Cross-aldol condensation of alkyl aldehydes using solid-supported bronsted acid catalysts

Abstract

Processes include cross-aldol condensations of alkyl aldehydes that include providing a reactant mixture containing a first alkyl aldehyde and a second alkyl aldehyde, the first alkyl aldehyde having a lower carbon number than the second alkyl aldehyde, and the feed stock having a molar ratio of 1:1 or more of the first alkyl aldehyde to the second alkyl aldehyde; and contacting the reactant mixture with a solid-supported Bronsted acid catalyst in a reactor; operating the reactor to generate a product mixture containing a cross-aldol product of the first alkyl aldehyde and the second alkyl aldehyde.

Description

[0001] CROSS-ALDOL CONDENSATION OF ALKYL ALDEHYDES USING SOLID- SUPPORTED BRONSTED ACID CATALYSTS

[0002] FIELD

[0003] Embodiments relate to processes for the generating branched aldehyde products by cross-aldol condensation using solid- supported Bronsted acid catalysts.

[0004] BACKGROUND

[0005] Cross-aldol condensation is a type of organic reaction that involves the formation of a carboncarbon bond between two carbonyl compounds (aldehydes or ketones). In cross-aldol condensation, carbonyl compounds combine to form a P-hydroxy carbonyl compound product under acidic or basic conditions. For example, a basic catalyst, such as hydroxide (OH-) or amine, may be used to deprotonate an a-carbon of one carbonyl compound, producing an enolate capable of subsequent nucleophilic attack. The enolate species can then combine with another carbonyl compound to generate a condensation product.

[0006] Cross-aldol condensations proceed readily under standard catalytic conditions, but traditional catalysts offer little control over selectivity for the cross-aldol product. Standard reaction conditions utilizing hydroxide catalysts also involve combination of the carbonyl reactants with an aqueous phase, which must be removed from the resulting product mixture. In addition to the target cross- aldol, product mixtures often contain significant amounts of self-aldol condensation byproducts, which lead to increased costs in terms of starting materials and time and processes to remove undesired products, catalysts, and caustic aqueous waste. Previous approaches to cross-aldol condensation include few ways to increase cross-aldol selectivity, and have focused predominantly on the use of activated substrate molecules having different rates of catalyst activation for aldol condensation. For example, selectivity may be introduced through the use of aldehydes having higher rates of reactivity in the presence of ketones, or the use of steric controls such as cross-aldol reactions between branched or functionalized aldehyde and linear aldehydes.

[0007] SUMMARY

[0008] Embodiments disclosed herein include processes for cross-aldol condensation of alkyl aldehydes that include providing a reactant mixture containing a first alkyl aldehyde and a second alkyl aldehyde, the first alkyl aldehyde having a lower carbon number than the second alkyl aldehyde, and the feed stock having a molar ratio of 1 : 1 or more of the first alkyl aldehyde to the second alkyl aldehyde; and contacting the reactant mixture with a solid-supported Bronsted acid catalyst in a reactor; operating the reactor to generate a product mixture comprising a cross-aldol product of the first alkyl aldehyde and the second alkyl aldehyde.

[0009] In another aspect, embodiments include a system for cross-aldol condensation of aldehydes that includes a reactor including a solid- supported Bronsted acid catalyst; a reactor input for providing a reactant mixture containing a first alkyl aldehyde and a second alkyl aldehyde, the first alkyl aldehyde having a lower carbon number than the second alkyl aldehyde; and a reactor output for obtaining a product mixture containing 20 wt% or more of the cross-aldol product of the first alkyl aldehyde and the second alkyl aldehyde.

[0010] DETAILED DESCRIPTION

[0011] Embodiments disclosed herein relate to processes for cross-aldol condensation using solid- supported Bronsted acid catalysts that generate branched aldehyde products from various alkyl aldehydes with improved selectivity and conversion. Methods may include batch or continuous processes that include reacting a first alkyl aldehyde and a second alkyl aldehyde having a higher carbon number in the presence of a solid- supported Bronsted acid catalyst. In some cases, processes may generate cross-aldol condensation products having a selectivity for the cross-aldol product of 2 or more over the concentration of the competing self-aldol products.

