Desorption Of Pyridine Research Articles

Synthesis, characterisation and catalytic evaluation of mesoporous AlPO supported phosphotungstic acid in the ethylation of ethylbenzene

Hexagonal mesoporous aluminophosphate (AlPO) was synthesised at room temperature using cetyltrimethylammonium bromide (CTAB) as the structure-directing agent. The mesoporosity of the synthesised material was confirmed from low angle XRD, TGA and nitrogen physisorption studies. Its textural properties were analysed with SEM and the framework vibrations by FT-IR. 27Al and 31P MAS NMR spectra revealed tetrahedral co-ordination of Al and P. The mesoporous AlPO was then impregnated with phosphotungstic acid, H 3PW 12O 40 (PW), of various loadings viz., 10, 20 and 40 wt% of PW. The PW loaded AlPOs were characterised by XRD, FT-IR, BET, DRIFT and 31P MAS NMR techniques. XRD patterns reveal the presence of Keggin structure in 20 and 40 wt% PW/AlPO. Temperature programmed desorption of pyridine by DRIFT studies indicate modified acidity values due to transfer of protons to the support AlPO. The catalytic activity of the materials was examined in the vapour phase ethylation of ethylbenzene with diethyl carbonate at temperatures between 200 and 275 °C. 20 wt% PW/AlPO was found to be more active than 10 and 40 wt% PW/AlPO catalysts. Among the alkylated products, the most significant product is hexaethylbenzene, obtained only with 20 wt% PW/AlPO. The selectivity of p-diethylbenzene is higher than for other alkylated products. The time on stream study over 20 wt% PW/AlPO catalyst indicates similar activity for about 4 h of stream. The effect of feed ratio and WHSV on conversion and product selectivity was also examined and the results are discussed.

Characterization of Transition-Metal Oxides Promoted with Oxoanions by Means of Test Reactions

The catalytic activity of some transition-metal oxides known to develop strong acidity by promotion with oxoanions was compared with a set of acid-catalyzed test reactions (isomerization of n-butane, isobutene, n-hexane, and cyclohexane). They were further characterized by thermally programmed desorption of pyridine and 2,4,6-trimethylpyridine, X-ray diffraction, and sortometry. SO42-−ZrO2, SO42-−TiO2, SO42-−Fe2O3, WO3−ZrO2, WO3−TiO2, WO3−Fe2O3, B2O3−ZrO2, B2O3−TiO2, and B2O3−Fe2O3 were synthesized by impregnation of the oxoanion over a fresh xerogel of the transition-metal hydroxide. Certain promoter-dependent orders, irrespective of the support, could be elucidated, mainly that the number of acid sites generated by promotion follows the order S, W > B. The proportion of strong acid sites is most important for S and B. The most active catalysts are always the zirconia ones, while the Fe2O3 catalysts are the least active.

Stability studies on undoped and doped Mg<Subscript>2</Subscript>TiO<Subscript>4</Subscript>, obtained by the polymeric precursor method

A series of AlMCM-41 molecular sieves were synthesized starting from a hydrogel keeping the following molar composition: 1CTMABr:4.58SiO 2 :(0.437+ X )Na 2 O: X Al 2 O 3 :200H 2 O. Tetramethylammonium silicate (TMAS) was used as silicon source and cethyltrimethylammonium bromide (CTMABr) was used as structure template. The obtained materials were characterized by nitrogen adsorption, XRD and TG/DTG. Acid properties were determined by pyridine thermodesorption. Model-free and Flynn-Wall kinetic models were used in order to correlate the total acidity with the apparent activation energy of pyridine desorption.

