CN111918856A

CN111918856A - Continuous process for the highly selective conversion of sugars to propylene glycol or a mixture of propylene glycol and ethylene glycol

Info

Publication number: CN111918856A
Application number: CN201780098192.6A
Authority: CN
Inventors: D·施雷克; R·克里斯曼; B·阿尔宾; N·克林顿; M·布拉德福德; D·邦宁
Original assignee: Iowa Corn Promotion Board
Current assignee: Iowa Corn Promotion Board
Priority date: 2017-12-01
Filing date: 2017-12-01
Publication date: 2020-11-10
Also published as: BR112020010866A2; EP3717445A4; JP6928178B2; JP2021504409A; WO2019108233A1; EP3717445A1

Abstract

A continuous process for the production of propylene glycol from ketogenic saccharides is disclosed which enhances selectivity to propylene glycol.

Description

Continuous process for the highly selective conversion of sugars to propylene glycol or a mixture of propylene glycol and ethylene glycol

Technical Field

The present invention relates to a process for the production of propylene glycol (1, 2-propanediol), in particular an efficient continuous process for the conversion of sugars to propylene glycol or a mixture of propylene glycol and ethylene glycol.

Background

Propylene glycol is a valuable commodity chemical with a wide range of uses, such as for freeze protection. Propylene glycol is currently produced from hydrocarbon feedstocks and by hydrogenolysis of glycerol.

An improved continuous process for the conversion of saccharides to ethylene glycol and propylene glycol is disclosed by Schreck et al in us patent 9,399,610B 2. They disclose the conversion of saccharides to glycols using a reactor having a first zone comprising a retro-aldol catalyst and a second zone comprising a retro-aldol reduction catalyst. In the case of an aldose feed, glycolaldehyde from the retro-aldol reaction is hydrogenated to ethylene glycol in the second zone of the reactor. They also disclose the use of ketoses as sugars for the production of propylene glycol.

However, challenges remain to further improve the selectivity of the conversion of saccharides to glycols. These challenges are not insignificant because an infinite number of reactions may occur under the conditions required for the retro-aldol reaction and hydrogenation, including, but not limited to, the hydrogenation of sugars to hexitols or pentitols (referred to herein as alkanols) and the formation of by-products such as methane, methanol, ethanol, propanol, glycerol, 1, 2-butanediol, threitol, and humus. While some of the by-products may be sold, their recovery to meet commodity grade specifications can be expensive.

Disclosure of Invention

The present invention provides a continuous process that increases the selectivity of the retro-aldol and hydro-conversion of saccharides to propylene glycol or a mixture of propylene glycol and ethylene glycol.

According to the present invention, rapid heating of the saccharide supplied to the reaction zone can reduce the production of alkanol and other by-products. The mechanism by which rapid heating of the saccharide feed results in improved selectivity for conversion to propylene glycol or a mixture of ethylene glycol and propylene glycol is not fully understood. Without wishing to be bound by theory, it is believed to some extent that the rate of retro-aldol conversion in the presence of a retro-aldol catalyst is sufficiently fast at temperatures above about 225 ℃, preferably above 230 ℃, that the intermediates are optionally converted to propylene glycol or ethylene glycol compared to the by-products.

Direct measurement of the heating rate of the saccharide feedstock is problematic due to the rate at which heating occurs. Heat and mass transfer through the heated fluid further confound the temperature measurement problem. The heat and mass transfer parameters within a given fluid will depend on many factors including, but not limited to, the heating method, the temperature differential, and the physical structure of the region where heating occurs. Moreover, analytical techniques for measuring the temperature in substantially all areas of the fluid are almost lacking. Thus, determining whether the heating rate is sufficient can only be performed with reference to the relative formation of certain compounds that are produced in the practice of the method. Nevertheless, it is believed that the heating rate is sufficient to raise the entire saccharide feed from about 170 ℃ to at least 225 ℃, preferably to at least 230 ℃, in less than about 10 or 15 seconds, and more preferably less than about 5 seconds, and in some cases less than about 3 seconds, and in some other cases less than about 1 second, especially in the isomerization of sugars (e.g., isomerization of aldoses to ketoses or isomerization of ketoses to aldoses) will be detrimental to ensuring the desired conversion to propylene glycol or a mixture of propylene glycol and ethylene glycol having the sought molecular ratio. However, the process of the present invention also includes the intentional isomerization of a saccharide feedstock, i.e., isomerization of aldoses to ketoses or isomerization of ketoses to aldoses, to obtain a product having the desired propylene glycol to ethylene glycol mass ratio.

The temperature range by which the process according to the invention rapidly heats the carbohydrate feed is from below 170 ℃ to above 230 ℃. In some cases, the saccharide feed may be at a temperature of less than about 150 ℃, or even less than about 100 ℃, when rapid heating is initiated. In some cases, it is preferred that where the saccharide feedstock comprises a retro-aldol catalyst, rapid heating of the saccharide feedstock begins before about 100 ℃.

According to one broad aspect of the invention, a continuous process for converting a saccharide-containing feedstock (containing at least about 40 mass% of at least one of aldopentoses, ketohexoses, and ketopentoses based on the mass of total saccharides in the saccharide-containing feedstock) to propylene glycol or a mixture of propylene glycol and ethylene glycol comprises:

a. continuously or intermittently passing the saccharide feedstock into a reaction zone having an aqueous hydrogenation medium containing a retro-aldol catalyst, hydrogen and a hydrogenation catalyst;

b. maintaining an aqueous hydrogenation medium in a reaction zone under hydrogenation conditions to provide a product solution comprising propylene glycol and an alkanol comprising at least one of a pentose and a hexitol and optionally ethylene glycol, said hydrogenation conditions comprising a temperature in a range between about 230 ℃ and 300 ℃, a ratio of a retro-aldol catalyst to a hydrogenation catalyst, and a hydrogen partial pressure, the combination of these conditions being sufficient to convert at least about 95% of the saccharide feed; and

c. continuously or intermittently withdrawing a product solution from said reaction zone,

wherein the saccharide feed is at least partially hydrated and at a pressure sufficient to maintain partial hydration; wherein the saccharide feed is at a temperature of less than about 170 ℃; and wherein the saccharide feed is heated to above 230 ℃ immediately prior to or in the reaction zone and the rate of heating of the saccharide feed from below 170 ℃ to above 230 ℃ is sufficiently fast to provide a product solution having a mass ratio of propylene glycol to ethylene glycol of greater than 1:5, preferably greater than about 1:2, sometimes greater than about 1.5:1, and a mass ratio of the total of propylene glycol and ethylene glycol to the alkanol of greater than about 10:1, preferably greater than about 20: 1.

