Synergistic Contribution of the Acidic Metal OxideâMetal Couple and

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Synergistic Contribution of the Acidic Metal Oxide-Metal Couple and Solvent Environment in the Selective Hydrogenolysis of Glycerol: a Combined Experimental and Computational Study Using ReOx-Ir as the Catalyst Jithin John Varghese, Liwei Cao, Christopher Robertson, Yanhui Yang, Lynn F. Gladden, Alexei A. Lapkin, and Samir Hemant Mushrif ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03079 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018

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Synergistic Contribution of the Acidic Metal OxideMetal Couple and Solvent Environment in the Selective Hydrogenolysis of Glycerol: a Combined Experimental and Computational Study Using ReOxIr as the Catalyst Jithin John Varghese,a 1, Liwei Cao,a,b 1 Christopher Robertson,a,b Yanhui Yang,a,c,d Lynn F. Gladden,a,b* Alexei A. Lapkin,a,b* Samir H. Mushrifa,c,e* a

Cambridge Centre for Advanced Research and Education in Singapore (CARES) Ltd., Campus

for Research Excellence and Technological Enterprise (CREATE), CREATE Tower, 1 CREATE Way, Singapore, 138602 b

Department of Chemical Engineering and Biotechnology, University of Cambridge, Philippa Fawcett Drive, Cambridge CB3 0AS, U.K. c

School of Chemical and Biomedical Engineering, Nanyang Technological University Singapore, 62 Nanyang Drive, Singapore, 637459

d

School of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing, China

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Department of Chemical and Materials Engineering, University of Alberta, 9211 - 116 St. NW, Edmonton, Alberta, T6G 1H9, Canada

Email: [email protected] (LFG), [email protected] (AAL), [email protected] (SHM)

1

Both authors contributed equally to this work.


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Abstract

Comprehensive mechanistic insights into the aqueous phase hydrogenolysis of glycerol by the ReOx-Ir catalyst were obtained by combining density functional theory (DFT) calculations with batch reaction experiments, and detailed characterization of the catalysts using XRD, XPS and FTIR techniques. The role and contribution of the aqueous acidic reaction medium was investigated using NMR relaxometry studies and were complemented with molecular dynamics and DFT calculations. At higher glycerol concentration, the enhanced competitive interaction of glycerol with the catalyst improved the conversion of glycerol. Sulfuric acid increased the concentration of glycerol within the pores of the catalyst and enhanced the propensity for dissociative adsorption of glycerol on the catalyst, explaining the promotional effect of acid during hydrogenolysis. Partially reduced and dispersed Brønsted acidic ReOx clusters on metallic Ir nanoparticles facilitated dissociative attachment of glycerol and preferential formation of the primary propoxide. The formation of the dominant product, 1,3-propanediol (1,3-PDO), results from the selective removal of the secondary hydroxyl of glycerol, with comparatively low activation barrier of 123.3 kJ mol-1 in the solid Brønsted acid catalyzed protonation-dehydration mechanism or 165.2 kJ mol-1 in the direct dehydroxylation mechanism. The formation of 1propanol (1-PO) is likely to follow a successive dehydroxylation pathway in the early stages of the reaction. Although 1,3-PDO is less reactive than 1,2-propanediol (1,2-PDO), it preferentially adsorbs on the catalyst in a mixture containing glycerol to form 1-PO. The thermodynamically favorable pathway involving dehydrogenation, dehydroxylation and hydrogenation elementary steps led to the dominant production of 1,2-PDO on pure Ir catalyst with high C-O bond cleavage barrier of 207.4 kJ mol-1. The optimum ReOx-Ir catalyst with Ir/Re ratio of 1 exploits the synergy of the sites of both the components. The detailed insights presented here would guide in the rational 3 ACS Paragon Plus Environment

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selection of catalysts for the hydrogenolysis of polyols and in the optimization of reaction parameters.

Keywords: Glycerol hydrogenolysis, multifunctional catalyst, reaction mechanisms, competitive solvent effects, DFT, NMR relaxometry


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1. Introduction Biomass is an abundant renewable resource to produce fuels and chemicals.1 Catalytic conversion of biomass-derived feedstock and platform chemicals to high value derivatives has attracted significant interest in the past decade.2-4 Glycerol is one such platform molecule1 which is a by-product of biodiesel manufacture and is generated in large volumes. Though it is a potential bottleneck in the bio-diesel process, there is an opportunity for its use as cheap chemical feedstock.5-9 Among the many processes for the transformation of glycerol to chemicals and fuels, catalytic hydrogenolysis or hydrodeoxygenation is an effective route to produce valuable diols.5, 10-16

Glycerol hydrogenolysis to 1,3-propandiol (1,3-PDO), a higher value derivative than 1,2-

propandiol (1,2-PDO), is highly desirable. 1,3-PDO is a monomer to produce various polymers, including polytrimethylene terephthalate (PTT).5 Although biochemical pathways to convert glycerol to 1,3-PDO have been demonstrated, the bio-processes are limited by low yield, complex process, tolerance to high substrate concentration and its purity,17 making inorganic catalytic hydrogenolysis a promising route for the conversion of glycerol to 1,3-PDO. In addition to noble metals, copper based catalysts have been demonstrated to be effective for the selective hydrogenolysis of glycerol to 1,2-PDO.18 However, the production of 1,3-PDO requires multicomponent-multifunctional noble metal catalysts, typically comprising metals like Ir, Rh, Ru, Pt, with Re, W, Mo species existing as finely dispersed oxides to selectively cleave the secondary C-O bond of glycerol to produce 1,3-PDO.15, 19-27 Metal oxides vary in their abilities to assist in hydrogenolysis reactions and these differences are attributed to the differences in their structures on the active metal.28 ReOx-Ir/SiO2 catalyst in the presence of sulfuric acid23 and IrRe/KIT-6 catalyst (Ir-Re alloy on silica) in the presence of a solid acid Amberlyst26 have been shown to be among the most effective catalyst systems in the liquid phase batch conversion of 5 ACS Paragon Plus Environment

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glycerol to 1,3-PDO. However, the exact mechanism of the formation of 1,3-PDO and the role of the acid promoter in improving the catalytic performance of these multicomponent catalysts have not been established. In spite of numerous efforts in optimization of the catalyst systems and reaction conditions, the yield of 1,3-PDO remains unsatisfactory for the commercialization of the process.26 Optimization of reaction conditions and engineering of the process for higher yields of 1,3-PDO require detailed understanding of the reaction mechanism and energetics. Additionally, development of alternative cheaper catalysts and rational design of catalysts for higher yield of 1,3-PDO are only possible with a comprehensive understanding of the pertinent active sites on the catalyst and the reaction mechanisms. Although many research groups have investigated the hydrogenolysis of glycerol to propanediols, there is no consensus on the mechanism of the formation of 1,3-PDO on a multicomponent metal oxide-metal catalyst like the ReOx-Ir system. The proposed mechanisms of glycerol hydrogenolysis are 1) dehydration-hydrogenation, 2) dehydrogenation-dehydrationhydrogenation, and 3) direct hydrogenolysis,11, 13-15, 29-31 as shown in Scheme 1. The formation of 1,2-PDO as the dominant product via acetol as a precursor18 (cf Scheme 1) is believed to be thermodynamically controlled.13,

32

On a catalyst like ReOx-Ir, acetol is not detected as an

intermediate, and the catalyst is believed to be multifunctional, with the metal oxide 1) providing anchoring sites for glycerol to facilitate a hydride attack at the secondary carbon through the metal oxide-metal interface in the direct hydrogenolysis mechanism

23

(cf. Scheme 1) or 2) providing

Brønsted acidity to facilitate protonation and dehydration of glycerol (cf. Scheme 1) via carbenium/oxocarbenium intermediates.27, 29


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Scheme 1. Mechanisms for the hydrogenolysis of glycerol to propanediols, as proposed in the literature. 11, 13-15, 29-31 In this context we have set out to identify the active sites on the multicomponent catalyst systems and elucidate the specific role of each component, to establish the reaction mechanism and pathway, with key intermediates, activation barriers and energetics of the elementary steps, and to rationalize the effects of the various reaction parameters, including the effect of sulfuric acid, in determining the conversion and product selectivity. Combining experiments and computations, we present a comprehensive analysis of the liquid phase batch hydrogenolysis of aqueous glycerol on


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the ReOx-Ir and Ir catalysts. To the best of our knowledge, this is the first comprehensive report revealing the role of acid promoter in enhancing hydrogenolysis, elucidating multiple competitive pathways for glycerol hydrogenolysis to various alcohols and explaining the selectivity to 1,3PDO on the metal oxide dispersed metal catalyst. These molecular mechanistic insights not only explain the observed trends in the conversion of glycerol and product selectivity on these catalysts, but also form guidelines for the rational design and development of improved catalysts and processes. The manuscript is organized as follows. The experimental and computational methods employed in this investigation are provided in Section 2. Results from the extensive characterization of the catalysts, together with the rationale for the chosen computational model systems are discussed in Section 3.1. The performance of different catalysts in the hydrogenolysis of glycerol is presented in Section 3.2. The reaction parameters and their influence on the hydrogenolysis of glycerol on the ReOx-Ir catalyst are presented and described in Section 3.3. The NMR relaxometry analyses of the solvent effect and the contribution of sulfuric acid in enhancing catalytic performance are presented and discussed in this section. The mechanisms of hydrogenolysis of glycerol on the ReOx-Ir catalyst and energetics of different elementary steps along multiple reaction pathways forming propanediols and propanols, as obtained from DFT calculations, are presented and discussed in Section 3.4. The NMR based displacement studies revealing the competition between glycerol and propanediols to interact with the catalyst surface and the influence of such competitive interactions on the product selectivity are presented in this section. The mechanisms in this investigation are also compared against those in the literature in this section. The mechanism of hydrogenolysis of glycerol on the Ir catalysts and energetics of the different elementary steps along different reaction pathways forming propanediols are presented and discussed in Section 3.5. The


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change in conversion of glycerol and product selectivity due to change in reaction mechanisms during hydrogenolysis with the ReOx-Ir catalysts containing different Ir/Re proportion is discussed in section 3.6. The results are summarized, and conclusions are drawn in Section 4.

