High Olefin Selectivity Research Articles

Hydrogen transfer reaction in butene catalytic cracking over ZSM-5

Hydrogen transfer reaction (HTR) is the pivotal side reaction in the catalytic cracking process of low carbon olefins. The intricate reaction pathways and product diversity of HTR directly impact the selective formation of ethylene and propylene. Therefore, elucidating the key HTR in various reaction pathways and defining the hydrogen transfer index (HTI) as a criterion lay a scientific foundation for precisely regulating HTR during olefin catalytic cracking process. Herein, the influence of the acid strength of ZSM-5 zeolites on the HTR degree was analyzed in butene catalytic cracking. Results showed that isobutane was the predominant component of HTR products, mainly derived from HTR during the dimerization-cracking of pentene (butene primary cracking product). Subsequent pentene catalytic cracking experiments validated this conclusion. Thus, the HTI in butene or pentene cracking process was defined as follows: for butene cracking process, HTI = Si–C4H10/SC5H10; for pentene cracking process, HTI = Si–C4H10/SC4H8. The HTI accurately reflected the extent of HTR with respect to the acid properties of the catalysts. Moreover, the reaction network of butene catalytic cracking process was optimized, providing a comprehensive explanation for the intriguing phenomenon of decreasing butene conversion with increasing reaction temperature when the acid strength of ZSM-5 was weak. Finally, a high-performance butene catalytic cracking catalyst, De-TS-1-0.25%P, was developed, exhibiting high olefin selectivity (92.33 %) and outstanding stability (307 h) in the conversion of butene.

Tuning selectivity in low-temperature Fischer-Tropsch synthesis by applying gas recycle mode

While hydrocarbons production by Fischer-Tropsch synthesis (FTS) is widely recognized, commercial interest in FTS products is mainly constrained by oil prices. The FTS-derived fuels, although higher quality and greener, cannot currently compete with cheaper fossil fuels, motivating to shift the FTS products distribution to higher value-added chemicals, such as olefins-C5+. The high olefin selectivity in FTS over Co-based catalysts is usually ascribed to Co2C on the catalyst surface with its inherent deficiencies. This study demonstrates a novel approach to alter low-temperature FTS selectivity by applying off-gas recycling. At slow gas recycling rates, C19+ selectivity increases due to re-adsorption and re-incorporation into the growing chain of gaseous olefins from the off-gas. High-intensity gas recycling facilitates C5+ olefin vapors removal from the reactor by enhanced gas flow with their subsequent cut-off in the cold trap, thus reducing residence time of condensed olefins and probability of their secondary reactions.

Control of metal-support interaction for tunable CO hydrogenation performance over Ru/TiO2 nanocatalysts.

The catalytic behavior of CO hydrogenation can be modulated by metal-support interactions, while the role of the support remains elusive. Herein, we demonstrate that the presence of strong metal-support interactions (SMSI) depends strongly on the crystal phase of TiO2 (rutile or anatase) and the treatment conditions for the TiO2 support, which could critically control the activity and selectivity of Ru-based nanocatalysts for CO hydrogenation. High CO conversion and olefin selectivity were observed for Ru/rutile-TiO2 (Ru/r-TiO2), while catalysts supported by anatase (a-TiO2) showed almost no activity. Characterization confirmed that the SMSI effect could be neglected for Ru/r-TiO2, while it is dominant on Ru/a-TiO2 after reduction at 300 °C, resulting in the coverage of Ru nanoparticles by TiOx overlayers. Such SMSI could be suppressed by H2 treatment of the a-TiO2 support and the catalytic activity of the as-obtained Ru/a-TiO2(H2) can be greatly elevated from almost inactive to >50% CO conversion with >60% olefin selectivity. Further results indicated that the surface reducibility of the TiO2 support determines the SMSI state and catalytic performance of Ru/TiO2 in the CO hydrogenation reaction. This work offers an effective strategy to design efficient catalysts for the FTO reaction by regulating the crystal phase of the support.

