Screening of solid acid catalysts for etherification of glycerol with isobutene under identical conditions

Article abstract of DOI:10.1016/j.cattod.2019.07.025

We compared the performance of more than 70 solid acid catalysts with Br?nsted acid function for the etherification of glycerol with isobutene under identical reaction conditions of 75 °C, 10 bar, 6 h and with an isobutene/glycerol molar ratio of 3/1. The catalysts were selected from a wide range of solid acid catalysts including ion exchange resins, zeolites, silica, and heteropolyacids, and their counterparts modified by simple treatments, with more than half of them being investigated for the first time for this reaction. For the modified commercial acid catalysts, the desired glycerol tertiary butyl ether (DTBGE and TTBGE) selectivity improved from 75 to 87 wt% upon partial cation exchange of the sulfonic acid protons in Amberlyst type ion exchange resins with Na⁺ cations; from 66 to 85 wt% by hydrothermal steam treatment of zeolite H-Y (SiO₂/Al₂O₃ = 80); and from 75 to 80 wt% with partial La⁺-exchange of zeolite H-Beta (SiO₂/Al₂O₃ = 300), all at high (90–100%) glycerol conversion. Impregnation of the heteropolyacids, tungstosilicic acid (TSA) and tungstophosphoric acid (TPA), on silica at a loading of 50 wt% provided a glycerol conversion of higher than 65 wt% and with a desired ether selectivity in the range of 62 to 76 wt%. In general, total ether selectivity increased with decreasing acid capacity for ion exchange resins, while the desired ether selectivity was enhanced with increasing acid strength for zeolites and supported heteropolyacids on mesoporous silica. Data present a detailed guideline for the selection of solid acid catalysts for the etherification of glycerol with isobutene.

Full text of DOI:10.1016/j.cattod.2019.07.025

Accepted Manuscript
Title: Screening of Solid Acid Catalysts for Etherification of
Glycerol with Isobutene under Identical Conditions
¨
˘
Authors: Ozge Deniz Bozkurt, Nur Baglar, Serdar C¸ elebi,
Alper Uzun
PII:
S0920-5861(19)30385-2
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2019.07.025
CATTOD 12354
To appear in:
Catalysis Today
Received date:
Revised date:
Accepted date:
31 December 2018
12 June 2019
19 July 2019
¨
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Please cite this article as: Bozkurt OD, Baglar N, C¸ elebi S, Uzun A, Screening of Solid
Acid Catalysts for Etherification of Glycerol with Isobutene under Identical Conditions,
Catalysis Today (2019), https://doi.org/10.1016/j.cattod.2019.07.025
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Screening of Solid Acid Catalysts for Etherification of Glycerol with Isobutene under
Identical Conditions
Özge Deniz Bozkurt1,2,3, Nur Bağlar3, Serdar Çelebi3, and Alper Uzun*1,2,4
1Department of Chemical and Biological Engineering, Koç University,
Rumelifeneri Yolu, Sariyer, 34450, Istanbul, Turkey
2KÜTEM (Koç University TÜPRAŞ Energy Center), Koç University
Rumelifeneri Yolu, Sariyer, 34450, Istanbul, Turkey
3Turkish Petroleum Refineries Co. (TÜPRAŞ) R&D Center,
Guney Mah. Petrol Cad., 41790 Korfez, Kocaeli, Turkey
4Koç University Surface Science and Technology Center (KUYTAM), Koç University,
Rumelifeneri Yolu, Sariyer, 34450 Istanbul, Turkey
*Corresponding Author: auzun@ku.edu.tr
Graphical abstract
Highlights
1

 Over 70 catalysts were screened for etherification of glycerol with isobutene.
 Desired ether selectivity enhanced with decreasing acid capacity in ion exchange
resins.
 High desired ether selectivity was obtained with high acid strength zeolites and
heteropolyacids.
 Results present a comprehensive guideline for catalyst selection.
Abstract
Here, we compared the performance of more than 70 solid acid catalysts with Brønsted acid
function for the etherification of glycerol with isobutene under identical reaction conditions of
75 °C, 10 bar, 6 h and with an isobutene/glycerol molar ratio of 3/1. The catalysts were
selected from a wide range of solid acid catalysts including ion exchange resins, zeolites,
silica, and heteropolyacids, and their counterparts modified by simple treatments, with more
than half of them being investigated for the first time for this reaction. For the modified
commercial acid catalysts, the desired glycerol tertiary butyl ether (DTBGE and TTBGE)
selectivity improved from 75 to 87 wt% upon partial cation exchange of the sulfonic acid
protons in AmberlystTM type ion exchange resins with Na+ cations; from 66 to 85 wt% by
hydrothermal steam treatment of zeolite H-Y (SiO2/Al2O3=80); and from 75 to 80 wt% with
partial La+-exchange of zeolite H-Beta (SiO2/Al2O3=300), all at high (90-100%) glycerol
conversion. Impregnation of the heteropolyacids, tungstosilicic acid (TSA) and
tungstophosphoric acid (TPA), on silica at a loading of 50 wt% provided a glycerol
conversion of higher than 65 wt% and with a desired ether selectivity in the range of 62 to 76
wt%. In general, total ether selectivity increased with decreasing acid capacity for ion
exchange resins, while the desired ether selectivity was enhanced with increasing acid
strength for zeolites and supported heteropolyacids on mesoporous silica. Data present a
2

detailed guideline for the selection of solid acid catalysts for the etherification of glycerol
with isobutene.
Keywords: Glycerol; Etherification; MTBE, Fuel oxygenate; Brønsted acid catalyst
1. Introduction
Methyl tert-butyl ether (MTBE) is the globally used gasoline additive. However, it is
considered as a contaminant for the ground water resources because of the leaks from the
underground petroleum storage tanks [1,2]. Therefore, the usage of MTBE has been banned
or limited in many states in the US [3]. Hence, there is an ongoing research for finding green
alternatives.
Biodiesel industry is associated with glycerol surplus which needs to be valorized in
alternative pathways. One of these routes is the production of glycerol-based fuel additives,
via etherification of glycerol [4,5] to produce glycerol tertiary butyl ethers (GTBE) that can
potentially replace MTBE [6,7]. This industrially promising reaction has been studied for
almost 20 years. The performance of different heterogeneous catalysts including ion exchange
resins [8–12], zeolites [9,10,12–18], mesoporous silica [14,19,20], and supported
heteropolyacids [21,22] have so far been reported for glycerol etherification with isobutene to
produce GTBE [23].
When glycerol and excess isobutene are in contact on a Brønsted acid site, glycerol
etherification, isobutene oligomerization, and isobutene hydrolysis reactions occur
simultaneously (Scheme 1). Glycerol (G) etherification proceeds in three steps with one, two,
and three levels of butylation of glycerol with isobutene (IB), forming mono-tert-butyl
glycerol (MTBGE), di-tert-butyl glycerol (DTBGE), and tri-tert-butyl glycerol (TTBGE),
respectively [9].. The reaction medium is initially a two-phase mixture at low glycerol
conversions. These two phases dissolve in each other with decreasing glycerol concentration
3

