Modulation of the cobalt species state on zincosilicate to maximize propane dehydrogenation to propylene

Dispersing metals from nanoparticles into clusters or single atoms often exhibits unique properties such as the inhibition of structure-sensitive side reactions.Here,we reported the use of ion exchange(IE)methods and direct hydrogen reduction to achieve high dispersion of Co species on zincosilicate.The obtained 2Co/Zn-4-IE catalyst achieved an initial propane conversion of 41.4%at a temperature of 550℃in a 25%propane and 75%nitrogen atmosphere for propane dehydrogenation.Visualization of the presence of Co species within specific rings(alpha-α,beta-βand delta-δ)was obtained by aberration-corrected scanning transmission electron microscopy.A series of Fourier transform infrared spectra confirmed the anchoring of Co by specific hydroxyl groups in zincosilicate and the specific coordination environment of Co and its presence in the rings essentially as a single site.The framework Zn for the modulation of the microenvironment and the presence of Co species as Lewis acid active sites(Co-O4)was also supported by density functional theory calculations.

The role of pentacoordinate Al^(3+)sites of Pt/Al_(2)O_(3)catalysts in propane dehydrogenation

Pentacoordinate Al^(3+)(Al^(3+)_(penta))sites on alumina(Al_(2)O_(3))could anchor and stabilize the active site over the cata-lyst surface.The paper describes the specific effect of Al^(3+)_(penta) sites on the structure and the catalytic performance of Al_(2)O_(3)supported Pt catalysts by modulating the quantity of Al^(3+)_(penta) sites.The Al^(3+)_(penta) site content of Al_(2)O_(3)exhibits a volcano-type profile as a function of calcination temperature due to the structural rearrangement.The loading of Pt and subsequent calcination can consume a significant portion of Al^(3+)_(penta) sites over the Al_(2)O_(3)support.We further find that,when the calcination temperature of the impregnated Al_(2)O_(3)is higher than the cal-cination temperature of Al_(2)O_(3)precursor,the structural rearrangement of Al^(3+)_(penta) sites could make Pt partially buried in Al_(2)O_(3).Consequently,this partially buried structure leads to relatively low conversion but high stability for propane dehydrogenation.This work further elucidates the stabilization mechanism of the Al^(3+)_(penta) site over Al_(2)O_(3)support.

PtSnNa/SUZ-4:An efficient catalyst for propane dehydrogenation

The structure and catalytic properties of PtSn catalysts supported on SUZ-4 and ZSM-5 zeolite have been studied by using various experimental techniques including XRD,nitrogen adsorption,NH3-TPD,TG,H2-TPR and TPO techniques combined with propane dehydrogenation tests.It has been shown that SUZ-4-supported PtSnNa（PtSnNa/SUZ-4） was determined to be a better catalyst for propane dehydrogenation than conventional catalysts supported on ZSM-5,owing to its higher catalytic activity and stability.Dibenzothiophene poisoning experiments were performed to investigate the detailed structures of the two supported catalysts.The characterization of the two catalysts indicates that the distribution of Pt on the porous support affects the activity.In contrast to ZSM-5-supported catalysts,Pt particles on the PtSnNa/SUZ-4 are primarily dispersed over the external surface and are not as readily deactivated by carbon deposition.This is because that the strong acid sites of the SUZ-4 zeolite evidently prevented the impregnation of the Pt precursor H_2PtCl_6 into the zeolite.In contrast,the weak acid sites of the ZSM-5 zeolite led to more of the precursor entering the zeolite tunnels,followed by transformation to highly dispersed Pt clusters during calcination.In the case of the PtSnNa/ZSM-5,the interactions between Sn oxides and the support were lessened,owing to the weaker acidity of the ZSM-5 zeolite.The dispersed Sn oxides were therefore easier to reduce to the metallic state,thus decreasing the catalytic activity for hydrocarbon dehydrogenation.

The effect of ethanol on the performance of CrO_x/SiO_2 catalysts during propane dehydrogenation

The effects of ethanol vapor pretreatment on the performance of CrOx/SiO2 catalysts during the dehydrogenation of propane to propylene were studied with and without the presence of CO2.The catalyst pretreated with ethanol vapor exhibited better catalytic activity than the pristine CrOx/SiO2,generating 41.4% propane conversion and 84.8% propylene selectivity.The various catalyst samples prepared were characterized by X-ray diffraction,transmission electron microscopy,temperature-programmed reduction,X-ray photoelectron spectroscopy and reflectance UV-Vis spectroscopy.The data show that coordinative Cr^3＋ species represent the active sites during the dehydrogenation of propane and that these species serve as precursors for the generation of Cr^3＋.Cr^3＋ is reduced during the reaction,leading to a decrease in catalytic activity.Following ethanol vapor pretreatment,the reduced CrOx in the catalyst is readily re-oxidized to Cr^6＋ by CO2.The pretreated catalyst thus exhibits high activity during the propane dehydrogenation reaction by maintaining the active Cr^3＋ states.

