Activation of n-pentane while prolonging HZSM-5 catalyst lifetime during its combined reaction with methanol or dimethyl ether

Highlights

• The combined reaction of n-pentane with methanol or dimethyl ether is studied.

• Synergies between cracking and dual cycle mechanisms over HZSM-5 are analyzed.

• The catalytic cracking of n-pentane is activated by the presence of the oxygenates.

• Dimethyl ether is more reactive than methanol for the paraffin activation.

• Catalyst lifetime during the conversion of oxygenates is enhanced with n-pentane.

Abstract

This work explores the synergies during combined reactions of n-pentane (nC₅) with oxygenates (methanol or dimethyl ether, OX). The experimental runs have been carried out in a packed bed reactor at 500 °C, using a high silica HZSM-5 zeolite-based catalyst with different oxygenate-to-n-pentane (OX/nC₅) ratios in the feed. A significant enhancement of the n-pentane conversion occurs for low OX/nC₅ ratios in the feed (0.1−0.25), especially when using dimethyl ether (DME). In addition, the presence of n-pentane reduces the rate of catalyst deactivation by coking during the conversion of oxygenates. These results have been explained on the grounds of a mechanistic interaction between the reactants: (1) the fast formation of methoxy and olefin intermediates from oxygenates, particularly from DME, could explain the promotion of n-pentane cracking, by facilitating the activation of the alkane by hydrogen transfer reactions; (2) the attenuation of deactivation during the conversion of oxygenates could be related to a lower extent of the arene cycle in the dual-cycle mechanism (limiting the polymethylbenzene formation). The analyses of used catalysts by means of temperature-programmed oxidation and confocal fluorescence microscopy have pointed out the higher reactivity of DME than that of methanol also for yielding coke structures.

Introduction

The catalytic cracking of paraffins is one of the most studied alternatives for producing olefins from the naphtha cut of the refineries (C₅₋₁₂) [1], because it has lower energy requirement and higher propylene selectivity than those of the steam/thermal cracking [2]. Catalytic cracking is carried out over acid microporous sieves, being HZSM-5 zeolite the most studied due to its shape selectivity that maximizes the olefin production. Promising and encouraging results have been reported on the cracking of paraffins of relatively long carbon chains, such as C₆₋₁₀ [[3], [4], [5]]. Nonetheless, the lightest paraffins contained in the naphtha (C₅) or in gaseous fractions (C₁₋₄) are much more difficult to crack [6]. High temperatures are recommended for promoting cracking but avoiding side reactions [7]. Steam catalytic cracking (SCC) has also been proposed for naphtha processing with enhanced selectivity to ethylene and propylene in the presence of water and high temperatures (up to 700 °C) [8]. However, the high temperature and presence of steam could also lead to an irreversible deactivation of zeolites by dealumination, with the corresponding increase of the catalyst makeup [9].

Within the possible process modifications for activating light paraffins and promoting a selective cracking to light olefins, Martin et al. [10,11] proposed the so-called coupled methanol and hydrocarbon cracking (CMHC). The first goal was to take advantage of the exothermic character of the methanol-to-olefins (MTO) process in order to overcome the energetic requirements of the catalytic cracking. The synergies between both reactions were also previously observed for a feed mixture of n-butane/methanol in the temperature range of 500−550 °C and low space time values [12]. The clearest evidences of CC paraffinic bond activation using methanol were reported by Yu et al. [13] in experiments with propane and labeled carbon reactants. They saturated the HZSM-5 zeolite with labeled methoxy species at very controlled conditions and made them react directly with propane, which confirmed a promoted hydride transfer pathway between both species. The combined reactions of paraffins and methanol were studied aiming for the intensification of propylene in the methanol-to-propylene (MTP) process. A significant enhancement of its yield was reported by co-feeding i-butane at 470−500 °C under conditions of full conversion of methanol [14]. Likewise, a positive synergy was found when the C₄₋₆ hydrocarbon byproducts were recirculated in the MTO process [15].

Coupling the reactions of paraffins and methanol means the coexistence of the two well-established mechanisms on the same acid sites of the HZSM-5 zeolite: the carbocationic mechanism of paraffin cracking [[16], [17], [18]] and the dual-cycle mechanism of MTO [19]. Chang et al. [20] related both mechanisms in combined reactions and suggested a faster route for the catalytic cracking of n-hexane due to the formation of intermediates from methanol. On the one hand, paraffin cracking mechanism follows a pathway involving the monomolecular cracking (via penta-coordinated carbonium ions that evolve towards light paraffins), the bimolecular cracking (via hydride transfer between the paraffin and an adsorbed carbenium ions) and the oligomerization-cracking (toward higher hydrocarbons and finally coke) [16,21,22]. On the other hand, in the dual-cycle mechanism of oxygenates, the methylation/β-scission of the olefin primary products (alkene cycle) is coupled with the methylation/dealkylation of methylbenzenes (arene cycle) through hydrogen transfer and cyclization pathways [19,23,24]. In this case, the formation of coke takes place from two different stages [25,26]: (i) the evolution of methylbenzene intermediates towards polyaromatic structures of coke and (ii) the olefin cyclization and condensation, which requires a high development of the first stage. Then, the acid properties (density and strength of the sites) and topology of the catalyst have a strong influence on the extent of each stage [27,28]. In this regard, HZSM-5 zeolite favors the diffusion of these methylbenzenes, thus allowing the two stages of coke formation to be distinguished [26].

Traditionally, dimethyl ether (DME) has been considered in thermodynamic equilibrium with methanol, assuming similar reactivities for both in the MTO process. Nonetheless, different studies have demonstrated that DME is more reactive than methanol [29]. Indeed, in-situ IR measurements pointed out its faster ability to form methoxy species, attributed to its higher affinity towards the Brønsted acid sites [30,31]. Despite the discrepancies in the literature about the formation of the first CC bond, Li et al. [32] reported that DME reacts faster than methanol with methoxy species, forming methoxy-methyl cation intermediates. Moreover, this is applicable to the reaction of methoxy species from DME with other hydrocarbons, as was observed for the methylation of benzene [33]. The reactivity of DME presents a renewed interest in the context of its transformation into olefins (DTO, dimethyl ether-to-olefins process) [34].

In this work, we have analyzed the combined reactions of n-pentane with methanol or DME, comparing the reactivity of both oxygenates. Our aim is to progress towards the understanding of the advantages of co-feeding paraffins (n-pentane) and oxygenates, linking the mechanisms that drive their individual reactions. For this, a catalyst based on a high silica HZSM-5 zeolite embedded in a mesoporous matrix of γ-Al₂O₃ has been used. The benefits of coupling paraffin and oxygenate conversions on the activation of the n-pentane (selected as the lightest and less reactive compound in naphtha) and on the attenuation of coke formation from oxygenates have been studied by comparing the reactivity of individual and combined reactions in a packed bed continuous reactor at 500 °C. The effect of the oxygenate-to-paraffin ratio in the feed on conversion, product distribution and on their evolution with time on stream is discussed.