Kinetics of Synthesis of Polyoxymethylene Dimethyl Ethers from Paraformaldehyde and Dimethoxymethane Catalyzed by Ion-Exchange Resin
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Chemical Engineering Science 134 (2015) 758–766 Contents lists available at ScienceDirect Chemical Engineering Science journal homepage: www.elsevier.com/locate/ces Kinetics of synthesis of polyoxymethylene dimethyl ethers from paraformaldehyde and dimethoxymethane catalyzed by ion-exchange resin Yanyan Zheng, Qiang Tang, Tiefeng Wang n, Jinfu Wang n Beijing Key Laboratory of Green Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China HIGHLIGHTS Kinetics of synthesis of PODEn from paraformaldehyde and methylal was studied. The transient molecular size distribution of PODEn follows Schulz–Flory model. The rate constants are the same for reversible propagation reactions of PODEn. The sequential reversible reactions to produce PODEn are exothermic. article info abstract Article history: Polyoxymethylene dimethyl ethers (CH3–O–(CH2O)n–CH3, PODEn, n41) are new concerned environ- Received 1 February 2015 mental benign alternative components for diesel fuels. This work aimed to investigate the kinetics of Received in revised form synthesis of PODEn from paraformaldehyde and dimethoxymethane catalyzed by ion-exchange resin 27 May 2015 NKC-9. Experiments were conducted in a designed space, namely reaction temperatures (60, 70, and Accepted 30 May 2015 80 1C) vs. reaction times (2, 5, 10, 20, 30, 60, and 90 min), in a batch stirred autoclave. The transient Available online 12 June 2015 molecular size distribution of PODEn compounds from paraformaldehyde and methylal followed Schulz– Keywords: Flory distribution model. In this system, the concentration of formaldehyde in the homogenous solution Polyoxymethylene dimethyl ethers (CF) was nearly constant. The sequential reversible reactions to produce PODEn were verified to follow a Kinetics second-order kinetics for propagation and a first-order kinetics for depolymerization. The rate constants Schulz–Flory distribution of propagation (k ) and depolymerization (k ) and the reaction equilibrium constant K were the same Sequential reversible reactions p d n for the series of PODEn synthesis reactions. Respecting to 5 wt% dosage of NKC-9 resin catalyst, the pre- 7 –1 –1 exponential factors Ap for propagation and Ad for depolymerization were 1.84 Â 10 L mol min and 6 –1 –1 5.36 Â 10 min , respectively. The activation energy Ep (39.52 kJ mol ) for propagation was lower than –1 Ed (52.01 kJ mol ) for depolymerization, validating that the reversible reactions of producing PODEn compounds were exothermic. & 2015 Elsevier Ltd. All rights reserved. 1. Introduction matter with an aerodynamic diameter less than 2.5 mm) in air usually exceeds 100 mgm–3, which is seriously detrimental to The combustion of petroleum-derived fuels is an important human health (Lin et al., 2009). Reducing the emissions of source of key precursors to the formation of secondary organic precursors to secondary aerosol from gasoline and diesel vehicle aerosol (Hallquist et al., 2009; Volkamer et al., 2006; Worton et al., emissions is important for controlling the PM 2.5 levels and 2014). The accumulation of secondary organic aerosol in air improving environmental and air qualities in China (Huang et al., contributes to particulate pollution during haze events (Huang 2014). Diesel engines have a higher thermal efficiency than gaso- et al., 2014). In China, the concentration of PM 2.5 (particulate line engines. However, diesel exhaust is seven times higher in forming aerosol than gasoline exhaust (Gentner et al., 2012). Generally, the combustion of diesel is responsible for 65À90% of n vehicular-derived secondary organic aerosol (Gentner et al., 2012). Corresponding authors. Tel.: þ86 10 62794132; fax: þ86 10 62772051. – – – – E-mail addresses: [email protected] (T. Wang), Polyoxymethylene dimethyl ethers (CH3 O (CH2 O)n CH3, [email protected] (J. Wang). PODEn, where n41), which are environmental benign alternative http://dx.doi.org/10.1016/j.ces.2015.05.067 0009-2509/& 2015 Elsevier Ltd. All rights reserved. Y. Zheng et al. / Chemical Engineering Science 134 (2015) 758–766 759 components for diesel fuels, are receiving increasing attentions in proposed an equilibrium molecular size distribution model based recent years (Burger and Hasse, 2013; Burger et al., 2010, 2012, on a sequential reaction mechanism, but did not study the kinetics 2013; Lumpp et al., 2011). Compared with the reference diesel fuel, in detail. The synthesis of PODEn from PF and DMM involved the the PM emissions are reduced by 18% with 10% blend and by 77% depolymerization of solid PF raw materials and sequential rever- with pure PODEn (Pellegrini et al., 2013). Among the PODEn sible chain propagation reactions, thus