and increasing ether concentration as the reaction proceeds. MTBGE formed at the beginning
of the reaction acts as a good solvent enhancing the mass transfer between glycerol and
isobutene phases. Merging of these two phases was observed at a glycerol conversion of 60-
70% [11,24]. Oligomerization of IB takes place as a side reaction, forming mainly di-
isobutene (DIB). Reaction of isobutene with the water molecules in the medium forms tert-
butyl alcohol (TBA). Less number of remaining hydroxyl groups in DTBGE and TTBGE
products, namely the desired ethers, provides them a less polar character, thus they can be
dissolved in non-polar hydrocarbon fuels and utilized as fuel additives [6].
Selectivity to the desired products depends on the catalyst structure at fixed reaction
conditions. The highest yield for DTBGE and TTBGE were achieved in macroreticular type
ion exchange resins (e.g. Amberlyst 15TM and 35TM) with large pore diameter, while no
TTBGE was produced over zeolite Beta because of the steric hindrance [9]. Increasing water
content in ion exchange resins was associated with higher MTBGE formation [9]. Initial
etherification rate over ion exchange resins and zeolite type catalysts increased with the
catalyst acidity [10]. Accessibility of the reactants to the active sites is controlled by internal
diffusion [12]. Moreover, acid site density is another factor controlling the selectivity. For
instance, in a study focusing on comparing the performance of resin catalysts with zeolites, it
was inferred that differences in product selectivity over Amberlyst 35TM and zeolite Beta was
because of the much lower concentration of acid sites in zeolite Beta. This difference led to a
higher DTBGE selectivity, but because of the presence of Lewis acidity in zeolite Beta, DIB
selectivity was also high [12]. Similarly, the higher desired ether selectivity compared to that
of Amberlyst 15TM was obtained from the lower surface acid site density of the
peflurosulfonic acid based Hyflon® catalyst [25].
4

There are also several studies examining the effect of modifications on a certain type
of catalyst. When Amberlyst 15TM protonic sites were partially exchanged with different
cations, isobutene affinity of the catalyst was altered at different degrees. Moreover, changing
the extent of cation exchange influenced the product selectivities as well [23]. Selectivity to
the desired ethers (DTBGE and TTBGE) increased with the modifications on zeolites, such as
acid wash [13], sulfonic acid functionalization [15], hierarchical synthesis and/or fluorination
[18], and rare earth cation exchange [16] because of the strengthened Brønsted acidity,
improved textural properties (increased surface area, pore size, pore volume, and additional
mesoporosity), and higher accessibility of the Brønsted acid sites. Similarly, the presence of
functional groups with higher acid strength were revealed to enhance the selectivity towards
desired ethers in the functionalized mesoporous silica samples [19]. Modified zeolites with
enlarged pores were associated with increased DIB formation [18].
However, other than the catalyst structure, the reaction conditions, such as
temperature, IB/G mole ratio, catalyst loading, and reaction time, can also induce an influence
on the higher ether selectivity. For instance, a higher temperature is associated with more di-
isobutene production [9] as isobutene dimerization rate is more temperature sensitive [10].
Moreover, the selectivity of isobutene to form glycerol ethers is optimum at an IB/G mole
ratio of 3:1 and at 80 °C, while exceeding this stoichiometric ratio results in excessive DIB
production [11]. MTBGE selectivity was observed to decrease with glycerol conversion at
longer batch runs [12]. Elevated reaction times were shown to increase the DIB and TTBGE
selectivity [14,19]. Hence, the selectivity to di- and tri- ethers as the desired products depends
not only on the type of catalysts but also on the reaction conditions. However, because the
studies reported previously for glycerol etherification using catalysts from different families
were performed mostly under incomparable conditions, drawing a definitive structure-
performance relationship is not possible. Thus, there is a strong need for comparing the
5

catalytic performance of different types of catalysts under identical conditions. Hereby, we
present the results of our comprehensive catalyst screening study considering more than 70
different solid acid catalysts including ion exchange resins, zeolites, and heteropolyacids, and
their counterparts modified by simple treatments. Some of these catalysts were used in the
literature under various, and most of the time incomparable, conditions. However, more than
half of the catalysts considered here are reported for the first time in the literature for the
etherification of glycerol with isobutene. The results presented here offer a guideline for the
rational design of solid acid catalysts for producing green alternatives to MTBE from
glycerol.
2. Material and Methods
2.1.Catalyst preparation
2.1.1. Cation exchange resins
Amberlyst 15TM, Amberlyst 35TM (dry), and Amberlyst 36TM (Dow Chemical Co.),
were used as received. The water content for these as-received resin catalysts were less than 3
wt% (as noted by the vendor). Additionally, Amberlyst 15TM was subjected to cation
exchange treatment to partially exchange the protons on the [–SO3⁻H⁺] groups with different
cations. Aqueous cation exchange solutions of 0.2-1.0 M were prepared using metal nitrates
or chlorides: NaNO3 (Aldrich, 99.995%), NaCl (Merck, 99.5%), LiNO3 (Aldrich, 99.99%),
KNO3 (Aldrich, 99.999%), Ca(NO3)2.xH2O (Aldrich, 99.997%), Ni(NO3)3.6H2O (Aldrich,
99.999%), CsNO3 (Aldrich, 99.999%), La(NO3)3 (Aldrich, 99.99%) and Ce(NO3)3.6H2O
(Aldrich, 99.999%). Approximately 2.5 g of as-received Amberlyst 15TM (dry) beads was
treated in 15 ml metal solution for 12 h at 50 or 310 rpm at room temperature. After
treatment, the excess solution over the intact ~0.5 mm beads of averagely half mm size was
removed using a Pasteur pipette, the beads were washed two times with 15 ml deionized
6

