Catalytic synthesis of diethyl carbonate with supported Pd-Cu bimetallic nanoparticle catalysts:Cu(Ⅰ) as the active species

负载型Cu-Pd双金属纳米氧化物催化剂催化合成碳酸二乙酯:Cu(Ⅰ)作为活性中心（英文）

Cupric oxide （CuO） and copper-cuprous oxide （Cu-Cu2O） nanoparticles were prepared by a simple hydrothermal method for the synthesis of diethyi carbonate （DEC） from ethanol. During these syntheses, varying NaOH and glucose concentrations were applied to explore and pinpoint the active species. It was found that PdCl2/CuO and PdCI2/Cu-Cu2O both catalysts exhibited good thermal stability and morphology. The results of catalytic tests showed that the catalysts prepared with 5 mol/L NaOH show superior catalytic performances because of their lower extent of agglomeration. It is noteworthy that the PdC12/Cu-Cu2O catalysts were the most active, especially the PdCl2/Cu-Cu2O catalyst prepared with 10 mmol glucose and having a higher Cu2O concentration. In Pd（ll）-Cu（II） （PdCl2/CuO） catalysts, there is an induction period, during which Pd（II） is reduced to Pd（0）, that must occur prior to electron transfer between Pd and Cu, and this can slow the catalytic reaction. To further pinpoint the active species, PdCl2/Cu-Cu2O catalysts with different Cu2O contents were prepared by controlling the dosages of glucose. The maximum DEC yield obtained with these catalysts was 151.9 mg.g-1.h-1, corresponding to an ethanol conversion of 7.2% and 97.9% DEC selectivity on an ethanol basis. Therefore, it was concluded that Cu＋ was the active species in this catalytic system, possibly because a higher proportion of Cu＋ reduces the Pd2＋ concentration and limits the CO oxidation side reaction, thus increasing DEC selectivity. In addition, Cu＋ promotes electron transfer between Pd and Cu without an induction period, which could also promote the catalytic activity.

催化反应活性与催化剂活性组分的存在价态密切相关,所以探讨催化剂在反应过程中的活性中心及其价态变化,对于催化反应机理和催化剂的研究都显得十分重要.目前对于氧化羰基合成碳酸二甲酯催化剂的机理的探讨很多,主要存在的争议是Cu+还是Cu2+作为活性中心,以及铜物种的配位状态.大多体系都是以分子筛为载体的铜基催化剂,其活性中心的研究存在铜离子在分子筛中的定位问题,而且催化活性也会受到分子筛结构的影响.采用这种方法研究活性中心的影响因素较多,存在一定的局限性.因此,直接制备纳米级的铜基氧化物用于本催化体系,有利于更直观简单地探索其活性中心.纳米级金属氧化物材料是一种新型的功能性材料,而纳米铜基氧化物(CuO和Cu2O)因其独特的物化性质和结构而引起广泛关注.我们采用水热法制备纳米CuO及其它氧化物,研究了NaOH浓度对催化剂的催化性能的影响;葡萄糖是一种还原性较强的还原剂,其用量必定会对所制备的氧化物的物种有所影响.为了探究Cu0和Cu+在本体系中的作用,采用不同葡萄糖用量制备了具有不同Cu2O含量的PdCl2/Cu-Cu2O催化剂.在上述研究基础上,我们采用X射线衍射、场发射扫描电子显微镜、热重分析、等离子体原子发射光谱等表征手段研究了负载型纳米铜基氧化物催化剂在合成碳酸二乙酯反应中催化性能差异的原因,旨在直接考察活性中心主要是Cu+还是Cu2+,避免分子筛等体系中载体结构的影响,研究结果更具参考性.结果发现,NaOH浓度为5 mol/L时制备的PdCl2/CuO和PdCl2/Cu-Cu2O催化剂的性能优于其他浓度下制备的催化剂,这可能是由于不同浓度的碱溶液会对铜离子的沉淀过程产生不同的影响;相同NaOH浓度下制备的催化剂中,PdCl2/Cu-Cu2O催化剂的催化性能明显优于PdCl2/CuO催化剂,这可能是由于PdCl2/Cu-Cu2O催化剂更有利于反应过程中电子的传递,从而表现出更好的催化性能,我们推测Cu0和Cu+可能更有利催化乙醇氧化羰基合成DEC;表征分析发现PdCl2/CuO和PdCl2/Cu-Cu2O均具有很好的热稳定性,两种催化剂中PdCl2负载量几乎相同,因此,主要影响催化性能的因素是载体CuO和Cu-Cu2O中铜的价态.采用不同葡萄糖用量制备了含有不同Cu2O含量的PdCl2/Cu-Cu2O催化剂,其中,PdCl2/Cu-Cu2O-2催化剂中含有更多的Cu2O,在反应中乙醇转化率达到了7.2%,DEC的选择性为97.9%,DEC的时空收率可达到151.9 mg·g–1·h–1.由此可见在乙醇气相氧化羰基合成DEC体系中,Cu+是主要的活性中心.