Enhanced asymmetric induction for the copolymerization of CO2 and cyclohexene oxide with unsymmetric enantiopure salenCo(III) complexes: synthesis of crystalline CO2-based polycarbonate

Enantiopure metal-complex catalyzed asymmetric alternating copolymerization of CO(2) and meso-epoxides is a powerful synthetic strategy for preparing optically active polycarbonates with main-chain chirality. The previous studies regarding chiral zinc catalysts provided amorphous polycarbonates with moderate enantioselectivity, and thus, developing highly stereoregular catalysts for this enantioselective polymerization is highly desirable. Herein, we report the synthesis of highly isotactic poly(cyclohexene carbonate)s from meso-cyclohexene oxide using dissymmetrical enantiopure salenCo(III) complexes in conjunction with bis(triphenylphosphine)iminium chloride (PPNCl) as catalyst. The presence of a chiral induction agent such as (S)-propylene oxide or (S)-2-methyltetrahydrofuran significantly improved the enantioselectivity regarding (S,S)-salenCo(III) catalyst systems. Up to 98:2 of RR:SS was observed in the resultant polycarbonates obtained from the catalyst system based on (S,S)-salenCo(III) complex 4d bearing an adamantyl group on the phenolate ortho position, in the presence of (S)-2-methyltetrahydrofuran. Primary ONIOM (DFT:UFF) calculations, which were performed to investigate the effect of the competitive coordination of (S)-induction agent versus cyclohexene oxide to Co(III) center on enantioselectivity, suggest that the (S)-C-O bond in cyclohexene oxide is more favorable for cleavage, due to the interaction between oxygen atom of (S)-induction agent and (S)-C-H of the coordinated cyclohexene oxide. The highly isotactic poly(cyclohexene carbonate) is a typical semicrystalline polymer, possessing a melting point of 216 °C and a decomposition temperature of 310 °C.     

Solvent‐free ring‐opening polymerization of cyclohexene oxide catalyzed by palladium chloride

In this study, we have for the first time demonstrated that palladium chloride (PdCl₂) is an efficient catalyst for ring‐opening polymerization of cyclohexene oxide in a solvent‐free condition. The polymerization product was in atactic structure, and reaction conditions, such as reaction temperature, time, and catalyst amount, showed effects on polymerization conversion yield, turnover number, and number‐average molecular weight of the resulting poly(cyclohexene oxide). PdCl₂ catalysis follows a cationic ring‐opening mechanism. The polymerization result is highly determined by the chemical structure of the monomers.     

基团转移聚合中分子量和分子量分布的研究

<正> 基团转移聚合的主要优点是能在室温下进行甲基丙烯酸酯、丙烯酸酯和丙烯腈等极性单体的控制聚合,得到实测分子量和理论分子量相近的、多分散指数较小的聚合物。对于每一种新发现的引发剂和催化剂组成的引发-催化体系,亦是在高转化率的前提下,视其实测分子量和理论分子量的相符程度和多分散指数的大小而评判其优劣。目前已报道的有关控制分子量及多分散指数的工作,大多限于研究催化剂含量的影响。而反应条     

High-activity,single-site catalysts for the alternating copolymerization of CO2 and propylene oxide

Despite recent advances regarding catalysts for CO2/epoxide copolymerization, the development of high-activity catalysts for alternating polymerization of CO2 and commodity epoxides, such as propylene oxide, remains a challenge. A new class of unsymmetrically substituted beta-diiminate zinc complexes is reported that exhibits unprecedented activity for CO2/propylene oxide copolymerization. The polymers formed are of high molecular weight (Mn approximately 35 kg/mol) and have narrow polydispersities (PDI approximately 1.1), consistent with a living polymerization.     

