Solid Acid Sites Research Articles

Enhancements of activity and hydrothermal stability of FeTi catalyst from solid acid sites for selective catalytic reduction of NOx

Enhancements of activity and hydrothermal stability of FeTi catalyst from solid acid sites for selective catalytic reduction of NOx

Metal-assisted low-temperature cracking of n-hexane over Rh-encapsulated ZSM-5 catalysts

Naphtha cracking is the most common process for producing light olefins such as ethylene, propylene, and butene. This process consumes enormous amounts of energy, so decreasing the energy consumption and operating temperature is an urgent issue. To decrease the reaction temperature for naphtha cracking, we focused on the metal-assisted cracking reaction, in which paraffin is first dehydrogenated into the corresponding olefin on a metal catalyst, and the produced olefin is then decomposed into light olefins. To effectively realize metal-assisted cracking, we considered metal-encapsulated zeolite catalysts to be useful. In metal-encapsulated zeolite catalysts, the dehydrogenation reaction proceeds inside the zeolite particles, and the dehydrogenated intermediates can access the solid-acid sites frequently. In this study, Rh nanoparticle encapsulated ZSM-5 catalysts (Rh@ZSM-5) were employed for the metal-assisted cracking of n-hexane. Rh@ZSM-5 exhibited significantly high activity for n-hexane cracking below 450 °C owing to the metal encapsulation structure and close proximity between the metal and solid-acid sites. Furthermore, the effects of reaction conditions, reaction temperature, amounts of metal and solid-acid sites, and contact time on metal-assisted n-hexane cracking over the Rh@ZSM-5 catalysts were investigated. The highest light-olefin yield of 38.6 carbon mol% was achieved by the conversion of n-hexane over Rh@ZSM-5 using 0.3 wt% Rh loading, an Si/Al ratio of 100, temperature of 450 °C, reaction time of 0.5 h, and W/F of 2 h.

NH3-probe X-ray photoelectron spectroscopy method to characterize solid acidity of clay minerals

Accurate determination of solid acidity is essential for evaluating the catalytic activity of clay minerals. However, the conventional Fourier-transform infrared (FTIR) method using NH3 as the probe encounters challenges in measuring the solid acidity owing to the presence of water on the surface or within interlayer spaces of clay minerals as well as the influence of environmental water during the detection process. To mitigate water interference, a NH3-probe X-ray photoelectron spectroscopy (XPS) approach was adopted as an alternative to the FTIR method for the identification and semi-quantification of solid acidity of clay minerals. Following NH3 adsorption, the types of solid acid sites in clay minerals were identified through the chemical shift in N1s XPS spectra. The ratio of Brønsted and Lewis acid sites in a given sample and the relative concentration of acid sites across different samples were determined by examining the specific N1s peak area ratio. According to the XPS data, the ratios of Brønsted and Lewis acid sites in Ca-montmorillonite (SAz-2) and Na-montmorillonite (SWy-2) were nearly 1:1, whereas that for the illite–smectite mixed-layer mineral (ISCz-1) was 4. The acid site contents in SAz-2 were more than twice those of SWy-2. This work highlights the promising application prospects of the NH3-probe XPS method in solid acidity investigations of clay minerals.

High efficiency Al modified mesoporous SBA-15 catalyst for biomass-derived γ-valerolactone decarboxylation to butene

