Silicalite Catalyst Research Articles

Lignin conversion into C4 dicarboxylic acids by catalytic wet peroxide oxidation using titanium silicalite-1

Lignin valorisation towards added-value products has become a relevant topic to consolidate a future circular bioeconomy. In this context lignin oxidation to C4 dicarboxylic acids (C4-DCA) by catalytic wet peroxide oxidation is emerging as a value-added strategy, supported by the extensive use of these building blocks in several industrial fields. In this work, lignins from different sources and processes (Indulin AT, Lignol, alkali and E. globulus kraft lignins) were oxidised using H2O2 and titanium silicalite-1 catalyst (TS-1) under different operating conditions (temperature, pH, time, H2O2, and TS-1 load). Indulin AT was the lignin leading to the highest succinic acid yield (11.3 wt%), and TS-1 catalyst enhanced its production four times over the non-catalysed reaction. Malic acid was also produced at high yields, especially for Lignol lignin. The other lignins (E. globulus kraft, and alkali lignins) also produced these C4 acids but at lower yields. The catalyst remained stable at the used experimental conditions, and showed potential to be reused for several cycles without being deactivated. Overall, the catalytic conversion of lignin to C4-DCA can help to guide the pathway to renewable chemicals production.

Kinetics and Mechanism of Allyl Alcohol Epoxidation with Hydrogen Peroxide on a Titanium Silicalite Catalyst TS-1. Formulation and Discrimination between Hypothetical Mechanisms

Kinetics and Mechanism of Allyl Alcohol Epoxidation with Hydrogen Peroxide on a Titanium Silicalite Catalyst TS-1. Formulation and Discrimination between Hypothetical Mechanisms

Tungsten-substituted Silicalite-1 with an interconnected hollow structure for catalytic epoxidation of cyclohexene

Abstract A novel hierarchical tungsten-substituted Silicalite-1 zeolite with the highly interconnected hollow structure (HWS-1) was synthesized for the first time by post-treating the purely microporous tungsten-substituted Silicalite-1 (WS-1) through simultaneous desilication and tungstation, in which a combination of sodium hydroxide and tetrapropyl ammonium hydroxide (TPAOH) was used to achieve a systematic balance between dissolution and recrystallization and further promote the synchronous tungstation with using sodium tungstate dihydrate as a tungsten source. Under an optimum addition ratio of NaOH/TPAOH, the resulted HWS-1 exhibited not only well retained microporosity and crystallinity but also obviously enlarged external surface area and mesopore volume in contrast to the parent WS-1. Furthermore, the effective tungstation also facilitated the substitution and increased the amount of coordinated W species in zeolite frameworks. The catalytic performance of HWS-1 was evaluated by the epoxidation of cyclohexene with using H2O2 as oxidant. Benefited by the highly interconnected hollow structure and more accessible active sites on the enlarged external surface, the HWS-1 catalyst revealed a higher catalytic activity with improved cyclohexene conversion than WS-1. Moreover, it was found that a significant positive effect on promoting epoxidation was achieved by tungsten-substituted Silicalite-1 (WS-1 and HWS-1) catalysts reflecting at a much higher selectivity of epoxide (>79%) in comparison with titanium-substituted Silicalite-1 (TS-1) (~50% selectivity of epoxide), due to their superior hydrophobicity and suitably weaker acidity, which further demonstrated the potential application of the developed hierarchical tungsten-substituted Silicalite-1 as a candidate catalyst for epoxidation and other selective oxidation reactions.

