A Recyclable Cu-MOF-74 Catalyst for the Ligand-Free O ... - CSIC Digital

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A Recyclable Cu-MOF-74 Catalyst for the Ligand-Free O-Arylation Reaction of 4-Nitrobenzaldehyde and Phenol Pedro Leo 1 , Gisela Orcajo 1 , David Briones 1 , Guillermo Calleja 1 , Manuel Sánchez-Sánchez 2 and Fernando Martínez 1, * 1

2

*

Department of Chemical and Energy Technology, ESCET. Rey Juan Carlos University, C/Tulipan s/n, 28933 Mostoles, Spain; [email protected] (P.L.); [email protected] (G.O.); [email protected] (D.B.); [email protected] (G.C.) Instituto de Catálisis y Petroleoquímica, ICP-CSIC. C/Marie Curie 2, 28049 Madrid, Spain; [email protected] Correspondence: [email protected]; Tel.: +34-91-488-7182

Academic Editors: Patricia Horcajada and Sergio M. F. Vilela Received: 3 May 2017; Accepted: 9 June 2017; Published: 16 June 2017

Abstract: The activity and recyclability of Cu-MOF-74 as a catalyst was studied for the ligand-free C–O cross-coupling reaction of 4-nitrobenzaldehyde (NB) with phenol (Ph) to form 4-formyldiphenyl ether (FDE). Cu-MOF-74 is characterized by having unsaturated copper sites in a highly porous metal-organic framework. The influence of solvent, reaction temperature, NB/Ph ratio, catalyst concentration, and basic agent (type and concentration) were evaluated. High conversions were achieved at 120 ◦ C, 5 mol % of catalyst, NB/Ph ratio of 1:2, DMF as solvent, and 1 equivalent of K2 CO3 base. The activity of Cu-MOF-74 material was higher than other ligand-free copper catalytic systems tested in this study. This catalyst was easily separated and reused in five successive runs, achieving a remarkable performance without significant porous framework degradation. The leaching of copper species in the reaction medium was negligible. The O-arylation between NB and Ph took place only in the presence of Cu-MOF-74 material, being negligible without the solid catalyst. The catalytic advantages of using nanostructured Cu-MOF-74 catalyst were also proven. Keywords: MOF; catalyst; recyclable Cu-MOF-74; ligand-free; O-arylation reaction; 4-nitrobenzaldehyde; phenol; 4-formyldiphenyl ether

1. Introduction Diaryl ethers are very valuable organic compounds in the synthesis of biologically natural products and in the pharmacological and polymer fields [1,2]. The traditional synthesis is based on the Ullmann cross-coupling reaction of phenols with aryl halides, catalyzed by copper salts [3]. However, homogeneous copper-mediated coupling reactions are limited by the large amount of catalyst needed and harsh reaction conditions [4–7]. The high concentration of the catalyst is an important environmental drawback, as large quantities of hazardous copper-based waste are produced. Additionally, it has been reported that additives or ligands are needed for promoting the ether formation at milder reaction conditions [8,9]. The synthesis of diaryl ethers with palladium as a catalyst has been also reported [10], but the need of expensive palladium amounts and complex ligands limit its feasibility. The development of copper heterogeneous catalysts for Ullmann C–O cross-coupling reactions is therefore a motivating challenge for a more sustainable process, as they could be easily recoverable and reusable and probably not requiring assisting ligands.

Nanomaterials 2017, 7, 149; doi:10.3390/nano7060149

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Nanomaterials 2017, 7, 149

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Up to date, very few works have been addressed to this issue [11,12]. Magnetic CuFe2 O4 nanoparticles showed remarkable performances for the C–O cross reaction of phenols with aryl halides, although acetylacetone as ligand was necessary [13]. More recently, metal-organic framework (MOF) materials have been evaluated. MOF materials are crystalline solids with surface areas surpassing activated carbons and zeolites, composed by metal nodes connected by organic linkers through strong chemical bonds [14] to lead to high order and uniform porous networks for chemical-, size- and shape-selective catalysis [15–19]. Phan and co-workers have paid attention on ligand-free copper-catalyzed coupling reactions of phenols with aryl iodides or nitroarenes [20] using MOF-199 and Cu2 (BDC)2 (DABCO) (BDC = 1,4-benzene dicarboxylate and DABCO = 1,4-diazabicyclo[2.2.2]octane), respectively. Maity and coworkers have also investigated the O-arylation of aryl alcohols with aryl bromides using a Zn-based isoreticular metal organic framework (IRMOF-3) in which copper(II) was anchored following a post-synthetic modification method [21]. The most stimulating features of MOF materials for catalytic applications are their high surface area, tunable pore size, scaffold flexibility, and diversity of structural functional sites like the accessible metallic centers [22]. In this work, the Cu-MOF-74 material, characterized by exhibiting unsaturated copper sites pointing to the pore channels, was studied as a catalyst of the free-ligand O-arylation reaction of phenol with 4-nitrobenzladehyde. This Cu-MOF-74 material was previously tested in the acylation of anisole with phenol, exhibiting remarkable catalytic activity and structural stability [23]. The influence of different reaction variables such as temperature, type of solvent, catalyst concentration and type and concentration of base in the absence of assisting ligands was evaluated in order to determine the best operation conditions for the synthesis of diaryl ethers. The recyclability of the catalyst was also studied along several successive catalytic runs, checking its resistance to activity loss. Finally, this MOF material was compared to the nanostructured homologue Cu-MOF-74 in order to study the influence of the catalyst’s crystal size over its catalytic performance. 2. Results and Discussion 2.1. Characterization of Cu-MOF-74 Cu-MOF74 was synthesized by the solvothermal method previously described [24] with a yield of 74%. Conventional techniques were used for the physico-chemical characterization of Cu-MOF-74 material as shown in Figure 1. Powder X-ray diffraction (PXRD) pattern revealed the typical reflections of the MOF-74/CPO-27 phase [25], discarding the presence of secondary crystalline phases (Figure 1a). SEM micrographs also revealed the expected large needle-shaped crystals that have been previously reported for this Cu-MOF-74 material [24] (Figure 1b). Both PXRD patterns and SEM pictures confirmed the crystallinity of the Cu-MOF-74 material. Elemental analysis by using ICP-OES indicated a copper loading of 6.1 mmol/g, which is close to the theoretical content from the molecular structure (6.23 mmol/g). The porosity of the material was measured by nitrogen adsorption at −196 ◦ C (Figure 1c). The isotherms revealed a microporous material with a BET specific surface area around 1100 m2 /g, a pore volume of 0.57 cm3 /g, and an average pore diameter of ca. 10.1 Å. Thermogravimetric analysis (TGA) under N2 atmosphere evidenced a first weight loss at ca. 100 ◦ C corresponding to the solvent removal from the porous framework as well as a high thermal stability until 370 ◦ C, when the decomposition of the organic ligand takes place (Figure 1d).

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Figure 1. Catalyst characterization of Cu-MOF-74: (a) PXRD patterns; (b) SEM images; (c) nitrogen Figure 1. Catalyst characterization of Cu-MOF-74: (a) PXRD patterns; (b) SEM images; (c) nitrogen adsorption–desorption isotherms at −196 ◦ C with an inset showing the micropore size distribution adsorption–desorption isotherms at −196 °C with an inset showing the micropore size distribution curve as generated by NL-DFT method, and (d) TGA under inert atmosphere. curve as generated by NL-DFT method, and (d) TGA under inert atmosphere.