A series of x wt%Al-SBA-15 catalysts prepared by modified wetness impregnation was investigated for γ-valerolactone (GVL) decarboxylation to butene. It was found that the impregnation of Al species not only remained the ordered mesoporous structure of parent SBA-15, but also created large amounts of Brønsted and Lewis acid sites on the surface of catalyst. The considerable amount of solid acid sites and the suitable Brønsted/Lewis acid ratio endowed 2.23 wt%Al-SBA-15 catalyst an excellent catalytic performance. Under the optimized conditions, the GVL conversion and butene yield can reach 99.8% and 96.6% at 320 °C, respectively. Mechanism studies indicated that the decarboxylation of γ-protonated PEA controlled speed of the whole reaction. Moreover, the catalytic activity of regenerated 2.23 wt%Al-SBA-15 catalyst was satisfactory after eight cycle tests, and the butene yield could still reach 92.2%. GVL conversion of 99.8% with excellent butene yield of 96.6% was obtained over the 2.23 wt%Al-SBA-15 catalyst prepared by simple modified wetness impregnation method at the corresponding react conditions. • High active 2.23 wt%Al-SBA-15 for GVL to butene was prepared by simple method. • Suitable amount of acid site and B/L ratio endowed catalyst excellent performance. • Butene yield maintained 92.2% after 8 cycle tests over 2.23 wt%Al-SBA-15 at 320 °C. • Decarboxylation of γ-protonated PEA was found to be the speed control steps.

The Structure and Properties of Sepiolite with Partial Lattice Ions Substituted by Aluminum Ions.

Sepiolite was modified with Al3+ via hydrothermal reaction. The substitution amount of Al3+ for Mg2+ and Si4+ located at sepiolite lattice and the influence of substitution amount on the structure, specific surface area, and surface acidity of Al-modified sepiolite were investigated. On this basis, indigo–sepiolite composite pigments were prepared by Al-modified sepiolite and indigo via grinding method to evaluate the influence of Al-modified sepiolite on the structure, bonding strength, and weather resistance of composite pigment. The crystal structure of Al-modified sepiolite had no obvious change after modification. Al3+ mainly substituted Mg2+ located at the octahedron of the sepiolite lattice, and the substitution amount was positively related to the dosage of Al3+. The specific surface area of Al-modified sepiolite decreased and the distribution of channel size became wider after Al modification. In addition, the absolute value of zeta potential decreased as well as the solid acid sites increased with the increase of Al substitution in Al-modified sepiolite. For indigo–sepiolite composite pigments, the structure of Al-modified sepiolite had no obvious change as well. The adsorption amount of indigo in composite pigment after treating by DMSO and Al content as well as weak acid amount in Al-modified sepiolite presented linear correlation, indicating that Al modification could enhance the bonding strength between indigo and Al-modified sepiolite by increasing the amount of coordinated water with Al. For indigo, Al-modified sepiolite could brighten the color and reduce the weather resistance of the prepared composite pigment. The results of this study provide a new idea and basis for regulating the structure and properties of clay and for studying the preparation of composite pigment and clay functional materials.

Modulating the Interaction of NiSO4 and Nb2O5 Boosts the Dimerization of Propylene

Propylene dimerization is an attractive way to branch out the bulk chemicals to high-value-added fine chemicals. Based on the coordinatively unsaturated surface sites of nickel and solid acid sites for boosting highly selective dimerization of propylene, a series of NiSO₄/Nb₂O₅ catalysts were synthesized by tuning the synergistic effect through adjusting the calcination temperatures. Under the conditions of 70 °C, 2.5 MPa, and LHSV = 3 h–¹ in a fixed-bed reactor, the NiSO₄/Nb₂O₅-400 catalyst exhibits the highest propylene conversion of ca. 89% with 75% selectivity to C6 products (mainly including 4-methyl-2-pentene and 3-hexene). A possible Cossee–Arlman pathway has been proposed for the dimerization of propylene. In the long-term stability testing, an induction period has been observed, which may be attributed to the coordination of propylene molecules to unsaturated Ni²⁺ cations generating some active nickel species. Subsequently, the NiSO₄/Nb₂O₅-400 catalyst presents a stability up to 30 h. The interesting results inspired us to modulate the interaction of metal (salt) and acid sites to strengthen the selective dimerization of low-carbon olefins.

