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\begin{document}

\title{Regulating thermal dynamics and gas transport of MOF glass through
silver composites}



\author[1]{Muhammad Yazid Bin  Zulkifli}%
\author[1]{Ruiqi Chen}%
\author[2]{Rijia  Lin}%
\author[1]{Yuqi Yao}%
\author[1]{Milton Chai}%
\author[3]{Dominique Appadoo}%
\author[1]{Wupeng Wang}%
\author[4]{Zhong-Hua Zhu}%
\author[5]{Kang Liang}%
\author[6]{Vicki Chen}%
\author[1]{Jingwei Hou}%
\affil[1]{University of Queensland}%
\affil[2]{Affiliation not available}%
\affil[3]{Australian Synchrotron}%
\affil[4]{The University of Queensland}%
\affil[5]{University of New South Wales South Western Sydney Clinical School}%
\affil[6]{University of Technology Sydney}%


\vspace{-1em}



  
  \date{May 24, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Metal-organic framework (MOF) glass is a versatile and processible
material that retains its porous nature while transitioning between
liquid and solid states. Incorporating composites into MOF glass can
enhance its functionalities, but the effect on its state conversion
remains understudied. This research explores the impact of silver ions
on the properties of ZIF-62 glass, revealing their influence on phase
conversion at varying ligand concentrations. The presence of silver
enhances gas separation processes, particularly improving CO2 and
hydrocarbon separation selectivity compared to pure ZIF-62 glass. These
findings provide valuable insights for composite integration and
highlight the potential of Ag-doped ZIF-62 glass in gas separation
applications. Furthermore, this work paves the way for processing
functional Ag-doped ZIF-62 glass films, enabling diverse applications
such as gas separation, film catalysis, and antimicrobial glass.%
\end{abstract}%



\sloppy


\textbf{Regulating thermal dynamics and gas transport of MOF glass
through silver composites}

\emph{Muhammad Yazid Bin Zulkifli, Ruiqi Chen, Rijia Lin, Yuqi Yao,
Milton Chai\textsuperscript{*}, Dominique Appadoo, Wupeng Wang, John
Zhu, Kang Liang, Vicki Chen and Jingwei Hou\textsuperscript{*}}

M.Y.B Zulkifli, R. Chen, R. Lin, Y. Yao, M. Chai, W. Wang, V. Chen and
J. Hou

School of Chemical Engineering, University of Queensland, St Lucia, QLD,
4072, Australia E-mail: milton.chai@uq.edu.au; jingwei.hou@uq.edu.au

M.Y.B Zulkifli

Group Research \& Technology (GR\&T), Project Delivery \& Technology
(PD\&T), PETRONAS Research Sdn Bhd, Block E, Lot 3288 \& 3289, Off Jalan
Ayer Itam, Kawasan Institusi Bangi, Kajang, 43000, Malaysia

D. Appadoo

Australian Synchrotron, 800 Blackburn Road, Clayton, VIC 3168, Australia

K. Liang School of Chemical Engineering, The University of New South
Wales, Sydney, NSW 2052, Australia

V. Chen

University of Technology Sydney, 15 Broadway, Ultimo, NSW 2007 Australia

Keywords: MOF glass, MOF composites, silver composite, gas separation
membranes, light hydrocarbon separation



\textbf{Abstract}

Metal-organic framework (MOF) glass is a versatile and processible
material that retains its porous nature while transitioning between
liquid and solid states. Incorporating composites into MOF glass can
enhance its functionalities, but the effect on its state conversion
remains understudied. This research explores the impact of silver ions
on the properties of ZIF-62 glass, revealing their influence on phase
conversion at varying ligand concentrations. The presence of silver
enhances gas separation processes, particularly improving
CO\textsubscript{2} and hydrocarbon separation selectivity compared to
pure ZIF-62 glass. These findings provide valuable insights for
composite integration and highlight the potential of Ag-doped ZIF-62
glass in gas separation applications. Furthermore, this work paves the
way for processing functional Ag-doped ZIF-62 glass films, enabling
diverse applications such as gas separation, film catalysis, and
antimicrobial glass.



\textbf{1. Introduction}

Silver (Ag) is an incredibly versatile material utilized in numerous
applications, both in its ionic and elemental forms. Its exceptional
catalytic properties have been demonstrated across a wide range of
fields, including gas hydration, medical treatments, sensors,
photovoltaics, drug delivery, textiles, paints, and
imaging\textsuperscript{{[}1--4{]}}. Particularly in organic synthesis,
silver has proven to be highly effective as a catalyst, contributing to
its extensive application in chemical
synthesis\textsuperscript{{[}4--7{]}}. Furthermore, silver ions and
nanoparticles have shown significant potential in gas separation
techniques, particularly when incorporated into membrane
systems\textsuperscript{{[}8--12{]}}. To ensure precise control over
silver nanoparticle size and prevent aggregation, researchers have
developed encapsulation and stabilization methods. By utilizing high
surface area, porous materials like metal-organic frameworks (MOFs) and
zeolites, silver nanoparticles can be effectively
encapsulated\textsuperscript{{[}13--16{]}}. Moreover, silver has been
successfully incorporated as a dopant in diverse porous frameworks,
thereby imparting its remarkable catalytic and antimicrobial properties
to the porous structure \textsuperscript{{[}17--19{]}}. This strategy
allows for the synthesis of multifunctional materials with enhanced
performance and applications in various industries.

MOF is one of the most studied porous materials due to it being a very
versatile material owing to not onlitx exceptional porosity but also
high tunability. MOF's capability to change its respective ligands and
metal nodes within its structure allows for tunable pore characteristics
to suit different applications.\textsuperscript{{[}20{]}} A recent study
has shown a new phase of this highly tunable material, at which some
species of zeolitic imidazolate frameworks (ZIFs), a family of MOF, can
be melted to form a glassy phase.\textsuperscript{{[}21,22{]}} This
highly processible material has been shown to have good applications in
gas separation and has also been shown to be able to form excellent
composites with other materials such as perovskite and other MOF for gas
separation, photocatalysis and oxygen reduction reaction. The ease of
processibility for this material also allows its application in coatings
of previously hard-to-coat substrates such as non-functionalized carbon
cloth.

One of the widely studied MOF glass is ZIF-62 due to its ultrahigh glass
forming capabilities and large melting-to-decomposition temperature
difference.\textsuperscript{{[}21{]}} Although ZIF-62 thermal behaviour
and applications have been studied in the form of composites (with ionic
liquid), metal substitution and ligand substitution, not much research
has been done on the effect of functional guest molecules within its
structure on its thermal and melting behaviour, as well as their gas
separation applications.\textsuperscript{{[}23--26{]}} In this paper, we
aim to understand and explore the effect of silver doping in ZIF-62
glass, its effects on dynamic thermal changes, and the resulting
interactions of Ag within the glassy structure towards engineered gas
separations.

\textbf{2. Materials and Methods}

Materials

All the materials were purchased from Sigma Aldrich unless otherwise
stated. For the synthesis of different variations of zeolitic
imidazolate frameworks (ZIF-62) the following chemicals were used: zinc
nitrate hexahydrate
(Zn(NO\textsubscript{3})\textsubscript{2}\selectlanguage{ngerman}·6H\textsubscript{2}O),
imidazole (Im), benzimidazole (bIm), All these chemicals were used as is
without further purification. The following solvents were used either
for chemical synthesis, dispersing synthesised particles, or washing:
methanol and N, N-dimethylformamide (DMF). All these solvents were used
as is without further purification. Polymers for the synthesis of mixed
matrix membranes were purchased and used directly from suppliers.
6FDA-DAM ((4,4'-Hexafluoroisopropylidene) diphthalic dianhydride (6FDA)
and 2,4,6-trimethyl-1,3-diaminobenzene (DAM)) is a type of polyimide
that was obtained from Akron Polymer Sytems (USA) at a molecular weight
of \textasciitilde{}370k.

