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Selectivity control through fundamental mechanistic insight in the conversion of methanol to hydrocarbons over zeolites
Microporous and Mesoporous Materials 136 (2010) 33–41
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Selectivity control through fundamental mechanistic insight in the conversion of methanol to hydrocarbons over zeolites Shewangizaw Teketel a, Unni Olsbye a, Karl-Petter Lillerud a, Pablo Beato b, Stian Svelle a,* a b
inGAP Center for Research Based Innovation, Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern, N-0315 Oslo, Norway Haldor Topsøe, Nymøllevej 55, DK-2800 Kgs. Lyngby, Denmark
a r t i c l e
i n f o
Article history: Received 18 June 2010 Received in revised form 16 July 2010 Accepted 24 July 2010 Available online 30 July 2010 Keywords: Alkenes Reaction mechanism Methanol-to-gasoline Zeolites ZSM-22
a b s t r a c t In this paper, the methanol to hydrocarbon (MTH) reaction mechanism is studied over H-ZSM-22 zeolite catalyst. The H-ZSM-22 catalyst is chosen due to its narrow one-dimensional 10-ring straight channels which are likely to suppress the space demanding hydrocarbon pool type mechanism and favor product formation from alkene cracking and methylation. The study was performed using isotope switch experiments from 12C to 13C methanol at different times on stream. The reactivity of the gas phase products and hydrocarbons trapped in the channels of H-ZSM-22 is determined. The result obtained for the HZSM-22 catalyst is compared with other zeolite/zeotype topologies. The study shows that the aromatics based hydrocarbon pool mechanism, which produces sizable amounts of light alkenes including ethene is suppressed over H-ZSM-22 catalyst. The methanol conversion and selectivity is controlled by the alkene cracking and methylation pathway, yielding an aromatics free product, rich in branched alkenes. This contribution addresses the possibility of controlling product selectivity based on intimate knowledge about the MTH reaction mechanisms over zeolite/zeotype materials. Reaction mechanism is shown to be one parameter that determines the selectivity in the MTH reaction, and this hint could be useful in rational design of catalysts based on mechanistic insight, thereby enhancing the selectivity for the desired products to the ultimate goal of 100%. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Developing a process in which only the desired products are formed is a challenge in catalysis. Thus, in this century the main focus in catalysis is shifting towards ‘‘green chemistry”; that is obtaining 100% selectivity for the desired product (s). In contrast to the last century where catalyst activity was the central issue; which was aimed at increasing turnover rates [1,2]. This is due to environmental impacts and cost of disposing of undesired products. This makes selectivity control a key issue in catalysis. Recent studies have revealed that several molecular components such as surface structure, surface composition, oxidation state, charge transport, reaction intermediates, adsorbate mobility and adsorbate-induced restructuring influence the selectivity of heterogeneous reactions on single-crystal model surface and collide nanoparticles [3]. The conversion of methanol to hydrocarbons (MTH) over zeolite catalysts has received increasing attention during the last decade. This is due to the potential of the process as the final step in the upgrading of any gasifiable carbon-rich feedstock, such as natural gas, coal, and biomass into value added products. The technology is versatile; depending on the catalyst and process * Corresponding author. Tel.: +47 22 85 54 54; fax: +47 22 85 54 41. E-mail address: [email protected] (S. Svelle). 1387-1811/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2010.07.013
conditions, the products may be light alkenes (methanol to olefins – MTO, or methanol to propene – MTP), or high octane gasoline (MTG) [4]. In the past years, several reaction mechanisms have been proposed for the MTH reaction over zeolitic materials [5]. Dessau studied the MTH reaction over H-ZSM-5 catalyst and reported that the reaction is cocatalyzed by the presence of alkenes and the net methanol conversion proceeds through methylation and cracking of the alkenes [6,7]. According to Dessau’s mechanism, an existing alkene molecule is repeatedly methylated by methanol molecules and forms large alkenes which then undergo cracking reaction. The cracking products may either oligomerize, undergo methylation by methanol or desorb from the catalyst, illustrated in Fig. 1 (cycle II, right panel). It was suggested that as little as a single alkene molecule could be enough to start the methanol conversion. In agreement with Dessau’s observation, Song et al. studied the MTH reaction over H-ZSM-5 and H-SAPO34 catalysts using highly purified reagents and observed that trace amounts of impurities in the methanol feed, in the carrier gas or in the solid catalyst itself play an important role for the MTH reaction [8]. Dessau’s MTH reaction mechanism considers ethene as a product obtained from secondary re-equilibration of primary olefins and not as a primary product obtained from methanol [7]. Another point to mention on Dessau’s mechanism it that aromatic species formed during the MTH reaction are only presented as products
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Fig. 1. Reaction mechanism for methanol conversion over H-ZSM-5 catalyst: cycle I, hydrocarbon pool type reactions via the lower methylbenzenes, exemplified by trimethylbenzene/toluene (left panel) and cycle II, alkene methylation/cracking via long chain alkenes, exemplified by hexene/propene (right panel), adapted from [9].
resulting from hydrogen transfer reaction, the mechanism does not explain the contribution of the aromatics to the product formation. Studies by Mole and coworkers over the H-ZSM-5 catalyst showed the cocatalytic effect of toluene for the MTH reaction [10,11]. Later, further investigation by Dahl and Kolboe pointed out the importance of retained species for the MTH reaction over H-SAPO-34 catalyst [12–14]. The study showed that species trapped in the cages of the catalyst act as reaction centers for the MTH reaction. The observation led Dahl and Kolboe propose the hydrocarbon pool reaction mechanism. According to the hydrocarbon pool mechanism organic species trapped in the zeolite/zeotype materials are repeatedly methylated with methanol molecules followed by light alkene splitting such as ethene and propene, as illustrated in Fig. 1 (cycle I, left panel). The mechanism explains formation of alkenes through an indirect route rather than direct coupling of methanol molecules. Detailed studies on the identity and activity of the hydrocarbon pool species have shown that polymethylbenzenes (methylated benzene molecules) act as the main reaction centers for the MTH reaction [9,15–20]. Unlike Dessau’s mechanism, light alkene formation including ethene from the hydrocarbon pool species is well documented and there is general consensus about the importance of the hydrocarbon pool mechanism over a limited number of materials studied so far [15–19]. In the H-SAPO-34 catalyst methylnaphthalene (methylated naphthalene molecules) were shown to have activity lower than methylbenzenes, suggesting that depending on the catalyst topology other aromatic species can act as hydrocarbon pool species [21,22]. The activity of the methylbenzene hydrocarbon pool species is also dependent on the catalyst topology. For example, for H-SAPO-34 and H-beta catalysts higher methylbenzenes intermediates (penta and hexamethylbenzene) were shown to be more active than the lower methylbenzene intermediates (toluenetrimethylbenzene) [20,24]. This is ascribed to the relatively large space found in the catalysts giving enough room for the higher methylbenzene intermediates. Contrary to this, for H-ZSM-5 zeolite the lower methylbenzenes were found to be more active intermediates than the higher methylbenzenes [20,24], this is due to the steric limitation imposed by the relatively narrow pores of HZSM-5 catalyst. It is important to note that the alkenes formed from the hydrocarbon pool are controlled by the identity of the methylbenzene intermediate involved. For H-beta higher methylbenzene intermediates favor the formation of propene and butenes [20], and for H-ZSM-5 catalyst lower methylbenzene intermediates favor the formation of mainly ethene and some of propene [24]. Note that the accessibility/activity of the hydrocarbon pool species decreases with increasing catalyst deactivation, and they will transform into deactivation products [23] Recently, detailed mechanistic investigations of the process over the archetype H-ZSM-5 catalyst have shown that both the aromatic based (cycle I) and al-
kene based (cycle II) mechanisms operate simultaneously, the socalled dual cycle concept [9]. Cycle I, which is based on aromatic reaction centers located in the zeolite voids in agreement with the generally accepted hydrocarbon pool mechanism [12–14], is the principal source of ethene. Cycle II, which is based on repeated alkene methylation and cracking steps according to the scheme originally proposed by Dessau [6,7], yields C3+ alkenes, but only minor amounts of ethene. Thus, cycle I must contribute significantly to the product formation in catalysts that yield sizable amounts of ethene. Herein, we manipulate the relative contribution from these cycles to the product formation, thereby controlling the selectivity. The H-ZSM-22 catalyst was chosen as a model system due to its one-dimensional non-interacting 10-ring pore structure with dimensions of 0.46 0.57 nm [25] (Fig. 2), which is likely to impose severe sterical constraints on the more space demanding cycle I. We exemplify the reaction mechanism as one parameter that controls selectivity for the MTH reaction, which could be used as a hint for rational designing of catalysts for the desired products with ultimate goal of ‘‘green chemistry”. 2. Experimental 2.1. Catalyst synthesis and characterization Details on the synthesis of H-ZSM-22 catalyst is presented in the literature [26,27]. Briefly, aqueous solutions of potassium hydroxide, aluminum sulfate and diaminooctane were mixed and
Fig. 2. The monodimensional 10-ring H-ZSM-22 (TON) topology.
