Loss Of Acid Sites Research Articles

Tuning the morphology of CoMn2O4 for efficient alkali-resistant catalytic reduction of NOx

To improve the NH3-SCR catalytic performance of CoMn2O4, we altered their geometric morphological and structural characteristics, developing the hollow spherical and hollow cubic morphologies of CoMn2O4 by employing hard-templating and self-templated strategy. The influence of catalyst morphology on NH3-SCR activities and alkali metal resistance was discussed specifically, and the physicochemical properties of fresh and used catalysts poisoned by alkali metals were investigated by XRD, FTIR, N2 physisorption, H2-TPR, NH3-TPD, XPS and in situ DRIFTS. Interestingly, the CoMn2O4 hollow spherical (CoMn2O4-S) outperformed hollow cubic (CoMn2O4-C) and the reference solid sphere with much better low-temperature activity, the widened temperature window and higher poisoning-resistant ability. Loss of acid sites, drop of active oxygen species and reducibility accounted for the poor low-temperature activity of alkali poisoned catalysts. CoMn2O4-S catalysts with abundant acid sites and hollow configuration can decelerate the process of alkali metal poisoning. With the aid of in situ DRIFTS investigation, it was confirmed that the SCR reaction over fresh or poisoned CoMn2O4 catalysts follows an Eley-Rideal (E-R) mechanism.

Spotlight on Cu/SAPO-34 with high hydrothermal stability induced by a small amount of SSZ-39

The stable structure is key to improving the performance of the selective catalytic reduction of nitrogen oxide (NOx) with ammonia (NH3-SCR) on Cu/zeolite catalysts after the high-temperature hydrothermal aging. In this work, the SSZ-39 seed with a stable texture property was introduced into the precursor of SAPO-34 to fabricate the zeolite with a hybrid microstructure of SAPO-34 + SSZ-39 (simplified as SA + SZ). The Si distributions of the original Al-rich SAPO-34 zeolite (simplified as SA) were optimized by the hybrid microstructure of SA + SZ. The results of solid nuclear magnetic resonance (NMR) and ammonia (NH3) adsorption indicate more Si(1OAl) structures with strong acidity on Cu/(SA + SZ) catalyst rather than on Cu/SA. The NH3-SCR performance within the reaction region of 150–550 °C is enhanced on the former. In addition, the Si(nOAl) (n = 2–4) structures of the original SAPO-34 zeolite are retained on Cu/(SA + SZ), which optimizes the low-temperature catalytic performance of Cu/SSZ-39. Furthermore, the SSZ-39 seed induces Cu/(SA + SZ) to form higher content of stable Si(0OAl) structures than Cu/SA. After hydrothermal aging, more isolated copper species can be formed on SA + SZ hybrid zeolite with advanced structural stability compared with Cu/SA, which better activates NOx on the former. And the loss of acid sites is also weakened on Cu/(SA + SZ) than on Cu/SA. These improvements on Cu/(SA + SZ) compensates for its hydrothermal deactivation to some degree. Hence, SA + SZ hybrid zeolite can be the new-generation candidate for NH3-SCR catalyst by introducing the second zeolite SSZ-39 in the synthesis process of SAPO-34. And the catalytic performance of Cu/(SA + SZ) is superior to Cu/SA.

Catalytic Isomerization of α-Pinene Epoxide Over a Natural Zeolite

Isomerization of α-pinene epoxide was carried out over a Colombian natural zeolite as potential geocatalyst with heulandite, chabazite and clipnotilolite as main crystallographic phases; the heulandite was identified as the active phase. Over synthetic zeolites, isomerization of α-pinene epoxide depended on Si/Al ratio, unit cell and the kind of structure. The best solvent was toluene and not isomerization activity was observed in presence of solvents with carboxyl groups. Complete α-pinene epoxide conversion and 45% selectivity to campholenic aldehyde were obtained at 70 °C, with fencholenic aldehyde, carveol, and p-cymene as main by-products. Decrease of activity of natural zeolite was associated with loss of acid sites. A reaction mechanism based on experimental and computational data was proposed including adsorption of α-pinene epoxide on Fe or Al sites of the natural zeolite; a reaction rate constant of 4.02 × 10–4 mol g−1 min−1 was estimated from a pseudo homogeneous kinetic model.

The alkali resistance of CuNbTi catalyst for selective reduction of NO by NH3: A comparative investigation with VWTi catalyst

