Article pubs.acs.org/JPCC
CO-Mediated Deactivation Mechanism of SiO2‑Supported Copper Catalysts during Dimethyl Oxalate Hydrogenation to Ethylene Glycol Jianwei Zheng,† Junfu Zhou,† Haiqiang Lin,† Xinping Duan,† Christopher T. Williams,*,‡ and Youzhu Yuan*,† †
State Key Laboratory of Physical Chemistry of Solid Surfaces, National Engineering Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters, Collaborative Innovation Center of Chemistry for Energy Materials, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, PR China ‡ Department of Chemical Engineering, University of South Carolina, Columbia, South Carolina 29208, United States ABSTRACT: Selective hydrogenation of dimethyl oxalate (DMO) derived from syngas to ethylene glycol (EG) over copper (Cu)-based catalysts is an important transformation of modern syngas chemical industry. Methanol, as a product or a solvent, can dissociate on the Cu surfaces by forming adsorbed CO under H2 atmosphere at 473 K. A small amount of adsorbed CO accelerates Cu redox processes, thus inhibiting catalytic activity with a negative kinetic reaction order. The strong interaction between CO and Cu blocks active sites and disrupts the synergy of Cu+ and Cu0 species, which are vital in DMO hydrogenation. The Ostwald ripening of Cu crystallites is induced by CO, resulting in aggregation of Cu crystallites. The imbalance of active species and crystallite aggregation lead to deactivation of the Cu catalysts during DMO hydrogenation to EG.
1. INTRODUCTION Ethylene glycol (EG) is an important raw material primarily used as intermediate to manufacture polyethylene terephthalate resins, polyester fibers, and fabrics. EG can also be used as antifreeze, coolants, heat-transfer solvents, and so forth.1 The EG market is expected to grow at around 6% each year through at least 2018, with the Asia Pacific region having the strongest growth due to high and increasing demand. EG is currently produced by direct hydration of ethylene oxide (EO),2 and more recently through an indirect path involving hydration of ethylene carbonate intermediate formed from EO and CO2.3 Notwithstanding these relatively mature processes, development of an EG production system by using coal-based syngas with dimethyl oxalate (DMO) as the intermediate has gained increasing attention as a promising process to balance the EG supply and demand.4 It is an integrated technology that consists of the coupling of syngas-derived methanol with CO to form DMO and subsequently hydrogenation to yield EG or ethanol. The first step was successfully scaled up into commercial production in 2010, making the production of EG a promising prospect.5 This process is also a plausible approach for EG synthesis from syngas derived from abundant shale gas and biomass resources. Selective hydrogenation of DMO to EG involves two sequential steps, namely, formation of methyl glycolate (MG) and further hydrogenation to EG.6 Both steps involve C−O bond hydrogenolysis and carbonyl group hydrogenation. Copper (Cu)-based catalysts are relatively inactive during hydrogenolysis of C−C bonds but facilitate hydrogenation of C−O bonds.7 Currently, silica-supported Cu-based catalysts (Cu/SiO2) have been widely investigated for chemoselective © 2015 American Chemical Society
DMO hydrogenation to EG because of their high activity and environmentally benign property. However, the short lifespan and ambiguous deactivation mechanism of Cu/SiO2 catalysts severely hinder their practical industrial applications.8 Cu-based catalysts have been extensively used in various applications, such as methanol synthesis, selective hydrogenation, coupling reaction, and click chemistry.9−12 Nevertheless, severe catalyst deactivation, similar to DMO hydrogenation, is frequently observed during these reactions. The short lifespan of conventional Cu catalysts inhibits their potential applications because of facile migration and coalescence of Cu nanoparticles under elevated reaction conditions. In general, deactivation of Cu-based catalysts has been ascribed to two main factors:13,14 loss of the active Cu surface sites through migration and aggregation of Cu nanoparticles and changes in the valence state of surface Cu species under reaction conditions. Migration and aggregation of Cu species accompanied by loss of catalytic performance have been observed in working catalysts for DMO hydrogenation.15,16 The loss of the active Cu surface areas caused by aggregation further exacerbates the deactivation. Previous work on the active sites of Cu-based catalysts assigns Cu0 species as the site for H2 dissociation and Cu+ species as Lewis acidic or electrophilic sites to stabilize acyl species by polarizing the C− O bond via an electron lone pair in oxygen.6,17 Synergistic cooperation between Cu0 and Cu+ species requires a suitable amount of these active sites with an optimal ratio. Wang and Received: April 14, 2015 Revised: May 26, 2015 Published: June 1, 2015 13758
DOI: 10.1021/acs.jpcc.5b03569 J. Phys. Chem. C 2015, 119, 13758−13766
Article
The Journal of Physical Chemistry C
the precursor was primarily reduced under 5% H2−95% N2 atmosphere at 623 K for 4 h to obtain the catalyst. 2.2. Catalyst Characterization. 2.2.1. Properties. The properties of the porous structures were determined from N2 adsorption−desorption measurements at 77 K with a Micromeritics ASAP 2020M+C system. The sample was outgassed under vacuum at 573 K for 3 h before the adsorption of nitrogen. The Brunauer−Emmett−Teller (BET) method, based on the Kelvin equation, was applied to evaluate the mesopore size distribution from the desorption branch. The micropore size was evaluated by the Horvath−Kawazoe (H−K) method. 2.2.2. TEM. TEM was performed on a Philips Analytical FEI Tecnai 30 electron microscope at an acceleration voltage of 300 kV and fitted with an ultrahigh resolution pole piece. The catalyst powders were lightly ground and reduced in a sealed flask. After cooling to room temperature, ethanol was injected into the flask and the catalyst powders were ultrasonically dispersed without air exposure. The as-obtained suspension was then loaded onto holey Cu grid supported with carbon films. Mean crystallite sizes were obtained by counting more than 100 nanoparticles in a spherical model. 2.2.3. XPS. XPS spectra were determined using a Quantum 2000 Scanning ESCA Microprob spectrometer (Physical Electronics) with an Al Kα X-ray radiation source (hv = 1486.6 eV) at a pressure of