2. Experimental and Computational methods 2.1. Catalysts synthesis, characterization and batch hydrogenolysis reactions Catalysts were synthesized by the wet impregnation method, using mesoporous silica as the support and the corresponding Ir and Re metal salts. The impregnation process was followed by drying, calcination in air and reduction pre-treatment in hydrogen. Synthesized catalysts included 1) Ir alone, 2) ReOx alone, 3) physical mixture of Ir and ReOx (Ir + ReOx) and 4) the sequentially impregnated ReOx-Ir catalyst (ReOx-Ir). Details of the catalyst synthesis process and of all the materials and apparatuses used are given in Section S1.1 of the Supporting Information. The surface area of the catalysts were measured using the BET method and the pore volumes were calculated using the BJH method. The chemical composition of the synthesized catalysts was determined using the ICP-OES analysis. XRD patterns were recorded to identify the crystalline states of both Ir and Re metals in the catalysts. XPS chemical shift analysis was done to identify the nature of the metals and their oxidations states in the catalysts. Adsorption of pyridine followed by DRIFT spectroscopy was done to identify and differentiate the nature of acid sites on the catalysts. Details of the characterization methods employed in this paper are provided in Section S1.2 of the Supporting Information. The hydrogenolysis of glycerol was performed in a stainless-steel autoclave using catalysts which were reduced in hydrogen atmosphere at 473 K. The standard conditions for the reaction, unless specified otherwise, were as follows: reaction temperature - 393 K, initial hydrogen pressure - 8 MPa, reaction time - 24 h, glycerol amount - 2 g, water amount - 8 g, H2SO4 (H+/Ir = 1) - 1 mg


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and catalyst amount - 100 mg. The reaction mixture was analyzed by a gas chromatograph-mass spectrometer (GC-MS) equipped with a flame ionization detector and a HP-5 capillary column (Agilent Technologies, Inc.) and liquid chromatograph with C18 column (Agilent Technologies, Inc.). The major products in the hydrogenolysis of glycerol were 1,3-PDO, 1,2-PDO, 1-propanol (1-PO) and 2-propanol (2-PO) while the degradation products such as ethanol, ethane, methanol, and methane were detected in the gas phase. The conversion and the selectivity were calculated on a carbon basis as follows: 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 (%) = 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 =

(𝑚𝑚𝑚𝑚𝑚𝑚 𝑜𝑜𝑜𝑜 𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐) (𝑚𝑚𝑚𝑚𝑚𝑚 𝑜𝑜𝑜𝑜 𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔 𝑐𝑐ℎ𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎)

× 100,

(𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 𝑜𝑜𝑜𝑜 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝) × (𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛 𝑜𝑜𝑜𝑜 𝑐𝑐𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 𝑖𝑖𝑖𝑖 𝑡𝑡ℎ𝑒𝑒 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝) (𝑠𝑠𝑠𝑠𝑠𝑠 𝑜𝑜𝑜𝑜 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 𝑜𝑜𝑜𝑜 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔)

Further details of the catalytic reactions and performance evaluation are provided in Section S1.3

of the Supporting Information. 2.2. NMR methods 1

H NMR experiments were carried out on a Bruker DMX 300 MHz spectrometer.

Characterization of the relaxation behavior within each sample was achieved using a standard T1T2 pulse sequence making use of inversion recovery to encode for T1, as shown in Figure 1a. Measurements were performed using 8 repeat scans and 32 delays ranging from 1 ms up to 6 s. Characterization of the diffusive behavior within each sample was achieved using a standard D-T2 pulse sequence making use of APSGTE to encode for D, as shown in Figure 1b. Measurements were performed using a gradient pulse duration of 2 ms, an observation time of 20 ms and 32 gradient strengths linearly spaced between 0.001 and 11 T/m. Inversion of the 2D correlation data sets was achieved using Tikhonov regularization with the GCV method for optimizing the choice of smoothing parameter.33 This combination has been shown to be, in general, the best method for


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inverting 2D NMR data sets in terms of both speed and robustness.34 The L1 regularization method, with a manually selected smoothing parameter, has also been used in select cases to improve the resolution of peaks.35

Figure 1. a) T1-T2 pulse sequence using a CPMG echo train to encode for T2 and inversion recovery to encode for T1. b) D-T2 pulse sequence using a CPMG echo train to encode for T2 and APGSTE to encode for D. To allow for consistent sample preparation, the < 100 μm particles were compressed into cylindrical disks (diameter = 2 mm and length = 10 mm) and subsequently broken into ≈ 1 mm diameter particles. Samples were prepared for NMR experiments by soaking the ≈ 1 mm diameter particles in the relevant liquid for at least 24 hours. The pellets were removed from the liquid prior to experimentation and any excess liquid dried from the surface. For the displacement studies, the pellets were soaked in the displacing liquid for the amount of time specified. Again, any excess liquid on the pellet surfaces was dried prior to any NMR experiments. 11 ACS Paragon Plus Environment

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2.3. Computational methods Simulations were performed with the plane-wave pseudopotential implementation of Density Functional Theory (DFT)36-37 using the ab initio total-energy and molecular-dynamics program VASP (Vienna ab-initio simulation program) developed at the Fakultät für Physik of the Universität Wien.38 The opt-B88-vdW exchange correlation functional,39 which incorporates the long range van der Waal’s interactions was used. Previous reports have recommended the incorporation of van der Waal’s corrections for investigating the interaction of glycerol with transition metal surfaces.40-42 The Projector Augmented Wave (PAW)43 method for the treatment of the inner core, with a plane wave cut-off energy of 400 eV were used in all calculations. The nudged elastic band (NEB) method implemented in VASP was used to locate the transition state (TS). Complete vibrational analysis was performed on all the identified TSs and the existence of only one imaginary frequency confirmed the TS nature of the saddle point. The vibrational mode corresponding to the imaginary frequency was visually inspected to ensure that it represents the relevant reaction coordinate in each case. Using the opt-B88-vdw functional, the bulk lattice constant of Ir was optimized to 3.888 Å, which is close to the experimentally determined value of 3.839 Å. Description of the model systems and the rationale for the choice are provided in Section 3.1.2. Additional details on the computational methods employed in this investigation are presented in section S2 of the Supporting Information. The activation energy barriers reported in this article were calculated as the difference in the energy of the TS and its corresponding initial state (IS), which was the physisorbed glycerol on the catalyst surface, or the stable intermediate from the previous elementary step. The reaction energies were calculated as the difference in energy of the stable intermediate/product and the corresponding IS/ reactant. All energies reported are electronic energies unless specified otherwise. Activation free energy barriers at 393 K, 12 ACS Paragon Plus Environment

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calculated based on the vibrational frequencies obtained within the rigid rotor harmonic oscillator approximation are reported for selected rate determining steps.

3. Results and Discussion 3.1. Catalyst characterization and the rationale for the chosen model catalyst systems 3.1.1. Catalyst characterization Physical characterization of all the catalysts suggests that the Ir and ReOx components are highly dispersed on the silica support. Some catalyst particles can be ascertained to be located inside the mesoporous channels of silica as there was a decrease in the pore volume and surface area of the silica, measured by N2-sorption experiments, as can be seen in Table 1. Table 1 Physical properties of the synthesized catalytic materials Sample

Ir/Re loading SBET [% wt] a [m2g-1]

VP [cm3g-1]

dp [nm]

davg. [nm] b

SiO2

--

700

0.6

6

--

Ir

4.0/--

552

0.39

5.6

1.9

ReOx

--/3.8

609

0.48

5.1

--

ReOx-Ir

4.1/3.9

560

0.43

5.0

2.0

a Actual metal loading on sample catalysts are determined by ICP-OES. b Mean metal particle size was estimated from XRD pattern. The XRD patterns for the ReOx-Ir catalyst, before and after reduction, are shown in Figure 2a. Analysis of the patterns from the as-synthesized and pre-reduced catalysts show that the reduction pre-treatment completely reduced IrO2 to metallic Ir, while the peaks due to ReOx species were not detected, suggesting their high dispersion. The characteristic dominant peak corresponding to Ir(111) was observed for the reduced Ir catalyst as well as ReOx-Ir catalysts with different Ir/Re 13 ACS Paragon Plus Environment

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metal loading ratios, as shown in Figure 2b. Further details regarding the XRD patterns are provided in section S3.1 of the Supporting information. The metal oxidation states for the ReOx-Ir catalysts after the reduction pre-treatment were investigated by ex situ XPS in 4f region of Ir and 4f region of Re. The presence of Ir 4f doublets at binding energies of 60.5 and 63.3 eV, as shown in Figure 2c, is the characteristic of Ir0 (4f7/2 = 60.3 ± 0.2 eV, 4f5/2 = 63.4 ± 0.2 eV). Combining the observations from XRD and XPS, it can be claimed that IrO2 particles on the sample catalyst were completely reduced into Ir during the reduction pre-treatment. The existence of ReOx species on the catalysts was confirmed by the XPS analysis of the binding energies of the 4f electrons of Re. The Re 4f region indicates the presence of several Re oxidation states, predominantly around 0, +4 and +7, as shown in Figure 2d. Because of the oxophilicity of Re, it is possible that Re metal is oxidized during XPS sample preparation due to contact with air and this could be one of the reasons for Re being detected in the higher oxidation states, as suggested in the literature.26 The FT-IR spectra of the chemisorbed pyridine on the Ir, ReOx, Ir + ReOx and ReOx-Ir catalysts, as shown in Figure 2e, were used to distinguish different kinds of acid sites on each catalyst. The band at 1457 cm-1 is attributed to pyridine interaction with Lewis acid sites. The bands at 1540 cm1

and 1635 cm-1 on the ReOx, Ir + ReOx and ReOx-Ir catalysts correspond to pyridinium ions

formed at Brønsted acid sites on these catalysts, while the band at 1490 cm-1 could be attributed to both Lewis and Brønsted types of acidic sites.44 No IR absorption corresponding to pyridine adsorption at the Brønsted acid sites was detected on the Ir catalyst. Summary of the analysis of the pyridine chemisorption FT-IR spectra of different catalysts, as shown in Figure 2e, is given in Table S1 of the Supporting Information. Based on this data, it is reasonable to conclude that the partially reduced ReOx species are the primary contributors of the Brønsted acidity of the catalysts.


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Figure 2. XRD patterns of a) the as synthesized and pre-reduced ReOx-Ir catalyst before catalytic reactions and b) the pre-reduced ReOx-Ir catalyst with different Ir/Re metal loading ratios. High resolution XPS scan of ReOx-Ir catalyst after the reduction at 473K, c) in the Ir 4f region and d) in the Re 4f region. e) DRIFT spectra of adsorbed pyridine on the single component (Ir and ReOx) and multicomponent catalysts (Ir + ReOx catalyst, ReOx-Ir catalyst). 15 ACS Paragon Plus Environment

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3.1.2. Model computational systems Based on the catalyst characterization presented in Section 3.1.1, we developed the model catalyst systems for DFT calculations, as shown in Figure 3. Figure 3a is a schematic representation the overall structure of the ReOx-Ir catalyst comprising the Ir nanoparticle on which the ReOx clusters are finely dispersed. The atomistic models used in this investigation are shown in Figures 3b and 3c. During glycerol hydrogenolysis, the silica support does not play a direct role in the reaction chemistry and hence was not included in the computational model system. Since all catalytic performance tests were performed with the pre-reduced catalyst, and our XRD and XPS analyses showed that Ir existed as metallic Ir nanoparticles, with majority Ir(111) sites, the model catalyst was chosen to comprise of Ir(111) surface. Ir(111) surface was modelled as a 4x4 supercell with three atomic layers (48 Ir atoms) and 12 Å of vacuum over the surface. 3x3x1 MonkhorstPack45 K-point mesh was used to sample the Brillouin zone. The ReOx dispersed Ir nanoparticle was modelled using an Ir(111) surface as a 5x5 supercell with three atomic layers (75 Ir atoms) with 15 Å of vacuum over the surface. 3x3x1 Monkhorst-Pack K-point mesh was used to sample the Brillouin zone. Literature suggests that ReOx on Ir is likely to be a three dimensional cluster.46 However, we have chosen an Re3O6 based planar model structure for the following reasons. The Re3O6 cluster, with three Re=O species and three Re-O-Re species has Re in +4 oxidation state and is representative of the +4 oxidation state of Re identified in our XPS analysis. Its initial gas phase geometry was obtained from the literature.47


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Figure 3. a) Schematic illustration of the ReOx-Ir composite catalyst showing ReOx clusters dispersed around the Ir nanoparticle. The black dashed box is the representaiton of the model catalyst system used in the computational investigation. Models of the ReOx clusters dispersed on the Ir catalyst represented by b) Re3O6H2 cluster on Ir(111) surface, and c) Re3O6H3 cluster on Ir(111) surface. Blue balls represent Ir, orange Re, red oxygen and white hydrogen.