State-of-the-art catalysts for direct dehydrogenation of propane to propylene

With growing demand for propylene and increasing production of propane from shale gas,the technologies of propylene production,including direct dehydrogenation and oxidative dehydrogenation of propane,have drawn great attention in recent years.In particular,direct dehydrogenation of propane to propylene is regarded as one of the most promising methods of propylene production because it is an on-purpose technique that exclusively yields propylene instead of a mixture of products.In this critical review,we provide the current investigations on the heterogeneous catalysts(such as Pt,CrOx,VOx,GaOx-based catalysts,and nanocarbons)used in the direct dehydrogenation of propane to propylene.A detailed comparison and discussion of the active sites,catalytic mechanisms,influencing factors(such as the structures,dispersions,and reducibilities of the catalysts and promoters),and supports for different types of catalysts is presented.Furthermore,rational designs and preparation of high-performance catalysts for propane dehydrogenation are proposed and discussed.

Synthesis of a new ordered mesoporous NiMoO_4 complex oxide and its efficient catalytic performance for oxidative dehydrogenation of propane

Highly ordered mesoporous NiMoO4 material was successfully synthesized using mesoporous silica KIT-6 as hard template via vacuum nanocasting method. The structure was characterized by means of XRD, TEM, N2 adsorption-desorption, Raman and FT-IR. The mesoporous NiMoO4 with the coexistence of a-NiMoO4 and fl-NiMoO4 showed well-ordered mesoporous structure, a bimodal pore size distribution and crystalline framework. The catalytic performance of NiMoOa was investigated for oxidative dehydrogenation of propane. It is demonstrated that the mesoporous NiMoO4 catalyst with more surface active oxygen species showed better catalytic performance in oxidative dehydrogena- tion of propane in comparison with bulk NiMoO4.

Effect of aluminum modification on catalytic properties of PtSn-based catalysts supported on SBA-15 for propane dehydrogenation

The catalytic properties of PtSn-based catalysts supported on siliceous SBA-15 and Al-modified SBA-15, such as Al-incorporated SBA-15 （AlSBA-15） and alumina-modified SBA-15 （Al2O3/SBA-15）, for propane dehydrogenation were investigated. Al2O3/SBA-15 was prepared either by an impregnation method using aluminum nitrate aqueous solution, or by the treatment of SBA-15 with a Al（OC3H7）3 solution in anhydrous toluene. N2-physisorption, FT-IR spectroscopy, solid-state 27Al MAS NMR spectroscopy, hydrogen chemisorption, XRF, NH3 temperature-programmed desorption, X-ray photoelectron spectroscopy and TPO were used to characterize these samples. Among these catalysts, the PtSn-based catalyst supported on Al2O3/SBA-15, which was grafted with Al（OC3H7）3, exhibited the best catalytic performance in terms of activity and stability The possible reason was due to the high Pt metal dispersion and/or the strong interactions among Pt, Sn, and the support.

Effect of cerium addition on catalytic performance of PtSnNa/ZSM-5 catalyst for propane dehydrogenation

The effect of cerium addition on the catalytic performance of propane dehydrogenation over PtSnNaIZSM-5 catalyst has been investigated by reaction tests and some physicochemical characterization such as XRD, BET, TEM, XPS, NH3-TPD, H2 chemisorption, TPR and TPO techniques. It has been found that with suitable amount of cerium addition, the platinum dispersion increased, while the carbon deposition tended to be eliminated easily. In these cases, the presence of cerium could not only realize the better distribution of metallic particles on the support, but also strengthen the interactions between Sn species and the support. Additionally, XPS spectra confirmed that more amounts of tin could exist in oxidized form, which was advantageous to the reaction. In our experiments, PtSnNaCe （1.1 wt%）/ZSM-5 catalyst exhibited the best catalytic performance. After running the reaction for 750 h, propane conversion was maintained higher than 30% with the corresponding selectivity to propylene of about 97%.