water in each wash cycle, water was removed again by a Pasteur pipette, and finally the beads
were dried under vacuum at 70 °C for 12 h. Water content after the cation exchange
modification determined by TGA was 7.3 ± 2 wt%.
2.1.2. Zeolites
Zeolite H-Y (CBV780, SiO2/Al2O3=80 and CBV400, SiO2/Al2O3=5.1), H-Beta
(CP811C-300 , SiO2/Al2O3=300 and CP814E, SiO2/Al2O3=23), NH4⁺-Mordenite (CBV21A,
SiO2/Al2O3=20), and NH4⁺-ZSM-5 (CBV2314, SiO2/Al2O3=25 and CBV8014,
SiO2/Al2O3=80) (Zeolyst International) were used after calcination in static air at 500 °C for 5
h. Additionally, zeolite Y (CBV780) and Beta (CP811C-300) were subjected to different
modifications including steam treatment, acid treatment, and metal exchange. These zeolites
were calcined at 500 °C for 5 h in static air before and after the modifications.
Steam treatment: A system similar to that proposed by Agudelo et al. [26] was used to
treat the zeolites in steam flow. Two grams of calcined zeolite was placed into a quartz reactor
and supported on quartz wool. The reactor was heated up to 500 °C at a heating rate of 5
°C/min. A 100 ml three-neck flask filled with 75 ml distilled water was heated by a round
bottom heater to 120 °C. The first neck was closed with a lid, the second neck was attached to
N2 gas inlet (20 ml/min) and the third neck was connected to the top inlet of the reactor using
a 1/8-inch stainless steel line, wherein the line was heated to 100 °C with an external heating
wire to prevent water condensation. The steam flowed through the catalyst for 1 h or 3 h at
500 °C using 20 ml/min N2 as the carrier gas.
Acid treatment: Aqueous HCl and citric acid solutions were prepared using
hydrochloric acid (Merck, 37 vol%) and citric acid monohydrate (Merck, >99.5 vol%).
Zeolites were treated with 0.25 M HCl solution (20 ml solution/g zeolite) at 60 °C under 500
rpm for 3 h or with 1.0 M citric acid solution (20 ml solution/g zeolite) at 80 °C under 500
7

rpm for 3 h. The mixture was centrifuged at 5000 rpm for 5 min. The powder was washed
with distilled water until the pH became neutral, centrifuged again at 5000 rpm for 5 min, and
then dried under vacuum at 110 °C for 12 h.
Metal exchange: Aqueous solutions of metal nitrates were prepared using
La(NO3)3.6H2O (Aldrich, 99.999%) and Ce(NO3)3.6H2O (Aldrich, 99.999%). Zeolites were
treated with 0.4 M lanthanum or cerium nitrate solutions (25 ml solution/g zeolite) at 90 °C
under 500 rpm for 1 h. Centrifugation and drying were the same as in the case of the acid
treatment.
2.1.3. Heteropolyacids
Tungstosilicic acid (TSA), tungstophosphoric acid (TPA), and molybdatophosphoric
acid (MPA) all in hydrate form were purchased from Merck and used after drying at 100°C
for 12 h to remove any water content.
Preparation of cesium salt of heteropolyacids: 0.1 M aqueous heteropolyacid
solutions were prepared for each of the three heteropolyacids. Cesium nitrate solution of 0.1
M was prepared from CsNO3 (Aldrich, 99.999%). 15.6 ml of 0.1 M CsNO3 solution was
added dropwise in 6.2 ml 0.1 M heteropolyacid solution (to form 2 g of Cs2.5H0.5PW12O40).
White precipitates were dried at 110 °C under vacuum for 12 h and then calcined at 300 °C
for 3 h.
Wet impregnation on supports: SiO2 (Sigma Aldrich), MCM-41 (ACS Material), and
SBA-15 (ACS Material) were used as the silica-based supports. 0.5 g dried heteropolyacid
was dissolved in 10 ml water and the solution was added dropwise on 0.5 g support to form
50 wt% heteropolyacid loading. The mixtures were stirred at room temperature under 100
8

rpm for 24 h. The white precipitates were dried at 110 °C under vacuum for 12 h and then
calcined at 300 °C for 3 h.
2.2.Catalyst characterization
2.2.1. Acid-base titration
The catalyst sample (55±5 mg) was placed into 10 ml of 2.0 M sodium chloride (≥99.5
wt%, Merck) solution and stirred at 100 rpm for 24 h. Later, the liquid part was transferred
into a beaker and titrated with 0.01 M sodium hydroxide (diluted from 1.0 M NaOH, Merck)
solution using a syringe pump (Aladdin, World Precision Instruments) and monitoring the pH
with a pH-meter (WTW Inolab, pH electrode WTW Sentix 41) under stirring. When the pH
increased to neutral 7.0 ± 0.1, titration was stopped and the volume of NaOH solution was
recorded. Brønsted acid capacity per gram of catalyst in mmolH+/gcat unit was calculated
using Eq. 1.
푚푚표푙
푚퐿
(
)
(
)
푥 푣표푙푢푚푒 표푓 푁푎푂퐻 푠표푙푢푡푖표푛 푢푠푒푑 푚푙
푚표푙푎푟푖푡푦 표푓 푁푎푂퐻 푠표푙푢푡푖표푛
acid capacity =
(Eq. 1)
푐푎푡푎푙푦푠푡 푎푚표푢푛푡 (푔)
2.2.2. X-ray fluorescence
Zeolites H-Y (CBV780) and H-Beta (CP811C-300) were analyzed for their
SiO2/Al2O3 ratios by XRF. A Bruker S8 Tiger XRF spectrometer in standardless mode under
helium atmosphere with an 18 mm mask was used. The samples were loaded into an XRF
sample cup (Chemplex Industries Inc., Cat. No: 1430) with a thin-film support (Prolene film
with 4μm thickness, Chemplex Industries, Inc., Cat. No: 426). The loose powder method was
selected for the measurement together with “Best analysis” assuming oxide matrix.
SpectraPlus Eval2 V2.2.454 software was used for data interpretation. Concentrations of SiO2
and Al2O3 were calculated in wt% using KA1 lines for Si and Al, then converted into moles.
Lanthanum amount in La⁺³-exchanged H-Beta was calculated using La LA1 lines.
2.2.3. X-ray diffraction (XRD)
9

