Influence of carbonization temperature on cobalt-based nitrogendoped carbon nanopolyhedra derived from ZIF-67 for nonoxidative propane dehydrogenation

Propylene is a significant basic material for petrochemicals such as polypropylene,propylene oxide,etc.With abundant propane supply from shale gas,propane dehydrogenation(PDH)becomes extensively attractive as an on-purpose propylene production route in recent years.Nitrogen-doped carbon(NC)nanopolyhedra supported cobalt catalysts were synthesized in one-step of ZIF-67 pyrolysis and investigated further in PDH.XPS,TEM and N_(2) adsorption-desorption were used to study the influence of carbonization temperature on as-prepared NC supported cobalt catalysts.The temperature is found to affect the cobalt phase and nitrogen species of the catalysts.And the positive correlation was established between Co0 proportion and space time yield of propylene,indicating that the modulation of carbonization temperature could be important for catalytic performance.

Sub-nanometer Pt_(2)In_(3) intermetallics as ultra-stable catalyst for propane dehydrogenation

Pt-based catalysts are the typical industrial catalysts for propane dehydrogenation(PDH),which still suffer from insufficient lo ng-term durability due to the structu ral instability and coke deposition.A commercial γ-Al_(2)O_(3) supported thermally robust sub-nanometer Pt2In3intermetallic catalyst with atomically ordered structure and rigorously separated Pt single atoms was fabricated,which showed outstanding robustness in 240 h long-term operation at 600℃ with the deactivation rate constant kdas low as0.00078 h^(-1), ranking among the lowest reported values.Based on various in situ characterizations and theoretical calculations,it was proved that the catalyst stability not only resulted from the separated Pt single-atom sites but also significantly affected by the distance of adjacent Pt atoms.An increasing distance to 3.25 A in the Pt_(2)In_(3)could induce a weak π-adsorption configuration of propylene on Pt sites,which facilitated the desorption of propylene and restrained the side reactions like coking.

A mini review on oxidative dehydrogenation of propane over boron nitride catalysts

Oxidative dehydrogenation of propane is an attractive route for the synthesis of propylene due to its favorable thermodynamic and kinetic characteristics, however, it is challenging to realize high selectivity towards propylene. Recently, it has been discovered that boron nitride (BN) is a promising catalyst that affords superior selectivity towards propylene in oxidative dehydrogenation of propane. Summarizing the progress and unravelling the reaction mechanism of BN in oxidative dehydrogenation of propane are of great significance for the rational design of efficient catalysts in the future. Herein, in this review, the underlying reaction mechanisms of oxidative dehydrogenation of propane over BN are extracted;the developed BN catalysts are classified into pristine BN, functionalized BN, supported BN and others, and the applications of each category of BN catalysts in oxidative dehydrogenation of propane are summarized;the challenges and opportunities on oxidative dehydrogenation of propane over BN are pointed out, aiming to inspire more studies and advance this research field.

C-H bond activation of propane on Ga_(2)O_(2)^(2+)in Ga/H-ZSM-5 and its mechanistic implications

Propane dehydrogenation(PDH)on Ga/H-ZSM-5 catalysts is a promising reaction for propylene production,while the detail mechanism remains debatable.Ga_(2)O_(2)^(2+)stabilized by framework Al pairs have been identified as the most active species in Ga/H-ZSM-5 for PDH in our recent work.Here we demonstrate a strong correlation between the PDH activity and a fraction of Ga_(2)O_(2)^(2+)species corresponding to the infrared GaH band of higher wavenumber(GaHHW)in reduced Ga/H-ZSM-5,instead of the overall Ga_(2)O_(2)^(2+)species,by employing five H-ZSM-5 supports sourced differently with comparable Si/Al ratio.This disparity in Ga_(2)O_(2)^(2+)species stems from their differing capacity in completing the catalytic cycle.Spectroscopic results suggest that PDH proceeds via a two-step mechanism:(1)C-H bond activation of propane on H-Ga_(2)O_(2)^(2+)species(rate determining step);(2)β-hydride elimination of adsorbed propyl group,which only occurs on active Ga_(2)O_(2)^(2+)species corresponding to GaHHW.

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.

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.

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.

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.

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.

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.

