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A Combined Theoretical and Experimental Approach for Platinum Catalyzed 1,2-Propanediol Aqueous Phase Reforming Roberto Schimmenti, Remedios Cortese, Lidia Godina, Antonio Prestianni, Francesco Ferrante, Dario Duca, and Dmitry Yu. Murzin J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 16 Jun 2017 Downloaded from http://pubs.acs.org on June 16, 2017

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The Journal of Physical Chemistry

A Combined Theoretical and Experimental Approach for Platinum Catalyzed 1,2-Propanediol Aqueous Phase Reforming Roberto Schimmenti,† Remedios Cortese,† Lidia Godina,‡ Antonio Prestianni,† Francesco Ferrante,† Dario Duca,∗,† and Dmitry Yu. Murzin∗,‡ Dipartimento di Fisica e Chimica, Universit` a degli Studi di Palermo, Viale delle Scienze Ed. 17, I-90128 Palermo, Italy, and Laboratory of Industrial Chemistry and Reaction Engineering, Process Chemistry Centre, ˚ Abo Akademi University, Biskopsgatan 8, FIN-20500 ˚ Abo/Turku, Finland E-mail: [email protected]; [email protected]

Abstract Decomposition pathways of 1,2-propanediol (1,2-PDO) on platinum were investigated by means of experiments and quantum-mechanical calculations. Different reaction paths on a Pt(111) model surface were computationally screened. Gas and liquid phase products distribution for aqueous phase reforming of 1,2-PDO solutions was experimentally analyzed. A mechanistic approach was used to trace the preferred paths according to calculated activation barriers of the elementary steps; in this way presence or absence of some hypothesized intermediates in the experiments was computationally ∗

To whom correspondence should be addressed Università degli Studi di Palermo ‡˚ Abo Akademi University †

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rationalized. Hydroxyacetone was demonstrated to be among the most favored decomposition products. The competition between C−H, O−H and C−C bond cleavages was investigated, revealing that shortening of the carbon chain occurs most likely via decarbonylation steps.

Introduction Aqueous Phase Reforming (APR) is one of the most interesting options for sustainable production of hydrogen from renewable feedstock. Besides the wide interest towards methodologies that exploit low cost and non-toxic feedstocks for obtaining fuels, 1 the practical advantages of the process itself make it a valuable candidate for the large scale production of H2 or alkanes. Working with aqueous solutions at relatively mild conditions, especially if compared with other H2 production methods, combined with the possibility to couple the reaction with the water-gas shift (WGS), maximizing the yield of the overall process, was recognized as the mainstays of this strategy, since it was first introduced by Dumesic and coworkers in 2002. 2 Despite these premises and many studies in the literature regarding APR, or more generally deoxygenation of biomasses, several concerns related to overall process economics as well as poor hydrothermal stability of the catalysts and limited understanding of the involved reaction mechanisms, hinders the implementation of APR. The latter has been actually explored by both experimental and computational approaches 3–5 and recently, Godina et al. 6 demonstrated that, in Pt catalyzed APR of sorbitol, more than thirty liquid phase products could be formed as the result of a very complex reaction network, involving retro-aldol transformations along with either more common dehydrogenation/decarbonylation or dehydrogenation and hydrogenation steps. Thus, computational investigations could be really useful to unveil the metal-catalyzed reaction mechanism of different polyols. As a consequence, a large number of independent computational works on the topic is found in the literature, widely diversified from the 2


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viewpoint of computational methods (periodic DFT calculations above all), models and substrates used. 7,8 Ranging from ethylene glycol 5 to glycerol, 9 as substrates suitable to be reformed, the majority of these studies is largely based on the use of linear energy relationships for the calculation of transition state energies. These theoretical approaches, when capable to provide useful insights for guidelines catalyst screening, require an exhaustive benchmarking and a proper statistical analysis, as clearly explained by Zaffran et al. 10 In recent investigations we demonstrated the possibility to use purposely shaped clusters for determining reaction mechanisms in the case of dehydrogenation of lignan-derivitives on gold 11,12 and of 2-methyl-3-butyn-2-ol hydrogenation on palladium. 13 The same approach was established to be valid also in the analysis of catalyst modification and structure-activity relationships. 14 Due to its computational efficiency, the approach above can be considered as a reasonable compromise between the direct determination of reaction intermediates and transition states — obtained, employing heavy periodic DFT calculations — and the use of Brønsted-Evans-Polanyi (BEP) 15,16 or Transition State Scaling (TSS) 17 relationships; therefore, it could be suitable for studying complex and tangled reaction paths, as those of APR. Symmetrical polyalcohols such as ethylene glycol and glycerol were thoroughly discussed in the literature while limited attention has been devoted to other possible feedstock molecules, such as 1,2-PDO, even if the latter is an intermediate in the glycerol reforming. On the contrary, 1,2-PDO could be used as a representative model for more complex polyalcohols, bearing C−C, C−O, C−H and O−H bonds. Moreover, this molecule has an appropriate size to carry out an extensive computational study since it gives a relatively simple ensemble of decomposition products traceable with experimental methods. According with these viewpoints, the present paper discusses the detailed study of 1,2-PDO decomposition pathways on a Pt catalyst model, obtained by means of DFT based calculations, combined with the experimental analysis of the decomposition products of APR. The molecular events of the decomposition process were analyzed and framed within a complex reaction network

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while most of the species identified experimentally were connected to specific molecular pathways. The use of a combined experimental and theoretical approach is expected to provide a comprehensive picture of the 1,2-PDO decomposition reaction useful for understanding the behavior of more complex oxygenated molecules.