Since the pyridine-adsorption and FT-IR analyses suggested that the partially reduced ReOx species are the primary contributors of the Brønsted acidity of the catalysts, all reactions were investigated on the Re3O6 cluster, which was modified to form Re3O6H2 by replacing two Re=O species by Re-OH as shown in Figure 3b. Re3O6H3 cluster, as shown in Figure 3c, is formed by replacing two Re=O species by Re-OH and one Re-O-Re species by Re-OH-Re. These species represent the Brønsted acid sites on the ReOx clusters. The reduction pre-treatment of the ReOx-Ir catalyst is believed to be one of the reasons for the acidity of the catalyst, as it introduces Re-OH


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species;27 higher temperatures during the reduction are believed to generate acidic species of higher strength.48 The other possible reason is the dissociation of water on the Re site, which also leads to the formation of Re-OH.26 Both these scenarios were investigated using DFT simulations and the results, which suggest the likelihood of both the pathways for the formation of different kinds of hydroxyls on the ReOx clusters, are presented in Section S3.2 of the Supporting Information. Pyridinium ion formation at the Re-OH and Re-OH-Re species in our model catalysts was studied. The characteristic vibrational frequencies obtained from the DFT calculations match those obtained from the FT-IR analysis of pyridine adsorption confirming that the hydroxyls on ReOx are the Brønsted acid sites. The details of these simulations and results are also presented in Section S3.2 of the Supporting Information. In the optimized Re3O6H2/Ir(111) and Re3O6H3/Ir(111) systems, each Re atom occupies the fcc hollow site or the bridge site on the Ir(111) surface, as shown in Figure 3b and 3c, in agreement with the literature.46 The Re oxidation state between +3 and +4 in both our model structures, Re3O6H2 and Re3O6H3, agrees with our XPS analysis and with the literature reporting it to be between 2 and 4 in the reduction temperature range of 450 to 500 K.46 The discussion presented here points to the validity of the model computational systems to represent the partially reduced ReOx clusters in the ReOx-Ir catalyst. 3.2. Performance of various catalysts in the hydrogenolysis of glycerol with and without sulfuric acid promoter Ir, ReOx, Ir + ReOx and ReOx-Ir catalysts were individually tested for the hydrogenolysis of glycerol, both, in the presence and absence of sulfuric acid. Conversion of glycerol and product selectivity at the end of 24 hours are reported in Table 2.


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Table 2. Conversion of glycerol and product selectivity obtained from its hydrogenolysis on four different catalytic systems, in the presence and absence of the sulfuric acid. Reaction conditions are described in Section 2.1. No .

Cataly st

Conversion [%]

Selectivity to [%]

H2SO No 1,3-PDO 1,2-PDO 1-PO 2-PO H2SO 4 H2SO No H2SO No H2SO No H2SO No 4 H2SO 4 H2SO 4 H2SO 4 H2SO 4 4

4

4

4

1

Ir

7

4

12

5

74

79

12

5

2

3

2

ReOx

2

2

8

1

56

58

2

1

1

1

3

Ir + 9 ReOx

5

10

7

71

76

10

7

3

4

4

ReOxIr

28

35

33

18

24

32

20

10

10

50

The Ir catalyst, the ReOx catalyst and the Ir + ReOx catalyst resulted in very low conversions of glycerol, the highest being 9% for the Ir + ReOx catalyst, in the presence of sulfuric acid. The ReOx-Ir catalyst on the other hand gave 50% conversion of glycerol in the presence of sulfuric acid. The inability of the metal, the metal oxide and their physical mixture to give satisfactory yield of 1,3-PDO highlights the significance of the synergy between the chemically interacting metal and the dispersed metal oxide in carrying out the chemistry. This synergy is discussed further in section 3.4.2. Conversion of glycerol is much lower in the absence of sulfuric acid for all the catalytic systems and this trend is in agreement with the literaruture.23 In all cases, except the ReOx-Ir catalyst, the single dominant product is 1,2-PDO, while 1,3-PDO and 1-PO are produced in negligible amounts. On the ReOx-Ir catalyst, 35% 1,3-PDO and 32% 1-PO were obtained as


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dominant products. The addition of sulfuric acid does not affect selectivity to the major products in any of the cases. The ReOx-Ir catalyst being the most effective, detailed investigations on the influence of various reaction parameters on the kinetics of glycerol hydrogenolysis and product selectivity on this catalyst system were conducted and the results are reported in Section 3.3. Analysis of the liquid phase after filtration and removal of the catalyst from the reaction mixture showed that leaching of the metal components of the catalyst is not significant and the catalysts could be reused up to two times without significant loss in activity and selectivity. 3.3. Influence of various reaction parameters on kinetics of glycerol hydrogenolysis and product selectivity on ReOx-Ir catalyst Conversion of glycerol was found to be influenced by the initial concentration of glycerol, the reaction temperature and the presence of sulfuric acid promoter, in addition to the nature and composition of the catalyst. The conversion of glycerol under different reaction conditions and changes in product selectivity are shown in Figure 4 and are discussed in this section. 3.3.1. Concentration of aqueous glycerol Conversion of glycerol increased with the increase in its initial concentration, but beyond the 20% initial concentration there seems to be no enhancement, as can be seen from Figure 4a. Although the conversion was much lower, a similar trend was observed in the reactions without the sulfuric acid promoter and this is shown in Figure S3 of the Supporting Information. Since the product selectivity does not change with the initial concentration of glycerol, the increase in glycerol concentration is believed to favorably influence the interaction of glycerol with the catalyst without any change in the reaction mechanisms and pathways.


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Figure 4. Variations in the conversion of glycerol and product selectivity during the sulfuric acid promoted hydrogenolysis of glycerol on the ReOx-Ir catalyst at different a) initial glycerol concentrations, b) reaction temperatures, c) reaction times and d) metal-metal oxide loading ratios. Standard reaction conditions, except for the parameter under investigation are: reaction temperature - 393 K, initial hydrogen pressure - 8 MPa, reaction time - 24 h, glycerol amount - 2 g, water amount - 8 g, H2SO4 (H+/Ir = 1) - 1 mg and catalyst amount - 100 mg. To understand the interaction of glycerol with the catalyst at varying concentrations of glycerol in the aqueous solutions, NMR relaxometry studies were performed. Comparison of the individual T1/T2 ratios of adsorbed species have been shown to characterize the relative interaction strengths of a molecule with different catalyst surfaces,49 and the relative strengths of different molecules with the same catalyst surface.50 Furthermore, D’Agostino et al.51 proposed that the T1/T2 ratio of the reactant to that of the solvent be considered when investigating competitive interactions of the 21 ACS Paragon Plus Environment

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solvent and the reactant with the catalyst surface. In simple terms, a large value of this ratio implies that the surface interacts more favorably with the reactant, whereas a small value implies that the solvent will preferentially adsorb on the surface. While the work of D’Agostino et al.51 looked at the solvent and reactant when adsorbed separately, the principle has been extended here to quantify the relative ability of glycerol to interact with the surface in aqueous solutions. Thus, the parameter 𝛽𝛽, given by equation (1) will be used to compare the ability of glycerol to interact with the catalyst surface as a function of its concentration in the mixture in which catalyst pellets were soaked. 𝛽𝛽 =

[𝑇𝑇1 /𝑇𝑇2 ]𝐺𝐺 [𝑇𝑇1 /𝑇𝑇2 ]𝑊𝑊

(1)

Where [𝑇𝑇1 /𝑇𝑇2 ]𝐺𝐺 represents the 𝑇𝑇1 − 𝑇𝑇2 ratio of glycerol and [𝑇𝑇1 /𝑇𝑇2 ]𝑊𝑊 represents the 𝑇𝑇1 − 𝑇𝑇2 ratio

of water. The T1-T2 distributions for the samples prepared by soaking the Ir-ReOx catalyst particles in 2, 5, 20 and 80 wt% glycerol solutions are shown in Figure 5. The peaks A and B, shown in Figure 5a, have been assigned based on comparison with the relevant D-T2 distribution. An example is shown in Figure 5c where it is clear that peak B is slower diffusing than peak A. This is characteristic of glycerol in aqueous solutions and as such peak A is assigned to water and peak B to glycerol.


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Figure 5. T1-T2 distributions for samples prepared by soaking the Ir-ReOx catalyst particles in a) 2%, b) 5%, d) 20% and, e) 80 wt% aqueous glycerol solutions. c) The D-T2 distribution for the sample prepared by soaking the Ir-ReOx catalyst particles in a 5% aqueous glycerol solution. The peaks A and B have been assigned to water and glycerol respectively based on comparison of the relative rates of diffusion in the D-T2 distribution. The parameter 𝛽𝛽 was compared with conversion as a function of the concentration in the mixture

used to soak the catalyst pellets in Figure 6. Increasing the glycerol concentration increases 𝛽𝛽, suggesting an enhancement in the interaction of glycerol with the catalyst in comparison to that of water. This dependence plateaus at glycerol concentrations above 20% in a similar manner to the conversion of glycerol. Intra-pellet compositions have also been obtained by integrating peaks A and B in each distribution in Figure 5 and the intra-pellet glycerol concentrations of each sample are also presented in Figure 6. In general, the trend in the competitive interaction parameter 𝛽𝛽 23 ACS Paragon Plus Environment

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correlates well with the trend in conversion and does not correlate with the monotonous increase in the intra-pore glycerol concentration. This suggests that competitive adsorption of glycerol and water is responsible for the trend in the overall glycerol conversion that is observed with changes in glycerol concentration.

Figure 6. Glycerol conversion (right axis) versus 𝛽𝛽 (left axis) as a function of glycerol

concentration in the mixture in the absence of sulfuric acid. The actual intra-pellet compositions (glycerol wt%) obtained via integration of the T1-T2 distributions are shown as intrapore for each sample in black. 3.3.2. Reaction temperature and batch reaction time The reaction temperature had a pronounced effect on both, the conversion of glycerol and the product selectivity, as shown in Figure 4b. Although 1,3-PDO was obtained with high selectivity (57%) at 373 K, the conversion of glycerol was very low (~11%). The conversion of glycerol increased sharply when temperature was increased from 373 to 393 K, although with a drop in the selectivity to 1,3-PDO. The observed trend suggests that 1,3-PDO is a kinetically preferred product. Further increase in temperature increased the conversion of glycerol, but, at the cost of 24 ACS Paragon Plus Environment

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selectivity to 1,3-PDO and the formation of degradation products, gases like methane and ethane which were observed. Based on the trends in Figure 4b, a temperature of 393 K was found to provide a balance between the conversion and 1,3-PDO selectivity. An increase in the conversion of glycerol was observed with the increase in the batch reaction time from 24 to 48 hours, as shown in Figure 4c. This resulted in a slight decrease in the selectivity to 1,3-PDO, suggesting subsequent reactions of the 1,3-PDO. This is discussed further in Section 3.4.3. 3.3.3. Ir/Re ratio in the ReOx-Ir catalyst To understand the effect of Ir/Re ratio on the conversion of glycerol and product selectivity further experiments were performed by changing Ir and Re loadings in the ReOx-Ir catalyst. The conversion of glycerol and product selectivity changed significantly with the change in the composition of the catalyst, as shown in Figure 4d. Conversion of glycerol was very low at low Ir/Re ratio with 1,2-PDO as the dominant product. Conversion increased with the increase in the Ir/Re ratio and peaked at the Ir/Re ratio of 1 with 4% metal loading; this composition also gave the highest selectivity towards the desired 1,3-PDO. Further increase in the Ir/Re ratio decreased the conversion and the selectivity to 1,3-PDO. The drastic changes in the conversion of glycerol and product selectivity with change in the Ir/Re ratio implies change in reaction mechanisms and these are discussed further in Section 3.6. 3.3.4. H2SO4 promoter Irrespective of the type of catalyst, the presence of the acid primarily influenced the conversion of glycerol without significantly affecting the selectivity, as shown in Table 2. Hence, we believe that it serves to improve the interaction of glycerol with the catalyst and does not affect the reaction mechanism. The possibility of sulfuric acid facilitating acid catalyzed dehydration of glycerol was


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investigated using DFT calculations and was found to be unlikely and this is discussed in Section 3.4.2. To understand the interaction of glycerol with the catalyst in the presence and absence of the acid in the aqueous solutions, NMR relaxometry studies were performed. The T1-T2 distributions for the samples prepared by soaking the ReOx-Ir catalyst particles in 5, 20 and 80 wt% glycerol solutions with added H2SO4 (0.1% by mass) are given in Figures 7 a-c. Comparisons between the 𝛽𝛽 values and glycerol conversion are provided, both, with and without the addition of acid in Figures 7d and 7e respectively. Intra-pellet liquid compositions are also shown for comparison.