Synthesis of ZIF-62 and AgZIF-62

Generally, ZIF-62 particles were synthesised using solvothermal methods
reported by Frentze-Beyme et al. with slight
modifications.\textsuperscript{{[}27{]}} ZIF-62 particles are usually
reported with an x value which represents the benzimidazole ratio in the
particle according to the formula below:

\begin{equation}
x\ =\ 2c(bim)/(c(im)\ +\ c(bim))\ \nonumber \\
 \end{equation}

whereby c is the molar concentration

Different amounts of
Zn(NO\textsubscript{3})\textsubscript{2}\selectlanguage{ngerman}·6H\textsubscript{2}O, Im, bIm
and DMF as per were measured into a Schott bottle and stirred until
fully dissolved. Once fully dissolved, the Schott bottle was tightly
sealed and left to react in a 130 \textsuperscript{o}C oven for four
days. The resulting solids were then soaked and washed with methanol
three times before being dried in a 70 \textsuperscript{o}C oven
overnight before use.

Table 1: Precursor amount for ZIF-62 synthesis.\selectlanguage{english}
\begin{longtable}[]{@{}llllll@{}}
\toprule
\textbf{Sample} & \textbf{Reported x value} &
\selectlanguage{ngerman}\textbf{Zn(NO\textsubscript{3})\textsubscript{2}·6H\textsubscript{2}O,
g} & \textbf{Imidazole, g} & \textbf{Benzimidazole, g} & \textbf{DMF,
mL}\tabularnewline
\midrule
\endhead
ZIF-62-a & 0.05 & 2.67 & 1.98 & 0.0267 & 200\tabularnewline
ZIF-62-b & 0.12 & 2.67 & 1.951 & 0.08 & 200\tabularnewline
ZIF-62-c & 0.24 & 2.67 & 1.867 & 0.231 & 200\tabularnewline
\bottomrule
\end{longtable}

The synthesised ZIF-62 particles were fully activated in the vacuum oven
at 150 \textsuperscript{o}C for 24 hours before Ag adsorption to ensure
maximal adsorption of Ag\textsuperscript{+} into the ZIF-62 pores. The
activated particles were then lightly crushed and mixed using a small
pestle and mortar to reduce possible agglomeration during activation and
drying. The lightly ground ZIF-62 particles were then dispersed into a
0.05M methanol solution of AgNO\textsubscript{3} and stirred at 300 rpm
using a magnetic stirrer for 1 hour. The resulting suspension was then
centrifuged at 9500 rpm for 15 minutes using Cence H1850 high-speed
centrifuge and dried at 70 \textsuperscript{o}C for 5 hours before being
stored in a dark and cool condition. The Ag-doped ZIF-62 can be further
melted at a temperature ranging between 400
\textsuperscript{o}C-450\textsuperscript{o}C using a furnace with an
Argon environment for 5 minutes to form the glassy/thermally treated
version of these materials.

Synthesis of melted ZIF-62 and Ag-ZIF-62 mixed matrix membranes

In the formation of the 6FDA-DAM-based membrane, the polymer percentage
in the solvent-polymer mixture was retained at 7.7 wt\%. The melted
ZIF-62 or Ag-ZIF-62 filler was first dispersed in chloroform for at
least 12 hours before the addition of 6FDA-DAM. Occasional
ultrasonication was also performed using an ultrasonic bath during the
12-hour filler dispersion period. This step is essential to ensure
better dispersion of fillers and reduced chances of agglomeration before
the addition of the polymer of choice. After filler dispersion, 3 wt\%
of the full amount of 6FDA-DAM (7.7 wt\%) was added to the dispersed
filler chloroform solution and left to stir for 24 hours. This step is
important as a priming step to ensure full dissolution and dispersion of
6FDA-DAM polymer while having good contact with the fillers. Occasional
ultrasonication was also performed using an ultrasonic bath during these
24 hours to ensure efficient dispersion and reduced agglomeration. After
this period, the remaining amount of 6FDA-DAM was added slowly to reduce
the formation of agglomerates and then left to be stirred overnight. The
highly viscous 6FDA-DAM-filler solution was then sonicated for 20-30
minutes to remove any remaining trapped air in the solution. The
solution was then cast on a glass slide using a casting blade at the
height of 500 \selectlanguage{greek}µ\selectlanguage{english}m. The glass slide was then immediately covered in a
solvent-saturated enclosed container and left to evaporate overnight
slowly. The resulting membrane was then slowly peeled off and activated
in a vacuum oven at 30 \textsuperscript{o}C for 24 hours before use. A
lower activation temperature of 30 \textsuperscript{o}C was used for
Ag-doped fillers to minimise the possibility of Ag nanoparticle
formation during activation. All the drying/activation processes for
Ag-doped fillers were also performed in a light-enclosed environment to
minimise possible changes in the filler structure, such as further Ag
nanoparticle formation.

Material characterization

SEM was used to analyze the morphology of particles and membranes in
this study. SEM-EDS was performed to study the chemical composition
within the Ag-ZIF-62 particles and membranes. Samples were prepared by
mounting them onto an aluminium SEM sample stub using carbon tape.
Particulate samples were dispersed onto a silicon wafer before being
secured onto the SEM sample stub. The samples and the stub were dried in
the oven overnight at 70 \textsuperscript{o}C under a vacuum to remove
any residual solvents. The samples were thinly coated with platinum for
morphological analysis (FE-SEM and conventional SEM) (ca. 15 nm
thickness) or carbon for Energy Dispersive Spectroscopy analysis
(SEM-EDS) (ca. 30 nm thickness) using Quorum Q150T high-resolution
sputter coater. Scanning Electron Microscopy was performed either using
JEOL JSM-7001F or JEOL JSM-7100F for Field Emission Scanning Electron
Microscopy (FE-SEM) and Hitachi SU3500-A for conventional SEM imaging.
All SEM analysis for morphological analysis was performed under
secondary electron imaging mode at 5kV and a working distance of 10mm.
Point Energy Dispersive Spectroscopy (SEM-EDS) were performed using JEOL
JSM-7100F and JEOL JSM-7001F. Mapping Energy Dispersive Spectroscopy
(SEM-EDS) was performed using Hitachi SU3500-A. All SEM-EDS analyses
were performed at 20 kV and a working distance of 10 mm.

FT-IR and synchrotron THz-IR analysis was performed to understand the
underlying bonds within the particles and membranes. Fourier-Transform
Infrared (FT-IR) spectra were obtained using Nicolet 6700 by Thermo
Scientific equipped with a diamond attenuated total reflection (ATR)
objective. All spectra were collected between 600
cm\textsuperscript{-1}to 4000 cm\textsuperscript{-1}. THz/Far-IR
absorption spectra were collected at the THz/Far-IR beamline at the
Australian Synchrotron with a Bruker IFS 125/HR Fourier Transform (FT)
spectrometer. The bolometer was cooled with liquid helium to reduce the
background noise. A 6 \selectlanguage{greek}µ\selectlanguage{english}m thick Multilayer Mylar beam splitter was
applied. The measurement was performed with an attenuated total
reflection (ATR) stage. Powder samples were mounted on the surface of
the diamond crystal window. In situ spectra were gathered with the ATR
heating stage under a flowing argon environment (ca. 20 mL
min\textsuperscript{-1}). For data processing, the Extended ATR
correction algorithms in the OPUS 8.0 software, together with NumPy
module v1.15 together with Python v3.9, were applied for spectral data
correction and peak fitting.

ICP-OES were performed to study the Ag to Zn ratios within the Ag-ZIF-62
particles. The Ag/Zn molar ratio of Ag-doped ZIF-62 was analysed with a
Thermofisher iCap Pro inductively coupled plasma optical emission
spectrometer (ICP-OES) instrument. The samples were digested with 35
wt\% nitric acids before the ICP-OES test.

X-ray diffraction (XRD) was performed to analyse the crystalline and
glassy structure of the ZIF-62 and Ag-doped-ZIF-62. Room temperature
X-ray diffraction measurements were carried out under ambient conditions
with Rigaku Miniflex 600 Benchtop X-Ray Diffractometer and a Cu
K\selectlanguage{greek}\textsubscript{α} \selectlanguage{english}(\selectlanguage{greek}λ \selectlanguage{english}= 1.5406 \selectlanguage{ngerman}Å) radiation source. Samples in particle
or small solid form are ground using a pestle and mortar before being
dispersed onto a zero-background holder. Membrane or film samples were
cut into squares with a size of ca. 2 cm x 2 cm before being secured
onto a zero-background holder using a blue tack on the edge of the
holder. The secured samples were then mounted onto the sample stage for
X-Ray diffraction measurements. The 2\selectlanguage{greek}θ \selectlanguage{english}range was 5\textsuperscript{o} to
50\textsuperscript{o}, with a step size of 0.03\textsuperscript{o} and a
1.5\textsuperscript{o} min\textsuperscript{-1} scan speed.

