NH3-TPD Studies Research Articles

Phenol alkylation with isobutene — influence of heterogeneous Lewis and/or Brønsted acid sites

Acidic solid catalysts with different types of acidity were used to study the liquid-phase alkylation of phenol with isobutene. A phosphonium ionic liquid immobilized on silica type carrier exhibiting pure Lewis acidity, Amberlyst 15 with pure Brønsted acidity as well as WO 3/ZrO 2 with both types of acid sites were used for this study. The active sites are postulated based on pyridine-FT-IR and NH 3-TPD studies, BET analyses, MAS NMR and XRD measurements. The different properties of the chosen catalysts are mirrored in the product distribution of the reaction mixture. It was found that WO 3/ZrO 2 is a very active and selective catalyst for the production of 2,4-di- tert-butylphenol under mild reaction conditions.

Synthesis and characterization of divalent cobalt-substituted mesoporous aluminophosphate molecular sieves and their application as novel heterogeneous catalysts for the oxidation of cycloalkanes

Thermally stable divalent cobalt-substituted hexagonal mesoporous aluminophosphate, CoHMA molecular sieves were hydrothermally synthesized and systematically characterized by several analytical and spectroscopic techniques. XRD, TEM, ED, ICP-AES, TGA-DTA, N 2 sorption, DR-UV–vis, XANES, and NH 3-TPD studies showed isomorphous substitution of divalent cobalt in a tetrahedral framework of a mesoporous aluminophosphate matrix. Furthermore, the blue color of CoHMA, before and after thermal treatment, further confirmed the tetrahedral environment of divalent cobalt in the matrix. As a result, the CoHMA catalyst showed excellent activity for the oxidation of cycloalkanes, that is, cyclohexane, cyclooctane and cyclododecane, under mild reaction conditions. Unlike the many other cobalt-based heterogeneous catalysts reported so far, CoHMA does not show any dislodgement or segregation of cobalt with calcination or any other postsynthesis treatments. In this study, the performance of CoHMA was also compared with the cobalt-containing microporous aluminophosphate, that is, CoAPO-5 with AFI topology, as well as with both microporous and mesoporous analogues of cobalt silicate molecular sieves that have MFI or MCM-41 structures.

Mesoporous H-AlMCM-48: highly efficient solid acid catalyst

Mesoporous cubic Na-AlMCM-48 molecular sieve catalyst with a Si/Al (molar) ratio of 60 was synthesized hydrothermally and characterized by various analytical and spectroscopic techniques. 27 Al MAS-NMR results indicate that the presence of aluminum in tetrahedral framework (Brönsted acid site) in both as-synthesized and calcined samples. However, in the case of the latter, a small amount of aluminum was found to be in octahedral coordination. This observation is well supported by NH 3-TPD studies over protonated catalyst (H-AlMCM-48), where desorption profile at higher temperature shows features characteristic of Lewis acid sites. Further, these studies also indicate that the presence of high concentration of moderate-to-strong Brönsted acid sites at lower temperature, which are more suitable for the para-selective tertiary-butylation reaction of phenol. Hence, in the present investigation, the reaction was carried out H-AlMCM-48, which however showed much higher activity as compared to the hexagonal H-AlMCM-41. In addition, it was also found that the former does not get deactivated owing to three-dimensional pore system while the latter is susceptible to deactivation on account of one-dimensional pore system.

Evaluating the Catalytic Performances of SAPO-34 Catalysts for the Oxidative Dehydrogenation of Ethane

Acid silicoaluminophosphate SAPO-34 catalysts with a chabasite-related (CHA) structure were tested for the oxidative dehydrogenation of ethane in the temperature range 550-700 °C achieving very interesting catalytic performances (about 70% C2H4 selectivity at 45% ethane conversion) which were related to both Lewis and Bronsted acid sites, as found by a NH3-TPD study.

