High Propylene Selectivity Research Articles

CATALYSTS OF ACID-BASE PROCESS ON THE BASIS OF THE MODIFIED CARBON FIBER

The functionalization of the carbon fiber based on polyacrylonitrile with sulfur-containing groups of high acidity was carried out in order to obtain the acid-base processes catalysts. Fibers were treated with sulfur vapors in the temperature range of 400-800°C, followed by surface oxidation with 30% hydrogen peroxide solution. Modified samples were investigated by chemical analysis, thermo-programmed desorption with mass spectrometric registration of products, IR spectroscopy and thermogravimetry. It is shown that the obtained materials contain SO3H-functional groups and oxygen-containing groups (carboxyl, lactone, phenolic, etc.) formed in the surface layer during the oxidation of the fiber surface. The chemical analysis showed that the concentration of sulfur in the samples of the modified fiber is 1.6-6.5 mmol/g. The synthesized samples have a satisfactory thermal stability.
 The synthesized catalysts were investigated in the model reaction - gas phase dehydration of isopropyl alcohol. It was found that obtained SO3H-containing carbon fibers were catalytically active and had high propylene selectivity. For all the samples obtained there is a complete conversion of alcohol into propylene. The activity of modified carbon fiber samples in the reaction indicated is a fairly high, temperatures of the total conversion of alcohol into propylene are in the range of 160-190°C. During the study of synthesized catalysts in several cycles of catalysis it have been shown that within repeated use (3 cycles) of all modified fiber samples, the yield of propylene does not decrease, the activity remains stable - the temperature of the dehydration reaction remains unchanged or increases insignificantly (by 5-10ºС). The temperatures of complete conversion of isopropyl alcohol in propylene for synthesized catalysts are lower than the temperatures of destruction maxima of surface sulfogroups. Thus, modified carbon fibers can be used as low-temperature catalysts of acid-base processes, in particular dehydration of alcohols.

Modification of Acidic Properties of the Support for Pt–Sn/BEA Propane Dehydrogenation Catalysts

Platinum–tin catalysts for propane dehydrogenation supported on a zeolite of the BEA framework type are studied. The presence of acid sites on the support leads to the occurrence of side reactions, which inhibit the achievement of a high propylene selectivity. To decrease the acidity of the support, the zeolite is subjected to dealumination with the preservation of the crystalline structure. The deposition of tin on this support leads to the incorporation of tin atoms into the zeolite framework, as confirmed by IR spectroscopy of adsorbed deuterated acetonitrile. The subsequent deposition of platinum leads to the occurrence of the Pt–Sn interaction, which provides a high selectivity for the target product and an increase in the agglomeration resistance of the active phase. In the presence of samples containing 0.5% of platinum and 0.5–1.0% of tin, at a reaction temperature of 550°C, a propane conversion of 26–28% at a propylene selectivity of up to 98.8% is achieved.

Fluidized bed propane dehydrogenation technologies challenge established fixed and moving bed processes

Ga-based catalysts are promising for use in propane dehydrogenation (PDH) because of the relatively superior activity, but the conventional Ga-based catalysts usually suffer from serious deactivation and unsatisfactory propene selectivity. Here, ultrafine bimetallic Ga-Pt nanocatalysts encapsulated into silicalite-1 (S-1) zeolites ([email protected]) were synthesized by a facile ligand-protected direct H2-reduction method. It is indicated that this catalyst is composed of confined ultra-small GaPt alloy nanoclusters and a part of isolated tetrahedral coordination of Ga species. The confined GaPt alloy nanoclusters are the active sites for PDH reaction, and their high electron density could boost the desorption of products, resulting in a high propene selectivity of 92.1% and propene formation rate of 20.5 mol g−1Pt h−1 at 600 °C. Moreover, no obvious deactivation was observed over [email protected] catalyst even after 24 h on stream at 600 °C, affording an extremely low deactivation constant of 0.0068 h−1, which is much lower than that of the conventional Ga-based catalysts. Notably, the restriction of the zeolite can enhance the regeneration stability of the catalyst, and the catalytic activity kept unchanged after four consecutive cycles.

