Acidity and catalytic activity of mesoporous ZSM-5 in comparison with zeolite ZSM-5, Al-MCM-41 and silica–alumina

Acidity of mesoporous HZSM-5 prepared using amphiphilic organosilane template molecules was measured. Brønsted acid sites were observed in the prepared sample, and the number and the strength of Brønsted acid sites were determined quantitatively by a method of infrared-mass spectroscopy/temperature-programmed desorption (IRMS-TPD) of ammonia. ΔH for ammonia adsorption as an index of the strength was ca. 150 kJ mol⁻¹ that was almost the same as on usual HZSM-5, but the number was smaller than that of HZSM-5. From the measured acidity, it was concluded that the mesoporous materials contained a smaller concentration of Brønsted acid site notable on the structure of HZSM-5. Measurements of turnover frequency (TOF) in the catalytic cracking of octane supported the conclusion. Density functional calculations showed that the defect sites Al–OH and Si–OH attached to the Brønsted acid site changed the strength of the acid sites to show some possible structures of the weak and strong Brønsted acid sites included in the mesoporous HZSM-5. Acidities of Al-MCM-41 and silica–alumina were also measured, and the difference in the solid acidities of these materials was discussed.     

Cracking of n-heptane over Brønsted acid sites and Lewis acid Ga sites in ZSM-5 zeolite

A series of gallium-containing ZSM-5 zeolites prepared by wet impregnation, ion-exchange and chemical vapor deposition (CVD) methods are compared in the cracking of n-heptane. Impregnation results in the dispersion of some of the gallium oxide clusters into the zeolite pore network as charge-compensating Ga species after calcination. Reduction of impregnated Ga/HZSM-5 catalysts leads to complete transformation of the oxidic Ga precursors to charge-compensating Ga⁺ and GaH²⁺ species. A small amount of divalent GaH²⁺ species can be stabilized; however, with increasing Ga/Al ratio monovalent cations dominate. While a model Ga/HZSM-5 catalyst prepared by CVD of Ga(CH₃)₃ containing mainly charge-compensating Ga cations displays high selectivity to dehydrogenated products (olefins, toluene and coke), catalysts with a lower Ga/Al ratio display improved activity with a product mixture resulting from contributions of Ga sites (dehydrogenation, aromatization, olefin cracking) and of Brønsted acid sites (protolytic cracking, olefin cracking). The synergy between Ga dehydrogenation sites and Brønsted acid sites is proposed to improve the dehydrogenation rate: the high acidity of the zeolitic proton facilitates hydrogen recombination and concomitant removal of product olefin from the Ga active sites. Ion-exchanged Ga/HZSM-5 catalyst which combines a difficult to reduce gallium oxide phase and high Brønsted acidity has the highest activity with relatively weak coke formation.     

Preferential occupation of molybdenum on the thin alumina film: Characterization by CO titration

A thin alumina film, the alumina model surfaces modified by the metallic molybdenum and by the MoO₃ are titrated by CO chemisorption. Two types of CO adsorption sites, namely octahedrally and tetrahedrally coordinated Al³⁺ sites, are present on the thin alumina film. The thin alumina film prepared under UHV conditions can be used to simulate the conventional high-surface-area alumina supports in real catalysis. The deposited metallic molybdenum preferentially occupies octahedrally coordinated Al³⁺ site and suppresses CO chemisorption on this site, and oxidation of the surface molybdenum species enhances this suppression.     

Effect of zeolite pore geometry on isomerization of n-butane

A comparative study of isomerization of n-butane over bifunctional catalysts based on zeolites with different pore geometry (ZSM-5 and mordenite) and with metal particles located inside and/or outside the zeolite main channel system has been carried out. 1,1,1,3,3,3-hexamethyldisilazane was used in order to obtain catalysts with external Brönsted acid sites poisoned. TPDA, FTIR, TPR and TEM techniques were used to characterize the catalysts. ZSM-5 catalysts were quite resistant to poisoning, mainly when metal sites were located inside the pore system. However, isomerization activity was virtually suppressed for the poisoned mordenite catalysts. It is proposed that the isomerization via C₈ formation cannot take place inside the one-dimensional pore system of mesopore-free mordenite catalysts, but this is not the case for isomerization on ZSM-5.     

