Highly active catalysis of methanol oxidative carbonylation over nano Cu2O supported on micropore-rich mesoporous carbon
Graphical Abstract
Introduction
Dimethyl carbonate (DMC) is such an environment-friendly chemical, which is widely used as nontoxic intermediate for methylation [1], carbonylation [2] and transesterification reaction [3], [4], and a fuel additive to improve combustion and reduce engine emissions [5], as well as green solvent in coating, adhesives and electrolytes [6]. Lots of routes have been developed to synthesize DMC with different feedstocks, such as phosgene, methyl nitrite, ethylene (propylene) carbonate, or urea [7]. Among these, the oxidative carbonylation of methanol with O2 and CO to DMC is highly atom-economical and clean due to the water as the only by-product, so called “green route” [7], and the reaction is more thermodynamically favorable rather than others [8]. It is found that CuCl is the most efficient catalyst, but in the reacting process, the chlorine is easy to form HCl causing chlorine loss [9], which makes the industrial application surfer from several environmental drawbacks, such as chlorine-containing wastewater, equipment corrosion, and product purification as well as catalyst deactivation [10]. To develop chlorine-free Cu-based catalysts has drawn considerable attentions in recent years.
Nano Cu catalyst supported on porous carbon has shown potential as one of candidates for the chlorine-free Cu-based catalyst, which is often prepared by impregnation with nitrate or acetate of copper as chlorine-free copper source [7], [8]. The activated carbon (AC) supporting Cu catalyst was reported to reach of DMC space-time yield based on per unit of catalyst (STYcat) [11]. If converted to the STY based on per unit of Cu (STYCu), it is calculated as 2.4 g for Cu/AC catalyst. However, it was characterized that the Cu particles in the catalyst seriously aggerate with average particle size as large as 12.0 nm [11], which far exceeds the AC micropore diameter (<1nm). It thus indicated that the as-formed Cu particles only located at the scarce external surface of AC rather than the inner surface of micropores. But, the ordered mesoporous carbon (OMC), rich in uniform mesopores with large mesopore surface, was reported to confine Cu nanoparticles as small as average 2.9 nm in the mesoporous channels. Even so, the STYCu over this Cu/OMC catalyst was calculated as only [12]. The similar result was observed with mesoporous carbon spheres (MCS) as support. It was found that ~2.0 nm Cu nanoparticles were uniformly dispersed inside the mesopores, but STYCu was only 2.3g g−1Cu h−1 [13]. A fact is clearly shown that the DMC yield in the methanol oxidative carbonylation failed to be enhanced even if the Cu particles are highly dispersed as small as 2.0 nm.
Except for the Cu particle dispersion, the oxidation state of Cu particles also influences the catalytic activity in the methanol oxidative carbonylation. It was shown that the Cu2O is the most efficient active species for the reaction [14]. On the carbon supports, Cu2O is formed from CuO auto-reduction by the carbon itself, and even over-reduced to metallic Cu under thermal treatment. In the reported Cu/AC catalyst, most of CuO was auto-reduced to not only Cu2O but also partial metallic Cu with 11.0% Cu loading calcinated at 350 °C [14]. But only 45.5% of CuO was auto-reduced to Cu2O in the above Cu/OMC catalyst at a low Cu loading of 8.7% [12] and 60.2% in Cu/MCS catalyst at 9.4% Cu loading [13] under the same treating condition. Besides, only 44.5% of CuO was auto-reduced to Cu2O for Cu particles of 3.0 nm confined in the inner mesopores of carbon nanotube (CNT) at 7.1% Cu loading [15]. It is obvious that the auto-reduction from CuO to Cu2O is more difficult on those mesoporous carbon supports than that on microporous AC, which inspires that the micropores in AC material play a role in the auto-reduction behavior to form active Cu2O species, further influence the DMC yield.
Moreover, if the turnover frequency (TOF) of DMC based on a single surface Cu+ site calculated, the Cu/AC catalyst shows TOF value as 32.4 h−1, which is significantly higher than 8.3 h−1 for the Cu/OMC catalyst [12] and 6.7 h−1 for the Cu/MCS catalyst [13]. It is clearly shown that one Cu+ site supported on microporous carbon materials give much higher catalytic activity than that supported on mesoporous carbon materials. It reveals that the micropores are also particularly relevant to the chemical environment of the as-formed Cu+ sites to influence the catalytic activity.
Therefore, it is valuable to prepare a micropores-rich mesoporous carbon (MMC) supporting nano Cu catalyst to improve the catalytic performance in the methanol oxidative carbonylation. And the effect of the micropores on the formation of the Cu2O species and its special role in the chemical environment of the as-formed Cu+ sites are so desired to be investigated.
Section snippets
Chemicals
The commercial AC (1643 m2 g−1) made from coconut shells is purchased from Xinsen Chemical Industry Co., Ltd. The hard template SBA-15 (XFF01, 550–600 m2 g−1, 0.65–0.7 cm3 g−1, relative crystallinity ≥ 90%) was purchased in bulk from Nanjing XFNANO Materials Tech. Co., Ltd. The water used was double-distilled. The H2SO4 (98 wt%), HF (35.35 wt%), HNO3 (65–68 wt%) solution, KOH, Cu(NO3)2·3H2O and methanol were manufactured by Tianjin Kermel Chemical Reagent Co., Ltd. Among, HF solution is so
Textural properties of the MMC supports
The textural properties of the MMC supports resulting from KOH etching are analyzed by N2 physisorption and the results are summarized in Table 1. As shown, the total pore volume of OMC reaches up to 1.22 cm3 g−1, of which the micropores account for only 0.04 cm3 g−1. With KOH/OMC mass ratio increasing from 1 to 3, the total pore volume, mesopore volume and external and mesopore surface area decline, but the micropore volume and area respectively increase from 0.04 to 0.35 cm3 g−1 and 62–847 m2
Conclusions
Micropores were introduced into ordered mesoporous carbon by KOH etching to fabricate a micropore-rich mesoporous carbon, which is further used as support to prepare a highly active Cu2O catalyst for improving catalytic activity of methanol oxidation carbonylation. The microporosity was adjusted by controlled the KOH/OMC mass ratio to investigate the role of the micropores. It is found that introducing micropores causes vacancy-type defects at graphite crystallite in the framework of the carbon
CRediT authorship contribution statement
Jiajun Wang: Conceptualization, Methodology, Software, Investigation, Writing – original draft, Formal analysis, Visualization. Tingjun Fu: Data curation. Fanhui Meng: Data curation. Dan Zhao: Assisted characterizations. Steven S. C. Chuang: Writing – review & editing. Zhong Li: Supervision, Project administration, Funding acquisition, Resources, Writing – review & editing.
Acknowledgment
This work was supported by the National Natural Science Foundation of China (U1510203); and the Key Research and Development Project of Shanxi Province (International Science and Technology Cooperation Program) (201803D421011). The authors are grateful to Prof. Huayan Zheng, Guoqiang Zhang, Mingsheng Luo and Dr. Nilesh Narkhede for their kindly academic discussion and language help.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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