Vapor-phase Aldol Condensation Research Articles

Influence of Al Oxides on Cs-SiO2 Catalysts for Vapor Phase Aldol Condensation of Methyl Acetate and Formaldehyde

Influence of Al Oxides on Cs-SiO₂ Catalysts for Vapor Phase Aldol Condensation of Methyl Acetate and Formaldehyde

Coking and Deactivation Behavior of Ba–La-Modified Alumina in the Aldol Condensation of Methyl Acetate with Formaldehyde: Effect of the Reactant Composition

Catalyst deactivation caused by coking has always been a concern in the aldol condensation reaction. In this work, Ba–La-modified alumina has been characterized and tested as a catalyst in the vapor-phase aldol condensation process. The coke formation, acid–base properties, and deactivation behavior of the spent catalysts were studied through a series of characterization techniques: N2 adsorption–desorption, pyridine FT-IR, TG-DTA, TPO-MS, as well as NH3, CO2-TPD, and UV Raman spectroscopy. It was found that the main components of coke are polycyclic aromatic hydrocarbons, which can be removed by high-temperature roasting under an air atmosphere. The overactivation of formaldehyde accelerates the rate of coke deposition and ultimately aggravates the decrease in the stability of the catalyst. Furthermore, adding a proper amount of methanol to the reactants can greatly inhibit the hydrolysis of esters and improve the selectivity of the catalyst for the target reaction. Under the optimum reaction condition of Ma/FA/CH3OH = 6:2:8, the pairs of medium acidic and weakly basic sites enable Ba–La/Al2O3 to achieve the highest yield of methyl acrylate.

Support effects of Cs/Al2O3 catalyzed aldol condensation of methyl acetate with formaldehyde

In this work, the support effects on vapor-phase aldol condensation of methyl acetate (MeOAc) with formaldehyde (HCHO) over cesium catalysts are understood by the employment of Al2O3 with different morphologies and crystal phases. The structural and electronic properties of these supported Cs catalysts are characterized by multiple techniques, such as XRD, HAADF-STEM, EDS-mapping, XPS, NH3-TPD and CO2-TPD. The Cs species are found to be highly dispersed on the Al2O3 surfaces, and their compositions vary with the identity of Al2O3. A combined catalyst characterization and testing results indicates a balance between acidic and basic sites, i.e., an acid-base synergy, for the production of MA: the excess acidic sites lead to the lower MA selectivity due to the promoted further condensation, while the excess basic sites with the lower MeOAc conversion due to the insufficient activated HCHO species. These insights could be valuable for the design and optimization of catalysts for this reaction.

Amorphous SiO2 catalyst for vapor-phase aldol condensation of butanal

Vapor-phase aldol condensation of butanal to form 2-ethyl-2-hexenal was carried out over several oxide catalysts such as SiO2-Al2O3, Al2O3, ZrO2, and SiO2. Catalysts with moderate and strong acid sites such as Al2O3 and SiO2-Al2O3 were active for the reaction in the initial period, whereas they deactivated rapidly. In contrast, SiO2 with weak acidity showed a low but a stable catalytic activity for the formation of 2-ethyl-2-hexenal. Thermogravimetric analyses of the samples used after the reactions indicate that SiO2 has the smallest amount of carbonaceous species that contributed to its stable activity among the tested catalysts. SiO2 catalysts with different pore sizes and specific surface areas were examined: SiO2 with a mean pore diameter of 10 nm and a surface area of 295 m2 g−1 showed the best catalytic performance and gave a 2-ethyl-2-hexenal selectivity of 90% at a conversion of 48% at 240 °C. In the catalytic test using deuterated SiO2, which was prepared by contacting SiO2 with deuterated water before the reaction, it was confirmed by a mass spectrometer that the deuterium atom of SiOD was transferred to a 2-ethyl-2-hexenal molecule during the reaction. It is indicated that silanol groups on the SiO2 surface played a role as an active site.

