Nonoxidative Dehydrogenation of Methanol to Methyl Formate through Highly Stable and Reusable CuMgO-Based Catalysts Item Type Article Authors Yuan, Ding-Jier; Hengne, Amol Mahalingappa; Saih, Youssef; Huang, Kuo-Wei Citation Yuan D-J, Hengne AM, Saih Y, Huang K-W (2019) Nonoxidative Dehydrogenation of Methanol to Methyl Formate through Highly Stable and Reusable CuMgO-Based Catalysts. ACS Omega 4: 1854–1860. Available: http://dx.doi.org/10.1021/ acsomega.8b03069. Eprint version Publisher's Version/PDF DOI 10.1021/acsomega.8b03069 Publisher American Chemical Society (ACS) Journal ACS Omega Rights This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Download date 23/09/2021 15:25:39 Item License http://pubs.acs.org/page/policy/authorchoice_termsofuse.html Link to Item http://hdl.handle.net/10754/631004 This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Article Cite This: ACS Omega 2019, 4, 1854−1860 http://pubs.acs.org/journal/acsodf Nonoxidative Dehydrogenation of Methanol to Methyl Formate through Highly Stable and Reusable CuMgO-Based Catalysts † Ding-Jier Yuan, Amol M. Hengne, Youssef Saih, and Kuo-Wei Huang* KAUST Catalyst Center and Division of Physical Science and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia ABSTRACT: Nonoxidative dehydrogenation of methanol to methyl formate over a CuMgO-based catalyst was investigated. Although the active site is metallic copper (Cu0), the best reaction conditions were obtained by tuning the ratio of Cu/Mg and doping the catalyst with 1 wt % of Pd to achieve a very specific activity for methyl formate synthesis. On the basis of the CO2 temperature- programmed desorption study, the basic strength of the catalyst plays a role in the efficient conversion of methanol to methyl formate via dehydrogenation. These CuMgO-based catalysts show excellent thermal stability during the reaction and the regeneration processes. Approx. 80% methanol conversion with constant selectivity to methyl formate was achieved even after 4 rounds of usage for a total reaction time exceeding 200 h, indicative of their potential for practical applications. ■ INTRODUCTION several homogeneous systems have been developed for dehydrogenative homocoupling of primary alcohols to the Methyl formate (MF) is an essential reactive intermediate in − − 9 16 C1 chemistry.1 3 It is widely used for the synthesis of corresponding esters, the formyl group of MF from methanol tends to undergo further dehydrogenation reac- numerous value-added products in the chemical industry, 17,18 including ethylene glycol, N,N-dimethyl formamide, methyl tions. On the other hand, for heterogeneous systems, the glycolate, acetic acid, methyl propionate, and formaide. MF is products obtained through dehydrogenation of methanol can be diverse, and many factors can influence the catalytic also a highly valuable chemical that can directly be used as an 19−21 4 performance. For example, an acidic support leads to the antiseptic, solvent, and gasoline additive. Several modes of 22 synthesis of MF have been reported in the literature. They increases in dehydration processes (e.g. dimethyl ether), and involve the selective oxidation of methanol,3 dehydrogenation higher temperatures favor gaseous products such as CO or 3 5 23 CO2. It has been demonstrated that copper was the active of methanol, hydrogenation of CO2 to MF, dimerization of 24−29 30−33 formaldehyde,6 carbonylation of methanol,7 and esterification species for MF syntheses and previous studies also of methanol with formic acid,8 with the first two synthetic showed that metallic copper was the major active species for routes being the most popular for the commercial production. the methanol dehydrogenation reaction. Besides copper, there In comparison to the selective oxidation reaction, the are other materials that can act as catalysts for this reaction, 34 nonoxidative dehydrogenation reaction not only avoids the including palladium, nickel, and platinum. However, despite See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. Downloaded via KING ABDULLAH UNIV SCI TECHLGY on January 30, 2019 at 08:26:16 (UTC). addition of an oxidant, meaning that it is not only a safer the high number of studies on the nonoxidative dehydrogen- reaction to lower the production costs, but it also produces ation of methanol to form MF, the formation rates of MF hydrogen as a value-added byproduct that can be used as a fuel obtained so far are not sufficiently high for practical use, and a or reducing agent in various industries. catalyst with a practically long lifetime still needs to be The direct dehydrogenation reaction of methanol to discovered. produce MF is described in details in Scheme 1. Although