Figure 7. T1-T2 distributions for samples prepared by soaking the Ir-ReOx catalyst particles in a) 5%, b) 20% and c) 80 wt% aqueous glycerol solutions with 0.1% (by mass) sulfuric acid added. Glycerol conversion (right axis) versus 𝛽𝛽 (left axis) as a function of glycerol concentration in the original mixture d) without sulfuric acid and e) with the addition of acid. The actual intra-pellet

compositions obtained via integration of the T1-T2 distributions are shown for each sample as intrapore in black.


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While there are slight changes in the value of 𝛽𝛽 when acid is added, the most marked difference

between the two scenarios is the increase in the intra-pellet concentration of glycerol. Hence, at lower concentrations of glycerol in the mixture, it can be said that acid facilitates the presence of higher concentration of glycerol within the silica pores than the bulk solution, which may contribute to an enhancement in the conversion of glycerol. Silica surfaces have been shown to exhibit high proton/hydronium ion mobility52 and it is possible that hydronium ions resulting from the added sulfuric acid are found concentrated along the silica-liquid interface within the silica pores. The possible reason for the increase in the intrapore concentration of glycerol in the presence of acid was investigated using a combination of ab initio molecular dynamics, DFT simulations and classical molecular dynamics simulations. The presence of hydronium ions was found to enhance the interaction of glycerol with the surrounding water molecules, evidenced by a higher interaction energy of glycerol with the water molecules in acidic medium compared to the aqueous medium (without acid). Details regarding the simulations and the obtained results are presented in Section S3.4 of the Supporting information. This increase in the interaction strength of glycerol with the aqueous medium in the presence of acid is attributed to the enhanced hydrogen bonding interaction in such systems compared to purely aqueous systems, where a slight increase in the lifetime of hydrogen bonds between glycerol and water was observed in classical MD simulations performed on such systems. Details regarding these MD simulations and the obtained results are also provided in section S3.4 of the Supporting information. The likely presence of higher concentration of hydronium ions in the silica pores and the enhanced solvation of glycerol in the presence of the hydronium ions are possible reasons for the enhancement of glycerol concentrations within the pores in the samples soaked in acid as seen in Figure 7. The increase in the glycerol concentration within the silica pores, where the active catalyst components are hosted,


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can potentially lead to an enhancement in glycerol interaction with the catalyst when acid is present, and this is discussed in Section 3.4.1. In addition to enhancing local concentration of glycerol within the silica pores, the added sulfuric acid promoter may influence the nature of the active sites on the ReOx-Ir catalyst. The Brønsted acidic Re-OH-Re species described earlier may form a localized hydrated hydronium ion in the aqueous medium, as this is a typical tendency of solid Brønsted acidic materials in water.53 DFT simulations suggested that deprotonation of the Brønsted acidic Re-OH-Re species to form a free hydronium ion, in the presence of an existing free hydronium ion in the vicinity (acidic medium) is relatively less favorable than in aqueous medium (non-acidic). The details of these simulations and the obtained results are presented in Section S3.4 of the Supporting Information. This implies that the Re-OH-Re species will be available as active sites for a “protonation dehydration” mechanism that will be discussed in Section 3.4.2 in the presence of sulfuric acid promoter. This would positively influence the conversion of glycerol, contributing further to the enhancement that is observed in the experiments (Table 2). In summary, our investigation reveals that the contribution of sulfuric acid in enhancing the conversion of glycerol may be due to a variety of indirect influences such as 1) increase in intrapore concentration of glycerol, 2) higher propensity for the formation/retention of Re-OH-Re species on the ReOx cluster and 3) enhanced dissociative attachment of glycerol on the ReOx cluster (discussed in section 3.4.1). Additionally, we show in section 3.4.2 that the homogeneous acid may not contribute towards increasing the conversion of glycerol due to the direct acid catalyzed dehydration mechanism.


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3.4. Reaction mechanisms, pathways and energetics for glycerol hydrogenolysis on the ReOxIr catalysts 3.4.1. Interaction of glycerol with the catalyst and its dissociative attachment DFT calculations of the molecular adsorption of glycerol on ReOx-Ir and Ir catalysts show that the interaction of glycerol with the model Re3O6H2-Ir catalysts is 27 kJ mol-1 stronger than its interaction in the same configuration with the Ir(111) surface. This can be attributed to the hydrogen bonding between the hydroxyls of glycerol and the oxygen species on the ReOx clusters. Based on the observed product selectivity towards 1,3-PDO during the hydrogenolysis of glycerol on the ReOx-Ir catalyst, the attachment of glycerol on the ReOx cluster through its primary hydroxyl group has been previously suggested.23 Similar primary propoxide formation from glycerol on metal oxides like γ-alumina, TiO2, ZrO2, CeO2 has been demonstrated using combined experimental and computational investigations.54-55 Our DFT calculations confirm this hypothesis that the attachment through the primary hydroxyl alone, forming a 2,3-dihydroxy propoxide is indeed the most stable configuration (highest adsorption energy) and is shown in Figure 8a. The relative stability of the alkoxides formed was found to follow the trend of primary > secondary (+18.1 kJ mol-1) > both primary (+53.1 kJ mol-1) > primary and secondary (+124.6 kJ mol-1), where the energies are relative to the primary alkoxide. These configurations are shown in Figures 8 a-d.


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Figure 8. Different modes of attachment of glycerol on Re species of the Re3O6H2 cluster through it’s a) primary hydroxyl, b) secondary hydroxyl, c) both primary hydroxyls, d) primary and secondary hydroxyl. This is facilitated by the hydrogen abstraction from the hydroxyl groups of glycerol by the Re-OH species on Re3O6H2 cluster and formation of a water molecule in each case. Attachment of two glycerol molecules on the same Re3O6H2 cluster through their e) primary hydroxyls, and f) secondary hydroxyls are also shown. Indicated in each case is the energy relative to the most stable configuration of primary propoxide. Color code is same as in Figure 3 with grey balls representing carbon. In addition to increasing the local concentration of glycerol within the silica pores as discussed in Section 3.3.4, the presence of sulfuric acid may also increase the tendency of glycerol to undergo dissociative adsorption on the ReOx clusters. Glycerol near a hydronium ion cluster (H3O+-H2O-8 as shown in Figures S6a and b) was found to undergo dissociative adsorption on the Re3O6H2/Ir(111) cluster with a reaction energy which is 18 kJ mol-1 more favorable than in the 30 ACS Paragon Plus Environment

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absence of the hydronium ion (cluster of nine water molecules as shown in Figures S6 c and d). Details regarding these simulations and the obtained results are presented in Section S3.5 of the Supporting Information. With higher local concentration of glycerol in the silica pores and higher propensity for dissociative adsorption on the ReOx-Ir catalyst in the presence of sulfuric acid, multiple glycerol molecules may attach on the clusters as shown in Figures 8e and f. As with the attachment of a single glycerol molecule, attachment of two glycerol molecules on the same ReOx cluster via the primary hydroxyl is more stable than the attachment via the secondary. Intuitively, the saturation of the ReOx clusters due to the dissociative adsorption of glycerol should have led to an increase in the value of the relative adsorption strength of glycerol on the catalyst, reflecting in the value of the parameter 𝛽𝛽 shown in Figure 7. However, the 𝛽𝛽 values for the samples containing acid, as shown in Figure 7e are lower than those in the samples without the

acid, as shown in Figure 7d. Dissociative attachment of multiple glycerol molecules on the same ReOx clusters led to a marginal decrease in the adsorption energy of glycerol. This may be responsible for the lowering of 𝛽𝛽 observed for the samples soaked in the acidic glycerol solution and this behavior is also discussed in Section S3.5 of the Supporting Information.

3.4.2. Reaction mechanisms and pathways for the formation of propanediols Reaction pathways for the hydrogenolysis of glycerol on Re3O6H2-Ir(111) and the Re3O6H3Ir(111) catalysts were investigated with two pre-adsorbed hydrogen atoms on the Ir surface. In experiments, the desorption of hydrogen on Ir(111) surface is reported to have a barrier of ~63 kJ mol-1.56 The free energy change during dissociative chemisorption of hydrogen on Ir(111) surface, generating active hydrogen atoms, was calculated as -22.5 kJ mol-1 at 393 K, further suggesting that the surface is likely to have chemisorbed hydrogen.


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The adsorbed glycerol forming a propoxide, as previously discussed, has free hydroxyls which can interact with the Ir surface, making C-O cleavage likely to be facilitated by the Ir surface. Hence, glycerol hydrogenolysis was investigated through the following elementary steps: 1) dissociative chemisorption of glycerol on Re3O6H2 / Re3O6H3 clusters and formation of the propoxide, followed by 2) C-O cleavage. Two mechanisms were investigated for the C-O cleavage step: 1) direct dehydroxylation (removal of OH) facilitated by the Ir(111) surface and 2) protonation-dehydration mechanism or proton assisted dehydration mechanism where the ReOx cluster is the source of the proton and these are depicted in Scheme 2. Glycerol hydrogenolysis proceeds further with 3) hydrogenation of the CHx* (x=1 or 2) species resulting from the C-O cleavage and 4) hydrolysis of the propoxide, to form propanediol. The mechanisms and energy profiles for the formation of propanediols and propanols via these two mechanisms are shown in Figure 9.

Scheme 2. Schematic representation of glycerol hydrogenolysis on ReOx-Ir catalyst via a) direct dehydroxylation mechanism and b) protonation and dehydration mechanism where the Re-OH-Re species is the proton source. The species indicated by OH* adsorbs on the Ir surface.


