Elsevier

Applied Catalysis A: General

Volume 646, 25 September 2022, 118860
Applied Catalysis A: General

Tuning the acidity by addition of transition metal to Mn modified hollow silica spheres and their catalytic activity in ethanol dehydration to ethylene

https://doi.org/10.1016/j.apcata.2022.118860Get rights and content

Highlights

  • Zirconia-silica hollow spheres with Mn core were prepared by a two-step method.

  • Tunning the Mn core with transition metal induces changes in the material acidity.

  • MnMe@SiZr catalysts show good performance in ethanol dehydration to ethylene.

  • MnNi@SiZr presents the best catalytic results and the highest number of acid sites.

Abstract

Due to the currently worldwide petrochemical feedstock shortage, the ethylene synthesis from renewable non-oil sources becomes of high interest. The catalysts were prepared in two steps: (i) formation of spheres containing the carbon-coated Mn core by hydrothermal method, (ii) formation of the Si-Zr oxide shell by sol-gel method. The prepared catalysts were characterized by N2 physisorption, SEM-EDX, XRD, XPS, and NH3-TPD. The catalytic results have shown that Fe, Zn or Ni modified Mn core exhibited superior activity compared to the catalysts containing only Mn in the core. With 75% yield and 98% ethanol conversion at 350 °C and WHSV of 1.4 h–1, MnNi@SiZr was the best catalyst. These results are due to an increased number of acid sites compared to the other materials and an optimal ratio of weak/medium acid sites. Our findings suggest new lines for developing active and stable catalysts for ethylene synthesis from ethanol.

Introduction

Dehydration of ethanol (EtOH) to ethylene is of major interest for sustainable development. [1], [2] Ethylene is the most widely produced organic compound in the chemical industry and represents the feedstock for about 30% of all petrochemicals worldwide production. [3] Thus, the ethylene synthesis from renewable non-oil feedstocks [4] is of high interest and represents an environmentally friendly alternative.

The most common route of dehydration of EtOH is carried out in gas phase in the presence of solid acid catalysts, such as metal oxides, [5], [6] zeolites [7], [8], [9], [10] and heteropolyacids. [11], [12], [13], [14] The first studies on the dehydration of ethanol to ethylene used γ-alumina as catalyst. [15] The ethanol dehydration reaction is an endothermic reaction, and its industrial application requires the use of high temperatures (between 180 and 500 °C). Obviously, research is going in the direction of lowering this temperature, which has an increased energy cost, through the use of catalysts, but the biggest drawback is that at lower temperatures there are also concurrent reactions with the generation of diethyl ether or acetaldehyde which reduce the ethylene yield. [16], [17], [18], [19], [20] According to literature, diethyl ether (DEE) is formed by EtOH dehydration at low temperatures, while ethylene formation is favoured by EtOH dehydration at higher temperatures. [21] Therefore, the main challenge for the scientific community and industry is to obtain ethylene at lower temperature, avoiding the formation of DEE. This can be done by tunning the acidity of the catalysts, since ethylene formation occurs by cleavage of two types of bonds C–O and C–H on a pair of acid (H+) and base (B) sites, while DEE needs only the Brönsted acid sites. [22], [23] For instance, Cosimo et al. showed that the presence of Lewis and Brönsted site pairs on MgO–Al2O3 catalysts was mandatory for the conversion of ethanol to ethylene and DEE. [24] The strength of these sites is also very important, because the strong acid sites promote the coke formation. One possibility to avoid this is to design catalysts to generate more weak acid sites (Lewis’s acid sites) or to decrease the amount of strong acid sites (Brönsted acid sites).

From the data presented, it is obvious that the type and strength of acid centers involved in the catalytic conversion of alcohols is still a matter of intense debate in the scientific community. On one hand, the strong acid centres needed for high alcohol conversion are highlighted [25], [26], while on the other, further works pointed out the importance of a certain ratio of Lewis/Brönsted sites. [27] Thus, the discussion related to the reaction mechanisms for ethanol dehydration on acidic heterogeneous catalysts, as well as the type and nature of the acid sites, remains open.