In situ thermal XRD was also performed to understand the structural
transformation between solid, liquid and cooling phases. In situ thermal
x-ray diffraction was performed using Anton Paar BTS 500 benchtop
heating stage to study the thermal dynamics of different particles and
thin film MOFs. The particles were dispersed onto a zero-background
holder before being secured onto the benchtop heating stage for
analysis. The heating rate was fixed at 20 \textsuperscript{o}C
min\textsuperscript{-1}. All in situ thermal XRD analyses were performed
under inert conditions via nitrogen gas flow. Samples within the
benchtop heating stage were purged for 30 minutes before the temperature
was increased for analysis. The collected XRD data were transformed into
xy data format using PowDLL software (N. Kourkoumelis, ICDD Annual
Spring Meetings (ed. Lisa O'Neill), Powder Diffraction, 28 (2013)
137-48) before being analysed using Excel. Literature comparison CIF
data were obtained from the Cambridge Crystallography Data Centre
(CCDC). The CIF data were processed using Mercury software obtained from
CCDC, and the XRD data were extracted in xy data type format.

Thermogravimetric analysis was performed to understand the thermal
decomposition behaviour of particles and membranes. Thermogravimetric
analysis (TGA) was carried out using a METTLER TOLEDO TGA/DSC 1 STARe
System. The sample was heated to 900 \selectlanguage{ngerman}°C at a rate of 10 °C
min\textsuperscript{-1} under flowing air or nitrogen as required (20 mL
min\textsuperscript{-1}).

Differential Scanning Calorimetry was used to study the melting
temperature (T\textsubscript{m}) and glassy transition temperature
(T\textsubscript{g}) of the samples. Differential scanning calorimetry
(DSC) analysis was conducted using a separate Mettler Toledo DSC 1 STARe
system. To determine the melting temperature (T\textsubscript{m}), the
samples were heated from 40 °C to 450 °C at 20 °C
min\selectlanguage{english}\textsuperscript{-1} under a nitrogen atmosphere. For determining the
glass transition temperature (T\textsubscript{g}), each sample was
heated above the inherent T\textsubscript{g} and then cooled back to
room temperature at a ramping rate of 10 degC to eliminate the effect of
thermal history on glass transition temperature (T\textsubscript{g})
determination. Then the T\textsubscript{g} was determined from the
following up-scans.

XPS was performed to study the different elemental states on the surface
of the ZIF-62 and Ag-ZIF-62 crystals and melted variations. X-ray
photoelectron spectroscopy (XPS, AXIS Supra+, KRATOS Analytical) was
performed using a mono Al X-ray gun with an emission current of 10.00 mA
and pass energy 160 eV for survey scan and 20 eV for high-resolution
spectra, respectively. The C--C peak position was set to 284.8 eV and
used as an internal standard.

\textsuperscript{1}H liquid state NMR was performed to study the molar
ratio of benzimidazole to imidazole in the as synthesised ZIF-62
particles. 5.0 mg of sample was digested in a mixture of DMSO-d6 (0.5
mL) and DCl/D\textsubscript{2}O (35\%; 0.1 mL). The \textsuperscript{1}H
NMR measurement was performed with a Bruker Avance 500 high-resolution
NMR spectrometer interfaced to an 11.7 Tesla 51 mm bore magnet
system.\textsuperscript{13}C solid-state NMR was performed to study the
molecular interaction surrounding carbon atoms within both the
crystalline and melted variations of ZIF-62 and Ag-ZIF-62. Solid state
NMR for \textsuperscript{13}C studies was performed using Bruker 300 MHz
Avance III solid-state (SS) NMR spectrometer with 7.41 Tesla (300 MHz)
wide bore superconducting magnet interfaced with a two-channel Bruker
Avance III spectrometer.

Adsorption isotherms were measured using Micromeritrics TriStar II 3020.
Samples were degassed for 24h under a high vacuum before adsorption
analysis. CO\textsubscript{2} isotherms at 273K were performed from high
vacuum up to 130 kPa through one adsorption-desorption cycle.

\par\null\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

Figure 1: Flow diagram of gas permeation rig.

All pure gas membrane permeation tests were performed in a variable feed
pressure and constant volume permeation system, as shown in Figure 1.
All gas permeation tests were performed at room temperature. The
membrane module utilizes a sealing o-ring directly onto the selective
polymeric membrane surface. The membranes were secured in the gas
permeation module before tests. The membranes were held under vacuum for
approximately 5 minutes before being exposed to selected gas on the feed
side at a selected pressure of around 2 bars. The permeate side of the
membrane (including the buffer tank) was left in a vacuum before the
start of the gas permeation test. The pressure of the feed and permeate
side of the membrane were measured at intervals of 5 s, and the steady
rate of pressure increase on the permeate side was calculated. The
effective membrane area for each test was also recorded.

The permeation coefficient was calculated using the following equation:

\begin{equation}
P=\ \frac{273.15\ \times 10^{10}}{760\times A\times T}\frac{V\times L}{\frac{P_{0}\times 76}{14.7}}\frac{\text{dp}}{\text{dt}}\nonumber \\
 \end{equation}

Where \(P\) is the permeation coefficient in barrer (1
barrer =1 \selectlanguage{ngerman}× 10\textsuperscript{-10} cm\textsuperscript{3} (STP) cm
cm\textsuperscript{-2} s\textsuperscript{-1} cm
Hg\textsuperscript{-1}),\(A\) is the effective area of the
membrane (cm\textsuperscript{2}),\(T\) is the absolute
temperature (K), \(V\) is the dead-volume of the permeate
side (cm\textsuperscript{3}), \(L\) is the membrane
thickness (cm), \(P_{0}\) is the feed pressure (psi),
and\(\frac{\text{dp}}{\text{dt}}\) is the steady rate of pressure increase in the
permeate side (mm Hg s\textsuperscript{-1}). All permeation values and
error bars were obtained from multiple measurements of the samples.

The ideal selectivity for different gas pairs was calculated using the
following equation:

\begin{equation}
\alpha=\ \frac{P_{A}}{P_{B}}\nonumber \\
 \end{equation}

Where \(P_{A}\) and \(P_{B}\) are the permeation
coefficient of pure gas A and B, respectively.

\textbf{3. Results and Discussions}

\textbf{3.1. Silver adsorption and effects on ZIF-62 crystal structure}

The pure ZIF-62 crystal structure was prepared in three different
benzimidazole to imidazole ratios using a previously reported method
with slight changes.\textsuperscript{{[}27{]}} This is to study the
effects of silver interactions on the thermal dynamics of these
structures at different melting points. Thermal dynamics in this context
can be defined as the structural dynamics that occur upon the
introduction/removal of heat. The as-synthesized ZIF-62 were labelled as
ZIF-62-a, ZIF-62-b, and ZIF-62-c in increasing benzimidazole ratio
within the structure. The benzimidazole (bIm) to imidazole (Im) ratio in
ZIF-62 can usually be denoted in the form of
Zn(Im)\textsubscript{2-x}(bIm)\textsubscript{x}. The actual
benzimidazole to imidazole ratio was measured using liquid state NMR as
per Table 2.