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Figure 9. Reaction pathways for the formation of propanediols and propanols during hydrogenolysis of glycerol via the a) direct dehydroxylation mechanism on Re3O6H2-Ir(111) and b) protonation-dehydration mechanism on Re3O6H3-Ir(111). The activation barriers (kJ mol-1) are shown in red font and reaction energies (kJ mol-1) are shown in black font for each step. The activation free energy barriers (kJ mol-1) at 393 K for the C-O cleavage steps along the most favourable routes (2,4 and 5), are reported in square brackets in blue font. The O* label (orange font) indicates oxygen atom attached to the ReOx cluster and * refers to attachment of CHx (x=1 to 2) on the Ir surface. 33 ACS Paragon Plus Environment

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Dissociative attachment of glycerol on a Re3O6H2 cluster through its primary and secondary hydroxyls was facile with low activation energy barriers of 18.4 kJ mol-1 and 25.1 kJ mol-1, respectively and was energetically favorable, as shown in Figure 9a. The formation of the primary propoxide leads to two potential reaction routes, as shown in Figure 9a. Route 1 leads to the formation of 1,2-PDO upon elimination of the primary hydroxyl and Route 2 formed 1,3-PDO upon elimination of the secondary hydroxyl. The secondary propoxide presented only one primary route and this is shown as Route 3 in Figure 9a, which leads to the formation of 1,2-PDO. The removal of the free primary hydroxyl in Route 1 had a high barrier of 217.9 kJ mol-1 while it was 185.6 kJ mol-1 in Route 3. The removal of the secondary hydroxyl of glycerol in Route 2 had a lower activation barrier of 165.2 kJ mol-1 with the corresponding activation free energy barrier at 393 K of 158.4 kJ mol-1. The resulting CH2* species in Routes 1 and 3 and the CH* species in Route 2 were stabilized by their interactions with the Ir surface making dehydroxylation energetically downhill. The hydrogenation of the CH2* (barrier: 97.1 kJ mol-1 Route 1), CH* (barrier: 90.6 kJ mol-1, Route 2) and CH2* (barrier: 111.5 kJ mol-1 Route 3) species attached to the Ir surface by the active hydrogen was energetically uphill. Finally, hydrolysis of the propoxide by the water molecule interacting with the Re species released 1,2-PDO (activation barrier of 50.3 kJ mol-1) in Route 1, 1,3-PDO (activation barrier of 73.1 kJ mol-1) in Route 2 and 1,2-PDO (activation barrier of 53.8 kJ mol-1) in Route 3 as the products. The hydrolysis process regenerated the OH species on the Re3O6H2 cluster. The energy profiles and transition states corresponding to each of these steps are presented in Figs. S7 and S8, respectively, of the Supporting Information. Since C-O cleavage and the removal of the hydroxyl group of glycerol had the highest activation barrier in all the three reaction routes, this was ascertained to be the rate determining step in glycerol hydrogenolysis. The comparatively lower barrier for the removal of the secondary


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hydroxyl of glycerol (Route 2) explained the formation 1,3-PDO as the dominant product in this kinetically preferred pathway, as discussed in Section 3.3.2. Comparison of Routes 1 to 3, as shown in Figure 9a, suggested that the formation of 1,2-PDO is most likely to proceed via the attachment of glycerol on the ReOx clusters through its secondary hydroxyl. The activation barrier here is lower than when it is attached through the primary hydroxyl. The higher barrier for the removal of the primary hydroxyl explains the lower selectivity to 1,2-PDO. Since ReOx clusters are Brønsted acidic, protonation of free glycerol and its dehydration is a possibility. The role of oxophilic metals like Re in bringing about the acid catalyzed ring opening and hydrogenolysis of esters and polyols has been discussed in the literature.27,

29, 57

The

deprotonation of Re-OH-Re on Re3O6H3/Ir catalyst was found to be more favorable than the deprotonation of Re-OH on the same catalyst by 35.5 kJ mol-1, suggesting that the Re-OH-Re is more acidic that the Re-OH species. Hence, the Brønsted acid (Re-OH-Re) catalyzed protonationdehydration mechanism for molecularly physisorbed glycerol on the Re3O6H3/Ir catalyst was investigated. The activation barrier for the dehydration of glycerol at the secondary carbon of glycerol was calculated to be 196.9 kJ mol-1, as shown in Figure S9 of the Supporting Information. The high barrier suggested that C-O cleavage would preferably happen only after the dissociative attachment of glycerol on the ReOx cluster and such reaction pathways are shown as Routes 5 and 6 in Figure 9b. The formation of primary and secondary propoxide had activation barriers of 37.3 kJ mol-1 (Route 5) and 19.5 kJ mol-1 (Route 6) respectively. The activation barriers here are slightly different from those on Re3O6H2 due to the additional Re-OH-Re on Re3O6H3. The vicinal acidic Re-OH-Re hydroxyl allows protonation of the free secondary hydroxyl (Route 5) and primary hydroxyl (Route 6), facilitating a protonation-dehydration mechanism, leading to elimination of a


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water molecule, as shown in Scheme 2b and Figures S10 and S11 of the Supporting Information. The resulting CH* (Route 5) and CH2* (Route 6) species were stabilized by the Ir surface. The activation barrier for the protonation-dehydration at the secondary carbon of glycerol was 123.3 kJ mol-1 (Route 5) with the corresponding activation free energy barrier of 100.7 kJ mol-1. The activation barrier for the protonation-dehydration at the primary carbon of glycerol was 167.7 kJ mol-1 (Route 6). These activation barriers are lower than those calculated in Routes 2 and 3. The formation of a water molecule in Routes 5 and 6 made the reaction energies in these routes much more favorable than the direct dehydroxylation mechanism in Routes 2 and 3. Steric effects and the stability of the CH* in Route 5 made its subsequent hydrogenation a high barrier process with activation barrier of 120.7 kJ mol-1, which is equivalent to the barrier for the dehydration. This is quite unlike the cases reported earlier, where the C-O cleavage was distinctly the rate determining step. Hydrogenation of CH2* in Route 6 had an activation barrier of 98.1 kJ mol-1. The hydrolysis of the propoxide in Route 5 formed 1,3-PDO with an activation barrier of 50.8 kJ mol-1 and the same in Route 6 formed 1,2-PDO with a barrier of 67.9 kJ mol-1. As discussed for the direct dehydroxylation pathway, hydrolysis regenerated the Re-OH species on the Re3O6H3 cluster. However, the Re-OH-Re was not intrinsically regenerated and became an Re-O-Re species in the catalytic cycle. The energy profiles and transition states corresponding to each step are presented in Figures S10 and S11 respectively of the Supporting Information. To verify if sulfuric acid can facilitate the acid catalyzed dehydration of glycerol, the protonation dehydration mechanism for C-O cleavage was also investigated, with the homogeneous acid being the proton source. Only the rate determining C-O cleavage step was investigated, with glycerol attached to Re3O6H2/Ir(111) as a primary propoxide. An E1 mechanism of elimination of a near neutral OH2 species from the protonated alcohol complex was reported as the lowest energy


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pathway in the homogeneous acid catalyzed dehydration of cyclohexanol to cyclohexene.58 The protonation dehydration of the free secondary hydroxyl of glycerol, leading to the removal of the OH2 species was found to have an activation barrier of 200.1 kJ mol-1 as shown in Figure S12 of the Supporting Information. Details regarding the simulations are provided in Section S3.8 of the Supporting information. As the activation barrier for this process was significantly higher than the corresponding step in the Re-OH-Re facilitated protonation-dehydration and in the direct dehydroxylation mechanisms shown in Figure 9, the homogenous acid catalyzed dehydration is less likely. This further confirmed that the homogeneous acid does not alter reaction pathways and energetics and merely serves to increase the local concentration of glycerol within the silica pores as demonstrated in Section 3.3.4. This conclusion agrees with the literature23 where the homogeneous acid catalyzed dehydration-hydrogenation route for the formation of 1,3-PDO was ruled out based on the observed trends in glycerol hydrogenolysis with varying acid content. 3.4.3. Reaction mechanisms and pathways to form propanols and effect of competitive interactions of different components Route 4 shown in Figure 9a presents the removal of the primary hydroxyl immediately after the removal of the secondary hydroxyl of glycerol. The removal of the primary hydroxyl following the removal of the secondary hydroxyl had an activation barrier of 141.6 kJ mol-1 with the corresponding activation free energy barrier at 393 K of 125.2 kJ mol-1. This was significantly lower than the barrier for the removal of primary hydroxyl as reported in Route 1 (217.8 kJ mol-1) and Route 3 (185.6 kJ mol-1). Two sequential hydrogenation steps along Route 4, hydrogenating the CH2* (barrier: 74.2 kJ mol-1) and the CH* (barrier: 73.8 kJ mol-1), followed by the hydrolysis of the propoxide (barrier: 64.5 kJ mol-1) released 1-PO as the product. Hydrolysis regenerates the Re-OH species on the Re3O6H2 cluster, as discussed earlier. The energy profile and transition states 37 ACS Paragon Plus Environment

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corresponding to each step described above are presented in Figures S13 and S14 respectively of the Supporting Information. The direct successive dehydroxylation, analogous to Route 4, forming 1-PO, from the dehydrated intermediate in Route 5 and 2-PO from the dehydrated intermediate in Route 6 are also possible. However, these steps were not explicitly investigated. The low selectivity of 3% to 1-PO against 58% to 1,3-PDO at 373 K, as shown in Figure 4b can be attributed to the higher barrier for successive dehydroxylation of glycerol via Route 4 in Figure 9, compared to the protonation dehydration mechanism via Route 5 which forms 1,3-PDO. With an increase in temperature to 393 K, the successive dehydroxylation pathway becomes competitive and can produce 1-PO, explaining the dip in selectivity of 1,3-PDO and increase in the selectivity to 1-PO, as seen in Figure 4c (data for 6 hours). The successive dehydroxylation as shown in Route 4 may not be the only pathway for the formation of 1-PO. Figure 4c shows an increase in the selectivity to 1-PO as a function of time suggesting the possibility of hydrogenolysis of propanediols. 1,3-PDO can only form 1-PO while 1,2-PDO can form both 1-PO (attachment through primary hydroxyl and removal of secondary hydroxyl) and 2-PO (attachment through secondary hydroxyl and removal of primary hydroxyl). The activation barrier for the removal of the free secondary and free primary hydroxyls of 1,2PDO were calculated to be 178.0 kJ mol-1 and 185.9 kJ mol-1 respectively, as shown in Figure 9 and in Figure S15 of the Supporting Information. Experiments with 1,2-PDO substrate yielded 1PO with ~85% selectivity, suggesting that 1,2-PDO primarily forms 1-PO while hydrogenolysis of 1,3-PDO showed 99% selectivity to 1-propanol. The results of hydrogenolysis of both the diols are presented in Table S2 of the Supporting Information. The removal of the free primary hydroxyl of 1,3-PDO had a barrier of 216.1 kJ mol-1, as shown in Figure 9 and Figure S15c of the Supporting Information. These calculated activation barriers correlate with the experimental trends which


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show that 1,2-PDO is significantly more reactive than 1,3-PDO and this agrees with the literature.29 These results suggest 1,2-PDO to be the precursor for the formation of 1-PO in a parallel pathway to successive dehydroxylation described earlier. However, Figure 4c shows a slight dip in the selectivity to 1,3-PDO as a function of reaction time (data at 48 hours), whilst selectivity to 1,2-PDO does not vary noticeably, suggesting that 1,3PDO may contribute to the formation of 1-PO, despite being less reactive than 1,2-PDO. To investigate this disparity, NMR displacement experiments have been performed. Figure 10 shows the D-T2 distributions of the catalyst samples soaked initially in glycerol and subsequently in 1,3PDO for different periods of time to investigate the displacement behavior. The duration of soaking time in the displacing liquid is defined as the displacement time and values of 10, 30, 90, 270 685 and 1100 minutes have been used here. It can be seen from Figure 10 that the diffusive and relaxation behavior of both glycerol and 1,3-PDO are so similar in each case that they cannot be resolved in the regularization of the data. Nevertheless, as the displacement time increases, the peak shifts towards faster diffusivities. Since the diffusivities of 1,3-PDO and 1,2-PDO are much higher than that of glycerol, mixtures of glycerol and PDO will have diffusivities between that of pure PDO and pure glycerol. The observed increase in diffusivities is in line with an increase in the intra-pellet concentration of 1,3-PDO and is clear evidence of the displacement of glycerol by 1,3-PDO within the pores.