Experimental Details Aqueous-phase reforming of 1,2-PDO was performed in a continuous reactor on a Pt carbonbased catalyst at 498K and 29.7 bar. Metal was supported on a polymer-based spherical activated carbon (PBSAC) via ion exchange according to the method described by Klefer. 18 Catalyst surface area (2104 m2 g−1 ) was measured by means of nitrogen physisorption. The metal content was 0.46 wt % as confirmed by inductively coupled plasma analysis (ICP). Fresh and used catalytic samples were characterized by means of temperature-programmed oxidation (TPO) and thermal gravimetric analysis (TGA). An experimental set-up and experimental procedure were described previously. 6 Continuous reactor was used in a trickle-bed mode. The catalyst bed (1 g) was located in the middle of a stainless-steel tube, and surrounded by sand beds. A stainless-steel mesh together with quartz wool prevented from filler leakage. The reactor was placed in a furnace. Gas and liquid feed inlets provided continuous flows of a carrier gas (nitrogen with 1% He) and an aqueous solution of 1,2-PDO (5 wt %). Helium in nitrogen played a role of an internal gas standard. The gas flow was 20 mL min−1 ; the liquid flow varied from 0.1 to 0.3 mL min−1 . An outlet for liquid samples was located under the reactor. A gas-liquid separator had an outlet for online gas sampling, and was combined with a liquid waste bottle. The catalyst was reduced in-situ under an excess hydrogen flow (60 mL min−1 ) at 673 K during two hours. Nitrogen was used for hydrogen removal and reactor pressurized to 29.8 bar and slowly cooled down to 498 K. Aqueous solution of 1,2-PDO was fed with flow rates 0.1-0.3 mL min−1 , which corresponds to the weight hour space velocities of 0.35-

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1.05 h−1 calculated as the mass of the substrate per mass of the catalyst per hour (gsubst · gcat −1 h−1 ). Quantification of gas products was conducted via an online analysis by means of a micro-GC instrument (Agilent Micro-GC 3000A) equipped with four columns: Plot U, OV-1, Alumina and Molsieve. Quantitative analysis of liquid products was performed by an HPLC instrument (Agilent 1100) equipped with an Aminex HPX-87H column and a refractive index detector (RI). The analysis was performed at 45◦ C under isocratic conditions (0.005M H2 SO4 /H2 O) and a flow rate of 0.6 mL min−1 . The column was quantitatively calibrated for the either anticipated or suggested DFT calculations reaction products (external standard method). Peak identification was based on retention times. In addition, identification of liquid-phase products was improved by a qualitative GC-MS analysis. Dry hexanol extracts were analyzed by means of a Hewlett-Packard 6890/5973 gas chromatograph coupled to mass selective spectrometer detector. Helium was used as a carrier gas. The Agilent 19091J-002 capillary column (25 m x 0.20 mm x 0.11 µm) was hold at 310 K for 5 min, heated up to 383 K with heating rate 3 K min−1 , and then to 503 K with 20 K min−1 . The substrate conversion is determined by:

conversion(%) =

N(f eedout) 1− N(f eedin)

· 100

(1)

where Nf eedin is an incoming molar flow of the substrate (mol min−1 ), and Nf eedout is an outcoming molar flow of the unreacted substrate (mol min−1 ). The yield to products is determined by:

yield(%) =

N(product) · 100 N(product) · x

(2)

where Nproduct is a molar flow of the product (mol min−1 ), and x is a stoichiometric coefficient showing theoretically possible product molar amount formed by one mole of feed. This coefficient is equal to 4 for H2 , to 2 for CO and CO2 , to 0.5 for alkanes C4-C7 and to 1 for 5



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all other substances. Molar flows are determined by:

Ni = C i · ν i

(3)

where Ci is the concentration of a substance (mol L−1 ) and νi its volumetric flow (L min−1 ).

Computational Details All the calculations were performed by the Gaussian 09 package, 19 using the M06-L exchangecorrelation functional, which has been recognized to be efficient for treating transition metals complexes as well as for reproducing dispersion interactions. 20 For all the reported calculations the Los Alamos LANL2 effective core potential and the corresponding double-ζ basis set was used for Pt atoms while, for lighter atoms, the D95 basis set with the addition of the LANL2dp polarization functions was employed. Through a proper cut of the fcc bulk platinum lattice, a three-layers Pt30 cluster showing both the (111) and (100) surfaces was obtained. The former was used as a model surface for the reactivity study. Even if treating a molecular cluster, this model should ensure the same sites topology as the periodic (111) surface. In order to search for a cluster structure that, together with the lowest energy, would retain the initial symmetry, the cluster was fully relaxed trying different spin multiplicities. This resulted in the conclusion that multiplicity of seventeen is the one that fulfils these criteria. This is in line with the findings of Goddard et al. on Pt clusters of comparable sizes. 21 Reaction intermediates and transition states were fully optimized using the same spin multiplicity, checking for the nature of the stationary point on the potential energy surface by calculation and inspection of vibrational frequencies. All the energy values were corrected for the zero-point vibrational contribution. For the optimization of all the reaction intermediates produced by the first dehydrogenation step, it was ensured that the Pt−C or Pt−O bond formed, would involve the central Pt atom of the selected (111)-like facet; this should allow to exclude non-negligible edge effects on the 6


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reaction energetics. The reported energies of the reaction intermediates are referred to the absolute energy value of the most hydrogenated species in the same reaction scheme, while the activation barriers are taken with respect to the reactant of the elementary step to which the transition state is associated. Hydrogen produced along with the dehydrogenation process was assumed to easily diffuse on the cluster. Hence, every time a couple of chemisorbed hydrogen atoms was produced, it was removed from the metal cluster, hypothesizing that its presence — not considering here hydrogen assisted desorption processes — would not influence the following surface mechanisms, before the occurrence of the H2 desorption step. Binding energy was calculated through the following equation:

BE = Etot − EP t30 − Eads

(4)

where Etot is the total energy of the system made up of cluster and adsorbate, EP t30 is the absolute energy of the bare Pt30 cluster while Eads is the gas-phase energy of the considered adsorbate. The obtained value was corrected for the basis set superposition error (BSSE), estimated by the application of the counterpoise method as suggested by Boys and Bernardi. 22 In the analysis of the 1,2-PDO decomposition mechanism, alternative routes that differ by more than 30 kJ mol−1 from the lowest activation barrier in a given step, were ruled out as energetically unaffordable. This has been proposed on the basis of the benchmarking study of Zhao and Truhlar who have estimated for the functional used in the current work an averaged mean unsigned error (AMUE) in the determination of transition states of approximately 15 kJ mol−1 ; 23 thus, considering twice this reference value should be enough to discriminate between different kinetic pathways.

