Catalytic Conversion of Biomass-derived Compounds to C4 Chemicals

As society considers ways to transition to more sustainable ways of living, how to produce the chemicals that enable the high standards of living of modern society needs to be considered. Currently, the vast majorities of chemicals ultimately come from non-renewable petroleum feedstocks. An alternative is to use renewable biomass resources to produce chemicals. This is attractive because of the vast resources of biomass^1,2  and the potential impact on rural communities.³  For this reason, research on thermochemical, biochemical, and catalytic processes for converting compounds derived from biomass to valuable fuels and chemicals has been intense in recent years.⁴ 

Processes to convert biomass to chemicals must start from a small list of starting chemicals that are most prevalent in biomass, including cellulose, hemicellulose, lignin, and fatty oils. From there, however, a wide variety of directions can be followed. Recognizing similarities between how a petroleum refinery operates and how biomass could potentially be converted to a wide variety of fuels and chemicals, the concept of a “biorefinery” has received considerable attention.⁵  With this model, primary biomass compounds are converted to platform chemicals that can then be converted to a wide variety of final products. An influential report from the US Department of Energy highlighted a number of potential chemical intermediates that could serve as platform chemicals in such a biorefinery.⁶ 

One class of chemicals that could be particularly interesting to produce from biomass are C4 chemicals (chemicals with four carbon atoms). C4 chemicals find uses in numerous areas, including polymer production, fuel additives, and food additives. Biomass is well suited to produce C4 chemicals since the major components of biomass are comprised of more than four carbon atoms (six in the case of cellulose, the most commonly considered starting material). In addition, efficient fermentation routes to produce C4 oxygenates like succinic acid and 2,3-butanediol (23BDO) are well known. The C4 oxygenates could serve as platform molecules to make a variety of final C4 chemicals.

There may also be economic arguments for using biomass resources to produce C4 chemicals. In the United States, the increasing production of shale gas has shifted the chemical industry to rely more heavily on natural gas and less heavily on crude oil and naptha for chemical production. This shift impacts the availability of different hydrocarbons for chemical production. For instance, the increased emphasis on cracking natural gas-derived ethane to produce ethylene in recent years has impacted 1,3-butadiene (BD) production, which has historically been produced from ethylene crackers that use naphtha as the feedstock.⁷  This has led to historically high BD prices, suggesting a potential market for a biomass-derived product.

This chapter considers catalytic chemistry for producing a variety of C4 chemicals, including succinic acid, C4 diols, γ-butyrolactone (GBL), tetrahydrofuran (THF), BD, butene, butanol, isobutanol, 2,3-butanedione and methyl ethyl ketone (MEK). In particular, it considers the catalyst properties that favor production of C4 chemicals, the reaction mechanisms importance in catalytically producing C4 chemicals from biomass-derived compounds, and the practical implications of research in the area of C4 chemical production.

This review considers catalytic approaches for producing C4 chemicals from biomass. However, direct conversion of biomass to C4 chemicals at high selectivity is quite challenging. For this reason, research in producing C4 chemicals from biomass typically follows a hybrid approach, where biochemical processes first convert biomass-derived sugars to key platform chemicals, which can then be converted to the desired chemical via a catalytic process. Such an approach, where 23BDO is considered as the platform chemical, is depicted in Fig. 1.

In this section, several potential platform chemicals for producing C4 chemicals are described.

Succinic acid is one of the most promising bio-derived platform chemicals because it could potentially replace the current maleic anhydride C4 platform. Succinic acid can be used as a precursor of many industrially important chemicals including adipic acid, 1,4-butanediol (14BDO), THF, N-methyl pyrrolidinone, 2-pyrrolidinone, succinate salts and GBL and can also be converted into biobased polymers such as nylons or polyesters.⁸  In 2004, succinic acid was identified as one of the 12 most promising bio-based chemical building blocks by the US Department of Energy.⁶  The market potential for succinic acid and its immediate derivatives has been projected to be as much as 245 000 t a⁻¹, with an estimated market size for succinic acid-derived polymers being as high as 25 000 000 t a⁻¹.⁹ 

Succinic acid is industrially produced by chemical process from n-butane/BD via maleic anhydride, utilizing the C4-fraction of naphtha. Recent investigations using engineered Mannheimia succiniciproducens gave high yields of succinic acid with little or no formation byproducts.¹⁰ 

2,3-Butanediol (23BDO) is colorless, odorless, strongly hydroscopic, oily liquid or crystals with sweet taste.¹¹  Commercially, the key downstream products of 23BDO have a global annual production of 32 million metric tons, approximate value of US$43 billion.¹²  23BDO has potential applications to produce printing inks, perfumes, fumigants, moistening and softening agents, explosives, plasticizers, foods, and pharmaceuticals.¹³  Meanwhile, it can be readily dehydrated into MEK (an important organic solvent)¹⁴  and BD.¹⁵  It can also be dehydrogenated into acetoin and 2,3-butanedione, which are flavoring agent and margarines and cosmetics.¹⁶ 

Fermentation of biomass-based glucose and xylose by Klebsiella pneumodae produces 23BDO at high yields.¹⁷  23BDO can also be fermented from substrates such as glycerol,¹⁸  starch¹⁹  and cellulose hydrolysates.²⁰  Alternatively, 23BDO can be produced by nonpathogenic bacteria utilizing CO from industry waste gas or syngas as a carbon source.¹² 

1,3-Butanediol (13BDO) has been used as a building block for the production of industrial chemicals including pheromones, fragrances, insecticides²¹  and BD which is an important building block to produce synthetic rubber.²²  Microbial production of 13BDO has been hindered because there aren't precursors or structural analogs for 13BDO in the major metabolic pathways. Microbial productions of 13BDO have been reported in patents by engineered microbial organisms.²³ 

Kataoka reported the construction of recombinant Escherichia coli that could efficiently produce (R)-13BDO from glucose.²⁴  The same group optimized the fermentation conditions to further improve 13BDO production by the recombinant strain.²⁵ 

Kim proposed a novel biosynthetic pathway for 13BDO production, which is based on the condensation of two acetaldehyde molecules by a 2-deoxyribose-5-phosphate aldolase (DERA) with the subsequent reduction of the produced 3-hydroxybutanal to 13BDO using an aldo-keto reductase (AKR). Several important aldol-keto reductases were identified (shown in Fig. 2).²⁶ 

Once the above platform chemicals are produced, further processing can convert them to desirable C4 molecules. With this approach, biomass-derived molecules can directly replace their petroleum-derived counterparts because they are chemically the same. This section considers nine different C4 products and the catalytic chemistry used in their production.

