(363ak) Mesoporous Organosilica As Catalyst Support for Aqueous Hydrogenation of Phenol: The Effect of Aromatic Content and Amine Loading of the Support

Research Interests - Heterogeneous Catalysis, Catalyst Synthesis, Catalyst characterization, Multiphase reaction system, Reaction Engineering, Energy and Sustainability, Material Science

Bridged polysilsesquioxanes are highly versatile silica-based organic-inorganic hybrid (OIH) materials derived from precursors consisting of an organic bridging group and at least two trifunctional silyl groups. Chemical and textural properties of the material can be modified by appropriate choice of precursor. This offers a great scope for synthesizing catalyst scaffolds with tailored morphologies and functionalities suitable for a particular reaction. In this research, we have used bis (trimethoxysilyl ethyl) benzene (BTEB) as a precursor to synthesize the silica based OIH material or organosilica (OS). Owing to the presence of organic groups, BTEB based OS is hydrophobic in nature. Results of static vapor phase adsorption experiment with acetone and water, established the hydrophobic nature of the material. Ratio of adsorbed acetone to water at equilibrium was higher for our OS compared to conventional inorganic silica such as SBA-15. Due to hydrophobic nature of OS, it has selective affinity to organics in aqueous solutions and it enhances the local concentration of the organic reactant near the active sites, hence, promoting the reaction kinetics. OS being hydrophobic also preserves the mechanical integrity of the catalyst during aqueous phase reaction and protects the active metal sites, from water, sulphate and chloride poisoning, unlike conventional supports like Al₂O₃. This has been established from the previous hydrodechlorination of trichloroethylene (groundwater pollutant) work of Ozkan laboratory where the swellable counterpart of OS referred as SOMS was used as a support. These properties of the OS material allow us to use environment friendly and abundantly available water as a solvent.

This research is focused on reactions including bio-oil derived molecules obtained from lignocellulosic biomass such as phenols and its derivatives. Lignocellulosic bio-oil is an emerging source of useful chemicals and fuels. In this research, we have studied aqueous phase hydrogenation of phenol, a model molecule of lignin bio-oil. Hydrogenation of phenol yields cyclohexanone and cyclohexanol. These chemicals are used as solvents and as starting materials for manufacturing nylon and esters. BTEB-based hybrid material is used as a support for a palladium (Pd) catalyst in aqueous phase hydrogenation of phenol. Pd was incorporated on the supports using incipient wetness impregnation (IWI) followed by reduction using NaBH₄. The reactions were performed in a 300 ml autoclave batch reactor at 150^oC-200^oC and 50 bar H₂ pressure. The activity of BTEB based organo-silica supported catalysts was compared with Pd supported on conventional mesoporous inorganic silica (Pd/SBA-15) and commercial Pd/activated carbon(AC) catalyst. BTEB based organo-silica supported catalysts showed significantly better conversion than Pd/SBA-15 and the commercial Pd/AC catalyst. The higher activity of our catalysts could be attributed to the presence of organic groups in the support which improved Pd dispersion and phenol adsorption on the catalyst. TEM results showed that Pd dispersion on OS catalysts was uniform with average particle size of 5-8 nm while on Pd/SBA-15 large agglomerated Pd particles with non-uniform dispersion were observed. Ex-situ phenol - temperature programmed desorption(TPD) experiment was performed with infrared spectroscopy and results revealed higher strength of adsorption of phenol on OS supports compared to SBA-15. Thus, these research findings proved that OS could be an excellent support for synthesizing noble metal catalysts with better dispersion and better aqueous phase phenol hydrogenation activity.

The effect of aromatic content of the support on the activity of the catalyst was explored in the study. Bis (trimethoxy silyl) ethane was used to change the aromatic content of the support. Activity of the catalyst increased as the aromatic content of the support decreased from 100% to 60%. ²⁹Si and ¹³C NMR revealed that the extent of crosslinking decreased with decreasing the aromatic content of the support. This resulted in an increase in the Si-OCH₃ groups present on the surface which could improve adsorption of phenol and thus, improve the catalytic activity. Further reduction of the aromatic content of the samples to below 60% led to a decrease in phenol conversion. This could possibly be due to higher hydrophilicity of these materials. To deconvolute the interactions of the polar hydroxyl group of phenol and the aromatic ring of phenol with the supports, a non-polar probe molecule, toluene was used to study the interactions with the support. At lower aromatic content, decreased Ï€- Ï€ interaction between toluene and the phenyl group of the support showed a significant reduction in toluene adsorption. This study showed that an optimum amount of aromatic content of the support is necessary to obtain good activity of the catalyst in phenol hydrogenation reaction.

Selective phenol hydrogenation to cyclohexanone is of particular interest as at higher catalytic activity, further hydrogenation of cyclohexanone can lead to cyclohexanol formation and low cyclohexanone yield. The presence of basic sites on the support has shown to enhance the catalytic activity of phenol hydrogenation and its selectivity to cyclohexanone by improving adsorption of phenol and metal-support interaction. We have investigated the effect of amine-site incorporation on phenol hydrogenation selectivity to cyclohexanone. The basic sites were incorporated using bis[(3-trimethoxysilyl)propyl]amine (BTMSPA) as a co-precursor along with BTEB during the sol-gel synthesis process. The supports were characterized using infrared spectroscopy, nitrogen physisorption. To confirm the presence of amine sites on the support and their accessibility, the catalyst was characterized using surface sensitive X-ray photoelectron spectroscopy. Phenol hydrogenation in aqueous phase showed that as the amount of amine sites on the catalyst support increased up to 20%, selectivity to cyclohexanone increased with no significant change in the phenol conversion. In conclusion, the current work has showed that BTEB based hybrid OS can be an excellent support for upgrading the aromatics of the bio-oil to valuable chemicals. Simple synthesis procedure of OS allows facile incorporation of different functionalities.