甲醇制烯烃反应机理和动力学研究进展

甲醇制烯烃（MTO）反应是重要的生产低碳烯烃技术，对于其反应机理的研究，尤其是第一个C—C键生成过程的探讨备受争论。近年来，Hydrocarbon Pool平行反应机理逐渐得到较广泛的认可，并在此基础上出现了很多MTO反应动力学的研究报道以及对该快速失活反应的催化剂积炭与失活现象的研究。针对在SAPO-34等分子筛催化剂的MTO反应情况，综述了近些年来国内外对于其反应机理、反应动力学、积炭和失活动力学的研究进展情况。     

SAPO-34分子筛催化剂制备及发展现状

乙烯和丙烯作为重要的化工原料,在经济发展中的需求量越来越大。在石油资源越来越匮乏的今天,甲醇制烯烃作为一种可以代替常规石油路线生产低碳烯烃的新工艺受到广泛关注。SAPO-34分子筛因为高甲醇转化率和优良烯烃选择性成为当前甲醇制烯烃工艺催化剂的研究重点。合成SAPO-34分子筛的影响因素有模板剂、合成原料和反应条件等。通过调节分子筛粒径尺寸、酸性、金属改性可以实现分子筛的性能优化。介绍了SAPO-34分子筛催化剂常用的制备方法和一些分子筛催化剂改进的专利。使用一定时间后催化剂由于积炭而失活,再生工艺目前主要采用烧焦再生。2011年,神华煤制烯烃示范工程进入工业化运行,近年陆续有多套甲醇制烯烃装置投产和在建,煤制烯烃正在改变中国聚烯烃市场格局。     

Kinetic modeling of methanol to olefins process over SAPO-34 catalyst based on the dual-cycle reaction mechanism

A kinetic model for methanol to olefins (MTO) process over SAPO-34 catalyst was established based on the dual-cycle reaction mechanism. Simplifications were made by assuming olefins-based cycle as virtual species S, and aromatics-based cycle as R, where the former mainly accounts for the production of higher olefins, while the latter for lower olefins. Transformation of S to R was considered with the participation of methanol and olefins. Meanwhile, a phenomenological deactivation model was developed to account for the deactivation process. With the proposed model, the evolution of methanol conversion and product selectivity with time on stream could be predicted, and key reaction characteristics, such as the autocatalytic nature of the reaction, could also be captured due to its mechanism-based nature. Further simulations of MTO reactors at different scales validated the robustness and applicability of the current model in MTO process development and optimization. © 2018 American Institute of Chemical Engineers AIChE J, 65: 662–674, 2019     

ZSM-5分子筛在甲醇转化制烯烃领域应用的研究进

综述了ZSM-5分子筛在甲醇制烯烃反应特别是在甲醇制丙烯反应中的应用。介绍在接近工业反应条件下甲醇在ZSM-5分子筛上生成烯烃的反应机理,在较高反应温度下高碳数烯烃裂解是导致轻烯烃形成的主要反应路径;分析ZSM-5分子筛酸性及粒径对产品选择性及催化剂寿命的影响,由于ZSM-5分子筛酸性较强,直接用于甲醇制烯烃反应时低碳烯烃的选择性不高,而粒径小的ZSM-5分子筛扩散性能好,因而丙烯选择性得到提高。重点介绍小晶粒ZSM-5分子筛的研究进展,指出目前ZSM-5分子筛的研究方向一是对其进行酸性改性,二是制备酸度适中、粒径合适和具有介孔的多级结构ZSM-5分子筛。     

甲醇制烯烃催化剂SAPO-34分子筛的改性研究进展

甲醇制烯烃（MTO）被认为是最有希望以煤或天然气为原料替代石油制取烯烃的技术路线。具有CHA结构的SAPO-34分子筛是MTO反应生产乙烯和丙烯最理想的催化剂,但在甲醇转化过程中,芳香烃类中间体受到SAPO-34分子筛八元环微孔结构的限制,使催化剂孔道堵塞并覆盖其酸性位点,造成催化剂积炭失活。为了提高SAPO-34分子筛催化剂的寿命和低碳烯烃的选择性,改善传质并延缓焦炭的沉积至关重要。从构建多级孔结构、减小晶粒尺寸及调控分子筛酸性3个方面出发,总结了SAPO-34分子筛在MTO反应中的研究进展,并对今后催化剂的粒度、孔尺寸、酸性质等方向的改进及发展进行了展望。     