Hydrothermally Stable Mesoporous Aluminosilicates with Strongly Acidic Sites

Mesoporous aluminosilicates were synthesized through S+X-I+ route using preformed beta zeolitic nanoclusters as building units in acidic media and subsequent ammonia hydrothermal treatment. The ordered mesoporous aluminosilicates were characterized by XRD, N-2 adsorption and desorption, HRTEM, SEM, NMR and NH3-TPD methods. The results indicated that the sample had the same hexagonal mesostructures as normal MCM-41 synthesized in acidic media. The presence of strongly acidic sites was substantiated by NH3-TPD study, and the hexagonal mesoporous structure was maintained even after 120 h treatment in boiling water. The synthesized materials were superior to conventional MCM-41 with respect to their stronger acidity and higher hydrothermal stability. The short period of assembly in acidic media and the ambient temperature reduced the dealumination effect considerably. The retention of zeolite-like connectivity of tetrahedral AlO4 and SiO4 units upon assembling the nanoclusters into mesostructured materials and the thicker framework pore walls contributed to the especially good hydrothermal stability greatly, which also gave rise to the presence of strong acidity.

Characterizations of Iron-Containing MCM-41 and Its Catalytic Properties in Epoxidation of Styrene with Hydrogen Peroxide

Iron-containing mesoporous molecular sieves (Fe–MCM-41) synthesized by both direct hydrothermal (DHT) and template-ion exchange (TIE) methods have been characterized and used as catalysts for the liquid-phase epoxidation of styrene with diluted H2O2. The characterizations with XRD, diffuse reflectance UV–vis, ESR, and EXAFS suggest that iron cations are highly dispersed and tetrahedrally coordinated with oxygen in the DHT samples with iron content lower changes as meant? if not, please recast sentence for clarity than ca. 0.9–1.1 wt% (Si/Fe=105–86). This coordination environment is similar to that in a ferrisilicate zeolite with MFI structure and contributes to the increase in relative strong acidic sites, as indicated by NH3-TPD studies. On the other hand, the TIE samples mainly contain small iron oxide clusters. The conversion of styrene over the DHT catalyst increases significantly with increasing iron content up to 1.1 wt%. The selectivity to styrene oxide and the efficiency of H2O2 for the epoxidation can be improved by adding H2O2 slowly to keep the H2O2 concentration low during the reaction. As compared with the DHT catalyst, the TIE catalyst shows a much poorer performance for the epoxidation of styrene. It is suggested that the atomically isolated iron sites account for the epoxidation reaction with hydrogen peroxide. The iron cations incorporated inside the framework of MCM-41 do not leach during the reaction, whereas the small iron oxide clusters leach out into the liquid phase and do not contribute to the catalytic reaction.

Thermal stability of porous MoO x and its catalytic property for the conversion of 2-propanol

Thermal stability of H 2-reduced MoO 3 was investigated in connection with its catalytic property for the conversion of 2-propanol. MoO 3 reduced at 623 K for 12 h had a surface area of about 180 m 2 g −1 and pores with the diameter of 0.6–3.0 nm. The surface area and the porous structure of H 2-reduced MoO 3 were retained even after treatment at 773 K in a N 2 atmosphere, indicating its high thermal stability. The dehydration and the dehydrogenation activities of H 2-reduced MoO 3 in the conversion of 2-propanol were enhanced by thermal treatment in a N 2 atmosphere. The highest dehydration activity was obtained after treatment at 723 K. An increase in the amount of acid sites by thermal treatment was confirmed by NH 3-TPD study.

Use of V2O5/Ti-PILC Catalyst for the Reduction of NO by NH3.