According to another broad aspect of the invention, a continuous process for converting a saccharide-containing feedstock (the feedstock containing at least about 40 mass%, and sometimes at least about 50 mass% aldohexoses, based on the mass of total saccharides in the saccharide-containing feedstock) to propylene glycol or a mixture of propylene glycol and ethylene glycol comprises:

wherein the saccharide feed is at least partially hydrated and at a pressure sufficient to maintain partial hydration; wherein the saccharide feed is at a temperature of less than about 170 ℃; and wherein the saccharide feed is heated to above 230 ℃ immediately prior to or in the reaction zone and the rate of heating of the saccharide feed from below 170 ℃ to above 230 ℃ is sufficiently slow to provide a product solution having a mass ratio of propylene glycol to ethylene glycol of greater than 1:5, preferably greater than about 1:2, sometimes greater than about 1.5:1, and a mass ratio of the total of propylene glycol and ethylene glycol to the alkanol of greater than about 10:1, preferably greater than about 20: 1.

When the saccharide feedstock contacts the aqueous hydrogenation medium, the saccharide feedstock contains at least one of an aldopentose and a ketose-producing saccharide. Aldopentoses produce glycolaldehyde and glyceraldehyde upon retro-aldol cleavage. Glyceraldehyde can eventually be converted to propylene glycol. The ketose-producing saccharide may itself be a ketose, or may be a disaccharide or polysaccharide or hemicellulose that produces a ketose or ketose precursor upon hydrolysis. Within the broad scope of the present invention, the saccharide feedstock may also comprise aldose-producing saccharides or other aldose-producing saccharides. It is understood that aldoses can isomerize to ketoses, and thus aldoses are ketogenic sugars. Conversely, ketoses may isomerize towards equilibrium with aldoses. The equilibrium and kinetics of the isomerization reaction are influenced by the presence and type of isomerization catalyst, the type of aldose or ketose to be isomerized, the temperature, the interaction with other components present (possibly catalytic isomerization conditions), and other conditions of the feed as is well known in the art. In an optional embodiment of the process of the present invention, the saccharide feed is maintained under isomerization conditions for a time sufficient to achieve the desired degree of isomerization. Typically, the retro-aldol catalyst also catalyzes the isomerization. Thus, the isomerization may, but need not, be carried out in the presence of a retro-aldol catalyst. The temperature of the isomerization is generally less than about 170 ℃ and preferably in the range of about 100 ℃ to about 150 ℃ or 160 ℃.

In many cases, at least about 60 mass%, and in some cases at least about 70 mass% of the total amount of aldopentoses, ketogenic saccharides, and aldose-producing saccharides is converted to propylene glycol and ethylene glycol. The mass ratio of propylene glycol to ethylene glycol will depend on the relative amounts of ketogenic saccharide, aldopentose, aldose-producing saccharide present in the saccharide feed and the degree of isomerization. Typically, the mass ratio of propylene glycol to ethylene glycol is greater than about 1:5, for example, greater than about 1:2, and sometimes greater than 1: 1. When the saccharide-containing feed is at least about 40 or 50 mass% of a ketogenic saccharide (e.g. sucrose), the mass ratio of propylene glycol to ethylene glycol is greater than about 1.5:1, such as greater than about 3: 1. A saccharide feed that provides typically between about 120 and 700 or 800, preferably between about 150 and 500, for example between 200 and 400 grams total saccharide per liter of aqueous hydrogenation medium provides a product solution with a favorable ratio of propylene glycol to ethylene glycol, and reduced co-production of alkanol and 1, 2-butanediol.

In many cases, the mass ratio of 1, 2-butanediol to total propylene glycol and ethylene glycol in the product is less than about 1:30, more preferably less than about 1: 50.

The saccharide feedstock may be mixed with the retro-aldol catalyst prior to heating to a temperature of at least 225 ℃, preferably at least about 230 ℃, or may be substantially free of any retro-aldol catalyst. In some aspects of the invention, the saccharide feedstock will comprise a retro-aldol catalyst. In these aspects, a retro-aldol reaction may occur during heating.

Heating of the saccharide feedstock may be achieved by indirect heat exchange, direct heat exchange, or a combination of both. In embodiments of the process of the present invention in which the saccharide feedstock is heated at least in part by direct heat exchange, the heating may be carried out immediately prior to or in the reaction zone. The hotter fluid used for direct heat exchange may be any suitable fluid and typically comprises water. The temperature and amount of this hotter fluid, in combination with any other heating source, is sufficient to bring the saccharide feed to a temperature of at least 230 ℃. Typically, the hotter fluid is above 230 ℃ and in some cases above 235 ℃. The saccharide feed may be introduced into the reaction zone for hydrogenation or, if used, into a previous retro-aldol reaction zone in the substantial absence of hydrogenation catalyst and the aqueous medium contained in such reaction zone is used as the hotter fluid. Alternatively, a hotter fluid may be combined with the saccharide feed and the combination then introduced into the retro-aldol reaction zone or the combined retro-aldol and hydrogenation reaction zone.

The initial contact between the saccharide feed and the retro-aldol catalyst may occur in the reaction zone containing the hydrogenation catalyst or in a separate reaction zone. In the case where the contacting is initiated in a separate reaction zone, all or part of the saccharide may be reacted in the separate reaction zone. In some cases, all or a portion of the ketose and aldose (if present) are subjected to retro-aldol conversion in a reaction zone containing a hydrogenation catalyst, e.g., at least about 10 mass%, sometimes at least about 20 mass%, to substantially all of the ketose in the saccharide feedstock is subjected to retro-aldol conversion in a reaction zone containing a hydrogenation catalyst. In certain embodiments of the process of the present disclosure, the retro-aldol catalyst is a homogeneous catalyst and the hydrogenation catalyst is heterogeneous. Thus, the dispersion of the retro-aldol catalyst within the area occupied by the hydrogenation catalyst can provide an intermediate that can provide propylene glycol adjacent to the hydrogenation site.

The amount of hydrogenation catalyst required in a given case will depend on the relative activity of the catalyst and the mass transfer of hydrogen and glycolaldehyde and intermediates to the catalyst. Preferred hydrogenation catalysts are supported nickel-containing hydrogenation catalysts, especially nickel catalysts containing one or both of rhenium and iridium. The ratio of retro-aldol catalyst to hydrogenation catalyst is preferably sufficient so that alkanol production from the hydrogenation of, for example, ketose-producing saccharides and aldose-producing saccharides is minimized. However, it is preferred that the hydrogenation catalyst has a density in the reaction zone sufficient to result in hydrogenation of glycolaldehyde and other intermediates before competing reactions of the intermediates are capable of producing products other than propylene glycol and ethylene glycol.

In some cases, the saccharide feed may be a molten solid, in which case it should remain at least partially hydrated to avoid caramelization during heating. Preferably, the saccharide feedstock is provided in the form of an aqueous solution.

Drawings

Figure 1 is a schematic diagram of a facility capable of using the process of the present invention, according to some embodiments.

Detailed Description

All patents, published patent applications, and articles cited herein are hereby incorporated by reference in their entirety.