流态化技术在甲醇制低碳烯烃装置中的应用

本文总结了甲醇制低碳烯烃(MTO)的发展历程,根据MTO反应机理及反应特点,评述了各类MTO工业反应型式的适用性,认为流态化反应器和再生器之间的循环适合于MTO反应热效大、催化剂寿命短的特点。中国神华煤制油化工有限公司包头煤化工分公司1.80×106 t/d的MTO工业示范装置近3年的运行数据表明流化床技术非常适合于MTO反应。     

甲醇制烯烃多级串联流化床反应器模拟

甲醇制烯烃(MTO)是现代煤化工发展的新技术,合理的甲醇制烯烃反应器操作模式可以有效提高催化剂的效率和低碳烯烃选择性。采用在工业SAPO34催化剂上实验得到的反应与失活动力学和流化床动态两相模型,结合颗粒停留时间分布模型,对MTO单一流化床反应器、二级串联及三级串联反应器进行了模拟,考察了催化剂停留时间、气固并流和气固逆流对甲醇制烯烃反应的影响。模拟结果表明:采用多级串联反应器有利于减小颗粒的返混,使出口积炭量分布更加均匀,催化剂寿命延长;二级气固逆流操作可以提高低碳烯烃选择性及出口催化剂积炭量,催化剂单位生产能力比单级反应器提高24.4%;三级串联、气固逆流反应器可以充分利用各级反应器的不同功能,使总的低碳烯烃选择性提高到79.36%(质量分数),比单级反应器提高1%,同时单位催化剂单程生产能力比单级提高31.1%。     

双模板剂法SAPO-34分子筛的合成及其MTO催化性能调变

以四乙基氢氧化铵（TEAOH）和二乙铵（DEA）为混合模板剂，在低投料硅铝比[n (SiO₂) ∶n (Al₂O₃)=0.2]及低模板剂用量[n (模板剂) ∶n (Al₂O₃)=1.9]下，考察了两种模板剂比例的调变对合成的SAPO-34分子筛物化性能及其催化甲醇制烯烃反应（MTO）催化性能的影响。研究发现，通过改变两种模板剂比例，可以明显调变SAPO-34分子筛晶粒尺寸、硅分布（晶粒表面和体相的硅分布）、硅原子的配位环境，从而影响其MTO催化反应的效果。在低模板剂用量制备的SAPO-34产品中，晶粒尺寸是影响其催化寿命的最主要因素，小晶粒分子筛因其扩散路径短有利于延长催化寿命。此外，硅分布也是影响催化寿命的因素之一，表面富硅的分子筛导致外表面积炭程度大于晶内积炭，积炭趋势由外向内发展，加速分子筛“假性”失活。硅分布还影响MTO反应产物分布，表面富硅分子筛外表面更易发生非择形催化，显著提高C₄～C₆等产物的选择性，不利于目标产物双烯（乙烯+丙烯）选择性的提高。     

Effect of temperature in the conversion of methanol to olefins (MTO) using an extruded SAPO-34 catalyst

The methanol-to-olefin (MTO) reaction was investigated in a bench-scale, fixed-bed reactor using an extruded catalyst composed of a commercial SAPO-34 (65 weight percentage, wt-%) embedded in an amorphous SiO₂ matrix (35 wt-%). The texture properties, acidity and crystal structure of the pure SAPO-34 and its extruded form (E-SAPO-34) were analyzed and results indicated that the extrusion step did not affect the properties of the catalyst. Subsequently, E-SAPO-34 was tested in a temperature range between 300 and 500 °C, using an aqueous methanol mixture (80 wt-% water content) fed at a weight hour space velocity (WHSV) of 1.21 h⁻¹. At 300 °C, a low conversion was observed combined with catalyst deactivation, which was ascribed to oligomerization and condensation reactions. The coke analysis showed the presence of diamandoid hydrocarbons, which are known to be inactive molecules in the MTO process. At higher temperatures, a quasi-steady state was reached during a 6 h reaction where the optimal temperature was identified at 450 °C, which incidentally led to the lowest coke deposition combined with the highest H/C ratio. Above 450 °C, surges of ethylene and methane were associated to a combination of H-transfer and protolytic cracking reactions. Finally, the present work underscored the convenience of the extrusion technique for testing catalysts at simulated scale-up conditions.     