Synthesis of Rh nanoparticles encapsulated in ZSM-5 and its application for methanol to olefin over acid sites with simultaneous production of hydrogen over Rh

Metal-loaded zeolites possess metal and solid acid catalysis and therefore can be applied for complex reactions where different reactions proceed on different type of active sites. The optimization of each active sites, such as amount and strength of active sites, are indispensable to improve the yield of the desired product in complex reaction. On the other hand, in metal-loaded zeolite catalysts, the acid site amount is still difficult to be controlled independently because metal particles may be located on the acid sites, leading to the decrease in the acidity. To realize independent control of the each active sites of metal-loaded zeolite, we herein synthesized Rh nanoparticles encapsulated in ZSM-5 catalysts (Rh@ZSM-5) in a water-in-oil microemulsion solution by modifying the synthesis method for metal nanoparticles encapsulated in zeolite in our previous literature. In this method, the formation of metal nanoparticles encapsulated in amorphous silica was followed by the transformation of amorphous silica into zeolite and thus the amounts of solid acid and metal active sites are expected to be controlled independently. Rh@ZSM-5 was found to possess Rh nanoparticles inside of zeolite and the metal particle size and the amount of the solid acid sites were almost independent of the Rh loadings. Reaction of methanol over Rh@ZSM-5 was carried out to elucidate the bifunctional catalysis and its effect on the product selectivity. During the methanol conversion over Rh@ZSM-5, methanol to olefin reaction over solid acid sites and methanol decomposition over Rh metal proceeded in parallel. Rh@ZSM-5 exhibited the larger yield of propylene and butenes than ZSM-5. It could be because that hydrogen atoms produced in situ over Rh@ZSM-5 effectively inhibited the formation of aromatic intermediates to produce ethylene and aromatic compounds.

Direct Production of 2,5-Dimethylfuran with High Yield from Fructose over a Carbon-Based Solid Acid-Coated CuCo Bimetallic Catalyst.

A carbon-based solid acid, which functionalized with p-toluenesulfonic acid (TsOH), an encapsulated non-noble CuCo multifunctional heterogeneous catalyst was for the first time developed and used to catalyze the one-pot direct conversion of fructose into 2,5-dimethylfuran (2,5-DMF) without purification of 5-hydroxymethylfurfural (5-HMF) from the reaction solutions. Fructose was first transformed into intermediate 5-HMF over the outer shell carbon-based solid acid sites via dehydration, and subsequently 5-HMF was further converted to produce 2,5-DMF over the non-noble metal active sites in the core. As high as 71.1 mol % yield of 2,5-DMF was achieved in tetrahydrofuran at 220 °C and 3 MPa H2 for 10 h, which is higher than the yield reported for the direct conversion of fructose to 2,5-DMF. Besides, the carbon-based solid acid-coated CuCo catalyst could be reused up to five times.

Catalytic dehydration of ethanol-to-ethylene over Rho zeolite under mild reaction conditions

Abstract A small-pore zeolite, Rho having 8-membered ring opening and 3-dimensional (3-D) channels is synthesized via a greener process, without the use of a structure-directing agent. The zeolite Rho is crystallized by a conventional hydrothermal reaction under static conditions. XRD pattern showed phase purity of RHO-type crystalline topology, and SEM images depicted polyhedral particles with bimodal size distribution. The average size of small and large sized particles are 200 nm and 1000 nm, respectively. The amount of total solid-acid sites on Rho zeolite is 2.534 mmol g−1 and BET specific surface area reached to 812 m2 g−1. The zeolite is tested for a gas-phase conversion of ethanol-to-ethylene in the temperature range of 200–400 °C. The catalytic efficiency and stability of Rho zeolite are compared with the materials extensively used in the dehydration of alcohols; a control zeolite (ZSM-5) and three different samples of γ-Al2O3. For ethanol dehydration under mild reaction conditions, the overall catalytic efficiency of Rho is superior to that of the ZSM-5 zeolite and the γ-Al2O3 materials. The strength and concentration of solid-acid sites are the main driving factors in the low-temperature dehydration of ethanol-to-ethylene.