XRD measurements were performed using a Bruker D8 Discover X-Ray Diffraction
system with a Cu Kα1 radiation source with the wavelength of 1.5418Å. A Vantec-1 detector
with a 11 mm slit was used. The power rating of X-ray generator was 40 kV and 40mA. The
2θ range of the measurements was 10–90° with a step size of 0.01263°. 10 mg sample was
fixed on a glass slide and attached on the sample holder. Phase identification was done using
the ICDD PDF-4 2014 database.
2.2.4. Scanning electron microscopy (SEM)
SEM images were retrieved by using a Zeiss Ultra Plus field emission scanning
electron microscope. Pristine and Na+-exchanged Amberlyst 15™ samples were analyzed
under ultra-high vacuum with 10 kV accelerating voltage and at 5.0 mm working distance. A
secondary electron detector was used for imaging.
2.2.5. Ammonia temperature programmed desorption (NH3-TPD)
Zeolite and heteropolyacid catalysts having a considerably high reaction performance
were analyzed by NH3-TPD measurements to compare the acid strength. A Micrometrics
AutoChem II 2920 instrument with a thermal conductivity detector (TCD) connected to an
MKS Cirrus II mass spectrometer was used for these measurements. Approximately 100±5
mg catalyst was loaded into the sample holder of the instrument. The catalysts were first
flushed with He for 1 h at 500 °C for zeolites and 250 °C for heteropolyacids, then treated
with 10 vol% NH3-He for 1 h at room temperature, flushed again in He flow at 110 °C to
remove any physisorbed ammonia, and then, the TCD signals were recorded during a
temperature ramp in He flow at a heating rate of 10 °C/min up to 800 °C for zeolites and at a
heating rate of 3 °C/min up to 700°C for heteropolyacids. TCD signals obtained as a function
of temperature (°C) were deconvoluted using Fityk (0.9.8) using Voigt function [27]. For
10

zeolites, obtained peaks at low temperature region (<400 °C) region were assigned to weak
Brønsted acid sites; while those at high temperature region (>400 °C) were assigned to strong
Brønsted and Lewis acid sites [28]. Amount of the acid sites were determined by the
calibrated desorbed ammonia peak areas.
2.2.6. Brunauer-Emmett-Teller (BET) pore volume and surface area analysis
BET analyses were done by using a Micrometrics ASAP 2020 Physisorption
Analyzer. Prior to each analysis, 100 mg of the catalyst sample was dried at 110 °C under
vacuum for 1 h. Degassing was performed at 300 °C for 5 h using helium. Volumetric N2
adsorption/desorption isotherms were measured between P/P0=0.01–0.95 at liquid nitrogen
temperature. BET surface areas were calculated between P/P0 =0.05−0.3 using liquid nitrogen
(10 data points). Pore distribution was obtained by the Barrett, Joyner, and Halenda (BJH)
method using Harkins and Jura t-curve.
2.3.Catalytic performance measurements
2.3.1. Batch reactor measurements
Catalytic performance tests were performed using a Parr Series 5000 Multiple Reactor
System with six stainless steel autoclave reactors of 75 ml each. 6.0 g glycerol (≥99.5%,
Sigma Aldrich) and 0.45 g catalyst, corresponding to a catalyst loading of 7.5 wt% of
glycerol, were loaded into the reactor and the reactors were sealed. Then the reactors were
pressurized in nitrogen to 8 bar from the gas inlet port, followed by the dosing of 18 ml (11 g)
isobutene (99.9%, Messer) using a system enabling the transfer of a desired volume of liquid
isobutene with the help of nitrogen. When the transfer of isobutene was finished, the reactor
was heated to 75 °C. Initial gas pressure in the reactor at the beginning of the reaction at 75
°C was measured as 10±1 bar. The reaction was run at autogenous pressure (the gas pressure
varied depending on the alterations in the liquid volume as the reaction proceeds). Stirring
11

was performed at 1200 rpm using magnetic stirring bars. The reaction was carried out for 6 h
on each catalyst. The final pressure for each run was approximately 5 bar. At the end of each
run, the reaction mixture was transferred into a glass tube and examined in terms of liquid
volume compared with the initial liquid volume of glycerol to determine whether glycerol
conversion was high or not. High glycerol conversion was associated with an increase in the
liquid volume. To refresh the used catalyst, solid catalyst particles were first filtered and
mixed with ethanol, the liquid portion was removed, and the solid part was washed twice with
methanol and dried at 70 °C under vacuum overnight.
2.3.2. Product analysis
Liquid products (approximately 15 g for each run) having high glycerol conversions
were separated from the catalyst by filtration using a syringe filter of 0.25 μm pore size, and
then the liquid part was homogenized by vortexing, prior to the gas chromatographic (GC)
analysis. Liquid products with very low glycerol conversions associated with low liquid levels
(less than 65 wt% glycerol conversion) were not analyzed. High glycerol conversion was
associated with an increase in the liquid volume as glycerol ethers has a lower density than
glycerol and the gas phase isobutene is converted into liquid products as well. In a standard
GC analysis, 510±1 mg was taken from the homogenized product and added to 165.0±0.5 mg
octanol (≥99.0%, Merck) in a volumetric flask and diluted to 10 ml with ethanol (≥99.8%,
Sigma Aldrich). After vortexing, a part of the homogenized solution was moved into a 2 ml-
GC-vial and placed in the autosampler. Composition analysis was done in a gas
chromatograph-mass spectrometer system (Agilent 7890B GC equipped with a 5977A Series
GC/MSD) equipped with an HP Innowax column (30 m x 0.25 mm x 0.25 µm). Octanol was
used as the internal standard for quantification. Split mode was run for the injection (1 µl
sample, split ratio of carrier gas:sample was 100:1). The oven temperature increased from 40
12

to 240 °C at a ramp rate of 10 °C/min. Peak assignments of all products were confirmed by
GC-MS. Calibrations for glycerol, mono-tert-butyl glycerol ((±)-3-tert-butoxy-1,2-
propanediol, ≥97.0 vol%, Aldrich), and di-isobutene (2,4,4-trimethylpentene, ≥90.0 vol%,
Merck) were done using commercially available compounds. Quantifications of di-tert-butyl
glycerol, tri-tert-butyl glycerol, tri-isobutene, tetra-isobutene, and tertiary butyl alcohol were
done by using a GC-FID response factor method [29]. The glycerol conversion was
determined once the mole-based amounts of glycerol, MTBGE, DTBGE, and TTBGE in the
GC sample were calculated (see Supplementary File “Product analysis: Determination of
component amounts and glycerol conversion”).
3. Results and Discussion
3.1. Structural properties of the catalysts
All prepared regular and modified catalysts were subjected to acid-base titration to
determine the number of Brønsted acid sites (the acid capacity). Acid capacities of these
catalysts are given in Table 1. Here, we note that the catalysts which do not provide a high
glycerol conversion were not characterized in detail.
According to Table 1, ion exchange resins are the heterogeneous catalysts with the
highest acid capacity, except for the MPA catalysts. When the Amberlyst 15TM was subjected
to cation exchange under 310 rpm stirring, modified catalysts were obtained in powder form.
Whereas, a stirring rate of 50 rpm conserved the original bead size. Acid capacities of powder
and bead form of the resin catalysts were similar, probably because the pore size of the ion
exchange resins is large enough to provide efficient mass transfer of the cations. For instance,
the powder and bead form of 310 rpm and 50 rpm stirred Amberlyst 15TM in 0.4 M NaNO3
solution had an acid capacity of 2.5 ±0.3 mmol H+/g cat acid capacity, almost within the error
13

