Gas-phase Dehydration Of Glycerol Research Articles

Acidic property and gas-phase glycerol-dehydration activity of WO3/Al2O3 catalysts

The acidic property of WO3/Al2O3 catalysts loaded with WO3 (5–37 wt%) and their activity for gas-phase glycerol dehydration were investigated. Temperature-programmed desorption of NH3 revealed the formation of weak acid sites (the enthalpy of NH3 desorption; ΔHNH3 <110 kJ mol–1) on WO3/Al2O3. Infrared spectra of adsorbed pyridine indicated that a part of Lewis acid sites of WO3/Al2O3 was converted to Brønsted acid sites by water vapor treatment. The ΔH‡–ΔS‡ relationship for glycerol dehydration over WO3/Al2O3 and H-ZSM-5 indicated a weaker interaction of WO3/Al2O3 catalyst with the reaction intermediate than that of H-ZSM-5. The coke species deposited over WO3/Al2O3 and H-ZSM-5 were confirmed to consist of C3–C4 compounds, acrolein, benzene, toluene, xylene, glycerol, indenes, naphthalenes, and anthracenes (or phenanthrenes); fewer polyaromatic compounds were formed over WO3/Al2O3. For WO3/Al2O3, the weak acid strength of the Brønsted acid sites resulted in high durability for glycerol dehydration.

Microwave-Assisted Facile Sol–Gel Synthesis of WO3-Based Silica Catalysts for Enhanced Activity in Glycerol Dehydration

WO3 is a promising catalyst for the production of acrolein from glycerol due to its high activity and selectivity. In this study, various metal oxides (W, Zr, and Nb) over colloidal silica (cSiO2) catalysts were obtained using a novel microwave-assisted synthesis method. The enhancement in the sol–gel grafting of metal oxides onto the cSiO2 support was achieved by microwave irradiation (MW) and compared to the conventional heating (CH). WO3-cSiO2 catalysts synthesized by microwave-assisted facile sol–gel synthesis were tested in the gas-phase glycerol dehydration reaction. The best catalytic performance (52.4% acrolein yield) was obtained over a 25%W-cSiO2 sample prepared by the MW method. The best yield results were obtained with a B/(B + L) ratio over 0.50 and with an optimum medium-strength acidity. The activity loss was correlated with the high-strength surface acidity.

Glycerol Dehydration to Acrolein Catalyzed by Silicotungstic Acid: Effect of Mesoporous Support

To facilitate value-added chemical production from renewable glycerol, gas-phase glycerol dehydration to acrolein was conducted using supported silicotungstic acid as solid acid catalysts, focusing on the effects of mesoporous catalyst supports on the catalytic performance. One alumina (Al) support with average mesopore size of 30 nm and two silica supports (Si1254 and Si1252) with mesopore size of 6 nm and 11 nm, respectively, were comparatively evaluated in this study. It was found that the Si1254 silica support with the smallest pore size (6 nm) deactivated the fastest, decreasing both the glycerol conversion and acrolein selectivity along the time-on-stream. The other silica support Si1252 with 11 nm pore size provided an acrolein yield comparable to the Al support over the tested 7.5 h time-on-stream (73.9 mol% for Si1252 vs. 74.1 mol% for Al). However, the mechanisms for achieving the comparable yield are different. Si1252 showed higher acrolein selectivity than Al, but it also deactivated faster than Al due to its quicker coking. On the other hand, Al showed more stable performance in terms of glycerol conversion rate and less coking, but it had lower acrolein selectivity and a higher selectivity to byproducts, especially the undesired byproducts of acetaldehyde and propionaldehyde, which posed difficulties in downstream separation.

Restraining deactivation of β-zeolite by modifying with MgAl2O4 spinel in gas-phase glycerol dehydration

MgAl2O4 dispersed on HBEA zeolite with different spinel contents were successfully synthesized and their glycerol dehydration performances were presented. The different solids were characterized by XRD, infrared (IR) and Raman spectroscopy, N2 physisorption, SEM-FEG, acidity by TPD-NH3 and DRIFTS-pyridine adsorption. It was not possible to observe the spinel phase in the diffractograms due to its high dispersion, while the reflections of the HBEA were evidenced in the different samples. The Raman and infrared spectra suggested the presence of stretches concerning the MgAl2O4 spinel. N2 isotherms and low-angle XRD indicated that the pore structure and textural properties were altered after impregnation, but the micro-mesoporosity of zeolite was maintained. The HBEA morphology did not undergo significant changes by modification with spinel and the metals are evenly distributed according to SEM and EDS spectral mapping. The addition of spinel on zeolite caused a change in the acidic properties of HBEA related to the type, number and strength of the sites according to the TPD profiles and the DRIFTS spectra. The 2% of MgAl2O4 on HBEA catalyst exhibited the best catalytic performance due to the effects of its acidic and textural properties, reaching 60% of glycerol conversion, 66% of acrolein selectivity in 6 h and 16% of coke deposited in 10 h, restricting deactivation compared to pure zeolite. The reaction pathway for the different products formed depending on the type of acid site were detailed in the proposed mechanism and correlated with electrostatic potential maps as well as the reactions for coke formation were described according to compounds identified by GC-MS after coke extraction.

