AdsorbentâAdsorbate Interactions in the Oxidation of HMF Catalyzed

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Adsorbent-Adsorbate Interactions in the Oxidation of HMF Catalyzed by Ni-Based MOFs: A DRIFT and FT-IR Insight Carlo Lucarelli, Simona Galli, Angelo Maspero, Alessandro Cimino, Claudia Bandinelli, Alice Lolli, Juliana Velasquez Ochoa, Angelo Vaccari, Fabrizio Cavani, and Stefania Albonetti J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05428 • Publication Date (Web): 19 Jun 2016 Downloaded from http://pubs.acs.org on June 21, 2016

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The Journal of Physical Chemistry

Adsorbent-adsorbate Interactions in the Oxidation of HMF Catalyzed by Ni-based MOFs: a DRIFT and FT-IR Insight

Carlo Lucarelli,a,b* Simona Galli,a,b* Angelo Maspero,a Alessandro Cimino,a Claudia Bandinelli,c Alice Lolli,c Juliana Velasquez Ochoa,c Angelo Vaccari,c Fabrizio Cavani,c Stefania Albonettic

a

Dipartimento di Scienza e Alta Tecnologia, Università dell’Insubria, Via Valleggio 11, 22100 Como, Italy.

b

Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali, Via Giusti 9 50121 Firenze, Italy.

c

Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy.

*

Corresponding

authors:

CL:

[email protected],

+39-031-2386620;

SG:

[email protected], +39-031-2386627

ACS Paragon Plus Environment

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ABSTRACT The three Ni-based metal-organic frameworks (MOFs) Ni(BDP), Ni(BPEB) and Ni3(BTP)2 [H2BDP = 1,4-(4-bispyrazolyl)benzene; H2BPEB = 1,4-bis(1H-pyrazol-4-ylethynyl)benzene; H3BTP = 1,3,5-tris(1H-pyrazol-4-yl)benzene], possessing square planar, potentially accessible metal sites, were preliminary tested as catalysts in the base-free selective oxidation of hydroxymethylfurfural to DFF. While Ni(BDP) undergoes degradation, Ni3(BTP)2 is the most active of the three MOFs, yielding 27% 2,5-diformylfuran after 24 h with a selectivity close to 100% in relatively mild reaction conditions (120 °C, 30 bar O2, water as solvent). Upon flowing a model probe, in situ DRIFT and FT-IR spectroscopy were employed to rationalize the different performances of Ni(BPEB) and Ni3(BTP)2 in terms of adsorbate-adsorbent interactions: not only hydrogen bonds between the hydroxyl functionality of the probe and the pore walls of the MOF are at work, but also, and more importantly, bands ascribed to Ni-OR stretching are detected, denouncing the insurgence of Ni-probe interactions. The different intensity of these bands in the two cases confirms the different accessibility of the metal centers, as suggested by crystal structure analysis and catalytic tests.


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INTRODUCTION Biomass-derived chemicals1,2 and fuels3 possess a high economic and environmental impact. 5hydroxymethylfurfural (HMF) is one of the key platform chemicals obtained through the transformation of cellulose-derived carbohydrates.4 Its aerobic oxidation leads to a number of highly valuable chemicals, such as 2,5-diformylfuran (DFF), 5-hydroxymethylfuran-2-carboxylic acid (HMFCA), 5-formyl-2-carboxylic acid (FFCA), and 2,5-furandicarboxylic acid (FDCA) (Scheme 1).5,6 DFF, inter alia, is a precursor in the synthesis of furanic polymers,7 pharmaceuticals and antifungal agents,8,9 as well as renewable furan-urea resins.10 Several attempts have been carried out to prepare DFF through one-pot syntheses directly from fructose, using both homogeneous and heterogeneous catalysts in aprotic polar solvents like dimethylsulfoxide and dimethylformamide.11,12,13 Regrettably, the high boiling points of the solvents raised difficulties in isolating DFF and reusing the solvents themselves. As an alternative, selective oxidation of HMF to DFF has been explored testing a range of oxidizing agents and solvents.5,6 The vast field of metal-organic frameworks (MOFs)14,15 is the object of a continuous development due to their potential use in a number of relevant functional applications, ranging from gas storage to gas or liquid separation, drug delivery and imaging – to mention only a few. Heterogeneous catalysis is another, industrially important application for which MOFs have shown promising performances,16,17,18,19 emerging as versatile alternatives to the traditional all-inorganic materials for a number of organic syntheses. Indeed, MOFs join the periodic organization of catalytically active centers (the metal nodes) to a modulation of the catalytic performances through the electronic and steric properties of the spacers, to size- and shape-selectivity. Up to now, a number of MOFs have been used as scaffold to encapsulate large Broensted-acidic catalysts to be employed in oxidation reactions to prepare HMF:20 for example, phosphotungstic acid, H3PW12O40, encapsulated into MIL-101(Cr) [Cr3(O)X(bdc)3(H2O)2, where: H2bdc = benzene-1,4-dicarboxylic acid, X = OH or F] successfully catalyzed the production of HMF from fructose or glucose.21 On the other hand, MOFs have been rarely employed for the oxidation of HMF. One example in this respect is the work by ACS Paragon Plus Environment

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Fang and colleagues,22 who reported the use of MIL-45(Fe,Co) (K[M3(BTC)3]·5H2O, M = Fe, Co) as a sacrificial template to produce a non-noble Fe-Co based catalyst for the oxidation of HMF to DFF (> 99% yield at 100 °C and 10 bar O2). To the best of our knowledge, no reports on the use of MOFs as the catalyst (not the scaffold, nor the template) in the oxidation of HMF have ever appeared in the literature. Even more important, to rationalize the behavior of the MOF catalyst and promote engineering and preparation of new-generation, optimized species is the elucidation of the mechanism, with the specific aim of identifying the active sites. In this respect, DRIFT and FT-IR spectroscopy are a powerful tool, successfully applied in the recent past also in the case of MOFs.23,24,25,26 In this context, as a case-study we selected the three Ni-based MOFs Ni(BDP)·S,27 Ni(BPEB)·S28 and Ni3(BTP)2·S29 [H2BDP = 1,4-(4-bispyrazolyl)benzene; H2BPEB = 1,4-bis(1H-pyrazol-4ylethynyl)benzene; H3BTP = 1,3,5-tris(1H-pyrazol-4-yl)benzene, Scheme 2]. In the following, we will adopt the label Ni(L)·S to indicate non desolvated MOFs, while the label Ni(L) to address the thermally activated, desolvated counterpart. Despite featuring two different structural motifs, closely depending on the hapticity of the ligand, when thermally activated all the three MOFs possess square planar, potentially accessible metal sites, which might be involved in adsorbentadsorbate interactions beneficial, e.g., for gas adsorption or separation,30,31,32 and catalysis.16,33 This favorable aspect is further valorized by the high thermal stability imparted, to the whole material, by the poly(pyrazolato)-based spacers, with decomposition temperatures, in air, above 400 °C. In addition, Ni(BPEB) is stable for 5 days if exposed to water vapors at room temperature (r.t), 2 days if suspended in boiling water, 5 days if suspended in acetone at r.t.28 Definitely more remarkable, Ni3(BTP)2 is stable under reflux at least for 14 days, both in water and in acidic (HCl or HNO3, down to pH 2) or basic (NaOH, up to pH 14) aqueous solutions, preserving both its structural features and its specific surface area.29 Finally, while no investigation on the chemical behavior of Ni(BDP)·S was ever carried out, Ni(BDP) preferentially traps trace (30 ppm) tetrahydrofurane


