Quantified Database for Methane Dehydroaromatization Reaction
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AbstractDirect conversion of methane to aromatics, known as methane dehydroaromatization (MDA) is a very promising onestep conversion route that could significantly reduce CO2 emissions and simplify the process. There is an increasing interest in this reaction evident by a large number of literature and patents published so far. Although this review examined 500 scientific articles studying the MDA reaction, the ones considered to be suitable to create a quantified database did not exceed 150articles. This resulted in more than 300experimental studies carried out at different combinations of catalyst compositions, and operating conditions for the reaction and the regeneration phases. This work provides a quantitative database in a unified manner, which sets it apart from other literature reviews in the field. A set of definitions are used to quantify the cyclic operation of this complex reaction that included deactivation and regeneration. Most of the studies in this work utilize the commonly used Mo/ZSM5 based catalyst. Other zeolites such as Mo/IM5, Mo/MCM22, and Mo/SiO2 are also included in this database with different metal compositions. This database is limited to experiments carried out using fixedbed reactors to allow a fair comparison. The quantified database generated many valuable results. The validity of different proposed research claims is tested against all collected experimental studies to verify the effect of Mo content, reaction temperature, reaction space velocity, and particle size. Most of the reported claims aligned with the reported data; however, they were only satisfied within a very narrow range of conditions which demonstrates how critical the correlation between the parameters is on the catalytic performance. This work proposed a simple decay model to quantify the rate of deactivation for the different studies. Additionally, figures of merit were generated from the quantified database to guide future experiments in this field, map out the performances achieved so far, as well as create a benchmark to identify bestperforming catalysts and operating conditions. A global yield is defined by taking into account the effect of deactivation and regeneration. This factor proved to be valuable as a benchmark of the enhancement in the catalyst stability and productivity. Some of the studies achieved high values for both the initial yield and the global yield. One study used a continuous feeding of CO2 in the feed and added Mg to the Mo/ZSM5 catalyst. Another study added minor amounts of C2H4 and C2H6 to the feed which help reduce regeneration and increase the yield. Studies that used periodicswitch feeding with H2, showed promising results as they maintained the performance for a longer time on stream for the same initial yield; thus, maximizing the global yield. A correlation analysis was performed on the MDA database to further statistically investigate the existence of correlations between input operating conditions (reaction temperature, reaction space velocity, and Mo. content) and the outputs (methane conversion, benzene yield, and decay factor). The absolute values of the correlation coefficients were categorized as weak [00.3], moderate [0.30.7], and strong [0.71]. The main highlights from this analysis are: (i)the conversion and yield are strongly correlated; (ii)Mo content has a nonlinear relation to the CH4 conversion. It has a positive moderate correlation for Mo content within the range of 1 to 3wt% while a negative moderate correlation within the range of 4 to 6wt%; (iii)space velocity and yield are strongly and negatively correlated; and (iv)there is a weak correlation between temperature and decay rate. The correlation analysis is efficient to analyze available data and provided similar conclusions as that of the knowledgebased approach. However, much more valuable as it quantifies the strength of the correlation. Besides, it imposes the need to study the operating conditions space more comprehensively. The correlation matrix, on its own, does not always provide decisive conclusions as shown by the effect of Mo content on CH4 conversion due to the nonlinear relationship between the variables. The real value is reached when the correlation matrix analysis is combined with the prior expert knowledge.