range. Sodium modification was previously performed in literature [23,30], wherein 51%
cation exchange in meq H+ basis resulted in 2.4 meq H+/g cat for Amberlyst 15TM.
Consistently, the exchange ratio of protons was in the range of 23-71 mol% when using 0.2-
1.0 M of metal solutions and the corresponding acid capacity was between 3.5-1.3 mmol
H+/gcat (Table 1). Metal cation exchange on Amberlyst 15TM reduced the acid capacity by i)
increasing the concentration of metal salt solution (from 0.2 M to 1.0 M), ii) increasing the
atomic size of the metal cation (from Li+ to Cs+), and iii) increasing the metal’s oxidation
state (from Li+ to La+3). Acid capacities of pristine ion exchange resins followed as Amberlyst
15TM (4.5 mmol H+/g cat) < Amberlyst 35TM (4.8 mmol H+/g cat) < Amberlyst 36TM (5.0
mmol H+/g cat), consistent with the values reported by the producer. Acid capacities of
zeolites were much lower than that of ion exchange resins, with zeolite Beta
(SiO2/Al2O3=300) having a higher acid capacity than zeolite Y (SiO2/Al2O3= 80), 0.22 and
0.13 mmol H+/g cat, respectively. Zeolite modifications didn’t have significant or reasonable
effects on acid capacity. For pristine zeolites, acid capacity increased with decreasing
SiO2/Al2O3 ratio, as expected. Heteropolyacids in bulk forms had high acid capacities in the
following order; TSA (1.95 mmol H+/g cat) < TPA (4.0 mmol H+/g cat) < MPA (12.9 mmol
H+/g cat). However, supporting them on mesoporous silica leads to an almost two-fold
decrease in their acid capacity.
SEM images of the pristine and 0.4 M Na⁺-exchanged Amberlyst 15TM samples
indicated that the resin morphology did not change significantly upon cation exchange
modification (Fig. 1 a-f). Acid capacities of the used ion exchange resins were similar to the
fresh catalysts in the 10% error range, so we do not anticipate leaching of the [–SO3⁻H⁺]
groups from the structure. However, size reduction because of cracking during mixing was
observed in the used catalyst (Fig. 1 g-i).
14

Acid capacities of zeolites were much lower than that of ion exchange resins, with H-
Beta having higher acid capacity than H-Y. For pristine zeolites, acid capacity increased with
decreasing SiO2/Al2O3 ratio, as expected (Table 1). Zeolite modifications did not indicate any
detectable effect on acid capacity determined by acid-base titration. Acid-base titration is
manual thus more prone to errors especially for the low acid capacity zeolites. Moreover, it is
an indirect method of counting the acid sites, as it counts the protons exchanged with sodium
in the high molarity sodium chloride solution. Some protons might not leave the structure in
the case of microporous materials. Thus, zeolite H-Y (CBV780) and H-Beta (CP811C-300)
were further analyzed by NH3-TPD for the number of desorbed ammonia molecules, related
to the number of Brønsted acid sites, and by XRF to determine their SiO2/Al2O3 ratios.
The number of desorbed NH3 molecules on pristine H-Y which was 0.185 mmol
NH3/gcat dropped to 0.178 and 0.177 mmol NH3/g cat upon 1 h and 3 h steaming, respectively.
NH3-TPD thus appears to be a better method for counting the number of Brønsted acid sites
on zeolites. Besides, SiO2/Al2O3 ratios of steam treated zeolite H-Y, determined by XRF,
were slightly higher than that of the pristine zeolite. For instance, SiO2/Al2O3 = 100 of zeolite
H-Y (CBV780) increased to 104 and to 116, upon 1 h and 3 h steam exposure, respectively,
indicating slight dealumination. We also checked the crystal structure of H-Y by XRD before
and after steaming. Crystallinity of the pristine and steam treated zeolite H-Y samples were
almost identical (Fig. 2), therefore the crystal structure was mostly preserved after the steam
exposure.
Crystal structure of H-Beta, on the other hand, was altered following 1 h steam
treatment (Fig. S1). XRF analysis did not yield in any meaningful change in the SiO2/Al2O3 =
242 of the pristine H-Beta, probably because it covered both framework and extra-framework
aluminum species. Steam exposure creates extra framework aluminum species via framework
dealumination, yet bulk SiO2/Al2O3 ratio may remain unchanged while nuclear magnetic
15

resonance (NMR) spectroscopy determined framework SiO2/Al2O3 ratio increases [26].
However, the drop in the desorbed ammonia from 0.257 mmol NH3/gcat in the pristine zeolite
to 0.215 mmol NH3/gcat in 1 h steamed zeolite upon steaming indirectly referred to the
removal of some Brønsted acidity related Al2O3 species from the framework. In the case of
0.4 M La+3-exchange, 0.404 mmol NH3/gcat implied the formation of new Brønsted acid site.
This phenomena is thought to occur in the presence of water in the zeolite pores with
+3
+2
La(H2O)2 ⇌ La(OH)(H2O)2 + H+ ⇌ La(OH)2 + 2H+ reaction [16]. Therefore 1 La is
associated with the formation of 1.5 H⁺s on average. XRF analysis suggested the La:Al ratio
to be 0.35 mol:mol. Assuming that the framework Brønsted acid sites do not change in
number, we can convert the La:Al=0.35 mol:mol to the ratio of La and Al associated H+s,
giving 0.53 mol:mol. This number is similar to (0.404-0.257)/0.257=0.57 mol:mol, which
explains the contribution of lanthanum in the formation of new Brønsted acid sites.
SiO2/Al2O3 ratio upon La+3 and Ce+3-exchange were similar to that of the pristine zeolite.
Fig. 3 presents the NH3-TPD signals of the zeolite and heteropolyacid samples
associated with high catalytic performance, as will be explained in Section 3.2. Steam
treatment improved the strength of the weak Brønsted acid sites of H-Y and H-Beta samples,
recognized from the elevated peak temperatures between 168-355 °C (Fig. 3 a-b). Framework
dealumination was deduced by our XRF and NH3-TPD analysis. The five or six coordinate
extra framework Al species with Lewis acidity forms when four coordinate Al is removed
from the framework. This Lewis acid sites might draw electrons from the O–H Brønsted acid
sites, resulting in diminished O–H bond strength and associated stronger Brønsted acidity
[31]. La+3-exchange in H-Beta blue-shifted the high temperature NH3 desorption peaks to
more than 30 °C (Fig. 3 b). The enlarged peaks prove the formation of new Brønsted acid
sites upon lanthanum introduction. Zeolite treatments with lanthanum was claimed to improve
16

