Search

Gas-phase hydrogenation of a wide range of ketones to alkanes, including hydrogenation of aliphatic ketones and acetophenone, was investigated using bifunctional metal–acid catalysis. The catalysts were comprised of a metal (Pt, Ru, Ni, and Cu) supported on acidic caesium salt of tungstophosphoric heteropoly acid Cs2.5H0.5PW12O40 (CsPW). The reaction occurred via a sequence of steps involving hydrogenation of ketone to alcohol on metal sites followed by dehydration of alcohol to alkene on acid sites and finally hydrogenation of alkene to alkane on metal sites. Catalyst activity decreased in the order: Pt > Ru >> Ni > Cu. Pt/CsPW showed the highest catalytic activity, giving almost 100% alkane yield at 100 °C and 1 bar pressure. Evidence is provided that the reaction with Pt/CsPW at 100 °C is limited by ketone-to-alcohol hydrogenation, whereas at lower temperatures (≤60 °C) by alcohol dehydration yielding alcohol as the main product. The catalyst comprised of a physical mixture of Pt/C + CsPW was found to be highly efficient as well, which indicates that the reaction is not limited by migration of intermediates between metal and acid sites in the bifunctional catalyst.

The alkylation of benzene with propane to yield isopropylbenzene proceeds with high selectivity over bifunctional metal–acid catalysts comprising Pt and Keggin heteropoly acid in a fixed-bed reactor at 250–350 °C and 1 bar pressure. Most efficiently the reaction occurs over Pt/H4SiW12O40/SiO2 catalyst at 300 °C, giving isopropylbenzene with 90–93% selectivity at 6–8% benzene conversion, significantly exceeding the efficiency of previously reported Pt/HZSM-5 catalyst. The alkylation proceeds through bifunctional reaction pathway including dehydrogenation of propane to propene (1) on Pt sites followed by benzene alkylation with propene (2) on acid sites. At Pt loadings above 0.5%, step 1 is at fast quasi-equilibrium and step 2 is the rate-limiting one.

Bulk Zn(II)-Cr(III) mixed oxides with a Zn/Cr atomic ratio of 1:1–20:1 were found to be active catalysts for the gas-phase ketonisation of carboxylic acids (acetic and propionic) to form acetone and 3-pentanone, respectively, at 300–400 °C and ambient pressure. Zn-Cr (10:1) oxide showed the best performance, significantly exceeding that of the parent oxides ZnO and Cr2O3. The catalytic activity was further enhanced by supporting Zn-Cr (10:1) oxide on TiO2 and γ-Al2O3. With 20%Zn-Cr/Al2O3, ketonisation of propionic acid occurred with 97% selectivity to 3-pentanone at 99% conversion at 380 °C. Zn-Cr oxides were characterised by BET, XRD, DRIFTS of pyridine and acetic acid adsorption and microcalorimetry of ammonia adsorption. From DRIFTS, carboxylic acid adsorbed dissociatively on Zn-Cr oxide to form a surface metal carboxylate in bidentate bridging bonding mode. A mechanism for ketonisation of carboxylic acids via β-ketoacid intermediate route has been proposed.

Polyisobutylene (PIB) oligomer-bound amines render Keggin polyoxometalates (POM) heptane soluble, making them efficient and recyclable catalysts for environmentally benign biphasic oxidations with hydrogen peroxide. This is illustrated using both oxidative desulfurization and alkene epoxidation reactions that can be efficiently carried out in a heptane–water two-phase system occurring through facile phase transfer of POM by the amine terminated PIB oligomer.

Dehydration of ethanol was studied at a gas–solid interface over a wide range of solid Bronsted acid catalysts based on Keggin-type heteropoly acids (HPAs) in a continuous flow fixed-bed reactor in the temperature range of 90–220 °C focussing on the formation of diethyl ether (DEE). The catalysts included H3PW12O40 (HPW) and H4SiW12O40 (HSiW) supported on SiO2, TiO2, Nb2O5 and ZrO2 with sub-monolayer HPA coverage, as well as bulk acidic Cs salts of HPW (Cs2.5H0.5PW12O40 and Cs2.25H0.75PW12O40) and the corresponding core–shell materials with the same total composition (15%HPW/Cs3PW12O40 and 25%HPW/Cs3PW12O40, respectively) comprising HPW supported on the neutral salt Cs3PW12O40. The ethanol-to-DEE reaction was found to be zero order in ethanol in the range of 1.5–10 kPa ethanol partial pressure. The acid strength of catalysts was characterised by ammonia adsorption microcalorimetry. A fairly good correlation between the catalyst activity (turnover frequency) and the catalyst acid strength (initial enthalpy of ammonia adsorption) was established, which demonstrates that Bronsted acid sites play important role in ethanol-to-DEE dehydration over HPA catalysts. The acid strength and the catalytic activity of core–shell catalysts HPW/Cs3PW12O40 did not exceed those of the corresponding bulk Cs salts of HPW with the same total composition, which contradicts the literature claims of the superiority of the core–shell HPA catalysts.

The alkylation of benzene by propane to yield isopropylbenzene (iPrPh) was studied using bifunctional Pd-acid catalysts, such as Pd-heteropoly acid and Pd-zeolite, in a fixed bed reactor at 300 °C and 1 bar pressure. Keggin-type tungstosilicic acid H4SiW12O40 (HSiW) and zeolite HZSM-5 were used as the acid components in these catalysts. The reaction occurred most efficiently over 2%Pd/25%HSiW/SiO2, giving iPrPh with up to 88% selectivity. The Pd-HSiW catalyst was more selective than the Pd-HZSM-5; the latter gave only 11–18% iPrPh selectivity. The reaction proceeded via a bifunctional mechanism including the dehydrogenation of propane to form propene on Pd sites, followed by the alkylation of benzene with the propene on acid sites. The effect of Pd loading in Pd-HSiW and Pd-HZSM-5 catalysts indicated that the first step reached quasi-equilibrium at 1.5–2% Pd loading and the second step became rate limiting. The addition of gold to Pd-HSiW enhanced the activity of this catalyst, although the Au-HSiW without Pd was inert.