Definition of

The following definitions and methods are provided to better define the present invention and to guide those of ordinary skill in the art in the practice of the present invention. Unless otherwise indicated, the terms are to be understood in accordance with their ordinary usage by those of ordinary skill in the relevant art.

Where ranges are used herein, only the endpoints of the ranges are recited to avoid verbose recitation and description of each and every value included in the ranges. Any suitable intermediate values and ranges between the endpoints may be selected. For example, if a range between 0.1 and 1.0 is recited, all intermediate values (e.g., 0.2, 0.3.6.3, 0.815, etc.) are included as are all intermediate ranges (e.g., 0.2-0.5, 0.54-0.913, etc.).

The use of the terms a and an are intended to include one or more of the described elements.

Mixing (admixing or admixed) refers to the formation of a physical combination of two or more elements that may have a uniform or non-uniform composition throughout and includes, but is not limited to, solid mixtures, solutions and suspensions.

Aldoses are those containing only one aldehyde group (-CH ═ O) per molecule and having the general chemical formula C_n(H2O)_nA monosaccharide of (a). Non-limiting examples of aldoses include aldohexoses (all aldehyde-containing six-carbon sugars including glucose, mannose, galactose, allose, altrose, idose, talose, and gulose); aldopentose (all five-carbon sugars containing aldehydes, including xylose, lyxose, ribose, and arabinose); aldotetroses (all four-carbon sugars containing aldehydes, including erythrose and threose) and aldotrioses (all three-carbon sugars containing aldehydes, including glyceraldehyde).

An aldose-producing saccharide refers to an aldose or a disaccharide or polysaccharide or hemicellulose that can produce an aldose upon hydrolysis. Most sugars are cyclic structures at ambient conditions, and therefore the aldose form occurs under the conditions of the process of the present invention. For example, sucrose is an aldose-producing saccharide, and ketose is produced even when it is hydrolyzed.

Alkanols are pentoses and hexitols. Hexitol is an empirical formula C₆H₁₄O₆A six carbon compound having one hydroxyl group per carbon. Hexitols may have different stereo configurations, such as sorbitol and mannitol. Pentitol is an alcohol of empirical formula C₅H₁₂O₅A pentacarbon compound having one hydroxyl group per carbon. The pentitols may have different steric configurations. Although this definition is limited to pentose and hexitol, it is understood that the saccharide feedstock may comprise one or more sugar alcohols of higher or lower carbon number, including mono-, di-and trisaccharide alcohols ("other-sugar alcohols"). These other-sugar alcohols provide additional sugars for conversion to propylene glycol and/or ethylene glycol.

Aqueous and aqueous solutions mean that water is present, but water is not required as a major component. For purposes of illustration and not limitation, a solution of 90% by volume ethylene glycol and 10% by volume water would be an aqueous solution. Aqueous solutions include liquid media containing dissolved or dispersed components such as, but not limited to, colloidal suspensions and slurries.

By a saccharide feedstock of biological origin is meant a product comprising all or a substantial portion of the saccharide derived from, or synthesized from a biological product or renewable agricultural feedstock (including but not limited to plant, animal, bacterial and marine feedstocks) or forestry material.

By initial contact is meant that the fluid is initially contacted with the component (e.g., a medium containing a homogeneous or heterogeneous catalyst), but not all molecules of the fluid are contacted with the catalyst.

The composition of the aqueous solution is determined using gas chromatography for the lower boiling components, typically the components having 3 or less carbons and a normal boiling point of less than about 300 ℃, and high performance liquid chromatography for the higher boiling components, typically having 3 or more carbons.

The conversion efficiency of ketose to propylene glycol or aldohexose to ethylene glycol is reported as mass percent and is calculated as the mass of ethylene glycol contained in the product solution divided by the mass of ketose or aldose as the case may be theoretically provided by the saccharide feed.

High shear mixing involves providing fluids that travel at different velocities relative to adjacent regions, which can be achieved by fixed or moving mechanical devices that affect shear to promote mixing. As used herein, components subjected to high shear mixing may be immiscible, partially immiscible or miscible.

Hydrodynamic dispensing refers to the dispensing of an aqueous solution in a vessel, including contact with any catalyst contained therein.

Immediately before means that there is no intervening unit operation requiring a residence time of more than 1 minute.

Intermittent means at intervals and possibly at regular or irregular intervals.

Ketose refers to a monosaccharide containing one keto group per molecule. Non-limiting examples of ketoses include hexulose (all ketone-containing six-carbon sugars including fructose, psicose, sorbose, and tagatose), heptulose (ketone-containing heptasaccharide), ketopentose (all ketone-containing pentasaccharide including xylulose and ribose), ketotetrose (all ketone-containing four-carbon sugars including erythrulose), and ketotriose (all ketone-containing three-carbon sugars including dihydroxyacetone).

A ketose-producing saccharide is taken to mean ketose or a disaccharide or polysaccharide or hemicellulose which, on hydrolysis, can produce ketose or a ketose precursor. Most sugars are ring structures at ambient conditions, and so the ketose form occurs under the conditions of the present method. For example, sucrose is a ketogenic saccharide, and even when it is hydrolyzed, aldose is produced. For purposes herein, unless the context requires otherwise, sugars that produce aldoses and ketoses will be considered ketose-producing sugars.

The pH of the aqueous solution was measured at ambient pressure and temperature. In determining the pH of, for example, an aqueous hydrogenation medium or product solution, the liquid is cooled and allowed to stand at ambient pressure and temperature for 2 hours before the pH is determined.

The pH control agent refers to a buffer and one or more of an acid or a base.

Pressure sufficient to maintain at least partial hydration of the saccharide means that the pressure is sufficient to maintain sufficient water of hydration on the saccharide to prevent caramelization. At temperatures above the boiling point of water, the pressure is sufficient to retain the water of hydration on the carbohydrate.

Rapid diffusion mixing is mixing where at least one of the two or more fluids to be mixed is finely dispersed to promote mass transfer to form a substantially uniform composition. Fractal mixing is rapid diffusion mixing.

The reactor may be one or more vessels in series or in parallel, and the vessel may contain one or more zones. The reactor may be of any suitable design for continuous operation, including but not limited to a tank and tube or tubular reactor, and may have fluid mixing capabilities if desired. Types of reactors include, but are not limited to, laminar flow reactors, fixed bed reactors, slurry reactors, fluidized bed reactors, moving bed reactors, simulated moving bed reactors, trickle bed reactors, bubble columns, and loop reactors.

Soluble means capable of forming a single liquid phase or forming a colloidal suspension.