Soft-templating routes for the synthesis of mesoporous tantalum phosphates and their catalytic activity in glycerol dehydration and carbonylation reactions

The craftsmanship in catalyst synthesis becomes very fascinating when the catalytic activity of the material can be enhanced by tuning the synthesis conditions. The underlying fundamental strategy is presented here in tailoring the structural and chemical behavior of the tantalum phosphates (TaP) by using different structure directing agents (SDAs). We found that by employing appropriate template assisted pathway it is possible to tune the morphology and the nature of active sites of TaP. The synthesised catalysts were comprehensively characterised by N2-physisorption, FESEM, HRTEM, powder XRD, FTIR, UV–Vis, XPS, NH3 and CO2-TPD, Py-adsorbed IR, elemental analysis and 31P{1H} NMR techniques. The FESEM and HRTEM analysis revealed the transition of non-spherical to rod-shaped particles upon moving from ionic to non-ionic surfactants. The TPD and Py-IR studies revealed the variation in amount, strength and nature of the acidic-basic sites with the change in SDA. The resultant TaP catalysts were investigated for gas phase glycerol dehydration to acrolein and liquid phase glycerol carbonylation to glycerol carbonate. Among these catalysts, TaPC synthesized by using CTAB as SDA exhibited promising catalytic activity in both gas and liquid phase reactions. The active site elucidation in TaPC catalyst was done by adopting basic probe molecule 2,6-lutidine. The deactivation kinetic study was carried out for the gas phase glycerol dehydration reaction. The investigation on the spent catalyst showed it is stable under both gas-phase and liquid-phase reaction conditions. This study hence provides a pathway for analytical architecture and construction of highly efficient TaP catalysts for glycerol transformation reactions.

Catalytic dehydration of glycerol to acrolein over unsupported MoP

Catalytic dehydration of glycerol over solid catalysts provides a sustainable and economic route to producing acrolein. Unsupported MoP catalyst was prepared by temperature-programmed-reduction method and used for the gas-phase glycerol dehydration. The catalyst was characterized by means of XRD, SEM, TEM, NH3-TPD, Py-IR, XPS, TG-DSC, and N2 adsorption-desorption. It is indicated that the acidic POH and MoOH groups, which are usually associated with the weak or moderate strength of Brønsted acid sites, were presented on the surface of MoP. The effects of reaction temperature and carrier gas flow rate on glycerol conversion and acrolein selectivity were investigated over MoP. The selectivity to acrolein was up to 89.0 % and kept constant in a 50-h run at 240 °C. The gradual decrease of glycerol conversion might be related to the insoluble carbonaceous materials deposited on the catalyst surface.

Gas Phase Dehydration of Glycerol to Acrolein Over Nickel Phosphate Catalysts.

We investigated the catalytic performance of glycerol conversion to acrolein on nickel phosphates samples (NiP-T (T = 300,400,500,600, and 700 °C)). The textural property, acidity of the fresh catalyst and carbon content of the used NiP-500 were also determined. The results showed that NiP was amorphous under the appropriate calcination temperature. The textural property, acid amount and strength were important in this reaction. Glycerol conversion was proportional to the acid amount of the sample. After 2 h on stream, NiP-500 with the largest pore size, largest acid amount and largest number of moderate acid sites had the maximum catalytic performance (89% glycerol conversion and 64% acrolein selectivity). NiP-700 showed the lowest performance (48% glycerol conversion and 34% acrolein selectivity), which is due to the lowest surface area, pore size and the lowest acid amount of NiP-700. Moreover, the catalyst deactivation was ascribed to carbon deposition on phosphates during the reaction.