Hydrodeoxygenation (HDO) of 3-pentanone to yield n -pentane was carried out using bifunctional metal-acid catalysis at a gas-solid interface at 40–80 °C and 1 bar H 2 pressure in a fixed-bed microreactor. The bifunctional catalysts studied comprised Pt as the metal component and Cs 2.5 H 0.5 PW 12 O 40 (CsPW), an acidic Cs salt of Keggin-type heteropoly acid H 3 PW 12 O 40 , as the acid component. The bifunctional HDO pathway included hydrogenation of ketone to secondary alcohol on metal sites followed by dehydration of the alcohol to alkene on acid sites and finally alkene hydrogenation to alkane on metal sites. Addition of gold to the Pt/CsPW was found to increase both catalyst hydrogenation activity (turnover rate at Pt sites) and catalyst stability to deactivation, although the Au alone without Pt was almost totally inert. The enhancement of catalyst performance is suggested to be caused by PtAu alloying. Scanning transmission electron microscopy–energy dispersive X-ray spectroscopy (STEM-EDX) and X-ray diffraction (XRD) analysis of the PtAu/CsPW catalysts indicated the presence of bimetallic PtAu nanoparticles with a wide range of Pt/Au atomic ratios (0.5–7.7).

The cesium salt of tungstophosphoric heteropoly acid, Cs2.5H0.5PW12O40, is an active and environmentally friendly solid-acid catalyst for the liquid-phase alkoxylation of widespread monoterpenes, such as camphene, limonene, α-pinene, and β-pinene. These reactions provide isobornyl or α-terpenyl ethers, useful as fragrances, in good to excellent yields. The reactions are equilibrium-controlled and occur with high catalyst turnover numbers (TONs) up to 1500–4200 without catalyst leaching. Heteropoly acid H3PW12O40 also efficiently catalyzes the alkoxylation of these monoterpenes under homogeneous conditions with TONs up to 1500–11 300.

Metal oxides such as Nb2O5, Cr2O3, and especially a ZnIICrIII mixed oxide are demonstrated to be highly active and recyclable heterogeneous catalysts for Prins condensation, which provides a clean, high-yielding route for the synthesis of nopol through the condensation of biorenewable β-pinene with paraformaldehyde. ZnCr mixed oxide with an optimum Zn/Cr atomic ratio of 1:6 gave 100 % nopol selectivity at 97 % β-pinene conversion, with the catalyst easily recovered and recycled. The acid properties of Nb2O5 and ZnCr mixed oxide were characterized by the diffuse reflectance IR Fourier transform spectroscopy of adsorbed pyridine and ammonia adsorption microcalorimetry. An appropriate combination of acid–base properties of ZnCr mixed oxide is believed to be responsible for its efficiency.

We demonstrate, for the first time, the application of FTIR spectroscopic imaging approach in situ to monitor the precipitation process of oil asphaltenes, induced by n-heptane under flow using microfluidic devices. The aggregation of asphaltenes induced by n-heptane flow with subsequent precipitation and formation of the deposits was studied for solution containing 5 wt % of asphaltenes in benzene. The FTIR spectroscopic approach with an array detector allowed us to assess the spatial distribution of the chemical components and their amounts in the system and display the process of aggregates formation with a spatial resolution of few micrometres. This chemical imaging technique shows that the flow of heptane results in non-equilibrium condition when the local content of the flocculants reaches the threshold amount needed for the asphaltenes aggregation. The relatively slow process of heptane diffusion into the benzene induces the asphaltenes aggregation followed by their precipitation and compaction. Fast asphaltene aggregation process under heptane flow leads to the formation of the deposit firmly fixed to the surface, while the precipitates formed during the diffusion process of heptane into the benzene can be easily taken from the channel. The FTIR spectroscopic imaging of the deposits formed revealed the spatial inhomogeneity of the precipitated asphaltenes in the direction of the heptane flow in microfluidic channel which is reflected in their different chemical composition and the presence of different functional groups (CH2–CH3, C O, S O). In particular, it was found that asphaltenes of different types and different CH2/CH3 ratios gradually start to precipitate in the direction of n-heptane flow. The data obtained demonstrate the importance of local concentration of flocculants as well as local characteristics of the system for asphaltenes aggregation, and overall precipitation process in flow regime.

Terpenes are an abundant class of natural products, which is important for flavor and fragrance industry. Many acid catalyzed reactions used for upgrading terpenes still involve mineral acids as homogeneous catalysts and/or toxic solvents. Heteropoly acids represent a well-established eco-friendly alternative to conventional acid catalysts. As these reactions are usually performed in the liquid phase, solvents play a critical role for the process sustainability. In the present work, we developed a catalytic route to valuable fragrance ingredients, dihydrocarvone and carvenone, from limonene oxide by its isomerization using silica-supported tungstophosphoric acid as a heterogeneous catalyst and dialkylcarbonates as green solvents. The reaction pathway can be switched between dihydrocarvone and carvenone (obtained in 90% yield each) simply by changing the reaction temperature. In addition, we developed an efficient method for kinetic separation of trans-limonene oxide from commercial cis/trans-limonene oxide mixture and stereoselective synthesis of trans-dihydrocarvone.