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contained in He:CO2:CH4 1:2.25:1 v/v/v flows even at 60% relative humidity, an occurrence which was explained in terms of (partial) hydrophobicity of the pores.27 In the following, we report upon the rationalization, in terms of adsorbent-adsorbate interactions, of the catalytic performances of the three Ni-based MOFs in the base-free selective oxidation of HMF to DFF by means of DRIFT and FT-IR spectroscopy.

EXPERIMENTAL SECTION Materials and Methods. All the solvents were dried and distilled under nitrogen by standard procedures.34 Unless otherwise specified, the reagents were obtained from commercial suppliers and used as received. All the reactions requiring an anhydrous or oxygen-free environment were performed in flame- or oven-dried glassware under nitrogen pressure. Elemental analyses were obtained with a Perkin Elmer CHN Analyzer 2400 Series II. Preliminary IR spectra were acquired over the range 4000-600 cm-1, either in nujol mull or in attenuated total reflectance (ATR) with a crystal of SeZn, by means of a Nicolet iS10 instrument, or in transmission mode on a FTIR Shimadzu Prestige-21 spectrometer; in the following, band maximum positions are reported in cm-1, while band shape and intensity are denoted as: s = sharp, br = broad, vs = very strong, s = strong, m = medium and w = weak. 1H and

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C(APT) NMR spectra were recorded at 400 and 100 MHz,

respectively, on a Bruker Avance 400 spectrometer. 1H and 13C NMR data are reported as follows: chemical shifts (in ppm, and referenced to internal TMS), multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet), integration. Powder X-ray diffraction (PXRD) data were acquired on a Bruker Axs D8 Advance diffractometer, equipped with a Cu-Kα X-ray tube (λ = 1.5418 Å), a filter of nickel on the diffracted beam, and a Bruker Lynxeye linear position-sensitive detector. The purity of all the batches of MOFs isolated for the present work was assessed by combining elemental analysis, IR spectroscopy and PXRD. As a general procedure, PXRD data were acquired in the 2θ range 3-35°, with a step of 0.02° and a time per step of 0.5 s. Le Bail’s


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refinements were carried out on the acquired PXRD data with the software TOPAS-R,35 using, as the starting point, the unit cell parameters already reported in the literature.27,28,29 Synthesis of the Ligands. 1,4-(4-bispyrazolyl)benzene (H2BDP) was prepared following the previously reported synthetic path.36 Anal. calc. for C12H10N4 (FW = 210.2 g/mol): C, 68.56; H, 4.79; N, 26.65%. Found: C, 67.98; H, 4.83; N, 26.26%. IR (KBr, cm-1): 3144(br), 1583(w), 1527(w), 1263(w), 1236(w), 1159(s), 1037(w), 965(w), 951(s), 866(s), 824(s), 719(w), 657(w), 627(w). 1H NMR (DMSO-d6) (δ, ppm): 7.58 (s, 2H), 8.05 (s, 2H), 12.5 (br s, 1H).

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C NMR

(DMSO-d6) (δ, ppm): 121.9 (C), 126.3 (HC-Ph), 131.3 (C), 137.0 (HC-pz). 1,4-bis(1H-pyrazol-4-ylethynyl)benzene (H2BPEB) was synthesized adopting the synthetic route optimized in the recent past.28 Anal. calc. for C16H10N4 (FW = 258.3 g/mol): C, 74.40; H, 3.91; N, 21.69%. Found: C, 74.08; H, 3.21; N, 21.32%. IR (nujol, cm-1): 3169(br), 2221(m), 1141(s), 1101(w), 1050(vs), 1037(vs), 1002(vs), 992(vs), 950(s), 941(s), 845(w), 868(s), 861(s), 834(vs), 798(br), 654(vs), 620(vs). 1H NMR (DMSO-d6) (δ, ppm): 13.3 (br s, 1H), 8.14 (s, 1H), 7.74 (s, 1H), 7.47 (s, 2H).

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C NMR (DMSO-d6) (δ, ppm): 133.2 (CH), 131.6 (CH), 123.0 (C), 101.3 (C), 89.7

(C), 84.5 (C). Analogously, 1,3,5-tris(1H-pyrazol-4-yl)benzene (H3BTP) was isolated through the published procedure.29 Anal. calc. for C15H12N6 (FW = 276.3 g/mol): C, 65.21; H, 4.37; N, 30.42%. Found: C, 64.55; H, 4.50; N, 29.97%. IR (neat, cm-1): 3164(br), 2941(br), 1605(vs), 1371(w), 1348(w), 1232(w), 1158(s), 1044(s), 994(vs), 947(s), 847(s), 792(s), 747(vs), 690(w), 656(s), 619(vs). 1H NMR (DMSO-d6) (δ, ppm): 7.68 (s, 1H), 8.26 (br s, 2H), 12.94 (br s, 1H). Synthesis of Ni(BDP)·S. Ni(BDP)·S was isolated according to the synthetic procedure recently reported in the literature.27 Before carrying out the catalytic tests, the as-synthesized batches were thermally activated by heating them for 18 h, at 150 °C and 10-6 bar. Elem. Anal. calc. for C12H8NiN4, Ni(BDP), (FW = 266.7 g/mol): C, 54.00; H, 3.02; N, 20.99%. Found: C, 53.70; H, 3.58; N, 20.87%. IR (nujol, cm-1): 1581(s), 1270(w), 1178(w), 1144(m), 1060(s), 957(m), 817(vs), 723(w). ACS Paragon Plus Environment

