Effects of diffusional constraints on lifetime and selectivity in methanol-to-olefins catalysis on HSAPO-34
Effects of diffusional constraints on lifetime and selectivity in methanol-to-olefins catalysis on HSAPO-34
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DOI：
10.1016/j.jcat.2018.10.031
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发表时间：
2019-01-01
影响因子：
7.3
通讯作者：
Bhan, Aditya
中科院分区：
文献类型：
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作者：
Hwang, Andrew;Le, Thuy T.;Bhan, Aditya
反应-输运形式表明，HSAPO-34 的晶粒尺寸、H+ 密度和 Si 形态对甲醇制烯烃催化中催化剂寿命和选择性的影响都是扩散约束的表现。在硅铝磷酸盐中，通过 Si 形态调节的结构催化剂特性（即微晶尺寸和 H+ 密度）和内在动力学常数（每个 H+）都会影响这些扩散限制的严重程度。 HSAPO-34 上的甲醇转化为烯烃是通过复杂的自催化反应网络发生的，其中时间梯度与连续流固定床反应器固有的空间梯度一起持续存在。在解释此类系统中的寿命和选择性趋势时调用反应速率需要在时间和空间上平均的定义量，因为测量的可观测值将瞬时反应速率与空间和时间梯度混为一谈。本文定义的量，即总周转率和累积选择性，提供了对寿命和选择性的严格评估，允许自催化反应的复杂网络内的反应速率与 HSAPO-34 的材料特性之间存在因果关系。总营业额随着扩散约束的增加而减少，因为脱氢环化反应比烯烃甲基化、芳烃脱烷基化和甲醇转移氢化经历更强的扩散约束。对链烷烃的累积选择性随着扩散约束的增加而增加，因为甲醇、乙烯和丙烯的转移氢化反应比复杂反应网络内的所有其他反应经历更强的扩散约束。本文详述的方法将过程结果趋势的化学和物理起源与以复杂反应网络和普遍的空间和时间浓度梯度为特征的反应系统的系统变量进行编码。 (C) 2018 Elsevier Inc. 保留所有权利。
Reaction-transport formalisms show that the effects of crystallite size, H+ density, and Si speciation of HSAPO-34 on catalyst lifetime and selectivity in methanol-to-olefins catalysis are all manifestations of diffusional constraints. Both structural catalyst properties-i.e., crystallite size and H+ density- and intrinsic kinetic constants (per H+)-regulated, in silicoaluminophosphates, by Si speciation-affect the severity of these diffusional restrictions. Methanol-to-olefins catalysis on HSAPO-34 occurs by a complex network of autocatalytic reactions in which temporal gradients persist along with spatial gradients inherent to continuous-flow, fixed-bed reactors. Invocation of reaction rates in the interpretation of lifetime and selectivity trends in such systems requires defined quantities averaged in both time and space because measured observables conflate instantaneous reaction rates with spatial and temporal gradients. Quantities defined herein, i.e., total turnovers and cumulative selectivity, provide such rigorous assessments of lifetime and selectivity that permit causative correlation between rates of reactions within the complex network of autocatalytic reactions and material properties of HSAPO-34. Total turnovers decreases with increasing diffusional constraints because dehydrocyclization reactions experience stronger diffusional constraints than olefins methylation, aromatics dealkylation, and methanol transfer hydrogenation. Cumulative selectivity to paraffins increases with increasing diffusional constraints because transfer hydrogenation reactions of methanol, ethylene, and propylene experience stronger diffusional constraints than all other reactions within the complex reaction network. The approaches detailed herein codify the chemical and physical origins of trends in process outcomes with system variables for reaction systems characterized by complex reaction networks and prevailing spatial and temporal concentration gradients. (C) 2018 Elsevier Inc. All rights reserved.