Methane combustion over palladium catalyst within the confined space of MFI zeolite

Isolated cationic Pd species encapsulated in MFI zeolite,i.e.,Pd@MFI,have been successfully prepared via in situ hydrothermal route followed by oxidative treatment.The as-prepared Pd@MFI samples are investigated as promising catalysts in the reaction of methane combustion.Typically,Pd@H-ZSM-5 shows remarkable activity in methane catalytic combustion with a low apparent activation energy value of 70.7 kj/mol as well as good catalytic stability even in excess water vapor.Detailed characterization results demonstrate the strong interaction between Pd sites and zeolite framework in Pd@ZSM-5 and the efficient stabilization of isolated Pd sites by zeolite thereof.Spectroscopy analyses reveal that the presence of BrΦnsted acid sites is beneficial to methane adsorption and its subsequent activation on adjacent Pd sites,constructing cooperation between Bronsted acid sites and Pd sites within the confined space of MFI zeolite toward high-efficiency methane catalytic combustion.The reaction mechanism of methane combustion catalyzed by Pd@H-ZSM-5 model catalyst is finally discussed.

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.

Coordinatively unsaturated sites in zeolite matrix: Construction and catalysis

Zeolites with ordered porous structure of molecular size are widely employed as commercial adsorbents and catalysts.On the other hand,the zeolite matrix is regarded as an ideal scaffold for hosting coordinatively unsaturated sites.Remarkable achievements have been made dealing with the construction,characterization and catalytic applications of coordinatively unsaturated sites in zeolite matrix.Herein,a literature overview of recent progresses on this important topic is presented from the specific view of coordination chemistry.Different strategies to construction coordinatively unsaturated sites in zeolite matrix,in zeolite framework or extraframework positions,are first introduced and their characteristics are compared.Then,spectroscopic techniques to determine the existing states of cation sites and their transformations in zeolite matrix are discussed.In the last section,the catalytic applications of coordinatively unsaturated sites in zeolite matrix for various important chemical transformations are summarized.

Stabilizing copper species using zeolite for ethanol catalytic dehydrogenation to acetaldehyde

The selective dehydrogenation of ethanol to acetaldehyde is a promising route for acetaldehyde production.Although Cu-based catalysts exhibit high activity in ethanol dehydrogenation,a rapid deactivation due to Cu sintering always occurs.In this study,highly dispersed Cu species were stabilized using the silanol defects in Beta zeolite(denoted as Beta)resulting from dealumination,and applied as robust catalysts for ethanol-to-acetaldehyde conversion.Typically,a long catalyst lifetime of 100 h with an acetaldehyde yield of^70%could be achieved over 5%Cu/Beta.The presence of Cu^+and Cu0 species and the agglomeration of Cu particles after a long-term reaction for 180 h were revealed by transmission electron microscopy,thermogravimetric analysis,and CO-diffuse-reflectance infrared Fourier transform spectroscopy,and were responsible for the deactivation of the Cu/Beta catalyst in the ethanol-to-acetaldehyde conversion.

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.

Bimetallic Cr-In/H-SSZ-13 for selective catalytic reduction of nitric oxide by methane

Bimetallic Cr-In/H-SSZ-13 zeolites were prepared by wet impregnation and investigated for selective catalytic reduction of nitric oxide by methane(CH4-SCR).Reduction-oxidation treatments led to close contact and interaction between Cr and In species in these zeolites,as revealed by transmission electron microscopy and X-ray photoelectron spectroscopy.Compared to monometallic Cr/H-SSZ-13 and In/H-SSZ-13,the bimetallic catalyst system exhibited dramatically enhanced CH4-SCR performance,i.e.,NO conversion greater than 90%and N2 selectivity greater than 99%at 550°C in the presence of 6%H2O under a high gas hourly space velocity of 75 000/h.The bimetallic Cr-In/H-SSZ-13 showed very good stability in CH4-SCR with no significant activity loss for over 160 h.Catalytic data revealed that CH4 and NO were activated on the In and Cr sites of Cr-In/H-SSZ-13,respectively,both in the presence of O2 during CH4-SCR.