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colloidal silica was added under vigorous stirring. The resulting gel, with a composition of 8.9 K2O: Al2O3: 90 SiO2: 3 K2SO4: 27.3 DAO: 3588 H2O, was aged for 24 h at room temperature, transferred to a Teflon lined stainless steel autoclave, and crystallized in an oven with an inset that tumbles the autoclave (25 rpm) for 3 days at 160 °C. The product was recovered by filtration, washed, dried and calcined as described in reference [27]. The catalyst was characterized using XRD, SEM, ICP–AES, BET, 27Al NMR, and FT-IR (see Supplementary material).
2.2. Catalytic tests Catalytic tests were carried out at 400 °C over H-ZSM-22 (Si/ Al = 30) in a fixed bed reactor (i.d. 10 mm). Before each catalytic test, the catalysts were calcined in situ at 550 °C with a flow of pure oxygen for 1 h as described in [27]. Methanol was fed by passing He through a saturation evaporator (pMeOH = 130 mbar), WHSV = 2 g g 1 h 1. By using two separate and identical feed lines, it was possible to switch from 12C methanol (BDH Laboratory Supplies, >99.8% chemical purity) to 13C methanol (ICON, 99%) after a predetermined time of 12C methanol reaction. The isotopic compositions of effluent compounds and the material retained within the catalyst pores (liberated by dissolving quenched catalyst in HF and extraction with CH2Cl2, see below) were determined at increasing 13 C methanol reaction times.
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3. Results and discussion 3.1. Catalyst characterization results Except for FT-IR measurement results, details on characterization of the H-ZSM-22 catalyst was presented in a previous publication [27]. Briefly, XRD confirmed the crystallinity and phase purity of the product. SEM revealed needle shaped crystals of 2–3 lm length. Al-NMR showed that, both for as-made and calcined/ionexchanged samples, Al is located exclusively in the framework. BET surface area of the catalyst was found to be 207 m2 g 1, which is in a good agreement with previous reports for the same structure [28,29]. ICP–AES revealed that the catalyst has a Si/Al ratio of 30. FT-IR measurement showed that the adsorption of CO led to a displacement of the frequency of the band associated to Brønsted acid sites by 320 cm 1. This shift is the measure of acid strength of the H-ZSM-22 catalyst and it is comparable with other zeolites [30–33], see Supplementary material. From characterization results, the material presented in this contribution is phase pure and has a high surface area. The crystal size and acid site density (Si/Al) of the material are in a desired range for the catalytic investigation of the methanol to hydrocarbon reaction and no extra framework Al was observed.
2.3. Analysis of reaction products 2.3.1. Online effluent analysis The reaction products were analyzed using automatic injection gas chromatograph connected directly to the reactor outlet by a heated transfer line. An Agilent 6890A GC with FID, equipped with a Supelco SPB-5 capillary column (60 m, 0.530 mm i.d., stationary phase thickness 3 lm) was used for the analysis. The temperature of the oven was programmed between 45 and 260 °C with a heating rate of 25 °C min 1 (hold time = 5 min at 45 °C and 16 min at 260 °C).
2.3.2. Offline effluent analysis The isotopic composition of effluent compounds was determined using a manual injection HP 6890 Gas Chromatograph equipped with a GS–GASPRO column (60 m, 0.32 mm) and a HP5973 mass selective detector. The temperature of the oven was programmed between 100 and 250 °C with a heating rate of 10 °C min 1 (hold time = 10 min at 100 °C and 15 min at 250 °C). The compounds were identified by comparing with the mass spectral library of the NIST98 database.
2.3.3. Analysis of retained material Organic species trapped in the channels of the H-ZSM-22 catalyst during the MTH reaction were analyzed following dissolution procedures as described in the literature [9]. Typically, 20 mg of the used catalyst was transferred into a screw-cap Teflon vial and dissolved using 1 ml of 15% HF. The organic phase was extracted by 1 ml of dichloromethane (CH2Cl2) having hexachloroethane (C2Cl6) as internal standard. The extract was analyzed using an Agilent 6890 N gas chromatograph connected to an Agilent 5793 mass selective detector equipped with a HP-5MS column (60 m, 0.25 mm i.d., stationary phase thickness 0.25 lm). The temperature of the oven was programmed between 50 and 300 °C with a heating rate of 10 °C min 1 (hold time = 3 min at 50 °C and 15 min at 300 °C). The compounds were identified by comparing with the mass spectral library of the NIST98 database.
Fig. 3. Methanol conversion (a) and Product selectivities (b) observed for H-ZSM-22 (400 °C and WHSV = 2 g g 1 h 1), data adapted from [27].
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3.2. Catalytic test results Fig. 3 displays the methanol conversion (a) and the product distribution (b) as a function of time on stream over H-ZSM-22 catalyst at 400 °C [27]. The catalyst converted methanol to hydrocarbons for several hours, however, compared to the well studied H-ZSM-5 catalyst [34], H-ZSM-22 catalyst was less stable towards deactivation for the MTH reaction. H-ZSM-22 catalyst showed at least >95% selectivity for C3+ fraction and with time on stream the selectivity for C3+ fraction increased up to >98%. The observed high selectivity for the C3+ fraction suggests that cycle II type mechanism is dominant over cycle I type mechanism which forms sizable amounts of ethene (C2). Correspondingly, very little aromatics (1%) were observed in the H-ZSM-22 product, in contrast to H-ZSM-5 and H-beta, which yield substantial amounts of aromatics. The selectivity for C5+ fraction was very high, ranging between 45% and 70% with slight deactivation of the catalyst as shown in Fig. 3. C5– C12 hydrocarbons are the main constituents in gasoline fuel [35], thus the observed high selectivity for the C5+ fraction over H-ZSM-22 catalyst can be beneficial in producing transportation fuel from carbon-rich feedstock. The composition of C5+ fraction was closely inspected and branched alkenes were the most abundant (>70%) and, linear and cyclic alkenes were the second and third most abundant products [27]. Branching of hydrocarbons is very essential to boost the octane number. For example, trimethyl-butane (C7) and n-heptane (C7) have a research octane numbers (RON) 112.1 and zero, respectively [36]. Thus the branched C5+ hydrocarbons obtained over the H-ZSM-22 catalyst and the absence of aromatics species makes the material competitive in developing a process for the production of environmentally friendly non-aromatic high octane gasoline. For direct comparison of the product distribution of the H-ZSM22 (Si/Al = 30) catalyst with other topologies, H-ZSM-5 (Si/Al = 50),
H-beta (Si/Al = 45) and H-SAPO-34 ((Al + P)/Si = 11) catalysts were tested for the MTH reaction under identical conditions, displayed in Fig. 4. All catalysts have a reasonably similar and high density of acid sites (Si/Al 30–50) except H-SAPO-34, which has the common density of acid sites corresponding to one per cage. H-ZSM5, which has three dimensional 10-ring system, and H-beta, which has three dimensional 12-ring system, catalysts produce significant amounts of aromatic compounds. On the other hand, HSAPO-34 catalyst which has large cages connected with narrow 8-ring windows with 3.8 3.8 Å channel dimension, produces a product mainly composed of light alkenes (ethene and propene). H-ZSM-22 catalyst produces mainly C3+ hydrocarbons without aromatic products. Comparing the C3–C2 ratio of the catalysts, H-ZSM22 produces the highest amount of C3 relative to C2, giving a C3:C2 ratio of 6.8. H-ZSM-5, H-beta and H-SAPO-34 give C3:C2 ratios of 2.8, 2.1 and 1.4, respectively. The formation of sizable amounts of C2 produced over H-ZSM-5, H-beta and H-SAPO-34 catalysts is ascribed to the significant contribution of the aromatic based hydrocarbon pool type reaction mechanism (Cycle I) for methanol conversion over these catalysts, but not over the H-ZSM-22 catalyst. In agreement with the observed aromatic formation for the H-ZSM-5 and H-beta catalysts, high hydrogen transfer indexes (HTI) were observed, 0.6 and 0.9, respectively. However, for H-ZSM-22 and H-SAPO-34 catalysts the HTI was very low, 0.06 and 0.04 respectively. NMR investigation of the retained material in the H-SAPO-34 catalyst has shown that considerable amounts isobutane and isopentane are trapped in the cages of the material [37]. Thus, the observed low HTI for the H-SAPO-34 catalyst calculated from effluent C4 alkanes:C4 total ratios do not mean that there is less formation of aromatic species for cycle I type mechanism. However, the low HTI for the H-ZSM-22 catalyst suggests that there is less formation of hydrocarbon pool species for product formation via the cycle I type mechanism, in agreement with the selectivity and C3:C2 ratio mentioned above.