缩酮胺合锌催化二氧化碳和氧化环己烯共聚

二氧化碳作为单体与环氧化物的共聚反应近年来越来越受到重视 .该反应不经过高耗能的还原过程 ,二氧化碳利用率高 ,所得到的产物聚碳酸酯在塑料、弹性体、涂料、胶粘剂、食品包装材料和生物降解材料等方面有广泛的应用 .二氧化碳与氧化环己烯的交替共聚物聚碳酸亚环己酯玻璃化转变温度较高 (Ｔｇ 为 1 2 0℃ ) ,热分解温度达 2 1 0℃以上[1] ,是一种性能优良的高分子材料 .Ｃｈｅｎｇ等于 1 998年首次报道了一种缩酮胺合锌催化剂[2 ] .它是由 β 二酮 (乙酰丙酮 )与 2 ,6 二异丙基苯胺为原料合成的 ,具有西佛碱结构 ,用于二氧化碳与氧化环己…     

Mechanistic Aspects of a Highly Active Dinuclear Zinc Catalyst for the Co‐polymerization of Epoxides and CO2

The dinuclear zinc complex reported by us is to date the most active zinc catalyst for the co‐polymerization of cyclohexene oxide (CHO) and carbon dioxide. However, co‐polymerization experiments with propylene oxide (PO) and CO₂ revealed surprisingly low conversions. Within this work, we focused on clarification of this behavior through experimental results and quantum chemical studies. The combination of both results indicated the formation of an energetically highly stable intermediate in the presence of propylene oxide and carbon dioxide. A similar species in the case of cyclohexene oxide/CO₂ co‐polymerization was not stable enough to deactivate the catalyst due to steric repulsion.     

卟啉铝催化下二氧化碳与氧化环己烯共聚反应研究

利用四苯基卟啉氯化铝(TPPAlCl)与双三苯基膦氯化铵(PPNCl)组成的二元催化剂催化二氧化碳与氧化环己烯共聚合,80℃下反应9h,二氧化碳-氧化环己烯共聚物(PCHC)的收率为97.2％,分子量分布窄(Mw/Mn=1.12),但数均分子量仅为6.8×103.研究发现溶剂的浓度和极性变化对聚合过程中的链转移反应影响...     

Comparative kinetic studies of the copolymerization of cyclohexene oxide and propylene oxide with carbon dioxide in the presence of chromium salen derivatives. In situ FTIR measurements of copolymer vs cyclic carbonate production

The catalysis of the reaction of carbon dioxide with epoxides (cyclohexene oxide or propylene oxide) using the (salen)Cr(III)Cl complex as catalyst, where H(2)salen = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexenediimine (1), to provide copolymer and cyclic carbonate has been investigated by in situ infrared spectroscopy. As previously demonstrated for the cyclohexene oxide/CO(2) reaction in the presence of complex 1, coupling of propylene oxide and carbon dioxide was found to occur by way of a pathway first-order in catalyst concentration. Unlike the cyclohexene oxide/carbon dioxide reaction catalyzed by complex 1, which affords completely alternating copolymer and only small quantities of trans-cyclic cyclohexyl carbonate, under similar conditions propylene oxide/carbon dioxide produces mostly cyclic propylene carbonate. Comparative kinetic measurements were performed as a function of reaction temperature to assess the activation barrier for production of cyclic carbonates and polycarbonates for the two different classes of epoxides, i.e., alicyclic (cyclohexene oxide) and aliphatic (propylene oxide). As anticipated in both instances the unimolecular pathway for cyclic carbonate formation has a larger energy of activation than the bimolecular enchainment pathway. That is, the energies of activation determined for cyclic propylene carbonate and poly(propylene carbonate) formation were 100.5 and 67.6 kJ.mol(-1), respectively, compared to the corresponding values for cyclic cyclohexyl carbonate and poly(cyclohexylene carbonate) production of 133 and 46.9 kJ.mol(-1). The small energy difference in the two concurrent reactions for the propylene oxide/CO(2) process (33 kJ.mol(-1)) accounts for the large quantity of cyclic carbonate produced at elevated temperatures in this instance.     

