Towards Efficient Acidic Catalysts via Optimization of SO3H-Organosilane Immobilization on SBA-15 under Increased Pressure: Potential Applications in Gas and Liquid Phase Reactions

In this paper, the optimization of the synthesis of catalysts based on acidic mesoporous silica of the SBA-15 type by post-synthesis immobilization of 3-(trihydroxysilyl)-1-propanesulfonic acid (TPS) under increased pressure up to 20 bar is reported. Sample structures and composition were examined by XRD measurement, low-temperature N2 adsorption/desorption and elemental analysis. The catalytic activities of the materials obtained were determined in both gas and liquid phase processes, i.e., by esterification of acetic acid and glycerol dehydration, respectively. The optimum pressure for modification leading to the highest number of acidic sites was found to be 10 bar. The final material was very active and stable in liquid phase processes; however, the stability in the gas-phase process was unsatisfactory due to the loss of sulphonic species from the catalyst surface.

Insight into Active Centers and Anti-Coke Behavior of Niobium-Containing SBA-15 for Glycerol Dehydration

Niobium containing SBA-15 was prepared by two methods: impregnation with different amounts of ammonium niobate(V) oxalate (Nb-15/SBA-15 and Nb-25/SBA-15 containing 15 wt.% and 25 wt.% of Nb, respectively) and mixing of mesoporous silica with Nb2O5 followed by heating at 500 °C (Nb2O5/SBA-15). The use of these two procedures allowed obtaining materials with different textural/surface properties determined by N2 adsorption/desorption isotherms, XRD, UV-Vis, pyridine, and NO adsorption combined with FTIR spectroscopy. Nb2O5/SBA-15 contained exclusively crystalline Nb2O5 on the SBA-15 surface, whereas the materials prepared by impregnation had both metal oxide and niobium incorporated into the silica matrix. The niobium species localized in silica framework generated Brønsted (BAS) and Lewis (LAS) acid sites. The inclusion of niobium into SBA-15 skeleton was crucial for the achievement of high catalytic performance. The strongest BAS were on Nb-25/SBA-15, whereas the highest concentration of BAS and LAS was on Nb-15/SBA-15 surface. Nb2O5/SBA-15 material possessed only weak LAS and BAS. The presence of the strongest BAS (Nb-25/SBA-15) resulted in the highest dehydration activity, whereas a high concentration of BAS was unfavorable. Silylation of niobium catalysts prepared by impregnation reduced the number of acidic sites and significantly increased acrolein yield and selectivity (from ca. 43% selectivity for Nb-25/SBA-15 to ca. 61% for silylated sample). This was accompanied by a considerable decrease in coke formation (from 47% selectivity for Nb-25/SBA-15 to 27% for silylated material).

Nonthermal Plasma Induced Fabrication of Solid Acid Catalysts for Glycerol Dehydration to Acrolein

The feasibility of fabricating better solid acid catalysts using nonthermal plasma (NTP) technology for biobased acrolein production is demonstrated. NTP discharge exposure was integrated in catalyst fabrication in air or argon atmosphere. The fabricated catalysts were characterized by Brunauer–Emmett–Teller surface area analysis, temperature-programmed desorption of ammonia, X-ray powder diffraction and Fourier-transform infrared spectroscopy of pyridine adsorption, in comparison to regularly prepared catalysts as a control. Further, kinetic results collected via glycerol dehydration experiments were compared, and improvement in acrolein selectivity was displayed when the catalyst was fabricated in the argon NTP, but not in the air NTP. Possible mechanisms for the improvement were also discussed.

Glycerol Hydrogenolysis with In Situ Hydrogen Produced via Methanol Steam Reforming: The Promoting Effect of Pd on a Cu/ZnO/Al2O3 Catalyst

The glycerol hydrogenolysis to produce 1,2-propanediol without using externally supplied hydrogen was investigated using methanol present in crude glycerol to provide in situ hydrogen via its steam reforming reaction. This paper focuses on the promoting effect of Pd on the reactivity of a Cu/Zn/Al2O3 catalyst. Adding 2 wt% Pd onto a Cu/ZnO/Al2O3 catalyst significantly improved the selectivity to 1,2-propanediol from 63.0% to 82.4% and the glycerol conversion from 70.2% to 99.4%. This enhancement on the catalytic activity by Pd is mainly due to the improved hydrogenation of acetol, which is the intermediate formed during the glycerol dehydration. The rapid hydrogenation of acetol can shift the reaction equilibrium of glycerol dehydration forward resulting in a higher glycerol conversion. The improved reducibility of the catalyst by Pd allows the catalyst to be reduced in situ during the reaction preventing any loss of catalyst activity due to any potential oxidation of the catalyst. The catalyst was slightly deactivated when it was firstly recycled resulting in a 5.4% loss of glycerol conversion due to the aggregation of Cu and the deactivation became less noticeable upon further recycling.