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Synthesis of Ni(BPEB)·S. Ni(BPEB)·S was isolated according to the synthetic path recently reported in the literature.28 Before carrying out the catalytic tests and spectroscopic measurements, the as-synthesized samples were thermally activated by heating them for 18 h, at 150 °C and 10-6 bar. Anal. calc. for C16H8NiN4, Ni(BPEB), (FW = 315.0 g/mol): C, 60.96; H, 2.54; N, 17.78%. Found: C, 60.48; H, 2.87; N, 17.07%. IR (nujol, cm-1): 2203(w), 1228(w), 1164(w), 1055(w), 1015(w), 1007(w), 840(w), 769(w), 719(w), 638(w). Synthesis of Ni3(BTP)2·S. Ni(BTP)·3DMF·5CH3OH·17H2O was isolated according to the synthetic path recently reported in the literature.29 Anal. calc. for C44H93Ni3N15O25 (FW = 1408.4 g/mol): C, 37.52%; H, 6.66%; N, 14.92%. Found: C, 37.66%; H, 5.95%; N, 14.35%. IR (neat, cm1

): 3370(br), 1655(s), 1609(vs), 1557(w), 1406(w), 1385(w), 1361(w), 1329(w), 1257(s), 1196(w),

1135(w), 1078(vs), 1015(s), 854(w), 761(vs), 685(w), 640(s), 461(w). To obtain the desolvated form for the catalytic tests and spectroscopic measurements, the as-synthesized samples were thermally activated by heating them for 18 h, at 150 °C and 10-6 bar. Synthesis of Cu3(BTP)2·S. Cu3(BTP)2·8CH3OH·10H2O was isolated according to the synthetic path recently reported in the literature.29 Anal. calc. for C38H70Cu3N12O18 (FW = 1173.70 g/mol): C, 38.89%; H, 6.01%; N, 14.32%. Found: C, 38.56%; H, 5.63%; N, 14.66%. IR (neat, cm-1): 3370(br), 1608(vs), 1557(w), 1426(w), 1385(w), 1354(w), 1322(w), 1243(w), 1180(w), 1126(s), 1061(vs), 1012(vs), 946(w), 832(s), 758(vs), 681(w), 637(w), 460(w). To obtain the desolvated form for the catalytic tests, the as-synthesized batches were thermally activated by heating them for 18 h, at 150 °C and 10-6 bar. Catalytic Tests. The following chemical products were used for the catalytic reactions: 5hydroxymethyl-2-furfural (HMF) (Alfa Aesar), 2,5-diformylfuran (DFF) (Toronto Research Chemicals), sodium hydroxide (pellets, Sigma Aldrich), benzyl alcohol (Sigma Aldrich). The oxidation of HMF was carried out using a Parr Instruments autoclave reactor of 100 mL capacity and equipped with a mechanical stirrer (0–600 rpm) and facilities to measure temperature and pressure. The reactor was charged with an aqueous solution (25 mL of distilled water) containing ACS Paragon Plus Environment

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the appropriate amount of HMF (205 mg, 1.8 mmol) and catalyst [180 mg, corresponding to 0.7, 0.6, and 0.7 mmol of Ni in Ni(BDP), Ni(BPEB), and Ni3(BTP)2, respectively]. The autoclave was purged 3 times with O2 (5 bar), then pressurized at 30 bar. The temperature was increased up to 120 ºC and the reaction mixture was stirred at ca. 400 rpm for 24 h. At the end of the reaction, the reactor was cooled down to r.t. and the suspension was filtered. Since the desired product (DFF) is not highly soluble in water, 25 mL of CH3CN were added to the reaction mixture before filtration. The solid catalyst thus recovered was washed with CH3CN in order to remove the product that could have remained absorbed onto it. The obtained solution was analyzed with an Agilent Infinity 1260 liquid chromatograph equipped with a 4.6×50 mm C18 Poreshell 120 column, using 40 vol. % of CH3CN and 60 vol. % of water as mobile phase. Identification of compounds was achieved by calibration using reference commercial samples. The oxidation of benzyl alcohol was carried out in a high pressure stainless steel autoclave (internal volume 50 mL). The reactor was charged with benzyl alcohol (512 mg, 47.0 mmol) and Ni3(BTP)2 (69 mg, 4.9×10-2 mmol). The autoclave was purged 3 times with O2 (5 bar), then pressurized at 30 bar. The temperature was increased up to 120 ºC and the reaction mixture was stirred at ca. 400 rpm for 1 or 18 h. The same procedure described above was used to analyze the final solution after filtration of the solid. In-situ DRIFT spectroscopy. DRIFT spectra were acquired in situ with a Bruker Vertex 70 instrument equipped with a Pike DiffusIR cell attachment. The spectra were recorded using an MCT detector after 128 scans and with a 4 cm−1 resolution in the region 4000−450 cm−1. As a general procedure, a sample of MOF was loaded and pre-treated at 150 °C under a flow of He (10 mL/min) for 45 min, in order to remove any molecules adsorbed onto it. Then, the sample was cooled down to 85 °C. The background was measured and, immediately after, a pulse of ethanol (1 µL) was introduced. IR spectra were acquired at 0.5-min time intervals to follow the adsorption process. Afterwards, the carrier gas was left to flow until weakly adsorbed ethanol was evacuated.


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The IR spectrum acquired after reaching this condition was used to compare the behavior of the catalysts. FT-IR Monitoring of Adsorption of a Probe Molecule in Vacuum. The FT-IR spectra were recorded using a Perkin-Elmer Spectrum One spectrometer after 36 scans with a resolution of 4 cm−1, in the region 4000−450 cm−1. Catalyst samples were pressed into self-supported wafers and activated in situ in the IR cell for 30 mi, at the maximum temperature of 150 °C and under vacuum (≤ 10−6 mbar). Then, at r.t., the activated samples were put into contact with an increasing amount of benzyl alcohol. Finally, desorption spectra were recorded at increasing temperatures, from r.t. up to 150 °C. PXRD Monitoring of Ethanol-impregnated MOFs. As a general procedure, a 20-mg sample was deposited on a Si zero-background sample-holder. A preliminary PXRD pattern was acquired in the 2θ range 3-35°, with a step of 0.02° and a time per step of 0.5 s. Then ethanol was added drop-wise with the incipient wet impregnation technique. The sample was then laid down again on the sampleholder. PXRD patterns were acquired, using the same conditions as above, at different time intervals. Le Bail’s refinements were carried out on the acquired data with the software TOPASR.35

