A critical review towards the causes of the iron-based catalysts deactivation mechanisms in the selective oxidation of hydrogen sulfide to elemental sulfur from biogas

Hydrogen sulfide(H_(2)S) not only presents significant environmental concerns but also induces severe corrosion in industrial equipment,even at low concentrations.Among various technologies,the selective oxidation of hydrogen sulfide(SOH_(2)S) to elemental sulfur(S) has emerged as a sustainable and environmentally friendly solution.Due to its unique properties,iron oxide has been extensively investigated as a catalyst for SOH_(2)S;however,rapid deactivation has remained a significant drawback.The causes of iron oxide-based catalysts deactivation mechanisms in SOH_(2)S,including sulfur or sulfate deposition,the transformation of iron species,sintering and excessive oxygen vacancy formation,and active site loss,are thoroughly examined in this review.By focusing on the deactivation mechanisms,this review aims to provide valuable insights into enhancing the stability and efficiency of iron-based catalysts for SOH_(2)S.

Styrene epoxidation catalyzed by polyoxometalate/quaternary ammonium phase transfer catalysts: The effect of cation size and catalyst deactivation mechanism

Catalytic epoxidation of alkenes is an important type of organic reaction in chemical industry,and the deep insight into catalyst deactivation will help to develop new epoxidation process.In this work,series of quaternary ammoniums bearing different cationic sizes,i.e.MTOA+(methyltrioctylammonium,[(C_(8)H_(17))_(3)CH_(3)N]+),HTMA+(hexadecyltrimethylammonium,[(C_(16)H_(33))(CH_(3))_(3)N]+) and DMDOA+(dimethyldioctadecylammonium,[(C_(18)H_(37))_(2)(CH_(3))_(2)N]+) were incorporated with polyoxometalate (POM) anions to prepare phase transfer catalysts (PTCs),which were used in the styrene epoxidations.Among them,(MTOA)_(3)PW_(4)O_(24)exhibits the best catalytic performance judged from the highest styrene conversion rate(52%) and styrene oxide selectivity (93%),during which the styrene epoxidation conditions were optimized.Meanwhile,the deactivation mechanism of this kind of PTCs was proposed firstly,i.e.in the case of low H_(2)O_(2) content,the oxidant can only be used in the styrene epoxidation,in which the catalyst can transform into stable Keggin-type POM.But when the content of H_(2)O_(2) is higher,the excess H_(2)O_(2) can reactivate the Keggin-type POM into active (PW_(4)O_(24))_(3)-anions,which can trigger the ring-opening polymerization of styrene oxide.Consequently,the catalyst is deactivated by adhered poly(styrene oxide)irreversibly,which was determined by NMR spectra.In this situation,the active moiety{PO_(4)[WO(O_(2))_(2)]_(4)}_(3)-in phase-transfer catalytic system can break into some unidentified species with low W/P ratio with the presence of epoxides.This work will be beneficial for the design of new PTCs in alkene epoxidation in fine chemical industry.

Oxidation of benzene to phenol with N_(2)O over a hierarchical Fe/ZSM-5 catalyst

Catalytic oxidation of benzene with N_(2)O to phenol over the hierarchical and microporous Fe/ZSM-5-based catalysts in a continuous fixedbed reactor was investigated.The spent catalyst was in-situ regenerated by an oxidative treatment using N_(2)O and in total 10 reaction-regeneration cycles were performed.A 100% N_(2)O conversion,93.3% phenol selectivity,and high initial phenol formation rate of 16.49±0.06mmol_(phenol gcatalyst)^(-1)h^(-1)at time on stream(TOS) of 5 min,and a good phenol productivity of 147.06 mmol_(phenol gcatalyst)^(-1)during catalyst lifetime of 1800 min were obtained on a fresh hierarchical Fe/ZSM-5-Hi2.8 catalyst.With the reaction-regeneration cycle,N_(2)O conversion is fully recovered within TOS of 3 h,moreover,the phenol productivity was decreased ca.2.2±0.8% after each cycle,leading to a total phenol productivity of ca.0.44 ton_(pheol kg_(catalyst)^(-1)estimated for 300 cycles.Catalyst characterizations imply that the coke is rapidly deposited on catalyst surface in the initial TOS of 3 h(0.28 mgc_(gcatalyst)^(-1)min^(-1)) and gradually becomes graphitic during the TOS of 30 h with a slow formation rate of 0.06 mgc g_(catalyst)^(-1)min^(-1).Among others(e.g.,the decrease of textural property and acidity),the nearly complete coverage of the active Fe-O-Al sites by coke accounts for the main catalyst deactivation.Besides these reversible deactivation characteristics related to coking,the irreversible catalyst deactivation is also observed with the reaction-regeneration cycle.The latter is reflected by a further decreased amount of the active Fe-O-Al sites,which agglomerate on catalyst surface with the cycle,likely associated with the hard coke residue that is not completely removed by the regeneration.