Synthesis of high-quality nano-H-ZSM-5 single crystals zeolite from montmorillonite and MTO catalytic studies

Clay is a rich aluminosilicate resource with unique advantages as a raw material for zeolites production. However, synthesizing nanosized single-crystalline zeolites through a simple approach using clay remains a significant challenge. In this paper, acid-washed montmorillonite (MSR) was used to prepare nano-H-ZSM-5 single crystals with high crystallinity, uniform grain size and permeable straight pores under the induction of a small amount of organic template tetrapropylammonium hydroxide (TPAOH). Through temperature-staged treatment, the nucleation and growth process of zeolite were decoupled, ultimately achieving precise control over the size of the nano-single crystals. Mechanistic study showed that the low-temperature aging process produces smaller structural units which probably derived from the geopolymer topology of the clay, providing more active sites for zeolite nucleation, leading to the rapid crystallization during the high-temperature process. The porous structure promotes material diffusion, inhibits aromatic cycling and reduces carbon formation in the pores. The large specific surface area and nanoscale multilayer pore structure delay carbon accumulation and increase carbon holding capacity. These properties make nano-ZSM-5 single crystals an excellent choice for high olefin selectivity and long catalytic life in the methanol-to-olefin reaction. This synthetic route provides a general strategy for preparing nanosized zeolite single crystals from clay.

Preparation of Fe-based catalysts from waste biomass as a carbon carrier and its catalytic performance in CO2 hydrogenation

Fe/LC composite catalyst was synthesized by one-step carbonization method and exhibited high olefin selectivity in CO2 hydrogenation.

Fischer-Tropsch synthesis to α-Olefins with low CO2 selectivity on a Co2C catalyst

A Co2C catalyst with low CO2 selectivity for the Fischer-Tropsch to Olefins (FTO) was derived from a precursor catalyst prepared by a “melting” method. Co-Mn composite oxide, MnCo2O4.5, was identified as the dominant composition in the catalytic material before it was activated. The catalyst showed a weak interaction between Co and Mn with the presence of Na. Therefore, Co could be liberated easily from MnCo2O4.5 with H2 reduction at a usually used condition for Co-based catalyst for the Fischer-Tropsch Synthesis (FTS). A clear carburization of Co0 to Co2C was observed during the initial period of reaction, and this transformation could be achieved within 6 h. Four different crystal facets, (210), (020), (111), and (101) for Co2C, were identified once Co0 has been successfully carburized during the reaction. A limited amount of Co0 also transformed to Co3O4 during Co2C formation when H2O was produced by the FT reaction. The prepared catalyst demonstrated outstanding performance in terms of product selectivity. Its selectivity to lower olefins (C2-C4) was as high as 60.26 % with an average olefin/paraffin ratio of 26.69. Additional to high olefin selectivity, the CO2 selectivity was as low as 11.49 %, and that of C1 (CO2 + CH4) could be controlled at 16.44 % with optimal Co/Mn ratio and operating conditions. It was also found that transient state Co0 with Mn and Na as promoters demonstrated similar high olefin to paraffin ratios when compared to Co2C.

Rationalizing kinetic behaviors of isolated boron sites catalyzed oxidative dehydrogenation of propane

Boron-based catalysts exhibit high alkene selectivity in oxidative dehydrogenation of propane (ODHP) but the mechanistic understanding remains incomplete. In this work, we show that the hydroxylation of framework boron species via steaming not only enhances the ODHP rate on both B-MFI and B-BEA, but also impacts the kinetics of the reaction. The altered activity, propane reaction order and the activation energy could be attributed to the hydrolysis of framework [B(OSi≡)3] unit to [B(OSi≡)3−x(OH···O(H)Si≡)x] (x = 1, 2, “···” represents hydrogen bonding). DFT calculations confirm that hydroxylated framework boron sites could stabilize radical species, e.g., hydroperoxyl radical, further facilitating the gas-phase radical mechanism. Variations in the contributions from gas-phase radical mechanisms in ODHP lead to the linear correlation between activation enthalpy and entropy on borosilicate zeolites. Insights gained in this work offer a general mechanistic framework to rationalize the kinetic behavior of the ODHP on boron-based catalysts.