Tandem Lewis acid catalysis for the conversion of alkenes to 1,2‐diols in the confined space of bifunctional TiSn‐Beta zeolite

The generation of multifunctional isolated active sites in zeolite supports is an attractive method for integrating multistep sequential reactions into a single‐pass tandem catalytic reaction.In this study,bifunctional TiSn‐Beta zeolite was prepared by a simple and scalable post‐synthesis approach,and it was utilized as an efficient heterogeneous catalyst for the tandem conversion of alkenes to 1,2‐diols.The isolated Ti and Sn Lewis acid sites within the TiSn‐Beta zeolite can efficiently integrate alkene epoxidation and epoxide hydration in tandem in a zeolite microreactor to achieve one‐step conversion of alkenes to 1,2‐diols with a high selectivity of>90%.Zeolite confinement effects result in high tandem rates of alkene epoxidation and epoxide hydration as well as high selectivity toward the desired product.Further,the novel method demonstrated herein can be employed to other tandem catalytic reactions for sustainable chemical production.

Unexpectedly selective hydrogenation of phenylacetylene to styrene on titania supported platinum photocatalyst under 385 nm monochromatic light irradiation

Conversion of alkynes to alkenes by photocatalysis has inspired extensive interest but it is still challenging to obtain both high conversion and selectivity.Here we first demonstrate the photocatalytic conversion of phenylacetylene(PLE)to styrene(STE)with both high conversion and selectivity by using the titania(TiO2)supported platinum(Pt)as photocatalyst under 385 nm monochromatic light irradiation.It is demonstrated that the conversion rate of PLE is strongly dependent on the content of Pt cocatalyst loaded on the surface of TiO2.Based on our optimization,the conversion of PLE and the selectivity towards STE on the 1 wt%Pt/TiO2 photocatalyst can unexpectedly reach as high as 92.4%and 91.3%,respectively.The highly selective photocatalytic hydrogenation can well be extended to the conversion of other typical alkynes to alkenes,demonstrating the generality of selective hydrogenation of C≡C over the Pt/TiO2 photocatalyst.

Metal-seed assistant photodeposition of platinum over Ta_(3)N_(5) photocatalyst for promoted solar hydrogen production under visible light

Cocatalysts play a vital role in accelerating the reaction kinetics and improving the charge separation of photocatalysts for solar hydrogen production.The promotion of the photocatalytic activity largely relies on the loading approach of the cocatalysts.Herein,we introduce a metal-seed assistant photodeposition approach to load the hydrogen evolution cocatalyst of platinum onto the surface of Ta_(3)N_(5) photocatalyst,which exhibits about 3.6 times of higher photocatalytic proton reduction activity with respect to the corresponding impregnation or photodeposition loading.Based on our characterizations,the increscent contact area of the cocatalyst/semiconductor interface with metal-seed assistant photodeposition method is proposed to be responsible for the promoted charge separation as well as enhanced photocatalytic H2 evolution activity.It is interesting to note that this innovative deposition strategy can be easily extended to loading of platinum cocatalyst with other noble or non-noble metal seeds for promoted activities,demonstrating its good generality.Our work may provide an alternative way of depositing cocatalyst for better photocatalytic performances.

Self-aldol condensation of aldehydes over Lewis acidic rare-earth cations stabilized by zeolites

The self-aldol condensation of aldehydes was investigated with rare-earth cations stabilized by[Si]Beta zeolites in parallel with bulk rare-earth metal oxides.Good catalytic performance was achieved with all Lewis acidic rare-earth cations stabilized by zeolites and yttrium appeared to be the best metal choice.According to the results of several complementary techniques,i.e.,temperature-programmed surface reactions,in situ diffuse reflectance infrared Fourier transform spectroscopy,ultraviolet-visible diffuse reflectance spectroscopy,the reaction pathway and mechanism of the aldehyde self-aldol condensation over Y/Beta catalyst were studied in more detail.Density functional theory calculations revealed that aldol dehydration was the rate-limiting step.The hydroxyl group at the open yttrium site played an important role in stabilizing the transition state of the aldol dimer reducing the energy barrier for its hydration.Lewis acidic Y(OSi)(OH)2 stabilized by zeolites in open configurations were identified as the preferred active sites for the self-aldol condensation of aldehydes.