Saccharide feedstock

The process of the invention uses a saccharide feed containing a ketose-producing saccharide. In some cases, the saccharide feed comprises at least about 40 mass%, preferably at least about 50 mass%, of ketose-producing saccharide based on the total saccharide in the feed. Where product solutions comprising high mass ratios of propylene glycol to ethylene glycol are sought, the saccharide in the feed comprises at least about 90 mass%, preferably at least about 95 or 99 mass% of a ketogenic saccharide. The carbohydrate feed typically comprises carbohydrate polymers (e.g. starch, cellulose) or partially hydrolyzed portions of such polymers, or mixtures of such polymers and partially hydrolyzed portions. Preferred ketoses are hexulose and ketopentose. The saccharide-containing feed may also contain other components, including those that can be converted to ethylene glycol or propylene glycol under the conditions of the process of the present invention. Glycerol is an example of a non-saccharide that can ultimately be converted to propylene glycol.

Most carbohydrate feedstocks of biological origin will produce glucose upon hydrolysis. The process of the present invention can be effectively used to convert glucose and glucose precursors to propylene glycol and mixtures of propylene glycol and ethylene glycol. Ketoses may be obtained from carbohydrate polymers and oligomers (e.g., hemicellulose, partially hydrolyzed forms of hemicellulose), disaccharides may be used, such as sucrose, lactulose, lactose, turanose, maltulose, palatinose, gentiobiulose, melibiose, and psyllium disaccharide or combinations thereof. However, the nature of these sugars may result in variable mixtures of ethylene glycol and propylene glycol.

As described above, the saccharide-containing feed may be subjected to isomerization conditions. Thus, an aldose-rich feed may be converted into a feed having a higher concentration of ketose, or a ketose-rich feed may be converted into a feed having a higher concentration of aldose. In some cases, the retro-aldol catalyst also promotes isomerization. Thus, the operator has the flexibility to use different feedstocks and adjust the molar ratio of propylene glycol to ethylene glycol, if desired. When isomerization is sought, it is typically carried out at a temperature of about 100 ℃ or 120 ℃ to about 220 ℃ for about 1 to 120 minutes. However, it is generally preferred that the duration of subjecting the saccharide-containing feedstock to a temperature of 170 ℃ or greater be maintained for less than about 10 seconds. In one aspect of the invention, the isomerization is carried out under conditions that enable the desired molar ratio of propylene glycol to ethylene glycol to be produced, which are generally milder conditions, e.g., a temperature of about 120 ℃ to 170 ℃, followed by rapid heating of the feed to at least 225 ℃ or at least 230 ℃ to attenuate further isomerization.

The saccharide feedstock may be a solid or liquid suspension, or dissolved in a solvent such as water. When the saccharide feedstock is in a non-aqueous environment, it is preferred that the saccharide is at least partially hydrated. More preferably, the saccharide feedstock is provided in the form of an aqueous solution. The mass ratio of water to saccharide in the saccharide feed is preferably in the range of 4:1 to 1: 4. Aqueous solutions containing 600 or more grams per liter of certain sugars, such as glucose and sucrose, are sometimes commercially available. In some cases, the saccharide feedstock may comprise a recycled aqueous hydrogenation solution or an aliquot or isolated portion thereof. When the saccharide feedstock comprises ethylene glycol or propylene glycol, the mass ratio of the total amount of ethylene glycol and propylene glycol to the saccharide is in the range of about 20:1 to 1: 20. It is within the scope of the present invention to add water to the saccharide feed prior to introduction to the aqueous hydrogenation medium. The saccharide contained in the saccharide feed is provided in an amount of about 120 to 700 or 800 grams, typically about 150 to 500 grams per liter of aqueous hydrogenation medium. Optionally, a separate reaction zone may be used which contains the retro-aldol catalyst in the substantial absence of a hydrogenation catalyst. In the case of a separate reaction zone containing a retro-aldol catalyst, it is preferred that the saccharide contained in the saccharide feed to the reaction zone provides about 120 to 700 or 800, typically about 150 to 500, grams of total saccharide per liter of aqueous medium in the separate reaction zone.

Rapid temperature rise

According to a first broad process of the invention, the saccharide feedstock is rapidly converted in a temperature zone of from 170 ℃ to 225 ℃ or 230 ℃, and preferably to at least about 240 ℃. The rapid temperature rise reduces the risk of caramelization of the saccharides and the production of products that reduce the conversion to propylene glycol or the desired ratio of propylene glycol to ethylene glycol product. In some cases, it has been found that rapid heating in the temperature region of 170 ℃ to 230 ℃ provides a relatively low mass ratio of glycerol to propylene glycol. In this case, the mass ratio of glycerol to propylene glycol is typically less than about 0.5: 1. The low yield of glycerol as a by-product is advantageous because the market value of glycerol is relatively low compared to ethylene glycol and propylene glycol.

During heating, other chemicals may be present in the saccharide feedstock. For example, hydrogen for hydrogenation may be supplied at least partially with the saccharide feed. Other adjuvants, such as pH control agents, may also be present if desired. In one embodiment, the saccharide feedstock comprises a retro-aldol catalyst, and in this case, the catalytic conversion of the saccharide occurs during heating. The degree of conversion of the saccharide during heating will be influenced by, inter alia, the duration of heating, the relative concentrations of saccharide and retro-aldol catalyst, and the activity of the retro-aldol catalyst.

As noted above, heating of the saccharide feedstock may be accomplished in any suitable manner, and one or more types of heating may be used. All, no, or partial heating of the saccharide feedstock may occur prior to introducing the saccharide feedstock into the aqueous hydrogenation medium. For example, but not limiting of, heating of the saccharide feed in a temperature zone of 170 ℃ to 225 ℃ or 230 ℃ may occur prior to introduction into the aqueous hydrogenation medium. In embodiments in which the heated saccharide feedstock is maintained in contact with the retro-aldol catalyst prior to introduction into the aqueous hydrogenation medium, the duration of such contact prior to introduction into the aqueous hydrogenation medium is typically less than about 15 seconds, preferably less than about 10 seconds, and in some cases less than 5 seconds. Generally, any hold time prior to introducing the heated saccharide feed into the aqueous hydrogenation medium is a result of equipment configuration, such as piping distance and residence time in ancillary equipment (e.g., fluid distributors from the heat exchange zone into the hydrogenation zone). It will be appreciated that the up and down operation will affect the inherent hold time.

The heat source used to heat the saccharide feed in the temperature region of 170 ℃ to 225 ℃ or 230 ℃ is not critical. For example, heating may be provided by radiation or microwave excitation, indirect heat exchange with other process streams, or direct heat exchange with process streams that are also passed through the aqueous hydrogenation medium, or a combination thereof.