Oxidative dehydrogenation of n-octane using Ba and Ga-modified faujasite type catalysts prepared by different methods

Ionic exchange and isomorphic substitution were used for the synthesis and modification of faujasite (zeolite and silicalite Y) catalysts. These two methods were used to synthesize 1 wt% extra-framework and framework gallium modified faujasite zeolites respectively, with a Si/M ratio of 2.6, where M = Al and/or Ga. For the silicalite catalyst, 3.5 wt% of Ga was used to maintain the Si/Ga ratio. A series of BaY, Ga-BaY(IS) and Ga-BaY(Sil) were prepared by isomorphic substitution, where (IS) and (Sil) represent Isomorphic Substitution and Silicalite respectively. Ga-BaY(IE) and BaGa-NaY(IE), were prepared by ionic exchange (IE) using the synthesised BaY and a commercial NaY for the latter. For all the prepared catalysts, barium was kept constant at about 4.2 wt% except for BaGa-NaY(IE), where Ba was 1.4 wt%. For the sodium containing catalysts, Na was 10.3 wt%. Pyridine IR and chemisorption techniques showed that the catalysts prepared had more Lewis acid sites compared to Bronsted acid sites. The Na containing zeolite had the highest total acidity compared to those containing only Ba. Aluminium free silicalite contained the least total acidity, which is consistent with previous studies. Catalytic data at iso-conversion (7 ± 1%) showed that the Ba incorporated by ionic exchange zeolites and the silicalite were more selective to octenes (36 and 33% respectively) and less selective to COx (35 and 33% respectively) compared to the isomorphically substituted Ba catalysts. Selectivity to cracked products was reduced when aluminium, responsible for the strong Bronsted acid sites, was eliminated from the synthesis.

Controllable Synthesis and Structure-Performance Relationship of Silicalite-1 Nanosheets in Vapor Phase Beckmann Rearrangement of Cyclohexanone Oxime

Bulky silicalite-1 shows high catalytic activity for vapor phase Beckmann rearrangement of cyclohexanone oxime. However, it often deactivates rapidly and the structure-performance relationship is unclear. Here, a series of well-crystallized silicalite-1 catalysts from three-dimensional micron particles (n < 10) to two-dimensional nanosheets (10 ≤ n ≤ 18) were controllably synthesized using diquaternary ammoniums [CnH2n+1-N+(CH3)2-C6H12-N+(CH3)2-C6H13](Br−)2 (n = 6–18) as the structure directing agent. The influence of morphology of catalysts on active sites distribution and the pore confinement effect on caprolactam selectivity and the catalytic stability were studied by XRD, SEM, TEM, In situ vacuum IR, 29Si MAS NMR and N2 adsorption/desorption isotherms. The silicalite-1 nanosheets with relatively high crystallinity exhibited superior catalytic activity to almost 100% and remarkably high caprolactam selectivity (92%). The catalytic lifetime of silicalite-1 nanosheets could be increased by 28 times in comparison to bulky silicalite-1 zeolite. The external surface area and mesopore volume of the synthesized catalyst dramatically increase with increasing the tail length of surfactant from C6 to C18, leading to high active sites and coke tolerance capacity. It was found the lifetime of the catalysts is almost linearly related to the product of area of silanol nest and volume of mesopore (Asilanol·nest*Vmeso) with correlation coefficient of 0.97, which strongly supports the excellent performance of nanosheets results from the synergetic effect of its high active sites and high coke tolerance capacity. This provides a theoretic guide for designing new catalysts with high catalytic activity. Silicalite-1 nanosheets were controllable synthesized through adjusting alkyl tail length of diquaternary ammoniums, resulting in large mesoporous volume and external surface area, which effectively improve high coke tolerance capacity and active sites of catalysts and exhibit much longer catalytic lifetime and high selectivity.

Mechanistic Insight into Propylene Epoxidation with H2O2 over Titanium Silicalite-1: Effects of Zeolite Confinement and Solvent.