[0012] Cross-aldol condensations disclosed herein include supported (or heterogenous) Bronsted acid catalysts having improved selectivity and conversion compared to alternative methods utilizing unsupported basic catalysts (i.e., aqueous hydroxide). Solid-supported Bronsted acid catalysts may include resins capable of catalyzing cross-aldol condensation between unmodified alkyl aldehydes without the requirement for aldehyde activation or functionalization to direct the reaction or increase reactivity. Moreover, the use of a heterogeneous catalyst reduces the need to introduce aqueous streams associated with unsupported basic catalysis, while also minimizing or omitting the need for post-reactor separation of the catalyst from the product stream.

[0013] Methods disclosed herein include reacting the at least two alkyl aldehydes in the presence of a solid- supported Bronsted acid catalyst under cross-aldol condensation conditions to yield a product mixture containing a fraction of longer-chain, branched aldehydes. Cross-aldol reactions proceed by an addition reaction shown in Eq. 1 that is followed by a condensation reaction shown in Eq. 2. In both equations, Ri and R2 represent unique alkyl substituents attached to the carbonyl groups.

[0014] RICH2C(H)=O + R2HC(H)=O + Catalyst R2CH(OH)CH(RI)C(H)=O (1) R2CH(OH)CH(R1)C(H)=O R2CH=C(R1)C(H)=O + H20 (2)

[0015] Eqs. 1 and 2 are shown as reactions between two aldehydes, however, one or both of the species may be ketones without departing from the scope of the disclosure.

[0016] The catalyst is a solid-supported Bronsted acid catalyst containing acidic functional groups capable of exchanging positively charged hydrogen cations with cations from the surrounding medium. Solid- supported Bronsted acid catalysts may be made from an inert matrix (e.g., polymer, silica) that is functionalized with varying levels of acidic functional groups (e.g., sulfonate, carboxylate, phosphonate). An exemplary reaction is shown in Eq. 3, depicting the reaction of butyraldehyde with nonanal to generate a mixture of cross-aldol C13 isomers and self-aldol products.

[0017] Methods may be applied in batch and flow reactor setups, including methods in which the catalyst is regenerated for repeated use. For example, following a batch reaction, the catalyst may be regenerated using a suitable solvent and then re-used in one or more successive reactions. Similarly, in continuous methods (e.g., flow reactor), the solid- supported Bronsted acid catalyst may be regenerated (continuously or intermittently) to allow for repeated catalyst use.

[0018] Cross-aldol condensations may be performed between two or more alkyl aldehydes. As used herein, alkyl aldehydes are differentiated as “first” and “second,” where the first alkyl aldehyde as a lower carbon number than the second alkyl aldehyde. The alkyl aldehydes may have the general formula of RCH2C(H)=O, where R is an alkyl group that may be linear and / or non-functionalized (i.e., include aldehydes having non-branching alkyl substituents). In some cases, the first alkyl aldehyde has a carbon number in the range of 2 to 6 and the second alkyl aldehyde has a carbon number in the range of 7 to 20. Suitable reactant aldehydes include acetaldehyde, propanal, n-butanal, n-pentanal, hexanal, heptanal, octanal, longer chain aldehydes, and the like. In one example, cross- aldol condensations are used to generate branched C13 aldehydes or alcohols from linear C4 and C9 aldehydes as shown in Scheme II.

[0019] Cross-aldol condensations may proceed from a reactant mixture (i.e., for a batch process) or feed (i.e., for a continuous process) containing a molar ratio of first alkyl aldehyde: second alkyl

[0020] - 3-

[0021] SUBSTITUTE SHEET (RULE 26) aldehyde in a range of 4:1 to 1:4, 3:1 to 1:3, or 2:1 to 1:2. Alkyl aldehydes may be provided as is suitable for the particular reaction (i.e., batch or continuous). In batch methods, aldehydes may be added at the same time or in sequence, and at the total concentration or added, respectively, into a batch reactor in one or more fractions.