Ordered mesoporous Sn-SBA-15 as support for Pt catalyst with enhanced performance in propane dehydrogenation

A series of Sn‐incorporated SBA‐15materials with high specific surface areas and highly orderedmesoporous structures were synthesized by a facile one‐pot method and used as catalyst supports.A reference sample was also prepared using a conventional impregnation method.The catalystswere characterized using various methods,and their activities in propane dehydrogenation wereinvestigated.The incorporation of Sn into the SBA‐15matrix led to strong interactions between Snspecies and the support,and these helped to maintain the oxidation states of Sn species during thereaction.Substitution with Sn changed the interfacial properties of the Pt species and improved thefunction and effect of the Sn promoter.The catalytic activities and stabilities of the Pt catalysts supportedon Sn‐incorporated SBA‐15were better than those of the impregnated sample.However,thecatalytic performance deteriorated when an excessive amount of Sn was introduced and the interactionsamong Pt,Sn species,and the support became weaker.The Pt/0.5Sn‐SBA‐15catalyst gavethe best propene selectivity,i.e.,98.5%,with a corresponding propane conversion of about43.8%.

Boosting selectivity and stability on Pt/BN catalysts for propane dehydrogenation via calcination & reduction-mediated strong metal-support interaction

Propane dehydrogenation(PDH) provides an alternative route for producing propylene. Herein, we demonstrates that h-BN is a promising support of Pt-based catalysts for PDH. The Pt catalysts supported on h-BN were prepared by an impregnation method using Pt(NH_(3))_(4)(NO_(3))_(2) as metal precursors. It has been found that the Pt/BN catalyst undergoing calcination and reduction is highly stable in both PDH reaction and coke-burning regeneration, together with low coke deposition and outstanding propylene selectivity(99%). Detailed characterizations reveal that the high coke resistance and high propylene selectivity of the Pt/BN catalyst are derived not only from the absence of acidity on BN support, but also from the calcination-induced and reduction-adjusted strong metal-support interaction(SMSI) between Pt and BN, which causes the partial encapsulation of Pt particles by BO_(x) overlayers. The BO_(x) overlayers can block the low-coordinated Pt sites and constrain Pt particles into smaller ensembles, suppressing side reactions such as cracking and deep dehydrogenation. Moreover, the BO_(x) overlayers can effectively inhibit Pt sintering by the spatial isolation of Pt during periodic reaction-regeneration cycles. In this work, the catalyst support for PDH is expanded to nonoxide BN, and the understanding of SMSI between Pt and BN will provide rational design strategy for BN-based catalysts.

Effect of Strontium Addition to Platinum Catalyst for Propane Dehydrogenation

PtSnSr/HZSM-5 catalysts with different amounts of strontium were prepared by sequential impregnation method, and characterized by BET analysis, TEM, NH3-TPD, Hz-TPR, TPO and TG techniques. The results showed that the addition of strontium could modify the characteristics and properties of both acid function and metal function of Pt-Sn-based cata- lysts. In this case, PtSnSr/HZSM-5 catalyst with an appropriate amount of Sr （1.2%） showed higher catalytic activity and lower amount of coke deposits than PtSn/HZSM-5 catalyst. However, excessive loading of Sr could facilitate the reduction of Sn, which was unfavorable to the reaction. Afterwards, 1.0 m% of Na was added into the PtSnSr（1.2%）/HZSM-5 catalyst to improve the catalytic performance in propane dehydrogenation, and this catalyst displayed the best catalytic performance during our experiments. After having been subjected to reaction for 5 h, the PtSnNa（1.0%）Sr（1,2%）/HZSM-5 catalyst had achieved a higher than 95% selectivity towards propene along with a corresponding propane conversion rate of 32.2%.

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 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%.

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.

Bimetallic PtSn nanoparticles confined in hierarchical ZSM-5 for propane dehydrogenation

A series of PtSn/hierarchical ZSM-5 catalysts were developed for propane dehydrogenation,in which the PtSn bimetallic particles are confined in the mesopores of hierarchical ZSM-5 zeolite.The synthesis of PtSn/hierarchical ZSM-5 catalysts was achieved via the loading of Pt and Sn species onto the hierarchical ZSM-5 catalysts that are obtained through a desilication of conventional ZSM-5.The PtSn/hierarchical ZSM-5 catalysts were fully characterized by XRD,N_(2) adsorption,STEM,XPS,and CO-IR techniques,which reveals that highly dispersed PtSn bimetallic nanoparticles are enclosed into mesopores of hierarchical ZSM-5.The catalytic performance of PtSn/hierarchical ZSM-5 is greatly affected by the concentrations of alkali solution in the desilication process and Sn/Pt ratios in PtSn bimetallic particles.The PtSn1.00/ZSM-5(0.8)catalyst shows the highest efficiency in propane dehydrogenation,which gives an initial conversion of 46%and selectivity of 98%at 570℃.The high efficiency in these PtSn/hierarchical ZSM-5 catalysts for propane dehydrogenation is mainly ascribed to the confinement of PtSn particles in the mesopores of hierarchical ZSM-5 zeolite.