Cascade adsorptive separation of light hydrocarbons by commercial zeolites

Adsorptive separation of light hydrocarbons by porous solids provides an energy-efficient alternative to state-of-the-art cryogenic distillation.However,an optimal balance between the cost,performance and stability of the sorbent material is yet to be achieved for industrial applications.Here,we report the efficient separation of C2 and C3 hydrocarbons by a faujasite zeolite(Na-X,Si/Al=1.23).A tandem configuration of two fixed-beds packed with Na-X affords complete dynamic separation of the ternary mixture of C_(2)H_(2)/C_(2)H_(4)/C_(2)H_(6)(1/49.5/49.5;v/v/v)under ambient conditions.Pressure-swing desorption on the latter fixed-bed gives ethylene(>99.50%,1.80 mmol g^(-1))and ethane(>99.99%,1.41 mmol g^(-1)).In situ synchrotron X-ray powder diffraction revealed the binding sites for C_(2)H_(2)and C_(2)H_(4)in Na-X.This study highlights the potential application of commercial zeolites for challenging industrial separations.

Robust cobalt oxide catalysts for controllable hydrogenation of carboxylic acids to alcohols

The selective catalytic hydrogenation of carboxylic acids is an important process for alcohol production,while efficient heterogeneous catalyst systems are still being explored.Here,we report the selective hydrogenation of carboxylic acids using earth‐abundant cobalt oxides through a reaction‐controlled catalysis process.The further reaction of the alcohols is completely hindered by the presence of carboxylic acids in the reaction system.The partial reduction of cobalt oxides by hydrogen at designated temperatures can dramatically enhance the catalytic activity of pristine samples.A wide range of carboxylic acids with a variety of functional groups can be converted to the corresponding alcohols at a yield level applicable to large‐scale production.Cobalt monoxide was established as the preferred active phase for the selective hydrogenation of carboxylic acids.

Application of ammonia probe-assisted solid-state NMR technique in zeolites and catalysis

Solid-state NMR(ssNMR)spectroscopy is a powerful technique for characterizing the surface sites of solid acids and organic intermediates formed during the acid catalyzed reaction.As a very useful probe molecule,ammonia is often utilized to determine the density of solidacids’surface sites by ssNMR spectroscopy.The present mini-review summarizes some of the latest research developments on the quantitative characterization of the acid sites and carbenium ions during the zeolite catalytic reaction by ammonia probe-assisted ssNMR spectroscopy.

Zeolites for separation:Fundamental and application

Material based emerging separation techniques are attracting more and more attention as alternatives to the traditional ones such as distillation and extraction,aiming to reduce energy consumption and pollutant emissions.Due to their structure characteristics,zeolites can act as versatile sieves and adsorbents for molecules and have been successfully applied in some very important separation processes.Herein,two major catalogues of zeolite separations,namely membrane separation and adsorptive separation,are discussed and their underlying mechanisms are focused.In the part of membrane separation,the synthesis strategies toward zeolite membranes are introduced and the uniformly-oriented zeolite membranes are emphasized.In the part of the adsorptive separation,the industrial and popular adsorptive separations with the corresponding zeolite adsorbents are summarized.Generally,membrane separation relies on the molecular diffusion behavior within zeolites while adsorptive separation relies on the guest–host interaction in principle.The key challenges and misconceptions in zeolite separations are highlighted throughout the article.

Zeolite-encaged gold catalysts for the oxidative condensation of furfural

The oxidative condensation between renewable furfural and fatty alcohols is a crucial avenue for producing high-quality liquid fuels and valuable furan derivatives.The selectivity control in this reaction process remains a significant challenge.Herein,we report the strategy of confining well dispersed gold species within ZSM-5 structure to construct highly active Au@ZSM-5 zeolite catalysts for the oxidative condensation of furfural.Characterization results and spectroscopy analyses demonstrate the efficient encapsulation of isolated and cationic Au clusters in zeolite structure.Au@ZSM-5(K)catalyst shows remarkable performance with 69.7%furfural conversion and 90.2%furan-2-acrolein selectivity as well as good recycle stability.It is revealed that the microstructure of ZSM-5 zeolite can significantly promote oxidative condensation activity through confinement effects.This work presents an explicit example of constructing zeolite encaged noble metal catalysts toward targeted chemical transformations.