SiO2-containing materials (quartz, Pyrex glass, silica gel, and H-Y zeolite) react slowly with methanol at 350 °C under both supercritical and gas-phase conditions. The amount of SiO2 reacted with supercritical methanol depends on the kind of the material and is varied from 0.05 wt% for quartz sand to 4.0 wt% for wide-porous silica gel for 5 h of the reaction. The main products of the reaction are methyl orthosilicates, mainly tetramethyl orthosilicate. N-Unsubstituted 1H-indole, 1H-benzimidazole, and 1H-indazole additives considerably increase the amount of reacted SiO2-containing material. Noteworthy, quartz sand “solubility” is increased ca 14-fold when indole is used at the same reaction conditions. These benzazoles provide a nucleophilic assistance to the reaction between SiO2 and methanol. During the assistance, the azole ring is methylated by methanol, and the participation of SiO2-surface or tetramethyl orthosilicate molecule facilitates the alkylation reaction of benzazole used.

The liquid-phase isomerization of limonene oxide was studied in the presence of heteropoly acid catalysts in aprotic solvents in homogeneous and heterogeneous systems. Among the catalysts were bulk and silica-supported tungstophosphoric acid H3PW12O40 and its acidic Cs salt Cs0.5H0.5PW12O40 (CsPW). The reaction gave dihydrocarvone, a valuable fragrance intermediate, as the main product with turnover numbers of up to 8000. The nature of the solvent had a strong effect on reaction rate and selectivity. CsPW (0.1 mol%) was found to be a highly efficient and truly heterogeneous catalyst for this reaction, providing 82% yield of dihydrocarvone in 1,4-dioxane as a solvent under ambient conditions. This simple catalytic method represents economically attractive route to industrially important compounds starting from bio-renewable substrates easily available from essential oils.

Reaction pathways of the gas-phase deoxygenation of propionic acid in the presence of heteropoly acid and bifunctional metal-loaded heteropoly acid catalysts were investigated in a fixed-bed reactor at 250–400 °C in flowing H 2 or N 2 . Silica-supported H 3 PW 12 O 40 (HPW) and bulk acidic salt Cs 2.5 H 0.5 PW 12 O 40 (CsPW), both in H 2 and in N 2 , exhibited ketonisation activity between 250 and 300 °C to yield 3-pentanone, CsPW being more selective than HPW. At 400 °C, HPW and CsPW were active for decarbonylation and decarboxylation of propionic acid to yield ethene and ethane, respectively. Loading Pd or Pt onto CsPW greatly enhanced decarbonylation in flowing H 2 but had little effect in N 2 . Similar performance exhibited Pd/SiO 2 and Pt/SiO 2 , giving almost 100% selectivity to ethene in H 2 . These results are consistent with hydrodeoxygenation of propionic acid on Pd and Pt, suggesting that hydrogenolysis of C C bond plays essential role. In contrast to the Pd and Pt catalysts, Cu catalysts, Cu/CsPW and Cu/SiO 2 , were both active in hydrogenation of C O bond to yield propanal and 1-propanol. Turnover rates of propionic acid conversion on metal catalysts followed the order Pd > Pt > Cu for both CsPW-supported and silica-supported metal catalysts.

It was demonstrated that iron molybdate catalysts for methanol oxidation can be prepared using Fe(II) as a precursor instead of Fe(III). This would allow for reduction of acidity of preparation solutions as well as elimination of Fe(III) oxide impurities which are detrimental for the process selectivity. The system containing Fe(II) and Mo(VI) species in aqueous solution was investigated using UV–Vis spectroscopy. It was demonstrated that three types of chemical reactions occur in the Fe(II)–Mo(VI) system: (i) formation of complexes between Fe(II) and molybdate(VI) ions, (ii) inner sphere oxidation of coordinated Fe(II) by Mo(VI) and (iii) decomposition of the Fe–Mo complexes to form scarcely soluble Fe(III) molybdate, Mo(VI) hydrous trioxide and molybdenum blue. Solid molybdoferrate(II) prepared by interaction of Fe(II) and Mo(VI) in solution was characterized by EDXA, TGA, DTA and XRD and a scheme of its thermal evolution proposed. The iron molybdate catalyst prepared from Fe(II) precursor was tested in methanol-to-formaldehyde oxidation in a continuous flow fixed-bed reactor to show similar activity and selectivity to the conventional catalyst prepared with the use of Fe(III).

Methyl isobutyl ketone (MIBK) can be viewed as a key intermediate for the conversion of biomass-derived acetone – the by-product of biobutanol production – to transportation fuel. Zeolite H-ZSM-5 doped with Pt nanoparticles was found to be a highly efficient catalyst for gas-phase hydrogenation of MIBK to methylpentanes with >99% yield at 200 °C. The reaction proceeds via bifunctional metal-acid catalysed pathway involving MIBK hydrogenation to 4-methyl-2-pentanol (MP-ol) on metal sites followed by MP-ol dehydration on acid sites to form olefin and finally olefin hydrogenation to 2-methylpentane (2MP) on metal sites, with all three steps occurring on a single catalyst bed. 2MP thus obtained underwent isomerisation over bifunctional Pt/H-ZSM-5 catalyst to give a mixture of 2- and 3-methylpentanes in a ratio of 83:17. The catalyst did not show any deactivation for at least 16 h on stream.

Methylation reaction of indole was studied in supercritical methanol (350 °C, 215 atm) catalyzed by TMOS and TEOS. Use of both orthosilicates shows the same result of indole methylation. The indole conversion is ∼30% for 5 h of the reaction with and without catalysts. But catalytic reaction is more high-selective on 3-methyl-1H-indole formation – 95–96 mol% against 60 mol% for non-catalytic methylation. Indole conversion in catalytic reaction increases by a quarter (with no change of selectivity) when two equivalents of water are added into reaction mixture. Methylating system was assumed to be the TMOS–MeOH, and methylation proceeds via cyclic, multi-centered transition state (TS). The TS involves molecules of indole, TMOS, methanol, and water (if used), which are orientated in a certain way to each other by hydrogen and coordinate bonding. The reaction does not occur in supercritical ethanol or isopropanol with and without using the catalysts.