Density functional theory (DFT) calculations were performed to investigate the effects of zeolite confinement and solvent on propylene epoxidation with H2O2 over the titanium silicalite-1 (TS-1) catalyst. The 144T and 143T cluster models containing typical 10MR channels of TS-1 were constructed to represent the tripodal(2I) and Ti/defect sites. It was found that the confinement of the zeolite pore channel not only impacts the adsorption stability of guest molecules but also alters reaction barriers, as compared to the results obtained based on small cluster models. When dispersion corrections were considered, an enhancement of the adsorption stability of guest molecules was observed because of the important contribution from van der Waals interactions, especially for propylene adsorption. An explicit protic methanol molecule was introduced into the catalytic system to probe the influence of the solvent on propylene epoxidation, based on which a significant enhancement of CH3OH-H2O2 co-adsorption was obtained owing to H-bond formation. More importantly, the energy barrier for H2O2 dissociation was largely reduced by ∼13 kcal/mol because of the participation of the methanol in the H-transfer process and the formation of H-bond network, resulting in an alteration of the rate-limiting step. By comparison, adding an aprotic acetonitrile solvent did not have substantial effect on reaction path and kinetics. The calculation results clearly demonstrate the important role of the protic methanol solvent, which not only strengthens the adsorption of guest molecules but also promotes the kinetics for propylene epoxidation with H2O2 over TS-1 catalyst.

Propylene oxide inhibits propylene epoxidation over Au/TS-1

The kinetics of propylene epoxidation over Au supported on titanium silicalite-1 (Au/TS-1) catalysts were measured in a continuous stirred tank reactor (CSTR) free from temperature and concentration gradients. Apparent reaction orders measured at 473 K for H2 (0.7 order), O2 (0.2), and propylene (0.2) for a series of Au/TS-1 with varied Au (0.02–0.09 wt%) and Ti (Si/Ti: 75–143) contents were consistent with those reported previously. Co-feeding propylene oxide enabled measurement of the apparent reaction order in propylene oxide (−0.4 to −0.8 order) and the determination that relevant pressures of propylene oxide reversibly inhibit propylene epoxidation over Au/TS-1, while co-feeding carbon dioxide and water had no effect on the propylene epoxidation rate. Analysis of previously proposed two-site reaction mechanisms in light of reaction orders for O2 (0.4), H2 (1), and C3H6 (0.4), corrected to account for propylene oxide inhibition, provides further evidence that propylene epoxidation over Au/TS-1 occurs via a simultaneous mechanism requiring two distinct, but adjacent, types of sites, and not a sequential mechanism that invokes migration of H2O2 formed on Au sites to PO forming Ti sites. H2 oxidation rates are not inhibited by propylene oxide, implying that the sites required for hydrogen oxidation are distinct from those required for propylene epoxidation.

An efficient Titanium silicalite-1 catalyst for propylene epoxidation synthesized by a combination of aerosol-assisted hydrothermal synthesis and recrystallization

Reducing the crystal size and creating intra/inter-crystalline mesoporosity or macroporosity are two options to enhance the mass transportation efficiency in microporous zeolites. In this study, an improved aerosol-assisted hydrothermal method was used to synthesize small-crystal Titanium silicalite-1 (TS-1) with low template amount. By using a more concentrated template agent the particle size of the TS-1 crystal can be reduced from 350-800 nm to 230–270 nm. A well-dispersion of the Ti species can be achieved under optimized synthesis condition. A post-treatment with tetrapropyl ammonium hydroxide (TPAOH) was used to further improve the diffusion property. The mesopore volumes and external surface areas are significantly increased due to the formation of the intracrystalline voids. All the TS-1 samples were evaluated in propene epoxidation. The conversion of H2O2 and the selectivity of Propylene Oxide are 99.9% and 98%, which are improved as compared to those of the parent TS-1.

Hydrogen Production from Coke Oven Gas by CO2 Reforming Over a Novel Ni-Doped Silicalite-1

A series of in-situ nickel atoms doped (Ni-doped) Silicalite-1 used as the catalysts in dry reforming of coke oven gas (COG) have been synthesized by one-pot hydrothermal method. These catalyst materials are characterized by electron microscopy, X-ray powder diffraction, and Nitrogen adsorption–desorption measurements, etc. And the catalytic results show that in-situ Ni-doped zeolites exhibit superior performance for dry reforming of COG. Moreover, the in-situ synthesized Ni-doped Silicalite-1 catalysts with trace amount of nickel behave quite considerable activity and outstanding anti-coking property in comparison with the traditional Ni-based catalysts. The method we adopt in this work is less complicated for the synthesis of Ni-doped Silicalite-1, which will make the in-situ Ni-doped Silicalite-1 catalysts have great prospects for hydrogen production from COG by CO2 reforming in the future.