The alkali metal poisoned CuNbTi and VWTi catalysts were prepared by wetness impregnation method. Poisoning effects of various K2O mass ratios (0.1%–2%) on CuNbTi and VWTi catalysts were studied, respectively. CuNbTi exhibited an excellent alkali metal resistance that 2%K2O-CuNbTi still remained 80% NOx conversion efficiency and 98% N2 selectivity while 2%K2O-VWTi was completely deactivated. Moreover, different alkali metals (K, Na, Ca, Mg) with the same molar ratio were doped on CuNbTi. The poisoned degree followed the order: K2O > Na2O > CaO > MgO. Then 2%K2O-CuNbTi and 2%K2O-VWTi were selected for follow-up study. To understand the poisoning mechanism, further investigations were performed by SEM, XRD, N2-physisorption, XPS, EPR, NH3-TPD, Py-IR, H2-TPR, in situ DRIFTS characterizations and DFT calculations. The particle of CuNbTi and VWTi agglomerated and the surface area decreased after 2%K2O loading. Loss of acid sites and drop of reducibility resulted in the deactivation of 2%K2O-VWTi. By contrast, experimental and computational results indicated that the alkali resistance of 2%K2O-CuNbTi was mainly due to the interaction between Ti2NbOx support and K atoms that K atoms were preferentially bound to NbOH and NbO with a lower bonding energy of −2.33 eV–−2.83 eV when Cu atoms were coordinated to TiO with a binding energy of −1.54 eV. This protected the active copper species from linking to K2O and the weak acid sites were preserved with the increasing isolated Cu2+. Ti2NbOx weakened the impact of potassium on NH3 adsorbing over the catalyst while the preserved copper species provided adsorption sites and redox ability for NH3-SCR reaction. Hence, the synergetic effect of copper and niobium species contributed to the alkali metal resistance. Ti2NbOx trapped the potassium and retained active copper species over 2%K2O-CuNbTi catalyst while potassium deactivated both TiO2 and active vanadium species on 2%K2O-VWTi catalyst. Meanwhile, both Eley-Rideal (E-R) and Langmuir-Hinshelwood (H-L) mechanisms with adsorbed NH3 coordinated to the Lewis acid sites and bidentate nitrate as the dominating intermediate species existed during the NH3-SCR reaction procedure over 2%K2O-CuNbTi at 225 °C.

Synthesis of mesoporous sulfated zirconia nanoparticles with high surface area and their applies for biodiesel production as effective catalysts

Sulfated zirconia (SZ) was employed as solid acid catalyst for biodiesel production via the transesterification of soybean oil with methanol. The effects of synthesis method, template/sulfonating agent and calcination temperature were investigated. Compared to conventional two-step route, one-step liquid-crystal template (LCT) method achieved the successful synthesis of mesoporous SZ nanoparticles (MNSZ) with larger surface area, favored the more acid sites, stronger acidity and smaller tetragonal ZrO2 (t-ZrO2). Sodium dodecyl sulfate (SDS), both as template/sulfonating agent, was superior to Hexadecylsulfonic acid sodium salt (CSS) in retarding the grain growth of t-ZrO2, formation of monoclinic ZrO2 phase and loss of sulfated groups. High calcination temperature resulted in the serious loss of acid sites, decrease of surface area and increase in crystal size of t-ZrO2, accelerated the transformation of active t-ZrO2 into monoclinic ZrO2, thus leaded to severe decline in catalytic activity. MNSZ, prepared via one-step LCT method at 550°C using SDS as template/sulfonating agent, was found to be the best catalyst. Over this catalyst of 4wt%, approximately 100% of soybean oil conversion and FAME yield of 94.9% could be achieved at 120°C in 4h, using the feedstock with methanol/soybean oil molar ratio of 12:1.

High calcium resistance of CeO2–WO3 SCR catalysts: Structure investigation and deactivation analysis

The SCR performances of CaO poisoning CeO2–WO3 and V2O5–WO3/TiO2 catalysts were prepared and compared for selective catalytic reduction of NO with NH3 (NH3–SCR). It was found that the CeO2–WO3 catalyst showed better calcium resistance than V2O5–WO3/TiO2 catalyst even when the CaO mass percentage reached up to 5wt%. In order to further understand the resistance mechanism, XRD, Raman, XPS, H2-TPR, NH3-TPD and in situ DRIFTS were used to analyze and explore the changes of CeO2–WO3 catalysts before and after the CaO deactivation. The results manifested that the introduction of Ca species not only affected CeO2 lattice but also led to CaWO4 formation on the surface. Additionally, although CaO increased the surface chemisorbed oxygen and Ce4+ species, the reducibility and Lewis acid sites, NH3 chemisorbed amount and Brønsted acid sites were greatly inhibited after deactivation. Furthermore, the enhanced NO2 adsorption capacity after the alkaline Ca species introduction could partially offset the negative effect in the loss of acid sites over the catalyst surface, which might be the primary reason for stronger CaO resistance of CW catalyst than traditional V2O5–WO3/TiO2 catalyst.