RESULTS AND DISCUSSION Overview of the Crystal and Molecular Structures. For the sake of comprehension of the following Sections, we report here the main structural features of Ni(BDP)·S,27 Ni(BPEB)·S28 and Ni3(BTP)2·S,29 both at r.t. and upon raising the temperature. Ni(BDP)·S and Ni(BPEB)·S are isostructural and represent an example of application of the socalled isoreticular approach.37 Square-planar NiN4 nodes (Figure 1a) are N,N’-bridged by the pyrazolato rings along 1-D chains of collinear metal ions (Figure 1b). Each chain is bridged by the bis(pyrazolato) ligands to four nearby ones, this occurrence bringing about the formation of a 3-D (4,4)-connected network with PtS topology (Figure 1c). The network features 1-D rhombic channels ACS Paragon Plus Environment

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(Figure 1c) 5.4×6.8 and 7.0×11.3 Å2 wide,38 in Ni(BDP)·S and Ni(BPEB)·S, respectively, accounting for an empty volume amounting to 57% and 70%.39 The two materials are stable, in air, up to 460 and 422 °C. Furthermore, as witnessed by VT-PXRD, both of them undergo a limited breathing40,41 when subjected to temperature increase: the unit cell volume experiences a maximum variation of 1.0 (in the temperature range 30-410 °C) and -0.8% (in the temperature range 30-290 °C), in Ni(BDP)·S and Ni(BPEB)·S, respectively, before decomposition. As a further proof of their porosity, N2 adsorption at 77 K after thermal activation allowed to estimate Langmuir specific surface areas of 1600 and 2378 m2/g, respectively, at 1 bar. Conversely, Ni3(BTP)2·S is a 3-D (4,6)-connected network with sodalite (sod) topology. Square Ni4N16 nodes (Figure 2a) are reciprocally connected by the BTP3- spacers within a polymeric architecture featuring both 1-D cylindrical channels 10.3×10.3 Å2 wide,38 running along all the three crystallographic axes, and small-aperture (1.9×1.9 Å2) octahedral cavities, the windows of which are decorated by the ligands, while the vertices are truncated by the Ni4 clusters (Figure 2b). Strictly speaking, in the as-synthesized material the metal centers show a square pyramidal NiN4O coordination sphere, the apical position of which is occupied by the oxygen atom of a solvent molecule (Figure 2b). At ambient temperature, the empty volume amounts to 66%39 of the unit cell volume. This MOF decomposes, in air, at 430 °C. Furthermore, as witnessed by VT-PXRD,29 upon raising the temperature the framework is rather rigid: the volume shrinks by only 0.5% (in the temperature range 30-410 °C), reasonably as the result of solvent release. After thermal activation, Ni3(BTP)2 porosity was assessed by N2 adsorption at 77 K, revealing a Langmuir specific surface area of 1900 m2/g at 1 bar. Catalytic Tests. Being sodium hydroxide considered to be fundamental in HMF oxidation reactions42,43 employing metal-supported catalysts, preliminary catalytic tests were performed in water with different amounts of NaOH, using Ni3(BTP)2 as the catalyst. As emerging from Table 1 (entries 1-3), the essayed experimental conditions (100 °C, 10 bar O2, 1-4 eq. NaOH) led to catalyst degradation (PXRD evidence), with concomitant formation of pitches, as previously reported by ACS Paragon Plus Environment

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some of us.44,45 For this reason, the catalytic reactions were performed in the absence of NaOH, which is a recognized added value46 in the search for environmental friendly catalytic systems. Temperature, oxygen pressure and reaction time were consequently increased with respect to the values adopted in the presence of the base. Before proceeding further, a test using the Ni(II) salt Ni(NO3)2·6H2O as catalyst was carried out (Table 1, entry 4), together with a blank test in the absence of any catalyst (Table 1, entry 5). As expected, the blank test gave no HMF conversion while, in the presence of the Ni(II) salt, HMF conversion was complete but the desired DFF product was not formed, nor were formed the main products of HMF oxidation.47 By comparing the catalytic performances of the Ni-based MOFs (Table 2), remarkable differences in their activity emerged. Ni3(BTP)2 was found to be the most active catalyst, yielding 3% of DFF in 8 h (Table 2, entry 1), and up to 27% of DFF in 24 hours (Table 2, entry 2). The selectivity was close to 100%, and no other products were formed. Furthermore, the catalyst was recovered intact (PXRD evidence) after the catalytic cycle. Notably, by using Ni3(BTP)2·S (i.e. an as-synthesized sample, without preliminary thermal activation to remove the clathrated guest solvent) a decrease in the product yield was observed, even if the selectivity still remained very high (Table 2, entry 3). This occurrence suggests that the coordinatively unsaturated Ni(II) centers of Ni3(BTP)2 play a not negligible role in the development of the catalytic reaction, as the complete48 saturation of their coordination sphere by the solvent molecules concurs to decrease the yield. Even if showing a selectivity close to 100%, after 24 h Ni(BPEB) yields only 3% of DFF. After the catalytic cycle, the catalyst has partially lost (the originally already limited) crystallinity (PXRD evidence). On the one hand, assuming that the ambient conditions structural features are strictly maintained, the different accessibility of the unsaturated metal sites in the two frameworks might play a key role. On the other hand, given the known framework flexibility of MOFs sharing the same topology of Ni(BPEB) in response to external stimuli,40,41 the existence of a closed-pore form in the essayed experimental conditions cannot be neglected. Further insight in this respect was achieved by DRIFT and FT-IR spectroscopy (see the Sections below). Lastly, even the strikingly different degree of ACS Paragon Plus Environment