Promoting the Photo-Fenton catalytic activity with carbon dots: Broadening light absorption, higher applicable pH and better reuse performance

A new visible and near-infrared light-responsive heterogeneous Photo-Fenton catalyst, Fe3O4@Cu2O/carbon quantum dots (CQDs)/nitrogen-doped carbon quantum dots (N-CQDs) (FCCN) was prepared under mild conditions with the structure of double quantum dots growing on the surface of Fe3O4@Cu2O nanoparticles. The prepared FCCN could be used for the degradation of azo compounds, such as methyl orange, acid orange II and mordant yellow 10, even at neutral and alkaline pH (pH: 7–12) with a shortest complete degradation time of 15 min. During the Photo-Fenton process, CQDs and N-CQDs acting as up-converting photo-sensitizers improved the utilization of the light, in which the contribution of N-CQDs was greater than that of CQDs. Besides, CQDs and N-CQDs could transfer photo-generated charges between Cu2O and Fe3O4 for the rapid decomposition of H2O2 to OH. CQDs as solid light-sensitive acid sites could consume OH− and enable the efficient decomposition of H2O2 under alkaline conditions. The magnetism of Fe3O4 made this catalyst easily separated and the insoluble layers of CQDs and N-CQDs allowed it to be repeatedly used without activity change even after 10 cycles. The degradation constant of methyl orange in FCCN/H2O2/light system is 5.4 times higher than that of the Fe3O4@Cu2O/H2O2/light system, indicating that the loaded quantum dots greatly enhanced the photocatalytic activity. The degradation constant in FCCN/H2O2/light system was 2.5 times higher than that of the simple mixture (Fe3O4@Cu2O + CQDs + N-CQDs)/H2O2/light system, which indicated these excellent catalytic performances should come from the synergism effect of all components in FCCN.

Catalytic Ethylene Oligomerization over Ni/Al-HMS: A Key Step in Conversion of Bio-Ethanol to Higher Olefins

Al-modified hexagonal mesoporous silica (HMS) materials were synthesized using dodecylamine as a template according to the methods reported in the literature. FT-IR spectra proved that Al3+ ions entered in the HMS framework in Al-HMS (prepared by sol-gel reaction) but Al3+ ions existed in the extra-framework in Al/HMS (prepared by post-modification). NH3-TPD indicated that either Al-HMS or Al/HMS had solid acid sites on the surface, and the acidic strength of Al/HMS was stronger than that of Al-HMS. For ethylene oligomerization at 200 °C under 1 MPa, Ni/Al-HMS showed an ethylene conversion of 96.3%, which was much higher than that over Ni/Al/HMS (45.6%). The selectivity for C4H8, C6H12, C8H16, and C8+ was 37.7%, 24.5%, 24.0%, and 9.1% for ethylene oligomerization over Ni/Al-HMS, respectively. Ni/Al-MCM-41, which has been reported as an effective catalyst for ethylene oligomerization in the literature, showed a high ethylene conversion (95.2%) similar to that of Ni/Al-HMS in this study. However, the selectivity for C8H16 over Ni/Al-MCM-41 (16.3%) was lower than that over Ni/Al-HMS (24.0%) in the ethylene oligomerization. For ethanol dehydration at 300 °C under 1 MPa, a commercial H-ZSM-5 catalyst showed a high ethylene yield (91.2%) after reaction for 24 h using a feed containing 90 wt.% ethanol and 10 wt.% water. In this study, a two-step process containing two fixed-bed reactors and one cold trap was designed to achieve the direct synthesis of higher olefins from bio-ethanol. The cold trap was used to collect the water formed from ethanol dehydration. By using H-ZSM-5 as a catalyst for ethanol dehydration in the first reactor and using Ni/Al-HMS as a catalyst for ethylene oligomerization in the second reactor, higher olefins were continuously formed by feeding a mixture containing 90 wt.% ethanol and 10 wt.% water. The yields of higher olefins did not decrease after reaction for 8 h in the two-step reaction system.

Optimal Synthesis of Environment-Friendly Iron Red Pigment from Natural Nanostructured Clay Minerals.