Mechanistic study of organic template removal from ZSM-5 precursors

The decomposition mechanism of tetrapropylammonium (TPA⁺) template cations in several commercial ZSM-5 precursors was studied by temperature-programmed desorption/oxidation using a mass spectrometer (TPD/TPO-MS). Different oxygen concentrations, gas flow rates and temperature ramp rates were examined to understand the effects of these factors on the organic template decomposition. The TPA⁺ template decomposition pattern is strongly dependent on the SiO₂/Al₂O₃ molar ratio (SAR) and the oxygen concentration. The TPA⁺ template cations may be associated with three different sites depending on the SAR: Lewis acid sites (surface Al–O⁻), Brønsted acid sites (framework Al–O⁻), and weakly acidic Si---O⁻ sites. The thermal decomposition of TPA⁺ templates at different sites produced different products, including water, ammonia, propylene, ethylene, methane, and n- and iso-propylamines. However, no significant heavy volatile compounds were detected, suggesting that secondary reactions (e.g., oligomerization of olefins) were extremely limited and negligible in a fixed-bed micro-reactor used for TPD/TPO. The TPD-MS results suggest that the thermal decomposition mechanism on Lewis acid sites is possibly a free radical type of reaction, while on Brønsted acid sites and Si---O⁻ sites, the decomposition undergoes both Hoffman elimination reaction and a radical type of reaction. The presence of oxygen significantly lowered the TPA⁺ decomposition temperature, which is also a function of oxygen concentration. The knowledge about the template decomposition pattern and the composition of the evolved volatiles obtained by the TPD/TPO-MS technique may be used to develop ideal organic template removal procedures for zeolites.     

Study of the acid character of some palladium-modified pillared clay catalysts: Use of isopropanol decomposition as test reaction

Preparation, textural and structural characterizations as well as acid properties of some aluminium, zirconium pillared montmorillonite (from Algerian bentonite) and including alumina or zirconium pillared montmorillonite supported palladium are reported. Heat resistant basal spacings of 1.7 nm, surface areas in the range of 250–300 m²/g and micropore volumes of about 0.1 cm³/g were obtained. The acid activation of montmorillonite prior pillaring conduces to a resulting material with significantly higher pore volume and acidity. The improvement in acidity is mainly of the Brønsted acid type. The modification of zirconium-pillared montmorillonite with sulfate ions affects the structural properties of the pillared sample but gives a material with strong acid properties and both Lewis and Brønsted acid types are enhanced. It is reported also that textural and structural properties are not affected by the impregnation of a metallic function (1 wt.% Pd loading) but the acid properties changed. The pillared montmorillonite supported palladium has more Brønsted acidity than does the pillared montmorillonite. Decomposition of isopropanol was studied on these systems at low reaction temperature.     

Investigation of pyridine sorption in USY and Ce/USY zeolites by liquid phase microcalorimetry and thermogravimetry studies

