The conversion of methane into valuable chemicals remains one of the major challenges in catalysis science. Both academia and industry are showing keen interest in developing direct one-step catalytic processes in contrast to the existing indirect technologies based on syngas (i.e., Fischer-Tropsch, Methanol-to-Hydrocarbons) that are highly energy-intensive and require high capital investment for the syngas preparation-compression units. Therefore, methane dehydroaromatization (MDA) catalysis on transition metal (i.e., Mo, W, Fe, V) loaded zeolites (i.e., ZSM-5, MCM-41, TNU-9) is still being studied by many researchers, as this process is considered to be one of the most promising alternatives despite the thermodynamic limitations and rapid catalyst deactivation. To develop stable catalysts and commercialize this process, one needs to understand the fundamentals of the reaction and investigate the possible process enhancement options. For instance, although several studies proposed the bifunctional pathway (CH4 coupling on activated Mo sites and oligomerization on Brønsted acids), the details of the mechanism are still ambiguous. Besides, there are options like H2O co-addition to enhance the performance and stability of catalysts against high coke deposition rates. However, a proper structure-function relationship during co-feeding was lacking in previously reported works. Furthermore, there can be alternative metals that may replace Mo. For example, tungsten oxides having similar chemical features with molybdenum oxides are thermally more stable; they can persist even during high-temperature air regeneration. However, W-supported catalysts cannot reach the activity levels that those based on Mo display. This performance difference between W and Mo should be investigated to improve the catalytic performance of W/ZSM-5. Also, the utilization of hierarchical zeolites in MDA catalysis has received a great deal of attention in the last two decades, since they have a great potential to help in improving catalysis performance and overall lifetime. However, when literature survey is done regarding this topic, it would be seen that there is a great inconsistency in many aspects (type of hierarchy, process performance, catalyst deactivation etc.) among the previous studies. Therefore, the hierarchical zeolite applications in MDA reactions should be revisited, and a more detailed discussions should be presented to catalysis community. Considering all these, we have developed new strategies to study MDA. First, we investigated the initial C-C bond formation mechanism during the early stages of MDA by applying “mobility-dependent” advanced ssNMR techniques on labeled methane (13CH4) treated Mo/ZSM-5 catalyst. We identified two mechanisms (mono- and bi-functional) leading to an initial C-C bond based on the detected species. Moreover, we elucidated the effects of H2O co-feeding over Mo/ZSM-5 by employing a battery of advanced characterization techniques. It has been found that water does not change the initial C-C bond formation mechanism but results in steam reforming reaction proceeding parallel to MDA. Also, we investigated W/ZSM-5 catalyst for MDA with a different perspective. The experiments conducted indicate that enhanced catalytic performance might be achieved if the dispersion and distribution of W sites on ZSM-5 can be tuned. Lastly, we revisited the hierarchical zeolite application in MDA catalysis. After analyzing our experimental results and the previous studies, a detailed discussion was presented to give some directions to those interested in hierarchy in zeolites.

Master of Science / Department of Chemical Engineering / Jennifer L. Anthony / Zeolite refers to a microporous material, which is also called a molecular sieve. There are three major industrial applications of zeolites: adsorbents, ion exchangers, and catalysts; and many other minor applications including: sensors, agriculture, medicine, veterinary, hydrogen storage, fuel cells, microreactors, membrane reactors, and racemic separations. Today, zeolite is not limited to aluminosilicate. Researchers are attempting to use other species (such as B, Ga, Ge, Ti, and Zn) to replace aluminum in zeolites framework to accomplish particular applications. In 1991, the first zincosilicate zeolite was synthesized by Annen et al.. Currently, only four zincosilicate zeolites have been synthesized. Theoretically, zincosilicate should balance divalent cations better than aluminosilicate zeolites to provide a stronger acid site especially for hydrogen cracking reactions. Large pore VET type VPI-8 (Li₁.₉₁₄Zn₁.₉₁₄Si₁₅.₀₈₆O₃₄) is the most thermal stable of all the zincosilicate zeolites and has low chemicals cost, however, a high crystallinity VPI-8 required prohibitively long synthesis times or seeded synthesis procedures. In this work, a seed-free short time synthesis zincosilicate zeolite VPI-8 is presented. Methane, also known as natural gas, had become the largest abundant carbon reserve today, more than the amount of the fossil fuel including conventional gas, oil, and coal. Unlike fossil fuel, methane can be recycled from landfill. Methane could be used to produce useful and/or expensive chemicals via syngas conversion to fuel, paraffin, methanol, alcohol, and dimethoxyethane. In addition to pathways via a syngas intermediate, methane could react directly to ethylene, formaldehyde, and aromatics. Because syngas preparation and compression usually expends 60-70% of the capital cost and consumes almost all the energy of operation, more and more researchers are exploring direct methane activation. However, the high stability of methane is one of the limitations, and coking is another limitation. In this work, methane dehydroaromatization (MDA) over zincosilicate zeolite Li-VPI-8 and ion exchanged Ni/Li-VPI-8 are investigated, due to the stronger acid site in zincosilicate than aluminosilicate zeolites. This is the first time to study using zincosilicate as catalyst, capitalizing on the more efficient synthesis methods demonstrated in this work.