2.2. Catalytic Study 2.2. Catalytic Study 2.2.1. Influence of Temperature 2.2.1. Influence of Temperature The influence of the temperature on the O-arylation cross-coupling reaction of The influence of the temperature on the O-arylation cross-coupling reaction of 44-nitrobenzaldehyde (NB) and phenol (Ph) to form 4-formyldiphenyl ether was evaluated at nitrobenzaldehyde (NB) and phenol (Ph) to form 4-formyldiphenyl ether was evaluated at 60, 80, 100, 60, 80, 100, 120, and 140 ◦ C (Figure 2). These reactions were carried out in N,N-dimethylformamide 120, and 140 °C (Figure 2). These reactions were carried out in N,N-dimethylformamide (DMF) as (DMF) as solvent, with an NB/Ph molar ratio of 1/2, 2 equivalents of K2 CO3 as a base, and 5 mol % of solvent, with an NB/Ph molar ratio of 1/2, 2 equivalents of K2CO3 as a base, and 5 mol % of catalyst, catalyst, conditions similar to those previously published [20]. A remarkable 88% conversion of NB conditions similar to those previously published [20]. A remarkable 88% conversion of NB was was achieved after 6 min and 100% conversion after 1 h for the highest reaction temperature (140 ◦ C). achieved after 6 min and 100% conversion after 1 h for the highest reaction temperature (140 °C). The The decrease of temperature reduced the catalytic activity of Cu-MOF-74, being less significant in decrease of temperature reduced the catalytic activity of Cu-MOF-74, being less significant in the the range of 120–140 ◦ C. At the lowest tested temperature (60 ◦ C), the NB conversion after 2 h was range of 120–140 °C. At the lowest tested temperature (60 °C), the NB conversion after 2 h was ca. ca. 60%. At the reaction temperature of 100 ◦ C, the Cu-MOF-74 showed a catalytic activity similar 60%. At the reaction temperature of 100 °C, the Cu-MOF-74 showed a catalytic activity similar to to another Cu-based MOF material (Cu2 (BDC)2 (DABCO)) reported in the literature [20]. However, another Cu-based MOF material (Cu2(BDC)2(DABCO)) reported in the literature [20]. However, Cu2 (BDC)2 (DABCO) evidenced a stronger loss of activity at lower reaction temperatures as compared Cu2(BDC)2(DABCO) evidenced a stronger loss of activity at lower reaction temperatures as compared to Cu-MOF-74. This behavior is attributed to the average pore diameter and diffusional constraints of to Cu-MOF-74. This behavior is attributed to the average pore diameter and diffusional constraints both materials when the temperature is decreased. The pore size diameter of Cu-MOF-74 (10.1 Å) is of both materials when the temperature is decreased. The pore size diameter of Cu-MOF-74 (10.1 Å ) larger than that reported for Cu2 (BDC)2 (DABCO) (6.4 Å), a difference that enables an easier diffusion is larger than that reported for Cu2(BDC)2(DABCO) (6.4 Å ), a difference that enables an easier of reactants within the porous structure of Cu-MOF-74, favoring its catalytic activity. On the other diffusion of reactants within the porous structure of Cu-MOF-74, favoring its catalytic activity. On hand, the Cu2 (BDC)2 (DABCO) material was not tested at temperatures above 100 ◦ C [20], whereas the the other hand, the Cu2(BDC)2(DABCO) material was not tested at temperatures above 100 °C [20], catalytic performance of Cu-MOF 74 is significantly enhanced. Therefore, the Cu-MOF-74 is active at whereas the catalytic performance of Cu-MOF 74 is significantly enhanced. Therefore, the Cu-MOFmild reaction temperatures (60 ◦ C) and becomes extremely active at 120 ◦ C, giving an NB conversion 74 is active at mild reaction temperatures (60 °C) and becomes extremely active at 120 °C, giving an close to 80% in few minutes. NB conversion close to 80% in few minutes. In order to evaluate the contribution of homogeneous catalysis by possible leached copper species In order to evaluate the contribution of homogeneous catalysis by possible leached copper from Cu-MOF-74, an additional catalytic run at 120 ◦ C was carried out, removing the solid catalyst species from Cu-MOF-74, an additional catalytic run at 120 °C was carried out, removing the solid from the mixture by hot filtration after 5 min of reaction. The reaction mixture was further maintained catalyst from the mixture by hot filtration after 5 min of reaction. The reaction mixture was further for 2 h under these conditions. Figure 3 shows the NB conversion profiles with the solid Cu-MOF-74 maintained for 2 h under these conditions. Figure 3 shows the NB conversion profiles with the solid catalyst present all the time (called “presence of catalyst”) and the so-called “after catalyst removal” Cu-MOF-74 catalyst present all the time (called “presence of catalyst”) and the so-called “after

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catalyst removal” with the Cu-MOF-74 catalyst removed after 5 min. The active role of the catalyst removal” with the Cu-MOF-74 catalyst removed after 5 min. The activeproven role of the heterogeneous Cu-MOF-74 catalyst in the O-arylation cross-coupling is clearly when with the Cu-MOF-74 catalyst removed after 5 min. The active role of reaction the heterogeneous Cu-MOF-74 heterogeneous Cu-MOF-74 catalyst in the O-arylation cross-coupling reaction is clearly proven when comparing both profiles. No further significant conversion of NB was detected once the solid catalyst catalyst in the O-arylation cross-coupling reaction is clearly proven when comparing both profiles. comparing bothfrom profiles. further significant conversion of NB was detected once the solid catalyst was removed theNoreaction mixture, which demonstrates a negligible contribution of No further significant conversion of NB was detected once the solid catalyst was removed from was removed catalysis from the mixture, which demonstrates a negligible contribution of homogeneous by reaction plausible coppera leaching. fact, dissolved copper species were not the reaction mixture, which demonstrates negligibleIncontribution of homogeneous catalysis by homogeneous by plausible copperanalyses. leaching. In fact, dissolved copper species were not detected in the catalysis reaction mixture by ICP-OES plausible copper leaching. In fact, dissolved copper species were not detected in the reaction mixture detected in the reaction mixture by ICP-OES analyses. by ICP-OES analyses.

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Figure 2. Kinetics of 4-nitrobenzaldehyde (NB) conversion catalyzed by Cu-MOF-74 at different Figure 2. Kinetics of 4-nitrobenzaldehyde (NB) conversion catalyzed by Cu-MOF-74 at Figure 2. Kinetics of 4-nitrobenzaldehyde (NB) conversion catalyzed by Cu-MOF-74 at different temperatures different temperatures temperatures

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Figure 3. NB conversion in the absence of catalyst (blank), in the presence of Cu-MOF-74 catalyst for Figure NBin conversion in the absence of catalyst (blank), in the presence of Cu-MOF-74 catalyst for 120 min3.and the presence of Cu-MOF-74 for 5 min and subsequent removal. Figure NBin conversion in the absence of catalyst (blank), in the presence of Cu-MOF-74 catalyst for 120 min3.and the presence of Cu-MOF-74 for 5 min and subsequent removal. 120 min and in the presence of Cu-MOF-74 for 5 min and subsequent removal. 2.2.2. Influence of 4-Nitrobenzaldehyde/Phenol Molar Ratio