Table 2: Measured benzimidazole to imidazole ratio for different
variation of ZIF-62 via NMR\selectlanguage{english}
\begin{longtable}[]{@{}lll@{}}
\toprule
\textbf{Sample} & \textbf{Benzimidazole ratio (x)} & \textbf{Imidazole
ratio (2-x)}\tabularnewline
ZIF-62-a & 0.0268 & 1.9732\tabularnewline
ZIF-62-b & 0.0936 & 1.9064\tabularnewline
ZIF-62-c & 0.225 & 1.775\tabularnewline
\bottomrule
\end{longtable}

Adsorption was chosen as the means to introduce silver ions into the
ZIF-62 structure as it is one of the more straightforward methods to
incorporate ions into porous structures. While some MOF encapsulation
methods involve a one-pot in situ encapsulation during the synthesis of
MOF, this method was not incorporated in this paper as the impact of
silver ions on the formation of ZIF-62 during the normal synthesis route
has not been studied well and will add complications which fall outside
the scope of this study - which is to understand the role of silver ions
towards the dynamics of ZIF-62 thermal behaviour. Methanol was also
chosen as the adsorption medium, both to dissolve silver ions and to
disperse ZIF-62 crystals, even though silver ions (from
AgNO\textsubscript{3} for this study) are much more soluble in aqueous
conditions. This is because ZIF-62 crystals are highly hydrophobic and
will not disperse well in water. A less toxic alternative such as
ethanol was not chosen due to the possibility of the formation of an
explosive byproduct when reacted with AgNO\textsubscript{3,} such as the
highly explosive silver fulminate\textsuperscript{{[}28{]}}. The
as-synthesized ZIF-62 particles were activated overnight before silver
adsorption to ensure that the pores were effectively vacant to maximize
the silver adsorption efficiency. The morphology of the crystal between
pure and silver-loaded ZIF-62 glass at different benzimidazole -
imidazole ratio as seen in Figure 2 also shows that there was no clear
morphological change in the shape of the resulting crystals.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

Figure 2: SEM images showing the crystal ZIF-62 morphologies of
a)ZIF-62-a, b)ZIF-62-b, c)ZIF-62-c, d)AgZIF-62-a, e)AgZIF-62-b, and
f)AgZIF-62-c. Black scale bar represents 1\selectlanguage{greek}µ\selectlanguage{english}m.

The presence of silver ions within the structure was confirmed using
SEM-EDS onto the resulting particles, as shown in Figure S 1. The peaks
at around 1.1 keV, 8.6 keV and 9.5 keV show the presence of zinc, which
is the metal node for the ZIF-62 framework. The two peaks around 3.0 keV
and 3.1 keV show the presence of silver within the particles. The peak
at around 1.8 keV represents silicon as the particles were dispersed
onto a silicon wafer. This shows that zinc and silver are both present
within the ZIF-62 structure indicating that silver has been successfully
introduced into the structure.

\par\null

Table 3: Measurement of Silver to Zinc ratio in silver-doped ZIF-62.\selectlanguage{english}
\begin{longtable}[]{@{}lll@{}}
\toprule
\textbf{Sample} & \textbf{Ag:Zn ratio during adsorption} & \textbf{Ag:Zn
ratio of resulting particles measured by ICP}\tabularnewline
AgZIF-62-a & 0.507 & 0.523\tabularnewline
AgZIF-62-b & 0.516 & 0.541\tabularnewline
AgZIF-62-c & 0.532 & 0.569\tabularnewline
\bottomrule
\end{longtable}

\par\null

ICP-OES was also used to further confirm the presence of silver ions in
the structure and to measure the silver to zinc ratio, as can be seen in
Table 3. The ratio of silver to zinc ions was shown to be around 0.5:1,
which is very similar to the zinc to silver ratio during the adsorption
process. Although the ratio calculated via ICP is slightly higher than
the calculated ratio during adsorption, the difference is reasonably
small and may have arisen from systematic and random errors throughout
the experimental synthesis process. This shows that the silver
adsorption into the structure was reasonably effective.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

Figure 3: XRD patterns of pure ZIF-62 and silver-doped ZIF-62 crystals

A further probe into the structure of the resulting Ag-doped ZIF-62 was
performed to understand the effect of silver adsorption onto the ZIF-62
structure. From the XRD results in Figure 3, it was found that there was
no significant difference between synthesized ZIF-62 particles with the
Ag-doped ZIF-62 particles, with the exception of the lower bIm ratio
(ZIF-62-a). For the Ag-ZIF-62 at this lower bIm ratio, some small new
peaks were seen, which may indicate the introduction of slight
structural change, though the bulk ZIF-62 peaks are still present. This
may also indicate that the silver interaction during adsorption towards
the overall crystal structure of the ZIF-62 is dependent on the
benzimidazole amount within the structure.

\textbf{3.2. AgZIF-62 thermal and melting behaviour}

The thermal decomposition behaviour of materials is an important
property to be studied for glass-related research. This is because a
number of ZIFs and MOFs such as ZIF-8 and ZIF-7 have a much higher
melting point as compared to their decomposition
point.\textsuperscript{{[}29{]}} This leads to the material not being
able to melt before being decomposed. Thus, it is important to measure
the thermal decomposition of AgZIF-62 variants to ensure that the
decomposition temperature lies above the typical ZIF-62 melting
temperature to ensure minimal decomposition from occurring during
melting.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

Figure 4: TGA analysis on AgZIF-62 variants in a) air and b)
N\textsubscript{2} flow

Thermogravimetric analysis (TGA) was performed on the AgZIF-62 variants
and compared with the as-synthesised ZIF-62-a, as can be seen in Figure
4. The decomposition temperature of AgZIF-62 variants is much lower than
ZIF-62-a when subjected to air, whereby the AgZIF-62 variants start to
decompose at around 400\textsuperscript{o}C as opposed to ZIF-62-a,
which only starts to decompose at around 600 \textsuperscript{o}C.
However, when subjected to high temperature under N\textsubscript{2}
flow, the decomposition of AgZIF-62 variants is almost similar to
ZIF-62-a and does not significantly decompose below 500
\textsuperscript{o}C. As the melting of ZIF-62 is usually performed
under inert gas conditions, this TGA analysis shows that the AgZIF-62
variants can be treated with temperatures up to 500 \textsuperscript{o}C
without triggering significant decomposition. We then continue to check
the thermal and structural behaviour of AgZIF-62 variants subjected to
temperature to see if there are any observational changes towards its
structural melting dynamics.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

Figure 5: XRD on the thermal structural dynamics of a)ZIF-62-a and
silver-doped ZIF-62 structures b)AgZIF-62-a, c)AgZIF-62-b, and
d)AgZIF-62-c. Ag peak marked with asterisk (*), and ZIF-zni peak marked
with hash (\#).

A dynamic study of the thermal effects of Ag-doped ZIF-62 crystals was
performed using in-situ thermal XRD in Figure 5 to further understand
the structural changes in Ag-doped ZIF-62. For a lower bIm ratio, Ag
nanoparticles start to form at a temperature of between 150
\textsuperscript{o}C to 200\textsuperscript{o}C. It is also interesting
to note that ZIF-zni was formed for a lower bIm ratio and at a much
lower temperature of below 200 \textsuperscript{o}C than that previously
reported for ZIF-4 and ZIF-62 via recrystallization
\textsuperscript{{[}30,31{]}}. ZIF-zni is a dense form of the
imidazolate (Im) based ZIF and the presence of benzimidazolate (bIm)
ligand usually prevents its formation due to the bulkier ligands
preventing the complete collapse of the frameworks. The temperature of
ZIF-zni formation here is close to the AgNO\textsubscript{3} melting
point, which may indicate that the AgNO\textsubscript{3} molecules were
mobilized at this temperature and started to interact with bIm in the
structure. This caused the rearrangement of imidazole parts to form a
ZIF-zni structure. However, at a higher bIm ratio, ZIF-zni formation was
not observed, and the glass was able to be melted fully and become fully
amorphous with Ag nanoparticle present. This shows that the Ag in the
structure may have interacted with bIm in the structure during heating,
leading to these observable changes. As the formation of ZIF-zni is seen
in both AgZIF-62-a and AgZIF-62-b, future analysis will be only focused
on AgZIF-62-a and AgZIF-62-c to compare and understand the interactions
and functionality of ZIF-zni-forming and glass-forming AgZIF-62.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

Figure 6: DSC heat flow analysis of pure and Silver doped ZIF-62
a)Initial upscan, and b) the second upscan. Small arrow indicating the
melting peak (T\textsubscript{m}) for the initial upscan and glass
transition peak (T\textsubscript{g}) for the second upscan. Highlighted
region in a) indicates a possible region for Ag nanoparticle formation.

