In situ DRIFT study of low-temperature methanol synthesis mechanism on Cu/ZnO catalysts from CO2-containing syngas using ethanol promoter

doi:10.1016/j.jcat.2004.08.017

Journal of Catalysis

Volume 228, Issue 1, 15 November 2004, Pages 23-35

https://doi.org/10.1016/j.jcat.2004.08.017 Get rights and content

Abstract

In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to study the reaction mechanism of a new methanol synthesis method on Cu/ZnO at low temperatures from syngas (CO/CO₂/H₂) using ethanol promoter. The adsorbed formate species were formed by exposing Cu/ZnO catalysts to syngas (CO/CO₂/H₂), and it reacted easily with ethanol in the gas phase to form ethyl formate in two states of species, gas phase and physisorbed, at low temperatures. Ethyl formate was the reactive intermediate, and it was reduced easily by hydrogen atoms on Cu to form gas-phase methanol. The reaction temperature was significantly decreased due to the catalytic action of ethanol and a new reaction route. In order to accelerate this reaction, a large amount of ethanol must be introduced into the reaction system. When there was no ethanol or little ethanol in the reaction system, this reaction was difficult to complete at a temperature as low as 443 K.

Introduction

Methanol, which is a fundamental chemical and fuel for fuel cells or vehicles, is being produced by 30 million tons per year around the world from CO/CO₂/H₂. Methanol is industrially produced under high temperature and high pressure, using copper–zinc-based oxide catalysts. However, the efficiency of methanol synthesis is severely limited by thermodynamics because methanol synthesis is an extremely exothermic reaction [1], [2]. For example, at 573 K and 5.0 MPa, the theoretical maximum of one-pass CO conversion is around 20% [3]. Therefore, developing a low-temperature process for methanol synthesis will greatly reduce the production cost and high CO conversion becomes available at low temperatures.

Brookhaven National Laboratory in the USA (BNL) realized this synthesis in the liquid phase at 373–403 K and 1.0–5.0 MPa, using a very strong base catalyst (mixture of NaH, alcohol, and acetate) and pure syngas (CO + H₂). However, a remarkable drawback of this process is that trace amounts of carbon dioxide and water in the feed gas (CO + H₂) or reaction system will deactivate the strongly basic catalyst soon [4], [5], which implies high cost coming from the complete purification of the syngas from the methane reformer or the gasification plant, as well as the reactivation process of the deactivated catalyst.

Methanol synthesis from pure CO and H₂ via the formation of methyl formate has been widely studied, where carbonylation of methanol and hydrogenation of methyl formate were considered as two main steps of the reaction [6], [7], [8], [9]. Wender realized this synthesis in the liquid phase using a mixed catalyst comprised of potassium methoxide and copper chromite under mild conditions of 373–453 K and 5.0–6.5 MPa, and gave high methanol synthesis rates and high one-pass CO conversions [7], [8]. CO + CH₃OH $\overset{RONa}{=}$ HCOOCH₃ HCOOCH₃ + 2H₂ $\overset{Solid Cat.}{=}$ 2CH₃OH - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - CO + 2H₂ = CH₃OH

Similar to the BNL method, in this liquid-phase reaction process, CO₂ and H₂O act as poisons to the alkoxide catalyst (RONa) and must be completely removed from syngas, making commercialization of low-temperature methanol synthesis impossible now.

The present authors proposed a new method of low-temperature synthesis of methanol from CO₂/H₂ on a Cu-based oxide catalyst using ethanol as a promoter, by which methanol was produced at 443 K and 3.0 MPa [10]. This new process consisted of three steps: (1) formic acid synthesis from CO₂ and H₂; (2) esterification of formic acid by ethanol to ethyl formate; and (3) hydrogenation of ethyl formate to methanol and ethanol. Considering the fact that water–gas shift reaction is easily conducted on Cu/ZnO catalysts [11], [12], a new route of methanol synthesis from CO/H₂ containing CO₂ is proposed, as a more practical way of methanol synthesis. It consists of the following fundamental steps:CO + H₂O = CO₂ + H₂ CO₂ + H₂ + ROH = HCOOR + H₂O HCOOR + 2H₂ = CH₃OH + ROH - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - CO + 2H₂ = CH₃OH

As formic acid was not detected in the products, the present authors suggested the reaction path as steps (5) and (6) above. The present authors reported that ethanol solvent, as well as Cu/ZnO catalyst, remarkably lowered the reaction temperature of methanol synthesis from syngas containing carbon dioxide [13]. Furthermore, as ethanol solvent contained a small amount of water, it is considered that this new process can use low-grade syngas containing carbon dioxide and water without purification as CO₂ and H₂O were involved in the reaction steps above. As expected, low reaction temperature realized high CO conversion as 50–80% [14], [15], [16].

