Guerbet Coupling Research Articles

Ni-Based Hydrotalcite (HT)-Derived Cu Catalysts for Catalytic Conversion of Bioethanol to Butanol.

Catalytic conversion of biomass-derived ethanol into n-butanol through Guerbet coupling reaction has become one of the key reactions in biomass valorization, thus attracting significant attention recently. Herein, a series of supported Cu catalysts derived from Ni-based hydrotalcite (HT) were prepared and performed in the continuous catalytic conversion of ethanol into butanol. Among the prepared catalysts, Cu/NiAlOx shows the best performance in terms of butanol selectivity and catalyst stability, with a sustained ethanol conversion of ~35% and butanol selectivity of 25% in a time-on-stream (TOS) of 110 h at 280 °C. While for the Cu/NiFeOx and Cu/NiCoOx, obvious catalyst deactivation and/or low butanol selectivity were obtained. Extensive characterization studies of the fresh and spent catalysts, i.e., X-ray diffraction (XRD), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Hydrogen temperature-programmed reduction (H2-TPR), reveal that the catalysts' deactivation is mainly caused by the support deconstruction during catalysis, which is highly dependent on the reducibility. Additionally, an appropriate acid-base property is pivotal for enhancing the product selectivity, which is beneficial for the key process of aldol-condensation to produce butanol.

Highly Selective and Stable Cu Catalysts Based on Ni-Al Catalytic Systems for Bioethanol Upgrading to n-Butanol.

The catalytic upgrading of ethanol into butanol through the Guerbet coupling reaction has received increasing attention recently due to the sufficient supply of bioethanol and the versatile applications of butanol. In this work, four different supported Cu catalysts, i.e., Cu/Al2O3, Cu/NiO, Cu/Ni3AlOx, and Cu/Ni1AlOx (Ni2+/Al3+ molar ratios of 3 and 1), were applied to investigate the catalytic performances for ethanol conversion. From the results, Ni-containing catalysts exhibit better reactivity; Al-containing catalysts exhibit better stability; but in terms of ethanol conversion, butanol selectivity, and catalyst stability, a corporative effect between Ni-Al catalytic systems can be clearly observed. Combined characterizations such as XRD, TEM, XPS, H2-TPR, and CO2/NH3-TPD were applied to analyze the properties of different catalysts. Based on the results, Cu species provide the active sites for ethanol dehydrogenation/hydrogenation, and the support derived from Ni-Al-LDH supplies appropriate acid-base sites for the aldol condensation, contributing to the high butanol selectivity. In addition, catalysts with strong reducibility (i.e., Cu/NiO) may be easily deconstructed during catalysis, leading to fast deactivation of the catalysts in the Guerbet coupling process.

Copper Supported on MgAlOx and ZnAlOx Porous Mixed-Oxides for Conversion of Bioethanol via Guerbet Coupling Reaction

The direct conversion of biomass-derived ethanol to high-valued-added chemicals has attracted widespread attention recently due to the great economic and environmental advantages. In the present study, the conversion of bioethanol through the Guerbet coupling process was studied in a fixed-bed reactor for MgAlOx and ZnAlOx mixed-oxides supported Cu catalysts. From the results, Cu adding into the system greatly enhance the dehydrogenation of ethanol and increase the H-transfer in the course of Guerbet coupling process. Simultaneously, the porous mixed-oxides provide the acid-base property of the catalysts for intermediate transformation. Notably, for Cu/MgAlOx, the main product of ethanol conversion is butanol, but for Cu/ZnAlOx, the primary product is ethyl acetate. Characterizations such as X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and CO2 temperature programmed desorption (TPD) were carried out to evaluate the structure and property of the catalysts. In combination with the catalytic performances with the characterization results, the synergistic catalytic effect between metal sites and acid-base sites were elaborated.

Cover Feature: High Performing and Stable Cu/NiAlOx Catalysts for the Continuous Catalytic Conversion of Ethanol into Butanol (ChemCatChem 17/2022)

The Cover Feature shows high-performing and stable Cu/NiAlOx catalysts derived from NiAl-hydrotalcite for continuous catalytic conversion of ethanol into butanol, which is one of the pivotal processes in biorefinery for the production of renewable fuels and value-added chemicals. In their Research Article, Y. Tan, W. Luo, Y.-J. Ding and co-workers report that significant enhancement in catalytic performance in terms of activity, selectivity and stability, can be attained by adding Cu properly into the NiAl-hydrotalcite. An optimal compromise between the ethanol conversion and butanol selectivity can be sustained over the optimal Cu/NiAlOx catalyst for 1000 h during the long-term stability test under moderate reaction condition, reflecting one of the state-of-the-art performances for non-noble metal-based heterogeneous catalysts in this Guerbet coupling process. More information can be found in the Research Article by Y. Tan, W. Luo, Y.-J. Ding and co-workers.