1,4-Butanediol (14BDO) is an important chemical commodity used in the production of solvents, fine chemicals and high performance polymers, such as spandex fibers and polybutylene terephthalate (PBT). PBT is an engineering-grade thermoplastic, heavily used in automobile and electronic industry to produce connectors, insulators, wheel covers, gearshift knobs, and reinforcing beams.²⁷  14BDO has a total annual production of approximately 2.5 million tons.²⁸  14BDO can be directly produced by an engineered E. coli from renewable carbohydrate feedstock or from succinic acid catalytic hydrogenation in aqueous solution. Succinic acid is first transformed into GBL by the two-step hydrogenation of succinic acid to 14BDO which is shown in Fig. 3.

Succinic acid hydrogenation has been carried out over many monometallic catalysts, including Pd,²⁹  Pt,³⁰  Rh,³⁰  Ru, Re.³¹  Reaction is carried out in the liquid phase with various solvents. Table 1 summarizes the reported catalytic results for a variety of catalysts. Rhenium has been considered as the most efficient monometallic catalyst for the selective formation of 14BDO.³²  Ly and coworkers found that rhenium addition to 2.0 wt% Pd/TiO₂ catalysts was essential for selective hydrogenation of succinic acid to 14BDO. They proposed that a synergetic interaction between Re and Pd enhanced the catalyst activity for succinic acid hydrogenation and also GBL hydrogenation into 14BDO. The highest selectivity to 14BDO was 83% over 3.4% Re–2.2% Pd/TiO₂ after 48h.³³ 

Chung examined the support effect on the catalyst Pd catalyzed succinic acid hydrogenation and found that large Pd particles located outside of the pore channel of MCM-41 with small pore size enhanced the production of 14BDO. A maximum 14BDO selectivity of 53% was achieved over Pd/MCM-41 at 250 °C and 10 MPa H₂ pressure with succinic acid conversion of 60%.²⁹ 

Kang systematically studied the bimetallic catalyst Re–Ru with different metal content supported on mesoporous carbon. The study showed that synergistic interactions formed by Re–Ru increased the amount of weak hydrogen-binding sites. The less stable weakly bonded hydrogen atoms could easily migrate on the catalyst surface and continuously provide hydrogen for hydrogenation of adsorbed succinic acid.³² 

Di proposed a synthesis method of carbon supported Re–Ru catalyst by using a microwave-assisted thermolytic method. The catalyst ReRu/C with the same metal concentration (total metal loading 4 wt%) gave the highest 14BDO selectivity at 160 °C, 8 MPa H₂ after 10 hours reaction.³⁴  Di also studied a Re–M/C (M=Pt, Rh) catalyst prepared with the same method, where the total metal loading was 2%. The highest 14BDO selectivity of 80% was achieved with succinic acid conversion over 99% at 160 °C and 8 MPa H₂ over Re–Pt/C after 10 hours of reaction. The kinetic study proved that the Re–M interaction increased the generation rate of 14BDO more than that of THF.³⁵ 

Liu and coworkers studied a series of Pd–FeO_x/C catalysts for succinic acid hydrogenation to 14BDO. The higher selectivity to 14BDO over 3% Pd–5% FeO_x/C were ascribed to the high acidity of the catalysts by the addition of Fe, the well dispersed Pd and the synergy between Pd and Fe species. They reported a 14BDO yield of over 70% under 200 °C and 5 MPa H₂ on this catalyst.³⁶ 

A biotechnological route to 14BDO has been patented by Genomatica utilizing engineered E. coli strains for 14BDO bioproduction from sugars such as glucose, xylose, sucrose, and biomass-derived mixed-sugar streams.²⁸  In one pathway, sugar is first converted into succinyl-CoA which is then further converted into 14BDO over 4-hydroxybutyrate and other intermediates.²⁷  The simplified scheme of the engineered metabolic pathway is shown in Fig. 4.

GBL is used as the starting material for the synthesis of N-methyl-2-pyrrolidone and N-vinylpyrrolidone, which are widely used in medicine. It can also be utilized as a solvent. Its annual production represents 250 000 t, approximately 20% of which originates from USA based production sites. GBL has been conventionally produced by direct hydrogenation of maleic anhydride.³⁷ 

Hong studied the hydrogenation of succinic acid to GBL over a palladium catalyst supported on an alumina xerogel (AX). The conversion of succinic acid and yield for GBL showed volcano-shaped curves with respect to the calcination temperature of the AX support. Conversion of succinic acid and yield of GBL increased with increasing acid density of Pd/AX catalyst. The catalyst Pd/AX850 gave a highest yield of 50.7% with 75% conversion of succinic acid.³⁸ 

THF is a cyclic ether used as a solvent in the manufacture of paint, adhesives, impression ink, and pharmaceutical products. It is also importantly used as a monomer for the production of polytetramethylene glycol (PTMEG), as a solvent in PVC cement, in pharmaceuticals and coatings, or as a reaction solvent. It is produced worldwide at a level of 439 000 t a⁻¹.³⁹  THF is mainly produced from 14BDO dehydration, which is achieved in the presence of an acid catalyst at temperatures above 100 °C and near atmospheric pressure agents include strong mineral acids,⁴⁰  heteropolyacids,⁴¹  zeolites,⁴²  sulfonic acids⁴³  and dimethyl sulfoxide (DMSO).⁴⁴  Strong mineral acids, aluminum silicates, and ion-exchange resins are the preferred commercial catalysts.

Hong studied the hydrogenation of succinic acid to THF over rhenium catalyst supported on H₂SO₄-treated mesoporous carbon. Both conversion of succinic acid and yield for THF showed volcano-shaped curves with respect to H₂SO₄ concentration. Re/MC-0.4 showed the smallest rhenium particle size due to the mesoporous structure of MC treated with 0.4M H₂SO₄ and gave the highest THF yield of 38% with full conversion of succinic acid.³¹ 

Hong also studied the hydrogenation of succinic acid to THF over a ruthenium–carbon composite (Ru–C) catalyst. A catalyst referred to as Ru-300C (calcined at 300 °C 4 h for pre-graphitization) showed the highest conversion of succinic acid (91.2%) and the highest yield for THF (46.4%). The excellent catalytic performance of the Ru-300C catalyst was due to the smallest ruthenium particle size.⁴⁵ 

BD is a commodity chemical with a worldwide production volume of over 14 Mt in 2015. Asia is the largest producing region in the world, accounting for nearly 52% of total production.⁴⁶  Because of the global BD production and its diverse applications, it is impacted by various dynamics, including changes in the production of ethylene, fluctuations in energy markets, and general economic cycles. Over 95% percent of BD is produced as a by-product of ethylene production from steam cracking process with feedstock such as ethane, propane, butane, naphtha, condensate or gas oil.

