Ammonia Adsorption Calorimetry Research Articles

Solid acid catalysts based on H 3PW 12O 40 heteropoly acid: Acid and catalytic properties at a gas–solid interface

Solid acid catalysts prepared by supporting 15 wt%H 3PW 12O 40 heteropoly acid (HPA) on TiO 2, ZrO 2 and Nb 2O 5 with a sub-monolayer HPA coverage were characterised at a gas–solid interface, regarding their acid properties and chemical structure of HPA on the catalyst surface and compared to “standard” HPA catalysts such as bulk and silica-supported H 3PW 12O 40 and Cs 2.5H 0.5PW 12O 40. In contrast to the parent acid, H 3PW 12O 40, possessing strong Brønsted acid sites, the catalysts supported on TiO 2, ZrO 2 and Nb 2O 5 have both Brønsted and Lewis acid sites, with the latter mainly originating from the oxide support. The strength of acid sites in these catalysts is weaker than that in H 3PW 12O 40 and Cs 2.5H 0.5PW 12O 40. The catalytic activity (turnover frequency) in gas-phase isopropanol dehydration decreases in the order: H 3PW 12O 40 > Cs 2.5H 0.5PW 12O 40 > 15%H 3PW 12O 40/SiO 2 > 15%H 3PW 12O 40/TiO 2 > 15%H 3PW 12O 40/Nb 2O 5 > 15%H 3PW 12O 40/ZrO 2, which is in line with the acid strength as determined by NH 3 adsorption calorimetry. Ammonia adsorption calorimetry, 31P{ 1H} MAS NMR and FTIR indicate increasing interaction between support and HPA in the following order of supports: SiO 2 < TiO 2 < Nb 2O 5 < ZrO 2.

Influence of the metal oxide support on the surface and catalytic properties of sulfated vanadia catalysts for selective oxidation of methanol

The selective oxidation of methanol to dimethoxymethane (DMM) was performed over a series of binary vanadia-based oxides (V 2O 5 TiO 2, V 2O 5 ZrO 2, V 2O 5 Al 2O 3 and V 2O 5 CeO 2) and the corresponding sulfated catalysts. The physicochemical properties of catalysts were characterized by BET, Raman, XPS, TPR–MS, ammonia adsorption calorimetry and sulfate TPD–MS techniques. The strength of the sulfate-support interaction depends on the nature of the oxide support and increases in the following order CeO 2 > Al 2O 3 > ZrO 2 > TiO 2. The catalytic reactivity was correlated with the nature of V O-support bonds. Sample V 2O 5 TiO 2 exhibits the highest intrinsic activity of methanol oxidation. With the addition of sulfate, the selectivity to DMM was enhanced whereas the turnover frequency (TOF) value of vanadium sites decreased, with a rate depending on the strength of sulfate-support bonds. The best catalyst ( V 2 O 5 TiO 2 SO 4 2 - ) with higher DMM yield presented higher reducibility, proper acidity and moderate strength of sulfate species.

Influence of the host oxide of sulfated-titania catalysts on partial oxidation methanol reaction

A series of sulfated binary titania-based (M x O y –TiO 2–SO 4 2−, M x = Cr, Mn, Fe, Co or Mo) catalysts were prepared by co-precipitation and tested for partial oxidation of methanol. The structural properties were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The acidic and redox properties were examined using ammonia adsorption calorimetry and temperature-programmed reduction coupled with mass spectrometry (TPR–MS) techniques, respectively. The surface M x species were essentially 100% dispersed on catalysts, as confirmed by XRD and Raman spectroscopy characterization. XPS results indicated the oxidation state of surface M x species. In addition, Ti and S species were present in their fully oxidized states for all the samples. The results of ammonia adsorption calorimetry showed a surface reaction between chromium oxide and ammonia. The addition of M x O y host oxide not only enhanced the redox properties of sample TiS but also brought changes in the sulfate reduction process, as investigated by TPR–MS. The best catalytic performance was obtained for sample MoO 3–TiO 2–SO 4 2− with high activity. Furthermore, the distribution of reaction products was examined to provide information about the surface acidity and redox properties simultaneously.

