27The role of acidity (nature, concentration, strength) and textural properties in the 28 etherification of glycerol with tert-butyl alcohol was studied for a wide range acid catalysts, 29 such as Amberlyst ® 15, silica, alumina, silica alumina and four type of zeolite, i.e. FAU, 30 MOR, *BEA and MFI. The etherification of glycerol by tert-butyl alcohol is a 31 thermodynamically limited reaction that occurs through a successive reaction sequence. We 32 found major evidence that glycerol etherification is not only a function of the amount of 33Brønsted acid sites, but that it further proceeds via a product shape selectivity mechanism. 34 Indeed, the formation of di-substituted ethers appears at very low conversions for zeolites 35 compared to meso-and macroporous acid catalysts. *BEA and MFI zeolites feature similar 36 confining voids and resulting thus in similar intrinsic acid strengths (as proved by n-hexane 37 cracking), but differ in the connectivity (4 vs. 6 channels) and access to these voids (0.54 38 vs. 0.67 nm), which leads to diffusion issues, notably for the MFI zeolite. 39 40 Key words: glycerol etherification, zeolites, confinement effect, auto-inhibition effect, 41 Brønsted acidity, product shape selectivity. 42 43 44 45 46 47 48 49 50 51 52 53 3 Introduction 54Glycerol is employed in over 1500 industrial applications and amounts to an annual 55 production of ca 160.000 tons [1]. By the year 2020, it is estimated that glycerol production 56 will exceed global demand by a factor of six [2]. Hence, the development of efficient 57 strategies for the glycerol conversion into value-added products represents a major issue as 58 far as glycerol disposal and the dealing with surplus production is concerned. A sustainable 59 strategy to valorize the polyol is its conversion into glycerol ethers, with widespread 60 applications, such as oxygenated fuel additives, intermediates in the pharmaceutical 61 industry, and non-ionic surfactants [3][4][5]. 62The etherification between two alcohols is promoted through acid catalysis. The use 63 of homogeneous catalyst such as strong acids (e.g. H 2 SO 4 ) [6] represents major 64 inconveniences causing corrosion and environmental issues. Solid acid catalysts are an 65 indisputable mean to overcome these drawbacks. A prominent family of solid acid catalyst 66 are ion-exchanged resins. Yet, these resins present important limitations, such as low 67 surface area and a poor thermal stability [7]. Zeolites are a class of solid acids that feature 68 strong Brønsted acidity with high thermal and mechanical stability; making them very 69 promising solid catalysts for the glycerol etherification [7, 8]. 70Gonzáles et al. [9] carried out the etherification of glycerol with tert-butyl alcohol 71 (TBA) at 348 K, on three commercial acidic zeolites: *BEA, MOR and MFI with 72 respective Si/Al molar ratios of 10, 6.5 and 20. The authors evidenced that *BEA zeolite 73 allowed to achieve highest conversion (75% with catalyst loadings of 5wt%) for this 74 transformation, which was furt...
Natural gas, the cleanest fossil fuel, is an abundant source of methane and expected to play an increasingly important role in powering the world’s economic growth over the energy transition of the coming decades. Methane has the potential to be a CO2-free feedstock to cogenerate hydrogen (H2) and added value “building-blocks” chemicals (e.g., olefins and aromatics) for petrochemistry. In this review, the two processes (i) the oxidative coupling of methane (OCM) for production of ethylene and (ii) the nonoxidative methane dehydroaromatization (MDA) producing hydrogen and benzene are discussed. Both routes convert methane directly into valuable products, an advantage over the several-steps syngas route. The performances of various a variety of catalysts reported during the last 25 years for OCM (MnNaW, La2O3, Li-MgO, etc.) and MDA (M/HZSM-5, M/TNU-9, M/IM-5, M/ITQ-2, M@SiO2, M@CeO2, TaH/SiO2, GaN/SBA15, single-site M@HZSM-5, bimetallic M-M′/HZSM-5, core–shell structures, M/Zr(SO4)2 with M = Mo, Fe, Pt) under similar reaction conditions are compared. The major drawbacks and the strategies used to mitigate the main challenges related with the performance of the catalysts in both OCM and MDA reactions are critically revealed. For instance, the overoxidation in the OCM is mitigated by optimizing of the operating conditions, using alternative oxidants, and the application of membrane reactor technology are discussed. In the MDA reaction, the major issue is the catalyst deactivation by coke formation and migration and sintering of metallic active phases. Strategies for robust catalysts, methods for mild coke removal, pretreatment under reductive atmosphere are presented. Approaches to improve aromatics yields over coke production by addition of promoters or co-feed reactants to the MDA catalysts are also discussed.
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