Effect of pH on the catalytic performance of PtSn/B-ZrO2 in propane dehydrogenation

Boron-modified ZrO2(B-ZrO2)was synthesized under various pH values(9,10,and 11)and used as the supports of PtSn catalysts(PtSn/B-ZrO2-x)for non-oxidative dehydrogenation of propane.The NH3-TPD and pyridine IR show that only Lewis acid is present and the acid strength increases with the synthesis pH.PtSn/B-ZrO2-10 exhibits the best catalytic performance with an initial propane conversion of 36%and a deactivation rate constant(kd)of 0.0127 h^-1.The XPS results indicate that the electronic properties of Pt and SnOx are affected not only by their interaction but also by the interaction with support.After a careful analysis of the oxygen storage capacity and activity in CO oxidation,it is hypothesized that the interaction between Pt and Sn becomes stronger following the order:PtSn/B-ZrO2-9<PtSn/B-ZrO2-11<PtSn/B-ZrO2-10.The characterization with TPO and Raman on spent catalysts exhibits that more hydrogen deficient coke forms on the support and less coke deposits on the metal surface of PtSn/B-ZrO2-10.The results reveal that the interaction between Pt and Sn is influenced by their respective interaction with the support and a moderate interaction between the metal species and the support is desired.

Promotion effect of sulfur impurity in alumina support on propane dehydrogenation

Alumina materials are widely applied either as a catalyst or support in various industrial catalytic processes. Impurities in alumina that are unfriendly to catalytic performance are inevitably present during the production processes. Facing this problem, we here report that the use of sulfur-containing alumina as the support can generate active alumina-supported platinum catalyst, which exhibits superior propylene selectivity and anti-coking ability during propane dehydrogenation. It demonstrated that the sulfur impurity in alumina is not entirely detrimental. During the reduction process, the formation of gas-phase sulfur species increased the electrons and poisoned unsaturated sites of platinum particles. The sulfur impurity in alumina can be removed through a hydrogen reduction process, and the degree of desulfurization is correlated with the operating temperature. This study demonstrated that the rational use of impurity will contribute to the design of a catalyst with high reactivity for potential applications.

CrOx supported on high-silica HZSM-5 for propane dehydrogenation

Industrial propane dehydrogenation(PDH)catalysts generally suffer from low catalytic stability due to the coke formation onto the catalyst surface to cover the active sites.The exploitation of an efficient catalyst with both high catalytic selectivity and long-term stability toward PDH is of great importance but challenging to make.Herein CrOx supported on high-silica HZSM-5 with a SiO2/Al2O3 ratio of 260(Cr/Z-5(260)is synthesized by a simple wet impregnation method,which exhibits high catalytic activity,good selectivity and excellent stability for PDH.At a weight hourly space velocity(WHSV)of 0.59 h-1,a propylene formation rate of 4.1 mmol g-1cath-1(~32.6% propane conversion and ~94.2% propylene selectivity)can be maintained over the 5%Cr/Z-5(260)catalyst after 50 h time on stream,which is much better than commercial Cr/Al2O3(Catofin process,catalyst life is several hours)at the same reaction conditions.With increasing the WHSV to 5.9 h-1,a high propylene formation rate of 27.9 mmol gcat-1h-1can be obtained over the 5%Cr/Z-5(260)catalyst after 50 h time on stream,demonstrating a very promising PDH catalyst.Characterization results and Na+doping experiments reveal that the Cr species combined with Br?nsted acid sites in Cr/HZSM-5 catalysts are responsible for the high catalytic performance.In particular,the Br?nsted acid sites in HZSM-5 zeolite could increase the propane adsorption and enhance the C–H bond activation.Furthermore,the high surface area and well-defined pores of HZSM-5 zeolite can provide a special environment for the dispersion and stabilization of Cr species,thus guaranteeing high catalytic activity and stability.