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Results and Discussion Theoretical modeling The binding energy of 1,2-PDO on the Pt surface has been estimated to be −46.9 kJ mol−1 . Oxygen belonging to the primary alcoholic moiety interacts atop with a Pt atom as showed in Figure 1, their bond distance being 2.51 ˚ A, while the secondary hydroxyl group oxygen A from the nearest acting as an intramolecular hydrogen bond acceptor, is placed at 3.40 ˚ metal atom. This is in agreement with previous findings suggesting that during adsorption of oxygenates on metal surfaces, the intramolecular hydrogen bond is very likely retained. 24,25 A similar conformation was found also by Kandoi et al. 5 and Gu et al. 26 in the case of ethylene glycol on a Pt(111) surface. With the computational approach chosen here, the binding energy of ethylene glycol on Pt30 is −43.3 kJ mol−1 , which suggests that, at least for the adsorption stage, the substitution of one of the hydrogen atoms with a methyl group does not have a significant influence. Furthermore, ethylene glycol coordination geometry is very similar to that reported in the literature and also the calculated value for the binding energy is in close agreement (−35.7 and −54.9 kJ mol−1 for the two studies 5,26 cited above). This comparison would suggest that the proposed computational settings, as well as possible border effects, scarcely influence the adsorption energetics. [Figure 1 about here.] Kinetics of the first H2 molecule loss Step I 1,2-PDO shows five chemically different hydrogen atoms namely methylic (bound to the methylic carbon, Cm), primary (bound to the primary carbon, C1), secondary (bound to the secondary carbon, C2) and those of the primary (O1) and secondary (O2) hydroxyl moieties. These, in principle, trace five different dehydrogenation routes. Four of them are reported in Scheme 1 with the respective activation barriers, whereas the optimized geometries of the reaction intermediates produced in the first dehydrogenation step are 8


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reported in the central panel of Figure 2. In the latter and in the scheme above, INTn (with n = 1, 9) represents a given intermediate produced along the dehydrogenation whereas LA and HA the originated lactic aldehyde and hydroxyacetone, respectively. The fifth possible reaction path, corresponding to the Cm−H bond breaking, requires overcoming a energy barrier of 126.5 kJ mol−1 : it will not be treated at this stage of the investigation since this barrier is too high if compared to that of the other paths. This behavior seems to be in accord with the study of Shibuta and co-workers that demonstrated through an ab initio molecular dynamics approach that the methylic dehydrogenation is not favored in the case of ethanol decomposition on a Pt32 cluster. 27 [Figure 2 about here.] The activation barrier for C1 dehydrogenation was calculated to be 83.2 kJ mol−1 . Comparing this value with the one associated to the Cm−H bond cleavage, a decrease of approximately 40 kJ mol−1 is revealed, which can be attributed to the effect of the hydroxyl group in C1 and to a more favorable orientation of 1,2-PDO for the C1−H breaking. A slightly higher barrier of 93.4 kJ mol−1 was calculated in the case of the C2 dehydrogenation. This difference may be ascribed to the different geometric features of the two involved transition states. Indeed, the transition state associated with the C1−H bond cleavage has a structure more similar to the adsorption configuration of 1,2-PDO with respect to that of the C2−H bond cleavage (see Figure 3), thus requiring a smaller energy for the structural rearrangement. [Figure 3 about here.] Dehydrogenation of O1 is nearly isoenergetic with that of C2, being the activation barrier 95.7 kJ mol−1 . The most favorable kinetic pathway involves dehydrogenation of O2, requiring only 74.7 kJ mol−1 . On the whole, the calculated activation barriers follow the order O2 < C1 < C2 ≃ O1. 9



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INT8 -28.2

H3C

CH3 H3C

CH

O

OH

OH CH

Pt CH

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O Pt

Pt

C

Pt

C INT7 H2

155.0

CH3

139.5

107.5 O

HO

Pt

INT1 -21.0

56.5

O Pt

C INT4 H2

C H2

INT6 15.5

62.2

CH OH

Pt

C H2

Pt

75.3

CH3 CH

O

O

5.3

54.4

95.7

CH3

OH

74.7

O CH HO

OH

HO

C C H2

1,2-PDO

CH3

H3C

LA -22.1

83.2 105.5

HA -74.9

133.5

Pt

77.7

OH OH

C

C H

93.4 OH

H3C

O

CH

C H2

CH

C H2 INT3

H3C

OH CH

90.7

INT2 -30.8 Pt

-59.3

H3C

OH CH

OH

109.0

H3C

O C

C H2

60.7

INT6 15.5

89.4

HO

INT8 -28.2 OH

OH C

C

H

ENOL INT5 -21.8

Pt

Pt

Pt

Pt H3C

Pt

CH

O

CH H3C

OH C

INT9 Pt -26.8 Pt

Scheme 1: Reaction scheme for steps I and II: the activation barriers (in kJ mol−1 ), calculated with respect to the associated precursor, are reported in black while the energy of each reaction intermediate or stable species, calculated taking the energy of the adsorbed 1,2PDO as reference, is reported in blue.

