Matthias Rubio

Joshua Näf

Franz Bühlmann

Philippe Brigger

Moritz Hüsser

Martin Inauen

Nicole Ospelt

Daniel Gisler

Marco Tognon

Roland Siegwart

Design of PrisMAV: An Omnidirectional Aerial Manipulator based on a 3-PUU Parallel Mechanism

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

I. INTRODUCTION

Micro aerial vehicles (MAVs) have gained a significant amount of market traction for personal and industrial use over the last years. Still, active interaction with objects or pick & place missions have not yet gained a foothold in industry, even though such tasks are commonplace for ground robots. Due to the increased mechanical and dynamic complexity of aerial manipulators (AMs) compared to conventional MAVs, this topic has been limited to academic research [START_REF] Ollero | Past, present, and future of aerial robotic manipulators[END_REF], [START_REF] Meng | Survey on aerial manipulator: System, modeling, and control[END_REF], [START_REF] Orsag | Aerial Manipulation[END_REF]. Possible applications of AMs are vast, ranging from power line maintenance, over welding and contact-based inspection of pipes on a refinery [START_REF] Tognon | A truly redundant aerial manipulator system with application to push-and-slide inspection in industrial plants[END_REF], to collecting objects on inaccessible rooftops and construction work on skyscrapers. In the future, AMs could provide efficient and safe techniques to carry out general labour tasks in remote and dangerous locations.

As of today, many implementations of AMs consist of a quad-or hexarotor morphology with an attached manipulator [START_REF] Bellicoso | Design, modeling and control of a 5-DoF light-weight robot arm for aerial manipulation[END_REF], [START_REF] Heredia | Control of a multirotor outdoor aerial manipulator[END_REF], [START_REF] Fumagalli | Developing an aerial manipulator prototype: Physical interaction with the environment[END_REF], [START_REF] Tognon | Dynamic decentralized control for protocentric aerial manipulators[END_REF]. This approach benefits from the ability to reuse off-the-shelf multirotors and manipulators, which require less mechanical development and thus facilitate the entry to the field. Note that the need for a robotic arm is not strictly necessary to physically interact with the environment. Instead, zero degrees of freedom (DoF) end-effectors could be used which would reduce the weight and mechanical 1 Autonomous Systems Lab, Department of Mechanical Engineering, ETH Zürich, 8092 Zürich, Switzerland, mrubio@ethz.ch complexity of the manipulator. An example is demonstrated in [START_REF] Mellinger | Design, modeling, estimation and control for aerial grasping and manipulation[END_REF], where they present different approaches of rigidly mounted grippers on a quadrotor platform for pick and transport missions. One downside of using a fixed endeffector is the loss of DoF which causes a decrease in the set of reachable grasping points around the platform's body. Depending on the application, the workspace of an AM is a crucial characteristic. In general, it is desired to maximize the workspace which can be achieved in different ways. One approach is presented in [START_REF] Hamaza | Omni-Drone: on the Design of a Novel Aerial Manipulator with Omni-directional Workspace[END_REF], where a quadrotor is equipped with a 2-DoF parallel manipulator to achieve an omnidirectional workspace.

In contrast to increasing the DoF of the manipulator, the workspace can also be expanded by exploiting the maneuverability of the chosen floating base. More specifically, by using an omnidirectional platform [START_REF] Hamandi | Design of multirotor aerial vehicles: a taxonomy based on input allocation[END_REF]. The additional DoF required for omnidirectionality can be achieved by placing fixed thrust actuators in a specific way [START_REF] Park | Odar: Aerial manipulation platform enabling omnidirectional wrench generation[END_REF], [START_REF] Brescianini | An omni-directional multirotor vehicle[END_REF] or by actuating the direction of the thrust axes with respect to the body by having tiltable propeller arms as shown in [START_REF] Kamel | The voliro omniorientational hexacopter: An agile and maneuverable tiltable-rotor aerial vehicle[END_REF], [START_REF] Bodie | Towards efficient full pose omnidirectionality with overactuated MAVs[END_REF]. Thereafter, the missing DoF incurred by using a fixed end-effector can be allocated on an omnidirectional platform to achieve the full 6-DoF. Nevertheless, a robotic arm can increase the AM's precision, since it can compensate for the slow flight dynamics.

It has to be mentioned that using a parallel manipulator can be advantageous since it generally has lower inertia and higher stiffness compared to a serial arm with equivalent DoF. The reason for this is that a parallel manipulator has its actuators located at the base rather than at each joint. Therefore, it can cover a higher bandwidth for error compensation while minimizing the resulting reaction forces on the floating base. The favourable weight distribution of a parallel manipulator also induces a reduction of the system's inertia which can increase the flying platform's agility. Additionally, a parallel manipulator is much less sensitive to errors in actuator position tracking compared to a serial manipulator, where the error is accumulated over all joints. This is also a desirable characteristic for high precision applications. A specific implementation of a parallel manipulator on an omnidirectional hexacopter was shown in [START_REF] Bodie | Dynamic end effector tracking with an omnidirectional parallel aerial manipulator[END_REF].

This work aims to showcase a novel AM design which incorporates the previously mentioned advantages of a parallel mechanism with the desired agility of an omnidirectional floating base. Following a top-down design approach starting from the end-effector, the manipulator and the flying platform were developed conjointly resulting in a unified and fully integrated system design. The presented AM PrisMAV (see Figure 1), is based on tilting rotors and a delta manipulator with three limbs having prismatic-universal-universal (3-PUU) joints. Especially the linear delta manipulator is chosen due to its enclosing structure which allows the robotic arm to be fully retractable into the main body of the AM. This keeps the design compact and allows the platform to land on its main structure. In the context of pick and place missions, the platform can retract grasped objects into its body and protect them during flight, particularly when landing or taking off. Furthermore, the proposed omnidirectional base platform uses only 4 tilting rotor groups compared to six on an omnidirectional hexacopter. The advantage of using a parallel robotic arm is shown by comparing the position tracking performance to a hypothetical fixed end-effector in static hover mode. Additionally, the design was validated in experiments by performing pick and place missions by using an adaptive end-effector. One mission was performed such that the gripper is pointing downwards and another where it is pointing to the side. In both missions, an object was grasped from a static location and released at another position. Finally, the critical parts of the design and possible improvements, as well as the impact on future applications in the field are discussed.

II. HARDWARE DESIGN

The focus of PrisMAV is to perform pick and place missions because this implies general tasks such as grasping an object and transporting it over small distances. For the missions, the test object to manipulate was defined as an aluminum cylinder of 0.5 kg with a diameter of 8 cm and a height of 20 cm. An example of an actual pick and place mission can be the positioning of delicate sensors in high or inaccessible areas on industrial sites. To achieve those tasks, the system has to be able to interact precisely with the object during the grasping and placing maneuvers. By analyzing the positional error data of an existing omnidirectional MAV [START_REF] Bodie | Towards efficient full pose omnidirectionality with overactuated MAVs[END_REF], the necessary workspace of the AM can be estimated. A spectral analysis of the positional error from the existing platform shows that a maximum error amplitude of 4 cm is reached at a frequency of approximately 1 Hz.

To compensate this positional error including an additional safety margin, the manipulator workspace should be designed to cover a cylindrical workspace with a radius of 5.6 cm and a depth of 10 cm. Furthermore the end-effector should achieve omnidirectionality to be able to pick and place the object in any given orientation. In the following subsections we present the design procedure starting from choosing the appropriate end-effector up to composing the complete AM PrisMAV, which can be seen in Figure 1.