In case the saccharide feed is at least partially heated in a temperature zone of from 170 ℃ to 225 ℃ or 230 ℃ by direct heat exchange with the aqueous hydrogenation medium, it is generally preferred that the retro-aldol catalyst is already present in the aqueous hydrogenation medium. As noted above, the heating rate will be affected by the heat and mass transfer parameters. It is generally desirable to promote mixing of the saccharide feedstock during heating to promote mass and heat transfer, thereby reducing the time required for the saccharide feedstock to pass entirely through this temperature zone. Such mixing may be effected in any suitable manner, including but not limited to mechanical and static mixing, and rapid diffusion mixing. The completeness of mixing can also affect the mass transfer of reactants, intermediates, catalysts and products, and thus the selectivity of conversion to propylene glycol and ethylene glycol, as well as the rate of formation of by-products.

A particularly useful stream for direct heat exchange with the saccharide feed is the withdrawn product solution (recycle). If a soluble retro-aldol catalyst is used in the aqueous hydrogenation medium, the recycle provides a large return of the retro-aldol catalyst to the reaction system. The recycle may be at a temperature of at least about 180 ℃, for example at a temperature in the range of about 230 ℃ to 300 ℃. The mass ratio of recycle to saccharide feed will depend on the relative temperatures of the two streams and the combined temperature sought. Typically where recycle is used, the mass ratio of recycle to saccharide feed is in the range of about 1:1 to 100: 1. The recycle may be an aliquot of the withdrawn product solution, or unit operations may be performed to separate one or more components from the recycle stream, such as, but not limited to, degassing to remove hydrogen and filtering to remove, for example, any entrained heterogeneous catalyst. Where the product solution is degassed to recover at least a portion of the hydrogen, the recycle is typically an aliquot of the degassed product solution. In direct heat exchange in operation, one or more components may be added to the recycle prior to being combined with the saccharide feed. These components include, but are not limited to, a retro-aldol catalyst, a pH control agent, and hydrogen. By using a recycle of the withdrawn product solution, the combined saccharide feed and recycle may comprise unreacted aldose-producing saccharide, ethylene glycol intermediates and ethylene glycol. When a saccharide feed that is not in an aqueous solution is used, such as a solid or melt, the recycle provides water to dissolve the saccharide and stabilize the saccharide from caramelization.

By kinetic effects, the heating rate can affect the degree of isomerization towards equilibrium of aldoses or ketoses in the saccharide feedstock, thereby affecting the ratio of propylene glycol to ethylene glycol in the product. The slower the heating rate, the longer the time required to drive the saccharide to equilibrium at the elevated temperature. While higher temperatures generally increase the rate at which equilibrium is reached, the equilibrium itself may be shifted. Thus, the operator needs to consider the relative duration of the saccharide feed at each temperature. For example, using ketoses as saccharide feedstock, longer residence times may be required at lower temperatures (where the equilibrium is at a higher aldose concentration) if higher ethylene glycol to propylene glycol contents in the product solution are sought. The saccharide feedstock is then heated very rapidly and subjected to hydrogenation conditions to prevent excessive conversion of aldoses to ketoses, which would otherwise occur at equilibrium at higher temperatures, with lower aldose concentrations. The presence of the isomerization catalyst increases the rate at which the feed progresses toward equilibrium. The retro-aldol catalyst generally acts as an isomerization catalyst. Thus, depending on the degree of isomerization desired during rapid heating, the saccharide-containing feed may or may not contain a retro-aldol catalyst. Other isomerization catalysts may be used in combination with or in place of the retro-aldol catalyst. Chemical catalysts for isomerization are well known to include homogeneous or heterogeneous acids or bases. It is well known that enzymatic processes also isomerize sugars; however, these processes must be carried out at temperatures below that which adversely affects the enzyme. Thus, for a thermophilic enzyme, a preferred temperature is less than about 80 ℃ or 85 ℃.

Transformation process

In the process of the present invention, the saccharide feedstock is introduced into an aqueous hydrogenation medium comprising a retro-aldol catalyst, hydrogen and a hydrogenation catalyst. The saccharide feedstock may or may not have been subjected to retro-aldol conditions prior to introduction into the aqueous hydrogenation medium, and the saccharide feedstock may or may not have been heated through a temperature zone of 170 ℃ to 225 ℃ or 230 ℃ when contacting the aqueous hydrogenation medium. Thus, in some cases, the retro-aldol reaction may not occur until the saccharide feedstock is introduced into the aqueous hydrogenation medium, while in other cases, the retro-aldol reaction may have occurred at least partially prior to the introduction of the saccharide feedstock into the aqueous hydrogenation medium. It is generally preferred that the saccharide feedstock is rapidly dispersed in the aqueous hydrogenation medium, especially where the aqueous hydrogenation medium is used to provide direct heat exchange to the saccharide feedstock. This dispersion can be accomplished by any suitable procedure, including but not limited to the use of mechanical and fixed mixers and rapid diffusion mixing.

The preferred temperature for the retro-aldol reaction is generally between about 225 ℃ or 230 ℃ and 300 ℃, more preferably between about 240 ℃ and 280 ℃. The pressure (gauge) is typically in the range of about 15 to 200 bar (1500 to 20,000kPa), for example between about 25 and 150 bar (2500 to 15000 kPa). The retro-aldol reaction conditions include the presence of a retro-aldol catalyst. The retro-aldol catalyst is a catalyst that catalyzes the retro-aldol reaction. Examples of compounds that can provide the retro-aldol catalyst include, but are not limited to, heterogeneous and homogeneous catalysts, including supported catalysts, including tungsten and its oxides, sulfides, phosphides, nitrides, carbides, halides, and the like. Also included are tungsten carbide, soluble phosphotungstic, tungsten oxide supported on zirconia, alumina, and alumina-silica. Preferred catalysts are provided by soluble tungsten compounds such as ammonium metatungstate. Other forms of soluble tungstates such as ammonium paratungstate, partially neutralized tungstic acid and sodium metatungstate. Without wishing to be bound by theory, the species exhibiting catalytic activity may or may not be the same as the soluble tungsten compound introduced as the catalyst. Specifically, catalytically active species may be formed during the retro-aldol reaction. The concentration of the retro-aldol catalyst used may vary widely and will depend on the activity of the catalyst and other conditions of the retro-aldol reaction, such as acidity, temperature and concentration of the saccharide. Typically, the retro-aldol catalyst is provided in an amount providing between about 0.05 and 100 grams, for example between about 0.1 and 50 grams, of tungsten calculated as metal elements per liter of aqueous hydrogenation medium. The retro-aldol catalyst can be added to the aqueous hydrogenation medium as a mixture with the saccharide feed or as a separate feed, or both.

Where the saccharide feedstock is subjected to retro-aldol conditions prior to introduction into the aqueous hydrogenation medium, preferably, the introduction into the aqueous hydrogenation medium occurs in less than 1 minute, sometimes less than about 0.5 minute, and in some cases less than about 0.1 minute, from the time the saccharide feedstock is subjected to retro-aldol conditions. At least about 10%, preferably at least about 20%, of the aldose-producing saccharide remains in the saccharide feed when introduced into the aqueous hydrogenation medium. The retro-aldol conversion of the saccharide is continued in the aqueous hydrogenation medium, whereby the duration between the retro-aldol conversion of the ketose or aldopentose to the start of contact with the hydrogenation catalyst can be reduced.