Kinetics of isopropanol dehydration at a gas–solid interface over a range of bulk and supported Bronsted acid catalysts based on H 3 PW 12 O 40 (HPW) Keggin-type heteropoly acid was studied in a continuous flow fixed-bed reactor in the temperature range of 55–135 °C and isopropanol partial pressure of 0.94–5.52 kPa. The bulk catalysts included HPW itself and its Cs acid salts Cs n H 3− n PW 12 O 40 (Cs n H 3− n PW). The supported catalysts comprised of 15 wt.% HPW supported on SiO 2 , TiO 2 , ZrO 2 and Nb 2 O 5 . Under the conditions studied, the reaction was found to be of zero order in isopropanol. The turnover rate decreased in the order HPW > Cs 2.5 H 0.5 PW ≈ Cs 2 HPW > HPW/SiO 2 > HPW/TiO 2 > HPW/Nb 2 O 5 ≈ HPW/ZrO 2 , which is in line with the acid strength of these catalysts. The true activation energies E and pre-exponential factors A obtained from zero-order kinetics, were found to exhibit a compensation linear relationship ln A = mE + c . Moreover, the bulk and supported catalysts exhibited different compensation plots. This is suggested to be due to the different chemical structure of Bronsted acid sites in these catalysts, resulting in differing reaction mechanisms. The bulk catalysts possess strong surface H + sites located on peripheral (bridging) oxygen atoms in the Keggin unit, whereas supported HPW catalysts have weaker H + sites probably located on oxygen atoms of support. Consequently, on the bulk catalysts, isopropanol dehydration is suggested to occur via E1 elimination pathway, whereas on the supported catalysts it might also involve a contribution of E2 elimination pathway.

Aryl alcohols are employed as feedstock for the Heck reaction. Keggin-type heteropolyacids catalyse the selective dehydration of the alcohols to styrenes, which, in one-pot, undergo palladium-catalysed Heck arylation with aryl bromides, affording broadly functionalised stilbenes. The choice of solvent is critical for the cascade dehydration–Heck reaction, with electron-rich aryl alcohols preferring a basic medium while electron-deficient ones demanding solvents of lower basicity.

With increasing biodiesel production, availability of glycerol is expected to increase. New processes are needed for converting this surplus glycerol to value-added chemicals. In this work, we used niobia catalysts for the liquid-phase acetalisation of glycerol without using any solvent. High conversions were achieved (∼80%), though water was present in the reaction system. The calcination temperature changed the strength as well as the nature of acidity of the samples. Samples with higher Bronsted acid strength exhibited higher catalytic performance. The results show that niobia is a water tolerant, reusable catalyst for glycerol acetalisation.

New data on enthalpy and entropy contributions to the energy barrier of β-pinene thermal isomerization were obtained. The rate of β-pinene conversion is higher in supercritical EtOH (P = 120 atm) than in the gas phase (P ≤ 1 atm, without solvent, or for inert carrier gas N2) at equal temperatures. The highest activation energy EΣ of total β-pinene conversion is also observed in reactions in the supercritical (sc) condition. Activation parameters ΔHΣ#, ΔSΣ#, and ΔGΣ# depend strongly on the reaction pressure. Thus, at P ≤ 1 atm (gas-phase reaction) the values of ΔSΣ# are negative, while at sc conditions at P = 120 atm is positive. The linear dependences lnkΣ0 − EΣ and ΔSΣ# − ΔSΣ# indicate an isokinetic relation (IKR) and enthalpy-entropy compensation effect (EEC). The isokinetic temperature was calculated (Tiso = 605.5 ± 22.7 K). It was shown that elevation of temperature reduces the value of ΔGΣ#(T) upon sc thermolysis only, whereas in all gas-phase reactions ΔGΣ#(T) increases. At equal reaction temperatures, the greatest values of Keq#(T) proved to be typical for thermolysis in sc-EtOH. We hypothesize that the rate of total β-pinene conversion increases dramatically due to a considerable shift in equilibrium toward higher concentrations of activated complex yTS#. A detailed analysis of activation parameters shows that the IKR and EEC coincide, evidence of a common mechanism of β-pinene conversion observed under different reaction conditions, including thermolysis in sc-EtOH.

Revision of the experimental data on α-pinene thermal isomerization attained in supercritical ethanol allowed us to expand the reaction scheme, which includes now six main products and eleven reversible reactions. The equilibrium constants of every reaction (K T, j and K Φ, j) were calculated to allow for reversibility of reactions. The thermochemical data of the pure compounds required to calculate constants K T, j and K Φ, j (standard enthalpy and entropy of formation Δf H° (298.15 K), Δf S° (298.15 K), heat capacity C p (T), critical parameters T cr and p cr, boiling point T b, and the acentric factor ω) were preliminary estimated using the empirical Joback and Benson methods. A kinetic model based on the new expanded scheme of reversible reactions was successfully identified and its kinetic parameters k j (600 K) and E j were determined. Detailed examination of the new kinetic model allowed us to refine the generally accepted mechanism of α-pinene thermal isomerization and to distinguish additional features of the multistep process.