Dehydrogenation of Propane on Pt or Pt Sn Catalysts with Al_2O_3 or SBA-15 Support

Dehydrogenation of propane on Pt or Pt Sn catalyst over Al2O3 or SBA-15 support was investigated. The catalysts were characterized by CO-pulse chemisorption, thermogravimetry, temperature-programmed-reduction of H2,and diffuse reflectance infrared Fourier transform spectroscopy of absorbed CO. The results show that the platinum species is in oxidation state in the catalyst on Al2O3 support, so the catalyst must be reduced in H2 before dehydrogenation reaction. Addition of Sn improves the Pt dispersion, but the catalyst deactivates rapidly because of the coke formation. The interaction of Pt and Al2O3 is strong. On SBA-15 support, the platinum species is completely reduced to Pt0 in the calcination process, so the reduction is not needed. Addition of Sn improves the activity and selectivity of the catalyst. The interaction of Pt and SBA-15 is weak, so it is easy for Pt particles to sinter.

Optimizing zeolite stabilized Pt-Zn catalysts for propane dehydrogenation

Propane dehydrogenation(PDH)provides an alternative route to non-petroleum based propylene and eligible catalysts with good overall performance are still being explored.Herein,we report the construction of zeolite stabilized Pt-Zn catalysts Pt-Zn/Si-Beta for PDH.Characterization results from transmission electron microscopy(TEM),ultraviolet-visible(UV-vis)and Fourier transform infrared(FTIR)spectroscopy reveal that highly-dispersed Zn species are stabilized by the silanols from zeolite framework dealumination,which then act as the anchoring sites for Pt species.The close contact between Pt-Zn species and the electronic interaction thereof make Pt-Zn/Si-Beta robust PDH catalysts.Under optimized conditions,a high propylene production rate of 4.11 molmol_(Pt)^(-1)s^(-1),high propylene selectivity of 98% and a sustainable deactivation rate of~0.02 h^(-1)can be simultaneously achieved at 823 K.Coke deposition is not the key reason for the catalytic deactivation,while the loss of Zn species and the resulting aggregation of Pt species under high temperatures are responsible for the irreversible deactivation of Pt-Zn/Si-Beta catalyst in PDH reaction.

Preparation of CeO_2-Modified Mg(Al)O-Supported Pt–Cu Alloy Catalysts Derived from Hydrotalcite-Like Precursors and Their Catalytic Behavior for Direct Dehydrogenation of Propane

A series of PtCuCeMgAl quintuple hydrotalcite-like compounds with different Ce contents were synthesized by one-pot method. After calcining and reduction, CeO_2-modified Mg(Al)O-supported Pt–Cu alloy catalysts were obtained. To understand the effect of Cu and Ce, the structure and physico-chemistry properties of the catalysts were characterized and analyzed, and the catalytic behaviors were investigated in a direct dehydrogenation of propane to propene. The results show that the Pt^(4+), Cu^(2+), and Ce^(3+) ions can be incorporated into the brucite-like layers and the Ce content significantly affects the interaction strength between Pt and Cu and the dehydrogenation performance of propane. Under the reaction conditions, the highest propane conversion(45%) with 89% selectivity to propene and a 40% propene yield were achieved with a 0.3 wt% Ce-modified PtCu/Mg(Al)O catalyst. The improved catalytic performance is related to the easy formation of Pt–Cu alloy phase, excellent resistance to sintering, and coke deposits of active components modified by CeO_2.

Oxidative dehydrogenation of propane over Ni-Mo-Mg-O catalysts

In this work, a series of Ni-Mo-Mg-O catalysts with mesoporous structure prepared by sol-gel method were investigated for the oxidative dehydrogenation of propane （ODHP）. The techniques of temperature-programmed reduction with H2 （H2-TPR）, N2 adsorption-desolption, Fourier transform infrared spectroscopy （FT-IR）, X-ray diffraction （XRD） and X-ray photoelectron spectra （XPS） were employed for catalyst characterization. It is found that the activity of the catalysts for ODHP increases first and then decreases with the increase of Mo content. The catalyst with a Mo/Ni atomic ratio of 1/1 exhibits the best catalytic activity, which gives the propene selectivity of 81.4% at a propane conversion of 11.3% under 600 ~C and maintains the good catalytic performance for 22 h on stream. This is related not only to its high reducibility and dispersion as revealed by TPR and XRD, but also to the formation of more selective oxygen species on the MoOz-NiO interface as identified by XPS.