Reaction chemistry of ethanol oligomerization to distillate-range molecules using low loading Cu/MgxAlOy catalysts

We study ethanol oligomerization to higher alcohols and other oxygenates with a 0.3 wt. %Cu/Mg2.9AlO catalyst. This reaction involves more than 130 products in a complicated reaction network. The selectivity towards diesel fuel precursor compounds (hereafter ‘DFPC’) increased with conversion until reaching a plateau at an ethanol conversion of ~70 %. Alcohol selectivity was found to follow Schultz-Flory distribution at all studied conversions. Larger sized alcohols then are formed mostly by chain-growth mechanisms via surface reactions of adsorbed oligomers with ethanol-based monomers. Higher esters are formed from alcohols and aldehydes in a series reaction mechanism. Moreover, C6+ ester and C4+ ketones selectivities increase as conversion increases. We also found that C4+ alcohols most likely undergo Guerbet coupling with the studied catalyst to form even higher alcohols once, and that the oxygen of these alcohols is active as a nucleophile, resulting in the selective formation of esters if the starting alcohol is branched. Finally, we performed several cofeed studies varying ethanol-to-H2 inlet partial pressures, and adding acetaldehyde and ethyl acetate as cofeeds to ethanol at different concentrations. We conclude from these experiments that acetaldehyde concentration controls reaction chemistry, with conditions favoring larger concentrations of the molecule promoting both alcohol coupling and ester formation, and conditions leading to lower concentrations of acetaldehyde resulting in higher alcohol selectivity at the expense of esters and higher aldehydes.

Guerbet coupling of methanol catalysed by titanium clusters

Alcohol coupling, also known as the Guerbet reaction, is a potentially important process to increase the value of short chain hydrocarbons. We present here an insight of C-C bond formation in the coupling of methanol to higher alcohols by Ti clusters. We have synthesized Ti13 clusters by laser ablation in methanol, and studied the catalysis for methanol coupling reactions using high resolution mass spectroscopy and also density-functional theory calculations. The experimental findings concurs with the calculated kinetic- and thermodynamic- allowed reaction pathways, helping to understand the catalytic mechanism of alcohols conversion chemistry on the transition metal catalysts.

Copper and lanthanum mixed oxides as catalysts for ethanol Guerbet coupling: The role of La3+ on the production of long‐chain alcohols

AbstractMixed oxides derived from the calcination of modified hydrotalcites precursors with copper and lanthanum were synthetized by the co‐precipitation method and characterized and applied as catalysts for ethanol Guerbet coupling reactions. The oxides obtained were used to convert ethanol into 1‐butanol and long‐chain alcohols, mostly 1‐hexanol. The selectivity for 1‐butanol and 1‐hexanol reached 62% and 10% for Cu‐MgAl(O) at 280°C. The addition of 1 mol% of La3+ (CuLa‐MgAl(O)) increased the number of basic sites comparing to Cu‐MgAl(O) changing the selectivity for 1‐butanol and 1‐hexanol reaching 16.70 and 41.7% at 280°C. The addition of lanthanum and consequently the increase on the catalyst basicity was related to the obtainment of long‐chain alcohols due to the enolate formation during the aldol condensation of Guerbet coupling.

Catalyst Performance Studies on the Guerbet Reaction in a Continuous Flow Reactor Using Mono- and Bi-Metallic Cu-Ni Porous Metal Oxides

Higher alcohols like 1-butanol are considered important biofuels with superior properties compared to the more readily available bio-ethanol. An attractive route to prepare 1-butanol from ethanol is the Guerbet reaction. We here report the use of hydrotalcite-derived mono- (Cu-PMO or Ni-PMO) and bi-metallic (CuNi-PMO) porous metal oxide catalysts for the Guerbet coupling of ethanol to 1-butanol in a continuous flow reactor (320 °C, 0.1 MPa, LHSV = 15 mL g−1 h−1) at extended times on stream (~160 h). Two distinct regimes with different product distributions were observed for the Cu-PMO and CuNi-PMO catalyst with time on stream. At the start of the run, the initial conversion of ethanol dropped from about 85% to less than 20% after 60 h and acetaldehyde was the main product (regime 1). At prolonged times on stream (60–160 h), fairly constant low conversions of ethanol (14%) were observed and 1-butanol was the main product (regime 2). Performance of the monometallic Cu-PMO catalyst in terms of 1-butanol yield and stability was lower compared to the bi-metallic CuNi-PMO. Detailed catalyst characterization studies (XRD, H2-TPR, sorption of acrylic acid, TGA, TEM, HAADF-STEM, and EDS mapping) on both fresh and spent CuNi-PMO taken at various times on stream was performed to determine the changes in catalyst morphology and composition during a run, and particularly to obtain information on changes in catalyst structure operating in regime 1 or 2. The change in chemoselectivity is in line with an increase in basicity of the catalyst at extended runtimes.