Self-evolved BOx anchored on Mg2B2O5 crystallites for high-performance oxidative dehydrogenation of propane

Oxidative dehydrogenation of propane (ODHP) is a promising process for producing propene. Recently, some boron-based catalysts have exhibited excellent olefin selectivity in ODHP. However, their complex synthetic routes and poor stability under high-temperature reaction conditions have hindered their practical application. Herein, we report a self-evolution method rather than conventional assembly approaches to acquire structures with excellent stability under a high propane conversion, from a single precursor-MgB2. The catalyst feasibly prepared and optimized exhibited a striking performance: 60% propane conversion with a 43.2% olefin yield at 535°C. The BOx corona pinned by the strong interaction with the borate enabled zero loss of the high conversion (around 40%) and olefins selectivity (above 80%) for over 100h at 520°C. This all-in-one strategy of deriving all the necessary components from just one raw chemical provides a new way to synthesize effective and economic catalysts for potential industrial implementation.

Boosting CO2 hydrogenation of Fe-based monolithic catalysts via 3D printing technology-induced heat/mass-transfer enhancements

The direct transformation of CO2 into valuable chemicals with the aid of sustainable energy-generated H2 has attracted enormous interest owing to the integrated functions of carbon elimination and non-fossil fuel-derived products supply. As one of the most promising catalysts for CO2 hydrogenation, the chemical properties of Fe-based catalysts, such as electronic structure and coordination environment, have been widely studied. However, the mass/heat transfer effects in Fe-based catalysts are also crucial to the targeted product selectivity and catalytic stability, but have been rarely investigated due to the lack of facile fabrication protocols. Herein, we precisely fabricate and tailor the architecture of the Fe-based monolithic catalysts by a direct ink writing (DIW)-type three-dimensional (3D) printing technology under the guidance of the computationally controlled printing procedure. The Fe-based monolithic catalyst with spiral-type architecture delivers extremely high light olefins selectivity (52.6%) and space time yield (STY, 451.8 gCH2 kgcat−1 h−1) from CO2 hydrogenation. Based on the mass-transfer simulation, the spiral-structured channels in the Fe-based monolithic catalyst lower the coverage of intermediates and products on the catalytic interface due to the optimal mass-transfer effect, thus maximizing the utilization of active sites and timely terminating the carbon-chain growth. More in-depth, the density functional theoretical (DFT) simulations verify that the relatively electron-rich catalytic interface with low intermediate coverage could facilitate the desorption of olefins and decelerate the C-C coupling step, which synergistically guarantee the enhanced light olefins synthesis performance. Furthermore, the enhanced heat-transfer effect endowed by the 3D architecture prolongs the life-time of catalyst by avoiding undesirable active site aggregation and carbon deposition. The powerful strategy for catalyst fabrication not only provides a new concept of regulating the CO2 hydrogenation performance of Fe-based catalysts but also holds a great promise to spread into other catalytic systems for targeted synthesis.

An investigation on the effect of cross-linking state of silicon species on the distribution of framework Al atoms in ZSM-5 zeolite and its catalytic performance for MTO reaction

Controllable regulation of Al atoms distribution in ZSM-5 zeolite framework can effectively modulate product selectivity for MTO reaction. Here, the Al atoms distribution of ZSM-5 zeolite can be adjusted by controlling the cross-linking state of silicon species. Detailed explorations show that, the surface of ZSM-5 zeolite is rich in silicon when low cross-linking state silicon species is silicon source, while the surface of ZSM-5 zeolite is rich in aluminum when high cross-linking state silicon species is silicon source. When the cross-linking state of silicon species presents a “gripper-like” states, Al atoms of ZSM-5 zeolites are homogeneously distributed, and thus, Al atoms tends to be distributed in straight or sinusoidal channels, resulting in high olefin selectivity and low double cycle index in the catalytic MTO reaction process. This work would be beneficial to the design of ZSM-5 zeolite catalyst with desirable acid site distribution for specific reactions.