USY (ultrastabilized Y) and Ce/USY (5 wt.% supported) zeolite acidities were characterized by microcalorimetric and adsorption studies of pyridine using liquid phase (Cal-Ad), thermogravimetry, and infrared analysis. The average adsorption enthalpies determined by microcalorimetry were −125.0 kJ mol⁻¹ for USY and −97.2 kJ mol⁻¹ for Ce/USY. A heterogeneous distribution of acid sites with heats of adsorption ranging from −134.0 (maximum heat value for USY) to −73.5 (minimum heat value for Ce/USY) kJ mol⁻¹ was found for both zeolites. A two-site model was best fitted by the Cal-Ad method for HUSY (n₁ = 0.1385 mmol g⁻¹ with ΔH₁ = −134.0 kJ mol⁻¹, and n₂ = 0.7365 mmol g⁻¹ with ΔH₂ = −101.5 kJ mol⁻¹) and Ce/HUSY (n₁ = 0.0615 mmol g⁻¹ with ΔH₁ = −117.6 kJ mol⁻¹, and n₂ = 0.7908 mmol g⁻¹ with ΔH₂ = −83.6 kJ mol⁻¹). DRIFTS measurements after pyridine adsorption showed that USY zeolite possesses only Brønsted acidity and that cerium impregnation leads to the appearance of Lewis sites. Based on these results, three families of acid strength were distinguished: (i) strong Brønsted sites (ΔH > −130 kJ mol⁻¹); (ii) Brønsted sites with intermediate strength (−100 < ΔH < −130); and (iii) weak Brønsted and Lewis sites (ΔH < −100). Thermogravimetric analysis showed that the strongest sites were able to retain pyridine up to 800 °C and that cerium incorporation leads to a more stable zeolite. A loss of strength was observed after impregnation. The total number of sites desorbed after gas adsorption (0.88 and 0.95 mmol for HUSY and Ce/HUSY, respectively) supports the Cal-Ad results (0.88 and 1.19 mmol for HUSY and Ce/HUSY, respectively) and indicates that not all Al sites are available to pyridine. The methodology used in this work for solid acid characterization (Cal-Ad) proved to be efficient in the evaluation of acid strength, total number and distribution of acid sites. XRPD, ICP-AES, ²⁷Al NMR, and FTIR were used for additional structural characterization.     

Inhibition effect of H2O on V2O5/AC catalyst for catalytic reduction of NO with NH3 at low temperature

The inhibition effect of H₂O on V₂O₅/AC catalyst for NO reduction with NH₃ is studied at temperatures up to 250 °C through TPD, elemental analyses, temperature-programmed surface reaction (TPSR) and FT-IR analyses. The results show that H₂O does not reduce NO and NH₃ adsorption on V₂O₅/AC catalyst surface, but promotes NH₃ adsorption due to increases in Brønsted acid sites. Many kinds of NH₃ forms present on the catalyst surface, but only NH₄⁺ on Brønsted acid sites and a small portion of NH₃ on Lewis acid sites are reactive with NO at 250 °C or below, and most of the NH₃ on Lewis acid sites does not react with NO, regardless the presence of H₂O in the feed gas. H₂O inhibits the SCR reaction between the NH₃ on the Lewis acid sites and NO, and the inhibition effect increases with increasing H₂O content. The inhibition effect is reversible and H₂O does not poison the V₂O₅/AC catalyst.     

氨水沉淀-胶溶-EISA法合成介孔氧化铝

以廉价且无毒的无机铝盐为铝源,十六烷基三甲基溴化铵(CTAB)为模板剂,结合氨水沉淀-胶溶-溶剂挥发自组装(EISA)方法,合成介孔氧化铝。考察胶溶过程中n(HNO3)∶n(Al~(3+))及n(CTAB)∶n(Al~(3+))对合成介孔氧化铝的孔径、孔容及比表面积等结构的影响。结果表明,加入HNO3胶溶和CTAB模板剂均可以提高合成介孔氧化铝的孔道有序度。当n(HNO_3)∶n(Al~(3+))=0~0.5时,氧化铝的有序度随着n(HNO_3)∶n(Al~(3+))的增加而增加。而有序度随着n(CTAB)∶n(Al~(3+))先增后降,当n(CTAB)∶n(Al~(3+))=0.1时有序度最好。n(CTAB)∶n(Al~(3+))过高,导致孔径分布范围变宽。氧化铝的孔径、孔容和比表面积主要受n(CTAB)∶n(Al~(3+))的影响。     

NO ions as IR probes for the location of OH groups and Na ions in main channels and side pockets of mordenite

NO⁺, generated from NO and O₂, is an infrared probe for the differentiation of Brønsted sites in the main channels and side pockets of mordenites, and for the location and accessibility of sodium ions in the structure. Na⁺ ions in cationic positions in the main channels are replaced by NO⁺, and sodium nitrate is formed. The band assignment was checked by isotopic experiments.     