BD is mainly used in the production of polymers and chemical intermediates for manufacturing polymer products. These polymers are widely used as components of automobile, construction materials appliance parts, computers, telecommunications equipment, clothing protective and clothing. The major end uses for BD are listed in Table 2. The largest consumption of BD is in the production of synthetic rubber, such as styrenre-butadiene rubber, polybutadiene rubber and other polymers. BD is also used for manufacturing of adiponitrile, which is an intermediate chemical used in the manufacture of nylon 6,6.

In the past years, 23BDO was used as feedstock to make BD for synthetic rubber. However, with the development of the petroleum industry, BD is now produced directly from heavy oil, such as naphtha. Recently the price volatility of petroleum and the abundance of shale gas has led to an increase in ethane usage in the US cracking industry from 46 to 65% during 2005–2011.⁴⁷  Use of ethane for cracking results in a lower BD yield compared with naphtha cracking, upsetting the supply of BD. Therefore, the shale gas revolution provides an opportunity for the bio-based, on-purpose production of BD to meet the growing BD demand.⁷ 

Due to the industrial importance of BD, a number of feedstocks have been proposed as starting materials to make BD, including ethanol, 23BDO, 13BDO, and 14BDO.

Ethanol is an important building block that can be converted into BD. Lebedev proposed a direct process to produce BD from ethanol over a mixture of zinc oxide and alumina at 400 °C. BD selectivity is 18% directly from ethanol according to following reaction:⁴⁸ 

Ostromyslensky proposed an indirect two-step pathway from ethanol and acetaldehyde over alumina or clay catalysts at 440–460 °C. The BD yield was 18%. This process includes the partial dehydrogenation of ethanol into acetaldehyde, which further reacts with ethanol to form BD.⁴⁹ 

Both the Lebedev and Ostromyslensky synthesis methods involve crotonaldehyde as an intermediate. As shown below, two acetaldehyde molecules condense into the acetaldo, then the aldol dehydrated into crotonaldehyde.

The general overall reaction mechanism from ethanol to produce BD is shown in Fig. 5.⁵⁰  The first step is the ethanol dehydrogenation to acetaldehyde, followed by two acetaldehyde molecules undergoing aldol condensation to produce the aldol, which is then dehydrated into crotonaldehyde. The hydrogenation of crotonaldehyde into crotyl alcohol proceeds in the presence of excess ethanol. Further dehydration of crotyl alcohol leads to BD.

There is some disagreement in the literature about the overall rate-determining reaction step of ethanol conversion to BD. Niiyama and coworkers compared the reaction for different reactant mixtures, including ethanol, acetaldehyde, crotonaldehyde, ethanol+acetaldehyde and ethanol+crotonaldehyde over MgO–SiO₂. The results indicated that BD formation substantially increased when 1% acetaldehyde was mixed with 10% ethanol. It was inferred that aldehyde formation was the rate-limiting step in this case.⁵¹  Sushkevich and coworkers found that aldol condensation was the rate-determining step over Ag/ZrO₂/SiO₂ catalyst.⁵² 

Pomalaza and coworkers recently reviewed the progress on catalyst development for converting ethanol to BD in one or two-steps. Table 3 summarizes the catalyst performance of ethanol condensation into BD. Redox and basic active sites are thought to participate in the dehydrogenation of ethanol to acetaldehyde, while the acid and basic sites have been reported to be active in the condensation and dehydration.⁵³ 

Natta studied the Cr-modified magnesia-silica catalyst 2% Cr₂O₃–59% MgO–39% SiO₂. On this catalyst, the BD yield and selectivity were 43% and 52%, respectively, while the Cr-free MgO–SiO₂ catalyst gave a BD yield and selectivity of 35% and 41%, respectively. The possible reason is that Cr can promote the dehydrogenation of ethanol to form acetaldehyde, which ultimately leads to higher BD selectivity.⁵⁴ 

Patil and coworkers investigated catalyst prepared by the incipient wetness impregnation method with metal oxides (ZnO: 1 wt%, ZrO₂: 5 wt%) and alkali-promoters (Li₂O, Na₂O, K₂O, Cs2O) using fumed silica. A BD yield of 54.6% with ethanol conversion of 97.7% was obtained over 0.5Cs₂O–1ZnO–5ZrO₂/SiO₂ catalyst at 400 °C and WHSV=1 h⁻¹. The author proposed that only Cs provides the appropriate balance of acid–base pairs with the suitable strength to carry out the condensation step effectively, resulting in a higher BD yield.⁵⁵ 

Sushkevich studied the use of metal-containing (M=Ag, Cu, Ni) oxide catalysts (MO_x=MgO, ZrO₂, Nb₂O₅, TiO₂, Al₂O₃) as promoters on silica. The highest selectivity was achieved over the 0.3 wt% Ag/4 wt% ZrO₂/SiO₂ catalyst: 73.8% towards BD with 30% ethanol conversion.⁵⁶  Sushkevich also investigated the molecular-level details of BD formation from ethanol over silica supported silver promoted zirconia catalyst Ag/ZrO₂/SiO₂, using kinetic measurements. It was shown that BD synthesis involves two independent catalytic cycles responsible for: (1) ethanol dehydrogenation into acetaldehyde over Ag/Si–OH sites and (2) acetaldehyde/ethanol transformation into BD over Zr Lewis acidic sites.⁵² 

Dai and coworkers studied the zeolite structural confinement effects on ethanol conversion into BD. The researchers compared a bicomponent 5%Zn–5%Y cluster loaded on different substrates (zeolite H-beta, MCM-41 and SiO₂) and found that the highest initial BD productivity of 0.52 gBD/gcat/h was achieved on 5%Zn–5%Y/beta. They suggested that the close contact between different functional sites (acid and base) leads to a higher chance of acetaldehyde condensation rather than desorption. The highest BD yield of 75% was achieved with 100% ethanol conversion over 2%Zn–8%Y/beta at 350 °C, and WHSV=0.3 h⁻¹.⁵⁷ 

Nearly 70 years ago, Winfield demonstrated that 23BDO could be dehydrated over thoria to produce BD for synthetic rubber manufacturing.⁵⁸  However, due to the abundance of petroleum resources, BD has been almost exclusively produced from hydrocarbon feeds, such as naphtha.⁵⁹  Recently, there has been renewed interest in producing industrial chemicals from biomass resources.⁶⁰  In this regard, catalytic BD production from renewable resources is an appealing strategy, which ensures ample supply and reduces price volatility of the final product.⁶¹ 