2.2.2. Influence of 4-Nitrobenzaldehyde/Phenol Molar Ratio influence of the reactants ratio in the cross-coupling 2.2.2.The Influence of 4-Nitrobenzaldehyde/Phenol Molar Ratioreaction was also investigated, testing the Themolar influence reactants ratioand in the reaction waswere also carried investigated, NB/Ph ratiosofofthe 1/1, 1/1.5, 1/2, 1/3cross-coupling (Figure 4). These reactions out at testing 120 ◦ C The influence of the reactants ratio in the cross-coupling reaction was also investigated, testing the ratiosand of 1/1, 1/1.5, 1/2, and (Figure 4). These reactions were carried out at 120 °C withNB/Ph DMF molar as solvent 2 equivalents of K1/3 2 CO3 as base in the presence of 5 mol % of Cu-MOF-74 the NB/Ph molar ratios of 1/1, 1/1.5, 1/2, and 1/3 4).in These reactions were carried out at 120 °C with DMF as solvent and 2 equivalents of K 2CO 3 as base the presence of 5observed. mol % ofThe Cu-MOF-74 catalyst. A significant effect of the NB/Ph ratio(Figure on the conversion is clearly increase with DMF as solvent and 2 equivalents of K 2CO 3 the as base in the presence of 5 mol % of Cu-MOF-74 catalyst. A significant effect of the NB/Ph ratio on conversion is clearly observed. The increase of of phenol proportion until NB/Ph = 1/2 produced a significant enhancement of the NB conversion. catalyst. A noted significant effect ofresults the NB/Ph ratio on conversion is clearly observed. increase of phenol until NB/Ph = 1/2 produced a the significant enhancement of the NBThe conversion. It It must proportion be that similar were reported using other copper-based MOF materials, like phenol proportion until NB/Ph = 1/2 produced a significant enhancement of the NB conversion. It must be noted that similar results were reported using other copper-based MOF materials, like Cu2 (BDC)2 (DABCO), being the best NB/Ph molar ratio 1/1.5 [20]. This result can be explained by the must be noted that similar results were reported using other copper-based MOF materials, like Cu 2(DABCO), being the best NB/Ph molar ratio 1/1.5 [20]. This result can be explained by the role2(BDC) of phenol as a ligand in the catalytic mechanism, which accelerates the coupling with aryl halides, Cu 2(BDC) 2(DABCO), being thecopper best NB/Ph molar ratio 1/1.5 [20]. This result can bewith explained by the role of phenol as a ligand in the catalytic mechanism, which accelerates the coupling arylCu-MOF halides, as observed in homogeneous catalytic systems [26]. In the case of heterogeneous role of phenolinashomogeneous a ligand in thecopper catalytic mechanism, which accelerates theofcoupling with arylCu-MOF halides, as observed catalytic systems [26]. In the case heterogeneous as observed in homogeneous copper catalytic systems [26]. In the case of heterogeneous Cu-MOF


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Nanomaterialsthe 2017, 7, 149 of using phenol in excess is also probably related to the primary interaction 5 of of 15 materials, benefit Nanomaterials 2017, 7, 149 5 of 15 phenolates with available copper sites followed by coupling with NB, instead of complexation of NB as anmaterials, electrophilic reactant and later coupling with phenolate as a nucleophilic agent. the benefit of using phenol excess is also to to thethe primary interaction of materials, the benefit of using phenol ininexcess is alsoprobably probablyrelated related primary interaction of phenolates with available copper sites followed by coupling with NB, instead of complexation of NB phenolates with available copper sites followed by coupling with NB, instead of complexation of NB as an electrophilic reactant and later coupling with phenolate as a nucleophilic agent. as an electrophilic reactant and later coupling with phenolate as a nucleophilic agent. 100

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Figure 4. NB NB conversion vs.vs. reaction time given by Cu-MOF-74 Cu-MOF-74catalyst catalyst at different different NB/Ph ratios. Figure 4. vs. reaction catalyst NB/Ph Figure 4. conversion NB conversion reactiontime timegiven given by Cu-MOF-74 at at different NB/Ph ratios.ratios.

2.2.3. Influence of Concentration Influence of Catalyst Concentration 2.2.3.2.2.3. Influence of Catalyst Catalyst Concentration In order to assess influenceofofthe thecatalyst catalyst concentration, a new reaction runrun waswas performed In to the influence concentration, aa new reaction performed In order order to assess assess thethe influence of the catalyst concentration, new reaction run was performed with DMF as solvent, at 120 °C,NB/Ph NB/Phmolar molar ratio ratio of 2,2,two equivalents of K 2CO , and catalyst with DMF as solvent, at 120 °C, of two equivalents of K 23CO 3, and catalyst ◦ with DMF as solvent, at 120 C, NB/Ph molar ratio of 2, two equivalents of K2 CO3 , and catalyst concentrations of1,0, 3, 1, 5, 3, and 5, and 7mol mol%%(Figure (Figure 5). 5). Catalyst concentration waswas calculated based on concentrations of 0, 7 Catalyst concentration calculated based concentrations of 0, 1,ratio 3, 5,values. and 7Amol %NB (Figure 5). Catalyst concentration was calculated based on on the Cu/NB molar low conversion (ca. 10%) was achieved in the absence of solid the Cu/NB molar ratio values. A low NB conversion (ca. 10%) was achieved in the absence of solid the Cu/NB values. low NBofconversion 10%)provided was achieved in theinitial absence catalyst molar (blank)ratio (Figure 3). TheApresence Cu-MOF-74(ca. catalyst a remarkable rateof in solid catalyst (blank) (Figure 3). The presence of Cu-MOF-74 catalyst provided aa remarkable initial rate catalyst (blank) (Figure 3). The presence of Cu-MOF-74 catalyst provided remarkable initial rate in in a mere 5 min of reaction, with values of NB conversion from 58% (1 mol % catalyst concentration) to aa mere 5 min of reaction, with values of NB conversion from 58% (1 mol % catalyst concentration) to mere 5 (7 min of%reaction, with values of NB conversion fromtime 58%and (1 mol % catalyst concentration) 81% mol of catalyst concentration). After 2 h of reaction catalyst concentrations of 1 81% (7 % of 284% h of timetime and and catalyst concentrations andmol mol %,catalyst ofconcentration). NB conversion After were respectively. An catalyst increase of catalyst of 1 to 81% (73 mol % ofyields catalyst concentration). After 2and hreaction of94%, reaction concentrations and 3concentration mol %, %, yields NB conversion were 84% respectively. of catalyst up toof5% led toconversion 100% conversion in 90and min, but interestingly, anAn increase beyond of 1 and 3 mol yields of NB were 84% and94%, 94%, respectively. An increase increase of 5% catalyst concentration up to conversion in but interestingly, increase did not produce any significant improvement. results reported byan Chen et al. beyond for NB 5% concentration up to to 5% 5% led led to 100% 100% conversion Similar in 90 90 min, min, butwere interestingly, an increase beyond 5% conversion when using 5 mol %improvement. of (Cu(OAc)2·HSimilar 2O) as a catalyst and similar reaction conditions [12]. did not produce any significant results were reported by Chen et al. NB did not produce any significant improvement. Similar results were reported by Chen et al. for for NB However, it must be pointed Cu content foras reaching a complete conversion of NB as shown [12]. conversion mol % %out of that (Cu(OAc) 2·H2O) a catalyst and similar reaction conditions conversion when when using using 55 mol of (Cu(OAc) 2 ·H2 O) as a catalyst and similar reaction conditions [12]. before is significantly lower compared to conventional Ullman reactions, where an average value of However, it must must be be pointed pointed out out that that Cu content for of NB However, it Cu content for reaching reaching aa complete complete conversion conversion of NB as as shown shown 10 mol % Cu-catalyst concentration is needed [27,28]. before is significantly lower compared to conventional Ullman reactions, where an average before is significantly lower compared to conventional Ullman reactions, where an average value value of of 10 mol % Cu-catalyst concentration is needed [27,28]. 10 mol % Cu-catalyst concentration is needed [27,28]. 100

