Research Article pubs.acs.org/acscatalysis
Robust Iridium Coordination Polymers: Highly Selective, Efficient, and Recyclable Catalysts for Oxidative Conversion of Glycerol to Potassium Lactate with Dihydrogen Liberation Zheming Sun,† Yaoqi Liu,† Jiangbo Chen,† Changyu Huang,† and Tao Tu*,†,‡ †
Department of Chemistry, Fudan University, 220 Handan Road, Shanghai, 200433 China State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin, 300071, China
‡
S Supporting Information *
ABSTRACT: Along with the rapid expansion of the biodiesel industry to deal with the world energy crisis, inexpensive glycerol is also produced in large scale as the main byproduct in biodiesel production via transesterification. Much attention has been paid to the development of environmentally benign technologies for the transformation of glycerol to valuable DL-lactic acid and its derivatives. Herein, a series of NHC-Ir coordination polymers were readily synthesized via reaction of some structurally rigid bisbenzimidazolium salts with iridium precursors under alkaline conditions and were successfully applied as robust self-supported catalysts in the oxidative dehydrogenation of glycerol to potassium lactate with dihydrogen liberation. Extremely high activity and selectivity were attained in open air under the mild reaction conditions even with ppm-level loadings of the catalysts, which were readily recovered after reaction by simple filtration and reused for up to 31 runs without obvious loss of activity or selectivity. Probably owing to the effective suppression of inactive binuclear iridium species in a homogeneously catalyzed reaction, the catalysts assembled via self-supported strategy exhibited high selectivity and productivity for potassium lactate, with up to 1.24 × 105 turnover numbers (TON) being attained even in large-scale reactions of neat glycerol at an elevated temperature. The high catalytic activity, recyclability, and scalability of the robust self-supported catalysts highlight their potential toward the development of practical technologies for transformation of glycerol to value-added chemicals. KEYWORDS: coordination assembly, iridium, lactate, oxidative dehydrogenation, self-support catalyst
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INTRODUCTION Due to indiscriminate consumption of fossil resources, currently there is an ever-increasing interest in the development of environmentally benign approaches to convert abundant biomass into biofuels and fine chemicals.1,2 However, the direct utilization of lignocellulosic biomass (including cellulose, hemicellulose, and lignin) still constitutes a major challenge in this field.3 On the basis of the recent achievements in the oxidative dehydrogenation of simple alcohols,4 the relatively less-explored selective conversion of sugar alcohols (polyols) into value-added commodity chemicals along with hydrogen generation was targeted as an upgrading strategy for the biomass transformation.5,6 As one of the main byproducts in the biodiesel production process and soap industry,7,8 glycerol is a cheap bulk chemical readily available in large scale and has been identified as one of the most important renewable carbon sources.9 To date, a number of oxidative protocols for glycerol transformation have been explored, to produce a variety of value-added fine chemicals, such as acrolein,10 dihydroxyacetone,11−13 glyceric acid,14,15 hydroxypyruvic acid,16,17 and lactic acid, in a cost-effective manner. Although some spectacular © XXXX American Chemical Society
progresses have been made in this vein, the selectivities and efficiencies of most reported protocols still need to be improved for their practical applications.18,19 As one of the most appealing biomass-derived platform chemicals, lactic acid (LA) and its derivatives have found numerous applications in chemical, pharmaceutical, food, and detergent industries.20 There was a steady increase in the annual consumption of lactic acid over the past decades, largely as a result of the increasing global demands for biodegradable polylactide materials.21 Whereas current industrial processes for lactic acid production generally involve various bacteriamediated carbohydrate fermentation routes, they are still plagued by disadvantages such as poor productivity/scalability, complicated downstream process, and/or waste disposal.22 Hence the development of alternative approaches to lactic acid production (e.g., from biomass waste products like glycerol) are highly desirable. Although great efforts have been made to Received: August 14, 2015 Revised: September 28, 2015
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DOI: 10.1021/acscatal.5b01782 ACS Catal. 2015, 5, 6573−6578
Research Article
ACS Catalysis develop viable routes from sugars/cellulose to lactates,23−25 documented examples on the selective conversion of glycerol into lactic acid are scarce. In 2010, Liu and co-workers reported a bimetallic Au−Pt catalyst supported on TiO2 that successfully attained the selective aerobic oxidation of glycerol to lactic acid.26 Since then, many efforts have been devoted to the development of new heterogeneous catalysts for this reaction.27−32 However, major difficulties including low productivities, poor selectivity, and harsh reaction conditions have largely hampered their industrial implementation. Inspired by the unexpected performance of bifunctional ruthenium pincer complex 1 (Scheme 1) in the catalytic
that preserves the structural features of its homogeneous catalyst counterpart but is insoluble in common solvents and thus obviates the need for an external support.38−41 Following our recent interests in exploring the applications of novel transition metal complexes in catalysis, supramolecular chemistry, and material sciences,42−46 herein we would like to report the preparation of main-chain iridium coordination polymers from rigid bis-benzimidazolium salt precursors and their application as self-supported catalysts in the oxidative conversion of glycerol to lactic acid.