Typically, the aqueous hydrogenation medium is maintained at a temperature of at least about 225 ℃ or 230 ℃ until at least about 95 mass%, preferably at least about 98 mass%, or in some cases substantially all of the aldopentose, ketogenic saccharide and aldose-producing saccharide are reacted. Thereafter, the temperature of the aqueous hydrogenation medium may be reduced, if desired. However, hydrogenation proceeds rapidly at these higher temperatures. Thus, the temperature of the hydrogenation reaction is typically between about 225 ℃ or 230 ℃ and 300 ℃, for example between about 235 ℃ or 240 ℃ and 280 ℃. The pressure (gauge) is typically in the range of about 15 to 200 bar (1500 to 20,000kPa), for example between about 25 to 150 bar (2500 to 15,000 kPa). The hydrogenation reaction requires the presence of hydrogen and a hydrogenation catalyst. Due to the low solubility of hydrogen in aqueous solutions, the concentration of hydrogen in the aqueous hydrogenation medium will be determined primarily by the partial pressure of hydrogen in the reactor. The pH of the aqueous hydrogenation medium is typically at least about 3, for example between about 3.5 and 8, and in some cases between about 4 and 7. Adjuvants having the effect of adjusting the pH may also be used.

The hydrogenation is carried out in the presence of a hydrogenation catalyst. The hydrogenation catalyst may also be referred to as a reduced metal catalyst, and is a catalyst for reducing carbonyls. Typically, the hydrogenation catalyst is a heterogeneous catalyst. It may be applied in any suitable manner including, but not limited to, fixed beds, fluidized beds, trickle beds, moving beds, slurry beds, and structured beds. Nickel, palladium and platinum are the more widely used reduced metal catalysts. However, many reduction catalysts will work in this application. The reduction catalyst may be selected from a variety of supported transition metal catalysts. Nickel, Pt, Pd and ruthenium are the main reducing metal components, known for their ability to reduce carbonyls. One particularly preferred catalyst for the reduction catalyst in this process is a Ni-Re catalyst supported on silica alumina. Due to the good selectivity of the conversion of the glycolaldehyde formed into ethylene glycol, similar forms of Ni/Re or Ni/Ir can be used. Nickel-rhenium is a preferred reducing metal catalyst and may be supported on alumina-silica, silica or other supports. Supported Ni-Re catalysts having B as a promoter are useful. The reduced catalyst may be pretreated or treated during hydrogenation to moderate or deactivate inappropriately active catalyst sites. The treatment may include one or more of limited sintering, coking, or contact with agents that can passivate or block active sites, such as molybdenum, tungsten, and rhenium. When the treatment is carried out during hydrogenation, the soluble salts of these reagents may be added first, continuously or intermittently. In some cases, an adjuvant may help to maintain the retro-aldol catalyst in an active form.

Typically the hydrogenation catalyst is provided in an amount of from about 0.1 to 100 grams, more typically from about 0.5 or 1 to 50 grams, per liter of aqueous hydrogenation medium in a slurry reactor, whereas in a packed bed reactor the hydrogenation catalyst comprises from about 20 to 80 volume percent of the reactor.

In general, the retro-aldol reaction proceeds faster than the hydrogenation reaction, and therefore the residence time of the saccharide feed in the hydrogenation reactor is selected to reflect the desired degree of hydrogenation. In some cases, the weight hourly space flow rate is between about 0.01 and 20hr-1, and often between about 0.02 and 5hr-1, based on the total sugars in the feed. In some cases, it is desirable to maintain the aqueous hydrogenation medium sufficiently dispersed to ensure a relatively uniform concentration of intermediates therein relative to ethylene glycol.

Retro-aldol and hydrogenation environments can lead to undesirable reactions. See, for example, Green chem.,2014,16, 695-707. Table 1 on page 697 and table 4 on page 700 of the article report the product composition from subjecting various aldoses to retro-aldol and hydrogenation conditions. The major by-products they reported include sorbitol, erythritol, propylene glycol and glycerol.

In the process of the present invention, the combination of reaction conditions (e.g., temperature, hydrogen partial pressure, concentration of catalyst, hydraulic distribution, and residence time) is sufficient to convert at least about 95 mass%, typically at least about 98 mass%, and sometimes substantially all of the aldopentose, ketopentose-producing saccharide, and aldohexose-producing saccharide in the feed. One of ordinary skill in the art, having the benefit of the disclosure herein, will well determine the set or sets of conditions that will provide the sought conversion of the saccharide.

Without wishing to be bound by theory, it is believed that the formation of intermediates by the retro-aldol reaction requires time immediately following the hydrogenation of those intermediates to propylene glycol so that they are hydrogenated before a large amount of intermediates can be consumed in a competing reaction. Thus, for a given retro-aldol catalyst and a given hydrogenation catalyst, the balance between the retro-aldol catalyst and the hydrogenation catalyst can be determined under reaction conditions to achieve high conversion efficiency to propylene glycol and mixtures of propylene glycol and ethylene glycol. In addition, it is believed that the rapid heating of the saccharide feedstock makes it easier for the inverse aldol reaction rate to match the hydrogenation reaction rate at the temperature at which the feedstock is exposed.

It is believed that the ratio of retro-aldol catalyst to hydrogenation catalyst can also be used to attenuate alkanol production by minimizing the presence of sugars and providing intermediate concentrations of propylene glycol and ethylene glycol that preferentially enter active hydrogenation sites. One mode of operation of the process according to certain embodiments uses a homogeneous retro-aldol catalyst and a heterogeneous hydrogenation catalyst such that the retro-aldol catalyst can be physically located in the vicinity of the hydrogenation catalyst. Intermediates (which are smaller molecules) diffuse to the catalyst site faster than larger saccharide molecules, and the mass transfer rate of hydrogen to the hydrogenation catalyst is believed to regulate the hydrogenation reaction due to the limited solubility of hydrogen in the aqueous hydrogenation medium. Preferably, the mass ratio of the total amount of propylene glycol and ethylene glycol to alkanol in the product solution is greater than about 10:1, and in some cases greater than about 20:1 or 25:1 or even greater than about 40:1 or 50: 1. As noted above, providing total carbohydrate in the carbohydrate feed in an amount of about 120 to 700 or 800 grams, or 150 to 500 grams per liter of aqueous hydrogenation medium can be used to reduce the production rate of 1, 2-butanediol.