Highly Catalytic Activity of Ba/γ-Ti–Al2O3 Catalyst for Aldol Condensation of Methyl Acetate with Formaldehyde

In this work, titanium-doped mesoporous Al2O3 (γ-Ti–Al2O3) was prepared by an evaporation-induced self-assembly method and used as a carrier of Ba/γ-Ti–Al2O3 catalyst to catalyze the aldol condensation of methyl acetate with formaldehyde to methyl acrylate in a fixed-bed reactor. The catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, N2 adsorption–desorption, pyridine absorption performed via Fourier transform infrared spectroscope (Py-IR), and NH3 and CO2 temperature-programmed desorption (NH3 and CO2-TPD). Experimental results indicated that the doping of the titanium species into the frame work of mesoporous Al2O3 (γ-Ti–Al2O3) had a significant influence on the catalytic activity via modifying the acid–base surface properties of the catalyst. Furthermore, the Ba/γ-Ti–Al2O3 catalyst demonstrated excellent catalytic performance, with a methyl acetate conversion rate of 50% and methyl acrylate selectivity up to 90.2%. Compared with the Ba/Al2O3 catalyst, the Ba/γ-Ti–Al2O3 catalyst had better catalytic activity, stability and potential for practical application, which was likely due to an increased number of Lewis acid sites, especially the medium acid sites. Barium supported mesoporous γ-Ti–Al2O3 catalyst was found to be an effective catalyst for vapor phase aldol condensation of methyl acetate with formaldehyde.

Experimental and Theoretical Study for Vapor Phase Aldol Condensation of Methyl Acetate and Formaldehyde over Alkali Metal Oxides Supported on SBA-15

Alkali metal oxides supported on SBA-15, M/SBA-15 (M = Li, Na, K, Rb, and Cs), prepared by the impregnation method were characterized and tested for the aldol condensation of methyl acetate and formaldehyde to methyl acrylate (MA). M/SBA-15 catalysts are active for the formation of MA while the support is inert. The supported alkali metals react with hydroxyl groups on SBA-15 by replacing the H atoms and generate weak and medium basic sites. The latter plays an important role in promoting the reaction. Coke formation results in deactivation of Cs/SBA-15, and the deactivated catalyst can be regenerated by burning off the coke. Two possible reaction pathways for the formation of MA were explored using quantum calculations. The predicated activity for M/SBA-15 is consistent with the experimental observations. A pathway with adsorbed enol molecule as an intermediate was suggested to be predominant for the formation of MA.

Synthesis of Methyl Acrylate by Aldol Condensation of Methyl Acetate with Formaldehyde Over Al2O3-Supported Barium Catalyst

Vapor phase aldol condensation of methyl acetate with formaldehyde was first studied over Al2O3 with different calcination temperatures and Al2O3-supported barium with different amounts of barium catalysts. The catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, N2 adsorption–desorption, the pyridine absorption performed via Fourier transform infrared spectroscope, NH3 and CO2 temperature-programmed desorption. The results indicated that the calcination temperature affected the strength and the number of surface acid and base sites over the alumina catalysts. In addition, the moderate Lewis acid and weak base sites were critical to promote this aldol condensation reaction. Adding barium species, which could effectively modified the acid–base properties of Al2O3, clearly improved the catalytic activity and selectivity. The stability and regeneration of the optimum catalyst were also investigated and did not exhibit an obvious decrease in efficiency. Al2O3-supported barium catalyst was found to be an effective catalyst for vapor phase aldol condensation of methyl acetate with formaldehyde as a result of the appropriate intensity of acid and base.

Vapor phase aldol condensation of methyl acetate with formaldehyde over a Ba–La/Al2O3 catalyst: the stabilizing role of La and effect of acid–base properties

Vapor phase aldol condensation of methyl acetate with formaldehyde was studied over Ba–La/Al2O3 with different amounts of lanthanum catalysts.