[0022] In batch methods, the combined aldehyde concentration in the reactant mixture as a percent by weight (wt%) ranging from 5 wt% to 99.9 wt%, 25 wt% to 95 wt%, or 40 wt% to 80 wt%. The resulting product mixture of a batch reaction may contain the cross-aldol product at a percent by weight (wt%) of 20 wt% or more, 30 wt% or more, or 40 wt% or more, or ranging from 5 wt% to 99.9 wt%, 25 wt% to 95 wt%, or 40 wt % to 80 wt%. In flow or continuous methods, the combined aldehyde concentration in the reactant mixture as a percent by weight (wt%) ranging from 5 wt% to 99.9 wt%, 25 wt% to 95 wt%, or 40 wt% to 80 wt%. The resulting product mixture of a flow reaction may be provided as an exit stream containing the cross-aldol product at a percent by weight (wt%) of 5 wt% or more, 10 wt% or more, or 20 wt% or more.

[0023] Cross-aldol condensations disclosed herein may be catalyzed by one or more solid-supported Bronsted acid catalysts, including acidic ion exchange resins. Ion exchange catalysts may include a heterogeneous catalyst support that contains or is functionalized with one or more types of acidic moieties, such as sulfonic acid, carboxylic acids, phosphonic acid, and the like.

[0024] Catalyst supports include any solid substance that is inert under the reaction conditions and can be modified with the selected acidic functional group. The support material can be inert materials including various polymers (e.g., crosslinked and non-crosslinked) including vinylaromatics such as styrene divinylbenzene, and the like, or oxides such as silica, alumina, and titania. The catalyst support can be in the form of powder, granules, pellets, or the like that are dimensioned for operation in the selected reactor. Examples of solid-supported Bronsted acid catalysts include sulfonated resins, sulfonic acid derivatives, carboxylic acid resins, phosphonic acid resins, sulfonimide resins, phenolic resins and derivatives, and the like.

[0025] Solid- supported Bronsted acid catalysts may be stable for any suitable operation temperature, including temperatures up to 200 °C or up to 300 °C, including at temperatures of 60 °C or more, or in the range of 40 °C to 200 °C, 50 °C to 150 °C, or 60 °C to 100 °C.

[0026] Solid-supported Bronsted acid catalysts disclosed herein may have an acid site concentration in the range of 0.0001 eq / mL to 10 eq / mL. Solid-supported Bronsted acid catalysts may be added at a molar percent of moles of acid content of the catalyst to total moles aldehydes (mol%) up to 5 mol%, up to 10 mol%, or up to 20 mol%, or in a range of 1 mol% to 20 mol%, or 1 mol% to 10 mol%.

[0027] Solid-supported Bronsted acid catalysts may be stored and / or placed into a reactor with a compatible solvent that is not water and unreactive in the reaction conditions. Solvents may include one or more hydrocarbons, including alkanes and alkenes having 4 to 15 carbon atoms. Solvents may be present in the reaction mixture and / or the product mixture at a percent by weight (wt%) of 10 wt% or more, or in a range of 10 wt% to 80 wt%.

[0028] In some cases, solid- supported Bronsted acid catalysts may undergo some level of deactivation during the reaction. Without being limited by theory, this may be due to the association with water or other polar species with the active sites of the catalyst that function to block active site access. In order to regenerate the catalyst, a regenerating solvent may be contacted with the catalyst and used to wash away aqueous and polar deactivating contaminants. Regenerating solvents may include glycol ethers such as anisole (methyl phenyl ether), tert-butyl methyl ether, dibenzyl ether, diethyl ether, dioxane, diphenyl ether, methyl vinyl ether, tetrahydrofuran, triisopropyl ether, diethylene glycol diethyl ether, diethylene glycol dimethyl ether (diglyme), diethylene glycol monobutyl ether, diethylene glycol monomethyl ether, 1,2-dimethoxyethane (monoglyme), ethylene glycol monobutyl ether, triethylene glycol dimethyl ether (triglyme), triethylene glycol monomethyl ether, acetone, diisobutyl ketone, methyl n-propyl ketone, methyl ethyl ketone, methyl isobutyl ketone, methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, n-butyl acetate, and combinations thereof.