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Figure 10. D-T2 distributions for samples prepared by soaking the Ir-ReOx catalyst particles in glycerol and then subsequently in 1,3-PDO for displacement times of a) 10 minutes, b) 30 minutes, c) 90 minutes, d) 270 minutes, e) 685 minutes, and f) 1100 minutes. Similar experiments have been performed for 1,2-PDO displacing glycerol as well as glycerol displacing 1,2-PDO and 1,3-PDO. An indication of the extent and rate of displacement in each case has been achieved by comparing the diffusivity of the peak in the D-T2 distributions as a function of displacing time and is shown in Figure 11. Part a and c in Figure 11 are from experiments of glycerol displacing 1,2-PDO and 1,3-PDO, respectively, while part b and part d are from experiments where, 1,2-PDO and 1,3-PDO, respectively, displace glycerol.


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Figure 11. Diffusivity of the intra-pellet mixture as a function of displacing time for a) glycerol displacing 1,2-PDO, b) 1,2-PDO displacing glycerol, c) glycerol displacing 1,3-PDO and d) 1,3PDO displacing glycerol. Red dotted lines represent the diffusivity of the pure displacing liquid within the pores and the red shaded region indicates the error on that measurement. There are two main conclusions that can be drawn from the results in Figure 11. Firstly, comparison of Figures 11a and c shows that 1,2-PDO was displaced more rapidly than 1,3-PDO by glycerol, as can be seen from the rapid convergence of the diffusivity to the bulk diffusivity of glycerol (red dotted line). This suggests that glycerol can more easily displace 1,2-PDO than 1,3PDO. Secondly, the plateau in diffusivity in Figure 11b at a value below that of pure 1,2-PDO within the pores suggests that 1,2-PDO cannot fully displace glycerol. 1,3-PDO, however, is capable of fully displacing glycerol as can be seen in Figure 11d. 41 ACS Paragon Plus Environment

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The outcomes of the displacement experiments and observed trends in the product selectivity as a function of time (Figure 4c) suggest that 1-PO is formed either via the successive dehydroxylation pathway or by the hydrogenolysis of 1,3-PDO which is relatively less reactive. The discussion here emphasizes the significance of understanding competitive interactions in liquid phase reactions, suggesting that reactivity of pure species/intermediates alone may not dictate reactivity/product selectivity. 3.4.4. Comparison of proposed mechanisms with those in the literature Based on the mechanisms and pathways presented so far, the roles of the ReOx and Ir components in ReOx-Ir/SiO2 catalysts are summarized here. The partially reduced ReOx clusters (represented by Re3O6H2 and Re3O6H3 in this article) performed the following functions: 1) attachment of glycerol (facilitated by the Re-OH species), 2) protonation-dehydration of glycerol (facilitated by the vicinal Re-OH-Re species), and 3) hydrolysis of the attached propoxide to release the product and to regenerate the Re-OH species. Ir on the other hand played the following roles: 1) C-O cleavage and stabilization of CH* and CH2* species after the C-O cleavage, 2) H2 dissociation, and 3) hydrogenation of CH* and CH2*. Although glycerol is adsorbed on the ReOx clusters, the sub-nanometer size of these clusters (which are finely dispersed on the Ir nanoparticles) facilitates the interaction of the attached glycerol molecule with Ir atoms. Hence, the ReOx-Ir interface enables a peculiar synergy of the sites on ReOx as well as Ir, explaining why single component Ir or ReOx catalysts and a physical mixture of these catalysts could not achieve satisfactory conversion of glycerol, as shown in Table 2. The mechanisms and pathways presented here differ slightly from those proposed in the literature. Dumesic and co-workers attributed the preferential removal of the secondary hydroxyl of polyols during the hydrogenolysis reaction on ReOx dispersed Rh catalyst to the higher stability 42 ACS Paragon Plus Environment

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of the carbenium ion resulting from the dehydration step catalyzed by the Brønsted acidic ReOx.29 The hydrogenolysis reactions facilitated by the Brønsted acidic hydroxyls on oxophilic ReOx clusters on active metals was discussed by Davis and co-workers,27 as well as Neurock and coworkers.57 However, we believe that the Brønsted acid catalyzed dehydration mechanism, which we refer to as the protonation-dehydration mechanism, will proceed with glycerol attached as a propoxide on the ReOx cluster and with the resulting CHx* species stabilized by the Ir sites. Hence, the concerted hydride shift from the α CHx-OH, believed to accompany acid catalyzed dehydration, forming a more stable oxo-carbenium intermediate29 may not be relevant in this mechanism. The protonation-dehydration mechanism presented here has lower activation barriers for the C-O cleavage and more favorable reaction energies than the direct dehydroxylation mechanism, making it the preferred mechanism for hydrogenolysis. Tomishige and co-workers on the other hand proposed a direct hydrogenolysis mechanism involving the hydride attack at the ReOx-Ir interface as the mechanism for the selective elimination of the secondary hydroxyl.23 We are unable to confirm a concerted direct hydride attack and OH removal mechanism and we believe the direct mechanism involves dehydroxylation by C-O cleavage and subsequent hydrogenation, as described in Section 3.4.2 and as shown in Figure 9a. 3.5. Reaction pathways for the hydrogenolysis of glycerol on Ir(111) surface Hydrogenolysis by direct C-O cleavage of the molecularly adsorbed glycerol on Ir(111) surface was investigated and the structures corresponding to the IS, TS and FS are shown in Figure S16 of the supporting information. The direct removal of the secondary hydroxyl of glycerol has an activation barrier of 176.7 kJ mol-1 while the removal of the primary hydroxyl of glycerol has a much higher barrier of 206.2 kJ mol-1. Due to the large activation barriers for the direct C-O


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cleavage, we believe that the dissociative adsorption of glycerol on the surface, which has a lower activation barrier, will precede the C-O cleavage step. Dissociative adsorption via dehydrogenation of polyols on noble metals like Ru have been proposed via an alkoxide pathway, forming a carbonyl species attached to the surface.59 A combined experimental and computational investigation confirmed that the hydrogenolysis of glycerol on supported Rh catalyst is initiated by dehydrogenation rather than dehydration.60 Hence, we investigated a reaction pathway for glycerol hydrogenolysis initiated by the dissociative adsorption of glycerol on the Ir(111) surface by two step dehydrogenation. A co-adsorbed water molecule which can assist in O-H cleavage in alcohols is not considered in this case as the effect of such assistance is believed to be lesser on metals like Ir.61 Following the dehydrogenation, the hydrogenolysis reaction proceeds further by dehydroxylation (C-O cleavage) and hydrogenation steps. Hydrogenolysis initiated by the dissociative attachment of glycerol on the Ir surface through the primary hydroxyl may lead to formation of 1,3-PDO (removal of a secondary hydroxyl) as well as 1,2-PDO (removal of the primary hydroxyl) as shown in Route 7 and Route 8 in Figure 12. Hydrogenolysis from adsorbed glycerol through the secondary hydroxyl leads to formation of 1,2PDO as shown in Route 9 in Figure 12.


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Figure 12. Summary of the various reaction pathways for the formation of propanediols during hydrogenolysis of glycerol on Ir(111) catalyst. The activation barriers are shown in red font and reaction energies are shown in black font for each step. The activation free energy barrier (kJ mol1

) for the C-O cleavage step via route 9 at 393 K is reported in square brackets in blue font. The *

refers to attachment on the Ir surface. The O-H cleavage in the primary hydroxyl groups of glycerol has a relatively high activation barrier of 104.8 kJ mol-1 and 114.2 kJ mol-1 and the same in the secondary hydroxyl is 78.8 kJ mol1

as shown in Routes 7, 8 and 9 respectively in Figure 12. The slight difference of around 10 kJ

mol-1 in the activation barriers and reaction energies for the cleavage of the O-H bond in the primary hydroxyl of glycerol in routes 7 and 8 is due to the different starting configurations of glycerol used to facilitate the subsequent C-O bond cleavage. The lower activation barrier for the secondary O-H cleavage compared to the primary could be because of the intramolecular hydrogen bonding involving the primary and secondary hydroxyls, as can be seen in Figure S18 of the 45 ACS Paragon Plus Environment

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Supplementary Information, and as reported in the literature.62 The subsequent C-H activation in each of these three routes has barriers which are slightly less than half of that of the previous step, forming glyceraldehyde in Routes 7 and 8 and 1,3-dihydroxyacetone in Route 9 as shown in Figure 12. 1,3-Dihydroxyacetone is 44 kJ mol-1 more stable than glyceraldehyde. The activation barriers for the removal of the free secondary and primary hydroxyls in Routes 1, 2 and 3 respectively are high at 184.3 kJ mol-1, 198.4 kJ mol-1 and 207.4 kJ mol-1 (free energy barrier of 185.4 mol-1 at 393 K) as shown in Figure 12. Hydrogenation of CH2* in Route 7 and CH* in Route 9 forms 3hydroxypropanal and hydroxyacetone (acetol) respectively. Acetol is generally accepted to be the precursor for the formation of 1,2-PDO and the higher stability of acetol over the equivalent precursor for 1,3-PDO formation, 3-hydroxypropanal, is generally believed to be the controlling factor for the preferential formation of 1,2-PDO during glycerol hydrogenolysis on metals.12 1,3Dihydroxyacetone is nearly 30 kJ mol-1 more stable on the Ir surface than acetol and hence may remain a surface intermediate which goes undetected as an intermediate in the liquid media, unlike acetol. Hydrogenation of the C* and O* in the carbonyl intermediates lead to the formation of 1,3PDO, 1,2-PDO and 1,2-PDO as the products in Route 7, 8 and 9 respectively. The energy profiles for the complete reaction via these intermediates are shown in Figures S17 and S18 of the Supporting Information. Among the glyceraldehyde pathways (Route 7 and 8), formation of 1,3-PDO (Route 7) may have a marginal preference over 1,2-PDO as can be seen from Figure S17 of Supporting Information. Analysis of all the three pathways for hydrogenolysis on Ir catalyst as shown in Figure 12, and Figures S17 and S18 of the Supporting Information suggests that formation of 1,2-PDO via the 1,3-dihydroxyacetone intermediate (Route 9) is the most preferred. With two pathways (Route 8 and 9) for the formation of 1,2-PDO and with one of them being thermodynamically the most