The isomerisation of α-pinene was studied in the gas phase over solid heteropoly acid (HPA) catalysts in a fixed-bed continuous flow reactor at 200 °C and ambient pressure. The catalysts included bulk and SiO 2 -supported H 3 PW 12 O 40 and bulk Cs 2.5 H 0.5 PW 12 O 40 possessing strong Bronsted acid sites, as well as composites prepared by supporting 15 wt% H 3 PW 12 O 40 on Nb 2 O 5 , ZrO 2 and TiO 2 possessing both Bronsted and Lewis acid sites of moderate strength. The reaction yielded camphene as the main product in a mixture with monoterpene by-products such as limonene, terpinolenes, terpinenes, β-pinene, p-cymene and others. The HPA catalysts with strong Bronsted acid sites exhibited high initial activities, but suffered from catalyst deactivation, resulting in low camphene yields. Conversely, the HPA catalysts supported on Nb 2 O 5 , ZrO 2 and TiO 2 , although weaker acids, showed more stable performance in α-pinene isomerisation. The HPA catalyst supported on TiO 2 gave a camphene yield of 51% and a total camphene and limonene yield of 58%.

Solid acid catalysts prepared by supporting 15 wt%H 3 PW 12 O 40 heteropoly acid (HPA) on TiO 2 , ZrO 2 and Nb 2 O 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 3 PW 12 O 40 and Cs 2.5 H 0.5 PW 12 O 40 . In contrast to the parent acid, H 3 PW 12 O 40 , possessing strong Bronsted acid sites, the catalysts supported on TiO 2 , ZrO 2 and Nb 2 O 5 have both Bronsted 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 3 PW 12 O 40 and Cs 2.5 H 0.5 PW 12 O 40 . The catalytic activity (turnover frequency) in gas-phase isopropanol dehydration decreases in the order: H 3 PW 12 O 40 > Cs 2.5 H 0.5 PW 12 O 40 > 15%H 3 PW 12 O 40 /SiO 2 > 15%H 3 PW 12 O 40 /TiO 2 > 15%H 3 PW 12 O 40 /Nb 2 O 5 > 15%H 3 PW 12 O 40 /ZrO 2 , which is in line with the acid strength as determined by NH 3 adsorption calorimetry. Ammonia adsorption calorimetry, 31 P{ 1 H} MAS NMR and FTIR indicate increasing interaction between support and HPA in the following order of supports: SiO 2 2 2 O 5 2 .

Silica-supported H3PW12O40 (PW), the strongest heteropoly acid in the Keggin series, is an efficient, environmentally friendly heterogeneous catalyst for the liquid-phase isomerization of styrene oxide into phenylacetaldehyde, an industrially important intermediate for fine chemical synthesis. The reaction occurs in cyclohexane as a solvent under mild conditions at 25–70 °C with low catalyst loadings and without PW leaching in solution. At 60 °C, the yield of phenylacetaldehyde reaches 92% at 97% styrene oxide conversion, with a catalyst turnover number of 19 600. The catalyst can be recovered and reused.

Caesium 12-tungstophosphate, Cs2.5H0.5PW12O40 (CsPW), possessing strong Bronsted acid sites is an active catalyst for the dehydration of glycerol to acrolein in the gas-phase process at 275 °C and 1 bar pressure. The initial glycerol conversion amounts to 100% at 98% acrolein selectivity, however, decreases significantly with the time on stream (∼40% after 6 h) due to catalyst coking, without impairing acrolein selectivity. Doping CsPW with platinum group metals (PGM) (0.3–0.5%) together with co-feeding hydrogen improve catalyst stability to deactivation, while maintaining high selectivity to acrolein. The enhancing effect of PGM was found to increase in the order: Ru ∼ Pt < Pd. The catalyst 0.5%Pd/CsPW gives 96% acrolein selectivity at 79% glycerol conversion, with a specific rate of acrolein production of 23 mmol h−1 gcat−1 at 275 °C and 5 h time on stream, exceeding that reported previously for supported heteropoly acids (5–11 mmol h−1 gcat−1 per total catalyst mass). Evidence is presented regarding the nature of acid sites required for the dehydration of glycerol to acrolein, supporting the importance of strong Bronsted sites for this reaction.

The most important information concerning thermal conversions of vegetable terpenes (α-pinene, β-pinene, turpentine, and cis-verbenol) in supercritical lower alcohols is systematized. The kinetics of selected reactions is reported and is compared with the kinetics of the same reactions in the gas and liquid phases. Thermodynamic calculations of the phase states and kinetic parameters are presented for a number of multicomponent multiphase systems containing terpenes and lower alcohols. The effect of the supercritical solvent pressure on the rate and selectivity of the selected reactions is reported.

H 3 PW 12 O 40 (PW), the strongest heteropoly acid in the Keggin series, is an active and environmentally friendly catalyst for the liquid-phase conversion of β -caryophyllene ( 1 ) to β -caryolanyl acetate ( 2 ) in homogeneous and heterogeneous systems. An efficient and clean method for the synthesis of 2 , providing a mixture containing two stereoisomeric β -caryolanyl acetates 2a and 2b , 2a/2b = 80/20 mol/mol, with 100% GC yield, has been developed using PW as a homogeneous catalyst under mild reaction conditions. The reaction occurs at 25 °C with a catalyst turnover number of 2000. The catalyst can be recovered without neutralization and reused without loss of activity and selectivity.

The paper presents mathematical models and calculation methods for solving particular research problems related to the thermodynamic characteristics of multicomponent and multiphase mixtures. The special features of chemical and phase equilibria in such mixtures are considered in the ideal gas approximation and taking nonideality into account. The conditions of equilibrium phase stability are studied for multiphase systems. The results of calculations of characteristic phase diagrams and binodal and spinodal are given for model systems with a fixed chemical composition, and a new interpretation of the mathematical model for localizing the critical point of a multicomponent mixture with a given composition is presented. A new interpretation of the well-known classic homotopy method is suggested for solving complex nonlinear systems of equations. Some anomalies of phase portraits and critical curves that are necessary to take into account in selecting (planning) experimental conditions and calculating chemical processes and reaction parameters are considered separately. The possibility of calculating thermodynamic and thermophysical properties (entropy, enthalpy, heat capacity, heat effects of reactions, and adiabatic heating) is demonstrated for the example of particular multicomponent nonideal mixtures. The conclusion is drawn that cubic equations of state can be used for predicting the deviations of these properties from the ideal gas state and their anomalies in the vicinity of the critical points of mixtures.