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The type of solvent can be crucial in coupling Ullmann reactions due to its influence on the 2.2.4.mechanism Influence of and the Solvent reaction solubility of reactants. Therefore, the O-arylation reaction of Ph with NB was carried out different solvents as DMF, toluene,reactions and acetonitrile, constant Theintype of solvent can be crucialnitrobenzene, in coupling Ullmann due to itskeeping influence on the the ◦ C), the NB/Ph molar ratio (1/2), 2 equivalents of K CO as a base, and 5 mol % temperature (120 2 3 reaction mechanism and solubility of reactants. Therefore, the O-arylation reaction of Ph with NB of catalyst concentration. As shown in Figure 6, toluene was thetoluene, worst solvent, with onlykeeping 20% of NB was carried out in different solvents as DMF, nitrobenzene, and acetonitrile, conversion 2 h of reaction. Under the same conditions, showed a relatively constantafter the temperature (120 °C), the NB/Ph molar ratio (1/2), nitrobenzene 2 equivalents ofalso K 2CO 3 as a base, and mol % of catalyst concentration. As shown in Figure 6, atoluene the conversion worst solvent, withalthough only low 5conversion (around 43%), whereas acetonitrile led to much was higher (75%), of than NB conversion 2 h of reaction. same conditions, showed a are still 20% lower the 100% after conversion attainedUnder with the DMF in hardly 1.5 hnitrobenzene of reaction. also These results relatively low conversion (around 43%), whereas acetonitrile led to a much higher conversion (75%), in good agreement with the literature using other MOF materials [20], homogeneous catalysts [12] lowerreactions than the 100% conversion attainedhas with DMF hardly 1.5effect h of reaction. These and although differentstill Ullman [4,29,30]. The solvent also an in important on homogeneous resultsO-arylation are in goodreactions agreement with thewith literature using other MOF materials [20], in homogeneous Ullmann of phenol aryl chlorides using copper bromide the presence of catalysts [12] and different Ullman reactions [4,29,30]. The solvent has also an important effect on 1,10-phenantroline as ancillary ligand (L) [31]. In this case, the halogen atom transfer-based mechanism homogeneous Ullmann O-arylation reactions of phenol with aryl chlorides using copper bromide in is sometimes proposed, but others based on oxidative addition, σ-bond, methathesis, or single electron the presence of 1,10-phenantroline as ancillary ligand (L) [31]. In this case, the halogen atom transfertransfer mechanisms have also been suggested. Hartwig et al. [32] described an equilibrium between based mechanism is sometimes proposed, but others based on oxidative addition, σ-bond, neutral LCu–OAr complex as a consequence of the base neutralization) and methathesis, or (Cu(I)-phenoxide single electron transfer mechanisms have also been suggested. Hartwig et al. [32]ionic + cation and Cu(OAr) − anion, as a result of disproportionation of LCu–OAr forms of the Cu(L) 2 described an equilibrium between neutral 2LCu–OAr (Cu(I)-phenoxide complex as a consequence of species. Thisneutralization) equilibrium isand controlled by the solvent the use2− of polarassolvents the base ionic forms of the Cu(L)polarity. 2+ cation Thus, and Cu(OAr) anion, a result favors of the formation of ionic complexes, ionic forms lead to the atom transfer of aryl disproportionation of LCu–OArsuggesting species. Thisthat equilibrium is controlled byhalogen the solvent polarity. Thus, the usefor of the polar solvents favors the ethers formation of ionic complexes, suggesting that ionic forms lead to chlorides formation of diaryl [33]. the transfer of aryl chlorides forwith the formation of diaryl ethers [33]. of aryl chlorides, this In halogen the caseatom of the O-arylation of phenol nitrobenzaldehyde instead In the case theresponsible O-arylation of with nitrobenzaldehyde instead chlorides, this mechanism can be of also forphenol the formation of ionic species over of thearyl unsaturated copper mechanism can be also responsible for the formation of ionic species over the unsaturated copper metal sites of the Cu-MOF-74, which promotes the coupling reaction with NB. Besides, it is also very metal sites of the Cu-MOF-74, which promotes the coupling reaction with NB. Besides, it is also very important to point out the solvent influence on the solubility of reactants, based on its polarity. important to point out the solvent influence on the solubility of reactants, based on its polarity. The The solubility of NB in the reaction conditions ranged from 0.5 to 1.7 M for toluene and DMF, solubility of NB in the reaction conditions ranged from 0.5 to 1.7 M for toluene and DMF, respectively. respectively. These results are in agreement with their polarity indexes, ranging from 2.3 to 6.4 These results are in agreement with their polarity indexes, ranging from 2.3 to 6.4 for toluene and for toluene and DMF, respectively. It is thus that the effect solvent on thecould solubility of NB DMF, respectively. It is thus concluded thatconcluded the effect of solvent on theofsolubility of NB also be could also bethe affecting catalytic performance of the heterogeneous Cu-MOF-74 catalyst [31]. affecting catalyticthe performance of the heterogeneous Cu-MOF-74 catalyst [31].

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Figure 6. NB conversion byCu-MOF-74 Cu-MOF-74catalysts catalysts different solvents. Figure 6. NB conversionvs. vs.reaction reactiontime time given given by in in different solvents.

Influence of the Base 2.2.5.2.2.5. Influence of the Base influence thebase baseon on the the catalytic of of Cu-MOF-74 was was also studied. Chen and co- and TheThe influence ofof the catalyticactivity activity Cu-MOF-74 also studied. Chen workers proved that Cs 2CO3 is more efficient than other bases for the coupling reaction between co-workers proved that Cs2 CO3 is more efficient than other bases for the coupling reaction between phenol 4-nitrobenzaldehyde.However, However, due due to ofof CsCs 2CO3, other more phenol andand 4-nitrobenzaldehyde. to the the relatively relativelyhigh highcost cost 2 CO3 , other more readily available and inexpensive bases such as K2CO3, Na2CO3, K3PO4, and Na3PO4 were tested readily available and inexpensive bases such as K2 CO3 , Na2 CO3 , K3 PO4 , and Na3 PO4 were tested under the usual conditions: DMF as solvent, 120 °C, NB/Ph molar ratio of 2, two equivalent of base, under the usual conditions: DMF as solvent, 120 ◦ C, NB/Ph molar ratio of 2, two equivalent of and 5 mol % of catalyst. Figure 7 shows the NB conversion obtained for the different bases. An

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additional blank experiment with no base is also included. As observed, a significant NB conversion Nanomaterials 2017, 7, 149 7 of 15 of 60% was achieved the absence of7base, butthe much values were obtained when thebases. base base, and 5 mol % of in catalyst. Figure shows NB higher conversion obtained for the different was present. The catalytic activity of Cu-MOF-74 is quite influenced by the type of basic anion additional blank included. AsAs observed, a significant NB NB conversion An additional blank experiment experimentwith withno nobase baseisisalso also included. observed, a significant conversion (carbonate or phosphate) and the counter-cation. Carbonates evidenced a better catalytic of 60% was achieved in the absence of base, but much higher values were obtained when the base was of 60% was achieved in the absence of base, but much higher values were obtained when the base was present. The catalytic activity of Cu-MOF-74 is quite influenced by the typeThe of basic anion performance than phosphates for a given cation (either sodium or potassium). higher kinetic present. The catalytic activity of Cu-MOF-74 is quite influenced by the type of basic anion (carbonate (carbonate or phosphate) and the counter-cation. Carbonates evidenced a better catalytic diameter of phosphate anions is probably affecting the catalytic process of deprotonation of phenol or phosphate) and the counter-cation. Carbonates evidenced a better catalytic performance than performance than phosphates a given cation (either sodium or potassium). The higher kinetic inside the porous structure of (either thefor Cu-MOF-74 the usediameter of potassium instead phosphates for a given cation sodium orframework. potassium).Additionally, The higher kinetic of phosphate diameter of phosphate anions is probably affecting the catalytic process of deprotonation of phenol of sodium salts also showed a remarkable enhancement of the catalytic performance of Cu-MOF-74. anions is probably affecting the processframework. of deprotonation of phenol inside the porous structure inside the porous structure of catalytic the Cu-MOF-74 Additionally, the use of potassium instead This effect is attributed to the lower electronegativity of potassium cations. A higher electronegativity of the the use of potassium instead of sodiumofsalts also showed of Cu-MOF-74 sodium salts framework. also showed aAdditionally, remarkable enhancement of the catalytic performance Cu-MOF-74. a stronger affinityoftothe deprotonated phenoxideofanions, which would hinder the formation apromotes remarkable catalytic performance Cu-MOF-74. effect is attributed to the This effect enhancement is attributed to the lower electronegativity of potassium cations.This A higher electronegativity of Cu–phenoxide complexes and consequently the following transmetallation step for the formation lower electronegativity of potassium cations. A higher electronegativity promotes a stronger affinity promotes a stronger affinity to deprotonated phenoxide anions, which would hinder the formation of deprotonated the diaryl ether compound. of Cu–phenoxide complexes and consequently thehinder following step for the formation to phenoxide anions, which would thetransmetallation formation of Cu–phenoxide complexes the diaryl ether and of consequently thecompound. following transmetallation step for the formation of the diaryl ether compound.