Effects of Support for Vanadium Phosphorus Oxide Catalysts on Vapor-Phase Aldol Condensation of Methyl Acetate with Formaldehyde

Vapor-phase aldol condensation of methyl acetate with formaldehyde over vanadium phosphorus oxide (VPO) deposited on SiO2, TiO2, ZrO2, Nb2O5, Sb2O3, γ-Al2O3, and n-γ-Al2O3 (γ-Al2O3 treated with H3PO4) for methyl acrylate production was developed. The VPO/n-γ-Al2O3 was found to have the best performance based on the evaluated experiments in a fixed-bed reactor. The physicochemical properties of the catalysts were investigated by different characterization methods such as the Brunauer–Emmett–Teller method, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, H2 temperature programmed reduction, Fourier transfrom infrared spectroscopy, and NH3- and CO2-temperature programmed desorption. The influence of P–OH intensity and acid strength on activity was obtained through systematic studies over the formation process of VPO phases and the acidity distribution. Preparation and reaction parameters were investigated and optimized.

Formation of Methyl Acrylate from Methyl Acetate and Formaldehyde Condensation over Solid Base Catalysts

The vapor-phase aldol condensation of methyl acetate (MeOAC) with formaldehyde (HCHO) to form methyl acrylate was investigated over solid base catalysts. A variety of solid base catalysts were prepared by loading Cs2O over SiO2 or Al2O3 as the support materials. The properties of the solid base catalysts were characterized by XRD, BET and CO2-TPD. The results of the vapor-phase aldol condensation indicated that SiO2 and Al2O3 modified with Cs2O showed high conversation of methyl acetate and high selectivity of methyl acrylate. 22.6% conversation of methyl acetate and 16.5% yield of acrylic acid and methyl acrylate were obtained over Cs-Al2O3. In addition, Cs-SiO2 exhibited about 42% selectivity to methyl acetate. The high yield of acrylic acid and methyl acrylate is attributed to the high base property of catalysts.

Formation of methyl methacrylate by condensation of methyl propionate with formaldehyde over silica-supported cesium hydroxide catalysts

The vapor-phase aldol condensation of methyl propionate (MP) with formaldehyde (HCHO) to form methyl methacrylate (MMA) was studied with a HCHO/MP molar ratio of 0.2, using formalin as a source of HCHO over solid base catalysts. Cesium hydroxide was the best base catalyst and silica gel was the sole compound effective as the support. The optimum Cs/Si atomic ratio was in the range of 2.0/100 to 2.5/100. Na cations present in the silica gel as an impurity decreased the yield of MMA. The addition of water into the reaction feed did not affect the selectivity, though the reaction rate decreased a little. With an increase in the HCHO/MP molar ratio, the yield increased, but the selectivity decreased. The catalytic activity decreased very slowly with the time-on-stream. The deactivated catalyst was completely regenerated by calcinations in air at 400 °C.

Production of methacrylic acid by vapor-phase aldol condensation of propionic acid with formaldehyde over silica-supported metal phosphate catalysts

The vapor-phase aldol condensation of propionic acid (PA) with formaldehyde (HCHO) to form methacrylic acid (MAA) was performed with a PA/HCHO molar ratio of 2 using various silica-supported metal phosphate catalysts. In the absence of water, a silica-supported tin phosphate with Sn/Si/P atomic ratio of 1/8/2.3 showed the same level of performance as a silica-supported vanadium phosphate with V/Si/P atomic ratio of 1/8/2.2 did. The one-pass yield of MAA reached 58% based on the HCHO feed. In the presence of water, the performances of almost all catalysts fell. However, the Sn/Si/P catalyst showed a relatively high performance even in the presence of water. The performance is almost identical to that obtained from a silica-supported cesium oxide catalyst.

Vapor phase aldol condensation over fully ion-exchanged montmorillonite-rich catalysts

The vapor phase aldol condensation of formaldehyde with acetaldehyde has been investigated over montmorillonite-rich (Mt-rich) materials exchanged with Na +, Cs +, Mg 2+, Ni 2+, La 3+ or UO 2 2+ cations. The main products are acrolein (via cross-condensation), and crotonaldehyde (via self-condensation), indicating that aldol processes effectively take place over such catalysts. The results showed that ion-exchange induces different catalytic activities depending on the nature of the cation introduced and the reaction temperature. Both the acid and base properties exert strong effects upon the total conversion of acetaldehyde and the acrolein:crotonaldehyde ratio. It was also found that cross-condensation takes place exclusively over weakly acidic sites. The highest cross-selectivities were obtained over Na, Cs and Mg-containing catalysts.