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The exact quantification of these activation barriers seems to be, however, at least controversial. This fact could be in part due to the different approaches employed in their determination. As an example, taking into account glycerol on Pt(111), Liu and Greeley 7 stated that the energy required for the first dehydrogenation step follows the trend C2 < O2 < O1 < C1, being their values 75.5, 79.4, 82.1, 97.2 kJ mol−1 , respectively, and being the first three values within a critical range of 6 kJ mol−1 . Chen et al. 9 on the contrary, calculated activation barriers for C1, C2 and O2 dehydrogenation of 36.3, 37.2 and 101.9 kJ mol−1 , respectively while more recently, Rangarajan et al. proposed, using an automatized approach based on the combination of both empirical and DFT derived correlation schemes, the following bond cleavage behavior: C1 < C2 < O1, namely showing the energy values 20.9, 49.5, 74.1 kJ mol−1 . 28 These latter findings seem to follow the energetic behavior found in the here presented investigation, even if disagreements in the values of the energy barrier occurs. Step II The optimized structures obtained in the second dehydrogenation step, leading to the release of one H2 molecule, are reported outside the central panel of Figure 2. Each surface intermediate, obtained from the first step, could in principle lead to four other species, being either surface intermediates or stable products. However, each of these could be obtained as product of different pathways depending on the order followed in the two consecutive dehydrogenation reactions. As an example, enolic intermediate (INT5), shown in Scheme 1, can be either obtained from INT2 or INT3. Moreover, with respect to the Cm−H bond breaking, only in the case of INT3 the methylic group could effectively interact with the metal surface. It was evaluated that this dehydrogenation requires 154.0 kJ mol−1 thus, this particular C−H bond cleavage could be excluded from the reaction mechanism network hence, the number of involved intermediates to be analyzed decreased further. Formation of HA, from INT1 and both enolic INT5 and lactic aldehyde, LA, from INT2 are by far the most favorable routes, with activation barriers of 56.5, 60.7 and 77.7 kJ mol−1 , respectively

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(see Scheme 1). Reactivity of 1,2-PDO on palladium (111) surface and ethylene glycol on platinum (111) were previously investigated by HREELS. 29,30 Based on the corresponding results, it is possible to argue that the formation of 1,2-dioxy species is more likely to occur on Pd, than on Pt. The calculations of this work confirm that the dehydrogenation of both alcoholic moieties is very unfavorable on the Pt surface. Indeed, the activation barriers of the molecular events leading from INT1 and INT4 to INT7 are the highest in the reaction path, i.e. 155.0 and 139.5 kJ mol−1 , and as a consequence, INT7 was excluded from further analyses. All the dehydrogenation paths discussed so far involved an alternation of C−H or O−H bond cleavages. In fact, 1,2-PDO could as well undergo a double dehydrogenation on the primary carbon leading to INT9 via INT2. For the production of the latter intermediate an activation barrier of 89.4 kJ mol−1 was found. It is interesting to notice that this process requires approximately 30 kJ mol−1 more than that consisting of the alternate C−H bond breaking leading to INT5. This trend is in agreement with the study of Chen et al. 9 which, for glycerol, estimates an alternate dehydrogenation path as more feasible compared with that associated to consecutive dehydrogenations on the same carbon. Energetics of Steps I and II Intermediates INT3 was evaluated to be the most stable intermediate of step I, being 59.3 kJ mol−1 more stable than adsorbed 1,2-PDO, followed by INT2, which is 30.8 kJ mol−1 more stable than its precursor. The exoergicity of the two molecular processes leading to INT3 and INT2 is most probably due to the substitution of C−H bonds with stronger C−Pt bonds. In INT3, whose optimized structure is reported in Figure 4B, the dehydrogenated C2 is atop coordinated, with a C−Pt bond distance of 2.13 ˚ A. This interaction geometry is characterized by a tetrahedral arrangement that allows both the interaction of the primary hydroxyl group with the surface and the retaining of an intramolecular hydrogen bond in which O1 acts as donor. A similar geometry is adopted by INT1 (Figure 4A), except for a slightly longer

12


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intramolecular O···H bond distance of 2.32 ˚ A, compared with the 2.30 ˚ A one found for INT3. The INT1 intermediate, originated from O1−H bond cleavages in step I, is 21.0 kJ mol−1 more stable than the reference state, while formation of INT4, derived from O2−H bond breaking, is slightly endoergic. It is interesting to note that energetic differences between these two intermediates were usually not encountered in the previous studies performed on glycerol. 5,9 Probably, this is a consequence of a better description of hydrogen bond interactions provided in the present work using the M06-L functional, which stabilizes INT1. It should be also mentioned that the unsaturated O2 moiety of INT1 interacts with a Pt atom at the edge of the cluster (see Figure 4A); in fact, the structure in which O2 interacts with the Pt atom at the center of the (111)-like surface (INT1r, see Figure S1) resulted 40.0 kJ mol−1 less stable than INT1. This destabilization may be due to the combined effects of the adsorption site change and to the lack of interaction between the −OH moiety and the cluster, which instead takes place in INT1. For the sake of this destabilization along with a conformation not suited to the successive dehydrogenation (see Figure S1), INT1r was not further investigated. [Figure 4 about here.] HA is by far one of the most stable product formed in step II, being 74.9 kJ mol−1 more stable than the reference state. INT8, LA and INT5 can be considered approximately isoenergetic, being respectively 28.2, 22.1 and 21.8 kJ mol−1 more stable than the adsorbed 1,2-PDO. Noticeably INT9, being the product of double dehydrogenation on the primary carbon, is stabilized by 26.8 kJ mol−1 . The remaining two intermediates, namely INT6 and the 1,2-propanedioxy species INT7, are the only one less stable than the considered reference state, being respectively 15.5 and 75.5 kJ mol−1 higher in energy. Further dehydrogenations In view of the calculated energy barriers, it seems that the most preferred products of the first and second dehydrogenations of 1,2-PDO are HA, LA and INT5. However, it is possible 13