[0029] Methods may include batch and continuous processes that contain the general steps of providing a reactant mixture containing a first alkyl aldehyde and a second alkyl aldehyde, the first alkyl aldehyde having a lower carbon number than the second alkyl aldehyde, and the feed stock having a molar ratio of 1:1 or more of the first alkyl aldehyde to the second alkyl aldehyde; and contacting the reactant mixture with a solid- supported Bronsted acid catalyst in a reactor; operating the reactor to generate a product mixture containing a cross-aldol product of the aldehydes.

[0030] The cross-aldol condensations described herein may be carried out in any reactor of suitable design, including batch, semi-batch reactors, and continuous flow reactors, without limitation as to design, size, geometry, flow rates, etc. (e.g., plug-flow reactors, continuous stirred-tank reactors, and the like). Example reaction parameters are given below and in the examples. Generally, reaction pressures run from atmospheric to about 100 atm, with temperatures ranging roughly from 0° C to 300° C. Cross-aldol condensations may be performed at suitable temperatures. For example, a reaction mixture or stream may be maintained at 60 °C or more during the condensation reaction.

[0031] Aqueous fluids may deactivate the solid-supported Bronstcd acid catalyst by associating with active acidic sites. Prior to cross-aldol condensation, solid-supported Bronsted acid catalysts may be activated by drying at 90°C to 100 °C (or higher depending on the catalyst) to remove adsorbed water. Moreover, the solid- supported Bronsted acid catalysts may be regenerated in batch or continuous settings by contacting the catalyst (in situ or removed from the reactor) with a regenerating solvent. The selected catalysts are then loaded into the desired reactor and the reactant mixture is added or fed into the reaction zone at the specified temperature for the specified time.

[0032] The cross-aldol product may be separated from a reaction mixture by any suitable method, such as solvent extraction, crystallization, distillation, vaporization, wiped film evaporation, falling film evaporation, phase separation, and filtration. For example, in a continuous process the liquid reaction mixture (containing aldehyde product, etc.), i.e., reaction fluid, removed from the reaction zone can be passed to a separation step, e.g., distillation column, wherein the desired aldehyde product can be separated via distillation, in one or more stages, under normal, reduced or elevated pressure, from the liquid reaction fluid, and further purified. The portion of the liquid reaction mixture that is not removed in the product stream from the separation step, containing aldehyde products, aldehyde feed, solvent, water, reaction byproducts, feedstock impurities, and the like, may then be recycled back to the reactor either in total or in part.

[0033] Systems for forming cross-aldol condensates may include a reactor (batch or continuous) containing a solid-supported Bronsted acid catalyst; a reactor input for providing a reactant mixture containing a first alkyl aldehyde and a second alkyl aldehyde, the first alkyl aldehyde having a lower carbon number than the second alkyl aldehyde; and a reactor output for obtaining a product mixture comprising 20 wt% or more of the cross-aldol product of the first alkyl aldehyde and the second alkyl aldehyde.

[0034] Methods disclosed herein may be performed in batch or continuous conditions with a cross- aldol selectivity, defined as the concentration of the cross-aldol product in the product mixture over the concentration of the self-aldol product of the second alkyl aldehyde in the product mixture (or stream), of 2.3 or more, 2.5 or more, or 3.0 or more, or in a range of 2.3 to 4.0, or 2.3 to 3.5.

[0035] The numerical ranges disclosed herein include all values from, and including, the lower and upper value and all values in between. Unless stated to the contrary, implicit from the context, or customary in the ail, all parts and percentages are based on weight and all test methods are current as of the filing date of this disclosure.