Dehydration of 23BDO can readily occur on zeolites,⁶²  and with mineral acids such as sulfuric acid.⁶³  However, MEK, rather than BD, is the dominant product due to the keto–enol tautomerization and pinacol rearrangement. Only a few studies resulting in successful selective production of BD have been reported. Among them, Winfield, who studied the catalytic dehydration of 23BDO to BD over thoria, reported a single pass conversion of 60% to BD at 500 °C.⁵⁸  Shlechter obtained BD from the esterification of 23BDO with acetic acid, followed by pyrolysis of the diacetate as shown in Fig. 6.^64,65  BD production started at temperatures above 475 °C. The optimal condition appeared to be at a temperature of about 585 °C and contact time of 7.1 seconds, where the BD yield was 84.9% under such conditions in his study. Also, Baek and coworker reported the process of BD production from 23BDO etherification with formic acid and acetic acid. They proposed using glucose fermentation liquor as the starting material with external addition of acetic acid (23BDO : formic acid : acetic acid=1 : 0.5 : 2.5). 70% yield of BD was achieved. Sulfuric acid was used as the catalyst for esterificaiton. The pyrolysis step did not evolve with catalyst.⁶⁶ 

Recently, 23BDO dehydration over rare earth metal oxides has been studied and acid/ basic properties were modified by introducing CaO, SrO, BaO, and MgO into monoclinic ZrO₂ can enhance the 3-buten-2-ol (3B2OL) selectivity.^67–70  Among all the catalysts tested, BaO addition (molar ratio of BaO/ZrO2=0.0452) gave the highest 23BDO conversion (72.4%) and 3B2OL selectivity (74.4%) in the initial stage of 5 h at 350 °C.

Duan and coworker also studied 23BDO conversion on a two-bed catalyst system (cubic Sc₂O₃+Al₂O₃), the BD selectivity was 94% with 100% 23BDO conversion.⁷¹  The Sc₂O₃ converts the 23BDO into 3B2OL and then acidic Al₂O₃ converts the 3B2OL into BD. Zeng and coworkers also studied 23BDO dehydration over tetragonal Sc₂O₃+an acid or base catalyst. The BD selectivity with a basic catalyst in the second bed (ZrO₂ for example) was much lower than for acidic catalysts (Silica-alumina and Al₂O₃).⁷² 

Kim and coworker studied the 23BDO dehydration over silica-supported Na/P catalysts and found that product distribution depended on the Na/P ratio. This ratio was important to provide a balance of acid and base sites. The optimal ratio for BD production was found to be 1.8–1.9 and the elimination product (BD+3B2OL) exceeded 60%. The authors explained that this ratio gave the best combination of acidic and basic sites for the production of the elimination product from 23BDO. XPS measurements showed that sodium cations migrated from the surface to the bulk of silica probably due to the competition of acidic silanols for sodium when Na/P>1. The dehydration mechanism is interpreted in terms of E1cb mechanism. The dehydration product of 2-butandiol over the same catalyst is 1-butene, which is a good indication of E1cb mechanism.⁷³ 

The literature suggests that acid–base properties are important in 23BDO dehydration. In particular, BD selectivity can be influenced in the presence of alkaline species. For example, Kim and Lee reported that cesium doped silica can promote BD selectivity which increased from 51% to 62% as the Cs₂O/Al₂O₃ mass ratio increased from 11% to 40%.⁷⁴  Tsukamoto and coworkers studied the 23BDO dehydration over silica gel supported alkali metal dihydrogen phosphate (MH₂PO4; M=Li, Na, K, Rb, Cs). They found that the SiO₂-supported CsH₂PO₄ catalyst gave the highest BD selectivity (above 90%) in a single-bed catalyst system. Catalysts containing Ca(H₂PO4)₂ and MgHPO4 mainly produced MEK and 2-methylpropanal (MPA). Researchers speculated that the high BD selectivity is probably due to combination of the proper acid–base activity of Cs phosphate and large ionic radius of Cs⁺.⁷⁵  However, other researchers have found that 10 wt% Cs₂O and alkali metal oxides (Na₂O and K₂O) loaded on silica gave 2,3-epoxybutane as the main product, while BD was not observed.⁷⁶  All these experiments indicated that the acid/base property of the catalyst had a crucial impact on the product distribution and will ultimately affect the BD selectivity.

Zhao and coworkers studied 23BDO dehydration over a series of P/HZSM-5 (Si/Al=360) samples with various phosphate contents. The phosphate content had a large impact on MEK and MPA selectivity. At 180 °C, the MEK to MPA ratio increased from 5.1 to 37.5 when the content of phosphate increased from 0.5 to 8.0. The characterization results demonstrated that the phosphate modification of HZSM-5 not only reduced the strong and medium acid sites but also produced new weak acid sites. Strong acid sites and high reaction temperatures promoted the formation of MPA through methyl group migration via carboniums. The strong acid sites and high reaction temperature accelerated the methyl group shift during the carbonium rearrangement process, which led to MPA formation.⁷⁷ 

Zeng and coworkers compared 23BDO dehydration over two forms of commercial γ-alumina (denoted by the manufacturer as SCFa and F-200) and found that the acid and base properties greatly affect the product distribution. The BD selectivity decreased from 24.7% over SCFa to 1.9% over Na-modified SCFa (SCFa–Na-7.6%) at 400 °C. Na-modified alumina catalyzed the 3B2OL dehydrogenation to form methyl vinyl ketone (MVK) as opposed to dehydration to BD. Basic sites catalyzed the retro-aldol condensation of MVK, which produces acetone and formaldehyde via cleavage of the CC bond. The research indicated the acid/base properties greatly affect the 23BDO dehydration product distribution and will ultimately affect the BD selectivity.¹⁵ 

Sato studied the 13BDO dehydration over commercial CeO2. 2-Buten-1-ol and 3B2OL were produced with the selectivity of 41.1% and 58.1%, respectively, at 325 °C. Further dehydration leads to BD using an acid catalyst.⁷⁸  The same group studied 13BDO dehydration over SiO₂–Al₂O₃, Al₂O₃, TiO₂ and ZrO₂ Between 200 °C and 375 °C, the maximum BD selectivity 36% was observed over SiO₂–Al₂O₃ at 250 °C and WHSV=11.4 h⁻¹. Al₂O₃ catalyzed the transformation of 13BDO into other products: the selectivity to formaldehyde and 4-methyl-1,3-dioxane were 28% and 18% respectively. The main products of 13BDO dehydration over TiO₂ and ZrO₂ catalyst were un-saturated alcohols.²² 

Gotoh extensively studied 13BDO dehydration over rare earth oxides (REOs) calcined at different temperatures at 325 °C and WHSV=6.67 h⁻¹. 3B2OL and 2-buten-1-ol were preferentially produced together with dehydrogenated products such as MEK, 3-buten-2-one, while BD formation was not reported. Cubic bixbyite REOs selectively produced the unsaturated alcohols such as 3B2OL and 2-buten-1-ol, while monoclinic REOs were less active and less selective than cubic REOs. In the REOs, cubic fluorite CeO₂ showed the highest formation rate of the unsaturated alcohols.⁷⁹ 