possessing very different oligomers, PODE3–5 compounds are the most ideal diesel additives, kinetic properties from other PODEn synthesis systems. while PODE2 does not fulfil the security criteria due to its low flash In the present work, the synthesis of PODEn from PF and DMM point, and PODEn 45 precipitate at low temperatures due to their catalyzed by NKC-9 acidic ion-exchange resins was conducted in high melting points (Zheng et al., 2013). Compared with methanol, the designed space, namely reaction temperatures (60, 70, and dimethyl ether (DME) and dimethoxymethane (DMM), the physi- 80 1C) and reaction times (2, 5, 10, 20, 30, 60, and 90 min), in a cochemical properties of PODE3–5 match well with that of petro- batch stirred autoclave. To the best of our knowledge, this article is leum diesel, thus allowing the use of PODE3–5 in modern diesel the first report on kinetics of synthesis of PODEn from DMM and PF engines without any change of the engine infrastructure catalyzed by ion-exchange resin. A kinetic model was developed (Pellegrini et al., 2013; Zheng et al., 2013). The application of assuming that the sequential reversible reactions to produce PODEn is promising to relieving both air pollution and petroleum PODEn compounds follow a second-order kinetics for propagation shortage. and a first-order kinetics for depolymerization. Rate constants at The PODEn compounds are prepared from end-group (–CH3, – different temperatures were obtained by least-squares aggression O–CH3) provider (DMM or methanol) and chain-group (–CH2O) of the experimental concentration of DMM as a function of provider (paraformaldehyde (PF), trioxane, or formaldehyde solu- reaction time. The Arrhenius parameters, including the pre- tion) over acid catalysts. Recent papers on synthesis of PODEn have exponential factor and activation energy, were calculated from focused on catalyst preparation and characterization, and process the rate constants at different temperature. optimization. Various acidic catalysts, including acidic ion- exchange resins (Wang et al., 2014; Zheng et al., 2013, 2014; Burger et al., 2012), ionic liquids (Wu et al., 2014), molecule sieves 2. Experimental (Zhao et al., 2011), and solid superacids (Li et al., 2015; Zhang et al., 2014), have been employed in synthesis of PODEn. Burger et al. 2.1. Materials (2012) proposed that water reacts with ethers and form alcohols 4 during the synthesis of PODEn from DMM and trioxane in the acid- DMM (analytic reagent grade, 99 wt%) was purchased from catalyzed system. To illustrate the effect of water on the synthesis Alfa Aesar-Johnson Matthey. PF (polymer grade, 496 wt%) was of PODEn from DMM and PF, we investigated the product dis- purchased from Sinopharm Chemical Reagent Co., Ltd. The NKC-9 þ tribution with different dosages of water added in this work, as acidic ion-exchange resin (H type) was provided by Tianjin shown in Table 1. The results showed that water induced the Bohong Resin Technology Co., Ltd. – 4 hydrolysis of DMM and PODEn compounds forming methanol and PODE2 5 (industrial grade, 95%) were provided by a 10 kt/a formaldehyde. In particular, when the added dosage of water PODE industrial plant in Shandong Yuhuang Chemical (Group) Co., exceeded 5 wt%, the hydrolysis reactions became crucial, and Ltd. in China using the technology developed by our research fl significantly decreased the yield of PODEn products and increased group. In this technology, a uidized bed reactor was used to the complexity of product purification. To avoid introducing or produce PODEn from DMM and PF over solid acid catalyst. These fi generating water, DMM is a better end-group provider than PODE2–5 samples were further puri ed to analytic reagent grade methanol, while PF and trioxane are better chain-group provider (499%) and used as standard samples in quantitative analysis of than formaldehyde solution. PODEn product. Kinetics is of great importance for the molecular size distribu- tion regulation, process optimization and reactor design for 2.2. Experimental setup and procedure synthesis of PODEn. However, the studies on the kinetics of synthesis of PODEn are very limited. Burger et al. (2012) reported The schematic of the experimental setup for synthesis of PODEn the reaction kinetics of the heterogeneously catalytic formation of from PF and DMM catalyzed by NKC-9 is shown in Fig. 1. Previous PODEn from DMM and trioxane, and proposed an adsorption- works (Zheng et al., 2013, 2014) showed that NKC-9 had a high based kinetic model to describe the results. Zhang et al. (2014) catalytic activity and good stability for the production of PODEn synthesized PODEn from methanol and formaldehyde solution and from PF and DMM. Therefore, NKC-9 was used in this work with a proposed a kinetic model based on an elimination mechanism. In dosage of 5.0 wt%