甲醇制烯烃催化剂SAPO-34分子筛的制备和喷雾成型

SAPO-34分子筛用于催化甲醇转化制烯烃,乙烯和丙烯选择性高,是很好的甲醇制烯烃催化剂。由于SAPO-34分子筛失活速率快,甲醇制烯烃反应器通常是连续循环再生的流化床反应器,SAPO-34分子筛必须喷雾成型并达到一定抗磨强度后才能使用。在50 L反应釜合成了SAPO-34分子筛,并在中试喷雾装置上,以SAPO-34为活性组分喷雾成型甲醇制烯烃催化剂。结果表明,喷雾成型甲醇制烯烃催化剂的抗磨损指数为1.58%·h-1,抗磨性能达到工业应用要求,与两种工业甲醇制烯烃催化剂对比,喷雾成型甲醇制烯烃催化剂寿命最长,达260 min,乙烯、丙烯选择性以及乙烯+丙烯总选择性在对应的各个反应时间点均最高,260 min分别达到49.09%、35.05%和84.98%。     

Busting the efficiency of SAPO-34 catalysts for the methanol-to-olefin conversion by post-synthesis methods

As an effective non-petroleum based process for producing light olefins, the methanol-to-olefin(MTO) route has become an indispensable alternative to the industrial production of light olefins. The silicoaluminophosphate SAPO-34 zeolite(CHA-type structure) has proven to be an efficient industrial catalyst for the production of ethylene and propylene by the MTO reaction. However, the inherent structure and related diffusion limitations of SAPO-34 limit the mass transport and thus cause rapid deactivation of the catalyst. Fabrication of hierarchical SAPO-34 zeolite is one of the most effective strategies to address the intrinsic diffusion limitation. As simple, inexpensive, and efficient approach, the post-synthetic route has attracted considerable attention and widely used to introduce secondary meso-/macropores into the microporous SAPO-34 material. Significant effort has been dedicated to the development of post-synthesis strategies to prepare hierarchical SAPO-34 zeolite, thereby enhancing its catalytic performance in the MTO process. This mini-review addresses the post-synthesis preparation of hierarchical SAPO-34 catalysts and their MTO performance. Furthermore, some current problems and prospects of the post-synthesis route to hierarchical SAPO-34 catalysts are also revised. We expect this minireview to inspire the more efficient preparation of hierarchical SAPO-34 catalysts for the MTO process.     

Busting the efficiency of SAPO-34 catalysts for the methanol-to-olefin conversion by post-synthesis methods

As an effective non-petroleum based process for producing light olefins, the methanol-to-olefin (MTO) route has become an indispensable alternative to the industrial production of light olefins. The silicoaluminophosphate SAPO-34 zeolite (CHA-type structure) has proven to be an efficient industrial catalyst for the production of ethylene and propylene by the MTO reaction. However, the inherent structure and related diffusion limitations of SAPO-34 limit the mass transport and thus cause rapid deactivation of the catalyst. Fabrication of hierarchical SAPO-34 zeolite is one of the most effective strategies to address the intrinsic diffusion limitation. As simple, inexpensive, and efficient approach, the post-synthetic route has attracted considerable attention and widely used to introduce secondary meso-/macropores into the microporous SAPO-34 material. Significant effort has been dedicated to the development of post-synthesis strategies to prepare hierarchical SAPO-34 zeolite, thereby enhancing its catalytic performance in the MTO process. This mini-review addresses the post-synthesis preparation of hierarchical SAPO-34 catalysts and their MTO performance. Furthermore, some current problems and prospects of the post-synthesis route to hierarchical SAPO-34 catalysts are also revised. We expect this minireview to inspire the more efficient preparation of hierarchical SAPO-34 catalysts for the MTO process.     

The separation of micron-size particles from air by diffusiophoresis

The continuous separation of micron-size particles from air by diffusiophoresis in a parallel plate collector was studied theoretically. Two models for the particle behavior were considered. Model I was based on the assumption that the particles adopt the local fluid velocity whereas the Schmitt-Waldmann equation was used to determine the diffusiophoretic velocity in Model II. The latter also included thermophoretic and gravitational effects. The two models yielded similar results when diffusiophoresis was predominant. The theoretical results were also used in the sizing of an experimental particle collector. The collector, which operated under laminar flow conditions, was capable of removing the particles effectively and its performance conformed with the theoretical work.     

MTO催化剂离心喷雾干燥制备过程的数值模拟

采用CFD模型对MTO催化剂喷雾干燥过程进行了非稳态数值模拟,得到了10s内热空气的湿含量、温度、速度、速度向量分布,以及催化剂颗粒的运动轨迹、停留时间和粒径分布.通过与试验结果的对比分析表明,模拟结果可信,模拟离心式喷雾干燥过程可行.     