Highly active and selective AlSBA-15 catalysts for the vapor phase tert-butylation of phenol

Hexagonally ordered mesoporous AlSBA-15 catalysts having n Si/ n Al ratios from 7 to 215 have been synthesized hydrothermally using a cheap non-ionic block copolymer as the structure-directing agent. The obtained materials were analyzed by XRD and nitrogen adsorption to determine the structural order and the textural properties. It has been observed by 27Al MAS NMR spectroscopy that aluminum atoms are exclusively in tetrahedral coordination for all samples except AlSBA-15(7), where octahedral aluminum has been detected to some extent. Temperature-programmed desorption of pyridine showed that AlSBA-15(45) has a higher number of strong acid sites as compared to other mesoporous materials such as FeMCM-41, AlMCM-41 and FeAlMCM-41. The catalyst AlSBA-15(45) showed superior performance in the acid-catalyzed tertiary butylation of phenol employing tert-butanol as the alkylation agent. A high phenol conversion of 86.3% is observed for this catalyst at a reaction temperature of 150 °C. Over AlSBA-15(45), the 4-TBP yield amounts to 40.5% and the 2,4-DTBP yield corresponds to 37.9%.

Linear Metal Atom Chain on GaN(0001) by Chemical Vapor Deposition

The adsorption, reaction, growth, and the bonding configuration of a trinuclear linear metal atom chain complex, tetrakis(2,2‘-dipyridylamino)chromium(VI) chloride (DPCC, Cr3(μ3-dpa)4Cl2), on the GaN(0001) surface at 110 K were studied using synchrotron-induced X-ray photoelectron spectroscopy (XPS) and static secondary ion mass spectrometry (SIMS). XPS studies revealed three Cr chemical states of equal surface population present on the sample surface exposed to DPCC of low doses. The SIMS spectra showed main signals contributed from the sputter desorption of pyridine and the dipyridylamino ligand. Studies on small model molecules, such as pyridine and dipyridylamine (Hdpa), exposed to the sample surface showed that upon adsorption at 110 K, these small molecules were present on the sample surface in the molecular form only. They yielded the molecule-Ga adducts formed during SIMS detection. The absence of these adduct signals in the SIMS spectra taken from the DPCC-exposed surface indicated that DPCC chel...

Synthesis of highly acidic and well ordered MgAl-MCM-41 and its catalytic performance on the isopropylation of m-cresol

Highly ordered mesoporous MgAl-MCM-41 materials with different n Si/ n (Mg+Al) ratios have been successfully prepared using magnesium acetate, sodium aluminate and sodium silicate as the source of magnesium, aluminum and silicon, respectively. The samples are labeled MgAl-MCM-41( x) where x denotes the molar n Si/( n Mg+ n Al) ratio in the synthesis gel. The obtained materials have been characterized by powder X-ray diffraction, nitrogen adsorption, 27Al MAS NMR spectroscopy and AAS analysis. XRD and nitrogen adsorption measurements show that the hexagonal structure and textural parameters of the MgAl-MCM-41 materials are similar to MCM-41 materials. The wall thickness and pore diameter of the MgAl-MCM-41 samples can be controlled by varying the magnesium content in the synthesis gel. 27Al MAS NMR analysis clearly confirms that most of the aluminum in the calcined product occupies the tetrahedral coordination state. Temperature programmed desorption (TPD) of pyridine indicates that strong and medium acid sites of MgAl-MCM-41 are much larger as compared to AlMCM-41 and increase upon increasing the Mg content. The catalytic activity of MgAl-MCM-41 materials with different n Si/ n (Mg+Al) ratios has been investigated using isopropylation of m-cresol with isopropyl alcohol. MgAl-MCM-41(20) shows more m-cresol conversion and higher selectivity towards 2,4-diisopropyl-5-methyl phenol (2,4-DI-5MP) as compared to the AlMCM-41(23) catalyst under optimized reaction conditions. Moreover, the above catalyst registers a very high m-cresol conversion and high selectivity of 2,4-DI-5MP at a very low reaction temperature of 200 °C, due to strong acid sites generated by divalent Mg 2+ ions incorporation in the silica walls.