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to notice from Scheme 1 that the formation of INT8 and INT9 would require energy barriers just slightly higher (ca. 12 kJ mol−1 ). Therefore, it is difficult to discard a priori them from the reaction network. For this reason almost all the reaction intermediates optimized in the previous molecular steps were used as starting point of further investigations, with the exception of HA, LA and INT7. Hence, while the first two are stable products the latter has prohibitively high activation barriers for its formation. In Scheme 2 the suggested reaction paths for the C−H and O−H bond breakings of INT5, INT6, INT8 and INT9 are shown while the optimized structures of their decomposition products are displayed in Figure 5. It has to be underlined that the dehydrogenation paths of INT5 and INT8 are connected through the INTa intermediate, while INTb links INT5 to INT6. INTd leads from INT5 to INT9, but the two paths were retained independent. [Figure 5 about here.] The enolic INT5 has four chemically different hydrogen atoms, which could in principle lead to an equal number of intermediates, namely INTa-d. INTa was evaluated to be the most favored product, having an activation barrier of 10.1 kJ mol−1 for the O−H bond cleavage of the secondary alcoholic group and being evaluated the final product to be 39.4 kJ mol−1 more stable than the same INT5. The activation barriers leading to the other intermediates were 38.1 (INTb), 57.8 (INTc) and 69.5 (INTd) kJ mol−1 . On the basis of this energy barriers, it seems quite probable that INT5 will proceed to successive dehydrogenation via INTa. This intermediate could be produced also from INT8 through the secondary carbon dehydrogenation requiring only 25.2 kJ mol−1 . It is worth noting that the same INT8 could otherwise lose its remaining alcoholic hydrogen to form INTg, its activation barrier being 85.3 kJ mol−1 . Even if conversion of INTg to pyruvic aldehyde (PA) should be really easy (the associated transition state is approximately isoenergetic with INTg) and the formed product is considerably more stable than INT5, we can rule out formation of PA. In fact, both INTa and INT8 do prefer to follow other paths.

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INTa intermediate could follow four alternative routes. The primary carbon dehydrogenation, with an activation barrier of 84.7 kJ mol−1 , gives rise to INTh which is 31.8 kJ mol−1 more stable than INT5. Furthermore, contrary to what already reported for 1,2-PDO, the Cm−H bond cleavage seems to be much easier, showing an activation barrier of 47.1 kJ mol−1 to produce INTe, whose optimized structure is reported in Figure 6A. It is noteworthy that, as reported above, also the Cm−H bond rupture of INT5 shows a similar but slightly higher activation barrier (57.8 kJ mol−1 ). This is not a coincidence since in both cases the methylic group is directly bound to unsaturated C−C or C−O fragments, which should promote its dehydrogenation. [Figure 6 about here.] Moreover, as reported in Figure 7A-B, both the transition states leading to INTc and INTe respectively from INT5 and INTa have quite similar structures characterized by the atop coordination of the primary carbon to a Pt atom and by η 2 coordination of the remaining carbon atoms. This interaction geometry, along with the presence of the intramolecular hydrogen bond, should be behind lower activation barriers associated to Cm−H bond cleavages if compared to the same molecular event for the 1,2-PDO case. On the basis of this favorable geometric arrangement on the Pt surface, INTe, whose optimized structure is reported in Figure 6A, gives the most stable reaction intermediate appeared in this stage of the decomposition mechanism, experiencing a decrease in energy of 65.7 kJ mol−1 with respect to INT5. In order to account for the influence of the Pt atoms at the edge of the cluster on the stability of INTe, this was re-optimized changing its coordination sites. The resulting structure (INTer, see Figure S2) shows the Cm interacting with the Pt atom at the center of the (111)-like surface, while the C1 with that at the edge of Pt30 . Since INTe and INTer resulted almost isoenergetic (in the limit of 4 kJ mol−1 ) it could be suggested that the effect of the edge coordination sites could be neglected in this context. Finally, the step leading from INTa to PA would is the least energetically favorable, with the energy barrier of 98.0 kJ mol−1 . 16


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[Figure 7 about here.] Although the discussion so far seems to individuate INTa as the starting point of the main branch of the reaction, the dehydrogenation path of INTb was investigated as well. Indeed, INTb could be also obtained from INTa following a rather easy intramolecular hydrogen shift, requiring 38.3 kJ mol−1 to occur. Three different dehydrogenation paths can be traced starting from INTb, leading to PA, an enolic aldehyde (EA) and INTf, reported in Figure 6B and characterized by activation barriers of 71.2, 59.2 and 39.9 kJ mol−1 , respectively. Comparing the reaction mechanism branches originating from INTa and INTb, it turns out that the formation of INTf is competitive with that of INTe. While the formation of EA could be to some extent feasible, also in this case the reaction path leading to PA has to be ruled out. It seems therefore that PA cannot be produced starting from 1,2-PDO, at least within the reaction network reported in this work. Finally, with respect to INT9 intermediate, only dehydrogenation of the primary hydroxyl group was analyzed: O1−H cleavage indeed would determine the formation of an intermediate showing a terminal C−O moiety, which according to the conclusions concerning INTf should be particularly favored. As presumable, a nearly non-existent activation barrier was found for this bond breaking, suggesting an easy to occur process, also characterized by a resulting intermediate, INTχ, 52.0 kJ mol−1 more stable than its precursor. The fate of INTe and INTf Through the examination of the above mentioned decomposition steps, both INTe and INTf were shown to be among the easiest reaction intermediates to be formed. Indeed, all the others surface species or products at the same level of dehydrogenation, namely INTh, INTg, EA and INTr, could be formed only through reaction branches that involve higher activation energies. Thus, it is worth discussing the fate of only INTe and INTf surface species, as reported in Scheme 3. Both surface intermediates could contribute to the overall H2 production with four more hydrogen atoms. However, with respect to INTe, only the primary carbon and the hydroxyl group dehydrogenation were 17



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investigated, as the related hydrogen atoms are thought to be more accessible than those of the methylenic fragment. O C

OH

H2C

O

C Pt

90.7

Pt

OH

C

INTβ 6.4

Pt O O

H2C

C H

C

Pt

O Pt

INTe

H2C

O

Pt Pt

74.7 C

C

H2C

H Pt

70.7

C Pt

INTω -15.3

Pt

INTα 21.4 O

O

HO C

C

O

53.0

INTj -2.4

C H3C

H3C Pt

Pt Pt

Pt

INTf

Scheme 3: Reaction scheme for further dehydrogenation steps starting from INTe and INTf. Activation energies (in kJ mol−1 ) calculated with respect to the associated precursor are reported in black, while in blue is indicated the energy of each reaction intermediate or stable species, taking as references INTe and INTf. This choice is in agreement with previous studies suggesting that, in the case of ethanol, the methylene dehydrogenation occurs only when all the other hydrogen atoms were already removed. 8 Activation barriers of 90.7 and 74.4 kJ mol−1 were found individually for C1−H and O1−H bond ruptures. Both processes resulted endoergic, being INTα and INTβ 21.4 and 6.4 kJ mol−1 less stable than INTe, respectively. These reaction intermediates would produce INTω, which is represented in Figure 8B. [Figure 8 about here.] Only further dehydrogenation originating from INTα was treated. The latter was produced by the path with the lower activation barrier. Moreover, the adsorption geometry 18