Unraveling the role of GaZrOx structure and oxygen vacancy in bifunctional catalyst for highly active and selective conversion of syngas into light olefins

The direct conversion of syngas to light olefins (STO) over metal oxide-zeolite (OX-ZEO) bifunctional catalyst has aroused wide interest. However, due to unmatched reaction temperature for CO conversion to oxygenated hydrocarbons over metal oxide (200–300 °C) and C−C coupling reaction over zeolite (350–450 °C), it is still a great challenge to achieve both high light olefins selectivity and high CO conversion; moreover, the reaction mechanism needs to be further studied. Herein, the Ga-Zr oxide is taken as a probe oxide to mix with SAPO-34 zeolite for STO. A high light olefins selectivity of 88.6% is achieved at an extremely high CO conversion of 51.6% with a stable activity for 200 h. The incorporation of Ga into ZrO2 generates the GaZrOx solid solution structure with abundant concentration of oxygen vacancy and hydroxy group, the strong synergetic effect between Ga and Zr in GaZrOx benefits the CO activation and CH3O* formation at high temperatures (280–430 °C), providing a compatible temperature match for C−C coupling over SAPO-34. The GaZrOx is a promising and potential catalyst for industrial application in direct conversion of syngas to olefins, aromatics, and liquid fuels.

Hydrophobic interfaces regulate iron carbide phases and catalytic performance of FeZnOx nanoparticles for Fischer-Tropsch to olefins

Fischer-Tropsch to olefins (FTO) with controllable iron carbide phase and high carbon utilization efficiency for Fe-based nanocatalyst attracts great interest but remains challenge. Herein, we report that the metastable Fe7C3 phase can be effectively tailored by altering shell thickness of hydrophobic SiO2 over a core-shell nanostructured FeZn@SiO2-c nanocatalyst. χ-Fe5C2 was the dominated iron carbide phase for FeZnOx and FeZn@SiO2 nanocatalysts, while numerous Fe7C3 phases existed for hydrophobic FeZn@SiO2-c nanospheres. Compared with FeZnOx, the CO2 selectivity of FeZn@ 4.1-SiO2-c decreased by > 70%, while 1.7-fold higher olefins selectivity was simultaneously obtained during syngas conversion process. The hydrophobic interface greatly suppressed the water-gas-shift reaction by promoting the quick diffusion of water, and the as-formed H2-lean and CO-rich local chemical environment benefits the formation of Fe7C3 and olefins products. This work provides a promising strategy to design phase-stable Fe7C3 with improvement of carbon efficiency for Fe-based nanocatalysts.

Hydrophobic modification for CO photo-hydrogenation to olefins with low CO2 selectivity

CO photo-hydrogenation is currently attracting a lot of interest as a low energy input technology for the sustainable production of olefins. Herein, a novel photo-driven Fischer-Tropsch to olefins (FTO) catalyst was synthesized by physical mixing a layered-double-hydroxide-derived cobalt catalyst (LD-Co) with a hydrophobic polydivinylbenzene (PDVB). The obtained LD-Co/PDVB catalyst synergistically harnessed the photothermal and catalytic properties of LD-Co with the hydrophobic properties of PDVB to deliver outstanding photothermal FTO performance. Under ultraviolet-visible irradiation, hydrophobic LD-Co/PDVB delivered a higher CO photo-hydrogenation activity than hydrophilic LD-Co, together with a high olefin selectivity (42.72%−50.49%) and greatly suppressed CO2 selectivity (2.89%−11.50%) compared to LD-Co (12.65%−30.61%) at 180–195 °C. Water vapor adsorption-desorption experiments and water-gas shift (WGS) reaction tests revealed that the hydrophobic modification acted to suppress the WGS reaction, thereby boosting the olefin production and suppressing CO2 selectivity. The findings demonstrated that a simple hydrophobic modification can be used to regulate the activity and selectivity of catalysts for photothermal FTO reactions, opening new vistas towards improved photo-driven olefin production.

Antiexfoliating h-BN⊃In2O3 Catalyst for Oxidative Dehydrogenation of Propane in a High-Temperature and Water-Rich Environment

Hexagonal boron nitride (h-BN) is regarded as one of the most efficient catalysts for oxidative dehydrogenation of propane (ODHP) with high olefin selectivity and productivity. However, the loss of the boron component under a high concentration of water vapor and high temperature seriously hinders its further development. How to make h-BN a stable ODHP catalyst is one of the biggest scientific challenges at present. Herein, we construct h-BN⊃xIn2O3 composite catalysts through the atomic layer deposition (ALD) process. After high-temperature treatment in ODHP reaction conditions, the In2O3 nanoparticles (NPs) are dispersed on the edge of h-BN and observed to be encapsulated by ultrathin boron oxide (BOx) overlayer. A novel strong metal oxide-support interaction (SMOSI) effect between In2O3 NPs and h-BN is observed for the first time. The material characterization reveals that the SMOSI not only improves the interlayer force between h-BN layers with a pinning model but also reduces the affinity of the B-N bond toward O• for inhibiting oxidative cutting of h-BN into fragments at a high temperature and water-rich environment. With the pinning effect of the SMOSI, the catalytic stability of h-BN⊃70In2O3 has been extended nearly five times than that of pristine h-BN, and the intrinsic olefin selectivity/productivity of h-BN is well maintained.