A DSC analysis (Figure 6) was used to further probe the behaviour of
these particles. For the lower bIm ratio samples, there were additional
peaks at temperatures ca. 179 \textsuperscript{o}C and
238\textsuperscript{o}C, which may indicate the interaction of Ag within
the structure, leading to the formation of ZIF-zni. The dip at around
200 \textsuperscript{o}C to 330 \textsuperscript{o}C in Figure 6a may
indicate the formation of Ag nanoparticles, which coincides with the
previous XRD structure. These peaks are also not present in the pure
ZIF-62, which further indicates the role of silver in the observation of
this behaviour. It is also interesting to note that the peaks seen at
earlier temperatures for the lower bIm ratio of silver-doped AgZIF-62
were not observed at the higher bIm ratio, which may indicate the
absence of ZIF-zni formation. The melting peak can be seen for all cases
in the first DSC upscan, with each respective ZIF-62 and AgZIF-62 of the
same benzimidazole ratio having an almost similar melting point. The
ZIF-62-a and AgZIF-62-a can be seen to have a melting peak, with
T\textsubscript{m} at around 374 \textsuperscript{o}C and
373\textsuperscript{o}C, respectively, while ZIF-62-c and AgZIF-62-c
have a melting temperature, T\textsubscript{m} at 413
\textsuperscript{o}C and 419 \textsuperscript{o}C respectively. A
clearer melting peak can also be observed for the higher benzimidazole
ratio ZIF-62, which may indicate clearer and better melting at a higher
benzimidazole ratio. An exothermic peak was also seen at around 373
\textsuperscript{o}C for AgZIF-62-a, after the melting peak ends, which
may indicate the recrystallisation back into the dense crystalline
ZIF-zni phase right after the melting process occurs. This peak forms
immediately after the observed melting point, which may also indicate an
accelerated process of the conversion of the remaining ZIF-62 to ZIF-zni
during melting, most probably due to the better flowability of silver
interacting with the whole ZIF-62-a structure.

This analysis shows that the Ag to bIm ratio is very important to
control the melting of ZIF-62, either to form a complete melt or to form
a recrystallized and more thermodynamically stable ZIF-zni. It is also
important to note that the Ag compounds in all AgZIF-62 variants start
to form Ag nanoparticles at temperatures between 150\textsuperscript{o}C
to 200 \textsuperscript{o}C.

\textbf{3.3. Ag interaction with Zn ZIF-62 crystal and glass framework}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

Figure 7: In situ THz IR analysis on ZIF-62 and AgZIF-62 at a
temperature range between 30\textsuperscript{o}C to 300
\textsuperscript{o}C for sample a) ZIF-62-a, b) ZIF-62-c, c) AgZIF-62-a,
d)AgZIF-62-c. For each plot, from bottom to top, the sample is heated
from 30\textsuperscript{o}C to 300\textsuperscript{o}C, before cooling
back down to 30\textsuperscript{o}C.

In order to further understand the silver interactions within the
structure of the ZIF-62 and its role in the ZIF-62 structural
transformation, the samples were probed using THz IR (Figure 7). There
were not many changes or differences between different benzimidazole
ratios of ZIF-62 particles with no silver loading (ZIF-62-a and
ZIF-62-c). Only three main clear peaks can be observed in both cases, a
peak at around 280-330 cm\textsuperscript{-1} which indicates the Zn-N
stretching movements, a peak around 650-700 cm\textsuperscript{-1,}
indicating in-plane and out-of-plane aromatic ring deformations, and a
peak at around 750-800 cm\textsuperscript{-1} indicating C-H bending. In
the presence of silver, however, some peak intensifies with new peaks
forming at different benzimidazole ratio. For the lower benzimidazole
ratio (AgZIF-62-a), the peaks responsible for all three groups present
in the normal ZIF-62-a start to intensify greatly above
200\textsuperscript{o}C. A clear peak broadening for the peaks
responsible for Zn-N stretching movements (280-330
cm\textsuperscript{-1}) and C-H bending can be seen, which may indicate
additional interaction at these sites. Additional stronger peaks also
start to emerge between 100-200 cm\textsuperscript{-1}, which represents
the 4-membered and 6-membered ring collective motions, especially with
the peak around 150-200 cm\textsuperscript{-1}. This may indicate the
formation of ZIF-zni-related interactions as this formation was not
observed for AgZIF-62-c. AgZIF-62-c also undergoes similar
intensification for the three main peaks observed for the normal
ZIF-62-c. Some intensifications of the peaks between 400-600
cm\textsuperscript{-1} may also indicate a stronger interaction with the
benzimidazole ligand within the structure, as data from the literature
shows no significant peak between this wavenumber region for Zn and
Imidazole precursors.\textsuperscript{{[}26{]}} This may also indicate
that the silver is strongly interacting with the benzimidazole ligands
at higher temperatures, which further explains the change of thermal
dynamics between lower and higher benzimidazole ratio AgZIF-62
(AgZIF-62-a forming ZIF-zni while AgZIF-62-c was able to fully melt).
The stronger interaction of Ag with the benzimidazole ligand may have
promoted the stacking of imidazolate ligand at a lower temperature, thus
promoting the formation of ZIF-zni in the environment with a lower
benzimidazole amount. An additional peak can also be seen for AgZIF-62-c
at a wavenumber range between 650-700 cm\textsuperscript{-1} at
300\textsuperscript{o}C, which may indicate an additional interaction
within the aromatic ring, leading to an additional aromatic ring
deformation peak. This additional peak is highly likely to be attributed
to the interaction of Ag within the AgZIF-62-c structure as this
additional peak was not seen in any of the normal ZIF-62 structures
(ZIF-62-a and ZIF-62-c), and also from THz data reported in the
literature on ZIF-62 variants.\textsuperscript{{[}26{]}} This proves
that Ag is strongly interacting with the ligand structure within the
AgZIF-62 crystals, but at different sites depending on the final
formation results (ZIF-zni for AgZIF-62-a and fully melted ZIF for
AgZIF-62-c).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}

Figure 8: FTIR analysis on ZIF-62 and AgZIF-62 particles

FTIR analysis was also performed on the pure ZIF-62-a and ZIF-62-c, as
well as the silver-loaded AgZIF-62-a and AgZIF-62-c and their melted
components (denoted as the mAgZIF-62) (Figure 8). The FTIR analysis
shows peaks for the C-C in-plane and out of plane bending (600-700
cm\textsuperscript{-1}), C-H out of plane bending (700-1000
cm\textsuperscript{-1}), C-H in-plane bending (1100-1300
cm\textsuperscript{-1}), C-N stretching (1300-1370
cm\textsuperscript{-1}), and C=C stretching (1390-1650
cm\textsuperscript{-1}), which are all present in the ligands used for
the formation of ZIF-62. However, no clear FTIR peak changes between the
silver-loaded ZIF-62 (AgZIF-62-a and ZIF-62-c), its melted components
(mAgZIF-62-a and mAgZIF-62-c), and pure ZIF-62 variants (ZIF-62-a and
ZIF-62-c) can be seen. Thus, it is most likely that these interactions
were not impacted by the presence of silver ions/molecules in the
structure. An extra peak for mAgZIF-62-a that is present at
775cm\textsuperscript{-1} could be attributed to the out-of-plane
deformation of the imidazolate linker contributed by the presence of
ZIF-zni. This extra peak was also present in FTIR data reported in the
literature for the ZIF-zni phase.\textsuperscript{{[}32{]}}

An XPS analysis was also performed to understand the bonding occurring
on the surface of the particles. The wide scan XPS analysis, as can be
seen in Figure S 2a, shows no obvious new peaks between all the
crystalline and melted variations, except for the variations containing
Ag. This is consistent with the sample and demonstrates that the Ag has
been successfully embedded within the sample. The intensity of Ag peaks
relative to other peaks was also diminished in the melted sample when
compared to the crystalline sample, which indicated that the Ag species
may be embedded within the ZIF-62 melted structures rather than being on
the surface. This is because XPS is usually a surface-sensitive
analysis, and with the same penetration power, the XPS will not
penetrate as deeply into the denser melted/zni forms when compared to
the crystalline forms, thus reading less Ag that is situated deeper
within the structure. The summary of the XPS peaks found for different
species can be found in Figure S 3. No clear changes can be seen for all
other species except for Ag. A deeper look into the Ag peak shifts can
also be seen in Figure S 2b, in which the non-melted AgZIF-62 variants
indicate that the Ag may exist in an ionic AgNO\textsubscript{3} form,
while the melted mAgZIF-62 variants show a possible oxidised Ag. This
may be due to the exposure of Ag on the surface to air, thus resulting
in the formation of AgO on its surface. As explained above, the lower
penetration depth and surface sensitivity of XPS into the denser
heat-treated ZIF-62 may be a reason for the lack of Ag nanoparticle
peaks, as the Ag nanoparticle may be deeper embedded within the
structure. Thus, the actual state of silver within the melted/zni
particles should closely resemble the results seen in previously
collected XRD data which shows the conversion of silver ions into silver
nanoparticles only at higher temperatures (above
150\textsuperscript{o}C).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}

\par\null

Figure 9: \textsuperscript{13}C solid-state NMR on ZIF-62 and
silver-loaded ZIF-62 a)crystals and b)glasses.