Titania-pillared interlayer clays (Ti-PILCs) exhibited peculiar physicochemical characteristics as catalyst supports compared to that of titania for the reduction of NO by NH3. Korean natural bentonite (KNB) was employed as a basic component of PILC due to its high pillaring capacity. For the freeze-dried Ti-PILC, the development of needle-like crystallites revealing the formation of a “house-of-cards” structure by delamination of long-ranged layered structure of Ti-PILC has been observed. Freeze-drying creates a unique pore structure of Ti-PILC catalyst such as multi-modal pore size distribution simultaneously containing micropores and meso- or macropores in the pore network. In particular, V2O5/Ti-PILC catalyst exhibits high performance of NO removal by NH3 mainly due to the strong catalyst surface acidity and redox properties examined by NH3 TPD and TPR-TPO studies, respectively. Without the addition of the catalyst promoter, WO3 and MoO3 to the catalyst, V2O5/Ti-PILC catalyst shows competitive NO removal activity to a commercial one.

Effect of H 2 reduction on the catalytic properties of molybdenum oxides for the conversions of heptane and 2-propanol

Conversions of heptane and 2-propanol were carried out using MoO 3 reduced with H 2 at 623 K. The parent MoO 3 was inactive for the isomerization of heptane. H 2 reduction, however, enhanced the isomerization activity, which became almost constant after reduction for 24 h. The reduced MoO 3 exhibited a higher isomerization activity than 0.5 wt.% Pt/USY zeolite, while there was no appreciable difference in product distribution between these two catalysts. The catalytic activities of MoO 3 for dehydration and dehydrogenation of 2-propanol also increased for longer periods of H 2 reduction. NH 3-TPD studies showed that the acidity of MoO 3 was strongly affected by H 2 reduction. We suggest from these results that H 2 reduction of MoO 3 at 623 K generated the bifunctional property, which was responsible for a high isomerization activity.

Synthesis and characterization of tantalum silicalite molecular sieves with MFI structure

Tantalum silicalite-1 (TaS-1) with MFI structure was prepared using the hydrothermal synthesis method. X-ray diffraction, scanning and transmission electron microscopies, Fourier transform infrared spectroscopy, ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy, 29Si magic-angle spinning nuclear magnetic resonance spectroscopy, temperature-programmed desorption of ammonia (NH 3-TPD), physical adsorption of nitrogen and elemental analysis were then performed to evaluate its physico-chemical properties and provided evidence for the incorporation of Ta 5+ into the zeolite framework. The unit cell volume of tantalum silicalite-1 increased linearly with increasing tantalum content, suggesting isomorphous substitution of Si 4+ by Ta 5+ in the lattice framework. The characteristic framework infrared spectra shifted to lower frequencies as Ta 5+ was incorporated into the silicalite lattice and showed a characteristic absorption band at 965 cm −1, probably due to SiOTa linkages. In the UV–vis spectrum, the absorption assigned to the presence of Ta 5+ in the zeolite framework was observed at around 200 nm. In addition, NH 3-TPD studies revealed weak acidity in tantalum silicalite-1 due possibly to [Ta⋯(OH)Si] sites in the framework.

NH 3-TPD and XPS studies of Ru/Al 2O 3 catalyst and HDS activity

The effects of pretreatment of catalyst on its surface properties and the HDS activity of a 0.49% Ru/Al 2O 3 catalyst were studied in a single-pass, differential microreactor. The surface properties of the catalyst were measured by NH 3-TPD and XPS analysis. The Ru/Al 2O 3 catalyst was pretreated in three ways: reduced in H 2 (Ru-R catalyst), oxidized in air and subsequently reduced in H 2 (Ru-OR catalyst), or sulfided in H 2S/H 2 (Ru-S catalyst). Three types of peaks (low, middle, and high temperatures) were observed in the NH 3-TPD study. The predominant high-temperature peak was observed for both the Ru-OR and Ru-S catalyst, pretreated at 300°C. Mass spectrometry showed that the high-temperature peak in NH 3-TPD consisted of N 2 and H 2 formed from the decomposition of NH 3 on the ruthenium sites. NO adsorption of unsaturated Ru species was related to the low-temperature peak in the NH 3-TPD. The XPS analysis showed that the peaks at 279.9 eV, 280.6 eV, and 282.5 eV were ascribed to metallic ruthenium, RuO 2, and RuO 3, respectively. The low-, middle-, and high-temperature peaks were assigned to RuO 2, acid sites on alumina, and metallic Ru, respectively. Metallic ruthenium was effective in the HDS of thiophene and the decomposition of NH 3.

