A Proposed Mechanism for Silica Supported Chromium HDPE Catalyst Activation
A Proposed Mechanism for Silica Supported Chromium HDPE Catalyst Activation
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二氧化硅负载铬 HDPE 催化剂活化的拟议机制
DOI：
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发表时间：
1996
期刊：
影响因子：
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通讯作者：
J. Blitz
中科院分区：
文献类型：
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作者：
S. Augustine;J. Blitz
采用等温动力学、程序升温反应（TPRxn）和变温漫反射红外光谱（VT-DRIFTS）研究了Cr 3（C2 H3 O2）7（OH）2 /SiO2在氧化、惰性和还原环境中的分解。基于这些结果，提出了二氧化硅负载的碱式醋酸铬的活化机理。在N2和CO/N2混合气体中的分解非常相似。活化能在实验误差范围内，并且都表现出相对于表面乙酸盐物种的二阶速率依赖性。TPRxn结果表明，Cr化合物影响载体的脱羟基反应，但VT-DRIFT谱没有发现Cr与SiO2表面键合的证据。当CO存在时，二氧化硅的脱羟基作用似乎是通过水煤气变换型反应进行的，该反应产生CO2和H2，而不是H2O。在氧气中，Cr化合物分解发生在比惰性或还原环境低90°C的温度下。氧的反应级数为1/2，表面乙酸盐的反应级数为1。活化能与其他两种介质的计算值相当。VT-DRIFT光谱表明，氧诱导更大程度的羟基去除和形成的Cr-O/Cr=O键的同时和随后记录的分解温度。以这种方式，他们支持TPR的结果，并建议Si-O-Cr键的形成。看来活化机制中的限速步骤是除去乙酸酯甲基。在氧气中，这涉及在Cr中心上的氧的解离活化和随后的燃烧。在其他环境中，迁移是必要的，以允许两个乙酸酯基团之间的氢转移形成甲烷并留下烃片段。这解释了不同的温度分解在各种介质和二阶速率依赖于乙酸在N2和CO/N2。在非氧化环境中没有明显的Cr-SiO2键的形成，使得Cr化合物的表面迁移成为可能。
Abstract The decomposition of Cr 3 (C 2 H 3 O 2 ) 7 (OH) 2 /SiO 2 in oxidizing, inert, and reducing environments was studied using isothermal kinetics, temperature-programmed reaction (TPRxn), and variable-temperature diffuse-reflectance infrared spectroscopy (VT-DRIFTS). Based upon these results a mechanism is proposed for the activation of silica-supported basic chromium acetate. The decompositions in N 2 and CO/N 2 mixtures appear very similar. The activation energies are within experimental error, and both exhibit second-order rate dependencies with respect to the surface acetate species. TPRxn results show that the Cr compound influences support dehydroxylation, but VT-DRIFT spectra show no evidence of Cr bonding to the SiO 2 surface. When CO is present, silica dehydroxylation appears to proceed via a water–gas shift type of reaction producing CO 2 and H 2 rather than H 2 O. In oxygen, Cr compound decomposition occurs at temperatures 90°C lower than in inert or reducing environments. The reaction orders are 1/2 for oxygen and 1 for the surface acetate species. The activation energy is comparable to that calculated for the other two media. VT-DRIFT spectra show that oxygen induces a greater degree of hydroxyl removal and formation of Cr–O/Cr=O bonds concurrent with and subsequent to recorded decomposition temperatures. In this way they support TPR findings and suggest Si–O–Cr bond formation. It appears that the rate-limiting step in the activation mechanism is removal of the acetate methyl group. In oxygen this involves dissociative activation of oxygen on the Cr center and subsequent combustion. In the other environments migration is necessary to allow hydrogen transfer between two acetate groups to form methane and leave behind a hydrocarbon fragment. This explains the different temperatures of decomposition in the various media and the second-order rate dependence upon acetate in N 2 and CO/N 2 . The lack of apparent Cr–SiO 2 bond formation in nonoxidizing environments allows surface migration of Cr compounds which makes this mechanism feasible.