Mechanistic insights on the ethanol transformation into hydrocarbons over HZSM-5 zeolite

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Abstract

HZSM-5 zeolite was found to be a very stable catalyst for the ethanol transformation into hydrocarbons at 350 °C and 30 bar total pressure. It was found to maintain high activity for C3+ hydrocarbons formation with time-on-stream in spite of a near total loss of Brønsted acidity, 92% loss of microporosity and high coke content deposited inside its micropore volume. The same solid, passivated with TEOS was tested in the same conditions and it was found that the treatment slightly improved the catalytic performance of the zeolite, even if similar losses of acidity and microporisty were determined after reaction. This shows that C3+ hydrocarbons’ formation does not occur at the external surface. Alkyl aromatic hydrocarbons were found occluded in the zeolite structure after reaction, detected by IR spectroscopy analysis and by CH2Cl2 extraction after solubilization of the structure with HF solution. EPR-CW analysis of both coked samples proved existence of free radicals. This last technique could provide us further enlightening of the ethanol transformation mechanism.

Introduction

HMFI zeolite is a well-known catalyst for the transformation of methanol into olefins (MTO) [1] and gasoline (MTG) [2], [3], [4]. It is also one of the most studied and claimed as one of the best catalysts for ethanol transformation into ethylene (BTE) [5], [6] and/or into higher hydrocarbons [7], [8], [9]. In methanol's case, the most appealing suggested mechanism nowadays is the hydrocarbon pool mechanism [10], [11]. It has been shown that the adsorbate representing the hydrocarbon pool may have common characteristics with ordinary coke [11] and therefore a deeper study of the coke nature might help elucidating the C3+ hydrocarbons formation mechanism.

In our previous study [12], in comparison with HBEA and HFAU zeolites, having similar number of Brønsted acid sites, HMFI zeolite was also found to be the more stable and performing for ethanol transformation into C3+ hydrocarbons (particularly aliphatic and aromatic families). The best catalytic property of this zeolite is due to its capacity to maintain high activity in C3+ hydrocarbons with time-on-stream and this in spite of very low residual acidity and high coke content deposited inside its micropore volume.

In the present work, ethanol transformation into hydrocarbons was carried out on a HMFI zeolite having a large amount of Brønsted acid sites, its aim being to go thoroughly into the comprehension of the mechanism of these hydrocarbons’ formation. In the 80s a free radical mechanism for the methanol transformation was suggested and supported by EPR evidence [13] but contested, for it remained unclear if they were essential to the reaction or not. It seems clear that a further study of the deactivation process and composition of the carbon deposit nature could be helpful to the enlightening of the ethanol transformation mechanism.

The subject of our work was approached by studying the nature of the products and of the carbonaceous species by using different spectroscopy techniques, such as IR and EPR analysis.

Section snippets

Experimental

HMFI (Si/Al ratio = 16) zeolite is a commercial material from Zeolyst International. The sample was compacted, crushed and sieved to achieve 0.2–0.4 mm homogeneous particles. Before catalytic testing, the solids were calcinated in situ under a nitrogen flow rate of 3.3 L h−1, at 773 K and a total pressure of 30 bar.

Passivation of the external surface of the previously mentioned HZSM-5 zeolite was done using Tetra Ethyl Ortho Silicate (TEOS). The parent zeolite was previously calcinated at 823 K during 1

Results and discussion

Under our operating conditions ethanol was totally converted into ethylene, C3+ hydrocarbons, water (stoichiometric quantity from ethanol dehydration—about 39 wt%) and traces of diethyl ether (Fig. 1). Initially, ethanol is converted into ethylene and diethyl ether by dehydration reactions, followed by their transformation into higher hydrocarbons.

During all the run (25 h), no deactivation for ethanol conversion was observed. Ethylene began to be detected after 5 h of Time-On-Stream (TOS) and its

Conclusions

Ethanol transformation into hydrocarbons has been studied on HZSM-5 (16) zeolite passivated and non-passivated with TEOS. It was found that TEOS passivation slightly improves the catalytic properties of HZSM-5 (16) zeolite for C3+ hydrocarbon formation by limiting the deactivation of the catalyst. These zeolites presented high carbon content after 25 h reaction but they still keep high activity for C3+ hydrocarbon production, despite a near complete loss of Brønsted acidity and microporosity.

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