Until now, many different views have been proposed regarding the reaction mechanism and the nature of the surface active sites of Cu-based catalysts for methanol synthesis reactions [17], [18], [19], [20], [21]. Especially, for the important Cu/ZnO-based catalysts, different types of interaction between Cu and ZnO have been proposed [22], [23], [24], [25], [26], [27], [29]. Many studies showed that the reaction mechanism and the nature of the surface active sites of Cu/ZnO for methanol synthesis reaction depended on the component of catalyst system and the preparation conditions of the catalysts [28], [29], [30].

Although many studies showed a reaction mechanism of methanol synthesis on Cu/ZnO catalysts, the methanol synthesis at low temperatures from syngas (CO/CO₂/H₂) on Cu/ZnO using ethanol promoter is a new process, and study of its mechanism has not been done until now. In the present work, the reaction mechanism of low-temperature methanol synthesis above is investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). We find that low-temperature methanol synthesis from syngas (CO/CO₂/H₂) using ethanol promoter is proceeded via reactive intermediates of ethyl formate, which is produced by the reaction of formate with ethanol on Cu/ZnO, and the formation of ethyl formate is a key step of this reaction.

Section snippets

Experimental

The catalyst was prepared by the conventional coprecipitation method. An aqueous solution containing copper and zinc nitrates (Cu/Zn in molar ratio = 1) and an aqueous solution of sodium carbonate were added simultaneously to 300 of ml water with constant stirring. The precipitation temperature and pH value were maintained at 333 K and 8.5, respectively. The obtained precipitate was filtrated and washed with distilled water, followed by drying at 393 K for 6 h and calcination in air at 623 K

Adsorption of syngas on Cu/ZnO

The catalyst was exposed to syngas (CO/CO₂/H₂/Ar) at 443 K and atmospheric pressure for 3 h, and then swept in helium for 20 min. The DRIFT spectra were recorded for different adsorption times in Fig. 2. Assignment of the bands for adsorption species was made by analogy with the spectra of known compounds and by comparison with published literature. A summary of assignments is given in Table 1. The bands of the bidentate formate species on ZnO (H single bond COOZn), the bidentate formate species on Cu (HCOO

Formation and reactivity of formate adsorption species on Cu/ZnO

The studies of Fisher and co-workers [31], Fujita et al. [21], and Waugh [17] show that the formate and carbonate species were formed when Cu-based catalyst was exposed to CO₂/H₂ or CO/CO₂/H₂ at low temperatures, and carbonate species were transformed to stable formate species by hydrogenation reaction with temperature increase. In the study, when Cu/ZnO catalyst was exposed to CO/CO₂/H₂, the reactions proceeded according to Eqs. (7), (8). The carbonate and formate species were formed by the

Conclusions

The reaction mechanism of a new low-temperature methanol synthesis method from syngas (CO/CO₂H₂) using ethanol promoter was studied by using DRIFTS. The results show that formate adsorption species are formed by exposing Cu/ZnO catalyst to syngas (CO/CO₂/H₂), and the reactive intermediate ethyl formate is formed by the reaction of adsorbed formate with ethanol in the gas phase. Finally, gas-phase and physisorbed ethyl formate are reduced by hydrogen atoms on Cu to form gas-phase methanol.

The

Acknowledgment

Y.F. is thankful for the support from Venture Business Laboratory of Toyama University.

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In situ DRIFT study of low-temperature methanol synthesis mechanism on Cu/ZnO catalysts from CO2-containing syngas using ethanol promoter

Abstract

Introduction

Section snippets

Experimental

Adsorption of syngas on Cu/ZnO

Formation and reactivity of formate adsorption species on Cu/ZnO

Conclusions

Acknowledgment

Chem. Eng. Sci.

Appl. Catal.

Appl. Catal.

Fuel Process. Technol.

Appl. Catal.

J. Catal.

Catal. Commun.

Catal. Today

J. Catal.

Appl. Catal.

Catal. Today

J. Catal.

J. Mol. Catal. A: Chem.

J. Catal.

Appl. Catal. A

Appl. Catal.

J. Catal.

Appl. Catal. A

J. Mol. Catal.

J. Catal.

J. Catal.

A.T. Bell. J. Catal.

In situ DRIFT study of low-temperature methanol synthesis mechanism on Cu/ZnO catalysts from CO₂-containing syngas using ethanol promoter