Importance of glycerol
Currently, it is estimated that the uses of glycerol are more than 1500 1, 2, highlighting its participation in the production of pharmaceuticals, cosmetics, personal hygiene, in the manufacture of tobacco, food, in the production of resins, among others 3, Figure I. 2. Due to this wide variety of potential uses, glycerol is considered one of the most versatile chemicals that exist, thanks to its three hydroxyl groups, glycerol is completely soluble in water and other alcohols and in turn is insoluble in hydrocarbons. The high boiling point of glycerol (563 K at atmospheric pressure) and the high viscosity, originate from the formation of intra and inter-molecular hydrogen bonds. These properties are used to use glycerol as a softener or lubricant in resins and plastics. On the other hand, since glycerol is a non-toxic substance with a sweet taste, it can find pharmaceutical and/or food applications.
For the year 2012, the amount of glycerol used in all its applications was close to 160,000 tons and according to the projection made for that then of a growth of 2.8% 1, the current use should be around 185,000 tons.
Although glycerol has great versatility, the progressive increase in the production of biodiesel (whose by-product is glycerol), has considerably increased glycerol to such an extent that by 2020 the production of glycerol will be six times the demand.
Problem of glycerol over-production
While the outlook is good economically for the industry that directly consumes glycerol in its processes due to the imminent over-production, the outlook is contrary to the environment who has seen that biodiesel producers have chosen not to refine glycerin obtained and instead, burn it 2, since it is more profitable to make this process with the by-product than to invest in the purification and subsequent sale, Figure I. 3.
The direct combustion of glycerin produces relatively small amounts of acrolein (~ 15 ppb) and other volatile organic compounds 6 and its emissions are comparable to those of natural gas combustion, but the opportunity to obtain other molecules could be carried out by means of the chemical transformation of glycerol. In this sense, the literature reports a large number of reactions that can be generated with glycerol as the main actor 7-9, Figure I. 4.
Among many of the possible reactions of glycerol, the etherification of glycerol is of great interest, which can be chemically carried through three different routes 10:
1) Reaction of sodium glycerate with alkyl halides by the Williamson synthesis process;
2) Reaction of glycerol with active alkenes;
3) Reaction of glycerol with aliphatic alcohols by a condensation reaction.
Of these syntheses, the last two are used industrially, where the most common alkene is isobutene and the alcohols are generally short chain. Although it is possible to obtain these ethers with the use of isobutene as an etherifying agent of glycerol, the major drawback is the need for high reaction pressures to guarantee the solubility of the reactants. With the use of alcohols in the same phase of glycerol (liquid phase), the use of these high pressures is suppressed. In this context, the present document focuses on the use of the etherification of glycerol with alcohols in liquid phase.
The main application of these ethers is as fuel additives, becoming an alternative to the typical methyl tert-butyl ether (MTBE), that is widely used throughout the world despite its high toxicity to the environment. With respect to the market for these ethers, historically its use has been increasing considerably in the last three decades, where at the end of the 90s the global consumption of fuel additives was approximately 600,000 tons, with petrol additives being the most desired. with more than 50% of the total. For the year 2000, the global demand was estimated at 200,000 metric tons.11 This increase shows that the production of fuel additives can be a good alternative to transform glycerol into ethers.
Etherification of glycerol with alcohols
Activated alcohols such as tert-butyl alcohol or benzyl alcohol have been mostly studied in the etherification of glycerol. The catalytic etherification of glycerol with less activated alcohols, such as aliphatic alcohols, is thermodynamically much more difficult.12 In recent years, some research has focused on this type of reactions obtaining low reaction yields. Pariente and co-workers reported the catalytic etherification of glycerol with ethanol over various solid acid catalysts.13 A maximum yield of 40 % of mono-ethoxy glyceryl ethers was obtained at 433 K by using sulfonated polystyrene resins and zeolites with a Si/Al ratio of ca. 25. Melero and co-workers also studied the etherification of glycerol with ethanol using mesostructured functionalized silicas, where the best reaction conditions with the highest conversion and yield found were: T = 473 K, ethanol/glycerol molar ratio = 15/1, and catalyst loading = 19 wt%. Under these reaction conditions, 74% glycerol conversion and 42% yield to ethyl ethers have been achieved after 4 h of reaction but with a significant presence of glycerol by-products. In contrast, lower reaction temperatures (T = 433 K) and moderate catalyst loading (14 wt%) in presence of a high ethanol concentration (ethanol/glycerol molar ratio = 15/1) are necessary to avoid the formation of glycerol by-products and maximize ethyl-glycerols selectivity.