Detailed Mechanistic Studies using in situ Spectroscopic Analysis: A Look at Little‐Known Regions of the Heck Reaction

The mechanism of the Heck reaction using palladium complexes with large phosphoramidite ligands is investigated. The catalyst precursor is an inactive dimer that equilibrates with the active monomeric species. A series of kinetic models is introduced and compared with concentration profiles obtained from FT‐NIR spectroscopy. First, an analytical solution of the differential equations for a simplified mechanism is considered. This fits well at low conversions but deviates at higher conversions with increasing deactivation. Formation of palladium clusters and palladium black is then included, with the simplification that all the deactivation processes are represented by a single first‐order process. This results in a five‐step mechanism that describes the dimer‐monomer equilibrium, the Pd(II)/Pd(0) catalytic cycle, and the catalyst deactivation process. The model can be used to monitor the transient concentrations of the virtual Pd(0), Pd(II), and dimer catalyst species, and helps to explain the effects of water. The high resolution of the measurements and low error levels of the models render this approach a powerful tool for mechanistic studies in homogeneous catalysis.     

Spatial confinement effects of cage-type SAPO molecular sieves on product distribution and coke formation in methanol-to-olefin reaction

Three kinds of 8-membered ring silicoaluminophosphate (SAPO) molecular sieves with different cage structures, SAPO-34, SAPO-18 and SAPO-35, were employed in methanol-to-olefin (MTO) reaction. The main products over SAPO-34 and SAPO-18 were propene and butenes, whereas ethene and propene especially ethene were predominantly generated over SAPO-35. Coke species formation greatly depended on reaction temperature and varied systematically with cage size. The differences in production distribution and generated coke species in the MTO reaction suggest great spatial confinement effects imposed by cage structure of SAPO catalysts.     

Simulation of liquid and vapour phase xylene isomerization over deactivating H-Y-Zeolite

A steady-state simulation model for xylene isomerization over H-Y-Zeolite was developed. Liquid and vapour phase reactions were considered. The fixed bed catalytic reactor was simulated as a plug flow reactor that operates either isothermally or adiabatically. Four temperature levels were investigated taking into consideration catalyst deactivation. The main reactions considered in the model were the isomerization reactions and the disproportionation of xylenes to toluene and trimethylbenzene. The model predicts the concentration and temperature profiles for a given input of operating conditions. The simulation results revealed the similarity in the reactor behaviour if it operates either isothermally or adiabatically with respect to the distribution of the reaction products and catalyst deactivation. The study also indicated that performing the reaction in the liquid phase is more favourable than in the vapour phase due to better selectivity, relative productivity, efficiency and approach to equilibrium. © 1998 Society of Chemical Industry     

Reaction scheme and kinetic modelling for the MTO process over a SAPO-18 catalyst

A kinetic model for simulation of the MTO process over SAPO-18 catalyst in a wide range of operating conditions has been proposed. The kinetic model predicts the experimental evolution of reaction products with time on stream, which follows three consecutive periods: initiation (where olefin production increases), a period of maximum olefin production and a period in which this production decreases. The kinetic scheme takes into account these three steps that evolve with time on stream: formation of active intermediate compounds, an step where olefins are formed by reaction of oxygenates (methanol/DME) with these intermediates and deactivation of intermediates by degradation to coke. The presence of water in the reaction medium attenuates the reaction rate of these steps. Discrimination of kinetic equations and calculation of the parameters of best fit have been carried out by solving the mass conservation equations of the individual components of the kinetic scheme together with the kinetic equation for deactivation and taking into account the effect of water on the kinetics of each step.     

Organosilane surfactant-assisted synthesis of mesoporous SSZ-39 zeolite with enhanced catalytic performance in the methanol-to-olefins reaction

SSZ-39 zeolite with AEI framework structure is a good catalyst candidate for the methanol-to-olefins (MTO) reaction. However, the diffusion limitation and coke formation often results in fast deactivation of the SSZ-39 zeolite catalyst. One solution for this challenge is to introduce mesoporosity in the SSZ-39 zeolite. Herein, we report the synthesis of mesoporous SSZ-39 zeolite using an organosilane surfactant, N,N-dimethyl-N-(3-(trimethoxysilyl)propyl)octan-1-aminium chloride, as a mesopore template and N,N-dimethyl-cis-2,6-dimethylpiperidinium as a micropore template. The obtained zeolites were characterized by X-ray diffraction, N₂ sorption, scanning electron microscopy, temperature programmed desorption of ammonia, and magic angle spinning nuclear magnetic resonance of ²⁷Al. The results show that the mesoporous SSZ-39 zeolite has high crystallinity, meso/microporosity, high surface area, cuboid morphology, and abundant acidic sites. More importantly, this mesoporous SSZ-39 zeolite exhibits enhanced catalyst lifetime in the MTO reaction due to the presence of mesoporosity for fast mass transfer, compared with a conventional SSZ-39 zeolite without mesoporosity.