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RESULTS AND DISSCUSION The synthesis of NHC-Ir coordination polymers 6a−e was straightforward, as shown in Scheme 2. A number of bis-
Scheme 1. Representative Noble Metal Complexes for Oxidative Dehydrogenation of Glycerol and Related Transformations (TON = Turnover Number)
Scheme 2. Syntheses of NHC-Ir Coordination Polymers 6a− e
benzimidazolium salts (5a−d) with a conformationally rigid core structure were employed as precursors for the corresponding bis-NHCs. This is to ensure that upon treatment with a base, bis-NHCs with the two carbene sites geometrically well-constrained and oriented opposite to each other would be generated in situ by deprotonation.47−49 Such a bis-NHC would bind to different Ir centers to generate an extended supramolecular structure via coordination copolymerization with an Ir precursor. Thus, condensation of commercial available 1,2,4,5-tetraaminobenzene hydrochloride with formic acid, followed by double alkylation of the resulting bisbenzimidazole with iodomethane or n-butyl bromide, afforded the corresponding bis-benzimidazolium salts 5a and 5b, respectively, in nearly quantitative yields.50 Subsequent anionexchange of 5a and 5b with Et3OBF4 furnished BF4− salts of bis-benzimidazolium 5c and 5d, respectively, in very good yields. With these ditopic NHC precursors 5a−d in hand, coordination polymerization with selected iridium(I) salts were then carried out via a solution phase self-assembly process. However, attempts to prepare polymer 6a via silver carbene route51 were unsuccessful, as the precipitated Ag-NHCs species can hardly react further with the Ir precursors. After a comprehensive survey of reaction conditions (see Supporting Information), lithium bis(trimethylsilyl)amid (LiHMDS) was eventually found to be adequate for this purpose, and the in situ-generated bis-NHC reacted smoothly with different iridium(I) precursors in DMF, respectively, to give NHC-Ir coordination polymers 6a−e in very good yields (93−99%). The polymeric solids 6a−e were virtually insoluble in all tested organic solvents and water, and their compositions were consistent with the expected structures by IR, solid-state 13C NMR, gel permeation chromatography (GPC), and elemental analysis (see Supporting Information). Morphological studies of 6b with a combined uses of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) indicated that the bulk solids are composed
oxidative dehydrogenation of simple monoalcohols,4 recently Beller and co-workers reported a homogeneous catalytic reaction for conversion of glycerol to lactic acid along with hydrogen generation, albeit with only a moderate selectivity (67%).33 Simultaneously, in light of the successful utilization of glycerol as both hydrogen donor and solvent in the catalytic transfer hydrogenation of carbonyl derivatives in the presence of N-heterocyclic carbene (NHC) iridium complexes 2 (Scheme 1),34,35 Crabtree and co-workers accomplished the solvent-free catalytic transformation of glycerol to lactates in very high selectivity (95%) with micromolar amount of NHC-Ir complex 3 as the catalyst.36 It is noteworthy that a binuclear NHC-Ir complex 4 was detected in the reaction, which slowed the glycerol transformation. We envisaged that immobilizing NHC-Ir complexes of the type 3 may effectively block the formation of inactive binuclear species and enhance the catalytic efficiency. The immobilization might also provide an opportunity to recycle the precious iridium catalyst, which is often essential for a large-scale transformation of biomass feedstock. In this context, the recently developed “selfsupporting” strategy37 constitutes an attractive approach for immobilization of homogeneous catalysts. Different from the conventional approaches using various solid supports for homogeneous catalyst immobilization, the self-supported catalysts were constructed via supramolecular assembly of a multitopic bridging ligand with an appropriate metal salt. A coordination polymer is generated, with catalytic active sites 6574