A. End-Effector

To grasp objects of varying sizes and geometries, we equip PrisMAV with commercial adaptive fingers. To actuate them, a rack and pinion-based mechanism is used thanks to its low complexity and high actuation speeds, which are beneficial when trying to grasp an object in flight. An explosion view of the model can be seen in Figure 2. The mechanism is actuated by a servo motor which is recessed into the gripper base which connects the manipulator linkages and the endeffector. The servo itself is rigidly mounted into the gripper base and hence serves as additional structural support. For the racks and the pinion, a light-weight and wear-resistant plastic is used. To increase the grip of the adaptive fingers on smooth materials they are coated by a layer of silicone on their contact surfaces. This reduces the necessary torque that the servo needs to produce when grasping an object. The minimum opening width is set to be at least 10 cm for the cylinder of 8 cm in diameter in order to account for uncertainties in the error compensation of the manipulator.

B. Manipulator as Base Structure

To be able to compensate the expected positional error of the platform, a parallel manipulator is chosen due to its fast dynamics and centralized weight distribution which leads to an overall lower inertia of the platform. Specifically, a 3-PUU linear delta manipulator is selected, which was highly inspired by similarly constructed 3D-printers. The ability of those printers to perform their desired tasks uniquely inside their structure was a key feature in the decision process. Their outer structure could be used as the body of the whole AM, setting the design apart from the state-ofthe-art approach of composing an existing flying platform with a manipulator. Furthermore, the manipulator and the object can be easily retracted into the structure providing a protective enclosure for delicate objects, such as sensors or expensive tools. One can even think of performing some task on the object inside the structure during the transportation from one location to another, e.g., by evaluating a collected ground sample by using spectroscopy or a computer vision algorithm to analyze the composition of the collected sample. In future applications, this could reduce the number of flights necessary to gather the desired information and hence the overall mission time. In our case, having a fully retractable manipulator makes the design more compact and it enables PrisMAV to land on its structure, as all the other faces of the AM are obstructed by propellers.

The base structure consists of three carbon linear guides for the manipulator, giving the system the shape of a triangular prism. To hold the linear guides in place and to rigidly attach the rotor groups to the system, T-shaped carbon plates are used which are connected together by aluminum parts as shown in Figure 3. This rigid arrangement prevents the guides from bending and facilitates precise operation of the manipulator during interaction. The manipulator is actuated by three brushless DC motors located at the end of the linear guides. The rotational motion is translated into linear motion via a toothed belt drive. The belts are attached to three low friction polymer sliders gliding along the carbon linear guides. The 3D-printed end-effector base is then connected to the sliders using three linkages each made up of two carbon tubes and bilaterally ending on ball joints. The dimensions for the arm linkages and the linear guides are chosen such that i) the manipulator is fully retractable into the prismatic structure, ii) the end-effector extension is at least 10 cm outside of the surrounding structure, and iii) the required workspace can be fully covered. As visualized in Figure 4, the workspace computed with the dimensions of the actual manipulator exceeds the required workspace.

C. Electronics

The electronic components are located on the back plate, which houses batteries, the IMU and the integrated computer which features an Intel core i7 and an STM32 micro controller. The on-board computer can be accessed through a Wi-Fi connection from a remote operator. Damping elements made out of silicone decouple the back plate from the rest of the system. Making use of the high inertia of the batteries, this damping method attenuates high frequency vibrations generated by the actuators of PrisMAV which would negatively affect IMU measurements and may strain and damage the electronic components in the long term. The electronic speed controllers for the thrust generating actuators are connected to the computer via a CAN bus interface. The power for the whole system comes from two 6200 mAh batteries at 22 V which provide an estimated mean flight time of 10 min in hover mode. Table I gives an brief overview of the electrical components and actuators used on PrisMAV.

D. Towards an Aerial Manipulator

The platform features four rotor arms allocated at the vertices of a tetrahedron, each carrying a propeller group which can tilt around its respective axis (see Figure 1). Note that this arrangement together with the extended delta arm ensures a safe distance to the operating point at the endeffector to prevent possible collisions of the propellers or the structure with the environment. A possible scenario could be PrisMAV bringing a tool to a human construction worker who is operating at height, where a safety margin is crucial. To increase the thrust and to cancel out the drag torque, each propeller group consists of two coaxial, counter-rotating rotors and is able to exert a maximum thrust force of 40 N. The tilting mechanism of the rotor groups is actuated by servo motors as can be seen in detail in Figure 5. The wiring for the propellers yields enough slack to allow for up to two full rotations in both directions, making almost any maneuver possible. The design of the tilt mechanism and the propeller group arrangement was inspired by existing platforms such as the one demonstrated in [START_REF] Papachristos | Towards a high-end unmanned tri-tiltrotor: design, modeling and hover control[END_REF]. In contrast to this platform, our system involves an additional tiltable propeller group, leading to three propeller groups in the front. This extra propeller group enables PrisMAV to also rotate around its roll axis (i.e., the direction of the end-effector), granting the desired six degrees of freedom and hence omnidirectional flight and manipulation. Summarizing the hardware design, PrisMAV is composed of a 3-PUU delta manipulator to compensate for the positional inaccuracies of the flying platform. An adaptive gripper is used as the end-effector, which enables manipulation of variously shaped objects in the context of pick and place missions. To make the system fly omnidirectionally, four tiltable propeller groups are placed at the vertices of a tetrahedron. This allows the AM, and hence the endeffector to hover in any arbitrary orientation in space. The final design of PrisMAV has a diameter of about 0.95 m while being 1.33 m tall. Furthermore, the prototype weighs approximately 7.7 kg.

III. SYSTEM MODELING

To allow for decoupled development and testing, the model is split into two separate subsystems: the flying platform and the manipulator. In the following, it is assumed that the bodies are rigid and that no mechanical play or friction is present in the moving parts.

A. Coordinate Frames

To describe the full configuration of the platform in the inertial frame W, a body-fixed, a manipulator and a rotor coordinate frame are introduced (see Figure 6). The bodyfixed frame B is located at the IMU and aligned accordingly. The manipulator frame M is placed at the geometric center of the equilateral triangle spanned by the three linear guides. Its z-axis is parallel to the linear guides and the origin of its z-coordinate is at the height of the sliders when they are fully retracted into the structure. The four rotor frames R i , i ∈ {1, 2, 3, 4} are attached to the rotor groups, where the x-axis is coincident with the corresponding thrust vector and the z-axis of the rotor frame is centered in the corresponding carbon tube.