Herein, we report that the formation rate of the MF can be enhanced by the addition of a basic material (MgO) and the Scheme 1. Dehydrogenation of Methanol catalytic performance can be improved by tuning the ratio of Cu/MgO. Compared to the traditional impregnated copper- based catalysts,4 the CuMgO-based catalysts show a significant improvement of methanol conversion (11.7−16.7%) and MF selectivity (62.5−88.1%). Furthermore, the incorporation of Pd to CuMgO was found to prevent the deactivation and Received: November 4, 2018 Accepted: January 14, 2019 Published: January 23, 2019 © 2019 American Chemical Society 1854 DOI: 10.1021/acsomega.8b03069 ACS Omega 2019, 4, 1854−1860 ACS Omega Article Table 1. Textural Properties of Catalysts 2 3 a catalyst SBET (m /g) Vp (cm /g) Dp (nm) particle size of Cu (nm) H2 up take (mmol/gcat)CO2 up take (mmol/gcat) Cu1MgO9 50.1 132.1 0.54 0.12 0.81 Cu2MgO8 32.7 128.1 0.20 0.33 0.74 Cu3MgO7 49.6 129.4 0.75 9.46 0.54 0.59 1Pd/Cu3MgO7 47.5 122.9 0.23 8.84 0.61 0.55 Cu5MgO5 67.4 152.6 0.68 11.88 0.72 0.21 1Pd/Cu5MgO5 20.4 151.7 0.38 11.02 0.78 0.11 Cu7MgO3 62.4 330.0 0.60 23.63 1.12 0.07 aCalculated by Scherrer equation. Figure 1. XRD patterns for: (a) fresh catalysts and (b) spent catalysts after the catalytic test for 20 h. Figure 2. TEM images for: (a) Cu3MgO7-fresh; (b) Cu3MgO7-spent; (c) 1Pd/Cu3MgO7-fresh; (d) 1Pd/Cu3MgO7-spent; (e) Cu5MgO5-fresh; (f) Cu5MgO5-spent; (g) 1Pd/Cu5MgO5-fresh; (h) 1Pd/Cu5MgO5-spent; (i) Cu7MgO3-fresh; and (j) Cu7MgO3-spent. increase the stability of the catalyst for methanol dehydrogen- the palladium heteroatom, as shown in 1Pd/Cu3MgO7 and ation. 1Pd/Cu5MgO5. To better understand the effects of Pd and the Cu/MgO ■ RESULTS AND DISCUSSION ratio, several techniques were employed to characterize the properties of the catalysts. In Figure 1a, the wide-angle X-ray Characterization of Catalysts. Texture properties (Table − diffraction (XRD) patterns show that the intensity of CuO 1) of the catalysts show that they all have similar Brunauer ° ° Emmett−Teller (BET) surface areas (50.1−62.4 m2/g) and peaks (35.6 , 38.8 , JCPDS card no. 01-073-6023) increased pore volumes (0.54−0.60 cm3/g), except the highest copper- with increasing copper loading, whereas the peaks of MgO (36.8°, 42.7°, 61.9°, JCPDS card no. 1-071-1176) decreased. containing catalyst (Cu7MgO3) which has a larger pore size. − The spent catalysts all showed Cu (43.2°, 50.3°, JCPDS cards The N2 adsorption desorption isotherm plot shows that the adsorption amount increased slowly with increasing pressure no. 00-004-0836) and MgO peaks but without those of CuO during the initial stage, and after the pressure reached the or Cu2O(Figure 1b). These results indicate that the metallic 0 saturated vapor pressure, the adsorption amount increased state of copper species (Cu ) can be maintained after the rapidly. These pores were all provided by the crystal stacking dehydrogenation reaction and the catalysts were not oxidized (type III, nonporous material). These results allow one to during the catalytic test. The particle sizes of metallic copper conclude that Cu7MgO3 produces the largest particles, increased with increasing copper loading because of the resulting in a larger pore size. These large particles might be aggregation of copper species. Moreover, the addition of Pd a result of copper aggregation, triggered by the higher copper likely inhibits the growth of copper crystal,35 as the Pd- loading. After the addition of palladium, the BET surface area containing catalyst shows smaller particle sizes than those of and pore volume both decreased, because of the presence of catalysts without Pd. 1855 DOI: 10.1021/acsomega.8b03069 ACS Omega 2019, 4, 1854−1860 ACS Omega Article Transmission electron microscopy (TEM) images (Figure reactivity in the reduction reaction. The amount of H2 2) show the significant differences in morphology between the consumption increased with the increasing copper loading, high copper-containing and low copper-containing catalysts. but the reduction temperature almost stayed the same (250 The low copper-containing catalysts (Figure 2a,c,e,g) show a °C); these findings suggest that our preparation processes homogeneous mixture of copper nanoclusters and MgO with using the coprecipitation method can successfully enhance the only small copper particles distributed on the MgO flakes. For amount of surface copper, and all the copper-based catalysts ff the high copper-containing catalysts (Figure 2i), the small have similar reduction sites with di erent amounts of H2 copper nanoparticles become larger copper cluster, consistent consumption (Table 1). The H2-TPR measurement also with the results from the XRD analysis.