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of INTβ shows the hydroxyl hydrogen directed outward the surface and forming an intramolecular hydrogen bond with the carbonylic oxygen. Therefore, INTβ is not prompt to be dehydrogenated. The last step from INTα to INTω requires 70.7 kJ mol−1 to occur, with the final product more stable of 15.3 kJ mol−1 than INTe. In the case of INTf the methylic group points out from the Pt cluster and is stucked in this configuration because of the presence of two C−Pt bonds as a consequence, only the secondary hydroxyl dehydrogenation was considered. For this, an activation barrier of 53.0 kJ mol−1 was calculated and the product, INTj, is nearly isoenergetic with INTf, being only 2.4 kJ mol−1 more stable. C−C bond breaking The C−C bond breaking is the key step that, in the case of APR process, leads from the starting feedstock to shorter polyols. Thus, the C1−C2 bond cleavage, at different reaction steps, was studied for selected reaction intermediates, which showed different dehydrogenation degree (dHd). According to Liu and Greeley 7 the C−C bond cleavage should be more favorable if less hydrogenated compounds are considered. The results of our calculations, reported in Table 1, would seem to confirm this hypothesis in some cases. Indeed, while the C−C bond breaking for INT3 requires the overcoming of an activation barrier of 224.0 kJ mol−1 , the same process is much more feasible in the case of the enolic INT5, having an activation barrier of 95.7 kJ mol−1 . [Table 1 about here.] It is interesting to note that when a terminal C−O moiety is present in an intermediate, the C−C rupture becomes easier, in agreement with the literature, 7 as it is evident by comparing the EaC1−C2 values of INTχ and INTb. Furthermore, even if a general decrease in the activation barriers characterizing the C−C bond cleavage along with the increasing of dHd values is observed, it is difficult to find a direct relationship between the number of hydrogens lost and the energetic requirement for the reforming. As an example, taking into account 19



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INTa and INTb, which have the same dehydrogenation degree, reaction barriers of 109.6 and 132.3 kJ mol−1 were observed, both being higher than in the case of a more hydrogenated intermediate, namely INT5. INTe and INTf can be considered as two limiting cases: despite having the same dehydrogenation degree, they have different activation barriers, 211.3 and 63.6 kJ mol−1 . This large difference reflects the thermodynamic stabilization of INTe and INTf. In particular, due to its geometry and unsaturations, INTe seems to be very well suited to fit in the fcc site available on the metallic surface (Figure 6). Hence unexpectedly high activation barrier may be explained considering that the C−C bond breaking would disrupt this stable conformation. Besides the analysis of the activation barriers behavior as a function of the dehydrogenation degree, it can be instructive to frame the C−C bond scissions within the energetic network of the already considered dehydrogenation processes. Comparing the activation barriers of the dehydrogenation and of the C1−C2 bond scission into given intermediates, it is clear that only for INTf, INTj and INTω the latter becomes a competitive hence a feasible process.

Potential energy curve for 1,2-PDO dehydrogenation on Pt(111) Due to these findings, it is useful to review, in the form of potential energy curve (PEC), the whole reaction mechanism. In Figure 9 the 1,2-PDO dehydrogenation PEC is reported. As a matter of fact the distinction between reaction paths, as it was already shown for the first dehydrogenation steps, could be controversial in some cases. These peculiar/borderline cases were as well reported, along with the path following the lowest activation barriers. Other paths characterized by prohibitive activation energy were conversely discarded. [Figure 9 about here.] Besides the reaction path leading to HA from INT1, which has been already recognized as a quite probable process, 1,2-PDO could convert to INT2 or INT3 overcoming activation barriers of 83.2 and 93.4 kJ mol−1 . With respect to the latter energy values, it is worth noting 20


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that despite the slightly higher activation barrier, INT3 is 28.5 and 38.3 kJ mol−1 more stable than INT2 and INT1, respectively. However, the path leading to INT5 from INT2 seems to be more affordable. A similar case is found for the conversion of INTa to INTb via hydrogen shift or of INTa to INTe via dehydrogenation. The former process is endoergic, being INTb 22.4 kJ mol−1 less stable than INTa, but requires a lower activation energy than the latter although, in this, the produced intermediate, namely INTe, is 39.4 kJ more stable than its precursor. In any case, at this level of investigation it cannot be predicted which factors, i.e. activation barrier values or final product stabilities, are more significant in determining either whole or local reaction pathways, especially when the energetic values involved are not too different. For this purposes more comprehensive methods such as microkinetic modeling would be required. BEP relationships The very high number of transition states required for having an overall picture of even small biomass derived molecules justifies the research of efficient strategies for activation barrier calculations, starting from pure thermodynamic quantities; in this sense, BEP relationships could be a valuable guide for a fast screening of empirically obtained activation barriers, to be refined with more accurate approaches. 10 For this reason, the correlation between the activation energy (Ea ) of different bond breaking events and the formation energy (∆E) of the associated surface reaction intermediates from its precursor, were evaluated for the case presented here. This relation was found, even if with different extents, for C-H, O-H and C-C bond breakings, as shown in Figure 10A; a similar trend was also found considering C-H and O-H bond breakings as part of the same data set (Figure 10B). [Figure 10 about here.] Finally, the following equations were obtained through a linear regression procedure:

Ea = (0.77 ± 0.14)∆E + (71.6 ± 4.1) 21


(C−H)

(5)


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Ea = (0.82 ± 0.09)∆E + (76.8 ± 4.6)

(O−H)

(6)

Ea = (0.79 ± 0.12)∆E + (132.8 ± 8.1)

(C−C)

(7)

(C−H + O−H)

(8)

Ea = (0.83 ± 0.07)∆E + (74.2 ± 2.8)

According to Vlachos and co-workers, 31 the correlation coefficient (R2 ) has a minor role as quality descriptor for these kind of linear energy relationships while it is important to quantify how well they predict activation energies for selected molecular events. In general a more complex statistical analysis should be taken into account, probably not suited in this case due to the limited data collected. Nevertheless a comparison between calculated and predicted activation barriers could be used as a reasonable descriptor of the accuracy of the relationships found. In our case most part of the calculated deviations is smaller than the AMUE of 16.2 kJ mol−1 associated to transition states determination by the M06-L functional. 23 The highest errors were found in the case of C-C bond cleavages, but in the worst case scenario, the deviation was estimated to be 20.2 kJ mol−1 , only 4.0 kJ mol−1 higher than the AMUE. This level of accuracy could be suitable for the purpose of fast initial screening of feasible bond scissions of more complex polyols.

