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44 M. Bjørgen et al. / Applied Catalysis A: General 345 (2008) 43–50 the methanol to gasoline process; in which methanol is converted to gasoline over ZSM-5 derived catalysts. A plant for an annual gasoline production of 600 000 tons over a H-ZSM-5 catalyst was started up in 1986, but due to the later fall in crude oil prices, only the methanol synthesis step was left on stream [2]. The Topsøe Integrated Gasoline Synthesis (TIGAS) process of Haldor Topsøe AS, also based on H-ZSM-5 as catalyst, was demonstrated on pilot scale in the mid-1980s [3]. However, the TIGAS process was never scaled up due to the situation in the global energy market. Later, Norsk Hydro and UOP jointly developed the methanol to olefins (MTO) variant of the reaction, where ethene and propene are the main products formed over H-SAPO-34 zeotype catalyst systems [4]. The latest addition to this field is the methanol to propene (MTP) alternative, currently offered by Lurgi [5]. Both the MTO and MTP processes have been proven in demo plants [5,6]. Zeolites are crystalline aluminosilicates which possess Brøns- ted acidic sites located within ordered micropores of molecular dimensions. For example, the H-ZSM-5 catalyst has a medium size pore system, i.e. pores which circumference is defined by rings consisting of 10 T-atoms. Straight channels (5.1 A ̊ 5.5 A ̊ ) are intersected by zigzag channels (5.3 A ̊ 5.6 A ̊ ), thus creating a three-dimensional network [7]. It is the presence of these micropores that gives rise to the use of zeolites as shape selective catalysts, but it is the same pore systems that impose limits on the applicability of zeolites as catalyst, due to diffusion limitations and a corresponding inefficient use of the entire zeolite crystal. Hence, improvements of the diffusion properties might lead to improved catalyst performance. As outlined by Hartmann [8] and Groen et al. [9], several fundamentally different approaches have been pursued towards this goal, and these may be described as post-synthetic or related to direct synthesis. This report focuses on post-synthesis modification of the zeolite crystals by treatments in alkaline media, i.e. desilication. Several recent publications by Groen et al. [10–16] report on the formation of accessible intracrystalline mesopores in H-ZSM-5 as a result of selective removal of framework Si by controlled desilication with NaOH, effectively creating two hierarchical pore systems. The thus created mesopores have been characterized extensively by sorption techniques [11,12,14] and electron microscopy [13,15], and direct demonstration of improved diffusion properties has indeed been reported [15]. However, only a few reports discussing the catalytic performance of desilicated H-ZSM-5 exist. Ogura et al. [17] have reported both improved overall conversion and activity per Al-site in the cracking of cumene over alkali-treated H-ZSM-5. Enhanced catalytic performance and coke-resistance has also been observed for Mo/H-ZSM-5 catalysts for methane aromatization, and the beneficial effects were attributed to formation of mesopores and improved mass transfer [18,19]. Jung et al. [20] observed changes in product selectivities when n-octane was reacted over alkali- treated H-ZSM-5. Finally, Song et al. [21] observed improved catalytic stability for the aromatization of butane over NaOH- treated H-ZSM-5. It should also be mentioned that Le Van Mao et al. [22–25] have studied the effects of post-synthesis treatment of ZSM-5 in NaOH solutions or Na2CO3 + NaOH solutions. The initial studies discussed the creation of mesopores [22,23], but in the later work by this group, the effects of the post-synthesis base treatment were predominantly described as a slight enlargement of the micro- pores, from 5.3 to 5.9 A ̊ , and the formation of mesopores was not mentioned. Yet, of some relevance is the report of Ohayon et al. [25], where ZSM-5 catalysts (Si/Al = 21) subjected to a treatment with 0.8M Na2CO3 were tested in the MTH reaction. This procedure influenced the product selectivities and the aromatics distributions, but no data on catalyst lifetime and activity were given. Lietz et al. [26] have also studied the modifications of H-ZSM-5 catalysts (Si/Al = 18) by NaOH treatment. It was observed that refluxing the catalyst (at 100 8C) in 0.08 M NaOH for 2 h led to a slight removal of framework aluminum. Methanol was reacted over the catalysts at full conversion, and the main effect of the treatment was a stabilization of the selectivity towards aromatics and the propene/propane ratio, which was interpreted as an increase in catalyst lifetime. In this study we have investigated the effects of alkaline treatment of an H-ZSM-5 sample on catalyst performance in the conversion of methanol to gasoline. A catalyst sample with relatively large crystals and a significant concentration of crystal defects, and thus a fairly short catalyst lifetime, was specifically chosen in order to maximize the potential for improvement. The treatment of this sample results in significant improvement in catalyst lifetime and a strongly enhanced selectivity towards the desired gasoline fraction. Extensive characterization reveals that, for this sample, these effects may be attributed to the development of Lewis acidity, improved crystallinity and mesopore formation. 2. Experimental 2.1. Catalyst synthesis The H-ZSM-5 catalyst was synthesized using tetrapropylam- monium (TPA) as the structure directing agent. First, 20 g TPABr (Fluka 98%) was dissolved in 20 g de-ionized water followed by addition of 25 g Ludox LS (solution A). Solution B was prepared by dissolving 0.25 g Al(OH)3 (Aldrich), 0.11 g (NaOH, Merck 99.98%), and 0.41 g KOH (Merck). Solution A was then added to solution B and the pH was adjusted to 11 by drop-wise addition of 2 M H2SO4 under stirring for 2 h at room temperature. The gel was then transferred to a 250 mL Teflon lined steel autoclave and the zeolite was crystallized under static conditions at 170 8C for 14 days. The solid product thus obtained was filtered and washed with de- ionized water until no Br was detectable in the washing water (precipitation with Ag+). Final molar gel composition was 78 SiO2:1.0 Al2O3:2.3 K2O:0.86 Na2O:47 TPABr:4000 H2O (Si/Al ratio 39). 2.2. Desilication and ion exchange procedure Prior to the treatment, the as-synthesized zeolite was calcined in static air at 550 8C for 24 h in order to remove the template. The calcined sample was treated with 0.05 or 0.20 M NaOH solutions (about 20 mL/g) for 2 4 h at 75 8C, followed by washing with de- ionized water for 1 h at 75 8C. After the first 4 h period, the samples were filtered and then submitted to another 4 h of treatment with a fresh NaOH solution of the same concentration. After drying overnight at room temperature, the samples were ion exchanged with a 1 M NH4NO3 (about 20 mL/g) for 3 2 h at 75 8C followed by calcination in static air at 550 8C for 24 h, thereby obtaining protonated samples. Data for three representative samples will be presented in this report, designated H-ZSM-5-PARENT (no treat- ment), H-ZSM-5-0.05M (treated with 0.05 M NaOH), and H-ZSM- 5-0.20M (treated with 0.20 M NaOH). 2.3. Catalyst characterization The elemental composition of the catalyst samples were measured by inductively coupled plasma atomic absorption spectroscopy (ICP-AES) using a Varian Vista AX CCD axial ICP-AES. Total acidity was evaluated using NH3-TPD and titration as the quantification method according to the following procedure. About 100 mg of the sample (particle size 150–300 mm) was dried atPDF Image | Methanol to gasoline over zeolite H-ZSM-5
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