Comparison of a monolith and a confined Taylor flow (CTF) reactor for propene epoxidation

Heterogeneous catalytic epoxidation of propene to propene oxide with hydrogen peroxide was investigated in a monolith and a confined Taylor flow (CTF) reactor in which titanium silicalite (TS-1) catalyst was coated on the walls. The influence of gas and liquid superficial velocity on the hydrodynamic characteristics of the monolith and CTF reactor was also investigated under Taylor flow regime at atmospheric and high pressure. The reactors showed distinctly different hydrodynamic properties which in turn led to different performance for propene epoxidation. The production rate of propene oxide was higher in the monolith reactor due to its larger catalyst coating area, larger mass-transfer surface area and more frequent recycling of liquid flow. A variation of reactor column structures confirmed that the propene oxide production was highly dependent on the catalyst coating area and cross-sectional area of the reactor column. High operating pressure made a significant impact on the length of Taylor bubbles and the propene oxide production rate was found to increase in proportion to the operating pressure.

Mesoporous/Microporous Titanium Silicalite with Controllable Pore Diameter for Cyclohexene Epoxidation

A mesoporous/microporous titanium silicalite with controllable pore diameter was synthesized in an easy and new route by using cetyltrimethylammonium bromide (CTAB) as a mesoporous template and tetrapropylammonium hydroxide (TPAOH) as a microporous template. This route leads to the formation of mesopores prior to the crystallization of microporous MFI topology. The porosity formation sequence makes the two types of channels, which are micropores with MFI topology and mesopores with wormlike morphology, distribute homogeneously. The pore diameter of the mesopores can be adjusted from the maximum center of 2.6 nm to that of 6.9 nm by tuning the molar ratio of CTAB to silicon from 0.125 to 0.20. The mesoporous/microporous titanium silicalite catalysts were evaluated in the epoxidation of cyclohexene, and showed excellent catalytic activities, with respect to the conventional microporous TS-1, because of the enhanced diffusion property in the mesopores and higher titanium content near the external surface of ...

Synergetic photo-epoxidation of propylene with molecular oxygen over bimetallic Au–Ag/TS-1 photocatalysts

Au–Ag bimetallic nanoparticle-supported microporous titanium silicalite-1 catalysts were prepared via a hydrothermal-immersion method, and their structures were examined. These materials serve as efficient catalysts for the photosynthesis of propylene oxide via the epoxidation of propene. The Au/Ag mass ratio and reaction temperature were demonstrated to have significant effects on the catalytic activity and selectivity of propylene oxide. The optimal formation rate (68.3 μmol/g·h) and selectivity (52.3%) toward propylene oxide were achieved with an Au:Ag mass ratio of 4:1. Notably, the strong synergistic effect between Au and Ag resulted in superior photocatalysis of the bimetallic systems compared with those of the individual systems. A probable reaction mechanism was proposed based on the theoretical and experimental results.

Epoxidation of propene in a confined Taylor flow (CTF) reactor at atmospheric pressure

Heterogeneous catalytic epoxidation of propene to propene oxide with hydrogen peroxide as oxidant was investigated in a confined Taylor flow (CTF) reactor, a continuous monolith reactor, containing a long alumina rod coated with titanium silicalite (TS-1) catalyst in the centre of the reactor column. The effect of gas and liquid superficial velocity on the hydrodynamics of CTF reactor was also investigated under Taylor flow regime at atmospheric pressure. The variation of hydrodynamics had a profound impact on the production of propene oxide. When liquid superficial velocity was constant, the concentration of propene oxide produced decreased with increasing gas superficial velocity as Taylor bubble length and bubble rise velocity increase. However, when gas superficial velocity was constant, the concentration of propene oxide produced had no linear dependency on liquid superficial velocity as Taylor bubble length decreases but bubble rise velocity increases.