Synthesis of ZSM-5 zeolite in fluoride media: an innovative approach to tailor both crystal size and acidity

ZSM-5 zeolites were prepared at neutral pH by fluoride-mediated synthesis. The degree of crystallinity, the specific surface area and the crystal size are higher than those of the zeolites prepared under alkaline conditions. Moreover, by varying the F/Si ratio from 0.3 to 1.6, the H–[F]–ZSM-5 zeolites with crystal size ranging from 10 to 75 μm and the specific surface areas from 60 up to 350 m 2/g were obtained. Temperature programmed desorption of pyridine was applied to characterize the zeolite acidity. A decrease of the number of Brönsted acid sites with a higher F/Si ratio in the starting synthesis solution was observed. Based on the Hammett acidity function H 0, a model has been developed to correlate the Brönsted acidity of the resulting zeolitic material with the amount of fluoride anions introduced. Thus, ZSM-5 zeolites with tailored properties both at the molecular and microscopic levels are obtained.

Pyridine adsorption and reaction on Mo(1 1 0) and C/N–Mo(1 1 0): experiment and modeling

The adsorption of pyridine (C 5H 5N) on clean and carbon/nitrogen-modified Mo(1 1 0) surfaces has been investigated using temperature-programmed desorption (TPD) and density-functional theory (DFT) calculations. Pyridine adsorbs and decomposes on Mo(1 1 0) and C/N–Mo(1 1 0) at temperatures less than 530 K. Molecular pyridine desorption on clean Mo(1 1 0) occurs at temperatures of 375 ± 5 K ( E ads = 23.5 ± 0.3 kcal mol −1) and 530 ± 5 K ( E ads = 33.6 ± 0.3 kcal mol −1). At higher exposures, a multilayer physisorption peak was also observed at ≈260 K ( E ads = 10.4 ± 0.3 kcal mol −1). Pyridine decomposition on Mo(1 1 0) results in surface carbon and nitrogen accumulation, and gaseous dihydrogen. The energetics of various pyridine adsorption modes were calculated with DFT in order to identify the bonding modes resulting in molecular desorption and surface decomposition. The DFT calculations suggest that pyridine initially adsorbs as η 6-Py-0° coordination followed by η 1(N)-Py-90° adsorption on clean Mo(1 1 0), and surface heating is suggested to result in the transformation of a portion of η 6-Py-0° pyridine to surface α-pyridyl species (μ 2,η 2(N,C2)-Pyridyl, η 2(N,C2)-Pyridyl) that further decompose to surface carbon and nitrogen and desorbing dihydrogen on clean Mo(1 1 0).

Synthesis, characterization and catalytic evaluation of zirconia-pillared montmorillonite for linear alkylation of benzene

Zirconia-pillared montmorillonite was synthesized by an ultrasonication method and characterized by XRD, FT-IR of adsorbed pyridine, XPS, XRF and low temperature nitrogen adsorption techniques. The basal spacing of d(0 0 1) reflection of the pillared clay estimated by XRD was found to be 20 Å. XRD study showed well distribution of pillars without any non-pillared portion of the clay. FT-IR studies showed the formation of both Lewis and Brönsted acid sites due to pillaring. Pyridine desorption studies revealed that the Brönsted sites were stronger than Lewis acid sites. The activity of the Zr-PILC for the alkylation of benzene with long chain α-olefins [C 10–C 13] was investigated. The Brönsted acid sites played major role in this reaction.

Study of the Acidic Sites and Their Modifications in Mesoporous Silica Synthesized in Acidic Medium under Quiescent Conditions

The number and nature of silanol groups could be estimated in mesoporous silica prepared with cetyltrimethylammonium bromide (CTMBr) in acidic medium to obtain a material appropriate as a stationary chromatographic phase, using thermogravimetric analysis (TGA) and diffuse reflectance infrared spectroscopy (DRIFT). The temperature-programmed desorption of pyridine in the DRIFT cell was quantitatively correlated to the change in intensity of the two IR bands, 1595 and 1446 cm-1, assigned to pyridine interacting respectively with hydrogen-bonded and free silanol groups, after the determination of the absorptivities for the two bands. For calcined or solvent extracted samples, the silanol density (3−4 groups/nm2), is represented mainly by hydrogen-bonded silanol groups. A significant increase is obtained by rehydroxylation. Activation by heating above 200 °C promotes the condensation of the vicinal silanol groups.