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crystallinity of the two materials [as witnessed by averaged crystal sizes of 100 and 20 nm for Ni3(BTP)2 and Ni(BPEB), respectively] could influence the catalytic activity: indeed, the lower the degree of crystallinity, the shorter the coherence domains in which the catalytically active centers are periodically and homogeneously distributed. Finally, Ni(BDP) is inactive. A PXRD analysis of the solid recovered from the reaction mixture revealed that, in the reaction conditions essayed, Ni(BDP) undergoes degradation to a rather low-crystallinity phase, possibly inhibiting the catalytic activity of this MOF. In order to demonstrate the importance of having nickel-based active sites, Cu3(BTP)2, isostructural to Ni3(BTP)2, was also tested. Cu3(BTP)2 catalyzed the formation of DFF but with lower yield and selectivity (Table 2, entry 6) than Ni3(BTP)2, with the concomitant formation of still uncharacterized49 by-products, confirming that nickel is fundamental for the selective oxidation of HMF using MOF catalysts. To further test its catalytic activity in oxidation reactions, Ni3(BTP)2 was used as catalyst for the model reaction of benzyl alcohol oxidation to benzaldehyde: 7% and 13% yields were obtained after 1 and 18 h of reaction. In situ DRIFT spectroscopy. In order to explain the different performances of Ni3(BTP)2 and Ni(BPEB) as catalysts in terms of adsorbent-adsorbate interactions, we recurred to in situ diffuse reflectance infrared (DRIFT) spectroscopy flowing the model probe ethanol on the two MOFs. Figure 3 collects the spectra acquired at 85 °C during ethanol adsorption on the clean surface of thermally activated Ni3(BTP)2, while Table 3 lists the most relevant bands detected during the experiment, together with their assignment. On the whole, three events appear to take place during adsorption. First of all, the progressive growth of the bands centered at 3582 and 3465 cm-1 demonstrates that the adsorbate interacts with the MOF through hydrogen bond interactions. Moreover, as suggested by the insurgence and progressive growth of the bands in the 1090-1040 cm-1 region, which can be confidently ascribed50 to the C-O and C-C stretching of ethanoate (ethoxy species), the alcohol partially dissociates. Finally, and definitely more interesting in the frame of ACS Paragon Plus Environment

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catalysis, the new band peaked at 879 cm-1 can be interpreted as the Ni-O(H) stretching,50 indicating that the probe interacts with the MOF not only by means of hydrogen bond interactions, but also through its oxygen atom with the metal centers. The insurgence of Ni-O(H) interactions between Ni3(BTP)2 and the Lewis-basic probe adopted in the present work can be tentatively explained on the basis of previously reported theoretical calculations and experimental observations:51 by means of FT-IR spectroscopy, Shearer and colleagues could not observe the insurgence of Ni(II)-probe interactions when probing Ni3(BTP)2 with the weak Lewis base CO. This evidence was ascribed to the absence of a positive electrostatic potential at the Ni4 nodes when Ni(II) is in low-spin state [typical for Ni(II) in square planar coordination]. A positive region could be conversely calculated under the assumption that the metal centers were in high-spin state. If a switch from low- to highspin state is at work when the Ni-O bond is formed, more basic probes like ethanol/ethanoate52 might establish Ni(II)-probe interactions energetic enough to overcome the 75 kJ51 necessary to a mol of Ni(II) ions to undergo the switch, in spite of the higher kinetic diameter of ethanol vs. CO (4.5 vs. 3.3 Å, respectively). In the case of thermally activated Ni(BPEB), while the bands peaked at 1088 and 1045 cm-1 (Figure 4b) indicate partial dissociation of the alcohol also in this case, the intensity of the bands centered at 3593, 3492 (witnessing the formation of hydrogen bond interactions) and 881 cm-1 [indicating the formation of Ni-O(H) interactions] is lower than in the case of Ni3(BTP)2, suggesting less pronounced interactions between the adsorbent and the adsorbate. The insurgence of Ni-O(H) bonds between Ni(BPEB) and the Lewis-basic probes ethanol/ethanoate can be qualitatively explained on the basis of previous theoretical calculations and experimental observations on the isostructural MOF Ni(BDP):53 Albanese and colleagues explained the lack of insurgence of metal-probe interactions between Ni(BDP) and the apolar probe H2 or the weak Lewis acid CO2 in terms of the presence of a positive electrostatic potential around the low-spin state metal centers, the negative cavities near the Ni(II) ions being partially shielded by the ligands. In spite of the positive electrostatic potential around the metal centers, also the weak base CO underwent only ACS Paragon Plus Environment

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physisorption: this occurrence was ascribed to the amount of energy [65 kJ per mol of Ni(II) ions] required by Ni(II) to switch from low- to high-spin state. Given the fact that Ni(BPEB) and Ni(BDP) are isostructural and their metal centers possess very similar stereochemistry and second shell environment, the features just described for Ni(BDP) could be qualitatively extended to Ni(BPEB). On the basis of this assumption, if a switch from low- to high-spin state is at work when the Ni-O bond is formed, stronger bases like ethanol or ethanoate might establish Ni(II)-probe interactions energetic enough to promote the switch. In order to further confirm the insurgence of Ni(II)-ethanol interactions on Ni3(BTP)2 and Ni(BPEB), ethanol was adsorbed, under the same experimental conditions, also over NiCl2. As expected, FT-IR monitoring of the adsorption on NiCl2 (Figure 4c) revealed the insurgence and progressive growth of a band centered at 877 cm-1. On the other hand, the higher difficulty faced by ethanol to be adsorbed on Ni(BPEB) rather than on Ni3(BTP)2 confirms the different accessibility of the metal sites, as suggested by the ambient conditions crystallographic features. As a matter of fact, in Ni(BPEB), despite the square planar coordination sphere shown by the metal centers, to approach to the free coordination sites is somehow hampered by the two nearest metal ions along the 1-D chain (Figure 5a). Conversely, in desolvated Ni3(BTP)2, the coordination site freed by the solvent48 shows a higher accessibility (Figure 5b). To further shed light on this aspect, samples of thermally activated Ni(BPEB) and Ni3(BTP)2 were impregnated with ethanol and monitored by PXRD. As evident from Figure 6a, Ni3(BTP)2 is not degraded by impregnation. As suggested by the variation of the relative intensity of the low-angle peaks, the probe enters the pores, yet the unit cell volume is almost unaffected (shrinking by only -0.1%). On the other hand (Figure 6b), as highlighted by Le Bail’s refinements, in the case of Ni(BPEB) impregnation brings about an initial unit cell volume increment (by 2.1% after 2 h), followed by a shrinkage (by -2.8% after 6 h), this confirming that breathing is at work, so that the existence of a closed-pore form in the conditions adopted for the catalytic tests is not unrealistic. ACS Paragon Plus Environment