Recent exciting materials that have sparked particular interest include hollow mesoporous silica spheres (HMS). These materials have an inner cavity protected by the mesoporous silica shell, which gives them unusual structural features such as low density, high surface area and good permeability. [28], [29] The main potential applications of hollow silica spheres are related to drug storage and delivery, separation and catalysis. [30], [31], [32], [33] In catalysis, these materials are more interesting due to their mesoporous shell structure and hollow interior, which allow reactants to diffuse more rapidly to active sites found on both the inner and outer surfaces of the shell. [34], [35] Furthermore, it was demonstrated that hollow silica–alumina composite spheres showed a surprisingly Brönsted acidity that promotes them as solid acid catalyst with high activity for hydrolytic dehydrogenation of ammonia borane. [36].

In order to tune the acidity of hollow sphere catalyst, this study investigates zirconia-modified silica hollow spheres having in the inner cavity bimetallic nanoparticles of the type Mn-Me (1–1 molar ratio), where for Me, low-cost first-row transition metals (Fe, Ni, Zn) were considered.

The synergy between all the material components generates new material properties, assisting a certain direction in the reaction and therefore different reactivity and selectivity. The acidic properties of these catalysts have been systematically characterized by temperature-programmed desorption of ammonia (NH3-TPD), complemented by other characterization techniques to create an overall picture of the catalyst properties (i.e., BET surface area, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The catalytic activity of the modified HMS in the dehydration of ethanol to ethylene was investigated.

Section snippets

Preparation of MnMe carbon spheres (MnMe@C)

In a stainless-steel autoclave with a Teflon jacket of 120 mL, D-glucose monohydrate (C6H12O6·H2O, 18.9 mmol) was dissolved in 30 mL of distilled water to form a clear solution. The mixture was hydrothermally treated (HT) at 180 °C for 4 h. After cooling to room temperature, depending on the desired material to be obtained, 0.945 mmol manganese acetate (Mn(Ac)2) and 0.945 mmol Me nitrate (Me: Fe, Ni, Zn) were added to the glucose solution and followed by another round of HT at 180 °C for 12 h.

Morphological, textural and structural properties of Mn@SiZr hollow spheres

The morphology of the materials was investigated using SEM and TEM techniques. As revealed by SEM, the materials prepared by the hydrothermal carbonization method (MnMe@C) presented diameters of the spheres in the range 100–300 nm, while in the case of MnMe@SiZr materials the dimension of spheres increases (see Fig. 1). This behaviour could be anticipated, given that the MnMe@C spheres are re-coated with a new layer of zirconia-modified silica, which causes the size of the spheres to increase.

Conclusions

In this study, silica hollow spheres with Mn core were prepared to be studied in ethanol dehydration. Preparation of hollow spheres was successful, as confirmed by SEM images and according to XRD and XPS analyses, the outer shell of the spheres being formed by a mixture of silica, zirconia and SiZrO4 oxides in miscellaneous ratio. The modification of Mn core with Zn, Fe, Ni determined different ratios between the oxidation states of Mn, and also different ratios between the oxides which form

CRediT authorship contribution statement

A.B, M.F and F.N. synthesized the materials and performed all the catalytic tests. M.M.T, F.N, S.N, M.F have performed the characterization (XRD, TPD, BET, XPS, RAMAN) and A.K. obtained the TEM micrographs. All the authors were involved in writing the paper.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by a grant of the Ministry of Research, Innovation and Digitization, CNCS/CCCDI – UEFISCDI, project number PN-III-P1–1.1-TE-2016–2116 and PN-III-P1–1.1-TE-2019–1969, within PNCDI III and to the Core Program 2019–2022 (contract 21 N/2019).

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