the number, distribution, and strength of the Brønsted acid sites [16,32]. We consider La+3
cations as sources of Brønsted and Lewis acidity, wherein the Brønsted acid strength is
enhanced by the existence of Lewis acidity.
Steam treatment and La+ exchange altered the catalyst acid strength. We performed
BET analysis to further understand the modification of the corresponding surface properties
(Table 2). Previously, H-Y was reported to have higher surface area than H-Beta (average of
700 m2/g and 600 m2/g, respectively) [23]. Consistently, the surface area and pore diameter of
our H-Y samples were higher than H-Beta samples. BET analysis for untreated and 1 h steam
treated H-Y samples indicated almost two-fold increase in average mesoporous pore diameter
upon steam treatment (8 nm to 21 nm), while the surface area decreased. For H-Beta, no
meaningful change in the mesoporous pore diameter upon steam exposure was observed.
However, there could have been changes in the microporous region, which is not shown in
our analysis. Synthesis of desilicated zeolite Beta could be suggested to achieve a higher
mesopore volume and improved acid strength [33].
Acid capacity of the bulk TSA was higher than TPA, however their 50 wt%
impregnation on mesoporous silica supports yielded in similar acid capacities in the range of
0.5-0.8 mmol H+/gcat, based on the acid-base titration and NH3-TPD data (Table 1). In a
recent study, the total number of acid sites in 50 wt% TPA loaded on MCM-41 were
determined as 0.54 mmol H+/gcat experimentally, and 0.52 mmol H+/gcat theoretically [34].
Therefore, we inferred that all HPA joined the porous structure. Fig. 3 c presents the NH3
desorption signals of TSA and TPA supported on mesoporous silica MCM-41 and SBA-15.
We considered ammonia desorption in the low temperature region (first peak in Fig. 3 c)
because heteropolyacid decomposition is pronounced at temperatures above 370 °C [34].
According to Fig. 3 c, NH3 desorbed at higher temperatures for TPA, compared to the TSA
17

based catalysts, pointing out to higher acid strength of TPA. Indeed, the Brønsted acid
strength of TPA is higher than TSA, with the corresponding deprotonation energy (DPE)
values of 1085 kJ/mol and 1105 kJ/mol, respectively [35–37]. DPE indicates the energy
required to remove a proton from a Brønsted acid, and strong acids have lower DPE values.
Thus, with a lower DPE, the Brønsted acid strength of TPA is higher than TSA. In our recent
study, we have demonstrated the association of low DPE values with the Brønsted acid sites
of higher strength [30]. The unmodified SiO2, MCM-41, and SBA-15 supports were
associated with significantly low acid capacities (< 0.1 mmol H+/g cat). 50 wt%
heteropolyacid loaded SiO2 support had comparable number of acid sites with the mesoporous
silica supports. According to Table 1, increasing the heteropolyacid loading from 5 to 50 wt%
in TSA/MCM-41, TSA/SBA-15, TPA/MCM41, and TPA/SBA-15 increased the number of
acid sites. Cs salt of heteropolyacids had lower acid capacities compared to the bulk form.
Impregnating the CsHPAs in mesoporous silica decreased the acid capacities further. Acid
capacities of heteropolyacids dropped after the reaction to approximately half of the initial
values, indicating the leaching from the support during the reaction under vigorous mixing of
1200 rpm. Therefore, both heterogeneous and homogeneous forms of heteropolyacids might
be acting in the reaction.
3.2.Comparison of the catalytic performance
All of the catalysts presented in Table 1 were tested for glycerol etherification with
isobutene in a batch reactor system under identical conditions: 75 °C temperature, 7.5 wt% of
glycerol catalyst loading, an isobutene/glycerol ratio of 3 mol/mol, an initial pressure of 10
bar, 1200 rpm stirring rate, and 6 h reaction time. Fig. 4 demonstrates two different product
mixtures obtained at the end of 6-h run period using two different catalysts. Product mixtures
with high liquid level as show in Fig. 4 a were further subjected to GC/MS analysis.
18

However, in the case of reactions with low glycerol conversion, as resembled by Fig. 4 b, the
final mixtures were viscous, liquid levels were low because of very low conversion; and the
catalyst was difficult to separate from the viscous phase. Such samples were not subjected to
the gas chromatographic analysis. The products of the performance tests performed on 42 out
of 71 of the catalysts considered were subjected to GC/MS analysis. Results (as presented in
Table 3) indicated that these catalysts provided more than 65 wt% glycerol conversion.
Desired ether selectivity may either be described in mole basis or in weight basis. The
former one expresses the utilization of glycerol, indicating the extent of glycerol transformed
into DTBGE and TTBGE among all three types of ethers. The second term, however,
considers not only glycerol, but also isobutene by conveying the weight percentage
information of DTBGE and TTBGE in the overall product mixture including ethers,
oligomers, and TBA. Thus, we mainly consider the weight based desired ether selectivity in
this study.
Previous literature studies using Amberlyst 15TM in the bead form have yielded
between 70-80 wt% desired ethers, representing the sum of the weight based DTBGE and
TTBGE selectivities in the overall product mixture, and 10-15 wt% di-isobutene selectivities
in the final product mixture at >95% glycerol conversion under various reaction conditions,
such as 60-90 °C, 5-7.5 wt% catalyst loading, IB/G = 3-4, 4-50 h reaction time [8,9,19]. Our
data indicated a selectivity of 75 and 12 wt% for desired ether and di-isobutene, respectively,
19

consistent with the literature. Out of five separate batch reactor experiments with pristine
Amberlyst 15TM, the error associated with individual product selectivities were smaller than 3
wt%, while the error range in selectivity for the desired ethers was less than 2 wt%.
Amberlyst 15TM counterparts in the powder and bead forms had comparable performance.
Desired ether selectivity (in mole basis) for ion exchange resins in our study are very similar
to each other with slight variations: Amberlyst 36TM < Amberlyst 35TM ~ Amberlyst 15TM
(72.5 wt% < 74.8 wt% ~ 74.9 wt%). Less isobutene dimers were formed over the first two
catalysts, which could be because of the higher acid strength favoring the etherification route
instead of the oligomerization route. The sulfonic acid sites in Amberlyst 35TM and 36TM were
previously shown by ammonia adsorption flow microcalorimetry to have higher acid strength
than that of Amberlyst 15TM [38,39].
Cation exchange treatments on the best performing ion exchange resin Amberlyst
15TM with lithium nitrate, potassium nitrate, sodium nitrate, and cesium nitrate yielded in final
reaction mixtures with no observable glycerol phase, thus the products obtained on these
catalysts were further analyzed by GC/MS. Results indicated a glycerol conversion of higher
than 90%. According to Table 3, cation exchange treatments reduced the selectivity of
isobutene dimers, which supports the previous literature studies on partial neutralization of
Amberlyst 15TM acid sites by cation exchange [23,30]. Selectivity of glycerol to its desired
ethers improved with increasing the degree of sodium exchange up to 0.3 M Na+ (both 87.2
wt% for Amberlyst 15TM and 36TM). At Na+ solution concentrations of 0.8 and 1.0 M, glycerol
could not be converted effectively, as the final reaction mixture was associated with a glycerol
phase only. We speculate that low glycerol conversion was associated with the very low acid
capacity of Amberlyst 15TM samples prepared by the treatment of pristine Amberlyst 15TM
catalysts at high sodium nitrate concentrations (< 1.7 mmol H+/g cat). For 0.4 M metal cation
20