Synthesis and properties of template-free mesoporous alumina and its application in gas phase dehydration of glycerol

Mesoporous alumina was prepared by varying the crystallization time and acid concentration and evaluated in the glycerol dehydration. The samples were characterized by X-ray diffractometry (XRD), specific surface area (SBET), NH3-Temperature-programmed desorption (NH3-TPD), and Temperature-programmed Oxidation (TPO). The gas phase dehydration of glycerol was performed in a fixed-bed quartz tubular reactor at 500 °C using a 10% glycerol aqueous solution. The specific surface area, the crystallinity and the acidic properties of the samples were dependent on both the crystallization time and the acid concentration. The catalytic properties are related to the specific surface area and acidic characteristics. Samples with higher crystallization time showed lower surface area and lower amount of acid sites. Samples with the highest acid concentration solution had the higher surface area. Conversions of glycerol above 92% were obtained for all samples. The selectivity for dehydration products was strongly related to the amount and strength of acid sites.

Gas-Phase Dehydration of Glycerol into Acrolein in the Presence of Polyoxometalates

The activity of catalytic systems based on polyoxometalates in the gas-phase dehydration of glycerol into acrolein has been investigated. The catalysts synthesized have been characterized using FTIR spectroscopy, X-ray phase analysis, scanning electron microscopy, and low-temperature nitrogen adsorption/desorption (BET method). It has been shown that the nature of the surface acid sites of catalysts has a significant effect on the direction of glycerol transformation. An increase in the fraction of Bronsted acid sites in the catalyst leads to an increase in the yield of acrolein. At the same time, an increase in the fraction of Lewis acid sites in the catalyst leads to an increase to the yield of acetol, which is formed through a competing route. Among the catalytic systems considered, the best results have been obtained with the silicomolybdic acid anion supported onto alumina (SiMo/A). The main patterns of the gas-phase glycerol dehydration have been studied using SiMo/A as a catalyst, and the conditions responsible for the highest yield of acrolein have been determined.

Gas-phase dehydration of glycerol to acrolein over different metal phosphate catalysts

We conducted a comparative study of gas phase dehydration of glycerol to acrolein over aluminium phosphate, iron phosphate and nickel phosphate catalysts prepared by a simple replacement reaction method (AlP, FeP and NiP). The textural properties, acid amounts, acid types, and coke contents of the samples were studied. The results showed that all metal phosphate catalysts remained in an amorphous state. The glycerol conversion was proportional to the acid amount of metal phosphate catalyst in the glycerol dehydration reaction. Higher value of B/L was more likely to produce acrolein. Among the metal phosphate catalysts, FeP showed superior performance due to its suitable textural and acid properties. After 2 h on stream, high glycerol conversion (96%), acrolein selectivity (82%) and acrolein yield (79%) were achieved on the FeP catalyst at 280 °C. The catalyst deactivation was ascribed to carbon deposition on the catalyst surface blocking the active sites during the glycerol dehydration reaction.

Preparation of alumina with different precipitants for the gas phase dehydration of glycerol and their characterization by thermal analysis

Mesoporous aluminas were prepared using different precipitants at different pH of precipitation and evaluated in the gas phase dehydration of glycerol. Samples were characterized by thermal analysis techniques (TGA, NH3-TPD and TPO), specific surface area measurements, XRD and SEM. The gas phase dehydration of glycerol was performed in a fixed bed quartz tubular reactor at 773 K using a 10% glycerol aqueous solution. Thermogravimetric analysis has revealed the temperature regions of decomposition for the prepared alumina catalyst precursors. The specific surface area and the crystallinity of the samples were dependent on both the pH and the used precipitating agent. Samples precipitated with NaOH presented higher density of acid sites than the samples prepared with Na2CO3, regardless of precipitation pH. The catalytic properties of the prepared aluminas are mainly related to the specific surface area and to acidic characteristics. Conversions of glycerol above 85% were obtained for all samples. The selectivity for glycerol dehydration was strongly related to the amount and strength of acid sites. The best result for dehydration was obtained for samples prepared with NaOH and precipitated at pH = 5. These results are related to the higher specific surface area, greater amount of acid sites and the higher ratio of weak acid sites. TPO revealed the amount of carbon deposited on the catalysts. Samples that showed higher carbon formation also showed a higher production of light olefins, indicating that the formation of carbon is related to the formation of these byproducts. NH3-TPD has shown the ratio of different acid sites on the surface of alumina samples that makes possible to estimate the correlation between the acidity and the catalytic properties.

Highly active and durable WO3/Al2O3 catalysts for gas-phase dehydration of polyols.