高聚物负载双金属催化剂催化二氧化碳-氧化环己烯的共聚反应

由CO2 氧化环己烯 (CHO)配位催化共聚制得高Tg 的脂肪族聚碳酸亚环己基酯 ,并用IR、NMR和DSC等进行了表征 ,用TG对聚合物的热稳定性进行分析 .加入环氧丙烷 (PO)三元共聚并分析PO/CHO摩尔比对Tg 的影响 .加入异氰酸苯酯有提高产物特性粘数的作用     

Copolymerization of cyclohexene oxide and carbon dioxide using (salen)Co(III) complexes: synthesis and characterization of syndiotactic poly(cyclohexene carbonate)

Synthetic routes to a series of new (salen-1)CoX (salen-1 = N,N'-bis(salicylidene)-1,2-diaminoalkane; X = halide or carboxylate) species are described and the X-ray crystal structures of two (salen-1)CoX (salen- = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-diaminocyclohexane; X = Cl, I) complexes are presented. (R,R)-(salen-)CoX (X = Cl, Br, I, OAc, pentafluorobenzoate (OBzF(5))) catalysts are active for the copolymerization of cyclohexene oxide (CHO) and CO(2), yielding syndiotactic poly(cyclohexene carbonate) (PCHC), a previously unreported PCHC microstructure. Variation of the salen ligand and reaction conditions, as well as the inclusion of [PPN]Cl ([PPN]Cl = bis(triphenylphosphine)iminium chloride) cocatalysts, has dramatic effects on the polymerization rate and the resultant PCHC tacticity. Catalysts rac-(salen-)CoX (salen- = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-diaminopropane; X = Br, OBzF(5)) have high activities for CHO/CO(2) copolymerization, yielding syndiotactic PCHCs with up to 81% r-centered tetrads. Using Bernoullian statistical methods, PCHC tetrad and triad sequences were assigned in the (13)C NMR spectra of these polymers in the carbonyl and methylene regions, respectively.     

二氧化碳与环氧环己烷共聚的研究进展

综述了国内外用于二氧化碳与环氧环己烷共聚的各类催化剂的研究现状、进展及研发新动向.指出二氧化碳作为一种廉价、无毒、可循环利用的理想原料,利用其和环氧环己烷共聚可合成具有良好生物降解性能的脂肪族聚碳酸酯.但二氧化碳具有较高的热力学稳定性,其和环氧环己烷共聚过程在一般情况下难以实现,故迫切需要开发高效、高选择性的催化剂.     

Laser-initiated polymerization of epoxies in the presence of maleic anhydride

Laser-initiated polymerization of cyclohexene oxide in the presence of maleic anhydride was investigated. The influences of solvents laser irradiation time and the monomer feed ratio on the polymer yield and composition were evaluated. The rate of polymerization increased with an increase in the molar concentration of maleic anhydride in the monomer feed. Short irradiation times of 1–3 min duration gave very high yield of epoxy polymer (>80% conversion). Infrared spectral studies of the polymer product indicated the formation of polyether linkage at lower levels of conversion and an adduct of polyether and maleic anhydride at higher polymer conversions. The quantitative chemical analyses results also showed similar results. The results indicated that the polymerization was initiated by the excited charge transfer complex between the electron donor, cyclohexane oxide, and the electron acceptor–maleic anhydride. In the initial stages of polymerization, cyclohexene oxide undergoes a cationic polymerization in the presence of the radical anion of maleic anhydride. Laser-initiated polymerization of cyclohexene oxide/maleic anhydride is several hundred times more efficient than UV-initiated polymerization, as measured by the energy absorbed by the polymer system.     