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For the sake of completeness, FT-IR monitoring of ethanol adsorption was performed, in the same experimental conditions, also on Ni3(BTP)2·S, still containing solvent within the pores. The comparison of the spectra acquired on Ni3(BTP)2 and Ni3(BTP)2·S after the ethanol pulse (Figure 7) highlights that, on the whole, the same events described above for Ni3(BTP)2 occur, namely: insurgence of hydrogen bond interactions, partial deprotonation of the probe, and formation of NiO(H) bonds. Nonetheless, while the bands ascribed to ethanoate are more intense for the nonactivated adsorbent, the one witnessing the presence of Ni-O(H) bonds is less intense: this evidence concurs to confirm the active role of the metal sites in the catalytic activity of the MOF, Ni3(BTP)2·S being consequently less active than the evacuated counterpart. FT-IR Monitoring of Adsorption of Benzyl Alcohol in Vacuum. As a complementary investigation to that reported in the Section above, an FT-IR study was carried out under vacuum in the presence of a probe adsorbent. A first attempt was undertaken using HMF as the probe molecule. Unfortunately, because of the high boiling point of HMF (114-116 °C at 1 mmHg), performing the planned experiment with our instrumental setup was unfeasible. Hence, benzyl alcohol (b.p. 205 °C at 1 bar) was chosen as the model probe: owning both to its aromaticity and the O-H functional group, it is the organic molecule better resembling HMF. In order to promote the desorption of water eventually adsorbed from air by the MOF samples, a preliminary step of thermal activation was carried out on both Ni3(BTP)2 and Ni(BPEB), heating the samples up to 150 °C under vacuum and maintaining them at this temperature for 30 min. The FT-IR spectra collected for Ni(BPEB) during activation (not shown), did not highlight significant modifications of the IR bands of the MOF during heating. Conversely, not negligible changes were observed while activating Ni3(BTP)2 (Figure 8). Upon heating, Ni3(BTP)2 does desorb water, as demonstrated by the disappearance or loss of intensity of the bands in the 3700-3200 cm-1 region, where the O-H stretching lays (Figure 8a). In particular, the intensity of the band centered at 3577 cm-1 decreases with increasing the temperature, while the broad band initially peaked at 3390 cm-1 lowers in intensity and shifts to 3411 cm-1. Consequently, the bands peaked at 3151, 3141, 3077 and ACS Paragon Plus Environment

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3051 cm-1, due to the C-H stretching of the aromatic carbon atoms, become more clearly visible, their intensity increasing by raising the temperature. Focusing on the 1800-450 cm-1 range (Figure 8b), temperature increase brings about a sharpening and an ipsochromic shift (to 1675 cm-1) of the broad peak initially centered at 1668 cm-1. Finally, modifications can be also noticed in the 1050750 cm-1 region (Figure 8b): a severe intensity drop or even a complete disappearance is observed for the bands peaked at 995, 967, 806 and 781 cm-1. After the preliminary thermal activation, adsorption of benzyl alcohol was carried out on both catalysts at r.t. and under vacuum, followed by desorption, performed by increasing the temperature up to 150 °C. The adsorption and desorption spectra are collected in Figure 9 for Ni(BPEB) and Figure 10 for Ni3(BTP)2, together with the spectra of benzyl alcohol, and of thermally activated Ni3(BTP)2 and Ni(BPEB), as references. When benzyl alcohol was fed over Ni(BPEB), no adsorption was observed (Figure 9). This occurrence is in agreement with what resulted from the catalytic tests, which highlighted a rather low activity of Ni(BPEB) toward HMF. The different behavior of Ni(BPEB) toward ethanol and benzyl alcohol can be explained in terms of the different accessibility of the two organic probes to the pores of the MOF (kinetic diameter of one-aromaticring compounds 6-7 Å54 vs. 4.5 Å for ethanol). Conversely, when the adsorption experiment was carried out on Ni3(BTP)2, a notable perturbation of the bands was observed, as evident from Figure 10. Table 4 collects the main IR bands detected for pure, thermally activated Ni3(BTP)2, pure benzyl alcohol and Ni3(BTP)2 after adsorption, together with their interpretation. In the case of benzyl alcohol, the interpretation was carried out by comparing our spectrum to that reported in reference 55, whereas the assignment of the vibrational frequencies of Ni3(BTP)2 was performed on the basis of what already reported in reference 51. Comparing the spectrum of Ni3(BTP)2 after adsorption with those of pure benzyl alcohol and pure, thermally activated Ni3(BTP)2 (Figure 10 and Table 4) four main phenomena can be observed: i) The appearance of new bands (highlighted in blue in Table 4), absent in the spectrum of Ni3(BTP)2, that can be attributed to benzyl alcohol vibrational frequencies. These bands persist, ACS Paragon Plus Environment

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upon temperature increase, up to 100 °C. With respect to their position for pure benzyl alcohol, upon adsorption they undergo none or very modest shifts, suggesting that the alcohol is barely perturbed when interacting with Ni3(BTP)2. ii) Significant changes in the form and intensity of the bands of the catalyst. Worthy of note, their original intensity is completely restored during desorption, already at 100 °C. This phenomenon, indicating the insurgence of reversible adsorbent-adsorbate interactions, is particularly evident for the bands centered at: • 3152 and 3052 cm-1 in the aromatic C-H stretching region (Figure 10a); • 1359 and 1328 cm-1, respectively corresponding to C-C symmetric + C-N symmetric and asymmetric stretching of the pyrazolate ring (Figure 10b); • 496 cm-1, relative to the Ni-N asymmetric stretching (Figure 10c). On the other hand, a reversible increase of the intensity of the band peaked at 462 cm-1, corresponding to N-Ni-N bending in plane, was recorded. Perturbation of the latter two bands upon adsorption suggests that the coordination sphere of Ni(II) is perturbed upon benzyl alcohol adsorption, which supports the formation of interactions between the probe and the metal centers. The pristine coordination sphere is completely restored after desorption. iii)The frequency shift, towards higher values, of some bands belonging to the catalyst, being particularly pronounced for bands originally peaking at: • 1188 cm-1, corresponding to the N-C-H bending out of phase in plane; • 679 and 633 cm-1, respectively due to the aromatic C-C-C bending out of phase out of plane and the inter-ring C-C-C bending. These changes point out the fact that, not only the pyrazolate ring, but also the phenylic one are perturbed when interacting with the adsorbate. iv) The occurrence of new bands, which cannot be related neither to the pure catalyst nor to benzyl alcohol, namely: • 3550 cm-1, in the O-H stretching frequency region, attributable to free O-H stretching; ACS Paragon Plus Environment

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• 3128 cm-1, reasonably assigned to an N-H stretching vibrational mode; • 838 cm-1, that can be related to the formation of a Ni-OR bond due to the interaction of Ni3(BTP)2 metal sites with the oxygen atom of the adsorbate. This interpretation is in agreement with the results obtained performing ethanol feeding on Ni3(BTP)2. The concomitant occurrence of new bands centered at 3128 and 838 cm-1, combined with the modifications of the band at 496 cm-1, strongly support that benzyl alcohol deprotonates upon bonding to the metal center of the MOF with its oxygen atom, as depicted in Figure 10. The interactions which develop between benzyl alcohol and Ni3(BTP)2 are fully reversible, as confirmed by the complete restoration of the original spectrum upon raising the temperature up to 150 °C to trigger desorption (Figure 11). These evidences of a reversible adsorption of the probe molecule, together with the rather low desorption temperature, strongly suggest that the interactions are rather weak. The detailed interpretation of the phenomenon is out of the scope of this work; nonetheless, the adsorption experiments here presented have proved to be beneficial to elucidate the catalytic behavior.