The estimation of thermal endurance for some heteropoly acidic catalysts from thermogravimetric decomposition data

The calculation of thermal endurance and the estimation of lifetime at certain temperatures based on thermogravimetric data for the tungstophosphoric acid, H3[PW12O40]·H2O (H3PW) and its Cs acidic salts, pure or doped with Pd, were done. The calculation of catalyst’s lifetime with high degree of confidence needs reliable values for activation energy. For this reason, the activation energy values were calculated with ASTM E 1641-04 “Standard Test Method for Decomposition Kinetics by Thermogravimetry” based on Flynn–Wall isoconversional method and with Friedman isoconversional method (differential model-free) for checking, because the last provides accurate values of activation energies compared with the integral model-free methods. The thermal decomposition of the H3PW and some of its acidic Cs salts, pure or doped with Pd, occurs through the constitutional water release [the water formed of the protons and the oxygen from the [PW12O40]3−] having as result the loss of acid sites (protons) and its catalytic activity diminution at the same time. The activation energies are reliable values for the conversion range of 5 ≤ α ≤ 20, but the value for α = 10 % is recommended for the calculation of the lifetime. The estimation of lifetimes was done with ASTM E 1877-00 “Standard Practice for Calculating Thermal Endurance of Materials from Thermogravimetric Data” method. The estimated thermal life of the H3PW and some of its acidic Cs salts pointed out a very good thermal endurance at temperatures below 573 K. The Pd doping decreases drastically the thermal endurance in all cases.

Catalytic behaviour of thermally aged Ce/Zr mixed oxides for the purification of chlorinated VOC-containing gas streams

In the present paper the thermal deactivation of a series of Ce/Zr mixed oxides (CeO2, Ce0.8Zr0.2O2, Ce0.68Zr0.32O2, Ce0.5Zr0.5O2, Ce0.15Zr0.85O2 and ZrO2) was investigated. In order to simulate long-term operation, samples were calcined at three different temperatures, namely 550, 750 and 1000°C in air for 4h. Structural, morphological and physico-chemical changes caused by high-temperature treatment were analysed by X-ray diffraction, BET measurements, NH3-temperature-programmed desorption and temperature-programmed reduction with hydrogen, and the behaviour in the oxidation of chlorinated volatile organic compounds (1,2-dichloroethane and trichloroethylene). The catalytic properties of Ce/Zr mixed oxides could be accounted for on the basis of their promoted redox, as characterised by the percentage of oxygen vacancies, and acidic properties due to the incorporation of zirconium in the ceria lattice. An increase in the calcination temperature led to a progressive decrease in the catalytic activity as a result of the modifications provoked by induced thermal aging (decrease in surface area, larger crystal sizes, reducibility at higher temperatures and loss of acid sites). Ce0.15Zr0.85O2 and Ce0.5Zr0.5O2 showed the best resistance to deactivation with combustion temperatures still notably lower in comparison with the homogeneous reaction even after calcination at 1000°C. Also slight changes in selectivity were evident resulting in favoured yields of hydrogen chloride, which was environmentally beneficial, and incomplete combustion products such as carbon monoxide and chlorinated intermediates.

1-Pentene Isomerization over Zeolites Studied by in situ IR Spectroscopy

The skeletal isomerization of 1-pentene at 1.2 bar total pressure and 30 mbar 1-pentene partial pressure was investigated by in situ IR measurements over FER and BEA zeolites. If the reaction was carried out isothermally a small loss of Bronsted acid sites due to coke formation, a constant conversion and a high selectivity to isopentenes was found over FER. A loss of acid sites and a decrease in conversion was found over BEA, but the selectivity to isomerization increased remarkably. The pore size of FER favored the monomolecular reaction of 1-pentene and suppressed bimolecular reactions. In the bigger pores of BEA initially dimerization–cracking and hydrogen transfer reactions leading to a coke layer took place then the skeletal isomerization of 1-pentene prevailed.

Fourier-transform infrared study of pyridine sorbed on zeolite HY

Adsorption of pyridine on zeolites can show three modes of sorption: physisorption, sorption on the LF hydroxyls and sorption on the HF hydroxyls. In this study a separate IR spectrum was obtained for each of the three sorbed pyridine species and major differences were observed, especially in the ring-bending and stretching region (2000 to 1200 cm–1). A comparison of the spectra showed that pyridine sorbed on the LF hydroxyls is not physisorbed, but protonated to some degree. H/D exchange enabled the IR spectrum of the bonded Bronsted proton to be obtained for pyridine sorbed on the HF hydroxyl sites which proved to be a broad band from ca. 3600 to ca. 2600 cm–1. For pyridine sorbed on the LF hydroxyls, a broader more complex band was observed from ca. 3700 to ca. 2200 cm–1. Reversible sorption/desorption occurred for physisorbed pyridine and pyridine sorbed on the LF hydroxyls, but complete desorption of pyridine from the HF hydroxyls resulted in loss of acid sites.

Specific modification of the external surface of ZSM-5 zeolites by 12-tungstosilicic acid

A medium-pore zeolite ZSM-5 has been modified by impregnation with an aqueous solution of 12-tungstosilicic acid. Adsorption measurements, IR spectroscopy and catalytic probe reactions (ethylbenzene conversion and m-xylene isomerization) show that the Keggin unit [SiW12O40]4– cannot penetrate the ZSM-5 pore system but merely becomes deposited on the external zeolite surface. Thermal decomposition of the acid yields WO3 at high loading accompanied by a loss of acid sites in the channel system. This is not exclusively caused by further dealumination but has partially to be attributed to site blocking by WO3. Obviously, the decomposition of the acid involves mobile species of smaller size which are able to migrate into the interior of the zeolite.