Elsevier

Chemical Engineering Journal

Volume 378, 15 December 2019, 122052
Chemical Engineering Journal

Promoting CO2 hydrogenation to methanol by incorporating adsorbents into catalysts: Effects of hydrotalcite

https://doi.org/10.1016/j.cej.2019.122052Get rights and content

Highlights

  • Catalytic hydrogenation of CO2 and H2 to methanol studied.

  • Physical mixtures of adsorbents and catalysts prepared at different lengths scales.

  • Finely mixed catalysts and adsorbents show considerable improvement in methanol selectivity.

Abstract

CO2 hydrogenation, in which CO2 conversion to methanol plays a key role, is of increasing importance in mitigating the climate crisis. However, high reaction pressures are required to conduct methanol synthesis because of poor catalytic performance of conventional catalysts, leading to high energy consumption. In this study, a novel strategy was applied to promote methanol synthesis by adsorption-enhanced CO2 hydrogenation. Catalysts made from Cu-ZnO-Al2O3 mixed with different hydrotalcite contents (named CZA-HT) were prepared by physically mixing a commercial copper-based catalyst for methanol synthesis with hydrotalcite for high temperature CO2 adsorption. In these catalysts, only the commercial copper-based component contains the active part for CO2 hydrogenation and its copper surface area is 48.1 m2 g−1 with an optimal reaction temperature of 523 K. Hydrotalcite exhibits no catalytic activity, yet the catalytic performance of the CZA-HT catalysts were clearly facilitated by CO2 adsorption on HT. The sample containing 40 wt% hydrotalcite and 60 wt% CZA showed the highest methanol selectivity of 73.4% and a methanol yield of 4.4% among all samples. The reaction was conducted at a low reaction pressure of 30 bar (much lower than conventional pressure), so that the methanol yield was not high. However, it is observed that the methanol formation rate based on a unit mass of active CZA always increases as the hydrotalcite content in CZA-HT increases, confirming the promotion effects of CO2 adsorption on HT on catalytic performance. The mechanism of adsorption enhanced catalytic reaction was also analysed and discussed, in which the well mixed finer particles of CZA and HT perform better than CZA alone with a 73.9% higher methanol yield.

Introduction

Recent reports have heightened the urgency of immediate action to keep the global average temperature rise within 2 K due to current high and increasing CO2 emissions [1]. Considerable efforts have been made to ease the crisis by reducing usage of fossil fuels, but challenges still exist in reducing the great amount of CO2 produced from industry [2], [3]. An attractive strategy, with the continuously reducing cost of hydrogen production, is highlighted recently to convert CO2 and H2 to value-added chemicals (methanol, hydrocarbons, etc.) in the presence of catalysts, where CO2 to methanol (MeOH) is one of the cornerstones of these processes [4], [5]. The produced methanol can not only be directly used as a fuel or additive, but can also be further converted into other chemicals, converting the low energy density of H2 and reducing the emissions of CO2 [6], [7], [8].

Significant as it is, nevertheless, intractable problems remain to be solved to date. One of the most significant restrictions lies in the thermodynamic equilibrium. As the synthesis of methanol will be promoted at high pressure and low temperature, strict reaction conditions are required to attain acceptable methanol yields [9]. However, the equilibrium CO2 conversion and methanol selectivity remain at 26.6% and 71.6%, respectively, even at industrially relevant conditions of 523 K and 5.0 MPa. Poor catalytic activity of conventional catalysts also exists. To improve the methanol formation rate and methanol selectivity, modification and improvement of copper-based catalysts has taken place for decades. As shown in Table 1, copper-based catalysts derived from co-precipitation still dominate in CO2 hydrogenation. In traditional copper-based catalyst systems such as typical Cu-ZnO-Al2O3 and Cu-ZnO-ZrO2, ZnO serves as a physical spacer between copper particles and helps disperse copper particles and promote their activities [10], [11], [12], [13]; Al2O3 and ZrO2 benefit the stability of active copper [11], [14], [15]. Additionally, many transition metals are used to improve the catalytic activity in CO2 hydrogenation. Widely applied options include cerium (Ce), manganese (Mn), lanthanum (La), etc., which can fine-tune the metal-oxide interfaces efficiently [16], [17], [18], [19] Recently, many non-copper-based catalysts have also been developed, e.g., replacing copper with noble or rare metals as active components [20], [21], [22], achieving significant improvements in CO2 conversion and methanol selectivity. However, the high costs in the preparations of noble catalysts is an economic barrier for their application in industry. Alternatively, one straightforward means to enhance methanol synthesis is elevating reaction pressure at the expense of increased energy consumption [23].