DOI: 10.1021/acscatal.5b01782 ACS Catal. 2015, 5, 6573−6578
Research Article
ACS Catalysis
Solid 6a was first tested for its catalytic activity in the oxidative dehydrogenation of glycerol. The reaction was conducted in water (0.3 mL) at 115 °C, with 15 mmol glycerol, and 1.1 mol equiv of KOH in the presence of 2.3 mg of 6a (300 ppm) as the catalyst. With evacuation of the released dihydrogen gas, glycerol was selectively converted into potassium lactate in very high yield (92%) and selectivity (99%) after 36 h (Table 1, entry 1). This exciting result prompted us to further optimize the reaction parameters, including the water content, base additives, reaction temperature, and catalyst loadings, and the results were shown in entries 2−14 of Table 1. Only 45% yield and 90% selectivity of potassium lactate to diol and other trace side-products were obtained in the absence of water (entry 2), probably as a result of the bare solubility of KOH in neat glycerol. However, adjusting the water content to 1 or 0.1 mL, led also to less satisfactory yields (80% and 49%, entries 3 and 4), respectively, as compared with results in entry 1 under otherwise identical conditions. The alkalinity of the reaction media, as modified by the ratio of KOH base with respect to water, was found to have a substantial impact on the outcome of the transformation. Although the reaction with 0.1 molar equiv of KOH in 0.05 mL of water afforded only a 16% yield of lactate (entry 5), the yield approached 80% with 2.1 equiv of base in 1 mL of water (entry 6). No reaction occurred using K3PO4 or K2CO3 instead of KOH under conditions otherwise identical to those in entry 1 (entries 7 and 8), suggesting that these weak bases may not be effective in deprotonating the alcoholic OH of glycerol to the alkoxide for iridium coordination. On the other hand, the use of t-BuOK afforded no targeted product (entry 9). Reactivity with NaOH is similar to that of KOH; however, the chemoselectivity toward lactic salts formation is slightly inferior to that of KOH (90% vs 99%, entries 10 vs 1). Thus, subsequent trials were
of irregular particles with sizes of ca. 30−50 nm in diameter (Figure 1a,b). The energy dispersive X-ray (EDX) analysis
Figure 1. (a, b) SEM and TEM morphologies of the as-prepared NHC-Ir coordination polymer 6b; (c) TEM morphology of polymer 6a; (d−f) SEM images of the recovered polymer 6b after the 2nd, 4th, and the 15th run, respectively.
results of solid 6b were consistent with its elemental composition. The powder X-ray diffraction (PXRD) patterns did not show any obvious peaks and confirmed the amorphous nature of the polymeric solid 6b. Similar results were also obtained for solids 6a (Figure 1c) and 6c−e. Additionally, solids 6a−e were insoluble in all common organic solvents and stable against air, moisture, and remained intact after prolonged heating (100 °C, 120 h) in water, suggesting a potential utility as self-supported catalysts.
Table 1. Oxidative Dehydrogenation of Glycerol Catalyzed by NHC-Ir Coordination Polymersa
entry
[cat.] (ppm)
base (equiv)
H2O (mL)
temp (°C)
yield (%)b
select. (%)c
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15d 16 17 18
6a: 300 6a: 300 6a: 300 6a: 300 6a: 300 6a: 300 6a: 300 6a: 300 6a: 300 6a: 300 6a: 100 6a: 150 6a: 300 6a: 300 6b: 300 6c: 300 6d: 300 6e: 300
KOH (1.1) KOH (1.1) KOH (1.1) KOH (1.1) KOH (0.1) KOH (2.1) K3PO4 (1.1) K2CO3 (1.1) t-BuOK (1.1) NaOH (1.1) KOH (1.1) KOH (1.1) KOH (1.1) KOH (1.1) KOH (1.1) KOH (1.1) KOH (1.1) KOH (1.1)
0.3 0 1.0 0.1 0.05 1.0 0.3 0.3 0 0.3 0.3 0.2 0.3 0.3 0.3 0.3 0.3 0.3
115 115 115 115 115 115 115 115 115 115 115 115 165 80 115 115 115 115
92 45 80 49 16 80