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preferred among all the pathways (Route 9), 1,2-PDO is the dominant product during hydrogenolysis of glycerol on Ir catalyst as seen from our experiments (Table 1, entry 1). The dissociative attachment of glycerol on Ir(111) has significantly higher activation barriers than on the ReOx-Ir catalysts and has less favorable reaction energies. Moreover, the activation barriers for the C-O cleavage on Ir(111) are significantly higher than the barriers calculated on the ReOxIr catalyst (123.3 kJ mol-1 in Route 5 and 167.7 kJ mol-1 in Route 6 in Figure 9). The significantly higher barriers for the rate determining C-O cleavage step and the lower tendency for dissociative adsorption are possible reasons for the lower reactivity of Ir catalysts in comparison with the ReOxIr catalyst (7% glycerol conversion on Ir(111) against 50% on ReOx-Ir). 3.6. Change in reaction mechanisms with change in Ir/Re ratio in ReOx-Ir catalyst Figure 4d showed the conversion of glycerol and product selectivity at different Ir/Re ratios of the ReOx-Ir catalyst. At high Re loading (Ir/Re= 0.2), the Ir surface would be covered by the Re atoms/clusters, as suggested in literature.23 Unavailability of free Ir sites resulted in low conversion of glycerol due to restricted C-O cleavage and hydrogenation functions. However, the Brønsted acidic ReOx clusters facilitated the dehydration-hydrogenation mechanism proposed in the literature27, 29, 57 (Scheme 1) to produce 1,2-PDO, the thermodynamically preferred product. The product selectivity in this case is similar to that observed on the ReOx only catalyst (Table 2). With increase in Ir/Re ratio from 0.2 to 1, the observed increase in conversion and 1,3-PDO selectivity suggests the transformation of the dominant mechanisms to those described in Section 3.4.2 At high Ir/Re loading ratio of 5, the average particle size of the Ir nanoparticle reaches 5.1 nm from 2 nm at the optimum Ir/Re ratio of 1. The relatively lower amount of Re at Ir/Re of 2 and 5 implies that there are fewer ReOx clusters for glycerol to anchor to and that more Ir sites are exposed. With the observed dominant product/diol as 1,2-PDO, it may be inferred that the predominant pathway 47 ACS Paragon Plus Environment

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is the dehydrogenation-dehydroxylation-hydrogenation pathway we described for the Ir surface (Route 9 in Figure 12). Although this catalyst registers a drop in the conversion of glycerol and 1,3-PDO selectivity (Figure 4d), both values are much higher than those at the Ir/Re ratio 0.2. This suggests that the pathways described for the ReOx-Ir catalyst in Section 3.4 and for the Ir catalyst in Section 3.5 may operate in parallel for this catalyst. The discussion here demonstrates that although the catalyst recipe can be tuned to change the product selectivity, there exist inherent limitations in the recipes for optimum performance towards 1,3-PDO. It also reiterates the significance of the unique structure of the ReOx-Ir catalyst formed at the optimum recipes.

4. Summary and Conclusions Comprehensive insights into the hydrogenolysis of glycerol on the silica supported composite ReOx-Ir catalyst in the presence of sulfuric acid are presented, based on 1) analysis of data from batch hydrogenolysis of glycerol under different reaction conditions and catalyst recipes, 2) detailed characterization of different catalysts, 3) extensive DFT calculations exploring various reaction pathways, and energetics along these pathways, explaining the formation of different products and 4) NMR relaxometry analysis, unravelling the effects of the solvent, sulfuric acid, and competitive interactions of the reactant and products with the catalyst. Glycerol concentration higher than 20% by mass was found to be ideal for optimum conversion of glycerol to benefit from the higher relative interaction of glycerol with the catalyst compared to the solvent water. A sharp increase in the conversion of glycerol in the presence of sulfuric acid was found to be due to a combination of effects: 1) increase in concentration of glycerol within the silica pores, 2) enhanced propensity for the dissociative attachment of glycerol on the active catalyst components forming a primary propoxide, 3) likely presence of higher number of active sites for hydrogenolysis. Partially reduced ReOx clusters that are finely dispersed and directly interacting with the metallic Ir 48 ACS Paragon Plus Environment

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nanoparticles makes available multifunctional sites on both these components for hydrogenolysis through the following elementary steps: 1) dissociative attachment of glycerol on the ReOx cluster, 2) C-O bond cleavage, 3) hydrogenation of the CHx* formed during the C-O cleavage, and 4) hydrolysis to release the alcohol. The rate determining C-O bond cleavage step proceeds to selectively remove the secondary hydroxyl of glycerol with a comparatively low activation barrier of 123.3 kJ mol-1 in a protonation-dehydration mechanism or 165.2 kJ mol-1 in a direct dehydroxylation mechanism. This explains the preferential formation of 1,3-PDO over 1,2-PDO. 1-PO may form in a competing parallel pathway involving successive cleavage of C-O bonds of glycerol, eliminating the secondary hydroxyl followed by the primary hydroxyl. Although less reactive than 1,2-PDO, 1,3-PDO due to its ability to competitively displace the reactant glycerol from the catalyst in the reaction mixture, can undergo further hydrogenolysis to form 1-PO at long reaction times (high conversion). Hydrogenolysis of glycerol on pure Ir catalyst proceeds to selectively form 1,2-PDO in the thermodynamically preferred pathway involving 1,3-dihydroxy acetone and acetol intermediates, resulting from a dehydrogenation-dehydroxylationhydrogenation mechanism. For the ReOx-Ir catalyst, a recipe containing nearly 1:1 ratio of Ir and Re enabled optimum performance, with the conversion of glycerol and selectivity to 1,3-PDO dipping towards that of pure ReOx or a mix of pure Ir and ReOx-Ir respectively to the left and right of the optimum Ir/Re ratio of 1. This investigation highlights the significance of understanding the competitive interactions of the reactant and solvent with the catalyst as well as those of the reactant/ intermediates/ products with the catalyst during liquid phase reactions to understand and predict trends in conversion and product selectivity. Hydronium ions in acidic solutions may exhibit complex effects, both direct and indirect which can alter reactivity trends and product selectivity. The mechanistic insights


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from this investigation suggests that the search for alternative catalysts for hydrogenolysis of polyols may be limited to combinations of metal oxides, one of which is oxophilic and undergoes partial reduction while the other undergoes complete reduction and becomes active in hydrogenation, together forming closely interacting structures. There may be additional constraints on the nature/morphology of the partially reduced metal oxides for optimum performance as large clusters may not be able to capitalize on the synergy of the sites on both components while extremely small clusters may not restrict attachment of glycerol in any configuration. Although it may be desirable to operate the reaction in a flow mode from an engineering perspective, comparatively long batch operations for high conversion, within the narrow range of temperatures offering high selectivity to the desired product, at relatively high concentration of glycerol might be ideal for this process. Associated content Supporting information. Additional details regarding the experimental and computational methods, additional supporting results including Figures S1 to S18, Tables S1 and S2, and references. Acknowledgements This research was supported by the National Research Foundation, Prime Minister’s Office, Singapore under its CREATE program. The computational work for this article was fully performed on resources of the National Supercomputing Centre, Singapore (https://www.nscc.sg).


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References 1. Bozell, J. J.; Petersen, G. R., Technology Development for the Production of Biobased Products from Biorefinery Carbohydrates - The US Department of Energy's "Top 10" Revisited. Green Chem. 2010, 12, 539-554. 2. Alonso, D. M.; Wettstein, S. G.; Dumesic, J. A., Bimetallic Catalysts for Upgrading of Biomass to Fuels and Chemicals. Chem. Soc. Rev. 2012, 41, 8075-8098. 3. Besson, M.; Gallezot, P.; Pinel, C., Conversion of Biomass into Chemicals over Metal Catalysts. Chem. Rev. 2014, 114, 1827-1870. 4. Robinson, A. M.; Hensley, J. E.; Will Medlin, J., Bifunctional Catalysts for Upgrading of Biomass-Derived Oxygenates: A Review. ACS Catal. 2016, 6, 5026-5043. 5. Chaminand, J.; Djakovitch, L. A.; Gallezot, P.; Marion, P.; Pinel, C.; Rosier, C., Glycerol Hydrogenolysis on Heterogeneous Catalysts. Green Chem. 2004, 6, 359-361. 6. Soares, R. R.; Simonetti, D. A.; Dumesic, J. A., Glycerol as a Source for Fuels and Chemicals by Low-temperature Catalytic Processing. Angew. Chem. Int. Ed. 2006, 45, 3982-3985. 7. Pagliaro, M.; Ciriminna, R.; Kimura, H.; Rossi, M.; Della Pina, C., From Glycerol to ValueAdded Products. Angew. Chem. Int. Ed. 2007, 46, 4434-4440. 8. Behr, A.; Eilting, J.; Irawadi, K.; Leschinski, J.; Lindner, F., Improved Utilisation of Renewable Resources: New Important Derivatives of Glycerol. Green Chem. 2008, 10, 13-30.


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9. Zhou, C. H.; Beltramini, J. N.; Fan, Y. X.; Lu, G. Q., Chemo selective Catalytic Conversion of Glycerol as a Biorenewable Source to Valuable Commodity Chemicals. Chem. Soc. Rev. 2008, 37, 527-549. 10. Nakagawa, Y.; Tomishige, K., Heterogeneous Catalysis of the Glycerol Hydrogenolysis. Catal. Sci. Technol. 2011, 1, 179-190. 11. Tendam, J.; Hanefeld, U., Renewable Chemicals: Dehydroxylation of Glycerol and Polyols. ChemSusChem 2011, 4, 1017-1034. 12. Ruppert, A. M.; Weinberg, K.; Palkovits, R., Hydrogenolysis Goes Bio: From carbohydrates and Sugar Alcohols to Platform Chemicals. Angew. Chem. Int. Ed. 2012, 51, 2564-2601. 13. Vasiliadou, E. S.; Lemonidou, A. A., Glycerol Transformation to Value Added C3 Diols: Reaction Mechanism, Kinetic, and Engineering aspects. Wiley Interdiscip. Rev.: Energ. Environ. 2015, 4, 486-520. 14. Sun, D.; Yamada, Y.; Sato, S.; Ueda, W., Glycerol Hydrogenolysis into Useful C3 Chemicals. Appl. Catal. B: Environ. 2016, 193, 75-92. 15. Tomishige, K.; Nakagawa, Y.; Tamura, M., Selective Hydrogenolysis and Hydrogenation Using Metal Catalysts Directly Modified with Metal Oxide Species. Green Chem. 2017, 19, 28762924. 16. Chaudhari, R. V.; Torres, A.; Jin, X.; Subramaniam, B., Multiphase Catalytic Hydrogenolysis/Hydrodeoxygenation Processes for Chemicals from Renewable Feedstocks: Kinetics, Mechanism, and Reaction Engineering. Ind. Eng. Chem. Res. 2013, 52, 15226-15243.