Fig. 4. MTH reaction over different catalysts, GC–FID chromatograms for H-ZSM-5 (Si/Al = 50), H-beta (Si/Al = 45), H-ZSM-22 (Si/Al = 30), and H-SAPO-34 ((Al + P)/Si = 11) catalysts, reaction carried out under identical conditions, full methanol conversion, WHSV = 2 g g 1 h 1 and 400 °C. The C2 and C3 peaks are indicated using ° and * on top of the peaks, respectively.
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3.3.
12
C/13C isotopic switching experiments
Isotopic switching is a commonly used method to distinguish active and inactive species in a working catalyst. For 12C/13C switches, more reactive species in the working catalyst will show faster 13C incorporation, giving rise to higher total 13C content in the compounds. Here, isotope switching from 12C to 13C methanol was performed after predetermined times of 12C methanol reaction over H-ZSM-22 catalysts. The switching to 13C methanol was performed at both full and low methanol conversion. The switching to 13C methanol at full methanol conversion was performed after 2, 5 and 18 min of 12C methanol reaction, followed in each case by 0.5, 1.0 and 2.0 min of 13C methanol reaction, hereafter denoted as the 2, the 5 or the 18 min experiments. The switching to 13C methanol reaction at low methanol conversion was performed at 46% and 58% methanol conversion, hereafter referred as the 46% and the 58% conversion experiments. The results of the switching experiments are presented in Figs. 5 and 6. Fig. 5 displays the total 13C contents in the effluent and retained material when switching from 12C to 13C methanol over H-ZSM-22 at 400 °C. Note that under these conditions the catalyst had full
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methanol conversion and the only difference is that for the 2 and the 5 min experiments the switching was performed at the early stage of the reaction where new aromatic species were building up, while for the 18 min experiment this stage of the reaction was passed (see Section 3.3.2). 12 13 C/ C switch experiments performed over deactivated catalysts are shown in Fig. 6. The switching was performed after the catalyst converted 12C methanol to hydrocarbons for several hours until it dropped to 58% and 46% conversion prior to the switching. Note that for these experiments the switching was performed after substantial amounts of hydrocarbons (coke) was trapped in the channels of the material compared to the switching performed at full methanol conversion. GC–MS chromatogram of the rationed material as a function of time on stream is presented in the Supplementary material. The basic findings of the isotope switch experiments are discussed below. 3.3.1. Effluent Inspection of the isotopic composition of the gas phase products shows that the 13C content in the alkenes is much higher than in the retained material, suggesting that the alkenes are much more
Fig. 5. Total 13C contents in the alkenes and retained material after 2 min (left panel), 5 min (middle panel) and 18 min (right panel) of 12C methanol reaction followed by 0.5, 1 and 2 min of 13C methanol reaction over H-ZSM-22 at 400 °C, WHSV = 2 g g 1 h 1 and full methanol conversion.
Fig. 6. Total 13C contents in the effluent alkenes at 58% (left panel) and 46% (right panel) 12C methanol conversion followed by 0.5, 1 and 2 min of 13C methanol reaction over H-ZSM-22 at 400 °C.
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reactive towards 13C than the retained material (methylated benzenes), clearly indicating that the alkene driven cycle II is all dominant in the product formation for this catalyst. This observation is in agreement with the high selectivity for C3+ alkenes (up to >98%) mentioned previously. All C3+ alkenes show identical 13C contents, indicative of the equilibrating nature of cycle II, whereas ethene lies consistently marginally below, which could point to a very minor contribution from the aromatics based cycle I. Cracking of hexene over H-ZSM-22 catalyst at high temperatures has been shown to yield ethene as cracking product [38] and since ethene contains a lot more 13C than the retained material during the MTH reaction over the H-ZSM-22 catalyst, the majority of the ethene probably comes from the cracking of higher alkenes. Previous studies have shown that the methylation rate of 12C alkenes with 13 C methanol increases in the order of ethene, propene and butene [39,40]. The slightly slower incorporation of 13C in ethene than the other C3+ alkenes could be rationalized by its slower methylation rate than the C3+ alkenes. If we assume cycle I mechanism produces small amounts of light alkenes (ethene, propene and butene), the C3+ alkenes will be methylated instantly and enter cycle II and display rapid increase in 13C content. Whereas ethene being the slowest in getting methylated will show some delay. This
will result in slight variation in the total 13C content between ethene and the C3+ alkenes. The rates of incorporation of 13C in the effluent products were identical for the 2, the 5 or the 18 min experiments. This suggests that the relative amounts of retained material (hydrocarbon pool) formed during 2, 5 or 18 min of the 12C methanol reaction prior to the switch to 13C methanol has no significant effect on the rate of incorporation of 13C in the effluent products. This in turn could mean that at these short times on stream there were no significant amounts of retained material (coke) in the channels of the H-ZSM-22 catalyst to influence the rate of incorporation of 13C by diffusion limitation (see Supplementary material). For the switching performed during low methanol conversion (58% and 46%, Fig. 6), ethene again contains slightly less 13C than the other alkenes whereas the C3+ alkenes show identical 13C contents as in the case of full methanol conversion. Looking at the 13C incorporation rate at 100, 58 and 46% conversion, it is clear that the incorporation rate of 13C decreases as the conversion decreases. This suggests that at long times on stream, enough hydrocarbons are retained in the channels of the H-ZSM-22 catalyst to affect the 13C incorporation by diffusion limitation.
Fig. 7. The distribution of 13C in hexamethylbenzene in the retained material after 2 min (top panel), 5 min (middle panel) and 18 min (bottom panel) of reaction followed by 0.5, 1.0 and 2.0 min of 13C methanol reaction. White bar: observed distribution and gray bar: random statistical distribution.
