Direct hydrogenation of CO2 to dimethyl ether (DME) over hybrid catalysts containing CuO/ZrO2 as a metallic function and heteropolyacids as an acidic function

Reaction Kinetics, Mechanisms and Catalysis https://doi.org/10.1007/s11144-020-01778-9 Direct hydrogenation of CO2 to dimethyl ether (DME) over hybrid catalysts containing CuO/ZrO2 as a metallic function and heteropolyacids as an acidic function A. Kornas1,2 · M. Śliwa1 · M. Ruggiero‑Mikołajczyk1 · K. Samson1 · J. Podobiński1 · R. Karcz1 · D. Duraczyńska1 · D. Rutkowska‑Zbik1 · R. Grabowski1 Received: 6 February 2020 / Accepted: 10 April 2020 © The Author(s) 2020 Abstract Dimethyl ether (DME) is considered as a substitution of diesel oil. It can be used in diesel engines because of its high cetane number (> 55). The combustion process does not generate particle matter (PM) or sulphur oxides (SOx) pollutions. One of the methods to obtain DME is direct synthesis from a CO2 and H2 mixture. On the other hand, CO2 is an attractive reagent, which is a safe and economical source of carbon. The aim of this work was to obtain DME in the direct process from the mixture CO2 and H2 in the presence of hybrid catalyst. In these catalytic the CuO/ ZrO2 was selected as a metallic function. The montmorillonite K10 modified by heteropolyacids was selected as an acidic function. The catalysts were obtained by different preparation methods and contained various types of heteropolyacids. The catalysts were characterized by following methods: BET/BJH, XRD, SEM, DCS/ TG, NH3-TPD and FT-IR. The direct hydrogenation of C O2 was performed in the high pressure fixed-bed flow reactor connected online with GC equipped with TCD and FID detectors. It was shown that both synthesis method of metallic function and the type of heteropolyacids have influence on the total catalytic activity of the hybrid catalyst. The acidity and thermal stability of HPAs are identified as the most important parameters having a decisive influence on the overall catalytic activity of the samples. Keywords Dimethyl ether (DME) · CO2 hydrogenation · Hybrid catalyst · CuO/ ZrO2 · Heteropolyacids * A. Kornas agnieszka.kornas@jh‑inst.cas.cz 1 Jerzy Haber Institute of Catalysis and Surface Chemistry Polish Academy of Sciences, Niezapominajek 8, 30239 Krakow, Poland 2 J. Heyrovský Institute of Physical Chemistry, Czech Academy of Sciences, 182 23, Dolejškova 2155/3 Prague 8, Czech Republic 13 Vol.:(0123456789) Reaction Kinetics, Mechanisms and Catalysis Introduction In recent years, the concentration of CO2 in the atmosphere has been constantly increasing, having an impact on global climate change [1]. Therefore, a growing number of countries and researchers are interested in developing technologies to reduce CO2 emissions and developing efficient carbon dioxide utilization systems. Synthesis of C O2-derived chemicals includes synthesis of urea [2], CO [3], methane [4], methanol [5–7], dimethyl ether [8, 9], and other hydrocarbons [10]. Among them, dimethyl ether appears very promising because of its physicochemical properties and possibilities of applications. Dimethyl ether (DME) is considered as a substitution of diesel oil in diesel engines due to its high cetane number (> 55). Undoubted advantage of using DME as a fuel would be a significant reduction of air pollution. During DME combustion, soot impurities are not generated, which is an important problem in engines running on traditional diesel oil. No toxic sulfur oxides are also emitted and the amount of nitrogen oxides is considerably decreased. Nowadays, dimethyl ether is produced form syngas (1), but also CO2 and H2 mixture can be used as a feedstock (2). ΔH0 = − 244.9 kJ mol−1 (1) 2CO2 + 6H2 ↔ CH3 OCH3 + 3H2 OΔH0 = −122.2 kJ mol−1 (2) 3CO + 3H2 ↔ CH3 OCH3 + CO2 The DME synthesis proceeds in two steps: hydrogenation of CO/CO2 into methanol (3, 4) and subsequent dehydration of methanol into DME (5). All reactions are exothermic, therefore, low temperature facilitates shifting the reaction balance towards the products. Moreover, in accordance to the Le Chatelier’s principle an increase in pressure increases the amount of methanol produced. CO2 + 3H2 ↔ CH3 OH + H2 O CO + 2H2 ↔ CH3 OH ΔH0 = − 49.4 kJ mol−1 ΔH0 = − 90.4 kJmol−1 2CH3 OH ↔ CH3 OCH3 + H2 O ΔH0 = − 24.0 kJ mol−1 (3) (4) (5) Hybrid catalysts used in a direct dimethyl ether synthesis exhibit metallic and acidic functions. The metallic function is responsible for hydrogenation of carbon dioxide, whereas the acidic function contributes to dehydration of methanol. Mostly, CuO/ZnO catalysts with different additives [11–13] are used as the metallic functions. At the same time, zeolites [14, 15] or γ-Al2O3 [16] are commonly applied as the acidic function. In commercial production of methanol from syngas, the catalytic system CuO/ ZnO/Al2O3 is used. In this case, the elevated temperature (250–280 °C) and pressures up to 6–8 MPa are needed [17]. On the other hand, in industry production, the syngas is enriched with 5% of CO2, which causes increased activity of copper-zinc catalyst. The commercial catalytic system has low activity in synthesis 13 Reaction Kinetics, Mechanisms and Catalysis of methanol from C O2 and H 2 mixture under 250 °C [18] because of low reactivity of the C O2 molecule. The increase of temperature makes the C O2 activation much easier. At the same time, high reaction temperature facilitates the sintering of Cu crystals which leads to deactivation of the catalyst. Currently it is considered that copper plays a role of an active phase during catalytic synthesis of methanol. However, the type of the support must be very carefully considered, because metallic copper poorly interacts with CO2 molecule [19]. The TiO2–SiO2 modification of industrial catalyst improves C O2 conversion from 15.8 to 40.7% and yield to methanol from 3.7 to 16.7% [20]. Similarly, Inui et al. [21] claims that Ga2O3 and Pd added to CuO/ZnO/Al2O3 improves catalytic activity in both CO and CO2 hydrogenation. Currently, it is considered that ZrO2 addition facilitates CuO dispersion on the support surface. As a consequence, conversion of C O2 increases [22]. Fisher and Bell in their study state that ZrO2 improves copper’s catalytic activity during methanol synthesis both from CO/H2 and CO2/H2 mixtures [23]. What is more, during hydrogenation of C O2 a considerable amount of water is formed, leading to a decreased catalytic activity of Cu/ZnO/Al2O3 catalyst [24]. This effect is associated with oxidation of active copper to copper oxide. The dehydration of methanol into DME requires acidic centers on the catalyst’s surface. The methanol molecule can be adsorbed on both Brønsted and Lewis acid centers [25]. The total concentration of acidic centers on the surface and their strength have an important influence on the catalytic activity. It is known that mainly weak and medium acid centers are responsible for dehydration of methanol, while the presence of strong acid centers favors receiving of olefins [26]. Commercial catalyst, with Al2O3 as the acidic function, rapidly loses its catalytic activity beca (...truncated)