Organic reactions catalyzed over solid acids

Biomass pyrolysis oil has been reported as a potential renewable biofuel precursor. Although several review articles focusing on lignocellulose pyrolysis can be found, the one that particularly focus on lignin pyrolysis is still not available in literature. Lignin is the second most abundant biomass component and the primary renewable aromatic resource in nature. The pyrolysis chemistry and mechanism of lignin are significantly different from pyrolysis of cellulose or entire biomass. Therefore, different from other review articles in the field, this review particularly focuses on the recent developments in lignin pyrolysis chemistry, mechanism, catalysts, and the upgrading of the bio-oil from lignin pyrolysis. Although bio-oil production from pyrolysis of biomass has been proven on commercial scale and is a very promising option for production of renewable chemicals and fuels, there are still several drawbacks that have not been solved. The components of biomass pyrolysis oils are very complicated and related to the properties of bio-oil. In this review article, the details about pyrolysis oil components particularly those from lignin pyrolysis processes will be discussed first. Due to the poor physical and chemical property, the lignin pyrolysis oil has to be upgraded before usage. The most common method of upgrading bio-oil is hydrotreating. Catalysts have been widely used in petroleum industry for pyrolysis bio-oil upgrading. In this review paper, the mechanism of the hydrodeoxygenation reaction between the model compounds and catalysts will be discussed and the effects of the reaction condition will be summarized.

Lignin Pyrolysis Components and Upgrading—Technology Review

Titania–zirconia mixed oxides with various ZrO 2 content in TiO 2 (10, 50 and 90 wt.%) were prepared by the sol–gel method. High specific surface areas (77–244 m 2 /g) were obtained. Acidity determined by NH 3 -TPD and FTIR-pyridine adsorption showed that in mixed oxides the number of acid sites is dramatically increased; it varies from 173 μmol NH 3 /g for TiO 2 to 1226–1456 μmol NH 3 /g for the mixed oxides. FTIR-pyridine adsorption showed the presence of Lewis sites in the catalysts. Basic sites were identified by FTIR-CO 2 adsorption, suggesting the formation of mixed oxides with acid–basic properties. XRD spectra identified anatase in the TiO 2 rich region, amorphous material in the mixed oxide 50–50 TiO 2 –ZrO 2 and tetragonal and monoclinic crystalline phases in the ZrO 2 rich region. Activity in the isopropanol decomposition showed a good correlation between the acid–basic properties and the selectivity to propene, acetone and isopropyl ether. The latter was found as a product which mainly depends of the acid sites density.

Preparation of TiO2–ZrO2 mixed oxides with controlled acid–basic properties

Mesoporous CeO 2 and ZrO 2 were synthesized using hexadecyltrimethyl ammonium bromide (C 16 TMABr) as the template. Their physicochemical properties were investigated using X-ray diffraction (XRD) and N 2 adsorption–desorption methods. Palladium (Pd) 1–10 wt.% was loaded on the mesoporous CeO 2 and ZrO 2 by deposition–precipitation method. The Pd metal surface area, dispersion and crystallite sizes were determined by room temperature H 2 chemisorption on reduced samples. The catalytic activity of Pd supported mesoporous CeO 2 (Pd/CeO 2 -MS) and ZrO 2 (Pd/ZrO 2 -MS) were evaluated for the first time in the vapor phase hydrogenation of phenol in the temperature range between 160 and 230 °C at atmospheric pressure. The reaction over 3% Pd/CeO 2 -MS at 180 °C offered cyclohexanone as the major product (about 50%) along with some amounts of cyclohexanol (about 35%) and cyclohexane (about 15%). The influences of Pd loading, temperature, H 2 /phenol ratio, contact time, and nature of solvent on the catalytic performance were investigated systematically. A 3% Pd/ZrO 2 -MS offered slightly lower phenol conversion but very high selectivity of cyclohexanone (above 90%). The Pd/CeO 2 -MS exhibited a stable activity, while Pd/ZrO 2 -MS underwent deactivation during on-stream operation. Under similar operating conditions, the catalytic performance of Pd/CeO 2 -MS and Pd/ZrO 2 -MS was found to be far better than that obtained using MgO, Al 2 O 3 and MgO-Al 2 O 3 derived from hydrotalcites (MgAl-CHT) as supports. The benefit of using mesoporous oxide support to Pd was also demonstrated by comparing the catalytic activity with the activities of commercial oxide supports.

Vapor phase hydrogenation of phenol over palladium supported on mesoporous CeO2 and ZrO2

This short review summarizes some of the most recently published results concerning the applications of microcalorimetry to the study of the acid/base sites of zeolites, pillared clays and mesoporous materials. The efficiency of thermal analysis techniques for the characterization of the acid/base strength of zeolite materials and their ability to provide information consistent with the catalytic data are discussed.

Microcalorimetry Methods to Study the Acidity and Reactivity of Zeolites, Pillared Clays and Mesoporous Materials

Several zirconia catalysts, differing in their preparative procedures and/or in the addition of dopants, have been tested for the title reaction. The catalysts were characterized by means of calorimetric techniques. The heats of adsorption of the probe molecules, ammonia and carbon dioxide, were measured to obtain information on the acid–base properties of the samples. Both flow and pulse reactors were used to investigate the behaviour of the catalysts. 4-Methylpentan-2-ol undergoes dehydration to give 4-methylpent-1-ene, which is the desired product, and 4-methylpent-2-ene, in variable amounts; skeletal isomers of C6-alkenes can also form. Simultaneous dehydrogenation of the reactant to 4-methylpentan-2-one can also occur. The acid-base properties of the catalyst appear to play a key role in governing the reaction mechanism, a well balanced number of acid and base sites being needed to obtain a good selectivity to the alk-1-ene isomer.

Dehydration of 4-methylpentan-2-ol over zirconia catalysts

Lanthanum and cerium oxides have been tested for the title reaction at 623 K and atmospheric pressure in a flow reactor. Lanthanum oxide (prepared from the corresponding nitrate) gives mainly 4-methylpent-1-ene (80% of the products). Similar results are observed with cerium oxide obtained from the corresponding hydroxide, whereas cerium oxide prepared from nitrate is less selective towards alk-1-enes. In addition to dehydration, dehydrogenation to 4-methylpentan-2-one is also observed to a limited extent for all the catalysts. Information on the acid–base properties of the samples was obtained by adsorption microcalorimetry of ammonia and carbon dioxide and correlated to reaction selectivities. Possible changes in the oxidation state of cerium ions due to the reaction atmosphere are considered. The present results are compared with former data for zirconia catalysts. Modification of cerium oxide via immersion in NaOH solution does not appear to be useful for improving alk-1-ene selectivity.

P. Artizzu

Papers

Dehydration of 4-methylpentan-2-ol over zirconia catalysts

Dehydration of 4-methylpentan-2-ol over lanthanum and cerium oxides

Conversion of 4-methylpentan-2-ol over alkali–metal ion-exchanged X and Y zeolites: a microcalorimetric and catalytic investigation