The interaction of NO with Co/Co redox centres in CoAPOs catalysts: FTIR and UV–VIS investigations

The interaction of NO with Co²⁺/Co³⁺ redox sites in CoAPO-18 and CoAPO-5 catalysts was studied by means of FTIR and diffuse reflectance UV–Vis spectroscopy both at 298 and 85 K. Two families of Co²⁺ sites were found in the CoAPO-18 structure. (A) Ions in framework [Co²⁺(OH)P], associated with Brønsted acid sites which adsorb NO to produce dinitrosyls absorbing at 1903 and 1834 cm⁻¹; these dinitrosyl complexes are reactive, in that Co²⁺ is oxidized to Co³⁺ and N₂O is formed. (B) Structural defects Co²⁺ (Lewis acid sites) which stabilize dinitrosyls absorbing at 1900 and 1813 cm⁻¹. The NO adsorption both on reduced and, more significantly, on oxidised CoAPO-18 also leads to the formation of NO₂^δ+ adsorbed species. It was found that the two kinds of dinitrosyl complexes have different reactivity in presence of oxygen. Both families of sites are also present in CoAPO-5 catalysts on which, however, the redox reaction upon NO adsorption does not occur significantly.     

Isobutane alkylation with C4 olefins on a sulfonic solid acid catalyst system based on laminar clays

Natural clays were modified by means of the controlled attack of strong Brönsted acids in aqueous solution. The variations of the surface acidity of the solids, followed by IR of adsorbed pyridine and hammett indicators, indicated a population of Brönsted type sites above 800 μ mol/g, while the surface area increased from 23 up to 105 m²/g. A series of solids (ATZ-L, ATZ-A and ATZ-G) was activated using trifluoromethanesulfonic acid (CF₃SO₃H), then evaluated at the bench scale in a fixed bed reactor, both in liquid and gas phase, using an iC₄/C₄⁼ molar ratio, which led to 5.5% trimethylpentanes (TMP) against 35.9% of C₈⁼ and 53.4% of heavier C₉⁺ products for the catalysts (ATZ-L) having higher acid strength, i.e. Ho≈−9.3. For the solids having a moderate surface acid strength (Ho≈−4.4) the activation rate after 12 h runs was about 70%, but the solids having a higher acid strength, i.e. Ho=−5.6 and −9.3, showed a deactivation rate almost null after 24 h runs.     

The role of acidity in the decomposition of 1,2-dichlorobenzene over TiO2-based V2O5/WO3 catalysts

The influence of Lewis and Brønsted acid sites on the performance of V₂O₅/TiO₂ and V₂O₅–WO₃/TiO₂ catalysts in the total oxidation of o-dichlorobenzene was investigated. Catalytic activity of these materials resulted strongly affected by their acidic properties. The presence of Brønsted acid sites significantly increases the o-DCB conversion but also leads to the uncompleted degradation of chlorinated compounds, promoting the formation of partial oxidation products, as dichloromaleic anhydride. On the contrary, Lewis acid sites, acting as absorbing sites, promote the further oxidation of intermediates to CO and CO₂, without any by-products desorption.

Furthermore, the presence of water in the feed-stream was proven to decrease o-DCB conversion but also to play a positive role on process selectivity, increasing COx production. Plausible reasons for this effect are the reduction of Brønsted acid sites and the hydrolysis of anhydride during wet tests.     