A series of environment-friendly clay minerals—α-Fe2O3 iron-red hybrid pigments—were prepared by a simple one-step hydrothermal reaction process using natural nanostructured silicate clay minerals as starting materials. The influence of structure, morphology and composition of different clay minerals on the structure, color properties, and stability of the pigments was studied comparatively by systematic structure characterizations with X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmittance electron microscope (TEM), X-ray fluorescence spectroscopy (XRF), X-ray photoelectron spectroscopy (XPS) and CIE-L*a*b* Colorimetric analyses. The results showed that the clay minerals act as green precipitants during the hydrothermal reaction to induce in-situ transformation of Fe(III) ions into Fe2O3 crystals. Meanwhile, they also act as the “micro-reactor” for forming Fe2O3 crystals and the supporter for inhibiting the aggregation of Fe2O3 nanoparticles. The color properties of iron-red hybrid pigments are closely related to the surface charges, surface silanol groups, and solid acid sites of clay minerals. The clay minerals with higher surface activity are more suitable to prepare iron-red pigments with better performance. The iron-red hybrid pigment derived from illite (ILL) clay showed the best red color performance with the color values of L* = 31.8, a* = 35.2, b* = 27.1, C* = 44.4 and h° = 37.6, and exhibited excellent stability in different chemical environments such as acid, alkaline, and also in high-temperature conditions.

Low-temperature methanol dehydration to dimethyl ether over various small-pore zeolites

Eight-membered ring small-pore zeolites Rho and KFI have been synthesized, characterized and tested for dehydration of methanol to dimethyl ether at low-temperature, and compared with other zeolites and three different samples of γ-Al2O3. Both the zeolites were mainly crystallized from the synthesis gels with the Si/Al ratio of 5.0 by a conventional hydrothermal method without any agitation. The amount of total solid-acid sites was 1.65mmolg−1 and 2.53mmolg−1 for zeolite KFI and Rho, respectively. In addition, zeolites SSZ-13, RUB-13, and ZSM-5 were also employed for the reaction. Reaction conditions were optimized for a low-temperature catalytic dehydration of methanol selectively to dimethyl ether. Methanol dehydration efficiency of various zeolitic frameworks is discussed against the strength of solid-acidity, type of channel structure, specific surface area, and particle size. At temperatures≤200°C, the overall catalytic efficiency of the small-pore zeolites with appropriate medium-strong acidity and 3-D channels was superior to that of the reference γ-Al2O3 materials.

Understanding the Role of Designed Solid Acid Sites in the Low‐Temperature Production of ϵ‐Caprolactam

AbstractModern society is placing increasing demands on commodity chemicals, driven by the ever‐growing global population and the desire for improved standards of living. As the polymer industry grows, a sustainable route to ϵ‐caprolactam, the precursor to the recyclable nylon‐6 polymer, is becoming increasingly important. To this end, we have designed and characterized a recyclable SAPO catalyst using a range of characterization techniques, to achieve near quantitative yields of ϵ‐caprolactam from cyclohexanone oxime. The catalytic process operates under significantly less energetically demanding conditions than other widely practiced industrial processes.

Synergistic effect on flame retardancy and thermal behavior of polycarbonate filled with α‐zirconium phosphate@gel‐silica

ABSTRACTA new kind of flame retardant (ZS) with layered α‐zirconium phosphate disks (α‐ZrP) as the core and inorganic flame retardant (gel‐silica, GS) to shield solid acid sites on the surface of α‐ZrP as the shell, was synthesized via a facile method. The incorporation effects of ZS with silicone resin on the thermal properties and flame retardance of PC composites were investigated. The presence of ZS could improve the thermal stability of PC matrix. With the addition of ZS contents increased to 3 wt %, the limiting oxygen index (LOI) of the composite was 32.3% and the vertical burning (UL‐94) test reached a V‐0 rating. However, with more contents of ZS, the LOI value decreased, and without the GS layer, the LOI value was decreased significantly as well. The synergism between the α‐ZrP core and gel‐silica shell, also with the silicone resin were found. Based on these results, the flame‐retardant mechanism was proposed. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44829.