12
C methanol
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incorporation is lower for the methylnaphthalenes than for the methylbenzenes over the H-ZSM-22 catalyst. This suggests that the methylnaphthalenes are even less accessible and less active for alkene formation. The observation is very reasonable, considering the narrow channels of the material which imposes severe steric constraints as the retained material gets bigger. In the same manner as methylbenzenes, the accessibility of methylnaphthalenes decreases very rapidly with increasing deactivation of the H-ZSM-22 catalyst. We note that dimethylnaphthalene and trimethylnaphthalene display comparable total 13 C contents (accessibility) while methylnaphthalene was relatively more accessible. Fig. 8 displays the 13C content in the retained material after 18 min (for H-ZSM-5 (Si/Al = 50), H-beta (Si/Al = 120), and HZSM-22 (Si/Al = 30)) or after 25 min (for H-SAPO-34 ((Al + P)/ Si = 11)) of 12C methanol reaction followed by 2 min of 13C methanol reaction over four different catalysts. Note that slightly different reaction conditions and acid site densities have been employed in order to emphasize the representative activity of the methylbenzenes retained in the different catalysts. For H-ZSM-5, the lower methylbenzenes (xylene to trimethylbenzene) display the greatest reactivity towards 13C methanol, signifying their dominant role as intermediates in cycle I, whereas the opposite is observed for H-beta and H-SAPO-34, where the higher methylbenzenes (pentamethylbenzene and hexamethylbenzene) display the higher reactivity. As mentioned in the Introduction, the variation in the activity of the methylbenzenes is explained by the space offered by the catalyst topologies. For H-ZSM-22, however, the rate of 13C incorporation in the methylbenzenes is very low for all methylbenzenes, meaning that these species, and thus cycle I, play a minor role in alkene formation in this catalyst topology. Note that it is the highest methylbenzenes that display the greatest reactivity, as was seen for H-beta and H-SAPO-34, despite the narrow pore structure of H-ZSM-22. This could be rationalized by adsorption of the higher methylbenzenes on the external surface or at the pore mouths of the H-ZSM-22 catalyst, as previously proposed for hydrocracking reactions [41,42]. From the isotopic labeling data, we suggest that the aromatic species in the H-ZSM-22 catalyst, unlike other topologies, contribute only to a minor extent to the product formation and mainly behave as coke species which eventually leads to catalyst deactivation. The net methanol conversion proceeds through Cycle II mechanism, which is based on methylation and cracking of alkenes. The initial alkenes to start cycle II might originate from slight contribution of cycle I, or from impurities in the feed. There has been a question whether cycles I and II can run independently during the MTH reaction [9,20]. For H-ZSM-5 catalyst both cycles were shown to run simultaneously, the dual cycle concept. According to the report for the H-ZSM-22 catalyst, in order to sustain methanol conversion, cycle I needs continuous supply of hydrocarbon pool species through cyclizations and hydride transfers of alkenes from cycle II. This means that the two cycles cannot run independently on H-ZSM-22 catalyst [9]. In the present contribution, it is observed that cycle II can operate more or less independently over H-ZSM-22 catalyst. As a result of this the MTH
3.3.2. Retained material The isotope switch results of the retained material are presented in Figs. 5 and 7. Fig. 5 displays the rate of incorporation of 13 C in the effluent and retained material. Clearly, the retained material displays significantly slower incorporation of 13C than the effluent. This shows that cycle I type mechanism has a minor role over H-ZSM-22 catalyst, as mentioned in Section 3.3.1. The accessibility/activity of the retained material decreases with increasing the 12C methanol reaction time before the switching. The total 13C content in the retained material after 30 s of 13C methanol reaction (Fig. 5) was 20% for the 2 min experiment and decreased to 10% and less than 5% for the 5 and 18 min experiments respectively. A similar trend has previously been reported for H-SAPO-34 catalyst [23], except the decrease in accessibility is much quicker in the H-ZSM-22 catalyst. As mentioned in the introduction, the hydrocarbon pool species will transform into deactivation products with increasing the 12C methanol reaction time prior to the switching and this will lead to slow incorporation of 13C in the retained material [23]. Fig. 7 displays the distribution of 13C in hexamethylbenzene after 2, 5 and 18 min of 12C methanol reaction followed by 0.5, 1 and 2 min of 13C methanol reaction. Experimentally observed 13C distributions are shown in white bars and statistical random distributions calculated from the actual total 13C in hexamethylbenzene are shown in gray bars. The full 13C distributions for the rest of the retained material and effluent compounds are presented in the Supplementary material. The number of 13C in hexametylbenzene increases with increasing 13C methanol reaction time. Hexamethylbenzene consisting only 13C were more readily formed after short times on stream. For example, for the 2 min experiment after 1 and 2 min of 13C methanol reaction, hexamethylbenzene formed solely from 13C was observed, Fig. 7 top panel. For the 2, 5 and 18 min experiments, after 2 min of 13C methanol reaction, the percentage of newly formed hexamethylbenzene was 16%, 12% and 0%, respectively. This indicates that for the 2 and 5 min experiments the reaction was still at the early stage where aromatic species were building up. Whereas for the 18 min experiment, the early stage of the reaction was passed. This in turn means that the relatively higher total 13C content after the 2 and 5 min of 12C methanol reaction in the retained material (Fig. 5) is due to both the buildup of aromatic species solely from 13C methanol and the accessibility of the existing aromatic species. The accessibility of hexamethylbenzene decreases with increasing deactivation of the H-ZSM-22 catalyst. For the 2, 5 and 18 min experiments followed by 2 min of 13C reaction the percentage amounts of inaccessible hexamethylbenzene (without 13C) was found to be 5%, 11% and 35%, respectively, Fig. 7. Among the retained material in the narrow channels of H-ZSM-22 catalyst, methylnaphthalenes are the most abundant followed by methylbenzenes, see GS–MS chromatograms in the Supplementary material. As mentioned in Section 1, depending on catalyst topology methylnaphthalenes can have activity for product formation. Table 1 presents the total 13C content in the retained methylnaphthalenes in the 2, 5 and 18 min experiments. Clearly, the rate of 13C
Table 1 Total 13C contents in the retained methylnaphthalenes, 2, 5 or 18 min of 12C methanol reaction followed by 0.5, 1 and 2 min of 13C methanol reaction over H-ZSM-22 at 400 °C, full methanol conversion. Compound
MNaphthalene DiMNaphthal TriMNaphthal
2 min
12
C reaction
5 min
12
C reaction
18 min
12
C reaction
30 s
1 min
2 min
30 s
1 min
2 min
30 s
1 min
2 min
19.8 12.6 10.8
29.5 23.3 23.2
38.2 36.4 39.5
10.2 5.8 6.0
15.0 10.8 10.9
35.6 20.5 19.23
1.5 1.1 1.4
2.0 1.9 1.9
2.6 2.1 2.7
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Fig. 8. Total 13C contents in the retained hydrocarbons after 18 min (H-ZSM-5 (Si/Al = 50), H-beta (Si/Al = 120) and H-ZSM-22 (Si/Al = 30)) or 25 min (H-SAPO-34 ((Al + P)/ Si = 11)) of 12C methanol reaction followed by 2 min of 13C methanol reaction over the three catalysts, H-ZSM-5 and H-beta data adapted from [24] and H-SAPO-34 data adapted from [23]. Reaction over H-ZSM-22 was carried out at 400 °C and for the other topologies at 350 °C.
reaction product over H-ZSM-22 catalyst is mainly controlled by cycle II type mechanism, yielding very high selectivity for C5+ hydrocarbons and very little amounts of C2 hydrocarbons. The hindrance of the cycle I type mechanism over H-ZSM-22 catalyst could be ascribed to the narrow channels of the materials which provides small room for the space demanding hydrocarbon pool species.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.micromeso.2010.07.013.
References 4. Conclusion In this paper the methanol to hydrocarbon reaction mechanism over H-ZSM-22 catalyst is studied. Unlike H-ZSM-5, H-SAPO-34, and zeolite H-beta catalysts, the aromatic compounds in the channels of H-ZSM-22 catalyst are almost inactive for methanol conversion through the aromatic based hydrocarbon pool type mechanism. The effluent alkenes are much more reactive towards 13 C methanol than the retained hydrocarbons during the MTH reaction over the H-ZSM-22 catalyst. The alkene cracking and methylation mechanism controls the product selectivity, demonstrating the possibility of controlling the product selectivity based on intimate knowledge about the reaction mechanism for the conversion of methanol to hydrocarbons. By carefully choosing a catalyst topology likely to favor product formation via alkene cracking and methylation, we have discovered a system that yields a non-aromatic product consisting primarily of branched C5+ alkenes, offering additional product flexibility in this process. Along the lines presented in the Introduction, understanding the molecular components that govern the reaction selectivity is vital for selectivity control. Here, reaction mechanism is highlighted as one parameter that controls selectivity in the MTH reaction over zeolite material. The investigation is useful in the fundamental understanding of selectivity control in the MTH reaction and it can potentially be used in rational designing of new catalysts for the process. Acknowledgements This publication is part of the inGAP Centre of Research-based Innovation, which receives financial support from the Norwegian Research Council under Contract No. 174893.