B. Aerodynamics and Allocation

A simplified aerodynamic model is introduced where we assume a quadratic relationship between the force f i ∈ R 3 and drag torque τ i ∈ R 3 produced by the i-th rotor group and its angular velocity ω.

f Ri i = µω 2 i e Ri x τ Ri i = κω 2 i e Ri x , i ∈ {1, 2, 3, 4} (1)
where µ is the thrust coefficient and κ is the drag torque coefficient of a propeller group. They can be derived empirically by conducting appropriate thrust tests. According to the previously introduced convention, e Ri x denotes the thrust direction expressed in R i . The allocation matrix is formulated as proposed in [START_REF] Kamel | The voliro omniorientational hexacopter: An agile and maneuverable tiltable-rotor aerial vehicle[END_REF] and [START_REF] Bodie | Towards efficient full pose omnidirectionality with overactuated MAVs[END_REF]. Due to the co-axial propeller configuration the single rotor drag torques cancel each other out (κ = 0) and the following equations hold:

f B = i f B i = i R BRi f Ri i (2)
τ B = i τ B i =0 + i τ B f B i = i r B i × f B i (3)
where R BRi denotes the rotation matrix from the rotor coordinate frame R i to the body fixed frame B and r B i is the position of the i-th rotor group in B. The equations can be formulated as a matrix-vector equation which results in:

w = f τ B = A(α)    f1 . . . f4    = A(α) f (4) fi = µ • ω 2 i ∀i ∈ {1, 2, 3, 4} (5)
where A(α) ∈ R 6×4 is the angle dependent allocation matrix and w ∈ R 6 denotes the desired wrench. α is defined as the vector containing all α i which denote the rotation of the i-th rotor group around e Ri z . Due to the a priori unknown and variable tilt angles α, the allocation matrix is non-constant. Thus, the auxiliary variables u k for k ∈ {1, 2, . . . , 8} are defined which eliminate the angle dependency of A(α) and provide a constant allocation matrix  ∈ R 6×8 .

u k = sin α k/2 • µ • ω 2 k/2 if k odd cos α ⌊k/2⌋ • µ • ω 2 ⌊k/2⌋ if k even (6)
The auxiliary variables u = u 1 . . . u 8 ∈ R 8 can then be calculated by: u = Â+ w

Preprint version, final version at http://ieeexplore.ieee.org/ 4 2023 International Conference on Unmanned Aircraft Systems (ICUAS) It can be shown that the resulting constant allocation matrix with the given the system dimensions, has full row rank, rank  = 6. Hence, PrisMAV is capable of exciting each dimension of the wrench-space independently with the proposed propeller allocation. The auxiliary variables u can then be transformed back to the desired tilt angles and rotor velocities by the use of trigonometric identities and the empirically obtained thrust coefficient µ.

α i = atan2(u 2i-1 , u 2i ) (8)
f i = u 2 2i-1 + u 2 2i ∀i ∈ {1, 2, 3, 4} (9)
ω i = f i µ (10)
To examine the omnidirectionality and the possible configurations of PrisMAV, in Figure 7 we show the thrust and torque envelope of the platform. The thrust envelope demonstrates the maximum thrust which can be achieved along any direction by constraining the overall torque to zero. The torque envelope denotes the torque that can be achieved by the AM subject to the constraint of a static hover thrust along the body z-axis. The envelopes in Figure 7 show that the platform can exert sufficient amounts of force or torque in any arbitrary direction while compensating for gravity to achieve omnidirectional flight and manipulation. However, there are configurations with higher maximal thrust or torque magnitudes. For instance, approximately 1.4 times thrust can be exerted in tricopter configuration (body z-axis is aligned with gravity, i.e., the end-effector pointing towards the floor or the ceiling) compared to horizontal mode (the body z-axis is normal to gravity, i.e., PrisMAV is pitched by 90 • ).

It has to be mentioned that in certain orientations the AM finds itself in a singular configuration. For instance when the AM is flying in the tricopter configuration the top propeller servo angle is not uniquely defined as it is not needed for flight. A workaround to this issue is using an alternate allocation, where the propeller group in the singular configuration is not included. Nevertheless, in practice a low but non-zero RPM command is set to the top propeller to enable faster transition between the alternate and the regular allocation.

C. Rigid Body Dynamics

The equations of motion of the flying platform can be derived using the Newton-Euler equations as proposed in [START_REF] Bodie | Towards efficient full pose omnidirectionality with overactuated MAVs[END_REF]. The resulting rigid body dynamics can be written as:

w = mI 3 0 0 J B AM p ω B + ω × m ṗ ω × J B AM ω B , ( 11
)
where I 3 is the 3×3 identity matrix, J B AM denotes the moment of inertia in the body-fixed frame B, p, ṗ, p denote the position, velocity and acceleration and ω, ω the angular velocity and angular acceleration of B, i.e. the flying platform.

Due to the moving arm and the grasped object, the inertia J B

AM and mass m of the system are not constant. This implies that the center of mass (CoM) is changing over time, resulting in an offset x B com from the body-fixed frame origin to the true CoM. In the nominal case, where the arm is fully retracted and no object is grasped, the nominal inertia J B AM,nm and CoM were identified with the CAD model of the platform. Using the parallel axis theorem, a change in inertia is characterized by:

J B AM = J B AM,nm + m • D, D =   y 2 m + z 2 m -x m y m -x m z m -x m y m x 2 m + z 2 m -y m z m -x m z m -y m z m x 2 m + y 2 m   ( 12
)
where m is the mass being moved away from the nominal CoM and x m , y m , z m are the corresponding displacements in the body-fixed frame B, which can be compactly written in the form of a displacement matrix D.

To model the change in inertia for PrisMAV, the sliders and end-effector assembly are assumed to be point masses, while the masses of the carbon tubes are neglected. Consequently the change of inertia equals to:

J B AM = J B AM,nm + m slider • D slider + m ee • D ee , (13)
where D slider is the displacement matrix for the sliders while D ee is the displacement matrix for the end-effector. They can be calculated by comparing the current end-effector position with the fully retracted end-effector position.

D. Manipulator Kinematics

The inverse kinematics for a 3-PUU delta arm are derived analytically by formulating a closed loop vector equation as shown in Figure 8. The position of the end-effector p M ee = x M ee y M ee z M ee ⊤ can be related to the joint positions q = q 1 q 2 q 3 ⊤ as:

p M ee = R j + q j e z + h j + d j + r j j ∈ {1, 2, 3} (14)
Resolving ( 14) for q j and taking the euclidean norm on both sides leads to the following equation:

q j = f j (p M ee ) = z M ee + d 2 -(x M ee -E j ) 2 -(y M ee -F j ) 2 (15) E j := (R -h -r) cos φ j ∀j ∈ {1, 2, 3} (16)
F j := (R -h -r) sin φ j (17)
Preprint version, final version at http://ieeexplore.ieee.org/ 5 2023 International Conference on Unmanned Aircraft Systems (ICUAS) where

φ j ∈ [0 • , 240 • , 120 • ] indicate the angles between e M
x and the linear guide with index j. R, h and r represent the magnitude of the corresponding vectors. Furthermore, the transform of the joint position q j to the actuator angle θ j with a radius r pulley to the pulley belt is:

θ j = q j r pulley . (18)
The inverse Jacobian mapping J -1 (p M ee ) relating the endeffector velocity ṗM ee to the joint velocities q is determined by analyzing the time derivative of q = f (p M ee ).

q = df (p M ee ) dt = ∂f (p M ee ) ∂p M ee dp M ee dt =: J -1 (p M ee ) ṗM ee (19)
IV. CONTROL

The control of the platform is split up into a flight controller and a manipulator controller (see Figure 9). This approach allowed disjoint development and testing of the two systems. Nevertheless, the influence of the manipulator motion on the flight controller (and vice-versa) is partially captured by adapting the CoM continuously as in [START_REF] Brescianini | An omni-directional multirotor vehicle[END_REF].

The setpoints for each controller are obtained by specifying a desired (denoted with a top bar) pose for the endeffector in the world frame, i.e., a position pW ee and a rotation φW ee described in Euler angles. The position setpoint for the flying platform pW is computed by a static offset h of f from the desired end-effector position such that when the arm is extended the sliders should be at 75% of their way out (as in Figure 8). The desired rotation for the flying platform is the same as φW ee since the parallel manipulator possesses only translational degrees of freedom. The position setpoint for the manipulator controller pM ee is computed by a coordinate transformation of pW ee to the manipulator frame. Note that this transformation depends on the current position and orientation of the flying platform.