Efficient Heterogeneous Hydroformylation over Zeolite-Encaged Isolated Rhodium Ions

Alkene hydroformylation is an extremely important industry process currently accomplished via homogeneous catalysis.Heterogeneous hydroformylation is being avidly pursued as a more economical and sustainable process.Herein,we report the construction of zeolite-encaged rhodium catalyst for efficient hydroformylation.Through a facile in situ hydrothermal strategy,isolated Rh^(δ+)(δ=2.5)can be encaged in faujasite and efficiently stabilized via interaction with framework oxygen atoms,producing a Rh@Y model catalyst with well-defined rhodium sites and coordination environment.Rh@Y exhibits high catalytic activity,perfect chemoselectivity,and recyclability in 1-hexene hydroformylation under mild reaction conditions,making it a robust heterogeneous catalyst for potential applications.A state-of-the-art turnover frequency value of 6567 molC=C/molRh/h for Rh@Y can be achieved in 1-hexene hydroformylation at 393 K,outperforming all heterogeneous catalysts and most homogeneous catalysts under comparable conditions.With the well-defined structure of Rh@Y,the detailed mechanism of alkene hydroformylation can be interpreted via theoretical calculations,and the advantages of heterogeneous hydroformylation are well explained.This work provides a promising solution toward efficient heterogeneous noble metal catalysis by encaging stable isolated ions in a zeolite matrix.

Dynamic single-atom catalysts toward ultrastable dehydrogenation

The increasing demand for light olefins,including ethylene,propyl-ene and butenes,which are key building blocks in chemical industry,has evoked a research boom of light alkane dehydrogenation[1].In com-parison with oxidative dehydrogenation,which shows limited olefin productivity due to the undesired formation of CO_(2)at high alkane con-versions,non-oxidative dehydrogenation can achieve very high olefin selectivity at high alkane conversion levels.

Homogeneous-like Alkyne Selective Hydrogenation Catalyzed by Cationic Nickel Confined in Zeolite

The selective hydrogenation of alkynes to their corresponding alkenes is an important type of organic transformation,which is currently accomplished by modified palladium catalysts.Herein,we report that coordinatively unsaturated Ni(II)sites confined in faujasite zeolite,that is,Ni@Y,can efficiently catalyze the selective hydrogenation of various alkynes in both gas-and liquid phase.Spectroscopic and kinetic analyses explicitly reveal that alkyne hydrogenation over Ni@Y follows the homogeneous associative mechanism instead of the classic heterogeneous Horiuti–Polanyi mechanism.Density functional theory calculations confirm that the preferential adsorption of alkynes on coordinatively unsaturated Ni(II)sites is crucial to initiate dihydrogen activation for the subsequent hydrogenation.Ni@Y represents a true heterogeneous catalyst acting as a homogeneous catalyst and shares the advantages of both heterogeneous and homogeneous catalysis.These findings may shed light on the rational design of robust catalysts and new catalytic routes by linking heterogeneous and homogeneous catalysis.

Multifunctional heteroatom zeolites:construction and applications

Multifunctional heteroatom zeolites have drawn broad attentions due to the possible synergistic effects in the catalytic reactions.Remarkable achievements have been made on the synthesis,characterization and catalytic applications of multifunctional heteroatom zeolite,while a review on this important topic is still missing.Herein,current research status of multifunctional heteroatom zeolites is briefly summarized,aiming to boost further researches.First,the synthesis strategies toward heteroatom zeolites are introduced,including the direct synthesis and postsynthesis routes;then,the spectroscopic techniques to identify the existing states of heteroatom sites and the corresponding physiochemical properties are shown and compared;finally,the catalytic applications of multifunctional heteroatom zeolites in various chemical reactions,especially in one-step tandem reactions,are discussed.

Progressive steps and catalytic cycles in methanol-to-hydrocarbons reaction over acidic zeolites

Understanding the complete reaction network and mechanism of methanol to-hydrocarbons remains a key chal-lenge in the field of zeolite catalysis and C1 chemistry.Inspired by the identification of the reactive surface methoxy species on solid acids,several direct mechanisms associated with the formation of the first C-C bond in methanol conversion have been recently disclosed.Identifying the stepwise involvement of the initial intermedi-ates containing the first CC bond in the whole reaction process of methanol to-hydrocarbons conversion becomes possible and attractive for the further development of this important reaction.Herein,several initial unsaturated aldehydes/ketones containing the C-C bond are identified via complementary spectroscopic techniques.With the combination of kinetic and spectroscopic analyses,a complete roadmap of the zeolite catalyzed methanol-to-hydrocarbons conversion from the initial CC bonds to the hydrocarbon pool species via the oxygen-containing unsaturated intermediates is clearly ilustrated.With the participation of both Bronsted and Lewis acid sites in H-ZSM-5 zeolite,an initial aldol-cycle is proposed,which can be closely connected to the well-known dual-cycle mechanism in the methanol-to-hydrocarbons conversion.