Tuning Fischer-Tropsch synthesis product distribution toward light olefins over nitrided Fe-Mn bimetallic catalysts

A series of Fe-Mn bimetallic catalysts with various pretreatment atmosphere including H2, CO and alternatively NH3, were employed for the Fischer–Tropsch synthesis (FTS). The structure of Fe-Mn bimetallic catalysts was revealed by using multiple characterization techniques, where Mössbauer spectra was used to define the iron species. With direct pretreatment, the reduction efficiency is in the sequence of H2 > CO > NH3. Fe-Mn bimetallic catalysts pre-reduced by H2 that permit an easy nitrogenation. To correlate the phase compositions of spent catalysts with FTS performance, the individual contribution of Fe, iron carbides (χ-Fe5C2, ε-Fe2(.2)C and θ-Fe3C) and iron nitride (Fe2N) to hydrocarbon selectivity was deconvolved by linear regression. It was discovered that C2=-C4= selectivity was proportional to the content of epsilon-iron carbide, which was promoted by the doping with Mn and further nitriding treatment. Successfully nitrided Fe-Mn bimetallic catalysts exhibit a better stability of C2-C4 O/P along time on stream, while either direct CO treatment or NH3 treatment followed by CO treatment result in a rapid decent of C2-C4 O/P ratio against time. Furthermore, Mn enhances long-chain hydrocarbon selectivity and suppresses CH4 formation, whereas the nitrogenation inhibits the chain growth ability, concentrating the hydrocarbon selectivity into C2-C4 fraction. Hence, the nitrided Fe-Mn catalyst is able to maintain a high light olefin selectivity. This study highlights that nitrogenation could provide an opportunity to tune the C2-C4 olefins by optimizing the phase structure of Fe-Mn catalysts.

Kinetic investigations of oxidative dehydrogenation of propane on boron oxide in confined spaces

Oxidative dehydrogenation of propane (ODHP) is a promising process for on-purpose propylene production with favorable thermodynamics. Boron-based catalysts exhibit high alkene selectivity in ODHP, but the understanding of catalytic mechanism remains incomplete, especially regarding the observed supra-linear reaction order of alkanes. In this work, we studied the impact of spatial confinement on the performance and kinetics of boron oxide supported on silica with different pore structures. The formation rate of propylene decreases with the degree of confinement of active boron sites derived from the support. Intriguingly, the apparent activation energy increases with the ODHP activity, and exhibits a positive correlation with the apparent propane reaction order. The apparent propane order of B2O3/SiO2 varies from 1 to 3, which is impacted by pore structure of supports, loading of B2O3, and partial pressure of propane. The trend in propane reaction order and apparent activation energy on B2O3 can be rationalized by the rate expression derived based on the proposed radical chain mechanism with three distinct pathways of propane activation.

Insight into Mn enhanced short-chain olefins selectivity in CO2 hydrogenation over Na-CuFeO2 catalyst