A. Flight Controller

Using the equations of motion formulated in (III-C), a PID controller for the flying platform can be formulated as in ( 22) and ( 23), where the desired forces f d and torques τ d

f d = m(R ⊤ BW e trans + (ω B × R ⊤ BW ṗB )) (22)
τ d = J B AM e rot + (ω B × J B AM ω B ) + (x B com × f d ) (23)
where R BW is the rotation matrix from the inertial world frame W to the body-fixed frame B, g the gravity vector along the inertial z-axis, m the mass of the platform, J B AM the inertia, ω B the angular velocity vector, p the desired acceleration and ṗB the velocity of the flying platform. The constants k p , k v , k R and k ω are the gains on position, velocity, rotation and angular rate errors. To eliminate a static lag error, integral terms on the position and rotation error are added and multiplied by the gains k pi and k Ri , respectively. The error terms e p , e v , e R and e ω for position, velocity, orientation and angular velocity as well as the integrated error terms e pi and e Ri for position and rotation are calculated as proposed in [START_REF] Kamel | The voliro omniorientational hexacopter: An agile and maneuverable tiltable-rotor aerial vehicle[END_REF].

B. Manipulator Controller

To control the delta manipulator such that the platform's positional error can be rejected, an inverse differential kinematic controller is proposed as follows:

q = J -1 p M ee ṗM ee FF + k P • e q + k I • e q dt + k D • de q dt PID ( 24
)
where e q = qq and J -1 p M ee denotes the inverse Jacobian of the manipulator evaluated at the current endeffector position p M ee . The desired slider setpoints q can be computed using [START_REF] Bodie | Towards efficient full pose omnidirectionality with overactuated MAVs[END_REF] and the desired end-effector position pM ee . Note that the desired velocity ṗM ee is computed using finite differences. The actual slider positions q are obtained from the motor controllers (DC drivers) and close the feedback loop. It can be seen that the control law is split into a feedforward (FF) and a PID action. The FF action anticipates further changes in the setpoint while the PID control action enables offset-free tracking.

V. RESULTS

The design was validated by testing it in an indoor environment. For these tests, an external motion tracking system was used to perform the state estimation of the platform. The motion tracking system runs at 100 Hz whereas the on-board IMU runs at 300 Hz. The pose of the platform is estimated by feeding both measurements into a multi sensor fusion framework [START_REF] Lynen | A robust and modular multi-sensor fusion approach applied to MAV navigation[END_REF] based on an extended Kalman filter. The AM was tethered to a safety rope during all the flight and manipulation tests to reduce the risk of damaging the AM. Since the safety rope was loosely attached, it had only a minimal influence on the results.

To quantify the enhanced precision of the delta arm, the error of the end-effector with compensation is compared to the error of a hypothetical fixed end-effector in Figure 10. During the test, the platform was in its tricopter configuration (i.e. the end-effector pointing downwards) and a trajectory was followed along the gripping direction. One can see that the system with compensation has a significantly lower upper bound on the absolute error compared to the fixed endeffector. In particular note that when the fixed end-effector experienced large disturbances the compensating arm showed the desired behaviour of filtering out these large deviations.

Pick and place missions in the tricopter and horizontal configuration demonstrate the AM's general capabilities to interact with the environment (see Figure 11). For the tricopter mission (A), PrisMAV took off from a box. After approaching the object, the manipulator started to compensate the positional error of the flying platform. The delta arm moved out of the drone structure and grasped the object. After a successful grasp, the arm was retracted back into the platform structure with the grasped object and stopped compensating. Finally PrisMAV flew to the desired drop-off location, where the object was dropped into a basket.

The mission procedure for the horizontal configuration (B) is similar as for the tricopter mission except that the MAV started in a horizontal position.

VI. DISCUSSION

This work presents PrisMAV, a novel aerial manipulator platform designed to perform pick and place missions. Pris-MAV shows the advantages of using a parallel manipulator and exploits the maneuverability of the platform by using tilting rotor groups. Especially, the system was designed such that the manipulator structure directly makes up the main body of the flying platform. This sets our platform apart from state-ofthe-art aerial manipulators which generally piece together a flying base platform and a robotic arm. Furthermore, it was shown that integrating a parallel mechanism into a flying platform improves precision compared to using a fixed endeffector, enabling precise interaction of the platform with its environment. Pick and place missions were executed in two different flight modes to show the capability of manipulating in different orientations. Still, a critical part is to reduce the mass of the platform and increase its compactness. One could think of integrating the electronics on the side of the linear guides to reduce the overall size of the structure instead of placing them on the back plate which lies behind the manipulator. On the software side, more sophisticated and combined control strategies could be developed such as fullbody control approaches or MPC-based control strategies which would include the dynamics of the manipulator. These more sophisticated control policies could lead to increased robustness of the platform in nominal flight, as well as close to singular configurations which could be avoided automatically. This increased robustness would also allow to remove the safety tether and the external motion tracking to perform extensive testing of pose transitions in realistic pick and place missions. The development of PrisMAV should contribute to aerial manipulation research by adding a new perspective on the composition of functionalities in the design process and should motivate the community to engineer unseen aerial manipulator designs.

Fig. 1 .

 1 Fig. 1. PrisMAV, the integrated aerial manipulator with a linear delta arm.

Fig. 2 .

 2 Fig. 2. Explosion view of the end-effector assembly.

Fig. 3 .

 3 Fig. 3. The main body structure of PrisMAV.

Fig. 4 .

 4 Fig. 4. Workspace of the manipulator (grey) and the desired compensation capability (green) as seen from the front view (gripping direction).

Fig. 5 .

 5 Fig. 5. Section view of the tilt mechanism.

Fig. 6 .

 6 Fig. 6. Coordinate frames used for system modeling and control.

Fig. 7 .

 7 Fig. 7. Force and torque envelopes of PrisMAV. Most thrust is available in tricopter configuration (yellow areas), least thrust in horizontal configuration (blue areas).

Fig. 8 .

 8 Fig. 8. Closed loop vector formulation for manipulator kinematics.

Fig. 9 .

 9 Fig. 9. Block diagram of the control structure adopted on the platform.

Fig. 10 .

 10 Fig. 10. Absolute positional error of a hypothetical fixed end-effector compared to the compensating delta arm. pee and pee denote the desired and the actual end-effector position respectively.

Fig. 11 .

 11 Fig. 11. Aerial manipulation missions in tricopter (A) and horizontal (B) configuration. A.1 Approach object. A.2 Grasp object. A.3 Retract arm. A.4 Place object. B.1 Approach object. B.2 Grasp object. B.3 Retract arm. B.4 Approach drop-off. B.5 Place object.

TABLE I OVERVIEW

 I OF THE ELECTRICAL COMPONENTS AND ACTUATORS.

Preprint version, final version at http://ieeexplore.ieee.org/ 2 2023 International Conference on Unmanned Aircraft Systems (ICUAS)

Preprint version, final version at http://ieeexplore.ieee.org/ 7 2023 International Conference on Unmanned Aircraft Systems (ICUAS)

ACKNOWLEDGMENT

The authors would like to thank Prof. Dr. Roland Siegwart, Dr. Marco Tognon, Dr. Jen Jen Chung, Michael Pantic, Julian Förster, Andreas Voigt, Karen Bodie and the ASL Staff for the valuable feedback and support during the project.


