverage went down. Another difference between the results at 300 and 350 °C is the FTIR CO signal (Figure 15), which was not present at lower temperatures. At 300 °C, the CO signal rises with a short delay but takes about 30 min to reach a plateau, much longer than the FTIR signal rise for the organic species and for the Ni absorption coefficient. At 350 °C, the CO signal rises with a much shorter delay and with a much faster approach to the plateau, although the plateau is only about 40% of the value at 300 °C. Again, these results are consistent with the faster kinetics and lower expected coverages at higher temperature. To summarize the overall results from the transient approach to steady state under high temperature conditions,

Figure 17. Possible surface reaction mechanism of the ring-opening reaction. I


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opening to form a 1-alkoxide species. Alternatively, the hydride species can perform an SN2 type nucleophilic attack on the open carbon (Figure 17b) to form a 2-alkoxide species. From the n-C5/n-C4 selectivity of 4 at 300 °C (Figure 10), the main cycle turns over four times to form pentane before the side reaction occurs to produce butane and CO. This explains the delay in the observation of CO. The reaction scheme gives an overall description of the overall transformation that is consistent with all of the observations.

4. CONCLUSIONS A complete study of the hydrodeoxygenation of 2-methyltetrahydrofuran (2-MTHF) was presented which encompassed characterization of the Ni2P/SiO2 catalyst, measurements of the catalytic activity, observations of adsorbed species by in situ Fourier transform infrared (FTIR) spectroscopy, and monitoring of the state of Ni by in situ X-ray absorption spectroscopy (XAS). The studies were conducted at atmospheric pressure and 200−350 °C. The results are consistent with initial interaction of 2-MTHF with hydroxyl groups on the surface of the catalyst, as seen by a rapid decrease in the O−H stretching frequency and a simultaneous fast growth in C−H bands. This is followed by transfer of the organic moiety to Ni centers, as observed by XAS, a process which is faster at higher temperatures. Reaction proceeds by ring-opening of the 2-MTHF ring to form adsorbed alkoxide species, which are deoxygenated to the major product npentane. One branch of the reaction leads to the formation of pentanal, which can undergo decarbonylation to form n-butane and CO. A portion of the CO is adsorbed on the surface and is observed by infrared spectroscopy. The simultaneous application of FTIR and XAS permits a reasonably detailed picture of the reaction.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Atsushi Takagaki: 0000-0002-7829-3451 S. Ted Oyama: 0000-0003-3409-1812 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the US Department of Energy, Office of Basic Energy Sciences, through grant DE-FG02963414669.

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Combined In Situ XAFS and FTIR Study of the Hydrodeoxygenation

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