Aromatics from Syngas: CO Taking Control

In this issue of Chem, Cheng et al. report a stable and highly selective one-step conversion process of syngas to aromatics in which CO plays the crucial role of a selectivity mediator and not only a reactant. In this issue of Chem, Cheng et al. report a stable and highly selective one-step conversion process of syngas to aromatics in which CO plays the crucial role of a selectivity mediator and not only a reactant. Aromatic molecules such as benzene, toluene, ortho-xylene, and para-xylene are widely deployed in the petrochemical industry for the manufacture of styrene-, aniline-, phenol-, and terephthalic-acid-based polymers such as nylon, polyester, polyurethane, polystyrene, and polycarbonate. They are recovered from petroleum by distillation or extracted from coal tar. The majority of aromatics are produced by the catalytic re-formation of naphtha over a catalyst consisting of platinum metal dispersed on a chlorinated alumina support, but recently several zeolite-based processes have also been proposed. Some of these are designed around a Ga/ZSM-5 catalyst and use liquefied petroleum gas or naphtha as feedstock.1Vermeiren W. Gilson J.-P. Top. Catal. 2009; 52: 1131-1161Crossref Scopus (700) Google Scholar A sustainable alternative to petroleum, biomass pyrolysis gas can also be converted over a Ga/ZSM-5 catalyst with good overall carbon yields and process energy efficiency.2Cheng Y.-T. Jae J. Shi J. Fan W. Huber G.W. Angew. Chem. Int. Ed. 2012; 51: 1387-1390Crossref PubMed Scopus (328) Google Scholar Cellulose breaks down to anhydrosugars that dehydrate to furans, which are small enough to enter the micropores of a silico-aluminate molecular sieve (named zeolite), where they are involved in a series of reactions that could lead to the formation of 57% useful (and interchangeable) products (44% aromatics and 13% olefins), 43% waste products (24% coke, 17% CO, and 2% CO2), and water.2Cheng Y.-T. Jae J. Shi J. Fan W. Huber G.W. Angew. Chem. Int. Ed. 2012; 51: 1387-1390Crossref PubMed Scopus (328) Google Scholar A bifunctional catalyst reported by Kang Cheng et al. in this issue of Chem enables the direct conversion of synthesis gas (syngas: a mixture of carbon monoxide and hydrogen) to aromatics with a selectivity as high as 80% and without loss of performance over time.3Cheng K. Zhou W. Kang J. He S. Shi S. Zhang Q. Pan Y. Wen W. Wang Y. Chem. 2017; 3: 334-347Google Scholar Syngas is an important platform in the petrochemical industry because, on the one hand, it can be produced from any carbon-containing feedstock—renewable or fossil—such as methane (natural gas, biogas, or shale gas), biomass, or coal, and on the other hand, it is consumed to make a number of important key chemicals such as methanol and ammonia or a broad spectrum of hydrocarbons and chemicals in the Fischer-Tropsch polymerization process. The timing of this report by Cheng et al. is auspicious indeed given ExxonMobil's announcement of the purchase of the largest aromatics plant in the world in Singapore, which will increase its own production volume to 3.5 × 106 tons per annum.4Tullo A.H. Chem. Eng. News. 2017; 95: 18Google Scholar Furthermore, ever since the shale oil and gas boom in the late 2000s, crackers have shifted to converting more ethane and less naphtha, making less co-product aromatics in the process and resulting in steadily increasing prices for these compounds.5Tullo A.H. Chem. Eng. News. 2017; 95: 20-21Google Scholar The article by Cheng et al. follows closely on their own recent breakthrough in the direct conversion of syngas to lower olefins. By separating CO activation on a Zn-Zr binary oxide and C–C coupling of the methanol and methoxy intermediates to lower olefins on a SAPO zeolite, they achieved a selectivity higher than that over a catalyst that both activates CO and polymerizes CHx on the same metal-metal-carbide surface.6Cheng K. Gu B. Liu X. Kang J. Zhang Q. Wang Y. Angew. Chem. Int. Ed. Engl. 2016; 55: 4725-4728Crossref PubMed Scopus (419) Google Scholar Almost simultaneously, another study demonstrated similarly high selectivity for lower olefins also by disconnecting CO activation (on Zn-CrOx) and C–C bond formation (on mesoporous SAPO zeolite).7Jiao F. Li J. Pan X. Xiao J. Li H. Ma H. Wei M. Pan Y. Zhou Z. Li M. et al.Science. 