From Biomass‐Derived Furans to Aromatics with Ethanol over Zeolite

AbstractWe report a novel catalytic conversion of biomass‐derived furans and alcohols to aromatics over zeolite catalysts. Aromatics are formed via Diels–Alder cycloaddition with ethylene, which is produced in situ from ethanol dehydration. The use of liquid ethanol instead of gaseous ethylene, as the source of dienophile in this one‐pot synthesis, makes the aromatics production much simpler and renewable, circumventing the use of ethylene at high pressure. More importantly, both our experiments and theoretical studies demonstrate that the use of ethanol instead of ethylene, results in significantly higher rates and higher selectivity to aromatics, due to lower activation barriers over the solid acid sites. Synchrotron‐diffraction experiments and proton‐affinity calculations clearly suggest that a preferred protonation of ethanol over the furan is a key step facilitating the Diels–Alder and dehydration reactions in the acid sites of the zeolite.

From Biomass-Derived Furans to Aromatics with Ethanol over Zeolite.

We report a novel catalytic conversion of biomass-derived furans and alcohols to aromatics over zeolite catalysts. Aromatics are formed via Diels-Alder cycloaddition with ethylene, which is produced in situ from ethanol dehydration. The use of liquid ethanol instead of gaseous ethylene, as the source of dienophile in this one-pot synthesis, makes the aromatics production much simpler and renewable, circumventing the use of ethylene at high pressure. More importantly, both our experiments and theoretical studies demonstrate that the use of ethanol instead of ethylene, results in significantly higher rates and higher selectivity to aromatics, due to lower activation barriers over the solid acid sites. Synchrotron-diffraction experiments and proton-affinity calculations clearly suggest that a preferred protonation of ethanol over the furan is a key step facilitating the Diels-Alder and dehydration reactions in the acid sites of the zeolite.

Expanding Beyond the Micropore: Active-Site Engineering in Hierarchical Architectures for Beckmann Rearrangement

The ability to engineer discrete solid-acid centers within hierarchically porous architectures that contain micropores with interconnected mesopores offers the potential to overcome the spatial restraints and diffusional limitations imposed by conventional microporous zeotype catalysts, which often lead to decreased catalyst lifetimes and restricted substrate scope. By employing a one-step soft-templating approach, coupled with detailed physicochemical and spectroscopic characterization, isolated, solid-acid sites can be suitably tailored and discretely modulated within the micropores and mesopores. The design strategy facilitates a synergistic enhancement in catalytic activity, selectivity, substrate versatility, and longevity, compared to analogous microporous zeotypes that have been extensively employed in the chemical industry, as solid-acid catalysts, for the production of e-caprolactam (precursor for Nylon-6).

Possible mechanism of structural incorporation of Al into diatomite during the deposition process I. Via a condensation reaction of hydroxyl groups

The structural incorporation of aluminium (Al) into diatomite is investigated by preparing several Al–diatomite composites by loading an Al precursor, hydroxyl aluminum polymer (Al13), onto the surface of diatomite and heating at various temperatures. The results indicate that Al was incorporated and implanted into the structure of diatomite by the condensation reaction of the hydroxyl groups of Al13 and diatomite, and the Si–O–Al(OH) groups were formed during the condensation reaction. Al incorporation by the condensation reaction of hydroxyl groups of Al13 with single silanols of diatomite occurred more readily than that with geminal silanols. The Al incorporation increased solid acidity of diatomite after Al incorporation. The acidity improvement was various for different types of acid sites, depending on the preparation temperature of the Al-incorporated diatomite. Both Brønsted and Lewis acid sites increased greatly after heating at 250 and 350°C, but only L acid sites significantly improved after heating at 500°C. These results demonstrate that the structural incorporation of Al3+ ions into diatomite can occur by the condensation reaction of the hydroxyl groups of the Al precursors and diatomite. Moreover, the rich solid acid sites of Al-incorporated diatomite show its promising application as a solid acid catalyst.

ChemInform Abstract: Predictive Design of Engineered Multifunctional Solid Catalysts

AbstractReview: 108 refs.