The effect of phase composition and crystallite size on activity and selectivity of ZrO2 in non-oxidative propane dehydrogenation

A series of bare ZrO2 materials composed either of monoclinic or tetragonal phase with the size of crystallites varying between 3.7 and 43.4 nm were prepared by precipitation, hydrothermal or thermal calcination methods. The samples were characterized by XRD, BET, CO-TPR, NH3-TPD, electrical conductivity measurements and operando UV–vis spectroscopy. Density functional theory calculations provided molecular insights into the kind of active site and individual steps of propane dehydrogenation to propene and hydrogen. Regardless of the phase composition, two Zr cations located at an oxygen vacancy, i.e. coordinatively unsaturated Zr cations (Zrcus sites) having, nevertheless, different chemical environment in tetragonal and monoclinic ZrO2, were concluded to be responsible for homolytic breaking of CH bonds in propane. Monoclinic ZrO2 showed, however, higher rate of propene formation and higher propene selectivity than ZrO2 stabilized in the tetragonal phase. Both the activity and the selectivity to propene increased with a decrease in the size of crystallites. The effects of phase composition and crystallite size on the PDH performance were related to the ability of ZrO2 to release lattice oxygen upon reductive catalyst treatment and thus to create Zrcus sites. The knowledge derived can be used for further optimizing PDH performance of ZrO2-based catalysts and also extended to other non-reducible metal oxides.

Downer fluidized bed reactor modeling for catalytic propane oxidative dehydrogenation with high propylene selectivity

This study reports a simulation of the catalytic propane oxidative dehydrogenation (PODH) in a circulating fluidized bed downer reactor. A relevant kinetics based on experimental work by the authors (Rostom and de Lasa, 2018), developed in a fluidized CREC Riser Simulator is considered. As well, Computational Particle Fluid Dynamics (CPFD) models featuring either “Particle Clusters” or “Single Particles” flows are implemented. The CPFD results obtained in a 20-m length unit show a 28% propane total conversion and 93% propylene selectivity using the “Single Particle” flow model. However, and once the more rigorous particle cluster flow is accounted for, the propane conversion is reduced significantly to 20%, while the propylene selectivity remains at 94% level. Thus, the obtained results show that, a PODH simulation using CPFD, requires one to account for “Particle Clusters”. This type of comprehensive model is required to establish unambiguously the downer reactor performance and is of critical value for the development of downflow reactors for other catalytic processes.

Pt–Cu Alloy Nanoparticles Encapsulated in Silicalite-1 Molecular Sieve: Coke-Resistant Catalyst for Alkane Dehydrogenation

Highly dispersed Pt–Cu alloy nanoparticles encapsulated in silicalite-1 were prepared by one-pot method. Taking propane dehydrogenation as a probe reaction, it was found that anti-coke ability was improved by encapsulation. Through H2-TPR, CO adsorption DRIFT spectra and X-ray Photoelectron Spectroscopy experiment, enhanced anti-coke ability was ascribed to the electronic environment change of encapsulated metal. And it was further confirmed by in situ coking experiment. Owning to limited acidity, metal encapsulated by silicalite-1 molecular sieve exhibited better anti-coke ability and higher propylene selectivity than that by stronger acidic ZSM-5 molecular sieve. Moreover it was found that the combination of Cu and Pt made better catalytic performance and accelerated dehydrogenation more effectively compared with monometallic catalysts. Therefore, Pt–Cu alloy nanoparticles encapsulated in silicalite-1 molecular sieve core–shell material was a potential coke-resistant catalyst for alkane dehydrogenation reaction.

ZnO Nanoclusters Supported on Dealuminated Zeolite β as a Novel Catalyst for Direct Dehydrogenation of Propane to Propylene

AbstractSmall ZnO nanoclusters supported on dealuminated β zeolite were prepared and evaluated for catalyzing direct dehydrogenation of propane to propylene (PDH), exhibiting high catalytic performance. N2 sorption, XRD, TEM, 27Al and 28Si MAS NMR, IR, XRF, DR UV‐vis, XPS, and NH3‐TPD techniques were employed to characterize the physicochemical properties of this novel catalyst system. It is found that the Zn species can be accommodated in the vacant T‐atom sites of dealuminated β zeolite due to the reaction of aqueous zinc acetate solution with silanol groups, and thus, producing massive small ZnO nanoclusters as active phases in PDH. Additionally, dealuminated β zeolite can greatly depress side reactions attributable to the absence of strong acid sites, thereby guaranteeing high catalytic activity, propylene selectivity and stability. As a result, the optimal catalyst of 10 wt% Zn loaded on dealuminated β zeolite exhibits a high initial propane conversion of around 53 % and a superior propylene selectivity of about 93 % at a space velocity of 4000 cm3 gcat−1 h−1, together with the high stability and satisfactory reusability. This study may open a new way to design and synthesize highly active PDH catalysts with high selectivity and stability.

