Is it permissible to combine the Dhuhr and Asr prayers with an early combination (jam' taqdim), and the Maghrib and Isha prayers with a delayed combination (jam' ta'khir), due to the difficulty of performing ablution and finding a place to pray at work?

We do not know the nature of your work, but performing prayer does not require effort or time that would constrain work hours. It is obligatory for you to strive to perform what Allah has enjoined upon you, and Allah has made our religion easy for us. It is not a condition that prayer be performed in mosques, for the Prophet, peace and blessings be upon him, said: "The earth has been made for me a place of prostration and purification." If you do not find water for istinja’ (washing after relieving oneself), istijmar (using stones or other purifying objects) suffices you, and for wudu’ (ablution), you can use handwashing areas. If you are completely unable to find water, you may perform tayammum (dry ablution), as Allah Almighty said: (and you find no water, then perform tayammum with pure earth). Combining prayers is not permitted except out of dire necessity.