金单原子催化剂上一氧化碳低温氧化

CO低温氧化对于基础研究和实际应用均具有重要意义.自上世纪八十年代日本的 Haruta教授发现氧化物负载金催化剂对 CO氧化的超高活性以来,负载金催化剂受到了广泛关注与深入研究,被认为是目前活性最高的 CO氧化催化剂.在诸多影响 CO氧化活性的因素中,纳米金的粒子尺寸是最重要因素之一.目前主流观点认为对于 CO氧化,纳米金有一个最优尺寸范围,在0.5–5 nm,而 Au原子/离子(Au3+, Au+)的活性则低一到两个数量级.因此,一般认为负载金单原子催
　　化剂对于 CO氧化反应的活性要比金纳米粒子和团簇低很多.然而,最近几年的理论与实验研究均表明,金单原子负载于合适的载体上可以显示出与金纳米粒子和团簇相当的活性.本文对这些新进展进行综述,阐述金单原子催化剂对 CO氧化的独特反应性能. Gates教授研究组进行了大量关于正价金对 CO氧化影响的研究,其中包括孤立的金原子(Au+).他们的研究发现, CO氧化活性随价态降低而降低,表明正价金对 CO氧化至关重要.此外,他们的研究也表明,孤立金原子对 CO氧化的活性(TOF)比金纳米粒子低一到两个数量级.然而,在他们的研究中,有几个因素可能导致催化剂的低活性.首先,他们一般采用非或弱还原性的载体.而载体的还原性对金催化剂上 CO氧化活性影响非常巨大.另外,他们所用的金原子前驱体为配合物,在催化剂制备与反应过程中配体并没有去除,可能也是导致催化剂活性低的原因之一.与此相反,张涛课题组近期采用氯金酸为前驱体,通过简单的吸附浸渍法制备了一系列负载金单原子催化剂.同时用相同的载体制备了负载金纳米粒子催化剂进行对比,可以排除载体等其它影响因素.对比结果显示,单原子催化剂均显示出与纳米粒子相当的 TOF(单位表面 Au原子)和更高的反应速率(单位重量金).首先制备了氧化铁负载金单原子催化剂,该催化剂在室温即展现出可观活性, TOF值与2–3 nm金粒子 TOF值相当(~0.5 s–1).更有趣也更重要的是,该催化剂在高温(200oC以上)展现出非常高的反应稳定性,在200oC反应100 h无失活.在300和400oC反应50 h也无失活发生,为开发高温稳定的金催化剂提供了新途径.其次制备了氧化钴负载金单原子催化剂,该催化剂以0.05%金负载量即可实现室温全转化,其 TOF值高达1.4 s–1.然而该催化剂在达到高活性之前必须首先在反应气氛中进行高温处理,这限制了其实用性.此外,催化剂需经反应气氛活化的原因尚待进一步研究.随之又制备了氧化铈负载金单原子催化剂,对富氢条件下 CO选择氧化不仅具有高活性,而且具有极高的 CO选择性.进一步研究结合理论计算表明,高选择性来自氧化铈负载的金单原子不能解离活化氢,对于氢气氧化活性极低,从而导致 CO氧化的高选择性.理论研究方面也有进展. Camellone等计算发现金原子可以取代 CeO2(111)面上的 Ce原子形成 Au+并促进 CO氧化.然而该金原子会扩散至氧空位形成带负电荷的 Auδ-,阻止 CO和 O2吸附,因而使催化剂失活.李隽课题组利用从头算分子动力学模拟首次发现氧化铈和氧化钛负载的 Au纳米粒子在 CO氧化过程中可以形成单原子的现象,并将之称为动态单原子催化剂. Yang等则计算了二维材料 BN负载 Au单原子催化 CO氧化并发现反应优先通过三原子 E-R机理进行.     