[0036] EXAMPLES The following examples are provided to illustrate the embodiments of the invention, but are not intended to limit the scope thereof. Materials used in the following examples are shown in Table 1. Solid-supported Bronsted acid catalysts are shown in Table 2, where all catalysts included sulfonic acid functional groups.

[0037] Cross-aldol condensation products were generated using batch or flow reactor (continuous) methods as described below. In these examples, conversion of a selected analyte (A) is defined as: Selectivity of product I from reactant A is: where x is the stoichiometric ratio required to convert A to i.

[0038] In the examples, “Cross-aldol selectivity” is denoted as a ratio of the cross-aldol product selectivity and the self-aldol product selectivity determined according to Eq. 5, respectively.

[0039] Unless otherwise specified in the examples, samples were tested under the following batch or continuous process conditions.

[0040] Batch Method

[0041] Batch reactions were performed on a Radleys Carousel 12 Plus Reaction Station with external overtemperature control with the provided reactor vials and lids. Cooling for reflux was provided by flowing water. In a standard run, catalyst was loaded in the reactor vial in an open lab. The premade aldehyde feed mixture of butanal, nonanal, decane, and 1-octene (37 / 37 / 3 / 23 wt%, respectively, “2:1 C4:C9”) was then added to the vial in a purge box. The vials were then capped and scaled, removed from the purge box and heated on the carousel reactor in a fume hood at the specified temperature with 500 rpm mixing and for the desired time. Following, the vials were removed from the carousel, uncapped, and the reaction mixture was filtered via syringe through a 0.2 pm filter and diluted in an approximately 2:1 isopropanokreaction mixture ratio for GC analysis.

[0042] Gas chromatography was performed on an Agilent 7890B GC equipped with a flame ionization detector using an Agilent J&W DB-17 column (Part Number 123-1732). The GC method involves a 5 min hold at 50°C followed by a 10°C / min ramp to 280°C and a 2 min hold for a total run time of 30 min.

[0043] Calibration of butanal, nonanal, 2-ethylhexanal, and 2-methylpentanal were completed by dilution of the pure materials in isopropanol to obtain standards of 40, 25 and 1 wt%. 2-ethylhexenal (89% purity) and octadecenal (83% purity) were calibrated at the same levels following synthesis of these materials by the self-aldol condensation of butanal and nonanal, respectively, using published methods described in Ostrowski, K. A.; Lichte, D.; Stuck, M.; Vorholt, A. J., Tetrahedron, 2016, 72 (5), 592. A response factor for the tridecenal (C13 product of the cross-aldol condensation of butanal and nonanal) was estimated based on linear interpolation from plotting response factors of the available aldehydes versus number of carbons. Decane was used as an internal standard for all components at 2 wt% decane and isopropanol was used as the diluent.

[0044] Flow Reactor Method

[0045] Flow reactions were performed in a custom-built flow reactor. Briefly, the reactor is a I / 2” OD stainless steel reactor connected up and downstream with 1 / 16” stainless steel tubing. There is PTFE tubing from selection valve 1 to the Gilson HPLC 305 pump equipped with a 5SC pump head. The reactor is placed vertically within a continuously N2 purged oven and solution flow is upwards. A catalyst bed is packed within the reactor by loading catalyst diluted with quartz beads to achieve an approximate 6” bed height. Above and below the catalyst bed, quartz wool was added to the reactor. A Type K thermocouple was placed in the reactor coaxial to the catalyst bed within the bed or at the top of the bed and the temperature was recorded continuously. Samples were collected periodically during a run for product analysis by GC.

[0046] Gas chromatography of flow reactor samples was performed on an Agilent 7890A GC equipped with a flame ionization detector using an Agilent J&W DB-17 column (Part Number 123- 1732LTM). The GC method involves a 5 min hold at 50°C followed by a 10°C / min ramp to 280°C and a 2 min hold for a total run time of 30 min.