Determining a suitable ratio of retro-aldol catalyst to hydrogenation catalyst is within the skill of one having the benefit of the disclosure herein. This ratio will depend, inter alia, on the relative activities of the two catalysts under steady state conditions. The relative activity is influenced by the inherent activity of the catalyst and the physical configuration of the catalyst. Thus, the ratio of these catalysts can vary widely within the range of retro-aldol catalysts and hydrogenation catalysts. However, for a given retro-aldol catalyst and hydrogenation catalyst, the desired ratio can be determined. If a retro-aldol reaction zone is used in the substantial absence of hydrogenation catalyst, conditions (including but not limited to hydraulic residence time and retro-aldol catalyst concentration) can be adjusted to achieve the desired conversion efficiency, as taught by Schreck et al in U.S. patent 9,399,610B 2. If desired, the reaction zone containing the hydrogenation catalyst may have a different ratio of retro-aldol catalyst to hydrogenation catalyst. For example, in a continuous stirred tank reactor using a homogeneous reversed aldol catalyst and a heterogeneous hydrogenation catalyst and introducing the saccharide feed at or directly below the surface of the aqueous hydrogenation medium, the stirring rate may be such that there is a density gradient of the hydrogenation catalyst. The lower concentration of hydrogenation catalyst on top of the aqueous hydrogenation medium allows the saccharide to undergo retro-aldol reaction before substantial hydrogenation occurs.

Post-reaction treatment

The product solution is continuously or intermittently withdrawn from the reaction zone. After the reactor, a portion of the withdrawn product solution may be separated for recycling back to the front of the process as described above. Preferably, at least a portion of the retro-aldol catalyst is recycled or recovered from the withdrawn product solution for recycling. The withdrawn product solution may be depressurized with a trapped gas to recover hydrogen and remove unwanted gaseous byproducts, such as methane and carbon dioxide.

On cooling, the portion of the less soluble catalyst dissolved from the bed or fed to the reactor is removed at a reduced temperature and the remaining liquid is transferred to the recovery portion of the process. Depending on the stability and solubility of the catalyst, the degassed reactor effluent may be recovered, with a portion of the volatile products being recovered, while the heavy residue is treated, for example, to recover the tungsten catalyst for reuse in the reactor.

In the recovery, low boiling point components such as ethanol and methanol are removed by distillation. Water is also removed by distillation and then propylene glycol and ethylene glycol are recovered.

It is possible that the separation of ethylene glycol from propylene glycol or other near boiling glycols requires additional, more complex separation techniques. One such option that may be used is simulated moving bed technology. The choice depends on the product quality required for the desired end use of the product.

Drawings

Reference is made to the accompanying drawings, which are provided to facilitate an understanding of the invention and are not intended to be limiting of the invention. The drawing is a schematic depiction of an apparatus, generally designated 100, suitable for practicing the immediately disclosed method. The drawing omits such smaller equipment as pumps, compressors, valves, instrumentation and other devices, the arrangement of which and the operation of which are well known to those skilled in the art of chemical engineering. The figure also omits the auxiliary unit operations.

The saccharide feed is provided via line 102. The saccharide feedstock may be a solid or a liquid, including a solution with water. For purposes of discussion, the saccharide feedstock is an aqueous sucrose solution. The retro-aldol catalyst is provided via line 104. In this process, the addition of the retro-aldol catalyst at this point is optional. For discussion purposes, the retro-aldol catalyst is ammonium metatungstate in aqueous solution, and the ammonium metatungstate is provided in an amount sufficient to have an ammonium metatungstate concentration of about 10 grams/liter.

The saccharide feed is then combined with a hotter recycle stream of the withdrawn product solution, as will be described later. This combination affects direct heat exchange to increase the temperature of the saccharide feed and provide a combined stream. The combined stream is then passed via line 106 to distributor 108 and reactor 110. The distributor 108 may be of any suitable design. For purposes of discussion, the distributor 108 is a spray head that distributes the combined stream as fine droplets onto the surface of the aqueous hydrogenation medium 112 in the reactor 110. The reactor 110 includes an agitator 114 to provide mechanical mixing of the aqueous hydrogenation medium 112. This mechanical mixing helps to disperse the fine droplets of the combined stream within the aqueous hydrogenation medium to further increase the rate at which the combined stream reaches the temperature of the aqueous hydrogenation medium. It also facilitates mass transfer of intermediates from the retro-aldol reaction to the hydrogenation catalyst. Reactor 110 also contains a particulate heterogeneous hydrogenation catalyst, such as a nickel/rhenium/hydroboration catalyst on a silica support, which is dispersed in the aqueous hydrogenation medium by mechanical mixing.

Hydrogen is supplied to reactor 110 via line 116. Hydrogen may be supplied through nozzles to provide small hydrogen bubbles in order to promote mass transfer of hydrogen into the aqueous hydrogenation medium. Additional retro-aldol catalyst and other adjuvants can be supplied to the reactor via line 118 if desired.

The aqueous hydrogenation medium is withdrawn from reactor 110 as a product solution via line 120. As shown, a portion of the product solution is passed as recycle through line 122 to line 106 to be combined with saccharide feed 102. The recycle will contain a homogeneous retro-aldol catalyst. Optionally, the recycle stream in line 122 can be heated in an indirect heat exchanger 124 to provide a higher temperature to the combined stream in line 106.

The following examples are provided to further illustrate the invention and are not intended to limit the invention. All parts and percentages are by mass unless otherwise indicated. The following general procedure was used in examples 1 and 2.

A 300ml Hastelloy C Parr reactor is equipped with a stirrer, one or two feed supply lines and a dip tube attached to the sample bomb. The end of the dip tube is positioned so that about 100 ml of solution remains in the reactor. The reactor was charged with a heterogeneous hydrogenation catalyst and an aqueous ammonium tungstate solution (1.0 mass%). Approximately 170 ml of aqueous solution was charged. The reactor was then sealed and purged to remove oxygen. Purging was accomplished through three cycles: the reactor was pressurized to 50psig (345kPa gauge) with nitrogen and then vented to atmospheric pressure. The liquid level in the reactor was reduced to about 100 ml by draining the dip tube. While stirring the aqueous solution, hydrogen was used to reduce the nitrogen concentration and then vented to atmospheric pressure, thereby conducting three additional purge cycles.

Agitation is initiated and is performed at a rate sufficient to maintain the heterogeneous hydrogenation catalyst in the slurry dispersion. The reactor was heated to 245 ℃ and pressurized to 10700 kilopascal gauge under hydrogen. When the reactor reached the operating temperature and pressure, the feed of sucrose solution was started and kept at a constant 1.0 ml/min during the run. During the run, the retro-aldol catalyst was continuously added at a constant rate of 1 mass%/liter. The reactor is discharged at regular intervals to a constant level determined by the position of the dip tube, whereby near continuous operation can be achieved. A filter attached to the end of the dip tube ensures that all heterogeneous catalyst particles remain inside the reactor. Every 10 to 15 minutes, the reactor pressure was adjusted to 10700 kpa by adding hydrogen or venting.