Kinetic Modeling of Alcohol Oligomerization over Calcium Hydroxyapatite

Guerbet coupling to oligomerize ethanol to heavy alcohols over a hydroxyapatite (HAP) catalyst is explored through the development and application of a reaction kinetics model that describes coupli...

Yttria-Stabilized Zirconia as a High-Performance Catalyst for Ethanol to n-Butanol Guerbet Coupling.

It has been shown that yttria-stabilized zirconia is an effective catalyst for ethanol to n-butanol Guerbet coupling. The variation of the calcination temperature allows an improvement in the catalytic characteristics of this material via stabilization of the tetragonal phase of zirconia, having higher basicity than the monoclinic one. The treatment of yttria-stabilized zirconia at an optimal calcination temperature of 500 °C induces the increase in surface basicity required for the aldol condensation step, along with a decrease in surface acidity, which is responsible for the side reaction such as ethylene formation. The catalyst obtained significantly exceeds in selectivity and n-butanol yield than individual zirconia and other oxide systems which have been studied in this reaction.

Correction: Catalytic synthesis of distillate-range ethers and olefins from ethanol through Guerbet coupling and etherification

Correction for ‘Catalytic synthesis of distillate-range ethers and olefins from ethanol through Guerbet coupling and etherification’ by Nathaniel M. Eagan et al., Green Chem., 2019, 21, 3300–3318.

Catalytic synthesis of distillate-range ethers and olefins from ethanol through Guerbet coupling and etherification

Ethanol can be converted to heavy diesel ethers and jet fuel precursor olefins through sequential Guerbet coupling and dehydration.

Hydrogen transfer reactions relevant to Guerbet coupling of alcohols over hydroxyapatite and magnesium oxide catalysts

The rates of double bond hydrogenation, deuterium exchange, and benzyl alcohol dehydrogenation were compared to those of ethanol coupling.

Influence of surface acid and base sites on the Guerbet coupling of ethanol to butanol over metal phosphate catalysts

Hydroxyapatite (HAP; Ca10(PO4)6(OH)2) is a well-recognized catalyst for the Guerbet coupling of ethanol to butanol. In an effort to explore the role of the anion components of the catalyst, steady-state, gas phase catalytic coupling of ethanol to butanol was investigated at 633K and atmospheric pressure over beta tricalcium phosphate (β-TCP; β-Ca3(PO4)2) and fluorine-substituted hydroxyapatite (FAP; Ca10(PO4)6F2). Both β-TCP and FAP catalysts were catalytically active for butanol formation, leading to ∼35% selectivity at low conversion, suggesting that the PO43− group contributes to the active acid-base site pair for butanol formation during ethanol coupling over HAP. Co-feeding water, a product of ethanol coupling, revealed weaker inhibition of the rate over HAP relative to MgO, confirming the potential negative influence of strong base sites on coupling catalysts. Catalytic reactions of ethanol over Mg3(PO4)2, β-TCP, and Sr3(PO4)2 catalysts demonstrated the importance of Lewis acidity of the metal phosphates on the reaction. Relatively strong Lewis acid sites on the Mg3(PO4)2 surface (Mg2+ cations) favored undesired ethanol dehydration to ethene (36% selectivity) and diethyl ether (52% selectivity) whereas the Sr3(PO4)2 catalyst predominantly catalyzed ethanol dehydrogenation to acetaldehyde (91% selectivity) at a rate significantly higher than that observed over the other catalysts. Evidently, the β-TCP exposes intermediate-strength Lewis acid sites provided by surface Ca2+ cations that enable the material to effectively convert ethanol to butanol with 35% selectivity.

