Kinetics of Valeric Acid Ketonization and Ketenization in Catalytic Pyrolysis on Nanosized SiO₂, γ‐Al₂O₃, CeO₂/SiO₂, Al₂O₃/SiO₂ and TiO₂/SiO₂

Abstract

Valeric acid is an important renewable platform chemical that can be produced efficiently from lignocellulosic biomass. Upgrading of valeric acid by catalytic pyrolysis has the potential to produce value added biofuels and chemicals on an industrial scale. Understanding the different mechanisms involved in the thermal transformations of valeric acid on the surface of nanometer-sized oxides is important for the development of efficient heterogeneously catalyzed pyrolytic conversion techniques. In this work, the thermal decomposition of valeric acid on the surface of nanoscale SiO , γ-Al O , CeO /SiO , Al O /SiO and TiO /SiO has been investigated by temperature-programmed desorption mass spectrometry (TPD MS). Fourier transform infrared spectroscopy (FTIR) has also been used to investigate the structure of valeric acid complexes on the oxide surfaces. Two main products of pyrolytic conversion were observed to be formed depending on the nano-catalyst used-dibutylketone and propylketene. Mechanisms of ketene and ketone formation from chemisorbed fragments of valeric acid are proposed and the kinetic parameters of the corresponding reactions were calculated. It was found that the activation energy of ketenization decreases in the order SiO >γ-Al O >TiO /SiO >Al O /SiO , and the activation energy of ketonization decreases in the order γ-Al O >CeO /SiO . Nano-oxide CeO /SiO was found to selectively catalyze the ketonization reaction.

Thermal Transformation of Caffeic Acid on the Nanoceria Surface Studied by Temperature Programmed Desorption Mass-Spectrometry, Thermogravimetric Analysis and FT–IR Spectroscopy

“…The interaction between the metal atom and the carboxyl group can be attributed to four types: bidentate chelate or bidentate bridge coordination, monodentate coordination and ionic interaction [50,51]. For bidentate chelate structures the difference between ν as COO − and ν s COO − (∆=ν as COO − − ν s COO − ) is less than 110 cm −1 , for bidentate bridges structures ∆ ≈ 140-190 cm −1 , while monodentate structures and non-dissociated acids are characterized by the highest values of ∆ (∆ = νC=O-νC-O~200-320 cm −1 ) [51].…”

Catalytic Pyrolysis of Aliphatic Carboxylic Acids into Symmetric Ketones over Ceria-Based Catalysts: Kinetics, Isotope Effect and Mechanism

“…Cinnamic acids on the silica surface transform into corresponding ketenes, vinyl-and acetylene-benzenes via three parallel pathways b, e and f: ketenization, decarboxylation, and decarbonylation, [51][52][53]; whereas over the nanoceria surface, only the decarboxylation (f) takes place [54]. A comparative study [41] of pyrolytic decomposition of valeric acid over SiO 2 , Al 2 O 3 , CeO 2 /SiO 2 , Al 2 O 3 /SiO 2 , and TiO 2 /SiO 2 catalysts showed that propylketene forms on the surfaces of all these oxides except for CeO 2 /SiO 2 . The formation of both ketene and ketone was observed on the surface of Al 2 O 3 .…”

“…Rather the loss of the CO 2 or the formation of a new C-C bond are the likely rate-determining steps [26,32,33,36]. Despite the enormous efforts spent on the establishment of the mechanism of this reaction, kinetic studies, isotope effect studies [26,27,36], quantum chemical calculations [30][31][32][33]36], studies of cross-ketonization reactions [28,30,33,38], 13 C/ 12 C exchange studies [34], H/D exchange studies on the catalyst surfaces [28,29,39], experiments with 13 C-labeled acetic acids [28,33,34], Fourier Transform Infrared Spectroscopy FT-IR studies [36,37,[39][40][41], etc., the question of establishing the full details of the ketonization reaction mechanism is still not completely closed [39]. However, linear free energy relationships (LFERs) have never been applied to study of the mechanism of ketonization.…”

“…As was previously established [43,[47][48][49], the ketenization reaction (pathway b, Scheme 3) proceeds on the silica at 300-450 °C with a high level of selectivity. Ketenes formation has also been observed over Al2O3/SiO2, TiO2/SiO2 [41] and aluminosilicates due to the presence of Brönsted acid sites on their surfaces [50]. Cinnamic acids on the silica surface transform into corresponding ketenes, vinyl-and acetylene-benzenes via three parallel pathways b, e and f: ketenization, decarboxylation, and decarbonylation, [51][52][53]; whereas over the nanoceria surface, only the decarboxylation (f) takes place [54] [44].…”

“…The acid-base and redox properties of ceria-based materials have been extensively investigated because the unique catalytic activity enables ceria to catalyze a variety of organic reactions [55][56][57][58]. The surface of ceria is characterized by very low Lewis and Brönsted acid site density and weak acid strength [39], while at the same time extremely high density [59] of strong Lewis-base sites [60], which interact with carboxylic acids to form different types of carboxylates (monodentate, bidentate: chelate and bridge) [40,41,61]. Subsequent transformation of the surface carboxylates leads to the formation of ketones [37,40,41,61].…”