Experiments and kinetics of the epoxidation of allyl chloride with H2O2 over organic base treated TS-1 catalysts

An efficacious approach to improve the catalytic performance of titanium silicalite-1 (TS-1) catalysts in epoxidation of allyl chloride with hydrogen peroxide has been developed. A series of modified TS-1 catalysts were prepared by corroding the classic TS-1 catalyst with different concentrations of tetrapropyl ammonium hydroxide (TPAOH) under controlled conditions. The resultant catalyst samples were characterized by XRD, ICP, N2-adsorption, FT-IR, UV–vis, SEM and TEM. The catalytic activity of the modified TS-1 catalysts is significantly improved, which could be attributed to the increase of the mesopore volume and pore diameter and decrease of the diffusion limitation but without obvious changes of the framework structure, framework Ti content, the outer surface shape and crystal size of the modified TS-1. Furthermore, the kinetics of the epoxidation of allyl chloride over TS-1-0.05 catalyst was also systematically studied, the apparent orders of reaction with respect to H2O2 and allyl chloride are 0.68 and 0.62, respectively, and the activation energy of the epoxidation reaction on the TS-1-0.05 catalyst is 57.4kJmol−1.

Better performance for gas-phase epoxidation of propylene using H2 and O2 at lower temperature over Au/TS-1 catalyst

Nano-gold supported on titanium silicalite-1 catalyst (Au/TS-1), prepared by the deposition precipitation (DP) method with Cs2CO3 as the precipitation agent, was tested for gas-phase epoxidation of propylene. Interestingly, the catalytic performances including PO selectivity and H2 efficiency at lower temperature (164°C) are better than those at higher temperature (200°C), which is mainly attributed to the change of Au particle size. However, the carbon deposits on catalyst surface as well as the surface acid amount of catalyst have little effect. 164°C may be a promising reaction temperature.

Catalytic performance of cerium modified Silicalite-1 molecular sieves in the conversion of methanol to propene

A series of cerium modified Silicalite-1 molecular sieves were prepared by incipient wetness impregnation and used as the catalysts in the conversion of methanol to propene (MTP). The cerium modified Silicalite-1 catalysts were characterized by X-ray diffraction (XRD), N2 sorption, pyridine adsorption Fourier transform infrared spectroscopy (Py-FTIR) and temperature-programmed desorption of NH3 (NH3-TPD); their catalytic performance in MTP was evaluated in a continuous flow fixed-bed micro-reactor at atmospheric pressure and 450°C, with a methanol weight hourly space velocity (WHSV) of 9.6 h−1. The results showed that Silicalite-1 exhibits superior catalytic performance in MTP in terms of catalytic stability and selectivity to propene, in comparison with the HZSM-5 zeolite with a SiO2/Al2O3 molar ratio of 200. The introduction of Ce can effectively adjust the acid properties and pore structure of Silicalite-1; modification with an appropriate concentration of Ce can reduce the amount of strong acid sites and then enhance the catalytic performance of Silicalite-1 in MTP. Over the cerium modified Silicalite-1 with a CeO2 content of 5.0%, the selectivity to propene is increased from 31.9% to 38.2% and the catalytic lifetime is extended from 51 to 72 h, in comparison with those over the parent Silicalite-1.

Clean Transformation of Ethanol to Useful Chemicals. The Behavior of a Gold-Modified Silicalite Catalyst.