Thermally induced chemistry and electron-mediated processes of pyridine on (2×1) and modified Si( [formula omitted]) surfaces: evidence of electron-induced condensation oligomerization

The room-temperature (RT) adsorption and surface reactions of pyridine on Si(1 0 0)2 × 1 have been investigated by thermal desorption spectrometry, low-energy electron diffraction, and Auger electron spectroscopy. In addition to molecular desorption, evidence of dissociation into smaller fragments, including C 2H 2 and atomic hydrogen, was also found upon thermal desorption of pyridine chemisorbed on Si(1 0 0)2 × 1 at RT. Other possible surface processes such as hydrogen abstraction, fragmentation and dissociative desorption have also been identified on Si(1 0 0)2 × 1 surfaces modified by sputtering, and pre- and post-exposure of molecular oxygen and of atomic hydrogen. Evidence of condensation oligomerization of pyridine at RT has been found on Si(1 0 0)2 × 1 upon irradiated by low-energy electrons and on an oxidized Si surface even without electron mediation.

Preparation of Ru metal nanoparticles in mesoporous materials: influence of sulfur on the hydrogenating activity

Hydrogenating catalysts were prepared by inserting Ru into the pores of mesoporous Al-MCM-41 materials by selective adsorption of [Ru(NH 3) 6] 3+. Ru/support catalysts were obtained after reduction with H 2. The activities of these catalysts in hydrogenation reactions were compared to those of Ru/HY and Ru/SiO 2. The catalytic properties in the absence of sulfur were tested in benzene hydrogenation, and the intrinsic activities of all the catalysts (either supported on mesoporous materials or on zeolites) were identical. It was concluded from this result that the dispersion of the Ru metallic phase was similar for all these catalysts. These samples were tested in the tetralin hydrogenation in pure H 2 and in the presence of H 2S (330 ppm of H 2S in H 2). They were found to be much less active than the zeolite-supported catalysts in the presence of H 2S. It is proposed that the lower activity of the catalysts supported on mesoporous materials is either due to their milder acidity, as evidenced by NH 3-TPD, cumene cracking and pyridine desorption experiments, or to the localization of the Ru nanoparticles on alumina islands.

Adsorption/desorption studies on nanocrystalline alumina surfaces

Two kinds of nanocrystalline alumina powders, boehmite (gamma-AlOOH, particle size d approximately 10 nm, BET surface area A(BET) = 180-200 m(2) g(-1)) and corundum (alpha-Al(2)O(3), d approximately 400 nm, A(BET) = 7 m(2) g(-1)) were used for comparative investigation by thermogravimetry (TG). The remarkable difference in the dehydration profiles between the two samples gives evidence for a distinct difference in their structures. In the following pyridine adsorption/desorption experiment, gamma-alumina was found to possess much more (20 times) and much stronger acidic sites than corundum. The activation energy of pyridine desorption was obtained from the respective minima in the first derivative of the TG-curves (DTG) at various heating rates (1-20 K min(-1)); the activation energy for pyridine desorption is smaller for gamma-alumina (61.5 kJ mol(-1)) than for corundum (78.8 kJ mol(-1)). Furthermore, the adsorption of water, carbon tetrachloride, and hexane on those alumina specimens provides evidence for the highly hydrophilic nature of their surfaces. The shift of T(max) to higher temperatures upon desorption of water was ascribed to the different adsorption coverage and the different energy required for removal of adsorbed water molecules.

Adsorption of Pyridine on Au(110) as Measured by Reflection Anisotropy Spectroscopy

The reflection anisotropy spectra of pyridine adsorbed onto a Au(110) surface in an electrochemical cell demonstrates that this system forms an ordered structure. The reflection anisotropy spectroscopy (RAS) of pyridine/Au(110) is attributed to transitions which are shifted from their position in gas-phase spectra by the effects of the interaction of the N lone pair orbitals with the Au(110) surface. The time dependence of the reflection anisotropy spectrum of the pyridine/Au(110) system shows that after the initial onset of pyridine adsorption it takes for the system to yield the spectrum of the equilibrium ordered structure. A similar time is required for the RAS signal to return to that of the Au(110) surface following the desorption of pyridine. © 2003 The Electrochemical Society. All rights reserved.