Solid State NMR has also been performed to help probe the interaction of
the carbon atoms within the ZIF-62 structure in the presence of silver.
The chemical shift at 125 ppm and 137 ppm could be attributed to the
carbon atoms available in the imidazole ligands, while the additional
peaks at 115 ppm, 120 ppm, 149 ppm and 160 ppm can be attributed to the
carbon atoms present in the benzimidazole ligands. This can be clearly
seen in both crystal and melted \textsuperscript{13}C solid-state NMR in
Figure 9, at which the peaks responsible for the benzimidazole ligands
are much more pronounced in higher benzimidazole ratio ZIF-62 variants.
The \textsuperscript{13}C solid-state NMR do not show a clear difference
between the detected C bonds, except for the case of mAgZIF-62-a, at
which an additional peak appears around 123 ppm. This peak can be
attributed to the presence of the ZIF-zni structure in the system of
mAgZIF-62-a. This is as, according to literature, when ZIF-4 was melted
into ZIF-zni, a new set of peaks appeared around the region between
127.8 ppm -- 128.3 ppm, which is slightly higher than the other
imidazole-related peak in ZIF-4 around 123.9 ppm -- 125.8
ppm.\textsuperscript{{[}33{]}} As this peak exists very close to the
peak responsible for carbon atoms in the imidazolate ligand, it
indicates that the presence of Ag may have strongly interacted with the
imidazolate ligand during melting/heating for the lower benzimidazole
ratio ZIF-62 (AgZIF-62-a).

A quick probe into the porosity behaviour of the synthesised ZIF-62
silver composites was also performed. CO\textsubscript{2} adsorption
behaviour was studied as previous reports for ZIF-62 have stated that
limited N\textsubscript{2} adsorption can be seen for ZIF-62 glasses due
to their relatively small pore windows and
cavities.\textsuperscript{{[}21{]}} From the
CO\textsubscript{2}adsorption behaviour seen in Figure S 4, it can be
seen that only a slight adsorption behaviour can be seen when silver was
doped into the ZIF-62 glass, indicating that minimal pore size change
occurred after silver adsorption. The slight increase of
CO\textsubscript{2} adsorption for ZIF-62-a variation when silver was
added may be due to the disruption of ultra-dense ZIF-zni structure by
the silver composite, which formed additional pores allowing for some
extra pore formation allowing for extra CO\textsubscript{2} adsorption.
The slight decrease of CO\textsubscript{2} adsorption capacity in the
ZIF-62-c variation upon the addition of silver composites may be due to
the filling up of some pores in the amorphous ZIF-62 glassy structure by
silver composite, leading to lower CO\textsubscript{2} adsorption.

\textbf{3.4. Utilisation of Ag Zn-ZIF-62 for gas separation processes}

In order to study the practical applicability of the synthesised
Ag-doped glassy materials, its capability to selectively separate gases
was tested. Silver in its different states, such as Ag ionic liquid, Ag
salts, Ag nanoparticles and even AgO, have been shown to have the
capability of selectively transporting different gas pairs such as
CO\textsubscript{2}/N\textsubscript{2} and
olefin/paraffin.\textsuperscript{{[}9,34--36{]}} In this section, we
attempt to demonstrate the accessibility of the silver nanoparticles
embedded within the glassy matrix by studying the gas separation
performance in a mixed matrix membrane set. 6FDA-DAM was chosen as the
polymer matrix due to its good capabilities in separating
CO\textsubscript{2}/N\textsubscript{2}, as well as its relatively good
selectivity for olefin/paraffin-based
separation.\textsuperscript{{[}37--39{]}} The melted AgZIF-62 and ZIF-62
particles were dispersed in a 6FDA-DAM polymer solution before being
cast and activated.

The membranes were characterised to understand their intrinsic
properties. SEM and SEM-EDS were used to study the contact between the
dispersed particles and the polymer matrix as can be seen in Figure S 5.
As can be seen in Figure S 5a and d, the contact between the filler
particles and the polymer matrix is considerably good. No clear void was
observed between the AgZIF-62 fillers with the 6FDA-DAM polymer matrix.
SEM-EDS showed the presence of the fillers embedded within the membrane,
with reasonable particle dispersion and particle size of around 5-10\selectlanguage{greek}µ\selectlanguage{english}m.
The silver atoms were also seen to be well dispersed within the regions
at which Zn is present, indicating that the Ag is well embedded in the
ZIF-62 glass/zni and does not leach into the main polymeric section of
the membrane. This would allow any improvement to be mainly attributed
to the filler rather than any leached silver ions in the 6FDA-DAM
matrix.

An analysis using XRD has also shown the successful incorporation of
different fillers at different loadings into the 6FDA-DAM matrix as can
be seen in Figure S 6a, as the filler retains its characteristic peaks
within the 6FDA-DAM matrix, with its intensity increasing with respect
to its loading in the membrane. As can be seen in Figure S 6b, all the
FTIR peaks present in pure 6FDA-DAM polymers over the range of 600
cm\textsuperscript{-1} to 4000 cm\textsuperscript{-1} were present in
all of the membranes, indicating that there were no significant changes
in the chemical bonds and structure of the polymer matrix were seen.
However, in the range between 600 cm\textsuperscript{-1} to 850
cm\textsuperscript{-1,} a few new peaks were seen in the MMMs. A peak
between 610 cm\textsuperscript{-1} to 615 cm\textsuperscript{-1} was
present only in AgZIF-62 variants of MMMs, which may indicate the
interaction of Ag with the polymer matrix. This is as the lower region
peaks are usually attributed to the interaction of metal
nanoparticles/ions with an organic ligand (typically seen in MOFs). A
peak at 665 cm\textsuperscript{-1} was seen in all AgZIF-62 and ZIF-62
loaded MMMs, which comes from the ZIF-62 particles themselves. Two peaks
at about 775cm\textsuperscript{-1} and 835 cm\textsuperscript{-1} were
seen only in AgZIF-62-a loaded MMMs, which is also present in the
AgZIF-62-a particles due to the presence of dense ZIF-zni
phase.\textsuperscript{{[}32{]}}

A TGA analysis, as can be found in Figure S 6c, demonstrates the
membrane decomposition in air at around 500 \textsuperscript{o}C, which
shows that the 6FDA-DAM polymer has a thermal decomposition behaviour
similar to the previous TGA analysis of the filler particles. The
6FDA-DAM neat polymer has a remaining weight of about 2\%, while the
10\%, 20\% and 30\% loaded mAgZIF-62-a membranes all have a residual
weight of 5\%, 12\% and 21\%, respectively. The 20\% mZIF-62-a membrane
has a slightly lower remaining weight of 9.5\% when compared to the
Ag-loaded variation of similar loading, which is also consistent with
the previous particle TGA analysis due to the absence of Ag
nanoparticles in the resulting weight. This demonstrates that these
membranes are relatively stable up to 500 \textsuperscript{o}C and can
be considered for higher temperature-based gas separation in the future.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image10/image10}

\end{center}
\end{figure}

Figure 10: Gas separation performance for AgZIF-62-a at different
composite loading for a) CO\textsubscript{2}, N\textsubscript{2}, and
CH\textsubscript{4} based separation, b )ethylene/ethane based
separation, and c) propylene/propane-based separation