The present thesis has focused on the intensive study of the methane dehydroaromatization process under non-oxidative conditions for producing benzene and H2 in a direct way. Nevertheless, MDA process is thermodynamically limited and, moreover, the catalyst quickly accumulates large amounts of carbonaceous deposits, which hinders its commercialization. Therefore, this thesis has as fundamental purposes the improvement of the catalytic activity and the stability of the catalyst on MDA reaction. The catalysts widely used on MDA reaction are Mo/zeolite, which are bifunctional, i.e., Mo sites are involved in the methane dehydrogenation and formation of CHx species, which are dimirized to C2Hy species, and Brønsted acid sites of the zeolite oligomerize these C2Hy species, forming mostly benzene and naphthalene. Thereby, different Mo/zeolite catalysts were prepared using commercial zeolites as well as zeolites synthesized on the laboratory. Thus, observing that the zeolite and the Mo content employed on the catalyst affected significantly the MDA performance. The topology and the channel dimensions of the zeolite as well as its Si/Al ratio and crystal size were also important on the MDA results obtained. Concretely, the best MDA performance was achieved by the 6%Mo/MCM-22 catalyst. Different catalyst activation procedures were tested, achieving the best MDA performance and catalyst stability using a gas mixture of CH4:H2, 1:4 (vol. ratio) during 1 h up to 700 ºC and maintaining this temperature for 2 h. This catalyst activation leads to the pre-carburization and pre-reduction of the Mo species, obtaining the most active and stable on MDA reaction. Moreover, the effect of the space velocity was studied in the present thesis. The best MDA results were reached at 1500 mL¿h-1¿gcat-1, as at higher space velocities methane barely can interact with the catalytic sites. While at lower space velocities the condensation of the heavy aromatic hydrocarbons is facilitated, causing higher coke accumulation on the catalyst. Furthermore, higher catalyst stability was obtained by co-feeding H2O, H2 and CO2 separately using the 6%Mo/HZSM-5 catalyst as well as the 6%Mo/MCM-22, due to the partial suppression of coke deposited. However, the catalytic activity was worsen by adding these co-reactants because of, on one hand, thermodynamically the addition of H2O, H2 or CO2 to the methane feed is detrimental and, on the other hand, H2O and CO2 partially re-oxidize the Mo species of the catalyst. Thermodynamically, H2 causes an equilibrium shift and, therefore, a decrease on the methane conversion; H2O favors the methane reforming reaction and coke gasification; and CO2 promotes the methane reforming reaction and the reverse Boudart reaction. The development and implementation of a catalytic membrane reactor (CMR) that integrates the 6%Mo/MCM-22 catalyst and the BZCY72 tubular membrane has been carried out on the present thesis. The MDA performance and the stability of the catalyst were exceptionally improved using this CMR by imposing a current to the electrochemical cell, changing or not the standard operating conditions. These good results were obtained due to the simultaneous H2 removal from MDA reaction side and O2 injection to this side through the BZCY72 tubular membrane. Thus, the H2 extraction results in the thermodynamic equilibrium displacement of MDA reaction, which causes the increase of the methane conversion and in turn of the aromatics yield. Moreover, the O2 injection involves the formation of H2O in low concentration, which reacts with coke accumulated (coke gasification), rising the stability of the catalyst. / La presente tesis se ha centrado en el estudio intensivo del proceso de deshidroaromatización de metano en condiciones no oxidativas para producir benceno e hidrógeno de forma directa. Sin embargo, el proceso de MDA está limitado termodinámicamente y, además, el catalizador acumula rápidamente grandes cantidades de depósitos carbonosos, lo que dificulta su comercialización. Por tanto, esta tesis tiene como objetivos fundamentales la mejora de la actividad catalítica y la estabilidad del catalizador en la reacción MDA. Los catalizadores Mo/zeolita son ampliamente utilizados en la reacción MDA, los cuales son bifuncionales, es decir, los sitios de Mo están involucrados en la deshidrogenación del metano y la formación de las especies CHx, las