Elsevier

Catalysis Today

Volume 351, 1 July 2020, Pages 68-74
Catalysis Today

Amorphous silica-alumina composite with regulated acidity for efficient production of hydrogen via steam reforming of dimethyl ether

https://doi.org/10.1016/j.cattod.2019.01.056Get rights and content

Highlights

  • Various amorphous silica-alumina composites were synthesized by hydrolytic method.

  • Amorphous silica-alumina composite was revealed an efficient solid acid for SRD.

  • A stable bifunctional catalyst with >99% DME conversion and H2 yield was obtained.

  • DME hydrolysis was revealed the rate-limiting step for two-step reactions of SRD.

Abstract

In this work, we demonstrate that the amorphous silica-alumina composite (ASA) is an efficient solid acid for the steam reforming of dimethyl ether (SRD). Thus, a series of ASA with different Si/Al molar ratios was synthesized via the modified hydrolytic method by using Al(NO3)3·9H2O and/or NaAlO2 as Al precursor and tetraethoxysilane as Si precursor, respectively. Irrespective of the synthesis conditions, all of ASA composites were in an amorphous nature. However, both the acidity and the surface area were significantly affected by the synthesis conditions, the extent of which depends on the specific precursor and Si/Al molar ratios. The ASA physically mixed with a commercial Cu/ZnO/Al2O3 was investigated as a bifunctional catalyst for SRD under the conditions of P=0.1 MPa and gas hourly space velocity of 4000 h−1. Results indicate that both the conversion of dimethyl ether (DME) and the hydrogen yield were strongly dependent on the acidic property of ASA and the ASA to Cu/ZnO/Al2O3 ratios. Following the two-step consecutive mechanism of SRD, the catalytic results were well explained based on the acidic property of ASA, and the synergetic effect between the two catalytic functions was found to play a crucial role in determining the activity and stability of the bifunctional catalyst. Moreover, the DME hydrolysis was revealed to be the rate-limiting step for SRD reactions in the case of ASA as an acidic catalyst. By optimizing the synthesis conditions of ASA and the composition of the bifunctional catalyst, greater than 99% of DME conversion and hydrogen yield was achieved, and was kept for a time on stream of 66 h without an observable decrease. Thus, ASA is a very efficient solid acid for SRD.

Introduction

In recent years, dimethyl ether (DME), which characterizes in the gas-like property and liquid-storage density, low cost, high hydrogen content (13 wt.%), and low or no toxicity, is recognized as a promising candidate to on-site supply hydrogen in mobile applications such as the fuel cell vehicles. Moreover, in comparison with other hydrogen-production routes from DME such as the catalytic partial oxidation, steam reforming of DME (SRD) not only produces high-quality reformate but also can be performed at lower temperatures of 473–673 K [1,2]. Thus, after the pioneering works of Sobyanin at the beginning of the 2000s [3,4], investigations on SRD in the aspect of thermodynamics analyses [5,6], catalyst screening [[7], [8], [9], [10], [11], [12], [13]], on-board dynamic analysis [14], and reaction engineering including types of reactors [[15], [16], [17]] and kinetics [[18], [19], [20]], are increasingly reported.

It is commonly accepted that SRD (Eq. (1)) occurs via two-step series reactions, i.e., DME hydrolysis to methanol (Eq. (2)) followed by the steam reforming of the intermediate product of methanol (Eq. (2), SRM) [2]. Moreover, undesirable side reactions (Eqs. (4), (5), (6), (7), (8), (9)) may also occur, the extent of which depends on the catalyst and reaction conditions.CH3OCH3 + 3H2O = 2CO2 + 6H2 ΔH° = 135 kJ mol−1 SRDCH3OCH3 + H2O = 2CH3OH ΔH° = 37 kJ mol−1 DME hydrolysisCH3OH + H2O = CO2 + 3H2 ΔH° = 49 kJ mol−1 SRMCO2 + H2 = CO + H2O Reverse water-gas shift (RWGS)CO + 3H2 = CH4 + H2O MethanationCH3OCH3 → CH4 + CO +H2 DME decomposition2CO = C + CO2 Boudouard reactionCH3OCH3 → hydrocarbons DME to hydrocarbons(DTH)CH3OH → hydrocarbons Methanol to hydrocarbons(MTH)

According to the mechanism, a bifunctional catalyst with active sites for catalyzing both the hydrolysis of DME and SRM are required to complete SRD. Moreover, high hydrogen yields can be reasonably expected when the reaction rates of the DME hydrolysis and SRM are sufficiently high and well matched [2]. Thus, the screening of the two types of active components is extensively performed to improve the SRD performance of the bifunctional catalyst. For the steam reforming of the intermediate product of methanol, the Cu-based catalyst is concentrated [[7], [8], [9],[11], [12], [13],21] as a result of its sufficiently high activity and low price although other catalysts such as Ga2O3 [22], Mo2C [10], noble metals of Au [23], Rh [24,25], Pt [25], Pd [14,26], and the transitional metal of Ni [27] are also reported.