Gas-phase glycerol dehydration over WO3/Al2O3 catalysts was investigated. WO3 loading on γ-Al2O3 significantly affected the yield of acrolein and the catalyst with 20 wt% WO3 loading showed the highest activity. The WO3/Al2O3 catalyst with 20 wt% WO3 loading showed higher activity and durability than the other supported WO3 catalysts and zeolites. The number of Brønsted acid sites and mesopores of the WO3/Al2O3 catalyst did not decrease after the reaction, suggesting that glycerol has continuous access to Brønsted acid sites inside the mesopores of WO3/Al2O3, thereby sustaining a high rate of formation of acrolein. Dehydration under O2 flow further increased the durability of the WO3/Al2O3 catalyst, enabling the sustainable formation of acrolein. In addition, the WO3/Al2O3 catalyst with 20 wt% WO3 loading showed high activity for the dehydration of various polyols to afford the corresponding products in high yield.

Extending Catalyst Life in Glycerol-to-Acrolein Conversion Using Non-thermal Plasma.

Booming biodiesel production worldwide demands valorization of its byproduct of glycerol. Acrolein, an important intermediate chemical, can be produced by gas-phase glycerol dehydration catalyzed by solid acids. Because catalysts that lead to high acrolein selectivity usually deactivate rapidly due to the formation of coke that blocks the active sites on their surface, one major challenge of this method is how to extend the service life of the catalyst. Silica-supported silicotungstic acid (HSiW-Si) is a good example of such a catalyst that shows good activity in glycerol dehydration to acrolein initially, but deactivates quickly. In this study, HSiW-Si was selected to probe the potential of using non-thermal plasma with oxygen-containing gas as the discharge gas (NTP-O2) to solve the catalyst deactivation problem. NTP-O2 was found to be effective in coke removal and catalyst regeneration at low temperatures without damaging the Keggin structure of the HSiW-Si catalyst.

Nanosheet MFI Zeolites for Gas Phase Glycerol Dehydration to Acrolein

To overcome the rapid deactivation of conventional ZSM-5, novel nanosheet MFI zeolites, with different Si/Al molar ratios were well fabricated. It was found that Si/Al molar ratios, do not just affect acid properties, but also determine the morphologies of nanosheet MFI zeolites by changing a-c plane areas of zeolite nanosheets. In reaction of gas phase glycerol dehydration to acrolein, the nanosheet MFI zeolites were much more active and stable than conventional ZSM-5 catalysts, owing to their suitable acidity and unique nanosheet structure. For nanosheet MFI zeolite, with Si/Al = 50 (NMZ-50), the conversion of glycerol is higher than 99% in the initial 12 h, with an acrolein selectivity of 86.6%, better than most previous reports. This superior stability of NMZ-50 can be ascribed to its low coke deposition rate and improved coke tolerance capacity. Additionally, it is interesting to find that Al contents do not just simply affect acid properties, but also determine morphologies of nanosheet MFI zeolites, and thus influence catalytic performance.

Gas phase dehydration of glycerol to acrolein over NaHSO4@Zr‐MCM‐41 catalyst

ABSTRACTThe NaHSO4@Zr‐MCM‐41 catalyst modified by dodecyltrimethoxysilane (DTMS) showed excellent performance during the dehydration of glycerol to acrolein. The glycerol conversion and acrolein selectivity and yield on NaHSO4‐DTMS@Zr‐MCM‐41 (Cat‐2) are higher than that of NaHSO4@Zr‐MCM‐41 (Cat‐1). The dispersity of NaHSO4 is remarkably improved by an appropriate amount of DTMS modification on Cat‐2 because of the steric effect between DTMS and NaHSO4. Therefore, the total acidic amount of Cat‐2 is more than that of Cat‐1, but its acidic intensity is weaker than that of Cat‐1. The more total amount of acidity is beneficial to improve the glycerol conversion, and weaker acid strength leads to less carbon deposition. The amount of Brønsted acid sites is more than that of Cat‐1; however, the amount of Lewis acid sites is less, so the B/(B + L) of Cat‐2 is higher, which is beneficial and improves the acrolein selectivity. The Cat‐2 catalyst has proper hydrophobicity by grafting the silane group on the support surface, which can suppress the leaching of NaHSO4 because the water that existed in the glycerol aqueous solution and that was produced in the reaction is not easily adsorbed on the catalyst surface. The appropriate hydrophobicity is beneficial to the quick desorption of products, which can also suppress the coke formation. The stability of Cat‐2 is obviously higher than that of Cat‐1 because the coke that was deposited and the S elemental leaching on Cat‐1 are more serious than that of Cat‐2. The reaction pathway of the dehydration of glycerol was proposed, and the reaction conditions were optimized on the Cat‐2 catalyst.