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Selectivity control through fundamental mechanistic insight in the conversion of methanol to hydrocarbons over zeolites Shewangizaw Teketel a, Unni Olsbye a, Karl-Petter Lillerud a, Pablo Beato b, Stian Svelle a,* a b
inGAP Center for Research Based Innovation, Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern, N-0315 Oslo, Norway Haldor Topsøe, Nymøllevej 55, DK-2800 Kgs. Lyngby, Denmark
a r t i c l e
i n f o
Article history: Received 18 June 2010 Received in revised form 16 July 2010 Accepted 24 July 2010 Available online 30 July 2010 Keywords: Alkenes Reaction mechanism Methanol-to-gasoline Zeolites ZSM-22
a b s t r a c t In this paper, the methanol to hydrocarbon (MTH) reaction mechanism is studied over H-ZSM-22 zeolite catalyst. The H-ZSM-22 catalyst is chosen due to its narrow one-dimensional 10-ring straight channels which are likely to suppress the space demanding hydrocarbon pool type mechanism and favor product formation from alkene cracking and methylation. The study was performed using isotope switch experiments from 12C to 13C methanol at different times on stream. The reactivity of the gas phase products and hydrocarbons trapped in the channels of H-ZSM-22 is determined. The result obtained for the HZSM-22 catalyst is compared with other zeolite/zeotype topologies. The study shows that the aromatics based hydrocarbon pool mechanism, which produces sizable amounts of light alkenes including ethene is suppressed over H-ZSM-22 catalyst. The methanol conversion and selectivity is controlled by the alkene cracking and methylation pathway, yielding an aromatics free product, rich in branched alkenes. This contribution addresses the possibility of controlling product selectivity based on intimate knowledge about the MTH reaction mechanisms over zeolite/zeotype materials. Reaction mechanism is shown to be one parameter that determines the selectivity in the MTH reaction, and this hint could be useful in rational design of catalysts based on mechanistic insight, thereby enhancing the selectivity for the desired products to the ultimate goal of 100%. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Developing a process in which only the desired products are formed is a challenge in catalysis. Thus, in this century the main focus in catalysis is shifting towards ‘‘green chemistry”; that is obtaining 100% selectivity for the desired product (s). In contrast to the last century where catalyst activity was the central issue; which was aimed at increasing turnover rates [1,2]. This is due to environmental impacts and cost of disposing of undesired products. This makes selectivity control a key issue in catalysis. Recent studies have revealed that several molecular components such as surface structure, surface composition, oxidation state, charge transport, reaction intermediates, adsorbate mobility and adsorbate-induced restructuring influence the selectivity of heterogeneous reactions on single-crystal model surface and collide nanoparticles [3]. The conversion of methanol to hydrocarbons (MTH) over zeolite catalysts has received increasing attention during the last decade. This is due to the potential of the process as the final step in the upgrading of any gasifiable carbon-rich feedstock, such as natural gas, coal, and biomass into value added products. The technology is versatile; depending on the catalyst and process * Corresponding author. Tel.: +47 22 85 54 54; fax: +47 22 85 54 41. E-mail address: [email protected] (S. Svelle). 1387-1811/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2010.07.013
conditions, the products may be light alkenes (methanol to olefins – MTO, or methanol to propene – MTP), or high octane gasoline (MTG) [4]. In the past years, several reaction mechanisms have been proposed for the MTH reaction over zeolitic materials [5]. Dessau studied the MTH reaction over H-ZSM-5 catalyst and reported that the reaction is cocatalyzed by the presence of alkenes and the net methanol conversion proceeds through methylation and cracking of the alkenes [6,7]. According to Dessau’s mechanism, an existing alkene molecule is repeatedly methylated by methanol molecules and forms large alkenes which then undergo cracking reaction. The cracking products may either oligomerize, undergo methylation by methanol or desorb from the catalyst, illustrated in Fig. 1 (cycle II, right panel). It was suggested that as little as a single alkene molecule could be enough to start the methanol conversion. In agreement with Dessau’s observation, Song et al. studied the MTH reaction over H-ZSM-5 and H-SAPO34 catalysts using highly purified reagents and observed that trace amounts of impurities in the methanol feed, in the carrier gas or in the solid catalyst itself play an important role for the MTH reaction [8]. Dessau’s MTH reaction mechanism considers ethene as a product obtained from secondary re-equilibration of primary olefins and not as a primary product obtained from methanol [7]. Another point to mention on Dessau’s mechanism it that aromatic species formed during the MTH reaction are only presented as products
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Fig. 1. Reaction mechanism for methanol conversion over H-ZSM-5 catalyst: cycle I, hydrocarbon pool type reactions via the lower methylbenzenes, exemplified by trimethylbenzene/toluene (left panel) and cycle II, alkene methylation/cracking via long chain alkenes, exemplified by hexene/propene (right panel), adapted from [9].
resulting from hydrogen transfer reaction, the mechanism does not explain the contribution of the aromatics to the product formation. Studies by Mole and coworkers over the H-ZSM-5 catalyst showed the cocatalytic effect of toluene for the MTH reaction [10,11]. Later, further investigation by Dahl and Kolboe pointed out the importance of retained species for the MTH reaction over H-SAPO-34 catalyst [12–14]. The study showed that species trapped in the cages of the catalyst act as reaction centers for the MTH reaction. The observation led Dahl and Kolboe propose the hydrocarbon pool reaction mechanism. According to the hydrocarbon pool mechanism organic species trapped in the zeolite/zeotype materials are repeatedly methylated with methanol molecules followed by light alkene splitting such as ethene and propene, as illustrated in Fig. 1 (cycle I, left panel). The mechanism explains formation of alkenes through an indirect route rather than direct coupling of methanol molecules. Detailed studies on the identity and activity of the hydrocarbon pool species have shown that polymethylbenzenes (methylated benzene molecules) act as the main reaction centers for the MTH reaction [9,15–20]. Unlike Dessau’s mechanism, light alkene formation including ethene from the hydrocarbon pool species is well documented and there is general consensus about the importance of the hydrocarbon pool mechanism over a limited number of materials studied so far [15–19]. In the H-SAPO-34 catalyst methylnaphthalene (methylated naphthalene molecules) were shown to have activity lower than methylbenzenes, suggesting that depending on the catalyst topology other aromatic species can act as hydrocarbon pool species [21,22]. The activity of the methylbenzene hydrocarbon pool species is also dependent on the catalyst topology. For example, for H-SAPO-34 and H-beta catalysts higher methylbenzenes intermediates (penta and hexamethylbenzene) were shown to be more active than the lower methylbenzene intermediates (toluenetrimethylbenzene) [20,24]. This is ascribed to the relatively large space found in the catalysts giving enough room for the higher methylbenzene intermediates. Contrary to this, for H-ZSM-5 zeolite the lower methylbenzenes were found to be more active intermediates than the higher methylbenzenes [20,24], this is due to the steric limitation imposed by the relatively narrow pores of HZSM-5 catalyst. It is important to note that the alkenes formed from the hydrocarbon pool are controlled by the identity of the methylbenzene intermediate involved. For H-beta higher methylbenzene intermediates favor the formation of propene and butenes [20], and for H-ZSM-5 catalyst lower methylbenzene intermediates favor the formation of mainly ethene and some of propene [24]. Note that the accessibility/activity of the hydrocarbon pool species decreases with increasing catalyst deactivation, and they will transform into deactivation products [23] Recently, detailed mechanistic investigations of the process over the archetype H-ZSM-5 catalyst have shown that both the aromatic based (cycle I) and al-
kene based (cycle II) mechanisms operate simultaneously, the socalled dual cycle concept [9]. Cycle I, which is based on aromatic reaction centers located in the zeolite voids in agreement with the generally accepted hydrocarbon pool mechanism [12–14], is the principal source of ethene. Cycle II, which is based on repeated alkene methylation and cracking steps according to the scheme originally proposed by Dessau [6,7], yields C3+ alkenes, but only minor amounts of ethene. Thus, cycle I must contribute significantly to the product formation in catalysts that yield sizable amounts of ethene. Herein, we manipulate the relative contribution from these cycles to the product formation, thereby controlling the selectivity. The H-ZSM-22 catalyst was chosen as a model system due to its one-dimensional non-interacting 10-ring pore structure with dimensions of 0.46 0.57 nm [25] (Fig. 2), which is likely to impose severe sterical constraints on the more space demanding cycle I. We exemplify the reaction mechanism as one parameter that controls selectivity for the MTH reaction, which could be used as a hint for rational designing of catalysts for the desired products with ultimate goal of ‘‘green chemistry”. 2. Experimental 2.1. Catalyst synthesis and characterization Details on the synthesis of H-ZSM-22 catalyst is presented in the literature [26,27]. Briefly, aqueous solutions of potassium hydroxide, aluminum sulfate and diaminooctane were mixed and
Fig. 2. The monodimensional 10-ring H-ZSM-22 (TON) topology.