The aldol condensation of lower aldehydes over MFI zeolites with different acidic properties

Various MFI type zeolites were synthesized taking into account two factors affecting zeolitic acidity “by synthesis”, namely the nature of the substituted heteroelement and the Si/Me 3+ ratio. The surface properties with respect to chemical composition and acidity were investigated by temperature-programmed reduction, X-ray photoelectron spectroscopy, temperature-programmed desorption of ammonia, Fourier transform infrared spectroscopy and microcalorimetry. Vapor phase aldol condensation of acetaldehyde and formaldehyde over these catalysts has been carried out, and it has been observed that the structural features of the catalysts do play an important role in controlling conversion and selectivity. A reasonable acidity and activity relationship is established. A mechanism for cross-aldol condensation involving both Brönsted and Lewis acid sites is proposed.

Vapor-Phase Aldolization of n-Butyraldehyde to 2-Ethyl-2-Hexenal over Solid-Base Catalysts

Vapor-phase aldol condensation of n-butyraldehyde to 2-ethyl-2-hexenal was studied at 1 atm and 150∼ 300°C in a fixed-bed, integral-flow reactor by using NaX, KX, γ-Al2O3 and Na/NaOH/γ-Al2CO3 catalysts. Ion exchange of NaX zeolite with potassium acetate solution results in a decrease of crystallinity and apparent lowering of surface area, whereas the basic strength is enhanced. Treatment of γ-Al2O3 with NaOH and Na causes a large decrease of the surface area but strong enhancement of the catalyst basicity. The catalytic activity on the basis of unit surface area is in the order Na/NaOH/γ-Al2O3 < KXU < KXW < NaX >γ-Al2O3, in accordance with the relative catalyst basic strength. The molar ratio of trimeric to dimeric products increases with increasing the reaction temperature and the catalyst basic strength except for Na/NaOH/γ-Al2O3. Very high selectivity of 2-ethyl-2-hexenal (>98.5%) was observed for reactions over NaX zeolite at 150°C. Based on the FT-IR and the catalytic results, the reaction paths are proposed as follows: self-aldol condensation of n-butyraldehyde, followed by dehydration produces 2-ethyl-2-hexenal, which then reacts with n-butyraldehyde and successively dehydrates to 2,4-diethyl-2,4-octadienal and 1,3,5-triethylbenzene. For the reaction over NaX, the calculated Arrhenius frequency factor and activation energy are 314 mol/g·h and 32.6 kJ/mol, respectively.

Influence of the acid–base properties of solid catalysts derived from hydrotalcite-like compounds on the condensation of formaldehyde and acetaldehyde

The vapor-phase aldol condensation of formaldehyde with acetaldehyde was studied using various oxide mixtures obtained in situ from hydrotalcite-like materials as catalysts. All samples exhibited catalytic activity, but the selectivity to both the aldol condensation and the acrolein formation is governed by the balance of acidic and basic properties. The oxides containing Mg and Al or Co and Al showed a high selectivity to aldol condensation (90% at 300°C) as well as to cross-condensation (80% at 300°C). The effects of both the reaction variables (temperature, mole ratio of reagents) and the catalyst variables (nature of cations, nature of anions) were studied.

Vapor phase aldol condensation of acetaldehyde on metal oxide catalysts

Vapor phase aldol condensation of acetaldehyde on silica supported alkali metal containing solid bases, silica supported tungsten and molybdenum oxides and other unsupported oxides were studied. It was found that silica supported alkali metal containing solid base catalysts can effectively catalyze the aldol condensation of acetaldehyde to produce crotonaldehyde as well as crotyl alcohol with a good selectivity (ca. 90%) and stability and moderate conversion. MoO 3/SiO 2 or MoO x/SiO 2 (reduced state) was found to be almost inactive for this reaction. Contrary to MoO 3(MoO x)/SiO 2, WO 3(WO x)/SiO 2 catalysts are active and more selective. Most of other unsupported oxides tested in this study are not attractive for the low activity and selectivity or poor stability. ZrO 2 or pre-treated ZrO 2 SO 4 2− was initially quite active, but deactivated quickly. Co-feeding hydrogen can enhance the activity of ZrO 2 or inhibit deactivation of pre-treated ZrO 2 SO 4 2−. The super acidic site associated with SO 4 2− species has a great contribution to coking but little to aldol condensation.