cuales se dimerizan a especies C2Hy, y los sitios ácidos de Brønsted de la zeolita oligomerizan éstas especies C2Hy, formando principalmente benceno y naftaleno. Por lo que, diferentes catalizadores Mo/zeolita se prepararon utilizando zeolitas tanto comerciales como sintetizadas en el laboratorio. Observando así que la zeolita y el contenido de Mo utilizados en el catalizador afectan significativamente el rendimiento de la reacción MDA. Tanto la topología y las dimensiones de los canales de la zeolita como su relación Si/Al y su tamaño de cristal son también importantes en los resultados obtenidos de la reacción MDA. Concretamente, el mejor rendimiento de MDA fue obtenido por el catalizador 6%Mo/MCM-22. Se probaron diferentes procedimientos de activación del catalizador, obteniendo el mejor rendimiento de la reacción MDA y estabilidad del catalizador usando una mezcla gaseosa de CH4:H2, 1:4 (relación en volumen) durante 1 h hasta 700 ºC y manteniendo esta temperatura durante 2 h. Esta activación del catalizador provoca la pre-carburización y pre-reducción de las especies de Mo, obteniendo las más activas y estables en la reacción de MDA. Los mejores resultados de MDA se obtuvieron con 1500 mL¿h-1¿gcat-1, ya que con mayores velocidades espaciales el metano apenas puede interaccionar con los sitios catalíticos. Mientras que con menores velocidades espaciales la condensación de los hidrocarburos aromáticos pesados se ve favorecida, provocando una mayor acumulación de coque en el catalizador. Por otra parte, co-alimentando H2O, H2 y CO2 por separado se obtuvo una mayor estabilidad tanto del catalizador 6%Mo/HZSM-5 como del 6%Mo/MCM-22, debido a la supresión parcial del coque depositado. Sin embargo, la actividad catalítica empeoró al añadir estos co-reactivos ya que, por un lado, la adición de H2O, H2 y CO2 a la alimentación de metano es perjudicial termodinámicamente y, por otro lado, el H2O y el CO2 re-oxidan parcialmente las especies Mo del catalizador. Termodinámicamente, el H2 provoca un cambio en el equilibrio y, por tanto, una disminución de la conversión de metano; el H2O favorece la reacción de reformado de metano y la gasificación de coque; y el CO2 promueve la reacción de reformado de metano y la reacción inversa de Boudart. En la presente tesis se ha llevado a cabo el desarrollo y la implementación de un reactor catalítico de membrana (CMR) que integra el catalizador 6%Mo/MCM-22 y la membrana tubular BZCY72. El rendimiento de la reacción MDA y la estabilidad del catalizador fueron excepcionalmente mejorados usando este CMR imponiendo una corriente a la celda electroquímica, cambiando o no las condiciones de operación estándar. Estos buenos resultados fueron obtenidos debido a la simultánea extracción de H2 del lado de reacción y la inyección de O2 a este lado mediante la membrana tubular BZCY72. Así, la extracción de H2 se traduce en un desplazamiento del equilibrio termodinámico de la reacción MDA, lo que causa el aumento de la conversion de metano y a su vez del rendimiento de aromáticos. Además, la inyección de O2 implica la formación de agua en baja concentración, la que reacciona con el coque acumulado (gas / La present tesi s'ha centrat en l'estudi intensiu del procés de deshidroaromatització de metà en condicions no oxidatives per produir benzé i hidrogen de forma directa. No obstant això, el procés de MDA està limitat termodinàmicament i, a més, el catalitzador acumula ràpidament grans quantitats de dipòsits carbonosos, el que dificulta la seva comercialització. Per tant, aquesta tesi té com a objectius fonamentals la millora de l'activitat catalítica i l'estabilitat del catalitzador en la reacció MDA. Els catalitzadors Mo/zeolita són àmpliament utilitzats en la reacció MDA, els quals són bifuncionals, és a dir, els llocs de Mo estan involucrats en la deshidrogenació del metà i la formació de les espècies CHx, les quals es dimeritzen a espècies C2Hy, i els llocs àcids de Brønsted de la zeolita oligomeritzan aquestes espècies C2Hy, formant principalment benzè i naftalè. Per tant, diferents catalitzadors Mo/zeolita es van preparar utilitzant zeolites tant comercials com sintetitzades al laboratori. Observant així que la zeolita i el contingut de Mo utilitzats en el catalitzador afecten significativament el rendiment de la reacció MDA. Tant la topologia i les dimensions dels canals de la zeolita com la seva relació Si/Al