Gan Ye

Yulong Gu

Christine Lancelot

Alain Rives

Carole Lamonier

Nicolas Nuns

Maya Marinova

Wei Xu

Yinyong Sun
email: yysun@hit.edu.cn

Synthesis of polyoxometalate encapsulated in uio-66(zr) with hierarchical porosity and double active sites for oxidation desulfurization of fuel oil at room temperature

Introduction

With the increasingly stringent regulations, desulfurization of fuel oil is of great importance for producing clean energy [1][2][3][4][5][6] . However, it is difficult to remove the sulfurcontaining molecules like 4,6-dimethyldibenzothiophene (4,6-DMDBT) in fuel oil due to the steric hindrance effect from alky-groups on the aromatic rings by industrial hydrodesulfurization (HDS) process [7][8][9][10] . Oxidative desulfurization (ODS) is regarded as one of the most promising processes to remove these aromatic sulfur-containing compounds due to its favorable features such as no hydrogen consumption and low cost of operation [11][12][13][14] . Thus, many research works have been focused on the development of ODS catalysts with high desulfurization efficiency [15][16][17] . Although some works about the preparation of novel ODS catalysts have been done, it is still rare that the catalysts can exhibit highly efficient ODS ability at room temperature and ambient pressure.

Polyoxometalates (POMs), as typical metal oxide cluster anions, have displayed great potentials in many applications [18][19][20][21] . However, due to their low surface area and easy solubility in solvent, it is difficult to utilize POMs as heterogeneous catalysts [22][23][24] .

To overcome this difficulty, many works were focused on POMs supported or encapsulated in porous materials [25][26][27][28] . As a result, the composite materials prepared by these methods can display heterogeneous character to some extent. However, the loss of POMs in these materials would still occur during the reactions [29,30] . Additionally, the accessibility of active centers could be poor due to the block of pore channels resulting from the introduction of PW species. For this, preparing a highly efficient heterogeneous catalyst with easy accessibility of POMs active sites maintains a great challenge.

In this work, we report that new composite materials of H3PW12O40 and UiO-66(Zr) have been prepared without the addition of any solvent by one-pot method (Scheme 1). The obtained catalysts were characterized by powder X-ray diffraction (XRD), N2 adsorption-desorption, scanning electron microscopy (SEM), High resolution high-angle annular dark-field scanning transmission electron microscopy (HR-HAADF), energy dispersive X-ray spectroscopy (EDX), Fourier transform infrared (FT-IR), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) techniques. Additionally, their catalytic performance was evaluated in the ODS reactions of benzothiophene (BT), dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) at room temperature. A possible reaction mechanism was proposed.

Experimental

Materials

ZrOCl2

2O, ZrCl4, 1,4-benzenedicarboxylic acid (BDC), phosphotungstic acid (H3PW12O40, PW), N,N'-dimenthylformamide (DMF), hydrogen peroxide (H2O2, 30 wt%), 5,5-dimethyl-1-pyrroline N-oxide (DMPO), ethanol, acetonitrile, tertiary butanol, p-benzoquinone and n-octane were purchased from Sinopharm Chemical Reagent Company. Benzothiophene (BT), dibenzothiophene (DBT) and 4,6dimethyldibenzothiophene (4,6-DMDBT) were purchased by Aldrich. All chemicals were analytical pure and used without further purification.

Synthesis of materials

UiO-66(Zr)-green and UiO-66(Zr)-solvent were prepared according to the previous references [31] .

Synthesis of PW/UiO-66(Zr)-green: ZrOCl2 2O (1.5 mmol) and BDC (1.5 mmol) were mixed and ground in a mortar by hand for 2 min at room temperature. Then, PW (100, 150, or 200 mg) was added and the resultant mixture was further ground for 8 min. After that, the obtained materials were transferred into 15 mL autoclave and crystallized at 130 °C for 12 h. After cooling to room temperature, the resulting samples were washed with 50 mL ethanol at 70 °C for 6 h once and 10 mL fresh ethanol twice.

Finally, the white solids were dried under vacuum for 12 h at 150 °C. The final samples were denoted as PW/UiO-66(Zr)-green-1, PW/UiO-66(Zr)-green, and PW/UiO-66(Zr)-green-2 based on the different weight of added PW in the process of synthesis.

Comparatively, a mechanically mixed sample named as PW+UiO-66(Zr)-green was prepared by grinding PW and UiO-66(Zr)-green.

Synthesis of PW/UiO-66(Zr)-solvent: 1 mmol ZrCl4 (233 mg) and 1 mmol BDC (166 mg) were dissolved in 30 mL DMF. Then, 300 mg PW was added into the above mixture with stirring. After that, the obtained mixture was transferred into a 40 mL Teflon-lined autoclave and crystallized at 120 °C for 24 h. After cooling to room temperature, the resultant white solid was washed with 50 mL ethanol at 70 °C for 6 h and 10 mL fresh ethanol twice. Finally, the sample was dried under vacuum for 12 h at 150 °C. The final sample was named as PW/UiO-66(Zr)-solvent.

Characterization

Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/Max-2550 diffractometer equipped with a SolX detector-Cu Ka radiation with wavelength of = 1.5418 Å. Nitrogen sorption isotherms were obtained at 77 K on a 3H-2000PS1 Gas Sorption and Porosimetry system. Samples were prepared for measurement after degassing at 423 K under vacuum until a final pressure of 1 × 10 -3 Torr was reached.

Scanning electron microscopy (SEM) images and energy dispersive X-ray spectroscopy (EDS) elemental mapping were recorded on SUPRA 55 operated with an acceleration voltage of 200 kV. Fourier transform infrared (FT-IR) spectra were recorded on a NicoLET iS10 spectrometer. High resolution high-angle annular dark-field scanning transmission electron microscopy (HR HAADF-STEM) analyses were performed using a FEG TEM/STEM system (Titan Themis FEI) operated at 200 kV, equipped with a monochromator and a probe Cs corrector. For HAADF acquisition, the spot size was 9 with a screen current of ~50 pA and a camera length of 115 mm, corresponding to inner and outer diameters of annular detector of 50 and 200 mrad, respectively. Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) measurements were performed with a TOF.SIMS 5 spectrometer (ION-TOF GmbH Germany) equipped with a bismuth liquid metal ion gun (LMIG). The compacted samples were bombarded with pulsed Bi 3+ primary ion beam (25 keV, 0.25 pA) rastered over a 500 × 500 m 2 surface area.

With 25 scans and 128x128 pixels, the total primary ion dose does not amount up to 10 12 ions/cm 2 ensuring static conditions. Charge effects due to primary ion beam were compensated by means of a 20 eV pulsed electron flood gun. Cycle time was fixed at 220µs in order to detect secondary molecular ions up to 4000 m/z. The mass resolution (m/ m) measured on our spectra was about 6000 at m/z = 232 for WO3 -. This excellent mass resolution allowed us to identify high m/z ionic fragments by their exact mass and attribution was always confirmed by the simulated isotopic pattern. X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 250 spectrometer. AlKα source was used for excitation. Pressure was in the 10 -10 Torr range during the experiments. The analyser was operated in constant pass energy of 25 eV.