Origin of Oxide sensitivity in gold-based catalysts: a first principle study of CO oxidation over Au supported on monoclinic and tetragonal ZrO2

The catalytic performance of Au/oxide catalysts can vary significantly upon the change of oxide species or under different catalyst preparation conditions. Due to its complex nature, the physical origin of this phenomenon remains largely unknown. By extensive density functional theory calculations on a model system, CO oxidation on Au/ZrO2, this work demonstrates that the oxidation reaction is very sensitive to the oxide structure. The surface structure variation due to the transformation of the oxide phase or the creation of structural defects (e.g., steps) can greatly enhance the activity. We show that CO oxidation on typical Au/ZrO2 catalysts could be dominated by minority sites, such as monoclinic steps and tetragonal surfaces, the concentration of which is closely related to the size of oxide particle. Importantly, this variation in activity is difficult to understand following the traditional rules based on the O2 adsorption ability and the oxide reducibility. Instead, electronic structure analyses allow us to rationalize the results and point toward a general measure for CO + O2 activity, namely the p-bandwidth of O2, with important implications for Au/oxide catalysis.     

Aluminum salen complexes and tetrabutylammonium salts: a binary catalytic system for production of polycarbonates from CO2 and cyclohexene oxide

A series of complexes of the form (salen)AlZ, where H2salen = N,N'-bis(salicylidene)-1,2-phenylenediimine and various other salen derivatives and Z = Et or Cl, have been synthesized. Several of these complexes have been characterized by X-ray crystallography. An investigation of the utilization of these aluminum derivatives along with both ionic and neutral bases as cocatalysts for the copolymerization of carbon dioxide and cyclohexene oxide has been conducted. By studying the reactivity of these complexes for this process as substituents on the diimine backbone and phenolate rings are altered, we have observed that aluminum prefers electron-withdrawing groups on the salen ligands, thereby producing an electrophilic metal center to be most active toward production of polycarbonates from CO2 and cyclohexene oxide. For example, the complex derived from H2salen = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-ethylenediimine is essentially inactive when compared to the analogous derivative containing nitro substituents in the 3-positions of the phenolate groups. This is to be contrasted with the catalytic activity observed for the (salen)CrX systems, where electron-donating salen ligands greatly enhanced the reactivity of these complexes for the coupling of CO2 and epoxides. While (salen)AlZ complexes are capable of producing poly(cyclohexene oxide) carbonate with low amounts of polyether linkage along with small quantities of cyclic carbonate byproducts, their reactivities, covering a turnover frequency range of 5.2-35.4 mol of epoxide consumed/(mol of Al x h), are greatly reduced when compared to their (salen)CrX analogues under identical reaction conditions.     

Solid-state structures of zinc(II) benzoate complexes. Catalyst precursors for the coupling of carbon dioxide and epoxides

Zinc complexes derived from benzoic acids containing electron-withdrawing substituents have been synthesized from Zn(II)(bis-trimethylsilyl amide)(2) and the corresponding carboxylic acid (2,6-X(2)C(6)H(3)COOH, where X = F, Cl, or OMe) in THF and structurally characterized via X-ray crystallography. The 2,6-difluorobenzoate complex crystallizes from THF or CH(3)CN as a seven membered zinc aggregate, where the metal atoms are interconnected by a combination of 10 mu-benzoates and mu(4)-oxo ligands, that is, [(2,6-difluorobenzoate)(10)O(2)Zn(7)](solvent)(2), solvent = THF (1) and CH(3)CN (1a). On the other hand, the 2,6-dichlorobenzoate zinc derivative crystallizes from THF as a dimer, [(2,6-dichlorobenzoate)(4)Zn(2)](THF)(3) (2), where the two zinc centers are bridged by three benzoate ligand. One of the zinc centers possesses a tetrahedral ligand environment where the fourth ligand is a unidentate benzoate, and the other zinc center has an octahedral arrangement of ligands which is accomplished by the additional binding of three THF molecules. Upon dissolution of complex 1 or 2 in the strongly binding pyridine solvent, disruption of these zinc carboxylates occurs with concomitant formation of mononuclear zinc bis-benzoates with three pyridine ligands in the metal coordination sphere. Complexes 1 and 2 were found to be effective catalysts for the copolymerization of cyclohexene oxide and carbon dioxide to afford polycarbonates devoid of polyether linkages, that is, completely alternating copolymers. Although these catalysts or catalyst precursors in the presence of CO(2)/propylene oxide afforded mostly propylene carbonate, they did serve as efficient catalysts for the terpolymerization of carbon dioxide/cyclohexene oxide/propylene oxide. The reactivities of these zinc carboxylates were very similar to those previously reported analogous complexes which have not been structurally characterized. Hence, it is suggested here that all of these zinc carboxylates provide similar catalytic sites for CO(2)/epoxide coupling processes.     