Modeling and Simulation of Coke Combustion Regeneration for Coked Cr2O3/Al2O3 Propane Dehydrogenation Catalyst

A heterogeneous model is developed for the regeneration of the Cr2O3/Al2O3 catalyst for the propane dehydrogenation process by considering the internal mass transfer and external mass/heat transfer during the coke combustion.Simulation shows that under practical operating conditions,multi-steady states exist for the catalyst pellets and the catalyst temperature is sensitive to gas temperature.However,at increased mass flow rate or lowered oxygen concentration,multi-steady states will not appear.Under the strong influences of film diffusion,the coke in the packed bed reactor will first be exhausted at the inlet,while if the film diffusion resistance is decreased,the position of first coke exhaustion moves toward the outlet of the reactor.

Mesoporous carbons as metal-free catalysts for propane dehydrogenation: Effect of the pore structure and surface property

Nanocarbon materials have been used as important metal-free catalysts for various reactions including alkane dehydrogenation.However,clarifying the active sites and tuning the nanocarbon structure for direct dehydrogenation have always been significantly challenging owing to the lack of fundamental understanding of the structure and surface properties of carbon materials.Herein,mesoporous carbon materials with different pore ordering and surface properties were synthesized through a soft-templating method with different formaldehyde/resorcinol ratios and carbonization temperatures and used for catalytic dehydrogenation of propane to propylene.The highly ordered mesoporous carbons were found to have higher catalytic activities than disordered and ordered mesoporous carbons,mainly because the highly ordered mesopores favor mass transportation and provide more accessible active sites.Furthermore,mesoporous carbons can provide a large amount of surface active sites owing to their high surface areas,which is favorable for propane dehydrogenation reaction.To control the surface oxygenated functional groups,highly ordered mesoporous carbons were carbonized at different temperatures(600,700,and 800℃).The propylene formation rates exhibit an excellent linear relationship with the number of ketonic C=O groups,suggesting that C=O groups are the most possible active sites.

Nature of active phase of VO_x catalysts supported on SiBeta for direct dehydrogenation of propane to propylene

The VOx catalysts supported on dealuminated Beta zeolite(Si Beta) with varying V loadings(from 0.5 to 10 wt%) are prepared and tested for their catalytic activities in the reaction of direct dehydrogenation of propane to propylene(PDH). It is characterized that the VSi Beta catalysts possess different kinds of vanadium species on the Si Beta support, including monomeric or isolated VOx species at a low V loading, and polynuclear VOx species in different polymerization degrees at higher V loadings. The 3 VSi Beta catalyst(V loading is 3 wt%), containing isolated VOx species in monolayer, shows around 40% of propane conversion with 90% of propylene selectivity(reaction conditions: 600 o C, 4000 m L g–1 h–1) which are comparable to VSi Beta catalysts with higher V loadings. The catalytic activity exhibits a good linear relationship with the amount of generated acidic sites, which are derived from the interaction sites between VOx species and Si Beta support, and keeps stable after several regeneration cycles. Thus, as the VOx species directly contact with Si Beta support via V–O–Si bonds, a reactivity enhancement can be achieved. While, the initial valence state of V does not seem to influence the catalytic performance. Moreover, the aggregation degree of VOx species determines the propylene selectivity and deactivation rate, both of which increase as raising the V loading amount.

Propane Dehydrogenation over a Commercial Pt-Sn/Al2O3 Catalyst for Isobutane Dehydrogenation： Optimization of Reaction Conditions

The applicability of a commercial Pt-Sn/Al2O3 isobutane dehydrogenation catalyst in dehydrogenation of propane was studied. Catalyst performance tests were carded out in a fixed-bed quartz reactor under different operating conditions. Generally, as the factors improving propane conversion decrease the propylene selectivity, the optimal operating condition to maximize propylene yield is expected. The optimal condition was obtamed by the experimental design method. The investigated parameters were temperature, hydrogen/hydrocarbon （HE/HC） ratio and space velocity, being changed in three levels. Constrains such as the susceptibility of the catalyst components to sintering or phase transformation were also taken into account. Activity, selectivity and stability of the catalyst were considered as the measured response factors, while the space-time-yield （STY） was considered as the variable to be optimized due to its commercial interest. A STY of 16 mol.kg^-1.h^-1 was achieved under the optimal conditions of T= 620 ℃, H2/HC = 0.6 and, weight hourly space velocity （WHSV） = 2.2 h^-1. Single carbon-carbon bond rupture was found to be the main route for the formation of lower hydrocarbon byproducts.