[0047] Calibration of butanal, nonanal, octenal, and octadecenal were completed by dilution of the pure materials in toluene to obtain standards of 40, 30, 15, 5 and 1 wt%. Octenal (92% purity) and octadecenal (88% purity) were calibrated at the same levels. A response factor for the tridecenal (product of the cross-aldol condensation of butanal and nonanal) was estimated based on linear interpolation from plotting response factors of the available aldehydes versus number of carbons. Decane was used as an internal standard for all components at 2 wt% decane and toluene was used as the diluent.

[0048] Example 1: Batch reaction using a solid- supported Bronsted acid catalyst

[0049] In this example, different solid- supported Bronsted acid catalysts were analyzed for conversion and selectivity performance during batch reaction. For each sample, the catalyst was dried overnight in a 90°C static oven prior to use. Reaction components were reacted at 60°C for 1 hour reaction time under 500 rpm mixing in the Radleys Carousel 12 Plus Reaction Station. The reaction formulation include a 2; 1 weight ratio of C4:C9 and a catalyst loading of 150 mg (5 mol% acid content of Amberlite FPX62 relative to total aldehydes) or 6.7 g solution / g catalyst. Results are shown in Table 3.

[0050] The results indicate that cross-aldol condensation of C4 and C9 alkyl aldehydes proceeds over acidic ion-exchange catalysts in comparison to a NaOH catalyzed solution (homogeneous, comparative). Importantly, in comparison to a NaOH catalyzed system, solid-supported Bronsted acid catalysts display a preference for the cross-aldol condensation of C9 with C4 to form C13 over the self-aldol condensation of C9 to form Cl 8. The ratio in selectivity for the cross-aldol condensation versus that of the self-aldol condensation is improved versus a NaOH catalyzed system. This also contrasts with prior ail as prior art demonstrates cross-aldol condensation with an aldehyde and a second reagent of different functionality (ketone or is additionally activated) to obtain a non- statistical product mixture.

[0051] Example 2: Temperature effects on batch reactions using solid- supported Bronsted acid catalysts

[0052] In this example, the effect of temperature on the rate of conversion of C9 to C13 aldehydes using solid-supported Bronsted acid catalyst Amberlite FPX62 was analyzed during batch reaction. Reaction components were reacted at temperatures ranging from 60°C to 100°C for 1 hour (unless otherwise indicated) reaction time under 500 rpm mixing in the Radleys Carousel 12 Plus Reaction Station. The reaction formulation include a 2:1 weight ratio of C4:C9 and a catalyst loading of 150 mg (5 mol% acid content of Amberlite FPX62 relative to total aldehydes). Prior to combination with the aldehydes, Amberlite FPX62 dehydrated in 90°C oven overnight. Results are shown in Table 4.

[0053] Results demonstrated that altering temperature and time provides some level of control over conversion rates of C4 and C9. respectively, and cross-aldol selectivity. Here, higher cross-aldol selectivity was obtained at relatively lower temperatures (particularly 60°C). Example 3: Flow reaction using solid- supported Bronsted acid catalysts In this example, the C4 and C9 conversion and C13 cross-aldol selectivity was studied for a flow reactor process including solid- supported Bronsted acid catalyst Amberlite FPX62. The flow reactor column was loaded to a 6” bed height with Amberlite FPX62 that was dried at 90°C. The reactor column was operated in flow at 0.15 mL / min (1 hour residence time), at 2:1 weight ratio of C4:Cg, 4.27 g catalyst, and temperature of 98°C. Results indicate that Amberlite FPX62 can be used in a flow reactor and deactivates in flow as observed by a drop off in C4 and C9 conversion.