Upon addition of gold to silicalite-1 pellets (a MFI-type zeolite), the vapor phase oxidation of ethanol could be addressed to acetaldehyde or acetic acid formation. By optimizing the catalyst composition and reaction conditions, the conversion of ethanol could be tuned to acetaldehyde with 97% selectivity at 71% conversion or to acetic acid with 78% selectivity at total conversion. Considering that unloaded silicalite-1 was found to catalyze the dehydration of ethanol to diethylether or ethene, a green approach for the integrated production of four important chemicals is herein presented. This is based on renewable ethanol as a reagent and a modular catalytic process.

NbOx/SiO2 in the gas-phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam: Influence of calcination temperature, niobia loading and silylation post-treatment

NbOx/SiO2 catalyst materials prepared by the incipient wetness impregnation method were studied in the industrially relevant gas-phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam. The catalytic experiments were carried out in a fixed bed reactor system at atmospheric pressure. Results have been complemented with Raman and FT-IR spectroscopy as well as N2 physisorption measurements. Optimal catalytic results were observed for a catalyst calcination temperature of 600°C and a niobia loading of 0.3wt.%. Raman spectroscopy revealed that isolated tetrahedral mono-oxo NbO4 surface species most probably play a key role in the catalytic reaction. Very positive effects were achieved by applying a catalyst silylation post-treatment. Firstly, the catalytic long-term stability increased very substantially as a consequence of a decreased coke deposition on the catalyst surface. Secondly, the harmful effect of water on catalytic performance was strongly suppressed even to such an extent that water could be added to the feed to enhance catalytic long-term stability. Cyclohexanone oxime conversion and ε-caprolactam selectivity could be maintained at constant high values >99% and around 95% respectively, for 26h time-on-stream. Finally, considering the Sumitomo gas-phase Beckmann rearrangement process as a benchmark, we made a rough comparison between the catalytic performances of our niobia/silica catalyst and the silicalite catalyst used by Sumitomo.

The role of acid sites induced by defects in the etherification of HMF on Silicalite-1 catalysts

Defects in Silicalite-1 (structure type MFI) generate acid sites in the form of (i) hydroxyl nests present inside the channels or (ii) Brønsted–Lewis acid pairs, associated with terrace step kink sites present on the external surface of zeolite crystals. The amount of the sites has been changed by controlling the thermal treatment and pH value during preparation, or by post-synthesis treatments (ionic-exchange, silylation). These sites have been quantified by NMR, XRD and FT-IR pyridine adsorption/desorption at room temperature and 150°C, and correlated with the catalytic etherification of 5-hydroxymethyl-2-furfural (HMF) with ethanol, a relevant reaction to produce diesel additives. The results show that the surface acidity of Silicalite-1 materials presents an heterogeneous distribution of Lewis and Brønsted acid sites with different degrees of strength. The amount and distribution of such active centers determine the catalytic performance in HMF etherification. It is also shown that adsorption of water, produced in situ by ethanol condensation, has a promoter effect at lower concentration and high temperature, due to the dissociation of S2R siloxanes bonds, and formation of Lewis and Brønsted acid centers with medium-high strength, responsible for the activation pathways of HMF conversion to 5-(ethoxymethyl)furfan-2-carbaldehyde (EMF) and ethyl-4-oxopentanoate (EOP), respectively.

Oxidative Desulfurization of Model Gasoline Over Modified Titanium Silicalite

The oxidative desulfurization of model gasoline consisting of thiophene dissolved in n-octane was investigated with a series of modified titanium silicalite (TS) catalysts in presence of hydrogen peroxide and formic acid systems, and the reaction mechanism of the oxidative desulfurization of thiophene was preliminarily researched. The results showed that the copper modified TS was an active catalyst for thiophene oxidation while the other metal modified TSs were less active catalysts. When Cu-TS at a Cu/Si molar ratio of 0.015 was used as a catalyst for oxidation of model gasoline, the conversion of thiophene was 94.1% at 120 min. The conversion of thiophene was easily enhanced by increasing reaction time or reaction temperature, and reduced with addition of xylene and cyclohexene.