Jing and coworkers studied the 13BDO low-temperature dehydration over Al-doped SBA-15 catalysts with different Si/Al ratios prepared using the evaporation-induced self-assembly method. They found that an Al-SBA-15 sample with a Si/Al ratio of 76 exhibited a BD yield of 59% with 13BDO conversion of 98% at 200 °C and WHSV=14 h⁻¹.⁸⁰ 

Fang investigated a series of catalyst Ce@MOR hybrids based on CeO_x nanoparticles (1–2.5 nm) encapsulated in mordenite. The genesis of acid sites with medium strength was directly correlated to the catalytic activity for the vapor-phase dehydration of 13BDO into BD. The hybrid with a Si/Ce atomic ratio of 50 displayed the highest BD yield of 46% with almost 100% 13BDO conversion at 350 °C and WHSV=14 h⁻¹, which was due to a higher density of acid sites with medium strength induced by encapsulated CeO_x nanoparticles.⁸¹ 

Sato and coworkers investigated the 14BDO gas phase dehydration over Al₂O₃, SiO₂–Al₂O₃, ZrO₂ and CeO₂ at 200–450 °C. 3-Buten-1-ol and THF were the major side products. Over the first three catalysts, BD selectivity was higher at lower temperatures (200 and 275 °C); however when the temperature increased to 425 °C, THF was the dominant product. Over CeO₂, the 14BDO conversion was only 6%, while the BD selectivity was close to 90% at 275 °C, while a 73% conversion was obtained at 425 °C but the selectivity shifted from BD to 3-buten-1-ol. THF selectivity was less than 10% over CeO₂ for all reaction conditions.⁸² 

BD can easily be produced from 3-buten-1-ol dehydration over acid catalyst. THF can undergo ring-opening dehydration to BD and water. Abdelrahman and coworker studied the THF dehydra-decyclization to produce BD in a packed-bed flow reactor within various temperatures and WHSV. The catalytic ring-opening dehydration of THF with phosphorus-containing siliceous self-pillared pentasil (SPP) or MFI structure exhibits high selectivity to BD (85–99%) at both low (9%) and high (89%) conversion of THF.⁸³ 

Stalpaert and coworkers investigated the 14BDO dehydration in ionic liquid Bu₄PBr. 0.5 mmol of 14BDO, 1.7 mmol of Bu₄PBr, 0.022 mmol of HBr, were loaded in a glass vial and sealed under N₂ atmosphere. The maximum BD yield of 94% was reached at 100% conversion at 220 °C after 120 min. The reactions were also carried out both in the absence of ionic liquid Bu₄PBr or acid HBr, and the results indicated that the ionic liquid and acid HBr both greatly affect the BD yield. Purely acid-catalyzed reaction in mesitylene and N-methylpyrrolidone led only to THF. However, the catalyst active site structure and role of phosphorus were unclear in this study.⁸⁴ 

Butene is an important chemical intermediate, with uses in both fuels and polymers. The uses for butene vary based on the isomer formed. The primary use of 1-butene is to copolymerize it with ethylene to form linear low-density polyethylene (LLDPE).⁸⁵  Recent research has also demonstrated the oligomerization of 1-butene to produce a mixture of heavy hydrocarbons with excellent potential as a fuel.^86–88  2-Butene can be alkylated with isobutane to produce high octane fuel.^89–91  Isobutene is copolymerized with isoprene to produce butyl rubber⁹²  as well as being used as an alkylation agent.

A number of recent papers have reported on the conversion of 23BDO to butene.^93–96  This conversion involves removal of two moles and water and addition of one mole of hydrogen (Fig. 7 below). Dehydration and hydrogenation chemistries require two different catalyst functionalities: acid sites for dehydration and metal sites for hydrogenation. For this reason, research has focused on bifunctional catalysts comprised of both acidic supports and metals active for dehydrogenation.

The primary function of the supported metal in the conversion of 23BDO to butene is to hydrogenate ketone and aldehyde intermediates (MEK and 3-methyl isopropanol) to alcohols which can further be converted to olefins. Copper has been found to be the best metal for conversion of 23BDO to butene because of its combination of good activity for hydrogenation with a lack of activity for C–C bond breakage.⁹⁷  This allows for butene selectivities of greater than 70% on a suitable acidic support,⁹⁵  as opposed to quite low selectivities for Ni (10.6%), Pt (2.3%), Pd (0%), and Rh (1.9%) on ZSM-5.⁹⁸  Metals other than copper give a wide variety of hydrocarbons, suggesting that C–C bond scission pathways are more favored. In addition, higher amounts of butane were found, demonstrating greater activity for hydrogenation of butene.

The catalyst support plays an important role in the conversion of 23BDO to butene both for its acid functionality but also for its physical properties (i.e. pore structure). The presence of acid sites is necessary to catalyze dehydration pathways: initially dehydration of 23BDO to MEK and MPA. Use of a silica support, with no measured Bronsted acidity according to ammonia TPD experiments, led to only trace amounts of butene.⁹⁴  On the other hand, too much acidity can also be detrimental. Experiments where the silica to alumina ratio was varied show that there is an optimal ratio for butene production, which implies that there is an optimal acidity. Table 4 shows the 23BDO conversion and major product selectivities for three different alumino-silicate supports with different silica to alumina ratios. As seen in this table, the maximum butene selectivities were achieved at silica to alumina ratios of 280, 100, and 50 for ZSM-5, Al-MCM, and Al-SBA, respectively. Supports with more and stronger acid sites may be inferior to supports with more modest acidity for two reasons. First, more acidic supports may catalyze additional reaction pathways to the desired one, including oligomerization and cracking pathways.⁹⁵  Second, more acidic supports have been found to deactivate quicker due to coke formation.⁹³ 

The porosity of the support also plays an important role in the conversion of 23BDO to butene. Mesoporous supports gave greater butene selectivitiy than microporous supports.⁹⁵  This was attributed to larger pores that allow products to escape before secondary reactions, such as cracking, could occur. The mesoporous materials also produced higher amounts of higher hydrocarbons, suggesting that butene could oligomerize within the mesopores and the large resulting molecules were able to diffuse out of the pores before being further reacted.

The reaction mechanism for conversion of 23BDO to butene has been explored by measuring the reaction kinetics for reactants and intermediates on different support copper catalysts.⁹⁴  The mechanism is summarized in Fig. 7. As seen in this figure, the route to butene involves an initial dehydration step to MEK or MPA, followed by hydrogenation to 2-methyl-1-propanol and 2-butanol and finally a second dehydration step. However, other reactions can also be important. Dehydrogenation of 23BDO to acetoin and subsequently to 2,3-butanedione were important side reactions. In fact, dehydrogenation to acetoin proceeded at a faster rate than dehydration. However, hydrogenation of acetoin to 23BDO also proceeds readily, so eventually 23BDO was converted to MEK/MPA and eventually to butenes.