A Two-Step Crystallization Route for Hierarchical SAPO-34 Molecular Sieves: Unique Structural Features and Catalytic Property for DTO

The hierarchical structure can significantly improve the diffusion efficiency of the catalyst and regulate the product distribution. Therefore, the preparation of hierarchical SAPO-34 molecular sieve has been a hot research topic. With Cetyltrimethyl Ammonium Bromide (CTAB) and Diethylamine (DEA) as templates, a two-step crystallization process was employed to synthesize hierarchical SAPO-34 molecular sieves. We found that the aging process is vital for the formation of pure phase SAPO-34. It was investigated the relationship of crystallinity trend and mesoporous content with the crystallization time. The results showed that the prolongation of crystallization time was beneficial to enhance the crystallinity of the molecular sieve, but unfavourable to the retention of mesoporous structure. The formation process of hierarchical SAPO-34 molecular sieve involved agglomeration, disintegration, crystallization, re-agglomeration and growth. The hierarchical SAPO-34 molecular sieve with a satisfactory crystallinity and considerable mesoporous structure could be obtained after 36 hours of crystallization. Moreover, the sample had the most suitable acid strength as well as acid amount. The catalytic activity was investigated by catalytic dimethyl ether (DME) to olefin (DTO) reaction. It revealed that the conversion of DME and the selectivity to olefins over the hierarchical SAPO-34 molecular sieve were significantly enhanced with comparison to that over microporous SAPO-34 molecular sieve. The amount of coke deposition of the hierarchical SAPO-34 molecular sieve (14.2%) was lower than that over the microporous molecular sieve (16.5%). Meanwhile, the propylene selectivity of hierarchical SAPO-34 was higher than that of microporous SAPO-34 in the whole reaction. In a word, the hierarchical SAPO-34 molecular sieve synthesized in this study showed a longer catalytic life, higher coke deposition resistance and higher propylene selectivity.

Dehydrogenation of Propane with СО2 on Supported CrOx/SiO2 Catalysts

A systematic analysis was performed of the physicochemical properties of amorphous silicas (Acros, Degussa, KSKG) and CrOx/SiO2 catalysts prepared from them with 2, 3, 5, 7, and 10 wt % chromium. The supports and catalysts were characterized by BET, XRD, and UV–Vis diffuse reflectance spectroscopy. In the oxidative dehydrogenation of propane with carbon dioxide, the highest propylene selectivity (95%) was observed for the 5% CrOx/SiO2 (Degussa) sample, and the highest propane conversion (53%) was reported on the 5% CrOx/SiO2 (Acros) catalyst. The structure of chromium oxide particles on the support surface was studied by UV–Vis spectroscopy. The catalytic properties were correlated with the state of chromium.

Comprehensive study on PVDF-HFP/BMImBF4/AgBF4 membranes for propylene purification

In this work, a comprehensive analysis of PVDF-HFP/BMImBF4/AgBF4 facilitated transport membranes for olefin/paraffin separation is presented. Previous works of our research group have reported high flux and propylene selectivity under dry conditions and using synthetic gas mixtures, highlighting the promising potential of these membranes for industrial applications. This work advances in the understanding of the phenomena involved in membrane performance and moves one step forward in the knowledge of the industrial viability of this membrane system. First, the internal interactions between the silver cations and the polymer backbone, the silver salt dissociation and the silver degradation have been studied using FTIR, Raman and XPS spectroscopic techniques. Secondly, the experimental membrane performance during 110 days and working at changing relative humidity conditions in the feed gas has been assessed. Thermogravimetric techniques helped determining the water uptake capability of the facilitated transport membrane. Thirdly, real gas mixtures from a fluid catalytic cracking unit were provided by the industry and used in permeation experiments to check the membrane behavior under industrial-like conditions. The results provide experimental evidence for the previously theorized facilitated transport mechanisms and reveal a major influence of feed gas humidity on membrane performance. On the other hand, the industrial gas mixture produces no deviation from synthetic feed conditions due to trace contaminants. Finally, the carrier deactivation in long-term permeation has been quantified through a mathematical expression.