100 80

80

NB conversion (%)

NB conversion (%)

100

60

60

K CO

40

2 3 K2CO 3 K3PO K3PO4 4 Na2CO3 Na2CO 3 Na PO 3 4 Na3PO 4 Without base Without base

40

20

20

00 00

20 20

40 40

60 80 60 80 Time(min) (min) Time

100 120 120 100

Figure 7. NB conversion reactiontime timegiven given by Cu-MOF-74 in in thethe absence of base and and in in Figure 7. NB conversion vs.vs.reaction Cu-MOF-74catalysts catalysts absence of base the presence of different bases. different bases. bases. the presence of different

influenceof ofthe theKK 2CO3 base concentration was also studied by varying the base TheThe influence 2CO3 base concentration was also studied by varying the base The influence of the K2 CO concentration was also studied by varying the base concentration 3 base concentration from 1 to 0.5 and equivalents. Figure 8 shows the NB conversion for the three catalytic concentration from21equivalents. to 0.5 and 2 2equivalents. Figure 8 shows the NB conversion for the three catalytic from 1 to 0.5 and Figure 8 NB/Ph shows NB conversion forThe theincrease three catalytic runs in runs in the usual conditions (DMF, 120 °C, =the 2 and 5 mol % catalyst). of the K2CO 3 ◦ C, runsusual in theconditions usual conditions (DMF, 120 °C, NB/Ph = 2 and 5 mol % catalyst). The increaseofofthe theKK2CO CO3 the (DMF, 120 NB/Ph = 2 and 5 mol % catalyst). The increase 2 3 concentration from 0.5 to 1 equivalents enhances the catalytic activity of Cu-MOF-74. However, the concentration from 0.5 to to 11 equivalents equivalents enhances enhances the the catalytic catalytic activity activity of of Cu-MOF-74. Cu-MOF-74. However, However, the concentration 0.5 increase upfrom to 2 equivalents does not produce further conversion improvement, revealing that one the increase up to to(base/Ph equivalents doesofnot not produce further conversion improvement, increase up 22 equivalents does revealing that one equivalent molar ratio 0.5)produce is able tofurther promoteconversion an efficient improvement, deprotonation ofrevealing phenol forthat the one equivalent (base/Ph molar ratio of 0.5) is able to promote an efficient deprotonation of phenol for formation of phenoxide with sites of Cu-MOF-74 material. equivalent (base/Ph molarcomplexes ratio of 0.5) is the ableCu-containing to promote an efficient deprotonation of phenol for the the

formation of of phenoxide phenoxide complexes complexes with with the the Cu-containing Cu-containing sites sites of of Cu-MOF-74 Cu-MOF-74 material. material. formation 100

100 NB conversion (%)

80

NB conversion (%)

80 60

60

2 equiv

40

1 equiv

2 equiv

40

20

20 0

0.5 equiv

1 equiv

0.5 equiv 0

20

40

60 80 Time (min)

100

120

0 0 20 40 60 80 100 120 Figure 8. NB conversion vs. reaction time given by Cu-MOF-74 catalysts at different K2CO3 Time (min) concentrations.

8. NB at different different KK22CO CO33 Figure 8. NB conversion conversion vs. reaction reaction time time given given by Cu-MOF-74 catalysts at concentrations.


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2.2.6. Influence ofofDifferent 2.2.6. Influence DifferentSubstrates Substrates

2.2.6. Influence of Different Substrates The catalytic was also also evaluated evaluatedfor fordifferent differentnitroarenes nitroarenes and The catalyticactivity activityofofCu-MOF-74 Cu-MOF-74 material material was and The catalytic activity of Cu-MOF-74 material was also evaluated for different ◦nitroarenes and phenol derivatives. The reactions were performed using DMF as solvent, at 120 C, nitroarene/Ph phenol derivatives. The reactions were performed using DMF as solvent, at 120 °C, nitroarene/Ph phenol derivatives. The reactions were performed using DMF as solvent, at 120 °C, nitroarene/Ph molar ratio of 1/2 equivalent of K2 CO . Figure 9 shows9 the NB conversions when 4-cyanophenol, molar ratio of and 1/2 1and 1 equivalent of 3K 2CO3. Figure shows the NB conversions when 4molar ratio of 1/2 and 1 equivalent of K2CO3. Figure 9 shows the NB conversions when 44-chlorophenol, 4-methoxyphenol, and Ph were used. reaction was accelerated cyanophenol, 4-chlorophenol, 4-methoxyphenol, and The Ph coupling were used. The coupling reaction for wasthe cyanophenol, 4-chlorophenol, 4-methoxyphenol, and Ph were used. The coupling reaction was accelerated for the phenol derivative with electron-donating groups such asattaining 4-methoxyphenol, phenol derivative with electron-donating groups such as 4-methoxyphenol, 87% of NB accelerated for the phenol derivative with electron-donating groups such as 4-methoxyphenol, attaining 87% NB conversion in a mere min. Phenol was slightly lesstimes activethan at early times than 4conversion in a of mere 5 min. Phenol was 5slightly less active at early 4-methoxyphenol, attaining 87% of NB conversion in a mere 5 min. Phenol was slightly less active at early times than 4methoxyphenol, but in both cases affording 100% after 90 presence min. The presence of electron-withdrawing but in both casesbut affording 100%affording after 90100% min.after The of electron-withdrawing groups methoxyphenol, in both cases 90 min. The presence of electron-withdrawing groups inderivative, phenol derivative, such as the of cases of 4-chlorophenol and 4-cyanophenol, decreased the in groups phenolin such as the cases 4-chlorophenol and 4-cyanophenol, decreased the phenol derivative, such as the cases of 4-chlorophenol and 4-cyanophenol, decreased theNB NB conversion significantly. conversion significantly. NB conversion significantly.

NB conversion (%) NB conversion (%)

100 100 80 80 60 60 4-methoxyphenol 4-methoxyphenol phenol phenol 4-chlorophenol 4-chlorophenol 4-cyanophenol 4-cyanophenol

40 40 20 20 0 00 0

20 20

40 40

60 80 60 (min)80 Time Time (min)

100 100

120 120

Figure time using usingdifferent differentsubstituted-phenols. substituted-phenols. Figure9.9.NB NBconversion conversionvs. vs. reaction reaction time Figure 9. NB conversion vs. reaction time using different substituted-phenols.

Figure 10 showed the the conversion of of different different nitroarenes compounds when 1-fluoro-4Figure nitroarenescompounds compoundswhen when 1-fluoro-4Figure1010showed showed the conversion conversion of different nitroarenes 1-fluoro-4nitrobenzene, 4-nitroacetophenone, and 4-nitrobenzaldehyde were used in the presence of of phenol. nitrobenzene, 4-nitroacetophenone, and 4-nitrobenzaldehyde were used in the presence phenol. nitrobenzene, 4-nitroacetophenone, and 4-nitrobenzaldehyde were used in the presence of phenol. The highest conversion was obtained for 1-fluoro-4-nitrobenzene affording almost 100% in only 30 The highest obtainedfor for1-fluoro-4-nitrobenzene 1-fluoro-4-nitrobenzene affording almost in 30 only The highestconversion conversion was obtained affording almost 100%100% in only min, followed by 4-nitrobenzaldehyde (100% in 90 min) and finally 4-nitroacetophenone (72% in 120 30min, min,followed followed 4-nitrobenzaldehyde (100% 90 and min) and 4-nitroacetophenone finally 4-nitroacetophenone (72% byby 4-nitrobenzaldehyde (100% in 90 in min) finally (72% in 120 min). This fact suggests that nitroarene compounds with electron-withdrawing groups, such as the This fact suggests that nitroarene compoundscompounds with electron-withdrawing groups, such asgroups, the in min). 120 min). This fact suggests that nitroarene with electron-withdrawing case of 1-fluoro-4-nitrobenzene, allow for an enhancement in the catalytic performance. In contrast, caseasofthe 1-fluoro-4-nitrobenzene, allow for anallow enhancement in the catalytic contrast, such case of 1-fluoro-4-nitrobenzene, for an enhancement inperformance. the catalytic In performance. electron-donating groups have shown the opposite effect, as has also been observed by other authors groups havegroups shown the opposite as has alsoeffect, been observed by other In electron-donating contrast, electron-donating have showneffect, the opposite as has also beenauthors observed [12,20]. These results are in agreement with those published by Phan et al. for this reaction using a [12,20]. These results are in agreement with those published by Phan et al. for this reaction using bycopper-based other authorsMOF [12,20]. These are in agreement with those published by Phan et al. afor catalyst (Curesults 2(BDC)2(DABCO)) [20]. However, Cu-MOF-74 evidenced a lower copper-based MOF catalyst (Cu 2(BDC)2(DABCO)) [20]. However, Cu-MOF-74 evidenced a lower this reaction using a copper-based MOF catalyst (Cu2character (BDC)2 (DABCO)) [20]. However, Cu-MOF-74 dependence on the electron-donating/withdrawing of the substituents of phenol and dependence on the electron-donating/withdrawing character of the substituents of phenol and of evidenced a lower on the electron-donating/withdrawing character of the substituents nitroarenes in thedependence overall catalytic performance. nitroarenes in the overall catalytic performance. phenol and nitroarenes in the overall catalytic performance.