Zn(II)–Cr(III) mixed oxide possessing acid and dehydrogenation functions is an efficient, noble-metal-free catalyst for the one-step dehydroisomerisation of α-pinene to p-cymene. This reaction is a good example of the use of heterogeneous multifunctional catalysis for the conversion of renewable feedstock into value-added chemicals. It involves acid-catalysed α-pinene isomerisation followed by dehydrogenation of p-cymene precursor(s). The reaction is carried out over a fixed catalyst bed in the gas phase at 350 °C. Amongst Zn–Cr oxides studied (Zn/Cr = 20:1–1:30), the preferred catalyst is Zn–Cr (1:1) oxide which produces p-cymene with a 78% yield at 100% α-pinene conversion. This catalyst shows stable performance for over 30 h without co-feeding hydrogen to the reactor.

Heterogeneous acid catalysis by heteropoly acids (HPAs) has the potential of great economic rewards and green benefits. Its application, however, has been limited to some extent because of a relatively low thermal stability of HPAs, hence difficulty of catalyst regeneration (decoking). The aim of this paper is to discuss approaches to the problem of catalyst deactivation that could be instrumental to achieve sustainable performance of solid HPA catalysts. These approaches include: developing new HPA catalysts possessing high thermal stability, modification of HPA catalysts to enhance coke combustion, inhibition of coke formation on HPA catalysts during operation, reactions in supercritical fluids and cascade reactions using multifunctional HPA catalysis.

Thermolysis of sulfate turpentine was studied in the pressure range of 40–280 atm. The conversion kinetics of main turpentine components (α-pinene, 3-carene) and their isomerization products (limonene, alloocimenes, pyronenes) was investigated and compared with the conversion kinetics of these compounds in supercritical ethanol. Thermodynamic parameters Δ H ¯ # , Δ S ¯ # and Δ G ¯ # of the reversible reaction “α-pinene-[TS]” were estimated, the observed discrepancies being interpreted in the context of transition state theory. The thermolysis rate of α-pinene as a turpentine component was compared to that in a supercritical diluted solution of ethanol versus pressure. The rate of α-pinene conversion was shown to depend on the density of working medium.

Bulk Keggin heteropoly acids (HPAs) H 3+ n [PMo 12− n V n O 40 ] ( n = 0–2) and their Cs + salts catalyse the gas-phase hydrogenation of hexanoic acid at 350 °C and 1 bar H 2 pressure, yielding hexanal together with 6-undecanone and C 4 –C 7 hydrocarbons as the main products. Catalyst acidity (controlled by Cs substitution) has crucial effect on reaction selectivity. Partially substituted Cs salts exhibit high aldehyde selectivity (up to 76% at 70% conversion). Further increasing Cs substitution in HPAs, increases the selectivity to 6-undecanone up to 71–76% at the expense of hexanal and hydrocarbons. Initially crystalline, the post-reaction catalysts become amorphous, with their surface area significantly reduced. This shows that the as-made heteropoly compounds are catalyst precursors rather than the true catalysts. The constituent V 2 O 5 and MoO 3 oxides make more active catalysts for hexanoic acid hydrogenation than the P–Mo–V heteropoly compounds. Under reaction conditions, the bulk MoO 3 reduces to MoO 2 , which performs steadily to give 75% hexanal selectivity at 95% conversion. The bulk V 2 O 5 , inactive itself, slowly, over 26 h, reduces to the active V 2 O 3 oxide, which shows excellent hydrogenation performance: >97% hexanal selectivity at 80–83% conversion over 72 h on stream.

Phase behaviors of two- and three-component mixtures ‘α-pinene–water’ and ‘α-pinene–ethanol–water’ were studied over a wide range of their composition, temperature, and pressure. Conditions were found that provide heteroazeotropic states of binary mixture ‘α-pinene–water’ and ternary mixture ‘α-pinene–ethanol–water’ depending on the ratio of ‘water/(α-pinene+ethanol)’ fractions at the ‘α-pinene+ethanol’ ratio of 1:1. Critical curves were calculated to demonstrate an anomalous dependence of these curves on the mixture composition, with the temperature minimum points typical of the mixtures that form heteroazeotropes.

Esterification of hexanoic acid and transesterification of ethyl propanoate and ethyl hexanoate with excess methanol (1:20 molar ratio) are tested at 60 °C and ambient pressure with a range of HPA catalysts in homogeneous and heterogeneous systems in comparison with conventional homogeneous and solid acid catalysts such as H2SO4, Amberlyst-15 and zeolites HY and H-Beta. The intrinsic catalytic activity (turnover frequency, TOF) of HPA catalysts is significantly higher than that of the conventional acid catalysts in these reactions. The TOF values decrease with decreasing catalyst acid strength in the order: H3PW12O40 ≈ Cs2.5H0.5PW12O40 > H4SiW12O40 > 15%H3PW12O40/Nb2O5, 15%H3PW12O40/ZrO2, 15%H3PW12O40/TiO2 > H2SO4 > HY, H-Beta > Amberlyst-15. The activity per unit catalyst weight falls in a different order: H2SO4 > H3PW12O40 ≈ H4SiW12O40 > Amberlyst-15 ≥ Cs2.5H0.5PW12O40 > supported H3PW12O40 > HY, H-Beta. Bulk cesium salt Cs2.5H0.5PW12O40 exhibits high catalytic activity as well as high stability to leaching. Supported HPA catalysts suffer from leaching and exhibit significant contribution of homogeneous catalysis by the leached HPA.