Isomorphism of bis(diethyldithiocarbamato)zinc(II) adduct with pyridine, [Zn(Py)(EDtc) 2]: hysteresis in the reaction of the adduct formation

The structure of the polycrystalline adduct bis(diethyldithiocarbamato)-pyridine zinc(II) depends on the pathway of physico-chemical conditions during the preparation procedure, as was revealed by solid state 15N CP/MAS spectroscopy in good correlation with known single crystal X-ray diffraction structures of this adduct. Two isomorphs of the adduct, namely α-[Zn(Py)(S 2CNEt 2) 2] and β-[Zn(Py)(S 2CNEt 2) 2], are the two molecules in the asymmetric unit of a single crystal (or polycrystalline) sample that can be obtained by recrystallization from toluene of the equimolar solution of the initial diethyldithiocarbamate zinc(II) complex and pyridine. The third isomorph, γ-[Zn(Py)(S 2CNEt 2) 2], can be obtained by recrystallization from pure pyridine of the diethyldithiocarbamate zinc(II) complex, or by its equimolar absorption of pyridine, or by desorption of pyridine from the clathrated adduct, [Zn(Py)(S 2CNEt 2) 2]·Py. Finally, the γ-[Zn(Py)(S 2CNEt 2) 2] isomorph recrystallizes from the melt into α/β-isomorphs of the adduct.

C 6-alkane conversion over γ-alumina supported palladium and platinum catalysts

A comparable study of n-hexane and 2,2-dimethylbutane reactions in excess hydrogen carried out on differently loaded Pd/Al 2O 3 and Pt/Al 2O 3 catalysts at ≤290 °C furnished information which is useful in assessing the role of activated alumina in catalyzing C 6-alkane conversion. In particular, it was confirmed that in effect of high temperature reduction (at 600 °C) Al 2O 3 gains a considerable acidity which must play an important role in transformation of C 6-alkanes. This effect is more clearly seen for palladium than for platinum catalysts, the role of palladium is even overshadowed by the action of activated alumina. Platinum exhibits higher intrinsic activity than palladium, so the respective changes associated with the presence of activated alumina are not so pronounced for Pt/Al 2O 3. Highly reduced catalysts showed completely different performance than lowly reduced samples. In the case of palladium, the high temperature reduction leads to a big enhancement of the catalytic activity and also vastly increases the isomerization selectivity. These variations accompanied by a big decrease of activation energy (from ∼50 to 20 kcal/mol) manifest an important modification of the reaction mechanism which changes from “metallic” to “acidic” one. The 2-methylpentane/3-methylpentane product ratio in n-hexane conversion represents another useful diagnostic parameter in assessing the role of activated alumina. “Regeneration” of highly reduced catalysts realized by oxidation of highly reduced samples with subsequent low temperature reduction (at 3000 °C) brings about a partial recovery of metal functioning. Temperature-programmed desorption of pyridine from variously pretreated catalysts appeared a promising method in diagnosing changes in surface acidity and metal state. A drastic decrease of chemisorption capability of palladium upon high temperature reduction of Pd/Al 2O 3, in combination with a positive correlation of catalytic activity with the amount of alumina in the sample, suggests that a dominant part of C 6-alkane isomerization takes place on acid sites of alumina, whereas the role of palladium is largely limited to supplying active hydrogen via spillover.