Charge compensation was applied to compensate charging effect occurring during the analysis. C1s, O1s, W4f-Zr4p and Zr3d regions were collected. Adventitious carbon was used as internal reference with the C1s peak at binding energy (BE) 284.6 eV. Peak fitting of the experimental photo peaks was carried out using Casa XPS software.

Quantification took into account a non-linear Shirley background subtraction.

Catalytic tests

The ODS reactions were performed by using a three-phase reaction system at room temperature. A calculated amount of BT, DBT or 4,6-DMDBT was dissolved in noctane (with a concentration of 1000, 1000 and 500 ppmw of sulfur for BT, DBT and 4,6-DMDBT, respectively) to act as model fuel. The reaction was performed under air in a closed 40 mL dram vial with a vigorous magnetic stirrer (800 rpm). In a standard run, the activated catalyst (50 mg) was added to model fuel (10 g) and acetonitrile (10 g), and the resulting mixture was stirred for 10 min at room temperature. The catalytic reaction process was initiated by the addition of hydrogen peroxide (H2O2, 30 wt. %) as oxidant with an O/S molar ratio of 6:1. The liquids in n-octane were withdrawn at regular intervals and analyzed by gas chromatography on an Agilent 7890A GC with an FID detector using a 30 m packed HP5 column. The removal rate (R) of sulfur compounds was calculated according to the equation R = (1-Ct/C0) × 100 %, where C0 and Ct stand for the initial concentration and reaction concentration of BT, DBT or 4,6-DMDBT in n-octane phase after t minute, respectively.

The reusability of catalyst was carried out at room temperature. After each cycle, the catalyst was recovered by centrifugation, washed with acetonitrile twice, dried under vacuum for 12 h at 150 °C and then reused in a new catalytic cycle under the same reaction conditions.

Results and discussion

Characterization

The XRD results of samples are shown in Fig. 1. As seen, all the PW/UiO-66(Zr)green samples with different PW contents exhibit well-resolved crystal diffraction peaks, which are in accordance with the crystal structure of UiO-66(Zr). This result indicates that the addition of PW during the synthesis did not prevent the formation of the UiO-66(Zr) structure. Notably, two diffraction peaks appeared at 6.4 o and 28.5 o of 2 theta degree cannot match well with those of UiO-66(Zr) (Fig. S1). Considering that the peak intensity increased with the addition amount of PW, the two diffraction peaks should result from the PW species although the peak positions shift toward the region of low angle compared with those of PW. This suggests that possibly the PW species exist as aggregated nanoparticles and are encapsulated into the interior of the UiO-66(Zr) crystals and a mutual effect on their crystal structure should occur.

Comparatively, PW/UiO-66(Zr)-solvent displays only well-resolved diffraction peaks corresponding to the UiO-66(Zr) structure. This means that the particle size of the PW species in PW/UiO-66(Zr)-solvent seem to be small and their distribution in sample could be relatively homogeneous. Moreover, a mechanically mixed sample (PW+UiO-66(Zr)-green) gives the XRD peaks whose positions are in accordance with those of pure PW besides those of UiO-66(Zr), suggesting that there should be no mutual effect on the crystal structure of this sample.

The N2 sorption isotherms of all the PW/UiO-66(Zr)-green samples show abrupt increase in the region of low relative pressure (<0.01) and second jump with hysteresis loop at the relative pressure (0.4~0.9), suggesting that they should be hierarchical porous materials (Fig. S2 andS3). In comparison, PW/UiO-66(Zr)-solvent and PW+UiO-66(Zr)-green exhibit type I isotherm and no mesopore size distribution (Fig. 2a andb). The detailed sorption data are listed in Table 1. Obviously, the BET surface areas for the samples prepared by green method gradually decrease with the increase of PW addition amount. However, PW/UiO-66(Zr)-green can still possess high BET surface area (843 m 2 /g) when the content of PW in PW/UiO-66(Zr)-green was 14.8 wt%. Actually, the difference in BET surface area is small for PW/UiO-66(Zr)-green, PW/UiO-66(Zr)-solvent and PW+UiO-66(Zr)-green with similar PW content.

Differently, PW/UiO-66(Zr)-green (0.57 mL/g) has higher mesopore volume than PW/UiO-66(Zr)-solvent (0.13 mL/g), which should be related to synthetic method.

Under the condition of adding no solvent, the concentration of PW should be high and PW species are easy to aggregate into nanoclusters in the synthetic system of the composite materials. Necessarily, it would occupy some rooms and thus prevent the formation of the perfect crystalline structure of UiO-66(Zr). Because the size of PW nanoclusters (about 1 nm per PW molecule) is big, mesopores can be easily created in the structure of PW/UiO-66(Zr)-green.

FT-IR spectra of various samples are shown in Fig. 3 andS4. All the samples with the addition of PW exhibit an absorption band at 1081 cm -1 assigned to the asymmetric vibration of the P=O bond from PW and three absorption bands at 982, 891, and 802 cm -1 corresponding to the asymmetric vibrations of the terminal W=O, corner and edge share of W-O-W bonds, respectively [32][33][34] . Comparatively, those absorption bands are absent in the samples of UiO-66(Zr)-solvent and UiO-66(Zr)-green. These results suggest that PW should exist in the composite materials of PW and UiO-66(Zr).

The SEM results are shown in Fig. 4 and S5. Clearly, UiO-66(Zr)-green exhibits a flake-like morphology and is mainly composed of crystals with a size of 50×150 nm (Fig. 4a). With the addition of PW, the crystal tends to aggregate each other and the average particle size is larger than that of UiO-66(Zr)-green (Fig. 4b andS5).

Comparatively, UiO-66(Zr)-solvent shows a morphology of spherical shape and its average particle size is about 200 × 200 nm (Fig. 4c). Similarly, after the introduction of PW, the crystal size of PW/UiO-66(Zr)-solvent is larger than that of UiO-66(Zr)solvent (Fig. 4d). Moreover, Fig. 4e and4f showed the EDS patterns of PW/UiO-66(Zr)-green and PW/UiO-66(Zr)-solvent. The results proved the presence of W species in both samples.

To clearly know the location and distribution of PW in samples, TEM images are taken (Fig. 5). As seen in Fig. 5a, the distribution of the element W in PW/UiO-66(Zr)green is not homogeneous and even no element W is detected in some crystal particles.

The results from EDS element mapping can further demonstrate this point (Fig. 5b andS6). Comparatively, the element W in PW/UiO-66(Zr)-solvent is uniformly distributed (Fig. 5c,d and Fig. S7). Moreover, the particle size of PW in PW/UiO-66(Zr)-green is bigger than that in PW/UiO-66(Zr)-solvent. These results reveal that PW species in PW/UiO-66(Zr)-green tend to form nanoclusters.

The XPS results of various samples are shown in Fig. 6. As seen, PW/UiO-66(Zr)green and PW/UiO-66-solvent present the binding energies of element W, suggesting that PW should exist in the UiO-66(Zr) materials. The binding energies of Zr 3d5/2, O 1s, and C 1s for all the samples are almost the same (Fig. S8-10). However, the binding energy of W 4f is somewhat different (Fig. 6a). As for pure PW, two binding energies at 36.1 and 38.2 eV is ascribed to W 4f7/2 and W 4f5/2, respectively. As for the composite materials of PW and UiO-66(Zr), the two binding energies shift toward low values (Fig. 6b). This means that the electron density of W in these composites was possibly increased after PW species captured electrons, suggesting that the electron transfer between UiO-66(Zr) and PW species might occur [35][36][37] .