Ordered macroporous boron phosphate crystals as metal-free catalysts for the oxidative dehydrogenation of propane

Ordered macroporous materials with rapid mass transport and enhanced active site accessibility are essential for achieving improved catalytic activity.In this study,boron phosphate crystals with a three-dimensionally interconnected ordered macroporous structure and a robust framework were fabricated and used as stable and selective catalysts in the oxidative dehydrogenation(ODH)of propane.Due to the improved mass diffusion and higher number of exposed active sites in the ordered macroporous structure,the catalyst exhibited a remarkable olefin productivity of^16 golefin gcat^-1 h^-1,which is up to 2–100 times higher than that of ODH catalysts reported to date.The selectivity for olefins was 91.5%(propene:82.5%,ethene:9.0%)at 515℃,with a propane conversion of 14.3%.At the same time,the selectivity for the unwanted deep-oxidized CO2 product remained less than 1.0%.The tri-coordinated surface boron species were identified as the active catalytic sites for the ODH of propane.This study provides a route for preparing a new type of metal-free catalyst with stable structure against oxidation and remarkable catalytic activity,which may represent a potential candidate to promote the industrialization of the ODH process.

Propane dehydrogenation catalyzed by in-situ partially reduced zinc cations confined in zeolites

Propane dehydrogenation(PDH), employing Pt-or Cr-based catalysts, represents an emerging industrial route for propylene production. Due to the scarcity of platinum and the toxicity of chromium, alternative PDH catalysts are being pursued. Herein, we report the construction of Zn-containing zeolite catalysts,namely Zn@S-1, for PDH reaction. Well-isolated zinc cations are successfully trapped and stabilized by the Si-OH groups in S-1 zeolites via in-situ hydrothermal synthesis. The as-prepared Zn@S-1 catalysts exhibit good dehydrogenation activity, high propylene selectivity, and regeneration capability in PDH reaction under employed conditions. The in-situ partial reduction of zinc species is observed and the partially reduced zinc cations are definitely identified as the active sites for PDH reaction.

Effect of Ga Addition on Catalytic Performance of PtSnNa/ZSM-5 Catalyst for Propane Dehydrogenation

PtSnNaGa/ZSM-5 catalysts with different contents of Ga were prepared and characterized by X-ray diffraction (XRD), nitrogen adsorption, hydrogen chemisorption, ammonia temperature-programmed desorption (NH3-TPD), hydrogen temperature-programmed reduction (H2-TPR), and temperature-programmed oxidation (TPO) techniques. The performances of these catalysts for propane dehydrogenation were investigated. The test results indicated that the addition of Ga not only could improve the catalytic stability and propene selectivity, but also could effectively prevent the catalysts from coking. It was found that the PtSnNaGa(0.5 m%)/ZSM-5 catalyst exhibited the best performance in terms of propene selectivity and propane conversion. The high catalytic performance was most probably attributed to the presence of Ga that could strength- en the interaction between metals and the support to stabilize the catalytic active sites.

Pt-Sn clusters anchored at Al_(penta)^(3+) sites as a sinter-resistant and regenerable catalyst for propane dehydrogenation

Pt-based catalysts are widely used in propane dehydrogenation reaction for the production of propylene.Suppressing irreversible deactivation caused by the sintering of Pt particles under harsh conditions and regeneration process is a significant challenge in this catalyst.Herein,a series of highly ordered mesoporous Al_(2)O_(3) supports with different levels of Al3+penta sites,are fabricated and used as the support to disperse Pt-Sn_(2) clusters.Characterizations of Pt-Sn_(2)/meso-Al_(2)O_(3) with XRD,NMR,CO-IR,STEM,TG,and Raman techniques along with propane dehydrogenation-regeneration cycles test reveal the structure-stability-re generability relationship.The coordinatively unsaturated pentacoordinate Al_(Al3+penta)^(3+)can strongly anchor Pt atoms via a formation of Al-O-Pt bond,and thus stabilize the Pt-based particles at the surface of Al_(2)O_(3).The stability and regenerability of Pt-Sn2/meso-Al_(2)O_(3) are strongly dependent on the content of Al3+penta sites in the Al_(2)O_(3) structure,and a high level of Al3+penta sites can effectively prevent the agglomeration of Pt-Sn2 clusters into large Pt nanoparticles in the consecutive dehydrogenation-regeneration cycles.The Pt-Sn2/meso-Al_(2)O_(3)-600 with the highest level of Al_(penta)^(3+) (50.8%)delivers the best performance in propane dehydrogenation,which exhibits propane conversion of 40%and propylene selectivity above 98%at 570℃ with 10 vol%C_(3)H_(8) and 10 vol% H_(2) feed.A slow deactivation in this catalyst is ascribed to the formation of coke,and the catalytic performance can be fully restored in the consecutive dehydrogenation-regeneration cycles via a simple calcination treatment.