Novel layered double hydroxides, [MAl4(OH)12](NO3)2⋅nH2O (M = (Li+)2, Co2+, Ni2+, Cu2+, Zn2+ ), impregnated with palladium , have proved to be precursors to active bifunctional mixed oxide catalysts for the one-step synthesis of methyl isobutyl ketone (MIBK) from acetone and H 2 in the gas phase. High acetone conversions of up to 76% and MIBK selectivities of up to 91% were recorded at a reaction temperature of 250 °C and atmospheric pressure. The palladium catalysts based on Li and Zn layered double hydroxides displayed efficient catalytic performances at the low reaction temperature of 120 °C. Powder XRD and 27Al MAS NMR showed that initially crystalline layered double hydroxides transformed to amorphous mixed oxide materials upon catalyst activation at 250 °C under H2.

Pd metal supported on Zn II –Cr III mixed oxide is an efficient bifunctional catalyst for the one-step synthesis of methyl isobutyl ketone (MIBK) from acetone and H 2 in the gas and liquid phases. The reaction involves acid-catalysed condensation of acetone to mesityl oxide, followed by its hydrogenation to MIBK. Diisobutyl ketone (DIBK) is a useful byproduct in this process. Zn–Cr oxides (Zn/Cr = 20:1–1:30) are prepared by coprecipitation of Zn II and Cr III hydroxides. The texture and acid properties (i.e., the nature, density, and strength of acid sites) of Zn–Cr oxides, as well as the Pd dispersion in the catalysts, are thoroughly characterised. For both the continuous gas-phase process and the batch liquid-phase process, the preferred catalyst formulation is 0.3 wt% Pd on the amorphous Zn–Cr (1:1) oxide ( S BET = 132 m 2 / g ) having Lewis acid sites (1.2 mmol/g density) with an enthalpy of NH 3 adsorption of − 155 kJ / mol . Both processes produce MIBK with a selectivity of 70–78% and 90% MIBK + DIBK total selectivity at 38–40% acetone conversion. Evidence is provided that hydrogenation of mesityl oxide to MIBK is the rate-limiting step in the gas-phase process.

Thermal isomerization of β-pinene and equimolar mixture of α- and β-pinenes in supercritical ethanol was studied experimentally. The reactivity of structurally similar α- and β-pinenes were compared in the same supercritical solvent. It was shown that both pinenes undergo thermal transformations independently of one another upon co-thermolysis in supercritical (SC) ethanol. Conversion of α-pinene yields monocyclic limonene as the main product, and β-pinene gives acyclic β-myrcene. This study allowed developing the first kinetic model of β-pinene thermolysis in SC ethanol.

Bulk Keggin heteropoly acids (HPAs) H3+n[PMo12−nVnO40] ( n = 0 – 2 ) and their Cs+ salts catalyse the vapour-phase hydrogenation of propanoic acid at 350 °C and 1 bar H2 pressure, yielding propanal together with 3-pentanone and propane as the main products. Catalyst acidity (controlled by Cs substitution) has crucial effect on the reaction selectivity. As the Cs content increases, the selectivity to propanal passes a maximum (74–76%). At the same time, the selectivity to propane sharply decreases, whereas 3-pentanone selectivity increases monotonously. This indicates that 3-pentanone is likely to form via Cs propanoate intermediate. Partial substitution of Mo(VI) by V(V) in the PMo12O3−40 anion has a small effect on the catalyst performance. Initially crystalline, the catalysts become amorphous after reaction, with their surface area significantly reduced. As evidenced by FTIR, H4[PMo11VO40] and its Cs salts, possessing a higher thermal stability, retain the Keggin structure in their bulk after reaction, whereas less stable H3[PMo12O40] and H5[PMo10V2O40] derivatives undergo decomposition. This shows that the as-made crystalline heteropoly compounds are catalyst precursors rather than the true catalysts. The reaction over Cs2.4H1.6[PMo11VO40] is zero order in propanoic acid with an activation energy of 85 kJ/mol. The formation of propanal is suggested to occur via a Mars–Van Krevelen mechanism.

Ruthenium-doped (5 wt%) acidic heteropoly salt Cs2.5H0.5[PW12O40] (CsPW) is an active bifunctional catalyst for the one-pot hydrogenolysis of glycerol to 1,2-propanediol (1,2-PDO) in liquid phase, providing 96% selectivity to 1,2-PDO at 21% glycerol conversion at 150 °C and an unprecedented low hydrogen pressure of 5 bar. Rhodium catalyst, 5%Rh/CsPW, although less active, shows considerable selectivity to 1,3-PDO (7.1%), with 1,2-PDO being the main product (65%).

The experimental study followed by mathematical processing of the data showed that supercritical ethanol is an effective reaction medium for thermal isomerization of α -pinene. Rate of the reaction in supercritical solvent is by several orders of magnitude greater than the rate observed under normal conditions, with selectivity to the target reaction products being retained. The study allowed us for the first time to obtain the kinetic model of α -pinene isomerization in supercritical ethanol that takes into account temperature and pressure effect on the reaction rate and selectivity.

Pd–H3PW12O40/SiO2 catalyzes the one-pot transformation of (+)-citronellal to menthol via acid-catalyzed cyclization followed by Pd-catalyzed hydrogenation, with 92% yield of menthol at 100% citronellal conversion and 85% stereoselectivity for the desired (−)-menthol.

Heterogeneous acid catalysis by heteropoly acids (HPAs) has the potential of substantial economic and green benefits. Its application, however, has been limited because of the difficulty of catalyst regeneration due to a relatively low thermal stability of HPAs. The aim of this paper is to discuss the perspectives of acid catalysis by solid HPAs, in particular focusing on several approaches that could help overcome the deactivation of HPA catalysts to achieve sustainable catalyst performance. These approaches include: developing novel HPA catalysts possessing high thermal stability, modification of HPA catalysts to enhance coke combustion, inhibition of coke formation on HPA catalysts during operation, reactions in supercritical fluids and cascade reactions using multifunctional HPA catalysis.