Yadav et al. 15, also studied the reaction between glycerol and ethanol using zeolites (H-beta, NaX, and NaY) and SRC-120 (strongly acidic cation exchange resin), where the SRC-120 catalyst, showed highest catalytic activity followed by H-beta. These results were related in terms of the Brønsted acidity difference (0.936 of the zeolite vs 4.5 mmol H+/g of SRC-120). The NaX and NaY zeolites showed no activity after 480 min. Based on these results, the authors conclude that there is an influence of the degree of acidity in the etherification of glycerol with ethanol. In this same work, the kinetics of the reaction was studied. The apparent activation energies of glycerol and ethanol were found to be 25.1 and 26.6 kcal/mol respectively. The high value of activation energy further confirms that the reaction is kinetically controlled.
Another short chain alcohol historically employed is n-butanol. Nandiwale Y. and co-workers presented for the first time in 2014 the etherification of glycerol with n-butanol.16 In this study, they used several acid solids, among which are the H-beta zeolite, ZSM-5 and K10. H-beta zeolite was found to be the most promising for the etherification reaction with glycerol conversion of 55% and a 98% selectivity towards mono-butyl-glycerol ether formation. The optimized process parameters were: catalyst loading of 15% (w/w) of glycerol, molar ratio of 1:9, reaction time of 4 h, reaction temperature of 433 K, and speed of agitation of 200 rpm. The optimized results were also used to develop a kinetic model for the etherification of glycerol. The activation energy and pre-exponential factor for etherification were 48.89 kJ mol-1 and 75.31 L h-1 mol-1, respectively.
One year later, Fang W. and co-workers reported for the first time an experimental result to prepare silica-immobilized Aquivion PFSA (copolymer based on tetrafluoroethylene and the sulfonyl fluoride vinyl ether produced by Solvay Specialty Polymers) as heterogeneous catalyst for the acid-catalyzed direct etherification of glycerol with n-butanol.17 The n-butanol conversion and yield to butyl glyceryl monoether reached values of 91% and 45%, respectively, at 423 K with a catalyst loading of 3 mol% H+, while the yield to by-product of dibutyl ether was discouraged (6%).
On the other hand, Canilla and his colleagues also studied the etherification of glycerol with n-butanol, using Amberlyst® 15 (A-15) with temperature variations of 343–433 K.18 The water produced in the reaction was continuously removed with the help of a permeable membrane. The results suggest a dependence of the reaction temperature with the conversion of glycerol. At 433 K, a conversion of 85.1% was obtained, with a yield of di and tri ethers of only 0.6%. The use of permeable membrane significantly increases glycerol conversion in A-15.
The etherification of glycerol with n-butanol at 413 K in the presence of sulfonated cation-exchange resins and zeolite catalysts in an autoclave reactor was studied by Samoilov and co-workers.19 It was demonstrated that the styrene-divinylbenzene ion exchange resins are effective catalysts for the production of glycerol n-butyl ethers: the conversion of glycerol was about 98% with a selectivity of n-butyl ether of about 88 % (413 K, 5 h, 5% by weight of Amberlyst 36 catalyst and 10% by weight of glycerol in n-butanol). The Y and β zeolites in the H+ form exhibited a comparable specific activity (conversion of glycerol of no more than 25% under similar conditions) in combination with a high selectivity for glycerol di-n-butyl ethers (up to 28%).