The activated ZIF-62@6FDA-DAM membrane was then tested for different
gases, including CO\textsubscript{2}, N\textsubscript{2},
CH\textsubscript{4}, ethane, ethylene, propane, and propylene. As
mentioned in earlier sections, silver, both in the elemental and ionic
state, has been shown in the literature to improve gas separation
performance. In the case of CO\textsubscript{2}/N\textsubscript{2} and
CO\textsubscript{2}/CH\textsubscript{4} separation, silver nanoparticles
embedded in Pebax mixed matrix membrane have been shown to consistently
help towards CO\textsubscript{2} permeance improvements and
selectivity.\textsuperscript{{[}40{]}} Similar behaviour can be seen in
this experiment, too, as per Figure 10 and Figure 11, whereby the
addition of mAgZIF-62 continually increases the permeability of
CO\textsubscript{2} and increases
CO\textsubscript{2}/N\textsubscript{2}selectivity until a maximum
loading of 20\%. At 20\% loading, a maximum
CO\textsubscript{2}/N\textsubscript{2} selectivity of 27.1\selectlanguage{ngerman}±1.7 and
CO\textsubscript{2} permeability of 864.3±4.4 barrer were achieved,
which is significantly higher than pure 6FDA-DAM with a selectivity of
17±0.9 and CO\textsubscript{2} permeability of 642.4±4.8 barrer. This
corresponds to a selectivity increase of 60\% and permeability of 34.5\%
as compared to a pure 6FDA-DAM membrane. However, an increase to 30\%
loading starts to lower the selectivity significantly despite a
continual increase in permeability, which may be caused by aggregation
of mAgZIF-62 particles in the membrane at higher loading and the
possible formation of interfacial voids. However, no clear increase in
CO\textsubscript{2}/CH\textsubscript{4} selectivity can be seen, which
may be caused by proportionally similar permeance improvements for both
CH\textsubscript{4} and CO\textsubscript{2} with increased loading.

Other than CO\textsubscript{2}-targeted separation, silver nanoparticles
have also been demonstrated to help with light hydrocarbon separation.
Although silver ions have been shown to result in good selectivity for
olefin/paraffin-based separation, they are unstable over an extended
period of time.\textsuperscript{{[}41{]}} The utilisation of silver
nanoparticles has been shown in the literature to be more stable over
longer periods, with good selectivity of olefin over
paraffin.\textsuperscript{{[}10,42{]}} This behaviour is also consistent
with our data, as can be seen in Figure 10 and Figure 11. Like previous
CO\textsubscript{2} -- targeted separation, the selectivity for
ethylene/ethane and propylene/propane both increases until mAgZIF-62
loading of 20\% before dropping, with a maximum selectivity of 3.57 and
10.21, respectively. This corresponds to ethylene/ethane and
propylene/propane selectivity increases of 32.8\% and 17.2\% when
compared to the pure 6FDA-DAM membrane. The permeability of both
ethylene and propylene also increases by 29.3\% and 18.4\% to a value of
44.4 and 31.1 barrer respectively at 20\% loading.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image11/image11}

\end{center}
\end{figure}

Figure 11: Gas separation performance for AgZIF-62 at different
composite states (ZIF-zni vs ZIF glass) for a)CO\textsubscript{2},
N\textsubscript{2} and CH\textsubscript{4} based separation,
b)ethylene/ethane-based separation, and c)propylene/propane-based
separation.

\par\null

Further analysis of the effect of Ag-loaded thermally treated ZIF type
(ZIF-zni and amorphous ZIF glass) on gas separation performance was also
studied, as can be seen in Figure 11. The performance of the presence of
Ag nanoparticles in ZIF-62 glass was also compared to pure ZIF-62 glass.
The selectivity of all gas pairs studied shows no significant increase
when the pure 6FDA-DAM polymer was loaded with mZIF-62-a, despite an
increase in gas permeability. This may be attributed to possible void
formation between mZIF-62-a towards its surrounding 6FDA-DAM polymer
matrix. The gas performance of all gas pairs for 6FDA-DAM mixed membrane
systems loaded with silver nanoparticles (mAgZIF-62-a and mAgZIF-62-c)
consistently show improvements in both selectivity and permeability no
matter the ZIF types (ZIF-zni or amorphous ZIF glass). This demonstrates
the accessibility of silver in thermally treated ZIF-62 irrespective of
phase, which is important to ensure that the silver can be accessed for
other applications too. The accessibility of silver for both ZIF-zni and
amorphous ZIF glass also demonstrates that silver nanoparticle
immobilisation/functionalisation can be applied for both ZIF material
types to suit their respective future applications.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image12/image12}

\end{center}
\end{figure}

Figure 12: Demonstration of reverse hydrocarbon selectivity for olefin
species with the presence of silver nanoparticles.

Another observation was that with similar carbon content, alkene/olefin
always has a much higher permeability than its alkane/paraffin
counterpart. This can be explained by the \selectlanguage{greek}π--\selectlanguage{english}Ag complexation allowing
the alkene bond (C=C) to overlap with the 5s and 4d orbitals present in
Ag.\textsuperscript{{[}10,42,43{]}}However, as can be seen in Figure 12,
the permeability trend with increasing carbon amount differs with the
presence of silver nanoparticles. An expectation of the permeability
trend of molecules with very similar physical and chemical properties is
that it should follow the trend of smaller particles having higher
permeability than their bigger counterpart. This trend can be seen for
paraffin molecules despite loading type and amount (CH\textsubscript{4}
\textgreater{} C\textsubscript{2}H\textsubscript{6} \textgreater{}
C\textsubscript{3}H\textsubscript{8}). However, in the presence of
silver, the permeability of alkene increases with an increase in
molecular size (C\textsubscript{3}H\textsubscript{6} \textgreater{}
C\textsubscript{2}H\textsubscript{4}), while the reverse of that
behaviour was seen in membranes and mixed matrix membranes with no
silver composites (C\textsubscript{3}H\textsubscript{6} \textless{}
C\textsubscript{2}H\textsubscript{4}). Although this behaviour has been
shown in terms of adsorption with the presence of silver ions, to our
knowledge, it has never been demonstrated for silver nanoparticles and
in terms of membrane permeability.\textsuperscript{{[}43{]}} The
adsorption capacity data of olefins for Ag\textsuperscript{+} decorated
particles from literature also follows the order of
C\textsubscript{3}\textgreater{} C\textsubscript{2} \textgreater{}
C\textsubscript{1}, which was explained to be due to the decreasing
sequence of the light hydrocarbon's critical temperatures
(T\textsubscript{c}). However, this behaviour was also reported to be
similar for paraffins, which is not the case for our silver loaded mixed
matrix membranes. Thus, this may demonstrate a facilitated
diffusion-based transport in our silver loaded membrane system, at which
the silver complexation helps improve the diffusion of olefin over
paraffin without strongly bonding to the olefin molecules.

A possible explanation for the observation of this behaviour may come
from the transport mechanism of the olefin itself. As each of the
propylene and ethylene has only one alkene bond (C=C), and the system
has a constant amount of \selectlanguage{greek}π \selectlanguage{english}bond site available on the Ag surface;
theoretically, the molar amount of \selectlanguage{greek}π--\selectlanguage{english}Ag complexation that is able to be
achieved by both propylene and ethylene should be relatively similar.
The \selectlanguage{greek}π--\selectlanguage{english}Ag complexation also occurs in the same electron orbital for both
propylene and ethylene, and thus, the complexation energy (for
adsorption and desorption) should be relatively close. Propylene has a
higher molecular weight and consequentially higher gas density when
compared to ethylene. Thus, when measured on a weight or volume basis, a
larger amount of propylene will be adsorbed than ethylene (when
considering that a similar molar amount was adsorbed for both species).
This higher weight/volume amount of propylene-Ag complex formed can then
be translated to a higher facilitated transport rate by weight/volume
when compared to ethylene, which is consistent with the result seen in
Figure 12. However, more experiments will have to be performed to
confirm this theory as many other factors also come into play, such as
the effect of propylene vs ethylene sizes which may reduce the molar
adsorption density on the Ag nanoparticle surface, and possible
differences of (C=C) \selectlanguage{greek}π--\selectlanguage{english}Ag complexation energies due to the presence of
extra alkyl chain in propylene when compared to ethylene.