Nitroarene (%) conversion(%) Nitroareneconversion

100 100 80 80 60 60 1-fluoro-4-nitrobenzene 1-fluoro-4-nitrobenzene 4-nitrobenzaldehyde 4-nitrobenzaldehyde 4-nitroacetophenone 4-nitroacetophenone

40 40 20 20 0 00 0

20 20

40 60 80 40 Time 60(min) 80 Time (min)

100 100

120 120

Figure 10. NB conversion vs. reaction time using different nitroarenes. Figure10. 10.NB NBconversion conversion vs. reaction Figure reactiontime timeusing usingdifferent differentnitroarenes. nitroarenes.


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2.2.7. Comparison with Other Other Cu-Catalysts Cu-Catalysts 2.2.7. Comparison with The catalytic phenol with 4The catalytic performance performance of of Cu-MOF-74 Cu-MOF-74ininthe thecross-coupling cross-couplingreaction reactionof of phenol with nitrobenzaldehyde was also compared with other Cu-based heterogeneous catalysts (CuO and 4-nitrobenzaldehyde was also compared with other Cu-based heterogeneous catalysts (CuO and HKUST-1)and andhomogeneous homogeneous catalysts (CuCl CuNO 3 salts). The catalyst concentration was HKUST-1) catalysts (CuCl andand CuNO 3 salts). The catalyst concentration was always always set to copper contents of 5 mol %. The experiments were carried using DMF as at set to copper contents of 5 mol %. The experiments were carried out using out DMF as solvent, at solvent, 120 ◦ C, an 120 °C, molar an NB/Ph 1/2 and 1 of equivalent of K2CO 3. Figure shows the best catalytic NB/Ph ratiomolar of 1/2ratio and 1ofequivalent K2 CO3 . Figure 11 shows the11best catalytic performance performance of Cu-MOF-74 (100% conversion at 2 h) compared to the other Cu-materials. The of Cu-MOF-74 (100% conversion at 2 h) compared to the other Cu-materials. The homogeneous homogeneous catalytic systems based on CuCl and CuNO 3 salts still show high NB conversions after catalytic systems based on CuCl and CuNO3 salts still show high NB conversions after 2 h, albeit below 2 h, albeit 100%. This decrease in activity attributed to possible Cl− and 100%. Thisbelow decrease in activity was attributed to was possible negative effects ofnegative Cl− andeffects NO3 −of anions in − NO 3 anions in the O-arylation cross-coupling reaction. This was confirmed by additional the O-arylation cross-coupling reaction. This was confirmed by additional experiments performed − experiments with mg/mL) an extra amount (1.165 mg/mL) of Cl− and NO3−, provided by potassiumwith an extraperformed amount (1.165 of Cl− and NO 3 , provided by potassium-based salts. These based salts. These experiments showed a significant decrease to in62% NB conversion 62% and and CuNO 76% for experiments showed a significant decrease in NB conversion and 76% fortoCuCl 3, CuCl and CuNO 3, respectively, demonstrating the strong interference of the anions in the reaction respectively, demonstrating the strong interference of the anions in the reaction course. On the other course.CuO On the othershowed hand, CuO catalyst showed the worst catalytic performance (65% conversion), hand, catalyst the worst catalytic performance (65% conversion), indicating the crucial indicating the crucial role of the microporous structure of Cu-MOF-74, which provides a higher role of the microporous structure of Cu-MOF-74, which provides a higher availability of copper sites 2/g). In the case of HKUST2 availability of copper sites compared to the very low porosity of CuO (34 m compared to the very low porosity of CuO (34 m /g). In the case of HKUST-1 (a well-known Cu-based 1 (a well-known Cu-based MOFopen material also containing open performance metal sites), the catalytic MOF material also containing metal sites), the catalytic was slightlyperformance lower than was slightly lower than for Cu-MOF-74. The lower activity is probably due to the higher specific for Cu-MOF-74. The lower activity is probably due to the higher specific surface area of Cu-MOF-74 2/g) compared 2/g) and the better 2 2 surface area of Cu-MOF-74 (1126 m to that of HKUST-1 (708 m (1126 m /g) compared to that of HKUST-1 (708 m /g) and the better accessibility to the open metal accessibility to the open metal sites of MOF-74 structure. sites of MOF-74 structure.

NB conversion (%)

100 80 60 Cu-MOF-74 HKUST-1 CuNO3 CuCl CuO

40 20 0

0

20

40

60 80 Time (min)

100

120

Figure 11. NB NBconversion conversionvs. vs.reaction reaction time given different homogeneous heterogeneous time given by by different homogeneous andand heterogeneous CuCu-based catalysts. based catalysts.

2.2.8. Effect of Nanostructured Cu-MOF-74 With the purpose thethe crystal size over the the catalytic performance of Cupurpose of ofstudying studyingthe theeffect effectofof crystal size over catalytic performance of MOF-74 material, nanostructured homologue was also in thisinreaction and compared to the Cu-MOF-74 material, nanostructured homologue was tested also tested this reaction and compared solvothermally synthesized microcrystalline material under thethesame to the solvothermally synthesized microcrystalline material under samereaction reaction conditions. conditions. Nanocrystalline Cu-MOF-74 was prepared as described elsewhere [34] following a general method Cu-MOF-74 for synthesizing different M-MOF-74 materials at room temperature [35]. XRD powder patterns of depicted in Figure Figure 12a. Both samples displayed the most important nano- and micro-Cu-MOF-74 are depicted reflexions of Cu-MOF-74 crystalline phase, the nanomaterial being obviously broader than the ones the conventional conventional micro-sized micro-sizedMOF MOFmaterial. material.NN2 2adsorption/desorption adsorption/desorption isotherms isotherms at at − −196 of the 196 ◦°C C are shown in inFigure Figure12b 12b and were used for estimating the textural properties of these Cu-MOF-74 and were used for estimating the textural properties of these Cu-MOF-74 samples samples (Table 1).(Table 1).

Nanomaterials 2017, 7, 149 Nanomaterials Nanomaterials 2017, 2017, 7, 7, 149 149

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a) a)

b) b)

1000 1000

3 3

STP) (cm Uptake STP) /g,/g, (cm N2NUptake 2

Relative Intensity (a.u.) Relative Intensity (a.u.)

Nano Cu-MOF-74 Nano Cu-MOF-74

Micro Cu-MOF-74 Micro Cu-MOF-74

5 5

10 10

15 15

20 20

25 30 252 (°) 30 2 (°)

35 35

40 40

45 45

800 800 600 600

Nano-Cu-MOF-74 Nano-Cu-MOF-74

400 400 200 200

Micro-Cu-MOF-74 Micro-Cu-MOF-74

0 00.0 0.0

50 50

0.1 0.1

0.2 0.2

0.3 0.3

0.4 0.4

0.5 0.5

P/Po P/Po

0.6 0.6

0.7 0.7

0.8 0.8

0.9 0.9

1.0 1.0

Figure 12. (a) PXRD patterns; (b) nitrogen adsorption–desorption isotherms at −196 °C nano sized◦ C of Figure 12. (a) PXRD patterns; (b) nitrogen 196 °C of nano nano sizedsizednitrogen adsorption–desorption adsorption–desorption isotherms isothermsat at− −196 of and micron sized-Cu-MOF-74. and micron sized-Cu-MOF-74. and micron sized-Cu-MOF-74. Table 1. Textural properties of nano- and micro-Cu-MOF-74 materials. Table 1. Textural Textural properties properties of of nanonano- and and micro-Cu-MOF-74 micro-Cu-MOF-74materials. materials.