The quantitative analysis from XPS results (Table S1) shows that the surface chemical composition of UiO-66(Zr) are in accordance with the chemical structure (Zr6O4(OH)4(bdc)6) and the structure is maintained for PW/UiO-66(Zr)-green and PW+UiO-66-green catalysts. It also should be related to the loss of organic linker in PW/UiO-66(Zr)-green and PW+UiO-66(Zr)-green as demonstrated for pure UiO-66(Zr)-green MOF in our previous works [31] . An excess of carbon, which could be attributed to lower loss of organic linker, is observed for PW/UiO-66(Zr)-solvent. The nW/nZr atomic ratio shows that the surface PW content is close to the bulk content for PW/UiO-66-green, compatible with an insertion of PW in the UiO-66(Zr) MOF framework or very small PW crystallites. A depletion of PW is observed at the surface for the mechanical mixture PW+UiO-66-green and PW/UiO-66(Zr)-solvent. It is in accordance with the large PW crystallites in contact with the UiO-66(Zr) solids.

PW/UiO-66(Zr)-green and PW/UiO-66(Zr)-solvent were analyzed by ToF-SIMS, in order to compare the interaction between HPW and support depending on the preparation method. Indeed, ToF-SIMS probes the top layers (1-3 nm) of the catalysts and allows obtaining molecular information about surface and interfaces. Fragments recorded are listed in Supplementary Information. For both catalysts, fragments containing W and Zr were recorded, WZrO 4+ and WZr2O 6+ , as illustrated in Fig. S11, the simulation of its isotope pattern confirming their identification. Considering that they contain both Zr and W, their presence indicated a close interaction between HPW and the UiO-66(Zr). However, it appears clearly that these mixed fragments are much more numerous in the PW/UiO-66(Zr)-green sample, proving that the without solvent method promotes the interaction between HPW and the support. It can be beneficial for the lifetime of the catalyst, thus avoiding leaching during reaction

Catalytic evaluation

The catalytic performance of the PW/UiO-66(Zr)-green samples with different PW contents were firstly evaluated by ODS of DBT at room temperature. As shown in Fig. S12, the removal of DBT over PW/UiO-66(Zr)-green-1, PW/UiO-66(Zr)-green and PW/UiO-66(Zr)-green-2 reached 91.5 %, 99.7 % and 98.2 % after a reaction time of 25 min, respectively. Notably, PW/UiO-66(Zr)-green displays best ODS performance among the three samples, which might result from a suitable combination of PW content and mesoporosity in PW/UiO-66(Zr)-green. Surprisingly, it is inactive when pure PW is used as ODS catalyst at room temperature (The removal of about 55% DBT resulted from the extraction by acetonitrile). Similarly, PW/UiO-66(Zr)-solvent also displays poor reactivity at the same reaction conditions. Interestingly, PW+UiO-66(Zr)-green exhibited better ODS performance than PW/UiO-66(Zr)-solvent, suggesting that probably the synergistic effect of PW and defective UiO-66(Zr)-green occurred (Fig. 7). Furthermore, in order to clarify whether DBT was oxidized or absorbed on PW/UiO-66(Zr)-green, the components in oil phase and acetonitrile phase after reaction were analyzed by GC-MS. These results show that only dibenzothiophene sulfone (DBTO2) can be detected in acetonitrile phase, verifying that DBT has been really oxidized into DBTO2 over PW/UiO-66(Zr)-green (Fig. S13).

To make a comprehensive comparison, the catalytic activities of several representative POM/MOF composites as catalysts for DBT removal are given in Fig. 8 and some data are listed Table S2. Obviously, the TOF number over reported catalysts is below 50 h -1 even if high reaction temperature is used. However, the TOF number over PW/UiO-66(Zr)-green can reach 221 h -1 although the reaction runs at room temperature. To our best knowledge, this value is the highest among the reported ODS catalysts under the reaction condition of room temperature. Considering that PW/UiO-66(Zr)-green could be an excellent ODS catalyst, the effect of some factors on its catalytic performance was investigated as follows.

Effect of reaction temperature

The effect of reaction temperature on the catalytic performance of PW/UiO-66(Zr)green is shown in Fig. 9a. Obviously, the ODS activity increases with reaction temperature. When the reaction temperature is increased to 50 °C, the removal of DBT reaches 99.8% after a reaction time of 10 min. The reaction rate constants at different temperatures are calculated according to the pseudo-first-order kinetics: ln (C0/Ct) = kt and ln k = -Ea/RT, where C0 and Ct represent the DBT concentration at the initial and t time, while k and Ea represent the rate constant and the activation energy, respectively.

The linear plots of ln(C0/Ct) versus reaction time are shown in Fig. S14 and the data are listed in Table S3. After calculation by Arrhenius equation, the apparent activation energy over PW/UiO-66(Zr)-green is 30.5 kJ/mol (Fig. 9b). The value is low compared with the data from the literatures [38][39][40] .

Effect of O/S molar ratio and DBT concentration

The effect of O/S molar ratio on the catalytic performance of PW/UiO-66(Zr)green is shown in Fig. 10a. Obviously, the ODS activity of PW/UiO-66(Zr)-green decreases with the O/S molar ratio. When the O/S molar ratio is 2:1, the removal of DBT can reach 80% after a reaction time of 30 min. Meanwhile, the oxidant efficiency increases with O/S molar ratio to 88.1%. Additionally, the effect of DBT concentration is also investigated. As shown in Fig. 10b, the removal of DBT is completed within 20 min when the concentration of DBT is 500 ppm.

When the concentration of DBT is increased to 1500 ppm, relatively long reaction time is needed for the complete removal of DBT. However, PW/UiO-66(Zr)-green is still active. The removal of DBT may reach 99.7% after a reaction time of 40 min at room temperature.

Effect of sulfur substrate

The effect of sulfur substrate on the catalytic performance of PW/UiO-66(Zr)-green is studied by the ODS reactions of BT, DBT, and 4,6-DMDBT. As seen in Fig. 11, the removal of 4,6-DMDBT over PW/UiO-66(Zr)-green is 99.8 % after a reaction time of 25 min, suggesting that PW/UiO-66(Zr)-green has great potential in deep ODS of fuel oil. BT was the most difficult to oxidize, with 81 % conversion at 25 min, in agreement with the lower electron density on the sulfur atom (5.739) compared to DBT and 4,6-DMDBT (5.758 and 5.760 respectively) [29,39] .

Reusability and structural stability of catalyst

The reusability of PW/UiO-66(Zr)-green is also evaluated by the ODS reaction of DBT. After each cycle, the catalyst is recovered by centrifugation, washing with acetonitrile, and reused in the next cycle under the same reaction conditions. As shown in Fig. 12, PW/UiO-66(Zr)-green could still maintain high catalytic ODS activity after four cycles. The results indicate that PW/UiO-66(Zr)-green is a heterogeneous catalyst and can be reused. Additionally, the structural stability of PW/UiO-66(Zr)-green after reuse is also assessed by XRD and N2 absorption techniques. As seen in Fig. S15, the XRD patterns of the reused sample are in accordance with those of the fresh one.

Moreover, the reused PW/UiO-66(Zr)-green displays similar N2 sorption isotherms (BET surface area of 833 m 2 /g) to the fresh one (843 m 2 /g) (Fig. S16), proving that the structure of PW/UiO-66(Zr)-green is stable after reuse.

Proposed reaction mechanism

To explore the reaction mechanism, quenching experiments were conducted.