Experimental discussion APR of 1,2-PDO on Pt/Al2 O3 was previously described by Bindwal and Vaidya. 32 Acetic and propionic acids, methanol, ethanol and propanal were found in small amounts as liquidphase products at 5 and 10 % level of conversion. It should be also mentioned that support acidity plays a crucial role in this process, thus a direct comparison of APR on acidic support with the experimental data generated in the current work may lead to different conclusions. Due to the different consecutive and parallel chemical reactions occurring in APR, the product distribution varies along with conversion. 2 In the current experiment 1,2-PDO conversion level is in the range between 14 and 17 %, which can be considered low enough to track such intermediates as ethanol, 1-propanol, hydroxyacetone, already indicated as HA, 22


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and lactic, propanoic and acetic acids, which can be referred as primary intermediates compared to final products: CO, CO2 , H2 and alkanes. The chromatograms of samples obtained at different flow rates are shown in Figure 11. [Figure 11 about here.] Hydroxyacetone is the main product, having the highest yield (Table 2). This is in agreement with the low activation barriers associated with the two dehydrogenation reactions leading to this species, as evident in Scheme 1. Significant amounts of propanal, PrA, were also observed. The presence of this aldehyde suggests that C−O bond cleavage may occur at the early stage of the reaction, before the C−C bond cleavage. [Table 2 about here.] Therefore, for selected intermediates with a low degree of dehydrogenation, the activation barriers for the C2−O2 bond cleavage, leading to precursors of PrA, were calculated. In particular for INT2, INT3 and INT5, they resulted in the following values 174.0, 116.0 and 68.7 kJ mol−1 , respectively. Hence, although the C−O scission seems to be favored with respect of the C1−C2 bond cleavage, the former is, in any case, not competitive with dehydrogenation events (Table 1). This means that according to the computational model previously discussed, C−O cleavages should not occur at the first stages of the reaction at least prior to dehydrogenation. In agreement with this inference, various examples in the literature confirm that, along with reforming processes on metallic surfaces, the C−O bond cleavage is not the most facile pathway. 33 As an example, for the sake of comparison, Rösch and co-workers, investigating different 1-propanol dehydration mechanisms over Pt extended surfaces, have found activation barriers for C−OH bond breaking not lower than 105 kJ mol−1 through hydrogenolysis at neutral pH conditions. 34 Even if the same barrier considerably lowers in the presence of HCl, via water elimination by a protonated intermediate, this pathway was demonstrated in any case to be less favored than dehydrogenation, in 23



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agreement with the current work. Additional GCMS analysis confirmed the presence of other intermediates, listed in Table 3. Interestingly, two different oxolanes were found, confirming that condensation reactions happen already at this stage. The discrepancy between the results of HPLC and GCMS analysis can be ascribed to poor ability of hexanol in extracting highly polar compounds from water and to the restrictions own of the HPLC analysis. [Table 3 about here.] It is worth discussing the case of lactic aldehyde, already indicated as LA. On the grounds of solely DFT calculations the formation of this sub-product was predicted to have moderate activation barriers; however, it was not detected among the products, as also confirmed by GC-MS analysis. As a consequence, it can be straightforwardly argued that LA can be easily oxydized to lactic acid. Understanding of APR process would be incomplete without analyzing gas-phase products, such as H2 , CO2 , CO and alkanes. Hydrogen in the APR process is formed in two different reactions, namely dehydrogenation of organic compounds and water gas shift, WGS, reaction. Additionally, hydrogen is consumed in multiple hydrogenation processes. According to the overall equation for APR, 1,2-PDO should give:

C3 H8 O2 + 4 H2 O −−→ 3 CO2 + 8 H2

[1]

where only H2 and CO2 in a stoichiometric ratio equal to 8/3 is formed. A more mechanistically adequate ratio equal to 2, would, at variance, correspond to the following equations:

C3 H8 O2 −−→ CH4 + 2 H2 + 2 CO

[2]

2 CO + 2 H2 O −−→ 2 CO2 + 2 H2

[3]

which comprise not only 1,2-PDO APR, resulting in syngas and methane but also incorporate WGS reaction. 24


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The experimental values of H2 /CO2 ratios varied from 7 to 24 (see Table 4), being higher than those characterizing both the stoichiometric values above. Such ratios, peculiarly, imply that hydrogen is primarily formed by dehydrogenation events. In addition, comparably high amounts of linear and branched alkanes C1 -C5 and C7 alkanes are observed, meaning that hydrogen is vigorously consumed by hydrogenation processes. The yield to the gas products is shown in Table 4: propane dominates in the gas mixture compared to other alkanes. [Table 4 about here.] Thermogravimetric oxidation of spent catalyst also revealed formation of condensation products showing a remarkably high peak at 700 K.