Implication of the acid–base properties of V/Ti-oxide catalyst in toluene partial oxidation

The work presents the effect of K-doping on V/Ti-oxides taking into account: the surface acid–base properties and the structure of surface vanadia species in respect to the catalyst performance and deactivation. The structure of active surface species determines redox properties, which are related to the catalytic performance by the Mars–van Krevelen mechanism. The reducibility of surface vanadia is studied by temperature-programmed reduction (TPR) in H 2. The molecular structure of surface vanadia is determined by FT-Raman spectroscopy in a controlled atmosphere. Surface acid–base properties are characterised via temperature-programmed desorption (TPD) of pyridine with mass spectrometric analysis of the products. Transient response techniques with continuous monitoring of the composition of gaseous phase are applied to follow the catalyst surface transformations. Evolution of benzaldehyde (BA) formed during interaction of toluene with the pre-oxidised catalyst (without gaseous oxygen) gives information about the nucleophilicity of surface oxygen. Addition of potassium to surface vanadia leads to an increased oxygen nucleophilicity, resulting in a higher selectivity towards BA formation. In general, increase in surface basicity decreases catalytic activity, but at the same time the catalyst deactivation due to coking is suppressed. This allows catalyst optimisation in view of a better control of the partial oxidation process.

Homogeneous OH groups in dealuminated HY zeolite studied by IR spectroscopy

The analysis of IR spectra of free OH groups in strongly dealuminated zeolite HY (Si/Al ≅ 100) restoring on desorption of ammonia or pyridine provides spectroscopic evidence for the homogeneity of SiO 1HAl groups. The same conclusion is obtained by analysis of the IR spectra of OH and OD groups interacting (by hydrogen-bonding) with nitrogen at 170 K. The homogeneity of OH groups in strongly dealuminated HY is explained by the presence of only (SiO) 3SiO 1HAl(OSi) 3 groups. By contrast, heterogeneous OH groups are found in non-dealuminated HY (Si/Al=2.56) containing three kinds of OH groups of various acid strength: (SiO)(AlO) 2SiO 1HAl(OSi) 3, (SiO) 2(AlO)SiO 1HAl(OSi) 3 and (SiO) 3SiO 1HAl(OSi) 3. Our dealuminated sample is the only known example of a zeolite with strongly acidic homogeneous hydroxyls. It may be a model catalyst for strongly acidic homogeneous OH groups or a model system for spectroscopic studies (IR, NMR etc.).

Thermal and Infrared Study of Desorption of Pyridine and Pyridine Derivatives from Ni-exchanged Montmorillonite

Thermal analysis (TG, DTG) and infrared absorption spectra were used to study desorption of pyridine (py), 4-methylpyridine (4-Mepy) and 4-ethylpyridine (4-Etpy) from Ni(II)-exchanged montmorillonite (Ni-MMT). It is shown that the bonds between the derivatives of pyridine (R-py) and the montmorillonite exhibit predominantly Lewis acid character. The thermal stability of studied samples and the total amounts of R-py inserted in the porous structure of Ni-MMT increase in the following sequence: py/Ni-MMT<4-Mepy/Ni-MMT<4-Etpy/Ni-MMT, i. e. the influence of the substituent on the pyridine ring is evident.

Evolution of the Texture and Structure of SiO2–Al2O3 Xerogels and Aerogels as a Function of the Si to Al Molar Ratio

SiO2–Al2O3 aerogels and xerogels with a Si to Al molar ratio rSi/Al varying from 0.25 to 20, were made by sol-gel process in acidic conditions at pH 2 and respectively dried by the CO2 supercritical method and by solvent evaporation. The Al precursor was also chelated with ethylacetoacetate, which made it possible to study the structure and texture of such gels in conditions favorable to the formation of mixed Al–O–Si bonds. Nitrogen adsorption isotherms according to the Brunauer, Emmett and Teller method (BET), 27Al magic angle spinning nuclear magnetic resonance (27Al MAS-NMR), Fourier Transformed Infrared Spectroscopy (FTIR) and Infrared absorbance spectra after Temperature Programmed Desorption (TPD) of pyridine, showed that the Si–O infrared asymmetric stretching vibration and the Bronsted acidity relative to the Lewis acidity, depended on the ratio of AlIV to Si atoms.