Carbon Supported Alkaline Catalysts for Guerbet Coupling of Bioethanol

Bioethanol valorization to butanol by continuous heterogeneous catalytic self-alkylation provides an option for the fuel industry. Dimerization of ethanol (E) was studied over activated carbon supported metal loaded and metal-free alkaline catalysts using a flow-through tube microreactor, applying WHSVE of 0.20-2.0 gE/h gcat. , 21 bar total pressure and 275-400°C reaction temperature. Activated carbon (AC) was impregnated with various alkaline salts as the catalysts of the aldol reaction. Different basicity of the investigated catalysts resulted in differing catalytic activities. Butanol was the product of highest yield. Longer chain primary aliphatic alcohols, having even number of carbon atoms were also obtained with yield decreasing significantly with increasing chain length.

Guerbet alkylation of acetone by ethanol and reduction of product alkylate to alkane over tandem nickel/Mg,Al-hydrotalcite and nickel molybdate/γ-alumina catalyst systems

Single and double Guerbet alkylation of acetone with alcohols and hydrodeoxygenation of the produced alkylates were investigated. A flow-through tube reactor system, comprised of two subsequent fixed catalyst beds, was used. The first catalyst bed contained the catalyst for Guerbet coupling, namely activated hydrotalcite-supported nickel or palladium catalyst, whereas the second catalyst bed contained γ-alumina-supported nickel molybdate catalyst. The catalysts, pre-reduced in H2 flow at 450 °C, were contacted with a mixture of acetone and ethanol in flowing H2 at a total pressure of 21 bar and at a temperature in the range of 150–350 °C. The molar ratio of the acetone-ethanol mixture was 1:2. The WHSV of the mixture was varied between 1 and 3 gAE h−1 g cat. −1 . In the first reactor, C5–C19 mono- or dialkylated ketones were formed. C4, C6, C8 aldehydes and alcohols were also detected. The oxygenate products leaving the first catalyst bed were reduced by H2 to the corresponding alkanes in the second reactor. Catalytic alcohol coupling and ketone hydrodeoxygenation were separately studied using ethanol and 4-pentanone as a model reactant over the catalyst of the first and the second catalyst bed, respectively.

Efficient Catalytic Conversion of Ethanol to 1-Butanol via the Guerbet Reaction over Copper- and Nickel-Doped Porous

The direct conversion of ethanol to higher value 1-butanol is a catalytic transformation of great interest in light of the expected wide availability of bioethanol originating from the fermentation of renewable resources. In this contribution we describe several novel compositions of porous metal oxides (PMO) as highly active and selective catalysts for the Guerbet coupling of ethanol to 1-butanol in the temperature range 180–320 °C. The novel PMO catalysts that do not contain any noble metals are obtained by calcination of a series of hydrotalcite precursors synthesized through modular procedures. In particular, catalyst compositions simultaneously containing Cu and Ni dopants have shown excellent catalytic activities. Up to 22% 1-butanol yield at 56% ethanol conversion was reached in a batch mode; recycling and leaching tests showed excellent robustness of the new catalysts. An extensive characterization by means of several techniques such as powder XRD, SEM, TEM, BET, and NH3- and CO2-TPD was performed...

Multiproduct Steady-State Isotopic Transient Kinetic Analysis of the Ethanol Coupling Reaction over Hydroxyapatite and Magnesia

The Guerbet coupling of ethanol into butanol was investigated using multiproduct steady-state isotopic transient kinetic analysis (SSITKA) in a comparative study between stoichiometric hydroxyapatite (HAP) and magnesia (MgO) catalysts at 613 and 653 K, respectively. The steady-state catalytic reactions were conducted in a gas-phase, fixed-bed, differential reactor at 1.3 atm total system pressure. Multiproduct SSITKA results showed that the mean surface residence time of reactive intermediates leading to acetaldehyde was significantly shorter than that of intermediates leading to butanol on both HAP and MgO. This finding may suggest that the dehydrogenation of ethanol to acetaldehyde is fast on these surfaces compared with C–C bond formation. If adsorbed acetaldehyde is a key reaction intermediate in the Guerbet coupling of ethanol into butanol, then SSITKA revealed that the majority of adsorbed acetaldehyde produced on the surface of MgO desorbs into the gas-phase, whereas the majority of adsorbed acetal...

Heterogeneous Catalysts for the Guerbet Coupling of Alcohols

Alcohol coupling, also known as the Guerbet reaction, is a potentially important process to increase the value of short chain alcohols. Metal oxides, metal phosphates, and supported transition metals, such as copper, are important components of heterogeneous catalysts for the reaction. However, the wide variety of catalyst compositions, reaction conditions, and reactor configurations used to study the reaction complicate a direct comparison of various catalysts. In this review, rates over different catalysts will be compared, the influences of the acid and base properties of the catalyst on product selectivity will be presented, and possible reaction paths to accomplish alcohol coupling will be discussed.