Deactivation Studies of the CrOx/Al₂O₃ Dehydrogenation Catalysts under Cyclic Redox Conditions

Abstract

In this work, the deactivation of a CrOx/Al 2 O 3 dehydrogenation catalyst in an industrial reactor has been systematically studied under cyclic dehydrogenation−regeneration conditions from start up to end of run. The results showed that the CrOx/Al 2 O 3 catalyst deactivation occurred through three major processes. The first process is related to the complex transformation of surface chromium species, the second process refers to the phase transformation and/or sintering of support alumina, and the third process is the interfacial CrOx/Al 2 O 3 transformation into the solid solution of α-(Al, Cr) 2 O 3 . In addition, the reduction of the chromium oxide surface area and the migration of Cr 3+ into the alumina support resulted in the increase of the surface Cr 6+ /Cr 3+ ratio at the end of the run which increased the specific catalyst acidity and reduced the catalyst selectivity. The main causes of irreversible deactivation of the CrOx/Al 2 O 3 catalyst are the decrease of chromium oxide surface area, the formation of α-(Al, Cr) 2 O 3 in the support, and the increase of surface Cr 6+ /Cr 3+ ratio.

The Effect of Transition Alumina (γ‐, η‐, χ‐Al2O3) on the Activity and Stability of Chromia/Alumina Catalysts. Part II: Industrial‐Like Catalysts and Real Plant Aging Conditions

“…However, Bol'shakov et al did not discuss the reasons for the difference in stability between various catalysts originally characterized by similar catalytic and physicochemical properties . A recent work by Fridman and Xing revealed some patterns of aging over 645 days on a stream of the old version of the chromia/alumina catalyst for the Catofin process . The results of the study by Fridman and Xing are consistent in most aspects with the conclusions to the study by Bol'shakov et al, although they are more detailed.…”

“…The lifetime of industrial chromia/alumina dehydrogenation catalysts varies from several months to several years; thus, the study of the irreversible deactivation (“aging”) of these systems is a very time‐consuming task and, as a consequence, the number of studies in this field is low. For example, Bol'shakov et al compared the properties of several industrial catalysts of unknown composition before and after operation in vacuum dehydrogenation of n ‐butane for about a year .…”

The Effect of Transition Alumina (γ‐, η‐, χ‐Al2O3) on the Activity and Stability of Chromia/Alumina Catalysts. Part I: Model Catalysts and Aging Conditions

“…According to the literature data, sintering of the support and the formation of chromia‐alumina solid solutions are the causes of irreversible deactivation of chromia/alumina dehydrogenation catalysts . Herewith, the formation of (Cr,Al) 2 O 3 may be a consequence of both the encapsulation of CrO x in the sintering process and the migration of Cr 3+ into Al 2 O 3 .…”

“…It should be noted that the catalysts used in this work are model and their aging conditions are artificial. Real industrial catalysts contain about 12–13 wt% Cr and are operated at temperatures below 800 °C for several years . In addition, under real conditions, the catalyst is not only exposed to the oxidizing atmosphere but also used in alternating reduction‐oxidation steps.…”

“…This is due to the fact that the dehydrogenation reaction is thermodynamically limited and endothermic . Such a high temperature is favorable for side reactions leading to coking . The coked catalysts are regenerated with air at temperatures up to 700 °C to restore their activity.…”