exchange, the smallest cation (Li+) gave the best performance, 80.2 wt% desired ether
selectivity. The largest cation (Cs+) was associated with high selectivity to mono-ether, as
well as low glycerol conversion, probably caused by steric hindrance. Double (Ca+2) and
triple (Ni+3, La+3, Ce+3) charged cations led to low glycerol conversions (<65 wt%) as their
acid capacities are significantly lower, below 1.4 mmol H⁺/g cat (Table 1). In order to
compare the catalytic performance of pristine and Na+-exchanged Amberlyst 15TM samples at
incomplete glycerol conversions, we performed batch reactor experiments for shorter
durations between 0.5-5 hours. At glycerol conversions exceeding 75 mol%, the desired
DTBGE + TTBGE selectivity of the 0.5 M Na+-exchanged catalyst kept higher than the
pristine catalyst (Fig. S2). In our recent study, we observed that when the H+s of the Brønsted
acid [–SO3⁻H+] sites were partially exchanged with Na+, the remaining Brønsted acid sites
improved in strength [30]. Stronger interaction of the MTBGE reaction intermediate with the
stronger acid [–SO3⁻H+] sites is envisaged to produce higher molecular weight ethers, if
isobutene is available in the reaction environment. This is enabled by a better mixing of
glycerol and isobutene phases above 60 mol% glycerol conversion [11], triggering the
formation of DTBGE and TTBGE at high acid strength. The pristine and treated resins had
less than 7.5 wt% water content. In a study elaborating the drying behavior of polystyrene
macroporous sulfonic acid resins, the final moisture content decreased to 13 wt% and 5 wt%
with drying under airflow for 1 h at 59 °C and 93 °C, respectively [40]. Although we
performed the drying in a vacuum oven at 70 °C for 12 h, the final moisture content is within
the expected limits. Considering the initial presence of 0.45 g catalyst with 10 wt% water (2.5
mmol) and 196 mmol (11 g) isobutene in the reactor, according to the IB + H2O  TBA
isobutene hydration reaction, only 2.5 mmol TBA would form, constituting approximately 1.3
wt% of the 15 g reaction mixture, which was observed with the metal treated resins.
21

Among the zeolites tested, zeolite H-Beta CP811C-300 (SiO2/Al2O3=300) yielded in
comparable total desired ether selectivity with pristine Amberlyst catalysts (~75 wt%
DTBGE+TTBGE); however, TTBGE production was very low on the former one. The cage
size of zeolite Y favors the formation of TTBGE, whereas the formation of this bulky ether is
almost hindered (<1 wt% in the product mixture) in the channels of zeolite Beta.
Consequently, zeolite H-Y CBV780 (SiO2/Al2O3=80) samples were associated with a three-
times lower selectivity for isobutene oligomers and relatively high selectivity for TTBGE.
The shape selectivity towards the di-ethers was also reported in the literature and has been
linked to the steric hindrance [9]. Isobutene dimerization on H-Beta was more pronounced
than that on H-Y in our study. Similar behavior of zeolite Beta was observed in literature,
possibly due to the diffusion limitation through narrow pores [12,41]. The maximum diameter
of a sphere that can be included in the pores of -Beta framework (BEA) is 6.68 Å, while this
number is 11.24 Å for the FAU structure of zeolite Y [42]. Considering the molecular size of
DTBGE as 9.4 Å, it possibly forms on the external surface of H-Beta, while it was forming on
both the external surface and inside the pores of H-Y. Besides the pore size, zeolite
SiO2/Al2O3 ratio was also observed to have an influence on the product selectivity. For
instance, the desired ether selectivity over H-Beta CP814E with SiO2/Al2O3=23 was lower
than that of its counterpart with SiO2/Al2O3=300, even though the acid capacity of the former
one was four-fold higher. Remembering that the Brønsted acid strength is a significant
parameter improving the selectivity to desired ethers [13,14,20,30], the higher SiO2/Al2O3
zeolite with higher acid strength might be producing the desired di-ethers at a higher
selectivity.
The effect of hydrothermal steam and HCl acid treatment on zeolites have not been
studied in the literature for the glycerol etherification with isobutene reaction. Our results
22

demonstrate that hydrothermal steam and HCl acid treatments on zeolite H-Y CBV780
(SiO2/Al2O3=80) considerably improved the desired ether selectivity in this reaction. Steam
treated H-Y had superior desired ether selectivity (85 wt% for 1 h steam treatment) to the
commercial ion exchange resins (72-75 wt%). BET analysis for the untreated and 1 h-steam-
treated zeolite Y samples indicated almost two-fold increase in average mesoporous pore
diameter upon steam treatment (8 nm to 21 nm), while surface area and pore volume
decreased. HCl treatment on H-Y improved the desired GTBE selectivity to 73 wt%. The
catalyst was not characterized in detail, but the reason might be dealumination and enlarged
pore size upon acid wash [13]. Hydrothermal treatment did not have any significant effect on
H-Beta CP811C-300, no meaningful change in the average pore diameter of this zeolite upon
steam exposure was observed (average 5 nm mesoporous pore diameter in all cases).
However, the treatments for lanthanum and cerium exchange on zeolite Beta had some effects
in decreasing the di-isobutene selectivity and in enhancing the desired ether selectivity
slightly from 75 to 80 and to 77 wt%, respectively. To achieve a higher mesopore volume in
zeolite Beta structure with improved acid strength, synthesis of its desilicated form could be
suggested [33]. Citric acid treated H-Y CBV780 and H-Beta CP811C-300 did not have any
detectable change in product selectivity, which might be because of the weak acid nature of
the citric acid.
The solid Brønsted acid catalysts in zeolite and heteropolyacid groups with improved
desired ether selectivity were further analyzed for their acid strength using temperature
programmed desorption of ammonia (NH3-TPD) (Fig. 3). Considering Table 3 and Fig. 3 a
together, it can be interpreted that the acid strength is a critical parameter in determining the
final product selectivity, because desired ether selectivity was substantially improved over
steam-treated zeolite H-Y CBV780 having a lower surface area and a higher acid strength,
probably providing mesoporous molecule highways allowing the transportation of desired
23