Generally, HO • or O2 •-is considered to be catalytic active species in the reaction. It is known that tertiary butanol or p-benzoquinone is a good quencher of HO • or O2 •-, respectively [41][42][43] . As shown in Fig. 13, when PW/UiO-66(Zr)-green is removed from the reaction system after a reaction time of 10 min, the removal of DBT remained constant after a reaction time of 25 min. With the addition of tertiary butanol, the removal content of DBT is slightly reduced, indicating that HO • should exist in the catalytic system. Moreover, the addition of p-benzoquinone decreases the removal of DBT from 99.8 % to 96.3 % after the same reaction time, suggesting that O2 •-should also play a positive role in catalytic performance. Furthermore, electron paramagnetic resonance (EPR) spin-trap method was utilized to probe the formation of radicals with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as traps. As seen in Fig. 14, the strong characteristic four peaks of DMPO-O2 •-signals with relative intensities of about 1:1:1:1 were observed in the mixture of PW/UiO-66(Zr)-green (5 mg), H2O2 (0.5 mL), and DMPO (50 L), confirming the formation of O2 •-radicals. However, it is difficult to know if the HO • radicals were generated because of EPR signals of DMPO-O2 •-and DMPO-HO • overlap. Interestingly, the addition of both tertiary butanol and pbenzoquinone can greatly inhibit the further removal of DBT, demonstrating that both HO • and O2 •-radicals should be generated in the ODS reaction system. Based on the above experimental results, a possible reaction mechanism was proposed over PW/UiO-66(Zr)-green (Scheme 2). The DBT molecule in model oil was extracted into acetonitrile phase with the catalyst and H2O2. Then, DBT and H2O2 can be adsorbed in the pores of PW/UiO-66(Zr)-green. It would be helpful for promoting the interaction between substrate, catalyst and oxidants. Meanwhile, partially adsorbed H2O2 decomposed into HO • radicals through electron transfer by the Zr-OH2 sites in UiO-66(Zr) [29,44] . Part of H2O2 could interact with the W sites in PW species into W(VI)-peroxo and then form O2 •-radicals. Both radicals can oxidize DBT into DBTO2 [14, 45-47] . Therefore, the existence of two types of active centers from PW species and defective UiO-66(Zr) plays a vital role in the formation of radicals, which should be responsible for the highly efficient ODS performance at room temperature.

In addition, the accessibility of active sites is also important. As shown in Figure 7, PW/UiO-66(Zr)-solvent is not active at room temperature in the ODS reaction of DBT although PW species are homogeneously distributed in the sample, which should be related to the accessibility of active sites. As seen from N2 sorption results, PW/UiO-66(Zr)-solvent had much lower pore volume than PW/UiO-66(Zr)-green and did not exhibit mesopore distribution. This means that it is difficult for sulfur compounds to access the active PW species in PW/UiO-66(Zr)-solvent due to the limitation of micropore size and the lack of mesoporosity. This could be main reason why PW/UiO-66(Zr)-solvent showed poorer ODS reactivity than PW/UiO-66(Zr)-green.

Conclusions

In summary, the composite materials of PW and UiO-66(Zr) have successfully been prepared without the addition of any solvent. The results from XRD and TEM indicate that PW species in PW/UiO-66(Zr)-green mainly consisted of nanoclusters. Nitrogen sorption measurement revealed that the obtained composite materials possessed hierarchical porosity with high mesopore volume. As ODS catalysts, they exhibited extraordinary catalytic performance in the ODS reactions of DBT and 4,6-DMDBT at room temperature. The TOF number for the removal of DBT reached 221.4 h -1 , which is much higher than that of the reported catalysts. Such outstanding catalytic performance could be mainly attributed to easy accessibility of active sites and the existence of two types of active centers from PW species and defective UiO-66(Zr).

Due to the simplification and environmental friendliness of synthetic processes, it will be of significance for potential applications.

Captions to

Figures Scheme 1 .Scheme 2 .

 12 Scheme 1. Diagram about the synthetic processes of PW/UiO-66(Zr)-green.

Figure 1 .

 1 Figure 1. XRD patterns of various samples.

Figure 2 .

 2 Figure 2. (a) N2 sorption isotherms and (b) pore size distribution curves of various samples.

Figure 3 .

 3 Figure 3. FT-IR spectra of (a) UiO-66(Zr)-green, (b) PW/UiO-66(Zr)-green, (c) UiO-66(Zr)-solvent, (d) PW/UiO-66(Zr)-solvent, (e) PW.

Figure 4 .

 4 Figure 4. SEM images of (a) UiO-66(Zr)-green, (d) PW/UiO-66(Zr)-green, (c) UiO-66(Zr)-solvent, (d) PW/UiO-66(Zr)-solvent, and EDS patterns of PW/UiO-66(Zr)green (e) and PW/UiO-66(Zr)-solvent (f).

Figure 5 .

 5 Figure 5. TEM images of (a) and (b) PW/UiO-66(Zr)-green, (d) and (e) PW/UiO-66(Zr)-solvent, and the corresponding EDS mapping of PW/UiO-66(Zr)-green (c) and PW/UiO-66(Zr)-solvent (f).

Figure 6 .

 6 Figure 6. XPS spectra of various samples: (a) survey spectrum and (b) W 4f.

Figure 7 .

 7 Figure 7. Catalytic activities of different catalysts with reaction time in ODS of DBT at room temperature.

Figure 8 .

 8 Figure 8. The TOF number over various catalysts in the ODS reaction of DBT.

Figure 9 .

 9 Figure 9. Effect of reaction temperature on catalytic performance of PW/UiO-66(Zr)green and arrhenius plot.

Figure 10 .

 10 Figure 10. Effect of different O/S molar ratio (a), and DBT content (b) on catalytic performance of PW/UiO-66(Zr)-green.

Figure 11 .

 11 Figure 11. Effect of sulfur substrate on catalytic performance of PW/UiO-66(Zr)-green.

Figure 12 .

 12 Figure 12. Reusability of PW/UiO-66(Zr)-green in the ODS reaction of DBT.

Figure 13 .

 13 Figure 13. Leaching test and quenching experiments in the ODS reaction of DBT over PW/UiO-66(Zr)-green.

Figure 14 .

 14 Figure 14. EPR spectrum of 5 mg PW/UiO-66(Zr)-green, 500 L CHP and 50 L DMPO.

  Figure 1

Table 1 .

 1 Textural properties of various samples.

	Samples	W content (wt.% ) a	BET surface area (m 2 /g)	Micropore volume (mL/g) b	Mesopore volume (mL/g) c
	UiO-66(Zr)-green	--	1131	0.48	0.46
	PW/UiO-66(Zr)-green-1	11.9	991	0.44	1.01
	PW/UiO-66(Zr)-green	14.8	843	0.35	0.57
	PW/UiO-66(Zr)-green-2	17.9	693	0.30	0.62
	UiO-66(Zr)-solvent	--	989	0.43	0.19
	PW/UiO-66(Zr)-solvent	14.4	746	0.32	0.13
	PW+UiO-66(Zr)-green	15.0	873	0.39	0.40

a Measured by EDS; b HK method; c BJH method.

Acknowledgments: The authors acknowledge the financial support from Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education, and Open Funding from the State Key Lab of Inorganic Synthesis and Preparative Chemistry, Jilin University. Chevreul institute (FR 2638), Ministère de l'Enseignement Supérieur et de la Recherche and Région Hauts-de-France are acknowledged for supporting this work.