In this presentation, Mn-promoted Na-CuFeO2 catalysts were prepared with a two-step method, hydrothermal synthesis of Na-CuFeO2 followed by impregnation of Mn. The developed material is explored for single-step CO2 hydrogenation towards short-chain olefins through a modified Fischer-Tropsch route. The addition of the appropriate amount of Mn over Na-CuFeO2 significantly limits the C5+ hydrocarbon formation while improving the short-chain olefins selectivity in the product stream. Among the different compositional catalysts, 2.5% Mn/ Na-CuFeO2 demonstrated excellent catalytic performance with a CO2 conversion of 36.6% with high C2-C4 olefins selectivity (35.7%), high O/P ratio (3.9), and a space–time yield of around 4.2 mmol gcat−1 h−1 at 320 °C temperature, 20 bar pressure and 3600 ml gcat−1 h−1 space velocity. The H2-TPR, XPS, and CO2-TPD of the catalysts indicate that adding the manganese increases the basicity which increases the CO2 conversion as well as reducibility of the catalyst and thus facilitates the formation of Hägg carbide (Fe5C2), the active catalysts site. DFT calculations were performed on the CuFeO2(102), Mn4O4-CuFeO2(102), and Na-Mn4O4-CuFeO2(102) surfaces, demonstrating that the MnO promotional effect lowers the energy barrier for oxygen vacancy generation. Also, a kinetic estimation of the experimental data under varying reaction conditions illustrates the promotional effect of Mn in lowering the activation energy of the direct CO2 hydrogenation compared to the unpromoted catalyst. The values of activation energy and rate constants at temperature 320 °C are 28.483 kJ/mol & 1.69 × 10−4 mol (min gcat bar1/3)−1 and 29.358 kJ/mol & 3.35 × 10−5 mol (min gcat bar1/3)−1, respectively for the Mn-promoted and unpromoted Na-CuFeO2 catalysts. The activation energy values derived from the experimental data presented herein are in good agreement with that reported (25–50 kJ/mol) in literature for iron catalysts.

Auto-accelerated dehydrogenation of alkane assisted by in-situ formed olefins over boron nitride under aerobic conditions

Oxidative dehydrogenation (ODH) of alkane over boron nitride (BN) catalyst exhibits high olefin selectivity as well as a small ecological carbon footprint. Here we report an unusual phenomenon that the in-situ formed olefins under reactions are in turn actively accelerating parent alkane conversion over BN by interacting with hydroperoxyl and alkoxyl radicals and generating reactive species which promote oxidation of alkane and olefin formation, through feeding a mixture of alkane and olefin and DFT calculations. The isotope tracer studies reveal the cleavage of C-C bond in propylene when co-existing with propane, directly evidencing the deep-oxidation of olefins occur in the ODH reaction over BN. Furthermore, enhancing the activation of ethane by the in-situ formed olefins from propane is successfully realized at lower temperature by co-feeding alkane mixture strategy. This work unveils the realistic ODH reaction pathway over BN and provides an insight into efficiently producing olefins.

Selective preparation of light olefins from cellulose-derived fermentation intermediates

Directional synthesis of bio-based light olefins has great significance for promoting sustainable development of chemical industry. Present work proves that light olefins can be selectively prepared from the cellulose-derived acetonebutanol-ethanol. This transformation has been achieved by coupling cellulose fermentation and acetone-butanolethanol catalytic dehydration over the Ce@SAPO-34 catalyst. The active sites and reusability of the catalyst were investigated. High acetone-butanolethanol conversion (91.9%) and high olefin selectivity (86.1%) are achieved. Based on the study of the individual components in acetone-butanol-ethanol, the reaction pathways are put forward.

Steam catalytic cracking and lump kinetics of naphtha to light olefins over nanocrystalline ZSM-5 zeolite.

This study investigates the reaction pathways and kinetics to comprehend the catalytic cracking of dodecane, a heavy naphtha model compound, over the nanocrystalline ZSM-5 catalyst in the presence and absence of steam with the aim of increasing olefin production. The nanocrystalline zeolite was characterized using XRD and BET, and the surface acidity was measured by NH3-TPD and Py-FTIR. The steam treated ZSM-5 contributed to an increase in pore volume with extra-framework alumina, resulting in highly catalytic active sites and hence higher olefin selectivity. The high conversion of dodecane (>90%) was achieved during catalytic cracking in the presence and absence of steam. In the presence of steam, the short pores of nano ZSM-5 led to an increase in the naphtha-to-olefin conversion with lesser dry gas and coke formation. The activation energies of primary cracking in the presence and absence of steam were slightly different. Lower activation energies through secondary cracking routes and higher reaction rate constants were obtained via assisted-steam catalytic cracking, promoted the selectivity towards light olefin products. Meanwhile the hydrogenation and alkylation reactions toward LPG and C5+ were favored in the absence of steam. Moreover, the ZSM-5 nano zeolite pores promoted more β-scission reactions, resulting in higher selectivity towards ethylene and dry gas.