ethers. In literature, hydrothermal steam treatment was observed to dealuminate the zeolites
and form extra framework aluminum species. Hydrochloric acid treatment was also applied as
a way of dealumination; and if done by following steam treatment, it is known to remove the
extra framework aluminum species, forming meso-sized pores [26]. However, the Brønsted
acid strength in the 3 h-steam-treated plus HCl-acid-treated H-Y did not improve, revealing
the possible role of extra framework aluminum species in glycerol etherification reaction. For
H-Beta CP811C-300 samples, steam treatment did not have any significant effect in terms of
enhancing the Brønsted acid strength; however, La+-exchange had an obvious effect (Fig. 3
b). Zeolite treatments with lanthanum and cerium ions are known to improve the Brønsted
acid strength of zeolites [16]. Shifting of the ammonia desorption peaks to higher
temperatures upon La+-exchange for zeolite Beta supports this statement. According to our
catalytic performance results, both La+ and Ce+-exchange resulted in lower selectivity to di-
isobutene (from 7.7 to 5.5 wt% upon cation exchange). The better performance of untreated
zeolite Beta compared to zeolite Y in terms of higher glycerol conversion and selectivity to
the desired ethers might arise from the presence of i) the higher number of acid sites and ii)
the larger proportion of strong Brønsted acid sites over weaker Brønsted acid sites, namely the
proportion of the areas larger than 400 °C to the areas smaller than 400 °C in TPD profiles
(Fig. 3 a-b). Besides, increasing the average pore size of zeolite Y by treatment results in
comparable performance with zeolite Beta. It can be interpreted that the number of acid sites,
acid strength, and pore size appear as the three crucial parameters controlling the glycerol
etherification reaction towards the desired products over heterogeneous catalysts [23].
Heteropolyacids were used for glycerol etherification in a very limited number of
studies in bulk and supported forms [21–23]. The reaction conditions employed were
significantly different from each other; for instance, they varied between 60-90 °C, 2-7.5 wt%
24

catalyst loading, 5-20 h reaction time, 2/1-4/1 of isobutene/glycerol in moles and with/without
emulsifier. TSA in homogeneous form provoked 47 wt% desired ether selectivity, while using
20 wt% TSA supported on clay significantly reduced this selectivity to 25 wt% [21]. We
evaluated different heteropolyacid based catalysts under identical conditions considering that
the product selectivities strongly depended on the reaction conditions. In our work, the desired
ether selectivity of bulk heteropolyacids followed as CsTSA (83 wt%) > TPA (72 wt%) >
TSA (71 wt%) (Table 3). Partial exchange with cesium has been shown to create higher
surface area and lower acid density, forming a heterogeneous heteropolyacid catalyst that is
more active than its bulk form [43,44], which supports the higher performance of CsTSA
compared to the bulk TSA. However, impregnation of CsTSA on SBA-15 yielded in lower
desired ether selectivity, probably because of low acid capacity and pore blockage. For
instance, high amount of MTBGE production indicates steric hindrance in the reaction
environment. Glycerol could not be converted considerably in the case of bulk and supported
forms of MPA. Inadequate activity of MPA might be linked to its low acid strength [45].
Desired ether selectivity increased from 41 to 73 wt% with an increase in TPA loading from
20 to 50 wt% over SiO2-support (Table 3). When mesoporous silica MCM-41 and SBA-15
were used as the support, TPA with two-fold number of acid sites and also stronger Brønsted
acidity when compared to TSA (lower deprotonation energy of TPA [37] resulted in halved
di-isobutene selectivity and improved yield of desired ethers. The MCM-41 supported TPA
with 50 wt% loading yielded in over 21 wt% TTBGE selectivity, outperforming the
commercial cation exchange resin and zeolite catalysts. High molecular weight ether
production is linked to the super acidity of heteropolyacids. Nevertheless, considering the
glycerol conversion and TTBGE selectivity together, the highest TTBGE yield was obtainable
with ion exchange resins (27 wt%, with 0.3 M Na+-exchanged Amberlyst 15TM). TTBGE
content has recently been demonstrated by us to be crucial in terms of gasoline compatibility
[46]. Thus, catalysts with high TTBGE yield will be promising for industrial utilization.
25

The catalytic performance of ion exchange resin, zeolite, and heteropolyacid groups in
terms of selectivity to glycerol ethers were plotted as a function of the most crucial structural
catalyst properties (Fig. 5). Total selectivity to MTBGE, DTBGE, and TTBGE gradually
increased with decreasing acid capacity of ion exchange resins, either by using a different
Amberlyst type or partially exchanging the protonic acid sites with different cations in the
same Amberlyst type (Fig. 5 a). The desired ether (DTBGE+TTBGE) selectivity could not be
correlated to the Amberlyst acid capacity in this study. Nevertheless, a recent study of us has
shown the relationship between acid strength and desired ether selectivity in Amberlyst 15TM
ion exchange resin by investigating the stretching frequency of chemisorbed ammonia on the
acid sites [30], as shown in Fig. 5 b. The high thermal stability of zeolites and mesoporous
silica supported heteropolyacids enabled the determination of acid strength by examining
desorption peak temperatures for a probe molecule, such as ammonia (Fig. 3). The desired
ether selectivities were well-correlated with the first desorption peak temperatures of the NH3-
TPD curves. For instance, increasing acid strength at higher peak temperatures provoked
higher desired ether selectivity (Fig. 5 c and d). The correlations with the second desorption
peaks were relatively weak. The first desorption in the 160-260 °C range could be assigned to
the weak acid sites. Strengthening of these weak sites is determinative in obtaining the desired
DTBGE and TTBGE products from the complex etherification network.
4. Conclusions
We performed a comprehensive catalyst screening study to test a large number of solid
acid catalysts (more than 70 different) with Brønsted acid functionality (about more than half
of them being reported for the first time), covering ion exchange resins, zeolites, and
heteropolyacids, as well as their modified forms under identical reaction conditions. The
relationships between the acidity and product selectivity of these catalysts were investigated.
26

Selectivity to glycerol ethers, especially to the higher molecular weight ethers increased,
while isobutene dimerization was suppressed with decreasing acid capacity for ion exchange
resins. Moreover, strengthening in Brønsted acidity upon zeolite Y steam treatment, zeolite
Beta La+-exchange or switching from TSA to TPA for mesoporous silica functionalization
yielded in higher selectivity to desired ethers.
Acknowledgements
This work was financed by the Scientific and Technological Research Council of
Turkey (TUBITAK) through Industrial R&D Projects Support Program (1501) with project
number 3130673. A.U. acknowledges the TUBA-GEBIP Award of Turkish Academy of
Sciences. Ö.D.B. and A.U. acknowledge the support from TARLA.
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