Cu-MOF-74

Cu-MOF-74 Cu-MOF-74 Nanocrystalline

Nanocrystalline

Crystal Size (nm)

CrystalSize Size (nm) Crystal 14 a (nm) 14 a

SBET (m22/g)

BET (m2 /g) SSBET (m /g) 1103

1103

SExternal cc (m22/g) SExternal (mc /g)2 SExternal 286 (m /g) 286 124286 124124

Nanocrystalline 14 a b 1103 Microcrystalline 10,000 1126 Microcrystalline 10,000 1126 b b Microcrystalline 1126 10,000 a Estimated b c Estimated by t-plot method. by Scherrer equation. Estimated by SEM. a Estimated by Scherrer equation. b Estimated by SEM. c Estimated by t-plot method. a Estimated by Scherrer equation. b Estimated by SEM. c Estimated by t-plot method.

Regarding the catalytic results depicted in Figure 13, nanostructured Cu-MOF-74 showed faster Regarding the catalytic results depicted in Figure 13, nanostructured Cu-MOF-74 showed faster Regarding catalytic results depicted in Figure 13, nanostructured Cu-MOF-74conversions showed faster reaction kineticsthe than its microcrystalline homologue, reaching nitrobenzaldehyde of reaction kinetics than its microcrystalline homologue, reaching nitrobenzaldehyde conversions of reaction than its microcrystalline homologue, nitrobenzaldehyde conversions of 93% and kinetics 76%, respectively, after 5 min of reaction. Thisreaching higher NB conversion at early times using 93% and 76%, respectively, after 5 min of reaction. This higher NB conversion at early times using 93% and 76%,can respectively, after min of reaction. This higher NBarea conversion times nanoparticles be attributed to its5 more than double external surface comparedattoearly the micronanoparticles can be attributed to its more than double external surface area compared to the microusing nanoparticles cananbeextra attributed to its than double external surface areaThese compared to the Cu-MOF-74, showing amount of more readily accessible copper active sites. promising Cu-MOF-74, showing an extra amount of readily accessible copper active sites. These promising micro-Cu-MOF-74, showing an extra amount of readily accessible copper system active sites. These promising results demonstrated that, using nanocrystalline catalysts, the reaction can exhibit the main results demonstrated that, using nanocrystalline catalysts, the reaction system can exhibit the main results demonstrated that, usinghighly nanocrystalline catalysts, thecatalysis reactionassystem exhibit the advantages of both strategies: active homogeneous well can as eco-friendly advantages of both strategies: highly active homogeneous catalysis as well as eco-friendly main advantages of both highlywith active catalysis of asusing well nanocrystalline as eco-friendly heterogeneous catalysis. Instrategies: good agreement ourhomogeneous results, the advantages heterogeneous catalysis. In good agreement with our results, the advantages of using nanocrystalline heterogeneous catalysis. In good agreement with our results,crystals the advantages of using nanocrystalline catalysts against their counterparts formed by micron-sized have been already evidenced for catalysts against their counterparts formed by micron-sized crystals have been already evidenced for catalysts against their counterparts byfor micron-sized have been some other MOF-based catalysts [36]formed and even these based crystals on M-MOF-74 [34]. already evidenced for some other MOF-based catalysts [36] and even for these based on M-MOF-74 [34]. some other MOF-based catalysts [36] and even for these based on M-MOF-74 [34].

NBconversion conversion(%) (%) NB

100 100 80 80

Nano Cu-MOF-74 Nano Cu-MOF-74 Micro Cu-MOF-74 Micro Cu-MOF-74

60 60 40 40 20 20 0 00 0

20 20

40 40

60 60

80 80

Time (min) Time (min)

100 100

120 120

Figure 13. NB conversion vs. reaction time given by nano- and micron-sized Cu-based catalysts. Figure 13. NB conversion vs. reaction time given by nano- and micron-sized Cu-based catalysts. Figure 13. NB conversion vs. reaction time given by nano- and micron-sized Cu-based catalysts.


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Reuse of of Cu-MOF-74 Cu-MOF-74 2.2.9. Reuse important feature feature of of chemical chemical processes processes based based on on organic organic reactions reactions from from the the environmental environmental An important reusability of theofcatalyst in a number of reaction Therefore, recyclability point of ofview viewis the is the reusability the catalyst in a number ofcycles. reaction cycles. the Therefore, the recyclability of the catalyst was also studied in the O-arylation cross-coupling reaction for five of the catalyst was also studied in the O-arylation cross-coupling reaction for five successive catalytic runs. Priorcatalytic to each catalytic run,tothe catalyst wasrun, recovered and washed several times with methanol successive runs. Prior each catalytic the catalyst was recovered and washed several times withatmethanol and 150conversion °C for 18 h. The conversion thecatalyst XRD patterns of five the and dried 150 ◦ C for 18 dried h. TheatNB and theNB XRD patterns and of the along the cycles are shown Figures 15, respectively. performance was above 90% after catalyst along the in five cycles 14 areand shown in Figures 14The and catalytic 15, respectively. The catalytic performance the five cycles, although a slight was observed after the third one (Figure 14). was above 90% after the five cycles,decrease althoughina activity slight decrease in activity was observed after the third one (Figure 14). XRD measurements of Figure 15 show that thephase crystalline phase of Cu-MOF-74 keeps XRD measurements of Figure 15 show that the crystalline of Cu-MOF-74 keeps unaltered, meaning that the catalyst is stable. The copper leaching was negligible, commented above, unaltered, meaning that structure the catalyst structure is stable. The copper leachingaswas negligible, as and the catalyst recovery after each cycle was after aboveeach 97%.cycle All these results97%. demonstrate the commented above, and the catalyst recovery was above All thesethat results demonstrate that the crystalline framework of Cu-MOF-74 material is stable under the testedthe reaction crystalline framework of Cu-MOF-74 material is stable under the tested reaction conditions, slight deactivationthe observed possibly being due to chemisorbed by-products along the reaction cycles that conditions, slight deactivation observed possibly being due to chemisorbed by-products along were not completely removed during the regeneration process. the reaction cycles that were not completely removed during the regeneration process.

NB (%) conversion (%) NB conversion

100 80 60 40 20 0

1

4 3 2 Number of reaction cycles

5

Figure Figure 14. 14. Catalytic Catalytic activity activity of of Cu-MOF-74 Cu-MOF-74 catalyst catalyst after after several several reaction reaction cycles. cycles.

Relative Relative Intensity Intensity (a.u.) (a.u.)