Nature of surface sites of [formula omitted] catalysts and reactivity in selective oxidation of methanol to dimethoxymethane

The selective oxidation of methanol to dimethoxymethane (DMM) over sulfated vanadia–titania catalysts, prepared by co-precipitation and calcined at different temperatures, was studied in the 393–473 K interval under steady state conditions. The catalysts were characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller isotherms (BET), inductively coupled plasma optical emission spectroscopy (ICP–OES), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy. The redox and acidic properties were examined using temperature programmed reduction (TPR), isopropanol probe reaction, ammonia adsorption calorimetry, and pyridine adsorption FTIR techniques. As evidenced by pyridine adsorption FTIR, some Brönsted acid sites transformed to Lewis sites upon removal of sulfate species by washing the samples with deionized water. A high sulfur content increased the number of Brönsted acid sites but reduced their strength. The best catalyst revealed the presence of amorphous polymeric VO x species with terminal V O bonds, and both redox and Brönsted acid sites, resulting from an adequate balance between the calcination temperature and the sulfate concentration. These are the key parameters for optimizing the DMM production.

Optimising catalytic properties of supported sulfonic acid catalysts

Siliceous mesoporous molecular sieves (SBA-15) have been functionalised with propylsulfonic acid groups by both co-condensing 3-mercaptopropyltrimethoxysilane with the solid at the synthesis (sol–gel) stage and by grafting the same compound to pre-prepared SBA-15, followed, in both cases, by oxidation to sulfonic acid. The acidic and catalytic properties of the supported sulfonic acids prepared in the two ways have been compared, using ammonia adsorption calorimetry and the benzylation reaction between benzyl alcohol and toluene. Using a combination of X-ray photoelectron spectroscopy and other analytical techniques, the level of functionalisation and the extent of subsequent oxidation of tethered thiol to sulfonic acid, both in the bulk and close to the surface of SBA-15 particles, have been assessed. The research shows that the co-condensing route leads to higher levels of functionalisation than the grafting route. The extent of oxidation of added thiol to acid groups is similar using the two routes, about 70% near the surface and only 50% in the bulk. Comparison is made with polymer supported sulfonic acid catalysts, Amberlysts 15 and 35, and Nafion. Nafion shows the highest acid strength and the highest specific catalytic activity of all materials studied. Amongst the other materials, average acid strengths are broadly similar but there appears to be a relationship between the concentration of acid sites on the catalysts and their specific activity in the benzylation reaction. A model is proposed to explain this, in which clustering of sulfonic acid groups, even to a small extent, leads to disproportionately enhanced catalytic activity.

Comparative study of transformation of linear alkanes over modified mordenites and sulphated zirconia catalysts: Influence of the zeolite acidity on the performance of n-butane isomerization

Zeolite mordenite was dealuminated by steam treatment followed by acid leaching and further impregnated with 1 wt.% of iron or sulphated. The various solids were characterized by XRD, 29Si and 27Al NMR, IR of adsorbed pyridine as well as ammonia adsorption calorimetry and evaluated in n-butane isomerization. BET surface area of non-dealuminated mordenite was 462 m 2 g −1 and this value showed marked decreases if compared with the dealuminated (310 m 2 g −1), iron impregnated (302 m 2 g −1) and sulphated mordenite (247 m 2 g −1). It suggested the pore plugging or blocking by EFAL species, further confirmed by NMR experiments. The dealuminated mordenite showed Brønsted and Lewis acid sites with medium strength. The sulphated modified mordenite showed highest acidity strength, however its isomerization activity remains modest. The activity in the transformation of n-butane to isobutane was markedly improved by dealumination and subsequently iron impregnation (from 4 × 10 −9 to 2 × 10 −7 mol s −1 g −1); a significant initial activity was observed at a temperature as low as 200 °C. At higher temperatures, the conversion level did not change. The positive effect of iron impregnation could be related to the increase in acid strength and/or to the participation of the redox properties of iron to the initial activation step of the reaction. Nevertheless, sulphated zirconia, used as a reference catalyst, presented the highest performance (7 × 10 −7 mol s −1 g −1) in comparison with modified mordenites.