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colloidal silica was added under vigorous stirring. The resulting gel, with a composition of 8.9 K2O: Al2O3: 90 SiO2: 3 K2SO4: 27.3 DAO: 3588 H2O, was aged for 24 h at room temperature, transferred to a Teflon lined stainless steel autoclave, and crystallized in an oven with an inset that tumbles the autoclave (25 rpm) for 3 days at 160 °C. The product was recovered by filtration, washed, dried and calcined as described in reference [27]. The catalyst was characterized using XRD, SEM, ICP–AES, BET, 27Al NMR, and FT-IR (see Supplementary material).
2.2. Catalytic tests Catalytic tests were carried out at 400 °C over H-ZSM-22 (Si/ Al = 30) in a fixed bed reactor (i.d. 10 mm). Before each catalytic test, the catalysts were calcined in situ at 550 °C with a flow of pure oxygen for 1 h as described in [27]. Methanol was fed by passing He through a saturation evaporator (pMeOH = 130 mbar), WHSV = 2 g g 1 h 1. By using two separate and identical feed lines, it was possible to switch from 12C methanol (BDH Laboratory Supplies, >99.8% chemical purity) to 13C methanol (ICON, 99%) after a predetermined time of 12C methanol reaction. The isotopic compositions of effluent compounds and the material retained within the catalyst pores (liberated by dissolving quenched catalyst in HF and extraction with CH2Cl2, see below) were determined at increasing 13 C methanol reaction times.
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3. Results and discussion 3.1. Catalyst characterization results Except for FT-IR measurement results, details on characterization of the H-ZSM-22 catalyst was presented in a previous publication [27]. Briefly, XRD confirmed the crystallinity and phase purity of the product. SEM revealed needle shaped crystals of 2–3 lm length. Al-NMR showed that, both for as-made and calcined/ionexchanged samples, Al is located exclusively in the framework. BET surface area of the catalyst was found to be 207 m2 g 1, which is in a good agreement with previous reports for the same structure [28,29]. ICP–AES revealed that the catalyst has a Si/Al ratio of 30. FT-IR measurement showed that the adsorption of CO led to a displacement of the frequency of the band associated to Brønsted acid sites by 320 cm 1. This shift is the measure of acid strength of the H-ZSM-22 catalyst and it is comparable with other zeolites [30–33], see Supplementary material. From characterization results, the material presented in this contribution is phase pure and has a high surface area. The crystal size and acid site density (Si/Al) of the material are in a desired range for the catalytic investigation of the methanol to hydrocarbon reaction and no extra framework Al was observed.
2.3. Analysis of reaction products 2.3.1. Online effluent analysis The reaction products were analyzed using automatic injection gas chromatograph connected directly to the reactor outlet by a heated transfer line. An Agilent 6890A GC with FID, equipped with a Supelco SPB-5 capillary column (60 m, 0.530 mm i.d., stationary phase thickness 3 lm) was used for the analysis. The temperature of the oven was programmed between 45 and 260 °C with a heating rate of 25 °C min 1 (hold time = 5 min at 45 °C and 16 min at 260 °C).
2.3.2. Offline effluent analysis The isotopic composition of effluent compounds was determined using a manual injection HP 6890 Gas Chromatograph equipped with a GS–GASPRO column (60 m, 0.32 mm) and a HP5973 mass selective detector. The temperature of the oven was programmed between 100 and 250 °C with a heating rate of 10 °C min 1 (hold time = 10 min at 100 °C and 15 min at 250 °C). The compounds were identified by comparing with the mass spectral library of the NIST98 database.
2.3.3. Analysis of retained material Organic species trapped in the channels of the H-ZSM-22 catalyst during the MTH reaction were analyzed following dissolution procedures as described in the literature [9]. Typically, 20 mg of the used catalyst was transferred into a screw-cap Teflon vial and dissolved using 1 ml of 15% HF. The organic phase was extracted by 1 ml of dichloromethane (CH2Cl2) having hexachloroethane (C2Cl6) as internal standard. The extract was analyzed using an Agilent 6890 N gas chromatograph connected to an Agilent 5793 mass selective detector equipped with a HP-5MS column (60 m, 0.25 mm i.d., stationary phase thickness 0.25 lm). The temperature of the oven was programmed between 50 and 300 °C with a heating rate of 10 °C min 1 (hold time = 3 min at 50 °C and 15 min at 300 °C). The compounds were identified by comparing with the mass spectral library of the NIST98 database.
Fig. 3. Methanol conversion (a) and Product selectivities (b) observed for H-ZSM-22 (400 °C and WHSV = 2 g g 1 h 1), data adapted from [27].
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3.2. Catalytic test results Fig. 3 displays the methanol conversion (a) and the product distribution (b) as a function of time on stream over H-ZSM-22 catalyst at 400 °C [27]. The catalyst converted methanol to hydrocarbons for several hours, however, compared to the well studied H-ZSM-5 catalyst [34], H-ZSM-22 catalyst was less stable towards deactivation for the MTH reaction. H-ZSM-22 catalyst showed at least >95% selectivity for C3+ fraction and with time on stream the selectivity for C3+ fraction increased up to >98%. The observed high selectivity for the C3+ fraction suggests that cycle II type mechanism is dominant over cycle I type mechanism which forms sizable amounts of ethene (C2). Correspondingly, very little aromatics (1%) were observed in the H-ZSM-22 product, in contrast to H-ZSM-5 and H-beta, which yield substantial amounts of aromatics. The selectivity for C5+ fraction was very high, ranging between 45% and 70% with slight deactivation of the catalyst as shown in Fig. 3. C5– C12 hydrocarbons are the main constituents in gasoline fuel [35], thus the observed high selectivity for the C5+ fraction over H-ZSM-22 catalyst can be beneficial in producing transportation fuel from carbon-rich feedstock. The composition of C5+ fraction was closely inspected and branched alkenes were the most abundant (>70%) and, linear and cyclic alkenes were the second and third most abundant products [27]. Branching of hydrocarbons is very essential to boost the octane number. For example, trimethyl-butane (C7) and n-heptane (C7) have a research octane numbers (RON) 112.1 and zero, respectively [36]. Thus the branched C5+ hydrocarbons obtained over the H-ZSM-22 catalyst and the absence of aromatics species makes the material competitive in developing a process for the production of environmentally friendly non-aromatic high octane gasoline. For direct comparison of the product distribution of the H-ZSM22 (Si/Al = 30) catalyst with other topologies, H-ZSM-5 (Si/Al = 50),
H-beta (Si/Al = 45) and H-SAPO-34 ((Al + P)/Si = 11) catalysts were tested for the MTH reaction under identical conditions, displayed in Fig. 4. All catalysts have a reasonably similar and high density of acid sites (Si/Al 30–50) except H-SAPO-34, which has the common density of acid sites corresponding to one per cage. H-ZSM5, which has three dimensional 10-ring system, and H-beta, which has three dimensional 12-ring system, catalysts produce significant amounts of aromatic compounds. On the other hand, HSAPO-34 catalyst which has large cages connected with narrow 8-ring windows with 3.8 3.8 Å channel dimension, produces a product mainly composed of light alkenes (ethene and propene). H-ZSM-22 catalyst produces mainly C3+ hydrocarbons without aromatic products. Comparing the C3–C2 ratio of the catalysts, H-ZSM22 produces the highest amount of C3 relative to C2, giving a C3:C2 ratio of 6.8. H-ZSM-5, H-beta and H-SAPO-34 give C3:C2 ratios of 2.8, 2.1 and 1.4, respectively. The formation of sizable amounts of C2 produced over H-ZSM-5, H-beta and H-SAPO-34 catalysts is ascribed to the significant contribution of the aromatic based hydrocarbon pool type reaction mechanism (Cycle I) for methanol conversion over these catalysts, but not over the H-ZSM-22 catalyst. In agreement with the observed aromatic formation for the H-ZSM-5 and H-beta catalysts, high hydrogen transfer indexes (HTI) were observed, 0.6 and 0.9, respectively. However, for H-ZSM-22 and H-SAPO-34 catalysts the HTI was very low, 0.06 and 0.04 respectively. NMR investigation of the retained material in the H-SAPO-34 catalyst has shown that considerable amounts isobutane and isopentane are trapped in the cages of the material [37]. Thus, the observed low HTI for the H-SAPO-34 catalyst calculated from effluent C4 alkanes:C4 total ratios do not mean that there is less formation of aromatic species for cycle I type mechanism. However, the low HTI for the H-ZSM-22 catalyst suggests that there is less formation of hydrocarbon pool species for product formation via the cycle I type mechanism, in agreement with the selectivity and C3:C2 ratio mentioned above.