Identification of cationic and oxidic caesium species in basic Cs-overloaded BEA zeolites

A parent acidic H-BEA with crystallites very small in size and high external surface area was used to prepare a series of materials loaded with increasing Cs⁺ contents by firstly ion-exchange and then impregnation with CsOH solutions. The monitoring of the ion-exchange process by chemical analysis and by IR spectroscopy in presence of CO or NH₃ reveals that a relevant amount of Brønsted acid sites in dehydrated H-BEA is related to framework Al sites that, in aqueous solution, turn into partially extraframework Al species unable to act any longer as sites of cationic exchange. This limits the exchange capacity in solution and higher levels of ion-exchange are attained by subsequent impregnation and calcination. A possible explanation for such a behaviour is proposed. The formation of carbonates by adsorption of CO₂, monitored by IR, confirms that the basic character induced on framework oxygen atoms by exchange of H⁺ with Cs⁺ is significantly weaker than that reached upon Cs-overloading. For the latter, the strong basicity is related to the presence of Cs₂O-like nanoparticles (also detected by EXAFS), dispersed within the zeolite pores (as shown by pore volume and TEM/EDX measurements). IR spectroscopy of adsorbed CO shows that Cs⁺ as countercations or as surface sites of occluded Cs₂O-like species exhibit a similar Lewis acid strength. Noticeably, in Cs-overloaded BEA, pairs of Cs⁺ sites (formed by two countercations and/or one countercation and a Cs⁺ at the surface of Cs₂O-like particles) are present, where CO can be adsorbed in a head–tail form, producing a distinct ν_CO band at 2145 cm⁻¹.     

The interaction of butenes with Cu ions in CuMCM-41 studied by IR spectroscopy

As CuMCM-41 was found to be active in skeletal isomerisation of n-butenes, we followed by IR spectroscopy the interaction of but-1-ene, cis-but-2-ene, and trans-but-2-ene with Cu⁺ sites in CuMCM-41. It has been revealed that Cu⁺ activated strongly CC bond, what resulted in a frequency shift of CC stretching band by about 100 cm⁻¹. Moreover, the vibration of CH bonds in CH₂ and CH groups neighbouring to the double bonds was also perturbed. We suppose that the activation of CC bond is a result of π-back donation of d electrons of Cu⁺ to π^* antibonding orbitals of butenes. IR results evidenced also the heterogeneity of Cu⁺ sites in CuMCM-41 and the presence of Cu⁺ sites of various electron donor properties. But-1-ene molecules bonded to Cu⁺ ions are less prone to accept protons from Brønsted acid sites than molecules bonded by hydrogen bonding to non-acidic SiOH groups, even though they are more negative than free or physisorbed molecules. The IR studies of coadsorption of CO and but-1-ene suggest that the interaction of CO with Cu⁺ is much weaker if but-1-ene is present, than without butene.     

The role of Brønsted acidity in the SCR of NO over Fe-MFI catalysts

The selective catalytic reduction (SCR) of NO with isobutane and with NH₃ was studied over Fe-MFI catalysts which differ strongly in Brønsted acidity but are similar in Fe content and structure of Fe sites, having shown similar activity in N₂O decomposition in related work. The catalysts were prepared by exchange of Na-ZSM-5 (Si/Al ca. 14) with Fe²⁺ ions formed in situ by acidic dissolution of Fe powder and by steam extraction of framework iron from Fe-silicalite or from H-[Fe]-ZSM-5 (Si/Al ca. 30). The characterization of acidic properties by ammonia TPD and by IR of adsorbed pyridine at different temperatures revealed marked differences in acidity between exchanged and steam-activated samples, the latter being (almost) void of strong Brønsted sites. The structural similarity of the iron sites was confirmed by UV–Vis and EPR spectroscopic results. The weakly acidic samples were inferior both in isobutane-SCR and in ammonia-SCR. With isobutane, dramatic differences over the whole range of parameters studied imply a vital role of Brønsted acidity in the reaction mechanism (e.g. in isobutane activation). In NH₃-SCR, large reaction rates were achieved with non-acidic catalysts as well, but a promoting effect of acidity was noted for catalysts that contain the iron in the most favorable site structure (oligomeric Fe oxo clusters). This suggests that an acid-catalyzed step (e.g. the decomposition of NH₄NO₂) may be rate-limiting at low temperatures.     