\textbf{4. Data Availability and Reproducibility Statement}

All plotted numerical data (XRD, TGA, DSC, THz-IR, FTIR, 13C SSNMR, Gas
permeation data, SEM EDS, XPS, and CO\textsubscript{2} adsorption data)
as shown in Figures 3 -- 12, and Figures S1, S2, S4 and S6 can be found
in the Supplementary Information. All data has been either calibrated,
or corrected to calibration where required, according to widely
practiced data analysis for specific techniques, such as carbon peak
calibration for XPS data, XRD sample calibration, and background data
removal for spectroscopic measurements. Where error bars are presented,
repeat of the analysis measurements were made and the mean value was
used, with the standard deviation reported as the error range/bar.

\textbf{5. Conclusion}

The presence of silver as guest molecules has been shown to provide a
significant impact on the melting behaviour of ZIF-62. The benzimidazole
ratio within the ZIF-62 structure was shown to directly impact the
resulting dynamic thermal behaviour. At a lower benzimidazole ratio, the
ZIF-62 tends to form ZIF-zni at a much lower temperature, while at a
higher benzimidazole ratio, ZIF-62 was able to melt fully into an
amorphous glass without any resulting ZIF-zni. An additional look into
the Ag interaction using THz-IR shows that the Ag may have significant
interaction with the ligands, especially the benzimidazole ligand, which
may explain the difference in behaviour at a lower benzimidazole ratio.
This chemical interaction analysis, coupled with solid-state NMR, also
indicates that the Ag may have interacted at different sites within the
ZIF-62 structure based on the initial benzimidazole ratio amount in the
structure. The resulting Ag-doped thermally treated ZIF-62, both in
ZIF-zni and amorphous glass form, forms accessible Ag nanoparticles
within the thermally treated ZIF-62 framework. This was shown by good
gas separation performance when the materials were applied as a gas
separation membrane in a mixed-matrix membrane setting. The gas
separation analysis also shows Ag nanoparticles' capability to reverse
the alkene selectivity as its size increase
(C\textsubscript{3}H\textsubscript{6} \textgreater{}
C\textsubscript{2}H\textsubscript{4}), which has never been demonstrated
before in a membrane.

The success in the formation of functional thermally treated ZIF-62 in
both ZIF-zni and amorphous ZIF-62 when doped with silver demonstrates
the suitability of this material to be composited with other functional
metallic molecules. The ease of processibility of these meltable ZIFs
also opens its usage in the form of a thin film for a variety of
real-life applications such as for gas separation or film-based
catalysis and functional films (eg. antimicrobial glass).

\textbf{Supporting Information}

Supporting Information is available from the Wiley Online Library or
from the author.

\textbf{Acknowledgements}

J.H would like to acknowledge the financial support from the Australian
Research Council (FT210100589), the Foundation Research Excellence Award
of the University of Queensland, and UQ Dow Centre. This research was
undertaken on the THz/Far-IR beamline at the Australian Synchrotron,
part of ANSTO. This research was supported by the AINSE Ltd. Early
Career Researcher Grant (ECRG).

Received: ((will be filled in by the editorial staff)) Revised: ((will
be filled in by the editorial staff)) Published online: ((will be filled
in by the editorial staff))

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Supporting Information

\textbf{Regulating thermal dynamics and gas transport of MOF glass
through silver composites}

\emph{Muhammad Yazid Bin Zulkifli, Ruiqi Chen, Rijia Lin, Yuqi Yao,
Milton Chai, Dominique Appadoo, Wupeng Wang, John Zhu, Kang Liang, Vicki
Chen and Jingwei Hou}

The numerical data for the plots in Figures 3 -- 12, and Figures S1, S2,
S4 and S6 are provided in a compressed ZIP file called ``Data for
publication'' in the Supplementary Information. The numerical data are
in an excel spreadsheet format, and the files included are:

\begin{itemize}
\tightlist
\item
  Figure 3, Figure S6a - XRD at room temperature (particles and
  membrane)
\item
  Figure 4, Figure S6c - TGA Data (Air and N2, particle and membrane)
\item
  Figure 5 - In-situ thermal XRD data
\item
  Figure 6 - DSC plot data
\item
  Figure 7 - THz-IR data
\item
  Figure 8, Figure S6b - FTIR data (particles and membrane)
\item
  Figure 9 - Solid State NMR 13C data
\item
  Figure 10, Figure 11, Figure 12 - Membrane gas permeation performance
  data
\item
  Figure S1 - SEM point EDS plot data
\item
  Figure S2 - XPS data
\item
  Figure S4 - CO2 adsorption data from Micromeritrics Tristar
\end{itemize}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image13/image13}

\end{center}
\end{figure}

Figure S 1: SEM-EDS analysis of Silver doped ZIF-62 crystal and glass.
The X mark indicates the location of point EDS. The scale bar indicates
a length of 1\selectlanguage{greek}µ\selectlanguage{english}m.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image14/image14}

\end{center}
\end{figure}

Figure S 2: a) Wide scan XPS analysis for all ZIF-62 and melted ZIF-62
composites. Ag3d peaks region were marked with the star symbol for
samples containing Ag. b) Focused XPS scan on Ag3d peak region for
samples containing Ag.\selectlanguage{english}
\begin{longtable}[]{@{}llllllll@{}}
\toprule
\textbf{Sample} & \textbf{XPS Binding Energy Peak Values (eV)} &
\textbf{XPS Binding Energy Peak Values (eV)} & \textbf{XPS Binding
Energy Peak Values (eV)} & \textbf{XPS Binding Energy Peak Values (eV)}
& \textbf{XPS Binding Energy Peak Values (eV)} & \textbf{XPS Binding
Energy Peak Values (eV)} & \textbf{XPS Binding Energy Peak Values
(eV)}\tabularnewline
\midrule
\endhead
& \textbf{C1s} & \textbf{N1s} & \textbf{O1s} &
\textbf{Zn2p\textsubscript{3/2}} & \textbf{Zn2p\textsubscript{1/2}} &
\textbf{Ag3d\textsubscript{5/2}} &
\textbf{Ag3d\textsubscript{3/2}}\tabularnewline
ZIF-62-a & 284.8 & 398.9 & 531.5 & 1021.5 & 1044.5 & N/A &
N/A\tabularnewline
ZIF-62-c & 284.8 & 398.8 & 531.4 & 1021.4 & 1044.4 & N/A &
N/A\tabularnewline
mZIF-62-a & 284.8 & 398.7 & 531.4 & 1021.5 & 1044.7 & N/A &
N/A\tabularnewline
mZIF-62-c & 284.8 & 399.1 & 531.6 & 1021.8 & 1044.7 & N/A &
N/A\tabularnewline
AgZIF-62-a & 284.8 & 398.9 & 531.9 & 1021.5 & 1044.5 & 368.4 &
374.4\tabularnewline
AgZIF-62-c & 284.8 & 398.9 & 531.7 & 1021.9 & 1044.9 & 368.7 &
374.7\tabularnewline
mAgZIF-62-a & 284.8 & 398.7 & 531.3 & 1021.2 & 1044.2 & 367.3 &
373.4\tabularnewline
mAgZIF-62-c & 284.8 & 399 & 531.5 & 1021.5 & 1044.7 & 367.6 &
373.7\tabularnewline
\bottomrule
\end{longtable}

Figure S 3: XPS peaks calculated by peak fitting from collected peaks
(C1s binding energy (eV) calibrated to 284.8).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image15/image15}

\end{center}
\end{figure}

Figure S 4: CO\textsubscript{2}adsorption data of different melted
ZIF-62 and ZIF-62 composites\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image16/image16}

\end{center}
\end{figure}

Figure S 5: SEM and EDS images of 20\% mAgZIF-62-a @ 6FDA-DAM on the top
layer (a, b, and c), and 20\% mAgZIF-62-c @ 6FDA-DAM on the bottom layer
(d, e, and f). a) and d) are SEM secondary electron images. b) and e)
are Zn EDS scans. c) and f) are Ag EDS scans.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image17/image17}

\end{center}
\end{figure}

Figure S 6: a)XRD analysis of melted mZIF-62 and mAgZIF-62 loaded into
6FDA-DAM at different loading, b)FTIR analysis of the loaded membranes,
and c) TGA analysis of filler loaded 6FDA-DAM.

\selectlanguage{english}
\FloatBarrier
\end{document}