Rational design of MoS_(2)-based catalysts toward lignin hydrodeoxygenation:Interplay of structure,catalysis,and stability

The MoS_(2)-based materials are a vital class of heterogeneous catalysts for the hydrodeoxygenation of lignin and its model compounds to produce value-added chemicals especially because of their unique selectivity to aromatics.The rational design of MoS_(2)-based catalyst greatly depends on the comprehensive understanding of its structure-activity relationship.However,an intensive summary and critical analysis are still scarce to date.In this review,we attempt to provide an in-depth understanding of the interplay of structure,catalysis,and stability of MoS_(2)-based catalysts for lignin hydrodeoxygenation.The recognition of intrinsic active sites on MoS_(2) structure was firstly discussed,followed by the illustration of MoS_(2)-catalyzed hydrodeoxygenation structural models.Afterward,based on the studies on the MoS_(2)-catalyzed lignin model compounds hydrodeoxygenation,the current active site modification strategies including structural modification of monometallic MoS_(2) catalysts and collaborative modification were summarized and emphatically discussed,which aims to elucidate the structure-activity relationship at the atomic-level.The deactivation mechanism and stabilization strategies were also illustrated to provide instructive suggestion for the rational design of efficient and stable MoS_(2)-based catalysts.Finally,the real lignin depolymerization over MoS_(2)-based catalysts was summarized to point out the advantages and difficulties.This review attempts to highlight the remaining challenges and provide some perspectives for the future development of MoS_(2)-based catalysts for lignin hydrodeoxygenation.

Deactivation mechanisms and anti-deactivation strategies of molecular sieve catalysts for NO_(x)reduction

Molecular sieve catalysts,owing to their unique chemical properties,are widely used as catalysts among various catalytic reactions.Abundant Br?nsted acid sites in molecular sieve catalysts usually enable active components to disperse well on the catalyst surface,and help to adsorb a large number of gas molecules to achieve maximum catalytic performance.Therefore,a variety of molecular sieve catalysts have been developed and used in the selective catalytic reduction of NO_(x)by NH_(3)(NH_(3)-SCR).For example,Cu molecular sieve catalysts such as Cu-SSZ-13 and Cu-SAPO-34 with wide temperature windows and stable structure are considered and applied as commercial catalysts for NO_(x)removal in diesel vehicles for a long time.Although molecular sieve catalysts possess many advantages,they still cannot avoid the serious deactivation caused by various factors in practical applications.In this review,reasons leading to the deactivation of molecular sieve catalysts for NO_(x)reduction in actual working conditions were concluded.The deactivation mechanisms of molecular sieve catalysts for NO_(x)reduction were analyzed and the corresponding anti-deactivation strategies were summarized.Finally,challenges and prospects of molecular sieve catalysts for NO_(x)reduction were also proposed.