The acidic cesium hetetropoly salt Cs2.5H0.5PW12O40 (CsPW) is an active solid acid catalyst for the liquid-phase Fries rearrangement of phenyl acetate, which, similar to other solid acid catalysts, suffers from deactivation due to coke deposition. Coke burning cannot effectively regenerate CsPW because of its relatively low thermal stability. Doping CsPW with Pt metal (0.3 wt%) allows full regeneration of catalyst activity and selectivity by the aerobic oxidation of coke at 350 °C and ambient pressure, which occurs without any structural damage to the catalyst. Pt doping practically does not affect the activity and selectivity of CsPW. As a coke-burning catalyst, Pt is much more efficient than Pd; the latter requires almost ten times higher a metal loading. The Pt/CsPW catalyst is characterised by XRD, TGA/TPO, FTIR and H2 chemisorption.

The isomerization of α-terpineol (1) catalyzed by heteropoly acid H3PW12O40 (PW) in homogeneous and heterogeneous systems yields 1,8-cineole (2) and 1,4-cineole (3), both useful for flavoring and pharmaceutical applications. In the homogeneous system, 2 and 3 were obtained with 25% and 23–27% selectivity, respectively, at 50–90% α-terpineol conversion (in a nitrobenzene solution, 40 °C). In the heterogeneous system, 35% of 2 and 25% of 3 were obtained at 70–100% conversion in a cyclohexane solution at 60 °C using silica-supported PW as a solid acid catalyst, and the catalyst could be recycled. PW showed a higher catalytic activity and selectivity than conventional acid catalysts such as H2SO4 and Amberlyst-15.

Pd metal (0.1–1 wt%) supported on ZnII–CrIII ( 1 : 10 ) mixed oxide was found to be an efficient bifunctional catalyst for the one-step synthesis of methyl isobutyl ketone (MIBK) from acetone and H2 in gas- and liquid-phase processes. The reaction involves the acid-catalysed condensation of acetone to form mesityl oxide, followed by its hydrogenation to MIBK. The gas-phase reaction produces MIBK with a selectivity up to 78% at 40–66% acetone conversion (200–350 °C, ambient pressure). Diisobutyl ketone (DIBK) is the main byproduct, with a total MIBK + DIBK selectivity of up to 93%. The catalyst reaches a steady state in ca. 1 h and shows constant activity and selectivity for at least 50 h on stream. The liquid-phase reaction yields MIBK with up to 83% selectivity and total selectivity to MIBK + DIBK up to 9 at 56% acetone conversion (200 °C, 5 bar H2 pressure). As a catalyst support, the amorphous Zn–Cr oxide calcined at 300 °C provides higher catalytic activity than the crystalline Zn–Cr oxide calcined at 400 °C, probably due to the lower surface area of the crystalline oxide. XRD of 0.3–1%Pd/Zn–Cr oxide showed no pattern of Pd metal, indicating a fine dispersion of Pd particles in the catalyst.

A novel one-pot catalytic synthesis of diisobornyl ether directly from camphene (1) has been developed. The reaction occurs under near ambient conditions in the presence of dissolved or silica-supported heteropoly acid H3PW12O40 (PW) as a catalyst to give diisobornyl ether (2) with up to 90% selectivity at 50–60% conversion, along with isoborneol (3). Ether 2 appears to be a new compound; it was isolated as a mixture of meso and dl stereoisomers and fully characterized by MS-GC, NMR and IR spectroscopy. The supported PW catalyst can be easily separated and reused; no PW leaching from support was observed during the reaction. Conventional acid catalysts such as H2SO4 or Amberlyst-15 showed no activity in this reaction.

Silica-supported heteropoly acid H 3 PW 12 O 40 (PW) has been found to be a very efficient and environmentally benign solid acid catalyst for the liquid-phase isomerisation of α-pinene oxide ( 1 ). The reaction has been performed in cyclohexane solutions at 15–40 °C to give campholenic aldehyde ( 2 ) and trans -carveol ( 3 ) as the main products in up to 98% total yield, with 70% selectivity to more valuable product 2 . Both 2 and 3 are expensive ingredients in the fragrance industry. No PW leaching has been observed during the reaction. The catalyst is easily separable from the reaction mixture and reusable without loss of its activity and selectivity.

Chemoselective catalysts in bulk or supported on γ-Al2O3 binary oxides RuIV–CoIII (Ru/Co = 1:1–1:2), prepared by co-precipitation, were used for liquid-phase oxidation of saturated and unsaturated primary and secondary alcohols to aldehydes and ketones with O2 or N2O. The catalysts can be separated by filtration and reused. No leaching of Ru or Co in solution was observed. The oxidation is enhanced by the presence of hydration water in the Ru–Co catalyst, which indicates the participation of active RuIV hydroxo species in the reaction. From XRD and TGA, the Ru–Co oxide can be approximated as a hydrous binary oxide comprising the amorphous RuO2 and heterogenite-3R cobaltic acid CoO(OH). The alcohol oxidation appears to occur by a nonradical mechanism, which may be viewed as an oxidative dehydrogenation of alcohols to form an aldehyde or ketone. H2-TPR shows that CoIII practically does not affect the oxidising ability of RuO2. This suggests that the cobalt is likely to enhance catalyst reoxidation by O2 rather than to play a significant role in the alcohol dehydrogenation. The alcohol oxidation by N2O exhibits a close similarity to the oxidation by O2 but is much less efficient. Much more active catalysts are required to make the oxidation with N2O synthetically useful.