However, great challenges are given to the DME hydrolysis as a result of the lower kinetics rate at lower reaction temperatures and the inhibiting effect of water [14,21,28]. Although much effort has been paid on the development of alumina or zeolites as acid catalysts for SRD [2,21,[29], [30], [31], [32], [33]], the lower SRD activity of alumina due to a smaller amount of acidic sites and easily coking of zeolites originated from the stronger acidic sites lead either lower hydrogen yield or quick decrease of the DME conversion [2,14,[29], [30], [31], [32], [33]]. In fact, amorphous silica-alumina composite (ASA) has long been practiced as a solid acid catalyst for many reactions in the petrochemical and other industries such as hydrocracking and isomerization [34]. Indeed, the ASA shows a moderate acidity in comparison with alumina and zeolites although the origin of the Brønsted acidity has not been unequivocally established [35,36]. However, it is conclusive that the acidity of ASA can be adjusted, to a certain extent, by varying the silica to alumina ratios, the calcination temperatures, and the preparation methods of the material [[36], [37], [38]]. Thus, quite a few works on the preparation and catalytic applications of ASA have been reported in recent years [34,37,38]. In a recent work, mesoporous Cu-Al2O3 and Cu-SiO2-Al2O3 prepared by the evaporation induced self-assembled method was comparatively investigated as a bifunctional catalyst for SRD [39]. The mesoporous Cu-SiO2-Al2O3 shows clearly higher DME conversion and H2 yield than Cu-Al2O3 although it is easily deactivated.

Based on these understandings, in this work, we demonstrate that ASA is a highly efficient and very promising solid acid for SRD. Thus, a series of ASA was synthesized, and its synthesis parameters and Si/Al molar ratio were optimized. According to the two-step mechanism of SRD, the bifunctional catalyst was made by physically mixing ASA with a commercial Cu/ZnO/Al2O3 considering its reasonably high activity and stability for methanol synthesis. With the optimal ASA, the DME conversion and H2 yield of >99% were achieved over the bifunctional catalyst, and were kept for a time on stream (TOS) of at least 66 h without any observable decrease.

Section snippets

Synthesis of ASA

The ASA was synthesized via the modified hydrolytic method as reported in the reference [40]. To regulate the porous property of ASA, cetyltrimethylammonium bromide (CTAB) was added as a template during the synthesis. For a typical synthesis, 2.0 g CTAB was dissolved in 50 ml deionized water at the ambient temperature. To the solution, 0.03 mol NaAlO2 was added under vigorous stirring. Then, the absolute ethanol solution of 0.03 mol tetraethoxysilane (TEOS) was slowly added. After aging at

Structural and textural properties

The XRD patterns of the ASA are shown in Fig. 1. For all of the samples, only a broad peak centered at 2θ of about 25° was clearly observed, which is easily assigned to the characteristic diffraction of the amorphous silica. Moreover, there was still no any diffraction from aluminum species even for the sample with the highest content of alumina (ASA-SN14), suggesting that aluminum species are well distributed in the matrix of silica. When the broad peak was concerned, its intensity and span

Conclusions

In summary, a series of amorphous ASA composites was synthesized, and the ASA physically mixed with a commercial Cu/ZnO/Al2O3 was studied as a bifunctional catalyst for SRD. Independent on the synthesis conditions, all of the ASA composites were amorphous, and no any crystallized aluminum species were formed even for the ASA with the highest amount of aluminum (Si/Al molar ratio of 1/4). However, the BET surface area (213 - 741 m2⋅ g−1), the total pore volume (0.55 - 1.78 cm3⋅ g−1), and the

Declarations of interest

None.

Acknowledgments

The financial supports by the National Natural Science Foundation of China (21636006), Shaanxi Innovative Team of Key Science and Technology (2013KCT-17), and the Fundamental Research Funds for the Central Universities (GK201901001) are highly appreciated.

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