The reaction mechanism highlighted in Fig. 7 shows that MEK is key intermediate in the process. An alternative to using a single process to convert 23BDO to butene is use an acid catalyst to convert 23BDO to MEK followed by a catalyst to convert MEK to butene. This eliminates the issue of competing reactions for the initial 23BDO reaction. With this approach the catalyst properties can be tailored towards hydrogenation of MEK to 2-butanol and dehydration of 2-butanol to butene. Copper was supported on Al₂O₃, sodium-modified zeolite Y, and H-form zeolite Y and tested for conversion of MEK to butene.⁹⁹  The most acidic support (H–Y) produced a low selectivity to butene (52.5%) at a modest conversion (39.7%) because it catalyzed a number of other reactions. Less acidic supports (Na–Y and Al₂O₃) gave extremely high selectivities to butene: 96.2% and 80.1%, respectively, at nearly complete conversion. These results suggest that it may be advantageous to separate out reactions of 23BDO to MEK and the subsequent reaction of MEK to butene, and that nearly 100% yield of butene might be possible from such a reaction scheme.

The above approaches all required large amounts of hydrogen. Kwok and coworkers took a different approach to converting 23BDO to butene, using a vanadium catalyst in the absence of hydrogen.⁹⁵  They found that a V/SiO₂ catalyst could achieve 45.2% butene selectivity at complete 23BDO conversion. The authors suggest that V/SiO₂ acts as a bifunctional catalyst with weak acid and polymeric VO_x sites that achieve both dehydration and transfer hydrogenation. They further proposed that transfer hydrogenation between 23BDO and MEK can occur, converting MEK to 2-butanol which can then dehydrate to form 2-butene. In this way, it was not necessary to add hydrogen to the feed.

2,3-Butanedione is a vicinal diketone (two adjacent carbonyl groups) which is a metabolic by-product of both yeast alcoholic (beer, wine and spirit) and malolactic (wine) fermentations.¹⁰⁰  2,3-Butanedione is a volatile compound whose buttery character is a key flavor compound in fermented diary food and alcoholic beverages. 2,3-Butanedione is also a bacteriostatic food additive, since it inhibits growth of some microorganisms.¹⁰¹ 

Beltramone studied the synthesis of 2,3-butanedione by selective oxidation of MEK in the presence of O₂ and H₂O₂ as oxidants. TS-1, Fe–Si, Ti-Beta, Fe-Beta, Ti-NCL-1, Ti-MCM-41 and VS-1 zeolite were synthesized. VS-1 (vanadium silicalite) was active and selective for the synthesis of 2,3-butanedione. The MEK conversion increased with temperature but the selectivity decreased notably. The reaction was performed at 200 °C with 7% MEK conversion and a 2,3-butanedione selectivity of around 80%.¹⁰² 

Anunziata also studied oxidation of MEK over numerous Fe, Ti and V modified zeolites to produce 2,3-butanedione. They found zeolites with V as the oxidation center were the most active catalysts. The 2,3-butanedione selectivity increased with decreasing temperature and higher oxygen partial pressure. The maximum 2,3-butanedione selectivity was 80% with reaction conditions T=200 °C, P_O2= 0.77 atm, W/F=24 g h mol⁻¹.¹⁰³ 

The oxidation of MEK was studied over V₂O₅ and V₂O₅ modified by Cs and K at 250 °C with various partial pressures of water in the feed. Acetic acid and acetaldehyde were the most abundant products. Other major products were 2,3-butanedione and carbon oxides. The addition of water to the feed increased the activities of the catalysts as well as the selectivities to acetic acid, acetaldehyde, and 2,3-butanedione. Modification of V₂O₅ with alkali metal ions increased the activity. The increase in activity upon water and alkali metal ion modification correlated well with the decrease in weight loss in thermogravimetric analysis (TGA) measurements in oxygen and with the surface carbon/vanadium ratios as determined by X-ray photoelectron spectroscopy (XPS). Thus, the main effect of water and alkali metal ion modification was to reduce the formation of carbonaceous deposits on the catalysts.¹⁰⁴ 

2,3-Butanedione can be synthesized as a byproduct in the fermentation of 23BDO from pyruvate. Intermediate includes α-acetolactate, acetoin, and 2,3-butanedione. The pyruvate is first synthesized from the glycolysis and is then converted into α-acetolactate. Further, α-acetolactate can be converted to acetoin by α-acetolactate decarboxylase (α-ALD) under anaerobic conditions. If oxygen is present, α-acetolactate can undergo spontaneous decarboxylation producing 2,3-butanedione.¹⁰⁵ 

Kim and coworker studied the 23BDO dehydration over silica-supported Na/P catalysts. The result showed that the optimal Na/P ratio for BD production was found to be 1.8–1.9 and the elimination product (BD+3B2OL) exceeded 60%. Meanwhile, more acidic catalysts like silica supported phosphoric acid(P/SiO₂) and 1Na_P/SiO₂(Na/P=1) had relatively high selectivity to 2,3-butanedione and acetoin. As the residence time increased, the butanedione and acetoin selectivity as nonselective reactions started to occur.⁷³ 

James studied electrochemical oxidation of 0.2M 23BDO in a divided cell using Pt anode and cathode using 0.5M NaCl under potentiostatic control. Acetoin and 2,3-butanedione were the only products. Furthermore, acetoin was reduced into 2,3-butanedione as the reaction time increased. When the reaction time was 10 hours, the 2,3-butanedione selectivity was close to 90% and 23BDO conversion was over 90%.¹⁰⁶ 

MEK is an industrially important solvent used in a variety of applications, including paints, coatings, adhesives, magnetic tapes, inks as well as for cleaning and extraction.¹⁰⁷  MEK was recently identified to be a promising fuel for spark ignition engines. Achieving the same engine efficiency as ethanol, efficiency gains up to 20% were measured at full load operation compared to gasoline due to a higher effective compression ratio enabled by the extreme knock resistance of MEK. In addition, MEK offers lower hydrocarbon emissions, less oil dilution, and better cold-start properties than gasoline alone.¹⁰⁸ 

The MEK market is estimated to increase to US$ 3.26 billion and to over 1754 kilo tons by 2020. MEK is currently synthesized from fossil C4-raffinates. The main commercial route to MEK is through the hydration of butene to produce secondary butyl alcohol (SBA) production and further SBA dehydrogenation to produce MEK.¹⁰⁷ 

MEK can also be obtained by dehydration of 23BDO. Emerson studied the liquid phase dehydration of aqueous 23BDO to MEK using a sulfuric acid catalyst at 138–180 °C with the MEK yield over 90%. The MEK formation mechanism is described by pinacol rearrangement.⁶³  Lee carried out a DRIFTS studies of 23BDO dehydration over ZSM-5, mordentite β- and Y-zeolites respectively. The peak at 1703 cm⁻¹ was assigned to MEK adsorbed on isolated silanol groups and the peak at 1688 cm⁻¹ and a corresponding shoulder at 1665 cm⁻¹ were assigned to MEK adsorbed on bridging OH groups of two different acidities. The results showed that dehydration to MEK is favored on ZSM-5.¹⁰⁹  Multer studied the fermentative 23BDO dehydration over HZSM-5 at 225 °C and found the MEK selectivity were greater than 90%. The only other product was MPA.¹⁴ 

