Transalkylation of alkylaromatics catalyzed by acid zeolites is a process widely employed in the petrochemical industry for upgrading aromatic fractions. The reaction mechanism is complex as it can proceed either by intermolecular alkyl-transfer involving dealkylation-alkylation steps with surface alkoxy species as reaction intermediates, or through the formation of bulkier diaryl intermediates. We have investigated how the possible formation of such bulky intermediates in the microporous channel system of different zeolite structures, together with their stabilization by confinement effects, can determine the preferential mechanism and, therefore, the selectivity of ethylbenzene disproportionation into benzene and diethylbenzene. For testing the concept, four zeolites, MCM-22 (3D MWW) with 10R pores, 12R cavities and external 12R hemicavities or "cups", DS-ITQ-2, a 2D MWW with the same 10R channels as MCM-22, no 12R cavities and much larger proportion of external "cups", a 10R ZSM-5 (MFI) and a 12R mordenite (MOR) have been used. The higher activity of DS-ITQ-2 and MCM-22 as compared to ZSM-5, at low temperature (573 k) and the high selectivity to diethylbenzene of the bidimensional material under all reaction conditions considered have been explained by means of DFT calculations. Contrary to what could be expected according to the available space at the external "cups" and at the 10R channels of the MWW structure, the bulkier diaryl intermediates are better stabilized within the 10R channel system than at the "cups" open at the external surface of the MWW materials. We show from this perspective how the channel structure and molecular confinement stabilization also explain the operating reaction mechanism in ZSM-5 and mordenite.

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Chemical looping has great potential for reducing the energy penalty and associated costs of CO2 capture from fossil fuel-based power and chemical production while maintaining high efficiency. However, pressurized operation is a prerequisite for maximizing energy efficiency in most proposed chemical looping configurations, introducing significant complexities related to system design, operation and scale-up. Understanding the effects of pressurization on chemical looping systems is therefore important for realizing the expected cost reduction of CO2 capture and speed up the industrial deployment of this promising class of technologies.This paper reviews studies that investigated three key aspects associated with pressurized operation of chemical looping processes. First, the effect of pressure on the kinetics of the various reactions involved in these processes was discussed. Second, the different reactor configurations proposed for chemical looping were discussed in detail, focusing on their suitability for pressurized operation and highlighting potential technical challenges that may hinder successful operation and scale-up. Third, techno-economic assessment studies for these systems were reviewed, identifying the process configuration and integration options that maximize the energy efficiency and minimize the costs of CO2 avoidance.Prominent conclusions from the review include the following. First, the frequently reported negative effect of pressure on reaction kinetics appears to be overstated, implying that pressurization is an effective way to intensify chemical looping processes. Second, no clear winner could be identified from the six pressurized chemical looping reactor configurations

Chemical looping reforming (CLR) is a promising method for achieving autothermal methane reforming without the energy penalty of an air separation unit that is required for partial oxidation (POX) or oxygen-blown autothermal reforming (ATR). Scale-up of the conventional dual circulating fluidized bed CLR configuration is challenging, however, especially under the pressurized operating conditions required for high process efficiency. The internally circulating reactor (ICR) concept has previously been proposed as a simplified solution for chemical looping, especially under pressurized operation. It assembles the chemical looping process into a single reactor with two sections connected by specially designed ports for oxygen carrier circulation. This study has successfully demonstrated CLR operation in a dedicated ICR test unit with a NiO oxygen carrier. Up to 3 kW of methane feed was reformed to syngas, achieving conversion efficiencies as high as 98%. The reactor behaved largely as expected over a range of CH 4 /O 2 ratios and in a case with steam addition. Autothermal reactor operation could also be achieved, illustrating the practicality of the ICR concept. Based on this positive first demonstration study, further study of the ICR concept is recommended.

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