Surface Properties and Activities of Ru/Al2O3 Catalysts for Hydrodesulfurization of Thiophene.

The activities of Ru/Al2O3 catalysts for hydrodesulfurization (HDS) of thiophene were studied, using a differential microreactor at atmospheric pressure. The surface properties of the catalysts were determined by NH3-TPD, XPS analysis, and IR spectroscopy. The Ru/Al2O3 catalysts were pretreated by three different methods: oxidized in air and subsequently reduced in H2 (Ru-OR catalyst), directly reduced in H2 (Ru-R catalyst), and sulfided in 10% H2S/H2 (Ru-S catalyst). All of the catalysts, pretreated at a temperature of 300°C, were among the most active, while the activity of Ru-S at 400°C was the least. Mild sulfidation at 300°C in flowing 10% H2S/H2 promoted the activity of the Ru/Al2O3 catalysts for the HDS of thiophene, while severe sulfidation at 400°C decreased the activity. Two NH3 peaks at low and middle temperatures and one peak of N2 and H2 at high temperature were observed for the three kinds of the catalysts in the NH3-TPD study. The XPS analysis showed that the low-, middle-, and high-temperature peaks in NH3-TPD were assigned to RuO2, acid sites on alumina, and Ru0 metal, respectively. The mass spectrometry indicated that the high-temperature peak of NH3-TPD consisted of N2, formed from the decomposition of NH3 on the ruthenium sites of the catalysts. The FTIR study indicated that the low-temperature peak of NH3-TPD was related to Lewis acid sites of the Ru catalysts. The HDS activity of the Ru/Al2O3 catalysts, for the HDS of thiophene, depended on the metallic ruthenium on the surface of the catalysts, rather than surface acidity measured in NH3-TPD.

NH3 TPD study and thermal behaviour of vanadium-exchanged titanium phosphates as catalysts

The ion exchange technique was employed for the preparation of VO2+ modified titanium phosphates as catalysts for the selective reduction of NO with NH3. The samples were prepared by contacting with a vanadyl sulphate solution different precursor materials, amorphous, crystalline or sodium half exchanged titanium phosphate. The vanadium contents of modified phosphates were in the range 0.08–2.3 wt%. XRD and thermal analysis TG/DTA showed that vanadium loading does not cause structural modification in hydrogen titanium phosphate. A vanadyl containing phase was obtained when half sodium titanium phosphate was employed. The NH3 TPD measurements indicated the presence of a wide distribution of NH3 adsorbing sites with medium-high strength. Catalytic activity measurements were performed under dilute conditions. It was found that the presence of vanadium even in low amounts strongly promote the catalytic activity.

Acid–base properties of modified aluminas

The acid–base properties of modified aluminas (SO2–4, PO3–4, Cl–, F–, Mg2+ and Na+, 0.2–2.0 wt.%) have been investigated by FTIR spectroscopy of probe molecules (pyridine and CDCl3), CO2 TPD and the dehydration of 4-methyl-pentan-2-ol. The results of these methods and those of previous NH3 TPD studies were compared in order to establish a classification of our aluminas. There is very good agreement between the results of the five methods. The various modifying ions have effects as summarised in the following acidity–basicity scale: Na+⩽ Mg2+ < pure γ-Al2O3 < PO3–4⩽ Cl– < SO2–4 < F–, increasing acidity →, â†� increasing basicity Fluorine is distinguished from the other ions as it is able to create Bronsted-acidic sites. CO2 TPD is an adequate method for basicity measurement. NH3 and CO2 TPD give complementary results. Furthermore, FTIR spectroscopy of adsorbed CDCl3 gives valuable information on the effects of ions on the basic strengths.