Breaking the scaling relationship via thermally stable Pt/Cu single atom alloys for catalytic dehydrogenation

Noble-metal alloys are widely used as heterogeneous catalysts. However, due to the existence of scaling properties of adsorption energies on transition metal surfaces, the enhancement of catalytic activity is frequently accompanied by side reactions leading to a reduction in selectivity for the target product. Herein, we describe an approach to breaking the scaling relationship for propane dehydrogenation, an industrially important reaction, by assembling single atom alloys (SAAs), to achieve simultaneous enhancement of propylene selectivity and propane conversion. We synthesize γ-alumina-supported platinum/copper SAA catalysts by incipient wetness co-impregnation method with a high copper to platinum ratio. Single platinum atoms dispersed on copper nanoparticles dramatically enhance the desorption of surface-bounded propylene and prohibit its further dehydrogenation, resulting in high propylene selectivity (~90%). Unlike previous reported SAA applications at low temperatures (<400 °C), Pt/Cu SAA shows excellent stability of more than 120 h of operation under atmospheric pressure at 520 °C.

Co-reaction of methanol with butene over a high-silica H-ZSM-5 catalyst

Co-reaction of methanol and butene was investigated over a high-silica H-ZSM-5 catalyst. It was found that the olefin methylation-cracking and butene dimerization-cracking routes co-occurred, and their contributions depended on the methanol/butene ratio. When more methanol was fed, the olefin methylation-cracking route played a dominant role, producing more propene. An increase in butene amount promoted butene dimerization and the subsequent cracking pathway, improving pentene yield at the expense of ethene, propene, and aromatics. A strong interaction of methanol with butene was observed at a methanol/butene molar ratio of 4:1, resulting in the highest propene selectivity. The spent catalysts were systematically studied by N2 adsorption/desorption, thermogravimetric analysis, Fourier transform infrared spectroscopy, diffuse reflectance UV–vis spectroscopy, Raman spectroscopy and temperature-programmed oxidation techniques. The results reveal that the coke species are mainly formed on the external surface through aromatics methylation and aromatics alkylation with alkene pathways. The increase in the catalytic stability from co-feeding butene is attributed to alleviation of aromatics formation and suppression of the aromatics methylation coking route.

Production of light olefins from methanol over modified H-ZSM-5: effect of metal impregnation in high-silica zeolite on product distribution

Improvement of H-ZSM-5 catalyst to convert methanol to light olefins was studied in this research. High-silica H-ZSM-5 zeolite (Si/Al = 200) was prepared by a hydrothermal method and modified by impregnation with different promoters (Cs, Mg, Ag, Mn, Fe, Ni, Ir, or P). The parent and modified catalysts were characterized using X-ray diffraction (XRD) analysis, inductively coupled plasma (ICP) spectrometry, scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) measurements, Fourier-transform infrared (FT-IR) spectroscopy, and NH3-temperature programmed desorption (TPD) measurements. Performance tests were performed for all samples in a fixed-bed reactor at 480 °C, 1 bar, and methanol weight hourly space velocity (WHSV) of 0.9 h−1, using feed with methanol to water weight ratio of unity. The results revealed that the parent catalyst was stable during the impregnation process and that the promoted catalysts exhibited nearly the same crystallinity and BET surface area compared with the parent sample. The acidity of the catalysts was decreased after impregnation of promoters except for Ag. It was found that the propylene selectivity was strongly dependent on the structure, texture, and acidic properties of the catalysts, being affected by the type of promoter. The phosphorus- and iron-promoted H-ZSM-5 catalysts showed high propylene selectivity (45.7 % and 43.7 %, respectively) and good catalytic stability, which was attributed to the moderate density and distribution of acidic sites over these catalysts.