Fig. 4. MTH reaction over different catalysts, GC–FID chromatograms for H-ZSM-5 (Si/Al = 50), H-beta (Si/Al = 45), H-ZSM-22 (Si/Al = 30), and H-SAPO-34 ((Al + P)/Si = 11) catalysts, reaction carried out under identical conditions, full methanol conversion, WHSV = 2 g g 1 h 1 and 400 °C. The C2 and C3 peaks are indicated using ° and * on top of the peaks, respectively.
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3.3.
12
C/13C isotopic switching experiments
Isotopic switching is a commonly used method to distinguish active and inactive species in a working catalyst. For 12C/13C switches, more reactive species in the working catalyst will show faster 13C incorporation, giving rise to higher total 13C content in the compounds. Here, isotope switching from 12C to 13C methanol was performed after predetermined times of 12C methanol reaction over H-ZSM-22 catalysts. The switching to 13C methanol was performed at both full and low methanol conversion. The switching to 13C methanol at full methanol conversion was performed after 2, 5 and 18 min of 12C methanol reaction, followed in each case by 0.5, 1.0 and 2.0 min of 13C methanol reaction, hereafter denoted as the 2, the 5 or the 18 min experiments. The switching to 13C methanol reaction at low methanol conversion was performed at 46% and 58% methanol conversion, hereafter referred as the 46% and the 58% conversion experiments. The results of the switching experiments are presented in Figs. 5 and 6. Fig. 5 displays the total 13C contents in the effluent and retained material when switching from 12C to 13C methanol over H-ZSM-22 at 400 °C. Note that under these conditions the catalyst had full
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methanol conversion and the only difference is that for the 2 and the 5 min experiments the switching was performed at the early stage of the reaction where new aromatic species were building up, while for the 18 min experiment this stage of the reaction was passed (see Section 3.3.2). 12 13 C/ C switch experiments performed over deactivated catalysts are shown in Fig. 6. The switching was performed after the catalyst converted 12C methanol to hydrocarbons for several hours until it dropped to 58% and 46% conversion prior to the switching. Note that for these experiments the switching was performed after substantial amounts of hydrocarbons (coke) was trapped in the channels of the material compared to the switching performed at full methanol conversion. GC–MS chromatogram of the rationed material as a function of time on stream is presented in the Supplementary material. The basic findings of the isotope switch experiments are discussed below. 3.3.1. Effluent Inspection of the isotopic composition of the gas phase products shows that the 13C content in the alkenes is much higher than in the retained material, suggesting that the alkenes are much more
Fig. 5. Total 13C contents in the alkenes and retained material after 2 min (left panel), 5 min (middle panel) and 18 min (right panel) of 12C methanol reaction followed by 0.5, 1 and 2 min of 13C methanol reaction over H-ZSM-22 at 400 °C, WHSV = 2 g g 1 h 1 and full methanol conversion.
Fig. 6. Total 13C contents in the effluent alkenes at 58% (left panel) and 46% (right panel) 12C methanol conversion followed by 0.5, 1 and 2 min of 13C methanol reaction over H-ZSM-22 at 400 °C.
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reactive towards 13C than the retained material (methylated benzenes), clearly indicating that the alkene driven cycle II is all dominant in the product formation for this catalyst. This observation is in agreement with the high selectivity for C3+ alkenes (up to >98%) mentioned previously. All C3+ alkenes show identical 13C contents, indicative of the equilibrating nature of cycle II, whereas ethene lies consistently marginally below, which could point to a very minor contribution from the aromatics based cycle I. Cracking of hexene over H-ZSM-22 catalyst at high temperatures has been shown to yield ethene as cracking product [38] and since ethene contains a lot more 13C than the retained material during the MTH reaction over the H-ZSM-22 catalyst, the majority of the ethene probably comes from the cracking of higher alkenes. Previous studies have shown that the methylation rate of 12C alkenes with 13 C methanol increases in the order of ethene, propene and butene [39,40]. The slightly slower incorporation of 13C in ethene than the other C3+ alkenes could be rationalized by its slower methylation rate than the C3+ alkenes. If we assume cycle I mechanism produces small amounts of light alkenes (ethene, propene and butene), the C3+ alkenes will be methylated instantly and enter cycle II and display rapid increase in 13C content. Whereas ethene being the slowest in getting methylated will show some delay. This
will result in slight variation in the total 13C content between ethene and the C3+ alkenes. The rates of incorporation of 13C in the effluent products were identical for the 2, the 5 or the 18 min experiments. This suggests that the relative amounts of retained material (hydrocarbon pool) formed during 2, 5 or 18 min of the 12C methanol reaction prior to the switch to 13C methanol has no significant effect on the rate of incorporation of 13C in the effluent products. This in turn could mean that at these short times on stream there were no significant amounts of retained material (coke) in the channels of the H-ZSM-22 catalyst to influence the rate of incorporation of 13C by diffusion limitation (see Supplementary material). For the switching performed during low methanol conversion (58% and 46%, Fig. 6), ethene again contains slightly less 13C than the other alkenes whereas the C3+ alkenes show identical 13C contents as in the case of full methanol conversion. Looking at the 13C incorporation rate at 100, 58 and 46% conversion, it is clear that the incorporation rate of 13C decreases as the conversion decreases. This suggests that at long times on stream, enough hydrocarbons are retained in the channels of the H-ZSM-22 catalyst to affect the 13C incorporation by diffusion limitation.
Fig. 7. The distribution of 13C in hexamethylbenzene in the retained material after 2 min (top panel), 5 min (middle panel) and 18 min (bottom panel) of reaction followed by 0.5, 1.0 and 2.0 min of 13C methanol reaction. White bar: observed distribution and gray bar: random statistical distribution.