After up to four hours of operation during the run, a sample of the aqueous medium was taken through the dip tube and sample bomb and cooled to room temperature. Samples were analyzed by High Pressure Liquid Chromatography (HPLC) and Gas Chromatography (GC). The HPLC was equipped with a refractive index detector and used a Hi-Plex H resin column available from Agilent technologies, Santa Clara, Calif. The GC analysis was performed using an HP 5890GC (agilent technologies, santa clara, california) using a flame ionization detector with 25:1 split injection. J & W DB-WAX 30m X0.32 mm X0.5 micron capillary columns (Agilent technologies, Santa Clara, Calif.) were used.

The hydrogenation catalyst is a nickel, rhenium and boron supported on silica alumina catalyst prepared using the procedure described in U.S. patent No. 6,534,441, column 8, line 62 to column 9, line 27. The silica alumina support was a 3 mm extrudate and had a surface area of about 125 square meters per gram and a pore volume of about 0.7 to 0.9 ml/g. Unless otherwise specified, the catalyst contained about 6.8 mass% nickel and the atomic mass ratio of nickel to rhenium to boron was about 5: 1.3: 1.

Example 1

In this example, sucrose was used as the saccharide feed and was provided at a concentration of about 32.4 mass% in the aqueous solution, unless otherwise specified. The sucrose-containing feed comprises a retro-aldol catalyst. The supply line used had an outer diameter of 1/16 inch (1.6 mm) and a length of 5 cm. The residence time per 2.5 cm length was about 0.4 seconds for a 1/16 inch supply line at a feed rate of 1 ml/min.

The sample contains propylene glycol and ethylene glycol in a mass ratio greater than about 3: 1; a ratio of the total amount of propylene glycol and ethylene glycol to 1, 2-butanediol greater than about 20: 1; and a ratio of total propylene glycol and ethylene glycol to alkanol of greater than about 10: 1.

Example 2

In this example, sucrose was used as the saccharide feed and was provided at a concentration of about 32.4 mass% in the aqueous solution, unless otherwise specified. When a single feed supply line is used, the glucose-containing feed contains a retro-aldol catalyst. The supply line used had an outer diameter of 1/16 inch (1.6 mm) and a length of 20 cm. The residence time per 2.5 cm length was about 0.4 seconds for a 1/16 inch supply line at a feed rate of 1 ml/min.

The sample contains propylene glycol and ethylene glycol in a mass ratio greater than about 1: 2; a ratio of the total amount of propylene glycol and ethylene glycol to 1, 2-butanediol greater than about 20: 1; and a ratio of total propylene glycol and ethylene glycol to alkanol of greater than about 10: 1.

Claims

1. A continuous process for converting a saccharide-containing feedstock containing at least about 40 mass% of at least one of aldopentoses, ketohexoses and ketopentoses, based on the mass of total saccharides in said saccharide-containing feedstock, to propylene glycol or a mixture of propylene glycol and ethylene glycol, said process comprising:

b. maintaining the aqueous hydrogenation medium in the reaction zone under hydrogenation conditions to provide a product solution comprising propylene glycol and an alkanol comprising at least one of a pentose and a hexitol and optionally ethylene glycol, the hydrogenation conditions comprising a temperature in a range between about 230 ℃ and 300 ℃, a ratio of a retro-aldol catalyst to hydrogenation catalyst, and a hydrogen partial pressure, the combination of these conditions being sufficient to convert at least about 95% of the saccharide feed; and

wherein the saccharide feedstock is at least partially hydrated and at a pressure sufficient to maintain partial hydration; wherein the saccharide feed is at a temperature of less than about 170 ℃; and wherein the saccharide feed is heated to above 230 ℃ immediately prior to or in the reaction zone and the rate of heating of the saccharide feed from below 170 ℃ to above 230 ℃ is sufficiently fast to provide a product solution having a mass ratio of propylene glycol to ethylene glycol of greater than 1:5 and a mass ratio of the total amount of propylene glycol and ethylene glycol to the alkanol of greater than about 10: 1.

2. The process of claim 1, wherein the feed comprises a ketose-producing saccharide and an aldose-producing saccharide and the mixture of propylene glycol and ethylene glycol is contained in the product solution.

3. The process of claim 2, wherein the mass ratio of propylene glycol to ethylene glycol in the product solution is greater than about 1.5: 1.

4. The method of claim 3, wherein the feed comprises sucrose.

5. The method of claim 1, wherein the product solution comprises a mass ratio of 1, 2-butanediol to propylene glycol and ethylene glycol combined less than about 1: 10.

6. The process of claim 1 wherein said aqueous solution is maintained at a temperature above about 170 ℃ and below about 230 ℃ for less than about 15 seconds prior to entering said aqueous hydrogenation medium.

7. The process of claim 1 wherein heating of the saccharide feed from below 170 ℃ to above 230 ℃ is carried out while contacting the saccharide with an aqueous retro-aldol solution comprising a retro-aldol catalyst in the substantial absence of a hydrogenation catalyst.

8. The process of claim 7 wherein said aqueous solution is maintained at a temperature above about 170 ℃ and below about 230 ℃ for less than about 5 seconds prior to entering said aqueous hydrogenation medium.

9. A continuous process for converting a saccharide-containing feedstock to propylene glycol or a mixture of propylene glycol and ethylene glycol, the feedstock containing at least about 40 mass% aldohexoses, based on the mass of total saccharides in the saccharide-containing feedstock, the process comprising:

wherein the saccharide feedstock is at least partially hydrated and at a pressure sufficient to maintain partial hydration; wherein the saccharide feed is at a temperature of less than about 170 ℃; and wherein the saccharide feed is heated to above 230 ℃ immediately prior to or in the reaction zone and the rate of heating of the saccharide feed from below 170 ℃ to above 230 ℃ is sufficiently slow to provide a product solution having a mass ratio of propylene glycol to ethylene glycol of greater than 1:5 and a mass ratio of the total amount of propylene glycol and ethylene glycol to the alkanol of greater than about 10: 1.

10. The process of claim 9, wherein the feed comprises a ketose-producing saccharide and an aldose-producing saccharide and the mixture of propylene glycol and ethylene glycol is contained in the product solution.

11. The process of claim 10, wherein the mass ratio of propylene glycol to ethylene glycol in the product solution is greater than about 1.5: 1.

12. The method of claim 11, wherein the feed comprises sucrose.

13. The method of claim 9, wherein the product solution comprises a mass ratio of 1, 2-butanediol to propylene glycol and ethylene glycol combined less than about 1: 10.

14. The process of claim 9 wherein said aqueous solution is maintained at a temperature above about 170 ℃ and below 230 ℃ for greater than about 5 seconds prior to entering said aqueous hydrogenation medium.

15. The process of claim 14, wherein heating of the saccharide feed from below 170 ℃ to above 230 ℃ is carried out while contacting the saccharide with an aqueous retro-aldol solution comprising a retro-aldol catalyst in the substantial absence of a hydrogenation catalyst.