Pd-membrane Integration in a Propane Dehydrogenation Process for Highly Selective Propylene Production

The forecasts in the industrial chemistry field evidenced the growing demand of propylene, and the necessity to develop effective processes able to sustain the market. Selective propane dehydrogenation is emerging as the most competitive technology for the production of propylene, on the other hand, the well-known drawback closely linked to the high temperature required to reach a sustainable propane conversion and the coke formation that suppress the catalytic stability still requires appropriate solutions. In this sense, the process intensification through the combination of hydrogen permselective membranes and the reduction of operating temperature could strike the targets of very high propylene selectivity and a quite high conversion value. Since the integration of membrane units in a process required a revision of the operating conditions, the effect of feed composition and temperature was investigated, in order to determine the optimal operating parameters window to operate the system, and assess the overall concept performance. A commercial membrane was characterized, in order to understand the suitability in the propane dehydrogenation process that may cause a severe membrane deactivation due to coke deposition. Finally, specific tests on a simulated process configuration, obtained in an open architecture, confirmed that the smart combination of PDH catalytic reactors and Pd-based membranes was able to allow a reduced operating temperature, so increasing propylene selectivity, and to overcome typical thermodynamic limitations related to a traditional configuration.

Pd-membrane Integration in a Propane Dehydrogenation Process for Highly Selective Propylene Production

The forecasts in the industrial chemistry field evidenced the growing demand of propylene, and the necessity to develop effective processes able to sustain the market. Selective propane dehydrogenation is emerging as the most competitive technology for the production of propylene, on the other hand, the well-known drawback closely linked to the high temperature required to reach a sustainable propane conversion and the coke formation that suppress the catalytic stability still requires appropriate solutions. In this sense, the process intensification through the combination of hydrogen permselective membranes and the reduction of operating temperature could strike the targets of very high propylene selectivity and a quite high conversion value. Since the integration of membrane units in a process required a revision of the operating conditions, the effect of feed composition and temperature was investigated, in order to determine the optimal operating parameters window to operate the system, and assess the overall concept performance. A commercial membrane was characterized, in order to understand the suitability in the propane dehydrogenation process that may cause a severe membrane deactivation due to coke deposition. Finally, specific tests on a simulated process configuration, obtained in an open architecture, confirmed that the smart combination of PDH catalytic reactors and Pd-based membranes was able to allow a reduced operating temperature, so increasing propylene selectivity, and to overcome typical thermodynamic limitations related to a traditional configuration.

In Situ Aluminum Migration into Zeolite Framework during Methanol-To-Propylene Reaction: An Innovation To Design Superior Catalysts

This work involves methanol-to-propylene (MTP) conversion over aluminosilicate MFI (ZSM-5), borosilicate MFI (B-ZSM-5), and all-silica MFI (Silicate-1). Both B-ZSM-5 and Silicate-1 were inactive in MTP reaction. However, extruded Silicate-1 sample prepared by extrusion with Al2O3 binder, followed by acid washing treatment, displayed a rapid increase in methanol conversion in the initial 30 h time on stream (TOS), and then retained 99% methanol conversion, higher propylene selectivity (52.2%), and higher propylene/ethylene ratio (11.3) for 400 h TOS. Silicate-1 modified with AlCl3 or Al(NO3)3 showed similar catalytic performance as the extruded samples. Several AlCl3-modified Silicate-1 samples after various TOS values were regenerated and characterized by NH3-termperature-programmed desorption (TPD), Fourier transform infrared (FT-IR) spectroscopy, and 27Al magic angle spinning nuclear magnetic responance (MAS NMR). The results point to continuous aluminum insertion into Silicate-1 framework during reacti...

Comparative study of CHA- and AEI-type zeolytic catalysts for the conversion of chloromethane into light olefins

Three pairs of CHA- and AEI-type zeolytic materials with similar crystallite size but with different framework atoms, i.e., silicoaluminophosphate SAPO-34 and SAPO-18, aluminosilicate SSZ-13 and SSZ-39, and titaniumin-corporated TiAPSO-34 and TiAPO-18, were prepared and their catalytic activities of chloromethane into light olefins were compared according to the structure types, as well as acidic properties. The AEI-type catalysts were found to have lower ethene/propene ratios, indicating relatively higher propene selectivity, because their aei-cage was larger compared to the cha-cage. However, all the CHA-type catalysts exhibited better activity and higher selectivity to light olefins. Both H-SAPO-34 and H-TiAPSO-34 showed good catalytic stability over all the reaction times studied here. When compared to H-SAPO-34, however, H-TiAPSO-34 exhibited ca. 20mol% higher selectivity to ethene and propene, despite a similar total density of acid site. This performance may be due to the higher strength of medium acid sites.