Gas phase dehydration of glycerol to acrolein over Cs2.5H0.5PW12O40/Zr-MCM-41 catalysts prepared by supercritical impregnation

The Cs2.5H0.5PW12O40/Zr-MCM-41 samples were synthesized by supercritical impregnation and investigated in gas phase dehydration of glycerol to acrolein. The Cs2.5H0.5PW12O40/Zr-MCM-41 sample prepared by conventional wet impregnation was also examined for comparison. The catalysts were studied using X-ray diffraction, N2 adsorption, the temperature-programmed desorption of ammonia, infrared spectroscopy after pyridine adsorption, thermogravimetric analysis, 31P MAS NMR, inductively coupled plasma-atomic emission spectrometry and CHNS analysis. According to the results of characterization, the preparation condition of supercritical carbon dioxide facilitated the improvement of the Cs2.5H0.5PW12O40 dispersion on Zr-MCM-41. The specific surface area, pore volume and pore size were increased, and the total amount of surface acid sites and the ratio of Brønsted acid sites to total acidity were increased. The strong acid sites were transformed into the weak and medium acid sites. The formation of coke was retarded. Therefore, the glycerol conversion, acrolein selectivity and yield, and the catalytic stability were obviously improved.

Glycerol dehydration over micro- and mesoporous ZSM-5 synthesized from a one-step method

Abstract In this work, ZSM-5 zeolites with low Si/Al ratios (25, 50 and 75) were synthesized with a one-step synthesis method and used for the gas-phase glycerol dehydration reaction. The catalysts were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR), N2 adsorption-desorption, scanning electron microscopy (SEM), thermogravimetric analyses (TGA) and temperature programmed desorption of ammonia (NH3-TPD). According to the characterization results, crystalline ZSM-5 zeolites with micro- and mesopores were obtained by the one-step synthesis method. Differences in acidity, related to the Si/Al ratios variation, were observed. The zeolite with Si/Al ratio of 25 showed the best performance during 4 h of the glycerol dehydration reaction with the acrolein selectivity remaining around 70%. The fast diffusion of species in the mesopores and high acidity of the synthesized ZSM-5 favored the acrolein formation from the glycerol dehydration.

Gas phase glycerol dehydration over H-ZSM-5 zeolite modified by alkaline treatment with Na2CO3

Modified H-ZSM-5 zeolite catalysts were studied for the dehydration of glycerol to acrolein in gas phase. The modifications were made by alkaline treatment using Na2CO3 solutions in different concentrations, followed by consecutive ion exchanges with NH4NO3 solution in order to recover the acidic form of the materials. The mild conditions selected (alkali concentrations, time and temperature) allowed to preserve the crystallinity and framework characteristic in the zeolite, selectively removing silicon while maintaining practically all the aluminum content. This process has important advantages over the alkaline treatment with NaOH (at an equivalent pH), such as high selectivity to remove silicon instead of aluminum, and in addition the yield of synthesized material respect to the initial mass is markedly higher (80% vs 50%). This treatment with Na2CO3 solutions leads to multiple positive effects as: increments in the mesoporosity and external surface area; and changes in the nature and accessibility to the acid active sites. The catalytic activity and selectivity were greatly improved. Further, the deactivation by coke deposition was attenuated.

Highly Efficient Dehydration of Glycerol to Acrolein Over Isomorphously Substituted Fe-MFI Zeolites

Gas phase dehydration of glycerol formerly in aqueous solution to acrolein was studied over iron MFI zeolites prepared by post-synthesis isomorphous substitution with different iron contents, Si/Al and substitution ratios. High Si/Al (60) ratios led to high conversion in presence of air and the insertion of iron in the zeolite framework greatly improved the catalytic performances by altering the amount and the nature of the coke components reducing dramatically the deactivation. Over the most efficient sample, acrolein yield reached a stabilized value of 68% with an excellent selectivity (80%), which ranked this catalyst among the best for the selective conversion of crude glycerol.

Acrolein Production by Gas-Phase Glycerol Dehydration Using PO₄/Nb₂O5 Catalysts.

In this study, modified niobium oxide were prepared to study the addictive effects on the catalytic performance for gas-phase glycerol dehydration. The catalysts were characterized by N2 adsorption/desorption, XRD, NH3-TPD, FT-IR. The amount of phosphoric acid was up to 50 wt% in niobium. As a result, the highest glycerol conversion was achieved over 20 wt% PO4/Nb2O5. It indicates that the optimal amount of phosphoric acid leads the catalyst to have appropriate acidity which is an important factor for gas-phase glycerol dehydration.