2016; 351: 1065-1068Crossref PubMed Scopus (882) Google Scholar Here, the authors report a catalyst that is composed of Zn-ZrO2 nanoparticles and H-ZSM-5 zeolite and further converts lower olefins into aromatics starting from syngas. CO is activated at the oxygen vacancies of the ZrO2 lattice, and H2 can be activated on well-dispersed –Zn–O– sites on the Zn-ZrO2 nanoparticles.3Cheng K. Zhou W. Kang J. He S. Shi S. Zhang Q. Pan Y. Wen W. Wang Y. Chem. 2017; 3: 334-347Google Scholar The optimal loading of Zn increases CO conversion without hydrogenating C2–C4 olefins to paraffin. Indeed, when pushed above the optimum temperature, prompting secondary reactions, the more strongly hydrogenating methanol synthesis catalysts such as Cu-Zn-Al oxide or Cr-Zn-Al oxide convert CH3OH/DME predominantly into methane rather than lower olefins, whereas Zn-ZrO2 generates lower olefins and methane in equal measure. The selection of H-ZSM-5 with mordenite frame inverted (MFI) topology seems self-evident given the available literature, but the literature has never seen a ZSM-5 zeolite that does not deactivate in methanol-to-olefin, methanol-to-aromatic, or methanol-to-gasoline chemistry. The authors provide extensive experimental and catalyst characterization data to support their claim that the correct choice of density of strong Brønstedt acid sites is a crucial factor in achieving this stability. Surpassing the optimal density of acid sites leads to hydrogenation of C2=–C4= rather than their conversion to aromatics. The proximity of zeolite and Zn-ZrO2 particles was shown to be important, and consequently nano-sized H-ZSM-5 was synthesized and employed. The dispersion of Zn-ZrO2 remained unaltered after many hours of operation. To further demonstrate their command of this novel catalyst system, the authors successfully augmented the benzene-toluene-xylene fraction of the total aromatics by a silylation treatment of the external surface of the zeolite, thereby exploiting one of the oldest tricks in the book: shape selectivity by confinement in the micropores. The authors have shown that Zn-ZrO2 is not impeded by 1 vol % CO2 at the most productive reaction conditions or affected by the common poison H2S. The advantages of a catalyst that does not deactivate in the design of a process cannot be over-emphasized. In contrast, the methane dehydroaromatization on Mo/H-ZSM-5 can produce a continued output of aromatics only by a cyclic combustion of built-up coke.8Kosinov N. Coumans F.J.A.G. Li G. Uslamin E. Mezari B. Wijpkema A.S.G. Pidko E.A. Hensen E.J.M. J. Catal. 2017; 346: 125-133Crossref Scopus (109) Google Scholar Or in oil-refining's flagship example of a complicated process, fluid catalytic cracking integrates productive and regenerative phases in a continuous operation by physically separating gasoline production and coke combustion. Perhaps the most intriguing discovery to be unveiled in this study is the peculiar role of CO beyond merely being the reagent for the formation of methanol that is then converted to lower olefins and ultimately aromatics. The authors found that administration of methanol in N2 or H2 to this catalyst—thereby skipping the methanol synthesis step from syngas CO and H2—surprisingly formed low amounts of aromatics. CO is indispensable, and high partial pressures of CO help to achieve high aromatics selectivities, but this occurs only on Zn-ZrO2/H-ZSM5 and not on H-ZSM-5 without the Zn-ZrO2 particles. The fact that isotope-labeled 13CO ended up in the aromatic products in the reaction with methanol gives further support to the CO control effect. The authors suggest a mechanism in which CO assists the dehydrogenative aromatization—which is believed to be the rate-determining step—by accepting the hydrogen species that are released and thereby reducing itself and joining the methanol moiety (Figure 1). The research reported by Cheng et al. should be of interest to both academia and industry. It is heartening to read of such a rich reward from such an extensive experimental study. This new process and its catalyst could play a major role in future sustainable production of aromatics. This discovery could also further spur development and application of other syngas-to-chemical processes,9Claeys M. Nature. 2016; 538: 44-45Crossref PubMed Scopus (21) Google Scholar which could become increasingly important during times of increased availability of natural gas and shale gas and growing use of biomass as a feedstock for chemical production. Bifunctional Catalysts for One-Step Conversion of Syngas into Aromatics with Excellent Selectivity and StabilityCheng et al.ChemAugust 3, 2017In BriefWang and colleagues successfully designed a powerful bifunctional catalyst composed of Zn-doped ZrO2 nanoparticles and zeolite H-ZSM-5 for one-step conversion of syngas into aromatics. Aromatics with 80% selectivity were obtained at 20% CO conversion. No catalyst deactivation was observed in 1,000 hr. Methanol and dimethyl ether were formed as reaction intermediates on Zn-doped ZrO2, which were subsequently transformed into aromatics on H-ZSM-5 via olefins. This work offers a highly selective and stable non-petroleum route for the synthesis of aromatics. Full-Text PDF Open Archive