To avoid the additional operation cost from elevating total system pressure, a novel strategy is to increase CO2 partial pressure to combine the catalyst with a CO2 adsorbent. Specifically, CO2 as a reactant can be adsorbed on the added adsorbent during the catalytic reaction, so that the CO2 concentration directly adjacent to the active catalytic site increases, which is equivalent to an elevated partial pressure of CO2 in the reaction system. For this purpose, Mg-Al types of hydrotalcite (HT) was adopted as a candidate as it possesses remarkable adsorption capacities for CO2 within the temperature range of methanol synthesis between 473 and 523 K [32], [33], [34]. Hydrotalcite consists of layered double hydroxides and presents a BET surface area of about 200 m2 g−1 after activation with favorable properties for CO2 adsorption though not for methanol production. This method may be applied in catalytic reactions where CO2 is involved as a reactant regardless of catalyst type.

In the present work, the commercial copper-based methanol catalyst (denoted CZA here) and high-temperature CO2 adsorbent hydrotalcite (denoted HT here) were used to confirm and investigate the influence of an imported adsorbent into the process of CO2 hydrogenation to methanol. A series of CZA-HT samples with incremental HT contents were prepared by physical mixing at a powder level, with subsequent pelleting, and thus the effect of CO2 adsorption on the catalytic performance can be assessed. CZA and HT were characterized by TGA, XRD, SEM, N2 physisorption, H2-TPR and N2O titration, and CO2 adsorption properties of the HT were studied by adsorption kinetics. Catalytic experiments with the CZA-HT samples were conducted in a fixed-bed micro reactor. In addition to these experiments, we also conducted control experiments in which mixtures of CZA pellets and hydrotalcite pellets were mixed at a bed level, and mixtures of quartz sand and CZA pellets were conducted at a bed level.

Section snippets

Materials

The copper-based catalyst (CZA) used for methanol synthesis was purchased from Alfa Aesar (Product No. 45776, Lot No. C18W019). The catalyst pellets were ground (with mortar and pestle) to powders with a diameter lower than 0.1 mm before usage.

The hydrotalcite PURAL MG50 (HT) was supplied by Sasol (Product No. 595050) [24]. It was activated by pretreatment at 673 K for 4 h in air.

Preparation of CZA-HT catalysts

CZA-HT catalysts were prepared by physical mixing. Typically, powders of CZA (diameter < 0.1 mm) and hydrotalcite

Thermal stability

Thermal stabilities of CZA and raw hydrotalcite (before activation) analysed by TGA and DTG are shown in Fig. 3. As shown in Fig. 3(a), the CZA only loses 2.4% of its total weight even at a high temperature of 1023 K. The hydrotalcite, by comparison, displays two obvious weight loss stages as temperature is increased, contributing to a total weight loss rate of 38.2%. From the DTG curves in Fig. 3(b), no peak is detected in CZA, but three peaks are distinguished in HT at 475.6, 573.6 and

Conclusion

In this study we assessed the influence of an adsorbent on CO2 conversion to methanol using hydrotalcite as the adsorbent and commercial Cu based catalyst. Catalysts (CZA-HT) containing different contents of hydrotalcite were prepared by physically mixing copper-based catalyst (CZA) and hydrotalcite. Among CZA-HT catalysts, the methanol formation rate based on unit mass of CZA increases along with the HT content in them. All methanol yields acquired with CZA-HT catalysts, though appear

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