MFI zeolite coating with intrazeolitic aluminum (acidic) gradient supported on SiC foams to improve the methanol-to-propylene (MTP) reaction

To hinder the deactivation and improve the propylene selectivity in the methanol-to-propylene (MTP) reaction, MFI coating with the intrazeolitic aluminum (acidic) gradient supported on SiC foam support (G-MFI/SiC foam) was proposed. The solid polycrystalline silicon was used in the synthesis of G-MFI/SiC foam catalyst provided a prolonged release of silica nutrient in the liquid phase and suppressed the precipitation phenomena. The resulting MFI coating showed the aluminum gradient along the surface normal direction of SiC foams with ZSM-5 layer (about 20 μm) near the SiC surface followed by the silicalite-1 layer (about 10 μm). The alumina (acidic) gradient in the MFI coating renders a passive outer layer of silicalite-1 with fairly large amount of weak and medium acid sites prevented the coke formation as well as promoted the selectivity to propylene in the MTP reaction. Compared to the conventional ZSM-5/SiC foam catalyst, the G-MFI/SiC foam catalyst showed excellent performance in the MTP reaction with good catalytic longevity (8 h vs. 76 h for >95% methanol conversion) and low coke deposition (6.7 × 10−3 wt.% h−1vs. 0.26 wt.% h−1), as well as high propylene selectivity (ca. 36% vs. 46%).

High-performance thin-felt SS-fiber@HZSM-5 catalysts synthesized via seed-assisted vapor phase transport for methanol-to-propylene reaction: Effects of crystal size, mesoporosity and aluminum uniformity

A series of thin-felt SS-fiber@HZSM-5 catalysts are fabricated via the seed-assisted vapor-phase transport (VPT) method. The crystal size and mesoporosity of zeolite shell are controllably tuned toward enhanced diffusion by increasing the silicalite-1 seeding gel (SG) amount used. The microstructured catalyst obtained with a high SG amount of 50% contains a smaller crystal size of ∼70 nm and higher mesopore volume of 0.21 cm3 gzeolite−1, which markedly prolonged single-run lifetime with a remarkable decrease in the coking rate in the MTP process. The adverse effect of non-uniform aluminum in the zeolite shell on the MTP stability is revealed. The use of aluminum-containing SG to replace the pure-silica one makes the acid sites of as-obtained ZSM-5 mounted on SS-fiber distributed homogeneously thereby leading to a marked suppression of hydrogen transfer, and as a result, the catalyst single-run lifetime is further increased by ∼40%. This promising SS-fiber@HZSM-5 catalyst is stable for at least 1600 h (>94% conv.) with a high propylene selectivity of ∼36% at 450 °C using a WHSV of 1 h−1 for a feed of MeOH/H2O molar ratio of 1:1, by taking advantage of improved diffusion from mesoporosity development and uniform aluminum distribution as well as our distinctive microfibrous-engineered design.

Stability Assessment of Regenerated Hierarchical ZSM-48 Zeolite Designed by Post-Synthesis Treatment for Catalytic Cracking of Light Naphtha

Hierarchical ZSM-48, a one-dimensional pore system zeolite with the presence of mesopores, was obtained by post-synthesis alkaline and acid treatments. Hierarchical ZSM-48 exhibited excellent hexane cracking activity compared to parent ZSM-48, which can be attributed to better diffusion as a result of the created mesoporosity. Moreover, the post-synthesis treatment allowed for manipulation of the distribution of active sites. Consequently, better stability and higher propylene selectivity were accomplished. The spent catalyst was regenerated by removing the deposited coke from the pores, and the regenerated catalyst was characterized again to investigate the recyclability of the hierarchical structure achieved. Parent ZSM-48 showed the same textural and acidic properties after regeneration, while the structure of the post-treated sample suffered from serious defects. The defects severely decreased the number of active sites as measured by pyridine Fourier transform infrared spectroscopy and caused major s...