[0054] Example 4: Regeneration of solid-supported Bronsted acid catalyst activity under flow conditions In this example, the C4 and C9 conversion and C13 cross-aldol selectivity was studied in a flow reactor process using Amberlite FPX62 in a flow reactor, comparing results for fresh catalyst and following catalyst regeneration with diglyme. The flow reactor column was loaded to a 6” bed height with Amberlite FPX62 that was dried at 90°C. The reactor column was operated in flow at 0.67 mL / min (15 minute residence time), at 2: 1 weight ratio of C4:Cg, 4.27 g catalyst, and temperature of 82.8°C. Following the run with fresh catalyst, diglyme regeneration was performed at 34°C and a flow rate of 0.33 mL / min for 2 hours.

[0055] Table 6: Cross-aldol flow reaction results before regeneration

[0056] Table 7: Cross-aldol flow reaction results after regeneration

[0057] The results shown in Tables 6 and 7 indicate that the solid- supported Bronsted acid catalyst can be regenerated in between multiple runs and conversion activity can be restored to that of fresh catalyst.

[0058] Example 5: Flow reaction using solid-supported Bronsted acid catalyst

[0059] In this example, the C4 and C9 conversion and C13 cross-aldol selectivity was studied for a flow reactor process including ion exchange resin Amberlyst 46. The flow reactor column was loaded to a 6” bed height with Amberlyst 46 that was dried at 90°C. The reactor column was operated in flow at 0.15 mL / min (1 hour residence time), at 2:1 weight ratio of C^Cy, 4.27 g catalyst, and temperature of 82.8 °C.

[0060] Table 8: Cross-aldol flow reaction results

[0061] Table 8 demonstrates the translation of a second solid- supported Bronsted acid catalyst Amberlyst 46 from a batch reaction to a flow reaction for selective cross-aldol condensation. While the foregoing is directed to exemplary embodiments, other and further embodiments may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

Claims1 . A process for cross-aldol condensation of alkyl aldehydes, comprising: providing a reactant mixture comprising a first alkyl aldehyde and a second alkyl aldehyde, the first alkyl aldehyde having a lower carbon number than the second alkyl aldehyde, and the feed stock having a molar ratio of 1 : 1 or more of the first alkyl aldehyde to the second alkyl aldehyde; and contacting the reactant mixture with a solid- supported Bronsted acid catalyst in a reactor; operating the reactor to generate a product mixture comprising a cross-aldol product of the first alkyl aldehyde and the second alkyl aldehyde.

2. The process of claim 1, wherein the solid- supported Bronsted acid catalyst comprises one or more of sulfonated resins, sulfonic acid derivatives, carboxylic acid resins, phosphonic acid resins, sulfonimide resins, and phenolic resins.

3. The process of claim 1, wherein the product mixture has a cross-aldol selectivity, defined as the concentration of the cross-aldol product in the product mixture over the concentration of the self-aldol product of the second alkyl aldehyde in the product mixture, of 2.3 or more.

4. The process of claim 1, wherein the first alkyl aldehyde has a carbon number in the range of 2 to 6 and the second alkyl aldehyde has a carbon number in the range of 7 to 20.

5. The process of claim 1, wherein the solid- supported Bronsted acid catalyst comprises a sulfonated resin.

6. The process of claim 1, wherein operating the reactor comprises maintaining the solid- supported Bronsted acid catalyst to at least 5 mol% acid content relative to the combined moles of the first alkyl aldehyde and the second alkyl aldehyde.

7. The process of claim 1, wherein the reactor is operated at a temperature of 60 °C or more.

8. The process of claim 1, further comprising regenerating the solid- supported Bronsted acid catalyst with a glycol ether.

9. The process of claim 1, the product mixture comprises 50 wt% or more of the cross-aldol product.

10. A system for cross-aldol condensation of aldehydes comprising: a reactor comprising a solid- supported Bronsted acid catalyst; a reactor input for providing a reactant mixture comprising a first alkyl aldehyde and a second alkyl aldehyde, the first alkyl aldehyde having a lower carbon number than the second alkyl aldehyde; and a reactor output for obtaining a product mixture comprising 20 wt% or more of the cross- aldol product of the first alkyl aldehyde and the second alkyl aldehyde.

11. The system of claim 10, wherein the reactor is a batch reactor.

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