Base catalysis for the synthesis of α,β-unsaturated ketones from the vapor-phase aldol condensation of acetone

The vapor-phase aldol condensation of acetone was studied over MgO promoted with 0.7–1.0 wt.-% of alkali (Li, Na, K and Cs) or alkaline earth (Ca, Sr and Ba) metal ions. The basic properties of the samples were characterized by chemisorption of carbon dioxide. The basicity of MgO increased on addition of the promoter following the basicity order of the promoter oxide: the stronger the electron donor properties of the promoter, the greater the generation of surface basic sites. Major reaction products were mesityl oxide (MO), isomesityl oxide (IMO) and isophorone (IP). The selectivity to (MO + IMO + IP) over unpromoted MgO was practically 100%, thereby showing that magnesium oxide is suitable for selectively obtaining α,β-unsaturated ketones. The reaction was totally inhibited by co-feeding acetic acid along with acetone whereas the co-injection of pyridine did not affect the acetone conversion. This indicated that the self-condensation of acetone over MgO-based catalysts is catalyzed by basic sites. The promoter addition increased the activity of the MgO catalyst and a good correlation was obtained between catalyst activity and the concentration of basic sites. Such a proportionality between activity and surface basicity was an additional evidence that the rate-determining step in the aldol condensation mechanism is controlled by the surface base property. All the catalysts exhibited similar IP/(IMO+MO) selectivity ratio, except the Li/MgO sample which produced substantially larger amounts of isophorone. Because the tricondensation of acetone to give isophorone requires strong basic sites, the higher selectivity toward isophorone was indicative of the presence of stronger surface basic sites in the Li/MgO sample. Results from carbon dioxide chemisorption confirmed that Li/MgO exhibited the strongest basic properties. The generation of high-strength basic sites was explained by assuming that the addition of lithium causes a structural promotion of the MgO sample by replacing the Mg 2+ ions by Li + in the MgO lattice. The replacement would result in strained Mg O bonds and formation of [Li +O −] species, which causes the generation of stronger basic sites.

Formation of Acrylaldehyde by Vapor-Phase Aldol Condensation I. Basic Oxide Catalysts

Abstract Vapor-phase aldol condensation of formaldehyde with acetaldehyde to form acrylaldehyde was studied with various oxides supported on silica gel using 35% formalin as the source of formaldehyde. The silica gel unsupported catalyst prepared from colloidal silica showed a relatively high activity and selectivity to form acrylaldehyde. The catalytic activity was markedly enhanced by the incorporation of a basic or amphoteric oxide into the silica gel, while it fell with the incorporation of an acidic oxide such as P2O5 or B2O3. The highest yields of acrylaldehyde were obtained with the Si–Mg, Si–Li, Si–Na, and Si–Zn oxide systems. Effects of reaction variables were also studied using a catalyst with an Si : Mg atomic ratio of 100 : 10.

Formation of Acrylaldehyde by Vapor-Phase Aldol Condensation II. Phosphate Catalysts

Abstract Vapor-phase aldol condensation of formaldehyde with acetaldehyde to form acrylaldehyde was studied with various oxide and phosphate catalysts using 35% formalin as the source of formaldehyde. Almost all of the tested oxides exhibited a certain activity as the catalysts. Acidic oxides, such as MoO3 and WO3 showed a relatively high selectivity to acrylaldehyde, though the activities were low. V2O5 and amphoteric oxides exhibited a much higher activity, but they promoted also the formation of CO2 and methanol. The catalytic performance was improved, to a certain extent, by combination of an acidic oxide with an amphoteric oxide. Further, the combination of phosphorus with V2O5 or an amphoteric oxide induced a large increase in the selectivity to acrylaldehyde. A good performance was obtained with the Ni–P, Mn–P, Fe–P, and V–P systems. The yield of acrylaldehyde reached 86 mol% on the AcH basis with a very slow formation of CO2 and methanol.