5° cycle 4° cycle 3° cycle 2° cycle 1° cycle 5

10

15

20

25

30 35 2 (°)

40

45

50

Figure 15. 15. XRD XRD patterns patterns of of Cu-MOF-74 Cu-MOF-74 catalyst catalyst after after several several reaction reaction cycles. cycles. Figure


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3. Materials and Methods 3.1. Catalysts Preparation Cu-MOF-74 was synthesized basically following the procedure previously published [24], in which a mixture of 2.2 g of 2,5-dihydroxyterephthalic acid (H2 dhtp, 11.2 mmol, Sigma-Aldrich, Madrid, Spain) and trihydrated copper nitrate(II) (5.9 g, 24.6 mmol, Sigma-Aldrich, Madrid, Spain) were added to a 250 mL solution of N,N-dimethylformamide (DMF) and 2-propanol (20:1 (v/v)) in a 500 mL screw cap bottle. The suspension was stirred until a homogeneous solution. Then, the resultant solution was placed in an oven at 80 ◦ C for 18 h. Thereafter, the sample was cooled down to room temperature and the mother liquor was separated from reddish needle-shaped crystals by vacuum filtration. Afterwards, the crystalline solid sample was washed with DMF and immersed in 100 mL of methanol for 4 days, renewing it by fresh methanol every 24 h. Prior to the catalytic runs, the solid material was dried at 150 ◦ C under vacuum (10−3 bar) for 5 h and stored under inert atmosphere. Other powdered materials used in this study with the purpose of comparison were HKUST-1 (Basolite C300, supplied by Sigma-Aldrich, Madrid, Spain) as a different copper-based MOF material and CuO. Likewise, homogeneous copper salts such as CuCl2 and Cu(NO3 )2 were used as homogeneous catalysts (Sigma-Aldrich, Madrid, Spain). Nanocrystalline Cu-MOF-74 was prepared at room temperature as described elsewhere [34]. 3.2. Catalyst Characterization X-ray powder diffraction (XRD) patterns were acquired on a PHILIPS X’PERT diffractometer using Cu Kα radiation. The data were recorded from 5◦ to 50◦ (2θ) with a resolution of 0.01◦ . Scanning electron microscopy (SEM) micrographs were obtained on a XL30 ESEM (Philips, Lelyweg, the Netherlands) electronic microscope operating at 200 kV. Nitrogen adsorption–desorption isotherms at −196 ◦ C were measured using an AutoSorb equipment (Quantachrome Instruments, Boynton Beach, Florida, USA). Samples were degassed at 150 ◦ C and high vacuum during 180 h. The micropore surface area was calculated by using the Brunauer–Emmett–Teller (BET) model [37]. The pore volume and diameter were estimated by non-local DFT calculations, assuming a kernel model of N2 at −196 ◦ C on carbon (cylindrical pores, NL-DFT equilibrium model) [38] and external surface was estimated by t-plot method and Harkins Jura equation [39]. Simultaneous thermogravimetry and derivative thermogravimetric analyses (TGA/DTG) were carried out under a nitrogen atmosphere with an N2 flow of 100 mL·min−1 at a heating rate of 5 ◦ C/min up to 700 ◦ C, using a SDT 2860 apparatus (TA Instruments, New Castle, DE, USA). 3.3. Reaction Procedure Cu-MOF-74 material was tested in the O-arylation cross coupling reaction of phenol (Ph) and 4-nitrobenzaldehyde (NB) to form 4-formyldiphenyl ether (FDE) (Scheme 1). All the catalytic experiments were carried out in a round bottom flask placed in a silicone bath under N2 atmosphere. The influence of the reaction temperature, the molar ratio of reactants (NB/Ph), the catalyst concentration, the type of solvent and both base nature and concentration, were evaluated according to preliminary conditions found in the literature [20]. The required amounts of reactants (Ph and NB) were added to 20 mL of the solvent. The base and catalyst concentrations were adjusted according to molar ratios of base/Ph and Cu/NB, respectively. The reaction was monitored by withdrawing aliquots from the reaction medium at different times ranging from 0 to 120 min. The NB and FDE were identified and quantified by gas chromatography, using a GC-3900 chromatograph with a flame ionization detector (FID) (Varian, Palo Alto, CA, USA). A CPSIL 8 CB capillary column was used as stationary phase of 30 m × 0.25 mm andfilm thickness of 0.25 µm (Agilent Technologies Spain, Madrid, Spain). The injector and FID temperatures were set to 280 ◦ C, and the oven temperature program started at 120 ◦ C for 1 min and continued until 280 ◦ C at 40 ◦ C/min−1 and 3 min at 280 ◦ C. Hexadecane was used as an internal standard, and all samples were analyzed in duplicate.


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Cu-MOF-74 K2CO3 DMF

Scheme 1. 1. C–O C–O cross-coupling cross-coupling reaction reaction of Scheme of phenol phenol with with 4-nitrobenzaldehyde. 4-nitrobenzaldehyde.

The relative NB conversion was calculated taking into account the maximum theoretical NB the 4-NB/Ph 4-NB/Ph ratio conversion, which depends on the ratio of of each catalytic run. The selectivity to diaryl ether 100% as as no no other other by-products by-products were were detected. detected. A similar assumption was FDE was considered of 100% reaction [20]. [20]. observed in the literature for this reaction 4. Conclusions Outstanding catalytic catalytic activity activity ofofCu-MOF-74 Cu-MOF-74ininthe theO-arylation O-arylationcross-coupling cross-coupling reaction reaction of of 44-nitrobenzaldehyde (NB) phenol evidenced. A catalyst concentration mol % nitrobenzaldehyde (NB) andand phenol has has beenbeen evidenced. A catalyst concentration of 5 molof%5exhibits ◦ C, exhibits a total conversion NB selectivity and 100% to selectivity to the desired after h at 120only a total conversion of NB andof100% the desired product afterproduct 2 h at 120 °C,2whereas whereas only 10% achieved absenceThis of catalyst. Thisreaction O-arylation only proceeds in the 10% is achieved inisthe absenceinofthe catalyst. O-arylation onlyreaction proceeds in the presence of presence of Cu-MOF-74 catalyst, discarding possible contribution of homogeneous to Cu-MOF-74 catalyst, discarding a possibleacontribution of homogeneous catalysiscatalysis due to due active active leached in the liquid The excess of phenol as apolarity high polarity the solvent leached speciesspecies in the liquid phase.phase. The excess of phenol as wellasaswell a high of the of solvent plays plays an important role in the catalytic performance of Cu-MOF-74. The best conditions for the reaction an important role in the catalytic performance of Cu-MOF-74. The best conditions for the reaction are are NB/Ph of 1/2, as solvent, andinexpensive an inexpensive K23 CO The catalytic performance of NB/Ph ratioratio of 1/2, DMFDMF as solvent, and an K2CO base. The catalytic performance of Cu3 base. Cu-MOF-74 dependent electron-donating/withdrawing characterofofthe thegroups groups of of phenol MOF-74 waswas dependent onon thethe electron-donating/withdrawing character derivatives and nitroarenes. The The catalytic catalytic activity activity of Cu-MOF-74 Cu-MOF-74 (above (above 90%) is higher than those shown by other homogeneous and heterogeneous Cu-based catalysts, and the material is stable and reusable during several reaction cycles. Nano Cu-MOF-74 catalyst exhibited faster reaction kinetics than its microcrystalline homologue, demonstrating the advantages of increasing the easy and quick accessibility of the reactants to the active active centres. centres. Acknowledgments: Acknowledgments: The The authors authors wish wish to to thank thank Spanish Spanish Ministry Ministry of of Science Science and and Innovation Innovation for for the the financial financial support to the CICYT Project (CTQ2015-64526-P). MSS acknowledges AEI and FEDER the financial support support to the CICYT Project (CTQ2015-64526-P). MSS acknowledges AEI and FEDER the financial support through the Project MAT2016-77496-R (AEI/FEDER, UE). through the Project MAT2016-77496-R (AEI/FEDER, UE). Author Contributions: Pedro Leo performed all the synthesis and catalytic experiments. David Briones performed Author Contributions: performed synthesis and catalytic experiments. David Briones samples characterization.Pedro GiselaLeo Orcajo analyzedall thethe samples characterization data and wrote the manuscript. Fernando Guillermo Calleja and analyzed the catalytic tests data data and assisted in the performedMartínez samples and characterization. Giseladesigned Orcajo analyzed the samples characterization and wrote manuscript Manuel prepared and characterized the the nanocrystalline Cu-MOF-74 manuscript.preparation. Fernando Martí nez Sánchez-Sánchez and Guillermo Calleja designed and analyzed catalytic tests data and sample. All the listed authors have contributed substantially to this work. assisted in the manuscript preparation. Manuel Sánchez-Sánchez prepared and characterized the Conflicts of Interest: The authors declare nolisted conflict of interest. nanocrystalline Cu-MOF-74 sample. All the authors have contributed substantially to this work.

Conflicts of Interest: The authors declare no conflict of interest.

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