In the study published by Gaudin and co-workers, the catalytic etherification of glycerol with various alkyl alcohols on Amberlyst® 70 (10 wt %): 1-butanol, 1-pentanol, 1-hexanol, 1-octanol and 1-decanol was evaluated.12 When starting from 1-hexanol the reaction medium became turbid. The conversion rate of 1-hexanol was two times lower than that of 1-butanol and 1-pentanol. After 24 h, 13 % 1-hexanol was consumed compared to 25 % when starting from 1-butanol and 1-pentanol, indicating that the catalyst activity dropped significantly with an increase of the alkyl alcohol chain length. The reaction medium was biphasic when starting from 1-octanol and 1-dodecanol inducing mass transfer problems. Conversion of the alkyl alcohol was very low while the glycerol conversion was increased.
According to the reports on the etherification of glycerol with short-chain alcohols, the results depend on the acid strength and the nature of the alkylating alcohol, that is, with activated alcohols such as tert-butyl alcohol, it is possible to generate more percentages high conversion and selectivity towards the di and tri-ethers, while with the C4-C6 primary alcohols, poor glycerol conversion results are achieved and the selectivity towards di and tri-ethers is very low.
Catalytic etherification of glycerol with tert-butanol
The obtaining of di- and tri-tert-butyl ethers of glycerol (DTBG, TTBG respectively) from the etherification of glycerol with tert-butyl alcohol and with potential use as additives for fuels,20 is a reaction whose first reports appear in 1994, 21 although the etherification reaction in the presence of acid catalysts dates back to 1934 with the main production of glycerol mono-tert-butyl ether (MTBG).
Previous studies described in the literature concerning the mechanism for this reaction, propose a rapid protonation of tert-butyl alcohol in acidic sites, thus generating a tertiary carbocation capable of reacting with glycerol. Additionally, etherification with tert-butyl alcohol produces isobutene and di-isobutene as byproducts by the dehydration of alcohol. The formation of these products is clear evidence of the competition between glycerol and tert-butyl alcohol for acid sites on the catalytic surface, 23 Figure I. 6.
These ethers can improve the detonation and the octane level of gasoline since they present values of 112-128 and 91-99 in BRON and BMON respectively (where these values indicate that they are suitable to be used as gasoline components),4 and in particular, the di-and tri-tert-butyl glycerol ethers, reduce the emission of particulate matter, hydrocarbons, carbon monoxide and aldehydes. As these ethers at room temperature are completely soluble in diesel, they can reduce the viscosity of this, for this reason, they are the desired products for this reaction. The mono-tert-butyl ether of glycerol becomes the least desired product for this application due to its low solubility in diesel.
Acid catalysts for the etherification of glycerol with tert -butyl alcohol
According to the mechanism described above (Figure I. 6), the presence of Brønsted-type acid catalysts is vital to promote the formation of the carbocation tert-butyl active species to carry out the etherification. In this sense, the first reports relate the use of H2SO4, 22 as a homogeneous acid catalyst in this reaction, although the selectivity that was obtained was mostly towards mono-tert-butyl ether and low for di-tert-butyl ether. On the other hand, it is possible to use heterogeneous type acid catalysts, with which an increase in the amount of di and tri-tert-butyl ethers has been evidenced, compared with their homogeneous similar. One of the first reports in the literature on the use of heterogeneous acid catalysts is that presented by Klepáčová et al., 28 where Amberlyst® 15 (acidic ion exchange resin) was compared with acid zeolites of large pores, H-Y and H-BEA. The results suggest a high conversion of glycerol using Amberlyst® 15, while the selectivity towards di-tert-butyl ether was favored by the presence of the micropores of the zeolite. Acid resins have been shown to be efficient in the etherification reaction of glycerol with tert-butyl alcohol. The work published by Pico et al., 29 is specific in the use of resins as catalysts in the etherification of glycerol with tert-butyl alcohol. In this work, the authors conducted a detailed study of Amberlyst® 15, Amberlite® 200 and Amberlite® IRC-50 in terms of conversion, selectivity and stability. The best results were obtained with the Amberlyst® 15 and were attributed to the high acidity and better texture properties over the other resins. With the Amberlyst® 15, a higher selectivity towards the production of di-ethers was also obtained. Regarding the stability study, this catalyst can be reused without significantly losing the catalytic activity. The studies in which Amberlyst® 15 is used as catalyst in the etherification of glycerol with tert-butyl alcohol are numerous and practically thanks to their good results, it has become the reference catalyst for this reaction; Table I. 1, summarizes some other studies that use this type of acid resin.