Gong found that MEK could be produced from sugar-derived levulinic acid decarboxylation via CuO oxidation in batch experiments. The highest yield of MEK of 67.5% was achieved at around 300 °C at pH=3.2.¹¹⁰ 

Penner conceptually designed a process for bio-based MEK production from 23BDO. This process included 23BDO separation from fermentation and catalytic dehydration of 23BDO into MEK. This article indicated that efficiency largely depends on the separation technology for 23BDO and its efficiency. Hybrid separation strategies showed a total product yield of nearly 38 wt% at a specific primary energy demand of 0.25 MJ/MJ MEK.¹¹¹ 

n-Butanol is used as an important chemical in paints, coatings, printing inks, adhesives, sealants, textiles, and plastics and is also considered as a superior liquid fuel with a number of advantages over ethanol, such as a higher energy density (29.2 MJ L⁻¹ vs 19.6 MJ L⁻¹), lower corrosiveness and being more suitable for distribution through existing pipelines.¹¹²  Blends of 16% butanol (by volume) with gasoline is allowed in United States which is higher than ethanol at 10%.

The dominant process for the manufacture of n-butanol is oxo-synthesis, where propylene is initially converted over a homogeneous catalyst to butyraldehyde via a hydroformylation with carbon monoxide and subsequently hydrogenated over a heterogeneous catalyst to n-butanol.¹¹³  98% propylene conversion per pass and n-butanol overall selectivities of 94% have been reported using Rh.¹¹⁴  n-Butanol can also be produced sustainably from bio-ethanol through the Guerbet reaction and direct synthesis. The following section describes both reaction mechanisms.

The Guerbet reaction is recognized as a useful synthetic tool to obtain higher alcohols from lower alcohols,¹¹⁵  which is shown in Fig. 8. The Guerbet reaction mechanism involves the coupling of two ethanol molecules with three steps: dehydrogenation of an alcohol into aldehyde, aldol condensation and hydrogenation of the unsaturated aldehyde.

Table 5 summarizes the Guerbet reaction of ethanol to n-butanol over heterogeneous catalysts. Jordison studied ethanol dehydration over La₂O₃ modified 8% Ni/Al₂O₃. The catalyst demonstrated a 55% conversion of ethanol and a yield of 39% to higher alcohols (C4–C8). Jiang and coworkers studied the upgrading of ethanol to n-butanol over Cu–CeO₂/AC catalyst with various Cu/Ce molar ratios. The highest catalytic activities to n-butanol were achieved at 4Cu1CeO₂/AC at 250 °C and 2 Mpa. The authors suggested that the highly dispersed Cu metals were beneficial for dehydrogenation/hydrogenation, while the presence of highly distributed CeO₂ species provided sufficient basic sites for the aldol condensation of acetaldehyde.¹¹⁶  Earley and coworkers studied solid acid supported copper catalysts for upgrading ethanol to n-butanol in the presence of supercritical CO₂. High surface area (HSA) CeO₂ supported Cu catalyst exhibited excellent performance with 67% ethanol conversion and up to 30% yield of n-butanol.¹¹⁷  The presence of CO₂ had a positive effect on n-butanol generation, probably by regenerating the active Ce⁴⁺ species lost by reaction with H₂ generated in the dehydrogenation step.¹¹⁸  Ogo studied the substituted hydroxyapatites, such as Sr₁₀(PO₄)₆(OH)₂, Ca₁₀(VO₄)₆(OH)₂, Sr₁₀(VO₄)₆(OH)₂, and Ca₁₀(PO₄)₆(OH)₂. Sr–P hydroxyapatite catalyst exhibited the highest n-butanol selectivity of 81% with ethanol conversion of 7.6% at 300 °C at atmospheric pressure. Ethylene was the major product over the Ca–V and Sr–V hydroxyapatite catalysts.¹¹⁹  The same group further optimized the Sr/P ratio increased to 1.7 which increased the n-butanol selectivity to 86.4% with the ethanol conversion of 11.3%.¹²⁰ 

Homogeneous metal catalysts for ethanol condensation into n-butanol are shown in Table 6, together with reaction conditions and catalytic results. Dowson studied the conversion of ethanol into n-butanol through the Guerbet reaction using a Ru complex catalyst with the ligand bis(diphenylphosphanyl)-methane in EtONa. n-Butanol yield was over 20% with 22% ethanol conversion. The TON and TOF were 221 and 55 at 150 °C for 4 h.¹²¹  Koda studied the ethanol conversion into n-butanol with an Ir complex catalyst in 1,7-octadiene and EtONa with the ligand 1,3-Bis(diphenylphosphino)propane. The ethanol conversion and n-butanol yield were 41% and 21%, respectively, with highest TON 1220 at 120 °C for 15 h.¹¹⁵  Chakraborty and coworkers used bifunctional iridium catalysts coupled with bulky nickel or copper hydroxides for ethanol conversion into n-butanol. These sterically crowded basic nickel and copper hydroxides catalyzed the key aldol coupling reaction of acetaldehyde to exclusively yield crotonaldehyde. Iridium-mediated dehydrogenation of ethanol to acetaldehyde has led to the development of an ethanol-to-butanol process. The n-butanol yields of Ni and Cu hydroxides were 34% and 28% at 150 °C for 24 h.¹²²  Fu and coworkers studied ethanol dehydration over homogeneous non-noble-metal Mn catalyst with EtONa as a base. Ethanol conversion of 11.2% and n-butanol selectivity 92% (yield 9.8%) was measured at 160 °C for 168 h and reaching the TON of 114 120 and TOF of 3078 h⁻¹.¹²³ 

n-Butanol can also be produced from ethanol via a direct synthesis method. Yang conducted the n-butanol production from ethanol over alkali cation loaded zeolites found the ethanol dehydration was inhibited when the crotonaldehyde was added in the reactant, which confirmed the reaction did not proceed through the Guerbet reaction mechanism. Yang proposed the direct and semi-direct synthesis of n-butanol from ethanol and ethanol/acetaldehyde as shown in Fig. 9. The ethanol C–H bond was activated in the β-position and the nucleophilic center generated attacks another ethanol molecule, resulting in OH displacement (dehydration), C–C coupling and hence formation of n-butanol and water.¹²⁴ 