Acid strengths and catalytic activities of sulfonic acid on polymeric and silica supports

The acidic and catalytic properties of sulfonic acids supported on polystyrene, on silica (via propyl and phenyl tethers) and on a fluorinated hydrocarbon polymer (Nafion) are compared. Surface acidities are characterised using ammonia adsorption calorimetry under flow conditions in which pulses of ammonia are introduced to the sample from a flowing carrier stream. The extent of adsorption and molar enthalpies of ammonia adsorption (Δ H ads°) are interpreted in terms of the abundance, accessibility and strength of surface acid sites. Catalytic activities are measured for the isomerisation of α-pinene. The Nafion catalysts show the highest Δ H ads°(NH 3) and the highest catalytic activities. Although both silica-supported and polystyrene-supported sulfonic acids show lower specific activities and lower Δ H ads°(NH 3) values, the correlation between activity and Δ H ads°(NH 3) is relatively poor for these supported forms of the acid. It appears that while Δ H ads°(NH 3) is certainly sensitive to the strength of acid groups on which ammonia is adsorbed, it can only be used to compare acid strengths in a meaningful way for structurally similar catalysts.

Liquid-phase alkylation of phenol with t-butanol over various catalysts derived from MWW-type precursors

Three types of MWW-based catalysts (MCM-22, MCM-36 and ITQ-2) were studied in the liquid-phase alkylation of phenol by tert-butanol (TBA). Ammonia adsorption calorimetry was applied to investigate the acid properties of zeolites with different framework topology and crystallinity. It was established that acid properties of MCM-22 family depend mainly of the solid Al-content (the Si/Al ratio varied from 9.1 to 46.0). Delamination of MCM-22 precursor (MCM-22(P)) leading to ITQ-2 results in the decrease of the total acidity and the increase of the intermediate acid sites concentration, while the pillaring process yielding MCM-36 affected mainly the total concentration of acid sites, the acid strength distribution being similar to the corresponding MCM-22 zeolite. The catalytic activity and selectivity of zeolite-based catalysts are discussed. The most active catalyst for the studied reaction was MCM-22 B (Si/Al ~ 15). The results obtained with MCM-22 as catalyst revealed that, despite the expectations due to the presence of 10-MR pores in the structure of this zeolite, no enhanced selectivity to p- tert-butyl phenol ( p-TBP) is observed. This has been rationalized by the assumption that most of reactions occurring in this system take place on acid sites at or close to the external surface. This assumption is also supported by the similar catalytic selectivity of the MWW-derived structures (viz. MCM-36 and ITQ-2), which expose a higher concentration of external pockets. However, it is noticeable the high catalytic activity of MCM-22 zeolites for the liquid-phase alkylation of phenol by TBA that is comparable to the gas-phase activity of large pore zeolites like as HY or HM. To cite this article: E. Dumitriu et al., C. R. Chimie 8 (2005).

Etude De L'acidité De Zéolithes Y Modifiées Par Différents Cations. Application à la réaction de reformage sec du méthane

A series of modified zeolite catalysts (HYM) was prepared by ion-exchange of a commercial faujasite Y by various cations (M=Ni, Cr, Fe, Li, Ce). The exchange level was calculated from the chemical analysis results. The acidity of the samples was determined by ammonia adsorption calorimetry which allowed the simultaneous determination of the number and strength of the acid sites. The order of acidity obtained was the following: HYCr>HY>HYNi>HYLi>HYFe>HYCe.

Effect of NNN Site Si/Al Substitution on the Acid Strength: Mordenite

The acidity variations upon Si/Al substitution at NNN sites were calculated for mordenite by the ab initio molecular orbital method. The conclusions obtained from our previous work on faujasite hold with mordenite, e.g., the effects of Si/Al substitution at NNN sites on the acid strength are not uniform. Mordenite was synthesized with various Si/Al ratios and the acidic properties were examined by means of ammonia adsorption calorimetry. The analysis of the experimental results based on the binomial distribution of Al indicates that not only 0NNN sites but some of 1NNN, 2NNN, etc. should also be strong acid sites, which is consistent with the calculation.