Page 53 of 60 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. Lee, C. S.; Aroua, M. K.; Daud, W. M. A. W.; Cognet, P.; Pérès-Lucchese, Y.; Fabre, P. L.; Reynes, O.; Latapie, L., A review: Conversion of Bioglycerol into 1,3-propanediol via Biological and Chemical Method. Renew. Sust. Energ. Rev. 2015, 42, 963-972. 18. Dasari, M. A.; Kiatsimkul, P. P.; Sutterlin, W. R.; Suppes, G. J., Low-pressure Hydrogenolysis of Glycerol to Propylene Glycol. Appl. Catal. A: General 2005, 281, 225-231. 19. Kurosaka, T.; Maruyama, H.; Naribayashi, I.; Sasaki, Y., Production of 1,3-propanediol by Hydrogenolysis of Glycerol Catalyzed by Pt/WO3/ZrO2. Catal. Commun. 2008, 9, 1360-1363. 20. Ma, L.; He, D., Influence of Catalyst Pretreatment on Catalytic Properties and Performances of Ru-Re/SiO2 in Glycerol Hydrogenolysis to Propanediols. Catal. Today 2010, 149, 148-156. 21. Nakagawa, Y.; Shinmi, Y.; Koso, S.; Tomishige, K., Direct Hydrogenolysis of Glycerol into 1,3-propanediol over Rhenium-Modified Iridium Catalyst. J. Catal. 2010, 272, 191-194. 22. Shinmi, Y.; Koso, S.; Kubota, T.; Nakagawa, Y.; Tomishige, K., Modification of Rh/SiO2 Catalyst for the Hydrogenolysis of Glycerol in Water. Appl. Catal. B: Environ. 2010, 94, 318-326. 23. Amada, Y.; Shinmi, Y.; Koso, S.; Kubota, T.; Nakagawa, Y.; Tomishige, K., Reaction Mechanism of the Glycerol Hydrogenolysis to 1,3-propanediol over Ir-ReOx/SiO2 Catalyst. Appl. Catal. B: Environ. 2011, 105, 117-127. 24. Arundhathi, R.; Mizugaki, T.; Mitsudome, T.; Jitsukawa, K.; Kaneda, K., Highly Selective Hydrogenolysis of Glycerol to 1,3-propanediol over a Boehmite Supported Platinum/Tungsten Catalyst. ChemSusChem 2013, 6, 1345-1347. 25. Nakagawa, Y.; Tamura, M.; Tomishige, K., Catalytic Materials for the Hydrogenolysis of Glycerol to 1,3-propanediol. J. Mater. Chem. A 2014, 2, 6688-6702. 53 ACS Paragon Plus Environment

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26. Deng, C.; Duan, X.; Zhou, J.; Zhou, X.; Yuan, W.; Scott, S. L., Ir-Re Alloy as a Highly Active Catalyst for the Hydrogenolysis of Glycerol to 1,3-propanediol. Catal. Sci. Technol. 2015, 5, 1540-1547. 27. Falcone, D. D.; Hack, J. H.; Klyushin, A. Y.; Knop-Gericke, A.; Schlögl, R.; Davis, R. J., Evidence for the Bifunctional Nature of Pt-Re Catalysts for Selective Glycerol Hydrogenolysis. ACS Catal. 2015, 5, 5679-5695. 28. Koso, S.; Watanabe, H.; Okumura, K.; Nakagawa, Y.; Tomishige, K., Comparative Study of Rh-MoOx and Rh-ReOx Supported on SiO2 for the Hydrogenolysis of Ethers and Polyols. Appl. Catal. B: Environ. 2012, 111-112, 27-37. 29. Chia, M.; Pagán-Torres, Y. J.; Hibbitts, D.; Tan, Q.; Pham, H. N.; Datye, A. K.; Neurock, M.; Davis, R. J.; Dumesic, J. A., Selective Hydrogenolysis of Polyols and Cyclic Ethers over Bifunctional Surface Sites on Rhodium–Rhenium Catalysts. J. Am. Chem. Soc. 2011, 133, 1267512689. 30. Feng, J.; Xu, B., Reaction Mechanisms for the Heterogeneous Hydrogenolysis of BiomassDerived Glycerol to Propanediols. Prog. React. Kinet. Mech. 2014, 39, 1-15. 31. Wang, Y.; Zhou, J.; Guo, X., Catalytic Hydrogenolysis of Glycerol to Propanediols: A Review. RSC Adv. 2015, 5, 74611-74628. 32. Coll, D.; Delbecq, F.; Aray, Y.; Sautet, P., Stability of Intermediates in the Glycerol Hydrogenolysis on Transition Metal Catalysts from First Principles. Phys. Chem. Chem. Phys. 2011, 13, 1448-1456.


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33. Wahba, G., Practical Approximate Solutions to Linear Operator Equations When the Data are Noisy. SIAM J. Numer. Anal. 1977, 14, 651-667. 34. Mitchell, J.; Chandrasekera, T. C.; Gladden, L. F., Numerical Estimation of Relaxation and Diffusion Distributions in Two Dimensions. Prog. Nucl. Magn. Reson. Spectrosc. 2012, 62, 3450. 35. Reci, A.; Sederman, A. J.; Gladden, L. F., Obtaining Sparse Distributions in 2D Inverse Problems. J. Magn. Reson. 2017, 281, 188-198. 36. Hohenberg, P.; Kohn, W., Inhomogeneous Electron Gas. Phys. Rev. 1964, 136, B864-B871. 37. Kohn, W.; Sham, L. J., Self-Consistent Equations Including Exchange and Correlation Effects. Phys. Rev. 1965, 140, A1133-A1138. 38. Kresse, G.; Furthmüller, J., Efficient Iterative schemes for Ab Initio Total-Energy Calculations Using a Plane-wave Basis Set. Phys. Rev. B – Condens. Matter Mater. Phys. 1996, 54, 11169-11186. 39. Klimeš, J.; Bowler, D. R.; Michaelides, A., Chemical Accuracy for the van der Waals Density Functional. J. Phys. Condens. Matter 2010, 22, 022201. 40. Tereshchuk, P.; Chaves, A. S.; Da Silva, J. L. F., Glycerol Adsorption on Platinum Surfaces: A Density Functional Theory Investigation with van der Waals Corrections. J. Phys. Chem. C 2014, 118, 15251-15259. 41. Cortese, R.; Schimmenti, R.; Armata, N.; Ferrante, F.; Prestianni, A.; Duca, D.; Murzin, D. Y., Investigation of Polyol Adsorption on Ru, Pd, and Re Using vdW Density Functionals. J. Phys. Chem. C 2015, 119, 17182-17192. 55 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

Page 56 of 60

42. Baltrusaitis, J.; Valter, M.; Hellman, A., Geometry and Electronic Properties of Glycerol Adsorbed on Bare and Transition-Metal Surface-Alloyed Au(111): A Density Functional Theory Study. J. Phys. Chem. C 2016, 120, 1749-1757. 43. Kresse, G.; Joubert, D., From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758-1775. 44. Barzetti, T.; Selli, E.; Moscotti, D.; Forni, L., Pyridine and Ammonia as Probes for FTIR Analysis of Solid Acid Catalysts. J. Chem. Soc., Faraday Trans. 1996, 92, 1401-1407. 45. Monkhorst, H. J.; Pack, J. D., Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188-5192. 46. Amada, Y.; Watanabe, H.; Tamura, M.; Nakagawa, Y.; Okumura, K.; Tomishige, K., Structure of ReOx Clusters Attached on the Ir Metal Surface in Ir-ReOx/SiO2 for the Hydrogenolysis Reaction. J. Phys. Chem. C 2012, 116, 23503-23514. 47. Zhou, Q.; Gong, W. C.; Xie, L.; Zheng, C. G.; Zhang, W.; Wang, B.; Zhang, Y. F.; Huang, X., Theoretical Studies on the Electronic Structures and Photoelectron Spectra of Tri-rhenium Oxide Clusters: Re3On- And Re3On (n = 1-6). Spectrochim. Acta A 2014, 117, 651-657. 48. Deng, C.; Leng, L.; Zhou, J.; Zhou, X.; Yuan, W., Effects of Pretreatment Temperature on Bimetallic Ir-Re Catalysts for Glycerol Hydrogenolysis. Chinese J. Catal. 2015, 36, 1750-1758. 49. D'Agostino, C.; Mitchell, J.; Mantle, M. D.; Gladden, L. F., Interpretation of NMR Relaxation as a Tool for Characterising the Adsorption Strength of Liquids Inside Porous Materials. Chem. Eur. J. 2014, 20, 13009-13015.


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ACS Catalysis

50. Weber, D.; Mitchell, J.; McGregor, J.; Gladden, L. F., Comparing Strengths of Surface Interactions for Reactants and Solvents in Porous Catalysts Using Two-dimensional NMR Relaxation Correlations. J. Phys. Chem. C 2009, 113, 6610-6615. 51. D'Agostino, C.; Feaviour, M. R.; Brett, G. L.; Mitchell, J.; York, A. P. E.; Hutchings, G. J.; Mantle, M. D.; Gladden, L. F., Solvent Inhibition in the Liquid-Phase Catalytic Oxidation of 1,4butanediol: Understanding the Catalyst Behavior from NMR Relaxation Time Measurements. Catal. Sci. Technol. 2016, 6, 7896-7901. 52. Lockwood, G. K.; Garofalini, S. H., Proton Dynamics at the Water–Silica Interface via Dissociative Molecular Dynamics. J. Phys. Chem. C 2014, 118, 29750-29759. 53. Hintermeier, P. H.; Eckstein, S.; Mei, D.; Olarte, M. V.; Camaioni, D. M.; Baráth, E.; Lercher, J. A., Hydronium-Ion-Catalyzed Elimination Pathways of Substituted Cyclohexanols in Zeolite H-ZSM5. ACS Catal. 2017, 7, 7822-7829. 54. Copeland, J. R.; Santillan, I. A.; Schimming, S. M.; Ewbank, J. L.; Sievers, C., Surface interactions of glycerol with acidic and basic metal oxides. J. Phys. Chem. C 2013, 117, 2141321425. 55. Copeland, J. R.; Shi, X. R.; Sholl, D. S.; Sievers, C., Surface Interactions of C2 and C3 Polyols with γ-Al2O3 and the Role of Coadsorbed Water. Langmuir 2013, 29, 581-593. 56. Engstrom, J. R.; Tsai, W.; Weinberg, W. H., The Chemisorption of Hydrogen on the (111) and (110)-(1 × 2) Surfaces of Iridium and Platinum. J. Chem. Phys. 1987, 87, 3104-3119. 57. Hibbitts, D.; Tan, Q.; Neurock, M., Acid Strength and Bifunctional Catalytic Behavior of Alloys Comprised of Noble Metals and Oxophilic Metal Promoters. J. Catal. 2014, 315, 48-58. 57 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

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58. Liu, Y.; Vjunov, A.; Shi, H.; Eckstein, S.; Camaioni, D. M.; Mei, D.; Baráth, E.; Lercher, J. A., Enhancing the Catalytic Activity of Hydronium Ions Through Constrained Environments. Nat. Commun. 2017, 8, 14113. 59. Hausoul, P. J. C.; Beine, A. K.; Neghadar, L.; Palkovits, R., Kinetics Study of the Ru/CCatalysed Hydrogenolysis of Polyols-Insight Into the Interactions with the Metal Surface. Catal. Sci. Technol. 2017, 7, 56-63. 60. Auneau, F.; Michel, C.; Delbecq, F.; Pinel, C.; Sautet, P., Unravelling the Mechanism of Glycerol Hydrogenolysis over Rhodium Catalyst Through Combined Experimental-Theoretical Investigations. Chem. Eur. J. 2011, 17, 14288-14299. 61. Zaffran, J.; Michel, C.; Delbecq, F.; Sautet, P., Towards more Accurate Prediction of Activation Energies for Polyalcohol Dehydrogenation on Transition Metal Catalysts in Water. Catal. Sci. Technol. 2016, 6, 6615-6624. 62. Michel, C.; Auneau, F.; Delbecq, F.; Sautet, P., C-H Versus O-H Bond Dissociation for Alcohols on a Rh(111) Surface: A Strong Assistance from Hydrogen Bonded Neighbors. ACS Catal. 2011, 1, 1430-1440.


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