镁铝离子共存硅酸体系的聚合行为

通过对镁铝离子共存硅酸体系中硅酸胶凝时间、单硅酸聚合反应速率以及凝胶热性能的研究,得出：在pH<;4的微酸性条件下,硅酸体系的胶凝时间随镁铝离子共存浓度的增大而减小,且铝离子在对硅酸体系的促凝过程中起主要作用。镁铝离子共存硅酸体系的单硅酸反应速率常数比不含金属离子硅酸体系的有所增大,计算得出MA_0.1+0.1硅酸体系的单硅酸平均速率常数为：k₀=7.28×10^-4,k₁=5.62×10^-4。镁铝离子共存硅酸体系凝胶的晶化转变温度与晶型转变温度,相较于不含金属离子硅酸体系的均有所降低,根据热重曲线计算出MA_0.1+0.1硅酸体系水合二氧化硅的化学式为：SiO₂·0.556 H₂O。     

Catalytic activity of Pd loaded on WO3/Al2O3 for NO–CH4–O2 in the presence of water vapor

Pd loaded on various kinds of monolayer supports was applied for selective reduction of NO by methane in the presence of O₂ and water vapor. Pd/WO₃/Al₂O₃ exhibited the highest conversion of NO to N₂ among Pd loaded monolayer supports. The catalyst was relatively tolerant and reversible upon the exposure of water vapor. This is due to the enhanced amount of Brønsted acid sites under the moisture as evidenced by the IR measurement of adsorbed pyridine. The Brønsted acid sites generated on WO₃/Al₂O₃ support were required to give the dispersed Pd species, similar to on the zeolite.     

Solid acidity of metal oxide monolayer and its role in catalytic reactions

Such metal oxide as SO₄²⁻, MoO₃, WO₃, and V₂O₅ spread readily on supports like SnO₂, ZrO₂, and TiO₂ due to the different properties between acid and base oxides to generate the acid site on the monolayer. Number, strength, and structure of the acid site were characterized by temperature-programmed desorption (TPD) of ammonia principally, together with various physico-chemical techniques, and its role for catalytic reactions was studied. Approximately, one to two acid sites were stabilized on 1 nm² of the surface, which consisted of four to eight metal atoms. The limit in surface acid site density was estimated on the monolayer based on the concept of solid acidity on zeolites. Sequence of the metal oxide to show the strong acidity was, SO₄²⁻>WO₃>MoO₃>V₂O₅, and for the support oxide to accommodate the monolayer, SnO₂>ZrO₂>TiO₂>Al₂O₃. From these combinations, the metal oxide monolayer to show the adequate strength of acid site could be selected. Brønsted acidity was observed often, however, the Lewis acidity was prevailing on the reduced vanadium oxide. The structure of acid site, Brønsted or Lewis acid site, thus depended on the oxidation state. Relationship of the profile of solid acidity with various catalytic activities was explained. The solid acid site on the monolayer will possibly be applied to environment friendly technologies.     

A theoretical study on Brønsted acidity of WO3 clusters supported on metal oxide supports by “paired interacting orbitals” (PIO) analysis

Ammonia adsorption on Brønsted acid sites of WO₃ cluster supported on metal oxide supports: SnO₂, ZrO₂, and TiO₂, is analyzed by PIO analysis. We employed (HO)(WO₃)₄(H) and (HO)(WO₃)₉(H) on (SnO₂)₁₂, (ZrO₂)₁₂, and (TiO₂)₁₂, respectively, as supported Brønsted acid models and examined two types of Brønsted acid sites, an edge type and a face type. We estimated ammonia adsorption strength by total overlap population (∑OP) of all PIOs between the Brønsted acid site and NH₃. The order of acidity (∑OP) of each model is as follows: edge type: SnO₂, 0.0096 > ZrO₂, 0.0048 > TiO₂, −0.0001  face type: ZrO₂, −0.0759 > TiO₂, −0.0761 > SnO₂, −0.0867. The edge type adsorption is far stronger than the face type one. This order in the edge type coincides with the experimental results. The reason of these results is explained by the difference of the influence of oxygen atoms sitting near the N atom of NH₃.