i el seu tamany de cristall són també importants en els resultats obtinguts de la reacció MDA. Concretament, el millor rendiment de MDA va ser obtingut pel catalitzador 6%Mo/MCM-22. Es van provar diferents procediments d'activació del catalitzador, obtenint el millor rendiment de la reacció MDA i estabilitat del catalitzador usant una mescla de gasos de CH4: H2, 1: 4 (relació en volum) durant 1 h fins a 700 ºC i mantenint aquesta temperatura durant 2 h. Aquesta activació del catalitzador provoca la pre-carburització i pre-reducció de les espècies de Mo, obtenint les més actives i estables en la reacció de MDA. A més, en la present tesi es va estudiar l'efecte de la velocitat espacial. Els millors resultats de MDA es van obtindre amb 1500 mL¿h-1¿gcat-1, ja que amb majors velocitats espacials el metà gairebé no pot interaccionar amb els llocs catalítics. Mentre que amb menors velocitats espacials la condensació dels hidrocarburs aromàtics pesants es veu afavorida, provocant una major acumulació de coc en el catalitzador. D'altra banda, co-alimentant H2O, H2 i CO2 per separat es va obtindre una major estabilitat tant del catalitzador 6%Mo/HZSM-5 com del 6%Mo/MCM-22, a causa de la supressió parcial del coc dipositat. No obstant això, l'activitat catalítica empitjorà en afegir aquests co-reactius ja que, d'una banda, l'addició d'H2O, H2 i CO2 a l'alimentació de metà és perjudicial termodinàmicament i, d'altra banda, el H2O i el CO2 re-oxiden parcialment les espècies Mo del catalitzador. Termodinàmicament, el H2 provoca un canvi en l'equilibri i, per tant, una disminució de la conversió de metà; l'H2O afavoreix la reacció de reformat de metà i la gasificació de coc; i el CO2 promou la reacció de reformat de metà i la reacció inversa de Boudart. En la present tesi s'ha dut a terme el desenvolupament i la implementació d'un reactor catalític de membrana (CMR) que integra el catalitzador 6%Mo/MCM-22 i la membrana tubular BZCY72. El rendiment de la reacció MDA i l'estabilitat del catalitzador van ser excepcionalment millorats usant aquest CMR imposant un corrent a la cel¿la electroquímica, canviant o no les condicions d'operació estàndard. Aquests bons resultats van ser obtinguts a causa de la simultània extracció d'H2 del costat de reacció i la injecció d'O2 a aquest costat per mitjà de la membrana tubular BZCY72. Així, l'extracció d'H2 es tradueix en un desplaçament de l'equilibri termodinàmic de la reacció MDA, el que causa l'augment de la conversió de metà i alhora del rendiment d'aromàtics. A més, la injecció d'O2 implica la formació d'aigua en baixa concentració, la qual reacciona amb el coc acumulat (gasificació de coc) / Zanón González, R. (2017). Intensification of methane dehydroaromatization process on catalytic reactors [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/83124 / TESIS

<p>  </p> <p>Cyclar process was previously developed to convert propane and butane into aromatics using gallium-promoted ZSM-5 zeolites (Ga/ZSM-5). However, it has two major limitations. Firstly, light gases (methane and ethane) limit the yield of higher molecular weight hydrocarbons for propane conversion. Secondly, ethane is unreactive on Ga/ZSM-5 catalysts. Relative rates and selectivity for propane conversion on two components, gallium (Ga/Al2O3) and acid ZSM-5 (H-ZSM-5) were investigated, and the results suggest that light gas was produced by propane monomolecular cracking on ZSM-5 due to the imbalance of alkane dehydrogenation and olefin conversion rates on two catalytic functions. A PtZn alloy catalyst, which has >99% propene selectivity and 30 times higher rate than Ga, was used for the dehydrogenation function. The bifunctional PtZn/SiO2+ZSM-5 catalyst has high yields of aromatics with low methane selectivity (<5%) at ~70% propane conversion. The results suggest methane can be minimized by utilizing the PtZn alloy and lowering the monomolecular cracking rate by ZSM-5. In addition, PtZn alloy increases aromatics selectivity. Aromatics formation pathway was investigated by studying the rate and selectivity of a model intermediate (cyclohexene) on ZSM-5, PtZn/SiO2 and Ga/Al2O3. Benzene is formed at similar rates on Ga/Al2O3 and ZSM-5 but cracking of cyclohexene on the latter is two orders of magnitude higher than the benzene formation rate, indicating cracking of cyclic hydrocarbons leads to low aromatization rate on Ga/ZSM-5. The benzene