Study of electronic effect in bifunctional catalysts for the copolymerization of CO2 and PO/CHO

Carbon dioxide (CO₂) is an easily available renewable carbon source that can be used as a comonomer in the catalytic ring-opening polymerization of epoxides to form aliphatic polycarbonates. Herein, a series of new Salen-Co(III) bifunctional catalysts were synthesized for the first time, and they were studied to catalyze the copolymerization of CO₂ and propylene oxide (PO)/cyclohexene oxide (CHO). At the same time, the effects of reaction conditions (electronic effect, temperature, time) on catalytic activity and selectivity were investigated. The results show that the Salen-Co(III) complexes with electron-withdrawing groups have higher selectivity and activity for propylene carbonate (PPC)/cyclohexylene carbonate (PCHC). At the same time, the Salen-Co(III) complexes can better catalyze the copolymerization of CHO and CO₂ than that of PO and CO₂. The catalytic efficiency of the four complexes increased with increasing temperature, and the best reaction condition is 80°C, 30 min and 2 MPa of CO₂.     

Is Cu instability during the CO2 reduction reaction governed by the applied potential or the local CO concentration?

Cu-based catalysts have shown structural instability during the electrochemical CO₂ reduction reaction (CO₂RR). However, studies on monometallic Cu catalysts do not allow a nuanced differentiation between the contribution of the applied potential and the local concentration of CO as the reaction intermediate since both are inevitably linked. We first use bimetallic Ag-core/porous Cu-shell nanoparticles, which utilise nanoconfinement to generate high local CO concentrations at the Ag core at potentials at which the Cu shell is still inactive for the CO₂RR. Using operando liquid cell TEM in combination with ex situ TEM, we can unequivocally confirm that the local CO concentration is the main source for the Cu instability. The local CO concentration is then modulated by replacing the Ag-core with a Pd-core which further confirms the role of high local CO concentrations. Product quantification during CO₂RR reveals an inherent trade-off between stability, selectivity and activity in both systems.

The stability of bimetallic AgCu and PdCu catalysts for electrochemical CO₂RR is investigated using the combination of operando and ex situ TEM. The local CO concentration is identified as the main link between activity, stability and selectivity.     

添加碱土金属化合物对CeO2甲烷氧化偶联催化性能的影响

天然气中甲烷的利用,特别是氧化偶联(OCM)制乙烯过程近年来成为多相催化研究的热点之一.自从1982年Keller和Bhasin提出该课题以来许多学者已进行了大量催化剂的研制工作.尽管在这方面已取得重要进展,但催化剂的性能距工业化要求还有较大差距.     

三元共聚物聚甲基乙撑-环己撑碳酸酯的合成与表征

报道了三元共聚物聚甲基乙撑-环己撑碳酸酯的制备与性能研究. 实验采用高活性的负载戊二酸锌作为催化剂, 在不添加任何溶剂的情况下, 以二氧化碳和环氧丙烷、环氧环己烷为原料, 制备了不同环己撑碳酸酯含量的三元共聚物, 并对其结构和热性能、力学性能进行了表征和分析. 结果表明这些三元共聚物具有较高的分子量, 且玻璃化转变温度随着主链上的环己撑碳酸酯含量增加而逐渐升高. 同时三元共聚物表现出比二元共聚物更好的力学性能.