CONCLUSIONS The accessibility of the metal center plays a key role when MOFs are used as catalysts. In this work, we have demonstrated that the active sites in the base-free selective oxidation of HMF catalyzed by case-study Ni-containing MOFs with exposed metal sites are not only the metal centers laying on the surface, but also those decorating the pore walls. The accessibility of these sites is strongly dependent on the crystal structure of the MOF, as highlighted by the different activity of Ni(BPEB) and Ni3(BTP)2. Moreover, the activity strongly decreases when the pores are filled with solvent, as proved by the response of Ni3(BTP)2 in the different conditions essayed. The different performances of the Ni-based MOFs herein investigated and the effective activity of their metal centers was rationalized by juxtaposing PXRD and FT-IR experiments, the latter showing the formation of reversible Ni-OR interactions of different strength in Ni(BPEB) and Ni3(BTP)2. Overall, only in ACS Paragon Plus Environment

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Ni3(BTP)2 the first and second coordination spheres of the metal ions are suitable for substrate entrance and interaction with the accessible metal ions themselves.

AUTHOR INFORMATION Corresponding authors *

Corresponding

authors:

CL:

[email protected],

+39-031-2386620;

SG:

[email protected], +39-031-2386627 Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS CL, SG, and AM acknowledge Università dell’Insubria for partial funding.


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functionalized carbon nanotube-supported Au–Pd alloy nanoparticles. ACS Catal. 2014, 4, 2175– 2185. (47) This catalytic test possibly yielded organics containing chromophores, as evidenced by UV-Vis spectroscopy. (48) The axial position opposite to the one occupied by the solvent is completely inaccessible, as it is internal to the Ni4 square (see the section devoted to the structural features). (49) None of the products typically expected from the oxidation of HMF was detected. (50) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds 3rd Ed., Wiley-Interscience Publication, John Wiley & Sons, 1978. (51) Shearer, G. C.; Colombo, V.; Chavan, S.; Albanese, E.; Civalleri, B.; Maspero, A.; Bordiga, S. Stability vs. reactivity: understanding the adsorption properties of Ni3(BTP)2 by experimental and computational methods. Dalton Trans. 2013, 42, 6450-6458. (52) The gas phase proton affinity of ethanol is 788 kJ/mol, to be compared with the value of 596 kJ/mol for CO. Lias, S. G.; Liebman, J. F.; Levin, R. D. Evacuated gas phase basicities and proton affinities of molecules; heats of formation of protonated molecules. J. Phys. Chem. Ref. Data 1984, 13, 695-808. (53) Albanese, E.; Civalleri, B.; Ferrabone, M.; Bonino, F.; Galli, S.; Maspero, A.; Pettinari, C. Theoretical and experimental characterization of pyrazolato-based Ni(II) metal–organic frameworks. J. Mater. Chem. 2012, 22, 22592–22602. (54) Baertsch, C. D.; Funke, H. H.; Falconer, J. L.; Noble, R. D. Permeation of aromatic hydrocarbon vapors through silicalite−zeolite membranes. J. Phys. Chem. 1996, 100, 7676–7679. (55) Silverstein, R. M.; Webster, F. X.; Kiemle, D. J. Spectrometric Identification of Organic Compounds, John Whiley & Sons, 2005.


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TABLES Table 1. Results of the preliminary catalytic tests carried out with Ni3(BTP)2 and with an inorganic source of nickel [Ni(NO3)2·6H2O] as catalyst, as well as of the blank test in the absence of any catalyst.

Entry

1

T

PO2

t

NaOH

HMF conv.

DFF yield

(°C)

(bar)

(h)

(eq.)

(%)

(%)

100

10

4

4

Catalyst

Ni3(BTP)2

0 HMF

2

Ni3(BTP)2

100

10

4

2

0 degradation

3

Ni3(BTP)2

100

10

4

1

0

4

Ni(NO3)2·6H2O

120

30

24

-

100

0

5

-

120

30

24

-

0

0

Table 2. Results of the catalytic tests carried out with the three Ni-based MOFs and with the copper-homologue of Ni3(BTP)2. Reaction conditions: 120 °C, 30 bar O2, 24 h, water as solvent.

Entry

a

HMF conv.

DFF yield

DFF sel.

(%)

(%)

(%)

Catalyst

1

Ni3(BTP)2a

3

3

>99

2

Ni3(BTP)2

27

27

>99

3

Ni3(BTP)2·S

11

11

>99

4

Ni(BPEB)

3

3

>99

5

Ni(BDP)

0

0

0

6

Cu3(BTP)2

38

11

29

reaction time 8 h.