Table of contents :
Chapter 1: Etherification of glycerol with alcohols
1.1. Importance of glycerol
1.2. Problem of glycerol over-production
2. Etherification of glycerol with alcohols
3. Catalytic etherification of glycerol with tert-butanol
4. Acid catalysts for the etherification of glycerol with tert-butyl alcohol
1. Fundamentals of the most common characterization techniques of zeolites and carbocatalysts
1.1. X-ray diffraction, XRD
1.2. Adsorption- desorption of N2
1.2.1. Isotherm classification
1.2.2. Specific Surface – BET surface
1.2.3. Pore volume
1.3. Scanning electron microscopy, SEM
1.4. Transmission electron microscopy, TEM
1.5. X fluorescence spectrometry, XRF
1.6. Inductively coupled plasma, ICP
1.7. Solid state nuclear magnetic resonance 27Al, NMR
1.8. IR (TOT band) infrared spectroscopy band structure
1.9. Thermodesorption of pyridine at 423 K followed by IR
1.10. Raman spectroscopy, RAMAN
1.11. X-ray photoelectron spectroscopy, XPS
1.12. Elemental analysis, EA
1.13. Boehm titration
1.14. Thermogravimetric analysis, TGA
1.2.1. Nature of the acid sites
1.2.2. Acid sites density
1.2.3. Acid sites strength
220.127.116.11. Ionic exchange degree
18.104.22.168. [T-O-T] bond angle
22.214.171.124. Electrostatic environment
126.96.36.199. Interaction with Lewis sites
1.2.4. Location of the acid sites
1.3. Shape selectivity
1.3.1. Reactant shape selectivity
1.3.2. Transition state shape selectivity
1.3.3. Product shape selectivity
1.4. Synthesis of zeolites
1.4.1. Silicon source
1.4.2. Aluminum source
1.4.3. Mineralizing agent
1.4.4. Structure directing agents (SDA)
1.5. Synthesis of hierarchical zeolites
1.6. Zeolites in nanocrystals and their synthesis
1.6.2. Synthesis from strongly alkaline gels
1.6.3. Alternative methods
2. Zeolites as catalysts in the etherification of glycerol with tert-butyl alcohol
5.1. Chemicals and catalysts
5.2.1. Chemical composition
5.2.2. Composition of the surface
5.2.3. Determination of the Si/Al ratio of the zeolite framework
5.2.4. Determination of the framework formula
5.2.5. Structural analysis
5.2.7. Textural properties
5.3. Glycerol etherification with tert-butyl alcohol
5.3.1. Catalyst regeneration and catalytic recycling
6.1. Characterization of catalysts
6.1.1. Chemical composition and Si/Al ratio
6.1.2. Structural results of zeolites
6.1.3. Morphology of the zeolites
6.1.4. Textural results of zeolites
6.1.5. Acidity of the catalysts
6.2. Glycerol etherification with tert-butyl alcohol
6.2.1. Thermodynamic analysis of reaction equilibrium
6.2.2. Kinetic model
6.2.3. Activity and stability
1. The carbon
1.1. Allotropic forms of carbon
2.1. Methods of obtaining graphene
2.1.1. Obtaining graphene from graphite oxide
2.1.2. Chemical reduction of graphene oxide
2.1.3. Functionalization of graphene oxide and reduced graphene oxide
3. Graphene oxide as a catalyst
4. Motivation of the study of graphene oxide in the etherification of glycerol with tert-butyl alcohol
5. Objective of the use of graphene oxide in the etherification of glycerol with tert-butyl alcohol
6. Experimental section
6.1. Chemical and catalysts
6.2.1. Textural properties
6.2.2. Chemical composition
6.2.3. Acid properties
6.3. Etherification of glycerol with tert-butyl alcohol and analysis
7. Results and discussion
7.1. Characterization of catalysts
7.1.1. Textural properties
7.1.2. Elemental Analysis
7.1.3. Acid properties
7.2. Etherification of glycerol with tert-butyl alcohol
7.2.1. Kinetic model