The reaction mechanism In light of these results, it is possible to trace a possible reaction mechanism for the 1,2-PDO transformations through APR, as reported in Scheme 4. In particular, considering as the most probable the overall reaction path formed by the calculated events having the lowest activation barriers (see Figure 9), the reaction should proceed in the order of the following molecular events: 1,2-PDO → INT2 → INT5 → INTa → INTb → INTf → INTj, being the latter two intermediates those in which the C−C bond breaking, via decarbonylation, becomes competitive with the dehydrogenation processes. It is worth mentioning that through the C−C bond cleavage, from INTf and INTj, hydroxyethylidene (HTE) and acetyl (ACT) fragments are formed, respectively. HTE and ACT surface derivatives might be the precursors of both the experimentally found ethanol and acetic acid, formed by hydrogenation or hydration while, their decomposition via decarbonylation could take place as well on platinum as reported by Neitzel et al. 35

25




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associated activation barrier for C−H, O−H and C−C bond cleavages, were analyzed. The results plainly suggested that the approach could be heuristically employed in future work for computational catalyst screening, starting from pure thermodynamic quantities computed using DFT.

Acknowledgement Funding by European Union Seventh Framework Programme (FP7/2007-2013) within the project SusFuelCat: “Sustainable fuel production by aqueous phase reforming – understanding catalysis and hydrothermal stability of carbon supported noble metals” GA: CP-IP 310490 (http://cordis.europa.eu/projects/rcn/106702 en.html) is gratefully acknowledged.

Supporting Information Available Optimized structures of additional reaction intermediates treated in the manuscript (INT1r, INTer); additional calculations on a Pt63 model catalyst, along with a comparison of the energy of INT1, INT2, INT3 and INT4 optimized on Pt30 , Pt63 and comparable intermediates on an extended Pt(111) surface.

This material is available free of charge via the Internet

at http://pubs.acs.org/.

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Simulation of Ethanol Decomposition on Platinum Cluster at Initial Stage of Carbon Nanotube Growth. Chem. Phys. Lett. 2015, 636, 110 – 116. (28) Rangarajan, S.; R. O. Brydon, R.; Bhan, A.; Daoutidis, P. Automated Identification of Energetically Feasible Mechanisms of Complex Reaction Networks in Heterogeneous Catalysis: Application to Glycerol Conversion on Transition Metals. Green Chem. 2014, 16, 813–823. (29) Skoplyak, O.; Barteau, M. A.; Chen, J. G. Reforming of Oxygenates for H2 Production: Correlating Reactivity of Ethylene Glycol and Ethanol on Pt(111) and Ni/Pt(111) with Surface d-Band Center. J. Phys. Chem. B 2006, 110, 1686–1694. (30) Griffin, M. B.; Jorgensen, E. L.; Medlin, J. W. The Adsorption and Reaction of Ethylene Glycol and 1,2-Propanediol on Pd(111): A TPD and HREELS Study. Surf. Sci. 2010, 604, 1558 – 1564. (31) Sutton, J. E.; Vlachos, D. G. A Theoretical and Computational Analysis of Linear Free Energy Relations for the Estimation of Activation Energies. ACS Catal. 2012, 2, 1624–1634. (32) Bindwal, A. B.; Vaidya, P. D. Toward Hydrogen Production from Aqueous Phase Reforming of Polyols on Pt/Al2 O3 Catalyst. Int. J. Hydrogen Energy 2015, 1–9. (33) Alcalá, R.; Mavrikakis, M.; Dumesic, J. A. DFT Studies for Cleavage of C-C and CO Bonds in Surface Species Derived from Ethanol on Pt(111). J. Catal. 2003, 218, 178–190. (34) Chiu, C.-C.; Genest, A.; Rösch, N. Formation of Propane in the Aqueous-Phase Processing of 1-Propanol over Platinum: A DFT Study. ChemCatChem 2013, 5, 3299–3308. (35) Neitzel, A.; Lykhach, Y.; Johánek, V.; Tsud, N.; Skála, T.; Prince, K. C.; Matol´ın, V.;

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Libuda, J. Role of Oxygen in Acetic Acid Decomposition on Pt(111). J. Phys. Chem. C 2014, 118, 14316–14325.

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Table 1: Activation barriers (Ea ) related to i) C1−C2 bond cleavages of selected reaction intermediates, showing different dHd values and to ii) several dehydrogenation processes occurring on various fragments of the same intermediates, summarized for comparison Intermediate

INT3 INT5 INTa INTb INTχ INTe INTf INTj INTω

Formula

CH3 COHCH2 OH CH3 COHCHOH CH3 COCHOH CH3 COHCHO CH3 CHOHCO CH2 COCHOH CH3 COHCO CH3 COCO CH2 COCO

dHd

1 2 3 3 3 4 4 5 6

Selected Activation Barriers [kJ mol−1 ] EaC1−C2

EaO1−H

EaO2−H

EaC1−H

EaCm−H

224.0 95.7 132.3 109.6 92.6 211.3 63.6 48.8 95.5

133.5 38.1 98.0 — ca. 0 — 74.7 — —

105.5 10.1 — 71.3 — 53.0 — — —

109.0 69.5 84.7 39.9 — — 90.7 — —

— 57.8 47.1 59.2 — — — — —

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Table 2: Liquid-phase product yields in 1,2-PDO APR Flow rate

WSHV

ml min−1

h−1

lactic acid

acetic acid

hydroxyacetone

Yield [%] propanal

ethanol

1-propanol

0.1 0.2 0.3

0.35 0.70 1.50

0.71 0.32 0.42

0.00 0.22 0.16

52.40 18.87 19.20

6.09 1.74 1.61

0.55 0.00 0.00

1.09 0.24 0.28

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Table 3: Substances identified via GCMS analysis in the 1,2-PDO APR Substance

Probability

ethanol propanal 1-propanol 2-methylbutanal hydroxyacetone 2,4-dimethyl-1,3-dioxolane 1,3-dioxolane-2-ethyl-4-methyl

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91 90 91 91 78 87 97



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Table 4: Gas-phase product yields and H2 /CO2 ratio in 1,2-PDO APR Flow rate

WSHV

mL min−1

h−1

0.1 0.2 0.3

0.35 0.70 1.50

H2 /CO2 8.04 18.74 24.55

Yield [%] H2

CO2

CO

CH4

C2 H6

C3 H8

C4 -C7 alkanes

7.42 2.67 1.49

1.90 0.36 0.12

0.85 0.20 0.09

0.26 0.06 0.00

0.85 0.24 0.15

1.75 0.66 0.51

0.04 0.01 0.01

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