formation rate on the PtZn/SiO2 is 200 times higher than that on ZSM-5, suggesting aromatics are formed by the metal pathway on PtZn/SiO2+ZSM-5. </p> <p>Unlike Ga/ZSM-5 catalysts, PtZn/SiO2+ZSM-5 catalysts also convert propane to aromatics at low temperature (350 ℃). The temperature effect on propane dehydroaromatization pathways on the PtZn/SiO2+ZSM-5 bifunctional catalysts was investigated to develop strategies for propane conversion to valuable liquid hydrocarbons. At high temperature (550 ℃), high dehydrogenation rates and lower monomolecular cracking rates are required to minimize methane formation, leading to primarily propene and BTX (benzene, toluene, and xylenes). By recycling propene in the propane conversion range of 30-45%, >80% BTX yields is likely achievable at full recycle. At mid temperature (400-450 ℃), the product has high selectivity to gasoline-blending hydrocarbons (butanes, C5+ hydrocarbons, toluene, and xylenes) at 15-25% propane conversions because dehydrogenation rates are moderately high, and oligomerization is more favored than cracking. At low temperature (350℃), ~25% propane conversion is achieved and has high selectivity (~60%) to butanes, but the propane conversion rates are likely too low to be practical. While methane formation by monomolecular cracking limits liquid yields at high reaction temperature, at mid and low temperatures, hydrogen co-produced at high propane conversions saturates light olefins to make undesired ethane, which becomes major yield-loss reaction on the PtZn/SiO2+ZSM-5. </p> <p>Finally, PtZn/SiO2+ZSM-5 catalysts can convert ethane to C3+ and aromatics but the methane selectivity increases rapidly at high ethane conversion. The roles of two catalytic function (Pt-Zn alloy and ZSM-5) in the dehydroaromatization pathways of ethane and propane will be further studied and their product distribution will be compared to have better understandings on the differences in the dominant yield-loss reaction and dehydroaromatization pathways. </p>

The increasing production of natural gas liquids attracts both academia and industry to develop on-purpose techniques converting those light alkanes to value-added chemicals. Dehydroaromatization is an alternative path for light alkane conversion to produce aromatics but still lacks active and stable catalysts. This work aims at the development of efficient dehydroaromatization catalysts by tuning the metal/acid bifunctionality of the Pt/HZSM-5 catalyst. Additionally, through co-processing light alkane with ammonia during the dehydroaromatization process, this study also proposes a new reaction system that could directly link the C-N bond for nitrile synthesis. The results suggested that the activity, selectivity, and stability of the monometallic Pt/HZSM-5 catalyst are highly dependent upon the Pt loading, the limit loading of 100 ppm is required to maintain sufficient metal functionality. To further minimize the Pt loading, the chemical properties of the Pt species were tuned by a second metal such as Zn or Cu. Consequently, the activity and stability of the catalyst are enhanced by orders of magnitude and the maximized metal functionality was achieved at Pt loading of 10 ppm. Characterizations show that Pt can be atomically dispersed as a hybrid [Pt1-Zn6] cluster in the Pt-Zn@HZSM-5 or forming single atom alloy type [Pt1-Cu4] ensembles in the Pt-Cu@HZSM-5. Specifically, the initial turnover frequencies of propane and ethane to BTX are up to 178.8 and 128.7 s-1 over the Pt-Cu@HZSM-5, up to 3-4 orders of magnitude higher than the state-of-the-art Pt-based catalyst. Furthermore, the deactivated catalyst can be continuously regenerated, demonstrating excellent stability of such a catalyst under hash oxidation conditions for coke burn-off. A new catalytic system named ammodehydrogenation (ADeH) for ethane selective conversion to acetonitrile, ethylene, and hydrogen over a bifunctional catalyst is proposed. Ethane ADeH over the Pt/HZSM-5 catalyst is active at low temperatures and atmospheric pressure for CH3CN production. The Pt/HZSM-5 shows high coke-resistibility during the ethane ADeH due to the strong interaction of NH3 with the acid sites of the catalyst. The catalyst can be further optimized by adding Co, the Pt-Co/HZSM-5 catalyst on ethane ADeH indicating that an appropriate balance between the metal and acid functionalities is critical for ethane ADeH.