12
C methanol
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incorporation is lower for the methylnaphthalenes than for the methylbenzenes over the H-ZSM-22 catalyst. This suggests that the methylnaphthalenes are even less accessible and less active for alkene formation. The observation is very reasonable, considering the narrow channels of the material which imposes severe steric constraints as the retained material gets bigger. In the same manner as methylbenzenes, the accessibility of methylnaphthalenes decreases very rapidly with increasing deactivation of the H-ZSM-22 catalyst. We note that dimethylnaphthalene and trimethylnaphthalene display comparable total 13 C contents (accessibility) while methylnaphthalene was relatively more accessible. Fig. 8 displays the 13C content in the retained material after 18 min (for H-ZSM-5 (Si/Al = 50), H-beta (Si/Al = 120), and HZSM-22 (Si/Al = 30)) or after 25 min (for H-SAPO-34 ((Al + P)/ Si = 11)) of 12C methanol reaction followed by 2 min of 13C methanol reaction over four different catalysts. Note that slightly different reaction conditions and acid site densities have been employed in order to emphasize the representative activity of the methylbenzenes retained in the different catalysts. For H-ZSM-5, the lower methylbenzenes (xylene to trimethylbenzene) display the greatest reactivity towards 13C methanol, signifying their dominant role as intermediates in cycle I, whereas the opposite is observed for H-beta and H-SAPO-34, where the higher methylbenzenes (pentamethylbenzene and hexamethylbenzene) display the higher reactivity. As mentioned in the Introduction, the variation in the activity of the methylbenzenes is explained by the space offered by the catalyst topologies. For H-ZSM-22, however, the rate of 13C incorporation in the methylbenzenes is very low for all methylbenzenes, meaning that these species, and thus cycle I, play a minor role in alkene formation in this catalyst topology. Note that it is the highest methylbenzenes that display the greatest reactivity, as was seen for H-beta and H-SAPO-34, despite the narrow pore structure of H-ZSM-22. This could be rationalized by adsorption of the higher methylbenzenes on the external surface or at the pore mouths of the H-ZSM-22 catalyst, as previously proposed for hydrocracking reactions [41,42]. From the isotopic labeling data, we suggest that the aromatic species in the H-ZSM-22 catalyst, unlike other topologies, contribute only to a minor extent to the product formation and mainly behave as coke species which eventually leads to catalyst deactivation. The net methanol conversion proceeds through Cycle II mechanism, which is based on methylation and cracking of alkenes. The initial alkenes to start cycle II might originate from slight contribution of cycle I, or from impurities in the feed. There has been a question whether cycles I and II can run independently during the MTH reaction [9,20]. For H-ZSM-5 catalyst both cycles were shown to run simultaneously, the dual cycle concept. According to the report for the H-ZSM-22 catalyst, in order to sustain methanol conversion, cycle I needs continuous supply of hydrocarbon pool species through cyclizations and hydride transfers of alkenes from cycle II. This means that the two cycles cannot run independently on H-ZSM-22 catalyst [9]. In the present contribution, it is observed that cycle II can operate more or less independently over H-ZSM-22 catalyst. As a result of this the MTH
3.3.2. Retained material The isotope switch results of the retained material are presented in Figs. 5 and 7. Fig. 5 displays the rate of incorporation of 13 C in the effluent and retained material. Clearly, the retained material displays significantly slower incorporation of 13C than the effluent. This shows that cycle I type mechanism has a minor role over H-ZSM-22 catalyst, as mentioned in Section 3.3.1. The accessibility/activity of the retained material decreases with increasing the 12C methanol reaction time before the switching. The total 13C content in the retained material after 30 s of 13C methanol reaction (Fig. 5) was 20% for the 2 min experiment and decreased to 10% and less than 5% for the 5 and 18 min experiments respectively. A similar trend has previously been reported for H-SAPO-34 catalyst [23], except the decrease in accessibility is much quicker in the H-ZSM-22 catalyst. As mentioned in the introduction, the hydrocarbon pool species will transform into deactivation products with increasing the 12C methanol reaction time prior to the switching and this will lead to slow incorporation of 13C in the retained material [23]. Fig. 7 displays the distribution of 13C in hexamethylbenzene after 2, 5 and 18 min of 12C methanol reaction followed by 0.5, 1 and 2 min of 13C methanol reaction. Experimentally observed 13C distributions are shown in white bars and statistical random distributions calculated from the actual total 13C in hexamethylbenzene are shown in gray bars. The full 13C distributions for the rest of the retained material and effluent compounds are presented in the Supplementary material. The number of 13C in hexametylbenzene increases with increasing 13C methanol reaction time. Hexamethylbenzene consisting only 13C were more readily formed after short times on stream. For example, for the 2 min experiment after 1 and 2 min of 13C methanol reaction, hexamethylbenzene formed solely from 13C was observed, Fig. 7 top panel. For the 2, 5 and 18 min experiments, after 2 min of 13C methanol reaction, the percentage of newly formed hexamethylbenzene was 16%, 12% and 0%, respectively. This indicates that for the 2 and 5 min experiments the reaction was still at the early stage where aromatic species were building up. Whereas for the 18 min experiment, the early stage of the reaction was passed. This in turn means that the relatively higher total 13C content after the 2 and 5 min of 12C methanol reaction in the retained material (Fig. 5) is due to both the buildup of aromatic species solely from 13C methanol and the accessibility of the existing aromatic species. The accessibility of hexamethylbenzene decreases with increasing deactivation of the H-ZSM-22 catalyst. For the 2, 5 and 18 min experiments followed by 2 min of 13C reaction the percentage amounts of inaccessible hexamethylbenzene (without 13C) was found to be 5%, 11% and 35%, respectively, Fig. 7. Among the retained material in the narrow channels of H-ZSM-22 catalyst, methylnaphthalenes are the most abundant followed by methylbenzenes, see GS–MS chromatograms in the Supplementary material. As mentioned in Section 1, depending on catalyst topology methylnaphthalenes can have activity for product formation. Table 1 presents the total 13C content in the retained methylnaphthalenes in the 2, 5 and 18 min experiments. Clearly, the rate of 13C
Table 1 Total 13C contents in the retained methylnaphthalenes, 2, 5 or 18 min of 12C methanol reaction followed by 0.5, 1 and 2 min of 13C methanol reaction over H-ZSM-22 at 400 °C, full methanol conversion. Compound
MNaphthalene DiMNaphthal TriMNaphthal
2 min
12
C reaction
5 min
12
C reaction
18 min
12
C reaction
30 s
1 min
2 min
30 s
1 min
2 min
30 s
1 min
2 min
19.8 12.6 10.8
29.5 23.3 23.2
38.2 36.4 39.5
10.2 5.8 6.0
15.0 10.8 10.9
35.6 20.5 19.23
1.5 1.1 1.4
2.0 1.9 1.9
2.6 2.1 2.7
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Fig. 8. Total 13C contents in the retained hydrocarbons after 18 min (H-ZSM-5 (Si/Al = 50), H-beta (Si/Al = 120) and H-ZSM-22 (Si/Al = 30)) or 25 min (H-SAPO-34 ((Al + P)/ Si = 11)) of 12C methanol reaction followed by 2 min of 13C methanol reaction over the three catalysts, H-ZSM-5 and H-beta data adapted from [24] and H-SAPO-34 data adapted from [23]. Reaction over H-ZSM-22 was carried out at 400 °C and for the other topologies at 350 °C.
reaction product over H-ZSM-22 catalyst is mainly controlled by cycle II type mechanism, yielding very high selectivity for C5+ hydrocarbons and very little amounts of C2 hydrocarbons. The hindrance of the cycle I type mechanism over H-ZSM-22 catalyst could be ascribed to the narrow channels of the materials which provides small room for the space demanding hydrocarbon pool species.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.micromeso.2010.07.013.
References 4. Conclusion In this paper the methanol to hydrocarbon reaction mechanism over H-ZSM-22 catalyst is studied. Unlike H-ZSM-5, H-SAPO-34, and zeolite H-beta catalysts, the aromatic compounds in the channels of H-ZSM-22 catalyst are almost inactive for methanol conversion through the aromatic based hydrocarbon pool type mechanism. The effluent alkenes are much more reactive towards 13 C methanol than the retained hydrocarbons during the MTH reaction over the H-ZSM-22 catalyst. The alkene cracking and methylation mechanism controls the product selectivity, demonstrating the possibility of controlling the product selectivity based on intimate knowledge about the reaction mechanism for the conversion of methanol to hydrocarbons. By carefully choosing a catalyst topology likely to favor product formation via alkene cracking and methylation, we have discovered a system that yields a non-aromatic product consisting primarily of branched C5+ alkenes, offering additional product flexibility in this process. Along the lines presented in the Introduction, understanding the molecular components that govern the reaction selectivity is vital for selectivity control. Here, reaction mechanism is highlighted as one parameter that controls selectivity in the MTH reaction over zeolite material. The investigation is useful in the fundamental understanding of selectivity control in the MTH reaction and it can potentially be used in rational designing of new catalysts for the process. Acknowledgements This publication is part of the inGAP Centre of Research-based Innovation, which receives financial support from the Norwegian Research Council under Contract No. 174893.
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