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Table 3. Assignment of the infrared bands observed during adsorption of ethanol at 85 °C on the clean surface of thermally activated Ni3(BTP)2 (see also Figure 3). Vibrational frequency (cm-1)

Vibrational mode

Assignation

3582

ν OH

Free OH (NH)

3465

ν OH

H-bonded Ethanol

2972

ν(as) CH3

Ethanoate

2892

ν(s) CH3

Ethanol/Ethanoate

1379

δ CH3

Ethanol

1275

δ OH

Ethanol

1094

ν(as) CO / ν(as) CC

Ethanoate

1043

ν(s) CO

Ethanoate

879

δ Ni-O(H)

Ethanol/Ethanoate-Ni


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Table 4. Assignment of the main FT-IR bands observed for pure benzyl alcohol, thermally activated Ni3(BTP)2 and Ni3(BTP)2 during adsorption of the alcohol in vacuum (see also Figures 8 and 10). The bands highlighted in blue appeared during adsorption. Vibrational frequency (cm-1) Vibrational modea

Ni3(BTP)2

Benzyl alcohol

after activation 3572

ν O-H ν O-H free ν intermolecular hydrogen bonded O-H

3332 (br)

3550 3412 (br)

ν N-H

after adsorption 3550 3396 (br) 3128

ν aromatic C-H ν aromatic C-H ν aromatic C-H

3088 3065

3152 3121 3077

3152 3087 3062

ν aromatic C-H ν aromatic C-H

3031

3052 3052

3052 3047

ν methylene C-H ν methylene C-H

2933 2874

2927 2859

2929 2871

1556

1556

ν inter-ring C-C ν aromatic C=C overlapped with δs CH2 at ca. 1471 cm-1 ν aromatic C=C overlapped with δs CH2 ν sym C-C + ν asym C-N ν sym C-N ν N-N stretching + δ ip C-H δ O-H possibly augmented by δ ip C-H δ op N-C-H δ ip phenylic H-C-C δ ip N-C-H bending ip ν C-O δ ip pyrazolic C-H + δ op phenylic ν Ni-O δ op phenylic C-C-C δ op aromatic C-H δ aromatic C=C δ op phenylic C-C-C δ inter-ring C-C-C ν asym Ni-N δ ip N-Ni-N a

1496

1496

1454

1453 1360 1328 1254 1208 1192 1134 1075 1016 856

1359 1328 1255 1209 1188 1135 1074 1020 855 762 736 698 679 633 496 462

838 761 735 699 683 638 496 461

op = out of phase; ip = in phase.


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SCHEMES AND FIGURES

O O

O

DFF OH O O

HMFCA

O O

OH

OH

O2, cat. OH

HMF

O O

O

FFCA OH

OH O

O

O

FDCA Scheme 1. Typical products of HMF catalytic oxidation, namely: 2,5-diformylfuran (DFF), 5hydroxymethylfuran-2-carboxylic acid (HMFCA), 5-formyl-2-carboxylic acid (FFCA), and 2,5furandicarboxylic acid (FDCA).


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Page 30 of 41

N

HN

N

H N

NH

N

H2BDP N

HN

HN

NH

N

H2BPEB

N N

N H

H3BTP

Scheme 2. Molecular structures of the three poly(pyrazolyl)-based ligands adopted in this study, namely: 1,4-(4-bispyrazolyl)benzene (H2BDP), 1,4-bis(1H-pyrazol-4-ylethynyl)benzene (H2BPEB), and 1,3,5-tris(1H-pyrazol-4-yl)benzene (H3BTP).


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Figure 1. Representation of the crystal structure of Ni(BDP): (A) the NiN4 node. (B) Portion of the 1-D chain of collinear metal ions. (C) Portion of the crystal packing viewed, in perspective, along the [100] direction: the 1-D rhombic channels can be appreciated. Carbon, grey; nickel, yellow; nitrogen, blue; oxygen red. The hydrogen atoms and the solvent molecules have been omitted for clarity.


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Figure 2. Representation of the crystal structure of Ni3(BTP)2·S: (A) the Ni4N16 node. (B) Portion of the crystal packing viewed along the [100] direction: both the octahedral cavities and one of the 1-D channels can be appreciated. Carbon, grey; nickel, yellow; nitrogen, blue; oxygen red. The hydrogen atoms and the solvent molecules have been omitted for clarity, except the oxygen atom of the solvent molecule bound to the metal center.


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Figure 3. Evolution of the DRIFT spectra acquired at 85 °C on the clean surface of Ni3(BTP)2 during ethanol adsorption.

Figure 4. DRIFT spectra acquired at 85 °C, after ethanol pulse and successive flushing, on the clean surface of (A) Ni(BPEB), (B) Ni3(BTP)2, and (C) NiCl2.


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Figure 5. Portion of the crystal structure of (A) Ni(BPEB), and (B) Ni3(BTP)2, showing the different accessibility of the square planar Ni(II) sites in the two cases, as indicated by the green arrows.


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Figure 6. PXRD monitoring of (A) Ni3(BTP)2 and (B) Ni(BPEB) after impregnation with ethanol. (A) trace (a), before impregnation; trace (b), after impregnation and drying; trace (c), after 30 min from drying; trace (d), after 2 h from drying; trace (e), after 4 h from drying; trace (f), after 24 h from drying. (B) trace (a), before impregnation; trace (b), after impregnation and drying; trace (c), after 30 min from drying; trace (d), after 2 h from drying; trace (e), after 4 h from drying; trace (f), after 6 h from drying; trace (g), after 24 h from drying.


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Figure 7. DRIFT spectra acquired at 85 °C, after ethanol pulse and successive flushing, on the clean surface of (A) activated and (B) non-activated Ni3(BTP)2.


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Figure 8: FT-IR spectra acquired during thermal activation of Ni3(BTP)2: (A) the 4000-2000 cm-1 range; (B) the 1800-450 cm-1 range. In both (A) and (B), spectrum (a), ambient temperature; spectrum (b), 100 °C; spectrum (c), 150 °C; spectrum (d), ambient temperature post treatment.


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Figure 9: FT-IR spectra acquired during adsoption and desoption of benzyl alcohol on Ni(BPEB). (A) the 4000-1850 cm-1 range; (B) the 1850-450 cm-1 range. In both (A) and (B), spectrum (a), pure benzyl alcohol; spectrum (b), thermally activated Ni(BPEB); spectrum (c), benzyl alcohol adsorption; spectrum (d), benzyl alcohol desorption at ambient temperature.


38

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Figure 10: FT-IR spectra acquired during the adsoption and desoption of benzyl alcohol on Ni3(BTP)2: (A) the 40002000 cm-1 range; (B) the 1800-1250 cm-1 range; (C) the 1250-450 cm-1 range. In (A), (B), and (C), spectrum (a), pure benzyl alcohol; spectrum (b), thermally activated Ni3(BTP)2; spectrum (c), benzyl alcohol adsorption; spectra (d) to (g), benzyl alcohol desorption at ambient temperature, 50 °C, 100 °C, and 150 °C, respectively.


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Figure 11. Schematic representation of the interaction between the Ni3(BTP)2 adsorbent and a molecule of adsorbate, as suggested by IR spectroscopy, in terms of insurgence of the Ni-O(H) and N-H(O) interactions.


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TOC GRAPHIC


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AdsorbentâAdsorbate Interactions in the Oxidation of HMF Catalyzed

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