Gines conducted isotopic tracer studies using ¹²C₂H₅OH–¹³C₂H₄O reactant mixtures over basic oxides which confirmed that condensation reactions proceeded via direct reactions of ethanol without the intermediate formation of gas phase acetaldehyde molecules. The β–H bond in ethanol was activated by the basic metal oxide and the activated ethanol molecule subsequently condensed with another molecule of ethanol by dehydration to form n-butanol.¹²⁵ 

Ndou studied the dimerization of ethanol to n-butanol over solid base catalysts MgO exhibited the highest activity: 56.14% ethanol conversion and 18.39% yield to n-butanol at 450 °C. The catalyst was modified by adding basic metal (Ca and Ba) and transition metals (Zn, Ce, Zr, Pb, Sn); however, the yield of n-butanol decreased significantly.¹²⁶ 

Tsuchida studied the direct synthesis n-butanol in one step from ethanol over nonstoichiometric hydroxyapatite with the Ce/P ratio 1.64. The highest yield of n-butanol was 25.7% with 57.4% conversion of ethanol at 400 °C.¹²⁷ 

Meunier studied thermodynamic equilibrium of ethanol condensation into n-butanol over metal-free hydroxyapatite Ca₃(PO₄)₂Ca(OH)₂ between 350 and 440 °C and reported that the condensation reaction did not proceed via acetaldehyde self-aldolization.¹²⁸  This observation is in contrast with the low temperature catalytic systems, which typically include a metallic phase, and which were shown to operate through aldol condensation.¹²⁹ 

Isobutanol is manufactured industrially mainly through propylene hydroformylation (oxo-synthesis) with subsequent hydrogenation of the aldehydes formed. It is used as a solvent and in synthetic resins. Isobutanol is also considered as an improved biofuel due to its better octane numbers and higher energy density (98% of that of gasoline) over n-butanol.¹³⁰  Isobutanol can be dehydrated with acid catalysts to produce the platform chemical isobutylene (annual production volume of fossil-based isobutylene is 10 million tons).¹¹⁴ 

Guerbet-type co-condensation of methanol (bio-sustainable sources) and ethanol is an attractive potential route. Using these substrates, methanol and ethanol are dehydrogenated to formaldehyde and acetaldehyde, which undergo aldol coupling to yield, after rehydrogenation, n-propanol. A further dehydrogenation, aldol coupling, rehydrogenation cycle with a second equivalent of methanol yields isobutanol (Fig. 10).¹³¹ 

Table 7 summarizes the Guerbet reaction of methanol/ethanol to isobutanol over various catalysts with reaction conditions and catalytic results.

Ueda reported the methanol/ethanol co-condensation over different metal oxide catalysts (MgO, ZnO, CaO, ZrO₂) at atmospheric pressure in a continuous flow reactor. MgO gave the best results at 360 °C, converting 60% ethanol to isobutanol with yield of 29%. The products distribution varied with temperature. Isobutanol was produced with higher selectivity at higher temperature with higher ethanol conversion, while propanol was the main product at lower temperatures.¹³² 

Carlini and co-workers investigated heterogeneous copper-based catalysts for the co-condensation of methanol with ethanol. The most productive catalyst (Cu-chromite) exhibited an ethanol conversion of 61% with 98% selectivity to isobutanol (isobutanol yield, 60%) at 200 °C for 6 hours.¹³³ 

Wingad reported on homogeneous ruthenium diphosphine complexes (1,1-bis(diphenylphosphino)methane ligands) for the production of isobutanol from methanol/ethanol mixtures. The highest isobutanol yield was 99.8% with ethanol conversion of 75% at 180 °C for 20 h.¹³⁴ 

Liu used Ir catalysts immobilized on N functionalized carbon materials for the upgrading of ethanol and methanol mixtures to isobutanol in air using water as a solvent. The isobutanol yield was 47% with 52% ethanol conversion at 160 °C for 32 h.¹³⁵ 

Olsen studied MgO impregnated on various carbon based catalysts. The reactions were carried out in a tubular reactor at 360 °C for 1 h. The highest isobutanol yield was 85% with complete ethanol conversion at 360 °C for 1 h. Ni containing MgO/C catalyst exhibited a higher isobutanol yield of 90% with full ethanol conversion.¹³⁶ 

Pellow studied the pre-catalyst trans-[RuCl2(dppm)2] which was tolerant to water and to the use of hydroxide rather than an alkoxide base. This catalyst system converted the reactant mixture (methanol : ethanol : water molar ratio=14.4 : 1 : 16.2) to isobutanol with a yield of 36% at high selectivity 78%. This level of water is typical of a fermentation broth; the ethanol content of which may be as high as 15 wt% from corn starch feedstocks.¹³⁷ 

This review summarized the research on converting biomass to platform C4 chemicals and on subsequent catalytic reactions of the platform molecules for production of C4 chemicals including succinic acid, C4 diols, GBL, THF, BD, butene, butanol, isobutanol, 2,3-butanedione and MEK. Bio-derived succinic acid can be catalytically transformed into interesting products, such as 14BDO, GBL, THF, which were widely used to synthesize polymers, solvents, and pharmaceutical products. 14BDO, 23BDO and 13BDO are important platform chemical used for production of chemical intermediates, building block compounds, and polymers. The three butandiols can all be biosynthesized from biomass-based carbohydrates, possibly enabling catalytic routes to produce BD sustainably. Acid–base properties greatly affect the 23BDO dehydration into BD. MEK formation via pinacol rearrangement dominates in 23BDO dehydration over acid catalysts. Rare earth oxides Sc₂O₃ and ZrO₂ convert 23BDO into 3B2OL, which can be further dehydrated into BD over acid catalyst. 14BDO dehydration into BD was carried out over Al₂O₃, SiO₂–Al₂O₃, ZrO₂ and CeO₂. Side products 3-buten-1-ol and THF can both be converted into BD. 13BDO dehydration into BD was achieved using SiO₂–Al₂O₃. CeO₂ selectively converts 13BDO into 2-buten-1-ol and 3B2OL, followed by dehydration to BD. Meanwhile, CeO_x nanoparticles encapsulated in mordenite tuned the acid sites with medium strength and was correlated with the catalytic activity of 13BDO dehydration into BD. 2,3-Butanedione can be synthesized from MEK oxidation and also 23BDO dehydrogenation. MEK can be sustainably produced from bio-derived 23BDO dehydration over acid catalysts with selectivity over 90%. Interest in n-butanol and isobutanol as advanced liquid biofuels has led to research on catalytic reactions based upon bio-derived chemicals. n-Butanol can be biosynthesized from ethanol dehydration via the Guerbet reaction and direct reaction. Isobutanol can also be produced from methanol/ethanol mixtures via the Guerbet reaction mechanism. The extensive work on producing C4 chemicals from biomass-derived compounds and notable successes indicate that this will continue to be an area of interest for catalytic scientists and engineers.