Deactivation mechanism of acetone to isobutene conversion over Y/Beta catalyst

The conversion of acetone derived from biomass to isobutene has attracted extensive attentions.In comparison with Brønsted acidic catalyst,Lewis acidic catalyst could exhibit a better catalytic performance with a higher isobutene selectivity.However,the catalyst stability remains a key problem for the long-running acetone conversion and the reasons for catalyst deactivation are poorly understood up to now.Herein,the deactivation mechanism of Lewis acidic Y/Beta catalyst during the acetone to isobutene conversion was investigated by various characterization techniques,including acetone-temperature-programmed surface reaction,gas chromatography-mass spectrometry,in situ ultraviolet-visible,and 13C cross polarization magic angle spinning nuclear magnetic resonance spectroscopy.A successive aldol condensation and cyclization were observed as the main side-reactions during the acetone conversion at Lewis acidic Y sites.In comparison with the low reaction temperature,a rapid formation and accumulation of the larger cyclic unsaturated aldehydes/ketones and aromatics could be observed,and which could strongly adsorb on the Lewis acidic sites,and thus cause the catalyst deactivation eventually.After a simple calcination,the coke deposits could be easily removed and the catalytic activity could be well restored.

Enhanced recycling performance of bimetallic Ir-Re/SiO_(2) catalyst by amberlyst-15 for glycerol hydrogenolysis

Recycling performance of heterogeneous catalysts is of crucial importance especially for a batch reaction system.In this work,we demonstrate a strategy for enhancing recycling performance of Ir-Re/SiO_(2) catalyst synergized with amberlyst-15 in glycerol hydrogenolysis to produce 1,3-propanediol.Comprehensive characterization results reveal that the Re sites in the Ir-Re/SiO_(2) catalyst undergo irreversible segregation and oxidation.These hinder the formation of active ReAOH species and thus contribute to a complete and irreversible deactivation.However,the introduction of amberlyst-15 into the reactant mixture can restrain the oxidation process of Re sites and favor the formation of ReAOH species,and thus significantly enhance the catalytic recycling performance.The results demonstrated here could guide the development of excellent bimetallic catalysts with the desirable recycling performances for the reaction.

Reconstruction of Cu-ZnO catalyst by organic acid and deactivation mechanism in liquid-phase hydrogenation of dimethyl succinate to 1,4-butanediol

A reconstructed Cu-ZnO catalyst with improved stability was fabricated by organic acid treatment method for the liquid-phase hydrogenation of dimethyl succinate to 1,4-butanediol.According to the characterization results of the fresh Cu-ZnO and reconstructed Cu-ZnO,three different forms of ZnO were suggested to be presented on the catalysts:ZnO having strong interaction with Cu species,ZnO that weakly interacted with Cu species and isolated ZnO.The first form of ZnO was believed to be beneficial to the formation of efficient active site Cu^(+),while the latter two forms of ZnO took the main responsibility for the deactivation of Cu-ZnO catalysts in the liquid-phase hydrogenation of diesters.The reconstruction of the Cu-ZnO catalyst by the organic acid treatment method resulted in a new Cu-ZnO catalyst with more Cu^(+)and less ZnO species that leads to deactivation.Furthermore,the deactivation mechanism of Cu-ZnO catalysts in liquid-phase diester hydrogenation in continuous flow system was proposed:the deposition of the polyesters on the catalysts via transesterification catalyzed by weakly interacted ZnO and isolated ZnO leads to the deactivation.These results provided meaningful instructions for designing highly efficient Cu-Zn catalysts for similar ester hydrogenation systems.

Recent advances in catalytic decomposition of ozone

Ozone(O3),as a harmful air pollutant,has been of wide concern.Safe,efficient,and economical O3 removal methods urgently need to be developed.Catalytic decomposition is the most promising method for O3 removal,especially at room temperature or even subzero temperatures.Great efforts have been made to develop high-efficiency catalysts for O3 decomposition that can operate at low temperatures,high space velocity and high humidity.First,this review describes the general reaction mechanism of O3 decomposition on noble metal and transition metal oxide catalysts.Then,progress on the O3 decomposition performance of various catalysts in the past 30 years is summarized in detail.The main focus is the O3 decomposition performance of manganese oxides,which are divided into supported manganese oxides and non-supported manganese oxides.Methods to improve the activity,stability,and humidity resistance of manganese oxide catalysts for O3 decomposition are also summarized.The deactivation mechanisms of manganese oxides under dry and humid conditions are discussed.The O3 decomposition performance of monolithic catalysts is also summarized from the perspective of industrial applications.Finally,the future development directions and prospects of O3 catalytic decomposition technology are put forward.